Valence Of Co Ions Research Articles

Influence of Co and Co-F co-doping on defect-induced intrinsic ferromagnetic properties of PbPdO2 nanoparticles

In this work, Co-doped and Co-F co-doped PbPdO2 nanoparticles with a body-centered orthorhombic structure were synthesized via sol–gel method. The measured results reveal that, Pb and O vacancies exist in all samples; the valence state of magnetic atom Co is identified as mixed Co2+ and Co3+; all samples exhibit both ferromagnetism and paramagnetism; the incorporation of fluorine results in an evident increase in the magnetic moment of PbPd0.875Co0.125O2@F; a jump of magnetization at about 265 K can be found in both FC and ZFC curves of PbPd1-xCoxO2 (x = 0, 0.05, 0.10 and 0.115). Moreover, the origin of the magnetism can be considered as follows: the ferromagnetism for PbPd1-xCoxO2 (0 < x ≤ 0.15) results from the combined effect of O1− and magnetic Co ions; fluorine substitution in an oxygen lattice site and the fluorine doping in the interstitial sites can induce both distinct roles on the valence of oxygen, Co and Pd ions, which changes evidently the magnetic properties of PbPd1-xCoxO2. Fortunately, the magnetic properties of PbPdO2, Co-doped PbPdO2, Co-F co-doped PbPdO2 can be verified by first-principles calculations. It is found that the enhanced magnetization results mainly from Pd(2−δ)+ and more O1− induced by F. Finally, the magnetization jump at about 265 K has been explained by the temperature-dependent valence transformation between O2− and O1−.

XAS and XPS analysis of double magnetic transition, canonical spin glass behavior and magnetoresistance in LaMn1-xCoxO3 (0.1 ≤ x ≤ 0.5) system

Polycrystalline LaMn1-xCoxO3 (0.1≤ x ≤ 0.5) samples were synthesized using conventional ceramic method. Rietveld refined X-ray diffraction pattern revealed the single-phase orthorhombic crystal structure of all the samples with the space group Pbnm. Temperature-dependent magnetic measurements performed in field cooled (FC) and zero field cooled (ZFC) mode at 102 Oe exhibit the onset of double transition in x = 0.3–0.5 compositions. The ordering temperature rises with an increase in Co concentration. FC and ZFC studies show the presence of glassy state below the ordering temperature in all samples; confirmed using a. c. susceptibility measurements. The a. c. susceptibility data are analyzed using power law and the existence of canonical spin glass is revealed. Magnetic hysteresis studies demonstrate the enhanced ferromagnetism amid the presence of unsaturated magnetization with an increase in Co doping. The presence of double transition and spin glass state is attributed to the competing ferromagnetic and anti-ferromagnetic interactions between the Co and Mn ions present in the system. The system also depicts the presence of appreciable value of magnetoresistance ~42% at 8 T magnetic field in x = 0.5 sample. These properties are interpreted through valence and spin states of Mn and Co ions, being confirmed from electronic structure studies using X-ray absorption spectroscopy (XAS) at L3,2- edges of respective ions along with O K-edge for all samples (0.1≤ x ≤ 0.5). After meticulous analysis and conjoining the results obtained from magnetization and XAS studies, it is found that cobalt is present in high spin Co2+ and high/low spin Co3+-state. Charge transfer multiplet calculation done at L3,2 edges of Mn and Co ions confirm the presence of Mn3+/Mn4+ and Co2+/Co3+ states consistent with XAS results. X-ray photoelectron spectroscopy performed at Mn2p, Co2p, and O1s -edges further substantiate the reasons behind the properties exhibited by the present system.

Oxygen Evolution Reaction in Ba0.5Sr0.5Co0.8Fe0.2O3-δ Aided by Intrinsic Co/Fe Spinel-Like Surface.

Among the perovskites used to catalyze the oxygen evolution reaction (OER), Ba0.5Sr0.5Co0.8Fe0.2O3-δ (BSCF) exhibits excellent activity which is thought to be related to dynamic reconstruction at the flexible perovskite surface due to accommodation of large amount of oxygen vacancies. By studying the local structure and chemistry of BSCF surfaces, in detail, via a range of transmission electron microscopy (TEM) methods, we show that the surfaces of the as-synthesized BSCF particles are Co/Fe rich, and remarkably, adopt a spinel-like structure with a reduced valence of Co ions. Post-mortem and identical location TEM analyses reveal that the Co/Fe spinel-like surface retains a stable chemical environment of the Co/Fe ions, although its structure weakens after electrochemical processing. Further, it is verified that prior to the onset of OER, the Co/Fe spinel-like surface promotes the formation of the highly active Co(Fe)OOH phase, which enhances the OER electrocatalytic properties of the underlying conductive BSCF perovskite. This study provides a detailed understanding of the fundamental transformations that oxide catalysts undergo during electrochemical processes and can aid in the development of novel oxide catalysts with enhanced activity.

