UNIVERSITY OF CALIFORNIA, IRVINE Synthesis of Gallium Zinc Oxynitride Solid Solution by Flame Spray Pyrolysis THESIS submitted in partial satisfaction of the requirements for the degree of MASTER OF SCIENCE In Chemical and Biomolecular Engineering by Ewa Richard Chukwu Thesis Committee: Assistant Professor Erdem Sasmaz, Chair Assistant Professor Iryna Zenyuk Associate Professor Ali Mohraz 2020 © 2020 Ewa Richard Chukwu DEDICATION To The Almighty God who only him knows the end of a journey right from the beginning. Do not fear, for I am with you; Do not be dismayed for I am your God; I will strength you and help you; I will uphold you with my right hand of righteousness. Isaiah 49:10 ii TABLE OF CONTENTS LIST OF FIGURES……………………………………………………………. .iv LIST OF TABLES……….………………………………………………………vi ACKNOWLEDGEMENT………………………………….…………………...vii ABSTRACT……………………………..……………………………………....viii CHAPTER 1: Introduction………………………………………………..…........1 CHAPTER 2: Literature Review…………………………..………….........….......4 CHAPTER 3: Experimentation..............................................................................27 CHAPTER 4: Results and Discussions……………...…………………...............34 CHAPTER 5: Conclusion and Recommendation….….…..……………..............50 CHAPTER 6: Future Work …………………..………………….…...................53 REFERENCES………..…...……………………………………….…….............69 APPENDIX A.............………………………………………………..…..............83 APPENDIX B.................……………………………………………..…..............86 ii LIST OF FIGURES Fig.1: Flame synthesis versus wet chemistry……………………………………....5 Fig.2: Different particle formation routes and .........................................................8 mechanisms for flame aerosol synthesis Fig.3: Different H2 production routes……………………………………….……12 Fig 4: Diagram showing the Z-system for photocatalytic water splitting…….…..13 Fig.5: (a) customized FSP set-up (b) set-up with the premixed flamelets…….….28 Fig.6: Schematic diagram of the FSP system………...………………………......29 Fig.7: XRD pattern of GZN2……....…………………………………………....37 Fig.8: Diffraction pattern of as-prepared zinc oxide………………………..........38 Fig.9. Fig. 9. XRD patterns of samples .................................................................40 made from urea and zinc nitrate solution only Fig.10. XRD patterns of gallium zinc oxynitride solid solutions made with….....43 methanol + butanol as solvents compared with pristine ZnO (GZN 3 (top) and GZN4 (middle) to Z3 (bottom)) Fig.11. XRD patterns of gallium zinc Oxynitride solid solutions made….............46 with DI H2O + butanol compared with ZnO (GZN5 (middle), GZN6 (top) and Z3 (bottom)) iii Fig.12. Tauc’s plot showing direct bandgaps GZN 3 (3.16 eV) ………………48 and (b) GZN4 (3.07 eV). Fig.13. Tauc’s plot showing the direct bandgaps GZN 5 (3.12 eV) …………...49 and (b) GZN6 (3.08 eV). Fig.14. Variation of flame height with (a) precursor flowrate (left)…….......….63 (b) equivalence ratio (right) at constant dispersion gas rate and pressure drop. Fig.15. Flame height versus equivalence ratio and precursor feed rate ……...…63 Fig.16. The influence of precursor rate on the particle size of FSP-made…...….64 ceria nanoparticles. Fig.17. Variation of flame height with oxygen flowrate at ..................................65 constant pressure drop and precursor rate. Fig. 18. Visualization of pressure drop effect on the flame at....…………....…..66 constant precursor and oxygen rates. (a) & (b) are at the same precursor rates and (c) & (d) are at the same oxygen rates Fig. B1: Heat formation as a function of the concentration of ZrO2 in ceria.........79 Fig. B2: Modelled heat of mixing as a function of the concentration of …….......79 ZrO2 in ceria compared with experimental work. Fig. C1: Variation of flame height with precursor...................................................80 flowrate at constant oxygen flowrate. Fig.C2: Variation of flame height with oxygen flowrate at ...…………................80 constant precursor flowrate and pressure drop. iv LIST OF TABLES Table1: Sample compositions and solvents..............................................................30 Table 2: Samples and their synthesis parameters.....................................................33 Table 3: Lattice parameters of the synthesized ZnO................................................38 Table 4: Estimated crystallite sizes of ZnO samples ................................................39 Table 5: Lattice parameters of ZN samples...............................................................40 Table 6: Estimated crystallite sizes of ZN samples ..................................................40 Table 7: Lattice parameters of GZN samples............................................................43 Table 8: Estimated crystallite sizes of GZN