State Of Co Ions Research Articles

A new approach to obtain calcium cobalt oxide by microwave-assisted hydrothermal synthesis

This study describes the effect of microwave-assisted hydrothermal synthesis on the crystalline phase-formation, microstructural characteristics, and thermoelectric performance of Ca3Co4O9. Synthesis parameters, such as time, temperature, KOH concentration, and oxidation states of Co ions, to produce this compound, were studied. X-ray diffraction and X-ray photoelectron spectroscopy showed that the most critical parameters for the phase formation are the oxidation states of Co ions. Ca3Co4O9 can be obtained by microwave-assisted hydrothermal synthesis at 503 K for 30 min and thermal treatment at 1173 K for 120 min. Scanning electron microscopy images showed that the sample exhibits randomly oriented particles with plate-like morphology and range size between 0.4 and 1.2 μm. From measurements of electrical resistivity, thermal conductivity, and Seebeck coefficient, we obtained a figure of merit of ~0.008 at 300 K.

Spin Dynamics and Unconventional Magnetism in Insulating La(1–2x)Sr2xCo(1–x)NbxO3

We study the structural, magnetic, transport and electronic properties of LaCoO$_3$ with Sr/Nb co-substitution, i.e., La$_{(1-2x)}$Sr$_{2x}$Co$_{(1-x)}$Nb$_{x}$O$_3$ using x-ray and neutron diffraction, dc and ac-magnetization, neutron depolarization, dc-resistivity and photoemission measurements. The powder x-ray and neutron diffraction data were fitted well with the rhombohedral crystal symmetry (space group \textit{R$\bar{3}$c}) in Rietveld refinement analysis. The calculated effective magnetic moment ($\approx$3.85~$\mu_B$) and average spin ($\approx$1.5) of Co ions from the analysis of magnetic susceptibility data are consistent with 3+ state of Co ions in intermediate-spin (IS) and high-spin (HS) states in the ratio of $\approx$50:50, i.e., spin-state of Co$^{3+}$ is preserved at least up to $x=$ 0.1 sample. Interestingly, the magnetization values were significantly increased with respect to the $x=$ 0 sample, and the M-H curves show non-saturated behavior up to an applied maximum magnetic field of $\pm$70 kOe. The ac-susceptibility data show a shift in the freezing temperature with excitation frequency and the detailed analysis confirm the slower dynamics and a non-zero value of the Vogel-Fulcher temperature T$_0$, which suggests for the cluster spin glass. The unusual magnetic behavior indicates the presence of complex magnetic interactions at low temperatures. The dc-resistivity measurements show the insulating nature in all the samples. However, relatively large density of states $\approx$10$^{22}$ eV$^{-1}$cm$^{-3}$ and low activation energy $\approx$130~meV are found in $x=$ 0.05 sample. Using x-ray photoemission spectroscopy, we study the core-level spectra of La 3$d$, Co 2$p$, Sr 3$d$, and Nb 3$d$ to confirm the valence state.

Site occupancy, valence state, and spin state of Co ions in Co-doped In2O3 diluted magnetic semiconductor

Abstract This study aims to investigate the 24 d and 8 b site occupancies, valence state, and spin state of Co ions in Co-doped In2O3 prepared by a chemical solution method. The Rietveld analysis of X-ray powder diffraction patterns of the products revealed that Co2+ and Co3+ ions tend to occupy the 24 d and 8 b sites, respectively. X-ray photoelectron spectroscopy analysis indicated that the Co2+ ions are dominant at low-doping region ( x ≤ 0.044 ) and that Co3+ ions appear at high-doping region ( x ≥ 0.052 ). Magnetic susceptibility measurements revealed that the effective Bohr magneton number p e f f is ~3.8 at the low-doping region, consistent with the value for Co2+ in the high-spin ( S = 3 / 2 ) state. The p e f f decreases at the high-doping region, where Co3+ ions appear. These results indicate that the Co3+ ions are in the low-spin ( S = 0 ) state or intermediate-spin ( S = 1 ) state.

