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Electrochemical Synthesis of Ceo2 and Ceo2/Montmorillonite Nanocomposites

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ANODIC ELECTROCHEMICAL SYNTHESIS AND CHARACTERIZATION OF NANOCRYSTALLINE CERIUM OXIDE AND CERIUM OXIDE / MONTMORILLONITE NANOCOMPOSITES Qi Wang, BS., MS. Dissertation Prepared for the Degree of DOCTOR OF PHILOSOPHY UNIVERSITY OF NORTH TEXAS August 2003 APPROVED: Teresa D. Golden, Major Professor Jeffrey A. Kelber, Committee Member Michael G. Richmond, Committee Member Nandika A. D’Souza, Committee Member, Department of Materials Science and Engineering Zhibing Hu, Committee Member Ruthanne D. Thomas, Chair of the Department of Chemistry Sandra L. Terrell, Dean of the Robert B. Toulouse School of Graduate Studies Wang, Qi, Electrochemical synthesis of CeO2 and CeO2/montmorillonite nanocomposites. Doctor of Philosophy (Chemistry), December 2003, 326 pp, 24 tables, 95 illustrations, 419 references. Nanocrystalline cerium oxide thin films on metal and semiconductor substrates have been fabricated with a novel electrodeposition approach – anodic oxidation. X-ray diffraction analysis indicated that as-produced cerium oxide films are characteristic face-centered cubic fluorite structure with 5 ~ 20 nm crystal sizes. X-ray photoelectron spectroscopy study probes the non- stoichiometry property of as-produced films. Raman spectroscopy and Scanning Electron Microscopy have been applied to analyze the films as well. Deposition mode, current density, reaction temperature and pH have also been investigated and the deposition condition has been optimized for preferred oriented film formation: galvanostatic deposition with current density of –0.06 mA/cm2, T > 50oC and 7 < pH < 10. Generally, potentiostatic deposition results in random structured cerium oxide films. Sintering of potentiostatic deposited cerium oxide films leads to crystal growth and reach nearly full density at 1100oC. It is demonstrated that in-air heating favors the 1:2 stoichiometry of CeO2. Nanocrystalline cerium oxide powders (4 ~ 10 nm) have been produced with anodic electrochemical synthesis. X-ray diffraction and Raman spectroscopy were employed to investigate lattice expansion phenomenon related to the nanoscale cerium oxide particles. The pH of reaction solution plays an important role in electrochemical synthesis of cerium oxide films and powder. Cyclic voltammetry and rotation disk electrode voltammetry have been used to study the reaction mechanisms. The results indicate that the film deposition and powder formation follow different reaction schemes. Ce(III)-L complexation is a reversible process, Ce3+ at medium basic 2+ pH region (7~10) is electrochemically oxidized to Ce(OH ) 2 and then CeO2 film is deposited on the substrate. CE mechanism is suggested to be involved in the formation of films, free Ce3+ - species is coordinated with OH at high basic pH region (>10) to Ce2O3 immediately prior to electrochemically oxidation Ce2O3 to CeO2. CeO2 / montmorillonite nanocomposites were electrochemically produced. X-ray diffraction and Raman spectroscopy illustrate the retaining of FCC structure for cerium oxide. Fourier Transform Infrared Spectroscopy and Differential Scanning Calorimetry of composites indicate the insertion of montmorillonite platelets into the structural matrix of cerium oxide. Sintering study of the nanocomposites demonstrates that low concentration of montmorillonite platelet coordination into cerium oxide matrix increases crystal growth rate whereas high concentration of montmoillonite in nanocomposites retards the increase of crystallite size during the densification process. ACKNOWLEDGMENTS I sincerely express my special gratitude to my academic advisor, Dr. Teresa D. Golden, for her kind guidance and great inspiration in research, also her continuous encouragement and wonderful support in my knowledge pursuit and working spirit development. I appreciate Dr. Nandika, D’Souza for her expertise and advice in the composite project. I wish to extend my acknowledgments to my committee members for their time and effort devoted to me and my research. I would like to thank Dr. Paul Braterman, Dr. Seifollah Nasrazadani (Engineering Department), Dr. Richard F. Reidy (Material Science Department), Dr. Robert M. Wallace (Material Science Department), Dr. Michael Lance (Oak Ridge National Laboratory), Ms. Laura Riester (Oak Ridge National Laboratory) and Mr. David Garrett (Biology Department) for their support and help in the material characterization. I am indebted to Dr. Oliver Chyan for his kindly providing silicon wafer for the research. Thank Mr. Williams A. Smith for making and adjusting glassware. I would like to thank Dr. Zhiping Xu, Hanjiang Dong, Dr. Prakaipetch Punchaipetch and Haritha Namduri for their helpful assistance. I truly appreciate my big sister-like