Nanoscale heterojunctions of transition metal oxide and silicon for high-efficiency oxygen evolution reaction

Abstract

Hydrogen fuel, storing solar energy by splitting water, is of great potential as efficient energy storage due to its sustainability, carbon-neutrality and high energy density per mass. One of major bottlenecks for the solar-driven energy storage into hydrogen, however, is oxygen evolution reaction (OER) because of its high overpoential and the complexity of surface structures and reaction mechanisms. To overcome these obstacles, researchers have approached in two ways: (i) searching for the best materials with the highest efficiency and (ii) devising schemes that can yield a higher efficiency, given materials. Considering that the efficiency improvement with inexpensive materials would be ultimately beneficial for future global energy requirements, we pursue the second approach and examine nanoscale heterojunctions of earth abundant materials. In this thesis, we employ density functional theory (DFT) calculations to investigate nanoscale heterojunctions of transition metal oxide and silicon (Si), which are commonly used for photo/photoelectocatalytic and photovoltaic materials, respectively. In particular, the heterojunction of anatase titanium dioxide (TiO₂) and Si is of our best interest. The heterojunctions of TiO₂ and Si have not only exhibit great synergies based on the bulk properties, but also have improved the photoelectrocatalytic efficiency experimentally. However, the mechanism for this improvement is unclear. Optimizing the catalytic activity of such systems requires a deeper understanding of the detailed atomic and electronic structure of the TiO₂/Si interface, the OER mechanism on TiO₂ surface, and how the TiO₂/Si interface affects the active TiO₂ surface, thus changing the OER overpotential. This thesis examines mainly four aspects of the heterojunctions of anatase TiO₂(001) and Si: (i) the thermodynamic stability of different local stoichiometry at the TiO₂/Si interface, (ii) the electronic structures induced by the different TiO₂/Si interface, (iii) how the TiO₂/Si interface influences OER on TiO₂ surface and its rate-limiting overpotential, and (iv) whether this scheme is transferrable to other oxides such as strontium titanate (SrTiO₃ perovskite) to improve the OER efficiency. We also propose a new OER pathway on anatase (001) surface that is plausible under realistic experimental conditions and compare it with the OER pathways that have been proposed earlier. This work, thus, has potential to deepen our understanding and insights of interface physics, surface chemistry and energy conversion.