Synthesis Of MoS₂ By Hydrothermal Method And Investigation For Its Photocatalytic Properties

Molybdenum disulfide (MoS2) belonged to the hexagonal system is a kind of semiconductor material with layered structure. Bue to its unique layered structure, there are a lot of useful properties of MoS2 such as lubrication, catalysis, photoelectric effect and anisotropy. Therefore, MoS2 has been extensively researched in different field, such as lubricants, transistors, photoelectric device, solid-state secondary lithium battery cathodes and industrial catalysts and so on.In this paper, we describe a simple hydrothermal method to synthesized specific morphology nano-MoS2, using sodium molybdate and thiourea as reactants, silicotungstic acid, Tetrabutyl ammonium bromide and oxalic acid as additive agent, respectively. The effects of the reaction time, reaction temperature and concentration of precursor solution on the morphology and structure of MoS2 were mainly studied. The morphology, crystallographic structure, chemical composition and the possible formation mechanism of the MoS2 were investigated by X-ray diffraction (XRD), Raman spectrum, high resolution transmission electron-microscopy (HRTEM), scanning electronic microscope (SEM) and EDS, respectively. At last, we evaluated the photocatalytic performance of flower-like MoS2 microspheres. The results of the study are summarized as follows:(1) A facile hydrothermal method was used to synthesize specific morphology nano-MoS2, using sodium molybdate and thiourea as reactants. The MoS2 with different morphology could be synthesized by adding silicotungstic acid, tetrabutyl ammonium bromide and oxalic acid, and controlling the synthesis process conditions. Without any additive agent, the flower-like 2H-MoS2 microspheres with 2～4μm in diameter were successfully prepared at 200℃ for 24 hours; MoS2 solid spheres with the diameter of 3～5 μm were prepared at 200℃ for 24 when the silicotungstic acid was added. The silicotungstic acid worked as an intermediate and it could help improving the crystalline; when tetrabutyl ammonium bromide was added, the two-dimensional block MoS2 powders could be prepared. One side of the block were assembled with hollow MoS2, while the other side looked smooth; when the oxalic acid was added, the flower-like MoS2 microspheres could also be prepared at 200℃ for 24 h. The oxalic acid acted as a reducing agent, which made it more easily to reduce the Mo (Ⅵ) to Mo (Ⅳ) and helped improving the crystalline.(2) Flower-like 2H-MoS2 microspheres, which are formed by several nanosheets gathering together perpendicular to the spherical surface, were synthesized by hydrothermal method at 200℃ for 24 h with the precursors of sodium molybdate, thiourea, oxalic acid and deionized water. The effects of oxalic acid concentration, growth time and reaction tempreture on the morphology and crystallographic structure of MoS2 were mainly investigated. It was showed that:with the oxalic acid concentration increasing, the diameter of flower-like MoS2 microspheres gradually decreased. When the oxalic acid concentration in reaction solution was increased to 0.1125 M, the flower-like MoS2 microspheres could not be synthesized, only small MoS2 particles could be formed; regardless of the growth time, the MoS2 powders were composed of large, flower-like nanosheets microspheres. The size of the flower-like MoS2 microspheres is uniform. The diameter of the flower-like microspheres increased with the increased growth time. However, the growth speed of the nanosheet microspheres slew down with increased growth time; the flower-like MoS2 microspheres couldn’t be synthesized under low-concentration conditions of precursor. Finally, the formation mechanism of the flower-like MoS2 microspheres was analyzed. With the increasing of oxalic acid concentration, the concentration of C2O42- and H+ that are both with reductive ability would increase, which would increase the reaction rate and speed up the nucleation rate. However, if the oxalic acid concentration was excessive, the reaction rate would be too fast. Meanwhile, it also would bring superfluous H+. During the crystal growing, the superfluous H+ would be attracted by Mo-S unsaturated bond to form "a charge shielding effect", which would lead to MoS2 nanosheets not to be grown. Therefore, flower-like MoS2 microspheres couldn’t be synthesized, and only small MoS2 particles could be formed when the oxalic acid concentration was excessive.(3) Finally, the photocatalysis of flower-like MoS2 microspheres using methyl orange was investigated. The results showed that:the degradation efficiency of the MoS2 catalyst was closely related to its particle size and morphology. When the flower-like MoS2 with 1.1μm in diameter and the MoS2 particles with 0.25 μm in diameter were used, the higher degradation efficiency could be observed, about 79% and 60.6% respectively. However, when flower-like MoS2 with 4.5μm and 3.9 μm in diameter were used, the degradation efficiency would decrease to 10.6% and 13.6%. Besides, both of the amount of MoS2 catalyst and the concentration of methyl orange would have a great influence on the degradation efficiency. Therefore, it is very important to adopt the reasonable catalytic process to improve the degradation efficiency and the utilization of the resources.