Tuning the Spin State in LaCoO3 Thin Films for

Letter pubs.acs.org/JPCL

Tuning the Spin State in LaCoO3 Thin Films for Enhanced HighTemperature Oxygen Electrocatalysis Wesley T. Hong,†,‡ Milind Gadre,#,‡ Yueh-Lin Lee,‡ Michael D. Biegalski,∥ Hans M. Christen,∥ Dane Morgan,# and Yang Shao-Horn*,†,‡,§ †

Department of Materials Science & Engineering, ‡Electrochemical Energy Laboratory, and §Mechanical Engineering Department, Massachusetts Institute of Technology, 31-056, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States # Department of Materials Science & Engineering, University of WisconsinMadison, 1509 University Avenue, Madison, Wisconsin 53706, United States ∥ Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: The slow kinetics of oxygen surface exchange hinders the efficiency of high-temperature oxygen electrocatalytic devices such as solid oxide fuel cells and oxygen separation membranes. Systematic investigations of material properties that link to catalytic activity can aid in the rational design of highly active cathode materials. Here, we explore LaCoO3 thin films as a model system for tuning catalytic activity through strain-induced changes in the Co spin state. We demonstrate that Raman spectroscopy can be used to probe the Co−O bond strength at different temperatures to determine the relative spin occupancies of LaCoO3. We find that strain can be used to reduce the spin transition temperature and promote the occupation of higher spin states that weaken the Co−O bond. The decrease in Co−O bond strength and increased spin moment of the thin films result in significant enhancements of the oxygen surface exchange kinetics by up to 2 orders of magnitude.

SECTION: Surfaces, Interfaces, Porous Materials, and Catalysis

C

Sm0.5Sr0.5CoO3−δ < PrBaCo2O5+δ < Ba0.5Sr0.5Co0.8Fe0.2O3−δ).7 However, such substitutions make it difficult to develop specific design principles for the optimal transition-metal electronic structure due to convolution of changes in both the oxidation state and the spin state.8−11 On a deeper level, the O p-band center is postulated to correlate with oxygen surface exchange because it acts as a descriptor for the energy of vacancy formation, and it was found that an increase in the O p-band center (catalytic activity) could be correlated with decreased energy of vacancy formation.7 This suggests that an alternative design criterion for SOFC cathodes is to simply decrease the oxide’s metal−oxygen bond strength, as proposed by Pavone et al.12 In this Letter, we explore methods for selectively tuning the metal−oxygen bond strength while preserving the oxide elemental chemistry in order to develop more detailed design principles for high-temperature oxygen electrocatalysis. The mechanical strain imposed in oxide thin films by heteroepitaxy is a well-known tool for accessing material states that would not otherwise be achievable in bulk.13 In particular, LaCoO3 (LCO) has demonstrated a rich and diverse physics in thin films due to the flexibility of the Co ion. In addition to changes in oxidation state, cobalt perovskites have demon-

atalysis plays a pivotal role in a number of prospective renewable energy conversion and storage technologies. The development of efficient and cost-effective catalysts for use in oxygen electrochemical energy conversion processes, including the oxygen evolution reaction (OER) and oxygen reduction reaction (ORR), is critical to the realization of such devices.1−4 At elevated temperatures, perovskite oxides (ABO3) have catalytic activities for oxygen electrocatalysis competitive with platinum,5 which makes them promising candidates for high-temperature (800−1100 K) clean energy technologies such as solid oxide fuel cells (SOFCs) and oxygen separation membranes (OSMs). However, slow oxygen surface exchange at the oxide surface has limited the conversion efficiency in SOFCs and high oxygen fluxes in OSMs.6 Moreover, the lack of fundamental understanding of the mechanism for oxygen surface exchange and the material properties that govern catalytic activity hinders the development of highly active catalysts. Density functional theory (DFT) studies by Lee et al.7 found that oxygen surface exchange is strongly correlated to the oxygen (O) p-band center of perovskites. In bulk, the O p-band center can be tuned through chemical substitution of the A-site cation, which influences the electronic configuration of the Bsite transition metal. For example, chemical substitution of La in LaCoO3 by Sr and Ba can increase the O p-band center, as well as the catalytic activity (LaCoO3 < La0.8Sr0.2CoO3−δ < © XXXX American Chemical Society

