Reversible Compositional Control of Oxide Surfaces by

Letter pubs.acs.org/JPCL

Reversible Compositional Control of Oxide Surfaces by Electrochemical Potentials Eva Mutoro,† Ethan J. Crumlin,† Hendrik Pöpke,‡ Bjoern Luerssen,‡ Matteo Amati,§ Majid K. Abyaneh,§ Michael D. Biegalski,# Hans M. Christen,# Luca Gregoratti,§ Jürgen Janek,‡ and Yang Shao-Horn*,† †

Electrochemical Energy Laboratory, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, Massachusetts 02139, United States ‡ Institute of Physical Chemistry, Justus-Liebig-University Giessen, Heinrich-Buff-Ring 58, 35392 Giessen, Germany § Sincrotrone Trieste Elettra, Str. Stat. 14, km 163.5, 34149 Basovizza Trieste, Italy # Center for Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States S Supporting Information *

ABSTRACT: Perovskite oxides can exhibit a wide range of interesting characteristics such as being catalytically active and electronically/ionically conducting, and thus, they have been used in a number of solid-state devices such as solid oxide fuel cells (SOFCs) and sensors. As the surface compositions of perovskites can greatly influence the catalytic properties, knowing and controlling their surface compositions is crucial to enhance device performance. In this study, we demonstrate that the surface strontium (Sr) and cobalt (Co) concentrations of perovskite-based thin films can be controlled reversibly at elevated temperatures by applying small electrical potential biases. The surface compositional changes of La0.8Sr0.2CoO3−δ (LSC113), (La0.5Sr0.5)2CoO4±δ (LSC214), and LSC214-decorated LSC113 films (LSC113/214) were investigated in situ by utilizing synchrotron-based X-ray photoelectron spectroscopy (XPS), where the largest changes of surface Sr were found for the LSC113/214 surface. These findings offer the potential of reversibly controlling the surface functionality of perovskites. SECTION: Surfaces, Interfaces, Catalysis

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function of applied electrical potentials at elevated temperatures (Figure 1). The potentials were applied to the depicted electrochemical cell of the LSC-based electrode films supported on a (001)cubic-oriented yttria-stabilized zirconia (YSZ) oxygenion-conducting electrolyte23 with a ∼5 nm thick buffer layer of (001)-oriented gadolinium-doped ceria (GDC) and a porous Pt counter electrode. Epitaxial LSC113 films were prepared using pulsed laser deposition (PLD), as reported previously.12,13,18 The epitaxial character, LSC 214(001)tetragonal/LSC113(001)pseudocubic/GDC (001)cubic/YSZ(001)cubic, and the crystallographic relationship of these structures depicted in Figure 1 were determined utilizing high-resolution normal theta−2theta and off-normal phi X-ray diffraction (XRD) scans, respectively (Figure S1, Supporting Information). One LSC113 film was surfacedecorated in part with epitaxial LSC214 of ∼5 nm in thickness (LSC113/214), shown in Figure 1,12 and a highly (001)-oriented LSC214 film supported on GDC(001)cubic/YSZ(001)cubic was prepared for comparison. Atomic force microscopy (AFM) (Figure 1) revealed that these films had low surface roughness with a root-mean-square (rms) of ∼1 nm.12

erovskite-based oxides such as ABO3−δ and A2BO4±δ have been used for a number of solid-state devices such as solid oxide fuel cells (SOFC),1−7 oxygen separation membranes,8 and sensors owing to their vastly flexible physicochemical characteristics9 such as catalytic activities1,3,5,10,11 and electronic/ionic conductivities.1,3 Knowing and controlling the surface composition of perovskites12−14 is critical to enhance the activity of oxygen surface exchange (O2 + 4 e− ↔ 2 O2−)1 and thus the device performance. Perovskites can transform into secondary phases on the surface, and the amount and composition of these surface phases can change as a function of temperature,15,16 oxygen partial pressure,16 humidity,17 and applied electrical potentials.14 The presence of surface secondary phases can greatly influence the activity of oxygen electrocatalysis12−14 as recent work has shown that the surface oxygen exchange activity of (001)-oriented La0.8Sr0.2CoO3−δ (LSC113 ) 18 films is highly dependent on the surface composition, where surface decoration with (La0.5Sr0.5)2CoO4±δ (LSC214)12 and Sr hydr(oxide)13 can result in enhancements of 1−3 orders of magnitude, while Co-based oxide(s) can reduce the activity by 1 order of magnitude.13 In this study, we utilized in situ synchrotron-based X-ray photoelectron spectroscopy (XPS)19−22 to examine how the surface compositions of an LSC113,18 an LSC214-decorated LSC113 (LSC113/214),12 and an LSC21412 thin film change as a © 2011 American Chemical Society

