Interstitial Oxygen Species Research Articles

A study on the evolution of hydrothermally synthesized MoS2 on thermal annealing and development of novel MoS2-MoO3 hybrid nanostructure for supercapacitor applications

The electrochemical capacitor also known as supercapacitor is a high-performance energy storage device with exceptional properties. We report the evolution of structural, morphological, optical, and thermal properties of MoS2 on thermal annealing along with its supercapacitor study. The pristine MoS2 is synthesized via the hydrothermal method at 200 °C and the material is annealed in the presence of air at 250 °C, 300 °C, and 350 °C. The 300 °C annealed sample has shown a hybrid MoO3-xMoS2-y phase characterized by morphology of mixed flower-like and nano brick-like structures, marking the first time report of this novel formation. The evolution of the novel MoO3-xMoS2-y phase can be attributed to the intervention of lattice and interstitial oxygen species during synthesis. Electrochemical analysis is employed to study the supercapacitor nature of the materials. The electrochemical study revealed that the hybrid structure has a higher charge storage capacity than MoS2 or MoO3. The calculated specific capacitance from the galvanostatic charge-discharge curve of MoO3-xMoS2-y in 0.1 M NaOH is observed to be 1.71 mF/cm2, five times greater than other samples. Furthermore, the capacitor behavior of MoO3-xMoS2-y is studied in different electrolytes such as KOH, NaOH, and Na2SO4 at a concentration of 3 M. The intercalation-assisted faradaic process is observed to be the major charge storage mechanism of the hybrid structure. Developing novel nanostructures with distinct properties is crucial for advancing supercapacitors to achieve optimal performance.

Oxygen transport in Pr nickelates: Elucidation of atomic-scale features

Pr2NiO4+δ oxide with a layered Ruddlesden–Popper structure is a promising material for SOFC cathodes and oxygen separation membranes due to a high oxygen mobility provided by the cooperative mechanism of oxygen migration involving both interstitial oxygen species and apical oxygen of the NiO6 octahedra. Doping by Ca improves thermodynamic stability and increases electronic conductivity of Pr2−xCaxNiO4+δ, but decreases oxygen mobility due to decreasing the oxygen excess and appearing of 1–2 additional slow diffusion channels at x ≥ 0.4, probably, due to hampering of cooperative mechanism of migration. However, atomic-scale features of these materials determining oxygen migration require further studies. In this work characteristics of oxygen diffusion in Pr2−xCaxNiO4+δ (x = 0–0.6) are compared with results of the surface analysis by X-ray photoelectron spectroscopy and modeling of the interstitial oxygen migration by the plane-wave density functional theory calculations. According to the X-ray photoelectron spectroscopy data, the surface is enriched by Pr for undoped sample and by Ca for doped ones. The O1s peak at ~531 eV corresponding to a weakly bound form of surface oxygen located at Pr cations disappears at ~500 °C. Migration of interstitial oxygen was modeled for a I4/mmm phase of Pr2NiO4+δ. The interstitial oxygen anion repulses the apical one in the NiO6 octahedra pushing it into the tetrahedral site between Pr cations. The calculated activation barrier of this migration is equal to 0.585 eV, which reasonably agrees with the experimental value of 0.83 eV obtained by the oxygen isotope exchange method. At the same time, for the model compound Ca2NiO4+δ, obtained by isomorphic substitution of Pr by Ca in Pr2NiO4+δ, calculations implied formation of the peroxide ion comprised of interstitial and lattice oxygen species not revealed in the case of incomplete substitution (up to PrCaNiO4+δ composition).Hence, calculations in the framework of the plane-wave density functional theory provide a realistic estimation of specificity of oxygen migration features in Pr2NiO4+δ doped by alkaline-earth metals.