Received: June 19, 2013 Accepted: July 15, 2013

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Figure 1. LCO thin film structural characterization by XRD and AFM. (a) XRD θ−2θ scans (logarithmic intensity scale) for the 110, 70, and 45 nm LCO thin films and (b) their constrained lattice parameters and volumetric strains. The dashed line in (b) designates the bulk lattice constant.9 (c) AFM images and the root-mean-square roughness of the LCO film surfaces.

strated low energetic barriers between spin states,9 and consequently, the nature of the Co spin configuration at room temperature still remains controversial.14−18 Recently, several authors have demonstrated that strain can be used to tune the magnetic properties of LCO in thin films by increasing the fractional occupancy of higher spin states in the ground state.19−22 For thermally activated spin transitions, an increase in spin state results in an increase in electron occupancy of the Co 3d−O 2p σ* band (Figure S1, Supporting Information), which is expected to weaken the metal−oxygen bond. This effect has also been shown by first-principle methods.18 However, few studies have been done on the change in metal−oxygen bonding accompanied by strain-induced spin transitions. On the basis of DFT calculations, Kushima et al.23 argued that the magnetoelastic effect of a low spin (LS)− intermediate spin (IS) transition may actually strengthen the Co−O bond due to a change in the symmetry of the charge density distribution around the Co ion and an increase in the Co−O hybridization. This would suggest that strain-induced spin transitions are somehow fundamentally different from thermally activated transitions; however, this has yet to be experimentally confirmed. These computational efforts motivate an investigation of how spin state can influence catalytic activity through its effect on the metal−oxygen bond strength. To this end, we conducted a systematic study of strained LCO thin films to shed light on the relationships between oxygen surface exchange, metal−oxygen bond strength, and spin state. LCO thin films were grown by pulsed laser deposition on single-crystal (001)-oriented Y2O3/ZrO2 (YSZ), which acted as both the film substrate and electrolyte. Although other substrates (e.g., LaAlO3, SrTiO3) can be used to access different strain states,20,22 only YSZ uniquely satisfies both the growth requirements (i.e., lattice mismatch) and electrochemical requirements (i.e., ionic conductor, electronic insulator). A ∼10 nm Gd2O3/CeO2 (GDC) film was deposited as an electrolytic buffer layer between the LCO and YSZ to prevent La2Zr2O7 decomposition reactions.24 The LCO films were grown at thicknesses of 45, 70, and 110 nm. The films were systematically characterized for quality using X-ray diffraction (XRD) and atomic force microscopy (AFM). XRD results (Figure 1a) confirmed the well-defined orientation of the GDC and LCO layers. Both the GDC and LCO films grow in the (001)/(001)pc orientation (where “pc” designates the pseudocubic notation). Phi scans showed that the LCO grows with a 45° in-plane rotation relative to the

GDC/YSZ substrate (Figure S3, Supporting Information), which is consistent with similarly prepared La1−xSrxCoO3−δ (LSCO) thin films on GDC/YSZ.25,26 The films are under inplane tensile chemical and mechanical strain at room temperature (apc, Figure 1b), resulting in a large volumetric strain of the unit cell relative to bulk (εv ≈ 1.6%). Interestingly, the unit cell dimensions of LCO grown at different thicknesses have negligible change as a function of thickness. In addition to the well-defined surface orientation, AFM images confirmed low surface roughness for all of the films, with root-meansquared roughness < 0.8 nm (Figure 1c). Additional details about the films can be found in the Supporting Information. Electrochemical impedance spectroscopy (EIS) measurements were conducted to probe the oxygen surface exchange kinetics of the films using geometrically well-defined microelectrodes. Details for the photolithographic fabrication of the microelectrodes and calculation of the electrical oxygen surface exchange coefficient, kq, and oxygen nonstoichiometry, δ, from EIS measurements can be found in the Supporting Information and previous studies.25−29 The electrical oxygen surface exchange coefficients (kq) for the different films were measured as a function of oxygen partial pressure at 800 K (Figure 2a). The thin films demonstrated up

