Fe Substitutions Improve Spectral Response of Bi2WO6-Based Photoanodes

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 are 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.