Atomistic Simulations of Al(100) and Al(111) Surface Oxidation

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Atomistic Simulations of Al(100) and Al(111) Surface Oxidation: Chemical and Topological Aspects of Oxide Structure Marcela E. Trybula, and Pavel A. Korzhavyi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b06910 • Publication Date (Web): 04 Dec 2018 Downloaded from http://pubs.acs.org on December 5, 2018

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The Journal of Physical Chemistry

Atomistic Simulations of Al(100) and Al(111) Surface Oxidation: Chemical and Topological Aspects of Oxide Structure

Marcela E. Trybula1,2* and Pavel A. Korzhavyi1,3

1Department

of Materials Science and Engineering, KTH Royal Institute of Technology,

Stockholm, Sweden 2Institute

of Metallurgy and Materials Science, Polish Academy of Sciences, Krakow,

Poland 3Institute

of Metal Physics, Ural Division RAS, 620219 Ekaterinburg, Russia

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Abstract

The chemical and topological aspects of short- and medium-range atomic ordering on oxidized Al(100) and Al(111) surfaces have been studied by employing Reactive Force Field based molecular dynamics simulations (ReaxFF-MD) as a function of O2 gas density at 300K. We found two oxide film growth regimes, compatible with experimental and recent modelling data. Trend of changes in oxide film thickness with increasing oxygen gas density agrees with available literature data, while slightly thicker oxide film forms on Al(100) substrate. Chemical descriptors of short- and medium-range correlation manifest difference in atom environment between two ultra thin oxide films as

[3,4]Al

and

[2,3]O

coordinated species dominate. In turn,

highly liquid-like structure of ultra thin oxide film develops on Al(100) surface compared to an amorphous nature of the Al(111) oxide film with slightly lower thickness. Three-dimensional analysis of oxide structures reveals a medium-range atomic order formed by the arrangement of dominating corner-sharing configurations over edge-sharing ones with some deviation from the ideal polyhedral units. 3-fold ring is in majority over 2-, 4- and 5- fold ones, in conjunction with a 2-fold ring forming the most frequent ring linkage. The high-n ring structure can be treated as a measure of a certain degree of ‘free volume’, incorporated in the oxide film during its growth on the Al(100) or Al(111) surfaces and can initiate nanostructure formation in anodic oxide film. Such diversity in ring abundance also explains the lower mass density of the oxide films compared to crystalline alumina compounds.


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1. Introduction A thin layer of native oxide spontaneously forms on an Al metal surface at ambient conditions. Usually its growth is an uncontrolled process that is accomplished in a series of competing steps leading to the formation of a complex amorphous structure. Obtaining knowledge about this structure is a challenging but necessary step in predicting the properties of thin oxide films. On the other hand, oxidation phenomena on metal surfaces represent a vast field of basic scientific research and play a key role in many technological and environmental applications ranging from electronic devices to mechanical tools. They are also important for heterogeneous catalysis, corrosion protection, and coating technology.

1–3

Low-

index Al surfaces, such as (111) and (100), have attracted much attention due to their superior thermodynamic stability compared to the (110) surface. 4,5 Even though many aspects of their oxidation have been studied, 6–8 the debate concerning the exact structure of oxide layers formed on low-index Al surfaces is still open. Even the most recent theoretical models cannot provide an atomic-level description of the structure of growing oxide depending on the crystallographic orientation of oxidized Al surface. 8,9 Therefore, it is of practical and fundamental interest to study the chemical and topological environment of atoms in oxide films grown on Al(111) and Al(100) surfaces at early and late oxidation stages depending on the exposure conditions. Most of the modelling and experimental studies made so far were devoted to understanding and describing the processes occurring upon thermal oxidation of various Al surfaces,9,10 Al nanoparticles11–13 (ANPs) and Al-based systems 14,15 as a function of temperature. These studies were mainly aimed at describing the consecutive oxide formation stages associated with the oxygen incorporation into aluminum and its alloy substrates and related to the oxide growth kinetics and morphology. Theoretical studies mostly provided molecular dynamics simulation data obtained by employing force field methods such as the Reactive Force Field13 and the electrostatic plus (ES+)11,12,16 approach. All these approaches gave relatively consistent views on the kinetics of oxide growth and the atomic-level oxidation mechanism. Interestingly, Hong and van Duin

13

reported the formation of ‘hot

spot’ regions on ANPs surface during the highly exothermic oxidation reaction caused by high temperature. These ‘hot spots’ facilitate oxygen penetration through the developed oxide film. In addition, their findings shed light on the relationship between thickness and void formation ability for ANPs and stacked Al surfaces as a function of increasing oxygen gas pressure and temperature. 13 On the other side, the structure of both amorphous and liquid alumina has been a subject of intensive studies for a long time, giving a consistent view on chemical aspects, discussing Al and O coordination environment as well as on the spatial network around Al and O species in amorphous and liquid state.17– 23

The attempts to tackle those issues required the use of sophisticated experimental techniques such as

neutron scattering23,24 and nuclear magnetic resonance (NMR) 18,25 to measure short- and medium-range order and to probe the evidence of triply coordinated oxygen18 ([3]O), constraining the glass-forming ability.With regard to the NMR-based study, [3]O and [4,5]Al coordination environments are found to be in majority in am-Al2O3 with distinct Al arrangements around edge-sharing oxygen atoms compared to 3 ACS Paragon Plus Environment


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crystalline Al2O3. 18 While other studies found surface-dependent local atom environment in amorphous alumina with the dominance of oxygen-bridged configurations on the outermost surface and increasing content of tetrahedrally coordinated aluminum atoms moving from the 1Å thick surface layer to bulk.26 Scant discussion of chemically different environment in the ultra thin oxide film growth on pure aluminum surfaces has been given theoretically as all the attempts have failed to capture the crystallographic surface orientation dependence. An Al-rich amorphous film forms at low temperatures and it attains the stoichiometry with increasing temperature to become crystalline γ-Al2O3.27 The structure of bulk γ-Al2O3, as it has been found long ago, has a highly defective spinel lattice that can transform into a pseudoamorphous structure.28 If grown at room temperature, alumina is most likely to be found in the amorphous state.20 Very recently, experimental studies confirmed its structural resemblance to bulk γ-Al2O3 indicating the unique character of [3]O and diminishing [5]Al site fraction in favor of increasing [4,6]Al site fraction.18 Indeed, it mainly consists of corner-sharing AlO4 polyhedral units forming planar 6-membered (6m) and 8-membered (8m) rings with a complex structure,20 dominating over the edge-sharing polyhedra. In comparison, the structure of α-Al2O3 consists only of 6m and 4m rings in which Al atoms are fourfoldand threefold-coordinated, whereas oxygen atoms are twofold- and threefold-coordinated.29 The present work is motivated by the absence of atomic-level understanding of crystallographic surface orientation dependence of the growing oxide film structure and by the significant difference in microstructural evolution of oxide films grown on Al(100) and Al(111) surfaces, as observed in HRTEM and XPS experiments. 30 According to Flötotto et al., 30 the difference may be attributed to the nucleation of an (1x1) structure of adsorbed oxygen and to the build-up of tensile stress on Al(111), generating ‘free volume’ during the oxide growth. The latter promotes the oxidation process through the growth of an oriented defective γ-Al2O3.28 The oxidation nucleation is described as a barrierless exothermic process in which a metal-oxygen cluster involving at least three metal ions is formed, which eventually leads to the formation of a defective and disordered structure of aluminum oxide on the initially bare Al(111) surface.28 The purpose of this work is to give a detailed atomic-level insight into the chemical and topological order in oxide films formed upon thermal oxidation of (100) and (111) aluminum surfaces at room temperature. We are particularly interested in describing the atomic-level differences in atomic short- and medium-range order that can potentially have an influence on glass-forming ability. First, we investigated the evidence of crystallographic orientation dependence of the structure of ultra thin oxide film grown on pristine Al surfaces at low temperature expressed in terms of statistical distribution and arrangements of Al species around O, and vice versa. Second, we extended our studies on analyzing the topological atom network in 3 dimensions by performing n-ring analysis to probe the structural resemblance of the ultra thin oxide film formed to stable and transient crystalline alumina compounds, amorphous and liquid alumina. The molecular dynamics simulations are performed using the Reactive Force Field method31 for


