Low temperature synthesis, characterization and substrate-dependent photocatalytic activity of nanocrystalline TiO2 with tailor-made rutile to anatase ratio

Abstract

TiO2 photocatalysts with predetermined phase composition (anatase-to-rutile ratio) have been prepared from strongly acidic aqueous HCl/TiCl4 mixtures with 0.1 M ≤ [HCl]TOT ≤ 5 M and 0.05 M ≤ [TiCl4]TOT ≤ 3 M (where the subscript TOT denotes total or analytical concentration). During the syntheses, TiCl4 was added to a cold-HCl solution. The initially precipitate free reaction mixture was boiled, resulting in the formation of large amount of solid, nanocrystalline (particle size 4–12 nm) TiO2 particles with large specific surface area (up to ∼240 m2/g). We found from the XRD patterns that at constant [HCl]TOT, the anatase content of the crystallized solid systematically increases with decreasing [TiCl4]TOT, while at constant [TiCl4]TOT, the rutile content was found to systematically increase with increasing [HCl]TOT. Thus the phase composition of the photocatalysts prepared was possible to be fine-tuned with the aid of these two simple initial parameters. The photocatalytic activity of the as-prepared catalysts was found to strongly correlate with the phase composition. In phenol decomposition the samples’ photocatalytic performance gradually and significantly increases with their anatase content: photocatalysts containing only anatase as crystalline phase were up to three times more efficient than rutile ones. Interestingly, in salicylic acid decomposition, rutile-only catalysts were found to show no activity at all, but some of our catalysts (both anatase-only and rutile–anatase mixtures) at pH ∼3 (but not at pH ∼7) displayed photocatalytic activity commensurable to that of Degussa P25. This can be explained in terms of the efficient surface chemisorption of salicylate ion on large specific surface area TiO2 particles in acidic solutions.