Distribution Of Co Ions Research Articles

Enhancement of localized superconductivity in BaFe2As2 films via Co-ion implantation

In this study, we present a novel approach to localized superconductivity induction in BaFe2As2 films via targeted implantation of cobalt (Co) ions. Primarily, our study focuses on the systematic distribution of Co ions and the subsequent evolution of superconducting properties in Co-ion-implanted BaFe2As2 films. Our observations show that Co-ion distribution in the films is congruent with the results of analytical methodologies employed in the semiconductor industry, as confirmed via transmission electron microscopy imaging. The temperature-dependent resistivity curves reveal the concurrent presence of superconducting and non-superconducting regions. Moreover, the superconducting domain demonstrates the typical diamagnetic behavior intrinsic in superconductors. Importantly, Co-ion concentrations of ∼1020 cm−3 can be achieved by finely tuning the beam energy and ion dose. This concentration is instrumental in establishing an effective superconducting percolation pathway within the films.

Super-exchange interaction model in DMOs: Co doped TiO2 thin films

This work presents a study of the structural, morphological, and magnetic properties of TiO2/Co thin films deposited by DC magnetron co-sputtering of Co and TiO2 targets, varying the concentration of the Co and TiO2 at room temperature. The formation of Co oxides binary phases was not observed. From XRD, μXRD, and Raman measurements were identified rutile and anatase phases, which evidence the formation of diluted magnetic oxide semiconductor (DMOs). Additionally, the TiO2 polymorphous and random location of Co ions are induced when the substrate is at room temperature, and in situ annealing process is applied. SEM, AFM, and MFM micrographs evidenced the formation of smaller grains (< 10 nm) in the surface without relationship with magnetic domains; values of the magnetic roughness were obtained between 199 and 442 pm. Magnetic measurements showed a ferromagnetic-like behavior with the appearance of a magnetic hysteresis loop. The hysteresis curves were associated with a combination of dipolar and super-exchange interactions between Co ions and mediated for oxygen ions based on Anderson’s model; this behavior was described from numerical simulations of isolated magnetic moments (Co atoms) diluted into a non-magnetic matrix (TiO2 structure). In this model, a random distribution of Co ions, contributing to dipolar magnetic interactions and super-exchange interaction, is considered, due to the hybridization sp3d2 of the Co–O bond and located randomly of Co ions. The experimental and theoretical results of this study present an explanation of ferromagnetic-like behavior for Co doped TiO2 thin films, as diluted magnetic semiconductor (DMS) for spintronic applications.

Controllable P Doping of the LaCoO3 Catalyst for Efficient Propane Oxidation: Optimized Surface Co Distribution and Enhanced Oxygen Vacancies.

The properties of LaCoO3 are modified by a controllable P doping strategy via a simple sol-gel route. It is demonstrated that appropriate P doping is beneficial for forming a relatively pure perovskite phase and hinders the growth of perovskite nanoparticles. The combined results of density functional theory (DFT), extended X-ray absorption fine structure (EXAFS), X-ray absorption near-edge structure (XANES), temperature-programmed reduction of hydrogen (H2-TPR), X-ray photoelectron spectroscopy (XPS), and temperature-programmed desorption of ammonia (NH3-TPD) reveal that appropriate P doping gives rise to more oxygen vacancies, optimized distribution of Co ions, and improved surface acidity, which are beneficial for the adsorption of active oxygen species and the activation of propane molecules, resulting in an excellent catalytic oxidation performance. Especially, LaCo0.97P0.03O3 exhibits more surface-active oxygen species, higher bulk Co3+ proportion, increased surface Co2+ species, and increased acidity, resulting in its superior propane oxidation performance, which is dominated by the Langmuir-Hinshelwood mechanism. In situ diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) confirms that the presence of P will accelerate oxygen mobility, which in turn promotes the oxidation rate. Moreover, the obtained LaCo0.97P0.03O3 catalyst displays excellent thermal stability during the 60 h durability test at 400 °C and strong resistance against 5 vol % H2O and/or 5 vol % CO2 for prolonged 150 h.

