Segregated chemistry and structure on (001) and (100) surfaces of

Supporting Information Segregated chemistry and structure on (001) and (100) surfaces of (La 1-x Sr x ) 2 CoO 4 override the crystal anisotropy in oxygen exchange kinetics Yan Chen a, Helena Téllez b,c, Mónica Burriel b,fan Yang d, Nikolai Tsvetkov a, Zhuhua Cai a, David W. McComb d, John A Kilner b*, Bilge Yildiz a,e* a Laboratory for Electrochemical Interfaces, Department of Nuclear Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA. b Department of Materials, Imperial College London, Royal School of Mines, Exhibition Road, London, SW7 2AZ, UK c International Institute for Carbon Neutral Energy Research (wpi-i2cner), Kyushu University, Nishi-ku, Fukuoka, 819-0395, Japan d Department of Materials Science and Engineering, The Ohio State University, Columbus, OH 43210, USA e Department of Materials Science and Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA, 02139, USA These three authors contributed equally to this work. * Corresponding authors: byildiz@mit.edu; j.kilner@imperial.ac.uk S1: Surface cleaning procedure before LEIS measurement The LEIS instrument is fitted with a reactive atomic oxygen source and a low-energy sputter gun to remove any adsorbed species that hinder analysis of the surface by either the formation of volatile species or sputtering, respectively. 1,2 The base pressure in the main chamber during the analysis was below ~2 10-8 mbar, assuring that only negligible deposition of any species from the residual gas took place during the analysis. In the case of thick LSC50 and LSC25 films (~81-84 nm), the surface was cleaned by light Ar + sputtering (500 ev@59, fluence < 1 10 15 ions cm -2 ), whereas atomic oxygen exposure was used for the thin LSC50 and LSC25 films (~39 to 45

nm). The progress of the cleaning procedure was monitored by analyzing the surface with 3 kev He + until no carbon peak was detected at ~846 ev and the surface spectrum did not change with further cleaning. S2: Isotopic exchange experiments In order to evaluate the orientation dependence of the oxygen diffusion and surface exchange properties of the LSC films isotopic exchange experiments in the transversal configuration (Figure S1 (a)) and the longitudinal configuration (Figure S1 (b)). 3 Prior to the exchange, the films were annealed at the testing temperature in research grade 16 O 2 ( 99.9995%) to ensure that equilibrium was established. Then the samples were exchanged with 18 O for a given time length as described in main context. All the anneals and exchanges described were carried out at a nominal pressure of 200 mbar. All the exchanged samples were then measured by time-of-fight SIMS equipped with a bismuth LMIG pulsed gun incident at 45º. A 25 kev Bi + primary ion beam was used to generate the secondary ions using burst alignment mode (eight pulses) for analysis and a Cs + beam (2 kev) incident at 45º for sputtering. For each exchanged sample the distribution of the oxygen isotopes ( 18 O and 16 O) as well as other characteristic relevant secondary ion species: LaO -, SrO -, CoO - (for the film) in addition to TiO - and LaO - (for the substrates) samples were acquired in selected regions. Figure S1: 18 O exchange, diffusion and SIMS measurement schematics: (a) transversal configuration, and (b) longitudinal configuration. The squared boxes represent the areas measured by SIMS.

