Atomistic Origin of the Complex Morphological Evolution of Aluminum

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Atomistic Origin of the Complex Morphological Evolution of Aluminum Nanoparticles during Oxidation: a Chain-Like Oxide Nucleation and Growth Mechanism Xingfan Zhang, Chengrui Fu, Yujie Xia, Yunrui Duan, Yifan Li, Zhichao Wang, Yanyan Jiang, and Hui Li ACS Nano, Just Accepted Manuscript • DOI: 10.1021/acsnano.8b07633 • Publication Date (Web): 20 Feb 2019 Downloaded from http://pubs.acs.org on February 22, 2019

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Atomistic Origin of the Complex Morphological Evolution of Aluminum Nanoparticles during Oxidation: a Chain−Like Oxide Nucleation and Growth Mechanism Xingfan Zhang, Chengrui Fu, Yujie Xia, Yunrui Duan, Yifan Li, Zhichao Wang, Yanyan Jiang* and Hui Li* Key Laboratory for Liquid−Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, People’s Republic of China * Corresponding author: [email protected] (Y. J.); [email protected] (H. L.) TOC Graphic

Abstract Metal nanoparticles usually show different oxidation dynamics from bulk metals, which results in various oxide nanostructures because of their size−related surface effects. In this work, we have found and investigated the chain−like nucleation and growth of oxides on the Aluminum nanoparticle (ANP) surface, using molecular dynamics simulations with the reactive force−field (ReaxFF). After nucleation, the chain−like oxide nuclei could stay on the ANP surface and continue growing into an oxide shell, or extend outward from the surface to form longer oxide

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chains, or detach from the ANP to generate independent oxide clusters, which is highly dependent on the oxygen content, temperature and nanoparticle size. Our results emphasize the complicated interplay between the surface structure of nanoparticles and environmental conditions in determining the formation of oxides, which provides insights into the atomic−scale oxidation mechanism of metal nanoparticles. Keywords ： oxidation; nanoparticles; chain−like oxide growth; aluminum; reactive molecular dynamic simulation

Oxidation can result in rearrangement of atoms on solid surfaces and have tremendous impacts on the properties and stability of materials. On one hand, corrosion occurs naturally on most metals when exposed in air or water, which has been spawning a great number of economic loss and environmental pollution throughout the world. Statistics show that corrosion costs industrial economies trillions of dollars each year,1 which has become one of the most urgent problems now confronting us. On the other hand, however, oxidation is valuable under controlled conditions because it provides a feasible way to produce metal/oxide composites. The oxidation process has significant impacts on the structure and component of the produced oxide films, which further affect their performance in passivation films against corrosion,2 catalytic activity as catalyst supports,3 and insulating properties in semiconductor devices.4 Therefore, a comprehensive understanding of the atomic reconstruction of materials during oxidation 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. Oxidation of most metals can be divided into two stages.5 First, a monolayer of oxides forms as rapidly as less than one second on the surface. Then, the oxide film grows thicker over longer period of time and finally reaches a limiting thickness. Existing theories of metal


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oxidation are mainly based on the growth kinetics of oxide films in the second stage, such as the generally accepted Cabrera−Mott (CM) model.5 According to this model, charged ions on the surface generate a Mott potential (VM) that drives oxidation. The Mott potential diminishes as the oxide film grows, which finally reaches a limiting thickness. However, one limitation is that this classic continuum model does not consider the complexity of the actual atomic configuration so that understanding the structural evolution at the atomic level during oxidation is vital for the atomic−scale control of the metal/oxide interface in many applications. With the rapid development of modern electron microscopes, in situ observations of the initial oxidation process of some less active metals have come true, which has addressed many fundamental questions in surface material science.6-8 Nevertheless, for highly active metals, such as aluminum, directly observing the rearrangement of surface atoms during initial oxidation still remains a great challenge. Molecular dynamics (MD) simulation serves as a favorite tool in providing detailed information of both the oxidation dynamics and the evolution of oxide structures.9-12 For example, Sankaranarayanan et al.9 discovered that the limiting thickness of oxide films predicted by the CM model can be enhanced by applying electric fields during oxidation. Their MD simulations showed that the presence of electric fields dramatically lowers the activation barrier and further enhances the ion migration through the oxides, which provided a theoretical guidance for the production of ultrathin oxide layers.13 Ciacchi et al.14 proposed an model for the aluminum oxide nucleation on Al(111) surface based on first−principles MD simulations. It shows that after the hot−atom dissociative adsorption, the O atoms can spontaneously penetrate through the Al surface layer, inducing the nucleation of oxides at a low oxygen coverage, which is in good agreement with experimental observations.15 Furthermore, the aluminum oxide film formed in the early oxidation stage is known to have an amorphous structure,5 but the understanding of the local atomic arrangement of amorphous alumina was lacking, thus limiting its applications such as catalytic supports and dielectric material. Gutiérrez et al.16 performed MD simulations to reveal that most of the Al atoms in amorphous aluminum


