Sol-Gel Materials Chemistry and Applications ADVANCED CHEMISTRY TEXTS A series edited by DAVID PHILLIPS, Imperial College, London, UK, PAUL O’BRIEN, University of Manchester, UK and STAN ROBERTS, University of Liverpool, UK Volume 1 Chemical Aspects of Photodynamic Therapy Raymond Bonnett Volume 2 Transition Metal Carbonyl Cluster Chemistry Paul J. Dyson and J. Scott Mclndoe Volume 3 Nucleoside Mimetics: Their Chemistry and Biological Properties Claire Simons Volume 4 Sol-Gel Materials: Chemistry and Applications John D. Wright and Nico A.J.M. Sommerdijk This book is part of a series. The publisher will accept continuation orders which may be cancelled at any time and which provide for automatic billing and shipping of each title in the series upon publication. Please write for details. Sol-Gel Materials Chemistry and Applications John D. Wright University of Kent, UK and Nico A.J.M. Sommerdijk Eindhoven University of Technology The Netherlands C RC P R E S S Boca Raton London New York Washington, D.C. Published in 2001 by CRC Press Taylor & Francis Group 6000 Broken Sound Parkway NW, Suite 300 Boca Raton, FL 33487-2742 C 2001 by Taylor & Francis Group. LLC CRC Press is an imprint of Taylor &: Francis Group No claim to original U.S. Government works 15 14 13 12 II 10 9 8 7 6 5 International Standard Book Number-10: 90-5699-326-7 (Hardcover) International Standard Book Number-13: 978-90-5699-326-9(Hardcover) This book contains information obtained from authentic and highly regarded sources. Reprinted material is quoted with permission, and sources are indicated. A wide variety of references are listed. Reasonable efforts have been made to publish reliable data and information, but the author and the publisher cannot assume responsibility for the validity of all materials or for the consequences of their use. No part of this book may be reprinted, reproduced, transmitted, or utilized in any form by any electronic, mechanical, or other means, now known or hereafter invented, including photocopying, microfilming, and recording, or in any information storage or retrieval system, without written permission from the publishers. Trademark Notice: Product or corporate names may be trademarks or registered trademarks, and are used only for identification and explanation without intent to infringe. Library of Congress Catakiging-in-Publication Data Catalog record is available from the Library of Congress Visit the Taylor & Francis Web site at http://www.taylorandfrancis.com and the CRC Press Web site at http://www.crcpress.com Contents Preface ix Chapter 1 Introduction 1 1.1 Colloid Stability 1 1.2 Control of Particle Nucleation 3 1.3 The Silicon Alkoxide Sol-Gel Process 3 1.4 Advantages of Sol-Gel Synthesis 4 1.5 Limitations of Sol-Gel Synthesis 5 1.6 Overall Structure of this Book 6 1.7 The Historical Development of Sol-Gel Processing 6 1.7.1 Origins 6 1.7.2 The Beginning of Sol-Gel Science 7 1.7.3 The Explosion of Sol-Gel Technology 8 References 14 Chapter 2 Silica Sol-Gels: Reaction Mechanisms 15 2.1 Hydrolysis 15 2.1.1 Acid and Base Catalysis 15 2.1.2 Precursor Substituent Effects 16 2.1.3 Hydrophobic Effects and Co-Solvents 17 2.1.4 Effect of Water: Alkoxide Ratio (R) 18 2.2 Condensation 19 2.2.1 Acid and Base Catalysis 19 2.2.2 Precursor Substituent Effects 20 2.3 Overall Kinetics of Hydrolysis and Condensation 20 2.4 Non-Hydrolytic Sol-Gel Processing 21 2.5 Gelation 22 2.5.1 Models of Gelation 23 2.6 Ageing 24 2.6.1 Cross-Linking and Syneresis 24 2.6.2 Coarsening and Ripening 24 2.6.3 Phase Transformations 25 2.6.4 The Significance of Ageing 25 2.7 Drying 26 2.7.1 The Constant Rate Period 26 2.7.2 The Critical Point 26 2.7.3 First Falling-Rate Period 26 SOL-GEL MATERIALS v CONTENTS 2.7.4 Second Falling-Rate Period 27 2.7.5 Consequences of Drying 27 2.7.6 Avoiding Cracking 27 2.8 Densification 28 2.8.1 Stages of Densification 28 2.8.2 Effects of Heating Rate and Gas Evolution 30 2.9 Conclusion 31 References 31 Chapter 3 The Chemistry of Sol-Gel Silicates 33 3.1 Introduction 33 3.2 Chemical Control of the Sol-Gel Process 33 3.2.1 The Hydrolysis and Condensation Reaction 33 3.2.2 Gelation 36 3.2.3 Ageing 36 3.3 Additives for Structuring and Processing 37 3.3.1 Drying Control Additives 37 3.3.2 Organic Templates 38 3.4 Ormosils 41 3.4.1 Modified Precursors 41 3.4.2 Entrapment of Functional Materials 43 3.5 Hybrid Materials 46 3.6 Surface Modification 48 3.6.1 Dehydroxylation 48 3.6.2 Surface Functionalisation 50 References 51 Chapter 4 Metal Oxide Gels 53 4.1 Introduction 53 4.2 Hydrolysis and Condensation Reactions of Metal Salt Precursors 54 4.2.1 The Partial Charge Model 54 4.2.2 Equilibrium Species in Aqueous Solutions of Metal Salts 55 4.2.3 Condensation and Polymerisation in Aqueous Solutions of Metal Salts 56 4.3 Effects of the Counter-Ion 59 4.4 Reactions of Metal Alkoxide Precursors 60 4.4.1 Hydrolysis and