Low Oxygen Vacancy Concentration Research Articles

Enhanced energy storage performance of 0.85BaTiO3–0.15Bi(Mg0.5Hf0.5)O3 films via synergistic effect of defect dipole and oxygen vacancy engineering

Dielectric capacitors are widely used in electronic devices due to their ultra-fast charge/discharge rate and ultra-high power density, but their lower energy density and poor thermal stability limit their further application. In contrast to the traditional strategy of suppressing defects, this work combines oxygen vacancies with defect dipoles to improve the breakdown strength and polarization behavior of ferroelectric films. Low concentration of oxygen vacancies and defect dipoles can trap charge carriers and increase breakdown strength, but if the concentration is too high, it can easily make films prone to breakdown. Moreover, the defect dipoles can reduce Pr by providing intrinsic restoring force for polarization switching, while excessive defect dipoles and oxygen vacancies can pin domain walls and increase Pr. By delicately controlling the concentration of oxygen vacancies and defect dipoles in the film, the BT-BMH film deposited at 0.135 mbar achieved the maximum breakdown strength and slim P-E loops, inducing the energy density to reach 108.9 J·cm-3 with an efficiency of 79.6 % at room temperature and excellent thermal stability in the wide temperature range of -100∼350 °C with the energy density of 69.1 J·cm-3. This work reveals the important significance of reasonable defect control for improving energy storage performance and provides an effective method for developing high-performance dielectric capacitors.

Strain-oxygen vacancies coupling in topotactic (La,Sr)CoO3-δ thin films

Oxygen defect engineering is a widely used approach for tuning physical properties in oxides. Multivalent transition metal oxide La0.7Sr0.3CoO3-δ (LSCO) shows oxygen vacancy-driven metal-to-insulator transition (MIT) due to topotactic phase transition and its high oxygen vacancy tolerance. Here, we introduce strain as a new degree of freedom to study the strain-oxygen vacancy coupling effects and elucidate its impact on the electronic property in oxygen-deficient LSCO epitaxial thin films grown on SrTiO3 (100) single crystal. By combining the experimental results with density functional theory plus U (DFT+U) calculations, we reveal that 2.1 % in-plane tensile strain can stabilize the insulating state of LSCO with a surprisingly low concentration of oxygen vacancies, <0.5 %. This study reveals that the MIT in LSCO is governed by the combination of oxygen vacancies and strain, offering the potential for additional tuning knob of the material's electronic properties.

Novel high-hardness and low-loss microwave dielectric ceramic LiZnMgGaTi2O8

Inorganic oxide ceramics are essential for microwave applications due to their stable structure and exceptional performance. LiZnMgGaTi2O8 ceramics were prepared using the solid-phase method. XRD and TEM confirmed that the ceramics possess spinel structures. The ceramics had a maximum relative density of 94.57 % and a Vickers hardness of 8.58 GPa when sintered at 1220 °C. The ionic polarizability of Ti4+ in the ceramics exceeds the Shannon value, leading to a 20.3 % deviation between εth and εr. The activation energy of the impedance spectrum fitting is 1.302 eV, indicating a low concentration of oxygen vacancies in the ceramic. Hence, the Q × f value of the ceramics is primarily influenced by relative density. The optimal microwave dielectric properties of LiZnMgGaTi2O8 ceramics are εr = 15.68, Q × f = 66,620 GHz, and τf = −30.9 ppm/°C. The test curves of εr and dielectric loss remain nearly parallel to the x-axis from −80 °C to 80 °C, demonstrating the excellent stability of LiZnMgGaTi2O8 ceramics.

