Synthesis of transition metal dichalcogenide-based heterostructures for efficient photocatalytic hydrogen evolution

Exploiting low-cost and earth-abundant photocatalysts is of great importance to achieve highly efficient photocatalytic water splitting.Transition metal dichalcogenides (TMDs) already show excellent performance using as co-catalysts for hydrogen evolution reaction (HER).However, the synthesis of TMD-based heterostructure photocatalysts which possess rich active sites for HER remains urgent.The aim of this thesis is to develop novel TMD-based heterostructures which exposed large amount of active edge-sites to improve the activity toward photocatalytic HER under visible light irradiation.To achieve this goal, the following works have been done.First, a facile one-pot method has been developed to synthesize nearly monodisperse CdS-MoS2 and CdS-WS2 heterostructures with an average diameter of about 6 nm.Monolayer MoS2/WS2 with a lateral size around 6 nm was grown on one side of bullet-like CdS particles.The direct contact of CdS and MoS2/WS2 monolayer improved the electron transfer and reduced the charge carrier recombination.What is more, these heterostructures possess lots of exposed edge sites in the MoS2/WS2 layers, which are also the active sites for HER.The photocatalytic activity of CdS-WS2 and CdS-MoS2 heterojunctions towards photocatalytic hydrogen production under light irradiation are about 16 and 12 times of that of pure CdS.The heterojunctions also shown improved durability, 70% of activity is still remained after long-time evaluation (total 16 h).Second, for the first time, controlled synthesis of a new type of heterostructure was reported, in which TMD nanosheets (NSs) (i.e., MoS2 and MoSe2) vertically grown along the longitudinal direction of 1D Cu2-xS nanowire (NW) in an epitaxial manner.The heterostructures were systematically characterized by the high-angle annular dark (bright)field scanning transmission electron microscopy, which demonstrated that the well match of crystal symmetries and lattice fringes between the TMD and Cu2S is critical for the construction of these epitaxial heterostructures.The architectures of the heterostructures can be well maintained after the composition and crystal structure of the original Cu2-xS Abstract ii nanowires were transformed by a well-known cation exchange method (e.g., Cu2-xS to CdS).The as-obtained CdS-MoS2 heterostructures with different loadings of MoS2 are used as photocatalysts for photocatalytic HER, exhibiting enhanced photocatalytic activity towards HER under visible light irradiation as compared to the pure CdS nanowires.This synthetic strategy opens up a new way for the controlled synthesis of TMD-based heterostructures which could have various promising applications.MoS2 heterostructure.(e) Dark-filed STEM image and corresponding EDS mapping images of CdSxSe1-x-MoS2 heterostructures.(f-g) XRD pattern of CdSxSe1-x-MoS2 heterostructure with different S/Se ratio…………………………………………………99