For the transverse configuration oxygen tracer depth profiles were measured from the exchanged surface penetrating down to the substrate by sputter depth profiling. For the longitudinal configuration, in which the oxygen exchange through the top surface is avoided by using an Au capping layer, secondary ion images for selected areas of the LSC films, including the edge of the opened trench, were acquired. Subsequently the number of counts was normalized to obtain the 18 O fraction corrected for the background isotope fraction and the isotope fraction of the annealing gas, as described by De Souza et al. 4 Tracer diffusion coefficients, D*, and surface exchange coefficients, k*, were determined by fitting the diffusion data to the solutions of the diffusion equation with surface limitation, given by Crank, for diffusion in a plane sheet model (transversal configuration) and in a semi-infinite medium (longitudinal configuration), and described in ref. 3 S3. LEIS surface spectra Figure S2 shows the LEIS spectra from the outmost surface of the LSC25 and LSC50 films. The peaks corresponding to the major isotopes of the constituent cations (i.e. 59 Co, 88 Sr and 139 La), are located at the theoretical scattering energies of 1377.7 ev, 2151.5 ev and 2950.1 ev, respectively, for the scattering of 5 kev Ne + ions. In the as-prepared state, all the constituent cations are present at the surface of the LSC25 (Figure S2 (a-b)), whereas on LSC50 films (Figure S2(c-d)) the transition metal, Co, is only minutely present at the surface.. An apparent decrease of the Sr peak while an increase of the La peak was observed (Figure S2 dashed lines). Annealing of the films at 400 C and 500 C gave rise to a mixed La-O and Sr-O dominated surface, with only a small amount of transition metal, Co, detected on the outer surface for any of the LSC compositions or orientations under study.

Figure S2: LEIS surface spectra for the scattering of 5 kev Ne + ion beam by (a,b) the LSC25 (001) and (100) and (c,d) the LSC50 (001) and (100) films, as-prepared (solid black line) and after annealing at 400 C (dashed red line) and 500 C (blue dotted line) for a total time of 10.25 h. The spectra are offset from each other for clarity. S4. LEIS depth profiling for thick LSC samples Figure S3 shows the LEIS depth profiling for thick LSC25 (001) and (100) films (~80 nm thickness). Sr cations were found to segregate towards the surface preferentially over the La cations, whereas the Co cations are located below the outer surface. The results are consistent with ones obtain on thin LSC25 films as shown in the main text.

Figure S3. Cation compositions obtained by LEIS on the thick LSC25 (001) and (100) films (81-84 nm) with (100) orientation (bottom row) and (001) orientation (top row). The depth profiles show the surface and near-surface restructuring as a function of the thermal history of the films (left to right, as prepared, 400 o C and 500 o C for 10.25 hours

S5. Surface structure characterized by AFM Figure S4. Surface topography (2D image) characterized by AFM for LSC25 (001) and (100) films in their as-prepared state and after annealing at 400 o C and 500 o C in O 2 for 10.25 hours. The corresponding 3D images are shown in Figure 7.

Figure S5: (a) Comparison of surface roughness for LSC25 (001) and (100) and for LSC50 (001) and (100) in their as-prepared state and after annealing at 400 o C and 500 o C in O 2 for 10.25 hours. (b) The average particle area (S), and the total coverage fraction of the particles at the surface after annealing at 400 o C and 500 o C in O 2 for 10.25 hours. The particles were identified as the regions with heights clearly above background roughness. The uncertainty arose from the different threshold height values in selecting the particles regions above the background. The total coverage of particles was estimated based on the density of particles and average area of those particles.

S6. Interaction of Sr-rich particles with electron beam Figure S6. Scanning electron microscopy image of the LSC25 (001) film annealed at 500 o C in O 2 for 10.25 hours (a) before and (b) after focusing of the 10 na, 20keV electron beam at the marked particle. S7. Lateral heterogeneities in surface cation composition detected by AES Figure S7: (a) Scanning electron microscopy image of the LSC25 (100) film annealed at 500 o C in O 2 for 10.25 h, and the corresponding AES elemental maps for (b) Sr, (c) Co, and (d) La. The scale bars show relative signal intensity and are not a direct measure of relative cation compositions.

