Morphology-Controlled Synthesis Of Nanosized MoS₂ And Its Catalytic And Lubrication Properties

Molybdenum disulfide (MoS₂) is a typical layered inorganic material with strong S?Mo?S covalent bonds and weak Van der Waals force between layers. The easy sliding between MoS₂ layers is generally regarded as a significant reason for its low friction coefficient. MoS₂ is often used as solid lubricants to reduce contact wear, save energy and maintain longer service life of the machines. Moreover, six sulfur atoms surround one molybdenum atom in a form of trigonal prismatic arrangement in MoS₂ layers, which produces a lot of Mo–S prismatic surfaces with high surface area and activity. Accordingly, MoS₂ presents very excellent catalytic properties and has been widely used as hydrodesulfurization catalysts in petro-chemical industry and photocatalysts in the process of waste water treatment. Both layered structure and weak Van der Waals force provide an opportunity for the intercalation of guests into MoS₂ via the restacking process of monomolecular layers, leading to MoS₂ intercalation compounds. This provides a novel method to resolve the dispersion problem of MoS₂ in matrixes. Taking these mentioned above into account, this dissertation investigated the preparation, intercalation, and properties of catalysis and lubrication of nano-MoS₂ particles. The results will possibly provide useful theoretical and application supports for the high-performance catalytic and lubrication materials.First, the dissertation studied the preparation of nano-MoS₂ via a precursor-synthesis method. The morphology of nano-MoS₂ was controlled by the synthesis process conditions. The results showed that thioacetamide (TAA) acted as both a reactant and a template reagent when HCl and TAA were selected as acidifying reagent and sulfur source respectively. The MoS₃ precursor grew along spherical surfaces forming hollow nano-balls with shells. Heating the obtained MoS₃ nano-balls under H2 at 780 oC led to MoS₂ nano-balls with an average diameter of ¹50 nm. Because high temperatures decreased the layer space of MoS₂, MoS₂ nano-balls became deformed and were changed into polyhedral structures at 960 oC. When HCl was replaced by H2SO4, the SO42- destroyed the template of TAA molecules through the hydrogen bond. Thus, it was difficult for MoS₃ nano-balls to form with H2SO4. Moreover, the MoS₃ precursor prepared from Na2S was found with bulk microsized and formless particles. This may be reasoned from the H2S production by a quick reaction between Na2S and acid, which reacted immediately with Na2MoO4 leading to bulk MoS₃ particles. Calcining the obtained MoS₃ bulk microsized particles resulted in MoS₂ nano-slices after desulfurization under H2. The sizes of the prepared nano-slices varied from 15 to 50 nm in length, and 5 to 10 nm in thickness.Secondly, the present dissertation studied the catalysis of the obtained MoS₂ nanoparticles with different morphologies in S2- oxidation into SO42- and degradation of methyl orange. It also investigated the relationship between the catalytic properties and the morphology of MoS₂, and the relative catalytic mechanism. The results showed the catalytic properties of nano-MoS₂ were affected obviously by the morphology and structure of nano-MoS₂. MoS₂ with closed layers presented very low catalytic efficiency for S2- oxidation into SO42-, while the opened layered MoS₂ nano-slices showed excellent catalytic properties. The active sites of the catalytic oxidation were located at the rim site of MoS₂. MoS₂ nano-slices with small sizes and wedge-like structure have higher BET surface area and more rim sites, which led to their excellent properties of catalytic oxidation. The experimental results showed that the curved basal surface of the MoS₂ nano-balls was also the active site in case of the catalytic degradation of methyl orange. Though the BET surface of the MoS₂ nano-balls was obviously lower than that of the nano-slices, they had a catalytic activity close to the nano-slices because of their larger absorbance in visible light region. The catalytic degradation of methyl orange could be improved by the increased MoS₂ content, acid solution and the low concentration of methyl orange. MoS₂ nano-slices not only had high catalytic activity, but