Synthesis and Applications of Inorganic Nanostructures 9783527340279

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Synthesis and Applications of Inorganic Nanostructures

Synthesis and Applications of Inorganic Nanostructures Huaqiang Cao

Author Professor Dr. Huaqiang Cao Tsinghua University Department of Chemistry 100084 Beijing China Cover Cover figures were kindly provided by the author.

All books published by Wiley-VCH are carefully produced. Nevertheless, authors, editors, and publisher do not warrant the information contained in these books, including this book, to be free of errors. Readers are advised to keep in mind that statements, data, illustrations, procedural details or other items may inadvertently be inaccurate. Library of Congress Card No.: applied for British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library. Bibliographic information published by theDeutsche Nationalbibliothek The Deutsche Nationalbibliothek lists this publication in the Deutsche Nationalbibliografie; detailed bibliographic data are available on the Internet at . © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany All rights reserved (including those of translation into other languages). No part of this book may be reproduced in any form – by photoprinting, microfilm, or any other means – nor transmitted or translated into a machine language without written permission from the publishers. Registered names, trademarks, etc. used in this book, even when not specifically marked as such, are not to be considered unprotected by law. Print ISBN: 978-3-527-34027-9 ePDF ISBN: 978-3-527-69816-5 ePub ISBN: 978-3-527-69817-2 Mobi ISBN: 978-3-527-69818-9 oBook ISBN: 978-3-527-69815-8 Cover Design Schulz Grafik-Design, Fußgönheim, Germany Typesetting SPi Global, Chennai, India Printing and Binding Printed on acid-free paper

Dedicated to my parents Dingqiu Cao and Yousong Zhang, and my wife’s parents Wei Liu and Xinlun Yang

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Contents Preface xvii Acknowledgments xix 1

Introduction 1

1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8

Wave-Particle Duality 2 Uncertainty Principle 3 Schrödinger Equation 3 Particle in a Potential Box 4 Fermi-Dirac Distribution and Fermi Energy 5 Density of States 7 Quantum Confinement 8 Top-Down and Bottom-Up Approaches to Construct Nanostructures 10 Nanostructured Materials Based on Dimension 11 Zero-Dimensional Nanostructures 11 One-Dimensional Nanostructures 13 Two-Dimensional Nanostructures 14 Three-Dimensional Nanostructures: Superstructures and Hybrid Structures 15 References 16

1.9 1.10 1.11 1.12 1.13

2

Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures 21

2.1 2.2

General Remarks 21 Synthesis, Characterization, and Bioapplication of Metal Ag Nanoparticles 21 Synthesis of GSH-Coated Ag NPs 22 Ag NPs Modification 22 Ag NPs and BSA Binding 22 SDS-PAGE of Ag NPs and BSA Binding 22 Cell Culture and Treatment 23 MTT (Thiazolyl Blue) Assay 23 Fluorescence Observation of K562 Cells Stained by Hoechst 33258 23

2.2.1 2.2.2 2.2.2.1 2.2.2.2 2.2.3 2.2.4 2.2.5

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Contents

2.2.6 2.2.7 2.2.8 2.2.8.1 2.2.8.2 2.2.9 2.3 2.3.1 2.3.1.1 2.3.1.2 2.3.1.3 2.3.1.4 2.3.2 2.3.2.1 2.3.2.2 2.3.2.3 2.3.2.4 2.3.2.5 2.3.2.6 2.3.3 2.3.3.1 2.3.3.2 2.3.3.3 2.3.3.4 2.3.3.5 2.3.3.6 2.3.4 2.3.4.1 2.3.4.2 2.3.4.3 2.3.4.4 2.3.4.5 2.4 2.4.1 2.4.1.1 2.4.1.2 2.4.1.3 2.4.1.4 2.4.1.5 2.4.2 2.4.2.1 2.4.2.2 2.4.2.3 2.4.2.4 2.4.3

Flow Cytometer Measurement 23 Characterization 24 Structure 24 Microstructure of Ag NPs 24 Binding of Ag NPs and BSA 25 Anticancer Activities of Ag NPs 29 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles 33 SnO2 Nanoparticles 33 Synthesis 35 Characterization 35 Photocatalytic Activity Test 35 Structure 35 ZrO2 Nanoparticles 45 Synthesis 45 Characterization 45 Photocatalytic Activity Test 46 Structure 46 Optical Properties of ZrO2 Nanoparticles 49 Photocatalytic Properties 51 In2 O3 Hollow Nanocrystals 52 Synthesis 52 Characterization 53 Photocatalytic Activity Test 53 Structure 53 Growth Mechanism of the rh-In2 O3 Hollow Nanocrystals 58 Photocatalytic Activity of the rh-In2 O3 Hollow Nanocrystals 61 Fe2 O3 Nanoparticles 68 Synthesis 69 Characterization 69 Measurement of Magnetic Properties 69 Structure 71 Magnetic Properties 73 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles 74 CdS Nanoparticles 74 Synthesis 76 Characterization 76 Structure 76 Growth Mechanism 80 Photoluminescence Properties 82 ZnS Nanoparticles and Microspheres 83 Synthesis of ZnS Nanoparticles and Microspheres 84 Characterization 84 Structure 84 Optical Properties 90 Ag2 S Nanospheres 92

Contents

2.4.3.1 2.4.3.2 2.4.3.3 2.4.3.4 2.4.3.5 2.5 2.5.1 2.5.1.1 2.5.1.2 2.5.1.3 2.5.1.4 2.5.2 2.5.2.1 2.5.2.2 2.5.2.3 2.5.2.4 2.5.2.5 2.5.2.6 2.5.2.7 2.5.2.8 2.6 2.6.1 2.6.2 2.6.3 2.6.4 2.6.5 2.6.6 2.6.7

Synthesis 92 Characterization 93 Structure 93 Optical Properties of Ag2 S Nanospheres 95 Growth Mechanism 96 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes 101 Fe2 O3 Nanocubes 101 Synthesis 101 Characterization 103 Structure 104 Magnetic Properties 106 Fe3 O4 Nanocubes 106 Synthesis 107 Characterization 107 Magnetic Behavior Measurement 107 Electrochemical Measurement 107 Structure 108 Growth Mechanism 110 Magnetic Properties 115 Applied as Anode for LIBs 117 Synthesis, Characterization, and Photocatalytic Application of Microspheres (Bi@Bi2 O3 Microspheres) 119 Synthesis 120 Characterization 121 Photocatalytic Test 121 Structure 122 Growth Mechanism 124 Optical Properties 126 Photocatalytic Activities 127 References 133

3

Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures 147

3.1 3.2

General Remarks 147 Synthesis, Characterization, and Magnetic/Electrical Properties of Metal Nanowires/Nanotubes 147 Fe Nanowire Arrays 147 Synthesis 148 Characterization 148 Structure 149 Magnetic Properties 150 Co Nanowire Arrays 150 Synthesis 151 Characterization 152 Structure 152 Magnetic Properties 152

3.2.1 3.2.1.1 3.2.1.2 3.2.1.3 3.2.1.4 3.2.2 3.2.2.1 3.2.2.2 3.2.2.3 3.2.2.4

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Contents

3.2.3 3.2.3.1 3.2.3.2 3.2.3.3 3.2.3.4 3.2.4 3.2.4.1 3.2.4.2 3.2.4.3 3.2.4.4 3.2.5 3.2.5.1 3.2.5.2 3.2.5.3 3.2.5.4 3.3 3.3.1 3.3.1.1 3.3.1.2 3.3.1.3 3.3.1.4 3.3.2 3.3.2.1 3.3.2.2 3.3.2.3 3.3.3 3.3.3.1 3.3.3.2 3.3.3.3 3.3.4 3.3.4.1 3.3.4.2 3.3.4.3 3.3.4.4 3.3.5 3.3.5.1 3.3.5.2 3.3.5.3 3.3.5.4 3.4 3.4.1 3.4.2 3.4.3 3.4.4 3.4.5

Ni Nanowire Arrays 154 Synthesis 154 Characterization 155 Structure 155 Magnetic Properties 156 Cu Nanowire Arrays 156 Synthesis 156 Characterization 157 Structure 157 Electrical Properties 159 Growth Mechanism for 1D Metal Nanostructures 160 Synthesis 161 Characterization 161 Structure 161 Growth Mechanism 164 Synthesis, Characterization, and Optical Properties of Metal Oxide Nanowires 167 In2 O3 Nanowires 167 Synthesis 168 Characterization 168 Structure 168 Photoluminescence Properties 169 ZrO2 Nanowires 170 Synthesis 171 Characterization 171 Optical Properties 173 SnO2 Nanowires 175 Synthesis 175 Characterization 176 Optical Properties 177 NiO Nanowires 179 Synthesis 179 Characterization 179 Structures 179 Optical Properties 180 Cr2 O3 Nanowires 181 Synthesis 182 Characterization 182 Structures 182 Optical Properties 183 Synthesis, Characterization, Electrochemical Properties, and Supercapacitor Applications of MoO3 Nanowires 185 Synthesis 186 Characterization 186 Wastewater Treatment Experiment 187 Electrochemical Measurement 187 Structure 187

Contents

3.4.6 3.4.7 3.5 3.5.1 3.5.2 3.5.3 3.5.4 3.5.5 3.5.6 3.5.7 3.5.8 3.5.9 3.5.10 3.6 3.6.1 3.6.2 3.6.3 3.7 3.7.1 3.7.2

Electrochemical Properties and Supercapacitor Applications 190 Adsorption Properties for Removal of RhB 192 Synthesis, Characterization, and Bioapplication of Hydroxyapatite (HAP) Nanorods 193 Synthesis 194 Characterization 194 Cell Culture and Treatment 195 MTT (Thiazolyl Blue) Assay 195 Fluorescence Observation of HeLa Cells Stained by Hoechst 33342 195 Flow Cytometer Measurement 195 Nanoindentation Test 196 Structure 196 Anticancer Activity 201 Mechanical Strength 205 Synthesis and Characterization of Sulfide (CdS) Nanowire Arrays 206 Synthesis 206 Characterization 207 Structure 207 Synthesis and Characterization of Fullerene (C70 ) Nanowires 208 Synthesis 209 Characterization 209 References 211

4

Synthesis, Characterization, and Applications of Two-Dimensional (2D) Graphene-Related Nanostructures 221

4.1 4.2

General Remarks 221 Synthesis, Characterization, and Applications of Graphene-Based Oxide Hybrid Nanostructures 221 Co3 O4 @Reduced Graphene Oxide 221 Synthesis 222 Characterization 223 Electrochemical Behavior Measurement 223 Structure 224 Electrochemical Properties 226 Fe3 O4 @Reduced Graphene Oxide 227 Synthesis 228 Characterization 228 Electrochemical Measurement 229 Structure 229 Magnetic Properties 233 Electrochemical Properties and Application as Anode for LIBs 235 SnO2 -Polyaniline-Reduced Graphene Oxide in LIBs 240 Synthesis 241 Characterization 242 Electrochemical Experiment 243

4.2.1 4.2.1.1 4.2.1.2 4.2.1.3 4.2.1.4 4.2.1.5 4.2.2 4.2.2.1 4.2.2.2 4.2.2.3 4.2.2.4 4.2.2.5 4.2.2.6 4.2.3 4.2.3.1 4.2.3.2 4.2.3.3

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Contents

4.2.3.4 4.2.3.5 4.2.4 4.2.4.1 4.2.4.2 4.2.4.3 4.2.4.4 4.2.4.5 4.2.5 4.2.5.1 4.2.5.2 4.2.5.3 4.2.5.4 4.2.5.5 4.2.5.6 4.2.5.7 4.2.6 4.2.6.1 4.2.6.2 4.2.6.3 4.2.6.4 4.2.6.5 4.2.6.6 4.2.7 4.2.7.1 4.2.7.2 4.2.7.3 4.2.7.4 4.2.7.5 4.3 4.3.1 4.3.1.1 4.3.1.2 4.3.1.3 4.3.1.4 4.3.1.5 4.3.1.6 4.3.1.7 4.3.2 4.3.2.1 4.3.2.2 4.3.2.3 4.3.2.4 4.3.2.5

Structure 243 Electrochemical Properties and Applied as Anode for LIBs 246 TiO2 @Reduced Graphene Oxide 249 Synthesis 250 Characterization 250 Electrochemical Measurement 250 Structure 251 Electrochemical Properties and Applied as Electrodes for Supercapacitors 253 Cu2 O@Reduced Graphene Oxide 259 Synthesis 259 Characterization 259 Adsorption Measurement 260 Electrochemical Measurement 260 Structure 261 Removal of RhB 265 Electrochemical Properties and Applied as Electrodes for Supercapacitors 267 ZnO@Reduced Graphene Oxide 269 Synthesis 269 Characterization 270 Photocatalytic Property Measurement 270 Structure 271 Electrochemical Behavior 273 Photocatalytic Properties 273 Fe2 O3 @Reduced Graphene Oxide 276 Synthesis 276 Characterization 277 Structure 277 Magnetic Properties 281 Removal of Rhodamine B Dye Molecules from Water 283 Synthesis, Characterization, and Applications of Graphene-Related Hydroxide Nanocomposites 285 β-Ni(OH)2 @RGO 285 Synthesis 286 Characterization 286 Electrochemical Measurement 287 Nickel–MH Battery Performance Measurement 287 LIB Performance Measurement 287 Structure 288 Electrochemical Properties and Applied in Ni-MH and LIBs 290 Mg(OH)2 @RGO 294 Synthesis 294 Characterization 294 Adsorption of Dye from Water 294 Structure 295 Applications 298

Contents

4.4 4.4.1 4.4.2 4.4.3 4.4.4 4.4.5 4.5 4.5.1 4.5.2 4.5.3 4.5.4 4.6 4.6.1 4.6.2 4.6.3 4.6.4 4.6.5 4.6.6 4.6.7 4.6.8 4.7 4.7.1 4.7.2 4.7.3 4.7.4 4.7.5

Synthesis, Characterization, and Applications of Graphene-Related Sulfide (SnS2 @RGO) Nanocomposites 301 Synthesis 301 Characterization 302 Electrochemical Measurement 302 Structure 303 Applied as Anode Material for LIBs 307 Synthesis, Characterization, and Applications of Graphene-Related Carbonate (MnCO3 @RGO) Nanocomposites 314 Synthesis 314 Characterization 315 Structure 316 Applications as Anode for LIBs 319 Synthesis, Characterization, and Applications of Graphene-Related Metal (Ni@RGO) Nanocomposites 324 Synthesis 324 Characterization 325 Adsorption Measurement 325 Electrochemical Measurement 326 Structure 326 Magnetic Properties 329 Removal of MB 331 Electrical Properties and Applied as Electrode for Supercapacitors 333 Synthesis, Characterization, and Applications of Graphene-Related Organic Nanocomposites (Adenine Modified Graphene, AMG) 334 Synthesis 335 Characterization 335 Electrochemcial Measurement 336 Structure 336 Electrical Properties 341 References 346

5

Synthesis, Characterization, and Applications of Three-Dimensional (3D) Nanostructures 363

5.1 5.2

General Remarks 363 Synthesis, Characterization, and Application of 3D Oxide Nanostructures 363 Boehmite Nanococoons 363 Synthesis 364 Characterization 364 Structure 364 Growth Mechanism 366 CL Properties 367 ZnO-CPP Nanostructures 368 Synthesis 369 Characterization 370

5.2.1 5.2.1.1 5.2.1.2 5.2.1.3 5.2.1.4 5.2.1.5 5.2.2 5.2.2.1 5.2.2.2

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Contents

5.2.2.3 5.2.2.4 5.2.2.5 5.2.2.6 5.2.2.7 5.2.2.8 5.2.2.9 5.2.3 5.2.3.1 5.2.3.2 5.2.3.3 5.2.3.4 5.2.3.5 5.2.3.6 5.2.3.7 5.2.3.8 5.2.3.9 5.2.4 5.2.4.1 5.2.4.2 5.2.4.3 5.2.4.4 5.2.4.5 5.2.4.6 5.2.4.7 5.2.4.8 5.2.5 5.2.5.1 5.2.5.2 5.2.5.3 5.2.5.4 5.2.5.5 5.2.5.6 5.2.5.7 5.2.5.8 5.2.5.9 5.2.5.10 5.3 5.3.1 5.3.1.1 5.3.1.2 5.3.1.3 5.3.1.4 5.3.1.5 5.3.1.6 5.3.1.7

Photocatalysis Experiment 370 Water Contact Angle (CA) Measurement 370 Structure 370 Possible Growth Mechanism of Zn-CPPs 375 Formation of ZnO from Zn-CPPs 376 Superhydrophobicity 377 Photocatalysis Properties 378 Co3 O4 3D Superstructures 379 Synthesis 379 Characterization 380 Magnetic Behavior Measurement 380 Catalytic Reaction 380 Structure 381 Optical Properties 381 Growth Mechanism 384 Magnetic Properties 386 Applied as Peroxidase 388 Mn2 O3 3D Superstructures 395 Synthesis 396 Characterization 397 Water Contact Angle (CA) Measurement 397 Measurement of Magnetic Properties 397 Structure 397 Growth Mechanism 401 Magnetic Properties 404 Superhydrophobic Properties 407 WO3 3D Snowflake-like Nanostructures 408 Synthesis 409 Characterization 409 Water Contact Angle Measurement 409 Photocatalytic Activity Test 409 Electrochemical Measurement 410 Growth Mechanism 410 Structure 417 Superhydrophobicity 420 Photocatalytic Activity 421 Improved Anode Performance for LIBs 425 Synthesis, Characterization, and Application of 3D Hydroxide Nanostructures 428 Ni(OH)2 3D Peonylike Superstructures 428 Synthesis 428 Characterization 428 Electrochemical Properties Measurement 429 Wetting Behavior Measurement 429 Structure 429 Growth Mechanism 433 Electrochemical Properties 435

Contents

5.3.1.8 5.3.2 5.3.2.1 5.3.2.2 5.3.2.3 5.3.2.4 5.3.2.5 5.3.2.6 5.3.2.7 5.3.2.8 5.3.2.9 5.3.2.10 5.4 5.4.1 5.4.2 5.4.3 5.4.4 5.5 5.5.1 5.5.1.1 5.5.1.2 5.5.1.3 5.5.1.4 5.5.1.5 5.5.1.6 5.5.1.7 5.5.1.8 5.5.1.9 5.5.2 5.5.2.1 5.5.2.2 5.5.2.3 5.5.2.4 5.5.2.5 5.5.2.6 5.5.2.7 5.6 5.6.1 5.6.1.1 5.6.1.2 5.6.1.3 5.6.1.4 5.6.1.5 5.6.1.6

Superhydrophobic Properties 437 Mg(OH)2 3D Nanoflowers 438 Synthesis 439 Characterization 439 Wetting Behavior Test 440 Fire Test 440 Mechanical Properties Test 440 Thermogravimetric Analysis (TGA) 440 Structure 440 Growth Mechanism 441 Hydrophobic Properties 445 Flammability 446 Synthesis, Characterization, and Application of 3D Sulfide (PbS) Nanostructures 450 Synthesis 451 Characterization 451 Structure 451 Growth Mechanism 458 Synthesis, Characterization, and Application of 3D Selenide Nanostructures 460 Ag2 Se 3D Plate-like Nanostructures 460 Synthesis 461 Characterization 461 Photocatalytic Activity Test 461 Wetting Behavior Test 462 Structure 462 Growth Mechanism 464 Optical Properties 465 Photocatalytic Properties 466 Superhydrophobic Properties 470 PbSe 3D Dendrite-like Nanostructures 472 Synthesis 473 Characterization 473 Water Contact Angle (CA) Measurement 474 Structure 474 Growth Mechanism 476 PL Properties 478 Superhydrophobic Properties 478 Synthesis, Characterization, and Application of 3D Carbonate Nanostructures 481 CaCO3 3D Superstructures 481 Synthesis 482 Surface Modification 482 Characterization 482 Structure 482 Growth Mechanism 486 Wettability 488

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Contents

5.6.2 5.6.2.1 5.6.2.2 5.6.2.3 5.6.2.4 5.6.2.5 5.6.2.6 5.6.3 5.6.3.1 5.6.3.2 5.6.3.3 5.6.3.4 5.6.3.5 5.6.3.6 5.6.3.7 5.6.3.8

BaCO3 3D Superstructures 490 Synthesis 490 Characterization 490 Water Contact Angle (CA) Measurement 490 Structure 491 Growth Mechanism 494 Wettability 494 MgCO3 ⋅3H2 O Viburnum Opulus-Like 3D Superstructures 496 Synthesis 496 Characterization 496 Water Contact Angle (CA) Measurement 497 Water Treatment Experiment 497 Structure 497 Growth Mechanism 502 Superhydrophobic Properties 504 Adsorption Ability 504 References 505

6

Summary Index 525

521

xvii

Preface Nanomaterials have been revolutionizing materials science for several decades, and one of the biggest challenges is how to synthesize and prepare nanoparticles with required dimensionalities which can enable key functionalities. As such, introducing the field in a systematic way of dimensionality and corresponding properties is very necessary, particularly for offering a broad view to materials scientists and chemists in the relevant field. In this book, Prof. Cao shares his 20 years’ experience in the synthesis and applications of inorganic nanostructures in a coherent tone, which makes the book stand out from and complement numerous books about the state of the art in inorganic nanomaterials. The book presents a unified summary of 0D, 1D, 2D, and 3D inorganic nanomaterials to bring together the synthetic methods and physical properties. Specifically, it covers a broad variety of inorganic materials, including metals, oxides, sulfides, carbonates, and even graphene. And, for each family of materials, their versatile functionalities and potential applications are also extensively explored. Given this breadth, it provides readers an introductory overview of several areas, and also offers them some specific examples for deep understanding, beneficial for both young professionals and experienced researchers. I believe that such a book will be an invaluable reference for the nanotechnology community. July 2017

Anthony K. Cheetham FRS University of Cambridge

xix

Acknowledgments The author Huaqiang Cao thanks the Department of Chemistry, Tsinghua University, the Department of Materials Science and Metallurgy, University of Cambridge, for financial support, and all his coworkers, whose names appear in the reference cited in this book, for their contributions. Special thanks are due to Professor A. K. Cheetham FRS, University of Cambridge, for his stimulating discussions on this topic and his encouragement. Financial support from the MoST (2016YFA0200200), the National Program on Key Basic Research Project (973 Program, No. 2013CB933804) and the National Natural Science Foundation of China (No. 21271112, 21231005) are acknowledged. July 2017

Huaqiang Cao FRSC, FIMMM At Tsinghua University & University of Cambridge

1

1 Introduction On 29 December 1959, the great physicist Richard P. Feynman, one of the Laureates for the Nobel Prize in Physics 1965 “for their fundamental work in quantum electrodynamics, with deep-ploughing consequences for the physics of elementary particles”, gave a far-reaching prophetic lecture entitled “There’s Plenty of Room at the Bottom” at the annual meeting of the American Physical Society at the California Institute of Technology (Caltech). He said “What I have demonstrated is that there is room – that you can decrease the size of things in a practical way. I now want to show that there is plenty of room. I will not now discuss how we are going to do it, but only what is possible in principle – in other words, what is possible according to the laws of physics. I am not inventing anti-gravity, which is possible someday only if the laws are not what we think. I am telling you what could be done if the laws are what we think; we are not doing it simply because we haven’t yet gotten around to it.” [1]. Feynman said that “The principles of physics, as far as I can see, do not speak against the possibility of maneuvering things atom by atom”. The past few decades have witnessed many inventions and discoveries in the preparation of nanoscale materials to authenticate his visionary prediction, and we can make nanoscale machine in the manner of arranging atoms one by one, and carry out chemical synthesis by mechanical manipulation. Nanotechnology is the term used to cover the design, preparation, and applications of nanostructured systems. Nanotechnology also includes fundamental the understanding of physical properties and phenomena of nanostructures. The typical dimension for nanostructures spans from subnanometer to several hundred nanometers [2]. One nanomater (nm, one-billionth of meter, 10−9 m), is approximately the length equivalent to 10 hydrogen, or 5 silicon, or 3 1/2 gold atoms aligned in a line. By convention, nanotechnology is taken as the structures at least one dimension in the range 1–100 nm following the definition used by the National Nanotechnology Initiative (NNI) in the US [3]. Nanotechnology has been becoming an important research fields, representing an assemblage of many sciences and technologies at the nanometer scale, which encompasses the synthesis and application of nanostructured systems with sizes ranging from individual atoms or molecules to submicron dimension, as well as the assembling the resulting nanostructures into larger systems [4]. The emergence of nanotechnology as a unique and powerful interdisciplinary Synthesis and Applications of Inorganic Nanostructures, First Edition. Huaqiang Cao. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.

2

1 Introduction

research activity with significant societal impact has affected almost all areas of science and technology. It has resulted in distinguished materials with novel and/or significantly improved physical properties (such as, optical, electrical, magnetic properties, etc.); chemical, mechanical, biological properties compared to those of their bulk analogues. The properties of nanostructured materials are quite different from those at a large scale. Nanotechnology today is a creative fusing of bottom-up chemistry approach and top-down engineering approach. We are currently witnessing an eruption of novel strategies for making and manipulating, visualizing and interrogating nanostructured materials. In 2000, United States federal government issued the NNI which is a program for the science, engineering, and technology research and development for nanoscale projects. Whereafter, the NNI initiated a nanotechnology windstorm around the world [3]. The cumulative NNI investment since fiscal year 2001, including the 2015 request, totals almost $21 billion. Most challenging in nanotechnology will be those areas that relate to nanofabrication, in particular the development of viable fabrication technologies which will lead to cost-effective nanomanufacturing processes [4]. In order to understand nanotechnology, we need to first understand some quantum mechanics. Quantum mechanism assumes importance as material size is diminished. This is especially true in nanoscience.

1.1 Wave-Particle Duality Before the beginning of the twentieth century, there was a disagreement about the true nature of light, with the followers of Isaac Newton supporting a corpuscular theory, whereas the followers of Christiaan Huygens approved light of a wave motion. Thomas Young performed a double-slit experiment in 1801, when he showed that interference patterns could be produced when light was passed through two closely spaced slits [5]. The modern double-slit experiment demonstrates that light and matter can display characteristics of both classically defined a wave and a particle. The possession of both wave and particle properties is known as wave-particle duality, which is at the heart of quantum mechanics. The same is true of atoms, molecules, and subatomic particles such as electrons, photons, and neutrons. The wave-particle duality of matter means that an electron is essential neither a wave nor a particle but its motion can be quantified using the mathematical equations appropriate to waves and particles [6]. This phenomenon is thus far absolutely impossible to explain by using any classical manner. And the wave-particle duality is regarded as the best explanation we have thus far. The double-slit experiment is one of the better ways to observe the quantum behavior of electrons in action. Based on the double-slit experiment, we can say about the electrons (the same being true of photons): they arrive one at a time, like particles, and their probability of arrival is subject to interference, like waves [7]. Detailed discussion of double-slit experiments can be found in the quantum mechanics book by Rae [8].

1.3 Schrödinger Equation

In 1924, Louis de Broglie postulated that matter exhibited a dual nature and proposed that the wavelength of a particular object of mass m is found from: 𝜆=

h h = mv p

(1.1)

where v is the velocity, mv is the momentum p of the object, h = 6.63 × 10−34 J s−1 is the Planck constant, and 𝜆 is called de Broglie wavelength here. The resulting waves are called matter waves. In the case of atomic structure, matter waves for electrons are standing waves that correspond to particular electron orbitals [6]. Electrons occupy regions of space, enclosing an atom’s nucleus, which is called orbitals. These orbitals are organized into levels and sublevels, depending on how much energy the electrons have. The closer the nucleus the electrons occupy, the lower energy levels they have.

1.2 Uncertainty Principle In relation to the double-slit experiment with electrons, the Uncertainty Principle, introduced first in 1927, by Werner Heisenberg, signifies that no device can be built to tell us which slit the electrons go through without also disturbing the electrons and ruining their interference pattern. Thus, it is not possible to know simultaneous both the precise position and the momentum of a microscopic particle, such as an electron or atom. It is the way of nature. The Uncertainty Principle quantifies the uncertainties of position and momentum as follows [6]: h ℏ = (1.2) 4𝜋 2 where Δx and Δpx are the uncertainties associated with these quantities, ℏ = h/2𝜋. The more precisely something’s position is determined, the less precisely its momentum is known. If Δx is very small, Δpx must be large, and vice versa. The x direction is picked, but it can also be applied for the y or z directions. ΔxΔpx ≥

1.3 Schrödinger Equation A particle of matter, such as electron, can be described by a generalized wavefunction 𝜓, which can determine the all of the measurable quantities of a particle, including its energy E and momentum p. The absolute value of the square of the wavefunction, ∣𝜓∣2 , is proportional to the probability that the particle occupies a given space at a given time. The Schrödinger equation applied to any confined particle and for motion in one dimension along the x-axis, shown as follows: d2 𝜓 −8𝜋 2 m = (E − V )𝜓 (1.3) 2 dx h2

3

4

1 Introduction

The total energy of a system is the sum of the potential energy (V ) and kinetic energy (KE), shown as follows: p2 +V (1.4) 2m where the KE is equal to p2 /2m, V is potential energy, m is the particle mass, p is the momentum. E = KE + V =

1.4 Particle in a Potential Box In order to understand nanotechnology, we must understand atoms, electrons, and photons, which are controlled by quantum mechanism [5, 9]. From the view of “the particle in a box” mode (i.e., a one-dimensional motion of a particle, such as an electron, is restricted between two rigid walls separated by a distance L between the opposite walls of the box) (also known as the infinite potential well or the infinite square well) in quantum mechanics [7], in which we can approximate a particle/wave at the bottom of an infinitely deep, square well. It means no particle can get enough energy to get out the square well, and the wavefunction outside the box must therefore be zero. It follows that the wavefunction inside the box must be zero at the points x = 0 and x = L. This is called the boundary condition. Therefore, we can expect that the solution to the time-independent wave equation will be of the form of a travelling wave: 𝜓(x) = A sin kx = A sin

2𝜋x 𝜆

(1.5)

where k = 2𝜋/𝜆. The boundary conditions associated with the walls are satisfied as long as k = n𝜋/L, or the allowed wavelengths of the electron are 𝜆 = 2L/n where n = 1,2,3,… ) ( n𝜋x (1.6) 𝜓(x) = N sin L where, N = 2iA, A is the amplitude for this wave function, L is the width of the well, x is the distance from one wall, and n = 1,2,3, . . . . This equation gives the allowed wave functions for a particle in an infinitely deep potential well. This means that the possible values of k are discrete, or quantized. This in turn has the consequence that also the energy levels have to be discrete. The KE, is equal to p2 /2m, where p = h/𝜆 from the de Broglie relationship and k = 2𝜋/𝜆. We thus can obtain: p2 (h∕𝜆)2 1 h2 h2 h2 k 2 KE = mv2 = = = = (1.7) = ) ( 2 2 2 2m 2m 2m𝜆 8m𝜋 2 2m 2𝜋 k With the substitution k = n𝜋/L this becomes: n2 h2 KE = 8mL2

(1.8)

1.5 Fermi-Dirac Distribution and Fermi Energy

The potential energy, V , is zero within the one-dimensional box, and therefore the total energy, E, equals KE. Thus: n2 h2 (1.9) 8mL2 Here, n = 1,2,3, . . . . The energy of the particle is quantized. This is what we want to expect about quantum mechanics. It should be pointed out that n = 0 is forbidden because this would lead to the wavefunction being zero everywhere. And this status will lead to losing particle. Thus, the particle must have a minimum energy of E1 = h2 /8mL2 when n = 1. This is known as the zero point energy. The next energy level up is n = 2, which gives E2 = 4h2 /8mL2 = 4E1 . Like standing waves, the electrons of an atoms can take on only very specific energies (or wavelengths). Electrons in the highest occupied energy level are called valence electrons. When an electron in an atom drops from one energy state to a lower state, a photon is emitted. Contrarily, a photon will be absorb after the electron jumps from one energy to a higher state. The photon which is regarded as massless chargeless “packlet” of electromagnetic radiation, behaves like a wave and like a particle at the same time. So do electrons. E = V + KE =

1.5 Fermi-Dirac Distribution and Fermi Energy Whether it is metal, semiconductor, or insulator, all materials have free electrons. Insulators have very few electrons, while conductors have many electrons. Semiconductors have electrons falling somewhere in between. These free electrons are not entirely free, however; they are beholden to the same laws as other electrons. The probability that a particular energy level is described by a probability density function f (E). The distribution of energies for T > 0 K is given by the Fermi–Dirac distribution function (or called Fermi function) (Figure 1.1): f (E) =

1 exp[(E − EF )∕kB T] + 1

(1.10)

Here, EF is the Fermi energy, k B is Boltzmann’s constant (1.38 × 10−23 J K−1 ), and T is the absolute temperature of the solid.

f (E)

f (E)

T=0K

0

T >0 K

0 EF

E

EF

E

Figure 1.1 The Fermi–Dirac distribution function for a material at T = 0 K and at non-zero temperature.

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The Fermi level is the highest filled electron level at absolute zero (0 K). The corresponding energy is called Fermi energy EF , that is, is the energy of the outermost occupied energy level at 0 K, and is simply the top-filled level in the band in a metal and is usually of the order of 5 eV for metals [6]. At temperature above absolute zero, some electrons in levels near to EF have sufficient thermal energy to be promoted to empty levels above EF . The Fermi energy is a characteristic of the material and depends upon the concentration of free electrons in the material. In essential, the Fermi energy is the zero-point energy. In the absolute zero temperature (T = 0) limit, f (E) = 1 for all E < EF , and f (E) = 0 for all E > EF . In other words, all electron states below the Fermi energy EF , are filled, while all those above EF , all the levels are empty. At non-zero temperature, f (E) describes the fact that some electrons are thermally excited from states just below EF to states just above EF . The highest filled electron energy range is called the valence band, while the conduction band is the lowest range of vacant electronic states. Separating the valence and conduction bands is a gap in allowed energies called the band gap. The valence band (VB) and conduction band (CB) are the bands closest to the Fermi level and thus determine the electrical conductivity of the solid. Conductors, semiconductor, and insulators are three solid forms. For a conductor, the conduction bands and valence bands are not separated and there is therefore no energy gap. The conduction band is then partially occupied (even at low temperatures). Insulators are materials with exactly enough electron to keep their energy levels fully occupied. Thus, the valence band of an insulator is completely full and stable. If the band gap of a material between the valence band and the conduction band is sufficiently small, is then called semiconductors. However, there is no so called “official” cutoff separating semiconductors from insulators (Figure 1.2). It should point out that EF for both metals and insulators does not correspond to any physical electron state. It is a theoretical concept and just an energy value that lies between the highest filled state and the next available state [6]. Again, because all materials have at least some free electrons, the probability density function applies to conductors, semiconductors, and insulators. In conductors, the Fermi energy is in the middle of the highest occupied band. In semiconductors and insulators, the Fermi energy is the band gap. The concept of the Fermi energy is a crucially important concept for the understanding of the electrical and thermal properties of solids. Detailed discussion can be found in the physics of electrons in solids book by Tanner [10]. The function f (E) gives the fraction of the allowed levels with energy E which are occupied [11]. It should point out that f (E) is merely a mathematical model; Conduction band Conduction band Conduction band Valence band

Valence band

Valence band

Conductor

Semiconductor

Insulator

Figure 1.2 The band gap of a conductor, an insulator, and a semiconductor.

1.6 Density of States

and because it is continuous, it indicates that energies inside the band gaps can be occupied by electrons. In reality, energies inside the band gap are forbidden to electrons, due to the discrete electron energies which are separated by the band gaps.

1.6 Density of States The small size of nanomaterials leads to them unique physical properties. One of the major ways in which nanomaterials differ from corresponding bulk solids in the number of available energy states, due to many of the optical, electronic, and magnetic properties of a nanostructure depend critically on the density of states (DOSs) g(E), represents the number of available states per unit of energy at an energy E [6]. Hence the nanostructured properties of nanomaterials exhibit a strong dependence on dimensionality. For example, in electronic transport the DOS determines the number of states on the number of available states into which they can be scattered. The DOS is not a constant within an energy band. The two-dimensional DOS is a piecewise-continuous approximation to the parabolic three-dimensional DOS in the relationship between DOS and energy, the diverging DOS for one-dimensional quantum wire as the energy approaches that of one of the subband minima, while zero-dimensional quantum dots exhibit “total confinement” discrete DOS function [12]. Further, the distribution of electrons within all the available states varies accompanied with temperature [6]. Detailed knowledge on electronic transport in nanostructures can be found in the quantum mechanics book by Ferry [12]. Optical transitions can occur between the electronic bands if they are allowed by the selection rules. The electronic and vibrational states of nanostructured materials are similar to free molecules and atoms which have discrete energies, thus both the electronic states and the phonon modes have discrete energies, but this is not the case in a solid. Obviously, this continuum of states leads to continuous absorption and emission bands for a solid [13]. The strength of an optical transition is proportional to the joint density states which account for the fact that both the initial and final electron states lie within continuous bands. The energy dependence of the absorption follows the joint DOSs, and therefore exhibits a very different form for nanostructures of different dimensionality [14]. Also the magnetic susceptibility – a dimensionless proportionality constant – the relationship between the magnetization and the applied magnetic field is affected by the DOS. A magnet with a higher DOS at the Fermi level will have more electrons that enter the conduction band. The more electrons in the conduction band, the higher susceptibility for such a magnet, due to the motion of electrons will create magnetic fields [7]. The DOS is a physical property of a material that can be calculated for electrons, photons, or phonons, depending upon the quantum mechanical system. It is commonly symbolized by N and can be given as a function g(E) of either energy E or a function g(k) of the wave vector k. For purely 1D, 2D, and 3D systems, the DOSs are proportional to E(n−2)/2 [6].

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N(E)

E

E Quantum dot (0D)

Quantum wire (1D)

N

N(E)

E Quantum well (2D)

Figure 1.3 DOS plots for 0D (discrete shape), 1D (inverse stepped parabola), 2D (stepped parabola), and 3D (parabolic shape).

N(E)

E Bulk semiconductor (3D)

The DOSs for 1D system: ( ) dN(E) L 2m 1∕2 −1∕2 g(E) = E (1.11) = dE 𝜋 ℏ2 where L is the length of the solid in real space, m is the particular object of mass, N(E) is the total number of states N with an energy less than our selected value of E and is written: N(E), while the DOS g(E) represents the number of available state per unit of energy at an energy E, m is the particle mass, ℏ is Planck’s constant h/2p (1.0545887 × 10−34 J⋅sec, or 6.582173 × 10−16 eV⋅sec). The DOSs for 2D system: dN(E) Am (1.12) = g(E) = dE 𝜋ℏ2 where A = L2 is the area in real space. And the DOS for a 2D solid is a constant. The DOSs for 3D system: ( ) dN(E) V 2m 3∕2 1∕2 g(E) = E (1.13) = dE 2𝜋 2 ℏ2 where V = L3 , is the volume in real space, E is selected value of energy. For 0D system (quantum dots), there is no continuous distribution of states and the DOSs takes the form of a spectrum of discrete energy values, which is similar to that found for individual atoms. The DOSs for zero-dimensional (quantum dots), one-dimensional (quantum wires), and two-dimensional (quantum wells) materials compared to that of a bulk semiconductor material are shown in Figure 1.3. Thus, we can say that bulk materials have enough quantum states, while the nanomateials have fewer quantum states. If we have even fewer quantum states, then we have the structures of the molecules, further on, the atoms [15].

1.7 Quantum Confinement When the volume of a solid is reduced to the small length in nanoscale system, the energy band structure will be directly affected and the energy levels inside

1.7 Quantum Confinement

will become discrete, accordingly, will lead indirectly to changes in the associated atomic structure. The electrons in the reduced-dimensional nanosystem tend to behave more like the description for “particle in a box.” Such kind of effect is known as quantum confinement [16, 17]. That means the electronic states are more like those found in localized molecular bonds rather than those in a macroscopic solid. There are three dimensions to confine, that is, quantum confinement in one dimension-called qauntum wells, quantum confinement in two dimension-called quantum wires, and quantum confinement in three dimension-called quantum dots. It should be confined at least one of these dimensions to less than 100 nm, or even to just a few nanometers, to obtain quantum confinement effect. The small length scales presenting in nanosystem can not only change the system total energy, but also change the system structure. As the system size decreases, the allowed energy bands become substantially smaller than those in an infinite solid. The electrons in this kind of reduced-dimensional system tend to behave more like the “particle in a box” description in which the energy of different states is dependent on the length of the box. Also as the system size decreases, the chemical reactivity will be changed, which will be a function of the structure and occupation of the outermost electronic energy levels. Accordingly, the physical properties, such as optical, electrical, and magnetic properties, which also depend on the arrangement of the outermost electronic energy levels, will be changed. The surface is important regardless how large or how small the material is. The surface of a material depends on its size and geometric shape. Both the surface area to volume ratio (S/V ) and the specific surface area (m2 g−1 ) of a system are inversely proportional to the size of particle and both increase remarkably for particles with the size less than 100 nm in diameter. If an atom is located at a surface then it is clear that the number of nearest-neighbor atoms are reduced, leading to differences in bonding and electronic structure. Obviously, in a nanosystem, a large proportion of the total number of atoms will be on the surface, while a bulk solid material will typically have less than 1% of its atoms on the surface. Thus, more dangling bonds will be present in the nanosystem, giving rise to more reactive active sites, because only the surface of an object is exposed to the reaction and participates in the chemical and physical processes. Therefore, such a reduced-dimensional nanosystem can be expected to have different physical and chemical properties compared with the bulk solid. If one dimension is reduced to the nanoscale range (i.e., one-dimensional confinement, 1D confinement) while the other two dimensions remain large, we generate a structure known as a “quantum well.” A quantum well is a three-dimensional structure in which two dimensions are large, a third is in the nanoscale range. If two dimensions are reduced to the nanoscale range and the other one dimension remains large (i.e., 2D confinement), the resulting structure is called a “quantum wire.” Nanotubes and other nanoscale wires can be quantum wires. When all three dimensions are reduced to the nanoscale range (i.e., 3D confinement) and quantum effects are observed, the resulting structure is termed as “quantum dot.” Because quantum wells and quantum wires each have at least one dimension in which the electrons are free to move,

9

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these structures are said to exhibit “partial confinement.” However, quantum dots exhibit “total confinement.”

1.8 Top-Down and Bottom-Up Approaches to Construct Nanostructures Nanostructures can be fabricated in numerous ways, however, they can be divided into two broad categories, that is, the bottom-up approach and the top-down approach. The top-down approach to fabricate nanostructures is extremely powerful and can generate effectively microscale objects, involving the removal or reformation of atoms to create the desired structures. In the top-down approach, we remove bulk material from one or more of the three dimensions (length, width, height) of a larger solid, or cut bulk material to fabricate the desired nanostructures with the appropriate properties. In the bottom-up approach, we build low-volume structures by utilizing growth and self-assembly to construct nanostructures from atomic and molecular precursors, even nanoparticles themselves used as the building blocks for the generation of complex nanostructures. We can also design properties and functionalities of the nanostructured solid system by adjusting the size of the building blocks and controlling their self-assembly processes. This approach is essentially highly controlled chemical synthesis processes. Although the bottom-up approach is nothing new, it plays an important role in the fabrication and processing of nanostructures. When structures fall into nanoscale, there is little choice for a top-down approach, because all of the tools from top-down approaches we have possessed are too big to deal with such tiny objects [2]. Chemical synthesis may be carried out in either the solid, liquid or gaseous state. More generally, liquid phase chemical synthesis involves the reaction of solution of precursor chemicals in either an aqueous or nor-aqueous solvent. Both of these methods can produce a structure small enough for quantum behavior to manifest. The bottom-up approach to fabricate nanostructures at the atomic and molecular scale is the original vision of Richard Feynman, possibly using self-assembly methods. Self-assembly can provide an effective spontaneous pathway for constructing desired three-dimensional new materials whose structure at all levels of construction, from the nanoscale to the macroscopic form [18]. It is regarded as the nanostructures synthesized by the bottom-up approach usually have less defects, more homogeneous chemical composition, and better short and long range ordering, which is driven mainly by the reduction of Gibbs free energy, so that nanostructures are the products in a state closer to a thermodynamic equilibrium state. By contraries, the top-down approach most likely introduced internal stress, besides surface defects and contaminations [2]. In order to exploit new applications, and understand the novel physical properties and phenomena of nanostructures, we must develop the ability for constructing nanostructures first. Many technologies have been explored to fabricate nanostructures. A major challenge of nanoscience and nanotechnology is to integrate the top-down solid-state physics ways of fabricating structures and the bottom-up

1.10 Zero-Dimensional Nanostructures

molecular-chemical methods of synthesizing structures, and develop various strategies to reliably construct complex systems over all of the length scales ranging from the molecular systems to the macroscopic systems that can interface with people [19]. For nanotechnology, the central problem is not only to obtain new structure and composition of material, but also to arrange and integrate building blocks into new structures with new form and scale, which constructs new materials with new properties, functions and novel applications. It is key to known which size and shape a material must have for it to possess a particular property, function and application, and to design and to construct nanostructures with desired length scale and shape.

1.9 Nanostructured Materials Based on Dimension According to the form of products, we can group the nanostructures as follows: (i) zero-dimensional (0D) nanstructures, such as nanoparticles and quantum dots, synthesized by colloidal processing, and so on, (ii) one-dimensional (1D) nanostructures, such as nanorods, nanofibers, nanowires, nanotubes, and nanoribbons, synthesized by template-based electroplating techniques, chemical vapor deposition (CVD) method, and so on, (iii) two-dimensional (2D) nanostructures, such as graphene or graphene oxide nanosheets, synthesized by the CVD method, or oxidation methods, and so on, (iv) three-dimensional (3D) nanostructures synthesized by self-assembly of nanosized building blocks. It is well known that nanostructured materials have different types ranging from zero-dimensional quantum dots to one-dimensional quantum wires, twodimensional quantum well, three-dimensional complex nanostructures which comprise of low-dimensional building blocks, where at least in one dimension, there is spatial quantum confinement facilitating size-dependent electronic properties [20].

1.10 Zero-Dimensional Nanostructures In the 1980s, two major breakthroughs initiated the boom period of nanotechnology. One landmark event is the discover of carbon fullerene C60 , which is reported by Sir Harold W. Kroto at the University of Sussex, Richard E. Smalley and Robert Floyd Curl, Jr. at the Rice University in 1985, who were together awarded the 1996 Nobel Prize in Chemistry [21]. Another landmark event is the invention of scanning tunneling microscope in 1981 by Gerd Binnig and Heinrich Rohrer at IBM Zurich Research Laboratory, who together won the 1986 Nobel Prize in Physics. Dr. John E. Kelly III, IBM, Senior Vice President, Cognitive Solutions and IBM Research, evaluated this event that, “This invention gave scientists the ability to image, measure and manipulate atoms for the first time, and opened new avenues for information technology that we are still pursuing today.” C60 is an isolated molecule made up of 60 carbon atoms, connected together as 60 apexes and 32 faces, among them 20 hexagonal and 12 pentagonal faces,

11

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1 Introduction

Figure 1.4 C60 structure with different views.

symmetrically arrayed to form a molecular ball, similar to a soccer ball. Every pentagon is surrounded by five hexagons. Each carbon atom is bonded to three other carbon atoms by sp2 hybrid orbital (Figure 1.4). However, because of the curvature of the surface there is about 10% sp3 character [22]. C60 is an extremely stable molecule with the diameter of 7.10 Å (0.710 nm) [21], in the kinetic sense, with a carbon atom placed at each of the 60 vertices of the structure, but it is not a stable molecule in the thermodynamic sense due to extreme bond strain, which is regarded as the first zero-dimensional nanoarchitecture [2]. Since it is very stable and does not require additional atoms for the C60 molecule to satisfy chemical bonding requirements, it is expected to own low surface energy. C60 has some unique properties, such as superconductivity [23], non-linear optical properties [24], reversible redox behavior [25], magnetism [26], and so on. Beside C60 and other fullerene molecules, there are many other zerodimensional nanostructures, such as nanoparticles, nanoclusters, nanocrystals, and quantum dots. 0D nanostructures are the simplest building block that may be used for designing and constructing 1D, 2D, and complex 3D nanostructures, and nanodevices. Nanoparticles, is the term for particles between 1 and 100 nm in size regardless of size and morphology, which generally comprise all 0D nanoscale building blocks. Nanoclusters usually refer to as those 0D nanostructures with at least one dimension between 1 and 10 nm and a narrow size distribution. Nanocrystals are usually referred to those ultrafine particles with at least one dimension below 100 nm, composed of atoms in either a single- or poly-crystalline arrangement. Quantum dots are often used to define those nanoparticle that the characteristic dimension is sufficiently small and quantum effects can be observed. Nanoparticles may or may not exhibit size-related properties which are quite different from those observed in fine particles or bulk materials [27]. Nanoparticles have a wide variety of applications, including as catalysts or catalytic supports [28], and building blocks for electronic nanodevices [29] due to their unique electronic properties, as fluorescent biological labels [30], and biomedical imaging [31] in biology and medicine due to their unique optical properties, as waste water

1.11 One-Dimensional Nanostructures

treatment [32] and medical diagnostics and treatments [33] and drug and gene delivery [34] for magnetic nanoparticles, and so on.

1.11 One-Dimensional Nanostructures Carbon nanotubes were discovered by Sumio Iijima of NEC Laboratory in Japan in 1991 [35]. In 1993, Iijima and Ichihashi at NEC Laboratory, and Donald Bethune of the IBM Almaden Research Center in California, independently discovered single-walled carbon nanotubes (SWCN) in 1993 [36, 37]. The individual single-walled carbon nanotubes have very small diameter, typically ∼1 nm in diameter, and are curled and looped rather than straight (Figure 1.5) [38]. A SWCN is defined by a cylindrical graphene sheet with a diameter of about 0.7–10.0 nm, though most of the observed SWCNs have diameters 200,000 cm2 V−1 s−1 at room temperature) [52], and high thermal conductivity. These unique properties of graphene nanosheets making them a wide range of applications. For instance, graphene find applications as a thin flexible ultra-strong film and extremely good conductor applied in flexible electronics due to the high thermal and electrical conductivities, the transparent, elastic, chemically inert, and stable character for graphene nanosheets [53], graphene

Figure 1.6 Single-layer and di-layer graphene nanostructures with different views.

