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First-principles studies on the adsorption of molecular oxygen on Ba(110) surface
Physics Letters A 352 (2006) 526–530 www.elsevier.com/locate/pla
First-principles studies on the adsorption of molecular oxygen on Ba(110) surface S.F. Li a,b,∗ , Xinlian Xue a , Pinglin Li a , Xinjian Li a , Yu Jia a a School of Physics and Engineering, Zhengzhou University, Key Laboratory of Material Physics of Ministry of Education, Zhengzhou-450052, PR China b Institute of Solid State Physics, Chinese Academia of Science, Hefei-230031, PR China
Received 28 September 2005; received in revised form 16 December 2005; accepted 17 December 2005 Available online 27 December 2005 Communicated by R. Wu
Abstract The adsorption of O2 on Ba(110) surface is studied with first-principles calculations based on density functional theory. Our calculations predict that O2 may prefer to dissociative adsorption on Ba(110) surface without obvious barrier. Also our results do not support the model of charge transfer from the surface to the molecule as a bond breaking mechanism. Instead, the increasing hybridization between O2 orbitals and the d states of Ba(110) surface may play an important role in the dissociation adsorption. © 2005 Elsevier B.V. All rights reserved. PACS: 68.43.Bc; 68.47.De Keywords: DFT calculations; O2 ; Adsorption; Surface; Orbital hybridization
1. Introduction The adsorption of molecules on metal surface is an active field of experimental and theoretical research [1–9]. For example, the dissociation of O2 is a key step in the epoxidation of ethylene. The chemical reactivity of molecules with metal surface is also important for understanding corresponding surface reactions, catalyzing process, etc. The present communication has been enlightened by our recent work [10], wherein we studied molecular oxygen adsorption on Ban (n = 2, 5) clusters and found that Ba clusters can bind strongly and dissociate O2 molecule into atomic state. The ground state of barium is 6s2 1 S0 and its first excited sate is 6s5d 3 D1 , which distinctly modifies the nature of the binding involved in alkaline-earth metal relative that of in alkali metal and transition metal. While diatomic molecule Ba2 are bound by Van der Waals forces, the bulk material is metallic, then a transition of the structural and electronic properties from Van * Corresponding author.
E-mail address: [email protected] (S.F. Li). 0375-9601/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2005.12.040
der Waals to metallic is thus expected as the cluster size increases. From these two points of view, the oxidization process of barium with molecular oxygen may exhibit a behavior different from those of both alkali metal and transition metal. In general, for O2 adsorption on metal clusters or surfaces, there are three reaction types with the substrate: a molecular physisorption state [9,11], a molecular chemisorbed state [9,11], and a dissociative (i.e., atomic) chemisorbed state [12]. It has been known that one molecular oxygen has a stable chemisorbed state on transitional metal surfaces with obvious barriers for dissociation [13–15]. Then, what about oxygen adsorption on the alkaline-earth metal? Unfortunately, at present we have found far less reports in this field. So, it is interesting to study the oxidization of barium surface by oxygen molecule. For the dissociative mechanism, one model is based on the charge transfer from the substrate to the adsorption molecule [1,5,7] and another different idea supports that the hybridization between the substrate state and that of the adsorption molecule [4,16]. In this Letter, we use density functional theory (DFT) [17–19] to study the reaction process and reaction mechanism for one O2 molecule adsorption on Ba(110) surface.
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2. Calculation details Our calculations have been performed using a spin-polarized version of the Vienna ab-initio simulation package (VASP) [20,21]. VASP performs an iterative solution of the generalized Kohn–Sham equations via an unconstrained minimization of the norm of the residual vector to each eigenstate and optimized routine for charge- and spin-density mixing. Nonlocal correction in the form of the generalized-gradient approximation (GGA) of Perdew et al. [22] has been included for the exchange and correlation function. The calculations are carried in a plane-wave basis with an energy cutoff of 400 eV, using PAW potential [23] to describe the electron–ion interaction. A grid of 4 × 4 × 1 Monkorst Pack special k-points [24] is used, which is tested to be enough to describe the Brillouinzone integrations. The surface is modeled by a supercell consisting of a slab of five atomic layers (60 atoms) and five ideal vacuum layers. Only the oxygen molecule and its nearest two layers of Ba atoms have been fully relaxed and the other three layers of Ba atoms are kept as ‘frozen’. All the calculations are converged with the atomic force less than 0.02 eV/Å. To test the accuracy of the plane wave basis set and that of the PAW potential used in the code, we have also calculated the bond-length of Ba2 and O2 dimers, our calculation results are 4.75 Å and 1.23 Å, which are in solid agreement with experimental results of 4.60 Å and 1.21 Å for Ba2 dimer [26] and oxygen molecule [27], respectively. From the bulk Ba calculation, the calculated equilibrium lattice constant is a = 4.99 Å, which fits well with experimental value of a = 5.02 Å [25]. The calculated binding energy 6.27 eV of the ground state O2 is overestimated compared to the corresponding experimental result of 5.11 eV [27], however, such overestimation will not make much effect on the adsorption energy for O2 molecule on Ba(110) surface, which has been supported by our previous study for O2 adsorption on Sin clusters [28].