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two low-index Al surfaces, i.e. (100) and (111) at 300 K and using three different values of initial O2 gas density (i.e. 0.12, 0.18 and 0.24 g/cm3). The present paper is organized as follows. In Section 2, we briefly review the ReaxFF-MD methodology. The simulation results obtained for Al(100) and Al(111) surfaces are presented and discussed in Section 3. First, we describe the reaction of oxygen incorporation into the surface oxide films in Subsection 3.1. Then, in Subsection 3.2 we discuss the time evolution of chemical and topological atom arrangements within the first and second coordination shells in the growing oxide depending on the initial oxygen density. The structure and chemistry of the formed oxides are analyzed in terms of static structure factor, pair correlation functions, coordination numbers and oxygen density profile through the oxide layer. We also discuss the effect of stress, incorporated into the surfaces during their oxidation, on the oxide film structure in the light of the structural differences observed. Finally, in Subsection 3.3 we analyze the statistics of n-ring structures formed at the early stage of oxidation and their influence on the overall structure of the developed oxide film. In Section 4 we summarize the findings of our modeling study of the structure and chemistry of oxide films grown on Al(111) and Al(100) surfaces at 300 K.

2.

Methodology

Reactive Force Field (ReaxFF) is a semiempirical force field that is fitted to a quantum mechanics-based calculation and uses bond order formalism to describe bond breaking and bond formation according to the methodology introduced by Tersoff and Brenner.

32

The bond order is instantaneously determined

from the interatomic distances and valence and torsion bond angles updated at each iteration. It also accounts for polarization effects determining the geometry-dependent charge distribution in a studied molecule or material, which is, in turn, derived using the electronegativity equalization method (EEM). 33 ReaxFF method also describes non-bonding interactions such as Coulombic and van der Waals terms, which are computed for each atomic pair. The mathematical formalism behind the ReaxFF method has been described in detail by van Duin at al. 31,34 Recently, its effectiveness and usefulness for the successful prediction of the overall characteristics of metal/oxide or metal/water interfaces have been demonstrated for several systems including silicon, 35 aluminum,13 zinc 36 and titanium. 37 In this work, molecular dynamics (MD) simulations were performed using LAMMPS code

38

and

employing a recent ReaxFF parametrization developed for Al/oxide interface by Hong & van Duin,

13

who had validated the force field by inspecting the temperature and oxygen gas pressure effects on the oxide growth kinetics on bulk Al(431) surface and on the surface of an Al nanoparticle. The scheme of simulation box, used for the present MD simulations, was similar to that previously employed by Hong and van Duin

13

or Hasnaoui et al.

8

We performed MD simulation of oxidation

reaction at 300K for Al(100) and Al(111) stacked slabs with the approximate dimensions 252525 Å and 202025 Å, respectively, each containing more than 800 Al atoms. Additional vacuum layers were added to the top and bottom of each box, and periodic boundary conditions were imposed in all three 5 ACS Paragon Plus Environment


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directions. In the vacuum boxes, 60, 100 and 120 O2 molecules were randomly distributed. Canonical ensemble with constant number of atoms, constant volume and constant temperature (NVT), was used in the present MD simulations. The Nose-Hoover thermostat was employed to maintain the Al-O system at 300 K and separate velocity-rescaling constraints were applied to oxygen and aluminum atoms to avoid melting the aluminum surface. The equations of motion were integrated using Verlet’s algorithm 39 with a time step of 0.5 fs and for the maximum simulation time of 1 ns. Positional correlations, expressed in terms of total structure factor 𝑆(𝑞), total and partial pair distribution functions (PDF), 𝑔(𝑟), as well as coordination number 𝐶𝑁𝛼𝛽(𝑅), were used to describe the chemistry of nearest-neighbor environment of ions in the growing oxide films.

40,41

They provide information on the

collective atom motion and statistical behavior of two-body correlations. The total 𝑆(𝑞) and 𝑔(𝑟) were computed from their respective partial contributions: 1

𝑆(𝑞) = ∑𝛼,𝛽(𝑐𝛼𝑐𝛽)2𝑆𝛼𝛽(𝑞)

(1)

𝑔(𝑟) = ∑𝛼,𝛽𝑐𝛼𝑐𝛽𝑔𝛼𝛽(𝑟)

(2)

The average coordination number 𝐶𝑁𝛼𝛽(𝑅) is given as an integral over the corresponding partial pair correlation function: 𝑅

𝐶𝑁𝛼𝛽(𝑅) = 4𝜋𝜌𝑐𝛽∫0 𝑔𝛼𝛽(𝑟)𝑟2𝑑𝑟

(3)

In the case of a system with charge-transfer effects, two-body charge-charge correlations are an important factor in describing the structure, and they can be represented by the charge-charge structure factor 𝑆𝑍𝑍(𝑞) which is defined as: 1

∑

𝑆𝑍𝑍(𝑞) =

𝑍 𝑍 (𝑐 𝑐 )2𝑆𝛼𝛽(𝑞) 𝛼,𝛽 𝛼 𝛽 𝛼 𝛽 ∑ 𝑍2𝛼𝑐𝛼 𝛼

,

(4)

Zi and ci are charge and concentration of α and β species, while 𝑆𝛼𝛽and 𝑔𝛼𝛽 are the corresponding partial structure factor and the pair distribution function, respectively. Bond-angle distributions were determined from the recorded MD trajectories, using a cutoff distance of respective α-β atom pair that was taken from the position of the first minimum of 𝑔𝛼𝛽(𝑟). Then, the typical bond angles α-β-α and β-α-β were calculated for 100 different oxide film structures. The topology of atom environment, representing a three-dimensional (3D) linkage of atoms beyond the nearest-neighbor distance, was investigated by performing the n-fold ring structure analysis for selected oxide film structures. An n-fold ring is defined as a ring consisting of 2n alternating Al-O units. For example, a 2-fold ring is a 4-member structure comprised of 4 Al-O bonds. n-fold ring analysis was performed for 50 inherent oxide film structures, extracted from MD trajectories, in which the atoms are brought to local minima of the potential energy surface by applying conjugated gradient algorithm, as implemented in LAMMPS, 38 to separate out the vibrational motion of atoms. Then, three different sets of Al-O bond distances were considered for each oxide film to collect their ring statistics, which were 6 ACS Paragon Plus Environment

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then expressed as a graph of abundance for 5 most abundant ring structures found in the ultra thin oxide film.

