Defect Chemistry of Pr Doped Ceria Thin Films Investigated by in Situ

Mar 6, 2017 - While oxygen defect equilibrium in functional oxide thin films has a strong impact on device performance and longevity, few methods exis...
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Defect Chemistry of Pr Doped Ceria Thin Films Investigated by in Situ Optical and Impedance Measurements Jae Jin Kim,†,∥ Sean R. Bishop,†,‡,# Di Chen,†,⊥ and Harry L. Tuller*,†,‡,§ †

Department of Materials Science and Engineering, and ‡Materials Processing Center, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, United States § International Institute of Carbon Neutral Energy Research, Kyushu University, Fukuoka 819-0395, Japan S Supporting Information *

ABSTRACT: While oxygen defect equilibrium in functional oxide thin films has a strong impact on device performance and longevity, few methods exist for evaluating it in situ. In this study, simultaneous in situ optical transmission and electrochemical impedance spectroscopy measurements are demonstrated as powerful and convenient means for this purpose, utilizing PrxCe1−xO2−δ (PCO) with multiple oxidation states for Pr (3+ and 4+) as a model system. The Pr4+ color center optical absorptivity increased linearly with the Pr4+ concentration, each independently derived from optical and chemical capacitance measurements, respectively, at elevated temperatures (550−700 °C) and under controlled pO2 (10−4−1 atm), validating the use of optical absorption as a convenient means for monitoring defect concentrations and oxygen nonstoichiometry. The extracted extinction coefficient for Pr4+ (εPr4+ = 5.86 ± 0.12 × 10−18 cm2 with y-axis intercept of 927.5 ± 207.3 cm−1) can now be utilized to study defect equilibria of PCO, and by extension other relevant oxide films, by optical means alone. This enables defect characterization at reduced temperatures where other characterization techniques, for example, chemical capacitance, may not be feasible.

1. INTRODUCTION A fundamental understanding of oxygen defect equilibria and transport kinetics is essential for achieving enhanced performance and longevity in many oxide-based device applications, such as solid oxide fuel cells (SOFCs), oxygen permeation membranes, oxygen storage materials used in emission catalysts, and solid oxide electrolysis cells (SOECs).1 In addition, many metal oxides in these applications experience significant changes in oxygen nonstoichiometry (e.g., changes of δ in MO2±δ) during operation at elevated temperatures and under reducing/oxidizing atmospheres. These deviations from stoichiometry can result in major changes in electrochemical, charge transport, and mechanical properties, which consequentially impact device performance.2,3 The ability to diagnose a material’s behavior under operating conditions, ideally in situ, is therefore especially beneficial. Oxide thin films not only serve as model systems,4,5 given their ability to be routinely reproduced with well-defined geometries and morphologies, but also have become of increasing interest in the fields of solid state ionic devices including micro-SOFCs,6,7 chemical sensors,8 and memristors.9 However, determining and monitoring changes in δ in thin films by conventional characterization methods are more challenging given their low mass, high surface to volume ratio, and interactions between film and support material. For example, while thermogravimetric analysis (TGA) and © 2017 American Chemical Society

coulometric titration directly measure changes in oxygen content (Δδ), upon oxidation or reduction, a reference value δref at a well-defined thermodynamic state is needed in order to establish δ(T, pO2). Given limited sensitivity of TGA and dilatometry, as conventionally applied, these methods are generally inappropriate for monitoring changes in mass and dimensions for thin film structures. Alternatively, electrical conductivity data can be related to changes in nonstoichiometry via a confirmed defect model. However, then the mobilities of the electronic and ionic charge carriers are needed to convert conductivity values to defect concentrations, which, even when known for bulk specimens, may differ in thin films. Besides knowledge of the thermodynamic or steady state values of the oxygen stoichiometry of films, it is also important for the purposes of establishing desired device properties, to characterize the kinetics associated with reaching such steady state values. Oxygen isotope tracer measurements allow the clear deconvolution of surface exchange and diffusion coefficients using quenched diffusion profiles; however, the measurements are time-consuming, utilize expensive instrumentation, are destructive, and cannot be performed in situ. On the other hand, surface exchange kinetics, derived from electrical Received: August 11, 2016 Revised: February 4, 2017 Published: March 6, 2017 1999

