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

Abstract

Fe/Mo single doped and codoped TiO 2 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 Ti 4+ reduction, Mo-doping caused Ti 4+ oxidation, and codoping did not alter the Ti valence. The XPS data also showed that the precursor valences of Fe 3+ and Mo 5+ 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 Ti 4+ played no role, Fe (3−y)+ , Mo (5−x)+ , and Mo (4−x)+ all acted as traps for the majority charge carrier electrons.

Design strategies for ceria nanomaterials: untangling key mechanistic concepts

“…7,55 Intervalence charge transfer (IVCT), involving electron exchange between dopant and matrix ions or between codopant ions, and multivalence charge transfer (MVCT), involving electron exchange between multiple-dopant ions and matrix ions, both without defect formation otherwise, represent underappreciated means of explaining valence changes. [56][57][58][59][60][61][62][63][64][65][66][67][68][69] Three types of colour centres (F 0 , F + , F ++ ) have been suggested by modelling 70 and validated experimentally for pure CeO 2Àx . 71 While, the latter were generated at room temperature, high-energy irradiation often is required to create the Schottky pairs that are involved in the generation of such F-centres.…”

Switching on/off molybdenum nitride catalytic activity in ammonia synthesis through modulating metal–support interaction

“…42 This slight shift to higher binding energy (∼0.3 eV) has been attributed to charge redistribution between Mo and Ti species. 38,43 However, after the activation step, a further shift to higher binding energies of Ti 4+ is observed (Ti 2p 3/2 : 459.51 eV and Ti 2p 1/2 : 465.23 eV). In our nitridation conditions, the formation of titanium oxynitrides/nitrides or reduction of Ti 4+ to Ti 3+ can be safely ruled out as both outcomes are accompanied by a marked shift in Ti 2p positions to lower binding energy.…”

“…38 Furthermore, in 10-Mo-O/TiO 2 , the binding energies of Ti 2p 3/2 and Ti 2p 1/2 were found at 458.89 and 464.61 eV, which are slightly higher when compared with values reported in the literature which further confirms the strong interaction of Mo species with titanium species. 42,43 Upon nitridation, a range of oxidation states have been detected. In both catalysts, a high proportion of Mo species were found to be in the nitride form (>60 at%).…”