Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications: Selected Proceedings of the 8th International Conference ... (Springer Proceedings in Physics, 264) 3030747999, 9783030747992

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Springer Proceedings in Physics 264

Olena Fesenko Leonid Yatsenko   Editors

Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications Selected Proceedings of the 8th International Conference Nanotechnology and Nanomaterials (NANO2020), 26–29 August 2020, Lviv, Ukraine

Springer Proceedings in Physics Volume 264

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Olena Fesenko · Leonid Yatsenko Editors

Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications Selected Proceedings of the 8th International Conference Nanotechnology and Nanomaterials (NANO2020), 26–29 August 2020, Lviv, Ukraine

Editors Olena Fesenko Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine

Leonid Yatsenko Institute of Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine

ISSN 0930-8989 ISSN 1867-4941 (electronic) Springer Proceedings in Physics ISBN 978-3-030-74799-2 ISBN 978-3-030-74800-5 (eBook) https://doi.org/10.1007/978-3-030-74800-5 © The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021, corrected publication 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 Switzerland AG The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface

This book highlights the most recent advances in nanoscience from leading researchers in Ukraine, Europe, and beyond. It features contributions from participants of the Eighth International Research and Practice Conference “Nanotechnology and Nanomaterials” (NANO-2020), held on 26–29 August 2020, in Lviv, Ukraine. This event was organized jointly by the Institute of Physics of the National Academy of Sciences of Ukraine, Lviv Polytechnic National University (Ukraine), University of Tartu (Estonia), University of Turin (Italy), and Pierre and Marie Curie University (France). Internationally recognized experts from a wide range of universities and research institutes shared their knowledge and key results in the areas of nanooptics and nanophotonics, nanoplasmonics, nanostructured surfaces, nanochemistry, nanobiotechnology, and nanobiotechnology for health care. Pushing optical, chemical, and physics interactions to the nanometer scale opens up new perspectives, properties, and phenomena in the emerging century of the nanoworld. Today, nanotechnology is becoming one of the most actively developing and promising fields of science. Numerous nanotechnology investigations are already producing practical results that can be applied in various areas of human life from science and technology to medicine and pharmacology. The aim of these books is to highlight the latest investigations from different areas of nanoscience and to stimulate new interest in this field. Volume I of this two-volume work covers important topics such as nanostructured interfaces and surfaces, nanochemistry and biotechnology, nanooptics and photonics, nanoplasmonics, and their applications. This book is divided into two parts: the first part—Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, and the second part—Nanomaterials and Nanocomposites, Nanostructure Surfaces, and Their Applications. Parts covering nanoscale physics, microscopy of nanoobjects, nanostructured interfaces and surfaces, and nanocomposites and nanomaterials can be found in Volume II: Nanomaterials and Nanocomposites, Nanostructure Surfaces, and Their Applications. The papers published in these five parts fall under the broad categories of nanomaterial preparation and characterization, nanochemistry and biotechnology, nanodevices and quantum structures, and spectroscopy and nanooptics. The book will help v

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readers to familiarize with current research and practical applications in nanoscience and thus promote further implementation of nanotechnologies into innovations according to public needs. We hope that both volumes will be equally useful and interesting for young scientists or Ph.D. students and mature scientists alike. Kyiv, Ukraine

Olena Fesenko Leonid Yatsenko

The original version of the book was revised: Chapter 22 has been added at the end of this book. The correction to the book is available at https://doi.org/10.1007/978-3-030-74800-5_22

Contents

Nanochemistry and Biotechnology The Kinetic Theory of the Width of Surface Plasmon Resonance Line in Metal Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . O. Yu. Semchuk, A. A. Biliuk, and O. O. Havryliuk

3

Optical and Electrical Phenomena Caused by the Lattice Defects in PbMoO4 Crystal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . T. Bochkova, D. Bondar, M. Trubitsyn, and M. Volnianskii

11

Effects of Eu3+ and F− Doping on Structure and Optical Properties of Zirconium Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . V. Chornii, V. Boyko, S. G. Nedilko, V. M. Prokopets, M. Slobodyanik, K. Terebilenko, and V. Sheludko

31

Electric and Spectral Properties of Solid Water-Nanocellulose Systems in a Wide Range of Temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . M. M. Lazarenko, S. G. Nedilko, S. A. Alekseev, S. Yu. Tkachov, D. O. Shevtsov, V. P. Scherbatskyi, V. A. Barbash, K. S. Yablochkova, M. V. Ushcats, V. I. Kovalchuk, D. A. Andrusenko, D. Izvorska, R. V. Dinzhos, and O. M. Alekseev

51

The Medium Influence on the Luminescence Intensity of SnO2 Nanoparticles’ Ensembles in a Porous Silicate Glass Matrix . . . . . . . . . . . S. A. Gevelyuk, I. K. Doycho, L. M. Filevska, and V. S. Grinevych

75

Spectrum of Localized Quasi-Particle Interacting with Three-Mode Phonons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . M. V. Tkach, Ju. O. Seti, O. M. Voitsekhivska, and V. V. Hutiv

83

Energy Spectra Dispersion of Vibrational and Electronic States in Layered Hexagonal γ-BN Crystals and Single-Layer Nitroborene (BN)L,1 . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Viktor Gubanov and Antonina Naumenko

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Contents

Nanochemistry and Nanobiotechnology The Influence of β-cyclodextrin on Biomembrane. The Molecular Dynamics Simulation Study . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 D. Makieła, M. Pabiszczak, and Z. Gburski Conjugate Formation in Films of Polyethylene Glycol and Polypropylene Glycol Nanocomposites with MultiWall Carbon Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143 M. A. Alieksandrov, A. M. Gaponov, T. M. Pinchuk-Rugal, O. P. Dmytrenko, Antonina Naumenko, V. M. Popruzhko, and M. P. Kulish Synthesis, Structure, Optical and Biomedical Application of Nanosized Composites Based on TiO2 , Fe3 O4 (Review) . . . . . . . . . . . . . 153 M. M. Zahornyi, O. M. Lavrynenko, O. Yu. Pavlenko, N. I. Tyschenko, M. A. Skoryk, and O. A. Kornienko Studying of Iron Oxyhydroxide Dehydration . . . . . . . . . . . . . . . . . . . . . . . . . 165 L. Frolova Surface Reactivity of Carbon Nanoporous Materials Studied with Chemical Bromination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 V. E. Diyuk, A. N. Zaderko, L. M. Grishchenko, A. V. Vakaliuk, R. Mariychuk, and V. V. Lisnyak Nano-, Micro- and Macrotransformations of Marine Sediments Under the Influence of Biocolloidal Processes and Aspects of Nanotechnologies of Their Enrichment and Application . . . . . . . . . . . . 207 A. V. Panko, I. G. Kovzun, V. A. Prokopenko, and O. M. Nikipelova Influence of Magnesium and Chrome on the Microstructure and Properties of the Al–Mg–Sc System as—Cast Alloys . . . . . . . . . . . . . . 223 Orest Ostash, Svitlana Polyvoda, Roman Chepil, Viktoriya Podhurska, and Bogdan Vasyliv Prospects for the Catalytic Application of Red Mud in CO Oxidation . . . 231 Olena Yanushevska, Tetiana Dontsova, Iryna Kosogina, Nina Vlasenko, and Oksana Balog The Mechanochemical Synthesis of Nanodispersed Bi2 Mo3 O12 Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 243 O. V. Sachuk, V. A. Zazhigalov, P. Dulian, W. B˛ak, P. Yu. Demchenko, O. A. Diyuk, and K. Wieczorek-Ciurowa Obtaining Nanostructured Materials by Heat Treatment of Amorphous Zirconium-Based Alloy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 257 Oleksandr A. Shcheretskyi, Anatolii M. Verkhovliuk, Ruslan A. Sergiienko, and Vladislav Yu. Zadorozhnyy

Contents

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Some Reasons of the Degradation of a Fine-Grained YSZ–NiO Anode Material During Intense Reduction and Reoxidation . . . . . . . . . . . 273 Bogdan Vasyliv, Viktoriya Podhurska, and Orest Ostash Effect of Nanobiopolymers on Morphofunctional State of Cryopreserved Fragments of Seminiferous Tubules of Testis . . . . . . . . . 287 Nataliia Volkova, Mariia Yukhta, Larisa Sokil, Ludmila Chernyshenko, Ludmila Stepanyuk, and Anatoliy Goltsev Ways to Create Promising Metal Oxide Catalytic Nanosystems for Selective Reduction of Nitrogen Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . 301 Artem Tyvonenko, Tetiana Dontsova, Olena Yanushevska, and Oleksii Skip Photochemical Properties of Side Chain Aurone Polymers . . . . . . . . . . . . . 313 Oksana Kharchenko, Vitaliy Smokal, Daria Shyrchenko, Oksana Krupka, Oksana Nadtoka, Natalia Kutsevol, and Mykhaylo Frasinyuk Hybrid Hydrogels with Biologically Active Dyes and Their Antibacterial Efficacy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323 O. Nadtoka, P. Virych, O. Krupka, V. Smokal, O. Kharchenko, S. Nadtoka, V. Pavlenko, and N. Kutsevol Correction to: Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications . . . . . . . . . . . . . . . . . . . . . . Olena Fesenko and Leonid Yatsenko

C1

Contributors

O. M. Alekseev Taras Shevchenko National University of Kyiv, Kyiv, Ukraine S. A. Alekseev Taras Shevchenko National University of Kyiv, Kyiv, Ukraine M. A. Alieksandrov Taras Shevchenko National University of Kyiv, Kyiv, Ukraine D. A. Andrusenko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Oksana Balog Kyiv, Ukraine V. A. Barbash National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, Ukraine A. A. Biliuk Chyiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kyiv, Ukraine T. Bochkova Experimental Physics Department, Oles Honchar Dnipro National University, Dnipro, Ukraine D. Bondar Experimental Physics Department, Oles Honchar Dnipro National University, Dnipro, Ukraine V. Boyko National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine W. B˛ak Institute of Technology, Pedagogical University of Cracow, Cracow, Poland Roman Chepil Department of Hydrogen Technologies and Alternative Energy Materials, Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Lviv, Ukraine Ludmila Chernyshenko Institute for Problems of Cryobiology and Cryomedicine, National Academy of Sciences of Ukraine, Kharkiv, Ukraine V. Chornii National University of Life and Environmental Sciences of Ukraine, Kyiv, Ukraine; Taras Shevchenko National University of Kyiv, Kyiv, Ukraine

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Contributors

P. Yu. Demchenko Department of Inorganic Chemistry of Ivan, Franko National University of Lviv, Lviv, Ukraine R. V. Dinzhos V.O. Sukhomlynskyi Mykolaiv National University, Nikolaev, Ukraine; Institute of Engineering Thermophysics of NAS of Ukraine, Kyiv, Ukraine O. A. Diyuk Institute for Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Kyiv, Ukraine V. E. Diyuk Taras Shevchenko National University of Kyiv, Kyiv, Ukraine O. P. Dmytrenko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Tetiana Dontsova Kyiv, Ukraine I. K. Doycho Odessa I.I. Mechnikov National University, Odessa, Ukraine P. Dulian Faculty of Chemical Engineering and Technology, Cracow University of Technology, Cracow, Poland L. M. Filevska Odessa I.I. Mechnikov National University, Odessa, Ukraine Mykhaylo Frasinyuk Institute of Bioorganic Chemistry and Petrochemistry NAS Ukraine, Kyiv, Ukraine L. Frolova Ukrainian State University of Chemical Technology, Dnipro, Ukraine A. M. Gaponov Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Z. Gburski Institute of Physics, Silesian Centre for Education and Interdisciplinary Research, University of Silesia in Katowice, Chorzów, Poland S. A. Gevelyuk Odessa I.I. Mechnikov National University, Odessa, Ukraine Anatoliy Goltsev Institute for Problems of Cryobiology and Cryomedicine, National Academy of Sciences of Ukraine, Kharkiv, Ukraine V. S. Grinevych Odessa I.I. Mechnikov National University, Odessa, Ukraine L. M. Grishchenko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Viktor Gubanov Faculty of Physics, Taras Shevchenko National University of Kyiv, Kiev, Ukraine O. O. Havryliuk Chyiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kyiv, Ukraine V. V. Hutiv Department of Theoretical Physics and Computer Simulation, Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine D. Izvorska Technical University of Gabrovo, Gabrovo, Bulgaria Oksana Kharchenko Kyiv Taras Shevchenko National University, Kyiv, Ukraine

Contributors

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O. A. Kornienko Frantsevich Institute for Problems of Material Science of NAS of Ukraine, Kyiv, Ukraine Iryna Kosogina Kyiv, Ukraine V. I. Kovalchuk Taras Shevchenko National University of Kyiv, Kyiv, Ukraine I. G. Kovzun F.D. Ovcharenko Institute of Biocolloid Chemistry of NAS of Ukraine, Kyiv, Ukraine Oksana Krupka Kyiv Taras Shevchenko National University, Kyiv, Ukraine M. P. Kulish Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Natalia Kutsevol Kyiv Taras Shevchenko National University, Kyiv, Ukraine O. M. Lavrynenko Frantsevich Institute for Problems of Material Science of NAS of Ukraine, Kyiv, Ukraine M. M. Lazarenko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine V. V. Lisnyak Taras Shevchenko National University of Kyiv, Kyiv, Ukraine; Faculty of Humanities and Natural Sciences, University of Prešov, Prešov, Slovakia D. Makieła Institute of Physics, Silesian Centre for Education and Interdisciplinary Research, University of Silesia in Katowice, Chorzów, Poland R. Mariychuk Faculty of Humanities and Natural Sciences, University of Prešov, Prešov, Slovakia Oksana Nadtoka Kyiv Taras Shevchenko National University, Kyiv, Ukraine S. Nadtoka Taras Shevchenko National University of Kyiv, 64, Volodymyrska str, Kyiv, 01033 Ukraine Antonina Naumenko Faculty of Physics, Taras Shevchenko National University of Kyiv, Kiev, Ukraine S. G. Nedilko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine O. M. Nikipelova Engineering and Technology Institute “Biotechnika” of NAAS of Ukraine, Odes’ka oblast, Ukraine; Odessa State Environmental University, Odessa, Ukraine Orest Ostash Department of Hydrogen Technologies and Alternative Energy Materials, Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Lviv, Ukraine M. Pabiszczak Institute of Physics, Silesian Centre for Education and Interdisciplinary Research, University of Silesia in Katowice, Chorzów, Poland A. V. Panko F.D. Ovcharenko Institute of Biocolloid Chemistry of NAS of Ukraine, Kyiv, Ukraine

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Contributors

O. Yu. Pavlenko Frantsevich Institute for Problems of Material Science of NAS of Ukraine, Kyiv, Ukraine V. Pavlenko Taras Shevchenko National University of Kyiv, 64, Volodymyrska str, Kyiv, 01033 Ukraine T. M. Pinchuk-Rugal Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Viktoriya Podhurska Department of Hydrogen Technologies and Alternative Energy Materials, Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Lviv, Ukraine Svitlana Polyvoda Department of Physical-Technological Processes of Aluminum Alloys Casting, Physico-Technological Institute of Metals and Alloys of the NAS of Ukraine, Kyiv, Ukraine V. M. Popruzhko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine V. A. Prokopenko F.D. Ovcharenko Institute of Biocolloid Chemistry of NAS of Ukraine, Kyiv, Ukraine; National Technical University of Ukraine «KPI», Kyiv, Ukraine V. M. Prokopets Taras Shevchenko National University of Kyiv, Kyiv, Ukraine O. V. Sachuk Institute for Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Kyiv, Ukraine V. P. Scherbatskyi Taras Shevchenko National University of Kyiv, Kyiv, Ukraine O. Yu. Semchuk Chyiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kyiv, Ukraine Ruslan A. Sergiienko Physico-Technological Institute of Metals and Alloys, National Academy of Sciences of Ukraine, Kyiv, Ukraine Ju. O. Seti Department of Theoretical Physics and Computer Simulation, Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine Oleksandr A. Shcheretskyi Physico-Technological Institute of Metals and Alloys, National Academy of Sciences of Ukraine, Kyiv, Ukraine V. Sheludko Oleksandr Dovzhenko Hlukhiv National Pedagogical University, Hlukhiv, Ukraine D. O. Shevtsov Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Daria Shyrchenko Kyiv Taras Shevchenko National University, Kyiv, Ukraine Oleksii Skip Kyiv, Ukraine M. A. Skoryk Nanomedtech, Kyiv, Ukraine M. Slobodyanik Taras Shevchenko National University of Kyiv, Kyiv, Ukraine

Contributors

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Vitaliy Smokal Kyiv Taras Shevchenko National University, Kyiv, Ukraine Larisa Sokil Institute for Problems of Cryobiology and Cryomedicine, National Academy of Sciences of Ukraine, Kharkiv, Ukraine Ludmila Stepanyuk Institute for Problems of Cryobiology and Cryomedicine, National Academy of Sciences of Ukraine, Kharkiv, Ukraine K. Terebilenko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine M. V. Tkach Department of Theoretical Physics and Computer Simulation, Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine S. Yu. Tkachov Taras Shevchenko National University of Kyiv, Kyiv, Ukraine M. Trubitsyn Experimental Physics Department, Oles Honchar Dnipro National University, Dnipro, Ukraine N. I. Tyschenko Frantsevich Institute for Problems of Material Science of NAS of Ukraine, Kyiv, Ukraine Artem Tyvonenko Kyiv, Ukraine M. V. Ushcats Admiral Makarov National University of Shipbuilding, Mykolayiv, Ukraine A. V. Vakaliuk Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Bogdan Vasyliv Department of Hydrogen Technologies and Alternative Energy Materials, Karpenko Physico-Mechanical Institute of the NAS of Ukraine, Lviv, Ukraine; Department of Mechanics and Automation Engineering, Lviv Polytechnic National University, Lviv, Ukraine Anatolii M. Verkhovliuk Physico-Technological Institute of Metals and Alloys, National Academy of Sciences of Ukraine, Kyiv, Ukraine P. Virych Taras Shevchenko National University of Kyiv, 64, Volodymyrska str, Kyiv, 01033 Ukraine; SE “Kolomiychenko Institute of Otolaryngology of the National Academy of Medical Sciences of Ukraine”, Zoologichna str. 3, Kyiv, 03680 Ukraine Nina Vlasenko Kyiv, Ukraine O. M. Voitsekhivska Department of Theoretical Physics and Computer Simulation, Yuriy Fedkovych Chernivtsi National University, Chernivtsi, Ukraine Nataliia Volkova Institute for Problems of Cryobiology and Cryomedicine, National Academy of Sciences of Ukraine, Kharkiv, Ukraine M. Volnianskii Experimental Physics Department, Oles Honchar Dnipro National University, Dnipro, Ukraine

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K. Wieczorek-Ciurowa Faculty of Chemical Engineering and Technology, Cracow University of Technology, Cracow, Poland K. S. Yablochkova Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Olena Yanushevska Kyiv, Ukraine Mariia Yukhta Institute for Problems of Cryobiology and Cryomedicine, National Academy of Sciences of Ukraine, Kharkiv, Ukraine A. N. Zaderko Taras Shevchenko National University of Kyiv, Kyiv, Ukraine Vladislav Yu. Zadorozhnyy National University of Science and Technology «MISiS», Moscow, Russia M. M. Zahornyi Frantsevich Institute for Problems of Material Science of NAS of Ukraine, Kyiv, Ukraine V. A. Zazhigalov Institute for Sorption and Problems of Endoecology, National Academy of Sciences of Ukraine, Kyiv, Ukraine

Nanochemistry and Biotechnology

The Kinetic Theory of the Width of Surface Plasmon Resonance Line in Metal Nanoparticles O. Yu. Semchuk, A. A. Biliuk, and O. O. Havryliuk

1 Introduction The optical spectra of metal nanoparticles are characterized by the presence in the range of visible light of a pronounced resonance band, which is called the local surface plasmon resonance band (LSPR). This phenomenon—appearance of local surface modes of oscillations inherent to free electrons at the surface of the metal nanoparticles—is of interest from both theoretical and practical points of view. The presence of enhancement of local fields in metal nanoparticles caused by SPR can significantly increase the efficiency of solar cells, create a new elemental base for the means of information transmitting, and processing [1]. In addition, this resonance interaction is accompanied by a number of nonlinear optical effects, such as increased light absorption efficiency, enhancement of luminescence, Raman scattering as well as others, that have been successfully used to enhance the resolution of microscopes [2], precision drug transportation, and treatment of tumor diseases [3]. It is known that location of the LSPR band is significantly influenced by the shape of nanoparticles and dielectric properties of the environment (dielectric matrix) [4]. These properties of metal nanoparticles can be used to improve the sensitivity of chemical and biological sensors [5]. Plasma structures are used to improve the efficiency of thin-film solar cells—placement of metallic nanoparticles onto the surface, inside, or between photosensitive layers of solar cells (SC) [6]. In these structures, metal (plasmon) nanoparticles can primarily act as additional scattering elements for the long-wave component of sunlight illuminating SC. In particular, in the presence of a reflecting rear metal contact, the light reflected in the SC surface direction will be partially redirected by the metal nanoparticles back to the semiconductor. Thus, the incident light will at least once more pass through the SC material, thereby increasing the length of its optical path and increasing the likelihood of increasing light absorption O. Yu. Semchuk · A. A. Biliuk (B) · O. O. Havryliuk Chyiko Institute of Surface Chemistry, National Academy of Sciences of Ukraine, Kyiv, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_1

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O. Yu. Semchuk et al.

by the light-sensitive semiconductor layer of SC. Also, to improve the efficiency of energy conversion in solar cells, a layer of organic PEDOT polymer, which contains metal nanoparticles, namely, gold and silver, is used [7, 8]. In addition, to increase the efficiency of thin-film SC, the effect of resonant excitation of plasmon modes in metal nanoparticles enclosed in a semiconductor matrix can be used. Then, the metal nanoparticles (5–20 nm in size) will serve as effective “antennas” for the incident light. Localized plasmonic modes will appear on the surface of the metal nanoparticles, due to the energy of which additional generation of electron–hole pairs in the semiconductor will occur, and thus the solar cell efficiency will increase. In this paper, the LSPR theory is based on the kinetic equation for the conduction electrons of nanoparticles. The developed theory is used to calculate the optical conductivity tensor for spheroidal metal nanoparticles. It is shown that oscillations of the LSPR line width can be observed in spherical metal nanoparticles. These oscillations are well expressed in nanoparticles with smaller radii and disappear for nanoparticles of larger radii. The magnitude of these oscillations increases with decreasing the nanoparticles radius and increases markedly with increasing the dielectric constant of the environment.

2 Kinetic Theory The absorption coefficient, and other optical characteristics of nanoparticles, for example, the width of LSPR, depend on the optical conductivity tensor σ j j (ω). For nanoparticles whose size is comparable to the free path of the electron, macroscopic electrodynamics becomes unsuitable because the relationship between the electric field strength E and current j is non-local. The electric field of a laser (electromagnetic) wave, acting on the charge carrier in the nanoparticle, causes a deviation f 1 , their distribution functions f equal Fermi function f 0 (ε): r , v), f ( r , v) = f 0 (ε) + f 1 (

f 0 (ε) =

1 . exp[(ε − μ)/k0 T ] + 1

(1)

 Here, ε = mv2 2—kinetic energy of the electron, μ—chemical potential, T— nanoparticle temperature, k0 —Boltzmann constant. The action of the field leads to the appearance of current  3  j = 2e m v f 1 ( r , v)dv . 

(2)

r , v) conduction electron distribuThus, the problem was to find the deviation f 1 ( tion functions in a nanoparticle from the equilibrium Fermi function f 0 (ε) under the action of the electric field of a laser (electromagnetic) wave. In the linear r , v) that satisfies the Boltzmann kinetic equation approximation is a function f 1 ( [9]

The Kinetic Theory of the Width of Surface Plasmon Resonance Line …

r , v) + v (γ − iω) f 1 (

∂ f 0 (ε) r , v) ∂ f 1 ( + ev El =0 ∂ r ∂ε

5

(3)

where γ —the frequency of bulk collisions of electrons in the nanoparticle. Equation (3) must be supplemented by boundary conditions for the function f 1 ( r , v). As such a boundary condition, we take the condition of diffuse electron scattering at the boundary of a metal nanoparticle. That is r , v)| S = 0; vn < 0. f 1 (

(4)

Here vn —the normal component of the electron velocity to the surface of the nanoparticle S. Equation (3) can be solved by the method of characteristics [10]. However, for ellipsoidal particles, this method requires modification [11]. Following the work [11], we proceed to the deformed coordinate system using the relation 3  xi2 = 1, Ri2 i=1

(5)

in which √ the ellipsoidal nanoparticle will take the form of a sphere with radius R = 3 R1 R2 R3 . In other words, we believe that xi =

xi R , γi = , γ1 γ2 γ3 = 1. γi Ri

(6)

Note that after such deformation, the volume of the nanoparticle will not change, only its shape will change. The concentration of electrons in the nanoparticle will also not change and the normalization of the distribution function will remain unchanged f. In the deformed coordinate system, equation will look like this: (γ − iω) f 1 + v

  ∂ f0 ∂ f1 = 0, + e El r v  ∂ r ∂ε

(7) (8)

In (7) and (8), we also introduced the “deformed” components of the electron velocity vector vi = γi vi .

(9)

Because along the characteristic (trajectory) d r = v dt  , then from (7) directly follows the following expression for the addition to the electron distribution function in the nanoparticle f 1 , caused by local fields:

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∂ f0 dt  . d f 1 = − (γ − iω) + e El v ∂ε

(10)

In addition, along a characteristic trajectory  r = v t  + R,

(11)

 where R—radius-vector, the end of which is at a given point on the surface of the sphere from which the trajectory begins. This parameter t  can be formally considered as the “time” of the electron’s motion along the characteristic trajectory. From (11), it is easy to determine the parameter by simple transformations t  : 

 2   1   2 2 2   t = 2 r v + R − r v + r v . v 

(12)

Characteristic (12) depends only on the modulus R and does not depend on the  This independence of the characteristic from the position orientation of the vector R. of a point on the surface became possible due to the transition to deformed coordinates (5). From (11), it is also seen that t  = 0 although r  = R. Given this circumstance, we find from (10), the type of function f 1 , which satisfies Eq. (7) with boundary condition (8): t  f1 = − 0

     ∂ f0   r − v t  − τ . dτ exp −(γ − iω) t  − τ ev El ∂ε

(13)

Taking into account that v j = vj R j /R from (13), we obtain:       ∂ f 0   1 − exp −(γ − iω)t r , v v El . f 1 ( r , v, t) = −e ∂ε γ − iω

(14)

The characteristic curve described by Eq. (20) depends only on the absolute value  but does not depend on its direction. Thus, the radius-vector R of the vector R, determines the position of the electron in the nanoparticle at time t  = 0. c Using a complex conductivity tensor σαβ r , ω), you can get the following connec( r , ω) and the electric field tion between the components of high-frequency current jα ( strength in the nanoparticle. jα ( r , ω) =

3  β=1

β

c σαβ r , ω)El . (

(15)

The Kinetic Theory of the Width of Surface Plasmon Resonance Line …

7

Now, taking into account (2) and (14), we can obtain the following expression for c the complex conductivity tensor σαβ r , ω) [12]: ( c r , ω) σαβ (

       m 3  ∂ f 0 1 − exp −(γ − iω)t  r , v vα −evβ = 2e dv . 2π  ∂ε γ − iω (16)

3 Oscillations of the Line Width of Localized Surface Plasmon Resonance We apply our theory to metallic spherical nanoparticles. In this case, for real part of the electrical conductivity of the spherical nanoparticles σsp (ω), we obtain the following expression (provided that ωs  γ , v  v F ): σsp (ω) =

 

2ωs2 3 ω2p v F ω 2ωs ω + 1 − cos . 1 − sin 16π ω2 R ω ωs ω2 ωs

(17)

For a spherical nanoparticle, taking into account (17), we obtain the following expression for the LSPR attenuation rate (LSPR line width) in the spherical nanoparticle Γ (ω, εm ):     ω 2 v ω ω 2ωs2 2ωs p F sin Γ (ω, εm ) = 4π σsp (ω) = + 2 1 − cos 1− . 2(2εm + 1) ω 2R ω ωs ωs ω 3

(18)    Considering only the first addition in (18), we obtain a well-known 1 R dependence for the LSPR attenuation rate in a spherical particle [13]: Γ0 =

 ω 2 v p F . 2(2εm + 1) ω R 3

(19)

As follows from (18) and (19), the lifetime of the LSPR (LSPR line width) in a spherical metallic nanoparticle depends on both the particle radius R and the frequency of the laser-excited SPR ω, and consists of two additives describing smooth Γ0 and oscillating:  

 ω 2 v 2ω ω ω 2ωs2 p F s , (20) − sin + 2 1 − cos Γosc (ω, εm ) = 2(2εm + 1) ω 2R ω ωs ω ωs 3

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√ For the frequency ω ≈ ωr es = ω p 2εm + 1, corresponding to the excitation of a surface plasmon in a spherical metallic nanoparticle contained in a dielectric matrix having a dielectric constant εm > 1 from relation (20) in energy units, the following parameter can be entered: 3 3 v F ≡ ωs (21) 4 R 2 √ and at the frequency SPR ωr es = ω p 2εm + 1 for the oscillating part of the line width can be obtained the following expression: Γ0r es =

3   v F 2  2εm + 1 4 ωp R √ 

 2Rω p 2Rω p v F 2εm + 1 , 1 − cos √ × −sin √ + 2Rω p v F 2εm + 1 v F 2εm + 1

r es Γosc (R, εm ) =

(22)

es provided that ωs  γ . The amplitude Arosc and period T of these oscillations can be estimated by the expressions:

3   v F 2  2εm + 1 4 ωp R π  T = 2εv + 1. ωp

es = Arosc

(23) (24)

Therefore, the total width of the full LSPR√ line for a spherical metal nanoparticle at 2εm + 1 in energy units Γr es (R, εm ) the plasmon resonance frequency ωr es = ω p r es consists of two additions that describe the smooth Γ0r es and oscillating Γosc (R, εm ) parts of the SPR line: 3   v F 2  3 v F r es + 2εm + 1 Γr es (R, εm ) = Γ0r es + Γosc (R, εm ) = 4 R 4 ωp R √ . 

 2Rω p 2Rω p v F 2εm + 1 × −sin √ + 1 − cos √ 2Rω p v F 2εm + 1 v F 2εm + 1

(25)

Consider the calculated dependences of the full and smooth width of the SPR line on the dielectric constant of the environment for the K and Ag nanoparticles shown in Fig. 1. It follows, in particular, that both the amplitude and the oscillation period of the SPR line width increase in small spherical K and Ag nanoparticles placed in a dielectric matrix with a larger dielectric constant εm . However, the amplitude of plasmonic oscillations decreases quadratically with increasing radius of the spherical nanoparticle. As can be seen from Fig. 1, as the magnitude increases εm , the width of the SPR line gradually increases and increases around its smooth part Γ0r es . The oscillating

The Kinetic Theory of the Width of Surface Plasmon Resonance Line … 0.30

20 nm

0.40

30 nm

Г, еV

Г, еV

0.15

20 nm

0.35

0.25

0.20

9

0.30 0.25

30 nm

0.20

50 nm

0.15

0.10 2

4

εm

a

6

8

10

50 nm 2

4

6

εm

8

10

b

Fig. 1 The dependence of full (solid lines) and smooth (dashed lines) components of the width of the LSPR line on the dielectric constant εm of the environment for spherical nanoparticles K (a) and Ag (b) with radii of 20, 30, and 50 nm r es additive to the SPR line width Γosc (R, εm ) is an important correction to Γ0r es , especially for small radius particles (the oscillations of the SPR line are well expressed for the K and Ag nanoparticles of small radii and practically disappear for large nanoparticles). The least admissible nanoparticle radius that can be considered in  the framework of the above theory is limited by some value Rmin  2π  mv F . For nanoparticle K, this value is Rmin  0.855 nm. Figure 1a, b also shows that as the radius of the spherical nanoparticle grows, the width of the SPR line decreases substantially and increases around a certain constant value in mediums of greater value εm . The magnitude of these oscillations is larger, the smaller the size of the nanoparticle, and the larger it increases εm .

4 Conclusions In this paper, we construct the theory of local surface plasmon resonance, which is based on the kinetic equation for the conduction electrons of nanoparticles. Our theoretical studies have shown that in metallic nanoparticles contained in a dielectric matrix, under the conditions of surface plasmon resonance, the full width of the LSPR line in a spherical metal nanoparticle depends on both the radius of the particle R and the excitation frequency of this LSPR laser ω radiation that describe the smooth and oscillating parts of the LSPR line width. The oscillations are well expressed for nanoparticles with smaller radii and disappear for nanoparticles of larger radii. The magnitude of these oscillations increases with decreasing nanoparticle radius and increases markedly with increasing dielectric constant of the environment. The optical conductivity tensor is associated with the width of the plasmon resonance line Γ j (ω), which determines the decay time of the plasmon resonance due to the scattering of electrons on the surface of the nanoparticle. Any deviation of the shape of the nanoparticles from the spherical one will lead to the splitting of the

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surface plasmon line in the spectra into several modes. In particular, if the transformation of the shape of the nanoparticle takes place in the direction of the spheroidal shape, then two such modes can appear: transverse (perpendicular to the axis of rotation of the spheroid) and longitudinal (parallel to this axis). Thus, we will observe two components of the width of the LSPR line—longitudinal and transverse. The obtained theoretical results can be important for the analysis of experimental data on the optical and transport characteristics of metal nanoparticles placed in different dielectric media.

References 1. 2. 3. 4. 5. 6. 7.

8.

9. 10. 11. 12. 13.

Maier SA (2007) Plasmonics: fundamentals and applications. Springer, New York, NY Valyanskii SI (1999) Microscope on surface plasmons. Soros Educ J 8:76–82 Atwater H (2007) The promise of plasmonics. Sci Am 296:56 Noguez C (2007) Surface plasmons on metal nanoparticles: the influence of shape and physical environment. J Phys Chem C 111:3806–3819 Jeffrey NA, Hall WP, Lyandres O, Shah NC, Zhao J, Van Duyne RP (2008) Biosensing with plasmonic nanosensors. Nat Mater 7:442–452 Atwater H, Polman A (2010) Plasmonics for improved photovoltaic devices. Nat Mater 9:205– 230 Wu J-L, Chen F-C, Hsiao Y-S, Chien F-C, Chen P, Kuo C-H, Huang MH, Hsu C-S (2011) Surface plasmonic effects of metallic nanoparticles on the performance of polymer bulk heterojunction solar cells. ACS Nano 5:959–967 Fleetham T, Choi J-Y, Choi HW, Alford T, Jeong DS, Lee TS, Lee WS, Lee K-S, Li J, Kim I (2015) Photocurrent enhancements of organic solar cells by altering dewetting of plasmonic Ag nanoparticles. Sci Rep 5:1–5 Dykman IM, Tomchuk PM (1981) Transport phenomena and fluctuations in semiconductors. Naukova dumka, Kiev Courant R (2011) Differential and integral calculus, vol 1, 2nd ed. Wiley, Hoboken Tomchuk PM, Tomchuk BP (1997) Optical absorption of small metal particles. J Exp Theor Phys 85:360–369 Grigorchuk NI, Tomchuk PM (2011) Optical and transport properties of spheroidal metal nanoparticles with account for the surface effect. Phys Rev 84:085448-1–085448-14 Kreibig U, Volmer M (1995) Optical properties of meta clusters. Springer, Berlin

Optical and Electrical Phenomena Caused by the Lattice Defects in PbMoO4 Crystal T. Bochkova, D. Bondar, M. Trubitsyn, and M. Volnianskii

Abstract PbMoO4 single crystals were grown from a stoichiometric as well as from a nonstoichiometric charge and with dopants of Bi2 O3 and BaO. The influence of composition and impurities on the optical transmission, photochromic, and photodielectric effect is considered. The formation of the structural defects caused by high-temperature treatment and UV irradiation of the crystals is discussed by assuming changes in the charge state of Pb and Mo ions.

1 Introduction Lead molybdate (PbMoO4 ) is an optical crystal widely used in functional electronics. Studies of this practically important material are carried out in various directions. Its crystal structure is well known and belongs to the scheelite type. PbMoO4 has tetragonal symmetry (spatial group I41 /a, Z = 4). The structure is built from PbO8 polyhedra and MoO4 tetrahedra linked by common vertices. Oxygen polyhedra PbO8 are connected by edges in spirals around the screw axis of the fourth order [001] [1–3]. The most extensively PbMoO4 crystals are used in acousto-optics. PbMoO4 has a wide range of transparency, high acousto-optical quality, and suitable acoustic properties [4–6]. The family of modern acousto-optic devices includes diffraction light modulators [7], spectrum analyzers [8], acousto-optic waveguides and acoustic wave sensors [9], scanning microscopes [10], etc. In this regard, it is of great importance to obtain PbMoO4 single crystals of high optical quality. In addition, PbMoO4 crystals doped with rare earth ions are grown to manufacture the solid-state Raman lasers used in Lidar systems (laser radar), since this material acts both as an active laser medium and as a Raman converter [11]. The growth of large-sized PbMoO4 single crystals doped with Nd and Yb for this purpose is reported in [12–16]. T. Bochkova · D. Bondar (B) · M. Trubitsyn · M. Volnianskii Experimental Physics Department, Oles Honchar Dnipro National University, Prosp. Gagarina 72, Dnipro 49010, Ukraine e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_2

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PbMoO4 crystals of high-quality came to be in demand in high energy physics for use as a cryogenic scintillator. The possibility of neutrinoless double β-decay for 100 Mo isotope combines in one material a source and a detector of a rare event that significantly increases the efficiency of the detector [17, 18]. It is also necessary to reduce the radiation background of the material due to the content of other nuclides. An unusual development for the use of archaeological lead purified by a special method is reported in [19]. Since the half-life of the 210 Pb radionuclide is about 22.3 years, in archaeological lead over hundreds of years its content has significantly decreased. This reduced the radiation background of the grown crystal. Realization of such projects emphasizes the uniqueness of PbMoO4 . The growth of PbMoO4 single crystals by the Czochralski technique was first reported in the early 1970s [5, 6]. PbMoO4 melts congruently in the interval 1338– 1343 K [20]. Sufficiently large yellow single crystals were grown from the melt under low thermal gradient conditions using radiofrequency heating. For acousto-optic applications, colorless crystals with high optical stability are required. However, PbMoO4 crystals grown by Czochralski technique in air, as a rule, are yellow and exhibit a significant photochromic effect, which indicates the presence of intrinsic and/or impurity defects. To eliminate the undesirable phenomena, one has to make clear the nature of the local centers and to develop the crystal growing technology. Further studies devoted to growth of PbMoO4 crystals were aimed precisely at solving this problem. A comprehensive analysis of the growth conditions for PbMoO4 crystal was carried out in [21–27]. To prepare the charge, both the solid-phase reaction method and the precipitation from aqueous solutions were used. Furnaces with high- and low-temperature gradients were tested. Attention was paid to purity of the initial reagents, evaporation of the components, and determination of the optimal directions of crystal pulling. The reasons for the appearance of such macroscopic defects as dislocations, twins, gas bubbles, and cracking were investigated. Considerable interest was aroused by the problem of point defects in these crystals, in particular, color centers. In a number of works [28–32], the color of PbMoO4 crystals was associated with the presence of uncontrolled impurities of iron group, in particular, Mn, Cr or even Pt impurity which could penetrate into the lattice due to the crucible dissolution. Another group of authors [33–35] studied PbMoO4 crystals subjected to heat treatment in PbO or MoO3 vapors, as well as in atmospheres of various compositions. They came to the conclusion that the dominant type of the point defects in PbMoO4 are cationic vacancies, in particular, vacancies of molybdenum ions (VMo ) associated in various complexes and forming hole color centers. There was a discussion about the assumption [36, 37] that the interaction with surrounding oxygen during the growth and annealing of PbMoO4 crystals affected the formation of such defects as Pb3+ and Mo5+ . The formation of photoinduced defects in PbMoO4 crystals is of particular interest. The defects of such kind were recently observed by EPR and thermally stimulated luminescence (TSL) in [38, 39]. The photoinduced defects of a complex nature (nanoclasters, dipole centers) can be connected with a recharge of

Optical and Electrical Phenomena Caused by the Lattice …

13

the cations, can act as emission centers, and cause such phenomena as luminescence, photochromism, and photodielectric effect [40–43]. According to [36], colorless crystals of PbMoO4 could be obtained by growing or annealing the crystals in a vacuum or inert gas. However, such crystals became even more colored under the action of high-energy radiation. Up to now, no method has been developed that would simultaneously reduce the natural color of as-grown crystals and the photochromic effect. In this work, we consider the effects of composition variations, high-temperature treatment in air, UV irradiation, and doping on optical and electrical properties of PbMoO4 single crystals. The phenomena observed are explained within the framework of a unified model of changes of the cations valence [36, 37]. These changes can occur in the crystal lattice under the action of both technological factors and external influences.

2 Experimental Details The single crystals of PbMoO4 were grown in air from the melt by Czochralski technique. The obtained crystals had a diameter up to 35 mm and a length about 50–60 mm. They were free from macroscopic inclusions (gas bubbles, cracks) and had a light yellow color [44]. The charge was prepared from the lead and molybdenum oxides of “high purity” grade using ceramic technology. The reagents (1 + x) MoO3 − (1 − x) PbO were taken both in a stoichiometric ratio (x = 0) as well as with the deviations toward an excess of MoO3 (x = 0.002; 0.004; 0.006; 0.01; 0.02; 0.03). Bi2 O3 and BaO dopants were introduced into the charge with x = 0.01 in an amount up to 1 wt% over stoichiometry. High-temperature treatment of the crystals in air was carried out in a muffle furnace at 1200 K for 7 and 87 h. The optical transmission spectra were measured at 295 K on PbMoO4 double polished plates using “Specord M-40” spectrophotometer. The thickness of the samples was about 10–13 mm. The polished planes were oriented perpendicular to the optical axis. The photochromic coloration was observed after irradiation by light of Hg lamp during 5–30 h. Colored samples were returned to the initial state by annealing at 700–950 K in air for 2 h. Permittivity ε and conductivity σ were measured in AC field by the bridge method at fixed frequency (f = 1 kHz) in the temperature interval 290–700 K. The samples were prepared as plane-parallel plates with dimensions 5 × 5 × 1 mm3 . The main planes of the samples were cut parallel to (001) and coated with platinum electrodes. Before each measuring cycle, the short-circuited samples were heat treated at 700 K for 10 min. Such procedure was used in order to eliminate the dependence of the electrical properties on the sample pre-history, similarly to the processing performed earlier for double Pb2 MoO5 [45]. After thermal treatment the samples were cooled

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to room temperature and irradiated through the side faces with UV light of Hg lamp. Permittivity and conductivity were measured on heating run with the rate of 8 K/min.

3 Results and Discussion 3.1 The Effects of Charge Nonstoichiometry and High-Temperature Treatment on the Optical Transmission It is known that annealing of PbMoO4 crystal at a low partial pressure of oxygen (3–4 orders of magnitude lower than atmospheric), in a vacuum or inert gas leads to crystal discoloration, while annealing in oxygen enhances the yellow color [33, 36]. Electronic and ionic conductivity, as well as self-diffusion, were studied in PbMoO4 [34, 35, 37] in assumption that these phenomena are associated with rather high diffusion coefficients. For measurements, the authors used both the as-grown “stoichiometric” PbMoO4 crystal and the crystals with deviations from stoichiometry caused by heat treatment under various conditions: in Pb2 MoO5 , PbO, or CaO powders; in vapors of MoO3 , as well as in different atmospheres with varying partial pressures of O2 . The experiments were carried out in order to affect the thermodynamic activity of MoO3 , to determine the dominant type of point defects, and to clarify the nature of the local centers responsible for the crystal coloration. Based on the dependencies of the electrical conductivity on the partial pressure of oxygen and MoO3 vapor, the authors of [33] declared a MoO3 deficiency in “stoichiometric” PbMoO4 crystals. They assumed that, due to the higher vapor pressure of MoO3 compared to PbO, in PbMoO4 single crystals grown from a stoichiometric charge, the number of molybdenum vacancies VMo exceeds the number of lead vacancies (VPb ). On the contrary, in [46], it was concluded that there was an excess of MoO3 in “stoichiometric” PbMoO4 crystals. The limits of deficiency and excess of MoO3 were also discussed in [47]. Using for research the sensitive modifications of X-ray phase analysis and differential thermal analysis, it was found that the homogeneity region of the PbMoO4 phase was in the range 49.85–50.50 mol% of MoO3 that in our notations corresponds to x = −0.003 − 0.01. Thus, studying the influence of nonstoichiometry and high-temperature treatment on the optical absorption of PbMoO4 crystal seems to be of current interest. Earlier, we have grown PbMoO4 single crystals from a charge with an excess of molybdenum oxide up to x = 0.03. The color centers in nonstoichiometric PbMoO4 crystals with an excess of Mo content and after high-temperature treatment in air were preliminary studied in [48]. The optical transmission spectra of PbMoO4 crystals with small deviations from stoichiometry in the charge (x = 0.002–0.004) differ slightly from each other. Figure 1 shows the optical transmission of the crystals grown from the charge of stoichiometric and nonstoichiometric compositions. The curves 4, 5 obtained by the spectra subtraction are presented too. With an increase of MoO3 excess in the charge up to

Optical and Electrical Phenomena Caused by the Lattice …

15

Fig. 1 The effect of the charge nonstoichiometry on the optical transmission of PbMoO4 crystals: 1—x = 0 (stoichiometry); 2—x = 0.01; 3—x = 0.03. The results of the spectra subtraction are shown by the curves 4, 5: 4 = 1–2; 5 = 2–3. The thickness of the samples was 10.5 mm

x = 0.01, the transmission in the range of 16,000–17,000 cm−1 increases by a few percent. At higher deviations from stoichiometry in the charge, the transmission of the crystals decreases. It is seen that an excess of Mo in an amount of x = 0.03 leads to the appearance of two broad absorption bands with maxima near 24,000 cm−1 and 16,000–17,000 cm−1 . High-temperature treatment in air also leads to coloration of PbMoO4 crystals, both “stoichiometric” and those grown from the charge of nonstoichiometric compositions (Fig. 2). The curves 4 and 5 representing the difference of the optical transmission spectra show that annealing at 1200 K for 7 h leads to the appearance of significant absorption in the region of 23,000 cm−1 and bleaching in the region of

Fig. 2 The effect of isothermal treatment in air (T = 1200 K) on optical transmission of the PbMoO4 crystals grown from the charge with x = 0 (a) and x = 0.006 (b): 1—initial state; 2—after heat treatment for 7 h; 3—after heat treatment for 87 h. The spectral differences are shown by the curves 4, 5: 4 = 1–2; 5 = 1–3. The thickness of the samples was 10.8 mm

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16,000–17,000 cm−1 . For the crystals with a slight excess of Mo (up to x = 0.006), a decrease of the light absorption intensity in the region of 25,000 cm−1 is also noticeable. An increase of the heat treatment time up to 87 h leads to a strong increase of the intensity of the absorption band at 23,000 cm−1 and a further decrease of the intensity of the light absorption in the region of 16,000–17,000 cm−1 . The reasons for the coloration of PbMoO4 crystals have been widely discussed in the literature and various assumptions have been made regarding the nature of color centers. Thus, the yellowish color of the crystals was attributed to the associates of holes with Mo (Pb) vacancies [13, 33], or to Pb4+ ions located in the Mo6+ positions [33]. As a rule, the substitution of Pb2+ ions for Mo6+ ions due to the difference in charge and ionic radii was considered unlikely; however, the authors believed that at an oxygen pressure close to atmospheric, some of Pb2+ could be oxidized to Pb4+ and replace Mo6+ ions. In a number of works [22, 28–31] the absorption in the region of 23,000 cm−1 was attributed to the impurity centers. The color of the crystals was explained by the formation of associates of Mn3+ ions with vacancies of Pb2+ ions [28], the presence of Mn2+ ions in the Pb2+ sites [29], and the presence of Cr6+ ions in the Mo6+ positions [30, 31]. However, we think that the most reasonable assumptions were made by Ballmann [36, 37]. He suggested that the absorption in the region of 23,000 cm−1 was due to the oxidation of Pb2+ ions to Pb3+ , and the wide absorption bands with maxima at about 25,600 cm−1 (390 nm) and 17,200 cm−1 (575 nm) were caused by intrinsic transitions in the Mo5+ ion. In addition, the study of luminescence in PbMoO4 made it possible to propose the following mechanism: an electron was captured by the MoO4 2– group, and a hole was captured by one of the nearest Pb2+ ions. The center of the glow was considered as a cluster: Pb3+ − MoO4 3− [40]. The Pb3+ paramagnetic centers in scheelites were observed earlier by EPR [49] and confirmed by analysis of hyperfine interaction with nuclei of 207 Pb isotope and superhyperfine interaction with the ligands. Perhaps, lack of the data on Pb3+ observation in PbMoO4 is due to their low concentration. The concentration of Pb3+ in as-grown yellow PbMoO4 crystals was estimated in [36] as about 1.4 × 1015 cm−3 . The presence of Mo5+ ions is also not unusual for scheelites. These centers were found in CaWO4 [50], CaMoO4 [36], PbMoO4 [36, 51]. Recent low-temperature studies of EPR in PbMoO4 crystals showed the existence of several types of photoinduced centers, including Mo5+ [38, 39]. Indeed, the absorption band at 23,000 cm−1 is detected in the optical spectra of the crystals grown from various raw materials and under various conditions. Thus, uncontrolled iron group impurities could not be associated with this band. In [36] the optical characteristics of Fe-doped PbMoO4 were studied. It was shown that iron group impurities could give rise to additional absorption bands, but the specific band near 23,000 cm−1 had a different nature. As for the associates of cationic vacancies with holes, complex hole centers such as two vacancies of molybdenum and a hole, and others which were discussed in [33, 34], their existence remained unproven. In [30, 31], it was proposed that yellow color of PbMoO4 crystals could be attributed to chromium impurity. Indeed, the crystal coloration intensified with an increase of Cr impurity concentration. EPR data showed the signal from Cr5+ centers

Optical and Electrical Phenomena Caused by the Lattice …

17

in Mo6+ sites and evidenced for strong covalent bond of Pb2+ with CrO4 3– tetrahedra. But the similar results for the host molybdenum ion itself were obtained in [36, 38, 39]. It means that intrinsic defects (Mo5+ ) and impurities (Cr5+ [30, 31] and Nb5+ [36]) located in the Mo6+ sites are very likely involved in the formation of Pb3+ − MoO4 3− nanoclusters. The effect of high-temperature treatment on the optical transmission of PbMoO4 crystals observed in this work can be well described within the framework of the Ballmann model [36, 37]. One can assume that annealing in air can lead to oxidation of Pb2+ according to the reaction: 4Pb2+ + O2 ↔ 4Pb3+ + 2O2− A certain amount of Mo5+ ions (dependent on the prehistory of the sample) is spontaneously formed in PbMoO4 [51] according to the reaction: Mo6+ + e− ↔ Mo5+ , These ions will also oxidize by interacting with oxygen of the surrounding atmosphere: 4Mo5+ + O2 ↔ 4Mo6+ + 2O2− . In [50], it was suggested that the presence of oxygen vacancies (VO ) near to Mo6+ ions favored to the electron capture. Since annealing in oxygen (and in air) decreases the concentration of VO , as a result, the number of Mo5+ ions also decreases. The increase of the optical absorption band intensity caused by Pb3+ ions and the decrease in intensity of the bands associated with Mo5+ ions (Fig. 2a) can be explained by these oxidative reactions. Let’s consider the manifestation of nonstoichiometry. One can assume that the deficiency of molybdenum arising during the crystal growth is compensated by excess of MoO3 in the charge up to x = 0.01. This is consistent with the data in [47]. However, even for a small excess of MoO3 , Mo5+ centers exist in the crystal. This is evidenced by the heat treatment of such crystals in air (Fig. 2b). First, after annealing, an increase of the transmission is observed in the range of 16,000–17,000 cm−1 . Second, for the sample annealed for 7 h, one can see a second absorption band in the UV region of the spectrum (25,000 cm−1 ). This absorption band was earlier observed by Ballmann for the reduced PbMoO4 after irradiation with X-rays [36]. It is not always visible at room temperature due to large thickness of our samples and is masked by overlapping wide band with a maximum of about 23,000 cm−1 . However, its behavior is similar to that of the absorption band at 16,000–17,000 cm−1 . This confirms the assumption [36] that absorption at 25,000 cm−1 also refers to intrinsic transitions in the Mo5+ ion. One should remember that an excess of MoO3 (for x > 0.01) means a deficiency of lead in the charge for growing PbMoO4 . Of course, the presence of VPb can result

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in a change of the charge state of some Pb2+ ions to Pb3+ . Besides, the excess charge of VPb can be compensated by appearance of VO . The latter can stabilize the located nearby Mo5+ centers. Thus, a deviation from the stoichiometric ratio toward an excess of molybdenum leads to the appearance of both a band with a maximum at about 23,000 cm−1 and a band at 16,000–17,000 cm−1 (Fig. 1). These results are consistent with the data in [52] where the influence of growth conditions on the structural parameters of PbTO4 (T = Mo, W) was studied. It was shown that the number of VO and VPb in the PbMoO4 structure depended on the atmosphere (air or nitrogen) in which the crystal was grown. The authors of [52] also assumed the possibility of Pb2+ → Pb3+ transfer as well as existence of Mo5+ centers in the crystals grown from nonstoichiometric charge.