Fig. 1. The total density of state (DOS) and local partial projected density of state (PDOS) for the relaxed bare Ba(110) surface. In the first panel the total DOS has been shown. The solid line, the dotted line and the long dashed line in the second panel correspond to the s, p and d electrons PDOS, respectively. The Fermi level has been shifted to the zero point.
3. Results and discussions First of all, the bare Ba(110) surface has been optimized and the properties of the surface have been analyzed before our comprehensive search for minima for oxygen molecule adsorption. Our results show that the outmost layer of Ba atoms relaxes outward about 3.0% compared with that of the ideal surface. From the density of state (DOS) analysis (see Fig. 1) we can get that the peak by the Fermi level are mainly contributed by d electrons and s electrons. The s–d hybridization is very significant and the d electrons contribute much in the bonding. These results predicate that barium bulk may show some properties of transition metal, which in turn implies that the d electron of Ba(110) surface may play an important role in the adsorption of oxygen. The adsorption products for O2 on Ba(110) surface are initial adsorption sites dependent. As presented in Fig. 2, nine different initial adsorption sites with the molecular axis both parallel and vertical to the surface have been considered in our calculations. In every of these initial cases the center of the mass of the
Fig. 2. Top view of the nine initial adsorption sites of O2 on Ba(110) surface. Only the two upmost layers of Ba atoms are shown. The bigger gray spheres and the black spheres are symbols of the first and the second layers of Ba atoms, and the smaller gray spheres are for oxygens, respectively.
oxygen molecule keeps about 3.5 Å above the relaxed Ba(110), followed by the relaxations to search for minima of O2 reacting with Ba(110) surface. The properties of all the nine final products corresponding to their initial adsorption sites have been presented in the Table 1. From Table 1 and Fig. 2, one can get that site (a) is the favorite initial adsorption structure obtaining the largest adsorption energy of 8.753 eV and the longest O–O distance of 5.17 Å, with each of these two oxygen atom occupying about 0.2 Å on one threefold hollow sites. We find that from (b) site O2 can also be dissociated, gaining 8.594 eV adsorption energy. In case (a), as the molecular oxygen has been dissociated, it seems that there should be no net magnetic moment. However,
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Table 1 The initial adsorption sites, the properties of the final products for the nine reaction cases are presented. Eads (eV) = −(E(Ba(110) + O2 ) − E(Ba(110)) − E(O2 )) (eV). RBa–O (Å) and RO–O (Å) refer to the nearest Ba–O bond length and the distance between the two oxygen atoms in the final products, respectively. Spin (μB ) defined as the difference between the number of spin-up electrons and spin-down electrons Sites
Eads (eV)
RBa–O (Å)
RO–O (Å)
Spin (μB )
a b c d e f g h i
8.753 8.594 8.605 8.672 8.594 8.680 1.059 8.384 4.089
2.39 2.44 2.32 2.32 2.35 2.37 2.34 2.40 2.45
5.17 4.98 3.04 3.26 3.06 3.23 1.38 3.36 1.48
1.84 0.33 0.00 1.95 0.11 0.00 0.00 0.00 0.00
we found that there is still about 1.8μB net magnetic moment in the final state. The local electronic structure analysis indicates that the net magnetic moments are mainly contributed by d electrons of Ba atoms, which can be seen clearly from Fig. 3. This result indicates that d electrons of the slab has been polarized or excited and Ba(110) surface shows some character of transition metal, where local d electrons dominate the properties by the Fermi level. However, no obvious barrier is found during the dissociative process for O2 on Ba(110) surface differs significantly from that of O2 adsorption on transition metal surfaces, which verifies that alkaline-earth metal barium may bridge the properties of alkaline metals and transition metals. The initial configurations (c), (d) and (h) are also found to be dissociative sites. For case (c), in the product the O2 has been dissociated with 3.04 Å O–O distance and each oxygen occupies one threefold site to which they initially point, giving 8.605 eV adsorption energy. As to the final relaxed configuration for case (d), one oxygen atom has punctured into the space between the first and second atomic layers, with another oxygen atom siting about 1.6 Å above the surface. And it seems that a local cluster or Ba2 O island has been constructed. We find that this product is fairly stable, giving about 8.67 eV adsorption energy. The product in case (h) is just like that of in (d) state, where one