Catalytic Dynamics and Oxygen Diffusion in Doped PrBaCo2O5.5+δ

Oct 19, 2015 - Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, United States. Sihyuk Choi, Areum Ju...
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Catalytic Dynamics and Oxygen Diffusion in Doped PrBaCo2O5.5+δ Thin Films Erik Enriquez, Xing Xu, Shanyong Bao, Zach Harrell, and Chonglin Chen* Department of Physics and Astronomy, University of Texas at San Antonio, San Antonio, Texas 78249, United States

Sihyuk Choi, Areum Jun, and Guntae Kim Department of Energy Engineering, Ulsan National Institute of Science and Technology (UNIST), Ulsan, 689-798, Korea

Myung-Hwan Whangbo Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States ABSTRACT: The Sr and Fe codoped double perovskites PrBaCo 2 O 5.5+δ (PrBCO) thin films of Pr(Ba 0.5 Sr 0.5 )(Co1.5Fe0.5)O5.5+δ (PBSCFO) were epitaxially grown for chemical catalytic studies. The resistance behavior of PBSCFO epitaxial films was monitored under the switching flow of reducing and oxidizing gases as a function of the gas flow time, t, using an electrical conductivity relaxation (ECR) experimental setup. The R(t) vs t relationships determined at various temperatures show the occurrence of two oxidation processes, Co2+/Co3+ ↔ Co3+ and Co3+ ↔ Co3+/Co4+. Mathematical fitting of the observed R(t) vs t relationships was carried out using Fick’s second law for one-dimensional diffusion of charge carriers to derive the diffusivity D(T) and τ(T) for the two processes at various temperatures, T. The D(T) vs T relationships were analyzed in terms of the Arrhenius relationship to find the activation energies Ea for each process. Oscillations in the dR(t)/dt plots, observed under oxidation reactions, were discussed in terms of a layer-by-layer oxygen vacancy exchange diffusion mechanism. Our work suggests that thin films of LnBCO (Ln = lanthanide) with their A and B sites doped as in PBSCFO are excellent candidates for the development of low or intermediate temperature energy conversion devices and gas sensor applications. KEYWORDS: transport properties, ion diffusion, surface catalysis, PrBaCo2O5+δ, MIEC

1. INTRODUCTION Catalysts for oxygen reduction operate on a seemingly simple principle of reducing molecular oxygen O2 into ionic species O2−, but this reaction has become a source of intense research interest due to its technological significance and complex chemical dynamics. These catalysts encompass a broad range of applications from energy production by oxygen separation membranes and solid oxide fuel cells (SOFCs)1−6 to environment-monitoring in gas sensors7 and solid state electrolysis devices.8,9 Reduction of oxygen generally occurs on the surface of a catalyst via the reaction: O2 + 4e− → 2O2−. The oxide ions O2− can diffuse through electrolyte materials to produce clean and efficient energy. Much of the recent research has focused on optimizing and tuning the properties of dense ceramic materials to understand the mechanisms of the complex surface exchange and diffusion reactions, which are aimed at lowering the operating temperature of oxygenreduction materials without sacrificing the required ionic permeation properties for these devices.10,11 Promising thin film materials of double perovskites AA′B2O5.5+δ (A = © 2015 American Chemical Society

lanthanide, A′ = alkaline earth, B = transition metal) such as LaBaCo2O5.5+δ (LaBCO) and PrBaCo2O5.5+δ (PrBCO) operate in the intermediate temperature range (300−600 °C) with superfast oxygen exchange kinetics.7,12,13 (Note that the double perovskites are related to the perovskites ABO3, which are made up of corner-sharing BO6 octahedra with the A-site ions occupying the 12-coordinate sites. Thus, in ABO3, the BO2 layers alternate with AO layers.) Tuning of the material transport properties and even chemical ordering has been achieved in these materials by interface-engineering.14−16 To better understand and optimize the properties of these materials for technological applications, many of the ratelimiting factors and surface exchange mechanisms must be explored further. Pure electronic conductors such as platinum have been widely studied as oxygen reduction catalysts, as well as other Received: August 22, 2015 Accepted: October 19, 2015 Published: October 19, 2015 24353

DOI: 10.1021/acsami.5b07688 ACS Appl. Mater. Interfaces 2015, 7, 24353−24359

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Figure 1. Simplified illustration for the designs of (a) three-phase boundary oxygen reduction catalysts and (b) MIEC catalysts.

