Effect of Sr Content and Strain on Sr Surface Segregation of La1

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Effect of Sr Content and Strain on Sr Surface Segregation of La SrCo Fe O as Cathode Material for Solid Oxide Fuel Cells 1-x

x

0.2

0.8

3-#

Yang Yu, Karl F. Ludwig, Joseph C. Woicik, Srikanth Gopalan, Uday B. Pal, Tiffany C. Kaspar, and Soumendra N. Basu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b07118 • Publication Date (Web): 20 Sep 2016 Downloaded from http://pubs.acs.org on September 24, 2016

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ACS Applied Materials & Interfaces

Effect of Sr Content and Strain on Sr Surface Segregation of La1-xSrxCo0.2Fe0.8O3-δ as Cathode Material for Solid Oxide Fuel Cells Yang Yua*, Karl F. Ludwigab, Joseph C. Woicikc, Srikanth Gopalanad, Uday B. Palad, Tiffany C. Kaspare and Soumendra N. Basuad* a

Division of Materials Science and Engineering, Boston University, Brookline, MA 02446, USA b Department of Physics, Boston University, Boston, MA 02215, USA c Material Measurement Laboratory, National Institute of Standards and Technology, Gaithersburg, Maryland 20899, USA d Department of Mechanical Engineering, Boston University, Boston, MA 02215, USA e Physical Sciences Division, Pacific Northwest National Laboratory, Richland, WA 99354, USA

*Corresponding Author: email: [email protected] *Corresponding Author: email: [email protected]; Tel.: +1 617 353 6728; fax: +1 617 353 5548

Abstract Strontium doped lanthanum cobalt ferrite (LSCF) is a widely used cathode material due to its high electronic and ionic conductivity, and reasonable oxygen surface exchange coefficient. However, LSCF can have long-term stability issues such as surface segregation of Sr during solid oxide fuel cell (SOFC) operation, which can adversely affect the electrochemical performance. Thus, understanding the nature of the Sr surface segregation phenomenon, and how it is affected by the composition of LSCF and strain are critical. In this research, heteroepitaxial thin films of La1-x SrxCo0.2Fe0.8O3-δ with varying Sr content (x = 0.4, 0.3, 0.2) were deposited by pulsed laser deposition (PLD) on single crystal NdGaO3, SrTiO3 and GdScO3 substrates, leading to different levels of strain 1 ACS Paragon Plus Environment


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in the films. The extent of Sr segregation at the film surface was quantified using synchrotron-based total reflection X-ray fluorescence (TXRF), and atomic force microscopy (AFM). The electronic structure of the Sr-rich phases formed on the surface was investigated by hard X-ray photoelectron spectroscopy (HAXPES). The extent of Sr segregation was found to be a function of the Sr content in bulk. Lowering the Sr content from 40% to 30% reduced the surface segregation, but further lowering the Sr content to 20% increased the segregation. The strain of LSCF thin films on various substrates was measured using high-resolution X-ray diffraction (HRXRD) and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. A model was developed correlating the Sr surface segregation with Sr content and strain effects to explain the experimental results.

Keywords Solid Oxide Fuel Cell, Strontium Doped Lanthanum Cobalt Ferrite, Sr Surface Segregation, Thin Film, Total Reflection X-ray Fluorescence, Hard X-ray Photoelectron Spectroscopy, High-Resolution X-ray Diffraction

2 ACS Paragon Plus Environment

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Introduction Lanthanum strontium cobalt ferrite, denoted as La1-xSrxCo1-yFeyO3-δ or LSCF, has been extensively investigated in the past decades as the cathode material for SOFCs operated at intermediate temperatures.1–5 The crystal structure, coefficient of thermal expansion (CTE), oxygen stoichiometry and electrical conductivity of LSCF can be altered by varying x and y values.6 La1-xSrxCo0.2Fe0.8O3-δ with x=0.4, 0.3 and 0.2 and y=0.8 are commonly used compositions for LSCF. These compositions have an electrical conductivity of ~200 S/cm and sufficient concentration of oxygen vacancies for intermediate-temperature operation of the solid oxide fuel cells (SOFCs).7 SOFCs convert chemical energy directly into electricity,8 and has advantages of high efficiency (~70%) achievable with cogeneration9,10, and environmental friendliness.11–13 Typical operating temperatures for SOFCs range from 600°C to 1000°C.14–17 At the high temperature end of this range, electrochemical reactions and ionic transport are sufficiently fast.18,19 On the other hand, high temperature operation presents challenges such as higher materials cost, higher stresses due to thermal expansion mismatch between SOFC components, and durability and reliability issues.20 Operation at the lower end of the temperature range leads to challenges related to slower reaction and mass transfer kinetics, requiring new materials with improved surface catalytic and bulk transport properties. Currently, SOFCs run at temperatures at the middle of the range (~800°C). However, at 800°C LSCF suffers from long-term stability issues, despite its 3 ACS Paragon Plus Environment


