Surface hydrophilic–hydrophobic property on transparent mesoporous silica thin films containing chromium oxide single-site photocatalyst

Using a sol–gel/spin-coating method with organic template, the transparent mesoporous silica thin films containing chromium oxide moieties can be prepared on quartz plate. The spectroscopic characterization has revealed that these thin films contain isolated and tetrahedrally coordinated chromium oxide moieties (single-site photocatalyst) in their frameworks. Even before UV light irradiation these Cr-containing mesoporous silica (CrMS) thin films have demonstrated a hydrophilic surface property more strongly than Cr-containing non-porous silica films and mesoporous silica thin films without chromium oxide. Furthermore, the Cr-containing mesoporous silica thin films showed the super-hydrophilic property not only under UV light irradiation but also even under visible light irradiation from a fluorescent lamp. In the presence of ethylene gas, the polyethylene was successfully formed on the surface of the CrMS thin film under UV light irradiation. After the formation of polyethylene the surface property of the CrMS thin film was converted into hydrophobic while keeping its transparency. The isolated and tetrahedrally coordinated chromium oxide moieties are responsible for these photo-induced surface reactions.