3.2 Photochromic Effect PbMoO4 crystals grown from stoichiometric and nonstoichiometric charges and studied in this paper, showed a distinct color change to dark grey under irradiation with UV light. This photochromic effect was reversible. Heating in air up to 700–950 K eliminated the absorption induced by UV irradiation. The optical transmission spectra of nominally pure PbMoO4 single crystals, grown from the compositions with optimal (x = 0.01) and maximal (x = 0.03) deviations from stoichiometry, before and after UV irradiation are shown in Fig. 3. The crystals were irradiated and optical spectra were measured at room temperature. One can see that after irradiation the optical transmission decreases significantly. Figure 3 shows also the photoinduced change of the optical transmission as the difference of the spectra of the initial and irradiated states. Three wide absorption bands with maxima near 25,000, 23,500, and 17,000 cm−1 can be distinguished in these spectra. No noticeable effect of excess MoO3 on the Fig. 3 The influence of UV irradiation on the optical transmission of PbMoO4 crystals grown from the charge with x = 0.01 (curves 1, 2) and x = 0.03 (curves 3, 4): 1, 2—before and 3, 4—after UV exposition. The curves 5, 6 demonstrate the spectral difference: 5 = 1–2; 6 = 3–4. The thickness of the samples was 10.5 mm

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photoinduced absorption was found, although the initial spectra themselves differed from each other. As already stated, an excess of molybdenum in an amount of up to x = 0.01 decreases the optical absorption in the region of 17,000 cm−1 . Further increase of the MoO3 excess amount causes appearance of the additional absorption of light in the region of 24,000 and 17,000 cm−1 (Fig. 1). The absorption band near 25,000 cm−1 can be masked by the thickness of the samples. The photochromic effect in PbMoO4 crystals most likely is caused by charge exchange between cations by means of redox reactions and within the model [36] can be described by the reaction: Mo6+ + Pb2+ ↔ Mo5+ + Pb3+ . Such reaction means the appearance of all three absorption bands after irradiation of PbMoO4 crystal with UV light. As already mentioned, formation of Mo5+ centers as a result of various processes in PbMoO4 (annealing at low oxygen partial pressure, irradiation with X-ray and UV light, deviation of composition from the stoichiometry) was confirmed by many authors [36, 41, 52] and was repeatedly observed by EPR [38, 39]. Presence of oxygen vacancies VO in the vertices of Mo–O tetrahedra facilitates formation of Mo5+ centers [50]. That is why, X-ray irradiation of PbMoO4 samples bleached by annealing in vacuum lead to appearance of the intensive absorption bands, including those attributed to Mo5+ ions [36]. The appearance of Pb3+ hole centers as well as involving lead electronic states in the creation of emission centers was actively discussed to interpret the luminescence in PbMoO4 caused by UV excitation [3, 40, 53]. It was suggested that photoluminescence was caused by radiative transitions in (MoO4 )2− complexes and transfer of an electron from Pb2+ ions to adjacent molybdenum groups. Van Loo offered the formation of Pb3+ – Mo5+ glow centers generated by UV irradiation [40]. The authors of [53] studied the luminescence and reflectivity spectra of lead and barium molybdate crystals using synchrotron radiation. They also inferred that the electronic states of lead participated in the formation of emission centers or took part in the transfer of energy to the centers responsible for the photoluminescence in PbMoO4 . This conclusion was confirmed in [38], where significant (of about 40%) mixing of lead orbitals into the ground state of the MoO4 complex was found.

3.3 Photodielectric Effect Among other effects, UV irradiation can induce electrical polarization. Figure 4 shows ε(T) and σ(T) dependencies for PbMoO4 single crystal irradiated previously with UV light at room temperature. One can see that UV irradiation resulted in appearance of ε(T) and σ(T) maxima which increased in amplitude for higher exposure times. The ε and σ anomalies could be detected only on the first heating run and disappeared after heating up to 450 K. For the subsequent temperature cycles

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Fig. 4 Dependencies of permittivity ε(T) (a) and conductivity σ(T) (b) for PbMoO4 crystal irradiated with UV light. Irradiation was performed at T = 290 K for the following times t: 1—0, 2—10, 3—20, 4—40 min. Measuring field frequency was f = 1 kHz

these anomalies were absent. The position of the broad σ(T) maximum (Fig. 4b) corresponds to the temperature of the dielectric anomaly ε(T) (Fig. 4a). Upon further heating (T > 400 K), the σ(1/T) dependence in Arrhenius scale linearizes and does not depend on exposure time. The appearance of ε(T) and σ(T) anomalies evidences that UV irradiation generates centers with electric dipole moment. Reorientations of the dipole centers in an external AC field give rise to ε(T) and σ(T) anomalies (Fig. 4). Heating up to 450 K vanishes the anomalies of ε and σ. Obviously, such effect can be ascribed to the thermal destruction of the dipole centers. These centers can be regenerated by subsequent UV light irradiation of the sample at room temperature. As can be seen from Fig. 4a the ε(T) anomaly represents nearly symmetrical peak which significantly differs from the typical Debye stepwise temperature dependence [54]. The Debye like behavior can be modified by accounting for the thermal decomposition of the dipole centers on heating. The decrease of the dipole centers concentration results in more sharp falling down the high-temperature wing of ε anomaly (Fig. 4a). Available experimental data allow to discuss the nature of the dipole centers responsible for the photodielectric effect. As it was shown by studying EPR and TSL in [38, 39], the irradiation induced three types of the centers Mo 1, 2, 3. All of them represented (MoO4 )3− complexes with trapped electrons e− and with different surroundings in nearest Pb sites. However, all these centers were stable only at low temperature and were destroyed by heating in the interval 40–120 K. Hence, Mo 1, 2, 3 cannot be responsible for the ε and σ anomalies (Fig. 4a, b) observed at much higher temperatures. Additional information can be found in earlier works [55–57] where photoinduced centers in isomorphous lead tungstate PbWO4 crystal were studied. In fact, in [38, 39] it was pointed out that the light-induced centers in PbMoO4 and PbWO4 were similar in the nature, since their EPR spectra were described by the same

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spin-Hamiltonian with very close parameters. Thus, the models of the photoinduced centers in PbWO4 [55–57] with certain reservations can be applied to interpret the data for PbMoO4 . In particular, it was shown, that irradiation of PbMoO4 (λ = 420 nm at T = 35 K [38]) and PbWO4 :Mo (λ = 260–330 nm at T < 50 K [56]) generated photoelectrons trapped by molybdenum ions in regular (PbMoO4 ) or irregular (PbWO4 :Mo) sites: e– + (MoO4 )2− → (MoO4 )3− . These polaron centers had a depth about Et = 0.05 eV below the conduction band bottom and were thermally destroyed on heating above 40–50 K. The destruction of (MoO4 )3− centers resulted in disappearance of EPR spectrum and TSL glow. The electrons released in the process partially recombined with holes and were partially captured by other deeper traps. Among such traps there were the thermally stable centers up to 180–190 K (Et = 0.55 eV). It was supposed that these centers were formed by oxygen vacancy VO and neighboring Pb2+ ion capturing an electron: Pb+ – VO . The deepest photoinduced centers (Et = 0.9 eV) were stable up to room temperature and even above. Detailed analysis of the EPR spectra anisotropy evidenced that such centers could be associated with W(Mo) – O tetrahedra distorted by vacancies VO and stabilized by the unknown defects in neighboring Pb site: (W(Mo)O3 )– – APb [56]. Accounting the EPR and TSL data in [38, 39, 55–57], the dipole defects, induced by UV light and giving rise to the anomalies of ε and σ (Fig. 4) can be attributed to photoelectrons captured by molybdenum ions within tetrahedra distorted by oxygen vacancies (MoO3 )− . An additional neighboring defects APb in lead sublattice could stabilize such complexes similarly to the situation in PbWO4 . Electric dipole moment of distorted (MoO3 )− tetrahedron is produced by excess charges of photoelectron captured by Mo and oxygen vacancy VO . Thermally activated VO hopping through the vertices of (MoO3 )− complex is accompanied by reorientation of the dipole moment. In external AC field such reorientations give rise to the anomalies of PbMoO4 electrical properties (Fig. 4). Apparently that reorientations of (MoO3 )− dipoles cause dielectric losses and contribute to conductivity peak observed around 340 K (Fig. 4b). For T > 400 K behavior of σ(T) does not depend on the time of UV irradiation. Thus, the photoelectrons generated by UV irradiation are captured by the other traps or recombined with the holes and do not participate in charge transfer. It should be noted that there are other possible traps for photoelectrons. For instance, Pb+ − VO centers which were found in PbWO4 [56]. Dipole moment of such centers could be reoriented by hopping of captured electrons between structurally equivalent lead sites. But in PbWO4 these F+ centers were stable only below 180 K. Consequently, in PbMoO4 the existence of such F+ centers above room temperature seems unlikely.

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3.4 Doping Effect on the Optical and Electrical Properties of PbMoO4 :Bi, Ba 3.4.1

Optical Transmission in PbMoO4 :Bi, Ba

The PbMoO4 single crystals doped with Bi and Ba were grown using standard technique. If the fraction of Bi2 O3 and BaO dopants in the charge was less than 1 wt%, the crystals grown had a yellowish color of varying intensity and did not contain visually observable cracks and inclusions. In Sect. 3.1 it was argued that the main absorption band at 23,000 cm−1 in the optical spectra could be associated with Pb3+ . Thus, it is of interest to study influence of doping with bi- and trivalent ions which have stable electronic configuration and substitute for Pb2+ host ions. Figures 5, 6 and 7 show the optical transmission spectra of PbMoO4 crystals doped with Bi3+ and Ba2+ ions before and after exposition to UV light. The unit cell of PbMoO4 lattice has the following parameters: a = 0.5431 nm, c = 1.2106 nm. The slightly distorted MoO4 tetrahedra represent the basic structural unit. The length of covalent Mo–O bonds is 0.177 nm. The MoO4 tetrahedra are linked through Pb–O bonds. Each molybdenum group is surrounded by eight lead ions: four at a distance of 0.3841 nm and other four at a distance of 0.4067 nm from the central Mo6+ ion. Each Pb2+ ion is surrounded by eight oxygen ions, belonging to one of eight MoO4 tetrahedra. Four of them are located at a distance of 0.261 nm from Pb2+ and four others at slightly greater distance of 0.263 nm [38, 58].

Fig. 5 The doping effect on the optical transmission in PbMoO4 :Bi (a) and PbMoO4 :Ba (b): the spectra 1, 2—undoped PbMoO4 ; 3, 4—PbMoO4 :Bi, Ba. The spectra were measured before (curves 1, 3) and after (curves 2, 4) UV exposition. The curves 5, 6 demonstrate the spectral difference: 5 = 1–2; 6 = 3–4. The fraction of Bi2 O3 in the charge was 0.1 wt%, BaO—0.15 wt%. The thickness of the samples was 10.5 mm (PbMoO4 , PbMoO4 :Bi) and 13 mm (PbMoO4 :Ba)

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Fig. 6 The optical transmission of undoped PbMoO4 (curve 1) and PbMoO4 :Ba (curves 2, 3) crystals grown from the charge with x = 0.01. The fraction of BaO in the charge was 0.15 wt% (curve 2) and 1 wt% (curve 3). The thickness of the samples was 10.5 mm (PbMoO4 ) and 13 mm (PbMoO4 :Ba)

Fig. 7 Photoinduced change of the optical transmission in undoped PbMoO4 (curve 1) and PbMoO4 :Ba (curves 2, 3, 4). The fraction of BaO in the charge was 0.15 wt% (curve 2), 0.60 wt% (curve 3), and 1 wt% (curve 4). The thickness of the samples was 10.5 mm (PbMoO4 ) and 13 mm (PbMoO4 :Ba)

We refused to dope PbMoO4 crystals with ions of transition groups for a number of reasons. First, to avoid the problems associated with heterovalence and recharge of the centers. Secondly, use of transition elements leads to appearance of d–d and f–f transitions, i.e., makes the optical spectra more complicated. Third, studies of PbMoO4 doped with iron group elements (Mn, Fe, Cu, Co, Cr) [28–31, 36, 58– 61] and rare earth elements (Gd, Nd, Eu, Yb) [12–16, 59–61] have already been carried out for various purposes. In these works it was shown that rare-earth ions isomorphically substituted for Pb2+ ions, while iron group ions could occupy both the sites of Pb2+ and Mo6+ ions and even interstitial sites in the lattice. Possibly, these features are caused both by weak solubility of iron group ions in PbMoO4 lattice and by the fact that the molybdates of these elements have mono- or triclinic symmetry, while scheelites have a tetragonal structure [61]. So, doping with trivalent Bi3+ ions enhances the color of the crystals (Fig. 5a). The band near 23,000 cm−1 is clearly visible in the transmission spectra of doped

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crystals. Irradiation of pure PbMoO4 crystal with UV light leads to enhancement of at least two broad absorption bands: at 23,000 and 17,000 cm−1 . The crystals darken and their transmission is sharply reduced. The crystals doped with Bi do not show a photochromic effect: the transmission spectra do not change after irradiation. Thus, doping with trivalent Bi impurity eliminates the photochromic effect, but intensifies yellow coloration of the crystals. The optical transmission spectra of PbMoO4 :Ba crystals with various concentrations of impurity are shown in Figs. 5b, 6 and 7. It can be seen (Fig. 6) that doping with Ba increases optical transmission in the region of 23,000 cm−1 . Therefore, the color intensity of the PbMoO4 :Ba crystals decreases. At the same time the photochromic effect in PbMoO4 :Ba crystals is still observed (Figs. 5b and 7). However, the magnitude of the photoinduced change of the optical transmission decreases with increasing Ba concentration (Fig. 7). This indicates that Ba2+ impurity prevents the appearance of Pb3+ centers in the lead sublattice. Probably the effect of doping with Ba is due to a decrease of VPb concentration in PbMoO4 :Ba crystal. As it was discussed above, disappearance of VPb lowered the content of Pb3+ centers and enhanced the optical transmission. However, studying the crystals grown from a mixture with excess of PbO did not give such results as BaO doping [26], possibly due to intense evaporation of lead oxide from the melt. Indeed, melting point of PbO is lower than that of PbMoO4 , whereas the melting point of BaO is much higher. In [59] it was shown that doping PbMoO4 crystals with Co led to decrease of the additional absorption in the region of 23,000 cm−1 , caused by high-temperature annealing in air. With the increase of Co concentration the effect was enhanced. Thus, doping with bivalent impurities, such as Ba and Co, reduces the yellowish coloration and enhances optical quality of PbMoO4 crystal.

3.4.2

Photodielectric Effect in PbMoO4 :Bi, Ba

The temperature dependencies of permittivity ε(T) and conductivity σ(T) for pure PbMoO4 crystal and PbMoO4 doped with Bi and Ba are shown in Fig. 8. One can see that doping with Bi3+ completely suppressed photodielectric effect. Taking into account the ratio of the ionic radii and charges, we consider that most Bi3+ ions probably substitute for Pb2+ host ions in the lattice. In this case Bi3+ introduces additional positive charge into lead sublattice. Therefore, doping with Bi3+ decreases the probability of the occurrence of positively charged intrinsic defects such as VO and Pb3+ . Consequently photodielectric effect as well as photochromism become suppressed in PbMoO4 :Bi crystal (see Sect. 3.4.1). At the same time doping with Ba sufficiently increased the magnitude of photodilectric anomaly (Fig. 8) in comparison with a pure crystal. As it was discussed in Sect. 3.4.1 photochromism in PbMoO4 :Ba was significantly reduced. So, doping with Ba has the opposite effects on photodilectric and photochromic effects. This phenomenon is not completely clear and requires further studies.

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Fig. 8 Permittivity ε(T) (a) and conductivity σ(T) (b) dependencies which demonstrate photodilectric effect for: 1—pure PbMoO4 ; 2—PbMoO4 :Bi (0.1 wt%); 3—PbMoO4 :Ba (1 wt%). The crystals were irradiated with UV light for 40 min at T = 290 K. AC field frequency was f = 1 kHz

4 Conclusions The nature of the structural defects was analyzed in PbMoO4 crystals which were grown from the stoichiometric and nonstoichiometric charge compositions, heat treated in air, and irradiated with UV light. It was found that an excess of MoO3 in the charge up to x = 0.01 increased the optical transmission of PbMoO4 crystal in the range 16,000–17,000 cm−1 . Vacancies VMo are filled and the transitions Mo6+ + e– ↔ Mo5+ , caused by the presence of VO , are improbable. An increase of MoO3 excess up to x = 0.03 in the charge is accompanied by the growth of VPb concentration. Accordingly, it stimulates some of Pb2+ ions to change their charge state to Pb3+ and presumably stabilizes Mo5+ centers through increase of VO concentration. Experimentally, the optical transmission of such crystals decreased in the region of 23,000 and 17,000 cm−1 . Heat treatment in air at 1200 K leads to filling in oxygen vacancies, and stimulates oxidation of Mo5+ ions to Mo6+ and Pb2+ to Pb3+ . The optical transmission of the heat-treated crystal sharply decreases in the region of 23,000 cm−1 and increases in the region of 16,000–17,000 cm−1 . Irradiation with UV light leads to a recharge of the cations in the crystal lattice. A reversible photochromic effect was observed. The irradiation decreases the transmission in the region of 23,000 and 17,000 cm−1 . Doping of PbMoO4 crystals with trivalent Bi impurity eliminates photochromism, but does not reduce the coloration of the crystals. The incorporation of a bivalent Ba impurity favors both crystal bleaching and decreasing the photochromic effect. The effect of doping with Ba is presumably connected with decrease of VPb concentration. We plan to find out the optimal amount of Ba impurity to dope and to enhance optical quality of PbMoO4 crystal. Since PbMoO4 and BaMoO4 are the iso-structural crystals, the solubility of the Ba impurity should be high enough.

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UV irradiation of PbMoO4 crystals also leads to the formation of the dipole defects. As a consequence, relaxation anomalies of permittivity are observed. It is suggested that the photodielectric effect can be attributed to (MoO3 )− complexes captured photoelectrons and stabilized by the unknown defects APb in neighboring lead site. In PbMoO4 crystals doped with Bi3+ , the photodielectric effect is absent, and conductivity is significantly reduced. At the same time, in PbMoO4 :Ba photodielectric effect becomes more complicated and is characterized by appearance of additional permittivity peak. To clarify the reasons of this phenomenon, further we plan to study optical and electrical properties of PbMoO4 :Ba crystal heat treated in various atmospheres.

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Effects of Eu3+ and F− Doping on Structure and Optical Properties of Zirconium Oxides V. Chornii, V. Boyko, S. G. Nedilko, V. M. Prokopets, M. Slobodyanik, K. Terebilenko, and V. Sheludko

Abstract A short review of literature as well as new results about structure and optical properties of oxide compounds of zirconium have been presented in this work. Peculiarities of crystal and electronic band structures of some zirconium oxides are analyzed. The effects of dopants concentration, particle sizes and morphology on photoluminescence characteristics have been analyzed and discussed. An influence of europium and fluorine doping on structures and optical properties of zirconium oxides are described in the example of zirconium dioxide (ZrO2 ) compound.

1 Introduction Modern science and technology are focused on the elaboration of environmentfriendly, cost- and energy-effective materials for various devices. A great attention has been paid to oxide crystalline materials due to a variety of their structures and chemical compositions as well as they reveal unique physico-chemical properties. Importantly, the various oxides retain their useful properties under the action of humidity, radiation, high light fluxes, temperatures, etc. Among oxides, the zirconium compounds have both highly practical and scientific values. From the practical point of view, zirconium oxides have been used (or can be used) in jewelry [1], ceramics [2], fuel cells [3, 4], gas sensors [5, 6], optical materials [7–9], catalyst applications [10–12], etc. The scientific interest in various zirconium oxides is related mainly to mechanisms of ionic conductivity [13, 14] and optical processes [15–20] in these compounds. It has been shown in numerous studies that both conductivity and optical V. Chornii (B) · V. Boyko National University of Life and Environmental Sciences of Ukraine, 15 Geroiv Oborony Street, Kyiv 03041, Ukraine V. Chornii · S. G. Nedilko · V. M. Prokopets · M. Slobodyanik · K. Terebilenko Taras Shevchenko National University of Kyiv, 64/13 Volodymyrska Street, Kyiv 01601, Ukraine V. Sheludko Oleksandr Dovzhenko Hlukhiv National Pedagogical University, 24 Kyjevo-Moskovs’ka Street, 41400 Hlukhiv, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_3

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properties of zirconium oxide compounds are sensitive to the content of intrinsic defects, e.g. oxygen vacancies [6, 15, 21–23]. The structural and physical properties of various zirconium oxide compounds can be modified through the substitution in cationic or/and anionic sub-lattice. In particular, their luminescent properties can be improved by doping with rare-earth (RE) ions that occupy cationic sites in structure (e.g. [24–26]). Regarding to substitution in anionic sub-lattice, there are some data on zirconium compounds (mainly ZrO2 films) doped with nitrogen that can be found in the literature [27–29]. At the same time, less attention was paid to the effect of fluorine, sulfur and chlorine on structure and physico-chemical properties of zirconium oxide compounds. The main problem in the studies of anion-substituted compounds is related with identification of content of light elements in the structure. The common features of structures and optical properties of various zirconium oxide compounds have been discussed in this paper. The effect of europium(III) and fluorine doping on crystal and electronic band structure as well as on luminescence properties of these compounds was analyzed on the example of micro/nanocrystalline powders of ZrO2 (zirconia).

2 Synthesis of Zirconium Compounds Due to the perspectives of abovementioned application of simple and complex zirconium oxides, numerous methods have been applied in order to produce these compounds in the form of either single crystals or polycrystalline samples and films. The crystallinity, crystal structure, size, phase and shape of the Zr-based materials play significant roles in properties, luminescent ones as well. The development of facile and efficient synthesis and doping strategies in a highly controlled manner is essential for tailoring chemical and physical properties. Generally, highly crystalline, well-defined and pure-phase or doped zirconium oxides can be obtained by using the conventional methods. The solid-state reaction has been utilized for obtaining ZrP2 O7 , NaZr2 (PO4 )3 , Ca2 ZrSi4 O12 , Ca3 ZrSi2 O9 , SrZrSi2 O7 , CaZrO3 and CaZr(PO4 )2 compounds [8]. The ZrOCl2 8H2 O, H3 PO4 , (NH4 )2 C2 O4 , NH3 ·H2 O, CaCO3 , SrCO3 , ZrO2 , SiO2 were used as raw materials for synthesis of noted compounds. The mixtures of initial ingredients have been calcined step-by-step in air or nitrogen atmospheres and at temperatures from 1273 K (ZrP2 O7 ) to 1673 K (Ca3 ZrSi2 O9 and CaZrO3 ). The Ca4 ZrGe3 O12 garnet can be obtained by solid-state reaction from CaCO3 , ZrO2 and GeO2 precursors in Pt crucibles by heating up to 1673 K [7]. This method also has been used for the synthesis of La2 Zr2 O7 and La2−x Yx Zr2 O7 , SrZrO3 , Sr4 Zr3 O10 , Sr3 Zr2 O7 and Sr2 ZrO4 zirconates from La2 O3 , SrCO3 , ZrO2 , and Y-stabilized zirconia (13 w/o Y2 O3 ) precursors by heating to 1723 K in the air [13]. The high-temperature solid-state reaction method has been used for the synthesis of some mixed-anion compounds of zirconium, e.g. Ca8 ZrMg(PO4 )6 (SiO4 ) [26], Zr2 (WO4 )(PO4 )2 [30] or Zr2 (MoO4 )(PO4 )2 [31]. The main advantage of this method for the synthesis of

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various zirconium oxide compounds is related with possibility to keep stoichiometry in system and can be used for doping of oxides in cationic and anionic sub-lattices. However, this method requires heating to high temperatures, and the synthesis process takes a lot of time (up to several days). The crystallization from melt (flux method) has been applied for the synthesis of ZrP2 O7 and KZr2 (PO4 )3 compounds [16]. The raw materials were (NH4 )H2 PO4 , Li2 CO3 , K2 CO3 , ZrO2 and ZrF4 . The cooling has been performed in 1373–900 K temperature range. The K2 BiZr(PO4 )3 compound has been obtained by this method from KPO3 , K4 P2 O7 , Bi2 O3 and ZrF4 precursors by cooling melted salts in 1173– 853 K range at a rate of 25 K/h [18]. Previously, flux method has been used for obtaining NASICON-type zirconium oxides with exceptional electric properties, e.g. Na5 Zr(PO4 )3 [32] and Na3 Zr2 Si2 PO12 compounds [33]. The pros and cons of this method are similar to those for solid-state technique except that requires additional study for melt composition estimation, but the annealing procedure takes less time and yields single crystals. Various modifications of the hydrothermal method have been intensively used for producing simple and complex zirconium oxides. In particular, this solution route has been used for the synthesis of La- and Sr-containing zirconates from La(C2 H3 O2 )3 ·5H2 O, ZrOCl2 ·8H2 O and Sr(NO3 )2 precursors with calcination at 873 K [13]. The wet synthesis hydrothermal method with annealing at 1273 K has been used to elaborate unpurified BaZrO3 with 0.5 mol% of different rareearth ions (RE = Yb, Er, Dy, Eu, Ce) [25]. The hydrothermal synthesis in alkaline media was utilized for the set of zirconium phosphates and arsenates of general formula Zr2 O3 MI MV O4 ) nH2 O (where MI = H, Na, K, NH4 ; MV = P, As, n = 1–3) [34]. The NASICON-type compound of zirconium, e.g. Na4 Zr2 Si3 O12 [35] and Na4 Zr2 (SiO4 )3 [36] also can be obtained by this method. Importantly, hydrothermal method has been considered as very cost-effective for producing oxide nanoparticles. Hydrothermal alkali leaching of commercial ZrO2 at T = 397 K has been used for the synthesis of zirconia nanopowders with grain size in 24–36 nm range [37]. In particular, the hydrothermal and microwave-driven hydrothermal methods have been applied in work [6] for obtaining of nanocrystalline zirconia from ZrOCl2 — NH4 OH and ZrOCl·8H2 O—NaOH solutions, respectively. In case of hydrothermal method a mixture of monoclinic/tetragonal zirconia nanoparticles with average grain size near 20 nm has been obtained after annealing at T = 873 K [6]. Typical size of nanoparticles synthesized by the second method after annealing at 1023 K was about 50.4 nm. The crystallite sizes for zirconia obtained by microwave-assisted hydrothermal synthesis were about 12 and 25 nm for calcining at 873 and 1273 K, respectively [38]. One of the main advantages of hydrothermal method is related with low temperature of reaction (near 400–500 K). However, for obtained powders annealing at high temperatures is usually required in order to obtain fine powders, in particular nanocrystalline ones. The sol–gel has been used for the synthesis of series of mixed phosphates M+ Zr2 (TO4 )x (PO4 )3−x (T–As, V; M+ —alkali metal) and M+ 1−x Zr2 (TO4 )x (PO4 )3−x (T–S, Mo) from aqueous solution at 363 K with further thermal processing in the temperature range 873–1173 K [39]. The set of La and Sr zirconates was

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synthesized by Pechini sol–gel technique at 423 K from C2 H6 O2 , C6 H8 O7 , SrCO3 , La(C2 H3 O2 )3 ·5H2 O, and ZrOCl2 ·8H2 O precursors [13]. The sol–gel synthesis of micro-powders and nanorods of Cs doped ZrV2 O7 was described in [40]. The paper [41] reports the preparation of metastable tetragonal ZrO2 nanoparticles and nanosheets with citric acid-assisted sol–gel method. Pure monoclinic zirconia nanopowders were synthesized via a simple, fast and low cost polyacrylamide gel method [42]. Zirconium oxide thin films in work [43] were prepared by the sol– gel dip-coating technique with annealing at different temperatures in air atmosphere. The synthesis of zirconia nanoparticles by microwave-assisted sol–gel technique was reported in [44]. In general, sol–gel and hydrothermal methods are the most commonly used routes for producing various nanoparticles including simple and complex zirconium oxide ones. The solid-state synthesis and flux methods have been used usually for the preparation of microcrystalline powders, which can be used as starting materials for further processing. At the same time, the solid-state route can be effectively used for substitution in cationic and anionic sub-lattices of various oxide compounds. In the present work, the solid-state technique was applied for the synthesis of a series of pure and doped zirconia samples: pure ZrO2 , ZrO2 :8%F, ZrO2 :0.5%Eu, and ZrO2 :0.5%Eu/8%F. The doped with Europium(III) and fluorine nanocrystalline powders of zirconia were prepared via a solid-state route using analytically pure zirconyl nitrate ZrO(NO3 )2 × 2H2 O, ZrF4 and Eu2 O3 as the precursors. Several procedures were applied to study the influence of Zr/F ratios on calcination temperature and morphology providing a set of solid solutions with formulae ZrO2 :0.005Eu3+ /xF− (x = 0, 0.02, 0.04, 0.08 and 0.10). To prepare fine powders and to get rid of gaseous co-products, the mixtures of initial reagents were preheated at 583 K for 4 h and afterward calcined at temperatures of 923, 1023, 1123, 1373 K for 20 h for each annealing temperature. The samples of ZrO2 , ZrO2 :8%F and ZrO2 :8%F/2%Y have been synthesized in the following way. The stoichiometric mixture of ZrO(NO3 )2 × 2H2 O, Y2 O3 and ZrF4 has been calcinated at 723 K to decompose the first ingredient and then heated step-by-step at 973, 1073, 1173 and 1273 K for 6 h at each temperature with intermediated regrinding. The final products have been obtained as white polycrystalline powders with a grain size about 50–150 nm.

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3 Some Aspects of Crystal Structure of Zirconium Oxides The only one crystalline phase was reported so far for the majority of Zr-containing oxide compounds. In case of the simplest and the most studied zirconium oxide compound, ZrO2 , the crystallization can be performed into three polymorphs— monoclinic, tetragonal or in cubic forms, depending on synthesis conditions, crystallite size and content of dopants. It is known the monoclinic zirconia is the stable phase when tetragonal and cubic ones require stabilization by dopants. The common features of various zirconium oxide compounds are the ZrOx polyhedra (where x ranges typically from 6 to 8). The characteristics of ZrOx polyhedra for some zirconium oxides are shown in Table 1. The data on characteristics of these structural components, namely, symmetry of Zr4+ position, number and length of Zr–O bonds, are very important from viewpoint of optical properties of zirconium compounds both pure and doped with various ions. In particular, the incorporation of rare-earth (RE) ions in zirconium oxide compounds often takes place in Zr cationic site. Thus, the parameters of ZrOx polyhedra should be similar to the parameters of EuZr Ox polyhedra in corresponding compounds. As it can be seen from Table 1 that the majority of zirconium oxide compounds has ZrO6 polyhedra with average Zr–O bond length about 2.0–2.2 Å. These values are somewhat smaller than Eu–O distances in coordination number 6 (e.g. Eu–O interatomic distance of 2.27 A was reported in [55] for [Eu(DDPA)6 ](ClO4 )3 ). It is worth noting that coordination number 6 is not typical for Eu3+ ions. The symmetry of Zr position is low in case of mixed-anion compounds but is high for zirconates (CaZrO3 and BaZrO3 ). In general, the substitution of Zr4+ cation by Eu3+ ions in zirconium oxide compounds should lower the symmetry of the corresponding position through elongation of EuZr -O distances. Requirements on charge compensation promote the creation of oxygen vacancies in the nearest surrounding of Eu3+ impurity that should affect optical properties of materials. At the same time, charge balance can be kept when zirconium oxide compounds are co-doped with europium and fluorine, if the latter one occupies oxygen sites in structure. The most interesting case from viewpoint of crystal structure changes during suchlike doping can be observed for ZrO2 . It is known, that stabilization processes of tetragonal and cubic phase of zirconia are related with oxygen vacancies. The amount of these vacancies can be increased by aliovalent substitution of Zr4+ with Y3+ , Ca2+ , Mg2+ etc. [56, 57]. In our previous paper [58], it was shown that monoclinic zirconia partially transformed in cubic phase when co-doped with 0.5 mol% of Eu3+ and x = 2 mol% of fluorine (see Fig. 1). The relative content of high-symmetry phase increases with an increase in fluorine concentration at same Eu3+ concentration. The doping with only fluorine in the range up to 10 mol% does not provide stabilization of high-symmetry zirconia (see Fig. 2). At the same time, doping with only 0.5 mol% Eu3+ leads to partial stabilization of zirconia. Thus, fluorine promotes structure transformation in ZrO2 only when additionally doped with Eu3+ .

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Table 1 Some parameters of the structure of zirconium oxides Compound

Space group

ZrOx

Zr site symmetry

Zr–O bond length (Å)

References

ZrO2 (monoclinic)

P 21 /c (#14)

ZrO7

C1

1.88, 1.91, 2.13(×2), 2.34, 2.36

[45]

ZrO2 (tetragonal)

P 42 /n m c (#137)

ZrO8

D2d

2.09(×4), 2.38(×4)

[46]

ZrO2 (cubic)

F m -3 m (#225)

ZrO8

Oh

2.20(×8)

[47]

ZrP2 O7

P a -3 (#205) ZrO6

S6

1.97(×6)

[48]

KZr2 (PO4 )3

R -3 c (#167) ZrO6

C3

2.06(×3), 2.07(×3)

[49]

K2 BiZr(PO4 )3

P 21 3, (#198)

I-ZrO6 II-ZrO6

C3 C3

2.16(×3), 2.17(×3) 2.16(×3), 2.23(×3)

[18]

CaZrO3

P m -3 m (#221)

ZrO6

Oh

2.01(×6)

[50]

BaZrO3

P m -3 m (#221)

ZrO6

Oh

2.10(×6)

[51]

NaZr2 (PO4 )3

R -3 c (#167) ZrO6

C3

2.04(×3), 2.08(×3)

[52]

Na4 Zr2 (SiO4 )3

R -3 c (#167) ZrO6

C3

1.87(×3), 2.11(×3)

[53]

Na3 Zr2 (SiO4 )2 (PO4 )

C 2/c (# 15)

ZrO6

C1

2.01, 2.02, 2.06, 2.10, 2.15 (×2)

[54]

Zr2 (WO4 )(PO4 )2

P n c a (#60) ZrO6

C1

2.04 (×2), 2.05, 2.06, 2.15, 2.19

[30]

Zr2 (MoO4 )(PO4 )2

P n c a (#60) ZrO6

C1

2.01, 2.03(× [31] 2), 2.04, 2.13, 2.16

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Fig. 1 XRD patterns of ZrO2 (1) and ZrO2 :0.005Eu3+ /xF− (x = 0.02 (2), 0.04 (3), 0.08 (4) and 0.10 (5)). Diagrams for standard tetragonal (JCPDS No. 88–1007), cubic (JCPDS No. 82–1246) and monoclinic (JCPDS No. 37–1484) polymorphs of zirconia are shown at the top and at the bottom parts of the figure (Reproduced from [58])

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Fig. 2 XRD patterns of ZrO2 (1), ZrO2 :0.10F− (2) and ZrO2 :0.005Eu3+ (3) nanopowders

Effects of Eu3+ and F− Doping on Structure and Optical Properties …

39

4 Electronic Band Structure Calculations of electronic band structure are considered as a valuable instrument for the explanation of electronic processes in various compounds. The calculations within the framework of density functional theory (DFT) were applied for various oxide materials, in particular zirconium-containing ones [16, 59–62]. It was found the top part of the valence band (VB) in simple and complex zirconium oxide compounds formed mainly by oxygen p states. The bottom of conduction band (CB) is formed mainly by Zr d states (Fig. 3). The exceptions are related with compounds those contain luminescent ions in the structure, e.g. K2 BiZr(PO4 )3 [18]. In the case of later compound, the states of bismuth are present at the top of VB (Bi s states) and at the bottom of CB (Bi p states). Thus, for the majority of zirconium oxide compounds, partial densities of electronic states (PDOS) related with ZrOx polyhedra play the main role in optical processes in energy region of band-to-band transitions. According to generally accepted terminology, light absorption in ZrOx polyhedra for energies

Fig. 3 Calculated PDOS of ZrP2 O7 and KZr2 (PO4 )3 crystals (Reproduced from [16])

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Fig. 4 Partial densities of states for oxygen vacancies containing ZrP2 O7 crystal: top part corresponds to vacancy in Zr–O–P bond, bottom part—vacancy in P–O–P bond (Reproduced from [21])

above band gap values realized through electron transition from O p states on Zr d states and has charge-transfer nature. It was marked above that Zr in zirconia has a higher coordination number than in most of other zirconium oxides. However, band-to-band absorption and emission processes in ZrO2 also have charge transfer character O2− → Zr4+ . So, the zirconia can be used as model object for the study of effects of europium and fluorine doping on physical properties of complex zirconium oxides, like phosphates and silicates. Importantly, oxygen vacancies are forming additional levels inside material band gap. The positions of these defect levels were studied by theoretical calculations in various approaches for all three polymorphs of zirconia [61, 63, 64]. The vacancy states are formed as a symmetric superposition of, in particular, the 4d orbitals centered at the four Zr atoms surrounding the vacancy [63]. It should be noted that for a more complex system, vacancy states are formed by superposition of states of all neighboring atoms (see Fig. 4). The peculiarity of ZrP2 O7 crystals is the presence of “bridge” oxygen that connects two PO4 groups. It is seen from Fig. 4, the states of two types of oxygen vacancies have a small difference in their position in respect to the top of valence band. However, these defect levels strongly differ in bandwidth and in relative contribution of O, P and Zr states. Thus, ZrO2 can be used as model of zirconium oxide compounds but some aspects in electronic structure may appear only when appropriate crystal structures have been used. The results of computational studies of nitrogen and fluorine impurity in zirconia have been reported in [65]. In case of nitrogen substation in anionic sub-lattice, the N 2p states appear near the top of valence band of material. The bottom of conduction band formed by Zr 4d states as in perfect zirconia with significant changes in PDOS with an increase in nitrogen impurity content. Somewhat different situation was observed in case of zirconia with fluorine impurity. The states of fluorine were located deep in VB at energy region 10 eV below Fermi level [65]. While ZrO2 :N remains insulator with Fermi level, EF , at the top

Effects of Eu3+ and F− Doping on Structure and Optical Properties …

41

of VB the ZrO2 :F have Fermi level located at the bottom of CB. The total energy calculations of Zr4 O7 F in monoclinic and tetragonal structures also suggest that the monoclinic phase is a true ground state as its energy is found to be ∼1634 meV less than that of the tetragonal structure. In comparison to this energy difference with that of ZrO2 and Zr4 O7 N, it can be admitted that the F doping is providing better stability to the monoclinic structure, which is a true ground state structure for all the concentrations studied in [65]. The fluorine impurity leads to appearance of defect level at a very bottom of CB of zirconia that occupied by Zr 4d states. At the same time, fluorine states are absent in this region that means fluorine does not participate directly in optical processes in ZrO2 :F compound. Figure 5 shows the calculated PDOS for monoclinic zirconia in supercell approximation for case of ideal crystal and crystal “co-doped” with Y/F and Eu/F (selected

2

ZrO2 Zr d Op

1

0

PDOSes, e/eV per atom

2

YZr7O15F

Zr d Yd Op Fp

1

0 2

Zr d Eu d Eu f Op Fp

EuZr7O15F

1

0

-6

-4

-2

0

2

4

Energy, eV Fig. 5 Partial densities of states for pure, Y/F and Eu/F co-doped monoclinic zirconia

6

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impurity concentrations roughly correspond to 12.5 mol% of Y or Eu and 6 mol% of F). Calculated PDOS of monoclinic zirconia is similar to the reported in the literature. The samples with double impurities have defect band near bottom of CB (marked by arrow in Fig. 5) that is similar to ZrO2 :F case described above. This band is formed by d states of zirconium atoms. Due to charge compensation by Y + F impurities adding the Fermi level remains at top of VB. It is seen from Fig. 5 that states of yttrium are almost absent near band edges of material. So, yttrium does not take part in optical processes of ZrO2 :Y, F compound. Any significant PDOS of fluorine in YZr7 O15 F can be found only at 0.5 eV below Fermi level and they dominate at energies 4 eV below EF . Thus, fluorine also should have no direct participation in optical processes with photon energies near band gap values. In case of zirconia co-doping with Eu3+ and F− impurities, the Fermi level moves to f states of Eu, those are located within band gap of zirconia. The O p and F p states have very small PDOS within material band gap where f states of europium have the highest PDOS. This result probably indicates small interaction of europium with near ligands. Except for europium f states, the rest of PDOS picture for EuZr7 O15 F is very similar to that for YZr7 O15 F case. Calculations had shown that fluorine impurity causes changes in the local structure of crystal. In particular, at the start of geometry optimization for YZr7 O15 F structure, a fluorine impurity had two zirconium atoms at distances 1.866 and 2.288 Å. These distances after geometry optimization changed to 2.164 and 2.166 Å, respectively. At the same time, distance from F to closest Y increased from 2.125 to 2.315 Å. Thus, it is likely that fluorine dopant has no direct influence on optical properties of fluorinated zirconia but rather acts through changes in structural elements, e.g. elongation of some Zr–O bonds and shortening other ones in ZrOx polyhedra.

5 Effect of Dopants on Luminescence Properties of Zirconia The luminescence of un-doped zirconium oxide compounds was observed at PL excitation at region of band-to-band absorption as well as for excitation into defect levels located in band gap. In particular, for ZrP2 O7 and KZr2 (PO4 )3 phosphates, an excitation with λex < 185 nm corresponds to charge transfer processes while at longer wavelengths, a defect-related luminescence excitation takes place. As Fig. 6 shows, the spectra considerably depend on λex . In case of the band-to-band excitation wide, the PL emission band with maxima near 310 nm was observed for both compounds. This band was ascribed to charge transfer processes Zr → O for these and some other zirconium phosphates and silicates [8, 16, 18]. The excitation with photon energy below Eg value had shown the presence of several bands in visible spectral range, those can be ascribed to intrinsic defects of zirconium phosphates. The intensity of these bands is considerably smaller than the intensity of the main UV band of the compounds mentioned above.

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Fig. 6 PL emission spectra of ZrP2 O7 and KZr2 (PO4 )3 crystals measured at T = 8 K (Reproduced from [16])

Complex luminescence bands are observed at low as well as at room temperature in case of “deep” defect levels excitation for ZrP2 O7 and KZr2 (PO4 )3 polycrystalline samples (Fig. 7). Both the intensity of the bands and their shape depends on temperature of the sample. Similar photoluminescence properties are inherent to un-doped zirconia [23, 57]. Concerning RE-doped zirconium oxide compounds, they were numerously studied, especially for the case of ZrO2 :RE nanocrystals [24–26, 66–68]. In our recent studies, [58] had been shown that fluorine doping improves luminescence properties of ZrO2 :Eu nanocrystals. In particular, the most intensive room temperature luminescence of Eu3+ ions in ZrO2 :0.005Eu was observed for doping with 8 mol% F. The mechanism of such increase of Eu3+ luminescence intensity was not clarified so far. In the mentioned work, the assumption was made that fluorine impurity affects some centers of intrinsic luminescence of zirconia that absorb light near 393 nm. Photoluminescence emission of these centers related with wideband in blue-green spectral region (maximum near 480 nm). It is commonly accepted that the visible luminescence of zirconia related to oxygen vacancies. According to literature data on theoretical calculations of oxygen vacancy-related level positions in band structure of zirconia and experimental spectroscopic studies, the PL band at

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Fig. 7 Photoluminescence spectra of un-doped zirconium phosphates obtained for λex = 337.1 nm

this spectral region can be ascribed to F+ and F0 centers (oxygen vacancy trapped one or two electrons) [17, 69, 70]. Taking into account the close packing of zirconia, it is reasonable to assume that fluorine should occupy oxygen position in structure. Requirements of charge balance lead to lowering of probability of oxygen vacancy creation. Thus, fluorination should lead to a decrease in oxygen vacancy content in zirconia. As a result, the quantity of some centers of intrinsic luminescence should decrease with an increase in fluorine content. The luminescence studies of fluorine doped zirconia were performed

Effects of Eu3+ and F− Doping on Structure and Optical Properties …

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Fig. 8 Photoluminescence spectra of pure and fluorine-doped zirconia obtained at λex = 393 nm (a) and 450 nm (b), T = 77 K

at low temperatures for case of excitation energy below zirconia band gap value (e.g. monoclinic phase Eg = 5.42 eV [63]). It was found that at room temperature, ZrO2 and ZrO2 :F samples reveal very weak luminescence that hard to register. At liquid nitrogen temperature, wide complex band of zirconia intrinsic luminescence is clearly observed (Fig. 8). The spectra in Fig. 8 are normalized for easier comparison of the band profiles. In case of luminescence excitation at 393 nm, there is clear evidence that fluorine impurity suppresses short-wavelength luminescence of zirconia at 440 nm (component around 2.8 eV). The spectra of both samples have a similar profile in 580–720 nm spectral range. The spectral profile does not change when PL of doped with fluorine zirconia is excited at 450 nm (deep defects excitation). So, fluorine has little effect on luminescence centers that correspond to PL component at 560 nm (2.2 eV). PL excitation spectra for green luminescence of these samples are shown in Fig. 9 for real intensities. It is seen that complex PL excitation band in 350–450 nm range retains its profile but the intensity of this band decreases after fluorine doping. So, doping with fluorine leads to suppression of intrinsic luminescence of zirconia in 425–600 nm spectral range. Such substitution in anionic sub-lattice can be

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Fig. 9 PL excitation spectra of pure and fluorine-doped zirconia obtained at λem = 500 nm, T = 77 K

performed for other zirconium-containing oxide compounds in order to improve their optical properties.

6 Conclusions The simple and complex zirconium oxide compounds possess good electrical and optical properties that determined mainly by processes in ZrOx polyhedra. Various procedures can be applied for producing these compounds such as solid-state technique, melt crystallization, hydrothermal and sol–gel methods. The latter two methods are commonly used for the synthesis of nanoparticles. The nanoparticles of pure, fluorine-doped and fluorine/europium co-doped zirconia were synthesized by solid-state technique and studied in this paper. It was found that fluorine promotes the stabilization of high-symmetry phases of zirconia when doped with europium. At the same time, zirconia retains a monoclinic structure when doped only with fluorine.

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According to geometry optimized calculation, a fluorine impurity causes changes in O–Zr bonds length. No states of fluorine are near band edges, so this dopant should have only an indirect influence on optical processes in the zirconia host. Doping of zirconia with fluorine leads to suppression of short-wavelength intrinsic luminescence that probably occurs through changing of oxygen vacancies’ content in the zirconia lattice structure.

References 1. Shigley JE (2012) Characterization of colorless coated cubic zirconia (Diamantine). Gems Gemol 48:18–30. https://doi.org/10.5741/GEMS.48.1.18 2. Manicone PF, Iommetti PR, Raffaelli L (2007) An overview of zirconia ceramics: basic properties and clinical applications. J Dent 35(11):819–826. https://doi.org/10.1016/j.jdent.2007. 07.008 3. Bae K, Jang DY, Choi HJ et al (2017) Demonstrating the potential of yttrium-doped barium zirconate electrolyte for high-performance fuel cells. Nat Commun 8:14553. https://doi.org/ 10.1038/ncomms14553 4. Rashid NLRM, Samat AA, Jais AA et al (2019) Review on zirconate-cerate-based electrolytes for proton-conducting solid oxide fuel cell. Ceram Int 45(6):6605–6615. https://doi.org/10. 1016/j.ceramint.2019.01.045 5. Can ZY, Narita H, Mizusaki J et al (1995) Detection of carbon monoxide by using zirconia oxygen sensor. Solid State Ionics 79:344–348. https://doi.org/10.1016/0167-2738(95)00085-K 6. Fidelus JD, Łojkowski W, Millers D et al (2007) Zirconia based nanomaterials for oxygen sensors–generation, characterisation and optical properties. Solid State Phenom 128:141–150. https://doi.org/10.4028/www.scientific.net/SSP.128.141 7. Blasse G, de Blank J, IJdo DJW (1995) The luminescence of the garnet Ca4ZrGe3O12. Mater Res Bull 30(7):845–850. https://doi.org/10.1016/0025-5408(95)00068-2 8. Kaneyoshi M (2006) Luminescence of some zirconium-containing compounds under vacuum ultraviolet excitation. J Lumin 121:102–108. https://doi.org/10.1016/j.jlumin.2005.09.017 9. Zhang H, Kim BN, Morita K et al (2011) Optical properties and microstructure of nanocrystalline cubic zirconia prepared by high-pressure spark plasma sintering. J Am Ceram Soc 94:2981–2986. https://doi.org/10.1111/j.1551-2916.2011.04477.x 10. Mercera PDL, Van Ommen JG, Doesburg EBM et al (1990) Zirconia as a support for catalysts: evolution of the texture and structure on calcination in air. Appl Catal 57:127–148. https://doi. org/10.1016/S0166-9834(00)80728-9 11. Xiao H, Liu S (2018) Zirconium phosphate (ZrP)-based functional materials: synthesis, properties and applications. Mater Des 155:19–35. https://doi.org/10.1016/j.matdes.2018. 05.041 12. Ishii T, Anzai A, Yamamoto A et al (2020) Calcium zirconate photocatalyst and silver cocatalyst for reduction of carbon dioxide with water. Appl Catal B 277:119192. https://doi.org/10.1016/ j.apcatb.2020.119192 13. Poulsen FW, van der Puil N (1992) Phase relations and conductivity of Sr-and La-zirconates. Solid State Ionics 53:777–783. https://doi.org/10.1016/0167-2738(92)90254-M 14. Yamazaki Y, Hernandez-Sanchez R, Haile SM (2010) Cation non-stoichiometry in yttriumdoped barium zirconate: phase behavior, microstructure, and proton conductivity. J Mater Chem 20:8158–8166. https://doi.org/10.1039/C0JM02013C 15. McCauley RA, Hummel FA (1973) Luminescence as an indication of distortion in A3+ 2 B4+ 2 O7 type pyrochlores. J Lumin 6(2):105–115. https://doi.org/10.1016/0022-2313(73)90046-X 16. Hizhnyi Y, Chornii V, Nedilko S et al (2013) Luminescence spectroscopy and electronic structure of ZrP2 O7 and KZr2 (PO4 )3 crystals. Radiat Meas 56:397–401. https://doi.org/10.1016/j. radmeas.2013.01.068

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Electric and Spectral Properties of Solid Water-Nanocellulose Systems in a Wide Range of Temperatures M. M. Lazarenko, S. G. Nedilko, S. A. Alekseev, S. Yu. Tkachov, D. O. Shevtsov, V. P. Scherbatskyi, V. A. Barbash, K. S. Yablochkova, M. V. Ushcats, V. I. Kovalchuk, D. A. Andrusenko, D. Izvorska, R. V. Dinzhos, and O. M. Alekseev Abstract We report gravimetric, electric, and spectral properties of the solid-state water-nanocellulose systems with different water content. The systems with the nanocellulose content of 0.15, 0.3, 0.6, 96.6, and 97.3% undergo dielectric relaxation at temperatures between −100 and 0 °C, whose nature differs from that of the dielectric relaxation in pure water. The relaxation process in water-nanocellulose systems was found to be due to the dipole thermal polarization. This process is determined by the behavior of the surface layers of the nanocrystals, surrounded by the hydrated shell; its mechanism is due to the conformational motion of the methylol groups on the surface of the nanocellulose crystals. M. M. Lazarenko · S. G. Nedilko · S. A. Alekseev · S. Yu. Tkachov · D. O. Shevtsov · V. P. Scherbatskyi · K. S. Yablochkova · V. I. Kovalchuk · D. A. Andrusenko · O. M. Alekseev Taras Shevchenko National University of Kyiv, Kyiv, Ukraine e-mail: [email protected] S. A. Alekseev e-mail: [email protected] D. A. Andrusenko e-mail: [email protected] O. M. Alekseev e-mail: [email protected] V. A. Barbash (B) National Technical University of Ukraine “Igor Sikorsky Kyiv Polytechnic Institute”, Kyiv, Ukraine e-mail: [email protected] M. V. Ushcats Admiral Makarov National University of Shipbuilding, Mykolayiv, Ukraine e-mail: [email protected] D. Izvorska Technical University of Gabrovo, Gabrovo, Bulgaria R. V. Dinzhos V.O. Sukhomlynskyi Mykolaiv National University, Nikolaev, Ukraine Institute of Engineering Thermophysics of NAS of Ukraine, Kyiv, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_4

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1 Introduction Composite materials, whose properties can be modified by adjusting the concentration of their components, enjoy wide recognition and find their applications in a variety of fields [1–4]. For instance, nanocellulose, a nano-sized additive derived from many natural sources [5–9], can significantly strengthen the mechanical properties of polymer composites [10] and make them biodegradable [11]. Nanocellulose is truly versatile: it is widely used in food production [12], fabrication of packing film [13], as means for drug delivery [14], in printed electronics [15], energy devices [16], and many other spheres [17]. The use of nanocellulose in all these products is due to its properties, which are determined, in particular, by its moisture content. It is no surprise then that the impact of moisture content on the physical properties of nanocellulose draws significant attention [18–21]. The systems under consideration include both nanocellulose hydrogels [22–24] and aqueous suspensions of nanocellulose [25]. The most obvious source of cellulose is, of course, plant matter. Plants’ cell walls contain structures called native microfibrils. A microfibril is covered by an external hydrophobic lignin shell and contains cellulose nanofibrils, separated by layers of hemicellulose. The degree of the polymerization of cellulose molecules in this structure is between n = 800 and 3000, and it varies for different plants. The transverse size of a native microfibril is between 20 and 40 nm, and the size of a nanofibril is between 3 and 6 nm. According to [9, 26], cellulose molecules in nanofibrils form linear sequences of mesomorphic and crystalline domains with a surface para-crystalline layer. The length of such crystalline domains is usually 50–100 nm. The mesomorphic domains are less ordered than the crystalline ones. To extract nanocellulose matter from the plant source, a sequence of steps must be performed. Nanocellulose extraction from the raw plant material begins with its acid or alkaline delignification (step 1). This step destroys the lignin shells of native microfibrils and hemicellulose layers, allowing for the lateral contacts between the nanofibrils and their microcrystallization, thus creating microfibrils. Next, weak acid hydrolysis (step 2) destroys molecular segments in mesomorphic domains, which, unlike the crystalline ones, are accessible to water and reagents. This, in turn, leads to the decrease in the polymerization degree of the cellulose molecules down to the so-called “level off” degree of polymerization, which is determined by the length of the crystalline domains in nanofibrils that become cellulose nanocrystals. These nanocrystals are cylindrical, with sharpened ends (whiskers), the aspect ratio of which depends on the cellulose source origin [9]. The transverse cross-section of the whiskers can be either hexagonal or rectangular, based on the geometry of the rosette terminal complex of the cell acting as the source of cellulose [9]. The formed nanocrystals are prone to aggregation in a liquid environment. The degree of such aggregation is determined by the chemical composition of the nanocrystal’s lateral surface. Nanocrystals least prone to aggregation have a negative charge of the surface SO3 H groups [9], which form after sulfuric acid is used in step 2 of cellulose preparation. In the final step

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of the nanocellulose preparation (step 3), water suspension of cellulose nanocrystals is formed after continuous rinsing and filtration using a centrifuge and ultrasound dispersion. The structure and the rheological properties of the resulting suspension are determined by the temperature and concentration of the cellulose nanocrystals in it. The presence of lyotropic liquid crystal phases in these systems has drawn significant attention to their rheological, optical, and structural properties in the liquid state [27–30]. However, the data on physical properties—electric properties in particular—of water-nanocellulose systems in the solid state are very sparse: the existing research focusing, mostly, on the properties of nanocellulose films with low moisture content. A significant feature of a solid water-nanocellulose structure is its special inhomogeneity due to aggregation and/or co-crystallization of nanocrystals formed when the water suspensions freeze or vaporize. The formed ion impurities concentrate in the inter-crystalline layers of ice or cellulose nanocrystal at different steps of sample preparation. Additionally, certain areas in the interface surfaces of the nanocrystals exhibit a variety of structural defects in the resulting polycrystals. Both nanosized spatial inhomogeneity and defects may significantly influence the phase transitions and relaxation process. It is thus especially important to study the solid-state water-nanocellulose properties in the vicinity of the phase transition temperatures. Here, we describe the electric and spectral properties of the solid-state waternanocellulose systems with different cellulose content. The analysis of these properties in a wide temperature range provides an important insight into the molecular mechanisms of relaxation processes in such systems.