3.

Results and Discussion

3.1. Oxidation mechanism and oxide growth kinetics The time evolution of the oxide films developing on Al(100) and Al(111) surfaces at 300 K for an initial oxygen gas density of 0.24 g/cm3 is presented in Figures 1a and 1b, respectively. The snapshots of the oxidation process show inward diffusion of oxygen and outward diffusion of aluminum atoms as the oxide films develop. The corresponding pictures of pressure-dependent thickness of oxide films for the two surfaces exposed to 0.12, 0.18 and 0.24 g/cm3 initial oxygen gas density are presented, respectively, in Figures S1a-c and S2a-c of the Supporting Information. In both analyzed cases, at very early oxidation stages, the adsorbed oxygen atoms go subsurface, thereby lifting up the outmost layers of aluminum atoms (see Figures S2 in the Supporting Information). This stage progresses relatively rapidly to increase the number of Al–O bonds, but some amount of oxygen bridging configurations remain in the outmost oxide layer. Then the oxidation reaction proceeds to the second (very slow) stage at which a diffusion controlled oxide growth process takes place. During the oxide layer formation, the concentration of O– O bonds at the oxide/gas interface is high, and this promotes the formation of ‘free volume’ in the structure of oxide, as marked in Figure 1c and Figure S2c. This ‘free volume’ is present in the form of voids in the structure of the oxide film, enabling easy transport of oxygen anions or aluminum cations through the oxide. The ‘free volume’ can bear some resemblance to the nanoporous structure that forms in anodically grown oxide layers. 42,43 However, the size of built-up voids in the ultra thin oxide films is smaller than the self-ordered pores in hexagonal array with diameter ranging between 10 and 400 nm, and height reaching tens of micrometers.44 Despite this fact the results of the present simulations may shed some light on the microscopic mechanism of the pore formation. A comprehensive description of oxygen adsorption onto Al surface and oxide growth is provided in the Supporting Information (Figures S3a-d).



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Figure 1. Snapshots taken from MD simulations of Al surface oxidation at 300 K and 0.24 g/cm3 initial oxygen gas density for a) Al(100) surface and b) Al(111) surface. Top view (c) on the two monolayers of an oxide film grown on Al(100) surface with ‘free volume’ at t = 1 000 ps. See text for more details.


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Time dependence of the oxide films thickness obtained from the present ReaxFF–MD simulations at 300 K starting from 0.24g/cm3 oxygen gas density is presented in Figure 2a. It can easily be seen that the oxide film grown on Al(100) surface is slightly thicker than that formed on Al(111) surface at 𝑡 = 1 000 ps. Importantly, the observed changes of ultra thin oxide film thickness are in agreement with the oxidation reaction mechanism (see Section 2S of the Supporting Information) as well as the trend obtained in the recent ES+- based MD study of Sankaranarayanan et al.15 for pure Al and its binary alloys. Basically, two kinetic regimes have been observed, a rapid oxidation during which the thickness of oxide film reaches 1.3 nm (between 150 ps and 210 ps) followed by a slow oxide growth regime until 1 000 ps. The present ReaxFF-based MD simulation results of oxide film growth as a function of increasing time on two Al surfaces exposed to 0.12 g/cm3 (triangles), and 0.24 g/cm3 (squares) initial oxygen gas densities are depicted in Figure 2b, where they are also compared to the results of recent MD simulations based on variable charge (ES+) interatomic potential enhanced by electric field.16 When comparing the present simulation results with experimental data,7 one should mention that much higher oxygen gas pressure and a shorter time-scale are considered in the simulations than it is typically used in the experiment. One can see that the oxide film thickness increases from ~0.5 nm to ~1.5 nm at 1 000 ps as the initial oxygen gas pressure is raised from 0.12 g/cm3 to 0.24 g/cm3. Considering the methodological differences between the two force fields, one can see that the oxide film thickness obtained in ES+ molecular dynamics simulations enhanced by electric field is higher than that achieved using the ReaxFF potential method. The different treatment of electric field effects in the ES+ based modelling does significantly affect the rate of oxide film growth and importantly, accelerates the overall kinetics of oxidation reaction even though no broken Al-O bonds appear at the oxide/gas interface. Sankaranarayanan et al.16 considered the effect of electric field at constant temperature and constant oxygen gas density while pressure-dependent oxide growth kinetics on Al substrate has been studied by Hasnanoui et al.9 A lower oxygen gas pressure results in a thinner oxide film formed upon oxidation, as demonstrated in Figure 2b and can be deduced from Ref.[9]. The ReaxFF-MD data for the highest oxygen gas density, used for oxidizing bulk Al surfaces (squares in Fig. 2b), correlate reasonably well with the results of ES+-MD simulations (solid blue line). Hence, the diffusion rate of cations and anions through a developing oxide film on a metal surface at low temperature may be accelerated by increasing the oxygen gas pressure and thereby increasing the chemical potential gradient.



a)

Al(100) Al(111)

oxide film thickness (nm)

1.5

1.0

0.5

0.0 0

200

400

600

800

1000

time (ps)

b) 2.5

oxide thickness (nm)

O2 gas density 0.12 gcm-3 0.24 gcm-3 Al(100) Al(100) Al(111) Al(111) ES+-MD study

2.0

1.5

1.0

0.5

10 00

80 0

60 0

40 0

20 0

0.0

0

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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time (ps)

Figure 2 a) Computed thickness of oxide films grown on Al(100) and Al(111) surfaces in the ReaxFFMD simulations at 300 K and 0.24 g/cm3 O2 gas pressure. b) Comparison of thickness between the present ReaxFF-MD simulations and ES+-MD simulations 16. See text for more details. 3.2

Chemical aspects of oxide film structure

Various structure descriptors exist for bulk materials and oxide films. Examples are structure factor 𝑆(𝑞) , pair distribution function 𝑔(𝑟), coordination number 𝐶𝑁 or three-body correlations such as bond-angle distribution function. Functions 𝑆(𝑞) and 𝑔(𝑟) give information about positional correlations arising from number-density fluctuations that can also be obtained from X-ray or neutron diffraction experiment.45–47 Below we discuss the structure of the oxide film formed in the simulations considering an initial oxygen gas density of 0.24 g/cm3, due to the fact that such film has the highest thickness, and to minimize the errors associated with the structural analysis of ultra thin films.

