ReaxFF-based Molecular Dynamics Study On The Nanoscale Surface Oxidation Of Aluminum And Silicon

Oxidation can result in the rearrangement of atoms on solid surfaces and have tremendous impacts on the structure and properties of materials.On the one hand,corrosion occurs naturally on most metals when exposed in air or water,which is the primary cause of materials failure.Oxidation and corrosion have been spawning a significant number of economic losses throughout the world,which is one of the most urgent problems in industries.On the other hand,however,oxidation is valuable under controlled conditions because it provides a valid route to produce metal/oxide composites and endow materials with versatile properties.For example,the oxidation condition is determinative to the structure and component of industrially produced ultra-thin oxide films,which further determines their performance in practical usages such as corrosion resistance as passivation films,stability as catalyst supports,and electronic transport properties in semiconductor devices.Therefore,a comprehensive understanding of the mechanism of materials oxidation at the atomic scale is of critical importance for the rational design of next-generation protective materials with enhanced corrosion resistance,as well as oxide-based functional materials with improved performance.However,due to the complexity and extremely high rate of the oxidation reaction on material surfaces,how the adsorption and dissociation of oxygen and nucleation of oxides occur remains unclear,which highlights the importance of using theoretical modelling methods to reveal the mechanisms of the microstructure evolution under reaction conditions.In this dissertation,we performed molecular dynamics simulations based on the reactive force-field(ReaxFF-MD)to explore how the atomic arrangement of solid surfaces and reaction condition corporately affect the oxidation mechanism and oxide growth pattern.We unraveled the distinct chain-like oxide growth mechanism on aluminum nanoparticles induced by surface active sites,as well as the promoting effects of structural defects in amorphous silicon on the oxygen dissociation and oxide film growth.The main findings are as follows(1)Owing to the size effect,metallic nanoparticles usually show different oxidation dynamics from their bulk counterparts and result in various oxide nanostructures.In this work,we discovered the interesting chain-like nucleation and growth of surface oxides on aluminum nanoparticles using ReaxFF-MD simulations.Our results revealed a strong site selectivity in oxidation induced by different atomic coordination environments on the nanoparticle surface,and predicted the compositional evolution and structural characteristics of the oxidized products under different reaction conditions.Under mild conditions,the chain-like oxide nuclei would coat the nanoparticle surface and slowly grow into a spherical metal/oxide core-shell structure.Under elevated temperature or oxygen concentration,the oxides would extend outward from the surface to form longer oxide chains,or detach from the nanoparticle matrix to generate smaller oxide clusters,which is induced by the synergistic effect of kinetics energy,internal stress,and lattice mismatch.The nanostructures of the final oxidation products are highly irregular,while the overall morphology depends on the reaction temperature,oxygen concentration,and nanoparticle size.(2)Precisely controlling the surface oxide layers of silicon-based materials is crucial for the fabrication of high-performance microelectronic devices and solar cells.The two common allotropes of silicon(crystalline silicon and amorphous silicon),have distinct atomic arrangements,which may result in certain differences in terms of their oxidation mechanism and surface oxide structure.In this work,based on ReaxFF-MD simulations,nudged elastic band(NEB)calculation,and fitting with the Cabrera-Mott theoretical model,we elucidate that the oxidation-resistance of amorphous silicon is the worse under the same reaction condition.Amorphous-structure-promoted oxidation of silicon is originated from the abundant structural defects,such as coordination defects,distortions,and ring defects.First,the surface defects of amorphous silicon could reduce the dissociation barrier of O2 by a "selective adsorption" mechanism and therefore enhance the reaction rate at the initial stage.Second,the surface defects could induce the island-like nucleation of oxides and increase the inhomogeneity of the oxide film.Third,the defects inside the amorphous silicon could provide an easier pathway for the penetration of oxygen,thus boosting the oxide film growth.Additionally,our simulations predicted the phase separation and thermal crystallization phenomena of the surface amorphous oxide films generated over 1500 K,which results in nano-sized polycrystalline SiO2 films.By comparison,because of the lower O/Si stoichiometry and rougher interfacial structure,surface oxides on the amorphous silicon tend to form smaller grains and a higher proportion of grain boundary during the structural transformationThis dissertation systematically elucidates the effects of the atomic structure of material surfaces on the growth mechanism and structural evolution of surface oxides,which unravels the site selectivity during oxidation,size effect,and the reaction condition dependence at the nanoscale.Our results provide theoretical insights into the atomic-scale mechanism of materials oxidation and have certain guiding significance to the preparation of advanced oxide-based nanomaterials.