Figure 2. LCO thin film electrochemical characterization by EIS at T = 800 K. (a) Electrical oxygen surface exchange coefficients, kq, at various partial pressures of oxygen, compared to bulk k* values extrapolated from Ishigaki et al.30 and Berenov et al.31 (b) Oxygen nonstoichiometry, δ, compared to bulk values reported by Mizusaki et al.32. 2494



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Figure 3. Raman spectra at various temperatures for the (a) 110 nm LCO thin film, (b) 70 nm LCO thin film, and (c) LCO polycrystalline pellet. Room temperature is given in black (300 K), temperatures below room temperature in blue (225 K, 275 K), and temperatures above room temperature in red/orange (325, 375, 425, 475, 525, 575, 675, and 800 K).

A comparison of spectra at some of the sampled temperatures for the LCO thin films and the polycrystalline ceramic pellet is provided in Figure 3. Minor differences between the 110 and 70 nm films are attributable to the additional contribution of the GDC/YSZ substrate spectrum (Figure S7, Supporting Information). The normal vibrational modes associated with the different Raman bands have been studied previously by polarized Raman spectroscopy on LCO single crystals.36 The lowest-energy Raman band is associated with the Eg La stretching vibration (peak 1), which shows minimal temperature dependence. In contrast, the various CoO6 vibrations are more sensitive to temperature. The two lowerenergy bands are due to the Eg bending (peak 2) and Eg quadrupole (peak 3) vibrations and share similar behavior with temperature, decreasing in spectral intensity and vanishing near 400 K. The highest-energy feature has been attributed to the A2g breathing mode of the oxygen ion cage (peak 4), which can have large scattering intensity due to strong electron−phonon interactions despite being symmetry-unallowed.36−38 As the temperature increases, this peak softens to lower energies, which will be discussed in greater detail later. While the thin films and bulk reference sample have similar features at room temperature and below, they deviate greatly at higher temperatures. In particular, the CoO6 vibrational modes in bulk become quenched and are no longer Raman-active above 400 K. At temperatures above 375 K, the bulk sample also introduces two new modes (200 and 400 cm−1, marked by stars) that are absent in the thin films. Peaks at these energies have been observed previously in LCO single crystals (albeit at much lower temperature) and were attributed to a break in symmetry selection rules due to the mixing of spin states.36 However, these changes in the bulk spectra and the difference in selection rules between the bulk and thin film are beyond the scope of this Letter and merit more detailed study (some additional discussion is provided in the Supporting Information). Instead, we focus on the differences in the vibrational energies of the thin films and bulk for the more definitively assigned La stretching (peak 1) and CoO6 breathing (peak 4) modes. The vibrational energies associated with these two modes are shown as a function of temperature in Figure 4. In addition to the polycrystalline (purple) and the 110 nm thin film (red) data measured in this study, data from the literature performed on single-crystal LCO36 is also provided (gray). The 70 nm film

to 2 orders of magnitude enhancement in activity compared to self-surface exchange rates (k*) extrapolated from bulk singlecrystalline30 and polycrystalline31 samples (kq and k* are assumed to be comparable for these systems33). It is interesting to note that the films demonstrate little thickness dependence in surface exchange kinetics, similar to the previously mentioned absence of thickness dependence in strain. Insight regarding the mechanism for the reaction can be obtained by examining the linear free-energy relationship, log kq ∝ log pO2. The slopes for the different thicknesses are roughly identical given the spread in the data, ranging from 0.29 to 0.57. This range is distinct from what we have observed previously in LSCO thin films, which have slopes in the range of 0.63− 0.89.25 This suggests that LCO thin films are mechanistically limited by a different step from LSCO. The lower slope has been previously attributed to a rate-determining step associated with atomic oxygen after dissociative adsorption, such as oxygen incorporation.34,35 The oxygen nonstoichiometry of the thin films is shown in Figure 2b. All of the films have roughly identical nonstoichiometry and are relatively insensitive to the partial pressure of oxygen. The thin films demonstrate larger nonstoichiometry relative to bulk samples reported in the literature.32 However, the low partial pressure dependence suggests that vacancy concentration is not the primary contributor to the enhanced oxygen surface exchange rate, which shows ∼2 orders of magnitude variation across the measured range of partial pressures. The low vacancy concentration in these films (on the order of 0.1%) also suggests that the cobalt oxidation state remains relatively unchanged and thus is unlikely to be a significant factor in the enhanced surface exchange kinetics. As mentioned earlier, the oxygen surface exchange coefficient has been linked to bulk material descriptions of the metal− oxygen bond strength. To probe this, we utilize Raman spectroscopy to gain insight into how strain influences the Co− O vibrational energies and consequently the Co−O bond strength. We measured Raman spectra for the 110 and 70 nm thin films. At 70 nm, the GDC/YSZ substrate begins to show small contributions to the spectral intensity, which prevents probing to thinner film thicknesses. A sintered polycrystalline ceramic pellet was used as a bulk reference sample. Spectra were collected at various temperatures ranging from 175 K to our operating temperature of 800 K under ambient air. 2495