oxygen has penetrated about 1.2 Å into the slab and another oxygen located about 2.0 Å above the surface with its two nearest neighbor Ba atoms being drawn out of the surface. The initial adsorption sites presented in Fig. 2(g) and Fig. 2(i), especially for case (g), are verified to be unbefitting sites to dissociate O2 . We obtain 1.48 Å O–O distance and 4.089 eV adsorption energy for case (i) and 1.38 Å O–O bonding length and 1.059 eV adsorption energy for case (g), respectively. Steering effect has been observed during the adsorption processes with the initial adsorption sites presented in both Fig. 2(e) and Fig. 2(f) (see Fig. 4). In Fig. 2(e) and Fig. 2(f), the axis of the molecule parallel to the surface with about φ = 54.7◦ . We find that the molecular oxygen adjusts to a fit orientation to dissociate without obvious barrier. The steering effect implies one obvious anisotropy character in the potential
Fig. 3. The total DOS and PDOS for the final state of O2 dissociation on Ba(110) surface with the initial adsorption site presented in Fig. 2(a). The Fermi level has been shifted to the zero point.
Fig. 4. The steering processes (a) and (b) corresponding to the adsorption of O2 in sites of Fig. 2(e) and Fig. 2(f), respectively.
surface for Ba(110) surface, which may be partially attributed to the behavior of d electrons by the Fermi level as presented in Fig. 1. In the following, the mechanism of dissociative process for the O2 adsorption on Ba(110) surface has been discussed. First, to analyze in more detail whether a similar charge transfer mechanism does work for O2 dissociation on Ba(110) surface, we study the charge density difference to examine the charge flow with three states along the dissociation path for the adsorption case of Fig. 2(f). The charge density difference has been defined as ρ = ρ Ba(110) + O2 − ρ Ba(110) − ρ(O2 ), where ρ(Ba(110) + O2 ) is the total relaxed charge density for the reaction product, ρ(Ba(110)) and ρ(O2 ) are the charge densities contributed by Ba atoms and O atoms with the same positions as that in calculating the total charge density, respec-
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Fig. 5. The DOS and PDOS for the reaction process of O2 adsorption on Ba(110) surface (the case of Fig. 2(f)). The first column, the second and the third column correspond to the initial state, intermedial state and final products, respectively. The Fermi level has been shifted to the zero point.
Fig. 6. The DOS and PDOS for the reaction process of O2 adsorption on Ba(110) surface (the case of Fig. 2(i)). The first column, the second and the third column correspond to the initial state, intermedial state and final products, respectively. The Fermi level has been shifted to the zero point.
tively. In order to estimate the total charge flow to the molecule, we integrate ρ within an equal strip around the molecule for each state that we selected. The first state corresponds to the initial adsorption structure and the last state corresponds to the final products, and the intermedial state corresponds to the onset of the dissociation, where the O–O distance has been lengthened to about 1.5 Å. In our system the charge transfers from the surface to the partially filled πg molecular orbital and weaken the O–O bond. For the first state and the intermedial state, we get that the total charge flows are about 0.1e and 0.6e, respectively. The quantity of total charge flow integrals obtained here fits well with that of O2 adsorption on Al(111) surface [4]. In the final product, we find that the oxygen exists with O− state. From the charge flow analysis, we can get that there is only very small, neither 2 nor 1 electrons transfer from the substrate to the oxygen molecule before it is to be dissociated. The charge flow analysis have also been done for all other cases, at most 0.5–0.6e charge flow has been gained before the molecule dissociation. Above results indicate that, firstly, the charge flow from the substrate to the oxygen molecule may be not enough to break the O–O bonds, as we have got that the bond length of one isolated O− 2 is no more than 1.33 Å, secondly, we cannot judge the charge flow simply from the magnetic moment. For example,
