The Synthesis And Photocatalytic Properties Of Micro-Nano ZnO-Noble Metal Materials Induced By Amphiphilic Calixarene

With the acceleration of global industrialization,the problem of water pollution in the environment becomes more and more serious.Under the irradiation of sunlight,semiconductor photocatalysts can degrade organic pollutants into mineral salts,carbon dioxide and water.Therefore,photocatalysis technology has been regarded as an effective means to solve the problem of water pollution in recent decades.Zinc oxide(ZnO)is a kind of direct wide band gap semiconductor,which is rich in resources and will not cause secondary pollution in the process of use.However,nano-sized ZnO has the disadvantages of photoetching and fast recombination of photogenerated electron-hole pairs,which seriously restricts the application of ZnO in photocatalytic degradation of pollutants in water.In this paper,the amphiphilic resorcinol calix[4]arene with different linear alkyl groups(denoted as ARCA)was used as an guide agent,dispersant and photosensitizer.The micro-nano structures such as ARCA-ZnO,HRCA-ZnO-Ag,HRCA-ZnO-ZnS-Ag,HRCA-ZnO-Au and PRCA-ZnO-Au-Ag were successfully prepared by atmospheric reflux.The size and morphology of ZnO-noble metal micro-nano composite materials were controlled,and the optical absorption range of ZnO-noble metal micro-nano composite materials was widened.Then the number of photogenerated carriers and the separation efficiency was increased.It provides a new idea and direction for the preparation of ZnO-noble metal photocatalysts.The main contents of this paper are as follows:(1)The amphiphilic resorcinol calix[4]arenes with different linear alkyl groups were synthesized according to the literature(the number of carbon atoms were 3,6 and 9,and abbreviated as PRC A,HRCA and NRCA,respectively).The results of structural characterization showed that all the synthesized samples were target products with yellow color.The color of HRCA-ZnO and NRCA-ZnO sample was pink,and the color of PRCA-ZnO was white.The results showed that the amphiphilic HRCA and NRCA with long alkyl chains at the lower edge could broaden the optical absorption range of ZnO samples.The photocatalytic activity of HRCA-ZnO and NRCA-ZnO was better than that of PRCA-ZnO.According to the experimental results,it was speculated that amphiphilic HRCA and NRCA could be used as shape directing agents and protective agents to prepare ZnO micro-nano materials,and show the photosensitization effect similar to dyes,and the photocatalytic performance of HRCA-ZnO was slightly better than that of NRCA-ZnO(2)In order to further improve the photocatalytic performance of HRCA-ZnO,a series of HRCA-ZnO-Ag nanocomposites were prepared by introducing AgNO3 into the reaction system.Due to the reduction ability of phenolic hydroxyl groups in HRCA,Ag+ions could be quickly reduced to metallic Ag,even prior to the formation of ZnO.The resulting silver seeds "induced" ZnO to precipitate and grow on their surface,thus successfully regulating the transformation of ZnO morphology from micronflower to nanosheet.HRCA-ZnO-Ag composite further broadened the optical absorption range of ZnO,increased the number of photogenerated carriers and improved their separation efficiency.However,due to the relatively small particle size(?nm),the hydrophobic HRCA-ZnO-Ag nanoparticles had a large buoyancy,and the samples were easy to be lost due to the difficulty of centrifugation in the recycling process(3)As mentioned above,HRCA-ZnO-Ag nanoparticles were difficult to recover in photocatalytic cycles due to their hydrophobic surface.We hoped to synthesize ZnO doped with Ag composites with large size,easy separation and high photocatalytic performance.The ZnO-ZnS-Ag microflowers coated with HRCA were synthesized by ion exchange and in-situ reduction.The products were different from the products of ZnO-ZnS-Ag2S which were obtained by ion exchange in the literature.These were attributed to the fact that HRCA reduced Ag+ions to metallic silver,guided the sample formation and protected the sample from aggregation and growth,absorbed and converted energy from visible light.The samples showed good photocatalytic activity during the recycling process,and the morphology and structure did not change significantly.(4)HRCA-ZnO-ZnS-Ag composites solved the problem of sample loss.However,the morphology of flower ball was relatively compact,under sunlight irradiation there were few active sites of the flower ball.Moreover,HRCA was wrapped tightly,which could inhibit the migration of photogenerated carriers to the environment.HRCA-ZnO-Au micro-nano composites were synthesized by a simple atmospheric reflux process.AuC14-ions were reduced to Au by phenolic hydroxyl in HRCA,and deposited on ZnO sheet,which prevented the new sheet of ZnO from depositing and growing at this position.Finally,the flake sample was obtained.In order to expose the surface of ZnO-Au,HRCA-ZnO-Au micro-nano composites were calcined at 400? to partially remove HRCA from the sample surface.A good synergistic effect was formed between ZnO and Au nanoparticles,and the photocatalytic activity of ZnO-Au micro-nano composites was significantly improved(5)HRCA was an effective shape directing agent and protecting agent for the synthesis of ZnO materials.However,its close coating on the surface of ZnO might inhibit the migration of photogenerated carriers to the environment.PRCA could synthesize the hydrophilic ZnO materials,a small amount of Au nanoparticles could control the morphology of ZnO;Ag had rich sources and strong electron transport ability.Combined with the advantages of PRCA,Au and Ag,ZnO-Au-Ag hydrophilic loose flower like composites were synthesized.The Au particles were evenly dispersed on the sample with relatively small size,while the Ag particles were relatively concentrated on some places with relatively large size.Under simulated sunlight,the photocatalytic performance of hydrophilic ZnO-Au-Ag with loose flower ball was significantly improved,and it had good photocatalytic cycle stability.Through this study,ARCA was successfully introduced into ZnO synthesis system to play the role of morphology orientation,protection and photosensitization.A series of ZnO micro-nano composites doped with noble metals(Au,Ag)were synthesized by ARCA had widened the optical absorption range of ZnO,increased the number of photogenerated carriers,and improved the efficiency of photogenerated carrier separation.In the future research work,we will continue to synthesize novel amphiphilic calix[4]arenes and apply them to synthesis of ZnO micro-nano composites with larger specific area,so as to further broaden the optical absorption range of ZnO,and improve the quantity and separation efficiency of photo generated current carriers.