1.13 Three-Dimensional Nanostructures: Superstructures and Hybrid Structures

transistors for high-frequency electronics [54], future ultrafast operation in optoelectronic technologies due to its tunable optical properties, broadband absorption ranging from UV to THz frequencies, and high electrical mobility [55], graphene-based (chemically doped graphene nanoribbons) transistors used in digital logic gates for [56], channel [57], resistive switch [58], storage layer [59] for digital nonvolatile graphene memories, graphene membranes and cantilevers for Mass sensors [60], graphene-based energy applications including supercapacitors [61], lithium ion batteries [62], Ni–MH batteries [63], fuel cells [64], and solar cells [65], and so on. Two-dimensional nanostructures can be defined as a structure where only one direction is restricted and is comparable to the exciton Bohr radius while the other directions is not restricted, leading to quantum confinement. Thin films are 2D nanostructures. There are many ways to form 2D nanostructures, for instant, evaporation, molecular beam epitaxy, chemical vapor deposition, atomic layer deposition, electrochemical deposition, electroless deposition, spincoating or dip-coating of gel, Langmuire–Blodgett films, and self-assembly monolayers [2, 16].

1.13 Three-Dimensional Nanostructures: Superstructures and Hybrid Structures Hierarchical nanostructures are three-dimensional materials, which gradually grow from one parent structure into a more complex form [15]. The self-assembly of nanosized building blocks can form 3D architectures [16]. And usually self-assembled aggregates exhibit hierarchical structure. Bio-mimetic self-assembly can not only form clearly defined hierarchical structures with greater strength against external stresses such as mechanical, electric, or magnetic force, but also exhibit higher stability against changes in environment conditions such as pH, temperature, and pressure. It is important that many self-assembly aggregates are strong enough to put up with unit operations and even perform mechanical action under the right conditions. For example, lipid bilayers formed by a self-assembly process have mechanical strength comparable to stainless steel of the same thickness, yet they extremely flexible [18]. This makes the self-assembly 3D structures (Figure 1.7) [66, 67] to find many applications. Self-assembly can form various three-dimensional structures. For instant, spherical micelles, cylindrical rod-like micelles, bilayer sheets, and other bicontinuous or tri-continuous structures can be generated from short chain amphiphilic molecules via strong hydrophobic attraction between hydrocarbon molecules [68]. Nanosized hierarchical structures are good candidates for use in medical applications, environmental greening, and renewable sources of energy [15], such as self-assembly delivery vehicles used as deliver materials of interest on the specific target [69], self-assembly nanoscale components into working electronics used in nanoelectronics [70], superhydrophobic surface [71], photocatalytic agents [72], and so on.

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2.5 μm

2.5 μm

(a)

(b)

20 nm

(c)

(d)

Figure 1.7 (a,b) SEM images of 3D WO3 nanostructures synthesized by a self-assembly manner (Yin et al. 2013 [66]. Reproduced with permission of American Chemical Society.); (c,d) SEM and TEM images of peonylike 3D Ni(OH)2 superstructures, respectively. (Cao et al. 2010. [67]. Reproduced with permission from Wiley-VCH Verlag Gmbh & Co. KGaA, Winheim).

An aim of the book is to describe the synthesis methods developed for synthesizing a range of nanoscale building blocks with strictly controlled dimension, size, shape, compositions, and corresponding physical properties, and applications, including optical, electric, magnetic applications, superhydrophobic, optical catalytic, energy-storage (such as lithium ion batteries, Ni–MH batteries, supercapacitors), bioapplications, and so on.

References 1 Feynman, R. (1960) There’s Plenty of Room at the Bottom. Eng. Sci., 23 (5),

22–36. 2 Cao, G. (2004) Nanostructures & Nanomaterials-Synthesis: Properties & Appli-

cations, Imperial College Press. 3 National Research Council (2002) Small Wonders, Endless Frontiers: Review of

the National Nanotechnology Initiative, National Research Council, National Academy Press, Washington, D.C. 4 Bhushan, B. (ed.) (2006) Springer Handbook of Nanotechnology, 2nd edn, Springer-Verlag, Berlin, Heidelberg. 5 Hayward, D.O. (2002) Quantum Mechanics for Chemists, The Royal Society of Chemistry. 6 Fischer-Cripps, A.C. (2008) The Materials Physics Companion, Taylor & Francis, Group.

References

7 Rogers, B., Pennathur, S., and Adams, J. (2008) Nanotechnology: Understand-

ing Small Systems, CRS Press, Taylor & Francis Group. 8 Rae, A.I.M. (1992) Quantum Mechanics, 3rd edn, Institute of Physics, Bristol,

England. 9 Heikkilä, T.T. (2013) The Physics of Nanoelectronics Transport and Fluctuation

Phenomena at Low Temperatures, Oxford University Press, UK. 10 Tanner, B.K. (1995) Introduction to the Physics of Electrons in Solids, Cam-

bridge University Press, UK. 11 Cox, P.A. (2005) The Electronic Structure and Chemistry of Solids, Oxford

University Press. 12 Ferry, D.K. (1997) Transport in Nanostructures, Cambridge University Press. 13 Fox, M. (2001) Optical Properties of Solid, Oxford University Press. 14 Kelsall, R.W., Hamley, I.W., and Geoghegan, M. (eds) (2005) Nanoscale Sci-

ence and Technology, John Wiley & Sons, Ltd.. 15 Hornyak, G.L., Dutta, J., Tibbals, H.F., and Ra, A.K. (eds) (2008) Introduction

to Nanoscience, CRS Press, Taylor & Francis Group. 16 Fahlman, B.D. (2011) Materials Chemistry, 2nd edn, Springer, Netherlands. 17 Yu, P.Y. and Cardona, M. (2001) Fundamentals of Semiconductors: Physics and

18 19 20

21 22 23 24 25 26 27 28 29 30 31 32

Materials Properties, 3rd edn, Springer-Verlag GmbH & Co. KGaA, Berlin and Heidelberg. Lee, Y.S. (2008) Self-assembly and Nanotechnology: A Force Balance Approach, John Wiley & Sons, Inc. Kittel, C. (2005) Introduction to Solid State Physics, 8th edn, John Wiley & Sons, Inc. Murugan, A.V. and Vijayamohanan, K. (2007) Applications of nanostructured hybrid materials for supercapacitors, in Nanomaterials Chemistry: Recent Developments and New Directions (eds C.N.R. Rao, A. Müller, and A.K. Cheetham), Wiley-VCH Verlag GmbH & Co. KGaA. Kroto, H.W., Heath, J.R., O’Brien, S.C., Curl, R.F., and Smalley, R.E. (1985) Nature, 318, 162. Haddon, R.C. (1993) Science, 261, 1545. Suzuki, S., Chida, T., and Nakao, K. (2003) Adv. Quantum Chem., 44, 535. Brusatin, G. and Signorini, R. (2002) J. Mater. Chem., 12, 1964. Echegoyen, L. and Echegoyen, L.E. (1998) Acc. Chem. Res., 31, 593. Arcon, D. and Prassides, K. (2001) Struct. Bonding, 100, 129. Cai, X.C., Anyaogu, K.C., and Neckers, D.C. (2007) J. Am. Chem. Soc., 129, 11324. Turner, M., Golovko, V.B., Vaughan, O.P.H., Abdulkin, P., Berenguer-Murcia, A., Tikhov, M.S., Johnson, B.F.G., and Lambert, R.M. (2008) Nature, 454, 981. Fan, H., Wright, A., Gabaldon, J., Rodriguez, A., Brinker, C.J., and Jiang, Y.-B. (2006) Adv. Funct. Mater., 16, 891. Bruchez, M., Moronne, M., Gin, P., Weiss, S., and Alivisatos, A.P. (1998) Science, 281, 2013. Zabow, G., Dodd, S., Moreland, J., and Koretsky, A. (2008) Nature, 453, 1058. Krieg, E., Weissman, H., Shirman, E., Shimoni, E., and Rybtchinski, B. (2011) Nat. Nanotechnol., 6, 141.

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33 Zhang, Y., Jeon, M., Rich, L.J., Hong, H., Geng, J., Zhang, Y., Shi, S., Barnhart,

34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51

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T.E., Alexandridis, P., Huizinga, J.D., Seshadri, M., Cai, W., Kim, C., and Lovell, J.F. (2014) Nat. Nanotechnol., 9, 631. Davis, M.K., Zuckerman, J.E., Choi, C.H.J., Seligson, D., Tolcher, A., Alabi, C.A., Yen, Y., Heidel, J.D., and Ribas, A. (2010) Nature, 464, 1067. Iijima, S. (1991) Nature, 354, 56. Iijima, S. and Ichihashi, T. (1993) Nature, 363, 603. Bethune, D.S., Kiang, C.H., De Vries, M.S., Gorman, G., Savoy, R., Vasquez, J., and Beyers, R. (1993) Nature, 363, 605. Harris, P.J.F. (1999) Carbon Nanotubes and Related Structures: New Materials for the Twenty-First Century, Cambridge University Press. Saito, R., Dresselhaus, G., and Dresselhaus, M.S. (1998) Physical Properties of Carbon Nanotubes, Imperial College Press. Rodriguez, N.M., Chambers, A., and Baker, R.T.K. (1995) Langmuir, 11, 3862. Dai, H., Rinzler, A.G., Nikolaev, P., Thess, A., Colbert, D.T., and Smalley, R.E. (1996) Chem. Phys. Lett., 260, 471. de Heer, W.A., Châtelain, A., and Ugarte, D. (1995) Science, 270, 1179. Kong, J., Franklin, N.R., Zhou, C., Chapline, M.G., Peng, S., Cho, K., and Dai, H. (2000) Science, 287, 622. Planeix, J.M., Coustel, N., Coq, B., Botrons, B., Kumbhar, P.S., Dutartre, R., Geneste, P., Bernier, P., and Ajayan, P.M. (1994) J. Am. Chem. Soc., 116, 7935. Darkrim, F.L., Malbrunot, P., and Tartaglia, G.P. (2002) Int. J. Hydrogen Energy, 27, 193. Salvador-Morales, C., Flahaut, E., Sim, E., Sloan, J., Green, M.L.H., and Sim, R.B. (2006) Mol. Immunol., 43, 193. Frackowiak, E. and Béguin, F. (2002) Carbon, 40, 1775. Nobel Foundation Announcement in 2010, www.NobelPrize.org. Haering, R.R. (1958) Can. J. Phys., 36, 352. Foa Torres, L.E.F., Roche, S., and Charlier, J.-C. (2014) Introduction to Graphene-based Nanomaterials, Cambridge University Press. Novoselov, K.S., Jiang, Z., Zhang, Y., Morozov, S.V., Stomer, H.L., Zeitler, U., Mann, J.C., Boebinger, G.S., Kim, P., and Geim, A.K. (2007) Science, 315, 1379. Bolotin, K.I., Sikes, K.J., Jiang, Z., Klima, M., Fudenberg, G., Hone, J., Kim, P., and Stormer, H.L. (2008) Solid State Commun., 146, 351. Torrisi, F., Hasan, T., Wu, W., Jung, S., Bonaccorso, F., Paul, P.J., Chu, D.P., and Ferrari, A.C. (2012) ACS Nano, 6, 2992. Liao, L., Lin, Y.C., Bao, M.Q., Cheng, R., Bai, J.W., Liu, Y., Qu, Y.Q., Wang, K.L., Huang, Y., and Duan, X.F. (2010) Nature, 467, 305. Mueller, T., Xia, F., and Avouris, P. (2010) Nat. Photonics, 4, 297. Farmer, D.B., Golizadeh-Mojarad, R., Perebeinos, V., Lin, Y.-M., Tulevski, G.S., Tsang, J.C., and Avouris, P. (2009) Nano Lett., 9, 388. Stützel, E.U., Burghard, M., Kern, K., Traversi, F., Nichele, F., and Sordan, R. (2010) Small, 6, 2822. Wu, C., Li, F., Zhang, Y., and Guo, T. (2012) Appl. Phys. Lett., 100, 042105.

References

59 Hong, A.J., Song, E.B., Yu, H.S., Allen, M.J., Kim, J., Fowler, J.D., Wassei, J.K.,

60 61 62 63 64 65 66 67 68 69 70 71 72

Park, Y., Wang, Y., Zou, J., Kaner, R.B., Weiller, B.H., and Wang, K.L. (2011) ACS Nano, 5, 7812. Kim, S.Y. and Park, H.S. (2010) Nanotechnology, 21, 105710. Cao, H.Q., Wu, X.M., Yin, G., and Warner, J.H. (2012) Inorg. Chem., 51, 2954. Li, B.J., Cao, H., Shao, J., and Qu, M.Z. (2011) Chem. Commun., 47, 10374. Li, B., Cao, H.Q., Shao, J., Zheng, H., Lu, Y.X., Yin, J.F., and Qu, M.Z. (2011) Chem. Commun., 47, 3159. Luo, B., Liu, S., and Zhi, L. (2012) Small, 8, 630. Yan, X., Cui, X., Li, B., and Li, L.-S. (2010) Nano Lett., 10, 1869. Yin, J., Cao, H., Zhang, J., Qu, M., and Zhou, Z. (2013) Cryst. Growth Des., 13, 759. Cao, H., Zheng, H., Liu, K., and Warner, J.H. (2010) ChemPhysChem, 11, 489. Desai, R.C. and Kapral, R. (2009) Dynamics of Self-organized and Self-assembled Structures, Cambridge University Press. Katherine Wang, C.H. (2011) Developing Gene Delivery Vehicles using Self-assembly, Gradworks. Heath, J.R. and Ratner, M.A. (2003) Phys. Today, 56, 43. Wu, S.S., Yin, S.F., Cao, H.Q., Lu, W.X., Yin, J.F., and Li, B.J. (2011) J. Mater. Chem., 21, 8734. Cao, H.Q., Xiao, Y.J., Lu, Y.X., Yin, J.F., Li, B.J., Wu, S.S., and Wu, X.M. (2010) Nano Res., 3, 863.

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures 2.1 General Remarks Zero-dimensional (0D) nanostructrues, including nanoparticles (NPs), or nanocrystals, quantum dots, and so on, are of major interest among nanomaterials, due to their unique electronic, optical, and magnetic properties, which make them broad applications. In view of the physicochemical properties and compositions, they are distinguished between several major cases: • • • • •

Synthesis and bioapplication of metallic Ag NPs Synthesis and optical properties of oxide NPs Synthesis and optical properties of sulfide NPs Synthesis and magnetic properties of oxide nanocubes Synthesis and photocatalytic application of microspheres.

Although there are many other related topics for 0D nanostructures, only the most basic examples are explained here. The synthesis methods cover liquid-phase reduction; hydrothermal synthesis, solvothermal synthesis, and microwave synthesis techniques; and so on. The anticancer activities of Ag NPs; photocatalytic activities of oxide and selenide NPs; magnetic properties of hematite NPs and with different shapes, magnetic properties, and anode behavior for lithium-ion batteries (LIBs) of magnetic oxide nanocubes; and the photoluminescence properties of sulphide NPs are discussed.

2.2 Synthesis, Characterization, and Bioapplication of Metal Ag Nanoparticles Noble metal silver (Ag) and Ag salts have been used as powerful antimicrobial agents for many years [1]. Recently, Ag NPs have attracted a great deal of attention in biomedical applications derived from its surface plasmon resonance effect and antibacterial activity. Interaction of single Ag NPs with light is the most efficient among the same-dimensional particles composed of other organic or inorganic chromophores [2]. Ag NPs have therefore been applied as the optical indicator for single-molecule detection in biological assays [3].

Synthesis and Applications of Inorganic Nanostructures, First Edition. Huaqiang Cao. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Inspired by the studies on biomineralization, biomolecules have been extensively utilized in the synthesis and assembly of nanomaterials [4]. Glutathione (GSH, γ-Glu-Cys-Gly) plays a crucial role in protecting intracellular components against oxidative damage and detoxifying heavy metal ions through the mercapto group in an organism [5]. In particular, GSH has two free −COOH groups and a −NH2 group to provide a hydrophilic interface and a handle for further reactivity with other functional molecules. GSH is thus considered an ideal capping molecule in the synthesis of metal and semiconductor nanocrystals, such as gold [6], silver [7], CdS [8], CdTe [9], and ZnSe [10] NPs, and so forth. Here, we introduce the synthesis of water-soluble and size-tunable Ag NPs using GSH as a capping agent [11]. The as-synthesized Ag NPs are ready to bind with other functional molecules (bovine serum albumin (BSA), used as a model protein here) and exhibit optical properties that are highly sensitive to changes in the particle size and surface modification. Furthermore, the stable and water-soluble Ag NPs, with an average diameter of about 6 nm, have been demonstrated to effectively inhibit the proliferation of human leukemic K562 cells in vitro. This implies that the Ag NPs are extremely promising as cancer therapeutic agents. 2.2.1 Synthesis of GSH-Coated Ag NPs

Ag NPs were synthesized according to a modified method [7, 12]. Briefly, an aqueous solution of GSH (25 × 10−3 M) was added dropwise into an aqueous solution of silver nitrate (3 × 10−3 M) under vigorous stirring. The molar ratio between GSH and silver nitrate (GSH/AgNO3 ) was adjusted from 1 : 1 to 1 : 2 and 1 : 10, respectively. After stirring for 15 min, the sodium borohydride aqueous solution (100 × 10−3 M) was rapidly added at a molar ratio of 8 : 1 between the reducing agent and silver nitrate under a vigorous stirring condition. Immediately, the color of the solution turned from yellow to deep brown. The reaction was carried out in the absence of light and continued for 4 h at room temperature (16 ± 1 ∘ C). The products were precipitated with absolute ethanol at a volume ratio of 4 and isolated by repeated centrifugation at 10 000 rpm for 5 min. 2.2.2 Ag NPs Modification 2.2.2.1 Ag NPs and BSA Binding

In a typical procedure, Ag NP aqueous solution (4 ml) of a designed concentration was added into a 7-ml plastic tube, and BSA aqueous solution (2.5 mg⋅ml−1 , 0.5 ml) and phosphate-buffered saline (PBS, 0.01 M, pH = 7.4, 0.4 ml) were added in turn. After the solution was shaken for 15 min at room temperature (16 ± 1 ∘ C), 100 μl of diluted glutaraldehyde aqueous solution (obtained by diluting 1.5 ml 25% glutaraldehyde solution into 10 ml) was added. The final concentration of Ag NPs was adjusted to 0.4, 0.6, 0.8, and 1.0 mg ml−1 , respectively. After another 5 min of shaking, the mixture was incubated at 37 ∘ C for 1 h. After incubation, the mixture was stored at −20 ∘ C. No subsequent treatment was adopted. 2.2.2.2 SDS-PAGE of Ag NPs and BSA Binding

Sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) was carried out using a Bio-Rad electrophoresis system [13]. A 50-μl sample was mixed

2.2 Synthesis, Characterization, and Bioapplication of Metal Ag Nanoparticles

with 10 μl of the sample buffer (50% glycerol, 10% SDS, 0.1% bromophenol blue, and Tris-HCl pH = 6.8). The sample mixture was denatured at 98 ∘ C for 1.5 min (3 and 6 min denatured times were employed for studying the thermostability of binding products). A 10-μl sample mixture was loaded for the electrophoresis separation. The gel was stained with Coomassie Blue after a suitable electrophoresis front was achieved. 2.2.3 Cell Culture and Treatment

The human leukemic K562 cell line was obtained from the Academy of Military Science of China and cultured in RPMI 1640 medium supplemented with 10% fetal calf serum, 100 U ml−1 penicillin, 100 μg ml−1 streptomycin, and maintained at 37 ∘ C in air with a mixture of 5% CO2 . 2.2.4 MTT (Thiazolyl Blue) Assay

A 200-μl suspension of K562 cells per well was seeded in a 96-well plate at a density of 5 × 104 cells ml−1 [14]. The cells grew for 12 h after seeding and were then treated with Ag NPs or GSH at the designed concentration (5, 10, 15, 20, and 25 μg ml−1 ) for different times (6, 12, and 24 h). The cells were resuspended in a flash culture medium after centrifugation at 1000 rpm for 10 min. Twenty microliters of MTT (5 mg ml−1 ) was added to each well, and incubation was allowed to continue for a further 4 h. Finally, all media were removed by centrifugation and 150 μl DMSO was added to each well and shaken for 10 min. The absorbance was read at a wavelength of 550 nm using a Benchmark Microplate Reader (Bio-Rad Corp., USA). 2.2.5 Fluorescence Observation of K562 Cells Stained by Hoechst 33258

To distinguish living cells from apoptotic and necrotic cells, K562 cells were stained with fluorescent dye [15]. A 2-ml suspension of K562 cells per well was seeded in a 12-well plate at a density of 1 × 105 cells ml−1 . The cells grew for 12 h after seeding, and were then treated with Ag NPs or GSH at the designed concentration (5, 15, and 25 μg ml−1 ) for different times (6, 12, and 24 h). After being washed with ice-cold PBS two times, the cells were fixed with 200 μl of methanol for 10 min at room temperature. Then, the cells were washed with PBS and resuspended in PBS, followed by adding 20 μl of Hoechst 33258 (100 μg ml−1 ) to an 80-μl cell suspension. The cells were stained at 37 ∘ C for 10 min. Finally, the cells were resuspended in 30 μl of PBS after being washed with PBS through centrifugation. A 20-μl cell suspension was spread on the glass scorer and observed, and pictures were taken using a DMIRB Inverted Fluorescence Microscope (Leica Corp., Germany). 2.2.6 Flow Cytometer Measurement

To assay the percentages of apoptotic and necrotic cells, fluorescein isothiocyanate (FITC)-annexin-V- and propidium iodide (PI)-stained cells were analyzed. A 2-ml suspension of K562 cells per well was seeded in a 12-well plate at a density of 8 × 104 cells ml−1 . Cells grew for 12 h after seeding and were then treated with Ag NPs or GSH at the designed concentration (5, 15, and 25 μg⋅ml−1 ) for different times (6, 12, and 24 h). After being washed with ice-cold

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

PBS two times, the cells were resuspended in 200 μl binding buffer (10 mM HEPES/NaOH, pH = 7.4, 150 mM NaCl, 5 mM KCl, 1 mM MgCl2 , 1.8 mM CaCl2 ) and co-incubated with 10 μl FITC-annexin V (25 μg ml−1 ) and 5 μl PI (50 μg⋅ml−1 ) in the absence of light for 15 min at room temperature. Finally, fluorescence intensities of cells stained were analyzed using a FACScalibur Flow cytometer (Becton Dickinson Corp., USA). 2.2.7 Characterization

The phase identification of samples was performed by a Bruker D8 Advance diffractometer (Cu K𝛼 radiation, 𝜆 = 1.5418 Å). Transmission electron microscopy (TEM) images of the samples were obtained with a Hitachi-800 transmission electron microscope operating at 120 kV. The surface structures of the samples were characterized with a Nicolet 560 Fourier transform infrared (FT-IR) spectrophotometer. UV–vis absorption spectra were recorded at room temperature using a UV-2102 PC UV–vis spectrophotometer (UNICO Corp., China). 2.2.8 Structure 2.2.8.1 Microstructure of Ag NPs

Figure 2.1 shows the typical TEM images of three samples synthesized with different reactant molar ratios (GSH/AgNO3 ). The size of the particles could be tuned by adjusting the GSH/AgNO3 molar ratio. When the GSH/AgNO3 molar ratio was changed from 1 : 1 to 1 : 2 and then to 1 : 10, the average particle sizes obtained from the TEM images were about 6, 8, and 11 nm, corresponding to parts a–c, respectively, of Figure 2.1. As shown in parts a–c, minimal aggregation was observed when the GSH concentration was increased in the synthesis process. This could be due to the relatively high adsorption of GSH molecules onto the particle surfaces.

50 nm

(a)

50 nm

(b)

50 nm

(c)

Figure 2.1 TEM images of Ag NPs synthesized at different GSH/AgNO3 molar ratio. (a) GSH/AgNO3 molar ratio at 1 : 1; (b) GSH/AgNO3 molar ratio at 1 : 2; (c) GSH/AgNO3 molar ratio at 1 : 10. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

2.2 Synthesis, Characterization, and Bioapplication of Metal Ag Nanoparticles

Figure 2.2 XRD patterns of Ag NPs synthesized at different GSH/AgNO3 molar ratio. (a) GSH/AgNO3 molar ratio at 1 : 1; (b) GSH/AgNO3 molar ratio at 1 : 2; (c) GSH/AgNO3 molar ratio at 1 : 10. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

(111)

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(222)

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2θ(°)

The crystalline phase of the as-synthesized samples was identified by X-ray diffraction (XRD). The (111), (200), (220), (311), and (222) planes shown in Figure 2.2 can be indexed to the cubic structure of silver (JCPDS 4-783). No peaks related to other crystalline phases were found. However, in part a of Figure 2.2, the (111) peak is significantly broadened along with the (200) peak (not observed). This is ascribed to a decrease in the particle size derived from the change in the GSH/AgNO3 molar ratio. This result is consistent with the TEM observation, as shown in Figure 2.1. Detailed information about the surface of the as-synthesized Ag NPs can be obtained from infrared (IR) spectroscopy (Figure 2.3). A sharp peak at 2524 cm−1 , corresponding to the S–H stretching vibration mode, appears in part d of Figure 2.3, whereas it disappears in parts a and b. This result strongly suggests that the GSH molecules are anchored on the surface of the silver particles through the sulfur atom of the mercapto group [16]. The sharp bands peaked at 3346, 3251, 3128, and 3026 cm−1 shown in part d of Figure 2.3 belong to N–H stretching vibration and are observed as three weaker broadening peaks centered at 3255, 3043, and 2926 cm−1 in part a of Figure 2.3 at 3242, 3050, 2926 cm−1 in part b of Figure 2.3, respectively. This provides strong evidence of hydrogen-bonding interactions between the adjacent GSH molecules [7]. Part c of Figure 2.3 is the FT-IR spectrum of the sample prepared with a GSH/AgNO3 molar ratio of 1 : 10 and displays weaker signatures compared with parts a and b of Figure 2.3. This suggests a poor adsorption of the GSH molecule on the particle surfaces due to the low GSH/AgNO3 molar ratio. 2.2.8.2 Binding of Ag NPs and BSA

The Ag NPs with an average particle size of about 6 nm (synthesized at GSH/AgNO3 molar ratio of 1 : 1) are stable in an aqueous solution for more

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Figure 2.3 FT-IR spectra of GSH and Ag NPs synthesized at different GSH/AgNO3 molar ratio. (a) GSH/AgNO3 molar ratio at 1 : 1; (b) GSH/AgNO3 molar ratio at 1 : 2; (c) GSH/AgNO3 molar ratio at 1 : 10; (d) pure GSH. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

(a)

Transmittance (%)

26

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(c) (d) 3346 3251 3128 3026

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than 6 months when it is stored in the dark at room temperature. Good water solubility is shown by the formation of the transparent aqueous solution when the as-synthesized Ag NPs are dispersed in water (the digital photograph of the transparent Ag NP solution shown in Figure 2.4), which is derived from the hydrophilic biomolecule (glutathione) coating and the nanoscale diameter (6–11 nm). Ag NPs are also ready to bind with the model molecule BSA via the glutaraldehyde linker. Two aldehyde groups of glutaraldehyde react with the amino group of GSH absorbed on the Ag NP surfaces and the lysine moieties on BSA molecules, respectively, to form a covalent linkage [13]. The schematic diagram of the binding procedure between the Ag NPs and BSA molecules is shown as part a of Figure 2.5, and the assembly was assessed using the SDS-PAGE technique. The gel electrophoresis has been widely utilized to characterize the conjugation of NPs and biomolecules [13]. As shown in part b of Figure 2.5, Figure 2.4 Digital photograph of the aqueous solution of Ag NPs with average particle size of about 6 nm (synthesized at GSH/AgNO3 molar ratio of 1 : 1) at a concentration of 1.5 mg ml−1 . (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

2.2 Synthesis, Characterization, and Bioapplication of Metal Ag Nanoparticles

NH2

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Figure 2.5 Schematic diagram, SDS-PAGE assay, and the UV–vis spectra of Ag NPs binding with BSA molecules via glutaraldehyde linker. (a) Schematic diagram of the binding procedure between the Ag NPs and BSA molecules via glutaraldehyde linker; (b) the results of SDS-PAGE assay. Channel 1: Ag NPs (0.6 mg ml−1 ); channel 2: Ag NPs (0.6 mg ml−1 ) + glutaraldehyde; channel 3: Ag NPs (0.6 mg ml−1 ) + BSA; channel 4: Ag NPs (0.4 mg ml−1 ) + glutaraldehyde + BSA; channel 5, 8, 9: Ag NPs (0.6 mg ml−1 ) + glutaraldehyde + BSA, samples were denatured for 1.5, 3, and 6 min at 98 ∘ C prior to electrophoresis, respectively; channel 6: Ag NPs (0.8 mg ml−1 ) + glutaraldehyde + BSA; channel 7: Ag NPs (1.0 mg ml−1 ) + glutaraldehyde + BSA; channel 10: BSA + glutaraldehyde; channel 11: BSA; channel 12: the standard protein ladder; (c) UV–vis spectra of (i) Ag NPs and (ii) the ensembles between Ag NPs and BSA via glutaraldehyde linker, corresponding to channel 1 and channel 5 in Figure 2.3b. Top right inset shows the magnitude of the first derivative of the absorption spectra in Figure 2.3c, (i) Ag NPs and (ii) the ensembles between Ag NPS and BSA via glutaraldehyde linker. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

the electrophoretic results show that Ag NPs (channel 1) traveled faster than the ensembles, even ahead of the electrophoretic front of bromophenol blue. Two independent bands shown in channel 3 representing the Ag NPs and BSA, respectively, suggest that the conjugation did not occur in the absence of the glutaraldehyde linker, whereas either Ag NPs or BSA alone formed the oligomers in the presence of the linker (channel 2 and channel 10, respectively). The dispersed bands shown in channel 10 traveled slower than BSA alone (channel 11) due to the formation of the BSA oligomers. Bands near the edge of the sample loading are ascribed to the oligomers, which can be minimized through optimizing the reaction conditions such as reaction temperature and reagent ratios [13]. In a primary experiment, a different amount of Ag NPs (0.4, 0.6, 0.8, and 1.0 mg ml−1 corresponding to channels 4, 5, 6, and 7, respectively) was employed for the binding procedure with the same amount of BSA and glutaraldehyde linker;

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and the results show that the larger assembly was not formed in the presence of excess Ag NPs, implying that the binding between Ag NPs and proteins is in a stoichiometric manner. That the obviously weakened bands of Ag NPs ahead of the dye front compared with channel 1 and the longer tailed bands of BSA compared with channel 10 strongly suggest the successful conjugation between Ag NPs and BSA via a glutaraldehyde linker. Longer denatured times of 3 and 6 min (channels 8 and 9, respectively) compared with 1.5 min (channel 5) at 98 ∘ C prior to electrophoretic separation were employed to assess the thermostability of the ensembles. Similar dispersed bands show a relative resistance of the ensembles to the high temperature. Therefore, the results shown in part b of Figure 2.5 adequately demonstrate the conjugation of Ag NPs and BSA. It is well known that the plasmon resonance of noble metal NPs is strongly dependent on their size, shape, and surface chemistry [3]. In this study, the size-dependent shift of the surface plasmon resonance positions from about 378 to 395 nm was observed when the particle size was changed from 6 to 11 nm (Figure 2.6), which is probably derived from the quantum size effect. The UV–vis absorption spectra of the Ag NPs and the ensembles of Ag NPs and protein (corresponding to channels 1 and 5 in part b of Figure 2.3) are shown in part c of Figure 2.5. The plasmon band at 378 nm is observed in the absorption spectrum of both the Ag NPs [part c (i) in Figure 2.5] and the ensembles between the Ag NPs and BSA via the glutaraldehyde linker [part c (ii) in Figure 2.5]. The peak at approximately 220 nm is due to the strong absorption from the Ag NPs, cuvettes, and the water. This peak is not real and is due to the absorbance reaching a maximum value in the UV–vis absorption spectrometer. The inset at the top right of part c of Figure 2.5 shows the magnitude of the first derivative of the respective absorption spectra, taken for 230–800 nm. The derivative of the absorption spectrum is more sensitive to fine changes and reveals the presence of a new feature at 283 nm for the ensembles between Ag NPs and BSA. This feature at 283 nm is attributed to the presence of the glutaraldehyde and BSA attached to Ag NPs. Few obvious changes have been observed in the UV–vis spectra of the mixture of Ag NPs and BSA in the absence of a glutaraldehyde

c

3.0

Figure 2.6 UV–vis spectra of Ag NPs dissolved in water at a concentration of 0.1 mg ml−1 . (a) GSH/AgNO3 molar ratio of 1 : 1, the average particle size at about 6 nm; (b) GSH/AgNO3 molar ratio of 1 : 2, the average particle size at about 8 nm; (c) GSH/AgNO3 molar ratio of 1 : 10, the average particle size at about 11 nm. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

a : λmax = 378 nm b : λmax = 389 nm c : λmax = 395 nm

2.5 Absorbance

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2.2 Synthesis, Characterization, and Bioapplication of Metal Ag Nanoparticles

Figure 2.7 UV–vis spectra of Ag NPs and BSA in different conjugating conditions. (a) Ag NPs alone; (b) Ag NPs and glutaraldehyde linker; (c) Ag NPs and BSA in the absence of glutaraldehyde linker; (d) Ag NPs and BSA in the presence of glutaraldehyde linker; (e) BSA and glutaraldehyde linker; (f ) BSA alone. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

(a) Ag NPs (b) Ag NPs + glutaraldehyde (c) Ag NPs + BSA (d) Ag NPs + glutaraldehyde + BSA (e) BSA + glutaraldehyde (f) BSA

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linker [part (c) of Figure 2.7], as well as the mixture of Ag NPs and glutaraldehyde linker [part (b) of Figure 2.7], compared with that of Ag NPs alone [part (a) of Figure 2.7]. The feature absorbance of the protein (BSA) at 280 nm [part (f ) of Figure 2.7] shifted to about 272 nm when BSA was mixed with the glutaraldehyde linker [part (e) of Figure 2.7]. The maximal spectra change is observed in the mixture of Ag NPs and BSA in the presence of a glutaraldehyde linker [part (d) of Figure 2.7], strongly demonstrating the conjugation between Ag NPs and BSA via the glutaraldehyde linker. 2.2.9 Anticancer Activities of Ag NPs

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To assess the anticancer activity of Ag NPs (the average particle size of about 6 nm), the chronic myeloid leukemia cell line (K562 cell) was used as an in vitro cell model. The suppression of K562 cell proliferations by Ag NPs was measured by an MTT (thiazolyl blue) assay, and the results are shown in Figure 2.8. After the K562 cells were incubated with Ag NPs for 6 h, a dose-dependent decrease of cell viability from 94.0% to 81.4% compared with the control was observed while increasing the Ag NP concentration from 5 to 25 μg ml−1 (part a of Figure 2.8).

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Figure 2.8 Effects of Ag NPs and GSH on cell viability of human leukemic K562 cells (n = 5). (a) Cell viability of human leukemic K562 cells treated with different concentrations of Ag NPs and GSH for 6 h; (b) cell viability of human leukemic K562 cells treated with different concentrations of Ag NPs and GSH for 12 h; (c) cell viability of human leukemic K562 cells treated with different concentrations of Ag NPs and GSH for 24 h. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Also, time-dependent decreases of cell viability can be observed in parts b and c of Figure 2.8: changing the Ag NP concentration from 5 to 25 μg⋅ml−1 , the cell viability decreased from 86.9% to 79.8% compared with the control when the incubation time was 12 h (part b of Figure 2.8). The viability further decreased from 79.9% to 66.8% compared with the control when the incubation time was prolonged to 24 h (part c of Figure 2.8). GSH can react with several heavy metals via complexation reaction, which can reduce the metal toxicity to cells [17]. In this study, the suppression of the K562 cell proliferation caused by GSH was significantly lower than that caused by Ag NPs, strongly suggesting that Ag NPs can effectively suppress the K562 cell proliferations. We also determined the apoptosis induced by Ag NPs or GSH using the fluorescent microscope and flow cytometer. Only a few apoptotic cells could be observed under the fluorescent microscope when cells were incubated with Ag NPs or GSH at 25 μg⋅ml−1 concentration for more than 12 h (the fluorescent images shown in Figures 2.9–2.11). These results were further confirmed by flow cytometer analysis. When K562 cells were incubated with GSH or Ag NPs at concentrations of 5, 15, and 25 μg ml−1 for 6 h, the percentages of early apoptotic cells were 1.09, 1.19, and 1.76 for GSH and 1.43, 1.44, and 2.17 for Ag NPs, respectively (Figure 2.12). The percentage of apoptotic cells in the control group was 1.21. When the incubation time was prolonged to 12 or 24 h, a slight increase in the percentages of

(a)

Control

(b)

(c)

(d)

(e)

(f)

(g)

GSH

Ag NPs

Figure 2.9 Fluorescent images of K562 cells treated with Ag NPs or GSH at different concentrations for 6 h and then stained with Hoechst 33258. (a) control; (b)–(d) cells treated with GSH at 5, 15, and 25 μg ml−1 , respectively; (e)–(g) cells treated with Ag NPs at 5, 15, and 25 μg ml−1 , respectively. Few apoptotic cells were observed at 6 h incubation with Ag NPs or GSH. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

2.2 Synthesis, Characterization, and Bioapplication of Metal Ag Nanoparticles

(a)

Control

(b)

(c)

(d)

(e)

(f)

(g)

GSH

Ag NPs

Figure 2.10 Fluorescent images of K562 cells treated with Ag NPs or GSH at different concentrations for 12 h and then stained with Hoechst 33258. (a) control; (b)–(d) cells treated with GSH at 5, 15, and 25 μg ml−1 , respectively; (e)–(g) cells treated with Ag NPs at 5, 15, and 25 μg ml−1 , respectively. Several apoptotic cells were marked with the white arrowheads. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

(a)

Control

(b)

(c)

(d)

(e)

(f)

(g)

GSH

Ag NPs

Figure 2.11 Fluorescent images of K562 cells treated with Ag NPs or GSH at different concentrations for 24 h and then stained with Hoechst 33258. (a) Control; (b)–(d) cells treated with GSH at 5, 15, and 25 μg ml−1 , respectively; (e)–(g) cells treated with Ag NPs at 5, 15, and 25 μg ml−1 , respectively. Several apoptotic cells were marked with the white arrowheads. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

31

32

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Control

(a)

GSH

(b)

(c)

(d)

(e)

(f)

(g)

Ag NPs

Figure 2.12 Flow cytometer analysis of K562 cells treated with Ag NPs or GSH for 6 h and then double labeled with FITC-linked Annexin V/PI. Dual-parameter dot plot of FITC-annexin V fluorescence (x-axis, FL1-H) versus PI fluorescence (y-axis, FL2-H) shows logarithmic intensity. Quadrants: lower left (FITC-annexin V− /PI− ) — viable cells; lower right (FITC-annexin V+ /PI− ) — early apoptotic cells; upper right (FITC-annexin V+ /PI+ )—necrotic or late apoptotic cells; upper left (FITC-annexin V− /PI+ )—damaged cells. (a) control; (b)–d) cells treated with GSH at 5, 15, and 25 μg ml−1 for 6 h, respectively; (e)–(g) cells treated with Ag NPs at 5, 15, and 25 μg ml−1 for 6 h, respectively. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

apoptotic cells was observed in GSH- and Ag-NP-treated groups (Figures 2.13 and 2.14). These results suggest that both Ag NPs and GSH only induced slight apoptosis or necrosis of K562 cells, in agreement with the observation from fluorescence microscopy. It was reported that Ag NPs with an average diameter of 15 and 100 nm exhibited cytotoxicity to the rat-liver- derived cell line (BRL 3A cells) through oxidative stress, induced significant depletion of the GSH level, decreased mitochondrial membrane potential, and increased cellular reactive oxygen species (ROS) level [18]. The authors also found that Ag NPs at about 15 nm in diameter were cytotoxic to the mouse spermatogonial stem cell line (C18-4 cell line), which drastically reduced mitochondrial function and increased membrane leakage [19]. In our investigations, the percentage of apoptotic or necrotic cells caused by Ag NPs is far lower than the decrease of cell viability. This result implies that the suppression of K562 cell proliferations by Ag NPs was not through the apoptosis or necrosis pathway, whereas other pathway(s) related to ROS formation may be involved [20–22]. It is noticeable that gold nanospheres, with a variety of surface modifiers at average diameters of about 4, 12, and 18 nm, respectively, could be taken up into K562 cells in the absence

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

Control

(a)

GSH

(b)

(c)

(d)

(e)

(f)

(g)

Ag NPs

Figure 2.13 Flow cytometer analysis of K562 cells treated with Ag NPs or GSH for 12 h and then double labeled with FITC-linked Annexin V/PI. Dual-parameter dot plot of FITC-annexin V fluorescence (x-axis, FL1-H) versus PI fluorescence (y-axis, FL2-H) shows logarithmic intensity. Quadrants: lower left (FITC-annexin V− /PI− ) — viable cells; lower right (FITC-annexin V+ /PI− ) — early apoptotic cells; upper right (FITC-annexin V+ /PI+ )—necrotic or late apoptotic cells; upper left (FITC-annexin V− /PI+ )—damaged cells. (a) control; (b)–(d) cells treated with GSH at 5, 15, and 25 μg ml−1 for 12 h, respectively; (e)–(g) cells treated with Ag NPs at 5, 15, and 25 μg ml−1 for 12 h, respectively. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

of detectable cytotoxicity [23]. Therefore, the detailed mechanism of the suppression of K562 cell proliferations by Ag NPs needs to be further investigated, including the comparison with known drugs and noncancerous cells.

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles 2.3.1 SnO2 Nanoparticles

Tin oxide (SnO2 ) is a direct wide-band-gap semiconductor (Eg = 3.6 eV) [24, 25] with excellent photoelectronic properties, gas sensitivity, and superior chemical stability; and it has already been used in sensors [25], solar cells [26], and LIBs [27]. Many methods have been reported to synthesize SnO2 nanocrystals. Well-aligned nanobox beams of SnO2 are generated by a simple vapor deposition route in an open atmosphere [28]. Highly crystalline SnO2 nanorods are prepared by a solvothermally heating method [29]. SnO2 nanoparticles are synthesized using a series of dendritic polymers [30]. Springs, rings, and

33

34

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Control

(a)

GSH

(b)

(c)

(d)

(e)

(f)

(g)

Ag NPs

Figure 2.14 Flow cytometer analysis of K562 cells treated with Ag NPs or GSH for 24 h and then double labeled with FITC-linked Annexin V/PI. Dual-parameter dot plot of FITC-annexin V fluorescence (x-axis, FL1-H) versus PI fluorescence (y-axis, FL2-H) shows logarithmic intensity. Quadrants: lower left (FITC-annexin V− /PI− ) — viable cells; lower right (FITC-annexin V+ /PI− ) — early apoptotic cells; upper right (FITC-annexin V+ /PI+ )—necrotic or late apoptotic cells; upper left (FITC-annexin V− /PI+ )—damaged cells. (a) control; (b)–(d) cells treated with GSH at 5, 15, and 25 μg ml−1 for 24 h, respectively; (e)–(g) cells treated with Ag NPs at 5, 15, and 25 μg ml−1 for 24 h, respectively. (Wu et al. 2008 [11]. Reproduced with permission of American Chemical Society.)

spirals of SnO2 nanobelts are prepared via a solid–vapor process [31]. SnO2 zigzag nanobelts are prepared by oxidizing Sn in an ambient atmosphere at temperatures ranging from 820 to 950 ∘ C [32]. Single-crystalline SnO2 nanorods are generated in an alcohol–water mixture at 150 ∘ C [33]. Highly ordered SnO2 nanorod arrays are synthesized by a hydrolysis condensation process [34]. Hollow SnO2 nanostructures are prepared by a template-free hydrothermal route [35]. SnO2 nanowires are prepared by a template route [24]. However, it is still a major challenge to prepare semiconductor nanocrystals with controllable size, shape, and doping. Especially, the size of nanoparticles under 10 nm still remains a big challenge [36]. To date, there are very few researches of the synthesis of SnO2 nanocrystals using a biomolecule-assisted hydrothermal approach. It has aroused our great interest in synthesizing new semiconductor nanocrystals using biomolecules [37]. Here, we introduce the biomolecule-assisted hydrothermal route to generate SnO2 with diameters 99% purity, 10 mmol) was dissolved in 20 ml of deionized water to form solution A. SnCl4 ⋅5H2 O (analytical reagent, AR, 2 mmol) was dissolved in 20 ml of deionized water to form solution B, followed by adding solution A into solution B with stirring for 12 h at room temperature. Then, the mixture was sealed into a 50-ml Teflon-lined autoclave, heated to a certain temperature (ranging from 120 to 240 ∘ C), and maintained at this temperature for a certain time (ranging from 5 to 15 h). After the autoclave was cooled down to room temperature naturally, the products were collected and washed with deionized water and then absolute alcohol, followed by drying at 60 ∘ C for 4 h. 2.3.1.2 Characterization

Samples were characterized using XRD with a Bruker D8 Advance diffractometer using Cu K𝛼 (𝜆 = 1.5418 Å) and operating at 40 kV and 40 mA. TEM images were obtained using a JEOL JEM-2100 transmission electron microscope, operating at an accelerating voltage of 100 kV, and high-resolution transmission electron microscopy (HRTEM, JEOL JEM-2010 F Electron Microscope) operating at 200 kV. Raman spectrum was recorded on a Renishaw RM-1000 with excitation from the 514-nm line of an Ar-ion laser with a power of about 5 mW. UV–visible measurement was carried out in a UV–vis spectrophotometer (UNICO Corp., UV-2102 PC). IR spectra measurements were carried out on a NICOLET 560 FT-IR spectrophotometer. 2.3.1.3 Photocatalytic Activity Test

The photocatalytic activities of the as-synthesized SnO2 nanoparticles were evaluated in terms of the degradation of RhB in an aqueous solution. A 250-W high-pressure mercury lamp (𝜆 > 365 nm, Beijing Huiyixin Electric Forces Technology Development Co., Ltd) was positioned inside a cylindrical vessel and surrounded by a circulating water jacket for cooling. A 50 mg of sample S-3 was suspended in 50 ml of an aqueous solution of 10–5 M RhB. The solution was continuously stirred for about 30 min at room temperature to ensure the establishment of an adsorption–desorption equilibrium among the photocatalyst, RhB, and water before irradiation with artificial solar light from the high-pressure mercury lamp. The distance between the light source and the bottom of the solution was about 10 cm. The concentration of RhB was monitored using a UV–vis spectrometer (UNICO Corp. UV-2102PC). The pH values of RhB solutions were adjusted by adding HCl or NaOH solutions. 2.3.1.4 Structure

A series of as-synthesized products with different concentrations of reactants, SnCl4 ⋅5H2 O/L-lysine molar ratios, reaction temperatures, and reaction times are listed in Table 2.1. The phase structure and purity of the as-fabricated sample were studied by powder XRD and Raman spectrum. The XRD pattern of the sample is shown in Figure 2.15. Five characteristic peaks can be indexed as the (110), (101),

35

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Table 2.1 Experimental parameters of the as-synthesized products. Sample

SnCl4 ⋅5H2 O/L-Lysine (molar ratio)

L-Lysine

(mmol)

Reaction temperature (∘ C)/time (h)

Size/d(110) (nm)a)

S-1

1:5

10

120/10

2.0

S-2

1:5

10

150/10

2.6

S-3

1:5

10

180/10

2.9

S-4

1:5

10

210/10

4.9

S-5

1:5

10

240/10

6.5

S-6

1:5

10

180/5

2.9

S-7

1:5

10

180/15

3.4

S-8

1:5

2.5

180/10

3.2

S-9

1:5

30

180/10

2.8

S-10

1:3

6

180/10

2.6

S-11

1:7

14

180/10

3.7

(301)

(101) (200)

(211)

a) The crystallite size is estimated using the Scherrer equation D = 0.9𝜆/B cos 𝜃. Source: Wu 2009 [38]. Reproduced with permission of American Chemical Society.

(110)

36

k j i h g f e d c b a

10

20

30

40

50

60

70

2θ (°)

Figure 2.15 XRD patterns of as-prepared samples with a reaction ratio of SnCl4 8H2 O/L-lysine of 1 : 5 and Sn4+ concentration of 2 mmol at (a) 120 ∘ C/10 h (denoted as S-1); (b) 150 ∘ C/10 h (denoted as S-2); (c) 180 ∘ C/10 h (denoted as S-3); (d) 210 ∘ C/10 h (denoted as S-4); (e) 240 ∘ C/10 h (denoted as S-5); (f ) 180 ∘ C/5 h (denoted as S-6); (g) 180 ∘ C/15 h (denoted as S-7); (h) a reaction ratio of SnCl4 ⋅8H2 O/L-lysine of 1 : 5 and Sn4+ concentration of 0.5 mmol, at 180 ∘ C/10 h (denoted as S-8); (i) a reaction ratio of SnCl4 8H2 O/L-lysine of 1 : 5 and Sn4+ concentration of 6 mmol, at 180 ∘ C/10 h (denoted as S-9); (j) a reaction ratio of SnCl4 8H2 O/L-lysine of 1 : 3 and Sn4+ concentration of 2 mmol, at 180 ∘ C/10 h (denoted as S-10); (k) a reaction ratio of SnCl4 ⋅8H2 O/L-lysine of 1 : 7 and Sn4+ concentration of 2 mmol, at 180 ∘ C/10 h (denoted as S-11). (Wu et al. 2009 [38]. Reproduced with permission of American Chemical Society.)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

(200), (211), and (301) planes of the rutile-type SnO2 , which is in agreement with reported data (Joint Committee on Powder Diffraction Standards, JCPDS card no. 72-1147). No impurity peaks are found in Figure 2.15, which suggests that the as-synthesized SnO2 nanocrystals have high phase purity. Comparing the samples S-1, S-2, S-3, S-4, and S-5, the enhancing relative intensities as well as the sharpening of the diffraction peaks are observed, accompanied by an increase of the reaction temperature changing from 120, 150, 180, 210, to 240 ∘ C gradually. This indicates that the crystalline grade is enhanced with the reaction temperature rising. The crystallite size is estimated using the Scherrer equation D = 0.9𝜆∕B cos 𝜃

(2.1)

where D, B, and 𝜃 represent the wavelength, the full width at half maximum (FWHM), and the Bragg angle, respectively [39]. Typically, Raman spectrum of S-3 exhibits bands at 473, 632, and 775 cm−1 in the range of 200–1000 cm−1 (Figure 2.16). The Raman peaks appearing at 473 cm−1 can be attributed to Eg mode, 632 cm−1 to A1g mode, and 775 cm−1 to B2g mode of SnO2 , respectively [40]. These modes, being related to the facet surface area of a crystal, arise from nanoscale SnO2 with small grain size, which demonstrates the very small size of the as-synthesized SnO2 nanocrystals [33]. Both XRD and Raman analysis demonstrate that the as-synthesized samples belong to pure rutile-type SnO2 phase. Figure 2.17 shows the low-magnification TEM images of the samples (S-1 to S-11). According to the observation, all these nanoparticles are monodisperse, with sizes smaller than 10 nm. Furthermore, Figure 2.18 shows the high-magnification TEM images of the samples S-1, S-2, S-3, S-4, and S-5 synthesized with different reaction temperatures at 120, 150, 180, 210, and 240 ∘ C, correspondingly. The inset of Figure 2.18 shows the HRTEM image of sample S-3. These results are in agreement with the XRD data, estimated using the Scherrer equation (Table 2.1). Figure 2.19 shows the FT-IR spectrum of the S-3 nanoparticle of SnO2 . The bands at 3400 and 1620 cm−1 can be ascribed to the O–H vibrations of H2 O absorbed in the sample. The absorption peak at 3400 cm−1 is low, suggesting that there are few O–H bonds on the SnO2 nanocrystals. The absorption peaks at 2920, 2860, and 1400 cm−1 are attributed to C–H vibrations. The C–H vibration 16000 Intensity (a.u.)