Density correlation functions A comparison of the structure factor calculated for the ultra thin oxide films developed upon Al(100) and Al(111) surface oxidation (present ReaxFF study), with experimentally determined in porous anodic 10 ACS Paragon Plus Environment

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aluminum oxide (PAA) film48 and liquid alumina24 is shown in Figure 3a. One striking feature is a small pre-peak accompanying the first highly intense peak of each 𝑆(𝑞) function. It is thought to be the first sharp diffraction peak (FSDP),49,50 which is an indicative of medium-range atomic order developing in amorphous and glassy state as it has been found for covalent glasses, such as SiO2 for example.51 The prepeak in the experimental data is due to the ordered pores that form the structure of an anodically grown oxide film as deduced from the explanation presented by Lamparter and Kniep.48 A feature looking like a small pre-peak is also seen in the structure factor of the oxide film obtained in the present ReaxFF simulations (solid and dashed lines), which is an indicative of a medium-range atomic ordering presence that will be discussed below. The presence of a pre-peak decreases the intensity of the first peak, as seen in Figure 3a, where the area under the pre-peak is similar to the reduced intensity of the first highly intense peak. Importantly, both shape and intensity of the first most intense peak of two ultra thin oxide films and PAA one (asteriks) differ from those obtained from synchrotron-based experimental measurement for liquid alumina (squares).24 Consequently, such a comparison illustrates the difference in atom environment and its spatial distribution between the two analyzed oxide films and liquid alumina that will be a subject of further discussion. At this point it is important to analyze whether there is any significant difference in the structure between the oxide films grown on Al(100) and Al(111) in the ReaxFF-MD simulations performed within this work.



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Figure 3. Computed neutron static structure factor of oxide films grown on Al(100) and Al(111) surfaces a) lines – ReaxFF-MD simulation results, asteriks – experimental data for porous anodic alumina (PAA) film48 and squares – liquid alumina.24 b) Charge-charge static structure factor of oxide films obtained by oxidizing Al(100) and Al(111) surfaces at 300K and initial oxygen gas density of 0.24 g/cm3.

To understand the atomic correlations and their impact on the structure, we analyze the charge distribution in the oxide films by computing the charge-charge static structure factor 𝑆𝑍𝑍(𝑞) 52 which is plotted in Figure 3b. In the calculations according to Eq. (4), the effective charges of +3Q and -2Q, have been assumed for Al and O atomic species in the oxide films, respectively. A detailed comparison of the 𝑆𝑍𝑍(𝑞) functions computed for the two analyzed oxide films, involving the calculated average ionic charges of Al and O in the oxides, is presented in Figure S4 of the Supporting Information. The charges were computed by using the electronegativity equalization method (EEM) as implemented in ReaxFF.33 A significant dissimilarity in the intensity between the first and the second main peaks of 𝑆𝑍𝑍(𝑞) is observed for ultra thin oxide films grown on both Al(111) and Al(100) surfaces, without any noticeable difference in shape between the two curves. The intensity distribution suggests the presence of medium12 ACS Paragon Plus Environment

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range atomic correlations, on a scale that ranges between 4 and 8 Å, in both thin oxide films. The atomic charge distribution through the oxide film grown on Al(100) substrate at 𝑡 = 1 ns is shown in Figure 3b as an inset. The positive charges (blue) correspond to aluminum atoms, while negative ones (red) represent the oxygen atoms. Atoms within the Al metal slab have neutral charges (white balls) as compared to the ionized atoms (colored balls) in the oxide films where the charges are distributed inhomogeneously. More specifically, the Al atoms close to the metal/oxide interface are weakly charged as compared to the Al atoms in the oxide interior whose charges are around +1.5e. The calculated negative charges of oxygen atoms are also high inside the film but attain less negative values at the oxide/gas interface. For keeping the oxide film charge neutral, the charges of oxygen atoms attain more negative values when approaching the Al metal/oxide interface. Very small negative values are recorded for the surface oxygen atoms, those not fully incorporated into the oxide, as compared to highly negative charges which are obtained for the oxide interior and metal-oxide interface. An analogous atomic charge distribution is found for the oxide film grown on Al(111) surface as it is seen in Figure S5 of the Supporting Information. Importantly, similar picture of atomic charge distribution through the oxide film gave other force fields, previously used for Al-Ni alloy surface oxidation modeling.15 Regarding the charge-density correlation, there is a very small difference between the 𝑆𝑁(𝑞) and 𝑆𝑍𝑍(𝑞) for the low-q region. The difference is too small to prove the FSDP presence arising from charge-density correlations. As it has been found for amorphous SiO2 and other covalent glasses,53 the density-density correlations and charge neutrality prevail in the intermediate-range between 4 and 8 Å and are dominant for those compounds. The peak around 3 Å1 in 𝑆𝑍𝑍(𝑞) structure factor involves contributions from AlAl and O-O, being partially cancelled by the negative contribution due to Al-O partial structure factor.

Bond length and coordination number Figure 4 shows the total pair distribution function, 𝑔(𝑟) of two oxide films formed at 1 ns and 0.24 g/cm3 O2 gas density, computed using Eq. (2). The peaks in this function correspond to coordination shells, therefore, the first most intense peak corresponds to the first (nearest neighbor) coordination shell, while the position of its maximum is related to the bond length. One can see a significant mismatch in the intensity of the first most pronounced peak between the present ReaxFF-MD simulation data and experiment, resulting from the difference in thickness between two thin oxide films. The experimentally studied porous anodic oxide (PAA) film had a larger thickness than that formed by electrochemical oxidation.54 Basically, a thicker anodic oxide film forms during electrochemical anodization due to high electric field created by the applied voltage when metal cations of the substrate and oxygen anions from liquid solution migrate through the oxide layer at longer distances.55 The most pronounced peak, predicted by ReaxFF method in the ultra thin oxide films grown on the two investigated Al surfaces, is located around 1.83 Å, probably due to the presence of five-coordinated Al atoms. Since it is assigned to Al-O correlation, it was found to be slightly higher compared to that measured in liquid alumina (1.77 Å).24 To 13 ACS Paragon Plus Environment


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the best of our knowledge, the surrounding of Al atoms can strongly affect the Al-O bond length in amorphous bulk Al2O3. It can change from 1.73 Å (O-[4]Al) through 1.83 Å (O-[5]Al) to 1.88 Å (O-[6]Al).26 At the same time, the sharp and narrow shape of the first two peaks is an indicative of strong spatial correlations among the Al and O atoms extending to the second coordination shell. These correlations may be viewed as an intermediate-range order, developing in supercooled liquids prior to glass formation, preventing crystal growth,56 in the bond network at distances of 3-8 Å,57,58 that originates from the presence of AlOx structural unit derivatives (x =3, 4, 5, 6) that will be discussed below.

