Valence State Of Co Ions Research Articles

Sr-substitution effects on crystal structure and thermoelectric properties of misfit layered cobaltate, [Ca2CoO3]pCoO2

Misfit-layered cobaltate [Ca2CoO3]pCoO2 is a promising thermoelectric (TE) material that can operate at high temperature in air. Partial substitution of Ca with Sr has been suggested to enhance TE properties. To comprehend the unresolved effects of Sr-substitution on the TE properties, we have prepared single-phase polycrystalline samples of Sr-substituted [Ca2CoO3]pCoO2, i.e., [(Ca1-xSrx)2CoO3]pCoO2, using a spark plasma sintering (SPS) method followed by O2 annealing. We analyzed the crystal structure by powder X-ray diffraction, evaluated the average valence state of Co ions by X-ray photoelectron spectroscopy, and measured temperature-dependent TE properties. Upon Sr-substitution, the average valence state of Co ions slightly increased. The electrical resistivity perpendicular to the SPS pressing direction increased, while that along the SPS pressing direction was almost unchanged. The Seebeck coefficient and the thermal conductivity were less affected by the Sr-substitution. The figure-of-merit zT perpendicular to the SPS pressing direction tended to decrease with Sr-substitution, reflecting the increase in electrical resistivity.

Temperature-Driven Anisotropic Mg2+ Doping for a Pillared LiCoO2 Interlayer Surface in High-Voltage Applications.

High-voltage lithium cobalt oxide (LiCoO2) has the highest volumetric energy density among commercial cathode materials in lithium-ion batteries due to its high working voltage and compacted density. However, under high voltage (4.6 V), the capacity of LiCoO2 fades rapidly due to parasitic reactions of high-valent cobalt with the electrolyte and the loss of lattice oxygen at the interface. In this study, we report a temperature-driven anisotropic doping phenomenon of Mg2+ that results in surface-populated Mg2+ doping to the side of the (003) plane of LiCoO2. Mg2+ dopants enter the Li+ sites, lower the valence state of Co ions with less hybridization between the O 2p and Co 3d orbitals, promote the formation of surface Li+/Co2+ anti-sites, and suppress lattice oxygen loss on the surface. As a result, the modified LiCoO2 demonstrates excellent cycling performance under 4.6 V, reaching an energy density of 911.2 Wh/kg at 0.1C and retaining 92.7% (184.3 mAh g-1) of its capacity after 100 cycles at 1C. Our results highlight a promising avenue for enhancing the electrochemical performance of LiCoO2 by anisotropic surface doping with Mg2+.

The pivotal role of Ag species on porous nanosheets in the significant reduction of soot ignition temperature

As a major technology, catalytic combustion is now widely used in reducing harmful soot emissions from diesel combustion. However, large amounts of soot are still emitted to atmosphere due to the lower exhaust temperature (below 200 °C) of diesel engines caused by frequent idling and cold start. Therefore, more and continued efforts are still needed to reduce the ignition temperature of T10 (at which 10 % of soot is converted) in terms of energy efficiency and environmental sustainability. In this work, an Ag-supported CoCe porous nanosheets catalyst was developed, through which a T10 < 230 °C was achieved under loose contact mode without the assistance of other atmosphere such as NOx. The detailed characterization of catalysts indicated that the motionless and mobile Ag particles, together with the regulation of valence state of Co ions by the abundant Ag+ species derived from Ag-Co interface are responsible for the ultra-low soot ignition temperature. Density functional theory (DFT) calculations proved that the electronic structure of Co can be tuned by the interactions between Ag and the support, which would promote the adsorption of O2. More importantly, the synergistic effect between Ag and Co, along with the formed interface can lower the dissociation energy barrier of O2, thus achieving a process with fast kinetics. In summary, the appropriate design of catalysts for soot combustion needs to consider both intrinsic activity and morphology, so that a significant reduction of soot emissions from vehicle exhaust can be achieved at low temperature.

Defect-induced modulation of magnetic, electronic, and optical properties of the double-perovskite oxide La2CoMnO6

Electron- and hole-doped La$_2$CoMnO$_6$(LCMO) are investigated using first principles DFT calculations. Hole and electron doping are achieved respectively by introducing Sr$^{2+}$ at La$^{3+}$ sites and by inducing O-site vacancies in LCMO. Electronic structure calculations suggest that hole doping alters the charge and valence state of Co ions, whereas electron doping influences the Mn ions. Introduction of defects is found to enhance antisite disorder(ASD) at Co/Mn site, which is expected to influence the magnetodielectric properties of the system. Our calculations suggest that while ASD and/or hole doping induces half-metallicity in LCMO, electron doping restores its insulating state. Mean-field calculations performed using exchange constants obtained by mapping DFT total energies of different collinear spin configurations onto the Ising Hamiltonian find that defects tend to reduce the Curie temperature ($T_C$). Interestingly, the calculated linear optical properties of the system suggest that the material becomes optically active with high values of birefringence in the presence of defects, a property that is highly sought-after in the optical communications and laser industry.