2 Experiment 2.1 The Choice of Samples and Their Preparation Nanocellulose was obtained by the hydrolysis of the cellulose prepared by the organosolv of Miscanthus x giganteus stems as described in [31–33]. Hydrolysis of the never-dried organosolvent Miscanthus pulp (OMP) was carried out by the solution of sulfuric acid with a concentration of 43%, at the liquid to solid ratio 10:1, at temperature 60 °C for 60 min. The calculated amount of sulfate acid with the corresponding concentration was slowly added into the flask with the OMP suspension. Upon expiration of the reaction time, the hydrolysis was stopped by tenfold dilution with the distilled water and the subsequent cooling of the suspension to room temperature. The hydrolyzed nanocellulose was rinsed three times with distilled water in a centrifuge at 4000 rev/min with subsequent dialysis until a neutral pH was reached. The samples of nanocellulose (NC) with a 0.6% concentration were obtained by their ultrasonic treatment in an ultrasound disintegrator UZDN-A (SELMI, Ukraine) at 22 kHz for 60 min. The nanocellulose suspension was placed in an ice bath to prevent

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overheating during treatment. The 1.5 g of NC/H2 O per 1 l of water (0.15NC/H2 O), 3 g/l NC (0.3NC/H2 O), and 6 g/l NC (0.6NC/H2 O) solutions were prepared. The NC films with a moisture content of 10.2% (89.8NC/H2 O), 3.4% (96.6NC/H2 O), and 2.7% (97.3NC/H2 O) were obtained by drying the 0.6NC/H2 O solution at 50 °C. Once dried, the films tend to absorb moisture from the surrounding air. Thus, the 90NC/H2 O sample was left exposed to the air for 24 h. The 96.6NC/H2 O was obtained by treating the 89.8NC/H2 O sample in a vacuum press-form at 110 °C and subsequently encased in a polyethylene capsule. Encapsulation of the sample prevented the adsorption of moisture from the surrounding air and maintained its constant moisture content throughout the experiment. The 97.3NC/H2 O was obtained from a dried film (not exposed to air for a prolonged time) in a vacuum press-form at 110 °C and subsequently encased in a polyethylene capsule as well.

2.2 Methods The moisture content in the samples was measured by a gravimetric method, as described in [34], using Derivatograf Q-1500D. The dried NC films’ moisture content was measured using AND MX-50 moisture analyzer. Nicolet-Nexus spectrometer was used to obtain IR spectra in the wavenumber range from 700 to 4000 cm−1 , allowing to control the moisture content of the encapsulated films of NC. The data for the specific electric conductivity of the samples was obtained with the help of a three-electrode capillary cell and an AC bridge P5083, which was used to measure the resistance between electrodes. The specific conductivity was then calculated using resistances measured at frequency 50 kHz at temperatures between 20 and 60 °C. [35]. The dielectric properties were studied in the 1–50 kHz frequency range and − 196–100 °C temperature range using an automated installation based on a P5083 AC bridge and a four-electrode thermostatted cell, with an option of a sample thickness control. The samples were placed into enclosed polyethylene capsules, which allowed us to perform measurements in a temperature interval that includes phase transition temperatures [36]. Luminescent characteristics were measured using a DFS-12 spectral monochromator equipped with two grates (1200 grooves/mm; linear dispersion 1 mm/nm) and FEU-79 photomultiplier. A diode pumping laser with a radiation wavelength of λex = 405 nm was used as a photoluminescence excitation source. All the PL spectra were corrected for the spectral response of the registering system.

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3 Results and Discussion 3.1 Moisture Content in NC Films The moisture content in the 89.8NC/H2 O sample was estimated by the gravimetric method and found to be 10.2%. Since the 96.6NC/H2 O and 97.3NC/H2 O samples were encapsulated, their moisture content could not be measured by the same method, as the temperature range for the water vaporization overlaps with the polyethylene melting temperature range. Accordingly, IR spectra were used to determine moisture content in these samples (Fig. 1). In particular, we compared the relative intensities of the 1640 cm−1 band in 89.8NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples. This band corresponds to the bending mode of water adsorbed in the NC films and its intensity is directly proportional to the water content in the samples [37]. Using the known moisture content for 89.8NC/H2 O sample, we could thus deduce the moisture content in the 96.6NC/H2 O and 97.3NC/H2 O samples (3.4 and 2.7%, respectively).

Fig. 1 IR spectra of NC films with a different moisture content near bending mode of the adsorbed water

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3.2 Photoluminescence A wide structural band was observed in the PL spectra of studied samples excited by λex = 405 nm light (Fig. 2a, b). We observed noticeable changes in luminescence intensity: the total intensity of luminescence emission decreases with an increased amount of adsorbed water molecules (Fig. 2a). An initial sharp decrease occurs as the amount of water increase from 0 to 4%. As the water content becomes higher, luminescence intensity decreases at a slower rate (Fig. 2b). The PL spectra of all samples reveal three important segments in the following wavelength ranges: 400–525, 525–600, and 600–700 nm (Fig. 2a). The maximum of the PL spectral band lies within the middle segment (λmax = ~547 nm), and its position does not change with increased water content. (The maximum intensity of all bands is marked by a vertical line in Fig. 2b). At the same time, the shape of the PL band undergoes certain changes as the water content increases; see spectra in Fig. 3. Undoubtedly, observed spectral behavior must be determined by the origin of the NC luminescence. Unfortunately, however, no unambiguous explanation of this origin exists as of now. Nonetheless, a certain insight into the nature of the luminescence of other cellulose materials has been described in [17, 38–40], and the spectra obtained in our experiments exhibit similar behavior to that described in the sources above. We can thus be relatively sure that the observed NC emission has the same origin as that reported in [17, 38–40]. It is generally believed that there are several sources of cellulose luminescence, all of them being molecular fluorophores embedded in the cellulose matrices. The cellulose molecules themselves, in their pure form, are not luminescent, due to their electronic and energy structure. Indeed, it is known that the absorption of light with wavelength λex = 405 nm is in the range of the NC bandgap [41], and therefore this absorption must be due to certain defects or impurities in the NC matrix. Thus, the luminescence caused by such excitation must be associated with certain objects, superfluous to the cellulose molecules: defects or impurities, specific to cellulosic materials. Possible candidates are carboxyl (–COOH) and carbonyl (C=O) groups of organic compounds, and single and conjugated hetero-aromatics, such as furan, oxopyran, and the like [42–44]. Moreover, the interaction of cellulose chains can cause additional absorption and associated luminescence [44]. If this is the case, then the peculiarities of the PL behavior we observed have the following explanation: it is well known that water molecules are good quenchers of the solid state’s luminescence. This is because they exhibit vibrations of high frequency (~1800 and 3600 cm−1 ). So, the energy of excited states of the above-mentioned fluorophores can be relaxed without radiation [45, 46]. Thus, when water molecules are present around a fluorophore, they cause a non-radiative quenching of its luminescence. As a result, the intensity of the NC luminescence decreases rapidly when the water molecules’ amount increases from zero to 10.2% (see Fig. 2a). Apparently, this number of water molecules is large enough to come into contact with most fluorophores. To test this assumption, the PL experiments were

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Fig. 2 a—Photoluminescence spectra of 89.8NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O; b dependence of the total luminescence intensity on water content; T = 290 K; λex = 405 nm

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Fig. 3 Normalized spectra of 89.8NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples; T = 290 K; λex = 405 nm

performed with NC films in a weak stream of water vapor for 1 (point 5%), 6 (point 10%), and 15 min (point 20%) (Fig. 2b). We found that with the further increase in the amount of moisture, the luminescence intensity decreases more slowly. Perhaps then (and it seems a likely supposition), different fluorophores react differently in the presence of water. As a result, the rate of their PL quenching differs. This is further manifested in the change in the shape of the luminescence band with the sample’s water content (Fig. 3).

3.3 Electric Conductivity Temperature dependences for the specific electric conductivity of different NC samples are presented in Fig. 4. The specific electric conductivity of the waternanocellulose system even in the lowest concentration (0.15NC/H2 O) was found to be three times higher than the specific conductivity of the bi-distilled water. Consequently, its concentration dependence is close to linear. We can thus infer the presence of an additional ionic component (besides cellulose) in our samples.

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Fig. 4 Temperature dependences of the specific conductivity for the bi-distilled water (H2 O) and 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O samples

3.4 Dielectric Properties We began the study of the dielectric properties of our systems by recording the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity for pure H2 O. The data were obtained in the temperature range from −175 to 55 °C at frequencies f = 5, 10, 20, and 50 kHz (Fig. 5). Keeping in mind these insights, we could then obtain the temperature dependences of the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity in the temperature range from −140 to 60 °C at the frequencies f = 5, 10, 20, and 50 kHz for the NC water solutions 0.15NC/H2 O (Fig. 6), 0.3NC/H2 O (Fig. 7), and 0.6NC/H2 O (Fig. 8). The temperature dependences of the real part of the complex dielectric permittivity for all three samples exhibit inflections (Figs. 6, 7 and 8). The position of the inflection shifts to higher temperatures with increasing frequency. The temperature dependency of the imaginary part of the complex dielectric permittivity shows maxima for all three samples as well. Similarly, their positions shift toward higher temperatures at an increasing frequency of field oscillation. All of these facts point to the relaxation process that occurs in this temperature range in 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O samples. A sharp increase in the dielectric permittivity

60 Fig. 5 Temperature dependence of the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity at different frequencies for the H2 O sample

Fig. 6 Temperature dependence of the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity at different frequencies for the 0.15NC/H2 O nanocellulose sample

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Fig. 7 Temperature dependence of the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity at different frequencies for the 0.3NC/H2 O nanocellulose sample

ε (T ) above the relaxation process indicates the melting of 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O samples. A somewhat different behavior of the real ε (T ) and complex ε (T ) parts of the complex dielectric permittivity in the temperature range from −175 to 60 °C at frequencies f = 5, 10, 20, and 50 kHz was observed in case of the 89.8NC/H2 O sample (Figs. 9 and 10). The temperature dependences of the real ε (T ) (Fig. 9) and imaginary ε (T ) (Fig. 10) parts of the complex dielectric permittivity for the 89.8NC/H2 O sample in the temperature range from −100 to 0 °C demonstrate an increase with a slight inflection. The sharp increase in both ε (T ) and ε (T ) between 0 and 60 °C is due to the evaporation of water from the NC film. Similar results are reported for the microcrystalline cellulose in [19, 38]. Based on the observed temperature dependences, it is difficult to resolve a relaxation process for this sample at temperatures between −100 and 0 °C. This may be due to a co-occurring high-temperature process involving moisture adsorbed on the NC film surface after drying. The 89.8NC/H2 O was consequently re-dried to obtain a 96.6NC/H2 O sample.

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Fig. 8 Temperature dependence of the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity at different frequencies for the 0.6NC/H2 O nanocellulose sample

The temperature dependences of the ε (T ) and ε (T ) parts of the complex dielectric permittivity for the 96.6NC/H2 O sample, obtained in the temperature range from − 150 to 60 °C at f = 5, 10, 20, and 50 kHz frequencies, are shown in Fig. 11. For the 96.6NC/H2 O sample, we observe weak changes in the real part ε (T ) of the complex dielectric permittivity with increasing temperatures. The ε (T ) temperature dependence, however, demonstrates a clear maximum. This maximum shifts to higher temperatures with increasing frequency. Yet again, a relaxation process is observed for the 96.6NC/H2 O sample. So re-drying of the NC film and encapsulating it removed the impact of the high-temperature process that was observed for the 89NC/H2 O sample. Instead, a low-temperature relaxation process became apparent. But what is the nature of the relaxation process observed in the NC samples? The intensity of this process has already been shown to decrease with the moisture content. It is possible that the moisture, adsorbed by an NC film, has not only attached itself to its surface but has also diffused inside the sample in the space between nanofibrils [21]. Thus, moisture would have remained in the sample at 110 °C in a vacuum press as we re-dried the sample.

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Fig. 9 Temperature dependence of the real ε (T ) part of the complex dielectric permittivity at different frequencies for the 89.8NC/H2 O nanocellulose sample

Fig. 10 Temperature dependence of the imaginary ε (T ) part of the complex dielectric permittivity at different frequencies for the 89.8NC/H2 O nanocellulose sample

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Fig. 11 Temperature dependence of the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity at different frequencies for the 96.6NC/H2 O nanocellulose sample

To resolve this issue, we have prepared another sample, in which we tried to eliminate as much moisture as possible. First, the 0.6NC/H2 O solution was dried at 50 °C to form an NC film. Once obtained, the film was immediately encapsulated between the polyethylene films in the vacuum press-form at 110 °C. Thus, a 97.3NC/H2 O sample was obtained. The temperature dependences of the ε (T ) and ε (T ) parts of the complex dielectric permittivity for this latest 97.3NC/H2 O sample were obtained at a temperature range from −150 to 60 °C at f = 5, 10, 20, and 50 kHz frequencies and are shown in Fig. 12. The real part of the complex dielectric permittivity for the 97.3NC/H2 O demonstrates a decrease with increasing temperatures. The complex part of its dielectric permittivity has a maximum, which we yet again relate to the presence relaxation process in the sample. Thus, even when the greatest amount of moisture is removed from the sample, the low-temperature relaxation persists. Its intensity, however, significantly decreases with the decrease in the moisture content (compare Figs. 5, 6, 7, 8, 9, 10, 11 and 12).

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Fig. 12 Temperature dependence of the real ε (T ) and imaginary ε (T ) parts of the complex dielectric permittivity at different frequencies for the 97.3NC/H2 O nanocellulose sample

4 Discussion We have observed a low-temperature dielectric relaxation for all water-nanocellulose systems under consideration. To compare the properties of these relaxation processes, the temperature dependences of the increment of the dielectric permittivity real part at 10 kHz were plotted for H2 O, 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O (Fig. 13). The plots indicate that the increment of the dielectric permittivity for the 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O samples increases with the increasing concentration of nanocellulose at temperatures above the relaxation process. No such dependence can be observed for the 96.6NC/H2 O and 97.3NC/H2 O samples’ increment of dielectric permittivity, and thus relaxation processes cannot be resolved for these samples. To determine the quantitative parameters of the relaxation process, the matter in our samples can be described as a system of identical relaxators, each having two positions of different energies, separated by a potential barrier [20, 47–50]. In such a model, the real and imaginary parts of the complex dielectric permittivity can be written as [51] ε (ω, T ) = ε∞ +

ε(T ) 1 + ω2 τ 2 (T )

(1)

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Fig. 13 Temperature dependence of the real  part of dielectric permittivity increment       ε = ε (T ) − ε∞ , where ε∞ = ε T =−175 ◦ C at 10 kHz for H2 O, 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples

ε (ω, T ) =

ε(T )ωτ (T ) 1 + ω2 τ 2 (T )

(2)

where ω = 2πf is the angular frequency of the electric field oscillation and τ is the relaxation time. The authors of [51] demonstrate that a single relaxation time attained in this model is given by    U exp kT 2π  , τ= ω0 1 + exp −V kT

(3)

and the increment of dielectric permittivity obeys the following relation:

ε = ε0 − ε∞

   −V exp kT Nμ · =   2 . 3kε0 T 1 + exp −V kT 2

(4)

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Table 1 Energy characteristics of the relaxation processes of the NC samples and water Sample

S/k

U, kJ mol−1

V, kJ mol−1

N μ2 3kε0 ,

H2 O

0

32.1 ± 3.6

3.7 ± 0.1

31,152 ± 180

0.15NC/H2 O

8.0 ± 1.8

49.3 ± 3.8

4.1 ± 0.1

41,488 ± 200

0.3NC/H2 O

8.9 ± 1.1

50.6 ± 2.3

4.0 ± 0.1

46,106 ± 112

0.6NC/H2 O

7.5 ± 0.3

47.2 ± 0.5

3.7 ± 0.1

53,153 ± 362

96.6NC/H2 O

9.4 ± 1.1

46.1 ± 2.4

–

–

97.3NC/H2 O

9.7 ± 1.2

47 ± 2.1

–

–

K

Here N—the concentration of the relaxators, μ2 —the root mean square value of the difference in dipole moments of relaxators in positions 1 and 2, V —the energy difference between positions 1 and 2, and k—Boltzmann constant. N μ2 By fitting the ε (T) dependence with expression (4), we obtain the values of 3kε 0 and V , summarized in Table 1. According to the data in Table 1, the increase in the concentration of cellulose nanoparticles leads to the increase of relaxators’ concentration, whose motion contributes to the dielectric relaxation. To elucidate the later point, temperature dependences of the imaginary part of the dielectric permittivity at 10 kHz were plotted for H2 O, 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples (Fig. 14). The maximum position of all curves is seen to shift toward lower temperatures with the increasing concentration of NC. If we take into account the influence of the activation entropy, the expression for the relaxation time can be rewritten as τ = τ0 exp ((U − T S)/kT, where τ 0 = 10–12 s, U—activation energy; T —absolute temperature; S—activation entropy [19]. The maximum of the relaxation process is attained whenever ωτ = 1. We can then derive that ln f = − ln 2π τ0 + S/k − U/kT. Then, plotting the Arrhenius equation ln f (1/T ) (Fig. 15) we can obtain the values of the activation entropy S/k and activation energy U (Table 1). The ln f (1/T ) dependences demonstrate that the dielectric relaxation processes shift toward lower temperatures at the increasing concentration of nanocellulose. Concurringly, as the cellulose content increases the activation energy increases as well. As the water is added to nanocellulose, activation entropy S/k is introduced. In our opinion, the activation entropy of the relaxation process is due to the formation of hydrogen bonds between NC and the surrounding molecules of water [52]. Identical values of S/k at different

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Fig. 14 Temperature dependences of the imaginary part ε (T ) of the dielectric permittivity at 10 kHz for H2 O, 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples

concentrations of NC are thus due to the formation of hydration shells around the NC crystals. Comparing the activation energies, we notice that their values are practically identical for 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples, but all of them are higher than the activation energy for the pure H2 O. We may then conclude that the nature of the relaxation process observed in 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples is different from that in H2 O, and it is due to the interference of two relaxation processes: one of them is related to the relaxation process in H2 O and the other to the relaxation in the NC crystals, surrounded by the hydrated shell.  intenThe 96.6NC/H2 O and 97.3NC/H2 O samples show a decrease in the εmax sity at 10 kHz with a decrease in water concentration (Fig. 16). Extrapolating the intensity of the imaginary dielectric permittivity peak to instances of lower water concentrations, we find that the intensity becomes zero at a water concentration of about 2%. We may then deduce that the relaxation will not occur in a film of NC completely devoid of moisture. The authors of [19, 20] demonstrate that the dielectric relaxation in the microcrystalline cellulose with low water content (0.4–16.4%) at temperatures between −100 and 0 °C occurs in the surface layers of cellulose crystallites, surrounded by a hydrated shell. This relaxation occurs due to the change in the orientation of the

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Fig. 15 Arrhenius equation ln f (1/T ) for H2 O, 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O samples

 Fig. 16 Intensity of the maximum of the imaginary part of the dielectric permittivity εmax for samples with different cellulose concentrations

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surface methylol groups of the cellulose molecule through tg ↔ tt conformation change. Thus, similar to the microcrystalline cellulose, the relaxation process in the 0.15NC/H2 O, 0.3NC/H2 O, 0.6NC/H2 O, 96.6NC/H2 O, and 97.3NC/H2 O solutions of the nanocellulose is also due to the conformational motion of the methylol groups on the nanocellulose crystals’ surface, surrounded by the hydrated shell.

5 Conclusions Dielectric properties of the nanocellulose solutions have been studied in detail to elucidate the mechanics of the relaxation processes that occur in these solutions. The 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O solutions, as well as 96.6NC/H2 O, 97.3NC/H2 O films, demonstrate a dielectric relaxation at temperatures between − 100 and 0 °C, whose nature differs from the nature of the dielectric relaxation in water: its mechanism is due to the dipole thermal polarization. The intensity of the dielectric relaxation decreases with the decreasing water content and vanishes as the concentration of moisture approaching zero. We may thus infer that the dielectric relaxation is absent in the dried NC crystals (crystals not surrounded by hydrated shells). Thus, the relaxation process in the 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O samples, as well as 96.6NC/H2 O, 97.3NC/H2 O films, is due to the surface layer of the nanocellulose particles surrounded by a hydration shell. A two-level model that assumes a single relaxation time for a solid system was applied and, thus, the energy and entropy characteristics of the relaxation process in NC solutions were calculated. The increase in the NC concentration was shown to cause an increase in relaxator concentration. These relaxators move between equilibrium positions, where the difference in energy levels is about V ≈ 4 kJ mol−1 , and overcome the energy barrier of value U ≈ 50 kJ mol−1 . The relaxation process observed for 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O samples is characterized by activation entropy contribution, which is absent in water. This activation entropy of the relaxation process is due to the possibility of the formation of hydrogen bonds between NC and the surrounding molecules of water. Identical values of this entropy for 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O samples hint at the same average number of water molecules surrounding the relaxators on the surface of the NC particles. We may thus conclude that in 0.15NC/H2 O, 0.3NC/H2 O, and 0.6NC/H2 O samples, the continuous hydrated shells are formed and that the relaxation process in water solutions of the nanocellulose is due to the reorientation of the surface methylol groups of the cellulose, which occurs by the change in their conformation from tg to tt.

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47. Lazarenko MM, Alekseev AN, Alekseev SA, Yablochkova KS, Bokhvan SI, Demidiuk OF, Lazarenko MV (2020) Topological solitons in aliphatic systems with a restricted translational mobility. Chem Phys 539:110959 48. Lazarenko MM, Alekseev AN, Alekseev SA, Hnatiuk KI, Demidiuk OF, Yablochkova KS, Atamas NO, Lazarenko MV (2020) Topological solitons in chain molecular crystals with stoichiometric obstacles and hydrogen bonds. J Phys Chem Solids 109514 49. Tkachev SY, Alekseev OM, Lazarenko MM, Lazarenko MV, Kovalov KM, Bokhvan SI, Grabovskii YE, Hoshylyk NV (2018) Topological solitons in branched aliphatic molecules. Mol Cryst Liq Cryst 665(1):166 50. Ananiadou A, Papamokos G, Steinhart M, Floudas G (2020) Nanometer confinement induces nematic order in 1-Dodecanol. J Phys Chem B. https://doi.org/10.1021/acs.jpcb.0c08403 51. Hoffman JD, Williams G, Passaglia E (1966) Analysis of the α, β, and γ relaxations in polychlorotrifluoroethylene and polyethylene: dielectric and mechanical properties. J Polym Sci C 1(14):173 52. Fröhlich H (1958) Theory of dielectrics: dielectric constant and dielectric loss. Clarendon Press, 192 p

The Medium Influence on the Luminescence Intensity of SnO2 Nanoparticles’ Ensembles in a Porous Silicate Glass Matrix S. A. Gevelyuk, I. K. Doycho, L. M. Filevska, and V. S. Grinevych

Abstract The sensitivity of SnO2 nanoparticles’ ensemble in a matrix of finely porous silicate glass with residual silica gel to the acidity of the environment has been studied. A sharp decrease in the intensity of the system’s luminescence was found when it was placed in an alkaline medium. In an acidic environment, luminescence attenuates slowly and smoothly in the same samples. The effect is explained by the unequal mechanisms of luminescence centers’ passivation in both media and different cross sections for the radiation quanta capturing by atmospheric polluting elements of acidic and alkaline media. A fairly rapid spontaneous recovery of the initial luminosity of the system upon its return to the ordinary atmosphere was noted. This result, together with the increased reliability of the system, due to the almost absolute chemical inertness of the nanoparticles of the ensemble, makes it possible to recommend it as an active element for the ammonia luminescent sensor.

1 Introduction Modern requirements for environmental safety determine the need for proximity sensors for various media [1]. Photoluminescence is one of the most convenient noncontact methods for environment composition detection [2]. However, the possibility of multi-use of the functional luminescent material essentially requires its stability in the detected medium. One of the most stable materials in a number of aggressive media is tin dioxide [3, 4]. Besides the said, this substance is highly sensitive to most environmental pollutants, and its nanoparticle ensembles glow in the visible range [5–12]. Thus, the SnO2 nanoforms are the sufficiently convenient material for the luminescent sensors’ active units of the environment composition. Porous silica glasses were used as a matrix to obtain such SnO2 nanoforms [8]. Various glass samples were impregnated with SnCl4 alcohol solutions of various concentrations. After this, nanoparticle ensembles were formed by thermosynthesis of SnO2 [9] directly in the pores. The best luminescent response was obtained by S. A. Gevelyuk · I. K. Doycho · L. M. Filevska (B) · V. S. Grinevych Odessa I.I. Mechnikov National University, Dvoryanskaya Str., 2, Odessa 65082, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_5

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using a matrix finely porous glass containing residual silica gel in the pores (socalled A-type of glass) [10, 11]. The most intensity of glow turns out for this type of glass because the residual silica gel functions as a separation material and prevents aggregation of synthesized particles, which are leakage channels [12]. Since SnCl4 is a strong glue for glass, a sufficiently high concentration worsens the separation properties of silica gel due to the partial gluing of its particles [13]. It was established that the radiation parameters of nanoparticles’ ensemble of SnO2 in A-type matrix were obtained by specified synthesis mode dependent on the environmental properties. That’s why it’s possible to create devices on the basis of this system for detecting such properties. The model medium with alkaline or acidic components was used. The determination of this media influence on the photoluminescence character of considered systems is the purpose of this study.

2 Results and Discussion The specific features of SnO2 nanoparticles’ ensembles in a porous glass matrix [8, 10] should be studied through the luminescence properties of this system. In these studies, the specimens obtained by the saturation of A-type matrix by SnCl4 solution with a concentration of 5 or 7.5% glow with the highest intensity [13]. An increase of concentrations of the saturating solutions results in attenuation of glow intensity of the system due to the “gluing effect”. The saturating solution with a lower concentration results in the formation of systems with an initially small number of nanoparticles, which are luminescence centers [8]. That’s why the specimens obtained, namely at the above-indicated concentrations of SnCl4 solution, were placed in a gaseous medium, which contained alkali (NH4 OH) or acid (HCl) pollutants for recording their photoluminescence spectra and for investigation, at the same time, the media’s influence on the glow intensity of such model systems. These studies showed that the photoluminescence intensity of the system has a number of specific dependences on the environment composition. Similar experiments with samples obtained at SnCl4 solution concentrations of 12.5 and 25%, which were performed for control, showed low sensibility of these systems to environmental pollutions. Such a result was expected. Figure 1 shows the glow intensity changes for ensembles of SnO2 nanoparticles obtained at SnCl4 saturating solution concentrations of 5 and 7.5% in alkaline and acidic environments. One can see that the luminescence intensity decreases by more than twice in a short time (about 30 s) at both concentrations in an alkaline medium, whereas in an acidic environment the luminescence intensity decreases smoothly and slowly (by about 5%) for the same samples during the same time. Probably, the decrease in intensity will be continuous in the last case, and in a few tens of hours it can reach the same level as in an alkaline medium. However, such a result is insignificant for environmental protection and, hence, it was not considered in this paper.

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Fig. 1 Kinetics of the photoluminescence intensity for ensembles of SnO2 NPs obtained at SnCl4 saturating solution concentrations of 5 and 7.5% in an alkaline and acidic medium

Figure 2 shows the irradiation kinetics changes, which were considered in detail during the first minute after placing specimens into a gaseous environment with pollutants for the nanoparticles’ ensembles of SnO2 , obtained at SnCl4 saturating solution concentrations of 5, 7.5, 12.5, and 25%. These measurements were performed to identify the ecological feasibility of using such systems as gas sensors. It is evident Fig. 2 Irradiation kinetics changes of SnO2 NPs’ ensembles obtained at SnCl4 saturating solution concentrations of 5, 7.5, 12.5, and 25% measured during the first minute after placing the samples into a gaseous environment with pollutant

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that for ensembles of SnO2 nanoparticles, obtained at the alcohol solution of SnCl4 equal to 25%, the curves are practically indistinguishable. In all the cases, the above results show that the decrease in the photoluminescence intensity begins after a certain latent period. Its duration depends both on the reaction of the medium and on the concentration of the initial solution, used at the formation of the nanoparticles’ ensemble. The results for both alkaline and acidic media are shown in the table below. As it can be seen from the table, the ensembles of SnO2 nanoparticles obtained at SnCl4 saturating solution concentrations of 5 and 7.5% are comparable in sensitivity to an alkaline pollutant, whereas the rest specimens are insensitive to such medium. It should be noted that in this case, an ensemble of SnO2 nanoparticles obtained at 5% is preferable as an ammonia sensor owing to its higher initial glow intensity. In the presence of an acidic pollutant, the latent period in the photoluminescence intensity decay does not depend on the concentration of the initial SnCl4 solution saturating the matrix. It must be noted that the nano-sized systems were obtained from 5 to 7.5% solutions sensitive to the acidic medium for 10 min. At the same time, those obtained at higher concentrations of initial solution do not change their glow practically. Thus, the considered nano-sized systems cannot be used as an acid medium sensor. It is important that the samples spontaneously restore their original glow completely in about 12 h after returning back to the pure atmosphere in both cases. It’s an advantage of these systems as an ammonia sensor. It is reasonable to underline that both the glow intensity of the initial specimens and the energy of their radiation quanta depend on the saturating solution concentration from which the system was formed [14]. At high concentrations, the sizes of SnO2 nanoparticles become larger due to their aggregation, hence the number of their luminescence centers, which concentrate on the nanoparticles’ surface, decreases. The luminescence intensity decreases too, but the energy of the emitted quanta increases. The aggregations generate the leakage channels [10, 12], and low-energy carriers recombine through them non-radiatively. This analysis explains the obtained result. In an ammonia medium, the ammonia molecules saturate the pores of the sample and form the unstable ammonia complexes of SnO2 [NH3 ] with SnO2 nanoparticles during the latent period. These complexes decompose continuously, but they continuously form again due to the presence of an ammonia atmosphere. It establishes a dynamic balance of formation and decomposition processes of these complexes. Such complexes are formed enough difficultly having much aggregations in the samples obtained at high concentrations of SnCl4 , so that the latent period is longer for these systems (see Table 1). The ammonia complexes capture cross section is large enough. Hence, they intensively absorb in the first turn the low-energy light quanta arising at carrier’s recombination [14]. The sharp decrease in the glow intensity of the desired specimens (Fig. 2) is explained by this. The systems obtained at the initial saturating solution concentrations of 5 or 7.5% irradiate primarily such quanta; that’s why the intensity of their glow decreases faster and is more noticeable than in the case of systems obtained at higher concentrations, where such quanta are absent practically.

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Table 1 Photoluminescence parameters of ensembles of SnO2 NPs in alkaline and in acidic media Concentration of SnCl4 at thermosynthesis (%)

Latent period in an alkaline medium, (s)

Decrease in luminescence Latent Decrease in luminescence intensity of SnO2 NPs in period in intensity of SnO2 NPs in an acidic an acidic medium an alkaline medium medium After 1 min, After After 1 min, After (s) (times/per 10 min, (times/per 10 min,

5

5

1.96

2.4

7.5

8

1.77

2.14

12.5

15

1.15

1.23

15

1.035

1.05

25

20

1.006

1.012

15

1.0005

1.001

min)

(times/per min)

min)

(times/per min)

15

1.086

1.6

15

1.04

1.46

In the atmosphere, polluted with HCl vapors, the pollutant’s molecules in the pores of the specimen dissociate with the formation of Cl− ions due to the action of atmospheric water, which is always present there. These ions do not form complexes with SnO2 nanoparticles of the ensemble because they are charged. Hence, the latent period is determined by the time of penetration into the pores and further dissociation of HCl molecules only. That’s why it doesn’t depend on the concentration of the initial saturated solution. Nevertheless, the formed Cl− ions as well as ammonia complexes of SnO2 [NH3 ] are centers of absorption of low-energy light quanta [14], but the intensity of this process is low because their capture cross section is smaller than for the mentioned complexes. As it was noted above, 12 h after returning the samples to the pure atmosphere are enough for the restoration of their initial ability to glow in both the cases because during such time, NH3 or HCl molecules leave the pores completely [15]. Comparing the luminescence spectra of the system formed by using 5% SnCl4 initial solution in a pure atmosphere and in 10 min after placing in ammonia medium shows that the glow intensity decreases shifting toward short wave length. It corresponds to an intense absorption of light quanta with low energy and confirms the described model.

3 Conclusions The changes in the luminescence intensity of SnO2 nanoparticles’ ensembles in a porous glass matrix in acidic and alkaline media were examined. The photoluminescence intensity’s different nature of the change, on our opinion, was determined by the nature of the interaction of alkaline or acidic ions with tin dioxide nanoparticles in the pores of silicate glass. The unstable SnO2 [NH3 ] ammonia complexes with a large capture cross section are formed in an alkaline medium. This is the reason for the recorded sharp drop in

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radiation intensity. In an acidic medium, a slow and smooth change in the photoluminescence intensity is due to the penetration of HCl molecules into the pores with their subsequent dissociation. The noticeable change in the luminescence intensity is observed in the SnO2 nanoparticles’ ensemble obtained by thermosynthesis upon impregnation of a porous matrix with a 5% SnCl4 alcohol solution. The mentioned above makes it possible to use the ensembles as an active medium for an ammonia fluorescent sensor. The almost absolute chemical inertness of the working substance and the possibility of spontaneous restoration of the sensor’s working parameters within a day after triggering are the advantages of such a system.

References 1. Ho CK, Robinson A, Miller DR, Davis MJ (2005) Overview of sensors and needs for environmental monitoring. Sensors (Basel) 5(2):4–37 2. Gfroerer TH (2006) Encyclopedia of analytical chemistry. In Meyers RA, McGuire GE (eds) John Wiley & Sons. https://doi.org/10.1002/9780470027318.a2510 3. Batzill M, Diebold U (2005) The surface and materials science of tin oxide. Prog Surf Sci 79:47–154 4. Das S, Jayaraman V (2014) SnO2 : a comprehensive review on structures and gas sensors. Prog Mater Sci 66:112–255 5. Periyasamy M, Kar A (2020) Modulating the properties of SnO2 nanocrystals: morphological effects on structural, photoluminescence, photocatalytic, electrochemical and gas sensing properties. J Mater Chem C 8:4604–4635 6. Gu F, Wang ShF, Song ChF, Lü MK, Qi YX, Zhou GJ, Xu D, Yuan DR (2003) Synthesis and luminescence properties of SnO2 nanoparticles. Chem Phys Lett 372(3–4):451–545 7. Kar A, Kundu S, Patra A (2011) Surface defect-related luminescence properties of SnO2 nanorods and nanoparticles. J Phys Chem C 115(1):118–124 8. Gevelyuk SA, Grinevich VS, Doycho IK, Lepikh YI, Filevska LM (2020) The radiation peculiarities of nanoscale SnO2 in a porous matrix. J Nano Electron Phys 12(3):03020 9. Uchiyama H, Shirai Y, Kozuka H (2011) Formation of spherical SnO2 particles consisting of nanocrystals from aqueous solution of SnCl4 containing citric acid via hydrothermal process. J Cryst Growth 319:70–78 10. Doycho IK (2015) Nonequilibrium processes in sensory structures. In Smyntyna VA (ed) Odessa, Odessa Mechnikov National University (in Russian) 11. Doycho IK, Grinevych VS, Filevska LM (2020) Porous silica glasses as a model medium for the formation of nanoparticles ensembles: review. In Bonˇca J, Kruchinin S (eds) Advanced nanomaterials for detection of CBRN. NATO science for peace and security series a: chemistry and biology. Springer, Dordrecht. https://doi.org/10.1007/978-94-024-2030-2_21 12. Tyurin OV, Bercov YM, Zhukov SO, Levitskaya TF, Gevelyuk SA, Doycho IK, RysiakiewiczPasek E (2010) Aggregation of dyes in porous glass. Opt Appl 40(2):311–321 13. Gevelyuk SA, Grinevych VS, Doycho IK, Lepikh YaI, Filevska LM, (2019) In 2019 IEEE 8th international conference on advanced optoelectronics and lasers (CAOL), Sozopol, Bulgaria, pp 416–419. https://doi.org/10.1109/CAOL46282.2019.9019433, https://ieeexplore.ieee.org/ document/9019433/metrics#metrics

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14. Doycho IK, Gevelyuk SA, Lepikh YaI, Rysiakiewicz-Pasek E (2017) Features of gas-sensibility of dyes on the base of 4-valence stannum complexes. Sens Electron Microsyst Technol 14(1):31–40. https://doi.org/10.18524/1815-7459.2017.1.96436 (in Ukrainian) 15. Doycho IK, Gevelyuk SA, YaI L, Rysiakiewicz-Pasek E (2019) Nature of gas sensitivity of dyes on the base of Sn(IV) complexes. Opt Appl 49(3):427–436

Spectrum of Localized Quasi-Particle Interacting with Three-Mode Phonons M. V. Tkach, Ju. O. Seti, O. M. Voitsekhivska, and V. V. Hutiv

Abstract Within the unitary transformed Hamiltonian of Fröhlich type, using the retarded Green’s function method, the exact renormalized energy spectrum of a quasi-particle strongly interacting with three-mode polarization phonons is obtained at T = 0 K in the model of a system with a limited number of its initial states. The exact analytical expressions for the average number of phonons in the main and all satellite states of the system are obtained. Their dependences on the magnitude of the interaction between quasi-particle and both phonon modes are analyzed.

1 Introduction Intensive development of nano scale physics during the last decades has revealed new physical properties of low-dimensional structures and stimulated the growth of different multi-layered nano heterostructures: quantum dots, wires, rings, etc. Also, it caused the experimental appearance of new unique nano devices which are widely used in medicine, ecology, communication and space, military and scientific investigations [1–4]. From the physical considerations, nano heterostructures are principally different with respect to bulk 3D structures, just due to the nano size of their composing elements. However, the effect of size quantization is always present in nano structures. It causes the multi-band spectra of quasi-particles (electrons, holes, excitons, etc.) and multi-mode spectra of polarization phonons [5–8]. It is well known that in order to optimize the operating parameters of modern nano devices functioning in infrared range, one should deeply understand the physical processes in multi-layered heterostructures being their main elements. The quantum theory of photon- and phonon-assisted tunneling, being developed now, will provide this knowledge. The main problem of the theoretical investigation is to take into M. V. Tkach (B) · Ju. O. Seti · O. M. Voitsekhivska · V. V. Hutiv Department of Theoretical Physics and Computer Simulation, Yuriy Fedkovych Chernivtsi National University, 2, Kotsyubinsky Str, Chernivtsi 58012, Ukraine e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_6

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account the multi-phonon processes, which essentially renormalize the quasi-particle spectra in a wide range of energies, which contains the bound-to-phonon states of the systems [8–10]. This problem cannot be solved in the framework of the perturbation method [10–12], since, the modern non-perturbative approaches should be used. Recently, non-perturbative approaches and methods, which correctly took into account multi-phonon processes, appeared in the theory of quasi-particle interacting with phonons: exact diagonalization (ED) for small systems, variational method (VM), momentum average (MA) approximation and so on [13–21]. Diagrammatic Monte Carlo (DMC) and bold diagrammatic Monte Carlo (BDMC) methods [22– 26] have played a particularly important role in the investigation of high-excited states, revealing reasons for different phenomena in electron–phonon systems. Both methods are essentially based on the computer algorithms for calculating high-order diagrams of the mass operator in Matsubara’s Green’s functions. The complicated theoretical problem of calculation of the renormalized spectrum of multi-band (or multi-level) quasi-particles interacting with multi-mode phonons was solved within the approximated methods in early Refs. [10, 27, 28] for the different models of the systems. The results obtained in these papers qualitatively correlated with experimental data and with each other. However, in the limit case (T → 0 K), the renormalized spectrum was decaying, due to used approximations that contradicted the physical considerations. It is to be mentioned that in [10], one of the approaches to study the renormalized spectrum of the two-state quasi-particle strongly interacting with polarization phonons was proposed. The retarded Green’s function was exactly calculated in general form within the unitary transformation of the Hamiltonian of Fröhlich type. However, the exact Fourier transformation and, since, the renormalized discrete spectrum were obtained only for the system with one phonon mode. Approximately calculated spectrum of the system with an arbitrary number of phonon modes revealed to be decaying both at T = 0 K and at T = 0 K. Such result for the latter case is evidently an artifact of not correct approximation because, from physical considerations, interaction only with virtual phonons (T = 0 K) cannot produce any decay. In recent papers [29, 30], the renormalized spectrum of multi-level quasi-particles interacting only with one-mode phonons was obtained within the modified method of Feynman–Pines diagram technique, using the Fröhlich-like Hamiltonian. This spectrum is discrete and non-decaying at T → 0 K, with respect to physical considerations. As far as the method developed in the cited papers has not yet been extended for the systems with several phonon modes, in this paper, we propose the approach which allows to solve exactly the problem of the renormalized energy spectrum of localized a quasi-particle interacting with three-mode polarization phonons at T = 0 K for the model of the system with a limited number of its initial states [10].

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2 Hamiltonian of the System. Exact Renormalized Spectrum and Average Number of Phonons in All States of the System The system under study consists of a localized quasi-particle (exciton, impurity, etc.), which strongly interacts with non-dispersive three-mode polarization phonons at T = 0 K. Its Hamiltonian in the representation of second quantization over all variables is written in Fröhlich’s form [8–12] Hˆ =

 k

E k Aˆ + Aˆ + k k

3   λ=1

+ ˆ λ Bˆ λ q + q Bλ

q

3   λ=1 kq

  + ϕλ ( q ) Aˆ + Aˆ Bˆ λq + Bˆ λ− q . k k (1)

→ = ε is energy of uncoupled localized quasi-particle, λ is energy Here, E − k   ˆ k q ) is a binding function. Quasi-particles Aˆ + , A of phonon mode (λ), and ϕλ (  k   + ˆ and phonon Bˆ λ operators of second quantization satisfy Bose commutative , B λ q q relationships. As in [10], we are studying the system for which the condition

nˆ 2 = nˆ =

 k

Aˆ + Aˆ k k

(2)

is fulfilled. It means that the eigenvalues of both operators (nˆ and nˆ 2 ) are either 0 or 1 and are interpreted as the condition of absence (0) or presence (1) of a “pure” quasi-particle state. To obtain a renormalized spectrum of such system, the Hamiltonian (1) is diagonalized within the transition from Aˆ k , Bˆ λq operators to a new aˆ k , bˆλq , using unitary operator ⎧ ⎫ ⎨  ⎬ S = exp σˆ aˆ k aˆ k+ , ⎩ ⎭

(3)

k

where σˆ =

3   λ=1

 + ∗ −1 q )bˆλ q )bˆλq . λ ϕλ ( q − ϕλ (

q

The Hamiltonian (1) in new operators has a diagonal form

(4)

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Hˆ =

 k

εk aˆ k+ aˆ k +

3   λ=1

+ ˆ λ bˆλ q, q bλ

(5)

q

where εk = E k −

3   λ=1

−1 q )|2 λ |ϕλ (

(6)

q

is an energy of new elementary excitation created by operator aˆ k+ . The two-time retarded Green’s function at T = 0 K is the same as causal Green’s function [10–12]  

    t = −iθ (t) 0 Aˆ k (t), Aˆ + (0) 0 (7) G k, k Taking into account (5), the relationship between old and new operators   Aˆ k = Sˆ aˆ k Sˆ + = aˆ k exp −σˆ

(8)

and using Weyl’s operator equality, we obtain an exact expression        

 iε t iε t  t = −iθ(t) 0eσˆ (t) eσˆ (0) 0 exp − k = −iθ(t) exp − k + g(t) . G k,  

(9)

Here     iλ t −1 , αλ exp − g(t) =  λ=1 3 

(10)

where α=

3  λ=1

αλ , αλ = −2 λ



|ϕλ ( q )|2

(11)

q

is a dimensionless parameter, which characterizes the binding energy of quasiparticle with λ-th phonon mode. Fourier image of Green’s function (9) is written according to the definition

Spectrum of Localized Quasi-Particle Interacting …

 ω + iη) = − i G(k, 

∞

87

 exp i(ω − −1 εk + iη)t +

3  λ=1

0

    iλ t − 1 dt. αλ exp − 

(12) Integral in expression (12) is calculated exactly using Newton’s binomial (at è = 1 and k = 0). As a result, we get  G(ω + iη) = exp −

3 

 αλ

λ=1

+

1 ω − ε0 + iη

3  ∞  λ=1 λ

αλλ  ! [ω − ε0 − λ λ + iη] =1 λ

∞  ∞  1 α11 α22 +  ! ! −   − ε [ω 0 1 1 − 2 2 + iη]  =1  =1 1 2 1

2

∞  ∞  1 α11 α33 +  ! ! [ω − ε0 − 1 1 − 3 3 + iη]  =1  =1 1 3 1

+

3

∞  ∞  2 =1 3

1 α22 α33  ! ! [ω − ε0 − 2 2 − 3 3 + iη] =1 2 3

 ∞  ∞  ∞  1 α11 α22 α33 + ,  ! ! ! [ω − ε0 − 1 1 − 2 2 − 3 3 + iη]  =1  =1  =1 1 2 3 1

2

3

(13) where ε0 = E 0 −

3 

αλ λ .

(14)

λ=1

According to the general theory [10–12, 28], the poles of the Fourier image of Green’s function determine the energy spectrum of the system. Therefore, from formula (13) it is clear that the renormalized spectrum for this model is given by an exact analytical expression E(1 , 2 , 3 ) = −

3 

αλ λ + 1 1 + 2 2 + 3 3 ; (1 , 2 , 3 = 0, 1, 2, ... ∞).

λ=1

(15) From (15), one can see that the spectrum of the system is real and discrete because the decay is absent. It contains the main renormalized level (at 1 = 2 = 3 = 0)

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E(0, 0, 0) = ε0 = E 0 −

3 

αλ λ

(16)

λ=1

which is shifted into the low-energy region with respect to the main level (E) of 3  uncoupled quasi-particle, at the magnitude αλ λ . Besides, there are two types λ=1

of groups of phonon satellite levels: mixed and non-mixed, which correspond to the bound states of the quasi-particle with all possible combinations of different numbers of phonons of all three modes. The group of non-mixed satellite levels contains three series of an infinite number of equidistant discrete levels, the energies of which are given by analytical expressions E(λ 1 ) = ε0 + λ 1 λ 1 ;

λ 1 = 1, 2, 3, . . . , ∞;

λ 1 = 1, 2, 3.

(17)

The group of mixed satellite levels contains two subgroups. The first one contains three series of an infinite number of equidistant discrete energy levels E(λ1 , λ2 ) = ε0 + λ1 λ1 + λ2 λ2 ; λ1 , λ2 = 1, 2, 3, . . . , ∞; λ1 = 1, 2 < λ2 = 2, 3

(18)

to which the double mixed states of phonon modes correspond. The second subgroup contains the infinite number of series with an infinite number of energy levels E(λ1 , λ2 , λ3 ) = ε0 + λ1 λ1 + λ2 λ2 + λ3 λ3 ; λ1 , λ2 , λ3 = 1, 2, 3, . . . , ∞; λ1 = 1; λ2 = 2; λ3 = 3,

(19)

to which the triple mixed states of phonon modes correspond. Obtained renormalized energy spectrum of the system, relationship between Fourier image of retarded Green’s function and mass operator M(ω) within Dyson equation [10–12] (è = 1) G(ω) =

1 ω − ε − M(ω)

(20)

gives an opportunity to define the average number of phonons in the so-called phonon «dress» of the quasi-particle in stationary states of the system. It is well known [9, 31] that these numbers, in the general case, are fixed by the expression.    −1 −1 N = 1 − Mω (ω = E St ) .

(21)

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89

In the model under study, with the above-presented stationary spectrum (E St ) (16– 19), the number of phonons in main (N 0 ) and all three groups of satellite (N 1 , N 2 , N 3 ) states are calculated analytically exactly:  N0 (α1 , α2 , α3 ; 1 = 2 = 3 = 0) = 1 − exp(−α);

α=

3 

 αλ

(22)

λ=1 λ

  αλ 1 N1 α1 , α2 , α3 ; λ1 = 1 − exp(−α) 1 ; λ1 = 1, 2, . . . , ∞; λ1 = 1, 2, 3; λ1 ! (23) λ

λ

  αλ 1 αλ 2 N2 α1 , α2 , α3 ; λ1 , λ2 = 1 − exp(−α) 1 2 ; λ1 !λ2 ! λ1 , λ2 = 1, 2, 3, . . . , ∞; λ1 = 1, 2 < λ2 = 2, 3; λ

λ

(24)

λ

  αλ 1 αλ 2 αλ 3 N3 α1 , α2 , α3 ; λ1 , λ2 , λ3 = 1 − exp(−α) 1 2 3 ; λ1 !λ2 !λ3 ! λ1 , λ2 , λ3 = 1, 2, 3, . . . , ∞; λ1 = 1; λ2 = 2; λ3 = 3.

(25)

Energy spectrum (16–19) and average number of phonons (22–25) prove that if there is only one phonon mode in the system, for example, with the energy 1 and coupling constant α 1 = 0, then, α 1 = α 2 = 0, since α 1 = α, λ1 = 1, 2, ..., ∞. Therefore, the renormalized spectrum and average number of virtual phonons are the following: E 0 = ε − α1 1 ; N0 (α1 ; 0) = 1 − exp(−α1 );

E(1 ) = E 0 + 1 1 ; N1 (α1 ; 1 ) = 1 − exp(−α1 )

(26) α11 ; 1 !

1 = 1, 2, 3, . . . , ∞.

(27)

These results, as it must be, are completely the same as the respective magnitudes for the one-mode system studied in [31].

3 Properties of Renormalized Spectrum and Average Number of Phonons in Main and Satellite States of the System The main properties of the renormalized spectrum and the average number of phonons in different states of the system are clear from the analytical expressions (16–19) and  (22–25), respectively. A detailed picture of a complete spectrum of the system ( )

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formed by partial interaction between quasi-particle and a different number of phonon modes is presented in Fig. 1. The calculated positions of satellite levels (with respect to themain renormalized one with E0 energy) are shown in this figure in the energy scale ε = (ω − ε0 )−1 1 . These satellite levels appear due to the interaction between quasi-particle and phonons of each separate mode (E(1 ), E(2 ), E(3 )), with each double mixed modes (E(1 , 2 ), E(1 , 3 ), E(2 , 3 )) and triple mixed modes (E(1 , 2 , 3 )). For the convenience, in Fig. 1, only indices λ are  written with their direction fixed by arrows. In the picture of complete spectrum ( ), index  fixes the number of the levels and s is a degree of degeneration. Analytical expressions (16)–(19) and Fig. 1 prove that the spectrum of the system is discrete and non-decaying. Herein, as it was above-mentioned (16), the main level E 0 is stronger shifted (with respect to ε) into the low-energy region; the bigger is the interaction between quasi-particle and all phonon modes. The position of satellite levels in the renormalized spectrum is independent of coupling constants but depends on the relationship between the energies of phonon modes. If both energies (2 , 3 ) are  multiples of the energy (1 ) of the first mode, then, the complete spectrum ( ) is equidistant (with step 1 ) and, except first   , all other levels are degenerate, Fig. 1a. If only one energy levels n = 2 −1 1 (2 or 3 ) is a multiple of the energy (1 ) of the first mode, then, the complete spectrum is non-equidistant and partially degenerate. If the ratios of energies of all three modes are irrational numbers, then, the spectrum is non-equidistant and completely non-degenerate, Fig. 1b. The properties of average numbers of phonons in the quasi-particle’s «dresses» in main and satellite states of the system are clear from the simple analytical expressions (22)–(25). Evidently, these numbers in all states are independent of the magnitudes of phonon energies λ but are the functions of coupling constants αλ . From Formula (22), it is clear that in the main state with the energy E 0 the number N 0 (α 1 , α 2 , α 3 ; 0) monotonously increases from 0 till 1 if α increases from 0 till α  1. Therefore, at α 1, the number N 0 (α 1 , α 2 , α 3 ; 0) ~ α, since the number of virtual phonons in the quasi-particle’s «dress» is small. Consequently, the “weakly dressed” quasi-particle in this state has the properties like those which were before its interaction with phonons. When coupling constants reach the vicinity of α = ln2, N 0 (α 1 , α 2 , α 3 ; 0) = 1/2 and, according to the classification of bound states proposed in [9], the quasi-particle has the properties characteristic to the phonon-quasi-particle complex. If α further essentially increases ln2, then, N 0 (α 1 , α 2 , α 3 ; 0) → 1 and in this case, even in the ground state, the quasi-particle already has predominantly phonon properties. Dependences of average numbers of phonons (N 1 , N 2 , N 3 ) in all satellite states on coupling constants (α 1 , α 2 , α 3 ) are qualitatively similar but essentially different from that for the main state. From the expressions (22)–(25), it is clear that at small coupling constants (α 1 , α 2 , α 3 1), the numbers N 1 , N 2 , N 3 ≈ 1 in all satellite states, while in the main state N 0 ≈ 0. If coupling constants (α 1 , α 2 , α 3 ) increase, all average numbers of phonons (N 1 , N 2 , N 3 ) in all satellite states at first decrease, with respect to 1, then approach minimal magnitudes bigger than zero and, further, again tend to 1.

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Fig. 1 Scheme of the lower part of the degenerate renormalized energy spectrum of the main and degenerate (a), and non-degenerate (b) phonon satellite energy levels of the system.  Main level ( ); satellite levels: non-mixed ( ), double mixed ( ), triple mixed ( ); —complete spectrum,  is a number of levels in complete spectrum, s is a degeneration degree; λ is a number of levels produced mode. Parameters of the system: 1 = 1; 2 = 3; 3 = 5 (a), √ by λ-th √ 1 = 1; 2 = 3; 3 = 5 (b)

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When coupling constants vary, the number N 1 (α 1 , α 2 , α 3 ; 1 = 1, 2 = 3 = 0) maximally differs from 1, compared to the other numbers (N 1 , N 2 , N 3 ). This number corresponds to the lowest satellite level. N1 (α1 , α2 , α3 ; 1 = 1, 2 = 3 = 0) = 1 − α1 exp(−α). It is clear that it is minimal at α 1 = 1; α 2 , α 3 1, since α ≈ 1, thus min N 1 = 1 − exp(−1) = 0.632. Therefore, according to the classification of bound states proposed in [9], it means that independently of coupling constants and magnitudes of phonon energies in all satellite states, where quasi-particle is bound to all possible combinations of numbers of phonons of different modes, it has predominantly phonon properties.

4 Main Results and Conclusions For the model of localized quasi-particle, which can be only in two initial states, interacting with three-mode polarization phonons at T = 0 K, the renormalized spectrum and average numbers of phonons are obtained for all states of the system. The exact calculation of the Fourier image of retarded Green’s function of a quasiparticle proves that the renormalized spectrum of such a system is discrete and nondecaying, with respect to physical considerations. It is established that the decaying spectrum at T = 0 K obtained in [10] was a consequence of the approximation used there for the calculation of the Fourier image of retarded Green’s function. However, it is well known from quantum mechanics that interaction of a quasi-particle only with virtual phonons cannot produce a decay of the spectrum, due to the energy conservation law. Now, we revealed that the stationary discrete spectrum of the system under study contains the main level, shifted into the low-energy region and two types of groups of satellite levels: mixed and non-mixed. They correspond to the bound states of quasi-particle with all possible combinations of different numbers of phonons of all three modes. It is established that the location of satellite levels in the renormalized spectrum is independent of magnitudes of coupling constants but depends on the relationship between the values of phonon energies. If the energies of both bigger modes are multiples of the energy of the smallest mode, the complete spectrum of the system is discrete and equidistant, with the step, which is equal to the smallest mode energy. All satellite phonon levels are degenerate, except those which are located lower than the first satellite level, which corresponds to the energy of the second, with respect to the energy, phonon mode. If only one of the bigger modes is a multiple of the smallest, then the spectrum is partially degenerate. If the ratios between the energies of all three modes are irrational numbers, the complete spectrum is non-equidistant and totally non-degenerate.

