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Temporal and spatial tuning of the refractive index of optical thin films is desired for flat optics applications. The redistribution of mobile ions in mixed ionic-electronic conductors (MIEC) has been demonstrated to serve as a viable means for achieving optical tuning down to the nanoscale. Here we studied the dynamic range of the optical tuning achievable in the refractive index, in the MIEC oxide – PrxCe1−xO2−δ (PCO), for x = 0.1, 0.2 and 0.4, at 500 °C, by in-situ spectrophotometry. Significant increases in the modulation of both the imaginary and real optical constants in the visible and the adjacent spectra were obtained for increased doping levels. Device employing an electrochemical titration method was implemented to modulate the oxygen concentration, and thereby the optical transmission of PCO. Incorporation of a patterned top electrode allowed for the demonstration of spatial control of PCO thin film properties by in-situ video imaging of the optical switching process. The electrochemically induced optical state is shown to remain non-volatile upon quenching the device to room temperature under applied bias. [ABSTRACT FROM AUTHOR]

The chemical expansion of Pr0.1Ce0.9O2–δ (PCO) and CeO2–δ thin films is investigated in the temperature range between 600 °C and 800 °C by laser Doppler vibrometry (LDV). It enables non-contact determination of nanometer scale changes in film thickness at high temperatures. The present study is the first systematic and detailed investigation of chemical expansion of doped and undoped ceria thin films at temperatures above 650 °C. The thin films were deposited on yttria stabilized zirconia substrates (YSZ), operated as an electrochemical oxygen pump, to periodically adjust the oxygen activity in the films, leading to reversible expansion and contraction of the film. This further leads to stresses in the underlying YSZ substrates, accompanied by bending of the overall devices. Film thickness changes and sample bending are found to reach up to 10 and several hundred nanometers, respectively, at excitation frequencies from 0.1 to 10 Hz and applied voltages from 0–0.75 V for PCO and 0–1 V for ceria. At low frequencies, equilibrium conditions are approached. As a consequence maximum thin-film expansion of PCO is expected due to full reduction of the Pr ions. The lower detection limit for displacements is found to be in the subnanometer range. At 800 °C and an excitation frequency of 1 Hz, the LDV shows a remarkable resolution of 0.3 nm which allows, for example, the characterization of materials with small levels of expansion, such as undoped ceria at high oxygen partial pressure. As the correlation between film expansion and sample bending is obtained through this study, a dimensional change of a free body consisting of the same material can be calculated using the high resolution characteristics of this system. A minimum detectable dimensional change of 5 pm is estimated even under challenging high-temperature conditions at 800 °C opening up opportunities to investigate electro-chemo-mechanical phenomena heretofore impossible to investigate. The expansion data are correlated with previous results on the oxygen nonstoichiometry of PCO thin films, and a defect model for bulk ceria solid solutions is adopted to calculate the cation and anion radii changes in the constrained films during chemical expansion. The constrained films exhibit anisotropic volume expansion with displacements perpendicular to the substrate plane nearly double that of bulk samples. The PCO films used here generate high total displacements of several 100 nm's with high reproducibility. Consequently, PCO films are identified to be a potential core component of high-temperature actuators. They benefit not only from high displacements at temperatures where most piezoelectric materials no longer operate while exhibiting, low voltage operation and low energy consumption. [ABSTRACT FROM AUTHOR]

Mixed ionic–electronic conductors offer chemical and electrical means for active tuning of their optical constants, e.g., with variations in oxygen non‐stoichiometry in Pr0.1Ce0.9O2–δ, enabling implementation of adaptive thin film optical devices. In situ chemo‐tuning of the extinction coefficient in Pr0.1Ce0.9O2–δ at elevated temperatures is demonstrated and a tuning model is provided that treats the interdependence of mobile oxygen vacancies and small polarons coupled to variations in optically active praseodymium ions. Furthermore, a new means for electro‐tuning of the optical constants of mixed ionic–electronic conductors is demonstrated experimentally and modeled for Pr0.1Ce0.9O2–δ thin films deposited on grid‐like electrode structures. Modeling of non‐steady‐state optical transmittance modulations in the latter allows for estimation of oxygen vacancy mobility that determines the switching speed of the device. Quenched‐in values of nr and k to room temperature become nonvolatile, providing a modulation range in the extinction coefficient of Δk ≈ 0.1 (change of ≈800%) and in the refractive index of Δnr ≈ 0.1 (relative to initial nr of ≈2.35). Key figures of merit, including transmission optical modulation of ≈0.04 per 1 mV nm–1, switching energy per area of 1.9 nJ µm–2, and switching times of seconds, are demonstrated, with further improvements possible. [ABSTRACT FROM AUTHOR]

