Construction Of Metal Sulfide Heterojunction System And Investigation Of The Photogenerated Carriers Transfer Mechanism

Environmental pollution and energy crisis are getting serious with the consumption of fossil fuels.To solve the problems and achieve the goal of carbon peak and carbon neutralization proposed by our country,it’s urgent to seek continuous and green clean energy for substituting.Using semiconductors for photocatalytic water splitting to produce hydrogen is considered extremely promising because it can convert solar energy to chemical energy for storage.Metal sulfide is attracted great attention for the suitable bandgap and adequate negative conduction band potential,but the rapid recombination of photogenerated charges and grievous photocorrosion restricts its further development.This work is aim to improve the separation and transfer of charge carriers through construction of heterojunction based on metal sulfides,thereby promoting the surface reaction dynamics and then achieving outstanding photocatalytic hydrogen evolution performance.The main content is as follows:（1）A novel amorphous monodisperse Co₃O₄ quantum dots/3D hexagonal Cd S single crystals（0D/3D Co₃O₄QDs/Cd S）p-n heterojunction was fabricated through a moderate solvothermal and electrostatic self-assembly method.The negative potential surface of hexagonal Cd S single crystals provides a good condition for the attachment of Co₃O₄ QDs with positive potential.With the synergetic effect of p-n heterojunction and the quantum confinement effect of Co₃O₄ QDs,it’s not only achieving the spatial separation of photoexciton but also limiting the charge recombination to some extent.Additionally,a small quantity of sulfur vacancy in the composite can serve as the electron trapper to promote charge transport.Thus,the Co₃O₄QDs/Cd S composite exhibits a high photocatalytic H₂ evolution rate of 17.5 mmol h^-1 g^-1with a turnover number of 4214,which is 10.3 times higher than that of the pure Cd S.（2）Defect engineering is an effective tactic to improve the photogenerated charges separation efficiency.Herein,S-scheme Zn Cd S/Zn S photocatalytic system with abundant zinc vacancies was derived from the precursor of Zn Cd S/MOF-5 in the Na₂S/Na₂SO₃ aqueous solution.The results of XPS and PL spectra confirmed the existence of zinc vacancy.The introduction of zinc vacancy can be acted as a capture center of photoinduced charges as well as form new defect levels to change the band structure of catalysts,thereby enhancing the charge transfer efficiency and the light absorption ability,and then improving the photocatalytic H₂ production activities.A mass of zinc vacancies in the Zn S is beneficial to form ohmic contact in the Zn Cd S/Zn S interface.The photogenerated electrons on the defect level of Zn S would combine with the holes on the conduction band of Zn Cd S via ohmic contact,thereby consuming the useless electrons and holes,forming the S-scheme charge transfer mechanism.As a consequence,accelerates the charge separation and transfer efficiency and restrains the photocorrosion.The maximum photocatalytic H₂ evolution rate of Zn Cd S/Zn S is 12.31mmol h^-1 g^-1 with a turnover number of 64.61,which is roughly 82.06-fold and 21.98-fold of the pure Zn S and Zn Cd S,respectively.This work provides new insight into the design of the S-scheme photocatalyst system regulated by zinc vacancies.（3）Using cocatalysts to improve photocatalytic performance has become a common strategy.Depositing oxidation and reduction cocatalysts on the semiconductors can effectually limit the recombination of charges and counterreaction.While they didn’t deposit on the expected locations in most situations,resulting in their roles couldn’t be fully utilized up to the hilt.In this work,a Cu/Cd S/Mn O_x（CSM）heterostructured photocatalyst was prepared through simple hydrothermal and in-situ photodeposition,realizing the spatial separation of photoinduced charge as well as the good H₂ evolution activities and stability.Cu nanoparticles（NPs）as the reduction cocatalyst are easy to capture electrons while the Mn O_x NPs as the oxidation cocatalyst tend to collect holes.The hexagonal Cd S crystal has the feature of different electron-rich and hole-rich facets,making Cu NPs and Mn O_x NPs deposit selectively corresponding crystal faces without mixing.The unique structure of CSM greatly promotes the transport of electrons and holes,thus accelerating the surface reaction kinetics.The results of PL spectra and photo-electrochemical characterization further proved the improved photocatalytic performance.Therefore,the optimized CSM sample reveals an excellent photocatalytic H₂ production rate of 5965.03μmol h^-1 g^-1with an apparent quantum efficiency of 4.5%under monochromatic light at 420 nm.This work shows that dual cocatalysts play a crucial role in improving charge transport and photocatalytic activity.