Handbook of Functionalized Nanostructured MXenes: Synthetic Strategies and Applications from Energy to Environment Sustainability 981992037X, 9789819920372

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Smart Nanomaterials Technology

Komal Rizwan · Anish Khan · Abdullah Mohammed Ahmed Asiri Editors

Handbook of Functionalized Nanostructured MXenes Synthetic Strategies and Applications from Energy to Environment Sustainability

Smart Nanomaterials Technology Series Editors Azamal Husen , Wolaita Sodo University, Wolaita, Ethiopia Mohammad Jawaid, Laboratory of Biocomposite Technology, Universiti Putra Malaysia, INTROP, Serdang, Selangor, Malaysia

Nanotechnology is a rapidly growing scientific field and has attracted a great interest over the last few years because of its abundant applications in different fields like biology, physics and chemistry. This science deals with the production of minute particles called nanomaterials having dimensions between 1 and 100 nm which may serve as building blocks for various physical and biological systems. On the other hand, there is the class of smart materials where the material that can stimuli by external factors and results a new kind of functional properties. The combination of these two classes forms a new class of smart nanomaterials, which produces unique functional material properties and a great opportunity to larger span of application. Smart nanomaterials have been employed by researchers to use it effectively in agricultural production, soil improvement, disease management, energy and environment, medical science, pharmaceuticals, engineering, food, animal husbandry and forestry sectors. This book series in Smart Nanomaterials Technology aims to comprehensively cover topics in the fabrication, synthesis and application of these materials for applications in the following fields: • Energy Systems—Renewable energy, energy storage (supercapacitors and electrochemical cells), hydrogen storage, photocatalytic water splitting for hydrogen production • Biomedical—controlled release of drugs, treatment of various diseases, biosensors, • Agricultural—agricultural production, soil improvement, disease management, animal feed, egg, milk and meat production/processing, • Forestry—wood preservation, protection, disease management • Environment—wastewater treatment, separation of hazardous contaminants from wastewater, indoor air filters.

Komal Rizwan · Anish Khan · Abdullah Mohammed Ahmed Asiri Editors

Handbook of Functionalized Nanostructured MXenes Synthetic Strategies and Applications from Energy to Environment Sustainability

Editors Komal Rizwan Department of Chemistry University of Sahiwal Sahiwal, Pakistan

Anish Khan Center of Excellence for Advanced Materials Research King Abdulaziz University Jeddah, Saudi Arabia

Abdullah Mohammed Ahmed Asiri Center of Excellence for Advanced Materials Research King Abdulaziz University Jeddah, Saudi Arabia Chemistry Department Faculty of Science King Abdulaziz University Jeddah, Saudi Arabia

Smart Nanomaterials Technology ISBN 978-981-99-2037-2 ISBN 978-981-99-2038-9 (eBook) https://doi.org/10.1007/978-981-99-2038-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 This work is subject to copyright. All rights are solely and exclusively licensed by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors, and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, expressed or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

During the last decades, 2-D nanomaterials; MXenes have been developed on a large scale. MXenes contain a novel combination of properties including great conductivity and mechanical, thermal features of transition metal carbide and nitrides; functionalized surfaces that endow MXenes hydrophilic characteristics and prepare them to bond with different species and its high zeta potential enable stable solution in aqueous media and they can absorb electromagnetic radiations effectively which made them versatile materials for various applications. They possess high surface area, novel morphology, and layered structure. Exposure of MXenes to surface functional groups (chemical functionalization) promote them as potential candidates for diversified applications such as electromagnetic shielding, energy, energy storage devices (super-capacitors, lithium-ion batteries, CO2 capture, optical switching, transistors), photocatalysis, drug delivery, implants, tissue engineering, water purification, and sensing applications. These MXene-based advanced architectures promote continuous innovations and provide a driving force in different fields particularly in environmental remediation and energy storage devices. During the last decade the MXenes have been synthesized largely and they have also been modified postsynthetically. Applications of MXene-based composites have been reported widely in different fields of life and this has helped scientists to get more insights into the fabrication and applications of these molecules. This book focuses on various aspects of MXenes such as their synthesis, structural analysis, functionalizations, and emerging trends in environmental and energy applications. Chapters Introduction to MXenes–Structural Design and Synthesis of Elemental Doped MXenes and MXenes-Based Composites focus on the introduction of MXenes, their structural design, properties, and synthesis including various etching and exfoliation methods has been discussed in detail. Hybrids of MXenes possess potentially enhanced properties in comparison to pristine MXenes due to coupling partner. Vacant d-orbital of early transition metals is advantageous, as skeletons of MXenes show great affinity to electron donors. Organic compounds with strong electron releasing moieties are incorporated, and original T groups could be replaced. Many organic/inorganic molecules such as polymeric materials, chalcones, carbon nanotubes, and metal oxides have been covalently bonded to MXenes v

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so far. Chapter Functionalized MXene-Based Polymer Composites presents the details of the fabrication of MXene-based polymeric composites, where structural features of MXene-based polymeric composites have been discussed. Furthermore, applications of these composites in various domains are discussed highlighting their promising potential in research and providing a vision of their future applications. Conductive hydrogels are receiving a lot of attention in the field of flexible and wearable soft strain sensors due to their great potential in electronic skins and customized healthcare monitoring. MXenes possess great mechanical characteristics, elasticity, and robust adhesive nature which made them eligible for hydrogel formation. MXenes stability which is frequently a limiting issue in many MXenebased applications may be greatly increased by the synthesis of MXenes into hydrogels. Chapter Fabrication and Structural Design of MXene-Based Hydrogels is focused on the fabrication and structural design of MXene-based hydrogels and their potential applications. MXenes possess great electrical properties due to which they have been widely examined in energy storage applications. Crystallinity, thickness, and layered structural features of MXenes provide great surface area with less energy barrier for transportation of electrons. Since the world is moving toward digitalization, the requirement for energy storage devices has increased to double in a short time. Chapters Emerging Trends of MXenes in Supercapacitors–Potential of MXenes in Photocatalysis provide the detailed role of MXenes in energy applications. The significant applications of MXenes in super-capacitors, lithium, sodium, lithium-sulfur ion batteries, hydrogen storage, energy storage, and photocatalysis have been presented in detail in this book. The environment around the globe has been polluted due to industrialization during the last decades. The emission of various organic and inorganic harmful toxic pollutants in waterbodies is destroying our underground water sources. There is a need to address this issue at immediate basis by designing new eco-friendly materials. MXenes and its composites have great potential to detect and remove toxic pollutants from the environment. Chapters Efficacy of MXene-Based Materials in the Removal of Gases – Potential Mitigation of Dyes Through Mxene Composites describe the photocatalytic and adsorptive potential of MXene and its composites for the removal of various environmental pollutants such as dyes, heavy metals, pharmaceuticals, radionuclides, and gases from various matrices. Chapters Potential Mitigation of Dyes Through Mxene Composites–Miscellaneous Applications of Other Mxene-Based Sensors present the role of MXene and its composites in sensing applications. Chapter Toxicology, Stability, and Environmental Impacts of MXenes and Its Composites is potentially presenting the toxic effects, associated stability problems, and environmental effects of MXene-based composites. A synoptic review of associated challenges and future perspectives has been presented in Chap. Challenges and Future Perspectives of Mxenes. Editors have tried their level best to deliver the comprehensive role of MXenes as energy tools and environmental applications for sensing and removal of toxic pollutants from the environment. This book provides a deeper look into tools, tricks, and challenges associated with techniques of fabrication of MXenes. Chapters incorporate fundamentals of different processes and this shall give a broad spectrum and

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inter-disciplinary audience of different academic and research institutions with the latest development and ideas on MXenes. This book is essential reading for all chemists, biologists, physicists, and environmental scientists working in the field of nanotechnology, energy, and environmental chemistry. It would help academics and professionals to polish their knowledge with the latest described data. It will also help the professionals for developing innovative technologies by keeping in mind the applications of functionalized nanostructured MXenes. We gratefully acknowledge all the authors for their contributions to the book. Sahiwal, Pakistan Jeddah, Saudi Arabia Jeddah, Saudi Arabia

Dr. Komal Rizwan Dr. Anish Khan Prof. Dr. Abdullah Mohammed Ahmed Asiri

Contents

Introduction to MXenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Zaeem Bin Babar, Esmat Sodagar, Komal Rizwan, and Gulshan Sikandar

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Structural Design, Properties, and Synthesis of Original MXenes . . . . . . Rabia Akhtar, Ameer Fawad Zahoor, Asim Mansha, Syed Makhdoom Hussain, Sajjad Ahmad, and Tahir Maqbool

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Structural Design and Synthesis of Elemental Doped MXenes and MXenes-Based Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Javeria Shoukat, Anila, Aqsa Iqbal, Muhammad Saleem Ashiq, Ataf Ali Altaf, and Samia Kausar

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Functionalized MXene-Based Polymer Composites . . . . . . . . . . . . . . . . . . . Umer Raza, Hafiz Abdul Mannan, Atif Islam, Tabinda Riaz, and Sidra Saleemi

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Fabrication and Structural Design of MXene-Based Hydrogels . . . . . . . . Asif Manzoor, Faisal Jamil, Abbas Washeel Salman, Farrukh Aslam Khalid, Umar Sohail Shoukat, and Muhammad Adnan Iqbal

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Emerging Trends of MXenes in Supercapacitors . . . . . . . . . . . . . . . . . . . . . . Memoona Qammar, Adeel Zia, and Omair Adil

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Recent Advancements in MXene-Based Lithium-Ion Batteries . . . . . . . . . Fozia Maqsood, Faisal Jamil, Umar Sohail Shoukat, and Muhammad Adnan Iqbal

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MXene-Based Sodium-Ion Batteries . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 Rabia Akhtar, Ameer Fawad Zahoor, Matloob Ahmad, Tanveer Hussain Bokhari, and Muhammad Naveed Anjum Design and Applications of MXene-Based Li–S Batteries . . . . . . . . . . . . . . 137 Saba Munawar, Ameer Fawad Zahoor, Muhammad Irfan, Sadia Javed, and Atta ul Haq ix

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Nanostructured MXenes for Hydrogen Storage and Energy Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 Sohaib Ahmad, Hafiz Abdul Mannan, Atif Islam, Rizwan Nasir, and Danial Qadir Diverse Applications of MXene Composites for Electrochemical Energy Storage . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 Muhammad Saleem Ashiq, Aqsa Iqbal, Javeria Shoukat, Anila, Samia Kausar, Komal Rizwan, and Ataf Ali Altaf Potential of MXenes in Photocatalysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 Muhammad Saeed, Nadia Akram, Akbar Ali, and Syed Ali Raza Naqvi Efficacy of MXene-Based Materials in the Removal of Gases . . . . . . . . . . 207 Zaeem Bin Babar, Nameer Urfi, Saeed ur Rehman, and Komal Rizwan Environmental Remediation of Heavy Metals Through MXene Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 229 Madeeha Batool and Hafiz Muhammad Junaid Advanced Approach of MXene-Based Materials in Removal of Radionuclides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 249 Abdul Rauf, Mashhood Urfi, Zaeem Bin Babar, Saeed ur Rehman, Shahid Munir, and Komal Rizwan Functionalized Mxene Conjugates in Removal of Pharmaceuticals and Other Pollutants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 Muhammad Saad Fasih, Shahzad Maqsood Khan, Saba Zia, Nafisa Gull, and Tanveer A. Tabish Potential Mitigation of Dyes Through Mxene Composites . . . . . . . . . . . . . 283 Jamil A. Buledi, Ali Hyder, Nadir H. Khand, Saba A. Memon, Madeeha Batool, and Amber R. Solangi MXene-Based Polymeric Nanocomposites for Pressure/Strain Sensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Ahmad Shakeel, Komal Rizwan, and Ujala Farooq Biosensing Applications of MXene-Based Composites . . . . . . . . . . . . . . . . . 325 Ayesha Sharif, Shahzad Maqsood Khan, Tanveer A. Tabish, Nafisa Gull, and Saba Zia Miscellaneous Applications of Other Mxene-Based Sensors . . . . . . . . . . . . 345 Fahd Jamshaid, Atif Islam, Abdul Mannan, Abdul Moqeet Hai, Nafisa Gull, Shumaila Fayyaz, and Rafi Ullah Khan

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Toxicology, Stability, and Environmental Impacts of MXenes and Its Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 357 Shumaila Fayyaz, Asma Khalid, Saba Urooge Khan, Atif Islam, Abdul Mannan, Saba Zia, Shahzad Maqsood Khan, and Rafi Ullah Khan Challenges and Future Perspectives of Mxenes . . . . . . . . . . . . . . . . . . . . . . . 377 Nafisa Gull, Atif Islam, Abdul Mannan, Tabinda Riaz, Asma Khalid, Shahzad Maqsood Khan, and Rafi Ullah Khan

About the Editors

Dr. Komal Rizwan is Assistant Professor in Chemistry Department—Faculty of Science University of Sahiwal, Pakistan. She earned Ph.D. in chemistry from Government College University Faisalabad, Pakistan in 2016 and also carried out Ph.D. research work at University of Pennsylvania, Philadelphia, United States. Currently, She is working as postdoctoral fellow at Department of Biochemistry and Biophysics, Perelman School of Medicine, University of Pennsylvania, Philadelphia, USA. Dr. Komal Rizwan is author of more than 100 research publications printed in world’s leading scientific societies and she is also serving as corresponding editor of many books of Springer and Elsevier. Her research group is interested to synthesize molecules that possess interesting functions including biological activity (natural products, drugs, drug-like compounds, etc.) or materials with useful properties, where she develop tools to perform efficient synthetic transformations in the creation of novel molecular architectures of medicinal or industrial relevance. In a nutshell, her interests are broadly in the area of synthetic organic chemistry, with a particular focus in the synthesis of bioactive natural products and novel molecular architectures (nanomaterials). Dr. Rizwan’s research group is also focusing on synthesis of nonlinear optical compounds, and computational part of her laboratory contributes to a fundamental understanding of the electronic structure and properties of nonlinear optical materials that are being considered in the design of electro-optics. Dr. Anish Khan is currently working as Assistant Professor, Chemistry Department, Centre of Excellence for Advanced Materials Research (CEAMR), Faculty of Science, King Abdulaziz University, Jeddah, Saudi Arabia, and has Ph.D. from Aligarh Muslim University, India, from 2010. He has a research experience of working in the field of synthetic polymers and organic–inorganic electrically conducting nano-composites. He completed Postdoctoral from School of Chemical Sciences, University Sains Malaysia (USM), in electroanalytical chemistry in 2010–2011. He has research and teaching experience of more than 250 research articles published in refereed international journal, more than 20 international conferences/workshop and 40 books published and 20 in progress and 70 chapters. He has around 20 research projects completed. He is Managerial Editor of Chemical and xiii

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Environmental Research (CER) Journal and Member of American Nano Society. His field of specialization is polymer nanocomposite/cation-exchanger/chemical sensor/microbiosensor/nanotechnology, application of nanomaterials in electroanalytical chemistry, material chemistry, ion-exchange chromatography, and electroanalytical chemistry, dealing with the synthesis, characterization (using different analytical techniques), and derivatization of inorganic ion-exchanger by the incorporation of electrically conducting polymers, preparation and characterization of hybrid nanocomposite materials and their applications, polymeric inorganic cationexchange materials, electrically conducting polymeric, materials, composite material use as sensors, green chemistry by remediation of pollution, heavy metal ion selective membrane electrode, and biosensor on neurotransmitter. Prof. Dr. Abdullah Mohammed Ahmed Asiri is Professor in Chemistry Department—Faculty of Science—King Abdulaziz University. He has Ph.D. (1995) from University of Wales, College of Cardiff, UK, on tribochromic compounds and their applications. He is Chairman of the Chemistry Department, King Abdulaziz University, currently and also Director of the Center of Excellence for Advanced Materials Research. He is Director of Education Affair Unit–Deanship of Community services and Member of Advisory Committee for advancing materials (National Technology Plan, King Abdul Aziz City of Science and Technology, Riyadh, Saudi Arabia). His area of interest includes color chemistry, synthesis of novel photochromic and thermochromic systems, synthesis of novel colorants and coloration of textiles and plastics, molecular modeling, applications of organic materials into optics such as OEDS, high performance organic dyes and pigments, new applications of organic photochromic compounds in new novelty, organic synthesis of heterocyclic compounds as precursor for dyes, synthesis of polymers functionalized with organic dyes, preparation of some coating formulations for different applications, and photodynamic thereby using organic dyes and Pigments Virtual Labs and Experimental Simulations. He is Member of Editorial Board of Journal of Saudi Chemical Society, Journal of King Abdul Aziz University, Pigment and Resin Technology Journal, Organic Chemistry Insights, Libertas Academica, Recent Patents on Materials Science, and Bentham Science Publishers Ltd. Besides that he has professional membership of International and National Society and Professional bodies.

Introduction to MXenes Zaeem Bin Babar, Esmat Sodagar, Komal Rizwan, and Gulshan Sikandar

Abstract MXenes are 2D transition metals carbides and nitrides. Owing to its extraordinary natural physicochemical characteristics (i.e., hydrophilic nature, variety of chemical functionalities, high ion-exchange capability, etc.), Mxenes possess vast applications in the research areas of adsorptive treatment of wastewater and membrane modifications. Therefore, they are considered as an appropriate material for the remediation of the environment. This chapter presents a detailed discussion of the structural characteristics and synthetic procedures for manufacturing of MXenes and their applicability to remove hazardous radioactive metals such as uranium and thorium, and dyes. The primary mechanisms for treating various elements and dyes have been considerably discussed. The incorporation of MXenes in membranes and their consequent augmented performance has been also explained. This chapter will aid in understanding prominent synthetic techniques for MXenes manufacturing and its subsequent utilization for treating hazardous pollutants. Furthermore, the future directions for MXenes synthesis and utility for the removal of contaminants have also been discussed. Keywords MXenes · Applications · Environment · Radionuclides · Dyes · Bioimaging · Sensing

Z. B. Babar · G. Sikandar Institute of Environmental Sciences and Engineering (IESE), School of Civil and Environmental Engineering (SCEE), National University of Sciences and Technology (NUST), Islamabad, Pakistan E. Sodagar Herman Ostrow School of Dentistry, University of Southern California, 925 W. 34th St., Los Angeles, CA, United States K. Rizwan (B) Department of Chemistry, University of Sahiwal, Sahiwal 57000, Pakistan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_1

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1 Introduction Variety of raw materials are utilized in industrial processes. Consequently, these materials enter the ecosystem in various ways (leakages, purge streams, waste effluent streams, etc.) after processing. Owing to the toxic effects of these materials, they severely affect human health and environment. To eliminate or limit their hazardous effects on the ecosystem, environmental laws and policies are designed and enforced to treat these waste streams using various scientific methods. In the last decade, a wide range of techniques are available including physical, chemical, and biological processes for their effective treatment (Rizwan et al. 2022a, b, c, d , e, f and g). Conventional techniques have been modified specifically in terms of utilizing efficient and novel materials. In order to overcome the drawbacks of conventional materials, MXenes have been fabricated. Furthermore, composites of MXenes were synthesized by combining with various polymeric materials, metal oxides and dichalcogenides (Zhan et al. 2020). MXenes, a class of two-dimensional (2D) composite materials based on transition metal carbides, carbonitrides, and nitrides, have generally attracted attention since 2011. It now contains around 30 compositions made of stacked transition metal carbides, carbonitrides, and nitrides that were synthesized from MAX phases. (Naguib et al. 2011; Verger et al. 2019). MXenes are a large family of compounds having the formula Mn + 1AXn, where n is a transition d-metal with a number between 1 and 3 (Sc, Y, Ti, Zr, Hf, V, Nb, Ta, Cr, Mo, or W), and A is a p-element (Al, S Si, etc.) X is either carbon or nitrogen, and A is an A-group element (usually IIIA and IVA, or groups 13 and 14). The crystalline structure of such phases, which is produced by the alternating of nanoscale layers of p-elements and d-metals, is one of their major characteristics. (Barsoum 2000; Sokol et al. 2019). MXenes offer unique properties as compared to their traditional 2D counterparts. It has electric conductivity equivalent to 2 × 105 S m−1 , comparable to that of graphene molecule. Also, it has enhanced surface characteristics including ability to be sufficiently functionalized and easy dispersion in water. Furthermore, it is conductive electrochemically which makes it very effective as an energy storing material (Ng et al. 2017). Due to these substantial electrochemical properties, it facilitates quick ion transportation within a certain framework and redox reactions for storing charge. Moreover, its extraordinary mechanical characteristics at nanoscale enable them to be utilized in large-sized lithium batteries (Liu et al. 2014). Furthermore, MXenes show excellent qualities such as stable chemical nature, high conductance, and eco-friendly behavior. They can be effectively used for treating environmental contaminants such as ions and molecules via adsorption. Their good adsorptive property is associated with hydrophobic characteristics and active functionalities located on its surface (Barsoum 2000). Despite all the advantages and superior physiochemical properties, MXenes possess certain drawbacks. Therefore, consistent research work is being done to overcome their potential disadvantages (Zhan et al. 2020). This

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chapter includes an in-depth analysis of synthetic procedures for MXenes manufacturing and their potential applicability in various domains of research and development such as sensing, tissue engineering, environmental remediation, drug delivery, and environmental remediation.

2 Synthesis of 2D MXenes Approximately 20 MXenes have been made so far via selective chemical etching in which agents were utilized to separate nitrides and carbides into several atomic layers. Aqueous salts containing fluorine are a common kind of etchant (Rizwan et al. 2022a, b, c, d , e, f and g). Initially, MXenes were separated from MAX constituents by soaking them in acidic solutions to destroy the M-A bonding. Absolute mixing and corrosion time are the two key elements in this procedure. The two primary ways utilized to build 2D Mxenes are bottom-up and top-down approaches. Topdown methods exfoliate huge crystal amounts into single-layered MXene sheets, whereas bottom-up methods produce MXenes from atoms or molecules. Bottom-up methods, sometimes known as alternative MXenes synthesis routes, have recently been created. Among them are plasma-enhanced pulsed laser deposition (PEPLD), the template approach, and chemical vapor deposition (CVD). MXenes produced using the bottom-up method are of greater quality than those produced using the top-down method. Additionally, bottom-up techniques may be used to create 2D nitrides and carbides of transition metals, such as WC, TaC, and TaN, and certain heterostructures, with stoichiometry that cannot be achieved by selective etching. It is important to recognize that bottom-up techniques have never generated singlelayer structures, but only ultrathin films with several layers. In recent years, topdown synthesis technique has been actively used to enable the transition of MXene production from laboratory to industrial production. This is in addition to the rapid advancement of numerous techniques for producing MXenes in scientific laboratories (Rasheed et al. 2022). These MXenes are more enticing in terms of their real biological and environmental effects, particularly when applying successful selective wet etching procedures. The A layers in MAX react with fluoride ions during chemical etching to generate AF. During this reaction, H2 gas is also liberated. In this way, layers of MXenes are exfoliated. In addition, the chemical functionalities such as –O, –F, and –OH are replaced with other atoms. This weakens the interaction between layers of Mn+1 Xn layers to generate graphite-like structural arrangements known as MXenes (Zhang et al. 2017). Furthermore, due to the reaction mentioned above, several surficial chemical functionalities are generated which imparts negative charge on its surface and can facilitate colloidal distribution (Naguib et al. 2016). MXenes in this form are called terminated MXenes and they are thermodynamically stable in comparison with their counterparts in pristine forms. Ren and colleagues treated Ti3 AlC2 using hydrofluoric acid (i.e., selective etching) (Ren et al. 2015). Among various MXenes, Ti3 C2 Tx is a well-known acquired after a selective etching process from different MAX systems. In addition, MXenes such as MO2 C, Ti2 C,

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Ti3 SiC2 , and Cr2 TiC2 are other examples synthesized via etching with hydrofluoric acid. Furthermore, the stable nitride MXenes produced from their related MAX systems via hydrofluoric acid etching are catchy owing to lower cohesive energy in their geometrical framework (Yorulmaz et al. 2016). Despite the benefits linked with the hydrofluoric acid-based etching, the noxious effect of utilizing hydrofluoric acid limits the further developments in such a method. However, this further paves a way for the utilization of other less hazardous etching agents (Khazaei et al. 2015). Considering this, Aluminum was etched using Ti3 AlC2 via a convenient single step etching process by utilizing less hazardous hydrochloric acid and lithium fluoride. In this process, MXenes were synthesized by incalating H2 O molecules with positive ions which further improved the gap between the layers, minimal crystal faults, and substantial yield (Peng et al. 2019; Zhang et al. 2020). On the other hand, the hydrothermal method is successful to produce 2D MXenes constituents with multiple patterns which also avoids the utilization of hydrofluoric acid. For instance, Cai and coworkers leached aluminum from MAX system via treating Ti3 AlC2 in NaOH solution at 85 °C for 100 h. Then, hydrothermal treatment was performed using 1 M H2 SO4 at 85 °C for 1.5 h (Cai et al. 2018). This method also avoids the corrosion of Ti3 AlC2 at the anode by providing fluoride free environment. Another method involves dispersing Ti3 AlC2 powder in an ammonium fluoride solution to create Ti3 C2 Tx, which is then treated hydrothermally (Wu et al. 2012). Also, Guo et al. (2017) avoided HF-etching during the synthesis of Nb2 C and Ti3 C2 via hydrothermal etching using hydrochloric acid and sodium boro-fluoride (Guo et al. 2017).

3 Application of MXenes MXenes have an increasing number of uses, and they are currently seen as potential solutions in a variety of areas, including optics, the manufacturing and energy sectors, biomedicine, and environmental remediation (Anasori and Gogotsi 2019; Soleymaniha et al. 2019; Szuplewska et al. 2020; Wang et al. 2020; Wang et al. 2019; Zhang et al. 2019). The remarkable properties of MXenes, including as their higher surface-to-volume ratio, greater electrical conductivity, and near-infrared absorption, facile surface functionalization with various polymers or nanoparticles and these all features contribute to MXene great functionalities. For the development of biosensors, cancer therapies, bioimaging, and drug delivery, MXenes are good nanoplatforms. Modifications made to the surface of MXenes can boost their in vivo effectiveness due to their less poisonous nature, extended internal circulation, and greater colloidal stability. MXenes created for biological activities have structure and dose-dependent antibacterial action. Additionally, they are appropriate for the development of implants, thermal therapy, targeted drug administration, and photoacoustic and optical imaging (Fig. 1) (Gazzi et al. 2019; Lin et al. 2018; Murugan et al. 2019).

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Fig. 1 Applications of MXenes in various areas of science and technology

The biological impacts of MXenes require further systematic study due to their enormous potential for biomedical and environmental applications. Two-dimensional (2D) nanomaterials have received a lot of scientific attention for approximately 20 years due to their exceptional physical and chemical characteristics as well as their practically limitless application possibilities. The following is a list of some of these programs:

3.1 Sensors The development of extremely sensitive gas sensors and biosensors is now based on Ti2 C and Ti3 C2 MXenes. Ti3 C2 MXenes electrochemical performance in mediatorfree H2 O2 biosensors has also been the multi-layered Ti3 C2 structure which proved to be a potential scaffolding for enzyme immobilization as hemoglobin can be absorbed through the nanolayers surface functional moieties and becomes fixed on their interior sides (Rizwan et al. 2022).

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3.2 Drug Delivery Applications Targeted delivery of the drug is the process of delivering a substance to a specified area without harming nearby tissues. One of the main issues with targeted medicine administration is bioavailability. A contributing element enhancing the bioavailability of medicines is their hydrophilicity. Because of this, hydrophilic MXenes are ideal candidates for the focused delivery system. Another significant benefit is the capability of the surface of MXenes to be functionalized with medicinal moities. The toolbox of cancer therapy strategies would benefit greatly from the addition of MXene-based structures for targeted drug delivery (Han et al. 2018). Combining chemotherapy and photothermal cancer treatment is offered by this nanoplatform. The substance contains large pores and a high water content (98%) which together account for its excellent drug loading efficiency (84%). The 3D hydrogel networks allow the regulated release of doxorubicin hydrochloride, which reduces the drug toxicity. The exceptional infrared absorption properties of the cellulose/MXenes composite hydrogels, according to the authors, are especially noticeable when illuminated at a wavelength of 808 nm. The response to illumination appears as a continuous dynamic process in water due to the pores expansion, which enhances drug release. After 5 min of laser irradiation, the hydrogel with 235.2 ppm MXene induced 100% non-relapsive death of the tumor cells with cell biodegradation in under two weeks.

3.3 Photo/Chemotherapy of Cancer One of the main objectives of anticancer research is the development of drugs that only attack tumor tissues. In this situation, chemotherapy and phototherapy are two very promising treatment options. These treatments use fluor-sensitizers that are safe for the surrounding tissues as well as bioavailable, non-toxic chemotherapeutic medicines. A promising base for such treatments is provided by MXenes. MXenes are brand-new, incredibly potent, and selective chemicals that can be used in photothermal therapy to cure cancer (Vasyukova et al. 2022).

3.4 Tissue Engineering To enhance or replace biological tissues, tissue engineering techniques may combine cells, synthetic materials, and the proper biochemical and physicochemical components. Tissue engineering calls for biocompatible materials with a specific set of mechanical characteristics. Making tissue engineering matrices is also another application that can effectively make use of the qualities of MXene, such as mechanical strength, biocompatibility, and good electrical conductivity. The osteoinductivity

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of Ti3 C2 Tx MXene multilayer films and the capacity for regulated bone regeneration in vitro and in vivo were studied. In vitro studies have revealed that MXenebased films are significantly cyto-compatible and encourage osteogenic development. Implantation of MXene-based films into the subcutaneous and calvarial defect-sites in rats exhibited excellent biocompatibility, osteoinductivity, and in vivo bone regeneration potential. The scientists noticed more activity in the macrophages that were linked to the MXene films, which may signal that the biodegradation of MXene has started (Iravani and Varma 2021).

3.5 Bioimaging A non-invasive technique for observing biological potential, bioimaging supports the analysis of the three-dimensional (3D) structure of samples without interfering with diverse biological functions. Quantum dots are necessary for systems for bioimaging. Due to their biocompatibility, quantum dots can be utilized in biological environments. Such quantum dots can be created using MXenes as a basis. The extraordinary properties of Ti3 C2 MXene-based quantum dots showed excellent potential for their application as fluorescence sensors in bioimaging, optical-sensing, and photoelectric conversion (Huang et al. 2021).

3.6 Antibacterial Agent It is crucial for antibacterial agents to only be poisonous to bacteria for preventing the development of antibiotic resistance. MXenes appear to have a lot of potential in these uses. MXenes and other 2D compounds are particularly regarded to be promising new antibacterial agents. Ti3 C2 /chitosan composite nanofibers are an excellent candidate materials for producing biodegradable wound dressings that are potentially effective against wound inoculation with both Gram-positive and Gram-negative bacteria (Khatami et al. 2022).

3.7 Environmental Applications Environmental pollution is responsible for posing bad health impacts on living beings and there is a need to find new sources to control environmental pollution (Rasheed et al. 2021; Rizwan et al. 2022a, b, c; Rizwan et al., b, g). Researchers are interested in the fast-growing field of applications for MXenes in environmental protection. There have been several recent review articles that have covered these subjects. The section below provides a comprehensive review of MXene’s broad range of applications in

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the study of wastewater treatment, particularly in relation to radionuclide and dye contamination.

3.7.1

Removal of Radionuclides

The nuclear waste is generally composed of radioactive elements such as cesium, palladium, strontium, and barium. They have lengthy half-lives, and their environmental remediation is critical owing to their hazardous impacts on human health and environment. They are highly carcinogenic and cause soil toxicity and infertility. Adsorption process is cost effective, highly efficient, and convenient to operate and handle, and more importantly, a wide range of biosorbents are available. Keeping in view of stringent rules and regulations to counter nuclear waste, researchers explored several adsorbents of organic, inorganic, and hybrid nature for treating radioactive elements (Deng et al. 2019). In addition, certain adsorbents exhibited lower efficiencies. Also, slow sorption and deficiency in selectivity are some other drawbacks in case of some adsorbents. Also, 2D materials including metallic dichalcogenides tend to exhibit lower adsorptive efficiencies due to mild van Der Waals (Gouveia et al. 2020). Over the past few years, MXenes managed to get significant attention in environmental remediation and are specifically relevant to managing nuclear waste owing to peculiar properties including a variety of surficial chemical functionalities, augmented ion-exchange capability due to large surface distribution, hydrophilic nature, extraordinary chemical and thermal stability, and alterable structural framework (He et al. 2020). Due to the hydrophilic nature of MXenes and sufficient spacing between its layers, wastewater treatment rates are higher and, therefore, quick removal of contaminants (i.e., radionuclides). In case of radionuclides’ remediation via MXenes, prominent mechanisms includes adsorptive removal mechanisms include chelation, ion-exchange, and d-layer alteration. However, ion-exchange is the most potent removal process for sequestering radionuclides from polluted water (Deng et al. 2019). For instance, Uranium is a traditional nuclear fuel and radioactive element with a half-life of greater than 4.45 × 109 years and poses threats to humans and environment (Rethinasabapathy et al. 2018). It is well established that Cr(VI) ions are of prime importance regarding their mobility in surface waters and form uranyl ion which allowed the researchers and scientists to develop a variety of sorbents for sequestering it from marine environment (Meshkian et al. 2018). In the last few years, different adsorbents including graphene oxide, MOF, activated carbon, etc., were investigated for the adsorptive removal of U(VI) from aqueous media. According to density functional modeling, hydroxylated titanium carbide seems to be the most promising sorbent for removing U(VI) from polluted waters and exhibited an adsorptive removal capability of ~595 mg/g. The prominent adsorptive removal mechanism included the binding of U(VI) ions to adsorption sites comprising of deprotonated oxygen located on the surface of titanium carbide (Zhang et al. 2020). Furthermore, Wang and colleagues 2020 (Wang et al. 2020a, b, c) reported that U(VI) ions were effectively captured using 2D V2 CTx nano sheets comprising multiple layers. The maximum

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adsorptive removal capability was 175 mg/g much that obtained for several inorganic materials.

3.7.2

Removal of Dyes

Dyes are chemically stable and possess complicated chemical structures. Due to their aromatic structural framework, they cause cancer and are considered mutagenic (Rajeshkannan et al. 2012). For instance, metal-based phthalocyanine dyes severely effect renal functions of human beings. In addition, wide spread of dyes in water bodies can cause imbalanced aquatic environment as they limit the penetration of sun rays (Natarajan and Manivasagan 2020). Therefore, due to inherent properties of dyes, traditional physical and biological techniques are ineffective in treating dyes (Rajeshkannan et al. 2011). However, adsorption method is reported to be effective in treating dyes containing polluted water (Rajan et al 2010). Due to the lack of economic feasibility associated with traditional adsorbents, utilization of MXenes can be feasible for treating dyes containing polluted water. Various investigations on 2D titanium carbides for removing dyes such as methylene blue and acid blue suggested their adsorptive removal as a key mechanism and in certain cases the adsorption capacities were found to appreciably higher than other 2D materials specifically in case of Methylene Blue, Methyl Orange, Methyl Red, Congo Red, and Evans Blue using Mxene (Feng et al. 2020; Jun et al. 2020).

4 MXenes Incorporated Membranes A wide range of materials including HCl-NaF, HF, HCl-LiF, etc., have been employed to fabricate membranes comprising of MXenes. Typically, nanomaterials including carbon nanotubes, metal–organic framework, and graphene oxides have been employed for the preparation of membranes incorporating nanomaterials (Jeon et al. 2020; Rezakazemi et al. 2021). The incorporation can be performed in three different ways: 1. Utilization of MXene as framework materials on which other structural arrangements can be performed to develop a lamellar structure. 2. Incorporation of other nanomaterials along with MXenes to alter the arrangement of base-matrix. 3. Utilizing MXenes as a coating material for surficial modification of membrane. Wang and colleagues developed a membrane possessing a lamellar structure using 2D nano-sized layers of Ti3 C2 Tx Mxene ( Wang et al., 2020a, b, c). The etching technique was used to etch Al from Ti3 AlC2 as AlF3 using fluoric acid to fabricate 2D MXene layers. Afterward, exfoliation, ultrasonication, and dissolution in iron hydroxide were performed to develop MXene-based membranes (Karthikeyan et al. 2021). Then, filtration was performed Al2 O3 membrane (pore size 0.2 microns)

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followed by treatment with hydrochloric acid for releasing iron hydroxide. Lastly, the prepared MXenes were characterized for surface morphology, chemical functionalities (i.e., –OH, –O, and –F), and conversion of Ti3 AlC2 to Ti3 C2 Tx via Scanning electron microscopy/transmission electron microscopy (SEM/TEM), X-ray diffraction (XRD), and Fourier Transform Infrared Spectroscopy (FTIR). In another study, the phase inversion technique was used to synthesize 2D Ti3 C2 Tx membrane (Jun et al. 2019). Polyethersulfone ultrasonication/polyvinylidene difluoride membranes were used for the filtration of Ti3 C2 Tx . These membranes have significant antibacterial resistance against bacterial species including Bacillus subtilis and E-coli. The thickness of the developed membrane was adjusted through a controlled accumulation of Ti3 C2 Tx on polyvinylidene difluoride (Zhao et al. 2021). Membranes manufactured utilizing non-solvent-based phase inversion tend to show reduced stability specifically in organic solutions (i.e., methanol, heptane, acetone, etc.).

5 Conclusions In this chapter, the structure of MXene has been explained in detail which aims to aid in selecting the best possible method regarding its fabrication. Literature review showed that substantial number of studies evaluated the MXenes functionalization. However, further studies are required to investigate the adsorptive potential of MXenes without the incorporation of chemical functionalities. Furthermore, the techniques for the preparation of MXenes including exfoliating their bulk quantities to MXene sheets composing of singular layers and their development from atomic and molecular units are discussed. Moreover, the applicability of MXenes in sensing, bioimaging, tissue engineering, and treating radioactive elements and dyes has been highlighted. In case of MXenes as suitable sorbents, the main removal process was found to be chelation, ion-exchange, physical and chemical adsorption, electrostatic forces, etc., In the case of MXene incorporated membranes, the development of membranes with significant structural and chemical integrity and hydrophilic and dissolution properties is still challenging. Therefore, further investigations are required to fabricate MXenes-based membranes with the inclusion of crosslinking agents, substrates, etc., to improve the aforementioned properties and performance of membranes.

References Anasori B, Gogotsi ÛG (2019) 2D metal carbides and nitrides (MXenes), vol 416. Springer Barsoum MW (2000) The MN+ 1AXN phases: A new class of solids: Thermodynamically stable nanolaminates. Prog Solid State Chem 28(1–4):201–281 Cai Y, Shen J, Ge G, Zhang Y, Jin W, Huang W, Dong X (2018) Stretchable Ti3C2T x MXene/carbon nanotube composite based strain sensor with ultrahigh sensitivity and tunable sensing range. ACS Nano 12(1):56–62

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Deng H, Li Z-J, Wang L, Yuan L-Y, Lan J-H, Chang Z-Y, Shi W-Q (2019) Nanolayered Ti3C2 and SrTiO3 composites for photocatalytic reduction and removal of uranium (VI). ACS Appl Nano Mater 2(4):2283–2294 Feng X, Yu Z, Long R, Sun Y, Wang M, Li X, Zeng G (2020) Polydopamine intimate contacted two-dimensional/two-dimensional ultrathin nylon basement membrane supported RGO/PDA/MXene composite material for oil-water separation and dye removal. Sep Purif Technol 247:116945 Gazzi A, Fusco L, Khan A, Bedognetti D, Zavan B, Vitale F, Delogu LG (2019) Photodynamic therapy based on graphene and MXene in cancer theranostics. Front Bioeng Biotechnol 7:295 Gouveia JD, Morales-García Á, Vines F, Illas F, Gomes JR (2020) MXenes as promising catalysts for water dissociation. Appl Catal B 260:118191 Guo X, Xie X, Choi S, Zhao Y, Liu H, Wang C, Wang G (2017) Sb 2 O 3/MXene (Ti 3 C 2 T x) hybrid anode materials with enhanced performance for sodium-ion batteries. J Mater Chem A 5(24):12445–12452 Han X, Huang J, Lin H, Wang Z, Li P, Chen Y (2018) 2D ultrathin MXene-based drug-delivery nanoplatform for synergistic photothermal ablation and chemotherapy of cancer. Adv Healthcare Mater 7(9):1701394 He P, Cao M-S, Cai Y-Z, Shu J-C, Cao W-Q, Yuan J (2020) Self-assembling flexible 2D carbide MXene film with tunable integrated electron migration and group relaxation toward energy storage and green EMI shielding. Carbon 157:80–89 Huang M, Gu Z, Zhang J, Zhang D, Zhang H, Yang Z, Qu J (2021) MXene and black phosphorus based 2D nanomaterials in bioimaging and biosensing: Progress and perspectives. J Mater Chem B 9(26):5195–5220 Iravani S, Varma RS (2021) MXenes and MXene-based materials for tissue engineering and regenerative medicine: Recent advances. Materials Advances 2(9):2906–2917 Jeon M, Jun B-M, Kim S, Jang M, Park CM, Snyder SA, Yoon Y (2020) A review on MXene-based nanomaterials as adsorbents in aqueous solution. Chemosphere 261:127781 Jun B-M, Yoon Y, Park CM (2019) Post-treatment of nanofiltration polyamide membrane through alkali-catalyzed hydrolysis to treat dyes in model wastewater. Water 11(8):1645 Jun B-M, Heo J, Taheri-Qazvini N, Park CM, Yoon Y (2020) Adsorption of selected dyes on Ti3C2Tx MXene and Al-based metal-organic framework. Ceram Int 46(3):2960–2968 Karthikeyan P, Ramkumar K, Pandi K, Fayyaz A, Meenakshi S, Park CM (2021) Effective removal of Cr (VI) and methyl orange from the aqueous environment using two-dimensional (2D) Ti3C2Tx MXene nanosheets. Ceram Int 47(3):3692–3698 Khatami M, Iravani P, Jamalipour Soufi G, Iravani S (2022) MXenes for antimicrobial and antiviral applications: recent advances. Mater Technol 37(11):1890–1905 Khazaei M, Arai M, Sasaki T, Ranjbar A, Liang Y, Yunoki S (2015) OH-terminated twodimensional transition metal carbides and nitrides as ultralow work function materials. Phys Rev B 92(7):075411 Lin H, Chen Y, Shi J (2018) Insights into 2D MXenes for versatile biomedical applications: current advances and challenges ahead. Advanced Science 5(10):1800518 Liu H, Neal AT, Zhu Z, Luo Z, Xu X, Tománek D, Ye PD (2014) Phosphorene: an unexplored 2D semiconductor with a high hole mobility. ACS Nano 8(4):4033–4041 Meshkian R, Dahlqvist M, Lu J, Wickman B, Halim J, Thörnberg J, Snyder J (2018) W-based atomic laminates and their 2D derivative W1. 33C MXene with vacancy ordering. Advanced materials 30(21):1706409 Murugan C, Sharma V, Murugan RK, Malaimegu G, Sundaramurthy A (2019) Two-dimensional cancer theranostic nanomaterials: Synthesis, surface functionalization and applications in photothermal therapy. J Control Release 299:1–20 Naguib M, Kurtoglu M, Presser V, Lu J, Niu J, Heon M, Barsoum MW (2011) Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv Mater 23(37):4248–4253 Naguib M, Saito T, Lai S, Rager MS, Aytug T, Paranthaman MP, Gogotsi Y (2016) Ti3C2T x (MXene)–polyacrylamide nanocomposite films. RSC Adv 6(76):72069–72073

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Natarajan R, Manivasagan R (2020) Effect of operating parameters on dye wastewater treatment using Prosopis cineraria and kinetic modeling. Environ Eng Res 25(5):788–793 Ng VMH, Huang H, Zhou K, Lee PS, Que W, Xu JZ, Kong LB (2017) Recent progress in layered transition metal carbides and/or nitrides (MXenes) and their composites: synthesis and applications. J Mater Chem A 5(7):3039–3068 Peng Q, Si C, Zhou J, Sun Z (2019) Modulating the Schottky barriers in MoS2/MXenes heterostructures via surface functionalization and electric field. Appl Surf Sci 480:199–204 Rajan R, Rajasimman M, Natarajan R (2010) Sorption of acid blue 9 on to wheat bran: Optimization, equilibrium and kinetic studies. Chem Prod Process Model 5(1) Rajeshkannan R, Rajasimman M, Rajamohan N (2011) Sorption of acid blue 9 using Hydrilla verticillata biomass—optimization, equilibrium, and kinetics studies. Bioremediat J 15(1):57–67 Rajeshkannan R, Rajasimman M, Rajamohan N (2012) Removal of malachite green from aqueous solutions using wheat bran: optimisation, equilibrium and kinetic studies. Int J Environ Eng 4(1–2):1–23 Rasheed T, Ahmad N, Ali J, Hassan AA, Sher F, Rizwan K, Bilal M (2021) Nano and micro architectured cues as smart materials to mitigate recalcitrant pharmaceutical pollutants from wastewater. Chemosphere, 129785. https://doi.org/10.1016/j.chemosphere.2021.129785 Rasheed T, Kausar F, Rizwan K, Adeel M, Sher F, Alwadai N, Alshammari FH (2022) Two dimensional MXenes as emerging paradigm for adsorptive removal of toxic metallic pollutants from wastewater. Chemosphere 287:132319. https://doi.org/10.1016/j.chemosphere.2021.132319 Ren CE, Hatzell KB, Alhabeb M, Ling Z, Mahmoud KA, Gogotsi Y (2015) Charge-and sizeselective ion sieving through Ti3C2T x MXene membranes. J Phys Chem Lett 6(20):4026–4031 Rethinasabapathy M, Kang S-M, Lee I, Lee G-W, Hwang SK, Roh C, Huh YS (2018) Layerstructured POSS-modified Fe-aminoclay/carboxymethyl cellulose composite as a superior adsorbent for the removal of radioactive cesium and cationic dyes. Ind Eng Chem Res 57(41):13731–13741 Rezakazemi M, Shamsabadi AA, Lin H, Luis P, Ramakrishna S, Aminabhavi TM (2021) Sustainable MXenes-based membranes for highly energy-efficient separations. Renew Sustain Energy Rev 143:110878 Rizwan K, Rahdar A, Bilal M, Iqbal HMN (2022) MXene-based electrochemical and biosensing platforms to detect toxic elements and pesticides pollutants from environmental matrices. Chemosphere 291:132820. https://doi.org/10.1016/j.chemosphere.2021.132820 Rizwan K, Bilal M, Iqbal HMN (2022) Chapter 1—Role of nanomaterials in sensing air pollutants. In: Assadi A, Amrane A, Nguyen TA (eds) Hybrid and combined processes for air pollution control. Elsevier, pp 1–17 Rizwan K, Babar ZB, Munir S, Arshad A, Rauf A (2022) Recent advancements in engineered biopolymeric-nanohybrids: A greener approach for adsorptive-remediation of noxious metals from aqueous matrices. Environ Res 215:114398. https://doi.org/10.1016/j.envres.2022.114398 Rizwan K, Rasheed T, Bilal M (2022) 10—Nano-biodegradation of polymers. In: Iqbal HMN, Bilal M, Nguyen TA, Yasin G (eds) Biodegradation and Biodeterioration At the Nanoscale. Elsevier, pp 213–238 Rizwan K, Rasheed T, Bilal M (2022) Chapter 21—Alginate-based nanobiosorbents for bioremediation of environmental pollutants. In: Denizli A, Ali N, Bilal M, Khan A, Nguyen TA (eds) Nano-Biosorbents for Decontamination of Water, Air, and Soil Pollution. Elsevier, pp 479–502 Rizwan K, Rasheed T, Bilal M, Iqbal HMN (2022) Chapter 7—Pulp and paper industry-based pollutants, and their adverse impacts. In: Bhat R, Kumar A, Nguyen TA, Sharma S (eds) Nanotechnology in Paper and Wood Engineering. Elsevier, pp 143–160 Rizwan K, Bilal M, Slimani Y, Show PL, Rtimi S, Roy A, Iqbal HM (2022) Hydrogen-based sonohybrid catalytic degradation and mitigation of industrially-originated dye-based pollutants. Int J Hydrog Energy Sokol M, Natu V, Kota S, Barsoum MW (2019) On the chemical diversity of the MAX phases. Trends in Chemistry 1(2):210–223

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Soleymaniha M, Shahbazi MA, Rafieerad AR, Maleki A, Amiri A (2019) Promoting role of MXene nanosheets in biomedical sciences: therapeutic and biosensing innovations. Adv Healthcare Mater 8(1):1801137 Szuplewska A, Kulpi´nska D, Dybko A, Chudy M, Jastrz˛ebska AM, Olszyna A, Brzózka Z (2020) Future applications of MXenes in biotechnology, nanomedicine, and sensors. Trends Biotechnol 38(3):264–279 Vasyukova IA, Zakharova OV, Kuznetsov DV, Gusev AA (2022) Synthesis, Toxicity Assessment, Environmental and Biomedical Applications of MXenes: A Review. Nanomaterials 12(11):1797 Verger L, Xu C, Natu V, Cheng H-M, Ren W, Barsoum MW (2019) Overview of the synthesis of MXenes and other ultrathin 2D transition metal carbides and nitrides. Curr Opin Solid State Mater Sci 23(3):149–163 Wang Y, Qiu M, Won M, Jung E, Fan T, Xie N, Kim JS (2019) Emerging 2D material-based nanocarrier for cancer therapy beyond graphene. Coord Chem Rev 400:213041 Wang Q, Pan X, Lin C, Gao H, Cao S, Ni Y, Ma X (2020) Modified Ti3C2TX (MXene) nanosheetcatalyzed self-assembled, anti-aggregated, ultra-stretchable, conductive hydrogels for wearable bioelectronics. Chem Eng J 401:126129 Wang Y, Feng W, Chen Y (2020) Chemistry of two-dimensional MXene nanosheets in theranostic nanomedicine. Chin Chem Lett 31(4):937–946 Wang H, Cui H, Song X, Xu R, Wei N, Tian J, Niu H (2020) Facile synthesis of heterojunction of MXenes/TiO2 nanoparticles towards enhanced hexavalent chromium removal. J Colloid Interface Sci 561:46–57 Wu HB, Chen JS, Hng HH, Lou XWD (2012) Nanostructured metal oxide-based materials as advanced anodes for lithium-ion batteries. Nanoscale 4(8):2526–2542 Yorulmaz U, Özden A, Perkgöz NK, Ay F, Sevik C (2016) Vibrational and mechanical properties of single layer MXene structures: a first-principles investigation. Nanotechnology 27(33):335702 Zhan X, Si C, Zhou J, Sun Z (2020) MXene and MXene-based composites: synthesis, properties and environment-related applications. Nanoscale Horizons 5(2):235–258 Zhang T, Pan L, Tang H, Du F, Guo Y, Qiu T, Yang J (2017) Synthesis of two-dimensional Ti3C2Tx MXene using HCl+ LiF etchant: enhanced exfoliation and delamination. J Alloy Compd 695:818–826 Zhang B, Fan T, Xie N, Nie G, Zhang H (2019) Versatile Applications of Metal Single-Atom@ 2D Material Nanoplatforms. Advanced Science 6(21):1901787 Zhang P, Wang L, Du K, Wang S, Huang Z, Yuan L, Chai Z (2020) Effective removal of U (VI) and Eu (III) by carboxyl functionalized MXene nanosheets. J Hazard Mater 396:122731 Zhang S, Liao S, Qi F, Liu R, Xiao T, Hu J, Min Y (2020) Direct deposition of two-dimensional MXene nanosheets on commercially available filter for fast and efficient dye removal. J Hazard Mater 384:121367 Zhao Q, Wang J, Li Z, Guo Y, Tang B, Abudula A, Guan G (2021) Two-dimensional Ti3C2TXnanosheets/Cu2O composite as a high-performance photocatalyst for decomposition of tetracycline. Carbon Resources Conversion 4:197–220

Structural Design, Properties, and Synthesis of Original MXenes Rabia Akhtar, Ameer Fawad Zahoor, Asim Mansha, Syed Makhdoom Hussain, Sajjad Ahmad, and Tahir Maqbool

Abstract Nowadays, two-dimensional materials have emerged speedily due to their unique characteristics. These ultrathin substances have a thickness of a few nanometers and give amazing services, especially in the electronic industry. Graphene is a renowned member of this class with potential applications fascinating the researchers to discover such types of 2D materials thereof, becoming the reason for the landmark discovery of MXenes. This class consists of metal carbides, nitrides, and carbonitrides obtained from MAX phases and have unique physical and chemical properties. Their hydrophilic nature, better surface chemistry with an ion-exchange feature, high Young Modulus, and excellent electrical conductivities make them ideal candidates for energy storage devices. Besides this, their use in catalysis and the biomedical field is also prominent. This chapter provides an overview of two-dimensional compounds and the latest progress in the development of MXene composites. First of all, their synthetic methods regarding etching and exfoliation are discussed in detail. Then their distinctive mechanical, electronic, and magnetic properties are elaborated. Moreover, their general atomic structure, oxidative or thermal stability, and hydrogen storage capacity are also deliberated. In the end, the role of MXene composites in energy storage devices such as lithium/sodium-ion batteries, sulfur batteries, and supercapacitors along with their biomedical applications are presented. Keywords Two-dimensional materials · Graphene · MXene · Etching · Exfoliation · Energy storage devices

R. Akhtar · A. F. Zahoor (B) · A. Mansha · T. Maqbool Department of Chemistry, Government College University Faisalabad, Faisalabad 38000, Pakistan e-mail: [email protected] S. M. Hussain Department of Zoology, Government College University Faisalabad, Faisalabad 38000, Pakistan S. Ahmad Department of Chemistry, University of Engineering and Technology Lahore, 38000-Faisalabad, Faisalabad-Campus, Pakistan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_2

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1 Two-Dimensional Materials A family consisting of two-dimensional (2D) materials is rapidly growing nowadays and it is estimated that around about 150 families related to these materials have been discovered up till now. The common examples of 2D materials are as follows: • • • • • • • • • • •

Graphene (Zhu et al. 2015) Transition metal dichalcogenides (Torrisi and Coleman 2014) Layered double hydroxides Hexagonal boron nitride (Liu et al. 2014) Black phosphorous Monoelemental compounds family (Xenes) Metal oxides Graphitic carbon nitride (Liu et al. 2016) Metal nitrides/carbides (MXenes) (Bao et al. 2018) Transition metal halides and metal oxides (Coleman et al. 1998; Sun et al. 2014) 2D polymers etc. (Boott et al. 2015)

Since the successful preparation of graphene (in 2004) which is the most famous material in the 2D family, several compounds have been discovered that can exhibit amazing applications than graphene. They are used in making several kinds of sensors and LEDs and also give a significant contribution to catalysis, medical, and environmental sciences fields. The outstanding performance of 2D materials as well as lower power use in electronic gadgets makes these materials more popular in the electronic industry. The enhanced physical and chemical properties of 2D materials are because of the distribution of atoms on the surface of these materials in an ultrathin form that enhances their surface area and effect their electronic, photonic, magnetic, and catalytic characteristics. In ongoing research, these materials are doped with other 2D materials or different kinds of metal oxides, etc. to improve their physical and chemical properties resultantly, generating a promising group consisting of 2D heterostructures. These hybrid structures display extraordinary device applications especially in optoelectronic devices because they can conduct electricity in a superfast way as compared to traditional circuit materials (Bagheri et al. 2016; Khan et al. 2019a, b, 2020; Nourbakhsh et al., 2019; Wang et al. 2015; Wu & Song 2018).

2 MXenes The landmark discovery of Ti3 C2 material in 2011 provide the pathway for the preparation of similar materials collectively known as MXenes (Naguib et al. 2011; Rizwan et al. 2022; Rasheed et al. 2021). To prepare such kinds of materials, the MAX phase is used from which the ‘A’ atomic layer is extracted by using some etching agent as a result desired 2D MXene material is obtained. It is obvious from the synthesis that the bond between M-A is weak as compared to the M-X bond so

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it can be easily broken via an etching agent. However, the usage of the etching agent stains some ions or molecules (such as OH, O, F, or other functional groups) on the surface of the MXenes, thereby making the general formula of MXenes, Mn+1 Xn Tx : Where, M stands for a transition metal, such as Ti, V, etc. X is carbon and/or nitrogen. T stands for surface functional groups (Das and Wu 2020; Sun et al. 2018).

2.1 Preparation of MXenes Several methods have been reported in various literature reports according to the nature of the etching process in which Gibbs free energy of the surface can be reduced by binding different end groups with M atoms for the completion of the coordination sphere. These methods are as follows:

2.1.1

Etching Methods

Hydrofluoric Acid Etching Method Of all the methods reported up till now, the hydrofluoric acid etching method is the most famous traditional way for the preparation of MXenes. This method was reported by Naguib et al. (2011) during the synthesis of two-dimensional Ti3 C2 layers by treating Ti3 AlC2 with HF (Naguib et al. 2011). The solution of HF/deionized H2 O easily removed the Al layer from Ti3 AlC2 by displacing Al with –OH, –F, or –O as a result, Ti3 C2 Tx material was obtained with the evolution of H2 . This synthetic method is affected by different parameters such as temperature, time, and the F ion density which considerably develop the quality of the MXenes layers. Research by Alhabeb et al. declared that to attain a good MXene layer, a high concentration of HF is used however when it is mixed with different acid solutions, the required results are not obtained (Alhabeb et al. 2017).

Combination of HF with Fluoride salts—A Modified Etching Method From the extraction of aluminum layers from the MAX phase, HF has been extensively used but the corrosive and toxic nature of HF diverts the attention of researchers to discover a more appropriate method of etching despite of traditional method. In this view, a combination technique consisting of a mixture of HF and fluoride salts (NH4 HF2 , LiF, NaF, KF, and FeF3 ) or HCl is common nowadays. This method significantly increases the interlayer spaces because the insertion of the cations resultantly

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is helpful in the ultrasonic processes. In this way, a few-layer MXenes can be synthesized in one step avoiding the complex multi-step synthetic pathways (Alhabeb et al. 2018; Ghidiu et al. 2014; Karlsson et al. 2015; Liu et al. 2017).

Molten Fluoride Salt Etching Method Likewise carbide MXenes, nitride MXenes also play a crucial role in different types of electronic devices. During their preparation, it was observed that the stability of Tin Nn-1 is greater than Tin Cn-1 so, there is less chance to extract aluminum from the Tin AlNn-1 system using traditional methods due to the strong bonding of aluminum atoms. Therefore, a more concise approach has been introduced by Urbankowski et al. utilizing molten fluoride salts such as lithium fluoride, sodium fluoride, and potassium fluoride as etchants to construct nitride MXenes (Ti4 N3 Tx ). But the poor crystallinity of Ti4 N3 Tx obtained via this molten fluoride salt etching method limits the use of this technique for the production of 2D metal nitrides (Urbankowski et al. 2016).

Non-Fluorine Etching Methods Electrochemical Method To avoid the problems faced during the usage of corrosive and toxic acidic solutions, a number of other environmentally friendly electrochemical etching methods which are safe and economical are discovered. For example, Yang et al. prepared Ti3 C2 Tx nanosheets by preparing a solution of 1.0 M NH4 Cl and 0.2 M TMAOH (tetramethylammonium hydroxide) as an electrolyte. Inserting Ti3 AlC2 in this electrolyte solution, Al atoms are fixed with Cl ions and the required Ti3 C2 Tx (T = O, OH) sheets are obtained with no fluorine groups which display crucial performance in supercapacitors due to their substantial area-specific capacitance (Sun et al. 2017; Yang et al. 2018). Hydrothermal Method In the hydrothermal method, an alkaline solution (NaOH) of high concentration is used mostly at high temperatures. This alkaline solution dissolves the A layer from the MAX phase and provides a multi-layer of Ti3 C2 Tx (T = O, OH) with a 92% quality score. In this method, certain issues are involved, e.g., it is obvious from the research that the alkaline condition disturbs the stability of the Ti3 C2 framework. Besides this, during the etching of aluminum, corrosion of Ti3 C2 also takes place in this technique. Therefore, to overcome these problems 270 °C temperature is provided with a 27.5 M concentration of NaOH which stables the Ti3 C2 layer and selectively extracts the Al from Ti3 AlC2 . On the other side, certain types of security issues are observed when using a high concentration of alkaline solution under hydrothermal conditions (Li

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et al. 2018; Peng et al. 2018). In this regard, Xuan et al. (2016) introduced a novel alkali etching method under mild conditions in which TMAOH is used as an etching agent (Xuan et al. 2016). Other Reported Methods Xu et al. (2015) utilized the chemical vapor deposition technique for the preparation of α-Mo2 C MXene with a large and clean surface area (Xu et al. 2015). Li et al. proposed the reaction of Ti3 AlC2 and Ti2 AlC in ZnCl2 solution to prepare Ti3 C2 and Ti2 C MXenes with Cl atoms termination. Similarly, for the synthesis of nitride MXenes from carbide MXenes, ammonia gas can be used at 550 °C. For example, Mo2 NTz and V2 NTz were synthesized from Mo2 CTz and V2 CTz , respectively, via this approach (Li et al. 2019).

2.1.2

Exfoliation

After passing through the etching process, the next step that comes is exfoliation. Some commonly used exfoliated methods are as follows: • Intercalation with large organic molecules • Intercalation with cations • Shaking/sonication The adaptation of these methods mainly depends upon the etching method which is used. For instance, when lithium fluoride is used in combination with hydrochloric acid, Li+ cations are fixed between the MXene layers, and in this case, exfoliation is done by shaking/sonication to attain the required MXene material. Similarly, by using HF as an etchant exfoliation can be done via the insertion of organic molecules or ions between the layers. The chemical composition of MXene also plays a crucial role to determine the exfoliation method as for the exfoliation of Ti3 C2 Tz and (Mo2/3 Ti1/3 )3 C2 Tz , DMSO is used while for many other MXenes tetrabutylammonium hydroxide (TBAOH) is used (Verger et al. 2019).

2.2 Properties of MXenes 2.2.1

Atomic Structure

The common atomic structure of MXene looks very similar to that of graphene when we see it from the top, i.e., rhombus unit cell. However, from the side view, it consists of three sheets, one layer of X which is inserted between two layers of M. Generally, hexagonal lattice geometry is observed in MXene. Transition metals that build two monolayers of MXene are linked with the X atoms by six chemical bonds. A literature survey reveals the significant thermodynamic stability of the functionalized MXenes

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in comparison with partially functionalized MXenes which can be judged by the accumulation of the termination groups (Ashton et al. 2016; Khazaei et al. 2014).

2.2.2

Mechanical Properties

Strong bonding between M-C and M–N composites greatly enhances the mechanical features of MXenes. It is observed that elastic parameters of MXenes are better (2 times greater) than MAX and other 2D composites but lower (2–3 times) than graphene. However, their excessive use to strengthen the different composites is increased because of their bending specificity, i.e., 1050 GPa. The strong interaction capacity of MXene with any type of polymer in comparison with graphene also increases its utility. Hydrophilicity with definite contact angles (25–40) can be observed in various titanium-based thin discs of MXene, e.g., Ti3 C2 Tx which has a contact angle of 35. Today a number of techniques are available which can characterize bulk materials, however, still, it’s a challenging task to assess 2D composites with their accurate mechanical features. For this purpose, the nanoindentation method is being used to determine the hardness or elasticity of the 2D materials. From this method, 333 ± 20 of Young’s modulus of a monolayer of MXene was observed that be closer to Ti3 C2 O2 (386 GPa) and higher to graphene oxide and MoS2 . Besides this, there is a dire need for such techniques which can estimate the exact mechanical characteristics of MXene and define its composition defect regarding deficiency in surface control ability of MXene and disturbance of geometric vacancies, etc. (Khazaei et al. 2016; Liang et al. 2017; Pargoletti et al. 2019; Zhang et al. 2020).

2.2.3

Electronic and Electric Properties

The electronic and electric nature of MXene can be included in the main characteristics of these composites which can be controlled by different angles. For example, alteration in its functional groups’ nature or position and material balance can change the efficacy of the MXenes. Pressed films of MXenes exhibit electrical conductivity similar to that of graphene but higher than carbon nanotubes and graphene oxide. Moreover, by the increment of different layers, resistivity also increases together with functional group hindrance and because of this, the conductivities of simulated functions are higher than the experimental observations. Wang et al. (2016a, b) estimated that electrical conductivities of MXenes can be altered from 850 to 9870 by the implication of different variations such as. • • • • •

Surface functionalities Concentration defect d-Spacing between MXenes particulates Delamination yield Lateral dimensions caused due to etching process.

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Studies show that during MXenes synthesis if the etching process is short with the usage of the minimum amount of fluoric acid then the electrical conductivity of these composites can be enhanced. Moreover, environmental moisture also greatly affects the electrical conductivities of MXenes. To improve (roundabout two-order increment) the electrical characteristics of these composites different alterations in surface area can be done under alkaline and thermal conditions. For example, by adding or extracting different surface functional groups and intercalated ions or molecules, an increment in the electrical conductivity can be possible (Centi and Peranthoner, 2010; Wang et al. 2016a, b; Zou et al. 2018).

2.2.4

Optical Properties

In photovoltaics, photocatalytic devices as well as transparent and optically conductive electrodes, UV/visible spectrum plays a significant role. Light energy absorbed by MXene films in the UV/visible region has been recorded from a wavelength in the range of 300–500 nm. 91% and 80% optical transmittance was observed using 10 and 15 nm thick films of Ti3 C2 Tx , respectively. According to the thickness of the film, the absorption band can be observed at around 700–800 nm, especially in the case of pale green films which make them operative in photothermal therapy. So, it is concluded that the transmittance percentage can be controlled by changing the thickness of the film and ion intercalation. For instance, the transmittance percentage is decreased by using urea, hydrazine, and DMSO while 75–92% transmittance is observed in the case of tetramethyl compound of ammonium hydroxide (Gao et al. 2021; Rasool et al. 2019).

2.2.5

Oxidative or Thermal Stability

The stability of MXene imparts a great effect on the efficacy of these composites which mainly depends on the presence of water or oxygen during their synthetic procedure. It is formulated that carbohydrates having hydroxylated components and irregularly distributed carbon/nitrogen display excellent stability. From a practical point of view, it can be seen that upon oxidation using water media under thermal conditions, pristine MXenes are decomposed. In addition to this, UV light on exposure to air also affects the stability of the MXenes. The decomposition of Ti3 C2 Tx layers on the contact of air (due to the presence of oxygen) with the conversion of MXene into titanium oxide is evidence of the above-mentioned facts as reported in various research articles. Although the stability of MXene can be seen up to 1300 °C under an argon atmosphere yet there are a number of stabilization techniques, e.g., carbon nano-plating and excess energy mechanical milling techniques in the dimethyl component of sulfoxide have been reported. These techniques effectively maintain the original structure of the MXenes even in oxidizing environments. In addition to this, the decomposition of the colloidal solution of MXene can be decreased by placing this solution in a dark place or refrigerator, or a vacuum (Yang et al. 2015).

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2.2.6

Magnetic Nature of MXenes

Magnetic properties of any substance are dependent upon the surface stretching especially in the case of MXenes where strong covalent linkages are present between M, X, and functional groups present on their surface. These linkages impart nonmagnetic characteristics in MXenes. On the contrary, different MXene composites with improved magnetic nature have been reported due to the increased electron density near the Fermi region. It is surface functionalization that reduces the electron density near the Fermi region and makes MXenes nonmagnetic in nature. Ferromagnetism and antiferromagnetism phenomena have been observed by a variety of 2D chromic carbides/nitrides and titanium carbides/nitrides, respectively.

2.2.7

Hydrogen Storage Characteristic

Extensive research has been conducted to develop such materials which can store hydrogen under atmospheric environments at maximum level by keeping in view the adsorption and desorption features in mind. Different organic materials preserve hydrogen at low temperature which is surrounded by liquid N2 . These materials are as follows: • • • • •

Covalent organic compounds Metal–organic composites Carbon nanotubes Graphene Fullerene etc.

