Stabilization of GaAs photoanodes by in situ deposition of nickel-borate surface catalysts as hole trapping sites

Although semiconducting gallium arsenide (GaAs) possesses an ideal band gap for efficient solar-driven fuel synthesis, it is extremely unstable in aqueous media, undergoing facile photocorrosion and therefore is seldom used. We have addressed this stability problem to some extent using a strategy of introducing a Ni–B surface catalyst onto p/n junction GaAs by in situ photoassisted electrodeposition. A monolithic layer of Ni–B/Ga(As)Ox was generated during the Ni–B deposition process, resulting in a Ni–B/Ga(As)Ox/GaAs photoanode structure. Such a structure was optimized by varying the GaAs surface architecture, electrolyte pH value and Ni–B deposition time to achieve optimal photoelectrochemical performance, together with improved stability. The optimized photoanode Ni–B/Ga(As)Ox/shallow GaAs with 0.5 h Ni–B deposition time (∼900 nm thickness of the Ni–B/Ga(As)Ox layer) exhibited a very high photocurrent, leading to a nearly 22 hour stable photocurrent density of 20 mA cm−2, while bare GaAs exhibits 60% photocurrent loss after three hours under continuous one sun illumination (100 mW cm−2) in alkaline media (pH = 14). This remarkable performance in both photocurrent and stability directly addresses the current severe limitations in the application of GaAs photoanodes for solar fuel synthesis, and they may be applicable to other unstable photoelectrodes.