CoPt Particles Research Articles

A Magnetocatalytic Propelled Cobalt–Platinum@Graphene Navigator for Enhanced Tumor Penetration and Theranostics

A Magnetocatalytic Propelled Cobalt–Platinum@Graphene Navigator for Enhanced Tumor Penetration and Theranostics

Size-Controlled Synthesis of L10-CoPt Intermetallic Fuel Cell Catalysts on Nitrogen-Doped Mesoporous Graphitized Carbon Support

Ordered intermetallic nanoparticles are promising electrocatalysts with enhanced activity and durability for the oxygen-reduction reaction (ORR) in proton-exchange membrane fuel cells (PEMFCs). However, after subjecting to a widely used postsynthesis annealing treatment, the particle size of intermetallic catalysts inevitably increases, leading to a poor performance in practical fuel cell application. In this work, ordered L10-CoPt intermetallic catalyst of small particle size was prepared through using nitrogen-doped mesoporous graphitized carbon (NMGC) support. The porous feature and nitrogen doping of NMGC support immobilized CoPt particles and prevented sintering during annealing treatment. These small and ordered L10-CoPt nanoparticles exhibit outstanding ORR activity (~ 1.18 A/mgPt) and superior high current density in fuel cell testing. Moreover, this L10-CoPt/NMGC catalyst performs remarkably enhanced stability in comparison with L10-CoPt/high surface area carbon (HSC) counterpart and commercial Pt/C catalyst, surpassing the DOE 2020 target. The enhancement in catalyst durability can be attributed to strong particles-support interaction via nitrogen heteroatoms and the strengthened p bonding. To better understand the support effect on particle growth, L10-CoPt particles on NMGC with different porosity/graphitization were prepared and their fuel cell performances were systematically studied. This study demonstrates an effective strategy to control the particle size of intermetallic catalysts with using NMGC support, which achieves excellent practical fuel cell performance.

Ordering and magnetic properties of nanostructured CoPt particles

The kinetics of ordering in nanostructured CoPt particles is investigated. The low activation energy (58 ± 2 kJ/mol) of the ordering process indicates that the kinetics of ordering is not limited by bulk diffusion. Coercivity grows along with the degree of ordering and reaches its maximum value of 14.2 kOe upon the complete ordering of CoPt alloy with a crystallite size of no more than that of a single domain.

Microstructures and Magnetic Properties of CoPt-C Nanogranular Films

CoPt-C films were fabricated on silicon substrate by dc reactive magnetron sputtering followed by vacuum annealing. The effects of C additions and annealing temperature on the microstructure and magnetic properties were investigated. The as-deposited films had flat, compact surfaces and face-centered cubic structure, which transforms into the face-centered tetragonal structure after thermal annealing at 700°C for 1 hour. The grain size of CoPt increased with the annealing temperature but decreased with increasing C content. No carbide appearing, the C content exists in amorphous state in the nanocomposite films, and it is homogeneous distributed between the CoPt nanograins, which have the benefit to restrain grain growth and obtain isolated CoPt particles with uniform size. The fct-CoPt films annealed at 700°C exhibited high in-plane coercivity, up to 4200 Oe at room temperature and better square degrees. In the CoPt-C granular films, the best doping amount is about 35 at.% C. These CoPt-C films with novel embedded structure and moderate coercivity have shown promise for high density magnetic recording medium.

The exchange interaction effects on magnetic properties of the nanostructured CoPt particles

Various manifestations of the exchange interaction effects in magnetization curves of the CoPt nanostructured particles are demonstrated and discussed. The inter-grain exchange constant A in the sponge-like agglomerates of crystallites is estimated as A=(7±1)pJ/m from the approach magnetization to saturation curves that is in good agreement with A=(6.6±0.5)pJ/m obtained from Bloch T 3/2 law. The fractal dimensionality of the exchange coupled crystallite system in the porous media of the disordered CoPt alloy d=(2.60±0.18) was estimated from the approach magnetization to saturation curve. Coercive force decreases with temperature as Hc~T 3/2 which is assumed to be a consequence of the magnetic anisotropy energy reduction due to the thermal spin wave excitations in the investigated CoPt particles.

