This dissertation is focused on the synthesis and characterization of semiconductor materials for photoelectrochemical (PEC) water oxidation and it is comprised of two sections. In the first section, synthesis and characterization of Ta3N5 thin films for photoelectrochemical water oxidation were investigated. Despite promising properties, the harsh synthesis conditions, involving high-temperature ammonolysis limits the PEC water oxidation efficiency on Ta3N5. From synthetic point of view, this method is highly energy intensive, produces a sizable quantity of chemical wastes, inefficient on chemical utilization, and provides highly reducing conditions, preventing to integrate it in the tandem cell. As the first study, the electrodeposition of tantalum oxide from aqueous solution followed by ammonolysis to synthesize Ta3N5 films was investigated. This is a promising approach as it requires fairly simple instrumentation, maximizes the chemical utilization, and allows to realize Ta3N5 on any conductive substrate. In order to eliminate the ammonolysis step, the atomic layer deposition (ALD) was utilized to directly deposit tantalum nitride on transparent conductive oxides (TCO) which otherwise require highly reactive (reducing) ammonolysis conditions. It was discovered that the low-temperature ALD (175- 280 °C) only results in amorphous TaOxNy films which still need to be crystalized/ nitridized in ammonia but practically at more moderate conditions. This further allowed to integrate Ta3N5 with a Ta-doped TiO2 – a newly developed TCO that is stable in reducing conditions- and to realize the first example of a Ta3N5 electrode on TCO. Lastly, a high temperature and fully automated ALD system was designed and built to directly deposit crystalline Ta3N5 on FTO. In the second section of this dissertation, hematite as another promising candidate for PEC water oxidation was investigated. The goal of this project was to understand how catalyst interfaces with underlying semiconductor and how it affects the performance of catalytically modified electrodes. To this end, the Ni1-xFexOy catalysts with various composition were utilized to coat hematite electrodes. A combination of structural and electrochemical techniques such as steady-state and transient photocurrent measurements, electrochemical impedance spectroscopy (EIS), intensity modulated photocurrent spectroscopy (IMPS), and dual-working electrode (DWE) measurements (in collaboration with Prof. Boettcher) were opted to elucidate the role of catalyst. The findings from these studies are of great importance as they provide a clear picture of the device under operando conditions. It was found that the catalyst layer acts as a hole storage layer where the photogenerated holes from underlying semiconductor are stored in the catalyst layer, causing the potential of catalyst to drop until a sustainable water oxidation is achieved. In addition, it was discovered that the effect of catalyst (improving or suppressing) on the PEC performance of the catalyst-coated electrodes strongly relates to the electronic conductivity of the catalyst and the morphology of the underlying hematite photoelectrode. For example, in case of highly conductive catalyst and porous hematite substrate (in presence of pinholes), the potential of the catalyst layer is pinned to the conductive substrate which subsequently limits the built-in potential in the catalyst layer.
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