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First-principles study of the initial stage of aluminum oxidation
Journal Pre-proofs Research paper First-principles study of the initial stage of aluminum oxidation Pingping Xu, Shouye Sun, Shiyang Sun, Xin Tan, Yuan Ren, Huiling Jia PII: DOI: Reference:
S0009-2614(20)30152-4 https://doi.org/10.1016/j.cplett.2020.137237 CPLETT 137237
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
4 November 2019 12 January 2020 15 February 2020
Please cite this article as: P. Xu, S. Sun, S. Sun, X. Tan, Y. Ren, H. Jia, First-principles study of the initial stage of aluminum oxidation, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/j.cplett.2020.137237
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© 2020 Published by Elsevier B.V.
First-principles study of the initial stage of aluminum oxidation Pingping Xu*, Shouye Sun, Shiyang Sun, Xin Tan, Yuan Ren, Huiling Jia. School of Mechanical Engineering, Inner Mongolia University of Science & Technology, Baotou, Inner Mongolia 014010, PR China *Corresponding Author: Pingping Xu Arding Street No.7, Inner Mongolia University of Science & Technology, Baotou 200240, PR China Tel. and Fax: +86 0472 5951574; E–mail: [email protected].
Abstract: The adsorption and migration of O atoms on Al (111) surface, and the infiltration of O atom to Al crystal was calculated by the first-principles method, to analyze the initial stage of aluminum oxidation. The results show that: the stable adsorption of O atom on the Al (111) surface is the Fcc position, and O atoms migrate on surface to form atomic islands, but the migration needs to pass through the Hcp position, reducing effectively the activation energy; until reaching some coverage, the adsorbed O atoms enter into Al crystal by the “abstract effect”. Keywords: Aluminum oxidation; First-principles; Adsorption; Migration barrier.
1. Introduction Because of its practical importance and apparent simplicity, the oxidation of Al (111) has been considered to represent a model system for metal oxidation in general [1,2]. Kiejna et. al. [3] had divided the oxidation of Al (111) into three stages: adsorption and dissociation of oxygen molecule on the surface, incorporation of oxygen into below-surface sites and the onset of oxide nucleation, and finally the oxide formation. For the chemisorption of oxygen molecule, the translational-energydependent initial sticking probability results determined by the molecular beam experiment depict a sigmoidal curve, which suggests the existence of an energy barrier preventing low-energy sticking [4-7]. Although the initial theoretical research did not support the conclusion, the more accurate exchange correlation function had recently been used to determine that important relationship between the activation energy and the initial position of oxygen molecule [8,9]. And our previous studies [10] also found that oxygen molecule would tend to rotate during approaching perpendicularly Al surface, and the rotational activation energy was consistent with the experimental values. This process is too fast to study the process of O2 molecular dissociation and oxide island formation by experimental methods.
After the dissociation of oxygen molecule, oxygen atoms formed (1×1) structure at the threefold fcc hollow position ( Fcc ) on Al surface, which had been confirmed by many theoretical [11,12] and experimental studies [13]. However, when the O atom penetrates into Al crystal, that is an important part of Al oxidation, it occupies the tetrahedral interstice [14]. The transition between these two structures involves not only the thermodynamic problems associated with system energy, but also the dynamics associated with the migration path. Jacobsen et al. [15] pointed out that the subsurface position was 1.86 eV higher in energy compared to the most preferred on-surface Fcc position. Kiejna et al. [16] believed that O atoms preferred to enter the Al subsurface once the oxygen monolayer coverage complied, but Colombi et al. [17] suggested that the process of penetration was beginning with the oxygen monolayer coverage of 20%. Ross et al. [18] found that oxygen atom occupied the tetrahedral interstitial position of Al crystal, but migrated between them through the octahedral interstitial position. Lanthony et al. [1] propose a migration mechanism in which oxygen atoms located on the outer surface extract aluminum atoms of the surface layers through local cooperation of other pre-adsorbed oxygen atoms. In summary, the research on the initial oxidation of Al surface is not enough, especially on the process of O atom penetrating into Al crystal, which is very important for the formation of various allotropic Al2O3 at the initial oxidation stage. In this paper, the first-principle calculation method was used to analyze the process and conditions of oxygen atoms penetrating into Al crystals, through studying the adsorption and migration behavior of oxygen atoms on Al(111) surface, and the comparison with entering Al crystal.
