Changes In Oxygen Stoichiometry Research Articles

Influence of Microstructure on Defect Chemistry - Transport - Chemical Strain Coupling of (Pr,Ce)O2-δ Nanoparticles

Solid oxide fuel/electrolysis cells (SOFC/SOECs) are promising and environmentally-friendly technologies for meeting ever-increasing electricity and energy storage demands. Nanostructuring of SOFC/SOEC electrodes can enable improvement of the reaction kinetics and reduction of the operating temperature, owing to factors including the enlarged surface area, short diffusion lengths, and unique surface & interface properties. While the kinetic benefit is apparent, the effect of nanostructuring on other device-relevant physico-chemical properties cannot be neglected. Especially, the coefficients of chemical expansion (CCEs) are an important metric for mixed ionic-electronic conductors that can undergo changes in oxygen stoichiometry with variations in the surrounding atmosphere, temperature, and voltages during processing/operation. Chemical strains can cause cracking of monolithic parts during fabrication and delamination of electrodes from electrolytes in operation. CCEs are typically measured on bulk, polycrystalline materials with large particle or grain sizes. Therefore, it is important to understand the coupled transport and chemical expansion behavior in nano-ionic materials with large interface/surface density. This work aims to uncover the role of microstructure and nanoscale effects on the chemical strain and transport properties, through the lens of defect chemistry, with the goal of ultimately developing design principles for zero-strain materials with improved device lifetime and performance.Pr0.2Ce0.8O2- δ (PCO20) was selected as a model system and prepared as nanoparticles by a surfactant-free, low temperature co-precipitation method. (Pr,Ce)O2-δ compositions are valuable model oxide ion conductors to study, since both Pr and Ce can undergo changes of valence states under different temperature and pO2 regimes to accommodate oxygen stoichiometry excursions. Prepared nanoparticle samples were first in-situ sintered in the dilatometer at three different temperatures (600 ⁰C, 725 ⁰C, 850 ⁰C) to induce and evaluate microstructure evolution. The chemical strain and electronic/ionic conductivity were then studied on stable microstructures with different average particle sizes at lower temperatures by simultaneous in-situ dilatometry and electrochemical impedance measurement (EIS) upon changing pO2 at different isotherms (550 to 400 ⁰C). At each isotherm, all the samples expand as pO2 decreases. From the conductivity vs. pO2 measurements, the maxima at certain pO2 indicate the dominant polaron mechanism of the electronic transport, consistent with the measured conductivity activation energy. However, samples with lower sintering temperatures exhibit less pO2 dependence of the conductivity, which implies a transition in the mixed ionic/electronic conduction behavior toward higher ionic transference numbers as particle size decreases. Correspondingly, the chemical strains associated with losing oxygen monotonically decrease with the decrease of the sintering temperature, and hence the particle size. The chemical strain vs. oxygen stoichiometry dependence and corresponding CCEs were quantified as function of particle size and temperature by combining thermogravimetric analysis (TGA) with the dilatometry results and in-situ X-ray diffraction. The particle size, chemical composition, valence states, and morphology of samples after CCEs measurements were also compared using XRD, SEM, and S/TEM-EELS, with the latter indicating the expected differences between bulk and surface defect chemistry. Implications of the results for size-dependent defect chemistry and its influence on chemical strains and redox CCEs will be further discussed. Keywords: redox, chemical expansion, defect chemistry, mixed ionic electronic conduction, ceria, chemo-mechanics, nanoparticle, thermogravimetric analysis, dilatometry, impedance

Protonic ceramics Ba5In2–xYxAl2ZrO13 with the perovskite-related hexagonal structure for solid oxide fuel cells: Synthesis, optical band gap and transport properties

The solid solution Ba5In2–xYxAl2ZrO13 (0 ≤ х≤0.50) with hexagonal structure (space group P63/mmc) was prepared by the solid-state reaction method. The effects of isovalent Y3+-substitution on the structure, hydration, bandgap and transport properties have been investigated. The introduction of yttrium was accompanied by lattice expansion, which led to an increase in the concentration of protons during hydration. The doping did not lead to a significant increase in oxygen-ion conductivity since there was no change in oxygen stoichiometry. At the same time, doping led to an increase in ionic transport numbers due to a decrease in hole conductivity. Proton conductivity contribution and the values of proton conductivity increase with the increase in yttrium concentration. The phases with yttrium content x > 0.2 were predominant proton conductors at the temperature below 600°С under wet air.

