Photoelectrochemical Hydrogen Generation: Theory, Materials Advances, and Challenges (Materials Horizons: From Nature to Nanomaterials) 9811672849, 9789811672842

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Materials Horizons: From Nature to Nanomaterials

Praveen Kumar Pooja Devi   Editors

Photoelectrochemical Hydrogen Generation Theory, Materials Advances, and Challenges

Materials Horizons: From Nature to Nanomaterials Series Editor Vijay Kumar Thakur, School of Aerospace, Transport and Manufacturing, Cranfield University, Cranfield, UK

Materials are an indispensable part of human civilization since the inception of life on earth. With the passage of time, innumerable new materials have been explored as well as developed and the search for new innovative materials continues briskly. Keeping in mind the immense perspectives of various classes of materials, this series aims at providing a comprehensive collection of works across the breadth of materials research at cutting-edge interface of materials science with physics, chemistry, biology and engineering. This series covers a galaxy of materials ranging from natural materials to nanomaterials. Some of the topics include but not limited to: biological materials, biomimetic materials, ceramics, composites, coatings, functional materials, glasses, inorganic materials, inorganic-organic hybrids, metals, membranes, magnetic materials, manufacturing of materials, nanomaterials, organic materials and pigments to name a few. The series provides most timely and comprehensive information on advanced synthesis, processing, characterization, manufacturing and applications in a broad range of interdisciplinary fields in science, engineering and technology. This series accepts both authored and edited works, including textbooks, monographs, reference works, and professional books. The books in this series will provide a deep insight into the state-of-art of Materials Horizons and serve students, academic, government and industrial scientists involved in all aspects of materials research.

More information about this series at https://link.springer.com/bookseries/16122

Praveen Kumar · Pooja Devi Editors

Photoelectrochemical Hydrogen Generation Theory, Materials Advances, and Challenges

Editors Praveen Kumar Indian Association for the Cultivation of Sciences Kolkata, India

Pooja Devi Central Scientific Instruments Organisation Chandigarh, India

ISSN 2524-5384 ISSN 2524-5392 (electronic) Materials Horizons: From Nature to Nanomaterials ISBN 978-981-16-7284-2 ISBN 978-981-16-7285-9 (eBook) https://doi.org/10.1007/978-981-16-7285-9 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 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

Of late, civil society’s utmost significant technical glitches are developing a long-term and sustainable energy economy. The available coal reserves and associated technologies to meet the energy demand have come up with calamitous social costs. It is, therefore, necessary for researchers and technologists to identify alternative environmentally sound and cost-effective energy solutions. Solar Photo Electro Chemical (PEC) hydrogen production is among the promising clean technologies that could theoretically produce energy carriers by taking advantage of the 120,000 TW of radiation that continually strikes the earth’s surface. In general, PEC technology combines the harvesting of solar energy and the electrolysis of water into a single device. Thus, intermittent solar energy is converted into an inherently more storable form of energy than chemical bonds. The critical role in defining the efficiency and acceptability of this approach is the material. The combination of affordable semiconductor nanomaterial’s architecture with light and water holds the potential of lab-based system translation into commercially viable technology without affecting the environment. Solar energy harvesting via PEC water splitting covers all themes utilizing synergistic technologies and systems, and engineering concepts to develop and improve photosystems of all kinds in energy-related research. To that end, we hope this textbook would provide meaningful insight into the energy crisis, current status, and challenges associated with materials from a device point of view. This certainly will help the peer community to take the next step for the desired objective. It will also provide enough base on theories behind solar PEC water splitting and practices being followed by researchers while investigating various classes of nanomaterials for the same. The book will be useful for undergraduate or university students, teachers, and researchers working in the field of material science, chemistry, and physics for energy-related research, especially energy harvesting and conversion. Kolkata, India Chandigarh, India

Praveen Kumar Pooja Devi

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Contents

1

Hydrogen: A Future Chemical Fuel . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nilima Sinha and Srimanta Pakhira

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2

Introduction to Hydrogen and World Energy Scenario . . . . . . . . . . . . Abhishek Anand, Pooja Devi, and Praveen Kumar

31

3

Electrochemical Water Splitting: H2 Evolution Reaction . . . . . . . . . . Shrish Nath Upadhyay and Srimanta Pakhira

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4

Design of Biomimetic Photocatalysts for the Solar Hydrogen Generation: An Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Niharika, Sweta Bastia, Rajeswari Kainda, Rajashree P. Mishra, and Yatendra S. Chaudhary

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Plasmonic Photocatalysts for Water Splitting . . . . . . . . . . . . . . . . . . . . 117 Francisco J. Peón Díaz, Rodrigo Segura del Río, and Paul Eduardo David Soto Rodriguez

6

Monoclinic BiVO4 -Based Photoanodes for Photoelectrochemical Water Splitting . . . . . . . . . . . . . . . . . . . . . . . . 175 Tatiana Santos Andrade, Izabela Campos Sena, Antero Ricardo Santos Neto, Mara Cristina Hott Moreira, Mariandry Rodriguez, and Márcio César Pereira

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Defect-Enriched Transition Metal Oxides Towards Photoelectrochemical Water Splitting . . . . . . . . . . . . . . . . . . . . . . . . . . . 207 Lalita Sharma and Aditi Halder

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Photoelectrochemical Water Splitting with Nitride-Based Photoelectrodes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225 Avishek Saha and Arindam Indra

9

Nanomaterial Assisted Photoelectrochemical Water Splitting . . . . . . 249 Subhavna Juneja and Jaydeep Bhattacharya

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Contents

10 Solar Hydrogen Production Using III-Nitride Nanowire Photoelectrode . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 Manish Mathew and Nikhil Deep Gupta

About the Editors

Dr. Praveen Kumar is working as an Assistant Professor at the Indian Association for Cultivation of Sciences (IACS), Kolkata and also a Chair of Marie Curie Alumni Association (MCAA) India Chapter funded by the European Commission. He is also a member of the National Academy of Sciences India (NASI) and Indian National Young Academy of Sciences (INYAS). He is also working as an Editorial Board Member of one of the reputed journal Materials Letters, Elsevier. He received his Ph.D. from the Department of Physics, Indian Institute of Technology, Delhi, in 2011, followed by the postdoctoral studies at ISOM, UPM Madrid. He is recipient of several recognized awards and fellowships, few of them are MRSI Medal 2021, Materials Research Society of India (MRSI), DAE Young Achiever Award (2019), BRICS Young Scientist Award (2017), Marie Curie Postdoctoral Fellowship (2012), INSPIRE Faculty Award (2013), 08 Best oral/poster awards and Gold Medal in M.Sc. (Physics) From Rajasthan University (2003). Dr. Kumar’s research contribution covers a broad spectrum of materials synthesis including III-V semiconductors, oxides, sulphides, carbon nanostructures, metal/semiconductor interfaces, etc, for LED emitters & solar cells, photoelectrodes, broad-band and self-powered photodetectors, and next-generation sensors applications. He has authored/edited 3 books, 81 publications in peer-reviewed international journals, more than 55 in conference proceedings, and delivered around 42 invited/oral talks at various conferences/institutes around the globe. Dr. Pooja Devi is currently working as a principal scientist at CSIR-CSIO, Chandigarh. She has done her Ph.D. in Engineering from the AcSIR, New Delhi, M.Tech. in Nanotechnology from IIT Roorkee with Gold Medal, B.Tech in Biotechnology with the first rank from Kurukshetra University. She is a well-established young scientist in the field of nanomaterials, sensors, and energy harvesting devices. She has published

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About the Editors

78 international high impact publications, 5 books, 18 Chapters, more than 75 conference papers, and delivered 31 invited/oral talks. Her research interest is towards techniques and technologies design and development for water pollutants monitoring using opto/electrochemical techniques and solar hydrogen production. She is recipient of several prestigious awards and fellowships including NASI- Young Scientist Platinum Jubilee Award (2021), IEI Young Engineer Award (2020-21), Haryana Yuva Vigyan Ratan Award (2019), Elected Member, National Academy of Science in India (NASI) (2021), Core Committee Member, Indian National Young Academy of Science (2021-23), Selection in Shanghai Cooperation Organisation (SCO) Conclave of Young Scientists (2020), INAE Young Engineer Award (2020), Associateship, Indian National Academy of Engineers (INAE, 2020), SERB Women Excellence Award (2020), Young Associateship, Indian Academy of Sciences (2019-22), MRSI GC Jain Memorial Award (2020), ISCA Young Scientist Award (2019), ISEES Young Scientist Award (2019), BRICS Young Scientist Award (2018), PITTCON-Travel Grant (2018), Indo-US WARI fellowship (2017), Young Scientist Project Award (2017), Canadian Commonwealth Fellowship (2010), and MHRD GATE Fellowship (2008).

Chapter 1

Hydrogen: A Future Chemical Fuel Nilima Sinha and Srimanta Pakhira

1 Introduction to Hydrogen Today, we are heading towards the development of human civilization which rapidly increases energy demands, so the uses of non-renewable fuels like natural gas, coal, and petroleum have been gradually increasing [1, 2]. It causes a threat to the environment as use of fossil fuel generates CO2 gas in the atmosphere and results in greenhouse gases such as CO2 and CO, climate change, and global warming. Moreover, all these fossil fuels are limited in the environment and one day it will disappear from the earth. Therefore, the limited fossil fuels in nature, growing global energy demand, and increment in the emission of pollutants like greenhouse gas forcing us to move towards a renewable and environment-friendly energy economy. So, it is very much important to discover renewable energy sources as a replacement of nonrenewable energy resources like gasoline, coal, natural gas, and nuclear energy in future [3]. It is a really a challenging task to discover a new cost-effective, efficient, and environment-friendly fuel as an alternative of carbon-based fossil fuel. This chapter tells us about hydrogen energy and requirements, world energy scenario, and the advantages of using hydrogen as a fuel. This chapter provides us with a deep

N. Sinha · S. Pakhira (B) Department of Metallurgy Engineering and Materials Science (MEMS), School of Engineering, Indian Institute of Technology Indore (IIT Indore), Simrol, Khandwa Road, Indore, Madhya Pradesh 453552, India e-mail: [email protected]; [email protected] S. Pakhira Department of Physics, School of Basic Sciences, Indian Institute of Technology Indore (IIT Indore), Simrol, Khandwa Road, Indore, Madhya Pradesh 453552, India Centre for Advanced Electronics, Indian Institute of Technology Indore (IIT Indore), Simrol, Khandwa Road, Indore, Madhya Pradesh 453552, India © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Kumar and P. Devi (eds.), Photoelectrochemical Hydrogen Generation, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-16-7285-9_1

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Fig. 1 Hydrogen atom with atomic number, symbol, and atomic mass in the chemical periodic table [6]

understanding of various energy sources and the emphasis on various technologies available to produce hydrogen in present and future with their viability. Hydrogen has the simplest structure containing a single proton in the nucleus and a single electron revolving around the nucleus, and it is the lightest chemical element in nature, and that’s why it is placed at first position in the modern periodic table (atomic number 1), as depicted in Fig. 1. H2 is available in the form of several complexes with an abundant amount in the earth’s interior but the diatomic gaseous form of H2 is very scarce in the atmosphere of the earth so it must be produced by some other methods [4]. The other natural elements or compounds which contain hydrogen are petroleum, coal, and water. It has been stated that hydrogen contains high-energy density per unit mass compared to any other conventional fuel, but, at the same time, it is the lightest element in the periodic table with the lowest energy density per unit volume [5]. Two-third of the whole universe is made up of this hydrogen element. H2 is a diatomic molecule, and it exists in nature in gaseous form. Hydrogen is highly reactive in nature which means it can easily react with the other elements in the periodic table formed chemical complexes/compounds [7]. It has density 1/40th of the air at room temperature and because of low density, it can quickly disperse in the air. At 20 K, H2 exists in a liquid state. It is one of the highest combustion energy densities among other energy resources (four times more than gasoline). Because of its higher combustion energy, it gathers enormous interest as a carrier of renewable energy [5]. As hydrogen is a noncarbon-based energy carrier, it comes under the clean and green energy. It can be the best solution to reduce the use of conventional natural fossil fuels and environmental problems. 71% of the earth surface is covered by water, i.e. H2 O, and water is the rich source of H2 . So, in future water can be

1 Hydrogen: A Future Chemical Fuel

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an energy provider by giving H2 and it will help in reducing the greenhouse gas emission. The greatest disadvantage of hydrogen is that it is highly flammable, but, at the same time, it is an advantageous point because we do not need to provide extra energy for fuel ignition. The gas-to-air volume ratio is 5–15% for the natural gas inflammable limit but for hydrogen, it ranges from 4 to 75%. Hydrogen is non-toxic and disperses rapidly when it is released. Despite knowing various applications of hydrogen, we will only focus on its application as a carrier of energy. More details about the hydrogen are listed in Table 1 [8–10]. Before studying hydrogen energy as a fuel, let us look in the past uses with discovery and development of hydrogen fuel. In 1776, the hydrogen was first discovered by Henry Cavendish, a British scientist [11]. He produced hydrogen through Table 1 Physical and atomic properties of the hydrogen (H2 ) atom [7] Properties of hydrogen Appearance

Colourless, odourless

Standard atomic weight

1.008

Atomic number

1

Group

1, s-block, (alkali metals)

Element category

Reactive non-metal

Electronic structure

1s1

Physical property Phase at standard temperature and pressure (STP)

Gas

Density (at STP)

0.08970 g/L

Melting point (M.P)

14 K

Density in liquid state (at M.P.)

0.076 g/cm3

Density in solid state (at M.P.)

0.0763 g/cm3

Boiling point (B.P)

20.271 K

Density in liquid state (at B.P.)

0.07099 g/cm3

Critical point

32.938 K, 1.2858 MPa

Molar heat capacity

28.836 J/(mol·K)

Heat of vaporization

0.904 kJ/mol

Heat of fusion

0.117 kJ/mol

Atomic properties Oxidation states

+ 1, −1 (an amphoteric oxide)

Crystal structure

Hexagonal

Ionization energies

1311.9 kJ/mol

Covalent radius

31.0 ± 5 pm

Electronegativity

2.20

Magnetic ordering

Diamagnetic

van der Waals radius

120 pm

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the reaction between hydrochloric acid (HCl) and zinc (Zn). During a demonstration in front of the Royal Society of London, he sparked the hydrogen gas which in turn yielded water. This invention later helped in the discovery of constituents of water which are found to be oxygen and hydrogen. The hydrogen is named by Antoine Lavoisier, a French chemist in 1788. The name is generated from the combination of Greek words, i.e. “Hydro” means water and “Genes” means born thus born of water. Anthony Carlisle and William Nicholson invented the electrolysis process in 1800 [1]. He produced oxygen and hydrogen in gaseous form by supplying electric current to the water. Christian Friedrich Schoenbein invented the fuel cell effect in 1838. It is an electrochemical reaction which produces electric current by mixing hydrogen and oxygen gas. After the invention of fuel cell effect, an English scientist William Grove secured the title “Father of the Fuel Cell” by confirming Schoenbein’s invention upon a practical scale and creating a “Gas Battery” in 1845 [12, 13]. Later on William Grove, Charles Langer, and Ludwig Mond built a coal gas and air-powered fuel cell in 1889. Various national and international organizations have been established for encouraging technology advancement of hydrogen energy, and some of the important organizations are enumerated in Table 2 [1, 8, 14].

1.1 World Energy Scenario United Nations predicted that by 2008, nearly half of world’s population will be living in big cities and by 2050, 70% population of the world will live in megacities (the total population of the city exceeding 10 million is also known as megacity) [15]. The hydrogen and its technologies will help in reducing pollutants like greenhouse gases, smog, fumes, and noise. Most of the countries around the world are trying to update their energy models by using a renewable energy source to reduce harmful emissions and pollutants. To analyse the worldwide need of hydrogen energy, it is very important to look at the world energy scenario. The world energy scenario of the past, present as well as future possibility’s data, facts, and figures are monitored and reported by the various international organizations like IEA, USDOE, etc. (listed in Table 2) [16]. The World Energy Outlook does not predict the future, but they provide us with various possibilities which can be used to improve the current scenario and establish a connection between various parts of the system [17, 18]. The report and statistical model presented by the IEA in the world energy outlook 2019 represent the current situation and future projections (till 2040) [16, 17]. The IEA released a graph (see Fig. 2) showing the total global energy generation by the different renewable and non-renewable sources with respect to years from 2000 to 2040 [19]. The graph shows that the power generated by renewable sources like wind, hydro, solar, etc. could be increased by time being. The global energy consumption data from 1990 to 2040 by different renewable and non-renewable sources has been collected and plotted in the International Energy Outlook-2019 (see Fig. 3) [20]. The study demonstrates that the global energy consumption is gradually increasing every year. The plotted data reveals that the rate of increment of renewable energy

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Table 2 List of international organizations working for the development of the hydrogen economy Year

Organization

Purpose

1923

World Energy Council

Encouraging sustainable usage and supply of energy all over the world

1958

National Aeronautics and Space Administration (NASA)

America’s organization for space program which uses maximum hydrogen for rocket propulsion in the whole world

1974

International Energy Agency (IEA)

Research to develop the technology related to hydrogen energy

1974

International Association for Hydrogen Energy (IAHE)

Research, development, and commercialization

1989

National Hydrogen Association (NHA)

An association established for coordinating social, industrial, and scientific aspects of hydrogen technology

1989

International Organization for Standardization Technical Committee

An international board for standardizing technologies related to hydrogen

1977

United State Department of Energy (USDOE) Fuel Cell Technologies Office (FCTO)

Emphasis on innovating advanced fuel cells of hydrogen through intensive research thus helps in making local economy more robust by using newly discovered technologies

2003

International Partnership for Hydrogen and Fuel Cells in the Economy

To promote the international collaboration for facilitating research and development for technology related to H2 economy and fuel cell technology and using this technology to fulfill energy needs and providing stability to the world economy

2017

Hydrogen Council

To bring together relevant private-sector players

consumption suddenly increases after 2006 compared to non-renewable energy. The most important part of the graph is the future projection which says renewable energy consumption will progressively increase and the energy consumption by coal will decrease. According to USDOE, production of hydrogen exceeds 10 million metric tons in a year. The IEA published a report in June 2019 indicating that the global annual demand for hydrogen energy is expanding gradually (see Fig. 4) [16]. The report shows that the global demand of hydrogen in 1975 was around 18 million tons and after 43 years the demand was increased to 75 million tons which means around 70 million tons of hydrogen is being used today in pure form. In future, it is anticipated that the demand of H2 energy will increase as the production and consumption of energy have been gradually rising.

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Fig. 2 Global power generation capacity in Gigawatt (GW) by different energy sources with respect to years (2000–2040). The graph represents current data as well as future projections [19]

Fig. 3 Graph of world energy consumption from the year 1990 to 2040. The graph represents current data as well as future projections [20]

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Fig. 4 The global annual energy demand for hydrogen. The “Refining”, “Ammonia”, and “Other pure” represent demand for specific applications that require hydrogen with only small levels of additives. “Methanol”, “Direct reduced iron steel production”, and “other mixed” represent demand for applications that use hydrogen as part of a mixture of gases, such as synthesis gas, for fuel, or feedstock [16]

2 Hydrogen as a Clean Fuel and Its Properties 2.1 Hydrogen Energy Unlike natural gas or petroleum oil, hydrogen is not directly available in nature, and therefore hydrogen cannot be considered as a primary source of energy. Thus, H2 is a secondary energy source, just like electricity. It can be used for transferring or storing energy. Worldwide, approximately 300 billion litres of hydrogen are produced annually by a variety of chemical processes to use in different purposes like industry, transportation, green vehicles, etc. [9]. Hydrogen has various industrial applications like in the production of methanol, dyes, plastics, fertilizers, and drugs, for the conversion of coal into gasoline and in the hydrogenation of fats and oils. The overall chemical reaction of the fuel cell comprises the oxidation of hydrogen with oxygen to give water. In addition, rather than allowing the two gases to react directly and produce heat energy, the fuel cell utilizes redox reactions to generate energy in the form of an electric current [21]. Several large-scale fuel cells have been tested in the United States. A 4.5-megawatt pollution-free fuel cell power plant has been successfully operated in Tokyo, Japan [22–24]. In 2014, a commercial power plant made by hydrogen fuel cell, “Gyeonggi Green Energy facility” with 59 MW capacity started its operation in Hwasung City of South Korea. Earlier, it has been reported that a company known as “Daesan Green Energy” has started the construction of another power plant, which is claimed to be the largest in the world and also they are pushing the agenda of increasing the overall capacity to 1 GW by 2030. Several methods have been proposed in which solar or wind energy could

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be applied to generate electricity for electrolysis of water resulting in H2 and O2 production, whereas the energy is stored and continuously supplied to a fuel cell power plant to produce electricity. In addition, its major combustion product with air is non-polluting water which is returned to the environment. Nitrogen oxides are also formed when hydrogen burns, but the level is lower than that produced during hydrocarbon combustion. Hydrogen energy is stored in underground tanks by forming chemical compounds in compressed state, when needed it is converted back to hydrogen by heating the compound. It can also be used in remote places where electricity cannot reach because hydrogen supply through pipes is cheaper than electric supply through wire [9, 10, 14, 15, 25].

2.2 Benefits and Limitations of Hydrogen Energy The H2 energy has various advantages and disadvantages which are simplified and summarized in this section.

2.2.1 1.

2.

3.

4.

5.

Benefits of Hydrogen Energy

Renewable energy source and generous in supply H2 is available in the vast amount in the earth, thus H2 is a rich energy supplier. It can be generated from various primary energy resources like fossil fuel, biomass, etc. So, it is possible that the hydrogen energy will replace the conventional fossil fuels such as, petrol, diesel, etc. [25]. Clean and green energy As hydrogen is a noncarbon-based energy source, the by-product of burnt hydrogen is completely safe, i.e. it has no side effects, and it does not generate any greenhouse gases. After the utilization of hydrogen, it transforms into water and it is so clean that it can be used as a drinking water. Therefore, it is known as a clean energy and green source [2]. Non-toxic As hydrogen is a clean energy source, and unlike other fossil fuels, it does not result in any harmful effect on our health. Other fossil fuels are extremely hazardous and destroying in nature. Therefore, it is preferred over other harmful and hazardous fuels [26]. Fuel efficient As a chemical fuel, hydrogen is superior to petroleum considering its combustion properties since it can provide more combustible energy per unit mass as the petroleum produces the combustion energy about 48 kJ/g, whereas hydrogen produces the combustion energy 142 kJ/g [8]. Because of its high fuel efficiency than other energy sources, the hydrogen vehicle can run more miles than other vehicles which have the same amount of fossil fuel [12, 26]. Can be used to power spaceships and vehicles

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6.

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Hydrogen energy can accelerate the spaceship rocket and so it is an ideal energy source for the spacecrafts. To complete the huge task of rocket propulsion, a lesser amount of hydrogen energy is needed. Nowadays, liquid hydrogen is the propulsion fuel for the spacecraft at NASA and other organizations, and hydrogen fuel cells drive the spaceship electrical system. Not only spacecraft, but it can also power up the aeroplane, bus, car, truck, boat, watercraft, etc. The main bottleneck to practical and commercial use of hydrogen is that we cannot directly use hydrogen to drive the car because the storage of hydrogen in tanks is very difficult at high pressure or cryogenic state [27]. Hydrogen fuel cells The fuel cell produces electrical energy, some heat, and water by utilizing the hydrogen’s chemical energy with high efficiency which is almost twice of conventional technologies based on combustion. These H2 fuel cells are alternatives to batteries which are normally used in many portable devices for power supply. These cells can act as an auxiliary source of power for traditional transportation technologies. In future, hydrogen can also be a better substitute for petroleum-based vehicles like buses, trucks, and cars. We already discussed previously that the fuel cells are used for supplying electricity to the space shuttle electrical system [26–29].

2.2.2

Limitations of Hydrogen Energy

Despite various advantages, it is not preferred as a cheap and clean energy source by various industries and governments. Hydrogen is highly volatile, which is an advantage as a fuel, but simultaneously great care must be taken while handling, storing, or using it. The hydrogen as an alternative green energy source has some disadvantages which are written below: 1.

2.

3.

Expensive fuel Steam reforming and water electrolysis are very expensive compared to fossil fuels or other energy sources. Thus, they are not preferred for hydrogen production in most of the places because of the production cost of the fuel. Nowadays, hydrogen is mostly used in powering hybrid vehicles. Thus, intensive innovation and research are needed to make the production process of hydrogen cost-effective [8, 12, 23, 24]. Storage complications Hydrogen needs to be liquified and stored at very low temperature because in liquid form, it requires less storage place and it can be used more effectively in this way. The hydrogen is liquified at very high pressure and stored in thick containers, thus careful handling is must required while transporting these containers that is why it is not feasible for common use. Safety concern The hydrogen is very volatile and odourless gas, thus in the situation of a leakage it is very hard to detect or smell the leak which makes it very unsafe to use. Thus, a suitable type of sensor is required which could detect the leaked hydrogen gas

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in the environment. There are various cases reported in the past which tell us that the accidents are frequent while using hydrogen as a fuel [3, 8, 12]. Challenging transportation Hydrogen transportation is very challenging for on-board vehicular applications because it has a very low density. Instead, coal transportation by trucks is easy and the fuel oil can be transported by pipelines or in oil containers. A large shipment of hydrogen fuel is very difficult due to its flammability and highpressure storage and thus it generally moved in lower quantities. Difficulty in fulfilling population needs H2 supply is very advantageous but the production cost prevents intensive uses in various areas. It must require a complete replacement of existing fossil fuelbased system to use hydrogen as fuel. So, it is a great challenge because fossil fuel-based systems are the most used systems in the world. If we can somehow make the hydrogen fuel sustainable and cheap, still we must transform the whole system to make it suitable for the use of hydrogen as easy as possible. This process will require huge capital to transform fuel stations and vehicles so they can utilize hydrogen as a fuel [12, 21, 29].

From the above-listed advantages and disadvantages of using hydrogen as an energy carrier, we understand that despite some disadvantages it also has major advantages. The next section will explain more about the challenges on developing of hydrogen energy, from the production to commercialization, and how to deal with those challenges.

2.3 Challenges and Hope The main challenges towards major industrial uses and the transportation are the supply and storage system with cost-effective and good efficiency. We have divided the challenges into two major categories, first one is technical challenges and another one is an economic concern.

2.3.1

Technical Challenges

The main technical challenge to develop the hydrogen energy is the difficulty in system integration. The system integration of an energy system is an approach to solving energy challenges that explores ways for energy systems to work more efficiently on their own and with each other. The development of hydrogen energy system faces various challenges such as robust storage and distribution system, inadequate durability, slow refuelling system, and the most serious challenge is high cost [26, 29]. Recently, hydrogen storage systems have excessive volume and weight which produce faulty end results compared to conventional storage system using fossil fuel like petroleum oil. To improve the driving range of a vehicle running on the hydrogen

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combustion engine, the liquified hydrogen is stored on the vehicle in a pressurized tank. Hydrogen can also be stored in empty oil fields or an underground structure as done by various chemical factories. Due to the safety and environment concern, storage and supply of the hydrogen are not an easy task. We should take special care for storage of H2 because even a minute leak can cause an explosion when sparked in the presence of air [27, 30]. Thus, hydrogen can be highly dangerous for underground closed spaces like tunnels or caves. To overcome this problem, it is very important to develop better infrastructure, and the previously designed infrastructures are very expensive. So, from the starting of the hydrogen technology, researchers are trying their best to develop a cost-effective integrated system to store H2 . The development of cost-effective infrastructure has been already started like Hydrogen Highway, nanomaterials for the storage of hydrogen, a good catalyst to help in hydrogen production, and many more so that everyone can adopt the renewable energy resources with good efficiency [31–38].

2.3.2

Economic Concern

According to Bose and Pierre, the estimated price of hydrogen production is 14–32 $/GJ and the total price of storage unit cost is 35–50 $/GJ with the distribution and transportation cost around 22 $/GJ [27]. According to EIA report, the average retail price of gasoline is 22.38 $/GJ, during 2010 to 2019 which includes the price of crude oil (13.2 $/GJ), refining cost (2.9 $/GJ), distribution and marketing cost (2.7 $/GJ), and taxes (3.58 $/GJ) [39]. The high cost of infrastructure and low efficiency of hydrogen energy result in poor commercialization. So, the better infrastructure with low cost can lead to more production supplied to the market in near future. Although the overall demand for hydrogen energy is getting increased by 5 to 10 million tonne per 5 years [40]. The production of hydrogen is possible from both renewable and non-renewable energy sources but it is very costly to produce hydrogen from renewable energy sources. Therefore, we must need to build up a cost-effective methodology for hydrogen production from renewable sources. The various international organizations are developing various policies and R&D targets to promote the hydrogen energy for the large-scale as well as small-scale applications [41, 42]. For the breakthrough result in the demand of the hydrogen energy and establishment of better commercialization of hydrogen technology, it is very important that the government, various organizations, industries, and various R&D organization should come together and work for the growth of hydrogen technology [28, 30].

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3 Timeline of Hydrogen Technology, Production and Storage Methods, and Applications 3.1 Hydrogen Technology Timeline The timeline of hydrogen economy describes the journey of hydrogen from an atom to fuel cell and a future chemical fuel (see Fig. 5) [42]. In the introduction section, we have already discussed the discovery of the H2 molecules and their properties. The practical realization of the fuel cell has been reported by various research groups. Furthermore, the main research towards the development of infrastructure to produce hydrogen energy has been already started in the last decade. The research in the field of developing infrastructure and storage unit for small-scale to large-scale applications was carried out in the years of 1950–2000. The emphasis on commercialization of the hydrogen energy along with the research on system integration was also done from the year 2000 to till date. Following the same trend, we have estimated that the market for hydrogen energy will see immense growth, and efforts will be made to substitute the conventional fuels by hydrogen energy from this current to 2040.

Fig. 5 Timeline diagram of the hydrogen economy

1 Hydrogen: A Future Chemical Fuel

13

3.2 Hydrogen Production and Storage It has found that approximately 95% of all hydrogen in industry is extracted by the steam reforming process, which is a conventional technique of hydrogen extraction from fossil fuels. Other than this, 5% of hydrogen is produced by electrolysis method. In electrolysis, water molecule splits into oxygen and hydrogen by the application of electric current [43]. H2 can be formed by several electrochemical reactions noted as H2 evolution reactions (HER) using various electrocatalysts such as transition metal dichalcogenides and their alloys [36, 44]. There are other hydrogen production methods available, but they are still in their initial phase of research. The section below contains different methodologies for hydrogen production.

3.2.1

Steam Methane Reforming

In USA, steam methane reforming process contributes 95% of total hydrogen produced and natural gas is the main source for this process. In presence of a catalyst, a reaction takes place between natural gas (CH4 ) and steam, and carbon monoxide (CO) and hydrogen are produced as a result. This process takes place at high temperature range from 700 to 1000 ◦ C and pressure range from 3 to 25 bar. Again, previously produced CO and steam reacts with each other and produces more H2 and CO2 . After the completion of the reaction, the remaining impurities and carbon dioxide are removed, and the pure hydrogen remains. The process mentioned above is known as “Pressure Swing Adsorption”. There are alternatives like petrol, diesel, propane, and ethanol which can be used in the place of methane. Nickel-based catalysts are the most suitable for this type of reaction. This process is called “Steam Methane Reforming”. Below equation shows the chemical reactions involved in this process [45]: CH4 + H2 O = CO + 3H2

(1)

CO + H2 O = CO2 + H2

(2)

The production of H2 using steam methane reforming method is very costeffective, thus it is the most preferred hydrogen production method used in the United States of America (USA). It emits a very low amount of greenhouse gases compared to other petrol-based methods. But this process has a major disadvantage that hydrogen production depends upon a non-renewable source of energy. Another method is the hydrogen production from methane derived from the biomass sources, in which the H2 production depends upon a non-renewable source of energy. The biomass or food waste is a green and renewable source of energy, and it would be a huge source of methane. The biomass is an important source of energy in many developing countries like India.

