Regulation of Carrier Properties in Zinc-chalcogenide Based Catalysts for Their Enhached Performance

Author:Zhou Yan Song

Supervisor:chen gang


Degree Year:2018





Building a clean energy society is crucial to solve the energy and environmental crisis nowadays.Semiconductor-based photocatalytic technology has become a hotspot in recent years due to its advantages such as the low cost,simple impletion over other solar energy conversion technologies.However,the fast recombination efficiency of photogenerated charge carriers in semiconductor photocatalyst is a key factor that restricts the performance.Therefore,it is of great significance to promote the charge carrier separation efficiency to improve the performance of photocatalyst.In this paper,the zinc-chalcogenide based photocatalystswas selected,a series of modified means were used to control the photogenerated carrier separation behavior of photocatalyst,and to achieve improved photocatalytic performance.The properties of photogenerated charge carriers in ZnS–based photocatalysts were regulated by means of N doping,formation of solid solution and graphene compositing.The results of DFT theoretical calculation and VB-XPS tests showed that N doping can increase the ZnS valence band maximum continuously,thus reducing the oxidation capacity of photogenerated holes.Results of XPS and other tests showed that the N-doped ZnS showed improved light stability,and had improved carrier separation efficiency and photocatalytic water-splitting performance.The optical absorption edge of ZnS was further extended to the visible light region by forming a solid solution with CdS.Simultaneously,the ZnS-CdS solid solution photocatalyst showed improve hydrogen performan ce under visible light illumination.By developing a novel synthetic method,well-composited ZnS-CdS/graphene(G)photocatalysts were synthesized,leading to the improved photogenerated charge carrier separation efficiency,which resulted in improved photocatalytic performance for both hydrogen evolution and methylene blue degradation over ZnS-CdS solid solution photocatalyst under visible light.The influence of space electric field on the separation behavior of the photogenerated charge carriers in ZnO and ZnO-Ga N solid solution were investigated by tunning the particle size and doping concentrations.A novel and facile synthesis method was developed to prepare the ZnO nanosheet photocatalyst with high crystallinity and clean surface.Analysis of the elect ric field distribution in ZnO with different particle size,results showed that the space electric field has a high volume ratio at smaller size,which improved the photogenerated charge carrier separation efficiency and lifetime.Using La as modifier,the space charge layer thickness of ZnO-Ga N solid solution photocatalysts were investigated with different La doping concentrations.The photogenerated charge carrier separation behaviors were systematically investigated using PL and photoelecchemical tests.The results showed that increasing the space charge layer thickness of the semiconductor will increase the lifetime of the photoelectric carrier for the certain penetration depth of the incident light.When the concentration of La doping was 3%,the ZnO-Ga N solid solution photocatalyst showed dramatically enhanced photocatalytic water-splitting performance.Combined with seed-mediated growth method and in-situ ion exchange technology,the PEDOT:PSS/CdS/ZnO/WS2 nano-photoelectrode was constructed using the CdS sensitized ZnO nano-rod as the major photoactive component.The structure and interface of the composite were analyzed using SEM,TEM,and EDX.The effects of various functional components on the photogenerated charge carrier separation behaviors in PEDOT:PSS/CdS/ZnO/WS2 nano-photoelectrode are systematically discussed.PL and photoelecchemical tests showed that a r ational and orderly combination of the functional components is the key to achieve high charge carrier separation efficiency in PEDOT:PSS/CdS/ZnO/WS2 nano-photoelectrode as well as imrroved photocatalytic performance.Under simulated sunlight illumination,the sample with the optimal compositing ratio showed outstanding photocatalytic that the highest hydrogen evolution rate of 1028mmol/h.Morever,PEDOT:PSS/CdS/ZnO/WS2 nano-photoelectrode can response to the long wavelength light,with 0.3%AQY of hydrogen evolution atλ=600 nm and detectable photoelectric signal atλ=700 nm.Nb3.49N4.56O0.44/ZnO-Ga N solid-state Z-scheme photocatalyst was constructed by analyzing the interfacisl properties of semiconductor junction.By testing and analyzing the semiconductor characteristics and band positions of Nb3.49N4.56O0.44and ZnO-Ga N solid solution using Mott-Schottky and VB-XPS,the Z-scheme transfer mechanism in the system of Nb3.49N4.56O0.44/ZnO-GaN was revealed.Nb3.49N4.56O0.44/ZnO-Ga N heterostructure system with the strong interface was synthesized using a high temperature solid phase method.PL and the photoelecchemical test showed that the photogenerated charge carriers were efficiently separated.With SPV and in-situ photochemical deposition Pt experiment,the Z-scheme transfer mechanism was demonstrated in Nb3.49N4.56O0.44/ZnO-Ga N heterostructure system.The Nb3.49N4.56O0.44/ZnO-Ga N solid-state Z-scheme photocatalyst shows excellent photocatalytic reduction Cr(VI)and oxidation water performance under simulated sunlight.