Diel change in inorganic nitrogenous nutrient dynamics and associated oxygen stoichiometry along the Pearl River Estuary

The reactive nitrogen (N) emitted from continents significantly perturbs the pristine N cycle around the land-ocean boundary resulting in eutrophication and hypoxia. As nutrients are transported downstream through an estuary, various types of biological processes co-occur to modulate nitrogen speciation to influence the biogeochemical habitats for downstream microorganisms. We surveyed the Pearl River Estuary to examine the N transfer dynamics among nitrogen species with considering process-specific oxygen production and consumption. By using 15N pulse-tracing techniques, we measured ammonia oxidation and uptakes of ammonium, nitrite, and nitrate simultaneously under dark and light conditions in parallel. Light strongly inhibited nitrification but enhanced N uptake, and such light effect was further considered in the calculation for nitrogen transformation rates over a diel cycle. We found both oxidation and uptake of ammonium decreased seaward as substrate decreased. The nitrifier and phytoplankton work in antiphase to draw down incoming ammonium rapidly. Contrary to ammonium uptake, uptake of nitrite and nitrate showed a seaward increasing pattern. Such an inverse spatial pattern implies a shift in N preference for phytoplankton. Such high ammonium preference inhibits nitrate/nitrite uptake allowing them to behave conservatively in the estuary and to travel farther to outer estuary. By integrating oxygen consumption and production induced by N transformation processes over the diel cycle, oxygen was produced although allochthonous ammonium input is high (∼250 μM). For most stations, ammonium was completely consumed within 2 days, some stations even less than 0.5 days, implying that although the water residence time is short (2-15 days), tremendous input of ammonium N from upstream was transformed into particulate organic or nitrate forms during traveling to modulate the biogeochemical niche, including substrate, organics and oxygen, of coastal microbes in water column and sediments.

Metallic networks and hydrogen compensation in highly nonstoichiometric amorphous In2O3−x

The unique response of amorphous ionic oxides to changes in oxygen stoichiometry is investigated using computationally intensive ab initio molecular dynamics simulations, comprehensive structural analysis, and hybrid density-functional calculations for the oxygen defect formation energy and electronic properties of amorphous ${\mathrm{In}}_{2}{\mathrm{O}}_{3\ensuremath{-}x}$ with $x=0$--0.185. In marked contrast to nonstoichiometric crystalline nanocomposites with clusters of metallic inclusions inside an insulating matrix, the lack of oxygen in amorphous indium oxide is distributed between a large fraction of undercoordinated In atoms, leading to an extended shallow state for $x<0.037$, a variety of weakly and strongly localized states for $0.074<x<0.148$, and a percolation-like network of single-atom chains of metallic In-In bonds for $x>0.185$. The calculated carrier concentration increases from $3.3\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at $x=0.037$ to $6.6\ifmmode\times\else\texttimes\fi{}{10}^{20}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at $x=0.074$ and decreases only slightly at lower oxygen content. At the same time, the density of deep defects located between 1 and 2.5 eV below the Fermi level increases from $0.4\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at $x=0.074$ to $2.2\ifmmode\times\else\texttimes\fi{}{10}^{21}\phantom{\rule{4pt}{0ex}}{\mathrm{cm}}^{\ensuremath{-}3}$ at $x=0.185$. The wide range of localized gap states associated with various spatial distributions and individual structural characteristics of undercoordinated In is passivated by hydrogen that helps enhance electron velocity from $7.6\ifmmode\times\else\texttimes\fi{}{10}^{4}$ to $9.7\ifmmode\times\else\texttimes\fi{}{10}^{4}$ m/s and restore optical transparency within the visible range; H doping is also expected to improve the material's stability under thermal and bias stress.