S8. Diffusion kinetics of oxygen quantified by 18 O SIMS analysis. The normalized oxygen diffusion profiles obtained through the thick films after the 18 O exchange for the 85 min are shown in Figure S8 (a). The low 18 O concentration values obtained in the first 5-7 nm of the top surface can be attributed to the presence of the Sr-rich particles described in the previous sections and to the back-diffusion of 16 O at room temperature. Therefore the concentration values for these regions have not been taken into account for the fit of the diffusion profiles. After this top region, the 18 O concentration slowly decreases with film thickness until the interface with the substrate is reached, where a sharp decrease down to the background level is observed. The diffusion profile corresponding to the LSC50 (100) presents a sharper decrease (lower D * ) and follows the shape of a normal diffusion profile down to the interface. A different behaviour can be observed for LSC50 (001), in which the shape of the diffusion profile clearly deviates from the profile expected for a constant D * value. This behaviour has previously been observed for other epitaxial R-P type films 3 and could be attributed to the effect of strain in the interface region (from 50 to 84 nm), which could result in a lower D * value in the region close to the substrate. This region has not been taken into account for the fit, and the extracted diffusion coefficient for this film corresponds therefore to the region where the LSC50 film is relaxed (from 5 to 50 nm from the surface). The normalized oxygen tracer diffusion profiles obtained along the film after the of 65 hours exchange are shown in Figure S8(b). In this case the oxygen exchange takes place only at the edge of an open trench, as the gold capping layer prevents the exchange at the top surface of the film (see configuration in Figure S1 (b)). It is clearly observed that the diffusion length for the films with (001) orientation is much longer than that for the films with (100) orientation.

Figure S8. Normalized isotope fraction of (a) the 81-84 nm thick LSC50 (001) (blue squares) and LSC50 (100) (red circles) films, exchanged with 18 O at 500 ºC for 85 minutes and analyzed by SIMS depth profile (transversal configuration depicted in Figure 3a), with the fits to the diffusion equation shown with solid lines, and (b) the 39-45 nm thick LSC25 (001) (blue up triangles) and LSC25 (100) (red down triangles) films, exchanged with 18 O at 500 ºC for 65 hours and analyzed by SIMS imaging mode (longitudinal configuration) to extract the line scans (where Distance=0 µm corresponds to the edge of the trench depicted in Figure S1 b) Table S1. Comparison of the oxygen tracer diffusion coefficient D* along the ab-plane and along the c-axis of LSC25 and LSC50 at 500 o C quantified using either the transverse configuration or the longitudinal configuration on the (100) and (001) films. Composition Diffusion direction D* (cm 2 /s) Configuration D * ab/d * c LSC50 along ab-plane 1.20 10-13 LSC50 (100) film, transverse along c-axis 2.97 10-14 LSC50 (001) film, transverse 4 LSC25 along ab-plane 3.58 10-12 LSC25 (001) film, longitudinal along c-axis 1.99 10-13 LSC25 (100) film, longitudinal 20

S9. Co oxidation state probed by XPS The Co 2p photoelectron spectra were recorded by XPS from the (001) and (100) LSC25 and LSC50 films: as-prepared and annealed at 400 o C and 500 o C. We did not find significant differences in the shape of the Co 2p spectra between the (100) and (001) LSC films. This indicates that the Co oxidation states at/near the LSC surface with different crystal orientation are quite similar at the experimental condition of this work. The representative examples of the Co spectra are shown below. Figure S9. Co 2p core-level photoelectron spectra from LSC25 (001) and (100) films, (a) as prepared, (b) after annealing at 400 oc for 10.15 h, and (c) after annealing at 500 oc for 10.15 h. References: (1) de Ridder, M.; van Welzenis, R. G.; Brongersma, H. H. Surface and Interface Analysis 2002, 33, 309. (2) Téllez, H.; Druce, J.; Hall, A.; Ishihara, T.; Kilner, J.; Rockett, A. Progress in Photovoltaics: Research and Applications 2014. (3) Burriel, M.; Garcia, G.; Santiso, J.; Kilner, J. A.; Richard, J. C. C.; Skinner, S. J. Journal of Materials Chemistry 2008, 18, 416. (4) De Souza, R. A.; Zehnpfenning, J.; Martin, M.; Maier, J. Solid State Ionics 2005, 176, 1465.