Enhanced Long-Term Cycling Performance of Single Crystalline LiCo0.95Ni0.05O2 cathode Material at High Cut-Off Voltage in Li-Ion Cell

A significant interest in developing high volumetric energy density and high power energy storage systems (ESSs) has facilitated development of ideal cathode material in Lithium-ion batteries. Among various cathode materials, LiCoO2 (LCO) is one of the promising candidates because of single crystalline morphology with relatively high electrode density (>3.9g cm-3) and superior electrochemical performance. However, Increased cut-off voltage, higher than the conventional operating voltage of LCO (~4.3 V vs. Li/Li+), to maximize energy storage capacity gives rise to severe electrochemical degradation at the electrode-electrolyte interface and irreversible structural deformation at highly delithiated states of cathode material, which result in a rapid decrease of capacity and nominal voltage in a continuous cycle. Herein, for an enhanced cycle life of LCO particularly in high cut-off voltage, we proposed LiCo0.95Ni0.05O2 (LCNO) which not only enabled to demonstrate remarkable cycling performance and but also to be synthesized as a single crystalline with layered structure(R-3m) by the solid-state method in air condition. For comparison, the electrochemical behaviors of active materials, LCO and LCNO, charged up to 4.45V, were investigated. Even though LCO showed an initial reversible capacity of ∼182 mAh/g, higher than that of LCNO (∼176 mAh/g), LCNO exhibited much better capacity retention of 93.6% than that of LCO (74.3%) under continuous 100 cycles. We identified the less formation of irreversible byproducts on the surface of LCNO, compared to that of LCO, indicating the reactivity of interface between LCNO-electrolyte was highly stabilized with the assistance of Ni-ions. Furthermore, the evolution of interfacial change between LCNO-electrolyte after electrochemical cycling has been also investigated by high-resolution scanning transmission electron microscopy (HR-TEM). Based on electron energy-loss spectroscopy measurements (EELS), we confirmed that Ni-ions in LCNO may have a potential ability to initiate the oxidation state of Co ions around the surface to be partially reduced to Co2+/Co3+ mixed-valence state during cycles with a change of surface structure from layered structure to spinel-like phase, thus reducing surface side reaction simultaneously.

Effect of film thickness and strain state on the structure, magnetic and transport properties of La0.9Sr0.1CoO3 films

In this paper, we investigate the effects of film thickness and strain state on the structure, magnetism and electrical transport properties of La0.9Sr0.1CoO3 (LSCO) thin films. The x-ray diffraction analysis shows that the out-of-plane lattice parameters of LSCO films grown on different substrates gradually approach those of the bulk LSCO with increasing film thickness, indicating that the strain induced by the substrates is partially relaxed in thick films, i.e. 100 nm. The film magnetism is gradually enhanced with increasing film thickness. For the 20-nm-thick film, it is almost non-magnetic. However, the film becomes ferromagnetic when its thickness increases up to 100 nm. The magnetism is stronger for the film under compressive strain than that under tensile strain. The change in the magnetic properties of films is attributed to the lattice strain, which can affect the spin state of Co ions. Regarding the conductivity, all the samples show a semiconducting-like behavior, and the resistivity is inversely proportional to the film thickness. The electrical transport mechanism of the films with different thickness transforms from a three-dimensional variable range hopping (VRH) to a small-polaron hopping conduction (SPC) with increasing temperature. Fascinatingly, the temperature for transport mechanism transformation decreases gradually with increasing film thickness. This work could shed light on the electrical transport mechanism of LSCO films with different thickness.

Tuning the structural and optical properties in cobalt oxide-doped borosilicate glasses

Borosilicate glass systems with composition xSiO2 - (50-x) B2O3 - 30Na2O - 19.5ZnO - 0.5CoO (x = 0, 5, 10 and 20 mol. %) have been successfully prepared following the traditional melt quenching technique. The incorporation of SiO2 into borate glass network was found to tune its properties at different aspects, as probed by a set of complementary experimental techniques. The density of such boraosilicate glasses was decreased by SiO2 addition, thus, rendering the glassy network less tightly packed. In addition, the BO4 structural units were found to increase with SiO2 content as obtained from comparative FTIR spectra. Interestingly, the optical properties of such borosilicate glasses could be tuned through SiO2 addition. In particular, the optical band gaps attain larger values together with noticeable suppression of the band tails associated with defective structures upon SiO2 addition. Further insights into the crystal field interaction and d-levels splitting were obtained by a detailed deconvolution procedure applied to the characteristic absorption bands of Cobalt. The outcome of such a process revealed noticeable reductions in the ligand field strength and the associated d-levels splitting with SiO2 additives. Furthermore, the octahedral symmetry contribution was increased in borosilicate compared to the pure borate glasses as inferred from the oxidation states of Co ions probed by ESR. The present system combines the advantageous of the pure borate and silicate glasses and, therefore, offers an alternative candidate for optical applications.