DOI: 10.1021/acs.chemmater.6b03307 Chem. Mater. 2017, 29, 1999−2007

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Chemistry of Materials

pO2), allows PCO to serve as a model system for investigating correlations between oxygen defect chemistry and various properties such as electrochemical activity,33 chemical expansion,34−36 elastoplasticity,37 and thermal conductivity.38 The key defect reaction, describing the generation of ionic and electronic defects upon reduction of PCO, written in KrögerVink notation, is described by

conductivity relaxation (ECR) measurements, while performed in situ, are likely to be affected by the catalytic activity of the metal current collector.10 We and others recently have demonstrated that the chemical capacitance (Cchem), obtained from in situ electrochemical impedance spectroscopy (EIS) measurements, can be used to quantify δ in dense oxide thin films over wide limits with high precision.11−17 Furthermore, we earlier demonstrated the ability to both control δ by applying a DC bias across an electrochemical cell and to study its defect chemistry by Cchem measurements even to high oxygen activities (up to 280 atm), not readily accessible by conventional means.18 Optical absorption has also been used to probe the oxidation state of multivalent cations and/or to investigate redox kinetics in oxide single crystals19−24 and thin films.25,26 Optical characterization techniques benefit from the ability to readily achieve in situ and contact-free measurements, and with high spatial resolution, due to the local nature of the interaction of electromagnetic radiation with solid matter. Our group demonstrated that this optical approach can be applied to PCO thin films for in situ recording of redox kinetics as well as the equilibrium defect concentrations ([Pr4+] and consequently δ), by monitoring the change in transmission spectra upon oxygen activity and/or temperature change.10,27 Moreover, with the novel arrangement of simultaneous in situ optical and EIS measurement on the one same film, our preliminary studies at one temperature (600 °C) demonstrated that both defect thermodynamics and surface exchange kinetics in PCO thin films can be characterized and modeled.28,29 In this study, we extend our preliminary study performed on PCO thin films as a function of pO2 (10−4−1 atm) at the single temperature of 600 °C29 to three additional isotherms at 50 °C intervals between 550 and 700 °C. The simultaneous examination of the films by in situ optical transmission and EIS measurement as a function of temperature enables direct investigation of nonstoichiometry in the film and thereby calibration of the extinction coefficient of the Pr4+ color center (εPr4+) as a function of temperature. We also examine the wavelength dependence of the absorption coefficient, originated from the interference fringes in the thin film transmission spectra, with respect to its treatment in the derivation and use of the extinction coefficient. Finally, we examine how, once derived, knowledge of εPr4+ enables one to investigate defect concentrations and nonstoichiometry in PCO and other appropriate thin films by optical studies alone and under circumstances where electrochemically derived parameters such as Cchem cannot be used.

× × 2Pr Ce + OO ↔ 2Pr′Ce + V ··O + 1/2O2 (g )

Pr×Ce,

Pr′Ce, O×O,

V··O

4+

(1)

3+

where and are Pr and Pr on cerium sites, oxygen on an oxygen site, and an oxygen vacancy, the latter with net positive charge of two. In the superscripts, ×, prime, and dot represent net charges of zero, −1, and +1, respectively. The corresponding mass action equation is given by [Pr′Ce]2 [V ··O]pO1/2 2 × 2 × [Pr Ce ] [OO ]

⎛ −Hr,Pr ⎞ ° exp⎜ = k r,Pr ⎟ ⎝ kT ⎠

(2)

where kr,Pr ° is a pre-exponential term, and Hr,Pr is the enthalpy for the reaction. Previously, Hr,Pr in Pr0.1Ce0.9O2−δ thin films was found to be lower than that in the bulk case, resulting in a larger δ for the film for given temperatures and oxygen partial pressures.16 In this work, the oxygen nonstoichiometry δ is defined by δ = [V ··O]/[PrxCe1 − xO2 ]

(3)

where [PrxCe1−xO2] is the concentration of PCO in #/cm3. Given that the concentrations of holes, reduced Ce, and oxygen interstitials are negligibly small under the investigated conditions at pO2s in the vicinity of air, relevant for SOFC cathode operation, the condition for charge neutrality is given by [Pr′Ce] = 2[V ··O]

(4)