Tuning ofL10atomic order in Co-Pt nanoparticles:Ab initioinsights

Monte Carlo simulation of the atomic configurational behavior of Co-Pt nanoparticles is performed in wide temperature-composition-size ranges. The simulation is based on bulk and local (inhomogeneous) cluster expansions calculated from first principles. A sharp drop of equilibrium $L{1}_{0}$ order is predicted for small equiatomic particles at low temperatures. This drop is explained by the interplay of two effects. First, the strong Pt and Co segregation to the first and second surface layers, respectively (core/Co/Pt or ``onion2shell'' profile) cause a depletion of Pt atoms within the core of small Co-Pt particles. Second, $L{1}_{0}$ is an adaptive structure on the fcc-restricted ground-state phase diagram of Co-Pt. Nevertheless, we show that a remarkable degree of $L{1}_{0}$ order can be restored by tuning the total composition of small Co-Pt particles.

Magnetic behavior of CoPt–AlN granular structure laminated with AlN layers

The magnetic behavior of CoPt–AlN granular structure laminated with AlN layers has been studied. Ultrathin multilayer structure, [CoPt0.5 nm/AlN0.5nm]4, is used as the precursor of the magnetic layers, which are separated by 5-nm-thick AlN layers. Upon thermal annealing, the ultrathin multilayer transforms into CoPt–AlN granular structure, and the thick AlN layers remain to be spacers. When the film was annealed at 400 °C, the out-of-plane direction becomes the easy axis of magnetization, although the coercivity remains small. TEM observation has proved that CoPt shows disklike shape at such an annealing temperature. When increasing the annealing temperature to 600 °C and above, the films show “isotropic” magnetic behavior due to the formation of equiaxial CoPt particles in the magnetic layers.

Synthesis and magnetic properties of size-selected CoPt nanoparticles

CoPt nanoparticles are widely studied, in particular for their potentially very high magnetic anisotropy. However, their magnetic properties can differ from the bulk ones and they are expected to vary with the particle size. In this paper, we report the synthesis and characterization of well-defined CoPt nanoparticle samples produced in ultrahigh vacuum conditions following a physical route: the mass-selected low energy cluster beam deposition technique. This approach relies on an electrostatic deviation of ionized clusters which allows us to easily adjust the particle size, independently from the deposited equivalent thickness (i.e. the surface or volume particle density in a sample). Diluted samples made of CoPt particles, with different diameters, embedded in amorphous carbon are studied by transmission electron microscopy and superconducting interference device magnetometry, which gives access to the magnetic anisotropy energy distribution. We then compare the magnetic properties of two different particle sizes. The results are found to be consistent with an anisotropy constant (including its distribution) which does not evolve with the particle size in the range considered.

Electrochemical preparation and magnetic properties of submicrometric core–shell CoPt–CoNi particles

Electrodeposition method has allowed the preparation of submicron CoPt particles covered with CoNi shells of different thicknesses. The magnetic properties of these particles have been analysed; a contribution of semi-hard magnetic behaviour and soft-magnetic behaviour, associated with the particles, and the CoNi shell, respectively, has been detected. Although a gradual decrease of the coercivity was observed by increasing the CoNi shell thickness, the CoPt core always dominates the magnetization–magnetic field curve shape.

Alloying Effect in CoPt Nanoparticles Probed by X-ray Photoemission Spectroscopy: Validity of the Bulk Phase Diagram

CoPt organized cluster assemblies are developed to produce ultra-high-density magnetic media that need the smallest sizes for recording particles. At the nanometer scale (3−4 nm in diameter in our case), surface atoms represent around 50% of the total number of atoms and the segregation effect could dominate the alloying effect. Our purpose is to describe the competition between segregation and alloying effects in as-deposited or embedded bimetallic CoPt clusters preformed in the gas phase. The segregation is studied by X-ray photoemission spectroscopy. First of all, it was found that satellite peaks at high energy disappear when particles are fully embedded in a matrix. Moreover, CoPt clusters exhibit a partial core−shell structure when the clusters are not chemically bonded to the matrix. Conversely, an alloying effect is reported when CoPt particles are embedded in a silicon matrix. These effects are analyzed on the basis of both the phase diagrams and the enthalpy formation in the bulk phase. The bulk...