Received: November 17, 2011 Accepted: December 7, 2011 Published: December 7, 2011 40

dx.doi.org/10.1021/jz201523y | J. Phys. Chem.Lett. 2012, 3, 40−44

The Journal of Physical Chemistry Letters

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Figure 1. Schematic of the experimental setup and the sample geometry (LSC113/214) showing the crystallographic orientations of the epitaxial thin-film system (determined by HRXRD, Figure S1, Supporting Information)12 and the surface morphology (determined by AFM).12

Upon heating the LSC113 film from 25 to 650 °C in an oxygen partial pressure, p(O2), of 1 × 10−6 mbar with no applied potential, the XPS analysis revealed that the number of non-perovskite phases was increased on the film surface. The temperature-dependent Sr 3d spectra are shown in Figure 2a, and corresponding spectra of La 4d, Co 3p, and O 1s are included in Figure S2 (Supporting Information). The Sr 3d region was separated (Figure 2a and b) into a “lattice” component at lower binding energy (Sr 3d5/2 at ∼131.9 eV, having a doublet energy separation of 1.74 eV24), which can be assigned to Sr2+ of the LSC perovskite lattice,15 and a “nonperovskite” (often referred to as “surface”) component at higher binding energy (Sr 3d5/2 at ∼133.5 eV), which can be assigned to Sr2+ in the termination layer of LSC (Sr 3d5/2: ∼133.0 eV)15 and/or in secondary surface phases (Figure S3, Supporting Information).25−27 These surface secondary phases may include Sr(OH)2 (Sr 3d5/2: ∼133.2 eV),25 SrO (Sr 3d5/2: 132.4−135.5 eV),15,25,28 or other phases such as LSC214 (Sr 3d5/2: 133.5 eV)27 (Figure S4, Supporting Information), where SrCO3 could be excluded due to the absence of a C 1s signal. The Sr lattice component decreased gradually from 25 to 450 °C and became absent at higher temperatures (Figure 2a and b). Similar changes were also found in O 1s spectra (Figure S2, Supporting Information), where the disappearance of the lattice component at a lower binding energy (O 1s: 528.7 eV) occurred at 450 °C. Unfortunately, the peak shapes of La 4d and Co 3p and the lack of changes upon heating did not allow the separation of lattice and surface components (Supporting Information). A table detailing the fitted XPS peaks, the component and region area intensities, and key fitting parameters can be found in Table S1 (Supporting Information). The disappearance of the LSC lattice in the Sr 3d and O 1s spectra with increasing temperature can be explained by the transformation of LSC113 into LSC214-like29−31 (Sr 3d5/2: 133.5 V, O 1s: 531.3 eV),27 CoOx-like,29 and SrO-like phases (Figure 2d), where the La/Sr ratio in the LSC214-like phase might be different from that in the pristine LSC113 film (Supporting Information, eqs S1 and S2). Interestingly, the LSC113 film surface became enriched in Sr, as revealed by examining how the intensities of the Sr, O, La, and Co regions changed with increasing temperature. The Sr

Figure 2. Surface composition changes of the LSC113 film with increasing temperature in a p(O2) of 1 × 10−6 mbar. (a) Sr 3d point spectra collected at 25, 450, 550, and 650 °C (black open circles: measurement; red line: sum of fits; gray line: background; Sr 3d5/2 and 3d3/2 surface (blue) and lattice (green)), showing (b) an increase of the surface component while the lattice vanishes (each component area intensity was normalized to the sum of both areas), where details about the error analysis can be found in the Supporting Information, and (c) strong increase in the total Sr intensity with increasing temperature (region area intensities of each element were normalized to the room-temperature condition). (d) Schematic depicting the proposed changes upon temperature increase (the blue shaded region depicts secondary phases already present at room temperature, which may form either a very thin film or small particles on the surface; the XPS information depth is ≤1 nm, Table S2 (Supporting Information); the gray bulk film region may be composed of either LSC113 or LSC214-based material).