Figure 4. Comparison of the total pair distribution function between the ReaxFF-MD data (solid line – Al(100) surface and dashed line – Al(111) surface) and bulk anodic aluminum oxide experimental ones (asteriks) 48.

Partial pair distribution functions (PDFs) 𝑔𝛼𝛽(𝑟) representing the local atomic structure of oxide film grown on Al(100) surface at 300K are shown in Figure 5a for three values of initial O2 gas density. Corresponding data for the oxide film on Al(111) can be found in Figure S6 of the Supporting Information. A comparison between the oxide films formed on the two Al surfaces at the highest oxygen gas density is made in Figure 5b. The PDFs for the Al-O pair correlations are shown as insets to Figures 5a and 5b. The small pre-peak accompanying the first most pronounced PDF peak disappears with increasing initial oxygen gas density. As Figure 5a also shows, the position of the first peak shifts towards shorter distances as a result of increasing the oxide film thickness with increasing the initial O2 gas density up to 0.24 g/cm3 (blue line). Despite the observed downscale shift from 3.2 Å to 2.98 Å, the nearest-neighbor O-O 14 ACS Paragon Plus Environment

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correlations predicted by ReaxFF-MD simulations significantly overestimate those registered for both liquid and amorphous alumina (2.8- 2.83 Å).

Figure 5. Partial O-O and Al-O (inset graphs) pair distribution functions: a) as a function of oxygen density for Al(100) surface b) for Al(100) and Al(111) at 1 000 ps.

An interesting feature in Figure 5b is a small intensity peak at around 1.8 Å for the O-O pair, which remains present in the oxide film grown on the Al(111) surface after 1 000 ps (FigureS6 of the Supporting Information). Its intensity is more pronounced at the early stages of the oxidation process and decreases simultaneously with decreasing intensity of the main peak at 2.89 Å. The presence of the small-intensity peak is likely due to the formation of 2D oxide clusters at an early oxidation stage28,59 that could introduce compressive stress to the surface as it was inferred from High Resolution Transmission Electron Microscopy (HRTEM) and X-ray Photoelctron Spectroscopy (XPS) analyses30 as well as from DFT-based calculations. 28 The build-up of compressive surface stress at the initial oxidation stage of Al(111) prevents the formation of a laterally continuous monolayer of oxide, and this, in turn, promotes the formation of ‘free volume’ associated with void-type defects.60 The build-up of tensile stress continues during oxide film thickening, when the structural relaxation is hindered during the film growth, due to continuous entry of oxygen into the oxide film and to reconstruction of the structural unit network. In comparison with 15 ACS Paragon Plus Environment


Page 16 of 43

Al(111), the oxide film developed on a bare Al(100) surface is almost stress-free due to the formation of a laterally continuous monolayer of oxide film at the metal/oxide interface, preventing against the incorporation of stress during oxide film growth. This should rather result in a partial rearrangement of the AlOx building blocks during film growth and in a decreased degree of structural disorder associated with the incorporation of voids in the oxide film. Quite similar explanation of the difference observed between growing ultra thin oxide films on Al(111) and Al (100) has been inferred by Flötotto et al. from HRTEM and XPS analyses.61 Similar shoulder, preceding the highly intense peak, was found in the 1 Å thick surface layer of amorphous alumina corresponding to the presence of edge-sharing configurations at the surface, while it disappears in bulk am-Al2O3 as a results of predominating corner-sharing configurations.26 The Al-O pair distribution function (shown as inset in Figure 5b) shows a narrow and highly intense first peak at 1.81 Å that is more noticeable for the oxide film grown on Al(111) surface, suggesting differences in the chemical and topological atom packing of the first two nearest-neighbor shells that we will discuss below. Regarding these differences, we analyzed the oxidation state of Al atoms in the two studied systems. Consequently, a transition from the reference Al metallic state to the various oxidation states of Al in the aluminum oxide layer after 1 ns of oxidation process is found. According to Hong & van Duin, and using their definition of oxidation states, 13 the sub-oxide is dominant at 1 ns for aluminum nanoparticles (ANP, see Figure 19 in Ref. 13) in contrast to our present finding revealing the prevalence of oxide state on both sides of the oxidized slab. Peroxide and sub-oxide states also appear, and the former predominates at the Al2O3/gas interface, while the latter occurs at the Al/Al2O3 interface. This leads to a oxygen content gradient between the exterior of the oxide film and the metal/oxide interface (M-O), which must be reflected in the evolving arrangement of the AlOx building blocks, associated with changing Madelung potential of the Al cations and/or a change of the Al-O bond ionicity.62 The oxygen content gradient decreases with decreasing oxygen pressure and when the oxide film narrows down to approach the M-O interface. A significant oxygen concentration gradient is found for the highest oxygen density used (0.24 g/cm3), where the oxide/gas interface is enriched in bonded oxygen atom pairs O-O, while the M-O interface is oxygen-deficient. This is clear from the O/Al ratio variation across the oxide films, changing from 1.2 at the oxide/gas interface to 0.98 at the M-O region. In general, substoichiometric and oxygen deficient ultra thin oxide films of amorphous nature with various degree of structural disorder are formed on Al(111) and Al(100) substrate. Amorphous nature of oxide film forms on Al(100) surface as it has been reported in previous ES+ modelling enhanced by electric field.16 Local atomic bonding analysis expressed in terms of average bond length and coordination number (CN) for O-O, Al-O and Al-Al pairs is depicted in Figure 6a. Bond lengths of Al-Al, O-O and Al-O pairs, given as red numbers in Figure 6, are barely sensitive to changes in the oxidizing atmosphere compared to much more articulated sensitivity exhibited by the average CN. Interestingly, the increase in oxygen gas density up to 0.24 g/cm3 significantly affects the CN values of the Al-Al pair and less the O-O pair in ultra thin oxide films for both surface orientations. The film thickness increases from 0.89 to 1.6 nm for Al(100) 16 ACS Paragon Plus Environment

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and from 0.80 to 1.3 nm for Al(111) as the initial oxygen density changes from 0.12 g/cm3 to 0.24 g/cm3. Interestingly, the calculated sequence of interatomic distances for the respective atomic pairs is in good agreement with the results of recent ES+ MD simulations,16 which predict O-O distances to be longer than Al-Al distances. However, the trend of bond lengths is opposite according to the experimental data which show that Al-Al distances are longer than those for O-O pairs.