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Analysis of average phonon numbers in the states of the three-mode system proves that independently of coupling constants, all satellite states, predominantly, have phonon properties. The properties of the main state essentially depend on coupling constants. In this state, at weak coupling regime (α ≤ 0.4), the properties of the quasi-particle are like those without interaction with phonons. The bound-to-phonon complex arises at intermediate coupling (0.4 ≤ α ≤ 0.6). At a strong coupling regime (α ≥ 0.6), the quasi-particle also has phonon properties, being even in the main state. We should mention that the proposed method of analytical calculation of renormalized energy spectrum and the average number of phonons in all states of the system can be generalized for an arbitrary number of phonon modes.

References 1. Faist J (2013) Quantum cascade lasers. Oxford University Press, Oxford 2. Belkin MA, Capasso F (2015) New frontiers in quantum cascade lasers: high performance room temperature terahertz sources. Phys Scr 90:118002. https://doi.org/10.1088/0031-8949/ 90/11/118002 3. Giorgetta FR, Baumann E, Graf M, Yang Q, Manz C, Kohler K, Beere HE, Ritchie DA, Linfield E, Davies AG, Fedoryshyn Y, Jackel H, Fischer M, Faist J, Hofstetter D (2009) Quantum cascade detectors. IEEE J Quantum Electron 45:1039. https://doi.org/10.1109/JQE.2009.2017929 4. Reininger P, Zederbauer T, Schwarz B, Detz H, MacFarland D, Maxwell Andrews A, Schrenk W, Strasser G (2015) InAs/AlAsSb based quantum cascade detector. Appl Phys Lett107:081107. https://doi.org/10.1063/1.4929501 5. Stroscio MA, Dutta M (2001) Phonons in nanostructures. Cambridge University Press, Cambridge 6. Tkach MV, Seti JO, Voitsekhivska OM (2015) Quasiparticles in nanoheterostructures: quantum dots, wires and layers. Books-XXI, Chernivtsi 7. Harrison P, Valavanis A (2016) Quantum wells, wires and dots: theoretical and computational physics of semiconductor nanostructures, 4th edn. Wiley, New York 8. Tkach MV, Seti JO, Voitsekhivska OM (2019) Diagram technique in the method of Green’s functions of quasiparticles interacting with phonons. Chernivtsi National University, Chernivtsi, p 164 9. Levinson YB, Rashba EI (1974) Threshold phenomena and bound states in the polaron problem. Sov Phys Uspekhi 16:892–912. https://doi.org/10.1070/PU1974v016n06ABEH004097 10. Davydov AS (1976) Theory of solids. Nauka, Moscow 11. Mahan GD (1981) Many-particle physics. Plenum, New York 12. Abrikosov AA, Gorkov LP, Dzyaloshinski IE (2012) Methods of quantum field theory in statistical physics. Dover, New York 13. Bonca J, Trugman SA, Batistic I (1999) Holstein polaron. Phys Rev B 60:1633. https://doi.org/ 10.1103/PhysRevB.60.1633 14. De Filippis G, Cataudella V, Marigliano Ramaglia V, Perron CA (2005) Static and dynamic polaron features in a coherent-state basis. Phys Rev B 72:014307. https://doi.org/10.1103/Phy sRevB.72.014307 15. Bieniasz K, Berciu M, Oles AM (2016) The Green function variational approximation: significance of physical constraints. Acta Phys Pol Ser A 130(2):659–663. https://doi.org/10.12693/ APhysPolA.130.659 16. Berciu M (2006) Green’s function of a dressed particle. Phys Rev Lett 97:036402. https://doi. org/10.1103/PhysRevLett.97.036402

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17. Covaci L, Berciu M (2007) Holstein polaron: the effect of coupling to multiple-phonon modes. EPL 80:67001. https://doi.org/10.1209/0295-5075/80/67001 18. Kornilovitch PE (2006) Polaron action for multimode dispersive phonon systems. Phys Rev B 73:094305. https://doi.org/10.1103/PhysRevB.73.094305 19. Ebrahimnejad H, Berciu M (2012) Trapping of three-dimensional Holstein polarons by various impurities. Phys Rev B 85:165117. https://doi.org/10.1103/PhysRevB.85.165117 20. Möller MM, Berciu M (2016) Discontinuous polaron transition in a two-band model. Phys Rev B 93:035130. https://doi.org/10.1103/PhysRevB.93.035130 21. Marchand DJJ, Stamp PCE, Berciu M (2017) Dual coupling effective band model for polarons. Phys Rev B 95:035117. https://doi.org/10.1103/PhysRevB.95.035117 22. Prokof’ev NV, Svistunov BV, Tupitsyn IS (1998) Exact, complete, and universal continuoustime worldline Monte Carlo approach to the statistics of discrete quantum systems. J Exp Theor Phys 87:310. https://doi.org/10.1134/1.558661 23. Mishchenko AS, Prokof’ev NV, Sakamoto A, Svistunov BV (2000) Diagrammatic quantum Monte Carlo study of the Fröhlich polaron. Phys Rev B 62:6317. https://doi.org/10.1103/Phy sRevB.62.6317 24. Marchand DJJ, De Filippis G, Cataudella V, Berciu M, Nagaosa N, Prokof’ev NV, Mishchenko AS, Stamp PCE (2010) Sharp transition for single polarons in the one-dimensional SuSchrieffer-Heeger model. Phys Rev Lett 105:266605. https://doi.org/10.1103/PhysRevLett. 105.266605 25. De Filippis G, Cataudella V, Mishchenko AS, Nagaosa N (2012) Optical conductivity of polarons: double phonon cloud concept verified by diagrammatic Monte Carlo simulations. Phys Rev B 85:094302. https://doi.org/10.1103/PhysRevB.85.094302 26. Mishchenko AS, Nagaosa N, Prokof’ev N (2014) Diagrammatic Monte Carlo method for many-polaron problems. Phys Rev Lett 113:166402. https://doi.org/10.1103/PhysRevLett.113. 166402 27. Toyozawa Y (1958) Theory of line-shapes of the exciton absorption bands. Prog Theor Phys 20(1):53–81. https://doi.org/10.1143/PTP.20.53 28. Agranovich VM (1968) Theory of exitons. Nauka, Moscow 29. Tkach M, Seti Ju, Pytiuk O, Voitsekhivska O, Gutiv V (2019) Spectrum of localized three-level quasiparticle resonantly interacting with polarization phonons at cryogenic temperature. Appl Nanosci 1–11. https://doi.org/10.1007/s13204-019-01002-8 30. Tkach MV, Seti Ju O, Voitsekhivska OM, Gutiv VV (2019) Method of successive separation and summing of multiplicative diagrams of mass operator for the multi-level quasiparticle interacting with polarization phonons. Condens Matter Phys 22, 3, 33707, 1–15. https://doi. org/10.5488/CMP.22.33703 31. Tkach MV, Seti Ju O, Voitsekhivska OM, Pytiuk O Yu (2016) Properties and temperature evolution of the spectrum of localized quasi-particles interacting with polarization phonons in two models. Condens Matter Phys 19:43701. https://doi.org/10.5488/CMP.19.43701

Energy Spectra Dispersion of Vibrational and Electronic States in Layered Hexagonal γ-BN Crystals and Single-Layer Nitroborene (BN)L,1 Viktor Gubanov and Antonina Naumenko

Abstract For the first time, for crystalline hexagonal graphite-like boron nitride γBN and its single-layer nitroborene (BN)L,1 , the projective classes were identified and the characters of projective representations characterizing the symmetries of vibrational and electronic excitations without taking the electron spin into account and taking it into account for different points of the Brillouin zones have been calculated. For the high-symmetry points of the corresponding Brillouin zones, distributions over the irreducible projective representations of vibrational excitations and electronic states determined by the structure of electronic π-zones have been found. The dispersion of electronic excitations and the structure and symmetry of their spindependent splittings due to the presence of electron spin are qualitatively established for these points.

1 Introduction Boron nitride crystallizes, like carbon, in the cubic polymorphic phase—sphalerite modification of α-BN, wurtzite modification of β-BN and graphite-like hexagonal modification of h-BN or, like graphite, modification of γ-BN. The physical properties of the corresponding polymorphic phases of carbon and boron nitride are close to each other. Regarding the symmetry of the crystal structures corresponding to the polymorphic phases, for example, the symmetries of the crystal structures of Bernal graphite [1] γ-C and hexagonal boron nitride h (γ)-BN are characterized by the same spatial symmetry group P63 /mmc (D46h ), despite the fact that graphite is a one-component atomic structure, and hexagonal boron nitride is two-component. The energy spectra of the electronic states of the atoms of both these structures (the electronic “skeleton” of their electronic shells) are determined by the same hybridization of the electronic states for C atoms and B− and N+ ions, which leads to the formation V. Gubanov · A. Naumenko (B) Faculty of Physics, Taras Shevchenko National University of Kyiv, 64/13, Volodymyrs’ka str., Kiev 01601, Ukraine e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_7

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of three strong σ-links for each atom bonds with adjacent atoms and a weak covalent π-bond between atoms of adjacent layers in the direction orthogonal to the planes of σ-bonds. It is sp2 -hybridization that is the electronic “skeleton” that determines the hexagonal structures of graphite and hexagonal graphite-like boron nitride. It was interesting to conduct a detailed theoretical and group analysis of the dispersion in the Brillouin zones of oscillatory and electronic excitations in the structures of hexagonal boron nitride and to determine the effect of spatial symmetry on its oscillatory and electronic energy spectra and on the manifestation of spin-dependent decays in its electronic spectra.

2 Crystal Structure of Hexagonal Graphite-Like Boron Nitride h(γ)-BN and Its Monolayer–Single-Layer Nitroborene (BN)L,1 , Their Brillouin Zones and Basic Elements of Symmetry Figure 1a shows a standard elementary unit cell of the crystal lattice of a hexagonal graphite-like boron nitride γ-BN. Atoms of this elementary unit cells are arranged as follows in such a way that the elements of symmetry of the crystal lattice for it coincide with the elements of symmetry, put in the form shown in Fig. 1b, more symmetrical in the arrangement of elements standard graph of the spatial group 4 ) [2], which defines standardization elementary cell, in of symmetry P63 /mmc (D6h Fig. 1, in the specified location and orientation of the elements of point symmetry group 6 mmm (D6h ).

a

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Fig. 1 Structure of a standard unit cell of hexagonal graphene-like crystals γ-BN (a), the standard 4 )(b), arrangement and orientation of the diagram of the spatial symmetry group P63 /mmc (D6h elements of the point symmetry group 6/mmm (D6h ) (c). The circles indicate the positions of nitrogen (transparent) and boron (dark) atoms

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Fig. 2 Brillouin zone of hexagonal graphite-like crystals γ-BN and its symmetry points

Figure 2 shows the Brillouin zone of crystals γ-BN, and its points of high symmetry are indicated. These points are indicated by the letters corresponding to the Herring for hexagonal structures [3, 4]. Figure 3a shows an elementary cell of single-layer nitroborene (BN)L,1 . Single-layer nitroborene (BN)L,1 contains as graphene C L,1 , two similarly located atoms in their three-dimensional diperiodic elementary cell but these atoms are different in chemical nature: one of them is an atom N; the other is an atom B. Spatial symmetry of monolayer nitroborene due to the difference in the types of

a

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a

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Fig. 4 Wigner–Seitz unit cell (a) and Brillouin zone of single-layer nitroborene (BN)L,1 (b)

atoms to its composition is lower than the spatial symmetry of graphene C L,1 . It is characterized by a diperiodic spatial group DG 78 (P− 6m2) [5], the graph of which 1 (P− 6m2). coincides with the graph of triperiodic group D3h The graph of the group DG 78, which corresponds to the choice of the origin shown in Fig. 3a, is shown in Fig. 3b. This graph of the group coincides with the standard graph of the group DG 78. This means that it is chosen in Fig. 3, and the elementary cell of single-layer nitroborene (BN)L,1 corresponds to the standard one and can be considered as standard. Figure 4 shows the elementary Wigner–Seitz cell (a) and the Brillouin zone (b) of single-layer nitroborene (BN)L,1 . In Fig. 4, solid lines, as for single-layer graphene C L,1 in [6, 7], schematically show the elementary cell of single-layer nitroborene (BN)L,1 , its vectors of basic translations a1 and a2 and indicate the used in the calculations the orientation of its symmetry elements in three-dimensional space, and the dotted line shows the corresponding orientation of the inverted lattice vectors b1 and b2 on an arbitrary scale and the position of the inverted lattice nodes in inverted space. In Fig. 4b, on the contrary, solid lines show the inverted lattice vectors, and the dotted line shows the straight lattice vectors. The elementary Wigner–Seitz cells of single-layer nitroborene in real space (Fig. 4a) and in inverted space (Fig. 4b), where such a cell coincides with the first Brillouin zone, are highlighted by a grey background. These cells are rotated relative to each other by an angle π/2. Figure 4b also shows the points of high symmetry of the Brillouin zone of a single-layer nitroborene, the points K and M. Equivalent points are indicated by one or two dashes. The spatial symmetry of the crystalline hexagonal natural graphite-like boron nitride γ-N, as well as the spatial symmetry of crystalline Bernal graphite γ-C (see above), 4 ), and f their crystal classes are describes a simple symmetry group P63 mmc (D6h by a point group 6/mmm (D6h ).

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4 Spatial group P63 /mmc (D6h ) belongs to the asymmetric groups and contains basic elements that are conveniently chosen in Seitz’s notation look like: h1 = (0|e), h2 = (0|c3 ), h3 = (0|c23 ), h4 = (0|(u2 )1 ), h5 = (0|(u2 )2 ), h6 = (0|(u2 )3 ), h7 = (a1 /2|c2 ), h8 = (a1 /2|c6 5 ), h9 = (a1 /2|c6 ), h10 = (a1 /2|(u 2 )1 ), h11 = (a1 /2|(u 2 )2 ), h12 = (a1 /2|(u 2 )3 ), h13 = (0|i), h14 = (0|ic3 ), h15 = (0|ic3 2 ), h16 = (0|i(u2 )1 ), h17 = (0|i(u2 )2 ), h18 = (0|i(u2 )3 ), h19 = (a1 /2|ic2 ), h20 = (a1 /2|ic6 5 ), h21 = (a1 /2|ic6 ), h22 = (a1 /2|i(u 2 )1 ), h23 = (a1 /2|i(u 2 )2 ), h24 = (a1 2|i(u 2 )3 ), where a1 is the main vector of the crystal lattice, directed along the axis 0Z (0z). To establish the dispersion of the energy spectra of vibrational and electronic excitations, this here we use the method developed in [6, 7] for constructing the irreducible projective representations of symmetry groups for different points of the Brillouin zones of periodic structures and the method of determining projective classes whose unambiguous corresponding excitations.

3 Distributions of Normal Vibrations by Types of Symmetry, Symmetry and Dispersion of Electron Bands Without Taking the Electron Spin into Account and Their Dispersion and Spin-Dependent Structure Taking the Electron Spin into Account for Different Brillouin Zone Points of Crystals of Hexagonal Graphite-Like Boron Nitride γ-BN and Their Single-Layer Nitroborene (BN)L,1 The spatial symmetries of the crystal structures of the hexagonal graphite-like boron nitride γ-BN and the Bernal graphite γ-C, as mentioned above, are described by 4 ) and their crystall classes, obvithe same spatial symmetry group P63 /mmc (D6h ously, describe by the same point symmetry group 6/mmm (D6h ), then, obviously, the irreducible projective representations for the corresponding points of their Brillouin zones, denoted by the same letters, and the projective classes identical for them must be the same. To determine the summary projective representations, it is necessary to take into account the specific atomic structure of the crystal, that is, the difference in the types of atoms included in the elementary cell of the crystal and the spatial arrangement of atoms of each type in its elementary cell. This information determines the characters of the projective representation of the equivalence of atoms—the projective representation Deq (r), where r is the symmetry element of the group of the crystal class. Higher-energy electronic valence π-bands and lower by energy π*-conductivity bands are formed by π-electrons formed as a result of sp2 -hybridization of atomic (for γ-BN crystals ionic) electron shells. This means that the transformation, splitting, and qualitatively dispersion of electron states of the same symmetric π-orbitals at

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different points corresponding to these structures of the Brillouin zones will have somewhat similar properties.

3.1 Hexagonal Graphene-Like Boron Nitride γ -BN Crystal Using the methods of constructing factor systems described in [6, 7], reducing them to a standard form, establishing projective classes and corresponding irreducible projective representations, we at first calculate the distributions of normal vibrations by types of symmetry for different points of the Brillouin zone of the graphite-like crystal γ-BN and determine the symmetries and the spin-dependent structure of its electronic bands: higher in energy from the valence bands–valence π-bands and lower in energy from the conduction bands–π*-bands. Point Γ . The factor group of the wave vector group by an invariant subgroup of translations in the point with a wave vector k = 0 for the γ-BN crystal is isomorphic to a point group that characterizes the symmetry of the crystal class—the group 6/mmm (D6h ). The characters of the representation of the equivalence of atoms for the crystalline hexagonal graphite-like boron nitride γ-BN at the point  of its Brillouin zone  eq , calculated by the method [6, 8], are given in Table 1, where the characters of the representation of the polar vector  vector =  r are also given; the representation of vibrations of the crystal lattice of the crystal γ-BN without division into acoustic and optical oscillations  vib ; representation of fundamental acoustic and optical oscillations  ac and  opt , respectively; representation  z , which determines the symmetry of the electron orbital pz without taking into account the electron spin; representation of electronic π-bands without taking the electron spin into account  π ; a two-valued spinor projective representation describing the transformation of an angular momentum with a full quantum number j = 1/2D1/2 + ; a two-valued spinor representation   z , which determines the symmetry of the electron orbital p z , taking into account the electron spin, and a two-valued spinor projective representation of electronic π zones, taking the electron spin into account  π . In the bottom part of the table the characters of irreducible two-valued spinor projective representations of the projec  +   − (1) (1) = 7+ and   2 = 8− are tive class K 1 of group 6/mmm (D6h )   1 additionally indicated (taken from [7, Table 1]). The symmetries of the highest-energy valence band without taking the electron spin into account, which corresponds to the highest-energy binding π-orbital, and the lowest-energy conduction π*-band without taking the spin of the electron into account, which corresponds to the anti-spin orbitals in the crystalline hexagonal graphite-like boron nitride γ-BN, as well as in the Bernal crystalline graphite [1], are determined by the representations  3 − (A3 − ) and  2 + (A2 + ) of the point group of the crystal class 6/mmm (D6h ), respectively. This is not difficult to establish on the basis of the distribution of irreducible representations of the representation of electronic zones Γ π and using the method of linear combinations of atomic orbits

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Table 1 Characters of the representations Deq , Dr , Dvib , fundamental vibrational modes  ac and  opt , Dz , electron π-bands without taking the electron spin into account Dπ , D1/2 + , Dz , and electron π -bands taking the electron spin into account Dπ for high symmetry points of Brillouin zone in hexagonal graphite-like boron nitride crystal γ-BN and single-layer nitroborene (B N ) L ,1

(continued)

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Table 1 (continued)

(continued)

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Table 1 (continued)

(continued)

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Table 1 (continued)

(continued)

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Table 1 (continued)

(LCAO method) [9]. It should be noted that the symmetry of the second valence band with a lower energy than the energy of the first valence band is determined by the representation  2 + (A2 + ), and the symmetry of the higher energy of the second π*-conductivity band is determined by the representation  3 − (A3 – ). Point A. Factor group of the wave vector group according to the invariant subgroup of translations at the point A of the Brillouin zone of crystals of hexagonal graphite-like boron nitride γ-BN, as well as the corresponding factor group of the group of the wave vector at the point A of the Brillouin zone of graphite crystal factor groups with invariant subgroups of translations at the points  of both of these crystal structures, isomorphic to the point group 6/mmm (D6h ). The star of the wave vector of the point A of the hexagonal boron nitride γ-BN, as well as the star of the wave vector at the point A of the graphite crystal γ-C, consists of only one vector k = −(1/2)b1 . In [6, 7, 9] it was shown that for crystal structures with a space group of symmetry 4 ), which include Bernal graphite γ-C and hexagonal graphite-like P63 mmm (D6h boron nitride γ-BN, the standard and projective equivalent to them systems for vibrational and electronic states without taking into account the electron spin ω1 ,A (r 2 , r 1 ) belong to the projective class K 5 , and standard and projectively equivalent factor systems for vibrational and electronic states with taking into account the electron spin belong to the projective class K 4 . This means that the representations of the projective class K 0 for vibrational and electronic states without taking the electron spin into account at the point  of the Brillouin zone of the hexagonal graphite-like boron nitride γ-BN pass into the representation A of its Brillouin zone into the representation of the projective class K 5 , the representation of the projective class K 1 at the point , since K 1 × K 5 = K 4 , in the representation of the projective class K 4 at the point A. One can see that the representation  eq at the point Γ belongs to the projective class K 0 , and the representation Aeq at the point A belongs to the projective

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class K 5 . The characteristics of the projective representation of the equivalence of atoms at the point A of the Brillouin zone of the hexagonal graphyte-like boron nitride γ-BN, Aeq are given in Table 1, and the distributions of the projective representation Aπ of projective class K 5 according to irreducible projective representations of projective class K 5 and π -zones A of projective class K 4 according to irreducible projective representations of projective class K 4 without taking the electron spin into account and taking it into account, respectively, in Table 3. Point K. The factor group of a group of a wave vector with respect to an infinite invariant subgroup of translations at the point K of the Brillouin zone of crystal of a hexagonal graphite-like boron nitride γ-BN is isomorphic to a point group 6m2 (D3h ). The star of the wave vector has two rays: (kK )1 = −(1/3) (2b2 − b3 ) and (kK )2 = (1/3) (2b2 − b3 ). Characters of irreducible projective representations of projective classes K 0 and K 1 of the group 6m2(D3h ), which correspond to standard factor systems ω (0) (r 2 , r 1 ) (unit factor system for one-valued irreducible vector representations) and ω (1) (r 2 , r 1 ), respectively, are given in Table 8 of [7], and the characters of two-valued (spinor) irreducible projective representations of the group 0.6m2(D3h ) (projective class K 1 ) are given in Table 9 of [7]. The characteristics of the representation of the equivalence of atoms at the point K of the Brillouin zone of the hexagonal graphite-like boron nitride γ-BN, K eq are given in Table 1, which also shows the characters of the representations of K r , K vib , K z , the representation of representations D1/2 + , K  z and projective representation of electronic π -bands taking into account the spin of the electron K  π . The distribution of the vibrational representation K vib by irreducible representations of the projective class K 0 is given in Table 2, and the distributions of the one-valued representation of electronic π-bands K π and the two-valued projective representation of electronic π -zones K π by irreducible representations of the corresponding projective classes are given in Table 3. Note that the symmetries of the valence band and the π*-conductivity band of the crystal of the hexagonal graphite-like boron nitride γ-BN without taking the electron spin into account at the point K of its Brillouin zone, as can be seen from Table 3, are characterized by the same two-valued irreducible representations 6m2 (D3h ) symmetry group—the representations of K 6(0) . Each of these unique twovalued double-degenerate orbitals of the projective class K 0 , when taking the electron spin into account, i.e., as a result of spin-dependent splitting, splits into two twovalued double-degenerate orbitals of the projective class K 1 —the orbitals (K  )(1) 2 and (K  )(1) . 3 Point H. As at the point K of the Brillouin zone of the hexagonal graphite-like boron nitride γ-BN, the factor group of the wave vector group by the invariant subgroup of translations at the point H of the Brillouin zone of crystal γ-BN is isomorphic to the group of equivalent directions—the point symmetry group 6m2 (D3h ). The star of the wave vector at this point contains, as at the point K, two vectors: (kH )1 = −(1/2)b1 − (1/3) (2b2 − b3 ) and (kH )2 = –(1/2)b1 + (1/3) (2b2 − b3 ).

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Table 2 Distribution of vibrational excitations at high-symmetry points of Brillouin zones over the irreducible projective representations of corresponding projective classes of hexagonal graphite-like boron nitride γ-BN and single-layer nitroborene (BN)L,1

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Table 3 Distribution of one-valued projective representations for π-bands of electronic excitations without taking the electron spin into account Dπ and two-valued projective representations for π bands of electronic excitations taking the electron spin into account D’π at high-symmetry points of Brillouin zones over the one-valued and two-valued irreducible projective representations of corresponding projective classes for structures of hexagonal graphite-like boron nitride γ-BN and single-layer nitroborene (BN)L,1

(continued)

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Table 3 (continued)

The characters of the projective equivalence representation of atoms at the point H of the Brillouin zone of the hexagonal graphite-like boron nitride γ-BN, like the characters of the projective equivalence representation of atoms at the point H of the Brillouin zone of Bernal graphite crystal γ-C (the spatial symmetry both of these structures is described the same spatial symmetry group P63/mmc (D46h ), belong to the projective class K 1 . The characters of the projective equivalence representation of atoms at the point H of the Brillouin zone of the hexagonal graphite-like boron nitride γ-BN, H eq are given in Table 1, where the characters of projective representations of the projective class K 1 projective representations H vib , projective representations for π-bands not taking the electron spin into account H π , projective representations D1/2 + and H  z and the representations of the projective class K 0 : H r , H z and projective representations of electronic π -zones H  π .

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Our calculation of the Herring criterion for the point H on the elements of symmetry corresponding to the equations g k = −k (see [7]) indicates that the twovalued one-dimensional spinor irreducible projective representations for point H of the Brillouin zone of the hexagonal graphite-like crystal of boron nitride γ-BN (H  )(0) 1  (0)  (0) and (H  )(0) 3 , as well as (H )2 and (H )4 , refer to the case b2 [10] and due to the invariance to the time inversion must merge in pairs, although our case corresponds to the merging of not complex conjugate but non-equivalent complex representations, while two-dimensional complex conjugate irreducible projective representa (0) tions (H  )(0) 5 and (H )6 refer to the case a2 [10] and, taking the symmetry to the time inversion into account, do not combine. An interesting feature of the correlations of electron excitations without taking the electron spin into account at the points K and H of the Brillouin zone of crystals of hexagonal graphite-like boron nitride γ-BN is that at both these points with the same point groups of symmetry, the valence band and the conduction band are the same, although at the point K the projective representations of the valence π-band and the π*-band of conductivity belong to the projective class K 0 and they are real and symmetrically identical (representation 2K 6(0) ), and at the point H the corresponding projective representations belong to the projective class (complex conjugate, i.e., different −H1(1) and −H2(1) . We assume that the projective representation of the valence band at the point H is the projective representation H1(1) (although this choice of two complex conjugate representations H1(1) and H2(1) is arbitrary), and the projective representation of the conduction band is the projective representation H2(1) . The middle part of Table 4b presents the characters of the products of projective representations H1(1) ⊗ D1/2 + , H2(1) ⊗ D1/2 + and H3(1) ⊗ D1/2 + , for by which it is easy to establish the conditions of compatibility of irreducible projective representations for the orbitals at point H without taking the electron spin into account—the orbitals Hi(1) of the projective class K 1 with two-valued irreducible projective representations of spinor (spin) projective representations (H )(0) of projective class K 0 corresponding to the orbitals with taking the electron spin into account. Thus, the sum of spin orbitals  (0)  (0) (H  )(0) 1 + (H )3 + (H )6 (corresponds to the orbital without taking into account the spin of the electron H1(1) , and the orbitals without taking into account the spin of  (0)  (0) the electron H2(1) is the sum of the spin orbitals (H  )(0) 2 + (H )4 + (H )5 ). In the bottom part of Table 4, the values of the characters of the projective representation of all considered electronic π-bands with taking the electronic spin into account (more precisely, π -zones) for the point H of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN are given equal to the sum of the corresponding values of the characters of all considered spin orbitals (H  )(0) i . Point P. The point group 3m (C 3v ) is a group of equivalent directions of the wave vector group at the point P of the Brillouin zone of crystals of hexagonal graphite-like boron nitride γ-BN, as well as of crystal graphite γ-C. The group 3m (C 3v ) has only one class of projective representations—this is the class K 0 , and therefore all projective representations of the group 3m (C 3v ) are p-equivalent to the vector ones. The star of the wave vector at the point P of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN contains four rays: (kP )1 =

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Table 4 Characters of the one-valued irreducible projective representations of projective class K 1 in point H of the Brillouin zone of hexagonal graphite-like boron nitride γ-BN and characters + of the two-valued spinor irreducible projective representation D1/2 for the point symmetry group 6m2(D3h ) (a) and characters of the two-valued irreducible projective representations in point H; (1)

products of the representations Hi

+ × D1/2 and characters of projective representation of electron

π -bands of crystalline hexagonal boron nitride with taking the electron spin into account Hπ of the projective class K o of the symmetry point group 6m2(D3h )(b)

−kz − (1/3)(2b2 − b3 ), (kP )2 = −kz + (1/3)(2b2 − b3 ), (kp )3 = kz −(1/3)(2b2 − b3 ) and (kp )4 = kz + (1/3)(2b2 − b3 ). The representations of the equivalence of atoms at the point P of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN, Peq , which, like all projective representations of the point group 3 m (C 3v ), belong to the projective class K 0 , are given in Table 1, where the characters of projective representations of the projective class K 0 are also given—the representations of Pr , Pvib , Pz , projective representation of electronic π-bands without taking the electron spin into account

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Pπ , D1/2 + , P z and projective representation of electronic π -bands when taking the electron spin into account P π . Table 5 shows the characters of one-valued irreducible representations of the projective class K 0 of the point symmetry group 3m(C 3v ) at point P of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN (a), characters of two-valued (spinor) irreducible projective representations also of projective class K 0 of this group, characters of spinor representation D1/2 + , products of projective representations P1(0) ⊗ D1/2 + , P2(0) ⊗ D½ + and P3(0) ⊗ D1/2 + and the characters of the projective representation of electronic π -bands taking the electron spin into account P π (b). In the bottom part of Table 5, the values of the coefficients u2 (r) are given, which lead the factor system of transformations of the spin variable ω2 (r 2 , r 1 ) to the standard form ω 2 (r 2 , r 1 ) ≡ω (0) (r 2 , r 1 ), and in the lower part of the whole Table 5, the value of the phase factor is indicated (the whole Table 5 is taken from [7]). Table 5 Characters of the one-valued irreducible projective representations of the projective class K 0 of the group 3m(C3v ) without taking the electron spin into account in the P point of Brillouin zone of hexagonal graphite-like boron nitride crystal (a) and characters of the two-valued (spinor) irreducible projective representations of electron π -bands taking the electron spin into account (b)

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The distribution of the vibrational representation Pvib by one-valued irreducible representations of the projective class K 0 is given in Table 2, and the distributions of the representation of electronic π-zones Pπ by one-valued irreducible representations of the projective class K 0 and the two-valued projective representation of electronic π -zones by P π class K 0 are given in Table 3. The calculation of the Herring criterion for the point P of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN by elements of symmetry satisfying the requirement g k = −k (see [7]) shows that two-valued one-dimensional  (0) projective representation (P  )(0) 1 and (P )2 refer to the case b2 [10] and due to the invariance to the time inversion are combined into a two-valued projective represen(0)  (0) tation ((P  )(0) 1 + (P )2 ), and one-valued irreducible projective representations P1 , (0) (0) (0) P2 , P3 and the two-valued spinor projective representation (P  )3 refer to the case a2 [10], taking into account the symmetry to the time inversion do not merge. It is also interesting to note that for the points P and H at the boundary of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN with absolute disregard for the electron spin, for example, for vibrational excitations, when the phase factor e−ikza1/2 at k z = −b1 /2 has the value of i, the sum of onedimensional characters unambiguous irreducible projective representations P1(0) and P2(0) , as representations of a complex conjugation them, pass for all elements of group 3 m (C 3v ) in the character of a two-valued representation H3(1) , which relates to the project active class K 1 of a higher symmetry group, than the symmetry group of points P. From Table 5, it would be easy to see that the orbitals without taking the electron spin into account P1(0) and P2(0) correspond to the spin orbitals (P  )(0) 3 , taking into account the spin of the electron, and the orbitals without taking the electron spin into  (0) account P3(0) —the sum of the combined spin orbitals ((P  )(0) 1 + (P )2 ) and the spin  (0) orbital (P )3 . Point M. The factor group of the wave vector group with respect to the invariant subgroup of translations at the point M of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN is isomorphic to the point group of symmetry mmm (D2h ), and the point M is point symmetry group of equivalent directions. The star of the wave vector at this point contains three vectors: (kM )1 = −(1/2)b3 , (kM )2 = (1/2)b2 and (kM )3 = –(1/2) (b2 − b3 ). From the three rays of the star of the wave vector at point M of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-N, we consider the ray (kM )1 , for which the symmetry elements that translate this ray into an equivalent, forming a point group mmm (D2h ) are (u2 )1 , c2 , (u 2 )1 , i, i(u2 )1 , ic2 , and i(u 2 )1 . In the role of generating elements of this group mmm (D2h ), we choose the elements a = (u2 )1 , b = c2 and c = i. Using the defining relations for the double group (mmm)  (D 2h ): a4 = e, b4 = e, c2 = e, ab = qba, ac = ca and bc = cb according to the method described in [6, 7], we find for group mmm (D2h ) factor system ω2 (r 2 , r 1 ), which characterizes the transformation of spin variables. This factor system belongs to the projective class K 1 , since for it α = −1, β = 1 and γ = 1 [6], and is reduced by means of the coefficients u2 (r) found for it to p-equivalent block-symmetric standard form.

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ω 2 (r 2 , r 1 ) ≡ ω (1) (r 2 , r 1 ), characteristic of standard factor systems of the projective class K 1 [6, 7]. The values of the coefficients u2 (r) make it easy to determine the characters of irreducible projective representations of the projective class K 1 at the point M of the Brillouin zone of a crystalline hexagonal graphite-like boron nitride 4 [7]. γ-BN with the symmetry space group P63 /mmc D6h Characters of representing the equivalence of atoms at the point M of the Brillouin zone of the crystal γ-BN the representation M eq , representations M r , M vib , M z , representation of the considered electronic π-zones without taking into account the electron spin M π , which belong to the projective class K 0 , and projective representations D1/2 + , M  z and projective representations of electronic π -bands taking into account the electron spin M  π , which belong to the projective class K 1 , are given in Table 1. The distribution of the representations M vib by irreducible representations of the projective class K 0 is given in Table 2, and the distributions of the representation of electronic π-bands without taking into account the electron spin by the irreducible representations of the projective class K 0 and the projective representation of electronic π -zones without taking into account the electron projective representations of the projective class K 1 are given in Table 3. Point L. The factor group of a group of a wave vector by an invariant subgroup of translations at the point L of the Brillouin zone of the crystalline hexagonal graphitelike boron nitride γ-BN is also isomorphic to the point group mmm (D2h ). The star of the wave vector of point P of the Brillouin zone of the crystal γ-BN also contains three rays: (kL )1 = −(1/2) (b1 + b3 ), (kL )2 = −(1/2) (b1 − b2 ) and (kL )3 = −(1/2) (b1 + b2 − b3 ). From the three rays of the star of the wave vector at point L of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN, as before for the point M of its Brillouin zone, we consider the ray (kL )1 , for which the symmetry elements e, (u2 )1 , c2 , (u 2 )1 , i, i(u2 )1 , ic2 and i(u 2 )1 are equivalent, forming a point group of symmetry mmm (D2h ). In the role of generating elements of this group mmm, as well as for the group of points M, we choose the elements a = (u2 )1 , b = c2 and c = i. As was shown in [6], the factor system ω1, L (r 2 , r 1 ), which is determined by the structure of the spatial symmetry group and describes the symmetric transformations of the wave functions of vibrational states and electronic states without taking the electron spin into account, for the point L of the Brillouin zone of a crystalline 4 ) belongs to the projective structure with a spatial symmetry group P63 /mmc (D6h class K 5 and is reduced to a block-symmetric standard form ω 1, L (r 2 , r 1 ) ≡ ω (5) (r 2 , r 1 ) using the coefficients u1,L (r) calculated in [6], and the factor system ω2 (r 2 , r 1 ) of the spinor transformation in symmetry operations of the group mmm (D2h ) belongs to the projective class K 1 and is reduced to a block-symmetric standard form ω 2 (r 2 , r 1 ) ≡ ω (1) (r 2 , r 1 ) by means of the coefficients u2 (r) found in [6]. This means that the standard factor system taking the electron spin into account for the point L ω 2, L (r 2 , r 1 ), which is the product of the standard factor systems ω 1, L (r 2 , r 1 ) (projective class K 5 ) and ω 2 (r 2 , r 1 ) (projective class K 1 ), i.e. factor system ω 2, L (r 2 , r 1 ) = ω 1, L (r 2 , r 1 ) ω 2 (r 2 , r 1 ), since K 5 × K 1 = K 4 [6], is a standard factor system of the projective class K 4 of the group mmm (D2h ) i, therefore, ω 2, L (r 2 , r 1 ) ≡ ω (4) (r 2 , r 1 ).

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Table 6 Characters of the one-valued (a) and two-valued (spinor) (b) irreducible projective representations of the L point of Brillouin zone of crystalline hexagonal graphite-like boron nitride 4 )) (taking from [6, 7]) γ-BN (spatial symmetry group P63 /mmc(D6h

Table 6 shows the characters of one-valued irreducible projective representations, and Table 6b shows the characters of two-valued (spinor) irreducible projective representations for vibrational and electronic excitations without taking the electron spin into account and taking it into account, respectively. at the point L of the Brillouin zone of the hexagonal graphite-like boron nitride γ-BN (spatial symmetry group 4 )) (taken from [6, 7]). P63 /mmc (D6h Characters of the representation of the equivalence of atoms at the point L of the Brillouin zone of the crystalline hexagonal graphite-like boron nitride γ-BN, which belongs to the projective class K 5 , as well as representations of oscillatory excitations L vib and electronic π-bands without taking the electron spin into account L π , which also belong to the projective class K 5 , representations L r and L z of the projective class K 0 , representations D1/2 + and L  z of the projective class K 1 , and representations of electronic π -bands taking the electron spin into account L  π of the projective class K 4 are given in Table 1. The distribution of the representation L vib according to the irreducible projective representations of the projective class K 5 is given in Table 2, and the distributions of the electronic π-bands without taking into account the electron spin L π according to the irreducible projective representations of the projective class K 5 and the electronic π -zones taking the electron spin into account effective representations of the projective class K 4 , respectively, are given in Table 3.

3.2 Monolayer of Boron Nitride γ -BN Crystal—The Single-Layer Nitroborene (BN)L,1 We proceed to determine the distributions of normal vibrions and states of electron π-bands without taking the electron spin into account and electron π -bands, taking it into account according to projective classes and types of symmetry for different

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points of the Brillouin zone of monolayer of a hexagonal graphite-like boron nitride γ -BN -- nitroborene (BN)L,1 . Point Γ . It is easy to see that the point group of symmetry of equivalent directions in a two-periodical spatial symmetry group of a single-layer nitroborene (BN)L,1 (the group DG 78 [5]) is a point group of symmetry 6m2(D3h ), which defines its macromolecular class that for two-periodic structures is an analogue of the crystal class for three-periodic crystal lattices. The location of the symmetry elements within the standard three-dimensional two-periodic elementary cell of a single-layer nitroborene (BN)L,1 is shown in Fig. 3a and is represented by the graph of the group DG 78 in Fig. 3b. Factor group of the diperiodic spatial group of symmetry of the layered package (monolayer) of the lattice of the crystal γ-BN according to a twodimensional invariant subgroup of translations, according to the irreducible representations of which the vibrational and electronic states of the monolayer, isomorphic to the point group 6m2 (D3h ). Note that in order to compare the results of the theoretical-group classification of the vibrational and electronic states of the crystal γ-BN and single-layer nitroborene (BN)L,1 , we must adhere to the same orientations of the symmetry elements in their groups of wave vectors. This means that at the point Γ —group of the wave vector with kΓ = 0—the classification of vibrational and electronic states should be performed by the group 6m2 (D3h ), which contains the elements e, c3 , c3 2 , 3u 2 , ic2 , ic6 5 , ic6, and 3iu2 , i.e., for the group 6m  2 , which is isomorphic to the group 6m2 with the elements e, c3 , c3 2 , 3u2 , ic2 , ic6 5 , ic6, and 3iu 2 . The isomorphism of the groups 6m2 and 6m  2 is achieved by replacing the elements (u2 )i with elements (u 2 )I , and conversely, the elements (u 2 )i are replaced by the elements (u2 )i (i = 1, 2, 3). Classification by an isomorphic group is important for determining the forms of normal oscillations and is important for establishing groups of a wave vector but does not affect the definition of types of symmetry of elementary excitations, for which only characters of irreducible projective representations are used, but not their matrix. The characters of the representation of the equivalence of atoms of single-layer nitroborene (BN)L,1 at the point  of its Brillouin zone  eq , calculated by the method [6, 8], are given in Table 1, which also shows the representation of the polar vector  r , the representations of the vibrations of the crystal lattice of single-layer nitroborene (BN)L,1 without division into acoustic and optical vibrations Γ vib , representations of the fundamental acoustic and optical vibrations Γ ac and Γ opt , respectively, representation  z , which determines the symmetry of the electron orbital pz without taking the electron spin into account; representations of electronic π-bands without taking the electron spin into account  π ; a two-valued spinor projective representation describing the transformation of an angular momentum with a full quantum number j = D1/2 + ; a two-valued spinor representation   z , which determines the symmetry of the electron orbital p y taking into account the electron spin, and a two-valued spinor projective representation of electronic π -bands taking the electron spin into account   π . In the bottom part of Table 1, the characteristics are additionally specified spinor irreducible projective representatives the group 6m2 (D3h ) of the projective class K 1 and then with [7], Table 9. The distributions of the vibrational representations  vib ,

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 ac ,  opt by the irreducible representations of the projective class K 0 are given in Table 2, and the distributions of the representations of the electronic π-bands without taking the electron spin into account  π are given over the irreducible representations of the projective class K 0 and the electronic π -zones by irreducible projective representations of the projective class K 1 —in Table 3. In this case, it is obvious that the equations   π =  eq ⊗  z =  π ⊗ D1/2 + hold for the representation   π . Note, in particular, that the symmetries of the valence band and the conduction band of nitroborene (BN)L,1 in pairs, both without taking the electron spin into account (electronic π- and π*-bands, respectively) and taking it into account (electronic π - i(π*) -zones), at the point  of its two-dimensional Brillouin zone coincide. Thus, without taking the electron spin into account of the wave function of each of these zones (electronic π- and π*-zones), according to Table 3, they are transformed by the irreducible representation of 4(0) of the point group of symmetry 6m2 (D3h ). We obtain the same result using the method of linear combinations of atomic (in this case, ionic) orbitals. According to this method, the highest energy-binding π-orbital corresponds to √ the valence band (electronic π-band) of nitroborene (BN)L,1 with  v (r) = 12 [ϕ1 (r) + ϕ2 (r)], where ϕ1 (r) and ϕ2 (r) are π-orbitals, respectively, of nitrogen and boron ions, which form a three-dimensional two-period elementary cell of single-layer nitroborene (BN)L,1 . Using the method of linear combinations of atomic orbits (LCAO method), it is easy to determine that both wave functions v(r) and c(r) are transformed by irreducible unambiguous representations of the projective class K 0 —representation 4(0) of the point symmetry group 6m2 (D3h ). Taking the electron spin into account, the spin wave functions of both electronic bands of nitroborene (BN)L,1 —the highest energy valence band (electronic π -band) and the lowest energy conduction band (electron (π*) —binary zones) are transformed irreducible two-dimensional representation of the projective class K 1 —projective representation 1(1) of the point group 6m2 (D3h ). Point K. The factor group of a diperiodic spatial group of symmetry of the monolayer of the γ-BN crystal—single-layer nitroborene (BN)L,1 over the two-valued invariant subgroup of translations, according to irreducible representations of which the vibrational and electronic states of single-layer nitroborene (BN)L,1 are classified as isomorphic to symmetry group at point K,—the point group 6(C3h ). The star of wave vector at this point has two rays: (kK )1 = −(1/3)(2b1 − b2 ) and (kK )2 = (1/3)(2b1 − b2 ). Table 7 shows the characters of one-valued irreducible vector representations of the point group of symmetry 6(C3h ). The group 6(C3h ) has only one class of projective representations—this is the class K 0 , and therefore all projective representations of the symmetry group 6(C3h ) are projectively equivalent (p-equivalent) to the vector ones. The irreducible one-valued vector representations of the group 6(C3h ) given in Table 7, as well as the irreducible vector representations of any point group of symmetry, correspond to the standard factor system ω (0) (r 2 , r 1 ), which consists only of the values “+1”, i.e., it is a single factor system. To such a unit factor system can

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Table 7 Characters of the irreducible one-valued vector representations of the projective class K 0 of the point group 6(C3h )

be reduced the factor system of any p-equivalent vector projective representation of the symmetry group with the help, in the general case, of complex coefficients u(r) with |u(r)| = 1, where r is an element of the symmetry group, the values of which should create a set, which is a function given on the group [10]. Let us dwell on the construction of a table of two-valued spinor irreducible projective representations of the point group of symmetry 6(C3h ), according to which the electronic states of single-layer nitroborene (BN)L,1 at the point K of its Brillouin zone must be classified. Let us verify that these projective representations really belong to the projective class K0 of the symmetry group 6(C3h ). Over the method given in [6], we construct the factor system ω2 (r 2 , r 1 ) which describes the transformation of spin variables for the group 6(C3h ). As the generating elements of group 6(C3h ), we will choose elements: a = c3 and b = ic2

(1)

Using the defining relations that satisfy the selected generating elements, we determine all the values of the factor system ω2 (r 2 , r 1 ). In this case, the defining  ) should be taken as the determining relations relations for the double group (6) (C3h [6]: a 6 = e, b4 = e, ab = ba.

(2)

Let us establish algebraic expressions for the elements of the double group  ) through their generating elements. Obviously that: (6) (C3h c32 = a 2 , ic65 = ba, and ic6 = qba 2 (ba 2 = ic2 c32 = ic67 = qic6 , i.e. ic6 = qba 2 ),

(3)

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Table 8 Factor-system ω2 (r 2 , r 1 ) for the point symmetry group 6(C3h )

where q is an additional geometrically abstract element of symmetry, which commutes with all symmetry operations and is interpreted as a rotation by an angle of 2π around an arbitrary axis, introduced by Bethe [12]. Such element changes the sign of the spinor, which characterizes quantum states, to the opposite, and therefore the corresponding representation of the symmetry group is called ambiguous. Calculated in this way for the group 6(C3h ) factor-system ω2 (r 2 , r 1 ) is given in Table 8. This factor system belongs to the projective class K 0 since for it α = ω2 (a, b)ω2 (b, a) = ω2 (c3 , ic2 )ω2 (ic2 , c3 ) = 1. The lower index near the values of the coefficients of the factor system ω2 (r, r 1 ) in Table 8, which contain numbers in parentheses, is a table of multiplication of elements of the group 6(C3h ) (numbers in parentheses indicate the numerical designations of the elements). Using the coefficients u2 (r) calculated in the lower part of Table 8, calculated by formulas (13.3), (14.18), and (14.19) [10], this factor system is actually reduced by the transformations: ω (r2 ,r1 ) =

ω(r2 ,r1 )u(r2 r1 ) u(r1 )u(r2 )

(4)

to the p-equivalent unit standard factor system ω 2 (r 2 , r 1 ) = ω (0) (r 2 , r 1 ) of the projective class K 0 . Note also that the values of the coefficients u2 (r) found coincide with the values of the coefficients of reduction of the factor system ω2 (r 2 , r 1 ) to the standard form ω (r 2 , r 1 ) u2 (r) for the corresponding group and for the corresponding element in particular, with the values of the coefficients u2 (r) for the group 6/mmm (D6h ) [6]. Table 9a shows the characters of unambiguous irreducible representations of the  ), additional one-valued irreducible representations double symmetry group (6) (C3h (additional to the usual vector one-valued irreducible representations of the symmetry group 6(C3h ), which can be obtained from irreducible representations of the group  ) by simple deletion from all relations of the element q, its products qr and (6) (C3h all values of characters for them and are two-valued or spinor irreducible projective

representations for symmetry group 6(C3h ) (b)

 ) (a) and the two-valued (spinor) projective Table 9 Characters of the one-valued irreducible representations of the double symmetry group (6) (C3h

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Table 10 Characters of the two-valued spinor irreducible projective representations for K point of the Brillouin zone of single-layer nitroborene (BN)L,1 (point symmetry group 6(C3h ))

representations of the symmetric group 6(C3h ). Spinor representations are denoted by the symbols E  i (i = 1, …, 6) in the Mulliken notation system or by the symbols  i (i = 7, …, 12) in the Koster notation system. Table 9a, b also show the union of complex conjugate representations in two-valued. In the top part of Table 8b, the values of the coefficients u2 (r) are given, which transform the factor system ω2 (r 2 , r 1 ) given in Table 8 to a standard form. The ordering of two-valued spinor irreducible projective representations in Table 9a, b was carried out by alternately multiplying one-valued irreducible vector representations by the values of the coefficients u2 (r) given in the top part of Table 9b. Table 10 presents the characters of two-valued spinor irreducible projective representations of the symmetry group 6(C3h ) for the point K of Brillouin zone of a single-layer nitroborene (BN)L,1 in our notation [7]. We take into account the invariance of states to the time inversion at K point of Brillouin zone of single-layer nitroborene (BN)L,1 using the Hering criterion [5, 6, 10], the calculation procedure of which is described in detail in [10, 11]. The calculation of the sum using this criterion is carried out by the elements g = (α|r  ) g = (α|r  ) of the group of the wave vector Gk corresponding to the condition g k = −k (r  k = −k), where r  is the “rotating” element of the point group of symmetry, α is the vector of nontrivial translation that accompanies the element r  . Note that since the spatial group of symmetry of a single-layer nitroborene (BN)L.1 (the group DG 78) is a symmorphic group, in contrast to the spatial group of symmetry of the crystal lattice of the hexagonal graphite-like boron nitride γ-BN all its symmetry elements do not contain non-trivial translations, i.e., for them all values of αr  = 0. For the point K of the Brillouin zone of a single-layer nitroborene (BN) L, 1, the conditions g k are satisfied by the elements: g 1 = (0|(u 2 )1 ), g 2 = (0|(u 2 )2 ), g 3 = (0|(u 2 )3 ), g 4 = (0|(iu 2 )1 ), g 5 = (0|(iu 2 )2 ), and g 6 = (0|(iu 2 )3 ). Obviously, the squares of these elements (g )2 = (r  α + α|(r  )2 ) at the calculations of oscillatory and electronic states without taking into account the electron spin are equal: (g 1 )2 = (0|e), (g 2 )2 = (0|e), (g 3 )2 = (0|e), (g 4 )2 = (0|e), (g 5 )2 = (0|e), (g 6 )2 = (0|e), and in the calculations of electronic states taking into the electron spin account spinor

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states: (g 1 )2 = (0|q), (g 2 )2 = (0|q), (g 3 )2 = (0|q), (g 4 )2 = (0|q), (g 5 )2 = (0|q), and (g 6 )2 = (0|q). Therefore, since at the point K k and −k they are included in one star of the wave vector, for states without taking the electron spin into account, all representations K 1 (0) , …, K 6 (0) belong to the case a2 ( (g ) 2 (g )2 = 1) and do not combine, and for states with taking the electron spin into account, all spinor representations K 1(0) , …, K 6(0) with taking it into account the factors u2 (r 2 ) [10, 11] and v(r 2 ) [10, 11] belong to the case c2 ( (g ) 2 ( (g ) 2 (g )2 = −1), i.e., they are complex and merge in pairs (according to the classification [10], where the cases a and c for spinor representations change places, these spinor representations refer to the case a2 , where they are also complex and combine in pairs). The characters of the representations of the equivalence of atoms at the point K of the two-dimensional Brillouin zone of a single-layer nitroborene (BN)L,1 of the projective class K 0 —the representations K eq are given in Table 1, where the characters of the representations of this projective class K r , K vib , K z and representations of electronic π-bands without taking the electron spin into account K π are also given, two-valued projective representations of the projective class K 1 : the representations D1/2 + , K  z and two-valued projective representations of electronic π -bands taking the spin electron into account K  π . The distribution of the vibrational representations K vib of a single-layer nitroborene (BN)L,1 over the irreducible representations of the projective class K 0 is given in Table 2, and the distribution of the representations of electronic π-bands without taking the electron spin into account K π by irreducible representations of the projective class K 0 and the projective representations of electronic π -bands taking the electron spin into account K π over the irreducible projective representations of the projective class K 1 is given in Table 3. Point M. Factor group of the diperiodic spatial symmetry group of a singlelayer nitroborene (BN)L,1 by a two-dimensional invariant subgroup of translations, according to the irreducible representations of which the vibrational and electronic states of the single-layer nitroborene (BN)L,1 is isomorphic to the symmetry group of equivalent directions showing in Fig. 4b its two-dimensional Brillouin zone the point symmetry group 2/m (C 2h ). The star of the wave vector at this point contains three rays: (kM )1 = −(1/2)b2 , (kM )2 = (1/2)b1 , and (kM )3 = −(1/2) (b1 − b2 ) (Table 11). From the three rays of the star of the wave vector at point M of the Brillouin zone of a single-layer nitroborene (BN)L,1 , we consider the ray (kM )1 , for which the symmetry elements that convert this ray to the equivalent form of the point symmetry group (2/m) (C 2h ), and there are elements e, (u 2 )1 , ic2 , and i(u2 )1 . An isomorphic group to a group of rotating elements of symmetry D2 , consisting of the symmetry elements e, (u2 )1 , c2 , (u 2 )1 , a group (2/m) (C 2 h), created by elements of symmetry e, (u 2 )1 , ic2 , (iu 2 )1 , contains the reflection plane i(u2 )1 . The presence of this reflection plane in the group (2/m) (C 2h ), as well as the presence of the rotational axis c2 in the group (D2 ), means that the rotational axes (u2 )1 in both of these groups, although the axis (u2 )1 in the group (2/m) (C 2h ) are only a rotating component of the symmetry element i(u2 )1 , bilateral (nonpolar), that is, the direction for the axes (u2 )1

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Table 11 Factor-system ω2 (r2 , r1 ) (a) and standard factor-system of the projective class K 1 for  (r , r ) (b) M point of Brillouin zone of single-layer nitroborene (BN)L,1 ω2 (r2 , r1 ) ≡ ω(1) 2 1

in each of these groups is one of the two antiparallel directions (as the direction of the axis (u2 )1 , and the direction of the axis (u2 )1 ) (Table 12). That is, using the formula f −1 cl(α)f = cf −1l (α) [10, 13] to describe the turns in double groups for the definition of the element (u2 )1 in the double group (2/m) (C  2 h ), it is possible, omitting near the designations of the axes (u 2 )1 and (u2 ) + the bottom indices “1”, to take the expression u2 = c4 u 2 qc4 3 , where the element f −1 = c4 transforms the axis u 2 (u 2 l0y) to the axis u 2 (u2 0x) (more precisely, to the direction u2 ). Therefore, u2 = c4 u 2 qc4 3 . It is also easy to see that qu 2 qc4 u 2 = c−l  (π/2), where l —the direction of axis c2 (c2l 0z). Taking into account that c−l (π/2) = qcl (3π/2) = qc4 3 , we have qu 2 c4 u 2 = qc4 3 or u 2 c4 u 2 = c4 3 . Multiplying both parts of the last expression by u 2 at left, Table 12 Characters of the irreducible projective representations for M point of Brillouin zone of single-layer nitroborene (BN)L,1 (point symmetry group 2/m (C2h ))

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Fig. 5 Dispersion of the energy electron π-bands in the hexagonal graphite-like boron nitride γ-BN crystal

obtain u 2 u 2 c4 u 2 = u 2 c4 3 . Because (u 2 )2 = q, qc4 u 2 = u 2 c4 3 or u 2 c4 3 = qc4 u 2 . So, finally u2 = qc4 qc4 u 2 = c4 2 u 2 = c2 u 2. Denoting symmetry elements u 2 = α and c2 = β, obtain u2 = βα. It is easy to find that u 2 c2 qu 2 = c−l (π) = qc2 or u 2 c2 u 2 = c2 . Multiplying both parts of the last expression by u 2 at left, obtain qc2 u 2 = u 2 c2 or u 2 c2 = qc2 u 2 , i.e., αβ = qβα. Multiplying the left and right parts of the equation c2 = β by inversion i, we receive that ic2 = iβ. Turning to the notation of the generating elements of symmetry group (2/m) (C 2h ) u 2 = α = a and ic2 = iβ = b, we get the expressions for the symmetry elements of double group (2/m) (C  2h ) iu2 = iβα = ba, and for its permutation determining relation αβ = qiβα, i.e., finally iu2 = ba and ab = qba.