Structural Distortion-Induced Charge Gradient Distribution of Co Ions in Delithiated LiCoO2 Cathode.

Layered LiCoO2 has drawn tremendous attention as a modeling cathode for Li-ion batteries, while its structural instability, especially in the high delithiation region, remains unsolved. With the aim of revealing the structural fundamentals, LiCoO2 electrodes are investigated at a long delithiation range using both in situ and ex situ techniques. In the highly delithiated LiCoO2 electrode, the unique charge compensation process leads to a spatial charge gradient of Co2+/Co3+/Co4+ ions from surface to bulk, which can be further manipulated by structural distortion, Li extraction, and surface side reactions. The coordinated surface oxygen is shown to be electrochemically active and fully reversible in participating in the charge compensation during cycling. Moreover, the active lattice O can be significantly stabilized by introducing the undesired surface Li-Co antisites, which also play an effective role in accommodating the internal stress induced by volume changes. These findings effectively bridge the structural changes with the Li+/e- migration kinetics to elucidate the degradation of LiCoO2 cathode upon delithiation, demonstrating a rewarding avenue for improving the electrochemical performance of LiCoO2 itself and developing high energy density cathodes for the battery community as well.

Poisson–Boltzmann theory with non-linear ion correlations

The Poisson–Boltzmann (PB) theory is widely used to depict ionic systems. As a mean-field theory, the PB theory neglects the correlation effect in the ionic atmosphere and leads to deviations from experimental results as the concentration or charge valance increases. A modified PB theory including ion correlation effect while retaining its simplicity is critical for many important applications in which ion correlation effect can be significant. In this paper, we present a new model to incorporate ion correlations into the original PB equation by utilizing the Green’s function with a non-linear form of the self-energy, which is different from the linear self-energy equation obtained by the field-theoretic (FT) approach. Both equations are solved numerically and compared with our molecular dynamics (MD) simulation. The co-ion distribution calculated by the FT approach deviates significantly from the MD simulation, while our results for both counter-ion and co-ion distributions are justified by the MD simulation.

Structural and electrical properties of an electric double layer formed inside a cylindrical pore investigated by Monte Carlo and classical density functional theory

We present the properties of an electrical double layer formed by ions inside a charged cylindrical pore studied by the grand canonical Monte Carlo simulation and classical density functional theory. The cylinder radius is 3000 pm. The wall is hard, perfectly smooth. The ions are modelled by hard spheres with a point electric charge at the centre. The hard sphere diameter is fixed at 400 pm. The monovalent ions are immersed in a continuous dielectric medium of the relative permittivity er. The temperature is 298.15 K and the electrolyte concentration takes the following values: 1.0, 2.5 and 4.0 M. The surface charge density varies in the range from − 1.0 to + 1.0 C/m2. The ion singlet distribution results show adsorption of counter-ions and desorption of co-ions from the cylindrical electrode. At high electrode charges the second layer of counter-ions is formed, while for high electrolyte concentration the co-ion distribution curve has a small maximum at some distance from the electrode surface. In comparison to the planar electrode, the concave one attracts stronger the counter-ions and repels the co-ions. At high electrolyte concentration, the profiles of the volume charge density have a positive hump, while those of the mean electrostatic potential have a negative minimum, which indicates the overscreening effect. For low electrolyte concentrations, the differential capacitance curve has a minimum at σ = 0 surrounded by two maxima. With increasing concentration, the minimum transforms into a maximum. The differential capacitance curves run above the curves for the planar electrode at small electrode charges and below them for high negative and positive charges. The very good agreement of all the grand canonical Monte Carlo to the classical density functional theory results presented in the paper indicates the reliability of the latter approach in cylindrical pore as well as planar geometry.