Solid‐state mixed ionic–electronic conductors (MIECs) in which ionic transport is commonly accompanied by predominant electronic conductivity underpin key technologies and require universal characterization methods for monitoring transport at the nanoscale, at both high and near ambient temperatures, the latter being especially challenging. In this study, a novel dynamic current–voltage analysis technique is utilized to decouple ionic and electronic transport properties from each other. The versatility of the method is demonstrated by enabling measurement of the oxygen vacancy mobility in Pr0.1Ce0.9O2−δ thin films, across an unusually wide temperature range, from 35 to 500 °C. Despite the presence of predominant electronic conduction, the oxygen vacancy mobility in Pr0.1Ce0.9O2−δ is measured, being 6.8 × 10−6 cm2 V−1 s−1 at 500 °C, decreasing by seven orders of magnitude down to 35 °C, and following a single thermal activation energy of 0.82 ± 0.02 eV. A comparison with previous reports on oxygen vacancy transport and with the one derived in this study from impedance spectroscopy, interpreted with the Jamnik–Maier model, further confirms the dynamic current–voltage analysis results. This method can more generally be applied to other types of MIECs, thereby enabling deeper insights into mobile ionic defect transport and accompanying thermodynamic properties. [ABSTRACT FROM AUTHOR]

The electrical and dilatometric properties of CeO2 nanopowders were examined as function of particle shape and size, including nanorods and nanocubes. Nanorods show continuous irreversible shrinkage, linked to particle reordering and compaction. Thermal expansion of CeO2 nanocubes was analyzed and was found to be consistent with literature data for microcrystalline ceria with no apparent nanosize effects. The electrical properties of the loosely compacted nanopowders were generally found to be characterized by n-type electronic conduction, except for proton conductivity contributions associated with adsorbed moisture at temperatures below 400°C. The PO2 and temperature dependences of the conductivity were examined in terms of defect chemical models. The lower effective enthalpy of reduction for nanorods (1.5 eV) in comparison with nanocubes (1.8 eV), both being much smaller than the value found for "bulk" ceria (4.7 eV), can be related to the larger surface to volume ratio of the nanorods, where oxide ion removal is more facile and less energy costly. [ABSTRACT FROM AUTHOR]

In this paper, we focus on the effect of processing-dependent lattice strain on oxygen ion conductivity in ceria based solid electrolyte thin films. This is of importance for technological applications, such as micro-SOFCs, microbatteries, and resistive RAM memories. The oxygen ion conductivity can be significantly modified by control of lattice strain, to an extent comparable to the effect of doping bulk ceria with cations of different diameters. The interplay of dopant radii, lattice strain, microstrain, anion-cation near order and oxygen ion transport is analyzed experimentally and interpreted with computational results. Key findings include that films annealed at 600 °C exhibit lattice parameters close to those of their bulk counterparts. With increasing anneal temperature, however, the films exhibited substantial compaction with lattice parameters decreasing by as much as nearly 2% (viz, Δd600-1100 °C: -1.7% (Sc+3) > -1.5% (Gd+3) > -1.2% (La+3)) for the annealing temperature range of 600-1100 °C. Remarkably 2/3rd of the lattice parameter change obtained in bulk ceria upon changing the acceptor diameter from the smaller Sc to larger La, can be reproduced by post annealing a film with fixed dopant diameter. While the impact of lattice compaction on defect association/ordering cannot be entirely excluded, DFT computation revealed that the main effect appears to result in an increase in migration energy and consequent drop in ionic conductivity. As a consequence, it is clear that annealing procedures should be held to a minimum to maintain the optimum level of oxygen ion conductivity for energy-related applications. Results reveal also the importance to understand the role of electro-chemo-mechanical coupling that is active in thin film materials. [ABSTRACT FROM AUTHOR]