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oxide have four O as nearest−neighbors, which was later confirmed by nuclear magnetic resonance (NMR) experiments.17 Other simulation results pointed out a coordination number difference between the surface layer and the interior layer of the oxide films.18-19 Such computational works provided us deeper atomic−scale insights into oxidation mechanism and structure of oxides. Most of these insights on bulk metal oxidation should be directly applied into the oxidation of nanoparticles, but some special features arise due to the geometry and confinement effects. During the oxidation of bulk metals, strain is homogeneously distributed across the oxide film and can elastically relax by the introduction of misfit dislocations.20 At the nanoscale, however, strain is inhomogeneous distributed and these dislocations are combined with large local strain fields.21 Hence, the metal/oxide interface becomes more unstable on nanoparticles, which could lead to different oxidation dynamics than their bulk counterparts and result in various oxide nanostructures. For example, Pratt et al.22 discovered that the cuboid iron nanoparticle experiences a geometrical evolution from cube to sphere during oxidation, which revealed that the strain gradients induced caused by the geometry in the oxide shell enhance the ionic transport. Furthermore, it has been well recognized that the existence of unsaturated surface sites on nanoparticles, such as corners, steps, and terraces could have higher reactivity in catalysis than those facet sites due to the reduced coordinated atoms.3 A similar phenomenon is expected to be found in nanoparticle oxidation, but much effort should be devoted to figuring out how these unsaturated surface sites affect the nucleation and growth of oxides on nanoparticles. Aluminum nanoparticle (ANP) has been recognized as one of the best energetic metals in the field of propellants and explosives because of its high energy density, low cost and environmentally friendly combustion products.23 Besides, ANP has also become a promising candidate in plasmonic materials and catalysts in hydrogen production.24-25 ANP is known to spontaneously form a passivation shell with a thickness of 0.6–4 nm on the surface, which has significant impacts on its combustion, optical, and catalytic performance.26-30 Hence, both


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experimental and simulated works have intensively studied the structure of the oxide shell. For example, experimental observations showed that the structure of the oxide shell on ANPs is predominantly amorphous, but partial crystalline regions were also found.31-32 Campbell et al.33-34 revealed the presence of large pressure gradients inside the oxidized ANPs using the MD simulations, which showed that the aluminum core is under positive pressure while the oxide shell is in negative pressure. The enormous pressure gradient at the interfaces could also lead to the rupture of the oxide shell.35-36 However, the detailed understanding of the transient surface structure evolution of ANPs as well as its interplay with the environmental conditions at the initial stage of oxidation is currently lacking. In this work, we performed MD simulations to show the oxidation regime of ANPs that describes a chain−like nucleation and growth mechanism of oxides on ANP surface. Our results would provide an important perspective on understanding the initial oxidation mechanism of metallic nanoparticles and have an instructive significance in the fabrication of oxide−based nanocomposites. Results and Discussion Figure 1 shows three typical oxidation processes of ANPs in the conditions of different oxygen contents at 300 K. The oxygen content is found to have a crucial influence on the oxidation dynamics and the morphology of the subsequent nanostructures. As shown in Figure 1a and Video S1, when the initial system has 2250 O2 (0.934 g/cm3), the oxides slowly accumulate and form several chain−like nuclei at first, and then grow into an oxide shell on the ANP surface. The formation of these chain−like oxide nuclei origins from the preferential oxygen dissociation at the corner and edge sites on the surface of ANPs. Further oxidation leads to the accumulation of oxides based on these chain−like nuclei, which results in an approximately spherical core−shell structure which consists of an amorphous oxide shell and an unoxidized aluminum core. Figure 1b and Video S2 show a different oxidation process when the oxygen content doubles (4500 O2, 1.868 g/cm3), in which the chain−like oxides epitaxially grow outward from the ANP surface. The phenomenon is induced by the heat released by rapid oxidation, which


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provides the surfaces atoms with high kinetic energy and induces surface instability. At 2000 ps, the oxide chains wrap around the aluminum core, but not bond closely with the core. The inner aluminum core keeps slightly oxidized, suggesting that the oxide chains also have the passivation effect to protect the inner aluminum core from being further oxidized. When the initial system has 6750 O2 molecules (2.801 g/cm3), the ANP explodes rapidly and eject small atomic chains in all directions (Figure 1c and Video S3). These chains are further oxidized and generate many oxide clusters with different sizes and shapes at the final stage. Once the oxide clusters eject from the ANP matrix, the passivation effect disappears, thus the ANP was completely oxidized in this condition without the remaining unoxidized aluminum core.