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Coal Gasification

Coal gasification is also an important alternative to produce hydrogen. Coal gasification produces energy, hydrogen, chemicals, and liquid fuels. In this process, a reaction takes place between coal, hydrogen, oxygen, and steam in a high-pressure environment to produce synthesis gas (H2 and CO) [1, 8]. Further reaction between steam and carbon monoxide produces more hydrogen as well as carbon, and this process is called as WGSR (water–gas shift reaction). After the completion of WGSR, pure hydrogen is extracted and the remaining carbon is also drawn out from the system (see Fig. 6) [46, 47]. The various reactions take place during the process which are written below: 3C + O2 + H2 O = H2 + 3CO . . .

(3)

CO + H2 O = CO2 + H2 . . .

(4)

Fig. 6 Block diagram of H2 production from coal gasification process [48]

Fig. 7 The electrolysis method of hydrogen production [48]

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Fig. 8 Schematic of photo-electrochemical decomposition of water for H2 production [50]

The advantage of this process is that we have enough coal available for 200 years but this method produces various environmental threatening greenhouse gases. Therefore, it is very essential to find out some improving factor for this process so that it will be eco-friendly. So, the first step is to minimize the production of carbon dioxide during this process or preventing the release of produced carbon in the atmosphere. The most promising way to do that is to find a substitute process for the cryogenic separation process of oxygen extraction from the air using membranes.

3.2.3

Electrolysis

In this process, water molecules (H2 O) split into its constituent’s oxygen (O2 ) and hydrogen (H2 ), when a high electric current is passed through the electrolysis system [36, 49]. The electrolysis or electrolyser system consists of cathode and anode dipped into aqueous KOH solution (electrolyte). This whole system is also called electrolyser. According to the production scale of the hydrogen, suitable electrolysers are selected from large scale to small scale. This technology is very advantageous for “Distributed Hydrogen Production”. Producing hydrogen with the help of electrolysis is an environment-friendly method because it produces a very low amount of greenhouse gases or can be zero greenhouse emission. The emission of greenhouse

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gases is depending on the sources which provide energy for the splitting of H2 O molecules from the solvent. Nuclear energy or wind energy can also be used to supply energy to the electrolyser. The cost of electrical energy transportation can be saved by using these kinds of energies. The various reactions take place during electrolysis, which are mentioned below: 4H2 O → 4H+ + 4OH− (Electrolyte) . . .

(5)

4H+ + 4e− → 2H2 (Cathode) . . .

(6)

4OH− → O2 + 2H2 O + 4e− (Anode) . . .

(7)

2H2 O → O2 + 2H2 (Overall) . . .

(8)

This process is very easy to use in industries because of its simple structure but also there are some improvements which can be employed in this method. The first one is to use polymer membranes of acidic nature in the place of an aqueous solution of an electrolyte. The second one is to carry out the electrolysis at a higher temperature because it takes less energy at high temperature compared to the lower temperature. The third one is to use a solar power-charged photovoltaic system to supply electric current to the electrolysis system because this system is very eco-friendly and it uses the renewable source of energy [44, 49]. But, the main drawback of using this method is that the manufacturing of such a system is very costly, and this system is not very efficient compared to other alternatives. Another challenging task is that we must interface the compressor, so we do not have to use another compressor which is desirable for storing hydrogen at high pressure and reduce the cost of a separate compressor.

3.2.4

Biomass Gasification

Biomass is going to be a potential energy option to produce hydrogen. Biomass gasification process is thermochemical in nature. In this process, industrial and organic wastes are decomposed to produce hydrogen, carbon dioxide, carbon monoxide, and methane in the presence of oxygen. This process takes place at high temperature 700 ◦ C. Carbon monoxide further transforms into hydrogen (H2 ) and carbon dioxide (CO2 ) with the help of WGSR. After the completion of this process, hydrogen is separated and stored. The one drawback of this process is that the steam requires external energy for increasing its temperature. The system used for biomass gasification is known as a gasifier. First, a chain reaction happens between steam and biomass, which produces H2 and CO [45, 46]. C + H2 O = CO + H2 . . .

(9)

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Then water–gas shift reaction happens between water and carbon monoxide CO + H2 O = CO2 + H2 . . .

(10)

This process has the main advantage of using renewable energy as its source with very low greenhouse gas emission and it takes CO2 from the environment during the process itself [45, 48, 50]. There are some extra hydrocarbon compounds generated during this process which are harmful to the environment. Thus, to overcome this problem some steps must be taken like using a catalyst to convert these compounds into useful synthesis gas [47].

3.2.5

Photoelectrochemical Decomposition of Water

This process uses water and sunlight which are the most easily available as renewable resources for producing hydrogen. The setup for this process consists of two electrodes: one of them is anode which is a semiconductor and another electrode is called cathode which helps in generating hydrogen [50, 51]. The semiconductor generates an electron when it is exposed to the sunlight. These electrons are sufficiently powerful to generate hydrogen from water. The cells used in this setup to generate hydrogen are called “photoelectrochemical cells”. According to the commercial point of view, this process is still in its development phase, so focussed research and more attention are needed to make it commercially viable [45, 48].

3.2.6

Thermochemical Water Splitting

This process involves a chemical reaction to generate enough heat energy to produce hydrogen at very high temperature in the range of 500–2000 ◦ C, shown in Fig. 9. The heat energy can be produced by the means of solar or nuclear energy. Thermochemical water splitting process involves a series of chemical reaction to produce hydrogen. During this process, only water gets consumed and the chemicals can be reused in the next cycle. The involved reactions are mentioned below [52]: 1 H2 SO4 (850◦ C) → SO2 + H2 O + O2 . . . 2

(11)

I2 (120◦ C) + SO2 + 2H2 O → H2 SO4 + 2HI . . .

(12)

2HI(450◦ C) → H2 + I2 . . .

(13)

This process is preferred for centralized and large-scale hydrogen production. It produces very pure hydrogen without emitting any greenhouse gases. But first we have to develop a nuclear reactor which can supply required heat energy at the

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Fig. 9 Schematic of thermochemical water splitting chemical reaction for H2 production [52]

desirable low-temperature range. It was found that using solar energy minimizes heat transfer medium and solar contractor’s cost [47, 49, 50, 53].

3.2.7

Biohydrogen Production

This process has become prominent because it uses waste materials as a source for producing hydrogen. In this process, micro-organisms like anaerobic bacteria, algae, etc. decompose the aqueous solution of the biological waste at ambient pressure and temperature. These types of processes are very useful in the areas where the biological waste or biomass is easily available because the transportation cost is saved, and the cost of raw materials is negligible. Although we must consider the availability and biodegradability of the biological material to produce hydrogen before establishing the setup at a prescribed location.

3.2.8

Hydrogen Storage

From the economy and ecological point of view, hydrogen storage is very important because it can be used as an alternative to petroleum fuel. There are various hydrogen storage techniques which have been already developed but none of them are practically efficient and able to achieve the on-board international target. The United States Department of Energy (USDOE) has revised the international target for H2 storage and its application in various energy technology, and, in general, they revise

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Table 3 United State Department of Energy Targets 2020 for on-board hydrogen storage for lightduty vehicles [39] Storage parameter

Units

2020

2025

Ultimate

System gravimetric capacity1

kwh/kg (kg H2 /kg system)

1.5 (0.045)

1.8 (0.055)

2.2 (0.065)

System volumetric capacity

kwh/kg (kg H2 /kg system)

1.0 (0.030)

1.3 (0.040)

1.7 (0.050)

Operating ambient temperature2

°C

-40/60 (sun)

−40/60 (sun)

−40/60 (sun)

Min/max delivery temperature

°C

-40/85

−40/85

−40/85

Min/max delivery pressure

Bar

5/12

5/12

5/12

Operational cycle life Cycles

1,500

1,500

1,500

Well-to-power-plant efficiency3

%

60

60

60

Fuel purity

% H2

99.7% (dry basis)

Storage system cost4 $/kWh net ($/kg H2 )

10 (333)

9 (300)

8 (266)

Fuel cost

4

4

4

$/gge at pump

the international target of H2 storage for every 5 years, to develop an efficient H2 storage system for light-duty vehicles. The USDOE revised the targets to 0.030 kg H2 /L volumetric capacity and 4.5 wt% gravimetric in 2017, which then became a new DOE 2020 target (see Table 3). USDOE, automotive research council of U.S., energy industries, and some organizations through partnership set these targets. For achieving such international targets, the modification and development of various storage technologies are being done. Storage technologies have three types: (1) (2) (3)

Physical methods, Chemical methods, and. Hybrid methods (see Fig. 10).

There are three types of physical methods of H2 storage which includes compressed gaseous, liquefied, and cryogenically compressed. There are two types of chemical methods which include chemical storage (off-board regenerable) and storage in solid state (on-board regenerable). There is also a hybrid technology available to store hydrogen which is a union of physical and chemical methods also 1

Capacities are the ratio of amount of hydrogen available for fuel cell and the whole mass or volume of storage system. 2 No allowable performance degradation from—20ºC to 40ºC. 3 Well-to-power-plant efficiency = off-board efficiency + on-board efficiency. 4 The cost at which FCEV (fuel cell electric vehicles) driven by hydrogen becomes competitive to HEV (hybrid electric vehicles) driven by gasoline on the basis of cost per mile.

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Table 4 The hydrogen storage methods with the description of storage conditions, capacity, advantages, and disadvantage Storage methods

Sub-division of methods

Physical methods

Chemical methods

Hybrid methods

Storage conditions

Gravimetric Advantages capacity (wt%) and volumetric capacity (g/L)

Disadvantages

Compressed 35 or gaseous hydrogen 70 MPa 300 K

2.8–3.8 wt.% and 16–18 g/L at 35 MPa, 2.6–4.4 wt.% and 19–25 g/L at 700 MPa

Most convenient method and a most practical option for automotive industry

Lower gravimetric and volumetric densities, lower compression work

Liquefied hydrogen

1 MPa

4.8–6.8 wt.% 31–39 g/L

Good gravimetric and volumetric storage densities

Low dormancy period up to 2 days and only attractive for short-term storage applications

Cryo-compressed hydrogen

70 MPa Co > Mo = W. Various types of electrocatalysts for HER are discussed below. (a)

Transition metal sulphides: For the marketable production of H2 , the advancement of bio-inspired catalysts is necessary [28, 29]. In a reference from the structure of nitrogenase and hydrogenase present in nature, scientists have explored and concluded that transition metal sulphides are efficient electrocatalysts for hydrogen generation by HER [30]. In the early 1970s, a few researchers started to study the molybdenum disulphide (MoS2 ) material for electrochemical H2 evolution, and finally concluded that MoS2 is an inefficient electrocatalyst for H2 evolution due to its bulk size and less reactivity [31]. Recently, it has been proved that the MoS2 is a novel electrocatalyst with superior electrocatalytic H2 evolution performance to replace the noble metal catalysts like Pt which are scarce and very expensive [32]. MoS2 has received tremendous attention in the field of nanotechnology and nanomaterials science due to its unique hexagonal structure. Nørskov and coworkers studied the equilibrium structure and properties of the MoS2 by applying first-principles based density functional theory (DFT), [33–41] and they investigated the possible application of the MoS2 material in H2 evolution reaction [42]. Their DFT calculations showed that MoS2 has many active edge sites and the basal planes, which are inert (i.e. inactive) for hydrogen evolution. They carried out an experiment by preparing MoS2 nanoclusters on graphite to confirm their DFT study. The scanning tunnelling microscope image of the MoS2 nanoclusters supported by graphite is shown in Fig. 5 [42].

There are various ways to enhance the electrochemical performance of the pristine MoS2 . For example, the electrochemical performance of MoS2 was improved by making more active sites to get exposed during reactions by doping with suitable metal atoms (such as transition metals) into the lattice structure and also by integrating MoS2 with electrically conductive materials such as graphene and carbon nanotubes [43]. Liu and coworkers synthesized the exfoliated MoS2 nanosheets from the bulk particles of MoS2 by using direct dispersion and ultrasonication methods [44]. These MoS2 nanosheets showed better electrochemical performance for HER by improving

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Fig. 5 Scanning tunnelling microscope (STM) images of MoS2 nanoclusters on graphite support [42]

the electrochemical parameters like overpotential, current density, etc. The electrochemical parameters have enhanced the electrical conductivity of the exfoliated MoS2 nanosheets compared to the bulk MoS2 , which increases the efficiency of electrocatalytic activity for H2 evolution [44]. Jin et al. experimentally found that the MoS2 has two phases: (i) 2H-MoS2 and (ii) 1T-MoS2 and the change of phase from 2H-MoS2 to 1T-MoS2 can significantly enhance the HER catalytic activity of the chemically exfoliated MoS2 . This research opens up a new technology in phase engineering science [45]. Balasubramanyam et al. showed that plasma-enhanced atomic layer deposition (PEALD) can be used as a new approach to nanoengineer and performance of WS2 can be enhanced for HER by maximizing the density of reactive edge sites [46]. They carried out DFT calculations to support their experimental observation and to explain how the addition of H2 to the H2 S plasma impacts the PEALD growth behaviour [46]. Both the pristine MoS2 and WS2 are transition metal dichalcogenides (TMDs) of the MX2 type with Mo and W as transition metals and S as a chalcogen anion in their structure. Two-dimensional layer structure materials of these pristine MX2 type of TMDs have a high surface area of one-atom-thick which can further enhance electrocatalytic activity [47]. Very recently, Lei et al. synthesized Wx Mo1-X S2 /rGO (where rGO = reduced graphene oxide) heterostructure films by using a simple wet chemical approach [25]. They found these TMD alloys (i.e. Wx Mo1-X S2 ) have efficient catalytic activity for HER, and they found a Tafel slope of 38.7 mV decade–1 at 96 mV onset potential when the heterostructure alloy was annealed at 300°C. Amongst all the TMD alloys, the heterostructure formed by reduced graphene oxide and W0.4 Mo0.6 S2 alloy is found to be more efficient than the pristine 2D WS2 and MoS2 materials. They carried out a computational investigation by applying a DFT method to analyse their experimental observation by investigating the detailed HER mechanism as depicted in Fig. 6. To find out the HER pathways, they computationally studied both the periodic model and a molecular cluster model, and their computational results are well harmonized with the experimental observation [47].

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Fig. 6 Complete HER mechanism and two-electron transfer reaction pathways on the surfaces of 2D MoS2 computed by DFT method (See the Supporting Information of [25])

Therefore, they concluded that the TMD alloys have better electrocatalytic performance than the pristine one and amongst all the compositions of the Wx Mo1-X S2 , the W0.4 Mo0.6 S2 alloy is the best Pt-free electrocatalysts for HER as shown in Fig. 7. Kong et al. experimentally found that the CoS2 has an excellent electrochemical activity for H2 evolution compared to FeS2 and NiS2 [48]. They continued to study first-row TMDs (ME2 , M = Fe, Co, Ni; E = S, Se) to test the performance of the electrocatalytic activity for HER in acidic electrolyte media, and interestingly they found that these TMDs exhibit excellent electrocatalytic activity for HER especially in

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Fig. 7 a Activation barrier for Volmer reaction (H+ migration), b Activation barrier for Heyrovsky reaction (hydrogen molecule formation) [25]

their nanoparticle form. In brief, structural modifications, phase change, and external metal-atom doping in the pristine TMDs are the special engineering technology to enhance the catalytic activity for H2 evolution. These compounds, especially pristine TMDs and their alloys expand and enrich the family of high-performance HER catalysts. At present, a lot of structural modifications like external doping of suitable metals to the pristine lattice structure of transition metal sulphides have been performed to increase the hydrogen adsorption reactivity of the electrocatalysts. The experimental electrochemical parameters of various TMDs and their alloys are reported in Table 2. Amongst these various TMDs, it can be seen that the Tafel slope of the 1T-Pt-MoS2 has lowest value compared to the others [51]. This lower value of the Tafel slope of the 1T-Pt-MoS2 shows that it has better electrocatalytic activity for H2 evolution. (b)

Transition metal phosphides (TMPs): Phosphides of transition metals like TMDs are another kind of novel material which are Pt-free electrocatalysts for electrochemical reactions, especially for H2 evolution. These kinds of TMPs catalysts have outstanding chemical stability and mechanical and electrical properties. Liu and Rodriguez computationally found that Ni2 P (nickel phosphide) is an excellent electrocatalyst for hydrogen evolution reaction as the strong H − Ni interaction on Ni2 P(001) can lead to poisoning of the highly active sites of the surface, which enhances the rate of the HER and makes it comparable to that of the [NiFe] hydrogenase [56]. Later, they observed that the catalytic activity of the Ni2 P is comparable to the Pt metal catalyst [56]. It has shown that the electrocatalytic activity for HER of the TMPs can be enhanced by proper utilization of phosphides and transition metals in the respective TMPs materials [57, 58]. Later on, a large number of TMP materials have been developed by a group of scientists keeping the NiFe hydrogenase

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Table 2 Electrochemical parameters of transition metal dichalcogenides (TMDs) and their alloys from experiments Electrocatalyst

Electrolyte media

Substrate

Tafel slope (mV decade-1 )

Overpotential (mV)

References

2H-MoS2 monolayer

0.5 M H2 SO4

silicon

60

170

[49]

W0.4 Mo0.6 S2 2D monolayer

0.5 M H2 SO4

gce

38.7

96

[47]

Porous 1T-MoS2 nanosheets

0.5 M H2 SO4

gce

43

153

[50]

1T’ -Pt-MoS2

0.5 M H2 SO4

gce

25

35

[51]

1T-WS2 nanosheets

0.5 M H2 SO4

graphite

70

140

[52]

FeS nanosheets

0.1 M KOH

carbon cloth

36.9

142

[53]

Ni-Mo-S

0.5 M H2 SO4

carbon cloth

85.3

200

[54]

Zn-MoS2

0.5 M H2 SO4

gce

51

300

[55]

gce-glassy carbon electrode

as a reference. Nickel phosphides nanoparticles (Nix Py ) with various configurations were experimentally synthesized by Chen et al. [59]. They experimentally found that the Nix Py has many structural phases like Ni5 P4 , Ni3 P, Ni12 P5 , Ni5 P2 , etc., and that all these materials are good catalysts for HER due to more activity of H atom adsorption on the surfaces of the 2D materials [59]. Ni12 P5 nanoparticles were experimentally synthesized using a cost-effective autocatalytic technique [60]. Interestingly, it was found that the electrochemical parameters like Tafel slope (~45 mV decade−1 ) improved a lot and is close to the noble metal catalysts. Thus, the electrochemical H2 evolution performance of the Ni12 P5 nanoparticles was enriched during water-splitting reactions. Ni3 P electrocatalyst has been proven to be an excellent electrocatalyst for HER, and it was computationally observed that Ni3 P is a chemically stable electrocatalyst which has high active sites for H2 evolution [61]. It has also been found from previous reviews that several transition metal phosphides (TMPs) like phosphides of cobalt, iron, and copper have shown the lowest energy barrier for hydrogen adsorption on their surfaces which indicates excellent catalytic activity for HER. For example, in acidic media, cobalt phosphides (CoPx ) have been found to be promising electrocatalysts for H2 evolution, and their performance of HER is quite high. CoP nanoparticles were synthesized using a thermal decomposition method. The nanoparticles were found to have overpotential Eg

OHe

h+

h+

+

h

+

h

h+

Gh< 0

h+

e-+h+

CB

Path A

H+ O2/H2O

VB 3.0

Eg

e-

-

O2

ΔG > 0

e-

h+

e-+h+ Path B Volume recombination

Fig. 3 a Electronic structure of semiconductor photocatalysts and Gibbs energy change in photocatalytic reactions, b Schematic of surface photocatalysis processes (i) formation of exciton (ii) charge transfer processes, (iii) bulk and surface recombination processes, (iv) electron and hole induced redox reaction on photocatalyst surface; A: electron acceptor, D: electron donor

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e− + A ↔ A− (A = electron acceptor)

(1)

  2e− + 2H+ ↔ H2 ; E0 = −0.401 eV, pH = 7

(2)

Oxidation half reaction: h+ + D ↔ D+ (D = electron donor)

(3)

  4h+ + 2H2 O ↔ O2 + 4H+ ; E0 = 0.828 eV, pH = 7

(4)

Overall Reaction:   1 H2 O ↔ /2 O2 + H2 ; G = 237 kJ/mol; E0 = 1.229 V

(5)

Recombination: e− + h+ ↔ light or heat

(6)

2.3 Solar Hydrogen Generation Pathways Inspired by the biological photosynthesis, two direct approaches are being explored for solar H2 generation [20]. One approach is called photoelectrochemical (PEC), where two electrodes acting as anode and cathode are wired to complete the circuit. The semiconductor(s) having appropriate energetics (bandgap and band edges position), used as an electrode, harvest the solar energy and concomitantly generate charge carriers (excitation of electron to the conduction band and leaving behind hole in the valance band), which subsequently drive the oxidation of water and proton reduction to H2 on anode and cathode, respectively. This integrates two processes, conversion of solar to electrical energy followed by the electrolysis of water, in a single step. In other approaches, the semiconducting photocatalysts (with or without cocatalysts) are suspended in an aqueous solution, where, these particulate photocatalysts harvest solar radiation creating charge carriers. These photo-generated charge carriers are drifted to the surface of these particulates, facilitating the water-splitting redox reaction at the catalytic sites on these suspended particulate surfaces. This approach is devoid of wiring and thus is simpler and has an economical photocatalytic reactor design. In both approaches, the electron donor (usually water) reduces the photo-generated holes, enabling continuous generation of H2 . Apart from the solar H2 generation, its storage is also a challenge due to its low gravimetric and high volumetric energy density, to realize its potential to meet the

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energy requirements. Currently, the cryo-adsorptive (storage of H2 in liquid state at higher pressure, 70 MPa) and chemical adsorptive storage (storage via binding with zelolites, MOFs, metal as hydrides and some light molecule such as CH3 OH, C2 H5 OH, NH3 ) are being exploited to store of H2 [21]. Since the storage of H2 is beyond the scope of this chapter, it is not discussed further in detail.

3 Enzymes as an Efficient Catalyst for Hydrogen Generation Green microalgal and cyanobacterial hydrogenases are different classes of enzymes having different metal clusters in their active site (reaction centre). These metalloenzymes called hydrogenases evolve separately, but drive the same enzymatic reaction through their respective metallo-clusters, that is the interconversions of protons and H2 [3, 22–24]. The hydrogenases contain an active site comprising non-precious metals (Ni and/or Fe) that are buried deep in the protein. The long range electron transfer in these active sites to and from redox partner is mediated by FeS clusters. Depending upon the metal present in the active sites, there are three types of hydrogenases: [FeFe] hydrogenase, [NiFe] hydrogenase, and [Fe] hydrogenase. The first two hydrogenases are known to bind and activate proton/hydrogen (H+ /H2 ) conversion. The enzymes evolved by nature prosses some outstanding attributes, which are (i) high conversion rate, with turnover frequencies (TOFs) of the order of thousands per second at room temperature being common [25, 26], for example, Allochromatium vinosum [NiFe]-hydrogenase shows kcat of the order of 6000 s− 1 for H2 oxidation and [FeFe]-hydrogenases exhibit 6000–9000 s− 1 for H2 production [27, 28], (ii) high substrate specificity, and (iii) excellent efficiencies with small overpotential requirement. The [FeFe] hydrogenase found in green microalgae (such as Chlorella) has an active site, called as H-cluster, and is composed of [FeFe] bonds with sulphur 179 bridges and 4Fe-4S residue [29]. The non-proteinous ligands CN and CO are attached to both Fe atoms, Fig. 4 [30]. The [FeFe] hydrogenase is a monomeric and has

Fig. 4 The [FeFe] active site with the dithiolate bridging ligand, CO and CN ligands, and Cys5 bridging one Fe to the [4Fe-4S] cluster [3]. The active site shown here is from D. desulfuricans; in C. pasteurianum a CO molecule bridges the two iron atoms in the as-isolated enzyme (Reproduced from Caserta et al. [30])

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molecular weight of around 45–50 kDa. “It is encoded by hydA gene in the nucleus and localized in the chloroplast after enzyme maturation” [29]. The [FeFe] active site, where electrons are transported from ferredoxin, is highly sensitive to oxygen, which restricts the production of H2 under oxygenic conditions. Nonetheless, the [FeFe] hydrogenase exhibits about 100-fold higher activity than that of other hydrogenases [31]. The [NiFe] hydrogenase constitutes the largest number of hydrogenases and may found in Cyanobacteria. It comprise of two compartments, the active site is the larger subunit (~60 kDa) having [NiFe] bonds; whereas the small subunit (~30 kDa) consist of a Fe-S (4Fe-4S or 3Fe-4S) cluster, Fig. 5. Four cysteine residues are bonded by sulphur bonds to metallogenic compartments. The small subunit transports electrons to the active site and protons are reduced to H2 . The studies show that both ferredoxins and flavodoxins act as electron donors to the cyanobacterial hydrogenase [32, 33]. Besides the main differences between [FeFe] and [NiFe]-enzymes as mentioned above, [NiFe] hydrogenases are more oxygen tolerant than that of [FeFe] hydrogenases, but the conversion frequency of [FeFe]-hydrogenase is usually 10–100 times more than that of [NiFe]-hydrogenase, partly because H2 binds tightly in the case of [NiFe]-hydrogenase and is a strong inhibitor. One exception is a subclass known as [NiFeSe] hydrogenase, where cysteine ligand to Ni is replaced by selenocysteine and produces H2 very efficiently and is less sensitive to oxygen. Furthermore, [FeFe] hydrogenases are irreversibly inhibited by O2 , whereas [NiFe] hydrogenases are reversibly inhibited by O2 [34].

Fig. 5 A ribbon representation of the X-ray-determined standard structure of the H2 producing enzymes, a [FeFe] hydrogenase, b [NiFe] hydrogenase (Reproduced from Khetkorn et al. [35])

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3.1 Enzyme-Based Catalysts for Solar Hydrogen Generation In order to simulate the natural photosynthesis, a variety of strategies are being explored. The enzyme that is an efficiecint catalysts has been coupled with sensitizers to develop artificial photosynthesis device/system to produce H2 . It should be noted that the integration of enzyme with light-harvesting sensitizer should be optimal such that it is electroactive (seamless electron transfer across sensitizer-enzyme assemblies) while maintaining the dynamic organization of enzyme. The commonly used sensitizer coupled with metalloenzymes are dye, semiconductor nanoparticles, PSI. There are four major approaches being used to link metalloenzymes to lightharvesters (Fig. 6) (i) non-specific interaction with light-harvesting components, (ii)

Fig. 6 Schematic of possible strategies for enzyme-light-absorber interaction, exemplified with hydrogenase enzyme, a “Photoenzyme”, b “fusion protein” comprising hydrogenase wired to Photosystem-I, c Hydrogenase immobilized on a semiconductor nanoparticle, d photoelectrochemical cell with hydrogenase attached to a semiconductor photocathode (Reproduced from Bachmeier and Armstrong [36])

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specific covalent-linkage, (iii) immobilization on porous semiconducting nanoparticles, and (iv) photoelectrodes [36]. In addition, a few multi-component (cascade) systems are also reported [37–39]. One early attempt was made to design a [Fe–Fe] hydrogenase-TiO2 (semiconductor) hybrid system to produce H2 [40]. Herein, different hydrogenase enzymes were isolated from Clostridium pasteurianum, Desulfovibrio desulfuricans strain Norway 4 and D. baculatus 9974 and subjected to the anatase TiO2 suspension. These enzyme-semiconductor assemblies produce H2 under illumination while using EDTA or methanol as electron donors. It was led by direct electron transfer from the conduction band (CB) of the TiO2 to the hydrogenase active site at a pH range 7–9. This mediator-independent charge transfer becomes more proficient when C. pasteurianum and D. baculatus 9974 hydrogenases were used in the presence of methanol as electron donor, exhibiting the rate of H2 generation about 1104 and 1083 μmol H2 g− 1 TiO2 h− 1 , respectively. It was further enhanced to 1422 μmol H2 g− 1 TiO2 h− 1 when the TiO2 surface was functionalized with Rhodium tris- and bis-bipyridyl complexes in the case of D. baculatus 9974 hydrogenase-TiO2 hybrid. The Rh complex used acts as an efficient e− carrier from TiO2 to the immobilized hydrogenase molecules. TiO2 being a wide bandgap semiconductor can only harvest solar radiation under UV region, therefore, a variety of sensitizer are used to harvest solar radiation in the visible region. These sensitizer could be a metal–organic framework (for example, ruthenium bipyridyl photosensitiser (Ru II(bpy)2 (4,4- (PO3 H2 )2-bpy)]Br2 (‘RuP’) [22, 23] or an polymeric semiconductor such as g-C3 N4 [41]. Reisner and coworkers immobilized Desulfomicrobium baculatum (Dmb [NiFeSe]–hydrogenase and RuP on TiO2 nanoparticles (P25 Degussa.). This hybrid evades the formation of oxidizing holes in the valance band (VB) of TiO2 while harnessing the visible solar radiation also. It exhibits the TOF of about 50 mol H2 (mol hydrogenase)− 1 s− 1 , under mild conditions (25 °C, pH 7.0). In another work, where polyheptazine carbon nitride polymer was used instead of RuP-TiO2 turnover number (TON) of about 5.8 × 105 mol H2 (mol Hydrogenase)− 1 was obtained after 72 h, Fig. 7 [41]. This increased activity is due to the formation of CNx -TiO2 charge transfer complex, facilitating effective electron transfer to hydrogenase, and the extended solar radiation adsorption. The molecular orientation of enzymes on the semiconductor has to be specifically optimized. Electrostatic control of complex formation was achieved by coupling Clostridial [FeFe]-hydrogenase CaI on MPA (3-Mercaptopropionic acid) capped CdTe nanoparticles, Fig. 8 [42]. The ligand used binds via thiol group interactions with the CdTe surface, leaving carboxyl group exposed to solvent. Above pH 4.3, the MPA undergoes deprotonation enabling negative charge, which subsequently facilitates direct binding to hydrogenase (as the enzyme surface around the distal [4Fe–4S] cluster contains an excess of lysine and arginine residue). This hybrid assembly shows TOF of 25 mol H2 (mol hydrogenase)− 1 s− 1 using ascorbic acid as a sacrificial electron donor, but the solar H2 generation reduces after 5 min of irradiation.

