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Surface segregation of core atoms in core–shell structures
Chemical Physics Letters 456 (2008) 64–67
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Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Surface segregation of core atoms in core–shell structures Gustavo E. Ramirez-Caballero, Perla B. Balbuena * Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, United States
a r t i c l e
i n f o
Article history: Received 20 December 2007 In final form 4 March 2008 Available online 10 March 2008
a b s t r a c t Density functional theory is used to calculate surface segregation tendencies of iridium atoms in Ir (core)–Pt (shell) systems with various shell thicknesses, on clean surfaces and under 1/4 monolayer of atomic oxygen adsorbed on fcc and hcp sites of (1 1 1) surfaces. Iridium shows strong antisegregation behavior in clean surfaces, but the behavior reverts dramatically in the presence of adsorbed oxygen. On Pt-skin surfaces, where the oxygen adsorption is the weakest, monolayers have better protective effect against segregation of the less noble metal, and the effect decreases as the number of layers in the shell increases. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Platinum is so far the best material to catalyze the oxygen reduction reaction (ORR) in acid medium, one of the electrode reactions in low-temperature fuel cells [1]. Such electrodes consist of an assembly of nanoparticles deposited on an electrically conductive substrate and surrounded by a hydrated polymer, the proton carrier. The ORR has a very slow kinetics, which added to the price and scarcity of platinum, makes the development of new catalytic materials one of the biggest challenges in fuel cell technology. It has been reported that bimetallic nanoalloys are promising candidate materials for ORR electrocatalysts [2–4]. However, the acid environment in which these nanocatalysts work triggers metal corrosion, which limits the catalyst lifetime and produces other undesirable effects such as the degradation of the polymeric membrane [5]. Such dissolution reaction usually causes the less noble alloy components to dissolve first, leaving a monolayer of Pt atoms on the surface [3]. This structure, also called ‘Pt-skin’ has shown excellent activity and durability properties. For this reason, structures such as M (core)–Pt (shell) are being tested as alternative ORR electrocatalysts. However, the stability of such Pt shell is crucial, and therefore the core elements should be such that remain in the core instead of segregating to the surface [6]. It has been shown that certain alloy elements may confer enhanced stability to Pt against dissolution, and Ir is among the most cited in this regard [7]. The Pt/Ir system was selected as a typical binary core–shell case; our current studies include Pt/Pd that has shown improved activity in comparison to pure Pt surfaces and other core/shell combinations. In this work, we study the thermodynamics of segregation of metal atoms to the surface of a core–shell * Corresponding author. Fax: +1 979 845 6446. E-mail address: [email protected] (P.B. Balbuena). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.03.008
model structure using a slab model to represent the ‘core–shell’ system; thus the shell are the surface layers, and the core is the substrate. We analyze the effect of shell thickness on atomic migration from the core to the shell layers, and that of the presence of adsorbed atomic oxygen on the same process. 2. Computational and system details Calculations were performed within the framework of density functional theory (DFT) using the Vienna ab initio simulation package (VASP) [8–13], which is a DFT code based on plane wave basis sets. Electron–ion interactions are described using the projectoraugmented wave (PAW) method [14], which was expanded within a plane wave basis setting up a cutoff energy of 400 eV. Electron exchange and correlation effects were described by Perdew– Burke–Ernzerhof (PBE) [15] GGA type exchange correlation functional. Spin polarization was included in every simulation. Three (1 1 1) surfaces, each composed of a four-layer Ir core and a Pt shell of n layers were modeled using 2 2 supercells. Brillouin zone integration was performed using 9 9 1 Monkhorst Pack grid [16] and a Methfessel–Paxton [17] smearing of 0.2 eV. The initial state was a slab model representing a core–shell system composed of a four-layer Ir core and Pt shells of different thicknesses: one, two, and three layers, respectively. The ‘inner’ two Ir core layers, those at the bottom of the slab, were fixed whereas the other two Ir layers (and the Pt layers) were allowed to relax (Fig. 1). Slabs were taken as infinite in the x and y directions and finite in the z direction; periodic boundary conditions were used in the three directions. A vacuum space of 42 Å (equivalent to 19 layers) is used to separate the slab from the slabs of the upper and lower cells, thus ensuring that there were no interactions between the adsorbed intermediates and the bottom surface of the next slab.
