Fundamental studies for the design of tantalum nitride photoanodes for solar water splitting [electronic resource]
Keywords: Chemical Engineering.

One of today's greatest challenges is meeting the increasing global energy demand using clean, renewable energy sources. The synthesis of chemical fuels such as hydrogen from sustainable energy sources such as solar or wind is an attractive option. Photoelectrochemical (PEC) water splitting is one promising route for hydrogen production. In PEC devices, semiconductor absorbers harvest solar energy to generate excited electrons and holes to drive the hydrogen and oxygen evolution reactions. The first part of this dissertation focuses on a technoeconomic evaluation of conceptual water splitting plants based on four different reactor designs with the aim of identifying key research needs in the field. A significant finding is that more efficient semiconductor photoelectrodes must be developed to make this technology cost-competitive with existing fossil fuel energy sources. Tantalum nitride (Ta3N5) is a promising photoanode candidate due to its nearly ideal band structure for solar water splitting. The remainder of the dissertation focuses on understanding which properties may limit its performance in order to ultimately design a higher efficiency oxygen-evolving photoanode of this material. An emphasis is placed on developing well-defined sample types and accurate measurement tools to systematically study the fundamental structural, optical, electronic, and photoelectrochemical properties of tantalum nitride. Photoelectrochemical measurements on tantalum nitride thin films grown via thermal oxidation and nitridation of Ta foils reveal that their photoactivity is strongly correlated with increased surface area, suggesting poor hole transport. While the thermal conversion of Ta foils is facile, it is difficult to control the surface morphology which hinders the systematic study of material properties. Work then shifts to the development of an improved sample architecture for the synthesis of flat, crack-free films of tightly controlled thickness to enable quantitative assessment of electronic conductivity and optical absorption. We also seek to control the synthesis of phase-pure materials through an understanding of the effect of nitridation temperature and the underlying substrate on film quality. We discover that temperature has little consequence on the crystallinity and absorption properties of tantalum nitride synthesized on fused silica but that the presence of mobile Ta atoms in Ta foil substrates can result in the formation of reduced nitride phases, as demonstrated by grazing incidence x-ray scattering measurements. We next turn our attention to a well-known issue with this nitride material, its degradation to an oxide under illumination. Several catalysts for the oxygen evolution reaction are deposited on the surface with the aim of stabilizing the material and improving the water oxidation kinetics. Lastly, the knowledge of the optimal synthesis conditions, hole and electron transport lengths, and absorption depth is combined to design a core-shell tantalum-tantalum nitride photoanode. Several approaches are explored for the nanostructuring of the Ta scaffold on which the nitride shell can be grown thermally or through anodization. In summary, this dissertation covers fundamental studies of the properties of tantalum nitride to design and develop a high performance photoanode which will hopefully enable more efficient solar water splitting devices for the production of hydrogen fuel.