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Fig. 7 a Schematic representation of photo-H2 production with Dmb [NiFeSe]-Hydrogenase (PDB ID: 1CC1)14 on CNx-TiO2 suspended in water containing EDTA as a hole scavenger, b irradiation of CNx-TiO2 can result in photo-induced electron transfer by three distinct pathways: i. TiO2 bandgap excitation, ii. Excitation of CNx (HOMOCNx-LUMOCNx), followed by electron transfer from LUMOCNx into the conduction band of TiO2 (CBTiO2 ), iii. Charge transfer excitation with direct optical electron transfer from HOMOCNx to CBTiO2 . The CB TiO2 electrons generated through pathways 1–3 are then transferred via the [Fe4S4] clusters to the [NiFeSe] hydrogenase active site (Reproduced from Caputo et al. [41])

Fig. 8 Functional and structural models for nc-CdTe-H2ase hybrids. The proposed scheme for light-driven H2 production by the nc-CdTe: hydrogenase complex. X represents a sacrificial hole scavenger. ET [FeS] clusters and the catalytic H-cluster are shown in VDW format. Protein structure images were visualized and rendered using the VMD software package.35,36. Atom colours: yellow, sulphur; green, iron; red, oxygen; blue, nitrogen; cyan, carbon. The Hydrogenase structures are homology models of C. acetobutylicum [FeFe]-H2ase to Clostridium pasteurianum [FeFe]hydrogenase CpI (PDB entry 3C8Y).37. b Electrostatic surface model of Hydrogenase generated with VMD using VolMap.35. Regions of positive and negative charge are shown in blue and red, respectively (Reproduced from Woolerton et al. [3])

In addition to the TiO2 , dyes were used as sensitizer to couple with hydrogenases by the Armstrong et al. group, the [NiFeSe]-hydrogenase, which is a reversible catalysts was coupled with viable light-harvesting CdS nanostructured thin film [43]. It shows a remarkable behaviour that the electronic state of the electrode used strongly biases the direction of H+ /H2 electrocatalytic interconversions, Fig. 9. It exhibits higher activities for both oxidation and reduction, with a notable shift in bias, in

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Fig. 9 Schematic representation of the formation of an accumulation layer at the surface of CdS or TiO2 due to the increased electron density at the surface when Eappl is lowered relative to EFB . The increased electron density and subsequent downward band bending facilitate efficient electron transfer to the enzyme active site via FeS clusters to catalyze H2 production or CO2 reduction. The term EF is the Fermi energy level of the semiconductor (Reproduced from Bachmeier et al. [43])

favour of H+ reduction, when enzyme is coupled with n-type semiconductor electrodes (viz., CdS andTiO2 nanoparticles). Such catalytic rectification effect may arise for a reversible electrocatalyst (viz., hydrogenases) when coupled to a semiconductor electrode, if the electrode converts between semiconductor- and metallic-like behaviour across same narrow potential range (1.6 eV are needed for water splitting. The most important measure in hydrogen generation for water splitting in PEC systems is solar-to-hydrogen conversion efficiency (STH) (see Table 2). From its expression, it can be deduced that higher STH ratios are obtained for high photocurrent values. The main problem is that semiconductors with the required bandgap for PC water splitting (like TiO2 or ZnO) can only absorb UV light from the solar spectra, leading to low photoconversion (photon to current) and low STH ratios [161]. In PEC cells, the generated photovoltage can only be over the required 1.23 eV when using two photoelectrodes, while for single photoelectrode cells an external bias should be applied. For achieving autonomous sunlight-driven water splitting efficiently, the photovoltage of the cell and the fill factor must be improved as well as photocurrent. At the same time, recombination, reflection, and photo-degradation are also problems to be solved in solar-driven water splitting. Many articles are available providing deep analysis of the key parameters governing water splitting and the reader is encouraged to find there more detailed information [162, 163]. Based on these articles, in Table 3, we highlight some of the main mechanisms and parameters. In summary, the need for an efficient photoelectrode may be stated as (1) high light harvesting directly from the sun, (2) long carrier lifetimes (to improve reaction time), (3) high reactivity favouring the redox water splitting, and (4) excellent transduction in the closed-loop photoelectrochemical cell. All these parameters are (in)directly enhanced by plasmonics as seen in the selected articles reviewed in Sect. 4. Direct comparison of articles published among the different fields: physics, chemistry, engineering, etc. is complicated as there is no real consensus on the parameters of merits for efficiency used. Although we already introduced the solar-to-hydrogen conversion factor STH (i.e. the overall efficiency for PEC water splitting exposed to broadband solar irradiance under zero-bias conditions), there are many others defined and reported, many forced due to the experimental conditions. Here a small overview for completeness is provided. No claim of comparison (being one better than another) is made as this currently is not defined and highly depends on the experimental conditions. Besides, the authors would not like to impose any bias on the reader. In general, two main categories may be identified, namely, benchmark efficiency (reflects the stand-alone water-splitting capabilities) and diagnostic efficiencies (enables material system/interface characterization and performance). STH belongs

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Table 2 Water-splitting benchmark and diagnostic efficiencies Benchmark efficiency Solar to hydrogen (STH)

1.23V·J(mA/cm2 ) Pin (mW/cm2 )

· neff

M(gr )·IC (A)·t (s) N ·F

= 2 AhANe a · 100%

Diagnostic efficiency Faradaic (neff ) Applied bias photon-to-current efficiency (ABPE)/half-cell solar-to-hydrogen (HC-STH) * External quantum efficiency (EQE)/incident photon-to-current efficiency (IPCE) Internal quantum efficiency (IQE)/absorbed photon-to-current efficiencies (APCE) Intrinsic solar-to-chemical conversion efficiency (ISTC) Apparent quantum efficiency (AQE)/apparent quantum yield (AQY)

j photo (m A/cm 2 )·(1.23V −Vbias ) PSolar,AM1.5 (mW/cm 2 )     j photo

mA cm 2

  ·1293.8(V ·nm)

Pmono (mW/cm 2 )·λ(nm)     j photo

  ·1293.8(V ·nm)   2 Pmono (mW/cm )·λ(nm) 1−10−A mA cm 2

n e f f ·1.23(V R H E ) U Dar k (V R H E )

·

J photo (m A/cm 2 )·V photo PSolar,AM1.5 (mW/cm 2 )

nR I

Here M stands for the molar mass (weight of the displaced element in grams), IC represents the current in Amperes, t is the time in seconds, N is the oxidation state (number of displaceable electrons per atom), and F = 96,485 C /mol represents Faraday’s constant. Ae indicates the number of electrons taking part in the water-splitting reaction, Ah is the amount of hydrogen, and NA is Avogadro’s constant (6.022 × 1023 mol−1 ). J is the short-circuit current and jphoto is the photocurrent (jlight − jdark ) normalized respect to the electrode area. Also, 1.23 V corresponds to the thermodynamic water splitting potential at 25 °C and 1239.8 V·nm represents a multiplication of h (Planck’s constant)   and c (the speed of light).PSolar,AM1.5 mW/cm2 and Pmono are the calibrated and AM 1.5 solar, monochromatic illumination power intensity in mW/cm2 , respectively, and λ (nm) is the wavelength at which the monochromatic illumination power is measured. neff is the Faradaic efficiency for hydrogen production. A is the absorbance and UDark (VRHE ) is the photoanode potential under dark conditions. Also, here, n corresponds to the number of interchanged electrons (2 for HER), R is the rate of hydrogen generation, and I is the rate of incident photons. * In the case of a three-electrode configuration Vbias = VRHE and instead of 1.23 eV, it is EO2 /H2 O for photoanode and EH+ /H2 for photocathodes considering their sign

to the first category, meanwhile in the second can be placed: the applied bias photonto-current efficiency (ABPE), the external quantum efficiency (EQE) which also is denominated incident photon-to-current efficiency (IPCE), the internal quantum efficiency (IQE) which is also denominated as absorbed photon-to current efficiencies (APCE) [162]. In Table 2, the mathematical expressions for each of these efficiencies are summarized. Another point for discussion is that most PEC experiments are performed in a three-electrode setup and the previous efficiency definitions are based on twoelectrode setups considering only working and counterelectrodes. For three-electrode setups, especially for tandem solar cells (in which the water splitting photoelectrode is combined in tandem with another photoelectrode) or photovoltaic cells (that provides

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Table 3 Water-splitting key parameters Mechanism

Parameters

Physical elements

Photon and exciton absorption

• Absorbance/reflectance/scattering

• Photoelectrode • Optical properties (w/r sunlight)

• Absorption coefficient • Absorption depth • Density of states • Effective mass • Dielectric constant/loss • Refractive index

Carrier diffusion and • Mobility transport • Diffusion coefficient

• Photoelectrode • Electronic properties

• Lifetime • Diffusion length • Concentration • Recombination kinetics Electrochemistry

• Exchange current density • Charge/electron transfer efficiency • Conductivity

• Photoelectrode • Electrochemical transducer properties

• Tafel slope • Activation energy Mass transfer

• Diffusion coefficient (ion size viscosity activity coefficient)

• Electrolyte/transducer properties

• Solution resistance Here, the main mechanism affecting the water-splitting performance, the main characteristic parameters to each mechanism, and the physical elements to be optimized to enhance them are described

the additional voltage needed to split water), there exists an intrinsic solar-to-chemical conversion efficiency (ISTC) [164] however criticized by some researchers [163]. Also note that in the case of ABPE, if the Vbias is taken as the electrode potential of the photoanode concerning the reversible hydrogen potential, it corresponds to three-electrode systems. In this case, ABPE would be equal to STH if the losses due to cathode overpotential and ohmic resistance were negligible (assuming that the three-electrode setup eliminates ohmic losses), this is also called half-cell solar-tohydrogen (HC-STH) and instead of the water-splitting potential, the corresponding oxygen and hydrogen energy generation is used depending on whether a photoanode or photocathode is considered, respectively. Several strategies can be developed to improve STH performance of PEC and PC cells, among them: doping the semiconductor material [165, 166], fabricating semiconductors heterojunctions [110, 167], Tandem cells [168], changing

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morphology and orientation design [169, 170], coupling with conductive nanomaterials [171–173], and coupling with quantum dots [174] or plasmonic nanomaterials [128, 170, 175].

3.2.2

Plasmonic Improvement of Semiconductors’ Photoresponse

One of the most recently studied strategies to overcome charge recombination and promote charge separation in semiconductors is coupling them with co-catalysts such as metal nanoparticles [176, 177]. The metallic nanoparticles in contact with the semiconductor material form an Ohmic or Schottky contact that creates an electrical field in the metal–semiconductor interface [157]. This electric field allows photogenerated electron–hole pairs to split better than in the semiconductor alone, so metals can act as “electron or hole traps”. Metal nanoparticles also may provide catalytic sites for HER or OER, improving water-splitting performance [178]. Pt-TiO2 composites are the most studied metal–semiconductor materials for photogenerated water-splitting reaction, but several other metallic nanomaterials have been also used such as Pd, Ni, Ru, Ir, Rh, Au, Ag, and Mn [178]. It has been proved by several studies that the presence of the metallic cluster embedded in the semiconductor material considerably increases its photocatalytic performance [115, 178, 179]. When plasmonic nanomaterials are coupled with semiconductors a better photocatalytic water-splitting performance is also observed [103]. Differing from single metal–semiconductors composites, in this case, the enhanced photocatalysis is not only referable to catalytic effects or electron trap sites. Here LSPR plays an important role in providing improved charge transfer, near-field enhancement, and other phenomena that will be described in this section. The enhanced photoconversion performance in plasmonic photocatalysis has a complex nature and can be related to several phenomena. The chronologic discoveries of these phenomena were previously described in Sect. 2.3.1. Here a more detailed description based on physicochemical phenomena will be given. We will mainly focus on those processes improving water splitting in plasmonic photocatalysts for simplicity and better understanding. More detailed analysis, mathematical descriptions, and full process discussion can be found in previously reported works [126, 128]. Direct charge carrier transfer When metal nanoparticles are exposed to light with energies capable of activating the LSPR phenomena, an increased electrical field is generated leading to high energy charge carriers (electrons and holes). If metal nanoparticles are coupled to a semiconductor, the photogenerated high-energy electrons in the metal (hot electrons) can easily migrate to the conduction band of the semiconductor material (Fig. 9a). This way, photons with energies corresponding to the LSPR frequency produce photogenerated electrons passing through the semiconductor in a phenomenon similar to the die-sensitization mechanisms [109, 180].

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Fig. 9 Photoresponse enhancement mechanisms in a semiconductor–metal nanoparticle composite. a Direct charge transfer processes (hot electron injection and LSPR resonant band bending. b Energy transfer by local electrical field enhancement and resonant energy transfer and c scattering and antireflection

Hot electron transfer may happen for photons with energy hν ≥ (Ec -Ef ), being Ec the energy of the bottom of the conduction band, and Ef the Fermi level of the semiconductor–metal system [126]. Interestingly, this phenomenon happens not only for the maximum in the LSPR band of the metal but also for less energetic light having low LSPR response [181]. Hot electrons are of special interest in photocatalytic applications and therefore in solar-driven water splitting [182]. Hot electrons transfer may not only provoke better photoconversion yield but also favour redox reactions in the surface of the semiconductor–metal composites [183]. At the same time, the electron–electron relaxation mechanism is also possible for hot photogenerated electrons (Fig. 9a). When hot electrons collide with electrons in the valence band of the semiconductor, it provokes electron excitation processes from the VB to the CB. This phenomenon, known as ¨LSPR band breaking¨, is then responsible for the generation of electron–hole pairs in the semiconductor material and can be understood as an LSPR direct transfer process. It is relevant to state that, for this mechanism to happen, the energy of the hot electron should be equal or higher than the bandgap [126]. The three LSPR-mediated reactive processes shown in Fig. 9 are also called as thermal, electronic, or antenna reactive processes in literature. Energy transfer by local electrical field enhancement When plasmonic nanoparticles interact with light of the same frequency of its LSPR, resonance is established between the plasmon electronic cloud and the incoming photons. As a result, the electric field in the surroundings of the nanoparticle

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is enhanced. If the nanoparticles are in contact with a semiconductor material, the enhanced electrical field can influence electron transitions from BV to CB in the neighbouring semiconductor. This provokes electron–hole pair formation in a phenomenon known as plasmon resonance energy transfer (PRET) (Fig. 9b) [184]. It has been found that PRET efficiency is dependent on metal nanoparticles’ size and shape. It is enhanced for particles with a low rate of dephasing, being maximum for particles of about 20 nm. Smaller particles than 20 nm have high electron scattering so PRET decreases [120]. Also, for shapes like nanorods even better PRET efficiencies have been obtained [185]. On the other hand, it also depends on the plasmonic material employed. It is desirable the overlapping of LSPR frequency with the absorption band of the semiconductor, so for a better PRET efficiency, the right plasmonic material should be chosen (to optimize absorption overlaps). For example, when coupling N-doped TiO2 with Au and Ag nanoparticles, due to the difference in LSPR frequencies between both metals, an enhancement in photocatalytic performance was only found for AgNPs (where absorption overlap was observed) [131]. PRET phenomenon has been described for metals in direct contact with semiconductors, but it has been proved that plasmonic nanoparticles can also enhance photoconversion in semiconductors even when they are not directly attached [122]. This phenomenon has been explained in terms of an indirect energy transfer process called near-field enhancement (NFE). Cushing et al. explained this local field enhancement through a dipole relaxation process and called it “resonant electron transfer” (RET). By this mechanism, localized surface plasmon dipoles in the plasmonic nanoparticles can non-radiatively induce electron–hole formation in the semiconductor (Fig. 9b) [123]. As discussed above, direct electronic transfer only enhances the semiconductor interband transition but does not allow the formation of more electron–hole pairs to longer wavelengths. However, as the RET phenomenon is not limited by charge equilibrium issues in the metal–semiconductor interface, it can create more electron– hole pairs at wavelengths above and under the bandgap value [123]. This fact is especially relevant when metallic nanoparticles can undergo a degradation process in contact with the semiconductor, so protective architecture layers can be designed and still maintain the LSPR enhancement of the photocatalytic performance [122]. Radiative transfer of photons: scattering and antireflection effect Previously discussed mechanisms (PRET, hot electron injection, RET) are all electronic transfer process. However, plasmonic nanoparticles can also improve watersplitting ratios in plasmonic photocatalysts through radiative (photon) transfer mechanisms [33]. In addition to LSPR, light scattering is a well-studied property of metallic nanoparticles, becoming more relevant for bigger particle sizes (see Sect. 3.1.2). As one of the most limiting properties of semiconductors towards solar energy utilization is their high reflection index, the presence of light scattering nanomaterials acts as an antireflective layer (Fig. 9c).

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It has been found that plasmonic particles above 100 nm act as nanomirrors improving the conversion rate of photons to electron–hole pairs in the semiconductor [184]. At the same time, changes in particle morphology can also improve light scattering. The relationship between scattering and antireflection effect with other mechanisms as PRET or hot electron generation is relevant and must be considered when fabricating plasmonic photocatalysts. As the size, morphology, and composition may have opposite effects on these mechanisms, optimization is needed to achieve the best photocatalytic performance of the semiconductor material [125].

4 Applications of Plasmonic Photocatalysts From the theory previously described, the potential of plasmonics in different technological areas has become clear. An important application is plasmonic-enhanced photocatalysts for hydrogen production which has been the focus in recent studies [186–188]. In the following, selected articles on water splitting are discussed and an overview is provided of materials used and their performances. In the theoretical section, band diagrams were used to highlight the performance enhancements, and in the following review and discussion on a set of selected articles, the same will be done as it simply highlights the underlying mechanism in uncommon material combinations [189]. Please note that no specifications on the exact energy levels will be provided and the intention is to provide a visual, schematic understanding of the underlying carrier transport mechanism.

4.1 Plasmon-Enhanced Ferroelectric-Based Photoanode A perfect example of a novel and unusual material combination presenting enhanced water-splitting performance is the combination of a ferroelectric n-type material, such as BiFeO3 (BFO), with a p-type metal oxide as Cu2 O to be used as the photocathode [190]. The latter is also unusual as most studies are dedicated to the photoanode instead. The idea behind combining these materials is to overcome their drawbacks. For example, Cu2 O is by itself unstable in aqueous solutions but BFO is highly stable and can be used as a protective/sacrificial layer. Although BFO is stable in the electrolyte, it has a high open-circuit potential and is photoresponsive under visible light (2.4 eV), by itself it presents rapid recombination of the photogenerated carriers that limit the photoelectrochemical (PEC) performance. So on, its combination with a well-designed p-type material such as Cu2 O (2 eV) allows improving the separation of photogenerated carriers. In Fig. 10a, a general band diagram is presented for a p-type and n-type semiconductor, and representative for the discussed study, with the corresponding hydrogen and oxygen levels. In Fig. 10b, the band diagram is shown for the case where Au is added between both semiconductors. Localized surface plasmon resonance

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Fig. 10 Plasmon-enhanced ferroelectric-based photoanode. Generic band diagrams for a present ferroelectric-based photoanode, where the p-type semiconductor (orange) would correspond to Cu2 O and BFO to the n-type semiconductor (green) and b the plasmon-enhanced system with a noble metal (Au) interlayer

(LSPR) of noble metal nanoparticles (i.e. coherent oscillations of conduction electrons) improves the transfer of photogenerated carriers at the heterostructure interface (see Sect. 3), commonly the main bottleneck for efficient carrier transport. Hot electrons being directly introduced into the conduction band of semiconductors avoid some important issues regarding the potential barrier in the interface, improving the energy band-edge alignment. Au then introduces an intermediate band that acts as a hole sink and electron injection hotspot enhancing the overall water-splitting performance. With the designed system, an electric field enhancement at 727 nm was shown attributed to the Au LSPR. An onset potential of 1.01 V versus RHE and photocurrent density of −91 μA/cm2 at 0 V versus RHE under 100 mW/cm2 illumination were reported. For all experiments, 0.1 M Na2 SO4 solution (pH = 7) was used as electrolyte.

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4.2 Multi-Interfacial Plasmonic Water-Splitting Enhancement As discussed, interfaces are important and may contribute either positively or negatively depending on whether the interface hinders or not the transport of charge carriers. Having several positively contributing interfaces can improve electron transfer as recently achieved by using multi-interfacial plasmon in multigap (Au/AgAu)@CdS core–shell hybrids [191]. Extinction tests together with numerical simulations proved a gap-depended light absorption for Au/AgAu hybrids as well as an enhanced local electric field owing to the multigap-induced multi-interfacial plasmon coupling. This phenomenon is similar to that seen with plasmonic nanovoids introduced in solar cells [192] and theoretically described by K. Sakai et al. [193]. They showed that a two-dimension periodically arrayed plasmonic gap efficiently collects light and enhances the nanogap field. For an array of gold slabs, a clear correlation is observed between array size and intensity (Fig. 11a), gap size (defined with its corresponding field in Fig. 11b), and both peak wavelength shift and intensity (Fig. 11c). Hetero-photocatalysts were designed by further coating CdS shells on multigap Au/AgAu cores which exhibited prominent gap-depended photocatalytic hydrogen production activity from water splitting under light irradiation (λ > 420 nm).

Fig. 11 Plasmon-enhanced nanogap-based photoelectrode. a Two-dimensional periodically arranged gold structures with nanogap and its size-dependent near-field intensity. b The twodimensional normalized electric field intensity of metal on a plane cutting through the middle of the nanoblock thickness. c The gap distance dependency on wavelength and intensity of spectral peak for different gap sizes. Adapted with permission from [193] Copyright (2020)

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Hydrogen generation rates of multigap (Au/AgAu)@CdS showed exponential improvement compared with pure CdS as the number of nanogaps increased. An optimum of four-gaps (Au/AgAu)@CdS core–shell catalyst displayed the highest hydrogen generation rate, which was 96.1 and 47.2 times of pure CdS and gapless Au@CdS core–shell hybrids, respectively. A representation of the optimum fourgaps (Au/AgAu)@CdS is presented in Fig. 12a. The irregular shaped multigap cores are presented near field hotspots at their corners (see the bottom image in Fig. 12b). In the study, hydrogen generation from water splitting was evaluated under light irradiation (λ > 420 nm) by using Na2 SO3 -Na2 S as sacrificial reagents. Under light irradiation (λ > 420 nm), the CdS shells can harvest visible light in a very small

Fig. 12 Plasmon-enhanced nanogap-based photoelectrode. a The optimized four-gap (Au/AgAu)@CdS core–shell structure showing the highest AQE(%) and the possible associated photoelectron transport mechanism. b The increase in peak intensity, wavelength shift, and full width half maximum (FWHM) and the near-field distribution for the different multigap structures (increments 1 to 4 from left to right). Adapted with permission from [191] Copyright (2020) API publishing Royal Society of Chemistry

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region (420–520 nm). The photogenerated electron–hole pairs in CdS can react with sacrificial agents and H2 O to produce hydrogen. On the other hand, the four-gap Au/AgAu core presents strong multi-interfacial plasmon coupling that broadens the harvesting wavelength region (500 ~ 1100 nm), see Fig. 12b, increasing so the light harvesting efficiency and offering abundant plasmonic energy. Both SPR generated resonance energy and hot electrons can be transferred from four-gap cores to the CdS shells, further boosting the photocatalytic reaction and conversion efficiency. Also, the strong near-field amplification in the intragap and peripheral corners of the metal cores contributes to exciting more electron–hole pairs in CdS and increases the rates of generation and separation of electron–hole pairs, thereby accelerating the H2 generation rate. As a result, AQE % of around 0.7% was found at a wavelength of 800 nm.

4.3 Modular System: Water Splitting and Pollutant Oxidation with Solar-To-Fuel Efficiencies up to 20% Systems presenting bifunctionality, i.e. the capability of water splitting and simultaneously providing a second functionality, is also being developed. A modular system capable of water splitting and pollutant oxidation with excellent performance has been recently introduced. Here, the dye-sensitized solar cells (DSSCs) concept (i.e. a synthesizer collects sunlight and rapidly injects electrons to a semiconductor conduction band) was applied. The usage of silver nanoparticles instead of the typical Au and functionalization by covalent bonding through the addition of a molecular linker together with the addition of a cocatalyst as Ru presented excellent results [194]. Here a titanium oxide photoelectrode was used as an n-type semiconductor, a molecular linker allowed effective electron transport from silver and a molecular regenerator was incorporated as shown in Fig. 13. The coating of silver nanoparticles with this regenerator was done directly during the synthesis by using pure Betanine as both reducing and capping agents. Betanin (Bn) and its derivatives from alkaline hydrolysis or oxidation, betalamic acid (HBt), cyclo-DOPA 5-O-glucoside (cDOPA), and neobetanin (neoBn) are expected to retard electron/hole recombination and are thus used as regenerators. The hot electrons generated under light irradiation have a very short lifetime ( 520 nm) through hot electron injection. In addition to the superior absorption capability of the plasmonic Au-BiVO4 design, the CoFe-PBA was added to improve even further their overall performance. At 1.23 V (vs. RHE) the photocurrent value for a bare BiVO4 photoanode was obtained as 190 mA/cm2 , whereas it was boosted to 295 mA/cm2 and 1800 mA/cm2 for Au-BiVO4 and Au-BiVO4 /PBA, respectively. A maximum of 43% abovebandgap IPCE and 1800 mA/cm2 (at 1.23 V vs. RHE) was obtained. Here, for the measurements, phosphate-buffered saline (0.5 M Na2 SO4 in 0.1 M PBS, pH 7 at 25 °C) was used as the electrolyte and the electrolyte solution was saturated with N2 gas (99.999% purity) for 30 min before each measurement to remove the dissolved O2 gas.

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4.11 Metal–organic Framework with a Hybrid Metallic Structure Provides Plasmon-Induced Water Splitting A hybrid bimetallic (Au/Pt) periodic structure modified with a metal–organic framework (MOF) MIL-101(Cr) was shown to present good overall water-splitting performance [210]. Utilizing MOF instead of common plasmon-active photocatalysts for water splitting (the semiconductor–noble metal composites) provides high novelty. The clear advantages are that the periodic network of MOF, with tunable sizes and functions, can be designed to maximize charge separation, while well-defined porous structures of MOF provide accessible active reaction sites and facilitates the transport of substrates and products. In these systems, a periodic gold structure is deposited on an SUV 8 polymer designed grating network, see schematics of Fig. 21a. The final periodic gold structure supports plasmon-polariton waves and allows exciting the hot electrons which are further injected into the Pt and MIL-101(Cr) layer. The Pt and MIL-101(Cr) structures introduce catalytic sites, which are able to generate hot electrons, and efficiently initiate water splitting and hydrogen production. Also, the MIL-101(Cr) serves for repelling generated hydrogen bubbles. The maximum hydrogen generation was achieved under plasmon resonance excitation in the NIR range, and it could be actively tuned by the applied LED wavelength and with seawater as a raw material source, see graph in Fig. 21a.

Fig. 21 MOF metallic hybrid for plasmonic-enhanced water splitting. a Illustrated representation of the Au/PtMIL-101(Cr) grating used as photoelectrode and the enhanced H2 generation at NIR illumination. b Graph and carrier transport schematics along the structure showing the effect of including a hole absorber like TEOA, improving even further the photoelectrode performance. Adapted with permission from [210] Copyright (2020) American Chemical Society

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Different MOF thicknesses were analysed to understand the influence of MOF thickness on H2 catalytic production. The highest amount of evolved H2 was recorded at MIL-101(Cr) thickness of 300 ± 10 nm. The introduction of MIL-101(Cr) onto the bimetallic surface resulted in “superhydrogenophobic” behaviour, i.e. the H2 bubbles approaching the Au/Pt-MIL(Cr) surface immediately repel off the surface and move away. The deposition of Pt and MIL-101(Cr) growth results in a broad absorption band in the 700–1100 nm wavelength range. For the photocatalytic H2 production, distilled (or seawater) H2 O (containing triethanolamine (TEAO)) was used after previously bubbling with N2 for 30 min. TEOA acts as an absorber of the holes that may be produced by the sample under illumination. The addition of TEOA resulted in a significant increase in H2 production, see schematics and graph of Fig. 21b. Interestingly in this system, the deposition of a thin Pt layer on the Au grating surface also leads to higher water-splitting performance (117 μmol/h/cm2 ). Therefore, the injected hot electrons into the Pt layer might initiate proton reduction on Pt active sites (alternative reaction pathway). As well, decoration with MIL-100(Cr) of the pristine Pt layer deposited on the polymer grating did not result in any H2 production, which manifests the crucial role of hot, plasmon-excited carriers. Structures were also designed to be flexible and it was shown that their bending does not lead to efficiency loss. The optimized system also exhibits superior stability, maintaining the same efficiency even after 20 h of water splitting. No efficiency parameter was provided however in this study.

5 Conclusion In this chapter, we have journeyed through the history of plasmonics, discussed its specific application and potential for photoelectrochemical application, overviewed the main parameters influencing and characterizing water-splitting systems, and provided an overview of different novel systems recently developed for water splitting applications (a comparison between articles presented is provided in Table 4). In general, plasmonics allows activating those systems that need improved charge carrier transport and carrier generation to initiate the water-splitting reaction. Hot electron injection or near-field enhancement is thus critical to boosting photoelectrode performance. Regarding the design of new materials for water splitting, it is important to highlight the importance of having a visual idea of the underlying mechanism in these systems, by, for example, the presented band diagrams as bandgap engineering is just as critical for enhanced performance as is the case in optoelectronics. Also, high care needs to be taken when comparing different photoelectrochemical watersplitting systems. The lack of a real consensus on the reported efficiency parameter hinders direct comparison between systems. In addition, a parameter that in many studies is just used and not optimized is the electrolyte of choice. The interaction of photoelectrodes with electrolytes may lead to

Plasmonic mechanism

HE

RE HE

HE

AR

HE

RE HE

HE

RE

RE HE

HE

Material

Cu2 O/Au/BFO

(Au/AgAu)@CdS

Bts-AgNPs_pABA _TiO2 : RuNPs

Au/XS2 /Au (X = Re, Mo)

FTO|SnO2 /AgNP/TiO2 |–RuP2 + /WOC

PtNPs/TiO2 -NB/CdS/CZTS

SiNW_AuCQDs

WO3 -based photoanode

InGaN/GaN MQWs @Ag/Au

Au-BiVO4 /PBA

0.5 M NaCl

0.5 M Na2 SO4 in 0.1 M PBS (pH 7)

λ > 520 nm

0.5 M H2 SO4

0.5 M H2 SO4 (pH 0.25)

0.2 M Na2 HPO4 /NaH2 PO4 (pH 3)

0.4 M NaClO4

Na2 SO3 (0.25 M) and Na2 S (0.35 M)

n/a

Na2 SO3 (0.25 M); Na2 S (0.35 M)

0.1 M Na2 SO4

Electrolyte

[370 -490] nm; [560–670] nm

560–1200 nm (depends on doping)

540 nm

~ 530 nm (Fig. 2c)

450 nm

600 nm

405 nm

500 -1100 nm (AQE% -800 nm)

727 nm

peak

Table 4 Overview of extracted parameters from discussed articles

0 vs RHE

-0.8 V vs RHE 1.23 VRHE

1.5 V versus RHE 1.23 V vs. RHE

17.2 mA/cm2

1.7 mA/cm2 0.79 mA/cm2

1.76 mA/cm2

1800 mA cm2

0.2 V vs. Ag/AgCl

n/a

~ 1.4 μA/cm2 2 mA cm−2

n/a

n/a

n/a

0 vs RHE

− 91 μA/cm2

n/a

Vw

Jphoto

100

100

100

100

n/a

100

n/a

n/a

n/a

100

P (mW/cm2 )

[198]

182.93 μmol h−1

43% IPCE n/a

(continued)

[209]

[208]

[199]

[197]

126.67 μmol h−1

n/a

[196]

[195]

23.04 μmol h−1 g−1 n/a

[194]

[191]

4.71 mmol g−1 h−1

150 nmol/min

[190]

Ref

1.5 umol/h (Fig. 5d)

Hrate

64% IPCE n/a

WO3 -Fe showed 0.1% ABPE

~ 0.8% ABPE

3.5% HC-STH

31.8% IPCE

n/a

19.9% STH

0.7% AQE%

90% Faradaic efficiency

ñ

160 F. J. P. Díaz et al.

HE

Au/Pt-MIL-101(Cr)

850 nm

peak

Distilled (or seawater) H2 O (with TEAO)

Electrolyte n/a

Jphoto n/a

Vw 15

P (mW/cm2 ) n/a

ñ Ref

[210]

Hrate 117 μmol/h/cm2

Here peak corresponds to the plasmon resonance peak; Vw is the working potential at which the best hydrogen amount or efficiency was produced; P corresponds to the optical power density of the laser/led used; ñ corresponds to the efficiency; H rate is the hydrogen production rate. HE: Hot electrons; RE: Resonance energy; AR: Antenna–reactor

Plasmonic mechanism

Material

Table 4 (continued)

5 Plasmonic Photocatalysts for Water Splitting 161

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band bending, thereby introducing an additional way to control the carrier transport at the interface. Even more, some material/electrolyte combinations may lead to etching current influencing the overall analysis and deteriorating the photoelectrode. Hence, the edge potential could be modified (in a small regime) by simply choosing the best electrolyte (Table 4). Scope As seen in most systems, the noble metal particles are the common choice for plasmonics, but other materials might provide better results depending on the design. Novel materials as ferroelectrics, metal chalcogenides, metal oxides, MOFs, widebandgap semiconductors, carbon quantum dots, etc. might bring new features and be combined with plasmonic elements (noble metal particles, nanovoids, gratings, etc.) and efficient WOC allows to design and fabricate superior photoelectrodes. In terms of absorption, plasmonic enhancement of III-nitride such as the high In-content InGaN is definitively worth investigating. Also, IR absorption might be improved by using upconverting nanoparticles which might benefit from plasmonics. Combining molecular linkers and regenerators is also very appealing due to their general lower costs and high efficiency. Also, modular systems showing multifunctionality by enhancing different chemical processes simultaneously show great potential. In summary, plasmonics has still a lot to offer for enhanced photocatalytic hydrogen generation and many pathways are still open to be explored. We hope to have provided the necessary tools and overview to stimulate the reader to design novel systems on improving photoelectrodes for water splitting using plasmonics. Acknowledgements Francisco J. Peón-Díaz wants to thank the financial support of the Chilean National Agency for Research and Development (ANID)/Programa de Doctorado Nacional 2020/Folio 21200362.

