Large Amount Of Oxygen Vacancies Research Articles

Oxygen vacancy engineering in ZnO/ZrO2 composite catalysts for highly enhanced hydrogen production by methanol steam reforming

The ZnO/ZrO2 binary catalyst system has been an emerging and promising catalyst for efficient production of H2 through methanol steam reforming (MSR). Herein, ZrO2 supports with different crystalline phases were prepared by the oxygen vacancy engineering strategy of Li doping, and then ZnO/ZrO2 catalysts were prepared by the isovolumic impregnation method. The different quantities of doped Li have been found to have a significant impact on dominant crystal phase of ZrO2 supports, which dictates the interaction between ZnO and ZrO2 and the number of oxygen vacancies. In comparison with ZnO/ZrO2-20Li and ZnO/ZrO2-40Li catalysts with predominate tetragonal and monoclinic crystal phase, ZnO/ZrO2-3Li with a mixed crystalline phase exhibits higher catalytic activity for methanol conversion (XCH3OH) and CO selectivity (SCO). Complete methanol conversion of the best performing ZnO/ZrO2-3Li catalyst was observed at a very low CO selectivity of 3.1 % at 375 ℃. The XRD, UV, and XPS characterization results indicate that the strong interaction between ZrO2 and ZnO leads to the appearance of large amount of oxygen vacancies at the ZnO/ZrO2 interface, which greatly improves the performance of the catalyst. ZnO/ZrO2 catalyst showed remarkable stability without obvious deactivation after 32 hours of continuous testing. This work highlights the effectiveness of oxygen vacancy engineering of the ZnO/ZrO2 catalysts for applications in methanol steam reforming and other heterogeneous catalytic systems.

Quaternary piezoelectric ceramics with ultra-high mechanical quality factor

The development of piezoelectric ceramics with large piezoelectric coefficient (d33), high Curie temperature (TC) and high mechanical quality factor (Qm) is still a key challenge for practical application for high-power device. Herein, a novel quaternary piezoelectric ceramics of (0.125-x) Pb0.98Sr0.02(Mg1/3Nb2/3)O3–0.445PbTiO3–0.43PbZrO3-xPb(Mn1/3Nb2/3)O3 (abbreviated as xPMnN-PSMN-PZT) were prepared by the solid-state sintering method. The hard dopant PMnN induced the morphotropic phase boundary (MPB) of PSMN-PZT into a tetragonal-rich region. In addition, the XPS and EPR analysis proved the existence of a large amount of oxygen vacancies (Vo••) in PMnN-PSMN-PZT ceramic system. The composition near tetragonal-rich MPB region and the appearance of defect dipoles formed a strong coupling effect on the domain wall motion and leaded to an ultra Qm value. The optimum piezoelectric properties were achieved at x = 9 mol% with d33 = 260 pC/N, Qm = 3880, tanδ = 0.009 and TC = 332 °C. A significant internal bias field (Ebias) of 8.83 kV/cm was also observed in 0.09PMnN-PSMN-PZT ceramics. The origin of this excellent piezoelectric properties was explained by the phase structure, piezoelectric and dielectric analysis. This work demonstrated a rational strategy to obtain ultra Qm value for high-power piezoceramics.

Enhancing Photoelectrochemical Water Oxidation on WO3 via Electrochromic Modulation: Universal Effects and Mechanistic Insights.

Although WO3 exhibits both electrochromic and photoelectrochemical (PEC) properties, there is no research conducted to investigate the correlation between them. The study herein reports the electrochromic enhancement of PEC activity on WO3. The electrochromic WO3 (e-WO3) exhibits a significantly enhanced activity for PEC water oxidation compared to raw WO3 (r-WO3), with a limiting photocurrent density three times that of r-WO3. The electrochromic enhancement of PEC activity is universal and independent of the type of cations inserted during electrochromism. Decoloring reduces the PEC activity but a simple re-coloring restores the activity to its maximum value. Electrochromism induces large amounts of oxygen vacancies and surface states, the former improving the electron density of WO3 and the latter facilitating the hole transfer across e-WO3/electrolyte interface. It is proved that the electrochromic enhancement effect is due to the significantly improved electron-hole separation efficiency and the charge transfer efficiency across the WO3/electrolyte interface.