2. Calculation method and details The Vienna ab-initio simulation package (VASP) [19] based on the pseudopotential plane wave approach and density functional theory was used. The electron projector-augmented wave method [20] was used for calculating the interaction between ions and valence electrons. The Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation approach [21] was used to determine the exchange-correlation function. The HSE06 [22] hybrid function was used when calculating single oxygen atom adsorption. The conjugate gradient method [23] was used to optimize the geometric structure of the model. The electronic occupancy was determined using the Methfessel–Paxton method [24], in which the smearing width was 0.05 eV. The constituent atoms were fully relaxed until the maximum Hellmann–Feynman forces were <0.02 eV/Å. The self-
consistent iterative matrix diagonalization uses the residual minimization scheme, direct inversion in the iterative subspace (RMM-DIIS) algorithm [25].All self-consistent loops were iterated until the total energy difference of the systems between the adjacent iterating steps was less than 1×10−5 eV. The cutoff energy for the plane waves was 400 eV. The integral of the Brillouin zone was divided using the Monkhorst–Pack method [26] of [5×5×1] k-points.
Figure 1. Top view of the adsorption and occupied position. There are three adsorption positions, namely the Fcc, Hcp and Top position. And three occupied sublayer positions, namely the Oct, Tet_up and Tet_dn position. The cyan ball represents an Al atom on the surface; the yellow ball represents an Al atom in the sublayer, the green ball represents an Al atom in the third layer, and the small red ball represents an O atom.
The Al surface model was shown in Figure 1. To avoid the influence of the periodic boundary, a vacuum layer was set at more than 18 Å. In the top view, the Al (111) slab was built as a rhombic cell of 4×4 primitive unit cells. The system contained 96 atoms distributed in 6 layers with 16 atoms corresponding to the slab dimensions of 8.58×8.58×35.03 Å3, shown in Figure 1. The adsorbed layer and the four uppermost surface layers were allowed to move freely, and the bottom three layers were fixed. The adsorption and occupied position were shown in Figure 1. There were three adsorption positions, namely the Fcc, Hcp and Top position. The Fcc was located in the three-fold hollow of surface and sublayer; the Hcp was located in the three-fold hollow of surface and on the top of sublayer atom; the Top was located on the top of surface atom. And there were three occupied sublayer positions, namely the Oct, Tet_up and Tet_dn position. The Oct was located in the octahedral interstice formed by the surface and sublayer atoms; the Tet_up was located in the tetrahedral interstice formed by one surface atom and two sublayer atoms; the Tet_up was located in the tetrahedral interstice formed by two surface atoms and one sublayer atom. The subscripts below indicated the positions of the atom between the layers when the O atom continued to diffuse into Al crystal.
Adsorption energy (Ead ) characterize the adsorption of O atom on Al surface, which is defined as equation (1) below:
Ead Esurf nEO Etot / n
（1）
where Esurf is the energy of the Al surface model; Etot is the total energy of the adsorption system; and EO is the O atom energies; and n is the numbers of O atoms. According to the transition-state theory, the migration activating energy (Esp) can be defined as the difference between and the saddle point energy, and the saddle point (SP) is the highest energy point in the migration path and the lowest energy point in other directions. To search the migration path, the Nudged elastic band method [27] was used to map the minimum energy path between the initial and final systems. The nine initial images were created by linear interpolation. To analyze atomic bonding and charge transfer, the charge density difference (CDD), density of states (DOS) and electronic population was studied. The bader method was used to calculate the electronic population, as detailed in reference [10,28].
