High Density Of Oxygen Vacancies Research Articles

Metastable pitting corrosion behavior and characteristics of passive film of laser powder bed fusion produced Ti–6Al–4V in NaCl solutions with different concentrations

This work investigated the metastable pitting corrosion mechanism of laser powder bed fusion (LPBF) produced Ti–6Al–4V. The passive films of samples were produced by potentiostatic polarization in NaCl solutions with different concentrations, and electrochemical measurements were employed to understand the influence of Cl- on the characteristics of passive films. The frequency of pitting nucleation and the pit dimension increase with increased Cl- concentration. The attack of Cl- promotes the dissolution of passive films. A higher density of oxygen vacancies is produced in passive film because of the ingressive Cl-, resulting in the condensation of voids and pitting corrosion.

Surface anchored atomic cobalt-oxide species coupled with oxygen vacancies boost the CO-production yield of Pd nanoparticles

The high density of oxygen vacancies and adjacent Pd domains promote CO2 activation and H2 dissociation, respectively.

Thermal effects on TiN/Ti/HfO2/Pt memristors charge conduction

TiN/Ti/HfO2/Pt resistive switching devices have been fabricated, measured, and modeled. After programming the devices in the low resistance state, the current–voltage characteristic below the reset switching voltage was measured at different temperatures (from 90 to 350 K). A weak but complex temperature dependence was obtained for several voltage regimes. These memristors belong to a wider set known as valence change memories, whose conductance is determined by the formation of conductive filaments (CFs) linked to a high density of oxygen vacancies in a dielectric sandwiched between two metal electrodes. This usually leads to ohmic conduction in the low resistance state. However, a non-linear current dependence has been also observed in the measured devices, in addition to symmetric current–voltage curves for positive and negative biases in the 0–0.6 V voltage range. Three different thermal dependences have been considered for explaining the whole set of experimental data. Two of them are linked to ohmic filamentary conduction; the CF shows a conductivity enhancement due to thermally activated mechanisms at low temperatures; on the contrary, a CF conductivity degradation is observed at the higher temperatures. Finally, an additional slightly higher value for the non-linear current component as the temperature rises has also been taken into account. A semiempirical compact model has been implemented including these conduction mechanisms and their corresponding temperature dependences, the device has been simulated in LT-Spice and the experimental currents have been correctly reproduced.

Tuning the photoconductivity of Co3O4 thin films by defect engineering

Light-stimulated phenomena, such as photoconductivity, allow access to the role played by structural and point defects of semiconductor oxides. We report on a systematic study of the room temperature photoconductivity of polycrystalline Co3O4 thin films grown by spray pyrolysis. Using different excitation wavelengths, we access the optical absorption edges related to Co2+ or Co3+. Structural and morphological characteristics, as well as their relation with the type of substrate, are also investigated. As-grown Co3O4 exhibits a high density of oxygen vacancies (VO), which can be modified by implementing defect engineering via annealing in different atmospheres. Regardless of the atmosphere, the XPS measurements reveal a similar drop in VO induced by thermal annealing. In addition, Co3+ ions are reduced to Co2+ on the films’ surface. The response time is tens of seconds for optical excitation above the Co3O4 absorption and increases for excitations closer to the Co2+ edge. The absence of saturation profile in as-grown Co3O4 indicates the influence of deep trap defect centers. In contrast, annealing inhibits the persistence effects of red excitations. The oxidizing atmosphere provides long response and recovery times, while annealing in Ar produces fast responses. The reduction in Vo on the film surfaces indicates that oxygen vacancies are not responsible for the deep traps.