Achieving water-floatable photocatalyst on recycled bamboo chopsticks

Disposable bamboo chopsticks (DBCs) are difficult to recycle, which inevitably cause secondary pollution. Based on energy and environmental issues, we propose a facile strategy to fabricate floatable photocatalyst (fPC) coated onto DBCs, which can be flexibly used in water purification. The photocatalyst of titania and titanium carbide on bamboo (TiO2/TiC@b) was successfully constructed from TiC-Ti powders and DBCs using a coating technique followed heat treatment in carbon powder, and the fPC exhibited excellent photocatalytic activity under visible light irradation. The analysis results indicate that rutile TiO2 forms on TiC during heat treatment, achieving a low-density material with an average value of approximately 0.5233 g/cm3. The coatings of TiO2/TiC on the bamboo are firm and uniform, with a particle size of about 20–50 nm. XPS results show that a large amount of oxygen vacancies is generated, due to the reaction atmosphere of more carbon and less oxygen, further favoring to narrowing the band gap of TiO2. Furthermore, TiO2 formed on residual TiC would induce the formation of a heterojunction, which effectively inhibits the photogenerated electron–hole recombination via the charge transfer effect. Notably, the degradation of dye Rhodamine B (Rh.B) is 62.4% within 3 h, while a previous adsorption of 36.0% for 1 h. The excellent photocatalytic performance of TiO2/TiC@b can be attributed to the enhanced reaction at the water/air interface due to the reduced light loss in water, improved visible-light response, increased accessible area and charge transfer effect. Our findings show that the proposed strategy achieves a simple, low-cost, and mass-producible method to fabricate fPC onto the used DBCs, which is expected to applied in multiple fields, especially in waste recycling and water treatment.

Enhanced co-production of high-quality syngas and highly-concentrated hydrogen via chemical looping steam methane reforming over Ni-substituted La0.6Ce0.4MnO3 oxygen carriers

Chemical looping steam methane reforming (CLSMR) is increasingly attracting more and more attention as a promising technology for co-production of syngas and hydrogen. However, low reactivity, poor selectivity and weak resistance to carbon deposition of oxygen carriers (OCs) probably discourage further commercial application of CLSMR. With this background, a novel strategy of incorporating Ce and Ni into LaMnO3 is advised to promote lattice oxygen migration rate and improve syngas selectivity. The redox activity and cycle stability of La0.6Ce0.4Mn1−xNixO3 for CLSMR are explored in a fixed-bed reactor coupling with various characterization methods. The characterization results show that La0.6Ce0.4Mn1−xNixO3 OCs still maintain the standard perovskite structures and well-ordered skeleton. The introduction of Ce3+ induces large amounts of oxygen vacancies on the surface of OCs and improve the syngas selectivity of methane reforming, leading to higher oxygen mobility and lower carbon deposition. Meanwhile, the experimental results also confirm that the doping of Ni into La0.6Ce0.4MnO3 can greatly improve the oxygen-donation ability and effectively provide more active sites for reaction with methane. While excessive Ni in perovskite will enhance the methane dissociation, which is the main reason of carbon deposits. The proper amount of Ni doping can well match the methane activation rate to migration rate of bulk lattice oxygen that is feasible to realize the balance between productivity and efficiency. In addition, the reduced metal elements along with great amounts of oxygen vacancies together contribute to more active sites for the following steam splitting which avails to produce high-purity H2. La0.6Ce0.4Mn0.7Ni0.3O3 oxygen carrier eventually achieve syngas yield of 2.8 mmol/g H2 and 1.4 mmol/g CO that is closed to the ideal value without obvious carbon deposition, and pure H2 yield of 3.8 mmol/g, exhibiting the best redox performance and good regenerability. Meanwhile, La0.6Ce0.4Mn0.7Ni0.3O3 can still maintain high redox activity and basic perovskite structure after 15 cyclic redox tests, illustrating a desirable stability and reliability. In summary, this study offers a valuable and effective strategy to tackle with the bottleneck of CLSMR, paving the way of doping transition metal as a pathway of modulating reaction performance and stability of OCs in chemical looping processes.

Reinforcement of methanol catalytic reforming for hydrogen production through Ru-based carbon-coated CeO2 catalyst

