Preparation and photocatalytic activity of TiO2 nanocomposites coated with monolayer-protected gold clusters

We report the preparation and photocatalytic activity of TiO2 nanocomposites modified with monolayer-protected gold clusters (MPCs). Highly monodisperse tiopronin-coated MPCs with core diameters of 2.2 +/- 0.2 nm were pre-prepared and then anchored onto a TiO2 surface using the bifunctional tiopronin linker. In this method, the gold core size was preserved after calcination at 260 degrees C as well as the anchoring process, and the gold loading on TiO2 could be precisely controlled by the experimental condition. The photocatalytic degradation of Rhodamine 590 (Rh-590) was carried out with thus prepared MPC TiO2 composites. Mechanistic study of the photocatalytic reactions revealed that the degradation of Rh-590 occurs via the oxidative pathway by photogenerated holes. The photocatalytic activity of the MPC TiO2 composites was found to increase significantly upon calcination at 260 degrees C, whereas the size of gold particles remained at their initial size. The photocatalytic activity of the composites, however, drastically decreased when the composites were calcined at 400 degrees C. X-ray photoelectron spectroscopy analysis of the calcined composites was conducted to understand the vastly different calcination results. A significant amount of oxidized sulfur remained in the composites after calcination at 400 degrees C, which appears to be responsible for the drastic decrease in the photocatalytic activity.