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outstanding mixed ionic and electronic conductivity21, decent catalytic activities for oxygen reduction reaction (ORR)22 and excellent CTE match with electrolyte materials.23

The formation of secondary phases as a result of Sr surface segregation has been observed on LSCF when exposed at SOFC operation temperatures.24–27 This Sr surface segregation phenomena is considered to be one of the main cathode degradation mechanisms in SOFCs.28 This phenomena has been previously studied as a function of temperature29, oxygen partial pressure30, gas composition24,31,32 and surface modification33,34. The present work examines the effect of Sr content by varying the ‘x’ values at 0.4, 0.3, and 0.2, as well as strain by depositing heteroepitaxial LSCF thin films on substrates with different lattice parameters, on the Sr surface segregation phenomena.

Experimental Thin film deposition (001)-oriented LSCF films with thicknesses of ~217 nm (Figure S1†) were deposited onto (110)-NdGaO3 (NGO), (001)-SrTiO3 (STO), and (110)-GdScO3 (GSO) single-crystal substrates (10 mm × 10 mm × 0.5 mm) using pulsed laser deposition (PLD). The films were deposited with 4 Hz laser repetition rate, under 10 mTorr O2 partial pressure with substrates kept at 550 °C. The LSCF powders used for making PLD targets were prepared by calcining a mixture of corresponding precursor powders that yield LSCF with Sr


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content ranging from 40% to 20%, confirmed using power X-ray diffraction (XRD, Figure S2†). The (001)-LSCF thin films were heteroepitaxially deposited on (110)-NGO, (001)STO, and (110)-GSO substrates and the surfaces of the as-deposited films were characterized by a Bruker* Dimension 3000 AFM. The RMS surface roughness of the asdeposited thin films were ~0.7 nm. The heteroepitaxy of the LSCF films were further confirmed by transmission electron microscopy (TEM). Detailed information on PLD target preparation and thin film characterization is provided in the Supporting Information† and Figure S3-5†.

Sample annealing and surface morphology characterization The samples were placed on a heater in a stainless steel chamber and were heated from room temperature to 800 °C in air in 40 min, and then held at temperature for 10 hours. After annealing, the samples were cooled rapidly to room temperature in the chamber, by turning off the power supply of the heater. The surface morphology of post-annealed sample was characterized by AFM.

Total reflection X-ray fluorescence (TXRF) TXRF measurements were carried out at X23A2 beamline at National Synchrotron Light Source-I (NSLS-I) at Brookhaven National Laboratory (BNL). TXRF is a surface-sensitive, non-destructive technique used for elemental analysis on surfaces of materials. TXRF is based on energy dispersive XRF spectroscopy, where the incident photon beam scans



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from a grazing incident angle below the critical angle towards increasing angles, while the detector collects the fluorescence signals resulting in depth profiling of all the elements. At large X-ray incident angles, the X-ray beam penetrates deep into the sample and the detector gets signals from the surface as well as the bulk of the thin film. At lower X-ray incident angles, the depth from which the signal is collected reduces. Below the critical angle, the X-ray beam is totally reflected and only the topmost nanometers (~10 nm) of the thin film fluoresces. The penetration depth of X-rays is defined as the depth at which the intensity inside the medium is reduced to 1/e, which is approximately 0.3 of the intensity at the surface.35 The X23A2 beamline is equipped with a Vortex* four-element SDD detector with a resolution of approximately 200 eV at 6 keV.36 The four-element detector enables simultaneously monitoring of La, Sr, Co and Fe signals.

