Interaction of La2NiO4 (100) Surface with Oxygen Molecule: A First

Jun 3, 2013 - In this report, we present the structural and energetic results of O2 adsorbed onto the perfect and defective La2NiO4 (100) surface to e...
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Interaction of La2NiO4 (100) Surface with Oxygen Molecule: A FirstPrinciples Study Jun Zhou, Gang Chen,* Kai Wu, and Yonghong Cheng State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University, Xi’an 710049, People’s Republic of China S Supporting Information *

ABSTRACT: Along the way to designing of new cathode materials for solid oxide fuel cells (SOFCs), an understanding of the mechanism of oxygen reduction reaction (ORR) plays a key role, especially the interaction between O2 molecule and surface of cathode. Recently, La2NiO4 with K2NiF4-type structure has been developed, and it has received great attention as an oxygen sensor and a potential cathode for SOFCs. However, the chemical activity of La2NiO4, in particular, the ORR on the surface, has not been studied so thoroughly. In this report, we present the structural and energetic results of O2 adsorbed onto the perfect and defective La2NiO4 (100) surface to elucidate the interaction mechanism between O2 molecule and cathode using atomistic computer simulation based on density functional theory. The results show that the surface structure and the adsorbed configurations are vital for O2 adsorption. and activation. The adsorbed species on the perfect surface are energetically less favorable than defective surface. The Ni site is preferred with adsorption energy of −1.25 (Ni-super) and −1.80 eV (Ni-per), much higher than these of La site, supporting the fact that transition-metal cations are more active than lanthanon metals in K2NiF4-type compounds. Surface oxygen vacancy is found to enhance the adsorption energy of O2 molecule on the La2NiO4 (100) surface; in addition, oxygen vacancy can be an active site in O2 adsorption. The most stable configuration is Ni−O−Ni mode, with the highest adsorption energy being −2.61 eV. This can be confirmed by the analysis of the local density of states (LDOS) and the difference electron density. These results have an important implication for understanding the ORR on La2NiO4 (100) surface. gas-separation membranes.10,11 In general, the K2NiF4-type oxides are structurally related to the perovskite oxides and consist of ABO3 (perovskite) and AO (rock salt) layers alternating in the c direction.12 In these materials, especially in La2NiO4+δ, the crystal structure is built of alternating rocksalt La2O2 and perovskite NiO2 layers and can accommodate a significant oxygen excess.13−15 The extra O2− anions are chargecompensated by the p-type electronic charge carriers and occupy interstitial positions in the LaO bilayers, while the concentration of oxygen vacancies in the perovskite sheets is very low. The information available on defect formation and transport mechanisms relevant to the oxygen permeation and catalytic behavior of La2NiO4-based phases is scarce and often contradicting.16 Among many factors affecting the chemical-electrical energy conversion, the oxygen reduction reaction (ORR) on cathode is the pivot in fuel cell.17−19 The ORR is a kinetically slow process,20 which dominates the overall performance of a fuel cell. For the design of novel low-cost cathode materials with

1. INTRODUCTION To meet the fast-growing global energy demand, solid oxide fuel cells (SOFCs) are one of the promising strategies to produce clean electricity with high efficiency and fuel flexibility.1−3 The SOFC performance is to a large degree determined by the catalytic properties of the cathode.4,5 One of the main research targets in the field of SOFC is the attempt to reduce the operating temperature from high temperature (∼1000 °C) to intermediate-temperature (IT) (500 to 800 °C). A reduced-temperature operation for SOFC has several advantages, which include broadening the material choice, improving the long-term performance, and reducing fuel-cell costs.3 However, the lower operating temperature results in a significant decrease in catalytic activity of cathode. Therefore, for the further improvement of the fuel cell performance, the development of high-performance cathode material is critical. Current highly performing cathodes are based on perovskite structure (ABO3) and related structures (such as K2NiF4). Lanthanum nikelate, La2NiO4+δ with K2NiF4-type structure (see Figure 1), exhibits high mixed oxide ion and electron hole conductivity in combination with relatively fast surface exchange kinetics.6−9 Thus, it is an interesting material candidate for cathodes used in IT-SOFCs and for oxygen © XXXX American Chemical Society

