Role Of Oxygen Vacancies Research Articles

The role of oxygen vacancies on the vibrational motions of hydride ions in the oxyhydride of barium titanate

Combined INS and DFT study on BaTiO3−xHx unravels the effect of oxygen vacancies on the vibrational dynamics of hydride ions.

Understanding the electronic properties of BaTiO3 and Er3+ doped BaTiO3 films through confocal scanning microscopy and XPS: the role of oxygen vacancies.

Photonic and electronic properties exist inherently in ferroelectric barium titanate (BaTiO3); severe luminescence quenching also exists due to the insufficient confinement of excitons. In this sense, high optical emission can only be achieved by its chemical and structural modification. Thin BaTiO3 and Er:BaTiO3 films were grown by the spin coating method on a glass substrate at room temperature. Self-trapping of excitons in the thin BaTiO3 film and its structural modification due to the doping with Er3+ ions (Er:BaTiO3) are verified using scanning confocal fluorescence microscopy (SCFM), where self-trapping excitons never occured in its pure state. By thermal treatment and doping (BaTiO3 and Er:BaTiO3) we obtained localization of the excitons, which would further induce lattice strain around the surface defects, to accommodate the self-trapped excitons. With such a self-trapped state, the structure of BaTiO3 generates broadband emission of several overlapping bands between 1.95 and 2.65 eV at room temperature, while the structure Er:BaTiO3 showed defined emission bands at 2.24 and 2.35 eV, with very weak contributions of the emission due to the self-trapping state. The influence of the variation of the excitation wavelength using 1PE and 2PE on the emission bands of BaTiO3 and Er:BaTiO3 is also investigated. The results of enhanced emission bands suggest a clear dependence of the emission intensity on the excitation energy, where a ∼3 fold enhancement in emission has been demonstrated under Er3+ (1.55 eV) excitation, which can be attributed to effective energy transfer between the Er3+ ions. As a result, it is concluded that the developed BaTiO3 and Er:BaTiO3 can pave the way for future photonic devices.

Insight into the effect of bromine on facet-dependent surface oxygen vacancies construction and stabilization of Bi2MoO6 for efficient photocatalytic NO removal

Surface oxygen vacancies (OVs) are believed as active sites for photocatalytic NO removal. The construction and stabilization of OVs on materials, especially from the point of view of exogenous ions towards different facets is vital for OVs-mediated photocatalysis. Herein, the impact of bromide ions (Br−) on OVs construction on different facets of Bi2MoO6 was systematically investigated. Theoretical calculations and characterizations demonstrated that Br− can boost and stabilize the OVs on {001} facets of Bi2MoO6, while hardly affect that on {010} facets. The Br- induced OVs on Bi2MoO6 caused a light-absorption until near-infrared region and enhanced the molecular oxygen activation, thus endowing the OVs concentration optimized Bi2MoO6 (BMO-001-Br) with superior activity on NO removal (62.9%) and selectivity of NO complete oxidation (93.61%) in a flow reaction. This work provides new insight into understanding the exogenous ions on construction and stabilization of OVs and the roles of OVs in photocatalytic NO removal.

Role of oxygen vacancies on the energy storage performance of battery-type NiO electrodes

In this study, the influence of the surface oxygen vacancies on the energy storage performance of electrodes based on nickel oxide (NiO) nanoparticles was investigated. NiO samples were synthesized by three facile and low-cost syntheses routes: nitrate calcination, citrate, and combustion methods. The concentration of surface defects in NiO powders was determined using XPS analyses, which showed a higher amount of oxygen vacancies for the sample obtained by nitrate calcination. According to the cyclic voltammetry (CV) and galvanostatic charge-discharge (GCD) curves, NiO-based electrodes were classified as battery-like. CV results suggest that redox reactions are diffusion-controlled processes with a faster diffusion rate for the sample obtained by the nitrate calcination method. This is in accordance with the GCD and electrochemical impedance spectroscopy (EIS) results, with higher specific capacity and higher electrical conductivity (lower equivalent series resistance) for the sample obtained by nitrate calcination. The results indicate that oxygen vacancies play an important role in the electrochemical performance of battery-type NiO electrodes.

