Inorganic and Metallic Nanotubular Materials: Recent Technologies and Applications (Topics in Applied Physics, 117) 3642036201, 9783642036200

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Topics in Applied Physics Volume 117

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Topics in Applied Physics Topics in Applied Physics is a well-established series of review books, each of which presents a com-prehensive survey of a selected topic within the broad area of applied physics. Edited and written by leading research scientists in the field concerned, each volume contains review contributions cover-ing the various aspects of the topic. Together these provide an overview of the state of the art in the respective field, extending from an introduction to the subject right up to the frontiers of contempo-rary research. Topics in Applied Physics is addressed to all scientists at universities and in industry who wish to obtain an overview and to keep abreast of advances in applied physics. The series also provides easy but comprehensive access to the fields for newcomers starting research. Contributions are specially commissioned. The Managing Editors are open to any suggestions for topics coming from the community of applied physicists no matter what the field and encourage prospective editors to approach them with ideas.

Managing Editor Dr. Claus E. Ascheron Springer-Verlag GmbH Tiergartenstr. 17 69121 Heidelberg Germany Email:[email protected]

Assistant Editor Adelheid H. Duhm Springer-Verlag GmbH Tiergartenstr. 17 69121 Heidelberg Germany Email:[email protected]

Tsuyoshi Kijima (Ed.)

Inorganic and Metallic Nanotubular Materials Recent Technologies and Applications With 172 Figures

123

Editor Prof. Dr. Tsuyoshi Kijima Miyazaki University Fac. Engineering Dept. Applied Chemistry Miyazaki 889-2192 Japan [email protected]

ISSN 0303-4216 ISBN 978-3-642-03620-0 e-ISBN 978-3-642-03622-4 DOI 10.1007/978-3-642-03622-4 Springer Heidelberg Dordrecht London New York Library of Congress Control Number: 2010921293 c Springer-Verlag Berlin Heidelberg 2010  This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover design: Integra Software Services Pvt. Ltd., Pondicherry Printed on acid-free paper Springer is part of Springer Science+Business Media (www.springer.com)

Preface

This book is written for those scientists and engineers who wish to understand the synthesis, physical and chemical properties, and applications of inorganic and metallic nanotubular materials. The original version of this book, written in Japanese, covered those of organic, inorganic, and metallic nanotubular materials or almost all the other nanotubular materials than carbon nanotubes. This English version is concerned with only the chapters of inorganic and metallic nanotubular materials. In most industries worldwide, recent attention is unexceptionally focused on the research and development of highly functional new materials or technologies leading to energetically highly efficient activities. Nanotubular materials are one of the materials with such technological potentials because of their nano-sized unique structures available, for example, functionalization at their internal and external surfaces. In 1991, Dr. S. Iijima discovered a tubular material of carbon and named it carbon nanotubes. Since then, worldwide attention has been focused on the basic and functional properties of the novel materials and in more recent times the research phase has developed into an advanced stage based on strategic researches toward various applications. Carbon nanotubes have thus become synonymous with nanotubular materials and still more a symbol of nanotechnology because of their unique, valuable, and versatile properties. In the field of inorganic materials, the discovery of mesoporous silica around 1990–1992 and the subsequent researches on mesoporous materials stimulated the synthesis of a wide range of inorganic nanotubular materials including oxides, sulfides, nitrides, and metals. Currently, nanotube ripples have spread from carbon to all categories of materials, inorganic, metallic, and organic materials. It might be not going too far to say nanotubular structures are obtainable from almost all of the main elements and main substances. However, a very few exhaustive books have appeared on inorganic, metallic, and organic materials excluding carbon nanotubes, although a great number of books have been published on the properties and applications of carbon

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nanotubes. In particular, there is no English version exhaustively concerned with inorganic and metallic nanotubular materials. This book covers a wide variety of inorganic and metallic nanotubular materials, ranging from metal oxides to fullerene and water nanotubes. This book is written by experts internationally known in the fields of nanomaterials and nanotechnology. Chapter 1 summarizes an outline of inorganic and metallic nanotubular materials, including their classification in structure, formation pathway and tube sizes, as well as their representative applications. Chapter 2 focuses on the synthesis and applications of titanium oxide nanotubes because they are the most extensively studied compared with all the other inorganic nanotubes. The first section, by Sekino, reviews the synthesis and functionalization of titanium oxide nanotubes and the second section, by Koyanagi, refers to its structural and photochemical characterization. In the succeeding section the synthesis and applications of titanium oxide nanotube thin-films are considered by Miyauch and Tokudome. The final section, by Yamanaka and Uno, is concerned with the synthesis and applications of titanium oxide nanohole arrays. Chapter 3 deals with the synthesis and applications of the single metal oxide nanotubes based on manganese, molybdenum, rare-earth metals, zirconium and ruthenium, by Feng, Suemitsu, and Yada and/or Inoue, respectively. The conversion of metal oxide nanosheets into their nanotubes is also considered by Ma and Sasaki, together with the synthesis and applications of mixed oxide nanotubes, by Ogihara, Sadakane, and Ueda. In Chapter 4, the synthesis, structure, and properties of imogolite nanotubes are discussed as well as their applications to heat-exchange materials and polymer nanocomposites, by Suzuki and Inukai and Ohtsuka and Takahara. Chapter 5, by Ohtani, reviews the synthesis and applications of chalcogenide nanotubes, and Chap. 6, by Miyazawa, considers the synthesis and functions of fullerene nanotubes. In Chapter 7, the synthesis, structure, and applications of noble-metal nanotubes are reviewed by Kijima and those of magnetic-metal nanotubes are considered by Nakagawa, Oda, and Kobayashi. Chapter 8, by Maniwa and Kataura, exhibits the formation and properties of water nanotubes. In the final chapter, several topics on the design, calculation, and/or manipulation of nanotubular materials are demonstrated using the titanium oxide and boron nitride systems by Hasegawa and by Golberg, Costa, Mitome, and Yoshio Bando, respectively. Finally I would like to deeply thank Dr. T. Shimizu, the first editor of the Japanese version for agreeing with the translation of the inorganic and metallic parts of the Japanese version into the current English version. I would also like to thank all the authors who participated in preparing this version.

Preface

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I would like to thank Mr. Claus for recommending the editing of this book. I would also like to thank Mr. Kitamura in Frontier Publishing Co. for kindly accepting the translation of the Japanese version into the current English version. Miyazaki, March 2010

Tsuyoshi Kijima

Contents

1 Introduction to Inorganic and Metallic Nanotubes Tsuyoshi Kijima . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2 Structural and Synthetic Bases for Inorganic and Metallic Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.1 Structural Types of Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . 1.2.2 Synthetic Strategies Toward Nanotubes . . . . . . . . . . . . . . . . . . 1.3 Structural and Dimensional Characteristics of Inorganic and Metallic Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1.4 Applications of Inorganic and Metallic Nanotubes . . . . . . . . . . . . . . . 1.5 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

8 11 12 13 15

2 Synthesis and Applications of Titanium Oxide Nanotubes Tohru Sekino . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2 Synthesis and Structure of Titanium Oxide Nanotubes . . . . . . . . . . . 2.2.1 Low Temperature Solution Chemical Processing . . . . . . . . . . 2.2.2 Nanostructures and Formation Mechanism . . . . . . . . . . . . . . . 2.3 Functions of Titanium Oxide Nanotubes . . . . . . . . . . . . . . . . . . . . . . . 2.3.1 Photochemical Properties and Photocatalytic Functions . . . 2.3.2 Novel Environmental Purification Functions . . . . . . . . . . . . . . 2.3.3 Multi-functionalized Titanium Oxide Nanotubes . . . . . . . . . . 2.4 Conclusion and Prospects . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

17 17 19 19 21 25 25 27 28 30 30 32

1 1 2 3 5

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3 Synthesis, Structural Analysis, and Applications of Titanium Oxide Nanotubes Tsuguo Koyanagi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2 Synthesis of Mono-Dispersed Titanium Dioxide Nanotubes . . . . . . . 3.3 Nanostructure Analysis of Titanium Dioxide Nanotubes . . . . . . . . . . 3.4 Applications of Titanium Dioxide Nanotubes . . . . . . . . . . . . . . . . . . . 3.4.1 Application for Dye-Sensitized Solar Cells . . . . . . . . . . . . . . . . 3.4.2 Application for Photo-Catalysis . . . . . . . . . . . . . . . . . . . . . . . . . 3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4 Synthesis and Applications of Titanium Oxide Nanotube Thin Films Masahiro Miyauchi and Hiromasa Tokudome . . . . . . . . . . . . . . . . . . . . . . . . 4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2 Titanate Nanotube (TNT) Thin Films via Alternate Layer Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.3 Thin Films of Vertically Aligned TNT Arrays and Their Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.4 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5 Synthesis and Application of Titanium Oxide Nanohole Arrays Shinsuke Yamanaka and Masayoshi Uno . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.2 Formation Mechanism and Properties . . . . . . . . . . . . . . . . . . . . . . . . . . 5.3 Application as an Electrode for Lithium Ion Batteries . . . . . . . . . . . . 5.4 Nanohole Arrays of Various Oxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6 Synthesis and Applications of Manganese Oxide Nanotubes Qi Feng . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.1 Single-Crystal Manganese Oxide Nanotubes . . . . . . . . . . . . . . . . . . . . 6.1.1 Crystal Structures of Layered Manganese Oxides . . . . . . . . . 6.1.2 Manganese Oxide Nanotubes Synthesized from Manganese Oxide Nanosheets . . . . . . . . . . . . . . . . . . . . . . 6.1.3 Manganese Oxide Nanotubes Synthesized from α-NaMnO2 . 6.1.4 Manganese Oxide Nanotubes Synthesized Directly from Mn(Ac)2 Solution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6.2 Polycrystalline Manganese Oxide Nanotubes . . . . . . . . . . . . . . . . . . . .

33 33 34 35 38 38 42 43 44 44

45 45 46 51 55 55 57

59 59 61 66 68 70 70 71 73 73 73 74 78 79 80

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6.3 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82 7 Synthesis and Applications of Molybdenum Oxide Nanotubes Maki Suemitsu and Toshimi Abe . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.2 Growth of Molybdenum Oxide Nanotubes . . . . . . . . . . . . . . . . . . . . . . 7.3 The Chemical Composition and the Crystal Structure of the Mo Oxide Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7.4 Growth Mechanism of the Mo Oxide Nanotubes . . . . . . . . . . . . . . . . . 7.5 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

83 83 84 85 89 94 95 96

8 Synthesis and Applications of Rare-Earth Compound Nanotubes Mitsunori Yada . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 97 8.2 Rare-Earth Compound Nanotubes Synthesized Using the Homogeneous Precipitation Method . . . . . . . . . . . . . . . . . . . . . . . . 98 8.3 Rare-Earth Hydroxide and Rare-Earth Oxide Nanotubes Synthesized Using the Hydrothermal Method . . . . . . . . . . . . . . . . . . . 102 8.4 Rare-Earth Oxide Nanotubes Synthesized Using Carbon Nanotube as a Template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107 8.5 Rare-Earth Oxide Nanotubes Synthesized Using Anodic Alumina Membrane as a Template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108 8.6 Cerium Phosphate Nanotubes with Blue Luminescence . . . . . . . . . . . 108 8.7 Rare-Earth Fluoride Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110 8.8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 113 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 114 9 Synthesis and Applications of Zirconia and Ruthenium Oxide Nanotubes Mitsunori Yada and Yuko Inoue . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 9.1 Zirconia Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 9.1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 117 9.1.2 Zirconia Nanotubes Synthesized Using Carbon Nanotubes or Nanofibers as Templates . . . . . . . . . . . . . . . . . . 118 9.1.3 Zirconia Nanotube Arrays Synthesized by Anodization of Metal Zirconium . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121 9.1.4 Zirconia Nanotubes Synthesized Using a Porous Membrane as a Template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 121

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9.1.5

Zirconia Nanotubes Synthesized Using a One-Dimensional Assembly Formed by Amphipathic Molecules as a Template . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 9.2 Ruthenium Oxide Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 9.2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126 9.2.2 Ruthenium Oxide and Ruthenium Oxide Hydrate Nanotubes Synthesized Using Anodic Porous Alumina Membrane . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 127 9.2.3 Ruthenium Compound Nanotubes Templated by Surfactant Assemblies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 130 9.3 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 131 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 132 10 Conversion of Metal Oxide Nanosheets into Nanotubes Renzhi Ma and Takayoshi Sasaki . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 10.1 Nanosheet and Nanotube . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135 10.2 Spontaneous Conversion of Nanosheets into Nanotubes During Exfoliation of Certain Layered Oxides . . . . . . . . . . . . . . . . . . . . . . . . . 138 10.3 Conversion of Nanosheets into Nanotubes via a Designed Soft Chemical Procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 141 10.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 145 11 Synthesis and Applications of Mixed Oxide Nanotubes Hitoshi Ogihara, Masahiro Sadakane, and Wataru Ueda . . . . . . . . . . . . . . 147 11.1 Conventional Fabrication Processes for Mixed Oxide Nanotubes . . . 147 11.2 Synthesis of Mixed Oxide Nanotubes Using Carbon Nanofibers as Templates . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149 11.3 Synthesis of Macro-Nanostructured Materials Using Template Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 152 11.4 Application of Macro-Nanostructured Materials to Catalytic Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 155 11.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 157 12 Synthesis and Applications of Imogolite Nanotubes Masaya Suzuki and Keiichi Inukai . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 12.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 159 12.2 Synthesis of Imogolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 12.2.1 Synthesis Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160 12.2.2 Influence of Heating Temperature and Elemental Substitution . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 12.3 Application of Imogolite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163

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12.3.1 Anti-dewing Material . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163 12.3.2 Heat Exchange Material in Adsorption-Type Heat Pump Systems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 165 12.4 Possible Future Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 166 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167 13 Structure and Properties of Imogolite Nanotubes and Their Application to Polymer Nanocomposites Hideyuki Otsuka and Atsushi Takahara . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 13.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169 13.2 Natural Imogolite Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171 13.3 Chemical Synthesis of Imogolite Nanotubes . . . . . . . . . . . . . . . . . . . . . 172 13.4 Structure of Imogolite Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173 13.5 Chemical and Physical Properties of Imogolite Nanotubes . . . . . . . . 175 13.6 Hybrid Materials Using Imogolite Nanotubes . . . . . . . . . . . . . . . . . . . 176 13.6.1 Surface Modification of Imogolite Nanotubes . . . . . . . . . . . . . 176 13.6.2 Polymer Hybrids with Imogolite Nanotubes . . . . . . . . . . . . . . 178 13.6.3 Imogolite Nanotubes as Templates . . . . . . . . . . . . . . . . . . . . . . 186 13.7 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 187 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 189 14 Synthesis and Applications of Chalcogenide Nanotubes Tsukio Ohtani . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 14.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191 14.2 Transition-Metal Dichalcogenides with Layered Structures . . . . . . . . 192 14.3 Chalcogenide Nanotubes (ChNTs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 193 14.3.1 Formation Mechanism of MS2 (M = W, Mo) Nanotubes . . . 193 14.3.2 Preparation of Different Chalcogenide Nanotubes (ChNTs) . 194 14.3.3 Properties and Applications of Chalcogenide Nanotubes (ChNTs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 197 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 198 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199 15 Synthesis and Functions of Fullerene Nanotubes Kun’ichi Miyazawa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 15.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 201 15.2 Liquid–Liquid Interfacial Precipitation Method . . . . . . . . . . . . . . . . . 202 15.3 Synthesis and Properties of Fullerene Nanotubes . . . . . . . . . . . . . . . . 205 15.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 212 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 214

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16 Synthesis and Applications of Noble-Metal Nanotubes Tsuyoshi Kijima . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 16.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215 16.2 Template-Free Synthesis of Base Metal Nanotubes . . . . . . . . . . . . . . . 217 16.3 Synthesis of Noble-Metal Nanotubes Using Solid Templates . . . . . . . 219 16.4 Synthesis of Noble-Metal Nanotubes Using Molecular Templates . . 221 16.4.1 Mixed Surfactant LC Templating Approach to Noble-Metal and Other Nanostructures Using Single- and Triple-Branched PEO-Type Surfactants . . . . . . . . . . . . . . . . . . 221 16.4.2 Synthesis of Silver Nanotubes Using Mixed Surfactant LC Templating Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 222 16.4.3 Synthesis of Platinum and Palladium Nanotubes Using Mixed Surfactant LC Templating Methods . . . . . . . . . . . . . . . 223 16.5 Applications of Noble-Metal Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . 229 16.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 231 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 234 17 Synthesis and Applications of Magnetic-Metal Nanotubes Masaru Nakagawa, Hirokazu Oda, and Kei Kobayashi . . . . . . . . . . . . . . . . 235 17.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 235 17.2 Template Synthesis of Nickel-Containing Tubular Materials by Electroless Deposition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236 17.3 Application to an Anisotropic Conduction Film . . . . . . . . . . . . . . . . . 239 17.3.1 Filler-Orientation-Type Anisotropic Conduction Film . . . . . 241 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 244 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 246 18 Synthesis and Applications of Water Nanotubes Yutaka Maniwa and Hiromichi Kataura . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 18.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 247 18.2 Water Containing Carbon Nanotube . . . . . . . . . . . . . . . . . . . . . . . . . . . 248 18.3 Liquid–Solid Phase Transition: Formation of Ice Nanotube . . . . . . . 250 18.4 Computer Simulations of Ice Nanotube . . . . . . . . . . . . . . . . . . . . . . . . 254 18.5 Physical Properties and Application of Water Containing SWCNT . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 256 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 258 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 259 19 Design and Synthesis of Titanium Oxide Nanotubes Akira Hasegawa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 19.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261 19.2 Crystal Structure Models and Electronic Structure Model Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 262 19.3 Highly Functional Nanotube: Conductivity Grant = Reduction of Resistivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 265

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19.4 Crystal Structure and Electric Conductive Property of Titanium Oxide . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 267 19.5 Synthesis of Titanium Oxide Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . 268 19.6 Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272 20 In Situ TEM Electrical and Mechanical Probing of Individual Multi-walled Boron Nitride Nanotubes Dmitri Golberg, Pedro M.F.J. Costa, Masanori Mitome, and Yoshio Bando . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 20.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 20.2 Electrical Probing Experiments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 275 20.2.1 Unfilled Multi-walled BN Nanotubes . . . . . . . . . . . . . . . . . . . . 275 20.2.2 Filled BN Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 278 20.3 Mechanical Probing of BN Nanotubes . . . . . . . . . . . . . . . . . . . . . . . . . 281 20.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 284 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 285 Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 287

1 Introduction to Inorganic and Metallic Nanotubes Tsuyoshi Kijima Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki, 889-2192, Japan [email protected] Abstract Nanotubular materials are classified into three types, single- and multiwalled ones, which correspond to one or more rectangular sheets rolled into their single or concentric cylindrical forms with no edge atoms and a scrolled one generated from a single straight sheet with keeping both free ends. In these nanotubular systems, the excess energy due to the sheet rolling or scrolling-induced strain and/or sheet surface and edge atoms is fully compensated by the energy decrease due to the loss of the sheet edges and/or the van der Waals interaction between neighboring cylindrical tubes. The pathway to nanotubes is based essentially on either the self-organization of constituent atoms or molecules or their tubular growth by the assistance of solid or molecular templates. The former processes are characteristic of the family of materials that are constructed by interatomic covalent bonding to stably form a 2D-layered structure as their typical phase, whereas the latter ones are applied to those with much less anisotropy in bonding character. This chapter also summarizes the structural and dimensional characteristics of inorganic and metallic nanotubes and finally refers to some representative applications and future applicabilities of the nanotubular materials.

1.1 Introduction In most industries worldwide, recent attention is unexceptionally focused on the research and development of highly functional new materials or technologies leading to energetically highly efficient activities. In order to fulfill such technological demands, extensive studies are in progress to generate core technologies in different hierarchies, e.g., through the fabrication, integration, and hybridization of nanomaterials as well as the optimal systemization of some functional units [1–12]. Since 1991 carbon nanotubes have attracted outstanding interest as the key material for nanotechnology because of their semiconducting, electron-emitting, high-strength, and other unique properties [13–15]. A study on nanostructured materials was also stimulated by another discovery of mesoporous silica MCM-41 [16] and FSM-16 [17] in around 1991, leading to the synthesis of a great number of elementally T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 1–16 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 1

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versatile nanomaterials. The new family of materials has been also morphologically extended from the initially prepared honeycomb structures to a variety of unique structures such as nanotubes, nanowires, nanorods, and nanobelts [2–12]. It is well known for silicate that nanotubular substances such as chrysotile and imogolite are naturally produced [18]. In addition to carbon nanotubes [13–15], there have been reports on a variety of synthetic inorganic nanotubes ranging from metal sulfides such as tungsten sulfide [19] to nitrides such as silicon nitride [20] and boron nitride [21], metal oxides such as vanadium oxide [22], titanium oxide [23, 24], silica [25], and rare earth oxide nanotubes [26], as well as metallic nanotubes such as platinum and silver [27]. Several reviews have already appeared for previous studies concerning various families of inorganic nanotubes, especially those for metal oxides and metal sulfides [28–33]. In Chaps. 2–20, the essential studies and topics are described for the synthesis, characterization, and properties of individual nanotubular compounds or families. This chapter refers to the fundamentals of nanotubular materials and the overview of inorganic and metallic nanotubes mainly from the view points of structure and synthetic methodology, as well as some representative applications and future applicabilities of inorganic nanotubes.

1.2 Structural and Synthetic Bases for Inorganic and Metallic Nanotubes Figure 1.1 shows the transmission or scanning electron microscope (TEM or SEM) images of titanium oxide nanotubes prepared under different conditions. Why can such nanotubular forms occur? The theoretical approach to the design of nanotubes is given in Chap. 19. Here we consider three structural types of nanotubes and synthetic bases for them.

(A)

(B)

(C)

Fig. 1.1. TEM (A) or SEM (B, C) images of titanium oxide nanotubes produced (A) on the surface of titanium wires under hydrothermal condition at 160◦ C [72], (B) from aqueous solution containing polylactic acid fibers [61], and (C) using aluminum anodic oxide as a solid template [65]

1 Introduction to Inorganic and Metallic Nanotubes

3

1.2.1 Structural Types of Nanotubes The structure of inorganic and metallic solids is determined by the manner of interatomic bonding dependent on the kind of constituent atoms and their composition. Generally, solids have a structural anisotropy along a given atomic plane or chain. This is true not only for the graphite-like system in which a group of the constituent atoms are bound through covalent bonds to form a sheet-like array but also for ionic crystals composed of spherical cations and anions and even for single metals composed of the same kind of atoms. Stacking or assembling of layered or linear chain atomic units yields a stable or quasi-stable phase leading to nanostructures with various shapes including tube, sheet, balloon, rod, wire, fiber, dot, whisker, ribbon, and belt. Nanotubes can be classified into three types, single-walled type (SWtype), multi-walled type (MW-type), and scrolled type (S-type), as shown in Fig. 1.2A. The SW-type nanotube corresponds to a rectangular sheet rolled into a cylindrical form and combined at both ends. The unrolled rectangular sheet is composed of atoms with the same arrangement as in bulk, except for the surface and edge atoms. In contrast, the SW-type nanotube has no edge atoms but undergoes a displacement of atoms due to the rolling of a straight sheet, which causes a strain energy. If we assume that a fully long rectangular sheet with a width of d consists of N atoms and is rolled into a single-walled tube with a diameter of D, the energies of the straight sheet and SW-type tube, Es and Et , are given by (1.1) and (1.2), respectively [34]: E s = Ni ε o + N e ε e Et = N ε o + N ε s

(N = Ni + Ne )

(1.1) (1.2)

where Ni and Ne are the numbers of internal and edge atoms, respectively, εo is the energy per one atom of an infinitely large sheet, εe is the energy per one atom associated with edge atoms, and εs is the energy per one atom due to the strain of atomic arrangement for the SW-type tube. Furthermore, the strain energy is correlated to the diameter of the tube through the relation (1.3). (1.3) εs ∼ 1/D2 Thus, if the tube diameter is beyond a critical value, the SW-type tube is energetically more stable than the corresponding straight sheet because the excess energy due to strain, N εs , is less than that due to the sheet edge, N εo [34]. For example, the stable form for MoS2 isostructural with WS2 is a SWtype nanotube when the total number of atoms per cross area is more than 223, i.e., the tube diameter D is more than 6.2 nm, according to a theoretical calculation by Tenne et al. (Fig. 1.3). In the case of MW-type nanotubes consisting of two or more cylindrical tubes, the van der Waals interaction between neighboring cylindrical tubes effects additional reduction in the net energy per atom of the

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Single-walled Multi-walled

Scrolled

Type-I

Type-II

Type-III

Fig. 1.2. Classification of (A) structural types of nanotubes and (B) pathways to nanotubes through (a) rolling of a sheet and deposition on (b) the surface of a core-type template or (c) the internal surface of a sheath-type template

nanotube. This leads to a decrease in diameter of the inside cylinder compared to the corresponding SW-type nanotube. Actually, seven-walled MoS2 nanotube can adopt a MW-type tubular structure with an inner diameter of ∼ 5 nm and an outer diameter of ∼ 12 m (N = 2, 000); the inner diameter of ∼ 5 nm is less than 6.2 nm for a critical diameter of SW-type MoS2 nanotube. The edge-based excess energy of a straight sheet could be also fully compensated by only the van der Waals interaction between neighboring tubes even if the straight sheet is scrolled with keeping both free ends. This leads to the formation of S-type nanotubes that are axially asymmetric (Fig. 1.2A).

1 Introduction to Inorganic and Metallic Nanotubes

(A)

5

(B)

(C)

Tube Tubular growth field

Sheet

Number of atoms

Fig. 1.3. (A) Straight-layered sheet and (B) single-walled nanotube of MoS2 and (C) energy as a function of number of atoms for these two structures [34]

1.2.2 Synthetic Strategies Toward Nanotubes Generally, the essential pathway to nanotubes can be categorized into two classes. Class 1 is based on the self-organization of constituent atoms or molecules under a given condition and class 2 is based on the template-assisted formation of nanotubular structure, as illustrated for a SW-type nanotube in Fig. 1.2B. In the class 1 systems, the SW-, MW-, or S-type nanotubular structure is obtained through either the cylindrical or the scrolled growth of atomic or molecular species or the phase transition from the initially formed straight nanosheet (Fig. 1.2B(a)). The nanotubes generated by the class 1 mode are unexceptionally assigned to the family of materials that are based on interatomic covalent bonding to stably form a 2D layered structure as their typical phase, as observed for carbon, metal chalcogenide, nitride, and some kinds of oxides such as MoO3 . In contrast, covalency-poor materials form their nanotubular phases by the assistance of templates, e.g., the deposition of chemical species on the surface of solid or supra-molecular core template or the internal surface of solid or supra-molecular sheath template: the latter two pathways

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to nanotubes are denoted as class 2a and 2b, respectively, as illustrated in Fig. 1.2B(a) and (b). The nanotube synthesis in the class 1 system mostly uses high-temperature reaction and hydrothermal processes, as typically applied to metal chalcogenide and titanium oxide, respectively. Intercalation of assistant species into layered compounds is also effective for the layered to nanotubular transition. Solid and molecular templating methods using core or sheath materials are extensively used for the class 2 systems including metals. These templating processes, which are similar to the formation of petrified silica templated by woods, are originated from the template synthesis of a series of zeolites such as ZSM-5 using short-chain organic cations such as tetrapropyl ammonium ions [35–37]. This approach was succeeded by the Mobil and the Waseda research groups who employed longer-chain surfactants as a templating agent to synthesize mesoporous silica: the enlargement of pore sizes in silicate frameworks was achieved by the templating effect of such large molecules [15, 16]. Table 1.1 summarizes the molecular and solid templating processes divided into various types of categories according to their methodologies. Table 1.1. Classification of the templates used for the synthesis of nano- and microstructured inorganic materials Template

Characterization of products

Type

Materials

Short-chain organic cations Micellar or liquid – crystalline molecular assemblies

Alkyl ammonium cations Single surfactant

Micro-phaseseparated copolymers

Structure and morphology

Pore or wire size Example

2D, 3D VS microchannels

Zeolite ZSM 5

Mesoporous cubic, 2D/3D hexagonal; nanotubular

S, M

Silica MCM-41, mesoporous Pt

Mixed surfactant

Nanotubular, rod like

S, L

Block copolymers

Mesoporous cubic, 2D/3D hexagonal; layered pores

M, L

Pt nanotubes, nano-holed Pt nanosheets, microporous– mesoporous bimodal silica Silicates with spherical pores of cubic order

1 Introduction to Inorganic and Metallic Nanotubes

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Table 1.1. (continued) Template

Characterization of products

Type

Materials

Gels

Organogels

Phaseseparated liquids Particlesaggregated lattices

Porous solids

Phaseseparated solids

Structure and morphology

Pore or wire size Example

Tubular, hollow- M fibrous, helical fibrous Microemulsions Reticulated L frameworks, spherical nanoparticles Monodisperse Photonic M, L polymer crystals latex, colloidal silica Carbon Nanowires S nanotubes Anodic Nanotubules, M, L aluminum nanowires oxide film Porous polycar- Nanotubules, M, L bonate nanowires film Mesoporous Nanowires, 3D S, M silica mesoporous MCM-41, MCM-48 Zeolite Honeycomb, 3D VS, S, M microporous Active carbons 3D porous VS, S, M Phase3D porous S, M separated alloys Phase3D porous S, M, L separated glasses Eutectic-phase 1D porous S decomposed textures

Helical silica fibers Reticulated calcium phosphate Photonic crystals of carbon

Ni nanowires Pt nanohole array, ultrafine carbon tubes Au nanotubules

3D mesoporous Pt, Pt nanowires Porous carbons Mesoporous Pt Nanoporous Au

Nanoporous and mesoporous glasses 1D mesoporous silica

VS, < 1 nm; S, 1–10 nm; M, 10–50 nm; L, > 50 nm.

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1.3 Structural and Dimensional Characteristics of Inorganic and Metallic Nanotubes The diameter, length, and structural types of inorganic or metallic nanotubular materials largely depend on the interatomic bonding character and composition of materials, the synthetic method, and some other factors, as seen for typical nanotubes in Fig. 1.4. Especially, the physical and chemical properties of nanotubes are highly sensitive to their diameters and wall thicknesses: nanotubes with a critical size show a quantum effect for their optical properties. As mentioned previously, a great number of studies have appeared on a wide range of inorganic nanotubes including metal sulfides, metal oxides, nitrides, and metals. These nanotubes can be characterized based on four groups categorized according to mainly their outer diameters: (1) under 20 nm, (2) 20–100 nm, (3) over 100 nm, (4) and NT array.

100nm

(A) chrysotile

(D) γ –Fe2O3

(B) VO2.4(C16H33NH)0.34

(E) BN

(C) BaTiO3

(F) GaN

20 nm

(G) MoS2

(H) Si

(I) Pt

Fig. 1.4. Examples of natural (A) and synthetic (B–I) nanotubes: (A) chrysotile [18], (B) VO2.4 (C16 H33 NH)0.34 [22], (C) BaTiO3 [67], (D) γ -Fe2 O3 [59], (E) BN [80], (F) GaN [53], (G) MoS2 [81], (H) Si [52], and (I) Pt [27]

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(1) NTs of under 20 nm This group is concerned with the smallest NTs including WS2 [19], MoS2 [37], BN [21], TiO2 [23, 24], VOx [22], L2 O3 (L = Er, Tm, Yb, Lu) [26], K4 Nb6 O17 [38], Pt [27], and Bi [39]. Especially, the NTs except for VOx and K4 Nb6 O17 are as extremely small as less than 10 nm. The discovery of carbon nanotubes in 1991 was followed by the synthesis of WS2 , MoS2 , and BN nanotubes using solid phase or solid–gas interfacial reaction at high temperatures. All of these materials are closely related to a layered structure: the former two sulfides are well known as a member of a family of layered compounds and boron nitride is isoelectronic with carbon so that the nitride phase called as hexagonal BN is a graphite-like compound with a layered structure. Tenne et al. found that the heating of tungsten films or MoO3 powders in H2 S at a high temperature resulted in the growth of WS2 nanotubes of 16 nm in outer diameter or MoS2 ones of 19 nm in outer diameter, in addition to fullene-like sulfide particles [19, 37]. Zettl et al. synthesized BN nanotubes of 6–8 nm outer diameter by a discharge of BN/W rod as an electrode [21]. Bando et al. also obtained BN nanotubes of 3–15 nm outer diameter by the laser heating of cubic BN in N2 [40]. Hydrothermal reaction is also highly effective for the preparation of nanotubular materials. Kasuga et al. reported the growth of TiO2 nanotubes with an inner diameter of ∼ 5 nm and an outer diameter of ∼ 8 nm by the hydrothermal treatment of rutile phase in an aqueous solution of NaOH at a temperature of as low as 110◦ C [23, 24]. It is believed that the 3D bulk phase of TiO2 is converted into a 2D nanosheet phase, followed by its scroll into a nanotubular form with an anatase framework [24, 41]. Mao et al. synthesized transition metal oxide BaTiO3 and SrTiO3 NTs with outer diameters of 8–15 nm by hydrothermal reaction using a TiO2 nanotube as a precursor [42]. The reduction of Bi(NO3 )3 with N2 H4 under a hydrothermal condition yielded multi-walled Bi nanotubes [39]. The nanotubular hemi-metal is produced by a similar scroll mechanism since metallic Bi usually forms a layered structure based on the stacking of hexagonal-networked puckered sheets. The above examples of nanotubes are based on the self-organization of atomic or molecular species or the re-organization of precursory solids. The basic or related materials for these nanotubes commonly form a layered structure as their characteristic phases. The structural change of such a straight sheet into a cylindrical or scrolled tube would be induced by the compensation of the edge-based excess energy. All of the layered materials do not always give a nanotubular phase as their alternate form. No nanotubular structure is self-organized for the materials based on a 2D framework are layered but too rigid to take a cylindrical or scrolled form. In such a layered system, it is required to weaken the interaction of layers and increase the curvature of their tubulated form through the intercalation of organic species leading to a significant

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decrease in the rigidity of the basic layered framework, that is, the strain energy of the tubulated form. Spar et al. synthesized VOx (C16 H33 NH2 )0.34 nanotubes with outer diameters of 15–100 nm by the hydrothermal treatment of a layered complex with an interlayer spacing of 3 nm prepared from a mixture of VO(C2 H5 )3 and ethanol–water–acetylamine solution [22]. This is the first example of nanotubes obtained by the plasticization of a rigid sheet. Similarly, the intercalation of butylamine into K4 Nb6 O17 resulted in the scroll of niobate sheets around the [100] axis into a nanotubular form [38]. It is unconceivable that metals with an isotropic structure such as a face- or body-centered cubic ones are self-organized into a straight sheet or nanotubular form. The use of a templating agent, however, enables us to obtain nanotubular metals, as observed for noble metal nanotubes with an outer diameter of as small as 6 nm [27]. (2) NTs of 20–100 nm This group is defined as a family of NTs with outer diameters of 20– 100 nm, including Au-doped WS2 [43], MgO [44], Al2 O3 [45], In2 O3 [46], SiO2 [25], L(OH)3 (L = Y, Tb) and L2 O3 (L = Y, Tb) [47], ZrO2 [48], NiCl2 [49], Pt [50], Pd and Au [51], Si [52], and GaN [53], in addition to the MW-type NTs related to most of the SW-type or thin NTs classified as the first group. The solid phase or solid–gas interfacial reaction at high temperatures with or without solid templates was extensively applied to the synthesis of the above NTs. Hacohen et al. synthesized helix-structured NiCl2 NT with an outer diameter of 70 nm by the successive calcination of NiCl2 in N2 at 400◦ C and in Ar at 900◦ C [49]. GaN is isostructural with ZnO with a Wurtzite-type hexagonal structure. Goldberger et al. prepared GaN NTs with inner diameters of 30–200 nm and with wall thicknesses of 5–50 nm by their epitaxial growth on the (110) plane of ZnO nanowire and the subsequent removal of the core template [53]. Hu et al. obtained Si NTs by a similar epitaxial growth of ZnS/Si based on the reaction of ZnS nanowires with SiO, followed by the removal of ZnS core materials [52]. The framework structure of wall for nanotubular materials is significantly affected by the wall thickness, δ, because the thickening of the tube wall tends to increase the strain energy of wall. For example, the MW-type MoS2 and WS2 nanotubes with wall thickness of as thin as δ < 100 nm grow by spiral rolling of the molecular layers with their hexagonal stacking, whereas their microtubular form with δ > 2μ m takes the rhombohedral stacking [54]. The Au-doped WS2 nanotubes with outer diameters of 75– 85 nm also have a concentric polycylindrical form, in contrast to a helix structure for those with smaller diameters [43]. Fang et al. found the growth of single-crystalline Y(OH)3 nanotubes with an outer diameter of ∼ 70 nm and with a hexagonal-structured framework by the hydrothermal treatment of Y2 O3 powder in 1 M aqueous solution of NaOH at 170◦ C [47]: on calcination the hydroxide NTs grown along the c-axis were converted into Y2 O3 NTs with a cubic-structured

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framework. A similar reaction was applied to the synthesis of Tb(OH)3 and Tb4 O7 NTs. Similarly to ZnO crystal, L(OH)3 (L = Y, Tb) has the hexagonal closest packing of (OH)3 triangular groups and L ions arranged alternately along the c-axis, which led to the 1D nanotubular growth of L(OH)3 . This process might proceed with an accompanying dissolution– redeposition of Y2 O3 particles in which the nanoparticles remaining undissolved serve as a templating agent for the growth of hollow nanowires. Pt, Pd, and Au NTs with outer diameters of ∼ 50 nm were obtained by the reduction of metal salts on the surface of Ag nanowires as a solid template, followed by the removal of the core material [50, 51]. (3) NTs of over 100 nm NTs with outer diameters of 100–200 nm were obtained for ZnS [55], ZrO2 [56], Ta2 O3 [57], Te [58], and γ-Fe2 O3 [59]. The ZnS NT is a single crystal with a hexagonal-shaped cross section, which was prepared by calcinations of ZnS powder in H2 O–Ar gas at 1,500–1,700◦ C [55]. Those with outer diameters of over 200 nm were prepared for NiS [60], TiO2 [61], Co3 O4 [62], and ZnO [63]. The ZnO NT with an outer diameter of 450 nm was synthesized by the hydrothermal treatment of a mixture of Zn(CH3 COO)2 /NH4 OH gel with C2 H5 OH [63]. (4) NT arrays NT array is an assembly of nanotubes in which a large number of uniformly sized nanotubes are arranged with their uniaxial orientation. The NT arrays are usually obtained by the sol–gel reaction or deposition reaction from gas phase within uniformly sized and arranged nanopores using solid templates such as nanoporous aluminum oxide or polycarbonate films. Previous studies have appeared on the synthesis of NT arrays including Ga2 O3 [64], In2 O3 [64], TiO2 [65], SiO2 [66], BaTiO3 [67], Co [68], and Sn/Pt [69]: the outer diameter of individual nanotubes in these NT arrays usually ranges from 50 to 200 nm.

1.4 Applications of Inorganic and Metallic Nanotubes The succeeding chapters describe the applications of various series of inorganic and metallic nanotubes. Here we refer to several examples of applications characteristic of nanotubular materials based on oxides, nitrides, and sulfides. Titanium oxide is one of the highly functional materials for its semiconductivity with Eg ∼ 3 eV. A wide range of studies have been carried out concerning the applications of titanium oxide in various fields including photocatalyst, dye-sensitized solar cell (DSC), lithium ion cell, and gas sensor. Much attention has been therefore focused on the functional properties of TiO2 nanotubes. For example, the TiO2 nanotubes used as a DSC electrode showed the efficiency of energy conversion a little more than that observed for TiO2 particles [70]. A few reports referred to the applicability of TiO2 nanotubes as a biomaterial [71, 72]. The details of applications for TiO2 nanotubes will be described in Chaps. 2–5. Vanadium oxide (VOx , 1.0 < x < 2.5),

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one of the mixed-valent metal oxides, is a layered compound consisting of weakly bound 2D oxide networks and known to show unique properties such as metal–insulator transition. The doping of VOx (C12 H28 N)0.27 (x ∼ 2.4) with lithium or iodine leads to electron- or hole-doped materials, indicating ferromagnetism with a Curie temperature of ∼ 500 K [73]. Boron nitride (BN) nanotubes are an insulating material with Eg = 5.5 eV, while the nitride nanotubes show transition into semiconductor on doping with fluoride [74]. The deformation of BN nanotubes caused a significant change of their electronic state into electrically conductive state [75]. BN nanotubes showed a sorption capacity of hydrogen up to 1.8–2.6 wt% at ∼ 10 MPa at room temperature, which is less than that for carbon nanotubes [76]. The BN nanotubes are useful as coating materials for electricconducting wires or fillers for fiber-reinforced plastics capable of coloring using [77]. Layered metal sulfides such as MoS2 are widely used as a solid lubricant since they are highly active in lubricity because of weakness in van der Waals interaction between their layered sheets. Compared to the bulk sulfides, the friction, fitness, and life-time properties of the lubricant materials are highly improved by the nanotubulation of sulfides [78]. This is due to synergistic effect of the interlayer lubricity and intertubular rolling. Open-edged MoS2 nanotubes are highly active as a catalyst for the CO → CH4 reaction: the reaction temperature for the nanotubular catalyst is lowered by more than 100◦ C in comparison with that for bulk one and the former catalyst is kept in activity for as long as 50 h [79]. Many other efforts have been extensively made to apply the electronic, electromagnetic, optical, and chemical properties of nanotubular materials to various fields, including nanothermometers based on a combined use of oxide nanotubes and low-melting metals.

1.5 Concluding Remarks As described above, a wide range of studies based on the versatility of elements has been carried out on the synthesis, characterization, and applications of inorganic nanotubular materials. The applications of MoS2 nanotubes as solid lubricants and BN nanotubes as filler for polymers would be a typical example for the effective use of elemental characteristics and geometrical functions of nanotubular materials. A future work will afford self-hybridized materials highly and differently functionalized at their internal and external surfaces, e.g., visible light-sensitive photocatalysts for the cleavage of water. The acceleration of global warming currently suggested demands the development of materials available for the transition of fossil resources and energy-based society to environmentally harmonized one. Some nanotubular materials are expected to play an important role for the solution of such serious problems.

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Index Au-doped WS2 nanotubes, 10 Boron nitride nanotubes synthesis, 9 Boron nitride synthetic inorganic nanotubes, 2

Inorganic nanotubes, structural and dimensional characteristics, 8–11 Inorganic nanotubes, structural and synthetic bases, 2–7

Carbon nanotubes discovery, 9 Chrysotile, 2, 8

Layered metal sulfides, 12

FSM-16 mesoporous silica discovery, 1–2

MCM-41 mesoporous silica discovery, 1–2 Metallic nanotubes, applications, 11–12 Metallic nanotubes, structural and dimensional characteristics, 8–11 Metallic nanotubes, structural and synthetic bases, 2–7 MoS2 nanotube synthesis, 9 Multi-walled type (MW-type), 3

Helix-structured NiCl2 NT synthesis, 10 Hexagonal BN, 9 Imogolite, 2 Inorganic nanotubes, applications, 11–12

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Nano- and microstructured inorganic materials synthesis, 6–7 Nanotubes (NT) array, 11 Nanotubes (NT), classification, 4 Nanotubes (NT), pathways to, 4 Nanotubes (NT), structural types, 3–5 Nanotubes (NT), synthetic strategies toward, 5–6 Plasticization of rigid sheet, 10 Rare earth oxide nanotubes, 2 Re-organization of precursory solids, 9 S-type nanotubes formation, 4 Scrolled type (S-type), 3 Self-organization of atomic or molecular species, 9 Seven-walled MoS2 nanotube, 4 Silica synthetic inorganic nanotubes, 2 Silicon nitride synthetic inorganic nanotubes, 2

Single-crystalline Y(OH)3 nanotubes synthesis, 10–11 Single-walled type (SW-type), 3 Solid phase or solid–gas interfacial reaction, 10 Straight-layered sheet, 4–5 Synthetic inorganic nanotubes, 2 Titanium oxide, synthetic inorganic nanotubes, 2 Titanium oxide, synthetic inorganic nanotubes synthesis, 9 Titanium oxide, synthetic inorganic nanotubes TEM/SEM images, 2 Tungsten sulfide, synthetic inorganic nanotubes, 2 Vanadium oxide, synthetic inorganic nanotubes, 2 WS2 nanotube synthesis, 9 Wurtzite-type hexagonal structure, 10

2 Synthesis and Applications of Titanium Oxide Nanotubes Tohru Sekino Institute of Multidisciplinary Research for Advanced Materials (IMRAM), Tohoku University, Aoba-ku, Sendai 980-8577, Japan [email protected] Abstract Titanium oxide nanotube (TiO2 nanotube, TNT) is synthesized by the low-temperature solution chemical method via the self-organization to form unique open-end nanotubular morphology with typically 8–10 and 5–7nm in outer and inner diameters, respectively. Because of the mutual and synergy combination of its low-dimensional nanostructure and physical-chemical characteristics of TiO2 semiconductor, properties enhancements and novel functionalization are expected in the TiO2 nanotube. In this chapter, synthesis, nanostructures, formation mechanism, various physicochemical characteristics, and prospects of future application for the TiO2 nanotube are described in detail. In such an oxide material, property control and enhancement is possible by tuning appropriate chemical compositions, crystal structures, and composite structures. Therefore, special emphasis is also placed to introduce modification of the nanotubes by doping and/or nanocompositing to meet the requirements as for the environmental friendly and energy creation systems and various functional devices.

2.1 Introduction After the discovery of carbon nanotube (CNT) [1], large attention has been paid to this unique low-dimensional nanostructured material because of its attractive various physical and chemical functions which arise from the synergy of low-dimensional nanostructure and anisotropy of carbon network, thus known as graphene structure. Till now, large numbers of not only fundamental studies on the structure, electrical, optical, mechanical, and physicochemical properties but also application-oriented research and development, such as single-electron transistor device, field emission device, fuel cells, and strengthening fillers of composites, have been extensively carried out. Besides CNTs, various inorganic nanotubular materials have been reported in nonoxide compounds, boron nitride (BN) [2] and molybdenum disulfide (MoSi2 ) [3]; in oxides such as vanadium oxide (V2 O5 ) [4–6], aluminum oxide (Al2 O3 ) [6], silicon dioxide (SiO2 ) [6, 7], titanium oxide (TiO2 ) [8–14]; and also in natural minerals like imogolite [15, 16]. T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 17–32 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 2

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Except natural mineral materials, fabrication of nanotubes is roughly classified into two methods; one is the template or replica method, in which some template materials are used to form tubular structure. Many efforts have been paid to fabricate tubular materials including nanotubes by attempting the template/replica method [4, 5, 8, 12–14]. The other one is based on the self-structuralization or self-organization of matter during chemical or physical synthesis/fabrication processes. Synthetic imogolite [16], sol–gelderived SiO2 nanotube [7], chemically prepared TiO2 nanotube [9, 10], and nanotube/nanohole arrays such as Al2 O3 [17, 18] and TiO2 [11, 19] prepared by electrochemically using anodic oxidation of metal films are the typical systems fabricated by the self-organizing process. Among them, titanium oxide nanotube (TiO2 nanotube, TNT) is one of the promising nanostructured oxides with tubular structure. TiO2 is well known as a wide gap semiconductor oxide. It is, however, inexpensive, chemically stable, and harmless and has no absorption in the visible light region. Instead, it is UV light responsible; electron and hole pair is generated by the UV irradiation, inducing chemical reactions at the surface. Therefore, the most promising characteristic of TiO2 lies in its photochemical properties such as high photocatalytic activity. Due to this reason, it has been widely studied by many researchers from 1950s to utilize TiO2 as a photocatalyst [20–22], an electrode of dye-sensitized solar cell [23], a gas sensor [24], and so on. On the other hand, Kasuga et al. [9, 10] have succeeded in the synthesis of nanotubular TiO2 , which has open-end structure with typically 8–10 and 5–7 nm in outer and inner diameters, respectively, using a simple and low temperature solution chemical processing. Various methods such as anodizing of metal substrates [11, 19], replica [8, 12, 13], and template methods [14] have been investigated to prepare tubular TiO2 . However, the synthesis method developed by Kasuga et al. is based on a self-organizing and templateless route that is achieved by low temperature process to form nanometer-sized tubular morphology. Using this so-called Kasuga method, many related investigations have been extensively carried out on structural analysis, process optimization, properties evaluation, and so on [25–27]. As mentioned above, not only fundamental interests in the formation mechanism and the unique nanotubular structures but also functions’ enhancements and novel functionalization are hence expected in the TiO2 nanotube because of the mutual and synergy combination of various factors lying in a nanotubular semiconductor: (1) crystal structure, (2) chemical bonding and (3) physical/chemical properties of the matter, and (4) low-dimensional nanostructures/nanospace/nanosurface and (5) self-organization/ordering of the structure. In this chapter, synthesis processing, nanostructures, various properties and prospects of future application for the TiO2 nanotube fabricated by the low temperature solution chemical route are described in detail. In such an oxide material, property control and enhancement are possible by tuning appropriate chemical compositions and crystal structures. Therefore, special

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emphasis is also placed to introduce modification of the nanotubes by doping and/or nanocompositing in order to meet the requirements as per the environmental friendly and energy creation systems and various functional devices.

2.2 Synthesis and Structure of Titanium Oxide Nanotubes As mentioned before, fabrication of nanotubular TiO2 is classified into two methods: template/replica route [8, 12–14] and direct synthesis (i.e., templateless) route. In the former method, some materials, such as organic, inorganic, and metal nanowires/nanorods/whiskers or nanotube/nanohole arrays such as Al2 O3 prepared by anodic oxidation of Al foil, are used as the templates. TiO2 is hence often synthesized by sol–gel or precipitation methods in solution, and then the templates are removed afterward. Therefore, the size of obtained materials can be easily controlled by the size of template used. Followed by these processing routes, however, the most as-synthesized nanotubes have an amorphous structure, and then they become nanocrystalline nanotubes after appropriate heat treatment. The latter (direct) synthesis route includes low temperature solution chemical method [9, 10] and electrochemical oxidation route from metal substrate or foil, i.e., anodic oxidation of titanium or titanium alloy [19] that also gives amorphous nanotubes. In the case of solution chemical route, crystalline TiO2 nanotube based on the TiO6 octahedron network can be obtained. In this section processing and structures of the TNT will be given. 2.2.1 Low Temperature Solution Chemical Processing Typical TNT is synthesized by the solution chemical route using highconcentration alkaline solution [9, 10]. Various titanium oxide powders including anatase- or rutile-type titania, their mixture, or titanium alkoxide can be used as the source materials of TNT. The raw material is refluxed in 10 M NaOH aqueous solution at around 110◦ C for 20 h or longer. The resultant product is washed many times by distilled water in order to remove sodium. Then 0.1 M HCl aqueous solution is added to neutralize the solution and again treated with distilled water until the solution conductivity reached 5 mS/cm. The product is then separated by filtering, centrifugation, or freeze drying technique and dried. This synthesis is carried out under the refluxing condition so that the pressure during synthesis is the same as that of ambient atmospheric pressure of 0.1 MPa; the synthesis temperature of around 110◦ C thus corresponds to the boiling temperature of high-concentration alkaline solution.

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On the contrary, hydrothermal synthesis using an autoclave, which provides closed reaction environment and hence the slightly higher pressure during the processing, can also be attempted to synthesize TNT [28]. Furthermore, not only TiO2 but also Ti metal can be used as the source material of TNT [29], in which process titanium is chemically oxidized in the alkaline solution. The size control of TNT also has attracted much attention. Various sized, especially thick TNT can often be synthesized by the hydrothermal method, because it gives higher synthesis temperature than 110◦ C. In addition, natural mineral source is also used for the TNT synthesis that may reduce the production cost of the TNT [30]. X-ray diffraction patterns showing phase development during the chemical processing are shown in Fig. 2.1, and corresponding transmission electron micrographs are represented in Fig. 2.2. After alkaline treatment, the product mainly consists of amorphous and crystalline phase corresponding to sodium titanate (Na2 TiO3 , Fig. 2.1b), but the shapeless matter is obtained (Fig. 2.2a). After the water and HCl treatment (Figs. 2.1c and 2.2b), sodium titanate disappears completely and another crystalline phase with low crystallinity is observed. In this step, nanometer-sized sheet-like morphology can be obtained, which is considered as the TiO2 nanosheet. Further, water washing provides fibrous product (Fig. 2.2c) with the length of several hundreds to several micrometers. Higher magnification TEM photograph shown in Fig. 2.2d clearly reveals that the outer and inner diameter of the final product is around 8–10 and 5–7 nm, respectively, and it has an open-end structure. The size of obtained TNT does not depend on the kind of raw materials used. In addition, when KOH is used as a reaction solution, TNT can also be produced with the similar size and morphology.

Fig. 2.1. X-ray diffraction patterns of products obtained in each chemical synthesis step: (a) anatase-type TiO2 raw material, (b) after alkaline reflux (10 M NaOH, 110◦ C, 24 h), (c) after water washing, (d) final product (after 0.1 M HCl and water washing)

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Fig. 2.2. TEM images showing morphological development of the products in each chemical synthesis step: (a) after alkaline reflux (10 M NaOH, 110◦ C, 24 h), (b) after 0.1 M HCl treatment, (c) final product, (d) high magnification image of obtained nanotubes

The surface area of the typical TNT is approximately 300∼ 350 m2 /g, and the value is in good agreement with the calculated theoretical surface area, 345 m2 /g, by assuming the tubular structure, the observed size, and the density of TiO2 crystal. However, recent investigation has revealed that the larger TNT with more than 10 nm in diameter can be obtained when larger titanium oxide powders with particle diameter in micrometer is used and when hydrothermal synthesis method is utilized.

2.2.2 Nanostructures and Formation Mechanism On the contrary to layered compounds like graphite, TiO2 has rigid crystal structure in which a lattice spreads out isotropically and three dimensionally, so that its crystal shape is usually equiaxial. However, solution chemical synthesis described above gives anisotropic and open-end nanotube structure in TiO2 . In order to identify the structural characteristic and also to understand the formation mechanism of TNT in relation to its synthesis process, much efforts for the structural analyses have been paid by using X-ray and

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Fig. 2.3. TEM image of TiO2 nanotube bundle (a) and selected area electron diffraction pattern (b)

neutron diffraction and high-resolution electron microscopy coupled with electron diffraction technique [9, 10, 28–39]. In the selected area electron diffraction (SAED) pattern of TNT bundle (Fig. 2.3), some diffraction spots with belt-like spreading are found, which is typically found in a fibrous compound. As summarized in Table 2.1, the interplanar spacing (d-spacing) of spots a (a ), b (b ), and d (d ) correspond to those of (101), (200), and (100) of typical anatase crystal of TiO2 , respectively [38]. From these facts, it is considered that the TNT basically has the similar crystal structure as the anatase type of TiO2 , and then the longitudinal direction of the nanotube corresponds to the a-axis [(100) direction] while the cross section is parallel to the b-plane [(010) plane] of the anatase crystal. On the other hand, the diffraction spot c (c ) provides the d-spacing of 0.87 nm, and corresponds to the broad diffraction peak found at 2θ of around 9◦ in the XRD patterns of Fig. 2.1d, and also corresponds to the spacing of 0.88 nm at the wall in Fig. 2.2d. The reflection of anatase crystal near to this value is (001) with d = 0.951 nm (Table 2.1); however, there is a slightly large deviation (approximately 8.5%) between these values and hence the spot c(c ) seems not to correspond directly to the (001) of anatase structure. This large interplanar distance is a typical characteristic in titanium oxide nanotube and closely related to the formation of the structures as described in the latter part. Thermogravimetry coupled with mass spectroscopic analysis for the assynthesized TNT exhibited the weight loss continued up to approximately 350◦ C and detected major species was H2 O. High-temperature XRD results

Table 2.1. Interplanar (d) spacing observed for TiO2 nanotube bundle (Fig. 2.3) and corresponding plane and d-spacing of anatase-type TiO2 Position

d (nm)

Anatase TiO2 , hkl/d (nm)JCPDS

a ∼ a b, b d, d c, c

0.37 0.19 0.28 0.87

(101)/0.352 (200)/0.189 (110)/0.268 (001)/0.951

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Fig. 2.4. High-temperature X-ray diffraction patterns of synthesized TiO2 nanotubes and corresponding structure change

(Fig. 2.4) demonstrated that the typical diffraction peak intensity found at 2θ around 9◦ decreased with increasing in test temperature up to around 400◦ C, while the peaks corresponding to anatase structure of TiO2 became to be the major crystalline phase and its crystallinity increased above the temperature. Annealing temperature dependency of the specific surface area for pure TNT is summarized in Table 2.2 (see also Fig. 2.8). High surface area was maintained up to around 400◦ C while sudden decrease occurred above the temperature and then reached to the value approximately 100 m2 /g at an annealing temperature higher than 450◦ C. From TEM investigation for the annealed TNT, its nanotubular structure was found to be kept up to around 450◦ C. These facts imply us that the as-synthesized TNT contains some amount of hydroxyl group (–OH) and/or structure water (H2 O) and has TiO6 octahedral network structure which is similar to common anatasetype structure of TiO2 crystal or, in another words, has titanate-like structure [38]. By the heat treatment (annealing) for the as-synthesized TNT, proton is released as H2 O and then the nanotube becomes to be the stoichiometric Table 2.2. Variation of surface area on the annealing temperature for the TiO2 nanotubes. The surface area is measured by the BET method Temperature (◦ C) Surface area (m2 /g)

RT 322

200 308

400 228

450 123

500 101

550 95.0

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Fig. 2.5. (010) projection of H2 Ti3 O7 unit cell (a) and structure model of nanotube by assuming a chemical composition as H2 Ti3 O7 (b and c) proposed by Chen et al. [31]. Reprinted with permission from [31]

TiO2 nanotube with an anatase structure as its base crystal structures at around 400◦ C. Detailed structure analyses have been carried out extensively. Chen et al. [31] investigated the structure of chemically prepared TNT by using highresolution transmission electron microscopy and reported that the TNT was titanate with the chemical formula of H2 Ti3 O7 and proposed the structure model as shown in Fig. 2.5. On the other hand, Ma et al. [32, 33] showed it was lepidocrocite which was one of the defect-containing titanate with the formula of Hx Ti2−x/4 x/4 O4 . Besides these structures, various compositions were reported, Na2 Ti2 O4 (OH)2 or its protonated titanate of H2 Ti2 O4 (OH)2 [34] and H2 Ti4 O9 [35]. These compounds, however, basically contain OH group and/or H2 O and can be described as (TiO2 )n ·(H2 O)m , which reasonably explains the fact that H2 O is released by the heat treatment of as-synthesized TNT as mentioned above. The reason why many plausible composition models are reported is considered as follows; synthesized TNT usually has a small diameter and hence the wall thickness is quite thin, around 1–2 nm, and also its crystallinity is rather low as shown in Fig. 2.1 by comparing with usual TiO2 crystalline particles. Furthermore, a large number of titanates are known in the series, and most of them have a layered structure with the similar structure. As mentioned before, TNT can be fabricated by using not only NaOH but also KOH, while the nanotubular matter is not synthesized in the case of LiOH solution; in this case more stable crystalline LiTiO2 is formed [38]. These facts imply us that the formation of alkaline titanate like Na2 TiO3 or its amorphous matter (see Fig. 2.1b) is an important intermediate compound

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for the formation of the nanotube. By considering these facts the formation of the TNT is thus regarded as follows: at first, titanate-containing alkali metals (alkali titanates) is formed during the solution chemical treatment. Then the alkali metal element is ion exchanged, and protonated titanate is formed as a nanosheet. In the final step, the nanosheet converts to be a tubular structure (Fig. 2.5) by scrolling process in order to lower the surface energy. Till now a large number of discussions on the actual structure models and formation mechanisms for the TiO2 nanotubes [37–39], and related investigations such as process development for controlling nanotubes length and diameter and extended research toward nanowires/nanorods, are continued by many research groups. Nevertheless, it should be noted that the crystal structure based on the three-dimensional framework of TiO6 polyhedron and low-dimensional nanostructure formation for the TiO2 nanotube is a quite unique and different from those of the carbon nanotube, which is built from the two-dimensional graphene sheet (carbon network).

2.3 Functions of Titanium Oxide Nanotubes Similar to common TiO2 powder, the TNT is also white colored powder. The optical bandgap energy calculated from the ultraviolet–visible light absorption spectra by assuming indirect transition of TiO2 is approximately 3.41 ∼ 3.45 eV for chemically synthesized TNT [38], which value is slightly larger than that of anatase (3.2 eV) and rutile (3.0 eV) crystals. This blue shift of the absorption edge wavelength is attributed to the quantum size effect of TiO2 semiconductor [40] in TNT because of very thin nanotube wall thickness of around 1 ∼ 2 nm. Recent materials design strategy of TiO2 nanoparticles focuses on the developed visible light responsible TiO2 photocatalyst [41] so that the enlarged bandgap seems to be disadvantageous; nevertheless TNT exhibits unique and excellent photochemical properties which contribute enhanced environmental purification performance. 2.3.1 Photochemical Properties and Photocatalytic Functions In order to clarify the photochemical characteristic of TNT, Tachikawa et al. [42] investigated the photocatalytic one-electron oxidation reaction of an organic molecule and related charge recombination dynamics during UV light irradiation on TNT using time-resolved diffuse reflectance spectroscopy. They observed remarkably long-lived radical cation and trapped e− for the TNT, approximately five times or more long lifetime than those for the nanoparticles. Further, they have observed that the electron generated by the steady-state irradiation of UV light could exist for longer time on the TNT surface, which phenomenon was usually not confirmed in TiO2 nanoparticles, and also the evidence of rapid reaction of trapped e− with organic halide pollutants such as CCl4 . These features are considered mainly due to the unique one-dimensional

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nanostructure of the TNT and are the reason of the good photocatalytic properties; TNT has very thin wall so that generated carriers can effectively move to the surface, and then charge recombination is inhibited due to its long one-dimensional structure, clearly suggesting morphological advantage of the TNT on the charge recombination dynamics. These may also be advantageous for the use of TNT as for the electrode of solar cell in which transfer characteristic is very important. In fact, longer lifetime while the similar diffusion coefficient of electron in TNT has been reported when it has used for the electrode of dye-sensitized solar cell [43]. As mentioned before, anatase-type TiO2 is well known as a promising photocatalytic material due to its photochemical characteristic. Figure 2.6 shows variation of hydrogen generation by UV light irradiation to as-synthesized and annealed TNTs and commercial TiO2 nanoparticles in water/methanol mixed solution (so-called water splitting test) [38]. As can be seen from the figure, as-synthesized TNT shows lower photocatalytic activity than the commercial TiO2 powders (P-25 and ST01). This low activity is considered due to the existence of many hydroxyls (–OH) and/or structural water (H2 O) and low crystallinity of the as-synthesized TNT. On the other hand, annealed (400◦ C) TNT can generate approximately two to three times higher amount of H2 than that of nanoparticles, when compared to H2 amount per unit mass of TiO2 photocatalyst. The enhanced hydrogen evolution performance of the annealed TNT is caused by the improved crystallinity (see Fig. 2.4) with maintaining its nanotubular structure and higher surface area, around 230 m2 /g (Table 2.2 and Fig. 2.8), than that of TiO2 nanoparticle (approximately 50 m2 /g). However, by comparing the generated amount of H2 per unit surface area of the catalysts, TNT exhibits around 44–65 % of nanoparticle system. This fact indicates that an approximately half of the surface may not act as for the active site of hydrogen generation, and hence the inner wall of the nanotube

Fig. 2.6. Hydrogen generation by the water splitting during UV irradiation to various TiO2 photocatalysts (P-25 and ST01, commercial TiO2 nanopowders, asprepared TNT, and annealed TNT at 400◦ C)

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may not contribute to the photocatalytic reaction, which is probably due to the diffusion limit of molecules within the inner part of the nanotubes during the reaction. Nevertheless, it is expected that the TNT would be one of the promising candidate as the high-performance energy creation materials and systems such as the excellent hydrogen generation catalyst. 2.3.2 Novel Environmental Purification Functions Photocatalytic performance is often evaluated by the removal test of organic molecules in water system. Figure 2.7 represents the variation of methylene blue (MB) concentration in TiO2 dispersed water system under dark and UV light irradiation conditions. In the case of commercial TiO2 nanoparticles, MB concentration is quickly decreased under UV irradiation while is not changed without the UV irradiation (hence under the dark condition). This clearly indicates that the TiO2 nanopowder is an excellent photocatalyst. However, in the case of as-synthesized TNT, MB decrease can be confirmed even under the dark condition and is enhanced further under the UV light irradiation. This fact indicates that the TNT has a molecule adsorption characteristic, and it is more obvious than the photocatalytic degradation under the UV irradiation. When TNT is annealed, the MB degradation under the dark condition is reduced; however, the photodegradation is higher than that of as-synthesized TNT. It is again considered that the increased crystallinity can enhance its photocatalytic performance. Generally, TiO2 including nanopowder has very low molecule adsorption capability compared to typical adsorbent materials such as zeolite, activated carbon, and clay minerals. Therefore, development of composite materials of TiO2 photocatalyst and some other adsorbents such as mesoporous silica

Fig. 2.7. Variation of methylene blue concentration under the dark and UV light irradiation conditions for the TiO2 nanoparticle, as-synthesized and annealed TiO2 nanotubes dispersed water system, and schematic drawing of adsorption/photochemical behaviors for the TNT

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[44] has been investigated. On the other hand, TNT has not only excellent photocatalytic property but also high capability for the molecule adsorption as a single phase material. This novel multifunctionality, hence the synergy of molecule adsorption and photocatalytic properties, is attributed to its unique crystal and nanostructures as well as material’s photochemical characteristic; as described before, TNT has a high surface area and layered compound-like structure such as clay minerals. These structural characteristics might be the reason of the high adsorption capability of the TiO2 nanotube. Therefore, TNT is regarded as a novel multifunctional nanostructured material and then expected to be an excellent candidate as for the advanced high-performance environmental purification system.