Anti-defect engineering of Pd/NiCo2O4 hybrid nanocatalysts for enhanced CO2 hydrogenation to formate

Efficiently converting CO2 into value-added chemicals remains a significant challenge due to its inert nature. Here, we present the rational design of Pd/NiCo2O4 hybrid nanocatalysts with diverse morphologies for highly efficient CO2 hydrogenation to formate. The synergistic combination of Pd and NiCo2O4 offers improved catalytic activity towards formate production. Remarkably, the observation of a morphology-dependent Pd-NiCo2O4 interaction, linked to the presence of oxygen vacancies in NiCo2O4, significantly contributes to our understanding of catalytic activity. The rose-like Pd/NiCo2O4 catalyst achieves an impressive formate yield (85.3 molformate moltotalPd−1 h−1) due to its low oxygen vacancy concentration and subsequent generation of less positively-charged Pd species, emphasizing the crucial role of oxygen vacancies in hybrid nanocatalyst performance. These findings were further validated through density functional theory calculations, providing valuable insights into the design and optimization of nanocatalysts for CO2 hydrogenation. This contributes to the development of efficient and sustainable processes for CO2 utilization in the production of formate and other valuable chemicals.

Oxygen Vacancy Mediation in SnO2 Electron Transport Layers Enables Efficient, Stable, and Scalable Perovskite Solar Cells.

Previous findings have suggested a close association between oxygen vacancies in SnO2 and charge carrier recombination as well as perovskite decomposition at the perovskite/SnO2 interface. Underlying the fundamental mechanism holds great significance in achieving a more favorable balance between the efficiency and stability. In this study, we prepared three SnO2 samples with different oxygen vacancy concentrations and observed that a low oxygen vacancy concentration is conducive to long-term device stability. Iodide ions were observed to easily diffuse into regions with high oxygen vacancies, thereby speeding up the deprotonation of FAI, as made evident by the detection of the decomposition product formamide. In contrast, a high oxygen vacancy concentration in SnO2 could prevent hole injection, leading to a decrease in interfacial recombination losses. To suppress this decomposition reaction and address the trade-off, we designed a bilayer SnO2 structure to ensure highly efficient carrier transport still while maintaining a chemically inert surface. As a result, an enhanced efficiency of 25.06% (certified at 24.55% with an active area of 0.09 cm2 under fast scan) was achieved, and the extended operational stability maintained 90% of their original efficiency (24.52%) after continuous operation for nearly 2000 h. Additionally, perovskite submodules with an active area of 14 cm2 were successfully assembled with a PCE of up to 22.96% (20.09% with an aperture area).

Advanced rare earth tantalate RETaO4 (RE=Dy, Gd and Sm) with excellent oxygen/thermal barrier performance

Thermal barrier coatings (TBCs) materials with lowered thermal and oxygen ion conductivity can provide thermal and oxidative protection for high temperature hot-end components in aeronautical engines and gas turbines. The rare-earth tantalate RETaO4 (RE = Dy, Gd and Sm) ceramics with monoclinic (m) phase were successfully synthesized via spark plasma sintering. Oxygen vacancies responsible for the thermal and oxygen ion conductivities of RETaO4 were demonstrated by atomic-resolution energy dispersive X-ray and X-ray photoelectron spectroscopy. Among the three samples, DyTaO4 has excellent oxygen/thermal barrier performance. Compared to the current service thermal barrier coating material ZrO2-8 wt% Y2O3 (8 YSZ), DyTaO4 has an ultra-low oxygen ion conductivity benefiting from low oxygen vacancy concentration and strong stretching force constants. The intrinsic thermal conductivity of DyTaO4 is 68.2% less than that of 8 YSZ. Additionally, the thermal expansion rate curves indicate that the phase transformation does not happen from room temperature to 1200 °C. The above results demonstrate that high-growth rate thermally grown oxide can be retarded by creating dense DyTaO4 coating with lowered thermal and oxygen ion conductivity.

Enhanced energy-storage properties in (Bi0.5Na0.5)TiO3 ceramics by doping linear perovskite materials Ca0.85Bi0.1(Sn0.5Ti0.5)O3