Synthesis and electrocatalytic property of cubic and spherical nanoparticles of cobalt platinum alloys

This paper describes the morphological control and electrocatalytic property of CoPt nanoparticles. Both cubic and spherical CoPt nanoparticles were made using cobalt carbonyl and platinum 2,4-pentanedionate under different reaction temperatures in the presence of capping reagents, which included adamantanecarboxylic acid and hexadecylamine. Effects of heterogeneous species on shape of the CoPt nanoparticles were examined by replacing cobalt carbonyl with silver acetylacetonate. Our results suggest that the formation of different shapes of CoPt particles could be attributed to the affinity between cobalt and platinum, and the effects of capping agents. The size and shape dependent electrocatalytic properties of these nanoparticles were examined based on the direct methanol oxidation reaction.

Core-shell morphologies of FePt and CoPt nanoparticles: An ab initio comparison

Large-scale first principles calculations for near-stoichiometric FePt and CoPt clusters with up to 923 atoms are presented including unconstrained structural relaxations. The energetic order and magnetic properties of 561 atom FePt and CoPt nanoparticles are evaluated with emphasis on segregated morphologies with Pt covered surfaces. The results imply that for CoPt particles segregation of Pt to the surface and the formation of a Pt depleted subsurface layer is dominant also for nanometer-sized single crystalline particles and may help to stabilize particles with partial L10 order, while for FePt multiple twinning is the most important mechanism at small particle sizes. The mixed (100) surfaces of the fully L10 ordered FePt and CoPt isomers exhibit a characteristic reconstruction.

Electrochemical preparation and characterisation of CoPt magnetic particles

CoPt particles of different size and modulate magnetic properties have been prepared by electrodeposition. Particles of growing size from 50 nm until continuous deposits have been obtained and their composition, crystalline structure and magnetic properties have been analyzed. The prepared CoPt particles from 50 nm to 250 nm showed ferromagnetic behaviour so did the continuous deposits. However, drastic changes in magnetism have been detected related to the size of the particles: the smallest particles presented lower coercivity which increases with increasing size, with a maximum value for particles of 150–250 nm diameter. The coercivity decreased when continuous deposits were attained due to the disordered growing and the loss of the surface anisotropy.

Formation of Close-Packed Nanoparticle Chains

The self-assembly of CoPt(3) particles (diameter 6 nm) into low-dimensional ordered arrays results from phase separation in a hexane solution containing nanoparticles, hexadecylamine, and water. The evaporation of hexane from the thin film of solution initiates the formation of a water layer on the solid substrate. Subsequent dewetting of this water layer leads to the formation of water droplets. The nanoparticles follow the motion of the contact line of the dewetting water layer and thus assemble into ordered arrays at the periphery of the water droplets.

The influence of Sb doping in achieving high magnetic coercivities in CoPt nanoparticles for micromagnet applications

The use of magnetic elements within microelectronic devices are increasingly required in the fabrication of miniature magnetic structures with high energy densities. A synthesis technique is reported that yields Sb-doped CoPt nanoparticles possessing magnetic coercivities as high as 1671 kA/m and magnetic remanences of 295 kA/m, providing an energy product of 20 kJ/m2. Antimony doping was shown to influence the atomic ordering within the alloy sublattices, which allowed the tetragonalization temperature of the nanoparticle structure to be lowered by 200 °C to 400 °C, thereby reducing crystallite growth and sintering during annealing. The “as-synthesized” particles had average diameters of 4.5 nm, which rose to 25 nm on annealing at 600 °C. Synthesis of the doped CoPt particles with high-energy products together with control of particle size distributions in the range of 25 nm allows fabrication of micromagnetic structures by conventional microfabrication techniques such as spin coating and ink-jet printing.

