Interaction Mechanism Study Between Two-dimensional Materials Graphene,molybdenum Disulfide And Environment

Graphene?G?and molybdenum disulfide?MoS₂?,two-dimensional materials,are widely used in the field of space corrosion resistance and lubrication due to their excellent physical and chemical properties such as their inert surface and special layered structure.The intrinsic two-dimensional materials,i.e.,G and MoS₂,have a complete surface morphology and show a certain inertia for the adsorption of most environmental factors.However,there usually are defects and together with the dangling bonds on the material surface,which may be becomed the outset points of corrosion reaction.For example,carbon monoxide,carbon dioxide,molybdenum trioxide and hydrogen sulfide?CO,CO₂ MoO₃ and H₂S?etc.are generated at the defects of G and MoS₂,which destroy the crystal structures to some extent and thus affects the properties of the materials.In this paper,based on the application of functionalized graphene in the field of atmospheric corrosion resistance,while the application of space?high-flux,high velocity,high activity oxygen atom?corrosion protection is neglected.Therefore,it is vital to study the interaction mechanism between graphene and space oxygen atom as to reveal their corrosion resistance in the application of space field.MoS₂,as a solid lubricating material,is generally applied in the field of lubrication.While the frictional performance is obviously decreased in atmospheric environment when the spacecrafts enters into low earth orbit from the atmospheric to vacuum environmens.Therefore,the mechanisms of MoS₂ failure and of MoS₂-Ti/Pb slowing down its failure,as well as of MoS₂-Ti/Pb affecting its friction performance are studied,which is crucial for the design and application of materials.Based on the first-principles,this thesis reveals the interaction mechanism between G,MoS₂ and different environments,the main research contents and conclusions are presented:1.Oxygen atoms corrosion resistance of pristine G is better than that of functionalized graphene.Functionalized group improves the adsorbed stability and diffusion migration rate of oxygen atoms on the surface of graphene,which means that functionalized graphene are easily destroyed by oxygen atoms.The reason is ascribted that functional groups destroy the perfect structure of graphene and generate active surface dangling bonds.Moreover,the vertical penetration resistance of oxygen atoms on the functionalized graphene surface is greater than that on graphene,which is due to the increase of electron cloud density on the graphene surface.The horizontal diffusion energy of oxygen atoms is lower than its vertical penetration energy,indicating that pristine and functionalized graphene are oxidized by means of surface adsorption and horizontal diffusion of oxygen atoms.2.Pristine MoS₂ basically does not react with O₂ or H₂O molecules,therefore the failure of MoS₂ is mainly caused by defects of surface.Moreover,the outset points of MoS₂ failure are MoS₂-V_S1 species in O₂ atmosphere and MoS₂-V_S2+Mo species in H₂O atmosphere,respectively.The failure process is presented:the phsisorption of O₂ or H₂O molecules?chemisorption?dissociation?diffusion of O or H adatoms on surface of MoS₂.In addition,MoS₂-nTi/Pb can effectively alleviate the damage of defective MoS₂,which is mainly due to the interaction between MoS₂-nTi/Pb and O₂,H₂O molecules even free H atoms.3.With respect to vacuum environment.The friction performance of MoS₂-Ti is lower than that of the pristine MoS₂,which means that MoS₂-Ti is not conducive to its interlayer sliding.While the friction performance of MoS₂-Pb is better than that of the pristine and Ti-doped MoS₂,thus MoS₂-Pb is conducive to its interlayer sliding.Those resons are mainly due to the decrease and increase of interlayer distance for MoS₂-Ti and MoS₂-Pb,resulting in the change of interlayered the interaction force and the work of separation,respectively.Besides above,the more important reasons are that Ti or Pb elements lead to the charge reconstruction between MoS₂ layers,and even the energy mutation of the systems.Whereas,as for atmospheric environment?i.e.,O₂ or H₂O molecules inserting into the interlayer of MoS₂ increase the interlayer distance,in which the interlayer distance is bigger than that of the van der Waals force boundary?.It is surperised that the friction performances both MoS₂-Ti and MoS₂-Pb are better than that of intrinsic MoS₂ and the performance of the former is improved obviously in contrast to one of the latter,meaning that van der Waals force is not only the factor affecting the friction performance of MoS₂,in which electrostatic repulsion force may also be one of the most important factors.Importantly,it is crucial that MoS₂-Ti and MoS₂-Pb may be suitable for atmosphere-vacuum environments,which expands the application of two-dimensional materials MoS₂ in lubrication fields.