Thickness-dependent magnetic and electrical transport properties of epitaxial La0.7Sr0.3CoO3 films

The thickness-dependent magnetic and electrical transport properties of nearly strain-free La0.7Sr0.3CoO3 (LSCO) films grown on (001)-oriented (LaAlO3)0.3 (Sr2AlTaO6)0.7 substrates were systematically studied. A crossover from ferromagnetic/metallic to non-magnetic/insulating behavior occurs at a critical thickness (∼8 nm) that is significantly smaller than LSCO films under larger strains in reported literature. X-ray absorption measurements revealed that the difference of functional properties at reduced film thicknesses was accompanied by changes in the valence state of Co ions at the film/substrate interface.

Characterization and evaluation of Ba-doped BaxSr1−xCo0.9Sb0.1O3−δ as cathode materials for LaGaO3-based solid oxide fuel cells

BaxSr1−xCo0.9Sb0.1O3−δ (BSCSbx, x = 0.0–0.8) are investigated as cathodes for intermediate temperature solid oxide fuel cells (IT-SOFCs). BSCSbx oxides are crystallized in a tetragonal structure with s.g. P4/mmm. XPS analysis shows that Ba doping reduces valence state of Co ions: Co in SrCo0.9Sb0.1O3−δ is prone to Co4+, while Co in Ba doped SrCo0.9Sb0.1O3−δ becomes more prone to Co3+. Oxygen deficiency δ increases with Ba doping even at high temperatures. Conductivity is found to decrease with Ba doping. Thermal expansion coefficient (TEC) decreases from 20.6 × 10−6 K−1 for x = 0.0 to 18.4 × 10−6 K−1 for x = 0.6 at 30–1000 °C. With increasing x from 0.0 to 0.6, polarization resistance (Rp) decreases monotonously, and then increases with further increasing x. The lowest Rp of 0.081 Ω cm2 at 700 °C is obtained for BSCSb0.6 cathode. At 700–850 °C, the Rp of BSCSb0.6 is mainly contributed from electron charge transfer; however, below 700 °C, ion charge transfer plays a more important role in Rp. Maximum power densities of the cell with BSCSb0.6 cathode on 300-μm thick La0.9Sr0.1Ga0.8Mg0.2O3−δ (LSGM) electrolyte attain 306–944 mW cm−2 at 700–850 °C. These results indicate that BSCSb0.6 is a promising cathode for application in IT-SOFC.

In-situ Growth of Ultrathin ZIF-67 Nanosheets on Conductive Ti@TiO2/CdS Substrate for High-efficient Electrochemical Catalysis

Efficient charge transport is a key to the successful design of electrocatalyst. In this work, we report the in-situ growth of ultrathin MOF material (ZIF-67) nanosheets on conductive Ti@TiO2/CdS substrate for high-efficient electrochemical catalysis due to optimized charge transport. The ultrathin ZIF-67 nanosheets were grown on Ti@TiO2/CdS nanowire array substrates resulting unique 3D hierarchical structures, which were investigated as electrocatalyst for the oxygen-evolution reaction (OER) and H2O2 oxidation. The nanowire-supported ZIF-67 nanosheet electrode shows remarkable electrocatalytic activity and excellent stability toward OER and H2O2 oxidation, which can be explained by the rapid electron transport along the 1D Ti@TiO2/CdS nanowire to Ti substrates, large electrochemical active surface area and the rising valence state of Co ions induced by electronegative N atom in imidazole ligands. Compared with other MOFs catalysts, we obtained a very small Tafel slope (42mV/dec) and a small overpotential (0.41V) at 10mAcm−2 for OER. When used for H2O2 detection, the electrode gave a high sensitivity of 1214.3μAmM−1cm−2, a wide linear range of 5μM–14mM and a low detection limit of 1.11μM. This research suggests that the in-situ grown nanostructure of Ti@TiO2/CdS/ZIF-67 hold great promise for advanced electrocatalytic electrode in water oxidation and H2O2 detection.