in case of Fig. 2(i), the final product is spin singlet, however, only about 0.5e has been charged by the oxygen molecule. This result is also in good agreement with that we found in O2 adsorption on small Ban cluster [10]. The p–d (p–s) orbital hybridizations between the molecular oxygen and the Ba(110) state may play an important role in the dissociative chemisorption process, which can be detected qualitatively with the density of state (DOS) and local partial projected density of state (PDOS) electronic structure analysis, as presented in Fig. 5 and Fig. 6. From the PDOS(O(p)) for the initial state, one can get that there are two main group of peaks, one is about 6.0–8.0 eV bellow the Fermi level, corresponding to the (1πu )4 orbital, and the other is about 1.0 eV bellow the Fermi level, corresponding to the (1πg∗ )2 orbital of O2 , respectively. When the molecule is attracted step by step to the Ba(110) surface, from the PDOS(O(p)) presented in the Fig. 5, we can get that the gap between these to group of peaks decreases gradually from initial state to the intermedial state, which indicates that the O–O bond break gradually. In the final product, these two peaks have almost merged with each other and the O–O bond has broken thoroughly. One can notice clearly that during the dissociative process, the d and s states of Ba surface begin to mix with p electrons orbital of O2 . In the final products O2 has been dissociated and oxygen bind
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with Ba atoms based on p–d (p–s) hybridization bonds, which can be easily deduced from the DOS. In fact, the peaks by the Fermi level in the final panel for the total DOS can be regarded as comprising of one main-peak corresponding to the p–d hybridization orbital and one additional sub-peak due to the p–s hybridization orbital, respectively. However, as to the molecular chemisorption in the case of Fig. 2(i) (see Fig. 6), neither in the intermedial state nor in the final product state, can we get significant hybridization between the O2 and Ba(110) surface. Above electronic structure analysis shows that p–d (p–s) hybridization between the p orbital of oxygen with d (s) states of Ba may play an important role in the dissociative chemisorption of O2 with Ba(110) surface. The hybridization may also play a key role in the steering effect during the adsorption process. From the viewpoint of the electronic configuration of O2 , for the axis of the oxygen molecule, the more it parallel to the Ba(110) surface, the easier and the more sufficient for both the π orbital hybridizing with the Ba states and the charge transfer from the substrate to the π orbital of the oxygen, so the initial adsorption site of Fig. 2(g) is the most unfitted site. Though the axis of the O2 in both Fig. 2(e) and Fig. 2(f) are all parallel with the Ba(110) surface, the d electrons of Ba atoms are localized, so the oxygen molecule will adjust its orientation by atomic force to a fit site for hybridization with d states of the surface when it is placed in a ‘bad’ initial adsorption site, then, steering effect occurs. 4. Summary In summary, we have presented first-principles calculations for O2 adsorption on the Ba(110) surface. Our results support direct dissociative chemisorption mechanism. In previous literatures, O2 dissociation on transition metals, the dissociations 2− are generally due to O− 2 and O2 states in which the antibonding orbitals become occupied and thus first weakens the O–O bonds and finally break it. In this Letter, we find that there is only very small charge transfer from the substrate to the oxygen molecule. Through electronic structure analysis, we owe the dissociation mainly to the p–d and p–s hybridization interaction and secondly to the charge transfer from the surface to the antibonding orbital. The Ba(110) surface shows some prop-
erties of transition metal, in which d electrons play a key role in the dissociation of the O2 . Acknowledgements One of the authors (S.F. Li) wishes to thank Professors X.G. Gong, Z. Zeng and Zongxian Yang for useful discussions. This research is partially supported by the National Science Foundation of China, the special funds for major state basic research and CAS projects. References [1] P.A. Gravil, D.M. Bird, J.A. White, Phys. Rev. Lett. 77 (1996) 3933. [2] J. Belher, B. Delley, S. Lorenz, K. Reuter, M. Scheffler, Phys. Rev. Lett. 94 (2005) 036104. [3] I. Popova, V. Zhukov, J.T. Yates Jr., Surf. Sci. 518 (2002) 39. [4] K. Honkala, K. Laasonen, Phys. Rev. Lett. 84 (2000) 705. [5] S.Y. Liem, J.H.R. Clarke, G. Kresse, Comput. Mater. Sci. 17 (2000) 133. [6] Y. Xu, M. Mavrikakis, J. Chem. Phys. 116 (2002) 10846. [7] Y. Xu, M. Mavrikakis, Surf. Sci. 494 (2001) 131. [8] G. Katz, Y. Zeiri, R. Kosloff, Surf. Sci. 425 (1999) 1. [9] K.C. Prince, G. Paolucci, A.M. Bradshaw, Surf. Sci. 175 (1986) 101. [10] S.F. Li, X.G. Gong, in preparation. [11] R.J. Guest, B. Hernnäs, P. Bennich, O. Björneholm, A. Nilsson, R.E. Palmer, N. Martensson, Surf. Sci. 278 (1992) 239. [12] L. Vattunone, M. Rocca, U. Valbusa, Surf. Sci. 314 (1994) L904. [13] I. Panas, P. Siegbahn, U. Wahlgren, J. Chem. Phys. 90 (1989) 