| The discovery of fullerenes and carbon nanotubes prompted the reserach of fullerene-like structures and inorganic nanotubes materials. Similarly, the discovery and characterization of graphene has created great interest to explore the synthesis of graphene analogues of layered inorganic materials. In recent years, two-dimensional (2-D) nanostructured materials have attracted tremendous research interest owing to their unique physical, chemical, electronics and mechanical properties. Among all these kinds of nanomaterials, the ultrathin molybdenum disulfide and tungsten disulfide are two important functional material, which are widely used in lubricants, semiconductor electronic devices, lithium-ion batteries, supercapacitors, hydrogen storage and other fields. This thesis mainly focused on exploring novel methods to prepare molybdenum/tungsten disulfide nanosheets and researching their properties. The ultrathin molybdenum disulfide (tungsten) nanosheets were prepared by high temperature sulfurization reaction and hydrothermal method, and the electrochemical performances as anodes in rechargeable lithium ion batteries and the tribological properties as lubricant additives of these nanosheets were also studied. The main points in this thesis are summarized as follows:(1) We have researched the preparation methods for ultrathin molybdenum disulfide (tungsten) nanosheets and the main content summarized as follows:Ultrathin MoS2nanosheets with thickness of5-10nm and lateral size of100-150nm were successfully synthesized by a facile high temperature sulfurization reaction method, using molybdenum trioxide (MoO3) and thiourea as the precursors, and then calcined at850℃under N2for1h. The influence of temperature, reaction time and the molar ratio of the starting material on the formation of MoS2nanosheets were investigated. The experimental results suggest that temperature plays a crucial role in the formation of MoS2nanosheets. MoO3is completely sulfurized into MoS2at600℃. And the lateral size of the nanosheets becomes bigger and the crystal morphology becomes clearer with the increase of annealing temperature. The reaction time and molar ratio of raw materials also have a great impact on the morphology of nanosheets. With increasing reaction time, the size of the nanosheets became larger. But the thickness of nanosheets were increased for too long reaction time.A possible reaction mechanism is proposed to explain the formation of MoS2nanosheets. During the reaction, thiourea rapidly decomposed at high temperature and produced of CS2, H2NCN, and NH3. Subsequently, CS2reduced Mo(Ⅵ) to Mo(Ⅵ). At850℃the reaction of MoO3and CS2was very fast and immediately formed an MoS2thin film on the surface of MoO3particles. And the MoS2layers were exfoliated by the impact erosion of gases and the MoS2nanosheets were formed.Ultrathin MoS2nanosheeets with thickness of5nm and lateral size of200-400nm were synthesized by hydrothermal method at220℃for24h using ammonium molybdate and thiourea as the precursors. The effects of reaction temperature and reaction time on the morphology of the prepared MoS2were researched. According to the experimental results, reaction temperature played an important role in the morphology of the products. At low temperature, only poor crystals were produced with almost no layer structure. When the reaction temperature reached220℃, the samples were mainly composed of well dispersed ultrathin nanosheets. In addition, the reaction time also have a great impact on the morphology of nanosheets. With a shorter reaction time, the prepared MoS2consists of three-dimensional micro-spherical structure which is constructed from2D nanosheets. With the extension of the reaction time, the size of the microspheres gradually decreased. Meanwhile, the nanosheets which composed of the microspheres began to dispersing and the most uniform dispersion will obtained when the reaction time prolonged to24h.Graphene-like WS2nanosheets with thickness of5nm and lateral size of100-300nm were successfully synthesized by a facile gas-solid reaction method, using tungsten trioxide (WO3) and thiourea as the precursors, and then calcined at850℃. The influence of temperature, reaction time and preparation process on the formation of WS2nanosheets were investigated. The experimental results show that WO3is completely sulfurized into WS2at800℃. And the lateral size of the nanosheets becomes bigger and the crystal morphology becomes clearer with the increase of annealing temperature. The reaction time also has a great impact on the morphology of nanosheets. With increasing reaction time, the size of the nanosheets became larger. A possible reaction mechanism is proposed to explain the formation of WS2nanosheets. During the reaction, thiourea rapidly decomposed at high temperature and produced of CS2, H2NCN,and NH3. Subsequently, CS2reacted with WO3. At850℃the reaction of WO3and CS2was very fast and immediately formed an WS2thin film on the surface of WO3particles. And the WS2layers were exfoliated by the impact erosion of gases and the WS2nanosheets were formed. (2) We have researched the electrochemical performances and the tribology performance of the ultrathin molybdenum disulfide (tungsten) nanosheets and the main content summarized as follows:The electrochemical performances of the ultrathin MoS2nanosheets prepared by high temperature sulfurization reaction method as anodes in rechargeable lithium ion batteries were investigated. And the result shows that during the first cycle, the ultrathin MoS2nanosheets deliver a discharge capacity of1213.2mAh/g and a reversible charge capacity of953.4mAh/g. After50cycles, its discharge capacity decreases to794mAh/g, which corresponds to capacity retention of79.8%.Different mass fractions of the MoS2nanosheets prepared by hydrothermal method were dispersed in the base oil and compared their friction reducing effect. The results show that the friction coefficient of base oil added with MoS2was smaller than the pure base oil obviously. And the base oil containing1wt.%ultrathin MoS2nanosheets presented good tribology performance under the condition of fixed speed with different loading and fixed load with different speed. Besides, the effect of friction reduction of the ultrathin MoS2nanosheets were compared with the commercially available MoS2. And the results also indicated that the ultrathin MoS2nanosheets’ friction reduction performance was stronger.The electrochemical performances of the ultrathin WS2nanosheets as anodes in rechargeable lithium ion batteries were investigated. And the results showed that during the first cycle, the ultrathin WS2nanosheets deliver a discharge capacity of755.8mAh/g and a reversible charge capacity of630.7mAh/g. After50cycles, its discharge capacity decreases to553.4mAh/g, which corresponds to capacity retention of86.5%.(3) Ti, V doped WS2nanosheets were successfully synthesized by high temperature sulfurization reaction and the morphology of the doped nanosheets were studied. The results show that the lateral size of the Ti-doped WS2nanosheets is obvious bigger than the pure WS2nanosheets.Titanium, vanadium doped tungsten disulfide nanosheets and pure tungsten disulfide nanosheets as lubricant additives were added to the base oil with1wt.%mass percentage and prepared three lubricants. The tribological properties of the lubricants were studied. The results showed that titanium-doped tungsten disulfide nanosheets’ antiwear performance was best and vanadium-doped tungsten disulfide nanosheets’ antiwear performance was worst. The antifriction mechanism of the nanosheets were analyzed and the reason mainly due to the deposition of nano-sheet film on the metal surface. Due to the larger lateral size and well dispersion of titanium doped tungsten disulfide nanosheets, resulting in a film on the metal surface is easy deposited and improve its antiwear performance. |
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