Synthesis, structure, and photocatalytic properties of ordered mesoporous metal-doped Co3O4

Efficient and robust water oxidation catalysts based on earth-abundant elements are crucial for the viability of solar fuel production. Recently, cobalt oxide spinel catalysts have shown promising visible-light-driven water oxidation activities. Although some efforts have been made to understand the role of the spinel structure in photocatalytic oxygen evolution from water, the underlying correlation is still unclear. Here, we have successfully synthesized a series of metal-substituted Co3O4 catalysts using a hard templating method and investigated their photocatalytic properties for water oxidation. Findings from this study suggest that Ni3+/4+ and Mn3+/4+ substitution create surface sites which bind oxygen too strongly or too weakly relative to Co3+, respectively, therefore decreasing oxygen evolution rates in the visible-light-driven image-persulfate system. On the other hand, mesoporous Mg-substituted Co3O4 exhibit very limited oxygen evolution activity in the first 2 min of reaction; while a high turnover frequency (TOF) of ∼1.6 × 10−4 s−1 per Co was observed after 30 min of photocatalysis. Detailed structural analysis has been done to reveal the mechanism of Mg–Co3O4 activation and the relationship to structural properties in metal-substituted Co3O4 oxygen evolution catalysts.