Sr2+, the cobaltates can have fractional Co oxidation states such as Co2+/3+ and Co3+/4+ and hence a higher ionic conductivity than those with nonfractional Co oxidation states such as Co2+ and Co3+, since the energy required for oxygen ion hopping is much lower in mixed-valence states than in a single-valence state.27 When oxygen vacancies occur randomly throughout the material, the strain created at the vacancy-defect sites is expected to be much larger than the case of ordered oxygen vacancies in double perovskite materials, and the oxygen nonstoichiometry generates structural phase transitions from the cubic structure of perovskite cobaltates to brownmilleritetype orthorhombic or hexagonal perovskite structures.28−30 These transitions have not been observed for epitaxially ordered double perovskite materials by previous research efforts.7 Doping of the A and A′ site cations in double perovskites AA′Co2O5.5+δ presents a new avenue of promise because it allows the retention of diffusion properties and stability of the A/A′-site charge-ordered structure, but modifies the electrical and transport properties by partially substituting with various divalent cations, which can create a charge imbalance leading to a possible increase in the oxygen vacancy concentration.22,31 The Co atom sites can also be doped with another element such as Fe, which exhibits multiple oxidation states similar to Co. This doping adds an element of potential control over the ionic/electronic conductivities as well as the thermal expansion coefficient. Sr-doping at the La site of LaCoO3 has been shown to increase its ionic conductivity and surface exchange coefficient. Similarly, Fe-doping at the Co site in various perovskite cobaltates has been shown to have a significant effect on the surface exchange properties.22 In the present work we study the resistance response of highly epitaxial Pr(Ba0.5Sr0.5)(Co1.5Fe0.5)O5.5+δ (PBSCFO) thin films under the alternating flow of oxidizing/reducing gases in the intermediate to high temperature range (463−1073 K). Our study shows the occurrence of layer-by-layer oxygen-vacancy exchange diffusion.

metallic and oxide materials with practically no ionic conduction component.17−19 These materials generally excel in catalyzing oxygen dissociation reactions, but they are limited by the need to rely on local three-phase boundary points (Figure 1a) or by the need for composite designs to transport the dissociated oxide ions (Figure 1b). Designs for SOFC cathodes utilizing pure electronic conductors generally adopt microporous or composite structures and require high operating temperatures. In this respect, mixed ionic/electronic conductors (MIECs) are desirable due to their ability to catalyze oxygen dissociation and permeation reactions at lower temperatures. This is the basis for many intermediatetemperature SOFC designs. Although MIECs represent significant potential for oxygen reduction and SOFC technology, oxygen reduction catalysts of MIEC type must be optimized to achieve high permeation while maintaining desirable surface catalytic properties.11,20,21 Thin films have generated interest in these endeavors due to their high surface to volume ratio. The characteristic thickness (Lc) of a material is defined as the ratio of the diffusion coefficient (D) to the surface exchange coefficient (k).22 Reports of the characteristic thickness for MIECs are generally of the order of 1−100 μm, although discrepancies exist among the reported values.23−25 Materials with a thickness on the order of Lc are considered to be of a mixed or diffusion limited regime, while thin films that are generally on the order of 102 nm and below are considered to be dominated by surface exchange reactions. In double perovskite cobaltates AA′Co2O5.5+δ, a large difference in the ionic radii of A3+ and A′2+ (e.g., La3+, Ba2+) makes AO sheets alternate with A′O sheets (Figure 2),26 with oxygen vacancies ordered such that they exist primarily on the AO sheets. Furthermore, with divalent cations such as Ba2+ and

2. EXPERIMENTAL SECTION Double perovskite PBSCFO thin films with c-axis preferential orientation were fabricated by pulsed laser deposition technique on (001) LaAlO3 substrates. The fabrication process in similar structures is described in detail in previous reports.32 To fabricate the PBSCFO target, the PBSCFO powder was synthesized by the Pechini method using Pr(NO3)3·6H2O (Aldrich, 99.9%, metal basis), Ba(NO3)2 (Aldrich, 99+%), Sr(NO3)2 (Aldrich, 99+%), Co(NO3)2·6H2O (Aldrich, 98+%), and Fe(NO3)3·6H2O (Aldrich, 98%) with the addition of ethylene glycol and citric acid as heterogeneous agents in distilled water. The synthesized PBSCFO powder was pressed into pellets and then sintered in air at 1423 K for 12 h. We employed the

