The effect of crystallinity on photocatalytic performance of Co3O4 water-splitting cocatalystsElectronic supplementary information (ESI) available: Additional XRD, XPS, EIS data. See DOI: 10.1039/c5cp07589k

Cocatalysts, when loaded onto a water splitting photocatalyst, accelerate the gas evolution reaction and improve the efficiency of the photocatalyst. In this paper, we report that the efficiency of the photocatalyst is enhanced using an amorphous cobalt oxide cocatalyst. The WO 3 film, when loaded with amorphous or nanocrystalline Co 3 O 4 , shows an improvement of up to 40% in photocurrent generation and 34% in hydrogen gas evolution. The effect of cocatalyst crystallinity on performance was systematically studied, and we found that the photocurrent deteriorates with the conversion of the cocatalyst to a highly crystalline phase at an annealing temperature of 500 °C. The mechanism of this effect was studied in detail using electrochemical impedance spectroscopy, and the enhancement effect produced by the amorphous cocatalyst is attributed to the large density of unsaturated catalytically active sites in the amorphous material. Amorphous/nano-crystalline Co 3 O 4 cocatalysts incorporated into WO 3 photoanodes show vastly superior performance compared to their highly-crystalline counterparts.