Recently, MXenes are introduced as a new material for the storage of hydrogen which follows three mechanisms to link hydrogen bonds. These mechanisms are as follows: • Physisorption • Chemisorption • Kubas type particle interaction. Osti et al. (2017) proposed that Ti3 C2 Tx (synthesized by utilizing 50% HF) under atmospheric conditions adsorb molecular hydrogen molecules (0.3% by weight) in the pores of the two-dimensional crystal structure of MXene composite. The efficacy of material penetrating hydrogen atoms with less energy can be accomplished by increasing carbon vacancies. Chen et al. (2019) proved that combining 4MgH2 LiAlH4 with Ti3 C2 composite greatly affects the hydrogen storage performance of the resulting material. Similarly, Xian et al. observed the promising hydrogen storage ability of the 2D Ti3 C2 material when combined with a mixture of LiH and MgB2 via the ball milling technique. This research proves MXene is a promising candidate for the storage of hydrogen with superior capacity (Chen et al. 2019; Huang et al. 2019; Osti et al. 2017; Ronchi et al. 2019; Rafiq et al. 2020; Tunesi et al. 2021; Xian et al. 2019).

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Biological Properties

By comparing the biological properties of MXene with graphene it is declared that graphene oxide depicts excellent antibacterial activity as compared to Ti2 C MXene. However, in the case of Ti3 C2 significant antibacterial results have been obtained against E. coli and B. subtilis. When the experiment was performed by applying 200 μg/mL colloidal solution of Ti3 C2 , cells of both species of bacteria were damaged by about 98% after 4 h. This cell death occurred due to the rupture of the cell membrane after applying MXene-based drug as a result cytoplasmic material was released which stopped the function of the bacterial cell. MXenes can also stop the biofouling process efficiently and kill 70% of the bacterial population on waste or sustainable water. When Ti3 C2 films are used along with polyvinylidene fluoride (PVDF) support, they damage the bacterial growth of both E. coli and B. subtilis after they are exposed to 24 h. Besides this, two-dimensional MXene sheets depict promising anticancer activity against a variety of cancer cell lines such as lung and human melanoma cancer cell lines (A549 and A375, respectively). Therefore, these composites can be effectively used in bioscience and biotechnology fields. Apart from this drug treatment, MXene materials play an important role in cancer therapy as they can be used as ceramic photothermal (PT) agents such as Ti3 C2 , Nb2 C, Ta4 C3 , and Ti2 C. Similarly, soybean phospholipid (SP) based Ti3 C2 composites can effectively be used in the identification and treatment of cancer. MRI function of MnOx /Ti3 C2 composites has also been proved by Dai et al. which might be helpful in PTT anticancer analysis. Moreover, when these composites were functionalized with soybean phospholipid, they not only depicted a high PT effect but also inhibited the growth of the tumor considerably. By functionalizing iron oxide nanoparticles with Ti3 C2 , the resulting Ti3 C2 -Fe3 O4 nanocomposite can be competently used to kill cancer cells as described in a past research report by Liu et al. Similarly, Hussein et al. combined the properties of gold and iron oxide with MXene by synthesizing Au-MXene and Au-Fe3 O4 -MXene materials and highlighted their significant photothermal activity. Szuplewska et al. reported polyethylene glycol (PEG) functionalized Ti2 C material and proved its momentous biocompatibility and NIR-induced ablation ability via in vitro study. By synthesizing the Mo2 C hybrid, Zhang et al. introduced a novel MXene-like structure and evaluated its 24.95% photothermal conversion efficiency that proving its synergic behavior in anticancer therapy. Further biological studies on MXenes reveal that in the neuronal biocompatibility process, MXene can be successfully used to cultural neurons as they easily adhere, grow on MXenes, and perform their functions effectively (Dong et al. 2020; Huang et al. 2018).

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2.3 Applications of MXenes MXenes have widespread applications in different fields because of their superior characteristics to graphene and other two-dimensional materials. The better surface chemistry of this family along with high Young Modulus and excellent electrical conductive properties make them a prominent candidate for various electronic devices such as lithium/sodium-ion batteries, supercapacitors, and fuel cells. The composites of this family also take a significant part in catalysis owing to their stable chemical nature, ion intercalation, and adjustable band gap characteristics. An overview of these applications is as follows (Papadopoulou et al. 2020; Rahman et al. 2022):

2.3.1

Energy Storage Devices

The usage of MXenes in different types of energy storage devices is increasing day by day. Among these devices, lithium and sodium-ion batteries are the most important. According to the survey, a number of authors presented their research reports in which they made MXene-based anode materials for these types of batteries. For example, Zhu et al. synthesized zirconium functionalized MXenes in which OH, F, and O terminations were replaced with sulfur. The authors’ study declares the synthesized composite an ideal candidate for lithium-ion batteries as this material depicted excellent electrical conductivity with a high cycling rate and gave the battery a better lifespan. Similarly, Shukla et al. and Wang et al. proved that S-terminated MXenes play a prominent role in lithium/sodium-ion and lithium-sulfur batteries. Recently, Li et al. declared the excellent performance of chalcogenated MXenes in lithium-ion batteries where they were used as anode material and displayed high lithium storage capacity. Supercapacitors are also valuable and used as an alternative to energy storage gadgets due to their cycle stability and excellent charge–discharge capability as compared to batteries. And in this case, MXene is also taking a substantial part by giving outstanding volumetric capacitances of 900 F/cm3 in acidic solution and 340 F/cm3 capacitance in the basic solution.

2.3.2

Used as Sensors

Another applicability of MXenes is their usage in different types of sensors. For example, hemoglobin is immobilized on MXene (Ti3 C2 ), and the resulting composite can be utilized as a nitrite biosensor as it can identify nitrite samples in a water medium. In the same way, the functionalization of MXene with gold nanoparticles provides such composites that can sense glucose in the presence of different electroactive materials. Montazeri et al. (2019) introduced MXene/GaAs/MXene photodetectors and proved their higher responsivity and quantum efficiency than

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the other existing photodetectors. Besides this, MXene-based composites can use as pressure sensors, detect humidity in the atmosphere, and also sense human activities, e.g., swallowing, coughing, etc.

2.3.3

MXenes Polymetric Matrix Composites

Day by day increasing demand for polymers in the industrial field, there is a dire need to enhance the electrical and mechanical properties of the polymer matrixes and MXenes play a vital role in this regard. The composites of this family form better linkages with polymers due to their hydrophilic nature as compared to graphene and therefore, provide a variety of MXenes-based polymetric materials through simple and economical pathways. Some examples of these types of materials that show outstanding electrical conductivity are as follows: • Ti3 C2 Tx -based PDDA (Polydiallyldimethylammonium chloride) composite • Ti3 C2 Tx -based PVA (Polyvinyl alcohol) composite • Ti3 C2 Tx -based PAM (Polyacrylamide) composite. 2.3.4

Removal of Heavy Metal from Water

MXenes have also been used to remove heavy metals from water. The authors utilized Ti3 C2 Tx to extract copper content from water molecules. Experimental results declared that copper reacts with surface functional groups and oxidizes the targeted MXene composite.

3 Conclusion MXene which belongs to the class of two-dimensional compounds have a profound interest in researchers since 2011. Their intrinsic characteristics regarding hydrophilicity, electrical conductivity, and oxidative or thermal stability make them prominent for the construction of a variety of energy storage devices. Besides this, their biomedical applications especially their usage in cancer therapy as photothermal agents increase the popularity of MXenes to a greater extent. This chapter summarizes all synthetic pathways (etching and exfoliation methods) involved to prepare MXene composites as well as describes their unique properties in detail. Moreover, an overview of MXenes applications in the electronic industry has also been discussed paving the pathway for chemists to develop unique two-dimensional MXene materials in the future.

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Structural Design and Synthesis of Elemental Doped MXenes and MXenes-Based Composites Javeria Shoukat, Anila, Aqsa Iqbal, Muhammad Saleem Ashiq, Ataf Ali Altaf, and Samia Kausar

Abstract A rapidly developing two-dimensional materials belonging to the family of transition metal carbides and nitrides are referred to as “MXenes” or “maxenes.“ MXenes are primarily made from their MAX or Mn + 1AXn predecessors in which M is a member of the first transition metal family, A is a member of the A-group, and together they form a huge class of multilayer hexagonal compounds (typically from groups 13 and 14), X is C or N, and n is 1 to 3. This process involves selective etching and exfoliation over the course of two steps. Classifications of MXenes that are frequently investigated include elementally doped MXenes (EDMs) and MXene-based composites (MBCs). A number of applications, including batteries, supercapacitors, catalysts, cocatalysts, contaminant removal, and sensors, have stimulated interest in MXene and its doped components and composites. In this chapter, the varieties of MXenes and their structural and synthetic characteristics, as well as any prospective uses for them in electrochemistry and energy storage, will be briefly discussed. Keywords MXenes · MXenes composites · Elemental doped MXenes · Electrochemical · Energy

1 What Are MXenes? Over the past 15 years, 2D materials have been the subject of intensive research, ever since single-layer graphene’s special physical characteristics were discovered. Due to this interest, numerous new 2D materials have been discovered in addition to a fresh research surge-on 2D materials already known to exist, such as metal boron nitride and dichalcogenides. While many of these materials continue to be studied only J. Shoukat · Anila · A. Iqbal · M. S. Ashiq · A. A. Altaf (B) Department of Chemistry, University of Okara, Okara, Pakistan e-mail: [email protected] A. A. Altaf · S. Kausar Department of Chemistry, University of Gujrat, Hafiz Hayat Campus, Gujrat 50700, Pakistan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_3

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for academic purposes, others have gained attention because of appealing features that have led to real-world applications (Rizwan et al. 2022a, b, c, d). These latter include the swiftly expanding class of 2D materials known as MXenes (pronounced “mxenes”), which consists of transition metal carbides and nitrides (Rasheed et al. 2021; Rizwan et al. 2022a, b, c, d). The 2D MXene flake has layers of C or N interwoven with early transition metals that have n + 1 (n = 13) layers (Gogotsi et al. 2019). Chemical delamination of MAX phases, which are 3D ternary (or quaternary) compounds, typically results in the formation of a wide variety of transition metal carbides, nitrides, and carbonitride compounds; these compounds are collectively referred to as “MXenes. However, additional layered compounds like Zr3 Al3 C5 and Mo2 Ga2 C can also be used to make MXenes. The general formula for MXenes is Mn + 1 Xn Tx (n = 1–3), where M stands for a transition metal (such as Sc, Zr, Ti, Nb, and others), X is carbon and/or nitrogen, and Tx is the oxygen, hydroxyl, or fluorine conclusions obtained from the synthesis techniques. Nearly 30 molecules have already been synthesized since the discovery of the Ti3 C2 Tx 2D complex in 2011 and many more have been theoretically anticipated (Ronchi et al. 2019). MXenes are available in a wide variety of compositions and topologies, upon which a large and rapidly growing family of 2D materials has been created. MXenes, their precursor MAX phases, and intercalated metallic ions in MXenes act as representations of the vital chemical concepts (Gogotsi et al. 2019). The MAX phase is the predecessor to MXenes, with the formula Mn + 1AXn, where n is one, two, or three, and “M” is a transition metal from the d-block, “A” is an element, such as Si, Ge, Al, or Sn, and “X” is either carbon, nitrogen, or both (Ronchi et al. 2019). The MAX phases are layered hexagonal materials with X atoms located in the centers of the octahedrons. These materials are joined by pure A layers and can be thought of as transition metal carbide/nitride sheets of octahedral blocks (Naguib et al. 2021). A variety of applications have been made possible by the unique amalgamation of properties that MXenes possess, taking into account the mechanical characteristics of transition metal carbides and nitrides, high electrical conductivity, surfaces with functionalities that render MXenes hydrophilic, and capable of forming bond with members of other species, high negative zeta-potential, which enables effective electromagnetic wave absorption and stable colloidal solutions in water (Gogotsi et al. 2019). In this chapter, we shall discuss MXenes, their classes (elemental doped MXenes and MXenes-based composites), their structure, properties, and applications.

2 Structural Design of MXenes Layered ternary MAX phase serves as precursors in the top-down etching method frequently used to create MXenes. Above 100 several kinds of metal carbides and nitrides, which have the formula Mn+1 AXn , are collectively referred to as MAX phase. 2D layered materials’ bonding and stacking create a 3D crystal structure.

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Here, the “M” stands for early transition metals, the “A” stands for the primary group elements (generally groups 13 and 14), the “X” can be either C or N, and the “n” ranges from 1 to 4. The deformed octahedron [XM6] spreads laterally in an edge-sharing configuration during the MAX phase to create the “M-X” layer structure. The “A” layers have metal connections between the “A” and “M” atoms and are situated on each side of the “M-X” structures. The 2D layered materials known as MXenes, which are created by eliminating the “A” layers from the MAX phase, have a structure that alternately consists of n + 1 layers of “M” and n layers of “X” with numerous surface endings, such as –OH, –F, –O, or –Cl (represented as Tx). Currently, effective synthetic attempts have been made with more than 30 distinct configurations, and more than 100 anticipated stoichiometric compositions of MXenes have been made (Wei et al. 2021).

3 Types of MXenes 3.1 Elemental Doped MXenes (EDMs) Recent research has demonstrated the efficacy of enhancing pure MXene’s characteristics (like magnetic, electrical, optical, etc.) using basic doping methods. The graphene’s elemental doping mechanism served as an inspiration for this technology. By altering the surface or substituting a lattice on the original MXene, element doping modifies the features of the material itself based on the atomic structure, greatly enhancing device performance and stability (Wang et al. 2022).

3.1.1

Doping in EDMs

MXenes, due to their heteroatom doping, may be roughly categorized into different three groups depending on where element atoms are added and how they operate (Fig. 1) (Wang et al. 2022). • Lattice substitution • Functional substitution • Surface adsorption. Lattice Substitution Due to the substitution of certain lattice structure locations or voids brought on by element doping in situ or ex situ, the lattice structure (partial lattice mismatch) is created, which significantly alters the electrical characteristics of the material (Lu et al. 2020).

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Fig. 1 Mechanism of doping

Functional Substitution Functional substitution, also known as the replacement of specific functional groups or chemical bonds on a material’s surface, is a doping substitution technique that gives MXenes particular surface functionalities, substantially improving their electrochemical performance (Liao et al. 2021).

Surface Adsorption The 2D network structure of MXenes and its active sites are where surface adsorption takes place. By modifying the surface termination of MXenes, it is possible to maximize the important physical and chemical properties based on electrostatic interaction (Schultz et al. 2019).

3.1.2

Multidirectional Coordination

Additionally, based on the various compound doping techniques stated above, multidirectional coordination helps to enhance the physiochemical characteristics of individual MXenes. It is important to note that computer simulation indicated the significant impact of heteroatom doping on the physicochemical characteristics of MXenes, which encouraged the quick development of element-doped MXenes (Fatima et al. 2020; Gul et al. 2021).

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Due to the practical approach, cheap preparation expense, and significant performance enhancement, element doping as a strategy for increasing the performance of MXenes has quickly emerged in current years (Fatima et al. 2020).

3.1.3

Preparation Methods of EDMs

A number of complementary doping strategies, such as solid-phase fusion, liquidphase fusion, and some cutting-edge techniques like ion beam bombardment, are introduced in order to bring out the remarkable features of MXene in electrochemistry, magnetism, electrochemistry, and mechanics (Agnoli et al. 2016; Yu et al. 2019; Zheng et al. 2016). The shape, dispersion, and size of the particles are coupled to various steerable doping techniques under specific experimental settings, which directly affect the intriguing performance of MXene. Direct synthesis and posttreatment are the two categories of doping in terms of the order. The former involves carefully eliminating A layers after combining MAX phase with doped components, while the latter refers to doping components being reasonably absorbed into ready MXene (Rao et al. 2014).

Solid-Phase Synthesis a. Thermal Sintering Treatment Atoms of doping and MXene undergo a substitution process during thermal sintering (Sun 2020). For instance, a novel molecularly-imprinted (MIP) quartz crystal microbalance (QCM) sensor used for chlorpyrifos recognition is produced at 1700 °C in Selcen et al. studies which also involved the synthesis of S-doped Ti3 AlC2 . Direct synthesis techniques use a greater sintering temperature than post-treatment. While the successive stages of removing A layers also have an impact on morphology and structure. Vacuum freeze-drying will fold the nanosheet to increase the surface area and expand the dopant layer gap (Kadirsoy et al. 2020). b. Thermal Annealing Treatment Internal tension and functional groups are both eliminated concurrently by thermal annealing (Iqbal et al. 2020). For instance, Mo2 CTx combined with phosphorus is annealed at 500 °C in a tube furnace with P substituted -F functional groups. Keeping the advantages of MXene’s intrinsic structure in consideration, this technique increases the activity of the material’s surface (Qu et al. 2018).

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Liquid-Phase Synthesis a. Solvothermal and Hydrothermal Approach Due to their straightforward operation and accommodating synthesis conditions, solvothermal and hydrothermal methods are now useful tools for producing nanomaterials (Tan et al. 2017). They are thus used with materials that have unique morphologies and surface functionalization. Due to the low-temperature environment, the solvothermal pretreatment is more protective and prevents structural frame deterioration (Yan et al. 2017). Yang et al., for instance, used solvothermal treatment in their innovative research of ex situ doping. Using methanol as an adjuvant and diethanolamine as a nitrogen supply, they hypothesized that solvothermal treatment would result in more defects in MXene film because of high internal pressure. Then, the auxiliary agent helps to widen the nitrogen source’s molecular channel, resulting in quicker charge transfer and tighter structure formation (Yang et al. 2019). b. Coprecipitation Method Comparing the coprecipitation approach to solid-phase synthesis, this has better ideal dispersion and homogeneity. The majority of the nanomaterials created with this technique have tiny particle sizes (Wang et al. 2012). As an illustration of the role that rare earth elements have played in the development of magnetic electronic devices, lanthanum, another rare earth element, is coprecipitated into Ti3 C2 . Peng et al. hypothesized that because of this, Ru3+ prefers to make contact with the surface of MXene before being restored by Mo vacancy in the combination of SA Ru-Mo2 CTX . Effective activation promotion and a rise in ammonia yield in the electrochemical nitrogen reduction reaction are achieved (Peng et al. 2020). c. Strong Electrostatic Adsorption Strong electrostatic adsorption occurs when negative-charged MXene and the protonated dopant precursor interact electrostatically, resulting in the production of products with a narrow size distribution (Ng et al. 2017). For instance, Amiri et al. ammonium chloride may be used in place of nitrogen supply, and the electrostatic force causes the MXene nanosheets to fold rather than piled on top of one another. The system functions for seawater desalination and capacitor deionization (Amiri et al. 2020).

Emerging Technology Additionally, there are numerous cutting-edge doping techniques, including ion implantation, plasma doping, and the potential cycling method (Sun et al. 2021; Xia et al. 2021; Yu et al. 2019). Pazniak et al. performed an ion establishment procedure beneath the probe current, proper room voltage, and voltage in order to produce a faulty surface. MXene’s surface is oxidized as a result of abrasive sputtering between atoms, which significantly changes the electrical structure. The most

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notable feature is that ion beam implantation’s disorder contributes to improving the surface functionalization of atoms and functional groups (Pazniak et al. 2021).

3.2 MXenes-Based Composites (MBCs) MXenes’ flexible chemistries enable them to have a wide range of intriguing mechanical, electrical, magnetic, and electrochemical properties. It is predominantly facile for the MXenes to make composites with different substances because of their extreme elasticity, 2-dimensional morphology, and layered structures. As a result, MXenes and its composites have gained significant scientific attention and hold great promise for a range of applications. High conductivity and remarkable electrochemical activity make MXenes and MXene-based composites ideal for use as electrode materials in energy storage devices such as supercapacitors, lithium-sulfur batteries, and sodium-ion batteries (Zhao et al. 2016). It is incredible that they have recently gained more notoriety in fields related to the environment (Sun 2020).

3.2.1

Synthetic Methods for MBCs

A desirable method for creating sturdy and adaptable materials in recent years is the fabrication of composites. MXenes are regarded as ideal prospects because of 2D shape, layered structures, and exceptional flexibility (Fig. 2) (Zhan et al. 2020). In recent years, making composites has become a popular technique for developing resilient and multipurpose materials. MXenes have so far been used to make various distinctive composites by combining them with a number of substances, including metal oxides, polymers, and carbon nanotubes. It is possible to anticipate MXene composites with much-enhanced capacity for charge storage by incorporating more inorganic nanostructures (Zhang et al. 2020).

Calcination The calcination temperature has an impact on the MXene crystal morphologies and structures. The thermodynamically metastable MXene can be calcined at a particular temperature and can be transformed into densified transition metal oxide composites that are more stable. For example, a Ti3 C2 /TiO2 /CuO ternary nanocomposite might be produced by giving heat to the cupric nitrate on the surface of Ti3 C2 at 500 °C in an atmosphere of Ar (Zhang et al. 2018).

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Fig. 2 Synthetic methods of MXene composite

Selective Etching Single or small atomic layers are frequently separated to create 2D materials in layered compounds when the connections between the layers are noticeably weaker than those between the individual layers. In order to create MXenes, one or more atomic layers can be removed from a layered material with a comparatively solid interlayer link. In general, the most frequent ancestors of MXenes are huge ternary nitride phases known as the MAX phases, which contain a variety of various carbides and topologies. Many scientists have claimed that the etching of single layers, mainly Al layers, produces MXenes.

Solvothermal Technique In the development of highly crystalline composite MXene-inorganic nanostructures, the (hydro-)solvothermal technique is equally as effective as thermal annealing. Pan et al. created a sandwich-like Ti3 C2 /CuS composite by reacting thioacetamide (TAA), Cu (NO3 )2 ·3H2 O, and Ti3 C2 precipitates in an ethylene glycol solution at 150 °C for 9 h. After solvothermal treatment, the positively charged Cu2+ ions are homogeneously attracted to the negatively charged Ti3 C2 surface due to the electrostatic interaction, creating a homogenous composite (Zhang et al. 2020).

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Exfoliation Hydrogen bonds or Van der Waals forces keep the Mn+1 XnTz multilayers (MLs) together, which are the building blocks of the material. Terminations of various types are used to replace the atomic layers that have been selectively removed by etching. Therefore, it is feasible to exfoliate or distort single layers to produce colloidal aqueous phase suspensions. In this case, methods like intercalation with cations, intercalation with large organic molecules, sonication, or shaking are used, which are frequently used to generate other two-dimensional materials. The exfoliation method to be utilized is determined by the etching circumstances (Coleman et al. 2011).

3.2.2

Different Types of MBCs

MXenes-Polymer Composites (MPCs) When used to create composites, MXenes have the potential to enhance the polymers’ mechanical and thermal properties by having excellent mechanical properties, metallic conductivity, and hydrophilic surfaces. Multi-layered MXenes are less compatible with polymers and have lower accessible surface hydrophilicity than single-layer MXenes. MXenes are therefore often delaminated before being combined with polymers (Zhan et al. 2020). Some MXenes-polymer composites are described below: a. Ti3 C2 Tx–UHMWPE composite According to reports, UHMWPE (ultrahigh molecular weight polyethylene) can combine with Ti3 C2 Tx to create composite materials. The enhanced surface Ti3 C2 Tx powders must be manufactured prior to the fabrication of the Ti3 C2 Tx-UHMWPE composite, and the adapted surface can enhance the dispersion stability and compatibility of Ti3 C2 Tx in UHMWPE (H. Zhang et al. 2016a, b). The fabricated Ti3 C2 TxUHMWPE composites with various Ti3 C2 Tx weights are stronger than UHMWPE (Zhang et al. 2016a, b). b. Other MXenes-Polymer Composites In addition to the aforementioned polymer, composites containing MXenes can also be made from poly (acrylic acid), poly (ethylene oxide), polyvinylpyrrolidone (PVP), and alginate/PEO. In general, MXene-polymer composites outperform isolated MXenes and polymers in terms of mechanical performance. They also exhibit strong electric conductivity, making several of them are capable of use in wearable electronic devices (Mayerberger et al. 2017).

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MXene Oxide Composites a. Nb2 O5 –Nb4 C3 Tx composite According to Zhang et al., oxidizing Nb4 C3 Tx powders in streaming CO2 at 850 °C for 0.5 h can easily produce a coated orthorhombic Nb2 O5 -Nb4 C3 Tx hierarchical composite. The ion diffusion routes are successfully shortened in the Nb2 O5 Nb4 C3 Tx composite due to the homogeneous distribution of Nb2 O5 at the Nb4 C3 Tx sheets’ edges and interlayers. The composite’s overall electronic conductivity is supported by the interior, unoxidized Nb4 C3 Tx. The high electrochemical and cycling capabilities of the Nb2 O5 -Nb4 C3 Tx composite are caused by the two aforementioned aspects (Zheng et al. 2016). b. Li4 Ti5 O12 –Ti3 C2 Tx composite Wang et al. provide a straightforward method to produce a composite made of Li4 Ti5 O12 and Ti3 C2 Tx. A potential anode substance for lithium-ion batteries (LIBs), the synthesized Li4 Ti5 O12 -Ti3 C2 Tx composite also possesses remarkable electrochemical characteristics. The migration routes of lithium ions are significantly shortened when the Li4 Ti5 O12 -Ti3 C2 Tx electrode is submerged in the electrolyte because Li4 Ti5 O12 with low lithium-ion dispersion fences develops on the exterior of the MXene. Additionally, Ti3 C2 Tx’s excellent electrical conductivity guarantees quick electron transport from the electrolyte to the electrode. Due to the quick electrical and ionic diffusion of the Li4 Ti5 O12 -Ti3 C2 Tx electrode, the performance of lithium storage has improved (Wang et al. 2018).

MXene-Carbon Nanotube (CNT) Composites a. Ti3 C2 Tx–CNT Composites Ti3 C2 Tx-CNT composites are created by Zhao et al. and used in electrochemical capacitors. Up till there are 6–10 layers in total, the CNT layers and Ti3 C2 Tx Each layer is placed alternately on top of the one below it which is a schematic of how the Ti3C2Tx-CNT composite is made. The microscopic microstructure of the artificial Ti3 C2 Tx-CNT composite reveals the sandwich-like superposition of the MXene and CNT layers. Comparing isolated MXene to the Ti3 C2 Tx-CNT, which serves as the supercapacitor’s electrode, reveals much greater volumetric capacitance and rate performance (Zhao et al. 2015).

MXene-Graphene Composites a. Ti3 C2 Tx/ rGO Composite Recently, Chen et al. created the 3D macroscopic hydrogel using a self-assembly mechanism facilitated by graphene oxide (GO). It has been discovered that GO can

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be reduced using the reduction capacity of Ti3 C2 Tx and that removing some of the surface oxygen-containing hydrophilic groups will help to create a balance between hydrophilic and hydrophobic groups. Through the interaction of hydrogen bonds, Van der Waals forces, and electrostatic interactions between interfaces, Ti3 C2 Tx is incorporated into the 3D rGO framework. These studies offer a solid foundation for the further design and development of flexible 3D materials based on MXenes to enhance their rate performance and volume capacitance (Liu et al. 2020).

3.3 Applications of MXenes As Pseudo Capacitors Pseudo-capacitors are able to store energy by a reversible Faradaic-type charge transfer on the thin surface layer of electrode materials. Pseudo-capacitors have received increasing attention recently due to their advantages over electrical doublelayer capacitors (EDLCs), such as their better energy density and higher power density. However, because of their poor surface-to-volume ratio and low electrical conductivity, most pseudo-capacitor electrodes have low-rate capability and poor cycle stability. Because of their redox-active surface locations and electrically conductive metal carbides and nitrides cores, MXenes have high capacitance. For instance, Ti3 C2 O0.84(6)(OH)0.06(2)F0.25(8) has a heat capacity of 615 C g1, but it can also satisfactorily answer the aforementioned issues, making it one of the most promising pseudo-capacitor electrode types (Bu et al. 2020). Lithium/sodium-ion Batteries On account of their potential for high-rate performance, batteries that store lithium and sodium ions in pseudocapacitive state mechanisms have drawn a lot of attention. In this instance, research has focused on ultrathin 2D layered materials which may respond to Li/Na, such as metal dichalcogenides, covalent organic frameworks, and black phosphorus. But such materials’ weak electric conductivity has significantly diminished their electrochemical qualities. MXenes have become one type of distinctive anode constituent for lithium/sodium-ion batteries by taking advantage of the enhanced electrical conductivity in redox-active metal carbides and nitrides. MXenes have a high theoretical capacity, according to even theoretical calculations. For instance, MXenes contain light transition metals (Sc, Ti, V, and Cr) with nonfunctionalized or O-terminated surfaces that have gravimetric capacities > 400 mAh/g. However, their electrodes currently only exhibit low-rate consummation, inferior capacity, and poor cycling steadiness. The fundamental reason for this behavior is the restacking of MXenes; as a result, permeable structures have been developed to enhance their electrochemical characteristics. This way, the electrochemical characteristics of MXenes can be significantly enhanced since ion transportation can be effectively encouraged, and active areas can be completely exposed (Bu et al. 2020).

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Piezoresistive sensors Piezoresistive sensors have garnered a lot of interest since they can be produced cheaply, collected signals easily, and have many essential uses. Some of them include skin-inspired electronic gadgets, a smart display, and portable healthcare monitoring. These sensors convert pressure from outside sources into a resistance signal by altering the contact of conductive fillers. Excellent-performance piezoresistive sensors based on MXenes have been proposed owing to the material’s high conductivity and processability. Pure MXenes, however, find it challenging to achieve high sensitivity due to thick MXene nanosheet stacking. In order to enhance their contact and subsequently conductivity, it is crucial to introduce MXenes into a variety of porous substrates (Bu et al. 2020). Cleaning the Planet One of MXenes’ promising qualities, water remediation, will be the main emphasis of this section. Because water resources are becoming more scarce worries about water contamination are growing. As a result, scientists are experimenting with various technologies to eliminate contaminants from water, such as radioactive chemicals, heavy metal ions (HMI), medicines, colors, and bacterial materials. From the aquatic setting and revive this valuable resource. In addition to the aforementioned water pollutants, electromagnetic waves are another type of contamination. Due to the presence of activated metallic hydroxide sites, MXenes have multiple active sites, a high surface area, and are hydrophilic and environmentally benign. Although these valuable materials have drawbacks like a propensity for aggregation and oxidation, by combining them with other materials and creating a composite, it is possible to increase the synergistic effect, lessen aggregation and increase stability, and ultimately improve the final properties. They are prone to pollutant remediation due to these characteristics. There are numerous methods for cleaning up contaminants, including adsorption, photocatalytic degradation, membrane separation, and others (Bilal et al. 2021; Mallakpour et al. 2021; Qamar et al. 2022; Rasheed, Rizwan, Shafi, & Bilal 2022; Rizwan, Bilal, et al. 2022a, b, c, d; Rizwan et al. 2022a, b, c, d; Shakeel, Rizwan et al. 2022). Supercapacitors In contrast to batteries, supercapacitors offer alternate energy storage with a fast power density but a lower energy density. So, the main focus of research has been on increasing their volumetric capacity, or energy density per volume. Supercapacitors are categorized as pseudocapacitors or electrical double-layer capacitors (EDLCs) based on their charge–discharge operations. The latter relies on quick and reversible surface redox reactions while maintaining rectangular-shaped CV curves. The earlier method is predicated on the reversible electrolyte ion accumulation at electrode–electrolyte interactions in the absence of redox processes. In general, pseudo capacitors have weaker cycle stability and greater volumetric capacitances. MXenes show themselves as interesting electrode materials for supercapacitors due to their 2D properties, huge surface areas, and well-defined shape (Mallakpour et al. 2021).

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Photocatalysts One of the most effective catalysts for the development of hydrogen is platinum. However, because of its high price and limited supply, platinum use is restricted. Because MXenes can function as effective catalysts or photocatalysts for HER, they have recently received a lot of attention from both theoretical and experimental researchers. Experimental measurements of the HER activities of Ti2 CTx, Ti3 C2 Tx, and Mo2 CTx (T: mixture of O and OH) have been made. When producing hydrogen from water, Mo2 CTx outperforms Ti2 CTx in terms of HER activity, but Ti3 C2 Tx can improve catalytic activity when producing hydrogen from ammonia borane (Khazaei et al. 2017). Thermoelectric Devices MXenes’ intrinsic ceramic nature makes them potentially attractive as thermoelectric materials for energy conversion applications at high temperatures. The dimensionless character of merit ZT, which has the formula S 2 T/K, is used to measure the performance of thermoelectric materials. σ, S, T, and K (= kl + ke) represent electrical conductivity, Seebeck coefficient, temperature, and thermal conductivity, respectively, with contributions from both the lattice and the electronic worlds. As an illustration, metallic MXenes for Nb2CF2 to have low power factor due to strong electrical conductivity but low Seebeck coefficient. The semiconducting MXenes, on the other hand, have great Seebeck coefficient despite having very poor electric conductivity, which leads to a reasonably strong power factor (Khazaei et al. 2017). As Biosensors The following components make up the majority of biosensing, which is mostly used to specifically identify particular compounds in the human body. (i)

A sensing component, such as a biomolecule immobilized, that recognizes its corresponding analytic. (ii) A transducer that can convert biological signals into electrical signals and other signals like optical signals. (iii) A unit for data interpretation. By reason of their expansive surface areas, great electrical properties, amazing hydrophilicity, 2D layered atomic structures, extraordinary optical properties, and an abundance of surface functional groups, MXenes have gained popularity as a material for biosensors (Huang, Li, Mao, & Li, 2021). Bioimaging Diagnostic imaging is made possible by the numerous physicochemical characteristics of 2D nanosheets, including element-enhanced contrast, intrinsic photothermal performance, quantum size effects, and active loading of functional contrast agents (CAs). This suggests that MXenes have the potential to enhance the diagnosticimaging performance. Diagnostic imaging, which includes photoacoustic imaging (PAI), X-ray computed tomography (CT), magnetic resonance imaging (MRI), and

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luminescence imaging, can identify the location of the tumor and direct PTT. Multimode imaging is occasionally used in conjunction with photothermal therapy (Huang et al. 2021).

4 Conclusion MXenes, also referred to as “MXenes,” are a new category of 2D material formed of transition metal carbides and nitrides. Through a two-step process that includes selective etching and exfoliation, MXenes are primarily created from their MAX originators. MXenes that have been thoroughly studied include those that have been doped with elements and those that are MXene-based composites. Interest in MXene and its doped components and composites has been sparked by applications in electromagnetic interference shielding, energy storage, biomedicine, sensors, supercapacitors, electrocatalyst, photocatalyst, electromagnetic shielding, seawater desalination, field-effect transistor, and in other domains. Other areas offer a wider range of possible applications due to their highly variable surface functional groups and metallic compositions. MXenes can be used to store energy electrochemically. In recent years, creating composites has become a popular method for creating resilient and adaptable materials. Due to their outstanding flexibility, layered structure, and two-dimensional morphologies, MXenes are regarded as a potential material for the creation of multifunctional composites. This has sparked an increase in research into MXene-based composites. MXenes have thus far been combined with a wide range of other materials, for instance, metal oxides, polymers, and carbon nanotubes, to create a variety of unique composites. MXenes have a wide range of fascinating mechanical, electrical, magnetic, and electrochemical properties thanks to their adaptable chemistries. MXenes and their composites are utilized in storing the energy, first as highly efficient electrode materials for sodium-ion, Li–S, and supercapacitor batteries. However, there is further progress needed in the development of more versatile MXene composites for use in electrochemical energy-storing devices. Conflict of Interest The authors declare no conflict of interest.

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Functionalized MXene-Based Polymer Composites Umer Raza, Hafiz Abdul Mannan, Atif Islam, Tabinda Riaz, and Sidra Saleemi

Abstract Mxene has attracted a lot of attention since their birth because of their outstanding and functional features related to their huge surface areas and electrical characteristics. Several Mxene composites have been considered over the past few years, but the structural and biological characteristics of these composites are not inspiring. This situation directs toward more innovative and flexible approaches. Polymers are an excellent alternative for making Mxene composites because of their versatility, compatibility, and low cost. This chapter discusses recent developments in MXene-based polymer composites. Structures and properties of various MXenebased polymer composites are discussed. Moreover, applications of these composites in various domains are discussed highlighting their promising potential in research and providing a vision of their future applications. Keywords Mxene · polymeric · Composites · Cost-effective

1 Introduction Owing to their huge surface area, extensive-bandgap modulation volume, excessive catalytic property, as well as other characteristics, 2D substances, and other nanomaterials are now a popular topic in the scientific community. In several industries including energy, catalysis, biomedical, medicine, optoelectronics, and biological and environmental defense, these molecules show enormous potential for attaining ad hoc applications and transformations (Rizwan et al. 2022a, b; Qamar et al. 2022; Rasheed et al. 2022; Shakeel et al. 2022; Bilal et al. 2022; Rizwan 2022). These substances lie under a broad group and include amalgams like double layered hydroxide (LDH), transition metal oxides, transition metal sulfides, and MXene as well as pure elements like borophene, silicene, graphene, and black phosphorus (Zhu et al. 2017; Zhou and Jiang 2017). The “A” layer element in MAX phase ceramics is U. Raza · H. A. Mannan (B) · A. Islam · T. Riaz · S. Saleemi Institute of Polymer and Textile Engineering, University of the Punjab, Lahore 54590, Pakistan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_4

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the newest addition to the 2D substance background because of its decreased detection. It is etched to produce transition metal, nitrides, or carbonitrides (Coleman et al. 2011). Exceptional surface chemistry, hydrophilicity, and conductivity (greater than 100 S/cm) are all characteristics of MXenes (CB and Anasori 2016; Khazaei et al. 2019). The surface conductivity of graphene is maintained even when it adsorbs or reacts with other chemical classes or materials, which is an obvious benefit (Mariano et al. 2016). Researchers are intrigued by MXene’s outstanding electrochemical, optical, magnetic, electrical, and water barrier characteristics, among its many other uses (Carey et al. 2019). The MAX phase is especially receptive to conversion to the equivalent (MXene) (Khazaei et al. 2014; Alhabeb et al. 2018; Sun et al. 2018) by acid solutions that produce fluoride, (Zhang et al. 2017; Ghidiu et al. 2014) at higher temperatures (Horlait et al. 2016) under some circumstances, such as those connected with creating hydrofluoric acid (HF). Because the M-A bonds are replaced by M-F, M-O, M-OH, and M-H bonds during the etching process, the MXenes can be represented as Mn+1 Xn or Mn+1 Xn Tx where T is O, or F, H, or OH, while M and X are as in the MAX phase. Moreover, 2D transition metal carbides as well as those made from precursors outside the MAX phase can be included. For example, 2D Mo2 C was produced via chemical vapor deposition using methane and molybdenum (CVD) (Gogotsi 2015; Xu et al. 2015). Since polymers have been for a while, it is recognized that they are a formidable rival to the 2D materials created before MXene. These extended carbon molecule chains are combined with other substances to change their chemical and mechanical properties. Polymers’ exceptional adaptability fuels their ongoing development as a scientific discipline. These materials are distinctive for their resilience to abrasion, impact resistance, fatigue resistance, fracture resistance, and corrosion resistance. They are also desirable for their low cost and simplicity of production. Due to changes made to the dissemination stage addition, such as flakes, particles, and fibers, as well as laminates the interfacial tension they provide, polymer matrix composites (PMC) have accomplished remarkable technological accomplishments. Research on polymers as an effective immobilizing surface for making composite surfaces, such as spray paint, has been ongoing for a while. Unquestionably, polymers provide an excellent composite material to be included in MXene (Basiron 2018). Using hydrophilic corona poly(2-ethyl-2-oxazoline) (POX) and hydrophobic inner poly(caprolactone), or stabilizing Fe3 O4 nanoparticles in aqueous media, it is possible to improve the refractive index of polystyrene (Safari et al. 2014). Conducting polymers like polypyrrole have undergone extensive research since MXene attracted significant attention in the transfer and energy storage fields in combination with polymers. MXene is added to many other polymers due to their 2D structure for their distinctive and intriguing properties. These investigations have been successful despite their flaws and have helped open the door for future development in technologies and scientific fields that had not yet been thoroughly investigated. Polymer composites that were produced and investigated by several research teams were employed for a variety of purposes, including wastewater treatment,

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photothermal conversion, and increasing mechanical properties (Gore and Kandasubramanian 2018; Gore et al. 2016; Rastogi and Kandasubramanian 2019). These composites were created shortly after employing a variety of manufacturing and grafting techniques, giving them their special characteristics, and enabling them to get beyond the constraints given by the current technology. Many MXene polymer composites are reviewed in this chapter, within an attention on their existing characteristics, as well as the amazing results they have achieved and how they vary from other 2D materials working in the same field.

2 Structures and Properties Due to the loss of a phase from MAX phase, MXene was formed along with the dispersion of their basal fans, giving them an accordion-like structure. Ti3 C2 Tx seems to be the most typical Mxene, therefore, Lipatov et al. (2016) studied the electrical characteristics of the Ti3 C2 Tx flakes (Lipatov et al. 2016). The results showed that the electron mobility of this uni-layer was 2.6–0.7 cm2 /V, and its significant conductance was 4600–1100 S/cm. According to studies on the flakes’ hardness and oxidation, they are balanced, and after their initial severe breakdown, the conductivity fell linearly as a result of the oxidation of their edges (Lipatov et al. 2016). The properties of MXene-based composites are dependent on their structure and constituents. In the following sections, structures and properties of a few MXene-based polymer composites are discussed.

2.1 Polyvinyl Butyral Composites of MXene Polyvinyl butyral (PVB), whose structural formula is shown in Fig. 1, is the result of the alcohol and aldehyde process. PVB has been widely employed in glass applications due to its clarity to create overlay glass sheets that are now often seen in cars and tall structures. Between two glass sheets, PVB is used to form a safety barrier against chipped glass, which is hazardous after car accidents. Additionally, it offers a stylish solution for adding anti-break and anti-shock characteristics to higher stand-up constructions. Therefore, glass composites and PVB have a wide range of uses, from frosted glass plates to artistic glass (Jimmy and Kandasubramanian 2020; McKeen 2017). These characteristics made it possible to realize that MXene polymer composites may be produced with adhesive capabilities similar to PVB. A composite made of MXene, PVB, Ba3 Co2 Fe24 O41 , and Ti3 C2 was produced by Yang et al., (2017) which have electromagnetic (EM) wave absorption capabilities across wider frequency limit. Successfully etching aluminum creates titanium carbide MXene, Co2 Z particles were attached to 41% volume MEK and 60% volume ethanol, and the mixture

50 Fig. 1 Chemical structure of polyvinyl butyral (PVB) (Jimmy and Kandasubramanian 2020)

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H2 C

H2 C HC

H C

O

O

C 3 H7

was ball-milled for four hours. PVB served as a plasticizer and binder for polyethylene glycol and diethyl phthalates. After ball milling, a thin sheet of the mixture with a thickness of just 200 μm was tape cast to make the multilayer polymer composite. These multilayered composites were incredibly good in attenuating and absorbing EM radiation. The polyvinyl butyral/Co2 Z/titanium carbide MXene composite had a maximum reflection loss (RL) of 46.3 dB at 5.8 GHz, with 1.6 GHz being the realistic absorption bandwidth with losses under 10 dB. In light of these findings, the polyvinyl butyral/Co2 Z/ titanium carbide MXene composite showed sophisticated as well as beneficent results in the side of low-weight electromagnetic wave absorption (Yang et al. 2017). The radar absorption technique now used in planes with limited eyesight will use these exceedingly small composite materials.

2.2 UHMWPE Composites of MXene A subtype of thermoplastic polyethylene with noticeably long chains and a molecular weight more than millions is referred to as “ultra-high molecular weight polyethylene” (UHMWPE). Thermoplastics may be reshaped using pressure and heat. These polymers, which may or may not be crystalline in nature, are made up of straight-chain or branching molecules that interact with one another intramolecular either weakly or strongly. These kinds of long chains make it easier to transfer loads and produce thermoplastic with a high impact strength. UHMWPE is an attractive alternative for polymer part in a MXene polymer composite due to its numerous advantageous properties, including its superior resistance to abrasion, decreased moisture absorption, and corrosion-resistant chemical composition. It may be used in a range of applications, including fishing nets and armors. The composite was created by evenly mixing Ti3 C2 and UHMWPE using a higher-speed mixer (Park and Seo 2011). The composite was then formed using a press vulcanize. Ti3 C2 has a lamellar structure like that of clay or graphite thanks to the existence of microparticles, which are visible in the material’s shape and numerous nano sheets. Researchers examining the morphology of the surfaces of nanocomposites found spherulites with diameters ranging from 150 to 200 nm using a SEM. The core of these spherulites was Ti3 C2 . Spherulites formed more quickly at greater Ti3 C2 concentrations, proving that these

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MXene particles served as polymer chain nucleation sites that eventually gave birth to numerous spherulites. The stiffness, crosslink density close to filler fields, as well as TS characteristics of composite also improved. Stress was released in two separate ways when UHMWPE chains were physically adsorbed to the MXene surface: first, by physically severing the cross-links holding them together, and second, by transporting stress from UHMWPE medium into titanium carbide particles, opening the door to controlling final mechanical characteristics. After MXene expansion was finished, the increased rehabilitative feature’s UHMWPE creep strain was decreased. Ti3 C2 makes the composite more crystalline, which lowers the coefficient of friction. Graphene and Ti3 C2 exhibit weak van der Waals bonding in multilayer systems, exfoliate into finer layers under high shear, and function as lubricants because of their similar morphologies. The concentration of MXene added to the composite is what gives a flexible polymer like UHMWPE improved mechanical characteristics. The Ti3 C2 /UHMWPE composite displayed all the louder qualities of MXene polymer composite in accumulation to a general improvement over the clear polymer matrix. Because of its reduced moisture absorption capacity, this composite is being used in systems such as marine-based equipment, conveyor belts, and even food processing.

2.3 Polyether Sulfone Composites of MXene When managed with standard plastics handling tools, polyether sulfone (PES) (Fig. 2) is categorized as a higher temperature thermoplastic as well as shapeless polymer within CAS. It has a high capacity to endure increasing temperatures in both water and air for lengthy periods of time due to its formlessness and low mold compression. In addition to having a high thermal resistance, PES also offers outstanding mechanical, chemical, electrical, and flame resistance. Additionally, it exhibits very strong resistance to hydrolysis and optical clarity (Zhang et al. 2016). Due to its high perm selectivity, heat resistance, and ultrafiltration (UF) properties, PES membranes have been widely used in waste water treatment and desalination (Wenten 2016). A membrane with a surface area of 48 cm2 was created by combining Ti3 C2 Tx MXene and PES matrix for utilizing in a dead-end filtering procedure. It was shown that the membrane flow was initially raised and then decreased when there was a high concentration of MXene on the membrane surface. In addition, dye rejected more effectively due to membrane’s hydrophobic behavior, which led to dye molecules adhering and reducing flow. On the other hand, the addition of MXene to membrane surface enhanced hydrophilicity of membrane or decreased fouling. Fig. 2 Chemical structure of polyether sulfone (Jimmy and Kandasubramanian 2020)

O O

S O

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The MXene particles’ multilayered structure and dense assembly increased the composite permanent selectivity and resulted in an acceptable flow. These composite membranes showed high perm selectivity at 0.3 g of MXene that allowed for extra investigation using a different dye, gentian violet. It was clear from the membrane’s higher rejection rate of 80.3% and flow of 117.69L/m2 .h that it was effective in treating and removing wastewater containing dyes (Han et al. 2017).

2.4 Chitosan Composites of Mxene Due to its biocompatibility and degradability, MXene was subjected to several attempts to be incorporated into polymers, which led the researchers to consider one of the essential amino polysaccharides known as chitosan (CS) as the polymer matrix in MXene-based polymer composites. These most important polymer composites have biological significance because CS possesses qualities that are antiinflammatory, antibacterial, hypoglycemic, and trap cholesterol and triglycerides (Chawla et al. 2015). Deacetylating the chitin of several crustaceans and insects produces CS. Due to their molecular weight and level of acetylation, natural polymer properties like biodegradability, solubility, substance-forming ability, etc., are also present in CS (available with different degrees of deacetylation) and desired for environmental engineering, agriculture and food applications (Lizardi-Mendoza et al. 2016). Due to its high conductivity and similarities to graphene, MXene is an excellent electrode material. An acetylcholinesterase (AChE)/CS-Ti3 C2 Tx /GCE biosensor was made by spreading Ti3 C2 Tx nano sheets in 0.20% CS solution and coating it with the AChE solution. Electrode with the largest electroactive surface area and the least amount of disturbance from electron migration was CS-Ti3 C2 Tx /GCE, which was created by further dividing the three electrodes. Acetylcholinesterase/CS-Ti3 C2 Tx /GCE showed an oxidation peak at 655 mV when this electrochemical activity was evaluated against 1.0 mM ATCl. The findings showed that the competition for the enzyme’s binding sites between citric acid, glucose, K+ , Cl− , PO4 3− , Fe3+ , and SO4 2− is insignificant. The AChE/CS-Ti3 C2 Tx /GCE biosensor was evaluated for storage durability using the fifth changed enzyme electrodes while in 1.0 mM ATCl, and it was able to keep 86.40% of its primary feedback next 37 days. Eventually, a Malathion improvement test on tap water revealed limits of 94–105%, showing the dependability of these types of sensors. As a result, the diagnostic limits of a biosensor for AChE/CSTi3 C2 Tx /GCE were reduced (Zhou et al. 2017). It also had a wide fine range, acceptable stability, high sensitivity, and outstanding repeatability. Owing to its excellent consistency and revealing value, MXene biosensor based on natural polymers may be regarded as state-of-the-art. The tracking of food processing might be included to the composite scope of use. Future in-vivo and in vitro applications might benefit from the design and technological advancements of this unique biosensor.

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3 Applications of MXene Polymer Composites As we have discussed in the earlier sections, MXene polymer composites combine the advantages of MXenes and polymers, increasing the interlayer spacing of MXene layers and improving structural stability and mechanical toughness. Considering this, MXene polymer composites can show noteworthy performance in a variety of domains, including energy storage, electromagnetic interference (EMI) shielding, sensing, photothermal conversion, and biomedical applications. In the following sections, some of the most promising applications of MXene polymer composites will be discussed.

3.1 Energy Storage Due to their higher specific area and higher conductivity, 2D materials like graphene (Zang et al. 2020; Zhang and Nicolosi 2019) MoS2 nano sheets (Liu et al. 2019; Choi et al. 2018) and MXene (Chen et al. 2019; An, et al. 2018) have been extensively investigated in the production of energy storage devices. MXene-based substances are the ideal contenders for electrochemistry energy storage gadgets, such as super capacitors (Javed et al. 2019; Fu et al. 2018) metal based ion batteries (Liu et al. 2018), and Li–S batteries (Bao et al. 2019; Zhao et al. 2019; Jiao et al. 2019; Zhang et al. 2020). Diffusion rate of Li as well as sodium ions can enhance higher interlayer spacing, which leads to higher conductivity and electric capacity (Zhao et al. 2020). Consequently, an expansive interlayer spacing of MXene is an excellent procedure which upgrades conductivity and electric capacity of MXene-based substances. A p-phenylenediamine 300/Ti3 C2 Tx composite was manufactured as an anode of Liion batteries (Dong et al. 2018). Hydrogen bonds were used to cover the PDA on the MXene surface. Inter layer space of MXene increased from 0.99 to 1.48 nm by intercalation of PDA. By enlarging ion-migration lanes, the wide interlayer spacing of MXene nano sheets improved lithium-ion migration. The conductivity and cycling stability were improved by the quick migration of lithium ions (82% retained after one thousand cycles). The composite electrode displayed a high specific capacity (1100 m Ah/g) and a significant rate execution (1190 m Ah/g at 50 mA/g) due to their better metallic conductivity and higher hydrophilic surface. Additionally, the cross-linked linkage among MXene and p-phenylenediamine has given the most energetic sites for Li ion storage. In order to enhance power density as well as energy density instantaneously, more ionic or electronic conductivity is required. The restacking of MXene is a key issue for producing energy storage devices, similar to other 2D materials, as it causes decreased ion availability, sluggish ion kinetics, or considerable volumetric variation through cycling. Because of the wide interlayer spacing and stable ordered parallel lamellar structure, MXene polymer-based ion batteries/super capacitors were able to achieve extremely extended cycle stability (He et al. 2021).

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By embedding the polymers between the MXene layers, in situ polymerization may be conducted to improve the interlayer spacing. Wu and others (Wu et al. 2019) used a distributed conductive polymer (PDT) and MXene to create an all-solid-state super capacitor using in situ polymerization on the surface of the material. Strong hydrogen bonding between the NH groups and the oxygen groups of Ti3 C2 Tx were facilitated by the scattered chains of PDT in PDT/MXene composites. Additionally, the combination of PDT and MXene can improve the structural stability of the PDT/MXene composite and provide precise lanes for charge transfer. These actions significantly improved the cycle stability and capacitance of the energy storage devices. A Ti3 C2 Tx /polypyrrole (PPy) composite film for energy storage was created by in situ polymerization by Zhu et al., (2016) . In order to increase the structural hardness of the film and give the film cycle robustness, hydrogen bonds were used. The Ti3 C2 Tx MXene and PPy have strong hydrogen bonds that create paths for charge-carrier movement, improving the pseudo-capacitive technique and ensuring a better conductivity. The PPy/Ti3 C2 Tx based super capacitor with PVA-H2 SO4 as the solid-state electrolyte has a higher capacitance due to the malleability of all the components (35 mF/cm2 ).

3.2 Biomedical Applications The research on biomedical applications of MXene/polymer nanocomposites is in the initial stages of development. We are aware that MXene has a variation of end groups (–oxygen, –hydroxyl group, –fluorine) on its surface, which contributes to its exceptional hydrophilicity, great surface sensitivity, high biocompatibility, and good electric and optical characteristics. MXene has, therefore, exciting potential for use in biological applications (Soleymaniha et al. 2019; Qin et al. 2021; Lin et al. 2018; Huang et al. 2018). In addition, it has been used in the treatment of cancer (Feng et al. 2019; Liu et al. 2017), as an antibacterial substance (Mayerberger et al. 2018; DJ 2017; Rasool 2016), in drug administration (Han et al. 2018) in between the other uses. Chen et al., (2017) investigated the cytocompatibility of Ti3 AlC2 , Ti3 SiC2 , and Ti2 AlN using the first principles theory. A Ti3 C2 Tx /polylactic acid (PLA) nanocomposite membrane was created for the leading bone changeover (Chen et al. 2018a). Berger et al., (2018) (Mayerberger et al. 2018) studied antiseptic characteristics of MXene polymer nanocomposites. Ti3 C2 Tz (0.75 wt. %) chitosan composite nanofibers with glutaraldehyde cross-linking were created via electrospinning. The calculations showed that the reduction in colony forming units of Escherichia coli (E. coli) and Staphylococcus aureus (S. aureus) was 95% and 62%, respectively. As a result, the fibers were found to be possessing the potential to be utilized as a bandage. A Ti3 C2 /cellulose composite hydrogel was presented by Xing et al., (2018) . They discovered that it is possible to combine MXene’s photothermal transmutation

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powers with cellulose’s exceptional biodegradability, biocompatibility, and drugloading properties. The first findings of the examining show that hydrogel and realistic drug release ability work well together. The anticancer medicine doxorubicin hydrochloride (DOX) may flow through these composite hydrogels, stopping the cycle of the illness for at least 48 h. It can be used in combination with chemotherapy since it has the ability to photo thermally scavenge tumors of up to 51–57 °C, sufficient to destroy tumor cells. It is difficult to eradicate all cancer cells with a single photo thermal or chemotherapeutic treatment due to the combination of the therapeutic effects on cancer. MXene/polymer nanocomposites’ medicinal potential was best proved by this study. Other MXene polymer complexes have also been used as well as proposed for use in identifying strong structural frameworks and biological sensing. A team at Texas A & M University (Wang et al. 2016) created a multilayer composite of MXene and polyelectrolytes for a humidity sensor. The way people breathed was also examined using these composites. The sensor’s ability to distinguish between changes in humidity brought on by humans walking and running was simple and elegant. A flexible gas sensor made of polyimide and Ti3 C2 Tx was developed. This technology may analyze human breathing to diagnose illnesses, such as how acetone could be analyzed to protect diabetics or NH3 to identify lung uneasiness (Lee et al. 2017). Zhang et al., (2018) produced a MXene/polyvinyl alcohol hydrogel within great human skin bonding abilities, outstanding stretch ability (>3400%), rapid selfhealing capability, and high conductivity. With the help of this newly developed sensor, it was possible to distinguish clearly between the motion of the human hand and the position and orientation of human faces (Zhang, et al. 2018). This sensor has a diversity of exhilarating potential uses in the arenas of state discrimination as well as human health. MXene has been used in a variety of biological applications; however, it is quite in its infancy in these areas (Chen et al. 2021). More dedicated research and development is needed to explore and use the potential of these marvelous materials.

3.3 Sensing Applications Due to their excellent conductivity (Zheng et al. 2019), a smaller band gap (Zhu et al. 2017), comfortably functionalization (Shao et al. 2017), and wealth of active sites (Zhan et al. 2020), it was feasible to utilize MXenes for long-range, high-sensitivity sensors (Guo et al. 2019; Yang et al. 2019; Kim et al. 2018). Therefore, at present, MXenes have been prepared in different types of sensors, such as electrochemical (Liu et al. 2015), gas (Kim et al. 2018), and photoluminescence (Chen et al. 2018b) sensors, exhibiting impressive identification and identifying capabilities. In the MXene sensing applications, the polymer’s malleability may be combined with those of other polymers to create malleable chemical (humidity, VOC) and mechanical (strain, pressure), parametric sensors (Chakraborty et al. 2014; Kim et al. 2017; Sajid et al. 2017; Yuan et al. 2018; Hu et al. 2019; Li et al. 2019). The

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environment, food, health, metallurgy, and other fields all benefit from humidity. Execution of a moisture sensor is affected by the impedance of the composites, which might change as a result of attachment, water molecule penetration, and absorption. Composite membranes and laminate humidity sensors using liquid-phase stripped two-dimensional Mo2 C, Cr3 C2 , and polymer (PAM, PVA) have been reported (Sajid et al. 2017). The best performance for linear and humidity sensing was offered by the MXene/ Polyacrylamide (PAM) laminate sensor (Chen et al. 2021). This was primarily because of PAM and MXene composite that responded well to progressively changing relative humidity levels. As a result, the broad range linear sensitive moisture sensing could be realized when polyacrylamide and MXene were layered creating a double-layer sensor. Volatile organic compounds are a type of hazardous as well as flammable air contaminants which is mostly produced by industrial methods. To stop industrial accidents, it is critical to find their root cause. Ti3 C2 Tx coating was applied to the polyvinyl alcohol/polyethylenimine (PVA/PEI) spinning skeleton by (Yuan et al. 2018) to produce a VOCs gas sensor that was more sensitive (0.10–0.17 ppm−1 ). Despite the fact that Ti3 C2 Tx was able to adsorb polar gas molecules via H-bonding, porous nature of the PVA/PEI backbone provides a sizable particular surface area that made process much easier. The test proved that the sensor’s identification limit range extended from ppb levels to saturated steam such that it could be utilized for analysis of a particular class of volatile organic gases. Additionally, it showed that both the sensor’s reaction and recovery times were under two minutes. Subsequently with 5000 + cycles of stretching at 3 mm/s, the electrical resistance of this sensor was noticeably high suggesting flexible strain sensors with wide identification limits and enhanced sensitivity.

4 Conclusion MXenes are emerging materials for future applications in various domains due to their unique features. They have a wide range of properties which make them exceptionally fit for a variety of applications. Over twenty different MXenes have been developed via selectively etching and exfoliating stacked ternary in recent years (carbonitrides, or carbides). MXenes have proved a wide range of fascinating electrical, magnetic, electrochemical, and mechanical capabilities thanks to their flexible surface chemistry. On the other hand, polymers are an excellent alternative for making Mxene-based composites because of their versatility, compatibility, and low cost. Polyvinyl butyral, UHMWPE, polyether sulfone and chitosan are some of the most widely used polymers in MXene-based polymers. MXenes and related composites have favorable properties and promising prospects for use in sensors, biomedical applications, and energy storage because of their distinctive morphologies, layered structures, and other features. A variety of combinations can be designed and tailored depending upon the applications and desired properties.

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Fabrication and Structural Design of MXene-Based Hydrogels Asif Manzoor, Faisal Jamil, Abbas Washeel Salman, Farrukh Aslam Khalid, Umar Sohail Shoukat, and Muhammad Adnan Iqbal

Abstract MXenes, also named two-dimensional transition metal carbides, carbonitride, and nitrides were reported in 2011 and have since increased in popularity in power storage, catalysis, electromagnetic interference shielding wireless communications systems, electronic sensors, and environmental biomedical applications. Mechanical characteristics, elasticity, and robust adhesive nature of MXenes play a critical part in practically most of the emerging applications, with their increased electrical conductivity and electrochemical activity. Hydrogels are of immense attention from researchers because of their many potential uses for them in a variety of applications. MXenes, which are two-dimensional (2D) transition metal carbides/nitrides, offer interesting and practical platforms for the creation of soft materials with a variety of adjustable application-specific characteristics, On the other hand, MXenes’ stability, which is frequently a limiting issue in many MXene-based applications, may be greatly increased by the synthesis of MXenes into hydrogels. Conductive hydrogels are receiving a lot of attention in the field of flexible and wearable soft strain sensors due to their great potential in electronic skins and customized healthcare monitoring. Typical conductive hydrogels that disperse their particles in pure water, on the other hand, will unavoidably freeze at subzero temperatures, reducing their conductivity and mechanical capabilities. Conductive hydrogels, on the other hand, are sometimes restricted by issues such as poor mechanical properties, difficulty in long-term use, and a limited operating temperature range. As a result, a high-strength hydrogel that can be used in a variety of temperatures is sought. Two-Dimensional MXenes possess a variety of fantastic properties, same is true for the hydrogels, but A. Manzoor · F. Jamil · U. S. Shoukat · M. A. Iqbal (B) Department of Chemistry, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan e-mail: [email protected] M. A. Iqbal Organometallic and Coordination Chemistry Laboratory, University of Agriculture Faisalabad, Faisalabad 38040, Pakistan A. W. Salman Department of Production, College of Agriculture, Wasit University, Wasit, Iraq F. A. Khalid Jinnah Burn and Reconstructive Surgery Centre, Allama Iqbal Medical College, Lahore, Pakistan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_5

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when MXenes are crosslinked with some other phase to form a hydrogel, and once the gelation is accomplished, then the behavior of the produced MXene hydrogel depends either on the sum of the merits of the MXene and hydrogel or come from the synergistic interactions between those parts, opening up a variety of potential uses. The current chapter is focused on the fabrication and structural design of MXene-based hydrogels and their potential applications. Finally, projecting a positive image is described briefly, which will aid researchers in identifying the existing scenario’s limitations and generating new strategic goals that focus on producing unique, efficient, and sensitive MXene-based hydrogels. Keywords Fabrications · MXene · Hydrogels · Nanomaterial · Potential applications

1 Introduction Two-dimensional (2D) materials, due to their unique electrical, mechanical, chemical, and optical capabilities, have emerged as a basis for vast potential in material research. Silicene, Graphene, Germanene, and Borophene are examples of materials that only contain one element, while dichalcogenides are examples of materials that only contain two elements (Wang et al. 2017). MXenes are comparatively new candidates in this family, first discovered by Naguib and his coworkers in 2011 by introducing Ti3 C2 . In this method, the authors used titanium carbide as the precursor in which the selective etching of aluminum atoms using hydrofluoric acid under normal conditions gave Ti3 C2 MXene, as represented in Fig. 1. (Naguib et al. 2011). MXenes, which have the general formula Mn+1 AXn , are coproducts of the threedimensional MAX phases, which serve as their template formula (Sivasankarapillai et al. 2020). M signifies elements from representative transition metals, A indicates an element from either group IIIA or group IVA, and X can be either carbon or nitrogen. Here “n” is an integer ranging from 1 to 3. MAX phase is a unique multilayered structure in which M and A are closely packed. The force of interactions between M and A is relatively weaker than between M and X (Lapauw et al. 2016). Electrically, magnetically (Schultz et al. 2019), optically (Tan et al. 2021), and electrochemically (Xie et al. 2019), MXenes are shown to be superior to their primary MAX phases. Compared to other 2D materials, MXenes are a better possibility due to their unique combination of metallic conduction of transition metal carbides or carbonitrides and the existence of hydrophilic terminating surfaces (Yu & Breslin 2019). A hydrophilic network of polymers called a hydrogel retains a lot of water. Based on their molecular architectures, hydrogels’ interaction may be divided into two main groups i.e., Covalent linkages, and physical crosslinking. These physical interactions are used to create hydrogels, such as electrostatic interaction, hydrogen bonds, host– guest interactions, etc. Due to hydrogels’ high-water content, they may be created with great biocompatibilities for various biological applications (Caló & Khutoryanskiy 2015). Numerous studies have recently been conducted on various hydrogels’

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HF Treatment

MAX Phase

HF Treated MAX phase

Sonication

Mxene Sheets

Fig. 1 MXene manufacturing technique using HF etching

characteristics and the resulted products are utilized in the creation of various biomedical devices. The best hydrogels for making hydrogel-based devices are those that are highly human-compatible and have mechanical characteristics similar to those of human skin or tissues (Jing et al. 2018). As most of the hydrogels are not good conductors of electricity and of low mechanical strength, so MXenes are incorporated to enhance these properties. Either MXenes or hydrogels do not have appreciable wound healing properties when they are applied individually, but when MXenes are incorporated with the hydrogels, these become good wound healers. In a recent study by Liu and coworkers in which they incorporated Titanium carbide sheet with polyaniline to strengthen the wound healing effects. In this way, a considerable increase in electrical conductivity causes it to act as Electro-stimuli which enhances cell-migration, proliferation angiogenesis and the deposition of collagen (Liu et al. 2022a, b, c). The hydrogels are composed of hydrophilic polymers that have undergone physical and chemical crosslinking to form one or more than one gel framework having as a minimum two polymeric chains. These forces of attraction may be covalent or noncovalent and can include ionic, hydrogen bonding, hydrophobic, and other types of interactions (Han et al. 2017). Despite their highly stretchy nature, hydrogels typically require two or more polymers to form (Myung et al. 2008). Rubbers and other highly stretchy and resilient elastomers, such as hydrogels, have high tensile strength and elasticity, while

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most of the hydrogels have comparatively low tensile strength and low elasticity. Due to their weak tensile strength, these hydrogels are easily broken when stretched, which therefore restricts the range of biomedical applications for which they may be used. A double network extremely stretchy hydrogel was initially found by Gong and his coworkers in 2003 (Gong et al. 2003). In 2012, Sun’s team developed a robust hydrogel that is extremely stretchy by weaving a network of polyacrylamide and alginate (Sun et al. 2012). Since then, stronger, more highly stretchy hydrogels have been created for a variety of applications. Using conducting MXene-based nanocomposite organic-hydrogel, Liao et al. described a wearable strain sensor (Liao et al. 2019). The synthesis was achieved by submerging MXene-based nanocomposite hydrogel in an ethylene glycol solvent and then allowing the hydrogel to form into polymeric hydrogel frameworks. This substance has advantages over conventional conductive hydrogels, which lose their capabilities when exposed to subzero temperatures and employ aqueous media as the dispersing agent. The authors found that MXene-based nanocomposite organic-hydrogel could detect human physiological actions over a wide strain range (up to 350% strain) and had exceptional properties like excellent anti-freezing capability (−40 °C), long-lasting moisture holding, outstanding self-healing ability, and improved mechanical strength. The most recent MXenes-based hydrogels and techniques for creating extremely stretchy hydrogels have been summarized in this study.

