Series of Sc/V co-doped rutile TiO2 with different Sc/V ratio was synthesized. Samples were characterized by XRD, SEM, XPS, BET, EPR, diffuse reflectance spectroscopy and Kelvin probe methods. EPR spectroscopy reveals a simultaneous increase of V4+ and Ti3+ as vanadium content grows. At the same time, an increase of vanadium concentration in co-doped samples results in stronger absorption in visible light range. However, a photocatalytic activity dependence on the co-dopant ratio demonstrates “volcano” plot behavior with maximum at 75/25 Sc/V ratio, while the work function dependence on Sc/V ratio demonstrates a negative correlation with photocatalytic activity resulting in a minimal value of work function at the same optimal ratio of co-dopant content. The analysis of the experimental results infers that alteration of Sc/V co-doping ratio leads to redistribution between shallow traps, which are not effective in charge carrier recombination, and deep traps, which act as effective recombination centers, with maximal shallow traps concentration corresponding to the optimal Sc/V ratio equal to 75/25, yielding the lowest recombination efficiency and therefore, the highest photocatalytic activity. Redistribution of defect states induced by co-doping should be distinguished as a primary factor of alteration of photocatalytic activity in co-doped TiO2. Presented results demonstrate that photoactivity of co-doped titania cannot be considered as result of either independent action of dopants or their additive effect.
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