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Chemical strain kinetics induced by oxygen surface exchange in epitaxial films explored by time-resolved X-ray diffraction Roberto Moreno, Pablo García, James Zapata, Jaume Roqueta, Julienne Chaigneau, and Jose Santiso Chem. Mater., Just Accepted Manuscript • DOI: 10.1021/cm401714d • Publication Date (Web): 03 Sep 2013 Downloaded from http://pubs.acs.org on September 8, 2013
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Chemistry of Materials
Chemical strain kinetics induced by oxygen surface exchange in epitaxial films explored by time-resolved X-ray diffraction. Roberto Moreno,a Pablo García,a James Zapata,a Jaume Roqueta,a Julienne Chaigneaua and José Santiso.*a,b a
ICN2, Institut Català de Nanociència i Nanotecnologia, Campus UAB, 08913 Bellaterra (Barcelona), Spain.
b
CSIC, Consejo Superior de Investigaciones Científicas, ICN2 Building , Campus UAB ,08193 Bellaterra (Barcelona), Spain. KEYWORDS: Oxygen surface exchange, solid state ionics, solid oxide fuel cells. ABSTRACT: We have developed a new method to study the oxygen surface exchange kinetics in oxide materials in the form of epitaxial thin films by analyzing subtle cell parameters variations induced by changes in the oxygen stoichiometry of the material. The method consists of continuously analyzing the X-ray diffraction pattern of particular film reflections with a linear X-ray fast detector in a static position, while exposing the sample to sudden changes in the pO2 of the atmosphere at elevated temperatures. With this method, we have been able to follow cell parameter changes as small as 2.10-4 Å in time intervals as short as 10 sec in La2NiO4+ epitaxial films and La2NiO4+ /LaNiO3- bilayers. This method provides a simpler and contact-less tool for dynamically analyzing oxygen surface exchange kinetics and diffusion in transition metal oxide compounds, and complements other currently used techniques like Electric Conductivity Relaxation (ECR) and Isotopic Exchange depth profiling (IEDP). In addition, this method is a unique tool to address oxygen transport across solid-solid interfaces in thin film heterostructures.
INTRODUCTION The study of oxygen in-take and out-take kinetics is relevant for the use of transition metal oxides in resistive chemical sensors.1,2 Additionally, it reveals the fundamental mechanisms for oxygen reduction end evolution reactions (ORR and OER, respectively) in oxide catalysts.3 It is also becoming increasingly important for the determination of oxygen surface exchange kinetics and diffusion in cathode materials used in solid oxide fuel cell (SOFC) technology.4 The overall mechanism for ORR at the cathode surface involves different steps, including chemical or physical adsorption, oxygen dissociation and incorporation, some of which follow concurrent paths.5,6 The rate determining step may depend on the material characteristics, surface microstructure and ambient conditions. In general, the oxygen dissociation/formation step consists of a reduction/oxidation reaction involving the adsorbed O2 species and the oxygen ionic defects (vacancies or interstitials), as well as the electronic charges (electrons or holes). Therefore, the kinetics for the oxygen surface exchange rates k in mixed ionic-electronic conductors may depend on various factors such as the concentration of the charged species, the enthalpies of the different steps, as well as the migration energies of the involved species towards and across the surface.