Methanol steam reforming for hydrogen production is a highly promising energy technology, due to the high hydrogen intensity of methanol and the appendant hydrogen capture from water. In order to reinforce the hydrogen yield in methanol steam reforming reaction, a series of Ru-based carbon-coated CeO2 catalysts were prepared in this work, using glucose, cellulose and activated carbon as different carbon sources via a hydrothermal method. The results showed that the Ru/CG-CeO2 (glucose) catalyst exhibited a superior catalytic performance at the reaction temperature of 400 °C, with a higher hydrogen production rate (0.276 mol/h) than Ru/CC-CeO2 (cellulose, 0.230 mol/h) and Ru/CA-CeO2 (activated carbon, 0.253 mol/h) catalysts. This was attributed to the synergistic effect between CeO2 and carbon, where the carbon layer formed by glucose enhanced the interaction between the active component and the CeO2 support, which also facilitated the electron transfer effects and resulted in a large amount of oxygen vacancies for Ru/CG-CeO2. This led to a strong adsorption capacity for CO and oxygen-containing species derived from the substrate, thereby improving hydrogen production. Additionally, the catalyst stability and the resistance to carbon deposition were evaluated for all the three catalysts, and Ru/CG-CeO2 catalyst also demonstrated a superior stability and anti-carbon deposition capability.

High surface proton conduction of a-axis oriented CeO2−δ thin film on (200) YSZ substrate

Abstract We have prepared a-axis oriented CeO2−δ thin films prepared on yttrium-stabilized zirconium (200) single crystal substrates by RF magnetron sputtering using a ceramic target. The amount of oxygen vacancies in the CeO2−δ thin films was quantitatively evaluated by X-ray absorption spectroscopy and defect chemical analysis. In terms of its electronic structure, the O 1s photoemission spectrum of the wet-annealed film shows an O–H bond peak, which suggests proton conduction. The a-axis oriented CeO2−δ thin film annealed at 600 °C in a wet atmosphere exhibit high proton conductivity of more than 101 Scm−1K at medium temperature range from 300 °C to 550 °C by thermal activation process and surface proton conductivity of σT = ∼7.0 × 10−1 Scm−1K by Grotthuss mechanism below 50 °C. The enhancements of proton conductivities are considered to be due to large amount of oxygen vacancies of ∼3.3% at the surface of wet-annealed CeO2−δ thin film. These results indicate that the wet-annealed a-axis oriented CeO2−δ thin films can be applied as an electrolyte for solid oxide fuel cell operating at medium temperature range or electric double layer transistor operating below 50 °C.

Tuning the superconducting performance of YBa2Cu3O7-δ films through field-induced oxygen doping.

The exploration of metal-insulator transitions to produce field-induced reversible resistive switching effects has been a longstanding pursuit in materials science. Although the resistive switching effect in strongly correlated oxides is often associated with the creation or annihilation of oxygen vacancies, the underlying mechanisms behind this phenomenon are complex and, in many cases, still not clear. This study focuses on the analysis of the superconducting performance of cuprate YBa2Cu3O7-δ (YBCO) devices switched to different resistive states through gate voltage pulses. The goal is to evaluate the effect of field-induced oxygen diffusion on the magnetic field and angular dependence of the critical current density and identify the role of induced defects in the switching performance. Transition electron microscopy measurements indicate that field-induced transition to high resistance states occurs through the generation of YBa2Cu4O7 (Y124) intergrowths with a large amount of oxygen vacancies, in agreement with the obtained critical current density dependences. These results have significant implications for better understanding the mechanisms of field-induced oxygen doping in cuprate superconductors and their role on the superconducting performance.

High proton conduction by full hydration in highly oxygen deficient perovskite

We report high proton conductivity (10 mS cm−1 at 235 °C) of stable BaSc0.8W0.2O2.8, which is attributed to (1) high proton concentration due to full hydration and large amount of oxygen vacancies and (2) high proton mobility due to reduced proton trapping.

Unveiling the red electroluminescence in LSMO-SRO thin film heterostructures

Herein, intense red electroluminescence (674 nm) in a La0.7Sr0.3MnO3(LSMO)-SrRuO3(SRO) thin films heterostructure obtained by RF-magnetron sputtering is reported for the first time. This novel phenomenon may open a new door in the field of optoelectronics and new generation devices based on multiferroic heterostructure. The structural properties were analyzed using grazing incidence x-ray diffraction demonstrating a rhombohedral structure for LSMO with (024) preferential orientation and an orthorhombic structure for SRO films with (121) preferential orientation. Raman and x-ray photoelectron spectroscopy (XPS) results demonstrated a mix of phases present in LSMO thin films. The thickness analysis was performed using scanning electron microscopy (SEM) in cross section and the thickness was found to be 123 nm and 49 nm for LSMO and SRO, respectively. XPS analysis for LSMO revealed that point defects associate to Mn cations and oxygen vacancies modify the total electrical conductivity and due to Mott transitions, suggest a p-type semiconductor behavior. For the SRO film, the XPS analysis demonstrated a change in the stoichiometry of the film surface, which provides a large amount of oxygen vacancies indicating a metal oxide transition with effects that involve multiple degrees of freedom (charge, spin, and lattice). The electrical behavior observed in the heterostructure corresponds to a metal-semiconductor junction showing a Schottky conduction mechanism in positive bias and a space charge limited conduction mechanism in negative bias. The red electroluminescence effect is discussed considering both a radiative recombination process occurred at the interface due to the presence of Mn2+ ions and an energy transfer mechanism assisted by the migration of the high amount of oxygen vacancies present in the interface.

