Bandgap Engineering

TiO2 is of great interest in the field of heterogeneous photocatalysis. TiO2 has the advantage of being cheap, nontoxic, and stable, all of which make it attractive for remediation of environmental organic pollutants. However, its wide bandgap (3.2 eV) means that is can only utilize just ~5% of the solar spectrum, all in the UV region. If this threshold energy could be reduced, visible light could then be used, opening up a much larger portion of the solar spectrum for potential photocatalytic work.
A large number of approaches have been taken to reduce the bandgap energy of TiO2, such as doping with transition metal cations, creating oxygen vacancies, or, most recently, doping with anions such as C, S, and N. The first report of N-TiO2 was by Asahi et al. in 2001, in which they bleached methylene blue (MB).
It was originally believed that mixing of nitrogen 2p states with lattice oxygen 2p states led to an overall reduction in the bandgap energy. However, more recent studies have shown both theoretically and experimentally that either substitutionally or interstitially bound nitrogen species result in localized N 2p states above the valence band.