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G.E. Ramirez-Caballero, P.B. Balbuena / Chemical Physics Letters 456 (2008) 64–67
from one layer to the next in the path toward the surface. It can be observed that the total segregation energy depends slightly upon the shell thickness and it is always positive, meaning that segregation of Ir through the Pt shell is not favorable, thus we could refer to this process as antisegregation. 3.2. Segregation of core atoms to the shell layers under 1/4 ML of adsorbed atomic O Fig. 1. Three (1 1 1) surfaces for a core–shell system composed of a four-layer Ir core (green) and Pt shell (grey) of different thicknesses: one, two, and three layers, respectively. The bottom two layers are fixed; all the others are allowed to relax. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The optimum bulk lattice constant of Ir was determined as 3.88 Å, a value 1.04% higher than the experimental (3.84 Å) [18]. The segregation process was studied by simulating the diffusion of one by one Ir atom from the core, through the shell, to the surface. Every time that the Ir atom was switched from one layer to the upper next, the system was allowed to relax. The segregation process was also simulated in two other systems containing surface atomic oxygen covering 1/4 of a monolayer, adsorbed in fcc and hcp positions, respectively. Results of Ir segregation with and without oxygen are compared. 3. Results and discussion 3.1. Segregation energy The total segregation energy is the energy cost of transferring an atom of Ir from the core to the surface; here it was calculated as the energy difference between two systems: one with the Ir atom located in one of the shell layers while switching positions with a Pt atom, and another with the same Ir atom located in the core. When the shell has n layers (n = 3 in Fig. 2), n segregation processes are studied, each one is calculated as the energy difference between systems that have Ir exchanging positions with Pt from two consecutive layers, until Ir reaches the surface. Table 1 displays the total energy for Ir segregation from the core to the surface (last column of Table 1) and the energy required to diffuse
Segregation of Ir through the Pt shell was also simulated in systems containing atomic oxygen adsorbed in the fcc and hcp positions. Although the most stable adsorption site for atomic oxygen is fcc, the adsorption energy difference between both sites is relatively small and therefore it is important to include the hcp case in the evaluation of surface segregation. Table 2 shows a significant decrease in the antisegregation energy for each system under reaction conditions where O is adsorbed on the surface. Energy decreases are in all cases higher than 60% with respect to the clean surfaces. It is noticeable that in the hcp case the antisegregation behavior of Ir turns into favorable to segregation (small negative value of the segregation energy). It is observed in all of the cases that the presence of oxygen in the hcp position causes the largest change in segregation energy. Also interesting is that the closer the Ir atom gets to the adsorbed oxygen atom, the positive (antisegregation) energies become smaller; that is, segregation becomes more favorable. This is clearly shown in the diagonal terms of Table 2, which correspond to the top surface layers in each case. This can be explained by the high affinity of Ir for oxygen, which is evidenced by the high adsorption energies when Ir reaches the surface layer, as discussed in the next section. The results of the segregation processes are summarized in Fig. 3. 3.3. Adsorption energetics under the segregation process Table 3 illustrates how the oxygen adsorption energies in hcp and fcc positions change during the segregation process. The adsorption energies, Ead, were calculated by the following equation: Ead ¼
ðEclean slab þ N EOxygen Þ Eslab w=adsorbed O N
ð1Þ
Fig. 2. Segregation process in a core–shell system with three layers of Pt shell; diffusion of one Ir atom (green) from the core, through the shell, to the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Segregation energies Eseg in eV, for the Pt–Ir core–shell system Core–shell systems
Eseg from the core to the 1st layer of the shell
Eseg from the 1st to the 2nd layer of the shell
Eseg from the 2nd to the 3rd layer of the shell
Total segregation energy
Core (Ir)–shell (1 layer of Pt)
0.62
–
–
0.62
Core (Ir)–shell (2 layers of Pt)
0.42
0.41
–
0.83
Core (Ir)–shell (3 layers of Pt)
0.25
0.22
0.31
0.78
The shell layers are numbered 1st, 2nd, 3rd from the inner to the outer layer.