Investigations of the valence states, cobalt ion distribution, and defect structures in Co-doped ITO films

Abstract

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.

Structural and magnetic properties of Fe1−xCoxSe1.09 nanoparticles obtained by thermal decomposition

A series of Fe1−xCoxSe1.09 (x = 0 to 1) nanoparticles were synthesized by thermal decomposition method. Particles in composition range Fe0.5Co0.5Se1.09 to CoSe1.09 crystallized in monoclinic structure of Co6.8Se8, while FeSe1.09 crystallized in hexagonal structure of FeSe achavalite. Magnetization dependences on temperature and external magnetic field reveal complicated magnetic behavior and correspond to the sum of paramagnetic and superparamagnetic response. Mössbauer spectra contain several paramagnetic doublets with parameters corresponding to nonequivalent positions of divalent and trivalent iron cations with low spin. The nonequivalent positions appeared due to inhomogeneous distribution of Co ions or metal vacancies in iron surrounding.

Dynamics of the incorporation of Co into the wurtzite ZnO matrix and its magnetic properties

Bulk Co-doped ZnO (Zn1−xCoxO) samples were prepared and studied with particular emphasis on their compositions, structures, and magnetic properties. A detailed microstructural analysis was conducted to investigate the nature of Co incorporation into the wurtzite ZnO matrix. The Zn1−xCoxO ceramic samples were prepared using the standard solid-state reaction method with different Co molar concentrations of up to 30%. A Co solubility limit of approximately 23% was determined. For samples with Co concentrations greater than 23%, a segregated phase identified as Zn-doped CoO (Co1−yZnyO with y=0.23) was observed. The magnetic characterization for the single-phase samples revealed a paramagnetic behavior with antiferromagnetic coupling of Co2+ ions within the ZnO matrix. In order to get more insight about the magnetic results Monte Carlo simulations were conducted for different magnetic couplings and different Co distributions over the volume of the samples. We discussed the observed magnetic behavior of our samples by considering the main theories regarding the magnetic properties of dilute magnetic oxides. An inhomogeneous distribution of Co ions within the grains was inferred from the microstructural and magnetic characterization.

Nano-X-ray diffraction study of single Co-implanted ZnO nanowires

This work reports on the long-range order of single as-implanted and annealed ZnO:Co nanowires studied using a hard X-ray nanoprobe. For all nanowires, nano-X-ray fluorescence maps reveal a homogeneous distribution of Co ions implanted along the nanowires. For the as-implanted nanowires, however, nano-X-ray diffraction data shows a larger deviation from the bulk ZnO lattice along the c-axis induced by the ion implantation process. For the thermally annealed nanowires, on the other hand, the c lattice parameter is similar to the one of bulk wurtzite ZnO. The structural analysis of individual nanowires highlights the importance of the subsequent thermal annealing and emphasizes the critical role of the unavoidable damage created by the ion-implantation process. Such defects ultimately limit the use of semiconductor nanowires.

X-ray absorption fine structure study of cobalt ion distribution in ferromagnetic Zn1−xCoxO films

We examined the distribution of Co ions in ferromagnetic n-type Zn1−xCoxO semiconducting films with Co concentrations of 0.0–0.07 using x-ray absorption fine structure (XAFS) measurements at the Co and Zn K edges. Extended XAFS (EXAFS) revealed that Co ions mainly occupied the zinc sites in the films. X-ray absorption near edge structure (XANES) spectra demonstrated that the pre-edge peak of the Co K edge was substantially affected by the second neighboring Co ions in the zinc sites due to their environmental potential distortion. From the pre-edge peak and EXAFS analysis using ab initio calculations, we found that Co ions uniformly occupied the zinc sites of the Zn0.93Co0.07O film, whereas the Co ions of the Zn0.97Co0.03O and Zn0.95Co0.05O films were substituted at the zinc sites with a non-uniform distribution. The ferromagnetic properties of the Zn0.93Co0.07O film may be induced by direct interaction between the magnetic dipoles of the Co ions with a mean distance of 4.3 Å, or by the Co conduction-electron mediation.

