Paramagnetic Oxygen Vacancies Research Articles

Mechanistic understanding of the thermal-assisted photocatalytic oxidation of methanol-to-formaldehyde with water vapor over Pt/SrTiO3.

Anaerobic thermal-assisted photocatalytic methanol conversion in the gas phase in the presence of water vapor has been suggested as an interesting way to generate formaldehyde as a valuable coupled product in addition to H2 production. Here, the reaction mechanism and photocatalyst deactivation are investigated in detail using in situ diffuse reflectance infrared fourier transform (DRIFTS) and electron paramagnetic resonance (EPR) spectroscopy. EPR shows that paramagnetic oxygen vacancies are not involved in the reaction mechanism over undoped SrTiO3. Instead, on an optimized 0.1 wt% Pt/SrTiO3 photocatalyst, methoxy species are formed by dissociative adsorption of methanol leading to formaldehyde formation while the formation of CO, CO2 (via a formate intermediate) and methyl formate occurs through three concurrent reactions from formyl species. Our findings suggest that CO adsorbed on Pt is a spectator species not perturbing the reaction kinetics, and deactivation is shown to be strongly correlated with the accumulation of formate groups on SrTiO3, which is more pronounced at high reaction temperatures. The mechanistic understanding provided here forms the basis for the further optimization of photocatalysts to increase methanol conversion and improve formaldehyde selectivity.

Reconstructing dust provenance from quartz optically stimulated luminescence (OSL) and electron spin resonance (ESR) signals: Preliminary results on loess from around the world

Quantitative provenance analysis studies are instrumental in understanding the tectonic and climatic processes that shape the earth's landscape. Although the most abundant mineral in the sedimentary system is quartz, almost all studies in provenance analysis investigate accessory minerals. Quartz crystals contain a vast number of point defects, intrinsic or due to impurities. For a signal to be an accurate indicator of provenance one needs to show that it is either dose independent or reaches a quantifiable steady state characteristic of the source rock. For signals used by trapped charge dating methods (optically stimulated luminescence (OSL) and electron spin resonance (ESR)), the latter option is the feasible one. By using quartz samples collected from the Chinese Loess Plateau (Luochuan loess-paleosol section), we show that the laboratory and natural dose response curves of E'1 and peroxy electron spin resonance signals of quartz (as defined later) overlap and reach a steady state for doses over about 1000 Gy. For E′1 signals we attribute this steady state to reaching an equilibrium state between diamagnetic oxygen vacancies (the oxygen deficiency centre (ODC), Si=Si) and paramagnetic oxygen vacancies (E′1). For sedimentary quartz irradiated naturally or artificially in this dose range we show a strong linear relationship with zero intercept between E′1 and peroxy signals for samples worldwide, supporting the hypothesis that these defects are Frenkel pairs. Further, we show significant correlations between the optically stimulated (OSL) sensitivity and the above two mentioned ESR signals. The very strong correlations (Pearson's r >0.9) between E′1, peroxy and OSL sensitivity remain valid after the samples have been heated for 15 min to 350 °C for E′1 to reach its maximum value, believed to be a result of the conversion of diamagnetic oxygen vacancies to E′1, clearly suggesting a relationship between OSL sensitivity and oxygen vacancies in general. Samples collected from different loess sites around the world can be distinguished based on both these OSL and ESR properties. An empirical increase in OSL sensitivity as well as oxygen related defect concentrations is observed in areas where the source material has components with older detrital zircon U–Pb ages, inferring a positive correlation between OSL sensitivity, as well as the signal intensity for E1’ and peroxy defects and the age of the source rocks.

Effects of oxygen vacancy concentration and sintering temperature on rechargeable Li-ion storage performance of titanium niobate anode materials

In this study, TiNb2O7 (TNO) was prepared by a simple solvothermal method and subsequently high temperature sintering process as anode materials for Lithium-ion batteries (LIBs). The surface morphology and oxygen vacancies of as-prepared TNO were investigated by using different sintering temperatures from 700oC to 1000 °C, namely TNO-700, TNO-800, TNO-900 and TNO-1000, respectively. The crystal structure, surface morphology as well as pore size distribution and oxygen vacancy content of a series of TNO anodes were characterized by X-ray diffraction (XRD), scanning electronic microscopy (SEM), high-resolution transmission electron microscopy (HRTEM), surface area and pore analyzer (N2 adsorption/desorption isotherms), electron paramagnetic resonance spectrometer (EPR) and X-ray photoelectron spectrometer (XPS), respectively. The condition-optimized TNO-800 electrode exhibited brilliant rate capability and the highest reversible capacity which resulted from the micro-spherical structure and enough oxygen vacancies in terms of 246 mAh/g after 250 cycles and 248 mAh/g at 20C, respectively. The XPS analysis proved that samples in lower sintered temperatures had more oxygen vacancies from Ti/Nb peak shifting and integrated area ratio of the main peak to the shoulder peak in the O 1s spectrum. In addition, they also exhibited a stronger EFP signal than that of other samples at g = 1.99 which means the signal of paramagnetic oxygen vacancies. These results indicated that the porous structure and high specific surface area, as well as the existence of oxygen vacancies, play important roles in the electrochemical performance of TNO anode materials for LIBs. Hence, it is an effective manner to enhance the Li-ion storage applications for anode materials from this research.