2.3.3 Multi-functionalized Titanium Oxide Nanotubes In order to enhance properties and/or to functionalize materials, doping some elements and/or compositing with the other materials is often utilized. For instance, doping to silicon can control its semiconductive properties and hence various devices are widely developed and used. In the case of TiO2 nanotube, these are also applicable techniques. For instance, when TNT is considered to be used as the chemical sensing device, electrode of solar cell, and so on, control and improvement of electrical properties are necessary and required to obtain higher conductivity, i.e., good carrier transfer properties and resultant better device performance. For this purpose, various metal cations have been doped into TNT via the chemical synthesis process [45]. When transition metal cations such as Cr3+ , Mn3+ , Co2+ , Nb5+ , and V5+ were doped, morphology, surface area, and optical bandgap of the doped TNT were almost as same as those of pure TNT. However, electrical conductivity of the doped TNT was around 1–2 orders of magnitude higher, for example, 1.0 × 10−4 S/cm for 0.08 mol% Cr-doped TNT, than those of TiO2 nanopowder (2.6 × 10−6 S/cm) or pure TNT (3.0 × 10−6 S/cm). Another effect of cation doping to the TNT was found in the thermal stability improvement as shown in Fig. 2.8; structural degradation of the nanotube and the resultant decrease of surface area began at around 400◦ C for the pure TNT (refer also to Table 2.2). However, the critical temperature was enhanced approximately 50 (Mn3+ , Co2+ , Nb5+ , V5+ ) to 100◦ C (Cr3+ ) for the doped TNT [38]. These facts indicate that the cation doping can enhance both electrical conductivity and thermal stability of the nanotube, which is regarded as one of the advantages when the TNT will be used as various devices, because most of these devices are fabricated by the pasting and the following sintering of the material to form films on the appropriate substrates. Furthermore, loading various metals and/or compounds into inside of the nanotubes and/or onto the surfaces in nanometer scale is possible. Figure 2.9 shows TEM images of metals and sulfide compound-loaded TNT nanocomposites which were prepared by using various physicochemical processing.

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Fig. 2.8. Temperature dependence of BET surface area of cation-doped TNTs (cation concentration ca. ∼ 0.1 mol%) and corresponding morphology change for pure and Cr-doped TNTs at 500◦ C

Fig. 2.9. Various TNT-metal nanocomposites. (a) Pd-loaded TNT prepared by sonochemical method, (b) Ag nanoparticles formed inside of the TNT, (c) Ni nanoparticles inside of the TNT, (d) ZnS-loaded TNT prepared by solution chemical route

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When Pd nanoparticles are composed with the TNT, hydrogen generation performance was much enhanced due to the promoting effect of the loaded noble metals [38]. For the properties enhancement and further multifunctionalization, nanocompositing of TNT with the other materials is suitable and advantageous method.

2.4 Conclusion and Prospects In this chapter, chemical processing, structure, physical, and chemical properties of TiO2 nanotube that can be prepared by the solution chemical route have been reviewed. Till now, a large number of fundamental studies and application-oriented researches and developments are extensively carried out by many researchers for this low-dimensional nanomaterial, because not only enhancement of various properties of TiO2 but also multifunctionalization due to the harmonization of materials properties and unique lowdimensional nanostructure is expected. As for the application of TNT, it has been used as the oxide electrode of the dye-sensitized solar cell, and better cell efficiency and structure-related characteristics on the charge transport phenomenon have been reported [43]. Also it is reported that TNT exhibits proton intercalation/de-intercalation and resultant electrochromism, size-selective adsorption of molecules [46], anion doping to develop visible light responsible TNTs [47], and biocompatibility [48, 49]. On the other hand, extensive challenges to develop various oxide and compound nanotubes have been continued. For instance, rare earth oxide nanotubes have recently been synthesized [50]. (Details on the variety of nanotube materials are introduced in other chapters.) All these facts imply us that the oxide nanotubes including TNT have multifunctionalities owing to the structure–property correlations. As mentioned before, one of the future research direction of the TiO2 nanotube might lie toward the application as the environmental and/or energy creating systems, which would become more important in the near future.

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Index Ag nanoparticles in TNT, 29 Carbon nanotube (CNT) discovery, 17 Cation doping to TNT, 28 Direct synthesis of TNT, 19 Environmental purification functions of TNT, 27–28 Formation mechanism of TNT, 21–25 High-temperature X-ray diffraction patterns of synthesized TNT, 23 Hydrothermal synthesis of TNT, 20 Interplanar spacing in TNT, 22 Kasuga method, 18 Low temperature solution chemical processing synthesis of TNT, 18–21 Ni nanoparticles in TNT, 29 Pd-loaded TNT, 29 Photocatalytic functions of TNT, 25–27

Photochemical properties of TNT, 25–27 Selected area electron diffraction (SAED) pattern of TNT, 22 Self-structuralization or selforganization, 18 Surface area variation on annealing temperature for TNT, 23 TEM images of TNT synthesis, 21–22 Temperature dependence of BET surface area of cation-doped TNTs, 29 Template or replica method, 18 Titanium oxide nanotube (TNT) mutual and synergy combination, 18 Titanium oxide nanotube (TNT) properties, 18 TNT-metal nanocomposites, 29 Water splitting test, 26 X-ray diffraction patterns of TNT synthesis, 20 ZnS-loaded TNT, 29

3 Synthesis, Structural Analysis, and Applications of Titanium Oxide Nanotubes Tsuguo Koyanagi New Business Research Center, New Business R&D Division JGC Catalysts and Chemical Ltd., Kitakyushu Operation Center, 13-2, Kitaminato-Machi, Wakamatu-ku, Kitakyushu-shi 808-0027, Japan [email protected] Abstract An explanation of the new synthesizing method for mono-dispersed titanium oxide nanotubes (TNT) is provided. By using the latest transmission electron microscopy method, the electric structure, crystal structure, and morphology of the TNT were analyzed. As a practical application, we studied the properties of TNT as the light electrodes of dye-sensitized solar cells and photo-catalysts in visible light. For DSC, light electrodes using nano-whiskers that were made from heat-treated TNT indicated high light harvest efficiency. Furthermore, we found that TNT has a high surface area and due to its interlayer structure which can insert hetero-atoms, N-doped TNT indicates high photo-catalysis in the visible light region.

3.1 Introduction The material Titania has attracted a great deal of public attention due to its important functions as an optical semiconductor material, for example as a photocatalyst, and in the next generation of cheap solar batteries within the fields of new energy and environmental improvement technology. The first photocatalyst research on Titania was the electrolysis of water in 1972 and this research used single-crystalline Titania for the electrode; this was called the Honda and Fujishima effect [1]. Afterwards, a new solar battery was invented in 1991 based on the idea of dye-sensitization to Titania covered with dye materials which was proposed by Prof. Graetzel (Switzerland EPFL) [2–4]. Since then R&D for the industrialization of the new solar cell has become active all over the world. In 1993 irradiation by comparatively weak ultraviolet rays was used for the purification of polluted air and polluted water using the Titania-photocatalyst reaction that Prof. Fujishima et al. proposed. In addition, Prof. Fujishima discovered the phenomenon of super-hydrophilicity of the surface of TiO2 . Recently, anion-doped Titania has been made practicable as a visible photocatalyst. Porous membranes made of colloidal Titania are known for their significant functionality. These functional films which have high specific surface areas and T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 33–44 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 3

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uniform porosity are important. Notably, the Titania nanotube (TNT) [5] has a specific surface area of about 300 m2 /g, and a nano hollow structure that is of about 10-nm internal diameter. Because of this unique nanostructure applied researches such as into photocatalysts and solar cells are advanced. The chief aims of this research are the structural analysis of TNT and changes through heat-treatment, as monitored by state-of-the-art electron microscope analysis techniques (FE-TEM), and this work also reports on the results of researching the applications in a dye-sensitized solar cell (DSC) and a visible photocatalyst.

3.2 Synthesis of Mono-Dispersed Titanium Dioxide Nanotubes To obtain good quality TNT we used mono-dispersed nano-Titanias that have a high crystalline Anatase particle size. The mono-dispersed Titanias were synthesized using a manufacturing process that separated the particle growth and the generating nucleus. The synthetic flow for the large particle Sol is shown in Fig. 3.1. The synthetic flow for the Titania nanotube using this large particle is shown in Fig. 3.2.

Fig. 3.1. Anatase TiO2 sol synthesis

Fig. 3.2. TiO2 nanotube synthesis

3 Synthesis, Structural Analysis, and Applications of Titanium Oxide

35

3.3 Nanostructure Analysis of Titanium Dioxide Nanotubes The nanostructural analysis of TNT was carried out using the state-of-theart of analysis technologies, i.e. with the field-emission electron microscope (FE-TEM). Figure 3.3 shows the result of high-resolution transmission electron microscopy (HRTEM) of TNT. The HR-TEM indicated that the TNTs have a crystal fringe interval of 0.363 nm. The interval fringe of the Anatase is a = b = 0.378 nm and that of the Rutile is a = b = 0.457 nm. Anatase and Rutile represent two of the crystalline morphologies of the Titania [6]. The fringe interval was different from either that of the Anatase or Rutile. On the other hand, the HAADF-STEM (high-angle annular dark-field scanning transmission electron microscopy) method is able to measure the difference in density and thickness of the nano domain from the intensity of elastic scattered electrons .The HAADF-STEM method is also called the “Z-contrast method,” because the intensity of elastic scattered electrons is proportional to the second power of the atomic number Z; the analytical result is shown in Fig. 3.4. It turned out that TNT has a very thin hollow structure with a very thin wall. Next, to clarify the electronic structure of TNT, EELS (electron-energyloss spectroscopy) was carried out. The Rutile structure of titanium oxide shows the symmetry of molecular orbital of D2h that degrade the Oh symmetry

Fig. 3.3. HRTEM images of TNT

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Tsuguo Koyanagi

Fig. 3.4. HAADF-STEM analysis

of the coordinated eight oxygen. The ELNES (electron energy-loss near edge structure) spectra of TiL of TNT, Anatase and Rutile are shown in Fig. 3.5. The A band is assigned to the electron transition from the 1s to the t2g of Oh symmetry orbital, on the other hand the B band transition is assigned the t2g symmetry orbital. There is no transition correspond to t2g and eg orbital transition in the TNT. Figure 3.6 shows the peak corresponding to the O-K edge. Anatase shows only four peaks though the Rutile shows five peaks. The spectrum for TNT showed a peak different from Anatase and Rutile, and showed an intermediate spectrum. Therefore, the result was that TNT had a different electronic structure to Anatase and Rutile. We investigated the change of the nanostructure of TNT by using a selected area electronic diffraction method. Figure 3.7 shows the results of the selected area electronic diffraction of TNT. It is shown that at 300◦ C the TNT has an amorphous structure and at 700◦ C it has a high Anatase crystalline

Fig. 3.5. EELS spectra (Ti)

3 Synthesis, Structural Analysis, and Applications of Titanium Oxide

37

Fig. 3.6. EELS spectra (O)

300°C

700°C

Fig. 3.7. HRTEM images of thermally treated TNT

structure. Also via heating XRD, we can confirm that the structural change occurs at 300◦ C and the crystal structure changes to an Anatase structure. Figure 3.8 shows the relation between the surface area (S.A.) and the crystal particle size when the TNTs are heat-treated at various temperatures. It is understood that the specific surface area decreases at 400◦ C or more, and

Fig. 3.8. Surface area vs. calcination temperature

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Tsuguo Koyanagi

300°C

450°C

600°C

Fig. 3.9. The morphology change of TNT during heat-treatment (using FE-SEM and STEM)

substantial change in the hollow construction is shown. The crystal diameter of Anatase grows this changing, too. It is ascertained that the TNT has the large surface area and the high Anatase crystallinity even at 600◦ C. The results that were measured using simultaneous high-resolution FETEM and STEM are shown in Fig. 3.9. The hollow structure still remained at 450◦ C, but at 700◦ C the TNT became a chain of rectangular particles.

3.4 Applications of Titanium Dioxide Nanotubes 3.4.1 Application for Dye-Sensitized Solar Cells Two application research examples that use TNTs are described herein. One is in a dye-sensitized solar cell. Figure 3.10 shows the structure of a dye-sensitized solar cell. A spongy Titania surface is formed on a transparent electrode, and the surface of this Titania is covered with dye materials. Similarly, the counter electrodes are covered with platinum on the conductive glass. To complete the solar battery an electrolyte comprised of iodine and iodide is enclosed between these two electrodes. The spongy porous TiO2 film shows good light conversion efficiency. Spongy porous TiO2 films that were made by the hydrothermal synthesis of TiO2 colloids have the highest light conversion efficiency. Instead of the colloidal TiO2 , we used the TNT for DSC. (The performance of the TNT is shown in Table 3.2.) We named the transformation of the TNT NanoWhisker (NW). (NWs were made from the TNT by heat-treatment at 600◦ C).

3 Synthesis, Structural Analysis, and Applications of Titanium Oxide

39

Fig. 3.10. Structure of DSC

Table 3.1. Characteristics of nanowhiskers and Titania nanotubes Surface area (m2 /g)

XRD

Shape (nm)

NW (650◦ C)

70

Anatase

TNT (110◦ C)

280 ∼ 300

Amorphous

S:10 ∼ 20 L:200 ∼ 400 S:16 ∼ 15 L:400 ∼

The NWs are better than the TNT (at 110◦ C) from the point of view of the light harvest conversion efficiency. The efficiency of the NWs is equal to that of the colloidal TiO2 . Table 3.1 lists the characteristics of NWs and TNTs. NW that was heat-treated at 650◦ C, has a higher electronic current than that of TNT. This means that the Anatase crystalline form is very important for DSC. The results of evaluation of the characteristics of the DSC are shown in Table 3.3 by using 15-nm TiO2 colloids in a hydrothermal synthesis. It turned out that the current value of the TNT was lower than that of the 15-nm colloidal TiO2 , and the conversion efficiency was not better. To analyze the cause, we measured the zeta potential of two types of TiO2 . A comparison between the NW and TNT is shown in Fig. 3.11. The dye adsorption was weak on the surface of the TNT. This reason was elucidated from the profile of the zeta potential which showed that the TNT became negatively charged in all pH range and the dye also carried negative charge. In this case the dye and the TNT are repulsive to each other. In conclusion, the dye adsorption on the surface of the TNT was so week that the electric injection from the dye to the TNT is not good.

Film thickness

11.5 11.0

Thickness

NW TNT

5.9 1.5

Efficiency 1/10S (%) 6.0 4.4

Efficiency 1S(%) 0.75 0.32

ff 1/10 0.68 0.65

ff 1S 1.21 0.94

I 1/10S (mA/cm2 )

12.6 9.8

I 1S (mA/cm2 )

Table 3.2. Comparison between TNT and NW (Nano-wisker; heat-treated at 650◦ C)

626 470

Pot. 1/10S (mV)

713 698

Pot. 1S (mV)

40 Tsuguo Koyanagi

15 nm Titania sol

TNT

3.2 μm

3.2 μm

Thickness 2.9 2.5 3.5 3.8

3.7

Efficiency 1S (%)

2.7 2.4 3.3

Efficiency 1/10S (%)

0.73

0.74 0.74 0.73

ff 1/10

0.69

0.72 0.71 0.69

ff 1S

0.71

0.51 0.45 0.64

I 1/10S (mA/cm2 )

7.3

5.2 4.5 6.6

I 1S (mA/cm2 )

681

690 700 680

Pot. 1/10S (mV)

Table 3.3. Comparison between the RNT and the 15 nm Ti02 colloid for the performance

760

772 787 762

Pot. 1S (mV)

3 Synthesis, Structural Analysis, and Applications of Titanium Oxide 41

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Tsuguo Koyanagi

Fig. 3.11. The zeta potential of TNT and TiO2 colloids

3.4.2 Application for Photo-Catalysis Next, we investigated the photocatalytic properties of TNT by a simple method. We prepared three samples. One sample is the dried TNT, the second is the TNT heat-treated at 300◦ C, and the third is the nitrogen-doped TNT that was modified by heat-treatment at 350◦ C mixing the TNT with urea. Figure 3.12 shows the X-ray photoelectron spectroscopy (XPS) of the N-doped TNT. A new peak was observed to 457.3 eV, and the strength of N1s in XPS also shows a big difference. In addition by EELS the mapping of Ti was carried out and the result is shown in Fig. 3.13. It turned out that Ns dope very uniformly along the tube shape.

TNT N-doped TNT

Fig. 3.12. XPS spectroscopy of TNT and N-doped TNT

3 Synthesis, Structural Analysis, and Applications of Titanium Oxide

43

Fig. 3.13. The N mapping by EELS of the N-doped TNT

Fig. 3.14. The photocatalytic properties (decomposition through irradiation by solar light)

The X-ray spectroscopy clarified that the N-doped TNT had an amorphous structure and confirmed that there was no change to Anatase. Next, the methylene blue bleaching examination that uses sunlight was used to determine photocatalyst characteristics and the result is shown in Fig. 3.14. The N-doped TNT has the highest photocatalytic activity compared to the TNT at 100◦ C and the TNT at 350◦ C (Anatase). The color of the N-doped TNT is yellow, and it is thought that the visible light absorption improved the photocatalytic activity.

3.5 Summary The colloidal large-sized particles of TiO2 and KOH were used as raw materials for the synthesized mono-dispersed TNT. The synthesized TNT is excellent with high purity and narrow sized-particle distribution. We found that TNT has a high surface area and due to its interlayer structure which can insert hetero-atoms, N-doped TNT indicates high photo-catalysis in the visible light region.

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For DSC, light electrodes using Nano-whiskers that were made from heattreated TNT indicated high light harvest efficiency. Acknowledgment Author thanks Dr. Koji Tanaka and Dr. Tomoki Akita of Sangyou-sougou institute Kansai center who advised and helped with the high-resolution TEM study.

References 1. 2. 3. 4. 5.

A. Fujishima, K. Honda, Nature 238, 37 (1992) 33 B. O’Regan, M. Graetzel, Nature 353, 737 (1991) 33 A. Hagfeldt, M. Graetzel, Chem. Rev. 95, 49–68 (1995) 33 K. Kalyanasandaram, M. Graetzel, Coord. Chem. Rev. 177, 347–414 (1998) 33 T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14, 3160– 3163 (1998) 34 6. D.-S. Seo, J.-K. Lee, H. Kim, J. Cryst. Growth 229, 248–432 (2001) 35

Index Anatase TiO2 sol synthesis, 34 Comparison between TNT and NW, 40 Dye-sensitized solar cell (DSC), 33 Dye-sensitized solar cell (DSC), TNT use in, 38–39 Electron-energy loss spectroscopy (EELS) of TNT, 35–37 Heat-treatment, morphology change of TNT during, 38 High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) method, 35–37 High-resolution transmission electron microscopy (HRTEM) of TNT, 35–37 Honda and Fujishima effect, 33 Methylene blue bleaching examination, 42 Mono-dispersed TNT synthesis, 34

N mapping by EELS of N-doped TNT, 43 Nanostructure analysis of TNT, 34–38 Nanowhiskers (NWs), 38–39 Photocatalytic properties of TNT, 39, 42–43 Rutile structure of Titania, 35–37 State-of-the-art electron microscope analysis techniques (FE-TEM), 33 Super-hydrophilicity of surface of TiO2 , 33 Titania, 33 Visible photocatalyst, 33 X-ray photoelectron spectroscopy (XPS) of N-doped TNT, 39, 42 Z-contrast method, 35–37 Zeta potential of TNT and TiO2 colloids, 42

4 Synthesis and Applications of Titanium Oxide Nanotube Thin Films Masahiro Miyauchi1 and Hiromasa Tokudome2 1

2

Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Higashi, Tsukuba, Ibaraki 305-8565, Japan [email protected] Research Institute, TOTO Ltd, Chigasaki, Kanagawa, 253-8577, Japan [email protected]

Abstract Layer-by-layer or vertically aligned TiO2 nanotube thin films were fabricated by using hydrothermally grown titanate nanotubes. These films were optically transparent and exhibited various functions. Layer-by-layer growth of TiO2 nanotubes on glass substrates was achieved by alternate layer deposition using an aqueous solution of colloidal titanate nanotubes and that of a polycation. These films exhibited photoinduced hydrophilic conversion, low-reflectivity, and significant electrochromism, owing to their unique one dimensional open-pore nanostructure. In addition, transparent thin films of vertically aligned TiO2 nanotube arrays were grown by a hydrothermal treatment of metal Ti thin film on glass substrates. These nanotube arrays were well adhered to the substrates and exhibited super-hydrophilicity even under the dark condition and the efficient electron field emission.

4.1 Introduction Titanium dioxide (TiO2 ) is known as an efficient photocatalyst, and it has already been applied to self-cleaning building materials and environmental purification [1, 2]. In addition to these environmental applications, TiO2 is also applied to the energy-related issue, such as hydrogen generation by water splitting [3] or dye-sensitized solar cell [4]. The TiO2 is nontoxic, stable, and resource abundant, and it is widely used as cosmetics and paint. Recently, TiO2 also becomes attractive in the field of transparent magnetic [5] or conductive materials [6], which can be used in electronic device or biosensor [7]. On the other hand, one-dimensional (1D) nanostructures of TiO2 have attracted great interest as they are expected to develop improved functional devices. There are four general approaches to the synthesis of TiO2 nanotubes, namely, chemical template synthesis [8], alumina template synthesis [9–11], electrochemical approaches (anodizing of Ti) [12], and the alkaline hydrothermal method [13]. Among these synthesis methods, hydrothermally grown titanate nanotubes have received a lot of attention due to their simple T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 45–57 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 4

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and economic fabrication process and unique one-dimensional nanostructures. The crystal structure and formation mechanism of titanate nanotubes have thoroughly been investigated [14–17]. The exact crystal structure is still controversial; however, some studies have indicated that the crystal structure of this nanotube is hydrogen titanate, which is composed of scrolled TiO2 sheets separated by H+ ions. Various properties and functions of hydrothermally grown titanate nanotubes have been reported, i.e., energy band structure [18, 19], photocatalysis [20], Li+ ion insertion [21–23], gas adsorption [24], proton conductivity [25], hydrogen storage [26], dye-sensitized solar cells [27], and sensors [28]. These properties were investigated on powder form of TiO2 nanotubes, but thin film fabrication of TiO2 nanotubes is very important to construct new functional devices for widespread industrial applications. In this section, two kinds of thin films for TiO2 nanotubes and their unique properties are reported.

4.2 Titanate Nanotube (TNT) Thin Films via Alternate Layer Deposition This section reports the preparation of a stable TNT-dispersed solution and fabrication of optically transparent nanostructured TNT thin films by the alternate layer deposition. These films display various functions, such as photoinduced hydrophilicity, low-reflection, and enhanced electrochromism [29]. TNTs were synthesized by a previously reported hydrothermal method. Typically, TiO2 (anatase) powder was added to 10 M NaOH aqueous solution in a Teflon-lined autoclave and heated at 120◦ C for 40 h. After the reaction, a white precipitate was filtered and washed with HNO3 aqueous solution and then washed with distilled water. The X-ray powder diffraction pattern was similar to that previously reported [14–17]. Nanotubular structures with approximately a 10 nm diameter and several hundred nanometres in length were observed by SEM and TEM. A colloidal suspension of TNT was prepared by stirring TNT powder in 1 M HNO3 aqueous solution for 15 h at room temperature. A gelatinous deposit was separated by centrifuging. Then adding this to 0.2 M tetra(n-butyl)ammonium hydroxide (TBAOH) aqueous solution, which was stirred for one night at room temperature, resulted in a translucent solution. Figure 4.1 shows the photos of colloidal solution and TEM images for (a) just after the hydrothermal reaction and (b) after the treatment by HNO3 and TBAOH solution. TNT powders just after the hydrothermal reaction were aggregated and could not be dispersed in water. In contrast, TNT particles were highly dispersed in water by the treatment of acidic and basic solutions. TNT powder could not be dispersed in TBAOH aqueous solution without an acidic treatment. Therefore, the acid treatment is indispensable for dispersing nanotubes in TBAOH solution. The acid treatment contributes to the protonation of the nanotube surface, which stabilized the nanotubes in the TBAOH aqueous solution by exchanging H+ for TBA+ .

4 Synthesis and Applications of Titanium Oxide Nanotube Thin Films

47

Fig. 4.1. Photos for colloidal solutions of TNTs and TEM images for TNTs

TNT thin films were fabricated by alternate layer deposition by using colloidal TNT solution and aqueous polycation solution. The zeta potential experiment indicated that the pH at the point of zero charge (pzc) of TNT was ca. 5.5; thus [29], the TNTs act as negatively charged particles in a neutral condition. Poly(ethyleneimine) (PEI) and poly(diallyldimethylammonium chloride) (PDDA) were used as a polycation. Alternate adsorption occurred by means of the Coulombic force between TNTs and polycations. A glass substrate previously coated by dipping into an aqueous solution of PEI and rinsing with pure water was used to prepare a positively charged surface. Then the substrate was exposed to the above-mentioned colloidal TNT–TBAOH aqueous solution, rinsed with pure water, and immersed in an aqueous solution of PDDA, and rinsed with pure water. This process can be repeated until the desired number of TNT layers is achieved. The TNT/polymer hybrid film was abbreviated as PEI/(TNT/PDDA)n−1 /TNT film, where n is the number of adsorption cycles. The PEI/(TNT/PDDA)n−1 /TNT films were optically transparent up to 10 adsorption cycles. UV-vis absorption spectra of PEI/(TNT/PDDA)n−1 /TNT films grown on glass substrate were shown in Fig. 4.2. Absorption in UV region was due to the interband transition of TNT and the absorbance value at 260 nm increased linearly versus the number of adsorption cycles (inset of Fig. 4.2). Film thicknesses after 5 and 10 cycles were ca. 50 and 100 nm, respectively. Therefore, the thickness for each adsorption cycle is ca. 10 nm, which corresponds to the outer diameter of TNT. These results suggest that the layer-by-layer growth of TNTs on the substrate was achieved by alternate adsorption with polycations. Figure 4.3 shows a tapping-mode AFM image of the film after the first deposition of TNTs (denoted PEI/TNT film) on a glass substrate. The surface was uniformly covered with monodispersed nanotubes. Height profile measurements indicated that the average height of nanotubes on the substrate was ca. 10 nm.

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Masahiro Miyauchi and Hiromasa Tokudome

0.45 10 layers

0.35 Absorbance

0.30 0.25

5 layers

0.5 Absorbance at 260 nm

0.40

0.4 0.3 0.2 0.1 0

0.20

0

2

4 6 8 10 Number of Layers

12

0.15 0.10

2 layers

0.05 1 layer

0.00 240 260 280 300 320 340 360 380 400 Wavelength (nm) Fig. 4.2. UV-vis absorption spectra of PEI/(TNT/PDDA)n−1 /TNT film. Inset shows a plot of absorbance at 260 nm vs. number of adsorption cycles

Fig. 4.3. AFM image of the first layer of TNTs deposited on the glass/PEI substrate

TNT has a photocatalytic activity, and it can decompose polycations by UV irradiation, resulting in a polymer-free TNT film. Figure 4.4 shows the change in water contact angle of the multilayered TNT/polymer film under UV irradiation. When initially irradiating, the contact angle decreased to nearly 0◦ within 60 min. Our X-ray photoelectron spectroscopy (XPS) measurements revealed that the N1s peak assigned to polymer (400 eV) became undetectable after UV irradiation, while the N1s peak assigned to NH+ 4 (401.5 eV) was observed after UV illumination. These results indicate that polymer is decomposed by the photocatalytic activity of TNT. NH+ 4 was not , which suggests that the ammonium cation stadecomposed to NOx or NO− 3 bly existed on the surface of anionic TNT. Moreover, the surface morphology

4 Synthesis and Applications of Titanium Oxide Nanotube Thin Films

49

40

Water contact angle (deg)

35 30 1st 25

2nd 3rd

20

Glass

15 10 5 0

0

20 40 60 UV illumination time (min)

80

Fig. 4.4. The change in water contact angle of TNT/polycation thin film under UV irradiation

remained unchanged after irradiating with AFM and SEM. Therefore, UV treatment converted the multilayered TNT/polymer film to a polymer-free TNT thin film. After the initial illumination, the substrate was stored in the dark and clean-air conditions for a month. Subsequently, the TNT thin film reproducibly demonstrated high photo-induced hydrophilicity in the second and third irradiation. In earlier literature, it has been reported that the photo-induced hydrophilic conversion of TiO2 (anatase or rutile) thin films is associated with the surface structure [30–34]. A similar surface reaction seems to occur on the TNT surface. Although the photocatalytic reaction proceeds on TNT, the band gap of TNT is larger than that of the bulk TiO2 materials because of its quantum size effect. Thus, it is not active when illuminating with visible light. Recently, the nitrogen-doped TNTs were synthesized by immersing TNTs into aqueous NH3 solution and subsequent annealing in air [35]. These nanotubes have an anatase crystal structure with nitrogen atoms substituted into oxygen sites of TiO2 . These nanotubes exhibited photocatalytic oxidation activity of gaseous contaminants under visible light irradiation. In addition to photocatalytic properties, the TNT thin film by an alternate layer deposition method has intriguing properties. One of the interesting properties of TNT films is the high transparency and low reflectance [36]. Generally, polycrystalline TiO2 has a high refractive index (anatase: nω = 2.56, nε = 2.49). Thus, it is difficult to obtain a low-reflective thin film of single component TiO2 . For a glass window application, thin films should be highly transparent and have a low reflectance. Figure 4.5 shows the reflectance of

50

Masahiro Miyauchi and Hiromasa Tokudome 50 45 40 Anatase film

Reflectance (%)

35 30 25 20 TNT film

15 10 5

Glass substrate

0 200

300

400 500 600 Wavelength (nm)

700

800

Fig. 4.5. Optical reflectance for thin films

TNT and anatase films. It is noteworthy that the reflectance of TNT film is much lower than that of polycrystalline TiO2 film. The calculated porosities of the TNT and polycrystalline anatase TiO2 films by ellipsometric analysis are 64 and 27%, respectively. The low reflective index of the TNT film is due to its high porous nanotube architecture. There are inner cavities in TNTs and void spaces between nanotubes. Further, there are not clear plane-parallel faces at the air/TNTs/substrate interfaces. Therefore, it is reasonable that the reflectance of the nanotube films is very low and that interference fringes are not observed in Fig. 4.5. High transparency and low reflectance of TNTs are originated in the nanoporous structure of TNT films. Another intriguing property of TNT is electrochromism [37]. Transparent TNT films could be coated on transparent electroconductive substrates (ITO-coated glass) by an alternate layer deposition. The TNT films were found to exhibit significant electrochromism under cathodic polarization in water and turned brown. Figure 4.6 shows the cyclic voltammograms for TNT electrode at various scan rates. Changes in the current were observed under cathodic polarization, and oxidation waves were observed between −1.0 and −0.5 V. Cathodic polarization causes electrons to accumulate in the semiconductor film, arising the creation of Ti3+ centers by intercalation of protons in the nanotubes. This reaction is reversible; therefore, an oxidation current is observed when the voltage is scanned in a positive mode. The current at the oxidation peaks increases linearly as the square root of the scan rate increases. These results are consistent with the Nernst equation, thus indicating that the redox process proceeds with proton diffusion. Diffusion coefficient of protons in TNT was much higher than that of polycrystalline anatase

4 Synthesis and Applications of Titanium Oxide Nanotube Thin Films Scan rate

51

10

400 mV/ sec 200 mV/ sec 5 100 mV/ sec 50 mV/ sec –2

–1.5

–1

–0.5

0

0

0.5

1

1.5

H+ H+ H+

2

Current (mA)

Voltage (V) –5

TNT –10

-15

Cathodic Polarization

Fig. 4.6. Electrochromism in TNT electrodes

TiO2 . The significant electrochromism of the TNT is attributed to its layered nanostructure. In this layered arrangement, a large surface area acts as H+ host as a result of its one-dimensional, open-pore nature. Therefore, protons can diffuse more easily into the layered nanostructure than into a densely packed polycrystalline TiO2 particle. New nanotube-based devices can be created from a stable, colloidal solution of TNTs and a thin film fabricated by an alternate layer deposition process. The transparent TNT films demonstrate intriguing properties, such as superhydrophilicity, high-transparency, low-reflectance, and significant electrochromism. An alternate layer deposition process does not need an annealing step; therefore, TNT films can be coated on various plastic substrates. This unique material has potential applications in various industrial components such as displays, sensor devices, and smart windows.

4.3 Thin Films of Vertically Aligned TNT Arrays and Their Applications Previous section reported the photo-induced hydrophilicity of TNT films synthesized by an alternate layer deposition method. This section reports the vertically aligned TNT films by hydrothermal reaction of metal titanium and

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various functions derived from aligned structures of these films. First, transparent titanate nanotube arrays were synthesized by a hydrothermal treatment of titanium films on sapphire substrates [38]. Metallic titanium films were deposited on sapphire substrates by sputtering. To grow the TNT arrays, these titanium films were immersed into 10 M aqueous NaOH solution, heated at 393 K, and subsequently washed with aqueous HNO3 solution and pure water. Figure 4.7 shows the cross-sectional SEM images for TNT arrays. TNT films have fibrous structures grown on to the substrates. The crystal structure of TNT was transformed from titanate to anatase by post-annealing in air at 773 K. Although the tubular structure was partially observed in the planeview image (Inset of Fig. 4.7), post-annealing causes transformation from titanate nanotubes to polycrystalline anatase rod structures. These vertically aligned TNT films were optically transparent, and the hydrothermal temperature and reaction time are very important to obtain transparent films. When the hydrothermal reaction time was longer or the temperature was higher, the nanotube arrays were etched away. There is a dense layer between TNT films and substrates, and this dense layer acts as a binder between the TNT arrays and the substrate, leading to good adhesion to the substrates. Figure 4.8 shows the change in the water contact angles for vertically aligned TNT films and polycrystalline rutile TiO2 film. The water contact angles were initially measured in the dark (for 860 h), then under UV illumination (1.5 h), and finally in the dark (2,500 h). Among these films, TNT

Fig. 4.7. Cross-sectional SEM image for vertically aligned TNT arrays on a glass substrate. Inset shows the cross-sectional TEM image for TNT

4 Synthesis and Applications of Titanium Oxide Nanotube Thin Films

53

UV illumination

dark

dark

70

Water contact angle (deg.)

Water contact angle (degree)

60 50 40 30 20

3.5 3.0 2.5 2.0 1.5 1.0 0.5 0.0 860

10 0

0

1000

under UV illumination

2000 Time (hour)

861 Time (hour)

3000

862

4000

Fig. 4.8. Change in the water contact angle for thin films. Open square: metal titanium, closed triangle: rutile TiO2 , open circle: TNT, and cross: annealed TNT (anatase). Inset shows changes in the water contact angle under UV illumination (from 860 to 861.5 h)

and annealed TNT films exhibited excellent sustainabilities for hydrophilicity even in the dark conditions. These superhydrophilicities are attributed to their surface morphologies. The surface roughness enhances the hydrophilicity of hydrophilic surfaces explained by Wenzel’s equation [39]. Although the vertically aligned TNT arrays have significant surface roughness, they are highly transparent in the visible light region. The nanostructural morphology of TNT arrays simultaneously provides concomitant properties of superhydrophilicity and high transparency. When a black light bulb was illuminated on these films, TNT films became superhydrophilic. The inset shows an expanded view of the contact angles for TNT and annealed TNT films under black light bulb illumination. The hydrophilicizing rate of the annealed TNT is faster than that of TNT because the crystal phase of the annealed TNT is anatase, which has a better crystallinity and more absorbed photons than the TNT film. Photo-induced hydrophilic conversion involves photo-excitation and diffusion of photo-generated charge carriers to a surface; thus, the hydrophilicizing rate depends on the light absorption property and the mobility of charge carriers. Band gap of anatase (annealed TNT) is narrower than that of titanate (TNT). Narrow band gap and high crystallinity are effective for an efficient hydrophilic reaction.

54

Masahiro Miyauchi and Hiromasa Tokudome

Another intriguing property of vertically aligned TNT arrays is the electron field emission, which is expected for field emission displays, compact electron/X-ray sources, and so on, because significant improvement in turn-on voltage and emission current can be attained by the electric field enhancement effects owing to the nanometer scale curvature radius of the emission surfaces. Thus, an aligned and vertically grown nanotube thin layer is a key to attaining an improved field emission. TNT arrays were synthesized by the abovementioned hydrothermal process on metal titanium substrate. Field emission experiments were carried out in a vacuum chamber at room temperature. A platinum collection electrode was placed at 100 μm apart from the nanotube surface. The backside of the substrate was polished by a filing paper to expose the metal Ti surface and was connected to a power supply. As prepared TNT arrays were not conductive, it did not show detectable emission. TNT arrays were followed by post-annealing in vacuum to convert the titanate nanotubes to conductive TiO2 (anatase) nanotubes [40]. Optical reflectance spectra in the ultraviolet-visible region revealed that the post-annealed nanotubes in vacuum show broad optical absorption spreading in the whole visible light region yielding dark color. Various energy states are distributed in the forbidden energy gap of the anatase nanotubes. It was reported that oxygen vacancies in TiO2 form energy levels at 0.75–1.18 eV below the conduction band bottom [41], which give the weak absorption bands observed. The formation of the oxygen vacancies generated electron carriers in the anatase nanotubes to maintain the charge neutrality. Figure 4.9 shows current-density–voltage (J–V) characteristics measured on the post-annealed TNT arrays in vacuum at room temperature. It is seen that the TNT arrays exhibited good electron emission. The J–V characteristics 180 –3.4

140

–3.6

120

log [J/ V2 (AV–2)]

Current density (μA/ cm2)

160

100 80

–4.0 –4.2 –4.4

60

–4.6

40

–4.8 0.5

20 0

–3.8

1.0

1.5

2.0

2.5

3.0

3.5

–1 1000/ V (V )

0

500

1000

1500

2000

2500

3000

Voltage (V)

Fig. 4.9. Electron emission characteristics from TNT arrays. Inset shows F–N plot for TNT arrays

4 Synthesis and Applications of Titanium Oxide Nanotube Thin Films

55

were repeatedly measured for three cycles and showed good reproducibility and stability. The turn-on voltage defined as the extraction voltage at a current of 1 μA/cm2 is estimated to be 280 V, corresponding to an apparent field as low as 2.8 V/μm. Inset of Fig. 4.9 shows the Fowler–Nordheim (F–N) plot for TNT arrays. J–V characteristics of the TNT arrays follow well the straight line, substantiating that the electron emission is dominated by the F–N tunneling in the measured voltage region. The field enhancement factor can be estimated to be 5,580 from the slope of the F–N plot. The estimated field enhancement factor is large enough to conclude that the low turn-on field and efficient electron emission is well enhanced by the small radius of the nanotube emitter tips. TNT arrays are promising for a field emitter because of their high thermal and chemical stabilities, nontoxicity, and resource abundance. In addition, although most of field emitters reported to date have been fabricated by vacuum thin film processes, the present hydrothermal process is one of the simplest ways to produce nanotubes and is possibly transferred to a large area process at low cost, possessing a potential for industrial production.

4.4 Conclusions TNT films by an alternate layer deposition and vertically aligned TNT arrays were successfully synthesized by simple methods. These films exhibited various functions, such as superhydrophilicity, high-transparency, low-reflection, significant electrochromism, and enhanced electron field emission. These intriguing properties are due to the unique nanostructure of TNT. Further, the hydrothermally grown TNT was recently known to be useful starting materials for synthesizing single crystalline rutile or anatase TiO2 nanoparticles. Titanate nanotubes are composed of scrolled TiO2 sheets, which are transformed into a particular titania polymorph by rearranging the layers in aqueous media [42–44]. In particular, we have successfully synthesized rectangular shaped nanorods [45] and bipyramidal shaped nanoparticles [46] from TNT, and they exhibited efficient photocatalytic activities. This section shows the unique functions of TNT films. Syntheses of TNTs fabricated by a hydrothermal method have received great attention because of its conspicuous process merits such as cost and productivity. The TiO2 is nontoxic, stable, and resource abundant; thus it is suitable a material for industrial mass-production process. TNTs may be potentially applied to various industrial items such as building materials, glass windows, sensors, displays, and photovoltaic devices.

References 1. A. Fujishima et al., J. Photochem. Photobiol. C 1, 1 (2000) 45 2. R. Wang et al., Nature 388, 431 (1997) 45

56 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39. 40. 41. 42. 43. 44. 45. 46.

Masahiro Miyauchi and Hiromasa Tokudome A. Fujishima, K. Honda, Nature 238, 37 (1972) 43 45 B. O’Regan, M. Gr¨ atzel, Nature 353, 737 (1991) 45 Y. Matsumoto et al., Science 291, 854 (2001) 45 Y. Furubayashi et al., Appl. Phys. Lett. 86, 252101 (2005) 45 H. Tokudome, Y. Yamada, M. Miyauchi, Appl. Phys. Lett. 87, 213901 (2005) 45 M. Adachi et al., Chem. Lett. 8, 942 (2000) 45 P. Hoyer, Adv. Mater. 8, 857 (1996) 45 H. Imai et al., J. Mater. Chem. 9, 2971 (1999) 45 A. Michailowski et al., Chem. Phys. Lett. 349, 1 (2001) 45 D. Gong, C. A. Grimes et al., J. Mater. Res. 16, 3331 (2001) 45 T. Kasuga et al., Langmuir 14, 3160 (1998) 45 Q. Chen et al., Adv. Mater. 14, 1208 (2002) 46 R. Ma et al., Chem. Phys. Lett. 380, 577 (2003) 46 A. Nakahira et al., J. Mater. Sci. 39, 4239 (2004) 46 J. Yang et al., Dalton. Trans. 20, 3898 (2003) 46 N. Sakai et al., J. Am. Chem. Soc. 126, 5851 (2004). 46 H. Sato et al., J. Phys. Chem. B 107, 9824 (2003) 46 T. Tachikawa et al., J. Phys. Chem. B 110, 14055 (2006) 46 L. Kavan et al., Chem. Mater. 16, 477 (2004) 46 J. Li et al., Chem. Mater. 17, 5848 (2005) 46 Y. Zhou et al., J. Electrochem. Soc. 150, A1216 (2003) 46 P. Umek et al., Chem. Mater. 17, 5945 (2005) 46 A. Thorne et al., J. Phys. Chem. B 109, 5439 (2005) 46 D. V. Bavykin et al., Chem. Mater. 109, 19422 (2005) 46 S. Uchida et al., Electrochemistry 70, 418 (2002) 46 A. Liu et al., Anal. Chem. 77, 8068 (2005) 46 H. Tokudome, M. Miyauchi, Chem. Commun. 8, 958 (2004) 46, 47 N. Sakai et al., J. Phys. Chem. B 107, 1028 (2003) 49 M. Miyauchi et al., Chem. Mater. 12, 3 (2000) 49 R. Nakamura et al., Langmuir 17, 2298 (2001) 49 A.Y. Nosaka et al., J. Phys. Chem. B 107, 12042 (2003) 49 K. Uosaki et al., J. Phys. Chem. B 108, 19086 (2004) 49 H. Tokudome, M. Miyauchi, Chem. Lett. 33, 1108 (2004) 49 M. Miyauchi, H. Tokudome, Thin Solid Films, 515, 2091 (2006) 49 H. Tokudome, M. Miyauchi, Angew. Chem. Int. Ed. 44, 1974 (2005) 50 M. Miyauchi, H. Tokudome, J. Mater. Chem. 17, 2095 (2007) 52 R. N. Wenzel, J. Phys. Colloid Chem. 53, 1466 (1949) 53 M. Miyauchi et al., Appl. Phys. Lett. 89, 043114 (2006) 54 D. D. Cronemeyer, Phys. Rev. 113, 1222 (1959) 54 M. Gateshki et al., Chem. Mater. 19, 2512 (2007) 55 D. V. Bavykin et al., Chem. Mater. 18, 1124 (2006) 55 J. N. Nian et al., J. Phys. Chem. B 110, 4193 (2006) 55 M. Miyauchi, H. Tokudome, Appl. Phys. Lett. 91, 043111 (2007) M. Miyauchi, J. Mater. Chem. 18, 1858 (2008)

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57

Index Alternate layer deposition, 46 Colloidal solutions of TNTs, 47 Cross-sectional SEM image for vertically aligned TNT arrays, 52 Current-density–voltage (J–V) characteristics for TNT arrays, 54–55

Poly(diallyldimethylammonium chloride) (PDDA), 47 Poly(ethyleneimine) (PEI), 47 Tetra(n-butyl)ammonium hydroxide (TBAOH) aqueous solution, 46 TNT thin films synthesis, 46–51

Electrochromism in TNT electrodes, 51 Electron emission characteristics from TNT arrays, 54

UV-vis absorption spectra of PEI/(TNT/PDDA)n-1 /TNT film, 47–48

Fowler–Nordheim (F–N) plot for TNT arrays, 54–55

Vertically aligned TNT films by hydrothermal reaction, 51–55

Hydrophilicity of TNT films, 53 Optical reflectance for thin films, 50

Water contact angle of TNT/polycation thin film, 49

5 Synthesis and Application of Titanium Oxide Nanohole Arrays Shinsuke Yamanaka1 and Masayoshi Uno2 1

2

Division of Sustainable Energy and Environmental Engineering, Graduate School of Engineering, Osaka University, Suita, Osaka 565-0871 Japan [email protected] Research Institute of Nuclear Engineering, University of Fukui, Fukui-city, Fukui 910-8507, Japan [email protected]

Abstract A titania nanohole array, consisting of an assembly of titania tubes with inner diameters of 200 nm and wall thicknesses of 30 nm, was successfully prepared by a liquid phase deposition method. Heat treatment at 900◦ C results in a titania nanohole array that consists of highly crystalline anatase and has good photocatalytic properties. It was found that dissolution of anodic alumina and the deposition of titania took place simultaneously, and alumina functioned as the nanostructure template as well as a fluoride ion scavenger. A titania nanohole array with one end closed was used as a positive electrode for a lithium ion battery, in which an electric capacity close to 300 mAh/g was obtained. In addition to titania, this preparation method is applicable to various oxides such as tin oxide, zirconium oxide, indium oxide, and iron oxide.

5.1 Introduction Material research in the synthesis of nanoscale ordered structures has been actively conducted to provide new functions such as an increase in specific surface area or the generation of quantum effects [1, 2]. There are numerous reports regarding methods for the preparation of nanostructured materials; however, a very large amount of energy and substantial labor are necessary for many of them. Therefore, a soft-solution process, which is a preparation technique for high-performance materials with minimal environmental burden, has been proposed [3]. A method for transferring the pattern of nanostructured self-organized anodic alumina [4–6] by a single simple solution process has been developed [7–9]. Anodic alumina forms, as shown in Fig. 5.1, as a selfordered hole array structure of submicron to nanometer scale, based on an electrochemical process. The basic units, called cylindrical cells, are ordered into a close-packed honeycomb structure. At the center of respective cells, T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 59–71 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 5

60

Shinsuke Yamanaka and Masayoshi Uno

Fig. 5.1. Schematic diagram of the fine structure of anodic alumina

holes with uniform diameters in the range of 5–300 nm are formed perpendicular to the substrate. The liquid phase deposition (LPD) method [10–12] has been used as the transfer method of the pattern. The LDP method is a preparation method for oxide thin films at ordinary temperature and pressure that utilizes the hydrolysis of metal–fluoro complex ions. In this method, a uniform oxide thin film or hydroxide thin film can be prepared, regardless of the substrate material and shape. The hydrolysis reaction of metal–fluoro complex ions is represented as follows: − + nH2 O = MOn + xF− + 2nH+ (5.1) MF(x−2n) x In the equilibrium state, both fluoride and oxide ions coexist. When aluminum ions or borate ions, which are fluoride ion scavengers, are added to this aqueous solution, free fluoride ions in the solution form more stable complex ions, as shown below: Al + 6HF = H3 AlF6 + 3/2H2 4−

H3 BO3 + 4HF = BF

(5.2) +

+ H3 O + 2H2 O

(5.3)

The concentrations on the right side of (5.1) then decrease, and as a result, the equilibrium is shifted to the right side, and an oxide thin film is obtained. The reaction in (5.1) is called a deposition reaction and the reactions in (5.2) and (5.3) are called driving reactions. Thus far, we have been developing a transfer method of an anodic alumina structure to other oxides using the LPD method [7–9]. This method allows

5 Synthesis and Application of Titanium Oxide Nanohole Arrays

61

transfer of the structure by a single process, unlike conventional transfer methods such as the sol–gel method. In the following section, the mechanism for formation of the prepared titania nanohole array and its application to electrodes is described.

5.2 Formation Mechanism and Properties The dimensions of the anodic alumina (Anodisc, Whatman Ltd.) starting material were a thickness of approximately 50 μm and a cell size of approximately 250 nm [8]. At the center, there is a through-hole with a radius of approximately 200 nm. A titania nanohole array can be obtained by immersing the anodic alumina in TiF6 solution, as shown in Fig. 5.2. A sample obtained by immersion in 0.1 M (NH4 )2 TiF6 solution at room temperature (20◦ C) for 1 h is described as an example. Field emission scanning electron microscopy (FESEM) images of the surface and cross section of the obtained titania nanohole array are shown in Fig. 5.3. The titania nanohole array has arranged tubular structures. The inner diameter and wall thickness of the tubes are approximately 200 and 30 nm, respectively. From the cross-sectional micrograph, the tubes that constitute the titania nanohole array consist of particles with a size of approximately 20 nm.

[TiF 6 ] 2− + 6H 2 O

[Ti(OH) 6 ] 2− + 6HF

H 3 BO3 + 4HF BF4 − + H3 O+ + 2H 2 O H 3 AlF6 + 3/2H 2 Al + 6HF LPD Fig. 5.2. Synthesis of a titania nanohole array

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Fig. 5.3. FE-SEM micrographs of a titania nanohole array; (a) surface and (b) cross section [7]

X-ray diffraction (XRD) patterns of the sample obtained by drying the titania nanohole array and for the samples obtained by additional heat treatment at various temperatures are shown in Fig. 5.4. The sample was amorphous immediately after drying; however, peaks due to anatase were observed for the sample heat-treated at 500◦ C. The crystallinity increased with an increase in temperature up to 900◦ C. Titania is a semiconductor photocatalyst; therefore, the photocatalytic performance was evaluated by the decomposition characteristics of acetaldehyde. The concentrations of acetaldehyde and CO2 were measured by gas chromatography under irradiation with a UV lamp of 1 mW/cm2 after introduction of 100 ppm of acetaldehyde into a 500 mL quartz vessel containing a sample with an area of 25 cm2 . The acetaldehyde gas was introduced and allowed to stand for 30 min until the gas concentration change due to adsorption ceased. The decomposition characteristics of acetaldehyde by the titania nanohole array are shown in Fig. 5.5. The decomposition characteristics were improved with an increase in the crystallinity of the titania nanohole array, as shown in the XRD patterns of Fig. 5.4. However, when the heat treatment temperature of the titania nanohole array was increased to 1,100◦ C, the decomposition characteristics of acetaldehyde deteriorated. The XRD pattern for the 1,100◦ C heat treatment in Fig. 5.4 shows peaks of α-alumina and some rutile. This indicates that the initial nanostructure has degraded and conversion of anatase to rutile has started. The mechanism for formation of the titania nanohole array is explained in the next section. The SEM micrographs of the surface and the crosssectional structures of the nanohole arrays prepared by various treatment times are shown in Figs. 5.6 and 5.7. According to the surface structures in Fig. 5.6, the anodic alumina starting material and the structure itself have not changed after 30 min immersion (Fig. 5.6c). However, the diameter of the hole has been enlarged from 200 to 270 and 300 nm after 15 and 30 min, respectively, and it is clear that the anodic alumina is dissolving. At 30 min, the deposition of titania particles is not observed; however, it can be seen that the anodic alumina holes are joined. On the other hand, the surface state has changed drastically after 45 min (Fig. 5.6d); the surface of the original anodic alumina is dissolved and the inside appears

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anatase rutile y-alumina a-alumina 1373 K

1173 K 973 K 773 K

573 K 373 K 20

30

40

2 θ

50

60

70

Fig. 5.4. XRD patterns for titania nanohole arrays treated at different annealing temperatures [9]

to be visible. After further elapse of 15 min, tubes of titania can be observed inside (Fig. 5.6e). At this point, the diameter of the hole is 250 nm, and the wall thickness is 40 nm. After 120 min, the diameter of the hole is decreased to 150 nm as the wall thickness increases to 70 nm. From the crosssectional SEM micrographs, the deposition of titania on the inner surface of the alumina hole was observed from 15 min after immersion (Fig. 5.7a), although the deposition of titania particles was not observed until 30 min after immersion in the surface SEM micrographs. This indicates that the dissolution of alumina is faster near the surface than the inside. Accordingly, it is considered that the deposition of titania is also more active on the surface. However, supposedly deposited titania dissolves; therefore, it is considered

Shinsuke Yamanaka and Masayoshi Uno

Acetaldehyde (%)

64

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

No Annealing 373 K 573 K 773 K 973 K 1173 K 1373 K

0

20

40

60

80

Time (min)

Fig. 5.5. Photocatalytic acetaldehyde decomposition characteristics using titania nanohole arrays treated at different annealing temperatures [9] (a)

(b)

(d)

(e)

(c)

(f)

Fig. 5.6. FE-SEM micrographs of the titania nanohole array surface structure prepared at 293 K for (a) 0 min, (b) 15 min, (c) 30 min, (d) 45 min, (e) 60 min, and (f ) 120 min [8]

that once titania is deposited near the surface, it will then drop off or redissolve. When the reaction progresses for 45 min, a state in which alumina holes are joined, due to the dissolution of alumina near the surface, can be observed on the surface SEM (Fig. 5.6d). On the other hand, in the cross-sectional SEM image, the formation of some titania tubes can be observed (Fig. 5.7c). Therefore, titania tubes are considered to be formed within 30–40 min after immersion. Thus, the mechanism for formation of a titania nanohole array can be summarized as shown in Fig. 5.8. Anodic alumina is different from highly crystalline alumina, and it contains anions that are present in the aqueous solution used for the preparation. Thus, anodic alumina is easily soluble in a weak acid, and it starts to dissolve gradually when immersed in a treatment solution containing Ti–fluoro complex ions, as shown in Fig. 5.8. Aluminum ions are then present in the aqueous solution, and as a result, the

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65

Fig. 5.7. FE-SEM micrographs of the titania nanohole array’s cross-sectional structure prepared at 293 K for (a) 15 min, (b) 30 min, (c) 45 min, and (d) 60 min [8]

Fig. 5.8. Mechanism for the formation of a titania nanohole array [8]

reaction shown in (5.2) occurs, and fluoride ions in the solution are removed. When the driving reaction of (5.2) takes place, the deposition reaction of (5.1) also takes place, and deposition of the desired oxide begins. Thus, anodic alumina functions not only as the starting material for the nanostructure but also as a fluoride ion scavenger, which promotes the reaction in the preparation process of the oxide nanohole array. Because a dense thin film of oxide can be prepared by the LPD method, a dense thin film is formed on the inner wall of the anodic alumina holes. Deposition of oxide particles and the dissolution of anodic alumina occur simultaneously. When observed from the surface, a state where the anodic alumina cannot be observed is generated, because the oxide deposition reaction takes place not only in the direction of closing holes but also in the direction of increasing the outer diameter of the tube, and the shortening of the distance between tubes is also characteristic. The joining of tubes is achieved by the remaining undissolved

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Shinsuke Yamanaka and Masayoshi Uno

Fig. 5.9. FE-SEM micrographs indicating control of the hole diameters according to reaction times

anodic alumina during the preparation. Therefore, the entire oxide that constitutes the nanohole array is not the desired oxide, but also contains anodic alumina. The structure of the oxide nanohole array prepared by this method can be easily controlled by changing the reaction conditions, and the degree of freedom is very high. As shown in Fig. 5.9, for example, it is possible to thicken the wall of the nanohole array and to decrease the diameter of the hole by lengthening the reaction time.

5.3 Application as an Electrode for Lithium Ion Batteries A lithium secondary battery has long-lasting energy capacity; however, output of instantaneous high power is difficult to achieve. Thus, the realization of a storage device having both large power density and long-lasting energy capacity is desirable. In order to realize both high energy density and high power density at the same time, improvement of the energy density was attempted by the combination of an electrical double layer capacitor and a pseudo-capacitor (supercapacitor) [13]. However, there are no examples in which a significant improvement of performance was achieved. However, for an oxide nanohole array a large specific surface area is expected due to its unique shape, and it is predicted to function efficiently as an electrical double layer capacitor. Thus, the application of a titania nanohole array as a positive electrode material for a lithium secondary battery was attempted. The template anodic alumina has through-holes, so that the titania nanohole array will also have through-holes, and if this configuration is used as an electrode, then the electrolyte and current collectors will contact directly and it will not function efficiently as a battery. Therefore, the synthesis of a titania nanohole array with one end closed was attempted by applying the LPD method once again after preparation of the titania nanohole array. FE-SEM images of the front surface and the rear surface of the titanium oxide nanohole array prepared for use as an electrode are shown in Fig. 5.10. The

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67

Fig. 5.10. FE-SEM micrographs of the titania nanohole array electrode; (a) front surface and (b) rear surface

holes appear to be closed on the structure of the rear surface. From the structure of the front surface, the tubes that constitute the titania nanohole array were confirmed to be double-walled. As a result, the specific surface area of the titania nanohole array was significantly increased from ca. 10 m2 /g before processing the electrode to ca. 120 m2 /g. In addition, the content of titanium oxide also increased dramatically from ca. 14.2% before processing to ca. 34.2%. When the titania nanohole array thus prepared was used as an electrode, it was found that the resistance was very high, due to the lack of a conductive additive. Therefore, an indium tin oxide (ITO) thin film between the first LPD and the second LPD was prepared by a sol-gel method, and the electrode resistance was successfully reduced. ITO is considered to be present between the double walls of the titania nanotubes. The addition of ITO in the titanium oxide nanohole array considerably decreased the resistance along the direction of film thickness. The charge–discharge characteristics of lithium ions were measured for the electrode and the results are shown in Fig. 5.11. The electric capacity without ITO was approximately 208 mAh/g; however, that with ITO as a conductive additive was approximately 272 mAh/g. 3.0

without ITO with ITO

2.8 2.6 2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0

50

200 100 150 Capacity (m A h\g)

250

300

Fig. 5.11. Charge–discharge characteristics of the titania nanohole array electrode

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Shinsuke Yamanaka and Masayoshi Uno

The reaction between titanium oxide and lithium ions is represented by the following equation, and it is known that there is a plateau in the vicinity of 1.8 V [14]. TiO2 + xLi+ + xe− → Lix TiO2 (0 < x < 1)

(5.4)

The theoretical capacity is known to be ca. 168 mAh/g when the capacity of titanium oxide is measured in the range of 1–3 V; however, the titania nanohole array considerably exceeded this value both with and without ITO. A plateau is observed in the vicinity of 1.6 V in the charge–discharge characteristics of the titanium oxide nanohole array. Thus, it is clear that a similar reaction to that of (5.4) has occurred. The period of the plateau is short, followed by a slight voltage decrease. This behavior is distinct for the charge–discharge characteristics of a system with electrostatic capacity, which is considered to be the reason why the electric capacity of a lithium secondary battery with a titania nanohole array electrode largely exceeds the theoretical capacity. In addition, the large electric capacity in the titania nanohole array containing ITO from an increase in the electrostatic capacity is considered to be due to a reduction in the resistance. When the resistance is high, it is expected that the electrostatic capacity due to the electrical double layer is formed only in the vicinity of the current collector. By reducing the resistance along the direction of film thickness, an electrical double layer is expected to be formed along the entire electrode. As a result, the electrostatic capacity will increase. Thus, it was found that another electric capacity could also be formed, in addition to that from the reaction between lithium and titania, when the oxide nanohole array is used as an electrode. Because the titania nanohole array contains alumina, it is still difficult to confirm that the material would be an effective electrode material for practical purposes. However, the present study has shown that a titania nanohole array could become an effective electrode material in the future, by the selection of a material with a large electric capacity and by reduction of the alumina content.

5.4 Nanohole Arrays of Various Oxides The structure, properties, and application of a titania nanohole array were described as an example. The preparation of nanohole arrays of zirconium oxide, iron oxide, tin oxide, and indium oxide has also been successfully achieved, in addition to titania [10]. SEM micrographs of tin oxide, iron hydroxide, tin/titanium layered oxide, and tin/indium layered oxide nanohole arrays are shown as examples in Fig. 5.12, and their synthesis methods are given in Table 5.1. Of these, the application of tin oxide and tin/indium layered oxide nanohole arrays as hydrogen gas sensors has been investigated [15].

5 Synthesis and Application of Titanium Oxide Nanohole Arrays

69

Fig. 5.12. FE-SEM micrograph of (a) tin oxide, (b) iron hydroxide, (c) Sn/Ti layered oxide, and (d) Sn/In layered oxide nanohole arrays

Table 5.1. Synthesis conditions for various oxide nanohole arrays Sample name

Solution

Concentration

Temperature (K)

Time

Tin oxide nanohole array

(NH4 )2 SnF6

0.10 M

293

1.0 h

Iron hydroxide nanohole array

NH4 F·HF FeOOH

1.0 M ⇒1/10 Saturation

293

1.0 h

Sn/Ti layered oxide nanohole array

(NH4 )2 SnF6 (NH4 )2 TiF6

0.10 M 0.10 M

293

0.5 h 1.0 h

Sn/In layered oxide nanohole array

(NH4 )2 SnF6 NH4F·HF In(OH)3

0.10 M 1.0 M Saturation ⇒ 1/10

298

40 min 10 min

Titania nanorod array

(NH4 )2 TiF6

0.10 M

338

1.0 h

Tin oxide nanohole array

(NH4 )2 SnF6

0.10 M

338

1.0 h

Zinc oxide nanohole array

ZnF2

Saturation

298

2.0 h

Network-type titania nanohole array

(NH4 )2 TiF6 H3 BO3

0.10 M 0.20 M

293

1.0 h

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5.5 Summary A titania nanohole array, consisting of an assembly of titania tubes with inner diameters of 200 nm and wall thicknesses of 30 nm, was successfully prepared by application of the liquid phase deposition method to anodic alumina, in which transfer of the nanostructure is achieved by only a single reaction. Heat treatment at 900◦ C results in a titania nanohole array that consists of highly crystalline anatase and has good photocatalytic properties. In the formation mechanism, the dissolution of anodic alumina and the deposition of titania take place simultaneously, and alumina functions as the nanostructure template as well as a fluoride ion scavenger. A titania nanohole array with one end closed was obtained by two liquid phase deposition reactions used to sandwich an ITO coating, and this structure was used as a positive electrode for a lithium ion battery, in which an electric capacity close to 300 mAh/g was obtained. In addition to titania, this preparation method is applicable to various oxides such as tin oxide, zirconium oxide, indium oxide, and iron oxide. We have reported the application of a titania nanohole array as a lithium battery electrode. Other applications of oxide nanohole arrays are also possible, such as the use of Sn oxide and Sn/In oxide nanohole arrays as gas sensor devices.

References 1. P. Ajayan, S. Iijima, Smallest carbon nanotube. Nature 358, 23 (1992) 59 2. S. Iijima, T. Ichihashi, Single-shell carbon nanotubes of 1-nm diameter. Nature 363, 603–605 (1993) 59 3. M. Yoshimura, Importance of soft solution processing for advanced inorganic materials. J. Mater. Res. 13, 796–802 (1998) 59 4. F. Keller, M.S. Hunter, D.L. Robinson, Structural features of oxide coatings on aluminum. J. Electrochem. Soc. 100, 411–419 (1953) 59 5. H. Masuda, K. Fukuda, Ordered metal nanohole arrays made by a two-step replication of honeycomb structures of anodic alumina. Science 268, 1466–1468 (1995) 59 6. H. Masuda, F. Hasegawa, S. Ono, Self-ordering of cell arrangement of anodic porous alumina formed in sulfuric acid solution. J. Electrochem. Soc. 144, L127– L130 (1997) 59 7. S. Yamanaka, T. Hamaguchi, H. Muta, K. Kurosaki, M. Uno: Fabrication of oxide nanohole arrays by a liquid phase deposition method. J. Alloys Compd. 373, 312–315 (2004) 59, 60, 62 8. T. Hamaguchi, M. Uno, K. Kurosaki, S. Yamanaka, Study on the formation process of titania nanohole arrays. J. Alloys Compd. 386, 265–269 (2005) 59, 60, 61, 64, 65 9. T. Hamaguchi, M. Uno, S. Yamanaka: Photocatalytic activity of titania nanohole arrays. J. Photochem. Photobiol. A 173, 99–105 (2005) 59, 60, 63, 64 10. S. Deki, Y. Aoi, O. Hiroi, A. Kajinami, Titanium(IV)oxide thin films prepared from aquous solution. Chem. Lett. 6, 433–434 (1996) 60, 68

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11. S. Deki, Y. Aoi, J. Okibe, H. Yanagimoto, A. Kajinami, and M. Mizuhata, Preparation and characterization of iron oxyhydroxide and iron oxide thin films by liquid-phase deposition. J. Mater. Chem. 7, 1769–1772 (1997) 60 12. S. Deki, Y. Aoi, Synthesis metal oxide films by liquid-phase deposition method. J. Mater. Res. 13, 883–890 (1998) 60 13. D. Li, H. Zhou, I. Honma, Design and synthesis of self-ordered mesoporous nanocomposite through controlled in-situ crystallization. Nat. Mater. 3, 65–72 (2004) 66 14. F. Bonino, L. Busani, M. Lazzari, M. Manstretta, B. Rivoltab, Scrosati: Anatase as a cathode material in lithium—organic electrolyte rechargeable batteries. J. Power Sources 6, 261–270 (1981) 68 15. T. Hamaguchi, N. Yabuki, M. Uno, S. Yamanaka, M. Egashira, Y. Shimizu, T. Hyodo, Synthesis and H2 gas sensing properties of tin oxide nanohole arrays with various electrodes. Sens. Actuators B Chem. 113, 852–856 (2006) 68

Index Anodic alumina structure, 60

Mechanism for formation of titania nanohole array, 64

Charge–discharge characteristics of titanium oxide nanohole array, 67–68 Cylindrical cells, 59

Oxide nanohole arrays, synthesis conditions, 69

Deposition reaction, 60 Driving reactions, 60

Photocatalytic acetaldehyde decomposition characteristics, 64

FE-SEM micrographs of titania nanohole array, 62, 64, 67

Soft-solution process, 59 Sol–gel method, 61

Indium oxide nanohole array, 68 Indium tin oxide (ITO) thin film, 67 Iron oxide nanohole array, 68

Tin oxide nanohole array, 69 Titania nanohole array synthesis, 61

Liquid phase deposition (LPD) method, 59–60 Lithium ion batteries, titania nanohole array as electrode in, 66

XRD patterns for titania nanohole arrays, 63 Zirconium oxide nanohole array, 68

6 Synthesis and Applications of Manganese Oxide Nanotubes Qi Feng Department of Advanced Materials Science, Faculty of Engineering, Kagawa University, Takamatsu 761-0396, Japan [email protected] Abstract Metal oxide nanotubes can be classified into single-crystal nanotubes and polycrystalline nanotubes. In the most cases, the single-crystal nanotubes have layered structure, where the metal oxide elementary layer of the layered structure is rolled into the nanotubular structure. This suggests that if a metal oxide can form layered compounds easily, its single-crystal nanotube could be expected. For the manganese oxide, many kinds of layered compounds have been reported, and the layered manganese oxides show excellent host–guest reactivities [1]. Their host– guest reactions have been applied to the development of new soft chemical processes for the preparation of new compounds and nanomaterials [2]. For the synthesis of the single-crystal nanotubes with layered structure, two types of processes have been reported. One is direct synthesis process, in which the nanotubes are prepared under the conditions where the layered structure is to be formed easily. The other is the soft chemical process, in which a layered compound is used as a precursor and is transformed to the nanotubular structure using the host– guest reactions. For the synthesis of polycrystalline metal oxide nanotubes, porous templates, e.g., porous anodic alumina and porous silicon, and one-dimensional templates, e.g., nanofiber and carbon nanotube, are used usually. This section describes the synthesis of single-crystal manganese oxide nanotubes by soft chemical process and direct synthesis process, synthesis of polycrystalline manganese oxide nanotubes by porous template process, and properties of the manganese oxide nanotubes prepared using these methods.

