Visible light induced paramagnetic sites in nitrogen-doped TiO2 prepared by a mechanochemical method

The radical sites generated by the visible light irradiation of nitrogen-doped TiO2 particles prepared by a mechanochemical method have been studied using cw-EPR spectroscopy. The powders prepared by planetary milling of the anatase TiO2 with urea clearly show an absorption in the visible region, indicating doping of nitrogen. Visible light irradiation with wavelengths longer than 420nm induces a nitrogen-centred radical site. The radical site is assigned to the interstitial NO22- radical. In the presence of air, a reversible response of the growth and decay of paramagnetic sites was observed during and after photolysis. In contrast, in degassed samples, the decay of the NO22- signals is accompanied by the concomitant growth of the neutral NO radical site after irradiation, suggesting breaking of the N-O bond.