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4 Dispersion of Electron Zones in Graphite-Like Boron Nitride γ-BN Crystal and Single-Layer Nitroborene (BN)L,1 without Taking the Electron Spin into Account and Taking it into Account. Spin-Dependent Splitting of Electronic States Figure 5 schematically shows the dispersion of the energy levels of the electronic π-zones of the hexagonal graphite-like boron nitride crystal γ-BN, and Fig. 6 shows the energy dispersion of the electronic π-zones of the monolayer of the γ-BN crystal (single-layer, nitroboren). The letters in Figs. 5 and 6 denote the points of the corresponding Brillouin zones, and the two-storey symbols from letters and indices separated by horizontal lines: over the lines denote one-valued irreducible projective representations characterizing the symmetry of electronic states without taking the electron spin into account and under the lines the notation of the two-valued irreducible projective representations, which describe the symmetry of electronic states taking the electron spin into account. The upper indices in parentheses indicate the projective classes of representations, and the lower indices near the letters indicate their numbers according to the tables of irreducible projective representations. The dashes next to the letters indicate that the representation is two-valued spinor, without the dashed mark—the representation is one-valued vector. The dispersion of electronic π-bands presented in Figs. 5 and 6 agrees well with numerical calculations without taking the electron spin into account, i.e., at a small value of the spin–orbit interaction energy obtained in [13, 14], but additionally includes also the qualitative behaviour of the dispersion of electronic π-bands

Fig. 6 Dispersion of the energy electron π-bands in the single-layer nitroborene (BN)L,1

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Fig. 7 Energy dispersions of electron π-zones in the K–P–H direction of the Brillouin zone of hexagonal graphite-like boron nitride γ-BN crystals without taking the electron spin into account (a) and π -zones with taking the electron spin into account (b). Fine spin-dependent structure of π zones in b is shown schematically with magnification of the splitting energies of electron π -zones by about 103 times

along the line  − − A of the Brillouin zone of crystalline hexagonal graphite-like boron nitride. Figure 7a shows the dispersion of electron π-bands along the line K−P−H of the Brillouin zone of crystalline hexagonal graphite-like boron nitride γ-BN without taking the electron spin into account, and Fig. 7b qualitatively presents very increased on the energy scale (approximately 103 times) to demonstrate the splitting of electronic states shown in Fig. 7b, that involves the application of methods of symmetric theoretical-group analysis to determine the dispersion of states of electronic π bands of crystalline hexagonal graphite-like boron nitride taking the electron spin into account. The magnitude of the spin-dependent splittings can be significant, for example, for transition metal dichalcogenides with the same spatial symmetry group but is small for crystalline hexagonal boron nitride, as well as for crystalline graphite with the same spatial symmetry group (1.0–1.5 meV according to estimates [15]), since it is due to the low energy of the spin–orbit interaction for atoms and ions of both nitrogen and boron and, as a consequence, for nitride-selective structures. We also note that from the symmetric theoretical-group analysis of the dispersion of π-electron excitations in the monolayer of a crystal of a hexagonal graphitelike boron nitride γ-BN single-layer nitroborene (BN)L,1 are not expected since its nondegenerate orbitals without taking the electron spin into account correspond to the orbitals K 6(0) and K 4(0) twice degenerate with the spin of the electron combined spinor orbitals ((K  )01 + (K  )04 ) and ((K  )02 + (K  )05 ), respectively.

5 Conclusions The following conclusions can be done from the work:

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2.

3.

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For the first time for crystalline hexagonal graphite-like boron nitride γ-BN 4 ) ) and its monolayer–single-layer (spatial symmetry group P63 /mmc (D6h nitroborene (B N ) L ,1 (diperiodic spatial symmetry group P6m2 (DG78) ) a symmetric theoretical-group description of the dispersion of electronic π-bands is given. For these structures, the conditions of compatibility of irreducible projective representations of symmetry groups of a wave vector without taking the electron spin into account and taking the electron spin into account and changes in projective classes for different points of high symmetry of their Brillouin zones are determined for the first time. Correlation of electronic excitations of crystalline hexagonal graphite-like boron nitride taking the electron spin into account with spinor excitations of singlelayer nitroborene is presented. For the first time, a symmetric classification of vibrational excitations of crystals of hexagonal graphite-like boron nitride and single-layer nitroborene at different points of high symmetry of their Brillouin zones is given. For the first time using the theoretical symmetry-group methods the spindependent splitting of energy spectra of electron π-bands due to taking the electron spin into account even for minor spin–orbital interaction is declared. This fine structure is the occurrence of insignificant (~1.0 ÷ 1.5 meV) according to estimates [15]) spin-dependent splitting of electronic π-bands at the K and H points of the Brillouin zone of crystalline hexagonal graphite-like boron nitride. For the first time the expected absence of fine spin-dependent splitting of energy electron spectra at points of high symmetry of the Brillouin zone of single-layer nitroborene is indicated.

References 1. Bernal JD (1924) The structure of graphite. Proc R Soc Lond A 106:749. https://doi.org/10. 1098/rspa.1924.0101 2. Hahn T (1983) International tables for crystallography. In: Reidel D (ed) Space group symmetry, vol A. https://doi.org/10.1107/97809553602060000100 3. Herring C (1937) Effect on time-reversal symmetry on energy bands of crystals. Rhys. Rev. 52:361. https://doi.org/10.1103/PhysRev.52.361 4. Herring C (1937) Accidental degeneracy in the energy bands of crystals. Phys Rev 52:365. https://doi.org/10.1103/PhysRev.52.365 5. Wood EA (1964) The 80 diperiodic groups in three dimensions. Bell System Tech J 43:541. https://doi.org/10.1002/j.1538-7305.1964.tb04077.x 6. Gubanov VO, Naumenko AP, Bilyi MM, Dotsenko IS, Sabov MM, Iakhnenko MS, Bulavin LA (2020) Energy spectra of electron excitations in graphite and graphene and their dispersion making allowance for the electron spin and the time-reversal symmetry. Ukr J Phys 65:342–366. https://doi.org/10.15407/ujpe65.4.342 7. Gubanov VO, Naumenko AP, Bilyi MM, Dotsenko IS, Navozenko OM, Sabov MM, Bulavin LA (2018) Energy spectra correlation of vibrational and electronic excitations and their dispersion in graphite and graphene. Ukr J Phys 63:431–454. https://doi.org/10.15407/ujpe63.5.431

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8. Dresselhaus MS, Dresselhaus G, Jorio A (2008) Group theory. Application to the physics of condensed matter. Springer 9. Gubanov VO, Naumenko AP, Dotsenko IS, Sabov MM, Gryn DV, Bulavin LA (2020) Fine spin-dependent splitting of electronic excitations and their dispersion in single-layer graphene and graphite. Ukr J Phys 65:625. https://doi.org/10.15407/ujpe65.7.625 10. Bir GL, Pikus GE (1974) Symmetry and strain-induced effects in semiconductors. Wiley 11. Balchuk DS, Bilyi MM, Gryschuk VP, Gubanov VO, Kononov VK (1996) Symmetry of vibrational modes, invariance of energy states to time inversion, and Raman scattering in 4H- and 6H-SiC crystals. 1. Classification of energy states in Brillouin zones. Ukr J Phys 41:146 12. Bethe HA (1929) Termaufspaltung in Kristallen. Ann Physik 395:133. https://doi.org/10.1002/ andp.19293950202 13. Doni E, Pastori Parravicin IG (1969) Energy bands and optical properties of hexagonal boron nitride and graphite. Nuovo Cimento B 64:117. https://doi.org/10.1007/BF02710286 14. Bassani F, Pastori Parravicini G (1975) Electronic states and optical transitions in solids. Pergamon Press 15. Katsnelson MI (2012) Graphene: carbon in two dimensions. Cambridge University Press, Cambridge

Nanochemistry and Nanobiotechnology

The Influence of β-cyclodextrin on Biomembrane. The Molecular Dynamics Simulation Study D. Makieła, M. Pabiszczak, and Z. Gburski

Abstract The molecular dynamics simulations were performed to assess the influence of β-cyclodextrin molecules on a phospholipid bilayer. To mimic the living cell, biomembrane is composed of 1,2-dimyristoyl-sn-glycero-3phosphocholine (DMPC) with and without cholesterol. The model biomembrane was placed in a water environment. The computer simulations show that the βCD molecules do not extract DMPC or cholesterol molecules from the membrane. Moreover, β-cyclodextrin molecules are not able to penetrate the hydrophobic core of cell membrane (phospholipid bilayer). This is a very desirable circumstance. The βCD molecules, while being neutral toward the biomembrane and not affecting its structure and functionality, increase the solubility of cholesterol in water. This observation favorable contributes to quest for the new, nanoscale tools suitable for the treatment of atherosclerosis disease.

1 Introduction The living cell is a basic component of all eukaryote organisms on Earth. It is protected by a cell membrane, efficient barrier for external molecules, proteins, and particles. The cell membrane is considerably a complex molecular system in terms of its structure as well as mechanical and thermophysical properties. Phospholipids are a class of lipids that are a major component of all biomembranes. The structure of the phospholipid molecule generally consists of two hydrophobic fatty acid “tails” and a hydrophilic “head” consisting of a phosphate group. The two components D. Makieła · M. Pabiszczak · Z. Gburski (B) Institute of Physics, Silesian Centre for Education and Interdisciplinary Research, University of Silesia in Katowice, 75 Pułku Piechoty 1a, 41-500 Chorzów, Poland e-mail: [email protected] D. Makieła e-mail: [email protected] M. Pabiszczak e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_8

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are joined together by a glycerol molecule. They can form lipid bilayers because of their amphiphilic character. Lipid bilayers occur when hydrophobic tails line up against one another, forming a membrane of hydrophilic heads on both sides facing the water. Biological membranes in eukaryotes also contain another class of lipid, sterols, interspersed among the phospholipids and together they regulate membrane fluidity and mechanical strength. Cholesterol is a special type of lipid, known as a sterol due to its molecular structure made of the steroid and alcohol. Cholesterol is present in all mammalian (including human) cell membranes with amounts varying from approximately 20% to about 50%. However, it is absent in the intracellular as well as prokaryote membranes. The so far performed experimental and computational studies have shown that cholesterol is one of the most important lipid molecules in biomembranes, due to its functional ability to modulate their physical properties. It is well-known, for instance, that the fluidity of the cell membrane is regulated by cholesterol concentration. The stiffening of the bilayer is a result of cholesterol appearance in the gaps between phospholipids. On the other hand, the increased fluidity of the bilayer is a consequence of the bending of hydrocarbon groups in phospholipid molecules which takes place when cholesterol is present at a very low concentration. Consequently, cholesterol is essential for cell viability with the maintenance of the appropriate cell membrane structure being its key function. Cholesterol also circulates with the bloodstream as a component of lipoproteins and can be found in the lymphatic fluid of the human body. There is an amount of literature on the role of cholesterol in biosystems [1–4]. Although cholesterol is essential for the functioning of cell membranes, its excess may prove unhealthy. Atherosclerosis is an inflammatory disease linked to elevated blood cholesterol concentrations. The search for new methods to remove the excess cholesterol molecules, precursors of plaque deposition in an early phase of atherosclerosis disease, is a vital subject of molecular medicine. Many studies have shown that β-cyclodextrin (βCD) increases cholesterol water solubility allowing potential use of βCD in the context of combating with atherosclerosis disease [5–8]. Our previous research also showed that βCD increases the solubility of cholesterol in an aqueous environment [9]. The βCD is oligosaccharide supramolecule with a unique toroidal (cup shaped) structure, and it is composed of seven D-glucose units bonded with α-1,4-glysosidic linkage [10]. The depth of the βCD pocket equals 7.8 Å, while its width varies from 7.8 (top) to 5.7 Å(bottom). The βCD is easily accessible from the natural sources, with good water-solubility and high biocompatibility. A stereochemical arrangement of the βCD toroid implies that its interior is hydrophobic, while the exterior is hydrophilic. This three-dimensional stereochemical arrangement enables the complexation of hydrophobic molecules (guests), in βCD inert cavity (hosts). The so-called host–guest inclusion complexes are formed [11]. As a result of this self-assembly, both the water solubility of the guest and the overall biocompatibility of the formed systems are increased. The cyclodextrins have already been used in pharmaceutical industry mainly as complexing agents to increase the aqueous solubility of poorly water-soluble drugs and to increase their bioavailability and

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stability [12–16]. The cavity diameter of β-cyclodextrin molecule has been found to be the most appropriate size for hormones, vitamins, and other compounds frequently used in tissue and cell culture applications. In the context of potential medical applications of βCD for atherosclerotic therapy, it is extremely important to study the βCD interaction with the cell membrane and cholesterol contained in it. In this work, we have used MD technique to study in detail the effect of βCD on phospholipid bilayer composed of the important living cell biomembrane constituent, namely, 1,2-dimyristoyl-sn-glycero3phosphocholine (DMPC) molecules with and without cholesterol 2 interspersed among the phospholipids [17–28].

2 Simulation Details The MD simulations were performed using NAMD 2.11 [29] simulation software with the all-atom CHARMM potential force field [30] and visualized with VMD 1.9.2 [31]. The studied systems are composed of 7 βCD and phospholipid bilayer composed of DMPC molecules with (bilayer 1) and without (bilayer 2) cholesterol interspersed among the phospholipids. Both bilayers are composed of 232 molecules of DMPC, and the first one contains additionally 48 cholesterol molecules. Hence, the first phospholipid bilayer is larger than the second. The structure of βCD [32] molecule is shown in Fig. 1, and the configuration of cholesterol and DMPC molecules in phospholipid bilayer is shown in Fig. 2. The cutoff distance for all non-bonding interactions was set to 10 Å. The systems were simulated in water environment with periodic boundary conditions. The initial simulation box was set to 113 × 92 × 110 Å and 114 × 101 × 122 Å. The number of water molecules include in this box was 23,833 and 33,563 for bilayer 1 and 2, Fig. 1 The structure of β-cyclodextrin—VMD visualization

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Fig. 2 Typical configuration of cholesterol and DMPC molecules in the phospholipid bilayer 1

respectively. In these simulation runs, pressure was controlled using Langevin barostat. The Langevin piston was used to control the pressure at 1 atm, with a piston period of 100 fs and a damping time constant of 50 fs. Long-range electrostatic forces were taken into account by means of the Particle Mesh Ewald 4 [33] approach. The equilibration in NPT (constant number of particles, constant pressure, and constant temperature) was performed. The integration time step was set to t = 1.0 fs for all simulation runs. The standard NAMD integrator (Brünger–Brooks–Karplus algorithm) [34] was used. Next, the “production” phases were started. The duration of the studied runs was set to 30 ns. During this stage, the data were collected every 2000 simulation steps (2 ps). The systems were simulated at six temperatures (T = 280, 290, 300, 310, 320, and 330 K). Five independent simulations runs were performed for each temperature. The temperature was maintained, employing Langevin thermostat with a damping coefficient of 1 ps−1 . All interactions in the simulated systems were described with CHARMM potential. The CHARMM force field includes intramolecular harmonic stretching Vbond , harmonic bending Vtotal = Vbond + Vangle + Vdi hedral + VvdW + VCoulomb

(1)

Vbond = Kr (r − r0 )2

(2)

where

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Vangle = K ( − 0 )2 Vdi hedral = Kϕ (1 + cos(nϕ − γ )) for n = 0 Vdi hedral = Kϕ (ϕ − γ )2 for n = 0   VvdW = 4 ∈ (σ/r)12 − (σ/r)6

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(3)

(4) (5)

The last two terms in the equation for the total potential energy Vtotal of the system come from the nonbonding interactions: van der Waals forces modeled with the standard Lenard-Jones 12–6 potential VvdW (with the Lorentz-Berthelot mixing rules) and electrostatic interactions VCoulomb . The standard TIP3P CHARMM model [35] was used in our simulations. All molecules used in our studies were modeled on the fully atomistic level.

3 Results We have constructed the initial configuration of the system by placing seven βCD molecules on phospholipid bilayer composed of the DMPC molecules with (system 1) and without (system 2) cholesterol interspersed among the phospholipids, then MD simulations were performed. In Figs. 3 and 4, we supply the sequence of snapshots visualizing the system 1 and 2, at T = 310 K. We observe that βCD molecules do not tend to adhere to the surface of the membrane and move over the entire volume of the system. We have calculated the root mean square displacement of atomic positions (RMSD) [36]. The calculations were performed separately for βCD and DMPC molecules. The RMSD plots are presented, as a function of temperature, in Fig. 5. The presented RMSD was determined as the average of the root mean square displacement without alignment, i.e., the elimination of translator and rotatory displacement of atoms [36], from the final part of the obtained trajectories (from 10 ns onward). Only the core atoms were taken into account (hydrogens were seven excluded from the RMSD calculations). The DMPC molecules in bilayer without cholesterol appear to be less mobile. We calculated average number of hydrogen bonds per one βCD molecule as a function of temperature. Hydrogen bonds are calculated between βCD–water, βCD– βCD, and βCD–DMPC molecules. A hydrogen bond is formed between an atom with a hydrogen bonded to it (the donor, D), and another atom (the acceptor, A) provided that the D-A distance is smaller than the cut-off distance (default 3.0), and the angle D-A-H is smaller than the cut-off angle (default 20°). Our calculations have shown that βCD molecules have greater affinity for water and other βCD molecules than for DMPC molecules. We did not observe any development of the structure based on hydrogen bonds between the βCDs and bilayer.

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Fig. 3 The sequence of snapshots visualizing the system 1 at T = 310 K. The water molecules are not shown for clarity

The next figure (Fig. 6) shows the thermal dependency of Lindemann δL index of βCD molecules. The values of Lindemann index are typical for liquid systems above the threshold value of 0.1. In case of the membrane with cholesterols, only very weak dependency between temperature and Lindemann index can be observed. The obtained results suggest that βCD-s are solvated in the water environment and do not strongly adsorb to the surface of the bilayer. The next figures (Figs. 7 and 8) show density profile in the z-axis direction for βCD molecules and phospholipid bilayers. The βCD molecules do not migrate into the membrane as well as do not extract DMPC or cholesterol molecules from the membrane. The detailed density profiles for βCDs show that the concentration of the βCD’s directly at the membrane surface decreases with the temperature. The none zero values of βCD density in the membrane region for higher temperatures suggest that with the increase of the temperature the mixing between hydrophilic parts of phospholipids and βCD slightly increases. Presented density profiles and visual inspection of the obtained trajectories suggest that the βCD molecules are not able to penetrate the hydrophobic part of phospholipid bilayer. The presence of cholesterol in the system 1 increases the stiffness of the membrane and reduces the penetration of the bilayer, effectively reducing the

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Fig. 4 The sequence of snapshots visualizing the system 2 at T = 310 K. The water molecules are not shown for clarity

penetration of the membrane by βCDs. The βCD molecules increase the solubility of cholesterol in water [13] while being neutral toward the membrane.

4 Conclusions We performed a series of MD simulations of the effect of βCD on phospholipid bilayer composed of 1,2-dimyristoyl-sn-glycero-3phosphocholine (DMPC) with and without cholesterols molecules placed among the phospholipids. These simulations were performed in a water environment. The simulations show that the βCD molecules do not extract DMPC or cholesterol molecules from the membrane. Moreover, βCD molecules are unable to penetrate the hydrocarbon core of cell membrane (phospholipid bilayer). This is a very favorable result. The βCD molecules, being neutral toward the biomembrane, increase the solubility of cholesterol in water

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Fig. 5 RMSD as a function of temperature for βCD and DMPC molecules

Fig. 6 The Lindemann index for βCD molecules

[9]. This observation documented above sets a ground for a future use of βCD in the context of searching for new, nanoscale tools of combating the atherosclerosis disease.

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Fig. 7 Density profile in the z-axis direction for βCD molecules and a bilayer 1 and b bilayer 2, at T = 310 K

Fig. 8 Density profile in the z-axis direction for βCD molecules for a system 1 and b system 2

Acknowledgements This research was supported in part by PAAD Infrastructure co-financed by Operational Programme Innovative Economy, Objective 2.3. We would like to thank the PL Grid supercomputers network for sharing computational resources.

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Conjugate Formation in Films of Polyethylene Glycol and Polypropylene Glycol Nanocomposites with MultiWall Carbon Nanotubes M. A. Alieksandrov, A. M. Gaponov, T. M. Pinchuk-Rugal, O. P. Dmytrenko, Antonina Naumenko, V. M. Popruzhko, and M. P. Kulish Abstract Films of polyethylene glycol (PEG) and polypropylene glycol (PPG) composites with multiwalled carbon nanotubes (MWCNT) were made by “the Doctor blade coating” method. The percolation dependences of electrical conductivity of composites of PENG-MWCNT, PPG-MWCNT films, photoluminescence (PL) emission, excitation photoluminescence in a wide range of BVNT concentrations before and after the percolation threshold were studied. Low values of percolation thresholds were obtained for both types of composites (ϕcPEG-MWCNT = 0.0003, ϕcPPG-MWNT = 0.0002). The electrical conductivity indicates the presence of tunnel jumps of charge carriers, which is associated with complex formation in composites. The presence of such complexes affects the intra-combinational and inter-combinational transitions of the excitation energy, which is manifested in the PL emission spectra of polymers and Van Hoff singularities of nanotubes.

1 Introduction Polymers when added to solutions of molecules, including water, can effectively affect their photodynamic properties [1]. This change in properties is especially important to increase the efficiency of molecules that are used as photosensitizers to solve various problems, including photodynamic therapy [2]. In the case of the use of small additives of polyacrylamide and its derivatives to solutions in water of hetero porphyrin molecules as effective photosensitizers, generating singlet oxygen, the destruction of aggregates of these molecules is observed due to binding to complexes of porphyrins with polymers. Thus, polymers act as molecular stabilizers that can significantly affect the content of the polynomial component of solutions and, as a consequence, the parameters of the internal and intercombination conversion of excited light. Polymers play an important stabilizing role in the synthesis of metal nanoparticles, including gold, in their solutions. In addition, the polymer shell M. A. Alieksandrov (B) · A. M. Gaponov · T. M. Pinchuk-Rugal · O. P. Dmytrenko · A. Naumenko · V. M. Popruzhko · M. P. Kulish Taras Shevchenko National University of Kyiv, Kyiv, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_9

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formed around the nanoparticles significantly affects the surface plasmon resonance bands of gold nanoparticles and, as a consequence, the photosensitizing efficiency of molecules [3, 4]. Polymers play an important role not only in the functionalization of the properties of metal nanoparticles, but also carbon nanostructures, such as single- and multiwalled carbon nanotubes [5–15]. It is known that in the spectra of electronic absorption of nanotubes of both types, bands with low intensity appear due to Van Hoff singularities and are associated with optical transitions M11 , S11 , S22, S33 . The optical virginity for these transitions is determined by the chirality (diameter) of the nanotubes. Photoluminescence spectra are not intense, their bands are mainly in the infrared region and are insufficiently studied. Significant photosensitization of nanotubes is achieved using conjugated systems (linear or branched dyes), the involvement of which allows to obtain effective emission of nanotubes with appropriate excitation and, as a consequence, organic sensors as a result of intercombination transitions of excitation energy from dyes to nanotubes. The aim of this work is to study the percolation and photosensitization properties and to establish their mechanisms in the films of nanocomposites of polyethylene glycol (PEG) and polypropylene glycol (PPG) with multiwall carbon nanotubes (MWCNT).

2 Experimental Studies PEG-400 powders, PPG, and MWCNT additives made by temperature catalytic conversion were used for the production of film composites. The structural formula of PEG-400 and PPG is shown in Fig. 1. MWCNT were weighed on scales with an accuracy of 0.0001 g. Weighted modifiers were poured into the flask, to which PEG was then added. The calculated masses of PEG and PPG were converted to ml and added to the flask with a syringe. Then, the stand with flasks was placed in an ultrasonic bath with warm water for 5 min. Films of pure PEG, PPG, and their composites with MWCNT were synthesized by the method of “Doctor blade coating” or “slot die coating.” The blade (barrier) is placed at a certain distance (height above) from the substrate. The thickness of the wet film can be adjusted by changing the distance between the blade and the substrate. In front

Fig. 1 Structural formulas of PEG-400 (left) and PPG (right)

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of the blade is a certain volume of solution in the tank, or in the form of a drop. Next, the blade moves along the substrate, leaving behind a wet film of a certain thickness. The accuracy of the drying temperature was set at ±4 ºC. The temperature of the evaporation process was set (adjusted) so that the rate of complete evaporation did not exceed 24 h. This temperature was about 180 ºC. The thickness of PEG films and PPG was 100–200 μm. Concentrations of PEG-MWCNT and PPG-MWCNT films were 0–1.0 wt.%. The optical absorption spectra of the films were recorded using a Lumsail 723PC spectrophotometer in the wavelength range 320–110 nm and a scanning step of 1 nm. The photoluminescence spectra of the films were recorded using a Horiba Scientific Fluoromax + Spectrofluorometer in the wavelength range 380–700 nm λex = 370 nm and 900–1400 nm λex = 660 nm.

3 Results and Discussion Polyethylene glycol (PEG) and polypropylene glycol (PPG) are high molecular weight compounds with low molecular weight macrochains. They are soluble in water, which contributes to their widespread use in pharmaceuticals and are used as binding macromolecules between gold nanoparticles and DNA (DNA), PEGlycolization of drugs, including for anticancer therapy [16]. One of the important properties of PEG is its ability to enhance the photoluminescent characteristics of conjugate systems, such as coumarin derivatives [17]. A special role is played by the formation of PEG conjugates with nanoparticles of graphene oxides on their interaction with drugs and conjugated molecules. This functionalization of graphene oxide nanoparticles is accompanied by an improvement in drug efficacy, a change in the confirmatory structure of proteins, which is a manifestation of the formation of hydrogen bonds and π-stacking [18, 19]. In addition, there are significant changes in the thermal conductivity of PEG immobilized on MSM-41 [20]. At the same time, taking into account the properties of carbon nanotubes and the special physical state of PEG and PPG polymers, the study of the percolation behavior of the electrical conductivity of nanocomposites of these polymers with MWCNT attracts attention. It is known that this behavior strongly depends on the methods of preparation of these nanocomposites. Thus, obtaining a segregated structure of nanocomposites allows the synthesis of bulk samples of PE and PVC with MWCNT with very small values of the percolation threshold [21, 22]. However, it was shown that the presence of nanotubes and radiation modification of nanocomposites, especially in the PVC-MWCNT system, leads to significant transformations of conjugated polyene structures, which are accompanied by cardinal changes in the photoluminescence spectra of these nanocomposites [23, 24]. Given the molecular structure of PEG and PPG polymers, which promotes their binding to conjugate systems, including nanotubes, we can expect not only the percolation behavior of MWCNT nanocomposites with these polymers, but also the photosensitization of the films of these systems upon excitation by light.

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In Fig. 2, percolation curves of electrical conductivity of PEG-MWCNT and PPG-MWCNT nanocomposite films are presented. It is seen that the percolation curves for both nanocomposites can be described by scaling dependences both before and after the percolation threshold. If we present the scaling dependence in the volume fractions of the content of nanotubes ϕ, and the percolation threshold is equal to ϕc vol. fractions, the scaling curves in the region ϕ > ϕc can be described by the equation: σ = σ0 ∗ (ϕ − ϕc )t

(1)

σ0 —electrical conductivity of the filler (nanotubes) in the polymer matrix. By the way, this value may differ significantly from its value for pure filler. In the

Fig. 2 Concentration dependences of electrical conductivity for films of nanocomposites PEGMWCNT (a) and PPG-MWCNT (b). (The inserts show the logarithmic dependences of electrical conductivity on lg (ϕ − ϕc), solid lines are the result of fitting in accordance with the scaling dependence)

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scaling equation, the exponent t is the critical factor associated with the nature of the distribution in this case of nanoparticles in the polymer matrix. (With the statistical distribution of the filler theoretical value t = 2). As a rule, only in some cases, the specified value is obtained for specific composites. The following electrical conductivity parameters were obtained for the PEGMWCNT nanocomposite from the scaling approximation: ϕc = 0,0003 vol. frac., t = 3, 2. At the same time, it can be noted that for both nanocomposites, very small values of the percolation threshold ϕc were obtained. Such values are inherent in the segregated distribution of the filler. Similar bulk conductivity behavior was also obtained for bulk nanocomposites of PEG and PPG with MWCNT, but the value of ϕc for them is much higher [25–28]. It should be noted that with the change in the content of nanotubes in the nanocomposite PEG-MWCNT, there are changes in the microstructure, phase transformations, percolation behavior, and temperature [29]. It can be assumed that these changes are a manifestation of the formation of conjugates between polymer macrochains and nanotubes. At the same time, the magnitude of the increase in electrical conductivity in the percolation interval for both systems differ. If for the PEG-MWCNT nanocomposite, it increases by about 9 orders of magnitude, then for the PPG-MWCNT system, the increase in electrical conductivity is much smaller and reaches about 6 orders of magnitude. In the post-percolation region, where contact phenomena have a significant role, for the PEG-MWCNT composite, the electrical conductivity continues to increase with increasing MWCNT content, and for the PPG-MWCNT system, it remains almost unchanged. It can be assumed that this change in electrical conductivity is due not only to the formation of a conductive cluster, but also to tunnel jumps of electrons in the case of conjugate formation. The formation of complexes between these structures will be accompanied not only by the influence on the conductivity, but also by changes in the optical properties, as a consequence of the transfer between the components of the nanocomposite of the excitation energy. Figure 3 shows the optical absorption spectra of pure PEG films and its nanocomposites with MWCNT, including the decomposition of the absorption bands into components. It is seen that with increasing MWCNT content, the optical density inherent in the PEG polymer decreases, and the absorption band expands toward greater wavelengths, with the emergence of new absorption components. The greatest expansion is the band of optical density for nanocomposite with 1.0 wt.% MWCNT. In this case, the optical density for almost all new components is approximately the same throughout the spectral region. This rearrangement of the optical absorption spectrum is due not only to the contribution to the absorption of nanotubes, but also to the formation of the percolation cluster and the resulting rearrangement of the electronic structure of PEG due to effective interaction between these components of PEGMWCNT nanocomposites. The mechanism of binding between PEG and MWCNT can be a stack interaction between π-conjugated elements of both components of this nanocomposite.

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For MWCNT, starting from a wavelength of about 300 nm, there is an optical absorption due to various transitions associated with the features of Van Hoff. The absorption bands for different singularities are concentrated in a wide range of wavelengths from 300 to 1800 nm. In this case, the widest interval corresponds to the singularities of the type S11 . It is seen that in the considered range, there are optical absorption bands due to the singularities S33 , M11 , S22 , S11 . Figure 4 shows the emission spectra of photoluminescence (PL) for pure and MWCNT and nanocomposite PEG-MWCNT. The emission spectra of PL for MWCNT and nanocomposites with different contents of nanotubes do not differ. With increasing concentration of MWCNT in nanocomposites, the intensity of the bands consistently increases. The set of PL emission bands corresponds to the singularities S11 for nanotubes of different chiralities. In the case of PPG-MWCNT nanocomposites, the PL emission spectra remain

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Fig. 4 PL emission spectra for pure PEG film (1), MWCNT powder (2), PEG-MWCNT nanocomposite film (3) (CMWCNT = 1 mass %, λex = 660 nm)

similar to their behavior for the PEG-MWCNT system, but the intensity of the bands becomes smaller as in Fig. 5. As for the PEG-MWCNT nanocomposite, photosensitization of nanotubes is observed for the PPG-MWCNT system, but it is less pronounced. It can be assumed that in this system of complexation between the macromolecules of the polymer matrix and nanotubes, which promotes tunneling conductivity and photosensitization is less than for the composite PEG-MWCNT. The PL excitation spectra obtained for composites with different MWCNT content have a set of PL peaks at 940, 950, 979, 992, and 1360 cm−1 . It is obvious that the considered PL emission bands for the specified λex = 640 nm are associated with the singularities manifested in this absorption range [41, 42]. Conjugation of PEG with nanotubes is accompanied by rearrangement of the emission spectra of the PL polymer, Fig. 6.

Fig. 5 PL emission spectra for pure PPG film (1), MWCNT powder (2), PPG-MWCNT nanocomposite film (3) (CMWCNT = 1 wt. %, λzb = 660 nm)

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Fig. 6 PL emission spectra for pure PEG film (1) and PEG-MWCNT nanocomposites containing 0.2 (2), 0.4 (3), 0.6 (4), 0.8 (5), 1.0 wt.% MWCNT (6) (λex = 370 nm)

It is seen that with a change in the content of nanotubes, a significant rearrangement of the PL spectrum in the region of smaller wavelengths is observed. If for PEG films and nanocomposite with 0.2 wt.% MWCNT at this scale, the intensities of PL peaks are not observed, then for nanocomposite with 0.4 wt.% MWCNT, PL bands appear at 412 and 436 nm. In addition, there is a PL arm of about 600 nm, and the value of the PL intensity in the entire wavelength range is the highest. As the concentration of nanotubes increases, these bands disappear. However, the complexation between PEG and MWCNT, which is accompanied by the transfer of excitation energy of the polymer to the nanotubes also changes the value of the internal conversion rate constant, because the behavior of PL with increasing nanotube content is nonmonotonic. At low concentrations of nanotubes (0.2 wt.% MWCNT), noticeable PL does not occur, and it appears only at 0.4 wt.% MWCNT, when the content of conjugates increases. In the case of further growth of their content, due to the increase in the concentration of MWCNT increases the role of inter-combinational transitions, which causes the quenching of the PL polymer. The PL excitation spectra obtained at λex at about 414, 437, 469, 607 nm indicate the appearance of absorption bands nearby 370 nm. Similar behavior of the PL emission and photoluminescence excitation spectra is also observed for the PPG-MWCNT system with different nanotube contents. Even at small excitation wavelengths (267, 320, 375 nm), starting from the wavelength of about 350 nm, PL peaks appear due to the Van Hoff singularities S33 , M11 . As for PEG-MWCNT nanocomposites, nonmonotonic behavior of the PL peak intensities of singular transitions is observed at different concentrations of nanotubes, which indicates a similar mechanism due to the influence of conjugate formation on internal and intercombination conversion. At the same time, the considered effects are weaker in comparison with the PEG-MWCNT system.

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4 Conclusion The study of films of polymer composites of polyethylene glycol (PEG) and polypropylene glycol (PPG) with multiwalled carbon nanotubes (MWCNT) shows the presence of a low-threshold percolation effect, which is characteristic of morphology with segregated distribution of nanocomposite components. For the PEG-MWCNT system in the post-percolation region, a slow increase in electrical conductivity with an increase in the content of nanotubes is observed. The conductivity behavior indicates the contribution due to tunnel charge transfer, which is facilitated by conjugation between polymer macromolecules and nanotubes. The presence of such complexes is accompanied by a significant increase in the intensities of PL photoluminescence peaks associated with Van Hoff singularities, especially for PEG-MWCNT nanocomposites. The formation of heteroassociates affects not only the intercombination transitions associated with the transfer of excitation energy from polymers to nanotubes, but also the internal conversion. Changing the characteristics of the internal conversion leads to nonmonotonic behavior of PL emission of polymers with increasing concentration of nanotubes. For nanocomposites with a lower content of nanotubes (up to 0.4 wt.% MWCNT), there is an increase in the effect of internal conversion, which is associated with the formation of complexes. With a further increase in the concentration of nanotubes, the intercombination conversion increases, which leads to a decrease in the PL intensity.

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Synthesis, Structure, Optical and Biomedical Application of Nanosized Composites Based on TiO2 , Fe3 O4 (Review) M. M. Zahornyi, O. M. Lavrynenko, O. Yu. Pavlenko, N. I. Tyschenko, M. A. Skoryk, and O. A. Kornienko Abstract Nowadays, different nature magnetite, based on nanocomposites, anatase doped by noble metal cations are widely used to create new kinds of biocompatible materials with unique physical–chemical properties. The magnetite nanoparticle coating by noble metals leads to their stabilization in corrosive biological media and effects on their electrical, magnetic, catalytic and plasmonic properties as well. In this paper, all recent studies in the field of producing hybrid composites based on nanosized particles (NPs) of different nature are shown or mentioned. The basic material preparation methods, their properties and the possible fields of materials application are summarized.

1 Introduction Today, an actual situation with world coronavirus COVID-19 pandemic situation, it is very important to attribute the spread of viral infections and ensure the population biological safety to the global problems of the twenty-first century. According to statistics, every year 15 billion people die in the world. Viral infections are divided into new (emergent) and secondary (re-emergent) ones. The second type includes avian influenza viruses (A (H5N1)) (1997), A (H9N2) (1999), A (H7N7) (2003), A (H7N3) (2004), A (H7N9) A (H10N8) (2013), including pandemic virus A (H1N1) pdm09 (2009), coronaviruses (SARS viruses, 2002, Middle Eastern respiratory syndrome MERS-CoV, 2012), etc. In general, the appearance of each new virus creates local or international emergencies, and the emergence of known diseases in new or altered forms and their spread in unusual nose areas accompanied by a rapid spread and atypical course of the disease. The urgency of the research topic is due

M. M. Zahornyi · O. M. Lavrynenko · O. Yu. Pavlenko (B) · N. I. Tyschenko · O. A. Kornienko Frantsevich Institute for Problems of Material Science of NAS of Ukraine, Kyiv, Ukraine M. A. Skoryk Nanomedtech, Kyiv, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_10

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to the need to develop the latest effective means of preventing the spread of viral pathogens that can lead to epidemics and pandemics. Metal oxides (TiO2 , ZnO, Fe3 O4 , etc.) have been utilized as gas sensors over the past several decades, but with some drawbacks, which include the inoperability of the sensors at room-temperature conditions (most of them are operative above 200 °C) and long-term instability. Among metal oxides, nanocrystalline TiO2 has shown auspicious results in the detection of hydrogen, ammonia and nitrogen dioxide. In addition to the use of metal oxides, a plethora of research has been focused on the development of gas sensors from conducting polymers. Conducting polymers (especially polyaniline, polypyrrole and polythiophene) have been widely researched as gas sensor materials due to their unique electrical properties and operability at ambient conditions. So, a combination of these components allows obtaining composite materials with unique optical, sensor, antimicrobial properties [1–9]. The microbial fuel cells’ (MFCs) based on oxide fillers obtaining is very actual nowadays [1]. For excellent electrode performance of MFCs, additive graphene was used during the polymerization in situ process on TiO2 particles. This is the reason to proceed with systematic studies to understand the mechanism impact inside the composite system. This allows the creation of inorganic particles and obtains composite materials with functional properties.

2 Preparation Methods and Composites’ Properties with TiO2 Nanoparticles The unique properties of TiO2 , especially nanoscale, are used to address important energy and environmental issues. This caused the recent big amount of scientific works on the synthesis and study of its antivirus properties and then the research for ways of its practical application [7]. The antiviral activity of TiO2 anatase nanoparticles against human adenovirus 5 serotypes was between 45 and 95%. The production of crystalline titanium dioxide with maximum photocatalytic activity and stability in corrosive media (organic substances) is a priority for many researchers in the field of photocatalysis [8–11]. The author’s many attempts have been made to dope titanium dioxide of the atom of sulphur, nitrogen, carbon, Ce, MnO2 to shift the absorption of the photocatalyst into the visible light and nearIR region [12–16]. For example, mixing the levels of p (nitrogen) with 2p (oxygen) makes it possible to narrow the width of the trapped zone of the oxide semiconductor. Doping with nitrogen has occurred when the powder is treated with nitrogen itself, ammonia or ammonium chloride and other nitrogen-containing compounds. Doping with metals is good because the Schottky barrier is created by contact of the metal with the surface of TiO2 , which acts as an electron trap, as a consequence of which the recombination of electron hole pairs is suppressed during photocatalysis. The authors of refs [12, 13] note that when doping with metals (rare-earth) elements, it is possible to shift to the visible region, but the photocatalytic activity

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may decrease, especially in the UV range. For the electronic interaction, nitrogen is good because electrons can pass for the dopant of the orbitals 2p or 3p to the 3d orbital Ti, and the width of the forbidden band decreases. The problem of doped oxide TiO2 powders is that photocatalytic activity in a pollutant environment can sharply decrease, especially in the visible region. An important role in the development of this area belongs to the conducting polymeric materials. It may be noted only a few basic functions that perform polymers TiO2 chemistry and other NPs [11, 16, 17]. There are several works devoted to research TiO2 in which an important part is given to polymers. But there are practically no studies on the kinetics of the synthesis of polymers in the presence of oxide NPs, particularly TiO2 [9, 17, 18]. The main task of nanocomposite materials manufacturing is to obtain the necessary level of controlled component dispersion and to prevent the aggregation of nanoparticles [19–22]. This requirement can be designed to meet the chemical methods of forming composites, where one of the components synthesized previously obtained another component. For polyacrylate-NPs, systems mainly use two methods. Sol–gel synthesis is carried out from the TiO2 precursors with polyacrylate solution, dispersion or a polymer film. In both embodiments, to regulate and stabilize the dispersibility of the resulting composite suggest the formation of strong physical and chemical bonds between components. The choice of method is due to the requirement to obtain a final material. In the processes for polyacrylate-TiO2 nanocomposites manufacturing as the binding agent phase chemical bonds were recently used in high-active alkoxysilanes. The siloxane component is obtained through ternary systems that perform several important functions.Alkoxysilane injection in small quantities is used to stabilize the nanoparticles and, in the result, better dispersed materials in an acrylate matrix [23–25] are received. The resulting polysiloxane on the surface of TiO2 particles protects the polymer matrix from the possible photocatalytic degradation. To obtain stable clear homogeneous nanocomposite precursors or a mixture of inorganic nanoparticles, dispersion introduced monomer mixture with subsequent polymerization, in which the final stage is completed at room temperature [24]. This method is obtained composites for use in nonlinear optics. It is known that in optical quality PMMA material has a small refractive index in the visible range (n = 1.49). Whereas TiO2 is n = 2.45 (anatase form) and 2.70 (rutile). Using these data, Chen and co-workers obtained a hybrid composite using a sol–gel process and the MSMA to form interfacial chemical bonds. The resulting composite films were accompanied by a wide range of refractive index in the range of 1.202–1.867. Thus, this area of the work allows to obtain polymeric materials with already combined unique properties, such as: optical (high transparency, high refractive index); catalytic (photochemical oxidation, photocatalysis), and some of the other valuable properties [26–42]. The presence of Ti3+ defects for the synthesized sample could be the reason for bandgap decrease in this semiconductor to 2.75 eV and growth of MO (methyl orange) anode oxidation currents under UV irradiation at high scan rates

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(above 50 mV/s) and potentials below 500 mV (SCE) comparing with standard samples [14]. The samples’ photoelectrocatalytic activity is defined by the nanoparticles’ nanodispersity form. Thus, the scientific and technical information nanocomposites indicate great potential in the synthesis of nanocomposites, especially PANI and different porous structures, the most of the photoactive properties of oxide semiconductors NPs, PANI as well as receive photocatalytic systems based on them [39–42]. The nature of the porous structure of the photocatalyst, in turn, determines the structure and properties of interfacial layers, as well as the availability of exposure to the active surface of the catalyst and the substances, which are subject to degradation. The strong interaction between NP’s oxide and polymer should be present for the efficient transmission of electronic processes. The reason is because it will strongly depend on the rate of hydroxyl radicals’ generation, which determines the speed and completeness of decomposition reactions of toxic organic substances. Photocatalytic degradation is an efficient and economical method that attracted increasing attention [26–29]. This is because it is particularly useful for cleaning biologically toxic or nondegradable materials such as aromatics, pesticides, petroleum constituents and volatile organic compounds in wastewater. The contaminant materials are converted to a large extent into stable inorganic compounds such as water, carbon dioxide and salts, i.e. they undergo mineralization.

3 Nanomagnetite Doped with Noble Metals Core and shell-type nanocomposites are attractive to practical application in biological and medical studies due to the combination of their useful physical–chemical properties such as (super) paramagnetic, optical (plasmonic), catalytic, accompanied with biocompatibility as well.

4 Typical Synthesis Methods for Obtaining Core and Shell Nanocomposites Type The core and shell-type nanocomposites obtaining are based on ferromagnetic cores covered with precious (noble) metal shells include the synthesis of iron oxide nanoparticles and the following formation of noble metal shell on their surface [43]. Nowadays, core and shell-type nanocomposites are usually formed via: • coprecipitation of ferric and ferrous salts in weak-alkaline water dispersion medium and reduction of noble metal shells on their surface [44]; • microemulsion method [45, 46]; • separate sedimentation of the nucleating seeds (magnetite and gold) and formation of corresponding composites using organic substance [47].

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Less commonly applied methods are closely connected with the X-ray emission [48], laser ablation [49], sonochemical reaction [50], “wet” chemistry [51] and photochemical reduction [52]. According to published data, the process of the formation of cores and shells is possible in two phases, system (microemulsion method) as well as in water [51] or organic medium only [52]. Recently, the combined method including the formation of ferromagnetic core in organic medium and formation of noble metal shell in water was proposed [53]. The carrying out of the surface separated reductive–oxidative reaction on the steel—solutions of noble metals’ interface may be referred to as alternative synthesis procedure to obtain core and shell-type nanoparticles [54]. In the first stage of the process, Fe(II)-Fe(III) layered double hydroxides (LDHs) are formed on the steel surface contacting with distilled water in the open-air system. Taking into account strong reductive properties of Fe(II)-Fe(III) LDHs, their following contact with water medium containing noble metal aqua forms leads to phase transformation of LDH into magnetite particles accompanied with reduction of precious metals on their surface or it leads to the inclusion of argentum, aurum, platinum or palladium cations into the crystal lattice of magnetite. The concentration of noble metals in the initial solutions influences degree of a core covering and also the thickness of the shell at all. The main advantages of the procedure lie in the simplicity of the method, the absence of necessity to use superficially active substances (SAS), various reducing agents and high concentrated ferric–ferrous solutions to form core particles as well. To obtain ferromagnetic cores usually apply co-precipitation in the water medium in the presence of inorganic ferrous and ferric salts and base solution NaOH or NH4 OH under standard conditions [55] or in the nitrogen atmosphere by addition of reducing agent, for example, sodium citrate [56]. Also, the reductant simultaneously plays the role of stabilizing substance [57]. Numerous synthesis methods of obtaining ferromagnetic nanoparticles were described in several reviews [58]. Nanosized iron oxides for biomedical applications usually obtain via polyol synthesis, preparation of microemulsions, co-precipitation, decomposition of organic species, etc. Thus, nanocomposites based on iron cores and silver shells were produced under the standard conditions in the solution containing argentum nitrate, ferrous salt, borohydride and sodium citrate [59]. As the determinative factors of obtaining nanocomposites, the sequence of solutions blending and time of reagents’ addition were found. Hydroxylamine, citrate or sodium borohydride are most commonly used as reducing agents [59]. To reduce the silver layer onto nanomagnetite surface, tartaric acid was used [60]. But, the application of various SAS and organic components to obtain core and shell-type nanocomposites is impossible for structures prepared for biomedicine, so in that case, there are preferable to use various bio-compatible substances. For example, Fe3 O4 and Au particles were synthesized via a combination of chemical and biological route [61]. Ethanolic extract of Eucalyptus camaldulensis was used to reduce aurum on the magnetite surface from a water solution of HAuCl4 . Today, we need to develop effective materials for preventing the bacteria from environmental because of pandemic spreading. The idea is the creation of functional

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composites based on magnetite and anatase with cations of Ag, Pt, Pd, which exhibit bactericidal and antiviral activity under UV irradiation for purification water and air. Functional nanocomposites based on metal oxides as TiO2 , Fe3 O4 doped with noble metals and rare-earth elements (REE) can be a perspective material for the new kind of bioactive photocatalysts creation. Whereas the presence of REE in the structure of the zinc and titanium oxides enhances their photocatalytic activity [43]. The inclusion of noble metal cations in the crystal lattice of iron oxides enhances their optical properties [44] accompanied by superparamagnetic properties and high catalytic activity for phospholipids. Our team synthesized bioactive magnetite and anatase-doped cations of Ag, Pt, Pd with a concentration in the interval of 0–5 wt% (Fig. 1). Noble concentrations influence surface structure and magnetic properties of as-prepared and UV irradiated nanocomposites doped with Ag, Pt, Pd. EDAX analysis testify composition TiO2 –Ag (Ti-58.6 wt%, O-36.69 wt%, Ag-4.12 wt%) and Fe3 O4 -Pd (Fe-50.02 wt%, O-44.92 wt%, Pd-5.06 wt%). The irradiation changes the spine quantity in the structure of the nanocomposites and shifts the characteristic lines to high energy. Raman spectroscopy performed the surface investigation of our pure TiO2 and TiO2 –Ag phases. We note an interesting region of 300–700 cm−1 . The Raman bands are observed at 395, 514 and 637 cm−1 for pure TiO2 and TiO2 –Ag (Fig. 2). Any peaks corresponding to silver oxide were not observed at even high doped samples. However, there are some slight shifts for Raman peaks with low intensity in comparison with TiO2 when silver ion content increases. Also, the boarding of the peaks has been noticed due to the particles size, nature defects, etc. Thus, we assume the presence of structural defects on the surface anatase leads to the creation of acceptor and donor centres in the oxide conduction band and holes in the valence band. There are several types of charge capture centres. The surface Fig. 1 SEM images: a TiO2 –Ag (×19,300); b Fe3 O4 -Pd (×16,900)

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Fig. 2 Raman spectra of TiO2 –Ag Raman intensity (arb. un.)

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defects create narrow donor zones of electronic energy states near the bottom of the conduction band and therefore belong to small traps. The deep traps are associated with disturbances in the structure of the crystalline phase of anatase TiO2 due to oxygen vacancies. The oxygen vacancies formed both in the volume of nanocrystals and on their surface. It is possible to form several localized electronic states for anatase. So, irradiation promotes the formation of surface traps due to disruption of oxygen stoichiometry, which reduces the recombination of captured electrons with holes and should increase the efficiency of photocatalysis for nanocrystalline TiO2 –Ag samples. The doping by Ag has got important work function to our TiO2 may be the transfer of electrons will take place from Ag to conducting band oxide, to achieve fermi level equilibrium following by localized surface plasmon resonance. Step-by-step we are going to discover all components and deliver all evidence of such a concept for the effective antimicrobial guard. Considering the methods of synthesis and study of nanocomposites properties with oxide NPs, especially of TiO2 , has not yet received information on the effectiveness of their use. The reason for all would be to use expensive alkoksititane precursors. Using cheap TiCl4 as the precursor of TiO2 does not allow to obtain the composite system with the required properties due to the complexity of the control of the hydrolysis process, the difficulties associated with the removal and the highly reactive reaction by-products (HCl). However, methods of surface modification of TiO2 in the synthesis of composites in most cases do not cause difficulties and could be successfully applied to industrial processes. Thus, it is important to create a composite photo generator based on TiO2 , Fe3 O4 to control the distribution of h + (generation) and then applying for the effective destruction of toxic organic substances due to the high concentration of oxide radicals.

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5 Summary Nowadays, nanosized core and shell-type composites containing iron oxide cores (magnetite and/or maghemite) and noble metal shells (in particular gold or silver) are one of the most perspective materials for biomedical applications due to the combinations of magnetic, optical, colloid-chemical properties as well as because of the possibility to biofunctionalization of the surface of the composite. The variation in the physicochemical characteristics of nanocomposites based on ferrimagnetic cores and noble metal shells opens great possibilities for their usage as a platform for the development of highly effective diagnostic and therapeutic tools with selectivity at the level of individual cells and biomacromolecules. The superparamagnetic properties of Fe3 O4 and Pd0 shell composite particles and the photocatalytic properties of TiO2 nanopowder in combination with the bactericidal and antiviral activity of both, which can be significantly increased under the influence of UV radiation, are the bases for the new protective composite materials creation. Variation in the material composition of such a material, the structure of oxide particles and the form of their composition of modifying components will optimize the photoactive composite system, which will show the greatest antiviral activity initiated by the action of UV radiation. The obtained nanocomposite (in the form of powder or film) can be used for the technical means of creation and prevention of the spread of infectious diseases in a confined air environment (transport, public places, hospitals). Summarized results show that the photocatalysis of composites TiO2 appears to be an interesting approach to water purification, offering the possibility of using sunlight as a sustainable and renewable source of energy. This technology is based on the presence of a semiconductor that can be excited by light with energy higher than its bandgap, inducing the formation of energy-rich electron-hole pairs that can be involved in ORR (oxygen reduction reaction). Recent progress has explored the chemical nature of nanoscale semiconductors with the object of improving their electronic and optical properties, enhancing their photoresponse to visible light. Nanomaterials typically have high reactivity and a high degree of functionalization, large specific surface area, size-dependent properties, etc. They can be used as an application in water purification. Acknowledgements This work was supported by the nano-program “The development of photocatalytic nanocomposites for a viruses inactivation in the air” (№ 40/20-H).