3. Results and discussion 3.1 Single oxygen atom Table 1. Occupation and situation of oxygen atom on the surface or inside Al crystal. Fcc
Hcp
Top
Oct
Tet_u(or Tet)
Tet_d
Etot (eV)
-357.20
-356.94
-354.35
--
--
-356.94
Ead (eV)
8.31
8.05
5.46
--
--
8.05
dAl-O(Å)
1.86
1.86
1.68/3.32
2.04
1.77
1.91/1.93
Obtain electric (e)
1.77
1.74
1.23
2.13
1.91
1.89
charge density difference The adsorption of single O atom at different locations on the Al (111) surface was shown in Table 1. The adsorption energy (Ead) at the Fcc position was the highest, that was similar to those reported elsewhere [29,30], and the one at the Top position was the lowest. The adsorptions of O atom at the Oct position and the Tet_u position were unstable, and would be converted to at the Tet_d position by relaxation. The Top position was only temporary stability, when the O atom was slightly offset, it would slip to the Fcc position easily, as shown in the migration path in Figure 2. The O atoms were very similar at the Fcc position and the Hcp position, such as the bond of O-Al
atoms and charge distribution, and the difference was the amount of charge exchange, details in the literature [10]. Oxygen atoms could occupy the Tet_d position stably, and the system free energy was very close to that at the Hcp position. According to charge density difference, the bond between Al-O at both positions had a very distinct ionic. And the O atom at the Tet_d position obtained more electrons based on charge population analysis by bader method. However, the bond length between Al-O at the Hcp position was shorter. The radius of O ion is 1.21 Å and the radius of Al ion is 0.39 Å [31], so the Al-O distance is more favorable for stability at the Hcp position. The same explanation applies to O atom occupying the octahedral interstice and tetrahedral interstice inside Al crystal. Although occupying octahedral interstice, O atoms can exchange more electrons with more Al atoms, the bonding distance between Al-O is too large, which makes the system less stable than that of the tetrahedral interstice.
Figure 2. Migration path of single O on the Al (111) surface. Red represents oxygen atoms migrate from the Top position to the Fcc position; translucent yellow represents migration from the Fcc position to the nearest Fcc position; and blue represents that from the Hcp position to the Fcc position. The height dimension represents the energy relative to that of at the Fcc position during migration of the oxygen atom.
The Fcc position was the most stable for adsorption of O atom shown in table 1. The migration of single O atom from the metastable position to the Fcc position on the Al (111) surface was shown in Figure 2. Oxygen atom migration from the Fcc position to the nearest Fcc position with migration activation energy 0.79 eV, yet, if transitioned by the Hcp position, the migration activation could be reduced to 0.69 eV. It was difficult for O atoms to enter the Al subsurface (Tet_d) from the Fcc position, and its migration activation energy was 1.15 eV. Therefore, a single O atom is stably
adsorbed on Al (111) surface at the Fcc position, and the transition between Fcc positions through the Hcp position, but it is difficult to penetrate into Al crystal.
3.2 Dimer oxygen atoms After single O atom adsorbed at the Fcc position, the stable adsorption position and migration path of the surrounding O atom were shown in Figure 3. In the context, F represented the Fcc position, H represented the Hcp position, d represented the Tet_d position, and the subscript indicates the neighbor order of the dimer O atoms.
Figure 3. Migration path and activation energy of the dimer O on Al surface.
When the dimer O atoms were far away (F4, F3 and F2), the migration was similar as that of single O atom on Al surface: the migration path went through the Hcp position, and was nearly symmetric with the Hcp position, and the migration activation energy was about 0.7eV. While the distance between the dimer O atoms was close (F2, F1), the migration path was no longer symmetrical although it still passed through the Hcp position. When the distance between the two O atoms was relatively close (F2, F1), although the migration still passed through the Hcp position, the migration path was no longer symmetrical. The migration path from F2 to H2 became steep and the activation energy increased. The migration from H2 to F1 became easier, with activation energy of only 0.15 eV. It can be considered that when dimer O atoms are adsorbed at the Fcc position and the distance become closest, the system energy is the lowest; while the distance of the dimer O atoms is larger, their adsorption energy and migration are the similar as those of single O atom,
which the migration path is symmetric with the Hcp position. The system energy has the highest value on the migration path of two O atoms from far away to near, that is, the maximum energy needed to overcome in the migration process is the activation energy in the migration from F2 to H2. Thereafter, the system energy decreases rapidly, and the activation energy of the migration from H2 to F1 becomes smaller. It was still difficult for dimer atoms to penetrate into sublayer from Al surface, and the migration activation energy from F1 to Tet_d1 was as high as 1.78 eV. The migration distance from F1 to Tet_d was the same as the distance from H2 to Tet_d. If the migration path from surface to Tet_d1 passed through H2 position, the activation energy would be reduced to 1.38eV. It indicates that when the two distant O atoms are close to each other, an O atom penetrated into Al crystal through the Hcp position, and the activation energy is lower than that of the single O atom, although the activation energy is still much higher than that on the surface.