6791. [14] A. Eichler, J. Hafner, Phys. Rev. Lett. 79 (1997) 4481. [15] H. Nakatsuji, H. Nakai, J. Chem. Phys. 98 (1993) 2423. [16] K. Kato, T. Uda, Phys. Rev. B 62 (2000) 15978. [17] M. Schliiter, L.J. Sham, Phys. Today 35 (1982) 36. [18] P. Hohenberg, W. Kohn, Phys. Rev. 136 (1964) B864. [19] W. Kohn, L.J. Sham, Phys. Rev. 140 (1965) A1133. [20] G. Kresse, J. Furthmüller, Phys. Rev. B 54 (1996) 11169. [21] G. Kresse, J. Furthmüller, Comput. Mater. Sci. 6 (1996) 15. [22] J.P. Perdew, J.A. Chevary, S.H. Vosko, K.A. Jackson, M.R. Pederson, D.J. Singh, C. Fiolhais, Phys. Rev. B 46 (1992) 6671. [23] G. Kresse, J. Joubert, Phys. Rev. B 59 (1999) 1758. [24] H.H. Monkhorst, J.D. Pack, Phys. Rev. B 13 (1999) 5188. [25] C. Kittel, Introduction to Solid State Physics, Wiley, New York, 1976. [26] V. Boutou, M.A. Lebeault, A.R. Allouche, C. Bordas, F. Paulig, J. Viallon, J. Chevaleyre, Phys. Rev. Lett. 80 (1998) 2817. [27] K.P. Huber, G. Herzberg, Molecular Spectra and Molecular Structure, Van Nostrand Reinhold, New York, 1979. [28] S.F. Li, X.G. Gong, J. Chem. Phys. 122 (2005) 174311.
First-principles studies on the adsorption of molecular oxygen on Ba(110) surface S.F. Li a,b,∗ , Xinlian Xue a , Pinglin Li a , Xinjian Li a , Yu Jia a a School of Physics and Engineering, Zhengzhou University, Key Laboratory of Material Physics of Ministry of Education, Zhengzhou-450052, PR China b Institute of Solid State Physics, Chinese Academia of Science, Hefei-230031, PR China
Received 28 September 2005; received in revised form 16 December 2005; accepted 17 December 2005 Available online 27 December 2005 Communicated by R. Wu
Abstract The adsorption of O2 on Ba(110) surface is studied with first-principles calculations based on density functional theory. Our calculations predict that O2 may prefer to dissociative adsorption on Ba(110) surface without obvious barrier. Also our results do not support the model of charge transfer from the surface to the molecule as a bond breaking mechanism. Instead, the increasing hybridization between O2 orbitals and the d states of Ba(110) surface may play an important role in the dissociation adsorption. © 2005 Elsevier B.V. All rights reserved. PACS: 68.43.Bc; 68.47.De Keywords: DFT calculations; O2 ; Adsorption; Surface; Orbital hybridization
1. Introduction The adsorption of molecules on metal surface is an active field of experimental and theoretical research [1–9]. For example, the dissociation of O2 is a key step in the epoxidation of ethylene. The chemical reactivity of molecules with metal surface is also important for understanding corresponding surface reactions, catalyzing process, etc. The present communication has been enlightened by our recent work [10], wherein we studied molecular oxygen adsorption on Ban (n = 2, 5) clusters and found that Ba clusters can bind strongly and dissociate O2 molecule into atomic state. The ground state of barium is 6s2 1 S0 and its first excited sate is 6s5d 3 D1 , which distinctly modifies the nature of the binding involved in alkaline-earth metal relative that of in alkali metal and transition metal. While diatomic molecule Ba2 are bound by Van der Waals forces, the bulk material is metallic, then a transition of the structural and electronic properties from Van * Corresponding author.
E-mail address: [email protected] (S.F. Li). 0375-9601/$ – see front matter © 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.physleta.2005.12.040
der Waals to metallic is thus expected as the cluster size increases. From these two points of view, the oxidization process of barium with molecular oxygen may exhibit a behavior different from those of both alkali metal and transition metal. In general, for O2 adsorption on metal clusters or surfaces, there are three reaction types with the substrate: a molecular physisorption state [9,11], a molecular chemisorbed state [9,11], and a dissociative (i.e., atomic) chemisorbed state [12]. It has been known that one molecular oxygen has a stable chemisorbed state on transitional metal surfaces with obvious barriers for dissociation [13–15]. Then, what about oxygen adsorption on the alkaline-earth metal? Unfortunately, at present we have found far less reports in this field. So, it is interesting to study the oxidization of barium surface by oxygen molecule. For the dissociative mechanism, one model is based on the charge transfer from the substrate to the adsorption molecule [1,5,7] and another different idea supports that the hybridization between the substrate state and that of the adsorption molecule [4,16]. In this Letter, we use density functional theory (DFT) [17–19] to study the reaction process and reaction mechanism for one O2 molecule adsorption on Ba(110) surface.