Photoelectrochemistry of Ferrites: Theoretical Predictions vs. Experimental Results

AbstractThis paper gives an overview about recent theoretical and experimental work on electronic and optical properties of spinel ferritesMFe2O4. These compounds have come into focus of research due to their possible application as photocatalyst material for photoelectrochemical water splitting. The theoretical background of state-of-the-art quantum-chemical approaches applied for predicting electronic and optical band gaps, absolute band positions, optical absorption spectra, dielectric functions and Raman spectra, is briefly reviewed. Recent applications of first-principles methods on magnetic and electronic properties of ferrites withM= Mg and the first row of subgroup elements Sc to Zn are presented, where it is shown that the fundamental band gap is strongly dependent on the spin state and the degree of inversion of the spinel structure. The observed variation of electronic properties may serve as an explanation for the large scattering of experimental results. The exchange ofMand Fe cations has also a pronounced effect on the Raman spectra of ferrites, which is analyzed at atomic scale from first principles. Calculated optical absorption spectra of ferrites are compared to experimental spectra. The electronic nature of the first excitations and the role of oxygen vacancies are discussed. For the calculation of absolute band positions, which have a significant impact on the photoelectrochemical activity of the ferrites, models of the most stable ferrite surfaces are developed that take into account their polar nature and the interaction with the solvent. Theoretically predicted valence and conduction band edges are compared to results from electrochemical measurements. The role of cation exchange on the surface electronic structure is investigated both theoretically and experimentally.

Asymmetric Oxygen Vacancies: the Intrinsic Redox Active Sites in Metal Oxide Catalysts.

To identify the intrinsic active sites in oxides or oxide supported catalysts is a research frontier in the fields of heterogeneous catalysis and material science. In particular, the role of oxygen vacancies on the redox properties of oxide catalysts is still not fully understood. Herein, some relevant research dealing with M1–O–M2 or M1–□–M2 linkages as active sites in mixed oxides, in oxide supported single‐atom catalysts, and at metal/oxide interfaces of oxide supported nanometal catalysts for various reaction systems is reviewed. It is found that the catalytic activity of these oxides not only depends on the amounts of oxygen vacancies and metastable cations but also shows a significant influence from the local environment of the active sites, in particular, the symmetry of the oxygen vacancies. Based on the recent progress in the relevant fields, an “asymmetric oxygen vacancy site” is introduced, which indicates an oxygen vacancy with an asymmetric coordination of cations, making oxygen “easy come, easy go,” i.e., more reactive in redox reactions. The establishment of this new mechanism would shed light on the future investigation of the intrinsic active sites in oxide and oxide supported catalysts.

Tuning the dielectric, ferroelectric and electromechanical properties of Ba0.83Ca0.10Sr0.07TiO3–MnFe2O4 multiferroic composites

We report the successful synthesis of Ba0.83Ca0.10Sr0.07TiO3–MnFe2O4 multiferroic composites showing significant improvement in electromechanical and magnetoelectric properties. All the composite samples have formed a diphasic perovskite-ferrite composite without the presence of any impurity or intermediate phase. The bare as well as composite samples have shown classical dielectric behavior even at higher ferrite substituted samples. The electrical characteristics of composite samples have shown slight deterioration, which is mainly attributed to non-ferroelectric MnFe2O4. However, the composites still exhibit high enough piezoelectric behavior and the modification in the electromechanical response of composites is mainly caused by a change in applied stress with MnFe2O4 addition. The M-H loops of composites have demonstrated a ferrimagnetic behavior with a substantial increase in saturation magnetization on increasing the ferrite concentration. Further, the composites have shown better coupling between the ferroelectric and ferrimagnetic phases, which has resulted in an improved magnetoelectric characteristic. The role of oxygen vacancies on ferroelectric and magnetic properties of prepared composites has been systematically studied.

