Preparation and Photocatalytic Properties of Highly Stable Mn:ZnO/Mn₂O₃ Nanocomposite Photocatalysts

Recently,with the growing problem of environmental pollution,a large number of emerging environmental purification technologies, especially the photocatalytic oxidation technique based on semiconducting metal oxide nanostructures,have been rapidly developed for the applications in degradation of organic pollutants from water and atmospheric environments. In the process of development,scientists around the world have recognized that the improvement in the visible light response, the separation efficiency of photogenerated charge carriers and the light stability of the catalysts are the key factors to promote the large- scale application of the photocatalytic oxidation technique. For this purpose,a series of new high-performance nanostructured composite photocatalysts have been continuously developed and investigated,but the process complexity and synthesis cost are gradually increasing. Also,it is difficult to realize the band structure regulation of the semiconductors and well-match energy levels between different semiconductors. These problems need to be solved urgently. At the same time,with the future demand for high selectivity of new technologies(such as selective sensing,selective gas separation,selective carbon dioxide reduction), the selective photocatalytic degradation of organic pollutants will also become a development direction in this field. In this study,a series of photocatalysts including pure ZnO,Mn-doped ZnO (Mn:ZnO),Mn2O3 coupled ZnO (ZnO/Mn2O3) and Mn2O3 coupled Mn- doped ZnO (Mn:ZnO/ Mn2O3)are successfully prepared by a modified polymer network gel method,and the characteristics of the catalysts for photocatalytic degradation of Rhodamine B (RhB) and Methylene Blue (MB) dyes under simulated sunlight irradiation are investigated. The phase compositions,microstructures,surface chemical states as well as optical and optoelectronic properties are analyzed through some material characterization approaches in detail. The results of X- Ray Diffraction(XRD),Scanning Electron Microscopy(SEM)and Brunauer-emmett-teller(BET) specific surface area shows that the particle sizes of Mn:ZnO and Mn: ZnO/Mn2O3 are decreased and the particle dispersibility are improved after trace (0.1 mol%)Mn doping and then coupling trace (0.2 mol%) Mn2O3,increasing BET specific surface area. Correspondingly,this results in the decrease of crystalline quality and the increase of crystal defects. Ultraviolet-visible(UV-vis) light absorption spectra indicates that the light absorption capacities of the composite photocatalysts in the visible region are significantly improved compared with pure ZnO and doped ZnO samples. Photoluminescence(PL)spectra suggests that trace Mn doping and trace Mn2O3 coupling jointly facilitate the separation of photogenerated electron- hole pairs. Combined with X-ray photoelectron spectroscopy (XPS),it is found that the enhanced visible light absorption capability and photogenerated electron-hole pair separation rate originates from the increase of oxygen vacancies on the catalyst surface and the formation of type II heterojunction structure between Mn:ZnO and Mn2O3. Therefore,Mn:ZnO/Mn2O3 exhibits the robust and superior photocatalytic activity for degradation of RhB dye under simulated sunlight irradiation. However,because of the narrow band gap(E-g similar to 1. 4 eV)of Mn2O3 and its higher valence band position than the redox potential of hydroxyl radical (center dot OH),the oxidation potential of photogenerated holes is too low to generate center dot OH with the stronger oxidizing power. This results in the lower RhB and MB dyes photodegradation efficiencies of the ZnO/Mn2O3 photocatalysts. In addition, Mn:ZnO/ Mn2O3 exhibits a selective photodegradation behavior towards RhB and MB dyes,that is,a significantly lower photodegradation efficiency for the easily degradable MB. Such a selective photocatalytic property is attributed to the difference between active species during the photocatalytic reactions and the relationship between the Zero Charge Point (ZPC) of the catalyst and the pH of the initial dye solution. In the photocatalytic processes,the strong oxidizing species(center dot O2-,center dot OH and h(+)) on the surfaces of the catalysts for the photodegradation of RhB dye are more than those(center dot OH and h(+)) for the photodegradation of MB dye. The ZPC value of Mn:ZnO/Mn2O3 closes to the pH value of the weakly acidic RhB (pH= 6.3 similar to 6.6) rather than MB(pH=6.0)dye aqueous solution,resulting in a higher adsorption ratio of cationic RhB dye molecules on the catalyst surface.