6.1 Single-Crystal Manganese Oxide Nanotubes 6.1.1 Crystal Structures of Layered Manganese Oxides Many kinds of layered manganese oxides have been reported, and some structures of the manganese oxides are shown in Fig. 6.1 [1]. The layered structures of Li1.09 Mn0.91 O2 , birnessite, and buserite contain manganese oxide layers of edge-shared MnO6 octahedral structural units and metal ions between the manganese oxide layers. The manganese ions in structures are Mn(IV) and Mn(III). The charge density of the MnO6 octahedral layer of these layered T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 73–82 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 6

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4.7 Å

(a) [ 1 × ∞ ] layered Li1.09 Mn 0.91 O 2

10 Å

7Å

(b) [ 2 × ∞ ] layered Birnessite

(c) [ 3 × ∞ ] layered Buserite Lior Mg 2+

5.3 Å

6.3 Å

Na+ H2O

(d) α-NaMnO2 layered

(e) β -NaMnO2 layered

Fig. 6.1. Structures of layered manganese oxides

compounds decreases in an order of Li1.09 Mn0.91 O2 > birnessite > buserite, corresponding to the amount of metal ion in the interlayer space, and their basal spacings are 0.47, 0.7, and 1.0 nm, respectively. The birnessite and buserite structures contain a single-crystal water sheet and a double-crystal water sheet between the MnO6 octahedral layers, respectively, while without crystal water in the Li1.09 Mn0.91 O2 -layered structure. The buserite structure can be transformed easily to the birnessite structure by dehydration. The structures of birnessite and buserite have a character that there are some Mn vacant sites in the MnO6 octahedral layers. Since the charge densities of the MnO6 octahedral layers of birnessite and buserite are low, ion exchange and intercalation reactions occur easily on the birnessite and buserite; therefore, these two types of layered manganese oxides are used usually in soft chemical synthesis. There are two types of layered structures in trivalent manganese oxide NaMnO2 . α-NaMnO2 structure has flat edge-shared MnO6 octahedral sheets similar to birnessite with Na+ ions between the sheets, but without Mn vacant sites in the MnO6 octahedral layers and without crystal water in the interlayer space. In the birnessite structure, Na/Mn molar ratio can be changed in a range of 0.2–0.7. When the molar ratio is larger than 0.7, the dehydration of crystal water in the interlayer space occurs, and the birnessite structure changes to the α-NaMnO2 structure. β-NaMnO2 has a zigzag MnO6 octahedral sheet and Na+ between the sheets. 6.1.2 Manganese Oxide Nanotubes Synthesized from Manganese Oxide Nanosheets The manganese oxide nanotubes can be prepared using a soft chemical process as shown in Fig. 6.2 [3, 4]. In this process, a Na+ -form birnessite-type manganese oxide is used as the precursor. The Na+ ions can be exchanged with H+ ions in acid solution and then an H+ -form birnessite can be obtained.

6 Synthesis and Applications of Manganese Oxide Nanotubes +

H -Birnessite

H + ion-exchange

Na+ -Birnessite d = 0.72 nm

75

d = 0.72 nm 0.1 M HNO3 0.1 M TMA OH – Intercalation +

Water washing Nanosheet

+

TMA -Birnessite d = 0.96 nm

Exfoliation Cationic surfactant Self-assembling

Nano-composite d = 2.48 nm

MnO6 octahedra

Nanotubes

Hydrothermal process

H2O

H

+

Na+

+

TMA

CTA+

Fig. 6.2. Synthesis process for manganese oxide nanotube from manganese oxide nanosheet

The H+ -form birnessite was treated with a tetramethylammoniun hydroxide (TMAOH) solution, and H+ ions were exchanged with TMA+ ions. The basal spacing increased from 0.72 to 0.96 nm after the ion exchange reaction. The TMA+ -form birnessite was washed with distilled water to remove unreacted TMAOH in the solution, and then large amount of water molecules intercalated into the interlayer space of the layered structure, which caused the exfoliation of the layered structure into manganese oxide nanosheets [5]. Next, a cationic surfactant solution was added into the nanosheet colloidal solution. The positively charged surfactant ions attract the negatively charged nanosheets together, resulting restacking of nanosheets into layered structure and formation of a nano-composite precipitate of manganese oxide and surfactant. The nano-composite was treated under hydrothermal conditions to transform the nano-composite to manganese oxide nanotubes. Four kinds of cationic surfactants, n-hexadecyltrimethylammonium chlorite (HeTAC, C16), n-dodecyltrimethylammonium chlorite (DoTAC, C12), n-decyltrimethylammonium chlorite (DeTAC, C10), and n-octyltrimethy lammonium chlorite (OTAC, C8), were used as the directing agent for the formation of nanotube in the process. When DeTAC solution was added into manganese oxide nanosheet colloidal solution, nano-composite of DeTAMnO2 nanosheet was formed. The nano-composite has a disordering layered structure with a basal spacing of 2.5 nm. Under hydrothermal conditions, the diffraction peaks corresponding to the layered structure became unclear

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gradually with increasing the reaction temperature up to 120◦ C. At above 140◦ C, a new phase of γ-MnOOH was observed. Figure 6.3 shows the SEM and TEM images of the precursor and the products in the reaction process. Na+ -form birnessite precursor has platelike particle morphology (Fig. 6.3a). The nano-composite prepared by reacting manganese oxide nanosheet solution and DeTAC surfactant solution has nanosheet-like particle morphology (Fig. 6.3b), suggesting the restacking reaction of nanosheets. Under hydrothermal conditions, first many cracks were formed on the nanosheet-like particles at 90◦ C (Fig. 6.3c), and then the nanosheet-like particles changed to fibrous particles gradually with increasing the reaction temperature (Fig. 6.3d). Figure 6.4 shows the high-resolution TEM images and SAED of the products after the hydrothermal reaction. Many stripes were observed on the sheet-like particles of the nano-composite at 110◦ C (Fig. 6.4a), indicating formation of cracks in the nanosheet-like particles. The sheet-like particles transformed to fibrous particles completely at 130◦ C. Some hollow fibers (nanotubes) with diameter of about 40 nm and length of about 1 μm were observed in the fibrous particles in a temperature range of 120–130◦ C. The nanotubes have a multilayer open-end structure with a basal spacing of 0.7 nm (Fig. 6.4b). The nanotubes show two sets of electron diffraction patterns (Fig. 6.4c). One corresponds to the layered structure with basal spacing of 0.7 nm. The other corresponds to the manganese oxide nanosheet that constructs the wall of the nanotube, which shows hexagonal pattern same as manganese oxide nanosheet. Surface area and pore size measurements indicated that this sample has a meso-porous structure with a mean pore size of about 4 nm and a large surface area of 215 m2 /g. At above 140◦ C, only solid fibers were observed (Fig. 6.4d). The nanofibers are single crystal

(a)

(b)

(c)

1 μm

(d) 90°C

1 μm

130°C

Fig. 6.3. SEM and TEM images of precursor and products of manganese oxides. (a) Na+ –birnessite precursor, (b) nano-composite of DeTA-MnO2 nanosheet, hydrothermally treated nano-composite at (c) 90 and (d) 130◦ C

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Fig. 6.4. TEM images and SAED pattern of the products obtained by hydrothermal treatment. (a) DeTA nanosheet at 90◦ C, (b) and (c) DeTA nanosheet at 130◦ C (nanotube), (d) DeTA nanosheet at 140◦ C (nanofiber), (e) OTA nanosheet at 120◦ C (nanotube), and (f ) HeTA nanosheet at 130◦ C (nanobelt)

of γ-MnOOH, suggesting that the nanotube changes its layered structure to γ-MnOOH structure above 140◦ C. The manganese oxide nanotubes can be obtained also using OTAC as the directing agent (Fig. 6.4e). The particle morphology change in the OTA-MnO2 nanosheet reaction system is similar to that in the DeTA-MnO2 nanosheet reaction system. However, the manganese oxide nanotubes were not obtained in DoTA- and HeTA-MnO2 nanosheet reaction systems. In these reaction systems, the nano-composites have high crystalline (well-ordering) layered structures [6]. Nanobelts with layered structure were obtained at 130◦ C (Fig. 6.4f). At above 140◦ C, the nanofibers of single-crystal γ-MnOOH were formed. A formation mechanism for the nanotube is given in Fig. 6.5. When the surfactant solution is added to the nanosheet colloidal solution, the surfactant cations attract the negatively charged birnessite nanosheets together, resulting in the restacking of the nanosheets to the layered structure and intercalation of the surfactant cations into the interlayer space, and thus in the formation of the nano-composite of surfactant-MnO2 nanosheet with sheetlike particle morphology. The surfactant ions in the interlayer space tend to form rod micelles. This tendency causes a bending force on the manganese oxide nanosheets and formation of the cracks on the nanosheet-like particles. With increasing the reaction temperature, the nanosheet-like particles are slit into strips, and the strips are curled into the nanotubes. It is a self-assembling process between the surfactant ions and the manganese oxide nanosheet. At temperatures higher than 140◦ C, the nanotubular structure collapses and transforms to γ-MnOOH nanofiber. The formation of the nanotube is strongly dependent on the structure of the surfactant-directing agent and reaction conditions. The electric and optical characteristics of the manganese oxide nanotubes were investigated.

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Hydrothermal

Nano-composite

S

elf-

g blin m e ass

Forming cracks

Structural transformation Nanotubes Multi-layered structure

Nanofibers of γ -MnOOH

Fig. 6.5. Formation mechanism of nanotubes and nanofibers from nano-composite under hydrothermal conditions

The manganese oxide nanotube shows semiconducting behavior with an indirect band gap of 0.69 eV. Ma et al. have reported a new method for the preparation of metal oxide nanotubular materials by curling metal oxide nanosheets. The manganese oxide nanotubes can also be prepared by this method, which is described in Sect. 3.7. 6.1.3 Manganese Oxide Nanotubes Synthesized from α-NaMnO2 Wang et al. have reported a synthesis process for manganese oxide nanotube using layered manganese oxide α-NaMnO2 as precursor [7]. In the process, first α-NaMnO2 precursor was prepared by solid-state reaction method. Then 0.3 g α-NaMnO2 was hydrothermally treated with 30 mL distilled water in a temperature region of 120–140◦ C for 4 days. After the hydrothermal reaction, a birnessite-type layered phase (monoclinic phase, space group C2/m (12), lattice constants a = 0.5149 nm, b = 0.2843 nm, and c = 0.7176 nm) with basal spacing of 0.72 nm was obtained. TEM study revealed that the product was multilayered manganese nanotube. The nanotube has morphology of about 20 nm in diameter, is several microns in length, and is open ended. It is a single-crystal nanotube constructed by about 10 layers of birnessite structure. In the α-NaMnO2 precursor, all manganese ions are trivalent (Fig. 6.1d). In acidic and neutral conditions, Mn(III) is unstable, and disproportionation reaction to Mn(IV) and Mn(II) occurs. Mn(II) formed in the disproportionation reaction dissolves into the solution phase, accompanying extraction Na+ ions from the interlayer space of the layered structure into the solution phase and

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Fig. 6.6. Transformation reaction from α-NaMnO2 structure to birnessite structure under hydrothermal conditions. The extraction of Na+ accompanies decrease of charge density of manganese oxide layer, resulting in intercalation of water molecules, exfoliation, and curling of the nanosheet into nanotube

intercalation of water molecules into the interlayer space (Fig. 6.6). Mn(IV) formed in the reaction remains in the solid phase. After the disproportionation reaction, the basal spacing of the layered structure increases from 0.53 to 0.72 nm (birnessite structure). In the reaction process, the extraction of Na+ results decrease of the charge density of MnO6 octahedral layer, and then the layered structure can be easily exfoliated to manganese nanosheets by the intercalation of water molecules. Under hydrothermal conditions, the nanosheet can be curled into multilayered nanotubular structures. A chemical composition analysis revealed without Na+ in the nanotubes prepared by this method. N2 adsorption study indicates that nanotube sample has porous structure with a pore size of 15 nm. The manganese oxide nanotubes described above are common in the layered structure with basal spacing of about 0.7 nm, open-end structure, without cations in the interlayer structure, and a chemical composition of Mn(IV)O2 . These manganese oxide nanotubes can be applied to one-dimensional semiconducting materials, porous adsorption materials, and cathodic materials for lithium ionic battery. 6.1.4 Manganese Oxide Nanotubes Synthesized Directly from Mn(Ac)2 Solution Wu et al. have developed electrochemical process for the preparation of manganese oxide nanotubes directly from manganese(II) acetate solution [8]. The manganese oxide nanotubes were prepared on a nickel substrate electrode by cycling the potential for 15 min between 0.4 and 0.1 V (vs SCE) (scan rate 0.5 V/s) at room temperature in a mixed solution of 0.1 M Mn(Ac)2 and 0.1 M Na2 SO4 . One side of the nickel substrate was covered with a polymer insulator and the other side was exposed in the solution. A Pt electrode was used as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. The product electrodeposited on nickel substrate was washed

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with distilled water, heat-treated at 300◦ C for 1 h, and then manganese oxide nanotubes were obtained. The nanotubes were grown densely on the nickel substrate vertical to the substrate surface and have particle morphology of pointy tip, average diameter of about 20 nm, and length in micrometer order. TEM study revealed that the nanotubes prepared by this method were single crystal, having a layered structure with a basal spacing of 0.35 nm and closed end. The chemical composition is MnO2 . Since the layered structure of manganese oxide with basal spacing of 0.35 nm has not been reported up to now, this layered structure may be a new type of structure. Field-emission characteristics of the prepared manganese oxide nanotubes were measured. The turn-on field is about 8.4 V/μm at a current density of 1 μA/cm2 . The emission current density and electric field follow Fowler–Nordheim behavior. The emission current density and its stability are close to that of ZnO nanofibers, suggesting possibility for application to field-emission filament material.

6.2 Polycrystalline Manganese Oxide Nanotubes The polycrystalline manganese oxide nanotubes can also be prepared using porous template similar to other metal oxides. Levy et al. have prepared a series of manganese oxide-based nanotubes with perovskite crystal structure, such as LaMnO3 , La2/3 Ca1/3 MnO3 , and La0.325 Pr0.300 Ca0.375 MnO3 , using porous polymer templates [9–11]. These compounds are widely known as excellent magnetoresistance and ferromagnetic materials. In this process, metal nitrates of stoichiometric mole ratio of the desired metal oxide were dissolved in a 0.5 M HNO3 solution. Porous polycarbonate films with passing-through holes of 1, 0.2, 0.1, and 0.05 μm diameters, respectively, were used as filters. The precursor solution was filled into the filter using syringe filtration system. The porous film filled with the precursor solution was microwave-heated for drying and denitration of the precursor, but the polycarbonate film was not decomposed at this stage. And then the sample was heat-treated at 800◦ C to decompose the polycarbonate film template, and the intermediate compounds (single or double oxides) were reacted to form the desired perovskite manganese oxide pure phase under the conditions. The polycrystalline perovskite nanotubes can be obtained when the passing-through pore size of the film was larger than 0.2 μm, while solid nanofibers were formed when the pore size was smaller than 0.1 μm [11]. The wall of the nanotubes and the nanofibers is constructed by nanocrystals with a size of about 20 nm. The magnetic characterization indicated that La0.325 Pr0.300 Ca0.375 MnO3 nanotubes have a positive Curie temperature at 170 K, showing ferromagnetic behavior below the Curie point and paramagnetic behavior above the Curie point [9]. An insulating to metal transition and nonvolatile magnetoresistive memory at 100 K and sizeable magnetoresistance for H < 1 T below 170 K were observed by an electric study. Such nanotubes have a potential

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application to spin-polarized injection, magnetic storage media, spin-sensitive scanning probe microscopy, and perhaps, ultimately, quantum computing, and also to cathodic materials for fuel cells [12].

6.3 Summary Many kinds of crystal structure and chemical compositions of manganese oxides have been reported. These manganese oxides exhibit excellent physical properties, e.g., semiconducting and magnetic properties, and chemical properties, e.g., redox, electrochemical, catalytic, and host–guest reaction properties. Fabrication of the manganese oxides into nanotubular materials will enhance the effects and discover new features, which may derive new types of functional materials.

References 1. Q. Feng, F. Kanoh, K. Ooi, Manganese oxide porous crystals. J. Mater. Chem. 9, 319–333 (1999) 73 2. Q. Feng, Soft chemical approach to synthesis and control of functional inorganic materials. J. Ion Exch. 14, 77–86 (2003) 73 3. Q. Feng, Z. Tian, N. Sumida, Synthesis of manganese oxide nano-fibers and nano-tubes by hydrothermal soft chemical process. 1st Workshop on Multidisciplinary Researches for Human Life and Human Support, Proceedings, 302–307 (2004) 74 4. Q. Feng, Z. Tian, Y. Kannabe, H. Itoh, Transformation of manganese oxide nano-sheets into manganese oxide nano-tubes and nano-fibers by hydrothermal soft chemical process. Joint Meeting of 8th International Symposium on Hydrothermal Reactions and 7th International Conference on Solvo-Thermal Reactions, Abstract, 68 (2006) 74 5. Z. Liu, K. Ooi, F. Kanoh, W. Tamg, T. Tomida, Swelling and delamination behaviors of birnessite-type manganese oxide by intercalation of tetraalkylammonium ions. Langmuir 16, 4154–4164 (2000) 75 6. Z. Tian, Q. Feng, N. Sumida, Y. Makita, K. Ooi, Synthesis of manganese oxide nano-fibers by self-assembling hydrothermal process. Chem. Lett. 33, 952–953 (2004) 77 7. X. Wang, Y. Li, Rational synthesis strategy. From layered structure to MnO2 nanotubes. Chem. Lett. 33, 48–49 (2004) 78 8. M. Wu, J. Lee, Y. Wang, C. Wan, Field emission from manganese oxide nanotubes synthesis by cyclic voltammetric electrode position. J. Phys. Chem. B 108, 16331–16333 (2004) 79 9. P. Levy, A.G. Leyva, H.E. Troiani, R.D. Sanchez, Nanotubes of rare-earth manganese oxide. Appl. Phys. Lett. 83, 5247–5249 (2003) 80 10. A.G. Leyva, P. Stoliar, M. Rosenbusch, P. Levy, J. Curiale, H. Troiani, R.D. Sanchez: Synthesis route for obtaining manganese oxide-based nanostructures. Physica B 354, 158–160 (2004) 80

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11. A.G. Leyva, H. Troiani, J. Curiale, R.D. Sanchez, P. Levy, Relationship between the synthesis parameters and the morphology of manganite nanoparticles-assembled nanostructures. Physica B 398, 344–347 (2007) 80 12. L. Hueso, N. Mathur, Dreams of a hollow future. Nature 427, 301–304 (2004) 81

Index Birnessite and buserite structures, 74 Cationic surfactants, 75 Crystal structures of layered manganese oxides, 73–74 n-decyltrimethylammonium chlorite (DeTAC), 75 Direct synthesis process, 73 n-dodecyltrimethylammonium chlorite (DoTAC), 75 Formation mechanism of nanotubes and nanofibers, 77–78 Fowler–Nordheim behavior, 80

Mn(Ac)2 solution, manganese oxide nanotubes synthesis from, 79–80 α-NaMnO2 , manganese oxide nanotubes synthesis from, 78–79 Nanosheet-like particle morphology, 76 n-octyltrimethy lammonium chlorite (OTAC), 75 Polycrystalline manganese oxide nanotubes, 80–81 Polycrystalline nanotubes, 73

n-hexadecyltrimethylammonium chlorite (HeTAC), 75

Saturated calomel electrode (SCE), 79 SEM and TEM images of precursor and products of manganese oxides, 76 Single-crystal nanotubes, 73 Soft chemical process, 73

Manganese oxide nanosheets, manganese oxide nanotubes synthesis from, 74–78 Manganese oxide nanotubes, 73

TEM images and SAED pattern of products of manganese oxides, 77 Tetramethylammoniun hydroxide (TMAOH) solution, 75

7 Synthesis and Applications of Molybdenum Oxide Nanotubes Maki Suemitsu1 and Toshimi Abe2 1

2

Research Institute of Electrical Communication, Tohoku University, Sendai, 980-8577, Japan [email protected] Department of Electronics and Intelligent Systems, Tohoku Institute of Technology, Sendai, 982-8577, Japan [email protected]

Abstract While bulk metal oxides already possess various intriguing electrical and chemical properties, their shape conversions into nanotubes even enrich their functionalities through enlargement in their surface areas, mechanical strength, and novel functionalities. This chapter describes formation of Mo oxide nanotubes having rectangular cross sections by use of a combustion-flame method.

7.1 Introduction As the size of the material shrinks down to nanoscale levels, drastically new properties, far from bulk ones, sometimes emerge. This is where increasing attention has been focused on nanomaterials. Some nanomaterials are accompanied by ultrathin walls like carbon nanotubes (CNT) [1] while others possess porous structures as well. These nanostructures are likely to present new functionalities featuring their enlarged surface areas, which can be used as material separation and storage [2] applications. These properties make nanomaterials promising in their use in environmental, energy storage/conversion issues, and as interface for biomaterials. Nanotube by far typifies the various forms of the nanomaterials. Aside from the well-known CNTs, several compounds show tubular forms. BN [3], MoS2 [4], and NiCl2 [5] indicate similar sp2 -based tubular structures like in CNTs showing zigzag and arm-chair chiralities. ZnS [6], GaN [7], and Si [8] also exhibit tubular structures, but with a sp3 -based, more bulk-like atomic arrangements. Recent attention has been focused on metal oxide nanotubes [9]. Since bulk metal oxides already possess various intriguing electrical and chemical properties, their shape conversions into nanotubes are expected to even enrich these functionalities through enlargement in their surface areas,

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mechanical strength, and novel functionalities. Examples can be found in TiO2 to be applied in photocatalyzers and photocells [10]; in VOx for gas sensors, cathode electrodes in Li-based batteries, and catalyzers [11]; and in ZnO as a wide-gap semiconductor [12] and for photoelectron nanodevices [13]. Numerous reports exist regarding the fabrication of metal oxide nanotubes. SiO2 nanotubes are synthesized through hydrolytic cleavage of tetra-ethylortho silicate (TEOS) in a mixture of ethanol, ammonia, water, and tartaric acid [14]. TiO2 nanotubes are fabricated by electrolysis of a titanium substrate in a hydrofluoric acid [10]. Nanotubes of V2 O5 are formed by annealing partially oxidized CNTs with vanadium oxide powders at temperatures higher than its melting point [15]. More direct use of CNT as a template for metal oxide nanotubes is also reported. By coating the surface of CNTs with TEOS and by removing the inner carbon atoms of the CNTs by oxidation, one can fabricate SiO2 nanotubes as well. Similar methods are reported for the formation of Al2 O3 , V2 O5 , and MoO3 nanotubes [9]. While many of the metal oxides are insulators, some of them, like MoO2 , RuO2 , and IrO2, show electrical conductivities [16]. These conductive oxides may find good applications as catalyzers, sensors, and recording materials [17], and the coverage of the applications will even expand when they are prepared in the form of nanotubes. In 2005, Suemitsu and Abe et al. [18] reported formation of Mo oxide nanotubes having rectangular cross sections. The metal oxide nanotubes were found on the backside of the molybdenum substrate used in the synthesis of microcrystalline diamonds using a combustion-flame method. Many applications are expected in molybdenum oxides (MoO2 , MoO3 ). MoO3 is a wide-gap semiconductor having a gap of about 3 eV and is therefore attracting recent attentions [19]. Molybdenum oxides can be a good catalyzer as well, and fabrication of their nanoparticles is being intensively investigated seeking for their use as a substitute for rare metals like platinum [20]. Molybdenum oxides also serve as a gas-sensing material [21]. Nanostructures of Mo oxide, therefore, will definitely enrich the applications of this material through increase of its surface area, which may include highly sensitive gas sensors and highly efficient catalyzers. In this respect, rectangular cross section of the molybdenum oxide nanotube is even more beneficial in obtaining densified arrays of the nanostructure in a limited area.

7.2 Growth of Molybdenum Oxide Nanotubes Figure 7.1 shows a schematic of the combustion-flame apparatus used in the growth of the molybdenum oxide nanotubes. It is exactly the same as what was used in the deposition of microcrystalline diamonds [22, 23]. The torch of the oxygen/acetylene flame is specially designed to enable an additional outer flow of an inert gas (typically Ar) surrounding the combustion flame

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Fig. 7.1. Schematic drawing of the growth apparatus using the combustion flame of acetylene and oxygen gases. MoO2 nanotubes form on the rear face of the Mo substrate while MoO3 powders form on the copper holder opposite to it. Microcrystalline diamonds form on the foreface of the Mo substrate

[23]. This outer flow protects otherwise unstable combustion flames produced at high oxygen/acetylene flow ratios and thereby protects the hydrocarbon radicals in the flame from being burnt out through mixing with the ambient air in the complete combustion. Molybdenum (99.95%) substrates with 20 × 20 × 0.2 mm3 in size are used in the experiments, which are attached to a water-cooled copper holder via a 0.6-mm-thick Si spacer. At the center of the spacer is a circular opening with a diameter of 12 mm. Molybdenum oxide nanotubes grow on the backside of the substrate within this opening. The Mo substrate is slightly bent to form a small gap (∼ 0.5mm) between the substrate and the Si spacer, without which no nanotubes grow. The substrate is directly heated by the combustion flame (2,500–3,000◦ C) itself, and its temperature was controlled at ∼ 1, 000◦ C by the water flow running through the copper holder. The substrate temperature was monitored by a pyrometer. The flow rates of acetylene, oxygen, and argon are 0.249, 0.438, and 1/30 l/min, respectively, unless otherwise stated.

7.3 The Chemical Composition and the Crystal Structure of the Mo Oxide Nanotubes Figure 7.2a, b shows typical scanning electron microscopy (SEM) images of the nanotube. The width of the rectangle cross section of the nanotube ranges from 100 to 10,000 nm and the length from 10 to 100 μm, with the aspect

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Fig. 7.2. SEM images of the MoO2 nanotubes with (a) low and (b) high magnifications. Nanotubes with rectangular cross sections are grown

ratio in the range of 4–100. To clarify the chemical composition and crystal structure of the nanotube, x-ray photoelectron spectroscopy (XPS), energy dispersive x-ray microanalysis (EDX), x-ray diffraction (XRD), transmission electron microscopy (TEM), and Raman scattering spectroscopy have been conducted. Figure 7.3 shows the (a) wide scan and (b) narrow scan of the XPS spectrum. The indium (In) peak seen in (a) is from the In substrate used to fix the nanotubes. The spectrum presents Mo and O peaks, suggesting Mo oxide as the central material produced in this experiment. XRD result indicates that the nanotubes are of molybdenum dioxide (MoO2 ) with a monoclinic structure. Figure 7.4a shows the XRD pattern

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(a) 10000

Intensity (cps)

8000 6000

O 1s

In 3d

4000

Mo 3d C 1s Mo 3p

2000 0

1000

500 Binding Energy (eV)

0

(b) Mo 3d5/2 Intensity (cps)

3000

C 1s Mo 3d3/2

2000

1000 300

280 260 240 Binding Energy (eV)

220

200

Fig. 7.3. XPS spectra of (a) wide and (b) narrow scans from the MoO2 nanotubes

from the nanotube, whose three major peaks are identified as diffractions from (01-1), (21-1), and (02-2) planes in the monoclinic MoO2 . Figure 7.4b illustrates the MoO2 structure. As shown, MoO2 takes a rutile structure consisting of six coordinated Mo atoms and three coordinated O atoms, with an octahedron of a molybdenum atom surrounded by six oxygen atoms being the unit structure. Figure 7.5 shows the TEM image of the nanotube. The image indicates that the nanotube is highly crystalline without significant amount of defects. The electron diffraction pattern in the inset indicates that the axis of the nanotube is of twofold symmetry. Raman scattering spectroscopy also identifies the nanotube to be MoO2 . Figure 7.6 compares the Raman spectra between the nanotube and the commercially available MoO2 bulk powder. Both spectra show identical sets of peaks, showing again that the nanotube is of MoO2 .

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Intensity (counts)

(01-1)

6000 4000 (21-1)

(02-2)

2000 0 0

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40 60 80 2 (deg) CuK'alpha'1

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(b)

Fig. 7.4. XRD pattern of the MoO2 nanotubes (a) indicating a monoclinic crystal structure (b)

Fig. 7.5. TEM image of the MoO2 nanotube

Raman scattering intensity (arb.unit)

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nanotube

MoO2 bulk powder

1000

800

600

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Raman shift (cm–1) Fig. 7.6. Raman spectra from the MoO2 nanotubes (above) and the MoO2 bulk powder (below )

7.4 Growth Mechanism of the Mo Oxide Nanotubes There are several crucial points to be noted in considering the growth mechanism of the nanotubes. The first point is the fact that the oxygen content of the nanotube increases with the growth time. Figure 7.7a shows the EDX O-peak intensity as a function of the growth time. The EDX measurement was conducted with an electron beam accelerated at 12 kV, aiming at the central portion of the sidewall of a nanotube. For the ease of the measurement, a rather large (width ∼10 μ m) nanotube has been chosen. Since the thickness of the nanotube wall is accordingly thick (∼ 1 μ m), we understand that this EDX result corresponds mainly to the surface portion of the nanotube wall and does not conflict with the MoO2 identification of the nanotube obtained from smaller nanotubes. The O-peak intensity is normalized with that of the Mo peak. In Fig. 7.7a, the relative O-peak intensity linearly increases with the growth time, suggesting coexistence of oxidation, in addition to the growth, of the nanotube during deposition. The blue and red dotted lines in Fig. 7.7a indicate the corresponding ratios from the MoO3 and the MoO2 bulk powders, respectively. We notice here that MoO3 has a high volatility at the growth temperature of around 1, 000◦ C. Molybdenum itself is a refractory metal having a melting point as high as 2, 617◦ C (2,890 K). As we oxidize Mo to MoO2 and to MoO3 , the compound becomes more and more thermally unstable [24], as evidenced by the lowered melting points (1,373 K for MoO2 and 1,068 K for MoO3 ). In Fig. 7.7a, the O/Mo ratio reaches that from MoO3 bulk powder at around 50 min of growth. This high oxygen content suggests highly unstable nature of the nanotube at this high temperature, which is actually confirmed by the SEM image shown in Fig. 7.7b. This image

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Intensity ratio O/Mo (arb.unit)

(a)

0.8 O/Mo (MoO3)

0.6 Thermally unstable

O/Mo (MoO2)

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0.2

10

20

30

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Growth time (min) (b)

Fig. 7.7. (a) Time evolution of the EDX O-peak intensity from typical MoO2 nanotubes. The upper (lower ) dotted line is the EDX O-peak intensity from commercial MoO3 (MoO2 ) bulk powders. The O-peak intensity is normalized by that of Mo. (b) SEM image of a MoO2 nanotube whose root is slightly etched

taken from a sample deposited for 60 min indicates occurrence of strong etching at the root of the nanotube. The second point is the importance of the gap between the Mo substrate and the Si spacer. This fact clearly indicates that some form of Mo precursor created at the foreface of the substrate is transferred through the gap to the rear face of the Mo substrate, where they form MoO2 nanotubes. The gap opening is also positively correlated with the amount of MoO3 white powders deposited on the copper holder opposite to the rear face of the Mo substrate. This fact unambiguously indicates that the Mo-bearing species transferred through the gap is Mo oxides. A most likely scenario is that the Mo oxide species produced as a result of chemical reactions between oxygen radicals in the combustion flame and the foreface of the Mo substrate are transferred to the rear face of the substrate, forming MoO2 nanotubes on the hot Mo substrate and MoO3 powders on the cold copper holder (Fig. 7.8). The third point is the fact that nanotubes do not form from the very beginning. Figure 7.9a shows the SEM image taken from a sample deposited for 15 min. A rectangular solid is found to grow. This picture suggests that the initially deposited bulk cluster may turn into a tube during the course of the deposition. Figure 7.9b indicates that this is actually the case. This image is

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Low temperature side MoO3 High temperature side MoO2 Si spacer

Mo substrate

MoO3 MoO2

O2+C2H2 combustion flame Ar

Ar

Fig. 7.8. Mass transport mechanism during the nanotube growth

(a)

2m

(b)

Fig. 7.9. SEM images of the MoO2 nanotubes: (a) a bulk cluster found in the initial growth, (b) a nanotube whose sidewall has a wide opening

taken from a sample that happened to have a collapsed side wall. Through the “window,” one can see clearly that the root portion of the nanotube is a bulk cluster, and a transition into a tubular growth occurs during the course of the growth. These results indicate that the growth of Mo oxide nanotube starts with a bulk cluster growth followed by a tubular growth, presumably caused by suppression of the growth at the central portion of the bulk cluster surface from some reason. One possible mechanism for this suppression is the selective

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∇

Fig. 7.10. Formation mechanism of the MoO2 nanotube

etching of the inner portion of the bulk cluster. Since the lateral distribution of the temperature at the top surface of the bulk cluster is mainly determined by the balance between the heating via thermal conduction from the substrate through the nanorod and by the cooling by the ambient gas molecules in touch with the cold copper holder, it is likely that the temperature is higher at the center than the outer region as shown in Fig. 7.10a. The Mo oxide then becomes thermally unstable (Fig. 7.10b) in the central region. Actually, we can see such a dimple in the central portion on the top surface of the cluster in Fig. 7.9a. EDX measurement indicates higher oxygen content in the central region, in harmony with the enhanced etching by oxygen suggested above. Another possible and probably additional mechanism is the suppression of the transport of the precursors to the bottom of the “well” as the walls of the nanotube become high enough (Fig. 7.10c) [25]. Final experimental result that is relevant in considering the growth mechanism is the presence of the optimum values in the oxygen/acetylene flow ratio. Figure 7.11 shows a series of SEM images as a function of the oxygen/acetylene flow ratio. With the flow ratio of 1.811 [(a) and (b)], no nanotubes grow on the rear face of the Mo substrate and microcrystalline diamond grows on the foreface. When the flow ratio is reduced to 1.803, Mo oxide nanotubes grow on the rear face and microcrystalline diamond and graphite grow on the foreface

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rear-face

(a)

(b)

(c)

(d )

(e)

(f)

Fig. 7.11. SEM images of the deposits for the oxygen/acetylene flow ratios of 1.811 [(a), (b)], 1.803 [(c) and (d)], and 1.643 [(e) and (f )]. The left and right columns are for the foreface and rear face of the Mo substrate, respectively

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Amount of deposited Mo-oxide

MoO3

Supply of precursor

Optimum flow ratio

MoO2 Etching by O radical O-poor (＜1 643)

Supply of precursor

O2 / C2H2=1.723 ~ 1.803

O-rich (＞1 811)

Fig. 7.12. The amount of deposited MoO2 and MoO3 schematically shown as a function of the oxygen/acetylene flow ratio

[(c) and (d)]. When the flow ratio is reduced further to 1.643, no nanotubes grow again on the rear face and only graphite grows on the foreface [(e) and (f)]. As a result of a scan, Mo oxide nanotube is found to grow for the flow ratio of 1.723–1.803. This tendency is understood as follows (Fig. 7.12). In the lower side of the oxygen/acetylene flow ratio, the positive correlation between the amount of the nanotube and the flow ratio is simply related to the increase of the Mo oxide precursors. In the higher side of the flow ratio, on the other hand, the negative correlation is understood to be caused by increase of the etching of the Mo oxide nanotubes by O-radicals such as atomic oxygen. Since the O-radical etching has an impact only on MoO2 , through the reaction of MoO2 + O → MoO3 , only MoO2 shows a peaking tendency while the amount of MoO3 powder continues to increase. This observation supports the above growth model of the nanotube involving O-related etching as a part of the mechanism.

7.5 Conclusion Growth and chemical/structural analyses of the MoO2 nanotubes have been described. Growth mechanism of these rectangular cross-section nanotubes has also been discussed using possible coexistence of oxidation, mass transport of the precursor, and temporal and process-dependent development of the nanotube shape. Understanding of physical properties of the MoO2 nanotube is still under way. Electrical, optical, chemical, and magnetic measurements in the future will evoke applications of this unique material. Combustion-

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flame method is convenient, but with limitations. To gain controllability over the size and density of the nanotubes, as well as to relax the limitation on the substrate, new growth processes must be developed by utilizing the knowledge obtained from the combustion-flame method. Acknowledgments This work has been supported in part by a Grant-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science, and Technology (16655075). This work was also supported by a collaborative research program in the Center for Interdisciplinary Research (CIR), Tohoku University. The authors are grateful to Prof. Yamane, Saida, and Matsushita at CIR for their help in our measurement of XRD and TEM, and to Mr. Handa for his SEM observation.

References 1. S. Iijima, Nature 354, 56 (1991) 83 2. S.M. Lee, Y.H. Lee, Appl. Phys. Lett. 76, 2877 (2000) 83 3. N.G. Chopra, P.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, Science 269, 966 (1995) 83 4. M. Remskar, A. Mrzel, Z. Skraba, A. Jesih, M. Ceh, J. Dems¨ yar, P. Stadelmann, F. Levy, D. Mihailovic, Science 292, 479 (2001) 83 5. Y.R. Hacohen, E. Grunbaum, R. Tenne, J. Sloan, J.L. Hutchison, Nature 395, 336 (1998) 83 6. J.Q. Hu, Y. Bando, J.H. Zhan, D. Golberg, Angew. Chem. Int. Ed. 43, 4606 (2004) 83 7. J. Golberger, R.R. He, Y.F. Zhang, S.W. Lee, H.Q. Yan, H.J. Choi, P.D. Yang, Nature 422, 599 (2003) 83 8. B.K. Teo, C.P. Li, X.H. Sun, N.B. Wong, S.T. Lee, Inorg. Chem. 42, 6723 (2003) 83 9. B.C. Satishkumar, A. Govindaraj, E.M. Vogl, L. Basumallick, C.N.R. Rao, J. Mater. Res. 12, 604 (1997) 83, 84 10. G.K. Mor, O.K. Varghese, M. Paulose, K. Shankar, C.A. Grimes, Sol. Energy Mater. Sol. Cells 90, 2011 (2006) 84 11. F. Sediri, F. Touati, N. Gharbi, Mater. Lett. 61, 1946 (2007) 84 12. B. Geng, X. Liu, X. Wei, S. Wang, Mater. Res. Bull. 41, 1979 (2006) 84 13. X. Liu, J. Wang, J. Zhang, S. Yang, Mater. Sci. Eng. A 430, 248 (2006) 84 14. H. Nakamura, Y. Matsui, Adv. Mater. 7, 871 (1995) 84 15. P.M. Ajayan, O. Stephan, Ph. Redlich, C. Colliex, Nature 375, 564 (1995) 84 16. B.C. Satishkumar, A. Govindaraj, M. Nath, C.N.R. Rao, J. Mater. Chem. 10, 2115 (2000) 84 17. J. Zhou, N.S. Xu, S.Z. Deng, J. Chen, J.C. She, Chem. Phys. Lett. 382, 443 (2003) 84 18. M. Suemitsu, T. Abe, H.-J. Na, H. Yamane, Jpn. J. Appl. Phys. 44, L449 (2005) 84 19. T. He, J. Yao, J. Photochem. Photobiol. C 4, 125 (2003) 84

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20. Y. Liang, Z. Yi, S. Yang, L. Zhou, J. Sun, Y. Zhou, Solid State Ionics 177, 501 (2006) 84 21. S. Barazzouk, R. P. Tandon, S. Hotchandani, Sens. Actuators B 119, 691 (2006) 84 22. T. Abe, M. Suemitsu, N. Miyamoto, N. Sato, Appl. Phys. Lett. 59, 911 (1991) 84 23. T. Abe, M. Suemitsu, N. Miyamoto, J. Appl. Phys. 74, 3531 (1993) 84, 85 24. R. S. Roth, J. R. Dennis, and H. F. McMurdie (eds.), Phase Equilibria Diagrams, vol. 6 (The American Ceramic Society Inc., Columbus, 1987), p.47 89 25. H. Handa, T. Abe, M. Suemitsu, e-J. Surf. Sci. Nanotech. 7, 307 (2009) 92

Index Chemical composition and the crystal structure of Mo oxide nanotubes, 86–89 Combustion flame of acetylene and oxygen gases, 84–85 Combustion-flame method, 84 Energy dispersive X-ray microanalysis (EDX), 86 Formation mechanism of MoO2 nanotube, 90–91 Gap between Mo substrate and Si spacer, Mo oxide nanotubes, 90 Gas-sensing material, 84 Growth mechanism of Mo oxide nanotubes, 89–94 Mass transport mechanism during growth of Mo oxide nanotubes, 91 Molybdenum oxide nanotubes growth, 84–86 Oxygen/acetylene flow ratio, 92–94 Oxygen content of Mo oxide nanotubes, 89–90

Raman scattering spectroscopy, 87 Raman spectra from MoO2 nanotube, 89 SEM images of deposits for oxygen/ acetylene flow ratios of Mo substrate, 93 SEM images of MoO2 nanotube, 86, 91 TEM image of MoO2 nanotube, 88 Tetra-ethylortho silicate (TEOS), 84 Time evolution of EDX O-peak intensity of Mo oxide nanotubes, 89–90 Transmission electron microscopy (TEM), 87 Ultrathin walls like carbon nanotubes (CNT), 83 XPS spectra of wide and narrow scans from MoO2 nanotube, 87 X-ray diffraction (XRD), 86 X-ray photoelectron spectroscopy (XPS), 86 XRD pattern of MoO2 nanotube, 88

8 Synthesis and Applications of Rare-Earth Compound Nanotubes Mitsunori Yada Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan [email protected] Abstract In this section, syntheses and applications of rare-earth compound nanotubes are described. The nanotubes are synthesized by the homogeneous precipitation method, the hydrothermal method, the template synthesis method using carbon nanotube and anodic porous alumina membrane, etc. Optical properties, surface modifications, and reactivities for the nanotubes are presented.

8.1 Introduction One example of the remarkable progress made in nanomaterials technology is the recent synthesis of various nanostructures from rare-earth compounds. An evaluation of the physical properties of these nanostructures has been undertaken. Applications of rare-earth compounds have been examined as follows: (1) applications based on magnetism or optical characteristics due to 4f-electron characteristics peculiar to rare-earth elements and (2) applications using crystal structures or chemical properties. As an example of the first, application of oxide-based materials, such as red phosphor Y2 O3 : Eu3+ and Y2 O2 S : Eu3+ , for luminescent materials can be cited. As an example of the second, application of CeO2 for automotive catalysts using the compound’s oxygen storage capacity, solid oxide fuel cells, ultraviolet-blocking agents, and abrasives can be cited. In order to improve the characteristics and explore the novel functions of rare-earth compounds, syntheses of rare-earth compounds having new nanostructures and morphologies should be performed. Therefore, syntheses of various nanostructures, such as nanowires, nanotubes, and mesoporous structures, have been investigated. My group first reported the synthesis of rare-earth compound nanotubes in 2001. Since then, nanotubes of rare-earth compounds, such as rare-earth hydroxides, rare-earth oxides, rareearth phosphates, and rare-earth fluorides, have been investigated. In this section, synthetic methods and the characteristics of rare-earth compound nanotubes will be introduced.

T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 97–115 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 8

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8.2 Rare-Earth Compound Nanotubes Synthesized Using the Homogeneous Precipitation Method Our group has successfully synthesized rare-earth compound nanotubes by the homogeneous precipitation method, using urea [1–3]. The nanotubes were synthesized as follows: (1) nitrate or chloride of rare-earth element, an anionic surfactant [sodium alkyl sulfate, such as sodium dodecyl sulfate (Cn H2n+1 OSO3 Na)], urea, and water are mixed; (2) by increasing the temperature of the mixture to 80◦ C, hydrolysis of the urea was accelerated; and (3) pH of the reaction solution was increased by the hydrolysis, and rare-earth compound nanotubes with uniformed outer diameters (approximately 6 nm) and inner diameters (approximately 2.5 nm) were synthesized (Fig. 8.1a). The nanotubes consisted of rare-earth hydroxides, carbonate ions, and the surfactant. The rare-earth compound phase (consisting of rare-earth oxy-hydroxides and carbonate ions) with positive electric charge was formed using a rod-like micelle with negative surface electric charge formed by the surfactant assembly as the core part. This process led to the formation of the nanotubes. In addition, the surfactant in the nanotube could be removed by ionic exchange with acetate ion. Then, we investigated the possibility of nanotube formation using various ions with negative electric charge. The results showed that when sodium sulfate 10-hydrate or sodium hypophosphite 1-hydrate was used as the anion source, nanotubes could be formed (Fig. 8.1b, c). Therefore, it was revealed that the nanotube-shaped structure could be formed by a combination of rare-earth hydroxides, carbonate ions, and a certain type of anion. Therefore, it appeared to be a reasonable assumption that the surfactant did not operate as the template; rather, the nanotube-shaped crystal structure of rare-earth compounds resulted from the potential characteristics of rareearth compounds with the slight aid of the surfactants. When Yb was used as the rare-earth element, the nanotubes synthesized using sodium dodecyl sulfate were similar to those obtained using sodium hypophosphite: both of them were single-walled nanotubes with uniformed outer diameters (6 and 8 nm, respectively), and their X-ray diffraction patterns were also similar to each other. On the other hand, when sodium sulfate was used as the anion source, double-walled nanotubes were observed in addition to single-walled nanotubes, and the outer diameters of these were distributed over a wide size range of approximately 6–30 nm, centering on 13 nm. Since the X-ray diffraction patterns obtained when sodium sulfate was used differed from those obtained when sodium dodecyl sulfate and sodium hypophosphite were used,  Fig. 8.1. TEM images of ytterbium compound nanotubes synthesized using various anion sources [3]. (a) Sodium dodecyl sulfate, (b) sodium sulfate 10-hydrate, and (c) sodium hypophosphite 1-hydrate. (Copyright Frontier Publishing. Reproduced with permission.)

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it was demonstrated that nanotubes with different crystal structures were formed according to which anion source was used. The specific surface and pore size distributions when sodium dodecyl sulfate was used were 253 m2 g−1 and 3 nm, respectively; those when sodium sulfate was used were 81 m2 g−1 and 11 nm (approximately 6–20 nm), respectively; and those when sodium hypophosphite was used were 248 m2 g−1 and 4.4 nm, respectively. Thus, a narrow pore size distribution was obtained when either sodium dodecyl sulfate or sodium hypophosphite was used. The large specific surface and the uniform inner and outer diameters, in a single nanoscale region (below 10 nm), of the nanotubes obtained using sodium dodecyl sulfate or sodium hypophosphite were significantly different from those of the nanotubes synthesized by the alternative method described below. We also confirmed that the ability of the rare-earth element to form nanotubes varies according to the anion source. It was demonstrated that elements with small ion radii, such as Y, Dy, Tb, Ho, Er, Yb, and Lu, could form nanotubes. Therefore, the formation of nanotubes depended on the coordinate structure relating to the ion radius or basicity of the elements [1–5]. It is important to control the aggregate structure of nanotubes in order to fully exploit their advantages efficiently, because this allows us to make use of their one-dimensional structure and large inner and outer surfaces. In the synthesis of nanotubes using sodium sulfate, when the synthesis was performed without stirring, spherical-shaped particles with approximately 2–3 μm in size were formed (Fig. 8.2a). These particles were formed as the nanotubes grew radially or dendritically from the central part (Fig. 8.2b,c). Many projections were observed on the particle surface, and each of them was a nanotube. When a similar synthesis was performed using a quartz plate soaked in the solution, a thin film of approximately 3 μm thickness was formed on the plate surface. This film consisted of nanotubes radially growing from the quartz plate surface, and the tips of the tubes were almost perpendicular to the plate. A similar thin film was formed on the gas–liquid interface. The nanotubes grew radially from the gas phase side to the liquid phase side and formed the film (Fig. 8.2d). A structure using nanotubes as building blocks, i.e., in a hierarchical structure, could be constructed only when sodium sulfate was used as the anion source. The luminescence properties of these nanotubes were also investigated. Wu et al. [4] formed nanotubes using sodium dodecyl sulfate following the method developed by us and reported that after the nanotube formation, Y2 O3 : Eu3+ nanotubes with 20–30 nm outer diameter and several nanometer tube wall thicknesses were formed by applying calcination for 3 h. However, since the X-ray diffraction patterns of the as-grown and calcined nanotubes were not reported, the nature of the crystal structure and its chemical composition are unknown. The luminescence properties of the Y2 O3 : Eu3+ nanotubes were compared with those of the Y2 O3 : Eu3+ nanoparticles with a 20 nm size. In the emission spectrum, using an excitation light at 394 nm, two peaks were detected at 610 nm (5 D0 →7F2 transition) and 590 nm (5 D0 →7F1 transition)

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Fig. 8.2. SEM images (a,b,d) of hierarchical microstructures formed by ytterbium compound nanotubes and a schematic representation of a spherical particle formed by ytterbium compound nanotubes (c) [2, 3]. (a) Spherical particles, (b) cross section of a spherical particle, and (d) thin film and its cross section. [(a, d) Copyright Frontier Publishing. Reproduced with permission. (b, c) Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.]

for the nanoparticles (Fig. 8.3). However, the intensity ratio between the peaks at 610 and 690 nm greatly changed, and a new peak was detected at 618 nm for the nanotubes. The reason for the difference in the emission spectrum between the nanoparticles and the nanotubes was reported to be due to the existence

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Fig. 8.3. Emission spectrum of Y2 O3 : Eu3+ nanotube [4]. (Reprinted with permission. Copyright 2003, American Institute of Physics.)

of the two types of Eu3+ sites with different symmetries inside the tube wall, and Eu3+ sites with various symmetries near the tube wall surface as the luminescent center. Sekita et al. [5] synthesized yttrium compound nanotubes, in which Eu3+ was doped, using sodium dodecyl sulfate, and the synthesized nanotubes were then calcined at 1, 000◦ C. The luminescence intensity of the calcined solid was stronger than that of the bulk at a wavelength near 610 nm.

8.3 Rare-Earth Hydroxide and Rare-Earth Oxide Nanotubes Synthesized Using the Hydrothermal Method The syntheses of rare-earth hydroxide and rare-earth oxide nanotubes using the hydrothermal method were reported from several groups almost at the same time, and it was demonstrated that nanotubes could be synthesized using almost all rare-earth elements. These reports commonly described that hexagonal rare-earth hydroxide nanotubes were synthesized by the hydrothermal method from the neutral to the alkaline region, and the synthesized nanotubes could be transformed into rare-earth oxide nanotubes by the calcination. The reaction mechanisms were considered to be almost identical [6–12].

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Fig. 8.4. TEM image of nanotube synthesized using hydrothermal method [6]. (Reprinted with permission from [6]. Copyright 2003 American Chemical Society.)

Xu et al. [6, 7] reported that after directly adding particles of rare-earth oxide (Dy2 O3 or Ho2 O3 ) to water, or after adding particles of rare-earth oxide (Tb4 O7 or Y2 O3 ) to a NaOH aqueous solution, the hydrothermal treatment was performed at 160–170◦ C for 48 h. Subsequently, hexagonal rare-earth hydroxide (Dy(OH)3 , Ho(OH)3 , Tb(OH)3 , and Y(OH)3 ) nanotubes were formed; these grew along [001] due to the dissolution/reprecipitation mechanism of oxide particles (Fig. 8.4). For example, Dy(OH)3 nanotubes with 40–500 nm outer diameters, 20–200 nm inner diameters, and 1–4 μm lengths were formed. Following a calcination at 450◦ C, Dy(OH)3 nanotubes could be transformed into cubic Dy2 O3 nanotubes while maintaining the nanotube structure. A similar result could be obtained in the Ho system. Tb(OH)3 nanotubes with 30–200 nm outer diameters, 20–100 nm inner diameters, and 2–6 μm lengths could be transformed into Tb4 O7 nanotubes with a face-centered cubic crystal structure following a calcination treatment at 450◦ C. Y(OH)3 nanotubes, with 30–260 nm outer diameters, 15–120 nm inner diameters, and 1–8 μm lengths, also could be transformed into Y2 O3 nanotubes. Wang et al. [8–10] successfully synthesized various rare-earth hydroxide nanotubes with the following procedure: (1) rare-earth oxides were dissolved into aqueous nitric acid solution; (2) pH was adjusted by KOH or NaOH; and (3) reaction was performed at 140◦ C for 12–24 h. The size of the nanotube formed depended on the ion radius of the rare-earth element used. When rare-earth elements with small ion radii (Y, Yb, Tm, Er, Ho, Dy, and Tb) were used, rare-earth hydroxide nanotubes with tens of nanometers – a little more than 100 nm in diameter and several micrometers in length – were synthesized as the compound Ln(OH)3 (Ln is a rare-earth element). When rareearth elements with large ion radii (Gd, Eu, Sm, Nd, Pr, and La) were used, rare-earth hydroxide nanotubes with diameters below 20 nm were synthesized.

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All of the synthesized nanotubes were hexagonal single crystals, and the tubes grew along [010]. Rare-earth hydroxide nanotubes could be used as precursors of various new types of rare-earth compound nanotubes. With a calcination at 700◦ C, rare-earth hydroxide nanotubes could be transformed into cubic rare-earth oxide (Ln2 O3 ) nanotubes. When rare-earth hydroxide nanotubes were calcined at 700◦ C in Ar or N2 atmospheres after mixing with sulfur, rare-earth oxysulfide (Ln2 O2 S) (which are currently used as luminescent materials) nanotubes were synthesized. When rare-earth hydroxide nanotubes were hydrothermally treated at 120◦ C for 12 h after being added to a solution containing F− , hexagonal single crystal (Y(OH)2.14 F0.86 ) nanotubes were also synthesized. Since the hydrophilic property of the rare-earth hydroxide nanotube surface was high, various liquids, such as ethanol, could be incorporated in the nanotube due to the capillary force (Fig. 8.5a). When the nanotubes were added to a hydrazine solution, a reducing agent, to make them adsorb hydrazine on their internal and external surfaces and were then soaked in a solution containing gold or silver ions, nanotubes coated with metal nanoparticles were obtained (Fig. 8.5b). Organic modification using nanotubes’ surface hydroxyl groups was also available, and surface modification with methyl methacrylate was successful (Fig. 8.5c). In terms of physical properties, both up-conversion luminescence and down-conversion luminescence were observed for Y2 O3 : Eu3+ , Y2 O2 S : Yb3+ , and Er3+ nanotubes (Fig. 8.5d). However, those nanotubes were not compared with bulk or nanoparticles, and so the luminescence property due to the nanotube was not discussed. Tang et al. [11] reported on the luminescence property of Tb2 O3 nanotubes; these were synthesized by the following procedure: (1) sodium dodecylbenzenesulfonate (SDBS) and terbium chloride 6-hydrate were added to water; (2) pH was adjusted to 12 by adding a NaOH aqueous solution, and hydroxides were obtained; and (3) the obtained hydroxides were hydrothermally treated at 120–180◦ C for 24–48 h, to obtain hexagonal Tb(OH)3 nanotubes with approximately 120 nm outer diameters, approximately 40 nm inner diameters, and 1–3 μm lengths, which grew along [101]. After calcination under a reducing atmosphere at 800◦ C for 2 h, single-crystalline Tb2 O3 nanotubes, which grew along [100], were obtained. Although Tang et al. reported that SDBS could act as a template, the role of SDBS was not exactly clarified when the above-mentioned results reported by Xu et al. [6, 7] and Wang  Fig. 8.5. Application of nanotube synthesized using hydrothermal method [8–10]. (a) TEM image of individual nanotube of Y(OH)3 containing ethanol inside. (b) TEM image of Y(OH)3 nanotubes coated with Au nanoparticles. (c) Y(OH)3 nanotubes, the surfaces of which were modified using methyl methacrylate. (d) Upconversion luminescence (excitation light at 980 nm) and down-conversion luminescence (illustration, excitation light at 310 nm) of Y2 O2 S/Yb3+ , Er3+ nanotubes. [(a, b, d) Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission. (c) Reproduced by permission of The Royal Society of Chemistry.]

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Fig. 8.6. Absorption spectrum of Tb2 O3 (a: Tb2 O3 nanotubes, b: Tb2 O3 bulk) [11]. (Reproduced by permission of The Royal Society of Chemistry.)

et al. [8–10] were taken into consideration. When the absorption spectrum was compared between bulk and the nanotubes, although an intense band at 225 nm (7 F6 (4f 8 ) → 7D(4f 7 5d)) and a weak broad band at 270–280 nm (7 F6 (4f 8 ) → 9D(4f 7 5d)) were observed in the bulk, these peaks shifted to 207 nm and 260–280 nm, respectively, and became broad in the nanotubes (Fig. 8.6). Tang et al. explained the reason for this as follows: (1) in comparison with the bulk, the nanotubes have much greater surface stress and strain on the inner or outer surfaces of the tubes. The strong stress and strain might slightly alter the coordination environment of Tb3+ in the surface, as the external environment may have more impact on the d–f transitions than on the f–f transitions. As a result, the band gap from the excitation to basal level in Tb2 O3 nanotubes was enhanced and the corresponding absorption peak shifted toward higher energy; (2) it is well known that the exact position of the 4f 8 − 4f 7 5d transitions is dependent on the crystal field of the lattice. In comparison with that of the bulk, the crystal field in the more anisotropic structure might be weakened, because inside or outside the nanotubes the coordination of ions close to these near-surface site ions is less than that for the bulk and the coordination of ions is not satisfied. These two factors might account for the blue shift of the absorption spectrum; and (3) the nanotubes have four different contact regions (the internal or external surfaces, the inside of the walls, or the tips of the nanotubes) that result in different degrees of

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stress and lattice defects in various positions, the difference will result in a wider distribution of the band gap and a corresponding broad distribution of the main peak. In the emission spectrum of the nanotubes using an excitation light at 325 nm, new peaks were observed at 607 nm (5 D4 −7F3 transition) and 568 nm (5 D4 −7F3 transition), although they were very weak, together with several peaks that were observed in the bulk. Tang et al. explained that the highly disordered surface might be responsible for the novel optical properties. Some Tb3+ ions are positioned in some new sites with lower symmetry and those sites might come from the surface or near the surface of the rim and tips of the nanotubes and these new peaks can be ascribed to strong splitting of the Stark level related to surface emission. In the above paper, Ce-based nanotubes were not reported. This is because the valence of cerium ion is easily changed during reaction. However, Tang et al. [12] succeeded in synthesizing hexagonal Ce(OH)3 nanotubes, which grew along [110], by preparing an oxygen-free condition in the reaction solution and atmosphere. Using CeCl3 as the Ce source, purple hexagonal Ce(OH)3 nH2 O (n = 0.5 − 2.5) nanotubes were synthesized by the hydrothermal reaction at 100–150◦ C in a 12 mol/l NaOH solution. When the reaction temperature was 150◦ C, their diameters became 20–500 nm and their lengths became several hundred nanometers. More than 50% of nanotubes had cavities in the particles, and cone- or cylinder-shaped cavities were frequently observed. When the reaction temperature was 120◦ C, 80% of the products were nanotubes with tens of nanometer in diameters and hundreds of nanometer–several micrometer lengths. Although the tube-shaped structure collapsed through natural oxidation and the calcination in air, hexagonal Ce(OH)3 nanotubes could be transformed into cubic CeO2 nanotubes by applying the temperature-controlled annealing under a reducing atmosphere.

8.4 Rare-Earth Oxide Nanotubes Synthesized Using Carbon Nanotube as a Template Liu et al. [13] synthesized Y2 O3 : Re3+ (Re = Eu, Tb, or Dy) nanotubes using multi-walled CNT as a template. The synthetic method was as follows: (1) CNTs were dispersed in a solution, in which Y(NO3 )3 , Re(NO3 )3 , and polyvinylpyrrolidone (dispersant) were mixed; (2) the solution was reacted at 80–90◦ C for 5 h; (3) the solution was filtered, and the obtained substances were washed with distilled water and dried at 100◦ C for 12 h; (4) CNTs coated with amorphous rare-earth compound phase with approximately 15 nm thickness were obtained; (5) the obtained CNTs were calcined at 900◦ C for 4 h to remove CNTs and to make the amorphous rare-earth compound phase being crystallized; and (6) cubic Y2 O3 : Eu3+ nanotubes with 50–80 nm outer diameters were obtained. Since the walls of the synthesized nanotubes consisted of nanoparticles, they were porous. The reason for this was reported to be due to the generation of CO2 caused by the burning of CNTs and the

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shrinking of the volume caused by crystallization of the amorphous compound. The emission spectrum of these porous Y2 O3 : Eu3+ nanotubes was similar to that shown in Fig. 8.3a. In terms of the luminescence properties, those due to the nanoparticles were considered to be greater than those due to the nanotubes.

8.5 Rare-Earth Oxide Nanotubes Synthesized Using Anodic Alumina Membrane as a Template Kuang et al. [14] synthesized rare-earth (Y, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, and Yb) oxide nanotubes by sol–gel processing using anodic porous alumina membrane as a template. The synthetic method was as follows: (1) rare-earth oxide was added to nitric acid and dissolved into distilled water; (2) the obtained solution was stirred at 100◦ C until white sol was formed; (3) anodic porous alumina membrane was soaked into the formed sol; (4) after being removed from the sol, the anodic porous alumina membrane incorporating the sol was calcined at 500◦ C; (5) the calcined anodic porous alumina membrane was soaked in an NaOH solution to remove alumina by dissolving; and (6) cubic rare-earth oxide nanotubes were obtained. The outer diameters of the synthesized nanotubes were 80 nm, which was equivalent to the pore diameter of the alumina; the lengths were several hundred nanometers, and the wall thicknesses were 5–15 nm. The synthesized nanotubes were polycrystalline substances consisting of nanoparticles with sizes between 5 and 10 nm. The majority of the synthesized nanotubes had bamboo-like morphologies, and their insides were partitioned with thin walls (5–10 nm) (Fig. 8.7a); some of them, however, had ordinary penetration holes. Controlling the ratio between these two types was reported to be difficult. Subsequently, the synthesis process was discussed (Fig. 8.7b). When the calcination was applied, after filling alumina pores with the sol, the sol changed to a gel through a shrinking process, accompanied by gas release. When there was a strong interaction between the gel and the alumina pores, the gel uniformly coated the pores’ internal surfaces, and nanotubes were formed. Whether the nanotubes had bamboo-like morphologies or penetration holes depended on the sol viscosity. When a sol with a specified viscosity was used, the released gas worked as a template, and bamboo-like gel appeared in the alumina pores due to the surface tension of gas foams.

8.6 Cerium Phosphate Nanotubes with Blue Luminescence Tang et al. [15] synthesized Ce(HPO4 )2 nH2 O nanotubes and transformed them into cerium phosphate nanotubes having the mixed-valence state

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Fig. 8.7. TEM image of Ho2 O3 nanotubes with bamboo-like structure (a) and schematic representation of the formation mechanism (b) [14]. (Reprinted from [14], Copyright 2007, with permission from Elsevier.)

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(Ce3+ /Ce4+ ) by a calcination in which both temperature and atmosphere were controlled. The following method was used for the synthesis: (1) a con− densed linear polyphosphate (Pn O3n+1 )(n+2) was obtained by heating a 6 mol/l aqueous phosphoric acid solution at 105◦ C; (2) a 0.02 mol/l aqueous solution of diammonium cerium(iv) nitrate was added dropwise to the phosphoric acid solution, and the mixture was allowed to react for 2 h; and (3) after washing with water, hexagonal Ce(HPO4 )2H2 O nanotubes with 20– 100 nm outer diameters were obtained. When the synthesized nanotubes were calcined under a reducing atmosphere, the nanotubes could maintain their fiber-shaped structures up to 900◦ C, although glass phase particles were partially formed. In the sample that was calcined at 900◦ C, no nanotubes were observed, but monoclinic CePO4 polycrystalline nanowires were observed. In the sample that was calcined at 600◦ C, although small amounts of substances, which were similar to the products at 900◦ C, were mixed, the majority was nanotubes with an amorphous structure (Fig. 8.8a). The valence of cerium measured by electron energy loss spectroscopy (EELS) was +4(Ce4+ ) before the calcination, +3(Ce3+ ) after the calcination at 900◦ C, and +3.34 after the calcination at 600◦ C. When the luminescence property of these products was investigated (Fig. 8.8b), no luminescence was observed in the sample before the calcination but strong ultraviolet (UV) luminescence was observed in the sample obtained by the calcination at 900◦ C. Moreover, a strong blue luminescence centering on 490 nm was observed in the nanotubes with mixed-valence state (Ce3+ /Ce4+ ) obtained by the calcination at 600◦ C (Fig. 8.8b). This blue luminescence was reported to be due to the charge transfer between the Ce4+ electron donation centers and the Ce3+ luminescence centers caused by the electron–photon interaction.