Role of oxygen vacancies and Mn4+/Mn3+ ratio in oxidation and dry reforming over cobalt-manganese spinel oxides

Spinel type cobalt-manganese oxides MnxCo3-xO4 (x = 0; 0.05; 0.10; 0.15) were prepared by co-precipitation method, from cobalt and manganese salts in the presence of ammonium carbonate. X-ray diffraction (XRD), texture measurements (BET/BJH), Fourier transform infrared spectroscopy (FT-IR), Scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS) were used for their characterization. Their catalytic behavior in ethanol combustion, dry reforming with CO2 and dehydrogenation was investigated. The results showed that the cubic structure of Co3O4, corresponding to spinel network, is not disturbed by adding manganese. Results obtained revealed that Mn0.15Co2.85O4 catalyst, with the highest content of Mn4+ cations in the octahedral sites and oxygen defects, display superior catalytic activity compared to the others. The apparent activation energies, calculated from Arrhenius plots, are as follows: Ea combustion < Ea non-oxidative dehydrogenation < Ea dry reforming, showing that cobalt-manganese spinel type oxides are good candidates for combustion.

Role of oxygen vacancies on photo-catalytic activities of green synthesized ceria nanoparticles in Cydonia oblonga miller seeds extract and evaluation of its cytotoxicity effects

Ceria nanoparticles (CN) were produced via green route in Cydonia oblonga miller (Com) seeds extract as capping and stabilizing agent. They were characterized by a variety of physico-chemical methods such as XRD, FT-IR, UV-Vis, FESEM, TGA/DTA, and photoluminescence (PL). Crystalline size of CN was increased with ascending the calcination temperature while band gap energy of them was descending from 2.4 to 3.1 eV. Cell viability was determined by MTT assay and results showed that CN had no significance toxicity on A549 cell line. Also, antioxidant effect of CN on the same cell line was performed via 2′,7′–dichlorofluorescin diacetate (DCFDA). All concentration of CN specially 15.6 μg/ml could neutralize the oxygen reactive species (ROS). Photo-catalytic study was performed to evaluate dyes degradation ability of CN under UV-A irradiation. Results illustrated that samples calcined in lower temperature showed better photo-catalytic activity. Also, PL study showed that the photoluminescence emission intensity of samples was diminished with increasing the calcination temperature. In conclusion, enhance the photo-catalytic activity and photoluminescence emission was related to presence of oxygen defects in CN structure which were increased in lower calcination temperature and responsible for improving the optical properties of CN.

Interaction of O2, H2O and H2 with proton-conducting oxides based on lanthanum scandates

An interaction of components from the gas phase, containing gaseous oxygen, hydrogen and water, with the La1–xSrxScO3–α oxides in the temperature range of 300–950 °С and partial pressure of 6.1–24.3 kPa 8.1–50.7 kPa has been studied using a high temperature thermogravimetric analysis. The effects of partial pressure of the gaseous oxygen, water and hydrogen on the apparent uptake level of protons with oxides has been found. The studies of incorporation processes of hydrogen from molecular hydrogen atmosphere into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates were performed in the temperature range of 300–800 °C and hydrogen pressure of 0.2 kPa by means of the isotope exchange method with the equilibration of isotope composition in the gas phase. The protons and deuterons concentrations were determined for the La0.91Sr0.09ScO3–α oxide. The existence of proton defects in the structure of studied oxides after the exposure in H2O and H2-containing atmospheres was revealed using the 1Н NMR. The role of oxygen vacancies in the proton incorporation processes is considered in the present work.

Charge trapping effect in HfO2-based high-k gate dielectric stacks after heavy ion irradiation: The role of oxygen vacancy

The charge trapping effect of HfO2 gate stacks under heavy ion irradiation was evaluated via capacitance-voltage (C-V) measurement and transmission electron microscopy (TEM). Ion irradiation introduced oxygen vacancies into the HfO2 dielectric layer, which located primarily at the interface between HfO2 and Si. The oxygen vacancies act as trapping centers dominating the degradation of device performance after irradiation. The role of the traps on charge trapping in HfO2 gate stacks and the trapped charge stability (i.e. detrapping process) as a function of time were also investigated in this work. The mechanism of how MOS capacitors are affected by irradiation provides a good foundation for future space application of HfO2 gate stacks based integrated circuits.