Co-ion specific effect on sodium halides sorption and transport in a cross-linked poly(p-styrene sulfonate-co-divinylbenzene) for membrane applications

Sodium halide sorption and transport in a cation exchange membrane based on cross-linked poly(p-styrene sulfonate-co-divinylbenzene) (CR61) were investigated experimentally and theoretically. At fixed co-ion valence, sodium halide partitioning between aqueous electrolyte solutions and ion exchange polymers is only minimally sensitive to co-ion chemical identity. NaCl, NaF and NaI exhibit similar solubility coefficients in CR61, which agrees reasonably well with predictions of the Donnan-Manning model, especially at high external salt concentrations. Ion activity coefficients in the membrane were evaluated and ion sorption thermodynamics were investigated. Ion diffusion coefficients in CR61 were predicted using a recently developed framework based on the Manning diffusion model, which accounts for electrostatic effects on ion diffusion, and the Mackie-Meares model, which accounts for tortuosity/area effects on ion diffusion in swollen polymers. Model predictions agree reasonably well with the experimental data, especially at high external salt concentrations. The observed decrease in salt diffusion coefficients in CR61 relative to values in aqueous solutions is due primarily to tortuosity/area effects. Salt diffusion coefficients correlate with co-ion hydrated radius and membrane water content. Finally, salt permeability coefficients predicted using the solution-diffusion model exhibit good agreement with experimental data, especially at external salt concentration greater than 0.1 mol/L.

Mixed-Valence Cobalt(II/III) Metal-Organic Framework for Ammonia Sensing with Naked-Eye Color Switching.

The construction of colorimetric sensing materials with high selectivity, low detection limits, and great stability provides a significant way for facile device implementation of an ammonia (NH3) sensor. Herein, with excellent alkaline stability and exposed N sites in molecule as well as with naked-eye color switching nature generated from changeable cobalt (Co) valence, a three-dimensional mixed-valence cobalt(II/III) metal-organic framework (FJU-56) with tris-(4-tetrazolyl-phenyl)amine (H3L) ligand was synthesized for colorimetric sensing toward ammonia. The activated FJU-56 demonstrates a limit of detection of 1.38 ppm for ammonia sensing, with high selectivity in ammonia and water competitive adsorption, and shows outstanding stability and reversibility in the cyclic test. The NH3 or water molecules binding to the exposed N sites with the hydrogen-bond are observed by single-crystal X-ray diffraction, determining that the attachment of guest molecules to the FJU-56 framework changes the valence of Co ions with a naked-eye color switching response, which provides an ocular demonstration for ammonia capture and a valuable insight into ammonia sensing.

Classical density functional theory and Monte Carlo simulation study of electric double layer in the vicinity of a cylindrical electrode

We have performed extensive Monte-Carlo simulations and classical density functional theory (DFT) calculations of the electrical double layer (EDL) near a cylindrical electrode in a primitive model (PM) modified by incorporating interionic dispersion interactions. It is concluded that (i) in general, an unsophisticated use of the mean field (MF) approximation for the interionic dispersion interactions does not distinctly worsen the classical DFT performance, even if the salt ions considered are highly asymmetrical in size (3:1) and charge (5:1), the bulk molar concentration considered is high up to a total bulk ion packing fraction of 0.314, and the surface charge density of up to 0.5 C m−2. (ii) More specifically, considering the possible noises in the simulation, the local volume charge density profiles are the most accurately predicted by the classical DFT in all situations, and the co- and counter-ion singlet distributions are also rather accurately predicted; whereas the mean electrostatic potential profile is relatively less accurately predicted due to an integral amplification of minor inaccuracy of the singlet distributions. (iii) It is found that the layered structure of the co-ion distribution is abnormally possible only if the surface charge density is high enough (for example 0.5 C m−2); moreover, the co-ion valence abnormally influences the peak height of the first counter-ion layer, which decreases with the former. (iv) Even if both the simulation and DFT indicate an insignificant contribution of the interionic dispersion interaction to the above three ‘local’ quantities, it is clearly shown by the classical DFT that the interionic dispersion interaction does significantly influence a ‘global’ quantity like the cylinder surface-aqueous electrolyte interfacial tension, and this may imply the role of the interionic dispersion interaction in explaining the specific Hofmeister effects. We elucidate all of the above observations based on the arguments from the liquid state theory and at the molecular scale.