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Table 2 Segregation energies Eseg in eV, for the Pt–Ir core–shell system with 1/4 ML O adsorbed on the fcc or hcp sites of the Pt(1 1 1) surface layer Core–shell systems with O adsorbed on the surface
Eseg from the core to the 1st layer of the shell
Eseg from the one layer to the 2nd layer of the shell
Energy segregation from the two layer to the 3rd layer of the shell
Total segregation energy
Core (Ir)–shell (1 layer of Pt)
fcc 0.17 hcp 0.02
–
–
fcc 0.17 hcp 0.02
Core (Ir)–shell (2 layers of Pt)
fcc 0.48 hcp 0.38
fcc 0.21 hcp 0.26
–
fcc 0.27 hcp 0.12
Core (Ir)–shell (3 layers of Pt)
fcc 0.38 hcp 0.32
fcc 0.09 hcp 0.11
fcc 0.27 hcp 0.33
fcc 0.19 hcp 0.10
When O is adsorbed in the hcp sites, the underneath atom is Pt. The shell layers are numbered 1st, 2nd, 3rd from the inner to the outer layer.
adsorption energies. For example, the second column of Table 3 reveals the effect of the shell thickness on the adsorption energy of Pt-skin surfaces of Pt–Ir core-shell systems. It is clear that the thicker the Pt shell the stronger the adsorption energy, which tends to become close to that of O adsorbed on a pure Pt(1 1 1) surface (4.2 eV). On the Pt-skin surfaces, the adsorption is weaker (values shown in italics in Table 3), with small differences depending on the composition of the subsurface layers. Comparing the results in Tables 2 and 3, we conclude that although the segregation trends are not monotonic, the stronger the adsorption energy, the Ir segregation energy tends to be more negative (or less positive), therefore the surfaces where Ir is already on the surface increase the tendency of Ir to segregate. See for example, the correlation between the adsorption energy values (in bold font) in the Table 3 diagonal (4.22, 4.40, 4.58) and the segregation energies in Table 2 (diagonal values: 0.17, 0.21, 0.27). On Pt-skin surfaces, the hcp site is the most favorable site for segregation, and given that the thinner the Pt shell the weakest the oxygen adsorption, the thinner shells are the most favorable for antisegregation of Ir to the surface. The main factors that affect the segregation energy are the relative size of the two elements, their surface energies, and the heat of formation of the alloy. In this case Pt has the largest size and the lowest surface energy and therefore has a tendency to segregate to the surface [6,19]. This may partially explain the antisegregation of Ir that prefers to stay in the core. Analyzing the second column of Table 1 we observe that when the shell has two or three layers the energy required for Ir to move from the core to the next upper layer is lower than that in the case of one-layer shell. There is another component to the segregation process that is given by the energy required for lattice deformation caused by such atomic motion. As we add more layers to the shell, that eases the relaxation of the shell layers, and such energy cost becomes reduced.
Segregation Energy (eV)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 0
1
2
3
4
Layers of Pt in the core-shell system Clean surface
with oxygen on fcc position
with oxygen on hcp position
Fig. 3. Total segregation energies in the systems of Fig. 1 for a clean surface, and for surfaces with oxygen adsorbed in fcc and hcp sites.