In oxide conducting materials, the characteristic oxygen diffusion lengths at moderate temperatures (T’s) are in the order of microns. In thin films with thickness below one micrometre, the diffusion process is very fast, thus oxygen stoichiometry changes are mostly governed by surface exchange mechanisms.7 These processes are generally addressed by means of impedance spectroscopy,8 electric conductivity relaxation (ECR),9 Isotopic 18O Exchange Depth Profiling (IEDP) experiments 10 and even optical absorption methods.11 However, some of these techniques show important drawbacks. In ECR, the variations in electric conductivity are a convolution of the conductivities of different parts of the sample, including possible sheet conductivities arising from surface adsorption layers of physical or chemical nature, or interface effects. These factors, as well as possible catalytic effects at the electrodes may hinder the interpretation of the results. In IEDP tracer experiments, the analysis of the 18O concentration profile in the sample after exposure to an 18 O-enriched gas atmosphere provides a very powerful tool to determine the surface exchange rate k* and oxygen diffusion D*. However, these experiments are relatively complex and time consuming. Despite the inherent limitations of these techniques, both are widely used for oxygen surface exchange rate characterisation, particularly for SOFC cathode materials optimisation. Nevertheless, the development of simpler and contact-less techniques is
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of general interest as complementary tools for this type of analysis. In transition metal oxides the variation in the oxygen stoichiometry is often accompanied by subtle changes in the cell parameters, i.e.: the so-called chemical expansion, caused by variations of the oxidation state of the cations, and consequently in their apparent ionic radii, as well as electrostatic interactions generated by the charged defects.12,13 As an example, in La2NiO4+ the incorporation of interstitial oxygen from = 0.02 to 0.18 causes a monotonous increase in the cell volume measured at room temperature from 378.5 to 379.96 Å3, respectively, around +0.4%.14 This change is approximately 103 times the precision of standard cell parameter determination by X-ray diffraction (XRD).15 Thus, the possibility to dynamically follow the cell volume variations by XRD at the same time scale than the oxidation/reduction process would provide a direct insight into the kinetics of the corresponding ORR/OER mechanisms. In some applications, such as in catalysis or oxygen sensing, the reaction rates take place typically in the milliseconds range. For this type of analysis, non-conventional time-resolved XRD techniques using pulsed X-ray sources (laser pumped) are readily attainable.16 In a much larger time scale of a few seconds, recent studies making use of synchrotron-source XRD have monitored in-situ the time-dependent chemical expansion produced by transient oxygen exchange in electrochemical cells at different applied voltages.17 For polycrystalline thin films with randomly-oriented crystallites the very weak intensity of the reflections limits the dynamic analysis unless synchrotron radiation is used. However, for highly textured or epitaxial thin films the Xray reflections may become intense enough to be monitored by conventional X-ray sources. In a previous study we analysed in-situ the cell parameter changes by XRD,18 but the limited accuracy and time scale resolution (of several minutes) of the former setup did not allow to study in detail the surface exchange dynamics in the temperature range of interest for SOFC applications. Therefore, the aim of the present study is to demonstrate the use of a conventional lab XRD setup for in-situ timeresolved measurements of the chemical expansion in epitaxial films after variation in the pO2 at elevated T’s. In the present study, a modification of the experimental setup along with the use of fast X-ray linear detector successfully validated the technique in a time scale of a few seconds. As a case study, we used epitaxial films of La2NiO4+ , which is a catalytically active material for oxygen reduction, and a promising candidate to be used as a cathode in SOFC technology.19 In order to determine the selectivity of the present technique, we also tested epitaxial La2NiO4+ and LaNiO3- bilayers analysing their individual response. EXPERIMENTAL La2NiO4+ epitaxial films were grown by pulsed laser deposition (PLD) on SrTiO3(001) single crystal substrates at T= 750 °C, pO2= 58 mTorr and laser fluence of 0.75
