Highly dispersed phase of SnO2 on TiO2 nanoparticles synthesized by polyol-mediated route: Photocatalytic activity for hydrogen generation

TiO2–SnO2 mixed oxides (Ti:Sn = 98:2 (TS2), 95:5 (TS5) and 90:10 (TS10) by atomic weight) of large surface area and small particle size, in which SnO2 is in a dispersed phase on TiO2, have been synthesized by a polyol-mediated route. Characterization by various techniques has shown that a highly dispersed phase of SnO2 on anatase TiO2 is formed in TS2 sample. No separate discernible phases corresponding to cassiterite SnO2 or rutile TiO2 is seen in TS2 sample, whereas rutile TiO2 and SnO2 are observed besides the anatase phase of TiO2 in TS5 and TS10 samples. The average particle size of the mixed oxide samples is ∼20 nm. All samples absorb visible light and the onset of absorption was ∼425 nm. These mixed oxides show emission from defect levels arising due to the anion vacancies present in TiO2. The visible light absorption of these samples is attributed to the presence of defect levels in the bandgap of TiO2. Photocatalytic activity of these samples for hydrogen generation from water using methanol as sacrificial reagent was studied under sunlight type radiation. The results indicate that mixed oxides have better activity compared to pure TiO2 synthesized by the same method and the activity decreases with increasing SnO2 concentration in TiO2. The enhanced activity of TS2 sample is ascribed to the efficient charge separation from TiO2 to SnO2 owing to the high dispersion of SnO2 in TiO2. The decreased photocatalytic activity with increased SnO2 concentration is due to the aggregation of SnO2 on TiO2, which results in relatively poor dispersion of SnO2 and decreased charge transfer efficiency, but still maintains better photocatalytic activity compared to TiO2. In addition loading Pd co-catalyst produces a pronounced increase in the hydrogen yield due to the accumulation of electrons in the metal from the TiO2 and SnO2 semiconductors and the increased reductive power of the Pd loaded mixed oxide nanoparticles.