Preparation And Photocatalytic Performance Of Cd_xZn_1-xS Composite

With the development of economy and the progress of urbanization,energy shortage and environmental pollution have become the two main problems that plague the development of various countries.As a new type of green and pollution-free energy,hydrogen can be used as a substitute for fossil energy.Therefore,the development of hydrogen energy has important strategic significance.Among them,the technology of photocatalytic hydrogen evolution does not rely on non-renewable resources,and therefore has good application prospects.Moreover,photocatalytic technology also has a good degradation effect on dye wastewater.Compared with other treatment methods,photocatalytic degradation can completely mineralize pollutants into relatively harmless substances without forming secondary pollution.In this study,CdS was mixed with different proportions of Zn²⁺ions to synthesize Cd_xZn_1-xS solid solutions by hydrothermal method.Various characterization methods and photocatalytic hydrogen evolution experiments were used to explore the optimal ratio of Zn²⁺ions incorporation.Then,two different types of composites were synthesized on the basis of Cd_xZn_1-xS solid solution with the best doping ratio.First,Cd_xZn_1-xS/Ni（OH）₂ composites were synthesized by chemical deposition.Second,Cd_xZn_1-xS/Bi₂Mo O₆ composites were synthesized by hydrothermal method.The structure,physical and chemical properties of the as-prepared composites were characterized by XRD,SEM,XPS,BET,UV-vis.At the same time,photocatalytic experiments were carried out by two composites to explore their photocatalytic hydrogen evolution and degradation properties,and the mechanism of the photocatalytic reaction was proposed.In addition,this study also used the response surface method to determine the optimum parameters for the degradation by the Cd_xZn_1-xS/Bi₂Mo O₆ composite.The main contents of this research are as follows:（1）Cd（CH₃COO）₂·2H₂O and Zn（CH₃COO）₂·2H₂O were used to control the molar ratio of Cd²⁺and Zn²⁺to synthesize Cd_xZn_1-xS solid solutions.It was found that increasing the doping of Zn²⁺can move the CB to the negative direction then improve the reduction ability of the photocatalyst,but at the same time,it has the adverse effect of increasing the band gap and narrowing the absorption wavelength.Through the photocatalytic hydrogen production experiment to explore its activity,it is found that when less Zn²⁺incorporated,the photocatalytic activity of the synthesized solid solution is significantly increased,and when the doping is greater than 50%,the photocatalytic activity decrease instead.Among them,Cd_0.5Zn_0.5S has the best photocatalytic hydrogen evolution performance,reaching 18.3 mmol·g^-1 within 3h under visible light irradiation,which is 1.5 times that of the pure CdS.（2）A series of Cd_0.5Zn_0.5S/Ni（OH）₂ composites were synthesized by a simple chemical deposition method.Compared with the structure and characteristics of pure Cd_0.5Zn_0.5S,it is found that the element valence state and band gap width of the composite change insignificantly,indicating that the Ni（OH）₂ only acts as a cocatalyst.The experiment explored the photocatalytic performance of the Cd_0.5Zn_0.5S/Ni（OH）₂composites,and found that the hydrogen evolution of the composites with all loading ratios were better than that of pure Cd_0.5Zn_0.5S.When the Ni（OH）₂ cocatalyst loading ratio is 10%,the composite has the best photocatalytic performance,and the hydrogen evolution reaches 46.6 mmol g^-1 h^-1,which is about 7.6 times that of pure Cd_0.5Zn_0.5S.Degradation of methylene blue under light to study its degradation performance,found that the composites have a better degradation effect on methylene blue,19%higher than pure Cd_0.5Zn_0.5S.The three-cycle experiments were conducted to explore its reusability,and it was found that the composites still had good hydrogen evolution and degradation properties after three cycles.（3）Cd_0.5Zn_0.5S/Bi₂Mo O₆ composites were synthesized by hydrothermal method.The composites were analyzed by XRD,XPS,PL and other characterization methods,and it was found that the composites were similar to the pure Cd_0.5Zn_0.5S in terms of morphology and band gap width.The photocatalytic performance of Cd_0.5Zn_0.5S/Bi₂Mo O₆ composite was examined by hydrogen evolution experiment,and found the composite with 10%doping has the best hydrogen effect(36.04mmol g^-1),which was about twice that of pure Cd_0.5Zn_0.5S.The degradation performance of the composite material was measured by photocatalytic degradation of methylene blue.The degradation rates of composites were significantly higher than that of pure Cd_0.5Zn_0.5S,and the degradation rates after 2 h were in the range of 80～95%.After cyclic testing,the composite has excellent stability.The experimental parameters were optimized by response surface method,and the results showed that when the photocatalytic reaction p H was 5.6,the initial dye concentration was 13.8 ppm,and the photocatalyst dosage was 0.41g/L,the composite had the best degradation effect.（4）The photocatalytic mechanism of Cd_0.5Zn_0.5S composite are proposed.On one hand,loading Ni（OH）₂ as a cocatalyst can promote the transfer of electrons and inhibit the recombination of photo-generated electrons and hole pairs.On the other hand,the cocatalyst can provide more reaction sites for photocatalytic reactions.The heterojunction is formed between Cd_0.5Zn_0.5S and Bi₂Mo O₆.Because of the energy level difference between the CB and VB,the photogenerated electrons migrate from the CB of the Cd_0.5Zn_0.5S to the CB of the Bi₂Mo O₆,and the holes migrate from the VB of the Bi₂Mo O₆ to the VB of the Cd_0.5Zn_0.5S,which effectively realizes the separation of photogenerated electrons and holes,then improve the photocatalytic activity.In addition,the Cd_xZn_1-xS solid solution has a serious photo-corrosion phenomenon,and these two composites also have a certain effect on improving the stability of the photocatalyst.