Synergistic Effect Of Nanocomposites With Multilevel Hierarchical Structures On The Photocatalysis

Water pollution is a worldwide widespread environmental problem,endangering ecosystem function and human health.The efficiently nanocomposites with low energy consumption for catalytic oxidation of water pollutants has become a major scientific issue.Nanomaterials with single component and single structure can not meet the multifunctional needs of advanced materials for their single performance.Controlling different structure and composition of nanomaterials with specific functions can be obtained.First of all,appropriate band structure can be obtained by controlling the composition of nanocomposites and thus the function of nanocomposites such as catalysis activity could be improved;Secondly,the forming space steric effect by gathering different low dimensional nanostructures can overcome the easily reunion problem of nanomaterials.Therefore,nanocomposites with multilevel hierarchical structures have obvious application advantages and have already become forefront and hot spots of nanomaterials research.This works mainly focus on sewage treatment and clean energy development.We put forward the design of“nanocomposites with multilevel hierarchical structures for the synergistic effect on the photocatalysis”and prepare core-shelled,double-shelled and sandwiched,double-shelled composite nanomaterials.We deeply discuss these three photocatalysts for photodegradation pollutants and hydrogen production,and used for sample pretreatment for the determination of COD and TOC in natural and waste water.1.Core-shelled Au@Cu₂O stellated polytope for adsorption and photodegradation of acid violet 43 and methyl blueAu nanoparticles were covered in a shell composed of Cu₂O nanoparticles and then uniform core-shelled and nanostructured Au@Cu₂O stellated polytope was synthesized by a facile aqueous solution approach.The samples were characterized by SEM,TEM,XRD,XPS,BET,FTIR,and UV-vis diffuse reflectance analysis.With excellent aqueous dispersibility,surface positive charge,and remarkable chemisorption capacity,Au@Cu₂O could be used for effective anionic dyes removal from solutions.Compared with Degussa P25,the adsorption of acid violet 43 and methyl blue(anionic dyes,5.0 mg L^-1)onto Au@Cu₂O was increased by90.12%and 50.8%,respectively.The photodegradation activity of methyl orange and methyl violet were in the declining order:Au@Cu₂O>Cu₂O-Au nanocomposites>Cu₂O>P25.The synergistic effect of coupling Au core with Cu₂O shell on the dyes photodegradation was observed.The photoexcited electrons from Cu₂O conduction band could be captured by Au nanoparticles,resulting in an improved electron-hole separation.Moreover,a Schottky barrier was assumed to form at the Cu₂O-Au interface and Au NPs as electron sink could reduce the recombination of photoinduced electrons and holes,facilitating the photocatalytic interface reaction.The geometry of Au@Cu₂O stellated polytope is effective in the design of Cu₂O-Au nanocomposites for adsorption and photocatalysis.2.Double-shelled and sandwiched TiO₂@Au@C hollow spheres for photodegradation of 4-nitroaniline and hydrogen generationA novel approach for the fabrication of double-shelled,sandwiched,and nanostructured hollow spheres was proposed,using hydrotherm reaction and calcination.The negatively charged nanoparticles（e.g.,Au）could be adsorbed successively onto the positively charged hollow spheres（e.g.,TiO₂）.The resulted nanocomposites（TiO₂@Au,as a proof-of-concept）were dispersed in glucose solution under hydrothermal conditions.After calcination,uniform double-shelled and sandwiched TiO₂@Au@C hollow spheres were obtained and Au nanoparticles were sandwiched between the shell wall of TiO₂ and C.The samples were characterized by SEM,TEM,XRD,XPS,BET,and UV-vis DRS.The photocatalytic activity for the degradation of 4-nitroaniline was in the order of TiO₂@Au@C>TiO₂@C>TiO₂/Au>P25.The visible-light photodegradation rate of 92.65%for 4-nitroaniline was achieved by TiO₂@Au@C,which exhibited an increase of 75%compared to Degussa P25TiO₂.Furthermore,no deactivation was occurred during catalytic reaction for three times,i.e.,the TiO₂@Au@C microspheres exhibited superior photocatalytic stability.TiO₂@Au@C microspheres could also enhance the photocatalytic activity for hydrogen generation from methanol/water solutions.The synergistic effect of coupling TiO₂ hollow spheres with Au nanoparticles and C shell on photocatalytic performance was proved by us.The photoexcited electrons from Au nanoparticles could be captured by the conduction band of TiO₂ and then the electron-hole separation was improved.Moreover,both the visible light absorption and the affinity between TiO₂ and pollutants could be improved by the coexistence of carbonaceous materials,which could facilitate the photocatalytic interface reaction.3.Double-shelled TiO₂@CeO₂/Au hollow spheres for photodegradation of methyl orangeA hydrothermal approach was proposed to prepare double-shelled,nanostructured hollow spheres with supported Au nanoparticles.The nanoparticles of TiO₂,CeO₂ and Au nanoparticles were coated successively onto the functionalized polystyrene（PS）template beads,the resulted PS@TiO₂@CeO₂/Au nanocomposites were calcined at elevated temperature and then double-shelled TiO₂@CeO₂/Au hollow spheres were obtained.The dispersity,morphology,size,and lattice of TiO₂@CeO₂/Au hollow spheres were tested by scanning electron microscopy（SEM）and transmission electron microscopy（TEM）.The presence of TiO₂ hollow sphere and CeO₂/Au shell was confirmed by electron energy loss mapping analysis and X-ray diffraction analysis（XRD）.The photodegradation activity for methyl orange was in the declining order of TiO₂@CeO₂/Au>TiO₂@CeO₂>CeO₂>P25.Under visible-light irradiation,the photodegradation rate of 91.13% was achieved for TiO₂@CeO₂/Au,which exhibited an increase of 59.32% compared with P25.The synergistic effect of coupling TiO₂ hollow spheres with CeO₂ shell and Au NPs on photocatalytic performance was proved by us.The photoexcited electrons from CeO₂ could be captured by TiO₂ and Au NPs,and then the electron-hole separation was improved.Moreover,the visible light absorption of TiO₂ hollow spheres was increased by the coexistence of CeO₂ and Au.Last,we offer a new way for facilitating electron-hole pair separation,using the Schottky barrier between TiO₂ and Au,and CeO₂ and Au,and then the major obstacle of heterogeneous photocatalysis could be resolved.