8.7 Rare-Earth Fluoride Nanotubes Liang et al. [16] synthesized hexagonal NaHoF4 nanotubes with a six-sided prism appearance, which grew along the c-axis, by performing a reaction at pH 3 at 140◦ C for 14 h, using NaF, Ho(NO3 )3 , NH4 HF2 , EDTA, and H2 O as the raw materials (Fig. 8.9A). They reported that similar rare-earth fluoride compounds could be synthesized using other rare-earth elements (except La– Pm). For example, when Sm was used, NaSmF4 nanotubes with 100–400 nm outer diameters and 10–100 inner diameters were synthesized. Partially, there were nanotubes in which nanorods were formed; the nanorods were held by six branch parts and grew along the c-axis. As shown in Fig. 8.9B, a hexagonal plate as a seed was formed, and the subsequent growth along the circumferential edges of the seed yield a tubular structure. The opposite growth toward [001] and [00T] proceeds simultaneously. The growth orientation of the edges (walls) is perpendicular to {001} plane of the seed. Up-conversion emissions of NaLnF4 (Ln = Y, Dy-Yb) were also observed [17]. For example, when

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Fig. 8.8. TEM image of cerium phosphate nanotubes (a) and luminescence property (b) [15]. (Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

excited by infrared light of 980 nm, NaYF4 tubes codoped with Yb3+ /Er3+ ions display strong green up-conversion emission as shown in Fig. 8.9C, which was much more intense than that of cubic NaYF4 or hexagonal NaYF4 nanoparticles [16–18].

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Wei et al. [18] synthesized multi-walled carbon nanotubes (CNTs) where approximately 80% of their surfaces were coated with EuF3 and TbF3 nanoparticles and examined their luminescence properties. The nanocomposites were synthesized by the following methods: (1) after dispersing CNTs in a 1 wt% sodium dodecyl sulfate (SDS) solution, an ultrasonic treatment was performed, and SDS/CNT composites were obtained; (2) these composites were dispersed in a Ln(NO3 )3 (Ln = Eu, Tb) solution; (3) a NaF solution was added; and (4) after the reaction for 24 h, the products were washed with distilled water and dried. The synthesis mechanism was reported to be as follows: (1) when dodecyl sulfate ions were adsorbed on the CNT surface by the hydrophobic interaction, composites having negative charge (−OSO− 3 ) on the surface were formed and (2) when rare-earth ions with a positive charge were electrostatically adsorbed on this negative charge and NaF was added, a EuF3 film with a thickness below 10 nm was formed on the CNT surface. When the excitation spectrum was set to be 330 nm, peaks at 628 and 430 nm were observed in the emission spectrum. In the bulk, the peak at 628 nm red shifted by 15 nm and the peak at 430 nm blue shifted by 10 nm.

8.8 Conclusion Among various inorganic compound groups, the rare-earth compound group is considered to be one of those capable of synthesizing the largest variety of nanostructures. As outlined in this section, various types of nanotubes, with various chemical compositions, crystal structures, inner and outer diameters, and lengths, have been synthesized as rare-earth compounds. By comparing physical properties of the nanotubes, focusing in particular on luminescence, the characteristics of nanotube structures will be clarified; these include the effects of crystal structure (single crystal, polycrystal, and amorphous), wall thickness (size effect), and one-dimensional structure. The study of rare-earth compound nanotubes will be progressed centering on the application study based on 4f-electron characteristics peculiar to rare-earth elements; in particular, use of rare-earth compound nanotubes as luminescent and magnetic materials will be vigorously investigated. Moreover, the study on ceria nanotubes, which are expected to be applied to various fields, will attract the attention of many researchers.  Fig. 8.9. (A) SEM image of NaHoF4 nanotubes [16], (B) schematic representation of the formation mechanism [16], and (C) up-conversion emission spectra of NaYF4 : Yb3+ /Er3+ excited by a 980 nm laser [17]: (a) cubic NaY0.94 Yb0.05 Er0.01 F4 , (b) hexagonal NaY0.94 Yb0.05 Er0.01 F4 , (c) hexagonal NaY0.79 Yb0.20 Er0.01 F4 , (d) hexagonal NaY0.78 Yb0.20 Er0.02 F4 , and (e) hexagonal NaY0.48 Yb0.50 Er0.02 F4 . (Reprinted with permission from [16]. Copyright 2004 American Chemical Society. Reprinted with permission from [17]. Copyright 2007 American Chemical Society.)

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Acknowledgment This work was partially supported by KAKENHI (16685021).

References

1. M. Yada, M. Mihra, S. Mouri, M. Kuroki, T. Kijima, Adv. Mater. 14, 309 (2002) 98, 100 2. M. Yada, C. Taniguchi, T. Torikai, T. Watari, S. Furuta, H. Katsuki, Adv. Mater. 16, 1222 (2004) 98, 100, 101 3. M. Yada, Org., Inorg. Metallic Nanotubular Mater., 148 (2008) 98, 100, 101 4. C.F. Wu, W.P. Qin, D. Zhao, J.S. Zhang, S.H. Huang, Appl. Phys. Lett. 82, 520 (2003) 100, 102 5. M. Sekita, K. Iwanaga, T. Hamasuna, S. Mouri, M. Uota, M. Yada, T. Kijima, Phys.. Stat.. Sol. 241, R71 (2004) 100, 102 6. A.-W. Xu, Y.-P. Fang, L.-P. You, H.-Q. Liu, J. Am. Chem. Soc. 125, 1494 (2003) 102, 103, 104 7. Y.-P. Fang, A.-W. Xu, L.-P. You, R.-Q. Song, J.C. Yu, H.-X. Zhang, Q. Li, H.-Q. Liu, Adv. Func. Mater. 13, 955 (2003) 102, 103, 104 8. X. Wang, X.-M. Sun, D. Yu, B.-S. Zou, Y.D. Li, Adv. Mater. 15, 1442 (2003) 102, 103, 104, 9. X. Wang, Y. Li, Chem. Eur. J. 9, 5627 (2003) 102, 103, 104, 106 10. W. Li, X. Wang, Y. Li, Chem. Commun. 164 (2004) 102, 103, 104, 106 11. Q. Tang, J. Shen, W. Zhou, W. Zhang, W. Yu, Y. Qian, J. Mater. Chem. 13, 3103 (2003) 102, 104, 106 12. C. Tang, Y. Bando, B. Liu, D. Golberg, Adv. Mater. 17, 3005 (2005) 102, 107 13. G. Liu, G. Hong, J. Nanosci. Nanotechnol. 6, 120 (2006) 107 14. Q. Kuang, Z.-W. Lin, W. Lian, Z.-Y. Jiang, Z.-X. Xie, R.-B. Huang, L.-S. Zheng, J. Solid State Chem. 180, 1236 (2007) 108, 109 15. C. Tang, Y. Bando, D. Golberg, R. Ma, Angew. Chem. Int. Ed. 44, 576 (2005) 108, 111 16. L. Liang, H. Xu, Q. Su, H. Konishi, Y. Jiang, M. Wu, Y. Wang, D. Xia, Inorg. Chem. 43, 1594 (2004) 110, 111, 113 17. J. Zhuang, L. Liang, H.H.Y. Sung, X. Yang, M. Wu, I.D. Williams, S. Feng, Q. Su, Inorg. Chem. 46, 5404 (2007) 110, 111, 113 18. X.-W. Wei, J. Xu, X.-J. Song, Y.-H. Ni, P. Zhang, C.-J. Xia, G.-C. Zhao, Z.-S. Yang, Mater. Res. Bull. 41, 92 (2006) 111, 113

Index Absorption spectrum of Tb2 O3 , 106 Anodic alumina membrane as template, rare-earth oxide nanotubes synthesis using, 108 Bamboo-like morphologies, 108 Carbon nanotube as template, rareearth oxide nanotubes synthesis using, 107–108

Cerium phosphate nanotubes with blue luminescence, 108–110 Cerium phosphate nanotubes and luminescence property, TEM image, 107 Cerium valency, 110 Condensed linear polyphosphate, 110 Cubic rare-earth oxide nanotubes, 108 Diammonium cerium(iv) nitrate, 110

8 Synthesis and Applications of Rare-Earth Compound Nanotubes Electron energy loss spectroscopy (EELS), 110 Emission spectrum of ytterbium compound nanotubes, 101–102 Homogeneous precipitation method, 98 Hydrothermal method, 102–107 Multi-walled carbon nanotubes (CNTs) synthesis, 113 Nanotube-shaped crystal structure of rare-earth compounds, 98 Rare-earth compound nanotubes synthesis, 98–102 Rare-earth fluoride nanotubes, 110–113 Rare-earth hydroxide nanotubes synthesis, 102–107 Rare-earth oxide nanotubes synthesis, 102–107

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SEM images of ytterbium compound nanotubes, 101 Shrinking process, 108 Sodium dodecylbenzenesulfonate (SDBS), 104 Sol–gel process, 108 TEM image of Ho2 O3 nanotubes with bamboo-like structure, 109 TEM image of nanotube synthesized using hydrothermal method, 103 TEM image of nanotube of Y(OH)3 containing ethanol inside, 104–105 TEM images of ytterbium compound nanotubes, 98–99 Terbium chloride 6-hydrate, 104 Ultraviolet (UV) luminescence, 110 Ytterbium compound nanotubes TEM images, 98–99

9 Synthesis and Applications of Zirconia and Ruthenium Oxide Nanotubes Mitsunori Yada1 and Yuko Inoue2 1

2

Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan [email protected] Department of Chemistry and Applied Chemistry, Faculty of Science and Engineering, Saga University, Saga 840-8502, Japan [email protected]

Abstract In this section, syntheses and applications of zirconia and ruthenium oxide nanotubes are described. The zirconia nanotubes are synthesized by the anodization of zirconium metal and the template synthesis method using carbon nanotube, anodic porous alumina membrane, porous polycarbonate membrane, and organic molecules as templates. Several applications of the zirconia nanotubes are also presented. Furthermore, the ruthenium oxide nanotubes are synthesized by the template synthesis method using anodic porous alumina membrane and the homogeneous precipitation method. Luminescence property and application to supercapacitor of the ruthenium oxide nanotubes are presented. Synthesis of ruthenium metal nanotube is also introduced.

9.1 Zirconia Nanotubes 9.1.1 Introduction Zirconia and zirconia-based solid solutions have been used in structural materials, catalysts, catalyst supports, adsorbents, and solid electrolytes (fuel cells, oxygen sensors, etc.). Similar to other oxides, syntheses and characteristic evaluations of various nanostructure materials containing zirconia have been studied. Particularly, many studies regarding zirconia nanotubes have been reported since Rao et al. [1] reported the synthesis of zirconia nanotubes using carbon nanotubes (CNTs) as templates in 1997. Zirconia nanotubes are generally synthesized using the following four methods: (1) After depositing zirconia using hard and stable substances with one-dimensional morphology, such as CNTs and cellulose fibers as templates (core parts), removal of the templates by burning to obtain zirconia nanotubes. (2) Formation of zirconia nanotube arrays using electrochemical techniques (anodization of metal Zr). (3) After depositing zirconia on pore walls using one-dimensional pores T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 117–133 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 9

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which are on porous membranes of anodic porous alumina or polycarbonate as templates, removal of the porous membrane to obtain zirconia nanotubes. (4) After depositing zirconia on a one-dimensional assembly formed by amphipathic molecules, removal of the amphipathic molecules to obtain zirconia nanotubes. In this section, the characteristics and applications of zirconia nanotubes synthesized using the above methods are described. 9.1.2 Zirconia Nanotubes Synthesized Using Carbon Nanotubes or Nanofibers as Templates Rao et al. [1] synthesized zirconia nanotubes using multi-walled CNTs as templates. Zirconia nanotubes were synthesized by the following procedure: (1) CNTs were refluxed in nitric acid to form acid points on the CNTs surfaces; (2) the obtained CNTs were mixed with Zr(OPrn )4 under the Ar atmosphere; (3) after ultrasonication, the mixture was kept under the Ar atmosphere for 48 h; (4) the obtained substances were washed with dilute hydrofluoric acid and methanol to remove excess alkoxide, and dried; (5) the obtained substances were calcined at 450◦ C, and the CNTs coated with zirconia were obtained; and (6) the obtained CNTs were calcined at 700◦ C, and zirconia nanotubes were obtained (Fig. 9.1a). The obtained nanotubes’ outer diameters were 40 nm, and the wall thicknesses were 6 nm. The crystal structure was the mixed phase of tetragonal phase:monoclinic phase = 50:50. Moreover, Rao et al. synthesized yttria-stabilized zirconia (0.92ZrO2 –0.08Y2 O3 ) nanotubes. The synthesis method was as follows: (1) ZrOCl2 8H2 O was dissolved into distilled water, and nitric acid, into which Y2 O3 was dissolved, was added; (2) the obtained solution was cooled down to 0◦ C, and pH was adjusted to 9 by adding NH3 to obtain a gel; (3) the gel was washed with distilled water, and dissolved into nitric acid; (4) acid-treated CNTs were added to

(a)

(b)

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Fig. 9.1. Zirconia nanotubes synthesized using carbon with various shapes as a template [1, 2]. (b, c) The scale bar: 200 nm. ((a) Reproduced with permission of The Royal Society of Chemistry. (b, c) Reprinted with permission from [2]. Copyright 2006 American Chemical Society.)

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the obtained solution, and the solvent was removed by slow evaporation; (5) the obtained substances were dried at 100◦ C, and calcined at 450◦ C to obtain CNTs coated with yttria-stabilized zirconia; and (6) the obtained CNTs were calcined at 700◦ C to remove carbon, and cubic yttria-stabilized zirconia nanotubes were obtained. The authors reported that when CNTs coated with yttria-stabilized zirconia were treated by ethylene glycol and nitric acid before the calcination at 700◦ C, the tube-shaped structure became stable. The obtained cubic yttria-stabilized zirconia nanotubes’ outer diameters were 40–50 nm, and the wall thicknesses were approximately 6 nm. Ogihara et al. [2] synthesized zirconia nanotubes with various shapes (linear and helical) using carbon nanofibers or nanocoils as templates (Fig. 9.1b, c). The synthetic method was as follows: (1) while dropping an ethanol solution containing Zr(On Pr)4 , the suction filtration was performed for carbon nanofibers or nanocoils to adsorb the ethanol solution on the carbon, and the excess ethanol solution was removed; (2) the obtained substances were dried in the air at 120◦ C to evaporate the ethanol and to make the alkoxide remain on the carbon surface; as alkoxide was hydrolyzed by moisture in the air, the carbon surface was coated with zirconia; (3) this coating process was repeated 10–40 times to obtain carbon completely coated with zirconia; and (4) the obtained carbon was heated at 750◦ C in the air to remove carbon, and zirconia nanotubes were obtained. The inner diameters of the obtained zirconia nanotubes were almost similar to the outer diameters of the carbon templates used, and their wall thicknesses could be controlled by the number of coating processes. For example, when carbon nanofibers with 100–200 nm diameters were used as templates, zirconia nanotubes with 100–200 nm inner diameters, approximately 30 nm wall thicknesses, and approximately 20 nm crystallite diameters were synthesized. Their crystal structures were mostly monoclinic, and a small amount of tetragonal was included. Sun et al. [3] synthesized nanocomposites (Fig. 9.2a) containing CNTs, on which zirconia was uniformly coated, by a reaction in a supercritical carbon dioxide/ethanol solution. These nanocomposites were synthesized by the following method: (1) after dispersing CNTs in an ethanol solution containing Zr(NO3 )4 5H2 O, the solution was placed in a high-pressure vessel; (2) CO2 was charged in the vessel to make the pressure in the vessel be 7.5 MPa at 25◦ C; (3) the vessel was heated at 120◦ C for 3 h; (4) the vessel was lowered to room temperature, and decompressed; and (5) the obtained substances were washed with anhydrous ethanol, and dried in a vacuum to obtain zirconia/CNT nanocomposites. Because only aggregates of zirconia were obtained under the condition without CNTs or CO2 , both CNTs and CO2 were thought to play important roles for synthesizing the nanocomposites. In this method, ethanol was used as a solvent for Zr(NO3 )4 , and CO2 was used to change the behavior of the fluid. As the wettability of the CNT surface increased in the supercritical state, Zr(NO3 )4 dissolved in the fluid easily reached the surface and adsorbed on it. During heating at 120◦ C, Zr(NO3 )4 was decomposed, and zirconia was obtained. The crystal structure of the obtained zirconia was amorphous. The thickness of the zirconia phase can be controlled within a range

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(a)

(b)

Fig. 9.2. TEM image (a) and the ethanol detection characteristics (b) of carbon nanotubes coated with zirconia [3]. (Reprinted with permission from [3]. Copyright 2006 American Chemical Society.)

of 1.5–10 nm by changing the material ratio of Zr(NO3 )4 5H2 O/CNTs. In the synthesis method using a supercritical carbon dioxide/ethanol solution, there were several interesting points; for example, a pretreatment was not required for CNTs, and uniform coating with zirconia could be achieved. Sun et al. further discovered that the zirconia/CNT nanocomposite could be used as a chemiluminescence sensor for ethanol. This sensor could detect ethanol with a concentration of 1.6 μg/ml at 195◦ C (Fig. 9.2b). Sun et al. also performed an experiment to detect ethanol using a zirconia nanoparticle instead of the zirconia/CNT nanocomposite. The result showed that the sensor using the zirconia/CNT nanocomposite was more sensitive than that using the zirconia nanoparticle; the detection temperature was lower, and the amount of the material was smaller in the zirconia/CNT nanocomposite than in the zirconia nanoparticle. Moreover, the zirconia/CNT nanocomposite could not act as a chemiluminescence sensor for hexane, cyclohexane, and carbon tetrachloride. Therefore, the zirconia/CNT nanocomposite was confirmed to be excellent in selectivity. The chemiluminescence mechanism was reported to be as follows: (1) zirconia acted as an oxidation catalyst for ethanol; (2) an intermediate was generated when ethanol was oxidized; and (3) by the chemiluminescence from the generated intermediate, ethanol was detected. Huang et al. [4] synthesized zirconia nanotubes using celluloses, natural fiber-shaped substances, as templates. These zirconia nanotubes were synthen n sized by the following method: (1) using a Zr(O Bu)4 solution, Zr(O Bu)4 was adsorbed into hydroxy groups on the cellulose surfaces; (2) excess alkoxide was removed by ethanol; (3) by adding water, alkoxide was hydrolyzed, and zirconia was formed; (4) by repeating steps (1)–(3), zirconia was formed on the cellulose surfaces; and (5) the obtained substances were calcined at

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450◦ C to remove celluloses, and zirconia nanotubes were obtained. The obtained nanotubes’ outer diameters were tens of nm, and the wall thicknesses were 10 nm. As celluloses are natural nanosize fiber-shaped materials, they have been processed into papers, cottons, and fibers. By using the synthesis method developed by Huang et al., an aggregate structure reflecting a paper, cotton, or fiber shape (i.e., a hierarchical structure) can be obtained.

9.1.3 Zirconia Nanotube Arrays Synthesized by Anodization of Metal Zirconium Tsuchiya et al. [5] revealed that zirconia nanotube arrays could be synthesized by anodization of metal zirconium. The morphology of the synthesized tube depends on the electrolyte used. In particular, in a 1 mol/l (NH4 )2 SO4 solution containing 0.5 wt% NH4 F, linearly grown cubic zirconia nanotube arrays with approximately 50 nm diameters and 17 μm lengths were synthesized (Fig. 9.3). When other electrolytes were used, nanotubes did not grow linearly. The reason for the linear growth was reported to be the inhibition of a change in pH in the tube during the growth due to a buffering action of (NH4 )2 SO4 and NH4 F. Tsuchiya et al. also synthesized oxide nanotube arrays using metal Ti, W, Nb, or Ta other than metal Zr via a similar method as that explained above. However, by anodization, only the product directly crystallized was obtained when metal Zr was used. When other metals were used, the amorphous phase was formed.

Fig. 9.3. SEM image of zirconia nanotube arrays synthesized by anodization [5]. ((a) From above, (b) cross section) (Reprinted from [5], Copyright 2005, with permission from Elsevier.)

9.1.4 Zirconia Nanotubes Synthesized Using a Porous Membrane as a Template Anodic porous alumina membrane [6–9], a polycarbonate membrane [10, 11], and a track etched membrane of polyester [12] are known to be used as porous membranes used to synthesize zirconia nanotubes.

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When anodic porous alumina is used as the template, the following three methods are known to deposit a zirconia component on pores’ internal surfaces. The method proposed by Bao et al. [6] is as follows: (1) after dissolving ZrOCl2 8H2 O into an ethanol/H2 O solution, the solution was stirred; (2) an ethanol solution containing HNO3 and acetylacetone was added to the stirred solution, and stirred again; (3) the aging treatment was performed on the stirred solution, and a zirconia sol was obtained; (4) anodic porous alumina membrane was immersed in the obtained zirconia sol; (5) after removing it from the sol, the anodic porous alumina membrane was dried at room temperature in the air; (6) the dried anodic porous alumina membrane was calcined at 500◦ C for 6 h to deposit zirconia on the alumina pore walls; (7) using a threeelectrode electrochemical cell, metal Co was electrochemically deposited on the pores’ internal walls, using a solution containing CoSO4 7H2 O and H3 BO3 ; (8) the obtained substances were immersed in a 6 mol/l NaOH solution to remove alumina membrane; and (9) the obtained substances were washed with distilled water to obtain zirconia nanotube arrays containing metal Co (Fig. 9.4a,b). The obtained zirconia had an amorphous structure, the nanotube wall thicknesses were 40 nm, and the nanotube lengths were equivalent to the membrane thickness of the anodic porous alumina membrane used. Moreover, the metal Co in the nanotubes formed hexagonal Co nanowires with 200 nm diameters and the aspect ratio was 250. The magnetic properties of these obtained zirconia nanotube arrays were examined (Fig. 9.4c). The results were as follows: the coercivities of the arrays were Hc ≈ 306 Oe and Hc⊥ ≈ 288 Oe, larger than that of metal Co in the bulk (10 Oe). The hysteresis loops revealed that the arrays exhibit the uniaxial magnetic anisotropy, with the easy axis perpendicular to the nanowires. The remanent magnetization of the arrays was very small (19% of the saturation magnetization). The reason for this was reported to be due to the interaction between densely existing Co nanowires. Let the angle between the Co nanowires and the applied magnetic field be θ. When θ = 30◦ , the coercivity was maximized. When θ = 90◦ , the ratio of the remanent magnetization to the saturation magnetization, Mr/Ms, was maximized. The role of the zirconia nanotube arrays was reported to act as templates for metal Co to one-dimensionally grow, to be regularly arrayed in high density, and to prevent the grown metal Co from being oxidized. Moreover, the application of zirconia nanotubes synthesized by Bao et al. was reported by Xu et al. [7] The applied method was as follows: (1) zirconia nanotubes were added to a (3-aminopropyl) trimethoxysilane (APS) solution; (2) by applying ultrasonication to the solution, APS was combined with zirconia nanotubes through hydroxyl groups on their surfaces; (3) the obtained substances were washed with distilled water and immersed in a gold colloid solution; and (4) nanocomposites, on the surfaces of which gold nanoparticles with a size of 15 nm were adsorbed through amino groups of APS combined with zirconia nanotubes, were obtained. Cochran et al. [8] synthesized tetragonal zirconia nanotubes via liquid phase deposition, using anodic porous alumina membrane with 60 and 250 nm

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(b)

(c) Fig. 9.4. TEM (a) and SEM (b) images and magnetic properties (c) of zirconia nanotube arrays containing metal Co [6]. (Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission.)

pore diameters as a template. The nanotube walls were polycrystalline consisting of 6 nm particles. The synthetic method was as follows: (1) A (NH4 )2 ZrF6 solution and a boric acid solution were mixed, and the pH was adjusted to 2.5 to prepare a zirconia precursor solution. The chemical reaction formula of zirconia generation is as follows: 3− − + − − + ZrF2− 6 + 2H2 O → ZrO2 + 4H + 6F , BO3 + 4F + 6H → BF4 + 3H2 O

(2) Anodic porous alumina membrane was immersed in the prepared zirconia precursor solution, and ultrasonication was applied. (3) After heating at

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45◦ C for 24 h, alumina was removed using a NaOH solution. (4) The obtained substances were washed with distilled water to obtain zirconia nanotubes. Moreover, using octadecyltetrachlorosilane-treated anodic porous alumina as a template, and by continuously reacting with a precursor solution containing (NH4 )2 TiF6 and a precursor solution containing (NH4 )2 ZrF6 , coaxial nanotubes with inner walls consisting of zirconia and outer walls consisting of titania were obtained (Fig. 9.5). Hou et al. [9] synthesized zirconia nanotubes, using anodic porous alumina membrane with 126 ± 10 nm pore diameters as a template. The synthetic method was as follows: (1) by alternatively immersing the membrane in 1,10-Decanediylbis(phosphonic acid) (DBPA) and ZrOCl2 · 8H2 O solutions, the pH of which were adjusted to 6.0, DBPA and ZrO2+ were alternatively deposited on the alumina pores’ inside surfaces; and (2) alumina was removed

Fig. 9.5. (a) Z-contrast TEM image of an isolated TiO2 /ZrO2 coaxial nanotube synthesized inside OTS-treated, commercial AAO template (10 mM precursor concentrations, 24 h each layer). (b) Schematic illustration of the TiO2 /ZrO2 coaxial nanotube. Electron spectroscopic (TEM/EELS) compositional maps of a TiO2 /ZrO2 coaxial nanotube (at same magnification): (c) Zr, (d) Ti, and (e) O [8]. (Reprinted from [8], Copyright 2007, with permission from Elsevier.)

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using phosphoric acid to obtain nanotubes with a layered nanostructure, consisting of DBPA and ZrO2+ . The nanotubes’ outer diameters were equivalent to the alumina’s pore diameter. The nanotubes’ wall thicknesses could be controlled by the number of depositions. By each deposition (DBPA/ZrO2+ ), the wall thickness increased by 1.7 nm. Only after ten depositions, nanotubes could be obtained by removing alumina. When the number of depositions was five, the nanotubes’ walls collapsed during the removal of alumina. Shin et al. [10, 11] synthesized zirconia nanotubes, using a porous polycarbonate (PC) membrane as a template, via the atomic layer deposition (ALD) method. The zirconia nanotubes were synthesized as follows: (1) OTSSAMs (octadecyltetrachlorosilane self-assembled monolayers) were deposited onto both sides of the PC membrane by the contact printing. The OTS-SAMs were used as the passivation layers. (2) While preventing zirconia from depositing on the both sides of the PC membrane other than the pore parts, zirconia was deposited on the pore’s inner surfaces of the PC membrane using the ALD process. Namely, the reaction temperature was 140◦ C, N2 gas was used as a carrier, and Ar gas was used to purge. Zr[OCH(CH3 )2 ]4 was flown for 2 s, Ar for 5 s, H2 O for 2 s, and Ar for 5 s, in this order, and these flows were considered to be one cycle. This cycle was repeated for predetermined times. In a typical experiment, ∼100–800 cycles of ALD were performed to yield the ZrO2 nanotubes. The tube wall thickness was reported to be controlled by the number of the cycles with a high accuracy of sub-angstrom levels. (3) The PC membrane was removed by soaking in a chloroform solution at 60 , and zirconia nanotubes were obtained. In the paper written by Shin et al., the transmission electron microscope (TEM) and the atomic force microscope (AFM) images of zirconia nanotubes after 300 cycles were shown. The zirconia nanotubes’ outer diameters (200 nm) and the lengths (12 μm) were equivalent to the PC membrane’s pore diameter and thickness, respectively. Moreover, the obtained zirconia was tetragonal polycrystalline. Chen et al. [12] synthesized zirconia nanotubes, using an oxygen-plasmatreated polyester track etched (PETE) membrane as a template. Because a non-treated PETE membrane is hydrophobic, when the non-treated PETE membrane was used as a template, zirconia nanotubes cannot be obtained. Via the oxygen-plasma treatment, oxygen is introduced (C OH, O C O, C O, etc.) to the PETE membrane’s pore surfaces. Consequently, the pore surfaces became hydrophilic, and oxide precursors easily adsorbed on the pore surfaces. The synthetic method was as follows: (1) distilled water was added to an ethanol solution of zirconium ethoxide containing acetic acid to accelerate the hydrolysis of alkoxide; (2) an oxygen-plasma-treated PETE membrane was immersed in the obtained solution to adsorb zirconia precursors on the membrane surface; (3) the obtained membrane was heated at 120◦ C for 12 h to accelerate the polycondensation reaction (this report explained that a zirconium species generated by the hydrolysis, such as [ZrOx (OR)4−2x ]n , was combined with the PETE membrane by hydrogen bonding, and consequently, the reaction was accelerated); and (4) the obtained membrane was calcined

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at 600◦ C to remove the PETE membrane by thermal decomposition, and zirconia nanotubes were obtained. Using PETE membranes with various pore diameters (50–500 nm) and thicknesses (6 and 12 μm), Chen et al. successfully synthesized nanotubes with outer diameters corresponding to the membrane’s pore diameter and lengths corresponding to the membrane’s thickness. It was demonstrated that the nanotubes’ aspect ratio could be controlled by the membrane’s pore diameter and thickness, and the wall thicknesses could be controlled by the precursor concentration. 9.1.5 Zirconia Nanotubes Synthesized Using a One-Dimensional Assembly Formed by Amphipathic Molecules as a Template Gundiah et al. [13] synthesized zirconia nanotubes using a tripodal cholamidebased hydrogel as a template. The advantages of this method are that various nanotubes containing SiO2 , TiO2 , WO3 , ZnO, ZnSO4 , and BaSO4 , in addition to zirconia can be obtained. Moreover, nanotubes can be synthesized without using alkoxide, which is difficult to handle. The zirconia nanotubes were synthesized by the following method: (1) after dissolving ZrOCl2 into H2 O, ammonia was dropped to obtain a gel; (2) the obtained gel was mixed with a gelator, CH3 COOH, and H2 O to obtain a white gel; (3) the obtained white gel was dried and washed with ethanol; and (4) after the calcination at 500◦ C in an O2 flow, zirconia nanotubes were obtained. The obtained nanotubes’ outer diameters were approximately 25 nm, the inner diameters were approximately 4–7 nm, and the lengths were several hundred nm. The crystal structure was the mixed phase of monoclinic, cubic, and tetragonal. Jung et al. [14] synthesized zirconia nanotubes, using a nanotube-shaped self-assembled structure formed by a steroid derivative as a template. The synthetic method was as follows: (1) a steroid derivative, which was molecularly designed and synthesized by Jung et al., was heated and dissolved into acetonitrile; (2) after cooling down to room temperature, Zr(OBu)4 and water were added to the obtained acetonitrile solution; (3) the mixture was heated at 200◦ C for 2 h, 500◦ C for 2 h under a nitrogen atmosphere, and then kept at 500◦ C under aerobic conditions for 4 h to obtain zirconia nanotubes. Because organic components were removed after depositing the zirconia on the internal and external surfaces of a nanotube-shaped aggregate structure formed by a steroid derivative, double-walled nanotubes (outer diameters, approximately 560 nm; inner diameters, 460 nm; distances between inner and outer nanotubes, 50 nm; and lengths, several μm) were synthesized.

9.2 Ruthenium Oxide Nanotubes 9.2.1 Introduction The use of ruthenium compounds, such as ruthenium oxide and ruthenium oxide hydrate, in electrocatalysts, catalysts for hydrogen production, CO oxidation catalysts, and supercapacitors has been investigated. Because the

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characteristics of ruthenium compounds are greatly affected by their surfaces and structures, an increase in the specific surface due to the nanostructurization and an excellent improvement in the characteristics due to the formation of unique surface structures are expected. Although precious metals, such as ruthenium, are scarce and expensive, it is thought that the nanostructurization can contribute to improving atom utilization in addition to the characteristics of precious metals and in reducing the usage, and consequently, cost production can be reduced. Regarding ruthenium compounds, various studies have been performed to improve their functions by controlling the nanostructures and morphologies. The nanotubular structure is one of the nanostructures, which have been attracting the attention of many researchers, also in ruthenium oxide. In this section, examples of ruthenium oxide nanotubes syntheses and their characteristics are introduced.

9.2.2 Ruthenium Oxide and Ruthenium Oxide Hydrate Nanotubes Synthesized Using Anodic Porous Alumina Membrane Ruthenium oxide (RuO2 ) nanotubes were first reported to be synthesized, using CNTs as templates, by Satishkumar et al. [15] However, in the TEM photograph shown in the paper, because the tube-shaped structure almost collapsed, they were hardly considered to be nanotubes. After that, Min et al. [16] successfully synthesized RuO2 nanotube arrays using anodic porous alumina as a template. The synthesis method is as follows: (1) Anodic porous alumina membrane with a 35 nm pore diameter was formed on a silicon wafer. (2) Carbon was deposited on the internal surfaces of the alumina pores by thermal decomposition of acetylene using N2 as a carrier gas at 600◦ C. (3) After removing the amorphous carbon residue from the surfaces via an ion mill, the alumina was etched using a mixed solution of phosphoric acid and chromic acid, and CNT arrays, in which CNTs with 30–50 nm inner diameters were partially exposed at heights of 70–100 nm, were obtained. (4) By the atomic layer deposition method using a Ru(od)3 /acetic acid n-butyl solution (od = octan-2,4-dionate) as a precursor gas of ruthenium and O2 as a reaction gas, polycrystalline metal Ru with 6 nm thickness was coated on the internal and external surfaces of CNTs. Namely, under the conditions that the pressure was 1 torr, the temperature was 300◦ C, and Ar was used as a carrier gas, a Ru(od)3 /acetic acid n-butyl solution was applied for 2 s, Ar for purge for 3 s, and O2 for 2 s in this order, and these applications were considered to be one cycle. This cycle was repeated 70 times, and metal Ru was deposited on the internal and external surfaces of the CNTs. Because the external surfaces were easier to expose to Ru(od)3 vapor than the internal surfaces, and because the internal surfaces were more difficult than the external surfaces to remove the gaseous by-product that was produced when metal Ru was deposited, metal Ru was more densely deposited on the external surfaces than on the internal surfaces. (5) By heating at 500◦ C in O2 , the CNTs were removed, and RuO2 nanotube arrays were obtained.

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Tan et al. [17] synthesized RuO2 nanotubes using anodic porous alumina as a template, as well as by using the introduction of Ru3 (CO)12 clusters into alumina pores and the transformation of RuO2 by thermal decomposition. The synthetic method was as follows: (1) anodic porous alumina membrane with 20 ± 10 nm pore diameters was immersed in a hexane solution containing Ru3 (CO)12 for 5 min; (2) the obtained anodic porous alumina membrane was dried in the air (at this time, dissociative adsorption of Ru3 (CO)12 clusters took place on the pores’ inner surfaces as Ru(CO)2 L2 (L: a ligand containing oxygen)); (3) the process of immersion and drying was repeated 50 times to introduce the sufficient amount of the precursor into the pores; (4) after removing excess Ru3 (CO)12 , the obtained anodic porous alumina membrane was calcined at 600◦ C in N2 ; (5) by the calcination, oxygen was supplied from the alumina, and RuO2 was synthesized; (6) by immersing in a NaOH solution, alumina was removed; and (7) by repeatedly washing with a low-concentration NaOH solution and distilled water, RuO2 nanotubes were obtained (Fig. 9.6a). The obtained nanotubes’ outer diameters were 25–40 nm, inner diameters were 15–20 nm, and lengths were 200 nm–3 μm. They were tetragonal RuO2 single crystals, and the major axis was along (110). The nanotubes’ lengths were shorter than the lengths of alumina pores (10 μm). This was due to the fragility of the nanotubes and the clogging of the alumina pores with Ru3 (CO)12 during the reaction. The color of the solution in which the nanotubes were dispersed was light green. When the excitation wavelength was set at 200–220 nm, the luminescence was observed in the ultraviolet–visible range (at λmax = 395 and 730 nm) (Fig. 9.6b), this was the first time for RuO2 . Hu et al. [18] synthesized hydrous RuO2 (RuO2 · xH2 O) nanotubular array (Fig. 9.7A, B) using anodic porous alumina as a template by the anodic deposition technique, for next generation supercapacitors. The synthetic method was as follows: (1) anodic porous alumina membrane-coated graphite or Ti substrates were prepared by using RuO2 · xH2 O nanocrystallites and

(a)

(b)

Fig. 9.6. SEM image (a) and emission spectrum (b) of RuO2 nanotubes [17]. (Copyright Wiley-VCH Verlag GmbH & Co. KGaA. Reproduced with permission)

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Fig. 9.7. (A, B) SEM top images of an RuO2 · xH2 O nanotubes arrayed electrode. (C) An ideal design of the electrode material for next generation supercapacitor, RuO2 · xH2 O nanotubular arrayed electrode [18]. (Reprinted with permission from [18]. Copyright 2006 American Chemical Society.)

polyvinylidene difluoride as binders between the membrane and substrates; (2) the substrates were coated with a thick film of epoxy resin with an exposed surface area of 1 cm2 ; (3) then RuO2 ·xH2 O nanotubes were electroplated from RuCl3 +CH3 COONa aqueous solutions at 1.0 V for 10–30 min in the pores; and (4) after deposition, the porous alumina membrane and epoxy resin were removed and the obtained RuO2 ·xH2 O nanotube array was degreased with pure water and finally dried at room temperature under a reduced pressure. The wall thickness of the 200◦ C annealed RuO2 · xH2 O nanotubes is ∼ 40 ± 5 nm which is very desirable for the supercapacitor application, although some particulates adherent to the wall are visible. The outer diameter of nanotubes is about 200 ± 20 nm corresponding to the diameter of anodic porous alumina membrane. RuO2 · xH2 O grains are adherently stacked to form the tubular structure. As schematically shown in Fig. 9.7C, the mesoporous architecture, hydrous nature, and metallic conductivity provide the proton and electron

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“superhighway” for the extremely rapid charge/discharge processes. The nanotube array simultaneously maintained the facility of electrolyte penetration, the ease of proton exchange/diffusion, and the metallic conductivity of crystalline RuO2 , exhibiting unexpectedly ultrahigh power characteristics with its frequency “knee” reaching ca. 4.0–7.8 kHz, 20–40 times better than that of RuO2 single crystalline, arrayed nanorods. The specific power and specific energy of the nanotubes measured at 0.8 V and 4 kHz is equal to 4,320 W kg−1 and 7.5 W h kg−1 , respectively. 9.2.3 Ruthenium Compound Nanotubes Templated by Surfactant Assemblies We also successfully synthesized ruthenium compound nanotubes using surfactant assemblies as templates via the homogeneous precipitation method using urea [19, 20]. Ruthenium chloride n-hydrate, C12 H25 SO3 Na, urea, and water were mixed at a predetermined ratio, and the mixture was heated at 70◦ C without stirring to obtain solids. By washing the solids with distilled water, fibrous substances were obtained. The lengths of the substances were mostly from several μm to tens of μm. The fibrous shape was helical, having regular pitches. They were either hollow or solid particles. The hollow helicoids can be said to be nanotubes (Fig. 9.8a–d). The inner diameters of these nanotubes were mainly tens of nm, and the nanotubes’ holes were also helical. The nanotubes were composed of hexagonal mesostructures consisting of the ruthenium compound phase having a positive charge and the C12 H25 SO− 3

Fig. 9.8. TEM images of the as-grown helical-shaped ruthenium compound nanotubes (a–d), a RuO2 nanotube (e), and metallic ruthenium nanotube (f ) [19]. (Reprinted with permission from [19]. Copyright 2008 American Chemical Society.)

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Fig. 9.9. Schematic representation of a helical ruthenium compound with a hexagonal structure templated by 1-dodecanesulfonate assemblies

phase having a negative charge (Fig. 9.9). The ruthenium compound phase is composed of ruthenium oxyhydroxide, NH3 , and CO2− 3 . When the synthesis was performed with stirring, helical particles scarcely formed, but only irregular-shaped particles were obtained. Therefore, it was demonstrated that helical nanotubes were formed by a self-organizing reaction of an Ru compound and C12 H25 SO− 3 under the static condition (without stirring). By calcination of the ruthenium compound nanotubes at 500◦ C or higher, the organic molecule components were removed by burning, and the ruthenium compound phase was crystallized to the RuO2 phase. Consequently, the helical ruthenium compound nanotubes were transformed into helical RuO2 nanotubes (Fig. 9.8e). These nanotubes consisted of RuO2 nanoparticles. These sizes were 4 and 11 nm when the ruthenium compound nanotubes were calcined at 500 and 700◦ C, respectively. On the other hand, when the ruthenium compound nanotubes were calcined at 700◦ C in a vacuum, the combustiondesorption of organic molecules and the reduction of the Ru compound phase to metal Ru progressed. Consequently, the ruthenium compound nanotubes were transformed into metallic ruthenium nanotubes while maintaining the helical form (Fig. 9.8f). It was confirmed that the ruthenium compound nanotubes were insulators, but RuO2 and Ru nanotubes were electric conductors.

9.3 Conclusion In this chapter, we introduced syntheses and applications of zirconia and ruthenium oxide nanotubes. Zirconia nanotubes will be applied to host materials or materials for sensor. Ruthenium compounds, such as ruthenium oxide and hydrous ruthenium oxide, nanotubes are one of the few conductive substances, although most of ceramics nanotubes are insulator or semiconductor. The study of the ruthenium compounds nanotubes will be progressed centering on the application study based on high conductivity as well as large specific capacitance and catalytic activity peculiar to the ruthenium compounds. In particular, the ruthenium compounds nanotubes are thus expected to be applied to electrode materials for supercapacitor.

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Acknowledgment This work was partially supported by KAKENHI (16685021, 19750172).

References 1. C.N.R. Rao, B.C. Satishkumar, A. Govindaraj, Chem. Commun. 16, 1581 (1997) 117, 118 2. H. Ogihara, M. Sadakane, Y. Nodasaka, W. Ueda, Chem. Mater. 18(21), 4981 (2006) 118, 119 3. Z. Sun, X. Zhang, N. Na, Z. Liu, B. Han, G. An, J. Phys. Chem. B 110(27), 13410 (2006). 119, 120 4. J. Huang, T. Kunitake, J. Am. Chem. Soc. 125(39), 11834 (2003) 120 5. H. Tsuchiya, J.M. Macak, L. Taveira, P. Schmuki, Chem. Phys. Lett. 410(4–6), 188 (2005) 121 6. J. Bao, C. Tie, Z. Xu, Q. Ma, J. Hong, H. Sang, D. Sheng, Adv. Mater. 14(1), 44 (2002) 121, 122, 123 7. J.-Z. Xu, W.-B. Zhao, J.-J. Zhu, G.-X. Li, H.-Y. Chen, J. Colloid Interface Sci. 290, 450 (2005). 121, 122 8. R.E. Cochran, J.-J. Shyue, N.P. Padture, Acta Materialia 55, 3007 (2007) 121, 122, 124 9. S. Hou, C.C. Harrell, L. Trofin, P. Kohli, C.R. Martin, J. Am. Chem. Soc. 126(18), 5674 (2004) 121, 124 10. H. Shin, D.-K. Jeong, J. Lee, M.M. Sung, J. Kim, Adv. Mater. 16, 1197 (2004) 121, 125 11. D. Jeong, J. Lee, H. Shin, J. Lee, J. Kimm, M. Sung, J. Korean Phys. Soc. 45, 1249 (2004) 121, 125 12. H. Chen, Y.A. Elabd, G.R. Palmese, J. Mater. Chem. 17(16), 1592(2007) 121, 125 13. G. Gundiah, S. Mukhopadhyay, U.G. Tumkurkar, A. Govindaraj, U. Maitra, C.N.R. Rao, J. Mater. Chem. 13(9), 2118 (2003) 126 14. J.H. Jung, T. Shimizu, S. Shinkai, J. Mater. Chem. 15(9), 3979 (2005) 126 15. B.C. Satishkumar, A. Govindaraj, M. Nath, C.N.R. Rao, J. Mater. Chem. 10, 2115 (2000) 127 16. Y.-S. Min, E.J. Bae, K.S. Jeong, Y.J. Cho, J.-H. Lee, W.B. Choi, G.-S. Park, Adv. Mater. 15, 1019 (2003). 127 17. H. Tan, E. Ye, W.Y. Fan, Adv. Mater. 18, 619 (2006) 128 18. C.-C. Hu, K.-H. Chang, M.-C. Lin, Y.-T. Wu, Nano Lett. 6, 2690 (2006) 128, 129 19. Y. Inoue, M. Uota, M. Uchigasaki, S. Nishi, T. Torikai, T. Watari, M. Yada, Chem. Mater. 20, 5652 (2008). 130 20. Y. Inoue, S. Ohtsuka, T. Torikai, T. Watari, M. Yada, Crystal Growth & Design in press, doi:10.1021/cg900261s. 130

Index (3-aminopropyl) trimethoxysilane (APS) solution, 122 Anodic porous alumina, 122

Anodization of metal zirconium, 121 Atomic layer deposition (ALD) method, 125

9 Synthesis and Applications of Zirconia 1,10-Decanediylbis(phosphonic acid) (DBPA), 124–125 1-dodecanesulfonate assemblies, 131 Emission spectrum of ruthenium oxide nanotubes, 128 Ethanol detection characteristics of CNT with zirconia, 120 Helical ruthenium compound representation, 131 Magnetic properties of zirconia nanotube arrays containing metal Co, 123 Next generation supercapacitor, design of electrode material, 129 One-dimensional assembly formed by amphipathic molecules as template, 126 OTS-SAMs (octadecyltetrachlorosilane self-assembled monolayers), 125 Oxygen-plasma treated polyester track etched (PETE) membrane as template, 125–126 Polycarbonate membrane, 121 Porous membrane as template, zirconia nanotubes synthesized using, 121–126 Proton exchange/diffusion, 130 Rapid charge/discharge processes, 129–130 Ruthenium compound nanotubes templated by surfactant assemblies, 130–131

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Ruthenium oxide hydrate nanotubes synthesis using anodic porous alumina membrane, 127–130 Ruthenium oxide nanotubes, 126 Ruthenium oxide nanotubes synthesis using anodic porous alumina membrane, 127–130 SEM image ruthenium oxide nanotubes, 128 SEM images of zirconia nanotube arrays containing metal Co, 123 SEM top images of RuO2 xH2 O nanotubes arrayed electrode, 129 Synthetic method, 123–124 TEM image of CNT with zirconia, 120 TEM images of as-grown helical-shaped ruthenium compound nanotubes, 130 TEM images of zirconia nanotube arrays containing metal Co, 123 Tetragonal zirconia nanotubes, 122–123 Track etched membrane of polyester, 121 Ultrasonication, 122 Z-contrast TEM image of isolated TiO2 / ZrO2 coaxial nanotube, 124 Zirconia/CNT nanocomposite, 120 Zirconia nanotube arrays synthesis by anodization of metal zirconium, 121 Zirconia nanotubes, 117 Zirconia nanotubes synthesis using carbon nanotubes or nanofibers as templates, 118–121

10 Conversion of Metal Oxide Nanosheets into Nanotubes Renzhi Ma1 and Takayoshi Sasaki2 1

2

National Institute for Materials Science, International Center for Materials Nanoarchitectonics, Tsukuba, Ibaraki 305-0044, Japan [email protected] National Institute for Materials Science, International Center for Materials Nanoarchitectonics, Tsukuba, Ibaraki 305-0044, Japan [email protected]

Abstract In this chapter, structural relationship and conversion between twodimensional (2D) nanosheets and one-dimensional (1D) nanotubes are reviewed. Nanotubes are spontaneously formed upon exfoliation of certain layered materials with a non-centrosymmetric or particular structure, such as K4 Nb6 O17 and some perovskite-type Ruddlesden−Popper phase K2 [An−1 Bn O3n+1 ](A = Na, Ca, Sr, La; B = Ta, Ti). On the other hand, colloidal centrosymmetric nanosheets represented by titanium oxide, manganese oxide, and calcium niobium oxide can also be successfully converted into their corresponding nanotubes through a simple ion intercalation/deintercalation procedure at ambient temperature. The conversion validates the hypothesis, in which directly rolling a nanosheet yields a nanotube. The close relationship is of fundamental importance in revealing the formation mechanism of nanotubes and may be used to realize a customized synthesis of nanotubes from a wide range of layered materials.

10.1 Nanosheet and Nanotube The coined term “nanosheet” is widely accepted as representing a type of nanomaterial in ultimate 2D anisotropy [1–4], exhibiting molecular thickness (∼1 nm) but submicron- or micron-scale lateral dimensions, which is generally produced by swelling and exfoliation/delamination of a layered host into individual layers in a suitable medium (Fig. 10.1). Nanosheet is typically obtained as a dispersed colloidal suspension. It is well known that some smectite clay minerals undergo spontaneous exfoliation in water. Similar behavior has been artificially achieved for several classes of layered materials such as certain layered oxides, dichalcogenides, and metal phosphates by controlling layerto-layer interactions via soft chemical procedures [1–4]. Graphene, created

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Fig. 10.1. Schematic diagram showing the generation of colloidal nanosheets via exfoliation of layered hosts into single layers

by exfoliating graphite, can be regarded as a new elementary nanosheet with high electrical and thermal conductivity and other peculiar electronic properties [5]. Particularly, layered transition metal oxides (e.g., Cs0.7 Ti1.825 O4 , K0.45 MnO2 , KCa2 Nb3 O10 ), prepared by a solid-state calcination method, can take up a high quantity of bulk organic ammonium cations between the layers after acid exchange. Under appropriate conditions, these layered hosts undergo osmotic swelling and exfoliation into nanosheets [2–4]. Figure 10.2 shows a representative structure of oxide nanosheets equivalent to one host layer. MO6 (M = Ti, Mn, Nb) octahedra are linked via edge- or corner-sharing to produce the host layer, and the thickness is about 0.75, 0.48, 1.44 nm, respectively. On the contrary, the lateral dimensions might be regarded as infinite in comparison with the thickness, e.g., up to several tens of microns. As a result, these oxide nanosheets have some unique characteristics: (1) inorganic

Fig. 10.2. Examples of some oxide nanosheets. (a) Titanium oxide (Ti0.91 O2 ); (b) manganese oxide (MnO2 ); (c) calcium niobium oxide (Ca2 Nb3 O10 ). Bold lines indicate unit cell.

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macroanions bearing negative charge; (2) 2D single crystal, in which constituent elements are still regularly arranged; and (3) ultimate surface area. Using oxide nanosheets with these unique characteristics as building blocks is attracting increasing attention. Research is being conducted to design nanocomposites or functional thin films utilizing these nanosheets [6–11]. In particular, as oxide nanosheet is negatively charged, nanosheets can be flocculated and restacked as wool-like deposits when an electrolyte is added into the colloidal suspension. Electrostatic interaction with various cationic materials, inorganic or organic, can be employed to prepare functional composite materials that have potential use in electrodes, fluorescent materials, and photocatalysts by combining the properties of both the nanosheet and the counter electrolyte [6–9]. Layer-by-layer assembly and the Langmuir–Blodgett method have also been used to build multilayer films based on nanosheets that, for example, demonstrate superior dielectric nature and magneto-optical properties [10, 11]. In addition, exfoliation of layered double hydroxide (LDH), a natural counterpart to cationic clays, yielded positively charged nanosheets: inorganic macrocations [12–14 and references therein]. Well-defined LDH nanosheets have been readily attained by synthesizing large LDH crystals in carbonate form via so-called homogeneous precipitation and subsequent exfoliation of the exchanged nitrate form in formamide [12–14]. A direct implication for the availability of LDH nanosheets is that direct electrostatic assembly of various anionic oxide nanosheets with cationic LDH nanosheets may be artificially realized at the molecular level [15]. Interesting structural correlations between 2D nanosheets and 1D nanotubes have recently been revealed. As described in this book, nanotubes constructed from carbon [16], boron nitride (BN) [17], metal chalcogenides (WS2 , MoS2 ) [18, 19], and various oxides (TiO2 , VOx ) [20–23], have been intensively studied. Generally, nanotubes can be structurally classified into two distinct categories. One is referred to as the Matryoshka doll/nesting type, represented by carbon, BN, and chalcogenide nanotubes [16–19]. The other is referred to as the scroll type reported for TiO2 and VOx nanotubes [20–23]. As schematically shown in Fig. 10.3, in the nesting-type nanotube, one concentric atomic plane is inserted into another. On the other hand, the scroll-type nanotube may be modeled as the rolling and folding up of an atomic plane onto itself. Under high-resolution transmission electron microscopy (HR-TEM), the number of layers on both walls of the hollow tube core is typically the same for a nesting nanotube. In contrast, a scrolled nanotube generally has one layer more or less on one of the sides. This difference in layer number can be effectively used to recognize the structure category of the nanotubes investigated. Also as shown in Fig. 10.3, compared to the nesting-type BN nanotube, the TiO2 nanotube is characteristic of the scroll type with one layer less on the right-hand side. Nevertheless, the category classification of a nanotube is not always defined by the constructing material. For example, even though carbon nanotubes are traditionally well-known to be formed in a nesting structure, it was recently

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Fig. 10.3. Structural models of nanotubes and typical examples. Left: nestingtype/BN nanotube; right: scroll-type/titanium oxide nanotube

discovered that they could also be synthesized as a scroll type through a newly developed process [24]. This indicates that synthetic conditions may affect the structure category to which the resultant nanotubes belong. Nanotubes are usually prepared under special conditions (chemical vapor deposition, hydrothermal synthesis, etc.). Nanotube formation is strongly dependent on synthetic parameters such as temperature, time, surfactants, and templates used. Under certain particular synthetic conditions, both sheet-like crystals and nanotubes have been observed [23]. As these sheet-like objects are naturally curled or bent at the edge, it is plausible that rolling or folding up of a sheet-like object (atomic plane/nanosheet) might generate a scroll-type nanotube, i.e., an artificial 2D to 1D structural transformation. However, the lack of definite data has prevented validation of the hypothetical process, in which directly rolling a nanosheet is intentionally realized and a nanotube is yielded.

10.2 Spontaneous Conversion of Nanosheets into Nanotubes During Exfoliation of Certain Layered Oxides In their attempt to exfoliate K4 Nb6 O17 when treating acid-exchanged K4 Nb6 O17 with tetrabutylammonium ions (TBA+ ), the Domen group [6] and Mallouk group [25] reported that abundant nanotubes were formed instead

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Fig. 10.4. Nanotubes obtained via exfoliation of K4 Nb6 O17 . (Reprinted with permission from [25]. Copyright 2000 American Chemical Society.)

of the expected Nb6 O17 nanosheets (Fig. 10.4a). As-produced nanotubes are usually 15–30 nm in diameter and 0.1–1 μm in length. Furthermore, lateral curling or bending was observed for some nanosheets (Fig. 10.4b), apparently occurring at the initial folding-up stage. The above observations strongly suggest that the nanotube formation might be induced by the rolling up of exfoliated Nb6 O17 nanosheets. The driving force for such spontaneous rolling-up behavior appears to be the relief of built-in strain in an individual Nb6 O17 layer due to its non-centrosymmetric architecture. Host layers of K4 Nb6 O17 (Nb6 O17 nanosheets) are not centrosymmetric in terms of arrangement and filling density of NbO6 octahedra at the top and bottom sides (see Fig. 10.5). As a result, the host material has two distinct interlayer environments. Being piled up in a bulk crystal, potassium ions (K+ ) are intercalated between the sheets. It is considered that, upon exfoliation, the nanosheets would lose their flatness and roll up into a tube. Figure 10.6 is a schematic diagram showing the possible exfoliation and subsequent rolling-up scenario for K4 Nb6 O17 . As a first step, about 80% of the potassium ions between K4 Nb6 O17 layers are substituted by protons via acid exchange. Solid acidity is generated and tetrabutylammonium ions come into the interlayers. High osmotic swelling is thus induced, promoting exfoliation. Taking into account the noncentrosymmetric nature of individual Nb6 O17 sheets, intercalation reactivity

Fig. 10.5. Crystal structure of K4 Nb6 O17 . There are two different interlayer environments.

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Fig. 10.6. Exfoliation of K4 Nb6 O17 and nanotube formation. (Adapted with permission from [25]. Copyright 2000 American Chemical Society.)

for tetrabutylammonium ions is supposed to differ in an adjacent double-layer pair. Sheet pairs with double-layer thickness were thus generated, reflecting the asymmetry at the top and bottom sides of K4 Nb6 O17 . These sheet pairs sandwich protons and potassium ions between the layers, which stabilizes the structure and maintains flatness. On the other hand, when exfoliation is further advanced, each in a pair is separated. Stress, derived from the asymmetry of a single-layer sheet, might cause curling/rolling up from the side with a higher filling density of NbO6 octahedra, eventually forming a nanotube. It is noteworthy that the formation of Nb6 O17 nanotubes may be influenced by experimental parameters, such as the concentration and pH of Nb6 O17 nanosheet colloidal solution, ultrasonic treatment [25]. The balance between nanotube (rolling up) and nanosheet (unraveling) appears to be subtly decided by the history of conditions, inferring a comparatively small energy difference between nanosheets and nanotubes. In addition, some protonated perovskite-type Ruddlesden–Popper phase H2 [An−1 Bn O3n+1 ] (A = Na, Ca, Sr, La; B = Ta, Ti) were also found to yield nanotubes upon exfoliation [26]. Unlike K4 Nb6 O17 , these host compounds have a centrosymmetric lamellar structure. It is difficult to explain why spontaneous formation of nanotubes also occurs with these compounds, although possible clues and speculations have been pointed out. For example, calcium and sodium cations selectively occupy the A sites in H2 CaNaTa3 O10 , which might become a source of internal stress and induce the rolling-up behavior as

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illustrated for K4 Nb6 O17 . The influence of chemical exfoliation reagents and the cooperative distortion of MO6 octahedra in H2 SrTa2 O7 and H2 La2 Ti3 O10 have also been speculated.

10.3 Conversion of Nanosheets into Nanotubes via a Designed Soft Chemical Procedure The previous section described nanotube formation induced by structural features of the exfoliated nanosheets, i.e., a spontaneous process. However, the spontaneous formation of nanotubes was found to be extremely limited. Conversely, a large variety of lamellar solids has been successfully exfoliated/delaminated into single-layer nanosheets via a soft chemical procedure, as shown in Fig. 10.2. These nanosheets exhibit top and bottom symmetry, namely centrosymmetry. It has become clear that these nanosheets can also be converted into nanotubes when a suitable soft chemical process is applied [27]. The conversion of centrosymmetric oxide nanosheets into nanotubes may be achieved by the following procedure. The colloidal nanosheets undergo flocculation/restacking by changing ionic strength. Typically, when the colloidal suspension is poured into a concentrated NaOH aqueous solution (e.g., 1 mol dm−3 ), wool-like precipitate is instantly yielded. The resulting material is filtered and rinsed with copious distilled water, and then shaken in pure water. A well-dispersed suspension is generally achieved after extended (e.g., 36 h) continuous shaking. The final product, which is collected from centrifugation and air-dried at room temperature, contains some nanotubes. Figure 10.7a depicts a typical TEM image of the starting Ti0.91 O2 nanosheets. The nanosheet exhibits a uniform and faint contrast, reflecting unilamellar thickness. The flocculation of colloidal nanosheets in NaOH solution leads to a turbostratic aggregate (Fig. 10.7b). X-ray diffraction characterizations of the flocculated products indicate the absence of three-dimensional order. This suggests that the flocculation results in turbostratic restacking of single-layer nanosheets, which accommodate Na ions and water molecules between the neighboring sheets. Based on microscopic characterization, turbostratic stacks of 5–20 nanosheets appear dominant in the flocculated product. After filtering and continuous shaking in water, restacked nanosheets are partially delaminated again and become well dispersed. Most of the dispersed nanosheets exhibit lateral curling. Some needle-shaped objects are observed by scanning electron microscopy (SEM) (Fig. 10.7c). TEM observation further demonstrates that the needle-shaped crystallites are nanotubes with an apparent hollow core (Fig. 10.7d, e). The diameter is found to remain constant for different tilting angles (0–30◦ ) along the tube axis, identifying a true tubular nature rather than wrinkling of the nanosheet. HR-TEM images also indicate that the tube walls are usually very thin, comprising only 3–6 layers (Fig. 10.7e).

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Fig. 10.7. Converted titanium oxide (Ti0.91 O2 ) nanotube. (a) 2D nanosheet; (b) flocculated aggregate; (c) SEM image of a needle-like object; (d) hollow nanotube (TEM image); (e) HR-TEM image of a nanotube

Similarly, as-converted manganese oxide and calcium niobium oxide nanotubes are displayed in Fig. 10.8. All three types of nanotubes have an outer diameter of 15–60 nm and a wall thickness of 3–6 layers. The dimensions imply that the nanotube may be constructed by wrapping a single 2D nanosheet which is 140–1,130 nm in lateral size, consistent with the lateral size of the starting nanosheets (0.1–1 μm). This fact indicates that there is no fracture or breakage of the starting nanosheets during the rolling up. Based on microscopy statistics, approximately 20% of titanium oxide and manganese oxide nanosheets are transformed into nanotubes. As niobium oxide nanosheets are thicker (1.44 nm) than titanium oxide (0.75 nm) and manganese oxide (0.48 nm) nanosheets and are supposed to be more rigid, their conversion into nanotubes brings a lower yield (∼10%).

Fig. 10.8. (a) Manganese oxide (MnO2 ) nanotube; (b) calcium niobium oxide (Ca2 Nb3 O10 ) nanotube

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Fig. 10.9. Schematic diagram illustrating a possible mechanism for the conversion of oxide nanosheets (Ti0.91 O2 , MnO2 , Ca2 Nb3 O10 ) into nanotubes

The formation of 1D nanotubes from 2D nanosheets under this designed protocol may be explained as schematically illustrated in Fig. 10.9. The starting unilamellar nanosheets are stabilized in a suspension with tetrabutylammonium ions. During the flocculation using NaOH solution, Na ions are substituted for tetrabutylammonium ions. Colloidal 2D nanosheets become unstable and are restacked incorporating Na ions. The Na ions are intercalated into the intersheet gallery and play a role in pinning up adjacent sheets. This results in an aggregate of fine crystallites of 5–20 turbostratic layered nanosheets. When shaken in water, the intersheet Na ions are gradually deintercalated/extracted. This deintercalation process is realized through the exchange with protons and water molecules (H3 O+ ), causing a variance in bonding characteristics between adjacent sheets. During shaking, the intersheet spacing might also be expanded due to the increase in water content. The electrostatic interaction between neighboring sheets is thus significantly reduced. As a result, the turbostratic restacked nanosheets become very loosely bonded and are granted a tendency to delaminate into individual nanosheets again. In pure water, the solution conditions are not perfectly suitable for attaining a well-dispersed colloid and individual nanosheets tend to flocculate and restack. However, a dilute nanosheet concentration makes it difficult to find counterparts to restack, and the nanosheet flocculates within itself to gain stabilization energy. In other words, it needs to be folded. If some particular optimum geometrical requirement is fulfilled, nanotube formation takes place. In a major case, the nanosheets form irregularly shaped objects. It might be reasonable to suggest that the deintercalation of Na ions would initially start at the edge sites in the turbostratic layered nanosheets. The reduction of electrostatic interaction therefore starts at the lateral edges of the nanosheets. Due to the reduced interaction with underlying layered sheets, the lateral edges of the topmost sheet might gradually curl up, similar to a thin film free from the underlying

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substrate [28]. As the edge curling-up process advances, a topmost sheet might be completely peeled off and rolled up into a multilayer nanotube. The above conversion process is not related to the type of structure a nanosheet might have. In a sense, it resembles a more general natural phenomenon, such as that seen when wood is planed and the shavings roll up at the edge once they are planed away from the wood. Utilizing this simple but important technique, it is possible to convert any type of single-layer nanosheets into nanotubes from various layered host materials.

10.4 Summary A close structural correlation was found between two types of nanomaterials which differ considerably in morphology, i.e., 2D nanosheet and 1D nanotube. The relationship is of fundamental importance in revealing the formation mechanism of nanotubes. On the other hand, in spite of a yield concern, the rolling-up process itself might be regarded as a new approach to generating nanotubes and will be useful for widening the scope of application of nanosheets. For example, H+ /K4 Nb6 O17 nanotubes are reported to exhibit high catalyst activity in the photolysis of water [25]. C3 F7 –Azo+ /K4 Nb6 O17 hybrid nanotubes show interesting photo-response property [29]. Titanium oxide nanotubes are being studied for ion exchange activity and functional thin film fabrication [30, 31]. Further striking developments are expected.

