Different Types Of Oxygen Vacancies Research Articles

Asymmetric oxygen vacancy promotes CO-SCR performance on defect-engineered Rh/CeCuOx catalyst

Selective catalytic reduction of NOx with CO (CO-SCR) is a process that purifies both NO and CO pollutants through a catalytic reaction. Specifically, the cleavage of NO on the catalyst surface is crucial for promoting the reaction. During the reaction, the presence of oxygen vacancies can extract oxygen from NO, thereby facilitating the cleavage of NO on the catalyst surface. Thus, the formation of oxygen vacancies is key to accelerating the CO-SCR reaction, with different types of oxygen vacancies being more conducive to their generation. In this study, Rh/CeCuOx catalysts were synthesized using the co-crystallization and impregnation methods, and asymmetric oxygen vacancies were induced through hydrogen thermal treatment. This structural modification was aimed at regulating the behavior of NO on the catalyst surface. The Rh/Ce0.95Cu0.05Ox-H2 catalyst exhibited the best performance in CO-SCR, achieving above 90% NO conversion at 162 °C. Various characterization techniques showed that the H2 treatment effectively reduced some of the CuO and Rh2O3, creating asymmetric oxygen vacancies that accelerated the cleavage of NO on the catalyst surface, rather than forming difficult-to-decompose nitrates. This study offers a novel approach to constructing oxygen vacancies in new CO-SCR catalysts.

A DFT study of CH4 adsorption on OMS-2 (1 1 0) surface with different types of oxygen vacancies

Cryptomelane-type octahedral molecular sieves (OMS-2) is considered as a promising non-precious metal catalyst, and its catalytic activity is largely related to oxygen vacancies (OVs). Hence, the effects of two types of OVs on CH4 adsorption on the surface of OMS-2 (110) were investigated based on density function theory (DFT). The cohesive energy, OVs formation energy, adsorption energy, density of states and Bader charge were calculated. The results show that OVs will preferentially form at sp3 oxygen sites on the surface. Methane tends to adsorb at the K site with three H atoms facing the surface. When the O-sp3 vacancy is introduced, the adsorption capacity of (110) surface for methane is enhanced, the electron transfer is increased, and the degree of CH activation is higher. Moreover, the adsorption capacity is further strengthened with increasing the number of O-sp3 defects. Conversely, the introduction of sp2 oxygen defect hinders the adsorption of methane by the catalyst due to the rearrangement of surface atoms. These conclusions can provide theoretical guidance for the development of efficient methane catalysts.

Electron-donating N−–Ti3+–Ov interfacial sites with high selectivity for the oxidation of primary C–H bonds

Element doping can fabricate different types of oxygen vacancies (O v ) on TiO 2 , but the effects of O v coordination and configuration on the oxidation reaction remain unclear. Here, we modify the local geometric ligand environment of O v by doping N and B into anatase TiO 2 (N-TiO 2 and B-TiO 2 ). N − –Ti 3+ –O v is induced in N-TiO 2 by substituting O with N, which has similar ionic size and electronegativity, and Ti 3+ –O v is found in TiO 2 and B-TiO 2 . Density functional theory calculations indicate that N − –Ti 3+ –O v is more reactive than Ti 3+ –O v toward O 2 activation. The resultant N-TiO 2 -0.25 enriched in N − –Ti 3+ –O v contributes to a rapid formation of superoxygen species (•O 2 − ), which increase the oxidation rate of primary C–H bonds in toluene, and thus exhibits much higher selectivity and yield. The fabrication of N − –Ti 3+ –O v develops doped catalysts with improved O v reactivity and enhanced primary C–H bond oxidation selectivity. • N − –Ti 3+ –O v is more reactive than Ti 3+ –O v toward O 2 activation to generate ⋅O 2 − • The generation of ⋅O 2 − is the rate-determining step in selective oxidation of toluene • The introduced photo energy does not change the thermal selective oxidation process • A higher yield of products is attributed to the increased selective oxidation rate Oxygen vacancies in TiO 2 influence catalytic activity differently depending on their coordination environment. Chen et al. explore N and B doping and show that N − –Ti 3+ –O v is more reactive than Ti 3+ –O v toward O 2 activation, enhancing the primary C–H bond oxidation of toluene.

A high-temperature 57Fe Mössbauer study of (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

An in-situ 57Fe Mössbauer study of mixed ionic electronic conducting (Ba0.5Sr0.5)(Co0.857Fe0.08Fe0.12)O3−δ (BSCF5582) has been made at temperatures up to 1000 °C. In the paramagnetic high-temperature phase (T ≥ 315 °C), quadrupolar interactions show that local symmetry at iron sites is lower than cubic. At 700 °C, 850 °C, and 1000 °C, spectra have been measured as a function of oxygen partial pressure, PO2 (1·10−5 ≲ PO2/bar ≤1). The high-temperature Mössbauer center shifts and their independence of oxygen partial pressure are discussed in respect to the average oxidation state and average local coordination of iron. The latter information has been derived from a thermodynamic analysis of the defect chemistry of BSCF5582 accounting for different types of oxygen vacancies as well as for multiple oxidation states of the transition metal cations. The isothermal decrease of high-temperature signal intensity observed with decreasing PO2 is attributed to changes in the vibrational properties of the highly oxygen-deficient material which have been analyzed in the framework of the Grüneisen model.

