Cobalt Atoms Research Articles

Structural and magnetic properties of SmCo5−XNiX intermetallic compounds

Modern technological applications in an extensive variety of fields require the use of Permanent Magnets (PMs). Intermetallic compounds such as SmCo5 are already used as high-performance PMs. Reducing the high content of the expensive cobalt in SmCo5 from low-priced transition metals can lead in a cost reduction. This study examines by computational methods the effect of substituting cobalt atoms in the crystal structure of SmCo5 by nickel atoms. The aim is to specify the structure that will be stable and at the same time will maintain high values of magnetization. A series of atomistic simulations are implemented based on Density Functional Theory calculations. Various simulations are performed by considering all possible crystallographic positions of Co and Ni atoms in a SmCo5−xNix compound. Based on energy minimization and maximizing the magnetization we pinpointed the interesting cases. An experimental implementation based on the sample with x = 1 is presented to translate the findings from atomistic simulations to realizable bulk materials. Interestingly, it is concluded that in many cases an energetically favourable atomistic configuration does not exhibit maximum magnetization. It should be noted that for the experimentally investigated case of SmCo4Ni, both the energetically favourable as well as the magnetically maximum configuration have been identified.

Electronic Structure of Graphene on Silicon Carbide Intercalated with Silicon and Cobalt Atoms

Electronic Structure of Graphene on Silicon Carbide Intercalated with Silicon and Cobalt Atoms

New insight of tailor-made graphene oxide for the formation of atomic Co-N sites toward hydrogen evolution reaction

The designed synthesis of graphene oxide (GO) for the formation of nitrogen-coordinated atomic metal sites (M−N−C) is crucial for developing efficient M−N−C catalysts. Here, the effect of basal plane oxygen functional groups of GO on the adsorption of cobalt ions and their Co-N transition is investigated by tailoring the oxygen content and composition of oxygen functional groups on the basal plane of GO sheets. The tailor-made GO shows that the specific oxygen functional groups, such as hydroxyl, epoxy, and carbonyl groups, can be efficiently incorporated with formation of defected structures on the basal plane of GO sheets rather than their edge. The adsorption of cobalt ions on the GO sheets and their transition to the Co-N state are systematically investigated by XPS and XAFS analyses. Annealing temperature-controlled experiments reveal that the incorporation of nitrogen around cobalt atoms is facilitated by the increased number of oxygen functional groups on the basal plane rather than those on the edge of GO sheets, which provide energetically favorable environments for Co-N formation. The hydrogen evolution reaction (HER) activities of the resulting cobalt atom-immobilized and annealed GO (Co/GOx-T) catalysts correlate to the degree of Co-N saturation, while also demonstrating excellent HER catalytic durability.

ASAXS measurements on ferritin and apoferritin at the bioSAXS beamline P12 (PETRA III, DESY).

Small-angle X-ray scattering is widely utilized to study biological macromol-ecules in solution. For samples containing specific (e.g. metal) atoms, additional information can be obtained using anomalous scattering. Here, measuring samples at different energies close to the absorption edges of relevant elements provides specific structural details. However, anomalous small-angle X-ray scattering (ASAXS) applications to dilute macromolecular solutions are challenging owing to the overall low anomalous scattering effect. Here, pilot ASAXS experiments from dilute solutions of ferritin and cobalt-loaded apoferritin are reported. These samples were investigated near the resonance X-ray K edges of Fe and Co, respectively, at the EMBL P12 bioSAXS beamline at PETRA III, DESY. Thanks to the high brilliance of the P12 beamline, ASAXS experiments are feasible on dilute protein solutions, allowing one to extract the Fe- or Co-specific anomalous dispersion terms from the ASAXS data. The data were subsequently used to determine the spatial distribution of either iron or cobalt atoms incorporated into the ferritin/apoferritin protein cages.

