Photocatalysis of Dye-Sensitized TiO₂ Nanoparticles with Thin Overcoat of Al₂O₃: Enhanced Activity for H₂ Production and Dechlorination of CCl₄

Dye-sensitized TiO 2 nanoparticles that were loaded simultaneously with Pt and Al 2 O 3 overlayer (Al 2 O 3 /TiO 2 /Pt) were synthesized and investigated for photocatalytic activity under visible light. Introducing a thin Al 2 O 3 overlayer (∼1 nm thick) on dye-sensitized TiO 2 markedly enhanced the visible light activities for the production of hydrogen (in the presence of EDTA as an electron donor) and the dechlorination of CCl 4 . The Al 2 O 3 /TiO 2 /Pt powder was characterized by HRTEM, EDX, and XPS. In agreement with the photocatalytic activity data, the photocurrent collected via electron shuttles on a Pt electrode immersed in an aqueous photocatalyst suspension under visible light was also enhanced in the presence of an Al 2 O 3 overlayer, which indicates an enhanced interfacial electron transfer despite the presence of an insulating surface layer. The initial H 2 and chloride generation rate increased from 0.4 and 5 μM min −1 on TiO 2 /Pt to 0.9 and 7.5 μM min −1 on Al 2 O 3 /TiO 2 /Pt, respectively. The visible light activity of the sensitized photocatalytic reactions highly depended on the thickness of the alumina layer and was optimized at a low level of Al loading (Al/Ti atom ratio ∼ 0.01), above which the activity was markedly reduced with thickening of the layer. It is suggested that the alumina layer retards the charge recombination between the electron injected from the excited dye and the oxidized dye. The slower charge recombination in the presence of an alumina overlayer was confirmed by time-resolved diffuse reflectance (TDR) spectroscopy.