Research On Preparation And Photocatalytic Properties Of ZnO-based Semiconductor Nanocomposites

The semiconductor ZnO has been widely used as a kind of photocatalysts for the degradation of organic pollutants due to its good high light response,easy preparation,easy morphology regulation,low cost and environmental friendliness.However,ZnO can only excited by the UV light because of its relatively high band gap energy.And it is very easy for the photogenerated electron-hole pairs to recombine in ZnO,which seriously limits the large-scale practical application of ZnO in the photocatalysis field.It is a feasible approach to combine ZnO with other materials to accelerate charge transfer and promote the separation of photogenerated electron hole pairs and and thereby improve the photocatalytic performance of ZnO.In this paper,ZnO was used as the research object,and reduced graphene oxide and graphitic carbon nitride were used to construct semiconductor composites with different heterostructures to promote the charge transfer of semiconductor materials and improve their photocatalytic degradation performance of organic pollutants.The relationship between the photocatalytic degradation performance and composition of ZnO-based nanocomposites was systematically studied through the characterization of the composites and the test of the photocatalytic degradation performance.The main research contents and conclusions are as follows:（1）With graphene oxide as the precusor,the ZnO/r GO semiconductor nanocomposites with different r GO contents were prepared by an ethanol-assisted one-step precipitation method.The microstructure and morphology of the obtained materials were characterized by various analysis methods,such as XRD,SEM,TEM,Raman and UV-vis DRS.The results showed that the composite of r GO and ZnO could refine the particle size of ZnO,and the position of the optical absorption edge of the composite samples showed a red shift compared with that of pure ZnO.The photocatalytic degradation performance of ZnO/r GO nanocomposites on the simulated pollutant methyl orange（MO）was effectively improved.And the photocatalytic degradation rate constant of the ZnO/r GO-1.0%was 0.02017 min^-1,which was 14 times higher than that of pure ZnO(0.00133 min^-1).The improved photocatalytic performance of the ZnO/r GO-1.0%nanocomposite was mainly due to the enhanced adsorption of ZnO/r GO on degradation substrates,the strengthened visible light absorption and the effective separation of charge carriers.（2）Graphitic carbon nitride（g-C₃N₄）was prepared by a classical thermal polymerization method,using melamine as the precursor.And zinc nitrate was used as zinc source to prepare ZnO nanoparticles by chemical bath deposition.The ZnO/g-C₃N₄ semiconductor composites were prepared by immersion in dimethyl sulfoxide solvent to fully mix ZnO and g-C₃N₄,followed by high temperature calcination.The characterization results showed that the light absorption edge of the ZnO/g-C₃N₄composite samples had a red shift compared with pure ZnO,which improved the utilization of visible light of the photocatalysts.The impregnation calcination process allowed the components to form a close combination,and the heterostructure with appropriate ratio promoted the effective separation of photogenerated electron hole pairs.The photocatalytic degradation rate of simulated pollutant methylene blue（MB）of 66.7-ZCN composite sample was 0.02335 min^-1,which was better than pure phase ZnO（0.01995 min-1）and pure phase g-C₃N₄(0.00663 min^-1).（3）With urea as the precursor,g-C₃N₄ was prepared by thermal polymerization,and ZnO nanoparticles were grown in situ on g-C₃N₄ nanostructures by solvothermal method.The results showed that the growth of ZnO on g-C₃N₄ could reduce the aspect ratio and crystallinity of ZnO nanoparticles and form a tight chemical bond between them.From the UV-vis DRS test,it can be seen that the position of the maximum absorption edge of the composite samples were red-shifted relative to the pure ZnO,and the utilization of light was effectively enhanced.Compared with pure ZnO and pure g-C₃N₄,the photocatalytic performance of ZnO/g-C₃N₄ nanocomposites for MB degradation under visible light irradiation was significantly improved.The photodegradation rate of the ZCN-40 composite sample with the optimal composition reached 0.2575 min^-1,which was 20 times that of pure ZnO(0.0114 min^-1)and 8.7times that of pure g-C₃N₄(0.0296 min^-1).The photocatalytic performance of ZnO/g-C₃N₄ nanocomposites was significantly better than that of pure materials,which was mainly due to the synergistic effect of energy band difference and built-in electric field in the composites,which promoted the effective separation of photogenerated charge carriers.