2 Overview of the MXene and MXene-Based Hydrogel The main goal of MXene fabrication pathways is to remove the more reactive “A” Elements from Mn+1 Xn layers without changing their structural integrity. This characteristic is explained by the fact that Element A is loosely held with M as contrasted to the X which is held strongly with M, which is a stronger type of metallic covalent interaction (Yu & Breslin 2019). Strong van der Waals interactions between MAX phases make exfoliation more difficult than with comparable 2D materials like graphene, phosphorene, etc. Hydrofluoric acid (HF) is typically used because it may dissolve and etch A layers under harsh acidic conditions. The A layer of Ti3 AlC2 was successfully etched from the material using hydrofluoric acid by Naguib and his colleagues as the first team of researchers (M Naguib et al.). Because HF is dangerous, researchers are concentrating on creating inventive substitute procedures that will produce MXenes with a high yield (Li et al. 2020). Here, we go over various practical and efficient ways to create MXenes that have appeared in recent literature. An excellent etching method was created by Yang and his colleagues employing Ti3 AlC2 anode-based corrosion with no need for fluorine in a double-electrolytebased aqueous solvent. Following the solubilization of Al, the authors used an in situ interpolation of NH4 OH, which led to a great yield extraction (more than 90%) of both mono and bi-layers of carbide lumps with a maximum size of 18.6 µm (Yang et al. 2018). Similarly, Sun and his colleagues proposed a mechanism based on an intercalation-alloying-expansion-micro explosion process for preparing layered

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fluoride-free Ti3 C2 Tx . Similar to this, Sun and his coworkers presented a system based on the intercalation-alloying-expansion-micro explosion process for creating layered fluoride-free oxy/hydroxy-based Titanium carbide MXenes (Sun et al. 2019). The specific corrosion of aluminum layers in titanium aluminum carbide was achieved by using this quick technique, which employed an electrochemical etching approach having no fluoride during the process. Malaki and his colleagues gave a thorough overview of the efficacious and effective method. They used to prepare MXene via ultrasonication (Malaki et al. 2019). The crucial step that must be taken into account while making MXenes is the correct dispersion and spalling of MXene flakes into layered sheets. As a result, layered structure, increased lateral size, decreased flaws, and the appropriate level of exfoliation all play a significant role in how MXenes behave. Pang and colleagues described a general method for the production of Titanium, Chromium, and Vanadium based MXenes based on a thermal-aided electrochemical etching approach. Moreover, Ti3 C2 was produced by Alhabeb and his colleagues by sequentially removing silicon from Ti3 SiC2 , which is thought to be the most prevalent MAX phase (Alhabeb et al. 2017). Recently, Reduced Ti3 C2 Tx MXene (r-Ti3 C2 Tx ) was synthesized by Limbu and his colleagues using an easy and environmentally friendly approach. In this approach, the authors treated L-ascorbic acid at ambient temperature (Limbu et al. 2020). MXene nanosheets coated with peroxide were used to achieve the polymerization of a variety of different acrylic-based monomers. Sonication-aided etching technique was used to create MXene nanosheets. Then MXene-polymer nanocomposite hydrogels polymerized subsequently and then gelated within a short period of time in an anaerobic environment. The fact that Ti3 C2 Tx (obtained without sonication) was unable to start the polymerization of acryl-based monomer shows the part played by peroxyl species linked to the surface of p-Ti3 C2 Tx nanosheets. By utilizing hydrogen bonds, polymer chain tangling, and hydrophobic contacts, it was possible to crosslink the hydrogel frameworks with the substrates. The Ti3 C2 Tx -polymer NC hydrogel has remarkable stability thanks to the covalently implanted polymer’s steric stability. MXene nanosheets have recently been employed as crosslinkers in place of conventional organic crosslinkers to create MXene-homopolymeric nanocomposite hydrogels, in addition to their capacity to start gelation. MXene nanosheets have mostly been used in most papers as the multipurpose nanofiller to give the substrate polymer hydrogel certain features. They continue to play a little role in the MXene-polymer hydrogel’s gelation. Tao et al. recently showed how MXene nanosheets may be used as an initiator to help different acryl-based monomers polymerize and aid in gel formation to create various MXene-polyacrylic-based nanocomposite hydrogels (Tao et al. 2019). A machine learning method was developed by Frey and colleagues that can accurately forecast which of the theoretically proposed MXenes is most likely to be successfully synthesized (Frey et al. 2019). With the use of elemental knowledge, they were able to do DFT studies on nitrides, carbides, and carbonitrides of two-dimensional D-block elements as well as their layered precursor MAX

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phases, which led to the discovery of 18 interesting MXenes that might potentially be produced. By using hydrogen fluoride to etch the Ti3 AlC2 MAX phase, multilayer MXene was created (Wan et al. 2020). In aqueous solvents, multilayer MXene is extremely unstable. So, Oligolayer MXene is produced by ultrasonic peeling and centrifugal processing of multilayer MXene. Since oligolayer MXene has superior water dispersibility than multilayer MXene, it is frequently used to make MXene hydrogels (Xiu et al. 2018). However, the dispersive stability of the MXene nanosheets in the aqueous solvent and generated hydrogel must still be taken into account during the synthesis of the oligolayer MXene composite hydrogel (Chen et al. 2018; Xiu et al. 2018).

3 Fabrication and Gelation Method of MXene-based Hydrogel MXenes, whether is an excellent two-dimensional material but because of limited available surface area and high resistance of ion diffusion, the electrical performance is lowered. Until now, almost all the documented MXene-containing gels have just been created with Ti3 C2 Tx which is the most studied MXene of all the MXenes reported to date. According to a study by Alhabeb and coworkers, all of the MXene nanosheets found in the MXene hydrogels were created by employing various top-down fluoride-containing etching techniques to selectively remove the A layers (i.e., primarily groups IIIA and IVA elements) from their primary layered 3D MAX phases (Alhabeb et al. 2017). Despite Ti3 C2 Tx ’s prevalence in MXene-based gels, its involvement during the gel formation differs noticeably from study to study. The continuous phase would mostly be contained within a 3D structured network that was physically and/or chemically crosslinked by a secondary phase, i.e., the gelator, in a standard MXene gel. Up to now, total MXenes (self-linking), graphene oxide, polymers, inorganic ions, or mixtures of any of them have been reported as the secondary phases to crosslink with the MXene to form hydrogels. Depending on the driving forces involved in the gelation process, the MXene nanosheet network’s interactions with the hydrogel in the presence or absence of additional gelators can take different forms. The three-dimensional MXene framework of the hydrogel or any of its derivatives is assembled and maintained by these driving factors, either physical or chemical. We have classified the MXenes hydrogels based on secondary phases, with which MXene is interacting, i.e., MXenes self-linking, MXenes linked with polymers to form composites, MXenes linked with metal ions, MXenes crosslinked with viscous polymeric substances and MXenes crosslinked with graphene oxide (GO).

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3.1 MXene Crosslinked with MXene to Form Hydrogel (Total MXene Hydrogel) It is difficult for MXenes to form a gel all by themselves due to the unavoidable accumulation and restacking propensity of MXene nanosheets as a result of the strong van der Waals interlayer interactions. Due to the higher hydrophilicity produced by the surface moieties of MXene, it even becomes much harder for the hydrogels (Anasori et al. 2017; Wu et al. 2020). Due to the hydrophilicity of MXenes, it is frequently required to include a second element in the hydrogel matrix to prevent this and maintain the 3D assemblage of 2D nanosheets. Nevertheless, despite all of the aforementioned difficulties, Lin et al. and Lukatskaya et al. were able to prepare all MXene hydrogels employing vacuum-assisted filtering of delaminated Titanium Carbide suspensions with Titanium Carbide as the only gelator (Lin et al. 2016; Lukatskaya et al. 2017). Even though they used a technique that is frequently used to create MXene “paper” films, they were able to create a Ti3 C2 Tx hydrogel by immediately unplugging the vacuum as soon as there was no longer any colloidal solution on the porous membrane. After that, the hydrogel film was temporarily submerged in acetone to aid in peeling off. The significant amount of pre-intercalated water molecules that were not removed caused a dynamic physical crosslinking between the MXene sheets, which is what gave the material its gel-like structure. The predominant van der Waals interlayer forces, however, cannot be maintained by this kind of force. Each of the previous researchers used a different approach to prevent the breakdown of the hydrogels that were created in order to stop the restacking of MXene nanosheets. Lin et al. immersed the undried hydrogel film in a thermally stable imidazolium NHC based ionic liquid to allow solvent exchange. Instead of a hydrogel, a Titanium Carbide ionogel was produced by vacuum drying at 80 °C while the ionic liquid was still present in the Titanium Carbide film. Thus, restacking was avoided, and interlayer space was increased. (Lin et al. 2016). In a different work, Lukatskaya et al. created an MXene hydrogel and retained its structure by soaking it in H2 SO4 electrolyte for three days before using it for direct electrochemical measurements. (Kayali et al. 2018; Lukatskaya et al. 2017).

3.2 MXene Crosslinked with Metal Ions to Form Hydrogel The electrical conductivity of MXene can generally be lowered when it is crosslinked to an external phase i.e., polymer. To further enhance the electrochemical properties of MXenes, it can be crosslinked to a metal ion. In order to create a well-organized MXene-metal hybrid hydrogel, Deng et al. encouraged the rapid gelation of Titanium carbide utilizing ferrous ions as crosslinkers. To join Ti3 C2 Tx nanosheets, ferrous ions were used as joints that depended seriously on contacts with OH surface groups. Negatively charged MXenes’ hydrophilicity is reduced, and its phase separation is facilitated when it is intercalated between ferrous ions and OH groups in a ferrous

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chloride tetrahydrate solution. Surprisingly, the process’s rapid gelation was effectively stopped from oxidizing MXenes by the bivalent metallic ion. Other polyvalent metallic ions like bivalent Magnesium, Cobalt, Nickel, and trivalent Aluminum were also studied in order to confirm the impact of metal ion dispersion on the gelation of MXene nanosheets. The electrostatic connections between MXene nanosheets were demonstrated to be broken by both bivalent and trivalent ions of the described metals, which then bonded the nanosheets together through surface OH groups. However, the monovalent potassium ions that were introduced did not start the gelation process and instead caused the MXene nanosheets to coagulate. This resulted from monovalent ions’ inferior hydration energy as compared to polyvalent ions. With the existence of accumulated MXene nanosheets, the MXene hydrogel supported with trivalent aluminum displayed a least-formed three-dimensional framework. The increased positive charges brought on by the trivalent ions quickly destroyed the electrostatic interactions between the MXene nanosheets (Deng et al. 2019). In order to create MXene-based hydrogels by an enhanced gelation method, Lin et al. combined two crosslinkers, namely Zn2+ ions and graphene oxide (Anasori et al. 2017). Zinc foil was used to deliver divalent zinc ions into the acidic Titanium Carbide-Graphene oxide dispersion in order to start the gel formation. The MXene-induced reduction of the graphene oxide layers was facilitated by the metal ions produced. An MXenerGO hydrogel was formed on the metal substrate as a result of the spontaneous assembly of anionic titanium carbide nanosheets with the aid of the diffusion of the metal ions. It is notable that the very conducting MXene-reduced graphene oxide nanocomposite hydrogels having features such as outstanding structural resilience, oxidation-opposed, and stability, were created by the divalent metallic ion-facilitated gelation process in the presence of a minute amount of graphene oxide. More importantly, some of the aforementioned hydrogel manufacturing procedures necessitate prolonged exposure to high temperatures, which might oxidize the MXene nanosheets and eventually impair the hydrogel’s functionality. The MXene composite hydrogel with ion crosslinking also has low stretchability, which may reduce its durability. To overcome these issues, Sheng and coworkers described that under high temperatures, the blended solution of MXene and graphene oxide can self-coagulate into an MXene/graphene oxide self-assembled hydrogel under the influence of various reducing agents (Shang et al. 2019). This method makes it easier for tiny molecules to create stable physical and chemical bonds with MXene nanosheets, which makes MXene the main structural component of the hydrogel and considerably increases the stability of the nanosheets inside it (Wang et al. 2020).

3.3 MXene-Based Micellar Hydrogels Despite the limited usage of single-molecule chemical crosslinkers in the creation of stretchy hydrogels, micelles, and covalent or noncovalent Nanostructures can be employed as chemical crosslinkers. Tao and coworkers synthesized MXene-based micellar hydrogel to study the antitumor activity of the produced gel system. Titanium

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carbide MXene nanosheet was prepared by conventional acid–base etching method. It was then loaded with vascular disrupting medium. This micellar hydrogel showed 99.6% loading efficiency, 41.4% photothermal conversion efficiency, excellent drug uptake of the cells and can remove the solid tumor thoroughly from an initial volume of 200mm3 (Tao et al. 2020). In another study, a stretchy hydrogel was created by copolymerizing acrylic acid (AA) with stearyl methacrylate, according to Gulyuz and his colleagues. With surfactants like the positively charged cetyltrimethylammonium bromide (CTAB) in which the long stearyl group on the network can form micelles (Gulyuz & Okay 2014). There’s one more similar system described by Sun and his coworkers, in which PEO99-PPO65-PEO99 diacrylate (F127DA) micelle functionalized with vinyl groups that copolymerized with acrylamide when the gel was created makes up the unique PAM-micelle gel (Sun et al. 2015). Later, based on both positively charged and negatively charged gels, they described two comparable systems (Gulyuz & Okay 2014). Depending on the applied electric field, the polycationic and polyanionic hydrogels twisted in the direction of the cathode or anode. Another illustration of a PAM-micelle gel is PAM-casein. (Y. Zhang et al. 2017). The milk protein casein, which forms submicelles in an aqueous solution, interacts with PAM by molecular processes such as electrostatic contact, hydrophobic interaction, and hydrogen bonding. In conclusion, introducing micelles to a hydrogel is a successful way to raise the strain of the hydrogels.

3.4 MXenes Crosslinked with Polymer to Form Hydrogels One method for making MXene hydrogels is to combine two-dimensional MXene with monomers to create an in situ MXene gel. In this way, MXene is integrated into gel systems. The inclusion of MXene sheets into polymer hydrogel frameworks, which swell significantly with water, has given them an exceptional degree of multifunctionality owing to their hydrophilicity. A number of MXene hydrogels have been created and used thus far in various applications (Lee et al. 2020; Zhang et al. 2020). Here we have carefully described the crosslinking of MXenes with highly stretchy and highly viscous polymeric materials.

3.4.1

MXenes Crosslinked with Alginate-Based Polymers (High Stretchy Double-Network Hydrogel)

Although a lot of work has been made on the stretchy double netwok hydrogels having excellent mechanical properties (Pourjavadi et al. 2020), strain sensing applications (Lu et al. 2020; Luan et al. 2022), self-healing features (Zheng et al. 2020), capable of 3D printing (Yang et al. 2017), intraoral ultrasound imaging (Yi et al. 2020), and for dye removing purposes (Bahrami et al. 2019), But one of the very first extremely stretchy hydrogels constructed of a double network of polyacrylamide (PAM) and alginate were described by Sun and coworkers in 2012 (Sun et al. 2012). In their

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studies, the extremely stretchy hydrogel was created by polymerizing acrylamide monomers with alginate using UV light. The hydrogel can be stretched up to 23 times its original length. Continuing with the discussion of the MXene-based hydrogels as described by Yuan and colleagues. To create a mechanically reliable and sensitive double-networked hydrogel strain sensor, they combined MXene nanosheets with a polyacrylamide-sodium alginate substrate. Strong contacts were created between the hydrophilic MXene nanosheets and the polymer substrate. These interactions gave the hydrogel superior tensile characteristics, uniform mechanical strength, long-lasting stability, and fatigue resistance (Yuan et al. 2021). This sensor attained extraordinarily high tensile and compression sensitivity attributed to the highly aligned MXene-based 3D conducting networks. Similarly, Kong and coworkers very recently introduced double-network-based MXene hydrogel sensor. In their study they composited MXene with polyvinyl alcohol/sodium carboxymethylcellulose system (Kong et al. 2022). It is worth mentioning that, apart from a few one-pot copolymerized gels, the majority of these double-network hydrogels are either produced through a two-step polymerization involving two different monomers or through the polymerization of a monomer in the presence of another polymer, such as PAM and alginate (Wang et al. 2018). First, transparent hydrogels were created by using tetrakis (hydroxymethyl) phosphonium chloride (THPC) as a covalent crosslinker for CCP. The stretchy hydrogel was created by further curing the hydrogel in a zinc sulfate solution to ionically crosslink MCPs. The quantity of pentapeptides in the protein domains influences the type of MCP. In summary, many of these stretchy hydrogels were made using a physical crosslinker, such as a metal ion, along with a chemical crosslinker, like MBAA.

3.4.2

MXenes Crosslinked with Viscous Polymeric Substances

The methods used by various researchers for creating MXene composite hydrogels with viscous polymeric substances have been described. MXene is combined with a very viscous polymer, and the mixture is then prepared for self-assembly using a crosslinker. Zhang and colleagues mixed MXene with polyvinyl alcohol and then utilized borax crosslinking to create a self-healing MXene-PVA hydrogel (Zhang et al. 2019a, b). Moreover, the Rafieerad group sought to incorporate twodimensional MXene into the honey-chitosan combination and then crosslink it to create a bioadaptable MXene-honey-chitosan hydrogel (Rafieerad et al. 2020). The thicker polymer typically has a high viscosity, which can stop the MXene nanosheets from stacking up again and increase the hydrogel’s dispersion stability. However, it is also challenging for MXene to connect uniformly among polymer networks because of the potent binding action among thicker polymers (Chen et al. 2017).

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3.5 MXene-Based Nanocomposite Hydrogel The characteristics of the MXene-based hydrogel are also influenced by nanomaterials, a class of substances that range in size from 1 to 100 nm, including graphene oxide. Graphene, carbon, polyaniline, and polypyrrole may all be used to make poly (3,4-ethylene dioxythiophene)-poly (styrene sulfonate, or PEDOT/PSS) (Tian et al. 2017). MXene-polymer nanocomposite hydrogels are synthesized using three essential components, which are a monomer, an initiator, and a crosslinker, in a manner similar to the creation of “traditional” polymer hydrogels (Triadhi et al. 2018). The 3D-formed frameworks are known as homopolymerized MXene nanocomposite hydrogels if just one monomer is used during the mechanism of gel-polymerization. MXene-copolymeric NC hydrogels, on the other hand, crosslinked networks are made up of two or more distinct monomer species and MXene nanosheets. By mixing polyvinyl alcohol, acrylamide, and titanium carbide nanosheets in an aqueous solution, Liao et al. produced copolymerized MXene hydrogels (Liao et al. 2019). The polyvinylalcohol chains were joined together by tetrahydroxyl borate ions, and NA2 B4 O7 ·10H2 O was utilized as a dynamic crosslinker. Thus, polyacrylamide framework was eventually achieved by the in situ polymerization of the acrylamide monomers at 60 °C. As a consequence of the polymeric chain tangling, MXene nanosheets were effectively linked into the hydrogel framework, where it functioned as an additional crosslinker through intramolecular connections much like hydrogen bonding between polyvinylalcohol and the hydrophilic external fractions of the MXene sheets. Various MXene homo-based polymeric nanocomposites hydrogels, in contrast, were created employing a variety of polymers, including polyvinylalcohol, chitosan, cellulose, and acrylic acid polymers (Lee et al. 2020; Zhang et al., 2018). One more hydrogel system composed of Titanium carbide-polyvinylalcohol was created by directly dispersing MXene nanosheets in Polyvinylalcohol and using water as a solvent (Zhang et al. 2019a, b). To prepare MXene hydrogel utilizing cellulose as a crosslinker, Xing et al. linked Titanium carbide nanosheets aqueous-suspension in the cellulosic solution by adding 1-chloro-2,3-epoxypropane crosslinker (Xing et al. 2018). The insertion of MXene nanosheets didn’t disrupt the cellulose chain’s crosslinking reaction with the crosslinker, permitting the production of well-structured Titanium carbide-cellulose hydrogels. MXene nanosheets were introduced by Liao et al. and Zhang et al. into a doublefunctional polymer hydrogel system, serving as both the hydrogel’s conductive substrate and nano reinforcers (Liao et al. 2019; Zhang et al. 2019a, b). This kind of monomer solution has a lower viscosity than thicker polymer solutions. Therefore, during the production of long-lasting hydrogels, it is important to emphasize MXene’s simple aggregation. Another form of MXene can be utilized to crosslink polymer hydrogels since the suspensions exhibit characteristics similar to those of clay. According to research by Wang and colleagues, the reducible face of MXene can stimulate the initiator through a redox process, producing a large number of hydroxyl radicals (OH) quickly to aid in the polymerization of the monomers (Wang

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et al. 2020). MXene can crosslink polymer chains fast and produce self-assembling nanocomposite hydrogels simultaneously. Here, MXene nanosheets significantly quicken the production of hydrogels and, in the end, stop the buildup of nanosheets during the formulation stage.

3.6 MXenes Crosslinked with Graphene Other physical and chemical interactions, in addition to the aforementioned gelation mechanisms, can contribute to the creation of MXene hydrogels and provide another way of classification. Although crosslinking of MXene with Graphene oxide section can be placed with polymers or nanocomposite hydrogels. But graphene itself is a major section and a lot of research has been made on the Graphenebased MXene hydrogel, that’s why we have placed it in a separate section. The gelation procedure depends on the crosslinker’s capacity to bring together freestanding MXene nanosheets dispersed in water into solidly attached 3D structures. As the MXene surface doesn’t contain many active sites for crosslinking purposes, So, the choice of a variety of crosslinkers has always been an issue (Lin et al. 2020). As a result, exposing more available crosslinking positions is critical for obtaining a well-established hydrogel framework. This might be accomplished with the use of another two-dimensional material-based gelator, that is graphene oxide, which would enable interfacial contacts with the MXene nanosheets rather than point-to-plane exchanges. By polymerizing acryl-based monomeric solution with dopamine-reduced Graphene Oxide, Jing and his coworkers created a highly stretchy nanocomposite hydrogel of reduced graphene oxide (PAArGO). The formation of strong intermolecular contacts with PAA by graphene oxide and the addition of Fe+3 led to greater mechanical strength. The reversible self-healing properties of the hydrogel are thought to be responsible for the physical interactions between PAA and Fe+3 . The conductive rGO not only supports dopamine but also enables electronic human motion sensing. A highly elastic and biocompatible hydrogel was later created by Jing et al. by polymerizing acrylamide monomer with talc (Mg3 Si4 O10 (OH)2 )intercalated and oxidized dopamine (Jing et al. 2018). Chen et al. reported the first instance of such precise and hierarchical interfacial crosslinking between titanium carbide MXene and reduced graphene oxide nanosheets via an organics-free selfconvergence mechanism. The numerous oxygen-containing surface species on the surface of the graphene oxide sheets were partially removed by Titanium carbide MXene, which allowed the hydrophilic graphene oxide to be reduced to its more hydrophobic reduced form when combined with graphene oxide solution. Its multivalent Titanium species, which can switch from low-valence to surface-terminated high-valence states, are thought to be the reason for Titanium carbide MXene’s extraordinary capacity to function as a reducing agent. Low electrostatic repulsion between titanium carbide and reduced graphene oxide, which occurs during gel formation, makes it easier for MXene nanosheets to self-assemble into the anisotropically assembled reduced graphene oxide network. This self-assembly is fueled by

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hydrogen bonds between MXene and reduced graphene. The formed hydrogel acted as a supercapacitor (W. Chen, Peng, Qiu, Zhang, & Xu 2022). By exploiting the redox properties of Ti3 C2 Tx in reducing Pd ions, which were progressively accumulated on the MXene sheets, similar GO-assisted interfacial crosslinking was also used to create Pd-grafted MXene hydrogel. Shang et al. used the interlayer incorporation of ethylenediamine to create chemical bonds between reduced graphene oxide layers and MXene nanosheets, which increased the obtainable surface area during the graphene oxide-driven gel formation procedure (Shang et al. 2019). Ethylenediamine encouraged the development of oxygen drooping bonds by opening the epoxy rings that were present on the graphene oxide sheets concurrently with the MXene-aided reduction of graphene oxide. Following the formation of MXene-rGO hybrid structures by joining MXene with these drooping bonds, the hybrid nanosheets spontaneously attracted one another to form hydrogels. MXene-reduced graphene nanocomposite hydrogels are thicker and more pliable than hydrogels made with sole-graphene oxide (Chen et al. 2020). Additionally, it should be mentioned that the addition of ethylenediamine as a spacer has decreased the analytical concentration of MXene needed to make a gel in order to produce an MXene-rGO nanocomposite hydrogel. There’s one more example in which Ming and colleagues designed an efficient Graphene/MXene hydrogel system for water purification (Ming et al. 2020). In this system, graphene aided the gelation of MXene with an aerogel. The efficient conversion of low-scale solar energy into high-scale steam energy was accomplished using this graphene-based MXene hydrogel. The principal behind this conversion was the solar-stimuli evaporation of water with an efficiency of 90.7% with the rate of 1.27 Kgm−2 by irradiating action of one sun.

4 Applications of MXene-Based Hydrogels MXenes own diversity of applications because of their two-dimensional structure, effective mechanical strength, great hydrophilicity, and exceptional surface features and hydrogel as well also possess a wide variety of application including stretchability and self-healing, etc. (Rasheed et al. 2021; Rizwan et al. 2022), A breakthrough happens when MXenes are combined with hydrogel. The features of the obtained MXene-based hydrogel are either added up or come from the synergistic interactions between those parts. MXenes and MXene-based hydrogel have shown numerous applications including energy storage (Zhang et al. 2022a, b), supercapacitors (Zhu et al. 2022), photoactuators (Zavahir et al. 2020), batteries (Liu et al. 2022a, b, c), water purification (Zhang et al. 2022a, b), strain sensing (Kong et al. 2022; Yuan et al. 2021), as an antifreeze (Li et al. 2022a, b), and as catalyst in chemical reactions (Chen et al. 2018; Li et al. 2022a, b). MXene hydrogels have a number of benefits over conventional 2D nanomaterialsbased hydrogels, including strong hydrophilicity, which encourages optimal dispersion and stability of MXene-derived photodynamic and photothermal mediators in physiological fluids. MXene hydrogels have recently demonstrated considerable

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potential for a variety of in-vivo biomedical applications, including cancer treatment (He et al. 2022), antibacterial activity (Y. Zheng et al. 2022), wound healing (Cai et al. 2022), and drug release (Liu et al. 2022a, b, c). Additionally, the partially charged terminal groups of MXene hydrogel make it simple to implant anticancer medications onto its surfaces, which is an additional benefit. By adjusting the bloating behavior of MXene hydrogels, effective anticancer medications can be produced, with loading capacities as high as 84% and a high release percentage (Sileika et al. 2013). Very recently, He and coworkers fabricated Titanium carbide MXene with DNA hydrogel along with doxorubicin as a loading agent that possessed the features of biodegradability and biocompatibility. Temperature triggering properties of this MXene hydrogel, upon irradiating the infrared radiation, cause the winding and unwinding of the DNA hydrogel that causes the delivery of the drug to the affected part (He et al. 2022). Rafieerad and coworkers have stated the development of a nontoxic sweetened Titanium carbide-chitosan nanocomposite hydrogel with a great swelling ratio, permeable composition, and precise degradation. This hydrogel outperformed pure chitosan-based hydrogels in terms of biocompatibility with medicinal signaling cells and cardiac muscle, opening the door for its use in tissue restoration. Additionally, this polyacrylamide-based nanocomposite Titanium carbide hydrogel displayed outstanding drug loading levels, durable release, and remarkable delivery proportions with improved mechanical and swelling capabilities (Rafieerad et al. 2020). Such as, it has been proven that encapsulating anticancer drugs inside the cellulose-based Titanium carbide nanocomposite hydrogel is a useful technique for treating cancer and controlled drug delivery. This was due to its dynamical polymer-like three-dimensional porous framework, which made doxorubicin (an anticancer drug) ingestion extremely high (Zhang et al. 2020). It is still very difficult to create adhesive polymers with good mechanical characteristics that mirror the functions of human skin. In order to enhance adhesion, common substances like chitosan and polydopamine containing free catechol groups have been added into hydrogels for wound healing. Electrical components may be included in wound healing patches to hasten the healing process. Xing and coworkers, in addition to dopamine, added Numerous polyphenolic substances, such as tannic acid (Xing et al. 2018). Tannic acid may establish potent noncovalent interactions with proteins and DNA, which can be exploited to create hydrogels with significant surface adherence. In the disciplines of medicine and healthcare, chronic wound healing is a crucial and fundamental problem. Hydrogel devices that respond to stimuli have nowadays gained popularity as potential drug delivery approaches for treating wounds. MXenes-based hydrogels have been used to administer drugs transdermally (Sileika et al. 2013). Recently, Mao and colleagues composited bacteria-based cellulosic hydrogel and Titanium carbide MXene nanosheets. This electrically conductive and biocompatible nanocomposite hydrogel possessed excellent mechanical strength, good flexibility, excellent water-uptake capability and efficient therapeutic potential these attributes lead to accelerate the wound healing processes (Mao et al. 2020). The rate of drug distribution out of a hydrogel patch may be controlled by altering the rate of diffusion of hydrogels in general. In addition to regulated diffusion rates, stretching of the skin and hydrogel, as demonstrated in PAM-cyclodextrin gel, can

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cause a drug release. These hydrogels can be used to create wound healing patches. There is an urgent need for novel hydrogel systems for efficient wound healing, nonetheless, given the limited treatment outcomes. Yang and colleagues described an MXene hydrogel system comprising optical and magnetic sensing capacities as well as adjustable drug delivery capabilities for the treatment of deep, persistently infected wounds. This sensing system is made up of MXene-wrapped magnetic colloids and poly(N-isopropyl acrylamide)-alginate double-network hydrogels. A promising method for deep wound therapy is provided by the combination of these drug carriers. Under alternating magnetic field and near-infrared, the system’s temperature can rise quickly, which can cause a controlled release of AgNPs. In models of diabetic rats with severely infected wounds, these drug carriers also demonstrated good performance. These stimuli-sensing MXene hydrogel approaches suggested the potential of these materials in the healing of deep persistent wounds and other biological applications (Yang et al. 2022). Substantial progress has been made in the creation of multifunctional MXene hydrogels with flexible mechanical characteristics, high detecting sensing, vast detection range, long-span stability under challenging circumstances, reliable degradation, and strong self-adhesion. Wearable electronics based on MXene hydrogel, however, are still in the early stages of study. Both hydrogels that conduct electricity on their own and hydrogels that can include electrical components can be made. This may consist of Motion sensors for tracking health (Hu et al. 2022). Hydrogels can include stretchable sensors that react to deformation in any way. A very sensitive sensor can discriminate between significant motions (such as joint bending) and minute motions (such as pulse and breath). Additional sensors, such as biosensors, optical sensors, tensile sensors, etc. can be fitted into hydrogels as well (Yang et al. 2022). Guo and colleagues recently incorporated polyacrylamide/alginate double-network hydrogel with MXene to enhance the electrical conductivity and the produced MXene hydrogel was utilized for motion sensing to detect the disorders in the joints (Guo et al. 2021). Flexible wearable electronics have advanced quickly in recent years, and the next generation of electronic gadgets will have capabilities like self-healing, stretchability, flexibility, and tactile sensing that are inspired by human skin (Chortos et al. 2016; Guo et al. 2016). In order to concurrently acquire high sensitivity and a broad detection range, 1-D polypyrrole nanowires and 2-D MXene nanosheets were successively spray-coated in an orthogonal orientation onto a hydrogel substrate (Bao and Chen 2016). After four rounds of stretching and releasing, samples of crumpled MXene nanosheets and spring-like polypyrrole nanowires were introduced in various orientations. The contact between MXene nanosheets and water molecules in the hydrogel was decreased by the hydrophobicity of polypyrrole nanowires. As a result, the MXene flakes can’t randomly slide along the water molecules, self-assemble, and eventually separate from the hydrogel surface. The MXene layer and the hydrogel substrate were connected, however, by a layer of polypyrrole nanowires that served as a nanobridging layer. The MXene flakes were able to move uniformly in the hydrogel under significant and repetitive deformation due to their large specific surface area, wrinkled shape, and relatively poor interaction with the polypyrrole nanowires. As a result, the electronic skin created from this MXene hydrogel has a very high sensitivity and wide detection

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range, making it the perfect foundation for next-generation highly flexible wearable electronics. Compressible conductive hydrogel derivatives based on MXene were exploited for sensing applications. For instance, MXene-rGO hydrogels were used by Ma Yanan and colleagues to develop a wearable pressure sensor (Morelle et al. 2018). In comparison to MXene-rGO filtrated films, the hydrogel’s 3D porous network has made it possible to sense a larger pressure range with higher sensitivity. Additionally, compared to pure rGO hydrogels, the mechanical characteristics of MXene and rGO nanosheets are improved. With a sensitivity as high as 22.56 per kPa, the ideal MXene-rGO hydrogel has demonstrated excellent stability over 10,000 sensing cycles. Such sensitivity can allow for the safe detection of minute but crucial signals that are important to human health, such as pulses.

5 Conclusion In this chapter, we have covered MXene-based hydrogels’ diverse fabrication methods. The process and materials for creating strong, extremely stretchy MXenebased hydrogels have been outlined. This study demonstrates that while there is still a great deal of room for research in the area of chemical crosslinking, a large amount of effort has been made into the creation of physical crosslinkers. Crosslinking two polymers or combining one polymer with micelles or nanomaterials are methods for creating variety of MXene-based hydrogels. To further enhance the characteristics of hydrogels, there is still a great deal of work that can be done in the sector. Although hydrophilic polymers are often utilized to create hydrogels, hydrophobic polymers have been successfully used to create tough hydrogels. Because hydrogels often have strong biocompatibility and have a high-water content in their network, the range of materials that can be employed is further expanded. By incorporating these biocompatible parts with devices, new technological applications will be made possible. MXenes, in particular, can perform a variety of tasks during the gelation process, including self-gelators, initiators, crosslinkers, and noncovalent nanofillers. It’s intriguing how the interactions between MXenes’ characteristics and those of other components of the hydrogel network have produced a wide range of advanced qualities, such as enhanced mechanical and electrochemical behaviors, better sensing capabilities, and biocompatibility. Thus, MXene-based hydrogels have demonstrated exceptional performance in a variety of applications, including energy storage/harvesting, medicines, catalysis, EMI shielding, sensing, wearable electronics, and drug delivery. Work is being done to address specific issues with each application. Although the applications of the MXene-based hydrogels are not the focus of this study, we have given a short overview of advancements in the application domains.

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Emerging Trends of MXenes in Supercapacitors Memoona Qammar, Adeel Zia, and Omair Adil

Abstract Ultracapacitors are emerging devices for the energy storage and are rapidly improving day by day. They have attracted the audience due to their faster charge storage properties and large operational window. The exceptional layered structure of MXenes facilitates with the redox reaction sites and an enhanced ion transportation. The role of two-dimensional (2D) transition-metal carbides, nitrides, and carbonitrides (MXenes) in the supercapacitors has been discussed in this chapter. This chapter will guide the reader with a general overview of electrochemical capacitors, their working principle, the role of MXenes in supercapacitors, and the recent progress in this field. Lastly, the chapter is concluded with a critical review and a few future suggestions. Keywords MXene · Supercapacitor · Energy storage devices

1 Introduction The electrical energy production, distribution, and efficient storage are among the most important challenges for the growing human population. One of the major hurdles to renewable energy sources is their limited availability in the hours of demand. Solar energy can only be harnessed in the daytime and wind energy can be exploited only when wind condition is favorable. To make this energy available around the clock, there must be some energy storage system that can hold a massive amount of energy, last a long time, cost effective to scale up, and quick to deliver M. Qammar Department of Chemistry, The Hong Kong University of Science and Technology (HKUST), Clear Water Bay Rd, Kowloon, Hong Kong (SAR) 999077, China A. Zia (B) Department of Chemistry, Virginia Tech, Blacksburg, VA 24060, USA e-mail: [email protected] O. Adil School of Chemical and Biomolecular Sciences, Southern Illinois University, Carbondale, IL 62901, USA © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_6

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and receive a significant volume of energy. Supercapacitors are among the promising candidates for this task (Raza et al. 2018; Hu et al. 2016). Traditionally, batteries are being used as an energy storage device. Batteries are electrochemical energy storage devices that can convert and store electrical energy in a chemical form. They are considered promising candidates for environment-friendly, efficient, and scalable energy storage devices (Ming et al. 2021; Dong et al. 2020). Their practical application is progressing robustly yet hampered by some essential elements like relatively low power operation, poor stability, slow reaction kinetics, and soaring prices (Sengupta et al. 2018). To address these issues, a lot of research effort is being offered in this field. Among many fronts, organic–inorganic hybrid electrode systems are one which researcher believes can make lithium-ion batteries more suitable for even a broader range of applications (Cheng et al. 2020). According to many studies, organic–inorganic hybrids can offer synergic behavior of their corresponding organic and inorganic parts. The organic part, in general, encourages to provide lower density, expands the range of available matrices, and can be utilized to regulate physical or chemical properties. On the other hand, the inorganic counterpart usually possesses better stability and contains electrocatalytic active species. The hybridization of organic and inorganic parts generates a range of interconnected porosity resulting in high surface area which can accelerate the diffusion of electrolyte, ions and increases the accessibility of the active sites. These benefits offer a substantial upgrade in the capacity and stability of the Li-ion battery (Iqbal et al. 2022). Metal oxides and transition-metal oxides have high theoretical capacitance, which is good for the electrodes, but they generally have poor conductivity (Wu et al. 2016). Hence in many studies, annealing is used to increase the conductivity of the materials. The annealing process makes organic–inorganic hybrid materials even more porous and hence aids in the diffusion of ions and buffer volumetric change. Therefore, calcinated organic–inorganic hybrids have been used for anodes. MXenes are two-dimensional (2D) layered materials first discovered by Gogosti and co-workers in 2011 (Naguib et al. 2011). The structure has a general formula of Mn+1 Xn Tx , where M = transition metals, X = carbon or nitrogen layers, T = terminal elements such as oxygen, fluorine, or chlorine and n = integer ranging from 1 to 4 (Gogotsi and Huang, 2021). So far, more than 30 MXenes have been synthesized experimentally, and more than 100 are reported theoretically (VahidMohammadi et al. 2021). The elements used for the most common four types of MXenes and their corresponding structures are depicted in Fig. 1. The versatility in composition provides a giant room for tuning the properties and applications of MXenes. A good mechanical conductivity, hydrophilicity, charge storage capacity, and potential for cations intercalation make them a good fit for charge storage applications like batteries and ultracapacitors (Lukatskaya et al. 2013; Ghidiu, et al. 2014; Naguib et al. 2013; Wang et al. 2015).

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Fig. 1 The elements used in the preparation of MXenes, color code represent the respective components. Four typical structures of MXenes are presented schematically. Adapted with permission from the American Chemical Society (Gogotsi and Huang 2021)

2 Synthesis of MXenes A room temperature top-down approach entitled as selective etching has been extensively deployed for the synthesis of MXenes since 2011 (Naguib et al. 2011). Wherein 2D MXenes are prepared by the selective etching of A material (for instance, Al, Ga, Si) from their respective MAX phase and replacing it with the terminator groups (Tx ) like OH, O, or F (Wei et al. 2021). However, the MAX phases possess threedimensional layered structure and ample metallic conductivity. These layers are strongly connected by metallic, covalent, and ionic bonds. For example, Ti3 C2 Tx has been prepared by the selective etching of Al from the parent Ti3 AlC2 MAX phase (Hu et al. 2016; Pathak et al. 2022). Pure MAX phases are usually prepared by reactive sintering in which precursors (MX and AX) are mixed and sintered in an inert atmosphere. The sintering reaction generates porous billets which can further be used for the synthesis of MXenes (Mathis et al. 2021; Rizwan 2022; Rasheed et al. 2021). A general protocol for the synthesis of MXenes is depicted in Fig. 2. Primarily etching is performed by dissolving the powdered MAX phase in HF under sufficient stirring. Lately, it is centrifuged or filtered and subsequently subjected to washing with DI water to achieve a pH of 6–7 for the suspension. This results in a loosely packed multilayer (ML) MXene product. If the number of layers is less than five then it will be called as few layer (FL) MXenes. As prepared ML MXenes can be deployed for certain applications but some applications require individual sheets. To meet this requirement intercalation is performed where solvent cations are intercalated between the layers. The choice of solvent depends upon the chemical properties of the surface. At the end delamination is performed via sonication (Abbas et al. 2022).

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Fig. 2 Preparation and schematic representation of MXene. Adapted with permission from reference (Hu et al. 2016; Zia 2022)

The facts about the hazardous nature, careful and tedious methodology of HF-based etching cannot be ignored. Apart from this HF etching technique, different MXenes for charge storage applications have been synthesized by alkali etching, Lewis acid molten salt etching, halogen etching, and electrochemical etching (Li et al. 2018, 2020a; Shi et al. 2021; Pang et al. 2019; Salim et al. 2019).

3 Supercapacitors Supercapacitors (SCs) or ultracapacitors being an astonishing invention of science are deemed to take over the energy storage applications. Ultracapacitors are the pulse current setup possessing much higher capacitive value than capacitor and are capable to meet high current and specific power demand for short span of time (Halper and Ellenbogen 2006). Like batteries they store electrochemical charge and are the bridging the gap between traditional capacitor and batteries (Raza et al. 2018; Pang et al. 2019). The seminal research paper and the patent for the first supercapacitor were published in 1957 (Kötz and Carlen, 2000). Despite of having analogous storage and operational mechanism like traditional capacitors, ultracapacitors have 100,000 times or even higher energy density and specific capacitance. Due to their robust charge storage capacity, high operating safety, better performance stability and superior cyclic durability, they have outperformed batteries as well. SCs are categorized into three classes, i.e., pseudocapacitors (PC), electrochemical double-layer capacitors (EDLC), and hybrid supercapacitors (HSC) on the basis of their storage mechanisms (Raza et al. 2018; Abbas et al. 2022). The behavior

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and operating voltage of ultracapacitors are highly dependent upon the type of electrolyte. Generally, three types of electrolytes are used, i.e., aqueous, organic, and ionic liquids (Shao et al. 2018). MXenes exhibit EDL or ion-intercalation capacitance with ionic or salt solution-based electrolytes and pseudocapacitance for acidic electrolytes (Zhan et al. 2018). The ultracapacitors are evaluated based on several parameters like cell capacitance (volumetric or gravimetric), operating voltage, series resistance, and energy density. Generally, cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electron impedance spectroscopy (EIS) are used for the evaluation of these parameters (Raza et al. 2018).

3.1 Electric Double-Layer Capacitors The energy storage in EDLCs takes place via the formation of an electric double layer between electrolyte and electrode (Tian et al. 2022). Despite having long cycle retention and good power density they are accompanied by drawbacks of low energy density and low capacitance as compared to pseudocapacitors. Their box-like voltammetric curve is shown in Fig. 3a along with the charge storage mechanism (Abbas et al. 2022; Yoon et al. 2000).

Fig. 3 Voltammogram and charge storage mechanism for a EDLC b PC and c HC. Adapted with permission from reference (Abbas et al. 2022)

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3.2 Pseudocapacitors They can store chemical energy with a very high charge/discharge rate. The pseudocapacitance originates from the electrosorption or redox reaction and ionic intercalation. The interaction between electrolyte and electrode is the key factor for their performance. They are capable to charge faster than batteries and possess more charge storage as compared to the EDLC (Ji et al. 2016). But they possess poor power densities owing to their insulating characteristics (Abbas et al. 2022). Their standard voltammogram and storage mechanism are depicted in Fig. 3b.

3.3 Hybrid Supercapacitors As can be assumed from their name they are the amalgam of the above two types of supercapacitors. They possess both kinds of materials and their voltammetric graph has both characteristic features (Fig. 3c). Being a fusion of both EDLC and PCs they are supposed to have improved power and energy density materials (Abbas et al. 2022; Muzaffar et al. 2019).

4 MXenes in Supercapacitors The nature and properties of electrode material are crucial for the performance of electrochemical capacitors. To meet the explosive desire for high capacitance, innovative electrodes with a sufficiently high number of redox sites, high conductivity, and high surface area are demanded. 2D materials are emerging as next-generation candidates in the field of clean energy owing to their high surface area, physicochemical properties, exclusive electronic architecture, robust ionic transmission, and superior electrochemical storage potential (Zia 2022; Chen et al. 2022). So far, graphene has achieved quite a higher capacitance of 200–350 F/cm3 versus activated carbon (60–100 F/cm3 ). But the electrochemical performance of graphene is restricted because of the limited carbon chemistry and the absence of redox reactions (Ghidiu et al. 2014). In contrast, MXenes are rich in redox reaction sites, ion transportation, and high surface area due to their unique layered structure. Both EDLC and pseudocapacitance energy storage mechanisms are possible in MXenes. The top layer possessing metal oxides provides sufficient redox reaction sites supporting pseudocapacitance while the channels for ionic transmission are provided by the interlayer spaces let the EDLC viable. Be indebted to these exceptional features researchers have already utilized them as electrodes for Li+ ion batteries and electrochemical capacitors (Lukatskaya et al. 2013). As MXenes have the potential for both EDLC and pseudocapacitance they are highly capable to generate excellent supercapacitors.

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The performance and some important parameters of MXenes-based supercapacitors are summarized in Table 1. As Ti3 C2 Tx is the oldest and widely studied representative material of the MXene family. Lately, Ma et al. published a review paper summarizing the contemporary advancements of Ti3 C2 Tx for supercapacitors (Ma et al. 2021). The cation intercalation in Ti3 C2 Tx is evident from the research report published by Lukatskaya et al., where they studied the intercalation of different cations (Li+ , NH4+ , Na+ , K+ , Mg2+ , and Al3+ ) in Ti3 C2 Tx layers. X-ray diffraction (XRD) depicts an increase in c (lattice parameter) value from 20.3 to 25.4 Å, simultaneously a downshift in (0002) peak is observed when Ti3 C2 Tx was immersed in potassium hydroxide (KOH) and ammonium hydroxide (NH4 OH) solutions. The electron dispersive spectroscopy (EDS) analysis proved the intercalation of cations only and expansion along c-axis is not affected by the anionic nature and radii. A similar phenomenon was observed for the other electrolytes tested. Few layered MXene paper depicted volumetric capacitance 340 Fcm−3 with KOH electrolyte solution which is superior to the capacitance acquired with other electrolytes like magnesium sulfate (MgSO4 ) and sodium acetate (NaOAc) (Lukatskaya et al. 2013). SEM image of 2D layered structure and effect of cationic intercalation of MXene is depicted in Fig. 4. As the MXene phase was synthesized by HF etching in the previous report (Lukatskaya et al. 2013), Ghidiu et al. reported the synthesis of Ti3 C2 Tx conductive clay by using HCL/LiF etching route. Upon the electrochemical performance testing the capacitance value was improved to 900 Fcm−3 at the rate of 2 mV/s. The availability of H+ cations and the enhanced accessibility of interlayer spacing in the clay resulted in the improvement of the capacitance. The volumetric capacitance is dependent upon the thickness of the electrodes. The highest capacitance value was obtained for an electrode having 5 µm thickness as compared to the electrodes with 30 and 75 µm thickness (Ghidiu et al. 2014). Two distinct types of MXene-based electrodes with prone redox sites were analyzed by (Lukatskaya et al. 2017). A microporous thin film with 13 µm thickness presented 310 Fg−1 , while the Ti3 C2 Tx hydrogels had 1500 Fcm−3 capacitances. These values surpass the best values reported for graphene and carbon electrodes-based capacitors (Chen et al. 2022). 3 M H2 SO4 was used as an electrolyte and the results of these scientific reports forecast the deployment of this material for the fabrication of a capacitor with a storage rate higher than 10Vs−1 . The better results in 3 M H2 SO4 instead of 1 M electrolyte can be ascribed to the availability large number of protons (Lukatskaya et al. 2017). Hu et al. have investigated the role of the nature of cation and an exhaustive mechanism of capacitance among Ti3 C2 Tx electrodes by introducing in situ Raman spectroscopy. Pseudocapacitance is observed predominately in the H2 SO4 while only the EDLC mechanism is observed for other electrolytes, i.e., NH4 SO4 and MgSO4 . In H2 SO4 the hydronium ions perform bonding/debonding during discharging and charging, respectively. TEM micrograph of Ti3 C2 Tx flakes along with a comparison of results and capacitance mechanisms in different electrolytes is presented in Fig. 5a, b, and c. The higher capacitance in the acidic mediums is due to pseudocapacitance and ion exchange storage system (Hu, et al. 2016). Mostly low capacitance and EDLC

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Table 1 Summary of electrochemical capacitance and some other factors of MXenes Material

Etchant

Electrolyte

Capacitance Retention %/Cycles

Ref

Ti3 C2 Tx

HCl/LiF

3 M H2 SO4

310 Fg−1

> 90/10,000

Lukatskaya et al. (2017)

Ti3 C2 Tx

HF

KOH

340 Fcm−3

100/10,000

Lukatskaya et al. (2013)

Ti3 C2 Tx

HCl/LiF

1 M H2 SO4

900 Fcm−3

100/10,000

Ghidiu et al. (2014)

Ti3 C2 Tx

HF

1 M H2 SO4

~390 Fg−1

– -

Hu et al. (2016)

Ti3 C2 Tx

HF

2 M KCl

369.5 Fg−1

95/5000

Gao et al. (2019)

Ti3 C2 Tx

HCl/HF

LiTFSI-PC

195 Fg−1

94/10000

Wang et al. (2019)

Ti3 C2 Tx

Lewis acid

LiPF6 (ethylene 264 Fg−1 carbonate/dimethyl carbonate)

90/ 2400

Li et al. (2020a)

Ti3 C2 Tx

6molL−1 HF H2 SO4

400 Fg−1

–

Hu et al. (2018)

Ti3 C2 Tx

–

H2 SO4

450 Fcm−3

100/10000

Dall’Agnese, et al. (2014)

Ti3 C2 Tx

HCl/LiF

3 M H2 SO4

140 Fg−1

20,000

Chen et al. (2018)

Ti3 C2 Tx /NiCo2 S4 HF

3 MKOH

1147.47 Fg−1

91.1/3000

Li et al. (2020a)

Ti3 C2 Tx @PANI*

HCl/LiF

3 M H2 SO4

1632Fcm−3 85/20,000 510 Fg−1

Li et al. (2020b)

Ti3 C2 Tx /rHGO*

HCl/LiF

3 M H2 SO4

1445 Fcm−3 438 Fg−1

93/10,000

Fan et al. (2018)

Ti3 C2 Tx /CNTs*

–

MgSO4

390 Fcm−3

100/10,000

Lipton et al. (2020)

Ti3 C2 Tx /rGO*

HCl/LiF

3 M H2 SO4

1040 Fcm−3 335 Fg−1

100/20,000

Yan et al. (2017)

Ti3 C2 Tx /rGO*

HF

PVA/H3 PO4

586.4 Fcm−3 327.5 Fg−1

100/10,000

Xu et al. (2017)

Ti3 C2 Tx /EG*

HF

PVA/H3 PO4

216 Fcm−3

–

Li et al. (2017)

Ti3 C2 Tx /AAC*

HCl/LiF

7 M KOH

378Fg−1

97.4/100,000 Li et al. (2020b)

Ti3 C2 Tx /OMC*

HF

3 M KOH

823 Fcm−3 329 Fg−1

117/10,000

Allah et al. (2019) (continued)

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Table 1 (continued) Material

Etchant

Electrolyte

Capacitance Retention %/Cycles

Ref

Ti3 C2 Tx /NC*

HF

6 M KOH

828 Fg−1

Zhang et al. (2019)

100/5000

* EG (Electrochemically exfoliated graphene), PANI (Polyaniline), rGO (Reduced graphene oxide), rHGO (Holey graphene), CNT (carbon nanotubes), AAC (Acid active carbon), OMC (ordered mesoporous carbon), NC (nitrogen doped carbon)

Fig. 4 a SEM micrograph of 2D Ti3 C2 Tx synthesized by HF etching b Depiction of intercalation of cations and an increase in d spacing upon intercalation. Adapted with permission from reference (Come et al. 2015)

mechanisms for charge storage are observed in KOH and neutral electrolytes. An exertion to improve the capacitance of Ti3 C2 Tx was made by Gao et al. where they doped the electrode with vanadium via hydrothermal method. This effort improved the gravimetric capacitance from 115.7Fg−1 to 365.9Fg−1 for a neutral electrolyte (2 M KCl) (Gao et al. 2019).

Fig. 5 a TEM micrograph of Ti3 C2 Tx flakes b CV scans at 20 mV/s in different electrolytes. c Capacitance variation at different scan rates for different electrolytes. Adapted with permission from the American Chemical Society (Hu, et al. 2016)

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Aside from these electrolytes, many organic electrolytes have been tried to study the role of electrolytes in supercapacitance. They are accompanied by higher operational windows (>2 V) so are the prospective candidates to improve energy densities. The potential windows for Ti3 C2 Tx are 2.4 V and 2.8 V in lithium bis-trifluoromethyl sulfonyl amine in propylene carbonate (LiTFSI-PC) (Wang et al. 2019) and lithium hexafluorophosphate (LiPF6) carbonate electrolytes (Li et al. 2020a), respectively. The electrodes revealed gravimetric capacitance values of 195 Fg−1 in LiTFSI-PC and 264 Fg−1 in LiPF6 electrolytes. Other than electrolyte the surface terminators on the electrodes play a vital role in electrochemical storage. The role of surface terminator groups (Tx = O, F, OH) for the Li+ cations storage for various MXenes was computed by Yu and co-workers via density functional theory (DFT) calculations. Theoretical Li+ cations storage capacity is directly proportional to the number of absorbed Li+ ions. Surfaces having O as the terminator group had the highest absorption capacity for Li+ ion as compared to other surfaces. The hydrophilicity, conductivity, and solubility of MXenes are also highly affected by the polarity of the surfaces (Xie et al. 2014). The effect of the concentration of HF on the performance of the electrode was investigated by Hu et al. The electrodes prepared with low concentration (6 molL−1 ) of HF were better candidates because of higher oxygen contents on the surfaces. Their pseudocapacitance was almost double of the electrodes prepared via high concentration (15 molL−1 ) HF. Another example for surface modification of Ti3 C2 Tx was stated by Dall’Agnese et al. The treatment of the electrode with KOH resulted in high surface oxygen content which ultimately led to a significant improvement in capacitance to 450 Fcm−3 from 180 Fcm−3 (Dall’Agnese et al. 2014). An improved charge storage results were obtained for the glycine functionalized free standing Ti3 C2 Tx film (Chen et al. 2018). The rectification of capacitance can be attributed to an increase interlayer distancing and strong Ti-N bonding. Though pure MXenes have excelled a lot in the field of supercapacitors still there is a room for improvement in the properties. To meet these yearning numerous composites are prepared. So far, many attempts have been made to prepare the hybrids of Ti3 C2 Tx with different materials like polymers, carbon-based compounds, graphene, and metallic components as well. Some of the hybrid electrode materials and their performances are mentioned in Table 1. Hence, it wouldn’t be a mistake to consider MXenes as a future promising candidate for energy storage devices.

5 Conclusion and Outlook Since 2011, Ti3 C2 Tx has attracted a lot of attention and has been extensively studied. In near future, other members of this family containing two or more elements and based on other transition metals will also get the limelight. Till now mostly topdown etching techniques are used for their synthesis but for commercialization,

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other methods having high yield, controllability, and safety will be desired. Industrialization will also need to consider mechanistic study and evaluation of new electrolytes. Regarding supercapacitance, MXenes have much higher volumetric capacitance (400–1500 Fcm−3 ) than other carbon-based 2D materials. This marvelous supercapacitance of Ti3 C2 Tx is on account of their high surface area, higher conductivity, superior mass density, and their unique storage mechanisms. There is still room for improvement to ameliorate the potential windows, oxidation stability, electrical conductivity and electrode structures. Overall, MXenes are sensational candidates and have progressed a lot during the decade. They are going to be a breakthrough in the field of supercapacitors and batteries. It can be predicted that various other variants will be discovered, evaluated, and deployed for supercapacitors.

References Abbas M et al (2022) Organic-Inorganic Nanohybrids in Supercapacitors. Hybrid Nanomaterials. Springer, pp 359–383 Allah AE et al. (2019) Auto-programmed heteroarchitecturing: Self-assembling ordered mesoporous carbon between two-dimensional Ti3C2Tx MXene layers. Nano Energy 65: 103991 Chen Y et al (2022) Mxene-Based Electrodes for Supercapacitor Energy Storage. 36(5):2390–2406 Chen C et al (2018) Effect of glycine functionalization of 2D titanium carbide (MXene) on charge storage. J Mater Chem A 6(11): 4617–4622 Cheng N et al (2020) Application of organic-inorganic hybrids in lithium batteries. 15:100289 Come J et al (2015) Controlling the Actuation Properties of Mxene Paper Electrodes upon Cation Intercalation. 17:27–35 Dall’Agnese Y et al. (2014) High capacitance of surface-modified 2D titanium carbide in acidic electrolyte. Electrochem Communications 48: 118–122 Dong Y, Shi H, Wu ZSJAFM (2020) Recent advances and promise of MXene-based nanostructures for high-performance metal ion batteries. Adv Funct Mater 30(47):2000706 Fan Z et al (2018) Modified MXene/holey Graphene Films for Advanced Supercapacitor Electrodes with Superior Energy Storage. 5(10):1800750 Gao ZW, Zheng W, Lee LY (2019) Highly enhanced pseudocapacitive performance of vanadiumdoped MXenes in neutral electrolytes. Small 15(40): 1902649 Ghidiu M et al. (2014) Conductive two-dimensional titanium carbide ‘clay’with high volumetric capacitance. Nature 516(7529): 78–81 Gogotsi Y, Huang (2021) MXenes: two-dimensional building blocks for future materials and devices. ACS Publications, pp 5775–5780 Halper MS, Ellenbogen JCJTMC (2006) Supercapacitors: A brief overview. McLean, Virginia, USA Hu M et al. (2016) High-capacitance mechanism for Ti3C2T x MXene by in situ electrochemical Raman spectroscopy investigation. ACS nano 10(12): 11344–11350 Hu M et al. (2018) Surface functional groups and interlayer water determine the electrochemical capacitance of Ti3C2 T x MXene. ACS nano 12(4):3578–3586 Iqbal A et al (2022) Organic-Inorganic nanohybrids in advanced batteries. Hybrid nanomaterials. Springer, pp 461–483 Ji X et al (2016) Probing the Electrochemical Capacitance of Mxene Nanosheets for HighPerformance Pseudocapacitors. 18(6):4460–4467 Kötz R, Carlen (2000) Principles and applications of electrochemical capacitors. Electrochimica acta 45(15–16): 2483–2498

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Li Y et al (2020a) A general lewis acidic etching route for preparing mxenes with enhanced electrochemical performance in non-aqueous electrolyte. 19(8):894–899 Li Y et al (2020b) Binder-Free Two-Dimensional MXene/acid Activated Carbon for HighPerformance Supercapacitors and Methylene Blue Adsorption. 34(8):10120–10130 Li H et al. (2017) Flexible all-solid-state supercapacitors with high volumetric capacitances boosted by solution processable MXene and electrochemically exfoliated graphene Li T et al. (2018) Fluorine-free synthesis of high-purity Ti3C2Tx (T= OH, O) via alkali treatment. 57(21): p. 6115–6119. Li H et al. (2020a) 3D hierarchical transition-metal sulfides deposited on MXene as binder-free electrode for high-performance supercapacitors. J Ind Eng Chem 82:309–316 Li K et al. (2020b) An ultrafast conducting polymer@ MXene positive electrode with high volumetric capacitance for advanced asymmetric supercapacitors. Small 16(4):1906851. Lipton J et al (2020) Layer-by-Layer Assembly of Two-Dimensional Materials: Meticulous Control on the Nanoscale. 2(5):1148–1165 Lukatskaya MR et al (2013) Cation Intercalation and High Volumetric Capacitance of TwoDimensional Titanium Carbide. 341(6153):1502–1505 Lukatskaya MR et al (2017) Ultra-High-Rate Pseudocapacitive Energy Storage in Two-Dimensional Transition Metal Carbides. 2(8):1–6 Ma R et al. (2021) Ti3C2Tx MXene for electrode materials of supercapacitors. J Mater Chem A 9(19): 11501–11529 Mathis TS et al. (2021) Modified MAX phase synthesis for environmentally stable and highly conductive Ti3C2 MXene.ACS nano 15(4): 6420–6429 Ming F et al (2021) Mxenes for rechargeable batteries beyond the lithium-ion. 33(1):2004039 Muzaffar A et al. (2019) A review on recent advances in hybrid supercapacitors: Design, fabrication and applications. Renewable and sustainable energy reviews 101: 123–145 Naguib M et al (2013) New Two-Dimensional Niobium and Vanadium Carbides as Promising Materials for Li-Ion Batteries. 135(43):15966–15969 Naguib M. et al. (2011) Two-dimensional nanocrystals produced by exfoliation of Ti3AlC2. Adv Mater 23(37): 4248–4253 Pang S-Y et al (2019) Universal strategy for hf-free facile and rapid synthesis of two-dimensional MXenes as multifunctional energy materials. 141(24):9610–9616 Pathak M, Polaki S, Rout CSJRA (2022) High performance asymmetric supercapacitors based on Ti 3 C 2 T x MXene and electrodeposited spinel NiCo2S4 nanostructures. RSC Advances 12(17): 10788–10799 Rasheed T et al. (2021) Two dimensional MXenes as emerging paradigm for adsorptive removal of toxic metallic pollutants from wastewater. Chemosphere: 132319 Raza W et al (2018) Recent advancements in supercapacitor technology. 52:441–473 Rizwan K et al. (2022) MXene-based electrochemical and biosensing platforms to detect toxic elements and pesticides pollutants from environmental matrices. Chemosphere 291 Salim O et al (2019) Introduction to MXenes: Synthesis and Characteristics. 14:100191 Sengupta AS et at. (2018) Supercapacitors outperform conventional batteries [energy and security]. IEEE Consum Electron Mag 7(5): 50–53 Shao Y et al (2018) Design and Mechanisms of Asymmetric Supercapacitors. 118(18):9233–9280 Shi H et al. (2021) Ambient-stable two-dimensional titanium carbide (MXene) enabled by iodine etching. 60(16): 8689–8693 Tian Y et al (2022) Two-Dimensional Hexagonal-Shaped Mesoporous Carbon Sheets for Supercapacitors. 7(32):27896–27902 Tian Y et al. (2022) magnesium hydroxide templated hierarchical porous carbon nanosheets as electrodes for high-energy-density supercapacitors VahidMohammadi A, Rosen J, Gogotsi YJS (2021) The world of two-dimensional carbides and nitrides (MXenes). Science 372(6547): eabf1581 Wang X et al (2015) Pseudocapacitance of MXene Nanosheets for High-Power Sodium-Ion Hybrid Capacitors. 6(1):1–6

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Recent Advancements in MXene-Based Lithium-Ion Batteries Fozia Maqsood, Faisal Jamil, Umar Sohail Shoukat, and Muhammad Adnan Iqbal

Abstract Lithium-ion batteries (LIBs)—are the most demanding technology of today. LIBs show a high energy density, high voltage capability, long life span, low discharge capacity, and higher power density than other technology. With their technical advantages, lithium-based batteries have the potential to transform the photovoltaic (PV) sector and renewable energy sources. Lithium-ion batteries have a lower cost and long life cycle than other batteries. MXene is a two-dimensional (2D) layered anodic material for Lithium-ion batteries, distinguished by its unusual physical and chemical features, as well as its applicability for high-power applications. MAX phase structure is used in a battery in which ‘M’ is the transition metal; ‘A’ for the 4th group of elements and ‘X’ may be nitrogen or carbon. In this chapter, we debated the current development of lithium-ion batteries. Also, discussed what are the changes required to compete with modern techniques. How MXene energy application cause changes in LIBs. The efficiency of LIBs is enhanced when modifying the surface structure of ionic species. Gel polymers are used in LIBs for the enhancement of mechanical performance, chemical and electrochemical stability, and excellent ionic stability. The revolutionization of portable electronics by LIBs sparked a surge in academic interest over the next few years. It also explains the key parts of material science in the advancement of LIBs. After two decades of Li-ion technology marketing for portable devices, LIBs are gaining attraction for a variety of new energy or power-intensive applications for both stationary and electro-mobility. Their performance is dependent on a precise balance of transport phenomena and interfacial processes. This chapter is based on the recent developments made in the field of Lithium-ion batteries. Keywords LIBs · Electrode · Separator · Gel polymer · MXene

M. A. Iqbal (B) Department of Chemistry, University of Agriculture Faisalabad, Faisalabad, Pakistan e-mail: [email protected] F. Maqsood · F. Jamil · U. S. Shoukat · M. A. Iqbal Organometallic and Coordination Chemistry Laboratory, Department of Chemistry, University of Agriculture Faisalabad, Faisalabad 38000, Pakistan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_7

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1 Introduction The demand for High-performance rechargeable batteries is increasing day by day. One could argue that the development of the transistor on a sufficiently tiny scale was what stimulated the search for improved rechargeable batteries. Mobile electronics frequently employ lithium-ion batteries (LIBs). The term ‘ion’ denotes the batteries’ lack of lithium metal. With high energy capacity, extended lifecycle also widespread use in consumer devices such as cell phones, computers, and full and hybrid electric cars, lithium-ion batteries have become quite popular as shown in Fig. 1 (Writer 2019). MXenes, a new class of 2D materials, have specific physicochemical characteristics that make them ideal for high-strength battery applications such as lithiumion batteries. Wet HF treatment of Aluminium containing MAX phases resulted in the formation of exfoliated transition metal carbides and carbonitrides known as ‘MXene’. Electric conductivities of freestanding, cold-pressed MXene discs were discovered to be equivalent to multilayer graphene (Rasheed et al. 2021; Rizwan et al. 2022). One of the most popular and least expensive MAX phases is Ti2 AlC.

Fig. 1 Applications of Li-ion batteries in devices

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The most common method for producing MXenes is through selective chemical etching of MAX phase precursor materials (Naguib et al. 2012). Early research on LIBs concentrated mainly on solid-state physics. But as nanotechnology advanced toward the last of the 20th period, scientists started to pay maximum attention to the morpho characteristics of electrode materials, such as surface coating, porosity, size, and form (Kesavan et al. 2018). Of all the metals, lithium is the lightest, has the highest voltage, and has the largest energy density. Researchers were attracted to lithium because of its strongly reducing nature (3.04 V compared to a conventional hydrogen electrode), low atomic mass, and wide voltage window of organic electrolytes in their hunt for a system with a greater energy density. Additionally, the high diffusion coefficient that Li-ions’ short atomic radius provided when utilized as the charge carrier, made them theoretically an actual gift method for meeting the excess power and energy density requirements of convenient energy-storing methods (Xu et al. 2014). In order to bring lithium-ion battery costs down to the desired objective. It is important to enhance materials’ processing and implement stringent quality control methods in the manufacturing process, as has already been done in other industries, such as semiconductor manufacturing (Li et al. 2011). With its high columbic efficiency and a large range of chemical potentials accessible with varied electrode designs, lithium-ion batteries (LIBs) are widely used as energy sources. It also contains low self-discharge and high energy density characteristics in a variety of applications. Li-rich (lithium, manganese, nickel, cobalt oxide with oxygen) and Ni-rich transition metal oxides (with lithium layered oxide), Lithium-S, high voltage spinel, and Li-O2 , are examples of high energy density battery technology. Additionally, they are less hazardous compared to the frequently used LIBs (e.g. LiCoO2 ). This cutting-edge innovation which is sometimes viewed as a self-containing power system offers excellent efficiency and dependable energy in a distant area. In the automobile sector, higher energy density LIBs are regarded as an optimum energy source for HEVs and EVs as shown in Fig. 1 (Kim et al. 2019). High energy elements are combined with LIBs including flammable electrolytes made of organic solvents. Some of the primary obstacles to the successful adoption of Li+ ion transportation technology and fixed electric energy storage include cost, life cycle, power, energy, and safety. These factors relate to the cathode and anode resources utilized in making Li-ion batteries (Nayak et al. 2018).