Effect of oxygen stoichiometry on the magnetization profiles and negative magnetization in LSMO thin films

The depth-dependent magnetization in thin film oxygen stoichiometric and oxygen-deficient La0.7Sr0.3MnO3 is investigated by using polarized neutron reflectivity and DC bulk magnetometry. The polarized neutron reflectivity results reveal that the stoichiometric sample shows enhanced interfacial magnetization relative to the rest of the film. The oxygen-deficient sample exhibits a reduced average magnetization from the optimized recipe. Both films show regions of suppressed magnetization at the surface regardless of the growth pressure. The oxygen stoichiometric film does not show an interfacial dead layer, whereas the oxygen-deficient film exhibits a dead layer whose thickness changes with temperature. At a low applied field, we observe striking differences in the depth dependence of the magnetic ordering, with the oxygen-deficient film exhibiting exchange spring behavior, while the stoichiometric film shows a constant magnetization direction across the film. These results suggest that the incorporation of oxygen vacancies during growth leads to an accumulation of vacancies at the interface, which is enhanced at higher temperature due to increased oxygen mobility, creating a region of reduced magnetism that couples to the rest of the film. These results offer insight into the complex behavior and role of oxygen vacancies in the magnetism of these systems. Additionally, the study reveals further details of the negative magnetization in La0.7Sr0.3MnO3 reported in previous studies, which are discussed here.

Interaction of H2O, O2 and H2 with Proton Conducting Oxides Based on Lanthanum Scandates

The research uses the method of high-temperature thermogravimetric analysis to study the processes of interaction of the gas phase in the temperature range 300–950 °C in the partial pressure ranges of oxygen 8.1–50.7 kPa, water 6.1–24.3 kPa and hydrogen 4.1 kPa with La1–xSrxScO3–α oxides (x = 0; 0.04; 0.09). In the case of an increase in the partial pressure of water vapor at a constant partial pressure of oxygen (or hydrogen) in the gas phase, the apparent level of saturation of protons is shown to increase. An increase in the apparent level of saturation of protons of the sample also occurs with an increase in the partial pressure of oxygen at a constant partial pressure of water vapor in the gas phase. The paper discusses the causes of the observed processes. The research uses the hydrogen isotope exchange method with the equilibration of the isotope composition of the gas phase to study the incorporation of hydrogen into the structure of proton-conducting oxides based on strontium-doped lanthanum scandates. The concentrations of protons and deuterons were determined in the temperature range of 300–800 °C and a hydrogen pressure of 0.2 kPa for La0.91Sr0.09ScO3–α oxide. The paper discusses the role of oxygen vacancies in the process of incorporation of protons and deuterons from the atmosphere of molecular hydrogen into the structure of the proton conducting oxides La1–xSrxScO3–α (x = 0; 0.04; 0.09). The proton magnetic resonance method was used to study the local structure in the temperature range 23–110 °C at a rotation speed of 10 kHz (MAS) for La0.96Sr0.04ScO3–α oxide after thermogravimetric measurements in an atmosphere containing water vapor, and after exposures in molecular hydrogen atmosphere. The existence of proton defects incorporated into the volume of the investigated proton oxide from both the atmosphere containing water and the atmosphere containing molecular hydrogen is unambiguously shown. The paper considers the effect of the contributions of the volume and surface of La0.96Sr0.04ScO3–α oxide on the shape of the proton magnetic resonance spectra.

