Nitrogen-doped and nitrogen–fluorine-codoped titanium dioxide. Nature and concentration of the photoactive species and their role in determining the photocatalytic activity under visible light

Two series of N-doped and N, F-codoped titanium dioxide samples having different loading of impurity were prepared using sol–gel synthesis and successive calcinations in air. Ammonium chloride and ammonium fluoride were used as source of dopants in the two cases. In all cases the insertion of nitrogen is observed which determines the optical absorption in the visible of the materials and their photocatalytic activity under solar light. This was investigated following the decomposition of methylene blue in aqueous phase. Two photoactive species (describable in terms of NO2− and NO3− bulk species) determine the optical absorption in the visible range. The former one (paramagnetic) is detectable by Electron Paramagnetic Resonance (EPR) and its intensity in all samples is roughly proportional to the optical absorption at 440 nm. The photocatalytic activity under solar light directly depends on the concentration of the nitrogen photoactive species in the solid till a saturation limit, which in turn, depends on the concentration of ammonium ions in the starting solution and on the heating rate during calcinations. The use of ammonium fluoride in the synthesis promotes N insertion and increments the photocatalytic activity due to the fact that, differently from Cl− ions, F− ions substitutes oxygen in the solid lattice giving rise to electronic effects which favour N photoactive species incorporation in the solid.