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range of temperatures studied (5−800 K when including the single-crystal reference data) is expected to include both LCO spin state transitions (∼100 and ∼500−600 K in bulk).9 It is apparent that the bond strength only decreases with increasing temperature in this system. In particular, a large softening of the CoO6 breathing energy was observed near 400 K, which is indicative of a thermally activated or second-order phase transition. We exclude the possibility of a vacancy ordered− disordered transition due to the low vacancy concentrations observed from EIS (∼10−3). Thus, it is expected that this transition feature is associated with the high-temperature spin state transition observed in LCO. In order to confirm this, we utilized a spin transition model developed by Bari and Sivardière, which can semiquantitatively describe the spin transition energies (expressed as a temperature) assuming a LS ground state.39 SQUID magnetometry confirmed that the films show no ferromagnetic transition at low temperatures, supporting application of the model (Figure S6, Supporting Information). The two most popular spin transition theories for LCO in the literature include a threestate low−intermediate−high spin (LS−IS−HS) system14−16 and a two-state low−high spin system with a spin-ordered state at intermediate temperatures.17,18 The three-state LS−IS−HS model was used to fit spin transition temperatures to the data and is provided here in the main text.40,41 The two-state transition with spin ordering was also modeled39 but was found to yield poorer fits and unphysical results (Figure S8, Supporting Information). The model defines the spin fractional occupancies for a given set of transition temperatures. To fit the Raman data, we assumed that the expected value for the breathing mode vibrational energy, ⟨EA2g⟩, could be described by an ensemble of noninteracting CoO6 vibrations with each spin state contributing a distinct breathing vibrational energy, ELS,IS,HS (taken to be identical for the thin film and bulk). The expected value is simply the sum of these individual vibrational contributions weighted by the spin fractional occupancies, ⟨nLS,IS,HS⟩ (eq 1).

Figure 4. Raman shift as a function of temperature for (a) the La Eg stretching mode and (b) the CoO6 A2g breathing mode. Open circles denote the bulk, while filled circles denote thin films. Single-crystal data reported previously in literature36 are also included for comparison.

⟨E A 2g ⟩ = E LS⟨nLS⟩ + E IS⟨nIS⟩ + E HS⟨nHS⟩

(orange) is included to demonstrate the similarity in the vibrational energy of the two films; however, it should be cautioned that due to the substrate contribution in the Raman spectra, the quantitative precision of the 70 nm data is less reliable. The three data sets show good quantitative agreement in the La Raman shifts, and the small degree of temperature dependence can be attributed to thermal expansion of the lattice. This behavior is in contrast with the CoO6 breathing mode energies. The two bulk sample data sets are again in agreement; however, the thin films have lower vibrational energies by comparison. This trend is similar to that observed in the surface exchange coefficient, for which single-crystal and polycrystalline samples behaved comparably while the thin films had much higher activity. The breathing mode frequency scales roughly with bond strength, and the relative Co−O strength for the bulk and thin films can be estimated by assuming harmonic vibrational behavior in a Lennard-Jones potential (see the Supporting Information for details). The thin films are found to have ∼7−11% reduction in Co−O bond energy at 300 K, which may explain the enhanced oxygen surface exchange kinetics. In addition to the reduction in metal−oxygen bond strength, the temperature profile of the breathing mode vibrational energy yields valuable insight regarding the LCO system. The