Adsorption of single metallic atoms on self-assembled molecular domain of terephthalic acid

Cobalt and gold atoms have been deposited on self-assembled molecular domains of terephthalic acid and are studied using low temperature scanning tunneling microscopy and DFT calculations. These two types of atoms present different adsorption properties as well as peculiar electronic structures. Co atoms are firmly adsorbed on the molecules while Au is highly dependent of the presence of diffusing hydrogen atoms. These latter can also favour the possible desorption of the adatoms or change their adsorption site depending on the presence of defective molecules. The results open the way to new investigations concerning the use of single atoms or molecules for chemistry at the subnanometer scale.

A Supramolecular Porous Organic Cage Platform Promotes Electrochemical Hydrogen Evolution from Water Catalyzed by Cobalt Porphyrins

AbstractWe report a supramolecular porous organic cage platform composed of cobalt porphyrins for catalyzing the electrochemical hydrogen evolution reaction (HER) from water at neutral pH. Owing to its permanent porosity, the supramolecular structure yields a catalyst film with a fivefold increase in the number of electrochemically active cobalt atoms and an improvement in Tafel slope from 170 mV/decade to 119 mV/decade compared to a planar cobalt porphyrin analog, reaching activities over 19,000 turnovers for HER over a 24‐hour period with 100 % Faradaic efficiency.

CALPHAD-assisted development of in-situ nanocrystallised melt-spun Co-Fe-B alloy with high Bs (1.57 T)

A thermodynamics-based approach, Calculation of Phase Diagram (CALPHAD), combined with topological instability parameters are proposed and experimentally evaluated, in order to optimise in-situ nanocrystallisation of rapidly quenched CoFeB alloys and exploit their remarkable Bs = 1.57 T. The high Ms of the alloy is attributed to the precipitation of the metastable Co7Fe3 nanocrystalline phase dispersed heterogeneously in the amorphous matrix. High Ms of Co7Fe3 phase can also be inferred from the high hyperfine magnetic field of the Fe nuclei deduced from Mössbauer spectra. It is worth noting that the in-situ nanocrystallisation is a volume phenomenon, instead of surface crystallisation at the air-side of ribbons owning to lower cooling rates. We judge, based on nucleation theory, that the formation of the metastable phase is kinetically favoured, when compared to the equilibrium phases, hence promoting the high Ms, when compared with conventional Co-rich amorphous alloys. The local atomic order of nanocrystallised phase was confirmed by X-ray and electron diffraction techniques. Using Mössbauer spectroscopy and the extracted distribution of the hyperfine magnetic field, it is asserted that cobalt atoms form clusters, as they attract each other to form ordered structures, and boron atoms undergo only short-range ordering, likely due to covalent bond formation, governed by the size and electronegativity differences with the atoms in the amorphous matrix. We suggest the proposed CALPHAD-assisted design of nanostructured alloys, along with an in-situ nanocrystallisation, provides a practical scheme to develop novel functional alloys with the best possible balance of coercivity and saturation, exclusively aimed for a high-tech application.

Cobalt doping of nickel ferrites via sol gel approach: effect of doping on the structural and dielectric properties

Abstract Cobalt doped nickel ferrites were fabricated by sol gel route and the dielectric constant, tangent loss and AC conductivity were investigated as a function of Co doping. The X-ray diffractometer characterization confirmed that the Co x Ni1−x Fe2O4 with doping concentration (x = 0.1, 0.2, 0.3, 0.4, and 0.5) have cubic spinel structure. In the XRD spectrum there appear extra peaks of Fe2O3 as an impurity that is gradually disappear by increasing doping ratio of cobalt ions, which is an indication of high crystallinity. The structural parameters (lattice constant, grain size, dislocation density, X-rays density and packing factor) are greatly influenced by the doping of cobalt atoms i.e., lattice constant increases. The crystal size increases from 30 to 42.26 nm by cobalt substitution in the pure nickel ferrites. The Fourier Transform IR Spectroscopy indicate shift in peaks to lower frequency region because cobalt doping reduced binding energy between metal ion and oxygen ions. Atomic structure of cobalt doped nickel ferrites examined by the Raman spectroscopy. Co x Ni1−x Fe2O4 shows Raman mode at ∼285, ∼477, ∼563, ∼624 and ∼704 cm−1. There is unnoticeable Raman shift due to the doping of cobalt’s atoms.