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39. Song XL, Yan CY, Huang ST, Zhang MW, Geng BY, Meng RB (2013) Synthesis of mesoporous polyaniline-TiO2 composite microspheres for gas sensing application. Adv Mater Res 750–752. https://doi.org/10.4028/www.scientific.net/amr.750-752.1098 40. Milani Moghaddam H, Nasirian S (2014) Hydrogen gas sensing feature of polyaniline/titania (rutile) nanocomposite at environmental conditions. Appl Surf Sci 317:117–124. https://doi. org/10.1016/j.apsusc.2014.08.062 41. Choquette-Labbé M, Shewa W, Lalman J, Shanmugam S (2014) Photocatalytic degradation of phenol and phenol derivatives using a nano-TiO2 catalyst: integrating quantitative and qualitative factors using response surface methodology. Water 6(6):1785–1806. https://doi.org/10. 3390/w6061785 42. Harada H, Onoda A, Uematsu T, Kuwabata S, Hayashi T (2016) Photocatalytic properties of TiO2 composites immobilized with gold nanoparticle assemblies using the streptavidin-biotin interaction. Langmuir 32(25):6459–6467. https://doi.org/10.1021/acs.langmuir.6b01073 43. Indira TK, Lakshmi PK (2010) Magnetic nanoparticles. Int J Pharm Sci Nanotechnol 3:1035– 1042 44. Lyon JL, Fleming DA, Stone MB, Schiffer P, Williams ME (2004) Synthesis of Fe oxide core/Au shell nanoparticles by iterative hydroxylamine seeding. Nano Lett 4(4):719–723. https://doi. org/10.1021/nl035253f 45. Nassireslami E, Ajdarzade M (2018) Gold coated superparamagnetic iron oxide nanoparticles as effective nanoparticles to eradicate breast cancer cells via photothermal therapy. Adv Pharm Bull 2:201–209. https://doi.org/10.15171/apb.2018.024 46. Kim DK, Mikhailova M, Toprak M, Zhang Y, Bjelke B, Kehr J, Muhammed M (2001) In-situ gold coating of superparamagnetic nanoparticles by microemulsion method. MRS Proc 704. https://doi.org/10.1557/proc-704-w6.28.1 47. Caruntu D, Cushing BL, Caruntu G, O’Connor CJ (2005) Attachment of gold nanograins onto colloidal magnetite nanocrystals. Chem Mater 17:3398–3402. https://doi.org/10.1021/ cm050280n 48. Kinoshita T, Seino S, Mizukoshi Y, Otome Y, Nakagawa T, Okitsu K, Yamamoto TA (2005) Magnetic separation of amino acids by gold/iron-oxide composite nanoparticles synthesized by gamma-ray irradiation. J Magn Magn Mater 1:106–110. https://doi.org/10.1016/j.jmmm. 2005.01.050 49. Kawaguchi K, Jaworski J, Ishikawa Y, Sasaki T, Koshizaki N (2007) Preparation of gold/ironoxide composite nanoparticles by a unique laser process in water. J Magn Magn Mater 310(2):2369–2371. https://doi.org/10.1016/j.jmmm.2006.11.109 50. He QG, Wu ZH, Hu R (2011) Platinum immobilization on iron oxide nanoparticles surface under sonication. Adv Mater Res 183–185:1989–1994. https://doi.org/10.4028/www.scientific. net/amr.183-185.1989 51. Lo CK, Xiao D, Choi MMF (2007) Homocysteine-protected gold-coated magnetic nanoparticles: synthesis and characterisation. J Mater Chem 23:2418. https://doi.org/10.1039/b61 7500g 52. Oliva BL, Pradhan A, Caruntu D, O’Connor CJ, Tarr MA (2006) Formation of gold-coated magnetic nanoparticles using TiO2 as a bridging material. J Mater Res 21:1312–1316 53. Wang L, Luo J, Maye MM, Fan Q, Rendeng Q, Engelhard MH, Zhong C-J (2005) Iron oxide– gold core–shell nanoparticles and thin film assembly. J Mater Chem 18:1821. https://doi.org/ 10.1039/b501375e 54. Lavrynenko OM, Pavlenko OYu, Shchukin YuS, Dudchenko NO, Brik AB, Antonenko TS (2020) Characteristics of nanocomposites formed on the steel surface contacting with precious metal solutions. In: Pogrebnjak AD, Bondar O (eds) Springer proceedings in physics microstructure and properties of micro- and nanoscale materials, films, and coatings (NAP 2019), Chapter 28, pp 297–306 55. Tamer U, Gündogdu Yu, Boyaci IH, Pekmez K (2010) Synthesis of magnetic core–shell Fe3 O4 – Au nanoparticle for biomolecule immobilization and detection. J Nanopart Res 12(4):1187– 1196

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Studying of Iron Oxyhydroxide Dehydration L. Frolova

Abstract The article discusses the process of dehydration of goethite. The mechanism of the process and its limiting stage have been established using X-ray phase and derivatographic analyses. The apparent activation energy is determined. The main factors influencing the specific surface of the formed hematite are determined. The optimal conditions for dehydration were selected.

1 Introduction Stable iron oxide hydroxides, such as α-FeOOH, γ-FeOOH, β-FeOOH, α-Fe2 O3 , γ-Fe2 O3 , Fe3 O4 , are widely used in many industries as catalysts, magnetic carriers, adsorbents, pigments, fillers [1–7]. As a rule, the starting materials for obtaining a fine oxide powder are hydroxides, oxyhydroxides obtained by calcination in different gaseous media [8], or different cationic compositions [9]. Often the specific surface and adsorption, photocatalytic properties are determined by the mode of conducting high-temperature stages in the technological chain. Since it is important not only the phase composition of the final product but also the dispersion, porosity, specific surface area of the obtained particles, there is a large number of works devoted to this issue [10, 11]. Numerous articles related to the study of the transformation of iron hydroxide– maghemite–hematite by thermogravimetric analysis, IR spectroscopy, Mössbauer spectroscopy [12–15]. To obtain active iron oxide, it is necessary to determine the temperature of treatment of oxyhydroxide. The mode of heat treatment of powders was chosen for the following reasons: all stages of chemical change of powder composition (removal of free, capillary, adsorption, crystallization, and chemically bound water) must be completed; the maximum specific surface area and a certain pore size distribution should be obtained while preserving the primary needle structure of the particles without sintering. L. Frolova (B) Ukrainian State University of Chemical Technology, Dnipro 49005, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_11

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The aim of this work was to study the effect of dehydration temperature on the dispersed, phase composition of the powder of the final products.

2 Experimental Iron oxyhydroxide powder, previously washed of impurities and dried in air at room temperature, was used for analysis. Iron oxyhydroxide was obtained by precipitation and its subsequent oxidation with air under the following conditions: the initial concentration of ferrous sulfate in a solution of CFeSO4 = 0.5 mol/l. The method of synthesis is described in detail in the article [16]. Optimal synthesis conditions are described in [17]. Derivatographic analysis was performed on a derivatograph of the Paulik Erdei system. X-ray phase analysis was performed on a diffractometer DRON-3. Microscopic analysis was performed on an electron beam microscope EMV-100L.

3 Results and Discussion For experimental evaluation of the degree of transformation, as a result of high temperature, solid-phase reactions use different methods: continuous and batch. Of the continuous, the most widespread method is thermogravimetric analysis (TGA), based on measuring the change in mass of the reaction mixture as a function of time in isothermal and nonisothermal, although their capabilities are not limited to only one type of solid-phase reactions. Since the process of dehydration of goethite is based on a chemical reaction, which is accompanied by a sharp change in the mass of the substance, the thermogravimetric method and X-ray phase analysis were used to calculate the kinetic characteristics. In addition, studies were performed under isothermal conditions in the temperature range of 150–800 °C. In ref. [18], the authors present the results of thermogravimetric analysis of αFeOOH, γ-FeOOH, Fe(OH)2 . However, it is known that the appearance of derivatographic curves depends on the phase composition, the dispersion of the powder. The selection of the kinetic equation, which more fully describes the process of dehydration of goethite and the calculation of the corresponding kinetic characteristics, is performed according to the method described in [19]. For a reasonable purposeful study of the process of dehydration of goethite, it seems appropriate to pre-evaluate the effect of temperature on the phase composition of the obtained powder by calculation methods. Experimental studies of the goethite dehydration process were performed. Derivatogram of goethite is shown in Fig. 1a. The curves of mass loss (TG), differential curves of mass loss (DTG), and temperature (DTA) are presented in Fig. 1a. At temperatures above 100 °C, the intensive weight loss of the sample begins, and the maxima of weight loss on the DTG curve

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а)

b)

Fig. 1 Dependences of a weight loss in % of the dehydration period at different temperatures 1–300 °C, 2–450 °C, 3–550 °C, b derivatogram of goethite powder

fall at 92, 243, 280 °C. The temperature corresponding to the transformation of goethite into hematite is 280 °C. The degree of transformation of goethite into hematite is determined by the formula: α=

Ni Ni,ucx

N i —where: N i ,int and N i N i —the number of moles of the i-th reagent, respectively, in the original system and at the time that has passed from the beginning of the interaction. The calculation results (Table 1) indicate the best linearization of the straight line in the coordinates α from τ, and the minimum value of S corresponds to the model described by equation D1. In order to determine the optimal decomposition time, studies of similar samples were carried out in an isothermal regime. On the basis of experimental studies, the dependence of the degree of conversion (α) on the duration of decomposition at different temperatures was constructed (Fig. 1b). Analysis of the data obtained allowed us to conclude that at 300 °C, complete dehydration does not occur even with prolonged heat treatment, which was confirmed by X-ray phase analysis (peaks corresponding to goethite were observed on the diffraction patterns). At a temperature of 450 °C, the time corresponding to an almost complete conversion degree is 12.2 min, for 550 °C—4.8 min (Fig. 1b). The dependence of the specific surface of hematite is extreme and depends on temperature. At a temperature of 500 °C, a specific surface area of 124.3 m2 /kg is achieved (Fig. 2). Based on the research, the following conclusions can be drawn:

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Table 1 Mathematical models of some reaction mechanisms, describing heterogeneous processes № Function Equation

The process that limits the reaction rate

rk

E, kJ/mol

1

N1

kτ = α

Nucleation according to the 0.97 power law, the nucleation rate determines the rate of the process, n = 1

62.1

2

N2

kτ = 2α1/2

Nucleation according to the 0.96 power law, the nucleation rate determines the rate of the process, n = 2

24.45

3

R2

kτ = 2[1 − (1 − α)1/2 ]

Interface reaction, cylindrical 0.95 symmetry

72.05

4

R3

kτ = 3[1 − (1 − α)1/3 ]

Interface reaction, spherical symmetry

0.95

78.16

5

F1

kτ = −ln(1 − α)

Accidental nucleation, one nucleus for each particle

0.88

41.6

6

A2

kτ = 2[−ln(1 − α)1/2 ]

Accidental nucleation Avrami 0.91 equation, n = 2

40.02

7

A3

kτ = 3[−ln(1 − α)1/3 ]

Random nucleation, Avrami Equation, n = 3

0.889

24.68

8

A4

kτ = 4[−ln(1 − α)1/4 ]

Accidental nucleation, Avrami equation, n = 4

0.86

9

D1

kτ = 0, 5α2

One-dimensional diffusion

0.991 132.12

10 D2

kτ = (1 − α)ln(1 − α) + α

2D diffusion, cylindrical symmetry

0.98

11 D3

kτ = 1, 5[1 − (1 − α)1/3 ]2

3D diffusion, spherical symmetry

0.967 170.21

12 D4

[1 − 2α/3] − (1 − α)2/3 = kτ Three-dimensional diffusion, the Gistling–Brunstein equation

Fig. 2 Dependence of the specific surface of hematite on temperature (dehydration period 1 h)

0.97

145.5

148.806

158.79

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• Complete dehydration, removal of all types of moisture occurs at a temperature of 400 °C. • At temperatures above 400 °C, hematite is formed, which differs in its microstructure. • The maximum specific surface corresponds to a dehydration temperature of 500 °C.

References 1. da Guarda Souza MO et al (2020) Production and in situ transformation of hematite into magnetite from the thermal decomposition of iron nitrate or goethite mixed with biomass. J Thermal Anal Calorimetry 139(3):1731–1739 2. Ammasi A (2020) Effect of heating rate on decomposition temperature of goethite ore. Trans Ind Inst Metals 73(1):93–98 3. Kar S, Equeenuddin SM (2019) Adsorption of chromium (VI) onto natural mesoporous goethite: effect of calcination temperature. Groundwater Sustain Develop 9:100250 4. Ding J et al (2020) Heterogeneously activation of H2 O2 and persulfate with goethite for bisphenol A degradation: a mechanistic study. Chemosphere 261:127715 5. Amrani MA et al (2020) Low-cost goethite nanorods for As (III) and Se (VI) removal from water. Appl Sci 10(20):7237 6. Yuan L et al (2019) Catalytic ozonation of 4-chloronitrobenzene by goethite and Fe2+ -modified goethite with low defects: a comparative study. J Hazard Mater 365:744–750 7. Frolova LA et al (2017) Recuperation of etching solutions with obtaining pigments on the basis of ferrum oxide. In: 2017 IEEE 7th international conference nanomaterials: application & properties (NAP). IEEE, pp 01NNPT09-1–01NNPT09-4 8. Frolova L, Butyrina T (2020) Investigation of color and anticorrosive properties of pigments in the Fe-Al-Mg-OH system by the simplex lattice method. Pigment Resin Technol 9. Frolova L, Pivovarov A, Butyrina T (2017) Synthesis of pigments in Fe2 O3 -Al2 O3 -CoO by co-precipitation method. Pigment Resin Technol 46(5):356–361 10. Hirokawa S, Naito T, Yamaguchi T (1986) Effect of atmosphere on the goethite decomposition and pore structure of product particles. J Coll Interf Sci 112(1):268–273 11. Liu H et al (2013) Thermal treatment of natural goethite: thermal transformation and physical properties. Thermochimica Acta 568:115–121 12. Kustova GN et al (1992) Vibrational spectroscopic investigation of the goethite thermal decomposition products. Phys Chem Min 18(6):379–382 13. Rizov B (2012) Phase transformations from goethite to hematite and thermal decomposition in various nickeliferous laterite ores. J Univ Chem Technol Metall 47(2):207–210 14. Liu K et al (2019) Pressure-induced phase transitions for goethite investigated by Raman spectroscopy and electrical conductivity. High Press Res 39(1):106–116 15. Valezi DF et al (2019) Enhanced magnetic component in synthetic goethite (α-FeOOH) and its relation with morphological and structural characteristics. Physica Status Solidi (B) 256(11):1800578 16. Frolova LA (2014) Production conditions of iron oxide black from pickle liquors. Metall Min Indus (4) 17. Frolova LA, Hrydnieva TV (2020) Influence of various factors on the ferric oxyhydroxide synthesis. J Chem Technol 28(1):61–67 18. Jia F, Ramirez-Muñiz K, Song S (2015) Mechanism of the formation of micropores in the thermal decomposition of goethite to hematite. Surf Interf Anal 47(4):535–539 19. Young DA (1966) Decomposition of solids, vol 1. Pergamon

Surface Reactivity of Carbon Nanoporous Materials Studied with Chemical Bromination V. E. Diyuk, A. N. Zaderko, L. M. Grishchenko, A. V. Vakaliuk, R. Mariychuk, and V. V. Lisnyak

Abstract An electrophilic bromination is used as a probe reaction to assess the reactivity of a carbon surface, as well as to prepare versatile precursors for multifunctional modifications. In this chapter, we consider the bromination of nanoporous carbon materials with liquid bromine and KBr3 . This bromination was performed under different reaction protocols. Here, we report the parallel oxidation and hydrolysis during bromination and show their effect on surface chemistry. Also, the pros and cons of the usage of the proposed reaction are considered. The surface oxidation and modification rules are discussed, and the amount of introduced bromine that can be substituted by a nucleophile is presented. In general, the potential for obtaining chemically uniform surfaces covered with amino groups by amination of the surface of brominated carbon materials has been shown.

1 Introduction Modifying the surface of carbon materials (CMs) with halogen to obtain a chemically uniform surface coverage is an important task when creating new sorbents, films, and electrodes [1–4]. Typically, electron-withdrawing halogen atoms bonded to the carbon surface assist in the enhancement of electrochemical properties [2, 5] . By bromination contrasted with iodation, one can produce new nanocomposite electrodes with unique properties. In this way, one can also improve the capacitance of activated carbons (ACs) for electrochemical energy storage [5–7]. Bromination is one of the methods of determining the carbon–carbon multiple bonds in the structure of organic matter [8]. We can use it for the quantitative determination of the active sites on the carbonic surface. Besides, currently, there are many bromination routes [9], including the direct [10–12], electrothermal [13], and V. E. Diyuk · A. N. Zaderko · L. M. Grishchenko · A. V. Vakaliuk · V. V. Lisnyak (B) Taras Shevchenko National University of Kyiv, Kyiv 01601, Ukraine e-mail: [email protected] R. Mariychuk · V. V. Lisnyak Faculty of Humanities and Natural Sciences, University of Prešov, Prešov, Slovakia © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_12

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hydrothermal [14] treatments. Also, bromination initiated by photolysis [15, 16], plasma [17–19], microwave sparks [20], and chemical vapor [21] is actively pursued to covalently functionalize the carbon surface with bromine with the aim to use in microelectronics and sensors. Sometimes, the substrate-mediated bromine condensation or bromine polycondensation in graphitic carbons can accompany bromination [22, 23]. Today, introducing electrophilic reactive sites into graphene and carbon nanotubes is required for the following functionalization with nucleophiles and multifunctional grafting [24– 26]. This introduction can, for instance, be accomplished by Grignard reactions [27] or by catalytic bromination using Lewis acids in appropriate solvents at an optimized temperature [28]. One of the most common synthetic approaches used in organic synthesis to produce a variety of derivatives is the initial preparation of a precursor by introducing a halogen atom into the organic molecule, which can then be substituted for a range of functional groups. This approach is universal for the synthesis of amines, thiols, oxygen-containing compounds, and their derivatives. Here, we will briefly focus on an approach to electrophilic sites. It relies on the well-known electrophilic aromatic substitution and on the addition of bromine to aliphatic structural units. Bromination is a promising modification method of high selectivity [29]. Surface bromination occurs as a radical process with elemental bromine that reacts with the carbons as a solution in chloroform. This bromination way is easy and well suited for graphene and its derivatives. Notably, brominated graphene can be a starting material for constructing other graphene-based functional materials. Despite the several studies on the bromination of carbon nanotubes and graphene in solutions, in polar and non-polar solvents [30], as well as in the gas phase [9–12], the nature of the surface C–Br bonds and their thermochemical properties remain poorly understood and badly documented. In the present chapter, we will consider the wet bromination method of nanoporous CMs, as the info on their bromination is rare. By the action of light on brominating solutions, bromine radicals are generated to attack the carbon surface. For the reduced reactivity of the carbon surface, however, just a partial bromination is achieved. A surface covering with bromine remains comparatively low level, at below 1 mmol g–1 . Typically, in the case of wet bromination, the solvent must not contain halogens for any complicating of the halogen identification in the modified nanoporous CMs. The solvent must be sufficiently volatile to be removed during drying and should not form halogen derivatives under reaction conditions, even in trace amounts, to exclude their adsorption in nanoporous structure and further identifying as chemisorbed bromine species. The solvent should also not be interfering with further chemical removal of physisorbed halogen that is carried out in aqueous solutions. By preventing access to dehalogenating agents, a hydrophobic organic solvent can block all small nanopores. Notwithstanding the low bromine content, a saturated bromine solution in water (bromine water) is almost the ideal solvent that meets all the requirements. A water solution of Br2 and KBr can be used to increase the concentration of the brominating agent. Here, the potassium bromide prevents the

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disproportionation of bromine in the water. Forming a Br2 ·KBr complex, this solution contains significantly higher concentrations of bromine than bromine water. Below, we disclose our achievements in optimizing the bromination conditions. This optimization leads to forming the required amount of Br groups, and they can be used for subsequent surface modification with amino groups. For this purpose, we studied the effect of bromination conditions on the chemical and thermodesorption properties of the resulting functionalized carbon surface.

2 Experimental 2.1 Materials and Chemicals Granulated ACs of Ukrainian trademarks (KAU, SCN1, and SCN2) were used for further modification. The granulated AC of the KAU trademark was obtained from peach stones by carbonization. Spherical nitrogen-containing AC of the SCN1 trademark was obtained by a standard method of carbonization and followed vapor activation of vinyl pyridine rubber. The SCN2 was prepared from the same raw material as SCN1. But, at the activation stage, the carbon burnout was at a level greater than 80%. Activated carbon fibers (ACFs) of ACFPAN and ACFBus trademarks were made based on polyacrylonitrile (PAN) and cellulose (Bus), correspondingly. The PAN fibers were carbonized and simultaneously activated with a water vapor atmosphere at 1,000 °C to prepare the ACFPAN. For the ACFBus of the Belarusian trademark Busofit, the final carbonation temperature of the technical viscose thread was 600 °C, and the activation was carried out with water vapor at 870 °C. Diethylamine (Et2 N, 95%), methylamine (MeNH2 , 95%), ethylenediamine (En, 90–95%), monoethanolamine (MEA, 99.5%), and sulfolanylethylenediamine (SuEn, 95%) were selected for amination and purchased from UkrOrgSynthesis (Kyiv, Ukraine). Liquid bromine (Br2 , 99.5%) was purchased from Sigma-Aldrich. Sodium hydroxide (NaOH, 97%, p.a.), hydrochloric acid (HCl, 37%), nitrate acid (HNO3 , 40%), sodium carbonate (Na2 CO3 , ≥ 99.5%), sodium hydrogen carbonate (NaHCO3 , ≥ 99.5%), potassium oxalate (K2 C2 O4 , 99.5%), potassium bromide (KBr, 99.999%), potassium bromate (KBrO3 , 99.5%), sodium thiosulfate (Na2 S2 O3 , 99%+), hydrobromic acid (HBr, 47.0%), silver nitrate (AgNO3 , 99.9995%), potassium thiocyanate (KSCN, 99.5+%), and ferric alum (FeNH4 (SO4 )2 ·12H2 O, 99.9+%) were purchased from Khimlaborreactive LLC (Brovary, Ukraine). Working solutions were prepared daily from the stock solutions by appropriate dilution with deionized (DI) water. All the chemical reagents were analytical grade and used without further purification. Gas cylinders with high purity argon (Ar, 99.99%) were supplied by Linde Gas Ukraine.

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2.2 Surface Modification Methods Before modification, the above mentioned CMs were pre-treated with 5 M HCl solution and then washed with double-distilled (DD) water from acid and soluble salts. The washing was ceased when the discharge wash water has a pH of 5.5–6.0. This procedure is aimed at removing interfering inorganic ash-forming components from nanoporous structures. For oxidation with nitric acid [31], a suspension of the selected pristine CM (2 g) in 5, 10, or 15% (w/v) HNO3 solution (60 ml) was refluxed for 2 h. The resulting oxidized CMs were thoroughly washed as reported above and dried at 120 °C. The prepared samples were labeled as CM-HNO3 (%), where the percent corresponds to the concentration of HNO3 solution. Thermal treatment (TT) was performed to detach a certain amount and type of oxygen-containing surface groups. In a typical experiment, the selected CM placed in a quartz flow reactor was subjected to TT in a dynamic argon atmosphere, with an argon flow rate of 50 ml per minute. The temperature was initially increased up to the selected temperature of TT (T TT ) of 350, 500, and 800 °C for 30 min, then kept at this temperature for 1 h. The resulting samples, hereafter designated as CM-T TT and CM-HNO3 (%)-T TT , were cooled to room temperature under an argon flow. After the TT, all prepared samples were transferred into a glass beaker for bromination. For bromination with KBr3 , 5 g of each solid carbon, including the samples of CM, CM-HNO3 (%), CM-T TT , and CM-HNO3 (%)-T TT series, were soaked with 50 ml of a mixture of 10% (w/v) Br2 and 15% (w/v) KBr solutions for 1 hour. After bromination, the brominated CMs were removed by filtration and immersed in 200 ml of water solution of 10% K2 C2 O4 . Once the gassing of CO2 has ceased, the sample was removed from the solution by vacuum filtering and thoroughly washed with double distilled (DD) water on a Whatman filter. When no bromide ions were detected in the washing water, the samples were designated as CM-KBr3 , CM-HNO3 (%)-KBr3 , CM-T TT -KBr3 , or CM-HNO3 (%)-T TT -KBr3 . For drying, the samples were collected and heated to 80 °C under a 0.06-torr vacuum for 2 h. For the first, in order to evaluate the acidic pH effect on the efficiency of bromination, an appropriate amount of HBr was added to the brominating Br2 ·KBr solution to gain a pH of 0, 1, 3, 4, and 8. For the second, to find out the basic pH effect, the same operation was done with 1 M solutions of NaHCO3 and Na2 CO3 to reach a pH of 8 and 12, respectively. The samples prepared by bromination with KBr3 at different pHs were denoted as CM-KBr3 (pH). Also, we treated CMs with a saturated aqueous solution of Br2 in water (bromine water) and KBrO3 solution in acidic (pH 0) and neutral (pH 7) media. The resulting samples were designated as CM-Br2 (H2 O), CM-KBrO3 (0), and CM-KBrO3 (7), correspondingly. For the direct bromination with molecular bromine, 5 g of either the pristine CMs or the modified CMs was brominated with 5 ml of distilled liquid bromine. After bromination, we performed purifying and drying operations as reported above, and then the resulting samples were labeled as CM-Br2 .

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For the amination of the brominated CMs, in a typical preparation procedure, a brominated sample of about 2 g was placed in a Teflon-lined autoclave contained 5 ml of an alcohol solution of the selected amine, e.g., Et2 N, En, MEA, or SuEn, and 0.25 ml of DD water. The autoclave was sealed, heated to 110 °C, and kept at this temperature for 12 h. After natural cooling to room temperature, the prepared samples were thoroughly washed with DD water and immersed in HCl for 2 days. After removing water-soluble organoammonium salts, the samples were filtered off using a Buechner funnel and then thoroughly washed on a filter with a soda solution and DI water to neutral (7.0–7.5) pH of washing waters. Finally, the aminated CMs were collected and dried at 120 °C.

2.3 Characterization Methods The brominated CM samples were collected for bromine analysis, and about 1 g of each CMs was pyrolytically decomposed in a NaOH–NaNO3 melt (3:1 molar ratio) at 400–500 °C [32]. After cooling, the melt was dissolved in water. The resulting solution was acidified, and then a back titration, the Volgard method, was used to determine the concentration of bromide ions in the solution. Attenuation total reflection Fourier transform infrared (ATR-FTIR) spectra were collected using IRAffinity-1S and IRPrestige-21 IR Fourier spectrophotometers. The spectra were analyzed using a standard LabSolutions IR software package and libraries containing approximately 12,000 spectra provided by Shimadzu. Before the examination, the samples were thoroughly dried and ground into powder. The spectra were recorded in the wavelength range 650–2,000 cm–1 , in the scan step of 0.5 cm–1 , in the mode of accumulation of 1,025 and 3,000 scans using the Basics™ and the PIKE MIRacle™ single reflection horizontal ATR modules equipped with prisms based on zinc selenide [33]. Thermogravimetric analysis (TGA) was performed in the temperature range of 30–850 °C to study the thermal stability of the prepared samples. The samples were heated at a heating rate of 10 °C min–1 in a dynamic argon atmosphere, at a flow rate of 50 ml min–1 . Thermogravimetric (TG) and differential thermogravimetric (DTG) curves were recorded by a custom-designed TG instrument, as reported elsewhere [31]. Thermoprogrammed desorption with IR spectrometric registration of gaseous products (TPD IR) coupled with the TGA method was used to determine the concentration of oxygen-containing surface groups (C FG ). As a result of heating in argon, the surface oxygen-containing groups are decomposed, and the decomposition products, which are carbon dioxide and carbon monoxide (CO2 and CO), were released. A stream of gaseous decomposition products diluted with argon was directed into the infrared cell of the IR spectrometer, where the concentrations of CO and CO2 (C(CO2 ) and C(CO)) were quantified. Both C(CO2 ) and C(CO) temperature profiles were decomposed in a sum of Gaussian components.

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The concentrations and the types of surface oxygen-containing groups were determined as in previous studies [34, 35], where the evolved gas analysis (EGA) was performed. Thermoprogrammed desorption mass-spectrometry (TPD MS) was used to determine the surface functional groups through a developed methodology of vacuum pyrolysis and thermal desorption combined with the direct analysis in a real-time mass spectrometry, which enables an unambiguous assignment of gaseous species. For such a purpose, a 0.1-g sample was heated from 30 to 800 °C in a vacuum of 10–4 Pa. The heating rate for the TPD MS experiment was 10 °C min–1 . The mass spectra were recorded in a cyclic manner, which allows obtaining a set of mass spectra of the gas phase at different sample temperatures. Further, for each m/z value, the dependence of the intensity in the mass spectrum on the sample temperature (thermal desorption profile) was constructed [36]. A monopole mass spectrometer MX 7304A from Selmi (Sumy, Ukraine) was used in this work [37]. The mass spectra were obtained by electron ionization at 70 eV. Different O-containing functional groups, which are accessible for ion exchange, were determined by the Boehm titration [38, 39]. Among them, acidic groups can be quantified using acid-base titration with a series of three basic water solutions of varying base strength. In the course of determination, dried, unmodified CM was weighted and portioned. The sample weighing 0.1 g was placed in a round flask with 10 ml of 0.05 M basic solution. The basic salts and a strong base used to prepare the basic solution were NaHCO3 , Na2 CO3 , and NaOH, correspondingly. It is assumed that the basic solutions neutralize acidic groups. Accordingly, the NaHCO3 solution primarily deprotonates the most acidic carboxyl (Cx) groups. The Na2 CO3 solution also reacts with lactone and anhydride (A and L) cycles, which show a weak acidity. In the prepared strongly basic NaOH solution, phenol (Ph) groups are additionally deprotonated. All prepared suspensions were shaken for 24 h to achieve adsorption equilibrium. After that, the solids were removed from the solutions. And then, a 2 ml aliquot was pipetted and titrated with 0.1 M HCl solution to determine the volume of the base, which reacted with the groups. At the same time, the comparison experiment was conducted as in [40]. Nitrogen adsorption-desorption isotherms for the degassed samples of unmodified CMs were measured on a TriStar Micromeritics (C10900A) porosimeter, up to p/p0 = 1 atm at –196 °C, to characterize the nanoporous structure. The BET-specific surface areas (S BET ) were determined using the Brunauer–Emmett–Teller (BET) equation. The total pore volumes (V S ) were derived from the amount of vapor adsorbed at a relative pressure close to unity (assuming pores are filled with liquid adsorbate). The micropore surface area (S mic ) and micropore volume (V mic ) were determined with the software supplied to the instrument. The pore volume versus pore width distribution was calculated by analyzing the adsorption branches of the isotherms. This analysis was done using two-dimensional models within the non-local density functional theory (2D-NLDFT) [41]. A 2D-NLDFT model for slit pore carbons with heterogeneous surfaces [42, 43] was used to obtain the pore width distribution with the computer program SAIEUS [43, 44]. The surface morphology of the unmodified CMs was observed using scanning electron microscopy (SEM). The SEM images

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were collected on a Zeiss EVO 60 scanning electron microscope, using the secondary electron mode with an accelerating voltage of 20 kV.

3 Results and Discussion 3.1 Characterization of Unmodified Carbon Materials Figure 1a–d shows the SEM micrographs of the unmodified CMs. The samples of ACs (KAU, SCN1, and SCN2) are granular materials. Macropores and transport channels are clearly visible in the KAU structure, as in [45]. The largest channel is about 10 microns in size; the size of small transport channels does not exceed 1 micron. Spherical granules of SCN1 and SCN2 are about 0.5–1 mm in size. This size of granules is typical of SKNs and SKSs [46–48]. Fibrous ACFBus and ACFPAN materials consist of separate fibers with a diameter of 5–7 microns. The cross-sectional shape of these fibers is almost round. Both ACFBus and ACFPAN are isotropic materials. However, the surface of ACFBus fibers is smoother; one can see some bright spots on the surface of ACFPAN fibers (Fig. 1c, d). Figure 1e shows the ATR-FTIR spectra of the unmodified CMs. They are similar, and, as in [49], by analyzing the absorbance peaks, one can easily discern the characteristic absorption bands of the aromatic and conjugated C=C bonds at 1552 cm–1 and 1520 cm–1 (C=C skeletal and stretching vibrations). Also, there is a broad asymmetric band in the 1450–1500 cm–1 region, and this band is appeared to be the superposition of several bands. Three absorption bands peaked at 1745 cm–1 , 1700 cm–1 , and

Fig. 1 SEM images of the unmodified CMs: a KAU, b SCN1, c ACFBus, d ACFPAN, and e selected ATR-FTIR spectra

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1649 cm–1 are assigned to the stretching vibration of the C=O bonds in the anhydride (A), lactone (L), and carboxyl (Cx) groups, correspondingly. A medium intense absorption band at (around) 1217 cm–1 (1165 cm–1 ) and a shoulder at 1363 cm–1 have been assigned to the O–H and C–OH vibrations of phenolic (Ph) groups, respectively. Bands at 1028 and 971 cm–1 can be attributed either to the C–O–H and O–H deformation (bending) vibrations in Cx and Ph groups or to the C–O stretching vibrations in the (C–O–C) fragments of the A-L groups. These bands are more intense in the spectra of SCN1 and ACFPAN than those in the spectrum of ACFBus. Their intensity increases with the oxygen content in the samples (Table 1). Nitrogen adsorption isotherms for the unmodified CMs are characterized by significant gas volume adsorbed in the range of low pressures at p/p0 < 0.1 (Fig. 2a). This feature is a typical characteristic of microporous adsorbents. For the ACFBus and ACFPAN sorbents, type I isotherms are observed. These isotherms show a slight increase in adsorption with increasing pressure and modest hysteresis. For the KAU, SCN1, and SCN2 sorbents, an increase in nitrogen adsorption with increasing p/p0 is substantial. Respective isotherms have a wide hysteresis. This hysteresis indicates a significant contribution of mesopores in the porous structure. Table 1 shows that the unmodified CMs have high S BET and V S simultaneously. As seen from Fig. 2b, all tested samples have a bimodal pore distribution in the range of 0–3 nm. The local maxima (peaks) are observed on the pore width distribution curves. For the KAU sorbent, they correspond to 0.62 nm and 1.23 nm wide pores. For the other granular CMs, these peaks are found at 0.50 and 1.64 nm (SCN1) and 0.58 and 2.17 nm (SCN2). For the fibrous CMs, these peaks are clearly seen at 0.48 and 1.52 nm (ACFPAN) and 0.61 and 1.55 nm (ACFBus). It should be noticed that almost all of these pores are micropores. For the granular materials, the mesopore width is in the range of 2–50 nm, and the respective peaks are seen at 3.78 nm (KAU) and 23.2 nm (SCN1), and, surprisingly, at 9.71, 21.7, and 40.7 nm (SCN2). When considering the texture parameters (see Table 1), their pairwise analysis showed that the studied sorbents could be divided into three groups. The first group includes the fibrous materials, the ACFBus and ACFPAN sorbents, which are microporous and have almost no other pores. The second group includes the KAU and SCN1 sorbents. They show preferential microporosity while having a considerable amount of mesopores. Contrasting to the KAU and SCN1 sorbents, the SCN2 sorbent is a micro-mesoporous material belonging to the third group. Figure 3 shows a typical temperature dependence of weight loss and the release of CO and CO2 gases. These gases are the products of the thermal decomposition of surface groups [50]. For the studied unmodified CMs, there is 2–5% total weight loss in the temperature range of 30–850 °C, which is explained by the low content of surface oxygen-containing groups. The first effect of weight loss between 30 and 180 °C is assigned to the thermal desorption of various physisorbed and chemisorbed water forms. High-temperature weight loss (mFG ) corresponds to the thermal decomposition of oxygen-containing surface groups and the release of CO and H2 O. Temperature dependences of the CO release showed a little difference for the studied pristine CMs (Fig. 3).

1,300

2,500

1,400

680

SCN1

SCN2

ACFBus

ACFPAN

675

1,390

1,695

1,195

0.34

0.68

3.06

1.09

0.33

0.65

0.67

0.56

0.41

V mic

94.2

97.5

97.3

95.4

95.2

VS

985

0.55

S BET

S mic

C

Specific volume, cm3 g–1

Specific surface area, m2 g–1

1050

EDX, at%

Texture parameters

KAU

Sample

5.0

2.1

2.7

3.5

4.8

O

Table 1 Texture parameters, EDX analysis, and surface chemistry of the unmodified CMs

0.9

0.4

–

1.1

–

N

0.038

0.020

0.023

0.043

0.049

0.19

0.10

0.07

0.24

0.35

Cx

mFG , g g–1

0.19

0.08

0.06

0.17

0.18

A-L

C FG (TPD IR), mmol g–1

TGA

Thermal analysis

0.60

0.42

0.41

0.53

0.52

Ph

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Fig. 2 Representative a N2 adsorption-desorption isotherms and b differential pore volume against pore width for the unmodified CMs

Fig. 3 TGA/EGA thermograms for SCN2 (a) and ACFBus (b). TGA: TG (1) and DTG (2), and EGA: CO2 (3) and CO (4)

Major CO2 desorption occurs between 200 and 400 °C, which corresponds to the thermal decomposition of Cx groups. In the temperature range of 400–600 °C, the release of CO2 and CO gases is small because of a low concentration of A-L surface groups. The intense release of CO occurs at temperatures above 600 °C, which is assigned to the thermal decomposition of high-temperature Ph and quinone (Qu) groups. Their presence is a common (main) feature for all CMs studied here. The temperature ranges of CO evolution are consistent with the corresponding effects of weight loss on the DTG curves. Therefore, the surface chemistry of the carbon fibers and activated carbons is similar. Excepting KAU, they contain a small amount of the oxygen-containing surface groups, which total concentration does not exceed 1 mmol g–1 . The highest weight losses and the maximum CO and CO2 release are shown by the KAU and ACFPAN samples, while the lowest amount of oxygencontaining groups is found in the case of the SCN2 and ACFBus samples.

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3.2 Properties of Brominated Carbon Materials Chemical analysis of different CMs brominated with aqueous KBr3 or liquid Br2 showed that the bromine content C(Br) varies from 0.4 to 1.0 mmol g–1 (Table 2). The action of both brominating agents gives similar C(Br) values within the 0.4–0.6 mmol g–1 range. Carbon fiber is somewhat better brominated with liquid bromine, in particular, C(Br) = 1.02 mmol g–1 for the sample of ACFBus-Br2 . Almost all characteristic absorption bands of the unmodified CMs are presented in the ATR-FTIR spectra of the brominated CMs (Fig. 4). Besides, there are additional Table 2 Surface chemistry parameters of the brominated CMs Sample

C.A

TGA

C(Br), mmol g–1

mFG , g g–1

mI , g g–1

T I , °C

C(Br), mmol g–1

C(CO2 ), mmol g–1

C(CO), mmol g–1

KAU-KBr3

0.62

0.126

0.046

293

0.29

0.69

1.41

KAU-Br2

0.52

0.118

0.044

283

0.24

0.72

1.44

SCN1-KBr3

0.42

0.115

0.034

302

0.18

0.55

1.89

SCN1-Br2

0.45

0.112

0.037

298

0.21

0.57

1.62

SCN2-KBr3

0.44

0.091

0.030

280

0.22

0.25

1.27

SCN2-Br2

0.45

0.090

0.031

287

0.23

0.28

1.13

ACFBus-KBr3

0.51

0.115

0.035

262

0.25

0.38

1.71

ACFBus-Br2

1.02

0.132

0.055

231

0.46

0.34

1.26

ACFPAN-KBr3

0.43

0.159

0.037

220

0.20

0.57

3.22

ACFPAN-Br2

0.49

0.160

0.045

230

0.25

0.60

2.92

Fig. 4 ATR-FTIR spectra of the pristine and brominated CMs

TPD IR

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Fig. 5 TGA/EGA thermograms for SCN2-KBr3 a TG (1) and DTG (2), and EGA: CO2 (3) and CO (4) and b negative correlations between C(Br)2 and C(CO) for the brominated CMs

absorption bands in the wavenumber region of 665–875 cm–1 . They, in our opinion, are originated from the vibrations of the C–Br bonds in the brominated CMs. For the brominated samples, the intensity of absorption bands increases in the range of 900–1230 cm–1 . In particular, the intensity of the bands peaked at 1217, 1171, 1028, and 971 cm–1 showed a significant increase. Considering the absorption of infrared radiation in the range of 1600–1800 cm–1 , the brominated CMs are not very different from the unmodified CMs. This statement is true when the intensive formation of new carbonyl-containing groups is restricted and means that both Cx and A-L groups almost do not form as a result of bromination. In this case, the intensity of infrared absorption in the range 900–1230 cm–1 is constantly increased with an increase in the surface concentration of Ph groups. Figure 5a shows typical thermograms of the brominated CM. In the temperature interval 150–850 °C, the thermal decomposition of functional groups caused weight loss (mFG ), which is much higher than that of the pristine (unmodified) CMs (cf. data in Tables 1 and 2). In the DTG curves of the brominated CMs, besides the release of the adsorbed water at 100 °C, there are two significant weight loss effects observed in the temperature ranges 170–450 °C and 450–850 °C, respectively. Releasing HBr between 170 and 450 °C is a reason for a 2–4 times increase in the weight loss (mI ), comparing with that for the unmodified CMs at these temperatures. This weight loss effect indicates detachment of chemisorbed bromine forms, as follows from the peak temperature (T I ) value. Since Cx groups are also decomposed in this temperature range, so the amount of detached bromine (C(Br)) is estimated from the difference between mI and the mass corresponding to the amount of CO2 released in this temperature range (Table 2). As can be seen, the value of C(Br) reaches only 0.17–0.46 mmol g–1 , which is significantly lesser than the value of C(Br). Probably, the thermal decomposition of bromine-containing surface groups is a staged process. First, at lower high temperatures, the weakly bound forms of chemisorbed bromine are decomposed, which is registered by the weight loss effect on the DTG curve. At higher temperatures, thermal decomposition of the strongly bound forms of chemisorbed bromine is accompanied by that of the functional oxygen-containing groups. This decomposition is indicated by a smoothed effect on

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the DTG curve between 400 and 600 °C. Bromination increased the release of CO from the prepared samples, indicating the course of the parallel oxidation reactions. These oxidation reactions cause a negligible increase in the CO2 -bearing groups (C(CO2 ) = 0.14–0.22 mmol g–1 ) and a significant increase in the CO-bearing groups (C(CO) = 0.66–2.52 mmol g–1 ) in the brominated CMs. Considering the data, we conclude that the physisorbed bromine may either chemisorb or desorb back into the liquid. The negative correlation between C(Br) and newly formed CO-bearing centers (C(CO)) corresponds to the intensive parallel oxidation during bromination (Fig. 5b). This correlation exists only in the case of additionally formed CO-bearing centers, and it is absent, for example, for the newly formed CO2 -bearing centers. The ratio between C(Br) and (C(CO)) varies, and this variation depends upon the surface composition and the preparation prehistory. However, for the CMs derived from natural sources, e.g., KAU and ACFBus, the slopes of the correlation lines in Fig. 5b are much steeper. As compared with the CMs obtained from polymeric compounds, the oxidation of the surface of KAU and ACFBus has a much stronger negative effect on the chemisorption of bromine. Bearing in mind the reactivity of added bromine, we will analyze below the data collected during thermal desorption experiments. Figure 6a presents the mass selective ion currents for fragments with a mass-to-charge ratio m/z of 79, 80, 81, and 82. In the temperature range 200–850 °C, one can see the thermal desorption of HBr (channels m/z 80 and 82). Here, we suggest the surface-mediated dehalogenation and recombination of HBr as in [10, 33, 36, 51]. Thermally-induced dehydrobromination is the preferred process at medium-high to high temperatures. Figure 6a presents the formation of Br+ ions (m/z 79 and 81) through the dissociation of HBr+ by an electron impact ionization. In the thermal desorption products, the pairs of positive ions (m/z 79 and 81 and m/z 80 and 82) are found to be in equal amounts, corresponding to the natural ratio of bromine isotopes. But, as well as in [33, 36, 51], the molecular ion of Br2 + (m/z 158, 160, and 162) was not found in the mass spectra. High-temperature thermal desorption has directly indicated that the HBr evolved during thermal decomposition is sourced from the detachment of different forms of

Fig. 6 Temperature-dependent mass selective ion current for a SCN2-KBr3 and b ACFBus-Br2

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Table 3 HBr thermal desorption parameters Sample

a HBr+

thermodesorption

β1

β2

Temperature range

Peak Peak Temperature Peak Peak temperature area range temperature area

T 1 , °C

T m1 , °C

A1 , %

KAU-KBr3

170–600

397

KAU-Br2

180–610

405

SCN1-KBr3

190–590

SCN1-Br2 SCN2-KBr3

T 2 , °C

T m2 , °C

A2 , %

48.8 250–820

532

51.2

48.5 260–830

544

51.5

394

44.9 240–790

520

55.1

170–580

382

43.6 260–770

513

56.4

180–600

402

48.7 250–880

557

51.3

SCN2-Br2

180–580

393

45.4 230–810

522

54.6

ACFBus-KBr3

250–560

397

48.1 330–700

505

51.9

ACFBus-Br2

220–530

369

46.9 300–670

476

53.1

ACFPAN-KBr3 190–490

338

47.2 270–620

437

52.8

ACFPAN-Br2

281

51.2 250–580

410

48.8

150–410

Vacuum: 1 × quartz ampule

a Conditions

10–4 –10–5

Pa, temperature range: 25–800 °C, heating rate: 10 °C min–1 ,

chemisorbed bromine. This model could explain the strong temperature dependence of the thermal desorption rate due to the relatively broad distribution of chemisorbed bromine centers. Figure 6 illustrates the surprisingly high reactivity of the brominated carbon surface in thermally stimulated dehydrohalogenation. By fitting with two Gaussian peaks, thermal desorption profiles of HBr+ can be formally deconvolved into (β1 and β2 ) components [10, 36, 51]. Table 3 sums up the thermal desorption parameters of HBr+ , including the temperature ranges (T 1 and T 2 ), the peak temperatures (T m1 and T m2 ), and the respective peak areas (A1 and A2 ), as % of the total area under the thermal desorption curve. Although the peak area A2 is slightly higher than 50%, while the peak area A1 is somewhat smaller, the content of both forms is almost the same. As seen from the tabulated data, the temperature regime of the HBr desorption widely differs for the studied samples. Meanwhile, the bromine is attached to the surface sites that have different bonding power. It is concluded that heating supplies the activation energy for the thermal desorption of HBr. The ability of sorbent to hold HBr was found to be related to the pore structure. Nanoporous CMs with larger pore diameters desorb HBr more rapidly than those with smaller pore diameters can. Besides the TGA method, the oxidation of the carbon matrix during bromination was confirmed by the TPD MS method that showed an increase in the intensity of CO2 , CO, and H2 O profiles. The formation of Cx and A-L groups and the redox processes during HBr release could explain an increase in CO2 and H2 O release between 200 and 500 °C. The most intensive CO release is observed between 550 and 850 °C because of the thermal decomposition of CO-bearing groups, mainly Ph

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Table 4 The surface chemistry changes in the ACFBus after reaction with KBr3 Dibromine concentration in KBr3 , C s (Br2 ), mol L–1

a Surface

chemistry

C(Br), mmol g–1

Related concentrations ω(H2 O)

ω(CO2 )

ω(CO)

0.4

0.34

7.1

1.7

9

0.5

0.41

7.7

1.7

11

0.7

0.51

8.6

1.8

13

1.0

0.53

10.4

3.9

64

a Note The bromine content C(Br) and the related concentrations of oxygen-containing groups in the form of ω(H2 O), ω(CO2 ), and ω(CO))

groups. This increased release of CO is caused by the formation of many new Ph groups after bromination. Here, we show the effect of dibromine concentration (C s (Br2 )) on the oxidation and bromination efficiency with aqueous KBr3 solutions. The results of the bromination are shown in the example of the selected CM (ACFBus). Thermodesorbed products of the vacuum pyrolysis were analyzed to estimate the oxidation intensity addressing the surface chemistry changes. The surface carbon oxidation was quantified from the analysis of the area under the temperature profiles of H2 O+ (m/z 18), CO+ (m/z 28), CO2 + (m/z 44), and HBr+ (m/z 80). The related concentrations with respect to that of HBr were found as follows ω(X) = S(X)/S(HBr),

(1)

where ω(X) and S(X) are the relative concentrations and the relative area under the thermal desorption curve of X = H2 O, CO, and CO2 , and S(HBr) is the area under the thermal desorption curve of HBr. Table 4 presents the surface chemistry variations with an increase of dibromine concentration, C s (Br2 ), in KBr3 . The bromination efficiency is gradually increased with the Br2 content in the KBr3 solution. However, the C(Br) value varies within the range of 0.34–0.53 mmol g–1 (Table 4). The bromination efficiency is largely offset by intense oxidation. The concentration of CO-bearing centers shows up to a 7.1-time increase, in the background of a minor increase in C s (Br2 ) values. As a result of bromination, the total number of CO-bearing centers reaches an enormously high value of 64. To a lesser extent, this is true for the CO2 -bearing centers. Their relative content increased by at least 2.3 times. These experimental results confirm the importance of determining the optimal conditions for successful bromination by molecular bromine in aqueous solutions.

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3.3 Influence of Bromination Conditions on Bromination Efficiency Regardless of the brominating agent used, we observed concurrent oxidation and bromination of CMs. The oxidation intensity depends on the chemical nature of the brominating agents. The reason for the oxidation is related to the structure of the active centers of the carbon surface. Noticeable oxidation with the brominating KBr3 mixture takes place in neutral aqueous solutions. The presence of surface water and or surface hydroxyl groups, such as Ph and Cx groups, can be a reason for the disproportionation reaction of dry Br2 . Bromine liquid added to water shows rapid hydrolysis, in only a few tenths of a second at 18 °C. As a rule, the pH level is a crucial variable in bromine–water systems, determining what form the bromine will take. In our group’s work, we suggest a pH as a regulator to decrease oxidation during bromination. We have also established the reaction conditions of bromination with molecular bromine. The high oxidative capacity of molecular bromine and the formation of HBrO, as the principal product of bromine disproportion in the aqueous medium, are likely reasons for the intense oxidation of carbon surfaces. Possible transformations in bromine solutions at different pHs can be described by the following scheme (2) Since HBr is a strong acid, the addition of liquid bromine to water lowers the pH level. The ratio between bromine, hypobromous acid, and hypobromite ion in solution depends on the pH level and somewhat on temperature. The important product of the reaction is HOBr, or hypobromous acid, which is unstable because the bromine molecule is lightly bound and, therefore, will react quickly. Hypobromous acid is aggressive against organic compounds. It is a weak acid, meaning that it tends to dissociate partially into H+ and OBr– ions. In water, this dissociation is incomplete at pH between 6 and 10, and both dissociated and molecular species are present to some degree. Since H+ ions are formed by dissociation of acid, so, their concentration is expressed by pH. It follows that changing the pH levels of water will influence the balance of this reaction and will change the availability of hypobromous and bromate acids for the oxidation reactions. In a water environment, the water’s pH will, therefore, affect the passage of the bromination reaction through its pH sensitivity. As mentioned above, the addition of KBr to Br2 does not prevent the oxidation of the carbon surface that occurs in neutral water solutions. Reducing the formation of HBrO can be done by adding HBr to the reaction mixture. The preset amount of HBr can be formed by adding the calculated amount of HCl to the Br2 ·KBr solution taken with an excess of KBr.

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Fig. 7 The bromine concentration C(Br) as a function of the bromination method

Figure 7 presents graphic evidence that the value of C(Br) depends on the bromination method. The highest amount of the added Br, of above 1 mmol g–1 , was found in the samples of SCN1-KBr3 (0) and SCN1-KBr3 (1). They have been obtained by bromination in a strongly acidic medium. Under the stated conditions, the carbon surface can chemisorb from 0.75 mmol g–1 to 1.15 mmol g–1 of bromine, depending on the type and level of graphitization of carbon support. Much less bromine, of 0.52–0.66 mmol g–1 , can be added with the same brominating agent if the bromination reaction occurs in weakly acidic water solutions at a pH of 4. These values are close to the data obtained by bromination at neutral pH values in water solutions (see Table 2). Assessment of the other (negative) results showed a sharp decrease in the bromination efficiency. Only trace bromine amounts of 0.03–0.07 mmol g–1 can be chemisorbed when the bromination takes place in bromine water as well as in alkaline and strongly alkaline bromine solutions. The bromination with KBrO3 in neutral water solutions doesn’t have a positive effect. However, one can add 0.12– 0.25 mmol g–1 of Br when the reaction occurs in a highly acidic medium. The abovementioned quantities are higher than those obtained during bromination with bromine water. By the bromination with KBrO3 in an acidic medium, the bromine addition occurs rapidly and efficiently only in the case of high reducing properties of the carbon surface, which decrease in the sequence SCN1 > KAU > SCN2. In the case of SCN1, the highest reducing ability and the highest surface concentration of active centers for bromination arise because of a low oxygen content at a medium burnout degree of sourced carbonaceous material. It is known that the reactive centers are homogenized and oxidized at high and low burnout degrees, respectively. In other words, for efficient bromination, the surface must contain as many active centers as possible, but they must be free centers. The Boehm titration method was used to determine the concentration of all types of groups having an acidic nature (C A ), including oxygen-containing groups C(OFG ), which are formed during bromination. Two types of processes occur when 0.1 M

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Fig. 8 Concentration of all types of acidic a and oxygen-containing groups b formed during bromination

NaOH solutions react with the brominated CMs. The first-type process is the neutralization of NaOH with all oxygen-containing groups of acidic nature, including Cx, A-L, and Ph groups. The second-type processes are hydrolysis and debromination; they take place when chemisorbed bromine reacts with NaOH. Upon knowing the value of C A (Fig. 8a) and accounting for the amount of residual bromine, one can estimate the concentration of oxygen-containing groups (Fig. 8b). Strongly deactivated aromatic cycles fused in the carbon matrix are gradually brominated with dibromine in a concentrated acid medium. This reaction affords good yields of surface bromo derivatives. Mild reaction conditions and a simple workup provide a practical and commercially viable route for surface bromination. However, the chemical composition of the surface after bromination undergoes changes. The products of free radical oxidation reactions are found on the carbon surface after the removal of HBr. For example, samples of the CM-KBrO3 (0) series show the highest acidity. The latter is attributed to approximately 2 mmol g–1 of oxygen-containing groups. In the samples of CM-KBrO3 (0), CM-KBr3 (8), and CM-KBr3 (12) series, over 95% of the oxygen-containing groups are acidic groups. The bromination with KBr3 solution at pH = 8 or with KBrO3 solution at pH = 0 causes the preferential oxidation of the carbon surface. In other words, the concentration of oxygen-containing groups after a tandem bromination–oxidation increases by several times, from 2 to 5 times. Increasing the acidity of the brominating solution increases the bromination efficiency. But, this slightly inhibits the oxidation of the carbon surface (Table 5). It can be seen that the concentration of oxygen-containing groups (C FG ) increases considerably after the acidic bromination of different CMs. For the samples belonging to the CM-KBr3 (0) and CM-KBr3 (7) series, if compared with the pristine CMs, the C FG value increases by 1.9–2.5 times and by 2.0–3.1 times, respectively. Thus, the oxidation intensity depends a little on the pH ranging from 0 to 7. The results of studying the nature of oxygen-containing groups indicate the predominant formation of Ph groups during bromination.