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2. Calculation details Our calculations have been performed using a spin-polarized version of the Vienna ab-initio simulation package (VASP) [20,21]. VASP performs an iterative solution of the generalized Kohn–Sham equations via an unconstrained minimization of the norm of the residual vector to each eigenstate and optimized routine for charge- and spin-density mixing. Nonlocal correction in the form of the generalized-gradient approximation (GGA) of Perdew et al. [22] has been included for the exchange and correlation function. The calculations are carried in a plane-wave basis with an energy cutoff of 400 eV, using PAW potential [23] to describe the electron–ion interaction. A grid of 4 × 4 × 1 Monkorst Pack special k-points [24] is used, which is tested to be enough to describe the Brillouinzone integrations. The surface is modeled by a supercell consisting of a slab of five atomic layers (60 atoms) and five ideal vacuum layers. Only the oxygen molecule and its nearest two layers of Ba atoms have been fully relaxed and the other three layers of Ba atoms are kept as ‘frozen’. All the calculations are converged with the atomic force less than 0.02 eV/Å. To test the accuracy of the plane wave basis set and that of the PAW potential used in the code, we have also calculated the bond-length of Ba2 and O2 dimers, our calculation results are 4.75 Å and 1.23 Å, which are in solid agreement with experimental results of 4.60 Å and 1.21 Å for Ba2 dimer [26] and oxygen molecule [27], respectively. From the bulk Ba calculation, the calculated equilibrium lattice constant is a = 4.99 Å, which fits well with experimental value of a = 5.02 Å [25]. The calculated binding energy 6.27 eV of the ground state O2 is overestimated compared to the corresponding experimental result of 5.11 eV [27], however, such overestimation will not make much effect on the adsorption energy for O2 molecule on Ba(110) surface, which has been supported by our previous study for O2 adsorption on Sin clusters [28].
Fig. 1. The total density of state (DOS) and local partial projected density of state (PDOS) for the relaxed bare Ba(110) surface. In the first panel the total DOS has been shown. The solid line, the dotted line and the long dashed line in the second panel correspond to the s, p and d electrons PDOS, respectively. The Fermi level has been shifted to the zero point.
3. Results and discussions First of all, the bare Ba(110) surface has been optimized and the properties of the surface have been analyzed before our comprehensive search for minima for oxygen molecule adsorption. Our results show that the outmost layer of Ba atoms relaxes outward about 3.0% compared with that of the ideal surface. From the density of state (DOS) analysis (see Fig. 1) we can get that the peak by the Fermi level are mainly contributed by d electrons and s electrons. The s–d hybridization is very significant and the d electrons contribute much in the bonding. These results predicate that barium bulk may show some properties of transition metal, which in turn implies that the d electron of Ba(110) surface may play an important role in the adsorption of oxygen. The adsorption products for O2 on Ba(110) surface are initial adsorption sites dependent. As presented in Fig. 2, nine different initial adsorption sites with the molecular axis both parallel and vertical to the surface have been considered in our calculations. In every of these initial cases the center of the mass of the
Fig. 2. Top view of the nine initial adsorption sites of O2 on Ba(110) surface. Only the two upmost layers of Ba atoms are shown. The bigger gray spheres and the black spheres are symbols of the first and the second layers of Ba atoms, and the smaller gray spheres are for oxygens, respectively.
