Selective formation of nanoholes with (100)-face walls by photoetching of n-TiO2 (rutile) electrodes, accompanied by increases in water-oxidation photocurrent

Nanometer-sized rectangular holes or grooves in the (001) direction were formed when single-crystal rutile-type n-TiO2 (001), (110), and (100) electrodes were illuminated in 0.05 M H2SO4 under anodic bias, accompanied by increases in water-oxidation photocurrents. Interestingly, only the (100) face was selectively exposed at the walls of the photoproduced holes and grooves, irrespective of crystal faces of electrode surfaces, though the (110) face for rutile TiO2 is known to be the thermodynamically most stable, that is, to have the lowest surface Gibbs energy. On the other hand, no (100) face was exposed at the walls of rectangular holes produced by chemical etching with hot H2SO4 and hydrogen reduction at elevated temperatures, indicating that the selective exposition of the (100) face is characteristic of photoetching in H2SO4 A possible explanation is given by assuming that the selective exposition of the (100) face is due to a kinetics-controlled mechanism. It is tentatively suggested that the photoetching, which makes the surface structure changeable, may be much slower than the water photooxidation at the (100) surface, though not so much slower at other surfaces such as (110) and (001).