FePt Alloy Nanoparticles Research Articles

Local Chemical Strain in PtFe Alloy Nanoparticles.

Precise deciphering of the atomic-insight chemical strain and distribution in nanosolids, especially in nanoalloys with disordered structure, is significant to correlate the structure and chemical properties. In this report, we found an abnormally enhanced negative thermal expansion (NTE) along the c axis ina 3.5 nm PtFe magnetic L10 alloy, which was usually weakened by the reduced contribution of invar behavior ina nanosized magnetic material. 57Fe Mössbauer spectra and extended X-ray absorption fine structure studies indicated the local disordered state of chemical coordination of the L10 phase. Pair distribution function and reverse Monte Carlo simulation revealed a surface A1-phase coating and atomic-level stretching chemical strain on the heteroatomic pair along the c axis. Local tensile chemical strain and increasing magnetic moments of Fe atoms on the surface of the L10 core made a dominant contribution to the enhanced NTE behavior. The present work brings an atomic reveal for the local chemical strain in bimetallic nanomaterials and provides an innovative perspective to tailoring the functional properties according to the local chemical environment.

Magnetically Coated Bioabsorbable Stents for Renormalization of Arterial Vessel Walls after Stent Implantation.

The insertion of a stent in diseased arteries is a common endovascular procedure that can be compromised by the development of short- and long-term inflammatory responses leading to restenosis and thrombosis, respectively. While treatment with drugs, either systemic or localized, has decreased the incidence of restenosis and thrombosis these complications persist and are associated with a high mortality in those that present with stent thrombosis. We reasoned that if stents could be made to undergo accelerated endothelialization in the deployed region, then such an approach would further decrease the occurrence of stent thrombosis and restenosis thereby improving clinical outcomes. Toward that objective, the first step necessitated efficient capture of progenitor stem cells, which eventually would become the new endothelium. To achieve this objective, we engineered intrinsic ferromagnetism within nonmagnetizable, biodegradable magnesium (Mg) bare metal stents. Mg stents were coated with biodegradable polylactide (PLA) polymer embedding magnetizable iron-platinum (FePt) alloy nanoparticles, nanomagnetic particles, nMags, which increased the surface area and hence magnetization of the stent. nMags uniformly distributed on stents enabled capture, under flow, up to 50 mL/min, of systemically injected iron-oxide-labeled (IO-labeled) progenitor stem cells. Critical parameters enhancing capture efficiency were optimized, and we demonstrated the generality of the approach by showing that nMag-coated stents can capture different cell types. Our work is a potential paradigm shift in engineering stents because implants are rendered as tissue in the body, and this "natural stealthiness" reduces or eliminates issues associated with pro-inflammatory immune responses postimplantation.

FePt nanoalloys on N-doped graphene paper as integrated electrode towards efficient formic acid electrooxidation

Pt-based bimetallic alloys have attracted intensive interests as highly efficient and stable heterogeneous catalysts for fuel cell applications, and searching flexible free-standing nanomembranes has been on the research focus because of their promising applications in catalysis, nanoelectronics, batteries, and sensing. Herein, a flexible free-standing paper composed of FePt alloy nanoparticles on nitrogen-doped paper (FePt–NG paper) has been developed as integrated electrode towards electrocatalytic oxidation of formic acid. It is found that, N doping is beneficial for the well dispersion of FePt nanoparticles on NG paper. The synergistic effect between Fe and Pt, as well as N doping effect simultaneously assures the improved catalytic activity of FePt–NG paper, making it the promising flexible electrode for application in fuel cell system. Moreover, the synthesis strategy could be used for the construction of other free-standing electrode towards catalysis, sensing, and battery applications.

