Research On Zn_xCd_1-xS-based Photocatalyst For Hydrogen Production From Water

Photocatalytic decomposition of water into hydrogen is expected to be one of the ideal ways to prepare hydrogen.It is a very promising research direction in the area of green energy.Development of high efficient,stable,and low cost semiconductor materials is crucial for the practical application of hydrogen production technology.CdS semiconductor materials absorbe in the visible light region,so they are visible light sensitive photocatalysts.But CdS is easy to undergo photocorrosion during the photocatalitic reactions.Intoduction of ZnS into CdS leads to the formation of zinc-cadmium sulfide(Zn_xCd_1-xS)solid solutions.The resulted ternary metal sulphide has the advantages of both CdS and ZnS with better capibilty to resist photocorrosion,visible light response,and the tunable forbidden band width.Most importantly,this ternary metal sulphide exhibited excellent activity for photocatalytic hydrogen production,therefore received extensive attentions in the past decade.The research works of this thesis focuse on the synthesis and catalytic activities of Zn_xCd_1-xS materials.The crytal structure and catalytic activityies were examined with X-ray diffraction experiments,electron microscope,UV-vis spectroscopy,electrochemical measurements,specific surface area determination,and photocatalytic H₂ evolution rate tests.The dependence of catalytic activity on the crystal structure,morphology,and specific area is discussed.The main research contents are as follows:1.Zn_0.5Cd_0.5S nanoparticles with phase homojunctions were prepared by a one-step hydrothermal method with thioacetamide as sulfur source.The effects of the various reaction conditions,such as initial concentrations of Zn and Cd ions,initial content of sulfur source in the reaction mixture,and the temperature rising rate,are investigated.The photocatalytic activities of these samples prepared at different reaction conditions are affected by the homogeneity and crystallinity of the nanoparticles.A faster reaction rate leads to the formation of Zn_0.5Cd_0.5S nanoparticles with bad crystallinity,but goo.d homogeneity.On the contrary,a slower reaction rate.results in a bad homogeneity,but better crystallinity.The highest rate of H₂ evolution achieved for these phase-homojunctions materials unnder visible-light irradiation???420 nm?reached 125.27 mmol·h^-1·g^-1 with an apparent quantum efficiency of 21.46%at 420 nm.Combination of good crystallinity and homogen eity in a Zn_0.5Cd_0.5S nanocomposite resulted in an excellent photocatalytic activity.2.A serious of novel visible-light-driven Zn_xCd_1-xS/ZnO photocatalysts were prepared by a one step cation exchange reaction.The residual trace of hydrazine in ZnS,the precursor of ion exchange rection,is found to be crucial for the formation of ZnO in situ.The intimate contacts between Zn_xGd_1-xS and ZnO segments leads to the formation of heterojunctions between Zn_xCd_1-xS and ZnO.The photocatalytic activity of Zn_xCd_1-xS/ZnO nanocomposites for H₂ evolution from water using Na₂S and Na₂SO₃ as sacrificial reagents were significantly promoted in comparison with that of Zn_xCd_1-xS along.The highest rate of H₂ evolution for these hybrid materials under visible-light irradiation???420 nm?is reached 35.51 mmol·h^-1·g^-1 photocatalyst and is about 14 times higher than that of pristine Zn_xCd_1-xS.The promoted catalytic activity of this hybrid material can be ascribed to the heterojunctions between Zn_xCd_1-xS and ZnO,which enhanced the separation of photon generated hole-electron pairs.