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 °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 V_O^•• 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 E_g (Mo_i^{•••••}, Mo_i^••••, Mo_i^•••, and V_Ti^′′′′), any effects from them on the performance would be mitigated by the annihilation of the charge-compensating V_O^••. 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.

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