Tuning the Co/Sr stoichiometry of SrCoO2.5 thin films by RHEED assisted MBEgrowth

Strontium cobaltite (SrCoO2.5+δ , SCO) is a fascinating material because of its topotactic structural phase transition caused by a change in oxygen stoichiometry. In the brownmillerite phase (δ = 0) it is an insulating antiferromagnet whereas in the perovskite phase (δ = 0.5) it is a conducting ferromagnet. In contrast, the impact of the varying Co/Sr stoichiometry on the structure has not yet been studied in SCO thin films. Using molecular beam epitaxy we have fabricated SCO thin films of varying Co/Sr stoichiometry. Films with Co excess exhibit a brownmillerite crystal structure with CoO precipitates within the thin film and on the surface. Co deficient films are amorphous. Only for 1:1 stoichiometry a pure brownmillerite structure is present. We find a clear dependence of the Reflection High Energy Electron Diffraction (RHEED) pattern of these thin films on the stoichiometry. Interestingly, RHEED is very sensitive to a Co excess of less than 12% while x-ray diffraction fails to reveal that difference. Hence, using RHEED, the stoichiometry of SCO can be evaluated and tuned in-situ to a high degree of precision, which allows for a quick adjustment of the growth parameters during a sample series.

Spectroscopic Ellipsometry for Simultaneous Thickness and Defect Concentration Changes in Oxide Thin Films

Measurements of strain and stoichiometry changes needed to determine coefficients of chemical expansion (CCEs) can be very time-intensive for bulk materials due to the long equilibration times needed at the multiple instruments and conditions. Additionally, bulk polycrystalline materials give macroscopic responses that include both lattice and microstructural contributions, and the responses are often therefore averaged over the randomized orientation of grains. Thin films provide an avenue to rapidly assess steady-state properties of such chemically expanding materials after short dwells due to their low volume and high surface area. Additionally, the controlled growth of thin films can be used to fabricate polycrystalline samples or single crystals with tailored crystallographic orientation, grain dimensions, and induced strain for controlled experiments. Although techniques exist for in situ analysis of thickness changes or defect concentration changes in thin films, there are few (if any) techniques that can measure both simultaneously. Further, the limited existing techniques to probe defect concentrations of thin films, e.g., in-plane electrical conductivity, cross-plane chemical capacitance, or in situ X-ray diffraction, require additional knowledge such as the dominant carrier mobility, the interface capacitance, or the coefficient of chemical expansion respectively (which is what we wish to determine).Spectroscopic ellipsometry has been widely used to analyze film thicknesses and optical properties, but we show that the optical properties obtained from the measurement can also be used to quantify defect concentrations, specifically oxygen non-stoichiometry. This work builds on our prior in situ optical absorption studies of oxygen concentrations and dynamics in thin films [1-4]. When we induce an oxygen stoichiometry change in a film, ellipsometry is, in principle, able to provide a measurement of the corresponding thickness change and oxygen stoichiometry change by a single technique.To evaluate the effectiveness of this technique initially, a SrTi0.65Fe0.35O3 thin film has been scanned after anneals in O2 and Ar. The modelled expansion and optical constants, derived from the data, were compared with previous work, and thicknesses were verified by X-ray reflectometry (XRR). For materials whose extinction coefficient corresponding to oxygen concentration is already known, the oxygen stoichiometry change can be estimated from the measured absorption coefficient change. Additionally, a variation of the Clausius-Mossotti equation is proposed to estimate the defect concentration in materials that lack prior optical extinction coefficient data corresponding to the defect of interest. These preliminary results suggest that spectroscopic ellipsometry may be used for the rapid acquisition of in situ data to quantify thin film CCEs in various environments.[1] Perry, N. H., Kim, J. J., & Tuller, H. L. (2018). Oxygen surface exchange kinetics measurement by simultaneous optical transmission relaxation and impedance spectroscopy: Sr (Ti, Fe) O3-x thin film case study. Science and Technology of advanced MaTerialS, 19(1), 130-141.[2] Perry, N. H., Pergolesi, D., Bishop, S. R., & Tuller, H. L. (2015). Defect chemistry and surface oxygen exchange kinetics of La-doped Sr (Ti, Fe) O3− α in oxygen-rich atmospheres. Solid State Ionics, 273, 18-24.[3] Chen, T., Harrington, G. F., Sasaki, K., & Perry, N. H. (2017). Impact of microstructure and crystallinity on surface exchange kinetics of strontium titanium iron oxide perovskite by in situ optical transmission relaxation approach. Journal of Materials Chemistry A, 5(44), 23006-23019.[4] Perry, N. H., Kim, N., Ertekin, E., & Tuller, H. L. (2019). Origins and Control of Optical Absorption in a Nondilute Oxide Solid Solution: Sr (Ti, Fe) O3–x Perovskite Case Study. Chemistry of Materials, 31(3), 1030-1041.