Figure 2.16 Raman spectrum of as-synthesized SnO2 nanocrystals (S-3). (Wu et al. 2009 [38]. Reproduced with permission of American Chemical Society.)

632

12000

775

8000

4000 200

473

400

600

800

Raman shift (cm–1)

1000

37

38

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

(a)

(b)

(c)

(d)

(e)

(f)

(g)

(h)

(i)

(j)

(k)

Figure 2.17 Low-magnification TEM image of as-synthesized SnO2 nanocrystals: (a) S-1, (b) S-2, (c) S-3, (d) S-4, (e) S-5, (f ) S-6, (g) S-7, (h) S-8, (i) S-9, (j) S-10, (k) S-11. (Wu et al. 2009 [38]. Reproduced with permission of American Chemical Society.) (a)

(b)

(c)

(d)

(f)

(g)

Figure 2.18 High-magnification TEM image of as-synthesized SnO2 nanocrystals: (a) S-1 with magnifying power x200 000, (b) S-2 with magnifying power x200 000, (c) S-3 with magnifying power x200 000, (d) S-3 with magnifying power x400 000, (e) HRTEM image of S-3. (f ) S-4 with magnifying power x200 000, (g) S-5 with magnifying power x200 000. (Wu et al. 2009 [38]. Reproduced with permission of American Chemical Society.)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

100 Transmittance

Figure 2.19 FT-IR spectrum of as-synthesized SnO2 nanocrystals (S-3). (Wu et al. 2009 [38]. Reproduced with permission of American Chemical Society.)

80 60 2400

40

1620

3400

1400 1000

2860 2920

20 660

550

0 4000 3500 3000 2500 2000 1500 1000 500 Wavenumbers (cm–1)

is attributed to the organic trace residuals. This suggests that carbon-related impurities are present in the sample, which may come from L-lysine. The absorption peak at 2400 cm−1 is attributed to the adsorption and interaction of atmospheric carbon dioxide with water [41]. The absorption peak at 1000 cm−1 is attributed to the vibration of different types of surface hydroxyl groups. The absorption peaks at 660 and 550 cm−1 are attributed to the Sn–O–Sn antisymmetric vibrations [42]. SnO2 is an n-type semiconductor with the free exciton Bohr radius of 2.7 nm [43]. The exciton radius can be taken as an index of the extent of confinement experienced by an NP. Two limiting regions of confinement can be identified according to the ratio of the dimension d of the NP to the exciton radius aeff , namely, the weak confinement regime with d > aeff (but not d ≫ aeff ) and the strong confinement regime d < aeff [44]. In our experiments, all the as-synthesized SnO2 nanocrystals are under 10 nm in size, and the sizes of the as-synthesized SnO2 are smaller than or close to the exciton radius aeff of SnO2 (2.7 nm), except sample S-5 (6.5 nm in size). It is known that a perfect crystal can be obtained only at absolute zero, hypothetically; at all real temperatures, crystals are imperfect. Crystals are invariably imperfect because the presence of defects up to a certain concentration leads to a reduction of free energy [45]. The presence of defects in semiconductors, even in small concentrations, often influences the properties of the semiconductors. Reaction and growth in the formation of oxide nanocrystals are more difficult to manipulate, since oxides are generally more stable thermally and chemically than most nonoxide semiconductors and metals [46]. The possible chemical mechanism for the hydrothermal formation of the SnO2 nanocrystals can be expressed as follows: SnCl4 + 4NH3 − (CH2 )4 − CH(NH2 ) − COOH + 4H2 O → Sn(OH)4 ↓ +4NH3 − (CH2 )4 − CH(NH3 + ) − COOH + Cl− Sn(OH)4 → SnO2 + 2H2 O

(2.2) (2.3)

Prior to the hydrothermal process, Sn(OH)4 precipitates are formed via reaction (2.2). Reaction (2.3) represents the hydrothermal formation of SnO2 nanocrystals during the hydrothermal stage. It is considered that traditional

39

40

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

hydrothermal routes are difficult to synthesize SnO2 quantum dots [47]. However, in our biomolecule-assisted hydrothermal route, the functional group of lysteine (Lys, C6 H14 N2 O2 , NH3 –CH2 –CH2 –CH2 –CH2 –CH(NH2 )–COOH) served as a ligand to coordinate with Sn4+ ions (six-coordination) which can terminate the growth of SnO2 nanocrystals in the hydrothermal stage. Therefore, the small-sized SnO2 nanocrystals under 10 nm in size can be successfully obtained without adding other surfactants. There are oxygen vacancies in the SnO2 crystal, and these can induce the formation of new energy levels in the band gap. So the oxygen defects in the SnO2 nanocrystals can be generated in the synthesis by the amino-acid-assisted hydrothermal route. After the heat treatment of Sn(OH)4 , the hydroxyls remaining in the structure will generate molecular water, while releasing the lattice oxygen and producing a neutral oxygen vacancy (V o ) in its position. Also, the high pressure produced in the hydrothermal process favors the removal of hydroxyls and produces oxygen vacancies. The process can be expressed as follows: n SnO2 ↔ SnO2−n + O2 (g) (2.4) 2 or 1 O×O ↔ O2 (g) + Vo (2.5) 2 These neutral vacancies may be singly ionized vacancies as Vo• or Vo•• [26]. Semiconductor photocatalytic activity has attracted great interest due to the potential applications in the degradation of environmental pollutants and organic pollutant transformation. Dye pollutants in effluents from the textile and paper industries are regarded as major environmental pollutants, because of their nonbiodegradability and toxicity [48]. The degradation of RhB, an organic dye, in aqueous suspension is used as a probe reaction to evaluate the catalytic activity of semiconductor photocatalytic performance [49]. However, to the best of our knowledge, there are few reports about the SnO2 used in RhB photodegradation. Li and coworkers report that mesoporous SnO2 /ZnO exhibits a significant enhancement of photocatalytic capability toward degrading RhB compared to undoped mesoporous SnO2 , which presents the degradation of RhB over 31.6% degradation rate within 200 min [50]. The photocatalytic activities of as-synthesized SnO2 nanocrystals with size under 10 nm are evaluated in terms of the degradation of RhB dye in aqueous solution under UV light irradiation. The characteristic absorption of RhB at 𝜆 = 553 nm is selected as monitoring the photocatalytic degradation process. Figure 2.20 shows the degradation of the RhB solution with a 250-W highpressure mercury lamp irradiation (𝜆 > 365 nm) under different pH conditions. After 6 h of adding the as-synthesized SnO2 nanocrystals in the RhB solution, the degradation of RhB reaches 56.4% and 74.0% under the condition of pH = 2 and pH = 6, respectively, while the degradation of RhB reaches 95.5%, close to 100% within 2.5 h of adding the as-synthesized SnO2 in the RhB solution under the condition of pH = 8 (Figure 2.20a). Evidently, the pH value of the RhB solution has a significant influence on the photocatalytic activity. Especially,

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

100 pH = 8

0.6 0.4

Absorbance Absorbance

800

a-6h

a-5h

(a)

a-3h

4

a-4h

2

500 600 700 Wavelength (nm)

a-2h

0

a-0h

0.0 400

40

a-1h

0.2

b-6h

0.4

600 700 Wavelength (nm)

b-5h

pH = 2

0.6

500

b-4h

50

Before adsorption After adsorption 1 h of irradiation 2h 3h 4h 5h 6h

b-3h

0.8

b-2h

0.0 400

60

800

b-1h

0.2

b-0h

Degradation rate (%)

0.4

c-2.5h

0.6

c-2.0h

70

500 600 700 Wavelength (nm)

Before adsorption After adsorption 1 h of irradiation 2h 3h 4h 5h 6 h pH = 6

c-1.0h

0.8

c-1.5h

0.0 400

c-0.5h

c-0h

0.2

80

Before adsorbtion After adsorbtion 0.25 h of irradiation 0.5 h 1.0 h 1.5 h 2.0 h 2.5 h

c-0.25h

Absorbance

0.8

90

800

6

8

10

12

14

pH 1.2 Light on

C/C0

0.8

Dark

1.0

0.6

0.4

pH = 2 pH = 8 0.2

pH = 6 pH = 6 no catalyst

0.0 0 (b)

100

200

300

400

Time (min)

Figure 2.20 (a) The photocatalytic degradation of RhB over different pH conditions of sample S-3. The inset shows the UV–vis spectral changes and the corresponding color changes of the RhB aqueous solutions in the presence of S-3 under UV irradiation (𝜆 > 365 nm). (b) RhB concentration decrease with UV irradiation time at different pH values. (Wu et al. 2009 [38]. Reproduced with permission of American Chemical Society.)

41

1.0

1.0 0.8

0.6 0.4 0.2 0.0 400

(a)

S-1 500 600 700 Wavelength (nm)

0.8

Before irradiation 30 min of irradiation 60 min 90 min 120 min 120 min

0.6 0.4 0.2

0.0 800 400 (b)

500 600 700 Wavelength (nm)

0.4 0.2 0.0 400

800 (c)

S-3 500 600 700 Wavelength (nm)

800

0.8 Before irradiation 30 min of irradiation 60 min 90 min 120 min 150 min

0.6 0.4 0.2

Absorbance

Absorbance

Before adsorbtion After adsorbtion 15 min of irradiation 30 min 60 min 90 min 120 min 150 min

0.6

S-2

0.8

S-4

0.6 0.4 0.2

Before irradiation 30 min of irradiation 60 min 90 min 120 min 150 min

S-5

0.0

0.0 400 (d)

Absorbance

Before irradiation 30 min of irradiation 60 min 90 min 120 min 150 min

Absorbance

Absorbance

0.8

500 600 700 Wavelength (nm)

800

400 (e)

500 600 700 Wavelength (nm)

800

Figure 2.21 The photocatalytic degradation of RhB of sample under UV irradiation (𝜆 > 365 nm) at pH = 8: (a) S-1, (b) S-2, (c) S-3, (d) S-4, (e) S-5. (Wu et al. 2009 [38]. Reproduced with permission of American Chemical Society.)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

the basic solution is favorable for the degradation process of RhB. This result confirms that the as-synthesized SnO2 nanocrystals indeed possess intrinsic photocatalytic activity under UV light irradiation. Under UV light irradiation, the pseudo-first-order rate constants for the photodegradation of RhB are 2.20 × 10−3 , 3.34 × 10−3 , and 2.19 × 10−2 min−1 , at different pH values 2, 6, and 8, correspondingly (Figure 2.20b). The blank experiments show that the degradation of RhB is negligible without UV irradiation or in the absence of SnO2 nanocrystal catalysts. In order to understand the effect of particle size and crystal structure on the photocatalytic activity of as-synthesized SnO2 nanocrystals, we carried out the photocatalytic degradation test of samples S-1, S-2, S-3, S-4, and S-5 (Figure 2.21 and Table 2.2), which presents the degradation rate of 63.9%, 80.2%, 95.5%, 89.1%, and 86.4%, correspondingly. All the samples synthesized in different reaction temperatures present photocatalytic degradation of RhB. Comparing the samples S-1 and S-2 with S-3, we found that the better the crystal structure (evaluated by the diffraction peaks of XRD pattern in Figure 2.15), the larger the degradation rate of RhB. This may be attributed to the higher synthesis temperature favoring better SnO2 crystalline structure. Comparing the samples S-3 and S-4 with S-5, we found that the smaller the particle size, the lower the degradation rate of RhB. This can be attributed to the larger surface area for the corresponding smaller SnO2 particle size. Combining crystalline structure and particle size, we found that S-3 was of best photocatalytic activity. In comparison, we also carried out the study of the degradation of RhB in aqueous solution under UV irradiation at the different pH values but without SnO2 nanocrystals (Figure 2.22), which demonstrates that the self-degradation rate of RhB solution of 99% purity, 4.32 mmol) was dissolved in 20 ml of deionized water with stirring for 10 min to form solution B, and then was added dropwise into solution A, while stirring for 13 h at room temperature. The mixture was sealed into a 50-ml Teflon-line autoclave, heated to a selected temperature (ranging from 170–260 ∘ C) and maintained at this temperature for a selected time (ranging from 10–48 h). After the autoclave was cooled down to room temperature naturally, the products were collected and washed with deionized water and then absolute alcohol. The cycle was repeated three times, followed by drying at 50 ∘ C for 3 h. 2.3.2.2 Characterization

The phase structure of the as-prepared products was characterized with XRD (Bruker D8 Advance) with Cu K𝛼 (𝜆 = 1.5418 Å). The morphology of the

45

46

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

as-prepared products was studied using HRTEM (JEOL JEM-2010 F Electron Microscope, operating at 200 kV). IR spectra measurements were carried out on a NICOLET 560 FT-IR spectrophotometer. UV–visible measurement was carried out in a UV–vis spectrophotometer (Shimadzu, UV-2100S). PL spectra were recorded using a fluorescence spectrophotometer (Perkin Elmer LS55). 2.3.2.3 Photocatalytic Activity Test

The photocatalytic activities of the as-synthesized ZrO2 NPs were evaluated in terms of the degradation of RhB in an aqueous solution. A 250-W high-pressure mercury lamp (𝜆 > 365 nm, Beijing Huiyixin Electric Forces Technology Development Co., Ltd) was positioned inside a cylindrical vessel and surrounded by a circulating water jacket for cooling. A 50 mg of sample Z-10 was suspended in 50 ml of an aqueous solution of 10−5 M RhB. The solution was continuously stirred for about 30 min at room temperature to ensure the establishment of an adsorption–desorption equilibrium among the photocatalyst, RhB, and water before irradiation with UV light from the high-pressure mercury lamp. The distance between the light source and the bottom of the solution was about 10 cm. The concentration of RhB was monitored using a UV–vis spectrometer (UNICO Corp. UV-2102PC). The pH values of RhB solutions were adjusted by adding HCl or NaOH solutions. 2.3.2.4 Structure

A series of as-synthesized products with different concentrations of reactants, ZrOCl2 •8H2 O/L-lysine molar ratios, reaction temperatures, and reaction times are listed in Table 2.3. Figure 2.24 shows the XRD patterns of the as-synthesized samples. The diffraction peaks shown in Figure 2.24 demonstrate that the samples belong to tetragonal (t-) (JCPDS card: 79-1769) and monoclinic (m-) Table 2.3 Experimental parameters of the as-synthesized products. Sample

ZrOCl2 ⋅8H2 O/L-Lysine (molar ratio)

n(L-Lysine)

Reaction temperature (∘ C)/time (h)

Size r(101) (nm)

Z-1

1 : 2.16

4.32

140/10

7

Z-2

1 : 2.16

4.32

170/10

7

Z-3

1 : 2.16

4.32

200/10

7

Z-4

1 : 2.16

4.32

230/10

6

Z-5

1 : 2.16

4.32

260/10

6

Z-6

1 : 2.16

4.32

140/16

6

Z-7

1 : 2.16

4.32

170/16

8

Z-8

1 : 2.16

4.32

200/16

6

Z-9

1 : 2.16

4.32

230/16

7

Z-10

1 : 2.16

4.32

170/24

8

Z-11

1 : 2.16

12.96

170/48

9

Z-12

1 : 2.16

21.60

260/20

8

Source: Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

T(101) M(111) M(111)

T(112)

T(110)

M(110)

T(211)

Intensity

Figure 2.24 XRD patterns of as-prepared samples with a reaction ratio of ZrOCl2 •8H2 O/L-lysine of 1 : 2.16, at (a) 140 ∘ C/10 h (denoted as Z-1), (b) 170 ∘ C/10 h (denoted as Z-2), (c) 200 ∘ C/10 h (denoted as Z-3), (d) 230 ∘ C/10 h (denoted as Z-4), (e) 260 ∘ C/10 h (denoted as Z-5), respectively. M: monoclinic phase of ZrO2 , T: tetragonal phase of ZrO2 . (Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.)

e d c b a 10

20

30

40

50

60

70

2θ (°) T(101) T(112) M(111) T(110)

T(211) d

Intensity

Figure 2.25 XRD patterns of as-prepared samples with a reaction ratio of ZrOCl2 •8H2 O/L-lysine of 1 : 2.16, at (a) 140 ∘ C/16 h (denoted as Z-6), (b) 170 ∘ C/16 h (denoted as Z-7), (c) 200 ∘ C/16 h (denoted as Z-8), (d) 230 ∘ C/16 h (denoted as Z-9), respectively. M, monoclinic phase of ZrO2 , T, tetragonal phase of ZrO2 . (Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.)

c b a

10

20

30

40

50

60

70

2θ (°)

(JCPDS card: 78-0047) phases of ZrO2 . In these mixtures, t-ZrO2 is the dominant phase, while the m-ZrO2 is produced by increasing the temperature from 140 to 260 ∘ C. We also studied the samples synthesized at 16 h with different temperatures from 140 to 230 ∘ C. Figure 2.25 shows the XRD patterns of these samples. All these samples belong to tetragonal and monoclinic phases of ZrO2 . It also demonstrates that t-ZrO2 is the dominant phase, while the m-ZrO2 is produced by increasing the reaction temperature. These data suggest that low temperature favors the generation of tetragonal ZrO2 under the L-lysine-assisted hydrothermal condition. The broad XRD peaks are attributed to the very small particle size, which is also demonstrated by the TEM observation. This research provides an interesting case on size- dependent ZrO2 phase transition from tetragonal to monoclinic nanocrystals at high temperatures. Figure 2.26 shows the TEM images of the samples (Z-1, Z-2, Z-5, Z-10, Z-11, and Z-12). According to the observation, the crystalline sizes of all these nanoparticles are ∼8 nm in diameter. That means the reaction temperature (comparing samples Z-1, Z-2, and Z-5), reaction time (comparing sample Z-2 and Z-10), and reaction concentration (comparing samples Z-10 to Z-11 and Z-5 to Z-12) have little effect on the size of as-prepared ZrO2 NPs. It suggests that lysine can control the growth of ZrO2 after the nucleation process. In order to understand

47

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

(a)

(b)

(c)

(d)

(e)

(f)

Figure 2.26 TEM images of (a) Z-1, (b) Z-2, (c) Z-5, (d) Z-10, (e) Z-11, (f ) Z-12, respectively. (Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.)

M(111) – M(111)

60

Intensity

48

T(112)

40 T(211)

T(110)

M(110) 20

20 nm

0 10

(a)

20

30

40 2θ (°)

50

60

70

(b)

Figure 2.27 (a) XRD pattern and (b) corresponding TEM image of ZrO2 without using L-lysine. (Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.)

the effect of L-lysine, we also carried out the compared experiment, that is, the same synthesis conditions but without any L-lysine. The diffraction peaks shown in Figure 2.27a demonstrate that the samples belong to tetragonal (JCPDS card: 79-1769) and monoclinic (JCPDS card: 78-0047) phases of ZrO2 . However, no dominant phase can be found in the mixture. The corresponding TEM image (Figure 2.27) shows that the product is composed of aggregated nanofibres about 50 nm in length and 5 nm in width. Obviously, the amino-acid L-lysine, with functional groups, –NH2 and –COOH, has a great influence on the size and shape of the final nanocrystals of ZrO2 . A similar phenomenon has been observed in our previous research work [62]. Figure 2.28 shows the FT-IR spectrum of the Z-10 NP of ZrO2 . The bands at ∼510 and 750 cm−1 can be attributed to the Zr–O vibrations [63]. The bands at 3410 and 1620 cm−1 can be ascribed to the O–H vibrations of H2 O absorbed in the samples. The band at 1360 cm−1 can be ascribed to the 𝛿 C–H of the carboxylate group [63]. This result suggests that carbon-related impurities are present in the sample, which may come from L-lysine.

Figure 2.28 The FT-IR spectrum of the as-synthesized Z-10 sample. (Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.)

Transimittance (%)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

4000 3500 3000 2500 2000 1500 1000

500

Wavenumber (cm–1)

2.3.2.5 Optical Properties of ZrO2 Nanoparticles

Absorption spectroscopy allowed us to characterize the quantum confinement effects in the nanocrystals with size 365 nm) under different pH conditions. After 5 h of different pH values (pH = 9, 6, and 2), the degradation of RhB reaches 21.40%, 30.90%, and 63.39%, correspondingly. In comparison, we also carried out the study of the degradation of RhB in aqueous solution but without ZrO2 nanocrystals (Figure 2.31), which demonstrates that self-degradation of RhB solution over a period of 5 h was negligible in the absence of ZrO2 nanocrystals. A similar phenomenon was observed by other groups [51]. These results suggest that the acid condition favors the degradation of RhB. Oxygen vacancy defects on the surface of ZrO2 NPs are generated due to the hydrothermal treatment. The oxygen vacancies can induce the formation of new energy levels in the band gap. The photocatalytic behavior of semiconductors is mainly dependent on the separation of photogenerated electron–hole pairs and the transfer of the

1.0 Before absorption After absorption 60 min 180 min 300 min

Absorbance (a.u.)

0.8

0.9 0.8 Absorbance (a.u.)

C/C0

0.7 0.6

0.6

0.2 0.0 400

500 600 700 Wavelength (nm)

0.4

800

0.2

400

Absorbance (a.u.)

500 600 700 Wavelength (nm)

800

Before absorption After absorption 60 min 180 min 300 min

0.8

0.3

0.4

0.0

0.5 0.4

Before absorption After absorption 60 min 180 min 300 min

0.8

0.6

0.6 0.4 0.2 0.0 400

500

0.2 1

2

600 700 Wavelength (nm)

3

800

4

5

6

7

8

9

10

pH

Figure 2.30 The photocatalytic degradation of RhB over different pH conditions of sample Z-10. The inset shows the UV–vis spectral changes of the RhB aqueous solutions in the presence of Z-10 under visible-light irradiation (𝜆 > 365 nm). (Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.)

51

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Figure 2.31 The self-degradation of RhB in the absence of ZrO2 nanocrystals: the UV–vis spectral changes of the RhB aqueous solutions in the absence of ZrO2 nanocrystals under visible-light irradiation (𝜆 > 365 nm). (Zheng et al. 2009 [61]. Reproduced with permission of American Chemical Society.)

1.0 Absorbance (a.u.)

52

Before absorption After absorption 60 min 180 min 300 min

0.8 0.6 0.4 0.2 0.0 400

500

600

700

800

Wavelength (nm)

separated electrons from the photocatalyst to the organic pollutants through the oxygen vacancy defects on the surface of the photocatalyst [75]. 2.3.3 In2 O3 Hollow Nanocrystals

Hollow nanocrystals [76] are important nanomaterials that exhibit many applications, including their use in catalysis [77], solar cells [78], magnetic resonance imaging (MRI) [79], drug delivery [80], nanoelectronics [81], and nanooptics [82]. In2 O3 is a polymorph, that is, the stable cubic phase (body-centered cubic, bcc-In2 O3 ) and the metastable rhombohedral phase (rhomb-centered hexagonal, denoted as rh-In2 O3 hereafter) [83]. Therein, rh-In2 O3 is regarded as a high-pressure polymorphic substance of cubic In2 O3 , which is a metastable phase at atmospheric pressure [84]. In2 O3 , being an important semiconductor with a wide band gap (2.93 ± 0.15 eV for bcc-In2 O3 and 3.02 ± 0.15 eV for rh-In2 O3 ) [85], has been applied in LIBs [86], solar cells [87], computer touchscreens [88], sensors [89], optoelectronic devices [90], photocatalysis [91], and so on. However, most of the researches of In2 O3 are focused on bcc-In2 O3 [87–93], whereas the metastable rh-In2 O3 has been seldom reported [94]. Li and coworkers reported bcc-In2 O3 hollow microspheres via a surfactant-free vesicle-template-interface route [91]. Gurlo et al. synthesized hollow bcc-In2 O3 microsphere and rh-In2 O3 NPs via a surfactant-free self-assembly route [95]. Here, we introduce the synthesis of pure and hollow single-crystalline rh-In2 O3 nanocrystals under mild synthesis conditions by annealing InOOH solid nanocrystal precursors at 400 ∘ C at atmospheric pressure [96]. 2.3.3.1 Synthesis

In(NO3 )3 •4.5H2 O (analytical reagent, AR, Sinopharm Chemical Reagent Beijing Co., Ltd) and L-proline (C5 H9 NO2 , >99%, Beijing Kebio Biotechnology Co., Ltd) were used without further purification. In a typical synthesis, In(NO3 )3 (1 mmol) and L-proline (8 mmol) were dissolved in 30 ml of a mixed solvent of ethanol-glycerol (v:v = 2 : 1) with stirring until complete dissolution, leading to solution A. NaOH (0.64 g) was dissolved in 10 mL of deionized water to form solution B. Then, solution B was added dropwise into solution A with stirring for 30 min, followed by transferring into a Teflon-lined stainless steel autoclave.

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

After the autoclave was heated at 240 ∘ C for 24 h and cooled down to room temperature naturally, the products were collected and washed with ethanol three times, followed by drying at 80 ∘ C for 3 h. After annealing at 400 ∘ C for 3 h in air under atmospheric pressure, the pure and hollow rh-In2 O3 nanocrystals can be obtained. 2.3.3.2 Characterization

The XRD measurement was carried out on an X-ray diffractometer (Druker D8 Advance) with Cu K𝛼 radiation (𝜆 = 1.54056 Å) in a 2𝜃 range from 10∘ to 70∘ . TEM and HRTEM measurements were carried out on a JEOL JEM-2010 F electron microscope, operating at 120 kV. UV–vis spectrum was monitored using a UV–vis spectrometer (UNICO Corp. UV-2102PC). FT-IR spectra were obtained on a Nicolet 560 FT-IR spectrophotometer. Raman spectra (Renishaw, RM 1000) were measured with excitation from the 514-nm line of Ar ion laser with a power of about 5 mW. Adsorption–desorption isotherms of nitrogen were recorded on BET Micromerities TriStar II 3020 equipment at 77 K. Thermal gravimetric analysis and differential thermal analysis (TGA/DTA) experiments were carried out on a TGA Q5000 V3.5 Build 252 in air atmosphere. 2.3.3.3 Photocatalytic Activity Test

The photocatalytic activity of the as-synthesized single-crystalline hollow rh-In2 O3 nanocrystals was evaluated by photodegradation of RhB and methylene blue (MB) dye aqueous solutions under UV irradiation. A 250-W high-pressure mercury lamp (Beijing Huiyixin Electric Forces Technology Development Co., Ltd), was set inside a cylindrical reactor and surrounded by a circulating water jacket to cool the lamp and minimize IR radiation. A 80 mg of rh-In2 O3 nanocrystals was suspended in 40 ml of aqueous solutions (RhB: 1 × 10−5 mol l−1 , MB: 2 × 10−5 mol l−1 ). The solutions were continuously stirred for about 30 min at room temperature to ensure the estabishment of an adsorption–desorption equilibrium among the photocatalyst, RhB or MB, and water, before irradiation with UV light from the high-pressure mercury lamp. The distance between the light source and the bottom of the solution was about 10 cm. The concentration of RhB and MB was monitored using a UV–vis spectrometer (Shimadzu UV-2100S spectrophotometer). 2.3.3.4 Structure

The typical preparation procedure is shown in Figure 2.32 (see Synthesis section for details). L-Proline-assisted solvothermal route was selected as the first step. A NaOH solution was added dropwise into a solution containing In(NO3 )3 , L-proline, and ethanol–glycerol mixed solvent with stirring for 30 min, followed by solvothermal treatment at 240 ∘ C for 24 h, and cooled down to room temperature naturally, as well as washing and drying at 80 ∘ C for 3 h, in succession. After annealing treatment at 400 ∘ C for 3 h in air under atmospheric pressure, pure and hollow rh-In2 O3 nanocrystals were obtained with a yield of ∼97%. The XRD pattern of the as-synthesized sample is shown in Figure 2.33a. The diffraction peaks are consistent with those of rh-In2 O3 (JCPDS no: 021-0406). No impurity peaks can be detected, suggesting the final product to be pure phase

53

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Step I: Solvothermal process

Step II: Annealing process

Ethanol–glycerol, L–Proline, NaOH

240 °C, 24 h In3+

InOOH Solid nanoparticle

InOOH@In2O3 Core–shell structure

rh-In2O3 Hollow nanoparticle

Kirkendall effect

163

50

60

3200 2400 1600 Wavenumbers (cm–1)

200

800

593

400 600 800 Wavenumbers (cm–1)

200

1000

Absorption Desorption

160

0.6 0.5

120 80

dV/dlog(w)

Absorbed quantity (cm3 g−1)

515

1050

480

1392 1277 1170 1050 1365

nOOH In2O3

70

(b)

2855

2934

3447 3447

Curve ii

4000

(c)

2703

Curve i

2002

40 2θ (°)

1636

30

1636

20

2403

10

502

220 272

JCPDS No. 021-0406

(a)

385

180

Intensity (a.u.)

(024) (116) (122) (018) (214) (300) (208) (10 10) (220)

(006) (113) (202)

(012)

Intensity (a.u.)

(104) (110)

Figure 2.32 The possible growth mechanism of as-synthesized hollow rh-In2 O3 nanocrystals. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

Transmittance (%)

54

0.4 0.3 0.2 0.1 0.0

40 0 0.0

(d)

0

20

40 60 80 Pore width (nm)

100

120

0.2 0.4 0.6 0.8 Relative pressure (P/P0)

1.0

Figure 2.33 Characterization of the as-synthesized In2 O3 : (a) XRD pattern, (b) Raman spectrum, (c) FT-IR spectrum, (d) nitrogen adsorption–desorption isothermals in nitrogen measured at 77 K. Inset of (d) is the corresponding pore-size distribution. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

of rh-In2 O3 . The XRD pattern of the precursor before an annealing treatment process shows that it belongs to orthorhombic InOOH (Figure 2.34, JCPDS no: 017-0549). It is known that the diffraction peak at 2𝜃 = 32.7∘ [(110)] is the characteristic peak of rhombohedral phase of In2 O3 , while the diffraction peak at 2𝜃 = 30.62∘ [(222)] is the characteristic peak of cubic phase of In2 O3 [97]. The average size of In2 O3 is about 15.1 nm based on the Scherrer formula (Table 2.4). The In2 O3 phase was further characterized by its Raman spectrum (Figure 2.33b). According to the crystallography data, the rh-In2 O3 crystal

(111) (210)

(211) (121) (220) (002) (301) (112) (130) (202)

(101) (011)

Intensity (a.u.)

Figure 2.34 XRD patterns of the intermediate product InOOH. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

(110)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

JCPDS No. 017–0549

10

20

30

40

50

60

70

2θ (°)

Table 2.4 The size of the as-synthesized hollow rh-In2 O3 nanocrystals based on the Scherrer formula. Lattice plane

The peak width at half intensity (in radian)

The Bragg angle corresponding to the lattice plane (∘ )

Size (nm)

(104)

9.59 × 10−3

30.52

14.8

(110)

7.15 × 10−3

32.078

19.4

(024)

1.25 × 10

−2

44.711

11.9

(116)

1.02 × 10−2

49.064

14.7

(214)

1.18 × 10−2

55.866

13.1

(300)

9.45 × 10−3

56.702

16.5

Average size (nm)

15.1

Source: Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.

structure belongs to space group R3c, with the point group D6 3d . Based on the group theory analysis, the optical modes have the irreducible representation, as shown here [98] Γopt = 2A1g + 5Eg + 2A1u + 2A2u + 3A2g + 4Eu

(2.17)

The A1g and Eg are Raman active, while the A1u , A2u , A2g , and Eu are IR active or Raman inactive. The Raman modes centered at 163, 180, 220, 272, 385, 502, and 593 cm−1 can be observed. The Raman modes at 163 and 502 cm−1 are attributed to the A1g mode, while the Raman modes at 180, 220, 272, 385, and 593 cm−1 are assigned to the Eg mode, which agree well with the reported values in the literature [99]. Most importantly, the Raman data further demonstrate that the hollow In2 O3 nanocrystals are indeed the single-crystalline structure of rh-In2 O3 [99]. Figure 2.33c shows the FT-IR spectra of the intermediate InOOH (curve i) and the final product, rh-In2 O3 (curve ii). In curve i, the broad peak centered at 3447 cm−1 , accompanied by a weak peak at 1636 cm−1 , is attributed to the O–H stretching vibration (𝜈 O–H ) from residual water in KBr discs [100], which also appears at curve ii, that is, the FT-IR spectrum of In2 O3 . The peaks at

55

56

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

2703 and 2002 cm−1 are typical O–H stretching vibrations (𝜈 O–H ) of InOOH. However, these peaks disappear in curve ii, suggesting that the transformation from InOOH to In2 O3 is complete [101]. The peaks at 1050, 1170, 1277, and 1392 cm−1 are attributed to typical O–H bending vibration, or deformation vibration (𝛿 O–H ) [98, 100]. The peak at 480 cm−1 is attributed to the In–O vibration [102]. The peak at 2403 cm−1 can be attributed to the absorption peak of adsorbed CO2 [103], coming from the heat decomposition of organic solvents and L-proline. However, in curve ii, the peak at 3447 cm−1 , accompanied by a weak peak at 1636 cm−1 , is due to the O–H stretching vibration (𝜈 O–H ) from residual water in KBr discs for FT-IR measurement [100]. The peaks at 1050 and 1365 cm−1 are attributed to typical O–H bending vibration (𝛿 O–H ) [98]. The peak at 515 cm−1 is attributed to In–O vibration [101]. The O–H absorption peaks at 2703 and 2002 cm−1 of InOOH after annealing treatment disappear in In2 O3 . This phenomenon further demonstrates that the transformation of InOOH to In2 O3 is complete. The peaks at 2855 and 2934 cm−1 may be attributed to 𝜈 s (CH2 ) and 𝜈 as (CH2 ), respectively [100], coming from the heat decomposition of organic solvents and L-proline. N2 adsorption−desorption analysis of the as-synthesized In2 O3 revealed a typical type-IV isotherm with an evident hysteresis loop in the range of 0.87 < P/P0 < 1.0 (Figure 2.33d). The characteristic hysteresis loop of the type-IV isotherm is indicative of small mesopores (i.e., 2−50 nm in pore sizes) [104]. Brunauer–Emmett–Teller (BET) measurement shows that the as-synthesized In2 O3 nanocrystals have a specific surface area of about 30.73 m2 g−1 . Pore size calculated from the adsorption branch of the N2 isotherm is ∼31.2 nm, agreeing with the HRTEM observation with a little error. The morphology and microstructure of the as-synthesized In2 O3 sample was further examined by TEM and HRTEM observations. Figure 2.35a–c shows

(c)

(a)

(b)

50 nm

20 nm (116)

(d)

(e)

(006)

5 nm

d = 0.287 nm (104)

(110) 5 1/nm

Figure 2.35 (a) and (b) TEM images, (c) and d) HRTEM images, (e) SAED pattern of rh-In2 O3 . (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

TEM and HRTEM images of rh-In2 O3 hollow nanocrystals; they have pale regions in the center in contrast to dark edges, indicating they are hollow spheres with 10–30 nm in overall diameter and 10 nm in inner diameter of holes (Figure 2.35a,b). The high-quality single-crystalline nature of the as-synthesized hollow rh-In2 O3 spheres is also confirmed by the HRTEM analysis and selected area of electron diffraction (SAED) (Figure 2.35c–e). The HRTEM images present well-defined lattice fringes with an interplanar distance of 0.287 nm, corresponding to the {104} interplanar distance of rh-In2 O3 , which demonstrates the hollow feature of the spheres. To study the void evolution process from orthorhombic InOOH solid nanocrystals to rh-In2 O3 hollow nanocrystals during the annealing process, we took the TGA/DTA experiment for the as-synthesized intermediate InOOH in air atmosphere, as shown in Figure 2.36. The TGA curve can be mainly divided into two weight-loss steps. The first step had taken place between ∼25 ∘ C and ∼240 ∘ C, which can be attributed to the great deal of physical water evaporation from the surface of InOOH. The weight loss of this step was ∼2%, and at the same time the DTA curve shows a negative temperature differential, suggesting an endothermic reaction. The second step of the TGA curve occurred between ∼240 ∘ C and ∼800 ∘ C, suggesting an endothermic decomposition reaction process, 2InOOH → rh-In2 O3 + H2 O↑. The weight loss of this step was ∼11.2%, consistent with the theoretical analysis (12.2%) within the error. Based on the TGA curve observation, we found that the metastable rh-In2 O3 was close to completely changing from InOOH after annealing the InOOH at 400 ∘ C under ambient pressure. The structure evolution process from orthorhombic InOOH to rh-In2 O3 nanoctystals can be further detected by XRD (Figure 2.37). It demonstrates that the phase transformation starts after annealing for 5 min, while the transformation is finished at 10 min. The strong and sharp diffraction peaks of InOOH (curve a in Figure 2.37) and rh-In2 O3 (curve f in Figure 2.37) indicate that both InOOH and rh-In2 O3 are well crystallized, which can be further demonstrated by the corresponding HRTEM observation (Figure 2.38).

0.00

2%

DTA –0.02

98

–0.04 TGA

96 94

DTA curve TGA curve

11.2%

–0.06 –0.08 –0.10

92

–0.12 –0.14

90 0

200

400

600

Temperature (°C)

800

Deriv. weight (%/°C)

100 Weight loss (%)

Figure 2.36 TGA and DTA curves of the InOOH powder tested in air at a temperature raising rate of 10 ∘ C min−1 . (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

57

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

∗

InOOH JCPDS No.17–0549

∗ ∗

In2O3

(f) 180 min∗ Intensity (a.u.)

58

JCPDS No.22–0336

∗

∗

∗

∗

∗

∗

∗

∗

∗

∗

∗

∗

∗

∗

∗

(e) 10 min ∗

∗

(d) 8 min ∗

∗∗

(c) 5 min

∗∗

∗

(b) 2 min

Figure 2.37 XRD patterns of the structure evolution by annealing InOOH nanocrystals at 400 ∘ C under ambient pressure with different annealing times, (a) 0 min, (b) 2 min, (c) 5 min, (d) 8 min, (e) 10 min, and (f ) 180 min. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

(a) 0 min

10

20

30

40

50

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2θ (°)

2.3.3.5 Growth Mechanism of the rh-In2 O3 Hollow Nanocrystals

The possible formation mechanism of rh-In2 O3 hollow nanocrystals is divided into two processes, that is, the formation of InOOH solid nanocrystals generated in the solvothermal reaction (Figure 2.38a) and the complete formation of rh-In2 O3 hollow nanocrystals after an annealing process (Figure 2.38c–e). In the first stage, the InOOH solid nanocrystals were formed by a hydrolysis reaction of In3+ under a solvothermal condition in the temperature range of 180–240 ∘ C. We found the glycerol (boiling point: 290 ∘ C)–ethanol mixed solvent was responsible for the formation of InOOH solid spheres, while cubic In(OH)3 was generated using other solvents, such as glycol (boiling point: 197.3 ∘ C)–ethanol mixed solvent, PEG (polyethylene glycol, M = 400) (boiling point: 250 ∘ C)–ethanol mixed solvent, and ethanol (boiling point: 78.3 ∘ C), under the same solvothermal condition (Eqs. (2.18) or (2.19)) (Figures 2.39 and 2.40). The detailed chemical reactions are suggested as follows: In3+ + 3OH− (in glycerol − ethanol solvent) → InOOH + 2H2 O

(2.18)

In3+ + 3OH− (in other solvents) → In(OH)3

(2.19)

or

Bcc-In2 O3 can be obtained after annealing the intermediate precursor In(OH)3 (2In(OH)3 → bcc-In2 O3 + 3H2 O), while rh-In2 O3 can be obtained after annealing InOOH (2InOOH → rh-In2 O3 + H2 O↑) (Figures 2.40 and 2.41). Obviously, the glycerol–ethanol mixed solvent plays a key role in the formation of the InOOH, and thus in the formation of rh-In2 O3 . The boiling point of glycerol is the highest among these solvents. Under the solvothermal conditions, glycerol will keep in liquid state, while others are in gas state. The liquid glycerol may form hydrogen bonds with InOOH, which may favor the stabilization of InOOH. The detailed process needs to be further investigated. In the second stage, the solid InOOH was transferred into hollow rh-In2 O3 . The Kirkendall effect leads to Kirkendall porosity through the supersaturation of vacancies into hollow pores [105]. The decomposition of surface InOOH

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

(a–i)

(a–ii)

50 nm

5 nm

(b–i)

(b–ii)

50 nm

5 nm

(c–i)

(c–ii)

50 nm

10 nm

(d–i)

(d–ii)

50 nm

5 nm

(e–i)

(e–ii)

50 nm

5 nm

Figure 2.38 (i) TEM and (ii) HRTEM images of the structure evolution by annealing InOOH nanocrystals at 400 ∘ C under ambient pressure with different annealing times, (a) 0 min, (b) 2 min, (c) 5 min, (d) 8 min, (e) 10 min, and (f ) 180 min. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

59

Intensity (a.u.)

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

InOOH (JCPDS No. 017–0549) 20

30

40 2θ (°)

50

60

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(d) InOOH

Intensity (a.u.)

10

InOOH (JCPDS No. 017–0549) (c) In(OH)3 (b) In(OH)3 (a) In(OH)3 In(OH)3(JCPDS No. 73–1810) 10

20

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50

60

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2θ (°)

Figure 2.39 XRD patterns of the intermediate samples synthesized in the solvothermal reaction with different solvents (a) ethanol–glycol, (b) ethanol, (c) ethanol–PEG, and (d) ethanol–glycerol. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.) Figure 2.40 XRD patterns of the final samples synthesized by annealing the precursors which were synthesized in the solvothermal reaction with different solvents composed of (a) ethanol–glycol, (b) ethanol, (c) ethanol–PEG, and (d) ethanol–glycerol. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

(d) rh-In2O3 rh-In2O3(JCPDS No. 022–0336)

Intensity (a.u.)

60

(c) bcc-In2O3 (b) bcc-In2O3 (a) bcc-In2O3 bcc-In2O3 (JCPDS No. 006–0416)

10

20

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50

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2θ (°)

occurred, and thus generated an In2 O3 layer coated on the InOOH core after an annealing process in 5 min. This process has been demonstrated by the XRD data (Figure. 2.37). Here, the anions exchange between OH− of InOOH with O2− of In2 O3 in a nanoscale InOOH@In2 O3 structure system, leading to hollow structures rather than solid nanoparticles. It is worth pointing out that the final product is a single-crystalline hollow structure of rh-In2 O3 . It is known that single-crystalline hollow structures are

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

(a)

(b)

(c)

(d)

Figure 2.41 TEM images of the samples synthesized with different solvents: (a) ethanol–glycol, (b) ethanol, (c) ethanol–PEG, and (d) ethanol–glycerol. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

rarely obtained [106]. Usually, poorly bonded atoms diffuse more rapidly than those that are similar in size and valence [107]. The In–OH bond in InOOH is weaker than the In–O bond in In2 O3 , based on the thermal stability of InOOH and In2 O3 [108]. So the diffusion rate of OH− (radii of OH− = 137 pm) is larger than that of O2− (radii of O2− = 140 pm) [109]. The anion in oxides usually has the smallest diffusion coefficient because it is usually larger than the cations [107]. Diffusion is a basic process involved in crystals. An ion can diffuse by moving into an adjacent vacancy of the same kind [110]. Inward ion diffusion is limited; but core species diffusion outward is forceful, which is favorable for the formation of a void space inside the nanocrystals. That means the faster diffusion of outward OH− than incoming O2− leads to the formation of a hollow structure, and H2 O vapor is released from the nanocrystals. Once the In2 O3 shell is generated after annealing InOOH, outward OH− diffusion will spontaneously proceed without the help of further anion exchange for releasing the interface energy in the InOOH@In2 O3 structure, and fully converting into In2 O3 hollow nanocrystals. To the best of our knowledge, this is the second report of Kirkendall effect in anion exchange [111]. HRTEM images demonstrated the complete conversion from the initial solid InOOH nanoparticles into In2 O3 hollow nanocrystals (Figure 2.38ii). 2.3.3.6 Photocatalytic Activity of the rh-In2 O3 Hollow Nanocrystals

The photocatalytic activities of the as-synthesized single-crystalline hollow rh-In2 O3 nanocrystals were evaluated using degradation of RhB (1 × 10−5 mol l−1 ) and MB (2 × 10−5 mol l−1 ) dyes in aqueous solutions under UV light irradiation at room temperature, respectively. The characteristic absorption peak of RhB at 𝜆 = 553 nm was selected as monitoring the photocatalytic degradation process of RhB. In the absence of rh-In2 O3 nanocrystals under UV light, the photodegradation of RhB was negligible (Figure 2.42a). The photodegradation in the presence of rh-In2 O3 nanocrystals in darkness was only about 13.7% after 8 h (Figure 2.42b). In comparison, the photolytic degradation of RhB in aqueous solution under UV irradiation in the presence of commercial In2 O3 (Tianjin Guangfu Research

61

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures 1.0 0 h of irradiation 2h 4h 6h 8h

0.8 0.6 0.4

RhB 1 × 10–5 M No catalyst UV light

0.2

Absorption (a.u.)

1.0 Absorption (a.u.)

62

0.0 400

(a)

0h 0.5 h 1h 2h 4h 6h 8h

0.8 0.6 0.4

RhB 2 × 10–5 M 2 mg ml−1 darkness

0.2 0.0

500

600

700

Wavelength (nm)

800

400

(b)

500

600

700

800

Wavelength (nm)

Figure 2.42 Temporal UV–vis absorption spectral changes of RhB in water (a) without catalyst and (b) in the presence of s-synthesized single-crystalline hollow rh-In2 O3 nanocrystals (2 mg ml−1 ) in darkness. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

Institute) was also carried out under the same condition. Figure 2.43a,b shows the photodegradation of RhB in the presence of rh-In2 O3 nanocrystals (content: 80 mg of rh-In2 O3 nanocrystals with the content of 2 mg ml−1 ) and commercial In2 O3 , respectively. The absorption maxima of the treated solution in the presence of rh-In2 O3 nanocrystals at various times exhibited hypsochromic shifts. After 4 h irradiation under UV light, the photodegradation of RhB dye solution with rh-In2 O3 nanocrystals was ∼92%, close to 100%, accompanied by the color change from violet red to colorless. However, if the catalyst content is decreased from 2 mg ml−1 to 1.5 or 1 mg ml−1 , the degradation times increase from 4 to 5 or 6 h with the degradation ratio reaching ∼92%, respectively (Figure 2.44a,b). It exhibits an inverse dependent relationship between catalyst content and degradation time (Figure 2.43c). Nevertheless, the photodegradation in the presence of commercial In2 O3 was only 48.2% after 8 h (Figure 2.43b). This is attributed to the large surface-to-volume ratio and the high-quality single-crystalline nature of rh-In2 O3 nanocrystals, which is favorable to increasing the photocatalytic reaction sites [112]. These results suggest that the as-synthesized rh-In2 O3 nanocrystals indeed possess intrinsic photocatalytic activity under UV light irradiation. Figure 2.43d shows the photodegradation efficiencies of RhB under different conditions. The linear relationship between ln c0 /c and t is shown in Figure 2.43e, confirming that the photodegradation reaction is indeed pseudo-first-order, which can be fitted by the equation: c ln 0 = kt (2.20) c where c0 and c indicate the initial RhB concentration and that after irradiation time t, and k is the reaction rate constant, respectively [113]. The pseudo-first-order rate constants for the degradation of RhB are 0.6020 h−1 and 0.0838 h−1 in the presence of rh-In2 O3 nanocrystals and commercial In2 O3 , respectively, while the pseudo-first-order rate constants for the degradation of RhB at the presence of rh-In2 O3 nanocrystals in darkness or in the absence of catalyst are only 0.0185 h−1 , or 0.0068 h−1 , respectively (Figure 2.45).

RhB 1 × 10–5 M 2 mg ml–1 UV light 0 h of irradiation

0.6

0.5 h 1h 2h 3h 4h

0.4 0.2

0.8

0 h of irradiation 2h 4h 6h 8h

0.6 0.4 0.2

0.0

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500

(a)

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5

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(b)

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6 Degradation time (h)

RhB 1 × 10–5 M 2 mg ml–1 UV light

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2.0

0.6 RhB self-degration rh-In2O3 nanocrystals in darkness Commercial In2O3 rh-In2O3 nanocrystals under UV irradiation

0.4 0.2 4

800

Wavelength (nm)

ln (C0/C)

Absorption (au.)

0.8

Absorption (a.u.)

1.0

y = 0.12351 + 0.60202x R2 = 0.98632 rh-In2O3 nanocrystals under UV irradiation Commercial In2O3 rh-In2O3 nanocrystals in darkness RhB self-degration

1.5 1.0

y = –0.01573 + 0.0838x R2 = 0.9956

0.5 y = 0.00834 + 0.0185x R2 = 0.96641

0.0

y = 0.00305 + 0.0685x R2 = 0.97069

0.0 1.0

(c)

1.5

2.0 −1

Catalyst content (mg ml )

0

(d)

2

4 Times (h)

6

0

8

(e)

2

4

6

8

Times (h)

Figure 2.43 (a) Temporal UV–vis absorption spectral changes and corresponding color changes (inset) of RhB in water in the presence of s-synthesized single-crystalline hollow rh-In2 O3 nanocrystals under UV irradiation. (b) Temporal UV–vis absorption spectral changes of RhB in water in the presence of commercial In2 O3 . (c) The relationship between rh-In2 O3 nanocrystals catalyst content and degradation time. (d) Photodegradation curves of RhB as a function of irradiation time. (e) The corresponding selected fitting results using pseudo-first-order reaction kinetics. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

0.6 0.4 0.2

–5

RhB 1 × 10 M –1 1.5 mg ml UV light

0.0 400

500

600

700

Absorption (a.u.)

Absorption (a.u.)

