Observation of Cathodic Photocurrents at Nanocrystalline TiO₂ Film Electrodes, Caused by Enhanced Oxygen Reduction in Alkaline Solutions

Abstract

Nanocrystalline TiO 2 (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 TiO 2 -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-TiO 2 electrodes. It is shown that the appearance of the cathodic photocurrents can be attributed to efficient electron transfer from the conduction band of TiO 2 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 TiO 2 particles are discussed in relation with the photocatalytic activity of particulate TiO 2 films.