Abstract: Undoped and acceptor doped cerium dioxide is known to exhibit non-stoichiometry induced chemical expansion at elevated temperatures and reducing environments with impact on the mechanical integrity of solid oxide fuel cells and permeation membranes. In this paper, the chemical expansion of Pr0.1Ce0.9O2−δ is measured and analyzed with respect to its defect equilibria and the chemical coefficient of expansion, analogous to thermal coefficient of expansion, is extracted. The addition of Pr to CeO2 leads to major deviations from stoichiometry, and correspondingly to large chemical expansions, under readily accessible experimental conditions (e.g. in air at elevated temperatures). Pr0.1Ce0.9O2−δ , therefore, serves as a model system for studying chemical expansion in ceria-based solid solutions in order to predict the conditions in which they exhibit suppressed chemical expansion. [Copyright &y& Elsevier]

Abstract: Determination of the ionic and electronic contributions to the total conductivity in mixed ionic–electronic conductors (MIEC) is central to understanding their properties, particularly in nanostructured ionic solids. The Hebb–Wagner blocking technique, commonly used to deconvolute ionic and electronic contributions in bulk MIECs, is susceptible to misinterpretation when applied to thin films. In this work, microfabricated electronic blocking electrodes consisting of porous Pt on dense thin yttria-stabilized zirconia (YSZ) films were applied to nanocrystalline CeO2 thin films. The validity of the blocking structure was expressly considered with respect to alternate current and gas phase reaction pathways, with criteria developed to aid in identifying spurious effects. The ionic partial conductivity in nanocrystalline CeO2 thin films was confirmed to be pO2-independent while the electronic partial conductivity was found to be pO2 dependent with a power dependence of −0.31±0.02. These results are compared with theoretical predictions of extrinsically-compensated ceria and previous results on bulk nanocrystalline ceria. [Copyright &y& Elsevier]

PrxCe1–xO2−δcan potentially serve as boththe cathode and anode in solid oxide fuel cells (SOFCs), given thatit exhibits significant mixed ionic-electronic conductivity (MIEC)under both cathodic and anodic conditions. While MIEC is stronglydependent on oxygen nonstoichiometry (δ), there have been fewways to extract this information reliably and in situ, particularlyfor thin films. In this work, this is achieved by analysis of chemicalcapacitance values extracted from impedance spectroscopy data obtainedon electrochemical cells of the form Pr0.1Ce0.9O2−δ/Y0.16Zr0.84O1.92/Pr0.1Ce0.9O2−δoperating at temperatures in the range of 450–700 °Cand over the oxygen pressure (pO2) rangeof 10–33–10–14atm. Incombination with our prior investigation, the oxygen nonstoichiometryover a very wide range of pO2values isderived. We found that δ in thin films differs from that ofthe bulk (δfilm> δbulk). Thisfollowsfrom a decreased enthalpy of reduction of 4.12 ± 0.25 eV forthe film, compared to 4.52 ± 0.37 eV for the bulk. [ABSTRACT FROM AUTHOR]

Abstract: Dense, nanocrystalline gadolinia doped ceria (CGO) thin films were prepared by sputtering on a variety of substrates. The CGO films were electrochemically characterized with the aid of interdigitated Pt electrodes by impedance spectroscopy as a function of operating temperature (175–340°C), CGO film thickness (157, 233, 315nm), deposition temperature (room temperature, 300, 500°C), and Pt electrode pattern geometry (line pattern spacing 10, 15, 25, 50, 100μm). The different parameter variations enabled the identification of five contributions to the impedance spectra attributed, respectively, to wiring and contacts, grain, grain boundary, current constriction at the triple phase boundary, and electrode effects. The ionic conductivity of the CGO films were characterized by the expression σ =(A/T)exp(−E A/kT), with A =108.66±0.72 Scm−1 K and E A =0.97±0.07eV with magnitude higher than values reported for both bulk and thin film CGO previously reported in the literature. The electrochemical impedance of the Pt electrodes was found to decrease markedly for CGO films grown at reduced temperatures. Explanations for these observations are presented. [Copyright &y& Elsevier]

