Origin of Surprising Attractive Interactions between Electronegative

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On the Origin of Surprising Attractive Interactions Between Electronegative Oxygen Adatoms on Aluminum Surfaces Matic Poberznik, and Anton Kokalj J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.6b08894 • Publication Date (Web): 17 Oct 2016 Downloaded from http://pubs.acs.org on October 27, 2016

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The Journal of Physical Chemistry

On the Origin of Surprising Attractive Interactions between Electronegative Oxygen Adatoms on Aluminum Surfaces Matic Poberˇznika,b and Anton Kokalja,∗ a b

Department of Physical and Organic Chemistry, Joˇzef Stefan Institute, Jamova 39, SI-1000 Ljubljana, Slovenia

University of Ljubljana, Faculty of Chemistry and Chemical Technology, Veˇcna pot 113, SI-1000 Ljubljana, Slovenia October 7, 2016

Abstract When electronegative atoms adsorb on a more electropositive metal surface, charge transfer occurs and adatoms become negatively charged, typically resulting in repulsive lateral interactions. However, in the case of O on Al surfaces the opposite occurs and the lateral interactions between adatoms are attractive. We demonstrate that the surprising attractive interactions are a consequence of a simple electrostatic stabilization that stems from an interplay between Coulombic interactions and geometric effects, i.e., there exists a critical adatom height above the surface below which the lateral interactions are attractive. We argue that this picture is generally applicable for electronegative adatoms on metal surfaces provided that (i) the adsorption bonding is sufficiently ionic and (ii) the adatoms are sufficiently small to come close enough to the surface.

∗ Corresponding

Author: Anton Kokalj, Tel: +386-1-477-3523; Fax: +386 1 251 93 85, E-mail: [email protected], URL: http://www.ijs.si/ijsw/K3-en/Kokalj

Introduction Aluminum is a very useful and important material due to its low density and good corrosion resistance and plays a prominent role in many industries, such as aerospace, transportation, and construction. It is well known that upon exposure to the atmosphere aluminum is passivated by a thin oxide layer that protects it from corrosion in many environments, in particular in the pH range of about 4 to 8. The oxidation of a metal generally begins with the dissociation of a molecular oxidizing agent. For oxidizing agents, such as O2 , this results in chemisorbed electronegative adatoms on the electropositive metal surface. Repulsive interactions between negatively charged adatoms are therefore expected and confirmed for many metal surfaces. 1 These repulsive interactions can be approximately described as dipole–dipole interactions by the classic method of images, i.e., a negatively charged adatom induces a positive image charge in the substrate and the two together can be seen as a dipole perpendicular to the surface. According to this picture the interactions should scale as 3 µ2 /R3 ∝ µ2 Θ 2 , where µ is the induced dipole, R is the

inter-adatom distance, and Θ is the surface coverage. An example of such a dependence is shown for O on Cu(111) in Figure S1 in the Supporting Information. However, for oxygen on aluminum it was observed that O adatoms group and form ordered islands at low coverage. 2–4 Furthermore, several studies based on DFT calculations reported that the magnitude of binding energy increases with increasing coverage. 5–8 Jacobsen et al. 5 explained this anomaly by an increased weight of oxygen p states at higher coverage that increases the charge transfer and makes the bond more ionic. They further draw attention to the observation that on transition and noble metals, where the interactions between O adatoms are repulsive, the s states are active in the bond formation, whereas on Al the contribution of p states opens new possibilities for hybridization that leads to stronger bonding configurations. However, the role of metal p states is questioned by the fact that similar attractive interactions have also been observed for the O step-edge decoration of silver (n10) surfaces 9 and more recently for N, O, and F on Mg(0001) surface, 10,11 where attractive interactions were accompanied by an adsorption induced decrease of work function—which is another anomaly because an increase would be expected for electronegative adatoms. Cheng et al. 11 attributed both anomalies to the response of a quantum mechanical electron gas represented by a highly polarizable electron spill-out in front of Mg(0001). In contrast, in this paper we show that neither the increased charge transfer to O adatoms at high coverage, nor