Figure 1. Structural evolutions of the ANPs over the course of oxidation at 300 K in the


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conditions of different initial oxygen contents. Blue and red atoms represent Al and O atoms, respectively. (a) When initial system has 2250 O2 molecules, the surface oxides tend to form several chain−like nuclei first, and then grow into an oxide shell on the surface of ANP. (b) When initial system has 4500 O2 molecules, some chain−like oxides epitaxially grow outward from the ANP surface. (c) When initial system has 6750 O2 molecules, the ANP explode very rapidly and finally form many oxide clusters with different sizes and shapes. In the above three conditions, the oxides tend to nucleate like a chain in the initial stage, then the oxides either grow on the ANP surface, or stretch outward from the surface. Why do these oxides prefer to nucleate like a chain? This behavior should be traced back to the atomic arrangement of the ANP surface, which is shown in Figure 2a. The structure of the ANP is firstly optimized by an energy minimization process using the conjugate gradient method,37 and then allowed to relax at 300 K for 300 ps to reach the equilibrium state. The surface atoms of the ANP are painted into different colors according to their coordination environments. The surface of the ANP is composed of many smooth facets and surface steps. On the surface steps, there are several kinds of unsaturated sites with different coordination numbers. In particular, atoms at the corner and edge sites (colored in dark and light blue) have lower coordination numbers than others, which often show higher reactivity in chemical reactions, such as in the heterogeneous catalysis.3 In the initial oxidation process, when an oxygen molecule moves close to the ANP, it will dissociate and generate two Al4O tetrahedrons on the ANP surface, in which oxygen anions are located in the tetrahedron voids of aluminum cations. After oxidation for 300 ps, it can be seen that most O2 molecules prefer to dissociate at the corner and edge sites (colored in blue), while the smooth facets (colored in yellow) almost keep unoxidized, suggesting that the initial oxygen dissociation has a selectivity on the ANP surface. As the oxygen continues to dissociate, the generated Al4O tetrahedrons connect into chains on the surface steps by sharing Al cations. Due to the lattice mismatch, these tetrahedron structures raise out of the aluminum matrix, which increases their potential energy and makes themselves more vulnerable to further oxidation.


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Hence, the growth of new oxides will base on the connected Al4O tetrahedron nuclei. When the initial system has 2250 O2 molecules, the oxides accumulate on the ANP surface to form several chain−like nuclei first (Figure 2b), then gradually grow into an amorphous oxide shell to compose a metal/oxide core−shell structure (Figure 2c). After oxidation for 2000 ps, owing to the selectivity of oxide nucleation, the initial surface steps of the ANP are covered with thicker oxides, while the initial smooth facets are covered with thinner ones. In particular, some Al(111) facets still keep unoxidized or slightly oxidized, which agrees well with the low dissociative sticking probability for O2 on Al(111) surface.38 Figure 2c shows the cross−section of the core−shell structure at 2000 ps. The radial distributions of the atomic charge and coordination number in Figure 2d illustrate a composition gradient at the metal/oxide interface. The atomic charge at the interface is strongly associated with the Al−O coordination number. With the increase in radius, the positive charge increases while the negative charge decreases, because the outer Al cations coordinate with more O anions than the inner ones. It is worth noting that there is a bleb−like oxide nucleus on the ANP surface formed at the initial stage of oxidation (Figure 1a), resulting in significant surface roughness of the oxidized ANP. We added another 10000 O2 molecules into the simulation box and simulated for another 2 ns to test whether the “blebs” will grow and extend outward as the conditions in high oxygen content. As shown in Figure S1, the “blebs” no longer grows even if the oxygen concentration is extremely high, because the passivation effect of the oxide shell significantly decreases the oxidation rate and stabilizes the surface structure.


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Figure 2. Nucleation and growth of surface oxides when the initial system has 2250 O2. (a) Atomic arrangement of the ANP surface. The surface atoms are painted into different colors according to their coordination environments. Atoms at the corner and edge sites have higher reactivity than others due to the reduced coordination numbers, which become the active sites for oxide nucleation. (b) Nucleation of chain−like oxides at 300 ps. The oxidized and unoxidized Al atoms are colored in blue and white, respectively. (c) Cross−section of the Al/oxide core−shell structure at 2000 ps. (d) Radial distributions of atomic charge and coordination number of the generated metal/oxide interface at 2000 ps, which show a gradual change of oxide components. The charge distribution shows the charge of each atom versus its position in the radius, and each atom is painted into different colors to show its nearest coordinated Al/O atoms. When the oxygen content goes higher, the chain−like oxide nuclei no longer stay on the ANP surface but induce the epitaxial growth of oxides outward from the ANP surface. Figure 3 shows the structural evolution of the ANP over the oxidation course with 4500 O2 molecules. At first, an absorbed O2 molecule dissociates on the surface step and forms two Al4O tetrahedrons