Structural, optical, photoluminescence, and EPR behaviour of novel Zn0·80Cd0·20O thick films: An effect of different sintering temperatures

Zn0·80Cd0·20O (ZnCdO) alloy coated screen-printed films on glass substrates were sintered in muffle furnace at different temperatures (500 °C, 550 °C & 600 °C) for 10 min duration and probed by different analytical techniques to reveal the effect of sintering on their structure, optical and electrical characteristics. X-ray diffraction (XRD) patterns exhibit polycrystalline phases of both hexagonal ZnO and cubic (CdO) crystal structures with a maximum intensity along (101) diffraction plane. UV-VIS and photoluminescence (PL) absorbance peaks exhibit red shift in their respective spectra. Zn–O, Cd–O stretching modes were observed in infrared transmission spectra of the raw and sintered films in 4000–400 cm−1 region, respectively. EPR studies exhibit the formation of oxygen vacancies and inherent defect in the Cd alloyed ZnO composite developed during synthesis process. All the parameters such as particle size, optical constants, activation energies and paramagnetic oxygen vacancies have been affected by sintering temperature.

Microspherical core-shell MoO2-graphitic C3N4 heterojunction promoted integration leading to Kröhnke pyridines and degradation of xylenol orange

In the current study, we fabricated a microspherical core-shell consisting of MoO2 and graphitic-C3N4 by a simple one-step calcination procedure. TEM analysis revealed MoO2 at the core (average diameter of 210 nm), surrounded by a graphitic-C3N4 shell. MoO2 of the core exhibited monoclinic symmetry. The Raman spectrum additionally confirmed the monoclinic MoO2. The sample was EPR active, indicating paramagnetic oxygen vacancies. It contained meso-sized pores and a high surface area. The core-shell’s optical bandgap was higher than the pure metallic MoO2. XPS results confirmed the existence of molybdenum in IV and VI oxidation states. The core-shell’s heterojunction catalyzed the degradation of xylenol orange (XO) dye into C8H9ONH3+, C6H5SO3− and C15H14ONH3+SO3−. The core-shell promoted the efficient synthesis of symmetrical and unsymmetrical 2, 4, 6-triaryl pyridines through the oxidative condensation of acetophenones with benzylamines in good to excellent yields using molecular oxygen as the oxidant. The catalyst is recyclable for both these reactions.

Ag-doping on ZnO support mediated by bio-analytes rich in ascorbic acid for photocatalytic degradation of dipyrone drug

The analytes such as ascorbic acid (AA) present in Sechium edule were extracted (294 mg AA kg−1 fruit) in an aqueous media for its potential application for Ag-doping onto wurtzite ZnO. The bandgap of ZnO was decreased to 2.85 eV at the optimal Ag-loading of 1.18% (w/w) against 3.13 eV for the control catalyst without using the analytes and, the commercial AA only could reduce the bandgap to 2.91 eV. The saturation photo-electrochemical current density (46.68 mA cm−2) at Eanode ≥ 0.31 V vs. Ag/AgCl was almost double than pristine ZnO under visible light illumination (λmean = 525 nm, 18 K lux) and, the current density was insignificant in the dark. The doped catalyst exhibited the maximum 79.5% degradation (71% COD removal) of an anti-analgesic drug, dipyrone (100 μg L−1 dipyrone, catalyst 100 mg L−1) resulted from the formation of O2•− radical (g-factor of 2.002–2.008) and paramagnetic oxygen vacancies (g-factor of 2.020) and, no effect of dye-sensitization was noted. The highest quantum yield was found to be 34.7%. The catalyst loss was 6% after the fourth cycle and the dipyrone degradation was reduced to 70.8%.