oxygen molecule keeps about 3.5 Å above the relaxed Ba(110), followed by the relaxations to search for minima of O2 reacting with Ba(110) surface. The properties of all the nine final products corresponding to their initial adsorption sites have been presented in the Table 1. From Table 1 and Fig. 2, one can get that site (a) is the favorite initial adsorption structure obtaining the largest adsorption energy of 8.753 eV and the longest O–O distance of 5.17 Å, with each of these two oxygen atom occupying about 0.2 Å on one threefold hollow sites. We find that from (b) site O2 can also be dissociated, gaining 8.594 eV adsorption energy. In case (a), as the molecular oxygen has been dissociated, it seems that there should be no net magnetic moment. However,
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Table 1 The initial adsorption sites, the properties of the final products for the nine reaction cases are presented. Eads (eV) = −(E(Ba(110) + O2 ) − E(Ba(110)) − E(O2 )) (eV). RBa–O (Å) and RO–O (Å) refer to the nearest Ba–O bond length and the distance between the two oxygen atoms in the final products, respectively. Spin (μB ) defined as the difference between the number of spin-up electrons and spin-down electrons Sites
Eads (eV)
RBa–O (Å)
RO–O (Å)
Spin (μB )
a b c d e f g h i
8.753 8.594 8.605 8.672 8.594 8.680 1.059 8.384 4.089
2.39 2.44 2.32 2.32 2.35 2.37 2.34 2.40 2.45
5.17 4.98 3.04 3.26 3.06 3.23 1.38 3.36 1.48
1.84 0.33 0.00 1.95 0.11 0.00 0.00 0.00 0.00
we found that there is still about 1.8μB net magnetic moment in the final state. The local electronic structure analysis indicates that the net magnetic moments are mainly contributed by d electrons of Ba atoms, which can be seen clearly from Fig. 3. This result indicates that d electrons of the slab has been polarized or excited and Ba(110) surface shows some character of transition metal, where local d electrons dominate the properties by the Fermi level. However, no obvious barrier is found during the dissociative process for O2 on Ba(110) surface differs significantly from that of O2 adsorption on transition metal surfaces, which verifies that alkaline-earth metal barium may bridge the properties of alkaline metals and transition metals. The initial configurations (c), (d) and (h) are also found to be dissociative sites. For case (c), in the product the O2 has been dissociated with 3.04 Å O–O distance and each oxygen occupies one threefold site to which they initially point, giving 8.605 eV adsorption energy. As to the final relaxed configuration for case (d), one oxygen atom has punctured into the space between the first and second atomic layers, with another oxygen atom siting about 1.6 Å above the surface. And it seems that a local cluster or Ba2 O island has been constructed. We find that this product is fairly stable, giving about 8.67 eV adsorption energy. The product in case (h) is just like that of in (d) state, where one oxygen has penetrated about 1.2 Å into the slab and another oxygen located about 2.0 Å above the surface with its two nearest neighbor Ba atoms being drawn out of the surface. The initial adsorption sites presented in Fig. 2(g) and Fig. 2(i), especially for case (g), are verified to be unbefitting sites to dissociate O2 . We obtain 1.48 Å O–O distance and 4.089 eV adsorption energy for case (i) and 1.38 Å O–O bonding length and 1.059 eV adsorption energy for case (g), respectively. Steering effect has been observed during the adsorption processes with the initial adsorption sites presented in both Fig. 2(e) and Fig. 2(f) (see Fig. 4). In Fig. 2(e) and Fig. 2(f), the axis of the molecule parallel to the surface with about φ = 54.7◦ . We find that the molecular oxygen adjusts to a fit orientation to dissociate without obvious barrier. The steering effect implies one obvious anisotropy character in the potential
Fig. 3. The total DOS and PDOS for the final state of O2 dissociation on Ba(110) surface with the initial adsorption site presented in Fig. 2(a). The Fermi level has been shifted to the zero point.
Fig. 4. The steering processes (a) and (b) corresponding to the adsorption of O2 in sites of Fig. 2(e) and Fig. 2(f), respectively.
surface for Ba(110) surface, which may be partially attributed to the behavior of d electrons by the Fermi level as presented in Fig. 1. In the following, the mechanism of dissociative process for the O2 adsorption on Ba(110) surface has been discussed. First, to analyze in more detail whether a similar charge transfer mechanism does work for O2 dissociation on Ba(110) surface, we study the charge density difference to examine the charge flow with three states along the dissociation path for the adsorption case of Fig. 2(f). The charge density difference has been defined as ρ = ρ Ba(110) + O2 − ρ Ba(110) − ρ(O2 ), where ρ(Ba(110) + O2 ) is the total relaxed charge density for the reaction product, ρ(Ba(110)) and ρ(O2 ) are the charge densities contributed by Ba atoms and O atoms with the same positions as that in calculating the total charge density, respec-
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Fig. 5. The DOS and PDOS for the reaction process of O2 adsorption on Ba(110) surface (the case of Fig. 2(f)). The first column, the second and the third column correspond to the initial state, intermedial state and final products, respectively. The Fermi level has been shifted to the zero point.
