Flame-made WO3/TiO2 nanoparticles: Relation between surface acidity, structure and photocatalytic activity

WO3/TiO2 composite nanoparticles have been synthesized by dissolving W and Ti precursors in a suitable solvent and spraying into a high temperature acetylene-oxygen flame using a reactive atomizing gas. Particles with controlled W:Ti ratios were produced at various flow rates of precursor solution and the resulting powders were characterized by Brunauer–Emmett–Teller (BET) surface area analysis, X-ray diffraction (XRD), transmission electron microscopy (TEM), Raman and attenuated total reflection infrared (ATR-IR) spectroscopy. Two-dimensional coordinatively unsaturated wolframyl species were well dispersed on the TiO2 surface for the samples with equal to or less than 3.6 mol% WO3 and contributed to an increase of the surface acidity. Crystalline WO3 was formed for samples with >3.6 mol% WO3. Formation of crystalline WO3 is attributed to the enhanced rate of condensation of W species with increasing loading of tungsten. Variation of λ (defined as the ratio of the actual oxygen-to-fuel ratio of the reactants to the stoichiometric oxygen-to-fuel ratio) influences the residence time of the particles in the high temperature flame and affects the type of surface species and thereby the resultant acidity. The photocatalytic activity of the composite particles was tested for the degradation of methylene blue (MB) and was compared with that of commercial Degussa P25-TiO2. The improved photocatalytic activity of the composite particles is attributed to the increased surface acidity and better charge separation due to the coupling of WOx species and TiO2 within the composite nanoparticles.