References 1. A.J. Jacobson, Mater. Sci. Forum 152, 1–12 (1994) 135 2. T. Sasaki, M. Watanabe, H. Hashizume, H. Yamada, H. Nakazawa, J. Am. Chem. Soc. 118, 8329–8335 (1996) 135, 136 3. Y. Omomo, T. Sasaki, L. Wang, M. Watanabe, J. Am. Chem. Soc. 125, 3568–3575 (2003) 135, 136 4. Y. Ebina, T. Sasaki, M. Harada, M. Watanabe, Chem. Mater. 14, 4390–4395 (2002) 135, 136 5. K.S. Novoselov, A.K. Geim, S.V. Morozov, D. Jiang, Y. Zhang, S.V. Dubonos, I.V. Gigorieva, A.A. Firsov, Science 306, 666–669 (2004) 136 6. R. Abe, K. Shinohara, A. Tanaka, M. Hara, J.N. Kondo, K. Domen, J. Mater. Res. 13, 861–865 (1998) 137, 138 7. T. Sasaki, Y. Ebina, T. Tanaka, M. Harada, M. Watanabe, G. Decher, Chem. Mater. 13, 4661–4667 (2001) 137 8. H. Xin, R. Ma, L. Wang, Y. Ebina, K. Takada, T. Sasaki, Appl. Phys. Lett. 85, 4187–4189 (2004) 137 9. K. Akatsuka, Y. Ebina, M. Muramatsu, T. Sato, H. Hester, D. Kumaresan, R.H. Schmehl, T. Sasaki, M.A. Haga, Langmuir 23, 6730–6736 (2007) 137 10. M. Osada, Y. Ebina, K. Takada, T. Sasaki, Adv. Mater. 18, 295–299 (2006) 137 11. M. Osada, Y. Ebina, H. Funakubo, S. Yokoyama, T. Kiguchi, K. Takada, T. Sasaki, Adv. Mater. 18, 1023–1027 (2006) 137 12. L. Li, R. Ma, Y. Ebina, N. Iyi, T. Sasaki, Chem. Mater. 17, 4386–4391 (2005) 137

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13. Z. Liu, R. Ma, M. Osada, N. Iyi, Y. Ebina, K. Takada, T. Sasaki, J. Am. Chem. Soc. 128, 4872–4880 (2006) 137 14. R. Ma, Z. Liu, L. Li, N. Iyi, T. Sasaki, J. Mater. Chem. 16, 3809–3813 (2006) 137 15. L. Li, R. Ma, Y. Ebina, K. Fukuda, K. Takada, T. Sasaki, J. Am. Chem. Soc. 129, 8000–8007 (2007) 137 16. S. Iijima, Nature 354, 56–58 (1991) 137 17. N.G. Chopra, R.J. Luyken, K. Cherrey, V.H. Crespi, M.L. Cohen, S.G. Louie, A. Zettl, Science 269, 966–967 (1995) 137 18. R. Tenne, L. Margulis, M. Genut, G. Hodes, Nature 360, 444–446 (1992) 137 19. Y. Feldman, E. Wasserman, D.J. Srolovitz, R. Tenne, Science 267, 222–225 (1995) 137 20. M.E. Spahr, P. Bitterli, R. Nesper, M. Muller, F. Krumeich, H.U. Nissen, Angew. Chem. Int. Ed. 37, 1263–1265 (1998) 137 21. T. Kasuga, M. Hiramatsu, A. Hosono, T. Sekino, K. Niihara, Adv. Mater. 11, 1307–1311(1999) 137 22. G.R. Patzke, F. Krumeich, R. Nesper, Angew. Chem. Int. Ed. 41, 2446–2461 (2002) 137 23. X. Chen, X.M. Sun, Y.D. Li, Inorg. Chem. 41, 4524–4530 (2002) 137, 138 24. L.M. Viculis, J.J. Mack, R.B. Kaner, Science 299, 1361 (2003) 138 25. G.B. Saupe, C.C. Waraksa, H. Kim, Y.J. Han, D.M. Kaschak, D.M. Skinner, T.E. Mallouk, Chem. Mater. 12, 1556–1562 (2000) 138, 139, 140, 144 26. R.E. Schaak, T.E. Mallouk, Chem. Mater. 12, 3427–3434 (2000) 140 27. R. Ma, Y. Bando, T. Sasaki, J. Phys. Chem. B 108, 2115–2119 (2004) 141 28. O.G. Schmidt, K. Eberl, Nature 410, 168 (2001) 144 29. Z.W. Tong, S. Takagi, T. Shimada, H. Tachibana, H. Inoue, J. Am. Chem. Soc. 128, 684–685 (2006) 144 30. R. Ma, T. Sasaki, Y. Bando, Chem. Commun. 948–950 (2005) 144 31. R. Ma, T. Sasaki, Y. Bando, J. Am. Chem. Soc. 126, 10382–10388 (2004) 144

Index Calcium niobium oxide nanosheets, 136 Calcium niobium oxide nanotube, 142 Colloidal nanosheets generation, 136 Conversion process, 144 Deintercalation process, 143 Designed soft chemical procedure, 141–144 Edge curling-up process, 143–144 Exfoliating graphite, 136 Flocculation of colloidal nanosheets, 141–143 Graphene, 135–136

Langmuir-Blodgett method, 137 Layer to-layer interactions, 135 Layer-by-layer assembly, 137 Layered double hydroxide (LDH) nanosheets, 137 Layered oxides exfoliation, 138–140 Manganese oxide (MnO2 ) nanotube, 142 Manganese oxide (MnO2 ) nanosheets, 136 Matryoshka doll/nesting type, 137 Nanosheet, 135 Nesting-type nanotube, 137

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Osmotic swelling, 136 Oxide nanosheets, 136 Oxide nanosheets conversion to nanotubes mechanism, 142–143

Sheet-like crystals and nanotubes, 138 Soft chemical procedures, 135 Solid-state calcination method, 136 Structural models of nanotubes, 138

Perovskite-type Ruddlesden-Popper phase, 135, 140

Titanium oxide nanosheets, 136 Turbostratic restacked nanosheets, 143

Scroll-type nanotube, 138

Well-dispersed colloid, 143

11 Synthesis and Applications of Mixed Oxide Nanotubes Hitoshi Ogihara1 , Masahiro Sadakane2 , and Wataru Ueda3 1

2

3

Department of Chemistry and Materials Science, Tokyo Institute of Technology, Meguro-ku, Tokyo 152-8552, Japan [email protected] Catalysis Research Center, Hokkaido University, Sapporo, Hokkaido 001-002, Japan [email protected] Catalysis Research Center, Hokkaido University, Sapporo, Hokkaido 001-002, Japan [email protected]

Abstract Metal oxide nanotube is one of the nanostructured materials. Templates method, which is the technique in which templates are covered with metal oxides and the templates were removed to form metal oxide nanotubes, is a typical synthesis method for metal oxide nanotubes. In this section, synthesis of mixed oxide nanotubes using carbon nanofibers (CNFs) as templates is described. Because CNFs with various shapes were used as templates, oxide nanotubes with various shapes such as straight and helical were formed. Successive adsorption of metal oxide precursors on CNFs resulted in the formation of mixed oxide nanotubes with specific composition. In addition, a nano-macrostructured material, silica fiber-immobilized nanofibrous LaMnO3 , was fabricated using this template process. In propane oxidation, the nano-macrostructured material showed superior activity to the conventional powder catalysts.

11.1 Conventional Fabrication Processes for Mixed Oxide Nanotubes Oxides containing more than two metal elements in their lattice structure are called mixed oxides. Perovskite-type compounds and ferrites are well known as typical mixed oxides. Synthesis and applications of mixed oxides have been vigorously investigated because they have a lot of attractive properties, such as ferroelectricity, superconductivity, ionic conduction, magnetism, and catalytic activity. By the way, since the discovery of carbon nanotubes, the nanotechnology has drastically developed and is expected to be the future important technology. Many researchers have attempted to apply the nanotechnology for various T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 147–158 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 11

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fields (e.g., medical care, biotechnology, electronics, and environmental science). One of the reasons why the nanotechnology has drawn great attention is that physical and chemical properties of nanoscale materials are different from those of bulk materials or molecules. Therefore, using nanoscale materials, it would be possible to achieve breakthrough on the issues that have been difficult to resolve through conventional processes. From these viewpoints, synthesis and applications of nanostructured materials are challenging research fields. Nanoscale materials are classified into nanotubes, nanowires, nanoparticles, and so on by their shapes. In this section, synthesis and applications of oxide nanotubes, especially mixed oxide nanotubes, will be focused on. Most of the mixed oxide nanotubes have been synthesized by template method or hydrothermal method. Hydrothermal method is a technique for crystal growth from high-temperature aqueous solutions at high vapor pressures, producing various types of crystals including oxides. Nanotubular materials can be synthesized by choosing appropriate conditions, and they can produce not only mono-oxide nanotubes but also mixed oxide nanotubes [1, 2]. However, synthesis of nanotubes through the hydrothermal method has not been entirely clarified, so that the design synthesis of desired oxide nanotubes through the hydrothermal process is difficult, leading to the limitation on the variety of formed mixed oxide nanotubes. Oxide compounds are often obtained from the decomposition of metal precursors under high temperature in the presence of oxygen, but it is impossible to control the shapes of formed oxides because oxides easily sinter under high-temperature atmosphere. In order to introduce nanostructures into the oxides, templates have been frequently used (template method). The strategy of template method is simple, that is, the coating of nanofibrous templates (e.g., nanowires) with oxides and the removal of the template bring about the formation of nanotubes. In fact, various oxide nanotubes have been synthesized by the template method. The sol–gel method is widely used to produce metal oxides and has often been employed to coat templates in the template method. The sol–gel method involves a hydrolysis reaction and a polymerization reaction of metal precursors in liquid phase. But, given the sol–gel reaction mechanism, metal oxides presumably form throughout the solution, not only on the template surface. Thus, in order to deposit the metal oxides on the template surfaces selectively, one needs to exploit the interactions between the templates and the metal oxides, such as electrostatic or hydrogen bonding interactions. For example, negatively charged silica species, formed through the hydrolysis of tetraethyl orthosilicate, could be adsorbed on positively charged templates, leading to the formation of SiO2 nanotubes [3–5]. Another selective deposition process was demonstrated by van Bommel and Shinkai [6]. The catalysts that promote the hydrolysis and polymerization reaction were deposited on the template surfaces to enable selective coating of the templates because the formation of metal oxide occurred only near the template surface. The sol–gel

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method is certainly an effective method for coating templates, but it has some shortcomings. The most serious is that the constituent elements of metal oxide nanotubes are limited. Metal alkoxides are the most common starting reagents for the sol–gel method, but the production of transition metal alkoxides involves complex, costly processes. Thus, it is difficult to fabricate transition metal oxide nanotubes using the sol–gel method. This has created a demand for the development of a fabrication process of metal oxide nanotubes from readily available precursors. In addition, mixed oxides with certain composition hardly form by the sol–gel method because deposition rate of oxides depends on the type of precursors. As a result, simultaneous deposition of more than one oxide scarcely occurs. In fact, the template method in liquid phase has produced a variety of mono-oxide nanotubes, whereas few mixed oxide nanotubes have been obtained by such processes. In the case of mixed oxide nanotubes, BaTiO3 , PbTiO3 , La0.6 Sr0.4 CoO3 , CoFe2 O4 nanotubes were fabricated through the introduction of gels of mixed oxides into cylindrical nano-pores of anodic aluminum oxide thin films, followed by heating to form mixed oxide phase and removal of the templates [7–10]. However, preparing the gel composed of several metal elements and filling high-viscosity gel within nano-pores are not easy processes.

11.2 Synthesis of Mixed Oxide Nanotubes Using Carbon Nanofibers as Templates Carbon nanofibers (CNFs) are the nanoscale fibrous carbonaceous materials. CNFs are synthesized through the decomposition of substances containing C atoms (e.g., hydrocarbon, carbon monoxide, and so on) over metal catalysts. In general, these synthesis processes are called chemical vapor deposition (CVD) process. One of the characteristics of CNFs is that their shapes can be controlled by CVD conditions [11]. Changing CVD conditions such as temperature and type of raw materials and catalysts, the various shapes of CNFs (e.g., straight, twisted, helical, and branched) have been synthesized. If CNFs can be used as templates for the synthesis of oxide nanotubes, oxide nanotubes with various shapes would be formed. From these viewpoints, we examined the synthesis of oxide nanotubes by using CNFs with various shapes, so that straight and helical nanotubes which reflected the shapes of CNF templates were obtained (as shown in Fig. 11.1) [12]. The advantage of CNF templates to the conventional templates is not only the variety of shapes but also their characteristic coating procedures, that is, metal oxides deposit on CNF templates without sol–gel method. The deposition process of metal oxides on CNF templates is as follows. CNF templates were placed in a suction filtering unit, and metal precursor (e.g., metal nitrate, metal chloride, and metal alkoxide) solution diluted with ethanol was dropped onto the templates. Immediately, the precursor solution infiltrated

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Fig. 11.1. TEM images of (a) straight CNFs, (b) ZrO2 nanotube synthesized using straight CNFs, (c) helical CNFs, (d) ZrO2 nanotubes synthesized using helical CNFs

into the porosity of the fibrous structure. The excess solution was removed by filtration. The obtained sample was dried to remove the ethanol in the pores of the fibrous structure. The metal precursors that were left behind adsorbed onto the template surfaces. In the case of metal alkoxides, they transformed into metal oxides through hydrolysis with water vapor in air. On the other hand, the adsorbed metal nitrates transformed into metal oxides through the subsequent heat treatment at 573 K. The above process was repeated to obtain CNFs covered with oxide layers. Finally, the CNF templates were removed by calcination in air at 773–923 K (C (CNF) + O2 → CO2 ), leading to the formation of metal oxide nanotubes. In this way, CNFs are covered with metal oxides without sol–gel method. Since this process allows the utilization of a variety of metal precursors as raw materials for nanotube synthesis, various metal oxide nanotubes (e.g., SiO2 , Al2 O3 , ZrO2 , Fe2 O3 , NiO, Co3 O4 ) can be obtained. In our method, the adsorption of metal precursors on CNF templates is the key process for the coating of templates. If adsorption rate for the different metal precursors is the same, it is expected that CNF templates would be covered with more than two metal precursors. So, we examined the synthesis of mixed oxide nanotubes by using raw solutions containing two metal precursors [13]. TEM images and XRD patterns of the resulting materials showed the formation

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Fig. 11.2. SEM and TEM images of (a, b) NiFe2 O4 , (c) LaMnO3 , (d) CoFe2 O4 nanotubes

of mixed oxide nanotubes such as NiFe2 O4 , CoFe2 O4 , LaMnO3 , SiO2 –Al2 O3 , and SiO2 –TiO2 (Fig. 11.2). Thus, one can conclude that the immersion of CNFs into the precursor solutions results in the simultaneous deposition of two oxides without precise control of reaction conditions or special equipments. We assume the coating mechanism as follows. The ethanol solution of metal precursors penetrates the CNF templates, filling the pores of the fibrous structure. During the drying process, metal precursors adsorb on the functional groups present on CNF surfaces. These metal precursors are transformed into metal oxides by hydrolysis or thermal decomposition, so that the CNF templates become coated with metal oxides. We consider that the functional groups present on CNF surfaces would play an important role for coatings because CNFs with less surface functional groups were not coated uniformly. However, after the second coating cycle, there must be no functional groups left on the templates because the CNF template surfaces are covered with thin metal oxide layers. Nevertheless, metal oxides continue to deposit on the templates after the second cycle. By the way, a surface sol–gel process for producing ultrathin films has been reported, where film growth is achieved by repeated saturation adsorption of alkoxides and subsequent regeneration of a uniform hydroxyl surface [14]. Alkoxides adsorb on the surface hydroxyl groups and are hydrolyzed. The oxide surfaces formed have hydroxyl groups so that repeated chemisorption can occur. In this surface sol–gel process, the hydroxyl groups that are regenerated on the metal oxide surface play an important role for the synthesis of the metal oxide film. We assume that

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a similar coating mechanism is at play in our case. During the drying process in the first cycle, metal precursors accumulate on the CNF surface because functional groups on CNFs act as adsorption sites. After the second cycle, the hydroxyl groups reproduced on the metal oxide layer surface act as adsorption sites for the metal precursors.

11.3 Synthesis of Macro-Nanostructured Materials Using Template Method In the chemical engineering field, macrostructured materials (e.g., monolithic, fibrous, and foaming materials) have various advantages in the pressure drop, mass/heat transfer, contacting efficiency, and separation process; therefore, the development of efficient macrostructured reaction system is demanded. Since macrostructured materials are almost inert for the chemical reactions, their surfaces must be modified with catalysts to use chemical reactions. If nanostructured materials with high catalytic activity could immobilize on the macrostructured materials, they would be the attractive macro-nanostructured catalytic system that shows characteristics of both nanostructured materials and macrostructured materials. Taking account for the growth mechanism for CNFs, CNFs could be immobilized on the macrostructured materials if metal catalysts which catalyze the CNF growth are present on macrostructured materials. Up to now, CNTs and CNFs have been immobilized on macrostructured materials such as cordierite monolith [15], Ni foam [16], and graphite felt [17–19]. CNFs immobilized on graphite felt were successfully used as a support for a high loaded Ir catalyst in the decomposition of hydrazine [17]. In addition, CNFs immobilized on graphite felt showed stable performance to oxidative dehydrogenation of ethyl benzene [19]. However, direct utilization of CNTs/CNFs for catalytic reaction systems has been regrettably limited because carbonaceous materials are primarily inert for most of the chemical reactions and are easily decomposed into gaseous compounds under oxidative or reductive atmosphere at high temperature [20]. In contrast, metal oxides are effective catalysts for a lot of heterogeneous catalytic reactions, and most of them are stable under the severe conditions. Therefore, we demonstrate the immobilization of nanofibrous metal oxides on macrostructured materials, combining CVD process and template method [21]. As mentioned above, one of the notable features is that CNFs can grow on the various macrostructured materials through CVD process. Using this feature of CNF growth, we attempted the immobilization of nanofibrous metal oxide on the macrostructure materials. Figure 11.3 shows a schematic representation of the procedure for fabrication of nanofibrous metal oxides immobilized on macrostructured materials. First, a small amount of Ni particles which catalyze CNF growth are attached

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Fig. 11.3. Fabrication process of nanofibrous metal oxides immobilized on microfibers

on the macrostructured materials. Contact of methane over macrostructured materials-supported Ni brings the growth of CNFs on macrostructured materials. Using these immobilized CNFs as templates for metal oxide nanotubes/nanofibers synthesis, development of macrostructured materialsimmobilized metal oxide nanotubes/nanofibers was examined. We chose silica fibers as macrostructured materials and LaMnO3 as metal oxides. The typical morphology of the silica fibers is straight, and their average diameter is ca. 5 μm (Fig. 11.4a). After CVD process, the weight of samples increased twofold. In addition, CVD process resulted in the drastic color change of samples. White silica fibers turned into black after CVD (insets of Figs. 11.4a and b), indicating the growth of CNFs over the silica fibers. To analyze the CNF growth, SEM images of the resulting samples were measured. The SEM image showed that a large amount of curved CNFs with ca. 50–100 nm of diameter covered over silica fibers (Figs. 11.4b and c). Subsequently, applying these CNFs as templates, the immobilization of LaMnO3 on silica fibers was demonstrated. A typical SEM image of prepared silica fibers-immobilized LaMnO3 is shown in Fig. 11.4d. It is obvious that the diameter of formed materials is much larger than that of the original silica fibers. This increase in the diameter clearly suggests that a thick layer of LaMnO3 was formed over silica fibers (the elemental analysis showed that no carbon atom (i.e., CNFs) was present in the samples). A high-resolution SEM image (Fig. 11.4e) shows that formed LaMnO3 is a nanofibrous shape and they are net-like structure. The detailed structure of the coating layer was examined by their cross-sectional images. Figure 11.5 shows the cross-sectional TEM image. In this TEM image, the absence of most of silica fibers at the inside of LaMnO3 layers is due to the breakage in silica fibers through the cutting by an ultramicrotome. LaMnO3 networks with 3–6 μm in thickness are uniformly covered around silica fibers, and the network structure becomes thinner on the outer part of them. This can be explained by the structure of CNF template

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Fig. 11.4. SEM images of (a) silica fibers (b, c) silica fibers-immobilized CNFs, (d, e) silica fibers-immobilized nanofibrous LaMnO3 . Insets show the micrographs of obtained samples

layers. As the CNFs grow away from silica fibers, the spaces where CNFs can grow become larger. Eventually, the outer layer of CNFs would be thin. Since the CNF layers were used as templates, formed LaMnO3 layers reflected the structure of CNF layers.

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Fig. 11.5. A cross-sectional TEM image of silica fibers-immobilized nanofibrous LaMnO3

11.4 Application of Macro-Nanostructured Materials to Catalytic Systems Next, the catalytic performance of silica fibers-immobilized LaMnO3 was examined [21]. Since LaMnO3 is well known as an effective catalyst for oxidation reactions, the oxidation of propane which is a model molecule of volatile organic compounds (VOC) was tested by using the silica fibersimmobilized LaMnO3 . As comparative catalysts, LaMnO3 powders with high catalytic activity were prepared by citrate method reported previously [22]. Figure 11.6 shows the propane conversion as a function of reaction temperature during propane oxidation over the silica fibers-immobilized LaMnO3 and LaMnO3 powder catalysts. Obviously, propane conversions over the silica fibers-immobilized LaMnO3 are larger than those over LaMnO3 powders. The propane conversions are 27.3% for silica fibers-immobilized nanofibrous LaMnO3 (579 K) and 18.0% for LaMnO3 powders (576 K), indicating that silica fibers-immobilized nanofibrous LaMnO3 showed ca. 1.5 times higher activity than LaMnO3 powders. Taking into account that the amount of LaMnO3 in both the catalysts was adjusted to be same, one can conclude that LaMnO3 immobilized over silica fibers has higher catalytic activity than LaMnO3 powders. This excellent catalytic activity can be explained on the

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Fig. 11.6. Propane oxidation over silica fibers-immobilized nanofibrous LaMnO3 (•) and LaMnO3 powder catalysts ()

basis of the difference of their specific surface areas. The specific surface areas estimated with nitrogen adsorption are 36 m2 /g for LaMnO3 immobilized over silica fibers and 26 m2 /g for LaMnO3 powders. It is easy to understand that LaMnO3 with higher surface areas shows higher catalytic activity. Citrate method is a typical synthesis process for perovskite-type oxide nanoparticles with high surface area (i.e., high catalytic activity). Therefore, the results in Fig. 11.6 strongly suggest that nanofibrous LaMnO3 catalysts with extreme high catalytic activity can be immobilized on silica fibers. That is to say, it is considered that they are effective nano-macrostructured catalyst systems.

11.5 Summary We focused on the synthesis of mixed oxide nanotubes by template method and their applications as macrostructured catalyst system functionalized with nanoscale fibrous mixed oxides. CNFs are coated with mixed oxide by the simple accumulation of precursors on CNFs. The formation of various types of nanotubes indicates that such synthesis method would be suitable for the preparation of mixed oxide nanotubes. In addition, nanofibrous LaMnO3 immobilized on microfibrous silica which is structured at nano- and macroscales could be synthesized. This macro-nanostructured material showed high catalytic activity due to their high surface area. Applying these synthesis and immobilization methods of mixed oxide nanotubes, it is expected to fabricate novel functional materials.

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References 1. Y. Mao, S. Banerjeea, S.S. Wong, Chem. Commun., 408 (2003) 148 2. T.-J. Park, Y. Mao, S.S. Wong, Chem. Commun., 2078 (2004) 148 3. K.J.C. van Bommel, A. Friggeri, S. Shinkai, Angew. Chem. Int. Ed. 42, 980 (2003) 148 4. Q. Ji, R. Iwaura, M. Kogiiso, J.H. Jung, K. Yoshida, T. Shimizu, Chem. Mater. 16, 250 (2004) 148 5. T. Seeger, Ph. Redlich, N. Grobert, M. Terrones, D.R.M. Walton, H.W. Kroto, M. R¨ uhel, Chem. Phys. Lett. 339, 41 (2001) 148 6. K.J.C. van Bommel, S. Shinkai, Langmuir 18, 4544(2002) 148 7. B.A. Hernandez, K.-S. Chang, E.R. Fisher, P.K. Dorhout, Chem. Mater. 14, 480 (2002) 149 8. G.B. Ji, H.L. Su, S.L. Tang, Y.W. Du, B.L. Xu, Chem. Lett. 34, 86 (2005) 149 9. J. Wang, A. Manivannan, N. Wu, Thin Solid Films 517, 582 (2008) 149 10. Y. Xu, J. Wei, J. Yao, J. Fu, D. Xue, Mater. Lett. 62, 1403 (2008) 149 11. K.P. de Jong, J.W. Geus, Catal. Rev. Sci. Eng. 42, 481 (2000) 149 12. H. Ogihara, M. Sadakane, Y. Nodasaka, W. Ueda, Chem. Mater. 18, 4981 (2006) 149 13. H. Ogihara, M. Sadakane, Y. Nodasaka, W. Ueda, Chem. Lett. 36, 258 (2007) 150 14. I. Ichinose, H. Senzu, T. Kunitake, Chem. Mater. 9, 1296 (1997) 151 15. E. Garc´ıa-Bordej´e, I. Kvande, D. Chen, M. Rønning, Adv. Mater. 18, 1589 (2006) 152 16. N.A. Jarrah, F. Li, J.G. van Ommen, L. Leffers, J. Mater. Chem. 15, 1946 (2005) 152 17. R. Vieira, C. Pham-Huu, M.J. Ledoux, Chem. Commun. 954 (2002) 152 18. F. Cesano, S. Bertarione, D. Scarano, A. Zecchina, Chem. Mater. 17, 5119 (2005) 152 19. J.J. Delgado, D.S. Su, G. Rebmann, N. Keller, A. Gajovic, R. Schl¨ ogl, J. Catal. 244, 126 (2006) 152 20. S. Takenaka, E. Kato, Y. Tomikubo, K. Otsuka, J. Catal. 219, 176 (2003) 152 21. H. Ogihara, M. Sadakane, Q. Wu, Y. Nodasaka, W. Ueda, Chem. Commun. 39, 4047 (2007) 152, 155 22. N.A. Merino, B.P. Barbero, P. Grange, L.E. Cad´ us, J. Catal. 231, 232 (2005) 155

Index Alkoxides, 151 Catalytic systems, macro-nanostructured materials application in, 155–156 Chemical vapor deposition (CVD) process, 149 Coating mechanism, 151 Drying process, 151–152

Fabrication process of nanofibrous metal oxides, 152–153 Fabrication processes, 147–149 Hydrothermal method, 148 Macro-nanostructured materials synthesis using template method, 152–155 Mixed oxide nanotubes, 147–149

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Mixed oxide nanotubes synthesis using carbon nanofibers as templates, 149–152 Propane oxidation, 156 Silica fibers-immobilized CNFs, SEM images, 153–154

Silica fibers-immobilized nanofibrous LaMnO3 , SEM images, 153–155 Sol-gel method, 148–149 Template method, 148 volatile organic compounds (VOC), 155

12 Synthesis and Applications of Imogolite Nanotubes Masaya Suzuki1 and Keiichi Inukai2 1

2

AIST, Institute for Geo-Resources and Environment, Tsukuba, Ibaraki 305-8567, Japan [email protected] AIST, Materials Research Institute for Sustainable Development, Nagoya, Aichi 463-8560, Japan [email protected]

Abstract Imogolite is a naturally occurring nanotube aluminum silicate. It is classified as a clay mineral, with tube dimensions of about 2 nm in outside diameter, 1 nm in inside diameter, and lengths ranging from tens of nanometers to several micrometers. Imogolite is remarkable not only for its nanoscale fibrous microstructure and its high surface area but also in its ability to adsorb water. So it can be expected to find a wide range of applications. In this chapter, synthesis method for imogolite and applications for anti-dewing materials and heat exchange material in adsorption-type heat pump systems are described.

12.1 Introduction Imogolite is a naturally occurring nanotube aluminum silicate, which is often found in soil originating from volcanic material such as pumice and volcanic ash. It is classified as a clay mineral, with tube dimensions of about 2 nm in outside diameter, 1 nm in inside diameter, and lengths ranging from tens of nanometers to several micrometers [1, 2]. It was first discovered in Japan and was named after soil originating in volcanic ash and glass from the Hitoyoshi region in Kumamoto Prefecture [3]. Research on imogolite has focused on the key role it plays in the movement of nutrients and water in the soil, the supply of nutrients and moisture to plants, the accumulation of harmful contaminants, etc. However, in recent years, imogolite has also received attention as a naturally occurring nanomaterial. It is remarkable not only for its nanoscale fibrous microstructure and its high surface area but also in its ability to adsorb water [4]. Imogolite can be expected to find a wide range of applications in such diverse areas as fuel storage media for natural gases [5], catalyst support [6, 7], polymer composites [8, 9], a humidity-controlling material [10, 11], an adsorption-type heat T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 159–167 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 12

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pump system using low-temperature heat sources such as solar heat and hot spring water [12], and a speed-dry desiccant due to its high adsorption and desorption rates [13]. However, the amount of naturally occurring imogolite is very small, so it is hoped that large-scale synthesis can be carried out. The following section describes a synthesis method for imogolite and applications that utilize its excellent water vapor adsorption characteristics.

12.2 Synthesis of Imogolite In the latter half of the 1970s the successful synthesis of imogolite was carried out for the first time from high-concentration solutions [14]. Subsequently, several industrial applications were proposed, although initial studies concentrated on determining the physical and chemical characteristics of the material. Here, a new synthesis method involving elemental substitution is described. 12.2.1 Synthesis Method The technique used to synthesize imogolite in this work is as follows. First, a 1.4 mmol/L solution of monosilicic acid solution and a 2.4 mmol/L aluminum chloride solution were mixed. Sodium hydroxide was then added to the solution and it was stirred vigorously to adjust the pH to a value of about 5. Next, a 1 mmol/L hydrochloric acid solution and 2 mmol/L acetic acid were added and the solution was heated at 95–100◦ C [14]. It has been reported that very little imogolite is produced by processes that raise the solution concentration such that the concentration of chloride ions becomes 25–30 mmol/L or more [14, 15]. This is because the synthesis of imogolite is obstructed by an increase in the concentration of the negative ions. Since the presence of anions limits the production of imogolite, techniques for lowering their concentration were investigated for high-concentration solutions. One method involves the use of organic silicon and aluminum compounds, where imogolite is obtained by hydrolyzing ethyl orthosilicate and aluminum-s-butoxide (Al(OCH(CH3 )CH2 CH3 )3 ) in perchloric acid followed by heating [16]. Although this method is similar to earlier techniques for reducing anion concentration using an inorganic reagent, it differs in its use of a desalination process by centrifugation [17, 18]. Figure 12.1 shows the flowchart for the synthesis method. A total of 200 mL of 60 mmol/L sodium orthosilicate solution and 200 mL of 150 mmol/L aluminum chloride solution were first mixed together. Sodium hydroxide was then added and the solution was stirred vigorously to adjust the pH to about 6. Next the desalination process was carried out three times using a centrifuge, and the precursor collected by the centrifuge was suspended in purified water, where this suspension was assumed to be 4 L. Following this, 20 mL of 1 mol/L hydrochloric acid was added to

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0.15mmol/L 200ml Aluminum Chloride

0.06mmol/L 200ml Sodium orthosilicate Si/Al=0.40

Mixing the solution pH6

Titration by NaOH(1N, 45ml) Centrifugation (three times) Dispersion in pure water(４L)

pH4∼4.5

Titration by HCl(1N, 20ml) Stir (10minutes)

Heating (98°C, 1week)

Fig. 12.1. Flowchart for the synthesis method of imogolite

the suspension, and it was stirred for 1 h. Well-separated imogolite can be obtained by heating this suspension for 2–4 days in a temperature-controlled bath at 100◦ C. Figures 12.2 and 12.3 show a transmission electron microscope image and an atomic force microscope image of the synthesized imogolite, respectively. The X-ray diffraction profile of the synthetic imogolite is shown in Fig. 12.4, where it can be seen that the diffraction peaks are shifted to

100nm

Fig. 12.2. XRD profile of synthesized imogolite

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Fig. 12.3. AFM image of synthesized imogolite

Fig. 12.4. XRD profiles of natural imogolite, synthesized imogolite, and germaniumsubstituted imogolite

lower angle compared to natural imogolite. This is due to a difference in the outside tube diameters in natural and synthetic imogolite, the diameter being 1.8–2.2 nm in natural imogolite [3] and 2.7–3.2 nm in synthetic imogolite [2]. This method to remove anions during the synthesis of imogolite involves heating the solution after the salt level has been decreased by dialysis in the

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desalination process [19]. This is currently the best method for synthesizing imogolite from high-concentration solutions during the heating process. To determine whether or not imogolite is produced, the precursor generated from a high-concentration solution is diluted and heated, although this is not a directly high-concentration synthesis method [20]. First, a 600 mmol/L sodium orthosilicate solution and a 1500 mmol/L aluminum chloride solution were mixed together. Following addition of sodium hydroxide, the solution was stirred vigorously to adjust the pH to about 6. As before, desalination was carried out three times in the centrifuge and the precursor was suspended in purified water until the concentration of Si was 5 mmol/L in the suspension. A total of 20 mL of 1 mol/L hydrochloric acid was then added, and the suspension was stirred for 1 h. When the hydrochloric acid was added for pH adjustment and the suspension was heated for 2 days at 100◦ C, it was confirmed that pure imogolite was produced. The fact that the precursor of imogolite was generated from a high-concentration solution suggests that there is a possibility that imogolite can be synthesized from a high-concentrate solution if anion removal during the heating process is possible. 12.2.2 Influence of Heating Temperature and Elemental Substitution The outside diameter of the imogolite nanotubes is found to depend on the temperature during the heating process [21]. Although synthesis is usually carried out at 95–100◦ C, it has been reported that outside diameters of 2.2– 2.4 nm, similar to natural imogolite, were obtained when the synthesis process involved spending 7 years at room temperature. Wada et al. also investigated synthesis of imogolite by substituting other elements for silicon and aluminum [22]. They successfully synthesized imogolite by substituting all of the silicon atoms with germanium and obtained an outside diameter of about 3.3 nm, which is larger than the case for silicon alone. Figure 12.4 shows the X-ray diffraction profile from germanium-substituted imogolite. The peaks in the profile are shifted to lower angle relative to synthetic imogolite made up of silicon since the tube diameter is larger. It should be noted that significant differences appear between the X-ray diffraction spectra of the germanium and silicon-containing imogolite. Synthesis of imogolite by partial substitution of aluminum with iron has also been reported [7]. In this case, only about a 5% substitution could be obtained.

12.3 Application of Imogolite 12.3.1 Anti-dewing Material Dewing has become a large problem in Japanese-style houses during the winter season. For this reason, it is highly desirable to develop reusable anti-dewing materials for walls and wallpaper.

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80

Desorption

Amount of adsor ption / wt%

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60

50

40

30

20

10

0

0

10 20

30 40 50 60 70

80 90 100

Relative Humidity / %RH

Fig. 12.5. Water vapor adsorption/desorption isotherm of the synthesized imogolite

One of the important properties of imogolite is its ability to adsorb water vapor. Figure 12.5 shows the water vapor adsorption isotherm of synthetic imogolite. It can be seen that both the adsorption and the desorption curves are very steep on the low and high humidity sides of the chart. The adsorption of water vapor at relative humidities in the range 0–10% is due to adsorption at the inside surfaces of the imogolite tubes, whereas adsorption at relative humidities of 10% or more is due to the pores between adjacent tubes. In particular, the amount of adsorption is large for relative humidities of 90%, exceeding 40 wt%, and the adsorbed vapor is re-released when the humidity decreases. This result shows that imogolite is a very promising anti-dewing material. Humidity control by inorganic materials is a result of capillary condensation at nanosized pores in the material, where water vapor condenses by capillary action on the surface of the material when the surrounding humidity rises, and the condensed water evaporates when the surrounding humidity becomes lower. From the Arai modification of Kelvin’s capillary condensation equation, the pore size of the adsorbed water vapor at a relative humidity of 90% is calculated to be about 22 nm in diameter. To use imogolite as an antidewing material, it is necessary to have controllable nanosize pores between

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the individual tubes. The best way to achieve this is to form one-dimensional crystals of imogolite where all the tubes are aligned in the same direction with a constant spacing. This would also have the advantage of being a highly flexible material. At present, although imogolite has the advantage of possessing two different pore sizes, it remains difficult to control the size of the pores formed between the tubes. This is an important direction for future research. 12.3.2 Heat Exchange Material in Adsorption-Type Heat Pump Systems Reduction of carbon dioxide emissions is an important issue related to global warming, and both energy conservation measures and new energy sources are being actively explored. While more efficient energy production methods such as fuel cells and cogeneration are being developed, techniques are also being pursued to make use of waste energy from factories, etc. However, warm water at temperatures of about 40–60◦ C produced by waste heat has found very little application since it is difficult to extract useful energy from it. For this reason, there has been great interest in developing adsorption-type heat pump systems that can efficiently extract thermal energy at relatively low temperatures [23, 24]. Although synthetic Mg89-A zeolites (where 89% of the exchangeable cation charge is due to Mg substitution in Na-A type zeolites) are assumed to have the highest heat exchange ability at present [25], the MgA type zeolites show peak performance when the dehydration temperature is 100◦ C or more, and thus a material with a large heat exchange ability at lower temperatures is being sought.

Amount of heat exchange (kJ/mol)

60 0

50 0

40 0

30 0

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Imogolite Allophane

10 0

Mg- A type Zeolite 0

20

40

60

80

100

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Dehydration temperature (°C ) Fig. 12.6. The total heat exchanged for each dehydration temperature of imogolite, allophane, Mg-A-type zeolite. Reprinted from [4]

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Imogolite is one material that exhibits low-temperature dehydration characteristics. The dehydration ratio of imogolite is 24.9 mass% when the dehydration temperature is 40◦ C and the dehydration occurs in a vacuum compared to 8.6 mass% for Mg-A type zeolites and 7.3 mass% for silica gel B type zeolites. Figure 12.6 shows the total heat exchanged for each dehydration temperature of imogolite [4]. The total heat exchanged is 405 kJ/kg when the dehydration temperature is 40◦ C, and this value is 2.2 times larger than the value of 182 kJ/kg for Mg-A type zeolites. Moreover, at 80◦ C the total heat exchanged is 468 kJ/kg for imogolite when the dehydration temperature is 80◦ C, a value 1.7 times larger than that of Mg-A type zeolites, which is 270 kJ/kg. For these reasons, imogolite shows strong potential as a heat exchange material for low-temperature waste heat.

12.4 Possible Future Problems Although in this chapter we have described a synthesis method for imogolite and determined its water vapor adsorption characteristics, practical use of this material is still not feasible at present. This is mainly because the cost of producing synthetic imogolite is still quite high. Expensive reagents are not required for synthesis since it is not necessary to use a template as in the case of synthesizing mesoporous silica. However, although the raw material costs are low and almost on the same level as zeolites, the problem is in the process itself and the upper limit of the precursor concentration during the heating phase. Therefore, it is essential to be able to synthesize imogolite using a simple process under high-concentration conditions. At the moment, it is not an exaggeration to say that there is a large potential market for a low-cost production process. Following this, the market would be driven by the development of wide-ranging applications of this material.

References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.

P.D.G. Cradwick et al., Nature. Phys. Sci. 240, 187 (1972) 159 S-I.Wada et al., J. Soil. Sci. 30, 347 (1979) 159, 162 N. Yoshinaga, S. Aomine, Soil Sci. Plant Nutr. 8, 22 (1962) 159, 162 M. Suzuki, Nendo kagaku 42, 144 (2003) 159, 165, 166 W.C. Ackerman et al., Langmuir 9, 1051 (1993) 159 S. Imamura et al., Ind. Eng. Chem. Res. 32, 600 (1993) 159 M. Ookawa et al., Clay Sci., 12, Supplement2, 280 (2006) 159, 163 K. Yamamoto et al., Trans. Mater. Res. Soc. Jpn. 29, 149 (2004) 159 K. Yamamoto et al., Polym. 46, 12386 (2005) 159 S. Tomura et al., Clay Science 10, 195 (1997) 159 M. Suzuki, Nano-fiber, NTS, p. 1016 (2006) 159 M. Suzuki et al., J.Ceramic Soc. Japan. 109, 681 (2001) 160 M.Suzuki et al., J.Ceramic Soc. Japan. 109, 874 (2001) 160

12 Synthesis and Applications of Imogolite Nanotubes 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25.

167

V.C. Farmer, A.R. Fraser, Intern. Clay Conf. Elsevier. p. 547 (1978) 160 M. Suzuki et al., Nendo Kagaku 40, 1 (2000) 160 V.C.Farmer et al. Clay. Miner 18, 459 (1983) 160 M. Suzuki et al., Rep. NIRIN 49, 17 (2000) 160 F. Ohashi et al., J. Material. Sci. 39, 1799 (2004) 160 R. Nakanishi et al., Nendo Kagaku 46, 112 (2007) 163 M. Suzuki et al., Shinkuu 49, 29 (2006) 163 S-I. Wada, Clays. Clay Miner 35, 379 (1987) 163 S-I. Wada, K. Wada, Clays.Clay Miner. 30, 123 (1982) 163 D.I. Tchernev et al., Natural Zeolite (Pergamon, Oxford, 1978). p. 479 165 T. Mizota et al., Thermochim. Acta 266, 331 (1995) 165 T. Kasai et al., Mineral J. 17, 170 (1994) 165

Index Adsorption-type heat pump system, 159 AFM image of synthesized imogolite, 162 Anti-dewing materials, imogolite application in, 164–165 Arai modification of Kelvin’s capillary condensation equation, 164

Imogolite, 159 Imogolite synthesis, 160–163

Catalyst support, 159

Speed-dry desiccant, 160 Synthesis method, 160–163

Dehydration ratio of imogolite, 166 Desalination process, 163 Flowchart for synthesis method of imogolite, 161 Fuel storage media for natural gases, 159 Heat exchange material in adsorptiontype heat pump systems, imogolite application in, 165–166

Mg-A type zeolites, 165–166 Polymer composites, 159

Total heat exchanged for dehydration temperature of imogolite, 165 Water vapor adsorption/desorption isotherm of synthesized imogolite, 164 XRD profile of synthesized imogolite, 161–162

13 Structure and Properties of Imogolite Nanotubes and Their Application to Polymer Nanocomposites Hideyuki Otsuka1 and Atsushi Takahara2 1

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Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan [email protected] Institute for Materials Chemistry and Engineering, Kyushu University, Nishi-ku, Fukuoka 819-0395, Japan [email protected]

Abstract Imogolite is a hydrous aluminosilicate nanofiber with the general formula of [SiO2 ·Al2 O3 ·2H2 O]. It forms a hollow nanotube with an external diameter of ca. 2.5 nm, an internal diameter of less than 1 nm, and lengths ranging from several hundred nanometers to several micrometers. This chapter describes the synthesis and properties of imogolite, along with its application to nanohybrids with polymers.

13.1 Introduction Imogolite is a hydrous aluminosilicate nanofiber with the general formula of [SiO2 ·Al2 O3 ·2H2 O]. It forms a hollow nanotube with an external diameter of ca. 2.5 nm, an internal diameter of less than 1 nm, and lengths ranging from several hundred nanometers to several micrometers (Fig. 13.1) [1]. The use of imogolite began in clay science in the 1960s and has spread to a wide variety of other scientific fields. The name “imogolite” originated from the name of the source layer. Imogolite was accidentally discovered by Yoshinaga and Aomine as one of the impurities of allophane, which is a hydrous aluminum silicate clay mineral with a spherical structure similar to that of fullerene (Fig. 13.2). It comes from naturally occurring weathered pumice beds of volcanic ash soil, which were called “Imogo-layers” in Kumamoto, Japan [2, 3]. Nowadays, imogolite can be collected from pumice beds in many places all over the world and is on the official list of the International Mineralogical Association-Commission on New Minerals, Nomenclature, and Classification (IMA-CNMNC) [4].

T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 169–190 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 13

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Fig. 13.1. Schematic representation of aluminosilicate nanofiber, “imogolite”

Fig. 13.2. Structural comparison of imogolite, allophane, single-walled carbon nanotube, and fullerene

The outer surface of imogolite is composed of aluminol (Al-OH) groups; therefore, the outer wall can be charged depending on the pH of the solution. Due to electrostatic repulsion, isolated units can form nanofibers under acidic conditions [1]. Imogolite with a unique tubular nanostructure has been discussed in the field of clay science for fundamental research [5, 6, 7] and in the field of materials science for various applications [8, 9]. Furthermore, it has been suggested that a polymer nanocomposite with imogolite could be expected to have unique physicochemical properties due to its extremely high aspect ratio and large specific surface area.

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Imogolite is often compared with single-walled carbon nanotubes (SWCNTs) [10, 11]. It has been reported that SWCNTs can be used as the reinforcement nanofiller of polymer nanocomposites. The strength, toughness, and anti flammability of polymers have been substantially improved by the addition of SWCNTs [12–15]. However, SWCNTs are not transparent; therefore, they cannot be used in transparent amorphous polymer materials. In contrast, since imogolite exhibits a refractive index that is similar to that of common polymers [16], it has the potential to act as a transparent polymer additive. Although imogolite preferentially aggregates into bundles, the effective utilization of imogolite in nanocomposite applications requires its homogeneous dispersion into a polymer matrix. There are various approaches to overcome this problem and produce polymer/imogolite nanohybrids. This chapter describes the synthesis and properties of imogolite, along with its application to nanohybrids with polymers.

13.2 Natural Imogolite Nanotubes Since imogolite exists as a water-swollen gel in soils, imogolite gel has a yellow or brown color in its raw form due to the presence of contaminants such as organic molecules, metal oxides, and metal hydroxides. Pure imogolite gel can be obtained by treatment with a hydrogen peroxide aqueous solution and the subsequent elimination of metal contaminants with a chelating agent such as ethylenediamine tetraacetic acid (EDTA). Pure imogolite gel can be dispersed in a weak acidic solution (pH = 3.0–4.0) by applying ultrasonic waves. Freezedrying this dispersion yields a white cotton-like solid (Fig. 13.3). The details of the purification of natural imogolite can be found elsewhere [16].

Fig. 13.3. Purification process for natural imogolite

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13.3 Chemical Synthesis of Imogolite Nanotubes As described above, natural imogolite can be collected from volcanic ash soil and has been used in many fundamental researches. Imogolite also has an advantage in that it can be prepared artificially, making it potentially useful in various practical applications. A synthetic pathway to obtain imogolite was first described in 1977, using dilute solutions of aluminum chloride and monomeric orthosilicic acid [17], and several investigations concerning synthetic methods for preparing imogolite have been reported since. Imogolite can be easily synthesized in a chemical laboratory from aluminum chloride and tetraethoxysilane, both of which are commercially available. In accordance with the typical procedure, as shown in Fig. 13.4, an aqueous solution of aluminum chloride hexahydrate [AlCl3 ·6H2 O] was mixed with an aqueous solution of tetraethoxysilane [Si(OEt)4 ]. The final solution had Al and Si concentrations of 2.4 and 1.4 mmol·L−1 , respectively. The solution was stirred for an hour in order to hydrolyze tetraethoxysilane. Aqueous sodium hydroxide (0.1 mol·L−1 ) was then slowly added until the aqueous solution reached pH 4.5–5.0. The solution was then reacidified by the addition of 1 mmol of hydrochloric acid and 2 mmol of acetic acid per liter of solution. The

Fig. 13.4. Process for synthesizing imogolite by Farmer’s method

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solution was then stirred at 369 K for 96 h. After being cooled to room temperature, the suspended material was gelated by a sodium chloride solution (8.6 mmol·L−1 ) and rinsed with distilled water using a 0.1-μm Millipore filter. The rinsed imogolite gel in an acidic aqueous solution was dispersed by sonication. A cotton-like white solid was obtained by freeze-drying the dispersed solution. The yield of the freeze-dried product was approximately 70%. The above-mentioned modified Farmer’s method must be carried out under diluted concentration conditions, and the reaction requires a long time. However, some improved methods have been recently reported. For instance, the chemical synthesis of imogolite under a high-concentration condition was reported [18]. In addition, the length of the imogolite can be controlled by additives [19]. Compared with natural imogolite, the advantages of imogolite synthesis are (i) a large amount of imogolite can be obtained at any time, (ii) no biological impurities are included, (iii) the length of the imogolite can be controlled by varying the reaction conditions, and (iv) the in situ synthesis of imogolite in a polymer solution (see Sect. 6.2) is possible.

13.4 Structure of Imogolite Nanotubes Imogolite has a unique tubular nanostructure with lengths ranging from several hundred nanometers to the micrometer scale. The detailed structure of imogolite was determined by Cradwick et al. in 1972, as shown in Fig. 13.1 [1]. The outer walls of imogolite consist of a single continuous Al(OH)3 (gibbsite) surface, with the inner hydroxyl group of the gibbsite replaced by orthosilicate (O3 SiOH) groups, as shown in Fig. 13.5. As a result, the orthosilicate groups are located on the inside of the cylindrical structure [1, 20]. The distance between the oxygen atoms is shorter in the orthosilicate groups (dO−O = 0.265 nm) than in the gibbsite sheet (dO−O = 0.32 nm). Focusing on the cross-sectional structure of imogolite shows that it is composed of 10–12 unit cells with a silicon atom. The sheets with unit cells are rolled to form a cylindrical structure. The nanofiber structure of imogolite can be directly observed by transmission electron microscopy (TEM). Figure 13.6 shows a TEM image of an imogolite sample prepared from a diluted imogolite solution. Homogeneous percolation of imogolite takes place at suspension concentrations below 0.2 vol% [21]. An electron diffraction measurement was carried out, and the (006) and (004) diffraction observed along the direction of the long axis revealed the presence of a 0.84 nm repeating unit [22]. In contrast, 1.2, 0.8, and 0.57 nm diffraction peaks in the perpendicular direction were observed, indicating the bundle structure of imogolite [2, 3]. The nanofiber structure of imogolite was also observed by atomic force microscopy (AFM) [23–25]. Figure 13.7 shows AFM images of imogolite on silicon wafers prepared from aqueous dispersions with a concentration of

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Fig. 13.5. Structural units of imogolite and gibbsite sheet

Fig. 13.6. TEM image of imogolite sample prepared from a diluted imogolite solution

0.05 wt% at pH = 7.0 and 3.0 [23]. The imogolite prepared from the aqueous dispersion at pH = 7.0 formed a solid aggregate. At pH = 3.0, however, the nanofiber structure of the imogolite was clearly confirmed by AFM observation, indicating that imogolite can form a nanofiber structure under acidic conditions. The line profile of the AFM image of imogolite shows rods with heights of 1.5–2.5 nm and widths of 20–30 nm. Since the convolution effect of

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Fig. 13.7. AFM images of the imogolite on silicon wafers prepared from an aqueous dispersion with a concentration of 0.05 wt% at pH = 7.0 and 3.0

the AFM tip leads to overestimation of the nanofiber width, the deconvolution of the observed diameter with the tip radius provides a molecular width of about 2–3 nm, which is in good agreement with the molecular diameter of imogolite evaluated from X-ray diffraction [26] and molecular dynamics simulation [27].

13.5 Chemical and Physical Properties of Imogolite Nanotubes Imogolite has a high specific surface area due to its unique nanostructure. Since imogolite contains adsorbed water molecules within its bundled structures, dried imogolite has micropores and shows a high gas adsorption ability. For example, the nitrogen adsorption isotherm of the synthetic imogolite corresponded to type-I adsorption behavior, which indicates the presence of micropores. In contrast, natural imogolite exhibits type-IV behavior. The specific surface areas of natural and synthetic imogolite were 297 and 222 m2 /g, respectively [28]. Most clay minerals possess a cation exchange ability, whereas imogolite possesses an anion exchange ability [29]. Only limited clay minerals such as imogolite, allophane, and kaolinite possess the anion exchange ability. As described above, the external surface of imogolite is composed of Al-OH groups; these groups are transformed into Al-OH+ 2 groups by protonation under acidic conditions. The anion adsorption ability of imogolite is due to the presence of these Al-OH+ 2 groups. In particular, imogolite can strongly adsorb phosphate ions [30]. The thermal properties of imogolite have been investigated by thermogravimetric (TG) analysis and differential thermal analysis (DTA). The TG curve for imogolite shows a significant weight loss at around 400◦ C, indicating the desorption of water. Furthermore, DTA results show endothermal peaks at 120–150 and 390–420◦ C [31].

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The various properties described above indicate the possibility of applying imogolite as a separation membrane, catalyst support, adsorbent, heat exchange material, gas storage material, and so on [32–34]. The properties of imogolite strongly depend on the state of its aggregation, which varies from a closely packed structure to single tubes. Interestingly, a lyotropic mesophase (liquid crystal structure) has been observed for an acidic aqueous dilute suspension of imogolite [35, 36].

13.6 Hybrid Materials Using Imogolite Nanotubes Imogolite, with its high surface area and aspect ratio, is expected to be used as a nanofiller for organic/inorganic hybrid materials due to its unique nanostructure. To obtain organic/inorganic hybrid materials, it is important to control the interface between the organic and inorganic substances. We focused on the surface chemistry of imogolite, with its aluminol groups, although the conventional approach of using organosilane molecules is not effective for the surface control of imogolite. 13.6.1 Surface Modification of Imogolite Nanotubes Imogolite is extremely hydrophilic because its outer surface is composed of aluminol groups. Due to the strong interaction between them, it is difficult to disperse imogolite in organic solvents and hydrophobic polymer matrices. To overcome this problem, the modification of imogolite was carried out to investigate the interaction between the surface of imogolite and some functional groups. For example, octadecylphosphonic acid (ODPA), an amphiphilic molecule, was employed for hydrophobidization, because imogolite can strongly adsorb phosphoric acid [23, 37, 38]. Figure 13.8 shows the infrared (IR) spectra of imogolite chemisorbed with ODPA. The absorptions at 995 and 935 cm−1 were attributed to the stretching vibration of Si–O–Al in imogolite. The absorption peaks at 2850–2853, 2921–2925, and 2956 cm−1 were attributed to the stretching of the alkyl chain in ODPA. In contrast, no significant interaction was observed for an alcohol derivative with a long alkyl chain. These results suggest that imogolite shows a strong interaction with a phosphonic acid group in ODPA, as shown in Fig. 13.9. The amount of ODPA adsorbed on the imogolite surface could be estimated by TGA. TGA curves of imogolite show a weight loss of 5–10% up to 523 K and 40–65% up to 732 K. The former weight loss is attributed to the dehydration of weakly adsorbed water, and the latter to the bound water and decomposed ODPA. The amount of ODPA adsorbed increased with the ODPA/imogolite ratio. Equilibrium was attained at an ODPA/imogolite ratio of approximately 5:1 (w/w). The estimated amount of adsorbed ODPA at adsorption equilibrium corresponded to the case of complete coverage of the imogolite surface with an ODPA monolayer [38].

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Fig. 13.8. Chemical structure of octadeylphosphonic acid (ODPA) and IR spectra of the imogolite and ODPA-modified imogolite

Fig. 13.9. Mode of interaction between imogolite surface and alkyl phosphonic acid

The surface modification of imogolite by ODPA was further confirmed by force–distance curve measurements between the cantilever tip and imogolite surface before and after the adsorption of ODPA. The adhesion force was estimated from the maximum attractive force, which corresponds to the minimum force in the force–distance curve. Figure 13.10 shows the adhesion force histograms observed on the surface of the imogolite and the ODPAadsorbed imogolite. The adhesion force between imogolite and the cantilever tip was much larger than that between ODPA-chemisorbed imogolite and the

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Fig. 13.10. Histograms of adhesion force observed between cantilever tip and imogolite surface before and after adsorption of ODPA

cantilever tip. The adsorption of ODPA onto imogolite changed the surface hydrophilicity since hydrophobic alkyl groups covered the imogolite surface. Indeed, although the hydrophobidized imogolite was dispersed in hexane and chloroform, the modified imogolite was precipitated in or floated on water and methanol because of the low surface energy of modified imogolite [23, 38]. 13.6.2 Polymer Hybrids with Imogolite Nanotubes Poly(methyl methacrylate)/Imogolite Hybrid The hybridization of poly(methyl methacrylate) (PMMA) with imogolite was achieved by using the surface modification method. Methacryloyloxyethyl phosphate (MOEP, Fig. 13.11), which has a polymerizable unit and a phosphoric acid group, was used as a surface modifier for imogolite. It is considered that MOEP is adsorbed onto the surface of the imogolite due to the presence of a phosphoric acid group, which interacts with the Al-OH groups on the imogolite surface. The chemisorption of MOEP onto the surface of the imogolite was confirmed by IR measurement. In the IR spectrum of the MOEP-chemisorbed imogolite, we observed absorption peaks at (a) 992 and 935 cm−1 corresponding to the absorption of imogolite and (b) 1722 and 1180 cm−1 corresponding to the ν(C=O) and νas (COC) of the MOEP. Furthermore, the intensity of the absorption peak at 1080 cm−1 , corresponding to the νas (PO2− 3 ) band, increased with the MOEP content in the imogolite/MOEP system [39]. To prepare a PMMA/imogolite nanohybrid, in situ MMA polymerization was achieved by the radical polymerization of MMA in the presence of an MOEP-modified imogolite with a high amount of MOEP adsorption (imogolite:MOEP = 1:1). The polymerization of MMA was confirmed by IR and

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Fig. 13.11. Chemical structure of methacryloyloxyethyl phosphate (MOEP) with a polymerizable unit and a phosphoric acid group and surface modification of imogolite with MOEP

nuclear magnetic resonance (NMR) measurements. The conversion of MMA was 95–98%, as estimated by NMR measurement. The number average molecular weight (Mn ) and polydispersity index (Mw /Mn ) of the soluble polymer contents were estimated by GPC. In order to confirm the formation of PMMA in the presence of imogolite nanofibers, both modified and unmodified imogolite were used. The GPC results indicated that 1.0 wt% imogolite had little influence on MMA polymerization. The optical and mechanical properties of the PMMA and PMMA/modified imogolite hybrid prepared through in situ polymerization were investigated. PMMA polymerized without imogolite was utilized to obtain a PMMA/imogolite blend film by simple solution blending. Since PMMA is usually used for transparent materials, it was important that the PMMA hybrid be transparent, even after utilizing reinforcing fillers in the PMMA matrix. Grafting the PMMA chains onto the imogolite surface can be expected to give rise to a high affinity between the imogolite nanofibers and PMMA matrix. To evaluate the optical properties of a PMMA/imogolite hybrid film, we prepared a hybrid film that included 1 wt% imogolite whose surface was modified with MOEP. As a reference, a blend film with the same imogolite content was prepared from a PMMA and unmodified imogolite mixed solution. Figure 13.12 shows the transparencies of the PMMA, PMMA/imogolite hybrid, and PMMA/imogolite blend films, as determined by light transmission measurements [39]. The results show that in comparison with the PMMA/imogolite blend film, the PMMA/imogolite hybrid film retained the transparency comparable to the matrix film. This difference in transparency can be attributed to the dispersibility of the imogolite nanofibers in the polymer matrix. One of the factors that controls this dispersibility is the affinity between the PMMA matrix and the surface of the PMMA-grafted imogolite.

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Fig. 13.12. Light transmittance of PMMA, PMMA/imogolite hybrid, and PMMA/imogolite blend films in the visible light region. The imogolite content was 1.0 wt%

To investigate the mechanical properties of the PMMA/imogolite hybrid film, the temperature dependence of the storage modulus (E’) and loss tangent (tanδ) was evaluated for three types of films: PMMA, PMMA/imogolite hybrid, and PMMA/imogolite blend films. The dynamic storage modulus of the PMMA/imogolite hybrid film was approximately 1.5 times as high as that of the PMMA film across the entire temperature range [39]. In addition, the αa -absorption temperature of the hybrid film was higher than that of the PMMA film. Furthermore, the tensile modulus and ultimate strength of the PMMA/imogolite hybrid were approximately 1.4 times as high as those of PMMA film. On the other hand, the tensile modulus and strength of the PMMA/imogolite blend film were similar to those of the PMMA film. The reinforcement effect of imogolite became significant in the case of the PMMA/imogolite hybrid film, because the dispersibility of imogolite in the PMMA matrix was improved. Poly(vinyl alcohol)/Imogolite Hybrid A polymer/imogolite hybrid was prepared from imogolite and poly(vinyl alcohol) (PVA), because PVA is hydrophilic and chemical modification is easily accomplished. Similar to the cases of surface modification of imogolite by ODPA and MOEP, the phosphonic acid group was introduced into the side chain of the PVA. The degree of phosphonization in the phosphonic acidmodified PVA (P-PVA) was ca. 20%. Figure 13.13 shows the temperature dependences of tan δ for the phosphonated PVA (P-PVA) and imogolite/P-PVA hybrid. The αa -absorption peak was located at a higher temperature for P-PVA than for unmodified PVA

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Fig. 13.13. Temperature dependences of tan δ for P-PVA and (imogolite/P-PVA) nanohybrid with 0.5 wt% imogolite at 11 Hz

because of the restriction of thermal molecular motion due to the inter- and intramolecular interactions of the phosphonic acid groups. The αa -absorption temperature of the P-PVA/imogolite hybrid was ca. 19 K higher than P-PVA. This suggests a strong interaction between the P-PVA and imogolite surface. Thus, a strong interface between imogolite and a matrix polymer can be realized by the introduction of phosphonic acid group into the matrix polymer. A more sophisticated PVA/imogolite hybrid system was created by an in situ preparation method. As described in Sect. 14.3, imogolite can be synthesized by Farmer’s method [17]. A PVA/imogolite nanohybrid was prepared by the in situ synthesis of imogolite in the presence of PVA in an aqueous solution. When incorporating hydroxyl groups, PVA can be expected to interact with the surface of imogolite. In the synthesis process for imogolite, an aqueous PVA solution was mixed with a dilute imogolite precursor solution and stirred at 369 K for 96 h. Mixed solutions were prepared with different mass– fraction ratios of 1:1, 1:5, 1:10, and 1:20 (imogolite:PVA). After cooling, the mixed solution of imogolite and PVA was reprecipitated with ethanol, and the precipitate was filtered using a 0.45-μm Millipore filter and then rinsed with water and ethanol. A white-colored product comprising the in situ synthesized imogolite/PVA hybrid was then obtained. Furthermore, a cast film was prepared from the in situ synthesized imogolite/PVA solution in order to measure

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its optical and mechanical properties. The concentration of the imogolite/PVA aqueous solution was 5 wt%. Films made of PVA homopolymer and a freezedried imogolite/PVA blend were prepared as reference samples. To confirm the formation of imogolite by the in situ synthetic method, IR measurements were carried out. Figure 13.14 (curve No. 4) shows the IR spectrum of the synthetic imogolite reacted for 96 h. This spectrum shows two sharp absorptions at 995 and 935 cm−1 attributable to Si-O-Al stretching vibration and a large absorption at 3440 cm−1 corresponding to OH stretching vibration. The IR spectra in this region were not sufficiently distinctive to allow for the presence of other aluminosilicates with a spherical structure, such as allophane. Figure 13.14 (curve No. 2) shows the IR spectrum of the imogolite/PVA hybrid sample prepared by the in situ method. The imogolite to PVA weight ratio was 1:1. The characteristic absorption peaks of PVA appeared at 2945 cm−1 [νa (CH2 )], 2915 cm−1 [νs (CH2 )], 1430 cm−1 [δ(CH2 )], and 1095 cm−1 [ν(C-O)] [40]. In addition, the IR spectra of the hybrid with imogolite showed two sharp peaks corresponding to Si-O-Al stretching vibration. The IR spectra of the imogolite/PVA blend and the imogolite/PVA hybrid were both quite similar to that of PVA, except for the characteristic imogolite absorptions at 995 and 930 cm−1 . These results suggest that imogolite was successfully formed in the PVA solution. The morphology of the in situ synthesized imogolite in the polymer matrix was evaluated by cyclic contact-mode AFM. Figure 13.15 shows cyclic

Fig. 13.14. IR spectra of imogolite/PVA blend, in situ synthesized imogolite/PVA hybrid, PVA, and imogolite

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Fig. 13.15. Cyclic contact-mode AFM images of in situ synthesized imogolite/PVA hybrid prepared with various weight ratios for the imogolite/PVA: (a) 1:1, (b) 1:5, (c) 1:10, and 1:20

contact-mode AFM images of the in situ synthesized imogolite/PVA hybrid prepared using various imogolite/PVA weight ratios ranging from 1:1 to 1:20. The brighter areas in these images correspond to the higher height region. All of the AFM images show the fiber morphology, indicating the formation of bundles of imogolite nanofibers. The average length of the synthesized imogolite fibers observed in the films prepared by the in situ method was several hundred nanometers. This value is smaller than that of typical natural and synthesized imogolite nanofibers. Previous papers have shown that the important factors governing the formation of imogolite are the pH, OH/Al, Si/Al molar ratio, temperature, and presence of additives in the solution [41, 42]. In particular, additives such as organic ligands, which can interact with the aluminum ions, play a decisive role in the formation of short-range ordered aluminosilicate [42]. The lengths of the imogolite nanofibers estimated from the AFM images were 689 ± 47 nm (imogolite:PVA = 1:1), 271 ± 21 nm (1:5), 224 ± 18 nm (1:10), and 178 ± 9.3 nm (1:20), suggesting that the growth of imogolite nanofibers along the axial direction was impeded by the presence of

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Fig. 13.16. Photographs of in situ synthesized imogolite/PVA hybrid films

PVA due to the interaction between imogolite and PVA. This intermolecular interaction was maintained by the formation of the lyotropic mesophase in the imogolite and PVA mixed system [43]. These results suggest that higher PVA concentrations inhibited the formation of the synthetic imogolite nanofiber. It therefore seems that there is a critical PVA concentration at which imogolite can be synthesized. Figure 13.16 shows photographs of the imogolite/PVA hybrid films. The hybrid film prepared by the in situ method was optically transparent. In contrast, the blend film prepared from a mixture of imogolite and PVA was not transparent. It appears difficult to redisperse the freeze-dried imogolite powder, since imogolite fibers aggregate during the process of air-drying or freeze-drying. However, imogolite formed via in situ synthesis in a polymer solution exhibited a fine dispersion because the PVA chains were adsorbed on the imogolite nanofibers during the synthesis process inhibited their aggregation. In addition, compared with the PVA film, the hybrid films showed increased elastic modulus and heat distortion temperature. The in situ synthesis method appears to facilitate the dispersion of imogolite nanofibers in the polymer matrix when preparing a homogeneous hybrid film, leading to considerable improvements in the mechanical and optical properties of the film. Enzyme/Imogolite Hybrid Since some enzymes such as pepsin have a phosphoric acid group, an imogolite/pepsin hybrid hydrogel was prepared (Fig. 13.17) [44]. The immobilization of pepsin was confirmed by IR measurements. Confocal laser scanning microscopy (CLSM) and field-emission scanning electron microscopy (FE-SEM) were used to evaluate the dispersion state of pepsin in the hybrid hydrogel and the network structure of the hybrid hydrogel. Pepsin was finely dispersed in the hybrid hydrogel, and fluorescent images of pepsin and imogolite showed a similar morphology due to the immobilization of pepsin onto the imogolite surface. Figure 13.18 shows an FE-SEM image of a hybrid hydrogel with a 99.7% water content. The 3-dimensional network structure of a hybrid hydrogel could

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Fig. 13.17. Preparation and photograph of imogolite/pepsin hybrid hydrogel

Fig. 13.18. FE-SEM image of a hybrid hydrogel with a 99.7% water content

be directly observed. The average pore size of the hybrid hydrogel in this image was 108 nm. The enzyme activity of the immobilized pepsin in the hybrid hydrogel was evaluated from the hydrolysis of hemoglobin at pH = 3.1. The immobilized pepsin in the hybrid hydrogel retained ca. 26% of its enzyme activity, compared with free pepsin in aqueous solution. The enzyme activity of immobilized pepsin was apparently decreased. Similar behavior has been observed for an enzyme immobilized at the inner surface of halloysite [45]. This decrease could be ascribed to the slow diffusion of the substrate into the network structure of the hybrid hydrogel, as well as to the inhibition of the diffusion of the substrate to the active site of the immobilized pepsin due to the steric hindrance of the imogolite network. When pepsin is immobilized in the hybrid hydrogel, it can be easily recovered from the reaction system and can repeatedly react with the substrate. The change in the enzyme activity was also investigated for the case of repeated reaction. The enzyme activity of immobilized pepsin was retained after four reactions, with a slight decrease in activity as the number of reactions increased.