Magnetic Properties of Co doped ZnS Diluted Magnetic Semiconductor

The magnetic properties of Zn0.9Co0.1S nanoparticles synthesized by solid state reaction method are presented. The nanoparticles are crystallized with zinc-blende structure. The variation of magnetic properties stems from the changes of the local structure around Co ions and the distribution of Co ions, given by X-ray absorption fine structures.

General solution to the electric double layer with discrete interfacial charges

We provide extensive molecular dynamics simulations of counterion and coion distributions near an impenetrable plane with fixed discrete charges. The numerical results are described by an explicit solution that distinguishes the plasma (√(A(c))/σ>3) and the binding regime (√(A(c))/σ<3) where σ is the ion diameter and A(c) = ∣e∕ν∣ (ν is the surface charge density). In the plasma regime, the solution consists of a product of two functions that can be computed from simpler models and reveals that the effects of the discreteness of the charge extends over large distances from the plane. The solution in the binding regime consists of a Stern layer of width σ and a diffuse layer, but contrary to standard approaches, the strong correlations between ions within the Stern layer and the diffuse layer require a description in terms of a "displaced" diffuse layer. The solution is found to describe electrolytes of any valence at all concentrations investigated (up to 0.4M) and includes the case of additional specific interactions such as van der Waals attraction and other generalizations. We discuss some open questions.

SUBTLE INTERPLAY BETWEEN HYDROGEN AND MAGNETISM IN Co DOPED ZnO

The effects of hydrogen, either interstitial (HI) or substitutional (Ho), on magnetic properties of Co doped ZnO (ZnO:Co) have been systematically investigated using first-principles density functional calculations. The study discovers the correlation between the distribution of Co ions and the hydrogen point defect and magnetism. It is found that Co ions and hydrogen have a strong tendency toward aggregation and hydrogen mainly contributes to the room temperature ferromagnetism observed experimentally in ZnO:Co . Furthermore, in ZnO:Co , the formation of Ho with four-fold hydrogenic bonds is favored over HI by 0.4 eV.

Subtle interplay between native point defects and magnetism in ZnO:Co

Distribution of Co ions and its effect on magnetic properties of ZnO:Co in the presence of native point defects, oxygen vacancy, and interstitial hydrogen, have been investigated using first-principles density functional calculations. The study provides a fundamental theoretical understanding on the correlation between magnetism and the distribution of magnetic ions and the native point defects in the semiconducting host. Results show that Co ions have a strong tendency toward aggregation via oxygen within ab plane in the presence of point defects. The room temperature ferromagnetism observed experimentally in ZnO:Co is mainly dominated by the interstitial hydrogen rather than oxygen vacancy.

The effect of synthesis solution pH on the physicochemical properties of Co substituted MCM-41

The adjustment of the initial synthesis solution pH affects the structure and stability of C16 Co-MCM-41. This pH effect was systematically investigated using N2 physisorption, TPR, in-situ FTIR, and X-ray absorption spectroscopy. Co-MCM-41 catalysts with the same pore size but greater porosity were synthesized with increasing pH from 10.5 to 12. The distribution of Co ions with respect to being on the pore wall or in the silica wall framework was changed by pH; higher pH produced Co ions mainly distributed just subsurface or in the interior of the silica wall. These pH effects significantly affected the reduction stability of the Co-MCM-41 sample similar to that of the pore radius of curvature effect (Lim et al, J. Phys. Chem. B, 109 (2005) 2285), resulting in stable and size controllable sub-nanometer Co clusters that are useful for catalyst design for specific reactions.