The mass and site conservation reactions for PCO are given by × [Pr′Ce] + [Pr Ce ] = [PrCe]total = x[PrxCe1 − xO2 − δ ]

(5)

2.2. Oxygen Nonstoichiometry Extracted from Cchem. The chemical capacitance (Cchem), in contrast to electrical capacitance, represents a measure of a material’s chemical storage capacity under an applied potential.11,12 In the PCO system, it reflects the formation and annihilation of oxygen vacancies, VO·· , upon a given change in oxygen activity commonly described in terms of oxygen partial pressure. Cchem can be expressed as16,17 Cchem = −

2. THEORY 2.1. Defect Chemistry of (Pr,Ce)O2−δ. A comprehensive defect equilibrium and transport model for PrxCe1−xO2−δ (denoted 100x·PCO) solid solution system was developed by the authors.30,31 Unlike fixed valent acceptor doped or undoped CeO2, multivalent doping using Pr4+/3+ leads to a pronounced mixed ionic electronic conductivity (MIEC) at high oxygen partial pressure (pO2 ∼ air), which is beneficial for SOFC cathode performance.32 The ionic conductivity arises from the formation of oxygen vacancies (acceptor behavior), and, for high enough Pr concentration (≥5 mol %), electronic conduction occurs via small polaron hopping of electrons between neighboring Pr cations (impurity band conduction). In addition, significant change in oxygen nonstoichiometry, at readily accessible experimental conditions (temperature and

8q2Vfilm ⎛ ∂[V ··O] ⎞ ⎜pO2 ⎟ kT ⎝ ∂pO2 ⎠

(6)

where q is the elementary charge of an electron, and Vfilm is the film volume, respectively. Then, the oxygen vacancy concentration in the material can be calculated at each pO2 by integrating eq 6 with respect to pO2 as [V ··O](pO2 ) =

kT 8q2Vfilm

∫ Cchemd ln pO2 + [V ··O](pO°2) (7)

[V··O]

where pO2° is a reference oxygen pressure at which is known. In the case of the PCO system, as demonstrated in the authors’ previous work,16 reference values for [V··O](pO°2 ) can be directly extracted from measurements of Cchem. This becomes possible when the mass balance and electroneutrality 2000

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Figure 1. Transmittance spectra of (a) as-prepared (oxidized) ceria and PCO thin films with different Pr concentrations and (b) oxidized and reduced 10PCO thin film on sapphire substrates. Oscillations in transmission with wavelength are due to expected thin film optical interference. (c) Energy band diagram of PCO thin films with illustration for color change in PCO thin films upon pO2 change. Reproduced with modifications from ref 27 for (a) and (b) and from ref 29 for (c), respectively.

Figure 2. (a) Schematic of the cell for simultaneous in situ optical transmission and electrochemical impedance spectroscopy (EIS) measurement. (b) Plot of optical transmittance and log oxygen partial pressure versus time at 600 °C. (c) Typical impedance spectra of symmetric cell PCO/YSZ/ PCO from EIS measurement. Variables are defined in the text.

Starting from [V··O](pO°2 ) obtained by eqs 10 and 11, values of δ at other pO2s not satisfied by the approximations as stated in eqs 8 or 9 can be calculated by using eq 7. It is worth noting that eq 6 and thus eq 7 are valid only for spatially uniform concentrations of defects. This condition is satisfied for the PCO films studied here, since surface oxygen exchange, as opposed to chemical diffusivity, limits oxygen transport into and out of the film, leading to rapid and uniform redistribution of defects within the film.33 2.3. Optical Properties of (Pr,Ce)O2−δ. A strong broad absorption is observed at 2.0−3.3 eV in the visible spectrum of PCO thin films, resulting in a red/orange coloration (see Figure 1 (a) and (c)). This absorption characteristic of the PCO system can be explained by considering Pr impurity level/band formation within the ceria optical band gap (∼3.5 eV39) and resultant optical transitions,27 as illustrated in the energy band

relations take on the approximations described in eq 8 when nearly all of the Pr ions are oxidized to Pr4+ (i.e., high pO2) or eq 9 when nearly all the Pr ions are reduced to Pr3+. × [Pr Ce ] ≈ [PrCe]total

(8)

[Pr′Ce] = 2[V ··O] ≈ [PrCe]total

(9)