Enhanced structural and magnetic ordering in as-synthesized Ca doped bismuth iron oxide nanoceramics

Calcium (Ca) added bismuth iron oxide (Bi1-xCaxFeO3) nanoparticles are synthesized using sol-gel method and studied under as-synthesized conditions. Calcium (Ca) concentration (x) is varied as 0.0, 0.1, 0.2, 0.3, 0.4 and 0.5. XRD results verify the formation of phase pure BiFeO3 at dopant concentration x = 0.0–0.3. Raman analysis and EDX spectrum also confirm the formation and incorporation of Ca in the lattice. At x = 0.4 and 0.5 peaks corresponding to calcium oxide are observed. With addition of Ca ions in the lattice, strain is induced, resulting in reduced symmetry and increased inhomogeneity in the crystal structure. XPS analysis indicates small number of Fe2+ cations whereas no change in oxygen stoichiometry is observed. Size distribution and surface morphology of nanoparticles is observed using SEM and TEM images. Ca added bismuth iron oxide nanoparticles demonstrate ferromagnetic behavior where saturation magnetization increases from 9.202 emu/g to 21.097 emu/g as x increases from 0.0 to 0.3. This increase in saturation magnetization is accompanied by reduced magnetocrystalline anisotropy. High temperature magnetization curves indicate increase in magnetic ordering temperature from 643.7 K to 697 K. Field-cooled and zero field-cooled curves exhibit magnetic ordering with blocking temperature of ∼89 K under applied field of 500 Oe. P-E loops show high saturation polarization of 3.58 × 10−4 μC/cm2 for Ca concentration of x = 0.3. It is important to mention here that nanoparticles are synthesized without the use of any post thermal treatment.

(Invited) Liquid Electrolyte Contacts to Semiconductor Nanostructures: Probing and Programming Local Defect Behavior

Replacing the metal top electrode of a metal-insulator-semiconductor (MIS) or –metal (MIM) junction with a liquid electrolyte contact can be used 1) to study the capacitance and conductance response of semiconductor nanostructures with a conformal liquid contact, and 2) to program, at higher voltages, populations of point defects in an insulator layer, as a means of investigating local resistance switching. This presentation will summarize recent results on both of these topics, beginning with a discussion of electrochemical spectroscopy (EIS) applied to semiconductor-liquid junctions as a means of performing quantitative interface state characterization on micro- and nano-structured samples. Using aqueous KCl solution as a blocking contact to a semiconductor electrode, electrochemical impedance spectroscopy measures the multifrequency capacitance-voltage and conductance-voltage characteristics of planar p-Si (100), planar p-Si0.55Ge0.45 (100), and p-Si crystals with surfaces that are textured to create nanoscale features.1 These substrates are coated with thin atomic layer deposited Al2O3 dielectrics. Analysis of impedance data obtained from the planar samples produces results that are in good agreement with solid-state multi-frequency C-V and g-V data from the same sample, including similar extracted values for density of interface states. Impedance data and the extracted interface trap properties of ALD-Al2O3 passivated silicon microwire arrays for application in solar cells will also be described. Liquid electrolyte contacts have also been used to study resistance switching and its relation to local oxygen deficiency in an initially insulating metal oxide layer.2 Local variations in electrical conductivity in amorphous TiO2 films prepared by atomic layer deposition (ALD) are examined using conductive atomic force microscopy. The local TiO2 conductivity is correlated with changes in oxygen stoichiometry revealed by electron energy loss spectroscopy performed in the electron microscope and by oxygen isotope tracer exchange. Both highly resistive ionic liquids and highly conductive electrolyte solutions are used as liquid top contacts to the TiO2 layers. In this way, the effects of field-driven migration of oxygen ions in and out of the TiO2 film can be distinguished from local accumulation of oxygen vacancies via Joule heating at high programming currents. We find that gating using a resistive ionic liquid contact at room temperature and positive gate bias produces an oxygen deficient ALD-TiO2 film with enhanced electronic conductivity, demonstrating the ease with which field-driven oxygen vacancy formation occurs at low temperature in these thin amorphous layers. However, conductive filaments were formed only when the TiO2 is programmed using a lower-resistivity electrolyte top contact. This indicates the decisive importance of programming current in localization of the point defects required for filamentary conduction. C. Meng, K. Tang, M. Braun, L. Zhang, and P.C. McIntyre, “Electrochemical Impedance Spectroscopy for Quantitative Interface State Characterization of Planar and Nanostructured Semiconductor-Dielectric Interfaces,” Nanotechnol. 28, 415704 (2017). Tang, A.C. Meng, F. Hui, Y. Shi, T. Petach, C. Hitzman, A.L. Koh, D. Goldhaber-Gordon, M. Lanza, and P.C. McIntyre, “Distinguishing Oxygen Vacancy Electromigration and Conductive Filament Formation in TiO2 Resistance Switching Using Liquid Electrolyte Contacts,” Nano Lett. 17, 4390–99 (2017).