For energy storage applications in Bi0.5Na0.5TiO3 (BNT)-based materials, the key challenges are the premature polarization saturation and low breakdown electric field (Eb), which confine the energy storage capacity of BNT and significantly restrict progress in advancing pulsed power capacitors. Hence, the cooperative optimization strategy of band structure and defect engineering was proposed, which successfully obtained a high recoverable energy density of 3.83 J/cm3 and an energy efficiency of 77.9 % in the Bi0.5Na0.5TiO3-0.12Ca0.85Bi0.1(Sn0.5Ti0.5)O3 (BNT-0.12CBST) ceramics. Doping the BNT matrix with CBST has been shown to not only reduce grain size but also enhance relaxor behavior, which collectively improves breakdown strength and delays polarization saturation. Furthermore, the x = 0.12 ceramic also exhibits a commendable discharge power density of 91.9 MW/cm3 and a transient discharge time of 40 ns. The higher resistivity and lower oxygen vacancy concentration are conductive to acquire superior breakdown electric field and energy storage performance. We determine the crystal structure, dielectric and ferroelectric properties of the studied ceramics in a wide range of temperatures and concentrations, and a phase diagram is constructed, which illustrates the complex phases present and their transformation behavior. Our work provides an effective strategy to optimize the energy-storage of dielectric ceramics, and shows great potential for practical applications in pulse power systems.

Improved resistive switching characteristics of solution processed ZrO2/SnO2 bilayer RRAM via oxygen vacancy differential

In this study, a solution-processed bilayer structure ZrO2/SnO2 resistive switching (RS) random access memory (RRAM) is presented for the first time. The precursors of SnO2 and ZrO2 are Tin(Ⅱ) acetylacetonate (Sn(AcAc)2) and zirconium acetylacetonate (Zr(C5H7O2)4), respectively. The top electrode was deposited with Ti using an E-beam evaporator, and the bottom electrode used an indium–tin–oxide glass wafer. We created three devices: SnO2 single-layer, ZrO2 single-layer, and ZrO2/SnO2 bilayer devices, to compare RS characteristics such as the I–V curve and endurance properties. The SnO2 and ZrO2 single-layer devices showed on/off ratios of approximately 2 and 51, respectively, along with endurance switching cycles exceeding 50 and 100 DC cycles. The bilayer device attained stable RS characteristics over 120 DC endurance switching cycles and increased on/off ratio ∼2.97 × 102. Additionally, the ZrO2/SnO2 bilayer bipolar switching mechanism was explained by considering the Gibbs free energy (ΔG o) difference in the ZrO2 and SnO2 layers, where the formation and rupture of conductive filaments were caused by oxygen vacancies. The disparity in the concentration of oxygen vacancies, as indicated by the Gibbs free energy difference between ZrO2 (ΔG o = −1100 kJ mol−1) and SnO2 (ΔG o = −842.91 kJ mol−1) implied that ZrO2 exhibited a higher abundance of oxygen vacancies compared to SnO2, resulting in improved endurance and on/off ratio. X-ray photoelectron spectroscopy analyzed oxygen vacancies in ZrO2 and SnO2 thin films. The resistance switching characteristics were improved due to the bilayer structure, which combines a higher oxygen vacancy concentration in one layer with a lower oxygen vacancy concentration in the switching layer. This configuration reduces the escape of oxygen vacancies to the electrode during RS.

Enhancement of oxygen separation performance through Pr0.6Sr0.4FeO3-δ perovskite by modulation of oxygen vacancies

Perovskite-type oxides are promising for oxygen separation purification as oxygen uptake materials. However, their separation performance is limited by the low concentration of oxygen vacancies. Hence, in this work, the low valence Cu is doped to introduce more oxygen vacancies, and a series of Pr0.6Sr0.4Fe1-xCuxO3-δ (x = 0, 0.05, 0.1, 0.15) oxides have been developed to investigate the effects of the Cu contents on the oxygen separation performance. The introduction of Cu in Pr0.6Sr0.4FeO3-δ effectively regulates the concentration of oxygen vacancies and significantly reduces the average metal–oxygen bond energy, thus promoting oxygen separation performance. Especially, Pr0.6Sr0.4Fe0.9Cu0.1O3-δ exhibited the enhanced oxygen adsorption capability (0.195 mmol·g−1) and average desorption rate (7.8 μmol·g−1·min−1) at 600 °C, which are 1.35 and 1.78 times of the parent Pr0.6Sr0.4FeO3-δ respectively. This work provides a successful strategy for optimizing the oxygen separation performance of perovskite materials.