3.3 Oxygen atoms islands Similar to the dimer O atoms, the formation process of three O atoms was calculated, and the result was also similarly. The migration of O atom between the Fcc positions was through the Hcp position on Al surface, and the activation energy of O atom entering into Al crystal was much greater than its migration on the surface. More O atoms were adsorbed to form the atoms island, and the migration and infiltration of O atoms were calculated.
Figure 4. Migration of O atom from surface to sublayer of Al (111).
When the coverage of O atoms reached 0.56 molecular layers (ML), the adsorption of O atom had greatly changes. Although the adsorption energy at the Top position was still minimal, the
adsorption at the Fcc position and the Hcp position became unstable, but both of them would converted to the bridge (Br) position, that is, the middle position of the two surface Al atoms. And we found that when the O atom was adsorbed on the O atoms island, it would produce an “abstract effect”, so that the surface Al atom was close to the adsorption O atom and far away from the surface, shown as the insert of Figure 4. The “abstract effect” had been revealed in the research of Lanthony et.al. [1] by the molecular dynamics method. We had calculated the migration process of
O atom into Al surface through the “abstract effect”. The results showed that O atom was easier to enter Al surface, and the activation energy was only 0.72 eV, as shown in Figure 4. In summary, due to the more charge exchange with surface atoms, the stable adsorption position of O atom on Al (111) surface is the Fcc position, and the adsorption energy is 8.31 eV. Although the adsorption energy is higher than that in Reference 1, the difference between the highly symmetrical positions is approximately the same. When O atom migrates on the surface, it can be transitioned through the Hcp position, which can effectively reduce the migration activation energy, from 0.79eV to 0.69eV. But, it is difficult for single O atom to enter Al crystal, and its migration activation energy is as high as 1.98 eV. In the process of the two adsorbed O atoms approaching each other, the migration activation energy is same as the migration of single O atom as the distance is far; the activation energy is slightly increased while the distance is close. So adsorbed O atoms form atomic islands by migrating on Al surface, but the migration needs to pass through the Hcp position. Although the migration activation energy is less than that of single O atom, the activation energy of O atom entering into Al crystal is still much larger than the migration on the surface. So according to the migration energy, O atoms easily aggregate on the Al surface and cannot penetrate into the crystal. However, while adsorbed atoms reaching some coverage, the surface O atoms will produce the “abstract effect”, which reduces the migration activation energy of O atoms entering into the Al crystal, but is still not lower than the migration activation energy on the surface. This phenomenon has been observed and studied by other researchers [1,6,32] through molecular dynamics method. The energy barrier to be overcome in this process is only 0.72eV, which can be provided by the dissociation of O2 on Al surface. Therefore, O atom is easily adsorbed on Al surface; and easily aggregates to form atomic islands; but after the atomic island reaching a certain degree of coverage, O atoms can penetrate into Al crystal by kinetics.
4. Conclusion The adsorption and migration behavior of oxygen atoms on Al (111) surface was studied by first-principles calculation method. And the process and conditions of oxygen atom penetrating into Al crystal were analyzed. The conclusions can be drawn: Oxygen atom can be stably adsorbed on the Al (111) surface at the Fcc position, and the adsorption energy is 8.31 eV. The migration path of oxygen atom on Al surface through the Hcp position can effectively reduce the activation energy, from 0.79eV to 0.69eV. Until the adsorbed atoms reach some coverage, the oxygen atoms penetrating into Al crystals by the “abstract effect”.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51702170 and 61765012), and the National Natural Science Foundation of the Inner Mongolia Autonomous Region (Grant No. 2015MS0554 and 2015MS0550).
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Research Highlights > The migration of O atom needs to pass through the Hcp position. > O atoms migrate on surface to form atomic islands. > O atoms infiltrate into Al crystals by the “abstract effect”.
