Cobalt phosphate modified TiO2 nanowire arrays as co-catalysts for solar water splitting

Abstract

Cobalt phosphate (Co–Pi) is photo-electrodeposited on TiO₂ nanowire arrays in Co²⁺ containing phosphate buffer. The resulting composite photoanode shows a generally enhanced photocurrent near the flat band potential region, and represents a 2.3 times improved photoconversion efficiency compared to that of pristine TiO₂ in a neutral electrolyte. A negative effect on the photocurrent generation is also observed when loading TiO₂ with a relatively thick Co–Pi layer, which is demonstrated to be due to the poor photohole transfer kinetics in the Co–Pi layer. Moreover, we find that Co–Pi can facilitate the photoelectrochemical performance of TiO₂ over a wide pH range from 1–14. This improved activity is studied in detail by optical and electrochemical analyses. It is suggested that the mechanism of the overpotential-demanding water oxidation reaction is changed to a facile pathway by the Co-based electrocatalyst. At the same time, the more significant band bending is induced by the Co–Pi catalyst decreasing the charge recombination. This work provides a feasible route to reduce the external power needed to drive water splitting by coupling an electrocatalyst with a photocatalyst, as well as mechanistic insights important for other Co–Pi modified photoelectrodes for solar-driven water splitting.