Synthesis and Electrochemical Characterization of Ternary Metal Oxides
Type of DegreePhD Dissertation
Chemistry and Biochemistry
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Metal oxides are an incredibly diverse class of materials that have found significant utility in the development of emerging solar conversion technologies. Their varied electronic properties allow them to function as charge transport materials, light absorbers, photocatalysts, and electrocatalysts, just to name a few. Many metal oxides consist of first row transition metals, which means they are cheap and abundant to use in these technologies. The structural diversity among metal oxides allows for the tuning of their electronic and optical properties, which is important for application of metal oxides in different devices. One particularly interesting way to tune metal oxide properties is to have two metal cations present in the material. These ternary metal oxides can be tuned in ways that binary oxides cannot. This thesis focuses on the investigation of two different ternary metal oxide materials and their electrochemical properties towards hole transport and catalysis. Chapter 2 and 3 focuses on delafossite CuGaO2 which has shown promise as a hole transport layer in p-type heterojunction solar devices. The properties of CuGaO2 as a nanocrystalline mesoporous film were characterized using electrochemical techniques such as cyclic voltammetry and electrochemical impedance spectroscopy and found that depending on the pH at which the particles were synthesized, the hole density of the material changed, shifting from metallic to semiconductive. The hole density could also be controlled by post synthetic annealing under different environments, such as O2, Ar, and H2. Chapter 4 explores the electrocatalytic properties of single crystalline epitaxial spinel MnFe2O4 towards the oxygen reduction reaction (ORR), which is an important catalytic process for fuel cell technologies. These single crystalline films were grown via MBE on conductive substrates such that their electrochemical properties could be assessed. We find that MnFe2O4 has great selectivity for ORR but suffers from large overpotentials. Lastly, the use of nanocrystalline materials in energy technologies has required significant investigation into different synthetic methods. Ternary metal oxides are more difficult to synthesize due to competition to form the individual binary oxide materials. While investigating a continuous injection nanocrystal synthesis for ternary metal oxide synthesis we discovered a unique pathway of metal oxide formation, where one metal precursor catalyzes the formation of the other binary oxide. Specifically, we found that Lewis acids, such as gallium and indium, can catalyze the formation of Cu2O, which previously could not be accessed with the continuous injection synthesis. The investigation of this catalytic pathway has revealed an interesting method to study nanocrystalline reaction kinetics, as well as access metal oxides that could be difficult to synthesize.