For TXRF measurements, the incident photon energy was set at 16305 eV which is 200 eV above the Sr K-edge.40 The fluorescence signals of La, Sr, Co, and Fe were counted individually as a function of incident angle of X-ray beam. The windows within the fluorescence peaks were selected to maximize counts and minimize peak overlapping effects (Figure S6†).

Hard X-ray photoelectron spectroscopy (HAXPES) HAXPES measurements were carried out on LSCF thin films both before and after annealing at beamline X24A at the NSLS-I of BNL. HAXPES was performed using a 6 ACS Paragon Plus Environment

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hemispherical analyzer with a monochromatic incident photon energy of 3000 eV. The incident beam was set at 5 degrees from the sample surface. The sample was kept in ultra-high vacuum (UHV) with chamber pressure less than 10-9 torr. Binding energies were calibrated using the binding energy of Ag 3d electron (Ag 3d5/2=368.27 eV) measured from a silver foil in electrical contact with the sample surface. Peak fitting was carried out using the XPSPeak* peak fitting software package,37 in a similar manner reported by van der Heide.38 A Shirley background was utilized and a convolution of Gaussian and Lorentzian functions was assumed for the peak shape. The Sr 3d is a doublet due to spin-orbit splitting.39 The Sr 3d doublet separation was set at 1.7 eV and area ratio (3d5/2 : 3d3/2) was set at 1 : 0.66. The full width at half maximum (FWHM) values of Sr 3d peaks were fixed at 1 eV and 1.4 eV for bulk-bound and surface-bound Sr, respectively.

High-resolution X-ray diffraction (HRXRD) Reciprocal space mapping (RSM) was performed to characterize the strain state and to calculate the in-plain and out-of-plane lattice constants of the LSCF films on different substrates. RSM measurements were carried out in a Bruker* D8 Discover diffractometer. The diffractometer uses a Cu X-ray tube as its source and is equipped with a 0-dimensional scintillation detector with Pathfinder variable optics. The optics for the primary beam path includes a 4-bounce monochromater coupled with a Goebel mirror for generating monochromatic parallel beam in line focus. The RSM scans were



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carried out at room temperature to measure the in-plane and out-of-plane strain in LSCF-6428, LSCF-7328 and LSCF-8228 on NGO and STO, and LSCF-6428 on GSO. The RSM scans consisted of multiple asymmetrical ω-2θ scans with an ω relative start, to locate (204)-LSCF/(44-4)-NGO, (-204)-LSCF/(-204)-STO, and (-204)-LSCF/(44-4)-GSO peaks in reciprocal space, respectively, for films grown on NGO, STO and GSO substrates. More RSM related information can be found in the Supporting Information†.

Results and Discussion Figure 1a shows the processed data from TXRF measurements for LSCF thin films on NGO substrates with x=0.4, 0.3 and 0.2, plotted as Sr/(Sr+La) ratio versus the X-ray beam incident angle. The data processing procedures can be found elsewhere.24 The profiles for each composition include measurements taken both before annealing (asdeposited) and after annealing (post-annealed), represented by blue and red lines, respectively. Starting from the right of critical angle position marked by the black dashed line in Figure 1a, moving towards lower angles to the left gives the profile of Sr/(Sr+La) ratio going from bulk towards the surface of thin film. The Sr surface segregation as a result of the 10h anneal at 800°C is given by the difference of the postannealed and as-deposited profiles for each composition. There appears to be a depletion of Sr near the very top surface in the as-deposited films. This is attributed to experimental issues related to measurements at very low grazing angles and not to an actual surface depletion of Sr. However; the difference in the profiles at very low 8 ACS Paragon Plus Environment

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incidence angles remains a valid indicator of Sr enhancement at the surface after annealing.

Figure 1a shows that for all three compositions, Sr segregates to the surface of LSCF thin films after annealing while the bulk compositions remain unchanged. Figure 1a is then replotted as Sr/(Sr+La) ratio versus X-ray beam penetration depth in Figure 1b (according to the relationship shown in Figure S7†). The areas under the post-annealed and as-deposited plots in Figure 1b, as well as their differences (shaded areas in Figure 1b, corresponding to the extent of Sr segregation) are tabulated in Table 1. Table 1 shows that the extent of Sr surface segregation decreased from LSCF-6428 to LSCF-7328, then increased from LSCF-7328 to LSCF-8228.

