CdS-based Heterostructures:Construction,Characterization And Photocatalytic Performance

CdS,as a traditional visible-light-driven semiconductor,has been extensively explored due to some merits like high charge exciting efficiency and possibility of water splitting.However,there are some disadvantages that suppress the photocatalytic performances,such as fast charge-recombination in bulk materials synthesized by calcining method,photocorrosion during irradiation and so on.Designing heterostructures by employing various coupling techniques in nanoscale can enhance the charge separation efficiency.In this thesis,to improve the photocatalytic activity of CdS,our research is focused on the construction of CdS-based binary/tenary heterostructures.Various CdS-based composites were prepared and the photocatalytic performances were evaluated by the photodegradation of organic pollutants and the hydrogen evolution reaction(HER).Finally,the potential photocatalytic mechanisms were proposed based on the band theory and the photoelectrochemical characterizations.The main research contents are summarized as follows:Firstly,CdS-rGO nanocomposites were fabricated by wrapping CdS particles with reduced graphene oxide(rGO)sheets through a facile mixing process using ethylene glycol as solvent and reducing agent.The photocatalytic behavior of the CdS-rGO nanocomposites was evaluated by the photocatalytic degradation of Congo red under visible light irradiation.The photocatalytic activity of CdS-rGO was 1.67 times higher than bare CdS with enhanced stability.The radical trapping test disclosed the dominant active oxidation species in the CdS-rGO photocatalytic process.Based on the results,a possible photocatalytic mechanism was proposed.Secondly,the heterostructured CdS/BiVO₄ nanocomposites were fabricated in a low-temperature water bath system.The uniform CdS nanoparticles with an average size of20 nm were homogeneously interspersed on BiVO₄ nanosheets.The Rh B degradation and water splitting test confirmed the coupling of BiVO₄ and CdS nanoparticles could notably promote the photocatalytic activity.The photodegradation rate of composites was 3.38 and4.84 times higher than CdS and BiVO₄,respectively,and the H₂-production rate was about0.57 mmol h^-1 under visible light irradiation,which was 5.18 times higher than that of pure CdS nanoparticles.The dominant active species in the photocatalytic system were also confirmed by the radical trapping test.Based on the calculation and redical trapping test,a all-solid-state Z-scheme photocatalytic mechanism was proposed,which was further verified by the electrochemical impedance spectroscopy and the cycling test.Thirdly,a novel three-component heterostructured photocatalyst of BiVO₄/Ag/CdS was synthesized by a two-step in-situ deposition method in water system.Based on the electrostatic absorption theory,we explained the selective growth mechanism of Ag nanoparticle,which deposited on the(040)planes of BiVO₄.The photodegradation rate of BiVO₄/Ag/CdS was 4.26,6.60 and 1.31 times higher than CdS,BiVO₄ and BiVO₄/CdS,respectively under visible light,and was variable with the loading of Ag.Finally,we proposed the mechanism of all-solid-state ternary Z-scheme charge migration in the BiVO₄/Ag/CdS system through electrochemical and photoluminescence analysis.Forthly,novel CdS/ZnFe₂O₄ composites were prepared through a two-step hydrothermal process.The homogeneous ZnFe₂O₄ nanoparticles were decorated on the self-assembled CdS spheres.As compared with pure CdS and blank ZnFe₂O₄,the photodegradation rate of the magnetically separable CdS/ZnFe₂O₄ composites was increased by 4.57 and 2.65 times,respectively with enhanced stability under visible light.The results of photoluminescence and electrochemical impedance spectroscopy further validated that the performance enhancement resulted from the construction of heterojunction structure,leading to high charge separation efficiency.On the basis of the calculation and the trapping test,a type-II heterojunction photocatalytic mechanism was proposed.Finally,ZnFe₂O₄-decorated CdS 0D/1D composites were successfully prepared by a low-temperature hydrothermal method.The existence of ZnFe₂O₄ nanoparticles could greatly prevent the CdS nanowires from agglomerating.Compared with pure CdS nanowires and ZnFe₂O₄ nanoparticles,the photodegradation rate of the 0D/1D CdS/ZnFe₂O₄ composite nanowires was increased by 2.40 and 6.96 times,respectively,and was also 4.03 times higher than that of 3D CdS/ZnFe₂O₄ under visible light.A potential photocatalytic mechanism was proposed and verified by the radical trapping test.The long lifetime and the high separation efficiency of the photogenerated carriers in the composites were further confirmed by the photoluminescence and the electrochemical impedance spectra.