Simultaneous Optical Transmission Relaxation and Impedance Spectroscopy Measurements of Thin Film Oxygen Surface Exchange Kinetics

The rate of oxygen exchange across mixed conducting oxide surfaces, often described via measurements of the surface exchange coefficient k, is a key metric impacting high temperature electrochemical device efficiency. Continuous measurement over time of native surface k values in realistic operating conditions can be accomplished by the contact-free optical transmission relaxation (OTR) approach, which relies on changes in oxygen stoichiometry being proportional to changes in optical absorption that can be detected by in situ optical transmission measurements. Conventional approaches to measuring k may suffer from being non-continuous (e.g., tracer exchange with ex situ secondary ion mass spectrometry), and therefore unable to observe changes in k over time, or require metal current collectors (e.g., ac-impedance spectroscopy measurements of area-specific resistance, or electrical conductivity relaxation), which could modify the surface reaction mechanisms and/or kinetics being probed. It is therefore of interest to compare k values obtained by ac-impedance spectroscopy (AC-IS) using metal current collectors and those obtained by OTR on native surfaces to examine the impact of current collectors and different measurement approaches. While k values obtained by various methods are already available in the literature, comparisons of such values measured on different samples may not provide much insight, as k can vary orders of magnitude depending on surface chemical composition, which is influenced by measurement conditions, processing, and thermal history. Therefore, in the present study, the two techniques were compared simultaneously on the same sample. A thin film of the mixed conductor SrTi0.65Fe0.35O3-x (STF35) was prepared by pulsed laser deposition on a transparent, ionically conducting (Zr,Y)O2-δ single crystal substrate. Its k values were measured by AC-IS on one region and by OTR on another region of the film, at 600 °C as a function of oxygen activity, controlled by gas partial pressure and/or dc bias applied across the substrate. Upon changing the oxygen activity, optical transmission relaxations resulted, corresponding to changes in the film oxygen stoichiometry and corresponding concentration of optically absorbing oxidized Fe (~Fe4+) species; such relaxation profiles were successfully described by the equation for surface exchange-limited kinetics appropriate for the film geometry, to obtain kchem values. From AC-IS, the fitted resistance and capacitance of the low frequency arc were used to determine both kchem (from the time constant) and kq (from the resistance), and additionally kchem values were estimated by multiplying kq by the thermodynamic factor (γ). Thin film γ values, determined from the film’s apparent volumetric chemical capacitance, were typically higher than the bulk values, determined from thermogravimetric measurements of non-stoichiometry, and varied with current collector material used (porous Au vs. porous Pt), suggesting a possible interfacial capacitance contribution. The AC-IS-derived kchem values and kq values multiplied by the thermodynamic factor (both bulk and thin film) were consistently orders of magnitude higher than the kchem values obtained by OTR, regardless of current collector material (Au or Pt). The results suggest a possible enhancement in k by the metal current collectors, which has similarly been observed previously for films of the mixed conductor (Pr,Ce)O2-δ. Additionally, long-term degradation in kchem and kq values obtained by AC-IS was also attributed to deterioration of the current collector, while hardly any degradation was observed in that time period in the optically-derived kchem values. The results suggest that, while the current collector could modify surface exchange coefficient evaluation by AC-IS, the contact-free OTR method offers a continuous, in situ approach to evaluate native surface exchange behavior.