First principle study of tritium trapping at oxygen vacancies in Li4SiO4

Tritium trapping at point defects is a major issue concerning tritium extraction efficiency in solid state breeder materials in future fusion reactors. Here, tritium trapping behaviors at oxygen vacancies in Li4SiO4 have been investigated by density functional theory simulations. Formation energies have been calculated to determine the stability of defects in various charge states. Density of states, charge distribution and atomic charge has been calculated to investigate the mechanism of defect formation. The results showed that the neutral and 2 + charged oxygen vacancies are the most stable species and are both diamagnetic. The 1 + charged paramagnetic oxygen vacancy (E′ center) is less stable. In addition, the tritium-trapped O-vacancy is most stable when the defect complex is 1 + charged. The trapped tritium is bonded to Si forming a T-SiO3 unit. Other charge states of the complex may lead to unstable defect structures and high formation energies, forcing tritium to escape the vacancy.

Benign Synthesis of Black Microspheres of Anatase TiO2 with Paramagnetic Oxygen Vacancies through NH3 Treatment.

Coloured TiO2 is coveted for its ability to extract energy from the visible region of electromagnetic spectrum. Here a facile synthesis of black anatase titania microspheres (B-TiO2 ) through a two-step process is reported. In the first step, amorphous white TiO2 microspheres (W-TiO2 ) are obtained by hydrolysing titanium tetraisopropoxide by ammonia vapours in ethanol. In the second step, the W-TiO2 is thermally annealed at 500 °C to obtain B-TiO2 . The diffuse reflectance analysis showed that B-TiO2 absorbs across visible spectrum with absorption extending well into NIR region. Raman scattering together with EPR analysis showed compelling evidence of the existence of oxygen deficiency within the crystal in B-TiO2 that induces black colouration in the sample. The defects present in the black anatase sample were confirmed to be single-electron-trapped (or paramagnetic) oxygen vacancies (Vo ⋅) by XPS and EPR studies. The magnetic susceptibility studies showed existence of antiferromagnetic interactions between these unpaired electron spins.

ZnO gap states investigated using magnetic circular dichroism

We investigate gap states in a series of nonmagnetic ZnO films grown by molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) using magnetic circular dichroism. This technique is shown to be sensitive enough to investigate defect states in thin films of disordered wide-gap semiconductors by differentiating between a loss of transmission due to scattering and absorption. The method is first applied to Zn- and O-terminated films grown by MBE. The O-terminated film shows a broad peak centred at ~2.4 eV which corresponds to the green PL signal, whereas the Zn-terminated film shows a magnetic circular dichroism (MCD) signal that increases slowly with energy. The method was then extended to PLD films. Oxygen vacancy states in both MBE and PLD-grown films were identified, and the introduction of low concentrations of Al was shown to quench the MCD signal at low energy by neutralising the paramagnetic oxygen vacancies. In PLD-grown ZnAlO we observed a negative MCD that increased in magnitude as the energy approached the band edge indicating a much more pronounced spin-split impurity and conduction band. These examples illustrate the versatility of the MCD technique in identifying gap states in a variety of different types of ZnO samples.

Oxygen defect assisted paramagnetic to ferromagnetic conversion in Fe doped TiO2nanoparticles

Here we report the magnetic properties of Fe doped TiO2 nanoparticles annealed in air and vacuum. Fe doped TiO2 nanoparticles display paramagnetism during air annealing and ferromagnetism when the samples are annealed under vacuum. Ferromagnetism in vacuum annealed pure TiO2 nanoparticles is because of the presence of a high density of paramagnetic oxygen vacancy centers. Ferromagnetism in vacuum annealed Fe doped TiO2 is because of the strong exchange interaction of lattice site Fe3+ ions with the nearest paramagnetic oxygen vacancies by forming bound magnetic polarons (BMPs). A collective contribution of the magnetic moment from the bound magnetic polaron on the lattice site and from the paramagnetic oxygen vacancies on the surface and on the grain boundary results in the conversion of paramagnetism to ferromagnetism in vacuum annealed Fe doped TiO2.

Facile Synthesis of Thermal‐ and Photostable Titania with Paramagnetic Oxygen Vacancies for Visible‐Light Photocatalysis