Eslab w/adsorbed O is the total energy of the interacting core–shell system and the adsorbed oxygen atom; N is the number of oxygen atoms on the surface; for all simulations N was equal to one (corresponding to 1/4 monolayer O); Eclean slab is the total energy of the bare Pt–Ir slab; Eoxygen is the energy of one oxygen in vacuum. Positive Ead values indicate favorable (exothermic) adsorption. In all of the cases the adsorption of oxygen in fcc and hcp sites is higher when Ir is located on the surface (see bold values in Table 3); however small differences in the compositions of the surface and subsurface layers translate in variation of the corresponding
Table 3 Adsorption energies Eads in eV, for core (Ir)–shell (Pt) systems with 1/4 ML O adsorbed on the fcc or hcp sites of the Pt(111) surface layer Core–shell systems
Eads core (Ir)– shell (Pt)
Eads core (Ir)–shell (Pt); one atom of Ir in the 1st layer of the shell
Eads core (Ir)–shell (Pt); one atom of Ir in the 2nd layer of the shell
Eads core (Ir)–shell (Pt); one atom of Ir in the 3rd layer of the shell
Core (Ir)–shell (1 layer of Pt)
fcc: 3.77 hcp: 3.39 (Fig. 1a, Pt/Ir core)
fcc: 4.22 hcp: 4.03 (Pt3Ir/PtIr3/Ir core)
–
–
Core (Ir)–shell (2 layers of Pt)
fcc: 3.84 hcp: 3.51 (Fig. 1b, Pt/Pt/Ir core)
fcc: 3.79 hcp: 3.56 (Pt/Pt3Ir/PtIr3/Ir core)
fcc: 4.40 hcp: 4.23 (Pt3Ir/Pt/PtIr3/Ir core)
–
Core (Ir)–shell (3 layers of Pt)
fcc: 4.00 hcp: 3.61 (Fig. 2a, Pt/Pt/ Pt/Ir core)
fcc: 3.87 hcp: 3.54 (Fig. 2b, Pt/Pt/Pt3Ir/PtIr3/Ir core)
fcc: 4.00 hcp: 3.65 (Fig. 2c, Pt/Pt3Ir/Pt/PtIr3/Ir core)
fcc: 4.58 hcp: 4.29 (Fig. 2d, Pt3Ir/Pt/Pt/PtIr3)
The shell layers are numbered 1st, 2nd, 3rd from the inner to the outer layer.
G.E. Ramirez-Caballero, P.B. Balbuena / Chemical Physics Letters 456 (2008) 64–67
The results shown in Table 2 where Ir may segregate to the surface of core–shell surfaces covered by 0.25 ML of atomic oxygen, along with the corresponding strong oxygen adsorption energies when Ir is on the surface (Table 3) explain why Pt-skin layers over Ir cores were not found as effective ORR catalysts as other core– shell combinations [20].
67
Acknowledgements This work is supported by the Department of Energy, Grants DE-FG02-05ER15729 and DE-FG36-07G017019-A000. Computational resources from NERSC are gratefully acknowledged. References
4. Conclusions Surface segregation of Ir in bimetallic Ir (core)–Pt (shell) systems are studied with DFT methods to determine the tendency of Ir to segregate to the surface on clean surfaces and under 1/4 monolayer of adsorbed atomic oxygen. It is found that Ir does not segregate to clean Pt(1 1 1) surfaces of core–shell systems, and the antisegregation tendency is not a strong function of the number of layers in the Pt shell. When 1/4 monolayer of O is adsorbed on the surface, the behavior reverts, and a borderline behavior between antisegregation and segregation of Ir is observed. When O is adsorbed on segregated surfaces containing Ir atoms, the adsorption energies are strong, and it is found that the adsorption strength correlates with the Ir segregation tendency. On Pt-skin surfaces where the adsorption strength is significantly lower, similar behavior takes place: the weakest the adsorption, the least favorable the tendency of Ir to segregate to the surface. Therefore, skin monolayers have better protective effect against the segregation of the less noble metal, and the effect decreases as the number of layers in the shell increase.
[1] H.A. Gasteiger, S.S. Kocha, B. Sompalli, F.T. Wagner, Appl. Cat. B: Environ. 56 (2005) 9. [2] M.H. Shao, K. Sasaki, R.R. Adzic, J. Am. Chem. Soc. 128 (2006) 3526. [3] V.R. Stamenkovic, B. Fowler, B.S. Mun, G. Wang, P.N. Ross, C.A. Lucas, N.M. Markovic, Science 315 (2007) 493 (Washington, DC, United States). [4] J. Zhang, Y. Mo, M.B. Vukmirovic, R. Klie, K. Sasaki, R.R. Adzic, J. Phys. Chem. B 108 (2004) 10955. [5] Y.Y. Shao, G.P. Yin, Y.Z. Gao, J. Power Source 171 (2007) 558. [6] Y. Ma, P.B. Balbuena, Surf. Sci. 602 (2008) 107. [7] F. J. Luczak, C. T. Glastonbury, US5013618: Ternary Alloy Fuel Cell Catalysts and Phosphoric Acid Fuel Cell Containing the Catalysts, International Fuel Cells Corporation, South Windsor, CT, USA, 1991. [8] G. Kresse, J. Furthmueller, Phys. Rev. B 54 (1996) 11169. [9] G. Kresse, J. Furthmuller, Phys. Rev. B 54 (1996) 11169. [10] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15. [11] G. Kresse, J. Furthmuller, Comput. Mater. Sci. 6 (1996) 15. [12] G. Kresse, J. Hafner, Phys. Rev. B 48 (1993) 13115. [13] G. Kresse, J. Hafner, Phys. Rev. B 47 (1993) 558. [14] P.E. Blochl, Phys. Rev. B 50 (1994) 17953. [15] J.D. Lawrence, H. Li, T.B. Rauchfuss, M. Benard, M.-M. Rohmer, Angew. Chem., Int. Ed. 40 (2001) 1768. [16] H.J. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1976) 5188. [17] M. Methfessel, A.T. Paxton, Phys. Rev. B 40 (1989) 3616. [18] J. I. Gersten, F. W. Smith (Eds.), The Physics and Chemistry of Materials, 2001. [19] A.V. Ruban, H.L. Skriver, J.K. Norskov, Phys. Rev. B 59 (1999) 15990. [20] J. Zhang, M.B. Vukmirovic, Y. Xu, M. Mavrikakis, R.R. Adzic, Angew. Chem., Int. Ed. 44 (2005) 2132.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Surface segregation of core atoms in core–shell structures Gustavo E. Ramirez-Caballero, Perla B. Balbuena * Department of Chemical Engineering, Texas A&M University, College Station, TX 77843, United States