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Chapter 6

Monoclinic BiVO4 -Based Photoanodes for Photoelectrochemical Water Splitting Tatiana Santos Andrade, Izabela Campos Sena, Antero Ricardo Santos Neto, Mara Cristina Hott Moreira, Mariandry Rodriguez, and Márcio César Pereira

1 Introduction The sun is the source with the highest potential to supply the growing demand for energy by modern society. Solar energy can be converted into electricity using different cells such as silicon solar cells [1], dye-sensitized cells [2], perovskite cells [3], photocatalytic fuel cells [4], etc. However, the intermittent nature of the sun requires that sunlight storage technologies be developed to make the best use of this energy source. Among the various energy storage technologies, water-splitting photoelectrochemical cells (PECs) stand out because they can collect solar energy using semiconductor electrodes to produce oxygen and hydrogen from water, as shown in Eqs. 1 and 2. Thus, solar energy is converted into chemical energy (H2 fuel) that can be used at any time of the day or night [5]. Cathodic reaction: 4H+ + 4e− → 2H2

(1)

Anodic reaction: 2H2 O → O2 + 4H+ + 4e−

(2)

Overall reaction: 2H2 O → 2H2 + O2 Go = 238 kJ mol−1

(3)

A PEC cell is formed by at least one semiconductor electrode that can be a photoanode or a photocathode. Different configurations of PEC cells have been described in the literature: (i) photoanode–dark cathode [6], (ii) photocathode–dark

T. S. Andrade · I. C. Sena · A. R. S. Neto · M. C. H. Moreira · M. Rodriguez · M. C. Pereira (B) Institute of Science, Engineering, and Technology (ICET), Federal University of Jequitinhonha and Mucuri Valleys (UFVJM), Campus Mucuri, Teófilo Otoni 39803-371, Brazil © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Kumar and P. Devi (eds.), Photoelectrochemical Hydrogen Generation, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-16-7285-9_6

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anode [7, 8], and (iii) photoanode–photocathode [9]. Alternatively, both configurations can be coupled to a photovoltaic device to provide the electromotive force for the water-splitting reaction [10]. The water-splitting mechanism in PEC cells starts with the photon absorption by the semiconductor photoelectrode with energy greater than or equal to its bandgap energy, causing excitation of electrons from the valence band to the conduction band. In n-type semiconductors (photoanode), electrons migrate from the bulk to the conductive substrate and then move to the dark cathode to reduce H+ ions in H2 . Simultaneously, the photogenerated holes migrate to the photoelectrode surface to carry out the water oxidation reaction. As can be seen in Eq. 3, water splitting is a non-spontaneous reaction that requires a minimum energy of 1.23 eV. In practice, higher potentials are required due to kinetic overpotentials and ohmic losses [11]. From a thermodynamic point of view, an ideal semiconductor should have a valence band energy more positive than 1.23 VNHE and a conduction band energy more negative than 0 VNHE . In addition, the semiconductor must absorb light in the region of the visible spectrum to make use of sunlight, have good separation efficiency and charge transfer to decrease losses by electron–hole recombination, high surface area to provide enough active sites, rapid O2 evolution kinetics, high photochemical stability in water, and be composed of abundant elements in the earth’s crust [12, 13]. Among the several photoanodes studied for the O2 evolution reaction, BiVO4 has stood out as a promising photocatalyst due to the valence band energy being located around 2.4 VRHE , relatively small bandgap energy (Eg = 2.4 − 2.5 eV), and be made up of relatively abundant elements. Also, its theoretical solar-to-hydrogen conversion efficiency is close to 10% under standard AM 1.5 sunlight irradiation. Despite this, it has some disadvantages, such as poor stability for several operation hours, slow water oxidation kinetics, and slow charge transport, which results in high electron–hole recombination rates. Also, the conduction band energy level is slightly more positive than the H+ /H2 reduction potential, indicating that it is necessary to apply an external potential for the H2 evolution reaction to occur [14]. In recent years, several studies have been conducted to minimize problems related to the use of BiVO4 photoanodes. Those studies involve different strategies such as metal and non-metal doping [15, 16], morphology control [17, 18], crystal facet engineering [19, 20], surface modification with O2 evolution catalysts [21, 22], manufacture of heterojunctions [23–25], combination with plasmatic nanoparticles [26], combination with ferroelectric materials [27, 28], and deposition of underlayers and overlayers [29]. In this chapter, we review the main strategies to increase BiVO4 photoelectrochemical performance from 2018 to 2020. The recent use of BiVO4 in PEC cells for unbiased water splitting is also reviewed.

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2 Main Features of BiVO4 2.1 Crystal Structure In nature, BiVO4 is found in three minerals: pucherite (orthorhombic), dreyerite (tetragonal), and clinobisvanite (monoclinic) [30]. The pucherite orthorhombic structure (Fig. 1a) consists of VO4 tetrahedrons and BiO8 triangulated dodecahedrons, in which each tetrahedron shares an edge with a tetrahedron and two edges with neighbouring dodecahedra [31]. The tetragonal structure of dreyerite (Fig. 1b) is formed by regular VO4 tetrahedrons and slightly distorted BiO8 dodecahedrons sharing corners and edges [32]. The monoclinic structure of clinobisvanite (Fig. 1c) contains 4 vanadium atoms, 4 bismuth atoms, and 16 oxygen atoms per unit cell. Distorted VO4 tetrahedra and BiO8 dodecahedra form the basic structural unit of BiVO4 . VO4 tetrahedrons are connected to BiO8 dodecahedrons by sharing the vertices of oxygen atoms. The atoms of V and Bi are arranged alternately along the crystallographic axis, thus forming a BiVO4 layered structure [33]. The distortion of VO4 tetrahedrons produces centres of positive and negative charges not located at the same point, thus generating an internal electric field that is beneficial for the separation of the photogenerated electron–hole pairs in the monoclinic structure of BiVO4 . In orthorhombic and tetragonal BiVO4 structures, polyhedra are completely symmetrical, which may partially explain the low photoactivity of those structures under visible light [33]. To date, there are no reports on the synthesis of orthorhombic BiVO4 in the laboratory. On the other hand, the zircon-type tetragonal BiVO4 can be synthesized at room temperature from the mixture of NH4 VO3 and Bi(NO3 )3 solutions [34]. The scheelite-type monoclinic BiVO4 has been synthesized by various solid-state or solution methods [35]. The scheelite-type tetragonal BiVO4 (Fig. 1d) exists only in

Fig. 1 BiVO4 crystal structures a orthorhombic (pucherite), b tetragonal zircon (dreyrite), c monoclinic scheelite (clinobisvanite), and d tetragonal scheelite. BiVO4 crystal structures were designed from crystallographic information files available at Inorganic Crystal Structure Database: ICSD (BDEC) (http://bdec.dotlib.com.br) and using the CrystalMaker® for Windows version 2.2.4

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synthetic form. It has been synthesized from the mixture of Bi(NO3 )3 and Na3 VO4 at controlled pH using Na2 CO3 or NaHCO3 solutions [36]. The scheelite-type monoclinic BiVO4 is the most thermodynamically stable phase among its polymorphs. At temperatures above 300 °C, the zircon-type tetragonal BiVO4 is irreversibly converted to scheelite-type monoclinic BiVO4 [37]. Above 500 °C, orthorhombic BiVO4 is irreversibly converted to monoclinic BiVO4 [31]. The scheelite-type tetragonal BiVO4 can be irreversibly converted to monoclinic BiVO4 in an acid medium [38]. At 255 °C and external pressure, monoclinic BiVO4 can be reversibly converted to scheelite-type tetragonal BiVO4 [38].

2.2 Electronic Structure and Optical Properties The optical bandgap energy of the scheelite-type monoclinic BiVO4 (2.4–2.5 eV) is slightly higher than that of the scheelite-type tetragonal BiVO4 (2.34 eV) and significantly lower than that of the zircon-type tetragonal BiVO4 (2.90 eV) [36, 39]. Kudo et al. [39] showed that under visible light illumination, monoclinic BiVO4 oxidizes water molecules more efficiently than the zircon-type BiVO4 due to its more exceptional ability to absorb visible light. Although the scheelite-type monoclinic and tetragonal BiVO4 electronic structures are similar, the photocatalytic response of monoclinic BiVO4 is superior to that of tetragonal BiVO4 due to the better charge separation, resulting from the distortion of the Bi-O polyhedrons caused by the 6s2 lone pairs of Bi3+ in monoclinic BiVO4 [36]. Those results have pointed out that scheelite-type monoclinic BiVO4 is the semiconductor more suitable for photocatalytic applications than its polymorphs. The optical nature of scheelite-type monoclinic BiVO4 is still a matter of debate. Some authors have shown through density functional theory (DFT) calculations that BiVO4 is a direct bandgap semiconductor. The conduction band is formed mainly by V 3d states with contributions from O 2p and Bi 6p, and the top of the valence band consists of 6s O 2p anti-ligand states [40]. Other studies show that BiVO4 is an indirect bandgap semiconductor with the top of the valence band formed by O 2pπ and Bi 6s anti-ligand states and the bottom of the conduction band consisting of V 3dx 2 -y 2 and 3dz 2 anti-ligand states [33]. More recently, it has been shown that the valence band consists mainly of O 2p states with non-bonding O 2pπ and Bi 6s states. The conduction band is formed by V 3d states with dx 2 -y 2 and dz 2 character at the lowest energy edge and Bi 6p at the upper region [41]. Also, it was determined that Fermi energy is located at 2 eV above the maximum of the valence band, indicating that BiVO4 is an n-type semiconductor [33]. Additionally, the effective masses of electrons and holes in the monoclinic BiVO4 are smaller than those of other oxides [33, 40]. A smaller effective mass means that the separation of charges must be more efficient since the drift speed of electrons and holes is inversely proportional to the effective mass [42]. Thus, photogenerated holes can more easily reach the BiVO4 surface to carry out oxidation reactions.

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3 Strategies to Improve BiVO4 Photoelectrochemical Performance Different strategies to increase BiVO4 performance have been reported. Here, studies carried out in the period from 2018 to 2020 are described, which include metal and non-metal doping, homojunction and heterojunctions, crystal facet engineering, annealing treatment, texture control, combination with plasmatic nanoparticles, O2 evolution catalysts, interfacial charge mediators, ferroelectric materials, use of underlayers and overlayers, and electrolyte effect.

3.1 Metal Doping Shi et al. [43] identified that optimal BiVO4 doping with 2% W produces a higher charge donor density and surface state concentration, which leads to a film with high conductivity and reactive sites for water oxidation. The surface states act as reactive sites because the V5+ /V4+ reversible redox process in the semiconductor bulk forms water oxidation intermediates through electron trapping. In contrast, the irreversible reduction of VO2 + to VO2+ through electron trapping increases surface recombination. W doping alters the electron trapping process by forming a high concentration of surface states as reactive sites and less surface recombination, thus improving the photoelectrochemical properties of BiVO4 . Zhang et al. [44] demonstrated that the increase in charge carrier efficiency of Mo:BiVO4 films is due to the lowering of the polaron hopping barrier. This effect is so expressive that larger photocurrents can be achieved with thinner Mo:BiVO4 films, despite the decrease of light absorption efficiency. Lu et al. [45] used Bi and V polyoxometalate precursors to synthesize BiVO4 . The developed method allowed to incorporate doping metals such as Co, Ni, Cu, or Zn in the BiVO4 structure. BiVO4 films doped with Co or Zn showed a higher photoelectrochemical response with photocurrents above 1 mA cm−2 at 1.23 VRHE . This synthesis method was used to produce a 300 cm2 Co:BiVO4 photoanode, which generated a photocurrent of up to 67 mA at 1.23 VRHE in borate buffer solution (pH 8.5). Doping BiVO4 with Zn produces oxygen vacancies that act as electron donors. Also, the Zn-doping shifts the position of the conduction and valence bands of Zn:BiVO4 to more negative values compared to undoped BiVO4 . Through DFT calculations, it was found that the Zn-doping and oxygen vacancies stimulate the Bi-sites to adsorb water, thus favouring the water-splitting reaction. The Zn:BiVO4 photoanode generated a photocurrent of 1.85 mA cm−2 at 0.6 VRHE and an IPCE of 34% at 400 nm in phosphate buffer solution (pH 7) [46]. The partial replacement of In3+ by Bi3+ in the BiVO4 structure increases the photocurrent by about 200% when compared to undoped BiVO4 . The In-BiVO4 photoanode produces a photocurrent of 1.56 mA cm−2 at 1.23 VRHE in Na2 SO4 . The

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isomorphic substitution of In3+ by Bi3+ does not change the morphology, phase, and bandgap energy of BiVO4 , but causes a positive shift of the flatband potential and increases the efficiency of charge separation [47]. Zr-doped BiVO4 photoanode has shown a photocurrent of 0.32 mA cm−2 at 1.23 VRHE in phosphate solution (pH 7.5) [48]. Prasad et al. [49] found that the isomorphic replacement of Bi3+ by Y3+ or Yb3+ in BiVO4 improves light absorption (λ ≤ 550 nm) due to a decrease in bandgap energy but decreases charge transport when compared to V5+ doping by W6+ . Despite the replacement of Bi3+ by Er3+ in the BiVO4 structure extending light absorption up to 680 nm, Er:BiVO4 photoanodes have shown very low photocurrents [50]. A T5 molecular nanocluster containing Cu+ , Ga3+ , and Sn4+ was used as a dopant source for BiVO4 . The Cu, Ga, Sn:BiVO4 thin film produced a photocurrent of 2.5 mA cm−2 at 1.23 VRHE, which was almost three times greater than that of BiVO4 . The enhanced performance of the tri-doped photoanode was due to the increase in the charge carrier density, greater charge separation efficiency, and faster kinetics for the reaction of oxygen evolution [51]. Doping of BiVO4 photoanode with Li improves the water oxidation photocurrent by more than 20 times. With the use of catalysts, the Li:BiVO4 photoanode reaches a photocurrent of 4.2 mA cm−2 at 1.23 VRHE in phosphate solution. DFT calculations suggest that the formation of inter-bands with a reduction of bandgap due to interstitial Li doping contributes to increasing the BiVO4 photocurrent [52].

3.2 Non-metal Doping N-doped BiVO4 nanosheets have been prepared by a hydrothermal method using NaN3 as a nitrogen source [53]. The N atoms partially replace the O atoms in the BiVO4 structure, forming Bi-N and V–N bonds. Doping led to an increase in the photocurrent from 0.39 mA cm−2 in non-doped BiVO4 to 0.78 mA cm−2 in N-doped BiVO4 at 1.23 VRHE in Na2 SO4 electrolyte. N doping improves light absorption by BiVO4 , as the bandgap energy of N-doped BiVO4 has decreased from 2.23 eV in undoped BiVO4 to 2.01 eV. Arunachalam et al. [54] prepared F-doped BiVO4 photoanodes using the sol–gelassisted spin-coating method. The F-doping slightly shifted the BiVO4 absorption edge by 30 nm to the red region. The heat treatment of the F:BiVO4 in an argon atmosphere produced a mixture of crystalline-amorphous phases on the film surface that improves conductivity through the formation of more active electrochemical sites from the amorphous phase. Also, F-doping forms shallow donor states, which increases the carrier concentration due to the generation of oxygen vacancies. The F:BiVO4 produced a photocurrent of 0.9 mA cm−2 against 0.3 mA cm−2 of undoped BiVO4 at 1.23 VRHE in Na2 SO4 solution.

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3.3 Homojunction Fabrication Zn:BiVO4 /Mo:BiVO4 homojunction has produced a photocurrent of 2.5 mA cm−2 at 1.23 VRHE using a potassium phosphate buffer solution (pH 7) as the electrolyte. It was identified that Zn and Mo move the position of the valence and conduction bands of BiVO4 towards the Fermi level, thus forming n-n+ -type II homojunction that facilitates the separation of charges between the two layers. Besides, Mo increases the concentration of electron donors, and Zn increases the number of oxygen chemisorption sites, which facilitates the charge transfer process [55].

3.4 Heterojunction Fabrication 3.4.1

BiVO4 /WO3

Spectroelectrochemical and transient absorption (TA) measurements have provided direct evidence of electron spillover from the intra-bandgap state of BiVO4 to WO3 when the WO3 /BiVO4 heterojunction is formed. Operating PEC-TA measurements reveal that the charge carriers in BiVO4 and WO3 /BiVO4 are strongly affected by the application of external potential, with the heterojunction being more sensitive due to the formation of an internal electric field. In the WO3 /BiVO4 heterojunction, the hole trapping speed increases by about 50%, and the electron–hole recombination speed decreases by almost half on a short time scale (below 1 ns). This effect is amplified by the application of an anodic potential, leading to an accumulation of surface-trapped holes that are beneficial for water oxidation [56]. Transparent WO3 /BiVO4 /NiTCPP (Ni (II) meso-tetra (4-carboxyphenyl) porphyrin) photoanodes with 50 nm thickness exhibited a photocurrent of 0.26 mA cm−2 at 0.6 VRHE and an ABPE of 0.25% at 0.82 VRHE in phosphate solution (pH 6.86) [57]. A WO3 /BiVO4 /FeOOH photoanode prepared by foaming-assisted electro-spraying BiVO4 and photodeposition of FeOOH layer exhibited a 4.4 mA cm−2 photocurrent at 1.23 VRHE Na2 SO4 /Na2 SO3 solution [58]. Zhang et al. [59] prepared a heterojunction from conformal BiVO4 /WO3 nanoplates modified with Co-Pi cocatalyst. The BiVO4 /WO3 /Co-Pi film generated a 1.8 mA cm−2 photocurrent at 1.23 VRHE in phosphate solution (pH 7). Heterojunctions of 1D WO3 nanowires (3.5 μm thickness) covered with a shell of 30 nm BiVO4 nanoparticles produced a photocurrent density of 3.8 mA cm−2 at 1.23 VRHE using a 0.5 M H2 SO4 solution as an electrolyte. IPCE at the bias of 1 VSCE was 62.2% at 420 nm [60]. 3D brochosomes-like TiO2 /WO3 /BiVO4 photoanodes generated a photocurrent of 3.13 mA cm−2 at 1.23 VRHE in Na2 SO4 /Na2 SO3 solutions. After the deposition of a FeOOH/NiOOH cocatalyst, the photocurrent increased to 4.27 mA cm−2 . The IPCE was 65.1% at 1.23 VRHE and 430 nm. The good performance of that photoanode was attributed to 3D ordered hollow porous brochosomes structure, which improved light absorption, charge transport, and charge separation

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efficiencies [61]. Photoanode of WO3 nanorods epitaxially grown and decorated with BiVO4 has shown a 3.87 mA cm−2 photocurrent at 1.23 VRHE in a phosphate buffer solution containing Na2 SO3 as a hole trap [62]. WO3 /(Y, W)-BiVO4 /Fe-NiO/CoPi photoanode showed almost complete suppression of charge recombination (98%), efficient charge transfer (75%), and improved light absorption efficiency (85%). The WO3 /(Y, W)-BiVO4 /Fe-NiO/CoPi photoanode produced a photocurrent of 5.77 mA cm−2 at 1.23 VRHE in K2 HPO4 electrolyte. The IPCE was 97% under the same conditions [49]. WO3 /(Er,W)-BiVO4 /CoPi photoanode has shown a photocurrent of 4.1 mA cm−2 at 1.23 VRHE in phosphate solution (pH 8) [50].

3.4.2

BiVO4 /V2 O5

Yaw et al. [63] showed that the coupling of BiVO4 with V2 O5 formed efficient heterojunctions for water oxidation. The BiVO4 /V2 O5 photoanode generated a photocurrent of 1.53 mA cm−2 at 1.5 VAg/AgCl using Na2 SO4 electrolyte, which was significantly higher than bare BiVO4 (0.22 mA cm−2 ) or V2 O5 (0.21 mA cm−2 ). They observed that the type II heterojunction improved the separation efficiency of photogenerated charges and, consequently, the photocurrent. Multijunctions of W-BiVO4 randomly combined with V2 O5 rods have produced a photocurrent of 7.8 mA cm−2 at 1.23 VRHE in Na2 SO4 solution. The W-BiVO4 /V2 O5 photoanode exhibits high charge separation efficiency (98.9% at 1.23 VRHE ) and light absorption (94%). The excellent performance of the W-BiVO4 /V2 O5 photoanode was attributed to the reduction of charge recombination in the film bulk due to the V2 O5 being an excellent hole carrier and improved light absorption due to the BiVO4 and V2 O5 bandgap energies complementing each other [64].

3.4.3

BiVO4 /Fe2 O3

BiVO4 layers deposited on Fe2 O3 form type II n–n heterostructures with photoactivity superior to their bare components. In NaOH electrolyte (pH 13.6), the Fe2 O3 /BiVO4 photoanode produced a 1.3 mA cm−2 photocurrent at 1.8 VRHE . Using an OER catalyst, the photocurrent increases to 1.7 mA cm−2 [65].

3.4.4

BiVO4 /ZnO

Photoanode of 3D bicontinuous ZnO combined with CoPi-modified BiVO4 has shown an onset potential of 0.09 VRHE and a photocurrent of 3.4 mA cm−2 at 0.6 VRHE in Na2 SO4 solution (pH 6.5). The unmodified ZnO/BiVO4 photoanode produced only 0.92 mA cm−2 at 0.6VRHE [66].

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183

BiVO4 /NiO

NiO combined with BiVO4 forms p-n heterojunctions that facilitate charge separation and improve the water oxidation kinetics. BiVO4 /NiO photoanode produces a photocurrent of 2.75 mA cm−2 at 1.23 VRHE and an ABPE of 0.72% at 0.76 VRHE in phosphate solution [67].

3.4.6

BiVO4 /AgVO3

BiVO4 modified with 3–5 nm AgVO3 forms p-n heterojunctions that help in the separation of photogenerated charges. As a result, a photocurrent of 1.93 mA cm−2 at 1.23 VRHE was achieved by a Mo-BiVO4 /AgVO3 electrode in a Na2 SO4 solution. In comparison, BiVO4 photoanode produced only 0.25 mA cm−2 [68].

3.4.7

BiVO4 /FeVO4

The BiVO4 /FeVO4 heterojunction reached a photocurrent of 0.4 mA cm−2 at 1.23 VRHE against 0.06 mA cm−2 of BiVO4 film in Na2 SO4 solution. An increase in ICPE from 2.5% in BiVO4 to 13% at 450 nm was also observed [69].

3.4.8

BiVO4 /Ag3 PO4

Ag3 PO4 nanoparticles deposited on BiVO4 nanoplates facilitate the hole transfer from BiVO4 to Ag3 PO4 and, consequently, improve the separation of photogenerated charges. BiVO4 /Ag3 PO4 films produce a photocurrent of 3.78 mA cm−2 at 1.23 VRHE in phosphate buffer solution (pH 7) [70]. Nanoparticles of Ag3 PO4 deposited on ZrBiVO4 film have demonstrated a photocurrent of 2.3 mA cm−2 at 1.23 VRHE in phosphate buffer solution (pH 7.5) [71].

3.4.9

BiVO4 /Ag2 S

BiVO4 and Ag2 S have been shown to form type II heterostructures [72]. The heterostructure not only facilitates the separation of photogenerated charges but also improves the surface hole injection. BiVO4 /Ag2 S photoanodes produce a photocurrent of 1.91 mA cm−2 at 1.23 VRHE in Na2 SO4 aqueous solution, which is about three times greater than the photocurrent of bare BiVO4 . Also, the films showed good stability, retaining 92% of the photocurrent after 2 h operation. The IPCE at 460 nm was 23% at 1.23 VRHE .

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BiVO4 /Cu2 S

BiVO4 /Cu2 S photoanode covered with Co(OH)x shows simultaneous improvement in charge separation and water oxidation kinetics due to heterojunction and catalyst, respectively. BiVO4 /Cu2 S/Co(OH)x heterojunction films show a charge separation efficiency of 79% and a photocurrent of 3.51 mA cm−2 at 1.23 VRHE in phosphate solution [73].

3.4.11

BiVO4 /Bi2 S3

A type II heterojunction of BiVO4 nanorods and Bi2 S3 nanowires was synthesized using solution and hydrothermal methods. The BiVO4 /Bi2 S3 /CoPi photoanode produced a photocurrent of 1.43 mA cm−2 at 1.23 VRHE in aqueous Na2 SO4 solution. In addition to the improved charge separation provided by the heterojunction, Bi2 S3 improves the absorption of visible light due to its small band energy (1.5 eV) [74]. BiVO4 /Bi2 S3 nanosheets prepared from BiOI templates have shown a photocurrent of 0.35 mA cm−2 . The surface modification of BiVO4 /Bi2 S3 with FeOOH can increase the photocurrent to 0.8 mA cm−2 at 0.4 VSCE in Na2 SO4 [75].

3.4.12

BiVO4 /FeF2

n-BiVO4 /p-FeF2 heterojunction produced a photocurrent of 2.49 mA cm−2 at 1.23 VRHE and an IPCE of 28% at 450 nm, which were more than twice as high as that produced with BiVO4 film [76].

3.4.13

BiVO4 /BiOCl

The BiVO4 /BiOCl heterojunction was prepared by treating the BiVO4 surface with HCl. During treatment, the V–O bond at the BiVO4 surface is replaced by the covalent Cl-O bond. The formation of heterojunction eliminates the enrichment of Bi on the BiVO4 surface, which negatively affects the photocurrent. Therefore, an increase in the photocurrent from 1.27 mA cm−2 in BiVO4 to 1.83 mA cm−2 at 1.23 VAg/AgCl in the BiVO4 /BiOCl heterojunction was observed [77].

3.4.14

BiVO4 /Carbon Structures

Prakash et al. [78] found that carbon nanotube, reduced graphene oxide, or graphitic carbon nitride improves light absorption and reduces the overall impedance of BiVO4 during the photoelectrochemical water oxidation. Also, the incorporation of carbon nanomaterials in the BiVO4 film increases the surface porosity, which leads to an improvement in light absorption. The BiVO4 photoanode covered with graphene

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oxide or C3 N4 has the same charge separation efficiency in the bulk, but C3 N4 helps with better charge transfer and light absorption in the 350–505 nm range. Xu et al. [79] prepared photoanodes combining BiVO4 with graphene oxide (GO) and Cu-porphyrin (CuTCPP). The CuTCPP/GO/BiVO4 photoanode produced a photocurrent of 5 mA cm−2 at 1.23 VRHE in a borate buffer solution (pH 9). Also, the onset potential decreased by 400 mV when compared to unmodified BiVO4 , and the IPCE reached 70% at 1.23 VRHE in the 420–560 nm range. The authors found that the improvement in photoactivity was due to the excellent conductivity of graphene oxide, which also acts as a BiVO4 protective layer against photocorrosion, and Cuporphyrin improves the O2 evolution kinetics. Soltani et al. [80] also observed an increase in the photocurrent produced by BiVO4 photoanode after the heterojunction formation with RGO. The BiVO4 /RGO photoanode produced a photocurrent of 0.5 mA cm−2 at 1.2 VAg/AgCl in Na2 SO4 electrolyte, which was about five times greater than that produced by the bare BiVO4 photoanode. Luan et al. [81] prepared polymeric/inorganic nanojunctions using 3D C3 N4 and BiVO4 . The 3D C3 N4 nanonetworks have a positive surface charge, which favours their coupling with the negatively charged BiVO4 particles. The intimate contact between the two semiconductors promoted by electrostatic attraction improves the separation of charges and increases the area of the liquid–solid junction. As a result, a photocurrent of 4.87 mA cm−2 at 1.23 VRHE and ABPE of 1.55% at 0.7 VRHE were obtained with the BiVO4 /C3 N4 /FeOOH/NiOOH photoanode in phosphate buffer solution (pH 7). Zeng et al. [82] prepared a Mo:BiVO4 /g-C3 N4 heterojunction that produces a photocurrent of 3.11 mA cm−2 at 1.23 VRHE , an IPCE of 45.5% at 430 nm, and an ABPE of 0.74% at 0.78 VRHE in phosphate solution (pH 7). Z-scheme heterojunctions of C3 N4 /BiVO4 prepared by electrodeposition of BiVO4 on spin-coated C3 N4 films have produced a photocurrent of 0.42 mA cm−2 at 1.23 VRHE in Na2 SO4 (pH 7). It has been found that interplanar hydrogen atoms in C3 N4 induce the formation of V4+ , which improves the mobility of charge transport through the formation of oxygen vacancies in BiVO4 [83]. γ irradiation has slightly decreased the C3 N4 bandgap energy from 2.82 to 2.76 eV. Therefore, the γ-irradiated C3 N4 /BiVO4 film produces a photocurrent of 1.38 mA cm−2 at 1.23 VRHE in Na2 SO4 solution, which was superior to the photocurrent achieved by the non-irradiated heterojunction (0.53 mA cm−2 ) [84]. C3 N4 /BiVO4 films prepared by an in situ method have generated a photocurrent of 4.06 mA cm−2 at 1.23 V that was approximately three times greater than the current produced by the BiVO4 photoanode. The modification of the C3 N4 /BiVO4 photoanode with a NiOOH catalyst increased the photocurrent to 5.44 mA cm−2 at 1.23 VRHE in phosphate buffer solution (pH 7) [85]. Heterojunctions of BiVO4 /covalent triazine-based polymers with a naphthyl group and covered with a FeOOH/NiOOH cocatalyst have shown a photocurrent of 4.05 mA cm−2 at 1.23 VRHE in phosphate buffer (pH 7). This excellent performance was attributed to the type II band alignment and the rapid hole extraction from BiVO4 due to the electric field generated between BiVO4 and the polymer [86].