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J/cm2 (KrF excimer laser, = 248 nm, 2 Hz repetition rate). Under these conditions the films are purely c-axis oriented and morphology is flat with roughness (rms) = 2 nm. For the gas exchange experiments, the films were exposed to 500 cm3/min flow of either dry air (21.0% O2) or N2 gas (oxygen activity was about 50 ppm O2 as measured by a YSZ oxygen sensor placed a short distance before the sample). Fast switching from one atmosphere to the other is achieved by means of a pneumatic valve in less than 100 msec. No temperature changes were observed in the samples during gas switching. For the X-ray diffraction analysis we used a X’Pert PRO MRD diffractometer (from PANalytical) with a fast linear solid-state PIXcel detector. This multichannel solid-state detector allows the simultaneous measurement of a 2 range of 2.51° (255 channels with a 2 resolution of about 0.01° in static mode) for the 320 mm goniometer radius. In a typical XRD measurement on polycrystalline nontextured samples and 2 goniometer angles are fixed to the initial values of a particular symmetric hkl reflection (incidence angle is half the value of 2 diffracted beam angle in reflection geometry). The measurement of the diffracted intensity over a linear detector in static-mode (without any variation in the goniometer angles) provides information of a 2 angle range (2.51° for the present detector type and goniometer radius) and the variation in the peak angles are directly correlated with the average cell parameters through the Bragg’s law. This is depicted in Fig 1.a, where the detector is tangent to the Ewald sphere construction. A small variation of the cell parameters of a polycrystalline material is equivalent to a variation of the radius of the corresponding Laue circle for that hkl reflection. The blue short line corresponds to the equivalent position of a linear detector. The intersection between the detector position and the Laue circle indicates that Bragg condition is satisfied, and therefore the shift in the 2 angle is measured by the detector. However, for epitaxial films a similar static measurement over a symmetric 00l reflection will follow a trajectory over the
Figure 1. Reciprocal space projection along with Ewald sphere construction to evidence diffraction conditions for (a) a polycrystalline film; (b) symmetric reflection for epitaxial film, and (c) largely asymmetric reflection for epitaxial film. The filled spots for the epitaxial films and the continuous circle for the polycrystalline film represent the equilibrium film structure, while the empty spots and the dashed line correspond to the compressed cell structure.
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Chemistry of Materials
Table 1. Angular positions of different hkl film and substrate reflections La2NiO4 hkl -1 0 7 -1 0 9 -2 0 10 -3 0 9 -1-1 10 -2-2 10
(º)
2 (º)
53.15 55.63 79.91 101.75 66.96 96.89
56.015 71.16 93.21 108.29 84.08 112.00
(º) 0 0 0 0 45 45
exit angle 2 - (º) 2.86 15.53 13.3 6.54 17.12 15.11
reciprocal space as depicted in Fig. 1.b. Therefore, a reduction of the out-of-plane parameter (expansion in the reciprocal lattice) makes the 00l reflection to lie out from the detector, and deviate from Bragg conditions. In epitaxial films one may consider that coherent growth with substrate makes the in-plane parameter to match that of substrate (at least below a certain critical thickness from what progressive strain relaxation is expected). Therefore, any variation in film cell volume due to changes in oxygen stoichiometry will be limited to variations in film out-ofplane parameter. This corresponds to a vertical axis in the reciprocal space representation. The only measuring conditions where a 2 scan follows an almost vertical line in the reciprocal space are those of asymmetric reflections ( ) with grazing exit angle, which correspond to ~ 2 , as depicted in Fig. 1.c. The selection of the proper hkl reflection for monitoring the out-of-plane parameter variations depend in the availability of sufficiently intense reflections at high 2 angles in the grazing exit angle geometry. For instance, in epitaxial c-axis oriented La2NiO4+ films on SrTiO3(001) substrates, a suitable choice of hkl reflections will correspond to the reflections in table 1. La2NiO4 hkl reflections are indexed following the I4/mmm space group with a=b= 3.861 Å, c=12.684 Å.14 However, it is convenient to simultaneously measure the possible variations in the substrate cell parameters to use it as an internal reference. The table also indicates the reflections for a cubic perovskite like SrTiO 3 (space group Pm-3m, a= 3.905 Å) in close proximity (2 , and angles) to the corresponding