The surfaces of pre- and post-annealed samples were characterized by AFM (Figure 2 left). No surface precipitates were seen on pre-annealed samples (Figure S4 † ). Secondary phases were formed on the surfaces of post-annealed samples for all three compositions. The surface coverage (in area %) of the secondary phases were quantified using ImageJ*41 (Figure 2 right), and the results are tabulated in Table 2. The table shows that the surface coverage (in area %) of secondary phases decreased when the Asite Sr content in LSCF was decreased from 40% to 30%, but then increased again when the Sr content was further decreased to 20%. This is in agreement with the trend measured by TXRF.



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Figure 1. Processed TXRF data for LSCF-6428, 7328 and 8228, before and after annealing, plotted as Sr/(Sr+La) ratio versus X-ray beam incident angle (a), as well as versus the Xray beam penetration depth in logarithm scale (b). The dashed lines in (a) show the locations of the critical angle corresponding to a penetration depth of 10 nm.

Table 1. Areas under curves for TXRF ratio vs. depth profiles. As-deposited

Post-annealed

Shaded area

Normalized by LSCF-6428

LSCF-6428

11.3

20.4

9.1

1.00

LSCF-7328

8.6

11.0

2.4

0.26

LSCF-8228

5.4

9.5

4.0

0.44


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Figure 2. AFM images (2 x 2 µm2) (left) and corresponding ImageJ* analysis (right) of surface coverage on post-annealed (a) LSCF-6428, (b) LSCF-7328 and (c) LSCF-8228 samples. Table 2. Surface coverage analysis of post-annealed surfaces of LSCF on NGO samples using ImageJ. Total Area

Average Size

Area

[um ]

[um ]

%


LSCF-6428

1.152

0.002

28.8

1.00

LSCF-7328

0.62

0.002

15.5

0.54

LSCF-8228

0.764

0.002

19.1

0.66

2

2



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The bonding environments at the surfaces of the LSCF thin films were examined using HAXPES. Figure S8† shows the HAXPES survey scans obtained from a LSCF-6428 surface before and after annealing, using a photon energy of 3000 eV. High resolution scans of Sr 3d portion of the spectra of the as-deposited and post-annealed LSCF-6428, 7328 and 8228 films are shown in Figure 3. The spectra are fitted as a bulk-bound (LSCF lattice) doublet (Sr 3d5/2 and Sr 3d3/2), and two surface-bound doublets representing surface strontium oxide and surface strontium carbonate species, respectively.38 The Sr 3d5/2 peak at binding energy of 131.7 eV can be identified as Sr in the bulk perovskite phase. The high resolution spectra of other elements present in LSCF sample were unchanged and details can be found in Figure S9†.

Figure 3. Sr 3d core level and the peak fittings for as-deposited and post-annealed samples of LSCF-6428, 7328 and 8228.


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To understand the evolution of each component of the Sr 3d core level spectra after annealing, the fractional area (area under individual peak/total area) of each component was calculated using the fitting results of the spectra. The ‘excess’ (defined as: area fraction after annealing - area fraction before annealing) amount of each species after annealing is tabulated in Table 3 and plotted in Figure 4. The result shows the excess total surface Sr decreased from LSCF-6428 to LSCF-7328, and then increased from LSCF-7328 to LSCF-8228, in agreement with the TXRF and AFM results. The excess surface SrCO3 peak areas also show a similar trend (LSCF-7328 has the lowest value). This trend, however, is reversed for the excess surface SrO (LSCF-7328 has the highest value). This trend can be explained by the nature of Sr-rich phases on the LSCF surface, as discussed elsewhere42. The surface phases were found to be SrO covered with a capping layer of SrCO3, presumably due to the reaction between the surface SrO phase and atmospheric CO2. It is possible that the thickness of the capping layer of SrCO3 varies with Sr content, that the SrCO3 capping layer is the thinnest for the surface precipitates on LSCF-7328. Since HAXPES is a surface sensitive technique that probes the top few nanometers of the sample determined by the mean free path of the escaped electrons, a thinner capping layer of SrCO3 in LSCF-7328 could explain a higher oxide signal.



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Table 3. Excess amount of various Sr components after annealing for LSCF samples. (unit: %) Bulk

SrO

SrCO3

Total


LSCF-6428

-31.5

8.7

22.8

31.5

1.00

LSCF-7328

-17.3

10.6

6.7

17.3

0.55

LSCF-8228

-20.6

-0.3

20.9

20.6

0.65

Figure 4. Excess amounts (fractional area differences between the annealed and asdeposited cases) of various species extracted from Sr 3d core level fittings.

