Marked promotive effect of iron on visible-light-induced photocatalytic activities of nitrogen- and silicon-codoped titanias

Nanocrystalline Si-doped titanias synthesized by the thermal reaction of titanium tetraisopropoxide and tetraethyl orthosilicate (glycothermal method) and three TiO2 without Si modification, i.e., TiO2 prepared by the glycothermal method, commercially available TiO2, P-25, and ST-01, were nitrified by heating in an NH3 flow at high temperatures followed by annealing in air. Onto these samples, small amounts of Fe were loaded, and their photocatalytic activities for decomposition of acetaldehyde under visible-light irradiation were investigated. The Fe addition to non-nitrified samples gave no visible-light-induced photocatalytic activities. On the contrary, marked improvements were attained for the nitrified catalysts: the addition of small amounts of Fe (Fe/Ti of 0.005-0.7) to N- and Si-codoped titanias caused more than 10 times enhancement of the visible-light-induced photocatalytic activity. The electron spin resonance (ESR) studies of Fe-loaded N- and Si-codoped titanias suggested that Fe(III) species loaded on the nitrified samples accepted the photoexcited electrons and then transferred them to oxygen molecules. These processes facilitate electron-hole charge separation thus suppressing the recombination of the photogenerated electrons and holes and consequently contributed to the improvement in the photocatalytic activity.