Origin of the Selective Cr Oxidation in CoCr Alloy Surfaces - The

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Origin of the Selective Cr Oxidation in CoCr Alloy Surfaces Janina Zimmermann*,†,‡ and Lucio Colombi Ciacchi†,§ †

Faculty of Production Engineering and Bremen Center for Computational Materials Science, University of Bremen, D-28359 Bremen, Germany, ‡Fraunhofer Institute for Mechanics of Materials IWM, D-79108 Freiburg, Germany, and § Fraunhofer Institute for Manufacturing Technology and Applied Materials Research IFAM, D-28359 Bremen, Germany

ABSTRACT CoCr alloy surfaces are known to oxidize with the formation of stable Cr2O3 protecting films. However, the inherent mechanisms initiating the selective oxidation of one component, commonly observed in most metal alloys exposed to an oxidizing environment, are still largely unknown. Here we present a study of the early oxidation stages of a Co0.67Cr0.33(0001) alloy surface by means of firstprinciples molecular dynamics based on density-functional theory. We observe the initial formation of a cobalt-rich amorphous oxide matrix, which crucially acts as the promoter of a selective chromium oxidation already at coverages below 2 ML. In this process, the outward diffusion of Cr atoms leads to vacancy formation beneath the surface and enables the diffusion of oxygen atoms into inner atomic layers. This mechanism differs from the oxidation of either pure Co or Cr surfaces, revealing that selective oxidation within CoCr is triggered by a cooperative interaction between the components. SECTION Surfaces, Interfaces, Catalysis

independently and each similarly to the pure metal surfaces.22 The growing oxide reveals features of CoO, Cr2O3, and CoCr spinels, and to a minor extent Co3O4.3,21,23 However, the physical driving forces leading to selective oxidation, and the chemical mechanisms governing the initial oxide nucleation on a bare CoCr surface remain open questions. In particular, information about the atomistic structure and the composition of ultrathin native oxide films formed on CoCr at low temperature is still lacking. The oxidation mechanisms of the separate elements, necessary to understand their individual influence on the oxide formation in the alloy, have been studied in ref 24. Oxide nucleation on Co(0001) has been found to occur via spontaneous place-exchange of metal and oxygen atoms and with the growth of an open, pseudoamorphous oxide structure with Co3O4-like features. On the contrary, Cr(110) oxidizes via formation of perfect oxygen adlayers, following a thermally activated layerby-layer growth mechanism. In the present work, we study the initial oxidation of a Co0.67Cr0.33(0001) alloy surface at low temperatures by means of first-principles molecular dynamics (FPMD) simulations within density functional theory (DFT).25-30 Previous theoretical calculations reveal enrichment of cobalt in the alloy surface layer and of Cr in the subsurface layer under vacuum conditions, whereas segregation of Cr to the surface layer and of Co in the subsurface layer occurs in the presence of oxygen.31 Experimental data for a Co0.86Cr0.14 alloy reveal

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xidation phenomena on the surfaces of metal alloys determine their lifetime and performance in several technological applications. Concerning cobalt/ chromium alloys, the properties of perpendicular magnetic recording media are influenced by the interaction of the thin CoCr film with impurity gases, in particular oxygen.1,2 Moreover, the inherent biocompatibility of CoCr-based orthopedic implants is determined by the interaction of the oxidized surface with the physiological environment.3 Thus, the prediction and optimization of the materials behavior under oxidizing operating conditions requires a precise knowledge of the mechanisms of superficial oxide formation. A key feature of the oxidation of alloys is the occurrence of surface segregation and selective oxidation processes.4-7 Indeed, the chemical composition of the alloy surface is not only different from the composition in the bulk, but depends strongly on the atmosphere with which the surface is set in contact.8-11 Element-specific reactivity, e.g., toward oxygen, leads to adsorbate-induced segregation of one component to the surface and of vacancies at the alloy/oxide interface.12-15 This feature can be exploited to tune the mechanical and electronic properties and improve the oxidation resistance of metallic materials. In Co, Fe, or Ni alloys with Cr contents of 20-30%, the formation of a thin, corrosion-resistant oxide film mainly composed of Cr2O3 takes place.7,16,17 While a number of studies exist about the high-temperature oxidation of CoCr alloys,18-20 only a few deal with the initial oxide formation at low temperatures, and theoretical works are absent. Experiments by X-ray photoemission spectroscopy at room temperature observe a preferential oxidation of Cr, with the first atomic layer of oxide being formed as a mixture of Co and Cr ions.21 Some authors suggest that Co and Cr oxidize

r 2010 American Chemical Society

Received Date: May 27, 2010 Accepted Date: July 7, 2010 Published on Web Date: July 13, 2010