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13.6.3 Imogolite Nanotubes as Templates Controlling the structure of a polymer on the submicron or nanometer scale is a very important technique in the development of functionalized polymer devices in electronics and photonics. We focused on imogolite because of its high aspect ratio and large surface area, and the ability of its positively charged surface to bind to anionic polyelectrolytes. One of the most important properties of imogolite is its optical transparency. It is likely that hybrid films with important optical properties will be prepared from imogolite and polymers. Therefore, imogolite nanofibers were used as templates to assemble a conjugated polymer at the surface of the nanofibers by ionic interaction. Watersoluble poly(p-phenylene) (WS-PPP) [46] was employed as an anionic polyelectrolyte, which is water soluble and has a conjugated molecular structure. The LBL assembly and spin assembly of imogolite and WS-PPP were successfully performed. UV-vis measurements indicated that the average amounts of WS-PPP deposited per thickness differed slightly between the two assembly methods. The stepwise deposition process was followed by UV-vis spectroscopy. The morphology of the spin-assembled films showed a higher ordered orientation of imogolite as compared to those obtained using the conventional LBL method (Fig. 13.19) [47]. In other words, the nanofibers from the spinassembly method showed a planar alignment along the radial direction. Imogolite nanofibers were used as templates to order the conjugated polymer at the surface of the nanofibers by ionic interaction. This is a simple method for preparing a highly ordered hybrid film of nanofibers and conjugated polymers. Another approach to align imogolite nanofibers was reported. The hydrophobic nature of ODPA-modified imogolite aided in the dispersion of

Fig. 13.19. Schematic representation of morphologies of films prepared by the LBL assembly and spin assembly of imogolite and WS-PPP

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Fig. 13.20. Schematic representation of ODPA-modified imogolite on HOPG substrate

organic solvents, and the imogolite fibers aligned themselves at the air/water interface. The surface-modified imogolite fibers were aligned with a constant nanospacing, which was caused by tight molecular contact (Fig. 13.20) [48]. The aligned structure was clearly visualized by scanning tunneling microscopy (STM) and the direction of the alignment was found to be perpendicular to the compressed direction at the air/water interface. Thus, imogolite can potentially be used as a template to control the system based on organic molecules and polymers.

13.7 Concluding Remarks In this chapter, we reviewed the synthesis, properties, and application to a polymer nanohybrid system. Imogolite is a nanotubular aluminosilicate with transparency and hydrophilicity; both characteristics are not observed in SWNT. Imogolite is a naturally produced nanofiber, but it can also be synthesized in a laboratory. The surface chemical modification, aggregation control, and nanohybridization of imogolite were successfully achieved; therefore, the importance of imogolite should increase in both academic and practical fields. Furthermore, imogolite is environmentally benign and shows promise as a next-generation inorganic nanomaterial.

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188 6. 7. 8. 9. 10. 11. 12. 13. 14. 15. 16. 17. 18. 19. 20. 21. 22. 23. 24. 25. 26. 27. 28. 29. 30. 31. 32. 33. 34. 35. 36. 37. 38. 39.

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J. Karube, Clays Clay Miner. 46, 583 (1998) S.-I. Wada, Y. Kakuto, Soil Sci. Plant Nutr. 45, 947 (1999) J.-C.P. Gabriel, P. Davidson, Adv. Mater. 12, 9, (2000) 170 L. Guimaraes, A.N. Enyashin, J. Frenzel, T. Heine, H.A. Duarte, G. Seifert, ACS nano 1, 362 (2007) 170 S. Iijima, Nature 354, 56 (1991) 171 S. Iijima, T. Ichihashi, Nature 363, 603 (1993) 171 J.C. Kearns, R.L. Shambaugh, J. Appl. Polym. Sci. 86, 2079 (2002) 171 A. Dufresne, M. Paillet, J.L. Putaux, R. Canet, F. Carmona, P. Delhaes, S. Cui, J. Mater. Sci. 37, 3915 (2002) 171 T. Kashiwagi, E. Grulke, J. Hilding, R. Harris, W. Awad, J. Douglas, Macro. Rapid Commun. 37, 761 (2002) 171 X. Zhang, T. Liu, T.V. Sreekumar, S. Kumar, V.C. Moore, R.H. Hauge, R.E. Smalley, Nano Lett. 3, 1285 (2003) 171 N. Miyauchi, S. Aomine, Soil Sci. Plant Nutr. 12, 187 (1966) 171 V.C. Farmer, A.R. Fraser, J.M. Tait, J Chem. Soc., Chem. Commun. 462 (1977) 172, 181 F. Ohashi, S.-I. Wada, M. Suzuki, M. Maeda, S. Tomura, Clay Mineral. 37, 451 (2002) 173 S.-I. Wada, C. Sakimura, Clay Sci., 11, 115 (2000) 173 L.A. Bursill, J.L. Peng, L.N. Bourgeois, Phil. Mag. A 80, 105 (2000) 173 A.P. Philipse, A.M. Wierenga, Langmuir 14, 49 (1998) 173 S. Mukherjee, V.M. Bartlow, S. Nair, Chem. Mater. 17, 4900 (2005) 173 K. Yamamoto, H. Otsuka, S.-I. Wada, A. Takahara, Chem. Lett. 1162, (2001) 173, 174, 176, M. Tani, C. Liu, P.M. Huang, Geoderma 118, 209 (2004) 173 Y. Ohrai, T. Gozu, S. Yoshida, O. Takeuchi, S. Iijima, H. Shigekawa, Jpn. J. Appl. Phys. 44, 5397 (2005) 173 K. Wada, N. Yoshiinaga, Am. Mineral. 54, 50 (1969) 175 P. I. Pohl, J.-L. Faulon, D. M. Smith, Langmuir 12, 4463 (1996) 175 F. Ohashi, S. Tomura, K. Akaku, S. Hayashi, S.-I. Wada, J. Mater. Sci. 39, 1799 (2004) 175 R.L. Parfitt, A.D. Thomas, R.J. Atkinson, R.St.C. Smart, Clay Clays Miner. 30, 143 (1982) 175 R.K.G. Theng, M. Russell, G.J. Churchman, R.L. Parfitt, Clay Clays Miner. 30, 143 (1982) 175 S.-I. Wada, Jinko Nendo 20, 2 (1993) 175 S. Tomura, M. Maeda, K. Inukai, F. Ohashi, M. Suzuki, Y. Shibasaki, S. Suzuki, Clay Sci. 10, 195 (1997) 176 W.C. Ackerman, D.M. Smith, J.C. Huling, Y.-K. Kim, J.K. Bailey, C.J. Brinker, Langmuir 12, 4463 (1996) 176 J. Park, J. Lee, S. Chang, T. Park, B. Han, J.W. Han, W.Yi, Bull. Korean Chem. Soc. 29, 1048 (2008) 176 K. Kajiwara, N. Donkai, Y. Hiragi, H. Inagaki, Makromol. Chem. 187, 2883 (1986) 176 K. Kajiwara, N. Donkai, Y. Fujiyoshi, H. Inagaki, Makromol. Chem. 187, 2895 (1986) 176 K. Yamamoto, H. Otsuka, A. Takahara, Polym. J. 39, 1 (2007) 176 K. Yamamoto, H. Otsuka, S. -I. Wada, A. Takahara, J. Adhesion 78, 591 (2002) 176, 178 K. Yamamoto, H. Otsuka, S.-I. Wada, D. Sohn, A. Takahara, Polymer 46, 12386 (2005) 178, 179, 180

13 Structure and Properties of Imogolite Nanotubes 40. 41. 42. 43. 44. 45. 46. 47. 48.

189

S. Krimm, C.Y. Liang, G.B.B.M. Sutherland, J. Polym. Sci. 22, 227 (1956) 182 S.-I. Wada, A. Eto, K. Wada, J. Soil Sci. 30, 347 (1979) 183 K. Inoue, P.M. Huang, Soil Sci. Soc. Am. J. 50, 1623 (1986) 183 H. Hoshino, T. Ito, N. Donkai, H. Urakawa, K. Kajiwara, Polym. Bull. 29, 453 (1992) 184 N. Inoue, H. Otsuka, S.-I. Wada, A. Takahara, Chem. Lett. 35, 194 (2006) 184 D.G. Shchukin, G.B. Sukhorukov, R.R. Price, Y.M. Lvov, Small 1, 510 (2005) 185 S. Kim, J. Jackiw, E. Robinson, K.S. Schanze, J.R. Reynolds, Macromolecules 31, 964 (1998) 186 N. Jiravanichanun, K.Yamamoto, H. Yonemura, S.Yamada, H. Otsuka, A. Takahara, Bull. Chem. Soc. Jpn. 81, 1663 (2008) 186 S. Park, Y. Lee, B. Kim, J. Lee, Y. Jeong, J. Noh, A. Takahara, D. Sohn, Chem. Commun., 2917 (2007) 187

Index AFM images of the imogolite on silicon wafers, 175 Alkyl phosphonic acid and imogolite, mode of interaction between, 177 Allophane, 170, 182 Aluminosilicate nanofiber representation, 170 Anion exchange and adsorption ability of imogolite, 175 Chemical synthesis of imogolite nanotubes, 172–173 Cyclic contact-mode AFM images of in situ synthesized imogolite/ PVA hybrid, 183 Differential thermal analysis (DTA), 175 Enzyme/imogolite hybrid, 184–185 Farmer’s method, 172–173 Fullerene, 170 Gibbsite sheet structural units, 174 Hydrophobidization, 176 “Imogo-layers,” 169 Imogolite nanotubes structure, 173–175 Imogolite/pepsin hybrid hydrogel, 184–185

International Mineralogical AssociationCommission on New Minerals, Nomenclature, and Classification (IMA-CNMNC), 169 IR spectra of imogolite/PVA blend, 182 Methacryloyloxyethyl phosphate (MOEP), 178–179 Natural imogolite nanotubes, 171 Octadecylphosphonic acid (ODPA), 176–177 ODPA-chemisorbed imogolite, adhesion force observed between, 177–178 ODPA-modified imogolite on HOPG substrate representation, 186–187 PMMA/imogolite hybrid, 178–180 Poly(methyl methacrylate) (PMMA), 178–179 Poly(vinyl alcohol) (PVA), 180 Polymer hybrids with imogolite nanotubes, 178–186 Purification process for natural imogolite, 171 PVA/imogolite hybrid, 180–184 Single-walled carbon nanotubes (SWCNTs), 170–171

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TEM image of imogolite, 174 Thermal properties of imogolite, 175 Thermogravimetric (TG) analysis, 175

Type-IV adsorption behavior of imogolite, 175 Water soluble poly(p-phenylene) (WS-PPP), 186

14 Synthesis and Applications of Chalcogenide Nanotubes Tsukio Ohtani Laboratory for Solid State Chemistry, Okayama University of Science, Ridai-cho 1-1, Okayama 700-0005, Japan [email protected]

14.1 Introduction In 1930, Pauling stated that if two faces of a constituent layer of a layered crystal, such as serpentine, are not equivalent, there would be a tendency for the layer to bend owing to the strain induced by the structural mismatch between adjacent layers, resulting in the formation of a cylindrical structure [1]. Similar cylindrical structures were found in misfit-layer chalcogenides that are built up with alternating MCh2 (M = transition metals; Ch = S, Se, Te) sandwiches and M’Ch (M’ = Pb, Sn, Bi, Lanthanides, etc.) double layers [2]. Figure 14.1 shows a scanning electron microscope (SEM) image of a microtube of a misfit-layer compound, (GdS)1.20 TaS2 , showing a scroll-like appearance [3]. It was observed that either the TaS2 layers or the GdS layers form regular lattices, and the neighboring layers are forced to be distorted. In terms of the morphology, these microtubes have relatively large diameters, their stacked layers are not concentric, and both ends of the tubes are open. These characteristics are different from those of carbon nanotubes (CNTs) discovered by Iijima in 1991 [4], indicating that the formation mechanism presented by Pauling is different from that of the CNTs. In fact, Pauling’s mechanism does not explain the formation of fullerenes and CNTs from graphite with a symmetric unit cell along the c-axis. Chalcogenide nanotubes (ChNTs) possessing a CNT-like structure were first found by Tenne et al. in 1992 [5]. Prior to the discovery of CNTs by Iijima, Tenne’s group had noticed some unusual tungsten disulfide particles during the preparation of thin films of tungsten disulfide, WS2 . After Iijima’s report, they reinvestigated these particles and found that they were CNT-like nanotubes of WS2 . Subsequently, chalcogenide nanotubes have been found in many transition-metal dichalcogenides (MCh2 ) and metal chalcogenides. The formation of CNTs is attributed primarily to the large surface energy induced by the presence of numerous dangling bonds at the rims of the graphene T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 191–199 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 14

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Fig. 14.1. SEM image of a microtube of (GdS)1.20 TaS2

sheets. WS2 -NTs are considered to be formed by a mechanism analogous to that of CNTs [5]. Various review articles on ChNTs can be found in [6–10].

14.2 Transition-Metal Dichalcogenides with Layered Structures Transition-metal dichalcogenides, MCh2 (M = Ti, Zr, Hf, V, Nb, Ta, Mo, W, etc.; Ch = S, Se, Te), possess typical layered structures. The structures are composed of sandwiched layers of (Ch-M-Ch) stacked along the hexagonal c-axis, where both the M and the Ch layers have closed-packed hexagonal planes [11]. The neighboring Ch layers are bound by weak van der Waals force, and thus the compounds have high antifriction properties. Thus, dichalcogenides such as MoS2 and WS2 are widely used as solid lubricants. Transition metal (M) atoms occupy either octahedral (O) or trigonal prismatic (TP ) sites, which are located between two chalcogen layers. The Group 4 metals, Ti, Zr and Hf, and Group 5 metals, V and Ta, occupy the O sites. The Group 5 metals, Ta and Nb, and Group 6 metals, Mo and W, occupy the TP sites. A number of stacked polytypes composed of various combinations of the two coordination types also exist. They are expressed using notations of 1T, 2H, 3R, etc. In addition, many transition-metal dichalcogenides show structural changes as a function of temperature, pressure, chemical composition, etc. These chalcogenides are usually obtained by heating the elemental mixtures with desired ratios [12]. Figure 14.2 shows the schematic structures of WS2 (TP coordination) and graphite with an emphasis on layer stacking [5]. There are many dangling bonds at the rims of the layers in both cases. With decreasing size of the molecular sheets, the relative number of rim atoms with unsaturated bonds increases. The Ch-M-Ch sandwiches therefore become unstable toward bending

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Fig. 14.2. Schematic illustrations of the structures of WS2 (prismatic coordination) and graphite. A number of dangling bonds are present at the rims of the layers for both structures

and have a high tendency to roll into curved structures, forming hollow nanotube structures [13]. This is also the case for the dichalcogenides with the O coordination.

14.3 Chalcogenide Nanotubes (ChNTs) 14.3.1 Formation Mechanism of MS2 (M = W, Mo) Nanotubes Figure 14.3 shows transmission electron microscopy (TEM) images of (a) a MoS2 nanoparticle with onion-like fullerene type structure and (b) a WS2 nanotube with an external diameter of 16 nm and length of ∼200 nm, obtained by the Tenne group [14]. The WS2 -NT has a morphology analogous to that

Fig. 14.3. TEM images of (a) fullerene-type nanoparticle of MoS2 and (b) WS2 nanotube [14]

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Fig. 14.4. Schematic view of nanotube formation by the folding of single MoS2 planes

of CNTs. Tenne et al. reported that these crystals were occasionally found in WS2 products obtained by annealing tungsten films on quartz substrates at 1,000◦ C in a H2 S atmosphere [5]. It was revealed, by electron diffraction studies, that the WS2 -NTs are three-dimensionally constructed with several sheets of WS2 . MoS2 -NTs with a similar structure were also found by the Tenne group in 1995 [15]. In addition, they found zigzag, armchair, and chiral (only right hand) MoS2 -NTs [16]. Figure 14.4 shows a schematic of the folding of a single WS2 plane in the process of formation of nanotubes [10]. A zigzag MS2 -NT is formed when an M atom along [100] axis (vector [n,0]) coincides with an M atom at the origin of the system of axes by folding the MS2 sheet into a tube. The armchair and chiral nanotubes are formed for the cases of vectors [n,n] and [n,m], respectively. Such situations are almost identical to those of CNTs formed by the folding of graphene sheets. 14.3.2 Preparation of Different Chalcogenide Nanotubes (ChNTs) Chalcogenide nanotubes reported thus far are listed below. Dichalcogenides: WS2 [5], MoS2 [15], WSe2 [17], MoSe2 [17], NbS2 [18], NbSe2 [19], TaS2 [18], TiS2 [20], ZrS2 [21], HfS2 [21] Monochalcogenides: InS [22], CdS [23], CdSe [24], ZnS [25], NiS [26] Other chalcogenides: Bi2 Se3 [27], Cu2 S [28], Ag2 Se [29], Se [30] (1) MCh2 (M = Mo, W) nanotubes [Method 1] Sulfurization of MO3 in (H2 /N2 + H2 S) stream Figure 14.5 shows a schematic of the preparation method of MoS2 -NTs [15]. First, MoO3 was reduced to MoO3−x in a stream of (95%N2 + 5%H2 ) gas in a tube situated inside the main tube. The sublimed MoO3−x was reacted with the main gas stream (H2 /N2 + H2 S). The reaction temperatures ranged

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Fig. 14.5. Schematic illustration of the preparation method of MoS2 nanotubes. The reactions were carried out between 800 and 950◦ C

from 800 to 950◦ C. To obtain high-quality samples, the size and shape of the nozzle of the inner tube should be carefully designed [15]. For the case of WS2 , an alternative preparation method was adopted using needle-like crystals of WO3−x as a precursor [31]. The proposed reaction mechanisms for M = Mo, W are given in reactions (14.1) and (14.2) [10]. Amorphous MS3 produced in reaction (14.1) was decomposed into MS2 -NTs in reaction (14.2). MO3−x (g) + 3H2 S(g) → MS3 (s) + (3 − x)H2 O(g) + xH2 (g) MS3 → MS2 (s) + S(g)

(14.1) (14.2)

[Method 2] Decomposition of MS3 in H2 stream In the above reactions, the formation of MS3 was revealed to be an important factor to obtain the ChNTs. In place of MoO3 , the reactions were also carried out using MS3 as the starting material [32]. MS3 was prepared by thermal decomposition of (NH4 )2 MS4 in an Ar stream at 400◦ C. The obtained MS3 was heated at 1,200–1,300◦ C in a H2 stream. A large amount of ChNTs can be obtained by the following reaction: MS3 + H2 → MS2 + H2 S

(14.3)

ChTNs were also obtained by heating (NH4 )2 MS4 in a H2 stream at 1,200–1,300◦ C. This method is useful for the preparation of MSe2 -NTs. Furthermore, when (NH4 )2 Mo1−x Wx S4 is used for the starting material, one can obtain nanotubes of Mo1−x Wx S2 (x = 0.15–0.5) [33]. [Method 3] Closed-tube chemical transport method MoSe2 or WSe2 was sealed in vacuo with I2 in a silica tube, which was situated in a furnace with a temperature gradient of 2 K/cm, with the higher

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temperature end of the tube (starting sample side) maintained at 1,000◦ C. The reaction was carried out for 22 days and was then slowly cooled to room temperature [34]. ChNTs can be obtained by this method together with platelike hexagonal crystals. (2) MCh2 nanotubes (M = Ti, Zr, Hf, Nb, Ta) MS3 was first prepared by heating an elemental mixture with the stoichiometric ratio in an evacuated silica tube at high temperatures. TiS2 -NTs were obtained by the decomposition of TiS3 in an atmosphere of H2 + He at 800◦ C [20]. ZrS2 and HfS2 -NTs were prepared by the decomposition of MS3 in a flow of Ar(95%) + H2 (5%) at 900◦ C [21]. NbS2 - and TaS2 -NTs were prepared by heating MS3 in a H2 flow at 1,000◦ C for 30–60 min [18]. (3) CdS and CdSe nanotubes For the preparation of CdSe-NTs, a suspension of fine powder of CdO was prepared in 20 ml of Triton X-100 (∼24 mmol). A solution of NaHSe was added under stirring to the suspension at 40◦ C in an Ar atmosphere [23]. CdS-NTs were obtained by using an aqueous solution of thioacetamide in place of the NaHSe solution [23]. (4) Bi2 Se3 nanotubes For preparation of Bi2 Se3 -NTs, 7.5 ml of H2 SeO3 solution (1 mol/l) was put into an autoclave (25 ml capacity), and 0.005 mol of BiCl3 was added into the H2 SeO3 solution. After ultrasonic agitation, 4 ml of hydrazine hydrate was poured into the reactants. The reactions were carried out in an autoclave for 150–210◦ C for 24 h [27]. (5) Ag2 Se nanotubes Hexagonal (or trigonal) Se nanotubes as-prepared were used as a template reagent as follows [29]. First, 0.1 mmol of trigonal Se nanotubes was added to an aqueous AgNO3 (0.3 mmol) solution under stirring. The mixture was then kept still in a water bath for 1 h at 60◦ C, thereby yielding the Ag2 Se nanotubes. (6) Nanotubes and microtubes of selenium Elemental selenium has many polytypes. The most stable phase of Se is hexagonal Se (h-Se) composed of spiral Se atoms [35]. Lu et al. have reported the preparation of Se microtubes with lengths of 1–15 mm and approximately 15 μm diameter [30]. These microtubes were obtained by solvothermal reaction by reacting a mixture of 0.2 g Se and 425 ml ethanol in a Teflon capsule in an autoclave at 180◦ C for 12 h. The Te microtubes can also be obtained by this method using stainless steel capsules instead of Teflon capsules [30]. The present authors prepared Se fibers and microtubes using a much simpler method [36]. When amorphous Se was exposed to organic molecules such as acetone with a high dielectric constant for a week at room temperature, the amorphous Se was converted into h-Se crystals with fibrous or needle-like morphology. In the case of organic molecules with a lower dielectric constant such as benzene, polyhedral crystals of α-monoclinic Se were obtained. The reactions are considered to take place by a catalytic effect chemisorbed on the surfaces of amorphous Se, since the reactions also occur in a gaseous

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Fig. 14.6. SEM images of microcrystals and microtubes of hexagonal Se obtained by exposing amorphous Se to acetone for 1 week at room temperature

atmosphere of organic compounds. Figure 14.6 shows microcrystals of h-Se obtained by exposing amorphous Se to liquid acetone at room temperature for 1 week. Some of these microcrystals have cylindrical structures. 14.3.3 Properties and Applications of Chalcogenide Nanotubes (ChNTs) 1. Catalytic properties: ChNTs typically have low densities, highly porous structures, and extremely large surface-to-weight ratios. Thus, the catalytic properties of ChNTs are much better than those of MCh2 . For example, a high catalytic activity of WS2 -NTs has been observed for hydrodesulfurization of thiophene [37]. 2. Electrical properties: It has been theoretically predicted that MoS2 - and WS2 -NTs have semiconductive conduction, and NbS2 - and NbSe2 -NTs metallic conduction [13]. Optical observations revealed that the band gap of semiconducting ChNTs decreases with decreasing diameter, owing to quantum size effects [14]. The armchair MoSe2 -NT was found to be an indirect gap semiconductor, and zigzag MoSe2 -NT was found to be a direct gap semiconductor. Monolayer MoSe2 -NTs were observed to show good conductivities, as high as that of graphite [38]. 2H-NbSe2 is known to exhibit superconductivity with a Tc of 7.2 K [39], and NbSe2 -NT also shows superconductivity at 8.3 K [40]. MoS2 -NTs show reproducible and stable field emission currents [41]. Also, WS2 -NTs can be used as sharp tips in scanning probe microscopy [42]. 3. Tribological properties: Fullerene-like WS2 and MoS2 nanoparticles and nanotubes were found to have excellent tribological properties: WS2 NTs outperforms currently used solid lubricants 2H-MoS2 and 2H-WS2 in every respect (friction, wear, and lifetime of the lubricant) [43, 44]. These superior tribological properties can be attributed to rolling friction

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allowed by the round shapes of the nanostructures. These nanotubes showed strong shock-wave resistance. Under similar shock conditions, WS2 nanotubes were found to be more stable than CNTs [45]. Thus, the most likely application of ChNTs is as a solid-state lubricant especially under harsh conditions.

References 1. L. Pauling, Proc. Nat. Acad. Sci. U.S. 16, 578 (1930) 191 2. G.A. Wiegers and A. Meerschaut, Mat. Sci. Forum 100 & 101, 101 (1992) 191 3. K. Suzuki, private communication (cf. K. Suzuki, T. Enoki, and K. Imaeda, Solid State Commun. 78, 73, (1991)) 191 4. S. Iijima, Nature 354, 56 (1991) 191 5. R. Tenne, L. Margulis, M. Genut, and G. Hodes, Nature 360, 444 (1992) 191, 192, 194 6. P.J. Harris, Carbon Nanotubes and Related Structures (Cambridge Univ. Press, Cambridge, 1999) p. 218 192 7. C.N.R. Rao, M. Nath, Dalton Trans., 1 (2003) 192 8. M. Remskar, Adv. Mater. 16, 1497 (2004) 192 9. C.N.R. Rao, A. Govindaraj, Nanotubes and Nanowires (RSC Publ. Cambridge, 2005), p.111 192 10. R. Tenne, M. Homyonfer, Y. Feldman, Chem. Mater. 10, 3225 (1998) 192, 194, 195 11. R.M. Lieth, J.C.M. Terhell, Physics and Chemistry of Materials with Layered Structures Vol 1, (D. Reidel Publ., Dordrecht, Boston, 1977) p.141 192 12. for example, T. Ohtani et al., Mater. Res. Bull 19, 1367 (1984) 192 13. G. Seifert et al., Phys. Rev. Lett. 85, 146 (2000) 193, 197 14. G.L. Frey et al., Phys. Rev. B 57, 6666 (1998) 193, 197 15. Y. Feldman, E. Wasserman, D.J. Srolovitz, R. Tenne, Science 267, 222 (1995) 194, 195 16. L. Margulis, P. Dluzewski, Y. Feldman, R. Tenne, J. Microscopy 181, 68 (1996) 194 17. M. Hershfinkel et al., J. Am. Chem. Soc. 116, 1914 (1994) 194 18. M. Nath, C.N.R. Rao, J. Am. Chem. Soc. 123, 4841 (2001) 194, 196 19. D.H. Galvan et al., Fullerene Sci. Technol. 8, 143 (2000) 194 20. J. Chen et al. Chem. Commun., 980 (2003) 194, 196 21. M. Nath, C.N.R. Rao, Angew. Chem. Int. Ed., 41, 3451 (2002) 194, 196 22. J.A. Hollingsworth et al., J. Am. Chem. Soc. 122, 3562 (2000) 194 23. C.N. Rao et al., Appl. Phys. Lett. 78, 1853 (2001) 194, 196 24. A. Govindaraji et al., Isr. J. Chem. 41, 23 (2001) 194 25. L. Dloczik et al., Appl. Phys. Lett. 78, 3687 (2001) 194 26. X. Ziang et al., Adv. Mater. 13, 1278 (2001) 194 27. H. Cui et al., J. Solid State Chem. 177, 4001 (2004) 194, 196 28. H. Lee et al., Nano Lett. 7, 778 (2007) 194 29. Sheng-Yi Zhang et al., J. Phys. Chem. C 111, 4168 (2007) 194, 196 30. J. Lu, Y. Xie, F. Xu, L. Zhu, J. Mat. Chem. 12, 2755 (2002) 194, 196 31. Y. Feldman et al., Science 267, 222 (1995) 195 32. A. Rothschild et al., J. Phys. Chem. B 104, 8976 (2000) 195 33. M. Nath, A. Achutharao, C.N.R. Rao, Adv. Mater. 13, 283 (2001) 195 34. M. Nath, K. Mukhopadhyay, C.N.R. Rao, Chem. Phys. Lett. 352, 163 (2002) 196 35. M. Remskar et al., Surface Sci. 433–435, 637 (1999) 196

14 Synthesis and Applications of Chalcogenide Nanotubes 36. 37. 38. 39. 40. 41. 42. 43. 44. 45.

199

T. Ohtani, N. Takayama, K. Ikeda, and M. Araki, Chem. Lett. 33, 100 (2004) 196 E. Furimsky, Appl. Catal. A 208, 251 (2001) 197 M. Remskar et al., Adv. Mater. 15, 237 (2003) 197 K. Ishida, Y. Nino, G.–q. Zheng, Y. Kitaoka, K. Asayama, T. Ohtani, J. Phys. Soc. Japan 6,5, 2341 (1996) 197 G. Seifert et al., Solid State Commun. 114, 245 (2000) 197 V. Nemanic et al., Appl. Phys. Lett. 82, 4573 (2003) 197 A. Rothschild, S.R. Cohen, R. Tenne, Appl. Phys. Lett. 75, 4025 (1999) 197 L. Rapoport et al., Nature 387, 791 (1997) 197 L. Rapoport, N. Fleischer, R. Tenne, Adv. Mater. 7–8, 651 (2003) 197 Y.Q. Zhu et al., J. Am. Chem. Soc. 125, 1329 (2003) 198

Index Ag2 Se nanotubes, 196

MS2 nanotubes formation mechanism, 193–194

Bi2 Se3 nanotubes, 196 Catalytic properties of ChNTs, 197 CdS and CdSe nanotubes, 197 Ch-M-Ch sandwiches, 192–193 Chalcogenide nanotubes (ChNTs), 191–193 Closed-tube chemical transport method, 195–196 Decomposition of MS3 in H2 stream, 195 Electrical properties of ChNTs, 197 Fullerene type nanoparticle of MoS2 and WS2 , 193–194

Selenium, nanotubes and microtubes, 196–197 Serpentine, 191 Sulfurization of MO3 , 194–195 Teflon capsule, 196 Transition-metal dichalcogenides (MCh2 ), 191, 196 Transition-metal dichalcogenides (MCh2 ) with layered structures, 192–193 Tribological properties of ChNTs, 197–198

15 Synthesis and Functions of Fullerene Nanotubes Kun’ichi Miyazawa National Institute for Materials Science, Fullerene Engineering Group, 1-1, Namiki, Tsukuba, Ibaraki, 305-0044, Japan [email protected] Abstract Fullerene nanotubes (FNTs) are the tubular needle-like crystals with diameters less than 1 μm that are composed of fullerene molecules such as C60 and C70 . Single crystalline FNTs can be synthesized by use of the liquid–liquid interfacial precipitation method. Up to now, C60 nanotubes, C70 nanotubes, and C60 − C70 two-component nanotubes (NTs) have been synthesized. The as-grown C60 and C70 nanotubes have solvated hexagonal structures and turn to face-centered cubic structures by losing solvent molecules upon drying. The C60 molecules of dried C60 nanotubes are bonded via weak van der Waals forces. The C60 nanotubes decompose at about 416◦ C in air, showing a high thermal stability. Various materials can be incorporated into the FNTs owing to their relatively large inner diameter on the order of 100 nm. The FNTs will find various applications in the field of transistors, solar cells, catalysts, chemical synthesis templates, MEMS devices, and so forth in future.

15.1 Introduction Carbon nanotubes (CNTs) are composed of rolled graphene sheets. On the contrary, the fullerene nanotubes (FNTs) are the tubular thin fibers that are composed of fullerene molecules such as C60 and C70. Liu et al. first synthesized C60 nanotubes (C60 NTs) using the holes of a porous alumina membrane as the synthetic templates [1]. They repeatedly coated the inner surface of the holes with a toluene solution of C60 , heat-treated the alumina membrane at high temperature of 500◦ C under an argon atmosphere, and obtained C60 NTs by dissolving the alumina membrane with an aqueous solution of sodium hydroxide. The C60 NTs thus prepared were polycrystalline tubular needlelike crystals with a length of about 60 μm that is determined by the thickness of alumina membrane. On the other hand, fine single crystalline nanofibers composed of C60 , “C60 nanowhiskers (C60 NWs),” were discovered by us in 2001 in a colloidal solution of lead zirconate titanate (PZT) added with a small amount of C60 [2]. C60 was used as an oxygen scavenger to hinder the formation of pyrochlore phase of PZT. The PZT sol contained both a good solvent of C60 (toluene) T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 201–214 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 15

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and a poor solvent of C60 (isopropyl alcohol=IPA). This combination of good solvent and poor solvent of fullerene is the basis of the liquid–liquid interfacial precipitation method (LLIP method) that can produce various forms of fullerene nanofibers. The fullerene nanowhiskers (FNWs) are the single crystalline nanofibers that are composed of fullerene molecules and usually have non-tubular morphologies. The LLIP method can produce the fullerene nanofibers with lengths ranging from a few micrometers to the order of millimeters. The FNTs with single crystalline walls were synthesized for the first time using C70 for the fullerene source by the LLIP method [3]. The C70 NTs were synthesized by self-assembly without using the templates like porous alumina membranes. The FNTs presented here are the single crystalline tubular nanofibers with a uniform diameter along most of the growth axis. The FNTs have an excellent recyclability and are expected to be applied variously in the fields of catalysts, chemical synthesis templates, electrodes, semiconductors, and so forth. Here the synthetic method, the functions, and potential applications of FNTs are shown.

15.2 Liquid–Liquid Interfacial Precipitation Method The LLIP method is a technique to synthesize the fullerene nanofibers and microfibers that nucleate at the interface between a fullerene-saturated organic solution and another solvent with a low solubility of fullerene, i.e., the LLIP method synthesizes the fullerene crystals through their nucleation and growth by adding a poor solvent of fullerene to a fullerene solution of good solvent. However, both the solvents in the LLIP method are usually miscible with each other. The LLIP method is also applicable to synthesize other crystals in addition to the fullerene crystals. For example, Kadota et al. reported that asymmetric NaCl crystals can be synthesized by the LLIP method using the interface formed between an aqueous NaCl solution and 1-butanol [4]. The diameter of FNWs is defined to be less than 1,000 nm and can be as small as about 80 nm at present [5]. The FNWs are usually single crystalline and their length ranges from a few micrometers to more than millimeters. Various FNWs such as C60 NWs, C70 NWs, C60 [C(COOC2 H5 )2 ]NWs, and two-component FNWs containing C60 derivatives with a composition of C60 -12.3 mass% C60 C3 H7 N have been synthesized [6–8]. Since the LLIP method is a simple process that just forms an interface between two solvents, it can be variously modified. The order of placing good solvent and poor solvent can be changed. Ultrasonic waves can be irradiated after forming the liquid–liquid interface to enhance the nucleation of seed crystals in the solution. The factors that mainly control the growth of C60 NWs are light [9–12], temperature [13], the quantity ratio of good solvent versus poor solvent, and

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impurity water contained in the solvents. In the system of C60 -toluene solution and isopropyl alcohol (IPA), a small amount of water contained in IPA assists the growth of C60 NWs. However, the addition of excess amount of water to IPA hinders the growth of C60 NWs [14]. This phenomenon is assumed to be owing to the change of isopropyl alcohol molecule clusters that is caused by the impurity water molecules which have a large dipole moment and form strong hydrogen bondings [15]. The length of C60 NWs can be controlled by changing the factors such as growth temperature, volume ratio between the good solvent and the poor solvent as well as the impurity water contained in the solvents. Figure 15.1

Fig. 15.1. (a) SEM micrograph of short C60 nanowhiskers and (b) the histograms showing the length (mean value, 10.3 ± 2.1 (m) and diameter (mean value, 631 ± 194 nm) of the short C60 nanowhiskers (by courtesy of Ms. Kayoko Hotta)

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shows an example of the short C60 NWs with similar lengths and diameters that were synthesized by use of a C60 -saturated toluene solution and IPA by controlling the growth temperature and time. The growth activation energy of C60 NWs was measured to be 52.8 kJ mol−1 in the system of toluene and IPA [13]. This activation energy is greater approximately by a factor of 4 than the activation energy of 13.1 kJ mol−1 in the diffusion of C60 in a mixed solvent of toluene and acetonitrile (4:1, v/v) [16]. This result shows that a high desolvation energy to remove solvent molecules from the solvated C60 molecules is necessary in order to complete the surface accumulation process of C60 molecules in the crystal growth. Figure 15.2 shows a transmission electron microscopy (TEM) image of the C60 NWs with a porous structure with pore sizes of several to tens of nanometers. Those porous C60 NWs with a high surface area of 376 m2 g−1 could be prepared by the LLIP method combined with ultrasonication, where IPA was added to a benzene solution saturated with C60 at a liquid temperature of 10◦ C, followed by the ultrasonication [5]. The pores are assumed to be formed by a densification of the matrix of C60 NWs during drying in the air. The LLIP method can be scaled down to the level of microchannel reactor. The first synthesis of FNWs using the microreactor was performed by us in

Fig. 15.2. TEM image of a porous C60 nanowhisker and its selected area electron c 2007 American diffraction pattern (inset). Reprinted with permission from [5]  Chemical Society

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Fig. 15.3. Synthesis of C60 fullerene nanowhiskers by use of the microchannel rec 2004 Elsevier actor. Reprinted with permission from [17], 

2004 [17]. As shown in Fig. 15.3, a microchannel with a width of 250 μm, a depth of 123 μm, and a length of 22.5 mm was formed on a silicon substrate that was covered by a glass layer with the holes a, b, c, and d. The holes a and b are the inlets of liquid, and the hole c is an outlet. The hole d is closed. When IPA is injected through the hole a and a C60 -saturated toluene solution is injected through the hole b, a liquid–liquid interface is formed in the microchannel and C60 NWs form vertically to the liquid–liquid interface. Subsequently, Shinohara et al. also fabricated a similar system and prepared a variety of morphologies of low-dimensional C60 crystals [18].

15.3 Synthesis and Properties of Fullerene Nanotubes The fullerene nanofibers can have two morphologies, i.e., tubular ones and non-tubular solid ones. The fullerene nanofibers with the tubular morphology are specially called fullerene nanotubes (FNTs). The FNTs can take both the polycrystalline and single crystalline walls. A model of single crystalline C60 nanotube (C60 NT) is shown in Fig. 15.4. Up to now, C60 NTs, C70 NTs, and C60 –C70 two-component nanotubes (NTs) have been synthesized by the LLIP method [3, 19, 20]. The FNTs can be synthesized by pouring IPA on a pyridine solution saturated with fullerene at a cool room temperature. A typical procedure to prepare C60 NTs is as follows. First, a pyridine solution saturated with C60 is poured into a transparent glass bottle. This glass bottle is irradiated by ultraviolet light for 24 h and IPA is added to form a liquid–liquid interface. Next, this glass bottle is irradiated by ultrasonic wave, manually mixed, and stored in an incubator at a cool room temperature typically lower

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Fig. 15.4. Model of a fullerene nanotube composed of C60 molecules

than 15◦ C [21] . The yield of C60 NTs changes depending on the wavelength of light illuminated on the pyridine solution of C60 [12]. The highest yield (30–38 mg/L) of C60 NTs was obtained in a mixed solution of C60 -saturated pyridine and IPA when the C60 -saturated pyridine solution was illuminated with the light of 370 nm wavelength corresponding to the maximum absorption peak of solid C60 . However, high yields (21–27 mg/L) of C60 NTs were also obtained in the wavelength range of 600–800 nm where the absorption of light by C60 is weak. This phenomenon is assumed to be related to the transient absorption of the triplet excited state of C60 in the wavelength region of 740 nm that is formed by the decay of photo-excited singlet C60 molecules through the intersystem crossing [22]. The illumination of C60 pyridine solution by the photons of special wavelengths is conjectured to produce the suitable cluster size of C60 molecules for the formation of C60 NTs. The fullerene nanofibers can have the tubular structure when the diameter is greater than about 240 nm in both the C60 nanofibers and C70 nanofibers owing to their finite wall thickness [3, 21]. The diameter of C60 nanofibers can be changed by changing the synthesis temperature and the mixing ratio between the good solvent and the poor solvent of fullerenes. In general, the diameter of C60 nanofibers decreases by increasing the concentration of C60 in the mixed solvents and the synthesis temperature [11]. It was also found that the uniformity of the diameter of C60 nanofibers increases by increasing the concentration of C60 and increasing the synthesis temperature [11]. This phenomenon may be explained by a formation of fine C60 nuclei with similar sizes. As shown in Fig. 15.5, the C60 NTs show a metallic brownish color with the polygonal cross sections (Fig. 15.6) and grow up to a length of millimeters or more with a constant diameter along the growth axis and without any

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Fig. 15.5. Optical micrograph of C60 nanotubes

Fig. 15.6. TEM image of a fractured C60 nanotube

bifurcation. The as-prepared C60 NTs have a solvated hexagonal crystal structure with the lattice constants a = 1.541 nm and c = 1.00 nm. This hexagonal structure turns to a face-centered cubic structure with a lattice constant of 1.424 nm by losing the contained solvent molecules upon drying [21]. The C60 NT (outer diameter 714 nm, inner diameter 416 nm) shown in Fig. 15.7 has a hole with a length of more than 100 μm, showing no clogging. Various nanomaterials can be stored in the holes of C60 NTs owing to their relatively large inner diameter. For example, PtCl4 , DNA, and KBr crystals

Fig. 15.7. TEM image of a C60 nanotube

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could be stored in the holes of C60 NTs [23–25]. Transportation of materials through FNTs is also possible. It was demonstrated that methyl alcohol flew inside a C60 microtube with an outer diameter of 2.5 μm at a line speed on the order of 50 μm s−1 [25]. Extinction fringes of electron beam are observed at the places marked by arrows (Fig. 15.7). The extinction fringes are continuous, and hence indicating that the wall of the C60 NT is single crystalline. Raman spectrometry is a very useful method to know the vibrational state of fullerene molecules. Figure 15.8 (a) shows a Raman spectrum of the C60 NTs well-dried at room temperature. Since this Raman spectrum of C60 NTs strongly resembles that of pristine C60 crystals of Fig 15.8b whose C60 molecules are weakly bound via van der Waals bonding forces at room temperature, the well-dried C60 NTs are known to be soluble in organic solvents like toluene. This soluble property of C60 NTs in organic solvents can be utilized in both the synthesis and recovery of fibrous substances. For example, single needle-like crystals of KBr were synthesized by injecting a methyl alcohol solution of KBr into the microtubes and nanotubes of C60 via the capillary force. After drying the C60 tubes with the KBr crystals inside, the C60 tubes were dissolved by toluene and the inside KBr needle-like crystals were collected [23].

Fig. 15.8. Raman spectra of (a) C60 nanotubes dried at room temperature and (b) C60 powder (99.5% purity)

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Fig. 15.9. TEM image of a C60 NT containing a precipitate of PtCl4

Figure 15.9 shows a TEM image of a C60 NT containing a precipitate of PtCl4 . The inclusion of PtCl4 precipitates was done by injecting an isopropyl alcohol solution of PtCl4 into the C60 NT via the capillary force. Before the injection experiment, the C60 NTs were cut by ultrasonication into short tubes with open ends. This example suggests that the C60 NTs may be used to carry various substances for medical and chemical applications. The agitation of the liquid–liquid interfaces by ultrasonication is not always necessary in the synthesis of FNTs. For example, C70 NTs can be obtained by keeping a glass bottle containing 3 volumes of IPA placed upon one volume of C70 -saturated pyridine at 10◦ C. As shown in Fig. 15.10, the C70 NTs have a fibrous morphology with shining metallic colors and like C60 NTs grow to the fibers with lengths on the order of millimeters and diameters upto a few hundred nanometers.

Fig. 15.10. Optical micrograph of C70 nanotubes. Reprinted with permission from c 2005 Elsevier Ltd [19], 

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Fig. 15.11. TEM image of (a) C70 nanotube, (b), (c), and (d) magnified images corresponding to the places marked by A, B, and C in photo (a), respectively [3]

A C70 NT was observed from its end by TEM as shown in Fig. 15.11a. In Fig. 15.11b, the tubular structure is terminated at the place marked by arrow A in photo (b), showing that the end of the tube is closed. The C70 NT becomes thicker along its growth axis. However, the tube diameter reaches a constant value of about 500 nm at the place of about 100 μm from its end. The average wall thickness of the tube obtained by the TEM image is 113 ± 18 nm. The granular images observed in Figs. 15.11b–d are the C70 particles that were formed during drying in air. The C70 NTs without any inclusion in their tubular space were also frequently found. The inner and outer diameters of the C70 NT of Fig. 15.12 are 240 and 420 nm, respectively. The magnified image of Fig. 15.13 clearly shows an open end of the C70 NT. The as-grown C70 NTs have a solvated hexagonal crystal structure with the lattice constants of a = 1.603 nm and c = 1.09 nm [21]. However, the C70 NTs turn to a face-centered cubic (fcc) structure (a = 1.495 ± 0.015 nm) by losing the contained solvents upon drying [3]. A high density of dislocations were observed in the dried C70 NTs that were introduced by the transformation from the hexagonal structure to the fcc structure. Since C60 can make a solid solution with C70 [26], C60 –C70 two-component nanotubes can be synthesized. Figure 15.14 shows a fullerene nanotube with a composition of C60 –15 mol% C70 that was synthesized by use of the unpurified fullerene powder with a composition of C60 –24 mol% C70 [3]. Since the price of crude fullerene powder is going down, the cheaper fullerene nanotubes prepared by use of the crude powder will find wider applications in the fields of

Fig. 15.12. TEM image of a C70 nanotube. Reprinted with permission from [19], c 2005 Elsevier 

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Fig. 15.13. Enlarged TEM image for the left end of the C70 nanotube of Fig. 15.12. c 2005 Elsevier Reprinted with permission from [19], 

Fig. 15.14. TEM image of a C60 –C70 two-component nanotube with a composition of C60 –15 mol% C70 [3]

catalysts, chemical synthesis templates, filters, battery electrodes, solar cells, and so forth. The Young’s modulus of C60 NTs has been measured to be 62–107 GPa by a transmission electron microscope equipped with a system of atomic force microscopy, which is close to 69 GPa of pure aluminum [27]. Since this value of Young’s modulus is by a factor of 1.1–3.3 greater than that of C60 nanowhiskers [28], the lighter and stronger C60 NTs will be more useful than the non-tubular C60 NWs for mechanical applications such as sensor cantilever beams. The thermal stability of C60 NTs in air was examined by the thermogravimetric (TG) analysis and their decomposition temperature (T0 ) was measured to be 416◦ C (Fig. 15.15a). Although the decomposition temperature of C60 NTs is lower by 35◦ C than the decomposition temperature 451◦ C of C60 NWs (Fig. 15.15b) [29], the thermal stability of C60 NTs is much higher than the typical organic semiconductor pentacene that decomposes at about 300◦ C in air [30].

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Fig. 15.15. Thermogravimetric analyses of (a) C60 nanotubes, (b) C60 nanowhiskers, and (c) C60 powder in air (heating rate 10◦ C min−1 ). Reprinted c The Institute of Electrical Engineers of Japan with permission from [29], 

15.4 Summary The synthesis of single crystalline FNTs has become possible for the first time by use of the liquid–liquid interfacial precipitation method. Since the FNTs are literally composed of fullerene molecules, innumerable kinds of FNTs may be produced by combining different fullerene molecules. Those new fullerene quasi-one-dimensional nanomaterials will find variety of applications in the field of semiconductor, catalyst, medicine, electronic devices, and so forth. Further intensive study is necessary on the synthesis and properties of FNTs for their practical application in future.

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References 1. H. Liu, Y. Li, L. Jiang, H. Luo, S. Xiao, H. Fang, H. Li, D. Zhu, D. Yu, J. Xu, B. Xiang, J. Am. Chem. Soc. 124, 13370 (2002) 201 2. K. Miyazawa, A. Obayashi, M. Kuwabara, J. Am. Ceram. Soc. 84, 3037 (2001) 201 3. K. Miyazawa, J. Minato, T. Yoshii, M. Fujino, T. Suga, J. Mater. Res. 20, 688 (2005) 202, 205, 206, 210, 211 4. K. Kadota, Y. Shiraka wa, I. Matsumoto, A. Shimosaka, J. Hidaka, Advanced Powder Technol. 18, 775 (2007) 202 5. M. Sathish, K. Miyazawa, T. Sasaki, Chem. Mater. 19, 2398 (2007) 202, 204 6. K. Miyazawa, J. Am. Ceram. Soc. 85, 1297(2002) 202 7. K. Miyazawa, T. Mashino, T. Suga, J. Mater. Res. 18, 2730(2003) 202 8. K. Miyazawa, T. Mashino, T. Suga, Trans. Mater. Res. Soc. Jpn. 29, 537(2004) 202 9. M. Tachibana, K. Kobayashi, T. Uchida, K. Kojima, M. Tanimura, K. Miyazawa, Chem.Phys.Lett. 374, 279(2003) 202 10. K. Kobayashi, M. Tachibana, K. Kojima, J. Cryst. Growth 274, 617 (2005) 202 11. C.L. Ringor, K. Miyazawa, Diam. Relat. Mater. 17, 529 (2008) 202, 206 12. C.L. Ringor, K. Miyazawa, “High yield preparation of fullerene nanowhiskers and nanotubes by solution route”, NANO 3, 329 (2008) 202, 206 13. K. Hotta, K. Miyazawa, “Growth rate measurement of C60 fullerene nanowhiskers”, NANO 3, 355 (2008) 202, 204 14. K. Hotta, K. Miyazawa, Effect of solvent on the growth of C60 nanowhiskers, Meeting Abstracts of the Physical Society of Japan 63, 765(2008) 203 15. J.-H. Guo, Y. Luo, A. Augustsson, S. Kashtanov, J.-E. Rubensson, D. K. Shuh, H. ˚ Agren, J. Nordgren, Phys. Rev. Lett. 91, 157401 (2003) 203 16. M. Wei, H. Luo, N. Li, S. Zhang, L. Gan, Microchem. J. 72, 115 (2002) 204 17. S.-H. Lee, K. Miyazawa, R. Maeda, Carbon 43, 887 (2005) 205 18. K. Shinohara, T. Fukui, H. Abe, N. Sekimura, K. Okamoto, Langmuir 22, 6477(2006) 205 19. K. Miyazawa, J. Minato, T. Yoshii, T. Suga, Sci. Technol. Adv. Mater. 6, 388(2005) 205, 209, 210, 211 20. J. Minato, K. Miyazawa, T. Suga, Sci. Technol. Adv. Mater. 6, 272(2005) 205 21. J. Minato, K. Miyazawa, Diam. Relat. Mater. 15, 1151 (2006). 206, 207, 210 22. T. Akasaka, Y. Maeda, T.W Akahara, M. Okamura, M. Fujitsuka, O. Ito, K. Kobayashi, S. Nagase, M. Kako, Y. Nakadaira, E. Horn, Org. Lett. 1, 1509 (1999) 206 23. J. Minato, K.Miyazawa, J. Mater. Res. 21, 529(2006) 208 24. K. Miyazawa, C. Ringor, Mater. Lett. 62, 410 (2008) 208 25. K. Miyazawa, S. Cha, C. Ringor, J. Okuda, A. Taniguchi, M. Watanabe, M. Tachibana, J. Minato, “Synthesis of fullerene nano and micro tubes for materials storage, delivery and recovery”, NANO 3, 335 (2008) 208 26. D. Havlik, W. Schranz, M. Halu¸ska, H. Kuzmany, P. Rogl, Solid State Commun. 104, 775 (1997) 210 27. W.D. Callister, Jr., Materials Science and Engineering : An Introduction, 3rd edn. (John Wiley & Sons, Inc., New York, 1994) 211 28. T. Kizuka, K. Saito, K. Miyazawa, Diam. Relat. Mater. 17, 972 (2008) 211 29. K. Miyazawa, J. Minato, K. Asaka, T. Kizuka, T. Mashino, S. Nakamura, T. Tachibana, T. Suga, The Papers of Technical Meeting on Physical Sensor, IEE Japan, PHS-07–14 (2007) 211, 212 30. Kagaku Jiten, Tokyo Kagaku Dojin, P. 1337, 1999 211

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Index As-grown C70 NTs, 210 C60 nanowhiskers (C60 NWs), 201–202 C60 –C70 two-component nanotubes, 210–211 C70 nanotubes optical micrograph, 209 Fullerene nanotubes (FNTs), 201 Fullerene nanowhiskers (FNWs), 202 Lead zirconate titanate (PZT), 201 Liquid–liquid interfacial precipitation method (LLIP method), 202–205

Optical micrograph of C60 nanotube, 207 Porous C60 nanowhisker TEM image, 204 PtCl4 precipitate inclusion in C60 NT, 209 Raman spectrometry, 208 TEM image of C60 nanotube, 207 Thermal stability of C60 NTs, 211 Thermogravimetric (TG) analysis of C60 NTs, 211–212

Model of fullerene nanotube composed of C60 molecules, 206

Well-dried C60 NTs, 208

Needle-like crystals of KBr, 208

Young’s modulus of C60 NTs, 211

16 Synthesis and Applications of Noble-Metal Nanotubes Tsuyoshi Kijima Department of Applied Chemistry, Faculty of Engineering, Miyazaki University, Miyazaki 889-2192, Japan [email protected] Abstract Metallic nanotubular materials can be formed in two different manners, self-organization or template-assisted organization, depending on their bonding natures. Base metallic Bi and Te with a 1D or 2D interatomic covalent bonding nature form a nanotubular phase by the reduction reaction of their salts at elevated temperatures through the cylindrical or scrolled growth of the metal atoms based on their bonding anisotropies. In contrast, the nanotubular phases of noblemetals with no covalency are formed by the assistance of soild or supra-molecular core and sheath templates. The solid templating studies demonstrated the deposition of Au, Pt and Pd nanotubes on the outer surface of Ag nanorods as a sheath template as well as those on the inner surface of nanoporous polycarbonate or anodic aluminum oxide films as a sheath template. The use of triple-branched polyoxyethylene (PEO)-based nonionic surfactant LCs as a core template successfully leads to the growth of Pt, Pd, and Ag nanotubes with an outer diameter of as small as 6–7 nm. In this system, the thin-walled nanotubular structure is inherited from the 2D metal clusters induced through the specific effect of triple PEO chains of surfactant molecules, coupled with their spatially controlled growth within the aqueous shells of cylindrical micelles. A few examples are also referred to for the applications of noble-metal nanotubes as a catalyst for polymer electrolyte fuel cells or biphenyl formation reaction.

16.1 Introduction Noble-metal nanoparticles have been extensively used as catalysts for a variety of chemical reactions in various fields including oil refining, petrochemical processes, depoisoning of exhaust gas from automobiles or factories, and drug or oil and fat manufacturing [1–3]. The Pt–Pd–Rh ternary catalysts as an industrial product are essential for depoisoning hydrocarbons, carbon monoxide, and nitrogen oxides (NOx) exhausted from gasoline engine and their total shipment amounts to about two quarters of that for all the catalysts produced in the world. Platinum and ruthenium (Ru) have served as the electrocatalysts essential for the practical use of polymer electrolyte fuel T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 215–234 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 16

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cells (PEFCs). The noble-metal catalysts are also expected to be applicable as catalytic materials for treating chlorine-based by-products and volatile organic compounds (VOC). However, the high cost and low supply of noblemetal catalysts, in marked contrast to their extremely extensive applications, have required much effort to improve their performance as highly functional catalysts. For example, recent studies in the field of PEFCs have been directed considerably to the preparation of Pt/C composites with the aim to lower the metal catalyst loading [3–6]. The catalytic activity of noble-metal particles generally depends on their structural and/or morphological properties such as particle size, shape, and the arrangement of surface atoms [7, 8]. This is the case for their use in electrocatalysis [9, 10] and actually a few studies are devoted to the loading of Pt nanostructures as the electrode materials of PEFCs [11–13]. Various methods have been developed for the control in nanometer-scales of Pt particles. Sizeand shape-controlled Pt nanoparticles, including cubic, tetrahedral, icosahedral, and cubo-octahedral nanocrystals, have been prepared by the polyol method or its combined use of capping polymers to induce anisotropic growth of their different facets [14–17]. The polyol process was also used to synthesize single-crystalline Pt nanowires through the control of reaction rate with a trace amount of Fe2+ (or Fe3+ ) [18]. High aspect ratio Pt nanowires of ca. 2 nm diameter were prepared by tuning the solvent polarity and amount of NaBH4 to prevent Pt (0) from aggregation [19]. Alternate approaches using a variety of templating agents have been extensively applied to the fabrication of unconventional Pt nanostructures, including nanowires or nanorods [20–23], nanosheets [24, 25], nanowire network [26], mesoporous solids [27–30], hollow nanospheres [31], nanodendrites [32], nanocages [34], and foamlike materials [34]. These processes used solid or molecular templating agents such as 2D mesoporous silica or alumina [20–23], graphite [24, 25], 3D mesoporous silica [26], surfactant molecular assemblies [27–30], Co nanoparticles as sacrificial templates [31], and surfactant solution or liposome [32–34]. On the other hand, outstanding attention is focused on nanosized tubular materials for their unique physical properties and potential applications as gas and fluid paths or reservoirs in catalysis, fuel cells, sensors, and separation systems [35–38]. Very few studies, however, have reported on noble-metal nanotubes, in spite of their extraordinary nanostructures characterized by catalytic sites located on both the outer and inner surfaces and edges. As reviewed in Chap. 1, the essential pathway to nanotubular structures can be categorized into two classes: The one is based on the self-organization of constituent atoms or molecules, e.g., either the cylindrical or scrolled growth of atomic or molecular species through their interatomic covalent bonding to stably form a 2D layered structure or the phase transition from the initiallyformed straight nanosheet, as observed for carbon, metal chalcogenide, nitride, and some kinds of oxides such as MoO3 . The other is based on the templateassisted formation of nanotubular structure. Covalency-poor materials such as metals form their nanotubular phases by the assistance of templates,

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e.g., the deposition of chemical species on the surface of solid or supramolecular core templates such as silver nanorods or the internal surface of solid or supra-molecular sheath templates such as anodic aluminum oxide films or nanoporous polycarbonate ones. In this chapter, we describe the synthesis of noble-metal nanotubes using solid or molecular liquid-crystalline templates, along with the template-free liquid phase synthesis of base metal nanotubes such as bismuth and tellurium nanotubes.

16.2 Template-Free Synthesis of Base Metal Nanotubes There have been reports on the nanotubular phases of two base metals, bismuth and tellurium [39–41]. Bismuth is a metalloid with a resistivity of 1.2×10−4 Ωcm at room temperature. Metallic bismuth has an antimony structure in which each bismuth atom has three sp 3 -type hybrid orbitals to form a hexagonal-networked but puckered sheet, leading to a layered structure based on the stacking of the graphene-like sheets [42]. Tellurium is a semiconducting material with the same structure as metallic selenium and the helical chains of covalently bonded tellurium atoms are arranged along the c-axis to form a trigonal pyramid [43]. Li et al. found the formation of thin-walled metallic bismuth nanotubes with uniform diameters of ∼5nm and lengths ranging between 0.5 and 5 μm through the hydrazine reduction of bismuth nitrate at room temperature, followed by the pH adjustment of the resulting mixture to 12–12.5 and the subsequent hydrothermal reaction at 120◦ C for 12 h (Fig. 16.1) [39]. High resolution transmission electron microscopy revealed that the metallic bismuth nanotubes are multiwalled ones with an interlayer spacing of 0.6 nm, in agreement with about half of the lattice constant along the c-axis of the bismuth structure or 1.185 nm determined by their X-ray diffraction patterns. These facts indicate that the hexagonal-networked bismuth sheets are curved to form a multiwalled or scroll-type nanotube. Mayer et al. synthesized tellurium nanotubes with diameters of ∼240 nm and lengths of ∼ 4 μm by refluxing a mixture of orthotelluric acid and ethylene glycol at 197◦ C (Fig. 16.2) [40]. It was also observed that tellurium cylindrical seeds grow from both their edges toward [001] and [00-1] directions along the longitudinal axis of tellurium spiral chains to form a pair of nanotubes connected together with the seed material. Mo et al. reported that the hydrothermal reaction of Na2 TeO3 in 25 wt% aqueous ammonia at ∼180◦ C led to the formation of tellurium single-crystalline nanotubes with diameters of ∼24,150–400 nm, wall thicknesses of 5–150 nm, and lengths of 5– 10 μm at yields of 10–15%, along with tellurium nanobelts at yields of 50–75% (Fig. 16.3) [41]. Metallic tellurium is formed through the disproportionation reaction (1) that occurs in the pH range of 8–14, more favorably, at pH12–12.5. In this reaction, initially-formed spiral nanobelts are likely to further grow into

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Fig. 16.1. TEM image of bismuth nanotubes synthesized under hydrothermal condition at 120◦ C. Reprinted with permission from [39]

nanotubular forms (Fig. 16.3B-b) or serve as a template for the nanotubular growth of tellurium deposited around the spiral core (Fig. 16.3B-a). 3NaTeO3 + H2 O → Te + 2NaTeO4 + 2NaoH

(1)

The above observations for bismuth and tellurium nanotubes clearly demonstrate that hexagonal-networked sheets or spiral chains of metallic atoms based on the covalent nature of both elements play a fundamental role for the formation of metallic nanotubes.

Fig. 16.2. TEM images of tellurium nanotubes synthesized by refluxing method using ethylene glycol at 197◦ C. Reprinted with permission from [40]

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Fig. 16.3. (A) TEM images of tellurium nanobelts (left) and nanotubes (right) synthesized by hydrothermal reaction of Na2 TeO3 and (B) the formation mechanism of both nanostructures. Reprinted with permission from [41]

16.3 Synthesis of Noble-Metal Nanotubes Using Solid Templates Solid or molecular templates have been used for the synthesis of nanotubes from materials with no or low covalent-bonding character. In the solidtemplate-assisted synthesis, chemical species were deposited on the internal surface of solid sheath templates such as anodic porous alumina and nanoporous polycarbonate films or on the surface of solid core templates such as silver nanorods. For example, nanotube arrays of TiO2 [44], SiO2 [45], and BaTiO3 [46], with their outer diameters of 170–300 nm were synthesized using anodic alumina films. Si nanotubes [47] and GaN [48] nanotube arrays were obtained by the high temperature growth on the surface of ZnS and ZnO nanowires as the template, respectively, and the subsequent removal of the core materials.

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Solid sheath and core templates were also applied to the synthesis of noblemetal nanotubes with no covalency in interatomic bonding. Early in 1994, Martin et al. synthesized gold nanotubes by a two-step electroless plating using nanoporous polycarbonate films [49, 38]. Masuda et al. used anodic aluminum oxide films as templates to obtain platinum nanohole arrays [50]. Steinhart et al. also prepared palladium polycrystalline nanotubes by wetting anodic aluminum oxide film with polymer/palladium acetate containing solutions, evaporating the solvent and then annealing for the reduction of Pd(II) to Pd(0) [51]. Sun et al. prepared silver nanowires by the polyol process using ethylene glycol as a reductant in the presence of polyvinyl pyrrolidone [52]. Refluxing of aqueous solution of HAuCl4 containing the silver nanowires resulted in the oxidation reaction of Ag into Ag+ and the reduction reaction of Au3+ into Au, leading to Au nanotubes with outer diameters of more than 20 nm (Fig. 16.4a) [53]. In this system, silver nanowires serve as not only the template but also the reductant for Au3+ into Au. The high-resolution TEM image taken from the edge of the gold nanotube (inset) showed well-resolved interference fringe spacing of ∼0.2 nm indicating the formation of a highly crystalline structure for the metallic wall. This is in marked contrast to noncrystalline or poorly crystalline structures observed for most of metal oxide nanotubes. Similar

Fig. 16.4. TEM images of (a) Au, (b) Pt, and (c, d) Pd nanotubes obtained using Ag nanowires as templating and reducting agents. Reprinted with permission from [53]

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reactions using Pt(CH3 COO)2 or Pd(NO3 )2 instead of HAuCl4 led to platinum (Fig. 16.4b) and palladium (Fig. 16.4c) nanotubes, respectively.

16.4 Synthesis of Noble-Metal Nanotubes Using Molecular Templates Ohshima et al. found that platinum and gold nanowires are converted into single-walled nanotubular forms on electron irradiation in a transmission electron microscope kept in high vacuo [54, 55]. However, none has been reported on thin-walled noble-metal nanotubes with diameters of 10 nm or below stably kept in non-vacuo. This is because metal nanoparticles in air are easily fused into aggregates for their high surface energy, in contrast to ceramics analogues. Therefore, for example, the reduction of platinum salts with alcohol in the presence of polyvinyl pyrrolidone was employed to prepare platinum colloids particles since the resulting metal nanoparticles are stabilized through the adsorption of polymer segments onto the particle surface [17]. More generally, amphiphilic molecules such as surfactants have been used to control not only the stability of metal nanoparticles but also their sizes, shapes, and atomic arrangements in nanometer-scales [56–62].