Colloidal Stability in Asymmetric Electrolytes: Modifications of the Schulze–Hardy Rule

The Schulze-Hardy rule suggests a strong dependence of the critical coagulation concentration (CCC) on the ionic valence. This rule is addressed theoretically and confronted with recent experimental results. The commonly presented derivation of this rule assumes symmetric electrolytes and highly charged particles. Both assumptions are incorrect. Symmetric electrolytes containing multivalent ions are hardly soluble, and experiments are normally carried out with the well-soluble salts of asymmetric electrolytes containing monovalent and multivalent ions. In this situation, however, the behavior is completely different whether the multivalent ions represent the counterions or co-ions. When these ions represent the counterions, meaning that the multivalent ions have the opposite sign than the charge of the particle, they adsorb strongly to the particles. Thereby, they progressively reduce the magnitude of the surface charge with increasing valence. In fact, this dependence of the charge density on the counterion valence is mainly responsible for the decrease of the CCC with the valence. In the co-ion case, where the multivalent ions have the same sign as the charge of the particle, the multivalent ions are repelled from the particles, and the surfaces remain highly charged. In this case, the inverse Schulze-Hardy rule normally applies, whereby the CCC varies inversely proportional to the co-ion valence.

荷電性リン脂質膜間に働く電気二重層相互作用：co-ionの価数依存性から

荷電性リン脂質膜間に働く電気二重層相互作用：co-ionの価数依存性から

Interactions between silica particles in the presence of multivalent coions.

Forces between charged silica particles in solutions of multivalent coions are measured with colloidal probe technique based on atomic force microscopy. The concentration of 1 : z electrolytes is systematically varied to understand the behavior of electrostatic interactions and double-layer properties in these systems. Although the coions are multivalent the Derjaguin, Landau, Verwey, and Overbeek (DLVO) theory perfectly describes the measured force profiles. The diffuse-layer potentials and regulation properties are extracted from the forces profiles by using the DLVO theory. The dependencies of the diffuse-layer potential and regulation parameter shift to lower concentration with increasing coion valence when plotted as a function of concentration of 1 : z salt. Interestingly, these profiles collapse to a master curve if plotted as a function of monovalent counterion concentration.

Hybrids of Cobalt/Iron Phosphides Derived from Bimetal-Organic Frameworks as Highly Efficient Electrocatalysts for Oxygen Evolution Reaction.

The electrochemical splitting of water, as an efficient and large-scale method to produce H2, is still hindered by the sluggish kinetics of the oxygen evolution reaction (OER) at the anode. Considering the synergetic effect of the different metal sites with coordination on the surface of electrocatalysts, the hybrids of Co/Fe phosphides (denoted as Co-Fe-P) is prepared by one-step phosphorization of CoFe metal-organic frameworks for the first time as highly efficient electrocatalysts for OER. Benefiting from the synergistic effect of Co and Fe, the high valence of Co ions induced by strongly electronegative P and N and the large electrochemical active surface area (ECSA) originated from exposed nanowires on the surface of Co/Fe phosphides, the resultant Co-Fe-P-1.7 exhibits remarkable electrocatalytic performances for OER in 1.0 M KOH, affording an overpotential as low as 244 mV at a current density of 10 mA/cm2, a small Tafel slope of 58 mV/dec, and good stability, which is superior to that of the state-of-the-art OER electrocatalysts. In addition, the two-electrode cell with Co-Fe-P-1.7 modified Ni foam as anode and cathode in an alkaline electrolyte, respectively, exhibits the decomposition potential of ca. 1.60 V at a current density of 10 mA/cm2 and excellent stability.

Chiral crystal-structure transformation ofR3Co4Sn13(R=La and Ce)

The compounds designated as ${R}_{3}{T}_{4}{\mathrm{Sn}}_{13}$ ($R$ = rare earth and $T$ = transition metal) are hypothesized to be strongly correlated electron systems. Some of these compounds exhibit structural phase transitions with speculated charge-density-wave (CDW) formations. We carried out x-ray diffraction measurements in order to investigate the second-order phase transition of ${R}_{3}{\mathrm{Co}}_{4}{\mathrm{Sn}}_{13}$ ($R=\mathrm{La} \text{and} \mathrm{Ce}$) by using single-crystalline samples synthesized by the molten Sn-flux method. Both compounds exhibit the structural superlattice transformations characterized by the wave vector $\mathbit{q}=(1/2,1/2,0)$ below ${T}_{\mathrm{D}}\ensuremath{\simeq}160$ K, which originate from electronic states owing to the common Co and Sn. The space group of the low-temperature phase is evidenced as a chiral cubic $I{2}_{1}3$. The magnetic-susceptibility enhancement and the Co-ion valence shift below ${T}_{\mathrm{D}}$ are signatures of a phase transition not fully attributed to the conventional CDW formation.