Nanocrystalline CeO2 films of 75 nm thickness were deposited on n-type (100) silicon substrates using pulsed laser deposition (PLD) technique to form a gate dielectric in the Pt/Si/CeO2/Pt MOS capacitor. XRD and AFM were used for characterization of the film crystal structure and grain size. Electrical properties of the MOS structure were examined by capacitance–voltage (C–V) and impedance spectroscopy measurements. Based on the impedance measurements and reported electron mobilities, we derive a rather high carrier density of the order of 6×1017 cm−3. Nevertheless, their overall high resistance enables the films to serve as dielectrics. In contrast to conventional MOS capacitors, we find an additional capacitive contribution under accumulation we attribute to the electron depletion in the CeO2 film. We present a model consistent with these results. [Copyright &y& Elsevier]

Cathodic electrochemical deposition, a simple and cost-effective route for fabricating ceria nanostructures, has attracted much attention in catalysis and renewable energy technologies. However, electrochemical nucleation and growth mechanisms that determine the nanoscale architecture of ceria films during deposition have not yet been clarified. In this study, we analyze current and mass-time transients and thin film microstructures as functions of applied potential and temperature. We then examine these results in light of analytical models for the determination of relative nucleation rate, nuclei density, and diffusivity of Ce ion during ceria electrocrystallization. As applied potential and temperature decrease, the reduced deposition rate allows nuclei to branch, leading to more 'petal'-like nanostructures with high specific surface areas. These optimized ceria nanostructures greatly improve the hydrogen electro-oxidation rate and reduced electrode resistance when coated onto a model electrode that simulates a Ni/yttria-stabilized zirconia composite anode utilized in solid oxide fuel cells. Image 1 • The nucleation and growth mechanisms of electrochemically deposited ceria nanostructures onto Ni electrodes are revealed. • The slower deposition rate allows nuclei to branch, leading to more 'petal'-like nanostructures with high specific surface areas. • Ceria overcoat, with highly controlled morphology, significantly improves the H 2 electrooxidation performance of Ni/YSZ fuel cell electrodes. [ABSTRACT FROM AUTHOR]

Abstract The effects of strain on the ionic conductivity of rare-earth substituted CeO 2 have been extensively studied, but the results have been inconsistent and focused upon the 'optimised' conductors such as Gd or Sm substituted CeO 2 where defect association is minimised. By thermally annealing epitaxial films deposited by pulsed laser deposition, we varied the strain systematically, whilst avoiding any influence from interfacial or grain boundary effects. The activation energy of the in-plane conductivity was found to increase with increasing compressive biaxial strain, which was quantitatively in excellent agreement with previous computational and experimental studies. These results provide a much needed quantitative consensus on the effects of lattice strain on ionic transport. Furthermore, we demonstrate that the change in the activation energy for Yb-substituted CeO 2 is around three times that for Gd or La substitutions for the same applied strain, indicating the important role played by defect association. These results have significant implications for ionic transport at reduced or ambient temperatures, where changes in conductivity due to strain may be several orders of magnitude larger for 'non-optimised' conductors compared with 'optimised' conductors. We rationalise our results by considering the defect-defect interactions in these materials and through force-field calculations. Graphical abstract Image 1 [ABSTRACT FROM AUTHOR]

