Preparation And Visible-Light Photocatalytic Performance Of Metal Sulfide-Based Photocatalysts

Nowadays,energy and environmental issues are globally important topics.To solve these issues,environmentally benign and renewable technologies for green energy production and environmental remediation must be developed.Photocatalysis technology,which utilizes sustainable solar energy to produce hydrogen from water splitting,reduces CO₂ into hydrocarbon fuels and degrades organic pollutants over the surface of photocatalysts,has attracted increasing research attention.The overall photocatalytic performance of the given photocatalyst is limited by three key steps:1)light harvesting properties;2)charge separation and transportation to the surface of semiconductor?active site/reaction site?;and 3)surface catalytic reduction and oxidation reactions.The solar-to-chemical energy conversion efficiency is determined by the balance of these thermodynamic and kinetic processes.For superior photocatalytic performance,the photocatalyst should simultaneously possess wide light absorption range,high charge separation efficiency,rapid charge migration rate and strong redox ability.Metal sulfides usually owing relatively narrow band gap,wide light responsive range and strong reduction ability,have been widely used in photocatalytic reduction reactions.However,given their narrow band gap,the recombination of photogenerated charge carriers is fast and photon conversion efficiency is low.In this dissertation,we mainly focused on the increasing the photoexcited charge separation efficiency of the sulfide-based photocatalytic materials,mainly from microstructure modification,loading cocatalyst,forming direct Z-scheme heterojuntion photocatalyst.The detailed contents were summarized as follows:Firstly,Co-Pi based oxidation cocatalyst was successfully deposited on the surface of CdS nanorods through a simple photochemical process in a neutral phosphate buffer solution containing Co²⁺ions.The prepared Co-Pi/CdS composite samples exhibited excellent photocatalytic H₂ production activity.According to the systematical characterizations,the enhanced visible-light photocatalytic H₂production activity was attributed to the hole trapping and collecting ability of Co-Pi cocatalyst,which could effectively suppress the recombination of photogenerated electron-hole pairs and increase the electron density for hydrogen production.At the same time,the photocorrosion of CdS could be effectively inhibited due to the hole trapping and collecting ability of Co-Pi cocatalyst,thus the photostability of CdS would be improved.Secondly,we successfully prepared two-dimensional CdS nanosheets by a low-temperature solvothermal strategy.And then,a novel metallic nickel nanoparticles encapsulated in few-Layer graphene cocatalyst was obtained by carbonization of metal–organic frameworks?MOF?and then and leaching treatment of hydrochloric acid.The resultant CdS-Ni@C nanocomposites are composed of CdS nanosheet with a thickness of several nanometers decorated with graphene-coated Ni?Ni@C?.The core-shell structure effectively separated Ni from the ambient to be contaminated and restrained the formation of Ni-hydrogen bonds which modulates desorption of hydrogen.Simultaneously,the graphene layers act as electron acceptor and medium.This structure exhibits many excellent properties,such as the superior conductivity,the suppression of the reverse reaction and the stability of the cocatalyst.Thus,the obtained CdS-Ni@C nancomposite displayed superior hydrogen production performance than that of pure CdS nanosheet.Thirdly,hierarchically nanostructured CdS composed of 4.7 nm-thick self-assembled ultrathin nanosheets was synthesised through a microwave-assisted solvothermal method.Ag₂S nanoparticles were deposited at the edge of the CdS nanosheets by an in situ ion exchange strategy.Given the difference in work functions between CdS and Ag₂S,electrons diffused from the CdS side to the Ag₂S side until the Fermi levels align after their contact.When the CdS–Ag₂S was illuminated,the photogenerated electrons on the conduction band of the CdS further migrated to Ag₂S,this migration promotes the efficient charge separation of CdS.This CdS–Ag₂S nanocomposite exhibits superior and durable photocatalytic H₂ production activity.The enhanced photocatalytic performance can be attributed to the effective charge separation and the low overpotential of Ag₂S,as well as the utilisation of the NIR light induced by Ag₂S.Lastly,besides CdS,which was widely applied in photocatalytic reduction reactions,other sulfides also have good visible light response and favourable photocatalytic performance.Therefore,this chapter mainly focuses on another narrow band gap semiconductor SnS₂,from its application in photocatalytic carbon dioxide reduction.A novel type of direct Z-scheme g-C₃N₄/SnS₂ heterojunction was constructed by depositing SnS₂ quantum dots onto the g-C₃N₄ surface in situ via a simple one-step hydrothermal method.X-ray photoelectron spectroscopy analysis indicated electron transfer from g-C₃N₄ to SnS₂ after deposition,forming an interfacial internal electric field directed from g-C₃N₄ to SnS₂.Driven by this internal electric field,electrons in SnS₂ conduction band would combine with holes in g-C₃N₄valence band under visible light irradiation,resulting in the Z-scheme pathway.The Z-scheme configuration enhanced the electron extraction for g-C₃N₄/SnS₂ and thus led to more efficient photocatalytic CO₂ reduction to CH₃OH and CH₄.