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(Figure 3a−b). Two picoseconds later, another O2 molecule dissociates nearby and generates a raised structure made up of connected Al4O tetrahedrons (Figure 3c), which is similar to the oxide nucleation process in low oxygen content. However, the difference is that these raised tetrahedrons subsequently extend outward from the ANP surface to form chain−like structures (Figure 3d). Such chain−like structures mainly consist of two parts: an oxide head made of several connected Al4O tetrahedrons and an unoxidized aluminum tail. These initially formed chains are unstable and can be further oxidized. In particular, the aluminum tail has high reactivity because the coordination numbers of these Al atoms are very low. As a result, the oxygen preferentially reacts with these highly reactive aluminum tail rather than oxidize the surface Al atoms of the ANP. Note that the connection between the ANP and oxide chains is very weak because there are only several Al atoms locating at the joints. As a result, when new oxygen attacks these aluminum tails, the dissociation of oxygen at these sites could lead to the detachment of the oxide chains into the environment, as shown in Figure 3e−f. Figure 3g shows that the remaining chains on the ANP surface could connect into rings by random collision with each other during the subsequent oxidation. Finally, the rings and chains merge into thicker oxide ones and wind around the unoxidized ANP (Figure 3h).


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Figure 3. Epitaxial growth of chain−like oxides out of the ANP surface over the course of oxidation with 4500 O2 molecules. (a−b) O2 molecules dissociate on the surface steps and form connected Al4O tetrahedrons. (c−d) Epitaxial growth of a chain−like structure from the raised Al4O tetrahedrons. (e−f) The oxide chain breaks up and separates from the ANP matrix because of the introduction of a new O2 molecule. (g−h) The residual small oxide chains connect into rings, merge into thicker ones and finally wind around the unoxidized ANP. Compared to the oxidation process with 2250 O2 molecules, the chain−like oxides generated under higher oxygen contents prefer to grow outward from the ANP surface rather than remain attached to the surface. This interesting phenomenon is originated from the exothermic nature of oxidation, which provides the surfaces atoms with high kinetic energy and produces large internal stress. In Figure 4a, the average atomic kinetic energy (KE) of Al atoms is shown as a function of nanoparticle radius. We firstly divided the Al atoms in the ANPs into different spherical shells with a thickness of 2 Å, and then calculated the average atomic kinetic energy in each shell. The results show a significant kinetic energy gradient along the radius, indicating that the surface atoms of ANPs attain more oxidation heat than the inner ones. The kinetic energy gradient is consistent with the melting studies of metallic nanoparticles performed by Liu et al.39, which reveals that the atoms located on the corner and edge sites attain higher kinetic energy than others when heated up and result in surface premelting below the melting points. The average kinetic energy of Al atoms increases with the initial oxygen content, suggesting that the oxidation of ANPs releases more heat in higher oxygen contents and further contributes to the surface disordering. Figure 4b shows the Al−Al pair distribution functions (PDF) of the surface Al atoms. We selected the Al atoms which have a distance of 20 Å~30 Å to the center−of−mass of the ANP to investigate the influence of oxidation on the surface structure. The outward cutoff is selected as 30 Å in order to include the unoxidized aluminum tails formed at the initial stage as shown in Figure 3e−f and at the same time exclude the oxidized Al atoms. Consider that the ANPs only absorb a little oxygen before 50 ps, the error of this approach is


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rather small. In the system with 2250 O2 molecules, the crystalline structure of the surface Al atoms is not damaged by the oxidation, which can be seen from the distinct peaks in the PDF curve. The released heat could not provide much kinetic energy to the surface energy because of the low oxidation rate. Therefore, the oxides gradually accumulate on the ANP surface and finally grow into an oxide shell. When the initial system has 4500 O2 molecules, the heat released by oxidation increases significantly due to the increased oxidation rate. The peaks in the PDF curve drop in comparison to the system with 2250 O2 molecules, indicating that the surface structure is more disordered and unstable. The increased kinetic energy of the Al atoms also induces the structural expansion and generates larger internal stress as shown in Figure 4c. Driven by the combined effect of the kinetic energy and internal stress, the oxide nuclei and some unstable surface Al atoms move outward, resulting in the epitaxial growth of chain−like structures. During the subsequent oxidation, these outstretched chains serve as the nuclei for further oxide growth because of the relatively low coordination numbers. Finally, an abnormal composite structure that oxide chains surround the slightly oxidized aluminum core is obtained. When the initial system has 6750 O2 molecules, the disappearance of the second peaks and the combination of other peaks reveal that the heat released by oxidation melt the surface of the ANPs. The high kinetic energy and internal stress drive the ANP to explode instantly, generating many unstable atomic chains in all directions. The length of the chains is also found to increase with the oxygen content, which can be seen from the snapshots in Figure 1. The chains are very unstable in higher oxygen content. Besides being more frequently attacked by the oxygen, the chains are prone to detach from the ANPs because of the extremely high kinetic energy. Hence, the final oxidation products in 6750 O2 molecules are scattered oxide clusters.