Epitaxial growth of Co3O4/In2O3 p-n heterostructure with rich oxygen vacancies for ultrahigh triethylamine sensing

Although metal oxides based gas sensors have been researched extensively in previous works, there are enormous challenges on improving the deficiencies in poor selectivity, sensitivity, humidity resistance and so on. Hence, we developed an epitaxial growth method to prepare dendritic Co3O4/In2O3-(1−3) heterostructures of distinct Co/In mass ratios through perfect lattice matching between the two crystal domains at the interface, along with introducing large amounts of oxygen vacancies. As a result, the Co3O4/In2O3-2 heterostructure based gas sensor showed enhanced sensing properties towards triethylamine (TEA, 100 ppm) with a high response value of Ra/Rg = 576.1 at an optimal working temperature of 200 °C, which is 93 times higher than that of pristine In2O3. Moreover, the Co3O4/In2O3-2 sensor exhibited excellent gas selectivity, quick recovery time, repeatable cycling, long-term stability, as well as outstanding humidity resistance. The improved TEA gas sensing performance can be attributed to the integration of interfacial synergies in Co3O4/In2O3 p-n heterostructure and generation of rich oxygen vacancies, exposing more active sites to facilitate the adsorption/desorption of gas molecules. This work provides a feasible pathway to regulate TEA sensing performance of metal oxide semiconductor (MOS) gas sensors by capitalizing on self-sacrificial template strategy and reconstruction of novel metal oxide heterostructure.

Effect of oxygen vacancies on the electrical transport properties of conductive Y3Fe5O12 films at high temperature

Oxygen vacancies doping in oxide materials is a very common means to modulate the electrical transport properties. In this work, Y3Fe5O12 (YIG) films with abundant oxygen vacancies were grown on Gd3Ga5O12 substrates by solution spin coating and high vacuum annealing method, and the effect of oxygen vacancies on the electrical transport properties was systematically studied. It was found that a large amount of oxygen vacancies doping could convert the YIG film from a good room-temperature insulator to an electrical conductor. At high temperature and high vacuum, a large number of oxygen vacancies increased the disorder of the system, resulting in the appearance of a band-tail state, thus forming a constant range hopping conduction. While when the sample was exposed to air, the oxygen vacancies in the sample would gradually recombine and disappear, and the conduction mechanism transferred to drift mode.

Waste eggshell to valuable Co3O4/eggshell material as an efficient catalyst for catalytic decomposition of N2O under simulated real tail gases

Co3O4 shows excellent catalytic activity for N2O decomposition, in which it is urgently needed to develop novel efficient catalysts/supports with cheap and environmentally-friendly properties. Herein, Co3O4 nanoparticles decorated eggshell catalysts (Co3O4/eggshell-GR, Co3O4/eggshell-IM, and Co3O4/eggshell-DP) were prepared using simple grinding, impregnation, and deposition-precipitation methods, where waste eggshells were used as an effective support and reaction-promoter. The results indicated that the preparation methods significantly influenced the catalytic activity of Co3O4/eggshell catalysts; the highest activity was obtained over the Co3O4/eggshell-DP. The activity of Co3O4/eggshell-DP was comparable to that of pure Co3O4. More importantly, its specific activity was about 7.3 times that of Co3O4 at 400 °C. The examined Co3O4/eggshell-DP also displayed outstanding stability and anti-poisoning property in NO, O2, and/or H2O-containing atmospheres. It was also verified that the excellent activity of Co3O4/eggshell-DP correlated with the unique structure of the eggshell and the close incorporation between CaCO3 and Co3O4. The Co3O4/eggshell-DP catalyst owned abundant Co3O4 nanoparticles, more Co3+ species, and excellent redox performance, promoting the conversion of N2O at a lower temperature. The presence of abundant surface basic sites and a larger amount of oxygen vacancies also facilitated the catalytic efficiency of Co3O4/eggshell-DP.

