TiO2 Nanoparticle Photocatalysts Modified with Monolayer-Protected Gold Clusters

TiO2 nanoparticle photocatalysts modified with uniform gold nanoparticles are prepared by anchoring bifunctional glutathione-coated monolayer-protected gold clusters (MPCs) onto TiO2. In this method, the MPC loading on TiO2 can be precisely controlled in the range of 1−5 wt % without incurring size change upon anchoring. The photocatalytic degradation of Uniblue A (UBA) is carried out with thus prepared MPC−TiO2 composites, which shows, however, no enhancement upon MPC anchoring. The MPC−TiO2 composites are thermally treated to activate the catalytic activity. When the MPC−TiO2 composites are calcined at 250 °C, the glutathione ligand on gold surface is partially removed and the photocatalytic activity of the composites significantly increases, highlighting the role of gold in the photocatalytic reactions. However, when the calcination temperature is raised to 400 °C, the photocatalytic activity of the composites drastically decreased. X-ray photoelectron spectroscopy analysis of the calcined composites reveals that significant amount of oxidized sulfur remains after calcination that appears to act as a recombination center for the photogenerated electrons and holes, resulting in a drastic decrease in the photocatalytic activity. These results emphasize the important role of ligands in the use of MPCs in photocatalysis.