48

The Al-O bond length observed in

octahedral configuration of α-Al2O3 ranges between 1.86 Å and 1.97 Å as compared to significantly shorter distances, i.e. 1.73 - 1.78 Å confirmed experimentally in liquid alumina and transient alumina compounds.24 While for the surface layer of 1 Å thickness in am-Al2O3, the Al-O distance reaches 1.71 Å in favor of substantial intensity increase corresponding to the increase in number of three- and fourcoordinated Al atoms.26 Very recent 27Al NMR-based experimental work by Lee and Ryu18 revealed the dominance of [4,5]Al and [3]O in am-Al2O3, where several possible ways of [3]O linkage to [4,5]Al are likely to occur such as [3]O-[4]Al3, [3]O-[4]Al2-[5]Al, [3]O-[4]Al-[5]Al2 and [3]O-[5]Al3 over a negligible percentage of O-[6]Al. Hence, the bond length values for the Al-O pair in the two ultra thin oxide films investigated herein, suggest that only few distinct configurations of [2]O-[3]Al [4]Al, [4]Al- [2]O-[3]Al 2

[2,3]O

around

[3,4,5]Al

can exist. These are mostly

and [3]O-[4]Al2[5]Al, which dominate over the scant [6]Al-O.

The CN values for Al-Al and O-O correlations for the Al(111) oxide film (open triangles) are lower than the CN values for the Al(100)-grown oxide. When compared to the data registered for liquid alumina,24 one can observe the significantly lower average CN values for the two self-atom pairs in the present ReaxFF-based MD simuation results. While, experimentally deduced coordination numbers in the PAA film for Al-Al, Al-O ad O-O pairs are 8.5, 6 and 4.1, and their respective interatomic distances are 3.2 Å, 1.8 Å and 2.8 Å.48 For the present ReaxFF-MD simulation results, the average CNs accounting for the Al-O correlations are around 4, while those determined experimentally in liquid, amorphous and porous anodic aluminum oxide (PAA) film are higher than 4.18,63,64 Importantly, AlO5 and AlO4 are the predominant structural units in both liquid

24

and amorphous alumina,17,18,20 while the present ReaxFF-

MD simulations manifest a majority of AlO3 and AlO4 over a lesser amount of AlO5 configurations. The decrease in the average Al-O CN observed in this work is due to the three-coordinated Al atoms which are absent in each of the aforementioned bulk alumina structures. The average distributions of AlOx and OAlx species (x = 3,4,5 or 6), contributing to gij(r) function, using a cutoff distance rcut= 2.5 Å, are drawn in Figures 6b and 6c. They show the dominance of three- and four-coordinated Al species with low content of

[5]Al,

and

[2,3]O

coordinated atoms in ultra thin oxide films grown on Al(100) and Al(111) surfaces.

Figures 6b and 6c also show the distribution of AlOx and OAlx units in liquid alumina24 and, in the 1 Å thick surface layer and bulk am-Al2O3 26 Both latest cases show the majority of AlO4, AlO5 and OAl3 structural motifs over AlO3 and AlO6 but a considerable number of AlO3 and OAl2 species was found in the surface layer. Indeed, surface effects become visible and hence, are the influential ones in an ultra thin oxide film of thickness smaller than 5 Å, where two-coordinated O atom is likely to form bridging configurations or an [3]O atom is shared among three AlOx units.26



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Figure 6. a) Average coordination number (CN) as a function of oxygen gas density for oxide films grown on Al(100) and Al(111). Distribution of b) AlOx (x = 3, 4, 5 or 6) and c) OAlx (x=2, 3, 4) units for a cutoff distance rcut = 2.5 Å. Al(100) –gray bars and Al(111) – blue bars, liquid alumina24 – asterisks, surface layer of 1Å thickness and bulk am-Al2O326 – open triangles and squares, respectively.


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3.3 Tolopogical aspects of oxide film structure Structural units connectivity In connection with the aforementioned discussion, an analysis of elementary unit network is presented in Figures 7a and 7b exhibiting the inter-tetrahedral (Al-O-Al) and intra-tetrahedral (O-Al-O) bond angle distribution in the two ultra thin oxide films, respectively. A similar analysis of Al-Al-Al, Al-Al-O, AlO-O and O-O-O bond angle distributions for two such oxide films is graphically presented in Figure S7 of the Supporting Information. We found that AlO4 are the predominant structural motifs building up the structure of oxide films developed on Al(100) and Al(111) substrates, while importantly AlO3 and AlO5 structural units are also present. Some distortion from the ideal tetrahedron (TO4, drawn in Figure 7c) can be noticed since the OAl-O angle distribution (Figure 7a) is peaked between 100° and 110° while the main peak should appear at 109.5° as for the ideal AlO4 tetrahedron. In addition, various ways of adjoining AlO4 units into a network exist. Two of them are the most common for amorphous oxide systems, namely the edge-sharing and corner-sharing of AlO4, AlO5 and AlO6 polyhedra,63,65 wich are illustrated in Figures 7d and 7e, respectively. Importantly, the splitting of the main peak for the Al-O-Al angle into two peaks and the absence of a shoulder above 150° are suggestive of these two ways of AlO4 network construction. In the case of corner-sharing polyhedra, the Al-O-Al angle distribution (Figure 7b) is peaked around 120° while the peak at 90° corresponds to edge-sharing polyhedra. On this basis, the oxide film structure of Al(111) substrate is seen as a mixed network of edge-sharing and corner-sharing AlOx structural motifs while amorphous alumina is composed mainly of corner-sharing oxygen confiurations with the highly-intense peak of Al-O-Al angle at 120°corresponding to them.20 In am-Al2O3, it correlates to the distinct pair network constituting of four- and six Al-O-containing rings. Interestingly, edge-sharing structural motifs occur in predominance over corner-sharing in the surface layer of amorphous alumina.26 While in γ-Al2O3 formed upon crystallization of am-Al2O3, the edge-sharing structural motifs, composed of

[3,4]O

atoms

linked to [6]Al, dominate over [3]O coordinated with [5]Al species.18 With regard to the observation made for amorphous bulk alumina by Gutierrez et al., 20 the occurrence of a small peak at ~ 90° for the Al-O-Al angle distribution in the oxide films grown on Al(111) and Al(100) surfaces can be attributed to the presence of AlO3 (more abundant) or AlO5 (less abundant) polyhedra. We found that the AlO3 units are mostly linked by a corner to AlO4 units (with ~1/3 abundance) or AlO3 units (~1/6), or by an edge to AlO5 units (~1/12).

[3]O

is privileged over

[2]O

in edge-sharing

configurations even if O-O pairs are present at the oxide/gas interface, and more importantly, the

[2]O-

type configuration does not take part in the formation of edge- and corner-sharing structures for the AlO-Al sequence as it has primarily been shown in the am-Al2O3 surface layer.26 Importantly, two types of edge-sharing configuration linkage for AlO5 and AlO4 units have also been identified in liquid alumina, and thus,

[5]Al-type

units are likely to form configurations between two other AlO5 units with ~1/3



Page 20 of 43

frequency as well as among AlO4 and AlO5 motifs with ~1/6 abundance. In addition, [3]O-[5]Al3 linkage type occurs in minority since [3]O species are likely to be with mixed [4]Al and [5]Al species.24

Figure 7. Distribution of a) intra-tetrahedral (O-Al-O) and b) inter-tetrahedral (Al-O-Al) angles in films grown on Al(100) and Al(111) surfaces at 300K and for 0.24 g/cm3 oxygen gas density. See more details in the text.

