Changes In Oxygen Vacancy Concentration Research Articles

Effect of manganese (Mn) doping on the structural, dielectric, ferroelectric, and ferromagnetic properties of ceria (CeO2)

The Ce1-xMnxO2 ceramics were prepared using a solid-state route, with Mn doping concentrations of (x = 0.00, 0.03, 0.08, 0.13, and 0.18). Examination of X-ray diffraction patterns confirms the effective insertion of Mn into the CeO2 lattice, resulting in discernible changes in oxygen vacancy concentration and lattice expansion. X-ray photoelectron spectroscopy (XPS) analysis confirmed the presence of Ce3+, Ce4+, Mn3+, and Mn2+ oxidation states. The Photoluminescence (PL) spectra reveal the existence of oxygen vacancies, as manifested by the appearance of peaks within the energy range spanning from 3.1 eV to 2.71 eV. Raman spectra reveal a symmetric vibration around 466.81 cm-1 in all samples. The surface morphology of the samples exhibited subtle changes in grain size upon the introduction of Mn into CeO2. The dielectric behavior, investigated across a range of frequencies with varying Mn doping concentrations, exhibits an increased dielectric constant in Mn-doped CeO2, particularly with a smaller dielectric loss at lower frequencies. The ferroelectric response revealed a maximum recoverable energy density of 0.56 mJ/cm2 with an efficiency of 47.86% for CeO2. The room temperature magnetic response of the samples improved as Mn concentration increased, which is well explained by the Bound Magnetic Polarons (BMPs) and exchange interactions model. Overall, the studies show the effect of structural, electrical, and magnetic properties after doping with Mn, opening avenues for diverse applications of these ceramics.

Evaluation of the effect of oxygen incorporation on the thermo–chemical–mechanical properties of the proton-conducting oxide BaZr0.8Yb0.2O3-δ

This study aims to evaluate the effects of multiple defect species produced by water and oxygen incorporation on the lattice parameters of BaZr0.8Yb0.2O3-δ (BZYb20) and model its chemical expansion considering these effects. In this study, the lattice constant of BZYb20 was investigated under oxygen partial pressure (pO2 = 1–10−3 bar), water vapor partial pressure (pH2O = 0.0023 bar and dry) and over a wide range of temperatures (473–1173 K) using two approaches: model calculations and high-temperature X-ray diffraction (HT-XRD). HT-XRD measurements revealed that the lattice expansion of BZYb20 changed within 700–900 K in a humidified atmosphere. In a nonhumidified atmosphere, this temperature range shifts lower. High-temperature gravimetric measurements of oxygen incorporation showed that the change in oxygen vacancy concentration was sufficiently small, and the effect of oxygen incorporation on the lattice constant was negligible. High-temperature gravimetric measurements of water incorporation yielded thermodynamic parameters for the hydration reaction. Based on these results, the HT-XRD results were reproduced by fitting a lattice constant model that neglects the effects of oxygen. Therefore, the effect of oxygen incorporation on chemical expansion can be ignored, and the chemical expansion can be estimated using the model developed in this study.

Enhancement of electrical characteristics of SnGaO thin-film transistors via argon and oxygen plasma treatment

The impact of Ar or O2 plasma treatment on the electrical characteristics of SnGaO thin film transistors (TFTs) fabricated via solution method was examined. The findings of the study revealed that Ar plasma treatment for an optimal duration enhances the electrical properties of SnGaO TFTs, including the improvement of mobility, subthreshold swing (S·S.), and on-off ratio. This result is considered to be due to the increased concentration of oxygen vacancy as donor in the channel layer. Oxygen plasma treatment has an opposite effect on the change in oxygen vacancy concentration, thereby reducing the off-state current and facilitating the adjustment of threshold voltage. However, excessive Ar or O2 plasma treatment duration led to an increase in deep defect states and a significant “hump" phenomenon in the output characteristic curve.