0.5 h 1h 2h 3h 4h 5h

0.8

(a)

1.0

0 h of irradiation

1.0

0.6 0.4 0.2

–1

1 mg ml

0.0

800

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0 h of irradiation 0.5 h 1h 2h 3h 4h 5h 6 h RhB 1 × 10–5 M

0.8

400 (b)

500

600

UV light

700

800

Wavelength (nm)

Figure 2.44 Temporal UV–vis absorption spectral changes and corresponding color changes (inset) of RhB in water in the presence of s-synthesized single-crystalline hollow rh-In2 O3 nanocrystals under UV irradiation with different catalyst content: (a) 1.5 mg ml−1 and (b) 1 mg ml−1 . (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

0.6 0.4 0.2

0h 0.5 h 1h 2h 4h 6h 8h

MB 2 × 10–5 M No catalyst UV light

Absorption (a.u.)

0.8 Absorption (a.u.)

64

0.0 400 (a)

0.8 0.6 0.4 0.2

0h 0.5 h 1h 2h 4h 6h 8h

MB 2 × 10–5 M –1 2 mg ml darkness

0.0 500

600

700

Wavelength (nm)

800

400 (b)

500

600

700

800

Wavelength (nm)

Figure 2.45 Temporal UV–vis absorption spectral changes of MB in water (a) without catalyst and (b) in the presence of as-synthesized single-crystalline hollow rh-In2 O3 nanocrystals (2 mg ml−1 ) in darkness. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

To further investigate the photocatalytic activity of rh-In2 O3 nanocrystals, we studied the photodegradation of MB molecules in aqueous solution under UV light irradiation at room temperature. The characteristic absorption peak of MB at 𝜆 = 664 nm was selected as monitoring the photocatalytic degradation process of MB. Figure 2.46a shows the photodegradation of MB in the presence of rh-In2 O3 nanocrystals (content: 80 mg of rh-In2 O3 nanocrystals with the content of 2 mg ml−1 ), the photodegradation of MB dye solution was about 92% after 3 h of irradiation under UV light; however, after 8 h of irradiation, the photodegradation of MB was only 42% in the presence of commercial In2 O3 with other parameters unchanged (Figure 2.46b). However, if the catalyst content is decreased from 2 mg ml−1 to 1.5 or 1 mg ml−1 , the degradation times increase from 3 to 6 or 10 h, with the degradation ratio reaching ∼92%, respectively (Figure 2.47a,b). It also exhibits an inverse-dependent relationship between catalyst content and degradation time (Figure 2.46c). The photodegradation efficiencies of MB under different conditions are shown in Figure 2.46d; there was only 19.2% degradation of MB dye solution after 8 h of irradiation under UV light

0.8 1.0

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0 h of irradiation 0.5 h MB 2 × 10–5 M 1 h 2 mg ml–1 UV light 2h 4h 6h 8h

0.0 500

10

y = –0.09269+0.8347x R2 = 0.98882

rh-In2O3 nanocrystals under UV irradiation Commercial In2O3 rh-In2O3 nanocrystals in darkness RhB self-degration

1.5 1.0

y = –0.02596+0.0699x R2 = 0.99281

0.5

y = –0.03416+0.0656x R2 = 0.92447 y = –0.01243+0.0272x R2 = 0.93727

0.0

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(c)

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ln (C0/C)

0.4

MB 2 × 10–5 M 2 mg ml–1 UV light

Absorption (a.u.)

Absorption (a.u.)

0 h of irradiation 0.6

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(d)

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Figure 2.46 (a) Temporal UV–vis absorption spectral changes and corresponding color changes (inset) of MB in water in the presence of as-synthesized single-crystalline hollow rh-In2 O3 nanocrystals under UV irradiation. (b) Temporal UV–vis absorption spectral changes of MB in water in the presence of commercial In2 O3 . (c) The relationship between rh-In2 O3 nanocrystals catalyst content and degradation time. (d) Photodegradation curves of MB as a function of irradiation time. (e) The corresponding selected fitting results using pseudo-first-order reaction kinetics. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

0.8 0.6 0.4 0.2 0.0 400

(a)

MB 2 × 10

–5

1.0

M

–1 0 h of irradiation 1.5 mg ml UV light 0.5 h 1h 2h 3h 4h 5h 6h

500

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Absorption (a.u.)

1.0 Absorption (a.u.)

66

0.8 0.6 0.4 0.2 0.0 400

800 (b)

0 h of irradiation MB 2 × 10–5 M –1 1 mg ml UV light 0.5 h 1h 2h 3h 4h 6h 8h 10 h

500

600

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Wavelength (nm)

Figure 2.47 Temporal UV–vis absorption spectral changes and corresponding color changes (inset) of MB in water in the presence of s-synthesized single-crystalline hollow rh-In2 O3 nanocrystals under UV irradiation with different catalyst content: (a) 1.5 mg ml−1 and (b) 1 mg ml−1 . (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

without rh-In2 O3 nanocrystals, and 43.4% degradation after 8 h of irradiation in the presence of rh-In2 O3 nanocrystals in darkness. Figure 2.46e shows the linear relationship between ln c0 /c and t, the corresponding pseudo-first-order rate constants for the degradation of MB dye solution with rh-In2 O3 nanocrystals and commercial In2 O3 were 0.8347 h−1 and 0.0699 h−1 , respectively, while the constant was only 0.0656 h−1 in the presence of rh-In2 O3 nanocrystals in darkness, and only 0.0272 h−1 without the catalyst. These results demonstrate that the as-synthesized single-crystalline hollow rh-In2 O3 nanocrystals have high photocatalytic activity, which can be utilized in the removal of dye molecules in aqueous solution. The degradation mechanism of the RhB over In2 O3 nanocrystals under UV light is shown in Figure 2.48, termed as photodegradation, which can create free radicals. Once this reaction starts, it sets off a chain reaction that accelerates degradation unless stabilizers are used to interrupt the oxidation cycle [114]. It is well known that In2 O3 is an important n-type semiconductor. The as-synthesized single-crystalline hollow rh-In2 O3 nanocrystals present a UV Figure 2.48 The degradation mechanism of dye molecules in aqueous solution (taking RhB as an example). (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

314 nm

Absorption (a.u.)

Figure 2.49 UV–vis spectrum of as-synthesized single-crystalline hollow rh-In2 O3 nanocrystals. (Yin and Cao 2012 [96]. Reproduced with permission of American Chemical Society.)

300

350

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450

500

550

600

650

Wavelength (nm)

absorption maxima around 314 nm (3.95 eV in photon energy), which is blueshifted in comparison with the band gap at 337 nm (3.67 eV in photon energy) of the bulk In2 O3 [115]. This blueshift is attributed to the existence of a weak quantum confinement effect (Figure 2.49) [116]. When absorbing UV light with an energy of hv matches or exceeds the band-gap energy, Eg , the rh-In2 O3 nanocrystals generate valence-band hole (h+ ) and conduction-band electron (e− ) pairs at the surface, which is shown in Eq. (2.21): 2In2 O3 + h𝜈 → In2 O3 (h+ ) + In2 O3 (e− )

(2.21)

And, the holes can react with water adsorbed on the surface of rh-In2 O3 nanocrystals to generate highly reactive hydroxyl radicals (•OH); at the same time, O2 acts as an electron acceptor to generate a superoxide anion radical (•O2 − ), which combines with protons to generate •OOH, as shown in Eqs. (2.21)–(2.23) [117]: H2 O + In2 O3 (h+ ) → •OH + H+ + In2 O3

(2.22)

O2 + In2 O3 (e− ) → •O−2 + In2 O3

(2.23)

O2 + H+ → •OOH

(2.24)

The •OH radical is a rather reactive species that degrades many classes of organic substrates [118]. It is considered that oxidative degradation of RhB and MB is generally caused by the subsequent attacks of •O2 − , •OH, and •OOH radicals through reaction (2.25) and reaction (2.26) [118, 119]. RhB + O2 ∕O−2 •OOH → (intermediates) → degraded products MB +

O2 ∕O−2 •OOH

→ (intermediates) → degraded products

(2.25) (2.26)

In comparison, hierarchical ZnO hollow spheroids (content: 0.5 mg ml−1 ) exhibited the photocatalytic ability for RhB (1 × 10−5 M) under UV irradiation of lower than 60% in 5 h (or 300 min) [120], while our rh-In2 O3 nanocrystals exhibited the photocatalytic ability for RhB (1 × 10−5 M) under UV irradiation of 92% in 4–6 h, with the catalyst content ranging from 2–1 mg ml−1 . The photocatalytic ability for RhB (1 × 10−5 M) in the presence of mixed-phase TiO2 nanocrystals (content: 1 mg ml−1 ) under artificial solar light irradiation

67

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

showed the degradation pseudo-first-order rate constants of 0.3531 (T1 sample), 0.1260 (T2 sample), 0.1299 (T4 sample), 0.1252 (T5 sample), and 0.1504 h−1 (P25 sample) lower than the constant of our rh-In2 O3 nanocrystals (0.6020 h−1 ) except the constant of 0.8971 h−1 for T3 [51]. TiO2 -coated alumina membrane exhibited photocatalytic ability for MB (0.01 mM, i.e., 1 × 10−5 M) under UV illumination of close to 90% after 20 h [121], while our rh-In2 O3 nanocrystals showed the degradation times ranging from 3 h to 6 or 10 h, with the degradation ratio of MB reaching ∼92% and with the catalyst content ranging from 2 mg ml−1 to 1.5 or 1 mg ml−1 , respectively. 2.3.4 Fe2 O3 Nanoparticles

Hematite (α-Fe2 O3 ), the most stable iron oxide under ambient conditions, is of scientific and technological importance [122, 123], mainly due to its magnetic properties and chemical stability. Developing new routes for the preparation of α-Fe2 O3 nanocrystals of various sizes and shapes and investigating their distinguished properties is of considerable interest. Within the past few decades the unusual magnetic behavior of α-Fe2 O3 has been studied extensively [124, 125], which usually relies on the sizes and shapes of α-Fe2 O3 particles. Preparation of nanomaterials with controllable shape has achieved limited success [76, 126]. 5000

(110) (116)

2000 (012)

(214)

(113)

1000 (122) (202)

(208) (1010) (220) (036) (312) (0210)

(300)

(024)

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3000 (116) (214) (113)

(012)

1000

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Intensity (a.u.)

8000 293 cm–1 615 cm–1 658 cm–1

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412 cm–1 246 cm–1

613 cm–1 658 cm–1

499 cm–1

4000 2000

200

(c)

(104)

(104)

Intensity (a.u.)

Intensity (a.u.)

4000

Intensity (a.u.)

68

400

600

800

Wavenumbers (cm–1)

200

1000

(d)

400

600

800

1000

Wavenumbers (cm–1)

Figure 2.50 XRD patterns of the as-synthesized samples synthesized with the same molecularity of reaction at different temperatures: (a) 200 ∘ C for 10 h and (b) 280 ∘ C for 10 h. Corresponding Raman spectra of the as-synthesized samples synthesized with the same molecularity of reaction at different temperatures: (c) 200 ∘ C for 10 h and (d) 280 ∘ C for 10 h. (Cao et al. 2006 [127]. Reproduced with permission of John Wiey & Sons.)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

Here, we introduce a new reaction route to obtain rice- and cube-shaped single-crystalline α-Fe2 O3 nanostructures via a reaction between Fe(NO3 )3 ⋅9H2 O and NH3 ⋅H2 O in ethylene glycol (EG) at 200 and 280 ∘ C, respectively. Herein, we demonstrate a facile method to obtain rice- and cube-shaped single-crystalline a-Fe2 O3 nanostructures [127]. 2.3.4.1 Synthesis Synthesis of Rice-Shaped 𝜶-Fe2 O3 Fe(NO3 )3 •9H2 O (AR), NH3 •H2 O (AR), ethylene

glycol (AR), and absolute alcohol (AR) were used without further purification. A 1 g Fe(NO3 )3 •9H2 O (2.475 mmol) was dissolved in 20 ml of deionized water to form solution A, and 10 ml NH3 •H2 O was dissolved in 10 mL EG to form solution B. Then solution B was added dropwise into solution A while stirring for 30 min at room temperature. The mixture was sealed into a 50-ml Teflon-lined autoclave, heated to 200 ∘ C, and maintained at this temperature for 10 h. After the autoclave was cooled down to room temperature naturally, the products were collected and washed with deionized water and then with absolute alcohol. The cycle was repeated three times, followed by drying at 80 ∘ C for 10 h (Figures 2.50a,c, 2.51a–c, and 2.55a). The different molecularity of the reactants (1 g Fe(NO3 )3 •9H2 O, 15 ml NH3 •H2 O, 10 ml EG, 15 ml deionized water) heated at 200 ∘ C for 24 h and 48 h can also result in rice-shaped α-Fe2 O3 nanocrystals (Figures 2.52 and 2.53). Synthesis of Cube-Shaped 𝜶-Fe2 O3 Fe(NO3 )3 •9H2 O (AR), NH3 •H2 O (AR), ethylene

glycol (AR), and absolute alcohol (AR) were used without further purification. A 1 g Fe(NO3 )3 •9H2 O (2.475 mmol) was dissolved in 20 ml of deionized water to form solution A, and 10 ml NH3 •H2 O was dissolved in 10 ml EG to form solution B. Then solution B was added dropwise into solution A while stirring for 30 min at room temperature. The mixture was sealed in a 50-ml Teflon-lined autoclave, heated to 280 ∘ C, and maintained at this temperature for 10 h. After the autoclave was cooled down to room temperature naturally, the products were collected and washed with deionized water and absolute alcohol. The cycle was repeated three times, followed by drying at 80 ∘ C for 10 h (Figures 2.50b,d and 2.51d). The different molecularity of the reactants (1 g Fe(NO3 )3 •9H2 O, 25 ml NH3 •H2 O, 10 ml EG, and 5 ml deionized water) heated at 280 ∘ C for 10 h can also result in cube-shaped α-Fe2 O3 nanocrystals (Figures 2.54 and 2.55b). 2.3.4.2 Characterization

The XRD measurement of samples was carried out using an X-ray diffractometer (Rigaku, D/max-RB, 40 kV × 100 mA). Resonance Raman spectra (Renishaw, RW 1000) were obtained with excitation from the 632.8 nm line of a He–Ne laser. The morphology of the reaction products was examined using TEM (JEOL, JEM-1200, operating at 120 kV) and HRTEM (JEOL JEM-2010, operating at 200 kV). 2.3.4.3 Measurement of Magnetic Properties

Magnetic properties of the as-synthesized α-Fe2 O3 were studied using a vibrating sample magnetometer (VSM, LakeShore 7307) at room temperature.

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

(a)

(b)

20 nm

200 nm

(c)

(d)

d = 0.250 nm (110)

5 nm

50 nm

Figure 2.51 (a) TEM image and (b,c) HRTEM images of rice-shaped α-Fe2 O3 synthesized at 200 ∘ C for 10 h. Inset of (b) is corresponding selected area electron diffraction. (d) TEM image of cube-shaped α-Fe2 O3 synthesized with the same molecularity of reaction at 280 ∘ C for 10 h. (Cao et al. 2006 [127]. Reproduced with permission of John Wiey & Sons.)

(104)

3000

(110) (116)

2000

(024) (113)

1000

(300) (214)

(012) (122)

200 nm

(1010) (208)

(220) (036) (312) (0210)

Intensity (a.u.)

70

0 20

(a)

(b)

30

40

50

60

70

80

2θ(°)

Figure 2.52 (a) TEM images and (b) corresponding XRD patterns of the α-Fe2 O3 nanocrystals synthesized at 200 ∘ C for 24 h. (Cao et al. 2006 [127]. Reproduced with permission of John Wiey & Sons.)

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

(104)

(110)

2000

(116) (024) (113)

1000

(300) (214)

(012) (122)

(1010) (208)

100 nm

(220) (036) (312) (0210)

Intensity (a.u.)

3000

0

(a)

20

30

40

(b)

50

60

70

80

2θ/°

Figure 2.53 (a) TEM images and (b) corresponding XRD patterns of the α-Fe2 O3 nanocrystals synthesized at 200 ∘ C for 48 h. (Cao et al. 2006 [127]. Reproduced with permission of John Wiey & Sons.) 4000 (104)

(110) (116)

2000

(300) (024)

1000

(214)

(113)

(012)

(220) (1010)

(122)

(208)

(202)

(036) (312) (0210)

Intensity (a.u.)

3000

0 20

30

40

50 2θ(°)

(a)

Intensity (a.u.)

8000

60

70

80

293 cm–1 226 cm–1

6000 411 cm–1

4000

611 cm–1 658 cm–1

246 cm–1

496 cm–1

50 nm

2000

200

(b)

400 600 800 Wavenumbers (cm–1)

1000

(c)

Figure 2.54 (a) XRD pattern, (b) Raman spectrum, and (c) TEM image of cube-shaped α-Fe2 O3 synthesized at 280 ∘ C for 10 h. Inset in Figure 2.54c is the corresponding selected area electron diffraction. (Cao et al. 2006 [127]. Reproduced with permission of John Wiey & Sons.)

2.3.4.4 Structure

Figure 2.50a,b shows representative XRD patterns of as-synthesized samples and demonstrates clearly that the products are crystalline, and the diffraction peaks can be indexed unambiguously to the rhombohedral phase of hematite (α-Fe2 O3 , JCPDS No. 79-0007). Hematite is a rhombohedrally centered hexagonal structure

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

0.4

0.4

0.2

0.2

M (emu g–1)

M (emu g–1)

72

0.0 –0.2

–10 000 –5000

–0.2 –0.4

–0.4

(a)

0.0

0 H (Oe)

5000

–10 000 –5000

10 000

(b)

0

5000

10 000

H (Oe)

Figure 2.55 Hysteresis loops (M–H curves) of (a) rice-shaped α-Fe2 O3 synthesized at 200 ∘ C for 10 h and (b) cube-shaped α-Fe2 O3 synthesized at 280 ∘ C for 10 h. (Cao et al. 2006 [127]. Reproduced with permission of John Wiey & Sons.)

of the corundum-type with a close-packed oxygen lattice in which two-thirds of the octahedral sites are occupied by Fe(III) ions [128]. It has been regarded that Raman spectroscopy is a fast and nondestructive tool to appreciate the quality of crystalline materials, including surface conditions and homogeneity [129]. Therefore, the structure characterization of the as-prepared samples is further studied using Raman spectroscopy. Raman spectra of the two typical samples at room temperature are presented in Figure 2.50c,d. The spectra contain bands at ∼226, 246, 293, ∼412, ∼500, ∼615, and 658 cm−1 in the range of 100–1000 cm−1 . These peak positions are in good agreement with α-Fe2 O3 [130–134]. The Raman peaks appearing at ∼226 and ∼500 cm−1 are attributed to A1g mode, and 246 (no 246 cm−1 present in Figure 2.50c), 293, ∼412, and ∼615 cm−1 are attributed to Eg mode [130–133]. The Raman peak appearing at 658 cm−1 is related to disorder effects and/or to the presence of nanocrystals of α-Fe2 O3 [134]. In most literatures on the Raman spectra of α-Fe2 O3 , the Raman peak at about 660 cm−1 is usually neglected, even if it, indeed, is presented in these literatures [130–133]. However, Bersani and coworkers have given a detailed study on the Raman peak at about 660 cm−1 for hematite, which cannot be attributed to the presence of trace of magnetite but appears to be typical of the α-Fe2 O3 itself [134]. The Raman results also demonstrate that the as-synthesized samples belong to the α-Fe2 O3 phase. The morphology of the products is studied using TEM (JEOL, JEM-1200, operating at 120 kV) and HRTEM (JEOL JEM-2010, operating at 200 kV). Figure 2.51a shows a representative TEM image and illustrates the monodisperse rice-shaped α-Fe2 O3 with well-defined facets. The average sizes of rice-shaped α-Fe2 O3 nanocrystals are major axis in length of approximately 84.9 nm and minor axis in length of approximately 41.2 nm. The length-to-diameter ratio is about 2. The strong spots of the SAED pattern depicted in the inset of Figure 2.51b confirm the α-Fe2 O3 to be single crystalline. More information about the crystal can be derived from the HRTEM images of the α-Fe2 O3 (Figure 2.51c). The lattice fringes (d = 0.250 nm) observed in the HRTEM image agree well with the separation between the (110) lattice planes. The angle between the lattice fringe

2.3 Synthesis, Characterization, and Optical Properties of Oxide Nanoparticles

of (110) plane and axial direction is 45∘ . This suggests that the growth direction is along [100]. The rice-shaped α-Fe2 O3 nanocrystals can also be obtained with different molecularity of reactions at 200 ∘ C for 24 h and 48 h. Corresponding data are presented in Figures 2.52 and 2.53. However, if the reaction temperature is raised to 280 ∘ C without changing other reaction parameters, it will result in cube-shaped single-crystalline α-Fe2 O3 . The TEM image in Figure 2.51d clearly illustrates that the morphology of the samples is cube shaped with a smooth surface. The edge length of cube-shaped α-Fe2 O3 nanocrystals is up approximately 67 nm. Cube-shaped α-Fe2 O3 nanocrystals with edge length of approximately 86 nm can also be obtained via changing the molecularity of reaction at 280 ∘ C for 10 h (Figure 2.54). The corresponding Raman spectrum of the as-synthesized sample is presented in Figure 2.54b and gives results similar to those presented in Figure 2.51c,d, except for changes in relative intensities and a slight shift. The inset in Figure 2.54c confirms the cube-shaped α-Fe2 O3 to be single crystalline. These facts indicate that the reaction temperature plays a key role in the morphology variation of the as-synthesized α-Fe2 O3 . The sensitivity to reaction temperature implies that the reaction route is under kinetic control rather than thermodynamic control [135]. The reaction between Fe(NO3 )3 solution and NH3 •H2 O is presented as follows: Fe(NO3 )3 + 3NH3 •H2 O → Fe(OH)3 + 3NH4 NO3

(2.27)

and α-Fe2 O3 particles will form through a two-step phase transformation [136]. Fe(OH)3 → β-FeOOH → α-Fe2 O3 (phase transformation)

(2.28)

EG will have an effect on preventing the Fe(OH)3 nanoparticles from agglomerating. Therefore, uniform α-Fe2 O3 nanocrystals can be obtained after being heated at 200–280 ∘ C to complete the phase transformation. 2.3.4.5 Magnetic Properties

Magnetic hysteresis measurements of α-Fe2 O3 samples are performed using a VSM (LakeShore 7307) at room temperature. The hysteresis loops (M–H) of four samples do not reach saturation up to the maximum applied magnetic field. [Only M–H curves of rice-shaped α-Fe2 O3 and cube-shaped α-Fe2 O3 synthesized at 200 ∘ C for 10 h and 280 ∘ C for 10 h are presented in Figure 2.55a,b, respectively. The corresponding TEM image of Figure 2.55a is presented in Figure 2.51a, while the corresponding TEM image of Figure 2.55b is presented in Figure 2.54c. Another two M–H curves are presented in Figure 2.56a,b. The related magnetic parameters, reaction conditions, and morphological characteristics are listed in Table 2.5. Field-dependent magnetization plots illustrate that all of the M–H curves of rice-shaped and cube-shaped α-Fe2 O3 are hysteretic features, which indicate that all the α-Fe2 O3 nanocrystals display weak ferromagnetism at room temperature [125]. The remanent magnetization (Mr ), squareness (Mr /Ms ), and coercivity (H c ) values are 0.040913 emu g−1 , 0.1074, and 149.80 Oe for rice-shaped α-Fe2 O3 and 0.088035 emu g−1 , 0.2041, and 335.16 Oe for

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

0.4

0.4

0.2

0.2

M (emu g–1)

M (emu g–1)

74

0.0 –0.2

–10 000 –5000

–0.2 –0.4

–0.4

(a)

0.0

0 H (Oe)

5000

10 000

–10 000 –5000

(b)

0

+5000 +10 000

H (Oe)

Figure 2.56 (a) M–H curve of the α-Fe2 O3 nanocrystals synthesized at 200 ∘ C for 24 h. (b) M–H curve of the α-Fe2 O3 nanocrystals synthesized at 200 ∘ C for 48 h. (Cao et al. 2006 [127]. Reproduced with permission of John Wiey & Sons.)

cube-shaped α-Fe2 O3 nanocrystals, respectively. These raw data are presented in electromagnetic units per gram of sample. An interesting result is observed from Table 2.5, that is, Mr , Mr /Ms , and H c values of all three rice-shaped α-Fe2 O3 are similar, although the reaction time and particle sizes are different. That means the particle sizes have little effect on the magnetic parameters. But Mr , Mr /Ms , and H c values of cube-shaped α-Fe2 O3 are >1.5 times and about 1.5 times and two times compared with those of all three rice-shaped α-Fe2 O3 nanocrystals, respectively. That means the particle shapes have a great effect on the magnetic parameters, which is attributed to shape magnetic anisotropy [137]. The antimagnetic field coefficients are different for ferromagnetic substances, with limited volume unless they are spherical. So magnetostatic energy will change, accompanied by spontaneous magnetization of gyration, which causes magnetic anisotropy [138]. The small coercivity of α-Fe2 O3 nanocrystals suggests that they belong to semi-hard magnetic materials, which may find applications as relays, switches, semi-fixed storage [138], and in magnetic-optical nanodevices [139]. The relationship between microstructures and magnetic properties of α-Fe2 O3 has been studied [124, 125]. Sahu and coworkers found that the coercivity values and remanent magnetization of α-Fe2 O3 particles increase with decreasing the particle size, which is attributed to the polycrystalline nature of the grains consisting of smaller subparticles [124]. Jing and coworkers found that the remanent magnetization values of α-Fe2 O3 particles are in the same range on the whole, while coercivity values are quite different, which is attributed to the surfactant effect [125].

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles 2.4.1 CdS Nanoparticles

CdS, one of the most important wide-gap semiconductors (Eg ≈ 2.5 eV for the bulk hexagonal wurtzite phase of CdS and Eg ≈ 3.53 eV for the “bulk” cubic zinc-blende phase of CdS) [140], has been extensively studied because of its

Table 2.5 The morphological characteristics, sizes, reaction conditions, and magnetic parameters of the samples. Samplea)

Shape

Particle size by TEM (major/minor axis) (nm)

Reaction temperature (∘ C)

Reaction time (h)

Coercivity Hc (Oe)

Remanent magnetization Mr (emu g−1 )

Squareness Mr /Ms

1#

Rice shaped

84.9/41.2

200

10

149.80

0.040913

0.1074

2#

Rice shaped

94.4/48.9

200

24

157.23

0.053262

0.1411

3#

Rice shaped

99.1/49.1

200

48

165.02

0.057514

0.1362

4#

Cube shaped

−86

280

10

335.16

0.088035

0.2041

a)

1#, corresponding XRD and TEM data in Figures 2.50a and 2.51a–c; 2#, corresponding TEM and XRD data in Figure 2.52a and 2.52b; 3#, corresponding TEM and XRD data in Figure 2.53a and b; 4#, corresponding XRD and TEM data in Figure 2.54a and c. Source: Cao et al. 2006 [127]. Reproduced with permission of John Wiley & Sons.

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

ability to tune emission in the visible range simply by changing its size or shape [141–147]. At present, it is one of the most important materials for nanoelectronics because it is possible to engineer the band gaps over a wide range from visible to ultraviolet [148]. The utilization of nanomaterials inevitably requires sufficient control of their structures and their assembly systems because the intrinsic properties of the nanomaterials are determined by their structures, including size, shape, and dimension. Nanocrystalline CdS has been synthesized by a variety of methods including a sol–gel template [141], a solvothermal route [142], an in situ micelle-template-interface reaction route [143], ion beam synthesis [144], ultrasonic irradiation in an aqueous solution [145], one-pot synthesis [146], a two-phase approach [147], and so on. The size, shape, and crystalline structure of semiconductor nanocrystallites are the important factors for determining their optical properties, which leads to considerable changes in the recombination of electrons and holes trapped at spatially separated donors and acceptors. Here, we introduce a new route of controllable synthesis of wurtzite-type CdS nanoparticles and nanowires in bulk quantities [149]. Organic molecule dithiol glycol (HSCH2 CH2 SH) is used as the sulfur source [150]. Through changes in the synthetic temperature and reaction time, CdS NPs and nanowires can be controllably synthesized. The mechanism of self-assembly of CdS from NPs to nanowires is discussed. The PL peaks change just because of the shape change of the CdS nanostructures. 2.4.1.1 Synthesis

In a typical procedure, dithiolglycol (HS-CH2 CH2 -SH, Eastgate, White Lund, Morecambe, England, 98% purity) and Cd(NO3 )2 ⋅4H2 O (AR) was dissolved in 5 ml of alcohol (AR). In a typical case, 5-ml dithiolglycol (0.6 mmol) alcohol solution was added dropwise to a 30-ml Cd(NO3 )2 (1 mmol) aqueous solution at room temperature and stirred for 15 min. A mixture was obtained, and then sealed in a 50-ml Teflon-lined autoclave and maintained at 220–280 ∘ C for 1–240 h. The light yellow products were washed with deionized water and then in alcohol, and the cycle was repeated three times. The products were dried at 50 ∘ C for 2 h. 2.4.1.2 Characterization

The products were characterized by XRD (Bruker D8 Advance), TEM (JEOL, JEM-1200 transmission electron microscope operating at 120 kV), and Raman spectra (Renishaw RM 1000). The Raman measurement was achieved using a microscopic confocal Raman spectrometer (Renishaw RM 1000) at room temperature. The 514-nm line of Ar-ion laser with a power of about 5 mW was used to excite the Raman spectra. The diameter of the laser spot focused on the sample was about 1 μm. PL spectra were recorded with fluorescence spectrophotometer (JASCO FP-6500). pH value was measured by acidometer (PHSJ-3 F). 2.4.1.3 Structure

Figure 2.57 shows XRD patterns recorded from the three typical samples synthesized at 220 ∘ C for 1 h (denoted as CdS-1) (Figure 2.57a), 120 h (denoted

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

(c)

(101) (100) (002)

(100) (b)

(112)

(103)

(101)

(002)

(a)

CdS-240 (220 °C/240 h)

(110)

CdS-120 (220 °C/120 h)

(110) (112) (103)

(002) (100) (101)

(203)

CdS-1 (220 °C/1 h)

(110)

(112)

(203)

5 10 15 20 25 30 35 40 45 50 55 60 65 70 75 80 85 90 2θ

Figure 2.57 XRD patterns of samples synthesized at 220 ∘ C for (a) 1 h (denoted as CdS-1), (b) 120 h (denoted as CdS-120), and (c) 240 h (denoted as CdS-240), respectively. (Cao et al. 2006 [149]. Reproduced with permission of American Chemical Society.)

as CdS-120) (Figure 2.57b), and 240 h (denoted as CdS-240) (Figure 2.57c), respectively. The XRD patterns of all samples can be consistently indexed to the hexagonal wurtzite-type CdS, in which the several prominent peaks correspond to the reflection (100), (002), (101), (110), (103), (112), and (203) (JCPDS 41-1049). The distinctive reflection peaks at 2𝜃 = 28.4∘ and 53∘ are evidence of a hexagonal CdS phase, while the absence of a reflection peak at 2𝜃 = 31.5∘ is evidence of no incorporation of the cubic zinc-blende phase CdS [151]. Nevertheless, the intensities of some diffraction lines were relatively enhanced with the reaction time increasing from 1 to 240 h. The broadening of diffraction peaks for CdS NPs (CdS-1) is obvious, which indicates the formation of ultrafine particles. However, the relative intensity of the diffraction peaks of CdS nanowires (CdS-120 and CdS-240) deviate from that of CdS NPs (CdS-1), suggesting a diffraction-oriented growth direction of the nanowire. The XRD patterns show a substantial texture effect in accordance with the crystal shape anisotropy and orientation. Comparing the intensities of the (100), (002), and (101) peaks of CdS nanowires (CdS-120 and CdS-240) with those of CdS NPs (CdS-1), it was found that the relative maximum intensity sequence was no longer (001) > (100) ≈ (101) for CdS-1, but (101) > (100) > (002) for CdS-240 and (101) ≈ (100) > (002) for CdS-120, implying that the nanowire growth occurs along ⟨101⟩ direction. These changes can be attributed to the preferential orientation of crystals.

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

CdS is usually known to exist in two modifications: zinc-blende- and wurtzite-type phases [152, 153]. Wurtzite-type structure is a uniaxial crystal with its optical axis parallel to the crystallographic axis c. In fact, wurtzite and zinc-blende have similar lattice structures. Both of them own the same tetragonally positioned first nearest neighbors and nearly identical secondary nearest neighbors [154]. Recently, it was also reported that a third phase, that is, a high-pressure rocksalt phase of CdS was observed [155]. It is well known that vibration spectroscopy is a very useful technique for the determination of the crystal phase. Furthermore, Raman spectroscopy of semiconductors is a fast and nondestructive tool to appreciate crystalline material qualities, including surface conditions and homogeneity. That is, crystalline samples present sharp Raman peaks while amorphous or polycrystalline samples show very broad Raman peaks [129]. So the structural characterization of the as-synthesized CdS nanostructures was therefore carried out using Raman spectroscopy. In Figure 2.58, Raman spectra of three typical samples – CdS-1, CdS-120, and CdS-240 – are displayed. The Raman peaks of CdS-1 (NPs) appear at 298 cm−1 attributed to the A1 (LO) mode with the FWHM about 22.7 cm−1 and its overtone at 596 cm−1 . The Raman peaks of CdS-120 (nanowires attached with NPs) appear at 301 cm−1 with the FWHM about 14.7 cm−1 and its overtone at 602 cm−1 . The Raman peaks of CdS-240 (nanowires) appear at 301 cm−1 with the FWHM about 15.2 cm−1 and its overtone at 603 cm−1 [150, 156]. The Raman spectra exhibit (c)

CdS-240 (220 °C/240 h)

603 cm–1

301 cm–1

(b) Intensity (a.u.)

78

CdS-120 (220 °C/120 h)

602 cm–1

301 cm–1

(a)

CdS-1 (220 °C/1 h)

298 cm–1

596 cm–1

200

300

400

500

600

700

800

Wavenumber (cm–1)

Figure 2.58 Raman spectra of samples synthesized at 220 ∘ C for (a) 1 h (denoted as CdS-1), (b) 120 h (denoted as CdS-120), and (c) 240 h (denoted as CdS-240), respectively. (Cao et al. 2006 [149]. Reproduced with permission of American Chemical Society.)

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

relatively sharp crystal-like peaks. The decrease of FWHM from NPs (CdS-1) to nanowires (CdS-120 and CdS-240) can be attributed to the improvement in the crystallinity of the CdS nanocrystals. It has been reported that defect-free crystalline CdS films have an FWHM of 8 cm−1 [157]. Also, this phenomenon has been demonstrated by the corresponding electron diffraction (ED) in TEM measurement, which is a single-crystalline structure for CdS nanowires (the insets in Figure 2.59d, e, and i). It is known that in a crystalline semiconductor or insulator, the observed Raman shifts usually correspond to the longitudinal optical (LO) phonons, whereas other modes such as the transverse optical (TO) and the surface phonon (SP) modes are not observable due to symmetry restrictions and (a)

(b)

(c)

50 nm

200 nm

200 nm

(d)

(e)

100 nm

100 nm

(h)

(i)

500 nm

100 nm

(f)

100 nm (j)

200 nm

(g)

200 nm (k)

500 nm

Figure 2.59 TEM images of as-synthesized samples at different conditions with the same concentration (a–j) for: (a) at 220 ∘ C for 1 h; (b) at 220 ∘ C for 2 h; (c) at 220 ∘ C for 3 h; (d) at 220 ∘ C for 4 h, inset is corresponding electron diffraction (ED); (e) at 220 ∘ C for 24 h, inset is corresponding ED; (f ) at 220 ∘ C for 36 h, (g) at 220 ∘ C for 48 h, (h) at 220 ∘ C for 120 h, (i) at 220 ∘ C for 240 h, inset is corresponding ED, (j) at 280 ∘ C for 240 h; and (k) 220 ∘ C for 240 h with the concentration rising three times compared with the above-mentioned, including samples (a–j). (Cao et al. 2006 [149]. Reproduced with permission of American Chemical Society.)

79

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

their low intensities [158]. Another observation is that the peak profile is almost symmetric for all three samples. It is also observed that the intensity of the Raman line of sample CdS-120 is lower than that of CdS-1, which is attributed to the quantity of the sample detected by Raman. It is known that the intensity of a Raman line is proportional to the number of scattering centers, due to Raman scattering being an incoherent process [159]. 2.4.1.4 Growth Mechanism

Assembly of nanocomponents is a key step for constructing nanodevices; thus, the shape of nanobuilding blocks is crucial for the assembly and device design. Shape-controlled synthesis of nanobuilding blocks is still a challenge technique wise for us. Generally, it is believed that Ostwald ripening [160] is the main manner for crystal growth. Recently, a new oriented attachment mechanism was found under hydrothermal [160–163] and refluxing solution conditions [164] to form high-quality oxide nanorods or nanowires from NPs, such as TiO2 [160], MnO3 [162], and ZnO [163, 164]. Weller et al. suggested that oxide NPs are very favorable for oriented attachment [164]. Obviously, the essential requirement for wirelike structure formation is anisotropic crystal growth, which can be usually realized when the free surface energy of the various crystallographic planes differs obviously. We carried out several experiments in different ways to probe the growth mechanism of as-synthesized CdS nanostructures. All the experiments described hereafter were performed under hydrothermal conditions. Details can be found in the experimental section. Figure 2.59a–c displays CdS NPs synthesized at 220 ∘ C for 1 h (denoted as CdS-1), 2 h (denoted as CdS-2), and 3 h (denoted as CdS-3) with an averaged particle size approximately 9.6, 17.2, and 17.9 nm, respectively. The corresponding powder XRD of CdS-1 is depicted in Figure 2.57 (curve a). It exhibits the typical size-broadened reflection from wurtzite-type CdS. The estimation of the primary crystallite size of this sample was ∼7.3 nm obtained by Scherrer linewidth analysis of the reflection peaks. This result is consistent with the TEM observation. As the reaction time is increased to 4 h at 220 ∘ C (denoted as CdS-4), we find that most of the CdS are in the form of nanowires attached with some NPs (Figure 2.59d). If the reaction time is increased to 24, 36, 48, and 120 h (denoted as CdS-24, CdS-36, CdS-48, and CdS-120) at 220 ∘ C, respectively, we also obtain nanowires attached with some particles (Figure 2.59e–h). The corresponding XRD of CdS-120 is shown in Figure 2.57 (curve b), which indicates that it belongs to hexagonal CdS. We find an interesting phenomenon, that is, all these samples present in the manner of nanowires attached with some particles, and the longer the reaction time, the fewer the particles attached to the nanowires. And, the wires belong to the single-crystalline structure demonstrated by the corresponding ED pattern in TEM images (insets in Figure 2.59d,e). When the reaction time reaches 240 h at 220 ∘ C (denoted as CdS-240) (Figure 2.59i), we can only obtain single-crystal nanowires, corresponding to the XRD presented in Figure 2.57 (curve c). And, under this condition, we can obtain the nanowires with maximum length reaching ∼2.3 μm, diameter ∼71 nm, and the aspect ratio ∼33. If we raise the reaction temperature to 280 ∘ C for 24 h (denoted as CdS-280-24), we can obtain wider CdS nanowires (Figure 2.59j) with the maximum length ∼0.6 μm, diameter

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

∼56 nm, and the aspect ratio ∼11. If we raise the concentration of the initial reactants three times compared with all the above-mentioned samples and keep the reaction temperature at 220 ∘ C for 240 h, we can obtain longer and wider CdS nanowires with the maximum length ∼6.5 μm, diameter ∼158 nm, and the aspect ratio reaches ∼42 (Figure 2.59k). However, if we reduce the reaction temperature to 190 ∘ C for 4 h or 24 h, we cannot obtain CdS nanowires. We believe that, before the hydrothermal reaction, the coordination occurs between HSCH2 CH2 SH and Cd2+ and forms a stable five-membered ring coordination cation, which is demonstrated by the changing of the pH value. After HSCH2 CH2 SH-alcohol solution (pH = 3.75) was added dropwise to Cd(NO3 )3 aqueous solution (pH = 5.30) with stirring at 15 min, the pH value of the resulting mixture was 1.45. This phenomenon was attributed to the coordination reaction between HSCH2 CH2 SH and Cd2+ (Figure 2.60). After hydrothermal reaction, the C–S bond in five-membered ring coordination Cd2+ was broken, and formed CdS. The coordination between –SH and Cd2+ may compete with the CdS crystal growth. If the reaction time is short, such as 1–3 h, we obtain CdS NPs (Figure 2.59a–c). It has been demonstrated that crystalline structures of NPs depend on their surface conditions due to their large surface-to-volume ratios [165, 166]. So the strong solvation of the solvent molecules HSCH2 CH2 SH to Cd2+ and the surface cadmium atoms of CdS NPs lead to the generation of hexagonal CdS NPs. After prolonging the hydrothermal reaction time, all

HSCH2CH2SH

+

Cd2+

CH2

CH2

(pH = 3.75)

(pH = 5.30)

2H+

+ S

S Cd2+

(pH = 1.45)

220 °C/ 1~3 h

220 °C/ 4~120 h

CdS nanowires attached with CdS nanoparticles

Reaction time prolonging

CdS nanoparticles

220 °C/ 240 h

CdS nanowires

Figure 2.60 Strategy for the preparation of CdS nanoparticles and nanowires, and the nanowire generated from nanoparticles via oriented attachment mechanism. (Cao et al. 2006 [149]. Reproduced with permission of American Chemical Society.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

the C–S bonds were broken. And, these CdS particles were assembled under hydrothermal conditions via “oriented attachment” mechanism [162–164], that is, spontaneous self-organization of adjacent particles so that these NPs share a common crystallographic orientation, followed by joining of these particles at a planar interface [161]. The results of the experiment for changing the reaction time provide strong evidence of the evolution process from the NPs to nanowires (Figure 2.59). The increase in the reaction time mainly leads to an increase in the elongation of the CdS particles. According to the presenting experiment results, we can obtain clear evidence that oriented attachment of preformed quasi-spherical CdS nanoparticles is a major reaction path during the generation of single-crystalline hexagonal CdS nanowires under hydrothermal conditions. Our results may further demonstrate that wurtzite structure itself favors oriented attachment no matter whether it is oxide [164] or sulfide. 2.4.1.5 Photoluminescence Properties

The PL emissions from CdS NPs (CdS-1) are shown in Figure 2.61a. All spectra exhibit the same emission maximum at 426 nm in wavelength violet light (2.91 eV in photon energy) and a similar peak shape with excitation wavelength range from 330–370 nm. Earlier reports indicate that CdS nanodots exhibit band-edge PL emission peak centered at ∼2.8 eV in photon energy (∼443 nm in wavelength) [167], and surface-capped CdS nanocrystals exhibit band-edge PL emission peak centered at ∼2.6 eV (476 nm in wavelength) [168]. Interestingly, emission at higher wavelengths (or lower energies), which has been attributed to deep trap states due to the surface [169], is not observed in our CdS nanoparticles, 300 λex = 330 nm λex = 340 nm λex = 350 nm λex = 360 nm λex = 370 nm

250

200

180

150

100

λex = 330 nm λex = 340 nm λex = 350 nm λex = 360 nm λex = 370 nm

160 140 PL intensity (a.u.)

PL intensity (a.u.)

82

120 100 80 60 40

50 20 0

0 400

(a)

450

500

550

Wavelength (nm)

600

400 (b)

450

500

550

600

Wavelength (nm)

Figure 2.61 PL spectra of the typical samples dispersed in ethanol: (a) CdS-1 and (b) CdS-240, respectively. (Cao et al. 2006 [149]. Reproduced with permission of American Chemical Society.)

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

indicating that it is high-quality size monodisperse CdS nanocrystals. In contrast, Figure 2.61b displays PL emission peaks from CdS nanowires (CdS-240) around ∼530 nm as well as ∼426 nm excited with 𝜆ex = 330–370 nm. The emission spectra are almost independent of the excitation wavelength used (𝜆ex = 330–370 nm) with the maximum PL intensity presenting at 𝜆ex = 360 nm. The results indicate that the PL emission comes from the CdS nanocrystals, not from other impurities, which is due to band-edge emission and radiative recombination of e− –h+ pairs at surface sulfur vacancy [170]. It is well known that CdS is an n-type semiconductor [152, 171]. The smaller the ion radius is, the easier it is to yield vacancies [171]. So it is easier to yield VS because of r(S2− ) < r(Cd2+ ). That means it will ionize electrons upon optical excitation, that is, VS ↔ VS•• + 2e−

(2.29)

VS is neutral sulfur vacancy, VS•• is doubly ionized sulfur vacancy, and e− is an electron in the conduction band, according to Kröger’s notation [171, 172]. The PL spectra of the CdS nanowires show a predominant band-edge emission at ∼426 nm and a weaker trap-state green light emission at ∼530 nm in wavelength, indicating a number of trap states, whose surface-to-volume ratio is higher than that in CdS nanoparticles, and this increases the occurrence of the surface trap states. A similar trap-state emission at 540 nm was also observed for CdS nanocrystals [173]. In fact, recent reports by Alivisatos and coworkers demonstrate that CdSe nanocrystals own polarized emission along the long axis, unlike spherical dots [126]. This indicates that the ability to control the shapes of semiconductor nanocrystals provides an opportunity to prepare materials with desirable optical characteristics from the point of view of application. 2.4.2 ZnS Nanoparticles and Microspheres

As one of the most important semiconductors, ZnS (Eg = 3.74–3.88 eV for hexagonal wurtzite phase of ZnS and Eg = 3.66 eV for the cubic zinc-blende phase of ZnS) [174] has been extensively investigated [175–177]. The cubic zinc-blende structure is a stable phase at low temperatures for ZnS, while the hexagonal (wurtzite) phase is the high-temperature polymorph of ZnS which can be generated at temperatures higher than 1296 K.8b,10 Many efforts have been devoted to the synthesis of wurtzite-type ZnS with different sizes and shapes in solution at low temperatures [174, 178–181]. Monodisperse ZnS spheres of tunable size may be of great application in the preparation of semiconductor photonic crystals as a complementary system for currently popular polymerand silica-based photonic crystals [182]. The tripeptide glutathione (GSH, γ-Glu-Cys-Gly) is widely distributed in living plant and animal cells, where it plays a crucial role in the protection of intracellular components against oxidative damage and in the capture or removal of toxic heavy metal ions through the thiol group [5]. GSH has, therefore, been considered to be an ideal sulfur source and an appealing candidate for the preparation of novel materials [183]. Here, we introduce a simple synthesis using a low-temperature self-assembly synthetic route to generate ZnS microspheres with several hundred nanometer diameters which are composed of hexagonal wurtzite ZnS nanocrystals [184].

83

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

Our strategy is to use biomolecular GSH as the sulfur source in the formation of ZnS where GSH functions both as a ligand to zinc ions and as a source of sulfur in the formation of ZnS, which is different from GSH being used as capping agent, as reported by Mehra et al. [175, 176]. 2.4.2.1 Synthesis of ZnS Nanoparticles and Microspheres

In a typical synthesis, biochemical grade (>98% pure) glutathione (0.500 g, 1.626 mmol) was added to a stirred solution of ZnCl2 (0.074 g, 0.542 mmol) in deionized water (35 ml). Analytical grade ethylenediamine (en, about 5 ml) was then added to the reaction mixture and stirred for 15 min (about 34 ∘ C). The resulting solution was transferred to and sealed in a Teflon-lined autoclave, heated to 160 ∘ C for 10 h, and finally allowed to cool to room temperature. The effects of reaction parameters on the size and shape of the products were studied through altering the time, temperature, reactant molar ratio (GSH/Zn2+ ), and reaction medium. When en was replaced with ammonia or sodium hydroxide (NaOH) solutions, the pH values of the reaction system were adjusted to the same as that of en. The precipitate was collected by centrifugation (10 000 rpm, 5 min), washed alternately with small amounts of deionized water and ethanol, and dried in air at 50 ∘ C for 1 h. 2.4.2.2 Characterization

The products were characterized by power XRD on a Bruker D8 Advance diffractometer using Cu K𝛼 radiation (𝜆 = 1.5406 Å) and operating at 40 kV × 40 mA. The Raman spectra of ZnS products were directly recorded with a RM2000 Microscopic Confocal Raman spectrometer (Renishaw Corp., U.K.) using the 514.5-nm line of an Ar ion laser in air at room temperature. TEM images were obtained with a JEOL TEM-1200EX transmission electron microscope using an accelerating voltage of 120 kV. TEM samples were prepared by spreading one drop of the ZnS colloid suspended in ethanol–water solution (at a volume ratio of 1 : 1) at a concentration of 0.1 mg ml−1 onto standard copper grids covered with perforated carbon films, and dried in air overnight. FT-IR spectra were measured with a NICOLET 560 Fourier Transform Infrared Spectrophotometer. HRTEM measurement was carried out on a JEM 2010 high-resolution transmission electron microscope using an accelerating voltage of 200 kV. UV–vis spectra were recorded on a UV–vis spectrophotometer (UV-2102 PC, UNICO Corp., China). PL spectra were recorded using a FP-6500 fluorescence spectrophotometer (JASCO Corp., Japan). 2.4.2.3 Structure

Monodisperse ZnS particles could be prepared using the ZnCl2 /GSH (glutathione)/aqueous amine under hydrothermal conditions, where GSH acts as a zinc chelating agent and sulfur donor. A summary of the results of the experiments is given in Table 2.6. As can be seen from Table 2.6 and Figures 2.62–2.64, the sizes of the microspheres can be tuned by varying the different parameters of the experiment, including GSH/Zn2+ molar ratio, solvent, reaction temperature, reaction time, and so on. Figure 2.62 shows the TEM and HTREM images of as-prepared product ZnS-1 microspheres. The average diameter of ZnS spheres

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

Table 2.6 Reaction conditions and sizes of the as-synthesized samples. GSH/Zn2+ molar ratio

Solvent

Temperature (∘ C)

Time (h)

Sphere size (nm)

ZnS-1

3:1

en-aqueous

160

10

∼597

ZnS-2

1.5 : 1

en-aqueous

210

10

∼377

ZnS-3

1.5 : 1

NH3 -aqueous

160

10

∼254

ZnS-4

1.5 : 1

NaOH-aqueous

160

10

Nanoparticles

Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.