Figure 2. Structures of double perovskites with three possible ordered oxygen vacancy structures: (a) LnBaCo2O6, (b) LnBaCo2O5.5, and (c) LnBaCo2O5. 24354

DOI: 10.1021/acsami.5b07688 ACS Appl. Mater. Interfaces 2015, 7, 24353−24359

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Figure 3. R(t) vs t plots measured under the switching flow of the reducing and oxidizing gases at various temperatures. The blue and red curves represent the R(t) values under the reducing and oxidizing gases, respectively.

then from Co3+ to Co2.5+.31 It is worth noting that although the fabricated PBSCFO thin films can undergo many cycles while largely retaining the resistance response shown in Figure 3, after cycling for approximately 100 h the samples begin to show a slightly higher base resistivity achieved in the fully oxidized state, which may suggest that structural defects or disorder drives introduced over the course of many redox cycles. A comprehensive structural study to analyze and quantify the changes occurring in these materials over time will be carried out in the near future. At temperatures below 673 K, the observed change occurs from the fully reduced oxidation state Co2.5+ under H2 to the fully oxidized state Co3.5+ under O2. First-principles density functional theory calculations performed in a previous study27 have suggested that the reactions on the film surface from Co3.5+ to Co2.5+ occur in the following steps:

deposition temperature of 1123 K under an O2 pressure of 250 mTorr with the energy density of about 2.0 J/cm2 and the frequency of 5 Hz for 30 min. A postannealing treatment was performed immediately after the deposition in a 200 Torr pure oxygen environment for 15 min at 1123 K, followed by a gradual cooling to room temperature at 5 K/ min. Films are approximately 100 nm in thickness and optimized to obtain highly epitaxial samples free of cracks or porous microstructures, as reported elsewhere.13,15,33 Surface resistance measurements using an electrical conductivity relaxation (ECR) experimental setup were performed on approximately 2 mm × 3 mm surface area sections of the PBSCFO thin films with a Lakeshore 370 AC resistance bridge. Samples were exposed to alternating oxidizing (O2) and reducing (a mixture of 4%H2 + 96%N2, which will be referred to as H2 for simplicity) gas environments with flow rates of 100 standard cubic centimeters per minute at atmospheric pressure in a tube furnace with an approximate volume of 1 000 cm3. The resistance R(t) of the PBSCFO thin films as a function of the gas flow time t was recorded at various temperatures within the range of 463−1073 K. At temperatures below approximately 553 K, it was observed that the reducing gas did not fully convert the PBSCFO thin films from the fully oxidized mixed-valence state of Co3+/Co4+ (abbreviated here as Co3.5+) (Figure 2a) to the fully reduced mixedvalence state of Co2+/Co3+ (abbreviated here as Co2.5+) (Figure 2c). To investigate the transition from the fully reduced to the fully oxidized state at a given temperature below 553 K, samples were saturated at 623 K in the H2 reducing gas, which was followed by lowering the temperature at the rate of 10 K per minute to the target temperature. Finally, this was followed by a switch to the O2 oxidizing gas.

LnBaCo2O6 [Co3.5+] → LnBaCo2O5(OH) → LnBaCo2O5.5(OH)[Co3+] + H 2O (1)

LnBaCo2O5.5(OH)[Co3+] → LnBaCo2O4.5(OH) → LnBaCo2O5[Co2.5+] + H 2O (2)

We note that the resistance R(t) of LnBaCo2O5+δ is highest when its Co atoms are in the single-valence state Co3+ and is reduced when its Co atoms are in the mixed-valence states (e.g., Co2.5+ and Co3.5+). For the oxidation reaction from Co2.5+ through Co3+ to Co3.5+ at temperatures below 623 K, a decrease in the operating temperature decreases the overall reaction rate steadily in both oxidizing and reducing processes (Figure 4). At 553 K the entire reaction after saturation under H2 to a near saturation under O2 occurs within 10 s, while at 463 K the same reaction requires more than 30 min to complete. At a given temperature the redox reaction rate observed for the PBSCFO thin films would be governed by its diffusivity, which is a measure of how fast its mobile ions diffuse within the lattice when a population gradient is created at the surface of the thin film. The responses of the resistance R to the flow time