Received: March 28, 2013 Revised: May 31, 2013

A

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k-points was used for the bulk unit cell, 3 × 2 × 1 k-points for the (100) surface. A relaxation is performed for the constructed slabs by using Broyden−Fletcher−Goldfarb−Shanno (BFGS) algorithm39 to minimize the energy with respect to atomic position. The tolerances for self-consistence are set at 1.0 × 10−6 eV/atom for total energy, 0.1 eV/Å for force, 0.2 GPa for maximum stress, 1.0 × 10−5 eV/atom for band energy, and 0.005 Å for the maximum displacement, respectively. Experimental surface energies, which are difficult to measure, are unavailable for La2NiO4. Previous work40 showed that the order of stability for the relaxed surfaces of undoped La2NiO4 is: {100} > {111} > {110} > {001} > {011}. Therefore, only the (100) surface of La2NiO4 was considered in this work. The (100) surface has the same termination, which includes all types of atoms (La, Ni, and O). To avoid the macroscopic dipole moment perpendicular to the polar surface, we used symmetrical slabs terminated on both sides in the same way. For 3D periodic boundary conditions, the metal oxide surfaces comprising the atomic layers were separated by a vacuum space equivalent of 10 Å in the direction perpendicular at the built surface. In all calculations, the atoms in the bottom layers were fixed, but the atoms in the three topmost layers were allowed to relax. Convergence with respect to k-point sampling, kinetic energy cutoff, and slab thickness was tested and found to be satisfactory. The geometry of the isolated O2 molecule was optimized using a large cubic cell of 8 Å × 8 Å × 8 Å, similar to previous study.41 The predicted O−O bond length was equal to 1.24 Å and was in good agreement with the experimental values of 1.21 Å. The calculated binding energy of free O2 molecule is 6.52 eV. After optimization, the LDOS and the difference electron density analyses were performed. These analyses were used to help to understand the mechanism of the bonding and the interaction between O2 and the La2NiO4 surfaces. In this work, the adsorption energies (Eads) were calculated via the following equation

Figure 1. Crystal structure of bulk La2NiO4.

high catalytic activity for oxygen reduction in IT-SOFCs, it is very vital to understand the complicated mechanism of ORR. Some works21−23 suggest that ORR at the surface of a mixed ionic−electronic conductor (MIEC) cathode consists of many elementary steps, which include adsorption of a superoxo(O2−) or peroxo-like (O22−) species and dissociation of diatomic oxygen species into the bulk lattice. Although numerous theoretical investigations on perovskite structures (ABO3) have been done,24−31 detailed mechanistic studies of ORR on the surface of La2NiO4 cathode by means of quantum chemical calculations are still lacking. We report the oxygen interaction mechanisms on La2NiO4 surfaces using first-principles calculations based on DFT and pseudopotential method. The structural sensitivity of La2NiO4 was studied by examining the adsorption of O2 molecule on perfect and defective (oxygen vacancy) La2NiO4 (100) surface. It was shown that La2NiO4 surface is indeed active for O2 activation, and this reaction is structure-sensitive. A variety possible binding configurations of O2 on the perfect and defective La2NiO4 (100) surface in terms of geometries, energies, and electronic structures were investigated. The adsorbing mechanisms of O2 on these surfaces were also discussed with examination of the local density of states (LDOS) and the difference electron density. We anticipate that results from the study of O2 adsorption on the perfect and defective La2NiO4 (100) surface will allow us to understand in great detail the chemical activity of the La2NiO4 surfaces.

±Eads = Esub − ad − Esub − Ead

(1)

where Esub and Ead are the total energy of substrate and adsorbate and Esubad is the total energy of the subad system in the equilibrium state, respectively. According to the above equation, a negative Eads value corresponds to an exothermic adsorption, and the more negative the Eads, the stronger the adsorption. Here a coverage of 10.5% was used for the oxygen molecule adsorbed on the perfect and defective La2NiO4 (100) surface. To estimate the vacancy formation energy of the oxygen vacancy in the crystal, we used the following expression EO − vac(F) = E(F) + 1/2E(O2 ) − E(perfect)

(2)

where E(O2) is the predicted electronic energy of triplet O2 in a 8 Å cubic box. E(F) and E(perfect) are energies of the defective and perfect crystal, respectively.

