Accumulation Of Oxygen Vacancies Research Articles

Corrosion behavior of carbon steel in chloride and bicarbonate ion‐enriched and CO2‐saturated solutions

AbstractThis article addresses the knowledge gap in the area of the synergistic effect between Cl− and by comprehensively evaluating the corrosion behavior of AISI 1020 carbon steel exposed to the CO2‐saturated solutions containing Cl− and/or over the time period of 6–72 hrs at 60°C. The results show a synergistic mechanism between Cl− and in the pitting corrosion of carbon steel. accelerates the localized precipitation of FeCO3 crystals by means of increasing pH and adsorption, which is a prerequisite for pitting initiation. Cl− contributes to the catalytic dissolution of the Fe matrix along with film breakdown via the local accumulation of oxygen vacancies, and the pitting corrosion of carbon steel is, thus, further exacerbated.

Significantly enhanced ferroelectric and dielectric properties in BaTiO3/LaNiO3 superlattices

BaTiO3/LaNiO3 (BTO/LNO) ferroelectric superlattices with different stacking periods have been prepared by pulsed laser deposition (PLD). Compared to the pure BTO films, the BTO/LNO superlattices have significantly enhanced ferroelectric and dielectric properties. This is because, in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current in the superlattices. The results indicate that the use of ultra-thin metallic oxide layers as the constituent material of the ferroelectric superlattices is a feasible and effective way to improve the properties of superlattices.

Interface-mediated local conduction at tubular interfaces in BiFeO3–CoFe2O4 nanocomposites

The electronic transport at the topological boundaries holds great promise for applications in next-generation devices. Amongst, tubular interfaces have become an emerging 2D topological state to achieve controllable charge transport path for data storage technologies. However, the mechanism and origin underlying the local conduction at tubular interfaces is still under debate. Herein, we fabricated BiFeO3–CoFe2O4 (BFO-CFO) nanocomposites using magnetron sputtering and systematically investigated the local conduction through variable temperature electrical measurements. We found that the observed conductivity can be well described within an interface-mediated Poole-Frenkel (PF) model combining interface tunneling and PF conduction. And the local current was attributed to the accumulated oxygen vacancies at the tubular interface, which was further confirmed by post-annealing experiments. This study provided further understanding of conduction behavior at complex oxide interfaces and lay the foundation for novel electronic devices.

Modulation of the resistive switching of BiFO3 thin films through electrical stressing

The resistive switching (RS) of Au/BiFeO3/SrRuO3 samples was shown to be controllable by using a thermal treatment and an electrical stressing method. Such a modulation of resistive switching effect can be associated to the oxygen vacancy movement and redistribution within the BiFeO3 thin film and the trapping/detrapping of charge carriers at the interfaces. After the application of a negative voltage to the thin film for a stressing period, a resistive switching reversal effect occurred and the current retention ability in the low resistance state increased, indicating an increase in the trap density at the interface and an enhancement of the charge carrier trapping ability. The trap density, trap level, and Schottky barrier height all display corresponding trends in their values as a result of the modulation of RS effect. The results indicate that the greater the accumulation of oxygen vacancies at any the film/electrode interface, when a reverse bias is applied the higher the resistance ratio was under reverse bias. Its diffusion process was likely to be hindered and the trapped charge carriers could be retained after a long time of electrical stressing.

Observation of Strong Polarization Enhancement in Ferroelectric Tunnel Junctions.

Ferroelectric heterostructures, with capability of storing data at ultrahigh densities, could act as the platform for next-generation memories. The development of new device paradigms has been hampered by the long-standing notion of inevitable ferroelectricity suppression under reduced dimensions. Despite recent experimental observation of stable polarized states in ferroelectric ultrathin films, the out-of-plane polarization components in these films are strongly attenuated compared to thicker films, implying a degradation of device performance in electronic miniaturization processes. Here, in a model system of BiFeO3/La0.7Sr0.3MnO3, we report observation of a dramatic out-of-plane polarization enhancement that occurs with decreasing film thickness. Our electron microscopy analysis coupled with phase-field simulations reveals a polarization-enhancement mechanism that is dominated by the accumulation of oxygen vacancies at interfacial layers. The results shed light on the interplay between polarization and defects in nanoscale ferroelectrics and suggest a route to enhance functionality in oxide devices.

