Effect of the TiO2 Crystallite Size, TiO2 Polymorph and Test Conditions on the Photo-Oxidation Rate of Aqueous Methylene Blue

This study systematically re-examines the titania-catalysed photo-oxidation of methylene blue (MB) in aqueous solution at 20 °C, placing particular emphasis on the effects of TiO2 crystallite size, TiO2 polymorph (anatase, brookite, rutile and combinations thereof) and experimental test conditions on the rate of MB photo-oxidation. For all TiO2 samples tested, the highest rate of MB photo-oxidation was observed at pH 6, slightly above the isoelectric point of TiO2 (~5.8 for P25 TiO2). Increasing the ionic strength at pH 6 induced MB dimer formation in solution, and lowered the rate of MB photo-oxidation by TiO2. For all TiO2 polymorphs, the surface area normalised rate increased with crystallite size reflecting the corresponding reduction in surface and bulk defects (electron–hole pair recombination sites). The optimum crystallite sizes were ~20–25 nm for anatase and ~50 nm for brookite. The photocatalytic activity of the different TiO2 powders followed the general order P25 > anatase > brookite ≫ rutile, with the high activity of P25 TiO2 providing strong evidence that anatase–rutile heterojunctions act as “hotspots” for MB photo-oxidation. Mixed phase anatase–rutile or brookite–rutile powders, each containing ~5 wt% rutile, demonstrated superior area normalized photocatalytic activities for MB photo-oxidation compared to pure phase anatase or brookite powders of comparable crystallite size. Finally, deposition of Pd, Pt or Au nanoparticles decreased the activity of P25 TiO2 for MB photo-oxidation. This paper clarifies long-standing confusion in the scientific literature about the photo-oxidation of aqueous MB over TiO2 and M/TiO2 (M = Pd, Pt and Au) photocatalysts.