Effect of the Valence State in Co-Based Cocatalyst on the Photocatalytic Oxygen Evolution Reactivity of WO3

WO3 is one of the most promising photocatalysts with a suitable energy band structure for water oxidation; however, the quick recombination of photoinduced charge carriers seriously limits the photocatalytic efficiency. Loading of suitable cocatalysts is considered as an effective method to significantly improve the photocatalytic activity. Herein, we used a facile one-step hydrothermal method and a "hydrothermal + photodeposition" method to synthesize xCo3O4/WO3 (x = 0.2, 0.4, 0.8) and yCoOOH/WO3 (y = 2, 3, 4), respectively. We found that, in comparison with the pristine WO3, both xCo3O4/WO3 and yCoOOH/WO3 exhibited remarkably increased photocatalytic O2 evolution activities owing to the loading of two Co-based cocatalysts Co3O4 and CoOOH, respectively. Moreover, the optimal O2 evolution rate achieved over 0.4Co3O4/WO3 (194 μmol·h–1) was ∼2.1 times that over 3CoOOH/WO3 (94 μmol·h–1). The superior photocatalytic O2 evolution activity of Co3O4/WO3 over CoOOH/WO3 could be attributed to the mixed valences of Co2+ and Co3+ in Co3O4, which was more favorable than CoOOH with the sole valence of Co3+ for promoting the separation and transfer of photoinduced electrons and holes. This work suggests a promising way to the development of noble-metal-free cocatalyst by using transition-metal oxide with mixed valence states for efficient photocatalytic water oxidation.