3. RESULTS AND DISCUSSION The R(t) vs t plots of the PBSCFO thin films measured by ECR in the range of 553−1073 K are presented in Figure 3. These R(t) vs t plots are highly reproducible in the investigated temperature range. At any temperature between 1073 and 673 K, the R(t) rises quickly and converges to a maximum value Rmax as the flow time t of the H2 gas increases. When the temperature is decreased from 1073 to 673 K, the Rmax under the reducing gas increases sharply. At a temperature below 673 K, the R(t) increases sharply to approximately 1.7 × 107 Ohms and is then followed by a gradual decrease with increasing flow time t of the H2. These variations of R(t) are related to the changes in the cobalt oxidation state from Co3.5+ to Co3+ and 24355

DOI: 10.1021/acsami.5b07688 ACS Appl. Mater. Interfaces 2015, 7, 24353−24359

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t of the redox gas suggests that two distinct diffusion stages occur at temperatures below 623 K with the resistance peak corresponding to Co3+. This two-stage diffusion activity indicates that a switch from the H2 to the O2 first creates a gradient of hydrogen partial pressure on the film surface. Hydrogen then diffuses outward, yielding the transition from Co2.5+ to Co3+. In a similar manner, oxygen begins to diffuse into the thin film, leading to the transition from Co3+ to Co3.5+. To quantify these transitions, we describe the observed R(t) vs t relationships in terms of Fick’s second law for onedimensional diffusion of charge carriers which, we assume, are created by an instantaneous change in partial pressure of a redox gas at time t = 0. Then, the conductance (i.e., 1/R) at time t can be written as34 ∞

2t 1 e[−(2i + 1) τ ] 2 (2i + 1)

∑ i=0

(4)

where D is the diffusivity of a specific charge carrier during the diffusion reaction and L is the total thickness of the thin film. According to eq 3, τ is approximately equal to the time Δt = t2 − t1 needed for the (σm − σ1)/(σ2 − σ1) value to become 0.702. The constant τ can be used to estimate the time to reach the monovalence state of Co3+ after a change in the redox gas partial pressure at t = 0 initiates the diffusion in the respective redox reactions. Using eq 3, we can see that, at t = τ(π2/2) ≈ 5τ, the total change in the mean conductance (σm − σ1) relative to the final state (σ2) is over 99%, so a relaxation time value of τeq = τ(π2/2) is used here. Table 1 below summarizes the τeq and D values for the oxidation-state change between Co2.5+ and Co3+ and that between Co3+ and Co3.5+ during the oxidation and reduction processes. For the oxidation change between Co2.5+ and Co3+ and for that between Co3+ and Co3.5+ during the oxidation and reduction processes, we describe the D(T) vs T data using the Arrhenius relationship

Figure 4. R(t) vs t plots obtained during the oxidation reactions at temperatures between 463 and 553 K.

σm(t ) − σ1 8 =1− 2 σ2 − σ1 π

L2 Dπ 2

D(T ) = D0 exp( −Ea /RT )

where Ea is the activation energy, and R, the gas constant. The plots of ln(D) vs 1/T, for all the redox reactions of the PBSCFO thin films observed in the investigated temperature range, are presented in Figure 5. In both oxidation and reduction processes, the diffusivity decreases with increasing temperature for the Co2.5+ ↔ Co3+ process above ∼593 K and for the Co3+ ↔ Co3.5+ process above ∼673 K, hence the calculation of negative Ea values. Such negative Ea values have been reported in oxygen reduction catalysts that are fabricated below the characteristic thickness (Lc) so that their redox reactions are dominated by surface exchange reaction rates, as reported by Armstrong et al.22 Although mechanistically bulk diffusivity increases with temperature, the dominating contribution of the surface exchange reactions reported by ECR measurements combines contributions of surface and bulk diffusion processes, which can increase or decrease depending on surface activity. On the thin film surface, which serves as the boundary between the solid and gas phases, the oxygen reduction reaction can occur as a precursor mediated

(3)

where σm is the mean conductance at time (t), σ1 and σ2 represent the conductance at the initial state at t1 and the final state at t2, respectively, and τ is the relaxation time given by

Table 1. Relaxation Time τeq (s) and Diffusion Coefficient D (cm2/s) for the PBSCFO Thin Films under the Switching Flow of the Redox Gases at Various Temperatures under O2

under H2

Co2.5+ → Co3+ T (K)