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An iterative method can then be employed to identify the IS and HS transition temperatures that yield a temperature profile for the breathing mode vibrational energy that most accurately agrees with the experimental data. The single-crystal reference data and polycrystalline data were treated together as a single data set to provide a wider range of temperatures for fitting. Only the 110 nm thin film was fitted due to the contribution of the substrate in the 70 nm film spectra. Details regarding both models, fitting methods, and fitting parameters can be found in the Supporting Information. The model yields excellent fits for both the bulk and thin film data, each with r2 ≈ 0.96 (Figure 5a). The fitted spin transition energies (ΔIS and ΔHS) in the thin film are significantly lower than those observed in the bulk by ∼300 K. It should be noted that these transition energies describe transition features in the fractional spin occupancy and are thus not reflected in any macroscopic measurement associated with the ensemble. In order to compare the validity of our transition temperatures with those reported in the literature from measurements such as electrical conductivity,9 heat capacity,9 Seebeck coefficient,9 or magnetic susceptibility,42 one needs to compare with the transition features in the measured vibrational energy (Figure 5a). Reported transition temperatures (∼100−200 and ∼500− 2496



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Figure 6. Ab initio energetics of LCO(001)pc for different spin states. (a) Bulk vacancy formation energy for LCO in different spin configurations. The gray line indicates the bulk LCO vacancy formation energy determined from thermogravimetric analysis reported in the literature.32 (b) Computed bulk O p-band center for several Co-based perovskites and its relation to the oxygen surface exchange rate (k). Open circles denote oxygen surface exchange rates of polycrystalline perovskite samples obtained from the literature: LaCoO3 (LCO),30 La0.8Sr0.2CoO3−δ (LSCO82),43 La0.3Sr0.7CoO3−δ (LSCO37), 4 4 PrBaCo 2 O 5 + δ (PBCO), 4 5 and GdBaCo 2 O 5 + δ (GBCO).46,47 LCO is shown in red, Sr-doped LCO in orange, and Co-based double perovskites in gray to indicate different families of oxidation state and spin state. The red closed circle denotes oxygen surface exchange rates measured for the LCO thin films in this work. Surface exchange rates are for T ≈ 1000 K and p(O2) ≈ 0.1 atm.

particular, GGA+U shows a monotonic decrease with magnetic moment, and the HS configuration energetics are consistent with that obtained from the literature at elevated temperatures.32 This trend demonstrates that the spin moment couples strongly to oxygen vacancy energetics, which is known to play a critical role in the surface exchange kinetics and therefore strongly supports a coupling of spin and oxygen surface exchange rate. Furthermore, although these calculations do not directly illustrate the connection between bond strength and vacancy formation energy, the spin state calculations implicitly capture this relationship, and Pavone et al. have already previously demonstrated the coupling between decreased metal−oxygen bond strength and decreased vacancy formation energy.12 The modification of the vacancy formation energetics and surface exchange kinetics through strain in LCO thin films is thus strongly mediated by its influence on spin state and bond strength. This effect is particularly pronounced in LCO due to its typically low oxygen vacancy concentration32 and single oxidation state in the bulk. Further studies are needed to examine the influence of strain on the spin state and transition metal bond strength of Sr-substituted cobalt-based perovskites, which have a higher concentration of oxygen vacancies.32 The decrease of vacancy formation energy with higher spin states (decreased bond strength) is also reflected as an increase in the bulk O p-band center (Figure S9, Supporting Information). This is consistent with previous reports that the O p-band center is an effective descriptor of the oxygen vacancy formation energy.7 Moreover, a linear relationship between the O p-band center and the surface exchange rate has been demonstrated for a variety of bulk Co-based perovskites (T ≈ 1000 K, p(O2) ≈ 0.1 atm).7 We measured the surface exchange rate for the 110 nm film under similar conditions and

Figure 5. (a) Fittings of the LS−IS−HS spin transition model, including the fitted transition energies (Δ) and r2 values. (b) Fractional spin occupancies obtained from the fit: LS (dotted), IS (dashed), HS (solid); bulk (gray), thin films (red).