Microstructure of Co-doped TiO2(110) rutile by ion implantation

Co-doped rutile TiO2 was synthesized by injecting Co ions into single crystal rutile TiO2 using high energy ion implantation. Microstructures of the implanted specimens were studied in detail using high-resolution transmission electron microscopy (HRTEM), energy dispersive x-ray spectroscopy, electron diffraction, and HRTEM image simulations. The spatial distribution and conglomeration behavior of the implanted Co ions, as well as the point defect distributions induced by ion implantation, show strong dependences on implantation conditions. Uniform distribution of Co ions in the rutile TiO2 lattice was obtained by implanting at 1075K with a Co ion fluence of 1.25×1016Co∕cm2. Implanting at 875K leads to the formation of Co metal clusters. The precipitated Co metal clusters and surrounding TiO2 matrix exhibit the orientation relationships Co⟨110⟩‖TiO2[001] and Co{111}‖TiO2(110). A structural model representing the interface between Co metal clusters and TiO2 is developed based on HRTEM imaging and image simulations.

Investigations on the nature of observed ferromagnetism and possible spin polarization in Co-doped anatase TiO2 thin films

High-quality epitaxial thin films of Co-doped anatase TiO2 (Co:TiO2) were grown epitaxially on SrTiO3 (001) substrates by using pulsed laser deposition with in-situ reflection high-energy electron diffraction. The oxygen partial pressure, PO2, during the growth was systematically varied. As PO2 decreased, the growth behavior altered from a two-dimensional layer-by-layer-like growth to a three-dimensional island-like pattern. Electrical conductivity and saturation magnetization increased, seemingly consistent with the picture of carrier-induced ferromagnetism. However, we also found that the spatial distribution of Co ions became highly nonuniform and the chemical state of Co ions changed from ionic to metallic. All of these PO2 dependences, even including the transport and the magnetic properties, can be explained in terms of the formation of cobalt clusters, whose existence was clearly demonstrated by transmission–electron–microscope studies. Our work clearly indicates that the cobalt clustering will result in the room-temperature ferromagnetism observed in our Co:TiO2 films. To check the possible spin polarization of carriers in Co:TiO2 films, we also fabricated a heterojunction composed of a ferromagnetic Co:TiO2, an insulating SrTiO3, and a ferromagnetic half-metallic (La,Ba)MnO3 layer. When the magnetic field was varied, we could not observe any changes in its I–V characteristic curves, which suggests that there might be little spin-polarization effect in the anatase Co:TiO2 layer.

Co 2+ ion siting in pentasil-containing zeolites, part 3. : Co 2+ ion sites and their occupation in ZSM-5: a VIS diffuse reflectance spectroscopy study

Siting of the Co 2+ ions in dehydrated Co–ZSM-5 is suggested based on a VIS optical study, the MFI topology and parallelism between the spectra of the Co 2+ ions and local framework structures in ZSM-5, mordenite and ferrierite. The proposed Co 2+ ion sites represent a first attempt to describe the cationic sites for transition metal ions in the MFI structure. Co ions exchanged in CoMeH–ZSM-5 zeolites (Me=Na +, K +, Ca 2+, Ba 2+) with compositions of Co 2+/Al molar ratios up to 0.5 and Si/Al molar ratios between 12 and 22 have been studied by diffuse reflectance VIS spectroscopy. Three characteristic spectral components, α, β and γ, i.e. a single band at 15 100 cm −1 (α), four bands at 16 000, 17 150, 18 600 and 21 200 cm −1 (β), and a doublet at 20 100 and 22 000 cm −1 (γ), have been ascribed to three Co ion types located at different cationic sites. The α-type Co ions are suggested to be coordinated to four oxygen atoms of the straight channel wall. The β-site is located at the deformed six-member ring at the intersection of straight and sinusoidal channels. The β-type Co ions predominate at all the Co loadings in Co–ZSM-5. The Co ions at ‘boat-shaped’ γ-site are located in the sinusoidal channel, and they represent a minor concentration of the Co ions in ZSM-5. A distribution of Co ions among those cationic sites was established.