SYNTHESIS AND NANOENGINEERING OF GALLIUM PHOSPHIDE NANOSTRUCTURES FOR PHOTOELECTROCHEMICAL SOLAR ENERGY CONVERSION by Wen Wen A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Chemistry) in The University of Michigan 2014 Doctoral Committee: Assistant Professor Stephen Maldonado, Chair Assistant Professor Bart Bartlett Associate Professor Peicheng Ku Professor Adam Matzger © Wen Wen All Rights Reserved 2014 DEDICATION For my parents, my husband and my daughter. ii ACKNOWLEDGMENT First, I would like to thank my advisor, Professor Stephen Maldonado. Thank you very much for your patient guidance during my last five years. I want to thank you for taking time for one-to-one weekly meeting with me for 5 years and preparing me for each important stage in graduate school (seminar, candidacy, data meeting and defense). I also would like to thank my committee member, Professor Adam Matzger, Professor Bart Bartlett and Professor Peicheng Ku. Dr. Matzger, I started my graduate school by rotating in your group. It is an invaluable experience. And I also want to thank you so much for letting using instruments in your lab. Dr. Bartlett, I took Inorganic Chemistry with you, I learned a lot from your class and it is very helpful in my PhD. research, thank you. Dr. Ku, thank you very much for your feedback and advice for my research during our meeting. Looking back to my past five years in graduate school, I think I could not feel any luckier to know my labmates. Thank you very much for your help and support. Sean Collin, for taking me to numerous SEM and TEM sections and you are always very patient to answer my question. Azhar Carim, for helping me with characterization of my samples and showing me a lot of tricks. Dr. Michelle Chitamber, for helping me out with the spectral response, even you left, your promote reply for my questions is very helpful. Dr. Jeremy Feldblyum, for your friendship and you are always there to encourage me. Sabrina Peczonczyk and Junsi Gu, my peers for five years, we went through each milestone of Ph.D life together and thank you so much for your support. Eli Fahrenkrug, for being a nice cubemate and your helpful advices in research and life. Besty Brown, for always being very sweet, no matter your candies or your kind comforts. Luyao Ma, Sudarat Lee and Tim Zhang, for taking lab duties and your support. This dissertation could not be done with the help from the department staffs. I would greatly thank Roy Wentz from glass shop, you are always there whenever I need to try new setups and offer me countless good ideas to make them happen. Stephen iii Donajkowski from machine shop, for always helping me fixing instruments promptly. Dr. Haiping Sun and Dr. Kai Sun from EMAL, for your generous help in solving technical problems. I would like to take this opportunity to thank my previous advisors, Professor Ranko Richert from Arizona State and Professor Kai Wu from Peking University, for your patient guidance and great support. Last but not least, I would like to thank my family. My parents, for giving me a family surrounded with love and raising me up to who I am with endless love and support. My husband Chen Ling, the love of my life. You are always there supporting me through all the frustrations and challenges I met. I could not imagine I can get to the point of what I am now without you. I am looking forward to share the journal of the rest of my life with you. iv TABLE OF CONTENTS DEDICATION................................................................................................................... ii ACKNOWLEDGMENT ................................................................................................. iii LIST OF FIGURES ....................................................................................................... viii LIST OF TABLES ......................................................................................................... xiv ABSTRACT ..................................................................................................................... xv CHAPTER I Introduction ................................................................................................ 1 I.1 Importance of Solar Energy ...................................................................................... 1 I.2 Solar Energy Conversion .......................................................................................... 1 I.2.1 Photosynthetic Energy Storage ........................................................................... 2 I.2.2 Photovoltaic Energy Conversion ........................................................................ 2 I.2.3 Photoelectrochemical (PEC) System .................................................................. 6 I.3 Gallium Phosphide Nanowires for PEC Application .............................................. 19 I.4 Content Description ................................................................................................ 20 I.5 References ............................................................................................................... 23 CHAPTER II Photoelectrochemical Behavior of n-type Gallium Phosphide Nanowires as Photoanodes ............................................................................................. 28 II.1 Introduction ............................................................................................................ 28 II.2 Methods ................................................................................................................. 29 II.2.1 Preparation of GaP Nanowires ........................................................................ 29 II.2.2 SEM/TEM Characterization of GaP Nanowires .............................................. 29 II.2.3 Diffuse Reflectance Measurement ................................................................... 29 II.2.4 Photoelectrochemistry ..................................................................................... 32 II.3 Results .................................................................................................................... 33 II.3.1 Morphology of GaP Nanowries ....................................................................... 33 II.3.2 Photoelectrochemical Response of as-Prepared Nanowire Films ................... 33 v II.3.3 Diffusive Reflectance Measurement of GaP Nanowire Films ........................ 38 II.3.4 Spectral Response of Nanowires in Different Electrolytes ............................. 38 II.3.5 Effect of Catalyst Residue in the Overall Quantum Yield Performance ......... 40 II.4 Discussion .............................................................................................................. 42 II.5 Conclusions ............................................................................................................ 46 II.6 References .............................................................................................................. 46 CHAPTER III Structural and Photoelectrochemical Properties of Gallium Phosphide Nanowires Annealed in NH3 ........................................................................ 49 III.1 Introduction .......................................................................................................... 49 III.2 Methods ................................................................................................................ 51 III.2.1 GaP Nanowire Film Preparation .................................................................... 51 III.2.2 Materials Characterization ............................................................................. 51 III.2.3 Photoelectrochemical Measurements ............................................................. 55 III.3 Results .................................................................................................................. 55 III.3.1 X-ray Diffraction ............................................................................................ 55 III.3.2 Diffuse Reflectance ........................................................................................ 57 III.3.3 Raman Spectroscopy ...................................................................................... 59 III.3.4 Transmission Electron Microscopy ................................................................ 65 III.3.5 X-ray Photoelectron Spectroscopy ................................................................. 69 III.3.6 Long Wavelength Photoelectrochemical Conversion .................................... 72 III.4 Discussion ............................................................................................................ 74 III.5 Conclusions .......................................................................................................... 77 III.6 References ............................................................................................................ 78 CHAPTER IV Zinc Doping of Gallium Phosphide Nanowire Films ......................... 82 IV.1 Introduction .......................................................................................................... 82 IV.2 Methods ................................................................................................................ 83 IV.2.1 Zinc Doping Methods .................................................................................... 83 IV.2.2 Photoelectrochemical Measurements ............................................................