Tri-iodide reduction activity of ultra-small size PtFe nanoparticles supported nitrogen-doped graphene as counter electrode for dye-sensitized solar cell

Efficient and cost effective counter electrode (CE) is pre-requisite for the commercialization of dye-sensitized solar cell (DSSC). Present work investigates ultra small size platinum-iron alloy nanoparticles dispersed over nitrogen-doped graphene (PtFe/NG) as an effective counter electrode for DSSC. Hereby we achieve low loading of Pt by alloying with Fe accompanied by superior electrocatalytic activity towards the iodide-triiodide (I−/I3−) mechanism. Enhancement in electrocatalytic performance of PtFe/NG has been shown by cyclic voltammetry, electrochemical impedance spectroscopy and Tafel polarization analysis. PtFe/NG counter electrode exhibits higher power conversion efficiency (∼6.12%) with lower charge transfer resistance, which helps in faster diffusion of I−/I3− ions as compared to NG and Pt/NG counter electrodes. The increased electrocatalytic activity of PtFe/NG is due to the collective effect of intrinsic electronic effects by alloying, uniform dispersion of small PtFe alloy nanoparticles over nitrogen doped graphene, and additional catalytic sites offered by nitrogen-doped graphene.

Structure and stability of Fe-Pt bimetallic nanoparticles: Initial structure, composition and shape effects

The aim of this work is to explore the effects of initial structure, composition and shape on structural stability of Fe-Pt bimetallic nanoparticles. Because the structures and stabilities of nanoparticles are closely related to their chemical and physical properties, an essential investigation of their structures and stabilities is of significance to their applications. To search the stable structures and examine the structural characteristics of Fe-Pt nanoparticles, a discrete particle swarm optimization algorithm has been proposed to perform their structural optimizations. The effects of initial structure, different compositions and shapes have been addressed in this article. The results show that our algorithm has a fast convergence rate and good optimal solution as compared with the Monte Carlo algorithm. The initial structures have a certain influence on the convergence rate of our algorithm while they have little effect on the optimized structures. Moreover, the optimized structures are relatively sensitive to Fe/Pt ratios since Fe compositions affect the atomic distribution of stable structures. Furthermore, we compared the lowest potential energies of different shaped nanoparticles with particle sizes from 1 to 5 nm, and found that the icosahedral shape is the best stable structure because it has the extraordinarily lowest energy among all the considered shapes, while the decahedron exhibits the worst structural stability. This study provides a significant insight to the structural stability and structural features of Fe-Pt alloy nanoparticles with different structures, compositions and shapes, and contributes to the design of bimetallic nanoparticles.

Infrared-active quadruple contrast FePt nanoparticles for multiple scale molecular imaging

A single nanomaterial with multiple imaging contrasts and functions is highly desired for multiscale theragnosis. Herein, we demonstrate single 1–1.9 μm infrared-active FePt alloy nanoparticles (FePt NPs) offering unprecedented four-contrast-in-one molecular imaging – computed tomography (CT), magnetic resonance imaging (MRI), photoacoustic (PA) imaging, and high-order multiphoton luminescence (HOMPL) microscopy. The PA response of FePt NPs outperforms that of infrared-active gold nanorods by 3- to 5.6-fold under identical excitation fluence and particle concentrations. HOMPL (680 nm) of an isolated FePt NP renders spatial full-width-at-half-maximum values of 432 nm and 300 nm beyond the optical diffraction limit for 1230-nm and 920-nm excitation, respectively. The in vivo targeting function was successfully visualized using HOMPL, PA imaging, CT, and MRI, thereby validating FePt as a single nanomaterial system covering up to four types (Optical/PA/CT/MRI) of molecular imaging contrast, ranging from the microscopic level to whole-body scale investigation.

Surface Segregation of Fe in Pt-Fe Alloy Nanoparticles: Its Precedence and Effect on the Ordered-Phase Evolution during Thermal Annealing.