Derivation of the inverse Schulze-Hardy rule.

The inverse Schulze-Hardy rule was recently proposed based on experimental observations. This rule describes an interesting situation of the aggregation of charged colloidal particles in the presence of the multivalent coions. Specifically, it can be shown that the critical coagulation concentration is inversely proportional to the coion valence. Here the derivation of the inverse Schulze-Hardy rule based on purely theoretical grounds is presented. This derivation complements the classical Schulze-Hardy rule, which describes the multivalent counterion systems.

The non-dominance of counterions in charge-asymmetric electrolytes: non-monotonic precedence of electrostatic screening and local inversion of the electric field by multivalent coions.

The asymptotic convergence of the thermodynamic and structural properties of unequally-sized charge-symmetric ions in strong electric fields was postulated more than thirty years ago by Valleau and Torrie as the dominance of counterions via the non-linear Poisson-Boltzmann theory [Valleau and Torrie, J. Chem. Phys., 1982, 76, 4623]. According to this mean field prescription, the properties of the electrical double layer near a highly charged electrode immersed in a size-asymmetric binary electrolyte converge to those of a size-symmetric electrolyte if the properties of counterions are the same in both instances. On the other hand, some of the present authors have shown that, in fact, counterions do not dominate the electrical properties of a spherical macroion in the presence of unequally-sized ions, symmetric in valence, if ion correlations and ionic excluded volume effects are taken into account consistently. These ingredients are neglected in the classical Poisson-Boltzmann picture. In the present work, we show the occurrence of the non-dominance of counterions in the opposite scenario, that is, when ions are equally-sized but asymmetric in valence. This is performed in the presence of highly charged colloidal surfaces of spherical and planar geometries for different ionic volume fractions. In addition to the phenomenon of non-dominance of counterions, our simulations and theoretical data also exhibit a non-monotonic order or precedence in the mean electrostatic potential, or electrostatic screening, at the Helmholtz plane of a charged colloid. This interesting behaviour is analyzed as a function of the coion's valence, the ionic volume fraction, and the charge and size of the colloidal particle. All these phenomena are explained in terms of the decay of the electric field near the colloidal surface, and by the appearance of a local inversion of both the electric field and the integrated surface charge density of the colloidal particle in the presence of monovalent counterions and multivalent coions.

Suppression and promotion of charge inversion in the presence of multivalent coions.

We report charge inversion using Monte Carlo calculations for a negatively charged surface in aqueous solutions involving coions of different charges and monovalent counterions. It is shown that a rise in the valence of coions at moderate concentrations can substantially promote charge inversion for the surface charge values of biological relevance, regardless of the representation of surface charges but dependent in a nontrivial way on polarization effects resulting from dielectric discontinuity. These obtained characteristics challenge the traditional belief that the coions are generally considered to suppress charge inversion and expose the important role of coions of higher valence in tailoring the effective interactions of biomolecules with the cell membrane.

Density Functional Analysis of Like-Charged Attraction between Two Similarly Charged Cylinder Polyelectrolytes

A systematic theoretical investigation is performed for electrostatic potential of mean force (EPMF) between two similarly charged rods (modeling DNA) immersed in a primitive model electrolyte solution. Two scientific anomalies are disclosed: (i) although a like-charge attraction (LCA) generally becomes stronger with bulk electrolyte concentration, the opposite effect unexpectedly occurs if the two rod surfaces involved are sufficiently charged and (2) contrary to what is often asserted, that the presence of multivalent counterion is necessary to induce the LCA, it is found that the univalent counterion induces the LCA solely only if bulk electrolyte concentration rises sufficiently and the rod surface charge quantities are high. On the basis of the system energetics calculated first by a classical density functional theory in three-dimensional space, a hydrogen-bonding style mechanism is advanced to reveal the origin of the LCA, and by appealing to fairly common-sense concepts such as bond energy, bond length, number of hydrogen bonds formed, and counterion single-layer saturation adsorption capacity, the present mechanism successfully explains the scientific anomalies and effects of counterion and co-ion diameters in eliciting the LCA first investigated in this work. To add weight to the hydrogen-bonding style mechanism, a theoretical investigation is further performed regarding the effects of the rod surface charge density, co-ion valence, relative permittivity of the medium, temperature, nonelectrostatic interion interactions, and rod diameter in modifying the EPMF, and several novel phenomena are first confirmed, which is self-consistently explained by the present hydrogen-bonding style mechanism.

