Highly Dispersed Cobalt Oxide on TaON as Efficient Photoanodes for Long-Term Solar Water Splitting

Photoelectrochemical water splitting into H-2 and O-2 over a semiconductor-based photocatalyst offers a promising way to achieve the sustainable harvesting and storage of solar energy. However, short diffusion lengths and inefficient separation of the charge carriers in the semiconductors following light absorption result in fast recombination of holes and electrons and eventually poor performance. Herein, we address this problem by integrating an efficient and robust water oxidation catalyst, cobalt oxide (CoOx), into screen-printed TaON photoanodes premodified with TiO2 coatings for better stability. SEM, TEM, and ICP-MS analysis of the Co deposits and electrochemical techniques were used to demonstrate the advantages provided by the photoassisted CoOx electrodeposition method. Specifically, this method allows the selective and facile functionalization of the TiO2-TaON surface with a uniform layer of near-(hemi)spherical CoOx particles having a diameter of 5-15 nm. In comparison to the TiO2-TaON photoanodes, the optimized CoOx/TiO2-TaON configuration provides an enhancement in the photocurrent densities of up to 2 orders of magnitude and a substantial improvement in the long-term stability on testing in borate buffer solutions (pH 9.2). The highest oxidative photocurrent density of 0.7 mA cm(-2) was achieved with CoOx/TiO2-TaON under visible light irradiation (lambda >400 nm; 100 mW cm(-2)) at 1.2 V vs reversible hydrogen electrode, and the system remained stable for at least 24 h. The Co loading in the best-performing photoanode is ca. 0.1 wt % with respect to TaON; higher and lower loadings result in poorer photocatalytic activity and stability. Comparisons of the performance of CoOx/TiO2-TaON with other representative inorganic water photoelectrooxidation systems are provided and discussed.