2. COMPUTATIONAL DETAILS AND SURFACE CONFIGURATIONS All calculations were carried out by periodic density functional theory (DFT) with the projector-augmented wave (PAW) method,32 as implemented in the CASTEP.33,34 The correlation interactions were described using the generalized gradient approximation (GGA)35 proposed by Perdew and Wang (PW91).36 Electronic wave functions were expanded in a plane-wave basis set, and ionic cores were described by ultrasoft pseudopotentials.37 Here a 340 eV plane-wave basis set the cutoff, and the cutoff energy used the whole calculations to ensure convergence. The Monkhorst-Pack38 grids of 3 × 3 × 1

3. RESULTS AND DISCUSSION 3.1. Bulk Properties. The stoichiometric nicklate La2NiO4 belongs to the Ruddlesden−Popper series with general formula An+1MnO3n+1 (n = 1, 2, 3, ...). In such a structure, n AMO3 perovskite layers alternate with AO rocksalt-type layer along the c crystallographic direction.42 As for the perovskite structure, the Goldschmidt tolerance factor43 can be used to account for the effects of the size mismatch between anions and cations in the A2MO4 structure. It is written as B

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vacancy sites of NiO6 octahedral were considered here, and they are a vacancy on the equatorial site (VO1) and a vacancy on the apical site (VO2), respectively. Their calculated formation energies are 3.73 and 4.01 eV, which are similar to other K2NiF4 structural material.40 However, the oxygen vacancy formation energies for bulk La2NiO4 are larger than those of perovskite-type cathodes-based LaMnO3.25−27 3.2. Structure of La2NiO4 (100) Surface. The surface energies and structures of the La2NiO4 (100) surface were first investigated using slabs of four (3.769 Å × 13.886 Å × 15.654 Å), six (3.769 Å × 13.886 Å × 19.423 Å), and eight (3.769 Å × 13.886 Å × 23.541 Å) atomic layers (Figure 2), maintaining the stoichiometric balance. The surface energy was calculated using the equation S = (ENslab − NEbulk)/2A,where A is the area of the surface, ENslab is the total energy of the surface slabs, N is the number of La2NiO4 units in the cell, and Ebulk is the energy per stoichiometric unit of the bulk.46 In Table 2, we report results for slab modes consisting of four, six, and eight layers, respectively. The predicted surface energies of various layers are in agreement with Read’s research (0.98 J/m2) of (100) surface.39 It is found that the surface energies of (100) converged to within 0.12 to 0.14 J/m2, suggesting that four layers for (100) surface is thick enough in calculated modes. Thus, to save computational time, we chose only a four-layer slab to study O2 adsorption in this work. To determine the surface relaxation of the La2NiO4 (100) surface, we optimized the clean surface, and no reconstruction was observed. (See the Supporting Information.) In general, only the uppermost one or two atomic layers exhibit the slight relaxation, indicating that our optimized modes can be used for further research. The oxygen vacancies are vital for the ORR in MIECs cathodes as they are likely to couple strongly to oxygen adsorption, dissociation, and transportation. To study the effect of oxygen vacancies on the O2−cathode interactions on the La2NiO4 (100) surface with different layers modes, we located an oxygen vacancy on the top layers. There are two different kinds of oxygen vacancy (VO1 and VO2). In the present work, we calculated the formation energies of the oxygen vacancies based on the reaction of La2NiO4 (bulk and surface) → La2NiO4−δ + 1/2O2(g). The calculated oxygen vacancy

Table 1. Calculated and Experimental Lattice Parameters of La2NiO4 computing methods

a = b (Å)

c (Å)

α = β = γ (deg)

V (Å3)

PW91 PBE rPBE experimenta

3.769 3.775 3.975 3.869

13.88 13.89 13.08 12.60

90 90 90 90

197.30 198.12 206.51 188.61

a

See ref 45.

t=

rA + r0 2 (rM + rO)

(3)

where rA, rM, and rO are the ionic radii of the An+, Mm+, and O2− ions, respectively. t = 1 corresponds to an ideal radii match. For La2NiO4, t = 0.89,44 indicating that the NiO2 plane is under pressure as a result of the stretched La2O2 layer. This strain may influence the oxygen vacancy formation of either NiO6 octahedral or La2O2 layers. Thus, two different oxygen vacancy sites were considered in this work. In our computing configurations, we calculated the formation energy of an oxygen vacancy based on the reaction of La2NiO4 → La2NiO4−δ + (1/2)δO2(g). To test the accuracy of our approach, we compared the optimized unit cell parameters obtained using PW91 with the experimental data and other GGA methods (PBE and rPBE). As shown in Table 1, the predicted unit cell parameters at PW91 level are a = b = 3.769 Å, c = 13.88 Å, and α = β = γ = 90°, which is in agreement with the experimental data42 more closely than the PBE and rPBE levels. For this reason, in the following sections, only GGA-PW91 energies are used in the discussion. The migration of the interstitial oxygen (OI) in La2NiO4 has been by far the most investigated because of the intrinsic features governing ionic conductivity for a direct interstitial mechanism of oxygen ions (OI).45 However, the oxygen vacancies are important for the ORR in SOFCs as they are likely to couple strongly to oxygen dissociation, transportation, and incorporation. Meanwhile, an oxygen vacancy has two effective positive charges. For bulk, two kinds of oxygen

