Preparation And Photocatalytic Properties Of ZnO-Based Heterojunction Nanocomposites

In this thesis,ZnO/Ag₂O and ZnO/Ag₂CO₃ p-n type heterojunction as well as ZnO/Ag₂CO₃/Ag₂O n-p-p type heterojunction nanocatalysts were successfully constructed by simple hydrothermal and co-precipitation methods.The controlled synthesis,crystal structure,morphology,photostability,photocatalytic performance and catalytic mechanism of the above series of nanocomposites have been thoroughly explored.The specific work consists of three main areas:（1）In order to obtain the optimum ratio of ZnO nanorods and Ag₂O nanoparticles,a series of ZnO/Ag₂O p-n type heterojunction photocatalysts with different molar ratios were prepared using hydrothermal and co-precipitation methods.The results showed that the optimum molar ratio of ZnO to Ag₂O was 8:3,and its photocatalytic degradation efficiency was about four and three times that of pure ZnO and Ag₂O.Subsequently,the catalytic mechanism,cycling stability and the effects of catalyst concentration,methylene blue solution concentration and p H of the solution on the catalytic activity of the optimal catalyst were further investigated.The results showed that the ZnO/Ag₂O p-n type heterogeneous photocatalytic degradation efficiency decreased by less than 8%after five cycles.The moderate catalyst concentration,small amount of pollutants and alkaline conditions were more favorable to enhance the photocatalytic degradation rate.This simple preparation method,high catalytic efficiency and good cycling stability of the photocatalysts obtained and its mechanism research has laid a certain theoretical and technical foundation for the research of photocatalytic degradation of pollutants in water.（2）ZnO nanorods and ZnO/Ag₂CO₃ p-n heterojunction were synthesized by a simple hydrothermal method and co-precipitation.A series of electrochemical（PL,TPC,EIS）tests showed that the photovoltaic properties of ZnO/Ag₂CO₃ p-n heterojunction were superior to those of the ZnO nanorods.The catalytic performance of the ZnO/Ag₂CO₃ heterojunction was investigated using malachite green,tetracycline and 2,4-dichlorophenol as degradants,respectively.The photocatalytic rate constants（k）for the degradation of these three pollutants by ZnO/Ag₂CO₃ heterojunction can reach 7.12,6.23 and 4.35 times that of ZnO and 2.54,2.72 and 2.27 times that of Ag₂CO₃.The degradation rate of ZnO/Ag₂CO₃heterojunction remained stable after five times of repeated degradation of tetracycline.and further explored that the photodegradation of tetracycline by this heterostructure was consistent with a Z-type charge transfer mechanism.Therefore,it can be tentatively shown that ZnO/Ag₂CO₃heterojunction has a bright future in the practical application of photocatalysis.（3）In order to further broaden the spectral absorption range and improve the photocatalytic efficiency,a series of ZnO/Ag₂CO₃/Ag₂O n-p-p heterojunctions were further synthesized on the basis of ZnO/Ag₂CO₃ p-n type heterojunctions,taking advantage of the easy decomposition of Ag₂CO₃ at high temperatures.The calcination time is closely related to the morphology,specific surface area and optoelectronic properties of the prepared ZnO/Ag₂CO₃/Ag₂O n-p-p heterojunction.A series of electrochemical（PL,TPC,EIS）tests showed that the photovoltaic properties of ZnO/Ag₂CO₃/Ag₂O n-p-p heterojunction were superior to those of ZnO/Ag₂CO₃ p-n heterojunctions and the ZnO/Ag₂O p-n heterojunctions.ZnO/Ag₂CO₃/Ag₂O heterojunction catalysts all completely degraded malachite green,tetracycline and 2,4-dichlorophenol with photocatalytic rate constants（k）up to 1.25,1.53and 1.31 times that of the ZnO/Ag₂CO₃ p-n type heterojunction.The effective degradation of2,4-dichlorophenol was maintained after five repeated photocatalytic experiments.The possible mechanism of photodegradation of 2,4-dichlorophenol by this heterojunction was investigated by analyzing the test results of radical trapping experiments and the energy band positions of the semiconductor.The above experimental results indicate that ZnO/Ag₂CO₃/Ag₂O n-p-p type heterojunction photocatalyst has high photocatalytic efficiency and stable photocatalytic performance,and has broad practical application prospects.