Study on the optical properties of CdWO4 crystal with oxygen vacancies

Cadmium tungstate crystals have attracted much attention because of its excellent scintillation performance. The existence of oxygen vacancies will reduce the luminous efficiency of the crystal, extend its luminous decay time and reduce light resolution of the crystal. However, the luminescence mechanism of the oxygen vacancies is not clear yet. Here, the optical properties of cadmium tungstate crystal with oxygen vacancies have been studied based on the Density Functional Theory (DFT). We calculated the defect formation energies of oxygen vacancies with three different charges (0, +1, +2), and corrected the band gap to make it more accurate. We obtained the absorption and emission peaks of the F center and F+ center for two different types of oxygen vacancies. The calculated absorption peaks of the F center of VOI and F+ center of VOII at 3.04 eV and 3.59 eV respectively, which are close to the experimental results (3.1 eV (400 nm) and 3.5 eV (350 nm)). So we speculate that these two obvious absorption peaks are caused by oxygen vacancies.

Molecular adsorption and dissociation of CO2 on TiO2 anatase (001) activated by oxygen vacancies.

A study on the influence of oxygen vacancies on the anatase (001) surface on the CO2 adsorption process is presented. For its realization, density functional theory (DFT) was used under the Perdew-Burke-Ernzerhof (PBE) generalized gradient and the spin-polarized approximations. Hubbard-U corrections and van der Waals interactions were also included. Three different types of oxygen vacancies were investigated at different sites on the anatase (001) surface; the formation energies in each case were 67.05, 113.84, and 93.16 kcal/mol, respectively. We identified a type of oxygen vacancy that could favor both the CO2 adsorption and dissociation. The differences on CO2 adsorption properties are due to electronic and structural causes, such as midgap states (Ti3+ polarons species) and changes in the structural properties on the TiO2 surface, generated upon the introduction of an oxygen vacancy. It is concluded that oxygen vacancies can play an important role in both CO2 adsorption and dissociation.

Theoretical calculation and experimental study of the electronic structure and catalysis of defected semiconductor

TiO2 photocatalysts with different types of oxygen vacancies were synthesized via a hydrolysis-precipitation method. The oxygen vacancies types, the charge carriers separation and the band structures of the photocatalysts were investigated by theoretical calculations and experimental characterizations. It is revealed that the energy level of oxygen vancancies with two trapped electrons is higher than that with one trapped electrons; the oxygen vancancies could act as charge carriers recombination centers. Therefore, TiO2 with two-electrons-trapped oxygen vancancies show enhanced visible-light photocatalytic activity, compared with the counterparts with one-electron-trapped oxygen vancancies. The photocatalyst with the lowest oxygen vacancies content shows the best photocatalytic activity under both UV and visible-light irradiation.

57Fe Mössbauer study into oxygen vacancy disorder in (Ba0.5Sr0.5)(Co0.8Fe0.2)O3−δ

A 57Fe Mössbauer study of mixed ionic electronic conducting (Ba0.5Sr0.5)(Co0.857Fe0.08Fe0.12)O3−δ (BSCF) has been made on samples of different oxygen content. The room-temperature spectra are found to depend strongly on oxygen stoichiometry. They reveal two magnetically and chemically different environments of the iron probes whose charge states and oxygen coordination are discussed. Analysis of spectral intensities provides confirmation of the close similarity of formation energies of different types of oxygen vacancies in BSCF. It is also shown that the distribution of oxygen vacancies on their sites of different coordination is far from random.

Anisotropic Effects of Oxygen Vacancies on Electrochromic Properties and Conductivity of γ-Monoclinic WO3

Tungsten trioxide (WO$_3$) is a paradigmatic electrochromic material, whose peculiar optical properties in the presence of oxygen vacancies or intercalated alkali atoms have been observed and investigated for a long time. In this paper we propose a rationalization of experiments based on first-principles calculations of optical and electrical properties of oxygen deficient (reduced) WO$_3$. Our approach is based on a parameter-free dielectric-dependent hybrid density functional methodology, used in combination with the charge transition levels formalism, for studying excitation mechanisms in the presence of defects. Our results indicate that oxygen vacancies lead to a different physics in $\gamma$-monoclinic WO$_3$, depending on the orientation of the W-O-W chain where the vacancy is created, thus evidencing strong anisotropic effects rooted in the peculiar structural properties of the original nondefective monoclinic cell. Different types of oxygen vacancies can hence be classified on the basis of the calculated ground state properties, electronic structure, and excitation/emission energies, giving a satisfactory explanation to a range of experimental observations made on oxygen deficient WO$_3$.