The present ReaxFF-MD simulation results for two oxide film structures indicate the development of a high structural disorder, stimulating defective amorphous structure formation with loose atom packing. This finding is supported by the aforementioned diversity in the local atom surroundings having an origin in the dense atom packing of both Al(100) and Al (111) planes. For the oxide film grown on Al(111) surface, the less-coordinated polyhedral units are dominant in a low-density system compared to the overcoordinated (AlO5, AlO6) ones being the most frequent elementary units in high-density crystalline structures. Difference in abundance of [3,4,5]Al and [2,3]O species between the two investigated aluminum substrates could be an indicative of differences in emerging structural disorder, and hence might have an origin in developing surface stress accompanying oxide growth and in the reduced density of the ultra thin oxide film. One could mention that the [5]Al site fraction can be used as a general factor to measure the degree of structural disorder in amorphous oxides, which generally increases with increasing

[5]Al

content.17 Numerous configurational states in the energy landscape was found contributing to amorphous oxides as compared to a narrow range found in the case of crystalline materials.17 Another quite reasonable scenario of preferentially disordered structure formation can be deduced from decreasing mobility of oxygen atoms as they are adsorbed on Al metal substrate, which prevents any significant structural reconstruction during oxide growth, or from the low adsorption energy of molecular O2 on


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Al2O3/Al(111).66 As a result, reconstruction of the chemisorbed oxygen to oxide species on the parent Al metal surface is hindered by the surface-induced stress.61

Ring structure distribution Figure 8a shows a graphical representation of four different ring structure types found in the two investigated ultra thin oxide films developed after 1 ns of oxidation at 0.24 g/cm3 O2. A topological analysis of the network unit connectivity is presented in Figure 8b as an abundance diagram for n-fold rings. Each ring is defined as the shortest path of 2n alternating Al-O bonds. 3-fold rings are dominant in both ultra thin oxide films (Al(100) and Al(111)) for the three different Al-O bond length values chosen for the present analysis. A different ring distribution is seen amongst 2-fold, 3-fold, 4-fold and 5-fold rings as the Al-O distance increases from 1.74 Å (gray bars) to 1.83 Å (blue bars). Then, 4-fold and 5fold rings prevail over 2-fold rings in the Al(100) oxide film as opposed to 2-fold ring structures predominant over the high-n rings only for Al-O distances shorter than 1.80 Å in the oxide film grown on Al(111) substrate. Importantly, only 2-fold and 3-fold rings occur in the corundum structure, while a more diverse variety of high-n rings is found in γ-Al2O3, with 2-fold and 3-fold rings prevailing over 4-fold and 5-fold rings.20 Based on that, we can conclude that oxide film grown on the Al(100) surface is likely to be highly liquidlike and it will be discussed in more detail below. An analogous statement could be made for the closely packed oxide film grown on Al(111) surface, as the statistical distribution of

[4,5]Al

and

[2,3]O

species

within it might bear some resemblance to that found in amorphous alumina. Particularly, it can be attributed to the presence of the [3]O-[4]Al2[5]Al connection.18 The qualitative structural difference between the two surface oxides reflects the changes in local chemical environment of Al cations and O anions that can result from low thickness of the oxide film developed, consequently hindering the elemental unit linkage expansion during oxidation.



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Figure 8. Calculated a) n-fold ring structures and b) n-fold ring statistics for Al(100) and Al(111) oxide films analyzed assuming three different Al-O bond lengths. Ring structures evolution In Table 1 we compare the ring statistics for several bulk phases of alumina, including crystalline, amorphous and liquid, with the ring statistics computed for the ultra thin oxide films of the present ReaxFF-MD simulations. The comparison is made with the purpose to quantitatively estimate the similarity of the two investigated oxide films to any of the bulk alumina compounds whose structural data are taken from the paper by Gutierrez et al.20 As the mass density of crystalline alumina compounds decreases from 3.98 g/cm3 of α-Al2O3 to 3.65 g/cm3 of θ-Al2O3, the degree of diversity in the ring statistics increases. Basically, crystalline α-Al2O3 consists mainly of 2- and 3- fold rings29 as compared to high-n rings, such as 4- and 5-fold contributing to θ-Al2O3 and γ-Al2O3 structures.20 In fact, the diversity of nfold ring structure statistics in θ-Al2O3 and γ-Al2O3 alumina is noticeably lower than the one found for am-Al2O3 or liquid alumina (Table 1). One can also notice that the diversity amongst five most abundant ring structures becomes substantial with decreasing the Al-O bond length to values typical of disordered alumina structures. Hence, we can conclude that high frequency of 5- and 6-fold rings contributes to a structural disorder being characteristic of loosely packed systems such as liquid and low-dense amorphous materials with no long-range order. Comparison of 4-fold and 5-fold rings frequencies between two ultra thin oxide films, obtained in this work, and the bulk alumina structures is gathered in Table 1. It shows some structural similarities of the 22 ACS Paragon Plus Environment

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oxide structures fomed during thermal oxidation - present ReaxFF-MD simulations with am-Al2O3 or liquid Al2O3 rather than with the crystalline alumina compounds. Prevalence of [3,4]Al sites over the [5]Al coordinated atom’s fraction is in disagreement with recent experimental discoveries revealing the importance of [5]Al species in the formation of the am-Al2O3 phase, where the latter content is resulting in ~4.5 value of average Al-O coordination number. Importantly, dominance of triply coordinated O with Al was found in the surface layer of 1 Å thickness in am-Al2O3, studied by Adiga et al.,26 and hence, this could be discerned as a new configurational state existing in ultra thin oxide films. One can see the two lowest n-fold rings, i.e. 2- and 3-fold, are more abundant in the ultra thin oxide film grown on Al(111) substrate than in the second analyzed oxide film – (Al(100) surface). In fact, the dominance of low-n rings is less pronounced for Al(111)-grown oxide film than for γ-Al2O3 (Table 1), while the comparison of 5-fold rings contribution to the ring statistics in both analyzed structures may indicate structural similarities between the Al(111) oxide film and γ-Al2O3, but negligible [6]Al site content is found in this ultra thin oxide film. Based on that, the structure of oxide film grown on Al(111) substrate is more likely to be similar to am-Al2O3. While for oxide growth on the Al(100) substrate, relatively high content of highly coordinated polyhydera (i.e AlO5), compared to oxide growth on Al(111), may explain the structural disorder developed during oxide growth. The present analysis of n-ring distributions reveals that the difference between two oxide films is lying in the type of structural units and their connectivity to form the structural network. This conclusion is consistent with the analysis of chemical local atom environment in the films. Importantly we demonstrate that the discussed structural differences cannot be deduced from an analysis of the total structure factor or total pair distribution function.



