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Fabrication, Modification Of CdxZn1-xS And Study Of The Enhanced Photocatalytic Hydrogen Evolution Under Visible Light

Posted on:2016-06-08Degree:MasterType:Thesis
Country:ChinaCandidate:Y H LvFull Text:PDF
GTID:2191330479994106Subject:Environmental Engineering
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The rapid development of the modern society leads to the evergrowing demand of energy. However, owing to the exploitation and use of the non-renewable fossil energy on a large scale, the world will face two challenges including the energy crisis and the environmental pollution issues. In recent years, photocatalytic H2 evolution from water splitting under solar irradiation has received much attention due to that hydrogen, which has the high combustion value and make no pollution when used, has been considered as an ideal candidate to replace the fossil fuels. In addition, in order to take advantage of the visible light, which accounts for a proportion about 44% in the solar spectrum, semiconductors responded to visible light, especially the metal sulfides have become a research focus.In order to fabricate the binary CdxZn1-xS solid solution photocatalysts with strong ability to absorb visible light, high catalytic activity and stability, in this thesis, research has been carried out as follows:Firstly, a series of novel visible-light-driven photocatalysts with low graphene content and high stability(GR-r) has been prepared by a thermolysis method. The graphene loaded was intimately connected with the semiconductor and it was beneficial for lowering the size of the crystal, improving the dispersibility of the particles and enhancing the absorbance of photocatalysts in visible-light region(≥534 nm). Furthermore, the results of photocatalytic H2 evolution experiments indicated that the 5 h average hydrogen generation rate of GR-1.0 is 2107 μmol h-1 g-1, exceeding that of GR-0(1132μmol h-1 g-1) by 1.86 times. The intimate contact of the graphene and the semiconductor can accelerate the the transfer of the electrons and prevent recombination of the photogenerated electrons and holes, thus enhancing the photocatalytic activity of the composites.Secondly, Cd0.5Zn0.5S solid solutions(CZS-r) with small crystal size, high dispersibility, large specific surface area, and especially the existence of unique stacking faults, were synthesized by an alkaline-assisted hydrothermal method. It has been found that CZS-0.5 possessed the 5 h average hydrogen production rate of 2154 μmol h-1 g-1, which is 29.92 times of that of CZS-0(72μmol h-1 g-1). The unique potential formed by the stacking faults induced on the surface can significantly improve the separation of the photogenerated electrons and holes, thus enhancing the photocatalytic activity. Furthermore, by loading carboxylic multiwalled carbon nanotubes modified with Cu2+(CNTs(Cu)), which was intimately connected with the semiconductor, the rate of photocatalytic hydrogen evolution of the composite was accelerated to 2995 μmol h-1 g-1, exceeding that of CZS-0.5 by 1.4 times.Finally, CdxZn1-xS solid solutions with high catalytic activity has been fabricated by an ethylenediamine-assisted hydrothermal method. The characterization of the samples indicated that, with the increase of Cd/Zn, the structures gradually turned to hexagonal wurtzite phase from cubic sphalerite phase, the size of the crystals became smaller and then larger, the specific surface area decreased slowly, the absorption edge showed a red shift, and the photocatalytic activity of the semiconductors decreased after the first increase. It has been found that Cd0.5Zn0.5S possessed the highest photocatalytic activity, with the 5 h average hydrogen production rate of 13539 μmol h-1 g-1, which is 15.3 times of that of Cd0.1Zn0.9S(882 μmol g-1 h-1) and 14.2 times of that of Cd0.9Zn0.1S(954 μmol g-1 h-1). Small amounts of transition-metal sulfides including Ni S, Cu S and Co S were loaded onto the surface of Cd0.5Zn0.5S to form MS/Cd0.5Zn0.5S heterojunction by the in situ precipitation method. It has been found that the photocatalytic performance of the composites first increased and then reduced with the increasing content of MS, among which the ones containing the MS of 0.25 wt.% show the highest activities, with the hydrogen evolution rate of 38187.7, 24691.7 and 20810.7 μmol g-1 h-1, respectively. the performance of Cd0.5Zn0.5S enhanced by co-catalysts was due to the intimate contact of them, which was beneficial for the fast transfer of electrons during the photocatalytic reaction.
Keywords/Search Tags:CdxZn1-xS solid solution, composite, photocatalyst, visible light, hydrogen evolution
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