Photoelectrochemical study of nitrogen-doped titanium dioxide for water oxidation

This paper describes the photoelectrochemical response in aqueous electrolyte of nitrogen-doped titanium dioxide, TiO2-xNx. Thin film electrodes were prepared by reactive DC magnetron sputtering in an environment of Ar, O-2, and N-2. A typical film thickness was 0.85 mum. The crystal structure of the photoelectrochemically active films was mainly of rutile character, and scanning and transmission electron microscopy showed a highly porous parallel penniform nanostructure. It was conclusively shown that dioxygen could be generated from water by illumination of the TiO2-xNx electrodes at moderate anodic potentials. The current density under 1000 W m(-2) visible light from a sulfur lamp was 0.2 mA cm(-2) at 0.55 V vs Ag/AgCl. Current-voltage characteristics under illumination were strongly dependent on the scan direction. Scanning the electrode from cathodic toward anodic potentials gave an onset potential similar to that of normal rutile TiO2, whereas a reversed scan gave an onset of photocurrent (depending on the light source) anodically upshifted by up to 0.8 V from its normal position. Moreover, a cathodic current was observed during the latter scans. This current was induced by the illumination at anodic potentials. This nonfaradic current was ascribed to photoinduced electron trap states distributed in an approximately 1.3 V wide range negative of the conduction band (CB) edge. These states also were active as electron-hole recombination centers. The density of this new set of states was similar to2 x 10(20) cm(-3), i.e., similar to the density of nitrogen atoms. They can be activated by light, even at wavelengths beyond 700 nm, and work as long-lived electron traps; hence, they have properties that are different from those of the earlier found Till (3d) states, also located below the CB of TiO2. The new states occur as a consequence of the nitrogen doping, but is not necessarily an intrinsic property of pure TiO2-xNx. Recombination via the new states-in conjunction with slow hole transport in the nitrogen-created band above the valence band edge-was suggested to be the cause of the large anodic shift of the onset potential for cathodic scans and of the moderate water oxidation efficiency of the TiO2-xNx thin film electrodes.