Catalytic oxidation of toluene over Co-Cu bimetallic oxides derived from CoyCu3−y-MOF-74

As a precursor material, CoyCu3−y-MOF-74 has been synthesized by using a cost-effective and environmentally friendly mechanochemical method. It had a uniform nano-morphology and a controllable stoichiometric composition. By calcinating the CoyCu3−y-MOF-74 material, a bimetallic Co-Cu oxide catalytic material, with the formula M-CoyCu3−yOx, could, thereafter, be prepared. It was later on used for catalytic oxidation of toluene. The effect of calcination temperature and Co/Cu molar ratio on the redox properties, catalytic activity, and structural properties of the catalyst, were also systematically studied. The M-Co2Cu1Ox catalyst showed a complete toluene conversion of 1000 ppm toluene at 225 ℃ and with a under weight hour space velocity of 30,000 ml/ (g h). The corresponding conversion temperature for 50% and 90% of the original amount of M-Co2Cu1Ox (i.e., T50 and T90) were 202 and 220 ℃, respectively. X-ray photoelectron spectroscopy (XPS), Raman spectroscopy, and electron paramagnetic resonance (EPR) results also showed a high density of oxygen vacancies in M-Co2Cu1Ox. Also, in situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) experiments indicated that benzaldehyde and benzoic acid were the main intermediates formed during the catalytic oxidation of toluene. In addition, the M-Co2Cu1Ox catalyst did also show an excellent durability and good water resistance by exposing it to a 5% water vapor for 24 h. Thus, the green and facile mechanical synthesis of the MOF-74 precursor has been proven to be very beneficial for the design and synthesis of a mixed metal oxide catalyst with a high toluene catalytic oxidation efficiency.

Enhanced resistive switching performance of hafnium oxide-based devices: Effects of growth and annealing temperatures

Although hafnium oxide-based resistive switching (RS) devices have been explored extensively, extremely high operating voltages and currents hinder their application in low power operations. In this study, the growth temperature and annealing conditions have been varied to significantly improve the characteristics of hafnium oxide-based RS devices. Deposited using a rf magnetron sputtering at different temperatures, HfOx films are subsequently subjected to vacuum annealing to create a high density of oxygen vacancies. It is observed that the crystallinity, defect density, and RS device performance of these films are influenced by these thermal treatments. The HfOx film annealed at 450 ºC exhibited bipolar RS with the substantial improvement in the device performance, such as electroforming, set/reset voltages, and operating currents as low as ~1 V, 0.5/− 0.5 V, and ~1 µA, respectively, in addition to large ON/OFF ratio> 103, good endurance (>103 cycles), and retention capability (>104 s). The localized conductive filaments composed of oxygen vacancies are found to be responsible for the RS behavior in Al/HfOx/FTO devices, and the simple vacuum annealing approach considerably increases the proportion of oxygen vacancies. With the insights into the influence of annealing temperatures on the switching parameters, this work paves the way for the design of low-power devices for data storage applications.

MgO as promoter for electrocatalytic activities of Co3O4–MgO composite via abundant oxygen vacancies and Co2+ ions towards oxygen evolution reaction

Oxygen evolution reaction (OER) is known as bottleneck problem during the water splitting process due to high energy barrier and non-availability of efficient nonprecious electrocatalysts. The cobalt oxide (Co3O4) in the spinel phase has limited OER activity and stability in the alkaline media. For this purpose, we have carried out the synthesis of Co3O4–MgO (CM) composite by wet chemical method and it offers abundant oxygen vacancies and Co2+ concentration for the efficient OER reaction. The effect of different amounts of MgO on the OER activity of Co3O4 was also studied. Despite inactivity of MgO towards OER, it creates high density of oxygen vacancies and favored the formation Co2+ ions at the surface, thus accelerated the OER kinetics. The physical studies were performed to investigate the morphology, crystalline structure, surface information and chemical composition using several analytical techniques. The optimized CM-0.1 composite produced an overpotential of 274 mV at 10 mAcm−2 which is lower in value than the pristine Co3O4. The significant enhancement in the OER activity was verified by the large value of electrochemical active surface area values 12.8 μFcm−2 and the low charge transfer resistance of 45.96 Ω for the optimized CM-0.1 composite. The use of abundance materials for the synthesis of CM composite revealed an enhanced OER performance, suggesting the dynamic role of MgO, therefore it could be used for improving the electrochemical properties of extended range of metal oxides for specific application especially energy conversion and storage devices.