The effect of active oxygen species in nano-ZnCr2O4 spinel oxides for methane catalytic combustion

The nano-ZnCr2O4 spinel oxides was synthesized by a ethylene glycol mediated solvothermal method. Catalytic combustion of methane test showed that an excellent activity over nano-ZnCr2O4 with T10% = 300 °C and T90% = 400 °C. The results of X-ray diffraction (XRD), scanning electron microscopy (SEM), N2 adsorption-desorption measurements (BET) indicated that a uniform nano-ZnCr2O4 spinel oxides particles with the high surface area (96.2 m2 g−1) was successfully synthesized. Oxygen temperature programmed desorption (O2-TPD) profile revealed there were two obvious desorption of oxygen species from nano-ZnCr2O4 in the range of 300–400 °C and 500–700 °C. It was clear that the desorption temperature range of the first oxygen species coincided with the methane catalytic combustion temperature. X-ray photoelectron spectroscopy (XPS) analysis exhibited that Cr6+ was present in the lattice of ZnCr2O4 apart from Cr3+. High valence cations of chromium in crystal lattice probable caused the presence of interstitial oxygen species in the structure to maintain the electroneutrality. Additionally, Raman spectra proved that there is the interstitial oxygen species in the crystal lattice of ZnCr2O4. Therefore, the excellent catalytic activity for methane combustion was contributed to the flexible interstitial oxygen in the ZnCr2O4.

Consequences of Nitrogen Doping and Oxygen Enrichment on Titanium Local Order and Photocatalytic Performance of TiO2 Anatase

Extended X-ray absorption fine structure (EXAFS) investigation of the oxygen-rich titania formed via the thermal treatment of N-doped TiO2 has revealed that the removal of N-dopants is responsible for the creation of defect sites in the titanium environment, thus triggering at high temperatures (500–800 °C) the capture of atmospheric oxygen followed by its diffusion toward the vacant sites and formation of interstitial oxygen species. The effect of the dopants on Ti coordination number and Ti–Oint and Ti–Nint bond distances has been estimated. The photocatalytic p-cresol degradation tests have demonstrated that the interband states formed by the N-dopants contribute to a greater extent to the visible-light activity than the oxygen interstitials do. However, under the UV irradiation the oxygen-rich titania shows higher efficiency in the pollutant degradation, while the N-dopants in N–TiO2 play the role of recombination sites. The presence of the surface nitrogen species in TiO2 is highly beneficial for the...

Solid-State Chemistry of Cuprous Delafossites: Synthesis and Stability Aspects

Cuprous delafossites exhibit exceptional electrical, magnetic, optical, and catalytic properties. Through the application of a battery of in situ and ex situ characterization methods complemented by density functional theory (DFT) calculations, we gathered an in-depth understanding of the synthesis of CuMO2 (M = Al, Cr, Fe, Ga, Mn) by the solid-state reaction of Cu2O and M2O3 and of their stability against oxidative disproportionation to CuM2O4 and CuO. TGA-DTA and XRD studies of the synthesis revealed that the nature of the M3+ cation strongly impacts (i) the formation temperature of the delafossite phase, which occurred at a much lower temperature for CuCrO2 than for the other metals (1073 versus 1273–1423 K), (ii) the mechanism of formation of the CuMO2 in different atmospheres, which was found to comprise up to four steps in air and a single step in N2, and (iii) the kinetics of the process, which could be significantly accelerated upon mechanochemical activation of the precursors by ball milling. The identification of unstable intermediate phases and, thus, a proper description of the synthesis mechanism was only possible by the application of in situ XRD. Electron microscopy, nitrogen sorption, and mercury porosimetry analyses of the precursor oxide mixtures at different stages of the synthesis in air revealed that particle agglomeration took place prior to the solid-state reactions forming the intermediate spinel phase and the delafossite, respectively, and that these led to a substantial drop in porosity and specific surface area. On the basis of XRD and He pycnometry, the resulting CuMO2 samples exhibit pure delafossite phase with rhombohedral structure (R3̅m), except for CuMnO2 which features a monoclinic structure (C2/m). Upon heating in air, CuCrO2 retained its structure up to 1373 K, while all other delafossites decomposed, CuAlO2 at 1073 K, CuGaO2 at 873 K, CuFeO2 at 773 K, and CuMnO2 at 673 K. The DFT-calculated surface phase diagram of CuCrO2 and CuAlO2 indicated that, at elevated oxygen pressures, the terminations with 1/2 and 0 ML of Cu are the most stable for the (0001) facet. The formation enthalpy for interstitial oxygen species in the bulk is endothermic for both delafossites, while that for oxygen insertion in subsurface layers of these terminations is still endothermic for CuCrO2 but slightly exothermic for CuAlO2. These results provide an improved understanding of the chemistry of these mixed oxides, enabling their optimization for specific applications.