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Figure 4. Structural evolution induced by the heat released by oxidation at the initial stage. (a) Average atomic kinetic energy as a function of nanoparticle radius. (b) Al−Al pair distribution function of the surface Al atoms. (c) Atomic stress distributions. These results are the average values of the atomic configurations from 45 ps to 55 ps. Figure S2a−b shows the comparison of the oxidation kinetics in the systems with different oxygen contents. The oxidation of ANPs experiences two distinct stages that an initial fast oxidation stage (t < 750 ps) follows by a slower one. The oxidation rates reach the peak at about 250 ps, and then gradually decrease in the following reactions. The growth of oxide chains significantly increases the contact area between aluminum and oxygen, which further leads to faster oxygen consumption in the initial stage. As a result, the rate of oxygen uptake at the initial stage increases with initial oxygen content and reaction temperature. At longer times, when the active sites of the ANPs (such as the corner, edge sites, and the outstretched chains) are covered with oxides, the oxidation enters the slower stage due to the reduced contact area. Figure S2c shows the numbers of oxide clusters as a function of time, which illustrates the fracture behaviors of the chain−like oxide nuclei. When the initial system has 2250 O2 molecules, few oxide clusters are generated because of the relatively low heat released by oxidation and internal stress. When the initial oxygen content goes higher, the oxidation reaction becomes violent and more oxide chains extend outward from the ANP surface. The larger internal stress drives the ejection of oxide clusters from the ANP matrix. After about 300 ps, the amount of oxide clusters


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gradually decreases because the oxide clusters tend to merge together when they collide with each other in the motion. To further figure out how the oxides are formed at the atomic scale, we investigated the evolution of the coordination numbers of Al atoms during oxidation. Figure 5a shows the stable configurations of the oxide components, in which Ali+ is used as a notation to describe the number of the nearest−neighbor O atom of the center Al atoms. Here, the notation Al, Al1+, Al2+, Al3+, Al4+ and Al5+, means that the Al atom has zero, one, two, three, four, or five nearest−neighbor O atoms, respectively. Al1+ exists in the Al4O tetrahedrons formed by initial oxygen dissociation on the exposed surface of the ANP. The structure of Al2+ is linear, which is the intermediate structure to connect two Al4O tetrahedrons and convert to chain−like oxide nuclei. Al3+ and Al4+ have relatively stable planar and tetrahedral structures, respectively, which are the dominated structures in the final products. Al5+ only appears after a long period of reaction when oxygen is superfluous. The actual configurations of the generated oxides could be distorted that the bond lengths and angles could deviate from the stable values due to the internal stress caused by oxidation. Figure 5b shows the evolution of Ali+ during oxidation. The amounts of Al1+, Al2+, and Al3+ increase firstly and then decrease with time, suggesting higher−coordinated oxides gradually replace the lower−coordinated ones in the process of oxidation. The amount of Al4+ increases before reaching the limit, and there are few Al5+ generated in the oxides, indicating that the Al4+ tetrahedron is predominant structure in amorphous aluminum oxides, which is in good agreement with other experimental or simulated works.9, 17 The oxides formed in the early stage of oxidation is amorphous in nature, which has been demonstrated by some research results.9-10 We found the oxide component is affected by their morphologies. The oxide shell is composed of all kinds of Ali+ (Figure S3a), the oxide chains are mainly composed of Al3+ and Al4+ (Figure S3b−c), and the small oxide clusters are mainly composed by Al4+ (Figure S3d).


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Figure 5. Evolution of oxide components in the conditions of different oxygen contents. (a) Stable configurations of the different coordinated oxides. Ali+ is used as a notation to describe the number of the nearest−neighbor O atom (i.e., Al, Al1+, Al2+, Al3+, Al4+ or Al5+, represent the Al atom has zero, one, two, three, four, or five nearest−neighbor O atoms, respectively). (b) The evolutions of Ali+ during oxidation. Oxidation of ANPs also shows a strong dependence of temperature. Figure 6a shows a statistical distribution of the coordination number of Al atoms in the initial unoxidized ANPs at different temperatures. As the tempearture increases, more atoms with low coordination numbers appear on the ANP surface, which can serve as the active sites for oxygen dissociation and oxide nucleation. Furthermore, the rise in temperature also induces the rise in the kinetic energy of both Al atoms and O2, which contributes to overcoming the activation energy barrier of oxidation40 and further facilitates the epitaxial growth of chain−like oxides during oxidation. To further illustrate the influence of temperature on the structural characteristics of oxidation products, we performed some oxidation simulations at various temperatures, in which the initial oxygen content remained constant (3375 O2 molecules). The oxidation kinetics at various