2 History of Lithium-Ion Battery Arfwedson and Berzelius found lithium in 1817 while studying petalite ore (LiAlSi4 O10 ). But Brande and Davy electrolyzed lithium oxide to produce the element in 1821. Harris’s work in 1958 marked the beginning of the first reported interest in lithium batteries. He investigated how well lithium dissolved in different electrolytes based on nonaqueous solvents, such as lactones, melted salts, and inorganic lithium perchlorate-solvated in propylene carbonate. Studies on the stability of

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lithium-ion batteries were sparked by his discovery that a passivation layer may form that could block the chemical interaction between Li and the electrolytes, while yet permitting ionic transfer across it (Harris 1958). Non-aqueous 3 V Liion primary batteries have been existing in the marketplace since the late 1960s, and in 1969, cathodes containing lithium sulfur dioxide Li/SO2 were added. Throughout the 1970s, many primary lithium batteries were being developed and commercialized. Matsushita began selling lithium-poly carbon monofluoride in 1973. The Sanyo corporation first commercialized lithium per manganese oxide batteries in 1975. The development of room temperature rechargeable systems gained a new boost when Exxon revealed its intentions to make marketable their lithium titanium disulfide system as a button-like small cell for electronic watches in the mid-1970s. Since then, numerous rechargeable systems with various anodes made of lithium metal or lithium compounds, different cathode materials, and different electrolytes have been researched and produced. Moreover, there is only one form of rechargeable battery that is not a coin cell and is widely accessible in the market. It’s a lithiumion cell made by Sony. It has liquid electrolytes inside. This arrangement seems to be headed toward becoming the norm. Cell phones, video cameras, and notebook PCs are all currently powered by it. Lithium compounds include lithium-iodine, LiMn2 O4 , LiSOCl2 , and Li(CFx)n. Lithium/sulfur dioxide systems were among the more popular ones. (Gnedenkov et al. 2012). Several efforts to create a rechargeable LB were made in the 1980s. But these efforts were withdrawn by problems concerning the recharging of the M-Li anode. Unlucky incidents involving safety did occur from time to time (audible with voicing and flash). As a result, Li inserted complexes were used as (+) electrodes to generate rechargeable lithium batteries at the end of the 1970s and earlier 1980s. In 1975, Wittingham and Gamble showed the intercalation of lithium into a variety of layered transition metal dichalcogenides using a wet chemical method (using n-butyllithium) (sulfides and selenides) (Whittingham & Gamble Jr, 1975). Once Moli and Exxon Energy attempted to market the schemes, the first cells of this sort emerged. The first commercial lithium-ion cell was made available by Sony in 1991. The cells’ working voltage was 3.6 V, while their open circuit potential was 4.2 V (Hwang et al. 2012).

3 Different Types of Lithium-Ion Batteries There are two types of lithium-ion batteries (Fig. 2). The first is called a primary/ non-rechargeable battery and the second is a secondary/rechargeable Li-ion battery. The primary and secondary batteries, which are electrochemical cells with terminals or connections for supplying electrical energy and are electrically coupled, have become extremely important for sustaining modern society (Julien et al. 2016).

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Fig. 2 Types of lithium-ion batteries

3.1 Primary Lithium-Ion Batteries Non-rechargeable batteries are lithium-ion primary batteries. Which is the result of the batteries’ irreversible electrochemical processes. Primary batteries are a significant class of energy storage devices with practices in mining, defense, and portable electronics. The battery can tolerate long-term storage at a high temperature of 71 °C (160°F). It includes electrical locks, emergency power sources, electronic counters, electronic measurement equipment, military, cameras, memory backup, and implantable therapeutic devices are just a few of the many areas where primary lithium batteries are frequently utilized. Excellent dependability and extended life protections are necessary for these applications. An F-based cathode, specifically carbon-fluoride is one of the appealing chemistries (Zhang et al. 2010). One of the exemplary primary batteries is the zinc-carbon battery (Yu et al. 2013). Based on the presumption that the predominant alkaline and zinc-carbon batteries would have been removed beforehand, LIBs and primary lithium were treated on large scale. To achieve leftover fractions that are demonstrative of a large-scale condition, all mechanical approaches were established on a medium scale. The lithium-sulfur dioxide (Li/SO) battery is a brand-new, portable primary battery system with great energy and temperature stability (Shu et al. 2019; Wang et al. 2021).

3.2 Secondary Lithium-Ion Batteries Secondary lithium batteries are another name for rechargeable lithium-ion (Li-ion) batteries (Chen et al. 2016). These batteries are electrochemical and may be recharged and discharged repeatedly. The reversible electrochemical reactions that occur in batteries are the source of this reusable function. Due to its particular high energy, large capacity, and long life span, the secondary lithium-ion battery is an excellent

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choice as a high energy source for electric vehicles and hybrid electric cars. Safety is particularly crucial for vast-scale LIBs (Eshetu et al. 2021). There was no metallic lithium present, and the lithium that was doped into the carbon layers was in an ionic state. Since LiCoO2 is an ionic bonded compound, it is obvious that all lithium species in the anodes and cathodes of LIB are in the ionic state (Zheng et al. 2016). Because of this, ‘lithium-ion secondary battery’ is named for this class of battery system. These batteries have a wide-range operative temperature, a small self-discharge ratio, high volumetric and gravimetric energy densities, no memory effect, and a high operating voltage (Nishi 2001). Despite having numerous benefits of LIBs e.g. no memory effects, extraordinary energy capacity, and excellent longer life cycle, with a 39 °C ambient temperature, the surface temperature of the secondary Li-ion batteries can reach approximately fifty (Poizot and Dolhem 2011). Due to warmness, there are two different types of Lithium batteries, i.e. lithium-ion and lithium polymer. The temperature properties of secondary LIBs are a crucial consideration when choosing a secondary battery for the implanted battery-operated system, and the surface of the lithium-ion secondary battery is a crucial problem. (Wang et al. 2014). Designing effective active materials with exceptional physical characteristics is important for developing highly effective LIB with superior energy storage capacity. It has gained popularity, recently, as a result of its outstanding properties, which include high thermal conductivity, electrical conductivity, and light transparency. Additionally, take into account its uses as conductive additives for batteries and transparent conductive films (Cui et al. 2010; Higgins et al. 2016).

4 Advantages of Lithium-Ion Batteries Li-ion batteries can store and use clean energy, that’s why they are commonly used in cleaner productions as a result of their excellent electrochemical properties and high mobility (Reddy et al. 2020; Tong et al. 2021). At the moment, technologies are being explored to reduce polarity in anode– cathode materials and improve battery achievements. Nanocrystallites have gotten a lot of interest recently as possible conductor resources for energy storage. There are several advantages of a decrease in the lithium-ion distribution path interval. So reduces polarity in electrode materials. For example, lithium iron phosphate, lithium cobalt phosphate (Nakayama et al. 2004), and lithium manganese(II) phosphate permit growth in charge/discharge level, such as a reduction in diffusion length of electrons in such nonconductive materials (Zheng et al. 2017). Co, Li, Cu, and Al are precious metals found in wasted LIBs that can be used in industrial and military systems. As a result, recycling precious metals from previous lithium-ion batteries is very desirable (Barik et al. 2016). MXenes offer unusual features such as high electronic conductivity, great hydrophilicity, and chemical durability (Zhang et al. 2020).

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Lithium-ion intercalation electrode materials for new markets must have a strong percentage capacity. So, this is connected with an electronic passage and lithium-ion diffusion path throughout the insertion/extraction route of intercalation compounds. Nanomaterials can give small diffusion path lengths for LIBs (Taberna et al. 2006).

5 Disadvantages Lithium-Ion Batteries Electric vehicles, mobile phones, laptops, power grids, and other devices all frequently use batteries, particularly Li-ion batteries. However, this is an important issue with the usage of lithium-ion batteries. When the battery is being charged or discharged, heat is produced, raising the battery’s temperature. The battery may enter thermal runaway if the heat cannot be removed promptly. A significant quantity of heat will be released, and the temperature could rise to 200 °C, causing combustion or perhaps an explosion (Lyu et al. 2020). Under abusive conditions, some LIBs might show a propensity to catch fire or produce hazardous fumes, posing a risk. Large-scale energy storage system incidents involving fire and explosion have also happened Diaz et al. 2020; Xu et al. 2021). Without proper disposal, these expended lithium-ion batteries may cause environmental issues such as soil and water pollution (Fu et al. 2019; Li et al. 2010).

6 Early Lithium-Ion Batteries Any device’s evolution is impacted by its predecessors. Lithium batteries are also related to it. Without its invention, we would not have been able to rely on multipurpose, continually improved smartphones, durable laptops, or listen to Mp3 music to brighten up our workouts (Clemm et al., 2016). After the innovation of alkali, Pb-acid, and Ni–Cd batteries, there was essentially little innovation in the market of batteries for more than a century because of only minor modifications to their interaction and configuration. These primary systems satisfied the needs of the expertise at that time. Battery technology didn’t change until the end of the 1960s when the need for portable energy led to a number of advancements. With the creation of batteries using a novel concept and Li as one of the electrode materials, a breakthrough was made (Yoshino 2022). Lithium has the highest electrochemical equivalent of any metal, which enables it to have a substantially greater definite capacity than Zn (such as 3860Ah/kg vs. 820Ah/kg). Since Li-metal is incompatible with H2 O, it was necessary to switch from convenient aqueous electrolytes to a more electrochemically stable organic medium, which was typically created by dissolving a Li-salt in a carbon-based solvent that is rich in carbonate. That transition from a Zn-based to a Li-based battery is simple to visualize the picture (Hwa et al. 2012; Wu et al. 2018).

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Following LIB’s release onto the consumer market, portable gadgets quickly adopted this new battery type’s adaptability. Early LIB had a high volumetric energy density of about 200WhL−1 , which was twice as high as rival technologies like hydride batteries such as Ni–Cd and Ni-metal at that time. Due in part to LIB, the size and weight of laptop computers, digital cameras, and cell phones quickly decreased in the 1990s. This made LIB the ideal solution for these devices. With a multi-billion dollar market and a global market share of more than 60%, LIB quickly surpassed the competition to become the main battery technology for portable devices (Khaligh et al. 2007). In Rome, Italy, the 1st international meeting on lithium batteries was organized in 1992 as a result of the continued high level of curiosity about lithium batteries and the rapid increase in the number of research laboratories engaged in Li-batteries’ research and development (Shi et al. 2019). Lithium-ion conducting solids have received a lot of attention. Li-I utilized in the pacemaker batteries serves as one example. However, the ionic conductivity level of this material (Li-I) in its pure form is too low to be employed for uses that require more energy as compared to a cardiac pacemaker (Hurzeler et al. 1980; Zhang et al. 2015).

7 Present Lithium-Ion Batteries Polymer electrolyte serves as an example of the system’s intricate structure. The improvement of Li-batteries, for use anywhere where the temperature is not a limiting aspect, is still made possible by the special benefits of polymer electrolyte, which include chemical strength, compatibility with the Li-M electrode, and reasonable rate. The lithium battery that guarantees the largest energy content is its polymer version. The creation of gel polymers electrolyte, in which a molten electrolyte is added to plasticize the polymer and to produce very conductive solid polymer electrolytes, was prompted by the little conductivity of polyethylene oxide-based solid polymer electrolytes (Gurusiddappa et al. 2016; Li et al. 2020a, b, c). Today, Li-batteries are utilized in smartphones, and other advanced technology consumer goods are built on modified versions of these gel-type electrolytes. Due to decreased hazards of liquid leakage, lithium-ion polymer batteries (LIPBs) are more shape-flexible than LIBs. As a result, an aluminum-laminated film rather than a metal box serves as the outside packaging for LIPBs. ‘Thin’ or ‘small’ shaperestricted mobile devices, such as thin notebook PCs and digital media players, are where LIPBs have become popular (Endo et al. 2000).

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8 Future of Lithium-Ion Batteries The choice of an anode in LIBs is graphite, but further tools are considered an effective replacement. Lithium titanium oxide (Li4 Ti5 O12 ) (LTO) is also an example of this substitute. In comparison to traditional graphite, the voltage level is higher, and the theoretical specific capacity is lower. Due to its unique characteristics, such as minor variations in the lattice arrangement that absorbs and releases Li-ions and an operating procedure evolving inside the stability sphere of the electrolytes, interest in lithium titanium oxide batteries remains high. In fact, long cycle lives and good reliability have been shown for LTO batteries. Since LTO is now a commercially available material, it might soon be used in large-scale marketable prototypes (Chou et al. 2011). The most promising (-) electrodes for modern LIBs and MXene are those made of Li-M alloys, such as lithium-tin and lithium-silicon alloys, which have a particular capacity that is significantly higher than that of lithium-graphite (Kang 2013). The cathode’s function is to ensure the electrochemical process and to receive lithium-ions back in a reversible manner, in order to ensure the battery’s longevity. Lithium nickel manganese oxide has also obtained significant attention. The theoretical specific capacity is comparable to that of LiCoO2 , a common form of lithium cobalt oxide. The high operational voltage is the main distinction, making LiNi0.5 Mn1.5 O4 a very competing cathode for the advancement of LIBs technology (Ma et al. 2016). These unusual power sources are now facing a new challenge. The urgent need for greater consumption of green, alternate sources like wind and solar energy, is being driven by the ongoing depletion of oil reserves and developing serious concerns about the decline of our planet’s climatic circumstances. Additionally, Li-batteries, in particular, are the better options for replacing polluting internal combustion engines with more effective, controlled-emission vehicles because they have high conversion efficiency and emit no harmful emissions when converting stored chemical energy into electrical energy (Rahman et al. 2011). As the number of new electric vehicles increases, so will the amount of expended lithium-ion batteries (LIBs). Li-ion batteries will account for around 60% of portable batteries worldwide, and the global market for LIBs was worth $50 billion by 2020 (Buttry et al. 2017; Wenxuan Zhang et al. 2017).

9 Important Elements of Lithium-Ion Batteries 9.1 Electrodes A typical current LIBs has two electrodes (anode or cathode). A LiCoO2 cathode and graphite (C6) anode, are separated by a spongy separator and submerged in an organic liquid electrolyte made of lithium hexafluorophosphate and a combination

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of ethylene carbonate (Jeong 2014; Kim 2015), at least one straight line carbonate chosen from DMC, DEC. Li-ions travel from the lithium cobalt oxide lattice structure toward the anode side during charging, creating graphite-containing lithium. These ions return to the cobalt oxide host structure during discharging, whereas electron is discharged to the outside circuit. Rocking-chair chemistry, or the process of shuttling, is what has changed modern living (Fathi 2015; Xie and Lu 2020). MXenes have received a lot of attention as anode materials for LIBs due to their high metallic conductivity, remarkable mechanical qualities, and huge specific surface area. Although bare MXenes have a larger theoretical specific capacity and a lower diffusion barrier than functionalized MXenes, they are unstable and tend to be functionalized. Because O-terminated MXenes have higher capacities than OHterminated MXenes, O-functionalized MXenes are better suited for the anode materials used in LIBs. In order to reduce the number of inactive modern separators and collectors needed per cell, cathode diameter, which now restricts the energy density and cell’s specific capacity, thus rise cell cost, should also be greatly improved. The unit-cell energy density is raised while the price of the battery pack is decreased by thickening the coating of the cathode and balancing it with a heavier anode. A graphite/NMC (Ni-Mn-Co Oxide) capacity would increase and its cost would be significantly decreased by doubling up the cathode thickness to around 200 m (Wood III et al. 2015). As an anode in LIBs, niobium pentoxide/MXene demonstrated remarkable electrochemical performance, including high reversible capacity. Because of their power density and higher energies, Li-ion rechargeable batteries with a lithium cobalt oxide cathode and carbon anodes are quickly displacing traditional batteries’ technology. The (+) electrode of lithium-ion batteries contains too much active material (Li + ). This is required for the electrode surfaces to develop a stable coating (Yang et al. 2020).

9.2 Separators A separator is a thin permeable membrane that separates the anode and cathode. The Separator’s principal role is to avoid physical contact between the anode and cathode while permitting ion flow in the cell. It is also important that no visible shrinkage at high temperatures takes place in separators. The trade-off between mechanical robustness and transport characteristics is the problem in constructing safe battery separators. In conventional LIBs, the separator is critical, and there is a close relationship between battery safety and the separator. The most important separator’s significance is self-evident. Separator failure would represent a significant hazard to the battery (Zhang et al. 2016). Following properties must exist in good separators. • The Separator between the anode and cathode serves as a barrier, as well as a reservoir for electrolytes to enable ion movement in the inner side of the

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battery (Deimede and Elmasides, 2015). So, their excellent construction must be important for a longer life cycle. Separators with excellent heat resistance and flame-retardant properties must be obtained for making the device environment-friendly (Kang et al. 2022). Good mechanical power is required to maintain the structural integrity of the separator. It does not matter whether it is used as either an individual portion or an integral component of lithium-ion batteries (Li et al 2020; Zhao et al. 2021). It must possess excellent qualities for preventing dendrite formation and improving charge/discharge capabilities in lithium-ion batteries (Na et al. 2016; Zhang et al., 2018). It should have higher chemical and electrochemical affinity to prevent the reaction between other battery parts and the separator (Mittal et al. 2021).

These properties must necessarily be present in separators to maintain the performance of the battery. Separator with higher safety that fulfills the mentioned performance standards should be extensively established and even placed at the forefront of battery-operated advances. At that time, high-safety separator research has been based on the change in existing polyalkene separators and other unique and new materials separators and structures. Multiple separators created by coating or imparting inorganic/organic compounds on surfaces have exceptional qualities, benefiting from the good chemical stability and mechanical power of commercial separators. Permeable separators created by solution forming have advanced considerably in recent years as a result of adjustable structure design. Nonwoven separators can have high thermal stability and mechanical strength by including inorganic and organic components within the fiber (Xu et al. 2020).

10 How to Secure Lithium-Ion Batteries For Li-ion batteries to be used in automotive applications, they must have excellent safety and aging characteristics. Thermal analysis is critically important for the research and designing of large-scale Li-ion batteries since safety is particularly critical for these batteries, and employing more stable electrolytes is an effective strategy to address the safety issues (Biensan et al. 1999). It is believed that it has a more close relationship with the internal portions, such as the anode and cathode material, electrolytes, and separator than other electronic devices. An efficient technique to comprehend how design and operational factors disturb the lithium-ion battery and thermal behavior in thermal modeling throughout charging and discharging. The geometry, structure, physical and chemical behavior, and electrochemical parameters must be outlined as precisely promising in the model, in order to produce an accurate depiction of a battery’s thermal behavior (Zhang et al. 2014). The performance of a battery can be impacted clearly by any modifications to the physical or chemical properties of the electrolyte. The selection of electrolyte

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systems must be cautious to ensure optimum, and smooth lithium deposition. The main element affecting the thermic stability of the entire Li-ion battery system is its flammable electrolyte. The next generation of secure lithium-ion batteries will require technological advancement in the electrolyte area. It is crucial to produce safer electrolytes, which may involve utilizing more stable lithium salts, adding additives, employing nonflammable electrolyte solvents, or creating watery electrolytes. (Wang et al. 2019). The development of an electrochemical reaction to lower heat generation or increase the thermal stability of battery function materials is the key solution to the thermal runaway problem. Effective battery thermal management solutions should be further developed to ensure the thermal safety of lithium-ion batteries because thermal concerns cannot be solved easily from the standpoint of material (Lyu et al. 2020). The measures to improve the safety of Lithium-ion batteries are inevitable. The introduction of MXene as an anode in lithium-ion batteries and some other measures, as mentioned below, has helped to overcome many of these drawbacks. MXene materials have electrochemical performance, as well as their thermal safety because the LIB anode is hidden. MXenes’ potential capabilities in generating safer LIBs may potentially aid their future applications. Differential scanning calorimetry reveals the MXene anode, including the reaction temperature and heat generation (Cai et al. 2021). Improving the thermal stability of the electrode materials and electrolytes can improve lithium-ion battery safety. The heat pipe, phase change material, liquid cooling, and air cooling were all employed to rise the security of Li-ion battery packs (Lebrouhi et al. 2022; Zhao et al. 2017). To reduce the likelihood of security mishaps for Li-ion batteries, positive temperature coefficient device RTS-based control, security, and current interrupt device were used (Fleming 2018). The combination of MXene addition and tungsten doping is critical in improving niobium pentoxide electrochemical performance at low operating temperatures (Chen et al. 2021). The Li-metal is replaced with a less destructive anode material (Zhang et al. 2022). To increase the electrochemical performance, Li3 VO4 was uniformly implanted between the highly conductive multilayered Ti3 C2 Tx MXene. MXene composite demonstrated increased specific capacity and greatly improved cycling performance.

11 MXene Energy Applications and LIBs’ Performance Enhancement 11.1 Organic Acid as a Lithium-Ion Reductant In this case, glucose is substituted instead of hydrogen peroxide as a reducing agent to recover cobalt and lithium from wasted lithium-ion batteries. In about 2 h, almost

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complete dissolution of cobalt and lithium was achieved in a 1.5 molL−1 phosphoric acid solution (chelating agent) and 0.02 molL−1 glucose at 80° centigrade. The extraction process went through a quick primary stage in about twenty minutes, followed by a sluggish phase. Glucose was oxidized and degraded throughout the leaching process, resulting in monocarboxylic acids, which aided in the extraction of cobalt from lithium cobalt oxide materials. Using oxalic acid after the leaching process, cobalt in solution was simply and selectively precipitated into cobalt(II) oxalate (Gerold et al. 2020). The regaining of cobalt and lithium from spent lithium cobalt oxide material is completed using a mixture solution of H3 PO4 and glucose (C6 H12 O6 ). In the leaching process of consumed active cathode material with H3 PO4 , glucose is used as a reducing agent. In comparison to other extraction, glucose was the most stable and common agent. It also has a minimal price from an economic standpoint. Due to its low consuming energy, low emissions of dangerous gasses, higher recovery ratio, and simple operation, recycling precious metals from spent lithium-ion batteries, is particularly desirable. As a result, the majority of studies have focused on the leaching process of lithium cobalt oxide with mineral acids such as H2 SO4 , HNO3 , and HCl, as well as H2 O2 as a reducing agent (Kaiser et al. 2021). On the other hand, these acids are expensive and emit pollutants like sulfur trioxide, Cl2 , and nitrogen oxides, which are hazardous to human health and the environment. Citric acid, malic acid, oxalic acid, aspartic acid, and succinic acid are mild organic acids. These organic acids were used to leach LiCoO2 materials, with hydrogen peroxide acting as a reducing agent. For the leaching of spent active cathode material, these acids are responsible for an environmentally responsive and satisfying outcome. Because H2 O2 is unstable in the acidic leaching process, it is critical to discover an effective and stable replacement for H2 O2 . Recently, sodium bisulfite and vitamin C(ascorbic acid) have been examined as alternatives to H2 O2 (Chi et al. 2019; Meng et al. 2017). Hybridizing Ti3 C2 Tx MXenes nanosheets with Cobalt Oxide may associate the complementing features of the two components. Because of the significant interfacial connection and electronic linking between the two parts, electron transfer speed is increased, and the structure is more stable during battery operation. CoO/Ti3 C2 Tx cathode has a greater discharge voltage plateau at 2.75 V than Ti3 C2 Tx and CoO has 2.7 V and 2.64 V, respectively, indicating that CoO/Ti3 C2 Tx has a better discharge capacity. Furthermore, the CoO/Ti3 C2 Tx cathode has an exceptional initial discharge capacity of 16,220 mAh g–1 , which is higher than Ti3 C2 Tx and CoO has 7230 mAh g–1 , 4140 mAh g–1 at 100 mA g–1 , respectively (Li et al. 2020a, b, c).

11.2 Titanium Carbide Lithium-Ion Battery MXenes have a distinctive two-dimensional (2D) shape, hydrophilic properties, excellent electronic conductivity, and good mechanical power. Because of the

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adequate interlayer space, alkali metal ions can be intercalated, giving them considerable potential as anode materials for secondary alkali ion batteries. Their electrochemical performance in LIBs has been effectively established theoretically as well as practically, with representative Ti3 C2 Tz MXenes demonstrating 447.8 mAh g–1 theoretical capacities. MXenes’ potential capabilities in generating safer LIBs may potentially aid their future applications. Ti3 C2 Tz with regulated surface-terminating groups as a safer LIB anode, after 50 cycles at 20 mA g–1 specific current, the specific charge capacity of immaculate Ti3 C2 Tz progressively decreases from 308 mAh g–1 to 170 mAh g–1 . Over 100 cycles, the annealed Ti3 C2 Tz retains a discharge capacity of 385 mAh g1, with an overall average columbic efficiency of 99% (Cai et al. 2021).

11.3 Nitrogen as an Anode to Enhance Lithium-Ion Batteries’ Capacity As anode materials, carbon nanotubes have the usual interest of researchers because of their exceptional qualities, which possess excellent electrical characteristics, higher mechanical strength, more chemical stability, and more active surface areas (Ren et al. 2013). A carbon nanotube is a hollow cylinder of pure carbon with nanochannels, large enough to accept Li+ . But, due to the maximum lithiated graphite stage of solid electrolyte, the theoretical capacity of carbon nanotubes is restricted to 372 mAh g−1 , which strictly limits the efforts to enhance its storage capacity. Nitrogen doping has been suggested to be a very gifted method for improving the lithium battery storage capabilities of C-based materials. By combining Ti3 C2 Tx and melamine using thermal treatment, nitrogen-doped MXene nanosheets were created. At 0.2 C, the crumpled nitrogen-doped MXene demonstrated a high reversible capacity of 1144 mAh g–1 . Using a hydrothermal reaction with urea, it creates a novel form of N-doped Nb2 CTx MXene. Nb2 CTx MXene has a nitrogen concentration of 4.5%. Nitrogen incorporation into MXene nanosheets raises the C-lattice parameter of Nb2 CTx MXene from 22.32 Å to 34.78 Å. Additionally, the specific surface area and electrical conductivity are increased. The reversible capacity of N-doped Nb2 CTx MXene is 360 mAh g−1 at 0.2 C, which is about 90% higher than the capacity of undoped Nb2 CTx MXene having 190 mAhg−1 at 0.2 C. Furthermore, the N-doped Nb2 CTx MXene has good cycling stability (Liu et al. 2019). Nitrogen element has been considered one of the most suitable heteroelements for carbon due to its similar atomic size. Nitrogen also has a high reversible capacity of over 400 mAh g−1 . High N-doping substances enhance Li+ storage performance on carbon nanotubes (Bulusheva et al. 2011). Floating catalyst chemical vapor deposition is one of the most competent ways of processing nitrogen atom inclusion in carbon nanotube walls in situ. as a result of the degradation of nitrogen-containing molecules. Their electrochemical performances as lithium-ion battery anodes showed that the discharge capacity of N-doped carbon nanotube-anodes improved with

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increasing nitrogen concentration. High reversible capacity and level of capabilities were also reported. Higher electrochemical performance could be attributed to enhancing a greater number of fault sites and electrical conductivity (Hu et al. 2004).

11.4 Zinc Anode for Enhancing the Capacity of LIBs Electrode materials for lithium-ion batteries with a higher specific capacity, rate performance, and extended life cycle are receiving a lot of attention. ZnCo2 O4 is appealing due to its several advantages, including low cost and toxicity, and good thermal stability. Particularly when the Zn2+ ion is reduced to Zn during the discharge process. Lithium-zinc alloy can be created under additional reduction of the corresponding metals. For the creation of the Li-Zn alloy, an additional discharge capacity was produced. Due to their high surface-volume ratio and outstanding electronic transportation capabilities, nanorods have shown promising outcomes for improving the electrochemical performance of LIBs. Hydrothermal synthesis is a good approach for making nanorods because of its simplicity, low consuming energy, and ease of experimental handling. Their first discharge capacity is 1509 mAh g−1 and the cyclability is outstanding. The capacity after fifty cycles is 767.15 mAh g−1 . The generated ZnCo2 O4 nanorods have a high potential for use as anode material in LIBs (Vattikuti et al. 2018). The anode is made up of alkalized MXene-Zn composite. The maximal specific capacity of Zn-MXene is 75.2 mAh g−1 and the energy density is 60.2 Whkg−1 . Even with a high-power density of 7.04 kW kg−1 , the device’s energy density remains at 21.08Wh kg−1 . Furthermore, it has an initial specific capacity of 92.5% after 10,000 charge–discharge cycles at 3.3 A g−1 (Li et al. 2021).

11.5 Strontium Anode in LIBs MXenes, not only provide direct ion-exchange sites, but they also effectively adsorb and immobilize radionuclides via chemical and electrostatic attraction. More crucially, because of their superior chemical and thermal stability, MXenes exhibit strong resistance to powerful radiations, which is required for a material to be a suitable adsorbent for the sequestration of dangerous radionuclides from nuclear waste. MXenes have high radionuclide adsorption capacities due to simple surface activation techniques. Li4 Ti5 O12 has received a lot of interest since it is naturally existing, cheap, environment-friendly, and thermally and chemically stable. When compared to graphite, the Li4 Ti5 O12 anode has a greater operative voltage window of about 1.55 V. But, high working voltage and low theoretical capacity of LIBs, result in a lower energy density. A new method for producing Li-ion conducting Ti+4 materials for lithium-ion batteries has been devised (Peng et al. 2018). SrLi2 Ti6 O14 , having a high theoretical capacity of about 262mAh g−1 , was the more promising

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of the two materials. SrLi2 Ti6 O14 has a three-dimensional (3D) structure that allows lithium-ions to travel fast, resulting in an outstanding high-power performance battery. Nanostructure materials have recently emerged as intriguing materials because they have a small lithium-ion addition/extraction distance, and easy strain relaxation during the electrochemical cycle. Have a large surface-volume ratio to contact with the electrolyte, all of which can increase the capacity and life cycle of lithium-ion batteries. Fabricating nanostructure SrLi2 Ti6 O14 using sol–gel methods and solidstate reaction is highly difficult because standard synthesis routes want a high temperature of about 900° centigrade for treatment to generate pure-phase SrLi2 Ti6 O14 . This time-consuming method results in undesired particle development and poor electrochemical performance. There are numerous ways for creating nanomaterials. Some of these include microwave treatment, electrodeposition method, template-based method, and hydrothermal approach. Electrospinning is the best, most basic, and most adaptable method for producing nanowires. Polymer solution containing the sol–gel precursors for nanowires made by electrospinning, followed by annealing at a specific temperature, a range of metal oxides with one-dimensional structures can be easily generated. Powerful 1-D nanowires’ architecture may maintain high operative contact areas while fully realizing the advantages of active materials at the nanoscale. Furthermore, self-aggregation of nanomaterials can be efficiently suppressed if one dimension of the nanocrystallites is as small as a few hundred micrometers or as large as a millimeter. As a result, an ultra-long 1D nanowire is one of the most promising cathodes or anode materials for high-performance lithium-ion batteries. At 0.1 C, strontium lithium titanate oxide nanowires have a high reversible capacity of 171.4 mAh g−1 and retain 96.2 mAh g−1 even at a high rate of 20 C. Furthermore, after 1000 cycles, the capacity can stabilize at 101 mAh g−1 (Li et al. 2015).

11.6 Vanadium Carbide MXene Anode for LIBs Naguib and coworkers discovered that oxidized MXenes have greater capabilities as anode materials for LIBs. MXenes were created through selective etching. MXenes with oxygen-containing surfaces often exhibit outstanding conductivity and offer tremendous promise for use as anodes in lithium-ion batteries. Because of their high capacities, improved cyclabilities, and relatively high operation voltages, bulk vanadium dioxide particles have been identified as one of the most promising electrode materials for LIBs. The presence of more oxygen-terminated groups and VO2 on the oxidized V2 CTx surface boosted the ability to collect Li+ . Their CV curves almost overlapped in subsequent cycles, indicating that oxidized V2 CTx electrodes are reversible. After 100 cycles, the V2 CTx enhanced with surface oxygen-containing groups and tetragonal VO2 showed high capacities of 318mAh g−1 at 100mAg−1 (Luo et al. 2020).

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11.7 FeOOH/MXene Enhances the LIBs MXene is formed by randomly combining Co3 O4 and MXene in situ growth processes. The in situ growth strategy results in a more uniform distribution of metal oxide/hydroxide nanostructures on the MXene substrate and stronger interaction effects in the hybrid electrode, resulting in improved electrochemical properties that are more efficiently improved. The well-designed NiCo2 O4 /Ti3 C2 Tx electrode produced a high reversible capacity of 330 mAh g−1 at high current rates up to 10 C with no capacity loss. The techniques for creating intimate interaction interfaces in MXene-based hybrids with anchored active nanoparticles are simple and effective. The strongly coupled interfaces between active nanomaterials and conductive frameworks are thought to play a crucial role in improving LIB performance. The FeOOH/MXene electrode demonstrates exceptional high-rate performance. It shows a charge capacity of 286.1 mAh g−1 at 10 mA g−1 charge current and cyclic stability. The ultra-small FeOOH quantum dots are uniformly coated with the surfactant on alkali-MX sheets. During repeated charging and discharging cycles. It provides an abundance of electrochemically active sites as well as structural integrity. Higher capacitive storage facilitates ion movement, which adds to high-rate Li storage (Ruan et al. 2021).

11.8 Lithium Complex Deposition on MXene Surface Due to their vast surface area and strong electronic conductivities, 2D materials have emerged as a research focus in energy storage devices. Because of their structural similarities to graphene, these newly created materials were introduced as MXenes. Ti3 C2 Tx MXene nanosheets have received the greatest attention among all types of MXene. MXene combines the unique properties of graphene (GN) and GN oxides, such as a 2D surface, exceptional mechanical flexibility, and Li+ transport capability. When MXene is used to build the hierarchical porous structure for lithiumfloridephosphate composites, it improves both lithium-ion diffusion and electronic conductivity, potentially leading to practical applications for high-power lithium-ion batteries. Hierarchically porous Ti3 C2 Tx MXene was prepared via an electrostatic self-assembly technique. As a cationic surfactant, the electrostatic self-assembly process positively charges the lithiumfloridephosphate surface, making it more receptive to joining with negative MXene sheets at low temperatures. MXene composites exhibit desired rate capability and very remarkable cycling performance, including a high discharge capacity of 139.5 mAh g−1 at an ultrahigh rate of 20 C and a high discharge capacity of 156.6 mAh g−1 at 1 C. The best rate performance is provided by lithiumfloridephosphate/MXene electrodes, which have discharge capacities of 165.9 mAh g−1 at 0.5 C i.e. increase in current and decrease in charge capacity (Zhang et al. 2021).

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11.9 Nanostructured Material of MXene Enhances the Effect of LIBs Although LIBs have achieved significant commercial success, there is still a probability for improvement by understanding the mechanism of depolarization and polarization in anode and cathode electrodes (Liu and Manthiram 2009) and correlated active materials during cycling, safety, improving thermal stability, and lowering its costs. Polarization, which causes inhomogeneous environments for LIB electrodes and related active materials during cycling, is a common problem for lithium-ion battery applications. Particularly in low electrical conductivity and Li-ion diffusivity, poor structural stability, and transition metal cation dissolution, all contribute to poor battery performance. Carbon coatings, device optimization, metal cation doping, and nanostructure are all used to reduce polarizability in anode–cathode material. Nanocrystallites have gotten a lot of attention due to energy storage in potential electrode materials. They gain from the reduction of the lithium-ion diffusion route distance, which considerably reduces polarization in electrodes material like LiFePO4 and allows for an increase in charge/discharge ratio, as well as the reduction of the electron diffusion length in nonconductive materials (Wang and Dai 2013). These are methods for inducing the reduction in polarization effect in electrode materials in order to improve the performance of Li-ion batteries. To achieve increased power density, the size of the lithium iron phosphate particles has been reduced to the nanoscale. Meanwhile, nanostructured LiFePO4 can benefit from shorter electron diffusion lengths. Many nanostructured LiFePO4 materials, including nanoplates, nanorods, nanosheets, and nanoparticles, have demonstrated reduced polarization and high-rate performance. When utilized as cathode materials for LIBs, all of them exhibit a considerable depolarization effect, and the potential difference between the anode and cathode peaks becomes much smaller. The depolarization effect is also affected by the shape of LiFePO4 nanocrystals. To increase the performance of LIBs, lithium iron phosphate nanostructures with a high ratio of (0:1:0) surface exposure would generate the most substantial reduction polarization impact. Furthermore, Li2 FeSiO4 is a novel cathode material with a huge theoretical capacity of 332 mAh g−1 . Nanostructured Li2 FeSiO4 particles are another effective way of increasing the depolarization effect and, hence improving electrochemical performance. Secondary nanopetals on Li2 FeSiO4 display a considerable discharge capacity of 327.2 mAh g−1 and a depolarization effect, nearing the full theoretical capacity with long life and high current performance (Zheng et al. 2017).

11.9.1

Lithium Trivanadate Cathode Increases the Capacity of LIBs

Because of its low price, high specific capacity, and great safety, lithium trivanadate compound has concerned the remarkable interest of many researchers as a potential alternative cathode. According to theoretical calculations, monoclinic LiV3 O8 may allow the intercalation of approximately 3 Li-ions while releasing a capacity of more

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than 280mAh g−1 . The amorphous wrapped LiV3 O8 nanorods, in particular, might have a very high lithium storage capacity of 388mAh g−1 . Despite having a high specific capacity, LiV3 O8 has low-rate capability and unexpected capacity decline. According to reports, the poor rate performance is primarily due to slow lithium diffusion kinetics. Many electrode designs have been created to improve the Li intercalation/deintercalation LiV3 O8 kinetics. Nanostructures may reduce the Li-ion diffusion path and enhance the number of electrode/electrolyte contact surfaces, resulting in increased rate capability (Li and Zhou 2012). Nanostructures may shorten the lithium-ion diffusion path and increase the number of electrode/electrolyte contact surfaces, resulting in increased rate capability. The suggested electrode, made of LiV3 O8 nanorod-assembled nanosheets generated via a hydrothermal process, exhibited a particular capacity of 124.5 mAh g−1 at 1500 mA g−1 . Despite the fact that several nanostructured materials have demonstrated effectiveness in enhancing the rate performance of LiV3 O8 , they still face significant obstacles. Large surface area may hasten side reactions on the surface of LiV3 O8 , assist vanadium solubility in nonaqueous electrolytes (Shinkle et al. 2014), and eventually lead to severe capacity decay and structure degradation during cycling. A resorcinol–formaldehyde sol–gel technique was used to create LiV3 O8 nanoparticles. The effect of nanorod shape on the physical–chemical and electrochemical properties of LVO was thoroughly investigated. After 500 °C calcination, the LiV3 O8 nanorods electrode material can provide a primary specific capacity of 275.8 mAh g−1 with outstanding capacity retention of 80.5% after five hundred cycles at 1 C (300 mA g–1 ), specifically, up to 6.4 A g–1 (21 C), the electrode can still hold a 138.4 mAh g−1 reversible capacity (Chen et al. 2017).

11.9.2

Graphite Anode for Enhancing the Power of LIBs

A large volume of studies has been conducted on improved anode materials with greater energy and power densities. Graphite is a popular commercially available anode material for lithium-ion batteries due to its extraordinary columbic efficiency and strong cycling performance. Graphene has a substantial particular surface area (2,630m2 g−1 ), unusual mechanical qualities, and strong electronic conductivity as a single-atom, thicker, honeycomb carbon lattice. As a result, graphene sheets may be an appropriate substrate for the phase transition of nanoparticles of metal oxide (Mukherjee et al. 2012). The ordinary commercial graphite anode, with its theoretical capacity of just 372 mAh g−1 , can hardly meet the growing need for high energy density and large capacities, especially, due to the vast development and expansion in lithium-ion batteries (LIB), self-charging power cells, and solar lithium-ion batteries power packs. Several transition metal oxide nanoparticles with flexible capacities ranging from 600-1000mAh g−1 have been investigated as LIB’s replacement materials. Due to its excellent electrical conductivity, environmental-friendly, and enormous theoretical capacity, Fe3 O4 has been broadly considered one of the most promising materials of the anode. The fundamental issue with these structures is the ease with which Fe3 O4 nanoparticles aggregate due to their tiny size and magnetic properties (Jiao et al. 2016).

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Nitrogen has a higher electronegativity than carbon, and its lone pair of electrons makes conjugation with the graphene system. Nitrogen doping can enhance the cycle performance and capacity rate of Fe3 O4 /graphene composites by improving electrical conductivity, ion permeability of the graphene layer, and transfer of charge at the boundary, among other things. Fe3 O4 nanorods were created in two phases using a binary precursor of ferric oxyhydroxide and FeF3 . Extended nitrogen-doped graphene sheets with Fe3 O4 nanorods boost electron exchange and transport at the interface. It allows a reversible increased capacity 929mAh g−1 . It also enhances cyclic stability in LIBs (Jiao et al. 2016).

11.9.3

Stannum Bi-Sulfide Layered Structure for Enhancing LIBs Performance

Several transition metal sulfides have recently been investigated as a potential applicant for lithium-ion battery anode materials due to their extraordinary theoretical capacities, small costs, high safety, and ease of manufacture. Among these, layered sulfides have piqued the interest of researchers due to their 2-Dimension structures and good electric characteristics, making them attractive for catalysis and energy storage. Tin sulfide is a CdI2 -type layered semiconductor in which each layer of tin atoms is sandwiched between 2-layers of hexagonal close-packed S atoms. This multilayer structure is ideal for quick Li+ extraction/insertion and has a reasonably big theoretical capacity. Unfortunately, due to losses in electrical conductivity, pulverization and degradation of the electrode materials, and volume increase during the charge and discharge process, the capacity of tin sulfide electrode degrades significantly during usage. First discharge capacity 1830–820 mAh g–1 . In 2015, Fang and Peng discovered and considered that this capacity is higher as compared to other tested materials (Fang and Peng 2015). Two viable strategies are available to control this problem. The first technique is to manage the size of the materials of the electrode to reduce straining during volume growth. The alternative technique is to manufacture transition metal sulfides as aggregates utilizing porous carbon, graphene, and cyanide nitrogen which not only cushion interior volume fluctuations during the cycling process but also strengthen the composites’ conductivity properties. Since it creates 1-atom-thick two-dimensional layered structures with amazing structural flexibility, outstanding electrical conductivity, extraordinary mechanical strength, and enormous surface areas, graphene is a suitable matrix for dispersing and confining active chemicals. Graphene (GN) uses a hydrothermal fabrication simple method. This reaction product can’t gain good dispersions on GN and frequently slides off readily, resulting in low electrochemical characteristics. Exploration of various simple and effective approaches could greatly improve the lithium-ion storage performances of GN/S2 nanocomposites. Because they have extra structural benefits and their threedimensional structures, constructed from 2-dimensional nanosheets have shown good lithium storing characteristics (Mahmood et al. 2014).

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Titanium Niobium Oxide Electrode for LIBs

Titanium niobium oxide electrode is prepared by the solvothermal method combined with the annealing procedure. It has sparked a lot of interest because of the following benefits: • A platform with a relatively high working voltage can inhibit the production of solid electrolyte interphase layers (Lee et al. 2022). • It has a higher theoretical capacity of 388 mAh g−1 than lithium titanate, due to multi-oxidoreduction couplings (Lin and Duh 2011). • It also contains excellent lithium-ion diffusion efficiency (Ramadesigan, Boovaragavan, Pirkle, & Subramanian, 2010). Doping or adding conductive layers on titanium niobium oxide nanostructures has resulted in a range of enhanced TNO electrodes. A simple solid-state process was used to create Ag-coated titanium niobium oxide composites with a theoretical capacity of 170 mAh g−1 at 30 C. Ru-doped titanium niobium oxide materials were made having a capacity of 181 mAhg−1 after 100 cycles at 5 C and capacity retention of 90.1%. Later 900 cycles, as described by (Yu et al. 2017), Titanium niobium oxide hollow nanofiber demonstrated exceptional electrochemical performance, with a reversible capacity of 158 mAh g− 1 at a current density of 10 C. The occurrence of shielding binders in the powder electrode would have a negative effect on electrochemical performance. Prepared composite carbon fiber/titanium niobium oxide electrode exhibits the enhanced high-rate performance of 245 mAh g−1 , as well as improved cyclability with a capacity of 150 mAh g−1 , this was all due to the advantages of the smart 3D structure and highly conductive CF., owing to all these attributes, an effective anode for high-power LIBs, a flexible and binder-free carbon fiber/titanium niobium oxide electrode was developed (Shen et al. 2017).

11.9.5

Cesium and Glass Fiber Enhances the Capacity of Lithium Batteries

Lithium dendrite is a branching or tree-like structure of lithium metal accumulating on the surface of the anode. To safeguard lithium metal, it is advantageous to change the Solid Electrolyte Interphase (SEI), which is created by natural interactions between electrolyte and Li-M (An et al. 2016). The addition of several electrolyte additives, as mentioned by Cheng and coworkers, such as LiC2 F6 NO4 S2 (LiFSI), Cs+ ions, C5 H4 F8 O an ether, trace quantity of water, halogenated salt, and concentrated electrolyte, is thought to improve the stability and homogeneity of the solid electrolyte interphase layer. The incomplete solid electrolyte interphase layer can’t offer continuous and well-organized security for the lithium deposits, resulting in little columbic efficiency and a short lifespan for LMBs. The cesium ions are used to enforce Li deposition to nearby anode regions rather than the initial growing tip of the Li protuberances. As a result, significant dendritic development is prevented. A threedimensional conductive matrix is an additional excellent approach for self-limiting

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the lithium deposits’ size to less than that of a nanostructured current collector while increasing columbic efficiency to 95%. At normal temperatures, nanostructured electrolytes with high mechanical modulus and ionic conductivity and low interfacial impedances limit dendrite development. Those tactics revealed a completely innovative approach to inhibit massive dendrite growth, in which the interaction between lithium-ions and the current collector is primarily regulated by physical concepts such as electric force. The polar functional groups of glass fiber can adsorb significant lithium ions to balance the electrostatic contacts and concentration diffusion between the Cu foil anode and the Li-ions, hence avoiding Li-I buildup. Li-ions form a dendrites-free deposit at a high rate of 10.0 mAcm−2 and a lithium high capacity of 2.0 mAhcm−2 . The use of GF cloth is an intrinsic technique for suppressing Li dendrite development. The technique, however, is not without flaws. To optimize the cyclic performance of lithium metal batteries than lithium-ion batteries, a far more complex design is necessary. The cells’ cycle of polarization at higher rates of 5.0 mAcm−2 and 10.0 mAcm−2 is substantially bigger (Cheng et al. 2016).

11.9.6

Lithium-Ion Battery Anodes with Nanorod of Stannum Oxide

A significant range of metal oxide nanomaterials, particularly tin oxide nanostructures, have also been broadly described to have high capacity as Li-ion battery anodes. Researchers and engineers are globally struggling to combat the two major issues with these batteries. The first issue is the poor cyclability induced by the substantial volume change in the course of Li insertion/extraction and the second one is the excessive irreversible capacity brought on by the creation of Li2 O. Numerous nano structures have been developed, and the first difficulty has been resolved because nanostructures can adapt the stress of change in volume (Lan et al. 2016). Due to the limitations of electrochemical reactions, the second challenge is extremely difficult to overcome. Reaction-type oxides have been shown to reversibly transform Li2 O to Li+ . As a result, if reaction-type oxide nanostructures may be coated with alloy-type oxide nanostructures, MoO3 + 6Li+ + 6e− ↔ Li2 O + Mo this is a promising development, and in this way, the reversible capacity of metals and Li2 O can be considerably increased (Wang et al. 2015). Iron (II) tungstate nanostructures are excellent options for reaction-type metal oxide cores. The capacity of iron (II) tungstate nanostructures ranges from 860–677 mAh g−1 after sixty cycles. Metal clusters of iron and tungsten can reversibly change Li2 O to lithium-ion. Thus, iron (II) tungstate nanorods were uniformly coated with tin oxide nanoparticles using a simple wet chemical method. Iron(II) tungstate-tin oxide core–shell nanorods’ reversible capacity is about 1286.9 mAh g–1 , which is substantially greater than that of bare iron (II) tungstate nanorods, and tin oxide has a reversible charge capacity of about 906.1 mAh g−1 and 782mAh g−1 , respectively. This phenomenon can be due to the synergistic action of nanomaterial iron (II) tungstate and SnO2 . Xing et al.,

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approved in 2014 that providing a synergistic effect is a good way to improve the reversible capacity of nanomaterial-based Li-ion battery anodes (Agyeman et al. 2016; Xing et al. 2014).

12 Heat Role in Lithium Batteries LIBs have several advantages over other commercial batteries. However, heat generation and temperature management, as elaborated by Nazari and Farhad, are critical concerns that should be solved before using LIBs for an application. Thermal management systems constructed on air cooling, liquid cooling, heating pipes, and phase-changing materials have already been used to disperse generated heat and control the temperature of lithium-ion batteries. But engineers and researchers still have questions about which system of thermal management is best for lithium-ion batteries with particular chemistry. Battery engineers must have fundamental knowledge and understanding of the amount of heat produced in different types of lithiumion batteries, in terms of battery interaction and minimal capacity at different cycles, in order to build an effective thermal system. It will also be beneficial to comprehend the primary heat sources created between all conceivable sources. Heat creation in cell components is caused by reversible heat generation in electrodes. But irreversible heat generation is caused by concentration, activation, and electronic and ionic ohmic polarizations. It consists of two major functions in heat management batteries, the first one is a Heat source and the second one is Heat management. The major goal is to calculate C-rates (charging/discharging rate) on the quantity of heat generated in lithium-ion batteries to build effective thermal management systems for these batteries. It also designs cell optimization to limit heat generation. The irreversible heat generation caused by concentration, activation, and polarizations in the anode accounts for the majority of the overall irreversible method in all LIBs. More than 35% of LIBs are formed by this irreversible heat (Nazari and Farhad 2017).

13 Conclusion Lithium-ion batteries find applications in various fields. That’s why, quality, enormous and exciting research is being conducted in this field. Researchers and engineers are struggling to find new ways to enhance the quality of Lithium-ion batteries. These are superior to other batteries, as well, in many ways. Lithium-ion batteries have large electrical conductivities and better columbic efficiency, high charge density, and long life span technology. Different transition metals combined with lithiumion electrodes enhance their capacity. Graphene nanocomposite lithium-ion batteries increase their storage capacity due to the hydrothermal fabrication method. Synergistic effects of iron tungstate improve the capacity of nanomaterial LIBs anodes. The temperature management system exerts a positive effect on the battery efficiency

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and its life cycle. SEI of glass fiber and nanorods reduces the dendrites’ growth and increases the deposition of Li+ ions. Glucose reductant, zinc, nitrogen, stannum, and graphite anode are used in lithium-ion batteries to enhance their efficiency. Stannum oxides also increase their activity. Moreover, the entry of MXenes into Lithium-ion batteries has brought a revolution in these batteries. Because of their 2D structures, these provide a large surface area for the storage of Lithium-ions and enhance the ion transport mechanism through the electrode. These various modifications in lithiumion batteries enhance their life cycle, theoretical capacity, and charge density, and they also, of course, are environmentally friendly and cost-effective. Acknowledgements The corresponding author is thankful to the Higher education commission of Pakistan for awarding research grants NRPU # 8396 and NRPU # 8198. The authors are also thankful to the University of Agriculture Faisalabad, Pakistan for providing the necessary facilities to complete this book chapter.

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MXene-Based Sodium-Ion Batteries Rabia Akhtar, Ameer Fawad Zahoor , Matloob Ahmad, Tanveer Hussain Bokhari, and Muhammad Naveed Anjum

Abstract Substantial consumption of electrochemical energy in a variety of gadgets increases the usage of energy storage appliances. The sodium-ion batteries take a prominent position in this regard due to the interesting characteristics of sodium metal which is low cost and abundantly present in nature. On the other side, sodium ions which have large ionic radii limited the excessive use of sodium-ion batteries. In addition to this, low energy density, low coulombic efficiency, and volume expansion of the material during cycling are the other main problems faced during the usage of sodium-ion batteries. To achieve the outstanding performance of these batteries, a variety of electrodes has been constructed which increase the efficacy significantly. MXene class is one of them (based upon 2D nitrides, carbides, and carbonitrides) when combined with other materials e.g. oxides, sulfides, carbon nanotubes, etc. give outstanding performance in many devices used to store energy. This chapter depicts the synthesis and performance of MXene and its related compounds which may be helpful for future researchers to prepare new MXenes with efficient characteristics. Keywords MXene · Two-dimensional · Electrical conductivity · Sodium-ion batteries · Supercapacitors

1 Energy Storage Devices Today, the energy crisis mainly depends upon the production of existing fuels and their usage for the generation of energy. This energy crisis is mainly due to the increasing population and their drastic use of gadgets such as mobile phones, laptops, cameras, etc. Therefore, it is a dire need to construct efficient energy storage devices R. Akhtar · A. F. Zahoor (B) · M. Ahmad · T. H. Bokhari Department of Chemistry, Government College University Faisalabad, Faisalabad 38000, Pakistan e-mail: [email protected] M. N. Anjum Department of Applied Chemistry, Government College University Faisalabad, Faisalabad 38000, Pakistan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_8

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(Chu and Majumdar 2012). Nowadays, batteries (Dunn et al. 2011), capacitors (Westerlund and Ekstam 1994), supercapacitors (Pandolfo and Hollenkamp 2006), etc. are being used for this purpose, however, their efficiency and cost affect their outcomes which are being improved by using a variety of noble metals. However, the high cost of these metals and their less accessibility divert the intention of scientists toward noble non-metals which are easily available but their poor performance as compared to noble metals limit their usage (Danilovic et al. 2014; Oh et al. 2016). To overcome this problem, fabrication techniques to construct various electrodes of carbon derivatives (e.g. carbon nanotubes (Gamby et al. and An et al. 2001), carbon aerogels (Zhang and Zhao 2009), graphene (Liu et al. 2010), etc.), metal oxides (ruthenium dioxide (Xia et al. 2012), magnesium oxide (Demarconnay et al. 2011)), and conducting polymers (polypropylene, polyaniline, and poly-ethyl dioxythiophene (Conway 1991)) are commonly used nowadays. Among all these, activated carbon has been effectively used since 2006 because of its efficient properties such as its abundance in nature (coal, wood, and coconut shell), surface area, and easy preparation method. But its use becomes also limited as during the activation process the controllability of different pore sizes is very limited (Laine and Yunes 1992). On the other hand, the study of carbon nanotubes and graphene reveals that graphene structure drastically affects the device capacitance because of the accumulation of graphene layers, as well as the challenging movement of the electrolyte ions (Yang et al. 2014). In addition to this, ruthenium oxide (RuO2 ) was also used as electrode material in the past few years but due to its high cost, it has been replaced with magnesium oxide (MnO2 ) having a low price in the market. Later on, its (MnO2 ) lower conductivity limited its use also (Yu et al. 2011). To overcome the limitations of previously reported electrodes, other new materials are discovered. In this regard, two-dimensional materials play a pivotal role such as black phosphorus (BP), etc. because of their unique physical and chemical characteristics (Hao et al. 2016; Ruiz et al. 2012; Tripathy et al. 2019). On the other hand, partial transportation of ions or electrons and poor specific capacitance and electrical conductivity are the disadvantages of these electrodes. Overall, the literature study reveals that electrodes that couldn’t efficiently give their performance in energy storage devices have the following common drawbacks: (Naguib et al. 2012). • Less conductivity • Hydrophobic nature • Surface oxidation

2 Sodium-Ion Batteries Among various energy storage devices, the battery system especially lithium-ion batteries (LIBs) are extensively used. High energy density, low maintenance, and excellent stability are the prominent features of these batteries. However, the high price of lithium, its scarcity, and the uneven distribution of Li resources have diminished the use of these batteries on a large scale (Li et al. 2018a, b). As compared to

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lithium, sodium which is abundant in nature, and has low cost with similar characteristics to that of lithium makes it an ideal candidate for the construction of sodium-ionbased batteries. On the other side, the main disadvantage of this system is the thermodynamically unfavorable behavior of sodium ions in graphite (Palomares et al. 2012). For the commercialization of sodium-ion batteries, anode materials with large interstitial spaces that can store sodium ions (1.02 Å) are necessary. The main problem to find out appropriate electrode composites for sodium-ion batteries is due to the large ionic radii of sodium ions. Besides this, low energy density, low coulombic efficiency, and pulverization of the material during cycling are the other main problems (Sun et al. 2019a, b, c). In this domain, 2D compounds have attained significant importance from researchers due to their physical and chemically stable nature. Moreover, different types of active sites are available on these materials which can store Na+ because of their open 2D ion transport channel and substantial surface area.

3 Anode Materials for Sodium-Ion Batteries For the construction of efficient sodium-ion batteries, their anodes must have the following characteristics: (Wu 2015). • Anode must be constructed of an element that has low atomic weight and low density. It must provide a significant area for the accommodation of a huge amount of sodium ions per formula unit having good cyclability to attain stable and high volumetric (mAhcm−3 ) and gravimetric (mAhg−1 ) capacities. • The potential of the anode must be in close range of pure Na metal. • It should be stable and doesn’t show any reaction or dissolution affinity in electrolyte solvent. • Overall, its cost should be low and show environmentally friendly behavior by giving significant electronic and ionic conductivity.

4 MXene Structure 2D metal carbides, nitrides, and carbonitrides which belong to MXene family have unique characteristics such as they possess high metallic conductivity (up to 8700 S cm−1 ), high charge storage capacity, and hydrophilicity. The general formula of MXene is as follows: Mn+1 Xn Tx (n = 1–3) (a sandwich structure) M = Transition metals. X and Tx = nitrogen and carbon with OH, O, or F groups, respectively (Barsoum 2013).

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Naguib et al. (2011) proved the efficacy of supercapacitors in which electrode material was made up of MXene (Naguib et al. 2011). The electrochemical efficacy of MXene is gradually increasing day by day via the fabrication of MXene with other materials, resultantly surface area has been increased for ion movements. Because low theoretical specific capacity and stacking phenomenon limited the use of MXenes in the past few years. Literature study reveals oxides and sulfides as suitable candidates for batteries because they are available at a low cost in the market and have significant theoretical capacity. But their drastic expansion in volume significantly decreases their rate performance. Hence, it is necessary to combine these composites with two-dimensional MXene materials for better electrochemical performance.

5 MXene-Based Sodium-Ion Batteries 5.1 Sodium Storage of Pure MXene Using MXene as an electrode may limit the electrolyte interaction because of its strong van der Waals forces (present in structure) which face-to-face assemble its sheets resultantly, providing low surface area for storage. To overcome this drawback of MXene, two-dimensional nanolayers of MXene are assembled into a threedimensional (3D) porous structure for easy diffusion and charge transportation (Wang et al. 2019). For example, the addition of the different polymer spheres or sulfur or nitrogen-based polymers into the slurries provided a 3D structure. These polymers are said to be sacrificial templates as these are removed afterward via thermolysis. In addition to this, additive manufacturing or three-dimensional printing of MXene-based inks are the other methods for the preparation of the required porous structure. Among all these methods, three-dimensional printing is an advanced method for the construction of modified electrodes simply by altering the shape or morphology, porosity, and mechanical stability of the material. In this regard, (Fan et al. 2020) reported a three-dimensional printed nitrogen-doped MXene-based anode (N-Ti3 C2 Tx ) for sodium-ion hybrid capacitors. In their 3D printing method, doping of MXene-based anode with nitrogen heteroatom followed by assembling of resulting nanostructure to a porous 3D network gave the required composite. The sacrificial template that is used in their approach is melamine–formaldehyde nanospheres. Enhanced redox reactivity, electrical conductivity, rate capability, and cyclic capability are the prominent features of this methodology. Zhao et al. (2018) defined the synthesis of alkali-based three-dimensional Ti3 C2 Tx nanoflakes simply by mixing a variety of bases with Ti3 C2 Tx and it was observed that sodium hydroxide gave better results as compared to LiOH, KOH TBAOH, etc. (Zhao et al. 2018). Such as Na-c-Ti3 C2 Tx and Li-c-Ti3 C2 Tx displayed 130 and 160 mAh g−1 specific capacities, after 500 and 300 cycles, respectively. This protocol is proved to be an important finding in the field of catalysis, biomedicine, etc.

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Yang et al. (2015) revealed that sodium-ion batteries made up of MXene material exhibit weak interaction between MXenes and sodium ions which was affected by different factors that interrupt the energy barrier of ion migration (Yang et al. 2015). Xiong et al. (2018) described the effectiveness of Ti3 C2 Tx which indicated 270 mAh g−1 after the first run discharge capacity (Xiong et al. 2018). Wu et al. constructed MXenes nano-tablets whose few layers displayed 267 mAh g−1 specific capacity (Wu et al. 2017a, b). Similarly, Lv et al. expanded the surface area of Ti3 C2 Tx layers by using alcohol and dimethyl sulfoxide to achieve good results (Lv et al. 2018).

5.2 Sulfide-Based MXene Materials to Store Na+ Zhang et al. (2018a, b) prepared MXene-based SnS materials via the hydrothermal method (Zhang et al. 2018a, b). Both of these materials improved the efficacy of the resulting electrode as SnS nanoparticles extended the surface area of MXene by increasing the layer spacing while MXene upgraded the conduction property of SnS. The required material was constructed by the combination of Ti3 C2 Tx with SnCl4 . 5H2 O to acquire a specific capacity of 255.9 mAh g−1 at 1000 mA g−1 . The two-dimensional layered material of MoS2 can store sodium ions at 0.01– 3.0 V. Conversion of MoS2 to molybdenum and sodium sulfide enhances the specific capacity of this material. On the other side, low conductivity and high volume expansion features are the major drawbacks of MoS2 . In this regard, Wu et al. combined MoS2 with Ti3 C2 -MXene by the preparation of MoS2 /Ti3 C2 -MXene@C material (Wu et al. 2017a, b). It was observed that the resulting material after 3000 runs gave a 95% capacity retention rate at 20 A g−1 . Similarly, the results of Du et al. research also proved the efficacy of molybdenum disulfide-based Ti3 C2 Tx composites (Du et al. 2019). In sodium-ion and lithium-ion batteries, excellent specific capacity values were observed.

5.3 Oxide-Based MXenes to Store Na+ The combination of Sb2 O3 nanorods with TiO2 made an effective composite (Sb2 O3 @TiO2 ) that significantly improved the ion conduction rate by reducing the volume expansion property of Sb2 O3 as reported by Wang et al. (Wang et al. 2018). This composite was used to construct anode which gave 593 mAh g−1 specific discharge capacity at 100 mA g−1 . In the same way, Guo et al. used the solutionphase method for the construction of Sb2 O3 @Ti3 C2 Tx material to get better electrical conductivity and Na+ diffusion process (Guo et al. 2017). 472 mAh g−1 capacity after 100 runs (at 100 mA g−1 ) was observed by using this composite. Huang et al. constructed a sandwich structure by combining nanoribbons of Na0.23 TiO2 with Ti3 C2 nanowafers (Huang et al. 2018). Both materials give significant properties to the electrode as high conductivity is associated with Ti3 C2 nanowafers while

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Na0.23 TiO2 exhibits a high magnification rate and good cycle stability. Discharge capacity and the capacity retention rate observed by this material were 56 mAh g−1 after 4000 runs and ~ 100%, respectively. In addition to this, a literature study reveals the importance of CoNiO2 (with high theoretical reversible capacity) in Li+ batteries. But its use in sodium-ion batteries has been rarely observed. In this regard, Tao et al. utilized an annealing treatment along with a hydrothermal process to prepare MXene-based CoNiO2 composite which showed significant electrochemical efficacy in sodium-ion batteries by displaying the following properties: (Tao et al. 2018). • 223 mAh g−1 Capacity after 140 runs • 98.7% Coulomb efficiency • 188 mAh g−1 Rate performance at 300 mA g−1 current density

5.4 Sodium Storage of MXene-Carbon Composites In today’s research field, the significance of carbon nanotubes has been extensively explored. To avail, substantial properties of both MXene and carbon nanotubes in one material Ren et al. (2016) described the synthesis of Ti3 C2 Tx -based carbon nanotubes composites (Ren et al. 2016). Flexible films were attained simply by mixing an aqueous solution of two-dimensional Ti3 C2 Tx with carbon nanotubes at room temperature. Similarly, Xie et al. explored the efficacy of carbon nanotubes-based Ti3 C2 composites for Na+ batteries (Xie et al. 2016). As this material exhibited good rate performance and stability by displaying 421 mAh cm−3 volume capacity.

5.5 Miscellaneous MXene Materials to Store Na+ Ren et al. described the synthesis of PANI-Ti3 C2 material via an in-situ polymerization technique that gives 164 F g−1 specific capacitance (at 2 mV s−1 ) and good cyclic stability (Ren et al. 2018). It can sustain its original capacitance up to 96% after 3000 runs. In addition, Zhang et al. combined Ti3 C2 Tx with red phosphorus via a ball milling technique to prepare MXene nanodots (Zhang et al. 2018a, b). Excellent specific capacity with a good cyclic performance of the required MXene composite make this methodology more effective. Later on, Zhao et al. prepared PDDA-BP/Ti3 C2 composite by self-assembly of its layers through the electrostatic adsorption technique (Zhao et al. 2019a, b). This composite speeds up the electrochemical kinetics significantly. The reversible capacity of this composite is 1112 mAh g−1 at 0.1 A g−1 current density.

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6 Conclusion In sodium-ion batteries, anodes that are made up of two-dimensional materials give excellent results. Among these 2D materials, MXene and its composites give a notable impact on the efficiency of Na+ batteries because of the remarkable safety, tunable interlayer spacing, specific surface area, and high electroconductivity of MXenes. This chapter describes a number of literature reports that consist of the synthesis and performance of the MXene and its related materials in sodium-ion batteries which may be helpful for researchers in near future to prepare novel MXene composites by adopting facile pathways.

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Design and Applications of MXene-Based Li–S Batteries Saba Munawar, Ameer Fawad Zahoor , Muhammad Irfan, Sadia Javed, and Atta ul Haq

Abstract Li–S batteries have gained spectacular momentum in the past few years as highly utilized energy storage devices on account of their low cost and high energy density. In spite of that, these devices face challenges of poor sulfur utilization, growth of dendrites on lithium anode, and formation of soluble discharge products leading to low cycling stability that reserve their commercial level applications. The use of freestanding architectures has contributed to solving challenges faced by Li–S batteries because of their tunable features, high mechanical properties, and strong electrical conductivity. However, the real solution lies behind the introduction of optimized substances possessing specific properties required to cope with the challenges faced by Li–S batteries. The family of MXenes derived from MAX phases have revolutionized the electrode development and functional interlayer synthesis for energy-storing devices attributing to their high energy density, and rich mechanical, electronic, and electrochemical properties. The MXenes retain metallic conductivity, rich surface functionalities, ultrathin structures with high surface area, and macrostructure-level adjustability that endows them to be promising candidates for administration in Li–S batteries. In this chapter, the progress of Li–S batteries from development of freestanding networks to introduction of MXenes along with their efficient synthetic approaches and wide applications in batteries has been discussed. Keywords Li–S batteries · Free-standing architectures · 2D MXenes · Sulfur host · Lithium polysulfides · Lithium dendrite

S. Munawar · A. F. Zahoor (B) · A. Haq Department of Chemistry, Government College University Faisalabad, Faisalabad 38000, Pakistan e-mail: [email protected] M. Irfan Department of Pharmaceutics, Government College University Faisalabad, Faisalabad 38000, Pakistan S. Javed Department of Biochemistry, Government College University Faisalabad, Faisalabad 38000, Pakistan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_9

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1 Introduction The energy resources on this planet Earth have been depleting due to excessive use in the power sector and other walks of life. The exorbitant use of energy resources i.e., fossil fuels have led to environmental decline and deprivation from non-renewable energy resources. This scenario has diverted attention towards the use of clean and renewable energy resources i.e., wind, solar, hydro, and geo-thermal energy. Nevertheless, these renewable energy resources are not continuous. So, researchers have to come up with better ways to exploit energy resources in the best possible manner. The key elements for this purpose are electrochemical energy storage devices that have solved the issues of the power sector. Li-ion batteries used to be the mainstream energy storing devices with their use in almost every handheld device and electric vehicle with the theoretical value of 400 Wh kg−1 energy density (Susai et al. 2018). Despite the fact that LIBs have played a pivotal role in the development of technologies for energy storage systems, it comes with their downsides too. Some of these are their high cost, rare metal consumption, and constrained charging rates (Dehghani-Sanij et al. 2019). To meet the growing demands of energy, Li-ion batteries have been upgraded to Li–S batteries with earthabundant sulfur as their main element. The history of Li–S batteries is as old as Li-ion batteries. A lot of intensive research has been carried out on Li–S batteries but, due to missing exploitable results on this topic, it has not gained that much attention until recent years (Balach et al. 2018). The Li–S batteries offer six times higher theoretical energy density than Li-ion batteries i.e., 2567 Wh kg−1 (Manthiram et al. 2014). These batteries provide low cost, high capacity, low equivalent weight, and environmentally benign features (Larcher and Tarascon 2015).