3d intensity was increased by ∼3 times, while La 4d (corrected for the overlapping Co 3s at ∼103 eV32 utilizing the photoionization cross sections33 for Co 3s and Co 3p), O 1s, and Co 3p intensities were reduced upon heating, as shown in Figure 2c. The increased Sr concentration suggests that the top film surface is covered primarily by surface SrO-like secondary phases with increasing temperature, presumably from the transformation of LSC113 and/or LSC214-like phases to yield Srdeficient LSC113-like and/or LSC214-like phases underneath, which were not detected by XPS, as shown in Figure 2d. It is interesting to note that increased Sr surface concentrations have been reported previously on (001)-oriented epitaxial thin La0.7Sr0.3MnO3 films at elevated temperature and low p(O2).34 The highly surface sensitive XPS data (information depth of ∼0.66 nm for LSC113 and ∼0.94 nm for Sr(OH)2 shown in 41



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Figure 3. Changes in the elemental surface composition of LSC113 and LSC113/214 with applied potential (V = ±0.5 to ±1.5 V) at T = 650 °C in p(O2) = 1 × 10−6 mbar. (a) Sr 3d spot spectra showing higher intensities upon cathodic (−, red) than upon anodic (+, blue) polarization, where the difference was found to be more pronounced for LSC113/214 than for LSC113. (b) Normalized peak region areas of O 1s (blue), La 4d (green, corrected for Co 3s), Sr 3d (red), and Co 3p (black) compared to those collected at −0.5 V. The filled circles represent LSC113, and open circles correspond to LSC113/214, where details about the error analysis can be found in the Supporting Information. The surface compositional changes of the LSC113 and LSC113/214 films normalized to those collected without potential bias are provided in Figure S6 (Supporting Information).

and spectra shapes of Sr 3d clearly show that only the nonperovskite or surface components were detected. During cathodic polarization, the Sr 3d intensity collected from the film surface increased, while those of O 1s and Co 3p (Figure S5, Supporting Information) decreased (Figures 3b and S6, Supporting Information). On the other hand, under anodic polarization, an opposite trend was found, that is, having decreased Sr and increased O and Co intensities. Interestingly, a similar trend in the compositional changes of Sr, O, and Co was found for the LSC113/214 surface upon cathodic and anodic polarization. It is worth mentioning that the magnitude of the potential-induced changes of O and Co was comparable for LSC113 and LSC113/214, while it was much more pronounced for Sr on the LSC113/214 surface. Although the intensities of La 4d collected from the LSC113 and LSC113/214 films appeared to change with anodic and cathodic polarization, the potentialinduced changes were small relative to experimental uncertainty. It should be mentioned that the observed composition changes appeared uniformly (at the submicrometer scale as defined by the SPEM lateral resolution) on the film surface (Figure S7, Supporting Information). Increased Sr 3d and reduced O 1s and Co 3p intensities can be explained by the hypothesis that the films developed increased amounts of SrO-like phases on the surface with