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Table 5 Bromine C(Br) and oxygen-containing groups concentration (C FG ) in the CMs brominated with KBr3 at pH = 0 and 7 Sample

a Boehm

titration, C FG , mmol g–1

a Sum,

Cx

A-L

Ph

KAU-KBr3 (0)

0.32

0.09

1.03

1.44

SCN1-KBr3 (0)

0.25

0.16

1.20

1.61

SCN2-KBr3 (0)

0.11

0.21

0.88

1.20

KAU-KBr3 (7)

0.33

0.09

1.08

1.50

SCN1-KBr3 (7)

0.27

0.19

1.53

1.99

SCN2-KBr3 (7)

0.13

0.23

0.89

1.25

a Note

C(OFG ), mmol g–1

C FG was calculated accounting for hydrolysis and debromination

These data indicate the high hydrolytic stability of added bromine in the samples of CM-KBr3 (0), CM-KBr3 (4), and CM-KBr3 (7) series. About 43–48% of the added bromine is removed during the day of storage in a medium of 0.1 M NaOH solution at room temperature. This result is important for the further use of these brominecontaining precursors and proves the possibility of their storage without significant loss of bromine. It is clear that the sum of C FG values for all oxygen-containing surface groups (C(OFG )) quantified by the Boehm titration is somewhat higher than the value of C(Br) in the samples of the CM-KBr3 (0) series. However, for the samples of the CM-KBr3 (7) series, the C(OFG ) value is 2–4 times higher than the C(Br) value. In this view, if accounting for the resulting C(Br) and C FG values, it is quite interesting considering the pH effect on the bromination and oxidation. The active surface centers for bromination and oxidation are the same. This conclusion can be deduced from the fact that the pH level of the brominating solutions negatively correlates with the C(Br) values but positively correlates with the C(OFG ) values (Figs. 9a, b). These correlations and an increase in the C(Br) + C(OFG ) values with decreasing pH level (Fig. 9c) mean that more active centers are involved in tandem of bromination–oxidation reactions in an acidic medium. Thus, a likely mechanism for the formation of oxygen-containing groups, among which Ph groups will prevail, is the hydrolysis of the most reactive part of chemisorbed bromine. In Fig. 10, we compare the ATR-FTIR spectra of the brominated KAU. These samples were prepared by bromination with KBr3 solution at different pH levels (KAU-Br2 (pH)), by bromination with bromine water (KAU-Br2 (H2 O)), and by treatment with acidic KBrO3 solution that has a pH of 0 (KAU-KBrO3 (0)). Increasing the pH of the brominating solution leads to a decrease in the absorption intensity between 665 and 830 cm–1 in the spectra of the resulting samples (Fig. 10, cf. spectra 2–4, 6, and 7). This intensity decrease is associated with a decrease in the amount of added bromine. However, a decrease in the pH of the reaction mixture does not prevent the formation of new Ph groups. Their formation can be confirmed by the ATR-FTIR spectra showing an increase in the intensity of the absorption bands in

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Fig. 9 Single correlations between pH and a C(Br), b C(OFG ), and c C(Br) + C(OFG ) Fig. 10 ATR-FTIR spectra of the brominated KAU

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Fig. 11 TGA: a TG and b DTG thermograms for the brominated SCNs

the range of 900–1,230 cm–1 , cf. them with the respective bands in the ATR-FTIR spectrum of the pristine KAU. Absorption bands ranged from 1600 to 1800 cm–1 are not very different for all samples. Therefore, the formation of new Cx and A-L groups does not occur in significant quantities. Bromination of KAU with a saturated solution of bromine at pH = 7 (bromine water) does not lead to the chemisorption of bromine. But it causes the surface oxidation and formation of Ph groups. Treatment of KAU with an aqueous solution of KBrO3 in a strongly acidic medium (pH = 0) results in the incorporation of dibromine. Apparently, the bromine in the KBrO3 was reduced on the carbon surface to Br2 . This reduction is accompanied by carbon surface oxidation under the action of KBrO3 . Figure 11 shows three weight loss effects for the brominated SCNs prepared by bromination with KBr3 solution at different pH values. The first effect is due to physisorbed water that desorbs in the temperature range of 30–180 °C. The second effect at 180–400 °C is assigned to the thermal decomposition of a part of the bromine groups and the least stable oxygen-containing groups (Cx groups and a part of AL groups). These bromine and oxygen-containing groups are detached in the form of HBr vapors detected in desorption products and as a mixture of CO2 /CO gases, respectively. The third effect at temperatures above 450 °C is because of the thermal decomposition of the most stable bromine and oxygen-containing groups, primarily Ph groups. As can be seen in Fig. 11a, the total effect of weight loss shows a decrease because of reducing the bromine content on the surface, with the increasing pH of the brominating solution. For the brominated SCNs prepared by acidic bromination, the weight loss in the temperature range of 180–400 °C is significant, which is associated with a large amount of the added bromine. For the SCNs brominated in an alkaline medium, mainly Ph groups are formed. Their thermal decomposition is accompanied by a significant weight loss in the temperature range from 400 to 800 °C (Fig. 11b). Considering the impact of surface oxidation on the bromination efficiency, we choose, as an example, the SCNs because they showed the largest parallel oxidation of the carbon matrix during bromination. The direct oxidation was performed with 5%,

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Table 6 Bromine C(Br) and oxygen-containing group concentration (C FG ) in the brominated CMs Volgard method, C(Br), mmol g–1

a Boehm

Cx

A-L

Ph

SCN1-KBr3

0.42

0.22

0.09

0.50

SCN1-HNO3 (5)-KBr3

0.08

0.36

0.14

0.85

SCN1-HNO3 (10)-KBr3

0.01

0.39

0.16

0.93

SCN1-HNO3 (15)-KBr3

~0

0.42

0.17

1.01

SCN2-KBr3

0.44

0.05

~0

0.36

SCN2-HNO3 (5)-KBr3

0.06

0.46

0.23

0.81

SCN2-HNO3 (10)-KBr3

0.01

0.62

0.29

0.98

SCN2-HNO3 (15)-KBr3

~0

0.68

0.31

1.12

Sample

a Note

titration, C FG , mmol g–1

C FG was calculated accounting hydrolysis and debromination

10%, or 15% (w/v) solutions of HNO3 . With the highest concentration of HNO3 , the content of Ph groups increases up to 1.9–2.3 times. Besides, Cx and A-L groups also appear on the surface. In sum, the resulting oxidized SCNs contain different types of oxygen-containing groups, namely, 55–65% of Ph groups, 25–35% of Cx groups, and about 10–15% of A-L groups. According to Table 6, the pre-oxidation of SCN1 and SCN2 markedly reduces the bromination efficiency. Even treatment with the most diluted HNO3 solution can be a reason to lose completely the ability to attach bromine (C(Br) < 0.1 mmol g–1 ). The surface reactivity can be restored by TT in an inert gas atmosphere, removing the oxygen-containing functional groups. We chose a TT temperature of 350 °C, 500 °C, or 800 °C in accordance with the range of thermal decomposition reported for the oxygen-containing functional groups [35, 52]. When the CMs were kept at 350 °C, Cx groups were completely decomposed. De facto, their detachment from the carbon surface results in the regeneration of a part of the active centers. At 500 °C, the thermal decomposition of both Cx and A-L groups occurs together with the regeneration of the respective active centers. As usual, the thermal decomposition is accompanied by the occurrence of uncompensated bonds for some surface carbon atoms. At 800 °C, the Cx, A-L, and Ph groups are decomposed, and the TT initiates and supports restoration processes in the nearsurface layer of the carbon matrix. After the TT at 800 °C, only Qu groups remain on the carbon surface. Table 7 shows the results of the bromination of the pristine and oxidized SCN1 and SCN2 samples after their TT in argon for 1 h. The bromine content in the SCN1-800-KBr3 and SCN2-800-KBr3 samples is 10–15% higher than that in the SCN1-T TT -KBr3 and SCN2-T TT -KBr3 samples, where T TT = 350 °C or 500 °C, respectively. The highest increase in the bromine content was observed for the brominated SCN1-HNO3 (15)-T TT and SCN2HNO3 (15)-T TT samples, while the smallest increase was found for the brominated SCN1-HNO3 (5)-T TT and SCN2-HNO3 (5)-T TT samples. We attribute this

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Table 7 Bromine content in the unmodified and oxidized SCN1 and SCN2 samples subjected to TT and subsequent bromination with KBr3 at pH = 7 Sample

Thermal treatment temperature (T TT ) 350 °C C(Br) (mmol

500 °C

800 °C

0.44

0.46

g–1 )

SCN1

0.43

SCN1-HNO3 (5)

0.23

0.25

0.39

SCN1-HNO3 (10)

0.25

0.28

0.42

SCN1-HNO3 (15)

0.26

0.29

0.44

SCN2

0.45

0.47

0.52

SCN2-HNO3 (5)

0.10

0.14

0.21

SCN2-HNO3 (10)

0.12

0.17

0.28

SCN2-HNO3 (15)

0.13

0.19

0.35

effect to the higher concentration of oxygen-containing groups in the samples oxidized with 15% (v/w) HNO3 . By removal of more groups, the TT can make the carbon surface heterogeneous for the most. This treatment activates it to further bromination. On the other hand, in such experiments, increasing the T TT naturally leads to the removal of more oxygen-containing groups. It increases the reactivity of the carbon surface towards the electrophilic addition of bromine. The reactivity of SCN1 is higher than that of SCN2. This difference is explained by the higher burnout degree at the production of SCN2. As a consequence, the prepared material shows a more homogeneous structure with lower content of active centers. Only in the case of SCN1-HNO3 (10)-800 and SCN1-HNO3 (15)-800 samples, the regeneration of the active centers of the surface is almost complete. After bromination, the bromine content in these samples is close to that in the sample of SCN1-800KBr3 . For other samples, this process is incomplete, and the resulting bromine content only approaches that in the samples of SCN1-T TT -KBr3 and SCN2-T TT -KBr3 series. Figure 12 shows positive linear relationships between an increase in the bromine content (C(Br)) and the number of removed oxygen groups (C(OFG )), as a result of TT, for all CM-HNO3 (%)-T TT -KBr3 samples. The observed linear dependencies prove that the nature of the removed groups does not influence the chemical nature and properties of the active centers that are formed after these groups’ thermal decomposition and detachment. Positive correlations in Fig. 12, in particular, indicate the regeneration of the surface centers active in oxidation reactions. In other words, if the carbon surface is oxidized, in any way, with the formation of an arbitrary ratio of oxygen-containing functional groups, then active centers can be regenerated by TT. However, it is crucial to find the optimal conditions for the TT from considering the types of groups that co-exist in the surface layer.

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Fig. 12 Positive correlations between C(Br) and C(OFG )

3.4 Chemical Properties and Hydrolytic Stability of Brominated Carbon Materials Despite the preparation method, the added (chemisorbed) bromine has high hydrolytic stability. It does not hydrolyze during long-term storage both in air and underwater at room temperature since no bromide ions are registered in the liquid phase. Short-term boiling in water causes no hydrolysis of the samples in the CMKBr3 (7) and CM-Br2 (7) series. The study on the reactivity of the surface bromine was carried out by immersing the brominated CMs in the basic solutions of Na2 S2 O3 , MeNH2 , and SuEn. In Table 8, we compare the bromine concentrations before C 0 (Br) and after debromination C 1 (Br), showing the residual bromine percentage (ωBr ). After short-term (40 min.) treatment with the basic solutions, the ωBr can accept values between 50 and 60% of C 0 (Br) regardless of the used bromination method and the base strength of dehalogenating solutions. The residual bromine comprises from ~40% to ~50% of the total added bromine, which can easily hydrolyze. Below, we will consider the hydrolytic stability of the brominated nanoporous CMs, where CM = KAU, SCN1, and SCN2, on the example of the samples of CM-KBr3 (0) and CM-KBr3 (4) series prepared at pH = 0 and 4, respectively. These samples (0.1 g) were immersed in 20 ml of 0.05 M aqueous solutions of NaHCO3 , Na2 CO3 , and NaOH and orbitally shaken for 24 h at room temperature. Figure 13 shows partial debromination according to the results of chemical analysis. Depending on the CM-KBr3 sample and the used basic solution, we found that from 36% to 46% of the added bromine can be successfully hydrolyzed to bromide ions. A large amount of bromine is easily cleaved in a NaHCO3 solution. This experimental fact shows the high reactivity of the added bromine. The halogenated surface can be a valuable precursor for adding S- and N-containing groups, bearing in mind its high reactivity. Here, we will consider changes in surface chemistry, omitting the carbon edge magnetism from consideration [53–58]. When brominating the carbon surface (I), the bromine molecule attaches to the active center and forms the dibromo derivative

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Table 8 Debromination in basic solutions and residual bromine percentage Sample

Basic solution

a Bromine

ωBr (%)

concentration (mmol g–1 )

C 0 (Br)

C 1 (Br)

KAU-KBr3

Na2 S2 O3

0.32

0.17

52

KAU-Br2

Na2 S2 O3

0.25

0.12

48

SCN1-KBr3

Na2 S2 O3

0.21

0.11

51

SCN1-Br2

Na2 S2 O3

0.27

0.17

61

SCN2-KBr3

Na2 S2 O3

0.24

0.13

55

SCN2-Br2

Na2 S2 O3

0.26

0.15

58

KAU-Br2

MeNH2

0.24

0.11

46

SCN1-Br2

MeNH2

0.22

0.11

50

SCN2-Br2

MeNH2

0.25

0.14

56

KAU-Br2

SuEn

0.31

0.19

60

SCN1-Br2

SuEn

0.26

0.15

59

SCN2-Br2

SuEn

0.27

0.16

60

aC

0 (Br)

and C 1 (Br) are the bromine concentrations before and after a 40-min treatment at 100 °C

Fig. 13 Decrease in C(Br) after debromination of a CM-KBr3 (0) and b CM-KBr3 (4) with basic aqueous solutions

(II) (Fig. 14). The active center is presumably formed by the edge HC=CH bonds, according to views on the reactivity of the edge bonds disclosed in [54]. The surface structure (II) can be converted to the monobromo derivative (III) by eliminating the HBr molecule. After the HBr elimination from the intermediate, the conjugated system of the carbon bonds is renewed. So this process is easy to carry out under moderate heating to 200–300 °C (see Figs. 5a and 11) or under the action of a weak base (see Table 8 and Fig. 13). The obtained brominated CMs (Tables 2 and 4) preferably contain the product (II) with an admixture of the product (III), which is formed at the stage of removing the adsorbed bromine. Cleavage of the second bromine atom from the product (III) occurs at high temperatures, probably by a radical mechanism. An alternative way of de-bromination is the total hydrolysis

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Fig. 14 Schematic diagram showing changes in carbon surface chemistry after bromination, oxidation, thermal treatment, and modification with N-containing compounds

of the product (II). This hydrolysis can produce dihydroxy groups, the product (IV). The formation of the product (IV) is also possible with the use of alkaline solutions in which Br2 disproportionates to bromide and hypobromite ions. This result can be reached when using KBrO3 as an oxidant. Thus, at acidic bromination, in the presence of KBr, the formation of Ph groups occurs by hydrolysis of the most active forms of chemisorbed bromine. The product (IV) is prone to the renewal of the conjugated carbon bonds (product (V)), which is achieved by the elimination of water. The product (V) does not lose the ability to attach Br2 , which, after addition, hydrolyzed to produce the product (VI). This product (VI) is more stable than product (IV) because the polycyclic aromatic system stabilizes the HOC=COH fragment. All products from (II) to (VI) are most represented on the brominated surface. Their ratio is determined by the nature of the CM, bromination conditions, subsequent thermal or chemical treatment. However, when bromination takes place in neutral and basic media, other oxidation products may be formed on the carbon surface. As a result of further oxidation of the dihydroxy derivative (VI), the products containing Qu (VIII), Cx (IX) and (X), and Ph (X) groups can be formed through the transition state (VII). Dehydration of the products (IX) and (X) can produce A and L groups, respectively. The oxidized surface, which can be represented by the products (VI), (VIII), (IX), and (X), does not react with the brominating reagents since it does not contain the corresponding active centers (I). Heating of the products (VI), (VIII), (IX), and (X) decomposes the surface groups resulting from hydrolysis of bromine groups and carbon oxidation.

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Depending on the temperature used at the TT, the result is the partial or almost complete renewal of surface reactivity toward bromination (see Table 7), which is caused by the formation of new active centers, namely, –HC=CH– fragments (XI). The scheme explains all the obtained results during bromination under different conditions. From considering the thermal desorption and chemical properties of chemisorbed bromine, this scheme discloses the features of the parallel oxidation process and the renewal of the activity of the carbon surface after TT. Other surface centers may also take part in bromination. For example, hydrogen atoms in isolated C–H fragments may undergo substitution by bromine atoms. However, under the experimental conditions, such processes don’t occur, probably due to the low reactivity of the C–H active centers at room temperature. The results from the conducted experiments support the fact that chemisorbed bromine has shown the expected behavior in elimination and nucleophilic substitution reactions. This predictability proves the involvement in bromination of only the C=C centers specified above.

3.5 Amination of Brominated Carbon Materials A principal result of the amination is the complete removal of bromine from the carbon surface. The bromine is quantitatively leached in the form of Br– ions into solutions. From the results of the elemental analysis (Table 9), it is found that 12 h is sufficient to complete the reaction between the brominated CMs and amines at 110–120 °C. This result confirms the high reactivity of the added bromine, which is important for surface modification. Table 9 confirms that nitrogen was introduced into the prepared samples. They can be arranged, by the nitrogen content, in descending order as follows: ACFBus-Br2 -En > ACFBus-Br2 -Su > ACFBus-Br2 -MEA > ACFBus-Br2 -Et2 N. For the brominated ACFBus modified with En and Su residues, the highest content of nitrogen is observed because there are two nitrogen atoms in each amine molecule. Table 9 Elemental analysis of the aminated CMs Sample

Elemental analysis, mass% aO

C

H

N

ACFBus-Br2 -En

85.7

1.6

5.1

7.6

b ACFBus-Br -SuEn 2

77.3

2.1

4.2

11.3

ACFBus-Br2 -MEA

86.3

1.6

3.1

9.0

ACFBus-Br2 -Et2 N

88.0

1.5

2.9

7.6

a The

oxygen content is found by difference According to the EDX and chemical analyses, the ACFBus-Br2 -SuEn contains 5.1 mass% of sulfur

b Note

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Fig. 15 ATR-FTIR spectra of selected KAU- and ACFBus-based samples

Figure 15 shows the ATR-FTIR spectra of the aminated CMs. As a result of amination, the absorption bands in the spectra disappear from the wavenumber region of 650–830 cm–1 . This observation is clear evidence of the complete removal of the surface bromine. As compared with the pristine CMs, an intense and broadband peaked at 1,177 cm–1 appears in the spectra of the aminated samples (cf. spectra 2, 4 with 1, 3 in Fig. 15). This band can be attributed to the absorption of the C–N bonds in the surface amino groups. Notably, the ATR-FTIR spectra collected for the aminated CMs differ from those of the pristine, oxidized, and brominated CMs. These observations confirm the passage of surface layer reactions with the formation of (XII) and (XIII) products (Fig. 14). Figure 16 shows typical TGA results for the brominated samples aminated with diethylamine. It is seen that the disappearance of the weight loss effect peaked at 270 °C, and the appearance of a new one peaked at 345 °C. For most of the aminated samples (Table 10), the peak temperature of the latter effect (T max ) oscillates around 320 ± 20 °C. The aminated CMs have higher thermal stability than the brominated

Fig. 16 TGA: a TG and b DTG for the selected SCN2-based samples

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Table 10 TGA data of the aminated CMs Sample

T max , °C

mN , g g–1

C N , mmol g–1

SCN1-Br2 -En

310

0.0324

0.54

SCN1-Br2 -SuEn

320

0.0979

0.55

SCN1-Br2 -MEA

330

0.0348

0.57

SCN1-Br2 -Et2 N

295

0.0329

0.45

SCN1-HNO3 (5)-800-KBr3 -Et2 N

330

0.0234

0.32

SCN1-HNO3 (15)-800-KBr3 -Et2 N

325

0.0300

0.41

SCN2-HNO3 (5)-800-KBr3 -Et2 N

340

0.0110

0.15

SCN2-HNO3 (15)-800-KBr3 -Et2 N

340

0.0234

0.32

ACFPAN-Br2 -En

260

0.0312

0.52

ACFPAN-Br2 -SuEn

250

0.0997

0.56

ACFPAN-Br2 -MEA

290

0.0336

0.55

ACFPAN-Br2 -Et2 N

280

0.0321

0.44

ACFBus-Br2 -En

310

0.0522

0.87

ACFBus-Br2 -SuEn

285

0.1175

0.66

ACFBus-Br2 -MEA

300

0.0580

0.95

ACFBus-Br2 -Et2 N

345

0.0453

0.62

CMs used for their synthesis. Removal of bromine during amination significantly reduces the weight loss at above 400 °C. The aminated surface layer was investigated by the TPD MS method. This method was used to estimate the thermal stability of the aminated surface and explore its decomposition mechanism. Apriori, the intensity of the molecular ion peaks for amines is very low, e.g., because of intensive thermal decomposition processes. In Fig. 17, one can see typical temperature profiles. They are for the particles that are most intensively desorbed from the aminated surface. Two of these profiles are from the amine series ions, namely, CH2 =N+ H2 (m/z 30) and CH3 –CH=N+ H2 (m/z 44) fragment ions. These ions are formed as a result of the α-cleavage of C–C bonds with subsequent rearrangements. For the ACFPAN-Br2 -En sample, the fragment ions (m/z 30 and 44) are from the particles desorbed over a wide temperature range (Fig. 17). Their rate of desorption reaches a maximum of about 300 °C. Two particles, CO+ 2 and CH3 –CH=N+ H2 , can contribute to the temperature profile m/z 44. The temperature profile m/z 30 attributed to CH2 =N+ H2 has a lower intensity and repeats that of m/z 44. This observation proves the main contribution of the CH3 – CH=N+ H2 fragment ions into the temperature profile m/z 44. In the temperature range of 200–400 °C, the temperature profile m/z 28 has a similar trajectory as m/z 44. This similarity is due to ethylenediamine CH2 =CH2 + (m/z 28) that is formed under the pyrolysis conditions. At above 500 °C, the temperature profile m/z = 28 is observed due to CO+ particles. They are produced during the vacuum pyrolysis of Ph groups. Our results demonstrate that physisorbed and chemisorbed water (m/z

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Fig. 17 Typical temperature-dependent mass selective ion current (m/z 18, 28, 30, and 44) for the sample of ACFPAN-Br2 -En

18) desorbs intensively below 600 °C. The temperature profile m/z 18 is partially overlapping with the thermal decomposition range of amino groups. Table 10 sums up the surface concentration of amino groups (C N ) estimated by the TGA method from the weight loss (mN ) between 200 and 400 °C. This temperature range corresponds to the thermal decomposition range of Cx groups. Here, when estimating the values of C N , we accounted for the release of CO2 and H2 O, so far as other thermal desorption processes in this range almost do not occur. For the aminated CMs, the C N varies from 0.4 to 1.0 mmol g–1 . This amount is sufficient for the further practical use of the aminated CMs. The obtained C N values most times are close to those of C Br in the brominated CMs. These data prove the efficient replacement of the bromine groups by the amino groups. The almost complete substitution of the bromine groups should give the product (XII) (see Fig. 14). The higher values of C N as compared with C Br (cf. data in Table 10 and Tables 2 and 7) are explained by the possible interaction of amines with acidic, mainly Cx, groups presented on the carbon surface. Because of parallel oxidation (products (IX) and (X)), these groups can also form during bromination, and they can neutralize amines by producing the corresponding organoammonium salts. Since the aqueous solutions of amines are weak bases, the partial hydrolysis of bromine groups causes a lower efficiency of amination. Hydrolysis is a reason for forming the surface product (XIII) or Ph groups (Fig. 14). The temperature range for the decomposition of the surface N-containing groups determined by the TGA method as 200–400 °C significantly exceeds the boiling points of the corresponding amines. This temperature range is independent of the studied carbon materials. It is important to note that the decomposition temperature of surface groups is a function of many parameters. Considering the concentrations and decomposition temperatures of amino groups, their dependence on the nature of the amine and its molecular weight had been less than expected. This observation can indicate the covalent bonding of amino groups. The aminated CMs have sufficient

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thermal stability and can be used in adsorption processes and for the creation of new functional materials.

4 Conclusions Well-controlled and characterized organic functionalization is a necessary precondition for the fabrication of many carbon-based molecular electronic and sensing devices. In this vein, of particular interest are methods that can help incorporate bromine into carbon materials. Heavy halogen atoms provide synthetic handles through which more chemically elaborate structures can be covalently bound to the carbon solids with different structural-sorption characteristics. The proposed bromination techniques are effective in the inclusion of reactive bromine groups into the composition of the carbon surface layer. Using miscellaneous brominating agents and varying pH of the reaction solution allows introducing the O-containing functional groups as well. By increasing the acidity at bromination with a KBr3 mixture to pH = 0, one can introduce more bromine, about 1 mmol g–1 of Br groups. Bromination in a neutral or alkaline medium gives a yield of mainly Ph groups. We found that about 2 mmol of Ph groups per gram of carbon material are enough to prepare the monolayer coverage with the maximal number of available adsorption centers. Notably, the oxidation of the carbon surface almost prevents the following bromination. Oxidation has a negative impact on the reactivity of the carbon surface and its ability to be brominated. After the thermal decomposition of the oxygencontaining functional groups, the reactivity of the deoxidized carbon surface can be, in part or in full, renewed to the level of that unmodified. Despite the nature of the oxygen-containing functional groups, only the number of removed groups determines an increase in the surface reactivity. Overall, the results of these experiments prove the brominated surface provides active sites needed for efficient functionalization with chemical methods. They can facilitate the production of nitrogen-containing coatings on the surface of bulk nanoporous carbons. Acknowledgements This work was funded by grants from the Ministry of Education and Science of Ukraine ([0111U006260], [0114U003554], and [0116U002558]).

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Nano-, Microand Macrotransformations of Marine Sediments Under the Influence of Biocolloidal Processes and Aspects of Nanotechnologies of Their Enrichment and Application A. V. Panko, I. G. Kovzun, V. A. Prokopenko, and O. M. Nikipelova Abstract Simultaneous nano-, micro-, and macrotransformations of marine ore and peloid sediments under the influence of microorganisms as a part of biogeocenosis composition have been investigated. It is shown that during the processes aided with biogeocenosis, the hydrational dispersion of sediment mineral compounds takes place. It happens with their stratification into several layers. The layers separated from each other differ in density, rheological, mineral, physicochemical, and geomechanical parameters. Moreover, it gives a possibility to develop the nanotechnologies of their enrichment and application on this basis. It is established that in disperse marine and salt lake sediments based on iron-aluminosilicate materials (such as peloids (therapeutic muds), turbidite-pelitic structures, and biocolloidal iron ore compositions), the processes of their further fracturing are significantly accelerated by the action of microorganism metabolic surface-active products. It is shown that deep synergic destructive transformations occur in mineral components containing iron and calcium-containing compounds and clay minerals. Mentioned complex mechanisms of biocolloidal transformation of marine iron-aluminosilicate sediments and regularities of their nanostructural transformation allowed to offer the basics of new effective enrichment technologies with separation of iron concentrate, technologies for receiving peloids with enhanced therapeutic properties for balneological needs

A. V. Panko (B) · I. G. Kovzun · V. A. Prokopenko F.D. Ovcharenko Institute of Biocolloid Chemistry of NAS of Ukraine, Ak.Vernadskogo Blvd. 42, Kyiv 03680, Ukraine e-mail: [email protected] V. A. Prokopenko National Technical University of Ukraine «KPI», Peremohy Ave., 37, Building 4, Kyiv 03056, Ukraine O. M. Nikipelova Engineering and Technology Institute “Biotechnika” of NAAS of Ukraine, Maiakska St, 26, Khlibodarske, Odes’ka oblast 67667, Ukraine Odessa State Environmental University, Lvivska Str., 15, Odessa 65016, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_13

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and for the regulation of ecological balance in seas and oceans in conditions of increasing negative anthropogenic processes.

1 Introduction Polymineral nano- and microdisperse iron-aluminosilicate systems and materials IASs are widespread in the Earth’s crust. They occupy an important place among natural dense and disperse rocks and their products. Colloid-chemical research of such systems and materials based on iron ores, sediments (sludges, oozes), and sands of different origins attracts great attention in recent decades [1–6]. Over the last years, they have been actively studied not only from the viewpoint of the influence on their properties of colloid chemical, nanochemical, and physicochemical geomechanics laws [3–7] but also to find out the mechanisms of complex biocolloidal interactions. The latter can be considered as interphase contact colloid-chemical and nanochemical transformations of IASs developed under the action of microorganisms’ metabolism products [3, 8, 9]. However, the interpretation of such influences on polymineral IASs and IASs in many specific cases is insufficient and not yet completed because of the complexity of multifaceted physicochemical, geomechanochemical, colloidchemical, and biocolloidal methods of their research and the ambiguity of relevant conclusions. In general, it is known that physicochemical, colloid-chemical, nanochemical transformations, and contact interphase interactions in IASs and claycontaining pelitic sediments (PS) aided with microbiological processes in biogeocenosis lead to the formation of new disperse polymineral nanostructural materials with new disperse and other properties. Such materials consist of iron, aluminum, and silicon oxides, and also, but in smaller quantities, of many other inorganic and organic components. The role of the last ones in similar processes is not clear yet [8–11]. For example, it has been recently established that solid microcrystalline iron quartzites (jaspilites), which are the basis of valuable iron ore deposits, started their formation two billion years ago [12]. It happened likely with the appearance of finely disperse polymineral compositions at first, then with their biocolloidal enrichment due to microorganisms’ activity, and with further compacting of enriched iron-aluminosilicate disperse formations. However, these processes are not yet fully established [4, 6, 13]. The practical importance of both IASs and nanostructural materials empirically extracted from them is growing permanently. That is why the attention to them of technologists and researchers studying not yet established fundamental transformation mechanisms of IASs and relevant materials is growing as well. Therefore, such investigations of problem questions are well-timed and reasonable. For example, there are no developed ideas about the role of physicochemical mechanics and geomechanics (as a part of colloidal and biocolloidal chemistry) for processes in sedimentary deposits of IASs (dispersion, transformation, following secondary interphase contact nanostructuring, and further compacting). The mechanisms of their particles’ secondary contact compacting have not been studied enough. The influence of

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microbiological processes on dispersion mechanisms and further hardening of IASs is known in general terms only. Nanochemical, mechanochemical, and nanostructural transformations of IASs have been studied quite superficially. These transformations in inorganic systems and in compositions with bio-originated materials have not yet found their generalized solution both from the theoretical consideration of corresponding processes and their experimental confirmation. Recommendations for creating various science-based effective ecogeotechnological solutions (for example, concerning the practical use of IASs in technogenic buildings, like dams) need further generalizations. Thus, the mentioned considerations show the significant actuality of the problem questions and show the necessity of their solution by conducting relevant systematic experimental and theoretical research of iron-aluminosilicate systems and materials to generalize them and form definite views on colloid-chemical, mechanical, and geomechanical transformations in IASs aided with nano-, ultra-, and microdisperse particles.

2 Materials and Methods Preliminary investigations of transformation processes of polymineral microdisperse and nanostructured iron-aluminosilicate systems and materials showed that as a result of many factors of influence on microdisperse and nanodisperse IASs, various interactions between their components could occur [3–6, 12, 13]. Such interactions can include reduction or oxidation, further dispersion or contact interparticle compaction, Ostwald ripening, change of surface interparticle or interaggregate nanostructured phase composition, chemical leaching, adsorption, adhesion, cohesion, ion exchange, and others. Such interactions give new properties and change characteristics of IASs. Concerning the said above, the choice of study materials was based on samples with typical colloid-chemical properties usual for most IASs. Earlier [3–6] there were chosen some examples of iron-aluminosilicate materials: different soils and clays; polymineral iron-aluminosilicate compositions extracted from iron ores; pelagic shallow-water and deepwater sediments and peloids of the Black Sea and the Azov Sea; bentonite and montmorillonite used in other publications [1, 2] too. The chemical composition of the materials used in the study is given in Table 1. Materials and compositions were purified by standard methods [7] and powdered up to particle sizes of 63 μm. XRD and X-ray fluorescence investigations of polymineral disperse systems and compositions [13] (Table 1, Figs. 1 and 2) showed that their structure includes minerals of kaolinite, illite, montmorillonite, glauconite, saponite, goethite, and others. The composition of fine fractions has a lower content of mixed layer formations of montmorillonite-illite and glauconite types with the advantage of illite-type ones.

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Table 1 Chemical composition of averaged typical iron-aluminosilicate materials (IASMs and NIASMs) Oxide content wt%

Polymineral iron-aluminosilicate composition

Saponite-goethite composition

The Black Sea ooze

Montmorillonite

SiO2

19.5

46.4

54.6

49.5

Al2 O3

2.9

5.2

11.4

21.1

FeO

2.6

2.8

–

2.7

50.1

Fe2 O3

22.2

5.5

–

CaO

2.87

4.1

3.7

5.7

MgO

0.6

0.5

1.5

1.6

MnO

7.9

0.4

0.1

–

Na2 O

0.3

0.9

0.2

0.4

K2 O

0.3

0.4

0.9

0.3

LOI

13.9

17.1

12.1

18.2

LOI loss of ignition

Fig. 1 XRD images and thermograms of (1) natural and (2) washed from clay minerals and their nanoparticles iron-aluminosilicate material

Investigations of IASs included using theoretical modeling methods and experimental methods: chemical, rheological, XRD, thermogravimetric, SEM, and biomedical methods [3, 4, 6, 8, 9, 13]. Chemical and X-ray fluorescence analysis used in sample tests was done by the known methodology of qualitative and quantitative methods of analysis of iron-aluminosilicate rocks. X-ray diffraction (XRD) sample

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Fig. 2 SEM images of natural (1) and washed (2) from clay minerals and their nanoparticles iron-aluminosilicate material

analysis was done using diffractometer Dron-UM-1 with CoKα emission and nickel filter at room temperature. The diffractometer was connected to a computer for diffraction pattern recording. The recording was conducted at 1° per minute speed. Registration of X-Ray emission was done by scintillation counter BDS-6. Electron microscopy of tested samples: electron microscopic images were received on electron microscope Selmi PEMU in light field mode. There were also used scanning electron microscopes TESLA BN, JEOL NeoScope JCM-5000, and JEOL JSM6490 LV with INCA ENERGY-450 (Oxford) energy-dispersion device. Thermogravimetric analysis (TGA) was done on thermogravimetric analyzer MOM Q-1500 D (Hungary). Rheological characteristics of investigated systems and materials were determined by rotational viscometer with coaxial cylinders Rheotest-2 connected to a computer.

3 Results and Discussion 3.1 Current Status of Iron-Aluminosilicate System (IASs) Studies Studying the laws of colloid-chemical, nanochemical, physicochemical, mechanical, and biocolloidal processes in IASs and IASs is the primary source of information about mechanisms of transformation of such systems and materials in natural and technogenic conditions. Tentatively substantiated ideas concerning this subject also allow the prediction of the behavior of natural dense micro- and nanostructural rocks [1–15] as the main compounds of relevant systems and materials in natural, technological, biomedical, and other processes. Thus, research of IASs is actual to create modern technological processes and further development of fundamental ideas concerning their natural transformations under the action of external factors. Widespread in the Earth’s crust iron-aluminosilicate materials and marine and lake disperse and ultradisperse systems on their basis take an important place among

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natural dense nanostructural but at the same time porous rocks and technogenic products of their technological processing. Their physicochemical, physicomechanical, and chemical transformations in the Earth’s crust and reservoirs, generally in seas and lakes, lead to the formation of disperse nano- and microstructured sedimentary materials of polymineral composition (Table 2). Their main components are oxides of iron, silicon, aluminum, and also many other compounds (Table 3). Besides, such compositions may also consist of nanoamorphic and fine-crystalline carbonates and silicates formed due to biocolloidal and mechanical transformations aided Table 2 Probability of finding minerals in samples of some IASs [3, 14, 16] Main inorganic components

Probability (%) Red clay

Radiolarian ooze

Diatom ooze

Field sparrows (sum)

76

90

60

73

Magnetite

89

–

100

80

Illite, montmorillonite

27

10

100

26

Quartz

30

–

80

42

9

–

20

11

Glauconite

Globigerina ooze

Manganese nodules

79

70

–

31

Number of analyzed samples

70

9

5

118

Table 3 Chemical composition of main pelagic sediment types [3, 14, 16]

Component Red clay, % Limestone ooze, % Silica ooze, % SiO2

53.93

24.23

67.36

TiO2

0.96

0.25

0.59

Al2 O3

17.46

6.60

11.33

Fe2 O3

8.53

2.43

3.40

FeO

0.45

0.64

1.42

MnO

0.78

0.31

0.19

CaO

1.34

0.20

0.89

MgO

4.35

1.07

1.71

Na2 O

1.27

0.75

1.64

K2 O

3.65

1.40

2.15

P2 O5

0.09

0.10

0.10

H2 O

6.30

3.31

6.33

CaCO3

0.39

56.73

1.52

MgCO3

0.44

1.78

1.21

Corg

0.13

0.30

0.26

Norg

0.016

0.017

–

Total Total Fe2 O3

100.20

100.17

9.02

3.14

100.10 4.98

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by microorganisms’ metabolism products according to the laws of physicochemical geomechanics. The main types of such materials are deep-sea and shallow (shelf) deposits of pelitic sediments, peloids (therapeutic muds), iron-containing (red) clays, and sedimentary disperse iron silicate carbonate ores. Soils are the main types of such materials on the land. Nano- and microdisperse crystalline iron quartzites (jaspilites), the products of secondary biocolloidal transformation of sediments under the action of digenesis processes in the Earth’s crust [3, 12], and other products [4–15] should be attributed to ore materials based on IASs. The mineralogical composition of similar sedimentary materials, which is partly given in Table 2, shows that they consist of feldspars (found in 73–90% of tested samples), magnetite (in 80–100% of the samples), illite-montmorillonite clays (in 6–40% of the samples), quartz (in 30–80% of the samples), glauconite (in 6–20% of the samples), manganese nodules (in 31– 70% of the samples), volcanic glass (in 31–79% of the samples), and other minerals. Magnetite and other oxide-hydroxide iron-containing minerals are the most common and present in most valuable sediments [3, 9–16]. So, it is possible that such minerals of sediments have a common effect on the properties of IASs, both in fine-disperse and in dense fine-, and nanocrystalline states. The data in Table 3 show that SiO2 amounts in samples of sediments are the highest and reach 53.93% in red clays, 24.23% and 67.36% in limestone and silica ooze, respectively. A significant amount of Al2 O3 (17.46, 6.60, and 11.33% in the appropriate oozes) also shows that the three mentioned oxides (Fe2 O3 , SiO2 , Al2 O3 ) play a general role in structural transformations of sediments. Limestone ooze, which contains up to 57% of CaCO3 , also has a significant biocolloidal and colloid-chemical effect of calcium minerals, and its ascertainment has just started [4]. The distribution of different sediments is presented in Table 4; some properties and structures of marine sediments are given in Table 5. The data in Table 5 evidence that marine sediments have a developed enough surface, and the sizes of some particles in Table 4 The area (in 106 km2 ) of dissemination of pelitic sediments [3, 14, 16] Atlantic Ocean

Pacific Ocean

Indian ocean

Total

Surface

%

Surface

Surface

%

Surface

% –

%

Limestone oozes Globigerina

40.1

–

51.9

–

34.4

–

–

Pteropodium

1.5

–

–

–

–

–

–

–

Total

41.6

67.5

51.9

36.2

34.4

54.3

127.9

47.7

Silica oozes Diatom

4.1

–

14.4

–

12.6

–

–

–

Radiolarium

–

–

6.6

–

0.3

–

–

–

Total

4.1

6.7

21.0

14.7

12.9

20.4

38.0

14.2

Red clay

15.9

25.3

70.3

49.1

16.0

25.3

102.2

38.1

Total

61.6

100.0

143.2

100.0

63.3

100.0

268.1

100.0

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Table 5 Chemical composition of averaged typical iron-aluminosilicate materials (IASMs and NIASMs) Type of sediment

Carbonate content

Total density

Actual specific surface

The average particle size of nano- and ultradisperse particles

mass%

g/cm3

m2 /g

nm

0.92

28.2

83.0

27.2

0.66

86.7

27.0

54.7

86.7

Diatom ooze

0.41

17.9

170.0

Shelf (terrigenous) sediment

0.91

12.4 (110 °C, 150 °C)

190.0

Red clay Red clay Red clay

the composition of red clays are 27.0–86.7 nm. That is why red clays can be attributed to nanostructural materials. However, since the establishment of the fact (until the 1970s), the nanoparticles’ specific role in the deep-sea oozes’ composition had almost no research [3, 14, 16]. Shallow-water oozes (diatomic and shelf ones) have a highly disperse microstructure, and the size of ultradisperse particles about 170–190 nm. Interestingly, the most widespread clays in oceans are red ones containing about 9% of iron oxides (Table 3). Their area in the World ocean reaches 540 · 106 km2 , including pelitic and terrigenous oozes [3]. Hence, the role of IASs in natural, technogenic, and anthropogenic processes is indisputable, but it has been investigated incompletely and irregularly. Thus, the role of IASs in natural and technological processes has been considered from ancient times, but without consideration of the influence of their nanostructural composition and nanostructuring processes on the properties of rocks, and marine, shelf, lake, and river sediments formed from them [3, 14, 16]. The latter partly refers to anthropogenic processes as well. Thus, the use of clays and peloids via empiric procedures in balneology and medicine has been known for a long time [1], and it continues today with traditional and new clay minerals, for example, glauconite. Recently, the studies of the influence of nanoparticles of iron oxides, carbonates, and red clays of land and underwater deposits on the medical properties are expanding too [1, 2, 4, 6]. However, they still do not have a generalized character, and therefore, they need new systematic research, first of all, using methods of nanochemistry, physicochemical mechanics, and geomechanics and also of biocolloidal approaches related to them. The predominant role of red clays (their total distribution area noticeably exceeds 540 million km2 in sum) in global environmental, catastrophic, and ore-forming processes should be noted. However, from the literature data [14, 16], it follows that these processes were considered very chaotically so far. Therefore, the need for their much broader study is undeniable and urgent.

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3.2 Analysis of Obtained Experimental Data Considering that our previous publications have already reviewed various aspects of the behavior of many IASs, this study mainly considers nano-, micro-, and macrotransformations of nanostructured red clays, which have so far been little studied by colloidal science. Based on the above analysis, they are components of almost all sediments of the oceans, and their color is due to the presence of iron in both chemically bound sate and in the form of oxide hydroxides. Such sediments generally consist of an inorganic mineral part and a small organic part, including biogeocenoses and products of their vital activity (Tables 6 and 7, Figs. 3, 4, 5 and 6). Red clays also belong to the materials of the IAS type, i.e., they are polydisperse, usually macro- and microstructured, iron-aluminosilicate systems and materials including feri- and ferro-aluminosilicates, as well as iron-containing compounds [3], with the general formula: (FeO)x (Fe2 O3 )y(Al2 O3 )z (SiO2 )n ·mH2 O. In the Azov-Black Sea region, sediments similar to red clays are located near the Kerch iron ore deposit, consisting of sedimentary biocolloidal red smectite-goethite deposits, as well as bottom deposits of Kuyalnyk Bay and the mine water storage pond in Kryvyi Rih. Their composition is similar to typical red clays, based on a comparison of the data given in Tables 1 and 2 with Tables 6 and 7, which agree with the results given in [3, 13, 17–19]. The processes of biocolloidal hydration autodispersion of IASs minerals to nanoscale state in the processes of obtaining, purification, and separation of IASs are Table 6 Microbiological research of sediments Type of bacteria

CFU/cm3 Initial

Used

After maturation

– aerobic

103

104

103

– anaerobic

101

101

101

Butyric-acid

104

102

104

Denitrifying

102

102

102

Sulfate-reducing

104

102

104

Nitrifying

106

105

106

Methane-forming

107

106

107

Iron-oxidizing

3·106

1·106

3·106

– aerobic

106

102

106

– anaerobic

102

102

102

Fat-decomposing

102

102

102

Sulfur-oxidizing

104

104

104

pH

7.4

6.8

7.3

Cellulose-decomposing

Ammonifying

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Table 7 Physico-chemical properties of bottom sediments Indicators

Storage pond (Kryvyi Rih) Kuyalnytskyi estuary

1. pH liquid phase

7.7

7.20

2. Redox potential

–

−227

3. Mass fraction of moisture, wt.%

56

58.1

4. Volume weight, g/cm3

1.33

1.47

5. Adhesiveness, Pa

–

833

6. Shear limit voltage, Pa

770

491

7. Content of particles with a diameter of more than 250 μm, %

1.2

0.61

8. Specific heat capacity, kJ/kg K

–

0.59

9. Content of H2 S, %

–

0.17

10. Content of Corg , %

–

2.06

11. Content of microparticles more than 1 μm, %

3.7

2.65

12. Content of nanoparticles less than 100 nm, %

0.5

0.34

Fig. 3 SEM images of pelitic carbonate sediments with microorganisms and some products of their metabolism: a—Foraminifera; b and c—cyanobacteria; d—hydrotroillite—a product of the activity of sulfate-reducing bacteria

Nano-, Micro- and Macrotransformations of Marine Sediments …

Fig. 4 SEM images of microorganism interaction with iron-aluminosilicate particles Fig. 5 The effect of “nonspecific bioflotation” (influence of biocolloidal processes induced by microorganisms [17]) on the enrichment and separation of low-grade ores (here on foto—the result after suspension exposure for 400 days)

Fig. 6 Simplified dispersion scheme with new phase formation [18]: A and B—initial products, C—surfactant (in our case—the products of microorganism metabolism)

217

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A. V. Panko et al.

accelerated by orders of magnitude [18] with the help of about 3000 peptide surfactants (products of microorganism metabolism [3, 17]). Thus, it should be expected that the obtained data (Figs. 1, 2, 3, 4, 5 and 6, Tables 6 and 7) will allow improving the processing methods of various IASs, including red clays of both deep-sea and land deposits. Based on the analysis of the obtained results (Figs. 7, 8, and 9), the colloidchemical ideas about stress and catastrophic phenomena on underwater slopes and the coastline of the seas and oceans (in the conditions of gravitational laminar-turbulent movement of sediments according to the laws of physicochemical geomechanics and laws of elasticity. plastic change in viscosity) was developed, taking into account the data provided in [3, 12]. The chemical mechanisms of the main processes in such sediments were also formulated: Fig. 7 Dynamics of changes in viscosity (ï), microbiological index (lgCFU), Fe(II) and Fe(III) concentration in a dispersion medium in the processes of the Black Sea deepwater ooze averaged suspensions maturation and reduction–oxidation

Fig. 8 Change of rheological behavior for pelagic sediment with increasing amounts of nano- and microparticles: (1)—IASs with humidity of 34%; (2)—after washing from nano- and microparticles (W = 34%); ●—increasing and ◯—decreasing of shear stress

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Fig. 9 Model of modern nanostructural marine transformations of IASs, pelitic sediments, and turbidites on slopes and abyssal plains, accompanied by catastrophic phenomena [3]

1.

Contact interactions in conditions of viscosity ultraanomaly effect:

2.

Biocolloid nanostructural nanochemical process:

Preliminary data in Table 8 simulate red clays’ behavior under mixing conditions during catastrophic movements on sea slopes. They give an idea of how the dispersion of various IASs goes, including red clays, under the action of surfaceactive metabolism products of microorganisms from biogeocenoses (Tables 6 and 7). Such transformations are accompanied by macrostructures’ transition into microand nanostructured sedimentary formations with their simultaneous separation into layers with different densities and mineralogical composition. They are forming under the influence of the vital activity of microorganisms from biogeocenoses. Some stages of such complex phenomena are shown in Figs. 1, 2, 3, 4, 5, 6, 7, 8, and

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Table 8 Probability of finding minerals in samples of some IASs [3, 14, 16] Suspension exposure (days)

Fractions, mm 5–1 mm

1–0.5 mm

0.5–0.05 mm

0.05–0.005 mm

SiO2 . Thus, TiO2 was more suitable for dispersing V2 O5 and enhancing redox capacity than other supports. In addition, in [32] obtained a series of catalysts V2 O5 /TiO2 with different ratios of rutile phase for the reduction of ammonia to nitrogen (II) oxide. Experimental results have shown that the conversion of NO in the presence of V2 O5 /TiO2 catalyst with a small amount of rutile phase TiO2 can significantly increase the degree of NO conversion (not less 80%) at temperatures below 270 °C. The results of experimental studies of comparing different modifications of titanium (IV) oxide TiO2 -A, TiO2 -B, and TiO2 -R (A—anatase, B—brookite, R—rutile) in CeO2 /TiO2 catalysts obtained in [33] showed that CeO2 /TiO2 -R catalyst demonstrates the best physicochemical properties and optimal catalytic characteristics for the reduction reaction of nitrogen oxides by ammonia. Therefore, analyzing literature information, namely, the use of Al2 O3 , SiO2, and TiO2 , last one is a promising support for create catalysts of the selective catalytic reduction of nitrogen oxide and the presence of rutile phase can significantly increase the degree of conversion. A mixture of TiO2 –SiO2 was also used as the catalyst support (matrix). Compared with pure TiO2 , this support has a slightly larger surface area and lower crystallinity of anatase, which leads to better dispersion of vanadium oxide [23]. V2 O5 –WO3 /TiO2 – SiO2 catalyst showed a high degree of NO reduction at temperatures from 270 °C to 550 °C, which was more than 80%. These results showed that the addition of silica is able to affect the formation of acid centers and increase the dispersion of V2 O5 on the surface of the support. In [34], the researchers showed that FeVTiOx catalyst had high catalytic activity (90%) and high resistance to SO2 in the process of nitrogen oxides reduction at temperatures of 200–400 °C. Catalysts based on copper oxide obtained by the sol–gel

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method are investigated in work [35]. The results of tests regarding the efficiency of selective catalytic nitrogen oxides reduction by ammonia indicated that the Sb/TiO2 catalyst shows a rather low efficiency of NOx conversion, which was only about 20% in the temperature range 150–400 °C. Cu/TiO2 catalyst had hight NOx removal efficiency, which reached 57% at 250 °C. However, with Sb introduction into the Cu/TiO2 catalytic system, there was a significant increase in its catalytic activity. The activity of CuSb/TiO2 catalyst reached more than 90% at temperatures of 200–300 °C. The authors explain the increase of catalytic activity during the Sb introduction into Cu/TiO2 catalyst by the increase in the surface adsorbed reagents and oxygen and the reactivity of the adsorbed reagents. The introduction of MoO3 into CeO2 /TiO2 catalyst, obtained by the sol–gel method, increases its catalytic activity compared to the catalyst without it [36]. In this case, the reduction of NO into N2 reached more than 90% (in the temperature range of 250–450 °C). This catalyst also showed better resistance to H2 O and SO2 . The authors attribute the improvement in the properties of the catalyst with the introduction of MoO3 to the increase in the amount of chemisorbed oxygen. According to scientists [37], titanium nanotubes (TNT) in the composition of catalysts can improve the process of selective catalytic reduction of nitrogen oxides. Scientists have synthesized a series of catalytic compositions with nanotubes of titanium M/TNT (M = Mn, Cu, Ce, Fe, V, Cr, and Co) using a hydrothermal method followed by wet impregnation. The obtained materials were investigated in the process of low-temperature selective catalytic reduction of NOx by NH3 in the presence of excess oxygen (10 vol. %). The prepared catalysts show a significant potential for denitrification at temperatures up to 100 °C and in a wide temperature range from 100 to 350 °C. Compared with ordinary modified manganese oxide TiO2 , the synthesized manganese catalyst Mn/TNT shows higher activity. The results of electron microscopy revealed the formation of a tubular structure of the layer structure in samples Mn/TNT and Ce/TNT. Interestingly, the surface texture and tubular morphology of the Mn/TNT catalyst significantly enhance the conversion of NOx in the temperature range of 100–300 °C, which is likely to contribute to the existence of a large number of surface Mn4+ ions. Vanadium-based catalysts V/TNT demonstrated a wide operating temperature window, which can be explained by the high dispersion of active substances on the support, where V5+ is the dominant degree of oxidation. After evaluation of monometallic catalysts, bimetallic composites of cerium and manganese oxide with different ratios of Mn/Ce were synthesized. It is concluded that the presence of cerium can improve the activity of Mn/TNT at high temperatures. Thus, we can conclude that the use of nanodispersed supports with a high specific surface area can significantly improve the NOx reduction process and, more importantly, reduce the temperature of catalytic activity of catalysts.

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3.3 CeO2 -Containing Catalysts Selective catalytic reduction of NOx by ammonia at low temperatures is the main subject of research on the use of catalytic processes to protect the environment. In study [38], catalysts with cobalt (II) oxide supported on CeO2 with variable Co/Ce weight ratios were obtained by the impregnation method. CoOx /CeO2 catalysts were studied for NO reduction and oxidation reactions. The test for the activity of these catalyst showed that the addition of cobalt oxide enhances both the activity of NO reduction and NO oxidation. The most active catalyst with a mass content of 10 wt. % Co showed an efficiency of 70% in NO reduction and 85% for NO oxidation. Additional physicochemical studies of catalyst characteristics have illustrated that Co is dispersed on cerium oxide surface for systems with low Co content, while Co3 O4 is formed on cerium oxide in systems with high Co content, which significantly affects the overall catalytic activity. It was found that catalysts using precious metals are more active at low temperatures in the range of 170–300 °C, but due to the high cost of precious metals they can be partially replaced by low-cost metals, such as Mn, Cu, and Co. At a temperature of 150 °C, it is shown that the Mn/TiO2 catalyst provides more than 90% NO conversion [39]. It is found that the combination of Co3 O4 with CeO2 is more active than Co3 O4 itself, due to the increase in surface area and reduction of cobalt (from Co3+ to Co2+ ). The authors concluded that CoOx /CeO2 catalyst with 10 wt. % Co showed the best results in a series of experiments, which may be due to the stronger interaction of cobalt–cerium. The interaction of cobalt with cerium leads to the formation of defects in CeO2 . The results represent that with increasing Co content, the surface of cerium itself becomes smaller, and the adsorption of NO, on a reduced surface of cerium, becomes larger, which improves the overall activity of catalysts.

