Al-doped TiO2 pigments:: influence of doping on the photocatalytic degradation of alkyd resins

Rutile white pigments are generally doped with Al2O3 to suppress their photocatalytic activity, the mechanism of this effect being however obscure. In this investigation, rutile microcrystals were used that had been doped with increasing amounts of Al2O3. Parts of the dopant were homogeneously dissolved in the crystal while the rests had segregated to the particle surface. To study the effects of dissolved Al2O3 alone, the Al2O3 at the surface could be removed by etching Photocatalytic activities of the pigments with and without surface-Al2O3 were determined by artificial weathering of pigmented alkyd paint films and a kinetic analysis in terms of Weibull statistics of failure. Results were interpreted using basic models of photosemiconductors. It is found that the reduction of photocatalytic activities by Al2O3 in the bulk is more important than at the surface although its amounts in the bulk are smaller. Bulk-Al2O3 dopant states provide efficient recombination sites for the electrical charges photogenerated within the TiO2 crystal. However, among the two modes of incorporation of Al2O3 into rutile only the one providing oxygen vacancies is effective. The slight activity reduction by surface-Al2O3 coincides with the reduction of surface hydroxyl concentration measured with these pigments. (C) 2001 Elsevier Science B.V. Ail rights reserved.