film peaks. Since the acceptance angle of the detector is of about 2.5 degrees, it is therefore clear that the only pair of reflections in the list with 2 < 2.5° is -1 -1 10 La2NiO4 and -1 -1 3 SrTiO3 reflections (this is the best choice for monitoring together film and substrate variations, but once demonstrated that substrate variations are negligible any other film peak in the list will successfully depict its cell parameter variations). Although the corresponding exit angle 2 - is the largest in the list, which means that 2 scan trajectory slightly deviates from the vertical axis in reciprocal space, the peak broadening in horizontal axis still allows the peak maximum to be followed without any additional error as depicted in Figure 2. The figure corresponds to reciprocal space maps for the -1 -1 10 La2NiO4 reflections along with 1 -1 3 STO reflections measured after stabilization at high temperature of 600 °C under N2 (left) and air (right) atmospheres. Substrate reflection position is not affected by
F(hkl) 106.1 22.1 47.7 15.5 68.6 42.6
hkl STO substrate -1 0 2 -1 0 3 -2 0 3 -3 0 3 -1 -1 3 -2 -2 3
2 52.73 57.03 79.02 101.81 66.1 97.7
52.34 77.18 90.67 113.63 81.72 108.8
2 -3.81 -6.02 2.54 -5.34 2.36 3.2
the change in atmosphere (differences in cell parameters c < 2 10-4 Å, corresponding to the instrumental error), which indicates that there are no substantial temperature variations between both measurements ( T < 1 °C, as estimated from the SrTiO3 expansion coefficient). However, film reflection shifts its position resulting in measured cell parameters c= 12.7627 Å, a= 3.9008 Å, and cell volume V= 194.14 Å3 for N2, while c= 12.7860 Å, a= 3.8987 Å, and V= 194.35 Å3 for air. These variations correspond to about c/c = +0.18 %, a/a = -0.05%, V/V = +0.1%, which are consistent with the relative variations reported for bulk material.14 In those measurements of bulk material at room temperature the incorporation of excess oxygen in interstitial sites of La2NiO4+ from =0.02 to 0.18 causes a cell expansion along c-axis (about +1.6%) while it shrinks along a,b-axis (-0.45%). The much larger expansion along c-axis (around four times larger than the contraction along a,b) causes an overall cell volume expansion of
Figure 2. XRD reciprocal space maps of a c-axis oriented La2NiO4+ epitaxial film deposited on SrTiO3(001) substrate obtained under N2 (left) and air atmosphere (right). The map contains -1-1 10 film reflection (upper) along with -1-1 3 reflection from the substrate (lower). The red lines indicate the line scan corresponding to the linear detector in static position. The black horizontal lines indicate the vertical shift of the reflection centroid corresponding to the c-axis parameter change.
about+0.4%. The absolute values of the cell parameters of film and bulk material are not directly comparable because the film is submitted to an additional tensile in-
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plane strain because of the epitaxial growth on SrTiO3. Still, their relative variations can be roughly estimated to correspond to a film oxygen stoichiometry variation of about = 0.04 from N2 to air atmosphere (four times smaller than that reported in bulk from =0.02 to 0.18). In a first approximation, provided the cell parameter variations are considered linearly dependent on the oxygen excess in the very narrow = 0.04 region, monitoring the c-axis relative variations would be a reasonable estimate of the oxygen stoichiometry changes. Because of the fast acquisition needed for kinetic measurements of film oxygen exchange the experiments concentrate in monitoring the out-of-plane c-axis variations. In a first approximation, the oxygen exchange rate at the surface is proportional to the concentration gradient imposed by the chemical potential in the different atmospheres
Where c(t) is the defect concentration at a given time, ceq is the equilibrium concentration, kexch is the surface exchange rate coefficient and d the film thickness. This holds for small step variations in pO2, below one order of magnitude, as generally used in ECR experiments. However, the precise determination of the out-of-plane cell parameter by XRD requires larger step changes. This may have some implications in the calculation of accurate k values that are discussed in detail in the following section, but still applies for qualitative k estimates. If we assume that the oxygen composition across the layer thickness is homogenous due to the small layer thickness (below hundred nanometres) compared to the characteristic diffusion length (several microns), the concentration will follow a simple exponential expression:
