Intervalence charge transfer and thermodynamic effects on the photocatalytic performance of Fe/Mo single and codoped TiO2 thin films

Fe/Mo single doped and codoped TiO2 thin films were spin coated on polished fused silica substrates and annealed in air at 450 °C for 2 h. The XPS data for the anatase thin films were distinctive in showing that Fe-doping caused Ti4+ reduction, Mo-doping caused Ti4+ oxidation, and codoping did not alter the Ti valence. The XPS data also showed that the precursor valences of Fe3+ and Mo5+ were reduced upon annealing. Analysis of the potential roles of thermodynamics and intervalence charge transfer (IVCT) demonstrates that the latter drives the equilibria both in solution and during annealing. Photocatalytic performance testing indicated that Fe-doping was slightly deleterious, Mo doping had a consistent positive effect, and codoping exhibited a clear negative trend as a function of doping concentration. These results are interpreted in terms of the effect of IVCT on the matrix valence, dopant valences, and charge carrier trapping. Fe doping exhibited reduced performance because both matrix Ti(4−x)+ and dopant Fe(3−y)+ acted as electron traps. Mo doping exhibited enhanced performance because the matrix Ti(4+x)+ acted as a hole trap and the dopant Mo(5−x)+ and Mo(4−x)+ acted as electron traps, thereby promoting charge separation. Codoping exhibited a clearly detected negative trend on the performance because, while Ti4+ played no role, Fe(3−y)+, Mo(5−x)+, and Mo(4−x)+ all acted as traps for the majority charge carrier electrons.