Effect of W concentration in the organized Ti-W alloy oxide nanotubes array on the photoelectrocatalytic properties and its application in the removal of endocrine disruptors using real water matrix

To evaluate the tungsten content on the photoelectrocatalytic properties of self-organized nanotubes grown by anodization on Ti-xW (wt%) alloys, where x is 0.5, 2.5 or 5.0, this work investigated the phase transformation of TiO2, the tungsten oxidation states in the oxide layers and their photoelectrocatalytic performance in the degradation of estrone (E1) and 17α-ethinylestradiol (EE2). These films were employed in photocatalytic and photoelectrocatalytic removals of hormones from synthetic and real water matrices. In synthetic water, E1 and EE2 removals, reached efficiencies of 92% and 71%, respectively, after 2 min under UV–Vis photoelectrocatalysis using NT/Ti-5.0W as catalyst. About 30 times longer was needed to degrade 77% of both hormones from the real water matrix due to the presence of other high organic charge. The high performance of the NT/Ti-5.0W was associated with the combination of doping (W-doped TiO2) and WO3 (W6+) heterojunction. However, this electrode had its stability compromised under long degradation times, mainly under visible light, due to a WO3 leaching process. As for NT/Ti-0.5W, the substitutional W-doped TiO2 contributed to its greater stability and efficiency for a long time. The low performance of NT/Ti-2.5W was justified by high density of oxygen vacancy and unfavorable position of its adsorption bands.

Trace Al component in ε-(AlxGa1-x)2O3 alloy films and film-based solar-blind photodetectors

ε-(AlxGa1-x)2O3 alloy thin films containing Al fractions of 0, 2.9 at%, and 4.7 at% with ε-Ga2O3 templates were fabricated on (0001) c-sapphire by tin-assisted PLD technique. Sn is a surface-mediated surfactant to stabilize the formation of epsilon phase. XRD reveals that the peak position of ε-(AlxGa1-x)2O3 shifts to a higher diffraction angle and that ε-(AlxGa1-x)2O3 is accompanied by a deterioration of crystal quality when the Al composition increases from 0 to 4.7 at%. With Al content increasing, XPS demonstrates the appearance of more aluminum atoms bound to oxygen atoms and higher oxygen vacancy density in ε-(AlxGa1-x)2O3 films. The optical bandgap for ε-(AlxGa1−x)2O3 thin films with various Al components are 4.97 eV, 5.09 eV and 5.16 eV, respectively. The solar-blind photodetectors were fabricated on ε-(AlxGa1-x)2O3 alloy thin films with various Al fractions. The photo-to-dark current ratio, responsiveness, detectivity and response time (rise time/decay time) for ε-(AlxGa1-x)2O3 (x = 2.9 at%) PDs were 2.7 × 103, 4.2 A/W, 1.23 × 1013 Jones and 2.09 s/0.49 s.

Interface engineered Sb2O3/W18O49 heterostructure for enhanced visible-light-driven photocatalytic N2 reduction

The direct photochemical N2 reduction reaction boasts the potential for clean and sustainable NH3 manufacturing, yet achieving this remains a significant challenge. To activate and cleave the nonpolar N≡N bond and accelerate the transport and separation of charge carriers during photocatalysis, the design and facile synthesis of active, robust, cheap, and earth-abundant photocatalysts is crucial. Here, we demonstrate efficient and stable visible light fixation of N2 to NH3 by constructing a novel heterostructure consisting of two-dimensional Sb2O3 nanosheets and one-dimensional W18O49 nanowires. A remarkable NH3 production rate of up to 731 μgNH3 h−1 gcat.−1 is attained in visible light over the nanocomposite without cocatalyst, 5.5-fold and 3.4-fold improvement over that of discrete Sb2O3 and W18O49 respectively, which also significantly exceeds many previous reported photocatalytic systems. The superior photocatalytic performance can be attributed to the unique structure of the composites, which provides ample exposed surface catalytic sites with a high density of oxygen vacancies, promoting the adsorption and activation of N2 molecules to form *NNH. Equally importantly, the interface between Sb2O3 and W18O49 greatly facilitates charge carrier migration and separation. Further density functional theory calculations manifest that the interfacial metal sites on Sb2O3/W18O49 serve as the main active centers which possess a strong donating electron ability to the empty N2 antibonding orbitals to trigger the following NH3 synthesis. This work provides a simple and effective method to modify metal oxides for enhancing N2 photofixation.