[V··O]

Under these circumstances, could be extracted directly from measurement of Cchem, by use of eqs 10 or 11 for the conditions described in eqs 8 and 9, respectively. Cchem =

2 4 q Vfilm ·· [V O] 3 kT

(10)

Cchem =

q2Vfilm ([PrCe]total − 2[V ··O]) kT

(11) 2001

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roughness of 0.7 ± 0.3 nm. From wavelength dispersive X-ray spectroscopy (WDS) measurements, the Pr concentration in 10PCO thin films was found to be 9.7 ± 0.3%, close to the nominal value of 10%. The standards used for Ce, Pr, and O were CePO4, PrPO4, and Fe2O3, respectively. In order to simultaneously perform both optical and EIS measurements on a single sample, one-half of the PCO coated wafer was covered by porous Pt current collectors prepared by reactive sputtering,44 while the other half was contact-metal free (see Figure 2(a)). An Ag counter electrode was sometimes painted on the opposite side of the YSZ wafer to form an asymmetrical PCO/YSZ/Ag electrochemical cell, by using Ag paste (SPI supplies, West Chester, PA). Alternatively, symmetrical cells of the PCO/YSZ/PCO were prepared and characterized. The samples were mounted into a specially designed sample holder, which supports the sample in a vertical orientation and enables contact with the Pt electrical leads and thermocouple. Further details of the novel experimental apparatus, which is designed to enable the simultaneous in situ measurements of the optical absorption and EIS spectra, are found in refs 29 and 45. The electrochemical impedance spectroscopy (EIS) measurements, covering the frequency range from 5−20 mHz to 65−70 kHz with an AC amplitude of 20−30 mV and zero DC bias, were performed using a combination of a Solarton 1260 impedance analyzer and 1286 potentiostat/galvanostat or a ModuLab system (Solartron Analytical) in conjunction with the FRA 1 MHz (2055A) Frequency Response Analyzer module. Zview software (Scribner Associates) was used to fit the data and construct equivalent circuits to analyze the impedance data. The oxygen partial pressure within the quartz tube was controlled by preparing Ar−O2 and H2−H2O−Ar gas mixtures with the aid of mass flow controllers (MKS instruments, Wilmington, MA) and monitored by a YSZ Nernst type oxygen sensor in an external furnace through which the same gas was flowed. Optical transmittance measurements were performed by passing a monochromated beam of light (532 nm) or scanning a wide range of wavelengths (420−560 nm) (Newport Apex Illuminator 70613NS with monochromator 74100). A beam splitter reflected part of the beam to a photodetector prior to passing through the sample to remove instantaneous and longterm variations in light source intensity. Glass band-pass filters (FGB37, 335−610 nm) were placed right before the photodetectors in order to minimize blackbody radiation transmitted from the hot furnace. The probe beam was mechanically chopped and detected using photodetectors connected to lock-in amplifiers, thus further isolating the transmitted light from background radiation. Measurements were performed between 550 and 700 °C.

diagram in Figure 1 (c). In addition to the intrinsic band to band O2p to Ce4f transition across the ceria band gap at UV wavelengths, optically induced transitions from the ceria valence band with O2p character to the Pr level with Pr4f character and from the Pr level to the ceria conduction band with Ce4f character, in principle, become possible. The latter transition though is likely very weak, being an f-f transition and therefore optically forbidden.40 The defect analysis performed previously on bulk PCO (TGA and conductivity measurements) indicated that the Pr impurity level lies 1.43 ± 0.03 eV below the bottom of the ceria conduction band.30,41 Considering this, together with the reported thermal band gap energy of undoped ceria (2.1−3.2 eV31,42,43), the Pr impurity level is placed at 0.67−1.77 eV above the upper edge of the valence band. This range of thermal excitation energy is smaller than the optical transition energy, which agrees with the Franck−Condon principle stating that the vertical optical excitation is greater than the thermal excitation between the relaxed states.40 The probability of optical transitions tied to the Pr level is expected to be pO2 dependent. At sufficiently reducing condition (low pO2), most of the Pr4+ become reduced to Pr3+, so that electrons occupy the Pr levels. Under this condition, because the impurity levels are occupied, the transition from valence band to Pr levels is suppressed, leading to the color change in PCO thin films from red to transparent.27 On the contrary, as Pr becomes oxidized to Pr4+ at high pO2, this optical transition becomes more probable, resulting in the increasing intensity of the red absorptivity of the PCO films (see Figure 1 (b) and (c)). This is also consistent with the observation in Figure 1 (a) that stronger absorption is observed for PCO thin films with higher Pr concentration. As demonstrated in the authors’ previous work,29 this absorption characteristic of the PCO system can serve as a means of quantifying the equilibrium Pr oxidation state (which can be 3+ or 4+) and, in turn, the oxygen nonstoichiometry (δ) via eq 4. With a novel cell design enabling joint in situ optical transmission and electrochemical measurements (Figure 2 (a)), the absorption coefficient related to the Pr4+ ions (αPr4+) extracted from optical measurements can be correlated with Pr4+ concentration independently obtained at the same pO2 values by Cchem measurements, as follows α Pr 4+ = ε Pr 4+[Pr 4 +]