Oxygen Exchange at the Internal Surface of AmorphousSiO2Studied by Photoluminescence of Isotopically Labeled Oxygen Molecules

The exchange between lattice and interstitial oxygen species in an oxide was studied by the 16O-18O isotope shift of the a1Deltag(v=0)-->X3Sigmag-(v=1) infrared photoluminescence band of the oxygen molecules (O2) incorporated into the interstitial voids of amorphous SiO2 (a-SiO2) by thermal annealing in 18O2 gas. A large site to site variation of the oxygen exchange rate, originating from structural disorder of a-SiO2, is found. The average exchange rate has an activation energy of approximately 2 eV, which is much larger than that for the diffusion of interstitial O2 (approximately 0.8-1.2 eV). The average exchange-free diffusion length of interstitial O2 exceeds approximately 1 microm below 900 degrees C, providing definite evidence that oxygen diffuses as interstitial molecules in a-SiO2.

Oxygen mobility in lanthanum nickelate catalysts for deep oxidation of propane

Nickelate catalysts were prepared by a modified Pechini method. Single phases of La 2NiO 4 (LN) and La 1.9Sr 0.1NiO 4 (LSN) were obtained after calcination at 1000 °C in air. O 2-temperature-programmed desorption (TPD-O 2) experiments have shown that, in spite of quite small specific area, this material exhibits extremely high bulk oxygen mobility since interstitial oxygen species can be exchanged with the gas phase at temperatures as low as 230 °C. Furthermore, these oxygen species can react with propane to mainly form CO 2 from 300 °C.

Diffusion and reactions of interstitial oxygen species in amorphous SiO2: A review

This article briefly summarizes the diffusion and reactions of interstitial oxygen species in amorphous SiO2 (a-SiO2). The most common form of interstitial oxygen species is oxygen molecule (O2), which is sensitively detectable via its characteristic infrared photoluminescence (PL) at 1272nm. The PL observation of interstitial O2 provides key data to verify various processes related to interstitial oxygen species: the dominant role of interstitial O2 in long-range oxygen transport in a-SiO2; formation of the Frenkel defect pair (Si–Si bond and interstitial oxygen atom, O0) by dense electronic excitation; efficient photolysis of interstitial O2 into O0 with F2 laser light (λ=157nm, hν=7.9eV); and creation of interstitial ozone molecule via reaction of interstitial O2 with photogenerated O0. The efficient formation of interstitial O0 by F2 laser photolysis makes it possible to investigate the mobility, optical absorption, and chemical reactions of interstitial O0. The observed properties of O0 are consistent with the model that O0 takes the configuration of Si–O–O–Si bond. Interstitial O2 and O0 react with dangling bonds, oxygen vacancies, and chloride groups in a-SiO2. Reactions of interstitial O2 and O0 with mobile interstitial hydrogen species produce interstitial water molecules and hydroperoxy radicals. Interstitial hydroxyl radicals are formed by F2 laser photolysis of interstitial water molecules.

Oxygen species in SiO 2: a first-principles investigation

Using a density functional approach, we determine the relative stability of various oxygen species in α-quartz. We considered interstitial species in the form of atomic and molecular oxygen, as well as network species, such as peroxyl and ozonyl linkages. The interstitial molecular oxygen species is found to be the most stable one. The ozonyl linkage lies higher in energy by 1.0 eV. Dissociation of the interstitial O 2 molecule into two peroxyl linkages or into two interstitial oxygen species occurs at even higher energies (1.6 and 3.2 eV, respectively). These results favor a long-range diffusion mechanism based on the interstitial O 2 molecule.

Determination of the visible range optical absorption spectrum of peroxy radicals in gamma-irradiated fused silica