Condensation 60 4.4.2 Acid and Base Catalysis 61 4.4.3 Steric Factors and Solvent Effects 61 4.4.4 Control of Metal Alkoxide Reactions 62 4.5 The Non-Hydrolytic Sol-Gel Method for Metal Oxides 63 SOL-GEL MATERIALS CONTENTS 4.6 Particle Growth and Aggregation 64 4.7 Preparation of MonodisperseP articles 66 References 67 Chapter 5 The Characterisation of Sol-Gel Materials 69 5.1 Introduction 69 5.2 Chemical Characterisation 69 5.2.1 Nuclear Magnetic Resonance 69 5.2.2 Vibrational Spectroscopy 71 5.3 Physical Characterisation 72 5.3.1 Nitrogen Adsorption Porosimetry 72 5.3.2 Mercury Porosimetry 74 5.3.3 Thermoporosimetry 74 5.3.4 NMR Spin-Spin Relaxation Measurements 76 5.3.5 NMR Spin-Lattice Relaxation Measurements 76 5.3.6 Small-Angle Scattering 77 5.3.7 Other Structural Techniques 80 5.4 Indirect Characterisation Methods 82 References 83 Chapter 6 Applications of Sol-Gel Silicates 85 6.1 Introduction 85 6.2 Optical Materials 85 6.2.1 Non-Doped Glasses 85 6.2.2 Doped Glasses 87 6.2.3 Contact Lenses 89 6.3 Chemical Sensors 89 6.3.1 Optical Chemical Sensing 89 6.3.2 Biosensors 90 6.4 Catalysts 91 6.5 Coatings 93 6.6 Membranes 94 References 94 Chapter 7 Applications of Metal Oxide Sol-Gels 97 7.1 Introduction 97 7.2 Ceramics 97 7.3 Catalysts 99 7.4 Electronic Materials 101 7.4.1 Ferroelectric Materials 101 7.4.2 Electrochromic Materials 102 7.4.3 Solid Electrolytes 103 7.4.4 Other Electronic Materials 103 SOL-GEL MATERIALS vii CONTENTS 7.5 Flammable Gas Sensors 104 References 106 Chapter 8 The Future 109 8.1 The Players 109 8.2 Market Prospects 109 8.3 Developments in Characterisation Methods 110 8.4 Composite Materials 110 8.5 New Precursors 111 8.6 New Processing Methods 111 8.7 Sol-Gel Supramolecular Chemistry 112 References 114 Index 115 Chemical Substance Index 121 viii SOL-GEL MATERIALS Preface Sol-gel processing methods were first used historically for decorative and construc­ tional materials. In the last century many new applications were developed, initially largely empirically but later on a more scientific basis as new characterisation tech­ niques became available. Today sol-gel methods are reaching their full potential, enabling the preparation of new generations of advanced materials not easily acces­ sible by other methods yet using mild, low-energy conditions. It is therefore appro­ priate that the topic should increasingly be included in advanced undergraduate, MSc and taught PhD courses in the areas of chemistry, physics and materials science. There is currently no concise introductory text which covers all the major areas of the subject. The aim of our book is to fill this evident gap in the market and to facilitate the development of new courses. It has been written to guide those who wish to join the growing ranks of sol-gel scientists, by providing an accessible introduction to the development, mechanisms, chemistry, characterisation methods and applications of the technique. It provides the reader with an extensive yet concise grounding in the theory of each area of the subject, as well as detailing the real and potential applications and the future prospects of sol-gel chemistry. The task of summarising such a vast and growing multi-disciplinary field into a volume of this size and price has proved demanding, yet we believe we have gone a long way towards achieving our aim of bridging the gap between an accessible textbook and a useful research resource. The basic ideas are described clearly for the newcomer, while their development to current research level is exemplified with numerous references. The references cited, while including many illustrative original papers as well as key reviews, are intended to facilitate further exploration of the literature rather than serving as a comprehensive bibliography. We apologise to authors of many excellent papers whose work could not be included: omissions are inevitable but not deliberate. Where possible, key fundamental ideas and other important sections of the text have been identified by shading the relevant sections. In some parts of the text, notably the chapters covering characterisation and applications, where it has been difficult to identify some sections as more important than others, little use has been made of the shading. In these areas, the absence of shaded areas should not be taken as an indication that the material is of lesser significance. We acknowledge a considerable debt to the international sol-gel community for assisting our own development in this area. In particular the conference series of International Workshops on Glasses and Ceramics from Gels has provided personal contacts and broad perspectives. In several countries the community has set up national sol-gel groups which provide valuable support and stimulus, especially for new workers in the field. The classic text Sol-gel Science: The Physics and Chemistry SOL-GEL MATERIALS