In situ topochemical carbonization derivative Co-Ni alloy@Co-Co2C for direct ethanol synthesis from syngas

In this work, Coδ+-Co2C decorated-Co-Ni alloy nanoparticles (NPs) loaded on Mg-Al-O complex oxides were fabricated via an in-situ reduction and topochemical carbonization strategy by using hydrotalcite as the precursor. The Coδ+-Co2C decorated-Co-Ni alloys were obtained via the following process: (I) after reduction, Co-Ni alloy was obtained and Co could donate electrons to Ni in the Co-Ni alloy NPs; (II) in syngas atmosphere, some of cobalt atoms on the surface of Co-Ni NPs reacted with CO and were converted to Co2C. Resultantly, in the reaction process, Co-Ni alloy NPs were transformed into Ni-Co alloy@Co-Co2C, a core-shell structure with Ni-Co alloy core and Co-Co2C shell. When used as the catalysts for direct ethanol synthesis (DES) from syngas, the bimetallic Co-Ni alloy@Co-Co2C catalyst performed excellent selectivity to ethanol, owing to the regulation of dissociatively and associatively adsorbed CO by Coδ+-Co2C active sites formed on the surface of Co-Ni alloy NPs. Moreover, the mutual dilution between Co and Ni in the Co-Ni alloy NPs inhibited the generation of heavy hydrocarbons, further improving the selectivity to alcohols. The total alcohol selectivity of 43.0% and ethanol occupied 45.2 wt% could be reached over the optimal catalyst.

Perfluorinated Phthalocyanines on Cu(110) and Cu(110)-(2 × 1)O: The Special Role of the Central Cobalt Atom

Interface properties of CoPcF16 on Cu(110) and Cu(110)-(2 × 1)O were investigated by X-ray photoemission spectroscopy (XPS), ultraviolet photoemission spectroscopy (UPS), X-ray absorption spectrosc...

Implanting Cobalt Atom Clusters within Nitrogen-Doped Carbon Network as Highly Stable Cathode for Lithium-Sulfur Batteries.

Realization of highly efficient sulfur electrochemistry, as well as the high capacity of lithium-sulfur (Li-S) batteries, can be achieved by the scientific construction of electrode host materials. In this study, using molten NaCl, a 3D porous nitrogen-doped carbon with uniformly embedded Co atom clusters (Co/PNC) is developed by pyrolyzing the precursors with NaCl at high temperatures. In the composite structure, a network carbon skeleton containing hierarchical pores acts as an advanced matrix for sulfur electrodes, and the doping of N and Co is subject to inhibit the shuttle of long-chain lithium polysulfides through chemical adsorption. The Co/PNC, with the optimized amount of Co, delivers an initial specific capacity of 1105.4 mAh g-1 at 0.2 C with a capacity drop of only 0.064% after the cell is charged and discharged for 300 cycles at 1 C, revealing its potential in promoting the large-scale application of Li-S batteries.

The Effect of Annealing On The Crystal Structure of the Surface of Silicon Doped With Iron And Cobalt Ions

The paper reports results of the study of the distribution profiles of implanted iron and cobalt atoms in silicon as a function of the radiation dose and annealing temperature that was performed by applying the Rutherford backscattering spectroscopy (RBS). The effects of thermal annealing on the distribution of iron, cobalt, and in particular oxygen were studied. The authors strongly suggest that under certain heat treatment conditions and by applying specific radiation doses, the so-called epitaxial silicides will build on the surface of a single crystal, which can play the role of conducting or metal layers. One could consider theRBS method for analysis of both the topological distribution of dopants and the interaction of impurities.

Engineering the Coordination Environment of Single Cobalt Atoms for Efficient Oxygen Reduction and Hydrogen Evolution Reactions

The coordination environment of single-atom catalysts (SACs) plays a crucial role in determining the energy conversion efficiency of related electrochemical devices. Herein, the coordination enviro...