Phase-field simulation on the interaction of oxygen vacancies with charged and neutral domain walls in hexagonal YMnO3

A three-dimensional model of the interaction between the charged or neutral domain walls and oxygen vacancies in the hexagonal manganite YMnO3 was proposed, and simulated using Landau–Ginzburg–Devonshire (LGD) theory, dynamic diffusion equation and Maxwell’s equation. The calculation proves that stiffness anisotropic factors can adjust the domain wall state and ultimately affect the distribution of oxygen vacancies. The head-to-head domain wall corresponds to low oxygen vacancy density, and the tail-to-tail domain wall corresponds to high oxygen vacancy density. The electrostatic field generated by the bound charge is the key factor leading to the change of oxygen vacancy distribution. Finally, e-index law N d = ae b*dP/dz can fit the relationship between the oxygen vacancy concentration and the polarization gradient along z direction. Our theory provides a new way to modulate the distribution of oxygen vacancies through domain wall morphology in hexagonal YMnO3.

Selective detection of sub-1-ppb level isoprene using Pd-coated In2O3 thin film integrated in portable gas chromatography

Monitoring breath isoprene concentration plays a role in non-invasive diagnosis of blood-cholesterol levels. In this study, we developed a highly sensitive and selective isoprene analyzer based on a Pd-coated In2O3 thin film integrated in a miniaturized gas chromatography column. To improve the sensing performance, the In2O3 thin films were fabricated with high uniformity and crystallinity using a dual-ion-beam sputtering system, and various metal catalysts (Au, Pt, and Pd) were loaded on the film surface. Among them, the Pd catalyst afforded the highest sensing reaction and the lowest detection limit (approximately 0.4 ppb), which is the best performance ever reported. It also significantly lowered the optimal operating temperature of the sensor from 432 °C to 196 °C. The excellent isoprene sensing performance of the Pd-coated In2O3 film can be attributed to the high density of oxygen vacancies, efficient reduction-reoxidation of PdO, and the Mars-van-Krevelen catalytic reaction. We further optimized the isoprene sensing performance of a Pd-coated In2O3 film by varying the Pd thickness and discovered that 1 nm of Pd deposition showed the optimal status of discontinuous islands for sensing. We expect that our device can be applied to a portable breath isoprene analyzer.

Oxygen‐Vacancies‐Induced Enhancement in Photocatalystic Performance of Sea Urchin‐Like Fe3O4@ZnO Hollow Core–Shell Structures

A novel sea urchin‐like Fe3O4@ZnO (S‐FZ) hollow core–shell photocatalyst is developed, which achieves about fourfold higher photocatalystic degradation efficiency (84.4%) than the waxberry‐type Fe3O4@ZnO (W‐FZ) photocatalyst due to its larger specific surface area. After postannealing at different temperatures under argon atmosphere, the photocatalystic degradation efficiency of S‐FZ is effectively improved. When the annealing temperature is 500 °C, the degradation efficiency of S‐FZ hollow core–shell structures is further improved to 99%, which is related to the synergistic effect of high density of oxygen vacancies. Such S‐FZ catalysts are easily separated with a magnet and reused with only 2% loss in photocatalystic degradation efficiency after five circles. Herein, a practical approach toward high‐efficient and recyclable photocatalysts is provided.