3.4 Catalysts Based on Mineral Materials The use of different types of catalytic supports, including natural origin, makes it possible to use the physicochemical features of their composition and structure, the variety of defective surface nature and the specificity of the porous structure of the carrier material. Mixed oxide catalysts of the perovskite type were used for the catalytic reduction of NOx [40]. The cubic crystal structure of perovskites has various physicochemical properties in a wide range of temperatures. It was observed that with increasing reaction temperature, the conversion of NO increases. LaFeO3 catalyst point the degree of NO conversion reached 80% at 350 °C and increased to 100% at 400 °C. Under the same reaction conditions, the perovskite catalyst LaFe1-x Cox O3 showed less activity. The higher specific surface area of LaFeO3 compared to LaFe1-x Cox O3 could be the reason for the higher catalytic activity of LaFeO3 catalyst. Replacement of more than 30% of Fe atoms with Co atoms in the structure of La/Fe/Co triple perovskites

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led to a decrease in activity compared to binary La/Fe or La/Co perovskites. The positive effect of adding Ce to perovskite-type catalysts on increasing the stability of the catalyst in the presence of SO2 for the reduction of nitrogen (II) oxide by carbon (II) oxide has also been reported. Another successful example of the application of natural materials for the catalytic reduction of nitrogen oxides is the use of composites “tourmaline/manganese-ferrumcerium oxide,” prepared by the hydrothermal method [41]. Subject to the addition of tourmaline in an amount of 2%, the composite catalyst demonstrated the best catalytic characteristics. NOx conversion of the composite catalyst was 100% at a temperature of 170–230 °C. According to the authors, tourmaline plays an important role in the reactions “solid–gas” because of its activating effect on gas molecules. Studies have performed that tourmaline has good piezoelectricity and thermoelectricity. Preliminary study of tourmaline properties has shown that the microelectric field generated on its surface can reduce particle size during catalyst preparation and limit particle agglomeration, which exposes more active catalyst sites and thus increases catalytic activity. It is shown in [42] that the properties of the mineral siderite also allow its use as a support for catalysts. Siderite is a mineral rich in transition elements and it is an ideal material for the catalyst preparation in the processes of selective catalytic NOx reduction by NH3 . In this work, siderite was doped with Mn and Ce and its efficiency was investigated. It is shown that the calcination temperature affects the nature of the active components, textural properties, and catalytic performance of siderite in denitrification reactions. It was found that doping of siderite with manganese and cerium increases the specific surface area and acidity of siderite and reduces the thermal stability of ammonium sulfate formed on the catalyst surface. As a result, siderite catalysts doped with Mn and Ce show high denitrification efficiency and high resistance to sulfur. For example, 3% Mn–siderite catalyst demonstrated NOx conversion that exceeds 90% at 180 °C and reaches 98% at 240 °C. In [43], a highly efficient galloisite-based denitrification catalyst modified with CeO2 was synthesized by the solvothermal method. The CeO2 /galloisite catalyst demonstrated high NOx reduction efficiency (>95%) over a wider temperature range (275–400 °C). In addition, the main gaseous product of NH3 oxidation to CeO2 /galloisite was N2 , which indicates a weakening of the effect of NH3 peroxidation. It was found that the use of galloisite contributed to the enrichment of reactive oxygen species and the appearance of nanocerium defects, which probably led to an increase in Brönsted acidity and increased reactivity of NH3 intermediates. Zeolite-based catalysts modified with metal oxides (Fe, Cu, Mn, and Ce) are attracting increasing attention as catalysts for the selective catalytic reduction of nitrogen oxides for mobile sources of pollution. Among such an extensive set of catalysts, zeolites modified with ferrum and copper are particularly interesting and widely studied [1]. For example, it was found that copper ions on mordenite of the catalyst create additional centers of adsorption of NO and NH3 , resulting in improved recovery process. Despite the large number of developments of catalysts for the selective catalytic reduction of nitrogen oxides, research, and improvement of these catalysts are still

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underway [44, 45]. The synergistic effect of composite metal oxides remains a topical issue in the analysis of catalyst synthesis methods. In addition, improving N2 selectivity and resistance to sulfur (IV) oxide poisoning of composite oxide catalysts is one of the most important goals of the study in the future.

4 Conclusions The article describes the existing ways of creating metal oxide catalytic nanosystems for the selective reduction of NOx . Analysis of the modern literature shows that the most promising method of neutralizing emissions with nitrogen oxides is a catalytic method of reduction by ammonia. It is presented that promising catalysts for catalytic reduction of NOx are materials based on V2 O5 (catalytic additive) and TiO2 (support). However, a more detailed literature search indicates the advantages of such catalytic additives as Fe, Ce, Sb, Co, Mn, Cu, CeO2 , CoO, and others. TiO2 and mineral materials have proven to be the most suitable supports. Despite numerous studies, the problem remains the creation of catalytically active systems that will be active at low temperatures. It is noted that the use of nanodispersed supports and the creation of composite catalytic additives is a promising direction in solving this issue. Acknowledgements The paper was prepared within the framework of the implementation of the project of the National Research Foundation of Ukraine (project No. 182/02/2020).

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Photochemical Properties of Side Chain Aurone Polymers Oksana Kharchenko, Vitaliy Smokal, Daria Shyrchenko, Oksana Krupka, Oksana Nadtoka, Natalia Kutsevol, and Mykhaylo Frasinyuk

Abstract Novel monomers based on (2Z)-2-(benzylidene)-6-hydroxy-1benzofuran-3 (2H)-one with different substituents were synthesized by reaction of acylation with methacryloyl chloride. Kinetic of homopolymerization was investigated by dylatometric method. The synthesis of copolymers based on novel aurone containing monomers and methylmethacrylate was conducted in 10% dimethylformamide solution with 2,2´-azobisisobutyronitrile as radical initiator at 80 °C (argon atmosphere). All structures of novel monomers were identified by 1H NMR and MS. The photochemical properties of synthesized aurone containing polymers have been investigated by UV VIS spectroscopy. It was found that aurone moiety in the side chain of polymer has been shown ability to Z-E-isomerization under irradiation with wavelength 365 nm. The half-reaction periods for Z-isomers of aurone containing polymers were calculated.

1 Introduction Design the structure of polymer material with predicted properties is highly relevant in modern polymer science. New polymers with different chromophore groups were intensive investigated in the last time due to superiority. The process of introducing the chromophore fragments does not change the polymer structure in mass and sometimes can be convenient and easiest way for acquiring necessary properties of polymer materials. Various types of optical polymers were synthesized with different active moieties in the side chain: polyalkylmethacrylates (PMMA), polyalkylvinylethers, polystyrenes, polyimides, polyacrylamides and others [1–3]. O. Kharchenko (B) · V. Smokal · D. Shyrchenko · O. Krupka · O. Nadtoka · N. Kutsevol Kyiv Taras Shevchenko National University, Volodymyrska 60, Kyiv 01033, Ukraine e-mail: [email protected] M. Frasinyuk Institute of Bioorganic Chemistry and Petrochemistry NAS Ukraine, Myrmanska 1, Kyiv 02000, Ukraine © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_21

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Among all types of polymers, PMMA has some important benefits, in particular great mechanical properties and low toxicity. The acrylic polymers are particularly useful as electro optical components in various electrical devices for processing optical signals including interferometers, optical switches, optical amplifiers, generators, computation devices [4–6]. Furthermore, PMMA has been applied as in situ drug delivery system for antibiotics [7, 8] and the one of the primary polymeric material that has been used for fabrication of microanalytical devices [9–11]. Recently, researches indicate that scientists from the all different types of chromophore containing derivatives drew attention to aurone containing compounds [12–14]. The aurone derivatives shown activity for several biological targets [15–18] and have anticancer [19], antioxidant [20] and antibacterial property [21]. In view of the above characteristics methacrylic polymers with aurone fragments are promising candidates for future investigations. It is well know that aminoaurones absorb and fluoresce in the visible region of the electromagnetic spectrum. The aurones have shown strong solvatochromic absorption and emission properties in solvents of different polarities. Some of these have shown high fluorescence quantum yields, which makes them potential compounds for sensing applications [12]. The spectral properties of these derivatives are dependent on the nature of the substituents [22]. We can suppose that changing of electron donating and electron withdrawing group in aurone fragment leads to significant changes in photochemical and photophisical properties. Moreover, π-conjugation, molecular planarity and length of conjugation bridge play an important role in changes of the molecule properties from the molecular engineering point of view. The study of photochromic polymers with aurone fragments is important since it allows estimate contributions from different structural fragments of a molecule on the photochemical and optical properties. This paper describes the synthesis details, characterization, optical results of methacrylic polymers incorporating aurone sidegroups.

2 Experimental 2.1 Methods 1

H NMR (400 MHz) spectra were recorded on a Mercury (Varian) 400 spectrometer tetramethylsilane was used as internal standard. Mass spectra were recorded on an Agilent 1100 mass spectrometer (atmospheric pressure chemical ionization). UV–VIS measurements were performed at room temperature in solution in a quartz liquid cell by VARIAN UV/VIS/NIR spectrometer in the range 200–600 nm. Air saturated aurone solutions were irradiated by lamp RR-818 (36 W, at λ = 365 nm).

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2.2 Materials The hydroxy derivatives were prepared by methods described previously [22, 23]. Compounds A1–A4 were synthesized according to the general procedure: 6-hydroxybenzofuran-3-one (1 eq) and aromatic aldehyde (1 eq) in mixture EtOH:DMF:50%KOH (aq) were refluxed 4–6 h. Reaction was monitored by thinlayer chromatography (TLC). Then reaction mixture was precipitated into distilled water and neutralized to pH 6–7. The precipitate was filtered off, washed several times with water, dried and crystallized from methanol/DMF. (2Z)-6-hydroxy-2-(4-methoxybenzylidene)-1-benzofuran-3-(2H)-one (A1). Yellow solid, Mp = 268–270 °C. Yield 85%. 1 H NMR (400 MHz, DMSO-d6 , δ, ppm): 3.81 (s, 3H); 6.71 (dd, 1H); 6.77–6.81 (m, 2H); 7.05 (d, 2H); 7.61 (d, 1H); 7.91 (d, 2H); 11.17 ppm (s, 1H, OH). Mass spectrum, m/z (I rel , %): 269 ([M + H]+ , 100). (2Z)- 2-benzylidene-6-hydroxy-1-benzofuran-3-(2H)-one (A2). Yellow crystals, Mp = 259–260 °C. Yield 90%. 1 H NMR (400 MHz, DMSO-d6 , δ, ppm): δ 6.61- 6.86 (m, 3H); 7.34–7.54 (m, 3H); 7.63 (d, 1H); 7.94 (d, 2H); 11.30 (s, 1H, OH). Mass spectrum, m/z (I rel , %): 239 ([M + H]+ , 100). (2Z)-6-hydroxy-2-(4-nitrobenzylidene)-1-benzofuran-3-(2H)-one (A3). Yellow crystals, Mp = 249–251 °C. Yield 90%. 1 H NMR (400 MHz, DMSO-d6 , δ, ppm): 6.75 (dd, 1H); 6.81 (d, 1H); 6.92 (s, 1H); 7.66 (d, 1H); 8.18 (d, 2H); 8.31 (d, 2H,); 11.39 (s, 1H, OH). Mass spectrum, m/z (I rel , %): 284 ([M + H]+ , 100). (2Z)-6-hydroxy-2-(pyridin-4-yl-methylidene)-1-benzofuran-3-(2H)-one (A4). Yellow crystals, Mp = 300–302 °C. Yield 91%. 1 H NMR (400 MHz, DMSO-d6 , δ, ppm): 6.73 (dd, 1H); 6.76 (s, 1H); 6.82 (d, 1H); 7.65 (d, 1H); 7.79–7.88 (m, 2H); 8.42–8.82 (m, 2H); 11.37 (s, 1H, OH). Mass spectrum, m/z (I rel , %): 240 ([M + H]+ , 100). A series of novel methacrylic monomers based on (2Z)-2-benzylidene)-6hydroxy-1-benzofuran-3 (2H)-one were synthesized according to the previously described procedure as for quinoline containing derivatives [24]. The compounds A1–A4 and triethylamine were dissolved in THF. The solution was kept in an ice bath for 10 min. A solution of distilled methacryloyl chloride was added dropwise to the reaction mixtures with catalytic amount of 4dimethylaminopyridine. The reaction mixture was stirred at 0–5 °C during 4 h and then poured into water. The monomers collected by filtration, washed with water and dried. The products M1–M4 (Fig. 1) were recrystallized from ethanole. (2Z)-2-(4-methoxybenzylidene)-3-oxo-2,3-dihydro-1-benzofuran-6-yl-2methylprop-2-enoate (M1). Bright yellow crystals, Mp = 165–166 ºC. Yield 72%. 1 H NMR (400 MHz, δ, ppm): 2.03 (s, 3H, CH3 ); 3.84 (s, 3H, OCH3 ); 5.97 (s, 1H, = CH2 ); 6.34 (s, 1H, = CH2 ); 6.95 (s, 1H); 7.14 (d, 1H); 7.50 (s, 1H, = CH–); 7.84 (d, 1H); 7.08–7.10 (d, 2H, Ar); 7.96–7.98 (d, 2H, Ar). Mass spectrum, m/z (I rel ,%): 337 ([M + H]+ , 100). (2Z)-2-benzylidene-3-oxo-2,3-dihydro-1-benzofuran-6-yl-2-methylprop-2enoate (M2). Bright yellow crystals, Mp = 129 oC. Yield 60%. 1 H NMR

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O a HO

b

O

O

O

O X R

A1-A4

M1-M4

c

C C C N

OCH3 H NO2 -

M1, P1, cop1 M2, P2, cop2 M3, P3, cop3 M4, P4, cop4

R

O

O

X

O

n

cop1-cop4

O X

O

n

P1-P4

O R

O O

X

R

Where X R

m

O

O

Fig. 1 Synthesis of aurones methacrylate monomers and polymers: a DCM or THF, 2 eq Et3N, DMAP (cat), 1 eq CH2 = C(CH3)COCl, 0 °C; b DMF, 80 °C, AIBN, 3 eq CH2 = C(CH3)COOCH3; c DMF, 80 °C, AIBN

(400 MHz, δ, ppm): 2.03 (s, 3H, CH3 ); 5.98 (s, 1H, = CH2 ); 6.34 (s, 1H, = CH2 ); 6.97 (s, 1H); 7.15 (d, 1H); 7.50 (m, 1H, = CH–); 7.86 (d, 1H); 7.52–7.54 (m, 2H, Ar); 7.47 (m, 1H, Ar); 7.99–8.01 (d, 2H, Ar). Mass spectrum, m/z (I rel , %): 307 ([M + H]+ , 98). (2Z)-2-(4-nitrobenzylidene)-3-oxo-2,3-dihydro-1-benzofuran-6-yl-2methylprop-2-enoate (M3). Orange powder, Mp = 220 ºC. Yield 80%. 1 H NMR (400 MHz, δ, ppm): 2.03 (s, 3H, CH3 ); 5.98 (s, 1H, = CH2 ); 6.34 (s, 1H, = CH2 ); 7.08 (s, 1H); 7.18 (d, 1H); 7.54 (s, 1H, = CH–); 7.88 (d, 1H); 8.2–8.22 (d, 2H, Ar); 8.31–8.33 (d, 2H, Ar). Mass spectrum, m/z (I rel , %): 352 ([M + H]+ , 96). (2Z)-3-oxo-2-(pyridin-4-ylmethylidene)-2,3-dihydro-1-benzofuran-6-yl-2methylprop-2-enoate (M4). Bright yellow crystals, Mp = 170 ºC. Yield 60%. 1 H NMR (400 MHz, δ, ppm): 2.03 (s, 3H, CH3 ); 5.97 (s, 1H, = CH2 ); 6.34 (s, 1H, = CH2 ); 6.92 (s, 1H); 7.17 (d, 1H); 7.54 (s, 1H, = CH–); 7.90 (m, 1H); 7.86–7.88 (m, 2H, Ar); 8.31–8.33 (d, 2H, Ar). Mass spectrum, m/z (I rel , %): 308 ([M + H]+ , 97). General procedure for kinetic investigation the reaction of homopolymerisation M1–M4. The polymerization ability of the new aurone containing monomers was investigated kinetically for free radical polymerization using the dilatometric method. The process was carried out in 10% DMF solution at 80 ºC, in argon atmosphere, with AIBN as initiator (1%); contractions were measured by KM-6 cathetometer. The obtained viscous solution was added dropwise into ethanol to precipitate polymeric materials. Polymers were purified of ethanol. The conversion rates were controlled gravimetrically. General procedure of free radical copolymerization. 1 eq of aurone containing monomer, 3 eq of methyl methacrylate (MMA) and 1 wt % of AIBN were added in

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a Shlenk flask. Reagents were dissolved in DMF to get 10% solution. The content of flask was degassed and filled with argon. Polymerization was carried out 24 h and then polymer was precipitated into methanol.

3 Results and Discussion The aurone derivatives with donating and withdrawing substituents have been obtained by the condensation of benzofuran-3-one with respectively benzaldehydes in solution of ethanol and DMF. The novel aurone containing monomers were synthesized by acylation the aurone alcohols with methacryloyl chloride. The polymers (P1–P4, cop1–cop4) were synthesized by free radical thermal initiated polymerization with AIBN as initiator. The polymerization ability of the new monomers M1–M4 was investigated kinetically for radical homopolymerization using the dilatometric method. The parameters of polymerisation such as conversion, speed of polymerization (V gr ), total constant of polymerization’s speed (K sum ) were calculated (Table 1). The kinetic curves of the radical homopolymerization for monomers M1–M4 are shown in Fig. 2. Monomers’ conversion during the homopolymerization process of M1 was 86% in 240 min, conversion of M2 was 54% in 240 min. As expected, for all novel monomers the speed of polymerisation was faster than for MMA and phenylmethacrylate (PhMA) due to influence of substituent in aurone fragment. Moreover, the speed of polymerization has been increased with amplifying donating effect of substituent. It was found that speed of polymerisation for M3 and M4 lower than for monomer M1. We can suppose that, decreasing the speed of polymerisation proceeds due to withdrawing effect of substituent that can act as trap for free radicals. The kinetic curve for polymerisation of MMA was added for comparison (Fig. 2). It was shown that speed of polymerization for novel aurone containing monomers in 3–5 times higher than for polymerization of MMA, and in 2–3 times higher than for PhMA (Table 1). Photochemical investigation. Table. 1 Kinetic’s parapeters of M1–M4 Yield for 4 h, %

Vgr × 104 , mol/l × s

K × 103 , mol/l × s

Monomer

Substituent

M1

OCH3

86

1,3

2,97

M2

H

54

1,22

2,94

M3

NO2

77

1,2

2,17

M4

–

68

1,09

1,77

MMA

–

33

1,08

0,51

PhMA

–

–

0,52

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1 2 3

Conversion, %

80 60

4

40

5

20 0

0

40

80

120

160

200

240

Time, min

Fig. 2 The kinetic curves of the radical polymerization of 10% methacrylic monomers in DMF at 80ºC (argon atmosphere, 1% of AIBN initiator): 1–M1; 2–M4; 3–M3; 4–M2; 5–MMA

Polymethylmethacrylate is a widely used material in optics due to many advantages such as transparency, film formation properties, and reasonable glass transition temperature. Copolymers with ration 1:3 were chosen for photochemical investigation in order to avoid steric hindrance during isomerization process. It is well known that, aurones can exist in Z-isomers and E-isomers forms. In particular, some of researches have reported that synthetic derivatives of aurone exist in thermodynamically more stable Z-isomer [21–23, 25]. Moreover, aurones and derivatives with aurone fragment have photoactive center—benzylidene moiety. This characteristic explained the opportunity for aurones enter into variety photochemical reactions. The photochemical reactions are always accompanied by changes in physical properties. The investigation of photochemical properties of aurone containing compounds have been made for charge-transfer molecules with electron-donating and electron-accepting moiety are connected by an extended π-electron system. These photochemical studies provide detailed information for understanding how the nature of p-substituted group in aurone fragment changed the photochemical properties. The photochemical properties have been investigated for polymers cop1–cop4 in THF solution. In the spectrum of absorption, polymers cop1–cop4 in THF have intensive long-wave band with vibration structure and approximate maxima of 300– 400 nm. The change of groups in benzylidene fragment lead to batochromic shift in case of electron-withdrawing group NO2 for the cop3 (Fig. 3, curve 2) and electron donating group -OCH3 cop1 (Fig. 3, curve 3) by 11 and 20 nm relatively cop2. The substitutions of phenyl fragment by pyridine cycle lead to hypsochromic shift of long-wave band for the cop4 (Fig. 3, curve 1) by 11 nm relatively cop2 (Fig. 3, curve 4). The absorbtion maxima λabs had been observed for the cop1 about 334 nm and

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1 2

1,6 1,4 Absorbance

1,2

3

1,0 4

0,8 0,6 0,4 0,2 0,0

250

300

350

400

450

500

550

Wavelenth, nm Fig. 3 Normalized absorbance spectra of polymers solutions in THF: 1–cop4, 2–cop3, 3–cop1, 4–cop2

392 nm, for cop2–308 nm and 372 and, cop3–318 nm and 381 nm, cop4–300 nm and 361 nm. The obtained aurones in vitro have Z-conformation along the double bond [25]. The Z-E-photoisomerisation with strong spectral changes took place during UV irradiation at 365 nm of the solution cop1–cop4 in THF. As an example, the cop1 (Fig. 4) has been demonstrated a strong decrease of the absorption in the regions of 300–350 nm and 370–410 nm and their increase in the region of 425–450 nm. During UV irradiation the speed of E-Z and Z-E-isomerisation has been coincide till the photostationary state (PS). The time of half-reaction (τ0,5 ) and rate constants (kZ-E ) of the photo Z-E-photoisomerization were obtained by the calculation of tgα for curve ln(D/D0 ) in the time of irradiation (Fig. 5). The k Z-E for cop1–cop4 have been determinate and the times of half-reaction for Z-E-photoisomerization have been calculated by the equation τ0,5 = ln2/kZ-E (Table 2). We suppose that constant and half-reaction period of photoisomerisation are not a constant speed of elementary photoprocesses. These constants characterized the speed of “disappearing” for Z-isomer under these experimental conditions (intensity of irradiation, temperature, and wavelength of irradiation). The results of our research confirmed that the nature of substitutions in benzylidene fragment almost does not influence on parameters of Z-E-photoisomerisation (kZ-E , τ0,5 ) for aurone containing polymers.

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1 2

1,2 1,0 Absorbance

0,8 0,6 14

0,4 0,2 0,0

250

300

350 400 450 Wavelenth, nm

500

550

Ln(D/D0)

Fig. 4 Changes in absorbance spectra for cop1 in THF: 1 – 0 s, 2–14 – 1 s-121 s of irradiation at λ = 365 nm

5 4 3 2 1 0 0

20

40

60

80

100

120

140

Time, s Fig. 5 Graphic dependence ln(Dt /D0 ) at wavelength 390 nm for cop1

4 Conclusion The present work describes the features of syntheses for novel aurone containing monomers and polymers. The polymerization ability of the new monomers M1– M4 was investigated kinetically for radical homopolymerization. The copolymers

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Table. 2 Constant’s values of speed Z-E-photoisomerization and times of half-reaction kZ-E × 102 , s−1

τ0,5 , s

392

3.2

21

308

372

12.5

5

318

381

3.1

22

300

361

3.5

20

Polymer

λabs , nm

cop1

334

cop2 cop3 cop4

with MMA and new aurone containing monomers were synthesized by thermoinitiated free-radical polymerization. Absorption spectroscopic properties for new aurone containing polymers in THF solutions were investigated with UV– Visible spectrophotometry. In all cases we notice that irradiation of polymers with aurone fragment lead to photoinduced Z-E-isomerization. It was found that introducing of electron donating or electron withdrawing group in aurone fragment has influence on photoisomerization process. From the obtained and presented results one can conclude that polymers with aurone fragments can be considered as promising materials for applications requiring photosensitivity in certain of range of wavelenght.

References 1. Celest S, Thierry V, Andre P (2000) Second-order non-linear optical polymers. Macromol Rapid Comm 21(1):1. https://doi.org/10.1002/(SICI)1521-3927(20000101)21:1%3c1::AIDMARC1%3e3.0.CO;2-X 2. Prasad PN et al (1991) Introduction to nonlinear optical effects in molecules and polymers. Wiley, New York 3. Che P, He Y, Wang X (2005) Hyperbranched Azo-polymers synthesized by Azo-coupling reaction of an AB2 monomer and postpolymerization modification. Macromolecules 38(21):8657. https://doi.org/10.1021/ma0511393 4. Derouiche Y, Koynov K et al (2012) Optical, electro-optical, and dielectric properties of acrylic tripropyleneglycol based polymer network systems including LCs. Mol Cryst Liq Cryst 561(1):124–135. https://doi.org/10.1080/15421406.2012.687149 5. Palessonga D, et al (2015). Tuning of microwave and optical properties of the electrooptic polymer PMMA-DR1 by loading with SiC nanoparticles for optimization of photonic microwave components. In: 23rd Telecommunications Forum Telfor (TELFOR), Belgrade, pp. 532–535. https://doi.org/10.1109/TELFOR.2015.7377523 6. Najjar R, Bigdeli E (2018) Synthesis of novel core-shells of PMMA with coumarin based liquid crystalline side chains and PMMA shell as electro-optical materials. Eur Polymer J 104:36–146. https://doi.org/10.1016/j.eurpolymj.2018.05.012 7. Kent ME, Rapp RP, Smith KM (2006) Antibiotic beads and osteomyelitis: here today, what’s coming tomorrow. Orthopedics 29(7):599–603. https://doi.org/10.3928/01477447-200 60701-02 8. Wentao Z, Lei G, Liu Y, Wang W, Song T, Fan J (2017) Approach to osteomyelitis treatment with antibiotic loaded PMMA. Microb Pathog 102:42–44. https://doi.org/10.1016/j.micpath. 2016.11.016

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9. Soper SA, et al (2002) Contact conductivity detection in poly (methyl methacylate)based microfluidic devices for analysis of mono-and polyanionic molecules. Anal Chem 74(10):2407–2415. https://doi.org/10.1021/ac011058e 10. Boone TD, et al (2002) The devices debuted in silicon and glass, but plastic fabrication may make them hugely successful in biotechnology applications. Anal Chem 74(3):78 A–86 A. https://doi.org/10.1021/ac021943c 11. Becker H, Locascio LE (2002) Polymer microfluidic devices. Talanta 56:267–287. https://doi. org/10.1016/s0039-9140(01)00594-x 12. Espinosa-Bustos C et al (2017) Fluorescence properties of aurone derivatives: an experimental and theoretical study with some preliminary biological applications. Photochem Photobiol Sci 16:1268–1276. https://doi.org/10.1039/C7PP00078B 13. Smokal V, Kharchenko O, Karabets Y, Iukhymenko N, Kysil A, Krupka O, Kolendo A (2018) Photochemical activities of polymers with aurone fragment. Mol Cryst Liq Cryst 672(1):11–17. https://doi.org/10.1080/15421406.2018.1542102 14. Zwick V et al (2014) Aurones as histone deacetylase inhibitors: Identification of key features. Bioorg Med Chem Lett 24:5497–5501. https://doi.org/10.1016/j.bmcl.2014.10.019 15. Detsi A et al (2009) Natural and synthetic 2’-hydroxy-chalcones and aurones: Synthesis, characterization and evaluation of the antioxidant and soybean lipoxygenase inhibitory activity. Bioorg Med Chem 17:8073–8085. https://doi.org/10.1016/j.bmc.2009.10.002 16. Hadjeri M, Beney C, Boumendjel A (2003) Recent advances in the synthesis of conveniently substituted flavones, quinolones, chalcones and aurones: Potential biologically active molecules. Curr Org Chem 7(7):679–689. https://doi.org/10.2174/1385272033486765 17. Bhasker N, Reddy MK (2011) Synthesis and antibacterial activity of prenyloxy chalcones and prenyloxy aurones, Indian. J Heterocy Ch 21:49–52 18. Kim JM et al (2013) Suppression of TPA-induced tumor cell invasion by sulfuretin via inhibition of NF-kappaB-dependent MMP-9 expression. Oncol Rep 29:1231–1237. https://doi.org/10. 3892/or.2012.2218 19. Kumar KS, Kumaresan R (2011) A quantum chemical study on the antioxidant properties of Aureusidin and Bracteatin. Int J Quantum Chem 111:4483–4496. https://doi.org/10.1002/qua. 22964 20. Siddaiah V et al (2007) Synthesis, structural revision, and antioxidant activities of antimutagenic homoisoflavonoids from Hoffmanosseggia intricata. Bioorg Med Chem Lett 17:1288– 1290. https://doi.org/10.1016/j.bmcl.2006.12.008 21. Shanker N et al (2011) Aurones: small molecule visible range fluorescent probes suitable for biomacromolecules. J Fluoresc 21:2173. https://doi.org/10.1007/s10895-011-0919-y 22. Popova A et al (2019) Aurones: synthesis and properties. Chem Heterocycl Compd 55(4/5):285. https://doi.org/10.1007/s10593-019-02457-x 23. Popova AV et al (2018) Efficient synthesis of aurone Mannich bases and evaluation of their antineoplastic activity in PC-3 prostate cancer cells. Chem Pap 72:2443–2456. https://doi.org/ 10.1007/s11696-018-0485-8 24. Guichaoua D et al (2019) UV irradiation induce NLO modulation in photochromic styrylquinoline-based polymers: computational and experimental studies. Org Electron 66:175–182. https://doi.org/10.1016/j.orgel.2018.12.022 25. Olyaei A, Javarsineh S, Sadeghpour M (2018) Green synthesis and Z/E-Isomerization of novel coumarin enamines induced by organic solvents. Chem Heterocycl Comp 54:934–939. https:// doi.org/10.1007/s10593-018-2376-x

Hybrid Hydrogels with Biologically Active Dyes and Their Antibacterial Efficacy O. Nadtoka, P. Virych, O. Krupka, V. Smokal, O. Kharchenko, S. Nadtoka, V. Pavlenko, and N. Kutsevol

Abstract The present article describes the synthesis, characterization, and sorption/desorption properties of cross-linked hybrid dextran–graft-polyacrylamide hydrogels. Polymers were characterized by scanning electron microscopy and Fourier transform infrared spectroscopy. The sorption/desorption ability of Methylene blue, Brilliant green, and Basic fuchsin into/out of hydrogel was studied and it was shown that these parameters were dependent on the hydrogel structure. Crosslinked hydrogels loaded with biologically active dyes were tested on antibacterial efficacy against Gram-positive and Gram-negative bacteria Staphylococcus aureus and Escherichia coli. The comparative study indicated that composites with Brilliant green could be considered as a potential antibacterial wound dressing.

1 Introduction The hydrogels, since their discovery by Wichterle and Lim in 1960 of poly(2hydroxyethyl methacrylate) [1], have been of great interest to biomedical scientists. Due to their unique properties [2–5], such as absorption capacity, swelling behavior, stability and degradation, bioadhesion and bioactivity, and permeability, hydrogels O. Nadtoka (B) · P. Virych · O. Krupka · V. Smokal · O. Kharchenko · S. Nadtoka · V. Pavlenko · N. Kutsevol Taras Shevchenko National University of Kyiv, 64, Volodymyrska str, Kyiv 01033, Ukraine e-mail: [email protected] P. Virych e-mail: [email protected] O. Kharchenko e-mail: [email protected] V. Pavlenko e-mail: [email protected] N. Kutsevol e-mail: [email protected] © The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_23

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show a very high potential for diverse applications. In theory, most of the features can be engineered into a hydrogel system. The ability of molecules of different sizes to diffuse into (loading) and out (release) of hydrogels permits the use of hydrogels as delivery systems. Composites based on chemically cross-linked hydrogels loaded with biologically active dyes attract attention as promising materials for medical, therapeutic, and diagnostic applications. They can be biologically active on their own, as well as used for photodynamic therapy (PDT). Today, a wide range of biologically active natural and synthetic dyes is well known [6]. Besides acridines and sulfonamide dyes, some other dyes such as Triphenyl methane, Brilliant green, Methylene blue, and Crystal violet are also used as skin disinfectants or in wounds and burns treatments [7]. Some of the dyes are photosensitizers and capable to produce free radical oxygen in the presence of the light source to immobilize the bacterial activity [8]. These biologically active dyes loaded into highly porous hydrogels structure may be delivered to the targeted tissues or wound surface. The transportation capability of hydrophobic/hydrophilic molecules is very important for a hydrogel-based delivery system. Since hydrogels have high permeability for water-soluble dyes, the most common mechanism of dye release in the hydrogel system is diffusion. Factors like polymer composition, water content, crosslinking density, and crystallinity can be used to control the release rate and release mechanism from hydrogels [9]. In this work, it has been aimed to create biologically active materials for the therapeutic delivery of biologically active dyes as universal antiseptic external use preparations from aqueous solutions. To enhance the sorption/desorption hydrogel response, hybrid polymer networks of dextran–graft-polyacrylamide in ionic and nonionic forms have been designed. The use of grafted copolymer for the creation of a 3D network has given advantages to produce the hydrogel with a more efficient structure for drug loading in comparison with single-network hydrogels. The sorption/desorption capacity of dextran–graft-polyacrylamide hydrogels in the presence of ionic dyes has been evaluated. The most well-known and versatile biologically active cationic dyes Methylene blue, Brilliant green, and Basic fuchsin were used for the study.

2 Experimental 2.1 Materials Acrylamide (AA) obtained from Aldrich was twice re-crystallized from chloroform and dried under vacuum at room temperature for 24 h. Cerium (IV) ammonium nitrate (CAN), and N,N’-methylene-bis-acrylamide (MBA) were purchased from Aldrich,

Hybrid Hydrogels with Biologically Active Dyes … Fig. 1 Chemical structures of the monomers and the dyes

325 O CH2

O CH2

O

O OSO3-

OH OH

-

OH

O3SO

n

Dextran

OSO3-

n

Dextran sulfate O

O NH2

O NH

Acrylamide

N,N-methylene-bis-acrylamide H3C

NH2+Cl-

H 2N

NH

N+

HSO4CH3

NH2

N

H3C

Basic fuchsin

CH3

Brilliant green N

H3C

N

S

CH3

+

N

CH3

CH3

Cl-

Methylene blue

without additional purification. Methylene blue (MB), Brilliant green (BG), and Basic fuchsin (BF) spectroscopic grades used in sorption studies were purchased from Fluka. Dextran (D) and dextran sulfate sodium salt (DSS) were purchased from Fluka, and the characteristics given by the manufacturer are Mw = 5 × 105 g/mol (designated as D500 and DSS500 throughout). The chemical structures of the monomers employed in polymerization as well as dyes are shown in Fig. 1. Distilled water was used throughout the experiments and as a polymerization medium during hydrogel synthesis.

2.2 Hydrogel Synthesis The dextran–graft-polyacrylamide (D500-PAA and DSS500-PAA) hybrid hydrogels were prepared by the following method [10]. The required amount of dextran or dextran sulfate sodium salt (0.0005 mM) and 25 mL of distilled water were charged into a beaker, equipped with a stirrer and He inlet, and then stirred over 20 min at

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ambient temperature (25 °C). A CAN solution (0.03 mmol mL−1 ) as an initiator was added and reacted for 2 min. Acrylamide (AA) (0.05 mol) and N,N’-methylene-bisacrylamide (MBA) (0.2% w/monomer) were added to the reaction mixture. Thereafter, argon was passed for 2 min, and the reaction mixture was left overnight. The formed hydrogel samples were taken out from the beaker to excess distilled water to remove unreacted AA (refilled fresh water for every 12 h for two days). Finally, the gels were dried at ambient temperature. The cross-linked polyacrylamides (PAAs) were prepared by the same method but without the addition of dextran.

2.3 Characterization of Hydrogel Composites The structure of cross-linked polyacrylamide and dextran–graft-polyacrylamide copolymers was confirmed using an FTIR-Spectrophotometer (MAGNA 550, Nicolet Instruments Corporation, USA). The sample was prepared at 0.25 mm thickness as KBr pellets with cross-linked PAA, D500-PAA, and DSS500-PAA and stabilized under reactive humidity before acquiring the spectrum. The spectrum was measured between 400 and 4000 nm. Also, the amounts of the dye adsorbed and released were obtained by measurement of the absorption maximum of the dyes (λmax = 670, 625, and 540 nm for MB, BG, and BF, respectively) using Lambda 35 UV–Vis spectrophotometer (Perkin-Elmer, CA). The morphology of the hydrogel membrane was observed by SEM mod. Stereoscan 440 (LEO), Cambridge, UK instrument. The cryogenically fractured film in liquid nitrogen was mounted vertically on the SEM stub by silver adhesive paste. The specimens were sputter coated with gold to avoid electrostatic charges and to improve image resolution before being examined by the electron microscopy.

2.4 Absorption and Desorption of Dyes Studies The dynamic study of dye diffusion into the hydrogel (sorption) was carried out by estimating the change in the absorption maximum of dyes solutions. In absorption experiments, the swollen cubic hydrogel samples with edge 1 cm (1 g) were immersed in 4 ml dye solutions of a certain concentration. The initial concentration C 0 of dye in solutions was 7 × 10–5 , 0.87 × 10–5, and 0.35 × 10–5 M for MB, BG, and BF, respectively. The samples were removed from the dye solution every 10 min until the equilibrium concentration of the dye in the solution was reached. The absorbance of the solution of non-absorbed dye was measured. Absorption/desorption experiments were carried out at room temperature in cylindrical glass vessels by using batch conditions.

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The diffusion of dyes out of the saturated hydrogels was similarly studied. In this case, the hydrogel samples loaded with dyes were put into distilled water. The mass ratio of loaded with dye hydrogels and water in all experiments was 1:4. At 10 min, the optical density of the solution of desorbed dye was measured. Further measurements were carried out at time intervals from 10 min to 150 min until the equilibrium concentration of the dye in solution was reached. The concentration of the desorbed dye was determined by the calibration curve. The amounts of absorbed dye per unit volume of absorbent [11] at time t (Qt, mol cm−3 ) and at equilibrium (Qeq, mol cm−3 ) were calculated by using the following expressions: Qt =

(C0 − Ct ) × V1 V2

(1)

Q eq =

(C0 − Ceq ) × V1 V2

(2)

where C o and C eq are the initial and equilibrium concentrations of dye (mol L −1 ), respectively, and C t is dye concentration at time t; V 1 is the volume of the solution added (L); and V 2 is the volume of swollen polymer (cm3 ).

2.5 Dynamic Study of Dye Absorption/desorption To examine the absorption/desorption process, kinetic models were used [12]. The equations were expressed as follows. The pseudo-first-order equation for absorption/desorption process is   ln Q eq − Q t = ln Q e − kt

(3)

where k (min−1 ) is the rate constant of swelling or absorption/desorption process.

2.6 Antibacterial Studies A disk diffusion method was applied to study the antibacterial activity of the hydrogels on a solid medium. Wild strains of Staphylococcus aureus and Escherichia coli were used as Gram-positive and Gram-negative bacteria models in the test. Wild strains of bacteria were obtained on an elective medium “Yolk-salt agar” and Endo agar [13]. The sensitivity of the selected strains to the action of dyes was carried out on a solid medium. A suspension of the bacteria (of approximately 105 CFU mL−1 ) was prepared to a particular standard and then spread evenly onto the Muller-Hinton agar

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in a Petri dish. The hydrogel samples and their composites with dyes were cut to 5 mm side squares, placed in Petri dishes with agar. Then the agar plates were placed in an incubating oven at 37 °C and left for 24 h. The antimicrobial activity of dyes-loaded hydrogels was assessed by analyzing the diameter of growth retardation [14].

3 Results and Discussion 3.1 Hydrogels Synthesis and Characterization Hydrogels as inert matrices with the three-dimensional structure were synthesized. As known, the porosity affords an opportunity for the rapid diffusion of water molecules into and out of the polymers and this property may be important for antibacterial applications. However, the cross-linking density strongly influences the mechanical properties of the hydrogels. As shown in our previous works, the samples obtained at a cross-linking agent concentration of 0.2% (w/monomer) have optimal mechanical and swelling properties. Therefore, the hydrogels used in the study have a cross-linking density of 0.2. The modification of the hydrogel network by grafting of polyacrylamide chain on polysaccharides [15] appears to be a determining factor for transporting bioactive molecules such as antibiotics or other pharmaceuticals to the destination. For modification of the hydrogel network, polyacrylamide chains were grafted on nonionized and ionized dextran macromolecules (D500 or DSS500). As was shown in our previous paper [16], the obtained branched macromolecules possess various internal structures: star-like and brush-like, respectively. With the addition of a cross-linker during grafting of PAA to dextran and dextran sodium sulfate, hybrid hydrogels with different structures could be obtained. For characterization of the internal structure of PAA, D500-PAA, and DSS500PAA 3D hydrogel networks, hydrogels SEM micrographs were used. As seen from Fig. 2, the studied hydrogels differ in the shape of pores. The hydrogels based on graft copolymers detect the cone-shaped pores in comparison with pores of randomly linked PAA hydrogel, wherein the pores of hydrogels based on ionic dextran are more voluminous. To assess the possible interactions between the hydrogel matrix and the adsorbed dye, FTIR spectra were recorded. Figure 3 shows the spectra of pure hydrogels and their composites with Methylene blue. To begin with, it should be noted that analysis of PAA, D500-PAA, and DSS500PAA hydrogels shows very similar FTIR spectra, in spite of that some differences were observed (Fig. 3a–c). The broad absorption from 3600 to 2850 cm−l corresponds to the stretching vibration of the hydrogen-bonded OH groups of dextran and dextran sulfate [17, 18] as well as the absorption maximum of water. Thus, this region of FTIR spectra cannot be used for comparative analysis of studied hydrogels.

Hybrid Hydrogels with Biologically Active Dyes …

(a)

(b)

329

(c)

Fig. 2 SEM images of cross-linked PAA a D500-PAA, b DSS500-PAA, and c hydrogels

The characteristic peaks of the PAA component for all hydrogels are registered at 1665 cm−1 (ν(C=O), amide I) and 1615 cm−1 (δ(N–H), amide II). The peak at 1450 cm−1 can be assigned to stretching vibrations in functional amide groups (ν(C–N), amide III) [17]. The band from 1480 to 1130 cm−1 contains five peaks, which are located at 1460, 1410, 1350, 1325, and 1132 cm−l . This region is known to contain the vibrations of the in-plane bending modes of associated and monomeric alcohols and of CH and CH2 groups. For aliphatic alcohols assigned the bands of 1410 and 1350 cm−1 to inplane deformation vibrations of hydrogen-bonded alcohols (“association bands”); the corresponding monomeric bands were thought to lie between 1330 and 1200 cm−1 . The 1410 cm−l vibration was assigned to both the deformation of C–OH groups and to the deformation of CH and CH2 groups (Fig. 3a). The broad subsidiary band within the C–O stretching band from about 1050 cm−1 till about 1000 cm−1 is assigned to the C–O stretching mode of solvated secondary alcohol groups (Fig. 3b) [17]. The presence of the sulfo group (SO2 ) can be determined by the bands at about 1267 cm−1 and 988 cm−1 originating from νas (S=O) and νs (S=O) vibrations, respectively (Fig. 3c) [18]. To prove the changes in hydrogels functional groups after MB adsorption, the FTIR was re-investigated again as shown in Fig. 3a–c. Methylene blue is known [19, 20] to have absorbance peaks at 3410, 1616, 1460, and 1177 cm−1 . The bands 1616 cm−1 and 1460 cm−1 belonged to the stretching band of C = N from the amide II. The peak at 1177 cm−1 indicated the bending band of N–H and from the amide III band functional group. The characteristic peaks of Methylene blue and polymers in the studied region coincide, and the spectra of pure hydrogel and composites with MB are very similar. Only Fig. 3c demonstrates changes in the spectrum of DSS500-PAA. The band corresponding to the sulfo group (SO2 ) at 1267 cm−1 has a significant shift. The general observation concludes that the MB dye effectively interacts with the functional groups of the adsorbent based on ionic hydrogel and almost does not form intermolecular bonds with PAA and D500-PAA. Negligible absorption into nonionic

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Transmittance, %

100 80

988 1132

60 40

1177

20 0 2500

1616 1662

2250

2000

1325 1410

PAA PAA + MB MB

1460

1750

1500 Wavenumber, cm-1

1250

1000

(a)

Transmittance, %

100 80

1012

60 40 D500-PAA D500-PAA + MB

20 0 2500

2250

2000 1750 1500 Wavenumber, cm-1

1250

1000

(b)

Transmittance, %

100 80 1025

60 40

1267

DSS500-PAA DSS-PAA500 + MB

20 0 2500

2250

2000

1750

1500

Wavenumber, cm-1

1250

1000

(c) Fig. 3 Infrared spectra of cross-linked hydrogels and their composites with MB: a PAA, b D500PAA, and c DSS500-PAA

Hybrid Hydrogels with Biologically Active Dyes …

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hydrogels is caused by the diffusion of dye molecules due to the difference in the concentration of solutions. Similar changes in FTIR spectra of hydrogels after adsorption of cationic dyes BG and BF were registered.

3.2 Absorption and Desorption of Dyes Absorption and desorption of dyes were studied using nonionized (PAA and D500PAA) and ionized (DSS500-PAA) hydrogels (Fig. 4). The concentration of dyes in hydrogels is inversely proportional to the concentration of the dye in solution and was determined by the calibration curve. The amounts of dye absorbed by the hydrogel were concluded (Table 1). As seen from Fig. 4a, c, and e, amounts of absorbed dyes depend on the chemical structure of polymer matrices. These differences may be caused by interactions of the dye with dextran. In all cases, ionized DSS500-PAA hydrogel possesses the highest absorption properties for MB, BG, and BF. Due to the positive charge of all dyes molecules, they can form ionic bonds with negatively charged sulfo group groups of DSS500 polysaccharide. A significant interaction of ionic polymer matrices and dye was demonstrated by FTIR spectra for Methylene blue as an example (Fig. 3c). An additional factor that increases the diffusion rate (k 1 as rate constant) and the amount of absorbed dye Q1eq (Table 1) during absorption is the expanded pores of the hydrogel based on DSS500-PAA due to the mutual repulsion of negatively charged functional sulfo groups inside the polymer matrix, as can be seen from Fig. 2c. On the contrary, hydrogels consisting of nonionized polymer matrices of PAA and D500-PAA almost do not differ in the amount of absorbed dyes and demonstrate lower values of k 1 and Q1eq (Table 1). The absorption of cationic dyes MB, BG, and BF is caused by the formation of hydrogen bonds with the polar OH, NH2, and C=O groups of dextran and polyacrylamide. For the biological use of hydrogels loaded with dyes, the concentration of the released active substance plays an important role. When studying desorption, it was found that nonionized PAA and D500-PAA hydrogels have higher values (%) of the released dyes. Despite the fact that they absorb less, the delivery of the active substance is more efficient. Some differences for the studied dyes can be explained by the differences in the internal network structure of hydrogels [21] and the steric factors of the dyes. At the same time, desorption of all dyes from the ionized hydrogel DSS500-PAA is negligible, and the resulting concentration of released active substances may be ineffective for antimicrobial use. It is obvious that ionic interaction between the anionic functional groups of the polymer matrix and cationic dyes is higher than the hydration energy of the dyes.

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8

Methylene blue

7

Desorption PAA D500-PAA DSS500-PAA

6 5 4 3 2

-5 Concentration (10 mol L-1)

Concentration (10 -5 mol L -1)

Absorption

Methylene blue 1,5

1,0

0,5

0,0

0

50

100

0

150

Time, min

PAA D500-PAA DSS500-PAA

0,6 0,4 0,2

Brilliant green

Concentration, 10-5 M

Brilliant green 0,8

50 100 Time (min)

150

(b)

(a)

Concentration, 10-5 M

PAA D500-PAA DSS500-PAA

0,12

PAA D500-PAA DSS500-PAA

0,08

0,04

0,00

0

50

100 time, min

0

150

50

100 time, min

(d)

(c)

Concentration, 10-5 M

PAA D500-PAA DSS500-PAA

0,25 0,20 0,15 0,10 0,05

Basic fuchsin Concentration, *10 -5 M

Basic fuchsin 0,30

150

0,06

PAA D500-PAA DSS500-PAA

0,04

0,02

0,00 0

50

100 time, min

((e)

150

0

50

100 time, min

150

f)

Fig. 4 a The change in concentrations of Methylene blue in solution at absorption of MB into the hydrogel [Co = 0.7 × 10–5 mol L−1 ; V1 = 4 × 10−3 L; V2 = 1 cm3 ] and b desorption out of the hydrogel [Co = 0 mol L−1 ; V1 = 4 × 10−3 L; V2 = 1 cm3 ]; c The change in concentrations of Brilliant green in solution at absorption of BG into the hydrogel [Co = 0.87 × 10–5 mol L−1 ; V1 = 4 × 10−3 L; V2 = 1 cm3 ] and d desorption out of the hydrogel [Co = 0 mol L−1 ; V1 = 4 × 10−3 L; V2 = 1 cm3 ]; e The change in concentrations of Basic fuchsin in solution at absorption of BF into the hydrogel [Co = 0.35 × 10–5 mol L−1 ; V1 = 4 × 10−3 L; V2 = 1 cm3 ] and f desorption out of the hydrogel [Co = 0 mol L−1 ; V1 = 4 × 10−3 L; V2 = 1 cm3 ]

Hybrid Hydrogels with Biologically Active Dyes …

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Table 1 Amounts of absorbed dye per unit volume of the hydrogel at equilibrium state after absorption Q1eq and desorption Q2eq and rate constants of absorption k 1 and desorption k 2 of dye Sample

Absorption

Desorption

Q1eq , × 10–8 [mol cm−3 ]

k1 [min−1 ]

Q2eq , × 10–8 [mol cm−3 ]

k2 [min−1 ]

Released [%]

Methylene blue PAA

32.44

0.3163

26.93

0.5566

17

D500-PAA

14.79

0.2720

10.07

0.4956

31.9

117.45

0.5597

0.2063

0.3

DSS500-PAA

117.1

Brilliant green PAA

26.68

0.3063

26.48

0.2638

0.75

D500-PAA

11.31

0.3320

10.90

0.5067

3.6

57.42

0.3597

57.03

0.4783

0.67

DSS500-PAA

Basic fuchsin PAA

0.348

0.2665

D500-PAA

2.117

0.3790

DSS500-PAA

3.248

0.4086

0.204

0.3098

41.4

1.913

0.4943

10.2

3.072

0.4321

5.4

3.3 Antibacterial Studies For biological research, the D500-PAA hydrogel was chosen as a more optimal matrix capable of releasing a sufficient concentration of biologically active dyes. The antibacterial susceptibility of prepared composites with dyes against Gram-positive and Gram-negative bacterial strains S. aureus and E. coli was evaluated by the disk diffusion method (Fig. 5). As was found, the sensitivity of gram-positive bacteria to dyes is much higher due to the peculiarity of the structure of the cell wall [22]. Antibacterial activity against S. aureus was revealed in hydrogels loaded with Methylene blue, Brilliant green, and Basic fuchsin in the range of dye concentrations 0.06–1 × 10–4 M (Table 2). As seen from Table 2, Methylene blue and Basic fuchsin do not affect the growth of E. coli at a concentration below 1 × 10–4 M. It is known that the Basic fuchsin plays the role of a selective factor in the differentiation of Gram-positive and Gram-negative microorganisms [23] and does not inhibit the growth of E. coli in vitro. Brilliant green released from the hydrogel is able to inhibit the growth of E. coli in vitro in an area with a diameter of 8–10 mm, depending on the concentration of the previously absorbed dye. It allows to conclude that hydrogel composites with adsorbed Brilliant green appear to exhibit bactericidal activity within the studied concentrations and are quite effective against both Gram-positive and Gram-negative bacteria. Thus, hydrogel composites loaded with cationic dyes can be used as universal bacteriostatic dressings to maintain the sterility of open wounds.

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E. coli

MB

BG

BF

S. aureus

MB

BG

BF

Fig. 5 Inhibition zone against S. aureus and E. coli using hydrogel composites loaded with dyes Table 2 Antimicrobial activity of dyes-loaded hydrogels Sample

1

Concentration in hydrogel, × 10–4 M 0.015625

Growth retardation, mm S. aureus

E. coli

MB

BG

BF

MB

BG

BF

–

–

–

–

8.6 ± 0.5

–

2

0.03125

–

–

–

–

9.4 ± 0.3

–

3

0.0625

–

10.5 ± 0.4

7.1 ± 0.3

–

9.6 ± 0.3

–

4

0.125

6.1 ± 0.3

10.1 ± 0.4

6.3 ± 0.4

–

9.6 ± 0.4

–

5

0.25

6.6 ± 0.2

9.8 ± 0.5

6.8 ± 0.3

–

10.3 ± 0.3

–

6

0.5

6.6 ± 0.3

9.9 ± 0.4

6.4 ± 0.5

–

10.5 ± 0.4

–

7

1

8.1 ± 0.3

10.5 ± 0.4

9.7 ± 0.3

–

12.5 ± 0.3

–

Hybrid Hydrogels with Biologically Active Dyes …

335

4 Conclusion The chemically cross-linked nonionized PAA, D-PAA, and ionized DSS500-PAA hydrogels were prepared by radical polymerization using dextrans in ionic and nonionic forms. Scanning electron microscopy has shown that the internal structure of the hydrogel network can be controlled by grafting PAA onto dextran. Hydrogels in ionic form have more voluminous pores caused by the mutual repulsion of negatively charged functional sulfo groups of the polymer matrix. Differences in the internal and chemical structure of hydrogels lead to different absorption and desorption properties. The ionized DSS500-PAA hydrogel was found to have the highest absorption properties for the cationic dyes Methylene blue, Brilliant green, and Basic fuchsin. The significant interaction of the ionic polymer matrix and the dyes was demonstrated using FTIR spectra. The antibacterial properties of hybrid hydrogels loaded with biologically active dyes were tested against Staphylococcus aureus and Escherichia coli. Satisfactory antimicrobial efficacy was registered for dye-loaded hydrogels in a humid environment. So, by the strategy of synthesis of hybrid polymer networks, the diffusion of biologically active molecules out of hydrogel matrices has been improved. Investigation of absorption and desorption of dyes by hydrogels confirms the potential of their application as a drug delivery system and antibacterial wound dressing.

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Correction to: Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications Olena Fesenko and Leonid Yatsenko

Correction to: O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5 The original version of this book was inadvertently published without including a chapter. The chapter titled “Hybrid Hydrogels with Biologically Active Dyes and Their Antibacterial Efficacy” has been added at the end of the book.

The updated version of this book can be found at https://doi.org/10.1007/978-3-030-74800-5

© The Editor(s) (if applicable) and The Author(s), under exclusive license to Springer Nature Switzerland AG 2021 O. Fesenko and L. Yatsenko (eds.), Nanooptics and Photonics, Nanochemistry and Nanobiotechnology, and Their Applications, Springer Proceedings in Physics 264, https://doi.org/10.1007/978-3-030-74800-5_22

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