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temperatures also experiences the stage transition from fast to slow as shown in Figure S4. We analyzed the oxide structure using the Ali+ distribution, atomic charge distribution, pair distribution functions (PDF) and bond−angle distribution functions (ADF). These results show the average configurations between 1991 ps and 2000 ps. Figure 6b reveals the distributions of Ali+ in the final products at different temperatures. With the increasing temperature, the amounts of Al, Al1+, and Al2+ in the products decrease while the amounts of Al3+, Al4+, and Al5+ increase, indicating that higher temperature promotes the formation of higher−coordinated oxides. Figure 6c shows the atomic charge distributions of the oxidized products at different temperatures. Each peak in the charge distribution curves corresponds to the atom with certain Al−O coordination number. The peaks of Al, Al1+, and Al2+ disappear over 600 K, suggesting that the ANP is completely oxidized at higher temperatures. Moreover, the positions of peaks are found to shift towards the higher charges as the temperature increases, suggesting the electron transfer during oxide formation is promoted at higher temperatures. The position of the first peak in gAl−O(r) in Figure 6d gives the Al−O bond length to be around 1.82 Å at all temperatures, which agrees well with the bond length in bulk amorphous Al2O3.41 Figure 6e−f shows the Al−O−Al and O−Al−O bond−angle distribution functions (ADF) of the oxidation products. The bond−angle distributions cover a wide range between 80° and 180°, which is a typical characteristic of amorphous and nonstoichiometric oxides. The Al−O−Al ADF in Figure 6e has a main peak at about 110°−120° and a small peak at 90°, which correspond to the corner−sharing and edge−sharing configurations of the Al4+ tetrahedra, respectively. The Al−O−Al ADF agrees well with previous MD simulation works which investigated the structure of amorphous alumina.16, 19 In Figure 6f, there are two distinct peaks in the O−Al−O ADF, which are approximately 90° and 110°, respectively. The small peak at 90° derives from the edge−sharing configurations of tetrahedra, while the main peak is the result of the mixture of trianglar Al3+ and tetrahedral Al4+ configurations in the oxides. With the increase in the oxidation temperature, the main peak gets higher and shifts towards 109°, indicating that Al4+ species become the dominant product at


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higher temperatures as shown in Figure 6b.

Figure 6. Temperature dependency of ANP oxidation. (a) Distributions of coordination numbers of the Al atoms in the initial unoxidized ANPs. (b) Distributions of Ali+ of the final oxidation products. (c) Atomic charge distributions of the final products. (d) Al−O pair distribution function (PDF) of the oxidation products. The insets are the edge−sharing and corner sharing configurations of the Al4+ tetrahedra which correspond to the two peaks, respectively. (e) Al−O−Al and (f) O−Al−O angle distribution function (ADF) of the oxidation products. The insets are the configurations which correspond to the two peaks, respectively. Size effect is also a nonnegligible factor for ANP oxidation. With the decline of the nanoparticle size, the ANPs become more reactive because of the increased surface−to−volume ratio. The proportions of the corner and edge sites of the surface atoms increase with the


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decreased curvature radius, which contributes to the chain−like oxide nucleation and growth. As shown in Figure 7, when D ≤ 40 Å, the oxidation process starts with the epitaxial growth of chain−like oxide nuclei and finally generates some oxides clusters. As the diameter increases, shorter chain−like oxide nuclei are generated and result in the formation of Al/oxide core−chain structure. When D > 60 Å, the chain−like oxide nuclei no longer extend outward, but attach to the ANP surface and finally generate the spherical Al/oxide core−shell structure. It is concluded that as the diameter increases, shorter chain−like oxide nuclei are generated and extend outward from the ANP surface in the initial oxidation stage, resulting in the morphology difference of the final products from the core−shell structure to the core−chain structure, then to independent oxide clusters.

Figure 7. Size effect of ANP oxidation. The snapshots show the oxidation processes and final products of the ANPs with various diameters in the same environment (3375 O2 molecules and at 300 K). Based on the above analysis, we plotted a schematic diagram in Figure 8 to reveal the relation between the morphology of the oxidized products of the 5−nm ANP and the environmental factors. The diagram can be divided into three regions according to the morphology of the oxidized ANPs: core−shell structure region, core−chain structure region and


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oxide cluster region. It can be seen that increasing the temperature and oxygen content can accelerate the growth of chains from the ANP surface, and at the same time, the initial generated chains are also more likely to break up and form some independent oxide clusters. As a result, in order to obtain the oxidized products with desired nanostructures, the temperature and initial oxygen content must be precisely controlled, because the overall morphology of the oxidation products is determined in the fast oxidation stage. For example, as shown in Figure S1, when the core−shell structure is exposed into higher concentration of oxygen, the overall morphology hardly changes after further oxidation. The subsequent slower oxidation stage could have certain effect on the thickness and detailed structure of the oxides, but no longer affect the overall morphology of the oxidation products.