Fig. 6. The DOS and PDOS for the reaction process of O2 adsorption on Ba(110) surface (the case of Fig. 2(i)). The first column, the second and the third column correspond to the initial state, intermedial state and final products, respectively. The Fermi level has been shifted to the zero point.
tively. In order to estimate the total charge flow to the molecule, we integrate ρ within an equal strip around the molecule for each state that we selected. The first state corresponds to the initial adsorption structure and the last state corresponds to the final products, and the intermedial state corresponds to the onset of the dissociation, where the O–O distance has been lengthened to about 1.5 Å. In our system the charge transfers from the surface to the partially filled πg molecular orbital and weaken the O–O bond. For the first state and the intermedial state, we get that the total charge flows are about 0.1e and 0.6e, respectively. The quantity of total charge flow integrals obtained here fits well with that of O2 adsorption on Al(111) surface [4]. In the final product, we find that the oxygen exists with O− state. From the charge flow analysis, we can get that there is only very small, neither 2 nor 1 electrons transfer from the substrate to the oxygen molecule before it is to be dissociated. The charge flow analysis have also been done for all other cases, at most 0.5–0.6e charge flow has been gained before the molecule dissociation. Above results indicate that, firstly, the charge flow from the substrate to the oxygen molecule may be not enough to break the O–O bonds, as we have got that the bond length of one isolated O− 2 is no more than 1.33 Å, secondly, we cannot judge the charge flow simply from the magnetic moment. For example,
in case of Fig. 2(i), the final product is spin singlet, however, only about 0.5e has been charged by the oxygen molecule. This result is also in good agreement with that we found in O2 adsorption on small Ban cluster [10]. The p–d (p–s) orbital hybridizations between the molecular oxygen and the Ba(110) state may play an important role in the dissociative chemisorption process, which can be detected qualitatively with the density of state (DOS) and local partial projected density of state (PDOS) electronic structure analysis, as presented in Fig. 5 and Fig. 6. From the PDOS(O(p)) for the initial state, one can get that there are two main group of peaks, one is about 6.0–8.0 eV bellow the Fermi level, corresponding to the (1πu )4 orbital, and the other is about 1.0 eV bellow the Fermi level, corresponding to the (1πg∗ )2 orbital of O2 , respectively. When the molecule is attracted step by step to the Ba(110) surface, from the PDOS(O(p)) presented in the Fig. 5, we can get that the gap between these to group of peaks decreases gradually from initial state to the intermedial state, which indicates that the O–O bond break gradually. In the final product, these two peaks have almost merged with each other and the O–O bond has broken thoroughly. One can notice clearly that during the dissociative process, the d and s states of Ba surface begin to mix with p electrons orbital of O2 . In the final products O2 has been dissociated and oxygen bind
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with Ba atoms based on p–d (p–s) hybridization bonds, which can be easily deduced from the DOS. In fact, the peaks by the Fermi level in the final panel for the total DOS can be regarded as comprising of one main-peak corresponding to the p–d hybridization orbital and one additional sub-peak due to the p–s hybridization orbital, respectively. However, as to the molecular chemisorption in the case of Fig. 2(i) (see Fig. 6), neither in the intermedial state nor in the final product state, can we get significant hybridization between the O2 and Ba(110) surface. Above electronic structure analysis shows that p–d (p–s) hybridization between the p orbital of oxygen with d (s) states of Ba may play an important role in the dissociative chemisorption of O2 with Ba(110) surface. The hybridization may also play a key role in the steering effect during the adsorption process. From the viewpoint of the electronic configuration of O2 , for the axis of the oxygen molecule, the more it parallel to the Ba(110) surface, the easier and the more sufficient for both the π orbital hybridizing with the Ba states and the charge transfer from the substrate to the π orbital of the oxygen, so the initial adsorption site of Fig. 2(g) is the most unfitted site. Though the axis of the O2 in both Fig. 2(e) and Fig. 2(f) are all parallel with the Ba(110) surface, the d electrons of Ba atoms are localized, so the oxygen molecule will adjust its orientation by atomic force to a fit site for hybridization with d states of the surface when it is placed in a ‘bad’ initial adsorption site, then, steering effect occurs. 4. Summary In summary, we have presented first-principles calculations for O2 adsorption on the Ba(110) surface. Our results support direct dissociative chemisorption mechanism. In previous literatures, O2 dissociation on transition metals, the dissociations 2− are generally due to O− 2 and O2 states in which the antibonding orbitals become occupied and thus first weakens the O–O bonds and finally break it. In this Letter, we find that there is only very small charge transfer from the substrate to the oxygen molecule. Through electronic structure analysis, we owe the dissociation mainly to the p–d and p–s hybridization interaction and secondly to the charge transfer from the surface to the antibonding orbital. The Ba(110) surface shows some prop-
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