The ultra-high electric breakdown strength and superior energy storage properties of (Bi0.2Na0.2K0.2La0.2Sr0.2)TiO3 high-entropy ferroelectric thin films

The electric breakdown strength (Eb) is an important factor that determines the practical applications of dielectric materials in electrical energy storage and electronics. However, there is a tradeoff between Eb and the dielectric constant in the dielectrics, and Eb is typically lower than 10 MV/cm. In this work, ferroelectric thin film (Bi0.2Na0.2K0.2La0.2Sr0.2)TiO3 with a dielectric constant of 115 is found to exhibit an ultra-high Eb = 10.99 MV/cm, attributing to the high-entropy effects that could result in dense nanostructures with refined grains, low concentration of oxygen vacancies, low leakage current and small polar nano-regions in the thin film. A recoverable energy storage density of 5.88 J/cm3 with an excellent energy storage efficiency of 93% are obtained for the dielectric capacitor containing the thin-film dielectrics. Remarkably, the dielectric capacitor possesses a theoretical energy storage density of 615 J/cm3 compatible to those of electrochemical supercapacitors. The high-entropy ferroelectric thin films with ultra-high Eb and superior energy storage properties are much promising dielectrics used in next-generation energy storage devices and power electronics.

Heterostructured electrodes for Cr-tolerant solid oxide fuel cells

A heterostructured electrode coated with a Cr-tolerant Sr-free material, that exhibits a low oxygen vacancy concentration and contains reducible sites, showed excellent performance and stability in a Cr atmosphere.

Relationship between defect and strain in oxygen vacancy-engineered TiO2 towards photocatalytic H2 generation

Photocatalytic H2 generation is an effective approach to convert solar energy into renewable H2 energy and has drawn great attentions. To improve the catalytic activity, lots of attentions have been paid to developing advanced strategies, of which oxygen vacancy engineering is considered as a promising one. However, oxygen vacancy induced coexistence of defect and strain makes understanding the essential mechanism of enhanced photocatalytic performance elusive and challenging. Here, the classic photocatalyst TiO2 is selected as a paradigm to reveal the relationship between the accompanied defect and strain in boosting H2 generation activity. It is found that oxygen vacancy concentration almost shows a volcano-type trend, while strain exhibits a monotonic decrease with increasing the calcination temperature. An optimal photocatalytic H2 yield of 788 μmol g−1 h−1 is achieved over the TiO2 with high oxygen vacancy concentration but slight strain. This value is 2.5 times higher than the counterpart with low oxygen vacancy concentration and large strain, indicating that oxygen vacancy induced defect instead of strain plays the dominate role in enhancing photocatalytic H2 generation, which is further verified by theoretical calculations and experimental carrier dynamics analysis. This work uncovers the relationship between defect and strain in oxygen vacancy-engineered photocatalysts.

Impact of Bottom Electrode in HfO2-Based Rram Devices on Switching Characteristics

Resistive random-access memory (RRAM) devices with hydrogen plasma treated HfO2 have shown low power switching (1) and good conductance quantization with programing pulsed operation (2) that qualify them to be used for in-memory computing. Engineering the distribution of defects or oxygen vacancies near the top and bottom electrodes has a significant impact on reducing the switching power and improving the multi-level cell (MLC) characteristics of the device. A graded distribution with higher concentration of oxygen vacancies closer to the top electrode (TE) due to hydrogen plasma treatment and lower concentration near the bottom electrode (BE) reduces switching energy (power) (1). However, the impact of the bottom electrode is not well understood. In this work we investigated three RRAM stacks as shown in Figure 1 that have the same top electrode metal (50nm PVD TiN/5nm ALD TiN) and dielectric (HfO2 with hydrogen plasma treatment at the mid-point) but with different BE for each stack: TiN (device A (control)), TiN plus Mo with thermal anneal (device B), and TiN plus Mo with plasma nitridation (device C). Choosing a more inert metal as the BE provides a lower oxygen vacancy concentration at the interface between the BE and the dielectric (3) which in turn reduces the switching power as noted with device C (TiN plus Mo with plasma nitridation) which switches at a minimum compliance current (Icc) of 500pA. It is possible that the presence of nitrogen prevents the increase in oxygen vacancy concentration near the BE. On the other hand, a higher concentration of oxygen vacancies at the BE and the dielectric interface leads to a larger magnitude of band bending and increases the height of Schottky barrier. This possibly leads to an increase in the switching power in device A and device B as compared to device C (3-4). Note that device A (TiN) switched at Icc=30µA and device B (TiN plus Mo with thermal anneal) switched at Icc=8µA, which is superior compared to device A in switching power. This suggests that the alloy of TiN with Mo is reducing the oxygen vacancies near the BE. To study the MLC characteristics of the devices, we performed a train of 36 RESET pulses with fixed amplitude Vp= -1V and pulse width starting at 4µs and increasing by 20µs every 3 pulses, while we applied Vr=0.1V and tr=10µs after each pulse to measure the conductance. Figure 2 shows that device B (TiN plus Mo with thermal anneal) has good MLC with no overlap between the states while device A shows unstable MLC behavior due to time constant variation. Figure 3a shows the results of device C for the same RESET pulse train that was applied to devices A and B. Since device C switches at very low current and voltage, the conductance quantization was unstable after 15 pulses due to overstress of the conductive filament. Figure 3b shows the results after selecting the proper pulse set Vp= -0.5V and pulse width starting at tp=100ns and increasing by 100ns every 3 pulses, while we applied Vr=0.05V and tr=100ns after each pulse, resulting in a better conductance quantization compared to the previous setup. Appropriate engineering of the BE and managing of the oxygen vacancy distribution by hydrogen plasma treatment can significantly reduce the switching power.