600 K in bulk)9 show good qualitative agreement with the features in our bulk LCO data, as well as the onset of IS and HS occupancy. This demonstrates the power of using Raman spectroscopy for semiquantitatively probing the spin state of the thin films; not only does it allow for spin measurement of samples that are too thin to be resolved by magnetic methods, it also allows for insight into the relative occupancies of different spin states by simple model fitting. The implications of these lower transition energies are readily seen in the fractional occupancies for the different spin states (Figure 5b). At low temperatures when LS states are dominant, the fractional occupancy of LS ions in the thin film decreases at lower temperatures than those of the bulk. Similar behavior is observed in the IS state. At the operating temperature, HS states are favored and have a larger contribution in the thin film as compared to that in the bulk. The enhanced oxygen surface exchange is thus attributed to weakening of the Co−O bond due to increased fractional occupancy of the HS state. To further explore the relationships between spin state, bond strength, and oxygen surface exchange, we performed DFT calculations of the oxygen vacancy energetics for LCO in different spin configurations. Both GGA+U and HSE calculations suggest that the energy of vacancy formation decreases dramatically as the spin state increases (Figure 6a). In 2497



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(2) Lewis, N. S.; Nocera, D. G. Powering the Planet: Chemical Challenges in Solar Energy Utilization. Proc. Natl. Acad. Sci. U.S.A. 2006, 103, 15729−15735. (3) Service, R. F. Hydrogen Cars: Fad or the Future? Science 2009, 324, 1257−1259. (4) Armand, M.; Tarascon, J. M. Building Better Batteries. Nat. Chem. 2008, 451, 652−657. (5) Adler, S. B. Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes. Chem. Rev. 2004, 104, 4791−4843. (6) Mogensen, M.; Hendriksen, P. V. High Temperature Solid Oxide Fuel Cells  Fundamentals, Design, and Applications; Elsevier: London, 2003. (7) Lee, Y.-L.; Kleis, J.; Rossmeisl, J.; Shao-Horn, Y.; Morgan, D. Prediction of Solid Oxide Fuel Cell Cathode Activity with FirstPrinciples Descriptors. Energy Environ. Sci 2011, 4, 3966−3970. (8) Takeda, T.; Yamaguchi, Y.; Watanabe, H. Magnetic Structure of SrCoO2.5. J. Phys. Soc. Jpn. 1972, 33, 970−972. (9) Señarís-Rodríguez, M. A.; Goodenough, J. B. Magnetic and Transport Properties of the System La1−xSrxCoO3−δ (0 < x ≤ 0.50). J. Solid State Chem. 1995, 118, 323−336. (10) Arnold, M.; Xu, Q.; Tichelaar, F. D.; Feldhoff, A. Local Charge Disproportion in a High-Performance Perovskite. Chem. Mater. 2009, 21, 635. (11) Frontera, C.; Caneiro, A.; Carrillo, A. E.; Oró-Solé, J.; GarcíaMuñoz, J. L. Selective Spin-State Switch and Metal−Insulator Transition in GdBaCo2O5.5. Chem. Mater. 2005, 17, 5439−5445. (12) Pavone, M.; Ritzmann, A. M.; Carter, E. A. QuantumMechanics-Based Design Principles for Solid Oxide Fuel Cell Cathode Materials. Energy Environ. Sci 2011, 4, 4933−4937. (13) Pertsev, N. A.; Zembilgotov, A. G.; Tagantsev, A. K. Effect of Mechanical Boundary Conditions on Phase Diagrams of Epitaxial Ferroelectric Thin Films. Phys. Rev. Lett. 1998, 80, 1988−1991. (14) Goodenough, J. B. An Interpretation of the Magnetic Properties of the Perovskite-Type Mixed Crystals La1−xSrxCoO3−δ. J. Phys. Chem. Solids 1958, 6, 287. (15) Korotin, M. A.; Ezhov, S. Y.; Solovyev, I. V.; Anisimov, V. I.; Khomskii, D. I.; Sawatzky, G. A. Intermediate-Spin State and Properties of LaCoO3. Phys. Rev. B 1996, 54, 5309−5316. (16) Maris, G.; Ren, Y.; Volotchaev, V.; Zobel, C.; Lorenz, T.; Palstra, T. T. M. Evidence for Orbital Ordering in LaCoO3. Phys. Rev. B 2003, 67, 224423. (17) Haverkort, M. W.; Hu, Z.; Cezar, J. C.; Burnus, T.; Hartmann, H.; Reuther, M.; Zobel, C.; Lorenz, T.; Tanaka, A.; Brookes, N. B.; et al. Spin State Transition in LaCoO3 Studied Using Soft X-ray Absorption Spectroscopy and Magnetic Circular Dichroism. Phys. Rev. Lett. 2006, 97, 176405. (18) Křaṕ ek, V.; Novák, P.; Kuneš, J.; Novoselov, D.; Korotin, D. M.; Anisimov, V. I. Spin State Transition and Covalent Bonding in LaCoO3. Phys. Rev. B 2012, 86, 195104. (19) Fuchs, D.; Pinta, C.; Schwarz, T.; Schweiss, P.; Nagel, P.; Schuppler, S.; Schneider, R.; Merz, M.; Roth, G.; Löhneysen, H. v. Ferromagnetic Order in Epitaxially Strained LaCoO3 Thin Films. Phys. Rev. B 2007, 75, 144402. (20) Fuchs, D.; Arac, E.; Pinta, C.; Schuppler, S.; Schneider, R.; Löhneysen, H. v. Tuning the Magnetic Properties of LaCoO3 Thin Films by Epitaxial Strain. Phys. Rev. B 2008, 77, 014434. (21) Rondinelli, J. M.; Spaldin, N. A. Structural Effects on the SpinState Transition in Epitaxially Strained LaCoO3 Films. Phys. Rev. B 2009, 79, 054409. (22) Choi, W. S.; Kwon, J.-H.; Jeen, H.; Hamann-Borrero, J. E.; Radi, A.; Macke, S.; Sutarto, R.; He, F.; Sawatzky, G. A.; Hinkov, V.; et al. Strain-Induced Spin States in Atomically Ordered Cobaltites. Nano Lett. 2012, 12, 4966−4970. (23) Kushima, A.; Yip, S.; Yildiz, B. Competing Strain Effects in Reactivity of LaCoO3 with Oxygen. Phys. Rev. B 2010, 82, 115435. (24) Mitterdorfer, A.; Gauckler, L. J. La2Zr2O7 Formation and Oxygen Reduction Kinetics of the La0.85Sr0.15MnyO3, O2(g)|YSZ System. Solid State Ionics 1998, 111, 185−218.

