Fabrication And Visible-light Photocatalytic Hydrogen Productionactivity Of Metal Sulfide-based Photocatalyst

The over-exploitation of fossil fuels by human beings has caused serious energycrisis and environmental pollution. Therefore, replacing non-renewable energy withclean and renewable energy has attracted a lot of attention in the scientific community.It has been found that photocatalytic hydrogen (H2) production from water splittingcan be achieved on semiconductors. The photocatalytic water splitting not only makesfull use of renewable solar and water resources, but also effectively solves the energycrisis and environmental pollution. Among the developed semiconductors, CdS is ahighly efficient visible-light-active photocatalyst due to its sufficiently negativeconduction band (CB) potential and narrow bandgap. However, the photogeneratedelectron-hole pairs of CdS usually suffer from serious recombination due to its directband gap, and CdS is highly susceptible to photocorrosion. Therefore, it is highlydesirable to find ways to improve the photocatalytic H2production activity andstability of CdS. In this dissertation, on the basis of previous work, valuableexplorations on the enhancement of the photocatalytic H2production activity ofsulphide were carried out, mainly from three aspects including loading co-catalyst,forming solid solution and compositing with hybrids. The following results are mainlyobtained:Firstly, the enhanced visible-light photocatalytic H2production activity of CdS wasachieved through in-situ deposition of cocatalyst. In the second chapter, cadmiumhydroxide nanoparticles (Cd(OH)2NPs)-CdS nanorods (NRs) hybrids were preparedby a hydrothermal method using cadmium oxide (CdO) and sodium sulphide (Na2S)as precursors in sodium hydroxide (NaOH) aqueous solution. It was found that asuitable amount of Cd(OH)2NPs largely enhanced the photocatalytic H2productionactivity of CdS NRs. When the molar content of Cd(OH)2in the hybrids was12.3%,the H2production rate reached the highest,15times higher than that of bare CdS NRs.The main reason is that the photogenerated electrons from the CB of CdS reducedCd2+(released from Cd(OH)2in aqueous solution) into Cd0. Thus, Cd0nanoclusterstogether with Pt NPs acted as co-catalyst to promote the seperation and transportationof photogenerated charge carriers, and also served as active sites for water or protonreduction.Secondly, the enhanced visible-light photocatalytic H2production activity of CdSwas achieved through forming solid solution with ZnS. In the third chapter, a highlyefficient photocatalytic H2production without the assistant of co-catalyst wasachieved using Zn1-xCdxS solid solutions as the visible-light-driven photocatalysts andthe mixed Na2S and Na2SO3aqueous solution as the sacrificial reagent. TheZn1-xCdxS samples were prepared by a simple zinc-cadmium-thiourea (Zn-Cd-Tu) complex thermolysis method, using thiourea, zinc acetate (Zn(Ac)2) and cadmiumacetate (Cd(Ac)2) as the precursors. The obtained Zn1-xCdxS solid solutions possesssmall crystallite size and precisely controllable band structure, which are beneficialfor the photocatalysis. When the Zn/Cd molar ratio is1:1, the prepared Zn0.5Cd0.5Ssample exhibits the highest H2-production rate, exceeding that of the pure CdS andZnS samples by more than24and54times, respectively, and even being much higherthan that of the optimal Pt loaded CdS. This high photocatalytic H2-productionactivity is attributed predominantly to the enough visible-light absorption capabilityand suitable CB potential of the Zn0.5Cd0.5S solid solution, which is further evidencedby the related theory calculations on the band structures of the Zn1-xCdxS solidsolutions. Moreover, the calculation on the Mulliken populations of Zn, Cd, and Satoms for the first time provides new insight into the deep understanding of thechemical shifts of element binding energies for the Zn1-xCdxS solid solutions and thedesigning of new multi-component photocatalytic materials.Thirdly, the visible-light photocatalytic H2production activity of Zn1-xCdxS solidsolutions was further enhanced by combining with graphene nanosheets. In the forthchapter, a high efficiency of visible-light-driven photocatalytic H2production withoutthe assistant of noble metal co-catalysts was achieved on the obtainedgraphene-ZnxCd1-xS composites with controlled stoichiometries. Thegraphene-ZnxCd1-xS composites were for the first time fabricated through a one-stephydrothermal method with thiourea as the organic S source. It is found that, during thepreparation process, thiourea facilitates the heterogeneous nucleation of ZnxCd1-xSand in situ growth of ZnxCd1-xS nanoparticles on graphene nanosheets. Such ascenario results in an abundant and intimate interfacial contact between graphene andZnxCd1-xS nanoparticles, sufficiently an efficient transfer of the photogeneratedcharge carriers and an enhanced photocatalytic activity for H2production. Incomparison, the graphene-ZnxCd1-xS composite photocatalyst prepared by using aninorganic S source, e.g. Na2S, exhibits a much lower photocatalytic H2-productionactivity. In the latter case, homogeneous nucleation of ZnxCd1-xS becomespredominant, causing an insufficient and loose contact with graphene backbones viaweak van der Waals force and a large grain size. This study highlights thesignificance of S source choice in the design and fabrication of advancedgraphene-based sulfide photocatalytic materials with enhanced photocatalytic H2production activity.Lastly, we found that carbon quantum dots (CQDs) is an excellent cocatalystcandidate for photocatalytic H2production from water splitting. In the fifth chapter,CQDs and platinum nanoparticles (Pt NPs) were utilized as dual co-catalysts ofZnIn2S4microspheres (ZIS MSs) for photocatalytic H2production. The CQDs and Ptco-loaded ZIS MSs exhibited a high photocatalytic H2production activity intriethanolamine aqueous solution under visible-light irradiation, which was much higher than that of pure ZIS, Pt loaded ZIS and CQDs decorated ZIS photocatalysts.Such a great enhancement was attributed to the integrative effect of the goodcrystallization, enhanced light absorption ability, high electrical conductivity of CQDsand the vectorial electron transfer from ZIS to CQDs and Pt NPs (ZISâ†'CQDsâ†'Pt).During the photocatalytic process, CQDs and Pt NPs on the surface of ZIS MSsperformed as charge intermediate and electron acceptor, respectively, whicheffectively enhanced the charge separation and transfer efficiency. This work providesnew insight into the potential application of CQDs in the field of energy conversion.