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DOI: 10.1021/jz1007164 |J. Phys. Chem. Lett. 2010, 1, 2343–2348

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Figure 1. Final snapshots at different coverages of consecutive FPMD simulations of the initial oxide formation on a Co0.67Cr0.33(0001) surface: (a-d) Model system CoCr_1, containing 1 Cr atom and 11 Co atoms in the surface layer; (e-h) Model system CoCr_3, with 3 Cr atoms and 9 Co atoms in the surface layer. Co, Cr, and O atoms are shown in blue, green, and red, respectively. Periodic boundary conditions are applied. Isosurfaces illustrate regions of positive (gray) and negative (orange) spin density.

at a higher coverage (0.83 ML) with a Cr atom leaving the surface and arranging in a tetrahedron-like coordination shell (Figure 1f). This mechanism strongly recalls the one observed at 1.0 ML for a pure Cr(110) surface at low temperature:24 in both cases, a single Cr atom is extracted from the almost perfect surface, to which it remains bound via three oxygen atoms in a chromate-like (CrO42-) structure. With the extraction of additional Cr and Co atoms at slightly higher coverages, this structure evolves into a dichromate-like (Cr2O72-) structure, consisting of two tetrahedra sharing an oxygen atom (visible in Figure 1g and ref 24). A tetrahedral oxygen coordination shell of Cr also builds up in the CoCr_1 system once the Cr atom leaves the surface plane (Figure 1c,d), and can be regarded as a typical feature of the early oxidation stages of Cr-containing surfaces, preceding the expected formation of Cr2O3. After the adsorption of the first incoming O2 molecules, the d-band depletion of the metal atoms with charge transfer into the p-orbitals of the adsorbed oxygen gives rise to a strong electrostatic dipole at the interface. This behavior is directly reflected in the changes of the work function upon oxidation: ΔΦ increases steadily up a certain “critical” coverage, as shown in Figure 2a. The onset of oxide formation after placeexchange between metal and oxygen is characterized by a sudden drop of the work function. For CoCr_1, analogously to what is observed for the oxidation of pure cobalt, ΔΦ first drops at 0.67 ML when a Co atom leaves the surface and

that the near-surface Cr content increases to up to 25% after vacuum annealing of the clean alloy surface.21 While the resolution of these experiments is limited to an average composition over several atomic layers, so that Cr enrichment may in fact be limited to subsurface sites, the presence of a certain amount of surface Cr atoms cannot be excluded. We thus perform our study using two different CoCr(0001) surface model systems with differing Cr content in the surface layer. Considering a six-layer slab with a surface unit cell of 12 atoms, we place 1, 5, and 6 Cr atoms in the surface, the second and the third layer in our first model system (called CoCr_1 throughout the paper) and 3, 4, and 5 in the second system (called CoCr_3), respectively.30 For both models, O2 molecules are placed one at a time above the surface, and the system is let free to evolve according to microcanonical CarParrinello dynamics,25,28 as described extensively in previous works.15,24 A series of consecutive simulations, whose representative final snapshots at different coverages are shown in Figure 1, are performed until reaching a final coverage of 1.5 ML. Both on the CoCr_1 and the CoCr_3 surfaces, the first incoming O2 molecules adsorb and dissociate spontaneously through a “hot atom” dissociative mechanism,32 binding preferentially to the Cr surface atoms (Figure 1a,e). Oxide formation on CoCr_1 takes place at 0.67 ML (Figure 1b) in a manner that resembles the nucleation on pure Co(0001), with extraction of Co atoms and incorporation of oxygen beneath the surface layer.24 Instead, on CoCr_3 oxide nucleation occurs


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Figure 2. (a) Evolution of the work function with respect to the bare surface as a function of the oxygen coverage, for the CoCr_1 and CoCr_3 systems (see Figure 1). (b) Cumulative energy gain after the consecutive adsorption of n O2 molecules on the same two model systems as a function of the oxygen coverage. For each slab we plot ΔE=Eslab(NCo, NCr, NO =2n) - nEO2 versus the corresponding coverage for n adsorbed O2 molecules. The dashed lines are given by n 3 ΔH0f (eV/O2), where ΔH0f is the enthalpy of formation per O2 for the bulk oxides Cr2O3 and Co3O4. All values for zero coverage were arbitrarily set equal to zero. (c) Surface energy per surface atom as a function of the oxygen coverage for both alloy models compared to pure Co and pure Cr surfaces,24 at 300 K and 1 atm pressure. (d) Same as in panel c, at 600 K and 10-12 atm.