Coupling electron microscopy techniques with in situ heating ability allows us to study phase transformations on the single-nanoparticle level. We exploit this setup to study disorder-to-order transformation of Pt–Fe alloy nanoparticles, a material that is of great interest to fuel-cell electrocatalysis and ultrahigh density information storage. In contrast to earlier reports, we show that Fe (instead of Pt) segregates towards the particle surface during annealing and forms a Fe-rich FeOx outer shell over the alloy core. By combining both ex situ and in situ approaches to probe the interplay between ordering and surface-segregation phenomena, we illustrate that the surface segregation of Fe precedes the ordering process and affects the ordered phase evolution dramatically. We show that the ordering initiates preferably at the pre-existent Fe-rich shell than the particle core. While the material-specific findings from this study open interesting perspectives towards a controlled phase evolution of Pt–Fe nanoalloys, the characterization methodologies described are general and should prove useful to probing a wide-range of nanomaterials.

Investigation of pyrolysis temperature in the one-step synthesis of L10 FePt nanoparticles from a FePt-containing metallopolymer

Ferromagnetic (L10 phase) FePt alloy nanoparticles (NPs) can be generated by single-step pyrolysis of metallopolymers. The influence of the pyrolysis temperature on the size and magnetic properties of these NPs were investigated.

Reconstructed PtFe Alloy Nanoparticles with Bulk-Surface Differential Structure for Methanol Oxidation

The high cost of the catalyst material is a large obstacle for the commercialization of Direct Methanol Fuel Cells. In present work, the non-noble metal Fe was added to the conventional Pt/C catalyst in order to lowering the catalyst cost without sacrificing the catalytic performance. A chemically dealloying procedure was adopted to leach out the surface iron atoms and thereby increasing the stability of the catalyst. Through these procedures, the PtFe bulk-surface differential nano-catalyst was obtained. The shift in diffraction angles according to XRD spectra confirms the formation of the alloy structure between Pt and Fe. TEM results indicate that the chemically dealloyed PtFe/C nanoparticles are uniformly-dispersed with optimized average size. The PtFe bulk-surface differential structure is inferred from XPS as no surface Fe atoms are detected in spite of the change in Pt electronic structure. The existence of Fe atoms inside the nanoparticles is further confirmed by the ICP characterization. Electrochemical characterizations show that the catalytic activity is increased to 2.8 times, compared to the Pt/C catalyst, which can be ascribed to the electronic effect of Fe. A further evidence for the electronic effect comes from the CO stripping spectroscopy, in which the onset potential for CO oxidation shifts dramatically toward the negative potential side compared with Pt/C. Owing to the bulk-surface differential structure, the chemically dealloyed PtFe/C catalysts exhibits nearly the same stability compared to that of the Pt/C catalyst.

FePt nanoalloys anchored reduced graphene oxide as high-performance electrocatalysts for formic acid and methanol oxidation

Reduced graphene oxide (RGO) supported FePt alloy nanoparticles are synthesized as high-performance electrocatalysts for methanol and formic acid (FA) oxidation. The microstructure, composition and morphology of the sample are systematically characterized with X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscope (SEM), transmission electron microscope (TEM), energy dispersive spectroscopy (EDS) and Raman spectroscopy. The electrocatalytic tests reveal that, in comparison with pure Pt nanoparticles supported on RGO sheets, the FePt nanoalloys anchored RGO sheets deliver remarkably enhanced electrocatalytic performance on FA/methanol oxidation, improved tolerance to CO poisoning, and superior catalytic durability.

Influence of ball milling and annealing conditions on the properties of L1 FePt nanoparticles fabricated by a new green chemical synthesis method

In this work, a new green chemical strategy for the synthesis of L10 FePt alloy nanoparticles is reported. The precursor is a polycrystalline molecular complex (Fe(H2O)6PtCl6), in which Fe and Pt atoms are arranged on alternating planes and milled with NaCl to form nanocrystals. Then the mixture was annealed under reducing atmosphere (5% H2 and 95% Ar) at temperatures varying from 350 °C to 500 °C for 2 h with a heating rate of 5 °C/min. After the reduction, the mixture was washed with water to remove the NaCl and L10 FePt nanoparticles were obtained. The X-Ray Diffraction pattern showed the presence of the characteristic peaks of the fct phase of FePt nanoparticles. Influence of precursor/NaCl ratio and ball milling time on particle size was investigated. Transmission electron microscopy images revealed that smaller precursor/NaCl ratio (10 mg/20 g) and longer milling time (15 h) lead to smaller particle size and narrower size distribution. Milling time does not influence the coercivity much but the decrease of the amount of precursor leads to a decrease of coercivity from 10.8 kOe to 4.8 kOe.

