Nanostructured Ti–M mixed-metal oxides: Toward a visible light-driven photocatalyst

In this report we investigate the structure–activity relationship in Ti–M (M = V, Mo, Nb, W) mixed-metal oxides with anatase structure used for the photoelimination of toluene under sunlight-type excitation. These systems were prepared by a microemulsion method, and their physicochemical properties were characterized by a multitechnique approach using X-ray diffraction–Rietveld, photoelectron spectroscopy, and Raman and UV–visible spectroscopy. The preparation method allowed the incorporation of up to around 20 at% of Mo, Nb, and W, whereas a significantly inferior solubility limit, below 5 at% was observed for V. The presence of nonpunctual defects, intimately related to the existence of MOx clustering, produced electronic mid-gap states involved in charge recombination and appeared to be the most negative factor influencing photoactivity. The maximization of photoactivity occurred for Ti–M samples with the highest doing level, together with a minimum structural disturbance of the anatase-type structure. The physicochemical bases for the photoactivity behavior of the Ti–M samples as function of the M content and, in particular, the presence of partial (for a specific Ti–M series) or global maxima are discussed.