Phase Transition Engineering of Vanadium Dioxide Induced by Oxygen Vacancies

A reconfigurable optical functional system is a pioneering and comprehensive approach for portable future flexible electronics, irrespective of traditional silicon-based technology. VO2 shows tremendous temperature-dependent structural and electrical phase transitions as a complex correlated metal oxide, presenting it as a potential candidate for bolometers, thermal camouflage, and optoelectronics applications. However, the structural stability and high transition temperature restrict room-temperature applications. The present study reports the oxygen vacancies-assisted phase transition engineering in the VO2/muscovite heterostructure. The ion bombardment alters the oxygen stoichiometry of VO2/muscovite thin film at room temperature. A significant decrease in lattice constant has been observed during the transition of the insulating M1 (monoclinic) phase into the metallic R (tetragonal) phase, which was confirmed through high-resolution XRD. The change in oxygen vacancy concentration determined using X-ray photoelectron spectroscopy (XPS) actively suppressed and stabilized the epitaxial thin film of VO2 near room temperature. We investigated whether applying increasing power-mediated ion bombardment for a few minutes affects the oxygen vacancy formation on the VO2/muscovite film’s surface. Consequently, a change in KPFM and 4 orders of magnitude change in resistance of VO2 thin film have been observed. This report reveals the fundamental understanding of controlling the MIT in epitaxial VO2 thin film via facile and clean phase transition engineering techniques for advanced flexible electronic devices.

In-depth understanding of the ‘‘promoting and binding’’ effects of Rh/CeO2-ZrO2 interaction regulation on TWCs performance

How metal-support interaction (MSI) affects three-way catalysts (TWCs) performance has always been a focus. This work regulated the interaction between Rh and CeO2-ZrO2 (CZ) by support pretreatment scheme, meanwhile deeply analyzed the effect on activity and stability. CZ after 1000 ℃ pretreatment weakened the MSI and made more active Rh on CZ surface, which significantly enhanced catalytic efficiency (C3H8 T90 decreased by 40 ℃). However, this weakened MSI prevented CZ from anchoring Rh, leading to the growth of Rh after aging and adversely affecting anti-aging ability (T90 increased by 90 °C). Fortunately, when CZ pretreatment at 900 °C, the catalytic performance did not decrease after aging due to the retained strong MSI. In addition, the change of oxygen vacancy concentration caused by CZ pretreatment was positively correlated with MSI strength. This work provided an in-depth understanding for MSI and a new idea for Rh-based TWCs design by elucidating the correlation between MSI and TWCs performance.

Improving the magnetic and dielectric properties of Cu1−xHoxFeO2 nanoceramics by tuning the vacancy defects and Fe valence state

In this study, Cu1−xHoxFeO2 nanoceramics were synthesized by sol-gel method to investigate the influence of the vacancy defects and Fe valence state tuning on the magnetic and dielectric properties of Cu1−xHoxFeO2 nanoceramics. XRD and Raman analysis indicated that Cu1−xHoxFeO2 nanoceramics with single-phase delafossite structure were synthesized by sol-gel method; Ho3+ ions had replaced the Cu ions in CuFeO2 and resulted in lattice deformation. XPS analysis indicated that Ho3+ substitution could tune the Fe valence state, but did not cause significant change in oxygen vacancy concentration. The results of positron annihilation lifetime spectroscopy revealed that cation vacancies concentration in the interface region could be effectively tailored by Ho3+ substitution. The magnetic measurements indicated that Ho3+ substitution enhanced the magnetic transition temperatures and magnetization of Cu1−xHoxFeO2 samples, which could be mainly attributed to the cation vacancies concentration in the interface region and Fe2+ concentration. Most interestingly, the Cu1−xHoxFeO2 nanoceramics exhibited giant dielectric characteristic, which was attributed to the charge ordering of Fe3+ and Fe2+.

Oxygen Vacancy Transition in HfOx‐Based Flexible, Robust, and Synaptic Bi‐Layer Memristor for Neuromorphic and Wearable Applications