Role of oxygen vacancies on optical, structural and photoluminescence properties of Sn0.97Cr0.03O2 nanoparticles co-doped with Cu

Cu doped Sn0.97Cr0.03O2 nanostructure have been synthesized by chemical co-precipitation method. Phase purity with excellent atomic-level of all ingredient elements and the substitution of Cu in Sn-Cr-O2 host was confirmed by x-ray diffraction (XRD) studies. The elevated crystallite size, lattice parameters, ‘d’ value and volume per unit cell at Cu = 4% recommended that Cu2+ ions with higher ionic radius is replaced in the position of Sn4+ ions of Sn-Cr-Cu-O lattice. Scanning electron microscopic (SEM) images revealed the changed over of hexagon shaped structure to fine and tiny spherical shaped particles by Cu-doping with un-even sized grains. The steady increase of energy gap from 3.51 eV (Cu = 0%) to 3.67 eV (Cu = 4%) is due to the creation of oxygen vacancies and the charge carriers. The modification in infrared band position and intensity confirmed the addition of Cu into the Sn-Cr-O2 lattice. The elevated visible green luminescence (GL) by Cu addition is due to the generation of oxygen vacancies in the inter-band levels which supports the increase of energy gap.

Efficient Charge Carrier Separation in l-Alanine Acids Derived N-TiO2 Nanospheres: The Role of Oxygen Vacancies in Tetrahedral Ti4+ Sites.

N-doped TiO2 with oxygen vacancies exhibits many advantages for photocatalysis, such as enhanced visible light absorbency, inhibition of the photogenerated charge carrier recombination, etc. However, preparation of N-doped TiO2 with oxygen vacancies under mild conditions is still a challenge. Herein, N-doped TiO2 nanospheres with tetrahedral Ti4+ sites were synthesized by using dodecylamine as template and assisted by l-alanine acids. The obtained samples were characterized by X-ray powder diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and UV–Vis diffuse reflectance spectra (UV–Vis DRS). It was found that the dodecylamine as a neutral surfactant controlled the structure of TiO2 spherical, while l-alanine acids provided a nitrogen source. The existence of tetrahedral Ti4+ sites in N-doped TiO2 was also confirmed. The N-doped TiO2 sample with tetrahedral Ti4+ sites exhibited significantly improved photocatalytic performance for degradation of methylene blue solution under UV light or visible light irradiation. A combined time-resolved infrared (IR) spectroscopy study reveals that the enhanced photocatalytic performance could be attributed to a large amount of photogenerated charge carriers and efficient charge separation. It is demonstrated that the shallow donor state produced by oxygen vacancies of tetrahedral Ti4+ sites can effectively promote separation of charge carriers besides capturing electrons.

Role of oxygen vacancies in vanadium oxide and oxygen functional groups in graphene oxide for room temperature CO2 gas sensors

The greenhouse effect is involving global heating and climate change within the world. Carbon dioxide (CO2) is one of the major gas at the origin of this effect, but also the byproduct of human activity. Therefore, monitoring the indoor/outdoor CO2 emission by gas sensors is one of the priorities for environmental preservation. In this paper, the sensing performance of CO2 towards two different O-rich films have been studied; graphene oxide (GO) and vanadium dioxide (VO2). The preparation of GO film has been carried out by spray pyrolysis on fluorine tin oxide (FTO) prepared by the modified Hummers method. While the VO2 film has been sol-gel spin-coated on a glass substrate. Both films have been characterized using XRD, SEM and electrical properties. The CO2 gas sensing mechanism and the role of oxygen vacancies in VO2 are addressing. The oxygen functional groups in GO play a main role in the CO2 gas the sensitivity level and response time. Their gas sensing performances have been investigated based on measuring the response vs recovery time, dynamic response curve analysis and sensitivity. In order to better understand the sensing mechanism, characterization has been done with different gas concentrations. Both GO and VO2 based CO2-sensors are acted as an n-type sensor. Sensing behavior of GO at RT has explained to be mainly mediated by the oxygen functional groups and a wide range of active sites. In the other hand, VO2 contains oxygen vacancies and more defect sites which play a main role in the RT sensing activity and low recovery time.