Figure 2. Side (a) and top (b) views of the La2NiO4 (100) surface. (Green circle denotes an oxygen vacancy.) C

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Table 2. Surface Energy and Oxygen Vacancy Formation Energy of Various La2NiO4 (100) Layersa

(100) surface plane a

layers

surface energy (J/m2)

EVO1 (eV)

EVO2 (eV)

4 6 8

0.37 0.49 0.51

8.89 8.72 8.75

5.92 6.04 6.11

EVO1 and EVO2 represent the oxygen vacancy formation energy of the La2NiO4 (100) surface.

Figure 3. Optimized adsorption configurations of O2 molecule at the perfect or defective (oxygen vacancy) La2NiO4 (100) surfaces.

formation energies for the La2NiO4 (100) surface with different layers are listed in Table 2. However, the EVO2 of all layers are larger than EVO1 for the La2NiO4 (100) surface, totally different

from the oxygen vacancy formation energy of La2NiO4 bulk structure. Meanwhile, Minervini et al.47 reported that several detailed mechanisms of charged defects including oxygen D

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atoms and oxygen vacancy site. (4) O2 interacts with both La and Ni sites, respectively. The optimized stable configurations are shown in Figure 3. The adsorption of O2 on the stoichiometric La2NiO4 (100) surface was investigated on two different adsorption sites: the first-layer La atoms and Ni atoms. For clarity, the two oxygen atoms of the adsorbed O2 were labeled as O1 (shorter bond length for O−X) and O2 (longer bond length for O−X) if they were not in equivalent positions. When O2 molecule is adsorbed on the stoichiometric perfect La2NiO4 (100) surface, the adsorption energy is a criterion to determine the stability of the adsorption. The oxygen sites were shown to be much less reactive with the respect to O2 molecule adsorption, and only super configuration was considered. The calculated results of adsorption energy (Eads), the equilibrium distance (dO‑X), and the O−O bond length (dO−O) are shown in Table 3. Calculated data show that the adsorption energy of La-super is −0.49 eV, which is smaller than that of La-per (−0.71 eV); meanwhile, the adsorption energy of Ni-super (−1.25 eV) is bigger than that of Ni-per (−1.80 eV). This is to say that per-mode has the bigger adsorption energy than the supermode including all of the atom sites, indicating that when O2 molecule is adsorbed on the stoichiometric La2NiO4 (100) surface the per-configuration is more stable. Moreover, for all adsorption configurations we studied, the Ni-per mode has the highest adsorption energy.

Table 3. Properties of the O2 Adsorption on the La2NiO4 (100) Surface configuration

ΔEads (eV)

d(O1−X) (Å)

d(O1−O2) (Å)

O2 La-super La-per Ni-super Ni-per O-super O-per Ni−O−La Ni−O−Ni Ov-super

−0.49 −0.71 −1.25 −1.80 0.60 0.87 −2.35 −2.61 −1.37

2.767 2.793 1.841 2.004 2.506 2.599 1.677 1.698 1.924

1.214 1.275 1.279 1.281 1.327 1.272 1.274 3.689 2.929 1.435

vacancy and oxygen interstitial were predicted. Only one type of oxygen vacancy (8e site) in bulk was considered in their study. Here only VO2 was considered in our calculations for defective configurations. 3.3. O2 Adsorption on Perfect La2NiO4 (100) Surface. In general, for O2 adsorption on metal oxide surface, there are several adsorbed configurations. On the basis of various bond interactions, modes can be obtained: (1) O2 only binds X (La or Ni) sites as superoxo-like species. (2) O2 forms a bond with X sites as peroxo-like species. (3) O2 interacts with oxygen

Figure 4. Local density of states (LDOS) for the adsorption of O2 on the prefect La2NiO4 (100) surface with various configurations. E

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Figure 5. Side views of the difference electron density (a for Ni-per, c for Ni-super) and the difference electron density maps (b for (010) plane of Ni-super mode, d for the (100) plane of Ni-per mode) highlighting the electron charge density redistribution due to the O2 adsorption.