AbstractIn this work, a reliable bilayer flexible memristor is demonstrated using TaOx/HfOx Bi‐layer (BL) to mimic synaptic characteristics by using oxygen concentration engineering in the oxide layers. Due to low Gibbs free energy of TaOx layer and stable properties of the single layer memristor, TaOx is inserted in the HfOx‐based memristor for making the BL flexible device. Such device exhibits stable gradual switching behavior with low set/reset voltages (1 V/−1 V) and multilevel cell characteristic making it favorable for synaptic application. The presence of oxide layers and change in oxygen vacancy concentration in two layers are examined by transmission electron microscopy and X‐ray photoelectron spectroscopy, respectively. Further, the device shows potentiation and depression epochs for more than 10 000 pulses and switching up to bending radius of 4 mm for 1000 bending cycles. The device mimics biological synaptic time‐dependent plasticity (STDP) operation when presynaptic and postsynaptic pulses are applied on top and bottom electrodes, respectively. The relationship between nonlinearity coefficient and control parameters in STDP is derived and established. It achieves more that 96% accuracy only after 20 iterations for neuromorphic application when a system of 2500 synapses incorporating 50 × 50 pixel image for recognition is deployed.

Ni/CeO2 catalysts for low‐temperature CO2 methanation: Identifying effect of support morphology and oxygen vacancy

AbstractIn order to investigate the effect of support morphology and oxygen vacancy on the low temperature catalytic activity of Ni‐based catalyst for CO2 methanation, three Ni/CeO2 catalysts were prepared by impregnation method, in which CeO2 materials with different morphology of flowerlike, nanowire, and bulk were used. The characterization results showed that the morphology of CeO2 exhibited a significant effect on the preferential exposed crystal planes, which led to the change of oxygen vacancy concentration and metal dispersion, resulting in the enhancement of catalytic activity at low temperature. Among all catalysts, the nanowire‐like Ni/CeO2 catalyst (Ni/W) showed the highest CO2 conversion due to its enhanced oxygen vacancy concentration and CO2 adsorption capacity of the catalyst. In situ DRIFTS experiment showed that the intermediate product of Ni/W catalyst reaction route was formate. In a 100 h–400°C–lifetime test, Ni/W catalyst also showed excellent long‐term stability. © 2021 Society of Chemical Industry and John Wiley & Sons, Ltd.

Annealing temperature dependent ferromagnetism in (In0.95Fe0.05)2O3 nanoparticles

Room temperature ferromagnetic (In0.95Fe0.05)2O3 nanoparticles were prepared using a sol-gel method at different annealing temperatures ranging from 400 to 800 °C. X-ray diffraction and X-ray photoelectron spectroscopy results indicate that Fe has been doped into In2O3 lattice without any precipitation. From the nonmonotonic changes with annealing temperature in lattice constant, O 1s spectra and band gap, we analyzed the corresponding changes in oxygen vacancy concentration, which is in accordance with the nonmonotonic change in ferromagnetic properties. We believe that annealing temperature can regulate oxygen vacancy concentration effectively, which is closely related to the ferromagnetism in our samples. The mechanism of ferromagnetic coupling can be explained by the bound magnetic polaron model.

Large domain-wall current in BiFeO3 epitaxial thin films

Large domain wall conductivity in insulating ferroelectric thin films provides a new idea for non-destructive readout of domain information of a nonvolatile ferroelectric domain wall memory. However, there are several challenges hindering its development, including the excessive leakage current and insufficient on-state read current. A lot of experimental and theoretical data have referred to the accumulation of oxygen vacancies at the domain wall regions of ferroelectric thin films as the extrinsic contribution of the wall conduction. In this work, we found that the wall current varied with the angle between the applied electric field and the initial polarization, and that the wall current could be increased to 1.9 μA for a bismuth ferrite thin-film device with the reduced oxygen vacancies in contrast to the previous value of 2 nA for the device with rich oxygen vacancies. The oxygen vacancy concentration can be controlled through the thermal annealing of the sample at two atmospheres of 5% hydrogen mixed with argon gas and pure oxygen gas. For the former, the enhanced oxygen vacancies within the annealed sample at the reduced atmosphere decreased the domain wall current by over a few orders of magnitude, in contrast to the significant enhancement of the wall current for the sample in the latter annealed at the oxidizing atmosphere. The change of oxygen vacancy concentration was implied from the Fe2+⇔Fe3+ transition by the X-ray photoelectron spectroscopy analysis. This change confirmed the rigid domain wall conduction mechanism as well as the extrinsic adverse defect compensation of the wall current, paving the way to the exploration of high-power domain-wall nanodevices with sufficient output currents.