Single, double and triple cobalt atoms confined in 2D regular framework for oxygen electrocatalysis

Many natural metalloenzymes consist of multi-nuclear metal sites. The multiple metal centers trigger a cooperative catalytic effect that decreases the reaction energy barrier. However, it remains a grand challenge in mimicking the metalloenzymes in heterogeneous catalysts. C2N is an emerging 2D material with a predictive regular holey framework. Its large hole with a diameter of ~8.35 Å contains three pairs of pyridinic nitrogen, allowing the capture of transition metals from single to triple atoms. Based on quantum chemistry calculations, we find that the introduction of double (Co2@C2N) and triple cobalt (Co3@C2N) atom clusters turns C2N nanosheet from semiconducting into a nearly metallic state. The confined double or triple atoms can work cooperatively toward oxygen reduction and evolution reaction (ORR and OER). Impressively, with the activation barrier of 0.57 eV, Co3@C2N is a competitive novel candidature that can substitute bulk Pt catalyst (~0.80 eV). In-depth analysis indicates that the formation of an oxo-doped field environment is the key to realize efficient oxygen electrocatalysis.

Embedding atomic cobalt into graphene lattices to activate room-temperature ferromagnetism

Graphene is extremely promising for next-generation spintronics applications; however, realizing graphene-based room-temperature magnets remains a great challenge. Here, we demonstrate that robust room-temperature ferromagnetism with TC up to ∼400 K and saturation magnetization of 0.11 emu g−1 (300 K) can be achieved in graphene by embedding isolated Co atoms with the aid of coordinated N atoms. Extensive structural characterizations show that square-planar Co-N4 moieties were formed in the graphene lattices, where atomically dispersed Co atoms provide local magnetic moments. Detailed electronic structure calculations reveal that the hybridization between the d electrons of Co atoms and delocalized pz electrons of N/C atoms enhances the conduction-electron mediated long-range magnetic coupling. This work provides an effective means to induce room-temperature ferromagnetism in graphene and may open possibilities for developing graphene-based spintronics devices.

Atomic cobalt anchored on covalent triazine frameworks with ultra-high performance towards oxygen reduction reaction

Single cobalt atom is promising non-precious metal catalyst instead of Pt in the oxygen reduction reaction (ORR). However, it is still a great challenge to develop a cost-effective, ultrastable and efficent single-atom cobalt catalyst for ORR, requiring efficient fabrication strategies and robust support to stabilize the single cobalt atom. Here, we prepared a highly active and stable atomically isolated cobalt catalyst via covalent triazine framework (CTF) support with Ketjen Black (KB) hybridization in scale. The prepared single Co catalyst (Co-CTF/KB) possesses high metal loading over 4 wt% and shows superior ORR performance with a half-wave potential (E1/2) of 0.830 V and a limiting current density of 6.14 mA cm−2 as well as high tolerance of methanol in an alkaline medium, which outperforms commercial Pt/C and most non precious metal catalysts reported to date. Benefiting from strong stabilization of Co atoms on CTF, Co CTF/KB shows outstanding stability with only 5 mV negative shifts after 10,000 cycles. Moreover, it also displays high catalytic activity for oxygen evolution reaction (OER), suggesting it is an efficient ORR/OER bifunctional catalyst. The present work provides a facile strategy for preparing single atom catalysts in bulk quantity and contributes to development of catalysts for electrochemical conversion and storage devices.

Kinetically Stable Nonequilibrium Gold-Cobalt Alloy Nanoparticles with Magnetic and Plasmonic Properties Obtained by Laser Ablation in Liquid.

Nonequilibrium nanoalloys are metastable solids obtained at the nanoscale under nonequilibrium conditions that allow the study of kinetically frozen atoms and the discovery of new physical and chemical properties. However, the stabilization of metastable phases in the nanometric size regime is challenging and the synthetic route should be easy and sustainable, for the nonequilibrium nanoalloys to be practically available. Here we report on the one-step laser ablation synthesis in solution (LASiS) of nonequilibrium Au-Co alloy nanoparticles (NPs) and their characterization on ensembles and at the single nanoparticle level. The NPs are obtained as a polycrystalline solid solution stable in air and water, although surface cobalt atoms undergo oxidation to Co(II). Since gold is a renowned plasmonic material and metallic cobalt is ferromagnetic at room temperature, these properties are both found in the NPs. Besides, surface conjugation with thiolated molecules is possible and it was exploited to obtain colloidally stable solutions in water. Taking advantage of these features, an array of magnetic-plasmonic dots was obtained and used for surface-enhanced Raman scattering experiments. Overall, this study confirms that LASiS is an effective method for the formation of kinetically stable nonequilibrium nanoalloys and shows that Au-Co alloy NPs are appealing magnetically responsive plasmonic building blocks for several nanotechnological applications.

