The Controllable Construction And Photocatalytic Properties Of SnIn₄S₈-based Core-shell Nanocomposites

Nowadays,the unreasonable discharge of urban and industrial wastewater seriously damages the quality of environmental water.Hence,it is of utmost importance to find an efficacious and sustainable way to remove these pollutants from water.In recent years,semiconductor-based photocatalysis,an environmentally friendly technology with high degradation rate and mineralization efficiency for pollutants,has shown great application prospects in wastewater treatment.In photocatalysis,the light-harvesting ability and charge carrier separation efficiency are two main determinants of the high photocatalytic performance.Generally speaking,photocatalytic reactions involve three processes:photon absorption,electron-hole charge generation and separation,and surface catalytic reactions.The main strategy to improve the light absorption capacity of materials is to broaden the light absorption range of semiconductors and develop light-responsive photocatalysts.Among them,ternary sulfide has been widely exploited owning to its unique electronic structure and excellent photoelectric properties.Indium tin sulfide（SnIn₄S₈）,as a typical ternary sulfide,with a narrow band gap and strong visible light response,large specific surface area to provide more reactive sites,an easy-to-control morphology and band gap,physical and chemical stability,low cost and non-toxicity,has aroused intense interest.However,the poor quantum efficiency obstructed by the rapid recombination of photogenerated electrons and holes still hance the practical application of single SnIn₄S₈.This paper combines the excellent visible light response ability of SnIn₄S₈,and uses the construction of a core-shell heterostructure as the main method to promote the rapid transfer of electrons through the close heterogeneous interface between the core and the shell,so as to achieve the goal of increasing the efficiency of carrier separation.A series of characterizations were carried out to prove the existence of the core-shell heterostructure.The photocatalytic activities of the as-prepared samples were evaluated by the photoreduction of Cr（VI）and the photooxidation of organic pollutants under visible-light.Finally,based on the photoelectric testing,capturing experiment and many other experiments,the photocatalytic reaction mechanism of performance improvement was carefully studied and revealed.The main research results are as follows:（1）The novel TiO₂-SnIn₄S₈（TOSIS）core-shell composite photocatalyst was prepared through a simple in-situ growth method,where the SnIn₄S₈ shell layer was uniformly grown on the surface of one-dimensional TiO₂ nanobelts.X-ray diffraction technology（XRD）,electron microscopy tests（SEM,TEM,HRTEM,etc.）,X-ray energy spectroscopy（XPS）and other characterization were tested to rigorously proved the successful construction of the core-shell heterostructure.The photocatalytic activity of the as-prepared sample was evaluated by the photoreduction of Cr（VI）and the photooxidation of MO under visible-light.The BTOSIS core-shell photocatalyst exhibited excellent photocatalytic activity.What is more,the TOSIS2 sample could effectively remove MO and Cr（VI）in just 40 minutes under visible light.Compared with TiO₂ and SnIn₄S₈ the photocatalytic performance of the core-shell composite photocatalysts had been greatly improved.The degradation rate of TOSIS2 in Cr（VI）reduction was 33.44 and 3.08 folds as those of the TiO₂ and SnIn₄S₈,and was 48.19 and5.50 times higher than those of TiO₂ and SnIn₄S₈in the degradation of MO.The photoelectric properties of the materials were tested by photoelectric current,impedance and fluorescence emission tests.Moreover,the photocatalytic reaction mechanism of the materials was studied by combining capture experiments to reveal the electron transport mechanism in the reaction process and the reasons for the improvement of photocatalytic performance.（2）In this chapter,a simple solvothermal method was used to prepare CdS-SnIn4S8（SISCS）core-shell composite photocatalyst.The results of electron microscopy（SEM,TEM,HRTEM,and AFM,etc.）showed that the shell of SnIn₄S₈with uniform thickness has been successfully loaded on the surface of CdS,and the core-shell heterostructure had been successfully constructed.Through the analysis of photocatalytic mechanism,we found that the SISCS core-shell composite photocatalyst follows the direct Z-type electron transport mode.The direct Z-scheme heterojunction was constructed ingeniously,via the intimate coaxial contact of CdS-SnIn₄S₈ core-shell nanostructure.The large and close coaxial contact between the core-shell and core-shell composite photocatalyst can effectively enhance the charge transfer.Moreover,the redox ability of the samples could be greatly maintained crediting to the Z-scheme heterojunction.What is more,the enlarged specific surface area and the super-hydrophilic of the surface both could ensure the excellent photocatalytic activity.Consequently,the efficiency of SISCS2 sample for reduction of Cr（VI）（20 mg/L）reached 100%within 30 min and got almost 100%within 24 min for degradation of MO（15 mg/L）.The apparent rate constants of Cr（VI）and MO photodegradation over SISCS2 was 3.5 and 7.5 times higher than bare CdS nanorods,4.5 and 5.6 folds as that of pure SnIn₄S_8,respectively.Furthermore,the Z-Scheme photocatalytic mechanism and the intermediate products of MO were also discussed in depth.（3）On the basis of the experiments in the first two chapters,a novel core-shell composite photocatalyst BaTiO₃-SnIn₄S₈（BTOSIS）with more uniform dispersion morphology was successfully prepared.According to the structural advantage of catalyst and the synergistic effect between the two materials,BTOSIS2 composite photocatalyst possesses excellent photocatalytic activity.Among them,BTOSIS2composite sample can completely remove MO and Cr（VI）within 30 min under visible light.In comparison with the two single samples,the photodegradation efficiency of the core-shell photocatalysts was greatly improved.In addition,after five photocatalytic cycle experiments,the photodegradation efficiency of the photocatalyst could still maintain at a high level,indicating the good photocatalytic stability of the BTOSIS core-shell composite photocatalysts.Finally,the mechanism of photocatalytic performance improvement was discussed and the photocatalytic reaction mechanism was studied with the aid of photoelectric performance test,capture and electron spin resonance（ESR）test and so on.