The characterization results from TXRF, HAXPES and AFM show the same trend, i.e., the Sr concentration/amount of Sr-rich phases on the surface reduces when the Sr content is reduced from 40% to 30%, but then increases on further lowering the Sr content to 14 ACS Paragon Plus Environment

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20%. The excess amount of Sr/Sr-containing phases analyzed using the different techniques described above, are all shown in the same plot with adjusted scales (Figure 5), with all the values normalized by the corresponding value for the case of LSCF-6428 on NGO. The figure shows excellent quantitative agreement between the three techniques used.

The non-monotonic relationship between the extent of surface segregation and the Sr content of the film (Figure 5) indicates that the Sr content in LSCF is not the only factor affecting Sr surface segregation. Another possible factor may be the lattice strain present in the LSCF thin films imposed by heteroepitaxial growth. To understand the effects of strain, LSCF films of the same composition were grown heteroepitaxially on substrates with different lattice parameters.

(001)-LSCF-6428 (40% Sr on A-site) thin films were grown on single crystal (110)-NGO, (001)-STO, and (110)-GSO substrates, leading to in-plane compressive, relaxed and tensile strains in the thin films, respectively. More information about selection of substrates can be found in Table S1†. The films were annealed in air at 800 °C for 10 hours. The surfaces of post-annealed samples were characterized for surface coverage by second phase precipitates by AFM (Figure 6 left) only, since all three characterization techniques agreed quantitatively (Figure 5). The surface coverages of the secondary phases were quantified using ImageJ* (Figure 6 right). The results are tabulated in Table 4, showing the surface coverage ratio of secondary phases increased with increasing in15 ACS Paragon Plus Environment


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plane strain (with compressive strain being negative and tensile strain being positive). The results showed that strain plays an important role in Sr surface segregation.

Figure 5. Comparison of excess amounts of Sr on the surfaces of LSCF samples of different composition after annealing, measured by different techniques.

Table 4. Surface coverage and strain of post-annealed LSCF-6428 on NGO, STO and GSO sample surfaces Total Area [um2] Average Size [um2]

Area [%]

In-Plane Strain

NGO

1.152

0.002

28.8

Compressive

STO

1.456

0.003

36.4

Relaxed

GSO

1.952

0.002

48.8

Tensile


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Figure 6. AFM images (2 x 2 µm2) (left) and corresponding ImageJ analysis (right) of surface coverage on post-annealed LSCF-6428 on (a) NGO (b) STO and (c) GSO substrates.

Reciprocal space mapping (RSM) analysis was performed to characterize the strain in the LSCF thin films deposited on the NGO, STO and GSO substrates. LSCF has a pseudocubic structure, with a in the range of 3.915-3.925 Å, depending on the Sr content.43 NGO has an orthorhombic structure with a=5.43 Å, b=5.50 Å, and c=7.71 Å.44 STO has a cubic structure with a=3.905 Å.45 GSO also has an orthorhombic structure with a=5.45 Å, 17 ACS Paragon Plus Environment


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b=5.75 Å, and c=7.93 Å.46 The heteroepitaxial relationships between the LSCF thin films and substrates were (001)-LSCF || (110)-NGO, (001)-LSCF || (001)-STO, and (001)-LSCF || (110)-GSO, respectively.

From the RSM results, the in-plane (a, b) and out-of-plane (c) lattice parameters of the LSCF thin films were extracted and the values are tabulated in Table 5. The in-plane (a, b) and out-of-plane (c) lattice parameters of LSCF films on NGO and STO substrates show the same trend, with the values decreasing going from LSCF-6428 to LSCF-7328, and then increasing from LSCF-7328 to LSCF 8228. Also, films under compressive in-plane strain are under tensile out-of-plane strain, and vice versa (Figures S11 and S12†).

