Preparation Characterization and Photocatalytic Activity of Polycrystalline ZnO/TiO2 Systems. 2. Surface, Bulk Characterization, and 4-Nitrophenol Photodegradation in Liquid−Solid Regime

Electron spin resonance spectroscopy (ESR), Fourier transform infrared spectroscopy (FTIR), and monitoring of pyridine (py) and boric acid trimethyl ester (BATE) adsorption for determining surface acidity and basicity, respectively, were used to carry out further characterization of mixed ZnO/TiO2 polycrystalline solids prepared by different methods. Moreover, the powders were tested in a batch photoreactor for a probe reaction, i.e., 4-nitrophenol photodegradation in aqueous medium. ESR results indicated the presence of signals attributable to Zn+ species in ZnO/TiO2 (anatase) solids, while in ZnO/TiO2 (rutile) samples the presence of zinc induced only the formation of signals probably due to Ti3+ centers. FT-IR spectra showed no significant differences of the surface hydroxylation degree of the various photocatalysts, whereas the surface acidic properties generally decrease by increasing the amount of ZnO. Coupling of ZnO and TiO2 semiconducting powders was not so beneficial, as expected on the basis of the intrinsic electronic properties, to enhance the photoreactivity for the studied reaction, although some of the powders showed photoactivities slightly higher than those of bare TiO2 and ZnO. Nevertheless, some of the samples (the mixed particles ZnO/TiO2 (rutile)) appear promising from an application point of view because no filtration was needed after the occurrence of the photoreactivity tests to separate them from the solution because they decanted easily. The mineralization of 4-nitrophenol was checked by determining the total organic carbon (TOC) and a complete photooxidation occurred in few hours in the presence of some of the samples.