a r t i c l e
i n f o
Article history: Received 20 December 2007 In final form 4 March 2008 Available online 10 March 2008
a b s t r a c t Density functional theory is used to calculate surface segregation tendencies of iridium atoms in Ir (core)–Pt (shell) systems with various shell thicknesses, on clean surfaces and under 1/4 monolayer of atomic oxygen adsorbed on fcc and hcp sites of (1 1 1) surfaces. Iridium shows strong antisegregation behavior in clean surfaces, but the behavior reverts dramatically in the presence of adsorbed oxygen. On Pt-skin surfaces, where the oxygen adsorption is the weakest, monolayers have better protective effect against segregation of the less noble metal, and the effect decreases as the number of layers in the shell increases. Ó 2008 Elsevier B.V. All rights reserved.
1. Introduction Platinum is so far the best material to catalyze the oxygen reduction reaction (ORR) in acid medium, one of the electrode reactions in low-temperature fuel cells [1]. Such electrodes consist of an assembly of nanoparticles deposited on an electrically conductive substrate and surrounded by a hydrated polymer, the proton carrier. The ORR has a very slow kinetics, which added to the price and scarcity of platinum, makes the development of new catalytic materials one of the biggest challenges in fuel cell technology. It has been reported that bimetallic nanoalloys are promising candidate materials for ORR electrocatalysts [2–4]. However, the acid environment in which these nanocatalysts work triggers metal corrosion, which limits the catalyst lifetime and produces other undesirable effects such as the degradation of the polymeric membrane [5]. Such dissolution reaction usually causes the less noble alloy components to dissolve first, leaving a monolayer of Pt atoms on the surface [3]. This structure, also called ‘Pt-skin’ has shown excellent activity and durability properties. For this reason, structures such as M (core)–Pt (shell) are being tested as alternative ORR electrocatalysts. However, the stability of such Pt shell is crucial, and therefore the core elements should be such that remain in the core instead of segregating to the surface [6]. It has been shown that certain alloy elements may confer enhanced stability to Pt against dissolution, and Ir is among the most cited in this regard [7]. The Pt/Ir system was selected as a typical binary core–shell case; our current studies include Pt/Pd that has shown improved activity in comparison to pure Pt surfaces and other core/shell combinations. In this work, we study the thermodynamics of segregation of metal atoms to the surface of a core–shell * Corresponding author. Fax: +1 979 845 6446. E-mail address: [email protected] (P.B. Balbuena). 0009-2614/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.cplett.2008.03.008
model structure using a slab model to represent the ‘core–shell’ system; thus the shell are the surface layers, and the core is the substrate. We analyze the effect of shell thickness on atomic migration from the core to the shell layers, and that of the presence of adsorbed atomic oxygen on the same process. 2. Computational and system details Calculations were performed within the framework of density functional theory (DFT) using the Vienna ab initio simulation package (VASP) [8–13], which is a DFT code based on plane wave basis sets. Electron–ion interactions are described using the projectoraugmented wave (PAW) method [14], which was expanded within a plane wave basis setting up a cutoff energy of 400 eV. Electron exchange and correlation effects were described by Perdew– Burke–Ernzerhof (PBE) [15] GGA type exchange correlation functional. Spin polarization was included in every simulation. Three (1 1 1) surfaces, each composed of a four-layer Ir core and a Pt shell of n layers were modeled using 2 2 supercells. Brillouin zone integration was performed using 9 9 1 Monkhorst Pack grid [16] and a Methfessel–Paxton [17] smearing of 0.2 eV. The initial state was a slab model representing a core–shell system composed of a four-layer Ir core and Pt shells of different thicknesses: one, two, and three layers, respectively. The ‘inner’ two Ir core layers, those at the bottom of the slab, were fixed whereas the other two Ir layers (and the Pt layers) were allowed to relax (Fig. 1). Slabs were taken as infinite in the x and y directions and finite in the z direction; periodic boundary conditions were used in the three directions. A vacuum space of 42 Å (equivalent to 19 layers) is used to separate the slab from the slabs of the upper and lower cells, thus ensuring that there were no interactions between the adsorbed intermediates and the bottom surface of the next slab.