Figure 3. (a) Evolution of charge, spin, and number of nearestneighbor oxygen atoms for the Cr atom of the surface layer of system CoCr_1 (labeled Cr1 in Figure 1) as a function of the oxygen coverage of the system. (b) Same as in panel a, shown for the Co atom of the surface layer, which first escapes from the surface plane during oxidation (labeled Co1). (c,d) Charge on all the Co and Cr atoms of the supercell of both alloy model systems at 1.5 ML with respect to their number of nearest-neighbor oxygen atoms. For comparison, the charges of the Co atoms of bulk Co3O4 and CoO are shown in blue and green, and the charges of Cr in Cr2O3 and Cr2O72- are shown in red and orange, respectively.

oxygen atoms are allowed to diffuse underneath. However, it increases again at 0.83 ML as a result of the formation of the above-mentioned chromate-like structure. The same structure leads also to a maximum of ΔΦ for CoCr_3 at 0.83 ML. For both systems, the subsequent development of the oxide structure leads to a drop of ΔΦ and to oscillations around a saturation value of about 0.7-0.8 eV, which is comparable to pure cobalt. A comparison with the data for pure Co and Cr24 reveals that the higher the Cr content in the surface layer, the larger the maximum value of ΔΦ, and the later its stabilization occurs at a lower constant value. In line with the larger enthalpic gain associated with the oxidation of Cr, the energy gain of the oxidation reaction increases according to the Cr content in the surface layer, i.e., in the order: Co, CoCr_1, CoCr_3, Cr, as shown in Figure 2b and reported in ref 24. However, at the small O coverages considered here, the energy gains of both alloy systems approach the enthalpy of formation per O2 of bulk Co3O4 (Figure 2b), as a first sign of the amorphous oxide matrix exhibiting Co3O4-like features. Furthermore, we compute the variation of the surface energy γ for increasing oxygen coverage as a function of the chemical potential of the oxygen gas following a standard ab initio thermodynamics formalism.33 The results are summarized in Figure 2c,d for two particular conditions of temperature and pressure, while full details are provided in the Supporting Information. Consistently with the results of ref 31,


at zero coverage, pure Cr presents the highest surface energy (1.1 eV/surface atom), followed by CoCr_3, CoCr_1, and Co (0.78, 0.73, and 0.71 eV/surface atom, respectively). This ordering is inverted already at low O coverages, indicating preferential oxidation of Cr. At environmental conditions (300 K and 1 atm pressure), γ decreases steadily with increasing O coverage for all systems, indicating a persistent driving force for superficial oxidation. However, at conditions typical of experiments in ultrahigh vacuum (600 K and 10-12 atm pressure), this driving force is reduced, especially at intermediate coverages around 0.75 ML, corresponding to the transition between an adsorbed layer of O atoms and the formation of an oxide network. Interestingly, in the case of pure Co and, less evidently, for the CoCr_1 system, a small energy barrier is present for coverages between 0.5 and 1.0 ML, which is reminiscent of a classical nucleation and growth mechanism. However, the small height of the barrier can be easily overcome at 600 K, so that spontaneous surface oxidation may be expected even under these conditions. The behavior of single Cr and Co atoms during the oxidation can be monitored by studying the development of their atomic Bader charge,34 magnetic moment, and structural coordination. Considering the single surface Cr atom in the CoCr_1 system, as a consequence of the electron donation

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Figure 4. (a) Model system CoCr_3 at 1.5 ML oxygen coverage (enlarged view of Figure 1h). The arrows indicate the direction of the driving forces on the respective atoms during oxidation. The levels of the first, second, and third layer are indicated, and Cr atoms originally lying in the first and second layer are labeled with 1 and 2, respectively. (b) Displacements of the atoms above their original layer for the system in panel a, after MD and geometry relaxation. The average equilibrium interlayer distance is shown for comparison, and the maximum displacements of two third-layer Cr atoms observed during the MD run are indicated with empty circles (see text).