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3.5 Crystal Facet Engineering Han et al. [87] have shown that BiVO4 with a preferred [001] growth orientation and exposed (001) facets exhibit exceptional charge transport and surface reactivity. The preferentially [001]-oriented BiVO4 photoanode was 16 times more photoactive than the randomly oriented BiVO4 photoanode. The Co-Pi modification of [001]oriented BiVO4 surface produced a 6.1 mA cm−2 photocurrent at 1.23 VRHE in phosphate solution. Kim et al. [88] observed through in situ PEC-XAS a contraction of the distance between the Bi and O atoms, and an elongation of the distance of Bi–VO4 units during the water oxidation. Coupled exposure extent of the (010)/(110) facets caused the Bi3+ ions to oscillate and lowered the energy barrier for charge transfer. Through Bi oscillations, each contraction and elongation stabilize the holes in the oxygen sites, which leads to a longer lifetime of the photogenerated charges. Song et al. [89] made epitaxial BiVO4 thin films with different crystallographic orientations. They demonstrated that the photoelectrochemical properties of the BiVO4 (010) are significantly higher than those of the BiVO4 (001). Also, the BiVO4 (010) has a more significant potential to be modified on the surface than the BiVO4 (001). Therefore, the photocurrent of 2.29 mA cm−2 at 1.23 VRHE produced by the BiVO4 film (010) was much higher than that of the BiVO4 (001) (0.74 mA cm−2 ).

3.6 Annealing Treatment Zhang et al. [90] have evaluated the influence of oxygen vacancies and hydrogen donors on the performance of BiVO4 photoanodes. H2 annealing produces more hydrogen donors as dominant defects, while carbon monoxide treatment generates more oxygen vacancies. It has been observed that oxygen vacancies are more effective than hydrogen donors to improve electron transport in both BiVO4 domains and along structural boundaries, thus producing higher front-illuminated photocurrent, higher film conductivity, and lower polaron hopping barrier. Safaei et al. [91] investigated the effect of microwave annealing on the BiVO4 properties. They found improved crystallization, higher charge carrier density, and more compact morphology compared to heat treatment in a conventional furnace. The photocurrent of BiVO4 annealed in microwaves was about twice that obtained with BiVO4 treated in a conventional furnace.

3.7 Texture Control BiVO4 with nanoworm morphology has been prepared through a hydrothermal treatment of scheelite tetragonal BiVO4 . The nanoworms were 3–6 nm in width and

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12–20 nm in length. The BiVO4 nanoworms produced a photocurrent of 9.3 μA cm−2 at 1.1 VAg/AgCl and an ABPE of 0.002% at 0.5 VAg/AgCl in phosphate solution [92]. Nanoporous BiVO4 films were synthesized by an in situ transformation method (WO3 → Bi2 WO6 → BiVO4 ). This method leads to the production of vertically grown BiVO4 nanoflakes with a worm-like morphology on FTO substrates. The films produced a photocurrent of 1.0 mA cm−2 at 1.23 VRHE in phosphate solution (pH 4.1) [93]. The morphology of BiVO4 has been controlled by varying the Bi/V molar ratio. Increasing the Bi/V ratio to 3 leads to the formation of sheet-like decagonal particles with (010) and (110) active surface facets that produced a photocurrent of 0.94 mA cm−2 at 1.23 VRHE in aqueous Na2 SO4 solution [94]. BiVO4 film with a rough surface, flower-like morphology, and enriched with oxygen vacancies generated a photocurrent of 2.23 mA cm−2 at 1.23 VRHE in borate solution. The flower-like morphology increases the surface area and reduces the solution-mediated interface recombination [95]. BiVO4 nanocones with high (040) to (121) ratio prepared by a solution method produce a photocurrent of 1 mA cm−2 at 1.23 VRHE in Na2 SO4 solution [96]. Mesoporous BiVO4 has been prepared using P123 as a pore marker. Mesoporous BiVO4 photoanode produces 2.19 mA cm−2 photocurrent at 1.23 VRHE and an IPCE of 39% at 420 nm in a phosphate solution, which was about twice as high as that of pristine BiVO4 . By modifying the mesoporous BiVO4 surface with a CoPi catalyst, the photocurrent and the IPCE are increased to 4.57 mA cm−2 and 65%, respectively [97]. A Mo-BiVO4 photoanode with high porosity has been prepared by a V resubstitution and alkaline dissolution method. In the phosphate solution, a photocurrent of 3.18 mA cm−2 at 1.23 VRHE has been observed. The method used produces a large contact area between photoanode and electrolyte, which favours the charge transfer at the electrode/electrolyte interface [98].

3.8 Combination with Plasmonic Nanoparticles Chen et al. [99] developed an antenna/spacer/reflector Au/BiVO4 /WO3 /Au photoanode using gold nanospheres of different sizes. A layer of Au (400 nm in diameter and 80–100 nm in thickness) deposited on the FTO substrate serves as a current collector and as a back reflector. The Au layer (20 nm in diameter and 200 nm in thickness) deposited on the surface of the film acted as antennas for absorbing incident and reflected light, thus concentrating the light in the BiVO4 layer. Due to the reflector and antenna coupling, a strong electric field is generated in the BiVO4 spacer, which resulted in more efficient charge separation. Thus, the Au/BiVO4 /WO3 /Au photoanode produced a photocurrent of 1.31 mA cm−2 at 1.23 VRHE in Na2 SO4 (pH 6.5), which was approximately three times greater than the photoanode without Au layers due to the combined plasmonic effect of Au nanospheres.

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BiVO4 covered with Au nanoparticles of different sizes improves the light absorption in bulk and the photocurrent through sub-bandgap hot electron injection. The Au-BiVO4 film produces a photocurrent of 0.30 mA cm−2 , while the BiVO4 film achieves only 0.19 mA cm−2 at 1.23 VRHE in phosphate buffer solution [100]. The plasmonic effect of Bi nanoparticles increases the photocurrent of BiVO4 films, which achieve 3.56 mA cm−2 at 1.23 VRHE in Na2 SO4 electrolyte. Bi/BiVO4 nanoparticles modified with reduced graphite oxide produce a significantly higher photocurrent of 6.05 mA cm−2 [101].

3.9 Combination with OER Catalysts 3.9.1

Fe-Based Catalysts

Fe-based catalysts are efficient in increasing the performance of BiVO4 for the water oxidation reaction. BiVO4 modification with FePi nanosheets shows a photocurrent of 2.28 mA cm−2 at 1.23 VRHE in borate solution, which is about 250% greater than that of BiVO4 [102]. A Mo:BiVO4 /MIL-101(Fe) core–shell film produces a photocurrent of 4.01 mA cm−2 at 1.23 VRHE in Na2 SO4 solution due to the catalytic effect of the MIL-101(Fe) layer [103]. BiVO4 hydrothermally covered with F:FeOOH exhibits a photocurrent of 2.7 mA cm−2 at 1.23VRHE in Na2 SO4 solution [104]. Zhang et al. [105] modified the BiVO4 surface with an ultrathin (2 nm) β-FeOOH layer using a solution impregnation method. The BiVO4 /β-FeOOH photoanode produced a current of 4.3 mA cm−2 at 1.23 VRHE in Na2 SO4 solution that was twice as high as the electrodeposited amorphous FeOOH. The greater photoactivity of the BiVO4 /β-FeOOH film was attributed to the ultrafine β-FeOOH crystalline structure and abundant oxygen vacancies, which improve hole transport/trapping and provide more active sites for the water oxidation reaction. Hu et al. [106] modified the BiVO4 surface with an amorphous FeSnOS catalyst. A photocurrent of 3.1 mA cm−2 at 1.23VRHE in Na2 SO4 was produced, which was 3.4 times greater than the photocurrent generated by unmodified BiVO4 . The surface modification of BiVO4 photoanode with a Co-LaFeO3 cocatalyst produced a photocurrent of 3.4 mA cm−2 at 1.23 VRHE in phosphate solution (pH 7), which was about four times greater than that of BiVO4 [107]. The catalytic effect of Fe2 TiO5 for the water oxidation reaction was demonstrated by Gao et al. [108]. BiVO4 /FeTiO5 photoanode led to a shift in the onset potential of 300 mV and produced a 3.2 mA cm−2 photocurrent at 1.23 VRHE in phosphate buffer solution (pH 7).

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Mn-Based Catalysts

Kaur et al. [109] studied the effect of different manganese oxide catalysts (MnO2 , MnOx —a mixture of Mn3+ and Mn4+ , Mn2 O3 , and Mn3 O4 ) on the photoelectrochemical activity of BiVO4 . The BiVO4 /MnOx photoanode showed the lowest onset potential of 0.33 VRHE and the highest activity among the oxides used. MnCo2 O4 and ZnCo2 O4 catalysts deposited on BiVO4 have exhibited photocurrents of 2.8 and 2.2 mA cm−2 at 1.23 VRHE in phosphate solution (pH 7), respectively [110]. A BiVO4 photoanode combined with a bioinspired Mn4 O4 -cubane catalyst exhibited a photocurrent of 2.5 mA cm−2 at 1.23 VRHE [111].

3.9.3

Co-Based Catalysts

Co-Pi, Ni-Bi, and Mn-Pi catalysts were deposited on the BiVO4 nanocones by an in situ two-electrode photodeposition method without applying bias. The best performance was obtained using the BiVO4 /Co-Pi photoanode, which reached a photocurrent of 0.76 mA cm−2 and a hole injection efficiency of 94.5% at 1.23 VRHE in Na2 SO4 solution [112]. The surface of BiVO4 has been modified with Co3 (PO4 )2 using a solidstate process. The BiVO4 /Co3 (PO4 )2 photoanode has shown a modest current of 0.30 mA cm−2 at 1.23 VRHE while bare BiVO4 produced only 0.13 mA cm−2 in phosphate buffer solution [113]. CoOOH has been attached to coral-like BiVO4 films to suppress surface states and increase water oxidation kinetics. The BiVO4 /CoOOH photoanode showed a low onset potential of 0.20 VRHE , a photocurrent of 4.0 mA cm−2 , and an IPCE of 65% at 450 nm, at pH 7 [114]. BiVO4 /MOF-CoNi photoanode generated a photocurrent of 3.2 mA cm−2 at 1.23 VRHE in Na2 SO4 solution [115]. Fluorination of BiVO4 films followed by the in situ formation of CoF2 catalyst leads to the generation of a 5.1 mA cm−2 photocurrent at 1.23 VRHE and an onset potential of 0.27 VRHE in borate solution [116]. BiVO4 photoanode modified with CoOx produced a photocurrent of 3.1 mA cm−2 at 1.23 VRHE in Na2 SO4 solution. The charge injection and charge separation efficiencies reached 70 and 90%, respectively, after modifying the BiVO4 surface [117]. The deposition of Co3 O4 and amorphous Co-Fe-layered double hydroxide (CoFeLDH) on BiVO4 accelerates the extraction and injection of holes for water oxidation. The BiVO4 /Co3 O4 /CoFe-LDH photoanode produced a 3.9 mA cm−2 photocurrent at 1.23 VRHE in phosphate solution (pH 7) [118]. Modifying the BiVO4 photoanode surface with a NiCo-LDH catalyst increases the photocurrent from 1.1 mA cm−2 in bare BiVO4 to 3.4 mA cm−2 at 1.23 VRHE in Na2 SO4 solution [119]. The deposition of CoP nanosheets on BiVO4 nanoporous led to a cathodic shift of the onset potential for water oxidation of 220 mV and a photocurrent of 4.0 mA cm−2 at 1.23 VRHE , which was about three times higher than the photocurrent of bare

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BiVO4 [120]. Fe-doped CoP has accelerated the oxidation of water by BiVO4 films, producing a photocurrent of 2.16 mA cm−2 at 1.23 VRHE in Na2 SO4 solution [121]. Co2 N0.67 prepared by a hydrothermal method followed by calcination in ammonia flow has shown an electrocatalytic response to the water oxidation reaction. In the dark, Co2 N0.67 produces a current of 10 mA cm−2 at an overpotential of 0.47 V, and a Tafel slope of 83 mV/dec. When the BiVO4 surface was modified with Co2 N0.67 , a photocurrent of 2.21 mA cm−2 at 1.23 VRHE in Na2 SO4 solution was produced [122]. Cobalt polyoxometalate evenly distributed over BiVO4 photoanode covered with N-doped carbon improves hole injection efficiency, leading to lower onset potential and improved kinetics for water oxidation. The modified photoanode produced an ABPE of 1.22% at 0.63 VRHE and a photocurrent of 3.30 mA cm−2 at 1.23 VRHE , which was five times greater than the photocurrent produced by unmodified BiVO4 [123]. BiVO4 photoanode modified with a WCoFe oxyhydroxide catalyst produced a 4.35 mA cm−2 photocurrent at 1.23 VRHE and a charge injection efficiency of 84.75% [124]. BiVO4 /CoMoO4 has produced a photocurrent of 3.04 mA cm−2 at 1.23 VRHE in a phosphate solution (pH 6.8) that is almost three times larger than that of the BiVO4 photoanode [125]. The BiVO4 surface modified with cobalt salophen complexes has exhibited a photocurrent of 4.27 mA cm−2 at 1.23 VRHE in a neutral solution. The Co complex (salophen) acts by accelerating the oxidation reaction of water and reducing surface recombination. Due to its hydrophobic nature, Co(salophen) can bind strongly to the surface of BiVO4 , thus improving its stability. BiVO4 /Co(salophen) films produced a stable photocurrent of 3.5 mA cm−2 at 1.23 VRHE for 3 h [126]. A cobalt cubane molecular catalyst has been immobilized on BiVO4 through an electrochemical polymerization method. With the introduction of vinyl phosphoric acid (VPA) as an anchoring linkage, Mo:BiVO4 /Al2 O3 /Vpa/poly-1/Co cubane photoanode shows a photocurrent of 4.5 mA cm−2 at 1.23 VRHE in Na2 SO4 (pH 7) [127].

3.9.4

Cu-Based Catalysts

Photoanodes prepared from the combination of BiVO4 and CuO nanosheets showed a negative shift in the onset potential of 280 mV and an increase of two times in the photocurrent when compared to BiVO4 . The BiVO4 /CuO photoanode produced a photocurrent of 0.9 mA cm−2 at 1.23 VRHE in an aqueous borate solution (pH 9.4). The current increase was attributed to the catalytic effect of CuO for O2 evolution [128]. CuCoO2 coupled to BiVO4 has produced photocurrent of 3.32 mA cm−2 at 1.23 VRHE against 1.21 mA cm−2 of bare BiVO4 . Also, the CuCoO2 helps in the stability

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of the photoanode. 79% of BiVO4 /CuCoO2 photoelectrochemical activity was maintained for 5 h operation at 0.8 VRHE , while only 9% of activity was retained using BiVO4 [129].

3.9.5

Ni-Based Catalysts

Chen et al. [130] modified the BiVO4 surface with different nickel, cobalt, and iron catalysts. Among the studied catalysts, the largest photocurrent of 2.1 mA cm−2 at 1.23 VRHE in Na2 SO4 /phosphate buffer solution was obtained with the BiVO4 /NiOOH film. An amorphous NiOx catalyst deposited on a Mo:BiVO4 film produced a photocurrent of 2.44 mA cm−2 at 1.23 VRHE in Na2 SO4 solution [131]. NiMoO4 also improves the photoelectrochemical activity of the BiVO4 surface. BiVO4 /NiMoO4 photoanodes produce a photocurrent of 6.40 mA cm−2 at 1.23 VRHE and an ABPE of 2.83% at 0.6 VRHE in Na2 SO4 /Na2 SO3 solution [132]. Zr-doped BiVO4 was modified with an amorphous Ni,Fe-based Prussian blue to produce core–shell structures. The Zr-BiVO4 /NiFePB photoanode produces a photocurrent of 3.23 mA cm−2 at 1.23 VRHE in a phosphate solution and excellent stability of more than 50 h operation [48]. A 75 cm2 BiVO4 photoanode was prepared using the electrodeposition method. The BiVO4 photoanode exhibited a 1.2 mA cm−2 photocurrent at 1.23 VRHE in Na2 SO4 solution (pH 5.5). Modifying the BiVO4 film surface with a NiOOH catalyst increases the photocurrent to 1.7 mA cm−2 and the IPCE to 24% [133]. Huang et al. prepared a 54.32 cm2 BiVO4 photoanode by electrodeposition that produced a photocurrent of 2.55 mA cm−2 for sulphite oxidation at 1.23 VRHE that was comparable to photocurrent of 1.66 cm2 BiVO4 photoanode (2.85 mA cm−2 ). After modifying the BiVO4 surface with a Ni2+ /Fe3+ catalyst in tannic acid, the large area photoanode showed a photocurrent of 2.23 mA cm−2 at 1.23 VRHE in a borate solution (pH = 9.35) [134]. A BiVO4 photoanode modified with a tannic acid coordinated with Ni and Fe catalyst has shown excellent stability in borate/boric acid solution, producing a photocurrent of approximately 1.5 mA cm−2 at 1.23 VRHE for 62 h [135]. Zr:BiVO4 modified with a nickel and silver hydroxyphosphate catalyst (AgNiOH-Pi) produces a photocurrent of 3.14 mA cm−2 at 1.23 VRHE with the stability of approximately 60 h [136].

3.10 Interfacial Charge Mediators Ning et al. [137] showed that the performance of BiVO4 photoanodes could be improved by using a Co-porphyrin as a mediator of interfacial charge transfer between BiVO4 and FeNi(OH)x cocatalyst. Co-porphyrin not only reduces the surface charge recombination but also speeds up the hole transfer, thus ensuring a longer lifetime

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for charge carriers at the photoanode/electrolyte interface. As a result, a photocurrent of 4.75 mA cm−2 at 1.23 VRHE in Na2 SO4 solution has been observed. Polyaniline has been used as a hole transport layer to improve the performance of BiVO4 . BiVO4 /PANI/NiOOH photoanode showed a photocurrent of 3.31 mA cm−2 and an IPCE of 83.3% at 430 nm, both 1.23 VRHE in phosphate solution (pH 7) [138]. BiVO4 has been modified with a polyoxometalate (H3 PW12 O40 ) that acts as an electron capture and transfer agent and with nickel (II) phthalocyanine tetrasulphonic acid as a hole extractor. This modification leads to an improvement of the photocurrent of approximately five times when compared with pristine BiVO4 [139]. BiVO4 covered with a Fe-phenolic (FTA) layer and with a molecular catalyst ([Co4 (H2 O)4 (HPMIDA)2 (PMIDA)2 ]6− ) achieved a photocurrent of 5.5 mA cm−2 at 1.23 VRHE in borate solution, retaining 93% of the current after 3 h operation [140]. Cobalt polyoxometalate (Ag10 [Co4 (H2 O)2 (PW9 O34 )2 ]) (PW9 Co) has been used as a hole extraction layer to improve the BiVO4 performance. The BiVO4 /PW9 Co photoanode produced a 3.06 mA cm−2 photocurrent at 1.23 VRHE in Na2 SO4 solution (pH 5.8) that was about three times the current generated by unmodified BiVO4 [141].

3.11 Combination with Ferroelectric Materials BiFeO3 deposited on the WO3 /BiVO4 surface improves charge separation due to the electric field generated at the n-BiVO4 /p-BiFeO3 interface and the self-polarization of the BiFeO3 layer [142]. Soltani et al. [143] prepared BiFeO3 /BiVO4 p-n heterojunctions by an ultrasonic method. The BiFeO3 /BiVO4 photoanode produced a photocurrent of 0.36 mA cm−2 under UV–vis light and 0.23 mA cm−2 under visible light at 1 VAg/AgCl , which was about three times greater than the photocurrent obtained with BiVO4 in Na2 SO4 solution (pH 6.5).

3.12 Underlayers and Overlayers Reddy et al. [144] improved the light-absorbing capacity and charge transport rate of BiVO4 photoanodes by introducing an inverse opal (IO)-SnO2 hole blocking underlayer and FeOOH-NiOOH overlayers. The IO-SnO2 underlayer improves charge separation and reduces the interface contact resistance due to the ordered electron migration path. Also, the IO-SnO2 underlayer improves light absorption through diffuse scattering and coherent multiple internal scattering. The IO-SnO2 /BiVO4 /FeOOH-NiOOH photoanode produced a photocurrent of 3.57 mA cm−2 at 1.23 VRHE with a stability of 10 h in phosphate buffer solution (pH 7). ABPE was 1.02% at 0.7 VRHE . BiVO4 films sandwiched between a SnO2 underlayer and a NiWO4 overlayer have exhibited a photocurrent of 0.93 mA cm−2 at 1.23 VRHE in a phosphate buffer solution (pH 7.5) [145].

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A 1.4 nm Lu2 O3 underlayer epitaxially grown on the FTO surface has contributed to improving BiVO4 charge separation efficiency and photocurrent. The Lu2 O3 underlayer modulates the electronic conduction pathway along the grain boundaries, thus improving the photoanode performance [146].

3.13 Effect of Electrolytes Firet et al. [147] have identified by XPS the formation of a borate layer on the BiVO4 surface, under lighting and open-circuit conditions. The borate anion of the electrolyte bound covalently to the bismuth ions in BiVO4 , forming a heterojunction near the surface. This heterojunction produces an improved band bending on the surface and, consequently, improves the charge separation and suppresses the surface recombination of the charge carriers. Under the same conditions in the dark, BiVO4 formed an OH layer on the surface, leading to a lower degree of band bending. Meng et al. [148] have shown that a post-synthetic treatment of dipping a BiVO4 film in a borate buffer solution improves photocurrent generation. Adsorption of [B(OH)4 ]− tetrahedral ions close to the active sites results in a change of BiVO4 at the molecular level. Those ions act as a regulating ligand and passivating, thus having an essential role in the water oxidation kinetics and charge trapping reduction on the BiVO4 surface. As a result, the modified photoanode produced a photocurrent of 3.5 mA cm−2 at 1.23 VRHE , and the onset potential was cathodically displaced by about 250 mV in borate buffer solution (pH 9.3). Favaro et al. [149] have verified through in situ ambient pressure X-ray photoelectron spectroscopy with “tender” X-rays (4.0 keV) that under illumination, a layer of bismuth phosphate is formed on BiVO4 films in potassium phosphate solution. The BiPO4 layer increases the density of negative charges on the film surface and acts by passivating surface states. Also, the formation of the BiPO4 layer forms a junction with BiVO4 , modifying the local density of states of the photoanode surface and influencing the cell performance. Interestingly, they noted that the changes at the BiVO4 /KPi interface are reversible after returning to dark conditions.

4 Unbiased Water-Splitting PEC Cells Using BiVO4 -based Photoanodes This section describes the latest advances in the development of unbiased watersplitting PEC cells to produce hydrogen.

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4.1 BiVO4 -CuBi2 O4 Cells Song et al. [150] have shown that the instability of unprotected CuBi2 O4 due to photocorrosion in aqueous solution is the main limitation of W:BiVO4 -CuBi2 O4 (photoanode–photocathode) cells. It prevents the production of detectable amounts of hydrogen even when a cocatalyst is used. To solve this problem, they protected the CuBi2 O4 film with a CdS/TiO2 layer. Also, the W:BiVO4 photoanode and CuBi2 O4 /CdS/TiO2 /RuOx photocathode produce photovoltages that are significantly lower than the bandgap energy of BiVO4 and CuBi2 O4 , respectively. Therefore, the W:BiVO4 /CoPi-RuOx /TiO2 /CdS/CuBi2 O4 cell needs an external potential to produce detectable amounts of hydrogen. Thus, hydrogen production from water splitting was observed at applied potentials ≥0.4 V. A cell made up of an anodized CuBi2 O4 /CuO photocathode and a BiVO4 /CoPi photoanode illuminated in parallel configuration generated a short-circuit photocurrent of 0.1 mA cm−2 and an open-circuit potential of 0.68 V in NaOH solution [151]. A tandem cell has been assembled using BiVO4 photoanode and CuBi2 O4 photocathode separated by a proton exchange membrane. Under illumination, the cell produced a short-circuit photocurrent of 36 μA cm−2 in a borate buffer/Na2 SO4 solution (pH 9.2), which corresponds to an STH (solar-to-hydrogen) efficiency of 0.04% [152]. Kim et al. [153] manufactured a membraneless tandem cell using Mo:BiVO4 /Co-Pi photoanode and CuBi2 O4 /Pt photocathode. The cell produced a short-circuit photocurrent of 0.15 mA cm−2 in potassium phosphate (pH 7).

4.2 BiVO4 -Si Cells A tandem cell manufactured using a BiVO4 photoanode, and Si/inverse opal TiO2 /hydrogenase photocathode produced 0.70 μmol cm−2 of H2 in 3 h under an applied potential of 0.4 V [154]. Liu et al. [155] prepared an inverted metal-insulating-semiconductor (I-MIS) Si photocathode with a 12.66% ABPE and stable operation for 108 h. By building a tandem cell with the I-MIS Si photocathode and a BiVO4 /FeOOH/NiOOH photoanode, a 1.55 mA cm−2 photocurrent and a 1.9% STH efficiency were achieved in an aqueous solution of potassium bicarbonate (pH 8.5). A two-compartment cell using pn+ Si/TiO2 /Pt photocathode dipped in HClO4 solution (pH 0) connected to a BiVO4 /CoOx photoanode in borate solution (pH 9) produced 1.7 mA cm−2 short-circuit photocurrent and 2.09% STH efficiency. This cell was stable for 1.5 h with 11% photocurrent drop [156]. Xue et al. [157] developed a BiVO4 /TiO2 /CoBi/TiO2 photoanode depositing cobalt borate (CoBi) between two layers of TiO2 . This configuration favours the deposition of CoBi, and the outside TiO2 layer stabilizes both BiVO4 and CoBi, improving the photoanode stability. Then, a tandem cell was manufactured by coupling the

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BiVO4 /TiO2 /CoBi/TiO2 photoanode (1 cm × 1 cm) with an a-Si photocathode (1 cm × 1 cm) that produces 1.6 V photovoltage. The cell produced a short-circuit photocurrent of 2.5 mA cm−2 in a borate buffer solution (pH 9.5), corresponding to an STH efficiency of 3%. A tandem cell consisting of a BiVO4 /Fe2 O3 /FeOOH/NiOOH photoanode coupled to Si solar cell and a Pt cathode generated an STH efficiency of 3.2% with an H2 production rate of 25.1 μmol cm−2 h−1 [158]. Feng et al. [159] have produced oxygen vacancies on the BiVO4 surface through pretreatment with a borate solution containing Na2 SO3 for 10 min under lighting without application of external potential. This pretreatment alleviates the effects of recombination in the bulk of the photoelectrode. Connecting the Ovac BiVO4 /FeOOH-NiOOH photoanode immersed in a borate solution (pH 9) to the Si photocathode immersed in HClO4 (pH 0) results in a cell with an STH efficiency of 3.5%. Ahmet et al. [160] demonstrated a large area PEC-PV cell using a 50 cm2 WBiVO4 /CoPi photoanode coupled to a two-series Si solar cell and Pt as a cathode. They found that the efficiency of the large area photoanode was significantly less than that of a small area (0.24 cm2 ). Only 10% of the voltage drop in the broad area cell was due to ohmic losses in the FTO substrate. Also, using an electrolyte concentration of 2 M instead of 0.1 M can increase the photocurrent up to 40%. Those factors did not affect the performance of the small area electrode. The performance of the large area device was also limited by H± OH transport close to neutral pH. Despite this, the large area cell produced an STH efficiency of 2.1%, while the small area cell showed an STH efficiency of up to 5.5%.

4.3 BiVO4 /Perovskite Cells A 0.25 cm2 tandem cell composed of a BiVO4 /TiCo photoanode coupled with a perovskite solar cell and a Pt cathode produced a 0.35% STH efficiency with high stability of 20 h. Scaling the cell to 10 cm2 leads to a small decrease in STH efficiency to 0.23% due to the increase in series resistance [161]. A tandem cell consisting of a halide perovskite solar cell/IO-TiO2 /[NiFeSe] hydrogenase photocathode coupled with a BiVO4 /TiCo photoanode reached an STH efficiency of 1.1% [162]. A tandem cell made up of a Mo:BiVO4 photoanode connected to a CsPbBrI2 perovskite solar cell and a Pt cathode produced a 2.13 mA cm−2 short-circuit photocurrent and an STH efficiency of 2.43% [163]. Wang et al. [164] prepared transparent BiVO4 films by electrodeposition with well-controlled oxygen vacancies that act as shallow donors. The FeOOH/NiOOHmodified BiVO4 photoanode produced a stable photocurrent of 5.87 mA cm−2 at 1.23 VRHE in a borate solution (pH 9.5). Cell manufactured from the coupling of the BiVO4 /FeOOH/NiOOH photoanode with a sealed perovskite solar cell and using a Pt cathode produces an STH efficiency of 6.5%.

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4.4 BiVO4 /Sb2 Se3 Yang et al. [165] developed an efficient Sb2 Se3 photocathode (Eg = 1.2 eV) that produces a photocurrent of up to 30 mA cm−2 . Covering the Sb2 Se3 with TiO2 and CdS layers raises the HC-STH efficiency to 3.4%. The Au/Sb2 Se3 /TiO2 /CdS photocathode was then combined with an H,Mo:BiVO4 /NiFeOx photoanode to form a tandem photocell. Both photoanode and photocathode showed a high IPCE of 70% for BiVO4 at 450 nm and 60% for Sb2 Se3 . The tandem cell operated without the application of external potential with an STH efficiency of 1.5% and high stability of 10 h in a phosphate buffer solution (pH 7) containing 0.01 M of V2 O5 .

4.5 BiVO4 -Cu2 ZnSnS4 Cell A cell manufactured from a BiVO4 photoanode coupled to a Cu2 ZnSnS4 /CdS solar cell (conversion efficiency of 7.66%) and connected to a Pt cathode performed unbiased water splitting with STH efficiency of 1.46% in a borate solution (pH 9.3) [166]. Cu2 ZnSnS4 /CdS/HfO2 /Pt photocathode in tandem with a BiVO4 photoanode showed an STH efficiency of 1.05% and stability of 10 h operation in phosphate solution (pH 6.5) [167].

4.6 BiVO4 -CuO Cells A photoelectrochemical tandem cell consisting of a BiVO4 /n-Se photoanode and a Ni:CuO/p-Cr2 O3 photocathode produced 0.03 mA cm−2 short-circuit photocurrent in Na2 SO4 (pH 7) [168].

4.7 BiVO4 -Cu2 O Cells A tandem cell formed by a BiVO4 photoanode and an H:Ti3 C2 Tx /Cu2 O photocathode produced an STH efficiency of 0.55% for illuminating from the photocathode and 0.34% for illuminating from the photoanode in phosphate solution [169].

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4.8 BiVO4 -WO3 Cells Wang et al. built a tandem cell with a WO3 photoanode and a black phosphorus photocathode decorated with BiVO4 . Under illumination, the cell produced an STH efficiency of 1.9% in Na2 SO4 solution [170].

5 Conclusions and Outlook Significant advances have been made in recent years to improve the photoelectrochemical efficiency of BiVO4 photoanodes for the water oxidation. Different strategies have been used to improve the charge separation and transport in the bulk of BiVO4 , such as doping with metal and non-metals, forming heterojunctions, using underlayers, and combining with ferroelectric materials. Due to the anisotropic properties of BiVO4 crystals, different facets have been shown to exhibit different photoelectrochemical activities. The control of BiVO4 morphology and porosity has also been shown to influence the photoelectrochemical response positively. The combination of BiVO4 with plasmonic nanoparticles and nanomaterials with bandgap energy less than 2.4 eV has been shown to improve the light absorption efficiency. The use of Fe, Ni, Co, Mn, and Cu catalysts and interfacial charge mediators is an extremely effective strategy to improve the charge transfer efficiency on the BiVO4 surface. Despite this, more efficient photoelectrodes need to be developed to scale BiVO4 films. Therefore, strategies to improve the efficiency of bulk charge separation and light absorption efficiency should be the key to achieve greater STH efficiency. Another issue that needs to be addressed is the stability of BiVO4 in an aqueous medium for continuous operation for several hours. Finally, we hope that this book chapter can inspire new researchers to develop increasingly efficient BiVO4 photoelectrodes for water splitting. Acknowledgements We are grateful to FAPEMIG and CNPq, which for several years has financed our research in photoelectrochemistry. We also thank CAPES (code 001), FAPEMIG, and UFVJM for the scholarships provided to our students.