Conduction mechanisms of TaN/HfOx/Ni memristors

The conduction mechanisms of TaN/HfOx/Ni memristors are investigated in the wide temperature range from 4.2 to 400 K. The hafnium oxide layer was deposited by ion-beam sputtering deposition (IBSD) at different partial oxygen pressures for various samples, for which resistive switching was possible. This range of partial oxygen pressures corresponded to the x range from 1.78 to 1.81 The analysis and fitting of the current-voltage (I−V) curves of low- and high-resistance states (LRS and HRS, respectively) at different temperatures revealed that, at temperatures above ∼70 K, the conduction is governed by the thermal generation of free carriers and space-charge-limited current (SCLC). Specifically, it is shown that trap-free and trap-mediated SCLCs govern the conduction for the LRS and HRS, respectively. At lower temperatures, the conduction in both states was found to be practically temperature-independent. At the same time, the TaN/HfOx/Ni memristor functioned properly at temperatures as low as 4.2 K. The x value is shown to influence the slope of the HRS I−V curves, and it correlates with the change in the SCLC power-law exponent (m, I ∝ Vm). TaN/HfOx/Ni memristors with low values of x ≤ 1.79 demonstrated the current power-law dependence on voltage in the HRS with m > 2 (3.0–6.5) and for the memristors with higher x values in the HRS of m ≈ 2. Within the suggested model, a local trap-rich conducting region was presumed to be formed in the oxide layer between the metal electrodes after the memristor forming process. The I−V curves modeling indicates that the LRS and HRS differ from each other by the thickness of this active region and the trap distribution. The role of oxygen vacancies and polyvacancies in HfOx, which are considered to act as charge carrier traps, is discussed.

Role of Oxygen Vacancies at the TiO2/HfO2 Interface in Flexible Oxide‐Based Resistive Switching Memory

AbstractFlexible memory is highly desirable for data storage in portable wearable electronics. Here, a flexible bilayer TiO2/HfO2‐architecture‐based resistive random access memory (RRAM) is fabricated a on polyethylene naphthalate (PEN) substrate, and exhibits outstanding uniformity, high durability, and excellent mechanical flexibility. The coefficients of variations for high and low resistance state are ≈3.2% and ≈3%, respectively. No performance degradation is observed under mechanical stress with bending radius ranging from 70 to 10 mm. Interestingly, the performance degradation after long‐term stability tests can be recovered. An asymmetric hourglass‐shaped oxygen vacancy (Vo) distribution at the HfO2/TiO2 interface plays a key role in high performance of this flexible RRAM device. The proposed flexible RRAM devices have potential applications in wearable electronics.

Oxygen Vacancies in Oxide Nanoclusters: When Silica Is More Reducible Than Titania.

Oxygen vacancies are related to specific optical, conductivity and magnetic properties in macroscopic SiO2 and TiO2 compounds. As such, the ease with which oxygen vacancies form often determines the application potential of these materials in many technological fields. However, little is known about the role of oxygen vacancies in nanosized materials. In this work we compute the energies to create oxygen vacancies in highly stable nanoclusters of (TiO2)N, (SiO2)N, and mixed (TixSi1−xO2)N for sizes between N = 2 and N = 24 units. Contrary to the results for bulk and surfaces, we predict that removing an oxygen atom from global minima silica clusters is energetically more favorable than from the respective titania species. This unexpected chemical behavior is clearly linked to the inherent presence of terminal unsaturated oxygens at these nanoscale systems. In order to fully characterize our findings, we provide an extensive set of descriptors (oxygen vacancy formation energy, electron localization, density of states, relaxation energy, and geometry) that can be used to compare our results with those for other compositions and sizes. Our results will help in the search of novel nanomaterials for technological and scientific applications such as heterogeneous catalysis, electronics, and cluster chemistry.

Deciphering Charge Transfer and Electronic Polarization Effects at Gold Nanocatalysts on Reduced Titania Support

Gold nanoparticles supported on reduced TiO2 (110) surfaces are widely used as catalysts for oxidation reactions. Despite extensive studies, the role of oxygen vacancies in such systems remains elu...