Interplay between External Strain and Oxygen Vacancies on Raman Spectra of SnO2

Comprehensive first-principle calculations on strained SnO2 crystal structure indicate that the formation energy of different types of oxygen vacancies depends on the external strain. Many novel Raman modes can be observed, their intensities and positions are strongly dependent on applied stain, which can be ascribed to crystal symmetry destruction by oxygen vacancies. Applied strain can compress/stretch distances between Sn and O atoms; therefore, Sn—O band vibration frequencies can be adjusted accordingly. Our calculated results disclose that the Raman spectra of SnO2 crystal structure with different types of oxygen vacancies are obviously different, which can be used to identify the oxygen vacancy types in strained SnO2 crystal structures.

Identification of oxygen vacancy types from Raman spectra of SnO2 nanocrystals

Raman spectra acquired from spherical SnO2 nanocrystals prepared by pulsed laser ablation and hydrothermal synthesis exhibit three oxygen‐vacancy‐related Raman modes at 234, 573, and 618 cm−1. The peak location and intensity vary with annealing temperature under O2 finally approaching those of bulk materials. Density functional calculation discloses that the three Raman modes stem from subbridging, in‐plane, and bridging oxygen vacancies, respectively. Raman spectra can thus be used to discern different types of oxygen vacancies in SnO2 nanocrystals. Copyright © 2012 John Wiley & Sons, Ltd.

Revealing the Surface Reactivity of Zirconia by Periodic DFT Calculations

ZrO2 is known in chemistry to be a stable material. However, some technological applications point to specific redox reactivity related to surface oxygen composition. In this paper, ab initio calculations are used to characterize structure–reactivity relationships on monoclinic zirconia surfaces as regards thermodynamic stability, electronic structure, and chemical reactivity in reducing conditions. It is shown that the formation of different types of oxygen vacancies is possible on the surfaces of monoclinic zirconia. 2-Fold vacancies are found to induce an important structural relaxation. Moreover, the presence of O vacancies affects the surface electronic structure in two ways: either zirconium sites are reduced or F-centers are formed; the final state depends on the local geometry around the vacancy. Finally, some oxygen vacancy sites are found to be highly reactive toward dihydrogen, leading to its spontaneous dissociation and formation of dihydride species. These results reveal a rich and complex su...

Diffusion of oxygen vacancies on a strained rutileTiO2(110)surface

Based on first-principles calculations of the rutile ${\text{TiO}}_{2}(110)$ surface with different types of oxygen vacancies, a phase diagram is constructed for the energetically favorable oxygen vacancy as a function of the externally applied in-plane strain. When the strain is relatively small, the bridging oxygen vacancy (BOV) is the energetically favorable one. The pathways and the energy barriers of surface diffusion of the BOV under different external strain are studied. For the cross row diffusion of the BOV along $[1\overline{1}0]$, a concerted diffusion mechanism mediated by the in-plane oxygen vacancy is found to be energetically more favorable than the hopping diffusion. The energy barrier of the concerted diffusion along $[1\overline{1}0]$ and that of the hopping diffusion along [001] are found to decrease with increasing strain. The former decreases more dramatically than the latter when the strain is applied along $[1\overline{1}0]$, which suggests a possible way of facilitating the diffusion anisotropy.

First-principles study of oxygen vacancies inMgxZn1−xOalloys

A first-principles study of oxygen vacancies in ${\text{Mg}}_{x}{\text{Zn}}_{1\ensuremath{-}x}\text{O}$ alloys is presented. Different types of oxygen vacancies are distinguished by their number of Mg and Zn nearest neighbors. The formation energy is found to be lowest for vacancies surrounded by four Zn nearest neighbors and is almost independent of the overall concentration in the alloy. Since this energy of formation enters the concentration via a Boltzmann factor, it implies that vacancies other than purely Zn surrounded are very unlikely even in relatively Mg-rich alloys. The defect energy level associated with the vacancy is a very deep donor level closer to the valence band than the conduction band. It does not follow the band gap with concentration but stays approximately fixed relative to the valence-band maximum. The defect level gradually increases with number of Mg neighbors.

Interplay between External Strain and Oxygen Vacancies on a RutileTiO2(110)Surface

Comprehensive first-principle calculations on strained rutile TiO2(110) indicate that the formation energy of different types of oxygen vacancies depends on the external strain. For the unstrained state, the energetically favorable oxygen vacancy (EFOV) appears on the bridging site of the first layer; when 3% tensile strain along [11[over ]0] is applied, EFOV moves to the in-plane site, while 2% compressive strain along either [001] or [11[over ]0] shifts EFOV to the subbridging site. We therefore suggest that the distribution of oxygen vacancies can be engineered by external strain, which may help to improve the applications of a TiO2 surface where oxygen vacancy plays an important role.