Oxygen vacancy migration along dislocations in SrTiO3 studied by cathodoluminescence

The impact of dislocations on oxygen vacancies in un-doped SrTiO3 single crystal has been investigated by cathodoluminescence (CL). The dominated luminescence peak located at ~2.8 eV is originated from oxygen vacancies. Enhanced luminescence has been observed at specific dislocations with the accumulation of oxygen vacancies. Under annealing either in oxidizing or reducing atmosphere, the diffusivity of oxygen ions at dislocation cores is nearly the same as that in the bulk. CL results suggest that the accumulation of oxygen vacancies is easily triggered by dislocations, while the diffusion of oxygen vacancies may not be significantly affected.

Resistive switching in FeNi/Al2O3/NiO/Pt structure with various Al2O3 layer thicknesses

Abstract Noble metals are generally used as electrodes in resistive switching devices. In this work, ferromagnetic metallic NiFe is used as top electrode on NiO films, and stable unipolar resistive switching (URS) behavior has been observed after a forming process. By inserting a thin Al2O3 layer between NiFe and NiO, URS persists with increasing the thickness of Al2O3 layer up to 20 nm. The URS behavior can be attributed to the formation and rupture of metallic Ni filaments in NiO layer, which penetrate through the thin Al2O3 layer, and have been confirmed by the observation of anisotropic magnetoresistance. With further increasing the thickness of Al2O3 layer up to 30 nm, forming free bipolar resistive switching (BRS) behavior is observed, which is due to the oxygen vacancy accumulation and dissipation at the interface because the thick Al2O3 layer blocks the penetration of Ni filaments. By inserting a thin Al2O3 layer with thickness below 20 nm, the distribution of the reset voltages in URS is significantly narrowed.

Iron-facilitated dynamic active-site generation on spinel CoAl2O4 with self-termination of surface reconstruction for water oxidation

The development of efficient and low-cost electrocatalysts for the oxygen evolution reaction (OER) is critical for improving the efficiency of water electrolysis. Here, we report a strategy using Fe substitution to enable the inactive spinel CoAl2O4 to become highly active and superior to the benchmark IrO2. The Fe substitution is revealed to facilitate surface reconstruction into active Co oxyhydroxides under OER conditions. It also activates deprotonation on the reconstructed oxyhydroxide to induce negatively charged oxygen as an active site, thus significantly enhancing the OER activity of CoAl2O4. Furthermore, it promotes the pre-oxidation of Co and introduces great structural flexibility due to the uplift of the oxygen 2p levels. This results in the accumulation of surface oxygen vacancies along with lattice oxygen oxidation that terminates as Al3+ leaches, preventing further reconstruction. We showcase a promising way to achieve tunable electrochemical reconstruction by optimizing the electronic structure for low-cost and robust spinel oxide OER catalysts.

Resistive switching and its modulating ferromagnetism and magnetoresistance of a ZnO-Co/SiO2-Co film

Abstract In this work, we report the resistance switching behavior and its modulating magnetic properties and magnetoresistance of an Au/ZnO-Co/SiO2-Co/Pt structure. The ZnO-Co/SiO2-Co bilayers prepared by magnetron sputtering at room temperature contain Co nanoparticles embedded in the oxide matrices. Repeatable bipolar resistance switching is observed in the Au/ZnO-Co/SiO2-Co/Pt structure, accompanied with a change of magnetization and magnetoresistance. At high/low resistance state, the sample exhibits weak/strong saturation magnetization and large/small magnetoresistance ratio. The accumulation of oxygen vacancies and the appearance of oxygen-deficient ZnO1-x, more Co atoms and Zn atoms in the switching areas due to the motion of oxygen ions under electric stimuli may be counted for the occurrence of the resistive switching effect. During the processes, the variations of the number of oxygen vacancies and Co atoms offer the opportunity to modulate magnetoresistance and ferromagnetism of the sample. This work shows that the resistance, magnetoresistance and magnetic properties of the Au/ZnO-Co/SiO2-Co/Pt structure can be electrically tuned, which may supply clues for designing multifunctional devices.