Tuning the size stability of MnFe2O4 nanoparticles: Controlling the morphology and tailoring of surface properties under the hydrothermal synthesis for functionalization with myricetin

In this work, manganese iron oxide nanoparticles (MnFe2O4NPs) were synthesized by hydrothermal method using sodium dodecyl sulfate, sodium hydroxide, iron chloride, and manganese chloride. Four morphologies (observed by electron microscopy) were selected to be studied: flakes, rough-octahedrons, regular-octahedrons, and icosahedrons. The four samples were functionalized with the myricetin flavonoid (Myr). Hence, tetraethyl orthosilicate was employed to coat a SiO2 layer on the nanoparticles, and then, Myr was added for obtaining the so-call MnFe2O4NPs/Myr. The fast Fourier transform infrared spectroscopy (FTIR) confirmed the SiO2 coating formation and the Myr-functionalization. In addition, both Raman spectroscopy, and X-ray photoelectron spectroscopy (XPS), demonstrated the high density of oxygen vacancies in the flakes and rough-octahedrons MnFe2O4NPs. This amount was higher than that observed in the regular-octahedrons and icosahedrons MnFe2O4NPs. Among the synthesized samples, the icosahedrons MnFe2O4NPs exhibited the maximum efficiency for Myr-functionalization due to 0.53 MnFe2O4NPs/SiO2 mass rate for coupling 1 mg of Myr. This efficiency was attributed to the low oxygen vacancies concentration on the surface of the nanoparticles. The Myr-functionalization promoted a size stability enhancement (about twice) in all the samples, with no significant changes in size across 112 days (stored at RT).

ZnO quantum dot based thin films as promising electron transport layer: Influence of surface-to-volume ratio on the photoelectric properties

ZnO quantum dots (QDs) with average particle size of 4.4 nm were prepared using a low temperature processing solvothermal route. ZnO QD based thin films were then prepared from the ZnO QD based solution using spin coating technique and annealed at 250, 350 and 450 °C. The average grain size and energy band gap of ZnO were respectively increased and decreased from 5.5 to 22.9 nm and 3.37 to 3.27 eV upon increasing the annealing temperature up to 450 °C. The photoluminescence analysis showed that the as-coated ZnO film and ZnO film annealed at 250 °C have high density of oxygen vacancies; these defects were reduced upon increasing the temperature to 350 and 450 °C. The photoelectric properties of the films were strongly affected by the grain size and the defects present in the films. The photo-to-dark current ratio (PDCR) was decreased from 3723 to 371%, whereas the responsivity was increased from 1.25 to 218 mA/W with the increase of temperature to 450 °C. As-coated and 250 °C-annealed films exhibited better photoresponse than others in terms of PDCR, rise time and fall time due to their larger surface-to-volume ratio, making them promising candidate as electron transport layer in perovskite solar cells.

Controllable design of defect-rich hybrid iron oxide nanostructures on mesoporous carbon-based scaffold for pseudocapacitive applications.

The controllable design of functional nanostructures for energy and environmental applications represents a critical yet challenging technology. The existing fabrication strategies focus mainly on increasing the number of accessible active sites. However, these techniques generally necessitate complex chemical agents and suffer from limited experimental conditions delivering high costs, low yields, and poor reproducibility. The present work reports a new strategy for controllable synthesis of a hybrid system including mixed iron oxide nanostructures enriched with non-stoichiometric Fe21.34O32 and Fe3+[Fe5/33+□1/32+]O4 phases, which possess a high concentration of oxygen and Fe2+ vacancies, and a mesoporous carbon-based scaffold (MCS), which was dervied from coffee residues, with graphitic surface and perforated architecture. The nanoperforates acted as trapping sites to localise the FexOy nanoparticles, thereby boosting the density of accessible active sites. Additionally, at the interfacial regions between the FexOy crystallites, a high density of oxygen vacancies with an oriented pattern was shown to create superlattice structures. The energy storage functionality of the defect-rich MCS/FexOy nanostructure with nanoperforated architecture was investigated, where the results exhibited a high gravimetric capacitance of 540 F g-1 at a current density of 1 A g-1 with outstanding capacitance retention of 73.6% after 14 000 cycles.