4. RESULTS AND DISCUSSION Figure 2 (b) shows the time dependent transmittance of the 10PCO film, following stepwise changes in pO2 at 600 °C. The spectra for other temperatures (550, 650, and 700 °C) are found in the Supporting Information (Figure S1). For all temperatures, the transmitted light intensity was recorded, while the film was reduced from the initial high pO2 (∼1 atm pO2) in a stepwise manner. As expected, at a given temperature, the 10PCO thin film becomes more transparent as reduced, consistent with decreasing Pr4+ concentration. After the film was reduced under highly reducing condition (pO2 ≤ 10−16), the film’s transmittance becomes constant upon further pO2 change within the regime where most of the Pr ions are reduced to Pr3+. With the transmittance recorded at this condition and ones at each equilibrium state at given pO2s, the optical absorption coefficients of the sample, i.e. Pr4+ color centers (αPr4+), are determined via the Beer−Lambert law, discussed in the authors’ previous work.29 The Pr4+ ion absorption coefficients obtained in this study are shown in Figure 3 as functions of temperature and pO2. As PCO reduces with increasing temperature and decreasing pO2, correspondingly, the absorption coefficient decreases from 550 to 700 °C

(12)

where εPr4+ is the molar extinction coefficient of Pr4+ ions.

3. EXPERIMENTAL DETAILS Details of the 10PCO thin film deposition by pulsed laser deposition (PLD) and characterization are discussed elsewhere.27,33 In summary, 10PCO thin films of 260 ± 9 nm thickness were deposited onto (001) oriented single crystal YSZ (8 mol % Y2O3 stabilized zirconia) substrates (10 × 5 × 0.5 mm3; MTI Corporation, Richmond, CA). The transparent YSZ substrates, polished both sides, were used in the optical transmittance measurements and also served as the electrolyte for EIS measurements. All samples were grown with the same deposition conditions, in the same PLD chamber, with the aim of achieving similar film thicknesses (determined by surface profilometry, KLA-Tencor P-16+ stylus profiler) and microstructures. The X-ray diffraction pattern (XRD; X’Pert PRO MPD, PANalytical) obtained from 2θ−ω coupled scans of the 10PCO films exhibited a highly (001) oriented texture. From surface analysis by atomic force microscopy (AFM; Digital Instruments Nanoscope IV), the grain sizes were estimated to fall within the range of 20−40 nm with a RMS surface 2002

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Figure 3. Plot of the pO2-dependent absorption coefficient for 10PCO thin films between 550 and 700 °C.

