Fe Substitutions Improve Spectral Response of Bi₂WO₆-Based Photoanodes

Abstract

The quest for a scalable solar fuel technology has stimulated a concerted effort to develop a metal oxide semiconductor for solar-driven photoelectrochemical water oxidation (oxygen evolution reaction) in an efficient and durable manner. So far, the search for such a metal oxide photoanode has highlighted the promise of Bi-based oxides, which have also been extensively studied for other photocatalyst applications. Bi₂WO₆ is a durable photocatalyst whose primary shortcoming is a 2.8 eV band gap that limits utilization of the solar spectrum. Improvements in visible photoresponse upon incorporation of Fe have been reported in the photocatalysis literature, motivating our use of high-throughput synthesis and photoelectrochemistry to determine the spectral photoresponse for Bi–W–Fe oxides synthesized under nonequilibrium conditions based on thermal oxidation of metallic films. Photoactivity down to 2 eV was achieved over a broad range of compositions, with detailed characterization of optimal compositions revealing that Fe incorporation increases the valence band position by 0.75 eV. Density functional theory calculations of Fe substitutions on W sites in Bi₂WO₆ re consistent with this valence band shift, providing a plausible explanation for the experiments. This Fe-mediated band tuning yields a ca. 2 eV band gap while retaining a turn-on potential for photoanodic current near 0.4 V versus RHE, which combined with the operational durability motivates continuous study and development of this promising class of metal oxide photoanodes.