ZnIn₂S₄-Based Composite Photocatalysts Towards Photocatalytic Cr（Ⅵ） Reduction

Cr（Ⅵ）is a common heavy metal pollutant in water and its reduction to Cr（III）via photocatalysis is an environmentally friendly and low-cost way.In addition,Zn In₂S₄（ZIS）is a relatively chemically stable,easily synthesized and visible light responsive metal sulfide,which is widely used in photocatalytic reduction of Cr（Ⅵ）.However,the wide utilization of ZIS still has some obstacles such as rapid combination of photogenerated electron-hole pairs,poor mobility of charge carriers and some other structural defects,which would affect its photocatalytic efficiency in Cr（Ⅵ）reduction.Combining ZIS with other materials to co-construct composite photocatalysts is one of the most effective ways to solve above problems.The morphological and physicochemical properties of ZIS supports can directly affect photocatalytic performance of ZIS-based composite photocatalysts.Therefore,the role of supports on photocatalytic reduction of Cr（Ⅵ）and their contributions to the enhancement of photocatalytic performance of ZIS-based composite photocatalysts were all studied,which is of great scientific value and research significance for designing high-performance ZIS-based composite photocatalysts.Accordingly,three different supports with distinguished dimensions,electron conduction and reception capabilities were selected as study subjects（such as g-C₃N₄nanotubes with one-dimensional structure,ZSM-5 molecular sieves with three-dimensional structure and nickel-cobalt layered double hydroxides（Ni Co-LDH）with two-dimensional structure）.These ZIS-based composite photocatalysts were obtained by situ growth of ZIS nanosheets on these supports and evaluated via photocatalytic reduction of Cr（Ⅵ）to study the support effects on photocatalytic performance,thus to investigate intrinsic relationships of support properties and photocatalytic performance,thus enhancing photocatalytic efficiency for Cr（Ⅵ）reduction.Research contents of this paper are as follows:Firstly,we constructed CN@ZIS composites for photocatalytic reduction of Cr（Ⅵ）using one-dimensional g-C₃N₄（CN）nanotubes as support（Chapter 2）.The morphological analysis showed that hollow CN nanotubes can play an important role on dispersing and bearing ZIS nanosheets,thus to prevent ZIS agglomeration.Performance tests showed that the optimal formulation of photocatalyst（CN@ZIS-100）achieved 100%Cr（Ⅵ）reduction within 10 min（with initial Cr（Ⅵ）concentration of 30 mg/L）under visible light irradiation（pure ZIS requires20 min to achieve almost 100%reduction of Cr（Ⅵ））.The reduction efficiency of CN@ZIS-100for Cr（Ⅵ）remained at 90%after four cycles,and ZIS nanosheets were tightly anchored on CN nanotubes after catalyst recyclability,demonstrating good catalytic stability.The mechanism showed that CN nanotubes can match energy band of ZIS and co-build type-Ⅰheterojunctions,which effectively inhibit the recombination of photogenerated electron and hole pairs,and the composite catalyst exhibits higher efficiency in photocatalytic reduction of Cr（Ⅵ）compared to pure ZIS.Secondly,granular ZSM-5 was used as support for the synthesis of ZSM-5@ZIS composites（Z5@ZIS）（Chapter 3）.Unlike one-dimensional g-C₃N₄,ZSM-5 is a better electron acceptor.Therefore,the results showed that Z5@ZIS-100 sample synthesized with the optimal ratio achieved 100%Cr（Ⅵ）reduction（with initial Cr（Ⅵ）concentration of 30 mg/L）within 7min under visible light,which is better than those of both pure ZIS and CN@ZIS-100.XPS characterization showed that chemical states of Zn 2p,In 3d and S 2p in Z5@ZIS-100 all shifted towards higher binding energies,whereas that of Si 2p shifted negatively,demonstrating the electron accepting ability of ZSM-5 and the photogenerated electron jumping from ZIS to ZSM-5.In addition,photocurrent tests further confirmed that ZSM-5@ZIS composite contains a longer separation time of photogenerated electrons and holes,which effectively suppresses the recombination of photogenerated electrons and holes in ZIS,thus achieving enhanced performance of photocatalytic reduction of Cr（Ⅵ）.Finally,two-dimensional Ni Co-LDH was used as support to construct Ni Co-LDH@ZIS composite with core-shell structure（Chapter 4）.Ni Co-LDH@ZIS was synthesized by Ni²⁺etching from Co-ZIF-L（a zeolite-like imidazole ester skeleton material with a lamellar structure）to prepare lamellar Ni Co-LDH agglomerates with a rough surface.This two-dimensional Ni Co-LDH not only facilitates ZIS loading,but also increases the interface contact area between LDH and ZIS due to its rough surface.It was found that energy bands of Ni Co-LDH and ZIS are matched to form type-II heterojunctions,which enables a"double transfer path",facilitating the transfer of photogenerated electrons from ZIS to Ni Co-LDH and suppressing the rapid recombination of photogenerated electrons and holes.The results showed that Ni Co-LDH@ZIS-100 achieved 100%reduction of Cr（Ⅵ）（with initial concentration of 30 mg/L）in 3min.In summary,among three types of supports,Ni Co-LDH@ZIS-100 showed the best efficiency for the photocatalytic reduction of Cr（Ⅵ）.It can be seen that support morphology and electron accepting ability can greatly affect photocatalytic efficiency of ZIS-based composite catalysts for Cr（Ⅵ）reduction.Therefore,the support should combine tightly with ZIS and co-build heterojunction with matching energy band,thus to inhibit the recombination of photogenerated electrons and holes,and increase the participation of photogenerated electrons in photocatalytic reduction of Cr（Ⅵ）to improve photocatalytic efficiency.