Structural and Microstructural Effects of Mo3+/Mo5+ Codoping on Properties and Photocatalytic Performance of Nanostructured TiO2 Thin Films

Mo3+/Mo5+ equally codoped TiO2 thin films (0.050-0.100 mol % individually) were fabricated by spin coating on polished fused silica glass substrates followed by annealing at 450 degrees C for 2 h. The highly transparent (>80%) films consisted of poorly crystallized single-crystal anatase, where interstitial solid solubility decreases the lattice parameters, with a reversed trend at the highest codoping level arising from supersaturation and Vo(center dot center dot) formation. The crystallinities correlate with the alternating trends of peak intensities and shifts in both the glancing angle X-ray diffraction and Raman data. X-ray photoelectron spectroscopy data reveal the absence of any valence changes, indicating that intervalence charge transfer is not relevant. Although there are four potential midgap states that may alter the Eg (Mo-i(center dot center dot center dot center dot center dot) Mo-i(center dot center dot center dot center dot), Mo-i(center dot center dot center dot), and V-Ti ''''), any effects from them on the performance would be mitigated by the annihilation of the charge-compensating V-O(center dot center dot). As the E-g values correlate with the lattice parameters, the principal driver of the E-g is the residual lattice compressive strain. However, the photocatalytic performance is principally a function of the crystallinity.