UC San Diego

Intrinsic drawbacks of Si, a well adapted material in photovoltaics, including the high valence band position compared to the proton reduction level, the unfavorable self-oxidation level compared to the one of water oxidation, and indirect energy band, prevent Si as a good candidate for applications in photoelectrochemical water splitting, despite its small band gap which matches the solar irradiance spectrum. Strategies need to be developed to overcome these problems. This dissertation presents a study of metal oxides integrated Si photoelectrode for solar driven water splitting. Different metal oxides including zinc oxide, titanium dioxide, indium tin oxide, and nickel oxide are coated on polished and nanotextured p -type and n-type Si from various methods. This heterogeneous uniform coating effectively replaces the problematic water Si interface which can be corrosive and oxidative to Si. The Schottky junction formed either using an n-type coating (ZnO and TiO₂) on p-type Si or a p-type coating (NiO) on n-type Si is able to effectively separate the photo-generated charges in Si. In addition, nanotextured Si is able to dramatically improve light absorption and provide enlarged surface chemical reaction sites. In this case, integration of metal oxides in the form of nanoscale structures on the nanostructured Si, a so-called branched nanowire structure or a "nanotree" array, is able to enhance the light absorption and reduce the radius of curvature to ease the gas evolution. Earth abundant transition metal, primarily Ni, was used to catalyze the water reduction and oxidation reaction. Solution-casted NiO from a cost-effective approach provides higher activity compared to the one prepared from gas phase sputtering. Surfactant and thermal treatment effect are studied to achieve an optimized catalytic activity. This catalyst is then successfully demonstrated on a surface inverted n-Si photoanode protected with a novel semitransparent conducting oxide. Considerable contributions have been made through this dissertation study. Suggestions on future improvements are provided