from Cr to O, the calculated positive charge on Cr increases considerably with increasing coverage, while the magnetization vanishes almost completely (Figure 3a). Both values reach a saturation at 0.83 ML, from where the coordination shell of Cr is stably composed by four nearest-neighbor oxygen atoms. This results in a high Bader charge (2.0 e), a high formal oxidation state (up to þ3.67), and an almost depleted d band with vanishing magnetic moment (0.1 μB). The surface Cr atoms in system CoCr_3 bear similar features, arranging preferentially in 4-fold coordination shells and presenting formal oxidation states higher than 3. Analogously, Figure 3b shows the evolution of the first Co atom that leaves the surface in the CoCr_1 system (labeled Co1 in Figure 1b-d). The onset of oxide formation leads not only to an increase of the spin polarization (from 1.1 μB at 0.5 ML to 1.74 μB at 0.67 ML), but gives rise to oscillations of the sign of μ. Such spin reversals are an indispensable and characteristic feature allowing for the growth of an antiferromagnetic oxide network on a ferromagnetic substrate, and have been observed also in the oxidation of Co(0001).24 At the largest coverages, the calculated charge of the Co atom (1.5 e) lies very close to the one of tetrahedrally coordinated Co atoms in Co3O4 (1.45 e), as found also for pure cobalt.24 Similar values are obtained for all 4-fold (4f)-coordinated atoms in both the CoCr_1 and CoCr_3 systems, while lowerlying Co atoms, still having bonds to the subsurface layer, present much lower Bader charges and bulk-like magnetic moments. These features are revealed in Figure 3c,d, where the atomic charges of all atoms of the simulation cell of system CoCr_1 and CoCr_3 at 1.5 ML of O coverage are displayed as a function of the number of oxygen neighbors. In the inner part of the slab (not yet involved in the oxidation process) the Co atoms carry a negative charge, and Cr atoms carry a positive charge, due to the electron transfer from Cr to the more electronegative Co.31 In the oxide films, the charges of both Co and Cr atoms vary almost linearly with the number of O neighbors, as found for the pure elements. The Cr atoms exhibit atomic charges larger than the Co atoms, reflecting


their larger oxygen affinity. In particular, the charges of 4f-Cr atoms are considerably higher than the one of Cr in Cr2O3, and approach the value obtained for the dichromate ion, consistently with the tethrahedral coordination and the almost depleted d-band. In summary, the oxidation of individual Co and Cr atoms of both CoCr alloy systems involves the same structural arrangements and electronic details as the oxidation of the separate element surfaces. Indeed, an independent oxidation behavior of Co and Cr at low O coverages has been suggested also by secondary-ion mass spectroscopy experiments on a Co86Cr14 alloy surface.22 However, the overall oxidation mechanism of the alloy system proceeds differently from what observed for the pure metals and bears remarkable features of a cooperative interaction between the components, as explained below. Despite the higher oxygen affinity of Cr, the initial oxidation of CoCr involves the formation of an amorphous oxide network rich in Co atoms of the first layer. This mixed oxide network facilitates the extraction of Cr atoms at an earlier stage than on the pure Cr(110) surface, where no place exchange between metal and oxygen atoms takes place up to coverages of about 1 ML.24 The Cr and Co vacancies left behind allow a number of O atoms to diffuse underneath the surface (Figure 1c,g). Here, they provide a driving force for the Cr atoms of the second (subsurface) layer to leave their lattice position, initiating a selective Cr oxidation process. Indeed, for system CoCr_3 we observe for the first time at a coverage of 1.5 ML the formation of vacancies in the second layer through extraction of only Cr atoms. This emerges clearly from Figure 4, where the final CoCr_3 oxide structure is presented along with the displacements of all atoms of the uppermost layers in the direction normal to the surface. Moreover, as a consequence of the Cr vacancies formation, an oxygen atom is able to diffuse underneath the subsurface layer and bind to the third layer. Interestingly, during the MD simulation reaching 1.5 ML we could observe evident upward displacements of Cr (and not Co) atoms of the third layer up to 1.0 Å (indicated with empty circles in Figure 4b). Similar displacements were observed, at lower coverages, prior to the selective extraction