τeq

463 473 493 513 533 553 573 623 673 773 873 973 1073

403.67 156.93 38.0 18.26 10.96 3.70 1.85 4.44

Co3+ → Co3.5+ τeq

D 1.24 3.19 1.32 2.74 4.56 1.35 2.70 1.13

× × × × × × × ×

10−13 10−13 10−12 10−12 10−12 10−11 10−11 10−11

1716.8 544.31 182.59 108.07 30.84 4.69 2.71 2.81 9.67 15.50 14.85 21.22 27.49

Co3+ → Co2.5+ τeq

D 2.91 9.19 2.74 4.63 1.62 1.07 1.84 1.78 5.17 3.23 3.37 2.36 1.82

× × × × × × × × × × × × ×

10−14 10−14 10−13 10−13 10−12 10−11 10−11 10−11 10−12 10−12 10−12 10−12 10−12 24356

155086 1170.5 435.25 3133.6

Co3.5+ → Co3+ τeq

D

3.22 4.27 1.15 1.60

× × × ×

10−16 10−14 10−13 10−14

22164 2715.6 1293.4 631.16 218.12 1322.5 1855.4 4969.3 5782.6

D

2.26 1.84 3.87 7.92 2.29 3.78 2.69 1.01 8.65

× × × × × × × × ×

10−15 10−14 10−14 10−14 10−13 10−14 10−14 10−14 10−15

DOI: 10.1021/acsami.5b07688 ACS Appl. Mater. Interfaces 2015, 7, 24353−24359

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Figure 5. ln(D) vs 1/T plots for the oxidation under O2 (left) and those for the reduction under H2 (right).

Figure 6. Dominant interactions of incident oxygen molecules on the surface at low temperatures (adsorption and dissociation) leading to positive activation energies (left), and those at high temperatures (elastic collision and desorption) leading to calculation of negative activation energies.

Figure 7. Evolution of oscillatory behavior of dR/dt curves with increasing temperature.

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4. CONCLUSIONS In summary, we prepared thin films of PBSCFO by doping the A- and B-sites of PrBCO thin films with Sr and Fe, respectively, and probed their diffusion and surface exchange kinetics of the redox reactions using ECR measurements under the switching flow of redox gases at various temperatures. Our study shows that the activation energy and the temperature of the peak diffusion activity of doped-PrBCO thin films suggests that LnBCO materials are excellent candidates for developing low or intermediate temperature energy-conversion devices and gassensor applications.

dissociation process. This process proceeds in two stages: (1) O2 approaches the surface and is adsorbed by a potential energy well, and (2) O2 is dissociated into individual oxide ions O2− and diffuses into the solid state medium. During the lifetime of the adsorbed O2 on the surface, it can either undergo a dissociation reaction or it desorbs and returns to the ambient gas environment. These two processes are in competition, and when the temperature is increased beyond a critical value, the incident O2 experiences more glancing collisions with the surface, since they possess enough kinetic energy to escape the local potential well at the surface of the thin film. The distinction between these two cases is illustrated in Figure 6. The rising frequency of elastic collisions decreases the available O2 molecules trapped on the surface, which in turn decreases the rate of diffusion and results in a measured negative activation energy. At temperatures within the range 463−653 K, the dR(t)/dt plot exhibits oscillatory behavior during the oxidation reactions, as can be seen in Figure 7. These oscillations occur during the oxidation change from Co2.5+ to Co3+ as well as from Co3+ to Co3.5+, as can be seen from Figure 8. Previous studies on

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the U.S. Department of Energy under DE-FE0003780 and also by the computing resources of the NERSC Center and the HPC Center of NCSU. X.X. and S.Y.B. would like to acknowledge the support from the “China Scholarship Council” for their PhD studies at UTSA. E.E. would like to acknowledge the support of NSF No. 1249284 LSAMP-BD.

