Crystal facet-dependent photocatalytic oxidation and reduction reactivity of monoclinic WO3 for solar energy conversion

The reactivity of a photocatalyst is basically influenced by its surface atomic and linked electronic structure. Tuning different crystal facets is becoming an important strategy to optimize the reactivity of a photocatalyst for targeted reactions. Here we report a facile and new route of synthesizing a quasi-cubic-like WO3 crystal with a nearly equal percentage of {002}, {200} and {020} facets, and a rectangular sheet-like WO3 crystal with predominant {002} facet by controlling acidic hydrolysis of crystalline WB. As a result of electronic structure effects induced by crystal facet, the quasi-cubic-like WO3 crystal with a deeper valence band maximum shows a much higher O-2 evolution rate in photocatalytic water oxidation than the rectangular sheet-like WO3 crystal. The latter, with an elevated conduction band minimum of 0.3 eV, is able to photoreduce CO2 to generate CH4 in the presence of H2O vapor.