Enhanced electrical correlation in the W-doped cobaltite Ca3Co4O9 ceramics

The effect of W-doping at Co-site on structural, magnetic, and transport properties in Ca3Co4−xWxO9 (0≤x≤0.4) polycrystalline samples has been studied. Based on the analysis of the structural parameter variations, the substitution is suggested to occur at Co-site in Ca2CoO3 layers. Magnetic results show that the low-temperature magnetic behavior of the series samples changes from a ferrimagnetic state of Ca3Co4O9 to a glass-like state for the W-doped samples. The result is suggested to originate from the variation of the average valence of Co ions induced by W-doping. All samples show metal-insulator transition at Tmin and Tmin increases monotonously with increasing W-doping level. The temperature range shows Fermi liquid transport behavior becomes narrower with increasing W-doping level, indicating an enhanced electron–electron correlation. Moreover, the resistivity increases and the transport mechanism changes from thermally activated model to the Mott’s variable range hoping model as W-doped into the system. The variation of the resistivity and the transport mechanism is ascribed to the increased distortion or disorder induced by W-doping.

Co doped ZnO semiconductor materials: structural, morphological and magnetic properties

Abstract Structural, morphological and magnetic properties of Zn1−xCoxO (x = 0.01 and 0.03) powdered materials are presented. XRD studies reveal a wurtzite-type structure, while the formation of a Co3O4 secondary phase was evidenced by Raman spectroscopy. A ferromagnetic behaviour with low Curie temperature was evidenced by Electron Paramagnetic Resonance (EPR) investigation. We suggest that the origin of the ferromagnetism in Zn1−xCoxO powders is probably due to the presence of the mixed cation valence of Co ions via a double-exchange mechanism rather than the real doping effect.

Spin-glass and spin-fluctuation in Mo-doped Ca 3Co 4O 9 system

Structural, magnetic, resistivity and thermal transport measurements have been performed to study the Mo-doping effect on a layered cobaltite Ca 3Co 4− x Mo x O 9 ( 0 ≤ x ≤ 0.4 ) system. The results indicate that the low-temperature magnetic behavior of the system changes from a ferrimagnetic state to a spin-glass-like state upon Mo doping, which is due to the decrease in the average valence of Co ions. Moreover, all the Mo-doped samples have a higher resistivity and larger thermopower S compared with the Mo-free sample. The variation in the resistivity and thermopower between the Mo-doped and the Mo-free samples is dominated by the change in the carrier concentration of the samples. In the Mo-doped samples with x ≥ 0.1 , both the resistivity and thermopower decrease gradually with increasing Mo-doping level, which is suggested to mainly originate from the variation in the carrier mobility of the samples. In addition, an obvious thermopower upturn is observed in the S ( T ) curve of all the Mo-doped samples, which can be explained by the enhancement of spin-fluctuation induced by Mo-doping.

High‐Performance Programmable Memory Devices Based on Co‐Doped BaTiO3

Non-volatile memory cells using Co-doped BaTiO3 as an active layer exhibit high-performance unipolar resistive switching characteristics, with a resistance ratio over 104, retention time longer than 7 × 104 s, endurance over 105 cycles, and switching speed less than 10 ns/70 ns for SET/RESET. Under high electric field and large Joule heating, the easily varied valence of Co ions, the pre-existing oxygen vacancies with sufficiently high density, and the local itinerant electrons introduced by the Co-doping all favor the local metal-insulator phase transition and therein the formation/rupture of conductive filaments, contributing to the stable resistive switching.

Generalization of a Classic Theory of the Low Frequency Dielectric Dispersion of Colloidal Suspensions to Electrolyte Solutions with Different Ion Valences

The classic Shilov-Dukhin theory of the low frequency dielectric dispersion of colloidal suspensions in binary electrolyte solutions was developed for symmetric electrolytes: equal counterion and co-ion valences. A rigorous generalization of this theory to asymmetric electrolytes, such that the valence of counterions is double or half the valence of co-ions, is presented. This generalization is possible because analytical solutions of the intervening integrals exist for these two particular cases but do not exist in the general case of different counterion and co-ion valences, a result apparently overlooked or ignored in the past.