Table S2, Supporting Information) collected in situ upon heating to 650 °C under 1 × 10−6 mbar of O2 demonstrate that the surface of the LSC113 develops a distinctively different composition from that at room temperature. At 650 °C at a p(O2) of 1 × 10−6 mbar, the surface composition of the LSC113 film was further changed by applying electrical potential biases. Upon cathodic polarization, the equilibrium oxygen partial pressure was reduced at the cathode following the Nernst equation, where oxygen molecules were reduced at the film surface (O2 + 4 e− → 2 O2−),1,35 oxygen ions diffused from the film surface to the film/ YSZ interface and migrated in the YSZ from the cathode to the anode side, and oxygen molecules were evolved at the anode. Upon anodic polarization, the processes are reversed. The exact oxygen partial pressures at the film surface for a given potential bias could not be estimated as the potential was applied across the cell, where the potential change at the cathode (film) surface relative to the open-circuit voltage was not known. However, for similar YSZ-based electrochemical cells, the oxygen exchange reaction has been reported under comparable experimental conditions and applied potentials.20,35 The point spectra of Sr 3d collected from the LSC113 film (Figure 1) as a function of cathodic and anodic potentials of increasing magnitude are shown in Figure 3a. The binding energies (Eb) 42



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reduced oxygen partial pressure34 upon cathodic polarization, which is also supported by the reduced stability of LSC113 at high temperature in low p(O2).29 Reduced O 1s intensities upon cathodic polarization can be rationalized by the growth of a SrO-like surface phase (0.049 mol/cm3 for SrO) having a lower molar volume concentration of oxygen ions at the expense of LSC214-like (0.073 mol/cm3 for LSC214) phases with a higher concentration. Similarly, reduced Co 3p intensities can be explained by the growth of surface SrO-like secondary phases, which reduced the detected XPS signals of Co 3p from the LSC214-like and CoOx phases underneath. It should be noted that we can exclude the complete decomposition29 of these films into insulating La2O3, SrO, and CoOx phases as the LSC113 and LSC113/214 surfaces remained electrochemically active and exhibited a characteristic Butler−Volmer-like behavior in the current−voltage profile based on the polarization experiments (Figure S8, Supporting Information), which requires both electronic and oxygen ion conductivities13 for the film surface and bulk. Strikingly, the potential-induced surface compositional changes of Sr, O, and Co were reversible upon anodic polarization (Figures 3b and S6, Supporting Information), where the coverage of surface SrO-like secondary phases was reduced, which is indicative of the re-formation of LSC214-like phases at the expense of surface SrO-like secondary phases upon increased oxygen partial pressure. Remarkably, such surface compositional changes were reversible upon cathodic and anodic polarization up to ±1.5 V. Further polarization to ±2.0 V resulted in YSZ reduction36 and large LSC compositional changes that deviated greatly from the trend shown in Figure 3b. The potential-induced composition changes of surface Sr found for LSC113/214 were much more pronounced than those for LSC113. The physical origin of this difference is not understood. Interestingly, the potential-induced surface composition changes observed for the LSC113 and LSC113/214 surface are significantly different from an LSC214 film surface (Figure S9, Supporting Information), which exhibited current instabilities at 1 V and an insulating character at ±1.5 V, indicative of the formation of a larger amount of secondary surface phases (Figure S8, Supporting Information) covering the entire surface and thus blocking oxygen surface exchange. These observations suggest that the LSC214-decorated LSC113 surface is fundamentally different from the LSC214 surface. The reversible changes in the surface Sr content of the LSC113/214 surface with greater magnitude than those of LSC113 might be responsible for its much enhanced activity for O2 electrocatalysis reported previously.12 In summary, we report reversible potential-induced compositional changes of perovskite-based oxide surfaces at elevated temperatures. This finding provides insights into developing promising strategies to actively control surface-related properties of complex oxides12,13 and tune the activity12,13,37,38 and selectivity of catalysts and sensors.