A positive electrode with thoughtful diffusion capacitance for solid flexible supercapacitor: N-6 doped porous carbon shell coated Cu doped Mn[sbnd]Co LDO core

The suitable proportion of diffusion contribution during the electrochemical process could influence the electrochemical performance of electrode materials. An effective method is that tuning the diffusion reaction contribution via doping heteroatom, introducing defects or surface modification. In this work, a unique core-shell structure material Cu-Mn-Co LDO-C is obtained. Benefitting from Cu pre-doped into MnCo LDO, large amount of oxygen vacancies is introduced to enhance electrochemical performance; N-6 doped porous carbon shell protects Cu-Mn-Co LDO core and improves electrochemical performance of Cu-Mn-Co LDO-C further. In addition, doping Cu broadens potential window of Cu-Mn-Co LDO-C surprisingly. Cu-Mn-Co LDO-C presents 5028.7 mF cm−2 at a current density of 1 mA cm−2 with a facilely reached potential window of 0–0.65 V finally, and retains 73.4 % diffusion contribution at a scan rate of 1 mV s−1, 18.3 % at 100 mV s−1. After 10,000 cycles at 2 mA cm−2, Cu-Mn-Co LDO-C keeps 90.3 % of initial capacitance. Using Cu-Mn-Co LDO-C as positive and Fe3O4-C as negative to fabricate a solid flexible asymmetrical supercapacitor (SAS), this SAS presents a considerable energy density that attribute to the high working potential of positive Cu-Mn-Co LDO-C. SAS could working stably at a potential window of 0–2.2 V, and presents the highest energy density 243.1 μWh cm−2 at a power density of 2.8 mW cm−2, remains 203.0 μWh cm−2 at the highest power density of 55.4 mW cm−2. A hand ring that was assembled by three SAS which contacted in series could light a highest rated voltage white LED more than 10 min. As a result, Cu-Mn-Co LDO-C is an ideal candidate for flexible positive electrode in the field of smart, portable energy storage.

Improving the Performance of Solution-Processed Quantum Dot Light-Emitting Diodes via a HfOx Interfacial Layer.

One of the major obstacles in the way of high-performance quantum dot light-emitting diodes (QLEDs) is the charge imbalance arising from more efficient electron injection into the emission layer than the hole injection. In previous studies, a balanced charge injection was often achieved by lowering the electron injection efficiency; however, high performance next-generation QLEDs require the hole injection efficiency to be enhanced to the level of electron injection efficiency. Here, we introduce a solution-processed HfOx layer for the enhanced hole injection efficiency. A large amount of oxygen vacancies in the HfOx films creates gap states that lower the hole injection barrier between the anode and the emission layer, resulting in enhanced light-emitting characteristics. The insertion of the HfOx layer increased the luminance of the device to 166,600 cd/m2, and the current efficiency and external quantum efficiency to 16.6 cd/A and 3.68%, respectively, compared with the values of 63,673 cd/m2, 7.37 cd/A, and 1.64% for the device without HfOx layer. The enhanced light-emitting characteristics of the device were elucidated by X-ray photoelectron, ultra-violet photoelectron, and UV-visible spectroscopy. Our results suggest that the insertion of the HfOx layer is a useful method for improving the light-emitting properties of QLEDs.

Highly Electron-Doped TaON Single-Crystal Growth by a High-Pressure Flux Method.

Transition-metal oxynitrides have a variety of functions such as visible light-responsive catalysts and dielectric materials, but acquiring single crystals necessary to understand inherent properties is difficult and is limited to relatively small sizes (<10 μm) because they easily decompose at high temperatures. Here, we have succeeded in growing platelet single crystals of TaON with a typical size of 50 × 100 × 10 μm3 under a high pressure and high temperature (6 GPa and 1400 °C) using a LiCl flux. Such a harsh condition, in contrast to powder samples synthesized under mild conditions, resulted in the introduction of a large amount of oxygen vacancies (x = 0.06 in TaO1-xN) into the crystal, providing a metallic behavior with a large anisotropy of ρc/ρab ∼ 103. Low-temperature oxygen annealing allows for a single-crystal-to-single-crystal transformation to obtain fully oxidized TaON (yellow) crystals. Needle-like crystals can be obtained when NH4Cl is used as a flux. Furthermore, black Hf2ON2 single crystals are also grown, suggesting that the high-pressure flux method is widely applicable to other transition-metal oxynitrides, with extensive carrier control.