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REFERENCES

(1) Sunarso, J.; Baumann, S.; Serra, J. M.; Meulenberg, W. A.; Liu, S.; Lin, Y. S.; Diniz da Costa, J. C. Mixed Ionic−Electronic Conducting (MIEC) Ceramic-Based Membranes for Oxygen Separation. J. Membr. Sci. 2008, 320 (1−2), 13−41. (2) Dyer, P. N.; Richards, R. E.; Russek, S. L.; Taylor, D. M. Ion Transport Membrane Technology for Oxygen Separation and Syngas Production. Solid State Ionics 2000, 134 (1−2), 21−33. (3) Yu, A. S.; Kim, J.; Oh, T. S.; Kim, G.; Gorte, R. J.; Vohs, J. M. Decreasing Interfacial Losses with Catalysts in La0.9Ca0.1FeO3−δ Membranes for Syngas Production. Appl. Catal., A 2014, 486, 259. (4) Choi, S.; Yoo, S.; Kim, J.; Park, S.; Jun, A.; Sengodan, S.; Kim, J.; Shin, J.; Jeong, H. Y.; Choi, Y.; Kim, G.; Liu, M. Highly Efficient and Robust Cathode Materials for Low-Temperature Solid Oxide Fuel Cells: PrBa0.5Sr0.5Co2−xFexO5+δ. Sci. Rep. 2013, 3, 2426. (5) Hashim, S. M.; Mohamed, A. R.; Bhatia, S. Current Status of Ceramic-Based Membranes for Oxygen Separation from Air. Adv. Colloid Interface Sci. 2010, 160 (1−2), 88−100. (6) Miller, C. F.; Chen, J.; Carolan, M. F.; Foster, E. P. Advances in Ion Transport Membrane Technology for Syngas Production. Catal. Today 2014, 228, 152−157. (7) Liu, J.; Collins, G.; Liu, M.; Chen, C. L.; Jiang, J. C.; Meletis, E. I.; Zhang, Q. Y.; Dong, C. A. PO2 Dependant Resistance Switch Effect in Highly Epitaxial (LaBa)Co2O5+δ Thin Films. Appl. Phys. Lett. 2010, 97 (9), 094101. (8) Hauch, A.; Ebbesen, S. D.; Jensen, S. H.; Mogensen, M. Highly Efficient High Temperature Electrolysis. J. Mater. Chem. 2008, 18 (20), 2331−2340. (9) Ni, M.; Leung, M. K. H.; Leung, D. Y. C. Technological Development of Hydrogen Production by Solid Oxide Electrolyzer Cell (SOEC). Int. J. Hydrogen Energy 2008, 33 (9), 2337−2354. (10) Adler, S. B.; Chen, X. Y.; Wilson, J. R. Mechanisms and Rate Laws for Oxygen Exchange on Mixed-Conducting Oxide Surfaces. J. Catal. 2007, 245 (1), 91−109. (11) Adler, S. B. Factors Governing Oxygen Reduction in Solid Oxide Fuel Cell Cathodes. Chem. Rev. 2004, 104 (10), 4791−4843. (12) Kim, G.; Wang, S.; Jacobson, A. J.; Yuan, Z.; Donner, W.; Chen, C. L.; Reimus, L.; Brodersen, P.; Mims, C. A. Oxygen Exchange

Figure 8. Resistance vs time plot obtained for the oxidation reaction at 503 K, with the central inset below the curve showing a detailed view of the curve with slight undulations occurring in the slope. The inset in the upper right corner shows the oscillatory behavior of the slope change throughout the reaction in a dR(t)/dt vs time plot, with dotted blue lines marking the local maxima with their corresponding points on the resistance vs time graph.

GdBCO material have confirmed that oxygen diffusion within the ab-plane is much easier, energetically, than diffusion along the c-axis direction.35 This suggests the possibility of diffusion along the c-axis of the LnBCO double perovskite thin films by an oxygen vacancy exchange mechanism, which acts between adjacent layers of LnO (Ln = La, Gd, Pr, Er, Nd, etc.) and CoO2 layers, as well as between BaO and CoO2 layers. Due to the charge discrepancy between Ln3+ and Ba2+ in double perovskite materials, it is energetically favorable for the oxygen vacancies to reside in the LnO layers rather than the BaO layers. This vacancy-site preference can lead to different average oxygen diffusion rates through the LnO and BaO layers. Previous investigations27 of ErBCO and PrBCO thin films show that this depends on the ratio between ionic radii of the A-site Ln3+ and Ba2+ atoms. The dR(t)/dt vs t plot for disordered perovskite materials such as LaSrCoO3+δ does not show oscillatory behavior, most probably because they do not possess a charge ordered structure. 24358

DOI: 10.1021/acsami.5b07688 ACS Appl. Mater. Interfaces 2015, 7, 24353−24359

Research Article

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DOI: 10.1021/acsami.5b07688 ACS Appl. Mater. Interfaces 2015, 7, 24353−24359