Nitrogen-doped ZnO nanowire arrays for photoelectrochemical water splitting.

We report the rational synthesis of nitrogen-doped zinc oxide (ZnO:N) nanowire arrays, and their implementation as photoanodes in photoelectrochemical (PEC) cells for hydrogen generation from water splitting. Dense and vertically aligned ZnO nanowires were first prepared from a hydrothermal method, followed by annealing in ammonia to incorporate N as a dopant. Nanowires with a controlled N concentration (atomic ratio of N to Zn) up to approximately 4% were prepared by varying the annealing time. X-ray photoelectron spectroscopy studies confirm N substitution at O sites in ZnO nanowires up to approximately 4%. Incident-photon-to-current-efficiency measurements carried out on PEC cell with ZnO:N nanowire arrays as photoanodes demonstrate a significant increase of photoresponse in the visible region compared to undoped ZnO nanowires prepared at similar conditions. Mott-Schottky measurements on a representative 3.7% ZnO:N sample give a flat-band potential of -0.58 V, a carrier density of approximately 4.6 x 10(18) cm(-3), and a space-charge layer of approximately 22 nm. Upon illumination at a power density of 100 mW/cm(2) (AM 1.5), water splitting is observed in both ZnO and ZnO:N nanowires. In comparison to ZnO nanowires without N-doping, ZnO:N nanowires show an order of magnitude increase in photocurrent density with photo-to-hydrogen conversion efficiency of 0.15% at an applied potential of +0.5 V (versus Ag/AgCl). These results suggest substantial potential of metal oxide nanowire arrays with controlled doping in PEC water splitting applications.