Facile Generation of L10‐FePt Nanodot Arrays from a Nanopatterned Metallopolymer Blend of Iron and Platinum Homopolymers

Hard ferromagnetic (L10 phase) FePt alloy nanoparticles (NPs) with extremely high magnetocrystalline anisotropy are considered to be one of the most promising candidates for the next generation of ultrahigh‐density data storage system. The question of how to generate ordered patterns of L10‐FePt NPs and how to transform the technology for practical applications represents a key current challenge. Here the direct synthesis of L10 phase FePt NPs by pyrolysis of Fe‐containing and Pt‐containing metallopolymer blend without post‐annealing treatment is reported. Rapid single‐step fabrication of large‐area nanodot arrays (periodicity of 500 nm) of L10‐ordered FePt NPs can also be achieved by employing the metallopolymer blend, which possesses excellent solubility in most organic solvents and good solution processability, as the precursor through nanoimprint lithography (NIL). Magnetic force microscopy (MFM) imaging of the nanodot pattern indicates that the patterned L10 phase FePt NPs are capable of exhibiting decent magnetic response, which suggests a great potential to be utilized directly in the fabrication of bit patterned media (BPM) for the next generation of magnetic recording technology.

FePt Alloy Nanoparticles for Biosensing: Enhancement of Vitamin C Sensor Performance and Selectivity by Nanoalloying

Electrocatalytic activity of supported FePt alloy nanoparticles (NPs) with different compositions (Fe25Pt75, Fe30Pt70, Fe35Pt65) for the electro-oxidation of vitamin C is investigated. These spherical FePt NPs with nanocrystallite size of 7-9 nm are found to consist of a nanoalloy core with a more Pt-rich shell. The FePt alloy NPs are superior catalysts than Pt NPs for vitamin C electro-oxidation, with a linear concentration range of 0.01-1 mM, a high sensitivity of 4.347 mA cm(-2) mM(-1), and a low detection limit of 0.1 μM (S/N = 3). By effectively reducing the overpotential for the electro-oxidation, these alloy NPs are significantly more selective to the detection of vitamin C against other common interference species, including dopamine, citric acid, uric acid, glucose, and NaCl. Enhancement in sensor performance can be attributed to the increase in surface area due to reduction of nanocrystallite size and to modification in the Pt electronic structure as a result of nanoalloying. These are supported by the X-ray diffraction data and binding energy shifts as observed by X-ray photoelectron spectroscopy, respectively. Alloying therefore represents a powerful approach to introduce synergetic properties for new biosensor applications.

Coalescence in the Thermal Annealing of Nanoparticles: An in Situ STEM Study of the Growth Mechanisms of Ordered Pt–Fe Nanoparticles in a KCl Matrix

Thermal annealing is essential for achieving ultrasmall size ferromagnetic properties in next-generation high performance nanocomposite magnetic materials. However, during the annealing process, growth and agglomeration of nanoparticles normally occurs, which destroys the narrow size distributions. Thus, the materials become less suitable for application in high-density magnetic recording. The mechanism of nanoparticle growth and sintering has been difficult to determine because of the lack of suitable in situ tools to probe subnanometer changes at the local level. Here we report a study using high-resolution scanning transmission electron microscopy (STEM) coupled with an in situ thermal annealing stage of surfactant-free, monodispersed superparamagnetic PtFe (cubic) alloy nanoparticles (≈2 nm in diameter) stabilized in or on a KCl matrix. Ex situ experiments confirmed that annealing produces PtFe (tetragonal) ordered intermetallic nanoparticles with a mean diameter of 5 nm, and the in situ study revealed that the mechanism of nanoparticle growth is dominated by particle–particle coalescence, although Ostwald ripening is also implicated in a few regions. In addition, to determine the time dependent evolution of the size distribution of an ensemble of over 400 nanoparticles, analysis of the in situ data also allows tracking of individual nanoparticles, distinguishing coalescence from Ostwald ripening, nanoparticle by nanoparticle. This approach has provided valuable insights into changes in crystal structure and sintering that occur during the thermal annealing of Pt–Fe nanoparticles.