Demonstrative operation of four-terminal memristive devices fabricated on reduced TiO2 single crystals

Resistive switching (RS) was demonstrated in four-terminal planar memristive devices fabricated on reduced TiO2 (TiO2−x) single crystal substrates. In the device, a pair of diagonally opposing electrode terminals is used to modify the distribution of oxygen vacancies in the region between another pair of diagonally opposing electrode terminals. This allowed microscopic visual observations of the oxygen vacancy distribution based on electrocoloring. The visual contrast observed in the TiO2−x reflects the oxygen vacancy concentration in the electrically active zone of the device, which can be modified by application of various external voltages to the electrodes. The current that flows in the device is significantly dependent on the modified oxygen vacancy distribution and the resultant resistance is switchable when the polarization of the applied external voltage is reversed. The crystallographic orientation of the TiO2−x substrate has a strong influence on the reversible RS phenomenon. Mechanisms behind the voltage-driven resistance change are elaborated with the aid of microscopic analysis for both crystalline and electronic structures in the electrically active zone of the device. Suppression of the formation of irreversible conductive structures comprised of accumulated oxygen vacancies is a key to establishing reversible RS in the device.

Degradation of Ni-YSZ Electrodes in Solid Oxide Cells: Impact of Polarization and Initial Microstructure on the Ni Evolution

Two types of solid oxide cells with different Ni-YSZ cermet microstructures have been aged in electrolysis and fuel cell modes for operating times ranging from 1000 to 15000 hours. The pristine and aged cermets have been reconstructed by synchrotron X-ray holotomography. Nickel agglomeration has been observed in the bulk of the operated samples inducing a significant loss of triple phase boundary lengths. The inspection of the microstructural properties has confirmed the stabilizing role of YSZ on Ni coarsening. Furthermore, the gradients of properties quantified at the electrolyte interface have revealed a depletion of Ni only in the electrochemically active region of the electrode. The process is strongly promoted for a coarse cermet microstructure when operated under electrolysis current. The evolution of the microstructural properties has been implemented in an in-house multiscale model. The simulations have shown that the loss of performance is dominated by the depletion of Ni in case of a coarse microstructure. Thanks to the computations, it has been shown that the Ni depletion is controlled by the cathodic overpotential. To explain this dependency, it has been proposed that the accumulation of oxygen vacancies in the double layer could deteriorate the Ni/YSZ interface and trigger the Ni depletion.

Significantly Enhanced Ferroelectric and Dielectric Properties in BaTiO <sub>3</sub>/LaNiO <sub>3</sub> Superlattices

BaTiO3/LaNiO3 (BTO/LNO) ferroelectric superlattices with different stacking periods have been prepared by pulsed laser deposition (PLD). Compared to the pure BTO films, the BTO/LNO superlattices have significantly enhanced ferroelectric and dielectric properties. This is because, in addition to the strain effect, the accumulated oxygen vacancies in each ultra-thin LNO layer can change the depolarization field and reduce the leakage current in the superlattices. Therefore, these results indicate that the use of ultra-thin metallic oxide layers as the constituent material of the ferroelectric superlattices is a feasible and effective way to improve the properties of superlattices.