(a)

(b)

(d)

(c)

d = 0.142 nm (104)

1 μm

50 nm

10 nm

Figure 2.62 (a–c) TEM images and (d) HRTEM image of ZnS-1 microspheres prepared at GSH/Zn2+ molar ratio = 3 : 1, T = 160 ∘ C, and t = 10 h in en-aqueous solution. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.) (b)

(a)

(c)

(d)

d = 0.142 nm (104)

1 μm

50 nm

10 nm

Figure 2.63 (a–c) TEM images and (d) HRTEM image of ZnS-2 microspheres prepared at GSH/Zn2+ molar ratio = 1.5 : 1, T = 210 ∘ C, and t = 10 h in en-aqueous solution. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

obtained from the TEM images is ∼597 nm. It is noticeable that the surface of the spheres is not smooth but rough (Figure 2.62b). The ED pattern (the inset of Figure 2.62c) confirms the ZnS-1 microspheres to be polycrystalline. The lattice fringes (d = 0.142 nm) agree well with the separation between the (104) lattice planes (Figure 2.62d, the boxed area in Figure 2.62c). These data confirm that the ZnS-1 microspheres consist of particles. Figure 2.63 shows the TEM and HRTEM images of ZnS-2 microspheres with an average diameter of ∼377 nm, which were synthesized with a GSH/Zn2+ molar ratio of 1.5 : 1 in the en aqueous solution at 210 ∘ C for 10 h. The two-dimensional

85

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

(a)

(c)

(b)

(d)

d = 0.142 nm (104)

50 nm

500 nm

10 nm

Figure 2.64 (a–c) TEM images and (d) HRTEM image of ZnS-3 microspheres prepared at GSH/Zn2+ molar ratio = 1.5 : 1, T = 160 ∘ C, and t = 10 h in ammonia-aqueous solution. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

ZnS microsphere arrays can be obtained by spreading one drop of the ZnS-2 colloid suspended in ethanol-aqueous solution (volume ratio of ethanol to water = 1 : 1) onto standard copper grids covered with carbon films and dried in air overnight, which suggests that these ZnS microspheres can be applied to construct two- and three-dimensional photonic crystalline materials [185]. To investigate the function of en in the synthesis of ZnS microspheres, we also adjusted the pH of the reaction system using aqueous ammonia (NH3 ⋅H2 O) and sodium hydroxide (NaOH) solutions, respectively. ZnS-3 microspheres with an average diameter of ∼254 nm were obtained when ammonia was used (Figure 2.64), while ZnS-4 nanoparticles not microspheres, were generated when NaOH (aq) was employed (Figure 2.65). These facts suggest that both organic base (en) and inorganic base (ammonia) play similar roles in the formation of ZnS microspheres. The phase characteristics of the products were examined by powder XRD. Figure 2.66a–c shows the XRD patterns of as-synthesized ZnS-1, ZnS-2, and ZnS-3 microspheres, respectively. The diffraction peaks at 28.5, 47.5, and 56.4∘ Figure 2.65 TEM image of ZnS-4 nanoparticles prepared in NaOH aqueous solution with GSH/Zn2+ molar ratio = 1.5 : 1, T = 160 ∘ C, and t = 10 h. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

100 nm

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

Figure 2.66 XRD patterns of ZnS microspheres: (a) ZnS-1; (b) ZnS-2; (c) ZnS-3. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

(002)

(110)

(c)

(112)

(002) (110)

(100) (101)

(b)

(112)

(002) (a) (100)

10

20

(101) (110) (112)

30

40

50

60

70

80

90

2θ (°)

can be indexed to the (002), (110), and (112) planes of hexagonal wurtzite-type ZnS, respectively (JCPDS Card No. 36-1450). These results suggest that the wurtzite-type ZnS can be synthesized at temperatures ranging from 160 to 210 ∘ C in the presence of either en or ammonia. The structural information of the as-prepared ZnS microspheres synthesized in organic base (en) system and inorganic base (ammonia) system was further investigated using Raman spectroscopy (Figure 2.67). A characteristic property of the wurtzite-type ZnS is that the vibrational modes are highly isotropic, that

(b) 346

255

410 Intensity

Figure 2.67 Raman spectra of ZnS microspheres (a) ZnS-1 and (b) ZnS-3 recorded in air using the 514.5-nm line of an Ar ion laser as the excitation source. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

(a)

220

346

255

410

700

600

500

400

150

220180150

300

Wavenumber (cm–1)

200

100

87

88

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

is A1 (transverse) = E1 (transverse) and A1 (longitudinal) = E1 (longitudinal) [186]. Figure 2.67a,b shows the Raman spectra of ZnS-1 and ZnS-3 microspheres, respectively. As shown in Figure 2.67a, there are two intense peaks at 346 and 255 cm−1 , which do not exactly match the corresponding LO and TO phonon modes of wurtzite ZnS (352 and 274 cm−1 , respectively) [186]. It was reported that small particles could result in frequency shifts of the maxima lines as compared with the massive crystals [187]. In the case of a large surface-to-volume ratio, surface scattering contributes more to the Raman signal than the volume scattering. Therefore, the frequency shifts of LO and TO modes observed in this study may be attributed to the smaller size and larger surface-to-volume ratio compared with the bulk ZnS. Several weak bands around 150, 180, 220, and 410 cm−1 may also be explained by the small size of the as-synthesized ZnS [186]. When en was replaced with ammonia (Figure 2.67b), two identical intense Raman peaks appeared at the same positions as those shown in Figure 2.67a, which suggests that the same structure is obtained independent of the nature of the added base. A slight difference exists in the weak bands between Figure 2.67a,b, which may be a consequence of the different size of the products. Thus, the Raman spectra provide further evidence that the wurtzite-type ZnS can be prepared using en or ammonia to control the pH. The as-prepared ZnS microspheres with different diameters were also characterized using FT-IR spectrophotometer (Figure 2.68). The sharp peak near 1620 cm−1 is assigned to the N–H deformation vibration that demonstrates the existence of en or ammonia on the surface of microspheres. In the case of en, the as-prepared ZnS microspheres with –NH2 group adsorbed on the surface can be utilized as a favorable matrix for the immobilization of –NH2 -containing biomolecules through the covalent binding [13]. Based on the results mentioned, we speculate that the present synthesis of wurtzite-type ZnS microspheres consists of a GSH-dominated nucleation process and en-/ammonia-dominated assembly process. The formation constants Figure 2.68 FT-IR spectra of ZnS microspheres: (a) ZnS-1; (b) ZnS-2; (c) ZnS-3. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

(a)

(b) (c)

N–H Deformation vibration

4000 3500 3000 2500 2000 1500 1000 Wavenumber (cm–1)

500

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

(K 1 ) of the GS− -Zn2+ and the en-Zn2+ (or ammonia-Zn2+ ) complexes are as follows [188]: GSH + Zn2+ → GS− − Zn2+ + H+ , K1 = 2.00 × 108 CH2NH2

(2.30)

CH2NH2 +

Zn2+

Zn2+

CH2NH2

K1 = 5.89 × 105

CH2NH2

(2.31) NH3 + Zn

2+

2+

2

→ Zn(NH3 ) , K1 = 2.34 × 10

(2.32)

It is obvious that GS− -Zn2+ complex is more stable than en-Zn2+ or ammoniaZn2+ , due to its larger formation constant. The GSH ligand also functions as the source of sulfur. ZnS nuclei can be formed after decomposing the GS-Zn2+ complex. This means that the nucleation process is controlled by GSH. Subsequently, the second ligand such as en or ammonia is adsorbed onto the surface of the incipient ZnS nuclei and thereby hinders the further growth of the nuclei. On the contrary, the en-/ammonia-coated ZnS NPs were assembled into the larger and monodisperse ZnS spheres. Thus, the monodispersity of ZnS microspheres is primarily determined by the ligands such as en and ammonia. Adjusting the content of en without changing other reaction parameters will directly determine the morphology of the products (TEM images are shown in Figure 2.69). In the absence of en (the pH value of the reaction system is 3.0), the unprotected ZnS NPs agglomerated into the irregular bulk structures (Figure 2.69a), while the ZnS NPs were obtained when the pH value of reaction system was in neutral (pH = 7.0, by adding about 0.2 ml en) or weak basic (pH = 9.0, by adding about 0.26 ml en) [corresponding to Figure 2.69b,c, respectively]. The monodisperse ZnS microspheres can be obtained when the pH value of the reaction system is at ∼11.40 (by adding 5 ml of en). Increasing the content of en further to pH = 13.0 (by adding 20 ml of en), the irregular bulk structures were obtained again (Figure 2.69d). According to TEM and FT-IR

1 μm

(d)

(c)

(b)

(a)

200 nm

200 nm

1 μm

Figure 2.69 TEM images of ZnS prepared with GSH/Zn2+ molar ratio = 1.5 : 1, T = 160 ∘ C, and t = 10 h in en-aqueous solution with different pH values: (a) pH = 3.0 (in the absence of en); (b) pH = 7.0; (c) pH = 9.0; and (d) pH = 13.0, respectively. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

data, it further suggests that the hydrogen-bond interaction between N–H in ligands en or ammonia adsorbed on the ZnS particles favor the assembly and formation of microspheres. 2.4.2.4 Optical Properties

The optical properties of semiconductor materials are directly determined by the size and shape of the particles. We investigated the optical properties of ZnS microspheres using a combination of UV–vis and PL technologies. The UV–vis absorption properties of the ZnS samples were investigated by ultrasonically dispersing the ZnS microspheres into aqueous solutions (Figure 2.70). For example, the absorption spectrum of ZnS-1 microspheres (Figure 2.70a) shows a very broad adsorption peak between 340 and 800 nm in wavelength consisting of several shoulder peaks centered at 390, 425, 475, and 555 nm in wavelength, a feature likely to reflect the change in the scattering efficiency of the spheres as a function of wavelength [189]. According to Mie theory, dielectric spheres, with a radius comparable to the wavelength of light, are efficient scatterers and the scattering cross section exhibits a number of resonances for a given particle [190]. The observation of a resonance peak in the UV–vis spectrum also provides strong evidence for the narrow size distribution of the ZnS microspheres because the short-range Mie resonance features are only observed when spheres are sufficiently monodisperse [189, 191]. Similar resonance peaks are also observed for ZnS-2 microspheres (Figure 2.70b); a main band 2.5 Original time t=5d t = 10 d

2.0 1.5 1.0

Absorbance

2.5 Absorbance

0.5 (a)

Original time t=5d t = 10 d

2.0 1.5 1.0 0.5

200 300 400 500 600 700 800 Wavelength (nm) (b)

200 300 400 500 600 700 800 Wavelength (nm)

3.5 3.0 Absorbance

90

2.5

Original time t=5d t = 10 d

2.0 1.5 1.0 0.5 0.0

(c)

200 300 400 500 600 700 800 Wavelength (nm)

Figure 2.70 UV–vis spectra of ZnS microspheres (a) ZnS-1, (b) ZnS-2, and (c) ZnS-3 in aqueous solution. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

centers at 352 nm in wavelength, which is consistent with data reported in the literature [190], accompanied by a shoulder at 388 nm in wavelength. When en was replaced with ammonia, the absorption maximum of ZnS microspheres (ZnS-3) shifted to 338 nm in wavelength accompanied by a weak shoulder in a shorter wavelength (∼215 nm in wavelength, Figure 2.70c). The differences between the UV–vis absorption spectra of the three samples prepared under different conditions may be attributed to the different sizes and dispersivity of ZnS microspheres [190]. The stability of the ZnS microspheres was assessed by ripening the products for different times such as original time and 5 and 10 days. There are no significant changes in peak positions after storing the aqueous suspensions for 10 days at room temperature (Figure 2.70). Only minor changes of absorbance are observed, and these are attributed to the ripening process. The PL properties of the as-prepared ZnS-1, ZnS-2, and ZnS-3 microspheres were measured at room temperature (shown in Figure 2.71). A blue emission peak at ∼415 nm in wavelength accompanied by two weaker peaks at ∼355 and ∼466 nm in wavelength are observed when excited with a wavelength of 260 nm. A slight redshift (∼6 nm) is observed when different excitation wavelengths are employed (𝜆ex = 260, 270, 280, and 290 nm), which is attributed to the coupling action among the ZnS microspheres and interactions between the ZnS microspheres and their surrounding environment [192]. Hu and coworkers have 90

90

75

λex = 270 nm

60

λex = 280 nm λex = 290 nm

45 30

λex = 260 nm

75 PL intensity

PL intensity

λex = 260 nm

15

λex = 270 nm λex = 280 nm

60

λex = 290 nm

45 30 15

0 350 (a)

400 450 Wavelength (nm)

500

350 (b)

400 450 Wavelength (nm)

500

90 λex = 260 nm

PL intensity

75

λex = 270 nm λex = 280 nm

60

λex = 290 nm

45 30 15 350

(c)

400 450 Wavelength (nm)

500

Figure 2.71 Photoluminescence spectra of ZnS microspheres (a) ZnS-1, (b) ZnS-2, and (c) ZnS-3 in aqueous solution. (Wu et al. 2006 [184]. Reproduced with permission of American Chemical Society.)

91

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

reported ZnS nanobelts that display fluorescence emission at 401 nm and also present shoulder peaks at 470 nm when excited with a 250-nm light source [193]. Kar and Chaudhuri have recently reported the synthesis of wurtzite-type ZnS nanowires and nanoribbons that show an intense blue emission consisting of two bands at 394 and 456 nm in wavelength when excited also at 250 nm [194]. Moreover, it has also been reported that ZnS colloidal dispersions present blue emission peaks centered at 428 and 418 nm in wavelength, which were attributed to the sulfur vacancy and interstitial lattice defects [195]. On the other hand, the emission bands around 480 nm in wavelength have traditionally been ascribed to the well-known luminescence of zinc vacancies [193, 196]. In our studies, the strong blue emission bands around 415 nm in wavelength can be assigned to the sulfur vacancies or interstitial lattice defects, whereas zinc vacancies could be responsible for the weaker peaks around 466 nm in wavelength. Additional weak peaks appearing near 355 nm in wavelength (3.49 eV in photon energy) can be assigned to the near-band-edge (NBE) emission [197]. The observed blue fluorescence emission of the monodisperse wurtzite-type ZnS microspheres could provide an interesting application in the development of novel luminescent devices. 2.4.3 Ag2 S Nanospheres

Ag2 S can take three forms: monoclinic α-Ag2 S (stable up to 178 ∘ C), bcc β-Ag2 S (178 to ∼600 ∘ C), and face-centered cubic (fcc) γ-Ag2 S (above 600 ∘ C) [198]. Among them, monoclinic α-Ag2 S is a direct and narrow band-gap semiconductor with a band gap of ∼1 eV at room temperature, a relatively high absorption coefficient [199], and excellent optical limiting properties [200]. Ag2 S has found a wide range of applications in optical and electronic devices such as photoconductive cells [201], IR detectors [202], superionic conductors [203], and solar-selective coatings [204]. A considerable amount of effort has been made to generate Ag2 S nanoparticles and nanocrystals using different methods [205, 206]. Ag2 S nanoparticles and nanorods have previously been synthesized under the direction of biomolecules. Yang et al. fabricated protein-conjugated Ag2 S nanorods using thioacetamide (TAA) as the sulfur source in a BSA solution [207]. Brelle et al. synthesized cysteine- and glutathione-capped Ag2 S colloidal nanoparticles using Na2 S as the sulfur source [208]. Here, we introduce a cysteine-assisted hydrothermal route to the synthesis of Ag2 S nanospheres using L-cysteine [Cys, HSCH2 CH(NH2 )COOH] as the sulfur source and ligand and explain their optical properties [209]. 2.4.3.1 Synthesis

All of the chemicals were of analytical grade and used without further purification. Deionized water was used in the sample preparation. In a typical procedure, 0.271 mmol AgNO3 was added to a stirring solution of 0.271 mmol L-cysteine in 40 ml ethanol, which made the Cys/Ag+ molar ratio to be 1 : 1. After stirring for 15 min, the mixture was transferred into a 50-ml-capacity stainless Teflon-lined autoclave. The autoclave was sealed and heated to 180 ∘ C for 10 h, then allowed to cool to room temperature naturally. The resulting precipitate was centrifuged and washed using deionized water and absolute ethanol several times, and finally dried at 60 ∘ C for 6 h.

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

2.4.3.2 Characterization

Products were characterized using XRD pattern on a Bruker D8 Advance diffractometer using Cu K𝛼 radiation (𝜆 = 1.5406 Å) and operating at 40 kV × 40 mA. TEM images were obtained with a JEOL JEM-1200 transmission electron microscope at an accelerating voltage of 100 kV. The X-ray photoelectron spectroscopy (XPS) spectra were recorded on a PHI Quantera SXM with an Al K𝛼 = 280.00 eV excitation source, where the binding energies were calibrated by referencing the C1s peak (284.8 eV) to reduce the sample charge effect. UV–vis measurement was carried out on a UV–vis spectrophotometer (UNICO Corp., UV-2102 PC). PL spectra were recorded with a fluorescence spectrophotometer (PERKIN Elmer LS55). FT-IR spectra were measured with a NICOLET 560 Fourier transform infrared spectrophotometer. The size distributions were measured with Malven Mastersizer 2000 laser particle size analyzer. 2.4.3.3 Structure

Ag2 S nanocrystals can be prepared via a simple hydrothermal route where cysteine acts as sulfur donor and a silver chelating agent. Figure 2.72a shows the XRD pattern of Ag2 S nanocrystals. All diffraction peaks could be indexed to the monoclinic Ag2 S phase (JCPDS card: 14-0072). The diffraction peaks in Figure 2.72a were appreciably broadened, which indicated that the particle size of the product was small. The calculated size based on the XRD patterns using Scherrer’s equation was approximately 20 nm. Although monoclinic Ag2 S is stable up to 178 ∘ C, our results suggest that the monoclinic Ag2 S can be synthesized at 180 ∘ C in the presence of L-cysteine. Figure 2.72b and c shows the TEM images of the as-prepared Ag2 S nanospheres. The average diameter of Ag2 S nanospheres observed from the TEM images was ∼20 nm, which was consistent with that obtained from XRD patterns. It can be seen from the TEM images that Ag2 S nanospheres were not monodisperse but cross-linked together. In the reaction system, the amount of cysteine was excessive. The free thiol of the excessive cysteine molecule binds to the surface of Ag2 S nanospheres [208, 210]. In addition, hydrogen bonds and S–S bonds potentially form between cysteine molecules. Therefore, the 180 121

134

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Intensity (a.u.)

150

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125 nm

0 10 (a)

20

30

40 50 2θ (°)

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50 nm

80 (b)

(c)

Figure 2.72 (a) XRD pattern and (b), (c) TEM images of Ag2 S nanospheres prepared at 180 ∘ C with a Cys/Ag+ molar ratio of 1 : 1. n(Ag+ ) = 0.271 mmol. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

O1s

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Intensity (k counts/s−1)

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Ag4d Ag4p Ag4s S2p S2s C1s

Intensity (k counts/s−1)

Ag2 S nanospheres would cross-link together via hydrogen-bond [211] and S–S bond interactions. The aggregation could affect the optical properties of Ag2 S nanospheres [212]. To further evaluate the sample purity and composition, the XPS spectra of Ag2 S nanospheres were also studied. The XPS spectra (Figure 2.73a) show the presence of Ag and S as well as C and O impurities. The C1s (284.8 eV) was chosen as the reference line. Part of the oxygen in the spectra was attributed to adsorbed gaseous molecules. The spectra of C1s were composed of three peaks located at about 284.7, 285.6, and 288.6 eV as shown in Figure 2.723b, which could be attributed to the –C–C, –C–N, and –COO− structures, respectively [213]. All of the three structures exist in the L-cysteine molecule, which suggested that there were L-cysteine molecules absorbed on the surface of the as-synthesized Ag2 S nanospheres. However, the FT-IR spectra could not detect other impurities (Figure 2.74). So the content of cysteine on the surface of Ag2 S nanospheres was not abounding. The peaks at 367.74 and 373.68 eV in Figure 2.73c could be assigned to the binding energy of Ag3d5/2 and Ag3d3/2 , respectively [214]. The doublet feature of the Ag3d spectrum was due to the spin–orbit separation. The spectrum of S2p shown in Figure 2.73d could be described as three peaks located at about 160.7, 161.9, and 163.7 eV, respectively. The peaks at 160.7 eV and 161.9 eV could be assigned to the binding energy of S2p3/2 and S2p1/2 , correspondingly, which was separated by a spin-orbit splitting of 1.2 eV [215]. And, the peak at 163.7 eV could be assigned to the elemental sulfur [216], which proved the presence of thiol, namely, cysteine molecule, on the surface of Ag2 S nanospheres.

Intensity (k counts/s−1)

94

280 285 290 Binding energy (eV)

295

S2p3/2

4000 S2p2/1

3500 3000 2500 2000 1500

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(c)

365 370 375 380 Binding energy (eV)

385

155

(d)

160 165 170 Binding energy (eV)

175

Figure 2.73 XPS spectra of (a) survey, (b) C1s, (c) Ag3d, and (d) S2p of Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 1 : 1. n(Ag+ ) = 0.271 mmol. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

a: Cysteine b: Ag2S Transmittance

Figure 2.74 The FT-IR spectra of (a) cysteine and (b) Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 1 : 1. 1585 cm−1 : carboxylate stretch vibration; 2551 cm−1 : S–H vibrational band; 2964 cm−1 : N–H stretch vibration. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

a b 2551 2964

1585 0

1000

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4000

Wavenumbers (cm–1)

The content of elemental sulfur was about 0.3 based on their peak areas. The actual Ag/S atomic ratio on the surface of the nanospheres was about 1.5 : 1, which indicated that sulfur is excessive on the surface of the product. It could be deduced that S–S bonds might be formed on the surface of the products. 2.4.3.4 Optical Properties of Ag2 S Nanospheres

50

2.2 2.1 2.0 1.9 1.8 1.7 1.6 1.5 1.4

Initial 24 h 48 h 72 h

PL intensity

Absorption (a.u.)

The size of the semiconductor nanocrystals can directly determine their optical properties. We investigated the optical properties of Ag2 S nanospheres using a combination of UV–vis and PL techniques. The UV–vis absorption properties were studied by ultrasonically dispersing the Ag2 S nanospheres into aqueous solutions. The spectrum of Ag2 S nanospheres showed a very broad absorption peak centered at about 515 nm (2.4 eV in photon energy) in wavelength (Figure 2.75a, top). The absorption spectrum was similar to that of Ag2 S nanocrystals reported by Zhang et al., which had a wide absorption band centered at 521 nm [217]. The broad absorption band was owing to the aggregation of nanospheres [212, 218]. The absorption band of the aggregated nanospheres was broad because different aggregate sizes would contribute

40 30

λex = 505 nm

λex = 500 nm

20 10 0

200 300 400 500 600 700 800 (a)

λex = 490 nm λex = 495 nm

Wavelength (nm)

600

(b)

700

800

900

Wavelength (nm)

Figure 2.75 (a) UV–vis spectra of Ag2 S nanospheres dispersed in aqueous solution after ripening for different times. (b) Photoluminescence spectra of Ag2 S nanospheres dispersed in aqueous solution. Cys/Ag+ = 1 : 1 (molar ratio), n(Ag+ ) = 0.271 mmol, 180 ∘ C. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

95

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

different absorption subbands [218]. A similarly broadened spectrum centered at about 467 nm has also been observed for Ag2 S dendrites [219]. Compared with bulk silver sulfide (Eg = 1 eV) [220], the absorption spectrum of as-prepared nanospheres exhibits a large blueshift, which suggests that the nanospheres are behaving within the quantum confined regime. We also investigated the stability of Ag2 S nanospheres by ripening the products over different lengths of time. Figure 2.75a displays the results by tracking the UV–vis absorption spectra of Ag2 S nanospheres over 72 h at room temperature. There were no significant changes in peak positions after storage of the aqueous suspension for 3 days at room temperature. The minor changes in absorbance can be attributed to the ripening process [184]. Figure 2.75b shows the PL spectra of Ag2 S nanospheres examined at room temperature. The PL spectrum shows an emission peak centered at ∼637 nm in wavelength accompanied by a weaker shoulder at ∼590 nm in wavelength when the sample was excited with a wavelength of 490 nm. A redshift (∼17 nm) was observed when different excitation wavelengths were employed (𝜆ex = 490, 495, 500, 505 nm), which could be attributed to coupling between Ag2 S nanospheres and interactions between the Ag2 S nanospheres and their surrounding environment [184, 192]. Yang et al. once reported that Ag2 S nanorods exhibit a PL emission peak at about 474 nm with the excitation wavelength of 378 nm [208]. A PL peak at ∼490 nm for Ag2 S clusters in zeolite was observed by Leiggener and Calzaferri, which depended on the co-cations [221]. The PL spectrum of Ag2 S/MWNTs (multiwalled carbon nanotubes) showed a strong peak at 398 nm and a shoulder band in the range of 300–450 nm [222]. Zhu et al. reported that polyacrylamide–Ag2 S nanocomposites showed an emission peak centered at ∼540 nm and the peak width was broadened due to a wider size distribution of Ag2 S particles [223]. In our study, the PL spectra might result from the recombination of electrons and holes in the surface state of silver sulfide [222]. The PL intensity was not very strong, which might be owing to the trapped states [224]. 2.4.3.5 Growth Mechanism

The effect of reactants’ concentrations and reaction temperatures on the sizes of Ag2 S nanospheres was also studied. The XRD patterns (Figures 2.76 and 2.77) confirmed the samples to be monoclinic Ag2 S. In order to investigate the Cys/Ag+ molar ratios’ effect on the products, we carried out a group of experiments at 180 ∘ C. The TEM images (Figure 2.78) show that the diameters of Ag2 S nanospheres increase to ∼50, 63, and 120 nm with corresponding variations of molar ratio to 2 : 1, 4 : 1, and 8 : 1. The sizes obtained from XRD patterns were 23, 53, and 62 nm, respectively. The size obtained from XRD patterns is grain size, while the size observed from TEM images is particle size. One particle may contain one or several grains. Consequently, the size obtained from the TEM image may be bigger than that calculated from XRD patterns. Figure 2.79 showed the TEM images of Ag2 S nanospheres obtained at different concentrations and temperatures. The average diameter of Ag2 S nanospheres increased to about 47 nm with decreasing reactants’ concentrations, as shown in Figure 2.79. The electron diffraction pattern (inset in Figure 2.79a) confirmed that the Ag2 S nanospheres were single crystals. If the concentrations of the

(c)

101 110 111 111 012 112 120 022 121 121 031 103 200 103 014 212 131 123 041 213 223 204 105 015 231 134

Figure 2.76 The XRD patterns of as-prepared Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of (a) 2 : 1, (b) 4 : 1, and (c) 8 : 1. n(Ag+ ) = 0.271 mmol. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

Intensity (a.u.)

2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

(b) (a)

10

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40

50

60

70

80

60

70

80

Figure 2.77 The XRD patterns of Ag2 S nanospheres synthesized at different temperatures with a Cys/Ag+ molar ratio of 1 : 1: (a) 180 ∘ C; (b) 180 ∘ C; and (c) 200 ∘ C. (a): n(Ag+ ) = 0.136 mmol; (b–d): n(Ag+ ) = 0.542 mmol. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

Intensity (a.u.)

2θ (°)

(c)

(b) (a)

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30

40

50

2θ (°)

100 nm (a)

100 nm (b)

200 nm (c)

Figure 2.78 TEM images of as-prepared Ag2 S nanospheres at 180 ∘ C with a Cys/Ag+ molar ratio of (a) 2 : 1, (b) 4 : 1, and (c) 8 : 1. n(Ag+ ) = 0.271 mmol. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

100 nm

50 nm

(a)

(b)

50 nm (c)

Figure 2.79 TEM images of as-synthesized Ag2 S nanospheres with a Cys/Ag+ molar ratio of 1 : 1 at different reaction temperatures: (a) 180 ∘ C; (b) 180 ∘ C; and (c) 200 ∘ C. (a): n(Ag+ ) = 0.136 mmol; (b), (c): n(Ag+ ) = 0.542 mmol. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

reactants were increased, the size of Ag2 S nanocrystals was about 25 nm, as shown in Figure 2.79b. It can be seen from Figure 2.79c that the products tended to aggregate together when the temperature was increased to 200 ∘ C. The size distributions of the products were also studied using water as a dispersant (Figure 2.80–2.86). The average sizes of the samples obtained from size distributions were bigger than those obtained from XRD patterns, which might be due to further aggregation of nanoparticles when water is used as a dispersant. Smaller nanoparticles have higher surface energy [225], which may lead to the aggregation of the nanoparticles. Cysteine consists of –SH, –NH2 , and –COOH functional groups, which enable it to coordinate with inorganic cations and form high-affinity metal–ligand clusters [208, 211]. During the process of metal sulfide nucleation, cysteine could compete for metal binding sites and inhibit inorganic sulfide formation, which makes the metal–biomolecule complexes good matrixes [208]. Cysteine has been proved to be an excellent nucleating reagent for the synthesis of metal sulfide nanoparticles [208, 210]. Burford et al. reported that metal ions could react with cysteine to form complexes [226]. The reaction between cysteine and Ag+ is well 12

Figure 2.80 The size distribution of Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 1 : 1. n(Ag+ ) = 0.271 mmol d(0.5): median size. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

d(0.5): 63 nm

10 Quantity (%)

98

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2.4 Synthesis, Characterization, and Optical Properties of Sulfide Nanoparticles

12

d(0.5): 62 nm

10 Quantity (%)

Figure 2.81 The size distribution of Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 2 : 1. d(0.5): median size. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

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Figure 2.82 The size distribution of Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 4 : 1. d(0.5): median size. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

8 6 4 2 0 0.01

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1

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12

d(0.5): 60 nm

10 Quantity (%)

Figure 2.83 The size distribution of Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 8 : 1. d(0.5): median size. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

8 6 4 2 0 0.01

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Diameter (μm)

known in thiol chemistry [227]. Pakhomov et al. reported that cysteine and silver nitrate can form gelatin in an aqueous solution [228]. At least 17 monomeric complexes of Ag and cysteine can theoretically be distinguished in the solution due to the protonation of amino and thiol groups of cysteine [228, 229]. So in our reaction, Ag+ could chelate with cysteine to form initial precursor complexes. In the process of hydrothermal treatment, the S–C bonds were broken due to the

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

12

Figure 2.84 The size distribution of Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 1 : 1. n(Ag+ ) = 0.136 mmol; d(0.5): median size. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

d(0.5): 61 nm

Quantity (%)

10 8 6 4 2 0 0.01

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1

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12

Figure 2.85 The size distribution of Ag2 S nanospheres obtained at 180 ∘ C with a Cys/Ag+ molar ratio of 1 : 1. n(Ag+ ) = 0.542 mmol; d(0.5): median size. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

d(0.5): 63 nm

Quantity (%)

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0.1

1

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Figure 2.86 The size distribution of Ag2 S nanospheres obtained at 200 ∘ C with a Cys/Ag+ molar ratio of 1 : 1. n(Ag+ ) = 0.542 mmol; d(0.5): median size. (Xiang et al. 2008 [209]. Reproduced with permission of American Chemical Society.)

30 25 Quantity (%)

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high temperature [215]. The possible formation mechanism of Ag2 S nanospheres could be formulated as follows: chelation

Ag+ + L-cysteine −−−−→ [Ag(L-cysteine)n ]+ S–C bonds rupture

[Ag(L-cysteine)n ]+ −−−−−−−−−−−→ Ag2 S Δ

(2.33) (2.34)

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

The strong coordination between cysteine and Ag+ can decrease the formation rate of Ag2 S nanocrystals. In other words, the chelation between L-cysteine and Ag+ competes with Ag2 S crystal growth, leading to the controlled growth of Ag2 S nanocrystals. Furthermore, the growth rate in all directions is nearly the same under the influence of L-cysteine, which leads to the final morphology being spheres. In our reaction system, L-cysteine was excessive. The free thiol of the excessive cysteine molecule binds to the surface of Ag2 S nanoparticles [208, 210] and hydrogen bonds and S–S bonds form between cysteine molecules. The Ag2 S nanospheres cross-link together via hydrogen-bond and S–S bond interactions [211]. In the case of a high cysteine concentration or a rather low Ag+ concentration, much less Ag+ is present in solution to form seed crystals; therefore, the fewer the seeds the larger the final particles. As a result, the sizes of Ag2 S nanospheres increased with the variations in Cys/Ag+ molar ratio to 2 : 1, 4 : 1, and 8 : 1 and the size of Ag2 S nanospheres also increased with the decrease of Ag+ concentration.

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes 2.5.1 Fe2 O3 Nanocubes

Many synthetic chemical methods for preparing nanocubes have emerged, such as Ag [76], Pd [230], CoFe2 O4 [231], α-Fe2 O3 [127], and PbS [232] nanocubes. A major challenge for nanotechnology is the directed organization of individual nanoscale building blocks into nanodevices. Nanocubes with well-defined crystal structure and controlled orientation are important for future electronic and memory devices [233]. Hematite (α-Fe2 O3 ) nanocrystals with different sizes and shapes have been extensively studied [127, 234, 235]. Previous results show that nanocrystalline building blocks can be stabilized temporarily and then used for mesoscale transformation into complex morphologies [236, 237]. Here, we introduce an amino-acid-assisted hydrothermal technique for the preparation of cubic α-Fe2 O3 nanocrystals [62]. The amino acid [238], with functional groups, –NH2 and –COOH, is regarded as the key to controlled crystallization of the α-Fe2 O3 nanocubes. 2.5.1.1 Synthesis

The synthesis of cubelike α-Fe2 O3 nanostructures was obtained using a biomolecule-assisted hydrothermal technique. FeCl3 ⋅6H2 O (AR, 3 mmol) was dissolved in 20 ml of deionized water to form solution A. L-arginine(α-amino-δguanidovaleric acid, C6 H14 N4 O2 , >99% pure, 3 mmol or 27 mmol) was dissolved in 20 ml of deionized water to form solution B, and was then added dropwise into solution A, while stirring for 30 min at room temperature. The ratios of FeCl3 ⋅6H2 O to L-arginine are selected at 1 : 1 (denoted as Sample-1, Figures 2.87–2.89) and 1 : 9 (denoted as Sample-2, Figures 2.87–2.89). The mixture was sealed into a 50-ml Teflon-lined autoclave, heated to 180 ∘ C, and

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

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Figure 2.87 XRD pattern of the samples (a) Sample-1. (b) Sample-2. Raman spectra of (c) Sample-1. (d) Sample-2. (Cao et al. 2008 [62]. Reroduced with permission of American Institute of Physics.) (a)

(b)

(c)

d = 0.36 nm (012) d = 0.36 nm (012) 1 μm (d)

1 nm (e)

(f) d = 0.36 nm (012)

100 nm

d = 0.36 nm (012)

1 nm

Figure 2.88 (a) TEM image, (b) and (c) HRTEM images of Sample-1. (d) TEM image, (e) and (f ) HRTEM images of Sample-2. (Cao et al. 2008 [62]. Reroduced with permission of American Institute of Physics.)

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes 0.008 0.004

M (emu g–1)

M (emu g–1)

0.008

0.000

0.004 0.000 –0.004

–0.004 –0.008 –10 000 –5000

(a)

0 H (Oe)

5000

–10 000 –5000

10 000

(b)

0 H (Oe)

5000

10 000

Figure 2.89 Room-temperature hysteresis loops (M–H curves) of (a) Sample-1 and (b) Sample-2. (Cao et al. 2008 [62]. Reroduced with permission of American Institute of Physics.) (a)

200 nm

(b)

500 nm

(c)

50 nm

(d)

100 nm

Figure 2.90 TEM images of the samples synthesized (a) at 180 ∘ C for 10 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 3 (Sample-3), (b) at 180 ∘ C for 24 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 3 (Sample-4), (c) at 180 ∘ C for 24 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 9 (Sample-5), and (d) at 180 ∘ C for 48 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 9 (Sample-6). (Cao et al. 2008 [62]. Reroduced with permission of American Institute of Physics.)

maintained at this temperature for 10 h. After the autoclave was cooled down to room temperature naturally, the products were collected and washed with deionized water and then absolute alcohol. The cycle was repeated three times, followed by drying at 80 ∘ C for 2 h. Also, we can obtain cubelike α-Fe2 O3 nanostructures with different sizes via changing the reaction conditions, including reacting at 180 ∘ C for 10 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1 : 3 (Sample-3, Figures 2.90a and 2.91a), at 180 ∘ C for 24 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine a) 1 : 3 (Sample-4, Figures 2.90b and 2.91b) , at 180 ∘ C for 24 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1 : 9 (Sample-5, Figures 2.90c and 2.91c), and at 180 ∘ C for 48 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1 : 9 (Sample-6, Figures 2.90d and 2.91d). 2.5.1.2 Characterization

The phase of the products was characterized by an X-ray diffractometer (Rigaku, D/max-RB) with Cu K𝛼 (𝜆 = 1.5418 Å; 40 kV × 100 mA) radiation. Resonance Raman spectra (Renishaw, RM 1000) were measured with excitation from the 632.8-nm line of a He–Ne laser. The microstructure of materials was determined using TEM (JEOL, JEM-1200, operating at 120 kV), HRTEM (JEOL-JEM-2010 F

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures 4000

(113)

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40 50 2θ (°)

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Figure 2.91 XRD patterns of the samples synthesized (a) at 180 ∘ C for 10 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 3 (Sample-3), (b) at 180 ∘ C for 24 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 3 (Sample-4), (c) at 180 ∘ C for 24 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 9 (Sample-5), and (d) at 180 ∘ C for 48 h with the molal ratio of FeCl3 ⋅6H2 O to L-arginine 1: 9 (Sample-6). (Cao et al. 2008 [62]. Reroduced with permission of American Institute of Physics.)

Field Emission Electron Microscope, accelerating voltage of 200 kV) and scanning electron microscopy (SEM, KYKY-2000). The magnetic properties of the as-synthesized samples were studied with a vibrating sample magnetometer (VSM, LakeShore 7307) at room temperature. 2.5.1.3 Structure

XRD and Raman analysis were used to determine the crystal structure and quality, including the surface condition of the as-prepared samples. XRD analysis (Figure 2.87a,b) shows that the products have the rhombohedral phase of hematite (α-Fe2 O3 , JCPDS No. 33-0664). No characteristic peaks from other impurities, such as γ- or ε-Fe2 O3 , can be detected from the XRD data, which suggests that the as-synthesized products have high phase purity. Raman spectra (Figure 2.87c,d) exhibit bands at 224, ≈243, ≈291, 409, 495, ≈609, and ≈656 cm−1 in the range of 100–1000 cm−1 . The Raman peaks appearing at 224 and 495 cm−1 are attributed to the A1g mode, and those at ≈243, ≈291, 409, and ≈609 cm−1 are attributed to the Eg mode [127, 130]. The Raman peak appearing at ≈656 cm−1 is related to disorder effects and/or the presence of α-Fe2 O3 nanocrystals. This was also observed in another work of ours [127], and in the detailed study by Bersani et al. [134]. Most reference works on Raman

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

spectra of α-Fe2 O3 neglect the Raman peak appearing at ≈660 cm−1 [130–133], which is attributed to α-Fe2 O3 and not to the presence of traces of γ- or ε-Fe2 O3 . Both XRD and Raman analysis demonstrate that the as-synthesized products belong to the pure α-Fe2 O3 phase. The shape and structure of the α-Fe2 O3 nanoparticles are characterized by TEM and HRTEM. Figure 2.88a shows a bright-field TEM image of monodisperse cubelike α-Fe2 O3 (Sample-1) nanoparticles with a very uniform size and pseudo-cube edge length ≈846 nm. The HRTEM image (Figure 2.88b) and the boxed area in Figure 2.88c show the single-crystalline nature of the α-Fe2 O3 nanocube. The lattice fringes (d = 0.36 nm) observed in this HRTEM image agree well with the separation between the (012) planes in α-Fe2 O3 . According to the TEM observation, the as-synthesized α-Fe2 O3 nanocubes are almost monodisperse and the majority of the cubes have a smooth surface. Monodisperse magnetic nanocrystals are considered as key materials for next-generation multiterabit magnetic storage media [239, 240]. Figure 2.88d shows the TEM image of monodisperse cubelike α-Fe2 O3 nanoparticles (Sample-2) obtained by changing the ratio of FeCl3 ⋅6H2 O to L-arginine. The nanoparticles have a very uniform size and pseudo-cube edge length of ≈71 nm. This is about 10 times smaller than that of Sample-1. The single-crystalline nature of these nanoparticles is demonstrated by the HRTEM presented in Figure 2.88e,f. This sensitivity to reaction conditions — that is, the ratios of FeCl3 ⋅6H2 O to L-arginine, reaction times, and temperature imply that the α-Fe2 O3 nanocube reaction is driven by kinetic rather than thermodynamic control [135]. In fact, both the crystal size and shape are typically manipulated by controlling the growth kinetics [232]. The reaction between FeCl3 ⋅6H2 O and L-arginine α-amino-δ-guanidovaleric acid, H2 NC(NH)(CH2 )3 CH(COOH)NH2 , which is basic [241] is shown in Eq. (2.35): FeCl3 + 3H2 NC(NH)NH(CH2 )3 CH(COOH)NH2 + 3H2 O → Fe(OH)3 + 3[H2 NC(NH)NH(CH2 )3 CH(COOH)NH+3 ]Cl−

(2.35)

α-Fe2 O3 nanocrystals are produced through a two-step phase transformation [Eq. (2.36)] [122, 127]: Fe(OH)3 → β-FeOOH → α-Fe2 O3 (phase transformation)

(2.36)

Here, the amino acid (L-arginine), with functional groups, –NH2 and –COOH, has a great influence on the size and shape of the final α-Fe2 O3 nanocrystals. By reducing the ratio of FeCl3 ⋅6H2 O to L-arginine (Sample-2), the number of available functional groups, –NH2 and –COOH, increases. This enhances the interaction between the amino-acid and the β-FeOOH surface via hydrogen bonds, which suppress the β-FeOOH nuclei assembly and growth. This leads to smaller sized α-Fe2 O3 nanocrystals (Sample-2), as compared with Sample-1. Shape and size control of nanocrystals has received great attention in recent years due to the strong correlation between the chemical, physical, optical, electronic, and magnetic properties. The magnetic properties of the α-Fe2 O3 nanocubes with different sizes can provide great insights into the fundamentals

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

of nanomagnetism. Cubic morphology with aspect ratio of almost 1 is magnetically quasi-isotropic, and, hence, the magnetic shape anisotropy should not have any effect on the cubic α-Fe2 O3 nanocrystals. So the difference in magnetic properties must come solely from the changes in sizes of the cubic α-Fe2 O3 nanocrystals [236]. 2.5.1.4 Magnetic Properties

Magnetic hysteresis measurements of the cubic α-Fe2 O3 nanocrystals were carried out using a vibrating sample magnetometer at room temperature. The hysteresis loops (M–H) of the two samples (Sample-1 and Sample-2) do not reach saturation, even at the maximum applied magenetic field (Figure 2.89). Field-dependent magnetization plots illustrate that the two M–H curves show hysteresis features. The remanent magnetization (Mr ), the squareness (Mr /Ms ), and the coercivity (H c ) values are 0.00325 emu g−1 , 0.4869, and 4212.6 Oe for Sample-1 and 0.024 emu g−1 , 0.2663, and 288.95 Oe for Sample-2, respectively. These raw data are presented in electromagnetic units per gram of sample. The saturation and remnant magnetization have shown an increase with increasing volume. Such increases have usually been regarded as a decreasing proportion of the pinned surface magnetic moments in overall magnetization as the nanocrystals grow in size [236]. An interesting phenomenon is observed in that the coercivity of Sample-1 is almost 15 times larger than that of Sample-2, while the Mr and Mr /Ms of Sample-1 are 1.4 and 1.8 times larger those of Sample-2. The size (edge length) of Sample-1 is over 10 times larger than that of Sample-2. The size-dependent magnetic properties of the cubic α-Fe2 O3 nanocrystals are distinguishable. Coercivity has to be considered together with the surface pinning of magnetic moments and the resulting surface anisotropy [236, 242]. It is well known that the coercivity force and squareness are dependent not only on the material itself but also on the variations in the crystal structure. High-coercivity materials have become one of the key materials in high-technology development [138]. There are two routes for enhancing coercivity, (i) enhancing the resistance of domain rotation whose prerequisite is a single domain particle and increasing the material’s magnetic anisotropy and (ii) enhancing the resistance of the domain wall displacement via enhancing the undulatory distribution of internal stress and increasing the volume concentration of impurity. The coercivity increase of Sample-1 as compared with Sample-2 may be attributed to the increase in the resistance of domain rotation [243]. This communication indeed demonstrates that chemical synthesis is an important tool to tailor the physical properties of materials; furthermore, biomolecules can help in constructing nanostructures. 2.5.2 Fe3 O4 Nanocubes

Magnetite (Fe3 O4 ), as the oldest and one of the most important magnetic materials, has aroused great interest for various applications such as low-field magnetic separation [244], LIBs [245], mimetic enzymes [246], a dual imaging probe for cancer [247], and two-photon fluorescence indicator [248]. Different morphologies and structures of Fe3 O4 nanostructures have been successfully

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

synthesized, such as nanoparticles [249], binary nanoparticle superlattices [250], hollow nanospheres [251], nanoprisms [252], nanowires [253], nanotubes [254], nanoflowers [255], and nanocubes [256]. The uniform size and shape of the nanostructures make it possible to distinguish the properties inherent to the material from other effects. So it is still great challenge to develop novel synthesis methods for generating monodisperse Fe3 O4 nanocrystals. Here, we introduce a simple synthesis of Fe3 O4 nanocubes by an amino-acid-assisted solvothermal process [257]. 2.5.2.1 Synthesis

FeCl3 ⋅6H2 O (analytical reagent, AR), L-serine [CH2 (OH)CH(NH2 )COOH]) (purity: >99%, Beijing Kebio Biotechnology Co., Ltd), glycol (AR, Beijing Chemical Works), and hexamethylenetetramine (HMT, also called urotropine, C6 H12 N4 , (CH2 )6 N4 , AR, Sinopharm Chemical Reagent Co., Ltd) were used without further purification. In a typical synthesis, 1 mmol FeCl3 ⋅6H2 O was dissolved in 15 ml of glycol with stirring for 10 min to form solution A. A 1.5 mmol hexamethylenetetramine and 1 mmol L-serine acid was dissolved in 25 ml H2 O with stirring for 10 min to form solution B. Solution B was added into stirred solution A in 30 min at room temperature. The resulting mixture was transferred to and sealed in a Teflon-lined autoclave, heated to 200 ∘ C, and maintained at this temperature for 10 h. After the autoclave was cooled down to room temperature naturally, the products were collected via a centrifugal method washing with deionized water and enthanol for three-time cycling, followed by drying at 80 ∘ C for 8 h. 2.5.2.2 Characterization

The X-ray powder diffraction measurement was carried out on an X-ray diffractometer (Druker D8 Advance) with Cu K𝛼 radiation (𝜆 = 1.54056 Å) in a 2𝜃 range from 10∘ to 80∘ . TEM and HRTEM measurements were carried out on a JEOL JEM-2010 electron microscope, operating at 200 kV. FT-IR spectra were obtained on a Nicolet 560 FT-IR spectrophotometer. Raman spectra (Renishaw, RM 1000) were measured with excitation from the 514-nm line of an Ar ion laser with a power of about 5 mW. 2.5.2.3 Magnetic Behavior Measurement

Magnetic properties of the sample were measured using a Physical Property Measurement System (PPMS-9 T) with temperature capabilities of 5–300 K and magnetic field up to 500 Oe for measuring the magnetization (M). 2.5.2.4 Electrochemical Measurement

Electrochemical experiments were performed using CR 2032-type coin cells assembled in an argon-filled glove box (MBRAUN). The working electrode was prepared by mixing the Fe3 O4 nanocubes and carboxymethyl cellulose sodium (CMC, 3%) at a weight ratio of 90 : 10, followed by pasting on pure Cu foil (15 μm). Celgard 2400 was used as a separator. Lithium foil was used as the counter electrode. The electrolyte consisted of a solution of LiPF6 (1 M) containing vinylene carbonate (2%) in ethylene carbonate/dimethyl carbonate/diethyl

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

carbonate (1 : 1 : 1, volume ratio). A galvanostatic cycling test of the assembled cells was carried out on a BS-9300 K system in the voltage range of 0.001–3.0 V (vs. Li+ /Li) at current density of 0.2 C (200 mA g−1 ), 0.5, 1.0, 2.0, and 5.0 C, respectively. The weight of Fe3 O4 nanocubes in the working electrode was used to estimate the specific discharge capacity of the LIBs, which was expressed in mA⋅h•g−1 of Fe3 O4 nanocubes. 2.5.2.5 Structure

The XRD pattern of the as-synthesized sample presents eight characteristic peaks at 18.31∘ (111), 30.36∘ (220), 35.77∘ (311), 43.35∘ (400), 53.76∘ (422), 57.16∘ (511), 62.98∘ (440), and 74.47∘ (533) (Figure 2.92), which correspond with the standard XRD data card of Fe3 O4 (JCPDS card. No. 85-1436). The Fe3 O4 nanocubes are approximately 14 nm in edge length of nanocubes, according to the Scherrer equation [258]. The strongest diffraction peak is on (311) lattice plane. D(311) = 𝜅𝜆/B cos 𝜃, where D, 𝜅, 𝜆, B, and 𝜃 represent the size of Fe3 O4 nanocubes vertical to the analyzed lattice plane with the Miller indices (hkl), the constant factor (= 0.9), the wavelength of the X-ray applied in the experiment, the peak width at half intensity (in a 2𝜃 intensity plot) of the (311) diffraction peak, and the Bragg angle corresponding to the (311) diffraction peak, respectively. Under ambient conditions, the Raman spectrum of the as-synthesized sample shows four peaks – at 215, 280, and 500 cm−1 , and a broad and main peak at 680 cm−1 (Figure 2.93). The main peak at 680 cm−1 is the characteristic peak for magnetite, attributed to A1g mode, that is, symmetric stretch of oxygen atoms along Fe–O bonds [259]. The weak peaks at 215 and 500 cm−1 are attributed to T2g (1) mode (translator movement of the whole FeO4 unit) and T2g (2) mode (asymmetric stretch of Fe and O), respectively [260]. And, the weak peak at 280 cm−1 is attributed to Eg mode (symmetric bends of oxygen with respect to Fe) [261]. The Raman data further demonstrate that the as-synthesized sample is pure Fe3 O4 phase. Figure 2.94 presents the FT-IR spectrum of the as-synthesized Fe3 O4 nanocrystals, which demonstrates that the Fe3 O4 nanocrystals are free of organic contaminants, such as L-serine and urotropine. The peaks at 1381, 1631, and 3404 cm−1 Figure 2.92 XRD pattern of Fe3 O4 nanocubes. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

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2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

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Figure 2.94 FT-IR spectrum of Fe3 O4 nanocubes. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