Experimental evidence of ferroelectricity in calcium doped hafnium oxide thin films

Ferroelectricity in calcium doped hafnium oxide (Ca:HfO2) thin films has been experimentally proved for the first time in this work. All films prepared by chemical solution deposition exhibited smooth and crack-free surfaces, which were observed using an atomic force microscope. After 104 field cycling, a maximum remanent polarization of 10.5 μC/cm2 was achieved in HfO2 films with 4.8 mol. % Ca content. Meanwhile, the breakdown of the film occurred after 7 × 106 electric cycles. A phase transition from the monoclinic phase to cubic/orthorhombic phases was observed with increasing Ca concentration. We suggest the change in oxygen vacancy concentration as the origin of phase evolution, which was confirmed by X-ray photoelectron spectroscopy analysis. These results open a new pathway for realizing ferroelectricity in HfO2-based films.

Resistive switching behavior in memristors with TiO2 nanorod arrays of different dimensions

In this work, memristors based on TiO2 nanorod arrays (TNAs) were fabricated using a hydrothermal synthesis method. The memristors had a structure consisting of a TNA sandwiched between gold and fluorine doped tin oxide (FTO) coated glass. Devices were constructed using TNAs containing nanorods of different height, diameter and area ratio, with different oxygen vacancy concentrations. The devices exhibit resistance switching behavior and the current through the devices is related to the oxygen vacancy concentration and the area ratio of TNAs. When the height of nanorods is around 3 μm or less, the devices perform well with a current that is as low as 10−8 A in the high resistance state (HRS) and 10−4 A in the low resistance state (LRS). It is found that the space charge limited current (SCLC) mechanism associated with the oxygen vacancies explains the current-voltage behavior of these devices in this three layer structure. A schematic model is presented to illustrate the changes in oxygen vacancy concentration and switching processes, including the formation of new oxygen vacancies (triggered vacancies) to explain the large increase in current at the end of the setting process in these devices.

Role of oxygen vacancies for resistive switching in noble metal sandwiched Pr0.67Ca0.33MnO3-δ

Non-volatile resistance change under electric stimulation in oxides is a promising path to next generation memory devices. However, the underlying mechanisms are still not fully understood. We report here on the study of switching in Pr0.67Ca0.33MnO3-δ (PCMO) films sandwiched by noble metal Pt electrodes, where electrode oxidation can be excluded. In order to develop an understanding of the switching induced oxygen migration, its initial concentration is modified by post-annealing of the deposited PCMO films. The oxygen distribution is obtained by manganese valence determination using spatially resolved electron energy loss spectroscopy in scanning transmission electron microscopy mode. We observe correlations between virgin state resistance, resistive switching properties, oxygen vacancy distribution, and stress/strain state of the PCMO films and propose a simplified interface resistance model based on the measured valence distribution. It assumes a linear correlation of oxygen vacancy concentration with conductivity and a metal to insulator transition above a critical vacancy concentration threshold. Our results suggest that resistance changes can take place at both interfaces of symmetric devices and only requires small changes in oxygen vacancy concentration.

Microstructure and electrical properties of (1−x)[0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3]-xBiCoO3 lead-free ceramics

The (1−x)[0.8Bi0.5Na0.5TiO3-0.2Bi0.5K0.5TiO3]-xBiCoO3 (x = 0, 0.02, 0.05, abbreviated as BNKT, BNKT-002Co, BNKT-005Co, respectively) lead-free ferroelectric ceramics were prepared via the solid state reaction method. The phase structure, microstructure, dielectric, ferroelectric, pyroelectric, and piezoelectric properties of the ceramics were investigated comparatively by using a combination of characterization techniques. All the samples exhibit typical X-ray diffraction peaks of ABO3 perovskite structure. The doping of BiCoO3 causes a decrease in lattice parameters and an increase in grain size of the ceramics. The Raman spectroscopy results suggest a lattice distortion due to the doping. It is found that BNKT-002Co and BNKT-005Co have higher depolarization temperatures compared with BNKT. The Curie-Weiss law and modified Curie-Weiss law explored a diffuse phase transition character for all the samples. The results of ultraviolet–visible diffuse reflectance suggests that BiCoO3-doped ceramics possess higher defect concentration. The impedance analysis shows a temperature dependent relaxation behavior, and the activation energy for the electrical responses varies with the change of BiCoO3 amount. The ferroelectric and piezoelectric properties of the ceramics decrease due to the doping of BiCoO3. Based on the results of the Rayleigh analysis, it was suggested that the differences in the electrical properties among the ceramics are closely related to the change in oxygen vacancy concentration.