16.4.1 Mixed Surfactant LC Templating Approach to Noble-Metal and Other Nanostructures Using Single- and Triple-Branched PEO-Type Surfactants Surfactants form a variety of self-organizing structures such as micelles, vesicles, microemulsions, and liquid crystals in aqueous systems, depending on the balance of the attractive and repulsive forces acting at the hydrophobic interface of the molecular aggregate [63]. On the basis of the self-organizing structures, surfactants have been thus used as the most potential templating agents for the synthesis of nanostructured materials [58–62]. Furthermore, the previous syntheses have almost unexclusively used a single surfactant, cationic, anionic, or nonionic, as templating agents. In the practical use of surfactants, however, the performance of the amphiphilic functions in a mixed surfactant system is often superior to that in a single surfactant system [64]. For example, the solubilization of oil and water in bicontinuous microemulsion is highly increased by the mixing of polyoxyethylene (EO)-type surfactants compared with a single homogeneous surfactant, and the mixing effect is especially enhanced by a combined use of surfactants with a big difference in HLBs [65]. When polyoxyethylene dodecyl ethers of different EO-chain lengths are mixed to form a liquid-crystalline (LC) aggregate, the repulsion between the hydrophilic moieties is considerably reduced and the surfactant molecules are more tightly packed in the aggregate [66, 67]. Such a molecular mixing effect may play a characteristic role in the materials synthesis. Actually, several

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studies have demonstrated the effective use of mixed surfactant systems for the synthesis of nanostructured silica materials [68–71]. Mixed solutions of cationic surfactants and nonionic poly(ethyleneglycol) or triblock copolymers were employed to prepare monolithic trimodal porous silica or mesoporous silica nanoparticles [68, 69]. Lyotropic mixtures of amphiphilic block copolymers of different lengths with hydrophilic linear EO chains were also applied to their nanocasting into bimodal micro-mesoporous silica of several hundreds nanometers or more in size [70, 71]. Kijima et al. also showed that lyotropic mixed surfactant nematic liquid crystals of cationic-chain-based and single EO-chain-based surfactants served as a 1D reaction medium to yield tin oxide or silver bromide microwires [72]. For noble-metal nanomaterials, Attard et al. first synthesized 2D-hexagonal structured mesoporous platinum (termed H1 -Pt) nanoparticles with a pore diameter of 3 nm by the reduction of platinum salts confined to lyotropic liquidcrystalline (LLC) templates consisting of a hexagonal array of octaethylene glycol monohexadecyl ether (C16 EO8 ) cylindrical micelles [27]. In contrast to the use of single-EO-chain-based ones, Kijima et al. has developed a templating approach to the synthesis of nanostructured noble-metal nanotubes using 2D or 3D LLCs of triple-branched polyoxyethylene (PEO)-based nonionic surfactants [73]. 16.4.2 Synthesis of Silver Nanotubes Using Mixed Surfactant LC Templating Methods Kijima et al. studied the hydrazine reduction of Pt salts confined to mixed LCs of AgNO3 , sodium dodecylsulfate (SDS), polyoxyethylene (20) sorbitan monostearate (Tween 60; Fig. 16.5), and aqueous dilute AgNO3 at a molar

Fig. 16.5. Chemical structures showing C12 EO9 and Tween 60

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ratio of 1:1:1:x. The reaction at x= 60 and at 20o C yielded silver nanotubes with an inner diameter of 4 nm and an outer diameter of 7 nm (Fig. 16.6a) [73]. The reaction at x = 60 and at 30◦ C and that at x = 80 and at 30◦ C produced hexagonal-structured and concentric-layered silver nanoparticles, respectively (Fig. 16.6b, c), whereas silver bulk particles were observed at x = 100 or above “Kijima, T. Yoshimura, unpublished data”. The morphological difference between the resulting products suggests that the precursory LCs have a hexagonal structure under the former two preparation conditions and a vesicle structure under the latter one. The reduction reaction (2) would proceed within the aqueous shell of the LC templates, leading to the growth of Ag atoms into hexagonal or vesicular nanostructures for the templating effects of the reaction media. Ag+ + NH2 NH2 → 4Ag + N2 + 4H+

(2)

At higher water contents, the ordered LC structures, hexagonal or vesicular, could be no longer maintained to convert to cylindrical or spherical micellar solution, resulting in the template-free growth of metal species.

16.4.3 Synthesis of Platinum and Palladium Nanotubes Using Mixed Surfactant LC Templating Methods Using compositionally almost the same LC templates as the above Tween 60-based ones, Kijima et al. succeeded in the first synthesis of platinum nanotubes with an inner diameter of 3 nm and an outer diameter of 6 nm by the hydrazine reduction of Pt salts (H2 PtCl6 ), as shown in Fig. 16.7 [73]. In the typical fabrication process, the mixed LC phase of H2 PtCl6 , nonaethyleneglycol dodecylether (C12 EO9 ), Tween 60, and H2 O at a molar ratio of 1:1:1:60 was treated with hydrazine. Palladium nanotubes with nearly the same dimensions were obtained by a similar procedure. XRD analysis showed that the single surfactant Tween 60/H2 O mixtures form layered LCs with an interlayer spacing of 7–8 nm. In contrast, the other single surfactant C12 EO9 /H2 O mixtures were observed to form a hexagonalstructured LC with a = 6.82 nm. As evidenced from the XRD patterns of the C12 EO9 /Tween 60/H2 O mixtures at a molar ratio of 1:y: 60 in Fig. 16.8a, even upon addition of much larger sized surfactant molecules the hexagonal structure is maintained over a wide range of y = 0-1.6 with keeping the parameter a in the narrow range of 6.8–7.2 nm. More interestingly, the parameter a corresponding to the rod-to-rod distance in an array of cylindrical rodlike micelles varies in sine waves with the Tween 60 content y and has a minimum of a = 6.9 nm at y ∼ 1 (Fig. 16.8c). These findings suggest that the greatly different-sized C12 EO9 and Tween 60 surfactant molecules are mixed at molecular scales to form a hexagonal array of cylindrical micelles and that their equimolar composition may afford the most stabilized phase. On addition of H2 PtCl6 to the y = 1 hexagonal composition, the XRD peaks

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Fig. 16.6. TEM images of (a) silver nanotubes [73], and (b) hexagonal-structured, and (c) concentric-layered silver nanomaterials [T. Kijima, T. Yoshimura, unpublished data], obtained by the hydrazine reduction of their precursory mixed surfactant LC phases in the AgNO3 /SDS/Tween 60/HNO3 (aq) system

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Fig. 16.7. TEM images of platinum nanotubes obtained by the hydrazine reduction of their precursory mixed surfactant LC phases in the H2 PtCl6 / C12 EO9 /Tween 60/H2 O system. Reprinted with permission from [73]

of the resulting phases considerably decreased in intensity (Fig. 16.8b): the precursory 1:1:1:60 phase containing PtCl2− 6 -ions was thus identified as a LC with a hexagonal but disordered array of mixed surfactant cylindrical micelles corresponding to a = 6.9 nm. Figure 16.9a shows a structure model for the precursory 1:1:1:60 LC proposed on the basis of the above X-ray observations along with the structural parameters of surfactant molecules and their mixed surfactant cylindrical micelles. The former molecular parameters (Table 16.1) were obtained by assuming that hydrophilic head groups of both surfactant molecules adopt a spherical conformation with the same packing coefficient of 0.681 as observed for a number of amorphous or poorly crystalline polymers [74]. The latter micellar parameters (Table 16.2) were calculated from some geometrical relations derived on the assumption that equimolar amounts of C12 EO9 and Tween 60 molecules are arranged side by side with their hydrophobic tail groups fully extended and tilted at the same angle to form a cylindrical rodlike micelle with their hydrophilic head groups directed to the outside and that the rodlike micelles are further assembled into a hexagonal array with a rod-to-rod distance equal to the observed value of 6.9 nm [75]. The structure model for the single surfactant C12 EO9 /H2 O LC was also calculated in a similar manner (Fig. 16.9b). The micellar structure models thus obtained were consistent

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Fig. 16.8. XRD patterns of (a) the 1:y: 60 mixtures of C12 EO9 , Tween 60, and H2 O, and (b) the x:1:1:60 mixtures of H2 PtCl6 , C12 EO9 , Tween 60, and H2 O at∼ 15◦ C (CuKα), and (c) plots of the rod-to-rod distance, a, as a function of Tween 60 content, y, for the former ternary mixtures. Reprinted with permission from [73]

with the structural change observed on addition or substitution of long chain alcohols to the above LCs [76]. The observed inner diameter of 3 nm for both the platinum and palladium nanotubes were in close agreement with 3.1 nm calculated for the hydrophobic core diameter of the mixed surfactant micelle (Table 16.2). It was also remarked that the aqueous shell of rodlike micelles in the C12 EO9 /Tween 60 mixed surfactant systems is about two times larger in thickness than those in the single surfactant C16 EO8 and the C12 EO9 systems (Table 16.2). It was thus concluded that equimolar amounts of Tween 60 and C12 EO9 surfactant molecules are combined into a hexagonal array

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Fig. 16.9. Schematic models for the formation of platinum nanotubes in the mixed surfactant templating system: (a) Mixed (C12 EO9 /Tween 60) and (b) single (C12 EO9 ) surfactant cylindrical rodlike micelles. (c) Pathway from micellar solution to metal nanotubes via the reduction of metals salts confined to the aqueous shell of mixed surfactant cylindrical micelles. The metal salts and water molecules are omitted in the illustrations. Reprinted with permission from [73]

of cylindrical rodlike micelles and the aqueous shell of the rodlike micelles is so thick that the reduced metal grows into nanotubes separately within the aqueous shell, as shown in Fig. 16.9c. On the other hand, the borohydride reduction of Pt salts using the same mixed surfactant LLC templates yielded nanogroove-network-structured single-crystalline Pt nanosheets [12] and the hydrazine reduction of Pt salts confined to the 2D hemicylindrical micelles of Tween 60 at aqueous solution/graphite interfaces produced singlecrystalline Pt nanosheets with a uniform thickness of ca. 3.5 nm and several

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Table 16.1. Structural parameters of C12 EO9 and Tween 60 molecules. Reprinted with permission from [73] Head group

Tail group

Molecule

Effect. Volumeb Surfactant Vh,i /nm3

Diameter Dh,i /nm

Volume Vt,i /nm3

Length Lt,i /nm

Volume Vm,i /nm3

Length Lm,i /nm

C12 EO9 Tween 60

1.04 1.30

0.214 0.324

1.61 2.53

0.803 1.623

2.65 3.88

0.589 1.299

a

The -CO2 -group of Tween 60 molecule is assumed to be a part of the tail group because the ester group is separated by CH2 unit from the triple-branched PEObased hydrophilic one. b Effective volume is given by the van der Waals volume/0.681 according to Slonimskii et al.

micrometer sizes [77]. These observations clearly indicate that the 2D and 3D LLCs based on Tween 60 hemicylindrical or cylindrical micelles serve as a templating agent for Pt metal to grow primarily into a thin nanosheet. This primary templating effect would be also conducive for the formation of the thin-walled Pt nanotubes because such thin-walled nanotubes are obtainable from materials based on 2D layered compounds such as carbon, metal chalcogenide, nitride, MoO3 , and V2 O5 , whereas 3D compounds such as SiO2 , TiO2 , or Al2 O3 often yield a thick-walled one to decrease the elastic strain energy [78, 36]. In the case of Pt metals with no 2D nature, the thin-walled nanotubular structure is believed to be inherited from the 2D Pt clusters induced through the specific effect of triple PEO chains of Tween 60 molecules, coupled with their spatially controlled growth within the aqueous shells of cylindrical micelles.

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Table 16.2. Structural parameters of surfactant cylindrical rodlike micelles. Reprinted with permission from [73]

Surfactant

Number of molecules per round N

Diameter of core Dc /nm

Thickness of shell δ/nm

C12 EO9 C16 EO8 C12 EO9 /Tween 60

17 21 12/12

4.73 5.72 3.12

1.04 1.00 1.93

16.5 Applications of Noble-Metal Nanotubes Very few studies have appeared on the applications of noble-metal nanotubes, except for a few examples, in contrast to those of many other nanostructured noble-metal materials to a wide range of fields including industrial catalysts and medical sensing. Sun et al. found that the palladium nanotubes described in 16.2 exhibit a high activity as the catalyst for the Suzuki coupling reaction, i.e., the formation of biphenyl by the reaction between phenylboronic acid and iodobenzene [53]. Yan and coworkers synthesized platinum nanotubes (PtNT) and platinumpalladium-alloy nanotubes (PtPdNT) as well as platinum small nanoparticles (2–5 nm) supported on amorphous carbon-particle aggregates (Pt/C) and studied their performance as the cathode catalyst for the oxygen-reduction reaction (ORR) in polymer electrolyte fuel cells (PEFCs) [79]. The former two supportless catalyst samples (50 nm diameter, 5–20 μm long, and 4–7 nm wall thickness) were prepared by a galvanic replacement reaction of silver nanowires developed by Xia and coworkers [52, 53]. Figure 16.10 shows typical ORR polarization curves of Pt/C, PtNT, and PtPdNT obtained at room temperature in O2 -saturated 0.5 M H2 SO4 using a rotating disk electrode (RDE) at 1,600 rpm. PtNTs were found to have a slightly higher mass activity but a significantly higher (3.8 times) specific activity than Pt/C at 0.85 V, as shown by the insets in Fig. 16.10. The mass and specific activities of PtPdNTs at 0.85 V were as high as 1.4 and 5.8 times more than those of Pt/C. The higher

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Fig. 16.10. ORR polarization curves of Pt/C, PtNT, and PtPdNT obtained at room temperature in O2 -saturated 0.5 M H2 SO4 using a RDE at 1,600 rpm. Data for specific activity at 0.85 V are compared in insets. Reprinted with permission from [79]

ORR kinetics of the PtNTs compared to Pt/C was attributed to the preferential exposure of certain crystal facets of the PtNTs; and the improved ORR kinetics of the PtPdNTs compared to PtNTs was attributed to changes in the bond lengths by alloying. The much smaller platinum nanotubes prepared by the mixed surfactant LC templating [73] might have much higher performance as the cathode catalyst for PEFCs compared to the PtNTs or PtPdNTs, but the former materials have not been obtained in high yields enough to evaluate their catalytic activity. The mixed surfactant LC templating method, however, was extended to synthesize nanogroove-network-structured platinum nanosheets by the borohydride reduction reaction using compositionally the same LC templates [12] and nanohole-structured platinum nanosheets by a similar reaction using polyoxyethylene (20) sorbitan monooleate (Tween 80) based single or mixed surfactant LC templates [80]. These two novel types of platinum nanomaterials were also found to exhibit fairly high electrocatalytic activity for ORR and a high performance as a cathode material for PEFCs along with their extremely high thermostability revealed through the effect of electron irradiation [12, 80]. Very few studies have been appeared on the applications of noble-metal nanotubes, except for the use of Pt–Pt alloy nanotubes as a cathode catalyst for fuel cells and Pd nanotubes as a catalyst for the Suzuki coupling reaction.

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16.6 Concluding Remarks In this chapter, worldwide studies on the synthesis of platinum and some other noble-metal nanotubes using solid or molecular liquid-crystalline templates were reviewed along with the template-free liquid phase synthesis of base metallic bismuth and tellurium nanotubes. It is not easy to synthesize noblemetal nanotubes because metal nanoparticles are readily fused into aggregates for their high surface energy, in contrast to non-metallic ones. No report has thus appeared on the catalytic and any other physical properties of noblemetal nanotubes with an outer diameter of as small as around 10 nm or below. The use of triple-branched PEO-based nonionic surfactant LCs as a core template also results in the growth of Pt, Pd, and Ag nanotubes with an outer diameter of as small as 6–7 nm, but in their yields of as low as 20% or below. Our recent study, however, suggests that the preferred orientation of the LC templates might be available for improving the yield of these noblemetal nanotubes.

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48. J. Goldberger, R.R. He, Y.F. Zhang, S.W. Lee, H.Q. Yan, H.J. Choi, P.D. Yang, Nature 422, 599 (2003) 219 49. C.R. Martin, Science 266, 1961 (1994) 220 50. H. Masuda, K. Fukuda, Science 268, 1466 (1995) 220 51. M. Steinhart, Z. Jia, A.K. Schaper, R.B. Wehrspohn, U.Gosele, J.H. Wendorff, Adv. Mater. 15, 706 (2003) 220 52. Y. Sun, Y. Xia, Adv Mater. 14, 833 (2002) 220, 229 53. Y. Sun, B. Mayers, Y. Xia, Adv Mater. 15, 641 (2003) 220, 229 54. Y. Oshima, H. Koizumi, K. Mouri, H. Hirayama, K. Takayanagi, Phys. Rev. B 65, 121401-1 (2002) 221 55. Y. Oshima, A. Onga, K. Takayanagi, Phys. Rev. Lett. 91, 20, 205503 (2003) 221 56. C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359, 710 (1992) 221 57. T. Yanagisawa, T. Shimizu, K. Kuroda, C. Kato, Bull. Chem. Soc. Jpn. 63, 988 (1990) 221 58. K.J.C. van Bommel, A. Friggeri, S. Shinkai, Angew. Chem. Int. Ed. Engl. 42, 980 (2003) 221 59. H. Terrones, M. Terrones, J.L. Moran-Lopez, Curr. Sci. 81, 25 (2001) 221 60. H. Colfen, M. Antonietti, Angew. Chem. Int. Ed. 44, 5576 (2005) 221 61. M.P. Pileni, J. Exper. Nanosci. 1, 13 (2006) 221 62. W.H. Suh, A.R. Jang, Y.H. Suh, K.S. Suslick, Adv. Mater. 18, 1832 (2006) 221 63. J.N. Islaelachvili, D.J. Mitchell, B.W. Ninham, J. Chem. Soc. Faraday Trans. 272, 1525 (1976) 221 64. K. Ogino, M. Abe (Ed.), Mixed Surfactant Systems (Marcel Dekker, Inc., New York, 1993) 221 65. H. Kunieda, A. Nankano, M. Akimaru, J. Colloid, Interface Sci. 78,170 (1995) 221 66. H. Kunieda, G. Umizy, Y. Yamaguchi, J. Colloid, Interface Sci. 218, 88 (1999) 221 67. H. Kunieda, H. Kabir, K. Aramaki, K. Shigeta, J. Mol. Liquids. 90, 157 (2001) 221 68. J.H. Smatt, S. Schunk, M. Linden, Chem. Mater. 15, 2354 (2003) 222 69. K. Suzuki, K. Ikari, H. Imai, J. Am. Chem. Soc. 126, 462 (2004) 222 70. B. Smarsly, S. Polarz, M. Antonietti, J. Phys. Chem. B 105, 10473 (2001) 222 71. C.G. Goltner, B. Smarsly, B. Berton, M. Antonietti, Chem. Mater. 13, 1617 (2001) 222 72. T. Kijima, T. Ikeda, M. Yada, M. Machida, Langmuir 18, 6453 (2002) 222 73. T. Kijima, T. Yoshimura, M. Uota, T. Ikeda, D. Fujikawa, S. Mouri, S. Uoyama, Angew. Chem. Int. Ed. Engl. 43, 228 (2004) 222, 223, 224, 225, 226, 227, 228, 229, 230 74. G. L. Slonimskii, A.A. Askadaskii, A.I. Kitaigorodskii, Vyskomol. Soyed. A 12, 494 (1970) 225 75. Supporting Information in ref. 73 225 76. T. Kijima, Y. Nishida, D. Fujikawa, M. Uota, T. Yoshimura, f. Sakai, J. Mol. Liq. 133, 54–60 (2007) 226 77. H. Kawasaki, M. Uota, T. Yoshimura, D. Fujikawa, G. Sakai, M. Annaka, T. Kijima, Langmuir 21, 11468 (2005) 228 78. M. Yada, S. Mihara, M. Mohri, M. Kuroki, T. Kijima, Adv. Mater. 14, 309 (2002) 228 79. Z. Chen, M. Waje, W. Li, Y. Yan, Angew. Chem. Int. Ed. Engl. 46, 4060 (2007) 229, 230 80. T. Kijima, Y. Nagatomo, H. Takemoto, M. Uota, D. Fujikawa, Y. Sekiya, T. Kishishita, M. Shimoda, T. Yoshimura, H. Kawasaki, G. Sakai, Adv. Funct. Mater. 19, 545 (2009) 230

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Index Base metal nanotubes, template-free synthesis, 217–218 Bismuth nanotubes, formation, 217–218 C12 EO9 and Tween 60, chemical structures, 222 C12 EO9 and Tween 60 structural parameters, 228 Concentric-layered silver nanotubes TEM images, 224 Hexagonal-structured silver nanotubes TEM images, 224 Liquid-crystalline (LC), 221–223 Lyotropic liquidcrystalline (LLC) templates, 222 Mixed surfactant LC templating method, 223–229 Molecular templates, noble-metal nanotubes synthesis using, 221 Nanogroove-network-structured platinum nanosheets, 230 Nanohole-structured platinum nanosheets, 230 Noble-metal nanoparticles, 215–217 Oxygen-reduction reaction (ORR), 229–230 Platinum and ruthenium (Ru), 215 Platinum nanotubes formation models, 227

Platinum nanotubes TEM images, 225 Polymer electrolyte fuel cells (PEFCs), 229 Polyol process, 216, 220 Polyoxyethylene (EO)-type surfactants, 221–222 Polyoxyethylene (PEO)-based nonionic surfactants, 222 Polyvinyl pyrrolidone, 221 Practical use of polymer electrolyte fuel cells (PEFCs), 215–216 Pt–Pd–Rh ternary catalysts, 215 Rotating disk electrode (RDE), 229 Silver nanotubes TEM images, 224 Single surfactant Tween 60/H2 O mixtures, 223 Solid templates, noble-metal nanotubes synthesis using, 219–221 Surfactant cylindrical rodlike micelles, structural parameters, 229 Surfactants, 221–222 Tellurium nanotubes, 217–219 Templating methods, silver nanotubes mixed surfactant using, 222–223 Tween 60 surfactant molecules, 223 Volatile organic compounds (VOC), 216 XRD patterns of Tween 60, 216

17 Synthesis and Applications of Magnetic-Metal Nanotubes Masaru Nakagawa, Hirokazu Oda, and Kei Kobayashi Institute of Multidisciplinary Research for Advanced Materials, Tohoku University, 2-1-1 Katahira, Aoba-ku, Sendai, Miyagi 980-8577, Japan [email protected] Abstract Preparation of magnetic metal-containing nickel–phosphorus (Ni–P) nanotubes by a template synthesis with advanced morphology-tunable and recyclable supramolecules and plausible application to anisotropic conduction polymer films using them as orientation and alignment-controllable magnetic conductive filler were introduced.

17.1 Introduction Organic, inorganic, and metallic tubular materials possessing an inner pore diameter of smaller than a submicrometer and a length of larger than a micrometer have attracted attention due to possibilities that they exhibit a unique physical and chemical property [1–3]. A method of template synthesis has been widely adopted for preparation of tubular materials in a facile, high-throughput, and cost-effective way. In the template synthesis, an inner wall of porous membranes made of alumina [4–7], polymer [4–7] and silica [8] and an exterior wall of lipid-derived rod-like molecular aggregates (cylindrical tubes and rod-like micelles) [9, 10], bola-amphiphile-derived fibrous molecular aggregates [11], tobacco mosaic virus [12], molecular aggregates of organic gelling agents [13, 14], carbon nanotubes [15, 16], and electro-spun polymer fibers [17] are used as a template. To date, tubular materials of oxides (SiO2 [18–23], TiO2 [24–28], Fe2 O3 [19], V2 O5 [29, 30]), sulfides (CdS, PbS) [19], metals (Al [31], Ni [32–34], Cu [35], Au [25, 39–41], Ag [36–38], Pt [39–41], Pd [41]), and conducting polymers [poly(pyrrole) and poly(thiophene)] [6, 42] have been prepared by template synthesis with condensation polymerization, electrochemical deposition, electroless deposition, and physical or chemical vapor deposition. In the case of an organic template mainly composed of carbon, hydrogen, and oxygen atoms, its exterior wall surface is covered with an oxide by a condensation polymerization or with a metal by electroless deposition to give a hybrid tubular material, and then the organic template is pyrolyzed at a T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 235–246 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 17

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high temperature or extracted with an organic solvent, eventually to give an oxide or metal tubular material. Pyrolysis at a high temperature is favorable for preparation of an oxide tubular material, while pyrolysis is unfavorable for preparation of a metal tubular material owing to contamination of metal oxides and generation of carbon oxides in addition to an energy-consuming process. Most of the organic templates are, in general, soluble to common organic solvents and insoluble to aqueous solutions in which metal electroless deposition proceeds. Therefore, extraction with an organic solvent is widely adopted to remove the organic template. However, in a standpoint of industry, it is desirable that a more sophisticated template for preparation of tubular materials is developed, which has no use for pyrolysis and extraction with any harmful organic solvents. The authors noticed that for conductive carbon nanotubes in a powder state prepared in a mass production way it is not easy for them to control their orientation and alignment and to gather themselves around a desired position in a simple manner. If a conductive nanotube composed of a magnetic metal element (Ni, Co, and Fe) is prepared, the conductive magnetic nanotube will show abilities to control its orientation and alignment and to accumulate itself in a simple way using a widely used conventional magnet. Taking current situations into consideration, the authors began to develop a conductive magnetic-metal nanotube which is obtainable in a template synthesis with an advanced template. Herein, the authors introduce mainly our results concerning two topics: (i) template synthesis of a conductive magnetic nickel-phosphorus (Ni-P) nanotube by electroless deposition with an advanced supramolecular template and (ii) application of the Ni-P nanotube to an anisotropic conduction polymer film.

17.2 Template Synthesis of Nickel-Containing Tubular Materials by Electroless Deposition The pioneering study to obtain nickel-containing tubular materials by electroless deposition was reported by Schnur and coworkers of the Naval Research Laboratories, USA in 1987. They used a rod-like micelle of a lipid molecule with polymerizable diacetylene units as a template for nickel electroless deposition [9, 10]. As another example, a nickel microtube is reported with a spun polymer fiber of several micrometers in diameter as a template [43]. In these template syntheses with organic templates, magnetic-metal nickel tubular materials are obtained by electroless deposition to coat an exterior surface of the templates with nickel-containing species followed by template removal by either pyrolysis or organic solvent extraction. In contrast to the templates mentioned above, the authors used a supramolecular template, having the advantages of a recycling ability in aqueous media and a morphology-tuning ability, and prepared Ni-P tubular materials by electroless deposition. The supramolecular template is formed

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C3H7 N

N

OH N

O

(CH2)5 O

Fig. 17.1. Chemical structure of an azopyridine carboxylic acid forming a hydrogenbonded fibrous molecular aggregate available as a recyclable template

from an amphoteric azopyridine carboxylic acid, as illustrated in Fig. 17.1. The amphoteric molecule possesses a basic pyridyl group and an acidic carboxyl group at respective molecular terminals. The amphoteric molecule is dissolved in an aqueous alkaline solution of sodium hydroxide, and to a thusobtained aqueous solution of azopyridine carboxylate an acidic substance such as a carbon dioxide is added gradually to increase the proton concentration of the solution. As a result of neutralization of the solution, the transparent aqueous solution turns into a dispersion, in which a fibrous aggregate 0.5 μm in diameter is formed. In the fibrous aggregate, the molecular longaxis of the amphoteric molecule is parallel to the long-axis of the fibrous aggregate. The fibrous aggregate is composed of a bundle of supramolecular polymers connected by successive intermolecular OH N hydrogen bonds between carboxyl and pyridyl groups in a head-to-tail manner [44]. The hydrogen-bonded fibrous molecular aggregate has the unique property that the outer diameter is changed on a 0.1 μm scale when the side-chain propyl group in the chemical structure shown in Fig. 17.1 is changed on a 0.1 nm scale. For examples, the diameter of a fibrous aggregate in the case of a methyl group is about 0.1 μm and the diameter in the case of a sec-butyl group is about 1 μm [45]. Moreover, the diameter of the fibrous aggregate is controllable even when the same amphoteric molecule shown in Fig. 17.1 is used. The diameter is influenced by acidic substances which neutralize the aqueous solution of azopyridine carboxylate. Neutralization by carbon dioxide gas gives a fibrous aggregate of 0.4 μm diameter, and neutralization by hydrogen chloride gas gives a fibrous aggregate 0.2 μm in diameter [46]. More detail on the one-dimensional growth of the fibrous supramolecular aggregate is given in the literature [47]. Interestingly, the hydrogen-bonded fibrous supramolecular aggregate formed from an azopyridine carboxylic acid shown in Fig. 17.1 is morphologically stable in an HCl-acidic aqueous solution of pH > 2 and in a NaOHalkaline aqueous solution of pH < 10. Taking notice of the stable fibrous morphology in a wide range of proton concentrations in aqueous solutions, the authors demonstrated that the hydrogen-bonded fibrous supramolecular aggregate is available as a template for electroless deposition [48]. The fibrous aggregate is immersed in an acidic aqueous solution containing tetrachloropalladinate of about pH = 2.5 and then in a nickel-phosphorus (Ni-P) electroless deposition bath of pH = 5.5. As a result, the exterior surface of the fibrous

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Fig. 17.2. Molecular recycle-type method for preparation of a nickel-phosphorus (Ni-P) nanotube consisting of template preparation, catalyzation, electroless deposition, and template removal

aggregate is covered with a thin film containing the magnetic metal nickel. When a nickel-coated fibrous aggregate is immersed in an alkaline aqueous solution of NaOH (pH = 14), the hydrogen-bonded fibrous aggregate as a core material is dissolved due to formation of azopyridine carboxylate to give a Ni-P tubular material having an inner pore diameter of 0.5 μm [45]. An alkaline aqueous solution dissolving the fibrous aggregate as azopyridine carboxylate repeatedly turns into a dispersion containing a hydrogen-bonded fibrous aggregate by the repeated addition of an acidic substance. Therefore, the hydrogen-bonded fibrous aggregate is available as a recyclable template. A Ni-P nanotube having an inner pore diameter of about 0.08 μm and an outer diameter of 0.13 μm is obtainable by the choice of the kind of azopyridine carboxylic acid and by turning preparation condition of a supramolecular template [49]. The mechanism of formation of a Ni-P thin film on a surface of a hydrogen-bonded fibrous aggregate is revealed by X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) [50]. First, tetrachloropalladinate (PdCl2− 4 ) in an HCl-acidic aqueous solution of PdCl2 is reacted with a pyridyl group at an outermost surface of the hydrogen-bonded fibrous aggregate [(i) coordination of Pd2+ species]. Second, a Pd–Cl bond

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of Pd2+ species coordinating a pyridyl group is hydrolyzed gradually in the HCl-acidic aqueous solution of PdCl2 at pH = 2.5, followed by condensation to form chloro/hydroxo-bridged Pd2+ condensation species. As a result of the condensation, an outer surface of the fibrous template is covered with a nano-sheet of the Pd2+ condensation species [(ii) formation of a Pd2+ nano-sheet]. Third, hypophosphite as a reducing reagent reduces the Pd2+ nano-sheet to give metal palladium nanoparticles of about 5 nm in diameter [(iii) formation of Pd nanoparticles]. Finally, a Ni-P thin film is formed by a reduction of Ni2+ → Ni0 in the presence of Pd nanoparticles as a catalyst [(iv) formation of a Ni-P thin film]. There is a pyridyl group every molecular period on an outermost surface of the hydrogen-bonded fibrous molecular aggregate. As a result of the situation, tetrachloropalladinate is chemisorbed in a uniform and dense manner, leading to nano-sheet formation of the Pd2+ condensation species. The Pd2+ nano-sheet plays a role in forming Pd nanoparticles entirely on a surface of the fibrous template and in giving a Ni-P tubular material by precise transcription of template morphology.

17.3 Application to an Anisotropic Conduction Film Downsizing, thinning, and lightening of electronic devices in addition to multifunction, low electricity expenditure, high speed, and environmental benignity are required in the field of modern electronics. In order to satisfy these requirements, the size of an electrode terminal and the wiring width in an electric circuit is still miniaturized, and the gap between laterally neighboring electrode terminals becomes narrower. In the bonding technology which is one of the constitutive technologies in electric device fabrication, an anisotropic conduction film (ACF) is widely used to connect simultaneously a multiple of confronted electrode terminals [51]. A typical ACF is a composite film homogeneously dispersing a conductive microsphere of a metal powder or a polymer particle covered with a metal by electroless deposition in an insulative thermosetting polymer resin such as an epoxy resin. The ACF is placed between printed-wiring circuit boards and bonds the circuit boards during pressing at 170◦ C for several seconds. The bonding with the ACF allows multiple confronted electrode terminals to become conductive connections and laterally neighboring electrode terminals to become insulative connections as illustrated in Fig. 17.3. Approximately 3,000–4,000 laterally neighboring electrode terminals are connected simultaneously by the bonding technology with the ACF. A conductive microsphere coated with nickel and then gold (Au/Ni-coated microsphere) having a diameter of about 5 μm with a narrow size distribution is used currently, allowing anisotropic connection at a wiring pitch of approximately 40 μm [52, 53]. In order to comply with a request in fabrication of liquid crystal displays (LCDs), it is urgently necessary to develop a highly reliable anisotropic conduction film corresponding to a narrower wiring pitch.

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conductive microsphere conductive-magnetic-fiber type (our study)

N S

N S

N S

N S

N S

N S

conductive magnetic nanotube

Fig. 17.3. Schematic illustrations of a currently used anisotropic conduction film containing conductive microspheres (top) and an anisotropic conduction film containing magnetically orientated and aligned conductive magnetic nanotubes (bottom)

As mentioned in the former section, nickel-phosphorus (Ni-P) tubular materials can be obtained by template synthesis with a supramolecular fibrous template via palladium-promoted electroless deposition. When the Ni-P tubular materials in an amorphous state are subjected to heating at 500◦ C under an inert argon atmosphere, crystalline phases of face-centered cubic Ni and Ni3 P are generated, so that annealed Ni-P tubular materials exhibit a weight saturated magnetization of about 10 emu/g, a coercive force of 0.1 kOe, and a powder volume resistivity of 101 Ωcm [54]. The annealed Ni-P tubular material is mixed with a thermosetting silicone resin, and the mixture is cured thermally under a magnetic field. Thereby, each long-axis of the Ni-P rod-like filler is oriented parallel to a line of the applied magnetic field and connected to each other. As a result of magnetic orientation, columns successively connecting the Ni-P rods are formed in the thermosetting resin, and

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respective columns are aligned with a spatial interval [54]. The event motivates us to use the annealed Ni-P tubular material as an advanced conductive magnetic filler in an ACF. In the next section, our study is introduced on an anisotropic conduction film containing magnetically oriented Ni-P rod-like filler, which is expected to correspond to a narrower wiring pitch as illustrated in Fig. 17.3. 17.3.1 Filler-Orientation-Type Anisotropic Conduction Film The conductive magnetic Ni-P rod-like filler is successfully obtained by the template synthesis with electroless deposition, followed by annealing. As can be seen in Fig. 17.4, the diameter of its inner pore is uniform, while the filler length is irregular in size in addition to having unfavorable grainy tubular materials. If a free-standing ACF of several tens of micrometers in thickness is prepared, the heterogeneity of the rod-like filler in size causes undesirable poor magnetic orientation of the filler, resulting in deterioration in the quality of the ACF. The authors improved heterogeneity in filler size by adding the two steps of breaking up the supramolecular fibrous template in a ball mill and fractionating the annealed Ni-P filler in size by a difference in precipitation speed in solution. In our study, the length and width of the Ni-P rod-like filler was measured to determine the average. The Ni-P rod-like filler having an average width of 0.35 μm and an average length of 2.6 μm in a length range of 1–10 μm was used in a composite film. long Ni-P tube

Ni-P grain

short Ni-P tube

Fig. 17.4. Scanning electron microscope (SEM) image of Ni-P nanotubes prepared by the molecular recycle-type method

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applied magnetic field: 0 kOe

θ = 90°

(a) θ = 90°

θ θ = 0° applied magnetic field: 3 kOe (b) θ = 90°

(c) θ = 70°

(d) θ = 50°

Fig. 17.5. Photographs of composite films containing Ni-P nanotubes and a thermosetting silicone resin prepared under an applied magnetic field of (a) 0 kOe and (b–d) 3 kOe at a view angle (θ) of (a, b) 90◦ , (c) 70◦ , and (d) 50◦

A wet thin film of a dispersion containing Ni-P filler and thermosetting silicone resin is bar-coated on an exfoliative polymer sheet and cured thermally under a magnetic field of 3 kOe, to give a composite thin film. As seen in Fig. 17.5, a composite film in the case of no applied magnetic field is dark and not transparent, while a composite film prepared under the applied magnetic field of 3 kOe is transparent and the film transparency shows a dependence on the view angle. The angle-dependent transmittance is highest when the incident angle of probing visible light (φ = 3 mm) at 633 nm is perpendicular to the film surface, indicating that the long-axis of the rodlike filler is oriented parallel to the direction of film thickness. In the case of observation for magnetically oriented Ni-P filler in a composite film with a scanning electron microscope, preparation of its cross-section is necessary. The measurement of optical transmittance is a nondestructive method. In addition to this advantage, there are other advantages including that a highest transmittance means an orientation angle of the long-axis of the rod-like filler and that the transmittance itself means a quality in the magnetically induced orientation of Ni-P filler. The conductive magnetic Ni-P filler can be oriented and aligned under a magnetic field of 3 kOe. The value of magnetic field is almost identical to the that for the Ni-P filler to show a saturated magnetization. Anisotropic conduction of the composite film is evaluated with a couple of glass substrates having indium-tin-oxide (ITO) line patterns, between which a filler-containing composite film was placed. ITO-patterned glass substrates with a line width and a space (L&S) of 5, 10, 20, 50 μm were used. It is

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a line of magnetic force

(a)

applying a magnetic field (b)

Fig. 17.6. Schematic illustration (a) of organization of Ni-P conductive magnetic nanotubes induced by applying a magnetic field resulting in formation of percolation conduction paths. SEM image (b) of a cross section of a composite film (Ni-P nanotube/thermosetting resin) prepared under a magnetic field of 3 kOe

confirmed that the composite film containing magnetically oriented Ni-P filler shows anisotropic conduction at L&S = 10 μm. The oriented composite film exhibits a volume resistivity of 101 –102 Ωcm in a direction of film thickness. The ratio of conduction in a direction of film thickness to that in a direction of film surface is estimated to be 109 –1010 . As illustrated in Fig. 17.6a, randomly dispersed Ni-P rods feel an applied magnetic field and their long-axis is oriented and they connect to each other in a suitable range of their volume fraction. Respective columns connecting Ni-P rods are aligned with a spatial interval induced by magnetic repulsion. The columns work as percolation conduction paths in the oriented composite film [55]. The authors have recently developed a UV-curable anisotropic conduction film containing oriented Ni-P rods [56]. A wet thin film including a photocurable polymer resin was successfully prepared on an exfoliative polymer sheet with a bar-coater or a micro-gravure coater. Using the exfoliative sheet as a carrier, the wet thin film is passed through sections of applying a magnetic field and fixing filler orientation by UV-exposure. A thus-obtained UV-curable anisotropic conduction film on an exfoliative sheet is in an incompletely cured state and can adhere to a glass substrate and be transferred readily from the exfoliative sheet, although the orientation of the Ni-P rods remains fixed. The photo-curable anisotropic film allows bonding by additional UV-exposure without heating at room temperature. The advanced ACF will be used in the field of film electronics. In summary, the authors introduced Ni-P tubular materials having both conductive and magnetic properties. Their template synthesis and application to an anisotropic conduction film were described. Noting orientation of the Ni-P rod-like filler under a magnetic field, the authors developed a new type of ACFs. The Ni-P rod-like fillers described herein were made through

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a combination of the research fields of molecular assembly and electroless deposition in bottom-up nanotechnology. It is possible to prepare readily the Ni-P rod-like filler on a scale of 10 g in a chemical laboratory and to obtain a sample from a company in Japan. Acknowledgment The authors thank the following coworkers: Prof. Kunihiro Ichimura, Prof. Takahiro Seki, Prof. Tomokazu Iyoda, Dr. Ke´ nichi Aoki, Dr. Daisuke Ishii, Dr. Nobuyuki Zettsu, Mr. Mitsuru Udatsu, Mr. Tomohiro Shimazu, Mr. Koichi Nagase, Mrs. Miki Tatsumi, Mrs. Naoko Ono, Mrs. Yumie Yamanobe, Mr. Taichi Nagashima, Mr. Shinichi Kawasaki, and Mr. Mitsuaki Yamada. The authors give thanks for financial support from the Joint Research Grant from Osaka Gas Co., Ltd. in Japan and by the Industrial Technology Research Grant Program in 2005 (05A25011d) from the New Energy and Industrial Technology Development Organization (NEDO) of Japan.

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F. Miyaji, S.A. Davis, J.P.H. Charmant, S. Mann, Chem. Mater. 11, 3021 (1999) 235 M. Harada, M. Adachi, Adv. Mater. 12, 839 (2000) 235 M. Zhang, Y. Bando, K. Wada, J. Mater. Res. 15, 387 (2000) 235 J.H. Jung, S. Shinkai, T. Shimizu, Nano Lett. 2, 17 (2001) 235 P. Hoyer, Langmuir 12, 1411 (1996) 235 P. Hoyer, Adv. Mater. 8, 857 (1996) 235 T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14, 3160 (1998) 235 H. Imai, Y. Takei, K. Shimizu, M. Matsuda, H. Hirashima, J. Mater. Chem. 9, 2971 (1991) 235 S. Kobayashi, K. Hanabusa, N. Hamasaki, M. Kimura, H. Shirai, Chem. Mater. 12, 1523 (2000) 235 H.-J. Muhr, F. Krumeich, U.P. Schonholzer, F. Bieri, M. Niederberger, L.J. Gauckler, R. Nesper, Adv. Mater. 12, 231 (2003) 235 S. Kobayashi, N. Hamasaki, M. Suzuki, M. Kimura, H. Shirai, K. Hanabusa, J. Am. Chem. Soc. 124, 6550 (2002) 235 M. Bognitzki, H. Hou, M. Ishaque, T. Frese, M. Hellwig, C. Schwarte, A. Schaper, J.H. Wendorff, A. Greiner, Adv. Mater. 12, 637 (2000) 235 J.M. Schnur, P.E. Schoen, P. Yager, J.M. Calvert, J.H. Georger, R. Price, US 4911981 (1990) 235 F.Z. Kong, X.B. Zhang, W.Q. Xiong, F. Liu, W.Z. Huang, Y.L. Sun, J.P. Tu, X.W. Chen, Surface and Coatings 155, 33 (2002) 235 L.-M. Ang, T.S. A. Hor, G.-Q. Xu, C.-H. Tung, S. Zhao, J.L.S. Wang, Chem. Mater. 11, 2115 (1999) 235 S.L. Browning, J. Lodge, P.R. Price, J. Schelleng, P.E. Schoen, D. Zabetakis, J. Appl. Phys. 84, 6109 (1998) 235 C.J. Brumlik, V.P. Menon, C.R. Martin, J. Mater. Res. 9, 1174 (1994) 235 B.C. Satishkumar, E.M. Vogl, A. Govindaraji, C.N.R. Rao, J. Phys. D., Appl. Phys. 29, 3173 (1996) 235 S. Demoustier-Champagne, M. Delvaux, Mater. Sci. Eng. C: Biomimetic and Supramolecular Systems C 15, 269 (2001) 235 Z. Liu, X. Lin, J.Y. Lee, W. Zhang, M. Han, L.M. Gan, Langmuir 18, 4054 (2002) 235 B. Mayers, X. Jiang, D. Sunderland, B. Cattle, Y. Xia, J. Am. Chem. Soc. 125, 13364 (2003) 235 T. Kijima, T. Yoshimura, M. Uota, T. Ikeda, D. Fujikawa, S. Mouri, S. Uoyama, Angew. Chem. Int. Ed. 43, 228 (2004) 235 M. Goren, R.B. Lennox, Nano Lett. 1, 735 (2001) 235 W.-H. Zhu, D.-J. Zhang, J.-J. Ke, J. Power Sources 56, 157 (1995) 236 K. Aoki, M. Nakagawa, K. Ichimura, J. Am. Chem. Soc. 122, 10997 (2000) 237 M. Nakagawa, D. Ishii, K. Aoki, T. Seki, T. Iyoda, Adv. Mater. 17, 200 (2005) 237, 238 D. Ishii, M. Udatsu, M. Nakagawa, T. Iyoda, Trans. Mater. Res. Soc. Jpn. 29, 889 (2004) 237 M. Nakagawa, in Magnetic metal nanotube, ed. by T. Takata. Application Technologies of Novel Ring-and-Tube-Based Supramolecular Materials (Japanese) (CMC Publishing, Tokyo (Japan), 2006) p. 150 237 D. Ishii, K. Aoki, M. Nakagawa, T. Seki, Trans. Mater. Res. Soc. Jpn. 27, 517 (2002) 237

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49. M. Udatsu, D. Ishii, M. Nakagawa, T. Iyoda, T. Nagashima, M. Yamada, Trans. Mater. Res. Soc. Jpn. 34, 1219 (2005) 238 50. D. Ishii, T. Nagashima, M. Udatsu, R.-D. Sun, Y. Ishikawa, M. Yamada, T. Iyoda, M. Nakagawa, Chem. Mater. 18, 2152 (2006) 238 51. K. Suganuma, The Current Technology of Conductive Bonding Agents (Japanese) (Kogyo Chosakai Publishing, Tokyo (Japan), 2004) 239 52. I. Watanabe, Polymers (Japanese, The Society Polymer Science, Japan) 53, 799 (2004) 239 53. I. Tsukagoshi, Hitachi Chemical Technical Report (Japanese) 41, 7 (2003) 239 54. M. Nakagawa, D. Ishii, T. Nagashima, Chemistry & Chemical Industry (Japanese, The Chemical Society of Japan) 58, 494 (2005) 240, 241 55. H. Oda, Y. Yamanobe, T. Iyoda, M. Nakagawa, The abstract of 56th Symposium on Macromolecules (The Society of Polymer Science, Japan), 2F16, 2007 243 56. M. Nakagawa, K. Kobayashi, T. Nagashima, S. Kawasaki, H. Ochiai, JP2009221360 243

Index Amphoteric azopyridine carboxylic acid, 237 Angle-dependent transmittance, 242 Anisotropic conduction, 242–243 Anisotropic conduction film (ACF), application, 239–241 Azopyridine carboxylic acid, 237–238 Chemical structure of an azopyridine carboxylic acid, 237 Conductive magnetic nanotube, 236

Mechanism of formation of nickelphosphorus (Ni-P) thin film, 238–239 Molecular recycle-type method for preparation of nickel-phosphorus (Ni-P) nanotube, 238 Naval Research Laboratories, 236 Nickel-phosphorus (Ni-P) nanotube, 236–237, 240–241 Pyrolysis, 236

Filler-orientation-type anisotropic conduction film, 241–243 Hydrogen-bonded fibrous supramolecular, 237–238 Indium-tin-oxide (ITO), 242–243 Liquid crystal displays (LCDs), fabrication, 239

Scanning electron microscope (SEM) image of Ni-P nanotubes, 242 Supramolecular template, 236–237 Template synthesis of nickel-containing tubular, 236–239 Transmission electron microscopy (TEM), 238 UV-curable anisotropic conduction, 243

Magnetic metal-containing nickel– phosphorus (Ni–P) nanotubes, 235

X-ray photoelectron spectroscopy (XPS), 238

18 Synthesis and Applications of Water Nanotubes Yutaka Maniwa1,2 and Hiromichi Kataura2,3 1

2 3

Department of Physics, Faculty of Science, Tokyo Metropolitan University, Hachioji, Tokyo 192-0397, Japan [email protected] JST, CREST, Kawaguchi, Saitama 332-0012, Japan [email protected] Nanotechnology Research Institute, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562, Japan

Abstract Characteristics of a material system strongly confined in a certain spatial region with nanometer dimension are not only dependent on the number of atoms (molecules) but rather extremely dependent on the shape and dimensionality of the limited space. This chapter reports that a new tubularshaped ice, referred to as an ice nanotube (ice NT) is formed in the cylindrical cavity of a single-walled carbon nanotube (SWCNT) and that the ice NT exhibits an abnormal melting point dependency on the cavity diameter. Possible applications and “exchange transition” found in gas atmosphere are also briefly discussed.

18.1 Introduction It is well known that the physical and chemical properties of liquid and solid materials consisting of many atoms and molecules change as the number of molecules (atoms) reduces, i.e., the size of the system reduces. For example, in solid particulates, the melting points decrease and the chemical activities increase as their sizes decrease. Prominent quantum effects, such as Kubo’s effect [1], also appear. Furthermore, when an atom or a molecule is confined in a limited space, characteristics that are strongly dependent on that particular shape are expected. In alkali metal clusters inside subnano cavities of zeolite, for example, novel magnetism [2] and thermal chromism phenomenon [3] are observed, which are not found in bulk. It is believed that the characteristics of a material system confined in a certain spatial region are not only dependent on the number of atoms (molecules) but rather extremely dependent on the shape and dimensionality of the limited space. This chapter reports that a new tubular-shaped ice, referred to as an ice nanotube (ice NT) is formed in the cylindrical cavity of a single-walled carbon nanotube (SWCNT) [4] and T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 247–259 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 18

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that the ice NT exhibits an abnormal melting point dependency on the cavity diameter [5, 6].

18.2 Water Containing Carbon Nanotube SWCNTs are seamless tubes formed by the rolling and connecting of single sheet graphite layer ribbons (graphene) (Fig. 18.1). Various diameters and spiral structures are possible depending on how graphene ribbons are structured. A typical SWCNT has a diameter of 10–20 ˚ A and a length in the order of microns, and usually several SWCNTs form a bundle. SWCNTs can be synthetically produced by arc discharge or laser evaporation of a carbon rod containing a metallic catalyst, and by the chemical vapor deposition (CVD) method using a precursor gas such as alcohol. The cross section of a SWCNT is considered to be close to a true circle in the case of a SWCNT with a typical diameter (14 ˚ A), but each tube cross section takes a polygonal shape when high pressure is applied to a bundle of such SWCNTs. A thick isolated tube can also easily deform to a flat shape. Ice NT was found in a bundle of SWCNTs each having an approximately true circular cross section with a diameter of approximately 10–15 ˚ A. SWCNTs have a hydrophobic graphene wall; therefore, the adsorption of water to the inside of the minute cavity of the SWCNT is of particular interest. The size of a water molecule is approximately 3 ˚ A, and the diameter of a SWCNT cavity with a typical diameter of 13.5 ˚ A is approximately 10 ˚ A (= 13.5 ˚ A – van der Waals diameter), so that most of the water molecules are in contact with the SWCNT wall inside a SWCNT. These water molecules on the interface are prevented from forming hydrogen bonds in the SWCNT wall direction, which is possible in the bulk state, so that it is questionable whether water can be adsorbed inside the SWCNT [7, 8]. Generally, in case of water adsorption to a minute cavity consisting of hydrophobic walls, water

Fig. 18.1. Schematic diagram of single-walled carbon nanotubes (SWCNTs). The structure of a SWCNT is defined by an index consisting of two integers (n, m). Top: a zigzag type tube with index (9, 0). Bottom: an armchair type tube with index (5, 5)

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is purged out from inside the cavity, due to a wet-dry transition as the cavity size becomes smaller [9]. The fact that water can be adsorbed to SWCNTs has been empirically confirmed by methods such as powder X-ray diffraction (XRD) [5, 6, 10] and neutron diffraction [11]. Although it is generally believed that the most straightforward method for confirming molecular inclusion into a SWCNT is observation using a high-resolution transmission electron microscope (TEM), conventional TEM observations are difficult when an evaporative sample such as water is placed under a high-vacuum environment. On the other hand, in case of the XRD method, the sample can be placed in a glass capillary, so that the environmental gas and the sample temperature are easily controlled, making it an effective method for determining the adsorption of gas and water. In addition, observation of the phase transition according to temperature can be easily achieved. The adsorption of water to a SWCNT has also been recently confirmed by nuclear magnetic resonance (NMR) of water [12]. Study of the adsorption characteristics by computer simulations has also been reported [13]. In order to utilize the internal cavity of a SWCNT, it is necessary to have “windows” provided on the tip or sidewalls of the SWCNT to allow molecules access. In the case of an as-grown SWCNT sample, such windows are not sufficiently formed, so that a window opening process is required. The method for preparing samples to be used in SWCNT water adsorption experiments for XRD analysis [5, 6, 10, 14] is summarized below: 1. SWCNT Syntheses: A SWCNT is produced by the arc discharge method using a carbon electrode mixed with nickel and yttrium catalysts, or by the laser vaporization method in an electric arc using a carbon rod mixed with nickel and cobalt catalysts. In case of the laser vaporization method in an electric furnace, the mean diameter of the SWCNT can be controlled by the electric furnace temperature; XRD analysis was performed on six different SWCNT samples with mean diameters of 11.7–14.4 ˚ A, confirming water adsorption and the formation of ice NT. These samples produced excellent growth of bundle structures, and high-order Bragg peaks could be clearly observed in the powder XRD patterns, as shown in Fig. 18.2. 2. Opening Process: Opening of a SWCNT can be achieved by the oxidation method, in which the samples are heated to 620–750 K. The heating temperature and time used are dependent on the SWCNT diameter and purity of the sample. 3. Degas Drying: A SWCNT sample that has undergone the opening process may adsorb various molecules, depending on the preservation conditions and other process performed thereafter. Organic solvents such as alcohols can be strongly adsorbed inside a SWCNT, so care must be taken. In order to degas such adsorbed molecules, it is necessary to heat the samples in a vacuum over 700 K. According to the XRD analyses, SWCNT samples may have a large amount of adsorbed substances after the refining process,

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Intensity (a.u.)

Empty SWNT bundles

∝1/R

10

2R 14.4 (Å) 13.8 13.6 13.4 13.0 11.7

1

0.4

0.6

0.8

1

1.2

1.4

1.6

Q (1/Å)

Fig. 18.2. Powder XRD patterns of six different SWCNT samples with different mean diameters 2R. The scattering vector Q is given by Q = 4π sin θ/λ, where λ is the wavelength of the X-ray, and 2 θ is the scatter angle. The curves are shifted vertically for clarity

so that it is necessary to heat the samples over 700 K in order to degas them completely under vacuum [15]. 4. Water Adsorption: Water can be adsorbed inside SWCNTs by introducing saturated water vapor at room temperature (approximately 300 K). Adsorption usually takes a relatively short period of time (several minutes to several tens of minutes); however, it may take a substantial time in some cases. It is not quite clear why this is the case, but adsorption can sometimes be improved by repeating the introduction of saturated water vapor and vacuum discharge several times, or repeating the process of lowering the sample temperature in water steam and then returning it to room temperature.

18.3 Liquid–Solid Phase Transition: Formation of Ice Nanotube The water structure and phase transition of water contained in a SWCNT can be examined by the powder XRD [5, 6]. As an example of the change in the XRD pattern due to water adsorption, the temperature dependence of a SWCNT sample with a mean diameter of 13.5 ˚ A is shown in Fig. 18.3. The sample is enclosed in a glass capillary together with saturated water vapor at 300 K. Since only the portion of the sample in the capillary is temperature controlled using a gas flow type device, the other end of the capillary is maintained at room temperature (300 K). Consequently, it is assumed that, if the temperature is higher than 300 K, the water vapor pressure is equal

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Intensity (arb. units)

10 peak

× 10

G002 330K 300K 100K 0.5

1

1.5 Q (Å)

2

2.5

Fig. 18.3. XRD patterns of a SWCNT sample with a mean diameter of 13.5 ˚ A encapsulated with saturated water vapor (300 K). Solid line: measured value. Dotted line: simulation result. A new peak at Q = 2.2 ˚ A−1 (indicating formation of ordered structure of water) appears at low temperatures. The strength of the 10 peak sharply increases in the vicinity of 320 K. At temperatures over 320 K the XRD pattern is the same as that of dry SWCNTs. The G002 peak is the peak for graphite-like impurities mixed in the sample

to the water vapor pressure (36 mbar) of the room temperature, while if the temperature is lower than 300 K, the water vapor pressure is equal to the saturated vapor pressure of the particular temperature. In Fig. 18.3, the XRD pattern changes sharply between 300 and 330 K. The XRD pattern at higher temperatures is the same as that of the dry SWCNT, which indicates that water adsorption to the SWCNT bundle does not occur at high temperatures. On the other hand, the XRD pattern at 300 K changes dramatically from that of the dry samples, indicating that water is adsorbed to the SWCNT bundle. Additionally, through analysis of the XRD pattern, it was determined that the water adsorption site is inside the SWCNT and the density is approximately 0.9 g cm−3 , which is close to that of bulk water. Because no new Bragg peak appears when water is adsorbed, it is suggested that water is in the form of a disordered structure. Furthermore, according to 1 H-NMR and 2 D-NMR of a SWCNT sample to which water and heavy water is adsorbed [16], it was confirmed that water molecules undergo fast translational and rotational motion at temperatures over 220 K, and it is conceivable that the water is in a liquid state down to 220 K when the XRD analysis results are also considered. When the water freezing phenomenon is examined by lowering the temperature, a new Bragg peak appears at Q = 2.2 ˚ A−1 , as shown in Fig. 18.3. This indicates that the water molecules are arranged regularly. The temperature at which this Bragg peak appears (or disappears) during decrease (or increase)

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of the temperature agrees within 5 K. From NMR analysis, the freezing (melting) of water molecule motion starts at a corresponding temperature [16], so that this temperature is assumed to be the freezing (melting) point of water inside a SWCNT. Structural information of the ice inside a SWCNT is obtained from a detailed analysis of low-temperature XRD patterns. The results indicate the formation of an ice nanotube (ice NT), which was theoretically predicted by Koga et al. [17, 18]. The ice NT is an ice tube, consisting of n (integer)pieces ring-shaped clusters one-dimensionally stacked, as shown schematically in Fig. 18.4. For six SWCNT samples with mean diameters of 11.7–14.4 ˚ A, ice NTs consisting of 5- through 8-membered rings were empirically found. In all of these ice NTs, each water molecule is bonded by hydrogen bonds to four adjacent water molecules, and each hydrogen atom participates in one hydrogen bond. In other words, the structure satisfies a similar ice rule to that for bulk water. The empirically observed Bragg peak of Q = 2.2 ˚ A−1 corresponds to the ˚ 2.9 A periodicity of the (n-membered) water ring of the ice NT, and is close to the hydrogen bond distance of 2.76 ˚ A in bulk ice. However, further investigation showed that the one-dimensional periodicity of the 8-membered ring

Fig. 18.4. Structural model of ice NT. Left: 5-membered ring ice NT inside a SWCNT. From top right, 6-, 7-, and 8-membered ice NTs. Red ball : oxygen, blue ball : hydrogen

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ice NT is significantly longer than those of the 5- through 7-membered rings. Consequently, a mixture of 8- and 7-membered ring ice NTs are present in the sample, due to the diameter distribution of SWCNTs in the sample with a mean diameter of 13.5 ˚ A, and the Bragg peak caused by the ice NT structure splits into two peaks. More detailed observation shows that the Bragg peak of Q = 2.2 ˚ A−1 is trailing in the direction that Q increases, as shown in Figs. 18.3 and 18.5. This is a peak shape characteristic of the one-dimensional (low dimension) structure, and can be explained by the weak correlation between the ice NT structures of different SWCNTs in a bundle [6]. Figure 18.5 shows the shape of the Bragg peak derived from the ice NT structure in the sample with a mean diameter of 11.7 ˚ A. Although a mixture of ice NTs composed of 5- and 6-membered rings is present in this sample, the asymmetric peak shape observed can be well reproduced by a model in which uniform rings with the same diameters are layered one-dimensionally. Inspection using infrared absorption spectroscopy showed a shift in the absorption spectrum due to inter-ring bonding in ice NT [19]. The melting point of an ice NT can be obtained from the temperature of the Bragg peak appearance at Q = 2.2 ˚ A−1 . Four discrete melting points were observed for six different SWCNT samples with mean diameters of 11.7–14.4 ˚ A. The melting points are derived from ice NTs of 5- through 8-membered rings. The dependency of the melting points on the SWCNT cavity diameters is shown in Fig. 18.6 in addition to the dependency of the bulk region melting points [20–22]. While the bulk region melting points

2R=11.7Å

4.2

2.2

2.4 Q (1/Å)

4.4

4.6 Q (1/Å)

2.6

4.8

2.8

Fig. 18.5. Diffraction peaks derived from the ice NT structure of a SWCNT sample with a mean diameter of 11.7 ˚ A. Thin dotted line: calculated result for 5-membered ring ice NT. Thin solid line: calculated result for 6-membered ring ice NT. Thick dotted line: calculated result for a mixture of 5- and 6-membered ring ice NTs

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n=5

300

Tm (K)

n=6 250

liquid ice nanotubes

Ih

200

150

glass tube

n=7

n=8

5

10

15

20

25

30

Capillary Diameter D (Å)

Fig. 18.6. Dependency of melting point on the diameter of the cylindrical cavity. The broken line is an extrapolation from the bulk region known to glass capillary. Insert (top) is a model of ice NT seen in the axial direction. Insert (bottom) is a structural model of bulk ice Ih

decrease with decreasing cavity diameter, the ice NT melting points increase as the SWCNT diameter decreases and are higher for smaller n. In other words, a reversal phenomenon of the melting point dependency on the cavity diameter is observed in the region where the SWCNT diameter is 10–20 ˚ A. This reversal phenomenon is considered to be due to a crossover phenomenon of the bulk region and the atomic-scale region. The water molecules in a SWCNT have the minimum potential energy at the surface of a cylindrical shape as a result of the interaction (e.g., Lennard–Jones type interaction) with the SWCNT walls. It is estimated that the stabilization of ring-shaped water clusters (n-membered clusters) is assisted by this cylindrical potential, to form a one-dimensional array leading to the formation of an ice NT. While the entire structure is determined by the shape and size of the interface in the atomic-scale region, in the bulk region it is assumed that a bulk-like or multiple layer structure in the radial direction of the cavity is formed.

18.4 Computer Simulations of Ice Nanotube Classical molecular dynamics (MD) calculations using a SWCNT model assumed to be a sheet of homogeneous cylinders indicated that an ice NT would be formed in a SWCNT [18]. While a constant pressure (50–500 MPa) was

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Fig. 18.7. Calculation based on classical MD for 84 water molecules contained in a (9, 9) SWCNT. The SWCNT is not shown. The TIP 3P model was used for the water molecules. After calculation for 1 ns at 300 K, the SWCNT was cooled to 100 K at a rate of 200 K ns−1 . (a)–(d): 300, 260, 200, and 100 K, respectively

applied in the axial direction in this calculation by Koga et al., Shiomi et al. recently conducted another MD calculation for a water cluster inside a SWCNT with a honeycomb structure to study the ice NT dependency on the SWCNT diameter, and the obtained result reproduced the experiment [23]. Figure 18.7 shows the result of the classical MD calculations by the author’s group. The temperature of 84 water molecules contained in a (9, 9) SWCNT was varied from 300 to 100 K at a rate of 200 K ns−1 . As a result, it was found that each water molecule has high speed rotational and translational motions at high temperatures (over 260 K). In addition, although no pressure was applied, the water agglomerates and water molecules are present even in the vicinity of the center axis of the SWCNT with a finite probability. However, when the temperature is lowered gradually, the distribution of water molecules starts to change at around 260 K, and forms a neat 6-membered ring ice NT at 100 K. Although the appearance of ice nanotubes with a multilayer ice tube structure [24, 25] and one-dimensional chain structure of water molecules is predicted, no experimental confirmation of the structure has been made yet.

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18.5 Physical Properties and Application of Water Containing SWCNT Although study of the physical properties of water contained in SWCNTs is important, very little has been reported to date. Of those, the following research reports are introduced. First, it was reported that the electrical resistance of a SWCNT mat (film) varied significantly according to water adsorption [10, 14, 26, 27]. It is believed that such a variation in electrical resistance is particularly conspicuous on a SWCNT film to which the opening process is applied, and the electrical resistance is sharply reduced as water is adsorbed to the inside of the SWCNTs. As a mechanism for the change in electrical resistance, it has been reported that a finite electrical charge transfer occurs between water molecules and SWCNTs [28]; however, a more detailed investigation is necessary. As noted from Figs. 18.4 and 18.7, ice NT may be able to adsorb gas molecules inside the cavity, as it has a tubular structure. Tanaka and Koga theoretically clarified that gas hydrates with a new structure can be formed

Fig. 18.8. Calculation of exchange transition based on classical MD. Water adsorbed on a (10, 10) SWCNT placed in methane gas (a) and neon gas (b) was studied using MD calculations. Water and methane were exchanged with time (300 ps) in methane. On the other hand, water remained stable in the SWCNT in case of neon. The number of molecules, temperature, and pressure were the same for both (a) and (b)

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by gas molecules in the cavity part of an ice NT [29]. However, the existence of the gas hydrate has not yet been confirmed by gas adsorption experiments of corresponding SWCNTs containing water. Instead, “exchange transition,” in which the water inside SWCNTs is replaced with ambient gas molecules, has been reported [14]. Figure 18.8 shows the result of the MD calculation of the exchange transition. Hundred water molecules were introduced into a (10,10) SWCNT that was then filled with methane or neon as an ambient gas. Methane and neon were maintained under the same conditions, and the temperature was increased from 100 to 200 K and held at 200 K for 0.3 ns. In the case of methane,

(I)

(II)

(III)

(IV)

Fig. 18.9. Structural models of four kinds of 7-membered ring ice NTs with proton arrangements of equivalent energy. Large balls: oxygen, small balls: hydrogen atoms (protons)

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the methane molecules purged water with time and completely replaced water after 0.3 ns (Fig. 18.8a). On the other hand, in the case of neon, the water is held inside the SWCNT in a stable manner (Fig. 18.8b). The exchange transition process strongly depends on the type, pressure, and temperature of the gas. In addition, the electrical resistance of the SWCNT film changes sharply with the progress of the exchange transition, so that it is expected that this system can be used for gas sensors, etc. Another proposed application that utilizes the exchange transition phenomenon is as a gas-selective nanovalve [14]. Finally, the characteristics of a water containing SWCNT as a dielectric material are interesting. As shown in Fig. 18.9, a plurality of structures is conceivable, particularly to an ice NT in which energetically degenerated or proximately degenerated structures with reference to the arrangement of hydrogen atoms are conceivable, and phenomena related to these structures are expected. For example, there is a possibility that an order–disorder transition may occur with respect to the arrangement of hydrogen atoms resembling bulk water. While it has been suggested from NMR observations that the motions of hydrogen molecules freeze in accordance with the ice NT transition [16], detailed studies of hydrogen ordering are very interesting from both base and application sides.