also could be regenerated conveniently. The catalysis of MoS₂ for methyl orange degradation did not need special ultraviolet lights, and especially the MoS₂ catalysts could be used for many times. Thus, the MoS₂ catalysts had some potential industrial application values.Subsequently, the preparation and the catalytic properties of nano-MoS₂/TiO₂ composites were studied. The results showed that MoS₃/TiO₂ could be prepared via the deposition of MoS₃ on the surface of nano-TiO₂. The MoS₃/TiO₂ composites would be changed into nano-MoS₂/TiO₂ catalysts after calcination and activation. The nano-MoS₂/TiO₂ composite catalysts were of high-activity catalysis for the degradation of methyl orange and had potential application in waste water treatment. Nano-MoS₂ was the active catalyst component in the nano-MoS₂/TiO₂ composite. The composite catalyst had higher activity than TiO₂ and pure nano-MoS₂. TiO₂ functioned as both a support and a cocatalyst. MoS₂ on the surface of the composite catalyst was composed of nano-slices with 10?30 nm length and ⁸ nm thickness. One of nano-MoS₂ edge surfaces linked with the matrix, and others were exposed to the environment. Thus, the composite catalyst has very high BET surface and excellent catalytic activities.Moreover, the intercalation behavior of MoS₂ with different morphologies was studied by the exfoliating and restacking method. The results showed that the MoS₂ slices had the highest intercalation activity while the MoS₂ nano-balls the lowest one. The exfoliating and restacking behaviors of the MoS₂ slices were different with those of the bulk MoS₂ platelets. The restacked products from the nano-MoS₂ monolayer suspension presented similar characteristics with the exfoliated nano-MoS₂. This indicated that nano-MoS₂ monolayers did not restack regularly along c axis, which was different from bulk MoS₂ platelets. The attention of transition metal ions or organic positive ions favored the restacking of monolayer nano-MoS₂ along c axis, leading to intercalation compounds (MoS₂?IC). However, the intercalation compounds of nano-MoS₂ still partly reserved the characteristics of monolayer. Moreover, POM/MoS₂ and PMA/MoS₂ intercalation compound could also be prepared by the intercalation of polyoxymethylene (POM) and polyamide (PMA) into MoS₂.In the end, it was investigated the lubrication properties of the nano-MoS₂ and MoS₂?IC as additives in the POM based self-lubrication materials, including the conventional tribology, vacuum tribology, micro tribology and their relative lubrication mechanisms. The results showed that MoS₂ nano-slices led to the obvious degradation of POM during the polymeric processing of POM with MoS₂ nano-slices. Thus, MoS₂ nano-slices were not proper to act as a solid lubricant in POM. However, the stability of POM was not affected by MoS₂ nano-balls, micro-MoS₂ and POM/MoS₂?IC during their compositing processes. Thus, they can be used as additives in POM. The POM/MoS₂ nano-balls plastic layer presented better tribological properties than the corresponding POM layer with micro-MoS₂ and POM/MoS₂?IC. Though the POM/MoS₂?IC composite had excellent dispersivity, the tribological behavior of POM/MoS₂-IC was not satisfactory. This is because that the intercalation of POM leads to the crystal structural transformation from 2H-MoS₂ with good tribological properties to 1T-MoS₂ with weak tribological properties. The results also indicated it was very advantageous that the content of nano-balls was at 1.0 wt% in the POM/MoS₂ nano-balls composite. The excessive nano-MoS₂ affected the crystallinity and thermal behavior of POM. Accordingly, the self-lubricating ability was also weakened as well. The MoS₂ nano-balls were partly exfoliated, destroyed, and oxidized into MoO3 during rubbing. However, the oxidation degree of the MoS₂ nano-balls was not obviously increased because of the chemical stability of spherical structure, compared to that of micro-MoS₂. It was discovered that wear debris clusters were formed on the surface of the composite during rubbing via scanning electron microscope and optical microscope. The wear debris clusters protruded from the surface and separated the friction pair. The debris clusters were produced and destroyed in turn, which led to a regular variation of friction coefficient.