The chemical expansion of Pr 0.1 Ce 0.9 O 2−δ (PCO) films was determined as function of oxygen activity at 720 °C by laser Doppler vibrometry (LDV) in combination with previously reported chemical capacitance studies that provided relevant oxygen nonstoichiometry δ data. The LDV method enabled detection of nanometer displacements, provided that displacement frequencies were above 1 Hz. The PCO films were deposited on yttrium stabilized zirconia (YSZ) substrates operated in the electrochemical oxygen pump mode at 720 °C to periodically adjust the oxygen activity within the PCO film. The potential across the YSZ was varied between 0 and 0.3 V, corresponding to oxygen partial pressures of about 0.2 bar and 10 −7  bar, respectively. The electrochemically induced expansion/contraction of the PCO film was observed to induce deflection in both film and substrate, as clearly demonstrated by scanning the sample surface by the LDV. A maximum displacement of about 18 nm at the center of the substrate plate was found during application of a periodic 1 Hz pumping voltage. A much lower displacement, on the order of 1 nm, was obtained when a pin-type sample support was applied, characteristic of chemical expansion of the film itself. These data were used to discuss correlations with similar recent data obtained by monitoring displacements of the PCO/YSZ sandwich structure with the aid of a specially modified nanoindenter. [ABSTRACT FROM AUTHOR]

In this study, YSZ-based potentiometric gas sensors of the type Au*/YSZ/Pt, where Au* represents either dense (d-Au), porous Au (p-Au) or porous Au/4MOx ( 4 wt% CeO 2 , Gd 2 O 3 , La 2 O 3 , Pr 6 O 11 , Sm 2 O 3 , Pr 0.1 Ce 0.9 O 2 or Ce 0.8 Zr 0.2 O 2 ) composite sensor electrodes (SEs), were fabricated, and their toluene-sensing properties examined over the temperature range of 400–500 °C. The roles of microstructure of the SEs, and improvements in sensing properties due to MO additions were examined with respect to their impact on catalytically enhanced toluene oxidation as well as electrochemical reactions at the SE/electrolyte interface. Au( 4 MO)/Pt sensors showed larger response to toluene than d-Au/Pt, with the Au( 4 CeO 2 )/Pt sensor showing the largest response among all of the Au( 4 MO)/Pt sensors. While the response of Au( 4 CeO 2 )/Pt was smaller than that of p-Au/Pt, its recovery time was much shorter. These observations were shown to result from Au( 4 CeO 2 ) exhibiting the largest catalytic activity of toluene oxidation among all of the oxides tested, leading to enhanced toluene oxidization during its diffusion through the SE, as well as the removal of residual toluene at TPBs, during the recovery process. This resulted in decreased response, but at the same time, faster recovery. The results demonstrated that both high electrochemical activity of toluene oxidation and low electrochemical activity of oxygen reduction are indispensable for achieving large sensor response. The Au( 4 CeO 2 )/Pt sensor also showed a nearly linear response vs logarithm of toluene concentration (0.5–50 ppm), with strong response down even to 0.5 ppm toluene (ca. +55 mV) in dry air. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

The chemical expansion of thin-film praseodymium‑cerium oxide solid solutions (Pr 0.1 Ce 0.9 O 2−δ , PCO10) is investigated at temperatures ranging from 500 °C to 800 °C and oxygen activities down to about 10−22. Differential Laser Doppler Vibrometry (D-LDV) in combination with periodic electrochemical pumping of oxygen is used to detect very small displacements of the sample surface. With decreasing pumping frequency, the film approaches chemical equilibrium, which is nearly reached at 700 °C below 0.01 Hz. D-LDV enables direct measurement of film thickness changes as well as the bending of the underlying substrates due electrochemically induced lateral mechanical stress. The substrate bending is found to be more than an order of magnitude larger than the film thickness change, increasing the detection limit by the same factor. The thickness change of the thin film is calculated from the measured displacement of the sample surface and compared with directly measured values. Thin-film literature data available for low oxygen activities up to 650 °C agree with our measurements. In addition, the thickness change of the film is determined from the displacement at different sample positions, which validates previous measurements. Finally, thin film chemical expansion is found to be larger by a factor of about 2 than the corresponding bulk value in agreement with predictions from a mechanical model that the out-of-plane expansion of a constrained film is increased. • Electrochemical adjustment of oxygen activity in the films. • Contact-less detection of chemical expansion in thin films up to 750 °C. • Differential Laser-Doppler Vibrometry reduced noise at low frequencies. • Detection of nm-scale motions at 0.001 Hz. • Strain in the films is found to be about twice as large as in bulk samples. [ABSTRACT FROM AUTHOR]