tion) under the following PLD conditions: 248 nm wavelength, 10 Hz pulse rate, ∼50 mJ pulse energy, 1.33 × 10−2 mbar of p(O2)PLD and cooling, and growth temperatures TGDC = 450 °C and T(LSC113,LSC214) = 675 °C. The partial coverage of the LSC113/214 sample was created by selectively etching (6% HCl, 60 s) the faster-dissolvable LSC214 film. The gold stripe was sputtered (T = 25 °C, p(Ar) = 4 × 10−3 mbar, t = 30 min) on the LSC films. HRXRD was performed in normal and offnormal configurations with a four-circle diffractometer (Panalytical). The surface morphology was examined by AFM (Veeco). In Situ XPS. XPS data were collected at the ESCA microscopy Beamline (Synchrotron ELETTRA, Italy).19 The samples were placed directly onto a Pt foil on a ceramic heater with a thermocouple close to the sample surface. The surfaces were cleaned from carbonaceous species (p(O2) ∼ 10 mbar, T ∼ 200 °C, t ∼ 2 h), and their removal was monitored with Auger electron spectroscopy (AES). At each condition, the following high-resolution XP spectra were taken (Ephoton ∼ 669 eV) in p(O2) = 1 × 10−6 mbar (collection time of about 3 h per sequence): La 4d, Sr 3d, Co 3p, O 1s, and Au 4f. The temperature was adjusted to ∼25, ∼450, ∼550, and ∼650 °C and held constant after each step for ∼1 h before starting to collect data. Electrical potentials (in 0.5 V steps and changing polarity between |0.5| and |1.5| V) were applied using a potentiostat (Gamry), and XPS spectra were collected after ∼1 h of holding at each potential step. Note that at 450 and 550 °C, applied potentials up to 0.4 V did not induce any detectable elemental surface composition changes (Figure S10, Supporting Information). As detailed in the Supporting Information, the following procedure for calibrating the Eb was used: the collected Au 4f7/2 was aligned to 84.0 eV for each temperature without an applied potential; the binding energy of the La 4d peak (a feature showing the least changes) at 650 °C was found to be 101.6 eV; all spectra collected during anodic and cathodic polarization at 650 °C were aligned to have the La 4d peak at 101.6 eV.

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ASSOCIATED CONTENT S Supporting Information * Details about the experimental methods (material and target synthesis, sample preparation, and characterization), XRD (normal and off-normal scans of the LSC113 and LSC214 sample), XPS (spectra, fitting parameters, literature reference values, binding energy calibration, and SPEM images), and electrochemical data. 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].

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ACKNOWLEDGMENTS This work was supported in part by DOE (SISGR DESC0002633), King Abdullah University of Science and Technology, and the King Fahd University of Petroleum and Minerals in Dharam (through the Center for Clean Water and Clean Energy at MIT and KFUPM). The German Research Foundation is acknowledged for financial support (E.M.: DFG research scholarship; H.P., B.L., and J.J.: LU1480/1-1 and JA648/17-1). The sample preparation performed at the Center of Nanophase Materials Sciences was sponsored by the

EXPERIMENTAL METHODS Sample Preparation. Pt ink (#6082, BASF) counter electrodes were sintered (800 °C, 1 h, air) on YSZ(001) single crystals (9.5 mol % Y2O3, Princeton Scientific, dimensions: 10 or 5 × 5 × 0.5 mm3). PLD films of GDC (500 laser pulses, ∼5 nm thickness), La0.8Sr0.2CoO3−δ (LSC113, 15 000 pulses, ∼85 nm), and LaSrCoO4±δ (LSC113/214, 900 pulses, ∼5 nm) or LSC214 (10 000 pulses, ∼50 nm) were grown using self-synthesized stoichiometric targets (details are in the Supporting Informa43