2 Electrochemical Concepts and Challenges for Lithium–Sulfur Batteries Generally, the Li–S batteries consist of Li metal anode, a separator, a sulfur-carbon composite cathode, and an ether-based liquid electrolyte. The redox procedure of these batteries proceeds through the formation of soluble polysulfides. The solidstate cyclic octatonic sulfur reacts with Li+ ions to form lithium polysulfides (Li2 Sx, 8 ≥ x > 4) and Li2 S/Li2 S2 as discharge products (Mikhaylik and Akridge 2004). These higher order lithium polysulfides are highly soluble in an electrolyte that triggers the liberation of active material from sulfur electrode. Soluble polysulfide intermediates move towards the anode side and reacts with lithium metal to form insoluble Li2 S2 /Li2 S that induces anode passivation. Some unreacted soluble polysulfides return to the cathode and form higher order polysulfides by reoxidation. This continuous exchange of polysulfides between electrodes is known as the shuttle effect. In addition, the stripping process (repeated dissolution of lithium) and platting

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process (heterogeneous deposition on the anode) leads to dendrite formation. It leads to short circuit life by over-decomposition of electrolyte that perforates the separator membrane and contacts the sulfur cathode. That is why Li–S batteries have been confronting issues of low sulfur utilization due to poor conductivity, severe capacity degradation, poor coulombic efficiency, fast capacity fading, and poor cycling performance. The commercialization of Li–S batteries has been stumbled by these problems. In the early years, electronically conductive porous carbon materials have been used to infiltrate molten sulfur to form a cathode that resulted in the physical entrapment of soluble polysulfides and interconnected conducting networks. Still, a mere entrapment was found to be insufficient for preventing shuttling and diffusion of polysulfides regarding long-term cell cycling. Besides this, many categories of sulfur host matrices have been employed to achieve long cell lifetime and suppress polysulfide diffusion i.e., nitrogen-doped carbon, reduced graphene oxides, metal–organic frameworks, metal oxides, and chalcogenides. However, many of these didn’t provide sufficient mechanical stability for high electronic conductivity necessary for electron transport and to maintain the electrode integrity required for long-term cycling. A lot of research has been done in the area of Li–S batteries to solve the above-mentioned problems and improve its performance. Free-standing networks are prepared by assembling of highly conductive building blocks. These building blocks have interconnected networks which amplify ion transportation boosting reaction kinetics. The unique properties of free-standing networks enable them to act as an interlayer between the separator and cathode. The functional interlayer acts as a barrier layer to stop the migration of polysulfides. It also acts as a conductive layer to recombine the active species. Additionally, a metallic current collector and a binder are not required while using free-standing networks, which remarkably improves the kinetics of Li–S batteries. In this way, the execution of these free-standing networks expresses many advantages in Li–S batteries (Du et al. 2020).

3 Free-Standing Networks for Li–S Batteries Numerous building blocks possessing high level conductivity and different characteristics are combined together to build a free-standing network. These free-standing networks are employed for boosting current density. These networks acquire interconnections that augment the transportation of electrons and ions and thus enhance reaction kinetics. These free-standing networks serve a great part in decreasing the shuttle effect by acting as a functional interlayer between the sulfur cathode and separator. This way, it blocks the movement of polysulfides by retaining the property of conduction. This attributes to the localized current density in the anode and suppression of dendrite formation. These properties explain the utility of free-standing networks in the construction of Li–S batteries (Du et al. 2020).

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3.1 Free-Standing Network for Sulfur Cathode A sulfur cathode is made up of sulfur-active material, conductive agents, and binders. It is integrated by a coating of the above-mentioned substances on current collectors which portray specific roles in the functioning of Li–S batteries. The active material of sulfur and current collectors are bridged by conductive agents. The binders maintain the stability and integration of electrodes. These are inactive substances in the electrode taking up to 20 wt% of whole electrode. If the weight of the metal current collector is also included, there will be only 20 wt% left for the sulfur electrode. This factor poses many problems in the effective functioning of sulfur cathode. The active layer of sulfur gets cracked when it is volatilized by solvent. These problems can be avoided by constructing free-standing networks with MXenes, graphene, CNTs, and 1D and 2D substances that provide high mechanical properties. The free-standing networks, obtained from these aforementioned materials can be synthesized with no requisite need of binder and current collectors. The carbon nanotubes, graphene, and MXenes have self-assembling properties, because of which these have been used in the construction of free-standing sulfur cathode. These free-standing structures do not require current collectors and binders and thus allow high utilization of sulfur wt%. These networks furnish plentiful space to cope with the problem of volume expansion to maintain the structural integrity of electrodes and contact between active sulfur specie and conductive skeleton. This methodology simplifies the manufacturing process of electrodes and improves sulfur content availability. The performance of free-standing electrodes can be further enhanced by the introduction of functional groups and heteroatom dopants. Free-standing cathodes of sulfur can be assembled by two approaches: • Post-loading of sulfur • Pre-loading of sulfur The former approach first involves the synthesis of a free-standing network followed by active sulfur loading. The latter approach first involves the growth of nanoparticles of sulfur on building blocks followed by assembling into a free-standing network. Carbon nanotubes possess all the properties that make them suitable choices for the construction of free-standing sulfur cathodes. Free-standing carbon nanotubes are constructed by a number of methods like vacuum infiltration, freeze-drying, and vapor deposition. These nanotubes are transformed into either foams or films possessing a different set of properties. The carbon nanotube film develops an intimate contact between the nanotube and the sulfur cathode. This phenomenon ensures the high utilization of sulfur exhibiting a good discharge capacity. The carbon nanotube foam is a highly porous structure that guarantees fast diffusion of mass and high loadings of sulfur. The properties of free-standing carbon nanotubes can be greatly enhanced by the introduction of dopants and heteroatoms to ensure high cycling stability and good specific capacity of the electrode (Zhou et al. 2012). The use

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of carbon nanotubes (CNT) as porous structures, combined with sulfur and graphene 3D composites has been reported and evaluated for their properties as free-standing networks (Sun et al. 2014; Peng et al. 2013; Shi and Zhao 2019). Graphene is a two-dimensional substance with a high surface area as compared to carbon nanotubes. Graphene oxide and reduced graphene oxide are derivatives utilized for the construction of free-standing sulfur cathodes. These free-standing networks manifest superb mechanical properties with a good conductive network. The presence of oxygen in the skeleton of free-standing cathodes endows them to have a strong ability to adsorb polysulfides and thus improve cycling stability. These free-standing cathodes can be assembled in the form of films or foams (Zhou et al. 2012; Zhou et al. 2013). The graphene films are prepared by the method of vacuum infiltration while graphene foam synthesis is accomplished by templatedirected chemical deposition method (Shi and Zhao 2019). To further enhance the confining effects of free-standing cathodes, polar anchoring positions, and porous C-materials can also be introduced (Zhou et al. 2015). MXenes are two-dimensional carbides or nitrides of transition metals that take part in the construction of free-standing sulfur cathodes. These MXenes possess a number of active metal groups and different functional groups which play an important role in adsorbing polysulfides and thus reducing the shuttle effect. The presence of highly conductive surfaces and metallic centers make MXenes excellent materials as building blocks. Nevertheless, these MXenes are restacked during the process of vacuum infiltration. Van der Wall forces are thought to be the main reason behind this factor. This problem is solved by adding spacers and carbon nanotubes which eventually improves the electronic conductivity and internal porosity.

3.2 Functional Interlayers Based on Free-Standing Networks The separation of the anode and cathode is achieved by a separator membrane to prevent short circuits in a Li–S battery. The commonly used separators are polypropylene and polyethylene membranes which allow the movement of ions between two electrodes. Despite the development of various strategies for the prevention of polysulfide movement, this has been an inevitable issue causing the shuttle effect and corrosion reactions of lithium. The introduction of a functional interlayer between two electrodes suppresses the movement of polysulfides enhancing cyclic stability and the specific capacity of Li–S batteries. The previously discussed free-standing substances i.e., graphene, carbon nanotubes, and MXenes are endowed with tunable properties, ample porous structure, and good conductive networks. These features are a good fit for a functional separator in a Li–S battery. For example, carbon nanotubes having nanopores allowed the movement of ions and blocked the movement of polysulfides ensuring high-rate capability and good specific capacity. The interlayer material based on graphene oxide (GO) and reduced graphene oxide (rGO) provided good cyclic performances of 1078 and 1065 mAh g−1 at interlayer loading of 3.5 and 0.1 wt% respectively (Su and Manthiram 2012; Kong et al. 2017).

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The reason for approximately the same results with different interlayer loadings lies in the different composite compositions inferring that CNT-based interlayers are more electrically conductive and aid in sulfur utilization more effectively. The interlayer composition with metal oxides and carbides also demonstrated good cycling performances (Huang et al. 2015; Sun et al. 2019; Song et al. 2016).

3.3 Anode Protection Based on Free-Standing Networks For a Li–S battery, commercially available lithium foil acts as an anode. The continuous charging and discharging process give rise to dendrite formation mainly because of hostless and highly active lithium. According to research, this issue can be resolved by increasing the surface area of electrodes. The utilization of interwoven freestanding structures in this respect is highly appreciable. By employment of these free-standing networks, the current density can be localized and volumetric variation can be accommodated. For example, the chemical steam deposition method is used to synthesize carbon nanotube-based ultrathin graphite foam (CNT-UGF). This process significantly improves lithium nucleation and lifespan. The free-standing anode architectures composed of carbon fiber, mechanochemically treated carbon nanotube, and nickel photonic crystal have been reported (Zhang et al. 2018; Jin et al. 2016; Lin et al. 2018).

4 Introduction of MXenes Besides, it has been found that the use of sulfur host materials with delaminated MXenes materials can play a decisive role. Delaminated MXenes possess rich surface characteristics and inherent high electronic conductivity. MXenes are twodimensional or three-dimensional transition metal carbides or nitrides or carbonitrides that were first reported in 2011 by Prof. Yury Gogotsi. These are represented by the formula Mn+1 Xn Tx where M represents early transition metal, X corresponds to carbon or nitrogen and T represents surface functional groups. MXenes are formed by selective etching of A layer derived from MAX phases, terminated with various functional groups such as hydroxyl(–OH), fluorine(–F), oxygen(–O), and chlorine(– Cl). These surface functional groups impart hydrophilicity to the MXene surfaces which is one of the reasons to use these in Li–S batteries.

4.1 2D/3D MXenes Two-dimensional transition metal carbides and nitrides MXenes feature good electric conductivity, structure controllability, sulfur affinity, and abundant surface FGs.

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These manifest multiple advantages such as polysulfide confinement, less volume changes, and good transport kinetics. The general formula for 2D MXenes is Mn+1 AXn with M representing an early transition metal, A representing group IIIA or IVA element, and X representing carbon or nitrogen. Numerous MXenes have been synthesized and exploited in LIS batteries with different etchants. These delaminated MXenes express electrical conductivity of 104 S cm−1 greater than graphite (1.04 S cm−1 ). Fast electron transfer and sulfur conversion are promoted by good electrical conductivity of MXenes. The metal-S interactions are responsible for anchoring polysulfides on terminal metal sites of MXenes. On a similar note, the dissolution of polysulfides is prevented by polar FGs like –F, –O, and –OH. However, these polar functional groups sometimes cause self-restacking of MXenes leading to long ionic pathways and deterioration of cell performance. On account of that, three-dimensional MXenes are introduced for effective electrical and mechanical properties. To suppress stacking behavior observed in two-dimensional MXenes, the construction of three-dimensional MXenes has been introduced with improved electrical and mechanical properties. The assembly of three-dimensional MXenes can be carried out with different strategies like 3D printing, intercalation agents, sacrificial templates, and self-assembly. The 3D MXenes express better features in comparison with 2D MXenes because of low density, high conductivity, more active sites, and abundant surface area. The 3D MXenes provide short ion distances and effectual electron pathways for the efficient consumption of sulfur material. 3D MXenes possess an interconnected structure that prevents the stacking of MXene sheets. Large pore volumes in 3D MXenes enable high sulfur loadings. Because of their abundant surface area, more functional groups are exposed to electrolytes facilitating their binding with polysulfides. 3D MXenes and their architectures include multilayered 3D MXenes, microsphere 3D MXenes and 3D MXene-based hollow structures. These are synthesized by 3D printing, template method, and electrospinning, etc. For example, a hybrid of Ti3 C2 MXene with reduced graphene oxide has been reported which exhibited an 1144.2 mAh g−1 initial capacity with 70% sulfur loading after 300 cycles (Wang et al. 2018). Similarly, an octahedral 1 T phase MoS2 has been reported which demonstrated high electrical conductivity and strong interactions between polysulfides and positive charges at 0.5 C with the cyclic performance of 300 cycles (Guo et al. 2019). An asymmetric double-sided polar heterostructure built by stacking of Ti2 C on WS2 followed by Ti2 C oxidation having O-terminal side which exhibited strong adsorption potential towards polysulfides and S-terminal side promoted the transformation of polysulfides because of low decomposition barriers and diffusion. 3D MXenes with porous structures possess large cavities to facilitate high sulfur loadings. These architectures also have interconnected brackets and strong shells that play the role of physical barrier to prevent polysulfide diffusion. Highly crumpled MXene nanosheets with nitrogen doping having high adsorption potential and abundant pore volume was reported (Wang et al. 2019). It was synthesized by employing a process of thermal annealing of MXene nanosheets with negative

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charges and melamine with positive charges. These nitrogen-doped MXenes exhibited cycling stability of 1000 cycles at 2 C with a high sulfur content of 5.1 mg cm−2 . Recently, a self-standing three-dimensional nanoribbon was introduced that aided in the high storage of sulfur and swift diffusion of Li+ ion. In addition, because of the self-standing structure of nanoribbon, the conventional current collector was not required that improved the specific capacity and energy density of Li–S batteries. Depending upon the requirement of Li–S battery, both 2D and 3D composites have been employed as sulfur host, interlayer membranes, and lithium host.

5 Electronic and Mechanical Aspects of MXenes In Li–S batteries, elemental sulfur changes its condition from solid state to solution state during discharging process and precipitates out as solid Li2 S/Li2 S2 at the end of the cell cycle. During this process, sulfur undergoes a large volume expansion due to Li2 S conversion. These volume changes pose many challenges in maintaining the structural integrity of the electrode. Thus, a robust sulfur host is required to provide mechanical support to the electrodes. MXenes consist of a strong M-X covalent bond that provides flexibility, good compressibility, and tensile strength. Guo et al. have studied the mechanical properties of MXenes by implementing first-principal calculations. Their results indicated that under uniaxial and biaxial strains, twodimensional TiC2 MXenes can put up with 9.5 and 18% large strains. It can be leveled up to 20 and 28% by introducing O-terminations. There are various factors that affect the mechanical properties of MXenes i.e., defect density, size, and incomplete MXenes nanosheet’s edge. Besides, weak interatom forces between MXenes macro films as compared with monolayers also lead to weak mechanical strength which can be significantly improved by adding binding agents. Besides mechanical properties, the electrical properties of MXenes depend upon the elemental compositions and surface terminations. This class of host material offers high electrical conductivity. Density functional theories have revealed that the electric properties of MXenes range from conducting to semi-conducting materials. The MXenes that are derived from heavier metals like molybdenum are supposed to be semi-conducting and MXenes with titanium compositions are metallic with a conductivity range of 1000–4600 S cm−1 . The electronic properties of multilayered stacked nanolayers are affected by the intercalation of reagent cations which can be controlled by post-processing modifications to achieve the target control of electrical properties.

6 MXene Interactions with Sulfur MXenes possess abundant functional groups that facilitate the adsorption of polysulfides and aid in the formation of electrical contact of insulating sulfur. MXenes

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are usually polar because of irregular chemical environments around metal atoms and the sulfur ring of S8 atoms s non-polar. Both of these moieties form a metalsulfur bond in the process of infiltration of molten sulfur. The presence of the Ti–S bond was identified by X-ray photoelectron spectroscopy. The chemical interaction between metal and sulfur host has also been demonstrated by density functional theory (DFT) calculations where a charge transfer of 0.26 e− between sulfur host and oxygen-terminated Ti2 CO2 MXene has been indicated. It is further illustrated by research that MXene bandgap doesn’t get changed by the adsorption of polysulfides (Anasori and Gogotsi 2019).

7 Fundamental Understanding of MXenes by Theoretical Calculations The proper functioning of MXenes depends upon the surface functional groups. These have been a growing family of two-dimensional compounds with 30 synthesized compositions and 25 theoretically calculated compositions reported till now (Sun et al. 2019). The theoretical calculation, an important factor, is used to find out negative enthalpies that help in the logical selection of MXenes-based cathode. The interaction of MXenes and sulfur species has been evaluated by considering pristine surfaces and surfaces with various other functionalities. The first principle calculations provide requisite information about binding energies which is defined as the difference of combined energies of host and polysulfide systems in comparison with individual energies of host and polysulfide specie. MXenes containing different surface functional groups are obtained depending upon the etchant conditions i.e., etchant concentration, nature, temperature and time, etc. The functional groups present on the surface execute a crucial role in chemical interaction with soluble polysulfide intermediates. The interaction of surface functional groups with soluble polysulfides can be calculated by DFT calculations (Giebeler and Balach 2021). For example, computational studies of O-functionalized MXenes reveal that the negative charge of oxygen binds with a positive charge of lithium ions of lithium polysulfide specie exhibiting 1–2 eV of binding energies. It infers that the chemical interaction between these LiPS and MXene surface functional groups is based on electrostatic forces of attraction. The MXenes with oxygen terminations interact in two ways i.e., in the case of higher order lithium polysulfides, elemental sulfur is obtained while in the case of lower order lithium polysulfides, these form weak interaction with MXenes keeping their configuration intact. Functionalized titanium carbidebased MXenes interacts strongly with polysulfide intermediates by the formation of Ti–S bonds which effectively immobilizes sulfur atoms. The strength of Ti–S bond can be explained on the basis of Lewis acid–base interaction. Spin-polarized DFT calculations have also been implied to analyze the behavior of Ti2 C-based MXenes that revealed behavioral patterns regarding F-functionalized and O-functionalized MXenes. F-functionalized MXenes adsorb lithium polysulfides in a parallel manner

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if they are higher ordered and vertically if they are lower ordered by Li–F interactions. Studies also revealed that TiCO2 MXenes with F-substitutional sites exhibit weak affinity for Li atoms of polysulfides changing the suppression mechanism of the shuttle effect while TiCF2 MXenes with O-substitutional sites show a great affinity for Li atoms promoting the interaction with soluble polysulfides. On a similar note, it was demonstrated by first-principal calculations that Ti2 CS2 exhibits a strong affinity for polysulfides than TiCF2 and TiCO2 . According to theoretical calculations, OH-functionalized MXenes exhibit higher binding energies as compared with F-functionalized and O-fuctionalized Mxenes mainly because of the dissociation of the hydroxyl group and formation of H–S bond. These hydroxyls terminated MXenes also coordinate with LiPS via Lewis acid–base interactions where LiPS behave as soft Lewis bases. The OH-functionalized MXenes interact with lithium polysulfides in two steps where the first step involves the formation of thiosulfate groups on MXene surface and exposing the Ti atoms. The second step involves the acceptance of further polysulfides and T–S bond formation. Additionally, MXenes with metal-based materials possess too high an affinity for polysulfides which leads to the permanent decomposition of polysulfides and less utilization of active materials (Giebeler and Balach2021; Anasori and Gogotsi 2019).

8 Synthesis of MXenes Various strategies have been developed to prepare MXenes with HF-forming etching, alkali etching, molten salt etching, and electrochemical etching. After etching, A atoms are selectively removed and some surface terminations are added, determined by etching agents. Ti3 C2 Xx -MXene was the first MXene discovered by Professor Yury Gogotsi in 2011. Since then, more than 30 types of MXenes have been synthesized and brought into applications in energy storage devices. Owing to some specific features i.e., metallic conductivity, water dispersibility, and high surface activity, MXenes have found many applications in fields of energy storage, biocatalysts, electromagnetic shielding Uniform, and biomedicine, etc. Specifically, MXenes exhibit great potential in energy storage devices i.e., Li-ion batteries, Li–S batteries, sodiumion batteries, zinc ion batteries by acting as highly conductive electrodes. The properties of MXenes can be controlled by varying their compositions, structure, and terminations. In the case of Li–S batteries, high performance can be achieved by employing MXenes as their high conductivity provides fast electron transfer for maximum utilization of sulfur. LiPSs are strongly adsorbed by surface terminations to inhibit the shuttle effect, nucleation and growth of Li can also be induced by MXenes to suppress the formation of Li dendrites. A volume expansion by loading a large number of Li or S can be provided by using a structural variety of MXenes (Anasori and Gogotsi 2019). The bulk synthesis of thin MXene films is achieved by following methods:

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Chemical Vapor Deposition Chemical vapor deposition methods involve chemical reactions at high temperatures and near thermodynamic equilibrium. It requires a much higher temperature as compared with other deposition methods. The deposition of Ti3 SiC2 MXene with chemical vapor deposition (CVD) has been the first thin film synthesis reported by now. In the case of chemical vapor deposition method, higher temperatures are required for the formation of complex nanolaminated structure due to the strong adsorption of surface species which confine its wide applicability to industrial-level synthesis. Physical Vapor Deposition This is the most common technique for the synthesis of thin films of MAX phases. It involves the deposition of MAX phases by sputtering techniques and cathodic arc deposition techniques. A higher temperature requirement for physical vapor deposition is a drawback for applications in industries and temperature-sensitive substrates. This area requires further research on the low-temperature deposition of different substrates. Solid State Synthesis Solid-state synthesis reactions of thin film MXene synthesis can be either between substrate and film or film/film interactions. The synthesis of Ti3 SiC2 MXenes is the most important example in this respect. Another category involves the individual deposition of M, A, and X in right proportions followed by high-temperature annealing that transforms it to MAX phases (Lai et al. 2018). Various thin film Ti3 AuC2 , Mo2 AuC2 , Ti3 Au2 C2 , and Mo2 AuC2 MXenes have been synthesized by employing solid-state synthesis approach. MXene Development by Selective Etching There are two synthetic approaches for the synthesis of 2D MXenes by selective etching as follows: • Top-down synthesis of MXenes • Bottom-up synthesis of MXenes A top-down synthesis method refers to a reduction of bulk material to a nano or micro-sized layer. Generally, the synthetic methods of MXenes involve the use of water as the main solvent and HF as etching agent which gives rise to intercalated water, poisonous gases, and hydroxyl groups over MXene surface. Nevertheless, 2D MXenes are widely synthesized on a commercial scale via HF etching because of unavailability of greener approaches. 2D MXenes are successfully synthesized by adding the powder of Ti3 AlC2 to a conc. HF solution (Ghidiu et al. 2014). The Al-layer is dissolved by HF and replaced by different functional groups like –F or –OH (Anasori and Gogotsi 2019; Li et al. 2021).

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Ta4 C3 ultrathin nanosheets are also synthesized by a similar strategy (Lin et al. 2018). As HF is considered hazardous and toxic for the environment, alternative methods have been tried and tested. So, instead of direct HF administration, in-situ produced HF is used for the etching process. This process produces high yields of single-layered flakes of MXenes. Wang et al. proposed a simple strategy for multilayer synthesis of MXenes by in situ generation of HF (Wang et al. 2016). The Ti3 AlC2 powder was added to a solution of NH4 F aqueous solution for 12 h at 150 °C. NH4 F produces HF by hydrolyzing in solution to etch the Ti3 AlC2 layer. Another alternative method involves the employment of water and fluoride salt as etchant component (Ghidiu et al. 2014). The synthesis of 2D MXenes by employing molten salts of zinc and copper chlorides can be carried out (Li et al. 2021). For this purpose, Ti3 AlC2 and ZnCl2 can be used as precursors. Other than the above-mentioned procedures, Wang et al. proposed a fluorinefree strategy to synthesize 2D MXenes by employing a hydrothermal procedure for etching the Ti3 AlC2 (Wang et al. 2016). By using this strategy, 82 wt% pure MXene sheets were obtained. Along with the top-down synthesis method, bottom-up synthesis methods are also considered efficient. Bottom-up synthesis methods are effective for the synthesis of MXenes which employs template method, chemical vapor deposition (CVD), and pulsed layer plasma-enhanced deposition. Xu et al. used a foil substrate of bilayer metal to synthesize an ultrathin and highquality 2D MXenes via CVD. A Cu foil was stacked onto a Mo foil and heated at 1085 °C facilitating the diffusion of Mo atom to Cu surface. Next, a carbon source was provided by methane to react with Mo atoms and gave a 2D Mo2 C layer (Li et al. 2021). A similar strategy was used to prepare 2D Mo2 C MXenes. The synthesis of ultrathin TaN with NH3 as a nitrogen source and Ta foil as a growth substance has been reported with improved qualities (Li et al. 2021). Numerous other techniques have also been proposed over the years to facilitate the synthesis 2D/3D MXenes. However, their execution on a commercial scale is still hindered by their high cost, time consumption, low yields, poor stability, and low quality. There is still room available for the exploration of cost-effective, scalable, and efficient MXenes synthesis methods. This goal can be realized by the discovery of easily available raw materials and environment-friendly etchants.

9 Assembling of MXenes The assembling of MXenes can be achieved by using various strategies. The presence of strong Van der Walls forces facilitates the assembling process. But, the presence of strong interactions causes the MXene sheets to restack or aggregate leading to a reduction of surface area and active material loss. This issue is resolved by introducing some additional components between MXene layers like graphene, carbon

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nanotubes, inorganic compound, or polymer. Given below are some of the commonly used methods for assembling of 2D/3D MXenes. • • • • •

Vacuum-assisted filtration Spin coating technique Electrostatic spinning Hot pressing Electrochemical deposition

The assembling of MXenes can be achieved via vacuum-assisted filtration by the formation of hydrogen-bonded interlayer to obtain wrinkle-like hybrid films. These hybrid films are spread over the entire surface of MXenes because of plentiful terminal groups of MXenes and distinctive ultrathin flakes. Ling et al. successfully fabricated polymer composite films from two polymers i.e., polyvinyl alcohol (PVA) and poly(diallyldimethylammonium (chloride) (PDDA). PDDA is a cationic polymer that strongly interacts with negatively charged MXenes. A similar strategy was used for the synthesis of PVA composite films that exhibited an electrical conductivity of 2.2 (104 S/m) (Ling et al. 2014). Spin coating method is an efficient and fast method for the synthesis of uniform MXene films. This method involves a rigorous stirring of a colloidal solution of MXene and additives to obtain a homogenous solution. Excess solvent is evaporated by spreading over a substrate which is then obtained as an independent composite film. Zhang et al. employed a spin coating technique to obtain conductive, transparent, and ordered Ti3 C2 Tx sheets. The Ti3 C2 Tx sheets were obtained via spin coating and vacuum annealing at 200 °C (Zhang et al. 2017). Besides, spin coating method and vacuum-assisted filtration methods, other methods are also available for MXene composite film preparation. As MXene are thermally stable than polymer, MXene/polymer films are also synthesized by the hot pressing method. It has been reported that the synthesis of MXene/polymer composite films has been achieved by a combination of hot pressing and melt blending methods to obtain MXene sheets with improved properties.

10 Administration of MXenes in Lithium–Sulfur Batteries 10.1 As a Sulfur Host The applications of porous carbon in sulfur hosts have been widely executed. Even so, poor interaction between polar polysulfide and non-polar carbon leads to the unavoidable loss of polysulfides causing capacity fading and loss of active material. To tackle this issue, a wide range of nanomaterials have been proposed listed as, metal–organic frameworks, carbides, sulfides, nitrides, and metal oxides. The use of Ti2 C-based MXenes is regarded as the first application of MXenes in this respect (Liang et al. 2015). Besides this, numerous MXene-based host materials have been

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studied and employed in Li–S batteries. Initially, these MXenes are synthesized by etching the Al layer from the Ti2 AlC MAX phase followed by HF treatment and DMSO-based solvent delamination. Next, sulfur nanoparticles are homogeneously incorporated on Ti2 C MXenes to make a sulfur-Ti2 C architecture having 70 wt% of sulfur. Liang et al. employed a carbon nanotube-based sulfur host with sulfur loading of 1.5 mg cm−2 , sulfur content of 63 wt%, and 13 µL mg−1 E/S ratio to obtain 930 mAh g−1 initial capacity (Liang et al. 2017). Bao et al. provided an efficient method for the synthesis of N-crumpled MXene nanosheets that exhibited 1140 mAh g−1 initial capacity with a sulfur loading of 1.5 mg cm−2 (Bao et al. 2018). The use of metal oxides as sulfur host material provided 1409 mAh g−1 initial capacity inferring that it could be a good option for sulfur host material (Du et al. 2019). The composites of metal carbides with carbon nanotubes, reported by Lv et al. (2018) exhibited good results in the form of 1235 mAh g−1 capacity value (Lv et al. 2018). Wang et al. reported a rational design for the synthesis of N-doped Ti3 C2 Tx MXenes hybridized with carbon nanotubes (Wang et al. 2019). The introduction of nitrogen to MXenes effectively improves their affinity, electrical conductivity, and binding energy of Ti atoms towards elemental sulfur and polar polysulfides. Even though, there have been much advancement in the field of MXene-based sulfur host designs for Li–S batteries, the use of Ti3 C2 Tx MXenes performs better by exhibiting a high capacity of 1477 mAh g−1 (Zhang et al. 2020a, b). The interaction process between MXenes and sulfur cathode involves either polysulfide chemisorption on superficial -OH groups and acidic Ti-sites or by the establishment of a strong transition metal-sulfur bond. Moreover, MXene nanosheets can also be combined with polysulfide mediators and nanostructured conductive materials to improve the chemisorption value for soluble polysulfides and to step-up redox reactions. Conclusively, the execution of MXenes in Li–S batteries notably amplifies the electrochemical performance of lithium–sulfur batteries by preventing the shuttle effect.

10.2 As Functional Separator Coatings A separator coating is used to separate the sulfur cathode from other battery parts. It is encapsulated in a cage built by a number of substances to ensure its leak-free stability and confinement. But sulfur somehow manages to escape and get leaked during battery operations. It could be attributed to a number of reasons i.e., high sulfur loadings or content and excessive use of electrolytes. Various endeavors have been made to resolve this issue. Either a multifunctional free-standing-based interlayer can be introduced between the sulfur cathode and separator or a separator modified with a coating of trapping material for polysulfide can be placed. Both interlayer and modified separator are endowed with migration retention and polysulfide capture properties which extend lifespan and augment electrochemical performance of Li– S battery. Another worth-mentioning solution to above-mentioned problem is the

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implementation of MXenes in the construction of separators on account of their welldefined morphology and polysulfide affinity. For functional interlayer separators, glass fibers, polymeric membranes, and commercial separators are coated only with Ti3 C2 Tx . The separators coated with Ti3 C2 Tx MXenes (Song et al. 2016), hybridized with polyethyleneimine functionalized carbon nanotubes, nitrogen, CO2 oxidized titanium carbides (Lee et al. 2020), and various other materials as interlayer membranes have reported in literature along with separator coating variables and sulfur cathode parameters. Li et al. reported the use of Ti3 C2 as functional interlayer coating on glass fiber by employing a simple vacuum filtration method (Lin et al. 2016). By using this method, a glass fiber is coated on one side with Ti3 C2 MXenes. The Ti3 C2 MXenes possess good electrical conductivity and polysulfide affinity that benefits the good capacity retention by Li–S batteries. Guo et al. reported the interlayer substances composed of Ti3 C2 Tx MXenes hybridized with polyethyleneimine functionalized CNTs and Li et al. reported Ti3 C2 Tx MXenes hybridized with CNTs to improve the physical and mechanical properties of interlayers (Guo et al. 2019; Li et al. 2019). The nitrogen-doped Ti3 C2 layer with MOF-derived carbon-based functional interlayer were reported by Jiang et al., which showed the reversible capacity of 1018 mAh g−1 (Jiang et al. 2019).

10.3 As Lithium Deposition Host MXene materials are considered excellent not only for negative electrode and functional interlayer formation but also for suppressing the formation of lithium dendrites. At high current densities, the growth of dendrites on the lithium anode is considered a great challenge for sulfur-loaded batteries. The formation of dendrites causes detrimental reactions with lithium polysulfides and short circuits in the cell. It leads to the passivation of the lithium anode by covering the whole surface with an insulating layer that results in cell failure. MXenes exhibit great affinity for lithium polysulfides and provide many nucleation sites to form Li–C bonds for the deposition of lithium. MXenes with lamellar structures possess nanoscale gaps which facilitate the controllable deposition of lithium. The demonstration of this concept was obtained by the preparation of a flexible Ti3 C2 MXene that provides 950 mAh g−1 high reversible capacity (Li et al. 2017). However, this concept is facing drawbacks too as dendrite formation will start again after the nanoscale gaps are filled. This avenue requires further investigation for efficient incorporation into Li–S batteries in a specific aspect of suppression of dendrite formation on lithium.

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11 Summary and Future Outlook The development of high-performance lithium–sulfur batteries from the perceptive designing of host substances to the apprehension of electrochemistry has been significantly advanced over the years. The strong mechanical strength, high electrical conductivity, and adjustable surface chemistry have made MXenes excellent materials as sulfur hosts. The surface interactions of MXenes effectively suppress the shuttle effect of polysulfides which is crucial for the long-term cell cycle of Li–S batteries. The practical applications of Li–S batteries require high sulfur loadings and facile Li+ ion/electron pathways which are made possible by the synthesis of flexible architectures from MXenes and other composites. It ensures the integrity of the electrode and the transfer of electron/Li+ ions to the active sulfur material. The MXene surface demonstrates a high affinity towards Li+ ions by furnishing abundant surface area for lithium deposition and mitigating the formation of dendrites. This concept is considered a new direction of exploration for the introduction of up-to-date improvements towards the applications of Li–S batteries. The preparation of MXenes with uniform surface functionalities and the same moieties is very important. The study of interactions between these surface groups and lithium polysulfides can be carried out via theoretical calculations. Until now, more than 100 MXenes and their compositions have been formulated but only Ti3 C2 -based MXenes are employed for practical applications. The selection of materials according to the interaction level with lithium polysulfides can be executed with computational studies. There are various other functional groups that possess a strong affinity towards LiPSs, are mechanically robust, and are highly conductive and can be employed as high sulfur loading electrodes. Moreover, there is a need for the development of Li–S batteries that require less electrolyte and can prove to be a new avenue of exploration in this field.

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Nanostructured MXenes for Hydrogen Storage and Energy Applications Sohaib Ahmad, Hafiz Abdul Mannan, Atif Islam, Rizwan Nasir, and Danial Qadir

Abstract The production of sustainable energy has now become a critical issue to maintain the existence of humankind. Efficient synthesis of renewable and clean energy such as H2 has become significant to fulfill future energy demands of the world. Today, H2 is commonly stored and utilized as highly compressed or liquefied gas. Storing H2 on solid-state materials is a better alternative because of the safety challenges of conventional storage technologies. In this framework, the prospects of high-performance lightweight materials such as MXenes for reversible H2 storage are discussed in this chapter. MXenes have emerged as an essential choice for newconcept energy storage systems (ESS) because of their unique characteristics. This chapter covers the most recent storage methods for H2 synthesis techniques, properties, and applications of MXenes when used as critical materials in energy storage devices. Keywords MXene · Hydrogen · Energy · Storage

1 Introduction The rate of environmental deterioration has taken a considerable pace in recent years because of population growth, fast industrialization, and vast fossil fuel consumption S. Ahmad · H. A. Mannan (B) · A. Islam Institute of Polymer and Textile Engineering, University of the Punjab, Lahore 54590, Pakistan e-mail: [email protected] S. Ahmad Research and Technology Professionals Private Limited, Lahore, Pakistan R. Nasir Department of Chemical Engineering, University of Jeddah, Asfan Road, Jeddah 23890, Saudi Arabia D. Qadir School of Computing, Engineering and Digital Technologies, Teesside University, Middlesbrough, UK © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_10

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(Rizwan et al. 2022a, b, c, d; Rasheed et al. 2020, 2021). Furthermore, until alternative energy sources are developed on a wider scale, the finite reservoirs of non-renewable fossil fuels are anticipated to result in an energy deficit. As a result, there is an increasing need for alternative energy sources that are sustainable, environmentally benign, emit zero emissions, and help to reduce environmental problems. Many nations, including South Korea, Germany, China, the USA, Japan, and others, have established goals for increasing renewable energy outputs. Furthermore, by 2020, the European Union (EU) planned to produce 30–40% of its electricity from renewable sources. Germany intends to generate 50% electricity from wind and solar energy plants by 2030 and 80% by 2050 (Preuster et al. 2017). Because of its lightweight, high gravimetric energy density (~33 kWh/kg), and high calorific value, hydrogen (H2 ) is regarded as the superior source of renewable power for helping to phase out the consumption of fossil fuels (Bhattacharyya and Mohan 2015). H2 can also help to minimize global warming because it emits no contaminants like CO2 when burned. The usage of H2 energy technology has attracted the interest of scientists and industries all around the world. The global market for H2 is growing and is predicted to increase by over 80 EJ by 2050. To this day, the storage and transportation of H2 remain a challenge, especially for H2 -powered vehicles. Due to its inflammable, explosive, and spreadable properties, storage and transportation pose serious challenges. This has greatly increased the interest of researchers in finding effective H2 storage materials. Hydrogen storage in solid materials is a practical approach, efficient, and cost-effective than storing it in liquid or gaseous forms (Hu et al. 2013). In this regard, the major focus is on various two-dimensional (2D) nanostructures that are projected to be possible candidates for hydrogen storage at near-ambient temperatures, such as carbon-based materials (carbon nanotubes, carbon nitride, graphene) (Li et al. 2019). 2D materials have gained considerable attention due to their exceptional physicochemical characteristics compared to their bulk counterparts. Graphene has been considered a viable option in many applications as an often-mentioned 2D material. Other 2D materials, i.e., analogues of graphene, are conceivably anticipated to have significant potential in next-generation energy devices. The structure of layered materials is typically thought to be comparable to that of graphene, with a planar geometry and ultrathin thickness (Sun et al. 2018). Transition metal dichalcogenides, transition metal oxides, hydroxides, silicone, phosphorene, MXenes, etc., are common graphene-like materials for storing energy. All materials have large surface areas but weak interactions with H2 molecules, making them unsuitable for use in H2 storage applications. Likewise, it is extremely difficult to increase hydrogen adsorption through metal decorating or doping nanostructures in industrial manufacturing (Li et al. 2019).

2 Importance of 2D Materials Due to their distinct and exceptional features, 2D materials are recognized as one of the most promising alternatives for energy and electronic applications. Current

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research on 2D carbon-based materials such as CNTs, graphene, and fullerene has revealed improved electrochemical performance due to their good thermal stability, large surface area, and outstanding mechanical properties. Additionally, 2D materials are widely known for exhibiting higher energy storage potential (Priyananda Singh and Herojit Singh 2021). It is important to talk about what makes 2D materials exceptional and different from their counterparts at this point (Mas-Balleste et al. 2011). For example, what accounts for the striking differences between 0D-fullerenes, 1D nanotubes, 2D graphene, and 3D graphite? The fact that dimensions and size have a role in determining a material’s qualities provides the answer. Size limitations in a certain dimension or dimensions significantly change the material’s mechanical, electrical, optical, and chemical properties. This is mostly caused by the electrons being confined to a small volume, the lack of interactions between layers, the high surface-to-bulk ratio, etc. The capacity to tune the material properties of 2D materials creates new opportunities for various intriguing applications.

2.1 MXenes: A Newfangled 2D Nanostructure Some of the most studied 2D materials are phosphorene, covalent organic frameworks, metal–organic frameworks, black phosphorus, hexagonal boron nitride, and transition metal dichalcogenides (Liu et al. 2018). In this regard, it has been claimed that some members of the MAX family have exfoliated into a novel class of 2D transition metal nitrides or/and carbides materials which are known as MXenes. MXenes are attractive prospects for applications such as transparent conductive electrodes, environmental monitoring, electromagnetic interference shielding, energy storage, etc., due to their hydrophilic nature, high surface area, availability of active sites, electron-rich density, and excellent electrical conductivities (Priyananda Singh and Herojit Singh 2021; Rasool et al. 2019). A new 2D class of nanostructures, “MXene (Ti3 C2 )” was discovered in 2011 at Drexel University (NBIC 2019). This material, which consists of a layered bulk substance resembling graphite, was synthesized by exfoliating Ti3 AlC2 MAX in HF at ambient temperature. The structural formula for the MAX class of ternary carbides is Mn+1 AXn , where M = transition metals (Cr, Zr, Nb, Sc, Hf, Ti, Ta, Mo or V), n = 1, 2, 3, A = III-A or IV-A group elements (P, Al, Pb, Si, Ga, etc.), and X = carbon, nitrogen or their mixtures, respectively. It should be noted that the functional groups (OH, F, and/or O) produced during etching frequently exist on the outside surfaces of exfoliated layers. Moreover, Mn+1 Xn Tx is the general formula for MXene materials with the aforementioned terminations, where Tx stands for the functional groups on the surfaces (Liu et al. 2018; Li et al. 2018a).

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2.2 Applications of MXenes MXenes have gained interest in recent years due to their unique qualities, such as safety, large interlayer spacing, superior biocompatibility, outstanding uptake capacity, environmental adaptability, strong surface reactivity, hydrophilicity, etc. These qualities provide access to applications in a wide range of fields (Fig. 1), including thermoelectricity, piezoelectricity, ferroelectricity, hydrogen storage EMI shielding (Li et al. 2017), batteries (ion, lithium ion, lithium-sulfur), photocatalysis, catalysts, gas sensors, optoelectronic devices, triboelectric nanogenerators, supercapacitors, etc., (Priyananda Singh and Herojit Singh 2021; Ibrahim et al. 2020). MXenes were first investigated for use in storing energy, and this constitutes a significant element of MXene activity today. Despite being a recent advancement, the application of MXenes in the biomedical area has quickly become one of the trendiest research topics, with studies on biosensors, photo-thermal treatment of cancer, neural electrodes, theranostics, and dialysis (Cheng et al. 2020; Ansori and Gogotsi 2019). MXenes research is also taking over from other nanostructures in electromagnetic applications, such as electromagnetic interference shielding and printed antennas (Sarycheva et al. 2018).

Fig. 1 MXene applications in different fields of life

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2.3 Characteristics of MXenes MXenes have a wide range of properties, such as high thermal and electrical conductivities, presence of functional groups (Zhang et al. 2017), high Young’s modulus, variable electronic band gap, highly negative zeta potential, allowing colloidal suspension solutions in water; and effective electromagnetic wave absorption, which has led to various applications (Ibrahim et al. 2020; Levitt et al. 2019). Moreover, the main properties of MXenes are their unique structure, electronic structure, optical properties, and magnetic states (Priyananda Singh and Herojit Singh 2021).

3 Methods for MXenes Synthesis 3.1 Hydrofluoric Acid Etching The hydrofluoric acid is a primary etchant to produce MXenes from equivalent MAX precursors. A group of researchers discovered in 2011 that in Ti3 AlC2 , the Al atomic layers might be selectively etched using hydrofluoric acid (50%) because of the strong reactivity between the F ions and Al-containing MAX phase. This led to the formation of an accordion-like Ti3 C2 Tx powder with hydrogen bonds and van der Waals interactions created between each layer by surface functional groups. Except for the Al atomic layer, the resulting Ti3 C2 Tx powder’s stoichiometry and crystal structure were identical to those of the corresponding Ti3 AlC2 MAX phase. Furthermore, this aqueous etching method produced MXenes with various (F, O, and OH) surface terminations. The following reactions might be used to describe the etching method (Wei et al. 2021; Kumar et al. 2021): Ti3 AlC2 + 3HF = AlF3 + 1.5H2 + Ti3 C2

(1)

Ti3 C2 + 2H2 O = Ti3 C2 (OH)2 + H2

(2)

Ti3 C2 + 2HF = Ti3 C2 F2 + H2

(3)

The produced products are strongly influenced by the etching duration, concentration of HF, and etching temperature during the etching process. After HF etching, washing is an important step that often involves centrifugation to eliminate excess acid and byproducts. After centrifugation, the colorless residue would be discarded while the accordion-like MXenes were precipitated. This procedure was repeated until the residue was almost neutral (Wei et al. 2021). Due to the corrosive nature of HF, raising the concentration and etching duration may increase the defect concentration while decreasing the lateral size of produced MXenes. Therefore, the required MXene characteristics and applications should be considered while choosing the

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etching parameters. MXenes with minimal flaws and big lateral flake sizes have demonstrated various uses in electromagnetism, electronics, and optics. For the applications mentioned above, using moderate etching conditions (low HF concentration and short etching duration) is the right decision (Sang et al. 2016). The MAX phase, which contains Al and a portion of the non-MAX phase, is best etched using HF etching technique since it is simple to use and has a low reaction temperature. However, HF etchant raises problems due to its highly corrosive character, operation hazards, toxicity, and negative environmental implications. Furthermore, high amount of –F groups are on the surface of etched products, which is undesirable for energy storage. Therefore, alternative etching procedures must be explored and developed to substitute the hydrofluoric acid etching process with softer, less hazardous, and ecologically friendly methods (Wei et al. 2021).

3.2 In-Situ Hydrofluoric Acid-Forming Etching Alternative etching procedures that can generate in-situ hydrofluoric acid etchant have also been investigated to resolve the corrosive issues of hydrofluoric acid etchant. Because of the strong reactivity of the Al-containing MAX phase and F− , the F− can interact with the Al atoms of MAX precursors during in-situ hydrofluoric acid-forming systems, generating H2 , fluoride, and the desired MXenes. Etching method provides advantages over traditional hydrofluoric acid techniques due to the avoidance of the direct use of hydrofluoric acid, including ease of use, low energy consumption, and lower chemical danger.

3.2.1

Etching with Acid-Fluoride Salt

The first study on using a solution containing HCl/LiF for etching Ti3 AlC2 at 40 °C was conducted in 2014. Ti3 C2 Tx conductive clay with robust plasticity that could be formed into a film via roller pressing was successfully made utilizing this technique (Wei et al. 2021). The free-standing rolled MXene clay has strong hydrophilicity, exceptional flexibility, and great toughness, and it could be readily twisted into an “M” form while maintaining conductivity of up to 1500 S cm−1 . The HCl/LiF etching method yields multilayered MXenes that resemble accordions comparable to HF etching. With a specific capacitance of 900 F cm−3 and minimal capacity loss after 10,000 cycles, the free-standing, rolled MXenes clay can be employed directly as the working electrode for supercapacitors. In this scenario, the interlayer gap among MXene layers will increase due to the insertion of cations like Li+ . Fluoride salts and acids have developed into a proficient etching technique for the MAX phases. The modification of fluoride salts can control the interlayer gap of MXenes to satisfy the necessary application criteria. Other fluoride salts besides LiF have also been applied as etchants (KF, LiF, NH4 F, NaF, FeF3 , etc.) to create a reaction mixture for etching

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Ti3 AlC2 . The outcomes showed that any mixed etchant might be used to develop MXenes at various temperatures and times (Liu et al. 2017; Wang et al. 2017). The HCl/LiF composition is still the most popular in-situ hydrofluoric acid etchant because it makes it easy to etch and delaminate the MAX phase. At the same time, other salt combinations produce MXenes that resemble an accordion and cannot yet be directly delaminated. The in-situ HF etching method provides MXenes surface functionalities (F, O, and OH) like the hydrofluoric acid etching process and this etching technique is quite friendlier and safer as compared to the hydrofluoric acid etching process (Wei et al. 2021).

3.2.2

Etching Using Bifluoride Salts (NH4 HF2 )

The use of NH4 HF2 etchant for cutting sputter-deposited epitaxial Ti3 AlC2 films at room temperature was first published in 2014 (Halim et al. 2014). The STEM image demonstrated the regular configurations of the Ti and C atom layers, proving that the Al atom layers were successfully extracted from the Ti3 AlC2 precursors. Negatively charged MXenes may increase the interlayer gap by absorbing the hydrated cations produced from the bifluoride salts during the etching process. The following could serve as a summary of the etching mechanism: Ti3 AlC2 + 3NH4 HF2 = (NH4 )3 ALF6 + Ti3 C2 + 1.5H2

(4)

Ti3 C2 + αNH4 HF2 + bH2 O = (NH3 )c (NH4 )d Ti3 C2 (OH)x Fγ

(5)

The etching capability of bifluoride salts quickly expanded to include Ti3 AlC2 powder and thin epitaxial Ti3 AlC2 film. Other bifluoride salts, such as KHF2 and NaHF2 are also identified as etchants to remove Ti3 AlC2 and produce Ti3 C2 Tx , in addition to NH4 HF2 . Bifluoride salts offer improved operational safety than HF because they are solid at room temperature. The approach has only been suggested so far for etching Ti3 AlC2 , and its applicability to other MAX phases has not been investigated.

3.3 Methods of Electrochemical Etching The electrochemical etching approach for MXene preparation comprises of separating the Al atomic layer at a specific voltage using the MAX phase as an electrode. It is possible to use HCl, HF, or NaCl as the electrolytic system for the electrochemical approach for carbide-derived carbon from the MAX phase (Failed 2014). The MA bond’s disruption facilitates the A-layer’s exfoliation in the MAX phase in the standard electrochemical etching process using cyclic voltammograms at 0 and 2.5 V. The M-layer is gradually removed, leading to the formation of amorphous

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carbon compounds. Selective deletion of A atoms can be accomplished by adjusting the etching potential (voltage) in the domain of the reaction potential between M and A layers and managing the proper etching time, enabling fine control of produced MXenes. Because the working electrode often consists of the MAX phase, the etching technique is first carried out on the MAX electrode’s surfaces, frequently leading to the creation of surface CDCs that impede the following etching process. To effectively etch the MAX phase, the etching power should be modulated. Although the electrochemical method has several benefits, including low react temperatures and low energy requirements with no need for corrosive acids, it is frequently criticized for the concurrent CDC layer atop MXenes that results in a low yield. A two-electrode setup employing Ti3 AlC2 MAX pieces is used as both the working and counter electrodes in conjunction with NaOH, H2 SO4 , FeCl3, NH4 Cl, and HNO3 , correspondingly, as electrolytes. Aluminum (Al) foil can be effectively corroded by chlorine (Cl) free acids (HNO3 , H2 SO4 ), however, in an electrochemical environment, these acids cannot erode the Al atomic layer from the MAX phase. On the other hand, the MAX phase’s strong interaction between Al and electrolytes, including Cl, makes it possible to sufficiently etch the Al layer. Based on the weight proportion of the etched result to the precursor, the etching output in this instance was roughly 40%. Intercalators can be used to intercalate the MAX phase to increase interlayer spacing and permit continuous electrolyte species diffusion. This increases the inner availability of the MAX phase, maintaining its contact with the electrolyte and guaranteeing a continuous etching process. For illustration, Ti3 C2 Tx was obtained using a mixed electrolyte of NH4 Cl (1 M) and TMAOH (0.2 M) at 5 V for an etching period of 5 h (Yang et al. 2018). The TMAOH may readily intercalate into the MAX phase’s interlayers, improving the Al layer’s availability to the electrolyte. As a result of the Ti–Al link being broken by Cl− and Al3 + , the MAX phase was effectively etched. The introduction of non-charged NH4 OH might also promote the inner MAX phases etching while expanding the margins of Ti3 AlC2 . The followings are the etching reactions: Ti3 AlC2 − 3e− + 3Cl− = Ti3 C2 + AlCl3

(6)

Ti3 C2 + 2OH− − 2e− = Ti3 C2 (OH)2

(7)

Ti3 C2 + 2H2 O = Ti3 C2 (OH)2 + H2

(8)

Although this method can lessen the impact of CDC layers during the etching process, intercalator toxic effects raise questions about experiment safety. To achieve effective etching without the need for intercalators, a new thermo-assisted electrochemical etching method was introduced (Pang et al. 2019). An energy-efficient, environmentally friendly synthetic process is electrochemical etching. Nevertheless, in addition to the low yield, there is also the issue of the CDC layer. Even while the MAX phase can be used as an electrode more than once, a common etching

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procedure only produces a small amount of MXenes, making it inappropriate for large-scale synthesis.

3.4 Methods of Alkali Etching The Al atom layers are typically etched using acids, as mentioned above. The alkali is also anticipated to achieve the MAX phase’s selective etching. To etch the MAX phase into Ti3 C2 Tx , a group of researchers described a two-step etching procedure that involved soaking the Ti3 AlC2 in NaOH (1 M) solution for 100 h and then in H2 SO4 (1 M) solution at 80 °C for 2 h (Xie et al. 2014). In this procedure, the surface-exposed Al atoms were removed by H2 SO4 , while the alkali was employed to eliminate the Al atoms from the MAX phases. The method made it possible to etch the MAX phase efficiently with small amounts of alkali. However, the MAX phase’s surface layer could be etched with a very low yield of MXene. Also, it was hard to separate the etched MXenes from their precursor. The reaction between the alkali and MAX phases will experience a significant shift when the alkali dosage and temperature are raised to a specific level. For instance, the Al layer of Ti3 AlC2 could be effectively removed utilizing NaOH (27.5 M) at 270 °C to provide 92% Ti3 C2 Tx (Li et al. 2018b). The key reaction route was the conversion of Al into Al hydroxides, accompanied with solubility in the alkaline media. The following reactions happened during the etching process: Ti3 AlC2 + OH− + 5H2 O = Ti3 C2 (OH)2 + Al(OH)− 4 + 2.5H2

(9)

Ti3 AlC2 + OH− + 5H2 O = Ti3 C2 O2 + Al(OH)− 4 + 3.5H2

(10)

In this instance, Al hydroxides (oxides) may be easily dissolved by concentrated NaOH at elevated temperatures, resulting in fluorine-free MXenes. It effectively etches the MAX phase using concentrated alkali because it produces an extremely hydrophilic product with F-free terminations. However, its suitability for largescale MXene synthesis is constrained by the risks associated with highly concentrated alkali and elevated temperatures. Additionally, the products are often multilamellar MXenes with accordion-like structures, necessitating additional intercalation/delamination to yield single-layer MXene nano-sheets. Table 1 outlines the MXene synthesis methods applied in various applications.

4 Current Methods for Storing H2 Recently, hydrogen (H2 ) storage has drawn a lot of attention as one of the most crucial technologies for creating H2 -powered cars. Due to its availability, effectiveness, and

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Table 1 MXene synthesis summary using various methods No. Synthesis method

Example

Reference

1

Wet etching method using Ti3 C2 HF

Development

Ti3 C2 Tx

Naguib et al. (2011)

2

Amine-assisted TBAOH method

Large-scale MXene

Mo2 TiC2 Tx , Cr2 TiC2 Tx Mo2 Ti2 C3 Tx

Anasori et al. (2015)

3

Wet etching method

Various types of new MXenes

(Ti,Nb)2 CTx , (V,Cr)3 C2 Tx , Ta4 C3 Tx

Halim et al. (2014)

4

Intercalation/delamination Isolated single-layer MXene

5

Clay method (LiF/HCl etchants)

Clay-like MXenes

Clay-like Ti3 C2 Tx

Ghidiu et al. (2014)

6

Mild method

Large flake MXenes

Single flake Ti3 C2 Tx

Lee et al. (2017)

7

Electrospinning method

Ti3 C2 Tx MXene/carbon Ti3 C2 Tx MXene flakes nanofibers

Levitt et al. (2019)

8

Oxygen-assisted molten fluoride salt method

Nano-layered Ti2 NTx MXene

Djire et al. (2019)

Naguib et al. (2015)

Ti2 AlN

maximum energy density per unit mass, H2 gas is considered as a near-perfect fuel in many situations. To use H2 storage systems for fuel cells and light-duty vehicles, molecular hydrogen must be kept in a gaseous, liquid, or solid form (Sun et al. 2018).

4.1 Physical Storage Compressed air and liquid storage are the two most widely used physical storage systems for H2 . In cylinders or tanks, hydrogen is frequently kept in a gaseous state and compressed at extreme pressures of 35–70 MPa in high-strength composite cylinders (Barthélémy et al. 2017). High energy (10 MJ/m3 ) of hydrogen gas at room temperature and pressure necessitates the need for a large container to hold the H2 . Therefore, it is crucial to employ tanks made of specific alloys or composite fibers. It can be kept at a pressure of 700 bar in a single tank with 4.2 wt% gravimetric density and 2.9 MJ/L volumetric density at the cost of about $15/kWh (Rivard et al. 2019). A compressed gas storage technology is simple, effective, and quick fillingreleasing rate, which assures commercial usage. However, it has several drawbacks, including high use of energy for compression, a low volumetric potential, heat control

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while filling, and high-pressure tanks. Such high-pressure tanks (35–70 MPa) pose several safety risks, including gas leaks, temperature increase, and vessel explosions. As there are numerous obstacles to the implementation of gas storage technology, researchers are still looking for effective ways to compress and store H2 gas. Liquefaction is the second most preferred method for storing H2 . Liquefied hydrogen (LH2 ) is kept in cryogenic tanks, which were initially used in the 1990s (Rivard et al. 2019). The first stage in the LH2 storage method is chilling to a low temperature (−253 °C) before transferring to a cryogenic tank (Alekseev 2016). The key benefit of the LH2 approach is the higher density (~70.8 kg/m3 ) than the pressurized gas storage density (35 kg/m3 ). High-density LH2 is mostly used in aircraft applications because it needs more energy and precautions to store. The cost of hydrogen liquefaction is predicted to be e1.72/(kg LH2 ). The biggest challenge with this method is reducing boil-off emissions, ranging from 0.1 to 3% per day. LH2 and compressed H2 storage techniques are widely understood and industrialized. In contrast, a combined cryo-compressed technique was created. In 2012, the BMW introduced the first cryo-compressed technology (Kunze and Kircher 2012). The fundamental benefit of cryo-compressed technology is that it achieves considerably large volumetric density, although it does not minimize overall cost of the system. The physical storage of hydrogen was examined through generating different kinds of solid-state materials through adsorption, i.e., clathrate hydrates, hyper crosslinked polymers, MOFs, COFs, CNTs, graphene, graphene oxides, and fullerene (Barthélémy et al. 2017; Kumar et al. 2018). Some of these materials (COFs, graphene, and fullerene) have H2 storage capacities (5–7, 7.0, and 9.0 wt%) which are greater than the targeted value (5.5 wt%) of the DOE. These materials’ operability and durability problems have prevented their commercialization despite their increased capacity (5.5 wt%). Therefore, for practical usage, other aspects such as system performance, operability and durability, volume and weight, charging–discharging, and storage expenses need to be taken into account (Rusman and Dahari 2016). The fundamental issue with physisorption is weak binding energy between H2 molecules and the substrate material. The interactions of an H2 atom with a substrate influence the amount of hydrogen that can be stored in solid-state materials. Solid-state materials can store H2 by physisorption, chemisorption, spillover, and Kubas-type interactions (Ren et al. 2017). In the former, the binding capacity or energy between substrate materials and H2 atoms is high (40–80 kJ/mol), making it difficult to discharge H2 at room temperatures for practical applications. Physisorption in graphitic materials and MOFs, on the other hand, exhibits poor binding energies (4–10 kJ/mol), which are similarly insufficient for practical uses. Theoretical investigations demonstrated that the binding capacity of H2 atoms on sorption-based materials could be increased by metal decorations (alkalis, alkaline earth, and transition metals). Metal decorations on sorbents, however, tend to create clusters, which may diminish H2 storage properties (Sigal et al. 2011).

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5 Chemical Storage H2 can be stored chemically by creating covalent bonds with liquid/solid-state components. Liquid organics, hydrocarbons (such as benzene and methane), and ammonia have already been investigated for H2 storage. Storage in this system requires the breakdown of raw material, such as storage in gaseous ammonia via a catalytic process. Ammonia-based substances have been shown to have 17.4 wt% H2 storage capacity (Zheng et al. 2011). Other storage materials like liquid organic heterocycles, ammonia borane, and alcohols have also been investigated. At ambient circumstances, low SiO2 type X zeolites with alkali metal cations demonstrated H2 storage capabilities of 1.6 wt% (Wang and Yang 2010). Metal hydrides are the most popular chemical H2 storage substances. Furthermore, complex hydrides (lithium borohydride and sodium borohydride) have been extensively studied and have storage capacities ranging from 14 to 18 wt% (Grinderslev et al. 2020). Unfortunately, reversible storage is hampered by high hydrogenation and dehydrogenation temperatures and slow kinetics (Li et al. 2011). The practical applications of these technologies for commercial use are limited. Many efforts have already been made to establish a safer platform for H2 storage by adsorption via solid-state materials. However, the strong interaction between H2 atoms and substrate materials is a significant barrier to H2 physisorption or chemisorption, respectively, limiting their practical applicability in H2 storage devices.

6 Storage of H2 in MXenes In comparison to other storage methods, the use of MXenes for H2 storage has received little attention. Hydrogen adsorption primarily occurs in two ways in solid-state storage materials: weak physisorption of hydrogen molecules and strong chemisorption of dissociated hydrogen atoms. The binding capacity of H2 molecules in physisorption is 70 types have been effectively fabricated as Mo2 C, (V0.5 Cr0.5 )3 C2 , TiNbC, Ta4 C3 , Ti3 CN, Ti2 C, and Ti3 C2 . These MXenes have been fabricated for

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a wide range of applications due to their outstanding characteristics. MXenes have enormous importance in supercapacitors and batteries for storage of electrical energy due to their excellent electronic features. Similarly, the research on MXenes have got much attention due to their hydrophilicity, structural constancy, and electrical conductivity. The metal-based MXenes have also enormous applications in photocatalysis due to lower Fermi level than other semiconductors. The MXenes have been used as co-catalysts in photocatalysis (Tang et al. 2018; Zhang and Nicolosi 2019; Lyu et al. 2019; Ran et al. 2017). For example, Ti3 C2 which is treated as MXene has been used as co-catalyst for the improvement of photocatalytic performances of g-C3 N4 , Znx Cd1−x S, TiO2 , CdS, and ZnS. Similarly, MXenes like Ti2 C, Ti3 C2 , and Nb2 C have been employed as co-catalysts for the photocatalytic splitting of water. The MXenes enhance the photocatalytic activities by formation of Schottky barrier at the interface of MXene and semiconductor. The Schottky barrier acts as sink for electrons, hence it separates the electrons the reduces the recombination of electrons and positive holes. The separation of electrons and positive holes results in an improvement of the photocatalytic performance (Ye et al. 2018; Wang et al. 2016; Yu et al. 2019; Sun et al. 2018). A huge literature is available on applications of photocatalysis in solving the problems in energy and environment. A large number of efforts have been made for fabrication of catalysts, and modifications in structure of catalysts for visible light response. In this regard, MXene-based photocatalysts have been reported for photodegradation of organic pollutants, production of hydrogen gas by water splitting, and reduction of carbon dioxide for production value added products.

2 Fundamental Principle of Photocatalysis Using the principles of natural photosynthesis, the sunlight can be utilized for resolving the energy crisis. The photocatalytic reactions comprised of the following major steps (Khan et al. 2021; Yasin et al. 2022; Saeed et al. 2016, 2021, 2022; Adeel et al. 2021). 1. Absorption of sunlight by photocatalyst 2. Formation of positive holes and electrons in valence band and conduction band of photocatalyst 3. Production of hydroxyl radicals/free radicals by series of reactions of electrons and positive holes The fast recombination of the positive holes and electrons decreases the photocatalytic performance. Therefore, it is very important to reduce the rate of recombination of the positive holes and electron. The use of a co-catalyst is one of the techniques that have been employed for the enhancement of photocatalytic performance. The electrons produced in conduction band of photocatalyst by absorption of light shift to the co-catalyst. Hence the electrons and positive holes accumulate in co-catalyst and valence band of the photocatalyst, respectively. This separation of electrons and

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positive holes reduces the rate of their recombination. The electrons then react with oxygen and produce superoxide anion radicals which further react with water and ultimately produce hydroxyl radicals. Similarly, the positive holes react with water and produce hydroxyl radicals. These hydroxyl radicals are highly reactive species and take part in further reactions. The hydroxyl radicals, positive holes, and superoxide anion radicals are the main oxidizing species that take part in photochemical reactions. The following reactions and Fig. 1 explain this mechanism. Zinc oxide (ZnO) and titanium dioxide (TiO2 ) are two substances that are commonly used as photocatalytic materials in photocatalysis. However, these semiconductor oxides have wide band gap energy, therefore these materials are ineffective in photocatalytic application in visible light irradiations. These oxides have very little photocatalytic activities in solar light irradiation because the solar light consists of only 10% ultraviolet light. This ultraviolet light is able to produce positive holes and electrons in the conduction band and valence band of Zinc oxide (ZnO) and titanium dioxide (TiO2 ). Various attempts have been made to produce visible light active Zinc oxide (ZnO) and titanium dioxide (TiO2 )-based photocatalysts by alteration in the structures of Zinc oxide (ZnO) and titanium dioxide (TiO2 ) (Prasad et al. 2019; Jo and Natarajan 2015; Yuan et al. 2015b; Wang et al. 2016). Recently, MXene materials have got attention in photocatalytic applications due to their large surface area and excellent electron conductivity. The 2D MXenes can be effectively employed as co-catalyst along with semiconductors metal oxide for the development of active photocatalysts by efficient separation of the charge carriers produced by absorption of light. When the MXenes-semiconductors composites are irradiated with light, positive holes and electrons are generated in valence band and conduction band of the semiconductor counterpart of the composite. As the MXenes have capability of trapping electrons, therefore the electrons excited to conduction Fig. 1 Mechanism of MXene-based photocatalytic process

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band flow towards MXenes. The Schottky barrier formed at the interface of semiconductor and MXene favors the flow of electrons. This Schottky barrier also prevents the flow of electron back to the conduction band of semiconductor counterpart of the composite. The flow of electrons towards the MXenes effectively separate the positive holes and electrons and thus reduces their rate of recombination. The electrons flowed towards MXenes react with oxygen and produce superoxide anion radicals. These superoxide anion radicals further react with water and produce hydroxyl radicals. Similarly, the positive holes react with water and produce hydroxyl radicals. Consequently, the construction of composite/heterojunction between semiconductor and MXene results in a photocatalyst that has enhanced photocatalytic performance under irradiation of light (Ye et al. 2018; Zhuang et al. 2019; Wang et al. 2016). These reactions are given below.   MXene − Photocatalyst + hv → MXene − Photocatalyst h + +MXene − Photocatalyst (e− )     MXene − Photocatalyst h + → Photocatalyst h +   MXene − Photocatalyst e− → MXene (e− )   Photocatalyst h + + H2 O → OH   MXene e− + O2 → O.− 2 O.− 2 + H2 O → OH

3 Applications of MXenes-Based Photocatalysts in Degradation of Organic Pollutants The rapid development in industrialization and significant increase in population of the world have severely increased the environmental pollution and energy-related problems world widely. The environmental protection and growth of industries are always contradictory during the last century. Now the people are conscious about the utilization of advanced and clean energy resources for solving the environmental problems. Textile industry is one of the industries that produces a huge amount of wastewater contaminated with a wide range of various organic dyes. These dyes are highly stable and toxic in nature. These dyes badly affect the aqueous ecosystem because these dyes stop the entrance of sunlight into the interior of the water body. It is reported that about 8 × 105 ton dyes are produced annually around the globe.