Fabrication of CoPt nanoparticles with high coercivity on a polymer film

L1 0 structured CoPt nanoparticles were fabricated on a polyimide film by depositing a 1.7-nm-thick Co–Pt alloy film, followed by annealing at 650–800 °C during which the Co–Pt alloy film was converted into a monolayer of 4 nm sized L1 0 phase CoPt particles. In order to produce well-separated larger particles, multiple depositions of 1.7-nm-thick Co–Pt alloy film were carried out. Heterogeneous nucleation on the pre-existing CoPt nanoparticles allowed the film deposition and annealing to be repeated up to four times to increase the average particle size from 4 nm to 7 nm as needed. The monolayer of 7-nm-szied CoPt nanoparticles exhibited a high coercivity of 9.8 kOe with a remanence ratio of 0.72. The method can be easily extended to a large area as well as other material systems to create a monolayer of metal nanoisland structures on a polymer film.

Weak ferromagnetism and exchange biasing in cobalt oxide nanoparticle systems

Cobalt oxide nanoparticle systems have been prepared by wet chemical processing involving the encapsulation of the nanoparticles by an organic ligand shell (oleic acid and oleylamine). CoO nanoparticles were easily prepared by this method, while the synthesis of the CoPt∕CoO nanocomposites was achieved using a two step polyol process. CoPt nanoparticles were first obtained by simultaneous reduction of cobalt acetate and platinum acetylacetonate and then used as seeds for the growth upon them of cobalt oxide using a second polyol process. The antiferromagnetic CoO nanoparticles, when field cooled to temperatures below 200K, show displacement of the magnetization curves (along the magnetization axis) characteristic of weak ferromagnetism phenomena that are attributed to the uncompensated surface magnetic moments. The transition temperature of the particles is considerably lower than the Néel temperature of CoO and it is followed by an upswing at low temperatures, which is attributed to spins that are loosely coupled to the antiferromagnetic core. In the CoPt∕CoO nanocomposites, magnetic measurements show the appearance of increased coercivity with respect to the as-prepared CoPt particles and unidirectional anisotropy (loop shift of Heb=1125Oe) at temperatures below 20K, as a result of exchange coupling between CoO and CoPt.

Novel fabrication method of IBICVD for FePt and CoPt particles

A novel fabrication method of ion beam-induced chemical vapor deposition with a focused ion beam system has been applied to fabricate FePt and CoPt particles. The size of the particles depends on the pattern size and frame number. The FePt particles with smallest diameter of about 110 nm were prepared. Both FePt and CoPt particles show fcc structures and soft ferromagnetic properties. Decreasing the Ga/sup +/ ion beam intensity may improve the crystallization structure of the particles. Concentric ring magnetic domain patterns have been observed in FePt and annealed CoPt particles with diameter of about 2 /spl mu/m.

Ion-beam-induced chemical-vapor deposition of FePt and CoPt particles

FePt and CoPt particles have been directly deposited on Si3N4 substrate by ion-beam-induced chemical-vapor deposition (IBICVD) with a focused ion-beam system. The as-deposited FePt particles are amorphous and crystallize into fcc structure after annealing at 600°C. The as-deposited CoPt particles and the CoPt particles annealed at 600°C are both in fcc structure, but annealing improves the crystallization of the CoPt particles. After applying a 20-kOe magnetic field perpendicular to the Si3N4 substrate, the magnetic force microscopy images of annealed FePt and CoPt particles both show concentric domain patterns, implying the alternately up- and downward perpendicular components of magnetizations. The successful fabrication of FePt and CoPt particles may lead to the potential application of IBICVD for nanoparticles and clusters which have not been synthesized yet.

Magnetic domain observations of FePt and CoPt particles fabricated by IBICVD

Magnetic domain observations of FePt and CoPt particles fabricated by IBICVD