Designing Chemically Robust Metal Oxides for Visible-Light Photocatalysis: Toward Solar Z-Scheme Water Splitting

Bart Bartlett, Department of Chemistry, University of Michigan

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Abstract

Solar driven water splitting for large-scale hydrogen fuel production from semiconductor photo-electrodes has the potential to provide energy on large scale from renewable, sustainable sources. Our research focuses on the kinetically more demanding oxygen-evolution reaction, and we prepare thin film metal oxide photoanodes by low-temperature, solution-based processes. One promising light absorber is TiO2:(Nb,N) where Nb and N substitute for Ti and O on their respective lattice sites in anatase. These materials are prepared by sol-gel processing followed by annealing in flowing ammonia. We observe a band-gap energy as low as 2.0 eV at 25% Nb and 2% N. In conjunction with a RuO2 catalyst, powdered TiO2:(Nb,N) evolves O2. A second class of materials we study is the transition-metal tungstates, and we have prepared our most promising candidate, CuWO4, by several routes: electrochemical deposition, sol-gel processing, and spray pyrolysis. These methods afford highly reproducible and robust CuWO4 thin-film electrodes on transparent conducting substrates. CuWO4 is an n–type semiconductor with a band-gap energy of ~2.4 eV. CuWO4 thin films photooxidize water with simulated solar radiation with a nearly quantitative Faradaic efficiency for O2 evolution at no applied bias in the presence of the sacrificial electron acceptor, [Fe(CN)6]3–. Most important, these thin-film electrodes are stable against photocorrosion when illuminated with visible light at neutral pH, a significant improvement to the more commonly studied photoanode, WO3. Current efforts are aimed at preparing complex tungstates that absorb lower energy light to improve the quantum yield.

This talk was given as part of the MITEI Seminar Series, made possible with the support of IHS-CERA.