Relation between the Co-O bond lengths and the spin state of Co in layered Cobaltates: a high-pressure study

The pressure-response of the Co-O bond lengths and the spin state of Co ions in a hybrid 3d-5d solid-state oxide Sr2Co0.5Ir0.5O4 with a layered K2NiF4-type structure was studied by using hard X-ray absorption and emission spectroscopies. The Co-K and the Ir-L3 X-ray absorption spectra demonstrate that the Ir5+ and the Co3+ valence states at ambient conditions are not affected by pressure. The Co Kβ emission spectra, on the other hand, revealed a gradual spin state transition of Co3+ ions from a high-spin (S = 2) state at ambient pressure to a complete low-spin state (S = 0) at 40 GPa without crossing the intermediate spin state (S = 1). This can be well understood from our calculated phase diagram in which we consider the energies of the low spin, intermediate spin and high spin states of Co3+ ions as a function of the anisotropic distortion of the octahedral local coordination in the layered oxide. We infer that a short in-plane Co-O bond length (<1.90 Å) as well as a very large ratio of Co-Oapex/Co-Oin-plane is needed to stabilize the IS Co3+, a situation which is rarely met in reality.

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.

Crystal field splitting and spin states of Co ions in cobalt ferrite with composition Co1.5Fe1.5O4 using magnetization and X-ray absorption spectroscopy measurements

Structural, magnetic and electronic properties of partially inverted Cobalt Ferrite with composition Co1.5Fe1.5O4 is discussed in the present work. Single phase (SG: Fd3m) sample is synthesized by co-precipitation technique and subsequent air annealing. The values of saturation magnetization obtained from careful analysis of approach to saturation in initial M(H) curves are used to determine spin states of Co ions in tetrahedral (TH) and octahedral (OH) sites. Spin states of Co3+ ions in TH sites, which has not been reported in literature, were found to be in high spin state. Temperature variation of magnetic parameters has been studied. The sample shows magneto-crystalline anisotropy with two clearly distinct pinning centers. Oxygen K-edge and Fe as well as Co L2,3-edge X-ray absorption (XAS) spectra have been used as complementary measurements to study crystal field splitting and core hole effects on transition metal (TM) 3d orbitals. The ratio of intensities of t2g and eg absorption bands in O-K edge XAS spectrum is used to estimate the spin states of Co ions at OH and TH sites. The results are in agreement with those obtained from magnetization data, and favors Co3+ ions in TH sites in high spin states. Normalized areas of the satellite peaks in TM L2,3-edge XAS spectra have been used to estimate 3dn+1L contribution in ground state wave function and the contributions were found to be significant.

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.

An energy competition of Co3+ and Co4+ ions during spin state transition in Ca3Co4O9 complex

The magnetic behavior of Ca3Co4O9 complex shows a very interesting phase transition at high temperature. It is probably that this ordering is due to the spin states of Co3+ and Co4+ ions. It is quite obvious that spin states of Co ions generally change as a function of temperature. We have shown that there is a strong correlation between oxidation states and ligand field parameters during spin state transitions.

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.

Theory of Spin-State Selective Nonlocal Screening in Co 2p X-ray Photoemission Spectrum of LaCoO3

The Co 2p X-ray photoemission spectrum (XPS) of LaCoO3 is investigated using a dp model simulating Co 3d and O 2p orbitals by means of a dynamical mean-field approach under the perovskite crystal structure. Across the spin-state transition from the low-spin to the high-spin state, the Co 2p3/2 main-line structure is substantially changed beyond expectation of a CoO6 cluster model calculation. In addition to the Coulombic multiplet effect, the origin of the spectral change is attributed to the nonlocal screening (NLS) from the correlated 3d band located on the top of the valence band to the core-excited Co site in the final state, where the NLS is practically active only for the high-spin state. The spin-state selectivity of the NLS is closely related to not only the spin state of the core-excited Co ion but also the spin and orbital character of the occupied Co 3d band in crystals. We emphasize that the Co 2p XPS can be an informative probe to investigate the spin state of Co ions in Co oxides, such as LaCoO3.