and from ∼1 atm to ∼10−4 atm pO2, respectively. One observes that the equilibration time for the change in optical transmittance is much longer than the time required for establishment of the new pO2 value. This optical relaxation upon pO2 step changes enables one to extract the oxygen surface exchange reaction constant from these kinetics, as discussed in detail elsewhere.10,28,46,47 Figure 2 (c) shows typical impedance spectra obtained at 600 °C in air for the symmetric cell, 10PCO/YSZ/10PCO. As shown in the inset, the spectra are represented by a resistor (R1) in series with an R2//CPE2 circuit (a resistor in parallel with a constant phase element [CPE]). CPEs are used to take into account any inhomogeneities in the electrodes resulting in “depressed” arcs not well represented by ideal capacitors.32 The equivalent circuit fits are observed to represent the data well. As discussed by the authors previously,33 R1 reflects the series YSZ ohmic contribution to the overall cell impedance, and R2 is the electrode resistance at the PCO/gas interface, limited by oxygen surface exchange kinetics. Lastly, CPE2 is the chemical capacitance (Cchem) of the PCO film. Typical phase angles for the CPE impedance were 88.6 ± 1.1 degrees, and thus CPE2 demonstrates near ideal capacitance. Additionally, the relatively large Cchem values (e.g., 3−35 mF/cm2) are indicative of chemical capacitance, as reported for other thin film MIEC systems.16,48,49 Details of impedance spectra analysis for the asymmetrical PCO/YSZ/Ag cell are found in the Supporting Information. In Figure 4 (a), the experimentally obtained Cchem values at 550−700 °C are plotted as a function of pO2. The solid curve represents the calculated values of Cchem based on eq 6 using oxygen vacancy concentration values from the thin film defect model with the thermodynamic parameters from the authors’ previous work.16 Good agreement between values from the present work and the predicted values is observed for all temperatures. Following the approach discussed above, oxygen nonstoichiometry values (δ) for the PCO thin films were extracted from experimentally obtained values for Cchem. The absolute stoichiometry was calculated by using eq 10 for 550− 650 °C and eq 11 for 700 °C at pO2 where eqs 8 and 9 are valid, respectively (as indicated with the red dotted circles in Figure 4 (a)). δ was then used to calculate the Pr4+ ion concentration by eq 13 obtained by substitution of eq 4 into eq 5. × [Pr Ce ] ≈ [PrCe]total − 2[V ··O]

Figure 4. (a) Isothermal dependence of volume-specific Cchem and (b) nonstoichiometry (δ) and Pr4+ ion concentration [Pr4+] derived from Cchem of 10PCO thin films on pO2 between 550 and 700 °C. The −1/ 6 and 1/4 indicate the slopes expected at high pO2 and low pO2 where eqs 8 and 9 are valid, respectively.16 The red dotted circles indicate capacitances used to derive absolute stoichiometry using eqs 10 and 11. Solid and dotted lines represent modeled data from ref 16.

The filled and empty dots in Figure 4 (b) show δ and the Pr4+ ion concentration extracted from Cchem, respectively. These results are compared with values calculated by using the thin film defect model (solid and dotted lines).16 Good agreement of the present data with both the magnitude and temperature/ pO2 dependence predicted by the thin film defect model confirms that thin films exhibit different defect formation energies as compared to the bulk, as shown at the authors’ prior study.16 The data in Figure 5 show the expected linear relationship between αPr4+ and [Pr4+] obtained from optical and electro-

Figure 5. Plot of experimentally obtained absorption coefficient of 10PCO as a function of Pr4+ concentration obtained experimentally from Cchem measurement. Data from a previous study at 600 °C29 (open hexagons) and a current study at 550−700 °C (solid symbols) are plotted. Dashed lines represent linear fits to the data.

(13) 2003

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Figure 6. (a) pO2 dependent transmittance spectra of 10PCO thin film measured at 650 °C. (b) Comparison of absorption coefficients calculated at the two different wavelengths indicated in the figure.

Figure 7. (a) Transmittance spectra of 10PCO thin films. Samples I and II represent the samples used in the previous29 and current study, respectively. The films were reduced at 1000 ppm of H2/Ar atmosphere at 600 °C (pO2 ≈ 10−23 atm) for 8 h, followed by rapid quenching to room temperature. (b) Modeled transmittance spectra of undoped ceria thin films grown on sapphire substrates with different thickness.

