Complex metal oxides (e.g., ternary) represent an exciting material class for solar energy conversion applications. To this end, BiVO4 is one of the highest performing metal oxide photoanodes for water oxidation. Progress in the past decade has improved the AM1.5 photocurrents of BiVO4 from hundreds of μA/cm2 to ~7 mA/cm2. With reported photocurrents already within 90% of the theoretical maximum, the solar-to-hydrogen (STH) efficiency of BiVO4 is now limited by the bandgap to &lt; 9% due to its relatively wide bandgap of 2.4-2.5 eV. Here, we demonstrate that this limitation can be alleviated by controlled anion substitution in BiVO4. We incorporated sulfur into BiVO4 by post-annealing in sulfur-rich atmosphere. As a result, the bandgap is reduced by up to ~0.3 eV, which increases the theoretical maximum STH efficiency to ~12%. We confirmed that sulfur substitutes oxygen in the lattice of BiVO4 by a series of structural and chemical characterization (e.g., XRD, Raman, XPS). Moreover, hard X-ray photoelectron spectroscopy reveals that the bandgap decrease is a result of an upward shift of the valence band maximum. Simultaneously, time-resolved microwave conductivity measurements reveal an improvement of charge carrier transport by the incorporation of sulfur; the mobility increases by a factor of ~5. Photoconductivity action spectroscopy measurements confirm the photoactivity of the sulfur-incorporated BiVO4 up to 560 nm, well beyond the bandgap of typical BiVO4 films. In the second part of this talk, recent results on α-SnWO4 will be presented. α-SnWO4 is a novel photoanode candidate with a nearly ideal bandgap of 1.9 eV. Our study on pulsed laser deposited α-SnWO4 films reveals that the performance of the photoanode is limited by the mismatch between the charge carrier transport and optical absorption, as well as the oxidation of Sn2+ to Sn4+ at the surface of the material. The poor carrier transport can be overcome by a high-temperature treatment that decreases grain boundary recombination, and a hole-conducting NiOx protection layer deposited on the surface of α-SnWO4 successfully prevents the surface oxidation. As a result, the AM1.5 photocurrent is enhanced by almost two orders of magnitude to 0.75 mA/cm2 at 1.23 V vs. RHE, which is a new benchmark for α-SnWO4. Fundamental properties and future outlook on this promising material will be discussed.

Complex metal oxide photoanodes for solar water splitting

2019 Spring National Meeting

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