compared the expected increase in the O p-band center for higher spin moment with the measured increase in the oxygen surface exchange rate relative to that of the bulk (Figure 6b), taking the thin films to have an O p-band center value between the IS value (−2.56 eV) and HS value (−2.20 eV). The expected shift in the O p-band center and the corresponding shift observed in the oxygen surface exchange rate agree well with the trend that exists for bulk polycrystalline samples. This consistency in the O p-band center shift and the predicted lower vacancy formation energy support our conclusion that an increase in higher spin Co ions in LCO thin films promotes oxygen electrocatalysis at elevated temperatures. In conclusion, we observed enhanced oxygen surface exchange kinetics in LCO thin films. This enhancement was found to be associated with a reduction in the Co−O bond strength. This insight sheds new light on the utility of metal− oxygen bond strength as a potential design principle for SOFC cathodes. We specifically demonstrate, through a combination of Raman spectroscopy and model fitting, that one possible way for tuning the bond strength is to tune the spin transition energy using highly strained thin films. Understanding the influence of intrinsic material properties on the oxide metal− oxygen bond strength, and consequently the oxygen electrocatalytic activity, could provide new guidelines for developing highly active catalysts.

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ASSOCIATED CONTENT

S Supporting Information *

Additional information on sample preparation, thin film characterization (RHEED, XRD, AFM, EIS, Raman, SQUID), the Raman model and fitting methods, and DFT computational details. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported in part by the U.S. Department of Energy (SISGR DE-SC0002633). Pulsed laser deposition was performed at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. This work made use of the MRSEC Shared Experimental Facilities at MIT, supported by the National Science Foundation under Award Number DMR-08-19762. We thank Dongkyu Lee and Zhenxing Feng for their help with sample preparation for this study and Justin Breucop for his help with developing the Raman measurement protocol.

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REFERENCES

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Tuning the Spin State in LaCoO3 Thin Films for

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