Highest-Efficiency Flexible Perovskite Solar Module by Interface Engineering for Efficient Charge-Transfer.

The electron-transport layer (ETL) plays an important role in improving the performance of flexible perovskite solar cells (F-PSCs). Herein, a room-temperature-processed SnO2 :OH ETL is demonstrated, that exhibits reduced defect density, in particular lower oxygen vacancy concentration, with better energy band alignment and more wettable surface for quality perovskite deposition. More importantly, an efficient electron-transfer channel is produced between the ETL and the perovskite layer due to the formation of hydrogen bonds at the interface, resulting in enhanced electron extraction from the perovskite. As a result, the efficiency of a large-area (36.50 cm2 ) flexible perovskite solar module based on MAPbI3 is increased to as high as 18.71%; this is thought to be the highest reported PCE value for flexible perovskite solar modules to date. In addition, it exhibits high durability while maintaining over 83% of its initial PCE after flexing test cycles. Further, F-PSCs with SnO2 :OH show remarkably long-term stability, owing to a high quality of the perovskite film and a strong coupling between the SnO2 :OH and perovskite layer caused by hydrogen bonds, which successfully inhibits moisture permeation.

Dielectric properties of BaTiO3 and Ba0.95Ca0.05TiO3 sintered in a reducing atmosphere

The dielectric materials utilized in high-performance barium titanate-based base-metal electrode multilayer ceramic capacitors (BME MLCCs) must satisfy stringent criteria such as high dielectric constant, low dielectric loss, high insulation resistivity, and reliability. In this investigation, the effects of raw materials, BaTiO3 (BT), and Ba0.95Ca0.05TiO3 (BCT) on densification, microstructure, and dielectric behavior were explored by introducing acceptor and rare earth elements. The findings revealed that BCT could attain densification at a lower sintering temperature of 1275 °C than BT. Moreover, BCT samples displayed relatively smaller grain sizes, resulting in a greater number of grain boundaries, which in turn increased the insulation resistivity. BCT had a lower oxygen vacancy concentration, thus resulting in lower leakage currents at high temperatures than BT. Substituting BCT for BT resulted in higher activation energies for conductivity in the core, shell, and grain boundary. This investigation demonstrates that BCT can enhance the dielectric properties, insulation resistance, and reliability of the BME MLCCs.

