Abstract
This paper systematically compares the activity of Au/TiO2 photocatalysts (Au loadings 0-10 wt.%) for H-2 production from ethanol-water mixtures under UV excitation. Degussa P25 TiO2 was used as the support phase. TEM analyses revealed that the average Au nanoparticle size at all loadings was 5 +/- 2 nm, with the Au nanoparticles preferentially located at the interfacial sites between TiO2 crystallites. XRD, XRF, XPS, and UV-Vis measurements established that metallic Au was the only gold species on the surface of the photocatalysts. The Au/TiO2 photocatalysts showed an intense absorption maximum centred around 560-570 nm due to the localised surface plasmon resonance (LSPR) of the supported gold nanoparticles: Photoluminescence measurements revealed that gold nanoparticles effectively suppress electron-hole pair recombination in TiO2, even at low Au loadings. All of the Au/TiO2 photocatalysts displayed high activity for H-2 production from ethanol-water mixtures under UV irradiation, with the highest activities observed in the Au loading range 0.5-2 wt.% (H-2 production rate 31-34 mmol g(-1) h(-1)). In order to deconvolute the role of the P25 TiO2 support in promoting H-2 production, anatase and rutile nanoparticles were isolated from P25 TiO2 by selective chemical dissolution and then functionalised with gold nanoparticles (3 wt.% loading, size 5 +/- 2 nm). The H-2 production activity of the resulting Au/anatase and Au/rutile photcatalysts was 22 and 10 mmol g(-1) h(-1), respectively, and substantially lower than the corresponding Au/P25 TiO2 photocatalyst (32 mmol g(-1) h(-1)). The data provide strong evidence that synergistic electron transfer between the TiO2 polymorphs and supported Au nanoparticles is responsible for the high rates of H-2 production observed in the Au/P25 TiO2 system. The interface between anatase and rutile crystallites, where gold nanoparticles preferentially deposit, is identified as a photocatalytic 'hot spot' for H-2 production. High Au loadings reduce the efficiency of such 'hot spots'. (C) 2013 Elsevier Inc. All rights reserved.