Page 24 of 43

Table 1. Calculated ring distributions in ultra thin oxide films grown on Al(100) and Al(111) surfaces in comparison with for the distributions in α, γ, θ, amorphous and liquid phases of alumina.20 phase

density

bond length

(g/cm3)

(Å)

n-fold ring fraction (%) 2

3

4

5

6

α-Al2O3

3.98

1.97, 1.85

40

60

-

-

-

γ-Al2O3

3.66

1.941

40

40

18.5

1.5

-

Θ-Al2O3

3.6-3.65

1.904

23

62

15

-

-

am-Al2O3

3.3

1.76

11

37

41

10

0.3

Al(100)ReaxFF

3.45

1.83

6.5

39

29.5

25

-

Al(111)ReaxFF

3.48

1.83

16

35

27

20

-

Liquid

3.175

1.75

13

25

32

23

7

For a better visualization of the discussed differences, in Figures 9a and 9b we present a side view of two oxide films grown at 1 ns with the rings marked by lines. Yellow dashed lines represent the rings containing an O-O atomic pair, while the blue solid lines indicate 2- and 3-fold rings. The time evolution of ring structure distributions in two ultra thin oxide films is presented in Figures S8a and S8b of the Supporting Information. The found ring structures reconstruct with time, in the course of the ongoing oxidation reaction, from high membered rings, containing Al-O alternating bonds and O-O bridges, to low-membered rings. The latter ones are 2- and 3-fold structures for which no disruption in the sequence of 2n alternating Al-O bonds is present for both oxide films formed at 300 K. Two regions can be distinguished in the oxide scale on Al(111) surface. The first region is composed of randomly linked 3fold rings, each of which contains at least one oxygen bridge as seen on the left-hand side of Figure 9b. In the second region, shown on the right-hand side of Figure 9b, the edge-linked 3-fold rings are built in two parallel rows of oxide film grown on Al(111). A totally different ring structure connectivity is observed for the Al(100) ultra thin oxide film, Figure 9a. It gives an excellent picture of randomly oriented ring structures that also proves that the structure is loosely packed, which is compatible with the chemical aspects of the oxide film discussed above. On the other hand, the occurrence of high-n rings can indicate the presence of ‘free volume’ or voids in the oxide film formed upon the oxidation of Al(100) substrate. These voids are built into the oxide film structure and make it liquid-like. Examples of ring structure connectivity samples taken from oxide film interior and from the oxide/gas interface at 1 ns are graphically presented in Figure 9c. 2-fold rings are linked to 3-fold rings to form a chain of 2-fold-3-fold alternating ring structures, excluding regions which contain high-n rings such as 4fold. Another possible type of ring structure linkage found in the oxide interior is 2-fold-2-fold type. A similar linkage type of ring structures is found in the oxide exterior (oxide/gas interface) which also contains O-O pairs breaking the sequence of alternating 2-fold and 3-fold rings and hence violating the 24 ACS Paragon Plus Environment

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ring structures network as it is seen in Figure 9c. Importantly, 2-fold-3-fold ring linkage type is found to be the most frequent kind of connectivity type whose abundance reaches 30% and 40% in the studied oxide films on Al(100) and Al(111).

Figure 9 Side view of 3D ring arrangements in the oxide films grown on: a) Al(100) and b) Al(111) at 1 ns, c) 3D representation of ring structures connectivity after 1 ns. Red balls – oxygen and gray balls – aluminum.



Page 26 of 43

4. Summary In summary, the structural and topological aspects of oxide growth on Al(100) and Al(111) substrates at 300 K have been studied for three initial oxygen gas densities by means of atomistic simulations based on the Reactive Force Field method. We also report on the oxidation mechanism and kinetics of oxide growth as a function of oxygen gas density (appended to Supporting Information). The structural differences between two ultra thin oxide films are discussed in the light of chemical and topological atom environment, which might originate from the ‘free volume’ development during the oxide growth. The ReaxFF-MD results give a new atomic-level insight into the structure of the oxide film grown on two crystallographic surfaces at room temperature. [3,4]Al

coordinations are dominant over

oxygen atoms are shared between

[5,6]Al

[4]Al,

species in the two ultra thin oxide films studied, while

and among

[3]Al

and

[5]Al

structural motifs mostly through a

corner. This would indicate some kind of structural disorder which is likely to be a surface effect due to very small oxide thickness and development of a new configurational state in ultra thin noncrystalline solids. In addition,

[2,3]O

and

[3,4]Al

coordinated species are prevailing in the two ultra thin oxide films,

where Al atoms are forming edge-sharing configurations. These findings are similar to those found in the surface layer of amorphous alumina, below the outermost surface, when [3,4]Al and [2]O are most abundant in am-Al2O3.26 The observed oxygen density profile through the oxide film shows the development of oxygen-deficient oxide resulting from the presence of excess oxygen atoms at the oxide/gas interface, which do not take part in any kind of configurations. Four different ring structure types are found to be most abundant structures building-up the investigated oxide films as it has also been discovered in both liquid and amorphous alumina compounds. In addition, 3-fold ring structures of those ultra thin oxide films prevail over 2-, 4- and 5- fold ones. Importantly, the presence of high-n ring structures (4- and 5-fold) in the formed oxide film can initiate the formation of nanostructures such as pores or voids that are built into the structure at an early stage and develop with time. A 2-fold-3-fold ring connectivity is found to be the most frequent type of ring connectivity in both oxide films studied.

5. Associated Content The Supporting Information is available free of charge on the ACS Publications website. It includes computational details, snapshots depicting oxide growth kinetics, and additional illustrations concerning ring structure evolution.

6. Acknowledgment Authors gratefully acknowledge financial support from the Carl Tryggers Stiftelse for Vetenskaplig Forskning (grant CTS 2016:253) and the Swedish Foundation for Strategic Research (SSF, project ALUX,


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RMA11-0090). P.K. acknowledges support from the Russian Science Foundation (grant 18-12-00366). Computer resources have been provided by the Academic Computer Centre CYFRONET AGH (Poland). 7. Author information and contributions *Corresponding Author E-mail: [email protected], [email protected] M.E.T. performed ReaxFF-MD simulations and their data analysis, made discussion and wrote the manuscript. P.A.K. discussed the results and corrected the manuscript. Notes The authors declare no competing financial interest. 8. References

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TOC Graphic



Graphical Abstarct 84x32mm (300 x 300 DPI)

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Figure 1 289x541mm (300 x 300 DPI)



Figure 2 264x354mm (300 x 300 DPI)


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Figure 3 264x304mm (300 x 300 DPI)



Figure 4 1134x932mm (72 x 72 DPI)


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Figure 5 188x227mm (300 x 300 DPI)



Figure 7 256x128mm (300 x 300 DPI)


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Figure 8 1257x1160mm (72 x 72 DPI)



Figure 9 359x627mm (72 x 72 DPI)


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Fig.6 213x431mm (300 x 300 DPI)


Atomistic Simulations of Al(100) and Al(111) Surface Oxidation

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