Controllable Synthesis, Characterization And Optical Properties Of ZnS Nanostructures

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 ZnS nanos pheres,and the influence of morphology, phase structure, and composition on the opticalproperties and photocatalytic activity were systematically investigated. The mainachievements are listed as follows:(1) ZnS nanospheres with cubic phase were successfully synthesized viachemical bath deposition. The size of ZnS nanospheres can be modulated only bytuning bath temperature. The as-prepared ZnS nanospheres are uniform in size andwell-dispersed. From the TEM micrographs, one can see that these spheres are fairlyrough and porous, illustrating that the spheres are composed by smaller particles withthe diameter about several nm, which is quite consistent with XRD analysis. The BETmeasurement is used to survey surface area of ZnS nanosphere samples. The specificsurface areas of ZnS nanospheres are43.1173m2/g (T=95℃) and12.5350m2/g(T=75℃) as determined by BET. Moreover, further comparative experiments werecarried out to investigate the effects of concentration of reactants、the PH value ofsolution and reaction time 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) To explore the potential applicability of the as-prepared ZnS nanospheres, the photocatalytic activity by choosing the degradation of MB solution is investigated atroom temperature. The characteristic absorption of MB at about664nm is chosen asthe monitored parameter for the photo-degradation process. ZnS nanospheres preparedat95℃possesses the highest catalytic activity, followed by the samples prepared at85℃,75℃, and65℃, respectively. The thorn cubic ZnS nanostructures also exhibitenhanced catalytic activity in visible light. Our results shed new light on thepreparation of highly efficient semiconductor photocatalysts used in environmentalremediation applications.(3) ZnS nanowires were synthesized by a modified chemical vapor deposition(CVD) method. The morphology, crystal structure and optical properties of ZnSnanowires were investigated by TEM、EDS、XRD diffraction, Raman scattering, andsurface photovoltage spectra. Characterization results prove that as-prepared ZnSnanowires possess high quality with cubic phase structure. Photoluminescence (PL)spectra measurements show that ZnS nanostructures exhibit near-bandedge emissions,with peaks wavelength at337nm. The Surface Photovoltage measurements of the ZnSnanowires show that the as-grown ZnS nanowires have a strong response. It is clearthat SPV response features a peak located at about334nm, which corresponds to thePhotoluminescence at337nm.