Defective Interfaces in Yttrium-Doped Barium Zirconate Films and

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Defective interfaces in Yttrium-doped Barium Zirconate films and consequences on proton conduction Nan Yang, Claudia Cantoni, Vittorio Foglietti, Antonello Tebano, Alex Belianinov, Evgheni Strelcov, Stephen Jesse, Daniele Di Castro, Elisabetta di Bartolomeo, Silvia Licoccia, Sergei V. Kalinin, Giuseppe Balestrino, and Carmela Aruta Nano Lett., Just Accepted Manuscript • DOI: 10.1021/acs.nanolett.5b00698 • Publication Date (Web): 19 Mar 2015 Downloaded from http://pubs.acs.org on March 23, 2015

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Defective interfaces in Yttrium-doped Barium Zirconate films and consequences on proton conduction Nan Yang1,2,3, Claudia Cantoni4, Vittorio Foglietti1,5, Antonello Tebano1,2,5, Alex Belianinov6, Evgheni Strelcov6, Stephen Jesse6, Daniele Di Castro1,5, Elisabetta Di Bartolomeo2,7, Silvia Licoccia2,7, Sergei V. Kalinin6, Giuseppe Balestrino1,5 and Carmela Aruta1,2,5*

1.

National Research Council CNR-SPIN, University of Roma “Tor Vergata”,

Rome I-00133, Italy. 2.

NAST Center, University of Roma “Tor Vergata”, Rome I-00133, Italy.

3.

Engineering Faculty, Università degli studi Niccolò Cusano, Rome I-00166,

Italy. 4.

Materials Science and Technology Division, Oak Ridge National Laboratory,

Oak Ridge, TN 37831, USA. 5.

Department DICII, University of Roma Tor Vergata, Rome I-00133, Italy.

6.

Center for Nanophase Materials Sciences, Oak Ridge National Laboratory,

Oak Ridge, TN 37831, USA. 7.

Department of Chemical Sciences and Technologies, University of Roma

“Tor Vergata”, Rome 00133, Italy.

1 ACS Paragon Plus Environment

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Abstract Yttrium doped Barium Zirconate (BZY) thin films recently showed surprising electric transport properties. Experimental investigations conducted mainly by electrochemical impedance spectroscopy suggested that a consistent part of this BZY conductivity is of protonic nature. These results have stimulated further investigations by local unconventional techniques. Here we use Electrochemical Strain Microscopy (ESM) to detect electrochemical activity in BZY films with nanoscale resolution. ESM in a novel cross sectional measuring set-up allows the direct visualization of the interfacial activity. The local electrochemical investigation is compared with the structural studies performed by state of art scanning transmission electron microscopy (STEM). The ESM and STEM results show a clear correlation between the conductivity and the interface structural defects. We propose a physical model based on a misfit dislocation network which introduces a novel 2D transport phenomenon, whose fingerprint is the low activation energy measured.

Keywords: ionic conduction, interface defects, doped barium zirconate, electrolytes, SPM, STEM, perovskite oxide thin films

Doped perovskite oxides have been widely investigated in recent years as proton conducting solid electrolytes for a variety of electrochemical devices,


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such as fuel cells, hydrogen sensors, electrolyzers and hydrogen pumps1, 2. Excellent chemical stability of BaZr0.8Y0.2O3-δ (BZY) makes it one of the most promising electrolyte materials for protonic fuel cells.3,

4

However, the

conductivity of BZY, reported by many groups varies widely introducing major uncertainties for its implementation in real devices5. Theoretical and experimental studies on doped barium zirconate proved that this material exhibits high proton conductivity when the grain boundary contribution to the transport properties is suppressed. 6, 7. This effect has been later investigated in epitaxial BZY, 1 µm thick films, grown on (100)-oriented MgO substrates confirming the theoretical predictions8 and showing a protonic conductivity, measured by Electrochemical Impedance Spectroscopy (EIS), higher than oxygen ion conductors, in the temperature range of 300 to 600 °C. These results stimulated further studies of BZY properties in epitaxial thin films especially focused on the interplay between structural and transport properties. Surprisingly, very high values of conductivity, at temperatures of 550 °C - 600 °C, have been recently measured by EIS in few tens of nanometers thick BZY film grown on (110) NdGaO3 (NGO)9, with a 10% lattice mismatch between the film and substrate. These results suggest that heavily strained interfaces may represent a key parameter for tailoring defect densities in thin epitaxial films, thus affecting their conductivity.

