Enhanced Charge Transfer And Photocatalytic Performance By Defect States In Semiconductor Photocatalytic Materials

Human beings vigorously advocate environmental protection so that the concept of sustainable development can be deeply rooted in the minds of the people,as the issue of environmental pollution and energy shortage gets increasingly serious nowadays.However,the higher recombination ratio of the photogenerated charge carriers and the lower charge transferability are still the critical puzzles that limit the development of photocatalytic technology.Based on the application of photocatalytic decomposition of hydrogen and photocatalytic degradation,this thesis aims to improve the performance of photo-generated charge transfer and reduce the recombination rate of photo-generated electron holes.Three kinds of photocatalytic materials with high efficient charge transfer performance were designed and developed by using different forms of defect states:A new photocatalyst material with surface vacancy defect and carbon intercalation structure was prepared by constructing the defective state in the co-catalyst.A new photocatalyst material with a micro doping defect structure and surface supported cocatalyst was prepared.A new photocatalytic material with an interface conjugation effect was designed and prepared by using vacancy defects to effectively connect TiO2 and MOFs.Through the combination of experiments and theoretical calculations,the effect of defect state on the performance of photo-generated charge transfer has been deeply explored,which provides a new method and new idea for the design of high-performance photocatalyst.(1)Carbon(C)-MoS2-x@CdS(C-MoS2-x@CdS)core-shell nanostructures with controlled surface sulfur(S)vacancies were prepared via a glucose assisted hydrothermal growth method.The glucose acted as a reducing agent of C-MoS2-x to partially reduce Mo4+ ions to Mo3+and served as a carbon source to insert the amorphous carbon into the layered MoS2-x simultaneously.Moreover,the inserted carbon in layered MoS2-x enhanced the electron mobility and decreased the resistance electron transfer.Density functional theory(DFT)calculation confirmed that the surface S-vacancies and the amorphous carbon increase the projected density of states at the conduct band edge,which could enhance the photo-absorption and photo-responsibility.The result is consistent with the photocatalytic H2 production experiment.C2-10%MoS2-x@CdS presented a high H2 evolution rate of 61.5 mmol h-1 g-1 under visible light irrigation(?? 420 nm),which is 1.98 times and 158 times higher than that of sample without S-vacancies(10%MoS2@CdS)and pure CdS,respectively.(2)Ni-doped CdS nanospheres with Ni2P loading(Ni2P@CdS-Ni)were successfully prepared via the two-step solventhermal and chemical reduction method.The Ni2P would accelerate the photo-generated electrons transfer from CdS-Ni to Ni2P and decrease the charges recombination.Moreover,DFT calculations results confirmed that Ni cations doped in the lattice of CdS lead to a doping level at the bottom of the conduct band,which furthernarrows the bandgap.The work function indicated that the doped Ni2+ also helps to enhance the capability of the photo-induced charge migration and separation.Base on the synergistic effects,3.7%Ni2P@CdS-Ni-3 presented a high H2 evolution rate of 176.6 mmol h-1 g-1 under visible light irradiation,which is 63 times higher than CdS.(3)Interfacial conjugation was employed to engineer preparation of NH2-MIL-101(Fe)@TiO2 heterojunction photocataysts through carboxylate bidentate linkage with TiO2 and NH2-MIL-101(Fe),which can enhance the electron transfer capability from MOFs to TiO2 and photocatalytic activity.The carbon nanospheres derived from glucose acting as reducing agent and template to synthesize oxygen vacancies TiO2 hollow nanospheres.Subsequently,NH2-MIL-101(Fe)was coordinating growth on the surface of TiO2.The conjugation effects between TiO2 and NH2-MIL-101(Fe)enhanced electron transfer capability and could also induce the band tail states to narrow bandgap of the composites.Thus,the photodegradability of methylene blue was remarkably enhanced under visible light irradiation.The degradation rate of 17%NH2-MIL-101(Fe)@TiO2 was 0.131 min-1,which was about 3.5 and 65 times higher than that of NH2-MIL-101(Fe)and TiO2,respectively.Through the research of this thesis,different forms of defect states are used to try to solve the problem of charge transfer performance in semiconductor photocatalytic materials in a progressive manner.Experiments and DFT theoretical calculations have verified that the construction of surface S vacancies on the surface of the cocatalyst,the construction of Ni element doping in the the mian catalyst and using oxygen vacancies to connect the main catalyst and cocatalyst can effectively enhance the photo generated charge transfer performance,which provides a new idea for the preparation and design of high-performance photocatalysts.