Synthesis Of Graphene-semiconductor Composite Photocatalysts And Study On Their Photocatalytic Performance And Mechanism

Recent years have witnessed the flourish of constructing graphene（GR）-semiconductor composite photocatalysts for solar energy conversion.Owing to that GR possesses unique two-dimensional（2D）sheet structure,excellent electron conductivity,high optical transparency and theoretically large surface area,it has been considered to be an ideal platform for assembling semiconductor components.The incorporation of GR with semiconductor has been widely proven to be effective for improving the efficiency of semiconductor photocatalysts,which provides a promising strategy for resolving the two key issues of low quantum efficiency and solar energy conversion efficiency that greatly hinder the development and wide application of semiconductor-based heterogeneous photocatalysis.Therefore,it is highly desirable to design new and efficient GR-semiconductor composites with optimized photocatalytic performance and exploit the photocatalytic mechanism,as well as extend their photocatalytic applications.This would significantly promote the development of photocatalytic solar energy conversion for eliminating the energy and environmental problems.This thesis mainly contains the following five parts:（1）We have synthesized a series of carbon（C60,CNT,and GR）-TiO₂ composite photocatalysts and carried out a comparative study on their photocatalytic performance for selective oxidation of benzylic alcohols to aldehydes.The results prove that,without the optimization of the composite photocatalyst system,GR is in essence the same as other carbon allotropes（CNT,C60）on enhancing the photocatalytic activity of TiO₂.This work highlights that although GR by itself has unique structural and electronic properties in comparison with other carbon allotropes,more efforts are keenly required to efficiently exert these unique properties of GR for fabricating high performance GR-semiconductor composite photocatalysts.（2）A series of GR-In₂S₃ composite photocatalysts with different interfacial contact effect have been synthesized via a surface charge modified method and applied to photocatalytic selective reduction of nitroaromatic compounds to amines.The results show that strengthening the interfacial interaction between In₂S₃ and GR is beneficial for efficient utilization of the electron conductivity of GR,which is able to promote the separation of photoexcited electron-hole pairs and increase the life span of photogenerated charge carriers,thus resulting in the photoactivity enhancement of the GR-In₂S₃ composites.（3）The ternary GR-CdS-TiO₂ composites have been synthesized by assembling GR and TiO₂ as dual co-catalysts into the semiconductor CdS matrix.The composites are able to serve as highly selective visible-light-driven photocatalysts for oxidation of saturated primary C-H bonds in toluene and substituted toluenes to corresponding aldehydes.The introduction of GR and TiO₂ as dual co-catalysts into CdS has promoted the effective electron transfer in the ternary composites significantly via the advantageous electrons relay,thus enhancing the photoactivity of CdS.（4）A simple,ultrasonic treatment of graphene oxide（GO）sheets with acid has been adoped to prepare GO-A with smaller lateral size dimensions and increased density of oxygenated functional groups and colloidal stability.This treatment remarkably improves the photoactivity of graphene oxide-derived graphene-CdS composites toward selective photoredox processes under visible light illumination.This is mainly because that GR-A obtained from the reduction of GO-A with smaller lateral sizes is less likely to aggregate,which thus displays higher electronic conductivity than GR derived from the reduction of GO without acid treatment.The findings highlight that the physico-chemistry features of GO play significant influence on the preparation,photoelectrochemical and photocatalytic activities of graphene oxide-derived graphene-semiconductor composites.（5）The GR-ZnO composites with intimate interfacial contact have been prepared and used as visible-light-driven photocatalysts for catalytic reduction of Cr（VI）in aqueous solution,while the semiconductor ZnO is unable to be band-gap photoexcited under visible light irradiation.The result proves that the visible light photoactivity of GR-ZnO is ascribed to the photosensitizer role of GR for semiconductor ZnO.Moreover,it has proposed that three basic principles are required for experimentally observing the photosensitizer role of GR in the GR-semiconductor composites under visible light:（i）the semiconductor by itself cannot be band-gap photoexcited;（ii）the proper interfacial interaction manner between GR and semiconductor should be achieved;（iii）the suitable probe reaction should be chosen.