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of Cr atoms from the second layer. Although the Cr atoms of the third layer relax back into their lattice position during geometry optimization at the end of the dynamics, this may be a hint toward a further selective Cr extraction during the next oxidation steps. In conclusion, the oxidation mechanism of the alloy is different from those observed in pure Co or pure Cr, where the oxide layer develops above the surface and exclusively involves atoms of the surface layer. At environmental temperature and pressure conditions we find a persistent driving force for the development of superficial oxides, which is more pronounced the higher the amount of Cr atoms in the surface layer. This tendency is enhanced in particular by a selective Cr segregation and further oxidation already at less than 2 ML. Namely, the larger energetic gain associated with the oxidation of Cr atoms leads to the formation of Cr vacancies, which enable O atoms to diffuse deeply into the metal bulk. The resulting gradient of the concentration of oxygen provides, in turn, a driving force for sustainable Cr segregation toward the outer part of the mixed CoCr-oxide network. Remarkably, this process is empowered by the initial formation of an amorphous Co-rich oxide matrix, which promotes the inward diffusion of vacancies and an easier oxidation than on a pure Cr surface.

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SUPPORTING INFORMATION AVAILABLE Technical details on the calculation of surface energies as a function of the oxygen chemical potential and Figure S1. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION Corresponding Author:

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*To whom correspondence should be addressed. E-mail: janina. [email protected].

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ACKNOWLEDGMENT We acknowledge useful discussions with A.

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Dianat and M. Bobeth. Computational time was allocated at the Zentrum f€ ur Informationstechnologie und Hochleistungsrechnung (ZIH) at the University of Dresden (Germany). This work was funded by the Deutschen Forschungsgemeinschaft under Grants CI-144/1-2 and CI-144/2-1.

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in the Brillouin zone, applying an energy smearing of kBT = 0.1 eV. The Car-Parrinello MD simulations28,29 were performed either microcanonically (at low O coverages) or by rescaling the velocity of the ions to impose simulation temperatures not exceeding, on average, 800 K (at the highest coverages). Oxygen molecules were adsorbed only on one surface of the slab models,30 and a dipole correction was included to prevent an electrostatic asymmetry in the supercell. Geometry optimizations were performed until the force components on the unconstrained atoms were less than 0.05 eV/Å. For further details, see also ref 24. Perdew, J. P.; Wang, Y. Accurate and Simple Analytic Representation of the Electron-Gas Correlation Energy. Phys. Rev. B 1992, 45, 13244–13249. Bl€ ochl, P. E. Projector Augmented-Wave Method. Phys. Rev. B 1994, 50, 17953–17979. Car, R.; Parrinello, M. Unified Approach for Molecular Dynamics and Density-Functional Theory. Phys. Rev. Lett. 1985, 55, 2471–2474. VandeVondele, J.; De Vita, A. First-Principles Molecular Dynamics of Metallic Systems. Phys. Rev. B 1999, 60, 13241–13244. The CoCr alloy was modeled by an hcp crystal lattice with the equilibrium√lattice constant of pure Co31 and by a 12-atom hcp-(3 2 3) surface unit cell. All (0001) surface supercells consisted of a slab of six stacked metal layers separated by a vacuum gap of the same thickness, in which the Cr and Co atoms were randomly arranged. The slab compositions were symmetric with respect to the slab center. Dianat, A.; Zimmermann, J.; Seriani, N.; Bobeth, M.; Pompe, W.; Colombi Ciacchi, L. Ab Initio Study of Element Segregation and Oxygen Adsorption on PtPd and CoCr Binary Alloy Surfaces. Surf. Sci. 2008, 602, 876–884. Colombi Ciacchi, L.; Payne, M. C. “Hot-Atom” O2 Dissociation and Oxide Nucleation on Al(111). Phys. Rev. Lett. 2004, 92, 176104. Reuter, K.; Scheffler, M. Composition, Structure, and Stability of RuO2(110) as a Function of Oxygen Pressure. Phys. Rev. B 2001, 65, 035406. Atomic charges and magnetic moments were computed according to a Bader decomposition of the total and spin charge density, respectively, using the grid-based algorithm of Henkelman et al.35 Henkelman, G.; Arnaldsoon, A.; Jønsoon, H. A Fast and Robust Algorithm for Bader Decomposition of Charge Density. Comput. Mater. Sci. 2006, 36, 354–360.


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Origin of the Selective Cr Oxidation in CoCr Alloy Surfaces - The

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