No abstract

Nanometer-sized rectangular holes or grooves in the 〈001〉 direction were formed when single-crystal rutiletype n-TiO 2 (001), (110), and (100) electrodes were illuminated in 0.05 M H 2 SO 4 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 TiO 2 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 H 2 SO 4 and hydrogen reduction at elevated temperatures, indicating that the selective exposition of the (100) face is characteristic of photoetching in H 2 SO 4 . 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).

Nanocrystalline TiO2 (rutile and anatase) film electrodes usually show anodic photocurrents in aqueous electrolytes. Detailed photoelectrochemical studies have revealed that they also show cathodic photocurrents under particular conditions, e.g., in alkaline solutions containing dissolved oxygen under illumination from the TiO2-film side at short wavelengths such as 300 nm. The cathodic photocurrents appear in a potential region about 0.5∼0.9 V more positive than the flat-band potential (U fb) of single-crystal n-TiO2 electrodes. It is shown that the appearance of the cathodic photocurrents can be attributed to efficient electron transfer from the conduction band of TiO2 particles to chemically adsorbed oxygen molecules, the density of which is largely increased in alkaline solutions through a charge-transfer interaction between surface anionic groups such as Ti−O- as an electron donor and oxygen molecules as an electron acceptor. The dependences of the cathodic and anodic photocurrents on the illumination wavelength, the illumination direction, the electrode potential, and the crystal form of TiO2 particles are discussed in relation with the photocatalytic activity of particulate TiO2 films.