band gap energy), often combined with high photon flux sources.51−54 Furthermore, light induced defects are most commonly generated close to a material’s surface and remain stable under conditions such as high vacuum and low temperatures. On the other hand, surface oxygen vacancies formed by light irradiation are easily reoxidized by introducing oxygen gas or by heating.51 The experiments in this study were performed with nonfocused visible light (2.21−2.95 eV) and under high temperatures (550−700 °C) and ambient pressure with relatively high pO2 (10−4−1 atm). Furthermore, no noticeable transient change in transmittance was observed due to blocking of the probe light within the data acquisition rate (∼102 Hz). Lastly, the measurements were both reversible and consistent with nonilluminated chemical capacitance measurements performed in parallel. It is therefore a safe assumption to ignore potential contributions from photoinduced defects from our analysis. Figure 6 (a) shows transmittance spectra of 10PCO thin film recorded at different pO2 and at the same temperature, 650 °C, during joint in situ optical and EIS measurements. Each spectrum was recorded after the transmittance reached a new equilibrium state following a pO2 change. Spectra in the 420− 560 nm range of wavelengths were selected considering the transparent window of the bandpass filter (FGB37, 335−610 nm). In the wavelengths of interest, the film is more absorbing (more [Pr4+]) when oxidized. At a highly reduced condition (pO2 = ∼10−21 atm), interference fringes are clearly observed. However, the amplitude of oscillation decreases as pO2 increases, and thus absorption by Pr4+ ions increases. Because Pr4+ ions induce absorption in a broad range of wavelengths,

chemical measurements, respectively. Following eq 12, the value of εPr4+, derived from the slope of αPr4+ vs [Pr4+] for all data at four temperatures, is 5.86 ± 0.12 × 10−18 cm2, while the y-axis intercept shows a small positive offset of 927.5 ± 207.3 cm−1, respectively. The εPr4+ values, extracted by linear fits at each temperature, remain constant, within the error bars, over the temperature range of 550−700 °C (see the Supporting Information). As described above, the absorption at 2.0−3.3 eV in the visible spectrum of the PCO system is characterized by the electronic charge transfer transition from the O2p valence band to that associated with the Pr impurity band formation. This “allowed” transition, while perhaps showing some shift in maximum wavelength at which absorption is initiated, is not expected to show any significant change in oscillator strength with temperature.50 The values of εPr4+ and y-axis intercept in the current study are close in magnitude to those obtained from the authors’ previous work measured at a single temperature (600 °C). There εPr4+ and the y-axis intercept were found to be 5.01 ± 0.14 × 10−18 cm2 and −474.5 ± 223.5 cm−1, respectively.29 While the source of the small y-axis intercept is not known, it may be that there is a source of weak absorption independent of the absorption due to Pr4+. The small discrepancy in εPr4+ values between the two studies, even though they were obtained for the same 10PCO film composition, may result from differences in the measured absorption coefficients, as discussed below. One needs also consider the possibility that additional optically active defects, such as oxygen vacancies, could be generated by irradiation. Photoinduced defects are generally associated with high energy photons (i.e., photon energies > 2004

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Chemistry of Materials any wavelength in this regime can, in principle, be used to extract the absorption coefficient of Pr4+ ions. However, as shown in Figure 6 (b), the absorption coefficient depends on wavelength. For example, the absorption coefficient calculated at 450 nm, where the oscillation reaches a maximum, is higher in magnitude (∼28%) than ones at 532 nm, even though the pO2 dependency of the absorption coefficient at the different wavelengths is identical. This wavelength dependence of the absorption coefficients results in different values of extinction coefficient as calculated from eq 12. The difference in values of absorption coefficients and corresponding extinction coefficients between samples, therefore, can be explained by this wavelength dependence, originating from the oscillatory nature of the spectra resulting from the formation of interference fringes. Even though the same wavelength was used for both previous and current studies during in situ optical measurements, the spectral shape of two samples can differ due to, for example, difference in thickness between samples. This is also the case in a given film for which the thickness of the specimen varies with position (originating, for example, from different distance from the target material during deposition). Indeed, the transmittance spectra of samples used for both studies, reduced and quenched, show different oscillations as shown in Figure 7 (a). The measured thickness difference between two samples ranges from 4 to 20 nm, considering the standard deviation of measured values. As a demonstration, the transmittance spectra of a thin film with different thickness are modeled in Figure 7 (b) by using the analytical expression developed by Swanepoel55 (The detail of analysis is found in the Supporting Information.). Because there are no reported refractive index values of Pr doped ceria, n(λ) of undoped ceria dense thin films, reported by Oh et al.,56 was used, while s(λ) of the sapphire substrate was extracted by measuring its transmittance (n(λ) and s(λ) are the wavelength dependent refractive index of the thin film and the transparent substrate, respectively). Only the 400−700 nm wavelength range was modeled because the analytical expression for the transmittance spectra is more applicable in the transparent regime. As clearly shown, even a 5 nm difference in thickness results in a considerable shift of the transmittance spectra. Therefore, careful attention is required when applying the absorption and extinction coefficients obtained from one sample in the analysis of another sample. The interferencefree spectra may be used for direct comparison between samples. This spectra can be calculated from the interference fringes given that optical parameters such as n(λ) and the optical band gap energy at the temperature and pO2 of interest are known.55 Once a reliable value for the extinction coefficient is derived, knowledge of εPr4+ opens up opportunities for determining defect concentrations and nonstoichiometry in thin films such as PCO as functions of temperature and pO2 from optical studies alone. This can be especially important at reduced temperatures where meaningful Cchem measurements may not be possible and where the quenched defect equilibrium is different from ones under high temperatures.