samples................................................43 Table 9: Bandgaps of GZN samples..........................................................................49 Table A1: Solution compositions and their synthesis conditions..............................77 GZN, Z1 -Z3 and ZN1 to ZN5 v ACKNOWLEDGEMENT I remain indebted to God, the giver and sustainer of life, for his continual grace and blessings. My appreciation goes to my parents, siblings and well-wishers for their encouragement, support, motivations and prayers. Special thanks to my Advisor and Chair of the thesis committee, Dr. Erdem Sasmaz, whose guidance and support saw to the successful completion of this work. Also, my sincere gratitude to my thesis committee members, Profs. Ali Mohraz and Iryna Zenyuk for their commitment and efforts. I am also grateful to the past and present Lab mates whose critiques, support and cooperation contributed in shaping my research thought process. My eternal gratitude goes to the United States’ Government for their Opportunity Fund Award through the LagosEducationUSA, Nigeria-that facilitated my graduate school dream. Worthy of mention are my EducationUSA advisers, Mrs. Uwadileke and Mrs. Adejumobi for their guidance and continual moral support. I would like to thank Dr. Ben Meekins of the University of South Carolina who carried out the UV-Vis spectroscopy and for his insights and suggestions. Additionally, I would like to acknowledge the SAGE, U of SC whose XRD machine was used for the characterization. Finally, I seize this opportunity to thank the CBE program of UCI for the enabling environment throughout my stay. God bless you all. vi ABSTRACT Synthesis of Gallium Zinc Oxynitride Solid Solution by Flame Spray Pyrolysis By Ewa Richard Chukwu Master of Science In Chemical and Biomolecular Engineering University of California, Irvine, 2020 Assistant Professor Erdem Sasmaz, Chair Photocatalytic water splitting is seen as a viable alternative to steam reforming for hydrogen fuel generation. One of the major keys to exploiting photocatalytic technologies as a commercial energy source lies in the development of catalytic materials that can drive photocatalytic reactions using the largest portion of the solar energy spectrum – the visible light region. Gallium zinc oxynitride solid solution has been recommended as a promising candidate for photocatalytic water splitting, however, the current synthesis methods are largely multi- steps and batch wet chemistry techniques that are not easily amenable to scale up. Flame spray pyrolysis (FSP) is a continuous, single step, fast, relatively cheaper and easily scalable nanomaterial synthesis method. This work reports the deployment of flame spray pyrolysis to the synthesis of gallium zinc oxynitride solid solutions. The solid solutions were prepared using two different precursor compositions expressed as urea to total metal ratio, solvent mixtures and synthesis conditions, and X-ray diffraction and UV-Vis spectra were employed for preliminary characterizations. Based on solvent screening results, no single solvent satisfied the solubility, boiling point and combustion enthalpy requirements. The XRD data shows that solvents of weak combustion enthalpy resulted in either amorphous or very low crystalline, vii bimodal and inhomogeneous phase solid solution. However, solvent mixtures of adequate combustion enthalpy yielded single phase and highly crystalline solid solutions with all peaks indexed to the hexagonal wurtzite ZnO structure irrespective of synthesis conditions. The absence of extra peaks coupled with the slight lattice contraction relative to pristine ZnO satisfied the necessary conditions to qualify the as-prepared samples as solid solutions of GaN and ZnO. Lattice contraction was favored as urea to total metal ratio decreased while peak broadening occurred as the urea to total metal ratio in the precursor solution increased while the crystallinity lowered. This could be possibly due to increased dopant concentration in the ZnO lattice. Samples made with DI H2O + butanol solvent mixture were more crystalline and of larger crystallite size than those made from methanol + butanol mixture which is attributed to the fact that the solvent mixture supplied higher rate of energy (1.17 kJ/s) than the methanol- butanol (1.12 kJ/s) to the flame. Tauc’s plots were used to estimate the direct bandgaps of the samples. With DI H2O + butanol mixture, the bandgaps are 3.08 and 3.12 eV with smaller bandgap belonging to the sample with higher urea to total metal ratio in the precursor solution. Comparing with the XRD data, the smaller bandgap corresponds with lattice contraction. But for the methanol-butanol mixture, the bandgaps are 3.07 eV and 3.16 eV with the smaller bandgap belonging to the