Phase transition, microstructure and electrical properties of K1‐xNaxNbO3‐based ceramic sintered in reducing atmosphere

AbstractLead‐free 0.955K1‐xNaxNbO3‐0.045Bi0.5Na0.5ZrO3+0.4% mol MnO ceramics (Abbreviated as K1‐xNxN‐0.045BNZ+0.4Mn) were prepared by a conventional solid‐state sintering method in a reducing atmosphere (oxygen partial pressure : 1 × 10−11 MPa). All K1‐xNxN‐0.045BNZ+0.4Mn samples show a pure perovskite structure with a polymorphic phase boundary (PPB) composed of rhombohedral (R) and tetragonal (T) phases. A high Na/K ratio and a low Na/K ratio can both induce an increase in the rhombohedral phase. The reverse piezoelectric coefficient and its temperature stability in K1‐xNxN‐0.045BNZ+0.4Mn ceramics can be improved by controlling the Na/K ratio. The increase in the Na/K ratio from x = 0.46 to x = 0.56 can decrease the A‐site cation vacancies. The activation energy of the grain is higher than that of the grain boundary due to the accumulation of oxygen vacancies at the grain boundary. K1‐xNxN‐0.045BNZ+0.4Mn ceramics with excellent piezoelectric properties (quasi‐static piezoelectric coefficient d33 = 326 pC/N, and = 472 pm/V at Emax = 25 kV/cm) were obtained at x = 0.52.

Local phenomena at grain boundaries: An alternative approach to grasp the role of oxygen vacancies in metallization of VO2

Vanadium dioxide (VO2) undergoes an insulator to metal transition (MIT) and an accompanied phase transition from a monoclinic (M) structure to rutile (R) structure near room temperature, forming the basis for many VO2-based functional devices. The MIT transition of VO2 and the functionality of VO2-based devices can be controlled by a variety of chemical and physical stimuli. With these external stimuli, defects, such as oxygen vacancies, are often inevitably introduced. However, due to the VO system-induced challenge to synthesize stable VO2 with different oxygen vacancy concentrations, the impact of oxygen vacancies on the resistance and transition of the VO2 is not fully understood. Oxygen vacancy, as one of the typical defects in VO2, is expected to concentrate at grain boundaries, and hence a concentration gradient of oxygen vacancies may exist between the grains interior and the boundaries, and this suggests a possibility to study the effects of oxygen vacancies on the transition of VO2 by probing local phenomena at the grain boundaries. For investigating local phenomena at the grain boundaries, Scanning Probe Microscopy (SPM) techniques are effective, which allows probing the structure and various properties at the nanoscale. In this work, a series of SPM techniques, including Atomic Force Microscopy (AFM), conductive-AFM (c-AFM), Electrochemical Strain Microscopy (ESM), and Kelvin Probe Force Microscopy (KPFM), are employed to measure variations of the surface structure, the resistance, the oxygen vacancy concentration, and the work function between the grain interior and the grain boundary. It has been demonstrated that, for most cases, both the resistance and the work function are lower at the grain boundaries as a result of the accumulation of oxygen vacancies at those positions. In addition, the resistance change induced by the electric field has been observed in the deposited VO2 thin films, which may be associated with the generation/annihilation of the oxygen vacancies, rather than charge injection. This work has demonstrated the effects of oxygen vacancies in the transition of VO2 by probing the local phenomena at grain boundaries, also provided a new insight into the resistance change of VO2 under an electric field.

(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).

Detection of Nanoscale Structural Defects in Degraded Fe-Doped SrTiO3 by Ultrafast Photoacoustic Waves

Strontium titanate, SrTiO3, has been intensively investigated as a model material in defect chemistry research. The underlying mechanism of the effects associated with a large variety of defects often requires microstructure imaging. In the present work, the distribution of nanoscale structural defects in electrodegraded Fe-doped SrTiO3 (Fe:STO) single crystals is directly revealed by ultrafast photoacoustic waves. We utilized time-resolved reflectance spectra to nondestructively characterize local structural distortions near the degraded anode and cathode interfaces in both the reduced and oxidized crystals along with transmission electron microscopy to image these defects. We show that an accumulation of oxygen vacancies resulted in significant structural deformations near the degraded cathode interface of the reduced crystal. The defect distribution shows a strong dependence on oxygen vacancy concentration and diffusion within the crystals.