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Chapter 7

Defect-Enriched Transition Metal Oxides Towards Photoelectrochemical Water Splitting Lalita Sharma and Aditi Halder

1 Introduction Increasing worldwide environmental problem has posed a serious threat to the humankind [1]. Depletion of the fossil fuels to fulfill the human needs has invoked the interest to find out the suitable alternative sources [2, 3]. We need to change the pattern of the energy consumption in order to utilize the renewable source of fuel in more effective way [4, 5]. Sunlight is providing infinite source of energy to the earth, [6, 7] but inefficient harvesting of solar energy will not fulfill the energy demands of the world due to discontinuous solar energy and variability of the light from day to night [8]. Thus, it is a necessary course of action to create technologies which harvest the sunlight in more effective way and convert it into transportable fuel or electrochemical energy [9]. Electrochemical water splitting is the most effective way to produce energy [10] as shown in Eq. (1). 2H2 O → O2 + 2H2 . . . . . . .

(1)

Hydrogen (H2 ) produced in chemical Eq. (1) can be further stored by efficient storage techniques and utilized later. It can be oxidized to release energy and regenerate water in order to produce carbon less or “zero carbon” fuel [11]. Electrochemical route is one of the effective and scalable routes to produce energy from water by utilizing electricity [12]. Electrical energy is used as an input in case of electrochemical water splitting but it is not only the single input to carry out the reaction. Indeed, the use of solar energy to drive Eq. (1) in photoelectrochemical system has proven to be a very interesting topic in the last decades [9]. Solar fuels are initiated by the “photon-chemical energy” conversion which proved as an effective alternative L. Sharma · A. Halder (B) School of Basic Sciences, Indian Institute of Technology, Mandi, Himachal Pradesh 175005, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Kumar and P. Devi (eds.), Photoelectrochemical Hydrogen Generation, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-16-7285-9_7

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of fuel. In precise, producing H2 from water by utilizing solar energy is an effective strategy for decades. Overall electrochemical reaction as depicted in Eq. (1) consists of two half reactions involving reduction at cathode and oxidation at anode. The two half reactions involving reduction and oxidation (against reversible hydrogen electrode (RHE) electrode) are represented in the given equations [13]. Reduction reaction  0  = 0.00VvsRHE . . . . . . 2H+ + 2e− → 2H2 E red

(2)

Oxidation reaction  0  = −1.23Vvs.RHE . . . . . . 2H2 O + 4H+ → O2 + 4H+ E ox

(3)

The two half-cell reactions consisting of hydrogen production at cathode and oxygen production at anode occur at the top surface of the photoelectrodes (i.e. photocathodes and photoanodes, respectively) which is in contact with electrolyte. Under the illumination of the sunlight there is the production of the charge carriers (i.e. holes and electrons) at the surface of photoelectrodes [14, 15] (Fig. 1). − + + hvb ...... Semiconductor → ecb

(4)

After the illumination of visible light on the surface of semiconductor it undergoes the four sequential steps: (i) absorption of the light, (ii) charge carriers separation, (iii) transport of the charge carrier (h+ ) to electrode surface (or e− to the back contact), and (iv) reactions occur at surface [10]. However, there is the big challenge to design new

Fig. 1 Schematic representation of artificial photosynthesis showing the band-edge positions of BiVO4 which act as photoanode and MoS2 as photocathode material and experimental setup of photoelectrochemical water splitting in which working and reference electrodes are in one chamber and counterelectrode is in the second chamber separated by the frit. Solar light is illuminated from the external source of light [16]. Reprinted with permission Copyright 2018 American Chemical Society

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semiconductor materials which are effective, stable without difficulties in handling high proton turnover rates [17].

2 Why Transition Metal Oxides as Photoelectrodes? Transition metal oxides (TMO) have tremendous applications as it is used in energy storage, magnetic, optical, electronic, and also in various other fields due to its unique physical and electronic properties [18]. It exhibits properties of ferromagnetic, ferroelectric, photoluminescence, and semiconductive behaviour. In TMO, s-orbital of the positively charged metallic ions is fully filled by the electrons and d-orbital is not always completely filled. So semi, half, and completely filled d-orbitals in different transition metallic ions provide a unique configuration to TMO [19]. In TMO, the nature of metal–oxygen (M–O) bond changes first close to ionic and then move to highly covalent or metallic. The remarkable properties of TMO arise from the nature of the filling of the electrons in the outer d-orbitals of metallic ions, respectively. However, in order to understand this concept in depth, it is very important to understand the correlation of the structure–property relationship which further depends on the description of the valence electrons. In order to explain structure of the system, ligand field theory and band theory play a very important role in solids. In the band theory, overlapping of the orbitals of the neighbouring atoms is large and less energy (U) is needed to transfer an electron from the other orbital having single electron present at an comparable site as compared to the bandwidth (W ). Whereas in coordination chemistry, where ligand field theory is applicable, U is very large as compared to W. In the third condition where U–W, there is strong correlation of the electrons present in the solids. In case of s- and p-orbitals, the overlapping with the neighbouring atoms is very large and described by the band model and in case of f-orbitals they are closely constrained with the nuclei and screened from the neighbouring atoms. This can be explained well by ligand filed theory based on the localized electron model. On the other hand, d-orbital has intervening character as they are not screened by the outer core electrons of the neighbouring atoms. Due to this property, d-electrons show different electronic properties and localized electron properties in TMO [20–23]. As a result, TMO act as fascinating functional materials. In the last 40 years, tremendous efforts have been devoted to find out the properties of TiO2 towards solar water splitting as it behaves as semiconductor with bandgap of 3.2 eV [19, 24]. When a semiconductor material is irradiated with the light having energy equal to or greater than the bandgap of semiconductor then there is the formation of the charge carriers at the surface (electrons (e− ) and holes (h+ )) [25, 26]. However, efficiency of chemical reaction which occurred at the surface decreases as large part of the charge carriers undergo recombination. In order to solve the problem, the augmentation of heterogeneous photocatalysis and electrochemistry is found to be very effective and their union resulted into photoelectrochemcial technique [27]. Photoelectrocatalysis involved the separation

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of charge carriers with the gradient potential. Conductive substrate coated with semiconductor material is utilized as photoelectrode. When this photoelectrode comes in contact to the redox electrolyte (semiconductor/electrolyte interface) there is generation of Schottky junction which makes changes in the Fermi level of the semiconductor so as to achieve equilibrium at the semiconductor/electrolyte interface. Hence, bending of the bands occurred at the semiconductor phase which depends on the fermi level of the semiconductor and electrolyte. The region from where band bending occurs is called the space charge region or depletion region and mostly characterized by the majority of the charge carriers. In n-type semiconductor (SC), electrons are the majority carriers and in p-type SC majority of the charge carriers are holes. As discussed in Fig. 2, condition where E is greater than the flat band potential (E fb ) the electrons are decreased and holes are accumulated at the electrode surface whereas in case where E = E fb there is no band bending and bands are flat due to which charge carriers end by recombination. Hence, overall solar-to-hydrogen conversion decreases as charge carriers are recombined [21]. However, numerous methods have been opted to produce H2 by utilizing solar energy which is renewable source, for energy conversion. There is particular interest in producing H2 by utilizing solar energy with semiconductors of adequate bandgap (hv ≥ bandenergy, E g ). Most frequently semiconductors are used at anode to produce hydrogen at cathode

Fig. 2 i n-type of SC band bending under E = E fb , E > E fb and E > E fb under light with hv > E bg . ii cross section and iii oblique SEM images of titanium nanotube (TiO2 ) arrays. Reprinted with permission [24] copyright 2008 American Chemical Society

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termed as photoelectrolysis. TMO are most frequently used semiconducting materials at anode due to its long-term stability in the harsh condition and its capability to undergo oxygen evolution even at high pH conditions [21]. In the history, Fujishima and Honda [28] utilized TiO2 -based single-semiconductor bandgap of 3.2 eV material at anode and platinum black electrode at cathode for PEC water splitting. When the surface of the TiO2 electrode was illuminated by visible light there was the flow of the current from the platinum black to TiO2 . It provides the confirmation of the oxygen evolution at TiO2 surface and hydrogen generation at the Pt surface. The mechanism of the reaction follows the steps as TiO2 → 2e− + 2h+ 2h+ + H2 O →

(after illumination in visible light)

1 O2 + 2h+ (at surface of anode electrode, i.e. TiO2 ) 2

2e− + 2h+ → H2

(at surface of cathode electrode, i.e. Pt electrode)

TiO2 has tremendous stability in the aqueous medium while it is not able to harvest the full spectrum due to the 3.2 eV bandgap which lies in UV range. Due to this reason, solar-to-hydrogen conversion efficiency (SHE) was very less and required to be further addressed by using semiconducting materials with suitable bandgap. Therefore, sufficient efforts have been drawn to design the effective PEC material. An appreciable SHE is only possible by combination of the semiconductor electrodes with adequate bandgap which allows utilizing the full energy of solar spectrum with appropriate conduction and valence band energy difference. However, it is more difficult to develop all the requirements in one semiconductor to utilize on the both sides of the PEC water splitting. As two separate half reactions (i.e. oxidation and reduction) are involved in PEC water splitting, it is more feasible to carry out the two half reactions with two light absorbers [29].

3 Different Routes of Defect Engineering in Transition Metal Oxides and Its Photoelectrochemical Applications Photoelectrocatalysis (PEC) technique is more efficient way to produce hydrogen using solar light and it is the union of the heterogeneous photocatalysis and electrochemical techniques [30]. PEC is a multidisciplinary field involving several other fields such as surface science, solid-state physics, electrochemistry, and optic knowledge. PEC water splitting involved two reactions and out of those one is oxygen evolution reaction (OER) which occurs at photoanode and other is hydrogen evolution reaction (HER) which occurs at photocathode with four electron transfer mechanism. Due to sluggish reaction of OER, it is very important to put some efforts to make an efficient photoanode. Various metal oxides, i.e. WO3 , Fe2 O3 , FeCoOx , BiVO4 ,

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Fig. 3 Calculation of internal energy of oxygen vacancies formation by DFT in α-MoO3−x . a for bulk b bipolaron of Mo5+ at surface and c bulk and d polaron of Mo4+ at surface. The configuration of bipolaron is built on the lowest energy configuration [34]. Reprinted with permission from the reference [35] Copyright 2018 American Chemical Society e dual etching and reducing mechanism of WO3 nanoflakes reprinted with permission [36] Copyright 2014 American Chemical Society f Electrochemical synthesis of semiconductor materials in order to utilize solar light directly for solar water splitting Reprinted with permission [14] Copyright 2015 American Chemical Society

TiO2 , etc. are demonstrated as photoanode material in PEC water-splitting reaction. However, photocurrent densities are still less than their theoretical values due to electron–hole pair recombination [31]. Generally, oxygen vacancies, heteroatom doping, crystal facet engineering, and heterojunction construction [32, 33] are effective strategies to improve photocurrent at some extent by adopting these techniques (Fig. 3). Defect engineering is also one of the accepted techniques to tune the properties of catalyst material for electrochemical applications which has been used widely to enhance the catalytic activities of an electrocatalyst. Chemical defects in the transition metal-based catalysts are vital in determining their chemical, physical, and electronic properties. Particularly, in transition metal oxides, oxygen vacancies are one of the most reactive species on metal oxide surfaces [36]. In metal oxides, it is very obvious to control and confine the oxygen vacancies in order to realize the full potential of the material towards the various applications. Oxygen vacancies behave as shallow donors for transition metal oxides (TMOs) and as a result produce low charge ions which implement high mobility of the charge carriers and improve the performance of TMOs towards various applications [37]. Various techniques have been followed so far to create oxygen vacancies in TMOs in a controllable manner [33]. Heat treatment in oxygen-deficient environment of either vacuum or inert atmosphere, hydrogen gas treatment, wet chemical reduction, and electrochemical reduction are the techniques to create oxygen vacancies in TMOs. In the below section, we will briefly discuss

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about different routes to insert oxygen vacancies in TMO which act as an effective material towards solar water splitting.

3.1 Thermal Annealing Thermal annealing of TMO in the oxygen-deficient environment gave rise to oxygen vacancies. As the thermal annealing starts in vacuum or inert atmosphere, oxygen escapes from the TMO system and forms nonstoichiometric metal oxides (MO1−x ) [33, 38]. MO = MO1−x +

x O2 2

Oxygen vacancy design by using thermal annealing has opened up a new research area for the researchers. Thermal annealing technique is not only utilized to make oxygen defects in TMO but also utilized to remove the defects from graphene sheets and form oxide functional materials. Defect-free graphene has fast temperature response and fast heat transfer capability as compared to defect-rich graphene [39]. Various TMO have been so far utilized as photoelectrode (WO3 , Fe2 O3 , TiO2 , VO2 , BiVO4 , CuFeO2 , etc.) [37, 40–43] for PEC water splitting (Fig. 4).

Fig. 4 i Oxygen vacancy incorporation in WO3 by thermal annealing in vacuum atmosphere showing excellent hydrogen production at 550 °C [37] ii synthesis of TMO on conductive substrates ITO (indium-doped tin oxide) by thermal treatment reprinted with permission [44] copyright 2015 American Chemical Society iii enhancement in ESR signal after thermal treatment of 24 h reprinted with permission [45] copyright 2012 American chemical society iv 2D nanosheets after heat treatment at 2200 °C reprinted with permission [39] copyright 2014 American Chemical Society

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Haematite (α-Fe2 O3 ) has been utilized as photoanodes because of the abundance, least cost, non-toxic nature, excellent stability, and most importantly its suitable bandgap 1.9–2.2 eV. However, overall SHE is very less and which is due to poor electrical conductivity, short hole diffusion length [46]. Hydrothermally synthesized Fe2 O3 followed by thermal annealing at different temperatures is able to create the oxygen vacancies which consequently help to increase the conductivity of TMO [47]. After thermal annealing, there is a significant effect on the morphology of the system as depicted in Fig. 5. Thermal annealing temperature varied from 550 to 800 °C with

Fig. 5 a–i SEM micrographs of α-Fe2 O3 NWs (nanowires) annealed at different temperatures j–l J-V curves under dark and light conditions annealed at different temperature conditions. Reprinted with permission [49] Copyright 2020 American Chemical Society

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different morphological structure has also different dopant of Sn. As haematite has been grown on the conductive side of fluorine-doped tin oxide (FTO) glass substrates and annealed from 500 to 800 °C, Sn from conductive substrate doped in situ during the thermal treatment. Sn doping in Fe2 O3 improves the electrical conductivity of the metal oxide and helps to increase the overall efficiency [48]. Photoconversion efficiency has been limited by number of factors, like annealing atmosphere, annealing temperature, and concentration of oxygen vacancies in the system. After optimizing the thermal conditions by operating the one-step (1# 600– 800 °C, 20 min, 2# 800 °C, 3–72 h) and two-step annealing (600–825 °C, 20 min), J–V curves show that in the first step annealing conditions, Fe2 O3 annealed at 800 °C has maximum photocurrent density. As the temperature increases up to 825 °C, there is the decrement in the photocurrent density [49]. So temperature, time, and ramp rate optimization in thermal annealing [50] are crucial parameters to control the oxygen vacancy concentration. To maximize the beneficial impact of oxygen vacancy, it is very important to optimize annealing time and temperature depending upon the reduction extent of metal ions. In order to characterize the oxygen vacancies in TMO ESR, XPS, and TEM are very powerful technique to do so.

3.2 Hydrogen Gas Treatment (Hydrogenation) Defect implantation has been adapted as the best technique to regulate physical and chemical properties of TMO. Various defects are created in the TMO of the different lattice sites of system which is further responsible to make a remarkable change in the optical, electronic, and magnetic properties. Another approach of creating defects in the crystal lattice is by reducing the metal ions by hydrogenation and introducing the oxygen vacancies into the TMO. Hydrogen treatment of TMO by varying temperature and adjustable pressure conditions is very usual strategy which is used to insert oxygen vacancies in TMO. Optimization of oxygen vacancies is critical issue which can be achieved by varying temperature, pressure, and gas composition so that oxygen vacancies could efficiently be produced [51]. As compared to heat treatment in inert environment, hydrogen gas treatment provides an easy way to insert oxygen vacancies in metal oxides and form nonstoichiometric MO as the final product [33] (Fig. 6). MO +

x H2 → MO1−x (OH)x 2

In 1950s, hydrogenation of TiO2 nanotubes was first time reported [53] which later boosted this idea to insert oxygen vacancies in TMO. After hydrogenation of TiO2 nanotubes bandgap narrowed to 1.54 eV which helps to increase electrical conductivity [52]. After hydrogenation, there is increment of hydroxyl groups at the top of the surface which is also one of the indications to confirm oxygen vacancies. Increment of hydroxyl groups along with the low valence state of the centre metal ions

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Fig. 6 a SEM micrographs of TMO (TiO2 ) b schematic illustration of CVD furnace used for H2 gas treatment c J–V curves of WO3 and after H2 treatment d IPCE conversion at applied voltage of 1.0 V versus Ag/AgCl for different WO3 nanoflakes. Reprinted with permission [52] Copyright 2011 American Chemical Society

is characterized by X-ray photoelectron spectroscopy (XPS) technique [54]. DFT calculations have also provided information about increment of oxygen vacancies due to hydroxyl groups at the metal oxide surface [55]. Oxygen vacancies in TiO2 material act as shallow donors [56]. PEC performance of TiO2 was drastically enhanced due to the presence of the shallow donors in TMO. Simultaneous creation of oxygen vacancies and hydrogenation results in the occupation of large density of states lying within the forbidden gap, deeper for the former (d-d bands attributed to metal–metal bond formation) and shallower for the latter (π* band near the Fermi level). It seems oxygen vacancies create the occupation of antibonding d-d* band, existed above the π* and within the conduction band, shifting the Fermi level towards higher energies and decreasing the oxide work function [57]. Chen et al. have demonstrated that black-coloured TiO2 has more defective surface which is collected after the hydrogenation treatment under hydrogen pressure of 20 bar and temperature conditions of 200 °C for 5 days [32]. In comparison to TiO2 , WO3 has small bandgap which is perfect for good visible light absorption. However, due to indirect bandgap, thick layer of WO3 is required to absorb significant amount of visible light. A thick layer of electrode material may cause several problems as electron–hole pair recombination, less photostability which decrease overall SHE.

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Additionally above pH >4 WO3 also suffers from chemical dissolution due to OH− in aqueous solution and also photocorrosion created during water oxidation due to the peroxo-species. Several other strategies, like coating of oxygen evolution catalyst over the WO3 surface, provide photostability and increase the efficiency at some extent. However, by using above method, penetration of light to the WO3 surface becomes challenging. Wang et al. revealed that insertion of oxygen vacancies not only enhances photostability of WO3 material but also helps to reduce electron– hole pair recombination which subsequently enhances solar-to-hydrogen conversion efficiency without any other catalyst coating [58]. Hydrogen-treated samples have shown twofold enhancement in the photocurrent density as compared to without any hydrogen treatment. IPCE studies disclosed that photocurrent is increased due to enhanced photoactivity which is below 480 nm. This indicated optical bandgap of hydrogen-treated WO3 and that without any treatment remain same which concludes that enhancement in the IPCE is only due to the charge collection [59] (electron/hole pair recombination rate decreases). Oxygen defective transition metal oxides MnO2−x [60, 61], Fe2 O3 [62], MoO3−x [63] have been synthesized by using hydrogenation and utilized in various energy storage and conversion applications.

3.3 Solution Reducing Treatment Although hydrogenation is an effective strategy to introduce defects in the TMO, there are some disadvantages of this method. Hydrogen is an explosive gas and dangerous to deal within lab at higher temperatures. For practical applications, simpler solutionbased attempts have to be made in which strong reducing agents such as N2 H4 [64], H2 O2 [65], NaBH4 [66], and ethanol [67, 68] have been used to introduce oxygen vacancies in TMO. Chemical reductants utilized to insert oxygen vacancies in TMO divide into two categories: organic reducing agents, e.g. imidazole, L-ascorbic acid, etc. and inorganic reducing agents, e.g. NaBH4 , H2 O2 , CaH2 , etc. [69] Zhou et.al have outlined the synthesis of defective TiO2 by using NaBH4 as reducing agent [70]. Aqueous solution of NaBH4 is commonly used reducing agent in order to convert MO into nonstoichiometric MO containing oxygen vacancies [33]. MO +

x x x NaBH4 + H2 O → MO1−x (OH)x + NaBO2 8 4 8

WO3 coupled with thermal treatment followed by reducing agent effect also becomes a new strategy. Zheng et.al reported annealed WO3 nanoflakes have been transferred to the mixture of PVP (polyvinyl pyrrollidone) and ascorbic acid in order to produce etched/reduced WO3 nanoflakes. As shown in Fig. 7, SEM micrographs (b and e) and TEM micrograph (c), respectively, have proved the formation of porous structure with holes of various sizes developed after etching. Ascorbic acid behaves as reducing agent and responsible to make the holey structure of WO3 nanoflakes. In order to confirm this, same experiment has been performed in absence of ascorbic acid

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Fig. 7 a WO3 system with and without oxygen vacancy schematic illustration of reduction followed by etching b WO3 SEM micrograph without treatment c TEM image after reducing e SEM micrograph depicting the holey structure after reduction treatment. Reprinted with permission [36] Copyright 2014 American Chemical Society

and no porous structure is formed which is suggesting reducing nature of ascorbic acid [36, 71]. In case of ethanol as reducing agent, WO3 nanostructure has been synthesized without any aid of surfactants [68]. As ethanol is very cheap, non-hazardous chemical, so use of such chemicals has posed a very simple, faster, and greener strategy to impose oxygen vacancies in TMO (Fig. 8).

3.4 Electrochemical Reduction Electrochemical synthesis and electrodeposition are effective ways for individual and integrated PEC devices due to their unique advantages. Previously discussed methods revealed insertion of oxygen vacancies in TMO created chemically in reducing environment. Researchers revealed that insertion of oxygen vacancies in TMO can be introduced by electrochemical reduction by applying ultrafast voltage in few minutes [32, 33]. As it is solution-based synthesis of TMO which proved its practical, inexpensive ways, at the end, photoelectrode/electrolyte interface is responsible for overall PEC performance. Materials that can be synthesized by using this technique have covered a wide range which includes semiconductors, metals, oxides, chalcogenides, and alloys which further can directly be utilized for various applications. In solution route, it is more feasible to tune the parameters pH, additives, type of solvents, and temperature which are responsible for different morphologies of nanostructures.

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Fig. 8 a Schematic illustration of TMO synthesis by using electrochemical technique “reprinted with permission [14] copyright 2015 American Chemical Society b photoconversion efficiency of different TMO” reprinted with permission [49] copyright 2013 American Chemical Society c photocurrent measurement under visible light for electrochemically synthesized TMO (WO3 /FeOOH/NiOOH) d TEM image depicting heterojunction formation. Reprinted with permission [73] Copyright 2018 American Chemical Society

In electrochemical reduction synthesis, metal ions take electrons from the external source to form metal ions with low oxidation state. For charge balance in solids, oxygen vacancies have been created, and cations from metal ions intercalate into metal oxides. Mechanism of the reaction is given below [32]: xC+ + MO y + xe− → Cx MO y−z Zhang et al. reported synthesis of TiO2 nanotubes by electrochemical reduction (ECR) method [72]. TiO2 nanotubes act as working electrode on which negative potential (-0.4 V Ag/AgCl) was applied in 0.1 M Na2 SO4 solution up to 30 min and reduction of Ti4+ to Ti3+ occurs after electron capture. WO3 is semiconductor with indirect bandgap of 2.2–2.4 eV capable of absorbing solar light and acts as efficient photoanode material. Synthesis of WO3 is very easy by electrochemical method as it is very stable in acidic conditions. WO3 films can be

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produced by applying cathodic deposition technique. In order to do this, plating solution is make ready by addition of W powder in 30% H2 O2 solution which plays both role of oxidizing agent and complexing agent to form dimeric tetraperoxoditungstate species. Peroxo bonds of W2 O11 2− during cathodic deposition play a crucial role in solubility of tungsten species which are broken and as a result coating of tungsten oxide films takes place [74]. In case of electrochemical reduction method to create oxygen vacancies, operating voltage, treatment time, and current are the descriptors. Considering example of Co(OH)2 , deposition of Co(OH)2 nanosheets happened on carbon cloth at voltage of −1.5 V for 10 min. Again converse voltage of 1.5 V was applied for another 10 min and reconstruction from Co(OH)2 nanosheets to defect-rich CoOOH takes place. Hence, reaction kinetics become faster after inserting defects in TMO [75]. From above reports, it is clear that electrochemical reduction method is facile technique to create defects or oxygen vacancies in TMO.

3.5 Plasma Treatment Defect engineering by using plasma technique is very clean, controllable, and scalable method [76]. Plasma is the fourth state of matter having highly energetic and reactive species, i.e. electrons, ions, and neutral radicals generated in plasma discharge process [77]. After plasma discharge, there is the formation of defect-rich edge structure which increases the density of exposed active sites which in turn enhances the electrochemical activities [78]. Figure 9a shows the treatment of W surfaces by using helium (He) plasma in which highly energetic He ions strikes the surface which in turn forms the open interconnected structure after irradiation. The open structure formation at W surface arises due to the formation and coalescence of He bubbles near to surface and inducing swelling effect [80]. Such structure formation is more frequent in cases of high particle flux at the surface (i.e. when the concentration of He ions near the surface is high which allows the more He bubbles grow and coalescence.) This porous and open frame nanostructure after He plasma irradiation absorbs 95–97% of visible light which in turn becomes more efficient in comparison to that without irradiation. As a result, after He irradiation WO3 films have 1 mA/cm2 photocurrent which is five times more as compared to without plasma use. In contrast to oxygen (O2 ) plasma, He plasma inserts only defects in the structure whereas in case of O2 plasma electrical properties also get changed [77]. In Fig. 9b, O2 plasma has been irradiated on MoS2 semiconductor producing defect-rich structure. DFT calculations provide the basis of O insertion increases the electrical conductivity by forming the heterojunction successfully after controlled exposure of O2 plasma for 30 s only. With the increased time of O2 plasma irradiation, there is the in situ formation of MoO3 species which will not contribute well towards the electrochemical performance. So controlled exposure is also one of the important factors [76, 81] that should be taken under consideration for the efficient activities.

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Fig. 9 a Schematic illustration of high flux plasma generator reprinted with permission [79] American Chemical Society 2013 b Defect engineering in MoS2 after oxygen plasma insertion forming MoS2 /MoO3 heterojunction [76]

4 Summary Photoelectrochemical water splitting utilizes various combination of transition metal oxides-based semiconductors as photoelectrode material. In order to enhance the photoconversion efficiency, promising materials are needed because photoelectrodes are the soul of the water-splitting devices. And the efficiency of the material could been increased by creating defects in the material by opting different strategies. In this chapter, we have explained the basic four strategies to create the defects in transition metal oxides. And the techniques could be extended also to other TMObased semiconductors. Introduction of the oxygen vacancies increases conductivity of the material which helps to ease of electron transfer in metal oxides. In case of SrTiO3 , after introduction of oxygen vacancies, electrical conductivity increases and which was very poor initially. Oxygen vacancies act as shallow donors and increase efficiency towards PEC water splitting [69]. Whereas in the second approach, oxygen vacancies have been created by hydrogen treatment method which helps to increase the photostability of WO3 material, which is one of the major challenges in PEC water splitting [52]. The enhanced photostability of WO3 is ascribed due to the formation of oxygen vacancy-rich WO3−x species which is very resistive towards peroxo-species-induced dissolution and re-oxidation. On the other hand,

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in more practical way, electrochemical synthesis of the semiconductor material for PEC water splitting has opened up a new research area. Electrochemical reduction of the material has certain advantages as it is solution-based synthesis which is quite feasible and controllable [14]. Improvement in the intrinsic properties of transition metal oxides after introduction of oxygen vacancies helps to increase the efficiency of the solar-to-hydrogen conversion. This is due to the fact that oxygen vacancies take part in the reaction process and help to improve the performance of the PEC water splitting. Acknowledgements The authors gratefully acknowledge Science and Engineering Research Board (SERB) for financial support.

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Chapter 8

Photoelectrochemical Water Splitting with Nitride-Based Photoelectrodes Avishek Saha and Arindam Indra

1 Introduction The increasing global energy demand has led to finding out a solar-based technology to produce renewable energy [1]. In this respect, photovoltaic and photoelectrochemical (PEC) devices have been designed to harvest solar energy [2]. Although photovoltaic cells have been widely explored in recent years, the large grid size of the energy storage devices, high cost, short time storage, and low energy density makes their use limited. Alternatively, PEC cells offer the advantage of producing chemical fuels by harvesting light [2]. As a result, solar energy can be stored at a large scale in the form of chemical bonds (Fig. 1) [3]. PEC utilizes semiconductor electrodes and converts photon energy into chemical energy in the presence of light and applied potential. Largely available small molecules like H2 O, CO2 , N2 , O2, etc. have been employed as the substrates for the reduction with photo-generated electrons [3]. For example, CO2 is reduced to form a series of C-containing chemicals like CO, methanol, formic acid, methane, etc. Under suitable conditions, C2 products (ethanol, ethanal, etc.), higher alcohols, and hydrocarbons are also produced [3]. These products can be directly consumed as fuel or can be mixed with fossil fuels to increase energy efficiency and to decrease the consumption of fossil fuels. Similarly, N2 is used to produce NH3 -a well-known feedstock for the production of fertilizer [3]. In this context, photoelectrochemical water splitting attains special importance to produce H2 —a carbon-free fuel [4]. H2 has the highest theoretical energy density A. Saha CSIR-CSIO, Sector 30-C, Chandigarh 160030, India e-mail: [email protected] A. Indra (B) Department of Chemistry, IIT BHU, Varanasi, Uttar Pradesh 221005, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Kumar and P. Devi (eds.), Photoelectrochemical Hydrogen Generation, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-16-7285-9_8

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Fig. 1 A schematic representation of the H2 production and application of H2 for power generation and chemical reactions

(~140 MJ kg–1 ) and burning it in air produces only water [3]. The produced H2 can be utilized as the energy carrier as well as energy-rich hydrocarbons can be synthesized by the reduction of CO2 [3].

