TiO2 Crystal Form-Dependence of the Au/TiO2 Plasmon Photocatalyst's Activity

The photocatalytic activities of Au nanoparticle-loaded anatase (Au/anatase) and rutile (Au/rutile) for green organic synthesis are compared under illumination of UV and visible light. Whereas Au/anatase shows a higher UV-light activity for the reduction of nitrobenzene than Au/rutile, the replacement of anatase by rutile greatly increases the visible-light activity of Au/TiO2 for the oxidation of alcohols to carbonyl compounds. The quantum efficiencies (molecules produced/incident photons) for the Au/rutile and Au/anatase systems for the selective oxidation of cinnamyl alcohol to cinnamaldehyde were calculated to be 1.4 X 10(-3) at lambda = 585 +/- 15 nm and 0.33 X 10(-3) at lambda = 555 +/- 15 nm, respectively. This superiority of rutile over anatase as the support of Au nanoparticle (NP) plasmon photocatalyst is also confirmed in the heterosupramolecular system consisting of Au/TiO2 and a cationic surfactant. In the system using Au/rutile, a quantum efficiency of 6.8 x 10(-3) at lambda = 585 +/- 15 nm has been achieved for the cinnamyl alcohol oxidation. Also, the plot of the visible-light activity versus Au particle size (d) for the Au/rutile system shows a volcano-shaped curve with a maximum at d approximate to 5 nm, while the activity of the Au/anatase system weakly depends on d. Photoelectrochemical measurements indicate that the Au/rutile system favors the localized surface plasmon resonance (LSPR) induced interfacial electron transfer from Au to TiO2, Further, intrinsic Fano analysis for the absorption spectra of Au/TiO2 suggests that the elongation of the LSPR lifetime with the Au NP loading on rutile is primarily responsible for the enhancement of the alcohol oxidation. We concluded that the optimum d value is determined by the factors of the LSPR absorption intensity, the interfacial electron transfer efficiency, and the surface area.