10

Here we

report nanoscale evidence of higher electrochemical activity at the interface


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by Electrochemical Strain Microscopy (ESM), in both in-plane and crosssection geometry. The ESM study, coupled with the scanning transmission electron microscopy (STEM) analysis, shows a clear correlation between the conductivity of our perovskite thin films and the defective interfaces. Crosssectional measurements with different scanning probe microscopy techniques already demonstrated to be a powerful tool to study the interface properties of complex oxide thin films11,

12, 13

. We extend such an approach to ESM to

directly visualize the interface reactivity and compare it with the crosssectional structural properties obtained by STEM. We demonstrate that the microscopic origin of such a high conductivity in BZY films is the strongly defective interface between film and substrate. Local electrochemical activity in electrolyte materials can be investigated at high spatial resolution by ESM14,15. This technique utilizes the SPM tip as a moving electrode as it scans over the sample surface. When bias is applied to the sample surface, at a sufficiently high probe bias, the electrochemical reaction at the tip-surface junction can be activated, resulting in generation or annihilation of mobile ionic species below the tip. The movement of those mobile species will result in a change in the molar volume (electrochemical strain). The associated dynamic surface deformation can be detected at the 2–5 pm level16,

17

. Protonic defects in BZY are formed from water gas phase

dissociation, where the hydroxide ion fills an oxide ion vacancy and the proton


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forms a covalent bond with lattice oxygen6. By ESM, further protons can be injected or extracted by electrochemical reactions (see below) giving rise to the surface deformation. The initial state of hydration of our samples can be estimated from the thermodynamic data available for bulk BZY (see Supporting Information). Our films are expected to be fully hydrated even at room temperature and in the range of experimental partial pressures used. Considering that the films in this study are much thinner than 1µm, the water uptake can approach equilibrium in less than a few hours, for the entire range of experimental conditions. In ESM the conductive tip, with a nanometer sized apex, confines the electric field to a small volume of material (comparable with tip contact area) in presence of a water meniscus at the point of contact between the hydrophilic tip and the film. When the tip is positively biased with respect to the grounded electrode, with a potential sufficiently higher than the redox potential V ~ 1.23 V, the water oxidation reaction occurs, generating protons, oxygen gas and losing electrons to the anode (see Supporting Information). Protons will migrate inside the film due to the high local electric field generated by the tip, resulting in the following chemical reaction:

4× + 2 → 4∙ + + 4

(1)

Under negative tip bias the water evolution reaction occurs:

4∙ + + 4 → 4× + 2


(2)

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These reactions have low redox potential and will be prevalent at low voltage tip bias. Further oxygen evolution and reduction reactions, involving creation and annihilation of oxygen vacancies inside the film, can appear at higher bias voltages (see Supporting Information and discussion below). Our measurements aimed at finding a correlation between strongly defective interface and electrochemical activity, and elucidating the microscopic origin of the high conductivity in BZY thin films9.

Figure 1. (a) Topography of a 1 × 1 µm2 area of 500 nm thick BZY film on NGO. The 600 × 600 nm2 region selected for the ESM measurement is highlighted by the black square. (b) Spatial distribution of the cantilever’s contact resonance frequency over the region indicated in (a). (c) Schematic of the cross-sectional ESM measurement. The second electrode not shown in the figure is grounding the film surface. (d) Hysteresis loops averaged over different 20 nm × 600 nm areas of the cross section map reported in (h): (A) is the film region close to the BZY film surface, (B) is close to interface with NGO substrate and (C) is within the NGO


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substrate region. (e)-(h), ESM hysteresis loop area maps at different maximal bias voltage: 24 V (e), 45 V (f), 52 V (g), 60 V (h). In panel (h) the vertical dashed black line indicates the interface between BZY and NGO. The colored dashed squares indicate the regions where the ESM loops of panel (d) were acquired.