Electron Mobility in γ−Al2O3/SrTiO3

SrTiO${}_{3}$-based interfaces with high electron mobility continue to attract interest, due to the possibility of combining quantum phenomena with the many functionalities exhibited by SrTiO${}_{3}$. The origin of the high mobility, however, remains poorly understood. The authors investigate the scattering mechanisms limiting the high mobility (over 10${}^{5}$ cm${}^{2}$/Vs) in \ensuremath{\gamma}-Al${}_{2}$O${}_{3}$/SrTiO${}_{3}$ heterostructures. Broken symmetry at the spinel/perovskite interface seems to play a key role, in terms of the accumulation of oxygen vacancies and spatial separation between donors and electrons. This deeper understanding paves the way for designing high-mobility oxide electronics at the nanoscale.

Analysis of Ti valence states in resistive switching regions of a rutile TiO2−x four-terminal memristive device

We have performed Ti valence state analysis of our four-terminal rutile TiO2−x single-crystal memristors using scanning transmission electron microscopy–electron energy loss spectroscopy (STEM–EELS). Analysis of Ti-L2,3 edge EELS spectra revealed that the electrocolored region formed by the application of voltage includes a valence state reflecting highly reduced TiO2−x due to the accumulation of oxygen vacancies. Such a valence state mainly exists within ∼50 nm from the crystal surface and extends along specific crystal directions. These electrically reduced surface layers are considered to directly contribute to the resistive switching (RS) in the four-terminal device. The present results add new insights into the microscopic mechanisms of the RS phenomena and should contribute to further development and improvements of TiO2−x based memristive devices.

Oxygen Release Induced Chemomechanical Breakdown of Layered Cathode Materials.

Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium batteries. Herein, we report a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge under thermal abuse conditions. We observe that the mechanical breakdown of charged Li1- xNi0.4Mn0.4Co0.2O2 materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles, i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, finite element modeling confirms our experimental observation that the crack formation is attributable to the formation of oxygen vacancies, oxygen release, and phase transformations. This study is designed to directly observe the chemomechanical behavior of layered oxide cathode materials and provides a chemical basis for strengthening primary and secondary particles by stabilizing the oxygen anions in the lattice.

(Invited) Phase Transformation and Chemomechanical Breakdown of Alkali Metal Ion Cathode Materials

Chemical and mechanical properties interplay on the nanometric scale and collectively govern the functionalities of battery materials. Understanding the relationship between the two can inform the design of battery materials with optimal chemomechanical properties for long-life lithium and sodium batteries. We found that chemomechanical properties of cathode materials are associated with phase transformation behaviors and can promote the cathode–anode crosstalk in full cells. In this presentation, we will first discuss an integrative approach of mapping valence states and constructing chemical topographies to investigate the redox phase transformation in layered oxide cathode materials at the secondary particle level under thermal abuse conditions. We discover that, in addition to the three-dimensional heterogeneous phase transformation, there is a mesoscale dynamic evolution of local valence curvatures in valence state topographies. The relative probability of negative and positive local valence curvatures alternates during the phase transformation. Such a behavior is likely associated with local chemical environments (e.g., grain boundaries, compositional variation) and can determine chemomechanical properties and ionic transport in cathode materials. Then we will discuss a mechanism of nanoscale mechanical breakdown in layered oxide cathode materials, originating from oxygen release at high states of charge. We observe that the mechanical breakdown of charged cathode materials proceeds via a two-step pathway involving intergranular and intragranular crack formation. Owing to the oxygen release, sporadic phase transformations from the layered structure to the spinel and/or rocksalt structures introduce local stress, which initiates microcracks along grain boundaries and ultimately leads to the detachment of primary particles; i.e., intergranular crack formation. Furthermore, intragranular cracks (pores and exfoliations) form, likely due to the accumulation of oxygen vacancies and continuous phase transformations at the surfaces of primary particles. Finally, we will highlight some methods that we develop to overcome chemomechanical breakdown.