Controlled suppression or enhancement of the photoactivity of titanium dioxide (rutile) pigment

The variation in the quantum yield of a photoreaction of an organic adsorbate on an n-TiO/sub 2/ pigment particle is considered. The yield depends on the ratio of the rate of oxidation of the organic adsorbate by holes to the sum of the surface and bulk electron-hole recombination rates. It is shown that at low bulk-recombination rate the quantum efficiency is controlled by the surface density of electrons. This density is determined by the barrier height, i.e., the Fermi level of the particles. Reduction of the particles raises their Fermi level, correspondingly increases the height of the potential barriers that repel electrons from the surface, reduces the rate of surface recombination, and thereby increases the quantum yield. Correspondingly, oxidation of the particles lowers their Fermi level and thereby the quantum yield. In reduced particles, i.e., when the surface-recombination rate is low, bulk defects dominate the recombination process. In this case the photoactivity of the particles decreases upon ball milling under clean conditions and increases upon removal of the lattice defects through etching by boiling mineral acids. When combined, oxidation/reduction and mechanical damage/etching allow controlled variation of the photoactivity of the 0.2-..mu..m particles by 2 orders of magnitude.