The stimulus for my research stems from the immediate need to reduce our planet’s dependence of carbon-based fuel in order to minimize the potential detrimental effects of climate change. It is known that the sun irradiates the surface of our planet with enough energy in one hour to meet our ever growing energy needs; with the potential to completely supplement our dependence on carbon based fuels. Since a majority of energy uses necessitate the need for liquid fuels, developing materials that have the ability to store energy in chemical bonds are of great interest. This work has focused on the fabrication, modification, characterization, and analysis of semiconductor metal oxides for photoanode materials, primarily hematite (α-Fe2O3). Hematite has ideal photoanode characteristics such as good light absorption, stable in contact with neutral and basic aqueous electrolytes, and has a low enough valence band energy to drive water oxidation via photogenerated holes. In addition, hematite is abundant, making it a cost effective material for potential scalability.Although hematite has many desirable characteristics as a photoanode material, its performance has been less than desirable. Water oxidation efficiency is controlled by three processes of the photoanode: light harvesting by the material, the transport of photogenerated holes to the solution interface, and hole collection via water oxidation at the electrode surface. Specifically, this work aimed to reduce the detrimental recombination of photogenerated holes on the surface of hematite before they are able to facilitate water oxidation by treating the surface with a known water oxidation catalyst, Ni(OH)2 using ALD. In my work, I have shown that mitigating this recombination has had a drastic effect of the performance of hematite as a photoanode, moving it closer to being a viable photoanode material. Once the recipe for Ni(OH)2 was established and reproducible, this was deposited onto well characterized thin films of Fe2O3 and was fabricated as a photoanode for photoelectrochemical studies. I was able to develop an electrochemical conditioning method for the electrodes that yielded stable, reproducible results. Using photoelectrochemical measurements such as cyclic voltammetry, transient spectroscopy, and impedance spectroscopy I was able to determine that the addition of Ni(OH)2 to the surface of Fe2O3 did in fact inhibit detrimental recombination of photogenerated holes. Ni(OH)2 acted as a charge storage medium (akin to that of a battery) that collected photogenerated holes from Fe2O3, which in turn oxidized Ni2+ to Ni3+, which then oxidized water at its surface. This result showed the greatest onset for water oxidation with a catalyst at the surface of hematite (with a shift in photovoltage on approximately 300 mV), which is vastly important for improving the efficiency for Fe2O3 as a photoanode material.
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