Synthesis And Photocatalytic Activity Of Graphene And Graphene-based Metal Sulfid Hybrid Materials

With the rapid development of industrial technology, the environmental pollutionproblems become increasingly serious, which have caused incalculable harm to thepeople’s health and life. Thus the pollution control and the environment protectionhave become one of the most concern problems. Semiconductor catalysis technologyhas been rapidly developed, which plays a decisive role for control environmentalpollution and purifying environment. II-VI metal sulfids have been widely used inphotocatalytic fields due to their energy band belonging to direct transition model,and having the special photoelectric properties. However, as with most of othersemiconductor catalysts, there are also some urgent problems for metal sulfides to besolved in the process of catalytic application, such as easy to clusters fornano-semiconductors, wide band gap, fast recombination of photogeneratedelectron-hole pairs, etc.Graphene, as a kind of novel carbon materials, has a two-dimensionalhoneycomb lattice structure, sp2-hybridized carbon atoms sheet, and exhibits somespecial properties, such as high specific surface area, strong adsorption capacity,excellent conductivity, etc. Because of these, researchers have tried to hybridizegraphene with nano-semiconductors by using various methods, to synthesizegraphene-based nano-semiconductor hybrid materials, which will be applied incatalysis field. Surprisingly, compared to pure nano-semiconductor, graphene-basednano-semiconductor hybrid materials can just solve the above problems in thecatalytic application. In addition, graphene oxide (GO) has often chosen as the starting material to synthesize graphene-based hybrid materials because that it has someoxygen-containing functional groups on the surface and edge, making GO easier toincorporate with nano-semiconductors through covalent, non-covalent, etc., and to fixnano-semiconductor on the GO surface.Based on the above analysis, we have selected GO as the starting material tosynthesize graphene-based metal sulfide hybrid materials by using solvothermal/hydrothermal method. And the morphology, structure, properties, and photocatalyticactivity of the hybrid materials have been studied. The main research contents andresults are listed as follows:In Chapter2, reduced graphene oxide/cadmium sulfide (RGO/CdS) hybridmaterials have been successfully synthesized by using one-step and simplesolvothermal process, in which CdS nanoparticles were uniformly distributed on thesurface of RGO. Compared to pure CdS, RGO/CdS shows an enhanced absorptionintensity in visible light region, and a gradual red shift of absorption edge, indicatingthe presence of RGO help CdS more effectively absorb and utilize visible light. Andthe phenomenon of fluorescence quenching is observed, indicating thatphotogenerated electron from CdS effectively transfer to RGO. As the weight ratio of5.0%for RGO, RGO/CdS (namely, RGO5/CdS) shows the highest photodegradationefficiency (94%) and removal efficiency (63%) of total organic carbon (TOC) for MBunder visible light irradiation, as well as excellent stability and reuse. The enhancedphotodegradation efficiency for RGO/CdS could be attributed to the RGO as electronacceptor and transporter that promotes photogenerated charges separation and transfer,as well as fix the CdS nanoparticles as support material. In addition, the appropriateaverage particle-particle’s distance (AP-PD)(14.8nm) increases the combinationchances of O2adsorbed on RGO surface and electrons, thus forming O2-and OHradicals. The above factors together improve the photocatalyst activity for CdS.In Chapter3, a series of reduced graphene oxide/zinc sulfide (RGO/ZnS) hybridmaterials has been synthesized by using solvothermal method. RGO/ZnS with thedifferent RGO contents show the cubic structure of sphalerite, the homogeneousdistribution of ZnS nanoparticles on the RGO surface, and the effective reduction of GO. The presence of RGO effectively enhances the UV absorption intensity of ZnS,but does not change the absorption band. The fluorescence quenching effect showsthat the RGO effectively transfer the photogenerated electrons from ZnS under UVirradiation. RGO/ZnS hybrids show the higher photodegradation activity for MB thanthat of pure ZnS under UV irradiation. When the weight ratio of RGO is6%,RGO6/ZnS hybrid material shows not only the optimal photodegradation activity andthe kinetics of degradation (93%, k=0.0312),2.7times more than pure ZnS, but also agood photostability.In Chapter4, based on the researches in the first two chapters, considering theinevitable photocorrosion of CdS and wide band gap of ZnS, to make up for theinadequacy of the two, we have successfully synthesized ternary metal sulfidZnxCd1-xS and ZnxCd1-xS/RGO (x=0,0.2,0.4,0.6,0.8,1.0) hybrid materials by usingthe green and eco-friendly hydrothermal method. The ZnxCd1-xS/RGO shows theporous structure, and ZnxCd1-xS nanoparticles evenly dispersed on the RGO surface(the minimum average particle size about8nm). With the decreasing Zn contents,ZnxCd1-xS/RGO exhibits the gradual red shift of absorption edge, and extends to thevisible light region, implying the formation of solid solution. And, the RGO onlyenhances the absorption intensity of ZnxCd1-xS in visible light region forZnxCd1-xS/RGO, does not change its band gaps. According to Kubelka-Munk function,the band gap energy of ZnxCd1-xS/RGO have been calculated from3.42to2.21eV,indicating that the band structure of ZnxCd1-xS/RGO can be tuned by changing themolar ratio of Zn/Cd. When x=0.4, namely Zn0.4Cd0.6S/RGO, shows the highestphotodegradation efficiency (98%) and the removing efficiency (67%) of TOC, aboutof3.6and9.6times higher than Z0CSG (CdS/RGO), respectively. The SO42-, NO3-,NH4+have been monitored in the final reaction liquid using ion chromatographanalysis, which further confirm the MB molecules were effectively mineralized intoinorganic molecules. In addition，the slight efficiency decrease for three cyclicphotodegradation experiments indicates the Zn0.4Cd0.6S/RGO is reusable. Meanwhile,no toxic Cd ions were detected, indicating that Zn0.4Cd0.6S/RGO is also stable, doesnot cause the secondary pollution. Furthermore, compared to Zn0.4Cd0.6S, the analysis of electrochemical impedance spectroscopy (EIS) and Mott-Schottky (MS) spectrumfor Zn0.4Cd0.6S/RGO show fast interface charge transfer and less the recombination ofcharge carriers due to the introduction of RGO. Based on the above discussion andanalysis, the mechanism of enhanced photocatalytic activity for Zn0.4Cd0.6S/RGO canbe attributed to the synergistic effect between the RGO and the Zn0.4Cd0.6S:(1) a optimal band gap and moderate conduction band position for Zn0.4Cd0.6S;(2) the efficient separation of photogenerated charge carriers；(3) relatively high surface area for Zn0.4Cd0.6S nanoparticles.