The BZY film on NGO substrate was fractured to perform a transverse measurement across the interface. The surface of the fractured BZY/NGO is not atomically flat, as it can be seen in the AFM topography of Fig. 1a. The height variation in the averaged topography line profile is found to be of the order of 10 nm. The main reason for the presence of a hill-valley morphology in the cross-sectional topography lies in the fracturing dynamics of two different materials18. ESM measurements on the cross section allowed direct visualization of the electrochemical interface activity, as shown in Fig.1. The schematics of the cross sectional ESM geometry is reported Fig. 1c. By comparing the AFM topography and the ESM contrast of Fig.1e-h, the interface can be precisely located and the left and right sides can be directly attributed to the BZY layer and NGO substrate. The ESM signal shows a hysteretic behavior, which arises from the positive and negative nucleation biases (PNB and NNB) required to activate the chemical processes of ionic injection or annihilation. The shape of the loops from different regions is shown in Fig.1d. The PNB and NNB are identified in the hysteresis loops as the voltage corresponding to the inflection points. In addition to the ESM signal, we have measured the resonance frequency shift


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of the tip, as shown in Fig. 1b. The tip resonance frequency gives direct information about the changes in mechanical properties of the tip-surface junction, which can depend on friction, viscous flow, and plastic deformation. The enhanced surface interaction experienced by the tip is equivalent to an increase in elastic modulus of the material, thus increasing the resonance frequency19. To rule out the topography effect, we can compare the resonance frequency map with the ESM response map. The spatial distribution of the resonance frequencies on the sample surface (Fig. 1b) shows a large variation that reflects changes in the topography. On the contrary, the ESM signal shows a bias dependent response which depends on the electrode interaction with the underneath material. At voltages just above the nucleation bias, a slight but well-defined contrast signal can be measured, as shown in Figure 1(e). A large variation of the ESM signal at the interface is clearly visible increasing the bias voltages, as shown in Fig. 1 f-h. We would like to stress that the ESM response depends on the distance from the interface and that the intensity at the interface exhibits a strong bias dependence. The hysteresis loop data of the cross-section map reported in Fig. 1d have been averaged in three regions: close to the surface (A), close to the interface (B) and within the substrate (C). Both ESM loop area and loop height increase when the cross-sectional measurement is performed in the region close to the interface. By increasing the applied bias the contrast can be improved


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suggesting that electrochemical reactions are occurring close to the interface. This is in agreement with the conductivity results obtained from previous electrochemical impedance spectroscopy measurements in BZY films on the NGO substrates9. Our cross-sectional ESM measurements confirm that BZY films can be viewed as consisting of two different regions, with the most electrochemically active located near the interface.

2

Figure 2. (a) Hysteresis loops averaged on a 1 × 1 µm surface area of two films with

different thickness, 300 nm and 20 nm. (b) Schematic of the measurements on the film surface where the probing depth of about 20 nm is highlighted. G indicates the grounding electrode.

Fig. 2 shows the ESM signal recorded in the standard planar geometry for two BZY films that are 300 nm and 20 nm thick, respectively. The probing depth of the ESM measurement is about 20 nm14, thus in the case of the thinner film


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we are more sensitive to the interface region. The ESM signal shown in fig 2a is clearly stronger for the thinner film, thus confirming that the electrochemical activity

is

enhanced

close

to

the

interface,

consistently

with

the

electrochemical behavior obtained in the cross-sectional geometry (Fig 1d). The similarity between the ESM measurements in the cross-section and planar configuration allows us to use the results obtained on the two films to study the electrochemical process close and far away from the interface. The ESM loop curves shown in fig 2 are relative to a bias tip ranging from -35 up to 35 V. In this region the shape of the two curves looks very different. In the 20 nm thick film, the shape of the hysteresis curves does not show any sign of saturation. These observations support the hypothesis that the measurement falls inside of the ohmic transport region16,

20,

where the

transport of charged defects, i.e. protons, is linearly related to the local electrical conductivity of the film. In this regime larger bias results in increased loop area. In the case of the 300 nm thick film, Fig. 2a shows that the ESM response saturates at around ±15V. There can be different mechanisms responsible for the saturation of the hysteresis loop. One possibility is related to a mass transport limited reaction; we rule this out, however, due to the fact that the thinner film does not show any saturation in the loop curve, under the same conditions as the thicker film. We believe that in our set-up a lower mobility of charged protons in thicker films, as compared to the thinner film,