2 The Basic Concept of Photoelectrochemical Water Splitting In PEC cells, semiconductor photoelectrodes are immersed in an electrolyte solution and irradiated with light while the potential is applied (Fig. 2). If the photon energy of the incident light is larger than the bandgap of the semiconductor; electrons are excited to the conduction band leaving holes in the valence band [2]. Besides, the bottom edge of the conduction band should be more negative than the reduction potential of H+ /H2 (0 V vs. NHE), as well as the top edge of the valence band should be of higher energy than that of O2 /H2 O oxidation potential (+1.23 V vs. NHE). Therefore, photo-driven water splitting can only be achieved if the theoretical bandgap of a semiconductor is more than 1.23 V. Further, an additional driving force is required to overcome the overpotential originated from the electron transfer processes at the semiconductor–liquid junctions. Therefore, an external bias is applied to overcome the overpotential and fasten the reaction kinetics [2]. In this context, the role of the

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Fig. 2 (Left) Photoelectrochemical water splitting with semiconductor photoelectrodes and (right) associated kinetic phenomena like electron–hole separation, charge transport, and recombination of the photo-generated charge carriers

electrocatalyst and its integration with the semiconductor are crucial. The efficiency of the PEC cell can be further improved by a “Z-scheme” system combining an H2 -evolving p-type semiconductor photocathode and an O2 -evolving n-type semiconductor photoanode. The photo-generated electrons in the conduction band reduce H2 O to form H2 while water is oxidized to O2 by the holes from the valence band [2]. For photoelectrochemical water splitting, an ideal semiconductor should execute the following requirements: (i) a suitable bandgap to absorb light in the visible region, (ii) favourable band positions, (iii) high electronic conductivity and charge transfer carrier, (iv) stability in the aqueous electrolyte solution, (v) facile method of synthesis, and (vi) inexpensive and environmentally friendly materials [2]. Similarly, the electrocatalysts should facilitate the water-splitting kinetics by increasing the rate of O2 and H2 evolution. The deposition of water oxidation electrocatalyst onto n-type semiconductors and hydrogen evolution electrocatalyst onto p-type semiconductors could be beneficial to enhance the rate of water splitting to a certain extent. However, other factors like charge carrier transport, recombination of photo-generated charges, band bending, interface-charge trapping, and transfer, optical and kinetic effects control the overall process of photochemical water splitting. Although Verne introduced the concept of artificial photosynthesis in 1874, it had been almost a hundred years to demonstrate it in the laboratory [5]. First, Boddy (1968) utilized n-type rutile (TiO2 ) for the light-driven oxygen evolution [6]. Subsequently, Fujishima and Honda (1972) demonstrated photoelectrolysis of water using n-type rutile (TiO2 ) photoanode and a platinum cathode [7]. Eventually, metal oxides have come out as the most studied materials in PEC cells due to their long-term stability [8]. However, metal oxides like TiO2 and ZnO suffer from inefficient light

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Fig. 3 Band edge positions of nitrides compared with other semiconductors for water redox reactions

absorption due to their large bandgap, short electron–hole lifetimes, and low mobility. As a result, band engineering by doping, substitution, composite formation, nanostructuring, etc. have been explored to improve the visible light absorption as well as enhance charge carrier transport. Though α-Fe2 O3 (2.2 eV) and WO3 (2.6 eV) have been found to show visible light activities, the photocurrent generated is moderate due to their low conductivity, short carrier diffusion length. Recently, promising photochemical water-splitting activity of BiVO4 photoanodes was reported by various researchers [8]. Alternatively, narrow bandgap semiconductors like Si, CdX (X = S, Se, Te), and Cu2 O are reported to show excellent visible light water-splitting activities but these materials are vulnerable to photocorrosion [2, 9]. In a continuous effort to develop semiconductors for PEC water splitting, metal oxy-nitrides, metal nitrides, and graphitic carbon nitrides have been explored (Fig. 3). Efficient utilization of the solar spectrum, stability in acid/base solutions, charge separation, and low cost make them suitable for photoelectrochemical water splitting. As the light irradiation on a semiconductor photocatalyst increases the concentration of holes, the quasi-Fermi level drops down from the electron level generating a photovoltage (V ph ) at the semiconductor-electrocatalyst interface (Fig. 4) [10]. The V ph largely depends on charge carrier transport in the semiconductor/catalyst interface and recombination. The injected charge from the semiconductor is accommodated by the electrocatalyst near the electrolyte-catalyst boundary. As a result, an electrostatic potential drop across the classical Helmholtz layer takes place. In the case of redox-active electrocatalyst (for example, Co(OH)2 + OH− → CoOOH + H2 O + e− ), oxidative redox chemistry simultaneously proceeds to accumulate the photo-generated holes [11]. From the above discussion, it is clear that PEC water splitting is highly complex compared to the photocatalytic route as the light absorption, charge separation, and catalytic reactions are not in close proximity. Therefore, the cost of the photoelectrochemical water-splitting system is more compared to the photocatalytic system with similar photocatalytic efficiency.

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Fig. 4 Schematic representation of the photoelectrochemical water-splitting process consisting of a photoanode and a metal cathode in the presence of light and applied potential

3 Metal Nitrides as Photoelectrodes Although seminal progress in p-type photocathodes for the efficient hydrogen evolution has been observed, the main concern persists in n-type photoanodes [12]. The onset potential for the anodic photocurrent is found to be unfavourably positive for water splitting. Therefore, a significant improvement in the designing of efficient photoanodes is highly desirable. Several metal nitrides (e.g. Ta3 N5 , GaN, InN, etc.) and metal-free nitride (graphitic carbon nitride) have been recognized as attractive semiconductors for photoanodes. These semiconductors have suitable bandgap and band positions for splitting water under visible light. Amongst the metal nitrides, Ta3 N5 has been widely explored as the photoanodes because of its visible light activity, simple composition, easy synthesis, and fabrication. Ta3 N5 is a d0 transition metal nitride with a suitable bandgap (~2.1 eV) for PEC water splitting [13–15]. Particulate materials, thin films, vertically grown films, nanorods of Ta3 N5 have been demonstrated as the photoanodes for water splitting. Ta3 N5 has an anosovite (Ti3 O5 ) structure consisting of a corner- and edge-sharing TaN6 octahedra (Fig. 5) [16]. The theoretical light conversion efficiency of Ta3 N5 is calculated to be 16% under 1.5 G AM irradiation. The valence band and the conduction band edges of Ta3 N5 straddle water oxidation and reduction redox potentials [17, 18]. However, pristine Ta3 N5 photoanode films are reported to require high onset potential for water oxidation because of low carrier mobility and fast electron– hole recombination (10% was reported for InGaN semiconductor.

4 Synthesis of Metal Nitrides As mentioned in the previous section, Ta3 N5 is the most studied metal nitride material for the PEC water splitting due to its promising activity and easy synthesis [20]. Although various routes were explored for the synthesis of Ta3 N5 , photoelectrodes can be prepared by depositing previously prepared particulate materials on conducting glass support, directly growing thin film on Ta, or growing nanorods on a solid support. Besides, the surface of the catalyst has been modified by loading cocatalyst(s). The synthesis of the particulate materials was carried out by the following methods [20]: (i)

Nitridation of metal oxide with ammonia In general, crystalline Ta3 N5 is synthesized by the direct nitridation of Ta2 O5 using NH3 gas as a nitrogen source. This high-temperature method produces nanoparticles with varying sizes (50–100 nm) and low surface area (10 m2 g−1 ). The materials contain anionic defects enhancing the recombination of the photo-generated holes and electrons. As a result, the hydrogen evolution activity of these materials is poor than that of the oxygen evolution.

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(ii)

Nitridation of metal oxide with nitride agent

(iii)

Graphitic carbon nitride (C3 N4 ) is used as the nitrogen source to synthesize Ta3 N5 from Ta2 O5 . The controlled release of N from C3 N4 ensures uniform size distribution of the nanoparticles as well as significantly increases the surface area. The photocatalytic H2 evolution activity is also improved by this method of Ta3 N5 synthesis. Urea can also be used as the nitrogen source to produce Ta3 N5 . Colloidal synthesis

(iv)

Hot injection of tantalum and nitrogen precursors into high boiling organic solvents under an inert atmosphere produce colloid Ta3 N5 . The size of the nanoparticles can be tuned by varying solvents and temperatures. Depending on the synthetic method, the band position and bandgap of the colloidal nanoparticles vary. The main concern of this method is the easy oxidation of the particles and low yield. Molten salt method Alkali metal salts are heated to produce a molten salt state at high temperature, reacted with Ta2 O5 followed by the nitridation. The introduction of alkali metal ions improve the crystallinity of Ta3 N5 and produce smaller particles to enhance photocatalytic activity. For example, Na2 CO3 molten salt was utilized to prepare Sc-doped Ta3 N5 nanorods [20].

Initially, it was a challenge to grow high-quality InGaN layers because of a large difference in lattice matching between InN and GaN resulting in inhomogeneity of In content across the film. High-quality InGaN film with precisely controlled thickness has been grown by metal–organic chemical vapour deposition (MOCVD). Even a few atomic layer thickness thin films can be developed by this method. In this method, trimethylindium, trimethylgallium, and ammonia are used as the source of In, Ga, and N while hydrogen is used as the carrier gas. The precursors are decomposed on a hot substrate and deposited as InGaN nanostructures. InGaN films with high purity can be grown under vacuum using the molecular beam epitaxy (MBE) method. N2 plasma-assisted MBE has been utilized for precise control of the film thickness. Deposition of InGaN film by plasma discharge radio-frequency magnetron sputtering of indium and gallium arsenide on a target has also been reported [20].

5 Fabrication of Metal Nitride Electrodes Fabrication of the photoelectrodes also plays a crucial role to improve the PEC activity [20, 21]. Formation of good electronic contact between semiconductor particles/layers and a conductive substrate and reduction of the resistance in the interface are the two main factors to achieve high photocatalytic activity. The direct fabrication of the photoelectrodes on conductive substrates was reported by nitridation of metals and metal oxides. Molecular beam epitaxy (MBE), radio-frequency (RF) magnetron

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sputtering, chemical vapour deposition (CVD), and vacuum evapouration are the other techniques often followed for the photoelectrode fabrication. (i)

Fabrication of the particulate materials

(ii)

Drop casting, doctors’ blade, and electrophoretic deposition have been utilized to immobilize powder metal nitride samples on conductive substrates. Poor catalyst-support interaction, low mechanical stability, and high interface resistance result in poor photocatalytic activity. For example, Ta3 N5 photoelectrodes were prepared by electrophoretic deposition (EPD) on F-doped SnO2 (FTO) optically transparent conducting glass. EPD method is useful to fabricate a thick film on the conducting surface applying a high potential difference between the two electrodes. A certain amount of control over the thickness of the film can also be achieved by varying the potential difference and time of deposition. Necking method

(iii)

In the necking method, the produced semiconductor film was treated with a metal salt solution followed by nitridation to improve the contact between the photocatalyst and conducting support. In general, Ta3 N5 films were treated with TaCl5 solution, dried in the air, and calcined in NH3 at high temperatures to get mechanically stable films. Sputtering method

(iv)

Films of Ta3 N5 photocatalysts can be fabricated by the surface oxidation of Ta foil followed by the nitridation. This route produces films with high crystallinity to increase the efficiency of the PEC system. However, control over the thickness of the films is poor, and the films contain a large number of cracks on the surface. In this respect, radio-frequency magnetron sputtering technology is found to be useful for the precise control of the composition and thickness of a film. Densely packed Ta3 N5 film is grown on Ta foil without having surface cracks or gaps between the product layer and the substrate. Thin film produced by this method also helps in the detailed study of the photophysical properties. Vertically grown nanorods on the conductive substrate

(v)

Vertically aligned Ta3 N5 nanorods can be fabricated by nitridation of vertically grown Ta2 O5 nanorods under a heated NH3 flow. Atomic layer deposition

(vi)

Uniform layer Ta2 O5 films can be grown by atomic layer deposition (ALD) on conducting support followed by nitridation. The use of ALD is beneficial over chemical or physical vapour depositions to obtain a uniform film. Plasma-assisted molecular beam epitaxy (PAMBE) Fabrication of the films of InGaN in slightly N-rich conditions was achieved by the PAMBE method.

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6 Band Edge Engineering in Nitride Materials In general, the bandgap of the nitride materials is determined from the diffuse reflectance spectrum [20]. In the case of Ta3 N5, a broad absorption band from the UV region to 600 nm was recorded for the transition of the electrons from the N 2p6 orbitals to the empty Ta 5d0 orbitals. The Tauc plot determined the direct bandgap as 2.1 eV. Another absorption in the range of 715–725 nm has been recorded due to the formation of metallic states below the original conduction band edge of pure Ta3 N5 . This metallic state is formed due to the substitution of nitrogen by oxygen to generate non-stoichiometry in the nitride materials [20]. Also, the Mott-Schottky studies revealed a flat band potential of −0.5 V versus RHE (at pH 13.5) for Ta3 N5 . As N 2p orbital possess negative potential in metal nitride compared to the O 2p orbital in oxide, the bandgap in nitride is also narrower than the corresponding metal oxides [20]. For example, Ta3 N5 has a bandgap 2.1 eV-significantly lower than that of Ta2 O5 (3.9 eV) [20]. Bandgap and the position of the conduction and valence bands of tantalum nitride can be significantly monitored by doping/substitution of alkali metals (Na, Ba, Mg), transition metals (Co, Zr), and non-metals (Ge, B) [20]. For example, the performance of Ta3 N5 photoelectrode has been improved by the addition of alkali metal ions (Ba2+ , Mg2+ ) and transition metal ions due to better charge transport [22– 24]. In principle, doping of metal ions in semiconductor photocatalyst may facilitate intervalence band absorption due to the creation of defects. However, such defects may also promote charge carrier recombination in bulk. Therefore, the mechanism of improved interfacial charge transfer is not properly understood in the context of PEC water splitting. Although doping of Na+ or Ba2+ in Ta3 N5 improves PEC water splitting, high loading leads to phase separation as the ionic radii of 6-coordinate Na+ (102 pm) and Ba2+ (135 pm) is larger than that of 6-coordinate Ta5+ ion (64 pm) [23]. Even the bandgap can be tuned by varying the ratio of the metals in bimetallic nitrides. Nanostructured GaN has been used as UV light active photoelectrode but with increasing In content in GaN, the bandgap is reduced from 3.4 to 0.65 eV [20]. The high absorption coefficient and enhanced charge carrier mobility of the nitride semiconductors facilitate photoelectrochemical water splitting. In general, photocorrosion of the defect-free crystalline nitride materials is low in the acidic, and neutral pH electrolyte solution and those are ideal to be used as photoanode. On the other hand, nitride materials containing a high density of defects are suitable for photocathodes. The n-type GaN photoelectrodes are stable in acidic solution whereas photocorrosion takes place in an alkaline medium. The conduction band edge of GaN can be tuned to polarize on Ga-centre or N-centre. As the conduction band edge of Ga-polar GaN is more negative than that of N-polar GaN, the first one is more efficient for photoelectrochemical hydrogen evolution while the later shows better performance as photoanode [20]. The introduction of dopants like Mg, In, Mn, W, etc. reduces the bandgap due to the formation of an intermediate band between the conduction band and valence band of GaN (Fig. 6). In acidic medium, In0.02 Ga0.98 N, Mn-doped GaN

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Fig. 6 (Left) Schematic representation of the energy levels formed by the doping of Mg in GaN nanowires. Mg doping leads to the formation of intermediate bands from nitrogen-vacancy (VN ) and Mg acceptor-related intra-gap states. (Right) Band edge manipulation of GaN because of Mg doping. Adapted from reference 20

photoelectrode showed better photoelectrochemical performance than GaN [20]. Although doping in GaN enhances the visible light activity, it increases photocorrosion due to the formation of structural defects in GaN. The Ta3 N5 photoanodes with high crystallinity were found to be more efficient to produce high photoelectrochemical activity in visible light as well as showed minimum photocorrosion. If we look at the wide variety of semiconductor materials, there are only a few choices like Ta3 N5 , TaON for overall water splitting. High visible light absorption (600 nm) and high theoretical solar-to-hydrogen energy conversion efficiency (17%) of Ta3 N5 make it the most studied material for overall water splitting. It should be mentioned here that in all these cases, photoactivity can be tuned by loading different cocatalysts, changing pH of the electrolyte solution, or making composite materials [20].

7 Photoelectrochemical Water Splitting with Metal Nitride Electrodes The following factors are directly correlated with metal nitride photoelectrodes to improve the stability, activity, and kinetics of PEC water splitting: i) integration of electrocatalyst on the surface of the nitride semiconductors, ii) doping of metal nitrides with metal ions, and iii) interface modification. For example, water oxidation electrocatalysts, such as iridium oxide, ruthenium oxide, cobalt phosphate (CoPi), and cobalt oxide, have been successfully utilized with Ta3 N5 photoanode to reduce the overpotential, improving the kinetics of the reaction, and facilitating electron– hole separation [20]. A size-dependent activity study of Rh nanoparticles loaded on (Ga1−x Znx )(N1−x Ox) showed that smaller Rh nanoparticles have better charge separation efficiency [25]. Deposition of CoPi on GaN thin film increased photocurrent by minimizing surface recombination [26]. In this respect, mixed valent metal oxide

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Co3 O4 was found to be beneficial to collect the photo-generated holes and to provide the active CoIV sites for O–O bond formation [27]. Transparent Co3 O4 nanoisland on GaN nanowire produced per metal turnover frequencies of 0.34–0.65 s−1 at an overpotential of 400 mV (Fig. 7) [27]. Mixed metal Fe–Ni–Co oxide was deposited on Ta3 N5 by photoassisted electrochemical deposition to avail stable current density for 2 h [28]. An IPCE of ~10% was achieved with IrO2 decorated Ta3 N5 nanotube arrays at 400 nm [13]. Further improvement was attained when overall water splitting was carried out with IrO2 loaded necking treatment made Ta3 N5 [29]. An IPCE of 31% was recorded at 500 nm when 1.15 V potential versus. RHE was applied in aqueous Na2 SO4 solution. During PEC water splitting, the photocurrent density and the stability of the photoanode were remarkably improved when Co(OH)2 was loaded as the electrocatalyst on Ta3 N5 surface [30]. Under PEC water-splitting conditions, photo-generated O• radicals mediate the chemical interaction between Ta3 N5 and Co(OH)2 to produce Ta-O-Co bonds in the interface. As a result, a boost in the photocatalytic activity was achieved along with improved stability of the photoanode (Fig. 8). Co(OH)x decorated surface exfoliated Ta3 N5 showed IPCE of 50% (400–700 nm) [31]. A solar

Fig. 7 SEM images of a GaN nanowires and b Co3 O4 (5 s) loaded GaN nanowires. c Current density versus potential curves for GaN and Co3 O4 (5, 10, and 15 s) loaded GaN nanowires under dark and AM 1.5G illumination. d Photocurrent density versus. time profiles for GaN and Co3 O4 (5, 10, and 15 s) loaded GaN nanowires under dark and AM 1.5G illumination (chopped light for 50 s) at 1 V versus RHE. Reprinted from reference 27 with the permission of Willey

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Fig. 8 a Schematic representation for the synthesis of Co(OH)2 loaded Ta3 N5 showing the formation of the new interface during PEC water splitting; b the mechanism of interface formation by the interaction of Ta3 N5 and Co(OH)2 during PEC water splitting, c current density versus. The number of cycles plot for Co(OH)2 loaded Ta3 N5 showing the improved stability of the photoanode during PEC water splitting, d current density versus time plots representing the enhanced stability of Ta3 N5 photoanode after Co(OH)2 loading and e current density versus potential plots show the improved activity of Ta3 N5 photoanode after Co(OH)2 loading due to the formation of Ta–O–Co interface. Reprinted from reference 31 with permission from CellPress

light absorption calculation for Ta3 N5 showed a theoretical maximum photocurrent of 12.9 mA cm−2 [32]. Attempts to reach the maximum theoretical current density gives rise to several efficient systems like CoPi-Ba-doped Ta3 N5 (6.7 mA cm−2 at 1.23 V vs. RHE) [17], CoPi/Co(OH)x –NiFe(OH)x –Ta3 N5 (6.3 mA cm−2 at 1.23 V vs. RHE) [33] and so on. Finally, a complex configuration by combining passivated Ta3 N5 with hole-storage layers [Ni(OH)x /ferrihydrite], bio-inspired molecular catalysts ([Cp*Ir(2,2 -bi-2-imidazoline)Cl]Cl and Co4 O4 (O2 CMe)4 (CNpy)4 ), and TiOx blocking layer produced photocurrent density 12.1 mA cm−2 at a potential of 1.23 V in a broad range of irradiation (400–550 nm) (Fig. 9) [34]. Dual absorber photocathode is another strategy to increase the photoconversion efficiency (Fig. 10) [35]. Double-band InGaN/GaN core–shell nanowire photoanodes were found to be highly stable acidic photoelectrochemical water-splitting systems [36]. GaN–InGaN core–shell photoanode exhibited a tenfold increase in photocurrent density compared to GaN. In another study, GaN–InGaN core–shell nanowire

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Fig. 9 a The schematic representation of the photoanode developed by integrating Ta3 N5 on Ta with TiOx protective layer, Ni(OH)2, and ferrihydrite (Fh) hole collector and molecular complexes catalytic centres. b Current density versus potential plots after loading the molecular complexes on Ni(OH)x /Fh/TiOx /Ta3 N5 photoanode under AM 1.5G simulated sunlight at 100 mW cm−2 in 1 M NaOH aqueous solution (pH = 13.6), the inset: the enlarged view of saturated photocurrent. c The applied bias photon-to-current efficiency (ABPE) determined by the related J–V curves for the Ta3 N5 (P) and complex 2/complex 1/Ni(OH)x /Fh/TiOx /Ta3 N5 (P) as photoanodes. d Photocurrent action spectra and corresponding estimated solar photocurrent of the complex 2/complex 1/Ni(OH)x /Fh/TiOx /Ta3 N5 (P) as photoanode at 1.23 V. e Oxygen evolution process with complex 2/complex 1/Ni(OH)x /Fh/TiOx /Ta3 N5 (P) as photoanode (reaction condition: 1 M NaOH solution, applied potential of 0.9 V versus RHE and AM 1.5G simulated sunlight irradiation). Reprinted from reference 34 with permission from RSC

produced 27% of incident photon-to-current conversion efficiency [37]. InGaN/GaN multiple quantum wells (MQWs) grown on hollow n-GaN nanowires (NW) has been designed to collect lateral carrier from different absorber layers for efficient carrier transport along the axial direction (Fig. 11) [38]. Doping is a mostly studied method to control the band edge of the photocatalysts for better transport of electrons to achieve high efficiency in PEC water splitting. La doping on ZnO–GaN led to the bending of the band to improve the separation of photo-generated carriers [39]. Ni [40], Mn [41], Mg [42], CrO [43] doping in GaN improves the charge separation efficiency and suppresses the recombination of carriers. On the other hand, Mn doping in GaN is not successful as it incorporates defects in the structure enhancing photodegradation [41] while W doping leads to band narrowing to enhance the hydrogen evolution activity [44]. Alkali and alkaline metal ion doping also improved the photocatalytic activity of Ta3 N5 [45]. A substantial effort has been provided to reduce the required external bias for PEC water splitting with metal nitrides integrating with photovoltaic devices. For example, the n-type InGaN electrode was integrated with GaAs solar cell to reach the photoconversion efficiency of 0.18–0.23% [46].

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Fig. 10 a Schematic representation showing the simple dual-photoelectrode system with parallel illumination showing the splitting of incident sunlight spatially and spectrally on the photoanode and photocathode. Each photoanode (or photocathode) is composed of several parallel-connected anodes (or cathodes), which are illuminated by a portion of the solar spectrum incommensurate with its energy bandgap and light absorption capacity. b Schematic illustration showing the GaN nanowire photoanode and InGaN nanowire photocathode arranged in a dual configuration and designed for the absorption of the UV and visible spectrum of sunlight, respectively. c Schematic representation for the GaN and InGaN nanowire connected in parallel and paired with the Si/InGaN photocathode. Reprinted from reference 35 with permission from ACS

8 Mechanism of Photoelectrochemical Water Splitting In a model photoelectrochemical cell, following inter-dependent, photophysical, and electrochemical processes occur [47]:

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Fig. 11 Schematic representative models for the InGaN/GaN MQWs grown on a solid and b hollow n-GaN NWs showing the reflection and absorption of incident light and electron–hole pair generation. Band diagrams for both kinds of heterostructures are also shown to explain the role of generated electron–hole pair in the hydrogen gas evolution of the PEC water-splitting experiment. Reprinted from reference 38 with permission from RSC

(i) (ii) (iii) (iv) (v) (vi)

Electron–hole pair generation in nitride semiconductor under light illumination. Separation of electron–hole pair (exciton dissociation). Transport of charge carriers to semiconductor/electrolyte or semiconductor/catalyst interfaces. Charge transfer at the catalytic interface followed by reduction or oxidation. Ionic transport through an electrolyte. Product separation.

However, a real photoelectrochemical cell has various other processes including radiative and non-radiative recombination, and trapping-detrapping of charge carriers, corrosion due to the transfer of photo-excited minority charge carrier at semiconductor electrolyte interface, etc. The study of the kinetics of charge recombination/transfer process in semiconductor nitrides promotes a fundamental understanding of efficiency in the photoelectrochemical cell. Delayed charge recombination than the charge separation process usually results in high solar to the hydrogen conversion efficiency. However, trapping of charge carriers in the surface or the bulk

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Fig. 12 Schematic representation of the mechanism of photoelectrochemical water splitting under light irradiation

defects of the semiconductor may precede the charge recombination, consequently influencing the efficiency of conversion. Apart from trapping, charge recombination can be further delayed by applying anodic bias and/or fabricating a heterojunction interface with another semiconductor, metals, or even dielectric insulators. Dynamics of charge carrier recombination can be probed using photoelectrochemical impedance spectroscopy, and time-resolved spectroscopic techniques including transient absorption/pump-probe, transient photoluminescence spectroscopy, transient photocurrent/photovoltage spectroscopy, etc. Amongst all these techniques, transient absorption spectroscopy (pump-probe) is the most relevant technique to probe kinetic processes related to separation and radiative/non-radiative recombination of charges on a wide-ranging timescale from hundreds of femtoseconds to even milliseconds. The interfacial charge separation at the heterojunction of electrodes depends on various factors including morphology, power fluency of laser, precursors, time of nitridation, etc. For example, nanowires or nanotubes usually have enhanced photoinduced charge separation, and slower recombination due to orthogonalization of directionality in nanowires [47]. An ultrafast transient spectroscopic study shows that electron–hole recombination in Ta3 N5 films prepared by 0.25 h of nitridation of potassium tantalate (KTaO3 ) features a second-order process (direct recombination of holes from valence band and electron from conduction band) whereas the recombination in film prepared by 10 h of nitridation features trap-assisted process [48]. Another disadvantage of Ta3 N5 photoanodes is the tendency of self-oxidation due to the presence of surface trapped holes under irradiation [49]. During the

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self-oxidation process, surface N-atoms are gradually substituted by O atoms. For example, He et al. have observed the formation of an amorphous oxide layer (~3 nm) during water oxidation using Ta3 N5 nanotube [50]. The addition of hole absorbing cocatalyst, such as Co3 O4 , NiFe layered double hydroxide facilitates interfacial charge separation and water oxidation [13, 33, 51– 53]. Microsecond transient absorption measurement can deduce electron lifetime >1 ms upon integration with CoOx cocatalyst [54]. For instance, long-lived (>1 ns) electron populations were observed in Ta3 N5 photocatalysts with Ru/Rh doped CoOx as hole acceptor. When photo-induced holes are transferred to Ru/Rh doped CoOx or pristine CoOx , the electron–hole recombination is retarded resulting in prolonged lifetime of electrons in the conduction band [51]. Similarly, photo-induced electrons can also be transferred to electron acceptor cocatalysts, for example, Rh metal particles, inducing longer lifetime of valence band holes [52]. Cobalt phosphate is also widely used as a cocatalyst for Ta3 N5 -based photoelectrode to enhance charge separation and water oxidation [13, 17, 55]. Another way to facilitate the photo-excited charge separation is to use a transparent conductive layer as a substrate for the deposition of Ta3 N5 . Several substrates were employed including FTO, n-type GaN/sapphire, Ta-doped TiO2 , TiOx [34, 56–58]. Liu et al. have reported a very large photocurrent (12.1 mA/cm−2 with a semi-stable photoelectrode system based on Ta3 N5 film on TiOx and (Ni(OH)x /ferrihydrite) holestorage layer with molecular photocatalyst [34]. Very recently, Domen et al. have demonstrated that crystalline Ta3 N5 on quartz can act as a stable photoanode and transparent conductive layer as well [59]. Amongst the d10 metal nitride system, gallium nitride is widely studied due to its notable application in solid-state light-emitting devices. However, similar to Ta3 N5 , a thin film of n-type GaN in the aqueous electrolyte solution is susceptible to photocorrosion due to the accumulation of holes on the surface of GaN under illumination [60]. GaN + 3H2 O + 3h+ → Ga(OH)3 + 1/2N2 + 3H+ Ga(OH)3 + 3H+ → Ga3+ + 3H2 O Various strategies have been developed to prevent corrosion including the incorporation of halide ion as sacrificial hole scavenger, nitrogen terminated GaN surfaces, wide bandgap metal oxides [61], metal ion doping [62], and addition of active cocatalysts [63]. For example, doping of GaN nanowires with Mg facilitates the transport of photo-excited charge carriers to the semiconductor/electrolyte interface via charge hopping through defect states [64]. Point to note that the type of conductivity of GaN nanowires could also be tuned from n-type to p-type by gradually increasing the concentration Mg dopants [65]. During p-type doping of GaN film with Mg, a hole trapping process is reported in the timescale of ~60 ps along with recombination in the timescale of ~150–600 ps [66]. The wide bandgap of GaN can be modified by

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using a solid solution GaN:ZnO (~2.4–2.8 eV) which has been used as photocatalyst and photoanode for water oxidation [67, 68]. With the addition of Rh2-y Cry O3 cocatalyst, the overall water-splitting efficiency of GaN:ZnO is notably improved. Time-resolved spectroscopic studies suggest an efficient electron transfer process from GaN:ZnO to Rh2-y Cry O3 cocatalyst in sub-μs timescale, resulting in a remarkably long lifetime (~30 s) of surface accumulated holes on GaN:ZnO [69]. To this end, cocatalyst deposition on conventional metal oxide-based photo(electro)-catalysts is yet to demonstrate such prolonged recombination timescale. Another ultrafast spectroscopic study was performed on InGaN/GaN nanowires with Rh/Cr2 O3 as cocatalysts. At low power excitation, pump-probe studies suggest that electron trapping and direct electron–hole recombination occur in the timescale of 3 and 167 ps, respectively [70]. Moreover, interfacial electron transfer occurs from the conduction band of In0.2 Ga0.8 N/GaN to Rh/Cr2 O3 in the timescale of ~50 ps. It is worth noting that, at high power excitation, however, charge carrier recombination in InGaN/GaN usually proceeds through three-carrier Auger recombination [70]. On the other hand, Atwater et al. developed a different strategy of interfacial charge separation based on the coupling of plasmonic gold nanoparticles with ptype GaN/sapphire. After plasmon photoexcitation ((λ > 495 nm), energetic holes are generated by plasmon decay and are injected from Au nanoparticles into p-GaN through interfacial Schottky barrier. The resulting photocurrent (due to the electrons residing on Au particle) is shown to be useful for the reduction of CO2 in a photoelectrochemical cell (λ > 495 nm, 50 mM K2 CO3 electrolyte) [71].