Electronic states of self stabilized L10 FePt alloy nanoparticles

The ligand capped bimetallic FePt alloys were prepared by using the chemical coreduction method in the presence of oleic acid and oleylamine. An X-ray photoelectron spectroscopy (XPS) study on the as prepared and annealed samples reveals the degradation of hydrocarbon capping with annealing temperature along with a phase transformation to a L10 phase. This degradation of organic capping results in formation of capping layer over FePt which has been observed using High Resolution Transmission electron microscopy (HRTEM). This capping layer over the FePt nanoparticles was further investigated with Raman studies confirming the presence of the graphitic carbon. The presence of the graphitic layer enhances the stability of FePt nanoparticles by protecting the surface against oxidation. This was confirmed by the magnetic measurements which show a high coercivity of 11.8 kOe, retained over a period of one year.

Low temperature synthesis of FePt alloy nanoparticles by polyol process

Low temperature synthesis of equiatomic FePt alloy nanoparticles with the FCC structure was carried out by the polyol process in the presence of NaOH at above 383 K. The Pt content in the FePt alloy particles decreased with decreasing reaction temperature and equiatomic FCC-FePt alloy nanoparticles were successfully obtained at 383 K. When the FCC-FePt alloy nanoparticles synthesized at 383 K were annealed at 573 K, a partially ordered phase with the L10 structure was observed. For comparison, FCC-FePt alloy nanoparticles were synthesized at 468 K without adding NaOH. Results showed that the degree of atomic ordering of the sample synthesized at 383 K with the addition of NaOH was higher than those obtained at 468 K after annealing at low temperatures. Thus, it is concluded that the temperature employed during the synthesis of FCC-FePt alloy nanoparticles affects the ordering transformation temperature.

Reduced graphene oxide supported FePt alloy nanoparticles with high electrocatalytic performance for methanol oxidation

Pt-based materials have been widely used as electrocatalysts in direct methanol fuel cells (DMFCs) due to their significant activity for methanol oxidation as well as their superior poison tolerance. In this study, a reduced graphene oxide (RGO) supported FePt alloy electrocatalyst is successfully synthesized by a facile in situ co-reduction route. The microstructure, composition and morphology of the synthesized materials are systematically investigated by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), Raman spectroscopy, energy dispersive X-ray spectroscopy (EDS), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). It is shown that the as-formed FePt nanoparticles with a size of 4 nm are well spread out on the RGO sheets and as a result, re-stacking of the RGO sheets is effectively inhibited. Their catalytic performance for electrocatalytic oxidation of methanol is investigated by cyclic voltammetry and amperometric method, which indicate that the RGO/FePt catalyst exhibits much higher catalytic activity and stability than the RGO/Pt nanocomposites. It is proposed that the addition of Fe, which increases the number of Pt active sites, is responsible for the improved catalytic performance. This result implies that the prepared RGO/FePt nanocomposites have great potential applications in DMFC.