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from one layer to the next in the path toward the surface. It can be observed that the total segregation energy depends slightly upon the shell thickness and it is always positive, meaning that segregation of Ir through the Pt shell is not favorable, thus we could refer to this process as antisegregation. 3.2. Segregation of core atoms to the shell layers under 1/4 ML of adsorbed atomic O Fig. 1. Three (1 1 1) surfaces for a core–shell system composed of a four-layer Ir core (green) and Pt shell (grey) of different thicknesses: one, two, and three layers, respectively. The bottom two layers are fixed; all the others are allowed to relax. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
The optimum bulk lattice constant of Ir was determined as 3.88 Å, a value 1.04% higher than the experimental (3.84 Å) [18]. The segregation process was studied by simulating the diffusion of one by one Ir atom from the core, through the shell, to the surface. Every time that the Ir atom was switched from one layer to the upper next, the system was allowed to relax. The segregation process was also simulated in two other systems containing surface atomic oxygen covering 1/4 of a monolayer, adsorbed in fcc and hcp positions, respectively. Results of Ir segregation with and without oxygen are compared. 3. Results and discussion 3.1. Segregation energy The total segregation energy is the energy cost of transferring an atom of Ir from the core to the surface; here it was calculated as the energy difference between two systems: one with the Ir atom located in one of the shell layers while switching positions with a Pt atom, and another with the same Ir atom located in the core. When the shell has n layers (n = 3 in Fig. 2), n segregation processes are studied, each one is calculated as the energy difference between systems that have Ir exchanging positions with Pt from two consecutive layers, until Ir reaches the surface. Table 1 displays the total energy for Ir segregation from the core to the surface (last column of Table 1) and the energy required to diffuse
Segregation of Ir through the Pt shell was also simulated in systems containing atomic oxygen adsorbed in the fcc and hcp positions. Although the most stable adsorption site for atomic oxygen is fcc, the adsorption energy difference between both sites is relatively small and therefore it is important to include the hcp case in the evaluation of surface segregation. Table 2 shows a significant decrease in the antisegregation energy for each system under reaction conditions where O is adsorbed on the surface. Energy decreases are in all cases higher than 60% with respect to the clean surfaces. It is noticeable that in the hcp case the antisegregation behavior of Ir turns into favorable to segregation (small negative value of the segregation energy). It is observed in all of the cases that the presence of oxygen in the hcp position causes the largest change in segregation energy. Also interesting is that the closer the Ir atom gets to the adsorbed oxygen atom, the positive (antisegregation) energies become smaller; that is, segregation becomes more favorable. This is clearly shown in the diagonal terms of Table 2, which correspond to the top surface layers in each case. This can be explained by the high affinity of Ir for oxygen, which is evidenced by the high adsorption energies when Ir reaches the surface layer, as discussed in the next section. The results of the segregation processes are summarized in Fig. 3. 3.3. Adsorption energetics under the segregation process Table 3 illustrates how the oxygen adsorption energies in hcp and fcc positions change during the segregation process. The adsorption energies, Ead, were calculated by the following equation: Ead ¼
ðEclean slab þ N EOxygen Þ Eslab w=adsorbed O N
ð1Þ
Fig. 2. Segregation process in a core–shell system with three layers of Pt shell; diffusion of one Ir atom (green) from the core, through the shell, to the surface. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)
Table 1 Segregation energies Eseg in eV, for the Pt–Ir core–shell system Core–shell systems
Eseg from the core to the 1st layer of the shell
Eseg from the 1st to the 2nd layer of the shell
Eseg from the 2nd to the 3rd layer of the shell
Total segregation energy
Core (Ir)–shell (1 layer of Pt)
0.62
–
–
0.62
Core (Ir)–shell (2 layers of Pt)
0.42
0.41
–
0.83
Core (Ir)–shell (3 layers of Pt)
0.25
0.22
0.31
0.78
The shell layers are numbered 1st, 2nd, 3rd from the inner to the outer layer.