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About 20% of the dyes employed in textile industries are wasted and released in wastewater to the environment (Kun-Min and Zong-Guo 2008; Guo et al. 2019; Prasad et al. 2017a, b, c, d). Consequently, it is very important to remove the dyes from wastewater. A number of methods have been tested for the removal of these dyes from the environment. Photocatalysis is the best, easy, and successful method for the complete removal of the dyes from the wastewater. A wide range of semiconductor materials have been employed as photocatalysts for the removal of dyes. The composite comprised of MXenes and semiconductors have exhibited best photocatalytic performance in the degradation of dyes in wastewater. For example, Iqbal and his co-workers (2019) have successfully prepared Ti3 C2 /Bi1−x Lax Fe1−y MnyO3 by sol–gel method. They used the prepared material as photocatalyst for photodegradation of Congo red dye. They have reported about 92% and 100% removal efficiency of Congo red dye under dark condition and under irradiation of light, respectively. The removal of Congo red dye under dark condition is attributed to adsorption on the MXene-based composite. The 100% removal of Conge red dye within 20 min has been attributed to photocatalytic degradation. Researchers have attempted to improve the performance and characteristics of MXenes through stabilization techniques. Wu and co-workers (2017) have used carbon nano plating technique for the stabilization and enhancement of photocatalytic performance of MXenes-based photocatalysts. They reported the preparation of two-dimensional MoS2 /C/Ti3 C2 MXene by coupling of MoS2 with Ti3 C2 -C MXene. The as-fabricated MXene-based material exhibited an effective synergetic effect. Similarly, a photocatalyst has been constructed between TiO2 and MXene for the photodegradation of dyes under irradiation of light (Wang et al. 2018b). The fabricated TiO2 -Ti3 C2 Tx had shown extra ordinary photocatalytic performance in photodegradation of methyl orange. The TiO2 -Ti3 C2 Tx had shown 98% photodegradation of methyl orange which was much higher than performance of bare TiO2 and Ti3 C2 Tx having 77 and 42% photodegradation of methyl orange. In another study, a silver decorated g-C3 N4 -Ti3 C2 has been reported with enhanced photocatalytic activity for photodegradation of aniline (Ding et al. 2019). About 81% photodegradation of aniline has been reported in this study. The enhanced photocatalytic performance of the composite has been ascribed to excellent absorption of visible light by the composite due to silver plasmon resonance effect and efficient separation of the charges. The Schottky barrier formed at the junction of g-C3 N4 and Ti3 C2 MXene accelerates the movement of photogenerated electrons from g-C3 N4 to Ti3 C2 MXene. Similarly, the free electron in silver was excited to higher energy state due to plasmon resonance by absorption of light as a result of irradiation of silver decorated g-C3 N4 -Ti3 C2 composite. After excitation to higher energy state, the free electron of silver migrates to Ti3 C2 and g-C3 N4 conduction band. The transfer of electrons in silver decorated g-C3 N4 -Ti3 C2 composite has been explained in Fig. 2. These electrons then participate in secondary reactions and produce hydroxyl radicals. Later on, Wojciechowski and co-workers (2019) have reported the synthesis of MXene-based photocatalytic material composed of Ti2 C couples with metal oxides, PdO, TiO2 , and Ag2 O. They have characterized the prepared samples by various techniques and then used as photocatalysts for photodegradation of salicylic acid.

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Fig. 2 Mechanism of separation of charges in silver decorated g-C3 N4 -Ti3 C2

The Ti2 C-Ag2 O and Ti2 C-Ag MXenes showed the best catalytic performance. The explained the separation of charges in MXene-based photocatalyst as given in Fig. 3. As well, Xie and team (2018) have reported an MXene-based photocatalyst comprised of CdS-Ti3 C2 Tx through an electrostatic self-assembly method. They reported that Ti3 C2 Tx counterpart of the composite acts as electron mediator that

Fig. 3 Separation of charge carrier by Ti2 C MXene tailored with PdO, TiO2 , Ag2 O

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assists the transfer of electrons from CdS. Due to the lower fermi level of Ti3 C2 Tx than CdS, the life span of photogenerated electron is higher in Ti3 C2 Tx than the life span of electron in CdS. Furthermore, the Ti3 C2 Tx has the ability to absorb the Cd2+ generated in photocatalytic reaction. As a result of these factors, CdS-Ti3 C2 Tx exhibited higher photocatalytic performance. Using the concept of separation of photoinduced charge carriers by formation of heterojunction in MXene-based binary composites, some researchers have worked on the development of MXenes-based ternary hetero composites as photocatalysts with enhanced photocatalytic performance. For example, Hou and co-workers (2018a) have developed ternary composite of MXene, Ti3 C2 Tx -In2 S3 -TiO2 by hydrothermal method. The prepared ternary composite was used as photocatalyst for photodegradation of methyl orange. The prepared various composites with various compositions. The composite with 16 mg of Ti3 C2 Tx showed excellent photocatalytic activity with a photodegradation rate of 0.04977 per minute. The observed rate was 3.2 and 6.2 times higher than the rate of photodegradation observed with In2 S3 and Ti3 C2 Tx , respectively. The enhanced photocatalytic performance of ternary composite was attributed to effective separation of charge carriers by construction of Schottky junction and type II heterojunction. These junctions created a large number of channels that help in separation of charges and ultimately enhanced the photocatalytic performance. The higher photocatalytic performance of ternary composite was explained by the involvement of synergy between visible light responsive In2 S3 , conductive Ti3 C2 Tx , and higher band of TiO2 . The separation of charge carriers in ternary MXene composite has been explained in Fig. 4.

Fig. 4 Separation of charge carriers in MXene-based ternary composites

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Similarly, Fang et al. (2019) have also reported a ternary MXene-based composite as photocatalyst for photodegradation of rhodamine B dye. They prepared ln2 S3 CdS-Ti3 C2 -OH composite by hydrothermal method. The prepared composite was found as visible light driven efficient photocatalyst. Furthermore, the ternary ln2 S3 CdS-Ti3 C2 -OH composite exhibited excellent photocatalytic performance for the degradation of mixture of methyl orange and rhodamine B dyes. The irradiation of ternary ln2 S3 -CdS-Ti3 C2 -OH under visible light excited the electrons from the valence band to the conduction band of CdS and In2 S3 . The existence of Ti3 C2 OH drive the flow of electrons from the conduction band of CdS and In2 S3 towards MXene (Ti3 C2 -OH). This flow of electrons towards MXene (Ti3 C2 -OH) separates the positive holes and electrons generated by irradiation. The separation of positive holes and electrons results in an enhanced photocatalytic performance of MXene-based ternary ln2 S3 -CdS-Ti3 C2 -OH composite. The Ti3 C2 -OH counterpart of composite has shown a vital role in the efficient separation of the positive holes and electrons. It has increased the life span of electrons and as a result the superoxide anion radicals are generated in higher concentration. Hence, the composite has shown outstanding photocatalytic performance. Furthermore, the electrons accumulated in the semiconductor’s conduction band are more energetic and produce superoxide anion radicals more effectively because the potential of conduction band of In2 S3 and CdS are more negative than the reductive potential of oxygen/superoxide anion radical. This factor also enhances the photocatalytic performance. The proposed mechanism has been explained with the help of Fig. 5.

Fig. 5 The charges separation mechanism in ln2 S3 -CdS-Ti3 C2 -OH composite

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4 Applications of MXenes-Based Photocatalysts in Production of Energy Sources The production of renewable and green energy is urgently needed due to rapid increase in greenhouse effect and industrialization. The sunlight is a big source of renewable energy. It is estimated that the amount of sunlight energy is higher than 5 MJ/m2 per year. Hence the sun is the largest source of energy (Wang et al. 2018c; Lin et al. 2019b). Fujishima and Honda are the pioneers of the production of hydron and oxygen by the splitting of water using sunlight as a source of energy in the presence of TiO2 as photocatalyst (Fujishima and Nature 1972). A wide range of photocatalysts have been reported for the production of hydron by water splitting. Recently, the MXene-based photocatalysts have been attempted successfully for production of hydrogen and oxygen. Wang and co-workers (2016) have reported a photocatalyst prepared by combination of TiO2 and Ti3 C2 Tx MXene to produce hydrogen by water splitting. The existence of Ti3 C2 Tx in developed photocatalyst played a role in the separation of positive holes and electrons generated by irradiation. It was found that 5 wt% TiO2 -Ti3 C2 Tx showed excellent photocatalytic performance in production of hydrogen. Furthermore, they also studied Ti2 CTx , and Nb2 CTx as co-catalysts for the development of efficient MXene-based photocatalysts for production of hydrogen. Similarly, Li et al. (2020) have synthesized an MXene-based photocatalyst comprised of BiOBr-Ti3 C2 . They used the developed material as photocatalyst for splitting of water under irradiation of visible light. They reported that Ti3 C2 acts as tank for positive holes due to its excellent conductivity. Furthermore, the work function of Ti3 C2 is more negative than conduction band of BiOBr therefore photogenerated electrons were confined to the conduction band of BiOBr and were blocked from moving to the Ti3 C2 . Lin and his team (2019a) have constructed a Schottky junction by combination of g-C3 N4 with Ti3 C2 . They prepared an MXene-based material comprised of O-doped g-C3 N4 and Ti3 C2 for efficient production of hydrogen gas. The combination of positively charged O-doped-g-C3 N4 with negatively charged Ti3 C2 resulted in a Schottky junction that played a key role in the separation of charges. The prepared composite showed twofold activity for production of hydron in comparison to activity of pristine components. The work function of Ti3 C2 is lower than the work function of g-C3 N4 therefore, the photogenerated electrons flow towards the g-C3 N4 counterpart of the composite. This flow of electrons leads to bend upward the energy band of g-C3 N4 . As a result, a Schottky junction is produced. Figure 6 explains the mechanism. Under irradiation of light, the electrons in conduction band of g-C3 N4 over-flow the junction and move to MXene due to interfacial contact between two components and formation of a built-in electric field. The electrons accumulated in MXene react with water and produce hydrogen. The existence of Schottky junction prevents the flow of electrons from MXene to g-C3 N4 . Later on, Zhuang and co-workers (2019) fabricated a 1D/2D composite composed of TiO2 /Ti3 C3 by electrostatic self-assembly method. The as-fabricated composite was used as photocatalyst for production of hydrogen gas. The prepared composite showed catalytic activity for production of hydrogen gas as 7 mmol per hour per gram

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Fig. 6 Mechanism of separation of charges in O-doped g-C3 N4 -Ti3 C2 composite

of the photocatalyst. The higher photocatalytic activity was attributed to formation of heterointerface between the components. Similarly, the MXene-based composites have been used as photocatalysts for reduction of CO2 as well. For example, Low and co-workers (2018) have employed TiO2 -Ti3 C2 as photocatalyst for reduction of carbon dioxide. The as-prepared composite showed 3.7 folds higher photocatalytic performance in reduction of carbon dioxide to methane in comparison to photocatalytic activity of TiO2 alone. More recently, Bi2 WO6 /Ti3 C2 heterojunction has been synthesized by in situ fabrication of sheets of Bi2 WO6 on sheets of Ti3 C2 . The as-fabricated material was employed as photocatalyst for reduction of carbon dioxide. The composite showed 4.6 times higher photocatalytic performance than individual components (Cao et al. 2018).

5 Conclusions In summary, this study over viewed the research work carried out related to MXenesbased photocatalysts for environmental decontamination and energy conversion applications. In this study we highlighted different MXene-based photocatalysts prepared by various techniques such as self-assembly method, ion exchange method, hydrothermal method, solvothermal method, etc. The 2D MXenes have got much consideration in photocatalysis due to their extraordinary characteristics. It was found that MXene component of the composite serves as pathway for the flow of electrons from one component to another component. This flow of electrons from one component to another component separates the electrons and positive holes and ultimately enhances the photocatalytic activity. The enhanced photocatalytic activity of MXenes-based photocatalysts has been attributed to the construction of Schottky junctions at the interface of two components. The Schottky junction drive the flow of

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electrons from one part to another part of the composite. The MXene-based composites have been used as photocatalyst under irradiation of light for both, the degradation of organic pollutants and production of hydrogen gas.

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Efficacy of MXene-Based Materials in the Removal of Gases Zaeem Bin Babar, Nameer Urfi, Saeed ur Rehman, and Komal Rizwan

Abstract An emerging class of 2D inorganic species is MXenes, which are wellknown for their superior morphological characteristics including extraordinary surface areas, excellent heat conductance, excellent mechanical behavior, superhydrophilic nature and outstanding inertness as compared to their conventional counterparts. As a result, MXenes are applicable to a variety of environmental and commercial relevant practices such as gas storing, purifying air, etc., and outperformed corresponding conventional materials in specific domains. Under ambient conditions of temperature and pressure, MXene-modified hybrid materials can be effective for the abatement of particulates, hazardous gases, airborne microbes, etc., via the sorption process. From an energy perspective, employing such materials solve the problem of gas storing and/or purification in substantially low energy utilization. In this chapter, a thorough literature is discussed to abate various gases from environment such as carbon dioxide (CO2 ), methane (CH4 ), hydrogen (H2 ), other harmful gases, and particulates by MXenes and MXenes-modified materials. Furthermore, the adsorptive uptake mechanisms regarding the removal of gas contaminants from air streams including chemical sorption, electrostatic interlinkages, and van der Waals attractive forces are also presented. Lastly, the limitations in MXenes usage and subsequent effective utilization of MXene from a futuristic perspective are also discussed. Keywords Mxenes · Air purification · Abatement · Toxic gases · Storage

Z. B. Babar · N. Urfi Institute of Environmental Sciences and Engineering (IESE), School of Civil and Environmental Engineering (SCEE), National University of Sciences and Technology (NUST), Islamabad, Pakistan S. Rehman High Temperature Energy Conversion Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of Korea K. Rizwan (B) Department of Chemistry, University of Sahiwal, Sahiwal 57000, Pakistan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_13

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1 Introduction Nanomaterials are an emerging class of materials which played a great role for environmental remediation (Babar et al. 2022; Bilal et al. 2022; Qamar et al. 2022; Rasheed et al. 2022a, b; Rizwan et al. 2021, 2022a, b; Shakeel et al. 2022). MXenes are a 2D material which was initially disclosed in 2011 and belongs to the group of metal carbides, nitrides, and carbonitrides (Gogotsi and Anasori 2019; Hart et al. 2019; Gao et al. 2020). Its generic chemical formula is Mm+1 Xm Ty where M stands for transition metals including Titanium (Ti), Molybdenum (Mo), Zirconium (Zr), etc., X can be carbon (C) or nitrogen (N), y represents the quantification of terminal functional moieties including OH, O, etc., (Jimmy and Kandasubramanian 2020; Jaya Prakash and Kandasubramanian 2021), and m is always greater or equal to 3. One of the common and widely utilized MXene is Ti3 C2 Ty where Ty is an O chemical functionality. They possess superior physicochemical characteristics including large surface area, excellent chemical inertness, good strength, superb heat conductance, and super-hydrophilic nature. These characteristics render them eco-friendly (Er et al. 2014; Han et al. 2020). As a result, they possess a variety of applications in multiple technical fields such as microchip fabrication (Hart et al. 2019), optical sensors (Hantanasirisakul and Gogotsi 2018) and wastewater treatment (Jeon et al. 2020). Additionally during past few years, they have been thoroughly investigated for air purification, antibacterial properties, vapor phase chemical sensing, and gas-storing purposes (Noor et al. 2023; Ding et al. 2018). Such as in recent studies, Ti3 C2 was employed for detecting and removing potentially hazardous gases such as ammonia, sulfur dioxide, oxides of nitrogen and hydrogen disulphide (Guo et al. 2020), volatile vapors (Yuan et al. 2018). Additionally, they have been used as significant adsorbents for capturing greenhouse gases due to the presence of a variety of chemical functionalities. Furthermore, their porous morphology and excellent hydrophilicity play a crucial role (Guo et al. 2020; Sun and Li 2021). In another study, Ti2 C nano-sized sheets were utilized in adsorbing H2 gas from contaminated air. The subsequent removal mechanisms were physical and chemical adsorption and Kubas-based attractions (Hu et al. 2013). Furthermore, air purification filters comprising of MXenes were reported to effectively remove particles with a size lower than 2.5 µm (PM2.5 ) with an efficiency of greater than 99%. This was relatively higher than those obtained for commercially available filters (Gao et al. 2019). Therefore, they were found promising specifically in purifying air from hazardous materials. In literature, substantial research work has been performed to evaluate the MXenes for various environment-related applications (Sun and Li 2021) including recovery of heavy metallic ions from polluted waters, storing energy, separating gases (industrial applications) focused on metallic organic frameworks and adsorption (Li et al. 2018), and microchip synthesis (Ronchi et al. 2019). There are also certain reviews focusing on the synthesis and morphological characteristics of MXenes specifically for constructing semiconductors (Ronchi et al. 2019). The literature analysis suggests that the critical review of investigations utilizing MXenes specifically for

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air purification and treatment of various air pollutants from air streams has been overlooked. Therefore, in this chapter the recent advances in the last four to five years in the application of MXenes-based materials for the treatment of toxic gaseous pollutants, volatile vapors, and respirable particles have been discussed. Initially, a thorough discussion of advancements in CO2 capturing is presented followed by an analysis on storing H2 and methane. Afterward, the application of MXenes for separating hazardous gases and particulates is provided. Then, recent studies discussing the regenerative capability of MXenes are presented. Lastly, an outlook on the application of MXene-based materials and their respective capabilities for purifying and storing various gases to address worldwide issues (i.e., climate change) has been presented. The review of MXenes’s application in this chapter is considered important keeping in view of COVID-19 which renders the utilization of air purification equipment for indoor environments such as residential buildings, institutions, offices, etc., extremely critical for a safe and healthy lifestyle.

2 Application of MXene-Based Materials for Gas Abatement 2.1 CO2 Abatement Due to rapid urbanization and industrial expansions (i.e., anthropogenic activities), the ambient concentration of CO2 increased from 280 ppm (in the pre-industrial era) to ~420 ppm in 2022 (National Oceanic and Atmospheric Administration (NOAA) 2022) which is significantly related to climate change. Initially, the focus was to capture CO2 from stationary sources including powerhouses (i.e., coal based), cement manufacturing units, and petrochemical and steel industries (Sanz-Pérez et al. 2016). However, CO2 capture is challenging due to the limitation in separating gases from the complex mixture (Sharma et al. 2021). Hence, direct capturing from air is another alternate. The main advantage of such alternate approach is utilizing technologies that do not contribute to carbon emissions (Breyer et al. 2019). Considering such significant aspects, the utilization of MXenes in direct capturing of CO2 from air provides another alternate although not yet competitive.

2.1.1

Separation of H2 /CO2 Before Combustion

Syngas is a gaseous combination of H2 , CO, and CO2 with additional contaminants such as CH4 and N2 . The CO can be oxidized after a water–gas shift process to produce a stream comprising 15–50% CO2 (Kanniche et al. 2010). The creation of highly purified H2 gas from an H2 /CO2 gaseous combination is required for precombustion CO2 collection. For this purpose, 2D materials such as graphene oxide

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(GO), zeolites, and MOF have been employed, with GO having the highest H2 permeability. The performance of a variety of MXenes-based membranes is shown in Table 1. Graphene oxide is created by reducing graphene and is commonly integrated into multi-layer GO membranes (Chuah et al. 2020a, b). Controlling defect development and desired space between the layers during GO development and lamination can be used to regulate GO gas permeability. Thin laminations based on GO are frequently employed as a separating layer in conjunction with a membrane framework with high porosity. Zeolites and MOF membranes operate as molecular sieves. Furthermore, they are not constrained by the trade-off based on permeation/selectivities; nevertheless, they are mainly constrained by their brittle behavior, which results from the manufacturing exfoliating stage (Ding et al. 2018). In the past few years, zeolites and MOF-based materials have been employed as filling agents in polymer frameworks, although the efficiency of these membranes is frequently restricted by the composition of the polymer framework (Chuah et al. 2020b). Extremely thin MXene nanosized sheets (width ~5 nm) demonstrated strong H2 permeance (36,000 GPU, where 1 GPU = 3.35 × 10–10 mol m−2 s−1 Pa) with H2 /CO2 selectivity of 5 (Fan et al. 2021). The closeness to the Knudsen selectivity (4.7) shows that the anodic aluminium oxide (AAO) substrate is not completely covered. A 10 nm MXene layer thickness exhibited H2 /CO2 selectivity of 8, while if the thickness increased from 20 nm, it resulted in an unchanged H2 /CO2 selectivity of 30, implying that 20 nm is an optimally thick membrane for selectivity with minimum influence on permeance (Shen et al. 2018). Shen and colleagues then spun-coated non-selective MXene defects with polyether block amide copolymer (PEBAX), and the selectivity improved beyond 05 coats. Because of the different kinetic diameters of H2 and CO2 (28.9 and 33 nm), molecular sieving is frequently used to separate these gases. To keep the space between the layered structures of MXene within this range, many techniques have been used. Fan and colleagues used vacuum-based filtering at high temperature, for example, to decrease d-space from 34 to 27 nm (at 25 °C) (Fan et al. 2019). In the case of separating gaseous streams after combustion, Ding and colleagues enhanced MXene membranes and reported H2 /CO2 selectivity of greater than 160 and strong H2 permeation (Ding et al. 2018). Membrane efficiency has also been increased by tuning intermediary spacing using tiny inorganic and bigger polymeric components. Fan et al. found that a unique method utilizing nickel intercalation enhanced H2 permeance by more than 50% (from 158 to 249 GPU). The corresponding selectivity almost increased by three times from 215 to 615 GPU (Fan et al. 2021). This might be because positively charged Ni2+ increases electrostatic interactions between neighboring MXene nano-sized layers, as has been observed with other interpenetrating cations (Lukatskaya et al. 2013). There is a substantial body of research covering GO, zeolite, polybenzimidazole (PBI), and other membranes, but there have been few contemporary publications on MXene and/or MXene-based membranes. In spite of the paucity of research, MXenes typically have greater H2 permeation and near to or equivalent H2 /CO2 selectivity. Furthermore, several MXene membranes exhibited remarkable permeation of greater than 1000 GPU. In the last few years, most of the research has concentrated on the production of ultra-thin MXene-based membranes.

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Table 1 MXenes-based materials for the separation of gas streams containing H2 and CO2 (This table is reprinted with permission from Dixit et al. 2022) Membrane type

Operating temperature (°C)

H2 permeability (GPU)

H2 to CO2 selectivity

Reference

9 mm GO

100

450

105

Li et al. (2013b)

MXene

40

4

5

Lin et al. (2021)

MXene borate (MB-75)

25

322

19.13

Shen et al. (2018)

20 mm MXene PEBA

1584

27

MXene PDMS

1612

25

200–400

20

Singh et al. (2014) Fan et al. (2021)

PBI hollow fiber membrane

20

MXene/Al2 O3 /Ni2+

25

MXene/Al2 O3

249

615

158

215

MFI zeolite/silica

450

1182

141

Tang et al. (2009)

Zeolite/boron

400

299

47

Hong et al. (2005)

PBI-H3 PO4 polymer

150

1.2

34

Zhu et al. (2018)

In the case of MXene, the great structural robustness and homogeneity during fabrication enable for the development of extremely thin membranes. Additionally, there are expectations that permeation and selectivity will increase to meet or exceed than those obtained for GO and other membranes in the future years.

2.1.2

Separation of CO2 /N2 After Combustion

After combustion in air, post-combustion separation processes of gas comprise the elimination of CO2 typically in lower concentrations from a main N2 gas. Because of CO2 ’s increased polarizability and quadrupole moment, this separation is achievable (Shamsabadi et al. 2019). Guan et al. employed a PEBAX membrane comprising 0.5 MXene (wt/wt%) as a filling agent to produce a CO2 /N2 selectivity of 105. The corresponding CO2 permeation about ~86 Barrer was reported (Guan et al. 2021). They reasoned that the oxygen-containing PE group’s polar ether oxygen promoted CO2 transportation, whereas the N-influenced PA part augumented CO2 transportation owing to Lewis acid–base interactivities. Another study showed that MXene as filling agent increased the efficacy of PEBAX membranes. They claimed that hydrogen bonding between the Ti3 C2 Tx and PEBAX constituents, resulting in an organized structure of MXene nano-galleries within the PEBAX constituent,

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allowing CO2 transit (Shamsabadi et al. 2019). Also, in another work, it was discovered that borate incorporation increased CO2 /N2 selectivity by crosslinking O-based functionalities of MXene (Shen et al. 2018). Others showed that borate crosslinking prevented bigger N2 (and CH4 ) molecules from passing through GO membranes, enhancing CO2 selectivity (Ding et al. 2018; Guan et al. 2021). There have been few reports using MXene membranes for separating CO2 /N2 , and their efficiencies are not high enough to outperform already developed commercial membranes. For example, the GO-brush membrane has a CO2 /N2 selectivity of 680, which is far greater than any other MXenes research in the literature (Zhou et al. 2017). While some contemporary MXene membranes have permeabilities greater than 100, there are numerous situations where this number is less than 100. Meanwhile, numerous alternative GO and zeolite membranes have permeabilities much beyond 100 barrer, while a membrane based on carbon nanotube has a susceptibility of 361 barrer (Zhao et al. 2014). MXene membranes generally take advantage of their super-thin active layers. Due to this the permeation increased in the past years without reducing selectivity. Recent cases include a PEBAX-based membrane comprising of 10% (wt/wt) with a porosity of 584 barrer and a sensitivity of 59 (Shi et al. 2021) and PEG-based MXene membrane with exceptional permeation and selectivity of greater than 800 barrer and 32, respectively (Wenjia et al. 2022). Although MXene-based membranes tend to catch up other membranes in terms of CO2 susceptibility and CO2 /N2 selectivity, the research indicates that further work is required for MXenesbased membranes to outsmart existing commercial membranes considering after combustion CO2 capturing (Table 2).

2.1.3

CO2 Capturing from Direct Air

In recent years, various techniques for capturing and sequestering CO2 were investigated (Choi et al. 2009; Rochelle 2009). In the context of utilizing adsorbents for this purpose, they must possess substantial adsorptive removal capability and augmented selectivity for CO2 . For instance, in the case of MOFs, the high interactive energy of CO2 with their surfaces is responsible for increased CO2 selectivity in comparison with other gases. Different M2C MXenes were utilized for the adsorptive abatement of CO2 (Morales-García et al. 2018). The MXenes-based material related to period IV exhibited higher CO2 adsorptive removal capabilities which were 8 mmol CO2 per gram of material employed in comparison to 4.5 mmol per gram and 2.4 mmol per gram obtained for MXenes-based materials associated with periods V and VI, respectively (Morales-García et al. 2018). Additionally, their adsorptive capacities were greater than 3.72 mmol per gram obtained for the case of Ca-A (zeolites) (Bae et al. 2013). Furthermore, they were even higher than 2.45 mmol per gram found for reduced graphene oxide-based material (i.e., a-RGO-950) (Chowdhury and Balasubramanian 2016). Yu and colleagues estimated the adsorptive removal of CO2 via physical adsorption. This estimation was based on theoretical evaluation which used typical factors for O-based terminal chemical functionalities located on the surface

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Table 2 MXenes-based materials for the separation of gas streams containing CO2 and N2 (This table is reprinted with permission from Dixit et al. 2022) CO2 permeation (barrer)

CO2 to N2 selectivity

Reference

0.275

111

97

Li et al. (2013b)

25.0

0.2

43.3

72.5

Liu et al. (2020)

Pebax/PRG

30.0

0.2

119.0

104

Dong et al. (2016)

Pristine MXene

25.0

–

6.8

44

142

77

Hong et al. (2022)

Membrane type

Operating conditions Temperature (°C)

P (pressure) (MPa)

Pebax/PRG

30.0

Pebax-MXene

MXene-ZIF-8 Pebax/MXene

25.0

0.4

86

105

Pebax/MXene 0.5

25.0

0.4

70.2 (gas)

93.3

Pebax/MXene (1% wt)

30.0

0.2

148

63

584

59

Pebax/MXene (10% wt)

Guan et al. (2021) Shi et al. (2021)

Pebax-4A zeolite

25.0

0.49

71.40

54.1

Surya Murali et al. (2014)

Pebax/graphene oxide

35.0

0.7

108

48.5

Zhao et al. (2015)

of MXene (Yu et al. 2015). The surficial defects within the layers of the MXenebased materials were found to be crucial as they provide extra surfaces, thus further enhancing interactivities with its surface (Khaledialidusti et al. 2020). This finding was well in agreement with those of Morales-García and coworkers as they found that for each nano-sized layered cell, 04 CO2 molecules were adsorbed within upper and lower layers (Morales-García et al. 2018). Furthermore, MXenes-based materials were modified in different ways to improve the CO2 adsorptive performance. For instance, in a recent study, MXene was modified by incorporating dimethylsulfoxide which enhanced its adsorptive removal capability by approximately four times from 1.33 to 5.79 mmol per gram (Wang et al. 2018). In another study, Shen and researchers modified MXene with borate and tested it for the adsorptive removal of CO2 (Shen et al. 2018). CO2 adsorptive removal was promoted via hydrated nano-sized channels as borate could convert CO2 to HCO3 − (Wang et al. 2016). The MXene-modified borate showed approximately 13% higher CO2 adsorptive capability than pure MXene. They also studied the adsorptive removal of CO2 by developing a modified MXene-based framework comprising of polyethylenimine and borate. It was found that the adsorptive removal of CO2 adsorption increased by 0.1 mmol per gram (Shen et al. 2018) in comparison with 0.8 mmol per gram found

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for MXene without polyethylenimine. The superior CO2 adsorptive capability of the modified matrix might be due to the development of nano-sized channels which effectively enhanced the selective transportation of CO2 molecules which were of relatively smaller size than molecules of N2 and CH4 and their transportation was hindered owing to large size. Lui and the workers utilized a modified polyether block amide with 0.15 wt% MXene and found that CO2 adsorptive removal capability enhanced by 50% in comparison with that obtained for polyether block amide without MXene (Liu et al. 2020). Consequently, previous studies have shown that MXenes when modified or combined with other materials yielded superior structural change within the resultant material. Thus, further significant enhancement in the adsorptive removal capability of CO2 for the case of MXenes-modified adsorbents can be foreseen futuristic perspective. Whereas, currently, MXene-based modified materials offer equivalent adsorptive capabilities as compared to conventional techniques which typically employ MOFs and zeolites. Generally, the packing density of the MXenes ranged between 2.2 and 4 g/cm3 (Ghidiu et al. 2015; Yoon et al. 2016), which is significantly higher than MOFs, zeolites, silica-constituted sorbents, and carbon-constituted sorbents which were found to be equivalent to 0.2–0.8 g/cm3 (Mueller et al. 2006; Purewal et al. 2012), 0.13–0.53 g/cm3 , 1.25 g/cm3 (Mueller et al. 2006; Balahmar et al. 2016), and 0.17–0.45 g/cm3 , respectively. For the case of MXenes-based materials (i.e., Ti3 C2 Tx ), the experimentally determined CO2 removal was 502 CO2 /MXene on the basis of volume-by-volume percent which is theoretically equivalent to the capacity of 3788 v/v and therefore higher than 400 v/v and 130 v/v obtained for commercially available polypyrole-derived carbons and MOF210, respectively (Cox and Mokaya 2017). Consequently, MXenes-based sorbents underperform than existing conventional sorbent regarding the adsorptive removal capability considering gravimetric analysis (per weight basis). This is due to their high packing density as described above. However, when comparison is done between MXenes and other traditional sorbents on volumetric analysis (per cm3 basis), their adsorptive removal capabilities for CO2 were found to be several times higher than those of their conventional counterparts.

2.1.4

CO2 Separation Mechanisms

The exchange between selectivity and permeability is seen as membrane separation’s “Achilles heel.” In principle, when pore size reduces to increase selectivity, total gas penetration drops, as suggested by the Robeson upper bound for polymeric gas separation processes (Robeson 2008). The creation of multilayer nanomaterials with atomic thicknesses was preceded by graphene membranes including augmented pore size dispersal. Soon after, two-dimensional membranes such as graphene oxide (GO) and reduced graphene oxide (rGO) (Li et al. 2013a), metal–organic frameworks (Biswal et al. 2016), covalent organic frameworks (Krishna and van Baten 2010), zeolite nanosheets (Karahan et al. 2020), and, very significantly, MXenes developed. MXenes allow for fine manipulation of the spacing between the layers

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(d-spacing), resulting in hundred times lag in molecular motions beyond this intermediate dimension, a characteristic that is being used to break the Robeson upper bound on perm-selectivity (Li et al. 2020). Jin and co-workers developed a computer model permeating gas inside an MXene membrane and discovered complete penetration with F Total = F Kn + F g where F Kn and F g describe permeations caused by Knudsen diffusion and intermediate spacing, correspondingly (Jin et al. 2019). Knudsen diffusion happens inside straight nanochannels formed by gaps in MXenebased monoliths, when the defect width (dD) is higher than the kinetic diameter of the gas molecule (dK). Selectivity is negatively proportional to the square root of the gas’s molecular weight. On the other hand, Knudsen diffusivity solely suggests a selectivity of just 4.7 for the gaseous mixture of H2 and CO2 , despite the fact that actual selectivities far surpass this number, demonstrating that Knudsen diffusion is not the major process (Karahan et al. 2020). When MXene faults are regulated, gas permeation by intermediate space offers a vital role in permeation (in a research by Jin et al., F g supplied 82% of total permeance). Gases transit tortuous nanochannels inside the MXene interlayer gap in the molecular sieve process (Jin et al. 2019). The space between the layers of the MXenes may be adjusted for higher selectivities, which is intriguing. High-temperature processing, for example, the incorporation of the filling agents between 2D layers, has increased selectivity. A recent study used computational models to vary intermediate distance in the range of 6–14, which is only greater than the kinetic diameters of H2 (28.9 nm), carbon dioxide (33 nm), and CH4 (38 nm) (Cecopieri-Gómez et al. 2007). As per calculations, the maximum selectivity was attained for distance between the layers of about 6.8 (thickness equivalent to 1–2 atoms) with selectivities of H2 /CH4 , H2 /N2 , and H2 /CO2 of 780, 129, and 238, correspondingly, confirming their experimental results. CO2 might also be aided transit by transiently adsorbing on the MXene surface, increasing diffusion over noninteracting gases. This improved CO2 adsorption is owing to its larger quadrupole moment relative to other gases, which is mediated further by the large accessible surface area (Ding et al. 2018).

2.2 Methane Abatement To meet energy requirements, methane (CH4 ) is an attractive option owing to its high energy content (i.e., octane number), high combustion efficiency (i.e., low CO formation), and low sulfur and nitrogen content which results in the generation of SOx and NOx in negligible quantities. Since, CH4 is gas at room temperature, therefore, transportation and storing of CH4 are quite challenging. According to the US Department of Energy, under standard conditions, the target for CH4 storing capability is approximately 263 cm3 /cm3 of sorbent which is equivalent to 699 cm3 /g of sorbent (USDOE). Over the past few decades, several conventional adsorbents were utilized for storing CH4 including zeolites, activated carbons, etc., (Li et al. 2019). Lately, MXenes are being investigated for CH4 storing purposes (Liu et al. 2016). Regarding

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CH4 storage, there are certain limitations associated with conventional adsorbents. For instance, the application of zeolites is not practical owing to lower specific areas, generally around 1000 m2 /g (Mason et al. 2014). Although activated carbons possess relatively high surface areas than zeolites, still their maximum CH4 storing capacity on a volume basis ranged between 100 cm3 /cm3 sorbent and 170 cm3 /cm3 at 5 MPa, substantially less than the targeted capacity. In the case of MOFs, the volume-based CH4 storing capacity at high pressures of less than 35 MPa was 200 cm3 /cm3 adsorbent, still lower than the desired target value of 263 cm3 /cm3 adsorbent (Li et al. 2016). Due to the superior surface and morphological characteristics such as exceptional surface areas, binding potential, and chemical functionalities, MXenes as state-ofthe-art adsorbents are expected to possess large gas sorption capabilities. For instance, on a theoretical basis, the surface area of Ti3 C2 Tx is around 496 m2 /g (Papadopoulou et al. 2020). Additionally, they possess extraordinary electric conductance (Liu et al. 2016). Lui and colleagues theoretically estimated that in the case of MXenes (Ti2 C), the adsorptive removal capability was 331.5 cm3 /g. This is significantly lower than the target value of 699 cm3 /g of sorbent. This adsorptive capacity might be due to physisorption in which the primary role might be played by oxygen-based chemical functionalities located on the surface of Ti2 C (Liu et al. 2016). It is also worthwhile to mention that practical investigations on determining the adsorptive removal of CH4 using MXenes are scant. However, experimental studies suggested that under MXenes have the capability for CH4 adsorption at high pressure and can conveniently liberate CH4 at relatively lower pressures (Liu et al. 2017). The adsorptive potential of Ti2 C can be exploited via its exfoliation using NH4 OH. It is extremely crucial as its thorough exfoliation can result in extraordinary surface areas and the corresponding adsorptive removal capability could be as high as approximately 1148 cm3 /cm3 which is roughly three times higher than the target value of 263 cm3 /cm3 of sorbent. Hence, on a theoretical basis, the adsorptive removal capabilities of MXenes for CH4 are way higher than those of traditional sorbents (i.e., activated carbon, zeolites, etc.).

2.3 Hydrogen Abatement For a futuristic perspective, H2 is an enormous energy source owing to its extraordinary energy content in comparison to other variants of conventional fuels. However, storing H2 in a safe manner is centric considering advances in H2 -based technology in the prominent fields of power storage and stationery and transportable power. However, the main limitation in practical utilization of H2 as an energy source is its low energy content based on volume since it is a gas under ambient conditions. Storing H2 is possible in both gas and liquid states. Storing H2 in pressurized cylinders typically requires pressure in the range of 35–70 MPa. Stored H2 under these elevated pressures and specifically for vehicular usages is considered as a serious safety hazard (Getty and Schlapbach 2009). In addition, the loading and distribution of H2 require even higher pressures. On the other hand, H2 can be stored in a liquid

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state at sub-freezing temperatures since it boils at 20 K and 1 MPa. Therefore, in the context of storing H2 under ambient conditions for industrial and commercial purposes, the adsorption process seems to be a feasible option at near-ambient conditions. It is noteworthy that as per the US Department of Energy, the targeted value of H2 adsorptive storage is 4.5 wt% which is equivalent to 0.030 kg H2 /L adsorbent (USDOE). Sorbents including metallic hydrides and organic liquids are generally effectual at elevated conditions of temperature and pressure. In most cases, the temperature is greater than 400 K and the pressure is in the range between 1 and 10 MPa. For commercial purposes, operations under these conditions are not viable. Moreover, in the case of conventional sorbents such as activated carbon, and MOFs, elevated pressures are required however, H2 sorption can effectively occur at lower temperatures. This superior characteristic is owing to their high pore diameter, less density, and morphological structure to yield exceptional surface areas. However, considering H2 storage, MXenes exhibited favorable results (i.e., excellent adsorptive removal capability under room conditions). This is crucial in storing and transporting H2 for commercially and industrially relevant utilization. The intrinsic morphological framework is an important aspect affecting the adsorptive storage of H2 (Sun et al. 2018). Hu and coworkers assessed the H2 adsorptive capability of various MXenes including Sc2 C, Ti2 C, and V2 C (Hu et al. 2013, 2014). It was found that molecules of H2 were adsorbed on top and bottom of the nano-sized layered structure of Ti2 C. The corresponding adsorptive storage capacity was 8.6 wt% H2 which was approximately two times the targeted value of 4.5 wt% (Hu et al. 2013). Three adsorptive removal mechanisms are established including physical adsorption, chemical adsorption, and Kubas binding. In the case of physical adsorption, the primary requirements are substantial surface area and favorable binding energy which typically is less than 0 eV for secure binding (Hu et al. 2013). It was found that under ambient conditions the binding energy for Ti2 C was high equivalent to 5.027 eV (i.e., low H2 binding strength). This was significantly lower than 0.109 eV and 0.272 eV obtained for Sc2 C and V2 C, respectively. On the other hand, it was reported that H2 storage contribution to the total adsorption (i.e., 8.6 wt%) for Kubas interlinkages and chemisorption was 3.4 wt% and 1.7 wt%, respectively. It is worth mentioning that the synchronized adsorptive H2 storage of chemical adsorption and Kubas binding was 5.1 wt% substantially higher than the targeted value of 4.5 wt% (Hu et al. 2013). Desorption investigations under ambient conditions suggested that H2 molecules which were adsorbed via chemical sorption were not conveniently released and physically sorbed H2 was not properly bound. Therefore, desorption was only successful for Kubas interlinkages which showed the capability to be sorbed and desorbed effectively. Although MXenes showed extraordinary potential for storing H2 still from a commercial utilization point of view, more research is required to explore and improve the physical adsorption abilities of MXenes.

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2.4 Other Gas Contaminants Except for the usage in elements related to energy like CH4 , CO2 , and H2 , there is also great potential for MXene utilization in the removal of pollutants of additional harmful elements like CO, NH3 , SO2 , NO, NO2 , H2 S, and VOCs (Li et al. 2018). Reportedly, the adsorption of other certain gases, that are toxic in nature using MXenes has received quite less attention, respectively, and various types of research are focused on MXene detection abilities toward these toxic gases. As a result, the effect of MXene traits on the capacity of adsorption of these gases is still to be discovered and described in the literature. Thus, this should be the aim of futuristic research. Nevertheless, we have compiled a summary of recent developments in this field. A significant point to be attended to is that the adsorption energy for harmful gases including NOx (−0.85 eV for NO on Sc2 CO2 ), SO2 (−0.65 eV on Sc2 CO2 (Ma et al. 2017; Yang et al. 2019)), and VOCs such as acetone (−0.77 eV on Ti3 C2 (OH)2 (Yuan et al. 2018)) are lesser than usually considered energy relevant compounds (Naqvi et al. 2020). MXenes can be customized to abate harmful gases via adsorption by emphasizing these promising Eads compared to the gases that are relatively less toxic. As a result, MXenes have a high capability for use in gas storage capacity and air decontamination materials in the upcoming advanced era. MXenes can also be studied to gain solving techniques for deteriorated air quality in smoggy cities or areas affected by emissions from wildfires. Particles, that is, tinier than 2.5 µm (PM2.5 ) comprising of tiny particles, liquid drops, and microbes [151] cause a bunch of respirational health problems (Merkel et al. 2010; Salthammer et al. 2018). A recent study used MXenes in the weight percentages between 0.005 and 0.080% and incorporated them with polyacrylonitrile (PAN) to develop electrospunbased air filtering devices in a revolutionary applicability of MXenes for air quality improvement (Gao et al. 2019). Utilizing atomic force microscopy, they concluded that MXenes incorporated filtering devices increased their interactivities with PM2.5 particles by threefold. The subsequent PAN/MXene filtering devices exhibited the ability to remove 99.7% of PM2.5 , far outperforming the approximately 80% efficiency acquired by a couple of commercial filtration systems. In addition, in the case of MXene/PAN-based filtering devices, the decline in pressure drop was low than those of commercial filtration systems. The nanofiber filter’s operation was maintained after 300 h of examination, emphasizing its stability. It was stated that typically van der Waals forces govern the interactions of PM2.5 and the PAN fibers, the addition of MXene comprising of polar chemical functionalities including O, OH, and F might augment the dipole–dipole and induced-dipole interactivities with PM2.5 particles (Gao et al. 2019). While this widely publicized discovery remains the lone use for PM2.5 filtration, MXenes appear ready to be used in airborne particle filtering.

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3 Cost Analysis Air purification systems, in addition, should preferably be affordable as well as carbon-negative. For example, using amine-containing standard CO2 adsorbing substances in direct capturing from air, prices for regenerating amine ranged between $95/ton of CO2 making use of natural gas-based electricity and $200/ton utilizing wind energy (Sanz-Pérez et al. 2016). In a cost–benefit study carried out by Merkel and colleagues, separation through the membrane was restricted by a pressure ratio of approximately ten between the feed and permeate side, demonstrating the significance of enhanced permeation of the membrane (Merkel et al. 2010). Nevertheless, expenditure for CO2 capture by the use of membranes typically depends on permeation and ranged between $32/ per ton at 1,000 gas permeation units to around $15 per ton at 4,000 gas permeation units. In accordance with CO2 permeance of 1,360 gas permeation units (Shamsabadi et al. 2019), the CO2 capturing expenditure by MXene-PEBAX-0.05 membrane is almost at $29 per ton of CO2 . Even though not yet used at a large scale or commercial level, utilizing MXenes may drastically decrease the expenditure of CO2 capturing from direct air. The cost–benefit studies in case of other harmful gases based on MXenes have not been published. The synthesis of MXenes is also a barrier to commercialization. Since the cost of graphite lies in the range of $430–20,000 per metric ton (Lee et al. 2019), for the synthesis of MXenes other substances such as HF acid that are dangerous are also required and their production at large scale becomes challenging (Mashtalir et al. 2013). As a result, further research is needed to ensure the safe development of MXenes and, eventually, to achieve regulated incorporation of chemical functionalities to deal with financial limitations. In a recent study, Shuck et al. exhibited the fabrication of reactor utilized to produce MXene. This addressed the threats related to its safe operation when mixing MAX powder and etchant (Shuck et al. 2020), a procedure which is being scaled up to produce MXenes in kilograms by Murata Manufacturing Co, Ltd (Shuck and Gogotsi 2020). Presently, the synthesis of MXene is not feasible at an industrial scale.

4 Membrane Longevity It is a significant effectiveness marker for the commercial use of MXenes in gas removal. MXene stability in ambient air conditions can rapidly decline. A number of observations presented in a relevant study were stated to enhance their strength for longer periods, but still do not seem to exhibit enough stability to rationalize their expensive and critical production. For utilizing MXene as a filling agent for manufacturing PEBAX membrane, “excellent and stable separation performance” was suggested by Liu et al., for experimentation time greater than 120 h (Liu et al. 2020). It was found that for a 15-day test, PEBAX-MXene membranes exhibited merely a 12% decline in permeation ability, though membranes retained performance after

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half a year of their storage under ambient conditions (Shamsabadi et al. 2019). Throughout experimentation at extreme temperatures (593 K), AAO-supported MXene membranes demonstrated uniform separating efficacy for a complete 200 h (Fan et al. 2019). Likewise, after a month of fabrication, the MXene nanofilm membranes indicated a small decline in permeation efficacy (Shen et al. 2018). Lastly, Ding and colleagues manufactured MXenes with a stable performance of approximately 700 h (Ding et al. 2018). Though stability in applications lasting over a hundred hours has been verified, this should be substantially extended before MXenes are viable for commercial applications. As a result, research in improving the lifetime of MXenes is critical for practical usage and should be considered for futuristic investigations.

5 MXene Reusability Initially, MXene was uncovered in 2011. Currently, they are in their developing stage. Most of the studies till now have emphasized their manufacture and use, but not their fate at the end of the life cycle. This critical manufacturing technique is quite significant to contemplate the probability to salvage spent MXene structural framework. A recent study adopted an attempt to re-create MXenes from the used-up constituents of a pulverized Ti3 C2 Tx they had created for treating polluted waters (employing methylene blue to show its adsorption capability) (Vakili et al. 2019). These researchers prepared a combination of spent MXene loaded with methylene blue and virgin aluminum powder. Then, a mechano-chemical (MC) restoration followed by etching was carried out to make nascent and fresh MXene. It was followed by X-ray diffraction (XRD) that generated comparable findings similar to that of pristine Ti3 C2 Tx , which suggested eminent re-development. Intriguingly, the authors suggested that their use of MC restoration without solvents constituted a “green” synthesis procedure, which is important for deeper understanding. Using a separate regeneration methodology by simplistic washing the existing MXene structure, Ghani and colleagues developed an MXene by the introduction of sodium ions into Ti3 C2 Tx to remove ciprofloxacin (CPX) (Ghani et al. 2021). Afterward, the regeneration of spent MXene constituting CPX was performed electrochemically: when immersed in a NaCl solution, the desorption of CPX was evident due to the concentration variation with the liquid-phase matrix. Then, electrochemical decaying of the aqueous CPX was performed to develop a concentration-based gradient. The corresponding regeneration was up to 99.7% efficient. This regeneration phenomenon of CPX was successful till five cycles. In every cycle, CPX was recovered to approximately 100%. After these five regenerative cycles, XRD displayed distortion of the Ti3 C2 Tx structural morphology with a simultaneous decline in a compositional constituent of titanium from 57 to 46 wt%. Any dangers or faults that may come from an interaction with these pollutants must be considered when selecting a regeneration, synthesis, and etching method. On the other hand, the concept of MXene regeneration is nearly as simple as its initial synthesis: spent MXenes are made up of MX+ impurities,

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hence adding “A” recreates the original MAX fabrication formula. The findings of the Ghani and coworkers regarding degradation convinced us to contemplate, though it is likely that the utilized substance survived five cycles without any decline in effectiveness. Another better way was extending the quantity of regeneration cycles and afterward, re-development should be done as per details given in Vakili et al. (2019).

6 Arguments for Potential Uses MXenes provide alternatives for reaching national carbon neutrality objectives by 2050. The separation of CO2 in the gaseous state from industrial sources and the storing of H2 , an alternate fuel with a reduced carbon emission than traditional gasoline, are the most viable. This evaluation looked at three CO2 separating applications: before combustion, after combustion, and directly capturing from air. MXenes have shown the best promise for before combustion CO2 collection. MXene membranes function similarly to typical membranes composed of graphene oxides or MOFs in terms of H2 /CO2 separation. However, with recent developments in super thin MXene membrane production, H2 permeance has grown considerably. MXenes may become a feasible choice for direct air capture, although they are currently more expensive than typical amine-based adsorbents. Improved MXene synthesis utilizing reagents such as borate, PEI, and PEBAX has significantly enhanced CO2 adsorptive ability to levels comparable to zeolites, MOFs, and carbon-based adsorbents, although additional progress is required for these new materials. MXenes were investigated for methane preservation since it is a lower-carbon fuel. Hypothetical studies revealed remarkable methane absorption capabilities surpassing 1100 cm3 /cm3 , much above the US DOE limit of 263 cm3 /cm3 . Conversely, there are few experiment-based trials, and extant studies suggest substantially low adsorption capabilities. Furthermore, another proposal that MXene abrasion might enhance the particular pore size dispersal and that advancements in abrasion could aid in achieving the promise. Keeping in view that currently, MXenes do not provide a suitable economic alternative for storing CH4 , however it can be expected soon in accordance with the interest of researchers in the economical utilization of MXenes. Likewise, MXenes carry the potential for hydrogen storage, another “green” energy. Another calculation projected that they might store H2 on both sides of the Ti2 C sheets, yielding 8.6 wt% storage (i.e., two times the DOE objective). However, due to the scant studies, MXenes are still not applicable for H2 storing on commercial scale. Rather, efforts should be directed on increasing particular surface area, presumably by enhanced MXene layer abrasion. Because the use of chemicals in hydrogen storage was less recorded than in methane storage, it is probable that comparable additions (borate, PEI, etc.) might increase storing H2 under an ambient environment. MXenes also have remarkable anti-bacterial capabilities, which have intriguing implications for incorporation into air purification procedures. However, there is, once again, a scarcity of literature on these findings. To summarize, scientific frameworks have projected that MXenes show considerable promise in a variety of gas

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separating and storing scenarios, but actual investigations, with a few outliers, have not yet accomplished this. The separation of CO2 before combustion is likely the most suitable use as of now, but we anticipate that the final contributions of MXenes will be in H2 storage in solid form. The safety advantages of storing H2 in sold form (as opposed to 70 MPa) can explain the high production cost of MXene-based storage in the future, which is now too costly for this application. MXenes are denser than zeolites or other adsorbent materials and possess large volume-based storing capacities. As a result, we anticipate that MXenes will first be used as ground-based hydrogen storage, such as in combined hydrogen-battery schemes. MXenes may also have a role in car hydrogen storage if the specific density is improved. This might be accomplished by experimenting with new etching methods or low-density fillers. Further issue is the absence of a clear path to commercially sized applications. Yet, most empirical results have been obtained on a laboratory scale. MXenes can be fragile when synthesized on their own, and the literature does not mention the production of huge MXene sheets. There is no understanding of scaling up from lab scale experimentations to cm3 -based CH4 adsorbing agents or CO2 separating monoliths for a blast furnace stack. Suitable alternatives such as covering a porous ground represent significant breakthroughs, but there is no clear road to large-scale manufacturing. It is crucial to account this, in combination with future breakthroughs for enhanced selectivity, for instance, possibly a polymer addition might increase efficiency suitable for commercial manufacturing.

7 Conclusions Limited experimentation-relevant literature is available regarding the practical utilization of MXenes and/or MXenes-based modified materials in storing and treating gases and gaseous contaminants. On the other hand, several theoretical evaluations highlighted the tremendous potential MXenes for futuristic practices in the domains of purifying air and storing gases. In the last decade or so, employing MXenes in specific gas purifying practices demonstrated their performance-based competitiveness. Whereas, in terms of economics, their potential practical usage is still not justified. However, scientists around the globe are in process of exploring the commercial viability of MXenes by investigating and improving the physicochemical characteristics suitable for storing and purifying gases. And it is highly expected that in the decade to come, there is a significant probability of employing MXenes with improved properties to capture H2 , CH4 , and CO2 on a commercial scale. In addition, MXenes can be substantially effective in abating harmful gases by selective permeation through modified membranes. Furthermore, MXenes can be modified via functionalization to impart excellent sorption potential. Moreover, their integration with other materials as fillers can be significant for a variety of environmental remediation-relevant purposes. In the context of large-scale applicability, they can be potent tools to abate PM2.5 and toxic gases and therefore, can be

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beneficial to improve urban air quality. Also, they can be utilized to treat and/or mitigate the toxic effects of smog and emissions from forest fires. Also, owing to their peculiar surficial and structural properties including O-based chemical functionalities, tremendous surface areas, and super-hydrophobicity, they provide substantial benefits such as relatively lower energy expenditures in storing and purifying gas in comparison with traditional sorbents such as MOFs. Mostly, studies on the MXenes are focused on evaluating their adsorptive removal capabilities and applicability as sensors which further warrant investigations on their cost justification and fabrication. Also, there is negligible data regarding the reuse and fate of already-used MXenes which needs exploration. In the last 10 years, MXenes have proved to be incredible materials with diverse applicability and the significant number of investigations provide evidence of their better performance as compared to traditional materials specifically relevant to gas storage and purification.

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Environmental Remediation of Heavy Metals Through MXene Composites Madeeha Batool and Hafiz Muhammad Junaid

Abstract Heavy metal pollution is a serious threat for life and therefore, the removal of these lethal entities is a burning issue of this silicon era. Although, a number of materials and chemical methods have been reported for their removal from various parts of environment but MXenes are up-and-comers in this field. MXenes belong to an emerging class of 2D inorganic layered compounds, which are composed of dmetal carbides, nitride and carbonitrides and are arranged in a few atom thick layered structures. MXenes have major applications in energy storage, super-capacitors, electrochemical sensing, photocatalysis and water splitting. They have gained considerable attention because of their miraculously large surface area, unique physical and chemical properties, chemical and thermal stability, high conductivity, hydrophilic surface and green nature which make them efficient adsorbents for heavy metal ions. Moreover, their variable valency as well as variable spin states of d-metals and surface moieties, i.e. hydroxyl (−OH) and oxygen (−O) impart metal ion adsorption properties to MXenes. Furthermore, heavy metal binding properties of MXenes can be tuned by introducing other electron donating groups, i.e. fluoride (−F), amino (−NH2 ), carboxyl (−COOH), etc., on their surface using various routes, i.e. chemical modification, composite formation or doping that further enhance their affinities toward metal ions. All these properties of MXenes make them efficient materials for environmental remedies of heavy metals. The proposed chapter is focused on the importance, application, mechanism and drawbacks of these materials in environmental remediation of heavy metals. Keywords MXenes · Heavy toxic metals · Inorganic sorbents · Environmental pollution · Mitigation of heavy metals

M. Batool (B) · H. M. Junaid Centre for Analytical Chemistry, School of Chemistry, University of the Punjab, Lahore 54590, Pakistan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_14

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1 Introduction Nanomaterials have played a great role in the remediation of environmental pollutants (Rizwan et al. 2022a, b, c, d; Shakeel et al. 2022; Rasheed et al. 2021a, b). Since their emergence in 2011 (Kim et al. 2019), MXenes have revolutionized the studies of physicists and chemists both theoretically and practically (Zhang et al. 2018) in the field of Adsorption (Jun et al. 2020a), Green Chemistry (Zhu 2017), Catalysis (Ling et al. 2016), Sensing (Zhu 2017), Photocatalysis (An et al. 2018), Polymerization, Energy storage (Ng et al. 2017), Electronics (Kim et al. 2019), Fillers in composites (Tu et al. 2018), purifiers (Zhang et al. 2018), hybrid nanocomposites (Satheeshkumar et al. 2016), Methane storage (Khazaei et al. 2014), Thermo-electric (Kumar and Schwingenschlögl 2016), Bio-medical (Chen et al. 2018) and Industry due to their marvelous physical, chemical, magnetic (Si et al. 2015) as well as optical (Lashgari et al. 2014) properties including their distinctive electronic structure, hydrophilic surface, large surface to volume ratios, chemical and thermal stability, high thermal and electrical conductivities, corrosion resistance and high mechanical efficiency (Ronchi et al. 2019). Chemically, MXenes can be defined as, “The two dimensional (2D) chemical entities composed of transitional metal carbides or nitrides.” These are represented by a general formula Mn+1 AXn (Khazaei et al. 2019). Where, M represents a d-metal belong to III-B, IV-B, V-B, VI-B and VII-B groups (usually Sc, Ti, V, Cr, Mn, Y, Zr, Nb, Mo, Hf and Ta), A represents the elements of group III-A or IV-A of elements of the periodic table (usually Al, Si, Ge and Sn) while X denotes for either C, N or both (Barsoum 2000). MXenes have been known for remediation of various environmentally concerned species, i.e. heavy metals, organic dyes, radio nuclides, gases, etc., by the process of adsorption (Zhang et al. 2018). Here we only discuss the adsorption of heavy metals by MXenes as discussion about other species is beyond the scope of this chapter.

2 Synthesis of MXenes As a thumb rule, the synthesis of any chemical species depends upon the strengths of bonds in precursor or reactant molecules which have to break. Similarly, the strengths of M–X and M–A bonds are critical in the formation of MXenes. Compared to M– X bonds, M–A bonds are weaker and can be broken easily (Khazaei et al. 2013, 2018; Zhou et al. 2017). Therefore, MXenes are synthesized by eliminating the Alayer from the original or parent MAX phase by the process of etching at room temperature. The acids containing aqueous fluorides are commonly etching agents for this purpose. Briefly, in this process the MAX phase powders and aqueous HF are stirred for a specific time at room temperature as a result of which strong bonds between MX-layer and A-layer are replaced with weak interactions with fluorides, oxygen or hydroxyl groups. After the stirring, the contents are centrifuged or filtered

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to separate the supernatant from solid followed by subsequent washings of solid with deionized or distilled water so that the pH of mixture reaches a value in range of 4–6. The formation of MXenes Ti3 C2 (OH)2 and Ti3 C2 F2 by the aqueous HF treatment of MAX phase Ti3 AlC2 can be summarized as (Naguib et al. 2011): Ti3 AlC2 + 3HF → Ti3 C2 + AlF3 + 3/2 H2 Ti3 C2 + 2H2 O → Ti3 C2 (OH)2 + H2 Ti3 C2 + 2HF → Ti3 C2 F2 + H2 MXenes can also be synthesized by etching treatment of the MAX phase at elevated temperatures. A nitride-based MXene was produced by this technique. Al layer of Ti4 AlN3 powder was etched using a fused fluoride salt mixture containing LiF (29%), NaF (12%) and KF (59%). Further treatment with tetrabutylammonium hydroxide (TBAOH)\results in the formation of mono-layered Ti4 N3 Tx (Urbankowski et al. 2016). Some extremely high temperature etching procedures have also been reported, i.e. etching of In-layer Ti2 InC by sublimation of In-layers at a temperature of about 800 °C and removal of Si-layer from Ti3 SiC2 using fused cryolite at 960 °C (Barsoum et al. 1999, 2002). However, the resulting structures were not perfectly 2D. A diagrammatic representation of various steps involved in the synthesis of MXenes is shown in Fig. 1.

3 Structure of MXenes Entailed for Heavy Metal Removal The structure of an MXene is based on n + 1 layers of transition metal element (M) which surround n layers of X (C or N element) arranged in (MX)n M pattern (Zhang et al. 2018). Computational studies are usually employed to evaluate the structure of MXenes (Naguib et al. 2011). These simulations have suggested six possible basic structures of MXenes which have the potential to adsorb heavy metal atoms or ions. All these structures are diagrammatically present in Fig. 2. These are discussed below (Ronchi et al. 2019).

3.1 Mono M Elements In monomer M elements MXenes, the transition metal atoms are arranged in a hexagonal pattern in which X-atoms occupy octahedral sites. However, the pattern of arrangement of atoms can be varied with stoichiometric ratios. MXenes of this type are represented as M2 X, M3 X2 , M4 X3 , etc., depending upon layers, where with suffix shows a number of layers.

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Fig. 1 Different steps involved in the synthesis of MXenes (treatment of MAX phase with aq. HF for etching followed by sonication to get MXenes sheets). This figure has been modified from that drawn by Naguib et al. (2012). This has been reprinted with permission from Naguib et al. (2012)

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Fig. 2 General structures of various MXenes employed for the adsorption of heavy metals. Reprinted with permission from Ronchi et al. (2019). License # 5383041387325

3.2 Solid Solutions In this type, two different metal atoms (M/ and M// ) are present which randomly hold the M-sites. These are generally represented by the formula (M/ and M// )3 X2 . Examples of such type include (Ti,V)3 C2 and (Cr,V)3 C2 .

3.3 Ordered Out of Plane Double M Elements In such structures, two different types of transition metals are arranged layer by layer with out of plane order. In this case, one of the two metals forms central layers while other constitutes external layers. These structures are presented as o-(M/ 2/3 and M// 1/3 )3 X2 .

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3.4 Ordered in Plane Double M Elements In this arrangement, two different types of transition metals are settled layer by layer with in-plane order and arranged in a basal plane. These MXenes are characterized by i-(M/ 2/3 and M// 1/3 )3 X2 .

3.5 Vacancies Ordered During the process of etching or exfoliation, vacancies in an MXenes are arranged in an orderly manner.

3.6 Vacancies Randomly Distributed Sometimes, etching or exfoliation of MAX phase results in the distribution of vacancies in an MXene randomly.

4 Properties of MXenes Involved in Heavy Metal Adsorption Key properties of those MXenes which are being involved in heavy metal remediation are listed below.

4.1 Surface Functional Moieties Functional moieties present on the surface of any chemical entity are responsible for imparting specific physical and chemical properties to that particular chemical species which make them suitable for a specific application. As a result of the etching of MAXphase by treating it with aqueous fluoride containing acids, the surface of an MXene usually contains Fluoro (–F), hydroxyl (–OH) and other oxygen containing functional groups (Salim et al. 2019). The hydrophilicity, high metal conductivities and chemical affinities for various polar chemical species like heavy metals are due to these surface functional groups (Kim et al. 2019). Furthermore, the 2D structure imparts a large surface area which enables MXenes for better chemical interaction with other chemical species as well as adsorption applications.

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4.2 Electronic Structure Electronic distribution in an MXene’s structural unit depends on two factors, type of transition metal atoms and surface termination (Tang et al. 2018). Studies revealed that MXenes exhibit high metallic conductivities due to high electron density near the Fermi-level which is because of d-electrons of transition metal (Ronchi et al. 2019). However, surface functionalization affects the electronic distribution. DFT calculations indicate that the band gap increases with atomic number of d-metal (Sarikurt et al. 2018).

4.3 Electrical Properties The electrical behavior of any MXene is based on its electronic structure, surface functional moieties and stoichiometric ratios. Electrical conductivities of MXenes are almost equivalent to those of multi-layer graphene, i.e. resistivity ranges from 22 to 339  (Naguib et al. 2014). Moreover, it has been observed that their resistivity values increase with the increase of number of layers as well as the presence of functional moieties (Wang et al. 2016). However, the desired electrical conductivities or resistivities and other electrical properties can be imparted by modifying the surface functional groups and stoichiometric ratios of various elements in an MXene.

4.4 Mechanical Properties Mechanical properties of any material depend upon the bond strengths of those bonds which are exist among its constituent elements. MXenes are of great mechanical interest due to the high bond energies of M–C and M–N bonds (Ronchi et al. 2019). Simulation studies reveal that their elastic constant is about 2 times better than that of MAX phase as well as that of other 2D materials (Kurtoglu et al. 2012). Their higher bond stiffness is proof of their utilization as reinforcement in composites. Subsequently, their Young Modulus value decreases with the increase in number of layers (Zhang et al. 2018). Experimental data suggest that nitride-based MXenes possess higher values of Young Modulus compared to carbide-based MXenes (Zhang et al. 2018).

4.5 Magnetic Properties Number of MXenes, i.e. Ti4 C3 , Fe2 C, Ti3 CN, Cr2 C, Ti2 N, Ti3 N2 , Zr2 C and Zr3 C2 have been proposed to possess certain magnetic moments. But reported magnetic

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moments have been calculated computationally and there is no experimental data available to support magnetic studies (Ronchi et al. 2019).

4.6 Thermal Properties Thermal properties of MXenes have not been experimentally understood yet. However, computational studies such as simulation calculations have predicted high thermal conductivities and lower co-efficient of thermal expansions for MXenes with respect to other 2D materials (Ronchi et al. 2019).

4.7 Optical Properties MXenes have the ability to absorb EMR in UV and Visible regions, i.e. Ti3 C2 Tx absorbs in UV/Vis.-region in the range of 300–500 nm. In addition, it also shows an absorption band at 700–800 nm. First principle calculations suggest that the surface functionalities are responsible for the UV/Vis. absorption in these 2D compounds (Hantanasirisakul et al. 2016). Moreover, their UV/Vis. absorption properties make them useful in photovoltaic, photocatalytic and optoelectronic applications (Ronchi et al. 2019).

5 Structural Modifications in MXenes for Heavy Metal Uptake The structure of an MXene can be tunable to impart desirable properties for a specific application. These modifications can be done by number of ways. Various modifications in MXene’s structures which enable them to adsorb heavy metals ions efficiently are discussed below.

5.1 Intercalation A distinctive property of layered substances is to house the smaller ionic species in the lattice. This process of accommodation of smaller ions in the lattice of layered compounds is termed as intercalation (Mashtalir et al. 2013). In MXenes, intercalation enhances the interlayer spaces resulting in an increase in the interaction between surface moieties and metal ions to be adsorbed (Peng et al. 2014).

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5.2 Delamination The chemical intercalation of multilayered MXenes using large organic molecules such as Tetrabutylammonium hydroxide (TBAOH), Cetyltrimethylammonium bromide (CTAB), Stearyltrimethylammonium bromide (STAB), etc., is designated as delamination (Luo et al. 2017). Like intercalation, delamination also brings an increase in interlayer spacing of MXenes which causes improvement in the metal absorbing capacities.

5.3 Surface Modifications The oxygen-containing surface functionalities of MXenes can be covalently modified with small organic amines, epoxides, acid anhydrides and acid halides (Chen et al. 2020). The introduction of such groups on the surface of MXenes elevates the chances of complexation or redox reaction between an MXene and metal atom or ion.

5.4 Doping Desired properties can also be incorporated into an MXene by doping. It is believed that the doping of nitrogen in an MXene improves its coordination abilities to attract metal atoms.

5.5 Composite Formation Composites of Mxenes with various organic molecules, polymeric materials as well as many inorganic species and metallic nanoparticles have been reported for metal adsorption through complexation, oxidation–reduction reaction or weak van der Waal’s interactions. For example, Cu2+ adsorption by alginate/MXene/CoFe2 O4 (Ren et al. 2021) and Ti3 C2 Tx -PDOPA (Gan et al. 2020).

