Microstructural Construction And Photocatalytic Performance Of CdS-based Nanocomposites

Due to the excessive consumption of fossil fuel and the emission of greenhouse gases,the energy shortage and environmental pollution have been urgent problems for human beings.Semiconductor photocatalytic technology is considered as a promising research direction since it can utilize the endless sunlight as the driving force to reach a variety of purposes,including clean energy conversion,pollutant degradation,and the synthesis of high value-added products.However,there are still many defects in the current photocatalytic industry,for example,the reaction molecules cannot be fully adsorbed on the catalyst;the catalytic reaction energy barrier is hard to reach;the exposure of the active sites on the catalyst is insufficient,and the catalyst may lose efficacy after long time reaction.In view of this,researchers are looking for suitable photocatalytic materials,meanwhile appropriate regulation or modification strategies should be employed so as to solve the above problems.As a classic photocatalyst,CdS possesses a band gap width of 2.4 e V and suitable band structure.Meanwhile,it has a good response to visible light.In recent years,CdS has been a hot topic in the field of photocatalysis.However,there are still some defects on the pristine CdS such as insufficient light absorption,fewer active sites,low degree of charge separation,and easy aggregation in the reaction process.Most importantly,the photocorrosion caused by the accumulation of photogenerated holes on CdS surface becomes a fatal problem which severely limits the large-scale application of CdS-based catalysts.Taking these defects of pristine CdS into consideration,this dissertation starts from three key perspectives:light response and corresponding absorption range;reactants molecular adsorption and activation;charge transfer and separation,to fabricate a series of CdS-based photocatalyst with high catalytic performance and stability.And further explore the photocatalytic mechanism,so as to promote the industrial application of CdS-based photocatalysts,thus providing new thoughts for exploring the structure-activity relationship between the structure of nanomaterials and their final properties.This research can be divided into the following four parts:（1）In order to promote the low charge separation efficiency of pristine CdS,cadmium zinc sulfide solution was prepared by replacing part of the cadmium source with zinc source,and in-situ solvothermal method was employed to anchor cadmium zinc sulfide solution on Ti₃C₂nanosheets.As a result,A dual-functional 0D/2D Cd_0.5Zn_0.5S/Ti₃C₂hybrid material with multiplied Schottky sites was obtained.The ultra-thin Ti₃C₂nanosheets are uniformly covered by Cd_0.5Zn_0.5S nanospheres and a tight interface is formed after the solvothermal process.The pores inside the 0D/2D structure enable the fully contact between the catalyst and sacrificial agent,thus rising the reaction completion.And the Ti₃C₂nanosheets with high-conductivity can greatly improve the charge separation.The photocatalytic test shows that the hybrid catalyst can oxidize benzyl alcohol into benzaldehyde while splitting water to produce H₂at the same time,making the photogenerated electrons and holes effectively utilized.When benzyl alcohol is used as sacrificial agent,the optimum hydrogen evolution rate of Cd_0.5Zn_0.5S/Ti₃C₂hybrid can reach 5.3 mmol/g/h,as the benzaldehyde production rate is 29.3 mmol/g/h.When lactic acid is used as sacrificial agent,the hydrogen evolution rate is up to 8 mmol/g/h.（2）The preparation process of the Ti₃C₂monolayer nanosheets in the previous chapter was complicated and the yield is relatively low.Meanwhile,the influence of the terminated-groups for Ti₃C₂on the interface-charge-flow had not been clearly revealed.In order to solve the above problems,multi-layer Ti₃C₂with high yield was obtained by etching the Ti₃Al C₂with HF,and the terminated-groups for Ti₃C₂were regulated by a serious of means.Finally,CdS/Ti₃C₂-T（T presents-F,-O and-OH）hybrid with different functional groups were prepared by chemical deposition method.0D CdS nanoparticles are evenly distributed on Ti₃C₂-T nanosheets and enter the interlayer,forming a good 0D/2D structure to avoid the agglomeration of CdS nanoparticles.The multilayer structure makes the absorbed light reflect and scatter inside,which increase the utilization rate of light.The Fermi level of Ti₃C₂-T will change with the variation of terminated-groups,and the CdS/Ti₃C₂-T hybrid reaches the highest H₂evolution（5.42 mmol/g/h）rate when the terminated-groups of Ti₃C₂-T are-OH,which is 6.8 times higher than pristine CdS（0.8 mmol/g/h）.This is because CdS/Ti₃C₂-OH has lower free energy of hydrogen evolution and better hydrophilicity.At the same time,a large area of ohmic contact is formed between Ti₃C₂-OH and CdS,which can greatly improve the rate of charge separation and avoid the recombination of electrons and holes.（3）In the previous chapter,we explored the influence of band structure on the electron flow direction,and found the importance of surface groups in photocatalytic reactions.Inspired by these,0D CdS nanospheres were uniformly anchored inside 3D flower-like CoAl-LDH,forming a uniform structure with tight interface.The 3D porous structure and abundant hydrophilic groups（-OH and-F）on LDH surface facilitate the contact between the catalyst and solution,thus improving the reaction completion.The analysis of band structure and free radicals proves that S-scheme heterojunction are formed between CdS and LDH.The internal electric field greatly promotes the flow of photoinduced carriers.The optimized CdS/LDH hybrid achieve a H₂evolution rate of 6.19 mmol/g/h,and the degradation efficiency of tetracycline hydrochloride attains 92.2%within 50 min,which realizes the purpose of clean energy production and pollutant control.In addition,the accelerated charge flow inhibits the accumulation of holes to some extent,thus improving the stability.（4）Although the works in previous chapters had accelerated the charge separation of CdS,and alleviated the photocorrosion while improving the photocatalytic performance,but the CdS in the hybrid catalyst may be eroded by sacrificial agent or oxidized by dissolved oxygen.What is more,there are still chances for the accumulation of photo-generated holes on CdS surface.To solve this problem,CdS/CoAl-LDH hybrid was used as precursor to fabricate CdS/CoP@amorphous aluminum hydroxide（CdS/CoP@AAH）ternary catalyst by in-situ phosphating method.In this hybrid material,the CdS nanorods are completely covered by AAH shell,which protecting CdS from the sacrificial agent.And the CoP nanoparticles embedded in the AAH shell act as a bridge for charge transfer,making the hydrogen evolution reaction originally occurring on the CdS surface transferred out of the shell,which will reduce the recombination of electron-hole pairs.The introduction of CoP@AAH can significantly promote charge separation and greatly increase the hydrogen evolution rate.The optimized CdS/CoP@AAH hybrid achieves a H₂evolution rate of 54.9 mmol/g/h,which is 15 times higher than that of pristine CdS,and the apparent quantum efficiency（AQE）reached 40.62%.The characterizations of the samples after catalytic reaction show that the sample has excellent stability.Finally,theoretical calculations are used to explore the influence of each component in the hybrid,and the mechanism of the increasing photocatalytic performance is further revealed.