Preparation And Modification Of ZnO/CuO Heterojunction Photocatalytic Materials

Semiconductor photocatalytic technology is a green,energy-saving,environmentally-friendly,and efficient technology,which has great potential in solving environmental pollution.Semiconductor ZnO is not only non-toxic and abundant in natural resources but also has stable physical and chemical properties,which is widely used in environmental protection,sewage treatment,photohydrolysis and hydrogen production,and solar cells.However,limited by the wide band gap of pure ZnO(3.20 eV),ZnO materials has two drawbacks:limited aborsption of sunlight and high photo-generated electron-hole recombination rate,resulting in low photocatalytic efficiency.In order to improve the photocatalytic activity of ZnO,ZnO needs to be modified on the basis of band engineering and interface design.In this paper,the in-situ template-free hydrothermal method was used to prepare ZnO/CuO hollow microsphere,in which the CuO’s band-gap was narrow around 1.7 eV.ZnO/CuO hollow microsphere were further modified by graphene oxide(GO)and zinc sulfide quantum dots(ZnS QDs)to obtain high photocatalytic efficiency materials.The structure and morphology of resultant materials were characterized by X-ray diffraction(XRD),photoelectron spectroscopy(XPS),differential thermal-thermogravimetric analyzer(TG-DTA),scanning electron microscope(SEM),transmission electron microscope(TEM).The optical characteristics were tested by Raman spectrum(Raman),Photoluminescence spectrum(PL)and ultraviolet spectrum(UV-vis).Furthermore,the catalytic mechanism was ananlyzed by electrochemical impedance spectroscopy(EIS)and photocurrent(i-t).The research had important theoretical guidance significance and practical application value,providing a simple and effective method for the design and preparation of multiple heterojunction nanostructures.First,the structure of ZnO/CuO hollow microspheres was constructed by hydrothermal method,and the effects of Zn/Cu molar ratio,hydrothermal temperature,hydrothermal time and calcination temperature on the structure,morphology and photocatalytic performance of the product were investigated.The results showed that when the ratio of Zn2+/Cu2+was 1:0.5,the hydrothermal reaction conditions were 6 hours,120℃,and the calcination temperature was 500℃,ZnO/CuO hollow microspheres about 5 μm could be obtained.The obtained microspheres had good photocatalytic performance under natural sunlight,and were able to degrade 20 mg L-1 rhodamine B solution in 120 minutes.Due to the special structure of CuO,the constructed p-n heterojunction between ZnO and CuO would effectively expand the spectral response range of ZnO and improve the separation efficiency of photogenerated electron-hole pairs.Second,GO modifying ZnO/CuO hollow microspheres was prepared by hydrothermal method,and the effects of the GO content and the compounding method on the microspherical appearance,structure and photocatalytic performance of ZnO/CuO hollow microspheres were investigated.The experimental results showed that in-situ compounding method could produce ZnO/CuO/GO(ZCGO)microspheres with uniform size and high catalytic activity.When the GO content was 6 mL,ZCGO3 sample with best photocatalytic performance in sunlight was obtained and it could degrade 99%rhodamine B solution in 100 minutes.The degradation rate of ZCGO3 sample was 1.4 times faster than ZnO/CuO.In-situ compounding method achieved the structure of the GO sheet embedded in ZnO/CuO microspheres,while the embedded structure of the GO sheet effectively reduced the resistivity of the composite material and increased the photocurrent.The photocurrent value was increased from 102 μA/cm2(ZnO/CuO)to 140 μA/cm2(ZCGO3).These phenomena showed that the embedded GO sheet in these microspheres was beneficial to reduce the recombination of photogenerated carriers and improve the photocatalytic activity.Finally,using ZnO/CuO/GO as a raw material and a solvothermal method to obtain a ZnS QDs-modified multiple heterojunction,the effect of ZnS QDs content on the photocatalytic performance of ZnO/CuO/GO composite was studied and the optimal sample was subjected to cyclic stability test.As a result,the multiple heterojunction ZnO/CuO/GO/ZnS QDs(ZCGS)performed stable photocatalytic performance under sunlight.ZnS QDs and ZCGO formed a heterogeneous structure,which significantly reduced their band gap from 2.68 eV(ZCGO)to 2.38 eV and improved their photocatalytic performance.When the ZnS QDs content was 40%,the ZCGS3 sample had the best photocatalytic performance,its degradation rate was 1.32 times to that of ZCGO,and the photocurrent reached 167μA/cm2.Because ZnS QDs were dispersed on the surface of ZCGO,ZCGS formed a unique multi-element heterojunction between ZnS QDs,ZnO,CuO,GO,and generated new energy states at the interface.This can not only extend the path of photo-generated electrons,but also avoid accumulation of the charge carriers,so that the high photocatalytic performance of ZCGS also showed good stability.