Figure 8. The relation between the morphology of the oxidized products of ANPs with a diameter of 5 nm and environmental parameter. The diagram is divided into three regions according to the morphologies of the oxidized products: core−shell structure region, core−chain structure region and oxide cluster region. Conclusion In summary, we performed reactive molecular dynamics simulations to systematically study the oxidation mechanism of ANPs. Our results reveal a chain−like nucleation model and the growth


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pattern of oxides on the ANP surface during oxidation, resulting in the structural diversity of oxidation products. Due to the reduced coordination number, the Al atoms located on the corner and edge sites of the ANP surface serve as the active sites to be oxidized preferentially and form chain−like oxide nuclei. Oxidation−induced heat release and internal stress are the factors which arouse the surface oxide nuclei to extend outward from the ANP surface and form chain−like structures during oxidation. Higher temperature, higher oxygen content and smaller nanoparticle size not only promote the growth of oxide chains, but also make these chains more likely to break from the ANP and generate small oxide clusters. As a result, under different environmental parameters, the final oxidation products have diverse shapes and can be divided into three categories: core−shell structure, core−chain structure, and oxide cluster structure. Our findings provide an important perspective on understanding the atomic−scale oxidation mechanism of metal nanoparticles, which could have a practical significance for controlling the production of the advanced oxide nanostructures. Methods The reactive force−fields (ReaxFF),42 which is based on quantum mechanics and employs a bond−order formalism as well as polarizable charge descriptions to precisely model the interatomic interactions, has been recognized as a reliable tool to describe chemical reactions in a wide range of systems.12, 43-45 Detailed information about the ReaxFF can be achieved in papers by van Duin and his co−workers.42 Reactive molecular dynamics (RMD) simulations using ReaxFF have been proved to gain a great insight into the rearrangement of atoms during oxidation at the atomic level.10-12 In this work, an Al/O ReaxFF developed by Hong and van Duin29 was used to study the oxidation mechanism of ANPs without change. This ReaxFF has been proven to qualitatively predict the oxidation kinetics of Al(431) slabs at various temperatures compared with previous experimental observations.29, 46 Douglas−Gallardo et al.30 combined RMD simulations and density functional theory (DFT) using this ReaxFF to reveal the influence of oxygen on the plasmonic properties of aluminum nanoclusters. Additionally, Hong


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and van Duin47 have developed this force−field into the Al/C/H/O ReaxFF and employed it in investigating the passivation effect of the carbon coating on the oxidation of ANPs, in which the bond dissociation and angle distortion energies of the Al/C/H/O systems are proved to agree well with quantum mechanics (QM) data. The RMD simulations were performed using the Large−scale Atomic/Molecular Massively Parallel Simulator (LAMMPS).48 The initial configurations of the ANPs are firstly optimized by an energy minimization process using the conjugate gradient method.37 Then, the ANPs are heated up to the required temperature and allowed to relax for 300 ps to reach the equilibrium state. The 300 ps relaxation is proved to be sufficient for stabilizing the structure of the ANPs (Figure S5). In an oxidation set−up, the 50.4 nm ×50.4 nm ×50.4 nm simulation box is comprised of an ANP with a diameter of 5 nm (about 4000 atoms) placed at the center and a certain amount of oxygen molecules. The initial atomic velocities were determined from a Maxwell−Boltzmann distribution based on the given temperature. Non−periodic boundary conditions with reflecting walls were imposed in all directions to prevent the atoms from reaching the box limit. The boundary conditions are found to have little influence on the oxidation behaviors as well as the resulting oxide structure. Detailed comparison of the oxidation simulations at 900 K with 3375 O2 using reflected boundary conditions (RBCs) and periodical boundary conditions (PBCs) is shown in Figure S5. The simulations of ANP oxidation were carried out in the canonical (NVT) ensemble using the Nosé−Hoover thermostat.49 The Newton's equation of motion was calculated using the velocity−Verlet algorithm with a time step of 0.5 fs. The atomic charges were calculated by the charge equilibration (QEq) method.50 The charge relaxation procedure used to minimize the electrostatic energy is very time−consuming so that the charges were updated every 10th time step, which is an effective method to save computing time and at the same time guarantee the accuracy of the simulation. We calculated the stress tensor for each atom during oxidation using the approach proposed in the reference.51 The tensor has 6 components for each atom and is stored as a 6−element


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vector in rectangular Cartesian coordinates: σxx, σyy, σzz, σxy, σxz, σyz. The Cauchy atomic stress tensor on atom i (σi) is defined as:

[

]

𝜎𝑖𝑥𝑥 𝜎𝑖𝑥𝑦 𝜎𝑖𝑥𝑧 1 𝜎𝑖 = 𝜎𝑖𝑦𝑥 𝜎𝑖𝑦𝑦 𝜎𝑖𝑦𝑧 = 2𝛺𝑖{∑𝑗𝐹𝑖𝑗 × 𝑟𝑖𝑗} 𝜎𝑖𝑧𝑥 𝜎𝑖𝑧𝑦 𝜎𝑖𝑧𝑧

(1)