Table 5. Lattice parameters for LSCF thin films on various substrates. [unit: Å] NGO

STO

GSO

In-plane Out-of-plane In-plane Out-of-plane In-plane Out-of-plane LSCF-6428

3.879

4.010

LSCF-7328

3.864

3.975

LSCF-8228

3.888

3.997

3.896

3.993

3.954

3.908

The main effect of strain on Sr surface segregation is on the diffusion of Sr cations within LSCF lattice due to the change of volume of the unit cells, which affects the activation energy of migration. The volumes of the unit cells and cubic root of the volumes (proxy for an effective lattice parameter of a cubic unit cell subjected to anisotropic strains) for 18 ACS Paragon Plus Environment

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LSCF-6428, LSCF-7328 and LSCF-8228 thin films on NGO, as well as LSCF-6428 thin films on NGO, STO and GSO substrates were calculated from the RSM results and tabulated in Table 6 and Table 7, respectively.

Table 6. Calculated volume and cube root of volume of unit cells for LSCF thin films on NGO substrates. [unit: Å] a

b

c

Volume [Å3]

Cubic root

LSCF-6428

3.879

3.879

4.01

60.337

3.922

LSCF-7328

3.864

3.864

3.975

59.349

3.901

LSCF-8228

3.888

3.888

3.997

60.421

3.924

Table 7. Calculated volume and cube root of volume of unit cells for LSCF-6428 on various substrates. [unit: Å] a

b

c

Volume [Å3]

Cubic root

NGO

3.879

3.879

4.01

60.337

3.922

STO

3.896

3.896

3.993

60.609

3.928

GSO

3.954

3.954

3.908

61.098

3.939

Taking the strain effect into consideration, the trend of Sr surface segregation observed in LSCF thin films with various Sr content on NGO substrates can be explained. Although 19 ACS Paragon Plus Environment


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the Sr content is decreasing monotonically from LSCF-6428 to LSCF-7328 to LSCF-8228, the cubic roots of volumes of unit cells follow the same trend as the Sr surface segregation (Table 6). However, the net tensile strain (magnitude of cubic root of unit cell volume) is the largest for LSCF-8228, resulting in an enhanced Sr surface segregation in LSCF-8228, despite the low Sr content.

A model is developed correlating the surface area coverage ratio with the Sr content of LSCF thin films and its related strain, and the details are presented in the Supporting Information† (pages S-10 – S-15). The model assumes that the equilibrium ratio of the Sr concentrations at the surface and bulk is fixed at a constant temperature. Two kinetic steps in series control the overall rate of surface precipitate formation. These are i) the rate diffusion of Sr from the bulk to the surface, and ii) the rate of surface reaction to form the precipitate. The diffusion kinetics is affected by the strain in the film due to its effect on the migration energy of the Sr atoms. This gives:

Af =

1 Xb K exp(− I ε ) + n

(1)

where the fractional area coverage of the surface precipitates, Af, can be expressed explicitly in terms of the bulk Sr concentration in the thin film, Xb, and the relative strain, ε, present in the LSCF lattice. Equation 1 has with 3 independent parameters K, I and n that are constant for the experimental conditions in this study, and can be extracted by fitting Eq. 1 with the data points used for the fitting tabulated in Table 8.


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Some conclusions can be directly made from Eq. 1. For a fixed relative strain, the surface area coverage ratio is linearly proportional to Sr content. The relative strain is affecting the surface area coverage ratio by affecting the Gibbs free energy of migration of Sr cations, where a positive relative strain will decrease the Gibbs free energy of migration, and vice versa. Consequently, for a fixed Sr content (Xb), the surface area coverage ratio Af increases with increasing relative strain, ε. The fitting result is shown in Figure 7, where area coverage ratio Af is plotted as functions of Sr content in LSCF and relative strain. The deviations of the fit from the data points were relatively small, indicating a good fit. The fitting parameters used in this model can be found in Table S3†.

Table 8. Data points used in modeling and the deviation of the fit. Xb

ε

Af (measured)

Af (fit)

Deviation

0.4

0

0.288

0.314

5.0%

0.4

0.00439

0.364

0.365

0.5%

0.4

0.01253

0.488

0.480

-1.6%

0.3

-0.01964

0.147

0.139

-4.8%

0.2

0.00191

0.181

0.171

-5.5%



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Figure 7. Plot of predicted surface area coverage by Sr-rich precipitates as a function of Sr content and relative strain in the LSCF thin films. The data points are marked as crosses in the figure, with solid cross representing data point above the surface and transparent crosses below the surface. The arrows represent the directions and magnitudes of the deviation of the fit from the data points.