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Spillover on Pt/YSZ Catalysts. Angew. Chem., Int. Ed. 2006, 45, 1473− 1476. (21) Janek, J.; Luerssen, B.; Mutoro, E.; Fischer, H.; Gunther, S. In Situ Imaging of Electrode Processes on Solid Electrolytes by Photoelectron Microscopy and Microspectroscopy  The Role of the Three-Phase Boundary. Top. Catal. 2007, 44, 399−407. (22) Zhang, C. J.; Grass, M. E.; McDaniel, A. H.; DeCaluwe, S. C.; El Gabaly, F.; Liu, Z.; McCarty, K. F.; Farrow, R. L.; Linne, M. A.; Hussain, Z.; Jackson, G. S.; Bluhm, H.; Eichhorn, B. W. Measuring Fundamental Properties in Operating Solid Oxide Electrochemical Cells by Using In Situ X-ray Photoelectron Spectroscopy. Nat. Mater. 2010, 9, 944−949. (23) Kumar, A.; Ciucci, F.; Morozovska, A. N.; Kalinin, S. V.; Jesse, S. Measuring Oxygen Reduction/Evolution Reactions on the Nanoscale. Nat. Chem. 2011, 3, 707−713. (24) Brookes, N. B.; Quinn, F. M.; Thornton, G. H2O Dissociation by SrTiO3(100) Catalytic Step Sites. Vacuum 1988, 38, 405−408. (25) Vasquez, R. P. X-ray Photoelectron-Spectroscopy Study of Sr and Ba Compounds. J. Electron. Spectros. 1991, 56, 217−240. (26) Bocquet, A. E.; Chalker, P.; Dobson, J. F.; Healy, P. C.; Myhra, S.; Thompson, J. G. X-ray Photoelectron-Spectra of Perovskite-Type Cobalt Oxides La1−xSrxCoO3−y (x = 0.4, 0.6). Physica C 1989, 160, 252−258. (27) Yang, X. M.; Park, D. W.; Kim, M. I. Selective Oxidation of Hydrogen Sulfide over LaCoO3 and LaSrCoO4 Mixed Oxides. Korean J. Chem. Engin. 2007, 24, 592−595. (28) Vandoveren, H.; Verhoeven, J. A. T. XPS Spectra of Ca, Sr, Ba and Their Oxides. J. Electron. Spectros. 1980, 21, 265−273. (29) Petrov, A. N.; Cherepanov, V. A.; Kononchuk, O. F.; Gavrilova, L. Y. Oxygen Nonstoichiometry of La1−xSrxCoO3−δ. J. Solid State Chem. 1990, 87, 69−76. (30) Morin, F.; Trudel, G.; Denos, Y. The Phase Stability of La0.5Sr0.5CoO3−δ. Solid State Ionics 1997, 96, 129−139. (31) Cherepanov, V. A.; Gavrilova, L. Y.; Barkhatova, L. Y.; Voronin, V. I.; Trifonova, M. V.; Bukhner, O. A. Phase Equilibria in the La-MeCo-O (Me = Ca, Sr, Ba) Systems. Ionics 1998, 4, 309−315. (32) McIntyre, N. S.; Cook, M. G. X-ray Photoelectron Studies on Some Oxides and Hydroxides of Cobalt, Nickel, and Copper. Anal. Chem. 1975, 47, 2208−2213. (33) Yeh, J. J.; Lindau, I. Atomic Subshell Photoionization CrossSections and Asymmetry Parameters − 1 ≤ Z ≤ 103. At. Data Nucl. Data 1985, 32, 1. (34) Fister, T. T.; Fong, D. D.; Eastman, J. A.; Baldo, P. M.; Highland, M. J.; Fuoss, P. H.; Balasubramaniam, K. R.; Meador, J. C.; Salvador, P. A. In Situ Characterization of Strontium Surface Segregation in Epitaxial La0.7Sr0.3MnO3 Thin Films as a Function of Oxygen Partial Pressure. Appl. Phys. Lett. 2008, 93, 1519041− 1519043. (35) Backhaus-Ricoult, M. SOFC  A Playground for Solid State Chemistry. Solid State Sci. 2008, 10, 670−688. (36) Poepke, H.; Mutoro, E.; Raiss, C.; Luerssen, B.; Amati, M.; Abyaneh, M. K.; Gregoratti, L.; Janek, J. The Role of Platinum Oxide in the Electrode System Pt(O2)/Yttria-Stabilized Zirconia. Electrochem. Acta 2011, 56, 10668−10675. (37) Mutoro, E.; Koutsodontis, C.; Luerssen, B.; Brosda, S.; Vayenas, C. G.; Janek, J. Electrochemical Promotion of Pt(111)/YSZ(111) and Pt-FeOx/YSZ(111) Thin Catalyst Films: Electrocatalytic, Catalytic and Morphological Studies. Appl. Catal., B 2010, 100, 328−337. (38) Vayenas, C. G.; Koutsodontis, C. G. Non-Faradaic Electrochemical Activation of Catalysis. J. Chem. Phys. 2008, 128, 1825061−18250613.

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Reversible Compositional Control of Oxide Surfaces by

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