6 Heavy Metal Remediation by MXenes By definition, heavy metals are those which have densities greater than 5 g/cm3 (Ali and Khan 2018). These belong to the d-block of periodic table. Heavy metals are thermally and biologically non-degradable and produce adverse effects on biosphere, quality of life and human health (Dong et al. 2019). As a result of which their control

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is one of the hotcakes, these days. A large number of techniques, i.e. chemical precipitation, coagulation, ultra-filtration, reverse osmosis, electrodialysis, etc., are available for the remediation of heavy metals from the environment especially aquatic systems (Basu et al. 2017), however, these methods are costly, time-consuming and required skills to be performed in an accurate manner. Adsorption is one of the most effective techniques to remove these heavy metals efficiently. Subsequently, the remediation of environmentally concerned heavy metals by adsorption using MXenes is given below.

6.1 Remediation of Cr6+ Chromium (Cr) is widely used in various industrial fields, i.e. textile, dyes, pigments, preservation of wood, chrome plating and as anti-fouling agent. Naturally, in aqueous media chromium exists in two forms, i.e. Cr (VI) and Cr (III) which have different biological, environmental and chemical properties (Dakiky et al. 2002). The Cr (VI) is an extremely toxic and life-threatening species. As per WHO guidelines, its permissible limits in drinking water is 0.05 ppm (0.05 mg/L) and in industrial effluents is 0.5 ppm (0.5 mg/L). The presence of Cr (VI) in water above its permissible limits is a potential cause of chronic diseases such as carcinoma, hepatic abnormalities and renal failure. Therefore, it is obligatory to eradicate Cr (VI) from drinking water (if contaminated) and industrial waste. Several MXenes have been reported for the environmental removal of Cr6+ ions. An MXene composite based on Ti3 C2 Tx and reduced graphene oxide (Ti3 C2 Tx /RGO presented) has been used as Cr6+ adsorbent with a removal capacity of 84 mg/g. XRD revealed the accumulation of Cr6+ ions in interlayer spaces of composite material (Xie et al. 2019). The uptake capacity of Cr6+ has been improved to 194.87 mg/g by the introduction of Iron intercalated titanium carbide Ti3 C2 (Alk-Ti3 C2 ). The surface adsorption phenomenon is involved in the remediation of Cr6+ by Ti3 C2 (Alk-Ti3 C2 ) as evident from XRD, XPS and EDS studies (He et al. 2020). Similarly, the uptake capacity of hexavalent chromium has been further enhanced by applying Ti3 C2 Tx -based MXenes and (Ti3 C2 (OH)0.8 F1.2 )/TiO2 –C (u-RTC) composite to a value of 250 and 225 mg/g, respectively (Ying et al. 2015; Zou et al. 2016). Ying et al. have proposed that Cr(VI) ions reduce to Cr(III) by surface –OH moieties of Ti3 C2 Tx (Ying et al. 2015). A titanium carbide-based (Ti3 C2 Tx ) MXene functionalized with poly (m-phenylenediamine), i.e. Ti3 C2 Tx /PmPD has exhibited an exceptional removal capacity of 540.47 mg/g. studies suggested that this removal efficiency is due to electrostatic interactions between Cr6+ ions and functional moieties of MXene composite which leads to the reduction of Cr6+ to Cr3+ and chelation of Cr3+ with these functionalities (Jin et al. 2020). MXene-based composites αFe2 O3 /ZnFe2 O4 @Ti3 C2 and Ti3 C2 /TiO2 have also been presented for green removal of Cr (VI) in the form of chromate ions owing to a reduction of Cr6+ to Cr3+ as confirmed by cyclic-voltammetry (Zhang et al. 2020; Wang et al. 2020). Likewise, another MXene composite (Bi2 MoO6 )/Ti3 C2 has been studied for its Cr6+ adsorption

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properties by DFT which proposed a polarization charge distribution to attract Cr6+ ions (Zhao and Cai 2020).

6.2 Remediation of Pb2+ Lead (Pb) finds its extensive applications in various processes like printing, Pbplating, smelting procedures, battery engineering and industrial wastes. These all operations are contribute to Pb pollution in our environment. Pb is one of the most toxic substances. WHO’s permissible limit of Pb in drinking water is 0.01 mg/L. Its existence in water and soil is lethal as it becomes a part of food chain which poses the boundless danger to our lives (Liu et al. 2016). Pb is a carcinogenic metal which can also cause a number of nervous system abnormalities including amnesia, tediousness and migraine (Feleafel and Mirdad 2013). Therefore lead remediation is compulsory in our environment. A morphologically nano-sheet structured MXene “Etched Ti3 AlC2 (e-TACSs)” has been presented for the removal of Pb2+ ions with an uptake capacity of 218.3 mg/g (Gu et al. 2018). Similarly, it has been observed that the uptake capacity could be further enhanced to 285.9 mg/g if the morphology of MXene changed to a nano-fibrillike structure, i.e. Etched Ti3 AlC2 (e-TACFs) (Gu et al. 2018). Likewise, another nano-fibril MXene composite “TAC@titanates (T-NTO)” has been devised for Pb2+ uptake having an uptake capacity of 328.9 mg/g (Gu et al. 2019). DFT calculations and XPS analysis have recommended a cation exchange and surface complexation mechanisms for this Pb2+ adsorption (Gu et al. 2018, 2019). An alkalized intercalated MXene (Ti3 C2 (OH)x F2−x ) has been proposed for the environmental removal of lead (II). The adsorption of Pb2+ on Ti3 C2 (OH)x F2−x is due to the exchange of H+ with Pb2+ ions as suggested by DFT and first principle calculations (Guo et al. 2015). A similar MXene, i.e. Ti3 C2 (OH/ONa)x F2−x has also been reported as Pb2+ adsorbent but the adsorption mechanism is based on complexation phenomenon as evident by FT-IR data. Guo et al. have introduced an MXene based on Ti2 C1 for Pb2+ ions removal which exhibited a miraculously exceptional uptake capacity of 2560 mg/g. MD-simulation method confirms the adsorption of Pb2+ ions on MXene surface. Various MXenes reported for Pb2+ remediation are enlisted in Table.

6.3 Remediation of Cu2+ Copper (Cu) is one of the micro-nutrient, required in traces for normal functioning of human body. Moreover, it is frequently used in the number of industrial operations like electrical wires production, alloy formation, catalysis, etc. Due to its broad-scale industrial applications, ca. 35,000–90,000 metric tons Cu is annually discharged as waste, anthropogenically (Moore 2012). Such a high amount of Cu contaminates soil and water table and hence entered our food chain and poses threat to life. Its

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permissible amount in drinking water described by WHO is 1–2 mg/L, therefore, control of copper as an environmental pollutant is very crucial. A polymeric composite of Ti3 C2 Tx and levodopa (Ti3 C2 Tx -PDOPA) has been reported for the adsorption of Cu2+ in having an uptake ability of 65.126 mg/g (Gan et al. 2020). Similarly, delaminated Ti3 C2 Tx sheets have also been present for Cu2+ removal with a removal capacity of 78.45 mg/g. this involves the reduction of Cu2+ to Cu+ by DL-Ti3 C2 Tx (Shahzad et al. 2017). Likewise, another Ti3 C2 Tx -composites, i.e. His@TiO2 @d-Ti3 C2 Tx further elevated the uptake capacity of Cu2+ 95 mg/g as a result of CuO NPs formation by oxidation upon the interaction of MXene composite and Cu2+ ions (Elumalai et al. 2020). An MXene based on Ti2 C1 has been introduced as an excellent adsorbent for Cu(II) ions as it can remove up to 660 mg of Cu2+ per gram of adsorbent. MD-simulation studies confirmed the adsorption of Cu2+ ions on the surface of MXene (Guo et al. 2016a).

6.4 Remediation of Hg2+ Mercury (Hg) pollution have drawn the serious attention of researcher because its traces produce adverse effects on human life. Therefore it is considered as critically hazardous pollutant (Zabihi et al. 2010). The permissible concentration of Hg in drinking water proposed by WHO is 6 μg/L. As a result of which removal of Hg from drinking water is one of the burning issues of this era. Titanium carbide MXene core (Ti3 C2 Tx ) shell aerogel spheres (MX-SA) have adsorption affinities for mercuric ions with an adsorption uptake capacity of 932.84 mg/g (Shahzad et al. 2019a). Similarly, another MXene composite Ti3 C2 Tx /Fe2 O3 has also been presented for Hg2+ removal with an improved adsorption capacity of 1132.41 mg/g. The chemistry behind the mercuric adsorption involves oxygen-based functional moieties, i.e. –O− and –OH and the negatively charged surfaces of composite (Shahzad et al. 2018).

6.5 Remediation of Cd2+ Cadmium (Cd) is a hazardous d-metal which is commonly utilized in paints, pigments, electroplating and Ni–Cd batteries and causes environmental pollution. Direct exposure to Cd cause flu-like symptoms in human beings. In the long run, it can seriously damage the lungs, kidneys, liver, bones and also leads to malignancy and hypertension (Xu et al. 2014). In drinking water, its permissible limits suggested by US-EPA are 3 μg/L. Therefore, Cd removal is extremely critical as it is a serious threat to life even in minute quantities. Jun et al. have introduced an MXene based on Ti3 C2 Tx as an adsorbent of Cd2+ ions (Jun et al. 2020b). Nano-structured fibril and sheet-based MXenes (Alk-Ti2 Cfibr and Alk-Ti2 Csheet ) have been devised for the Cd2+ sequestration in an aqueous medium.

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These MXenes have shown a maximum uptake capacity of 325.89 mg/g for Cd2+ ions (Shahzad et al. 2019b). Interactions between Cd(II) ions and surface hydroxyl groups (–OH) are responsible for the adsorption of Cd2+ on the surface of MXene as evident from FT-IR and XPS studies (Jun et al. 2020b; Shahzad et al. 2019b).

6.6 Remediation of Miscellaneous Heavy Metal Ions MXene and reduced graphene oxide composite (Ti3 C2 Tx /RGO) have been reported for the green adsorption of Au3+ and Pd2+ with adsorption capacities of 1241 and 890 mg/g in addition to Cr6+ ions (Xie et al. 2019). An MXene based on Ti3 C2 Tx has the ability to adsorb Zn2+ ions due to interaction with –OH moieties present on the surface of MXene (Jun et al. 2020b). Another MXene composite Ti2 CTx /poly(diallyldimethylammonium chloride) (PDDA) has been known for its adsorption affinity for Re (VII) in the form of ReO4 − with a removal capacity of 363 mg/g. The reason for this adsorption is the reduction of Re(VII) to Re(IV) (Wang et al. 2019). A hydrated MXene, i.e. Ti2 CTx -hydrated has been devised for the efficient removal of radio-active Th4+ ions with an adsorption capacity of 213.2 mg/g. The results obtained from SEM–EDS, XRD and XPS suggest an inner sphere complexation of Th4+ with surface hydroxyl functionalities (Li et al. 2019). The adsorption parameters of various heavy metals by MXenes are summarized in Table 1.

7 Mechanism of Adsorption Various heavy metal ions interact with MXenes in different ways. Some prominent mechanisms through which heavy metals adsorb on the surface of MXenes are explained below.

7.1 Inner-Sphere Complexation An inner sphere complex is the one in which there is a direct bond exists between the adsorbent surface and adsorbate (Rahnemaie et al. 2006). Upon interaction of Ti3 C2 (OH/ONa)x F2−x with Pb2+ , the peak of Ti–O peak in IRspectrum splits into two peaks at 653 and 618 cm−1 corresponding to Pb–O and Ti–O. This peak splitting suggests a strong interaction between Ti–O and Pb2+ ions which results in the formation of an inner-sphere complex (Peng et al. 2014). Similarly, Shahzad et al. also propose an inner sphere complex formation phenomenon for the removal of Cd2+ using Alk-Ti2 Cfibr and Alk-Ti2 Csheet (Shahzad et al. 2019b). Figure 3

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Table 1 Adsorption of heavy metals by MXenes MXene

Heavy metal

Adsorption capacity (mg/g)

Reference

Ti3 C2 Tx

Cr6+ as Cr2 O7 2−

250

Ying et al. (2015)

(Ti3 C2 (OH)0.8 F1.2 )/TiO2 –C (u-RTC) composite

Cr6+

225

Zou et al. (2016)

(Bi2 MoO6 )/Ti3 C2

Cr6+

Ti3 C2 Tx /PmPD

Cr6+

α-Fe2 O3 /ZnFe2 O4 @Ti3 C2

Cr6+

as Cr2 O7 2−

Zhang et al. (2020)

Ti3 C2 /TiO2

Cr6+ as Cr2 O7 2

Wang et al. (2020)

Iron intercalated titanium carbide Ti3 C2 (Alk-Ti3 C2 )

Cr6+

Ti3 C2 Tx /RGO

Cr6+

84

Xie et al. (2019)

Ti3 C2 Tx /RGO

Pd2+

890

Xie et al. (2019)

Ti3 C2 Tx /RGO

Au3+

1241

Xie et al. (2019)

Ti2 C1

Pb2+

2560

Guo et al. (2016a)

Ti2 C1

Cu2+

660

Guo et al. (2016a)

TAC@titanates (T-NTO)

Pb2+

328.9

Gu et al. (2019)

Etched Ti3 AlC2 (e-TACFs)

Pb2+

285.9

Gu et al. (2018)

Etched Ti3 AlC2 (e-TACSs)

Pb2+

218.3

Gu et al. (2018)

Ti2 C(OH)2

Pb2+

Guo et al. (2016b)

Ti3 C2 (OH/ONa)x F2−x

Pb2+

Peng et al. (2014)

Ti3 C2 (OH)x F2−x

Pb2+

Guo et al. (2015)

Ti3 C2 Tx

Pb2+ , Cu2+ , Zn2+ , Cd2+

Jun et al. (2020b)

His@TiO2 @d-Ti3 C2 Tx

Cu2+

95

Elumalai et al. (2020)

Delaminated (DL)-Ti3 C2 Tx

Cu2+

78.45

Shahzad et al. (2017)

Ti2 CTx /poly(diallyldimethylammonium chloride) (PDDA)

ReO4 −

363

Wang et al. (2019)

Alk-Ti2 Cfibr and Alk-Ti2 Csheet

Cd2+

325.89

Shahzad et al. (2019b)

Zhao and Cai (2020) 540.47

194.87

Jin et al. (2020)

He et al. (2020)

(continued)

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Table 1 (continued) MXene

Heavy metal

Adsorption capacity (mg/g)

Reference

Ti3 C2 Tx -PDOPA

Cu2+

65.126

Gan et al. (2020)

Ti2 CTx -hydrated

Th4+

213.2

Li et al. (2019)

Titanium carbide MXene core (Ti3 C2 Tx ) shell aerogel spheres (MX-SA)

Hg2+

932.84

Shahzad et al. (2019a)

MXene-derived titanate

Eu3+

Ti3 C2 Tx /Fe2 O3

Hg2+

Zhang et al. (2019) 1128.41

Shahzad et al. (2018)

Fig. 3 General representation of an inner-sphere complex formation upon the interaction of an MXene with a metal ion

represents the formation of an inner-sphere complex between a metal ion (Mn+ ) and an MXene.

7.2 Ion-Exchange Adsorption of a metal ion on the surface of MXenes may also occur due to the ion-exchange phenomenon. For example, adsorption of Pb2+ on the surface of TAC@titanates (T-NTO) follows an ion-exchange mechanism. The FT-IR spectrum of T-NTO exhibits two characteristic peaks at 886 and 917 cm−1 for Ti–O–Na, which merge upon the interaction of MXene with lead (II) ions. This merger of peaks and its shift to lower intensity is an evidence of exchange of Pb2+ ions with Na+ ions (Gu et al. 2019). Subsequently, DFT calculations also reveal that the uptake of Pb2+ by Etched Ti3 AlC2 Nanofibers and Nanosheets is also due to the exchange of surface Na+ ions of MXene with Pb2+ ions (Gu et al. 2018). This ion-exchange process is summarized in Fig. 4.

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Fig. 4 General representation of ion-exchange mechanism for the adsorption of metal ions on the surface of an MXene

7.3 Redox Reaction Adsorption of various chemical as well as bio-chemical species on the surface of adsorbent depends upon the oxidation–reduction reactions between adsorbent and adsorbate. Adsorption of metal ions by MXenes also involves such reactions. Interaction of His@TiO2 @d-Ti3 C2 Tx with Cu2+ ions results in precipitation. The UV/Vis./NIR data reveals a blue shift from 785 to 724 nm which is due to the oxidation of Cu2+ ions to copper oxide (Elumalai et al. 2020). Similarly, XPS data advocates that Cr6+ has been reduced to Cr3+ upon an interface with Ti3 C2 /TiO2 which is adsorbed on the surface of Ti3 C2 /TiO2 (Wang et al. 2020).

7.4 Multiple Chemical Combinations Jin et al. (2020) have described a mechanism for the removal of Cr (VI) using an MXene composite Ti3 C2 Tx /PmPD which involves multiple chemical combinations. According to FT-IR and XPS studies, Cr6+ ions interact electrostatically with protonated Ti3 C2 Tx /PmPD, resulting in the reduction of Cr (VI) into trivalent chromium which adsorbs on the surface of composite by complex formation.

8 Conclusion MXenes are emerging layered chemical compounds and became the apple of researcher’s eye since their discovery due to their unique electronic structure, electronic distribution, chemical reactivity, mechanical and thermal stability, hydrophilicity and high conductivity. Moreover, their large surface area and tunability of interlayer spacing as well as surface chemistry make them a rising star for the adsorption of heavy metals. Various MXenes were discussed in this chapter for their promising toxic heavy metals removing properties. However, there is still room to work with MXenes for the remediation of heavy metals as MXenes could be a complete package for this purpose. Surface properties of MXenes suggest that these could be used for the sensing of heavy metals along with their adsorption. In near the

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sensing and removal of heavy metals by MXenes may become a complete solution for the environmental remediation of hazardous heavy metals.

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Advanced Approach of MXene-Based Materials in Removal of Radionuclides Abdul Rauf, Mashhood Urfi, Zaeem Bin Babar, Saeed ur Rehman, Shahid Munir, and Komal Rizwan

Abstract MXenes are an incredibly interesting group of 2D layered transition metal carbides and/or nitrides that resemble graphene. They have great specific surface area, wide interlayered distance hydrophilic nature, and an abundance of significantly active chemical functionalities. These properties have allowed them to find applications in environmental remediation for the effective removal of dyes, heavy metals, and radioactive nuclides. They possess extraordinary resistance to deterioration by radiative action and exhibit outstanding thermal stability as well as high chemical compatibility because of their flexible nature in terms of chemical compositional surface engineering. Keeping economic and technological aspects in mind, they are known to possess the capability to perform as super adsorbents for various radionuclides such as uranium (238 U), strontium (90 Sr), barium (133 Ba and 140 Ba), thorium (232 Th), palladium (105 Pd and 107 Pd), and cesium (137 Cs). This chapter thoroughly analyzes current research on MXenes’ intriguing potential as an adsorbent for the environmental abatement of radioactive elements. It also highlights the current challenges being experienced when using these substances for the adsorptive cleanup of radionuclides and outlines their potential future utilization. Keywords Radionuclides · MXenes · Adsorption · Metals · Treatment · Removal

A. Rauf · M. Urfi · S. Munir Institute of Energy and Environmental Engineering, University of the Punjab, Lahore, Pakistan Z. B. Babar Institute of Environmental Sciences and Engineering (IESE), School of Civil and Environmental Engineering (SCEE), National University of Sciences and Technology (NUST), Islamabad, Pakistan S. Rehman High Temperature Energy Conversion Laboratory, Korea Institute of Energy Research, Daejeon 34129, Republic of Korea K. Rizwan (B) Department of Chemistry, University of Sahiwal, Sahiwal 57000, Pakistan e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_15

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1 Introduction The global resource demands have been increasing with population growth. In these circumstances, energy requirements are high and greatly pose threats to environmental conditions (Rasheed et al. 2020, 2021; Rizwan et al. 2022b; Shakeel et al. 2022). From the initial combustion of fuels to the disposal and management of waste, the whole procedure of energy extraction has become problematic for the authorities, industrialists, and environmentalists. Since, traditional fossil fuels are neither very abundant nor eco-friendly, nuclear power plays an important role in supplying energy demands. Although nuclear energy does not involve carbon and thus is sometimes categorized as safe and green energy for the global environment, it causes a unique environmental problem. Due to intensive operations and usage of nuclear energy, a substantial quantity of radionuclides having long durations of half-life are released into the environment. These highly mobile radionuclides include Barium (Ba), Uranium (U), Thorium (Th), Cesium (Cs), and Palladium (Pd) (Fig. 1). They constitute a long-term hazard to the habitats above and below the surface of the earth’s crust, as well as to human health, causing neurological diseases, birth deformities, genetic disorders, infertility, and numerous forms of cancer in various organs (Hwang et al. 2020). Consequently, we conclude that we must be careful with radioactive waste at every point, throughout the life cycle, and it is crucial to develop novel green adsorbents to remove radionuclides from the environment in an effective way. Among all the removal techniques including electrochemical methods, biological treatments, active oxidation, etc., adsorption is the most relevant being efficient, easy-to-use, easily available, and financially affordable (Rasheed et al. 2020, 2021). Although after extensive experiments and studies, researchers were convinced on using two-dimensional nanocomposites as adsorbents due to their amazing physiochemical properties and surface area to volume ratios, they lagged due to weak van der Waal’s forces (Rizwan et al. 2022a). MXenes are highly stable nanomaterials and can be used for a variety of applications (Rasheed et al. 2022; Rizwan et al. 2022c). For the first time, it was in 2011 when MXene was reported as two-dimensional material with outstanding properties for adsorption. Recently, MXenes have gained attention as an interesting group having layered 2D structures like graphene with hydrophilicity, a large number of active surface sites, and appreciable spacing between the layers. Due to these properties, they have a scope for ecological rectification because of their ability to separate dyes, heavy metals, and radionuclides. MXenes are chemically defined as Mn+1 Xn Tx (n = 1, 2, or 3), where M indicates d-block transition metals for example, Sc, Ti, V, Cr, etc. Furthermore, X denotes carbon (C) and/or nitrogen (N), and Tx designates surface groups such as –O and –OH (Alhabeb et al. 2017). In addition to compositional flexibility, MXenes can also withstand harmful radiations and have high thermal stability. Therefore, they are subjected to be used as adsorbents for a large variety of radionuclides with respect to economic and technological efficiencies (Hwang et al. 2020). In this chapter, we shall discuss the utilization and reusability of MXenes for the removal/separation of the radionuclides, as mentioned

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Fig. 1 Mxene-based materials for removal of radionuclides through different approaches

earlier as Barium, Uranium, Thorium, Cesium, and Palladium. Mxene-based materials remove radionuclides through ion exchange, physical adsorption, and chemical adsorption processes (Fig. 1).

2 Application of MXenes for Adsorptive Removal of Radionuclides 2.1 Removal of Cesium (Cs) The remediation of nuclear waste comprises the complete separation of half-life radionuclide components such as cesium. Keeping in mind the concerns with effective adsorbent utilization and large quantities of competing ions, selective separation of Cesium from nuclear effluent remains a difficulty. Radioactive cesium, 137 Cs is one of the most threatening species having a yield of fission as high as 6.09%. In addition to its volatility, high solubility, and gamma-ray emission, its interaction with water is also harmful since it can readily form compounds with other substances. Moreover, Cs has a life of 30.2 years (Chi et al. 2022). It is considered dangerous because of being carcinogenic and thus harmful to both human health and environment. To address these issues, efficient elimination of radioactive Cs ions from nuclear effluent is quite essential. Because of the facile operations, increased efficiency, low cost, and availability of many adsorbents, cesium removal by adsorptive technique has piqued the interest of many researchers. Because of the fast development of nuclear waste

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regulation, numerous organic/inorganic and organic/inorganic adsorbents and their components for radioactive retention have been developed. Furthermore, due to their poorer adsorptive effectiveness, weak selectivity, and slow dynamics of processes, the expanding utilization of adsorbing components has been limited. Nevertheless, efficient sequestration of cesium and other radionuclides has been remarkable with that of two-dimensional materials that comprise transition metal dichalcogenides, the moderate van Der Waals interaction defines them as displaying a strong adsorptive nature which tends to be of lesser adsorptive efficiency. Chi and the colleagues reduced the coverage of fluorine elements using alkali to modify MXene. This modified MXene was then used to remove Cs ions from the sample wastewater through adsorption. Their study, based on the Langmuir model, presented the exchange of ions between the positive ions of Cs and [Ti–O]− K+ /H+ . It also showed a combination of Cs and the oxygen-bearing groups [Ti–O] in Mxene by the formation of Cs2 O (Chi et al. 2022). Afterward, the MXene was also tested for its recyclability. It was found that the surficial sites had only partially adsorbed the Cesium ions. Thus, a value 0f 21% was obtained for the Cs+ removal. After treating with HCl solution, the adsorbent had achieved its initial capacity showing that adsorbed Cs ions can be alternated by H+ ions. The adsorbent was able to be used three times at almost full capacity after the regeneration treatment. Under 288 K and pH 7, the adsorbent showed 90% removal efficiency (Chi et al. 2022). MXene (Ti3 C2 Tx ) was explored for its performance in the removal of radioactive cesium ions from model wastewater. The prepared MXene substantially contained the following elements: Ti, C, O, and F. It was achieved by a fabrication process in which hydrogen fluoride (HF) was used to etch an aluminium layer from the pulverized form of Ti3 AlC2 . The surface area found by Brunauer–Emmett–Teller method, using a porosimeter was 10 m2 g−1 . This MXene showed outstanding results in 60 min (Jun et al. 2020a). The adsorption capacity was found to be 148 mg g−1 , when concentrations were kept at 5 and 2 mg L−1 and the pH was kept neutral at 7. Discussing about recyclability, Ti3 C2 Tx can give almost four cycles. Not just practically logical but MXenes are also cost-efficient to be used to treat wastewater in nuclear industries/power plants (Jun et al. 2020b). Rethinasabapathy and colleagues prepared a complex stack structure of aminopropyl-Isobutyl polyhedral oligomeric silsesquioxane (POSS-NH2 ) intercalated titanium carbide (Ti3 C2 Tx ) MXene material (Ti3 C2 Tx /POSS-NH2 ). Postintercalation technique was used for its formation targeted to achieve Cesium sequestration. The unparallel and extraordinary adsorption capacity was shown by Ti3 C2 Tx /POSS-NH2 for Cs ions noted as 148 mg g−1 . Since the equilibrium was achieved within 30 min, it was found that the reaction had quite a fast adsorption kinetics. This MXene material could be used three times by recycling while exhibiting 89% selection efficiency (Rethinasabapathy et al. 2021). The adsorptive mechanism seemingly was dependent upon encapsulation, electrostatic interactions, and formation of ion exchange complex. This study shows a strong affinity C–Ti–O compound for Cs ions. Moreover, the presence of hydroxyl also plays a crucial part being hexagonal traps as well as sites for ion exchange. Considering the studies, we can evaluate the performance of various types of MXene based on certain factors and

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Table 1 MXenes-based adsorbents for adsorptive removal of radioactive cesium (Cs+ ) Adsorbent

Radionuclide Adsorption mechanisms

[Ti–O]− K+ /H+

Cs+

Ion exchange 7.0

–

Chi et al. (2022)

Ti3 C2 Tx

Cs+

Electrostatic 6.0 attraction, ion exchange

148

Chi et al. (2022)

Ti3 C2 Tx /POSS-NH2 Cs+

pH

Adsorption Reference capacity (mg g–1 )

Electrostatic 5.0–11.0 148 attraction, ion exchange

Rethinasabapathy et al. (2021)

varying conditions. Table 1 presents adsorption potential of various MXene-based materials for the removal of radioactive cesium ions.

2.2 Removal of Palladium (Pd) Radioactive palladium, 107 Pd is a product of fission reaction mostly and usually from high level radioactive waste. The half-life of palladium is found to be 6.55 × 106 years (Hwang et al. 2020). Being a metal with high versatility, it has utilization and application in various industries like ornamental and electrical. The demand and need for this metal are continuously rising due to the speedy development of the electrocombination and automobile industries (Ahmaruzzaman 2022). Being carcinogenic of nuclear elements and heavy metals is now a well-known and well-recognized fact. Therefore, the nuclear effluents from industries must separate anyhow from the mainstream of water to eliminate the contact and further possible threats. For this purpose, the process of vitrification is adopted but the presence of palladium serves as an obstacle. It possibly forms a distinctive layer causing vitrification to fail. Thus, it is necessary to remove palladium from within to decrease the pollution of aquatic elements. With this course of considerations, several materials, and components such as carbon black, chitosan, lignin, etc., have been used to separate Pd from the wastewater stream. However, the usage of these adsorbents was restricted due to the non-affordability and less efficiency. Then, around the start of the second decade in the twenty-first century, the introduction of MXenes pulled a substantial attention of scientists and nuclear researchers (Alhabeb et al. 2017). They were found to have excellent hydrophilicity and facile availability and thus were at hand to be used in water treatment. Scientists treated Ti3 AlC2 MAX (MAX phases are layered, hexagonal carbides and nitrides having the formula Mn+1 AXn where n ranges from 1 to 3) against HF to develop a range of MXenes. They were named “MXene-25,” “MXene-35,” and “MXene-45”, respectively, after their specific surface area temperatures, i.e., 25°, 35°, and 45 °C. These were experimentally designed to remove Pd from a nitric acid solution (Mu et al. 2019). Although, every

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Table 2 Adsorptive potential of various MXenes-based adsorbents for treating radioactive palladium (Pd+2 ) Adsorbent

Radionuclide

Adsorption mechanisms

Mxene-45

Pd+2

MXene-35

Mxene-25

pH

Specific surface area (m2 g−1 )

Maximum adsorption potential (mg g–1 )

Reference

Ion exchange 0.1 mol L−1 in HNO3 HNO3 (pH 1)

76.42

184.56

Mu et al. (2019)

Pd+2

Ion exchange 0.1 mol L−1 in HNO3 HNO3 (pH 1)

65.42

163.82

Mu et al. (2019)

Pd+2

Ion exchange 0.1 mol L−1 in HNO3 HNO3 (pH 1)

19.78

118.86

Mu et al. (2019)

sample of MXene had a wide surface area but they were tested and compared for the effect of change in temperature. The values for adsorption capacities came out as 118.86, 163.82, and 184.56 mg g−1 . This revealed the one prepared at 45 °C as the best performing MXene. MXene-45, opposing the thermodynamic study according to which Pd adsorption is assisted at low temperatures, demonstrated five consecutive cycles of reusability without showing any noticeable decrease (Ahmaruzzaman 2022). Table 2 presents the adsorption potential of different MXene-based composites for adsorptive removal of radioactive palladium.

2.3 Removal of Barium Due to insufficient handling and dumping of radioactive waste, the most hazardous fission product that is frequently produced after nuclear accidents is barium 133 Ba which is present in considerable concentrations in radioactive liquid wastes (Fard et al. 2017; Mu et al. 2018). Barium isotopes which are highly radioactive in nature, are very harmful to the kidney, heart, and liver and cause breathing impairments (Fard et al. 2017). Therefore, capturing Ba2+ from the environmental field is a vital and pressing challenge (Jun et al. 2020b). A few techniques have been developed and put into use to remove barium from wastewater. Some of these methods include adsorption (Ghaemi et al. 2011), membrane filtration (Kayvani Fard et al. 2016b), precipitation (Kondash et al. 2013), and ion exchange (Mishra and Tiwary 1999). Some of these technologies have not been widely adopted and employed for the removal of barium from wastewater due to economic and operational restrictions, such as the formation of sludge, low efficiency, inapplicability, and high operating as well as capital cost. The precipitation procedure typically produces enormous amounts of low-density sludge that needs to be disposed off. Additionally, the process efficiency is decreased by the accumulation of precipitated hydroxides (Fu and Wang 2011;

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Kongsricharoern and Polprasert 1995). Because of the acidic environment, poisonous H2 S can evolve when sulfides are used as precipitating agents (Fu and Wang 2011). On the other hand, ion exchange technique uses synthetically produced resins that are mainly composed of organic and polymeric materials that make the process very costly. Furthermore, reduced efficiency of the process that is mainly caused by the presence of acid and fouling due to metals, grease, oils, and organic materials are the main drawbacks faced during the removal of barium from wastewater through ion exchange technique (Fu and Wang 2011; Kurniawan et al. 2006). Because there are so many pollutants in the generated water, membranes frequently foul and scale. Additionally, using acid media dramatically reduces the lifespan of membranes (Fu and Wang 2011; Kurniawan et al. 2006). On the other hand, adsorption procedures are among the most popular methods for the removal of pollutants because of their flexibility in design, high efficiency, reusability, low cost, and potential regeneration (Fu and Wang 2011). New organic and inorganic adsorbents have been discovered by researchers to extract barium from water. Examples include clays such as chloriteillite and kaolinite (Shahwan and Erten 2004) and natural dolomite (Ghaemi et al. 2011). To remove the barium from nuclear waste, there is a need for novel adsorbent materials with high capacity and efficiency of adsorption however, to address operational and environmental issues. This has motivated the researchers to explore carbon-based materials for the removal of heavy metals. Examples of such carbon-based materials include activated carbon, chitosan, fly ash, carbon nanotubes, and graphene derivatives (Ali et al. 2014; Kayvani Fard et al. 2016a). For the first time, a novel class of 2D nanomaterials also called MXenes which are developed to remove barium metal from nuclear wastewater effectively. These materials belong to a group of layered ternary carbides and nitrides also known as MAX phase. The standard chemical formula for these carbide and nitride materials is Mn+1 AXn , where n = 1, 2, or 3. M stands for an early transition metal (such as Ti, Cr, V, Nb, etc.), A is for an A-group element (such as Sn, Al, Si, In, etc.), and X can either be carbon or nitrogen. MXene compounds are presently being produced and studied for a variety of uses (Peng et al. 2014) because of their excellent chemical and structural stabilities, exceptional electrical conductivities, hydrophilic surfaces, and environmentally benign nature. Novoselov and colleagues reported extensive use of MXenes in a variety of applications (Novoselov et al. 2004), including optoelectronics (Aïssa et al. 2016), biological and antimicrobial activities (Rasool et al. 2016), supercapacitors and batteries (Kajiyama et al. 2016), chemical and biosensors (Yu et al. 2015), catalysts (Seh et al. 2016), solar cells (Koski and Cui 2013) and membranes for water treatment (Ren et al. 2015). For the first time, Fard and colleagues revealed the removal of barium using titanium(III) carbide(II) (Ti3 C2 TX ), which was produced by exfoliating and intercalating Ti3 AlC2 with HF (Fard et al. 2017). Barium ions showed a great adsorption potential of 9.3 mg g−1 (Table 3) according to the authors who proclaimed a 100% removal efficiency. With extremely high selectivity in comparison to other existing metals in the solution, they removed 90% of the barium in just 10 min. The investigations showed that surface terminal functional moieties –O, –OH (hydroxyl), and –F were responsible for the adsorption of Ba2+ ions on the MXene surface through physisorption

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Table 3 Sorption of radionuclide Ba2+ on titanium-based MXenes Adsorbent

Radionuclide

Adsorption mechanism

pH

Adsorption potential (mg g–1 )

Reference

Ti3 C2 Tx

Ba2+

Ion exchange, electrostatic interaction, and inner-sphere-complexation

7.0

180

Jun et al. (2020b)

Ion exchange with enhanced d-spacing

7.0

46.46

Mu et al. (2018)

Ion exchange through chemisorption

6.0–7.0

9.3

Fard et al. (2017)

and chemisorption as per the following reactions: Ba+2 + 2OH− → Ba(OH)2 Ba+2 + 2F− → Ba(F)2 Ti3 C2 Tx showed a very low adsorption potential (9.3 mg g–1 ) of Ba2+ ions, therefore its use for nuclear wastewater treatment is restricted. Mu and colleagues used the surface activation process and metal basic intercalation on Ti3 C2 Tx to boost its Ba2+ adsorption capability (Mu et al. 2018). The sodium (Na+ ) ions efficiently bonded into Ti3 C2 Tx ’s layered structure because of enhanced intercalation that greatly increases the c-lattice parameter of MXene (2.09 nm) and ultimately enhances the ability of functional groups to bind firmly on the surface of MXene layers which leads toward the high capacity of Ba2+ adsorption on MXene surface. Alkali-treated Ti3 C2 Tx (AlkTi3 C2 Tx ) was used and achieved 46.46 mg g−1 of Ba2+ ions adsorption capacity (Table 3). Hence, in order to boost the adsorption capability of MXene-based materials, NaOH activation treatment is a potential approach. Recent research has shown that 2D MXene (Ti3 C2 Tx ) may adsorb Ba2+ from wastewater (Jun et al. 2020b). According to their findings, Ba2+ ions were adsorbed onto the surface of Ti3 C2 Tx through electrostatic interaction, with exceptional adsorption potential (180 mg g−1 ) (Table 3). Additionally, they stated that chemical ion exchange, chemisorption, and inner complexation methods are used to adsorb Ba2+ , as shown by FTIR, XPS, kinetic, and isothermal analyses. More significantly, they showed that MXene-based materials can be recycled for up to four cycles, which is crucial for removing radionuclides from fracking liquid waste. Table 3 presents the adsorption potential of MXene-based composites for adsorptive removal of barium.

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2.4 Adsorptive Removal of Uranium The most crucial component of conventional nuclear fuel power plants is uranium which is highly radioactive having a half-life period of 4.47 × 109 years and is therefore extremely harmful to the environment and ecosystems. The highly mobile and readily dissolved uranyl ion (UO2 2+ ) that is produced by hexavalent uranium [U(VI)] has prompted scientists to develop a novel class of adsorbents to save the environment from the hazard of uranium (Yu et al. 2019). Many adsorbents, including MOF, activated carbon, COF, and graphene oxide, have been extensively studied in the past for aqueous U(VI) mitigation. According to density functional theory calculations, Zhang et al. were the first to propose that hydroxyl functionalized titanium carbide Ti3 C2 (OH)2 , with a sorption potential of 595.3 mg g−1 (Table 4), would be a suitable adsorbent for UO2 2+ ions (Zhang et al. 2016). They discovered that rather than protonated hydroxylated surfaces, the UO2 2+ ions preferentially interacted with the deprotonated O adsorption sites. Chemical and hydrogen bondings were the major influencers during these adsorption processes. Wang et al. (2016) reported that 2D multilayer V2 CTx nanosheets can adsorb uranium (U(VI)) with an adsorption potential of 174 mg g−1 , which is significantly greater than the adsorption capability of typical inorganic materials (Wang et al. 2016). They claimed that U(VI) capture was caused by the existence of heterogeneous adsorption sites in MXene, e.g., –F, –O, and –OH. Based on X-ray absorption and DFT calculations, the authors suggested that the adsorption of U(VI) in V2 C(OH)2 has a bidentate adsorption configuration with an adsorption potential of 536 mg g−1 (Table 4). To put it another way, the UO2 2+ ions prefer to create a bidentate inner-sphere complex with –OH groups available on V-sites. The OH moiety deprotonation after interacting with U(VI) indicates that an ion exchange mechanism supported the adsorption process. A simple method to increase the Ti3 C2 Tx interlayer space was proposed by Wang and colleagues and it involves the fabrication of pure and intercalated MXenes under hydrated circumstances (Wang et al. 2017). Radionuclide imprisoning technique was adopted by the authors and they achieved an exceptionally significant adsorption capacity improvement of Ti3 C2 Tx for radionuclide removal using a hydrated intercalation technique. They evaluated the possibility of encapsulating U(VI) between the layers of Ti3 C2 Tx by rationally controlling the interlayer space. When compared to the adsorption capacity of Ti3 C2 Tx -dry (26 mg g–1 ), the synthesized hydrated Ti3 C2 Tx MXenes coupled with DMSO (Ti3 C2 Tx -DMSO) shows improved results for U(VI) removal with adsorption capacity of 214 mg g–1 (Table 4). Using density functional theory and molecular dynamics calculations, Zhang and colleagues investigated hydroxylated vanadium carbide [V2 C(OH)2 ] nanosheet adsorption behavior toward UO2 2+ ions (Zhang et al. 2017). According to their findings, UO2 2+ may form stable bonds with hydroxylated MXenes with binding energies between 3.3 and 4.6 eV, indicating that MXenes might work well as UO2 2+ ions adsorbers. The formation of two U–O bonds with the hydroxylated V2 C MXene nanosheets gives a very strong interaction between U and O atoms at adsorption sites that leads toward a high adsorption capacity. Furthermore, high adsorption energy is produced because

Bidentate inner sphere complexation Ion exchange

– 4.5 ± 0.1

Addition of amidoxime functional groups

Hydrated intercalation strategy

Density functional theory computations

Density functional theory computations

Termination of V2 CTx with abundant –F, –O and –OH groups to increase heterogeneous sites

Ti3 C2 Tx –DMSO–hydrated

Hydroxylated titanium carbide [Ti3 C2 (OH)2 ]

V2 CTx

214

626

470

Adsorption capacity (mg g–1 )

174

536

Chemisorption, adsorption on 595.3 deprotonated O adsorption site upon hydroxylated (OH) surface, bidentate coordination, hydrogen bonding

Heterogeneous adsorption

5.0 ± 0.1 –

Bidentate chelation

5.0

Deprotonation ion exchange and bidentate binding

Amidoxime-functionalized Ti3 C2 Tx

3.0

Sorption–reduction immobilization

Adsorption mechanism

Ti2 CTx

pH

Strategy for adsorption

Adsorbent

Table 4 Titanium-based MXenes for the adsorptive removal of uranyl ions [U(VI)]

Wang et al. (2016)

Wang et al. (2016)

Zhang et al. (2016)

Wang et al. (2017)

Verger et al. (2019)

Wang et al. (2018)

Reference

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of the formation of –H bonds between the axial O atom and nearby H atoms. On the other hand, the F functional moiety may dramatically reduce V2 C nanosheets’ capacity to adsorb uranyl ions. At the adsorption site, U–F bonds were found weaker in comparison to the U–O while studying the impact of F termination on UO2 2+ ions adsorption capabilities of the V2 C nanosheets. This result suggested that applications involving UO2 2+ ions adsorption were less suited for F-terminated MXene. Despite titanium’s relatively low geographical occurrence, research has demonstrated that uranium is efficiently adsorbed on surfaces of titanium-rich minerals (Wang et al. 2018). In order to reduce U(VI) contents in environment, Wang and colleagues converted U(VI) to a less hazardous form U(IV) through sorption–reduction method by using Ti2 CTx , which has very good adsorption capacity of 470 mg g–1 at a pH of 3 (Wang et al. 2018). Their research showed that Ti2 CTx MXene could simultaneously adsorb and reduce U(VI) at a wide pH range and highlighted that Ti2 CTx MXene causes reduction-induced immobilization of U(VI), which is based on a pH-dependent reduction mechanism that may encourage usage of titanium incorporating materials for removal of other oxidized pollutants. For abatement of U(VI) from polluted environment, the selectivity of UO2 2+ ions was very limited in previous reports (Wang et al. 2017, 2018). Zhang and colleagues introduced the successful addition of amidoxime functional moieties on Ti3 C2 Tx MXene’s surface through grafting of diazonium salt to develop extremely stable amidoxime functionalized MXene nanosheets (F-TC) with high selectivity for UO2 2+ ions (Zhang et al. 2020a). In addition to amidoxime functional group, UO2 2+ ions are efficiently coordinated with Ti3 C2 Tx MXene in the form of stable chelate structure with high selectivity. This greatly increases their stability in aqueous solution to extract uranium more quickly and efficiently. Because of the MXene’s outstanding conductivity, authors showed that Ti3 C2 Tx nanosheets-loaded carbon cloth and functionalized with amidoxime gives an amazing adsorption capacity of 626 mg g–1 for uranium metal ions through an electric field application. Utilizing density functional theory results and calculations along with a large workload 6.2 eV, enhanced transportation ability of electrons and promising electrical conductivity of MXenes, Deng and colleagues synthesized Ti3 C2 /SrTiO3 material with hetero-structure through partially superficial oxidation of the Ti3 C2 , a multilayered precursor, based on basic hydrothermal crystallology (Deng et al. 2019). The synthesized Ti3 C2 /SrTiO3 material was successfully utilized as a photo-catalyst to remove U(IV) from aqueous solutions efficiently through a simple photocatalytic reduction. The findings illustrate that 77% of photo-catalytic UO2 2+ removal rate can be achieved by using just only 2 wt% Ti3 C2 /SrTiO3 , which is almost 38 times the results shown by SrTiO3 . The multilayered structure of Ti3 C2 helps in proper charge distribution and further prevents electrons recombination in the conduction band. This research demonstrated the enticing potential of creating doped perovskite oxide crystals with excellent sunlight responsiveness based on MXene Ti3 C2 for maximizing solar energy use.

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2.5 Adsorptive Removal of Thorium Nuclear energy is gaining popularity in fulfilling current energy demands due to its lower greenhouse gas emissions and better energy density. Further consideration of the utilization of 232 Th as a potentially fruitful material is necessary for the development of nuclear energy. Using 232 Th is significantly better than using 238 U, because it produces less long-lived minor actinides and can meet the world’s energy needs for a longer length of time (Arnold et al. 2014). According to reports, thorium is on average 3–4 times more common and available than uranium and coexists with rare earth elements in some ores (Lu et al. 2016). Therefore, the best 238 U substitute is thought to be thorium, which has the potential to be productive. Once it is enriched, it is highly dangerous and causes irreparable damage to the environment that eventually causes human health problems like cancer because of its highly radioactive nature that makes it life-threatening element even in trace amounts. Thus, thorium adsorption has long raised questions about the environment (Li et al. 2019; Zhang et al. 2020b). Therefore, the subject of thorium enrichment and separation is quite important. Modern research has focused a lot of interest on the MXene family of novel two-dimensional transition metal carbide/carbonitride/nitride materials. The creation of the new materials with the formula Mn+1 Xn Tx involves selectively etching A layers from the MAX phase, where A is mostly a group IIIA or IVA element, M stands for an early transition metal, and X is either C or N (Ghidiu et al. 2015; Naguib et al. 2011). Tx frequently denotes surface functional groups like O, OH, or F. MXenes have received much research due to their good electrical conductivity, high thermal stability, and great ion-intercalation capacity (Feng et al. 2018). These properties make them interesting materials for a range of applications, including energy storage, chemical sensing, and catalysis. They are also promising sorbents for a variety of molecular and ionic species due to their hydrophilic character and abundance of functional groups (Wei et al. 2018; Zhang et al. 2018). MXenes have been seen as the best candidates for use in nuclear wastewater disposal, in particular due to their excellent chemical compatibility with the harsh environment of molten salt and their capacity to withstand severe radiation (Mu et al. 2018; Wang et al. 2017). This knowledge serves as the driving force behind the importance of using Ti2 CTx MXene to capture tetravalent actinide Th(IV). Li and colleagues used lightweight material Ti2 CTx MXene for thorium(IV) removal from waste, taking into account the benefits of negatively charged surface, excellent dispersibility in water and multiple functional groups (Li et al. 2019). They used lithium salt method to synthesize Ti2 CTx MXene material under dry and hydrated conditions. Ti2 CTx MXene synthesized under dry conditions has low adsorption capacity as compared to Ti2 CTx MXene. This difference was because of the easy diffusion of Th(IV) ions into the large interlayer spaces between Ti2 CTx MXene material. A maximum of 213.2 mg g–1 of adsorption capacity was observed for 2-dimensional Ti2 CTx Mxene material synthesized under hydrated conditions. This extended value of adsorption rate is because of the strong binding that took place between Ti–OH and Th(IV). All these results represent that Ti2 CTx Mxene materials have high potential of removing

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Table 5 Sorption of radionuclide Th4+ on titanium-based MXenes Adsorbent Radio nuclide Mechanism of adsorption Ti3 C2 Tx

Ba2+

pH Adsorption capacity Reference (mg g–1 )

Ion exchange, 7.0 180 electrostatic attraction, and inner sphere complexation

Jun et al. (2020b)

and preconcentration of Th(IV). Table 5 presents adsorptive removal of Th4+ on titanium-based MXenes.

3 Conclusion and Outlook Among the most widely researched two-dimensional layered nanomaterials, the most promising materials are MXenes. They have major applications in environmental remediation, e.g., removal of the most hazardous radionuclide materials. The application section of this chapter describes the most recent advancements that have been carried out in MXene materials for the removal of radionuclides. From the above discussions, it can be concluded that MXenes have a very good capability of removing radionuclides and these materials have good performance as compared to any other two-dimensional conventional materials used for the removal of radionuclides. Although MXenes are thought to be very potential radionuclide adsorbents, but still, there are several obstacles to be overcome and numerous questions that need to be resolved before they can be used in real-world applications. (1) The number of MXenes that have been fabricated in the lab is small in comparison to the numerous MXene materials that have been forecasted by theoretical calculations. Apart from Ti3 C2 Tx , which is the sole contender that has been extensively explored for environment remediation applications for example heavy metals sequestration, radionuclide, and organic dyes adsorption; various MXenes are believed to be ideal for use as adsorbents. It is worthwhile to investigate and synthesize various kinds of MXenes with preferred physiochemical features, suitable functionalization, high absorbability, and stability in H2 O to advance environmental cleanup. Furthermore, to investigate MXenes as potential components for environmental redressals, the research should be focused on the fabrication of nonlayered carbides and hierarchical MXene structures having a high number of active metal components which is much more challenging. In order to enhance the environmental applications of MXenes, it would be better to study other structures of MXenes, e.g., nanocages and nanotubes instead of flat film structures. The factor that mainly affects the adsorption capacities of MXene is the active functional groups present at the surface of an

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(3)

(4)

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MXene. In contrast to actual synthesis, various kinds of modeling and simulation processes and theoretic calculations are mapped out assuming that the surface of MXene is ended up with uniform functional groups. These simulations, modeling and calculations are necessary to find out the best precursors and conditions to synthesize a MXene material with specific functional groups to remove a particular pollutant. For radioactive nuclides with hydrated ionic radii with larger d-spacing in between the MXene sheets, the adsorption process is restricted by wellstacked and small interlayer spaces between the MXene sheets. Through proper molecule cross-linking and adding a suitable intercalant, the interlayer spacing of MXene sheets can be controlled in a proper way to encapsulate the radionuclides with larger hydrated ionic radii that eventually increase the adsorption capacity of MXene. Delamination conditions and the type of functional group present at the surface of MXene are the major parameters that control the rate of adsorption of MXene. It is crucial to develop the latest techniques to generate functional groups with uniform properties because the MXenes obtained by synthetic approaches are sometimes randomly functionalized with more than one terminating group, which is a significant challenge. Surface metal atoms of non-terminated MXenes are more effective for the removal of radionuclides because of their extended reactive nature and it needs further experimental study with respect to adsorption applications. Hydrofluoric acid (HF) is most employed during the synthesis process of MXenes. Most of the time, resultant product includes fluorine in it. Thus, MXene synthesis with zero fluorine is another challenge that needs to be addressed. Topdown technique is the most used technique till date. In this technique, surface termination control is not so good. Therefore, bottom-up techniques need to be explored further. MXene obtained through the bottom-up technique has a specified structure and termination with very few defects. The long-term toxicity of MXenes and any possible environmental risks should be thoroughly examined considering the rapid consumption and expanding the availability of MXene materials. In order to broaden the scope of potential uses of MXene in environmental remediation, further research and development are required. MXenes manufacturing cost should be comparable to graphene and any other two-dimensional adsorbents in order to use MXene on industrial level to resolve environmental remediation problems. Thus, it is necessary to find less expensive phase precursors for MXene synthesis processes. Utilizing MXenes formed of naturally plentiful components is advised for reaching cost-effective environmental applications. In the near future, it is anticipated that MXene phases with improved hydrogen storage capability would be discovered. Under atmospheric conditions, these MXene phases have capability to store hydrogen reversibly. So, tritium should be the focus of research because it is a major part of nuclear waste that must be removed consistently in order to lower the expense of energy required for the process.

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(6) MXenes oxidize in water and disintegrate under various application circumstances. In order to assess them as an adsorbent, more thorough research on their stability for different solvents is essential. As compared to other pollutants, radionuclides are present in nuclear wastes in a very low concentrations, so MXenes must be designed in such a way that allows radionuclides to be selectively captured from the waste solution. It can be done by modifying the surface of MXenes with functional groups and manipulating the interlayer spacing by intercalation and delamination with the use of appropriate intercalates. Once adsorption is done, it’s very difficult to separate MXene from aqueous solution that limits its use in real time for the effective removal of radionuclides from large volumes of an aqueous solution. This aforementioned problem can best be resolved by using MXene-based composites containing magnetic nanostructures or adding MXenes to fibers.

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Functionalized Mxene Conjugates in Removal of Pharmaceuticals and Other Pollutants Muhammad Saad Fasih, Shahzad Maqsood Khan, Saba Zia, Nafisa Gull, and Tanveer A. Tabish

Abstract In today’s world there are a vast number of pharmaceutical industries which are working for humans and veterinary lifesaving therapies, but their by products are the major concerns. Despite many of their aquatic and smoked components are filtered, still our ecofriendly system is affecting and it is due to some major harmful products which cannot be cached and are released in the air and in water (oceans, rivers, etc.). In order to dispense the environment from toxic compounds we will regulate the two-dimensional transition metal carbides/nitrides material which will be a novel material for suiting the environment, these materials are known as MXenes. In delinquent with some unique properties of MXene (physical, electrical, optical, chemical), it avails an essential structure, biological compatibility, high surface area, and other physicochemical properties required to work as an efficient pollutant removal. Besides detecting the hazardous components in environmental matrix, the main advancements of MXene are to remove all those pollutants from the environment. The Mxene-based nanomaterial is further operated for the removal of bacteria, inorganic salts, organic waste, toxic metals, radionuclides, and many other contaminants. In this chapter, the credibility trends of Mxene nanostructure for the applicability and scalability is discussed. While moving through the outcomes of this chapter, it can be culminated that Mxene is the well next generation material for the water sustainability and can act as potential pollutant removal. Keywords Mxene · Pollutants · Pharmaceuticals

M. S. Fasih · S. M. Khan (B) · S. Zia · N. Gull Institute of Polymer and Textile Engineering, University of the Punjab, Quaid-e-Azam Campus, Lahore, Pakistan e-mail: [email protected] T. A. Tabish Radcliffe Department of Medicine, John Radcliffe Hospital, University of Oxford, Oxford OX3 9DU, UK © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_16

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1 Introduction A large number of industrial waste is discharged from the production units everywhere in the world. These wastes include many hazardous materials, but the major concern is that, if we sort out, the pharmaceutical wastes are more toxic than others. The reason behind is that these industries utilize synthetic raw material which cannot be absorbed directly or indirectly by our environment. During production, sometimes the improper use of such materials can cause the over efficient or inefficient process which become the basic reason to discharge these materials into the waste. Often accidental discharging of these materials is a major cause to emit these toxic wastes in the environment. Especially in Asian countries like Pakistan, India, and Bangladesh etc., the disposal of the discharged waste is settled by pharmaceutical companies. All of these wastes are destroying our ecosystem day by day. Engineers have set many of the scientific techniques to protect the environment from such sort of matter which include physical, chemical, and biological separation techniques for the recovery of wastes to remove toxics. The waste management should be aimed to protect the environment by the proper solution. Among the latest trends, MXene is becoming the trend to develop an active material for mitigating the novel materials from the waste (Kumar et al. 2022). MXene is owing some special and unique characteristics (physical, chemical, functional, etc.) and has a vast number of applications in a division of the industrial sectors. The family of the 2D material acquired by the graphene-based materials further produces metal oxides. The metal-based components of the carbides and nitride with some roughly distributed different types of the functional groups is studied with a better development (Jun et al. 2019). The metals obstruct these groups by their surface, using the formulae (Mn + 1 Xn Tx ) in which the transition metal is represented by M, the carbon or nitrides are showed by X and T promotes the functional groups like oxygen, fluorine, Hydroxyl, etc. whereas n is the integer (Jun et al. 2019). To reduce the edge of the contaminants from the ecosystem, MXenes are engineered candidates used to minimize the predominance from the environmental matrix (Zhang et al. 2021). During the last 10–15 years, a measureable increase is observed in the growth of the industrial sector, which means the expenditure of water is also increased. By experiencing the large increase in the growth ratio of the industrial products, it is clarified that the expenditure of the water is also increasing worldwide. But as the demand of the consuming water increases the concentration of the wastewater is also increased which is the major highlighting concern for every era. Water thus demands to be recycled and reused. In developed countries, the industrial wastewater is filtered and used in urban and rural areas for general purposes. Whereas, in underdeveloped countries, especially if we highlight the Asian countries, most of the industrial wastewater is released to the rural areas and forwarded to the farming fields. There is a need to prior the pharmaceutical sector to maintain the wastewater quality as it consists of the large amount of toxics and the defaulted materials which is hazardous for the environment (Cao et al. 2020). The emergent step assumed for the treatment of such toxicity of wastewater is use of MXenes nanoparticle which is a handsome choice for the advanced treatment for

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such queries. Besides some of the unique advantages of the MXenes like the hydrogen storage, good conductivity, and a handsome chemical stability, it also obliges some effective ecofriendly properties. Due to hydrophobicity and active functional groups on its surface, many of the molecular/ion species are manipulated by the MXenes due to its high groove adsorption which can also be used for the coverage of the environmental contamination or probably for the environmental sensing. In this chapter we will highlight one unique property of the MXenes to overcome the environmental remediation and structure engineered and fabrication method for the MXene. The properties that would be highlighted include optical, thermal, electrical, hydrophilic, mechanical, and all other general properties which are the reason to prefer these nanomaterials and how these properties help to summarize the applications of these MXenes. The main aim of the chapter is to highlight the applications of MXene and to explain the treatment of the pharmaceutical wastes that how the toxic pollutants, organic materials, antibodies, heavy metals, and other toxic materials are removed from the pharmaceutical waste by the MXene nanomaterials (Xia et al. 2018).

2 Synthesis Technique of the MXenes The fabrication of the MXenes depends upon the layer of the element grooved out at room temperature by the MAX phase or also known as the parent phase of the MXenes. The aqueous form of the hydrofluoric acid is mixed with the layer of the MAX phase which will be used in the powdered form. As a result of that mixing the layer present in the MAX is dissociated and the strong metallic bond is replaced by the weak bonds through elements A and MX layer particularly. There are some other functional groups like oxygen, fluorine, and hydroxyl group attached to the surface of ML MXenes by these weak interactions (Liu et al. 2020). Moreover, the solid is moved by the centrifugation and filtration process after making its washing from the deionized water whose pH would be almost 4–6 to maintain the isolated form of the supernatant, all this process is done to make the MXene having small layer less than 5 (Lopez-Pacheco et al. 2019). The pH of solutions pivots mainly the structure of the MXenes layer. If the pH of the solution decreases, the layers of the MXenes will break down. It means the acid like HF can be used for the etching of the A layers to make 2D MXenes. As shown in Fig. 1 generally Ti3 C2 Tx MXene is acquired by such type of case (Lopez-Pacheco et al. 2019). The additional part is etched in the non-Max precursor to fabricate the MXene e.g. the Ga is etched by the non MXene precursor that is Mo2 Ga2 C to form the MXene Mo2 CTx . Now in this case there would be two layers playing the role of “A” out of which one would be etched out same like that of Ga case and the other would remain in that MAX phase (Bilal et al. 2019). Another case in a similar way aluminum carbide (Al3 C3 ) layer is etched out from the Zr3 Al3 C5 whereas the aluminum (Al) remains to form MXene. The synthesis of the MXenes can also be done by the high-temperature etching process. Different era experienced the different methods of the synthesis of MXenes. For nitride-based MXene it was initially synthesized experimentally in 2016, with a molten fluoride

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mixture at 550 °C in an inert atmosphere. The Al layer in these conditions is etched from TiAlN3 powder and is forwarded to produce monolayers of Ti4 N3 Tx . MXene is delaminated through tetrabutylammonium hydroxide (Mohammadi et al. 2015). Another interesting fact is that some MXenes layers can be synthesized by sublimation at 800 °C in a vacuum. For example, the synthesis of TiCx can be done through the sublimation of the In from the Ti2 InC and Si layers which are sublimated through Ti3 SiC2 from molten cryolite at 960 °C (Cai et al. 2018a). Under all these certain conditions the 3D carbide can be acquired instead of 2D cubic. Inclosing with that, the metals treatment requires temperature below 800 °C. Related to their phase diagram, the metallic carbides are stable only below 800 °C which means that MXenes synthesis requires temperature lower than 800 °C or else the probability of 3D structure appearance will be absolutely acquired (Cai et al. 2018b). Many of the different methodologies are available like GaN and MoN of metal nitrides which are synthesized by the employment of the scalable salt-templated i.e. the technique of the migration-enhanced encapsulated growth technique. The ammoniated hexagonal oxides obliged to synthesize the nitrides like V2 N and W2 N with same procedure as of 2D nitrides. Urbankowski et al. in 2017 synthesized 2D V2 N and Mo2 N by making the V2 CTx and Mo2 CTx through ammoniation of the 2D precursors of the carbide. In this process the atoms of the C are replaced by the atoms of the N through the ammonization at the temperature of 600 °C. Eventually the 2D nitrides are obtained through the same method that is ammonization and shows the different structures in the finalized crystals (Pang et al. 2019; Fard et al. 2017).

Fig. 1 Pictorial view of the etching of the A layers through 3d models

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3 Structure Pattern of MXenes The modifications of modeling and the structure pattern play an important role to identify the structural properties of the MXenes. Fig. 2 presents the structure pattern of MXenes. Different functionalized surfaces are synthesized with functionalities like hydroxyl, oxygen, and fluorinated functional group with the function of etching the acid. The layered structure is in fact based on the modeling of the density functional theory (DFT) proposed by the first structure of that OH-terminated titanium carbide ML MXenes (Rasheed et al. 2020). While the data of the XRD gained from geometryoptimized and experimentally synthesized hydroxylated MXene was very close and in good accordance with each other. While the generation of the O, F, and OH functionalities is shown by the analysis of the XPS which shows that there is a mixed or incomplete termination present at their surface. There is also a probability of the intercalation of the water molecules into the layered structure of MXenes. After the etching of the HF acid, the analysis of the XRD pattern define the loss of crystallinity and the absence of Ti3 AlC2 MAX phase (Rashid et al. 2021). The hexagonal closely packed (HCP) structure of MXenes was observed mainly. Furthermore, with some different combinations, the sequence of the M atoms is also different. Same like the pattern of the HCP is exhibited by M in M2 X like that ABABAB. Whereas the face center cubic pattern was followed by the M3 X2 and M4 X3 like of that ABCABCABC (Guzzi et al. 2021). The arrangement done in bulk like molybdenum carbide and chromium carbide by the HCP pattern involves the M atom which is suitable for the MXenes. The DFT very firstly assembles the stability of the HCP pattern of carbide for the molybdenum in comparison to the counterparts in FCC (Huang et al. 2019). Although for some cases it was noticed from the energy point of view, due to the variation of the arrangements of the M atoms some MXenes are getting unstable. For the stabilization of their bond and for the production of the ordered double M element known as 2D carbides, titanium is inserted with the Carbon. In 2012, Enyashin and Ivanovskii studied and analyzed the structure of the free-standing and hydroxylated Ti2 C (OH)2 and Ti3 C2 (OH)2 . These were studied with three different configuration of the hydroxyl group that was with hollow space supposed to be A, situated at the top of carbon atom supposed to be B, or a mixture of these two configurations supposed to be C. It was observed that the configuration of the A was more stable with the free-standing and functionalized structure as compared to B and C. Due to most of the steric repulsion among T and C, the B configuration will be less stable and form the different functionalities the adoption forwarded by the configuration will be preferably low. However, by the ABCABCABC, the most stable configuration will be shown by the A and also as well as by the MX distinct position on T (Urbankowski et al. 2017).

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Fig. 2 Structure pattern of MXenes

4 Properties With some exceptional features of MXenes, it acquired some of the unique and precious properties like thermal conductivity, electric conductivity, Young’s modulus, hydrogen storage, hydrophilic behavior and tunable bandwidth, etc. The thermal conductivities enhance the hydrophilic nature of MXenes that makes them different from the other 2D materials like graphene. The features which are mainly associated with MXenes regarding their explosive performances are adjusted by the correlation of the composition, surface functionality, and the morphology/geometry of the structure (Ng et al. 2017).

4.1 Optical Properties The spectrum of the visible/UV light is essential for machines which run by some different optical properties of that photovoltaic, photo catalytic, transparent, and optically conducting electrode which makes them more capable to use. In the UV–visible region the films of the Ti3 C2 Tx might absorb light energy with a wavelength between 300 nm and 500 nm (Kumar and Schwingenschlogl 2016). Eventually 91% of the transmittance was recorded off the 10 nm film thickness. Furthermore, in order to depend upon the thickness of the film it can make the accumulation of the broad band of absorption and the strongest band of the absorption which is almost around 700– 800 nm which defines its enabling effective photo-thermal therapy usages with the

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formation of the pale green films (Lei et al. 2019a). It is observed that the percentage of the transmittance can be relatively optimized by assorting to its thickness and may also be with ion intercalation. In the case of the urea, hydrazine, and DMSO transmittance of Ti3 C2 Tx film decreased whereas tetramethyl compound of ammonium hydroxide presently used to make it more handsome for its accommodation from 75 to 92% (Wang 2016).

4.2 Mechanical Properties The stronger M–N and M–C bonds present in MXenes make a huge interest into its mechanical features. The study of the simulation has got priority which proposed the elastic parameters which are twice greater than that of the MAX systems as well as 2D components like cadmium sulfide (CdS2 ). Although they have 2–3 times lower elastic properties comparative to graphene, and their characteristics of bending (1050 GPa) are maximums which highlight their role as composite reinforcement materials for use (Elyassi et al. 2017). Comparable to the graphene, the MXenes have the great potential to interact with the polymeric material for composites formation of the advantaging owing to the involved functionalities. The thin discs of the MXenes which are titanium-based show the hydrophilic nature encroached between 25 and 40 definite contact angles such as contact angle of Ti3 C2 Tx is 35 (Quyen et al. 2021). Furthermore, the upcoming reports explained the supplementary gain in the layer numbers and decrement in the Young’s modulus of both MXene nitrides/carbides. While in comparison with the carbide MXenes, the nitride-based components have maximum values. The C11 parameter values are decreased severely by the obstruction of the terminations but also affect the increasing of the critical deformations. In comparison to graphene layers, these values of MXene are prone to be high which are considered as an excellent material for flexible-electronic devices. At present the mechanical existence encroached by the layers was established after a long direction. While at last, the negative binomial values determined for the W2 C and fortunately their applications were predicted in different sectors like as breakage resistance components applicable to the aircrafts and automobiles (Rajeshkannan et al. 2010). Although today the various techniques are available to characterize the bulk materials, the assessment of two-dimensional materials along with their mechanical features still remained a big challenge. The estimation was taken out by the nano indentation method for most of the experimental calculations, which engaged the application of force using AFM tip at the central point of 2D material. The experimental Young’s modulus of 333 to Ti3 C2 Tx monolayer which is closer to 386 GPa value of Ti3 C2 O2 but higher in comparison of GO and MoS2 is provided through this type of technique (Zhang et al. 2020). Despite the impacts of the mechanical estimation of MXenes, there are some difficulties in the measurement techniques of MXenes like the intrusion of geometric vacancies, interfaces among weak composite materials, and the deficiency of MXenes surface control. Although Mxenes acquire a unique mechanical property but still there is a need for the development of such

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methodologies to define the defects as well as the active functional groups and their components present and working in all areas. Still there is a need to find the complete estimate of the mechanical properties (Zhang et al. 2016).