Magnetization and Resistance Switchings Induced by Electric Field in Epitaxial Mn:ZnO/BiFeO3 Multiferroic Heterostructures at Room Temperature.

Electric field induced reversible switchings of the magnetization and resistance were achieved at room temperature in epitaxial Mn:ZnO(110)/BiFeO3(001) heterostructures. The observed modulation of magnetic moment is ∼500% accompanying with a coercive field varying from 43 to 300 Oe and a resistive switching ratio up to ∼10(4)% with the applied voltages of ±4 V. The switching mechanisms in magnetization and resistance are attributed to the ferroelectric polarization reversal of the BiFeO3 layer under applied electric fields, combined with the reversible change of oxygen vacancy concentration at the Mn:ZnO/BiFeO3 interface.

Operando reduction of elastic modulus in (Pr, Ce)O2−δ thin films

Non-stoichiometric oxides are key functional materials within technologies such as solid oxide fuel cells (SOFCs) and gas sensors that often exhibit interdependent electrochemical and mechanical properties affecting operando mechanical stability. Here, we explore this electrochemomechanical coupling experimentally and computationally for (Pr, Ce)O2−δ (PCO), a model SOFC cathode material. We quantified Young's elastic modulus E of PCO thin films in situ, at temperatures up to 600 ∘C and oxygen partial pressures pO2 down to 10−3 atm via environmentally controlled nanoindentation. The observed significant reduction (up to 40%) in E with increased temperature or decreased pO2 correlated with changes in oxygen vacancy concentration δ and lattice parameter a expected due to chemical expansion. We confirmed the trend of decreased E with increased δ and a via first principles calculations for bulk PCO. The experimentally observed decrease in E vs. pO2 and temperature was more extreme than predicted by bulk computations, and is anticipated from the higher concentration of vacancies in thin films relative to bulk. These results demonstrate that accurate models of deformation in thin-film devices comprising these chemomechanically coupled oxides may differ markedly from bulk counterparts, and must reflect significant, reversible decreases in elastic moduli resulting from increased temperature and decreased pO2.

Aliovalent Ba2+ doping: A way to reduce oxygen vacancy in multiferroic BiFeO3

This paper demonstrates the impact of Ba2+ substitution on the structural, dielectric relaxation and AC conductivity properties of Bi1−xBaxFeO3(0≤x≤0.25) ceramics. Ba doping incorporates rhombohedral to tetragonal structural transformation in perovskite BFO. XPS data shows change in oxygen vacancy concentration with Ba doping and it also suggests that schoimetry of the doped compounds is not maintained by creating mix valance state of Fe. Reduction in oxygen vacancy (OVs) in the doped samples is explained by Kroger–Vink notation. Arrhenius plot shows activation energy for dielectric relaxation of the doped samples lies between ~1.16 and 1.44eV. AC conductivity of material decreases as Ba ion substitution increases in the parent compound. Electrical conductivity is attributed to the correlated barrier hopping (CBH) motion of the oxygen vacancies in the samples. Coulombic potential barrier (WM) height, calculated from Elliott model for CBH motion of charge carriers shows correlation with the activation energy of AC conductivity at low temperature. Activation energy value obtained from the impedance measurements of the samples implies short range migration of oxygen vacancies dominates the frequency dependent conductivity while the frequency independent part of conductivity is the result of long range migration of oxygen vacancies.