length of the O−O bond is 1.327 Å. Compared with the O−O bond length of the free O2 is 1.214 Å, it is in agreement with the experimental value of about 1.207 Å.48 However, they are elongated after adsorption on the stoichiometric La2NiO4 (100) surface for both configurations (Ni-super and Ni-per). This phenomenon is related to the charge transfer between the O2 and the absorbed clusters, leading to the elongation of the bond.49 Such charge transfer is confirmed by the analysis of the electronic structure in the following parts. We also calculated the density of states (DOS) for the isolated O2 molecule (free O2) and for oxygen species absorbed on the more stable adsorption site on the Ni atom (Ni-per and Ni-super). Figure 4 shows the DOS of the Ni atom before and after adsorption with Ni-super and Ni-per configurations. After oxygen molecule adsorption, the d, p, and s orbits of Ni atom exhibit significant changes around the Fermi level for both of Ni-per and Ni-super configurations. The d orbits of Ni atom in these configurations split more orbits near the Fermi level. The intensity of p orbits of Ni atom decreases in the energy region of 0−5 eV but becomes stronger in the lower energy levels. Interestingly, the s, p, and d orbits of the Ni atom all participate in the bonding of the Ni−O. In addition, the overlap of DOS peaks of Ni-d and O-p orbit depicts a stronger hybridization between them, obviously illustrating that the d orbit of the Ni atom plays the main role for the bonding of the Ni-super and Ni-per configurations. We can also analyze the adsorption energies for adsorbed oxygen from electronic effects point of view. The hybridization can be seen more clearly in the cases where the oxygen is in the Ni-per configuration. What makes the Ni-per mode more favorable than the Ni-super mode is the fact that there is more occupation in the lower bonding state of the DOS for the Ni-per mode. Ni-per mode also seems to be more stable with a lower value in energy. Figure 5 shows the difference electron density maps due to the adsorption of O2 on La2NiO4 (100) surface with respect to

Figure 6. Local density of states (LDOS) for the adsorption of O2 on the defective La2NiO4 (100) surface with various configurations (a) Ni−O−La, (b) Ni−O−Ni, and (c) Ov-super.

This shows that the Ni site is the preferential adsorption site for stoichiometric La2NiO4 (100) surface, consistent with other similar prediction results in some cathodes with perovskite structures.26,27,40 Interestingly, for O-per and O-super configurations, the positive value of adsorption energy calculated from the eq 1 showed that it is difficult for oxygen molecule adsorbed on oxygen site. For the Ni-super configuration, the calculated equilibrium distance (d(O1−La)) between adsorbed O2 and the La site is 1.841 Å, which is short enough to form the Ni−O bond. The O−O bond length (d(O1−O2)) of the absorbed O2 is found to be 1.281 Å. However, the bond length of Ni−O elongated to 2.004 Å (d (O1−Ni)) for La-per configuration, while the bond F

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Figure 7. Side views of the difference electron density and the difference electron density highlighting the electron charge density redistribution in (a,b) the Ni−O−Ni mode, (c,d) the Ni−O−La mode, and (e,f) the Ov-super mode, respectively.

with oxygen vacancy. In the case of one atomic O adsorbed on an oxygen vacancy and another atomic O binding to Ni site, we found that configurations shown in Figure 3g (Ni−O−La) and Figure 3h (Ni−O−Ni) are much more active. For these configurations, the adsorption energy increased distinctly to the range from −2.35 to −2.61 eV, both larger than those of other configurations in this work. Compared with the perfectconfigurations, the Ni−O bond lengths (dNi−O1) of Ni−O− La and Ni−O−Ni configurations decreased to 1.677 and 1.698 Å after adsorption, but the O−O bond lengths distinctly elongated to 3.689 and 2.929 Å. It also proved that the Ni−O− La and Ni−O−Ni modes are more stable. Moreover, this phenomenon is associated with the charge transfer between the O2 molecule and the absorbate. This charge transfer weakens the O−O bond by lengthening its bond in the process of adsorption. For all adsorption configurations, the Ni−O−Ni has the highest adsorption energy and smallest dNi−O1, showing that this adsorbed configuration is most stable. Furthermore, the adsorption mechanism was examined by local densities of states (LDOS) calculations for the adsorbed oxygen species on an oxygen vacancy and another atomic O binding to Ni site, including Ni−O−La, Ni−O−Ni, and Ovsuper configurations. As shown in Figure 6, after adsorption, the d orbits of Ni atom and the p orbits of O atom show significant hybridization for these configurations. However, a