Two silicas (Russian KS-4V and an F-doped silica) with <1 ppm cationic impurities, OH contents <5 ppm, and Cl contents <20 ppm have been investigated in optical-fiber forms by differential-optical spectroscopy following 14-MGy(Si) 60Co γ irradiation at 27°C and isochronal anneals to 600°C. Bulk samples of the F-doped silica and of two other high-purity silicas (Suprasil F300 and F310) were studied by electron spin resonance (ESR) and isochronal annealing following exposure to (i) a γ-ray dose of 12 MGy(Si) and (ii) the same 12 MGy γ irradiation plus a subsequent fission-reactor irradiation. The strength of an optical band near 2 eV is shown to correlate with the intensity of the ESR spectrum of peroxy radicals (PORs) as a function of anneal temperature. The peak energy, band width, and oscillator strength of this band are determined to be E 0=1.97 ± 0.01 eV, W=0.17 ± 0.01 eV, and f POR=0.00057 ± 0.00015, respectively. This POR band overlaps with, and has previously defied separation from, those of the non-bridging-oxygen hole centers (NBOHCs), here fitted by two bands ( E 0=2.19 ± 0.01 eV, W=0.51 ± 0.01 eV and E 0=2.08 ± 0.01 eV, W=0.32 ± 0.01 eV) with an aggregate oscillator strength f NBOHC=0.00054 ± 0.00015. The annealing processes of the PORs and NBOHCs displayed a dose-rate dependence, tentatively linked to the effect of dose rate (1 Gy/s vis-à-vis 5.3 Gy/s) on the steady-state self-trapped-hole populations, which in turn determine the equilibrium of radiolytic interstitial oxygen species: O 0 + O 0 ⇌ O 2.

Oxidative coupling of methane over Ce4+-doped Ba3 WO6 catalysts: investigation on oxygen species responsible for catalytic performance

Ce4+ doped Ba3 WO6 complex oxides were used as catalysts for methane oxidative coupling (MOC), and characterized by XPS and O2-TPD-MS techniques. The results indicate that the ratio of electrophilic oxygen species O− and O 2 − to lattice oxygen on the surface is crucial for C2 selectivity. By adjusting the relative amount of cations in Ba-W-Ce complex oxides with perovskite superstructure interstitial oxygen species can be created which benefits C2 selectivity by raising the relative amount of (O− + O 2 − ) on the surface.

EPR study of structural defects in ion-implanted multicomponent silicate glasses

Radiation defects induced by ion bombardment of multicomponent silicate (MCS) glasses are investigated and discussed. In MCS glasses, implanted with B +, P +, N +, Ag +, Sb +, W + ions at energies varying between 30 and 380 keV and doses ranging from 10 13 to 10 18 ions/cm 2, two types of electron paramagnetic resonance (EPR) signal, differing from those observed in γ-irradiated samples, are found. Ond of them, denoted by S, is a narrow, almost symmetric signal with g-values ranging from 2.0018 to 2.0034 which is attributed to a vacancy-type defect with one dangling silicon bond. The other signal, denoted by A, is an anisotropic line with g 1 = 2.015, g s = 2.008 and g 3 = 1.999 (±0.001) which is ascribed to interstitial oxygen species. It is shown that the appearance of the S- and A-signals and their relative intensity depend on the energy, dose and atomic mass of the incident ions. It is also found that an anisotropic EPR spectrum with g = 2.29 ± 0.01 and g = 2.04 ± 0.01 appears after γ-irradiation and subsequent annealing of MCS glasses implanted with Cu + ions at energies ranging from 150 to 250 keV and doses varying between 3 × 10 14 and 1 × 10 15 ions/cm 2. It is assumed that this spectrum belongs to Cu + ions which have trapped a positive hole produced by γ-irradiation, becoming Cu 2+. MCS glasses treated by TiB + and TiP + ion-mixing with boron doses of 6.2×10 15 and 3×10 17 ions/cm 2 and phosphorus dose 2 × 10 17 ions/cm 2 were studied. Ti 3+ was found to have no EPR signal, although titanium results are present from Rutherford backscattering and secondary ion mass spectroscopy measurements.

Observation of migrating superoxide species in YBa 2Cu 3( 57Co)O 7−δ

The interaction of the migrating superoxide ion O − 2 with the central cobalt-57 atom of the four coordinate species at the chain site is probed with the help of emission Mössbauer spectroscopy. The bond formation between 57Co and O − 2 requires some thermal activation, and consequently the five-coordinate species is stable only above ∼ 350 K. The equilibrium shifts in its favor at higher temperatures, and the interconversion between the two species is completely reversible. The O − 2 ion can either attach to cobalt end-on with one of the oxygen atoms or interact equivalently with both. The amount of the five-coordinate species formed seems to be limited by the availability of O − 2. The interconversion ceases to occur after a prolonged thermal treatment of the YBa 2 Cu 3( 57 Co) O 7−δ pellet at 420°C under argon flow. All the interstitial oxygen species are presumably removed without any measurable loss of oxygen from the Cu-O chains.