An Atomic Level Understanding of Optimal Characteristics of TiO2 Protection Layers and Photoelectrode/TiO2 Interfaces for Efficient and Stable Solar Fuel Production

With the support of the Chemical Catalysis program in the Division of Chemistry, Professor Kyoung-Shin Choi of the University of Wisconsin-Madison and Professor Giulia Galli of the University of Chicago are studying semiconductor electrodes that can utilize solar energy to split water and produce hydrogen gas, a clean fuel. These semiconductor electrodes are called photoelectrodes, and they often need to be coated by a protection layer to enhance their performance and stability. To date, titanium dioxide (TiO2) has been the most extensively used material as a protection layer due to its inertness. However, the optimal characteristics of an effective TiO2 protection layer reported in the literature are inconsistent. Using combined experimental and computational approaches, Choi and Galli will work toward obtaining a microscopic understanding of how different photoelectrode-TiO2 interfaces influence overall photoelectrode performance. The teams aims to elucidate the photoelectrode-dependent characteristics of the TiO2 layer to optimally protect different types of photoelectrodes. This project is aimed at enabling the rational design of optimal photoelectrode/TiO2 assemblies for efficient and sustainable solar water splitting. In terms of broader impact, Choi and Galli will maintain a website that contains validated sets of data, which will be easily accessible and reusable by members of the community. This website will enhance the infrastructure for guiding researchers to study solar water splitting and other complex systems using combined experimental and computational approaches. This project will also train graduate students in a highly interdisciplinary environment and generate versatile researchers in the field of clean fuel production using renewable solar energy. Under this award, the Choi (U Wisconsin)/ Galli (U Chicago) team will study photoelectrodes that can utilize solar energy to split water and produce hydrogen gas, a clean fuel. . Choi and Galli will use tightly integrated experimental and computational investigations with the goal of achieving a comprehensive understanding of the photoelectrode-dependent characteristics of the TiO2 layer to optimally protect different types of photoelectrodes while maximizing photocurrent and photovoltage generation. The team will elucidate the impact of the photoelectrode/TiO2 interface on band alignments and electron-hole recombination. They will use n-BiVO4/TiO2, p-Cu2O/TiO2, and p-Si/TiO2 as model systems for oxide-based photoanodes, oxide-based photocathodes, and covalent photocathodes, respectively, to build toward a holistic understanding of photoelectrode/TiO2 interfaces. The team will vary the crystallinity, thickness, and deposition method of the TiO2 layer as well as the surface of the semiconductor electrode (composition and atomic arrangement) to systematically alter the interface and comprehensively investigate their impacts on the overall performance of the photoelectrodes. This project has the potential to provide critically needed rational guidelines for the preparation of optimal TiO2 protection layers for different types of photoelectrodes that can maximize the solar-to-fuel conversion efficiency and the stability of various photoelectrodes. Beyond this, the project will also offer strategies to combine experimental and computational investigations to study complex interfaces formed between metal oxides or between metal oxides and non-oxide semiconductors. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.