Where Ωi is the volume of atom i, and the sum is taken over all j−neighbors of atom i, with a distance of 𝑟𝑖𝑗 each and exerting a interatomic force 𝐹𝑖𝑗 upon atom i. Consider the volume of atoms is not well defined during oxidation, we followed the common practice that the atomic stress is expressed by σi · Ωi in the unit of atm × nm3.52 The average normal stress was used to determine whether the atom i is in tension (negative value) or compression (positive value), which is given as: 1

𝜎𝑖𝑚 = 3(𝜎𝑖𝑥𝑥 + 𝜎𝑖𝑦𝑦 + 𝜎𝑖𝑧𝑧)

(2)

Associated content Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Additional figures illustrate the passivation effect of the oxide shell, the oxidation kinetics in the conditions of different oxygen contents and temperatures, atomic charge distributions of the oxidation products, effect of the relaxation time on the structure of the ANP model, and the comparison of the simulation results using different boundary conditions. Additional videos show the structural evolution of the ANPs in the conditions of different oxygen contents. Acknowledgments The authors would like to acknowledge the support from the National Natural Science Foundation of China (Grant No.51671114, Grant No. U1806219). This work is also supported by the Special Funding in the Project of the Taishan Scholar Construction Engineering and Qilu


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Young Scholar Program of Shandong University. Additionally, Xingfan Zhang thanks Han Yu for her support over the course of this work. References 1. Li, X.; Zhang, D.; Liu, Z.; Li, Z.; Du, C.; Dong, C., Materials Science: Share Corrosion Data. Nature News 2015, 527, 441. 2. Popova, I.; Zhukov, V.; Yates Jr, J., Electrostatic Field Enhancement of Al (111) Oxidation. Phys. Rev. Lett. 2002, 89, 276101. 3. Liu, L.; Corma, A., Metal Catalysts for Heterogeneous Catalysis: From Single Atoms to Nanoclusters and Nanoparticles. Chem. Rev. 2018, 118, 4981-5079. 4. Bian, Z.; Tachikawa, T.; Cui, S.-C.; Fujitsuka, M.; Majima, T., Single-Molecule Charge Transfer Dynamics in Dye-Sensitized P-Type NiO Solar Cells: Influences of Insulating Al2O3 Layers. Chem. Sci. 2012, 3, 370-379. 5. Cabrera, N.; Mott, N., Theory of the Oxidation of Metals. Rep. Prog. Phys. 1949, 12, 163. 6. LaGrow, A. P.; Ward, M. R.; Lloyd, D. C.; Gai, P. L.; Boyes, E. D., Visualizing the Cu/Cu2O Interface Transition in Nanoparticles with Environmental Scanning Transmission Electron Microscopy. J. Am. Chem. Soc. 2017, 139, 179-185. 7. Luo, L.; Su, M.; Yan, P.; Zou, L.; Schreiber, D. K.; Baer, D. R.; Zhu, Z.; Zhou, G.; Wang, Y.; Bruemmer, S. M., Atomic Origins of Water-Vapour-Promoted Alloy Oxidation. Nat. Mater. 2018, 17, 514. 8. Xia, W.; Yang, Y.; Meng, Q.; Deng, Z.; Gong, M.; Wang, J.; Wang, D.; Zhu, Y.; Sun, L.; Xu, F.; Li, J.; Xin, H. L., Bimetallic Nanoparticle Oxidation in Three Dimensions by Chemically Sensitive Electron Tomography and in Situ Transmission Electron Microscopy. ACS Nano 2018, 12, 7866-7874. 9. Sankaranarayanan, S. K.; Kaxiras, E.; Ramanathan, S., Atomistic Simulation of Field Enhanced Oxidation of Al (100) Beyond the Mott Potential. Phys. Rev. Lett. 2009, 102, 095504. 10. Sen, F. G.; Alpas, A. T.; van Duin, A. C.; Qi, Y., Oxidation-Assisted Ductility of Aluminium Nanowires. Nat. Commun. 2014, 5, 3959. 11. Zou, C.; Shin, Y. K.; van Duin, A. C.; Fang, H.; Liu, Z.-K., Molecular Dynamics Simulations of the Effects of Vacancies on Nickel Self-Diffusion, Oxygen Diffusion and Oxidation Initiation in Nickel, Using the Reaxff Reactive Force Field. Acta Mater. 2015, 83, 102-112. 12. Sun, Y.; Zuo, X.; Sankaranarayanan, S. K.; Peng, S.; Narayanan, B.; Kamath, G., Quantitative 3D Evolution of Colloidal Nanoparticle Oxidation in Solution. Science 2017, 356, 303-307. 13. Tsuchiya, M.; Sankaranarayanan, S. K.; Ramanathan, S., Photon-Assisted Oxidation and Oxide Thin Film Synthesis: A Review. Prog. Mater. Sci. 2009, 54, 981-1057. 14. Ciacchi, L. C.; Payne, M. C., “Hot-Atom” O2 Dissociation and Oxide Nucleation on Al


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Atomistic Origin of the Complex Morphological Evolution of Aluminum

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