Methane Pulse and in-Situ Raman Study of Impregnated Ni/CGO As Anodes of SOFC

Nickel/gadolinium doped ceria (Ni/CGO) has been a popular SOFC anode material for decades for its superior mix conductivity and excellent oxygen storage capacity. However, the study of Ni/CGO in a realistic working condition is still lacking. Also, not much research has linked the chemical properties of CGO with the carbon resistance which is essential when using hydrocarbon as fuels. In this research, Ni/CGO powder was prepared via impregnation-calcination method. A novel pulse experiment has been conducted on impregnated powder. We did certain pulses of a fixed amount of methane into the sample at 600℃.The outlet gas was monitored by a quadrupole mass spectrometer. The product gas was composed of several species including hydrogen, steam, carbon monoxide, carbon dioxide, and methane that was not participated to the reaction. The major reaction was the partial oxidation of methane. With the increase of pulsing number, the product and shape of product peak changed a lot. Also, we observed a “tail” after each carbon monoxide production peak which was deduced to be the self-oxidation of carbon by the bulk migrated oxygen. After detailed quantitative analysis of the shape and intensity of the peaks and tails. We came up with a model in which the mechanism of methane partial oxidation was confirmed and so was the relation between carbon self-oxidation rate, surface oxygen and bulk oxygen migration rate of CGO. In-situ Raman spectroscopy was performed during the pulse experiment to monitor the change of CGO surface oxygen stoichiometry and deposited carbon. Thus we can confirm the conclusion deduced by pulse experiments and directly relate the carbon self-oxidation rate with bulk oxygen migration of CGO. The results would provide an insight into the design and modification of SOFC anode materials.

Methane Pulse Study on Nickel Impregnated Gadolinium Doped Ceria

Nickel/gadolinium doped ceria (Ni/CGO) has been a popular SOFC anode material for decades for its superior mix conductivity and excellent oxygen storage capacity. However, the study of Ni/CGO in a realistic working condition is still lacking. Also, not much research has linked the chemical properties of CGO with the carbon resistance which is essential when using hydrocarbon as fuels. In this research, Ni/CGO powder was prepared via impregnation-calcination method. A novel pulse experiment has been conducted on impregnated powder. We did certain pulses of a fixed amount of methane into the sample at 600℃.The outlet gas composition was monitored by a quadrupole mass spectrometer. The product gas was composed of several species including hydrogen, steam, carbon monoxide, carbon dioxide, and methane that was not participated to the reaction. The major reaction was the partial oxidation of methane that produced carbon monoxide and hydrogen. With the increase of pulsing numbers, the product and shape of product peaks changed a lot. Also, we observed a “tail” after each carbon monoxide production peak which extend with the number of pulsing. This was deduced to be caused by the self-oxidation of carbon by the bulk migrated oxygen. These changes revealed the changing of CGO during methane oxidation. After detailed quantitative analysis of the shape and integrated intensity of the peaks and tails. We confirmed that the majority of methane partial oxidation reaction on Ni/CGO follows a deposition-oxidation route rather than direct oxidation of methane. Also we were able to evaluate the ability of carbon self-oxidation with different ceria oxidation state. Consequently, a model was proposed to elucidate the relation between carbon self-oxidation rate, surface oxygen concentration and bulk oxygen migration rate of CGO, in forms of partial differential equation. In-situ Raman spectroscopy was performed during the pulse experiment to give a direct parallel evidence. We observed the change of CGO surface oxygen stoichiometry and deposited carbon. Thus we can confirm the conclusion deduced by pulse experiments to prove its validation, and directly relate the carbon self-oxidation rate with bulk oxygen migration rate of CGO. The results provided a novel yet facile method to evaluate the oxygen storage properties of materials and gave an insight into the design and modification of ceria based SOFC anode materials.

Achieving selectivity from the synergistic effect of Cr and Pt activated SnO2 thin film gas sensors

The enhanced selectivity due to the synergistic effect of Cr and Pt co-doping on gas sensing properties of SnO2 thin films, is reported. The undoped and Cr/Pt co-doped SnO2 thin films are deposited on the glass substrates by Ultrasonic Nebulized Spray Pyrolysis of Aqueous Combustion Mixture (UNSPACM). The as-deposited films are nanocrystalline in nature with crystallite size ∼3nm, as seen from x-ray diffraction and transmission electron microscopy (TEM). The chemical state of dopants is investigated using x-ray photoelectron spectroscopy. Further, the presence of separate Pt additives in SnO2 film is also confirmed using TEM studies. The gas sensing properties show that although Pt addition enhances the overall sensitivity, it does not improve the selectivity of the sensor. Nonetheless, Cr and Pt co-doped SnO2 films exhibit greater selectivity towards CO gas, while only Cr doped film exhibits selectivity towards NO2 gas at 275°C. The improved selectivity is attributed to the change in carrier concentration, oxygen stoichiometry and specific catalytic effect of the dopant ions.

