Controllable Synthesis, Characterization And Photocatalytic Properties Of ZnS Nanomaterials

Energy crisis and environmental problem have become a restricting factor in thesustainable development of human society in21st century. Therefore, exploiting greenenergy and controlling environment pollution is one of the most important subjects inscience and engineering fields at present. Semiconductor photocatalysis has beengained increasing attention in the field of environmental protection, especially in thedegradation of organic water pollutants due to the advantages of energy saving andenvironment friendly. As an important II-VI semiconductor material, ZnS has uniqueoptical property and the favorable thermal and chemical stability, and is an active andpotential photocatalysts. The photocatalytic reaction mainly takes place on the surfaceof the catalysts, so the photocatalyitc activity of ZnS is strongly dependent on themorphology and surface area. Recently, to improve the photocatalyitc activity bycontrolling the morphology and size has become one of the popular research subjectsin the field of semiconductor photocatalysis. In this thesis, a simple and low-costchemical bath deposition (CBD) procedure is presented for synthesis ZnSnanostructures, and the influence of morphology, phase structure, and composition onthe optical properties and photocatalytic activity were systematically investigated.The main achievements are listed as follows:(1) Porous ZnS nanospheres with cubic phase were successfully synthesized viachemical bath deposition. The size of the ZnS nanospheres is controllable byadjusting the pH of reaction solution. The as-prpared porous ZnS nanospheres areuniform in size and well-dispersed. BET analysis indicated that the specific surfacearea of the porous ZnS nanospheres was31.09m2/g and the pore size was about2-4nm. The photocatalytic activity of the ZnS nanospheres was evaluated byphotodegradation reaction of methylene blue. The porous ZnS nanospheres preparedat pH=8.5shows the highest catalytic activity, which is associated with the largerspecific surface area and favorable dispersibility. Moreover, further comparativeexperiments were carried out to investigate the effects of reaction time and the dosageof trisodium citrate on the morphology and size of the samples. On the basis ofexperimental results and reaction mechanism, the possible formation mechanism ofthe porous ZnS nanospheres was proposed.(2) ZnS nanostructures with two different morphologies were prepared in a binary solution made of ethanol amine and distilled water. The influence of ethanolamine on the morphology of ZnS nanostructure was investigated, the as-preparedstinging spheres with diameter of around50nm, which the dosage of ethanol aminewas125mL, presented as dark-gray and exhibit enhanced photocatalytic activity. Inour experiment, ethanol amine act not only as solvent, but also as complexing agentfor Zn2+and alkaline medium provider for promoting the decomposition of thiourea.It is interesting that the morphology of ZnS nanostructure can be easily tuned byincreasing the dosage of ethanol amine, and the size is decreased gradually, thesurface are becoming increasingly complex, the color of the sample turns dark.(3) The flower-like cubic ZnS nanostructures were successfully synthesized inthe mixed complexing agent made of trisodium citrate and tartaric acid, and theinfluence of Zn/S on the morphology of ZnS nanostructures were investigated. SEMimage shows that the ZnS flowers are constructed by numerous nanosheets. The PLspectra exhibit a strong emission peak centered at580nm, which is related todeep-level defects of ZnS. The increased concentration of zinc or sulfur source wouldlead to the increase of defects, which results in the enhanced defects emission.