References 1. R. Kubo, A. Kawabata, S. Kobayashi, Annual Review of Materials Science 14, 49 (1984) 247 2. Y. Nozue, T. Kodaira, T. Goto, Phys. Rev. Lett. 68, 3789 (1992) 247 3. Maniwa, Y., et al., Chem. Phys. Lett. 424, 97 (2006) 247 4. M.S. Dresselhaus, G. Dresselhaus, P.C. Eklund, Science of Fullerenes and Carbon Nanotubes (Academic Press, New York, 1995) 247 5. Y. Maniwa, et al., J. Phys. Soc. Jpn. 71, 2863 (2002) 248, 249, 250 6. Y. Maniwa, et al., Chem. Phys. Lett. 401, 534 (2005) 248, 249, 250, 253 7. G.. Hummer, J.C. Rasaiah, J.P. Noworyta, Nature 414, 188 (2001) 248 8. M.S.P. Sanson, P.C. Biggin, Nature 414, 156 (2001) 248 9. K. Lum, et al., J. Phys. Chem. 103, 4570 (1999) and references therein 249 10. Y. Maniwa, et al., Jpn. J. Appl. Phys. 38, L668 (1999) 249, 256 11. A. Kolesnikov, et al., Phys. Rev. Lett. 93, 035503 (2004) 249 12. H-J. Wang, et al. Science 322, 80 (2008) 249 13. A. Striolo, et al., J. Chem. Phys. 122, 234712 (2005) 249 14. Y. Maniwa, et al., Nat. Mater. 6, 135 (2007) 249, 256, 257, 258 15. Y. Maniwa, et al., Phys. Rev. B 64, 241402(R) (2001) 250 16. K. Matsuda, et al., Phys. Rev. B 74, 073415 (2006) 251, 252, 258 17. K. Koga, et al., J. Chem. Phys. 113, 5037 (2000) 252 18. K. Koga, et al. Nature 412, 802 (2001) 252, 254 19. O. Byl, et al., J. Am. Chem. Soc. 128, 12090 (2006) 253 20. K.A. Jackson, B. Chalmers, J. App. Phys. 29, 1178 (1958) 253 21. K. Morishige, K. Kawano, J. Chem. Phys. 100, 4867 (1999) 253

18 Synthesis and Applications of Water Nanotubes 22. 23. 24. 25. 26. 27. 28. 29.

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Z. Liu, et al., Phys. Rev. E 67, 061602 (2003), and references herein 253 J. Shiomi, et al., J. Phys. Chem C 111, 12188 (2007) 255 J. Bai, J. Wang, X.C. Zeng, Proc. Natl. Acad. Sci. USA 103, 19664 (2006) 255 D. Takaiwa, et al., Proc. Natl. Acad. Sci. USA 105, 39 (2008) 255 Y. Maniwa, et al., Mol. Cryst. Liq. Cryst. 340, 671 (2000) 256 A. Zahab, et al., Phys. Rev. B 62, 10000 (2000) 256 R. Pati, et al., Appl. Phys. Lett., 81, 2638 (2002) 256 Tanaka, H., & Koga, K., J. Chem. Phys., 123, 094706 (2005) 257

Index Bragg peak, 251–253 Computer simulations of ice nanotube, 254–255 Formation of ice nanotube, 250–254 Ice nanotube (NT), 252–253, 256–257 Liquid–solid phase transition, 250–254 Melting point of ice NT, 253–254 Molecular dynamics (MD) calculations, 254–257 Nuclear magnetic resonance (NMR) of water, 249 Physical properties and application of water containing SWCNT, 256–258

Powder XRD patterns of six different SWCNT samples, 250 Single-walled carbon nanotube (SWCNT), 247–258 Structural information of ice inside SWCNT, 252–253 Structural models of four kinds of 7-membered ring ice NTs, 257 Transmission electron microscope (TEM), 249 Water containing carbon nanotube, 248–249 Water freezing phenomenon, 251–252 X-ray diffraction (XRD), 249 XRD patterns of SWCNT sample, 251

19 Design and Synthesis of Titanium Oxide Nanotubes Akira Hasegawa Tsukuba Research Laboratory, Sumitomo Chemical Co., Ltd., Tsukuba, Ibaraki 300-3294, Japan [email protected] Abstract Long titanium oxide nanotubes are fabricated by dissolution and recrystallization. The length of the nanotubes is more than 120 μm and the diameter of the nanotubes is about 50 nm. The resistivity of titanium oxide can be decreased by doping and/or reduction. Long titanium oxide nanotubes are expected for many electrical device applications such as sensor, electrode, FET, photocatalyst, and dye-sensitized solar cell (DSC; Gr¨ atzel cells).

19.1 Introduction The carbon nanotube [1, 2] was discovered as a new carbon allotrope after fullerene [3]. Because the carbon nanotube not only has an interesting geometric structure but also interesting electrical properties, which change from metal to semiconductor [4–6], it attracts the interest of many researchers. There is much research to make nanotubes other than carbon. As boron nitride (BN) is a material which has carbon-like structures the BN nanotube is manufactured by a substitution reaction of the carbon nanotube [7–9]. For the application of quantum wires as conductive carbon nanotubes, gold nanotubes have been manufactured [10] and the relationship between structure and properties have been studied [11]. Researches to make similar nanotube structures with metal oxides have been intended. Template methods which make nanotubes of Al2 O3 , V2 O5 , and MoO3 with a diameter of around 10 nm, using the sol-gel method with the carbon nanotube as a mold have been reported [12]. Nanotubes of titanium oxide TiO2 with a diameter of 70–100 nm are provided by a replica method using alumina madreporite [13]. Kasuga et al. obtained titanium oxide nanotubes with an Anatase crystal structure, about 8 nm in diameter and about 100 nm in length, by chemical treatment of fine titanium oxide powder in 10-M sodium hydroxide water solution at 110◦ C for 20 h [14–16]. This is a simple and easy synthesis method for titanium oxide nanotubes at low temperature and with a chemical solution process by selforganization without a template. T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 261–273 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 19

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19.2 Crystal Structure Models and Electronic Structure Model Calculation Kasuga et al. reported that the crystal structure of the titanium oxide nanotube is Anatase type. Figure 19.1 shows the XRD pattern of the titanium oxide nanotube which was made with alkali processing. The XRD pattern of the titanium oxide nanotube closely resembles that of Anatase-type titanium oxide. The (200) peak in the vicinity of a plane of the octahedrite is very sharp, and the (101) peak in the vicinity of a plane

( 2 0 0 )

( 1 0 1 )

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The unit cell of Anatase

Inteensity(CPS)

400

300

200

100 0 21-1272> Anatase - TiO2

Anatase

21-1276> Rutile -TiO2

R il Rutile 10

20

30

40

50

60

70

2θ(deg)

Fig. 19.1. XRD pattern of the titanium oxide nanotube

Fig. 19.2. High-resolution TEM images of the titanium oxide nanotube

80

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of the octahedrite becomes very broad on the high angle side. The peak near the Anatase (200) peak is very sharp, and the peaks around Anatase (101) are very broad in the high angle direction. From this viewpoint, the unit cell of the crystal structure of the titanium oxide nanotube resembles that of the Anatase-type crystal structure but the peaks around the (101) Anatase peak show wide broadening because of the rotation of the unit cell along the side curve of the nanotube fiber. On the other hand, the peak near the (200) Anatase peak is very sharp because this peak is related to the axis direction of the nanotube fiber and is not affected by rotation of the nanotube fiber. We can see the continuous crystal lattice pattern in the axis direction of the nanotube fiber in the high-resolution TEM image in Fig. 19.2. From this point of view, the titanium oxide nanotube is made of a continuous crystal structure, and is not made of a micro-crystal assembly. The titanium oxide nanotube is a single crystal and its unit cells are rotated around the core of the titanium nanotube fiber. Q. Chen assumes that the structure of titanium oxide nanotubes consists of an arranged Ti-O octahedron coordination frame which is wound up [17]. This knowledge and the unit cell of Anatase led me to assume a spiral structured model. Furthermore, in order to simplify the calculation, I assumed the fourunit cell stacked model shown in Fig. 19.3 as the simplest model and calculated with the extended Hueckel method (YAeHMOP) [18–20].

8QP‡

Cross Section of the titanium oxide nanotube

Fig. 19.3. Slab model of the titanium oxide nanotube

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I calculated the model with added OH at Ti and added H at O, because just after synthesis the titanium oxide nanotube includes H and OH. The titanium oxide nanotube in Fig. 19.4 shows the electronic structure of the wide-gap semiconductor (an insulator). In addition, I calculated the structure without H, because H included in the structure is removed as H2 O by around 350◦ C. In Fig. 19.5, we can see a small band (gray in the figure) between the t2g band and eg band. This comes from the dangling bond of the Ti. The Ti3dz2 band is shifted down from the eg band, because the oxygen atom does not exist in the z-direction. This band does not appear in the electronic band structure of the hydrated Slab model in Fig. 19.3. The titanium oxide nanotube just after synthesis is well explained by the surface hydrated model, but the structure of the anhydrous titanium oxide nanotube should be explained by a more appropriate model. The band structure of the Anatase bulk crystal is shown in Fig. 19.6 for comparison.

Fig. 19.4. Band structure and DOS of the titanium oxide nanotube, Hydrated surface slab model

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Energy (eV)

19 Design and Synthesis of Titanium Oxide Nanotubes

EF

Γ

Γ

Fig. 19.5. Band structure and DOS of the titanium oxide nanotube, Anhydrous surface slab model

19.3 Highly Functional Nanotube: Conductivity Grant = Reduction of Resistivity Powdered titanium oxide has uses as a white pigment, a filler in polymers, and as a photo catalyst. When the particle diameter distribution of titanium oxide powder, which has a high refractive index, becomes the same as the wavelength degree of light (visible ray), light is scattered by the powder and the powder shows a white color. The refractive index of Rutile (2.72) is higher than that of Anatase (2.52), so Rutile has larger concealment power and is used as a white pigment. Titanium oxide is known as a superior material in photo catalytic activity. Titanium oxide shows strong oxidation/reduction power, when it is irradiated with ultraviolet rays [21]. An electron in the valence band is activated by light to the conduction band and a positive hole is made in the valence band. This positive hole shows very strong oxidation power and decomposes organics into carbon dioxide and water. In addition, the conductive electron activated to the conduction band shows strong reductive power. Particularly, the oxidation power is much stronger than chlorine, hypochlorous acid, hydrogen peroxide, and ozone, and shows an antibacterial

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Fig. 19.6. Band structure and DOS of Anatase

effect through bacterial decomposition and a purification effect through decomposition of organic matter stains. Because Anatase shows high photo catalyst activity, Anatase is used as a photo catalyst. As in the light of the sun and fluorescent light there are few parts of the near-ultraviolet radiation equal to or less than 380 nm, and the main part is a visible ray, one of the biggest targets in photo catalyst research is the search for a photo catalyst that acts in visible light. Therefore, band gap control has been tried via doping. In addition to these properties that are available for powdered titanium oxide, when the length of the titanium oxide nanotube becomes long enough, we can use properties along the length direction of the titanium oxide nanotube such as electrical conductivity. Furthermore, materials that have a high aspect ratio can be used as anisotropic materials by orientation control. Initially, the length of titanium oxide nanotubes were short at around 100 nm and a longer one was not provided by changes in synthesis conditions or raw material. The author has developed a titania nanotube whose length is more than 100 μm and reaches up to millimeters. One approach to make a highly functional material taking advantage of length is control of conductivity. The carrier can be increased by doping, i.e. valence control, as titanium oxide is an n-type wide gap semiconductor. This approach is commonly used in the field of semiconductors such as Si and in this approach a larger valence element than the host element is doped and substitutes the host site and increases the carrier of free electrons.

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In the case of titanium oxide (TiO2 ) and Nb2 O5 is dissolved, the Kroger– Vink notation is Nb2 O5 → 2NbTiX + 4OOX + 1/2 O2 2NbTiX → NbTi + e (electron carrier) That is, one doped atom of a larger valence element than the host element creates one carrier of free electrons. In addition, there is a method to increase the carrier of free electrons, as (TiO2 ) is an oxide. This method is not commonly used in semiconductors such as Si. Reduction of oxide is one approach to increase the carrier of free electrons. In the case of stoichiometry, titanium oxide (TiO2 ) has a high resistance, but when an oxygen vacancy is introduced by heating in a reductive atmosphere, the electron carrier increases. OOX → VOX + 1/2 O2 VOX → VO + e (electron carrier) V is the vacancy in Kroger–Vink notation (not vanadium). Furthermore, under severe reduction conditions, more and more oxygen is removed and becomes other oxide material with different Ti and O ratios. The valence of the titanium ion changes to smaller than 4 and oxide changes to a lower valence oxide. Of structural interest in the titanium oxide nanotube is the tunnel structure and spiral structure, which has a gap between the walls of sheets. These structures can contain metal atoms that are different from the usual dopant atoms substituted in the host site. The electronic structure of the titanium oxide nanotube varies with these metal dopants, and high functionality may be given [22, 23].

19.4 Crystal Structure and Electric Conductive Property of Titanium Oxide Titanium oxides whose titanium ion has a valence of 2, 3, or 4 are well known, and tetravalent oxide is called titania. There are three kinds of crystal structures in titanium oxide TiO2 . They are Rutile, Anatase, and Brookite, but Brookite is only of academic interest as there is no industrial use. Figures 19.6 and 19.7 show the band structure and density of state (DOS) of Anatase and Rutile of titanium oxide, respectively. In the figure, the dotted line with the letter EF indicates Fermi level. The Fermi level is located above the valence band and the band structure consists of a vacant conduction band and filled valence band. The band gap between the valence band and the conduction band is wide, and both Rutile and Anatase are wide-gap semiconductors. Because titanium oxide is a widegap semiconductor, the use of titanium oxide is as a white pigment. That is, single-crystal titanium oxide is transparent because the band gap does not

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Fig. 19.7. Band structure and DOS of Rutile

absorb visible light. Titanium oxide is almost an insulator at room temperature but it shows electrical conductive properties as an n-type semiconductor when the titanium site is replaced by a quinquevalent metal ion or an oxygen vacancy is introduced by reduction and/or heating.

19.5 Synthesis of Titanium Oxide Nanotubes Long titanium oxide nanotubes of more than 100 nm had not before been obtained. Therefore, in order to obtain long titanium oxide nanotubes, I used very fine titanium oxide powder and dissolved it under intense stirring. A part of the titanium oxide powder was dissolved in the solution and continuous crystal growth occurred and long titanium oxide nanotubes were synthesized [24, 25]. The left side of Fig. 19.8 shows the powdered raw material and the right side shows the synthesized titanium oxide nanotube. The synthesized product of titanium oxide has a different form than the powdered raw material and seems like a fiber-shaped material forming a bunch. Each fiber which forms the fiber bunch has a uniform diameter and is independent of fiber material. Figure 19.9 shows a TEM Image of the synthesized titanium oxide nanotube.

19 Design and Synthesis of Titanium Oxide Nanotubes Raw Material Powder

269

Titanium Oxide Nanotube

Fig. 19.8. Raw material powder and synthesized titanium oxide nanotube SEM Images

50 nm

50 nm

Fig. 19.9. Synthesized titanium oxide nanotube TEM Images

From the TEM image, the diameter of the titanium oxide nanotube was about 50 nm and in terms of diameter uniform titanium oxide nanotubes were obtained. In addition, from the TEM image, there are walls on both sides of the fiber and the inside is hollow, so the fiber has a tube structure. In order to determine the length of the titanium oxide nanotubes we used SEM. As the length of the titanium oxide nanotube increases, its confirmation becomes difficult. There is a loose titanium oxide nanotube in the horizontal direction of the SEM image shown in Fig. 19.10. The extended area is shown in Fig. 19.10. The length of the titanium oxide nanotube is more than 120 μm from comparison with the scale bar of 100 μm. The diameter of the titania nanotube is 50 nm, and the length is more than 120 μm, so the aspect ratio is above 2,400.

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end

magnified

end

magnified

end

Fig. 19.10. The length of the synthesized titanium oxide nanotube. Magnified image and scale

Figure 19.11 shows the titanium oxide nanotube after washing. There were lumps of titanium oxide nanotubes in the water of the beaker, and a lump was able to be picked up with tweezers. Furthermore, the lump of titanium oxide was able to be picked up in air. The full length of the titanium oxide nanotube cannot be confirmed, but it is close to the length of this lump. Figure 19.12 shows the electron diffraction pattern of the titania nanotube. An ordinary single crystal shows a spot pattern, but the electron diffraction pattern of the titanium oxide nanotube shows a striped pattern, whichis made

Picked up in the water

Picked up in the air

Fig. 19.11. Photograph of the synthesized titanium oxide nanotube, after washing

19 Design and Synthesis of Titanium Oxide Nanotubes

Direction of the streak

Direction of the nanotube

271

Direction of the streak

Direction of the nanotube

Fig. 19.12. Electron diffraction images of the synthesized titanium oxide nanotube

up of streaks of the single-crystal spot pattern. Below the electron diffraction patterns, TEM images are also shown. The angle between the fiber length direction in the titanium oxide nanotube and the streak of the electron diffraction was perpendicular (90◦ ). Another electron diffraction pattern and TEM image is shown in Fig. 19.12. The angle between the fiber length direction of the titanium oxide nanotube and the streak of the electron diffraction was also perpendicular (90◦ ). In addition, a piece of a nanotube with a different direction appears in the TEM image, and weak electron diffraction pattern.

19.6 Concluding Remarks Herein, I calculated the model structure of a titanium oxide nanotube with the extended Hueckel method (YAeHMOP). In the structure of the nanotube it is assumed that the layers of the Anatase unit cell are rotated around the axis of the nanotube fiber and a simplified Slab model of the four layers of the Anatase unit cells is further assumed. In the future, enhancement of computer performance and development of calculation algorithms will progress, and we will be able to calculate nanotube materials directly by a first principle method without any simplifications or assumptions.

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References 1. S. Iijima, Nature 354, 56 (1991) 261 2. S. Iijima, T. Ichihashi, Nature 363, 603 (1993) 261 3. H.W. Kroto, J.R. Heath, S.C. O’Brien, R.F. Curl, R.E. Smalley, Nature 318, 162 (1985) 261 4. N. Hamada, S. Sawada, A. Oshiyama, Phys. Rev. Lett. 68, 1579 (1992) 261 5. R. Saito, M. Fujita, G. Dresselhaus, M.S. Dresselhaus, Phys. Rev. B 46, 1804 (1992) 261 6. K. Tanaka, K. Okahara, M. Okada, T. Yamabe, Chem. Phys. Lett. 191, 469(1992) 261 7. E.J.M. Hamilton, S.E. Dolan, C.M. Mann, H.O. Colijn, C.A. McDonald, S.G. Shore, Science 260, 659 (1993) 261 8. A. Rubio, J.L. Corkill, M.L. Cohen, Phys. Rev. B 49, 5081 (1994) 261 9. X. Blase, A. Rubio, S.G. Louie, M.L. Cohen, Europhys. Lett. 28, 335 (1994) 261 10. Y. Kondo, K. Takayanagi, Science 289, 606 (2000) 261 11. A. Hasegawa, K. Yoshizawa, K. Hirao, Chem. Phys. Lett. 345, 367 (2001) 261 12. H. Nakamura, Y. Matsui, J. Am Chem. Soc. 117, 1651 (1997) 261 13. P. Hoyer, Langmuir 12, 141 (1996) 261 14. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Langmuir 14, 3160 (1998) 261 15. T. Kasuga, Materials Integration 12, 33 (1999)(in Japanese) 261 16. T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino, K. Niihara, Adv. Mater. 11, 1307 (1999) 261 17. Q.Chen, G.H. Du, S. Zhang, L.M. Peng, Acta Cryst. B 58, 587 (2002) 263 18. G.A. Landrum, YAeHMOP (Yet Another extended Huckel Molecular Orbital Package) Version 2.0, Cornell University, Ithaca, NY, 1996 263 19. E. Muller, Tables of Parameters for Extended Huckel Calculations ([email protected]), available in YAeHMOP 2.0 263 20. P. Pyykko, L.L. Lohr Jr., Inorg. Chem. 20, 1950 (1981) 263 21. A. Fujishima, K. Honda, Nature 238, 37(1972) 265 22. Japanese Patent No. 2003-261,331, “conductive metal oxide nanotube”, Akira Hasegawa 267 23. A. Hasegawa, “Nano Design-Design of titania tube”, Nano material SAIZENSEN, KAGAKU DOJIN (2002) 267 24. Japanese Patent No.2004-331,490, “titania nanotube and the manufacturing method”, Akira Hasegawa 268 25. A. Hasegawa, “long fiber titania nanotube”, nanomaterial engineering system, Vol. 1 new ceramics glass, Japanese Fuji techno system, p. 140 (2005) 268

Index Anatase bulk crystal, band structure, 264 Anatase-type titanium oxide, 262–263 Anhydrated surface slab model, 265

Band structure and DOS of anatase, 266 Band structure and DOS of Rutile, 268 Band structure and DOS of titanium oxide (TiO2 ) nanotube, 264–265 Boron nitride (BN), 261

19 Design and Synthesis of Titanium Oxide Nanotubes Crystal structure models, 262–265 Crystal structure of titanium oxide, 267–268 Diameter of titanium oxide (TiO2 ) nanotube, 269 Electric conductive property titanium oxide, 267–268 Electron diffraction images of titanium oxide (TiO2 ) nanotube, 271 Electronic structure model calculation, 262–265 High-resolution TEM images of titanium oxide nanotube, 262 Hydrated surface slab model, 264 Kroger–Vink notation, 267 Length of synthesized titanium oxide (TiO2 )nanotube, 270 Powdered titanium oxide (TiO2 ), 265

273

Raw material powder and synthesized titanium oxide (TiO2 ) nanotube, 268–269 Refractive index, 265 Rutile, 265 Slab model of titanium oxide nanotube, 263 Synthesis of titanium oxide (TiO2 ) nanotubes, 268–271 Titania nanotube, 266 Titanium oxide (TiO2 ), nanotubes, 261, 264 Titanium oxide (TiO2 ) nanotubes structure, 263, 267 Washing of titanium oxide (TiO2 ) nanotube, 270 XRD pattern of titanium oxide nanotube, 262

20 In Situ TEM Electrical and Mechanical Probing of Individual Multi-walled Boron Nitride Nanotubes Dmitri Golberg, Pedro M.F.J. Costa, Masanori Mitome, and Yoshio Bando National Institute for Materials Science (NIMS), WPI Center for Materials Nanoarchitectonics (MANA), Tsukuba, Ibaraki 3050044, Japan [email protected] Abstract Measurements of electrical and mechanical properties of individual multiwalled BN nanotubes inside a high-resolution transmission electron microscope are presented.

20.1 Introduction In situ experiments inside a high-resolution transmission electron microscope (HRTEM) using modern piezo-driven holders allow one not only to gain an insight into the electromechanical properties of a one-dimensional nanostructure but, and most importantly, make it possible doing so under a full structural control (at a high-spatial resolution natural for HRTEM) of all changes within a tested object during the complex measurements. Below we provide a brief review of several sound examples of successful experiments performed in our laboratory using the state-of-the-art STM-TEM and AFM-TEM holders, commercialized by “Nanofactory Instruments AB,” on multi-walled boron nitride (BN) nanotubes with and without fillings in their channels.

20.2 Electrical Probing Experiments 20.2.1 Unfilled Multi-walled BN Nanotubes Since recently there has been a growing interest in nanotube electrical probing and manipulation [1–17]. The usage of piezo-driven stages within the standard TEM and HRTEM setups is one of the efficient routes to accomplish such goals [12–17]. The prime advantage of this technique, opposed to the commonly used transport measurements using microscale pre-formed electrical T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 275–286 (2010) c Springer-Verlag Berlin Heidelberg 2010  DOI 10.1007/978-3-642-03622-4 20

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circuits containing nanotubes, is the possibility to morphologically, structurally, and chemically characterize a nanoscale object which is under testing, prior, during and after the electrical measurements. All standard TEM operations, e.g., high-resolution imaging with an atomic lattice resolution, spatially resolved nanobeam electron diffraction and chemical composition analyses using electron energy loss (EELS) or X-ray dispersion (EDS) spectrometers, could be selectively performed on exactly the same nanotube that is electrically probed. It is noted that to date most of standard experimental setups, like AFMor SEM-based techniques, suffer from limited magnifications and spatial resolutions. Even if the time- and effort-consuming electrical tests on an individual nanotube had been successfully accomplished, there was always a huge degree of uncertainty as to what particular structure, morphology, and/or nanotube chemical composition they were related to, and how the given nanostructure was affected during the data collection. The series of the in situ TEM highly informative experiments using piezostages have already been performed on standard carbon (or C-containing) nanotubes [1–8]. However, we emphasize that the analogous measurements on purely non-carbon nanotubes, e.g., those made of BN, have been lacking. Herein we review in situ TEM experiments on individual highly pure multiwalled BN nanotubes by means of a “Nanofactory Instruments” piezo-holders inserted into either a JEOL-3000F 300 kV or a JEM 3100 FEF 300 kV (JEOL) (Omega filter) field-emission high-resolution transmission electron microscope. The electronics and software from the “Nanofactory Instruments” were also used. The inertial sliding mechanism consists of a sapphire ball rigidly attached to a piezo-tube and a movable part with six springs that embraces the sapphire ball [13]. The entire experimental setup within the STM-TEM holder is sketched in Fig. 20.1. It should be noted that Cumings and Zettl [14] (UC Berkeley, USA) have for the first time evaluated field-emission and I–V properties of arc-discharge multi-walled BN nanotubes using an in situ TEM manipulation stage. Surprisingly, at that time BN nanotubes were found to possess stable field-emission currents, albeit they were detected to be insulating at a low bias. However, the tubes showed low (several nanoamperes), but stable, reversible breakdown current at a high bias. In this section we provide novel and unexpected results related to a reversible deformation-driven electrical transport in individual multi-walled BN nanotubes prepared in our laboratory using a high-temperature floating catalyst method [18, 19]. Figure 20.2a displays a deformation cycle recorded in TEM on an individual multi-walled BN nanotube [20]. Figure 20.2b shows that the insulating character of the tube has surprisingly changed under tube squeezing/bending between the two gold contacts inside TEM. A notable current of several dozens of nanoamperes may then pass through the tube (Fig. 20.2b). Most interestingly, such transport was found to be fully reversible and almost

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Fig. 20.1. A sketch summarizing the experimental setup of a “Nanofactory Instruments” piezo-driven STM-TEM holder for the in situ TEM analysis of the nanotube electrical and mechanical properties. The inset on the top-right displays the same setup as seen inside TEM

Fig. 20.2. (a) An individual multi-walled BNNT bent between the two approaching each other Au contacts inside the piezo-driven TEM stage of a HRTEM; and (b) consecutive transport I–V curves recorded during stages (1,2,3,4) of tube deformation. The current gradually increases while the tube is bent (curves 1–3), whereas the current almost totally disappears (curve 4), when the tube is reloaded under the electrode separation. Adapted from [20]

entirely disappeared after tube reloading. The transport was also found to be bias-polarity sensitive (a positive to a negative range sweeping or vice versa). A sort of hysteresis existed on the I–V curves taken of deformed BNNTs, implying that the polarization under tube deformation and related

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piezoelectric phenomena are important for the observed unusual BNNT electrical behavior. The reason behind these specific phenomena was then studied using EELS. It was found that, in fact, the electronic structure of a BNNT has significantly been changed during tube bending inside TEM, as manifested by notable changes in the EELS peak fine structure appearance. The normal view of peaks was restored after tube reloading implying the complete recovery of a tubular structure. Another important parameter that could not been ruled out for the observed electrical behavior is pumping-in of structural defects into a BN nanotube under deformation. Such defects may include N vacancies or divacancies, dislocations, interstitial B atoms, and missing BN planes. The defects are known to produce additional inter-band electronic states within a BN band gap and may significantly alter the resultant electrical performance of a tube. Interestingly, the defects were found to fully disappear after a pressure release due to intrinsically high BN tube elasticity and/or the effects of an electron beam which may additionally heal the tubular structure, as depicted in Fig. 20.3 [21].

Fig. 20.3. TEM images of a BN nanotube heavily deformed with a maximum applied degree of bending (a); and that fully structurally recovered after reloading (b). Adapted from [21]

20.2.2 Filled BN Nanotubes In this section we highlight the results obtained on BN nanotubes which were additionally pre-filled with a dielectric magnesium–oxygen–hydrogen containing phase [22]. The “Nanofactory Instruments” STM-TEM holder was again used to accomplish this work. In the course of the electrical measurements we serendipitously realized a rare opportunity to delicately adjust the filled BN

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Fig. 20.4. A layout showing the entire structural (a, b) and chemical composition (c) characterization of a Mg(OH)2 -ceramic-filled multi-walled BN nanotube gently squeezed between the two gold contacts (d); and electrically probed inside TEM using a two-terminal scheme within the STM-TEM “Nanofactory Instruments” holder. The filled nanotube possesses the characteristics of an electrical insulator with a resistivity exceeding ∼10 GΩ, as evidenced by the I–V curve shown in (e). Adapted from [22]

nanotube position and/or deflection within TEM through smooth tuning of a bias on a gold counter-tip (see Fig. 20.1). Figure 20.4 presents the synopsis of the obtained TEM and I–V data. It is visible that a representative BN nanotube of approximately 40 nm in diameter is indeed filled with the dark contrast matter (Fig. 20.4a) and is capped at the tip end. Detailed electron diffraction (Fig. 20.4b) and chemical composition analysis (Fig. 20.4c) verify that the filling is made of insulating hexagonal magnesium hydroxide Mg(OH)2 . While looking at the TEM images, it is clear that the nanotube surface is free from a distinct amorphous carbonbased residue, which may negatively affect the electrical measurements and lead to artifacts. The I–V curves were taken on a filled tube which was preliminary slightly squeezed between the two gold electrodes (Figs. 20.4d) in order to improve the physical contact between the tube and the gold leads. The resultant I–V curve is depicted in Fig. 20.4e. At a low bias there is no conduction at all (except a noise current). The regarded I–V curve is highly serrated showing the current discrepancy in the range of ∼1 nA. The voltage can be increased to up to ±30 V with not detectable current pass or tube burning-out/deterioration. In contrast, it has been known that when just a ∼2–4 V bias is applied to standard multi-walled C nanotubes, the latter are typically burned-out or collapsed [23]. Thus the present magnesium hydroxide filled BN nanotube is a perfect insulator up to the voltages of approximately ±25–30 V independent of

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deformation. The major contribution of a nanotube–gold contact resistance to the measured extremely high-resistant I–V curves could be surely ruled out. In parallel, we performed numerous test experiments on pure C nanotubes (with and without fillings) using the exactly similar setup: the latter nanotubes start to conduct just at a several millivolts bias already revealing passing currents of 10 nA or even more. At voltages of more than ±30 V the filled BN nanotubes exhibited small passing currents of several nanoamperes. The electron transport is reversible and does not lead to any morphological destruction of the nanotubes, similarly to the case of unfilled pure BN nanotubes. The breakdown occurs at nearly ±25–30 V, which is slightly higher than the values reported for the empty BN nanotubes (12–25 V) [14]. The marked serrations on the I–V curves (Fig. 20.4g) are thought to illustrate the specific varying electrostatic interactions between a dielectric BN nanotube and the Au counter-electrode during the bias sweeping. In line with the existence of such strong electrostatic interactions, Fig. 20.4a and b demonstrate two representative experiments in which on-demand dielectric-filled BN nanotube manipulation is shown. Initially, the nanotubes were kept out of contact with the Au tip before applying the bias voltage to it. Applying of a negative bias to the tip effectively pulls up the BN nanotubes from a debris on the sample side toward the Au tip side. Applying of a positive bias oppositely pushes the nanotubes out of the tip. Both modes are fully reversible and a given BN nanotube may repeatedly be manipulated in a circle-like fashion, as shown in Fig. 20.5b. It is worth noting that the present dielectric nanotubes can be gently pulled up, pushed down, and even bent reversibly (Fig. 20.5b) over many circles through smooth tuning of the applied bias voltage on the counter gold electrode. The filled BN nanotubes appear to be very flexible, tough, and elastic to a large degree of deformation, analogously to standard unfilled BN nanotubes. It should be admitted that sometimes a bias-assisted deflection was noticed for C nanotubes during in situ experiments, as well [23]. However, in the latter case the deflections are of sporadic character, since they solely relate to the bad nanotube-grounded holder electrical contact (which is unpredictably changeable and is difficult to control) rather than to a natural stable dielectric nanotube performance itself. The mechanism lying behind the regarded effective manipulation is very simple and is related to the electrostatic repulsive and attractive forces. Under an electron beam of the microscope the filled dielectric BN nanotubes on the grounded stage of the holder are positively charged. Applying a positive bias to a gold counter-electrode would repulse the nanotubes, whereas a negative bias would attract them. The phenomenon described in this section may allow one to reliably manipulate with any dielectric inorganic nanotubes inside TEM. It makes possible precise positioning and/or deflection of a given dielectric nanotube, e.g., BN, SiO2 , MgO, inside TEM and designing multifunctional mechanical actuators based on them.

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Fig. 20.5. TEM images illustrating the possibilities of an individual BN multiwalled nanotube pulling up (a) and pushing down (b) from a dense nanotube debris through smooth tuning of the bias voltage on a gold tip of the “Nanofactory Instruments” TEM holder. The dielectric nanotubes are attracted to the electrode at a negative bias and repulsed from it at a positive bias due to the prominent electrostatic interactions between the positively charged BN nanotubes (under 300 kV electron irradiation) and the gold electrode. Adapted from [22]

20.3 Mechanical Probing of BN Nanotubes It is known that a C nanotube possesses the extremely high Young’s modulus and yield strength [24]. The same was expected to be true for a BN nanotube. Several experimental studies and theoretical estimates of C nanotube deformation mechanics and physics have already been performed [23–31]. The experiments were attempted to measure the elastic modulus of C nanotubes through vibrating/blurring of individual CNTs under an alternating current (AC) field or by direct measuring of a reaction force for an imposed displacement and by nano-indentation. Generally, a nanotube has been treated as a

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homogeneously structured solid cylinder and its specific structural details, like defects, and possible fillings have not been taken into account at all. On the other hand, the importance of specific nanotube structural features for the deformation behavior and electronic structure changes caused by deformation have been well illustrated in several TEM studies [32, 33]. Such works related to BN nanotubular counterparts have been not available in the literature until the research of the present authors. A multi-walled BN nanotube may have a ∼1.2 TPa Young’s modulus, as was measured in a pioneering work by Chopra and Zettl using a thermal vibration amplitude technique [34]. To the best of our knowledge this work has been the only experimental research devoted to the BN nanotube mechanical response until the present authors launched the regarded project. The force measurement experiments were carried out by us using a new state-of-the-art AFM-TEM holder made by “Nanofactory Instruments AB.” The holder was arranged within a 300 kV high-resolution field-emission TEM JEM-3000F. An experimental setup is sketched in Fig. 20.6. A Si cantilever was attached to a fixed MEMS force sensor, whereas an Al wire with a mounted BNNT sample was placed on the piezo-movable side of the holder. The motion of the sample and force-acquisition parameters were programmed and controlled by the dedicated software and electronics from “Nanofactory Instruments AB.” During the experiments we have carefully monitored the development of the force-deformation process in individual BNNTs up to the bifurcation points during compressive bending/buckling. Based on the model adopted from the classical mechanics, the elastic modulus of BNNTs undergoing deformation has been calculated [35]. The real force–displacement (F –d) curves recorded on the BN morphologies are shown in Fig. 20.7. Based on the experimental F–d curves the elastic bending modulus of BN tubes was evaluated as 0.5–0.6 TPa. It is noted that

Fig. 20.6. A sketch summarizing the experimental setup of a “Nanofactory Instruments” piezo-driven AFM-TEM holder for the in situ TEM analysis of the nanotube mechanical properties. The inset on the top-right displays the same setup, as seen inside TEM. Adapted from [35]

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Fig. 20.7. The force (F ) versus sample wire (i.e., nanotube) displacement (d) curves recorded for the thick (a) and thin (b) multi-walled BN nanotubes. The left and right insets on each curve display the appearance of the starting and bent nanotube morphologies. The intentional directions of piezo-driven moves of an Al wire with the attached nanotube during the deformation are pointed out; The bending modulus was measured to be 0.5–0.6 TPa. Adapted from [35]

such value is in a good match with the elastic modulus measured using a free-standing beam-like BN nanotube under applied AC voltage [36]. It is remarkable that the nanotube bending-relief can be performed at least 20 times without tube failure. An in-plane measured bending angle may reach ∼70◦ or even more. It strongly contradicts the pre-existing beliefs on BN nanotube inherent brittleness due to a partial ionic character of chemical bonding between the B and N atoms. Another appealing issue is that the nanotube bending most frequently occurred at the analogous kink (or neck) point. This feature was persistent and independent of a cycle number and a mode of the bending deformation (direct or reverse). The analysis of kink HRTEM images (see Fig. 20.3, for example) clearly implies that in spite of the heavily deformed and almost entirely corrugated BN tubular shells in the vicinity of the kink, the BN nanotube is fully capable to restore its original shape after reloading. Therefore, a BN multi-walled nanotube is straw-like flexible: the striking result which was not a priori expected (if one keeps in mind a profound ionic-like bonding component in a layered BN). The pre-existing nanotube helicities, which are of the zigzag orientation, as a rule, and the overall B/N stoichiometry were also not affected by the deformation. The results presented in this section verify the bright prospects of multiwalled boron nitride nanotube applications in a variety of fields, for example, for the reinforcement of polymeric or ceramic composites, for structural use in the form of multi-tube ropes and for a design of novel MEMS and NEMS sensors.

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20.4 Summary To sum up this chapter, we describe the successful usage of novel TEM compatible STM and AFM piezo-driven holders which allow one to test an individual nanotube inside TEM under a full control of all morphological and structural changes occurring during the electrical/mechanical probing. Amazingly, multi-walled BN nanotubes exhibit the superb flexibility which was not a priori expected for a hexagonal, partially, ionic-like BN compound. The reversible bending deformation was found to be truly elastic with no traces of residual plastic deformation up to very large bending angles (in excess of 70◦ ). The highly corrugated BN tubular layers in the vicinity of a reproducibly appearing kink fully restore its original structure after reloading. A kink position on the nanotube was persistent. Remarkably, an original truly insulating BN nanotube performance may effectively be altered during tube bending inside TEM: a deformed BN nanotube may allow up to several dozens of nanoamperes current to pass through it. Interestingly enough, it repeatedly becomes insulating after re-bending due to a full recovery of a tubular shape and entire healing of all deformationinduced defects. Acknowledgment The authors are indebted for the experimental and technical support of Drs. O. Lourie, X.D. Bai, Y. Uemura, and Mr. K. Kurashima. The present studies became possible due to the Nanotube Project tenable at the Nanoscale Materials Center (NIMS) and to the WPI Center for Materials Nanoarchitectonics (MANA) Initiative of MEXT.

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Index Bias-assisted deflection for C nanotubes, 280 Bias-polarity sensitive, 277 Circle-like fashion of BN nanotube, 280 Dark contrast matter, 279

Dielectric-filled BN nanotube manipulation, 280–281 Electrical probing experiments, 275–281 Electron diffraction and chemical composition analysis, 276 Electron transport, 280

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Experimental studies and theoretical estimates of C nanotube deformation, 281–282

Multi-walled BNNT bent, 277 Multi-walled boron nitride (BN) nanotubes, 275–276

Filled BN nanotubes, 278–281

Nanofactory Instruments, 276–279, 281–282

Hexagonal magnesium hydroxide Mg(OH)2, 279 High-resolution transmission electron microscope (HRTEM), 275, 283

Pumping-in of structural defects into BN nanotube, 278

I–V curves of filled BN nanotubes, 279–280

Real force–displacement (F –d) curves, 282

JEOL/Omega filter field-emission high-resolution transmission electron microscope, 276

TEM experiments on multiwalled BN nanotubes, 276 TEM images of BN nanotube, 278

Mechanical probing of BN nanotubes, 281–283

Unfilled multi-walled BN nanotubes, 275–278

Index

Absorption spectrum of Tb2 O3 , 106 Adsorption-type heat pump system, 159 AFM images of imogolite on silicon wafers, 173 AFM images of synthesized imogolite, 162 Ag2 Se nanotubes, 196 Ag nanoparticles in TNT, 29 Alkoxides, 151 Alkyl phosphonic acid and imogolite, mode of interaction between, 177 Allophane, 170, 182 Alternate layer deposition, 46 Aluminosilicate nanofiber representation, 170 (3-aminopropyl) trimethoxysilane (APS) solution, 122 Amphoteric azopyridine carboxylic acid, 237 Anatase bulk crystal, band structure, 264 Anatase TiO2 sol synthesis, 34 Anatase-type titanium oxide, 262–263 Angle-dependent transmittance, 242 Anhydrated surface slab model, 265 Anion exchange and adsorption ability of imogolite, 175 Anisotropic conduction, 242–243 Anisotropic conduction film (ACF), application, 239–241 Anodic alumina membrane as template, rare-earth oxide nanotubes synthesis using, 108

Anodic alumina structure, 60 Anodic porous alumina, 122 Anodization of metal zirconium, 121 Anti-dewing materials, imogolite application in, 163–164 Arai modification of Kelvin’s capillary condensation equation, 164 As-grown C70 NTs, 210 Atomic layer deposition (ALD) method, 125 Au-doped WS2 nanotubes, 10 Azopyridine carboxylic acid, 237–238 Bamboo-like morphologies, 108 Band structure and DOS of anatase, 266 Band structure and DOS of Rutile, 268 Band structure and DOS of titanium oxide (TiO2 ) nanotube, 265–267 Base metal nanotubes, template-free synthesis, 217–219 Bi2 Se3 nanotubes, 196 Bias-assisted deflection for C nanotubes, 280 Bias-polarity sensitive, 277 Birnessite and buserite structures, 74 Bismuth nanotubes, formation, 217–218 Boron nitride (BN), 261 Boron nitride nanotubes synthesis, 9 Boron nitride synthetic inorganic nanotubes, 2 Bragg peak, 251–253

T. Kijima (Ed.): Inorganic and Metallic Nanotubular Materials. Topics in Applied Physics 117, 287–295 (2010) c Springer-Verlag Berlin Heidelberg 2010 DOI 10.1007/978-3-642-03622-4 

288

Index

Calcium niobium oxide nanosheets, 136 Calcium niobium oxide nanotube, 142 Carbon nanotube (CNT) discovery, 17 Carbon nanotubes discovery, 9 Carbon nanotubes as template, rare-earth oxide nanotubes synthesis using, 107–108 Catalyst support, 159 Catalytic properties of ChNTs, 197 Catalytic systems, macro-nanostructured materials application in, 155–156 Cation doping to TNT, 28 Cationic surfactants, 75 C60 –C70 two-component nanotubes, 210–211 CdS and CdSe nanotubes, 196 C12 EO9 and Tween 60, chemical structures, 222 C12 EO9 and Tween 60 structural parameters, 228 Cerium phosphate nanotubes with blue luminescence, 108–110 Cerium phosphate nanotubes and luminescence property, TEM image, 111 Cerium valency, 110 Chalcogenide nanotubes (ChNTs), 191–192 Charge–discharge characteristics of titanium oxide nanohole array, 68 Chemical composition and the crystal structure of Mo oxide nanotubes, 86–89 Chemical structure of an azopyridine carboxylic acid, 237 Chemical synthesis of imogolite nanotubes, 172–173 Chemical vapor deposition (CVD) process, 149 Ch-M-Ch sandwiches, 192–193 Chrysotile, 2, 8 Circle-like fashion of BN nanotube, 280 Closed-tube chemical transport method, 195–196 C70 nanotubes optical micrograph, 209 C60 nanowhiskers (C60 NWs), 201–202 Coating mechanism, 151 Colloidal nanosheets generation, 136

Colloidal solutions of TNTs, 47 Combustion flame of acetylene and oxygen gases, 84–85 Combustion-flame method, 84 Comparison between TNT and NW, 40 Computer simulations of ice nanotube, 254–255 Concentric-layered silver nanotubes TEM images, 224 Condensed linear polyphosphate, 110 Conductive magnetic nanotube, 236 Conversion process, 144 Cross-sectional SEM image for vertically aligned TNT arrays, 52 Crystal structure of layered manganese oxides, 73–74 Crystal structure models, 262–265 Crystal structure of titanium oxide, 267–268 Cubic rare-earth oxide nanotubes, 108 Current-density–voltage (J–V) characteristics for TNT arrays, 54–55 Cyclic contact-mode AFM images of in situ synthesized imogolite/PVA hybrid, 183 Cylindrical cells, 59 Dark contrast matter, 279 1,10-decanediylbis(phosphonic acid) (DBPA), 124–125 Decomposition of MS3 in H2 stream, 195 Dehydration ratio of imogolite, 166 Deintercalation process, 143 Deposition reaction, 60 Desalination process, 162–163 Designed soft chemical procedure, 141–144 Diameter of titanium oxide (TiO2 ) nanotube, 269 Diammonium cerium(IV) nitrate, 110 Dielectric-filled BN nanotube manipulation, 280–281 Differential thermal analysis (DTA), 175 Direct synthesis process, 73 Direct synthesis of TNT, 19 1-dodecanesulfonate assemblies, 131

Index Driving reactions, 60 Drying process, 151–152 Dye-sensitized solar cell (DSC), 33 Dye-sensitized solar cell (DSC), TNT use in, 38–39 Edge curling-up process, 143–144 Electrical probing experiments, 275–276 Electrical properties of ChNTs, 197 Electric conductive property titanium oxide, 267–268 Electrochromism in TNT electrodes, 50–51 Electron diffraction and chemical composition analysis, 276 Electron diffraction images of titanium oxide (TiO2 ) nanotube, 270–271 Electron emission characteristics from TNT arrays, 54 Electron energy loss spectroscopy (EELS), 110 Electron energy loss spectroscopy (EELS) of TNT, 35–37 Electronic structure model calculation, 262–265 Electron transport, 280 Emission spectrum of ruthenium oxide nanotubes, 128 Emission spectrum of ytterbium compound nanotubes, 101–102 Energy dispersive X-ray microanalysis (EDX), 86 Environmental purification functions of TNT, 27–28 Enzyme/imogolite hybrid, 184–185 Ethanol detection characteristics of CNT with zirconia, 120 Exfoliating graphite, 136 Experimental studies and theoretical estimates of C nanotube deformation, 281–282 Fabrication processes, 147–149 Fabrication process of nanofibrous metal oxides, 152–153 Farmer’s method, 172–173 FE-SEM micrographs of titania nanohole array, 61, 63–64, 67 Filled BN nanotubes, 278–281

289

Filler-orientation-type anisotropic conduction film, 241–244 Flocculation of colloidal nanosheets, 141–143 Flowchart for synthesis method of imogolite, 161 Formation of ice nanotube, 250–254 Formation mechanism of MoO2 nanotube, 91–92 Formation mechanism of nanotubes and nanofibers, 77–78 Formation mechanism of TNT, 21–25 Fowler–Nordheim behavior, 80 Fowler–Nordheim (F–N) plot for TNT arrays, 54–55 FSM-16 mesoporous silica discovery, 1–2 Fuel storage media for natural gases, 159 Fullerene, 170 Fullerene nanotubes (FNTs), 201 Fullerene nanowhiskers (FNWs), 202 Fullerene type nanoparticle of MoS2 and WS2 , 193–194 Gap between Mo substrate and Si spacer, Mo oxide nanotubes, 90 Gas-sensing material, 84 Gibbsite sheet structural units, 174 Graphene, 135–136 Growth mechanism of Mo oxide nanotubes, 89–94 Heat exchange material in adsorption-type heat pump systems, imogolite application in, 165–166 Heat-treatment, morphology change of TNT during, 38 Helical ruthenium compound representation, 131 Helix-structured NiCl2 NT synthesis, 10 Hexagonal BN, 9 Hexagonal magnesium hydroxide Mg(OH)2, 279 Hexagonal-structured silver nanotubes TEM images, 224

290

Index

High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) method, 35–37 High-resolution TEM images of titanium oxide nanotube, 35–37, 262 High-resolution transmission electron microscope (HRTEM), 275, 283 High-temperature X-ray diffraction patterns of synthesized TNT, 23 Homogeneous precipitation method, 98 Honda and Fujishima effect, 33 Hydrated surface slab model, 264 Hydrogen-bonded fibrous supramolecular, 237–238 Hydrophilicity of TNT films, 53 Hydrophobidization, 176 Hydrothermal method, 102–107, 148 Hydrothermal synthesis of TNT, 20

Ice nanotube (NT), 252–253, 256–257 “Imogo-layers”, 169 Imogolite, 2, 159 Imogolite nanotubes structure, 173–175 Imogolite/pepsin hybrid hydrogel, 184–185 Imogolite synthesis, 160–163 Indium oxide nanohole array, 68 Indium-tin-oxide (ITO), 242–243 Indium tin oxide (ITO) thin film, 67 Inorganic nanotubes, applications, 11–12 Inorganic nanotubes, structural and dimensional characteristics, 8–11 Inorganic nanotubes, structural and synthetic bases, 2–7 International Mineralogical Association-Commission on New Minerals, Nomenclature, and Classification (IMA-CNMNC), 169 Interplanar spacing in TNT, 22 Iron oxide nanohole array, 68 IR spectra of imogolite/PVA blend, 182 I–V curves of filled BN nanotubes, 278–280

JEOL/Omega filter field-emission high-resolution transmission electron microscope, 276 Kasuga method, 18 Kroger–Vink notation, 267 Langmuir–Blodgett method, 137 Layered double hydroxide (LDH) nanosheets, 137 Layered metal sulfides, 12 Layered oxides exfoliation, 138–141 Layer-by-layer assembly, 137 Layer to-layer interactions, 135 Lead zirconate titanate (PZT), 201 Length of synthesized titanium oxide (TiO2 ) nanotube, 270 Liquid crystal displays (LCDs), fabrication, 239 Liquid-crystalline (LC), 221–223 Liquid–liquid interfacial precipitation method (LLIP method), 202–205 Liquid phase deposition (LPD) method, 59–60 Liquid–solid phase transition, 250–254 Lithium ion batteries, titania nanohole array as electrode in, 66 Low temperature solution chemical processing synthesis of TNT, 18–21 Lyotropic liquidcrystalline (LLC) templates, 222 Macro-nanostructured materials synthesis using template method, 152–155 Magnetic metal-containing nickel–phosphorus (Ni–P) nanotubes, 235 Magnetic properties of zirconia nanotube arrays containing metal Co, 123 Manganese oxide (MnO2 ) nanosheets, 136 Manganese oxide nanosheets, manganese oxide nanotubes synthesis from, 74–78 Manganese oxide nanotubes, 73, 142 Mass transport mechanism during growth of Mo oxide nanotubes, 91

Index Matryoshka doll/nesting type, 137 MCM-41 mesoporous silica discovery, 1–2 Mechanical probing of BN nanotubes, 281–283 Mechanism of formation of nickel-phosphorus (Ni-P) thin film, 238–239 Mechanism for formation of titania nanohole array, 65 Melting point of ice NT, 251–252 Metallic nanotubes, applications, 11–12 Metallic nanotubes, structural and dimensional characteristics, 8–11 Metallic nanotubes, structural and synthetic bases, 2–7 Methacryloyloxyethyl phosphate (MOEP), 178–179 Methylene blue bleaching examination, 42 Mg-A type zeolites, 165–166 Mixed oxide nanotubes, 147–149 Mixed oxide nanotubes synthesis using carbon nanofibers as templates, 149–152 Mixed surfactant LC templating method, 223–229 Mn(Ac)2 solution, manganese oxide nanotubes synthesis from, 79–80 Model of fullerene nanotube composed of C60 molecules, 206 Molecular dynamics (MD) calculations, 254–257 Molecular recycle-type method for preparation of nickel-phosphorus (Ni-P) nanotube, 238 Molecular templates, noble-metal nanotubes synthesis using, 221 Molybdenum oxide nanotubes growth, 84–85 Mono-dispersed TNT synthesis, 34 MoS2 nanotube synthesis, 9 MS2 nanotubes formation mechanism, 193–194 Multi-walled BNNT bent, 277–278 Multi-walled boron nitride (BN) nanotubes, 275–276, 282

291

Multi-walled carbon nanotubes (CNTs) synthesis, 113 Multi-walled type (MW-type), 3 α-NaMnO2 , manganese oxide nanotubes synthesis from, 78–79 Nanofactory Instruments, 276–279, 281–282 Nanogroove-network-structured platinum nanosheets, 230 Nanohole-structured platinum nanosheets, 230 Nano-and microstructured inorganic materials synthesis, 6–7 Nanosheet, 135 Nanosheet-like particle morphology, 76 Nanostructure analysis of TNT, 34–38 Nanotubes, classification, 4 Nanotube-shaped crystal structure of rare-earth compounds, 98 Nanotubes (NT) array, 11 Nanotubes, pathways to, 4 Nanotubes, structural types, 3–5 Nanotubes, synthetic strategies toward, 5–6 Nanowhiskers (NWs), 38–39 Natural imogolite nanotubes, 171 Naval Research Laboratories, 236 n-decyltrimethylammonium chlorite (DeTAC), 75 n-dodecyltrimethylammonium chlorite (DoTAC), 75 Needle-like crystals of KBr, 208 Nesting-type nanotube, 137 Next generation supercapacitor, design of electrode material, 129 n-hexadecyltrimethylammonium chlorite (HeTAC), 75 Nickel-phosphorus (Ni-P) nanotube, 236–237, 240–241 Ni nanoparticles in TNT, 29 N mapping by EELS of N-doped TNT, 43 Noble-metal nanoparticles, 215–217 n-octyltrimethy lammonium chlorite (OTAC), 75 Nuclear magnetic resonance (NMR) of water, 249

292

Index

Octadecylphosphonic acid (ODPA), 176–177 ODPA-chemisorbed imogolite, adhesion force observed between, 177–178 ODPA-modified imogolite on HOPG substrate representation, 186–187 One-dimensional assembly formed by amphipathic molecules as template, 126 Optical micrograph of C60 nanotube, 207 Optical reflectance for thin films, 50 Osmotic swelling, 136 OTS-SAMs (octadecyltetrachlorosilane self-assembled monolayers), 125 Oxide nanohole arrays, synthesis conditions, 69 Oxide nanosheets, 136 Oxide nanosheets conversion to nanotubes mechanism, 142–143 Oxygen/acetylene flow ratio, 92–94 Oxygen content of Mo oxide nanotubes, 89–90 Oxygen-plasma treated polyester track etched (PETE) membrane as template, 125–126 Oxygen-reduction reaction (ORR), 229–230 Pd-loaded TNT, 29 Perovskite-type Ruddlesden-Popper phase, 135, 140 Photocatalytic acetaldehyde decomposition characteristics, 64 Photochemical properties of TNT, 25–27, 39, 42–43 Physical properties and application of water containing SWCNT, 256–258 Plasticization of rigid sheet, 10 Platinum nanotubes formation models, 227 Platinum nanotubes TEM images, 225 Platinum and ruthenium (Ru), 215 PMMA/imogolite hybrid, 178–180 Polycarbonate membrane, 121 Polycrystalline manganese oxide nanotubes, 80–81 Polycrystalline nanotubes, 73

Poly(diallyldimethylammonium chloride) (PDDA), 47 Poly(ethyleneimine) (PEI), 47 Polymer composites, 159 Polymer electrolyte fuel cells (PEFCs), 229 Polymer hybrids with imogolite nanotubes, 178–180 Poly(methyl methacrylate) (PMMA), 178–179 Polyol process, 216, 220 Polyoxyethylene (EO)-type surfactants, 221–222 Polyoxyethylene (PEO)-based nonionic surfactants, 222 Poly(vinyl alcohol) (PVA), 180 Polyvinyl pyrrolidone, 221 Porous C60 nanowhisker TEM image, 204 Porous membrane as template, zirconia nanotubes synthesized using, 121–126 Powdered titanium oxide (TiO2 ), 265 Powder XRD patterns of six different SWCNT samples, 250 Practical use of polymer electrolyte fuel cells (PEFCs), 215–216 Propane oxidation, 156 Proton exchange/diffusion, 130 PtCl4 precipitate inclusion in C60 NT, 209 Pt–Pd–Rh ternary catalysts, 215 Pumping-in of structural defects into BN nanotube, 278 Purification process for natural imogolite, 171 PVA/imogolite hybrid, 180–184 Pyrolysis, 236 Raman scattering spectroscopy, 87 Raman spectra from MoO2 nanotube, 89 Raman spectrometry, 208 Rapid charge/discharge processes, 129–130 Rare-earth compound nanotubes synthesis, 98–102 Rare-earth fluoride nanotubes, 110–113

Index Rare-earth hydroxide nanotubes synthesis, 102–107 Rare earth oxide nanotubes, 2 Rare-earth oxide nanotubes synthesis, 102–107 Raw material powder and synthesized titanium oxide (TiO2 ) nanotube, 269–271 Real force–displacement (F –d) curves, 282–283 Refractive index, 265 Re-organization of precursory solids, 9 Rotating disk electrode (RDE), 229 Ruthenium compound nanotubes templated by surfactant assemblies, 130–131 Ruthenium oxide hydrate nanotubes synthesis using anodic porous alumina membrane, 127–130 Ruthenium oxide nanotubes, 126 Ruthenium oxide nanotubes synthesis using anodic porous alumina membrane, 127–130 Rutile, 265 Rutile structure of titania, 35–37 Saturated calomel electrode (SCE), 79 Scanning electron microscope (SEM) image of Ni-P nanotubes, 242 Scrolled type (S-type), 3 Scroll-type nanotube, 138 Selected area electron diffraction (SAED) pattern of TNT, 22 Selenium, nanotubes and microtubes, 196–197 Self-organization of atomic or molecular species, 9 Self-structuralization or self-organization, 18 SEM images of deposits for oxygen/ acetylene flow ratios of Mo substrate, 93 SEM images of MoO2 nanotube, 86, 91 SEM images of ruthenium oxide nanotubes, 128 SEM images of ytterbium compound nanotubes, 101 SEM images of zirconia nanotube arrays containing metal Co, 123

293

SEM and TEM images of precursor and products of manganese oxides, 76 SEM top images of RuO2 xH2 O nanotubes arrayed electrode, 129 Serpentine, 191 Seven-walled MoS2 nanotube, 4 Sheet-like crystals and nanotubes, 138 Shrinking process, 108 Silica fibers-immobilized CNFs, SEM images, 153–154 Silica fibers-immobilized nanofibrous LaMnO3 , SEM images, 153–155 Silica synthetic inorganic nanotubes, 2 Silicon nitride synthetic inorganic nanotubes, 2 Silver nanotubes TEM images, 224 Single-crystalline Y(OH)3 nanotubes synthesis, 10–11 Single-crystal nanotubes, 73 Single surfactant Tween 60/H2 O mixtures, 223 Single-walled carbon nanotubes (SWCNTs), 170–171, 247–258 Single-walled type (SW-type), 3 Slab model of titanium oxide nanotube, 263 Sodium dodecylbenzenesulfonate (SDBS), 104 Soft chemical process, 73, 135 Soft-solution process, 59 Sol–gel method, 61, 108, 148–149 Solid phase or solid–gas interfacial reaction, 10 Solid-state calcination method, 136 Solid templates, noble-metal nanotubes synthesis using, 219–221 Speed-dry desiccant, 159 State-of-the-art electron microscope analysis techniques (FE-TEM), 33 Straight-layered sheet, 4–5 Structural information of ice inside SWCNT, 252–253 Structural models of four kinds of 7-membered ring ice NTs, 257 Structural models of nanotubes, 138 S-type nanotubes formation, 4 Sulfurization of MO3 , 194–195 Super-hydrophilicity of surface of TiO2 , 33

294

Index

Supramolecular template, 236–237 Surface area variation on annealing temperature for TNT, 23 Surfactant cylindrical rodlike micelles, structural parameters, 229 Surfactants, 221–222 Synthesis method, 123–124, 160–163 Synthesis of titanium oxide (TiO2 ) nanotubes, 268–271 Synthetic inorganic nanotubes, 2 Teflon capsule, 196 Tellurium nanotubes, 217–219 TEM experiments on multiwalled BN nanotubes, 276 TEM image of BN nanotube, 278 TEM image of C60 nanotube, 207 TEM image of CNT with zirconia, 120 TEM image of as-grown helical-shaped ruthenium compound nanotubes, 130 TEM image of Ho2 O3 nanotubes with bamboo-like structure, 109 TEM image of imogolite, 174 TEM image of MoO2 nanotube, 88 TEM image of nanotube synthesized using hydrothermal method, 103 TEM image of nanotube of Y(OH)3 containing ethanol inside, 104–105 TEM image and SAED pattern of products of manganese oxides, 77 TEM image of TNT synthesis, 21–22 TEM image of ytterbium compound nanotubes, 98–99 TEM image of zirconia nanotube arrays containing metal Co, 123 Temperature dependence of BET surface area of cation-doped TNTs, 29 Template method, 148 Template or replica method, 18 Template synthesis of nickel-containing tubular, 236–239 Templating methods, silver nanotubes mixed surfactant using, 222–223 Terbium chloride 6-hydrate, 104 Tetra-ethylortho silicate (TEOS), 84

Tetragonal zirconia nanotubes, 122–123 Tetramethylammoniun hydroxide (TMAOH) solution, 75 Tetra(n-butyl)ammonium hydroxide (TBAOH) aqueous solution, 46 Thermal properties of imogolite, 175 Thermal stability of C60 NTs, 211 Thermogravimetric analysis of C60 NTs, 211–212 Thermogravimetric (TG) analysis, 175 Time evolution of EDX O-peak intensity of Mo oxide nanotubes, 89–90 Tin oxide nanohole array, 68 Titania, 33 Titania nanohole array synthesis, 61 Titania nanotube, 266 Titanium oxide nanosheets, 136 Titanium oxide nanotubes properties, 18 Titanium oxide nanotubes structure, 263, 267 Titanium oxide nanotubes (TNT) mutual and synergy combination, 18 Titanium oxide, synthetic inorganic nanotubes, 2 Titanium oxide, synthetic inorganic nanotubes synthesis, 9 Titanium oxide, synthetic inorganic nanotubes TEM/SEM images, 2 TNT-metal nanocomposites, 29 TNT thin films synthesis, 46–51 Total heat exchanged for dehydration temperature of imogolite, 165 Track etched membrane of polyester, 121 Transition-metal dichalcogenides (MCh2 ), 191, 196 Transition-metal dichalcogenides (MCh2 ) with layered structures, 192–193 Transmission electron microscope (TEM), 87, 238, 249 Tribological properties of ChNTs, 197–198 Tungsten sulfide, synthetic inorganic nanotubes, 2 Turbostratic restacked nanosheets, 143

Index Tween 60 surfactant molecules, 223 Type-IV adsorption behavior of imogolite, 175 Ultrasonication, 122 Ultrathin walls like carbon nanotubes (CNT), 83 Ultraviolet (UV) luminescence, 110 Unfilled multi-walled BN nanotubes, 275–278 UV-curable anisotropic conduction, 243 UV-vis absorption spectra of PEI/(TNT/PDDA)n-1 /TNT film, 47–48 Vanadium oxide, synthetic inorganic nanotubes, 2 Vertically aligned TNT films by hydrothermal reaction, 51–55 Visible photocatalyst, 33 Volatile organic compounds (VOC), 155, 216 Washing of titanium oxide (TiO2 ) nanotube, 270 Water contact angle of TNT/polycation thin film, 49 Water containing carbon nanotube, 248–249 Water freezing phenomenon, 251–252 Water soluble poly(p-phenylene) (WS-PPP), 186 Water splitting test, 26 Water vapor adsorption/desorption isotherm of synthesized imogolite, 164 Well-dispersed colloid, 143 Well-dried C60 NTs, 208 WS2 nanotube synthesis, 9 Wurtzite-type hexagonal structure, 10

295

XPS spectra of wide and narrow scans from MoO2 nanotube, 87 X-ray diffraction patterns of TNT synthesis, 20 X-ray diffraction (XRD), 87, 249 X-ray photoelectron spectroscopy (XPS), 87, 238 X-ray photoelectron spectroscopy (XPS) of N-doped TNT, 39, 42 XRD patterns of MoO2 nanotube, 88 XRD patterns of SWCNT sample, 251 XRD patterns for titania nanohole arrays, 62 XRD patterns of titanium oxide nanotube, 262 XRD patterns of Tween 60, 226 XRD profile of synthesized imogolite, 161–162 Young’s modulus of C60 NTs, 211 Ytterbium compound nanotubes TEM images, 98–99 Z-contrast method, 35–37 Z-contrast TEM image of isolated TiO2 / ZrO2 coaxial nanotube, 124 Zeta potential of TNT and TiO2 colloids, 42 Zirconia/CNT nanocomposite, 120 Zirconia nanotube, 117 Zirconia nanotube arrays synthesis by anodization of metal zirconium, 121 Zirconia nanotubes synthesis using carbon nanotubes or nanofibers as templates, 118–121 Zirconium oxide nanohole array, 68 ZnS-loaded TNT, 29