Microstructure and Electrical Properties of Nonstoichiometric 0.94(Na0.5Bi0.5+x)TiO3–0.06BaTiO3Lead‐Free Ceramics

0.94(Na0.5Bi0.5+x)TiO3–0.06BaTiO3(x = −0.04, 0, 0.02; namedNB0.46T‐6BT,NB0.50T‐6BT,NB0.52T‐6BT, respectively) lead‐free piezoelectric ceramics were prepared via the solid‐state reaction method. Effects of Bi3+nonstoichiometry on microstructure, dielectric, ferroelectric, and piezoelectric properties were studied. All ceramics show typical X‐ray diffraction peaks ofABO3perovskite structure. The lattice parameters increase with the increase in the Bi3+content. The electron probe microanalysis demonstrates that the excess Bi2O3in the starting composition can compensate the Bi2O3loss induced during sample processing. The size and shape of grains are closely related to the Bi3+content. For the unpoledNB0.50T‐6BTandNB0.52T‐6BT, there are two dielectric anomalies in the dielectric constant–temperature curves. The unpoledNB0.46T‐6BTshows one dielectric anomaly accompanied by high dielectric constant and dielectric loss at low frequencies. After poling, a new dielectric anomaly appears around depolarization temperature (Td) for all ceramics and theTdvalues increase with the Bi3+amount decreasing from excess to deficiency. The diffuse phase transition character was studied via the Curie–Weiss law and modified Curie–Weiss law. The activation energy values obtained via the impedance analysis are 0.69, 1.05, and 1.16 eV forNB0.46T‐6BT,NB0.50T‐6BTandNB0.52T‐6BT, respectively, implying the change in oxygen vacancy concentration in the ceramics. The piezoelectric constant, polarization, and coercive field of the ceramics change with the variation in the Bi3+content. The Rayleigh analysis suggests that the change in electrical properties of the ceramics with the variation in the Bi3+amount is related to the effect of oxygen vacancies.

Elastoplastic Properties of (Pr, Ce)O2-δ Thin Films

Solid oxide fuel cells (SOFCs) rely on long term, consistent operation of non-stoichiometric oxide components under elevated temperatures and oxygen partial pressure gradients. Materials that exhibit the desired ion transport and reactivity properties generally also exhibit electro-chemo-mechanical coupling that can affect mechanical stability in operando. The importance of chemomechanical coupling such as chemical expansion of SOFC electrodes is increasingly relevant to the goal of reduced operating temperatures enabled by thin film electrodes. Here, we explore this coupling experimentally and computationally, for the model SOFC cathode material, (Pr, Ce)O2-δ. We quantified the Young’s elastic modulus E of (Pr, Ce)O2-δ thin films at temperatures up to 600ºC and oxygen partial pressures pO2 below 10-3 atm via environmentally controlled nanoindentation. The observed decrease in E with increased temperature or decreased pO2 correlated with the changes in oxygen vacancy concentration and lattice parameter expected due to chemical expansion. We compared these results with those we calculated for several compositions of bulk (Pr, Ce)O2-δ via density functional theory, and found that the experimentally observed reduction in mechanical stiffness of reduced (Pr, Ce)O2-δ thin films exceeded that predicted computationally for bulk counterparts. These results demonstrate that the in operando mechanical properties of non-stoichiometric oxides used in thin film SOFCs can differ significantly from those expected from characterization of bulk forms or at standard temperature and pressure. Thus, this chemomechanical coupling must be considered when designing mechanically robust SOFCs with thin film electrode components.

The influence of boric acid on improved persistent luminescence and thermal oxidation resistance of SrAl2O4:Eu2+

Persistent luminescence of SrAl2O4:Eu2+ has attracted considerable attention due to their high initial brightness, long-lasting time and excellent thermal stability. Here the influence of boric acid on the persistent luminescence and thermal oxidation resistance of SrAl2O4:Eu2+ was investigated in detail. Crystal structural analysis and scanning electron microscopy revealed that with the addition of boron, the unit cell volume decreased and the morphology of the particles became more irregular with sharp edges. Thermogravimetric analysis showed better thermal oxidation resistance accompanied by a change in oxygen vacancy concentration when boron acid is used. Photoluminescence spectra and afterglow decay curves confirm an improved afterglow performance for boron-added SrAl2O4:Eu2+. Thermoluminesence allowed monitoring the changes in the trap states due to the presence of B. Our results imply that the substantial improvement of afterglow performance and the thermal stability in SrAl2O4:Eu2+ can be attributed to the incorporation of boron into the aluminate network.