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.

Particulate TiO 2 film electrodes in aqueous electrolytes show photocurrent-potential curves similar to those for single-crystal n-TiO 2 electrodes, though the TiO 2 particles are insulating and simply deposited on conductive substrates without making any ohmic contact. The reason for such an apparently curious phenomenon has been investigated by examining the effect of changing the work function of the substrate (eφ w ). The onset potential of photocurrent (U on ) for particulate TiO 2 film electrodes remains nearly the same, irrespective of eφ w , indicating that a simple Schottky junction model cannot be applied to the TiO 2 /substrate contact. On the other hand, the deviation (∆U) of U on from the flat-band potential of single-crystal n-TiO 2 electrodes (U fb ) is nearly zero in the presence of ethanol, but large in the absence of it, especially in acidic solutions. The large ∆U implies that a photocurrent starts to flow by a positive shift of the electrode potential from U fb or, in other words, by formation of certain band inclination in some TiO 2 particles. Two plausible models, extended Schottky junction and Bardeen-type junction, are proposed for the electrical junction at the TiO 2 particle/substrate interface, and it is suggested that the Bardeen-type junction is more plausible, explaining all the experimental results.

Photoelectrochemical activity of nanocrystalline TiO2 film electrodes prepared by use of various TiO2 particles has been studied comparatively. It is confirmed that the photocurrent (or photoactivity) for anatase-type TiO2 is in general higher than that for the rutile-type. The photocurrent quantum yield (ηpc) for anatase-type TiO2 is nearly equal to that for rutile-type in long wavelengths but increases more and more with decreasing wavelength in contrast to the ηpc for rutile-type, and thus the ηpc for anatase-type becomes much higher than the ηpc for rutile-type in short wavelengths around 300 nm, indicating that the high photoactivity for anatase-type TiO2 is due to this high ηpc in short wavelengths. There are some discussions on the reasons for the high ηpc for anatase-type TiO2 in short wavelengths, including the possibility of contribution of hot electrons or holes in the TiO2 film.

Rutile-Type. -Five kinds of TiO 2 film electrodes are prepared by use of colloidal TiO 2 solutions. The photocurrent for anatase-type TiO 2 is in general higher than that for the rutile-type. The photocurrent quantum yield ηpc for anatase is nearly equal to that of rutile at long wavelength but increases more and more with decreasing wavelength in contrast to that of rutile. Thus, the ηpc for anatase becomes much higher than the ηpc for rutile at short wavelengths around 300 nm, indicating that the high photoactivity for anatase-type TiO 2 is due to this high ηpc at short wavelengths. -(SHIGA, A.; TSUJIKO, A.; YAE, S.; NAKATO, Y.; Bull. Chem. Soc. Jpn. 71 (1998) 9, 2119-2125; Dep. Chem., Fac. Eng. Sci.,