oxygen nonstoichiometry. The absorption coefficient of the Pr4+ color center, extracted from optical absorption measurements, was found to exhibit an expected linear dependence on Pr4+ concentration, as correlated with oxygen nonstoichiometry (δ) values obtained independently by chemical capacitance measurements. This allowed for the derivation of the extinction coefficient for Pr4+ (εPr4+ = 5.86 ± 0.12 × 10−18 cm2 with the yaxis intercept of 927.5 ± 207.3 cm−1). The derivation of the extinction coefficient of the Pr4+ color centers in ceria opens up future opportunities for investigating defect concentrations and nonstoichiometry in PCO and other appropriate thin films from optical studies alone. This could be especially important with nonconducting substrates57 or at reduced temperatures where meaningful Cchem measurements may not be possible. Furthermore, visible light optical measurements are nondestructive, particularly in reactive environments characterized by elevated temperatures and oxidizing/reducing oxygen partial pressures. This distinguishes this technique from widely used high energy X-ray and electron beam studies in which specimens are susceptible to beam damage.58,59 The noncontact optical transmission technique thereby provides an additional and quantitative insight into the defect equilibria of thin films, which often differ from those of bulk materials, while conventional characterization methods are often severely limited when applied to thin films. In addition, as shown in the authors’ other studies,10,28,46,47 with the novel measurement arrangement of in situ optical and electrochemical measurement techniques, better insights into surface reaction kinetics and oxygen diffusivity in oxide thin films can be obtained including, for example, the potential influence of variable surface chemistry, metal contacts, strain, and thermal history. This technique is now being extended by the author and colleagues to the very broad and technologically important perovskite structured oxides. In particular the Sr(Ti,Fe)O3 thin film system, that exhibits color change associated with the Fe3+ to Fe4+ transformation, is being extensively investigated.60 This approach also shows promise in detecting the onset of phase transformations driven by isothermal changes in oxygen stoichiometry,61 which bring with them associated changes in optical absorption, further demonstrating the versatility of the technique.

5. CONCLUSIONS A novel investigation method for simultaneously utilizing in situ optical transmission and electrochemical impedance spectroscopy measurements was performed over a wide range of temperatures (550−700 °C) and various values of pO2 (10−4− 1 atm) on the Pr doped ceria thin film model system exhibiting

*E-mail: [email protected].

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.chemmater.6b03307. Transmittance spectra of 10PCO thin films upon pO2 change at various temperatures (550−700 °C), electrochemical impedance spectra for the asymmetric cell, the molar extinction coefficient at each temperature, and the effect of interference fringes on determination of absorption coefficients (PDF)

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AUTHOR INFORMATION

Corresponding Author ORCID

Jae Jin Kim: 0000-0001-7709-3530 Present Addresses ∥

Chemical Sciences and Engineering Division, Argonne National Laboratory, Argonne, IL 60439, USA. 2005

DOI: 10.1021/acs.chemmater.6b03307 Chem. Mater. 2017, 29, 1999−2007

Article

Chemistry of Materials #

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Redox Power Systems, LLC, College Park, MD 20740, USA. Department of Materials Science and Engineering, Stanford University, Stanford, CA 94305, USA. ⊥

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This research was funded by the Department of Energy, Basic Energy Sciences under award DE SC0002633. J.J.K. thanks The Kwanjeong Educational Foundation for partial fellowship support.

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DOI: 10.1021/acs.chemmater.6b03307 Chem. Mater. 2017, 29, 1999−2007