S0009-2614(20)30152-4 https://doi.org/10.1016/j.cplett.2020.137237 CPLETT 137237
To appear in:
Chemical Physics Letters
Received Date: Revised Date: Accepted Date:
4 November 2019 12 January 2020 15 February 2020
Please cite this article as: P. Xu, S. Sun, S. Sun, X. Tan, Y. Ren, H. Jia, First-principles study of the initial stage of aluminum oxidation, Chemical Physics Letters (2020), doi: https://doi.org/10.1016/j.cplett.2020.137237
This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
© 2020 Published by Elsevier B.V.
First-principles study of the initial stage of aluminum oxidation Pingping Xu*, Shouye Sun, Shiyang Sun, Xin Tan, Yuan Ren, Huiling Jia. School of Mechanical Engineering, Inner Mongolia University of Science & Technology, Baotou, Inner Mongolia 014010, PR China *Corresponding Author: Pingping Xu Arding Street No.7, Inner Mongolia University of Science & Technology, Baotou 200240, PR China Tel. and Fax: +86 0472 5951574; E–mail: [email protected].
Abstract: The adsorption and migration of O atoms on Al (111) surface, and the infiltration of O atom to Al crystal was calculated by the first-principles method, to analyze the initial stage of aluminum oxidation. The results show that: the stable adsorption of O atom on the Al (111) surface is the Fcc position, and O atoms migrate on surface to form atomic islands, but the migration needs to pass through the Hcp position, reducing effectively the activation energy; until reaching some coverage, the adsorbed O atoms enter into Al crystal by the “abstract effect”. Keywords: Aluminum oxidation; First-principles; Adsorption; Migration barrier.
1. Introduction Because of its practical importance and apparent simplicity, the oxidation of Al (111) has been considered to represent a model system for metal oxidation in general [1,2]. Kiejna et. al. [3] had divided the oxidation of Al (111) into three stages: adsorption and dissociation of oxygen molecule on the surface, incorporation of oxygen into below-surface sites and the onset of oxide nucleation, and finally the oxide formation. For the chemisorption of oxygen molecule, the translational-energydependent initial sticking probability results determined by the molecular beam experiment depict a sigmoidal curve, which suggests the existence of an energy barrier preventing low-energy sticking [4-7]. Although the initial theoretical research did not support the conclusion, the more accurate exchange correlation function had recently been used to determine that important relationship between the activation energy and the initial position of oxygen molecule [8,9]. And our previous studies [10] also found that oxygen molecule would tend to rotate during approaching perpendicularly Al surface, and the rotational activation energy was consistent with the experimental values. This process is too fast to study the process of O2 molecular dissociation and oxide island formation by experimental methods.
After the dissociation of oxygen molecule, oxygen atoms formed (1×1) structure at the threefold fcc hollow position ( Fcc ) on Al surface, which had been confirmed by many theoretical [11,12] and experimental studies [13]. However, when the O atom penetrates into Al crystal, that is an important part of Al oxidation, it occupies the tetrahedral interstice [14]. The transition between these two structures involves not only the thermodynamic problems associated with system energy, but also the dynamics associated with the migration path. Jacobsen et al. [15] pointed out that the subsurface position was 1.86 eV higher in energy compared to the most preferred on-surface Fcc position. Kiejna et al. [16] believed that O atoms preferred to enter the Al subsurface once the oxygen monolayer coverage complied, but Colombi et al. [17] suggested that the process of penetration was beginning with the oxygen monolayer coverage of 20%. Ross et al. [18] found that oxygen atom occupied the tetrahedral interstitial position of Al crystal, but migrated between them through the octahedral interstitial position. Lanthony et al. [1] propose a migration mechanism in which oxygen atoms located on the outer surface extract aluminum atoms of the surface layers through local cooperation of other pre-adsorbed oxygen atoms. In summary, the research on the initial oxidation of Al surface is not enough, especially on the process of O atom penetrating into Al crystal, which is very important for the formation of various allotropic Al2O3 at the initial oxidation stage. In this paper, the first-principle calculation method was used to analyze the process and conditions of oxygen atoms penetrating into Al crystals, through studying the adsorption and migration behavior of oxygen atoms on Al(111) surface, and the comparison with entering Al crystal.
