Effects Of Semiconductor Coupling And Noble Metal Modification On Photocatalytic Activity Of Cu₂O

Smiconductor-based photocatalysts, activated by illumination at roomtemperature, have been used for degradation of organic pollutants by redox reaction ofphoton-generated carrier. In addition, the photocatalyst also have been applied forreducing metal ions, hydrogen production from water decomposition, deodorization,antisepsis, sterilization and polluted water processing as well. Currently, TiO₂asphotocatalyst can only be activated by ultraviolet （UV） light because of their broadband-gap （3.2eV）, for which only less than5%solar energy can be utilized. Therefore,exploring visible light response photocatalysts has become one of the most importantresearch areas for environmental pollution control and renewable energy sources.Narrow-band semiconductor like cuprous oxide （Cu₂O，Eg=2.2eV） has beenconsidered as the most promising photocatalysts besides TiO₂, for it can be easilyactivated by visible light, hence can sufficiently utilize solar energy. Due to the easyrecombination of photogenerated electrons and holes, pure semiconductor exhibitsvery low quantum efficiency. p-Cu₂O and n-Cu₂O were coupled with semiconductorZnO and modified with Ag. The photocatalytic properties and catalytic mechanism ofZnO/Cu₂O and Ag/Cu₂O for degradation of azo dyes were studied.ZnO, n-Cu₂O, p-Cu₂O, ZnO/n-Cu₂O, ZnO/p-Cu₂O films were synthesized byelectrodepostion method on ITO. Based on the Cu₂O, Ag/n-Cu₂O, Ag/p-Cu₂Ocomposite films were prepared through direct oxidation using AgNO₃as Ag source.X-ray diffraction （XRD）, scanning electron microscope （SEM） and energy-dispersivespectrometry （EDS） were applied to characterize crystal structure, morphology andcomposition of the films, respectively. The performance of the photocatalysis wasevaluated by degradation of methyl orang （MO） under visible light.The results indicated that ZnO, pure n-Cu₂O and p-Cu₂O films have nophotocatalystic activity under visible light, While, ZnO/n-Cu₂O exhibited higherphotocatalystic activity, but lower stability, which was related to the bottom thickness of ZnO and surface content of n-Cu₂O. The difference in photocatalytic activitybetween the two types of composite films is interpreted in terms of differentmechanisms for transfer of charge carriers photogenerated within the heterostructures.For p-Cu₂O, the adsorption capacity was enhanced by coupling with ZnO, but thephotocatalytic activity was not improved.Ag/n-Cu₂O and Ag/p-Cu₂O were synthesized by direct reduction method. Bothof the films appeared filiform structure and have stronger adsorption capacitycompared with pure Cu₂O. What’s more, the phtotcatalytic activity has beenobviously enhanced after modification with Ag, and more stable than ZnO/n-Cu₂O.The photocatalytic activity of the two films was closely related to modification timeof Ag and deposition time of Cu₂O. The optimum condition corresponding to thehighest photocatalytic activity was that modification time of Ag was2min anddeposition time of Cu₂O was3min. Ag/n-Cu₂O has higher photocatalytic activity butlower stability than Ag/p-Cu₂O. The reason for the higher photocatalytic activity canbe explained by the electron transfer at Ag/Cu₂O interface. The enhanced stabilitymay be ascribed to the competive consumption of hole by Ag.Ag/n-Cu₂O was used as ptotocatalyst to degrade MO, and the effect of somefactors on degradation was investigated, including initial concentration of MO, pH,H₂O₂, inorganic anions and methanol. The lower initial concentration was benefited todegradation rate; the highest degragation rate was achieved at pH=8.2; degradationrate was increased in the presence of H₂O₂,whereas decreased in the presence ofNO₃-、SO₄^2-, Cl-and methanol, respectively.