Hydroxyl defect effect on the resistance degradation behavior in Y-doped (Ba,Ca)(Ti,Zr)O3 bulk ceramics

Degradation behavior of acceptor (Y)-doped (Ba,Ca)(Ti,Zr)O3 ceramics sintered in moist reducing atmosphere and subsequently re-oxidized in dry or wet atmospheres was contrasted. The degradation behavior critically depends on the water vapor incorporation, the time to degradation systematically decreased with the incorporation of hydroxyl defects (OH)O•. The electrical conductivity was investigated by means of impedance spectroscopy (IS) to determine properties, such as grain and grain boundary conductivity, grain boundary potential barrier height, and space charge layer thickness. Furthermore, transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS) were applied to consider microstructure, microchemistry, and oxygen stoichiometry changes.

A van der Waals Density Functional Study of MoO3 and Its Oxygen Vacancies

The electronic structure of layered molybdenum trioxide MoO3 is highly sensitive to changes in oxygen stoichiometry as Mo6+ has an empty 4d shell. Applications of MoO3 are responsive to small changes in vacancy concentration, with some functions relying on a narrow window of oxygen nonstoichiometry. Difficulties in analyzing the energetics of oxygen vacancies by computational methods stem from the inability to accurately model the layered structure of MoO3. One unit cell parameter is governed by long-range forces across the structural gaps, and these dispersed interactions are not well described by conventional density functional theory (DFT) methods. With the exchange functional vdW-DF2, we accurately model the structure, in good agreement with experimental data. This basis allows exploration of the effect of oxygen nonstoichiometry on the electronic structure and properties of the oxygen-deficient material. The layered structure efficiently screens the structural perturbations caused by oxygen vacancies...

Strain Control of Oxygen Vacancies in Epitaxial Strontium Cobaltite Films

The ability to manipulate oxygen anion defects rather than metal cations in complex oxides can facilitate creating new functionalities critical for emerging energy and device technologies. However, the difficulty in activating oxygen at reduced temperatures hinders the deliberate control of important defects, oxygen vacancies. Here, strontium cobaltite (SrCoOx) is used to demonstrate that epitaxial strain is a powerful tool for manipulating the oxygen vacancy concentration even under highly oxidizing environments and at annealing temperatures as low as 300 °C. By applying a small biaxial tensile strain (2%), the oxygen activation energy barrier decreases by ≈30%, resulting in a tunable oxygen deficient steady‐state under conditions that would normally fully oxidize unstrained cobaltite. These strain‐induced changes in oxygen stoichiometry drive the cobaltite from a ferromagnetic metal towards an antiferromagnetic insulator. The ability to decouple the oxygen vacancy concentration from its typical dependence on the operational environment is useful for effectively designing oxides materials with a specific oxygen stoichiometry.

Characterization of oxygen-dependent stability of selected mixed-conducting perovskite oxides

For high-temperature energy applications, e.g., as solid oxide fuel cell cathodes or as dense ceramic oxygen-transport membranes, mixed ionic-electronic conducting (MIEC) perovskites, namely of the composition (AxSr1−x)(CoyFe1−y)O3−δ (A=La, Pr, Ba), are promising candidates given their excellent oxygen-ionic and electronic transport properties. A high level of oxygen non-stoichiometry is an important prerequisite for a high performance; however, the structural stability of the oxide is also affected by oxygen deficiency.By coulometric titration experiments performed on selected state-of-the-art MIEC powder samples in a tubular zirconia “oxygen pump” setup, facilitating precise control of the oxygen partial pressure in the entire range between pO2=10−18…1bar at temperatures between 700…1000°C, the low-pO2 stability limits can be determined. By monitoring the electrical current necessary for pumping oxygen through the solid electrolyte into or out of the measurement chamber, the amount of oxygen transported and, thus, changes in oxygen stoichiometry can be determined. The low-pO2 stability limits for all investigated high-performance MIEC oxides were similar (below 10−10bar at 900°C).

