ReaxFF-based Atomistic Simulations Of The Oxidation And Corrosion Mechanisms Of Carbon/Carbide Nanomaterials

Oxidation would damage the original atomic arrangement of materials,leading to internal defects which have great impact on their service performance.However,it is a promising route to produce oxides materials with better properties than the pristine ones if the oxidation process is controllable.With the rapid development of the nanotechnology,more and more kinds of carbon/carbide nanomaterials have been discovered,synthesized,and applied in many fields.Similar to the traditional bulk materials,carbon/carbide nanomaterials would also encounter the risk of oxidation and corrosion.One the one hand,due to the smaller size and larger specific area,it would be much easier for low-dimensional carbon/carbide nanomaterials to be oxidized and much difficult to conduct corrosion protection.On the other hand,controllable oxidation of the carbon/carbide nanomaterials would undoubtful lead to valuable oxide derivatives.Benefited from the development of quantum mechanics and computer science in recent years,first principles calculations based on the density functional theory and molecular dynamics simulation based on the reactive force field have become useful tools for the investigation of the atomic structure,properties,reactions of materials.In this dissertation,these two methods have been combined to explore the oxidation of a series of carbon/carbide nanomaterials.The typical oxidation mechanisms for each type of carbon/carbide nanomaterials and common oxidation mechanisms have been revealed.The main contents and results of this thesis are as follows:(1)The oxidation of graphene with variable defects have been explored.The oxidation would begin with a vacancy.In the initial stage of the oxidation,it proceeded in such a manner as to maintain the symmetry of the vacancy.During the oxidation of graphene sheets with various grain boundary defects,disordered rings were seen to selfrestructure in order to decrease potential energy and increase stability.Oxidation usually proceeds along the direction of the grain boundaries,which represent the weak point in the structure.The results highlight the importance of variable defects in determining the oxidation kinetics.(2)The stacking and oxidation of C18 rings have been explored.The parallel-selfassembling stacking behaviors of multilayer C18 has been revealed.By manipulating specific initial interlayer spacing and temperatures,several stacking structures are obtained such as conchoidal structures and tubular structures.Moreover,the oxidation mechanisms of C18,including the O2 adsorption properties and the oxidation kinetics were analyzed in detail.The significant center-capture effect has been found that the hollow rings would preferentially attract an O2 molecule into their centers,and the reactivity-enhancing effect that the O2 adsorption on the C atoms of O2-doped rings would be enhanced by the captured central O2.The center-capture effect reveals the possibility of applying C18 as selective adsorbent for O2.The electronic transport behaviors of C18 oxides are also investigated.For devices application,the "cC18_central_vertical" device is the most promising one among the 13 C18 oxides.The main transmission channel of this device mainly originates from the LUMO frontier orbital.It emphasizes the significance of the controllable oxidation that the oxide derivatives may have better properties than the pristine ones.(3)The oxidation of P-doped g-C3N4 has been explored.The phosphorusactivating oxidation of P-doped g-C3N4 has been revealed.The doping P atoms were seen to bond with O2,and were oxidized to phosphorus oxide.Subsequently,the phosphorus oxide broke away from the g-C3N4 sheet and leave a vacancy that would expand and lead to further oxidation.The phosphorus-activating oxidation was observed in all P-doped g-C3N4 with different doping sites.It emphasizes that the reactivity of two-dimensional materials could be tuned by heterogeneous doping atoms.(4)The oxidation of Tin+1Cn(n=1,2,3)MXenes and the reduction of TiO2 nanosheets and nanoparticles have been explored.The results demonstrate that the oxidation of MXene sheets preferentially begins with O2 adsorption on the surface Ti layers,which induces the lattice distortion of Ti layers and the fracture of Ti-C bonds.Without the constraint from Ti atoms,the free C atoms subsequently reassemble,generating some new carbon chains inside the sheets.Interestingly,the chains could erupt out,during which the reformation of Ti-C bonds and the interstitial O atoms in Ti layers would be the obstacles to the eruption.Under the same conditions,more layers would result in easier surface adsorption of O2,severer lattice distortion of Ti layers,and weaker eruption of carbon chains.The eruption could also be accelerated by the humid environment,high temperature,and less layer number.TiO2 would be obtained after the eruption of carbon chains.As for the reduction of TiO2 nanoparticles,the orientation-relative disordering has been found that the preferable disordered facets are along {001} for anatase,{100} for brookite,and {001} for rutile,causing anisotropic stress deformation.Oxidation and reduction reactions are a pair of reversible reactions.If these two reactions can be fully utilized,it would be promising to realize the mutual preparation of a variety of low-dimensional materials.Oxidation is a double-edged sword that would damage the materials when uncontrollable,but would be a useful route to synthesize high-performance oxide derivatives when controllable.Therefore,the exploration of the atomistic oxidation mechanisms of carbon/carbide nanomaterials would not only provide theoretical guidance for the corrosion prevention of carbon/carbide nanomaterials,but also innovate new ideas for the preparation of oxide derivatives of carbon/carbide nanomaterials.