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were performed at sample temperatures between 400 and 750 °C. Switching gas lines with stable flows of about 400 cm3/min guarantees a gas exchange response time of a few seconds for the small chamber volume (Anton Paar DHS-1100C). Any changes in the samples with response time above this range were attributed to the oxidation/reduction process. X-ray diffraction profiles of a 2 angle range of 2.5° (simultaneously measured by the 255 channel detector) are collected at acquisition time intervals from 10 to 40 sec, depending on the corresponding kinetics (the higher the temperature the faster the required acquisition time). The 2 window was centred at the initial peak position. The 2 scans for a 50 nm thick film La2NiO4 measured at 600 °C when gas is changed from air to N2 are depicted in Figure 3 in time intervals of 10 sec. The broad peak around 2 = 83° corresponds to the -1 -1 10 film reflection, while the narrow peak at 82.3° corresponds to a trace coming from the intense -1 -1 3 substrate reflection (although the substrate signal does not correspond to its maximum intensity it still serves as a reference). Since this substrate peak does not vary in position at a given temperature one may exclude any shift due to possible sample misalignments or temperature variations after gas exchange (equivalent temperature changes T < 1 °C). Film peak progressively shifts towards higher 2 angle after the gas atmosphere has been switched from air to N2 gas. This is consistent with a c-axis shortening (and therefore a volume decrease) due to oxygen stoichiometry reduction, in accordance with the expected behaviour in La2NiO4+ compound. The recorded XRD spectra were fit to single-peak pseudo-Voigt curves and peak positions were extracted for each time interval. The c-axis parameter was calculated by using the following equation:
where is the peak angle, = 67° (constant) is the incidence angle for the measurement, λ= 1.5406 Å is the Cu K wavelength, and l= 10 is the Miller index of the chosen reflection (-1 -1 10 in this case). If one assumes a direct correspondence between oxygen composition and film cell parameters, and neglecting any possible relaxation time due to the mechanical response of the structure, those would also follow a simple exponential time dependence with characteristic time response = d/kexch. The study of the surface exchange rates k at different temperatures may provide fruitful information about the activation energies for oxidation and reduction. ECR measurements were also performed on epitaxial La2NiO4+ films of the same thickness by using the same pO2 step change from N2 to air.
Fig. 4 depicts the c-axis parameter variation with time for the same sample in Fig. 3 after N2/air cycling at 600 and 700 °C. At 600 °C the parameter varies from c= 12.745 Å under N2 to c= 12.765 Å under air (corresponding to a 0.15% variation). Oxidation step from N2 to air is fast and it only takes about 3-5 min before full stabilization (measurement step is 16 sec), while reduction from air to N2 is clearly slower taking more than 1h to reach 95% of the full reduction. Still after a few hours the c-axis parameter seems to continue shortening at a very slow rate. At 700 °C both c-axis parameter values under N2 and air are larger because of the thermal expansion. They vary from c= 12.758 Å under N2 to c= 12.785 Å under air (0.21% variation). These values are in good agreement with those pre-
RESULTS AND DISCUSSION Time response of La2NiO4+ single film structure. Gas exchange experiments from N2 to air (or vice versa)
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Chemistry of Materials
Figure 5. Logarithmic representation of the reduced c-axis parameter variation with time for (a) oxidation and (b) reduction, at different temperatures. The lines correspond to the best fit to the experimental data of a double exponential decay dependence. (The data for each T have been shifted vertically for clarity). The insets correspond to a zoom of the first 500 sec. Figure 3. Line XRD scans obtained at different time (time intervals of 10 sec) when changing the atmosphere from air to N2 at 600 °C. The dashed line corresponds to the film peak position in air.
was not possible to separate both processes although a slight deviation from the linear dependence is still observed. The faster response taking place during the first minutes is depicted in the insets of Fig. 5 for both oxidation and reduction steps. In both cases it is clear that response times 1 become shorter when increasing temperature. However, oxidation response is always faster than reduction. Values of 1 and 2 ( 1