9 Graphitic Carbon Nitride Metal-Free Catalyst Despite several attempts, the required stability for the metal nitride photoelectrodes has not been achieved. The metal nitride photoelectrodes can produce a stable current for a few hours, far away from the commercialization where stable current for months is essential. The Discovery of graphitic carbon nitride (g-CN) further ensures the potential of solar-driven water splitting for H2 production [72]. Although most of the studies with g-CN revolve around photocatalytic H2 evolution with particulate matter, PEC water splitting is recently explored [73]. As a photocatalyst, g-CN possesses several advantages (i) visible light absorption, (ii) suitable band edge positions and bandgap, (iii) facile synthesis with commercially available cheap precursors, (iv) negligible photodegradation, and (v) excellent thermal and chemical stability [73]. Mostly thermal polymerization of substrates like cyanamide, dicyanamide, urea, thiourea, melamine has been used to produce g-CN (Fig. 13) [72]. The reaction proceeds through several temperature-dependent steps, as shown in Fig. 14 [72]. The polycondensation method generates a heptazine ring structure in g-CN to enhance physicochemical stability and tune the electronic band structure. The temperature, precursor source, the heating environment has a significant effect of control the physicochemical properties of g-CN. The surface area, bandgap, band edge, crystallinity, C/N ratio, etc. are widely varied with different synthetic methods. The

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Fig. 13 Synthesis of g-CN by polycondensation of different precursors

Fig. 14 The stepwise formation of g-CN from cyanamide as a function of temperature

photoelectrodes of g-CN were fabricated on transparent conducting oxide (TCO) glass or Si surface by drop-casting, EPD, doctors blade method, etc. [74]. However, all these methods provide very low mechanical stability, poor charge transport in the interface. As a result, new methods are adopted by direct depositing the precursor on the surface of TCO/Si and carrying out the condensation method [75]. Further, treatment by thermal vapour condensation was reported to improve the crystallinity and hence electron transfer. Micro-contact printing and solvothermal process have also been followed to deposit films of g-CN [74]. These methods provide the advantages of uniform microstructure, close contact with the support, continuous coverage, low particle aggregations, and a minimum amount of cracks. Therefore, improved PEC performance could be achieved. The bandgap of g-CN varies from 2.6 to 2.8 eV. Tuning of the band edge of g-CN was reported by doping elements like S, P, C, N, O, etc. [72]. Doping of non-metals in carbon nitride significantly changes the band edge maximum of the conduction band and valence band. As a result, the bandgap also deviates from the pure g-CN. For

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example, C and P doping in g-CN shifted the conduction band edge to a more negative position to improve the photocatalytic H2 evolution activity. B doping in g-CN film reduced the bandgap from 2.77 eV to 2.60 eV. As the electronic conductivity of pure g-CN is poor, the recombination of the charge carriers results in low photocatalytic activity [72]. A significant improvement in charge separation and transport has been observed by hetero-atom doping in g-CN. Electron donating polymer, N-doped quantum dots, carbon nanotube, and graphene have been introduced in the structure of g-CN to manipulate the conduction band and valence band positions as well as to enhance the electronic conductivity. The introduction of sp2 C by the phenyl group showed a change in optoelectrical properties. Synthesis of g-CN in the molten sulfur flux also integrates sp2 C in the g-CN structure [76]. All these approaches reduce the bandgap, increase visible light absorption or enhance the photo-generated charge carrier separation. However, for an efficient charge carrier separation and slow recombination, loading of cocatalyst in g-CN is essential. Besides, the cocatalyst provides the active centres for proton reduction and oxygen evolution. Therefore, cocatalyst loading leads to achieve high photocurrent density with improved kinetics [77]. Pt is widely used as the cocatalyst to collect the photo-generated electrons [78]. Non-noble cocatalysts like metal phosphides, sulfides, layered double hydroxides, oxides have also been recently explored to achieve maximum photocurrent [78]. Ni-bulk doping in g-CN produced 12 times higher photocurrent density compared to the pristine g-CN electrode (69.8 vs. 5.6 mA/cm2 ) [79]. Metal phosphides having the optimum Gibbs free energy of proton adsorption (Ni2 P, CoP, Cu2 P, MoP, RhP, etc.) have been efficiently utilized as the cocatalysts with g-CN [80]. Surface junction modification of the g-CN framework by Co enhances visible light absorption and charge separation [81]. Surface modification of g-CN with CoOx significantly reduces the overpotential of water oxidation [82]. Transient photoluminescence studies have been employed to understand the charge separation, carrier transport, and recombination processes during PEC water splitting with g-CN [82]. The lifetime measurements showed that high-temperature synthesis of g-CN resulted in a compact bulk packing to shorten the lifetime. Transient photoluminescence study also revealed that heptazine layers are mainly responsible for the charge separation and transport in g-CN and excitons are located at heptazine monomer [83]. However, consideration of the localized decay process is not helpful to understand the overall photophysical properties of g-CN [84]. A combination of the theoretical model and experimental data described a hopping mechanism of electrons and holes after separation of the initially generated singlet excitons [85]. This study confirms that charge transport is not only confined to the hydrogen-bonded planes of melon, but a major fraction moves perpendicular to these planes. Although significant progress in PEC water splitting has been achieved with g-CN electrodes, the performance is poor to the benchmark system [86]. Combination with other photocatalysts (BiVO4 , TiO2 , Cu2 O, black P, Fe2 O3, etc.) has been explored to improve the efficiency of g-CN [85, 86].

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10 Conclusions and Perspective As metal nitrides have emerged as the promising semiconducting materials for PEC water splitting, we have described the latest progress associated with metal nitride photoelectrodes. The synthesis of high-quality films, strong visible light absorption, and increasing stability of the photoelectrodes has been discussed with proper examples. The challenges for charge carrier separation, transport, and reduction of recombination have been addressed to improve the PEC activity. Amongst different photoelectrodes, Ta3 N5 is widely explored because of its high dielectric constant, low effective masses, suitable band position, and visible light absorption. However, the defects in the Ta3 N5 structure induce photodegradation. Surface modification with cocatalyst loading showed a significant improvement in the photocatalytic activity as well as the stability of the photoelectrode. InGaN showed comparatively better stability and band edge can be tuned by varying the ratio of In and Ga. Other techniques like metal ion doping, core–shell dual photocatalyst system, solid solution, double irradiation, etc. have been demonstrated to improve the activity. In fact, a solar to the hydrogen conversion efficiency of ~1% has been attained with Ta3 N5 . However, the use of a huge amount of potential and sacrificial agents is the major concern for the practical application. On the other hand, metal-free g-CN is found to be a highly stable catalyst with negligible photodegradation. The main drawback arises from the difficulty in high-quality film preparation, the high recombination rate of photogenerated charge carriers, and low electrical conductivity. Recently, thermal vapour condensation, screen printing, solvothermal method, and direct growth on transparent conducting oxide/Si, etc. have been demonstrated to fabricate high-quality films. Other ways of modification include functional groups grafting, hetero-atoms doping, and composite formation with different photocatalyst(s). Incorporation of conducting materials like graphene, carbon nanotube, N-doped carbon quantum dots have been explored to increase the conductivity, whereas effective charge separation is achieved by cocatalyst loading. However, these breakthroughs are not enough to commercialize PEC water splitting with g-CN as for the industrial production of H2 requires solar to the hydrogen conversion efficiency 10%. Therefore, it is essential to achieve further progress in this field. Nevertheless, the development of efficient PEC devices could be meaningful as they have attained the target efficiency in PEC devices and practical solar hydrogen production from PEC could be realized by resolving the cost and scalability issues. Besides, the application of the PEC system can be extended to N2 and CO2 reduction, biomass-derived chemicals production, C–H bond activation, and carrying out other organic reactions.

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Chapter 9

Nanomaterial Assisted Photoelectrochemical Water Splitting Subhavna Juneja and Jaydeep Bhattacharya

1 Introduction Translating a fossil fuel intensive growing economy into a clean energy one finds key contribution in efficient conversion and storage of its energy resources. Capitalizing on greener energy transitions finds an exploitable alternative in using renewable resources such as wind and solar. One of the most promising schemes of action developed over the years has been generation of fuel from renewable resources by splitting water. Breaking down of water molecules into hydrogen and oxygen over a photosensitive catalytic electrode simply defines a photoelectrochemical (PEC) water-splitting reaction [1]. The photoresponsive electrodes, commonly semiconductors, immersed in electrolytes produce photocharge carriers, both electrons and holes, when stimulated by an incoming photon. The photoelectron places itself in the conduction band of the semiconductor material while holes are left behind in the valance band. These charge carriers move to the surface active sites later, to be consumed in surface redox reactions, generating H2 by electrons (reduction), and O2 by holes (oxidation). Being an uphill reaction, the energy required to drive a PEC reaction is provided by light [2–4]. Progressive research directed towards societal sustainability has emphasized enough on advantages of using sunlight as the excitation resource. Effective utilization of sunlight is economical, renewable, and environmentally benign. Furthermore, solar hydrogen produced in the reaction is clean, storable, and an efficient viable fuel alternative capable of addressing current and future global energy crises [5].

S. Juneja · J. Bhattacharya (B) NanoBiotechnology Lab, School of Biotechnology, Jawaharlal Nehru University, New Delhi 110067, India e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Singapore Pte Ltd. 2022 P. Kumar and P. Devi (eds.), Photoelectrochemical Hydrogen Generation, Materials Horizons: From Nature to Nanomaterials, https://doi.org/10.1007/978-981-16-7285-9_9

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PEC is often interchangeably used with other light assisted splitting reactions such as photovoltaics and photocatalysis. Although similar in aspects limited to semiconductor characteristics and electrode systems, all the three methods are different from each other. In PEC, one or more photosensitive electrodes work together in an electrochemical cell setup to split water. While in a photocatalyst system, light active nanomaterials or particle in suspension relay redox reactions and in photovoltaics, the light-sensitive and electrochemical cell work in isolation to split water in the cell over a non-light absorbing electrode [6]. In contrast with other photocatalytic water-splitting reactions, PEC offers advantage in (a) suppression of charge carrier recombination due to external bias (b) easy separation of hydrogen and oxygen, as produced over different electrodes (c) easy upscaling due to advent of cost-effective photonanomaterials and lastly (d) low power consumption [7, 8]. Most commonly, efficiency of a PEC water-splitting system is dominated by the properties of the photocatalyst used to harness the solar light. The photoelectrodes take part in the electron transfer process through the electrolytic solution such that Fermi levels of the photoelectrode reach a state of equilibrium with respect to the redox potential of the electrolyte [4]. To realize commercialization of solar-driven PEC systems, the solar-to-hydrogen conversion efficiency should stand at 10% or higher, professing the role of the photocatalyst used. Starting with TiO2 , numerous wide bandgap, UV sensitive semiconducting materials have been well established as photocatalyst, although they are associated with high recombination efficiencies and limited response to visible light, which forms major part of the solar spectrum [9, 10]. Contemporary photocatalyst research has thus been directed majorly towards maximizing solar energy conversion by improving visible light responsiveness of the photocatalyst. For example, Oxy-nitrides have emerged as an alternate class of photocatalyst materials which are sensitive to visible light at short wavelengths. Metal sulphides are another alternative that responds to visible light however they are prone to photocorrosion. Hybrid nanomaterials, such as plasmonic or carbon functionalized photocatalysts and heterojunctions by far, have been the most reformed photocatalyst system to work with but requires functionalization and design optimization [11, 12]. Importance of nanoscale sizing has been emphasized enough over the last decade with applications ranging in fields as varied as medicine and automobile. Significantly different properties of nanomaterials, such that, the characteristics do not match the properties of the same matter at bulk or single molecule level makes it intriguing. Quantum confinement, size, and morphology-driven property tuning, high extinction coefficients often characterize material at the nanoscale [13]. For PEC watersplitting reactions per say, photoelectrode materials at nanoscale sizes significantly increase carrier charge collection efficiency by reducing scattering rates [14]. Strong absorption coefficients as a consequence of increased oscillator strength enable a higher conversion efficiency [15] making nanomaterials, favoured alternatives. Since bandgap is sensitive to photoelectrode light absorption, bandgap tunability in the nanoscale, over the whole spectrum of light, theoretically can be achieved by simply altering their sizes. The electronic band structures are also controlled and modified by attaching other nanomaterials chemically or through doping. Nanostructures are

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free to be used as suspensions/dispersions or electrode coatings to improve STH efficiency. A large surface area to volume ratio is advantageous as they allow better light absorption and quicker surface reactions owing to higher mass and charge transfer rates [16]. Unlike bulk thin film deposition which is prone to high-density defect formation, due to the dimensional constraints, strain relaxation in nanomaterials leads to the formation of electrodes devoid of defect states [17]. Presence of defect states leads to deteriorated signals as they can act as recombination centres both radiative and non-radiative. Lastly, bottom-up nanostructure growth has allowed to build bigger complex structures by assembling smaller constituting units, facilitating scalable fabrication on flexible substrates, which is both low cost and light weight [18]. Nanoscale materials thus hold promising future for revolutionizing the PEC research and instrumentation. This chapter is a narrative of recent advances made in the field of PEC with respect to the nanoscale electrode materials. Various classes, advantages, disadvantages, and modifications employed to address the key challenges associated with devising an efficient PEC water-splitting system are overviewed and discussed.

2 Major Factors Influencing Photoelectrode Activity Efficient photoelectrode design is a cumulative consideration of various factors such as photon absorption, separation of charge carriers, thermodynamic and kinetic factors, and the overall stability of the photoelectrode [19]. These electrode defining characteristics in turn are dependent on nanomaterial properties such as size, structure, crystallinity, surface active sites, etc. These factors when worked upon in optimization can lead to an efficiently designed photoelectrode. A synergistic balance between physical, chemical, and electrochemical features of the photoelectrode material is thus ultimate requirement for accomplishing efficient water-splitting reaction. (a)

Size and Morphology The photoelectrode morphology that allows maximum light capture is considered most ideal. Rough surface photoelectrodes are preferred over flat surfaces due to an increase in the horizontal light distribution due to light scattering [20]. Similarly, photoelectrodes made up of smaller particle-sized material are preferred due to their high surface to volume ratios. Higher surface areas enriched with multiple active sites promote surface reactions increasing conversion probability and the rate of reaction [21]. Photoelectrodes with higher active sites capture more water molecule in their surface lattice, promoting scattering, internal reflection, and eventually photoabsorption cross section [22]. Adversely, in smaller nanoparticles rates of recombination are fairly high due to low potential drop across the space charge region, forfeiting the advantage gained by increased surface area. The decreased carrier transport across the interface can then be improved by surface treatments leading to

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decreased surface defects [23]. Amongst nanomaterial of different dimensionality (0D, 1D, 2D, and 3D) one-dimensional nanostructures including nanowires, nanorods, and nanotubes are preferred photoelectrode material. Their high surface areas, small thickness, and easy charge transport have been observed to improve hydrogen generation efficiency [24]. 1D nanostructures characterized by rapid single direction diffusion results in retarded charge carrier recombination thereby improving splitting efficiency at the surface. Noticeable hydrogen generation efficiency has also been marked with the usage of 3D nanostructures. 3D nanostructures, pristine or modified, allow efficient light absorption by facilitating improved charge carrier diffusion at the interface, reduced recombination rate, and improved light sensitivity [25]. Bandgap The bandgap for efficient water splitting is estimated to lie between 1.6 and 2.2 eV [26]. Any increase in the bandgap, fails to absorb the requisite light intensity required to split the water. In the tabulated range, the band edge positions are accurate and charge carrier mobility is high. A non-ideal band positioning can lead to reduced photovoltage production, which serves as the driving force for the splitting reaction [27]. For efficient hydrogen generation, the photovoltage produced should be high enough to address the associated overvoltage (1.8 V). Photovoltage thus varies as a function of bandgap energy. Additionally, the water redox potential value should also lie within the bandgap of the photomaterial to allow splitting of the water molecule, which would otherwise be thermodynamically not feasible. Water-splitting efficiency also requires a sustained photovoltage output. Stable photovoltage generation under light excitation throughout the reaction finds dependence on photoelectrode surface and additional energy levels between the bandgaps [28]. Temperature and Pressure Although temperature and pressure are vital to mass transfer characteristics and thus to PEC splitting, it finds rather limited reportage. In a selective study, it was found that the incident photon to electron conversion efficiency was improved when the PEC was performed at low temperature [29]. An enhancement in bandgap was reasoned to be the source of improved splitting efficiency. Crystallinity Amongst amorphous and ordered, crystalline nanostructures with improved charge transfer and defect density are proven to be better photoelectrode materials. Crystalline materials show better charge carrier transfer efficiency and retarded recombination rates [19]. For an amorphous and calcined TiO2 nanotube study, a difference in PEC efficiency as high as twice was recorded [30]. pH A buffered electrolyte is an ideal solution where maximal splitting efficiency over nanomaterial photoelectrode can be achieved. Operational pH serves as the deciding factor whether the net absorbed charge at the interface would be neutral, positive, or negative. Water-splitting equilibrium is thus a function of electrolyte pH. Constant flow of ions can weaken the electrode surface however,

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nanomaterial infused photoelectrodes show stability over a range of pH conditions and can also be functionalized adequately to address photocorrosion at harsh pH conditions [19, 31].

3 Classification of Photoelectrode Materials for Water Splitting 3.1 Semiconductor Photoelectrodes The ability of a semiconductor material to absorb photons and convert it to free electrons makes them well suited as photoelectrode material. Thermodynamically, the minimum energy required for the splitting reaction to take place should be equal to the redox potential difference between the semiconductor CB and VB. Thereby, a material taking part in PEC must have a bandgap exceeding 1.23 eV [32]. The hydrogen and oxygen evolution reaction (HER/OER) potential thus must be more negative (OV RHE) and positive (1.23 VRHE), respectively. The half-reactions can be described as [6] (Fig. 1). H2 O(l) + 2h + (aq) → 2H+ (aq) + 1/2O2 (g)

(1)

2H+ (aq) + 2e− → H2 (g)

(2)

Fig. 1 a Schematic representation of a photoelectrochemical cell (Adapted from Refs. [31, 33]). b Bandgap positions of various metal oxide semiconductor photoelectrodes for PEC (Adapted from Ref. 34])

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Overall Reaction H2 O(l) → H2 (aq) + 1/2O2 (g) Amongst various semiconductors available as photoelectode materials, metal oxides are favoured for their band edge tunability, inherent band positions, cost effectiveness, easy synthesis, and chemical stability [35]. Metal oxide valence band consisting of O 2p orbitals allows positioning of metal oxide VB well below the Fermi level for the O2 /H2 O redox reaction enabling photosplitting. Ionic nature of O 2p and metallic s orbital bestows metal oxides such as ZnO (3.2 eV) with large bandgaps. Metal oxide systems comprising transition metal cations such as Fe2 O3 (2.2 eV, d5 configuration) or Cu2 O (2.1 eV, d10 configuration) have narrower bandgap enabling sensitivity to visible light. Metal oxides having cations ranging beyond the transition metals such as in case of PbO have further reduced bandgaps, high binding s-orbitals, and metallic p characters [36–38]. TiO2 has been the most versatile and widely explored metal oxide for PEC photoelectrodes for its low toxicity, stability, availability, and near visible light absorption [39]. However, the valence band placement for TiO2 lies so low that most of the absorbed energy from the photon is simply wasted, even if the overpotential is considered for water-splitting reaction [40]. In other cases, where bandgaps are narrower, H+/H2 redox above the conduction band of the semiconductor system such that an external bias voltage is required for the reaction to proceed [41]. Conclusively, the redox potential characterization renders numerous potential metal oxides as inefficient owing to (a) wide bandgap (b) limited light responsiveness (c) driving bias voltage [42]. Semiconductor materials where bandgap structures do not align with water-splitting redox potentials thus cannot be used for splitting reactions unless tuned with modifications. Bandgap engineering is a viable alternative to tailor metal oxide properties and has been achieved through functionalizing with metal nanostructures, carbon molecules, doping with cations, and introduction of defect states such that they can be used as efficient photoelectrodes [43]. In the following sections of the chapter each of these has been explained in detail.

3.2 Plasmon Modified Photoelectrodes Plasmonic nanostructures such as Ag, Au, and Cu have been functionalized over wide bandgap photoelectrode material to reduce the activation barrier for water splitting and improve the PEC performance. Hydrogen evolution on plasmon rich photoelectrodes can be accomplished through improved photoabsorption, increased charge separation, denser active site inclusion, and altered charge migration pathways [44– 46]. Plasmonic nanoparticles in isolation are incapable of achieving high solar energy conversion owing to short-lived intra-band transitions. However, when functionalized over wide bandgap photomaterials such as metal oxides, they function as antennas that capture light and transfer the energy to adjoining metal oxide system improving

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their photoconversion efficiency [47, 48]. Although the exact mechanism involved in improving the evolution yet remains controversial, direct electron transfer and plasmonic resonant energy transfer have been ascribed to be the possible pathways. The hot electrons, having energy higher than the Schottky barrier at the interface, under surface plasmon resonance effect can be injected into the metal oxide conduction band directly forming an energy level above the conduction band. A TiO2 /Au nanocomposite was tested to observe a two-order magnitude change in the lifetime of the generated hot electrons produced during TiO2 intra-band transitions [49]. In other words, the hot electrons from plasmonic nanoparticles had a higher thermodynamic force towards pulling a photocatalytic splitting reaction and a reduced recombination chance due to increased lifetime. Such an arrangement only works when a Schottky barrier is established at the metal–semiconductor interface. For an ohmic contact, i.e. when the work function of metal is lesser than work function of the metal oxide electronic transition to conduction band, charge recombination, and back electron transfer predominates [50]. In the second process, plasmon excitation in metal nanostructures forms a strong dipole that non-radiatively transfers to the semiconductor through dipole–dipole interaction leading to formation of electron–hole pairs in the metal oxide [51]. Although silver has comparable efficiency, owing to the oxidative resistance of gold it finds frequent use and is mentioned in the literature. Peerakiatkhajohn et al. synthesized a sandwich TiO2 1wt%Au@TiO2 /Al2 O3 /Cu2 O hybrid nanostructure with efficient PEC ability. It was found that with the addition of Au nanoparticles, an approximate 20-fold increase in the photocurrent density was obtained under solar illumination when compared to TiO2 -P25/Cu2 O photoelectrode. Improved charge separation and improved light absorptivity were ascribed as major contributing factors for the improved photoelectrode performance [52]. Pu and coworkers also established an extension of TiO2 nanowire photoactivity over the whole visible light spectrum by functionalizing TiO2 using gold nanoparticles and nanorods of different dimensions. The results were indicative of the fact that particle shape, size, and concentration had notable effect on the photoelectrode activity as it directly influenced the efficiency of hot electron injection (HEIE) [53]. Presence of surface defects however are often associated with reduced HEIE and increased Schottky barrier. Thereby, defect rich metal oxide require adequate pretreatments prior to surface functionalization with plasmonic centres. Zhang et al. reported the synthesis of well-aligned ZnO nanorods with controlled gold nanoparticle loading. For splitting reactions at wavelength above 420 nm photocurrent density of 30 μA cm−2 at 0.8 V versus Ag/AgCl was observed [54]. PEC performance superiority was attributed to synergistic effects of metal–metal oxide characteristics. The PEC performance was also found to be adjustable based on the amount of Au loading. Recently, Adam and coworkers used low-temperature hydrothermal method supplemented with SILAR to synthesize ZnO/Ag/Ag2 WO4 photoelectrode having visible light SPR behaviour [55]. The synthesized plasmonic nanocomposite showed better PEC performance when compared to bare ZnO nanorods. Generation of electron–hole pairs and shifting of absorption spectrum to visible region under solar illumination were proposed to improve PEC mechanism

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in the composite. Choi’s research group demonstrated a novel strategy to improve hydrogen production of a TiO2 /Ag photocatalyst by inducing thiocyanate mediated surface complexation on Ag [56]. By adding SCN-, photo-generated charge transfer facilitation resulted in a four times increase in the photocatalytic H2 production. A reduced overpotential for water splitting facilitated further enhanced the splitting efficiency of the electrode system. Zhang et al. studied the effect of chemical state of Ag on the PEC behaviour in a TiO2 /Ag nanocomposite system [57]. Between Ag(0)TiO2 and Ag(I)-TiO2 , Ag(0) showed noticeable photocurrent generation under visible light. It was demonstrated that Ag(0) were photo-excited due to their SPR, causing charge separation by translating electrons from Ag(0) to TiO2 and simultaneously converting into Ag(I). This interconversion between the 0 and I state in Ag leads to an increased charge separation time, reduced recombination frequency ensuring stable photocurrent generation. Improved light harvestation is key to the performance of plasmonic PEC photoelectrodes however, transitory hot electron lifetime retards charge injection from plasmonic centre to metal oxides and thus requires further experimental optimization (Fig. 2). Metal–semiconductor nanocomposites proven to show improved photocatalytic or photoelectrochemical reactions as observed in the above examples have been proposed to mechanize majorly through Schottky and SPR associated phenomenon such as PRET and scattering however conclusive mechanism mobilizing electronic flow is still an anomaly. Based on detailed report by Khan and coworkers, electron excitation from metal to semiconductor is well established however, back flow of electrons to metals as per the basic Schottky law is in ambiguity. Similarly, in plasmon resonance associated phototransfers, photoactivity of reactions are influenced by the optical properties of the metal, semiconductor, and the excitation source which would decide the direction of electron flow from metal to semiconductor or vice versa [58].

Fig. 2 TEM images of TiO2 /Ag photocatalyst and schematic representation of thiocyanate mediated surface complexation on Ag developed by Choi’s group [56]

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3.3 Heterojunction Photoelectrodes In annex to homogenous and plasmonic heterojunction formation for improving PEC performance, mixing different metal oxides forming heterogeneous nanostructures has also proven to be efficient alternatives as photoelectrode materials. The heterojunctions benefit by assisting charge separation, maximizing light absorption, and prevention of photocorrosion in narrow bandgap semiconductors [59, 60]. Principally, when two photomaterials with comparable bandgaps such as TiO2 (3.2 eV) and SrTiO3 (3.3 eV) combine to form a heterojunction, light absorption spectrum is rarely affected. However, the band offsets undergo alterations, shifting Fermi levels to more negative potential and accumulation of electrons, assisting charge separation by regressing recombination [61]. Common form of junctions formed, broadly classify as p-n diode, p-n with ohmic contact, and Schottky junctions. Dhara et al. described in their study the synthesis of p-CuO/n-ZnO heterojunction core–shell nanocomposites for application in PEC conversion reaction [62]. In 0.5 M polysulphide couple, 6.12% efficiency was achieved. Performance superiority was attributed to broadband adsorption and mutual electron–hole transfer, decreasing recombination. Liu and coworkers reported p-CuO/n-ZnO heterojunction prepared by annealing followed by facile chemical solution reaction. For the heterojunction formed, a fourfold increase in photocurrent (1.23 V) versus RHE was obtained in comparison to the bare ZnO nanorods [63]. A p–n junction heterostructure between n-type hematite thin film and p-type NiO were formed yielding a visible active photoelectrode for splitting water under solar light. The p-type material not only lowered the reaction threshold for OER by shifting the onset potential to negative value but it also facilitated accumulated hole extraction from hematite allowing reduced recombination [64]. Traditional wide bandgap metal oxides conjugated to narrow bandgap semiconductor quantum dots (QDs) have also been explored for PEC reactions for their bandgap modulated photosensitivity. In an ideal combination scenario, the narrow bandgap semiconductor valence and conduction band should possess higher energy compared to the wide bandgap semiconductor such that band offset allows efficient interfacial charge transfer and thus improved activity [65]. For a TiO2 -PbS heterojunction, it was established that smaller QDs with larger bandgaps and higher energy levels than TiO2 , allowed faster charge transfer. As the conduction band of QDs slipped below that of TiO2 , electron transfer was halted emphasizing on the importance of band alignment [66] (Fig. 3). Li et al. constructed a staircase alignment between CdS-anatase TiO2 -Rutile TiO2 such that a gradual electron transfer route is established. The charge transfer from CdS to rutile phase TiO2 is mediated through the anatase form as its conduction band lies lower than CdS but higher than the rutile phase. This arrangement also allows the formation of a potential gradient across the interface that bars reversed charge flow. Reported efficiency was twice than that of the binary nanostructure [67]. QDs offer wide photoselection but are also prone to photooxidation in water leading to corrosion. From the use of excess polysulphides as photohole absorbers or forming

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Fig. 3 Scheme representing the proposed electronic transfer pathway between TiO2 -CdSe (Adapted from Ref. [66]). Energy levels referenced to NHE

core–shell nanostructures with wide bandgap metal oxides forming outer layers, different strategies have been rationalized. To address the photoinstability in a ptype Cu2 O multi-layered FTO/Au/Cu2 O/Al-doped ZnO/TiO2 was fabricated, where TiO2 formed the outermost shell. A photocurrent density of 7.6 mA/cm2 at 1.23 V versus RHE was reported [68]. In similar attempts, n-Si/n-TiO2 nanowire core–shell nanocomposite was synthesized, where TiO2 formed the outer shell protecting Si nanowire from corrosion. Also upon sunlight illumination, absorbed UV light while allowing visible light to pass through Si [69]. Heterostructures can further be modified by introducing extrinsic (impurities) or intrinsic defects in the lattice. Doping models the electronic nature of the composite by reducing the bandgap. An’s group proposed efficiency boosting role of oxygen vacancies in the heterojunction nanostructures [70]. For a TiO2 /Bi2 WO6 composite system, by suppressing the intrinsic defect states in Bi2 WO6 and promoting presence of interfacial oxygen vacancies, a profound difference in photoelectrode activity can be observed. Through experimental and theoretical analysis, oxygen vacancy presence was proven to be profitable as it helped charge separation, prolonging recombination time, and availability of charge carriers for surface reactions. Annealing in argon was used to introduce oxygen vacancies into MoO3 /BiVO4 heterojunction film for improving its water oxidation activity. Compared to non-vacancy rich MoO3 /BiVO4 film, a 200% improvement in photocurrent density was reported. Moreover vacancy rich film showed better stability, faster kinetics, and consistent photocurrent generation for over 5 h [71]. Heterojunction formation is also associated with the formation of trap states and reverse charge transfer at the interface, leading to reduced charge transfer. Trap states particularly, use up photocharge carrier effecting auger scattering [72]. Using staircase heterojunctions as mentioned above or alternatively using metal nanostructures in the heterojunctions can suppress the charge transfer reversal forming a stable photocatalyst for the electrodes.

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3.4 Carbon Functionalized Photoelectrodes Semiconductors coupled with carbon in different forms comprising fullerenes, graphene, carbon nanotubes, amorphous carbon, and graphene oxide (GO) have been utilized as photocatalysts for enhanced PEC efficacy. In addition to providing an increased surface area, they serve as scaffolds, promote electron transportation, repress charge recombination and reduce semiconductor bandgaps [73–75]. In a 2017 study conducted by Huang and coworkers NiTiO3 /g-C3 N4 nanocomposite system was synthesized using facile calcination methodology. Photoelectrode with 50 wt% NiTO3 loading exhibited a 4.4 and 3.1 times increased photoconversion efficiency comparison drawn against NiTiO3 and g-C3 N4 respectively. Excellent PEC activity was ascribed to interfacial charge transfer and carrier separation [76]. Biroju et al. demonstrated that number of layers and their stacking sequence in a MoS2 /graphene heterostructure has the ability to influence PEC performance. The nanocomposite system showed improved hydrogen adsorption in the graphene rich region, as MoS2 induced p-type doping, shifting the GH towards 0 eV. PEC HER activity showed an indirect relation with the layer stacking in MoS2 , highest activity was reported for monolayer, reducing further as number of layers increased [77]. Current density and charge transfer resistance were influential factors in deciding the overall electrode efficiency. In a parallel study on a Fe2 O3 –reduced GO system, it was spectroscopically established that the charge recombination rates decreased upon photoexcitation owing to an improved charge transfer from Fe2 O3 into reduced graphene oxide [78]. Semiconductor dimensionality also has a role to play while accessing PEC output. For a TiO2 -graphene sheet composite system with TiO2 functionalized as nanoparticles, tubes, and sheet in separate samples, interfacial electron transfer rate was established to follow 0D-2D