A novel dopant-free TiO(2) photocatalyst (V(o)(.)-TiO(2)), which is self-modified by a large number of paramagnetic (single-electron-trapped) oxygen vacancies, was prepared by calcining a mixture of a porous amorphous TiO(2) precursor, imidazole, and hydrochloric acid at elevated temperature (450 °C) in air. Control experiments demonstrate that the porous TiO(2) precursor, imidazole, and hydrochloric acid are all necessary for the formation of V(o)(.)-TiO(2). Although the synthesis of V(o)(.)-TiO(2) originates from such a multicomponent system, this synthetic approach is facile, controllable, and reproducible. X-ray diffraction, XPS, and EPR spectroscopy reveal that the V(o)(.)-TiO(2) material with a high crystallinity embodies a mass of paramagnetic oxygen vacancies, and is free of other dopant species such as nitrogen and carbon. UV/Vis diffuse-reflectance spectroscopy and photoelectrochemical measurement demonstrate that V(o)(.)-TiO(2) is a stable visible-light-responsive material with photogenerated charge separation efficiency higher than N-TiO(2) and P25 under visible-light irradiation. The V(o)(.)-TiO(2) material exhibits not only satisfactory thermal- and photostability, but also superior photocatalytic activity for H(2) evolution (115 μmol h(-1) g(-1)) from water with methanol as sacrificial reagent under visible light (λ>400 nm) irradiation. Furthermore, the effects of reaction temperature, ratio of starting materials (imidazole:TiO(2) precursor) and calcination time on the photocatalytic activity and the microstructure of V(o)(.)-TiO(2) were elucidated.

Role of crystal defects in red long-lasting phosphorescence of CaMgSi2O6:Mn diopsides

CaMgSi2O6:Mn diopsides are used for in vivo long-lasting phosphorescence (LLP) imaging. Trapping defects involved in the LLP mechanism were investigated. On annealing Mn-doped diopsides at 1100 °C in an Ar–H2 atmosphere, Ca EXAFS/XANES and electron paramagnetic resonance (EPR) evidenced paramagnetic oxygen vacancies while X-ray diffraction, Mn XANES and EPR revealed SiO2 formation and significant evaporation of the Mn dopant. A thermally stimulated luminescence (TSL) peak at 475 K ascribed to electron trapping at oxygen vacancies was found responsible for LLP at RT. Most intense red LLP suitable for in vivo imaging was achieved by a trade-off between a high MnII content (favourable to MnIIMg red over MnIICa orange luminescence) and the formation of oxygen vacancies favourable to LLP and luminescence light yield. Mn XANES revealed an effective charge larger than 2+ for MnII, in line with the role of hole trap in the LLP mechanism. Compounds annealed at lower temperatures (800 °C and 900 °C) in Ar–H2 showed smaller particle size (60–70 nm) and maximum Mn content but poor luminescence and LLP due to surface quenching defects.

Point defects induced in yttria-stabilized zirconia by electron and swift heavy ionirradiations

We present an extensive study of point-defect creation in yttria-stabilized zirconia (ZrO2:Y) exposed to 2.5 MeV electrons and various heavy ions (from C to U) covering an energy rangefrom 100 MeV to several GeV. A synthesis of results from UV–visible optical absorptionspectroscopy and electron paramagnetic resonance spectroscopy is provided with specialemphasis on the respective roles of elastic collisions and electronic excitations. The colourcentre production and recovery are the main focus in this survey. It is concluded thatF + -type centres (involving singly ionized oxygen vacancies) are produced by elastic-collision processes.The large threshold displacement energy and defect volume hint that these colour centresmight actually be small paramagnetic oxygen vacancy clusters, most probably divacancies (i.e. F2 + centres). Such a picture is consistent with the⟨100⟩ axial symmetry, inhomogeneous broadening of the optical absorption band, lack ofhyperfine splitting, and weak spin–lattice coupling found for this defect.

Investigating the role of hydrogen in indium oxide tubular nanostructures as a donor or oxygen vacancy passivation center

Electron paramagnetic resonance (EPR) spectroscopy of hydrogen-doped indium oxide (IO) tubular nanostructures shows presence of paramagnetic oxygen vacancies (VO) at room temperature. For temperatures below 80 K, the EPR spectra exhibit two distinct split resonances correspond to S=12 hydrogen electron spin. Interestingly, presence of hydrogen EPR resonances is accompanied by absence of EPR signal of VO, which is restored above 80 K with the concomitant disappearance of signature resonances from hydrogen. The temperature dependent donor and passivation behavior of hydrogen has been directly observed in metal oxide. This could provide valuable explanations of various VO induced controversial properties of IO nanostructures.

Observation and manipulation of paramagnetic oxygen vacancies in Co-doped TiO2 nanocrystals

Electron paramagnetic resonance measurements were presented to investigate paramagnetic oxygen vacancies (F+ centers) in Co-doped TiO2 nanocrystals. Surface and interior F+ centers were manipulated by washing or/and annealing. Anisotropic surface F+ centers are so instable as to be destroyed at high temperature, while isotropic interior F+ centers are rather stable even at 430°C. The correlation of ferromagnetism with F+ centers was observed, which provided direct experimental evidence for F+-center-mediated coupling in diluted magnetic oxides.