4.3 Oxidative/Thermal Stability During the synthesis of the MXenes, the most important parameter which requires the efficient monitoring throughout the process is the stability of the presence of the water/oxygen. According to the old findings of the DFT, it is observed that components of the hydroxylase of the carbohydrates along with some random distribution of the carbon or nitride were settled to produce maximum stability for making thermodynamically stable product. According to practical exploration and pristine in MXenes, the oxidation of prolonged exposure to oxygen content decomposes the MXenes under the influence of the water medium using the higher temperature, besides all it is also affected by the exposure of the air by the UV irradiation. For example, the decomposition of the layered geometry caused by triggered nucleation of oxygen in the case of the Ti3 C2 Tx MXene aqueous solution with the condition of ambient air. Simultaneously, the precipitation of titanium oxide from black/greenish color to bluish white ensures that the MXene turns into titanium oxide on sheets of carbon material. Moreover, different studies show the anatase development of titanium oxide in TiO2 nanosheets at a maximum temperature of almost about 226.50 °C, which after converted to rutile titanium oxide at enhanced temperature. MXenes developed under argon (Ar) environment, can remain stable upto 1300 °C. Many of the several techniques for the stabilization are obliged like as the carbon nano-plating technique and excess energy mechanical milling procedure in dimethyl sulfoxide (DMSO) inhibited to hold the original framework and other features subjected in case of oxidizing environments. Generally, the stability of MXenes is improved through essential storage capability of MXene component in degassed oxygen-less water or dry air (Xiao et al. 2018). However, the rate of oxidation of the MXenes was influenced by the colloidal solution and the rate of reduction is influenced by possessing under the complete vacuum for storage and conserving in a refrigerator the dark maintained environment (Xiao et al. 2018).

4.4 Hydrophilic Properties MXenes exhibit a well-mannered hydrophilic property. It shows its wettability with different components but one of the most unique properties is the water contact angle. It was observed with the oxygen and fluorine terminal groups on MXene sheets. If we look forward for the hydrogen storage, then it acquires a great extent of absorption and desorption features for the hydrogen storage (Yuan et al. 2023).

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5 Applications The main field of the MXenes is environmental cleanup and the recovery of their resources. So, indeed, this is an ecofriendly product with a summary of novel adsorption phenomenon for the filtration of the water and the other wastes exhausted by industry. Its unique feature provides a vast application for the industrial and as well as the environmental sectors. Different industrial sectors like textile, printing, dying, paints, cosmetics, ordinance, pharmaceuticals, etc. have the handsome MXenes summary for either to remove their non-concerned components during the process or to filter out the waste material discharged by the industry. This may include many types of hazardous contents like nontoxic and toxic metals, organic wastes, radionuclide content, bacterial and antibiotic content, phenolics, etc. MXenes will be the preferred product for the environmental as well as for the industrial remediation due to its ridiculous behavior (Ma et al. 2022).

5.1 Application for the Removal of the Pharmaceutical Waste Today pharmaceutical industry is one of the main and important industries who serves the largest hazardous, toxic, and nontoxic waste to our environment. The pharmasector industry is the only one which has to work throughout the year. Especially if we review covid-19 period, the pharma was the only one industry which was working throughout the whole period. Therefore, there is a need to filter its waste to protect our environment and the MXenes are the main candidate for the treatment of waste to ecofriendly waste. Highlighting the pharmaceutical sector, there are two main types of waste which have to be focused: One is all that content present in solid and easy to access as a result of mishandling of the material, wrong formation of the product, rejected final products, etc. Many processes are available for their handling and can be maintained by the quality assurance. While another main part of the waste is the wastewater of the industry. Many of the pharmaceutical industries discharge many hazardous materials for the sake of the wastewater which require a stringent need of water treatment to protect the environment from those hazardous materials. The 95% of pharmaceutical waste is discharged out in the form of water (Xia et al. 2018).

5.2 Removal of Dyes The groups of the organic substance which are used in different industrial sectors are known as dyes. The main challenging aspect of the dyes which has enough complicated structure to degrade is the methylene blue which is approached by the hindering treatment which is photo catalysis based (Cai et al. 2018b). For the efficient sorption

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from the wastewater samples, the fabricated porous p-MXene/SWCNTs nanocomposite was used as a scalable and simple technique for the filtration of the organic dyes were recently referred by the Cao et al. (2021). The excellent efficiency showed by the displayed films of the p-MXene/SWCNT was of 1068.8 mg g−1 synthesized for the adsorption of the methylene blue. The glorious ability for recycling was noted up to the efficiency of 95.2% of the absorption after at least five numbers of repeated cycles. The vigorous hydrogen bonding and the effect of the electrostatic interaction were commanded by the adsorption of the high dye which assigns the considerable area of the MXene-based free-standing films. The adsorption was noted with the efficiency accommodating up to 168.93 mg g−1 of the methylene blue by using the oxidative dopamine hydrochloride as for the self-polymerization and the freeze-drying in the synthesis method of the functionalized cellulose/MXene bionano composite of aerogel. It is observed that after the five-time repeated consecutive cycles for the removal of the MB were efficient to 84% and the good stability was shown by the composite of the aerogel. The results of the SEM after the absorption of the dyes in the multiple cycles show the positive and enough good preservation for the interconnected 3D absorbed structure (Wang et al. 2018). The Langmuir isotherm model fits the accurate adsorption. The peak of the absorption, in many cases, was noted at 1593 cm−1 which may process the aromatic ring of the vibrating stretching methylene blue with the adsorption of the dye. The absorption of dyes regulates the electrostatic, hydrogen bonding and π–π interactions which are generally weak interactions. Highlighting the overall role of all the forces of the nanocomposite of the aerogel, most exclusive adsorption is forward by the aerogel. If we check out the high salted environmental conditions, the capability of adsorption of the salt containing water recommends for the use of an efficient dyes removal which is over almost 3% of NaCl. Another undesired common product which is the methyl orange with a lot of common uses can cause a lot of health issues like tissue necrosis, cyanosis, tachycardia, jaundice, and vomiting. In 2021 the excellent results of Ti3 C2 Tx MXene nanosheets for the adsorption through the aqueous matrix of the Cr4 and methyl orange were observed and reported by Karthikeyan et al. There was a handsome removing rate achieved rapidly of 94.8 and 104 mg/g respectively for the MO and Cr4 by the MXenes driven. The isotherm of the sorption was described very well by the isotherm of the Langmuir (Wang et al. 2019). The Ti3 C2 Tx MXene nanosheets have a great advantage of regeneration or recycle in different reactions. The nanosheets of the MXenes make the adsorption with different process complexion, the adsorption of the electrostatics, the ion exchanging, and the different interactions of the surface are mechanized for the commanding of MO and Cr4 (Wang et al. 2019).

5.3 Removal of Phenolics These are the highly toxic organic contaminants in industrial waste and announced as to be harmful pollutants by the US and European Union and need to prior to treat them before being discharged. In the extensive range of the industrial wastes, these

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are present in the lesser concentrations of few ppm range to few grams per liter. Their toxicity elements involve carcinogenetic, mutagenetic, and teratogenic which makes them enough harmful to affect the environment (Vakili et al. 2019). Due to all of these following elements, it is the foremost requirement to pay attention to filter out these phenolics from the waste stream of the industries. The photo catalytic degradation of the organic phenolics is done by using the surface components of the different constructed nanocomposites that include MXenes of the metals, metals oxide, and the oxysalt such as Au/Ti3 C2 Tx, etc. as a metal MXene, Fe3 O4 /Ti3 C2 Tx, etc. as metal oxide MXene, Ag3 PO4 /Ti3 C2 as oxysalt/metals MXenes. Under the observed based experiments, it is noted that the reductivity, hydrogen bonding, and the electrostatic attraction assign accumulation of the degradation of the phenolic compounds through the MXene-based composites. The strongest bioaccumulation and the acute toxicity are drawn by one of the most toxic pollutants known as the chlorophenols (Karahan et al. 2020), which is unfortunately alarming condition for the environment which therefore required a standard degradation. This concern was resolved by Ming et al. Although the uptaking of the hydrogen in 2017, alkalization treatment was used by removing all the layers of the Ti3 AlC2 . Scientists fabricated Rh/alk-Ti3 C2 X2 for the dispersed adsorption. The 4-chlorophenol hydro chlorination was highlighted by the stable catalyst as-fabricated of Rh/alk-Ti3 C2 X2 which assigns the small nanoscale particles of the Rh for the number of activities to perform (Ali et al. 2020). The ability of the uptaking hydrogen was improved through the efficient MXene of Rh/alk-Ti3 C2 X2 as catalysts (Parveen et al. 2021).

5.4 Removal of Antibiotics Another important class of the pollutants gains a lot of attention during the last two decades and become a major concern of effecting the environment. Many of the antibiotics are present for the extensive applications but the sulfonamides are a highly exploited antibacterial agent. Whereas tetracycline’s, lincomycin, and fluoroquinolones also preferred as curing agent for the bacterial infections and be a part of the toxic pollutants (Mashtalir et al. 2013). The main consequence in the environment was noted when the use of sulfonamides for the sake of food and medicine were making worldwide advancement. The aspects of environment and the human health is affected by the sulfonamides because of their persistence, water solubility, complex structure, and their distinct chemical stability of their metabolics (Bilal et al. 2019). The field of the environmental science requires the novel photo-assisted material with high performance for the efficient removal of the sulfonamides. Looking forward for the solution, the Cao et al. in 2021 derived the catalyst for the degradation of the sulfamethazine through visible light-assisted photo catalyst which was designed as MXene CuFe2 O4 and CFO/Ti3 C2 . Through the interruption of the photogenerated electron holes and the loading of Ti3 C2 , they extend the life of the photoinduced carriers (Borysiuk et al. 2015). The result was found up to the 60% removal of the sulfamethazine by the fabrication of the photocatalyst. The nanomaterials in results

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of the mechanism of the degradation produce some small molecular weight organic substances like methylpyrimidines, etc. The molecules with low weight and some other products like CO2 or H2 O made by the midway products (Borysiuk et al. 2015). Furthermore, the curing agent for bacteria is a basic need now a days not only for the human but also for the animals and plants as well, but the antibacterial agents is a non-tolerable product for the environment which need to be treated with some advancement phenomenon and the MXenes are the best and unique particles to treat such contents (Ding et al. 2020).

5.5 Removal of Radionuclides The radionuclide is used in a wide range for the remediation of the medical research and the medicines produced a lot of waste during production. This type of waste is a serious concern which requires some efficient curing (Xu et al. 2015). The hazardous materials in this waste ceases radioactivity and the waste is of half longer life and it is highly toxic chemical to extract such harmful waste from the ecofriendly environment. There is a need of some novel absorbent candidate to acquire such contents to filter out (Parra-Saldivar et al. 2020a). Although there is too much data available for the treatment of such concerns but still there is a scare data available to deal with the mitigation of radionuclide through the composite materials designed with the MXenes-based exhibition. MXenes are used in many of the techniques for the adsorption of the radionuclide materials from the waste such as inner-structure complexion, the ion exchanger and specially the manipulation of the d-layer process. The finalized pathway of the MXene was the etching of the –O, –OH, and –F for the collection of the radionuclide waste as well as it is also valuable to collect the metallic ions through the mechanism of the ion exchange (Kajiyama et al. 2016). The interlayers in between the MXenes exhibit d-spacing, are another complimentary command to encapsulate properly the radionuclides through adsorption. The radionuclide materials can coordinate more efficiently by the thinning of the nanosheets through delamination of the MXene layers. The choice of MXenes is prior due to its unique behavior managing the powerful radiation and its behavior with different molten salts of the environment through its chemical compatibility (Tran et al. 2018).

5.5.1

Removal of Uranium

The radionuclide possesses a longer shelf life of about 4.45 × 109 years approximately. Uranium is defined as the toxic pollutant of the environment due to its chemical toxicity and its radiological behavior which make the serious concerning treatment of this material. Different methodologies are applied for the treatment of the uranium through the carbides of chromium, vanadium, and titanium. Highlighting the studies, the MXenes of the titanium carbides are more efficient for the removal of the U4 as compared it with other one. The character of the titanium carbide MXenes

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processes its surface complexity and electrostatic attraction. The Vast study on this issue praises the MXenes titanium carbide for the treatment of uranium waste is amazingly efficient (Lei et al. 2019b).

5.5.2

Removal of Thorium

It is an important material as it can easily be replaceable with the uranium with some more radioactive properties due to which its waste became the important concern as it is too harmful for the human body parts as well as for the environment. The management of the thorium can cause cancer. Thus, there is a lot of need to illuminate such waste from the environment. For such treatment in 2019, Ti3 C2 Tx MXene component was used by LI et al. in small ratios for the removal of the thorium. The adsorbing metal ions on the MXenes surface through the chemical and electrostatic interactions make them more unique process to deal with such material (Lei et al. 2019b).

6 Conclusion Pharmaceutical industries are growing day by day as a result their production ratio is increasing every year. It means that its waste and the pollutants present in the waste are also increasing which are affecting our environment very speedily and there is a stringent need to resist it as soon as possible. Pharmaceutical waste can not be eliminated, but can be filtered and treated. All types of toxic and nontoxic components can be mitigated from the waste streams. Pharmaceutical industries exhibit the types of the waste including organic dyes, phenolics, antibiotics, radionuclides, and its products like (uranium, thorium etc.), etc. These components are not easy to extract from the waste. A handsome material with some unique abilities is therefore required. MXenes have proved themselves for the best novel materials for such wastes. To mitigate every toxic and nontoxic material, the MXene is operated due to its unique properties of the structure complexity, hydrogen bonding, ion exchange, electrostatic interactions, catalytic activation and removal, and the super adsorption behaviors. Despite the bundle of methodologies present for the MXenes nanomaterials but still there is a need to look forward for a more unique and effective MXene structure and its fabrications. The attitude of the layers present in MXenes are used to operate it with highly toxic materials but still there is a requirement to look forward for the more accurate layers for the MXenes nanoparticles. Overall MXenes are still having most of the unique features for making environment clean. There is a need to maintain the layers for the radionuclide case for adsorbing more percentage of the pollutant and to initiate the advancement in the MXenes for the sake of ecofriendly environment.

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Potential Mitigation of Dyes Through Mxene Composites Jamil A. Buledi, Ali Hyder, Nadir H. Khand, Saba A. Memon, Madeeha Batool, and Amber R. Solangi

Abstract MXenes are the inorganic materials of nitrides, carbides, or carbonitrides having two-dimensional structural features. MXenes are considered as superlative aspirants in divers’ modern applications because of having a few-atom-thick layered structure of inorganic entities which have exceptional photocatalytic properties and fascinating chemical composition with exposed metallic hydroxide sites. From the last decade, MXenes are being exploited as prominent materials for energy storage, supercapacitors, electrochemical sensing, and water splitting but the MXenes are considered as young materials for water purification applications. Thus, we mainly emphasized on updated published research on MXene composites for mitigation of dye from water by renowned processes such as adsorption/degradation/removal which are rarely reported. This book chapter is broadly focused on the new and updated model of MXene composites, photocatalysts, sorbents, electrodes, and membranes for dyes mitigation supported by different useful mechanisms. The synthesis protocols, characteristics of fabricated MXenes, and comparison of MXene composites with other photocatalysts, sorbents, and membranes for dyes mitigation would be highlighted as well. Along with potential applications of MXenes, the cytotoxicity of MXene composites in dye mitigation has also been discussed in this chapter. Though, there are various challenges and pitfalls in the development and utilization of MXenes for dye removal have been reported which should be mitigated to explore the practicality of MXenes in advanced applications and their exploitation in sustainable future societies. Keywords MXenes · Photocatalysts · Membranes · Inorganic sorbents · Water purification and mitigation of dyes

J. A. Buledi · A. Hyder · N. H. Khand · S. A. Memon · A. R. Solangi (B) National Centre of Excellence in Analytical Chemistry, University of Sindh, Jamshoro 76080, Pakistan e-mail: [email protected] M. Batool School of Chemistry, University of the Punjab, Lahore 54590, Pakistan © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_17

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1 Introduction In the last few decades, environmental pollution has been recognized as an important issue because of the extensive release of the synthetic organic and inorganic toxic pollutants into the aqueous environment without proper treatment (Bilal et al. 2022; Hyder et al. 2022; Lu and Astruc 2020; Qamar et al. 2022; Rana et al. 2021; Rasheed et al. 2021b; Rasheed et al. 2022a, b; Rizwan et al. 2021; Rizwan et al. 2022b; Shakeel et al. 2022a, b; Vörösmarty et al. 2010). The industrial effluents are being continually released into water bodies that are easily accumulated in human body through food chain that cause various types of serious diseases such as headache, heart diseases, respiratory issues, and other toxic health issues on the human beings as well as on the aquatic environment (Chaudhary et al. 2020; Iqbal et al. 2017; Pavan et al. 2008; Rafatullah et al. 2010). However, to evaluate the potential lethal effects and hazardous nature of synthetic organic dyes to obtain the information regarding their pollution levels and other toxic effects on human health as well as on water bodies (Zhu et al. 2018; Zhang et al. 2019b), it is an extremely important and urgent need to develop the cost-effective, sensitive, and selective methods for the removal of synthetic organic dyes from the aqueous environment to make the water safe for drinking purpose. For the treatment of synthetic organic dyes from the aqueous environment, enormous methods have been employed including reverse osmosis, coagulation, ultrasonic degradation, ion exchange, chemical oxidation, and flocculation techniques (Ain et al. 2020; Casbeer et al. 2012; Fang et al. 2019b). There are many drawbacks of these conventional methods, e.g., high cost, time-consuming, expensive instrument, sophisticated protocols, low sensitivity, and produced slugs as secondary waste (Oturan and Aaron 2014; Zhao et al. 2019). Thus, instead of these conventional methods, photocataylsis, membrane ultrafiltration, and adsorption techniques have attracted a lot of interest from the modern scientific community because of their high efficiency, low cost, time-saving, more effective removal of toxic substances from aqueous environment, high reusability of photocatalysis, high membrane ultrafiltration, and exceptional adsorption materials (Han and Wu 2019; Gunasundari et al. 2020; Keerthana et al. 2021). Recently, a fascinating family of two-dimensional (2D) nanomaterials comprising of transition metals carbides, nitrides, and carbonitrides are well-known MXenes (Barik et al. 2019; Deysher et al. 2019; Han et al. 2020). MXenes are the new class of exceptional and interesting 2D nanomaterials prepared first time by Gogotsi et al. in the year 2011 and got tremendous attention from the modern researcher’s community owing to their excellent physical and chemical properties such as large theoretical specific surface area, high electrical conductivity, favorable Fermi-level position, high hydrophobicity, mechanical properties, and tunable chemistry (Iqbal et al. 2020; Yun et al. 2020; Iqbal et al. 2021). Due to these outstanding physical and chemical properties of MXenes-based composite, these materials have several applications in various fields including electrocatalysis, energy storage, nitrogen fixation, electromagnetic shielding, adsorption of environmental pollutants, photocatalysis, and optoelectronics and electronics devices. The general formula of MXene is Mn+1 Xn Tx ,

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where n = 1, 2, 3, or 4, M stands for transition metals (including Cu, Ni, Zn, Ag, Cr, Ti, etc.), X represents either carbon or nitrogen, and Tx refers to surface functional groups including (–O, –OH, and –F) on the surface of MXene (Sun et al.; Nguyen et al. 2020). MXene is considered as well-organized 2D nanomaterial than graphene because of its highly conductive nature and semi-conductive properties (Li et al. 2019). Moreover, there are up to 70 MAX phases have been discovered (Hong et al. 2021). Due to these properties, a different form of the MXene-based nanocomposite has been employed for the remediation of various environmental pollutants from the aqueous bodies. The MXene-based nanocomposites are widely employed as the delicate adsorbent and co-photocatalyst material for the remediation of synthetic organic dyes from industrial wastewater (Jia et al. 2018; Hu et al. 2021).

2 Photocatalytic Degradation of Organic Dyes 2.1 Photocatalytic Degradation of Methylene Blue Through MXene Composites In an experimental study, the researchers (Alsafari et al. 2021) prepared the copper ferrite/MXene (CuFe2 O4 /Ti3 C2 ) monohybrids composite material by using an environmentally friendly ultrasonication method. The prepared composite material was characterized via employing various analytical techniques which revealed excellent morphological texture and crystalline structure of material. The prepared composite material was exploited for photocatalytic degradation of methylene blue (MB) from aqueous media. The developed photocatalyst material showed an outstanding crosslink network, high surface area, and optical properties. The degradation percentage of 94% of MB within 40 min exhibited exceptional reusability as well. Moreover, (Tahir et al. 2022) the synthesis of v novel Gd3 + doped V2 O5 nanostructure with GVO/MXene binary nanocomposite material was carried out via using sol–gel method. GVO/MXene-based nanocomposite material exhibited excellent photocatalytic performance for the degradation of methylene blue (MB) dye. Optical investigations of the material displayed the redshift with an admirable energy band gap of around 2.3 eV of prepared composite material. The GVO/MXene nanocomposite material revealed superlative photocatalytic activity in the presence of sunlight. The proposed photocatalyst degraded 92.30% of MB dye within 120 min from aqueous environment. Another research has been (Qu et al. 2022) to prepare 2D Ti3 C2 Tx MXene by using HF as etching agent and subsequently modified through alkalization to replace –F with –O containing terminal functional groups of fabricated nanocomposite. The ML-Ti3 C2 showed photocatalytic performance for the removal of MB dye for the treatment of wastewater contaminants. Particularly, the prepared photocatalyst material has an incredible photocatalytic activity for MB dye under visible-light irradiation and the developed photocatalyst removes the 81.2% of MB dye in 120 min from the

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contaminated water. Another research group synthesized (Zhou et al. 2022) novel 2D ZnO/MXene (Nb2 C and V2 C) heterostructure nanocomposite material by using the effective electrostatic self-assembly method to tackle environmental pollution. The produced heterostructure was considered as an alternative pathway to improve and enhance the photocatalytic activity of the MXene-based photocatalyst materials to promote interfacial electron transfer charge of prepared material. The established photocatalyst demonstrated tremendous photocatalyst properties for MB dye degradation from the aquatic environment. The photocatalytic degradation rate for MB dye reached 62.63% and 99.53%, respectively, under the irradiation of UV light for 120 min. This is due to the formation of strong coupling interface between the ZnO NPs and MXene molecule that stimulates the effective charge transfer from ZnO NPs to MXene which is responsible for carrier concentration of nanocomposite. Furthermore, Ti3 C2 Tx /Bi12 TiO20 composite material (Tang et al. 2022) was fabricated via applying a simple and safer protocol. The advanced Ti3 C2 Tx /Bi12 TiO20 -based photocatalyst shows the excellent removal of the MB dye from the aqueous environment with 85.4% within 120 min at an applied electric volt of 1 V. The NiFe2 O4 /MXenebased nanocomposite material (Rasheed et al. 2021a) has been fabricated by using a cost-effective and safer ultrasonication method. The as-prepared NiFe2 O4 /MXenebased nanocomposite was employed to eradicate the environmental pollutant MB dye from the industrial effluent. However, the author suggested that the NiFe NPs helped in preventing the re-stacking of MXene films and enhancing the surface area of nanocomposite material which is greatly responsible for the high photocatalytic activity toward the MB. The proposed photocatalytic material contains the utmost photocatalytic performance for MB with 74% degradation within 70 min which is 6.1 times better than NiFe, and MXene individually. The mechanism for the degradation of MB dye under light irradiation is given in Fig. 1. In addition, copper oxide nanoparticles functionalized MXene nanocomposite material through hydrothermal method has been reported (Alsafari 2022). The presence of copper oxide nanoparticles on the surface of MXene is responsible

Fig. 1 Degradation mechanism of MB dye under light irradiation

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for the enhanced catalytic activity. However, the prepared CuO/MXene nanocomposite material acts as an excellent photocatalyst for the remediation of MB from water bodies. The maximum degradation for MB was observed at 99% within 60 min under solar light irradiation. Luo et al. (2019) and colleagues synthesized Co3 O4 nanoparticles-anchored MXene-based nanocomposite material by employing solvothermal method. The Co3 O4 cube-like nanoparticles are well dispersed on the surface and inner layers of MXene films, which effectively prevent the restacking of MXene films and form a well-organized structure of the nanocomposite material. The prepared photocatalyst was employed for the degradation of MB from industrial wastewater. The prepared photocatalyst degrades the MB with high capacity 90% within 120 min with outstanding.

2.2 Photocatalytic Degradation of Congo Red (CR) Dye via MXene Composites A novel and outstanding 2D La– and Mn–Co-doped Bismuth Ferrite/Ti3 C2 MXenebased nanocomposite material has been fabricated via employing the cost-effective double-solvent sol–gel method (Iqbal et al. 2019a). However, the engineered nanocomposite material was employed to remove the Congo red (CR) dye from aqueous media in dark and photo-induced conditions. Moreover, the photocatalyst demonstrated that the BFO has the highest electron hole recombination rate as compared to all the codoped BFO/Ti3 C2 nanocomposites. Therefore, the higher electron–hole pair generation rate of photocatalyst material offers an appropriate environment for a fast and excellent degradation pathway for the synthetic organic dye from aqueous media. Also, the engineered La– and Mn–Co-doped Bismuth Ferrite/Ti3 C2 MXene-based nanocomposite materials have a unique energy band gap of 1.73 eV which improves and enhances the photocatalytic abilities of the photocatalyst. The prepared BLFO/Ti3 C2 and BLFMO-5/Ti3 C2 photocatalyst materials degraded 92% and 93% of CR dye from water in dark. Moreover, Gd3+ – and Sn4+ –Co-doped Bismuth Ferrite/MXene-based nanocomposite material has been prepared utilizing the co-precipitation method (Tariq et al. 2018). The presence of the various metal functionalities on the surface of prepared material provided a better pathway for electron to easily flow which offer large recombination time and also increases the photocatalytic activity for degradation of CR. The prepared Gd3+ – and Sn4+ –Co-doped Bismuth Ferrite/MXene-based nanocomposite material displayed 100% degradation CR dye from aqueous media within 120 min. Therefore, the prepared photocatalyst material is highly efficient for industrial application for the degradation of CR dye. In another study, furthermore, Ti3 C2 -MXene/Bismuth Ferrite nanohybrid composite material was synthesized through solvothermal method (Iqbal et al. 2019b). The prepared nanocomposite material showed exceptional physical properties which are observed by using

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different modern instruments. The prepared photocatalyst material showed 100% degradation for CR within 42 min from water bodies. Bismuth vanadate/MXene (BiVO4 /Ti3 C2 ) nanocomposite material was engineered through hydrothermal method (Qamar et al. 2022). The engineered BiVO4 /Ti3 C2 nanocomposite material displays brilliant structural, chemical composition, and optical properties. Also, the engineered BiVO4 /Ti3 C2 nanocomposite material was employed as an efficient photocatalyst for simultaneously degradation of azo dyes including CR dye from aqueous environment. Moreover, the prepared photocatalyst exhibited excellent 99.1% photodegradation efficiency for CR within 40 min.

2.3 Photocatalytic Degradation of Methyl Orange (MO) Dye Through MXene Composites A novel 2D silver nanoparticles functionalized MXene (Ag/Ti3 C2 Tx )-based nanocomposite material was reported by applying the reduction method (Qu and Fan 2010). The prepared silver nanoparticles functionalized MXene (Ag/Ti3 C2 Tx )based nanocomposite material shows excellent structural analysis, phase purity, size, and optical properties which were determined by employing numerous analytical techniques. The prepared silver nanoparticles functionalized MXene (Ag/Ti3 C2 Tx )based nanocomposite material showed excellent photocatalytic activity toward the MO from industrial wastewater. The obtained results show that the prepared photocatalyst has exceptional degradation efficiency for MO up to 99.3% under UV light irradiation within 60 min. Moreover, Hieu et al. (2021) engineered brilliant 2D Ti3 C2 –TiO2 -based nanocomposite material by using hydrothermal method. The prepared photocatalyst was used to degrade the MO from the aquatic environment. The prepared photocatalyst showed excellent degradation efficiency up to 99% for MO dye within 40 min with excellent reusability. The unique g-C3 N4 -loaded MXene-based nanocomposite material has also been reported (Quyen et al. 2021) through wet impregnation method. The engineered photocatalyst showed brilliant photocatalytic activity for MO dye in the presence of UV light irradiation. Also, the photocatalyst exhibits excellent 90% degradation efficiency for MO dye within 180 min from industrial wastewater. However, this photocatalytic performance was attributed to intimate interfacial contact and smooth photo charge carriers to transport between the g-C3 N4 and MXene compound. Furthermore, BET analysis also showed that the photocatalyst material has a high surface area and large active sites which is responsible for the enhanced photocatalytic activity of the prepared material. In addition, TiO2 /Ti3 C2 nanocomposite material was engineered through the electrostatic self-assembly method (Li et al. 2022). The synthesized TiO2 /Ti3 C2 nanocomposite was employed as a photocatalyst material for the degradation of MO dye from the aquatic environment. The photocatalytic showed superb 99.6% degradation efficiency for the MO dye from aquatic environment within 40 min.

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2.4 Photocatalytic Degradation of Rhodamine B (RhB) Dye via MXene Composites The hierarchical ZnO/MXene-based composite material was reported through the facile two-step chemical reaction (Rana et al. 2021). However, their structural and chemical composition was examined systematically. The prepared nanocomposite material was explored as essential photocatalyst for the remediation of harmful Rhodamine B (RhB) dye from the water bodies. The photocatalyst exhibited high degradation (97.5%) efficiency for RhB dye within 18 min from the water bodies under UV light irradiation. In another study, the preparation of α-Fe2 O3 /ZnFe2 O4 @Ti3 C2 MXene-based nanocomposite material via using onestep hydrothermal and ultrasonication methods has been reported (Zhang et al. 2020). High porosity, surface area, and oxygen functionalities on the surface of αFe2 O3 /ZnFe2 O4 @Ti3 C2 MXene-based nanocomposite materials were determined by using numerous analytical techniques. The α-Fe2 O3 /ZnFe2 O4 @Ti3 C2 MXene-based material was found to exhibit higher photocatalytic activity than the Fe2 O3 /ZnFe2 O4 heterostructures in eliminating RhB dye from water. The α-Fe2 O3 /ZnFe2 O4 @Ti3 C2 MXene-based nanocomposite showed exceptional photocatalytic ability with up to 90% degradation efficiency for RhB dye within 150 min. Xin Zhang and his coworkers (Zhang et al. 2022b) have reported novel α-Fe2 O3 @SnO2 /Ti3 C2 MXenebased core–shell nanocomposite material fabricated through modified Stober and hydrothermal methods. However, the prepared α-Fe2 O3 @SnO2 /Ti3 C2 MXene-based core–shell nanocomposite contains tremendous photocatalytic ability for the RhB dye from industrial wastewater. The prepared photocatalyst has brilliant 72.3% photodegradation efficiency for RhB dye within 140 min which is much higher than that of pure α-Fe2 O3 , SnO2 or Ti3 C2 MXene. Carbon nitride coupled with Ti2 C3 MXene-based nanocomposite material was fabricated using facile electrostatic self-assembly method (Rasheed et al. 2021b). The prepared material is employed as an efficient photocatalyst for the degradation of RhB dye from the aquatic environment. Moreover, the developed photocatalyst showed exceptional 97.2% degradation efficiency for RhB dye within 60 min from the aquatic environment and photocatalyst material also exhibited distinguished stability and reusability properties for potential application for wastewater treatment. In 2020, Quyen et al. (2021) reported TiO2 @Ti3 C2 MXene-based nanocomposite material prepared through hydrothermal method. The prepared nanocomposite material was employed as a potential photocatalyst for the removal of RhB dye from industrial effluent. In addition, the photocatalyst removed RhB dye with high (99%) degradation efficiency within 40 min. Huang et al. (2021) have engineered novel 2D Ag/g-C3 N4 /Ti3 C2 MXene-based nanocomposite material via a one-step calcining approach. The photocatalyst material effectively degraded the RhB dye up to 92.1% within 70 min. Furthermore, a research group (Fang et al. 2019a) has fabricated the unique (CdS/Ti2 C3 ) nanocomposite material by utilizing the facile electrostatic selfassembly and hydrothermal approaches. The photocatalytic results confirmed that

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the photocatalytic degradation activity of fabricated photocatalyst material was (97% for RhB dye within 100 min from aqueous atmosphere). The active species trapping experiments for fabricated photocatalyst material showed that superoxide radical (O2 − ) and holes (h+ ) were the predominant material for the photocatalytic degradation of RhB. Shi et al. (2019) reported the novel 2D BiOBrx I1−x /MXene composite material prepared by using etching method. The prepared BiOBrx I1−x /MXene composite material was employed as a photocatalyst for the removal of RhB dye from the industrial wastewater. The photocatalytic material exhibited 90% degradation efficiency for RhB dye within 40 min. Sulfide-rich bi-metallic-assembled MXene-based nanocomposite engineered by using a two-step hydrothermal method (Rizwan et al. 2022b). The developed photocatalyst degraded RhB dye to approximately 100% within 20 min under UV light irradiation from industrial wastewater. Concise degradation of methylene blue, methyl orange, rhodamine B, and Congo red are given in Table 1.

2.5 Removal of Dyes by Adsorption Through MXene-Based Composites In a recent study, fabrication of a Mxene/PPy nanocomposite by inducing in situ polymerization of polypyrrole in a few-layered solution of MXene is reported (Shi et al. 2022). The material was used for the adsorption of methylene blue (MB) dye and it has shown a maximum adsorption capacity of 553.57 mg/g for MB dye. Moreover, the reported material was also found highly selective for MB dye in a mixed solution containing cationic and anionic dyes. Furthermore, the author has claimed that the induction of polypyrrole particles in few-layered MXene not only has enhanced the adsorption capacity but also increased the adsorption stability of the material (Shi et al. 2022). Similarly, another study reports the synthesis of 2D stack-structured magnetic iron oxide-laden titanium carbide (Ti3 C2 Tx /Fe3 O4 ) MXene nanocomposite as an effective adsorbent for the removal of carcinogenic dyes MB and Rh B. The batch adsorption experiment was carried out and different parameters were optimized to achieve the maximum absorption capacity and dye removal rate. The experimental data fit well in the Langmuir adsorption isotherm and the maximum absorption capacities for MB and Rh B were reported as 153 and 86 mg/g, respectively. The study reports that Ti3 C2 Tx /Fe3 O4 shows the fastest absorption kinetic for MB and Rh B and attains equilibrium within 45 min. It also shows 91% selectivity for MB and 88% selectivity for Rh B. The author further reports that the material retains the removal efficiency of MB and Rh B to 82.88% and 71.16%, respectively, even after four consecutive cycles of adsorption–desorption which shows its god reusability/regeneration capability (Rethinasabapathy et al. 2022). Byung-Moon Jun et al. have used an ultrasonication-assisted MXene for the removal of dyes from wastewater. In this study, the dispersion of MXene was made at two different frequencies, 28 kHz and 580 kHz to study the feasibility of the

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Table 1 Summarized table of MXene composites for the removal of synthetic organic dyes Photocatalyst

Preparation method

Organic Irradiation Degradation Ref dye time efficiency (%)

CuFe2 O4 /Ti3 C2

Ultrasonication

MB

40

90

Alsafari et al. (2021)

GVO/MXene

sol–gel

MB

120

92.30

Tahir et al. I2022))

Ti3 C2 Tx

Etching with HF MB

120

81.2

Qu et al. (2022)

ZnO/MXene/Nb2 C and V2 C

Electrostatic self-assembly

MB

120

99.53

Zhou et al. (2022)

Ti3 C2 Tx /Bi12 TiO20

Ultrasonication

MB

120

85.4

Tang et al. (2022)

NiFe2 O4 /Ti3 C2 Tx

Ultrasonication

MB

70

74

Rasheed et al. (2021a)

CuO/MXene

Hydrothermal

MB

60

99

Alsafari (2022)

Co3 O4 NPs/Ti3 C2

Solvothermal

MB

120

90

Luo et al. (2019)

CR

20

92 and 93

Iqbal et al. (2019a)

BLFO/Ti3 C2 /BLFMO-5/Ti3 C Sol–gel BGFO/20Sn/MXene

Co-precipitation CR

120

100

Tariq et al. (2018)

Ti3 C2 -MXene/Bismuth Ferrite

Solvothermal

CR

40

100

Iqbal et al. (2019b)

BiVO4 /Ti3 C2

Hydrothermal

CR

40

99.1

Sajid et al. (2021)

Ag/Ti3 C2 Tx

Reduction

MO

60

99.3

Lv et al. (2022)

Ti3 C2 –TiO2

Hydrothermal

MO

40

99

Hieu et al. (2021)

g-C3 N4 /MXene

Wet impregnation

MO

180

90

Nasri et al. (2022)

TiO2 /Ti3 C2

Self-assembly

MO

40

99.6

Li et al. (2022)

ZnO/MXene

Ultrasonication

RhB

17

97.5

Khadidja et al. (2021)

α-Fe2 O3 /ZnFe2 O4 @Ti3 C2

hydrothermal and ultrasonication

RhB

150

90

Zhang et al. (2020)

α-Fe2 O3 @SnO2 /Ti3 C2

Hydrothermal

RhB

140

72.3

Zhang et al. (2021b)

Ti2 C3

Electrostatic self-assembly

RhB

60

97.2

Tu et al. (2022) (continued)

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Table 1 (continued) Photocatalyst

Preparation method

Organic Irradiation Degradation Ref dye time efficiency (%)

TiO2 @Ti3 C2

Hydrothermal

RhB

40

99

Quyen et al. (2021)

Ag/g-C3 N4 /Ti3 C2

Calcining

RhB

70

92.1

Huang et al. (2021)

CdS/Ti2 C3

Electrostatic self-assembly

RhB

100

97

Fang et al. (2019a)

BiOBrx I1 -x /MXene

Etching of HF

RhB

40

90

Shi et al. (2019)

Sulfide-rich Hydrothermal bi-metallic-assembled MXene

RhB

20

100

Vigneshwaran et al. (2021)

ultrasonication method in the treatment of wastewater. The physicochemical characterization results showed that the MXene ultrasonicated at 28 kHz has a high number of oxygenated functional groups as compared to the 580 kHz sonicated and original MXene. Both ultrasonication-assisted materials were used as adsorbents for the removal of two dyes, namely MB and Methyl orange (MO). The ultrasonicated MXene materials have shown better adsorption performance only for positively charged dye MB as compared to the stirring-assisted MXene after optimizing different factors. The adsorption mechanism of MXene and MB mainly depends on electrostatic (attraction) interaction. The material has shown a regeneration capacity of up to four cycles (Jun et al. 2020b). Another study was done by a chines group of researchers, Chong Cai and his co-workers. They have synthesized a self-assembled phytic acid/MXene nanocomposite by a hydrothermal route and utilized it for the adsorption of MB and Rh B dye to treat the sewage water. Different composites were synthesized with different hydrothermal reaction times (0.5 h, 3 h, 6 h, 12 h and 30 h) to study the comparison and structural transformation occurring with hydrothermal time. The characterization results show that different structural changes have occurred with different hydrothermal times. The adsorption study results show that the material that has been synthesized with 12 h of hydrothermal time has given the absorption capacity of 42 mg g−1 for MB and 22 mg g−1 for Rh B that is much better as compared to the rest of the materials. Furthermore, this material was also found highly stable, and it retained the absorption capacity of about 85% for MB to even after 12 cycles (Cai et al. 2020). A study published by Bin Sun et al. reports the synthesis of novel MXene (Ti2 CTx ) by selectively removing the Al layer from Ti2 AlC and then exfoliating by ultrasonication method. The as-prepared MXene was utilized for the removal of MB dye and a comparative study was carried out with graphene. The Mxene has shown a maximum absorption capacity of 2460.9 mg/g for MB as compared to the graphene which has shown an absorption capacity of 83.2 mg/g. The potential of MXene adsorbent for the removal of anionic dyes was also tested on MO dye and the maximum absorption capacity was observed to be 122.6 mg/g which is significantly lower than the absorption of MB that is attributed

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due the electrostatic interaction of cationic dyes with the negative active sites of the MXene. Furthermore, the adsorption data follows Langmuir isotherm and fit well in pseudo-second-order reaction (Sun et al. 2021). An alkalized mono/few layers of MXene nanosheets were grafted with acrylic acid to prepare an efficient AA-alkMXene adsorbent for the adsorption of anionic (congo red, CR) and cationic (methylene blue, MB) dyes from wastewater. The fabricated AA-alk-MXene adsorbent has shown excellent absorption efficiency (264.46 mg g−1 ) for CR and (193.92 mg g−1 ) MB dye and it maintained the removal efficiency to 81.77% for CR and 70.17% for MB after three cycles of regeneration. Compared to the cationic dye (MB) the absorption efficiency of novel fabricated adsorbent was greater for anionic dye (CR) which could be for the electrostatic interaction, hydrogen bonding or inter-layer forces which were the main reason for adsorption. The functionalization of MXene and MXene-derived composites have shown exceptional efficiency for the adsorption of organic dyes, the general mechanism for the functionalization of MXene and adsorption of dyes in given in Fig. 2. To study the adsorption process/kinetics the pseudo-first- and second-order models were studied and to check the isothermal adsorption process the Langmuir and Freundlich isotherms were studied, respectively. It was reported that the obtained data fitted well in the Langmuir isotherm model by following pseudo-second-order kinetics (Hao et al. 2022). Different nanocomposites of transition metal oxides with multilayered Ti3 C2 MXene are fabricated by facile hydrothermal method and utilized for the effective removal of MB dye. In this regard, three transition metal oxides namely ZnO, Bi2 MoO6, and SnO2 were utilized to prepare the composites with multilayered Ti3 C2 MXene. The structural characterization revealed that the prepared nanocomposites have porous accordion-like structures and 0D/2D heterostructures. The study reports

Fig. 2 Schematic layout of functionalization of MXene and adsorption of dyes

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that the coupling of MXene with oleophobic metal oxides increased their lipophilicity towards the water-soluble organic dyes and they show enhanced absorption performance for the pollutants. The maximum absorption capacities shown by the prepared ZnO/ML-Ti3 C2 -MXene, Bi2 MoO6 /ML-Ti3 C2 -MXene and SnO2 /ML-Ti3 C2 -MXene nanocomposites for MB are 155 mg g−1 , 163 mg g−1 and 152 mg g−1 , respectively (Zhang et al. 2022a). A porous MXene-based film was reported by a research group by Chenxue Yao et al. The p-MXene/SWCNTs Film was prepared by a scalable and easy hydrothermal approach and used for the effective electrosorption of cationic dye (methylene blue) from wastewater. The prepared film shows a maximum adsorption capacity of 28,403.7 mg/g with remarkable stability and reusability that it maintained the absorption to 95% even after five cycles of regeneration. Such outstanding absorption performance of the material is attributed to the large surface area of p-MXene/SWCNTs film, hydrogen bonding and powerful electrostatic interaction of MB with adsorbent (Yao et al. 2021). In a study, a three-dimensional magnetic nanocomposite of Mxene@Fe3 O4 -CS was fabricated by an ultrasonic method and used for the sorption of Congo red dye from an aqueous solution. The synthesized adsorbent has a high adsorption capacity (620.22 mg/g) for Congo red which retained 85% after five constructive cycles with a desorption rate of 92.8%. The adsorption mechanism mainly depends on the electrostatic interaction and hydrogen bonding, while the nature of the adsorption process was spontaneous, endothermic and entropically driven. Moreover, the adsorption kinetics fit well in the pseudo-second-order and the Langmuir isotherm model was followed, respectively (Xu et al. 2022). The summarized data for the adsorption of various organic dyes at MXene-based composites is given in Table 2.

3 Conclusion MXene is new 2D material of this modern era which has exceptional properties and outstanding applications in divers’ fields. The structural appearance and functional properties of MXene have been broadly discussed in the chapter. The stable 2D texture and viable characteristics of MXene depend on synthesis methods and functionalization with terminating groups. It is examined that both bottom-up and topdown methods are good enough to get different MXene composites either synthesized from exfoliation of large crystals or grown from molecules/atoms. The potential applications of MXene composites as photocatalysts for the degradation of organic synthetic dyes and as adsorbents for the adsorption of toxic dyes are discussed in detail. MXenes with different terminations and synthesis procedures showed 100% degradation of dyes in some experimental studies while as an adsorbent maximum adsorption capacity was noticed as 85%. MXenes are the new class of materials which have found their applicability in separation science as nanofilters. The nanofiltration membranes of MXene composites are examined exceptionally well with a maximum removal percentage of 99.9%. The thorough observation of the differently fabricated

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Table 2 Adsorption of organic dye at different MXene-based composite adsorbents Adsorbent

Types of dye

Absorption capacities mg g−1

Ref

P-M/MX-m

Methylene blue

168.93

Zhang et al. (2021b)

MXene-COOH@(PEI/PAA)n

Safranine T Methylene blue Neutral red

35.59 40.38 46.12

Li et al. (2018)

MXene/Al-MOF

Methylene blue, Acid Blue

140 200

Jun et al. (2020a)

2D-MX@Fe3 O4

Methylene blue

11.68

Zhu et al. (2019)

MXene@Fe3 O4

Methylene blue

11.68

Zhang et al. (2019a)

2D Mxene nanosheets

Malachite Green

4.6

Albukhari et al. (2022)

TiO2 /ML-Ti3 C2

Methylene blue

149.47

Zhang et al. (2021a)

Ti3 C2 Tx /SA beads

Methylene blue

92.17

Zhang et al. (2021c)

CPCM@MXene

Crystal violet

2750

Wu et al. (2021)

MXene/PEI@SA

Congo red

3568

Feng et al. (2021)

MXene composites has shown phenomenal response in removal as well as degradation of organic dyes. Additionally, certain factors such as stability and mechanical and electrical properties of MXene still need massive improvement. Moreover, extensive research is needed for the MXene-based nanofiltration membrane development for identifying the substrate and crosslinkers which greatly influence the performance of membranes. Conflict of Interest All authors declare no conflict of interest.

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MXene-Based Polymeric Nanocomposites for Pressure/Strain Sensing Ahmad Shakeel, Komal Rizwan, and Ujala Farooq

Abstract Pressure/strain sensors are a type of flexible sensor which have great applications in life due to their capability to monitor, respond, and transform mechanical motion in an electrical signal. Strain sensing applications have widened the scope of technology and permitted us to detect alterations in surroundings in distinct ways that can be barely imagined. To increase the efficacy and broaden the application scope of pressure sensors, MXenes have been incorporated into strain/pressure sensing devices due to their great electrochemical features. In this chapter, we have reviewed the progress in MXene-based pressure sensors. We have discussed the fundamentals, synthesis, sensing mechanism, and applications of MXene-based strain/pressure sensors. Strain/pressure sensors based on MXenes have the potential to overcome the limitations of conventional metallic strain gauges, such as poor sensitivity and low stretchability. We have reported the state-of-the-art MXene-based strain sensors with a particular focus on their fabrication strategies and efficiency (sensitivity and strain) of the strain sensor created. Keywords Polymer · MXene · Nanocomposites · Pressure · Strain

A. Shakeel (B) Laboratory of Process Engineering, NeptunLab, Department of Microsystems Engineering (IMTEK), Albert Ludwig University of Freiburg, 79110 Freiburg, Germany e-mail: [email protected] Freiburg Materials Research Center (FMF), Albert Ludwig University of Freiburg, 79104 Freiburg, Germany K. Rizwan (B) Department of Chemistry, University of Sahiwal, Sahiwal 57000, Pakistan e-mail: [email protected] U. Farooq Faculty of Aerospace Engineering, Department of Aerospace Structures and Materials, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, the Netherlands © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2023 K. Rizwan et al. (eds.), Handbook of Functionalized Nanostructured MXenes, Smart Nanomaterials Technology, https://doi.org/10.1007/978-981-99-2038-9_18

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1 Introduction Sensors based on nanomaterials have great applications in human life, and they are efficient, user-friendly, and cost-effective tools for the analysis of environmental samples (Rasheed & Rizwan 2022; Rasheed et al. 2022; Rizwan et al. 2022a, b, c; Shakeel et al. 2022a, b, c). The largest sensing organ in the human body is the skin. Skin perceives signals from the physical world. In recent years, various flexible devices which can fold and bend reversibly have been developed to replicate the features of human skin. These sensing devices have applications for electronic skin, health, and human–machine interaction (Liu et al. 2021a, b). Sensors have the potential to sense information from surroundings and transform it into the required type of signal, providing us with the information of surroundings. Pressure sensors may transform pressure signals into electrical signals, which are an important type of flexible sensors (Li et al. 2020a, b, c). Pressure/strain sensors have potential applications in soft robotics, artificial intelligence (AI), and monitoring of health (Yang et al. 2019a, b). There are two different strategies, such as engineering and chemical synthetic approach which are used to obtain flexibility of pressure sensors. The engineering strategy incorporates ultra-thin, high-performance sensing components on a soft substrate without affecting the efficacy of sensors. A decrease in thickness of these sensing components has purpose to decrease rigidity and strain created through bending as inorganic component has great modulus and brittleness (Li et al. 2017), while chemical synthetic approach may obtain flexible pressure sensing component directly and betow pressure sensors with great stretching capacity (Matsuhisa et al. 2019). Different flexible substrates, such as carbon-based nanomaterials, metal NPs, oxides, nitrides, and polymeric materials, have been used in flexible pressure sensors (Zhou et al. 2020). Among these nanomaterials, the usage of MXenes in sensing devices has emerged more rapidly in recent years due to great flexibility, conductivity, ultra-thin structure, cost-effectiveness, and commercial availability. In this chapter, we have discussed synthesis, sensing mechanism, the structure of MXene-based composites, and their applications in pressure/strain sensors.

2 Synthesis Routes Several methods have been reported in the literature to produce 1D, 2D, or 3D structures of polymer–MXene-based nanocomposites. These preparation techniques include in situ polymerization, coating methods, self-assembly methods, template methods, spinning methods, and additive manufacturing methods (i.e., 3D printing).

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2.1 In Situ Polymerization In situ polymerization approach involves the synthesis of macromolecules by mixing monomers, initiators, and curing agents with nanomaterials in the wet form (Chen et al. 2021a, b; Shakeel et al. 2022a, b, c). This wet blending approach provides homogenous dispersion of MXene in the polymer matrix, creating a strong interaction between MXene and polymer matrix (Tang et al. 2019). However, this method is not very environment-friendly, as it involves harmful solvents for incorporating MXene into the polymer matrix. This technique has been widely used to produce devices for energy storage and harvesting, pressure/strain sensing, EMI shielding, and other smart wearables. For instance, the synthesis of a composite strain sensor by drop coating the mixture of MXene, pyrrole monomer, and ethanol on the electrospun PVDF nanofibers, followed by polymerization, has been reported (Leong et al. 2022). The prepared sensor shows a conductive micro-cracks network, representing its strain sensing properties.

2.2 Template Methods This methodology involves the application of nanomaterials as a template to support MXene. In this approach, a template is typically mixed with a colloidal solution of MXene, followed by heat treatment or freeze drying to obtain solidification and crosslinking. In the end, the support material is removed. The commonly employed support materials are MgO nanoparticles (Zhu et al. 2020a, b), poly(methyl methacrylate) (PMMA) particles (Zhao et al. 2017), polystyrene (PS) microspheres (Zhu et al. 2020a, b), etc. The ice template method is a low-cost and green method along with environmentally friendly and simple operation (Ji et al. 2020).

2.3 Self-assembly Methods Self-assembly of materials into different macroscopic structures is typically achieved by the gelation phenomenon. MXene nanomaterial cannot make a gel-like structure on its own, and therefore, an appropriate carrier is needed. For example, the instant gelation of MXene was reported by using divalent metallic ions in a water solution (Deng et al. 2019). Likewise, the MXene/reduced graphene oxide hydrogels have been produced using hydrothermal treatment in the presence of graphene followed by forming porous freeze-dried gels (Saha et al. 2021). Self-assembly methods are vacuum filtration and electrochemical deposition.

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Vacuum-Assisted Filtration (VAF)

Vacuum-assisted filtration, as the name suggests, is a separation technique in which a dissolved solid is isolated from a solution by forcefully pushing the liquid through a filter using a vacuum pump (Gao et al. 2020). It is an extensively used approach in materials chemistry, particularly for incorporating MXene into the polymer matrix. In this method, the required amount of MXene and polymer is vigorously sonicated or agitated in a polar solvent, followed by vacuum filtration. For instance, Liu and colleagues integrated MXene into the conductive PEDOT:PSS polymer using the VAF approach for EMI shielding application (Liu et al. 2018). Likewise, Wang and colleagues produced an ultra-thin sandwich structure based on Ti3 C2 Tx /Fe3 O4 @PANI composite using Ti3 C2 Tx as a matrix by the VAF method (Wang et al. 2021a, b, c). In this study, the variation in film thickness was reduced by increasing the content of Fe3 O4 @PANI followed by VAF for even distribution of Fe3 O4 @PANI composite. The production of MXene–polymer-based nanocomposite using the VAF technique has several benefits, however, controlling the content of monomer is a bit difficult, as there is a chance to get rid of the water-soluble polymer (Ahmed et al. 2020).

2.3.2

Electrochemical Deposition (ED)

Electrochemical deposition is one of the most environmentally friendly and costeffective methods to prepare polymeric membranes or composites, along with efficient control of the morphology of fabricated structures by varying the deposition potential/current, time, and composition of electrolytes (Tong et al. 2020). This approach allows the homogenous coating of nano- and microparticles of different natures/types on the substrate (Gunputh & Le 2017). The morphology and homogeneity of the coating layer depend on the type of polymer or polymerization pattern. This technique is eco-friendly because there is no requirement for surfactants, capping agents, and/or dispersing agents. Furthermore, this method increases the interfacial bonding between the substrate and the coating material before sintering or heat treatment. However, sometimes this method results in large-sized nanoparticles along with a poor particle size distribution, leading to the lower properties of the fabricated composites (Kakaei et al. 2019). This method has drawn significant attention from researchers for producing polymer–MXene-based nanocomposites. For example, Tong and colleagues reported the fabrication of freestanding and flexible Ti3 C2 Tx /PPy composite film using the ED approach (Tong et al. 2020). In this study, Ti3 C2 Tx was first mixed into the pyrrole monomer solution, followed by the infusion of monomer into the MXene layers and then polymerization of pyrrole to form polypyrrole at the MXene surfaces.

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2.4 Coating Approaches Several coating strategies have been utilized to prepare MXene-based composite films such as spray coating, spin coating, inkjet printing, dip coating, etc. In spin coating, a thin film composite is formed from MXene dispersion by using a spin coater. The optimal conditions for producing a homogenized MXene film are found by selecting different spinning speeds (Zhang et al. 2019a, b). In dip coating, MXene is typically applied to the polymer substrate by using a dip coating machine, followed by the drying of the composite film by air or in a desiccator (Salles et al. 2019). The spray coating method utilizes a spray gun, along with the hot air gun, to spray the MXene dispersion on to the substrate and immediate drying. For example, Zheng and colleagues reported the fabrication of MXene/PANI-based composite fabric using in situ polymerization and spray coating methods (Zheng et al. 2022). Firstly, PANI nanoarrays were grown on the PET fabric using the chemical polymerization method, followed by the spray coating of MXene aqueous dispersion on the PANI/PET fabric.

2.5 Spinning Methods Typical spinning strategies include electrostatic spinning and wet spinning. The electrospinning method requires certain viscosity of the MXene solution, which can be obtained by adding polymer. For instance, Mayerberger and colleagues developed a nanofiber by mixing MXene with poly(acrylic acid) and poly(vinyl alcohol), which makes it viable for the electrospinning of MXene (Mayerberger et al. 2017). Likewise, Shao and colleagues successfully synthesized Ti3 C2 Tx /polyester nanofiber-based yarns via electrospinning for super-capacitors (Shao et al. 2018).

2.6 3D Printing This interesting method produces 3D structures with complex geometries by curing or connecting the prefabricated ink. Typical examples of this technique that have been used to produce 3D structures of polymer-MXene composite, include direct ink writing (Wu et al. 2021) and stereolithography (G. D. Huang et al. 2021). Direct ink writing involves the extrusion and curing of high-viscosity MXene-containing ink. On the other hand, the stereolithography technique utilizes photopolymers for curing. MXene nanosheets, as a photoblocker, enhances the overall printing quality of the final structure through suppressing the light scattering during printing and providing outstanding light-to-heat transformation. For example, Li and colleagues reported the synthesis of poly(vinyl alcohol) composites via binder jet printing of MXene–surfactant ink (Li et al. 2022a, b). By extruding highly conductive MXene nanoparticles onto a polymer matrix, the resulting material showed potential for

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strain sensing. In summary, the precise assembly of MXene with polymer is crucial for broadening application potential. The coating approach is facile and easy without any requirement of a tedious production environment and suitable for large-scale production. MXene-based structures produced by the spinning method typically have high conductivity but require the addition of a viscosity modifier (i.e., polymer). Selfassembly method, template method, and 3D printing method can produce complex 3D structures of polymer–MXene. The 3D framework provides excellent mechanical characteristics along with a higher specific surface area. Furthermore, it effectively circumvents the self-stacking behaviour of MXene nanosheets, which results in a homogeneous distribution of MXene into the polymer matrix. However, these techniques involve more complicated preparation, higher process complexity, and a tough production environment. In short, depending upon the application, a particular synthesis approach needs to be opted to enhance the efficacy benefits of MXene.

3 Sensing Mechanism The change in the resistance R of a conductor can be given as R=ρ

L A

(1)

where L is the length, ρ is the resistivity, and A is the conductor’s cross-sectional area. Hence, change in resistance, under the application of an external force, comes under the from the change in length and resistivity. Relative change in resistance ΔR R influence of a stress is given as (Amjadi et al. 2016) Δρ ΔR = (1 + 2υ)ε + R ρ

(2)

where υ is the Poisson ratio of the material, ε represents the strain, and Δρ is the ρ relative resistivity change, which shows the effect of the piezoresistivity of the sensor can material itself. In the case of conventional materials (i.e., metals), the term Δρ ρ

be ignored due to small values. On the other hand, the term Δρ cannot be ignored ρ for semiconductors. In addition to the above-mentioned basic mechanisms, there are several other structure-specific mechanisms. It is important to note that the changes in sensitivity or resistance of a piezoresistive sensor do not come from a single mechanism. In the case of piezoresistive polymer composites, three main mechanisms can derive the sensing behaviour: variations in the band structure of fillers, tunneling resistance between fillers, and variations in seepage path (Theodosiou & Saravanos, 2010). For MXene-based sensing materials, sources of resistance variation include producing wrinkled structures, relative sliding of MXene layers, and crack initiation and growth (Pu et al. 2020).

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Apart from the preparation and cost of the material, there are several other parameters that are required to be considered for evaluating the efficacy of a sensor, including stretchability, sensitivity, selectivity, durability, detection range, detection limit, response time, recovery time, etc. Sensitivity is a tool to measure the capability of a sensor to transform pressure into an electrical signal (Huang et al. 2019) and is given as S=

ΔR/R0 ΔP

(3)

where ΔR/R0 is the ratio of normalized resistance change and ΔP is the pressure variation. In the case of strain sensors, sensitivity is generally estimated through gauge factor (GF) (Ma et al. 2017): GF =

ΔR/R0 ε

(4)

where ε is the tensile strain.

4 Pressure/Strain Sensing Using MXene–Polymer Nanocomposites In literature, several nanomaterials have been reported to use as pressure/strain sensing materials, such as carbon nanotubes, graphene, and silver nanowires (Feng et al. 2020; Wang et al. 2020a, b). However, graphene possesses poor hydrophilicity due to the limitations of the production process, which restricts its processability. This hydrophilic character of graphene can be improved by post-processing but at the cost of making the final product expensive and adding complexity to the production process. In the case of silver nanowires, the sensing performance parameters such as sensitivity and detection limit need to be improved. On the other hand, MXene (particularly Ti3 C2 Tx )-based sensors display faster response time, higher sensitivity, and lower detection limit, which makes them excellent and popular candidates for pressure/strain sensors (Wang et al. 2022). Researchers are eager to combine MXene with other materials (i.e., polymers) or to design various exciting structures to increase the sensing performance of MXene-based sensors. On the basis of literature studies, MXene–polymer-based pressure/strain sensors can be divided into three main categories such as (i) 1D fiber structures, (ii) 2D planar structures, and (iii) 3D architectures. These categories are explained in the following sections.

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4.1 1D Fiber Structures Researchers have frequently attempted to combine yarn, which possesses a large surface area and high flexibility, with conductive materials to generate multifunctional wearable devices. The MXene surface is highly hydrophilic due to the presence of functional groups, and, therefore, it may be processed and coated onto yarn via spraying or printing. Zheng and colleagues used a simple polymerization and spray-coating technique for preparing a composite fiber possessing poly(3,4-ethylene dioxythiophene) (PEDOT) film and MXene sheet (Zheng et al. 2021a, b). Firstly, the fabrics were coated with PEDOT via vapor phase polymerization, followed by the spraying of MXene on the modified fabric. The fabricated composite displayed higher conductivity and excellent strain sensitivity due to the interconnected networks between PEDOT and MXene. This study shows that the combination of polymer, yarn, and MXene has incredible potential for producing wearable electronic devices. Likewise, Seyedin and colleagues reported the fabrication of MXene/polyurethane-based composite fiber via a wet spinning method. The prepared fibers displayed excellent electrical conductivity and higher stretchability, along with a high gauge factor of 12,900 and a large range of sensing strain up to 152% (Seyedin et al. 2020). During the stretching process as a strain sensor, MXene nanoparticles may lose their interconnection, which eventually results in lower sensitivity. Therefore, it is important to incorporate different materials to sustain this interconnection between MXene nanoparticles during sensing without losing its flexibility. For instance, Wang and colleagues deposited the MXene layers on the sheathed polyester yarn by intelligently using the capillary effect (Wang et al. 2020a, b). The results showed a gradual increase in the contact area (i.e., contact resistance) between poly(ethylene terephthalate) filaments during stretching, which causes an increase in the resistance of the composite yarn by increasing the strain from 0 to 120%. Moreover, the fabricated composite yarn exhibited a fast response time (120 ms) and great cyclic stability (>1 × 104 cycles). Uzun and colleagues effectively synthesized MXene-coated cellulose yarns via the dip coating method followed by drying (Uzun et al. 2019). The outcome of the study showed an excellent electrical conductivity of the modified yarn, i.e., up to 440 S cm–1 . Furthermore, the fabricated cotton yarn electrode displayed a specific capacitance of 759.5 mF cm–1 at 2 mV s–1 . Composites of MXene with other nanomaterials also exhibit better mechanical properties (i.e., tensile strength). For example, Li and Du combined materials of various dimensions, such as 0D silver nanoparticles (NPs), 1D silver nanowires (NWs), and 2D MXene nanosheets (Li & Du, 2019). The authors reported the successful preparation of MXene/silver NPs/silver NW-based composite yarn strain sensor. The results showed outstanding tensile properties of the composite fiber due to the better tensile stability of the yarn.

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4.2 2D Planar Structures Recently, high-performance and flexible pressure/strain sensors with 2D planar geometry have attracted significant attention from researchers for their significant applications in wearable electronics, smart robots, and electronic skin. For example, Chen and colleagues developed a double-layer pressure sensor by getting inspiration from human skin (Chen et al. 2022a, b). They utilized MXene for providing electrically conductive pathways and sandpaper for producing a micro-protrusion rough surface on polydimethylsiloxane (PDMS) film. The fabricated sensor exhibited higher sensitivity (2.6 kPa−1 ), a wide linear range of pressure sensing (0–30 kPa), fast response time (40 ms), and super repeatability. Moreover, the prepared sensor was used for the real-time detection of limb movement, artery heart rate, vocal cord vocalization, and handwriting. The outcome of this study revealed a new approach for manufacturing flexible pressure sensors for human–computer interaction, wearable electronics, health monitoring, and intelligent robots. In another study, (Qin et al. 2022a, b) fabricated a miniature flexible pressure sensor based on polyvinyl alcohol and MXene. The prepared sensor showed excellent sensitivity (2320.9 kPa−1 ), fast response time (~70 ms), lower detection limit (~6 Pa), and stability for more than 10,000 cycles. Moreover, the PVA/MXenebased sensor exhibited low cost, simple preparation, and easy industrialization. This design approach provides a nice solution to the urgent need for a miniaturized flexible pressure sensor having a wide sensing range and excellent sensitivity. A multifunctional, highly conductive, elastic, and breathable electronic fabric has been developed (Zheng et al. 2022). The authors utilized a combination of chemical polymerization and coating techniques to produce interconnected networks of polyaniline nanoarrays and MXene on the fabric. The prepared modified fabric displayed good elasticity, low electrical resistance, high energy density (8.08 µWh cm−2 ), and high specific capacitance (647 mF cm−2 ), which are significantly higher than other MXene-based fabrics. Furthermore, the polyaniline/MXenebased fabrics exhibited excellent strain sensing performance, such as higher sensitivity (gauge factor of 435.0), wide range (>0.9), and linearity (0–40.8%), which makes them an ideal candidate for wearable sensors for accurately detecting human motions. Zheng and colleagues utilized a dip-coating technique to prepare a conductive MXene-based cotton fabric, which was inserted between a PDMS film and an interdigital electrode (Zheng et al. 2021a, b). The prepared sensor was rich in MXene functional groups and hydroxyl groups on the fabric. The as-fabricated sensor showed higher sensitivity and wide sensing range, along with a fast response time (50 ms) and recovery time (20 s). The fabricated sensor was further used to check human health, such as wrist pulse, finger movement, and Parkinson’s early static tremor. This study offers a simple and cheaper preparation method to produce flexible pressure sensors with excellent performance. Inspired by the super-hydrophobic surface of the lotus, Wang and colleagues attempted to modify the surface morphology of cotton with the help of MXene, in

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order to produce a corrosion-resistant and self-cleaning sensor (Wang et al. 2021a, b, c). This interesting sensor was fabricated by incorporating the silica nanoparticles and MXene sheets on the cotton fabric. The results showed higher sensitivity (up to 12.23 kPa–1 ), wide sensing range, and stable response under multiple cycles. Moreover, the prepared sensor displayed higher Young’s modulus and good tensile strength, which were attributed to the convex surface of the cotton, the outstanding elasticity of MXene, and stable silicon–oxygen bonds formed between silica nanoparticles and the MXene sheets. Therefore, this sensor provides an exciting option for potential applications in wearable electronics under extreme environments. Sensitivity and flexibility are the two key parameters of scalable strain sensors. The flexibility of the sensor relies on the slip mechanism, while the sensitivity depends upon the crack growth mechanism, and both of these mechanisms are typically opposite to each other. However, a conductive MXene/PDMS film with high flexibility and sensitivity via a bottom-up approach has been fabricated (Chen et al. 2021a, b). The prepared film exhibited higher tensile capacity (100%) due to the micro-cracks produced by non-uniform deformation during pre-stretch and release cycles. In addition, MXene/PDMS film showed exceptional dynamic cyclic stability for more than 1000 pressure cycles along with higher sensitivity (66.3 nF kPa–1 ). Luo and colleagues fabricated a multi-core–shell structure by modifying the elastic textile with polydopamine (PDA) followed by decorating it with MXene and coating it with PDMS (Luo et al. 2021). The incorporation of MXene provided protection from oxidation, while PDMS imparted super-hydrophobicity to the textile, resulting in enhanced corrosion resistance. This smart fabric showed long-lasting and excellent electric-to-heat and light-to-heat conversion performance, along with a higher thermal resistance coefficient. Furthermore, the prepared sensor showed outstanding temperature sensing and strain sensing capabilities. Hence, the fabricated textile provides an interesting option for potential applications in multifunctional wearable devices. On the other hand, Jia and colleagues developed a flexible strain sensor by varying compositions of the substrate instead of only modifying the substrate surface with MXene (Jia et al. 2021). The authors incorporated polyacrylonitrile (PAN) into thermoplastic polyurethane, followed by electrospinning to produce a flexible PAN/TPU. This approach allowed to retain the outstanding conductivity of MXene. Furthermore, fabricated MXene/PAN/TPU sensor exhibited higher sensitivity (gauge factor of 9.69), lower detection limit (