Formation of Surface Complex Leading to Efficient Visible Photocatalytic Activity and Improvement of Photostabilty of ZnO

Abstract

Phenolic compounds (PCs) such as 4-chlorophenol, 2,4-dichlorophenol, and phenol, can form a surface complex with ZnO. The surface complex was characterized with FTIR, PL, DTG, DRUV−vis, and photoelectrochemical measurement. The surface 4-CP/ZnO complex binds to ZnO through phenolate Zn−O−Ph−Cl linkage, which is much stronger than the phenolate Ti−O−Ph−Cl linkage of the surface 4-CP/TiO2 complex. The formation of the surface complex shifts the absorption response of ZnO from the UV to the visible region through ligand-to-metal charge transfer with excitation of visible photon, and induces efficient visible photocatalytic activity. ZnO exhibits 3.7 times higher photocatalytic activity for the photodegradation of 4-CP than TiO2 under visible irradiation. The much higher visible photocataltyic activity of ZnO than TiO2 is attributed to the higher efficiency of the charge transfer in the former than in the latter evidenced by the much larger photocurrent of PC/ZnO than PC/TiO2. Moreover, the formation of the surface complex results in the substantial improvement of the photostability of ZnO as it leads to a considerable decrease of the surface defect sites of ZnO, and the photogenerated holes trapped on the surface of ZnO probably prefer reacting with the surface complex to reacting with the surface oxygen atom under UV−visible irradiation.