2. Calculation method and details The Vienna ab-initio simulation package (VASP) [19] based on the pseudopotential plane wave approach and density functional theory was used. The electron projector-augmented wave method [20] was used for calculating the interaction between ions and valence electrons. The Perdew–Burke–Ernzerhof (PBE) generalized gradient approximation approach [21] was used to determine the exchange-correlation function. The HSE06 [22] hybrid function was used when calculating single oxygen atom adsorption. The conjugate gradient method [23] was used to optimize the geometric structure of the model. The electronic occupancy was determined using the Methfessel–Paxton method [24], in which the smearing width was 0.05 eV. The constituent atoms were fully relaxed until the maximum Hellmann–Feynman forces were <0.02 eV/Å. The self-
consistent iterative matrix diagonalization uses the residual minimization scheme, direct inversion in the iterative subspace (RMM-DIIS) algorithm [25].All self-consistent loops were iterated until the total energy difference of the systems between the adjacent iterating steps was less than 1×10−5 eV. The cutoff energy for the plane waves was 400 eV. The integral of the Brillouin zone was divided using the Monkhorst–Pack method [26] of [5×5×1] k-points.
Figure 1. Top view of the adsorption and occupied position. There are three adsorption positions, namely the Fcc, Hcp and Top position. And three occupied sublayer positions, namely the Oct, Tet_up and Tet_dn position. The cyan ball represents an Al atom on the surface; the yellow ball represents an Al atom in the sublayer, the green ball represents an Al atom in the third layer, and the small red ball represents an O atom.
The Al surface model was shown in Figure 1. To avoid the influence of the periodic boundary, a vacuum layer was set at more than 18 Å. In the top view, the Al (111) slab was built as a rhombic cell of 4×4 primitive unit cells. The system contained 96 atoms distributed in 6 layers with 16 atoms corresponding to the slab dimensions of 8.58×8.58×35.03 Å3, shown in Figure 1. The adsorbed layer and the four uppermost surface layers were allowed to move freely, and the bottom three layers were fixed. The adsorption and occupied position were shown in Figure 1. There were three adsorption positions, namely the Fcc, Hcp and Top position. The Fcc was located in the three-fold hollow of surface and sublayer; the Hcp was located in the three-fold hollow of surface and on the top of sublayer atom; the Top was located on the top of surface atom. And there were three occupied sublayer positions, namely the Oct, Tet_up and Tet_dn position. The Oct was located in the octahedral interstice formed by the surface and sublayer atoms; the Tet_up was located in the tetrahedral interstice formed by one surface atom and two sublayer atoms; the Tet_up was located in the tetrahedral interstice formed by two surface atoms and one sublayer atom. The subscripts below indicated the positions of the atom between the layers when the O atom continued to diffuse into Al crystal.
Adsorption energy (Ead ) characterize the adsorption of O atom on Al surface, which is defined as equation (1) below:
Ead Esurf nEO Etot / n
（1）
where Esurf is the energy of the Al surface model; Etot is the total energy of the adsorption system; and EO is the O atom energies; and n is the numbers of O atoms. According to the transition-state theory, the migration activating energy (Esp) can be defined as the difference between and the saddle point energy, and the saddle point (SP) is the highest energy point in the migration path and the lowest energy point in other directions. To search the migration path, the Nudged elastic band method [27] was used to map the minimum energy path between the initial and final systems. The nine initial images were created by linear interpolation. To analyze atomic bonding and charge transfer, the charge density difference (CDD), density of states (DOS) and electronic population was studied. The bader method was used to calculate the electronic population, as detailed in reference [10,28].