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Table 2 Segregation energies Eseg in eV, for the Pt–Ir core–shell system with 1/4 ML O adsorbed on the fcc or hcp sites of the Pt(1 1 1) surface layer Core–shell systems with O adsorbed on the surface
Eseg from the core to the 1st layer of the shell
Eseg from the one layer to the 2nd layer of the shell
Energy segregation from the two layer to the 3rd layer of the shell
Total segregation energy
Core (Ir)–shell (1 layer of Pt)
fcc 0.17 hcp 0.02
–
–
fcc 0.17 hcp 0.02
Core (Ir)–shell (2 layers of Pt)
fcc 0.48 hcp 0.38
fcc 0.21 hcp 0.26
–
fcc 0.27 hcp 0.12
Core (Ir)–shell (3 layers of Pt)
fcc 0.38 hcp 0.32
fcc 0.09 hcp 0.11
fcc 0.27 hcp 0.33
fcc 0.19 hcp 0.10
When O is adsorbed in the hcp sites, the underneath atom is Pt. The shell layers are numbered 1st, 2nd, 3rd from the inner to the outer layer.
adsorption energies. For example, the second column of Table 3 reveals the effect of the shell thickness on the adsorption energy of Pt-skin surfaces of Pt–Ir core-shell systems. It is clear that the thicker the Pt shell the stronger the adsorption energy, which tends to become close to that of O adsorbed on a pure Pt(1 1 1) surface (4.2 eV). On the Pt-skin surfaces, the adsorption is weaker (values shown in italics in Table 3), with small differences depending on the composition of the subsurface layers. Comparing the results in Tables 2 and 3, we conclude that although the segregation trends are not monotonic, the stronger the adsorption energy, the Ir segregation energy tends to be more negative (or less positive), therefore the surfaces where Ir is already on the surface increase the tendency of Ir to segregate. See for example, the correlation between the adsorption energy values (in bold font) in the Table 3 diagonal (4.22, 4.40, 4.58) and the segregation energies in Table 2 (diagonal values: 0.17, 0.21, 0.27). On Pt-skin surfaces, the hcp site is the most favorable site for segregation, and given that the thinner the Pt shell the weakest the oxygen adsorption, the thinner shells are the most favorable for antisegregation of Ir to the surface. The main factors that affect the segregation energy are the relative size of the two elements, their surface energies, and the heat of formation of the alloy. In this case Pt has the largest size and the lowest surface energy and therefore has a tendency to segregate to the surface [6,19]. This may partially explain the antisegregation of Ir that prefers to stay in the core. Analyzing the second column of Table 1 we observe that when the shell has two or three layers the energy required for Ir to move from the core to the next upper layer is lower than that in the case of one-layer shell. There is another component to the segregation process that is given by the energy required for lattice deformation caused by such atomic motion. As we add more layers to the shell, that eases the relaxation of the shell layers, and such energy cost becomes reduced.
Segregation Energy (eV)
0.9 0.8 0.7 0.6 0.5 0.4 0.3 0.2 0.1 0 -0.1 0
1
2
3
4
Layers of Pt in the core-shell system Clean surface
with oxygen on fcc position
with oxygen on hcp position
Fig. 3. Total segregation energies in the systems of Fig. 1 for a clean surface, and for surfaces with oxygen adsorbed in fcc and hcp sites.
