The formation of nano-sized selenium-titanium dioxide composite semiconductors by photocatalysis

Nano-sized selenium (Se) particles were deposited onto titanium dioxide (TiO2) by the photocatalytic reduction of selenate (Se(VI)) and selenite (Se(IV)) ions. Se particles deposition on TiO2 was only observed in the presence of formic acid, which acted as the organic hole scavenger. The Se particles formed were crystalline. Se particles of different size could be formed onto the TiO2 particles by manipulating experimental parameters such as pH and the Se precursor used. When Se(VI) ions were used as the precursor, the Se particles formed on TiO2 were found to be spherical in shape, up to 6 times bigger than the TiO2 particles (up to 145 nm) and discretely formed on the TiO2 particles. The growth and sphericity of the Se particles were explained in terms of electron transfer across the p-n junctions formed by the p-type Se and n-type TiO2 semiconductors under illumination and the adsorption of the Se(VI) ions. The size of the Se particles were found to be dependent on the amount of Se(VI) photoreduced. When Se(IV) ions were used as the precursor for Se particles formation, the particles formed were much smaller than that of TiO2 crystals (less than 25 nm) and also more evenly dispersed on the TiO2 particles.