Enhanced soot oxidation by oxygen vacancies via K + doped CuFe 2 O 4 spinel catalysts

Soot particulate emitting from diesel engines are serious to both human health and environment. Efficient catalyst is vital in diesel particulate filter technology to decrease soot oxidation temperature. Cu1−xKxFe2O4 (x = 0, 0.05, 0.1, and 0.15) catalysts are synthesized and analyzed with XRD, Raman, BET, SEM, TEM, TG, EDS, XPS, catalytic activity tests, and kinetic analysis. The results suggests that many factors affect the catalytic activity, and the contents of oxygen vacancies are dominant. A large amount of oxygen vacancies which facilitate to the soot oxidation are created with the assistance of K+, and they increase with the increasing doping contents. Excessive surface concentration of K+ may lead to the partially covering of active sites of the catalysts. Therefore, Cu0.9K0.1Fe2O4 catalyst exhibits the best catalytic activity in soot oxidation, which decreases the Tmax of soot by 279°C. This decrease makes it highly promising for application in diesel particulate filter.

Perovskite SrCo1-xTixO3-δ as anion-intercalated electrode materials for supercapacitors

Perovskites oxides have recently been widely deployed in supercapacitors though they are suffering from poor electrochemical activity as electrode materials. Herein, a series of Ti-substituted perovskite SrCo1-xTixO3-δ (x = 0, 0.05, 0.10, 0.15, 0.20) are synthesized by solid-state reaction method for anion-intercalated supercapacitor electrode materials. The structural evolution, morphology and the electrochemical performance with the variation of Ti content are characterized in detail. The parent material SrCoO3-δ contains two phase: a hexagonal structure with space group R3¯:H (87.09 wt%) and a simple cubic structure with space group Pm3¯m (12.91 wt%). While a stable cubic structure with Pm3¯m space group is obtained after Ti substitution and the electrochemical performance is also significantly optimized. A highest specific capacitance (Cs) of 625.0 F g−1 at 1 A g−1 is achieved in SrCo0.9Ti0.1O3-δ sample. The superior electrochemical performance of SrCo0.9Ti0.1O3-δ is attributed to the stable cubic structure with higher conductivity and large amount of oxygen vacancy which directly contributes to the anion-intercalated energy storage. In addition, the asymmetric supercapacitor combined with activated carbon electrode exhibits a high specific capacitance of 114.4 F g−1 (50.8 mAh g−1) and 98.3 F g−1 (42.7 mAh g−1) at 5 mV s−1 and 1 A g−1, respectively. The device presents an energy density of 69.9 Wh kg−1 at power density of 1600 W kg−1 with a working voltage of 1.6 V.

Highly dispersible cerium-oxide modified Ni/SBA-15 for steam reforming of bio-mass based JP10

Ni/SBA-15 modified by highly dispersed cerium-oxide was prepared with the aid of sucrose for steam reforming of JP10 (C10H16). Their characterization showed that addition of appropriate amount ceria led to the formation of highly dispersed CeO2 and Ni, and the CeO2 covered smaller nickel particles like strawberry seeds to form much more interface between them. Their catalytic activity exhibited higher stability over time on stream of 6.5 h with conversion higher than 95% and higher carbon resistance (mass loss less than 4.5% by TG), which may derive from good properties below: (1) much more interface enhanced cooperation effect and increased turnover frequency at the interface; (2) the stronger interaction between Ni and ceria to suppress sintering by formation of Ni-O-Ce solid solution; (3) the large amount of oxygen vacancies from the formation of Ni-O-Ce solid solution and highly dispersed CeO2 to facilitate the water–gas–shift reaction and carbon removal.

Electric-field-induced epitaxial breakdown and emergent magnetoresistance due to strong oxygen reduction in Ca-doped BiFeO3

We study the structural and transport properties of a heavily oxygen-reduced region that is formed by electrical injection of a large amount of oxygen vacancies into a ${\mathrm{Bi}}_{0.7}{\mathrm{Ca}}_{0.3}{\mathrm{FeO}}_{3\ensuremath{-}\ensuremath{\delta}}$ thin film. In electroforming, the epitaxial as-grown state is transformed into a disordered, polycrystalline phase. Nonohmic current-voltage relations, which can be interpreted as a space-charge-limited conduction, appear at low temperatures. As temperature increases, the curvature of the current-voltage relationship gradually changes to be nearly ohmic at and above 100 K. The maximum value of magnetoresistance, as large as \ensuremath{-}2.6% at 90 kOe, is attained at the same temperature of \ensuremath{\sim}100 K, which is significantly larger than \ensuremath{-}0.1% at 25 K and \ensuremath{-}1.3% at 300 K.