3. Results and discussion 3.1 Single oxygen atom Table 1. Occupation and situation of oxygen atom on the surface or inside Al crystal. Fcc
Hcp
Top
Oct
Tet_u(or Tet)
Tet_d
Etot (eV)
-357.20
-356.94
-354.35
--
--
-356.94
Ead (eV)
8.31
8.05
5.46
--
--
8.05
dAl-O(Å)
1.86
1.86
1.68/3.32
2.04
1.77
1.91/1.93
Obtain electric (e)
1.77
1.74
1.23
2.13
1.91
1.89
charge density difference The adsorption of single O atom at different locations on the Al (111) surface was shown in Table 1. The adsorption energy (Ead) at the Fcc position was the highest, that was similar to those reported elsewhere [29,30], and the one at the Top position was the lowest. The adsorptions of O atom at the Oct position and the Tet_u position were unstable, and would be converted to at the Tet_d position by relaxation. The Top position was only temporary stability, when the O atom was slightly offset, it would slip to the Fcc position easily, as shown in the migration path in Figure 2. The O atoms were very similar at the Fcc position and the Hcp position, such as the bond of O-Al
atoms and charge distribution, and the difference was the amount of charge exchange, details in the literature [10]. Oxygen atoms could occupy the Tet_d position stably, and the system free energy was very close to that at the Hcp position. According to charge density difference, the bond between Al-O at both positions had a very distinct ionic. And the O atom at the Tet_d position obtained more electrons based on charge population analysis by bader method. However, the bond length between Al-O at the Hcp position was shorter. The radius of O ion is 1.21 Å and the radius of Al ion is 0.39 Å [31], so the Al-O distance is more favorable for stability at the Hcp position. The same explanation applies to O atom occupying the octahedral interstice and tetrahedral interstice inside Al crystal. Although occupying octahedral interstice, O atoms can exchange more electrons with more Al atoms, the bonding distance between Al-O is too large, which makes the system less stable than that of the tetrahedral interstice.
Figure 2. Migration path of single O on the Al (111) surface. Red represents oxygen atoms migrate from the Top position to the Fcc position; translucent yellow represents migration from the Fcc position to the nearest Fcc position; and blue represents that from the Hcp position to the Fcc position. The height dimension represents the energy relative to that of at the Fcc position during migration of the oxygen atom.
The Fcc position was the most stable for adsorption of O atom shown in table 1. The migration of single O atom from the metastable position to the Fcc position on the Al (111) surface was shown in Figure 2. Oxygen atom migration from the Fcc position to the nearest Fcc position with migration activation energy 0.79 eV, yet, if transitioned by the Hcp position, the migration activation could be reduced to 0.69 eV. It was difficult for O atoms to enter the Al subsurface (Tet_d) from the Fcc position, and its migration activation energy was 1.15 eV. Therefore, a single O atom is stably
adsorbed on Al (111) surface at the Fcc position, and the transition between Fcc positions through the Hcp position, but it is difficult to penetrate into Al crystal.
3.2 Dimer oxygen atoms After single O atom adsorbed at the Fcc position, the stable adsorption position and migration path of the surrounding O atom were shown in Figure 3. In the context, F represented the Fcc position, H represented the Hcp position, d represented the Tet_d position, and the subscript indicates the neighbor order of the dimer O atoms.
Figure 3. Migration path and activation energy of the dimer O on Al surface.
When the dimer O atoms were far away (F4, F3 and F2), the migration was similar as that of single O atom on Al surface: the migration path went through the Hcp position, and was nearly symmetric with the Hcp position, and the migration activation energy was about 0.7eV. While the distance between the dimer O atoms was close (F2, F1), the migration path was no longer symmetrical although it still passed through the Hcp position. When the distance between the two O atoms was relatively close (F2, F1), although the migration still passed through the Hcp position, the migration path was no longer symmetrical. The migration path from F2 to H2 became steep and the activation energy increased. The migration from H2 to F1 became easier, with activation energy of only 0.15 eV. It can be considered that when dimer O atoms are adsorbed at the Fcc position and the distance become closest, the system energy is the lowest; while the distance of the dimer O atoms is larger, their adsorption energy and migration are the similar as those of single O atom,
which the migration path is symmetric with the Hcp position. The system energy has the highest value on the migration path of two O atoms from far away to near, that is, the maximum energy needed to overcome in the migration process is the activation energy in the migration from F2 to H2. Thereafter, the system energy decreases rapidly, and the activation energy of the migration from H2 to F1 becomes smaller. It was still difficult for dimer atoms to penetrate into sublayer from Al surface, and the migration activation energy from F1 to Tet_d1 was as high as 1.78 eV. The migration distance from F1 to Tet_d was the same as the distance from H2 to Tet_d. If the migration path from surface to Tet_d1 passed through H2 position, the activation energy would be reduced to 1.38eV. It indicates that when the two distant O atoms are close to each other, an O atom penetrated into Al crystal through the Hcp position, and the activation energy is lower than that of the single O atom, although the activation energy is still much higher than that on the surface.
