Synthesis of V- and Mo-doped/codoped TiO₂ powders for photocatalytic degradation of methylene blue

V-doped, Mo-doped, and codoped anatase powders have been fabricated by hydrothermal synthesis at 150 °C for 20 h, followed by calcining at 500 °C for 4 h in air. The raw materials were titanium tetra-isopropoxide, VCl₃, and MoCl₅ and the doping levels were in the relatively low range of 0.02–0.08 mol% (metal basis). The samples were characterized by XRD, SEM, FESEM, BET, UV–Vis spectrophotometry, XPS, and MB degradation by UV–Vis and simulated-solar light spectrophotometry. All of the powders consisted of anatase only and the crystallinities decreased for all dopant levels. For all samples as a function of dopant level, the grain sizes were similar and in the range ∼1–5μm, with some dense agglomerates of size up to ∼ 20μm, although individual grains consisted of highly agglomerated nanoscale particles. For the doped samples, the crystallite sizes generally decreased, the surface areas were consistent, and the band gaps decreased for V doping but increased for Mo doping. The codoped samples appeared to be dominated by the V doping as they exhibited similar trends. These trends were interpreted largely in terms of the effects of irregular, clustered, or homogeneous dopant distribution on the nucleation and grain growth. The XPS data revealed the presence of V5+, Mo4+, Mo5+, Ti3+, and Ti4+. The presence of these dopants and their variable valence states was interpreted in terms of intervalence charge transfer (IVCT) and multivalence charge transfer (MVCT) and the associated introduction of midgap states, which represent key parameters in the semiconducting behavior of these doped materials. The irregular trends in photocatalytic performance as a function of dopant level and an increasing trend for the codoped samples revealed the competition between the positive effects of decreasing crystallite size and increasing role of midgap states versus the negative effects of decreasing crystallinity for the doped samples and the dominance of the former two factors for the codoped samples.