Simultaneous in situoptical absorption and electrochemicalimpedance spectroscopy measurements were performed, for thefirst time, at elevated temperature on a metal oxide thinfilm exhibiting oxygen nonstoichiometry, utilizing Pr0.1Ce0.9O2−δ(10PCO) asa model system. Chemical capacitance measurements, capable of providingexplicit values of δ, were used to determine the optically absorbingcenter (Pr4+) concentration and thereby the optical extinctioncoefficient for Pr4+. The absorption coefficient was foundto exhibit a linear dependence on Pr4+concentration, validatingthe use of optical absorption to examine defect concentration trendsand allowing derivation of the extinction coefficient εPr4+= 5.01 ± 0.14 × 10–18cm2. Values of Pr4+concentration derivedfrom the chemical capacitance and corresponding trends in opticalabsorption were found to be self-consistent, validating the thin filmdefect model for 10PCO, thereby confirming that the oxygen reductionenthalpy in thin film 10PCO is lower than that in the bulk. The non-contactoptical absorption technique thus provides an additional insitumethod for investigating the defect equilibria of thinfilms and is expected to aid in confirming whether and under whatconditions the defect thermodynamics of films differ from that oftheir bulk counterparts. [ABSTRACT FROM AUTHOR]

Abstract: Praseodymium doped cerium oxide (Pr x Ce1− x O2− δ ; PCO) exhibits mixed ionic and electronic conductivity at relatively high oxygen partial pressures (e.g. air) and enhanced oxygen storage capacity resulting from the ready reduction/oxidation of tri- and tetra-valent Pr. A change in the redox state of Pr results in a color change of PCO, and so serves as a means of investigating the Pr oxidation state, oxygen non-stoichiometry, and position of the Pr energy level within the ceria band gap. The wavelength dependent optical absorptivity was examined as a function of the Pr content (x) and oxygen partial pressure. Based on an analysis of the optical absorption spectra, the Pr level was found to lie approximately 1.6eV below the ceria conduction band for x =0.1 and 0.2, consistent with a defect model derived from electrical and thermogravimetric measurements on PCO. [Copyright &y& Elsevier]

• Pr 0.1 Ce 0.9 O 2 (PCO) films were fabricated by pulsed laser deposition (PLD). • The YSZ-based sensor using a PCO sensing electrode with a thickness of 2200 nm showed large response to 50 ppm toluene. • Interfaces between PCO/gas in the sensing electrode contributed to enhance the toluene response. • The sensor certainly detected 0.25 ppm toluene in dry air. Pr 0.1 Ce 0.9 O 2 (PCO) films, with columnar structure containing voids, were fabricated by pulsed laser deposition (PLD) and integrated into YSZ-based potentiometric gas sensor as a sensing electrodes (SEs). Sensors with PCO film thicknesses of 550, 1100 and 2200 nm, fabricated by controlling the number of PLD shots of 750, 1500 and 3000 respectively, were examined over the temperature range of 450–600 °C. The sensor output voltage, E, of all sensors showed increasing negative change with increasing toluene concentration. The sensor with the thickest PCO SE film (thickness: 2200 nm) showed a much larger toluene response at 450 °C than the others (PCO-SE thickness: 550, 1100 nm). Because PCO is a mixed ionic and electronic conductor and the morphology of SE was porous, the electrochemical oxidation reaction is viewed as occurring at the double phase boundaries – DPBs (interfaces between PCO/gas) as well as triple phase boundaries – TPBs (interfaces of PCO/YSZ/gas). Because a fraction of the toluene is catalytically oxidized in part during gas diffusion within the SE films, especially at lower operating temperatures, the much larger toluene response can be explained by the electrochemical oxidation reaction of the partially oxidized products together with toluene at the TPBs and DPBs. [ABSTRACT FROM AUTHOR]