Enhanced photocatalytic performance of nanostructured TiO2 thin films through combined effects of polymer conjugation and Mo-doping

Mo-doped TiO2 [≤ 0.20 wt% Mo; ≤ 0.10 mol% (metal basis)] with conjugated polyvinyl alcohol (TiO2/C-PVA) composite thin films was prepared by sol–gel dip coating on polished fused SiO2 substrates, followed by annealing at 180 °C for 4 h. These conditions were sufficient for solid solubility, despite the unusually low annealing temperature. The annealed thin films consisted of homogeneously distributed individual and slightly agglomerated anatase grains in a continuous C-PVA matrix characterized by the carbon double bond formed upon conjugation. The films exhibited drying shrinkage cracks, which increased consistently in extent with increasing Mo-doping concentration, effectively increasing the number of exposed TiO2 particles. Mo addition enhanced anatase nucleation, recrystallization, and growth at lower doping concentrations (up to ≤ 0.10 wt%), thereby increasing crystallinity. However, increasing doping levels (> 0.10 wt%) appeared to exceed the solubility limit, resulting in supersaturation and significant lattice destabilization. Mo-doping also caused the Ti2p XPS peaks to shift to lower binding energies and the Mo3d peaks to shift to higher binding energies. These data are consistent with thermodynamically unstable Ti4+ → Ti3+ conversion and thermodynamically stable Mo5+ → Mo6+ conversion, which are interpreted in terms of intervalence charge transfer (IVCT), in which charge compensation is achieved through majority Ti4+ → Ti3+ reduction plus Mo5+ → Mo6+ oxidation. Ti3+ concentration also reflects a direct correlation with the Mo-doping concentration and resultant IVCT within the Mo solubility limit and a reverse effect upon supersaturation. There is a correlation with the Eg but this can be attributed to recrystallization rather than a semiconducting effect. No effect of midgap state formation from enhancement of the $$ {\text{V}}_{\text{O}} ^{\cdot \cdot} $$ concentration is expected because IVCT is a redox effect only and dissolution of Mo5+ or Mo6+ would generate Ti vacancies. The methylene blue dye degradation data exhibited the same trend but at a significant level (90.6% degradation), thus indicating that the mechanism dominating the photocatalytic performance is the recrystallization of the anatase and/or the modification of the semiconducting properties induced by Mo-doping, as indicated by the trends in band gap.