3.3 Oxygen atoms islands Similar to the dimer O atoms, the formation process of three O atoms was calculated, and the result was also similarly. The migration of O atom between the Fcc positions was through the Hcp position on Al surface, and the activation energy of O atom entering into Al crystal was much greater than its migration on the surface. More O atoms were adsorbed to form the atoms island, and the migration and infiltration of O atoms were calculated.
Figure 4. Migration of O atom from surface to sublayer of Al (111).
When the coverage of O atoms reached 0.56 molecular layers (ML), the adsorption of O atom had greatly changes. Although the adsorption energy at the Top position was still minimal, the
adsorption at the Fcc position and the Hcp position became unstable, but both of them would converted to the bridge (Br) position, that is, the middle position of the two surface Al atoms. And we found that when the O atom was adsorbed on the O atoms island, it would produce an “abstract effect”, so that the surface Al atom was close to the adsorption O atom and far away from the surface, shown as the insert of Figure 4. The “abstract effect” had been revealed in the research of Lanthony et.al. [1] by the molecular dynamics method. We had calculated the migration process of
O atom into Al surface through the “abstract effect”. The results showed that O atom was easier to enter Al surface, and the activation energy was only 0.72 eV, as shown in Figure 4. In summary, due to the more charge exchange with surface atoms, the stable adsorption position of O atom on Al (111) surface is the Fcc position, and the adsorption energy is 8.31 eV. Although the adsorption energy is higher than that in Reference 1, the difference between the highly symmetrical positions is approximately the same. When O atom migrates on the surface, it can be transitioned through the Hcp position, which can effectively reduce the migration activation energy, from 0.79eV to 0.69eV. But, it is difficult for single O atom to enter Al crystal, and its migration activation energy is as high as 1.98 eV. In the process of the two adsorbed O atoms approaching each other, the migration activation energy is same as the migration of single O atom as the distance is far; the activation energy is slightly increased while the distance is close. So adsorbed O atoms form atomic islands by migrating on Al surface, but the migration needs to pass through the Hcp position. Although the migration activation energy is less than that of single O atom, the activation energy of O atom entering into Al crystal is still much larger than the migration on the surface. So according to the migration energy, O atoms easily aggregate on the Al surface and cannot penetrate into the crystal. However, while adsorbed atoms reaching some coverage, the surface O atoms will produce the “abstract effect”, which reduces the migration activation energy of O atoms entering into the Al crystal, but is still not lower than the migration activation energy on the surface. This phenomenon has been observed and studied by other researchers [1,6,32] through molecular dynamics method. The energy barrier to be overcome in this process is only 0.72eV, which can be provided by the dissociation of O2 on Al surface. Therefore, O atom is easily adsorbed on Al surface; and easily aggregates to form atomic islands; but after the atomic island reaching a certain degree of coverage, O atoms can penetrate into Al crystal by kinetics.
4. Conclusion The adsorption and migration behavior of oxygen atoms on Al (111) surface was studied by first-principles calculation method. And the process and conditions of oxygen atom penetrating into Al crystal were analyzed. The conclusions can be drawn: Oxygen atom can be stably adsorbed on the Al (111) surface at the Fcc position, and the adsorption energy is 8.31 eV. The migration path of oxygen atom on Al surface through the Hcp position can effectively reduce the activation energy, from 0.79eV to 0.69eV. Until the adsorbed atoms reach some coverage, the oxygen atoms penetrating into Al crystals by the “abstract effect”.
Acknowledgments This work was supported by the National Natural Science Foundation of China (Grant No. 51702170 and 61765012), and the National Natural Science Foundation of the Inner Mongolia Autonomous Region (Grant No. 2015MS0554 and 2015MS0550).
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Research Highlights > The migration of O atom needs to pass through the Hcp position. > O atoms migrate on surface to form atomic islands. > O atoms infiltrate into Al crystals by the “abstract effect”.