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may lead to the formation of a space charge region in the vicinity of the tip, preventing further injection of mobile species and thus explaining the saturation effects observed in the loop curve. Once the electric field is removed, there is a drift-diffusion of protons inside the BZY film, thus relaxing the strain signal. Analysis of the relaxation time reported in Supporting Information shows a different behavior for BZY films with different thickness. However, it does not allow for a full understanding of the proton dynamics, because it requires an accurate knowledge of the number of carriers injected and of the possible presence of compensating carriers for protons which would eventually enable ambipolar diffusion. To further elucidate the transport mechanism, we conducted humidity and temperature dependent ESM. Furthermore, to avoid the uncertainty related to the choice of the bias window in ESM and in order to separate the reaction and transport processes, we utilize the first order reversal curve (FORC) method21. FORC is a powerful tool to study the on-site transport process for a typical electrochemical reaction-diffusion system. It consists in progressively increasing the maximum bias voltage Vmax of the ESM measurements thus allowing systematic tracking of the evolution of loops. The envelope of probe bias is a bipolar excitation waveform with increasing amplitude. This allows multiple hysteresis loops with increasing bias to be collected at each spatial location.


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A demonstration of the proton injection by reactions (1) and (2) is given by the humidity dependence measurements reported in Figure 3. It can be observed that the loop area of the 20 nm thick BZY film increases with Vmax and this effect is enhanced by increasing the humidity RH from 10% to 80%. The inset of Fig. 3 shows the comparison between the thicker (300nm) and the thinner (20nm) films. Above the bias tip voltage of ~30-35 V, the ESM loop area of the

Loop area (a.u.)

300 nm thick film also increases.

3 3

Loop area (a.u.)

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2

3

300 nm 20 nm

2 1 40% RH 0 0

2

20

40

60

Bias (V)

1 10% RH 20% RH 40% RH 80% RH

1 0

0

10

20

30

40

50

60

Bias (V) Figure 3. Loop-area vs peak bias voltage for 20 nm BZY film with increasing Relative Humidity. Inset : comparison of the Loop-area vs peak bias voltage for the 20 nm and the 300 nm films, made at 40% RH (similar with ambient condition).

This is a consequence of oxygen evolution/reduction reaction:

× ⇆ ∙∙ + + 2

(3)

In fact, in case of 300 nm thick film, loop area does not depend on RH (see Supporting Information). Larger threshold voltage will be necessary to activate


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oxygen evolution/reduction reaction than the proton injection/water evolution reaction. This may explain the larger threshold voltage observed in the case of the 300nm thick film. (see Supporting Information).22 Temperature-dependent ESM allows us to describe the electrochemical activity in terms of the activation energy Ea, which is one of the key parameters of interest in describing ionic transport. In order to extract the activation energy of the ionic motion, we analyze the averaged loop area across a 1 µm2 area from room temperature to 110 °C. The temperature dependence of averaged loop area for BZY films under different bias voltages are given as ln(loop area) vs. 1/T in Figure 4.

Ea= 0.1 eV

-6

31 V 37 V 42 V

-7 300 nm 20 nm

-6

-8 -9

ln(loop area)

ln (loop area)

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-7 -8 -9

Vmax=31 V -3

-3

-3

2.7x10 3.0x10 3.3x10 -1

1/T(K ) -3

2.7x10

-3

3.0x10

-3

3.3x10

-1

1/T(K ) Figure 4. Temperature dependence of averaged loop area for the 20 nm BZY film under different bias voltages. Inset : comparison of the measurements for the 20 nm and the 300 nm films at 31 V peak voltage bias.

The behavior was fitted using an Arrhenius equation, shown by a solid line. 13 ACS Paragon Plus Environment

