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

Abstract

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/TiO 2 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/TiO 2 (anatase) solids, while in ZnO/TiO 2 (rutile) samples the presence of zinc induced only the formation of signals probably due to Ti 3+ 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 TiO 2 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 TiO 2 and ZnO. Nevertheless, some of the samples (the mixed particles ZnO/TiO 2 (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.

Modeling with Hybrid Density Functional Theory the Electronic Band Alignment at the Zinc Oxide–Anatase Interface

“…These semiconductors have very 4 similar band gaps; at temperatures close to 0 K the optical gaps are 3.42 eV for anatase 14 and 3.44 eV for ZnO 15 , while at ambient temperatures they are respectively close to 3.2 and 3.3 eV. Several works in the literature have studied photocatalysts constituted by anatase-ZnO composites and in several of them photoactivity improvements in comparison with the single phases have been reported 16,17,18,19,20,21,22 .…”

“…However, there is disagreement concerning whether the bands of anatase would lie at higher or lower energy than those of ZnO 16,17,23,24,25,26 , the arguments on this subject being usually based on data about the work function or flatband potential of the individual oxides. Although there are some measurements of the band alignment at the ZnO-rutile interface 27 , to this author's knowledge no experimental or theoretical work has been done which allows determining the band alignment between ZnO and anatase in a situation in which they are in direct contact.…”

Atomic layer deposition of TiO2-nanomembrane-based photocatalysts with enhanced performance

“…8,9,13 It is believed that the coupling of anatase and rutile TiO 2 with ZnO can achieve a more efficient electronhole pair separation under illumination and, consequently, a higher reaction rate. 14,15 Specifically, the electronic properties modification of the composite materials is invoked to explain this behavior: the electron transfer from the conduction band of ZnO to the conduction band of TiO 2 under illumination and, conversely, the hole transfer from the valence band of TiO 2 to the valence band of ZnO give rise to a decrease of the pairs recombination rate, i.e., to an increase of their lifetime. 13,15 This phenomenon increases the availability of the pairs on the surface of the photocatalyst and an improvement of the occurence of redox processes can be expected.…”

“…14,15 Specifically, the electronic properties modification of the composite materials is invoked to explain this behavior: the electron transfer from the conduction band of ZnO to the conduction band of TiO 2 under illumination and, conversely, the hole transfer from the valence band of TiO 2 to the valence band of ZnO give rise to a decrease of the pairs recombination rate, i.e., to an increase of their lifetime. 13,15 This phenomenon increases the availability of the pairs on the surface of the photocatalyst and an improvement of the occurence of redox processes can be expected. 15 In this study, TiO 2 and TiO 2 -ZnO were deposited by atomic layer deposition (ALD) onto the polyurethane sponge template, which then followed by calcination to form three-dimensionally porous structures consisting oxide semiconductor nanomembranes.…”

“…13,15 This phenomenon increases the availability of the pairs on the surface of the photocatalyst and an improvement of the occurence of redox processes can be expected. 15 In this study, TiO 2 and TiO 2 -ZnO were deposited by atomic layer deposition (ALD) onto the polyurethane sponge template, which then followed by calcination to form three-dimensionally porous structures consisting oxide semiconductor nanomembranes. After a pulverizing process, the formed nanomembranes were employed as photocatalyst with methyl orange (MO) solution as a typical pollutant.…”