UC Davis

Solar energy conversion offers a carbon free and renewable alternative to fossil fuel consumption. Current materials still fall short of the target solar-to-hydrogen efficiency needed to make the technology economically viable. In order to increase the efficiencies, a deeper understanding of charge generation, transfer and recombination, to further improve their photocatalytic activities. Within this dissertation n-type gallium phosphide is investigated for photocatalytic hydrogen production. P-type gallium phosphide and carbon nitride are investigated for photoelectrochemical solar energy conversion. Characterization of these materials consists of surface photovoltage spectroscopy, optical spectroscopy, electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy.Chapter 1 provides an introduction to the topic and important measurements. Chapter 2 investigates n-type gallium phosphide as a photocatalyst for hydrogen production, looking at defect states, space charge layer effects on the charge generation, cocatalyst effect on charge transfer and recombination processes to optimize activity for solar water splitting. A quantum efficiency of 14.8 % was achieved by 4 % (w/w) dinickel phosphide nanoparticles loaded onto gallium phosphide microparticles in an aqueous solution of 0.3 M sodium sulfide and 0.3 M sodium sulfite under 525 nm LED illumination. Chapter 3 reveals how the photovoltage and photocurrent of p-type gallium phosphide photoelectrodes is affected by a cadmium sulfide passivation layer, added platinum cocatalyst, altered electrolyte composition, and added hydrogen or oxygen. The champion photocathode drives hydrogen evolution with a quantum efficiency of 62 % at 0.0 V RHE and an open circuit photovoltage of 0.43 V at 250 mW/cm2 (400 nm). Finally, Chapter 4 explores the photovoltage of carbon nitride films prepared using different fabrication techniques and in different electrolytes. The best performance was achieved by a photoanode prepared via doctor blading in 0.1 M KOH with O2 purging. This film reached 200 µA/cm2 photocurrent at 1.23 V vs RHE and 1 sun illumination and a photovoltage of 1.1 V under 79 mW/cm2 illumination (405 nm).