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Although the used narrow temperature range of 100 °C limits the accuracy of the measurement, the activation energy (Ea) of the 20 nm film was clearly around 0.1 eV. The inset of figure 4 shows a comparison of the measured activation energies between the 20 nm and the 300 nm thick films, at Vmax =31 V and identical environmental conditions. While the value of Ea measured for the 20 nm thick film is surprisingly low, the film of 300 nm shows an activation energy of 0.3 eV which is much closer to the values reported in literature for bulk protonic conductors, namely about 0.4 eV6. The activation energy measured by ESM is also lower than the previous values obtained by EIS in the case of thin films8,9. However, the range of temperatures of ESM measurements, 20-100 °C, is different than that used to perform conventional EIS measurements, 400-600 °C. A consistent experimental scenario is that the transport phenomena prevailing at lower temperature, whose fingerprint is in the low activation energy, may be obscured at higher temperatures by a further conductive path with higher activation energy. Very recently it has been theoretically demonstrated that the expected value of the activation energy in the BZY bulk should be as low as 0.1 eV, but the actual higher value can be a consequence of the proton trapping23. Our experimental findings, as discussed below, suggest that the presence of defects at the substrate-film interface, mainly dislocations, may create new channels for protonic conduction where the trapping mechanism is possibly


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inhibited. We used state-of-art aberration-corrected STEM/EELS analysis to correlate the local information on the electrochemical activity obtained by ESM with the local structural information. The STEM/EELS results are shown in Fig. 5.

Figure 5. Structural and composition defects near the substrate interface. a) High resolution Z-contrast STEM images of a 10-nm-thick BZY with the NGO substrate, showing numerous dislocations at the interface and associated tilting of BZY film grains. b), c) and d) electron energy loss (EELS) maps for Ba, Zr and O, respectively. The maps were obtained by plotting the integrated intensity of Ba-N4,5, Zr-M4,5, and O-K in a spectrum image acquired in a region indicated by the yellow box in a).

The majority of defects observed in Fig. 5a close to the interface originate from the large mismatch with the NGO substrate. The cubic lattice parameter of BZY perovskite cell is a = 4.22 Å, to be compared with the pseudocubic lattice parameter of NGO which is 3.86 Å. There is a network of dislocations


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at the interface spaced ~ 10 u.c. apart . The amount of strain relieved by the interface dislocations can be estimated using the equation D = bZ/(fO-εO), where D is the average dislocation spacing, fO defines the original misfit between lattices (fO = 0.088), bZ is the Burgers vector component along the interface plane (bZ = 1/2 a), and εO is the residual misfit strain. The obtained value for εO indicates that a large part of the original misfit strain is relieved in the ~ 5 nm-thick interfacial layer containing the dislocations. The EELS spectra of Fig.5b-d show a Ba depletion and Zr accumulation close to the interface. It has been theoretically demonstrated that defects concentrations affect the proton incorporation in proton conductors.24 In particular, proton incorporation can be enhanced with increasing the cation vacancy concentration. In addition, the increased cation distances may reduce the trapping effects leading to a lower proton activation energy.6 However, we cannot discuss the exact interface stoichiometry variation because EELS spectra measurements at the Y edges were not feasible. In the following we concentrate our discussion on the structural defects and their effect on the proton conduction. To investigate the evolution of the strain above the 5 nm interfacial layer, we generated maps of the in-plane and out-of-plane BZY lattice spacing by calculating positions (xij, yij) of the center of mass of the Ba columns in the Zcontrast image of the region indicated in Fig. 6b by a dashed square. The c-


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parameter, given by yij-yi(j-1) is mapped in Fig. 6a and its average along the x direction is plotted in the graph of Fig. 6c. The figure shows that the c-lattice parameter close to the interface is smaller than expected on the basis of an in-plane compressive strain. After several tens of unit cells the bulk c-lattice value is recovered.

Figure 6. (a), c-spacing map of the BZY thicker film over the region indicated by the dashed box in the Z-STEM images of panel (b). (c), BZY c-spacing average along the x direction in a, as a function of y (distance from interface). Dashed line indicates the BZY bulk value.

ESM results suggest that the thinner BZY film possesses a larger electrochemical activity and lower transport activation energy. The structural characterization reveals a large density of the dislocations near the interface. The defects are clearly identified as misfit dislocations spaced ~ 10 unit cells apart. These dislocations reduce the film misfit within a highly defective 17 ACS Paragon Plus Environment

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interface layer of less than 5 nm in thickness. The remaining film strain, generated by lattice tilts and antiphase boundaries is relieved within a thickness of 50-60 unit cells or ~ 20 nm from the interface. There are many examples in literature, showing that dislocations can give a major contribution to modification of the transport properties of ionic and electronic conductors.25,