Tuning of multiferroic orders correlated to oxygen stoichiometry in magnetite films

Magnetite films are grown on Si(100) substrate using pulsed laser deposition technique. The deposition is performed at three fixed oxygen pressures of 0.4 × 10−6, 4.0 × 10−6, and 10 × 10−6 Torr keeping other conditions fixed. Raman and x-ray photoemission spectroscopies confirm formation of single magnetite phase for the first two pressures and a mixed phase composed of magnetite and hematite for the highest pressure. A trivial change in oxygen stoichiometry of magnetite is found crucial for the multiferroic orders at room temperature and below the charge ordering temperature. Possible mechanisms of multiferroic orders driven by change in oxygen stoichiometry are discussed.

Thermodynamics of oxygen in CaMnO3 − δ

The experimental data for equilibrium oxygen con- tent were used in order to extract increments of partial molar thermodynamic functions of oxygen with changes of oxygen stoichiometry in calcium manganite CaMnO3−δ .I t is shown that along with the oxygen exchange reaction, thermal exci- tation of Mn 4+ cations plays an important role in equilibration of charged manganese species that appear in response to the loss of oxygen at heating. The interrelation of partial molar enthalpy and entropy of oxygen with electron and ion defect formation parameters is obtained in approximation of the point defect model. The nearly linear changes of oxygen partial molar enthalpy are shown to directly reflect thermally driven changes in concentration of Mn 3+ cations.

Effects of A-site composition and oxygen nonstoichiometry on the thermodynamic stability of compounds in the Ba–Sr–Co–Fe–O system

The thermodynamic stability of the perovskite-type oxides in the BaxSr1−xCo0.8Fe0.2O3−δ (BSCF) system was investigated with varying Ba:Sr ratios (x=0.2, 0.4, 0.5, 0.6, 0.8) and correlated with the charge compensation mechanism and the change in the oxygen stoichiometry of the materials. Thermodynamic properties represented by the relative partial molar free energies, enthalpies and entropies of oxygen dissolution in the perovskite phase, as well as the equilibrium partial pressures of oxygen have been obtained in the temperature range of 823–1273K using solid electrolyte electrochemical cells (EMF) method. The influence of the oxygen stoichiometry change on the thermodynamic properties was examined using a coulometric titration technique coupled with EMF measurements. The temperature dependence of enthalpy increment (HT−H298) in the temperature range of 700–900K was measured by drop calorimetry. The energetic parameters allow for the correlation of the structural and electrical stability with the defect structures.

High-Temperature pO2 Stability of Metal Oxides Determined by Amperometric Oxygen Titration

For any mixed ionic-electronic conducting (MIEC) oxide an assessment of its basic material properties should include chemical stability as a function of oxygen partial pressure pO2. However, the stability range is not always easily experimentally accessible, but decomposition is linked to oxygen exchange between MIEC sample and surrounding atmosphere. A zirconia-based “oxygen pump” setup is introduced, which enables precise pO2 control of the atmosphere surrounding the MIEC sample. This is facilitated continuously over a pO2 range between 10−20…1 bar at temperatures between 700°C and 1000°C. By measuring the electric current necessary for pumping oxygen into or out of the sample chamber, the corresponding amount of oxygen is determined with a high precision. Furthermore, the time-scale of the continuous pO2 change during this so-called amperometric titration can easily be adjusted to the oxygen exchange/decomposition kinetics of the analyzed sample. In this way, the oxygen uptake or release of an MIEC oxide is determined as a function of pO2, temperature, and time. The amperometric titration setup presented here is discussed in detail and validated using a defect-chemically well-characterized SrTiO3 single crystal and a CuO powder sample. Furthermore, advantages as well as pitfalls are discussed. The subsequent assessment of a Ba0.5Sr0.5Co0.8Fe0.2O3−δ (BSCF) powder sample yields valuable information on oxygen stoichiometry changes during continuous pO2 changes and, finally at low-pO2 values, irreversible demixing phenomena. The stability limits of this MIEC material are thus determined within a temperature range of 700 to 900°C.