Intrinsic ferromagnetic coupling in Co3O4 quantum dots activatedby graphene hybridization

Activating ferromagnetic couplings of transition-metallic ions in the antiferromagnetic metal oxide semiconductors is desired for creating ferromagnetic semiconductors for spintronics applications. Here, we report intrinsic ferromagnetic coupling in a typical antiferromagnetic metal oxide Co3O4, by virtue of a hybrid structure that modifies the valence state of Co ions. The Co3O4 quantum dots exhibit ferromagnetism of 2.2 emu/g at 2 K after hybridization with reduced graphene oxide (RGO). In this hybrid structure, electron-transfer from RGO to Co3O4 occurs and Co3+ ions occupying the octahedral (Oh) positions are converted into Co2+. Then the super-exchange interactions between Co2+ ions at Td (tetrahedral) and Oh positions switch the magnetic coupling of Co2+(Td)–Co2+(Td) from antiferromagnetic to ferromagnetic. These results offer promise for tailoring the spin exchange interactions of oxide semiconductors for spintronics applications.

Ferromagnetic and Photocatalytic Properties of Layered Perovskite LaBaCo2O6 Nanostructures

Cationic ordered layered tetragonal perovskite phase LaBaCo₂O₆ (LBCO) nanomaterials with irregular shape, and an average diameter of about 100 nm were successfully prepared. A precursor material was annealed in an argon atmosphere at 1100 °C for 48 hours, and then heat treated in an oxygen atmosphere pressure at 800 °C for 12 hours. The resulting LBCO nanomaterials show ferromagnetic transition at about 175 K without any structural changes. They show metallic behavior at below 140 K, and adopt the behavior of soft ferromagnetic materials at 80 K. Cationic ordered LBCO nanomaterials display photocatalytic activities, as they successfully degrade MB solutions under both UV irradiation and visible light. The B-site cations act as the active center, and are located in the centre of BO₆ octahedron units. The various valence states of Co ions in LBCO could contribute to the enhancement of photocatalytic activies.

Influence of Tb substitution on electromagnetic and microwave absorption properties of barium hexaferrites

Rare earth element Tb substituted hexaferrite Ba1−xTbxCo2Fe16O27 (where x = 0·00–0·25) are synthesised by solid state reaction method. The samples present single W type phase and homogenous grain size. The substitution of Tb3+ decreases the lattice parameter c and cell volume of ferrite. The imaginary part of complex permittivity ϵ″ shows a tendency of decrease with the increasing Tb content. The imaginary part of complex permeability μ″ increases with the increasing Tb content in the frequency range of 0–12 GHz. The magnetic loss tan δm of sample, with Tb content x = 0·25, increases about 0·20 after being substituted by Tb3+. As a result, the microwave absorption properties of Tb substituted ferrites are enhanced in the investigated frequency. The X-ray photoelectron spectroscopy results indicate that the Fe exists as mixed valence state of Fe(II) and Fe(III), and the valence state of Co ions is unchanged as Co2+ after the substitution of Ba2+ with Tb3+ in hexaferrites.

Analysis of Charge Compensation Mechanisms in Pr1-xAxCoO3-δ(A = Ca, Sr) by X-ray Absorption Near-Edge Structure

The charge compensation mechanisms in polycrystalline Pr1-xCaxCoO3-δ and Pr1-xSrxCoO3-δ here synthesized by the solid-state reaction method have been investigated by analyzing the Pr-L3 and Co-K X-ray absorption near-edge structure (XANES) with the aid of first-principles calculations. The valence states of Pr ions in these materials were determined to be trivalent (Pr3+) and independent of the concentration of alkaline-earth ions observed in the Pr-L3 XANES profile. The Co-K XANES spectra shifted to the higher-energy side with an increase in alkaline-earth concentration and were examined in detail by first-principles calculations. Using calculated XANES results, we successfully determined that the valence state of Co ions is an intermediate value between 3+ and 4+ and increases with an increase in the concentration of alkaline-earth ions in Pr1-xAxCoO3-δ (A = Ca, Sr) accompanying oxygen vacancy.

Analysis of Charge Compensation Mechanisms in Pr1-xAxCoO3-δ (A = Ca, Sr) by X-ray Absorption Near-Edge Structure

The charge compensation mechanisms in polycrystalline Pr1-xCaxCoO3-δ and Pr1-xSrxCoO3-δ here synthesized by the solid-state reaction method have been investigated by analyzing the Pr-L3 and Co-K X-ray absorption near-edge structure (XANES) with the aid of first-principles calculations. The valence states of Pr ions in these materials were determined to be trivalent (Pr3+) and independent of the concentration of alkaline-earth ions observed in the Pr-L3 XANES profile. The Co-K XANES spectra shifted to the higher-energy side with an increase in alkaline-earth concentration and were examined in detail by first-principles calculations. Using calculated XANES results, we successfully determined that the valence state of Co ions is an intermediate value between 3+ and 4+ and increases with an increase in the concentration of alkaline-earth ions in Pr1-xAxCoO3-δ (A = Ca, Sr) accompanying oxygen vacancy.

