Effects of Au Nanoparticles on TiO2 in the Photocatalytic Degradation of an Azo Dye

The photocatalytic activity of Au modified titanium dioxide was evaluated in the photodegradation of the azo dye Acid Red 1 (AR1) under 254 nm irradiation. Noble metal nanoparticles were deposited on TiO2 either through deposition-precipitation (DP), or by immobilisation of preformed metallic sols (polyvinylalcohol (PVA)/NaBH4 or tetrakis(hydroxymethyl)phosphonium chloride (THPC)/NaOH systems). Gold nanoparticles on the photocatalyst surface had dimensions of around 3-4 nm in diameter, as determined by HRTEM analysis, and exhibited visible light plasmon absorption. THPC Au/TiO2 appears to be the most photoactive amongst the photocatalysts with a 1 wt.% Au loading, while among THPC samples with different Au loadings (0.5-20.0 wt.%) the maximum photoactivity was attained with 5 wt.% Au/TiO2. The higher AR1 photodegradation rate observed on Au/TiO2 at basic pH can be related to the higher concentration of hydroxyl anions at the interface: these are able to effectively scavenge photoproduced valence band holes, possibly in competition with Au0 oxidation to Au+.