Ab initio cluster calculations on the electronic structure of oxygen vacancies at the polar ZnO(0001̄) surface and on the adsorption of H2, CO, and CO2 at these sites

Oxygen vacancies at the polar O terminated (0001) surface of ZnO are of particular interest, because they are discussed as active sites in the methanol synthesis. In general, the polar ZnO surfaces are stabilized by OH groups, therefore O vacancies can be generated by removing either O atoms or OH or H2O groups from the surface. These defects differ in the number of electrons in the vacancy and the number of OH groups in the neighborhood. In the present study, the electronic structure and the adsorption properties of four different types of oxygen vacancies have been investigated by means of embedded cluster calculations. We performed ab initio calculations on F+ like surface excitations for the different defect types and found that the transition energies are above the optical band-gap, while F+ centers in bulk ZnO show a characteristic optical excitation at 3.19 eV. Furthermore, we studied the adsorption of CO2 and CO at the different defect sites by DFT calculations. We found that CO2 dissociates at electron rich vacancies into CO and an O atom which remains in the vacancy. At the OH vacancy which contains an unpaired electron CO2 adsorbed in the form of CO2-, while it adsorbed as a linear neutral molecule at the H2O defect. CO adsorbed preferentially at the H2O defect and the OH defect, both with a binding energy of 0.3 eV.

Electrochromic tungsten oxide: the role of defects

Tungsten oxide is today the most widely used electrochromic material, and the optical and transport properties have been extensively studied. The optical properties of polycrystalline tungsten oxide films are well understood in terms of polaron theory. However, there is still a lack of detailed physical understanding of the properties of amorphous tungsten oxide. In this paper we review the concept of defects in amorphous materials with a special emphasis on tungsten oxide coatings produced by sputter deposition. Different types of oxygen vacancies and interstitials are believed to be of importance depending on the deposition conditions of the films. Tungsten oxide exhibits a strong electron–phonon interaction, which makes the occurrence of doubly charged defects likely. Different defects give rise to transparent and coloured as-deposited amorphous tungsten oxide coatings. We also discuss the character of the localized electronic states giving rise to the electrochromic effect upon proton or lithium intercalation.

Fabrication and optical properties of large-scale uniform zinc oxide nanowire arrays by one-step electrochemical deposition technique

Semiconductor ZnO nanowire arrays were fabricated by one-step electrochemical deposition technique based on ordered nanoporous alumina membrane. Their microstructures were characterized by transmission electron microscopy, Raman spectrum, and scanning electron microscopy. The results indicate that the ZnO nanowire array is uniformly assembled into the nanochannels of anodic alumina membranes (AAM). It is found that photoluminescence spectrum of the ZnO/AAM assembly system depends on the excitation wavelength in the visible region, which is attributed to different types of oxygen vacancies in the ZnO nanowires.

Oxygen defects ordering in ?-Bi2O3: LMTO-ASA and FPLMTO calculations

Using the Full-Potential LMTO and LMTO-ASA methods we have carried out the calculations of electronic structure and chemical bonding for different types of oxygen vacancies ordering in δ-Bi2O3. Nonlinear changes of cohesive energy with the variation of vacancy concentration was found. The lattice with two vacancies per cell appears to be the most stable one with vacancies ordered along the 〈111〉 direction. The semiconducting gap formation is analyzed, and the suggestion is made that the gap corresponds to the structure with uniform distribution of oxygen atoms over all possible oxygen crystal lattice site. © 1995 John Wiley & Sons, Inc.

Photoluminescence and optical absorption studies of the effects of heat treatment on cuprous oxide

The defects responsible for the short-wave (720 nm) and medium-wave (820 nm) luminescence in cuprous oxide can be created by annealing the crystal at a temperature of 1050 °C under low oxygen pressures. The annihilation of these defects has been observed from the photoluminescence and optical absorption measurements after the crystal has been subjected to a second annealing in the temperature range of about 750 °C under a reducing atmosphere. From the photoluminescence measurements, the density of these defects is found to decrease exponentially as a function of annealing time, as manifested by the reduction in the luminescent intensity. The rate of reduction in intensity is also found to increase with annealing temperature.Because the defects responsible for the luminescence are ascribed to various forms of oxygen vacancies, we believe the decrease in luminescence is due to a reduction in the oxygen vacancies resulting from the formation of copper precipitates in the crystal. The short-wave and medium-wave emissions, which are ascribed to different types of oxygen vacancies, are found to have different activation energies of diffusion.