Preparation and characterization of supported magnetic nanoparticles prepared by reverse micelles

Monatomic (Fe, Co) and bimetallic (FePt and CoPt) nanoparticles were prepared by exploiting the self-organization of precursor loaded reverse micelles. Achievements and limitations of the preparation approach are critically discussed. We show that self-assembled metallic nanoparticles can be prepared with diameters d = 2–12 nm and interparticle distances D = 20–140 nm on various substrates. Structural, electronic and magnetic properties of the particle arrays were characterized by several techniques to give a comprehensive view of the high quality of the method. For Co nanoparticles, it is demonstrated that magnetostatic interactions can be neglected for distances which are at least 6 times larger than the particle diameter. Focus is placed on FePt alloy nanoparticles which show a huge magnetic anisotropy in the L10 phase, however, this is still less by a factor of 3–4 when compared to the anisotropy of the bulk counterpart. A similar observation was also found for CoPt nanoparticles (NPs). These results are related to imperfect crystal structures as revealed by HRTEM as well as to compositional distributions of the prepared particles. Interestingly, the results demonstrate that the averaged effective magnetic anisotropy of FePt nanoparticles does not strongly depend on size. Consequently, magnetization stability should scale linearly with the volume of the NPs and give rise to a critical value for stability at ambient temperature. Indeed, for diameters above 6 nm such stability is observed for the current FePt and CoPt NPs. Finally, the long-term conservation of nanoparticles by Au photoseeding is presented.

L1<sub>0</sub> FePt-Filled Carbon Nanotubes by Wet Chemical Synthesis

This paper reports on the synthesis of iron-platinum (FePt) alloy clusters within Multi Wall Carbon Nanotubes using a novel wet chemical method that allows the filling of the nanotube cavity keeping clear their external wall. In the present case, the procedure permits the filling of the nanotubes with precursor salts of Pt and Fe and, by direct heating at 700°C of the Fe and Pt mixed salts, magnetic hard, face-centred-tetragonal (fct) crystallographic (L10) phase FePt alloy nanoparticles are obtained without the need to pass through the formation of the magnetic soft, face-centred-cubic (fcc) FePt alloy phase. By a suitable choice of the concentration of the starting material solution, we obtained particles of two different sizes into the nanotubes with magnetic blocked and super-paramagnetic states at room temperature. Morphological, structural and magnetic properties of the filled nanotubes have been performed by magnetic measurements, X-ray diffraction and transmission electron microscopy in normal and high resolution mode

Enhanced electrocatalytic oxidation of formic acid by platinum deposition on ruthenium nanoparticle surfaces

PtRu alloy nanoparticles were prepared by the spontaneous adsorption of Pt(acac) 2 on the surface of freshly prepared octanethiolate-protected ruthenium (RuSC8) nanoparticles. UV–vis spectroscopic measurements suggested that the adsorption was facilitated by the strong affinity of the platinum metal center to the ruthenium surface. Scanning tunneling spectroscopic studies showed that the resulting PtRu nanoparticles exhibited enhanced conductance as compared to the original ruthenium nanoparticles, where the adsorbed Pt centers might serve as the effective sites for electron-tunneling across the tip/particle/substrate junction. The nanoparticles were then loaded onto a glassy carbon (GC) electrode and their catalytic activities in the electro-oxidation of formic acid were examined and compared. Remarkably, despite the presence of an organic protecting monolayer on the particle surface, apparent electrocatalytic activity was observed. Voltammetric measurements suggested that a Pt skin layer was formed on the Ru nanoparticle surface, most likely as a result of electro-reduction of the adsorbed Pt(acac) 2. On the basis of the onset potential and current density in the electro-oxidation of HCOOH, the resulting PtRu nanoparticles displayed much higher electrocatalytic activity than the Ru counterparts and commercial PtRu nanoparticle catalysts from BASF. It was also found that the electrocatalytic activity of the PtRu nanoparticles was comparable to that of FePt alloy nanoparticles and PtRu bulk electrocatalysts, despite a minimal loading of Pt in the catalysts. Consistent responses were observed in electrochemical impedance spectroscopic measurements, where the charge-transfer resistance on PtRu/GC was found to be at least an order of magnitude smaller than that on Ru/GC and commercial PtRu catalysts. These measurements suggest that the PtRu thin-layer nanoparticles might be used as a promising candidate for the electrocatalytic oxidation of formic acid.