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are attributed to Fe3 O4 , CO2 , and H2 O, respectively [262]. The peak at 582 cm−1 corresponds to the Fe–O bond of Fe3 O4 phase [263]. Figure 2.95 shows the crystal structure of Fe3 O4 . The relatively large O2− ions form an fcc-lattice and Fe cations occupy the interstitial tetrahedral position [264]. Fe3 O4 (≡ FeO⋅Fe2 O3 , [Fe3+ ]tet [Fe2+ ,Fe3+ ]oct O4 ) belongs to the inverse spinel structure ([B]tet [A,B]oct O4 ) (A-octahedral sites and B-tetrahedral and octahedral sites), where Fe3+ (d5 ) has no crystal field stabilization energies, (CFSEs) in either octahedral or tetrahedral sites; Fe2+ (d6 ) has a preference for Figure 2.95 Crystal structure of Fe3 O4 . (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

(a)

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Figure 2.96 (a) TEM image, (b) and (c) HRTEM images of as-synthesized Fe3 O4 nanocrystals. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

octahedral sites [45]. The Fe2+ and Fe3+ at octahedral sites are close together due to edge-sharing octahedra; thus, positive holes can migrate easily from Fe2+ to Fe3+ ions. Fe3 O4 is a good conductor, which is usually attributed to a half-metal with the highest known Curie temperature (858 K, 523 ∘ C) [265]. Fe3+ ions are in a state with spin S = 5/2 and zero orbital moment, while Fe2+ ions have a spin of 2, so Fe3+ and Fe2+ should contribute 5𝜇B and 4𝜇B , respectively. Magnetic order at the A and B sites is antiparallel, leading to ferromagnetic order with an excess magnetic moment of about 4𝜇B per formula and it has a high Curie temperature of 858 K [266]. The morphology and microstructure of the as-synthesized Fe3 O4 was studied by TEM and HRTEM (Figure 2.96). The TEM image shows the Fe3 O4 are cube-shaped structures. The edge length of cube-shaped Fe3 O4 nanocrystals is approximately 16 nm (Figure 2.96a), which is close to the size of D(311) calculated on the basis of the Scherrer formula. More information about the crystal can be derived from the HRTEM images taken on the Fe3 O4 nanocubes (Figure 2.96b,c). The lattice fringes (d = 0.30 nm) observed in the HRTEM image agree well with the separation between the (220) planes of Fe3 O4 . 2.5.2.6 Growth Mechanism

In order to understand the crystal growth process, a series of experiments were performed. The first study was the effect of the reaction time. Figure 2.96 and Figures 2.97a, 2.98a, and 2.99a show TEM images of the products synthesized in identical concentrations (Fe3+ concentration = 3 mmol/40 ml, L-serine concentration = 18 mmol/40 ml, HMT concentration = 1.5 mmol/40 ml) and the identical reaction temperature of 200 ∘ C, but for different reaction times, that is, 30 min, 1 h, 5 h, and 10 h, with these being denoted as Fe3 O4 -1, Fe3 O4 -2, Fe3 O4 -3, and Fe3 O4 -4 (i.e., the typical Fe3 O4 characterized by Figures 2.92–2.94 and Figures 2.96 and 2.103–2.105 in the text), correspondingly. All these samples belong to Fe3 O4 based on XRD analyses (Figures 2.97b, 2.98b, and 2.99b and Figure 2.92). However, the morphology and size are different. The samples Fe3 O4 -1 (Figure 2.97a) and Fe3 O4 -2 (Figure 2.98a) are nanoparticles with sizes of about ∼5 nm and ∼7.5 in diameter. Both Fe3 O4 -3 (Figure 2.99a) and Fe3 O4 -4 (Figure 2.96) are nanocubes with ∼10 and ∼16 nm in edge length, respectively. The phenomenon of enlarged sizes along with prolonging the reaction time is attributed to the Ostwald ripening process, in which larger particles are energetically favorable to growing at the expense of smaller, less stable particles [73]. When we carried out a similar experimental parameter but without L-serine, we

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

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Figure 2.97 (a) TEM image and (b) XRD pattern of the product synthesized in identical concentrations (Fe3+ concentration = 3 mmol/40 ml, L-serine concentration = 18 mmol/40 ml, HMT concentration = 1.5 mmol/40 ml) at 200 ∘ C for 30 min. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.) 60

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Figure 2.98 (a) TEM image and (b) XRD pattern of the product synthesized in identical concentrations (Fe3+ concentration = 3 mmol/40 ml, L-serine concentration = 18 mmol/40 ml, HMT concentration = 1.5 mmol/40 ml) at 200 ∘ C for 1 h. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.) (311)

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Figure 2.99 (a) TEM image and (b) XRD pattern of the product synthesized in identical concentrations (Fe3+ concentration = 3 mmol/40 ml, L-serine concentration = 18 mmol/40 ml, HMT concentration = 1.5 mmol/40 ml) at 200 ∘ C for 5 h. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

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Figure 2.100 (a) TEM image and (b) corresponding XRD pattern of the sample synthesized with similar experimental parameters but without using L-serine. The XRD pattern matches well with α-Fe2 O3 (JCPDS No. 79-0007). (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

just obtained a red product of rice-shaped α-Fe2 O3 nanocrystals (Figure 2.100), not black Fe3 O4 . Our previous work demonstrated that α-Fe2 O3 is formed in glycol-NH3 •H2 O system at 280 ∘ C under solvothermal treatment [127]. Even if glycol is the reducing agent, no Fe3 O4 can be observed, because Fe(OH)3 is easily formed via the deposition of free Fe3+ in NH3 •H2 O environment. Then, Fe(OH)3 is transformed into β-FeOOH, followed by α-Fe2 O3 through a phase transformation. Hou and coworkers demonstrated that α-Fe2 O3 cubic particles can be generated by a hydrothermal synthesis at 130 ∘ C from a solution of urotropine and ferric chloride. Urotropine slowly decomposed to generate formaldehyde and OH− . The OH− ions favor the deposition and crystallization of Fe3+ to form cubic α-Fe2 O3 particles, not Fe3 O4 particles [267]. This phenomenon can be attributed to free Fe3+ ions deposited with OH− ions (from the decomposed product of urotropine). This experiment suggests that free Fe3+ ions are not protected by L-serine via coordination at room temperature before the solvothermal process. However, when we carried out a similar experimental parameter but without using urotropine, we obtained a small quantity of Fe3 O4 with a low yield of 16% (Figure 2.101), which is quite lower than our glycol-L-sersine-urotropine solvothermal system with a yield of over 91%. However, the size of the products is multidisperse particles with different shapes including small spherelike particles and big cubelike particles. The experiment demonstrates that glycol can react with Fe3+ and generate Fe3 O4 . Xi and coworkers demonstrated that ethylene glycol (= glycol) functions as both reductive agent and solvent, and reacts with Fe3+ ions at 200 ∘ C in a solvothermal system, leading to the generation of Fe3 O4 [268]. This experiment suggests that glycol can function as a reducing agent, which can reduce Fe3+ to Fe2+ , and Fe3+ ions are free under high temperature of 200 ∘ C. When we carried out a similar experimental parameter but without using glycol, we obtained Fe3 O4 with a yield of ∼93% (Figure 2.102). However, the size of the products is multidisperse particles with different shapes including small spherelike particles and big cubelike particles. This experiment suggests that urotropine functions as a reductive agent (due to the generation of CH2 O)

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

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Figure 2.101 (a) TEM image and (b) corresponding XRD pattern of the sample synthesized with similar experimental parameters but without using urotropine. The XRD pattern matches well with Fe3 O4 (JCPDS No. 85-1436). (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

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Figure 2.102 (a) TEM image and (b) corresponding XRD pattern of the sample synthesized with similar experimental parameters but without using glycol. The XRD pattern matches well with Fe3O4 (JCPDS No. 85-1436). (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

and reacts with Fe3+ ions at 200 ∘ C in a solvothermal system, leading to the generation of Fe3 O4 . The higher yield than that of the experiment without using urotropine indicates that urotropine is a stronger reductive agent than glycol. Based on these control experiments, we know that L-serine functions as a coordination agent which can protect Fe3+ ions to be free of reacting with OH− at room temperature. However, the coordination complex can release Fe3+ under heat treatment at 200 ∘ C. Both glycol and urotropine function as reductive agents, but the reducing ability of urotropine is stronger than that of glycol. The possible growth mechanism of Fe3 O4 in our glycol-L-serine-urotropine solvothermal system is composed of three stages. First, Fe3+ ions cooperate with L-serine (HSer) and generate Fe(HSer)3+ complex [269] through the carboxyl oxygen, not the nitrogen-containing group of the amino acid [270]. It is known that all amino acids can exist as zwitterion structure (A) at physiological pH, and

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RCHCOO–

RCHCOO–

NH+3

NH2

(a)

(b)

Figure 2.103 (a) Zwitterionic form and (b) anionic form of amino acid. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

the complex-forming species with Fe3+ is the anion (B) (Figure 2.103) [270]. The zwitterionic structure (A) has less net attraction for Fe3+ than does the anionic structure (B), which lacks this hydrogen bond, while strong intramolecular hydrogen bonds are invariably formed between these ammonium groups (–NH3 + ) and carboxylate ion (–COO− ) in zwitterion structure (A). Urotropine decomposes to give ammonia and leads to the generation of OH− . So the mixed solvent is a weak basic solution (pH = 7.02 measured using an acidometer PHSJ-3F), being favorable for the formation of anionic structure (B) of L-serine which has strong affinity to complex with Fe3+ . This was demonstrated by an experiment: after forming the mixed solution (i.e., solution A mixed with solution B), we added KOH solution into the mixed solution, and we did not find any precipitation, suggesting the absence of free Fe3+ in the mixed solution due to the formation of coordination ions of Fe3+ at room temperature. In the second stage, Fe3 O4 nuclei are formed. Fe(HSer)3+ will decompose into Fe3+ under 200 ∘ C (Eq. (2.37)). Both urotropine and glycol function as reductive reagents under solvothermal treatment at 200 ∘ C, which will lead to the formation of Fe2+ from Fe3+ (Eqs. (2.38), (2.40), (2.41)). After Fe2+ /Fe3+ ions react with OH− (Eq. (2.39)), Fe3 O4 nuclei will be generated (Eqs. (2.42)–(2.45)) [268]. The formation of Fe3 O4 is therefore believed to proceed via the following steps: Free Fe3+ ions were released from its complex compound: Fe(HSer)3+ ↔ Fe3+ + HSer

(2.37)

HCHO and OH− were generated from (CH2 )6 N4 : (CH2 )6 N4 + 10H2 O → 6HCHO + 4NH3 •H2 O NH3 •H2 O ↔

NH+4

−

+ OH

(2.38) (2.39)

Fe2+ ions were formed via the reaction between reducing agent and Fe3+ : Fe3+ + HCHO → Fe2+ + HCOOH Fe

3+

+ HOCH2 CH2 OH → Fe

2+

+ HOCH2 COOH [268]

(2.40) (2.41)

Thus, the co-deposition of Fe2+ /Fe3+ will occur with OH− and generate Fe3 O4 [271]: Fe2+ + 2OH− → Fe(OH)2 Fe

3+

−

+ 3OH → Fe(OH)3

Fe(OH)2 + 2Fe(OH)3 → Fe3 O4 + 4H2 O

(2.42) (2.43) (2.44)

The overall reaction is shown as follows: Fe2+ + 2Fe3+ + 8H2 O → Fe3 O4 + 4H2 O

(2.45)

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

Also, because glycol is a strong reducing agent with a relatively high point of 246 ∘ C [272], Fe3+ reacts with glycol under the solvothermal system at 200 ∘ C, which can directly generate Fe3 O4 nanoparticles [268, 273]. In the third stage, the newly formed Fe3 O4 nuclei tend to grow bigger on the nucleation sites and form Fe3 O4 nanocubes. 2.5.2.7 Magnetic Properties

It is known that magnetite is a half-metal with the highest known Curie temperature of 858 K, and its conduction is only attributed to one spin channel, while the other spin channel exhibits a gap at the Fermi level [265]. The d electron of solid transition metal compounds is sensitive to both the crystal structure and the oxidation state of the transition metal. Figure 2.104 shows the temperature-dependent magnetization at 500 Oe between 10 and 300 K using zero-field cooling (ZFC) and field-cooling (FC) procedures, that is, M–T curves. The ZFC and FC curves are usually used to understand the information of the energy barriers. The blocking temperature (T B ), estimated on the basis of the peak maximum in the ZFC curve [274], is shown at about 163 K (for H = 500 Oe). At T B , the energy of the aligned magnetic moments is balanced with the thermal energy k B T of the particles, that is, the direction of the magnetic moments of the particles begins to wobble about the direction and the moments become disordered [43]. Below the blocking temperature T B , given by T B = KV /k B ln(𝛼t/𝜏 0 ) (where KV is an activation energy to frisk the particle’s magnetization from the direction at 𝜃 = 0∘ to 180∘ or from the direction at 𝜃 = 180∘ to 0∘ , K is the constant which quantifies the energy density associated with this anisotropy, V is the volume of the particle, k B is the Boltzmann constant of 1.3807 × 10−23 J K−1 , t is the experiment measuring time, 𝜏 is the relaxation time, and 𝜏 0 is typically 10−9 s, α = 100 when 𝜏 is “much longer” than t), each magnetic particle keeps to be locked into one of its two minima (i.e., from 𝜃 = 0∘ to 180∘ or from 180∘ to 0∘ ) [275]. Below T B , the ZFC curve shows an increase as the moments progressively reorient along the applied field at low temperature from 10 to 163 K, while the FC curve shows a decrease at this low temperature. Also, a transition point at about 51 K can be observed appearing with discontinuous changes in FC and ZFC curves, corresponding to the signature of the Verwey transition temperature (T v ), a designation of chemistry purity in magnetite. FC ZFC

45 40

45 44

35

M (emu g−1)

M (emu g−1)

Figure 2.104 Zero-field-cooled (ZFC) and field-cooled (FC) curves for as-synthesized Fe3 O4 particles measured with the field of 500 Oe between 10 and 300 K. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

30 25

43

Tv~51 K TB~163 K

42 41

FC ZFC

40 39

20 0

50

100

0

50 100 150 200 250 300 T (K)

150 T (K)

200

250

300

115

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

For bulk Fe3 O4 , T v ≈ 120 K [276], above which fast electron hopping between the Fe2+ and Fe3+ ions on the octahedral B sites occurs. Lower T v compared with the bulk Fe3 O4 have been reported in spherical Fe3 O4 nanoparticles with mean particle size of 150 nm (T v = 98 K) and 50 nm in diameter (T v ≈ 20 K) [277], granular Fe3 O4 films (T v = 75 K) [278]. The smaller T v value of 51 K and more broadened transition T v can be attributed to the smaller grain size [278]. In fact, it is complex to understand the magnetization behavior of Fe3 O4 at low temperature, although Verwey proposed that the electron can hop between the Fe2+ and Fe3+ ions on the octahedral B sites in an inverse spinel structure in a thermally activated process [276]. It has been demonstrated that there is a structural transformation from cubic to triclinic phase at low temperature [279]. So the as-synthesized Fe3 O4 nanocrystal systems undergo a structural transition accompanied by a sudden change of the magnetic and transport properties at T v . Above the T v , Fe3 O4 has an inverse spinel structure with cubic symmetry, while below the T v , the cubic symmetry of Fe3 O4 crystal is broken by a small lattice distortion [280]. The large divergence temperature of ZFC and FC is about 300 K, which can be attributed to the large amounts of magnetic isotropy energy contributed by the external magnetic field during the cooling procedure. The M–H curve (Figure 2.105) measured at room temperature shows nearly no hysteresis loop, which suggests that the as-synthesized Fe3 O4 nanocrystals possess strong superparamagnetic character, that is, single-domain particles that orient as large individual magnetic moments in the applied magnetic field. The saturation magnetization (Ms ), remnant magnetization (Mr ), and coercivity (H c ) for the as-synthesized Fe3 O4 nanocrystals are 67 emu g−1 , 3.5 emu g−1 (nearly no remanence effect), and 35 Oe, respectively, while the Ms and H c of bulk Fe3 O4 are 85–100 emu g−1 and 115–150 Oe, respectively. This difference is attributed to the as-synthesized Fe3 O4 nanocrystals consisting of small particles [281]. The Fe3 O4 nanocubes could be removed from solution (Figure 2.105). The initial rust-colored solution contained Fe3 O4 nanocubes homogeneously dispersed in water (1 mg ml−1 ) (inset of Figure 2.105). Once a magnet was placed near the solution, the solution became clear within 6 min, and Fe3 O4 nanocubes kept close to the magnet at the vial wall, where the magnetic field gradient

Figure 2.105 Field-dependent magnetization curve at room temperature. And, the inset on the left is the photograph of the responsive performance of Fe3 O4 nanocubes to an external magnet, inset on the right is the magnified section of the hysteresis loop. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

60 40 20 0 –20

M (emu g−1)

M (emu g−1)

116

–40

60 40 20 0 –20 –40 –60

–160 –80 0 80 160 H (Oe)

–60 –40 000

RT

–20 000

0 H (Oe)

20 000

40 000

2.5 Synthesis, Characterization, and Magnetic Properties of Oxide Nanocubes

is the largest. This phenomenon suggests that the Fe3 O4 nanocubes can find application in low-field magnetic separation [244]. 2.5.2.8 Applied as Anode for LIBs

Charge 1st 2nd 5th 28th 29th 30th

2.5 2.0 1.5 1.0

Discharge

0.5 0.0 0

300

600

900

1200

Specific capacity (mA hg–1) Specific capacity (mA hg–1)

(a)

(c)

1200

Charge Discharge

1000 800 600

0.2 C

0.5 C

400

2.0 C

200

5.0 C

0 0

5

(b)

10 15 20 Cycle number

800 600 400 200 0

Charge Discharge

0

10

0.2 C

1.0 C

20 30 40 Cycle number

50

60

110 100 90 80 70 60 50 40 30 20 10 0

25

30

Coulombic efficiency (%)

3.0 Capacity (mA hg–1)

Potential (V vs Li+/Li)

The as-synthesized Fe3 O4 nanocubes are used as anode material for LIBs to study the electrochemical properties (Figure 2.106). Figure 2.106a shows the potential profiles for the 1st, 2nd, 5th, 28th, 29th, and 30th cycles of the Fe3 O4 /Li cell. The first specific discharge capacitance is as high as 1200 mA h g−1 . The phenomenon that the first discharge capacity exceeds the theoretical capacity of Fe3 O4 (926 mA h g−1 , based on the reaction 8Li+ + Fe3 O4 → 3Fe + 4Li2 O, assuming the reduction of Fe3+ and Fe2+ to Fe0 during the Li+ intercalation) [282] has been observed wildly for transition metal oxide electrodes, such as nanostructured CuO [283], Co3 O4 nanowires [284], Co3 O4 nanoparticles [285], Co3 O4 nanobelts [286], Co3 O4 microspheres [287], Fe3 O4 nanoparticles [288], and so on, which has been attributed to the large electrochemical active sites and/or grain boundary area of the nanostructured oxide particles, as well as irreversible reactions (i.e., electrolyte decomposition occurring during the first discharge cycle), or the reversible formation of a Li-bearing solid-electrolyte interface (SEI) [284–288]. The discharge capacities of the Fe3 O4 electrode in the 2nd, 5th, 28th, 29th, and 30th cycles are 838.6, 660.4, 389.4, 376.6, and 366.4 mA h g−1 , respectively.

Figure 2.106 Electrochemical performance of the Fe3 O4 nanocubes/Li cells: (a) the discharge–charge profiles in the voltage range 0.01–3.0 V at the current of 0.2 C rate; (b) the rate performance with the cycling rate of 0.2–5 C; and (c) the cycling performance at 0.2 C rate. (Cao et al. 2011 [257]. Reproduced with permission of American Chemical Society.)

117

118

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

In the first discharge, there is a steep voltage drop from 3 to ∼0.73 V, which can be attributed to the reaction (2.46) Fe3 O4 + xLi → Lix Fe3 O4

(2.46)

An obvious potential plateau at 0.73 V corresponds to the conversion reaction (2.47) Lix Fe3 O4 + (8 − x)Li → 3Fe + 4Li2 O

(2.47)

The sloping part of the discharge curve between 0.73 and 0 V can be assigned to the reaction process of Fe and electrolyte to form a gel-like film and inorganic SEI layer [289]. The Fe3 O4 electrodes exhibit high specific capacity in the first cycle, which can deliver 1200 mA h g−1 in the discharge process and 810 mA h g−1 in the charge process. Fe3 O4 is among a group of metal oxides that abide by the “conversion reaction” mechanism involving the formation and decomposition of Li2 O upon subsequent cycling, accompanied by the redox of metal nanoparticles. Thus, the electrochemical reaction mechanism of Li with Fe3 O4 nanocubes in LIBs can be described as follows: Fe3 O4 + 8Li+ + 8e− ↔ 3Fe0 + 4Li2 O +

8Li ↔ 8Li + 8e

−

(2.48) (2.49)

0

Fe3 O4 + 8Li ↔ 3Fe + 4Li2 O

(2.50)

This mechanism of Li reactivity is quite different from the classic Li insertion/deinsertion or Li-alloying mechanisms, the catchwords of the past 30 years [283, 290, 291]. The cycling response at various rates is shown in Figure 2.106b, which shows the discharge of the material at various rates after a slow charge and hold at 3V to fully charge the material. A rate of nanocoulomb corresponds to a full discharge in 1/n h [292]. At a 0.2 C rate (corresponding to a time of 5 h to fully discharge the capacity), the Fe3 O4 nanocubes discharge to an average 695.1 mA h g−1 , ∼75.1% of theoretical capacity of Fe3 O4 , while the Fe3 O4 nanocubes reach about 51 mA h g−1 at the highest rate tested (5 C), corresponding to a time of 720 s to fully discharge the capacity. The capacity decreased stepwise along with the rate increase. When the rate returned to 0.2 C, the material discharges to 360 mA h g−1 , ∼38.9% of theoretical capacity of Fe3 O4 . It is worth pointing out that the average specific capacitance of 695.1 mA h g−1 at a current rate of 0.2 C is markedly higher specific capacity compared with that of commonly used graphite electrode (372 mA h g−1 ), that is, about twice that of current carbon-based negative electrode [290]. The cycling performance is shown in Figure 2.106c, which measured the long-cycle characteristics up to 60 cycles at a current rate of 0.2 C. The Fe3 O4 nanocube electrode delivers a capacity of 418.4 mA h g−1 at the 10th cycle with electrode coulombic efficiency of 94.4%, while 332.5 mA h g−1 of specific capacity and corresponding electrode coulombic efficiency of 95.9%, 306.1 mA h g−1 with coulombic efficiency of 97.4%, 252.1 mA h g−1 with coulombic efficiency of 98.5%, 231.4 mA h g−1 with coulombic efficiency of 98.8%, and 221.9 mA h g−1

2.6 Synthesis, Characterization, and Photocatalytic Application of Microspheres

with coulombic efficiency of 99.1%, that is, approaching 100%, corresponding to the 20th, 30th, 40th, 50th, and 60th cycles (Figure 2.105c). The coulombic efficiency of the material is above 95% since the 11th cycle in the subsequent cycles, which concludes excellent electrochemical stability. The as-synthesized Fe3 O4 nanocubes exhibit a gradual fade in the capacity during cycling, retaining ∼70.4% of its initial capacity after 60 cycles. This suggests that the small nanocubes are initially active, but are unstable under the harsh reduction–oxidation conditions during the electrochemical cycles. The continuous disintegration of nanocubes is a result of the volume change on cycling. This cracking and crumbling of the nanocubes during cycling may keep on generating new active surfaces that were previously passiviated by the stable surface. These new active surfaces consume or trap lithium ions. Thus, the repeated reaction between Fe3 O4 and electrolyte leads to fade of the capacity. Similar phenomena have been observed for bare α-Fe2 O3 and commercial Fe3 O4 fading from 250 and 300 to 105 mA h g−1 (42% of initial capacity) and 152 mA h g−1 (50.7% of initial capacity), respectively [293]. The Sn electrode fading from >1300 to ∼50 mA h g−1 after 30 cycles was attributed to the Sn disintegration [294]. Tin-nanoparticle-encapsulated elastic hollow carbon spheres (TNHCs) exhibited gradual fading from >1625 to 550 mA h g−1 (99.5% pure), and ethylene glycol (HOCH2 CH2 OH, EG, A R) were used without any purification. In a typical case, 1 mmol BiCl3 and 8 mmol DG were dissolved in 40 ml of EG solvent. The mixture was stirred vigorously for 30 min at room temperature and then

2.6 Synthesis, Characterization, and Photocatalytic Application of Microspheres

moved into a 50-ml Teflon autoclave. The heating process was conducted in a microwave oven (model: Glanz WD800, max power: 800 W). After heating the autoclave for 2.5 min with a power of 480 W, followed by cooling the autoclave down to room temperature naturally, the as-synthesized microspheres were obtained. Finally, the products were collected and washed with deionized water and then alcohol via using centrifuge. 2.6.2 Characterization

The phase characteristic of the as-prepared product was examined by powder XRD [Bruker D8 Advance diffractometer using Cu K𝛼 radiation (𝜆 = 1.54056 Å), operating at 40 kV × 40 mA]. Results were recorded with 2𝜃 ranging from 10∘ to 80∘ with a scanning step of 0.02∘ at a scanning rate of 4∘ min−1 . Typical SEM (KYKY-2800, China) images of the sample were obtained as follows: firstly, 4 mg of as-synthesized sample was dissolved in 4 ml 0.5% PEG (Mw = 3350, Sigma Corp.) solution, followed by ultrasonic treatment for 15 min, and then moved into a 4-ml plastic tube. Secondly, a cleaned microslide was placed horizontally into the plastic tube and kept still for 5 days at room temperature, followed by moving the microslide out of the plastic tube, and then drying in air. Finally, the as-prepared microslide was observed under SEM. Typical TEM (JEOL TEM-1200EX, operating at 120 kV) images of the product were obtained as follows: firstly, 1.2 mg of the product was dissolved in 4 ml of a mixture of deionized water and ethanol (v/v = 1 : 1). The solution was treated with ultrasonic treatment for 15 min, followed by dropping one drop of the colloid (concentration of 0.3 mg ml−1 ) onto standard copper grids covered with perforated carbon films, and dried in air overnight. Finally, the copper grids were observed under TEM. Particle distribution patterns of the as-synthesized microspheres were obtained with a particle size distribution analyzer (MasterSizer2000, Malvern Instruments Ltd. UK). The Raman scattering measurements were performed with a multichannel modular triple Raman system (JY-T64000) with confocal microscopy at room temperature using the 514.5-nm line of an Ar ion laser. A 50× microscope objective lens was used for focusing the laser beam and the collection of the scattered light. The spot diameter of the focused laser beam on the sample is about 1 μm and typical spectrum acquisition time was 50 s. The XPS data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W AlK𝛼 radiation. The base pressure was about 3 × 10−9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. UV–vis measurement was carried out in a UV–vis spectrophotometer (Shimadzu, UV-2100S). PL spectra were recorded using a fluorescence spectrophotometer (Perkin-Elmer LS55). Energy-dispersive X-ray spectroscopy (EDX) spectrum was measured using a JSM-6301F field emission SEM with Energy Dispersive Spectrometry to investigate the chemical composition. 2.6.3 Photocatalytic Test

The photocatalytic activity test of Bi@Bi2 O3 microspheres was investigated by degrading the dyes of RhB and MO aqueous solutions under UV irradiation,

121

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

respectively. The photoreactor was designed with an internal light source (250-W high-pressure mercury lamp with main emission wavelength 365 nm in the range of 315–380 nm along with a power of 830 mW cm−2 , Beijing Huiyixin Electric Forces Technology Development Co., Ltd) surrounded by a quartz jacket, where the suspension includes the as-synthesized Bi@Bi2 O3 catalyst and aqueous RhB or MO (50 ml, 5 × 10−5 M) completely surrounded the high-pressure mercury irradiation light source. Bi@Bi2 O3 microspheres (0.5, 1, and 2 g l−1 ) were suspended in an aqueous solution of 5 × 10−5 M RhB or MO, respectively. The solution was continuously stirred for about 30 min at room temperature to ensure the estabishment of an adsorption–desorption equilibrium among the photocatalyst, RhB or MO, and water before irradiation with UV light from the high-pressure mercury lamp. The distance between the light source and the bottom of the solution was about 10 cm. The concentration of RhB/MO was monitored using a UV–vis spectrometer (UNICO Corp. UV-2102PC). 2.6.4 Structure

Figure 2.107a shows a typical XRD pattern of the as-prepared product. All of the diffraction peaks can be indexed to a rhombohedral phase of bismuth [space group: R3m (166)] (JCPDS44-1246). Raman spectroscopy is a fast and (012)

100

(015) (113)

40

Intensity (a.u.)

60

20

69.1

500

(202) (024) (107) (116) (122) (214) (300)

80

(104) (110)

Intensity (a.u.)

95.4

400 300

183.8 126.9 60

70

80

90

100

200

0 40 2θ (°)

60

80

200 (b)

(101) (110) (102)

400

(002)

100

(001)

300 200

400 600 800 1000 Raman shift (cm–1)

(103) (200) (113) (211) (212)

20 (a)

Intensity (a.u.)

122

0 20 (c)

40 60 2θ (°)

80

Figure 2.107 (a) The XRD pattern and (b) Raman pattern of the as-synthesized product, the first-order peaks in dashed frame are magnified to the upper-right corner. (c) The XRD pattern of the BiOCl synthesized using deionzed water as solvent, substituted for ethylene glycol (EG) in the similar reaction system of Figure 2.107a. (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

2.6 Synthesis, Characterization, and Photocatalytic Application of Microspheres

nondestructive tool to determine the quality of crystalline materials [127, 149, 184]. So the structure characterization of the product is further measured by Raman spectroscopy (Figure 2.107b). Here, a low laser power of 0.062 mW was used to analyze the crystals because of the low melting point of bismuth [324]. As shown in Figure 2.107b, the first-order scattering bands located at 69.1 and 95.4 cm−1 can be definitely attributed to the Eg and A1g modes of the Bi [325–328]. The lower and wider peaks located at about 126.9 and 183.8 cm−1 are the corresponding second-order Raman scattering measurement [327]. However, if we substitute deionized water for EG as the solvent in the reaction system, we just obtain BiOCl (Figure 2.107c) but not Bi, due to the hydrolysis between Bi3+ and deionized water. Furthermore, glucose cannot reduce BiOCl to Bi. That is why we selected EG as the solvent and not deionized water. It is well known that the XPS detection depth of inorganic materials is about 2 nm, while the detection depth of organics is less than 10 nm [329]. The surface composition of the as-synthesized product is also studied by XPS (Figure 2.108). The XPS spectrum (Figure 2.108a) indicates the existence of Bi and O, as well as C from reference [330]. C1s was chosen as the reference line. According to the peak area ratios, the molar contents of O, C, and Bi are ∼21.1%, ∼69.4%, and 9.4%, respectively. The Bi4f peak is further examined by high-resolution XPS (Figure 2.108b). The peaks located at 157.1 and ∼162.4 eV are attributed correspondingly to the binding energies of Bi 4f7/2 and Bi 4f5/2 of pure Bi [331–333], while the peaks at ∼159.1 and ∼164.4 eV are assigned to Bi2 O3 [333, 334]. The 25.0k

60k 50k

Bi4f

40k

O1s Bi4p3/2 Bi4d

Relative intensity

Intensity (Kcounts/s−1)

70k

C1s

30k 20k 10k

Bi5d

4f7/2(BiO)

20.0k

4f5/2(BiO)

15.0k 4f7/2(Bi)

10.0k

4f5/2(Bi)

5.0k

0 0

300 600 900 Binding energy (eV)

Relative intensity

(a)

16 000 15 000 14 000 13 000 12 000 11 000 10 000 9000 8000

(b)

155 160 165 Binding energy (eV)

170

C–C C–O O–C=O

275 (c)

150

1200

280 285 290 Binding energy (eV)

295

Figure 2.108 XPS spectra of (a) survey, (b) Bi 4f, (c) C1s of as-synthesized microspheres. (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

123

124

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

XPS data suggest that the composition on the surface of the product is composed of Bi2 O3 . Obviously, the surface of the product consists of Bi2 O3 . In addition, the peak at 284.6 eV for C1s is observed. In the high-resolution XPS spectrum of C1s (Figure 2.108c), the peak at 284.6 eV is assigned to the C–C bonds and those at 285.4 and 287.8 eV suggest the existence of functional groups such as –OH and –CHO [330]. The molar ratio of Bi:O of 1 : 2.2, larger than 1 : 1.5, suggests that the surface of the sample is slightly rich in oxygen. The other part of oxygen in the sample was attributed to the existence of functional groups, such as –OH and –CHO. The trace of Bi2 O3 is attributed to the oxidation of bismuth in air. According to the analysis of XRD, Raman, and XPS data, the as-synthesized product can be attributed to a bismuth microsphere structure with a trace of Bi2 O3 on the surface, that is, Bi@Bi2 O3 . Also, no other impurities were observed on the surfaces of Bi@Bi2 O3 , suggesting that the as-synthesized Bi@Bi2 O3 microspheres are relatively pure. The morphology of the product is studied by SEM and TEM. Typical SEM images at different magnifications are shown in Figure 2.109. The images indicate that the microspheres are relatively homogeneous (about in 124 μm × 83 μm observed area of Figure 2.109a). The high-magnification image (Figure 2.109b) shows that the microspheres have smooth surfaces. Their average diameter is 1.60 ± 0.70 μm according to SEM observation. The shape of the product is further examined by TEM. According to the TEM observation (Figure 2.109c) the average diameter is about 1.42 μm, which is consistent with the SEM result. It is notable that the microspheres are too unstable to resist the electron beam due to the low melting point of bismuth (271.3 ∘ C) [324]. They can be broken into smaller particles under electron beam bombardment (Figure 2.109d). The particle distribution pattern of the sample is shown in Figure 2.109e. According to the statistical analysis of the quantity distribution, the size distribution is mainly in the range from 1.2 to 2.8 μm (Figure 2.109e), which is in good agreement with the SEM and TEM results. According to these analyses, we demonstrate that Bi@Bi2 O3 microspheres were obtained, which may be composed of even smaller particles. EDX (Figure 2.109f ) analysis of the as-synthesized Bi@Bi2 O3 microspheres: dominating Bi and a small quantity of O signals could be readily detected, which matches with the XRD and Raman data. 2.6.5 Growth Mechanism

The growth of Bi@Bi2 O3 microspheres is attributed to the use of microwave heating and EG used as a solvent in the reaction system. Firstly, it is believed that microwave volumetric heating can reduce the overall thermal gradients or even eliminate thermal gradients during the microwave reaction [316]. Obviously, a low thermal gradient is necessary to produce high-quality materials [335, 336]. So the use of microwave heating provides thermal gradient free and uniform reaction conditions for the nucleation and growth of the Bi@Bi2 O3 microspheres. Secondly, EG as a high permanent dipole solvent is an excellent medium of microwave irradiation, which can absorb the energy from the microwave field and instantaneously make the polar medium be heated up to high temperature [337]. We also carried out a control experiment, that is, using the same reaction

2.6 Synthesis, Characterization, and Photocatalytic Application of Microspheres

(a)

(b)

100 μm

1 μm (d)

(c)

10

Contents (a.u.)

Volume (%)

Bi

(f)

(e) 8 6 4 2

Si Au

CO

0 0.01

0.1

1 10 100 1000 Diameter (μm)

0

1

2 3 Energy (keV)

4

Figure 2.109 (a) SEM image at magnifying power ×900, (b) SEM image at magnifying power ×10 000. (c) TEM image of the as-synthesized product under a weaker electron beam. (d) TEM image of the as-synthesized microsphere which was broken into even smaller spheres under TEM electron beam bombardment because of the low melting point of bismuth. (e) The particle diameter distribution pattern of as-synthesized Bi@Bi2 O3 microspheres. (f ) EDX spectrum of as-synthesized Bi@Bi2 O3 microspheres. The C and Si peaks originate from carbon and Si wafer substrate, and the Au peak originates from plasma sputtering deposition. (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

125

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

conditions but without adding DG. We could not obtain any product under the same heating power and time. So we believe that the Bi3+ is reduced by the DG, not EG. The major oxidation–reduction procedures can be summarized by the following steps, which are similar to that of the reduction of Au3+ [338]. DG → Pryolysis radicals Bi

3+

(2.51)

+ reductive radicals → Bi

0

(2.52)

0

nBi → BM

(2.53)

We believe that, before the solvothermal reaction, the chelation occurs between HOCH2 CH2 OH [owing to two –OH groups] and Bi3+ ion, which leads to a stable five-membered ring coordination cation. This was demonstrated by the changing pH value in the reaction system. Before the addition of BiCl3 , the pH value of EG was about 6.42. However, after the BiCl3 was added and dissolved in HOCH2 CH2 OH, the pH value of the resulting mixture changed to about 0.72, as shown in reaction (2.54). After microwave irradiation, the five-membered ring coordination cation was broken, and then Bi3+ was reduced to Bi by DG. This has been demonstrated by XRD and Raman data. CH2

H2C HOCH2CH2OH

+ Bi3+

+ +

(2.54)

2H+

O

O

pH = 6.42

Bi

pH = 0.72

2.6.6 Optical Properties

The optical properties of as-synthesized Bi@Bi2 O3 microspheres were investigated using a combination of UV–vis and PL technologies (Figure 2.110). The UV–vis absorption property of the Bi@Bi2 O3 microspheres sample was investigated by dispersing Bi@Bi2 O3 microspheres in dimethyl formamide (DMF) (1 g l−1 ) (Figure 2.110a). From the UV–vis spectrum, an absorption peak can be found centered at about ∼294 nm in wavelength (4.21 eV in photon energy). This result is similar to the values of Bi2 O3 reported by Li and Wang, respectivley [339, 340]. 600

1.6 1.4

500

1.2 1.0 0.8 0.6

400 300

λem = 330 nm

λex = 284 nm λex = 279 nm λex = 274 nm λex = 269 nm λex = 264 nm

200 100

0.4 0.2 200

(a)

294 nm Intensity (a.u.)

Absorption (a.u.)

126

400 600 Wavelength (nm)

0 300

800 (b)

350 400 450 Wavelength (nm)

500

Figure 2.110 (a) UV–vis spectrum, (b) PL spectra of as-synthesized Bi@Bi2 O3 microspheres. (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

2.6 Synthesis, Characterization, and Photocatalytic Application of Microspheres

The room temperature PL spectra of as-synthesized Bi@Bi2 O3 microspheres are shown in Figure 2.110b. All the PL spectra exhibit an emission peak centered at about 330 nm (3.75 eV in phonon energy). The emission peaks are almost independent of the used excitation wavelengths (𝜆ex = 264, 269, 274, 279, and 284 nm) with the maximum PL intensity presenting at 𝜆ex = 284 nm. These results indicate that the PL emission comes from the Bi@Bi2 O3 microspheres and not from other impurities. A Stokes shift is observed; such an effect has been attributed to either band-edge emission or shallow-trap emission. Similar results were reported by Yang and Dong [341, 342]. 2.6.7 Photocatalytic Activities

The photocatalytic activities of the as-synthesized Bi@Bi2 O3 microspheres are evaluated in terms of the degradation of dyes, such as RhB and MO in aqueous solutions (5 × 10−5 M) (Figures 2.111–2.113). Figure 2.111a,d shows the absorption spectra of an aqueous solution of RhB and MO in the presence of the as-synthesized Bi@Bi2 O3 microspheres (content: 2 g l−1 ) under UV light irradiation for various durations at room temperature. The absorption maximum of the degraded solution at various times exhibited hypsochromic shifts. The temporal evolution of the spectral changes taking place during the photodegradation of RhB and MO over the as-prepared Bi@Bi2 O3 microspheres with different contents, that is, 0.5, 1, and 2 g l−1 , are displayed in Figure 2.112a and 2.112b and Figure 2.113a,b, respectively. As shown in Figure 2.111b, the photodegradation of RhB catalyzed with or without Bi@Bi2 O3 microspheres of different contents under UV irradiation as well as in darkness. In the absence of Bi@Bi2 O3 microspheres under UV irradiation, the photodegradation of RhB was negligible. The photolytic fade in the presence Bi@Bi2 O3 microspheres in darkness was about 5.2% in 4 h. However, the degradations of RhB were 49.3%, 50%, and ∼96.6% in the presence of Bi@Bi2 O3 microspheres with corresponding contents of 0.5, 1, and 2 g l−1 under UV irradiation in 4 h, respectively. The enhanced photocatalytic ability of Bi@Bi2 O3 microspheres is dependent on the contents of the Bi@Bi2 O3 catalysts. This result confirms that the as-synthesized Bi@Bi2 O3 microspheres indeed possess intrinsic photocatalytic activity under UV light irradiation. Under UV light irradiation, the corresponding pseudo-first-order rate constants for the photodegradation of RhB at the Bi@Bi2 O3 microspheres contents of 0.5, 1, and 2 g l−1 are 0.172, 0.180, and 0.801 h−1 , respectively (Figure 2.111c). The blank experiments show that the degradation of RhB is negligible without UV light irradiation or in the absence of Bi@Bi2 O3 microsphere catalysts. MO was chosen to further investigate the photocatalytic activity of Bi@Bi2 O3 microspheres. Figure 2.111c–e shows the residue of MO without and with Bi@Bi2 O3 microspheres as a catalyst. It can be seen that the photolytic fade without Bi@Bi2 O3 microspheres as catalyst under UV light irradiation or in the presence of Bi@Bi2 O3 microspheres in darkness was 0% or 5.2% in 4 h, suggesting the negligible activity. However, the degradations of MO were 14.2%, 40.7%, and ∼100% in the presence of Bi@Bi2 O3 microspheres with corresponding contents of 0.5, 1, and 2 g l−1 under UV irradiation in 4 h, respectively. MO was

127

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

1.0 0 h of irradiation 1h 0h 2h 1h 3h 2h 4h

1.5 1.0

0.8 C/C0

2.0

3h

0.5

4h

0.0 400

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 –0.5

ln (C0/C)

2 g l–1 1 g l–1 0.5 g l–1 Bi@Bi2O3 darkness No catalyst

No catalyst Bi@Bi2O3 darkness 0.5 g l–1 1 g l–1 2 g l–1

800

0

1

(b)

y = –0.2500+0.8010x R2 = 0.909 y = –0.0000+0.1800x R2 = 0.966 y = –0.0460+0.1724x R2 = 0.943 y = –0.0107+0.0120x R2 = 0.780

2 Time (h)

3

4

0.8 0 h of irradiation 1h 0h 2h 1h 3h 2h 4h 3h

0.6 0.4 0.2

4h

0.0 0

1

(c)

2 3 Time (h)

4

300

400 500 600 Wavelength (nm)

(d)

700

3.0

1.0

2 g l–1 1 g l–1 0.5 g l–1 Bi@Bi2O3 darkness No catalyst

2.5 ln (C0/C)

0.8 0.6 0.4 No catalyst Bi@Bi2O3 darkness 0.5 g l–1 1 g l–1 2 g l–1

0.2 0.0 0 (e)

0.4

0.0

600 700 500 Wavelength (nm)

(a)

0.6

0.2

Absorbance (a.u.)

Absorbance (a.u.)

2.5

C/C0

128

1

2 Time (h)

2.0 1.5 1.0 0.5

y = –0.1469+0.6741x R2 = 0.980 y = 0.0000+0.1200x R2 = 0.998 y = 0.0000+0.0400x R2 = 0.983 y = 0.0010+0.0100x R2 = 0.779

0.0 3

4

0 (f)

1

2 Time (h)

3

4

Figure 2.111 (a) Temporal UV–vis absorption spectral changes and corresponding color changes (inset) of RhB in water in the presence of Bi@Bi2 O3 microspheres under UV irradiation with different Bi@Bi2 O3 microsphere contents: 2 g l−1 ; (b) photodegradation curves of RhB as a function of irradiation time; and (c) corresponding selected fitting results using the pseudofirst-order reaction. (d) Temporal UV–vis absorption spectral changes and corresponding color changes (inset) of MO in water in the presence of Bi@Bi2 O3 microspheres under UV irradiation with the Bi@Bi2 O3 microsphere contents of 2 g l−1 ; (e) photodegradation curves of MO as a function of irradiation time; (f ) corresponding selected fitting results using the pseudo-first-order reaction. (g) Photodegradation of RhB and MO over different catalysts of Bi@Bi2 O3 microsphere contents. The insets show the UV–vis spectral changes of RhB and MO aqueous solutions in the presence of Bi@Bi2 O3 microspheres under UV light irradiation. (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

2.6 Synthesis, Characterization, and Photocatalytic Application of Microspheres

0.8

80

0 h of irradiation 1h 2h 3h 4h MO 5 × 10–5 M

0.6 0.4

catalyst 2 g l–1 UV light

0.2 0.0 300

60

400 500 600 Wavelength (nm)

40

20

700

Absorption (a.u.)

Absorption (a.u.)

Degradation ratio (%)

100

MO RhB

2.5

0 h of irradiation 1h 2h 3h 4h

2.0 1.5 1.0

RhB 5 × 10–5 M catalyst 2 g l–1 UV light

0.5 0.0 400

500 600 700 Wavelength (nm)

800

0 0.5

1.0

1.5

2.0

Catalyst content (g l–1)

(g)

Figure 2.111 (Continued)

2.5 2.0 1.5 1.0 0.5 0.0 400

(a)

500

Absorption (a.u.)

700

2.0 1.5 1.0 0.5 0.0 400

500

600

700

Wavelength (nm)

2.0 1.5 1.0 0.5

(b)

0 h of irradiation 1h 2h 3h 4h

2.5

0 h of irradiation 1h 2h 3h 4h

2.5

0.0 400

800

Wavelength (nm) 3.0

(c)

600

3.0 Absorption (a.u.)

0 h of irradiation 1h 2h 3h 4h

500

(d)

700

800

0 h of irradiation 1h 2h 3h 4h

3 2 1 0 400

800

600

Wavelength (nm) 4

Absorption (a.u.)

Absorption (a.u.)

3.0

500

600

700

800

Wavelength (nm)

Figure 2.112 Temporal UV–vis absorption spectral changes of RhB in water in the presence of Bi@Bi2 O3 microspheres under UV irradiation with different Bi@Bi2 O3 microsphere contents: (a) 0.5 g l−1 , (b) 1 g l−1 ; (c) Temporal UV–vis absorption spectral changes of RhB in water in the presence of Bi@Bi2 O3 microspheres (content: 2 g l−1 ) in darkness; (d) Temporal UV–vis absorption spectral changes of RhB in water in the absence of Bi@Bi2 O3 microspheres under UV irradiation. Initial RhB concentration was 5 × 10−5 mol l−1 . (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

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2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

1.0 0 h of irradiation 1h 2h 3h 4h

0.8 0.6 0.4 0.2

Absorption (a.u.)

Absorption (a.u.)

1.0

0.0 300 (a)

400

500

600

700

Wavelength (nm)

0.4 0.2

(b)

400

500

600

700

Wavelength (nm) 1.0

0.6 0.4 0.2

Absorption (a.u.)

0 h of irradiation 1h 2h 3h 4h

0.8

300

0.6

300

0.0

(c)

0 h of irradiation 1h 2h 3h 4h

0.8

0.0

1.0 Absorption (a.u.)

130

0 h of irradiation 1h 2h 3h 3h

0.8 0.6 0.4 0.2 0.0

400

500

600

Wavelength (nm)

700

300 (d)

400

500

600

700

Wavelength (nm)

Figure 2.113 Temporal UV–vis absorption spectral changes of MO in water in the presence of Bi@Bi2 O3 microspheres under UV irradiation with different Bi@Bi2 O3 microsphere contents: (a) 0.5 g l−1 , (b) 1 g l−1 ; (c) temporal UV–vis absorption spectral changes of MO in water in the presence of Bi@Bi2 O3 microspheres (content: 2 g l−1 ) in darkness; (d) temporal UV–vis absorption spectral changes of RhB in water in the absence of Bi@Bi2 O3 microspheres under UV irradiation. Initial MO concentration was 5 × 10−5 mol l−1 . (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

decolored as fast as RhB using Bi@Bi2 O3 microspheres as the catalysts. Under UV light irradiation, the corresponding pseudo-first-order rate constants for the photodegradation of MO at the Bi@Bi2 O3 microsphere contents of 0.5, 1, and 2 g l−1 are 0.04, 0.120, and 0.674 h−1 , respectively (Figure 2.111f ). The blank experiments show that the degradation of MO is negligible without UV light irradiation or in the absence of Bi@Bi2 O3 microsphere catalysts. Figure 2.111g shows a summary of the degradation ratio of RhB and MO under different contents of Bi@Bi2 O3 microspheres under UV light irradiation. It can be seen that MO decomposes slower than RhB under the same conditions. The degrading of both RhB and MO increases with the contents of Bi@Bi2 O3 microspheres in dye solutions from 0.5 to 1, and 2 g l−1 , reaching ∼100% degradation in 4 h. Soitah and coworkers reported Bi2 O3 thin films synthesized by a modified Pechini route on glass substrate and annealed at temperatures ranging between 400 and 700 ∘ C. Among these thin films, Bi2 O3 thin films annealed at 550 ∘ C had the best photodegradation of MO. The best degradation of MO (concentration: 10 mg l−1 , i.e., 3.05 × 10−5 M) under UV irradiation reaches 94.1% after 2 h in the presence of Bi2 O3 thin films with the degradation rate

2.6 Synthesis, Characterization, and Photocatalytic Application of Microspheres

of 1.17 h−1 [343]. Wang and coworkers reported Bi2 O3 nanofibers synthesized by an electrospinning route. The photodegradation of RhB (concentration: 20 mg l−1 , i.e., 4.18 × 10−5 M) under UV light irradiation reaches 93% after 2 h in the presence of Bi2 O3 nanofibers, with the degradation rate of 1.26 h−1 [340]. Zhou and workers reported Bi2 O3 hierarchical nanostructures synthesized by a template-free aqueous method. The photodegradation of RhB (concentration: 1 × 10−5 M) under visible light irradiation reaches 90% after 2.5 h in the presence of Bi2 O3 catalyst with the degradation rate of 0.92 h−1 [112]. After using the Bi@Bi2 O3 microspheres in the photodegradation of RhB or MO, we washed the Bi@Bi2 O3 microspheres using deionized water and absolute alcohol three times, followed by drying at 80 ∘ C for 4 h. Then, the photodegradation test was evaluated under the same condition mentioned earlier (Figure 2.114). The degradation of RhB and MO reaches ∼100% after 4 h of adding the used Bi@Bi2 O3 microspheres in the RhB or MO solution, similar to the results measured the first time. Being a semiconductor of Bi2 O3 at the surface of the Bi@Bi2 O3 microspheres, the photocatalytic mechanism of Bi@Bi2 O3 microspheres should be as described in Figure 2.115. The photocatalytic mechanisms of the Bi2 O3 for dyes RhB or MO are similar. So we explain the photocatalytic activity of Bi2 O3 as follows. Firstly, RhB is excited to their excited states (RhB* ) by visible irradiation. The excited states of RhB inject electrons into Bi@Bi2 O3 microspheres to generate the dye radical cation (RhB•+ ). Since the active oxygen species such as singlet oxygen (1 O2 ) can be formed by exciting the band gaps of semiconductors in air, this excitation plays an important role in photocatalytic reactions. Electron/hole pairs should be formed by the photoexcitation of Bi2 O3 . In the presence of oxygen, these photoinduced electrons are immediately trapped by the molecular oxygen to form ⋅O2 − , which can then generate active ⋅OOH radicals. At the same time, the holes are trapped by the water in the air to produce hydroxyl radicals [52]. The simple free radical, ⋅O2 − , which has one unpaired electron and possesses a low energy of −41.4 kJ mol−1 , is a doublet. The metastable singlet oxygen, which has a pair of electrons in one orbital leaving the second equal-energy orbital 0.8 0 h of irradiation 1h 2h 3h 4h

2.5 2.0 1.5 1.0 0.5 0.0 400

(a)

Absorption (a.u.)

