Synthesis And Photocatalytic Properties Of New Composites Based On Graphene And Semiconductor

As is known to all,semiconductor photocatalyst is the most widely used in photocatalytic field during the recent decades.However,the conventional semiconductor photacatalyst due to the wide band gap,the fast recombination of photo-generated charge carries and the low quantum efficiency limited its practical utilization.Therefore,in order to enhance the photocatalytic efficiency,more and more researchers tried to fabricate a new type of composite materials with the introduction of carbon materials,which can overcome the drawbacks of conventional semiconductor.And thus it has become a new research direction in photocatalytic field.Recently,graphene has drawn much attention due to its remarkable properties such as superior electrical conductivity,specific surface area and high carrier mobility because of a flat monolayer of hexagonally arrayed sp2-bonded carbon atoms tightly packed into a two-dimensional honey-comb lattice.It can serve as an ideal support material with improved interfacial contact and enhanced adsorption activity.In this thesis,we focused on preparing the pristine graphene and graphene derivatives（i.e.,nitrogen-doped graphene and reduced graphene oxide aerogel）modified the semiconductor materials with wide band gap,improving the visible light harvest and enhancing efficient separation of photogenerated electron-hole pairs.Thereby,it is aimed at improving the photocatalytic activity for the degradation of organic dyes under visible light irradiation.Besides,the photodegradation reaction mechanism was also discussed in depth.The main contents are shown as follows:1.A series of nitrogen-doped graphene–BiOBr（NG-BiOBr）nanocomposites with different weight addition ratios of NG were firstly prepared by a facile solvothermal method,and found to possess a higherphotocatalytic activity than pure BiOBr toward degradation of methyl orange（MO）under visible light irradiation.It is demonstrated that the NG nanosheets can serve as special ‘‘surfactant’’ and play very important role in controlling the final morphology of BiOBr.The NG-BiOBr composite with 1.76 wt% NG content exhibited the highest photodegradation efficiency of MO,its degradation rate was about 50,4.6and 3.8 times of P25,BiOBr microsphere and 1.76 wt% RGO–BiOBr composite,respectively.The enhanced photocatalytic performance could be ascribed to more visible light harvest and more effective separation of photogenerated electron–hole pairs.The degradation of MO over NG–BiOBr composites was mainly attributed to hydroxyl radicals（·OH）and superoxide radicals（·O₂^-）.2.A facile solvothermal route was developed to synthesize 3D reduced graphene oxide–BiOBr（3D RGO–BiOBr）composites with different weight addition ratios of RGO,which shows a higher visible light photocatalytic activity and stability toward degradation of MO in water than pure BiOBr.It is demonstrated that RGO:BiOBr=2:1 has the highest photodegradation efficiency for MO removal（85.8% in 2.5 h）,which was about 5.8 times that of pure BiOBr.The improved photocatalytic activity of 3D RGO–BiOBr composites was ascribed to the more effective separation of photogenerated electron-hole pairs and expanding the optical response range.The advantage of this strategy is that the composite is easily to be removed from the aqueous reaction systems for recycling.In addition,the control experiments implied that ·O₂^ˉ is the main active species during the photocatalytic reaction.3.We have developed a facile solvothermal approach to obtain NG–Bi₂WO₆ composites with weight addition ratios of NG,and used them in the photocatalytic degradation of Rh B aqueous solution under the visible light illumination.The 1.5 wt% NG–Bi₂WO₆ composite exhibited enhanced photoactivity and its degradation efficiency was about 3.5 times that of pure Bi₂WO₆.It is explored that the enhanced photocatalyticactivity of NG–Bi₂WO₆ composites can be mainly attributed to the more effective separation of photogenerated electron-hole pairs.At last,controlled experiment proved that the degradation of Rh B was mainly attributed to h⁺ and ·O₂^-,while hydroxyl radicals（·OH）only played a relatively minor role in the whole process.4.NG modified SnO₂ nanoparticles with ca.10–20 nm in size composites（NG–SnO₂）with different contents of NG were obtained by a facile solvothermal method,which displayed higher photocatalytic activity and stability than pure SnO₂ toward degradation of Rh B aqueous solution under visible light irradiation.The 7.5 wt% NG–SnO₂ composite possessed the highest photocatalytic efficiency.As a result,due to enhanced adsorption capacity and more effective separation of photogenerated electron-hole pairs,NG–SnO₂ composites showed improved photocatalytic activity.Finally,we examined the effects of different scavengers on the degradation efficiency,which indicated that the degradation of Rh B over NG–SnO₂ composite photocatalysts was mainly attributed to ·O₂^-or direct oxidization by h⁺.