The temperature (T) and oxygen partial pressure (pO2) used in the model were set at 1073.15 K (800 °C) and 0.21 atm, respectively. The depletion length of Sr, l, is assumed to be 10 nm.47 The height of Sr-rich surface phases, h, is the average height obtained by measuring the height of 10 particles from the AFM image of each of the surface of postannealed LSCF samples. The molar volume of the surface phases, Vm, is calculated based on the study of nature of Sr-rich phases on surface that can be found elsewhere42, where a fixed ratio in thickness of SrO and SrCO3 layers is assumed. The annealing time was fixed at 10 hours. The rest of the parameters were either obtained from references or extracted from the fitting parameters of the model. 22 ACS Paragon Plus Environment

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The parameters extracted from the fitting results were found to be in a physically meaningful range. The Gibbs free energy of migration of Sr cations in LSCF lattice was fitted at 180 kJ·mol-1, which is in between 145 kJ·mol-1 and 193 kJ·mol-1 reported by Huang et al.48 The diffusion coefficient (or diffusivity) of Sr in LSCF was calculated to be 4.4×10-21 m2s-1 (or 4.4×10-17 cm2s-1), which is of the same order of magnitude as reported by Kubicek et al47, where they studied cation diffusivity in LSC and its relevance to Sr segregation. The estimated time at which steady-state conditions could be achieved was also calculated using the following equation49:

l2 τ= DSr

(2)

Where τ is the estimated time to reach steady state, which is calculated to be 22,729 seconds in this study. The annealing time of 10 hours (36000 s), is greater than this value, validating the steady state assumption.

Conclusions In this study, the Sr surface segregation phenomenon was studied as a function of Sr content and strain of LSCF thin films. TXRF results showed that the Sr segregation to the surface was reduced by lowering the Sr content in LSCF from 40% to 30%, while it was enhanced by further lowering Sr content from 30% to 20%. The surface morphology and the nature of Sr-bonding of the Sr-rich secondary phases on the surfaces of post-



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Page 24 of 35

annealed LSCF samples were investigated by AFM and HAXPES, showing the same trend that agreed with the TXRF results. The strains in LSCF thin films with various Sr content on various substrates (NGO, STO and GSO) were measured with RSM using HRXRD and the Sr surface segregation was found to be reduced with compressive strain and enhanced with tensile strain present within the thin films. The strain in this study is defined as the relative strain and is determined by the fractional difference in volumes of unit cells in LSCF thin films at 800°C with the volume of an LSCF unit cell on NGO at 800°C.

A model was developed correlating the Sr surface segregation with Sr content and the relative strain. A surface was fitted based on the model and the physical parameters extracted from the fit agree well with the available data in the literature. It was found that Sr surface segregation increased linearly with increasing Sr content at a fixed relative strain, and that it increased monotonically with increasing relative strain at a fixed Sr content. The model indicates that the effect of bulk Sr content is to increase the equilibrium surface concentration of Sr, while the effect of strain is to modify the diffusivity of Sr cations by changing the Gibbs free energy of migration of Sr cations.


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Supporting Information† PLD target preparation; thin film characterization; TXRF spectra, raw data; HAXPES survey scan, fine spectra; RSM results; model details; substrate selection information; list of terms used in the model; and parameters used in the model.



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Page 26 of 35

Acknowledgements This

work

is

supported

through

the

DOE

SECA

program

under

Grant

DEFC2612FE0009656. Materials synthesis was performed using EMSL, a DOE Office of Science User Facility sponsored by the Office of Biological and Environmental Research and located at Pacific Northwest National Laboratory. Use of the National Synchrotron Light Source, Brookhaven National Laboratory, was supported by the U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under Contract No. DEAC02-98CH10886. Additional support was provided by the National Institute of Standards and Technology. The Bruker* D8 Discover diffractometer used for HRXRD studies was purchased through NSF-MRI Award #1337471. The authors would like to acknowledge the contributions of Mr. Barry Karlin at NIST, Dr. Jeff Bacon, Mr. Jeff Woodward and Dr. Alexey Y. Nikiforov at Boston University.


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Disclaimer * The inclusion of company names is for completeness and does not represent an endorsement by the National Institute of Standards and Technology.



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TOC Graphic


Effect of Sr Content and Strain on Sr Surface Segregation of La1

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