Synthesis Of Molybdenum (Tungsten) Disulfide Nanostructures And Their Properties

Besides in the field of lubrications and catalysts, molybdenum/tungsten disulfide nanomaterials were also widely used in lithium batteries, hydrogen storage, photoelectricity, engine and chemical industry. Due to the structural anisotropy, the differences of morphology and structure will affect the performances significantly. Therefore, it is necessary to improve the current preparation technologies and explore novel methods, which can control the preparation of microstructures, enhancing the final performances. And then, they can be used in more fields, which is very important for developments of theory and applications. The current thesis is focused on exploring novel methods to prepare molybdenum/tungsten disulfide nanomaterials, discussing the formation mechanism of the products'morphologies and structures, and exploring their potential applications.Molybdenum disulfide nanostructures with different morphologies were prepared by a surfactant-assisted method, adopting surfactants with different types, including anionic SDBS, non-ionic PEG and cationic CTAC. And the influences of various surfactants on the morphologies of final products were studied. It was shown that molybdenum disulfide hexagonal nanoparticles with a size of100nm was prepared via a liquid phase reduction method, adopting anionic SDBS, and hollow molybdenum disulfide microspheres were obtained using non-ionic PEG as a template. However, the cationic CTAC would attend the chemical reaction and promote the formation of hollow molybdenum disulfide nanospheres. If anionic SDBS and non-ionic PEG were used together, the final products were molybdenum disulfide nanorods with a length of200-400nm. Moreover, molybdenum disulfide solid nanosperes with a diameter of100nm could be synthesized by a hydrogen reduction process adopting PEG as the covering agent, in which molybdenum trisulfide nanopsheres were used as precursors and then reduced in an atmosphere of hydrogen at high temperatures.Based on the effect of instantaneous high-temperature and high-pressure during ball milling process, molybdenum disulfide nanoparticles were successfully prepared by a mechanochemical method, adopting molybdenum trisulfide as precursors, and the effects of ball-milling speed and time were also studied. Moreover, the influence of the ball milling pretreatment on general microparticles and their hydrodeoxygenation performances was discussed. It was shown that the precursors could be decomposed completely only at a rotation speed of over400rpm for more than24h, and the pretreatment of ball milling would crash the previous huge microparticles, creating a lot of multiple coordinatively unsaturated vacancies, which would rise the TOFs (turnover frequencies) and enhance the final catalytic activity. However, the pretreatment of ball milling did not change the selectivity of reaction route at all.Molybdenum disulfide nanosheets were successfully prepared by a solid micro-domain method, in which microparticles could be crashed into ultrafine particles and distributed homogeneously. The effects of annealing temperature, annealing time and the pretreatment of ball milling on the final products were also discussed. The formation mechanism of nanosheets and their solid self-assembly process at high temperatures were also studied. It was found that the nanosheets could be obtained at400-700℃, and it took only10min to finish the reaction at600℃. The pretreatment of ball milling helped distribute the ultrafine particles homogeneously and create lots of "micro-domains", which promoted the formation of the final nanosheets. Moreover, the molybdenum disulfide nanosheets would transform into larger regular hexagonal nanoplates through cohering and stacking at high temperatures.The lubricating properties of molybdenum disulfide nanosheets were tested by a four-ball tribometer, and compared with that of commercial ultrafine particles. The influences of dispersant and additive content on tribological performances were also investigated. The results showed that the molybdenum disulfide nanosheets could form stable lubricating films due to high surface activity, and showed lower friction factor, better anti-attrition and load-carrying capacity. The optimized content of additive was1.5wt%and the dispersant of Span-80could improve the dispersion of additives in basal oils, enhancing their antiwear capacity.Nanosheets with various stacking heights were obtained by adjusting the annealing temperatures. Then, the effect of annealing temperatures on molybdenum disulfide nanosheet catalysts and the relation bwteen the stacking height and hydrodesulfurization function were studied. It was found that the surface area and metal dispersion would decrease with the increasing of annealing temperatures and the relation between stacking structures and catalytic function could be explained by "Rim-Edge" model with a slight modification that there was a crucial value (7.7nm) reversing the trend.The influences of assistants (Ni and Co) on molybdenum disulfide nanosheet catalysts were investigated systematically, including their structures and catalytic functions. The results showed that the addition of Ni and Co would reduce the pore diameter significantly but improve their metal dispersion, and the addition of Ni would promote the crystal growth of molybdenum disulfide but Co would suspend it at relatively low temperatures. Both of them enhanced the final activity and preferred the reaction route of DDS. However, the effects of them on catalytic activity and selectivity were quite different with increasing annealing temperatures:the activity of NiMo catalysts would be decreased and their HYD selectivity would be increased; While, the activity of CoMo catalysts was increased consistantly and reached the maximum value at700℃, then decreased sharply due to the formation of segregations, but the HYD selectivity showed the reverse trend.Tungsten disulfide monodisperse nanosheets and inorganic fullerene structures were successfully synthesized by a solid micro-domain reaction method, and the effects of annealing temperatures, annealing time and the pretreatment of ball milling on the formation of final products were investigated. And their catalytic function for hydrogen evolution reaction (HER) was also tested. It was found that the tungsten disulfide nanosheets could only be obtained at600-700℃, and inorganic fullerene structures was prepared at800℃. The reaction must be kept for more than30min at600℃, or the final products could not be sulfurized completely. The pretreatment of ball milling not only made the whole reaction process faster and more direct, but also promoted the sulfurization of outer layers of tungsten oxide, inducing the formation of nanosheets and inorganic fullerene structures as a template. Due to the loosely stacking structures of nanosheets, their catalytic activity for hydrogen evolution reaction was much better (8.6times) than that of common molybdenum disulfide catalysts. The rate-limiting step was electrochemical desorption and the Volmer-Heyrovsky HER mechanism was operative in the HER catalyzed by the tungsten disulfide nanosheets.