the isolated molecule and clean surface. It clearly exhibits that the electron density increases between Ni atom and oxygen molecule in Ni-per and Ni-super configurations. Meanwhile, some electron density depletion is visible near the Ni atom, which oxygen molecule adsorbed on. The density isosurfaces clearly show charge transfer from Ni atom into the adsorbed oxygen species, with charge transfer in Ni-super configuration having a lesser extent into the oxygen than that in Ni-per. It shows that the more the charge transfer the more the O−O bond with lengthening the bond (Table 3). Such structural changes illustrate that these molecular states are likely precursors for oxygen molecule dissociation. 3.4. O2 Adsorption on Defective La2NiO4 (100) Surface. In this section, we studied O2 molecule adsorption on the defective La2NiO4 (100) surface with an oxygen vacancy (VO2). To study the defective (VO2) surface, one oxygen atom of the outermost surface is removed from the supercell configuration, which is shown in Figure 3. In this Figure, the location of VO2 is indicated by a green cycle. From our calculated results, the adsorption energy of the defective-Osuper configurations (Ov-super) is −1.37 eV, which is bigger than that of perfect La-super (−0.49 eV), La-per (−0.71 eV), and Ni-super (−1.25 eV). This result shows that oxygen vacancies are also active sites on the surface of metal oxides. Thus, in the following, we consider O2 adsorption at a surface G

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Notes

stronger hybridization can be found in Ni−O−Ni configuration, indicating that this configuration is more stable, in agreement with the adsorption energy analysis. This is also investigated by plotting the difference electron density and exhibiting at the regions of charge accumulation and depletion upon adsorption, as shown in Figure 7. Similarly, we can see that there is a distinct charge accumulation between the species and the Ni atoms in these configurations. It confirms the presence of a strong covalent bond between O2 and the surface. Atomic oxygen adsorption causes significant electron redistribution. In addition, an obvious charge accumulation occurred on the Ni−O−Ni configuration.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This project was financially supported by the Fundamental Research Funds for the Central Universities and the State Key Laboratory of Electrical Insulation and Power Equipment, Xi’an Jiaotong University.

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4. CONCLUSIONS For the understanding of the mechanism of ORR, periodic DFT calculations have been performed to study the oxygen molecule adsorption on the La2NiO4 (100) surface. We have considered the interaction of O2 with stoichiometric perfect and containing oxygen vacancy of La2NiO4 (100) surface. The surface structure, the vacancy formation energy, the most stable adsorption configuration, and the adsorption energy as well as the LDOS and the difference electron density for the interaction of O2 on the perfect- and defective-La2NiO4 (100) surface were systematically investigated. Compared with the oxygen vacancy in O1 site (VO1), the oxygen vacancy in O2 site (VO2) was created much more easily in La2NiO4 (100) surface by predicted the oxygen vacancy formation energy. For the perfect (100) surface, the most favorable oxygen adsorption sites are observed to be atop surface Ni atoms (Ni-super and Ni-per configurations), with adsorption energies being −1.25 and −1.80 eV, respectively. For defective La2NiO4 (100) surface, surface oxygen vacancy was found to play an important role and can significantly enhance the adsorption energy of O2 molecule with the metal-oxide’s surface: the intermediates on the defective surface are energetically more favorable than perfect surface for all configurations. Moreover, oxygen vacancies are also active sites on the surface of La2NiO4. The highest adsorption energy (−2.61 eV) can be obtained in the Ni−O−Ni configuration. Analysis of the LDOS and the difference electron density maps for all adsorbed O2 on La2NiO4 (100) surfaces shows that O2 accepted electrons from the surface, especially Ni atoms. This is also confirmed from the strong hybridization between Ni (3d) and O (2p) and shorter Ni−O bond length. We may conclude that our results demonstrate that the surface structure of La2NiO4 is vital for O2 adsorption. Oxygen vacancy not only was observed to distinctly modify the adsorption energies of O2 on La2NiO4 (100) surface but also participated to be an active site in O2 reduction reaction. This result is useful for further research of new cathode design in SOFC and gas sensor.

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ASSOCIATED CONTENT

S Supporting Information *

Ion displacement for the top layer of the La2NiO4 (100) surface. This material is available free of charge via the Internet at http://pubs.acs.org.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +86 029 82668493. Fax: +86 029 82668493. H

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