Tuning the photocatalytic properties of sol-gel-derived single, coupled, and alloyed ZnO-TiO2 nanoparticles

Existing industrial titania production lines and their intermediate products are economically attractive precursors for synthesis of highly concentrated TiO2 coating sols and soluble nanopowders. Refluxing of aliphatic alcoholic TiOCl2 solutions (with up to 500g TiO2/L) allows straightforward formation of nonaggregated anatase nanocrystals with size between 5 and 8nm. Surface modification of these nanoparticles with organosilanes shifts the pH value of point of zero charge from initially 5.3 down to around 2.8. At appropriate Ti/Si ratios, complete suppression of the anatase ultraviolet (UV) photoactivity is possible. Furthermore, we address the optical and photocatalytic data of transparent nanocrystalline layers produced from superconcentrated ZnxTiyOz heterosols. Depending on the Zn/Ti ratio and processing temperature, colorless cubic c-ZnTiO3 and c-Zn2TiO4 spinel nanophases or coupled r-ZnTiO3-ilmenite/r-TiO2-rutile heterojunctions (Zn/Ti=2/3) can be produced. The cubic spinel films are indirect semiconductors showing band gap transition of around 3.7eV. In contrast, r-ZnTiO3-ilmenite films have slightly lower band gap of around 3.6eV. Organic dye photodegradation studies reveal the highest catalytic activity for the coupled r-ZnTiO3-ilmenite/r-TiO2-rutile systems compared with colored (nitridated) or colorless cubic spinel film samples.