The Preparation, Characterization And Photocatalytic Performance Of ZnO/ZnS-Ag₂S Core-shell Nanorods

In this paper, a series of composite photocatalysts comprised of ZnO nanorods, ZnO/ZnS core/shell nanorods and ZnO/ZnS-PbS (ZnO/ZnS-PdS or ZnO/ZnS-Ag2S) heterostructural core/shell nanorods with different MS/ZnS molar ratios have been synthesized via the combination of a low-temperature hydrothermal growth and cation exchange technique. The characterization of XRD, SEM, TEM, EDX&mapping, XPS and UV-Vis DRS were used to reveal the morphologies, crystal structure, distribution of components, photo absorption, and photocatalytic performance of the ZnO/ZnS-MS core/shell nanorods. We also discuss the energy band structure of catalysts and the reaction mechanism of photocatalytic hydrogen production.The ZnO/ZnS-MS core/shell nanorods, with the diameters of about150nm and the lengths of ranging from a few hundred nanometers to several micrometers, were fabricated by coating the ZnO nanorods of about100nm with a layer of ZnS and MS composite shell mainly consisting of nanocrystals (diameter of5-8nm) with the thickness of about20-30nm. With the increase of MS/ZnS molar ratios, the surface of shell layer becomes rougher and finally the agglomerate of MS-ZnS into larger particles and surface exfoliation over the samples. The samples are composed of the related elements, which presents the chemical shift because of the strong synergetic effect among the ternary materials.The p-n heterostructures in the photocatalysts could efficiently improve the migration path of photo-generated carriers. Due to the coupling with low bandgap material PbS, PdS and Ag2S, the ZnO/ZnS-MS nanorods exhibite a much higher solar-simulated light absorption than that of ZnO/ZnS. The synergetic effect, the proper MS/ZnS molar ratios and composite interaction among the ternary structure could enhance the photocatalytic hydrogen production from glycerol aqueous solution under the UV light (Hg-Lamp) and solar-simulated light (Xe-Lamp) irradiation. The ZnO/ZnS-Ag2S nanorods exhibit a maximum H2production rate of4942.9μmol·h-1·gcat-1and650.4μmol·h-1·gcat-1under the UV light irradiation and the Xe-Lamp light irradiation, respectively.The photogenerated holes can be trapped by water molecules or surface hydroxyl to form high oxidative hydroxyl radical species (·OH), which will oxidize C3H8O3to C02, in the meantime, the electrons are captured by H+on the surface of ZnO, resulting in the formation of H2. As a result, the change of the energy band structure, the uniform distribution of components, and the reduced recombination of photo-induced charge carriers due to the addition of MS play an important role in enhancing the photo absorption, yield, and the lifetime of photo-induced charge carriers, which enhanced the photocatalytic performance for hydrogen evolution from glycerol/water mixtures.