Absorption (a.u.)

3.0

0 h of irradiation 1h 2h 3h 4h

0.6 0.4 0.2 0.0

500

600

700

Wavelength (nm)

800

300 (b)

400

500

600

700

Wavelength (nm)

Figure 2.114 Temporal UV–vis absorption spectral changes observed in the presence of reused Bi@Bi2 O3 microspheres after washing and drying treatment as a function of irradiation time for different dye molecule solutions: (a) RhB and (b) MO. (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

131

132

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

O2 ·OOH

H+

·O2–

UV-light ·OH + H+

0 kJ mol−1

O2 e– 2.75 eV h+

–41.4 kJ mol−1

·O2–

94.7 kJ mol−1

1O 2

H2O

Figure 2.115 Schematic pattern of the photoinduced charge transformation on Bi2 O3 . (Liu et al. 2011 [321]. Reproduced with permission of Nano Research.)

empty, possesses higher energy (94.1 kJ mol−1 ) than the ground-state triplet oxygen (0 kJ mol−1 ) [344]. Holes may trap one electron from the superoxide radical, ⋅O2 − , to produce a singlet oxygen or triplet oxygen. From the energy level, it is possible for the superoxide radical, ⋅O2 − , to be produced easily and quickly, while the singlet oxygen should be formed slowly and requires extra energy. Subsequent reactions between RhB•+ (or RhB) and active oxygen radicals (⋅OOH, ⋅OH, ⋅O2 − ) leads to the destruction of the dye chromophore and also results in the formation of even smaller decomposed fragments after a series of complex oxidative steps [345]. The possible reaction mechanism of photodegradation RhB under visible irradiation can be illustrated as follows [345, 346]: RhB + h𝜈 → RhB∗ ∗

(2.55) +

−

RhB + Bi2 O3 → RhB + Bi2 O3 (e ) RhB + +•OOH, •OH, or

• O− 2

(2.56)

→ intermediates → degraded products (2.57)

Generally, it is believed that the photocatalytic activity for the degradation of organic compounds is mainly attributed to two factors: the position of the valence band of the catalyst and the mobility of the photogenerated carriers [347, 348]. Maybe the Bi–O in the Bi@Bi2 O3 microspheres may serve as the active electron donor sites, which can enhance the electron transfer to O2 and dispel the recombination of the electron–hole pairs. However, for pure Bi2 O3 , organic dyes (RhB or MO) are degraded by indirect excitation pathway and the absence of suitable charge scavenger results in high recombination rate, leading to decreasing photocatalytic activity [306]. It is known that semiconductor Bi2 O3 can directly absorb phonon by its band gap and generate electron–hole pairs on its surface under UV light irradiation [340, 343], and the excitation of an electron from the valence band to the conduction band is initiated by light absorption with energy matching or exceeding the band gap of Bi2 O3 . When Bi2 O3 absorbed photons, electrons were excited from the valence band to the conduction band leaving holes behind. After the photo-generated electrons migrate to lower lying conduction band of Bi2 O3 , the holes will gather in the valence band of Bi2 O3 . And these electrons and holes have redox activities, such as the holes reacting with surface hydroxyl groups to produce the free radicals of ⋅OH. Also, the excited electrons migrate to the

References

conduction band of Bi2 O3 , are then captured by O2 to generate ⋅O2 − , and, finally, dye molecules (RhB or MO) are decomposed by the active oxygen species to intermediates and final CO2 , due to both ⋅O2 − and ⋅OH radicals being two very strong oxidizing agents [306, 307]. Our results suggest that the complex Bi@Bi2 O3 particles with a band gap of 3.75 eV may decrease the rate of electron–hole recombination, which speeds up charge transfer from the surface of the catalyst to the organic dye molecules.

References 1 Gupta, A. and Silver, S. (1998) Nat. Biotechnol., 16, 888. 2 Malynych, S. and Chumanov, G. (2003) J. Am. Chem. Soc., 125, 2896. 3 Schultz, S., Smith, D.R., Mock, J.J., and Schultz, D.A. (2000) Proc. Natl.

Acad. Sci. U. S. A., 97, 996. 4 Katz, E. and Willner, I. (2004) Angew. Chem. Int. Ed., 43, 6042. 5 Mehra, R.K., Tran, K., Scott, G.W., Mulchandani, P., and Saini, S.S. (1996) J.

Inorg. Biochem., 61, 125. 6 Kou, X.S., Zhang, S.Z., Yang, Z., Tsung, C.K., Studky, G.D., Sun, L.D., Wang,

J.F., and Yan, C.H. (2007) J. Am. Chem. Soc., 129, 6402. 7 Slocik, J.M. and Wright, D.W. (2003) Biomacromolecules, 4, 1135. 8 Barglik-Chory, C., Remenyi, C., Strohm, H., and Müller, G. (2004) J. Phys.

Chem. B, 108, 7637. 9 Qian, H.F., Dong, C.Q., Weng, J.F., and Ren, J.C. (2006) Small, 2, 747. 10 Zheng, Y.G., Yang, Z.C., and Ying, J.Y. (2007) Adv. Mater., 19, 1475. 11 Wu, Q.Z., Cao, H.Q., Luan, Q.Y., Zhang, J.Y., Wang, Z., Warner, J.H., and

Watt, A.A. (2008) Inorg. Chem., 47, 5882. 12 Jiang, X.C., Xie, Y., Lu, J., Zhu, L.Y., He, W., and Qian, Y.T. (2001) Langmuir,

17, 3795. 13 Mamedova, N.N., Kotov, N.A., Rogach, A.L., and Studer, J. (2001) Nano

Lett., 1, 281. 14 Mosmann, T. (1983) J. Immunol. Methods, 65, 55. 15 Peng, L.M. and Wang, Z.L. (2000) The Foundation and Clinic of Cell Apopto-

sis, People’s Medical Publishing House, Beijing, China. 16 Chen, S.H., Yao, H.S., and Kimura, K.S. (2001) Langmuir, 17, 733. 17 Singhal, R.K., Anderson, M.E., and Meister, A. (1987) FASEB J., 1, 220. 18 Hussain, S.M., Hess, K.L., Gearhart, J.M., Geiss, K.T., and Schlager, J.J.

(2005) Toxicol. in Vitro, 19, 975. 19 Braydich-Stolle, L., Hussain, S., Schlager, J.J., and Hofmann, M.C. (2005)

Toxicol. Sci., 88, 412. 20 Brown, J.M. and Attardi, L.D. (2005) Nat. Rev. Cancer, 5, 231. 21 Yoshimaru, T., Suzuki, Y., Inoue, T., Niide, O., and Ra, C. (2006) Free Radi-

cal. Biol. Med., 40, 1949. 22 Lovri´c, J., Cho, S.J., Winnik, F.M., and Maysinger, D. (2005) Chem. Biol., 12,

1227.

133

134

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

23 Connor, E.E., Mwamuka, J., Gole, A., Murphy, C.J., and Wyatt, M.D. (2005)

Small, 1, 325. 24 Cao, H., Qiu, X., Liang, Y., Zhang, L., Zhao, M., and Zhu, Q. (2006)

ChemPhysChem, 7, 497. 25 Kuang, Q., Lao, C., Wang, Z.L., Xie, Z., and Zheng, Z.L. (2007) J. Am. Chem.

Soc., 129, 6070. 26 Moreno, M.S., Varela, A., and Otero-Diaz, L.C. (1997) Phys. ReV. B: Con-

dens. Matter Mater. Phys., 56, 5186. 27 Park, M.-S., Wang, G.-X., Kang, Y.-M., Wexler, D., Dou, S.-X., and Liu, H.-K.

(2007) Angew. Chem. Int. Ed., 46, 750. 28 Lin, Y., Dong, J., and Liu, M. (2004) Adv. Mater., 16, 353. 29 Zhang, D.-F., Sun, L.-D., Yin, J.-L., and Yan, C.-H. (2003) Adv. Mater., 15,

1022. 30 Juttukonda, V., Paddock, R.L., Raymond, J.E., Denomme, D., Richardson,

A.E., Slusher, L.E., and Fahlman, B.D. (2006) J. Am. Chem. Soc., 128, 420. 31 Yang, R. and Wang, Z.L. (2006) J. Am. Chem. Soc., 128, 1466. 32 Duan, J., Yang, S., Liu, H., Gong, J., Huang, H., Zhao, X., Zhang, R., and Du,

Y. (2005) J. Am. Chem. Soc., 127, 6180. 33 Chen, B., Russell, J.M., Shi, W., Zhang, L., and Samuiski, E.T. (2004) J. Am.

Chem. Soc., 126, 5972. 34 Vayssieres, L. and Graetzel, M. (2004) Angew. Chem. Int. Ed., 43, 3666. 35 Yang, H.G. and Zeng, H.C. (2004) Angew. Chem. Int. Ed., 43, 5930. 36 Tang, K., Zhang, J., Yan, W., Li, Z., Wang, Y., Yang, W., Xie, Z., Sun, T., and

Fuchs, H. (2008) J. Am. Chem. Soc., 130, 2676. 37 Cao, H., Zhang, L., Liu, X., Zhang, S., Liang, Y., and Zhang, X. (2007) Appl.

Phys. Lett., 90, 193105. 38 Wu, S.S., Cao, H.Q., Yin, S.F., Liu, X.W., and Zhang, X.R. (2009) J. Phys.

Chem. C, 113, 17893. 39 Koo, H.-J., Kim, Y.J., Lee, Y.H., Lee, W.I., Kim, K., and Park, N.-G. (2008)

Adv. Mater., 20, 195. 40 Zhou, J.X., Zhang, M.S., Hong, J.M., and Yin, Z. (2006) Solid State Com-

mun., 138, 242. 41 Emirogla, S., Barsan, N., Weimar, U., and Hoffmann, V. (2001) Thin Solid

Films, 391, 176. 42 Li, Z., Shen, W., Zhang, X., Fang, L., and Zu, X. (2008) Colloids Surf., A,

327, 17. 43 Owens, F.J. and Poole, C.P. Jr. (2008) The Physics and Chemistry of

Nanosolids, John Wiley & Sons, Inc., New York. 44 Agekyan, W.T. (1977) Phys. Status Solidi A, 43, 11. 45 West, A.R. (1999) Basic Solid State Chemistry, 2nd edn, John Wiley & Sons

Ltd., West Sussex, England. 46 Cao, G.Z. (2004) Nanostructures & Nanomaterials: Synthesis, Properties &

Applications, London, Imperial College Press. 47 Zhu, H., Yang, D., Yu, G., Zhang, H., and Yao, K. (2006) Nanotechnology, 17,

2386. 48 Tao, X., Su, J., and Chen, J.-F. (2006) Chem. – Eur. J., 12, 4164. 49 Sun, X. and Lin, J. (2009) J. Phys. Chem. C, 113, 4970.

References

50 Wen, Z., Wang, G., Lu, W., Wang, Q., Zhang, Q., and Li, J. (2007) Cryst.

Growth Des., 7, 1722. 51 Xu, H. and Zhang, L. (2009) J. Phys. Chem. C, 113, 1785. 52 Hoffmann, M.R., Martin, S.T., Choi, W., and Bahnemann, D.W. (1995) Chem.

Rev., 95, 69. 53 Hidaka, H., Zhao, J., Pelizzetti, E., and Serpone, N. (1992) J. Phys. Chem., 96,

2226. 54 Zhang, C. and Yang, H. (eds) (2006) Introduction to Biomaterials, Beijing,

Chemical Industry Press. 55 Heuer, A.H. and Hobbs, L.W. (eds) (1981) Science and Technology of Zir-

56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77

conia, Advances in Ceramics; Vol. 3, American Ceramic Society, Columbus, OH. Subbarao, E.C. and Maiti, H.S. (1988) Adv. Ceram., 24, 731. Gole, J.L., Prokes, S.M., Stout, J.D., Glembocki, O.J., and Yang, R.S. (2006) Adv. Mater., 18, 664. Wang, C.M., Fan, K.N., and Liu, Z.P. (2007) J. Am. Chem. Soc., 129, 2642. Joo, J., Yu, T., Kim, Y.W., Park, H.M., Wu, F.X., Zhang, J.Z., and Hyeon, T. (2003) J. Am. Chem. Soc., 125, 6553. Cao, H.Q., Qiu, X.Q., Luo, B., Liang, Y., Zhang, Y.H., Tan, R.Q., Zhao, M.J., and Zhu, Q.M. (2004) Adv. Funct. Mater., 14, 243. Zheng, H., Liu, K.Y., Cao, H.Q., and Zhang, X.R. (2009) J. Phys. Chem. C, 113, 18259. Cao, H., Wang, G., Warner, J.H., and Watt, A.A.R. (2008) Appl. Phys. Lett., 92, 013110. Chen, S.G., Yin, Y.S., Wang, D.P., Liu, Y.C., and Wang, X. (2005) J. Cryst. Growth, 282, 498. Emeline, A.V. and Serpone, N. (2001) Chem. Phys. Lett., 345, 105. Lai, L.J., Lu, H.C., Chen, H.K., Cheng, B.M., Lin, M.I., and Chu, T.C. (2005) J. Electron Spectrosc. Relat. Phenom., 144, 865. Zhao, N.N., Pan, D.C., Nie, W., and Ji, X.L. (2006) J. Am. Chem. Soc., 128, 10118. Lin, C.K., Zhang, C.M., and Lin, J. (2007) J. Phys. Chem. C, 111, 3300. Becker, J., Hald, P., Bremholm, M., Pedersen, J.S., Chevallier, J., Iversen, S.B., and Iversen, B.B. (2008) ACS Nano, 2, 1058. Garvie, R.C. (1978) J. Phys. Chem., 82, 218. Shukla, S. and Seal, S. (2005) Int. Mater. Rev., 50, 45. Adschiri, T., Hakuta, Y., and Arai, K. (2000) Ind. Eng. Chem. Res., 39, 4901. Sue, K., Suzuki, M., Arai, K., Ohashi, T., Ura, H., Matsui, K., Hakuta, Y., Hayashi, H., Watanabe, M., and Hiaki, T. (2006) Green Chem., 8, 634. Hornyak, G.L., Dutta, J., Tibbals, H.F., and Rao, A.K. (2008) Introduction to Nanoscience, CRC Press, Boca Raton, FL. Vollath, D. (2008) Nanomaterials: An Introduction to Synthesis, Properties and Applications, Weinheim, Wiley-VCH. Chen, L.Y., Liang, Y., and Zhang, Z.D. (2009) Eur. J. Inorg. Chem., (7), 903. Sun, Y. and Xia, Y. (2002) Science, 298, 2176. Dhas, N.A. and Suslick, K.S. (2005) J. Am. Chem. Soc., 127, 2368.

135

136

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

78 Hung, L.-I., Tsung, C.-K., Huang, W., and Yang, P. (2010) Adv. Mater., 22,

1910. 79 Kim, Y., Momin, E., Choi, J., Yuan, K., Zaidi, H., Kim, J., Park, M., Lee, N.,

80 81 82 83 84 85

86 87 88 89 90 91 92 93 94 95 96 97

98 99 100

McMahon, M.T., Quinones-Hinojosa, A., Bulte, J.W.M., Hyeon, T., and Gilad, A.A. (2011) J. Am. Chem. Soc., 133, 2955. Shin, J., Anisur, R.M., Ko, M.K., Im, G.H., Lee, J.H., and Lee, I.S. (2009) Angew. Chem. Int. Ed., 48, 321. Zhang, W.F., He, Z.B., Yuan, G.D., Jie, J.S., Luo, L.B., Zhang, X.J., Chen, Z.H., Lee, C.S., Zhang, W.J., and Lee, S.T. (2009) Appl. Phys. Lett., 94, 123103. Zeng, J., Huang, J., Lu, W., Wang, X., Wang, B., Zhang, S., and Hou, J. (2007) Adv. Mater., 19, 2172. Farvid, S.S., Wang, T., and Radovanovic, P.V. (2011) J. Am. Chem. Soc., 133, 6711. Park, Y.-G., Seo, K.-H., Lee, J.-H., Kim, J.-J., Cho, S.-H., O’Conner, C.J., and Lee, J.-S. (2004) J. Electroceram., 13, 851. King, P.D.C., Veal, T.D., Fuchs, F., Wang, Y.C., Payne, D.J., Bourlange, A., Zhang, H., Bell, R.G., Cimalla, V., Ambacher, O., Egdell, R.G., Bechstedt, F., and McConvile, C.F. (2009) Phys. Rev. B: Condens. Matter Mater. Phys., 79, 205211. Kim, D.-W., Hwang, I.-S., Kwon, S.J., Kang, H.-Y., Park, K.-S., Choi, Y.-J., Choi, K.-J., and Park, J.-G. (2007) Nano Lett., 7, 3041. Park, K.-S., Choi, Y.-J., Kang, J.-G., Sung, Y.-M., and Park, J.-G. (2011) Nanotechnology, 22, 285712. Parthiban, S., Ramamurthi, K., Elangovan, E., Martins, R., and Fortunato, E. (2009) Appl. Phys. Lett., 94, 212101. Du, N., Zhang, H., Chen, B., Ma, X., Liu, Z., Wu, J., and Yang, D. (2007) Adv. Mater., 19, 1641. Cao, H., Qiu, X., Liang, Y., Zhu, Q., and Zhao, M. (2003) Appl. Phys. Lett., 83, 761. Li, B., Xie, Y., Jing, M., Rong, G., Tang, Y., and Zhang, G. (2006) Langmuir, 22, 9380. Chang, J., Lee, W., Mane, R.S., Cho, B.W., and Han, S.-H. (2008) Electrochem. Solid-State Lett., 11, A9. Pinna, N., Neri, G., Antonietti, M., and Niederberger, M. (2004) Angew. Chem. Int. Ed., 43, 4345. Epifani, M. and Siciliano, P. (2004) J. Am. Chem. Soc., 126, 4078. Gurlo, A., Miehe, G., and Riedel, R. (2009) Chem. Commun., (19), 2747. Yin, J.F. and Cao, H.Q. (2012) Inorg. Chem., 51, 6529. Kadir, A., Mukhopadhyay, S., Ganguli, T., Galande, C., Gokhale, M.R., Arora, B.M., Raychaudhuri, P., and Bhattacharya, A. (2008) Solid State Commun., 149, 361. Hollas, J.M. (2004) Modern Spectroscopy, 4th edn, John Wiley & Sons Ltd., West Sussex PO19 8SQ, England. Dong, H., Chen, Z., Sun, L., Zhou, L., Ling, Y., Yu, C., Tan, H.H., Jagadish, C.P., and Shen, X.C. (2009) J. Phys. Chem. C, 113, 10511. Williams, D.H. and Fleming, I. (2008) Spectroscopic Methods in Organic Chemistry, 6th edn, McGraw-Hill Higher Education, UK.

References

101 Chen, C., Chen, D., Jiao, X., and Wang, C. (2006) Chem. Commun., (44),

4632. 102 Chen, L., Ma, X., Liu, Y., Zhang, Y., Wang, W., Liang, Y., and Zhang, Z.

(2007) Eur. J. Inorg. Chem., (28), 4508. 103 Labuschagne, P.W., Kazarian, S.G., and Sadiku, R.E. (2011) Spectrochim.

Acta, Part A, 78, 1500. 104 Erbil, H.Y. (2006) Surface Chemistry of Solid and Liquid Interfaces, Blackwell

Publishing Ltd, 9600 Garsington Road, Oxford OX4 2DQ, UK. 105 Ozin, G.A. and Arsenault, A.C. (2005) Nanochemistry: A Chemical Approach

106 107

108 109

110 111 112 113 114

115 116 117 118 119 120 121 122 123

to Nanomaterials, The Royal Society of Chemistry, Cambridge CB4 0WF, UK. Dloczik, L. and Konenkamp, R. (2003) Nano Lett., 3, 651. Newnham, R.E. (2005) Properties of Materials: Anisotropy, Symmetry, Structure, Oxford University Press, Great Clarendon Street, Oxford OX2 6DP, UK. Zhu, H., Yao, K., Zhang, H., and Yang, D. (2005) J. Phys. Chem. B, 102, 20676. Atkins, P., Overton, T., Rourke, J., Weller, M., and Armstrong, F. (2006) Inorganic Chemistry, fourth edn, Oxford University Press, Great Clarendon Street, Oxford OX2 6DP, UK. Smyth, D.M. (2000) The Defect Chemistry of Metal Oxides, Oxford University Press, Great Clarendon Street, Oxford OX2 6DP, UK. Park, J., Zheng, H., Jun, Y.-W., and Alivisatos, A.P. (2009) J. Am. Chem. Soc., 131, 13943. Zhou, L., Wang, W., Xu, H., Sun, S., and Shang, M. (2009) Chem. – Eur. J., 15, 1776. Wu, J.M. and Zhang, T.W. (2004) J. Photochem. Photobiol., A, 162, 171. Ashby, M., Shercliff, H., and Cebon, D. (eds) (2007) Materials Engineering, Science, Processing and Design, Butterworth-Heinemann, Linacre House, Jordan Hill, Oxford OX2 8DP, UK. Zhu, H., Wang, X., Qian, L., Yang, F., and Yang, X. (2008) J. Phys. Chem. C, 112, 4486. Bigioni, T.P., Lin, X.M., Nguyen, T.T., Corwin, E., Witten, T.A., and Jaeger, H.M. (2006) Nat. Mater., 5, 265. Liu, H.F., Guo, B., Hong, L., and Jiang, H.X. (2005) J. Photochem. Photobiol., A., 174, 81. Chen, C., Zhao, W., Lei, P., Zhao, J., and Serpone, N. (2004) Chem. – Eur. J., 10, 1956. Lachheb, H., Puzenat, E., Houas, A., Ksibi, M., Elaloui, E., Guillard, C., and Herrmann, J.-M. (2002) Appl. Catal., B, 39, 75. Sinha, A.K., Basu, M., Pradhan, M., Sarkar, S., and Pal, T. (2010) Chem. – Eur. J., 16, 7865. Kemell, M., Pore, V., Tupala, J., Ritala, M., and Leskelä, M. (2007) Chem. Mater., 19, 1816. Chen, J., Xu, L., Li, W., and Gou, X. (2005) Adv. Mater., 17, 582. Cao, M., Liu, T., Gao, S., Sun, G., Wu, X., Hu, C., and Wang, Z.L. (2005) Angew. Chem. Int. Ed., 44, 4197.

137

138

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

124 Sahu, K.K., Rath, C., Mishra, N.C., Anand, S., and Das, R.P. (1997) J. Colloid

Interface Sci., 185, 402. 125 Jing, Z., Han, D., and Wu, S. (2005) Mater. Lett., 59, 804. 126 Peng, X., Manna, L., Yang, W., Wickham, J., Scher, E., Kadavanich, A., and

Alivisatos, A.P. (2000) Nature, 404, 59. 127 Cao, H.Q., Wang, G.Z., Zhang, L., Liang, Y., Zhang, S.C., and Zhang, X.R.

(2006) ChemPhysChem, 7, 1897. 128 a Zboril, R., Mashlan, M., and Petridis, D. (2002) Chem. Mater., 14, 969;b)

129 130 131 132 133 134 135 136 137 138 139 140

141 142 143 144 145 146 147 148 149

Wang, X., Chen, X., Ma, X., Zheng, H., Ji, M., and Zhang, Z. (2004) Chem. Phys. Lett., 384, 391. Nandakumar, P., Vijayan, C., Rajalakshmi, M., Arora, A.K., and Murti, Y.V.G.S. (2001) Phys. E, 11, 377. de Faria, D.L.A., Venâncio Silva, S., and de Oliveira, M.T. (1997) J. Raman Spectrosc., 28, 873. Fu, Y.Y., Wang, R.M., Xu, J., Chen, J., Yan, Y., Narlikar, A.V., and Zhang, H. (2003) Chem. Phys. Lett., 379, 373. Beattie, I.R. and Gilson, T.R. (1970) J. Chem. Soc. A, (6), 980. McCarty, K.F. (1988) Solid State Commun., 68, 799. Bersani, D., Lottici, P.P., and Montenero, A. (1999) J. Raman Spectrosc., 30, 355. Murphy, C.J. (2002) Science, 298, 2139. Sugimoto, T. (1987) Adv. Colloid Interface Sci., 28, 65. Carlier, D., Terrier, C., Arm, C., and Ansermet, J.-P. (2005) Electrochem. Solid-State Lett., 8, C43. Tian, M.B. (ed.) (2001) Magnetic Materials, Tsinghua University Press, Beijing, China. Ziolo, R.F., Giannelis, E.P., Weinstein, B.A., O’Horo, M.P., Ganguly, B.N., Mehrotra, V., Russell, M.W., and Huffman, D.R. (1992) Science, 257, 219. Madelung, O., Schulz, M., and Weiss, H. (eds) (1982) Semiconductor Physics of Group II–VI Compounds, Landolt-Börnstein, New Series, Group III, Vol. 17, Part b, Springer-Verlag, Berlin. Cao, H.Q., Xu, Y., Hong, J.M., Liu, H.B., Yin, G., Li, B.L., Tie, C.Y., and Xu, Z. (2001) Adv. Mater., 13, 1393. Tang, K., Qian, Y., Zeng, J., and Yang, X. (2003) Adv. Mater., 15, 448. Xiong, Y., Xie, Y., Yang, J., Zhang, R., Wu, C., and Du, G. (2002) J. Mater. Chem., 12, 3712. Kanemitsu, Y., Ando, M., Matsuda, K., Saiki, T., and White, C.W. (2002) Phys. Status Solidi A, 190, 537. Shao, M., Wu, Z., Gao, F., Ye, Y., and Wei, X. (2004) J. Cryst. Growth, 260, 63. Cao, Y.C. and Wang, J. (2004) J. Am. Chem. Soc., 126, 14336. Pan, D., Jiang, S., An, L., and Jiang, B. (2004) Adv. Mater., 16, 982. Liu, S.-M., Guo, H.-Q., Zhang, Z.-H., Li, R., Chen, W., and Wang, Z.-G. (2000) Phys. E, 8, 174. Cao, H.Q., Wang, G.Z., Zhang, S.C., Zhang, X.R., and Rabinovich, D. (2006) Inorg. Chem., 45, 5103.

References

150 Donahue, E.J., Roxburgh, A., and Yurchenko, M. (1998) Mater. Res. Bull., 33,

323. 151 Wang, W., Germanenko, I., and Ei-Shall, M.-S. (2002) Chem. Mater., 14,

3028. 152 Banerjee, R., Jayakrishnan, R., and Ayyub, P. (2000) J. Phys.: Condens. Matter,

12, 10647. 153 Lozada-Morales, R., Zelaya-Angel, O., Jiménez-Sandoval, S., and

Torres-Delgado, G. (2002) J. Raman Spectrosc., 33, 460. 154 Stampfl, A.P.J., Hofmann, P., Schaff, O., and Bradshaw, A.M. (1997) Phys.

Rev. B: Condens. Matter Mater. Phys., 55, 9679. 155 Chen, C.C., Herhold, A.B., Johnson, C.S., and Alivisatos, A.P. (1997) Science,

276, 398. 156 Arguello, C.A., Rousseau, D.L., and Porto, S.P.S. (1969) Phys. Rev., 181, 1351. 157 Froment, M., Bernard, M.C., Cortes, R., Mokili, B., and Lincot, D. (1995) J.

Electrochem. Soc., 142, 2642. 158 Nanda, K.K., Sarangi, S.N., Sahu, S.N., Deb, S.K., and Behera, S.N. (1999)

Phys. B, 262, 31. 159 Brunner, H. and Sussner, H. (1972) Biochim. Biophys. Acta, 271, 16. 160 Banfield, J.F., Welch, S.A., Zhang, H., Ebert, T.T., and Penn, R.L. (2000) 161 162 163 164 165 166 167

168 169 170 171 172 173 174 175 176

Science, 289, 751. Penn, R.L. and Banfield, J.F. (1998) Science, 281, 969. Lou, X.W. and Zeng, H.C. (2003) J. Am. Chem. Soc., 125, 2697. Liu, B. and Zeng, H.C. (2003) J. Am. Chem. Soc., 125, 4430. Pacholski, C., Kornowski, A., and Weller, H. (2002) Angew. Chem. Int. Ed., 41, 1188. Murakoshi, K., Hosokawa, H., Saitoh, M., Wada, Y., Sakata, T., Mori, H., Satoh, M., and Yanagida, S. (1998) J. Chem. Soc., Fraraday Trans., 94, 579. Tolbert, S.H. and Alivisatos, A.P. (1994) Science, 265, 373. Filatova, E.O., André, J.-M., Taracheva, E.Y., Tvaladze, A.J., Kraizman, V.L., Novakovich, A.A., and Vedrinskii, R.V. (2004) J. Phys.: Condens. Matter, 16, 4597. Okamoto, S., Kanemitsu, Y., Hosokawa, H., Murakoshi, K., and Yanagida, S. (1998) Solid State Commun., 105, 7. Murray, C.B., Norris, D., and Bawendi, M.G. (1993) J. Am. Chem. Soc., 115, 8706. Ramsden, J.J. and Grätzel, M. (1984) J. Chem. Soc., Faraday Trans., 80, 919. Yang, S., Wang, Z., and Wnag, J. (eds) (2004) Semiconductor Materials, Science Press, Beijing. Kröger, F.A. (1964) Chemistry of Imperfect Crystals, North-Holland, Ansterdan. Chan, W.C.W. and Nie, S.M. (1998) Science, 281, 2016. Yu, S.H. and Yoshimura, M. (2002) Adv. Mater., 14, 296. Torres-Martínez, C.L., Nguyen, L., Kho, R., Bae, W., Bozhilov, K., Klimov, V., and Mehra, R.K. (1999) Nanotechnology, 10, 340. Bae, W., Abdullah, R., Henderson, D., and Mehra, R.K. (1997) Biochem. Biophys. Res. Commun., 237, 16.

139

140

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

177 Steckel, J.S., Zimmer, J.P., Coe-Sullivan, S., Stott, N.E., Buloviæ, V., and

Bawendi, M.G. (2004) Angew. Chem. Int. Ed., 43, 2154. 178 Hu, J.S., Ren, L.L., Guo, Y.G., Liang, H.P., Cao, A.M., Wan, L.J., and Bai, C.L.

(2005) Angew. Chem. Int. Ed., 44, 1269. 179 Zhao, Y.W., Zhang, Y., Zhu, H., Hadjipanayis, G.C., and Xiao, J.Q. (2004) J.

Am. Chem. Soc., 126, 6874. 180 Banerjee, I.A., Yu, L.T., and Matsui, H. (2005) J. Am. Chem. Soc., 127, 16002. 181 Li, Y.C., Li, X.H., Yang, C.H., and Li, Y.F. (2004) J. Phys. Chem. B, 108,

16002. 182 Vlasov, Y.A., Bo, X.-Z., Sturm, J.C., and Norris, D.J. (2001) Nature, 414, 289. 183 Lu, Q.Y., Gao, F., and Komarneni, S. (2004) J. Am. Chem. Soc., 126, 54. 184 Wu, Q.Z., Cao, H.Q., Zhang, S.C., Zhang, X.R., and Rabinovich, D. (2006) 185 186 187 188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203 204 205 206

Inorg. Chem., 45, 7316. Jones, C.D. and Lyon, L.A. (2003) J. Am. Chem. Soc., 125, 460. Abdulkhadar, M. and Thomas, B. (1995) Nanostruct. Mater., 5, 289. Bobovich, Y.S. (1988) J. Appl. Spectrosc., 49, 869. Dean, J.A. (2003) Lange’s Handbook of Chemistry, 15th ed. edn, McGraw-Hill, New York. Liddell, C.M. and Summers, C.J. (2004) J. Colloid Interface Sci., 274, 103. Scholz, S.M., Vacassy, R., Dutta, J., and Hofmann, H. (1998) J. Appl. Phys., 83, 7860. Scholz, S.M., Vacassy, R., Lemaire, L., Dutta, J., and Hofmann, H. (1998) Appl. Organomet. Chem., 12, 327. Babayan, Y., Barton, J.E., Greyson, E.C., and Odom, T.W. (2004) Adv. Mater., 16, 1341. Hu, P.A., Liu, Y.Q., Fu, L., Cao, L.C., and Zhu, D.B. (2004) J. Phys. Chem. B, 108, 936. Kar, S. and Chaudhuri, S. (2005) J. Phys. Chem. B, 109, 3298. Becker, W.G. and Bard, A.J. (1983) J. Phys. Chem., 87, 4888. Zhang, H., Zhang, S.Y., Zuo, M., Li, G.P., and Hou, J.G. (2005) Eur. J. Inorg. Chem., 1, 47. Li, Y.J., You, L.P., Duan, R., Shi, P.B., Du, H.L., Qiao, Y.P., and Qin, G.G. (2004) Nanotechnology, 15, 581. Wen, X.G., Wang, S.H., Xie, Y.T., Li, X.Y., and Yang, S.H. (2005) J. Phys. Chem. B, 109, 10100. Dlala, H., Almouk, M., Belgacem, S., Girard, P., and Barjon, D. (1998) Eur. Phys. J. A, 2, 13. Sun, Y.P., Riggs, J.E., Rollins, H.W., and Guduru, R. (1999) J. Phys. Chem. B, 103, 77. Nikam, P.S. and Shinde, C.B. (1994) Pramana, 43, 55. Hodes, G., Manasen, J., and Cahen, D. (1976) Nature, 261, 403. Terabe, K., Hasegawa, T., Nakayama, T., and Aono, M. (2005) Nature, 433, 47. Bass, A.K. (1988) Sol. Energy Mater., 17, 375. Zhang, W.X., Zhang, L., Hui, Z.H., Zhang, X.M., and Qian, Y.T. (2000) Solid State Ionics, 130, 111. Gao, F., Lu, Q.Y., and Zhao, D.Y. (2003) Nano Lett., 3, 85.

References

207 Yang, L., Xing, R.M., Shen, Q.M., Jiang, K., Ye, F., Wang, J.Y., and Ren, Q.S.

(2006) J. Phys. Chem. B, 110, 10534. 208 Brelle, M.C., Zhang, J.Z., Nguyen, L., and Mehra, R.K. (1999) J. Phys. Chem.

A, 103, 10194. 209 Xiang, J.H., Cao, H.Q., Wu, Q.Z., Zhang, S.C., Zhang, X.R., and Watt, A.A.R.

(2008) J. Phys. Chem. C, 112, 3580. 210 Bae, W. and Mehra, R.K. (1998) J. Inorg. Biochem., 70, 125. 211 Panigrahi, S., Kundu, S., Basu, S., Praharaj, S., Jana, S., Pande, S., Ghosh,

S.K., Pal, A., and Pal, T. (2006) Nanotechnology, 17, 5461. 212 Zhong, Z.Y., Subramanian, A.S., Highfield, J., Carpenter, K., and Gedanken,

A. (2005) Chem. – Eur. J., 11, 1473. 213 Okpalugo, T., Papakonstantinou, P., Murphy, H., Mclaughlin, J., and Brown,

N.M.D. (2005) Carbon, 43, 153. 214 Wagner, D. (1979) Handbook of X-ray Photoelectron Spectroscopy,

Perkin-Elmer Corporation, MN. 215 Zhang, B., Ye, X.C., Dai, W., Hou, W.Y., and Xie, Y. (2006) Chem. – Eur. J.,

12, 2337. 216 Kartio, I., Laajalehto, K., Suoninen, E., Karthe, S., and Szargan, R. (1992)

Surf. Interface Anal., 18, 807. 217 Xu, C.Q., Zhang, Z.C., and Ye, Q. (2004) Mater. Lett., 58, 1671. 218 Mandal, S., Gole, A., Lala, N., Gonnade, R., Ganvir, V., and Sastry, M. (2001) 219 220 221 222 223 224 225 226 227 228 229 230 231 232 233

Langmuir, 17, 6262. Xiao, J.P., Xie, Y., Tang, R., and Luo, W. (2002) J. Mater. Chem., 12, 1148. von der Oster, M. (1982) Landolt-Bornstein, 172, 156. Leiggener, C. and Calzaferri, G. (2005) Chem. – Eur. J., 11, 7191. Wei, X.W., Song, X.J., Xu, J., Ni, Y.H., and Zhang, P. (2005) Mater. Chem. Phys., 92, 159. Zhu, J.F., Zhu, Y.J., Ma, M.G., Yang, L.X., and Gao, L. (2007) J. Phys. Chem. C, 111, 3920. Pileni, M.P., Motte, L., Billoudet, F., Mahrt, J., and Willig, F. (1997) Mater. Lett., 31, 255. Zhang, L.D. and Mou, J.M. (2001) Nanomaterials and Nanostructures, Beijing, Science Press. Burford, N., Eelman, M.D., Mahony, D.E., and Morash, M. (2003) Chem. Commun., (1), 146. Cheronis, N.D., Ronzio, A.R., and Ma, T.S. (1954) Micro- and Semimicro Methods, New York, Interscience. Pakhomov, P.M., Ovchinnikov, M.M., Khizhnyak, S.D., Lavrienko, M.V., Nierling, W., and Lechner, M.D. (2004) Colloid J., 66, 65. Cartmell, E. and Fowles, G.W.A. (1977) Valency and Molecular Structure, 4th Ed. edn, Butterworths, London. Gugliotti, L.A., Feldheim, D.L., and Eaton, B.E. (2005) J. Am. Chem. Soc., 127, 17814. Song, Q. and Zhang, Z.J. (2004) J. Am. Chem. Soc., 126, 6164. Cao, H.Q., Wang, G.Z., Zhang, S.C., and Zhang, X.R. (2006) Nanotechnology, 17, 3280. Murphy, C.J. (2002) Science, 298, 2139.

141

142

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

234 Cao, M., Liu, T., Gao, G., Sun, G., Wu, X., Hu, C., and Wang, Z.L. (2005) 235 236 237 238 239 240 241 242 243 244

245 246 247 248 249 250 251 252 253 254 255 256 257 258

Angew. Chem. Int. Ed., 44, 4197. Chen, J., Xu, L., Li, W., and Gou, X. (2005) Adv. Mater., 17, 582. Yu, H., Cölfen, K., and Antonietti, M. (2005) Nat. Mater., 4, 51. Guo, X.H., Yu, S.-H., and Cai, G.-B. (2006) Angew. Chem. Int. Ed., 45, 3977. Orme, C.A., Noy, A., Wierzbicki, A., McBride, M.T., Grantham, M., Teng, H.H., Dove, P.M., and DeYoreo, J.J. (2001) Nature, 411, 775. Sanchez, C., Arribart, H., and Guille, M.M.G. (2005) Nat. Mater., 4, 277. Sun, S., Murray, C.B., Weller, D., Folks, L., and Moser, A. (2000) Science, 287, 1989. Wu, Y., Li, W., and Shen, X. (eds) (2002) Organic Chemistry, University of Science and Technology of China Press, Hefei, China. Neel, M.L. (1954) J. Phys. Radium, 15, 225. Wan, D.F. and Ma, X.L. (eds) (1999) Magnetic Physics, Publishing House of Electronics Industry, Beijing, China. Yavuz, C.T., Mayo, J.T., Yu, W.W., Prakash, A., Falkner, J.C., Yean, S., Cong, L., Shipley, H.J., Kan, A., Tomson, M., Natelson, D., and Colvin, V.L. (2006) Science, 314, 964. Taberna, P.L., Mitra, S., Poizot, P., Simon, P., and Tarascon, J.-M. (2006) Nat. Mater., 5, 567. Gao, L., Zhuang, J., Nie, L., Zhang, J., Zhang, Y., Gu, N., Wang, T., Feng, J., Yang, D., Perrett, S., and Yan, X. (2007) Nat. Nanotechnol., 2, 577. Lee, H., Yu, M.K., Park, S., Moon, S., Min, J.J., Jeong, Y.Y., Kang, H.-W., and Jon, S. (2007) J. Am. Chem. Soc., 129, 12739. Jiang, J., Gu, H., Shao, H., Devlin, E., Papaefthymiou, G.C., and Ying, J.Y. (2008) Adv. Mater., 20, 4403. Chen, X., Li, L., Sun, X., Liu, Y., Luo, B., Wang, C., Bao, Y., Xu, H., and Peng, H. (2011) Angew. Chem. Int. Ed., 50, 5486. Talapin, D.V., Shevchenko, E.V., Bodnarchuk, M.I., Ye, X., Chen, J., and Murray, C.B. (2009) Nature, 461, 964. Hu, P., Yu, L., Zuo, A., Guo, C., and Yuan, F. (2009) J. Phys. Chem. C, 113, 900. Zeng, Y., Hao, R., Xing, B., Hou, Y., and Xu, Z. (2010) Chem. Commun., 46, 3920. Mathur, S., Barth, S., Werner, U., Hernandez-Ramirez, F., and Romano-Rodriguez, A. (2008) Adv. Mater., 20, 1550. Pitzschel, K., Moreno, J.M.M., Escrig, J., Albrecht, O., Nielsch, K., and Bachmann, J. (2009) ACS Nano, 3, 3463. Hu, F., MacRenaris, K.W., Waters, E.A., Schultz-Sikma, E.A., Eckermann, A.L., and Meade, T.J. (2010) Chem. Commun., 46, 73. Shavel, A., Rodríguez-González, B., Spasova, M., Farle, M., and Liz-Marzán, L.M. (2007) Adv. Funct. Mater., 17, 3870. Cao, H.Q., Liang, R.L., Qian, D., Shao, J., and Qu, M.Z. (2011) J. Phys. Chem. C, 115, 24688. Vollath, D. (2008) Nanomaterials: An Introduction to Synthesis, Properties and Applications, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim, Germany.

References

259 Buyukhatipoglu, K. and Clyne, A.M. (2010) J. Nanopart. Res., 12, 1495. 260 Shebanova, O.N. and Lazor, P. (2003) J. Chem. Phys., 119, 6100. 261 Guo, C., Hu, Y., Qian, H., Ning, J., and Xu, S. (2011) Mater. Charact., 62,

148. 262 Wu, X., Tang, J., Zhang, Y., and Wang, H. (2009) Mater. Sci. Eng., B, 157, 81. 263 Lan, F., Liu, K.-X., Jiang, W., Zeng, X.-B., and Wu, Y. (2011) Nanotechnology,

22, 225604. 264 Cornell, R.M. and Scwertmann, U. (1996) The Iron Oxides: Structure, Proper-

ties, Reactions, Occurrence and Uses, VCH, New York. 265 Stöhr, J. and Siegmann, H.C. (2006) Magnetism: From Fundamentals to

Nanoscale Dynamics, Spinger-Verlag, Berlin, Heidelberg. 266 Prozorov, R., Prozorov, T., Mallapragada, S.K., Narasimhan, B., Williams, T.J.,

267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285

and Bazylinski, D.A. (2007) Phys. Rev. B: Condens. Matter Mater. Phys., 76, 054406. Hou, B., Wu, Y., Wu, L., Shi, Y., Zou, K., and Gai, H. (2006) Mater. Lett., 60, 3188. Xi, G., Wang, C., and Wang, X. (2008) Eur. J. Inorg. Chem., (3), 425. Djurdjevi´c, P. and Jeli´c, R. (1997) Transition Met. Chem., 22, 284. Reddy, I.M. and Mahoney, A.W. (1995) J. Agric. Food Chem., 43, 1436. Dong, W., Li, X., Shang, L., Zheng, Y., Wang, G., and Li, C. (2009) Nanotechnology, 20, 035601. David, R.L. (1994) CRC Handbook of Chemistry and Physics, 75th edn, CRC Press, New York. Deng, H., Li, X., Peng, Q., Wang, X., Chen, J., and Li, Y. (2005) Angew. Chem. Int. Ed., 44, 2782. Jiang, W., McCloy, J.S., Lea, A.S., Sundararajan, J.A., Yao, Q., and Qiang, Y. (2011) Phys. Rev. B: Condens. Matter Mater. Phys., 83, 134435. Blundell, S. (2001) Magnetism in Condensed Matter, Oxford University Press, Great Clarendon Street, Oxford OX2 6DP, UK. Verwey, E.J.W. (1939) Nature, 144, 327. Goya, G.F., Berquó, T.S., Fonseca, F.C., and Morales, M.P. (2003) J. Appl. Phys., 94, 3520. Chang, M.-T., Chou, L.-J., Hsieh, C.-H., Chueh, Y.-H., Wang, Z.L., Murakami, Y., and Shindo, D. (2007) Adv. Mater., 19, 2290. García, J. and Subías, G. (2004) J. Phys.: Condens. Matter, 16, R145. Poddar, P., Fried, T., and Markovich, G. (2002) Phys. Rev. B: Condens. Matter Mater. Phys., 65, 172405. Liu, F., Cao, P., Zhang, H., Tian, H., Xiao, C., Shen, C., Li, J., and Gao, H. (2005) Adv. Mater., 17, 1893. Hamann, C.H., Hamnett, A., and Vielstich, W. (2007) Electrochemistry, 2nd version edn, Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim. Poizot, P., Laruelle, S., Grugeon, S., Dupont, L., and Tarascon, J.-M. (2000) Nature, 407, 496. Nam, K.T., Kim, D.-W., Yoo, P.J., Chiang, C.-Y., Meethong, N., Hammond, P.T., Chiang, Y.-M., and Belcher, A.M. (2006) Science, 312, 885. Wu, Z.-S., Ren, W., Wen, L., Gao, L., Zhao, J., Chen, Z., Zhou, G., Li, F., and Chen, H.-M. (2010) ACS Nano, 4, 3187.

143

144

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

286 Wang, Y., Xia, H., and Lin, J. (2010) ACS Nano, 4, 1425. 287 Wang, X., Wu, X.-L., Guo, Y.-G., Zhong, Y., Cao, X., Ma, Y., and Yao, J.

(2010) Adv. Mater., 20, 1680. 288 Ni, S., Wang, X., Zhou, G., Yang, F., Wang, J., Wang, Q., and He, D. (2009)

Mater. Lett., 63, 2701. 289 Laruelle, S., Grugeon, S., Poizot, P., Dollé, M., Dupont, L., and Tarascon,

J.-M. (2002) J. Electrochem. Soc., 149, A627. 290 Tarascon, J.-M. and Armand, M. (2001) Nature, 414, 359. 291 Du, N., Zhang, H., Chen, B., Wu, J., Ma, X., Liu, Z., Zhang, Y., Yang, D.,

Huang, X., and Tu, J. (2007) Adv. Mater., 19, 4505. 292 Kang, B. and Ceder, G. (2009) Nature, 458, 190. 293 Zhang, W.-M., Wu, X.-L., Hu, J.-S., Guo, Y.-G., and Wan, L.-J. (2008) Adv.

Funct. Mater., 18, 3941. 294 Hassoun, J., Derrien, G., Panero, S., and Scrosati, B. (2008) Adv. Mater., 20,

3169. 295 Zhang, W.-M., Hu, J.-S., Guo, Y.-G., Zheng, S.-F., Zhong, L.-S., Song, W.-G., 296 297 298 299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314

and Wan, L.-J. (2008) Adv. Mater., 20, 1160. Garcia, N. and Kao, Y.H. (1972) Phys. Rev. B: Solid State, 5, 2029. Heremans, J. and Hansen, O.P. (1979) J. Phys. C: Solid State Phys., 12, 3483. Boukai, A., Xu, K., and Heath, J.R. (2006) Adv. Mater., 18, 864. Cornelius, T.W., Toimil-Molares, M.E., Neumann, R., Fahsold, G., Lovrincic, R., Pucci, A., and Karim, S. (2006) Appl. Phys. Lett., 88, 103114. Chiu, P.H.P. and Shih, I.H. (2006) Appl. Phys. Lett., 88, 072110. Li, L., Yang, W.Y., Huang, X.H., Li, G.H., Ang, R., and Zhang, L.D. (2006) Appl. Phys. Lett., 88, 103119. Liu, H. and Wang, Z.L. (2005) J. Am. Chem. Soc., 127, 15322. Leontie, L., Caraman, M., Delibas, M., and Rusu, G.I. (2001) Mater. Res. Bull., 36, 1629. Leontie, L., Caraman, M., Alexe, M., and Harnagea, C. (2002) Surf. Sci., 507, 480. Li, W. (2006) Mater. Chem. Phys., 99, 174. Hameed, A., Montini, T., Gombac, V., and Fornasiero, P. (2008) J. Am. Chem. Soc., 130, 9658. Li, H.Y., Wang, D.J., Wang, P., Fan, H.M., and Xie, T.F. (2009) Chem. – Eur. J., 15, 12521. Lin, X.P., Xing, J.C., Wang, W.D., Shan, Z.C., Xu, F.F., and Huang, F.Q. (2007) J. Phys. Chem. C, 111, 18288. Lin, X.P., Huang, F.Q., Wang, W.D., and Shi, J.L. (2007) Scr. Mater., 56, 189. Hameed, A., Gombac, V., Montini, T., Felisari, L., and Fornasiero, P. (2009) Chem. Phys. Lett., 483, 254. Liu, Y., Xin, F., Wang, F., Luo, S., and Yin, X. (2010) J. Alloys Compd., 498, 179. Bian, Z., Huo, Y., Zhang, Y., Zhu, J., Lu, Y., and Li, H. (2009) Appl. Catal., B, 91, 247. Drache, M., Roussel, P., and Wignacourt, J.P. (2007) Chem. Rev., 107, 80. Shuk, P., Wiemhofer, H.D., Guth, U., Gopel, W., and Solid State, M. (1996) Ionics, 89, 179.