The Role of Oxygen Vacancies on the Microstructure Development and on the Electrical Properties of SnO2-Based Varistors

Variations on the microstructure development and on the electrical properties of SnO2-based varistors are discussed on the basis of the oxygen vacancies created or annihilated by the presence of different additives. Electron paramagnetic resonance (EPR) analysis of sintered samples evidenced a substantial increase in the paramagnetic oxygen vacancies concentration when Nb2O5 is added to the SnO2 ⋅ Co3O4 system. On the other hand, the observed diminution in the concentration of such species after the addition of Fe2O3 indicates solid solution formation. The quantification of paramagnetic oxygen vacancies allowed to confirm the proposed substitutions taking place in the lattice during sintering. These findings are supported by scanning electron microscopy, by density measurements and by current density versus electric field curves. The characterization of secondary phases through EDS assisted SEM and TEM is also reported in this work.

Nanostructured Pt-Doped Tin Oxide Films: Sol−Gel Preparation, Spectroscopic and Electrical Characterization

Nanostructured (3-6 nm) thin films (80 nm) of SnO 2 and Pt-doped SnO 2 were obtained by a new sol-gel route using tetra(tert-butoxy)tin(IV) and bis(acetylacetonato)platinum(II) as metal precursors. The results of glancing incidence X-ray diffraction (GIXRD) and X-ray photoelectron spectroscopy (XPS) investigations demonstrated that platinum substituted for tin(IV) in the cassiterite structure. Electron paramagnetic resonance (EPR) and XPS analyses showed that singly ionized paramagnetic oxygen vacancies (V O .) were formed on pure SnO 2 thin films by interaction with CO atmosphere; in Pt-doped SnO 2 films, the same defects V O . fully transferred their electrons to the noble metal so that Pt(IV) became Pt(II) and Pt(0). Such samples successively exposed to air, at room temperature, reduced O 2 to O 2 - . The behavior was well-detected by EPR measurements, which showed on thin films the presence of Sn(IV)-O 2 - species. The surface reactivity agrees with the results of the electrical measurements.

Theoretical characterization of charge-transfer reactions between N 2 and O 2 molecules and paramagnetic oxygen vacancies on the MgO surface

In this work we have studied computationally, at the Hartree–Fock and density functional levels, the interaction of O 2 and N 2 molecules with paramagnetic oxygen vacancy ( F S + centers) located at the (1 0 0) MgO surface. We have employed the molecular cluster approach, which describes the adsorption site by means of a limited number of ions embedded in a large array of point charges, as well as the more sophisticated perturbed cluster method, where the selected cluster is self-consistently embedded in the quantum-chemical description of the regular host crystal. We found that, at short distances from the surface, an electron transfer from the surface to the admolecules occurs, with formation of charge-transfer complexes F S 2+/N 2 − or F S 2+/O 2 − that are stabilized by the electrostatic interaction between the charged fragments. However, F S 2+/O 2 − is stable with respect to the dissociation into F S ++O 2 fragments, while F S 2+/N 2 − results to be a metastable product.

Disorder and thermal transport in undoped KTaO3

The thermal conductivity of nominally pure KTaO3 exhibits an unexpected strong phonon scattering around 8 K. The question arises whether this scattering is related to the distorted microregions observed by other experiments. We have investigated a set of undoped single crystals grown by different methods. By comparison with doped specimens, it is suggested that acoustic phonons are coupled to the soft optic mode via the local disorder. From complementary dielectric permittivity and electron paramagnetic resonance experiments, oxygen vacancies and rhombic Fe3+ appear to be incontestable candidates to account for the distorted microregions.

Modifications of the atomic and electronic structure of quartz by high-energy ion irradiation

Abstract the response of SiO2 quartz to ion-induced dense electronic excitations and the nature of the track-like damage are the main issues of the present work. The atomic structure of ion-irradiated quartz is probed by means of X-ray absorption spectroscopies (XANES, EXAFS). Modifications in the electronic band structure are investigated by X-ray photon spectroscopy (XPS) and bulk paramagnetic defects are observed by Electron Spin Resonance (ESR). Quartz targets irradiated by light ions are found to preserve their long-range order. E’ point defects, i.e. paramagnetic oxygen vacancies, are created along the ion path with a poor efficiency. For heavier ions, E’ defects essentially reside in amorphous zones. The apparent density of these defect-rich, amorphized regions is ∼3% larger than the density of common relaxed silica. EXAFS data suggest very drastic reductions of the Si-O-Si angle, probably resulting from the large internal strain caused by ion irradiation. This decrease of the bridging angle should...