Co-L3 X-ray absorption near-edge structure analysis of Pr1−xCaxCoO3−δ and Pr1−xSrxCoO3−δ

Abstract The valence state of Co ions in Pr 1− x Ca x CoO 3− δ and Pr 1− x Sr x CoO 3− δ has been investigated by an analysis of the Co-L 3 X-ray absorption near-edge structure (XANES) profile. The observed intensity distributions of Co-L 3 XANES change continuously with increasing concentration of alkaline-earth ions. To investigate the origin of this change in the XANES profile, charge transfer multiplet calculations were carried out, which could successfully explain the change in the spectral profile; they also suggest that the valence state of Co ions in Pr 1− x Ca x CoO 3− δ and Pr 1− x Sr x CoO 3− δ is between 3+ and 4+ and increases gradually with the concentration of alkaline-earth ions.

Thermoelectric and Magnetic Properties of [(Ca1-xPbx)2CoO3.1]0.62CoO2 (0≦x≦0.03)

We have prepared polycrystalline [(Ca1-xPbx)2CoO3.1]0.62CoO2 (0≦x≦0.03) samples by a conventional solid-state reaction method and investigated the effects of Pb substitution on the thermoelectric and magnetic properties of the samples. With Pb substitution, both electrical resistivity and Seebeck coefficient do not change markedly. This is attributed to the carrier concentration of the samples. Seebeck and Hall coefficient measurements reveal that the major charge carriers in the samples are holes, however, the carrier concentration does not change with increasing x. Neutron powder diffraction and magnetic susceptibility measurements also reveal that Pb ions are in a divalent state in a rock salt-type Ca2CoO3 block layer. The average valence state of Co ions in a CdI2-type CoO2 sheet is 3.1+ and that of Co ions in the block layer is 3.6+. The resulting dimensionless figure of merit for an x=0.02 sample at room temperature is 0.024, which is approximately equal to the corresponding value of a polycrystalline NaCo2O4 sample.

Observation of magnetization jump associated with a spin-state transition in oxygen-deficientY0.33Sr0.67CoO3−δ

dc magnetization, electronic transport, and structural properties are studied on the mixed valence rare earth Sr-doped cobaltites ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{3\ensuremath{-}\ensuremath{\delta}}$. Oxygen deficiency and alternative layered structure with $\mathrm{Co}{\mathrm{O}}_{6}$ octahedron and $\mathrm{Co}{\mathrm{O}}_{4}$ tetrahedral polyhedron lead to the mixed valence states of Co ions. We found a Curie temperature at $304\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and a magnetization jump around $200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ under $0.01\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ with a thermal hysteresis indicating a kind of magnetic memory effect. Temperature dependence of the electrical resistance exhibits nonmetallic with a thermal activation type. The observed magnetization jump is interpreted in term of a spin state transition of ${\mathrm{Co}}^{3+}$ ions in the octahedral site from the intermediate to low spin state. The intermediate spin configuration of ${\mathrm{Co}}^{3+}$ with ${t}_{2g}^{5}{e}_{g}^{1}$ may develop an orbital ordering which accounts for the nonmetallic transport. With increasing magnetic field, the magnetization jump gradually disappears and its onset temperature shifts to lower temperature. The oxygen rich compound ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{2.704}$ with 6% rich ${\mathrm{Co}}^{3+}$ population compared to the less oxygen compound ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{2.614}$ shows a relatively large magnetization jump as well as the enhancement of the ${e}_{g}$ electron localization leading to higher magnetic transition temperature from the intermediate to low spin state. Contrary, the less oxygen ${\mathrm{Y}}_{0.33}{\mathrm{Sr}}_{0.67}\mathrm{Co}{\mathrm{O}}_{2.614}$ shows the substantial low oxidation state as ${\mathrm{Co}}^{2+}$ which guides to a possible intermediate spin state stabilization.

Electronic structures of [formula omitted] delafossite oxides

Electronic structures of AgNi 1 - x Co x O 2 ( 0 ⩽ x ⩽ 1 ) delafossite oxides have been investigated by using photoemission spectroscopy (PES) and soft X-ray absorption spectroscopy (XAS). T 2p XAS ( T = Co , Ni) spectra of AgNi 1 - x Co x O 2 show that the valence states of Co ions are trivalent and do not change with x . In contrast, Ni ions are in the Ni 2 + – Ni 3 + mixed-valent states and the trivalent component increases with increasing x . This finding suggests that the metallic nature of AgNi 1 - x Co x O 2 for low values of x arises from the divalent Ni ions. The valence-band PES study of AgNi 1 - x Co x O 2 reveals that the Ni 3d states are more metallic like than the Co 3d states.

Valence state of cobalt ions in a sputtered ZnO film

The authors present x-ray photoelectron spectroscopy data for a Co(Li)-doped ZnO thin film and discuss the valence state of Co ions in the sputtered ZnO film.