Eslab w/adsorbed O is the total energy of the interacting core–shell system and the adsorbed oxygen atom; N is the number of oxygen atoms on the surface; for all simulations N was equal to one (corresponding to 1/4 monolayer O); Eclean slab is the total energy of the bare Pt–Ir slab; Eoxygen is the energy of one oxygen in vacuum. Positive Ead values indicate favorable (exothermic) adsorption. In all of the cases the adsorption of oxygen in fcc and hcp sites is higher when Ir is located on the surface (see bold values in Table 3); however small differences in the compositions of the surface and subsurface layers translate in variation of the corresponding
Table 3 Adsorption energies Eads in eV, for core (Ir)–shell (Pt) systems with 1/4 ML O adsorbed on the fcc or hcp sites of the Pt(111) surface layer Core–shell systems
Eads core (Ir)– shell (Pt)
Eads core (Ir)–shell (Pt); one atom of Ir in the 1st layer of the shell
Eads core (Ir)–shell (Pt); one atom of Ir in the 2nd layer of the shell
Eads core (Ir)–shell (Pt); one atom of Ir in the 3rd layer of the shell
Core (Ir)–shell (1 layer of Pt)
fcc: 3.77 hcp: 3.39 (Fig. 1a, Pt/Ir core)
fcc: 4.22 hcp: 4.03 (Pt3Ir/PtIr3/Ir core)
–
–
Core (Ir)–shell (2 layers of Pt)
fcc: 3.84 hcp: 3.51 (Fig. 1b, Pt/Pt/Ir core)
fcc: 3.79 hcp: 3.56 (Pt/Pt3Ir/PtIr3/Ir core)
fcc: 4.40 hcp: 4.23 (Pt3Ir/Pt/PtIr3/Ir core)
–
Core (Ir)–shell (3 layers of Pt)
fcc: 4.00 hcp: 3.61 (Fig. 2a, Pt/Pt/ Pt/Ir core)
fcc: 3.87 hcp: 3.54 (Fig. 2b, Pt/Pt/Pt3Ir/PtIr3/Ir core)
fcc: 4.00 hcp: 3.65 (Fig. 2c, Pt/Pt3Ir/Pt/PtIr3/Ir core)
fcc: 4.58 hcp: 4.29 (Fig. 2d, Pt3Ir/Pt/Pt/PtIr3)
The shell layers are numbered 1st, 2nd, 3rd from the inner to the outer layer.
G.E. Ramirez-Caballero, P.B. Balbuena / Chemical Physics Letters 456 (2008) 64–67
The results shown in Table 2 where Ir may segregate to the surface of core–shell surfaces covered by 0.25 ML of atomic oxygen, along with the corresponding strong oxygen adsorption energies when Ir is on the surface (Table 3) explain why Pt-skin layers over Ir cores were not found as effective ORR catalysts as other core– shell combinations [20].
67
Acknowledgements This work is supported by the Department of Energy, Grants DE-FG02-05ER15729 and DE-FG36-07G017019-A000. Computational resources from NERSC are gratefully acknowledged. References
4. Conclusions Surface segregation of Ir in bimetallic Ir (core)–Pt (shell) systems are studied with DFT methods to determine the tendency of Ir to segregate to the surface on clean surfaces and under 1/4 monolayer of adsorbed atomic oxygen. It is found that Ir does not segregate to clean Pt(1 1 1) surfaces of core–shell systems, and the antisegregation tendency is not a strong function of the number of layers in the Pt shell. When 1/4 monolayer of O is adsorbed on the surface, the behavior reverts, and a borderline behavior between antisegregation and segregation of Ir is observed. When O is adsorbed on segregated surfaces containing Ir atoms, the adsorption energies are strong, and it is found that the adsorption strength correlates with the Ir segregation tendency. On Pt-skin surfaces where the adsorption strength is significantly lower, similar behavior takes place: the weakest the adsorption, the least favorable the tendency of Ir to segregate to the surface. Therefore, skin monolayers have better protective effect against the segregation of the less noble metal, and the effect decreases as the number of layers in the shell increase.
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