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.

CaCo&amp;lt;sub&amp;gt;1-x&amp;lt;/sub&amp;gt;Ru&amp;lt;sub&amp;gt;x&amp;lt;/sub&amp;gt;O&amp;lt;sub&amp;gt;y&amp;lt;/sub&amp;gt;: Role of Ru/Co Ratio on Its Transport Properties

Calcium cobaltites, especially Ca3Co4O9 with a misfit layered structure, are promising thermoelectric materials due to their suitability for high temperature applications and low densities. The existence of low spin-state electronic configurations for both and species is one of the key parameter to explain the large thermopower values. Ruthenium oxide, with a layered structure, exhibits strong electron-electron correlation and the extended nature of their 4d electrons enhances orbital overlapping which is expected to influence the transport characteristics of CaCo1-xRuxOy (CCR) samples, by affecting the spin state of the 3d Co ions. The effect on thermopower and electrical resistivity due to partial substitution of Co by Ru ions, up to 0.33 moles, from 300 to 600 K was investigated. A sharp decline in resistivity and in thermopower was observed until a transition ion ratio (TIR), (Ru/(Ru + Co)), of 0.5 is reached, beyond which both the properties became less sensitive to TIR. These variations in the transport properties are explained by the presence of 4d Ru in close proximity to the Co, which could influence the spin and oxidation state of Co ions. The Co rich and Ru rich samples exhibit very distinct microstructures and phase assemblages.

Structure, Stoichiometry, and Electrochemical Performance of Li2 CoTi3 O8 as an Anode Material for Lithium-Ion Batteries.

Li2 CoTi3 O8 powder has been synthesized by a simple citric nitrate method and was evaluated as an anode material for lithium-ion batteries. The X-ray diffraction (XRD) Rietveld refinement and X-ray photoelectron spectroscopy (XPS) measurements indicate the existence of Ti-site deficiency and a mixed-valence state of Co ions in the Li2 CoTi3 O8 structure. The refined site-occupation result gave a nonstoichiometric chemical formula of Li2 CoTi2.682 O8 , which corresponds to a theoretical specific capacity of 322 mA h g-1 , which is much higher than that of the stoichiometric material (233 mA h g-1 ). The synthesized Li2 CoTi2.682 O8 material exhibits high reversible capacity (ca. 320 mA h g-1 ), and excellent cycling stability and rate capability. A specific capacity of about 240 and 160 mA h g-1 can be achieved at 2 and 6 A g-1 , respectively, by the Li2 CoTi2.682 O8 material. First-principles calculation demonstrates that Ti-site deficiency decreases the bandgap and thus facilitates the electron conduction.

Mediating Distribution of Magnetic Co Ions by Surfactant in Co-Doped SnO&lt;SUB&gt;2&lt;/SUB&gt; Nanorods

We demonstrate a method to control the distribution of magnetic Co ions in Co-doped SnO2 nanorods by the surfactant. The nanorods with nanoflower morphology have been synthesized by surfactant-assisted hydrothermal method. The X-ray absorption fine structure spectra at Co K-edge reveal that Co ions substitute for Sn site in SnO2 lattice. The X-ray absorption fine structure spectra at O K-edge with its simulated spectra have been performed to investigate the local environment of Co ions. In the experimental spectra, the pre-edge peak continues to decline with the addition of the surfactant. It shows the same trend when the number of clustered Co ions in the simulated spectra is reduced, which indicates that the surfactant suppresses the formation of Co clusters. The effect of surfactant in mediating Co distribution arises from the strong interactions between the Co2+ ions and the CTA+ ions. In the Co K-edge experimental spectra, the weakened pre-edge peak reflects the electron transfer from the CTA+ ions to Co 3d band. Such electron transfer changes the charge state of Co ions and modifies the electrostatic interactions among dopant ions, leading to the uniform distribution of substitutional Co dopants. These results provide experimental guidance in the synthesis of the homogeneous-doped metal oxide semiconductors.

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.