References

315 Landry, C.C. and Barron, A.R. (1993) Science, 260, 1653. 316 Gerbec, J.A., Magana, D., Washington, A., and Strouse, G.F. (2005) J. Am.

Chem. Soc., 127, 15791. 317 Gupta, M. and Leong, W.W. (2007) Microwaves and Metals, John Wiley &

Sons (Asia) Pte. Ltd.. 318 Zhu, J.F. and Zhu, Y.J. (2006) J. Phys. Chem. B, 110, 8593. 319 Jacob, D.S., Genish, I., Klein, L., and Gedanken, A. (2006) J. Phys. Chem. B,

110, 17711. 320 Li, D.S. and Komarneni, S. (2006) J. Am. Ceram. Soc., 89, 1510. 321 Liu, X.W., Cao, H.Q., and Yin, J.F. (2011) Nano Res., 4, 470. 322 Park, S., Kang, K.H., Han, W.Q., and Vogt, T. (2005) J. Alloys Compd., 400,

88. 323 Gao, Y.H., Niu, H.L., Zeng, C., and Chen, Q.W. (2003) Chem. Phys. Lett.,

367, 141. 324 Wang, J., Wang, X., Peng, Q., and Li, Y. (2004) Inorg. Chem., 43, 7552. 325 Mitch, M.G., Chase, S.J., Fortner, J., Yu, R.Q., and Lannin, J.S. (1991) Phys.

Rev. Lett., 67, 875. 326 Lannin, J.S. (1992) J. Non-Cryst. Solids, 141, 233. 327 Lannin, J.S., Calleja, J.M., and Cardona, M. (1975) Phys. Rev. B: Solid State,

12, 585. 328 Onari, S., Miura, M., and Matsuishi, K. (2002) Appl. Surf. Sci., 197/198, 615. 329 Huang, H.Z. (2003) Nanomaterials Analysis, Chemical Industry Press, Bei-

jing, China. 330 Chastain, J. (1992) Handbook of X-ray Photoelectron Spectroscopy,

Perkin-Elmer Corporation, Minnesota, USA. 331 Hüfner, S. (1996) Hotoelectron Spectroscopy, 2nd edn, Springer-Verlag, New

York. 332 Dharmadhikari, V.S., Sainkar, S.R., Badrinarayan, S., and Goswami, A. (1982) 333 334 335 336 337 338 339 340 341 342 343 344

J. Electron. Spectrosc. Relat. Phenom., 25, 181. Morgan, W.E., Stec, W.J., and Van Vazer, J.R. (1973) Inorg. Chem., 12, 953. Ding, P., Du, Y.G., and Xu, Z.L. (2004) Chem. Res. Chin. Univ., 20, 717. Qu, L., Shi, G., Wu, X., and Fan, B. (2004) Adv. Mater., 16, 1200. Chen, S., Liu, Y., Shao, C., Mu, R., Lu, Y., Zhang, J., Shen, D., and Fan, X. (2005) Adv. Mater., 17, 586. Thostenson, E.T. and Chou, T.W. (1999) Composites, Part A, 30, 1055. Rao, K.J., Vaidhyanathan, B., Ganguli, M., and Ramakrishnan, P.A. (2002) Chem. Mater., 11, 882. Li, C., O’Halloran, K.P., Ma, H., and Shi, S. (2009) J. Phys. Chem. B, 113, 8043. Wang, C., Shao, C., Wang, L., Zhang, L., Li, X., and Liu, Y. (2009) J. Colloid Interface Sci., 333, 242. Yang, B.J., Mo, M.S., Hu, H.M., Li, C., Yang, X.G., Li, Q.W., and Qian, Y.T. (2004) Eur. J. Inorg. Chem., (9), 1785. Dong, W.T. and Zhu, C.S. (2003) J. Phys. Chem. Solids, 64, 265. Soitah, T.N., Yang, C., Yu, Y., Niu, Y., and Sun, L. (2010) Curr. Appl. Phys., 10, 13720. Kanofsky, J.R. (1989) Chem.-Biol. Interact., 70, 1.

145

146

2 Synthesis, Characterization, and Applications of Zero-Dimensional (0D) Nanostructures

345 Liu, G.M., Li, X.Z., Zhao, J.C., Hidaka, H., and Serpone, N. (2000) Environ.

Sci. Technol., 34, 3982. 346 Kosani´c, M.M. and Triˇckovi´c, J.S. (2002) J. Photochem. Photobiol., A, 149,

247. 347 Shamaila, S., Sajjad, A.K.L., Chen, F., and Zhang, J. (2010) Appl. Catal., B,

94, 272. 348 Sato, J. and Saito, N. (2003) H. Nis6.hiyama, Y. Inoue. J. Phys. Chem. B, 107,

7965.

147

3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures 3.1 General Remarks In view of the physicochemical properties and compositions, 1D nanostructures have been distinguished between several major cases: • Synthesis and magnetic/electrical properties of metal nanowires/nanotubes and corresponding growth mechanism • Synthesis and optical properties of metal oxide nanowires • Synthesis and electrochemical properties and supercapacitor applications of MoO3 nanowires • Synthesis and bioapplication of hydroxyapatite (HAP) nanorods • Synthesis of sulfide (CdS) nanowires • Synthesis of fullerene (C70 ) nanowires. These nanowires can be synthesized by sol–gel template technique, or electrodeposition template technique, electroless template technique, or hydrothermal technique. The optical, magnetic, and electrical properties of metal nanowires; bioapplications of HAP nanorods; and adsorption properties for removal of rhodamine B (RhB) dye molecules have been studied and applied in supercapacitors for MoO3 nanowires, and so on. Especially, the “current-directed tubular growth” for 1D metal nanostructures has been proposed. This mechanism can guide the growth of 1D metal nanostructures. Furthermore, these 1D metal nanostructures can be oxidized or vulcanized, leading to 1D nanostructures of oxides or sulfides. Also put forward is the the concept of “second-order template,” which can be applied to synthesize 1D concentric cylinder-like nanostructures.

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties of Metal Nanowires/Nanotubes 3.2.1 Fe Nanowire Arrays

Great efforts have been devoted to the development of perpendicular recording materials [1–4]. The template synthesis method provides a versatile approach and has had considerable success in the preparation of arrays of Fe, Co, Ni, and Synthesis and Applications of Inorganic Nanostructures, First Edition. Huaqiang Cao. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA. Published 2018 by Wiley-VCH Verlag GmbH & Co. KGaA.

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alloy nanowires which have the easy axis perpendicular to the film plane [5–8]. However, magnetic metal nanowires are quite active and can be corroded in a corrosive atmosphere. In particular, iron nanowires can be easily corroded in humid air and this limits their application. Here, we introduce the synthesis of an array of iron nanowires by electrodeposition inside the polyaniline nanotubues (abbreviated to APF) [9]. The latter was synthesized in the pores of the commercially available alumina filter (anodic ) made by Whatman Inc. with a thickness of 60 mm. The magnetic properties of the APF are also described.

®

3.2.1.1 Synthesis

The synthesis process of the arrays of polyaniline nanotubes filled with iron (Fe) nanowires (APF) [9] is composed of two steps: the array of polyaniline nanotubes was prepared in the pores of an alumina membrane (Anopore Membrane, Whatman Anodisc ) by simply immersing the membrane in 10 ml of a solution of 0.3 M aniline and 1 M HCl. Ten milliliter of a solution of 0.5 M p-toluene sulfonic acid sodium acid salt solution, 0.12 M ammonium metavanadate, and 1 M HCl solution was successively added to the solution [10]. The mixture was bubbled with nitrogen before and throughout the polymerization. The reaction took 1.5–2.5 h at room temperature, then the membrane was removed from the solution, and the polyaniline on the surface of the alumina membranes was removed by polishing with alumina powder. After this step, the array of polyaniline nanotubes within the alumina membrane support was used as a “second-order template.” Before the electrochemical deposition step, silver paste was spread on one side of the second-order template membrane used as a working electrode in the electrochemical deposition process. The electrochemical deposition step, carried out using a solution of FeSO4 ⋅7H2 O 140 g l−1 , H3 BO3 50 g l−1 , and ascorbic acid 1 g l−1 as the electrolyte, was confined to the bare side of the “second-order template” membrane, so that the electrodeposition was initiated onto the silver film from within the polyaniline nanotubes. The iron was deposited via a bottom-up manner in the “second-order template” membrane so as to fill up the pores. The extra iron that overflowed from the pores was removed by chemical etching using FeCl3 solution.

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3.2.1.2 Characterization

The phase structure of the as-synthesized sample was characterized by powder X-ray diffraction (XRD) of the sample, which was recorded on a Rigaku X-ray diffractometer (Cu K𝛼 radiation, 𝜆 = 0.15418 nm). The as-synthesized sample was immersed in 6 M of NaOH solution to dissolve the alumina template and the silver electrode film was removed as far as possible by chemical and mechanical treatments. Then, the morphology and component of the as-synthesized sample was characterized by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) analysis measurements, recorded on a Hitachi X650 electron microscope assembled with EDAX PV91000. Magnetization loops were measured using a vibrating sample magnetometer (VSM, Lakeshore EM 7037) at room temperature.

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties

Figure 3.1 XRD pattern of the as-synthesized sample. (Cao et al. 2001. [9]. Reproduced with permission of Royal Society of Chemistry.) Ag Ag Fe(200)

Ag Fe(110)

40

50

60

Ag Ag Fe(211)

70

80

2θ (°)

Figure 3.2 EDX profile of the as-synthesized sample. (Cao et al. 2001. [9]. Reproduced with permission of Royal Society of Chemistry.)

Al(Ka)

Fe(Ka)

Au(Ma)

Au(La1) Fe(Kb) Au(Lb2)

0

2

4

6

8

10

12

14

Energy (keV)

3.2.1.3 Structure

From the XRD pattern (Figure 3.1), it can be seen that iron (ASPDF 6-0696) and silver (ASPDF 4-0784) have similar XRD diffraction peaks in the range of 38∘ < 2 𝜃 < 82∘ . And, the iron diffraction peaks can be indexed as (110), (200), and (211) body-centered cubic (bcc) iron metal, while there is no iron compound. However, in order to identify the presence of iron, the sample is further characterized by EDX measurement (Figure 3.2). From the EDX profile, it can be seen that there are three elements: iron, aluminum, and gold. Aluminum comes from the alumina template and gold comes from the thin film of gold sputtered before the SEM and EDX measurements, while the iron comes from the iron nanowires encapsulated within polyaniline nanotubes. The VSM measurement (Figure 3.4) can further demonstrate the presence of iron besides silver (Figure 3.1). The morphology of the as-synthesized sample is studied by SEM observation (Figure 3.3). Figure 3.3a presents the polyaniline nanotube arrays after dissolving a part of the alumina template, which shows the outer diameter of the tubes is about 200 nm, corresponding to the pore diameter of alumina. Figure 3.3b presents the iron nanowires encapsulated within the polyaniline nanotube array, from which can be observed that the as-synthesized sample stands on the substrate surface just like the bristles of a brush.

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3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

(a)

(b)

Figure 3.3 Scanning electron microscopic images of (a) polyamine nanotubes obtained after dissolving the top layer of alumina membrane, (b) iron nanowires filled within polyaniline nanotubes obtained after dissolution of the top layer of alumina template, showing nanowires of monodisperse length corresponding to the alumina membrane thickness (60 μm). (Cao et al. 2001. [9]. Reproduced with permission of Royal Society of Chemistry.)

3.2.1.4 Magnetic Properties

The M–H hysteresis curves of the as-synthesized APF sample are shown in Figure 3.4a, and shows that the easy axis lies in the membrane plane. The coercivity force increases from 119 to 210 Oe as 𝜃 increases from 0∘ to 90∘ , where 𝜃 is the angle between the surface of the membrane plane (Figure 3.4b) and the applied field. The squareness ratio (Mr /Ms ) of the array decreases from 0.108 to 0.056, with the maximum at 𝜃 = 15∘ (Figure 3.4c). The remanence of the array is 11% less than the saturation magnetization, because the results show that the array of iron nanowires encapsulated within the polyaniline nanotubes may be a candidate material for horizontal magnetic recording. 3.2.2 Co Nanowire Arrays

Martin et al. have been exploring a template synthesis method which entails synthesizing the desired materials within the pores of a nanopore membrane. This method has been shown to be a versatile approach for preparing nanomaterials. Using this template synthesis method, nanotubules and nanofibrils composed of metals [11], semiconductors [12], polymers [13], and various composites of these materials have been prepared. These materials often have useful optical [14], electronic [15], and magnetic [5] properties. For example, the distinctive anisotropic magnetic properties of an array of Ni and Co nanowires make this material a potential candidate for perpendicular magnetic recording. Also, the nanosize volume of this material reduces the turbulence current when a high-frequency field is applied, which is important for antenna materials in the high-frequency range. But the metal nanowires are very sensitive to air and moisture, which degrades the performance of the nanodevice. A polymer envelope would protect metal nanowires from oxidation and corrosion, giving good performance for a long time.

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties 220

1.0

200

Coercivity (Oe)

Mr /Ms

0.5

0° 90°

0.0 –0.5

160 140 120

–1.0 –10 000

180

–5000

(a)

0 Field (Oe)

5000

10 000

0

15

30

45

60

75

90

θ (°)

(b)

Squareness ratio

0.11 0.10 0.09 0.08 0.07 0.06 0.05 0

(c)

15

30

45

60

75

90

θ (°)

Figure 3.4 VSM (a) Magnetization curves with field applied parallel and perpendicular to the as-synthesized sample membrane recorded at room temperature, (b) the variation of coercive force, (c) the variation of squareness with the angle (𝜃) between the applied magnetic field and the APF membrane. (Cao et al. 2001. [9]. Reproduced with permission of Royal Society of Chemistry.)

Here, we introduce Co nanowires within polyaniline nanotube arrays by the template method [16]. 3.2.2.1 Synthesis

The synthesis process of the arrays of polyaniline nanotubes filled with cobalt (Co) nanowires (termed as APC) [16] is similar to the synthesis of the APF, shown in the Synthesis in Section 3.2.1; which is composed of two steps: the arrays of polyaniline nanotubes within the alumina template membranes (with quoted pore diameter of 20 nm, Anopore Membrane, Whatman Anodisc ) were synthesized firstly, then the alumina template membrane was removed from the solution, and the polyaniline on the surface of the alumina template membranes was removed by a polishing treatment with an alumina powder suspension liquid through a mechanical friction. The second step is the cobalt electrochemical deposition step, that is, the synthesized polyaniline nanotubes were used as a “second-order template.” Silver paste was spread on one side of the “second-order template” membrane, used as a working electrode in the electrochemical deposition process. The electrolyte solution containing 266 g l−1 CoSO4 ⋅7H2 O and 40 g l−1 H3 BO3 was confined to another bare side of the “second-order template” membrane, so that the electrodeposition was initiated on the silver electrode

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151

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3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

Figure 3.5 Scanning electron micrographic images of (a) polyaniline nanotubes and (b) polyaniline nanotubes filled with cobalt nanowires obtained after an etching treatment of alumina template membrane by using 6 M NaOH solution. (Cao et al. 2001 [16]. Reproduced with permission of John Wiley & Sons.) (a)

(b)

film from within the polyaniline nanotubes. The metal cobalt was deposited in a bottom-up manner within the pore of the “second-order template” membrane so far as to fill up these pores. The extra cobalt that overflowed from the pores was removed by a chemical treatment with a FeCl3 solution. 3.2.2.2 Characterization

The phase structure of the as-synthesized APC was studied by XRD using an X-ray diffractometer, which was recorded on a Rigaku X-ray diffractometer (Cu-K𝛼 radiation, 𝜆 = 0.15418 nm). The morphology of the APC was studied using SEM (Hitachi X650 electron microscope). 3.2.2.3 Structure

The morphology of the as-synthesized APC is shown in Figure 3.5. Figure 3.5a presents the polyaniline nanotube arrays which are used as the “second-order template” for the cobalt electrochemical deposition. Figure 3.5b presents the arrays of polyaniline nanotubes filled with cobalt nanowires, and the length of the nanowires corresponds to the alumina Anopore membrane thickness (i.e., 60 μm), suggesting that the as-synthesized nanowire arrays are mechanically stable under the sample preparation conditions. The typical XRD pattern of the as-synthesized APC, which is obtained after 2 weeks of being exposed to the air, is shown in Figure 3.6. The diffraction peaks come from silver (due to silver paste film as the working electrode in the electrochemical deposition) and cobalt. The cobalt diffraction peaks can be indexed as (100), (002), (101), and (110) of the hexagonal close packed (hcp) cobalt phase (Card number 5-0727). The preferred growth direction for the cobalt nanowires is the (110) direction – the orientation index, which can be identified by from the XRD pattern. 3.2.2.4 Magnetic Properties

The M–H hysteresis curves of the as-synthesized APC sample are shown in Figure 3.7: the applied magnetic field is parallel to the nanowire arrays (i.e., perpendicular to the surface of the APC membrane) and perpendicular to the nanowire arrays (i.e., parallel to the surface of the APC membrane). The

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties

Figure 3.6 XRD pattern of the as-synthesized the APC. (Cao et al. 2001 [16]. Reproduced with permission of John Wiley & Sons.)

Co(110)

Ag Ag Co(002)

Ag

Ag

Co(100) Co(101)

35

40

45

50

55

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65

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75

Ag

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85

2θ

1.0

Mr /Ms

0.5

0° 90°

0.0 –0.5 –1.0 –10 000

–5000

0

5000

10 000

Field (Oe)

Figure 3.7 Hysteresis loops of the as-synthesized APC varied for various angles, 𝜃, between the applied magnetic field and the membrane plane. (Cao et al. 2001 [16]. Reproduced with permission of John Wiley & Sons.)

hysteresis loops show that the array exhibits uniaxial ferromagnetic anisotropy with the easy axis parallel to the nanowire: the magnetization, M, being perpendicular to the membrane. By comparison with a few tens of oersteds of coercivities of bulk cobalt, enhanced values in the range of 180–209 Oe can be observed. It is well known that ultrafine ferromagnetic particles exhibit greatly enhanced magnetic coercivity, 2 orders of magnetic higher than the bulk material, because of their single-domain nature. Figure 3.8 shows the angular dependence of coercivity (H c ) and the remanent–saturation magnetization ratio (Mr /Ms ). Here, the 𝜃 angle indicates the angle between the surface of the membrane and the applied magnetic field. It can be found that the maximum of Mr is got when the applied magnetic field is parallel to the nanowires (i.e., 𝜃 = 90∘ ), while the H c of the array reaches a maximum when the applied magnetic field is perpendicular to the nanowires (i.e., 𝜃 = 0∘ ). Furthermore, the remanent magnetization, Mr , is always less than 20% of the saturation magnetization, which indicates very strong interaction among the nanowires because they are close to each other.

153

3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures 215 210 205 200 195 190 185 180 175

0.20 Squareness ratio

Coercivity (Oe)

154

0.18 0.16 0.14 0.12 0.10

0

15

30

(a)

45 θ (°)

60

75

90

0

(b)

15

30

45 θ (°)

60

75

90

Figure 3.8 Angular dependence of (a) coercive force and (b) squareness, with the angle, 𝜃, the angle between the surface of the membrane and the applied magnetic field. (Cao et al. 2001 [16]. Reproduced with permission of John Wiley & Sons.)

3.2.3 Ni Nanowire Arrays

Searson and Chien [5] recently reported the fabrication of an array of Ni and Co nanowires by the template synthesis method and revealed that the preferred magnetization direction of the array is perpendicular to the film plane with the enhanced coercivities and high remanent magnetization, which may make the array useful for high-density perpendicular magnetic recording [17]. However, the large surface area of the metal nanowires makes it very sensitive to moisture, oxygen, and corrosion gases, which lead to degradation of the performance of the nanodevice. The encapsulated metal nanowires with polymer nanotubules may be a suitable approach with which to overcome this trouble. In addition to this, the magnetic metal nanowires encapsulated with polymer nanotubules may be a potential candidate for magnetic antenna materials in high-frequency bands because their nanosize and increasing specific resistance reduce the turbulence current loss. We introduce here a method for the preparation of an array of magnetic metal nanowires encapsulated with polyaniline nanotubules and their magnetic properties [18]. 3.2.3.1 Synthesis

The synthesis process of the arrays of polyaniline nanotubes filled with nickel (Ni) nanowires (termed as APN) [18] is similar to the synthesis of the APF, shown in the Synthesis in Section 3.2.1; the arrays of polyaniline nanotubes within the alumina template membranes (Anopore Membrane, Whatman Anodisc ) were synthesized firstly, then the alumina template membrane was removed from the solution, and the polyaniline on the surface of alumina template membranes was removed by a polishing treatment with an alumina powder suspension liquid through a mechanical friction. The second step is the nickel electrochemical deposition step, that is, the synthesized polyaniline nanotubes were used as a “second-order template.” Before electrochemical deposition, silver paste was spread on one side of the second-order template membrane, used as a working electrode. The electrochemical deposition [5, 19, 20] was carried out using a constant current mode at 0.1 mA. The electrolyte solution containing NiSO4 ⋅6H2 O (270 g l−1 ), NiCl2 ⋅6H2 O (40 g l−1 ), and H3 BO3 (40 g l−1 )

®

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties

was confined to the bare side of the second-order template membrane so that the electrochemical deposition was initiated on the silver film electrode from within the polyaniline nanotubes. The nickel nanowires were generated by a bottom-up manner within the pores of the second-order template membrane far enough to fill up the pores. The extra nickel that overflowed from the pores was removed using a chemical treatment – reacting with FeCl3 solution. Then, the arrays of polyaniline nanotubes filled with nickel nanowires (termed as APN) was obtained. The length of the nickel nanowires corresponds to the thickness of the alumina membrane (about 60 μm). 3.2.3.2 Characterization

The phase structure of the as-synthesized APN was studied by XRD using an X-ray diffractometer, which was recorded on a Rigaku X-ray diffractometer (D/Max-RA, Cu K𝛼 radiation, 𝜆 = 0.15418 nm). The morphology of the APN was studied using SEM (Hitachi X650 electron microscope). 3.2.3.3 Structure

The morphology of the APN is shown in Figure 3.9. The typical inner diameter of the polyaniline nanotubes is ∼260 nm, which corresponds to the outer diameter of the nickel nanowires (Figure 3.9b). In order to present the nanowires, the alumina membrane was dissolved in 6 M of NaOH solution for 5 min. Figure 3.9a shows the SEM image of the APN, whose length corresponds to the membrane thickness (about 60 μm), suggesting that the as-synthesized nanowires are mechanically stable under the sample preparation conditions and the aspect ratio of the wires is more than 200. Figure 3.10 presents a typical XRD pattern of the as-synthesized sample. The diffraction peaks can be indexed to (111) and (200) of the face-centered cubic (fcc) nickel structure (card number: 4-0854), besides silver diffraction peaks due

(a)

(b)

(c)

Figure 3.9 SEM images of (a) polyaniline nanotubes, (b) a cross-section view, showing that the array of polyaniline nanotubes filled with nickel nanowires (APN) stand on the silver paste layers which are used as a working electrode in the electrochemical deposition, and (c) an aerial view, showing that the APN, obtained after dissolution of part of the alumina template membrane using 6 M NaOH solution. (Cao et al. 2001 [18]. Reproduced with permission of American Institute of Physics.)

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3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

Figure 3.10 XRD pattern of the as-synthesized APN. (Cao et al. 2001 [18]. Reproduced with permission of American Institute of Physics.)

Ni (111)

5000 Counts per second

156

4000 3000 Ni (200)

2000 Ag

1000

Ag

0 35

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50

55

60

65

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2θ (°)

to the silver film on the one side of the membrane used as the working electrode in the electrochemical deposition. The fcc-Ni, not nickel-related compounds, can demonstrate that the polyaniline envelope protects the nickel nanowires from oxidation, because the XRD measurement was carried out after 2 weeks for the APN exposed to the air. 3.2.3.4 Magnetic Properties

The M–H hysteresis curves of the APN are shown in Figure 3.11a. The coercivities, remanences, and saturation magnetization values were measured. The hysteresis loops reveal that the easy axis of the APN is parallel to the nanowires. By comparison with a few tens of oersteds of coercivities of bulk nickel, enhanced values in the range of 129–180 Oe were observed (Figure 3.11b). The minimum value appears at 𝜃 = 15∘ (𝜃, the angle between the surface of the membrane and the applied magnetic field) while the maximum value appears at 𝜃 = 90∘ (the applied magnetic field is parallel to the nanowires). The Mr /Ms − 𝜃 curve has a similar pattern with the minimum value at 𝜃 = 15∘ and the maximum value at 𝜃 = 90∘ and Mr /Ms increases from 0.0815 to 0.128 (Figure 3.11c). Furthermore, the remanent magnetization Mr is always less than 18% of the saturation magnetization, which indicates very strong interaction among the nanowires because they are very close to each other. 3.2.4 Cu Nanowire Arrays

A lot of methods have been reported to synthesize Cu nanowires, such as a DNA-templated construction method [21], a hydrogen arc method [22], a vapor–solid reaction growth method [23], a vacuum vapor deposition method [24], a polycarbonate template method [25], and so on. Here, we introduce an electrochemical deposition technique to synthesize the copper nanowire arrays (termed as CNAs) within the pores of alumina template and to understand current–voltage (I–V ) properties of the CNAs [26]. 3.2.4.1 Synthesis

®

We synthesized the CNAs using alumina template (Whatman Anodisc ) with a quoted pore diameter of 20 nm. Cu plate and silver or gold film covered on one

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties 190

1.0

180 Coercivity (Oe)

Mr/Ms

0.5

0° 90°

0.0 –0.5

160 150 140 130 120

–1.0 –10 000

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–5000

(a)

0 Field (Oe)

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0

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(b)

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45 60 θ (°)

75

90

Squareness ratio

0.18 0.16 0.14 0.12 0.10 0.08 0.06 0

(c)

15

30

45 60 θ (°)

75

90

Figure 3.11 (a) Magnetization loops with the applied magnetic field parallel (𝜃 = 90∘ ) and perpendicular (𝜃 = 0∘ ) to the APN recorded at room temperature using a VSM, (b) variation of coercive force and (c) the variation of squareness with angle between the applied magnetic field and the surface of the membrane. (Cao et al. 2001 [18]. Reproduced with permission of American Institute of Physics.)

side of the alumina membrane were used as anode and working electrode (cathode). The electrochemical deposition solution [CuSO4 ⋅5H2 O (23.8 g l−1 , analytical reagent, AR) and H2 SO4 (2.1 g l−1 , AR)] [25] was confined to the bare side of the alumina template. A current density of 0.2 mA mm−2 was applied between the two electrodes, and held for about 15 min until the pores of the template were filled with copper nanowires. 3.2.4.2 Characterization

The phase and morphology of the CNAs were studied by an X-ray diffractometer (Rigaku, D/max-RB), scanning electron microscope (KYKY-2000), and transmission electron microscope (JEOL, JEM-1200 EX TEM). The I–V curve measurement for the CNAs was carried out using a current-sensing atomic force microscope (CS-AFM; SPI 3800 Series SPA-400 scanning probe microscope system). 3.2.4.3 Structure

The XRD pattern (Figure 3.12) can be well indexed to (111), (200), and (220) reflection of the cubic copper (JCPDS 04-0836). The preferred growth for the copper nanowires is along the (111) direction. From the SEM images (Figure 3.13), we can find the products are well-aligned nanowires. In most cases, the length of the nanowires corresponds to the alumina template membrane thickness (about 60 μm).

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3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

Figure 3.12 XRD pattern of the sample. (Cao et al. 2006 [26]. Reproduced with permission of American Institute of Physics.)

Cu(111)

Intensity (a.u.)

158

Cu(200) Cu(220)

10

20

30

40

50

60

70

80

2θ (°)

(a)

(b)

10 μm

(c)

1 μm

10 μm

(d)

10 μm

Figure 3.13 SEM images with various magnifications of Cu nanowire arrays: (a) top-view image; (b)–(d) side-view images with different magnification. The top alumina template was removed using 3 M NaOH solution. (Cao et al. 2006 [26]. Reproduced with permission of American Institute of Physics.)

A large quantity of pure copper nanowires have nearly uniform diameters, ranging from 50 to 80 nm which corresponds to the pore sizes of the alumina template membrane, based on the TEM image observation (Figure 3.14a). A single copper nanowire with a diameter of about 65 nm is shown in Figure 3.14b. The electron diffraction (ED) pattern (inset in Figure 3.14b) demonstrates that the Cu nanowire belongs to a single-crystal structure.

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties

(a)

(b)

Figure 3.14 TEM images of the as-synthesized Cu nanowires: (a) low-magnification TEM image of Cu nanowires, to show the large quantity of nanowires; (b) a piece of Cu nanowire. The inset in part (b) is the corresponding electron diffraction pattern showing a single-crystal structure. The scale bars in (a) and (b) are 500 and 200 nm, respectively. (Cao et al. 2006 [26]. Reproduced with permission of American Institute of Physics.)

AFM tip

1.0

III

0.8

Cu nanowires

0.6

Current (nA)

V

II

0.4 0.2 0.0 –0.2

Au back electrode (a)

I

–0.4

(b)

–10 –8 –6 –4 –2 0 2 4 Voltage bias (V)

6

8

10

Figure 3.15 (a) Schematic pattern of current-sensing AFM measurement; (b) a typical I–V curve for the copper nanowire arrays. (Cao et al. 2006 [26]. Reproduced with permission of American Institute of Physics.)

3.2.4.4 Electrical Properties

Electronic transport through nanocontacts has been an active research area for a decade [27, 28]. The schematic pattern model in scanning probe microscope is shown in Figure 3.15a. With the CS-AFM and embedded copper nanowire arrays, the I–V curves of hundreds of individual copper nanowires can be investigated quickly. The I–V measurements are carried out placing a bias between the gold back-electrode layer and the Au-coated AFM tip with a spring constant of 0.14 N m−1 at room temperature in air. The I–V curves are obtained by measuring at different spots.

159

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3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

The electrical properties of copper nanowire arrays are obtained by recording five sets of I–V curves. After averaging these curves, we get the I–V curves shown in Figure 3.15b, which shows a typical I–V characteristic curve for copper nanowires with the contact force of −0.1 nN (“−” means repulsive force in the AFM system). This figure shows that the electronic transport behavior of Cu nanowires does not observe the Ohm’s law, while it has obviously nonlinear properties. The I–V curve can be divided into three regions, that is, I, II, and III, marked in the Figure 3.15b, respectively. The slopes (k = I/V ) for the regions I, II, and III are k I = 0.45, k II = 0.03, and k III = 0.18, with the corresponding resistances of ≫ 4.66 × 1010 , 9.15 × 109 , and 1.09 × 1010 Ω, respectively. k I = 15k II = 2.5k III , which means there is a quasi-plateau development in region II. The copper nanowire arrays exhibit highly nonlinear I–V behavior and a symmetric characteristic with respect to zero bias, which is a characteristic of tunneling through a barrier [28]. The simplest possibility for observing such a phenomenon is generation of a tunneling barrier at the wire-lead junction whose effect gradually collapses as a function of increasing bias voltage [29]. The nonlinear curves of copper nanowire arrays may be caused by the existence of impurities (such as oxide) near the wire-gold contact region. It has been demonstrated that the nonlinear I–V characteristic is the basis of functional electronic devices [30, 31]. Thus, we believe the as-synthesized copper nanowire arrays may find application in future functional electronic devices. 3.2.5 Growth Mechanism for 1D Metal Nanostructures

Since the discovery of carbon nanotubes by Iijima [32], nanotube materials have attracted great interest due to their distinctive physical properties and potential applications in nanodevices [33]. Many kinds of nanotubes composed of layered structures were synthesized, for example, metal disulfides (TiS2 , MoS2 , WS2 ) [34], WO2 Cl2 [35], and Bi [36] nanotubes. Recently, numerous methods have been used to grow inorganic nanotubes constructed of nonlayered materials, such as hexagonal structured TiS2 , AlN, Y(OH)3 , Dy(OH)3 , and Mg3 Si2 O5 (OH)4 nanotubes [37–41], orthorhombic structured Cu(OH)2 [42] and Mg3 B2 O6 nanotubes [43], cubic structured Si [39, 44] and Y2 O3 nanotubes [39], wurtzite-type structured InN nanotubes [45], anatase TiO2 nanotubes [46], and so forth. The template (including alumina and polymer membrane) synthesis method, explored by Martin, Searson, Chien, and coworkers [5, 47], is an important synthetic strategy. It has been demonstrated that template technology can be used to synthesize one-dimensional (1D) nanomaterials including nanowires and nanotubes composed of carbon [48], metals [49–52], semiconductors [53, 54], polymers [13], and some organic molecules [55, 56]. These 1D materials usually have useful optical, electronic, and magnetic properties. For example, 1D nanostructured metal materials, such as metal nanowires [9, 16, 18] and nanotubes [49–52, 57–59], have potential applications as magnetic storage media [5], nanoelectronic circuitry [60], and nanoscale sensors [61]. However, only a few reports

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties

on the synthesis of metal nanotubes have been published, which deal with the chemical reduction of metallic complexes and chemical vapor infiltration within a porous alumina template. Metal nanotubes, such as Au, Pd, Fe, Ni, and Co nanotubes, have been synthesized via a template-assisted synthesis method with chemical modification of the inner surface of the pores of the template prior to deposition [52, 57–59]. However, this technique might yield impurities [62–65]. To the best of our knowledge, the systematic preparation and detailed growth mechanism of metal nanotubes by an electrodeposition method have never been reported. This aroused our interest in probing the growth mechanism of metal nanotubes. Here, we introduce the systematic preparation of well-aligned metal (Fe, Co, Ni) nanotubes and their growth mechanisms [66]. 3.2.5.1 Synthesis

®

The alumina template membrane (Anopore Membrane, Whatman Anodisc ) was used to prepare metal (Fe, Co, and Ni) nanotube arrays using electrochemical deposition method. Before electrochemical deposition, gold film was evaporated on one side and used as a working electrode. The electrochemical deposition solution contained (i) FeSO4 ⋅7H2 O (140 g l−1 , AR), H3 BO3 (50 g l−1 , AR), and ascorbic acid (1 g l−1 , AR) [9]; (ii) CoSO4 ⋅7H2 O (266 g l−1 , AR) and H3 BO3 (40 g l−1 , AR) [16]; (iii) NiSO4 (270 g l−1 , AR), NiCl2 ⋅6H2 O (40 g l−1 , AR), and H3 BO3 (40 g l−1 , AR) [18], respectively. These solutions were confined to the bare side of the membrane to synthesize Fe, Co, and Ni nanotube arrays, respectively. All the preparations were carried out at room temperature. 3.2.5.2 Characterization

The phase structure of the as-synthesized samples were characterized by an X-ray diffractometer (XRD, Rigaku, D/max-RB, Japan) with Cu K𝛼 (𝜆 = 1.5418 Å, 40 kV × 150 mA) radiation. The morphology and microstructure of the sample were studied using SEM (SKYKY-2000 scanning electron microscope) and TEM (JEOL, JEM-1200 EX), operating at 120 kV. 3.2.5.3 Structure

Typical XRD patterns of the as-synthesized samples are shown in Figure 3.16. The XRD patterns indicate that the samples are of high crystallinity, and the diffraction peaks are broadened owing to the small size. The diffraction peaks can be indexed to the bcc iron structure (space group: Im3m) with a lattice constant a = 2.8664 Å (JCPDS 06-0696, Figure 3.16a), the hcp cobalt phase (space group: P63/mmc) with lattice constants of a = b = 2.514 Å, c = 4.105 Å, 𝛼 = 𝛽 = 90∘ , 𝛾 = 120∘ (JCPDS 1-1278; Figure 3.16b), and the fcc nickel structure (space group: Fm3m) with a lattice constant a = 3.5238 Å (JCPDS 4-850, Figure 3.16c), respectively. The broad spectral features in the 2𝜃 range of 15–35∘ originate from amorphous alumina after the removal of the top alumina layer using 3 N NaOH; no other phases are observed in the XRD patterns. This means that the metal nanostructures are stable at room temperature. It should be pointed out that the

161

Intensity (a.u.)

3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

Intensity (a.u.)

Fe(110) Al2O3(012) Fe(200)

0 10 20 30 40 50 60 70 80 90 2θ (°)

(a)

Co (100) Al2O3 (012)

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Co(002) Co(101) Co(110)

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(b) Ni (220)

Intensity (a.u.)

162

Al2O3 (012)

Ni (111)

Ni (200)

0 10 20 30 40 50 60 70 80 90 2θ (°)

(c)

Figure 3.16 XRD patterns of the as-synthesized nanotube arrays: (a) body-centered cubic iron nanotube arrays, (b) hexagonal close-packed cobalt nanotube arrays, (c) face-centered cubic nickel nanotube arrays on alumina template. (Cao et al. 2006 [66]. Reproduced with permission of John Wiley & Sons.)

Fe, Co, and Ni have nonlayered structures. In fact, in the overwhelming majority of cases, the metal crystalline structure belongs to one of three classifications: fcc, bcc, or hcp structures [67]. Thus, bcc-Fe, hcp-Co, and fcc-Ni are the three representative metal crystalline structures. (b)

(a)

10 μm

(d)

10 μm

(c)

1 μm

1 μm

(e)

(f)

10 μm

1 μm

Figure 3.17 SEM images of iron nanotube arrays. (a) top-view, magnifying powder ×5000. (b) top view, magnifying powder ×10 000. (c) top view, magnifying powder ×20 000. (d) side view, magnifying powder ×2000. (e) side view, magnifying powder ×5000. (f ) side view, magnifying powder ×10 000. (Cao et al. 2006 [66]. Reproduced with permission of John Wiley & Sons.)

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties

The morphology and microstructures of the metal nanostructures are characterized by SEM and TEM (Figures 3.17–3.19). From the SEM images (Figures 3.17, 3.18a,b, 3.19a,b), we find that the as-synthesized samples are clearly composed of well-aligned nanotubes. Typically, the length of the as-synthesized tubular structures can reach about 60 μm, which corresponds to the thickness of the alumina template membrane. Most of them have outer diameters of about 50–100 nm, which correspond to the pore diameters of the alumina template membrane, and inner diameters of about 30–50 nm. SEM observation reveals that well-aligned metal nanotubes are generated on the substrate face of the gold electrode. Figure 3.18c,d shows a typical TEM image for a single cobalt nanotube with inner and outer tubular diameters of about 30 and 65 nm, respectively. The ED patterns shown in the inset of Figure 3.18d reveals that the tube is composed of a single-crystalline structure. Figure 3.19c presents a typical TEM image for (a)

(b)

1 μm

1 μm

(c)

(d)

Figure 3.18 (a) SEM image of cobalt nanotube arrays in top view, magnifying powder ×10 000. (b) SEM image of cobalt nanotube arrays in side view, magnifying powder ×15 000. (c) TEM image of a piece of Co nanotube, magnifying powder ×20 000, scale bar is 200 nm. (d) TEM image of a piece of Co nanotube, magnifying powder ×40 000, scale bar is 200 nm. Inset is corresponding electron diffraction pattern. (Cao et al. 2006 [66]. Reproduced with permission of John Wiley & Sons.)

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3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

(a)

(c)

10 μm

(b)

1 μm

Figure 3.19 (a) SEM image of nickel nanotube arrays in upward view, magnifying powder ×1000. (b) SEM image of nickel nanotube arrays in top view, magnifying powder ×10 000. (c) TEM image of a piece of Co nanotube, scale bar is 200 nm. Inset: the corresponding electron diffraction pattern. (Cao et al. 2006 [66]. Reproduced with permission of John Wiley & Sons.)

a nickel nanotube with outer diameter of about 100 nm and inner diameter of about 50 nm; the corresponding ED pattern suggests it is also a single-crystalline structure. 3.2.5.4 Growth Mechanism

Figure 3.20 is the schematic diagram of the electrochemical deposition processes. At the beginning, metal ions (bivalent ions Fe2+ , Co2+ , and Ni2+ ) surrounded by a hydration layer move toward the cathode and are then reduced. The reduction process of metal ions in the electrochemical solution consists of three steps [68]: 1) The hydration number of metal ions decreases and the metal ions are rearranged in solution near the cathode surface; see Eq. (3.1): M2+ ⋅ mH2 O → M2+ ⋅ (m − n)H2 O (m > n) + nH2 O

(3.1)

2) Metal ions surrounded by partly discarding water molecules are reduced. In general, this process is conducted in a step-by-step manner; see Eqs. (3.2) and (3.3): M2+ ⋅ (m − n)H2 O + e− ⇌ M+ ⋅ (m − n)H2 O (adsorption)

(3.2)

M ⋅ (m − n)H2 O + e ⇌ M ⋅ (m − n)H2 O (adsorption)

(3.3)

+

−

3.2 Synthesis, Characterization, and Magnetic/Electrical Properties

3) Adsorbed metal atoms discard the surplus hydration layer, and then enter the crystal lattice; see Eq. (3.4): M ⋅ (m − n)H2 O (adsorption) → M (crystal lattice) + (m − n)H2 O (3.4) The movement rates of metal ions in the electric field (E field) are given by Eqs. (3.5) and (3.6): dE (3.5) dl dE 𝛾− = U− ⋅ (3.6) dl where 𝛾 + is the metal ion movement rate (m s−1 ), U + is the metal ion mobility (m2 s−1 V−1 ), 𝛾 − and U − are the negative ion movement rate and mobility, 𝛾+ = U+ ⋅

vII

vII

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v⊥

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+

– –

–

–

+

+ + +

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– +

– – + – + – + + + +

vII

+ – – – – + – + – + + + +

vII

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Cathode v⊥

(a) vII >> v⊥

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–

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– – + + +

Cathode v⊥

–

– +

vII

vII

Cathode

vII

v⊥

+

–

vII

Cathode

v⊥

v⊥_ (b) vII ≈ v⊥

Figure 3.20 Schematic drawing of the growth mechanism of 1D metal nanostructures via the template electrodeposition template method. (a) Metal nanotube growth steps. (b) Metal nanowire growth steps. (Cao et al. 2006 [66]. Reproduced with permission of John Wiley & Sons.)

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respectively, and dE/dl is the potential gradient (V m−1 ). After moving onto the surface of the cathode, the metal ions are reduced and enter the crystal lattice. In order to interpret this growth mechanism of metal 1D nanostructures, we propose the hypothesis that the junction between the cathode surface and the bottom edge of the template pore serves as a preferential site for the deposition of metal ions. Surface atoms of the nanochannels of the alumina template are distinguished as energetically favorable sites that enhance metal atom adsorption on the inner walls of the nanochannels due to the high surface area of the nanochannels [69]. This effect is demonstrated by the phenomenon of as-synthesized metal (Fe, Co, Ni) tubular structures. There are competitive growth rates for metal atoms entering the crystal lattice, that is, v|| (i.e., the growth rate is parallel to the applied current direction) and v⟂ (i.e., the growth rate is perpendicular to the applied current direction). The morphology of the 1D metal nanostructures depends on the competitive growth rates v|| and v⟂ . When a low current density (E field) is applied, it has little influence on the values of v|| and v⟂ , which means v|| is similar to v⟂ (i.e., v|| = v⟂ ). As time prolongs, metal atoms fill most of the template pores until the pores are completely filled, and wirelike 1D nanostructures can be obtained. This has been demonstrated by the Fe, Co, and Ni nanowires synthesized with a low applied current density (≤0.02 mA mm−2 ) [9, 16, 18]. When a large current density is applied (such as 0.2 mA mm−2 current density applied for Fe nanotubes), it leads to large values of dE/dl. A growth direction parallel to the current direction is the preferential direction, which means v|| ≫ v⟂ . In the direction parallel to the current direction, metal atoms deposit straightforwardly to generate a tubular structure. We termed this growth mechanism current-directed tubular growth (CDTG). The successful synthesis of nonlayered bcc-Fe nanotubes controlled by the CDTG mechanism demonstrates that the synthesis of 1D metal nanostructures can be successfully designed through increasing the current density. Directed by this idea, we also successfully generated nonlayered hcp-Co and fcc-Ni nanotubes under conditions similar to that for the bcc-Fe nanotubes. Obviously, these data fully demonstrated that nonlayered materials can also form tubular nanostructures under appropriate synthesis conditions. The as-synthesized metal (Fe, Co, Ni) tubular and wirelike nanostructures provide powerful evidence to support the above-mentioned hypothesis. The crucial points for metal nanotube growth are as follows: when a large current density is applied, the metal atoms are deposited preferentially on the junction of the surface of pore walls, not on the substrate. As deposition time prolongs, the metal nanotubes continue to grow straightforwardly until they reach the top end of the template. It should be pointed out that the electrochemical deposition method to generate metal nanotube does not require chemical modification of the inner surface of the template pore, which might decrease the possibility of introducing impurities. We believe that the CDTG mechanism is a general method for controlling synthesis of other metal 1D nanostructures, even compound 1D nanostructure

3.3 Synthesis, Characterization, and Optical Properties of Metal Oxide Nanowires

arrays. This general synthesis method could make metal nanotube arrays a material platform for systematic studies of magnetic recording properties, sensors, catalysis, and so on.

3.3 Synthesis, Characterization, and Optical Properties of Metal Oxide Nanowires 3.3.1 In2 O3 Nanowires

Short-wavelength blue-light lasers and ultraviolet lasers play important roles in laser printing and information storage [70–74]. It is well known that making the next generation of compact disk (CD) read-heads requires short-wavelength operating lasers. Because the density of storage in an optical memory system is inversely proportional to the square of the wavelength of operating lasers, the shorter the wavelength of the operating laser, the higher the informational storage density of the CD. The storage density of CDs based on GaAlAs lasers [recording wavelength (RW) = 780 nm] is 0.25 Gbit in−2 , while the storage density of the digital versatile disk based on GaAlInP is 2.0 Gbit in−2 (RW = 630/650 nm). Reference [70] indicated that if III–V nitrides were used as laser material, lasers in blue-light region (RW = 430–500 nm) can be obtained to reach a storage density of 10–20 Gbit in−2 . However, the greatest shortcoming of the lasers based on III–V nitrides is that nitrides can be oxidized in the air. The wide-bandgap semiconducting oxides with room temperature photoluminescence (PL)-emitting properties may be good candidate materials to replace nitrides, for the oxides have higher chemical stability and heat stability. It has been demonstrated that ZnO is a better short-wavelength semiconductor laser material than III–V nitrides [71, 72]. This has aroused our great interest in developing oxide semiconductor laser materials with short-wavelength PL-emitting properties, which may greatly improve the storage density of CDs in the future. One-dimensional (1D) nanostructures, including nanowires and nanotubes [71–75], represent the smallest dimension for efficient transport of electrons and optical excitations. Thus, these nanomaterials are ideal building blocks for optoelectronic nanodevices. Combining a 1D nanostructure with wide-bandgap semiconducting oxide laser material has become the focus of our study. Indium oxide (In2 O3 ) is an important wide-bandgap transparent semiconductor material (Eg = 3.6 eV) [76, 77], which has many possible applications in optical and electric devices [78], solar cells, liquid crystal devices, and so on [79, 80]. Most researches focus on the preparation and properties of In2 O3 films or nanoparticles [81, 82]. Recently, In2 O3 nanofibers synthesized by thermal evaporation–oxidation method [77] and In2 O3 nanowires synthesized by electrodeposition and oxidizing (EDO) method [83, 84] as well as laser ablation process [85] have been reported. Here, we introduce a facile way to fabricate room-temperature UV-emitting In2 O3 nanowires [86] by the template method [12].

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3 Synthesis, Characterization, and Application of One-Dimensional (1D) Nanostructures

3.3.1.1 Synthesis

First, we synthesized In(OH)3 sol. The In(OH)3 sol was prepared using a sol–gel method similar to that described by Yu et al. [87]. InCl3 ⋅4H2 O (analytical reagent, AR) and ethylenediamine (NH2 CH2 CH2 NH2 , AR) were used without further purification. In a typical case, 0.2 g InCl3 ⋅4H2 O was dissolved in 100 ml of deionized water, and then two drops of ethylenediamine were added to the InCl3 solution, which was stirred for 5 h at room temperature. Light-blue hydrated In(OH)3 sol was obtained. Then, we used the sol–gel template synthesis method to generate In2 O3 nanowires within the pores of alumina template membrane [86]. The alumina template membrane (Anopore Membrane, Whatman Anodisc ) with a quoted pore diameter of 20 nm [SEM observation shows that the pore diameters are in the range of 20–100 nm] and thickness of ∼50 μm, was immersed in In(OH)3 sol for 4–5 h under a pressure of ∼1.3 atm at ambient temperature. The template was then taken out from the sol and dried at ∼75 ∘ C for 30 min. After that, the template was treated by annealing under argon atmosphere ramping up to 600 ∘ C for 5 h, followed by ramping down to room temperature in 5 h. The In2 O3 nanowires within the pores of the template membrane were obtained.

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3.3.1.2 Characterization

The morphology and microstructure of the as-synthesized samples were characterized by SEM and TEM. SEM was carried out on a KYKY-2000 scanning electron microscope. TEM was taken with a Hitachi H-800, transmission electron microscope operating at 200 kV. 3.3.1.3 Structure

Figure 3.21a shows the SEM image of an as-synthesized sample. The nanowires have a length of over 10 μm and an average diameter of 80 nm. It shows that the nanowires lean against each other. Figure 3.21b is the TEM image of the morphology of a piece of In2 O3 nanowire. It shows that the diameter of the nanowire is ∼40 nm. The corresponding ED pattern (Figure 3.21c) proves that it is In2 O3 of a single-crystal cubic structure. The diffraction spots in the ED (a)

(b)

40 nm

(822) (600) (400) (200)

(c)

(422) (222)

(000)

10 μm

Figure 3.21 (a) SEM image of In2 O3 nanowires, (b) TEM image of a piece of In2 O3 nanowire, and (c) corresponding electron diffraction (ED) pattern. (Cao et al. 2003 [86]. Reproduced with permission of American Institute of Physics.)

Figure 3.22 Excitation spectra and PL emission spectra of In2 O3 nanowires embedded in alumina template. (Cao et al. 2003 [86]. Reproduced with permission of American Institute of Physics.)

Relative intensity

3.3 Synthesis, Characterization, and Optical Properties of Metal Oxide Nanowires

λEx1 = 274 nm

λEx2 = 305 nm

λEm1 = 398 nm (λEx1 = 274 nm)

λEm2 = 398 nm (λEx2 = 305 nm)

200 240 280 320 360 400 440 480 520 560 Wavelength (nm)

pattern can be indexed as (200), (222), (400), (422), (600), and (822), with d = 5.02, 2.86, 2.51, 2.008, 1.6595, and 1.1817 Å, respectively. These values match well with the cubic crystalline In2 O3 structure (JCPDS 76-0152). They are all within the error allowance limit (