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[29] Lauderback, L. L.; Lynn, A. J.; Waltman, C. J.; Larson, S. A. The Bonding Site Location of Chemisorbed Oxygen on Al(100) from Angle Resolved Secondary Ion Mass Spectrometry, Surf. Sci. 1991, 243, 323–333. [30] Non primitive overlayer structure are the following: √ (i) the 2/3 ML structure on Al(111), labeled as 2O ( fcc √ √3 × ◦ 3)R30 , which consists of 2 fcc O adatoms per ( 3× √ 3)R30◦ supercell; (ii) the 1 ML structure on Al(110), labeled as 2Ozigzag (2 × 1), which consists of 2 fcc O adatoms per (2 × 1) supercell adsorbed on the opposite side of the Al row thus forming a zigzag pattern along the [110] direction; and (iii) the 3/4 ML structure on Al(100), labeled as 3O(2 × 2), which consists of 3 hollow O adatoms per (2 × 2) supercell. [31] Michaelides, A.; Hu, P.; Lee, M.-H.; Alavi, A.; King, D. A. Resolution of an Ancient Surface Science Anomaly: Work Function Change Induced by N Adsorption on W 100, Phys. Rev. Lett. 2003, 90, 246103.

[32] Michel, R.; Gastaldi, J.; Allasia, C.; Jourdan, C.; Derrien, J. Initial Interaction of Oxygen with Aluminium Single Crystal Faces: A LEED, AES and Work Function Study, Surf. Sci. 1980, 95, 309–320. [33] Zhukov, V.; Popova, I.; Yates, J. T. Electron-Stimulated Oxidation of Al(111) by Oxygen at Low Temperatures: Mechanism of Enhanced Oxidation Kinetics, Phys. Rev. B 2002, 65, 195409. [34] Hofmann, P.; Wyrobisch, W.; Bradshaw, A. The Interac-


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tion of Oxygen with Aluminium Single Crystal Surfaces: Mainly ∆ϕ Aspects, Surf. Sci. 1979, 80, 344–351. [35] Kravchuk, T.; Akhvlediani, R.; Gridin, V. V.; Hoffman, A. Initial Stages of Surface and Subsurface Oxidation of Al(100) Studied by Photoelectron Spectroscopy and Low Energy Ion Scattering, Surf. Sci. 2004, 562, 83–91. [36] According to a mechanism proposed by Lanthony et al., 41 oxide nuclei can start to form even at islands as small as three O adatoms. Our calculations, that follow the mechanism of Lanthony show that some oxygen islands of moderate size decrease while others increase the work function. For example, a 15 atom O island (5 adatoms in the upper layer and 10 in the layer below) in a (6 × 4) supercell decreases the work function by about 0.3 eV at a net coverage of 0.6 ML, whereas a stable 2 ML thin oxide film on Al(111) decrease the work function by about 0.5 eV. [37] Gartland, P. O. Adsorption of Oxygen on Clean Single Crystal Faces of Aluminium, Surf. Sci. 1977, 62, 183–196. [38] Although oxygen charges beyond the −2 seem rather awkward from chemical point of view, such Bader charges have been reported in the literature for atomic adsorbates chemisorbed on electropositive base metals 10,42,43 . P [39] The N ∞ i6=j qi qj /|Ri −Rj | sum converges very slowly and care has been taken to obtain converged results. If the sum is truncated at a cutoff radius of Rcut from the reference −1 zero cell, the error is ∝ 2π(µ/A0 )2 Rcut , where µ is a dipole of a neutral O/Aln unit and A0 is the area of a supercell (in atomic units). [40] Kutzler, F. W.; Painter, G. S. Energies of Atoms with Nonspherical Charge Densities Calculated with Nonlocal Density-Functional Theory, Phys. Rev. Lett. 1987, 59, 1285–1288. [41] Lanthony, C.; Ducéré, J.; Rouhani, M. D.; Hemeryck, A.; Estève, A.; Rossi, C. On the Early Stage of Aluminum Oxidation: An Extraction Mechanism via Oxygen Cooperation, J. Chem. Phys. 2012, 137, 094707. [42] Kokalj, A. On the HSAB Based Estimate of Charge Transfer between Adsorbates and Metal Surfaces, Chem. Phys. 2012, 393, 1–12. [43] Zamora, R. J.; Nair, A. K.; Hennig, R. G.; Warner, D. H. Ab Initio Prediction of Environmental Embrittlement at a Crack Tip in Aluminum, Phys. Rev. B 2012, 86, 060101.




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Origin of Surprising Attractive Interactions between Electronegative

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