26

Dislocations have been reported to have an effect

of increased oxygen mobility in oxides,27 while a dislocation mediated mechanism reported recently has been shown to enhance the surface exchange coefficient, decreasing the relaxation time28. However, further experimental and theoretical studies do not confirm any evidence of fast diffusion of oxygen ions along the dislocation array29. In the case of proton conductors the situation is even more controversial because the theoretical studies reported in literature are mainly devoted to the space charge effects on the segregation energy of oxygen vacancies and protons in grain boundaries, and do not consider local spatial correlations between the defects within dislocation cores.30 Nevertheless, it is well known that proton transfer between octahedral sites determines the activation energy of the proton diffusion. One of the main factors affecting the activation energy is the distance between the cations and oxygen ions. However, density functional theory calculations of atomic-scale proton movements31 demonstrate that a structural variation of 5% in the octahedral oxygen distance of BZY should not


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substantially vary the energy barriers of proton migration, which remain in the range 0.4-0.5 eV. Therefore, the c-axis compression alone cannot explain the very low activation energy obtained in the case of the thinnest BZY film. Very recently, a strong dependence of the dislocations structure at the SrTiO3/MgO heterointerface on the ionic transport properties has been demonstrated32. The correlation between our ESM and STEM results suggests that similar mechanisms can be active is this case. A quantitative estimate of the contribution that such an array of dislocations can provide to proton conductivity must rely on ad hoc atomistic simulations. Nevertheless, at the film-substrate interface, the atomic arrangement is highly perturbed along every dislocation line, where chemical defects are also observed. Both cation vacancies and structural distortion may play a relevant role: charge unbalance due to cation vacancy can be compensated by proton incorporation,24 while the local distortions of oxygen octahedra, which coordinate Zr or Y, may reduce the trapping effect.23 Therefore, the presence of a two-dimensional (2D) dislocation network at the interface with the substrate, as shown in Fig. 7, provides easily accessible sites for anchoring H+ and allows for the low activation energy to be measured in our thinner BZY films.


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Figure 7. Schematic drawing of the interface between BZY film and NGO substrate with the dislocation network as deduced from the STEM structural characterization.

In conclusion, many experimental results reported in literature show unusual high values of ion conductivity in very thin films which are explained in terms of defective interfaces.33 In this framework, our study demonstrates a clear correlation between the local electrochemical investigation by ESM with the structural studies performed by STEM. Our results suggest a 2D transport mechanism occurring across the misfit dislocation network with a very low activation energy which gives to a contribution to the high values of proton conductivity obtained in our thin BZY films. However, we stress that our experimental results and the qualitative explanation proposed are related to the in-plane conductivity. Conventional solid oxide fuel cells (SOFCs) consist of a cathode and anode on the opposite surfaces of an electrolyte membrane. There are two separated and sealed gas chambers which supply the fuel and


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the oxidant gases. In the BZY-NGO system we discuss here, the nature of the high conductivity values requires the use of a planar technology without a membrane. Indeed, when the membrane is present, the transverse conductivity rules the transport properties. However, the use of free standing membranes is technologically very challenging considering the thickness of our BZY films. In perspective, to exploit the extraordinary properties of these strongly defective interfaces, dual chamber SOFCs using a planar technology should be fabricated using the state of the art microfluidic and microelectromechanical system technology.

Author information Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing financial interest.

Acknowledgements The authors acknowledge META—Materials Enhancement for Technological Applications

Project

(FP7-PEOPLE-2010-IRSES—Marie

Curie

Actions,

PIRSES-GA-2010-269182. Italian MIUR is acknowledged for support through the FIRB Project RBAP115AYN “Oxides at the nanoscale: multifunctionality and applications” and PRIN Project 2010-2011 OXIDE, “OXide Interfaces: emerging new properties, multifunctionality, and Devices for Electronics and Energy. The research at ORNL was conducted at the Center for Nanophase Materials Sciences, which is sponsored at Oak Ridge National Laboratory by the Division of Scientific User Facilities, U.S. Department of Energy, in the 21 ACS Paragon Plus Environment

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project CNMS2013-032 “Local electrochemical characterization of epitaxial thin films of proton conductor perovskite oxides”. CC acknowledges support by the U. S. Department of Energy, Basic Energy Sciences, Materials Science and Engineering Division.

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Defective Interfaces in Yttrium-Doped Barium Zirconate Films and

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