Co oxidation state at LSC-YSZ interface in model solid oxide electrochemical cell

Oxidation states of cations in a perovskite structure play an important role for conductivity in solid oxide electrochemical cells. For bulk materials oxidation states can be calculated for specific conditions, but is not well understood for micro−/nanostructured materials and at interfaces between materials. We present a fundamental study of interfaces between La0.6Sr0.4CoO3-δ (LSC) and yttria-stabilized zirconia (YSZ) in symmetric model solid oxide electrochemical cells. Nanoscale morphology as well as the chemical state of the LSC are investigated by scanning transmission electron microscopy (STEM) and electron energy loss spectroscopy (EELS). The experiments were performed in situ at temperatures up to 600 °C in high vacuum (ca. 10−7 mbar) and in 2 mbar oxygen. The measured LSC Co oxidation at room temperature is lower than that expected for bulk LSC, indicating a high oxygen vacancy density and possibly high ionic conductivity. However, the Co oxidation state increases with increasing temperature, both in oxygen and in vacuum. The results shows that the Co oxidation state approach that expected for bulk LSC for typical solid oxide fuel or electrolysis cell operation temperatures.

Resistive random access memory characteristics of NiO thin films with an oxygen-deficient NiO0.95 layer

Herein, we characterize resistive random access memory (RRAM) devices containing an oxygen-deficient layer. The RRAM devices consist of multiple layered NiO thin films that consist of an oxygen-deficient NiO0.95 layer, within a NiO/NiO0.95/NiO layer structure. The NiO thin-film layers were deposited on Pt/Ta/SiO2/Si substrates by radio frequency magnetron sputtering. The stoichiometry and thickness of the NiO triple-layer thin films were determined by Auger electron spectroscopy. We fabricated RRAM capacitors with a structure of Pt/NiO/NiO0.95/NiO/Pt to investigate the resistive switching characteristics. The presence of an oxygen-deficient layer significantly lowers the forming and SET/RESET voltages compared to a Pt/NiO/Pt RRAM capacitor. This reduces the energy required to form conducting filaments in the oxygen-deficient layer. Moreover, the forming and SET/RESET voltages exhibit reduced dispersion due to the relatively high oxygen vacancy density in the oxygen-deficient NiO0.95 layer.

Novel synthesis of Al-amorphized, flexible Fe2O3 nanofibrous membranes for enhanced electrocatalytic H2 evolution

The electrocatalytic H2 evolution reaction (HER) represents an efficient technology to obtain clean and renewable energy. To realize the practical applications of HER, it is necessary to develop cost-effective, high-activity electrocatalysts. In this work, Al-amorphized, flexible Fe2O3 nanofibrous membranes are synthesized for the first time, which are used as self-supported electrocatalysts for HER. It is found that the amorphization results in higher density of oxygen vacancies, which translate into enhanced electrocatalytic activities. Moreover, the amorphization endows the Fe2O3 nanofibrous membranes with robust flexibility, which allows for good self-regulating abilities according to the electrocatalytic conditions. In consequence, the resultant amorphous Fe2O3 nanofibrous membranes exhibit better HER performances than the crystalline counterparts.

Effect of Annealing Treatment on Performance of Ga2O3 Conductive-Bridging Random-Access Memory

The effect of annealing on the performance of conductive-bridging random-access memory (CBRAM) with an amorphous gallium oxide (a-Ga2O3) switching layer has been studied. After annealing at 200°C in N2, a significant improvement of the bipolar resistive switching characteristics was observed. The a-Ga2O3 CBRAM showed good memory performance, with high switching endurance (up to 7.5 × 102 cycles) and high thermal stability. The resistive switching stability and electrical uniformity were also obviously improved. The memory window remained above 105 over 7.5 × 102 endurance cycles at 85°C. Material characterization by x-ray photoelectron spectroscopy suggested that the improved uniformity of the resistive switching in the N2-annealed Ga2O3 films may result from a higher density of oxygen vacancies. These results indicate great potential for such devices in future high-density nonvolatile memory applications.