Structural, electronic and optical properties of BaTiO3-CoFe2O4 nanocomposites for optoelectronic devices

Structural, electronic and optical properties of BaTiO3 (BTO), CoFe2O4 (CFO) and their nanocomposites are presented. The XRD spectra of composites exhibited the superposed peak patterns of individual compounds with a consistent shift from BTO to CFO phase with the increase in CFO proportion. The XPS analysis revealed a lower concentration of Oxygen vacancies in the composite samples as compared to that in pure CFO. This finding is also confirmed by the consistent transformation of Co from +2 state in CFO to +3 state in the composite samples. The UV–Vis spectroscopy revealed that optical absorption of BTO is limited to the UV regime, however, CFO exhibited the absorption in visible and infrared (IR) region. However, after the formation of BTO-CFO composite, the optical absorption was broadened from UV to IR regions. The magnitude of absorption was further enhanced by increasing the CFO content in the composites.

Orthorhombic phase induced ultra-low operation voltage in La-doped BiFeO3 resistive switching devices

In this study, the effect of La doping on the resistive switching properties of BiFeO3 (BFO) is systematically discussed. It was found that La doping can not only inhibit the volatilization of Bi and thus diminish the intrinsic oxygen vacancy of BFO, but also improve the lattice symmetry of BFO for generating orthorhombic phase BFO. To confirm these microstructural changes and their relation to the resistive switching properties of BFO, the phase structures, chemical compositions, microstructures, energy band structures, and interfacial features of BFO with different La-doping concentrations are characterized. The resistances of BFO devices with different La doping concentrations also show significant differences, especially, the 15 at. % La-doping BFO device exhibits ultra-low operation voltage of only −0.15 V and good cycling consistency due to the integration of low oxygen vacancy concentrations and precipitation of the orthorhombic phase. The multiple effects of La doping on the resistive characteristics of BFO discussed in this paper provide a reference for the application of BFO in the field of resistive memories.

Influence of stabilizers on hydrothermal behavior of zirconia coatings by APS

In recent years, the degradation of zirconia in a humid environment has attracted the attention of researching. In this work, Mg0.09Zr0.91O1.91 (MSZ), Y0.09Zr0.91O1.955 (YSZ) and Ce0.09Zr0.91O2 (CSZ) powders were prepared by co-precipitation method, the corresponding zirconia coatings were fabricated by atmospheric plasma spraying (APS). All coating samples were subjected to hydrothermal treatment to investigate their degradation process in moisture. The evolution of phase composition and microstructure were characterized. Results show that the content of monoclinic phase (m) in MSZ, YSZ and CSZ coating samples increased after hydrothermal treatment, rising by 47 % in MSZ coating, by 31 % in YSZ coating and by 8 % in CSZ coating. The degradation of internal structure of coatings was more serious with the prolongation of hydrothermal treatment time. This is because different stabilizers will cause different oxygen vacancy concentration in the zirconia unit cell, which determines the hydrothermal stability of zirconia. Under the tested condition, CSZ has the lower oxygen vacancy concentration (4 %) than MSZ (11 %) and YSZ (7 %), exhibiting better hydrothermal stability.

Enhancing the photoelectrochemical activity of monoclinic BiVO4 by discretizing oxygen vacancies: insights from DFT calculations.

Monoclinic bismuth vanadate (BiVO4) has emerged as an excellent optically active photoanode material due to its unique physical and chemical properties. Experiments reported that the low concentration of oxygen vacancies enhances the photoelectrochemical (PEC) activity of BiVO4, but the high concentration of oxygen vacancies decreases the charge carrier lifetime. Using time-domain density functional theory and molecular dynamics, we have demonstrated that the distribution of oxygen vacancies has strong effects on the static electronic structure and nonadiabatic (NA) coupling of the BiVO4 photoanode. The localized oxygen vacancies create charge recombination centers within the band gap and enhance the NA coupling between the VBM and the CBM, resulting in fast charge and energy losses. By contrast, the discrete oxygen vacancies can eliminate the charge recombination centers and decrease the NA coupling between the VBM and the CBM, enhancing the PEC activity of monoclinic BiVO4. Our study suggests that the PEC performance of a photoanode can be improved by changing the distribution of oxygen vacancies.

Significantly enhanced piezocatalytic activity of BaTiO3by regulating the quenching process

Under the synergetic effect of a low oxygen vacancy concentration and rapid cooling rate, the air-cooled BTO powder achieved a significantly enhanced piezocatalytic activity, with a large degradation rate constantkof 0.077 min−1and a H2evolution rate of 1.43 mmol h−1g−1.