Photoadsorption and photocatalysis at rutile surfaces I. Photoadsorption of oxygen

A study has been made of the manner in which oxygen is adsorbed on the surface of titanium dioxide (rutile) under the influence of illumination in the near uv (λ > 300 nm). It has been shown that the presence of adsorbed water enhances the photoadsorption process, and that rutile which has been subjected to prolonged outgassing at high temperature followed by reoxidation in dry oxygen is inactive. The activity can be partially restored by exposing the dry specimen to water vapor. The kinetics of oxygen photoadsorption conform to a parabolic law and it is suggested that a surface-controlled diffusion process is the slow step. A mechanism involving OH− ions as traps for photo-holes produced by the illumination has been proposed as being the manner in which “adsorbed water” participates in the process, the photoelectrons remaining free to interact with weakly held molecular oxygen. An important observation is the occurrence of a “memory effect” by which the specimen recovers in the dark the activity it progressively loses during the primary illumination. If the specimen is left in the dark for sufficient time (>16 hr) the activity can be enhanced to a value greater than that of the original specimen. This effect is interpreted as being due to the formation, in the dark, of hole traps of a greater efficiency than the original OH− ions, and HO2− ions are considered to be the species responsible.