colleague, Charoendee Pingsuthiwong, for her invaluable help in the research and wonderful friendship during daily life. And help from other colleagues is also appreciative. Financial support from Robert A. Welch Foundation and UNT Faculty Research Grant are highly grateful. Thanks also go to Chemistry Department of UNT for financial support as a Teaching Assistant in my first four-year graduate study and research here. ii Last but not least, I deeply appreciate my dear parents and brother for their never-ending love, support and encouragement. iii TABLE OF CONTENTS Page LIST OF TABLES ……………………………………………………………………………..viii LIST OF ILLUSTRATIONS……………………………………………………………………..xi Chapter 1. INTRODUCTION AND LITERATURE REVIEW………………………………….1 1.1 Cerium Oxide Properties, Applications, Synthesis and Characterizations………..1 1.1.1 Structure and Properties of Cerium Oxide………………………………...1 1.1.2 Practical Applications of Cerium Oxide…………………………………..7 1.1.3 Production of Cerium Oxide……………………………………………..21 1.1.4 Characterization of Cerium Oxide 1.2 Cerium Oxide / Layered Silicate Nanocomposites ……………………………...32 1.2.1 Montmorillonite Nanoclays……………………………………………...32 1.2.2 Secondary Phase Reinforcement in Composites…………………………34 1.2.3 Layered Silicate Composites …………………………………………….42 1.2.4 Synthesis of Layered Silicate Composites ………………………………44 1.2.5 Characterization of Layered Silicate and Composites …………………..46 1.3 Chapter Summaries and Dissertation Introduction ……………………………...46 1.4 Chapter References………………………………………………………………48 2. ELECTROCHEMICAL SYNTHESIS AND CHARACTERIZATION METHODS…………………………………………………………………………..64 2.1 Electrochemical Synthesis and Other Electrochemical Techniques……………..64 2.1.1 Electrochemical Synthesis……………………………………………….64 iv 2.1.2. Cyclic Votammetry (CV)………………………………………………...79 2.1.3. Rotating Disk Electrode (RDE) Method…………………………………83 2.1.4 Corrosion Electrochemistry and Corrosion Measurement……………….85 2.2 Characterization and Analysis Methods…………………………………………88 2.2.1 X-ray Diffraction (XRD)………………………………………………...88 2.2.2 X-ray Photoelectron Spectroscopy (XPS)……………………………….95 2.2.3 Infrared Spectroscopy (FTIR) and Raman Spectroscopy……………….98 2.2.4 Scanning Electron Microscopy (SEM)………………………………...103 2.2.5 Brunauer-Emmett-Teller (BET) ……………………………………….104 2.2.6 Nanoindentation………………………………………………………..106 2.2.7 Sintering Technique……………………………………………………108 2.3 Chapter Conclusions……………………………………………………………110 2.4 Chapter References……………………………………………………………..112 3. ELECTROCHEMICAL DEPOSITION OF CERIUM OXIDE FILMS…………….118 3.1 Introduction……………………………………………………………………..118 3.2 Experimental……………………………………………………………………121 3.2.1 Electrochemical Deposition of CeO2 Films…………………………….121 3.2.2 Characterization of CeO2 Films………………………………………...122 3.2.3 Property and Application Studies ……………………………………...123 3.3 Results and Discussions………………………………………………………..124 3.3.1 Deposition Conditions Investigations ………………………………….124 3.3.2 Evaluation of Particle Sizes and Lattice Strains………………………..137 3.3.3 Determination of Oxidation State with XPS……………………………141 v 3.3.4 Morphology……………………………………………………………..149 3.3.5 Sintering Studies ……………………………………………………….152 3.3.6 Corrosion Test Results………………………………………………….165 3.4 Chapter Conclusions……………………………………………………………168 3.5 Chapter References……………………………………………………………..170 4. ELECTROCHEMICAL SYNTHESIS OF NANOCRYSTALLINE CERIUM OXIDE POWDERS…………………………………………………………………………175 4.1 Introduction …………………………………………………………………….175 4.2 Experimental …………………………………………………………………..178 4.3 Results and Discussions………………………………………………………..179 4.3.1 Electrochemical Synthesis of Cerium Oxide Powders…………………179 4.3.2 Nanosized Cerium Oxide Powder Sintering……………………………184 4.3.3 Lattice Relaxation of Nanocrystalline Cerium Oxide…………………..193 4.4 Chapter Conclusions…………………………………………………………...209 4.5 Chapter References…………………………………………………………….210 5. MECHANISM STUDIES ON ELECTRODEPOSITION OF CERIUM OXIDE FILMS AND ELECTROSYNTHESIS OF CERIUM OXIDE POWDER IN SOLUTION OF CE (III) COMPLEXES…………………………………………...214 5.1 Introduction……………………………………………………………………..214 5.2 Experimental …………………………………………………………………...216 5.2.1 Electrochemical Techniques……………………………………………216 5.2.2 FTIR…………………………………………………………………….216 5.3 Results and Discussions………………………………………………………...217 vi 5.3.1 Ligand-pH Effect on Film or Powder Formation………………………217 5.3.2 Mass Transfer Study using Rotation Disk Electrode…………………...224 5.3.3 Coordination Investigation Through Varying Ligand Concentration…..228 5.4 Chapter Conclusions……………………………………………………………236 5.5 Chapter References……………………………………………………………..238 6. ELECTROCHEMICAL SYNTHESIS AND CHARACTERIZATION OF CeO2 / MONTMORILLONITE NANOCOMPOSITES…………………………………...240 6.1 Introduction ……………………………………………………………………240 6.2 Experimental …………………………………………………………………...241 6.2.1 Materials and Electrosynthesis of Composites…………………………241 6.2.2 Composite Property
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