Oxidative Power of Nitrogen-Doped TiO_2 Photocatalysts under Visible Illumination

Abstract

Nitrogen doping was recently shown to extend the absorptivity of TiO_2 photocatalysts into the visible. We find that N-doped TiO_2 materials fail, however, to catalyze the oxidation of HCOO^- into CO_2^(•-), or of NH_3OH^+ into NO_3^-, under visible illumination. By N-doping anatase at ambient or high temperature according to the literature we obtained yellow powders A and H, respectively, that absorb up to ∼520 nm. Aqueous H suspensions (pH ∼ 6, 1 atm O_2) photocatalyze the oxidation of HCOO^- into CO_2^(•-) radicals at λ ∼ 330 nm, but the quantum yield of CO_2^(•-) formation at λ > 400 nm remains below ∼2 × 10^(-5) and is probably zero. A is similarly inert toward HCOO^- in the visible region and, moreover, unstable in the UV range. Thus, the holes generated on N-doped TiO_2 by visible photons are unable to oxidize HCOO^- either by direct means or via intermediate species produced in the oxidation of water or the catalyst. Reports of the bleaching of methylene blue (MB) on N-doped TiO_2, which may proceed by direct oxidative or reductive photocatalytic pathways and also by indirect photocatalysis (i.e., induced by light absorbed by MB rather than by the catalyst) even under aerobic conditions are, therefore, rather uninformative about the title issue.