Tunable cobalt doping titanium nitride (TixCoyN) interlaced nanotubes enable an enhanced electronic synergy on visible-light driven hydrogen evolution

Overcoming the limited light harvest and sluggish charge transfer efficiency has remained a long-standing challenge for the practical application of visible-light driven hydrogen evolution. In this work, a novel cobalt doped titanium nitride interlaced nanotube was synthesized through a facile hydrothermal approach and subsequent ammonia gas nitrification route. The optimized Ti0·75Co0·25N (molar ratio of Ti:Co:N = 0.75:0.25:1) catalyst delivered the highest hydrogen yield in visible-light-induced water splitting, which was almost 2 times higher than the pristine TiN did. Further structure characterizations have unraveled that the cobalt atoms were partially substituted the titanium atoms in crystallite lattice, generating a significant synergistic regulation on band structure and electronic state. As a result, the narrowed energy bandgap, improved charge carrier mobility and lowered over-potential contributed to the high electron-hole separation efficiency and smoothed interface charge transfer, yielding an enhanced photocatalytic activity. This work is a significant contribution to the design of efficient and stable visible light driven water splitting catalysts.

Spontaneous migration induced Co nanokarstcave encapsulated in N-doped carbon hybrids for efficient oxygen electrocatalyst

Despite the extensive application of porous nanostructures as oxygen electrocatalysts, it is challenging to synthesize single-metal state materials with porous structures, especially the ultrasmall ones due to the uniform diffusion of the same metal. Herein, we pioneer demonstrate a new size effect-based controllable synthesis strategy for the homogeneous Co nanokarstcaves assisted by Co-CN hybrids (CCHs). The preferential migration of cobalt atoms on the surface of small size zeolitic imidazolate framework (ZIF) with high surface energy during pyrolysis is the key factor for the formation of nanokarstcave structure. Furthermore, graphene can act as a diffusion barrier to prevent the agglomeration of nanoparticles in the synthesis process, which also plays an important role in the formation of porous nanostructures. In alkali media, CCHs achieve overpotential of 287 mV (@10 mA·cm−2) for oxygen evolution reaction (OER) and a half wave potential of 0.86 V (vs. RHE) for oxygen reduction reaction (ORR).

Synthesis, X-ray crystal structure and BVS calculation of Co(II) complex of pyrimidine derived Schiff base ligand: Approached by Hirshfeld surface analysis and TDDFT calculation

A new Co(II) complex [Co(L)(Cl)2]•(MeOH)2 (1) has been synthesized using a symmetrical Pyrimidine derived Schiff base ligand 2,2′-((2E,2′E)-2,2′-(butane-2,3-diylidene)bis(hydrazin-1-yl-2-ylidene))bis(4,6-dimethylpyrimidine) (L) and characterized by single crystal X-ray crystallography analysis. The solid-state structure of 1 is pseudooctahedral with two methanol molecules outside the coordination sphere. The said complex is stabilized through various intermolecular H-bonding incorporating non coordinated solvent methanol molecules. Bond Valence Sum (BVS) calculation confirms that the central cobalt atom in 1 exists as ‘+2′ oxidation state instead of ‘+3′. Hirshfeld surface analysis is performed to determine the intermolecular interactions and the crystal packing of the title complex. The 2D fingerprint plots associated with the Hirshfeld surface clearly present each significant interaction involved in the structure, by quantifying them in an effective visual manner. The electronic transition of complex 1 was recorded in methanol solvent and the electronic distribution of HOMO−LUMO can be rationalized theoretically (through time-dependent density functional theory (TDDFT)).