Direct chemical synthesis of L1(0)-FePtAu nanoparticles with high coercivity

Emerging Pt-based intermetallic nanoparticles for the oxygen reduction reaction

The advancement of highly efficient and enduring platinum (Pt)-based electrocatalysts for the oxygen reduction reaction (ORR) is a critical determinant to enable broad utilization of clean energy conversion technologies. Pt-based intermetallic electrocatalysts offer durability and superior ORR activity over their traditional analogues due to their definite stoichiometry, ordered and extended structures, and favourable enthalpy of formation. With the advent in new synthetic methods, Pt-based intermetallic nanoparticles as a new class of advanced electrocatalysts have been studied extensively in recent years. This review discusses the preparation principles, representative preparation methods of Pt-based intermetallics and their applications in the ORR. Our review is focused on L10 Pt-based intermetallics which have gained tremendous interest recently due to their larger surface strain and enhanced M(3d)-Pt(5d) orbital coupling, particularly in the crystallographic c-axis direction. Additionally, we discuss future research directions to further improve the efficiency of Pt-based intermetallic electrocatalysts with the intention of stimulating increased research ventures in this domain.

Intermetallic Nanocrystals for Fuel-Cells-Based Electrocatalysis

Electrocatalysis underpins the renewable electrochemical conversions for sustainability, which further replies on metallic nanocrystals as vital electrocatalysts. Intermetallic nanocrystals have been known to show distinct properties compared to their disordered counterparts, and been long explored for functional improvements. Tremendous progresses have been made in the past few years, with notable trend of more precise engineering down to an atomic level and the investigation transferring into more practical membrane electrode assembly (MEA), which motivates this timely review. After addressing the basic thermodynamic and kinetic fundamentals, we discuss classic and latest synthetic strategies that enable not only the formation of intermetallic phase but also the rational control of other catalysis-determinant structural parameters, such as size and morphology. We also demonstrate the emerging intermetallic nanomaterials for potentially further advancement in energy electrocatalysis. Then, we discuss the state-of-the-art characterizations and representative intermetallic electrocatalysts with emphasis on oxygen reduction reaction evaluated in a MEA setup. We summarize this review by laying out existing challenges and offering perspective on future research directions toward practicing intermetallic electrocatalysts for energy conversions.

Operating interfaces to synthesize L10-FePt@Bi-rich nanoparticles by modifying the heating process

A facile strategy to operate interfaces when synthesizing L1₀-FePt@Bi-rich nanoparticles (NPs) has been proposed. Two interfaces are indispensable to obtain the high ordering L1₀-FePt structure. One is the mismatched interfaces between the initial γ-PtBi₂ nuclei and the disordered fcc-FePt phase. The other is the in situ grown coherent interfaces between the L1₀-FePt and Bi-rich phases. Increasing the heating rates when the temperature rises from 120 °C to 310 °C benefits the formation of mismatched interfaces and improves the uniformity of phases and composition in NPs. Reducing the heating rate at higher temperature ensures sufficient time for Bi to diffuse across the coherent interface, which facilitates the disorder-order transition of L1₀-FePt NPs. This study provides a new perspective on operating interfaces during the wet-chemical synthesis process.

Chemically Induced Magnetic Dead Shells in Superparamagnetic Ni Nanoparticles Deduced from Polarized Small-Angle Neutron Scattering

Advances in the synthesis and characterization of colloidal magnetic nanoparticles (NPs) have yielded great gains in the understanding of their complex magnetic behavior, with implications for numerous applications. Recent work using Ni NPs as a model soft ferromagnetic system, for example, achieved quantitative understanding of the superparamagnetic blocking temperature-particle diameter relationship. This hinged, however, on the critical assumption of a ferromagnetic NP volume lower than the chemical volume due to a non-ferromagnetic dead shell indirectly deduced from magnetometry. Here, we determine both the chemical and magnetic average internal structures of Ni NP ensembles via unpolarized, half-polarized, and fully polarized small-angle neutron scattering (SANS) measurements and analyses coupled with X-ray diffraction and magnetometry. The postulated nanometric magnetic dead shell is not only detected but conclusively identified as a non-ferromagnetic Ni phosphide derived from the trioctylphosphine commonly used in hot-injection colloidal NP syntheses. The phosphide shell thickness is tunable via synthesis temperature, falling to as little as 0.5 nm at 170 °C. Temperature- and magnetic field-dependent polarized SANS measurements additionally reveal essentially bulk-like ferromagnetism in the Ni core and negligible interparticle magnetic interactions, quantitatively supporting prior modeling of superparamagnetism. These findings advance the understanding of synthesis-structure-property relationships in metallic magnetic NPs, point to a simple potential route to ligand-free stabilization, and highlight the power of the currently available suite of polarized SANS measurement and analysis capabilities for magnetic NP science and technology.

Magnetic Nanostructures: Rational Design and Fabrication Strategies toward Diverse Applications

In recent years, the continuous development of magnetic nanostructures (MNSs) has tremendously promoted both fundamental scientific research and technological applications. Different from the bulk magnet, the systematic engineering on MNSs has brought a great breakthrough in some emerging fields such as the construction of MNSs, the magnetism exploration of multidimensional MNSs, and their potential translational applications. In this review, we give a detailed description of the synthetic strategies of MNSs based on the fundamental features and application potential of MNSs and discuss the recent progress of MNSs in the fields of nanomedicines, advanced nanobiotechnology, catalysis, and electromagnetic wave adsorption (EMWA), aiming to provide guidance for fabrication strategies of MNSs toward diverse applications.

Effect of Ag evolution process on ordering transition for L10-FePt nanoparticles synthesized by Ag addition

As a typical element, Ag can effectively promote the ordering transition in the direct synthesis of L10-FePt nanoparticles (NPs). However, the role of Ag in the ordering process and the...

Magnetic Nanoparticles: Synthesis, Anisotropy, and Applications

Anisotropy is an important and widely present characteristic of materials that provides desired direction-dependent properties. In particular, the introduction of anisotropy into magnetic nanoparticles (MNPs) has become an effective method to obtain new characteristics and functions that are critical for many applications. In this review, we first discuss anisotropy-dependent ferromagnetic properties, ranging from intrinsic magnetocrystalline anisotropy to extrinsic shape and surface anisotropy, and their effects on the magnetic properties. We further summarize the syntheses of monodisperse MNPs with the desired control over the NP dimensions, shapes, compositions, and structures. These controlled syntheses of MNPs allow their magnetism to be finely tuned for many applications. We discuss the potential applications of these MNPs in biomedicine, magnetic recording, magnetotransport, permanent magnets, and catalysis.

Anisotropic Growth and Magnetic Properties of α″-Fe16N2@C Nanocones

α″-Fe₁₆N₂ nanomaterials with a shape anisotropy for high coercivity performance are of interest in potential applications such as rare-earth-free permanent magnets, which are difficult to synthesize in situ anisotropic growth. Here, we develop a new and facile one-pot microemulsion method with Fe(CO)₅ as the iron source and tetraethylenepentamine (TEPA) as the N/C source at low synthesis temperatures to fabricate carbon-coated tetragonal α″-Fe₁₆N₂ nanocones. Magnetocrystalline anisotropy energy is suggested as the driving force for the anisotropic growth of α″-Fe₁₆N₂@C nanocones because the easy magnetization direction of tetragonal α″-Fe₁₆N₂ nanocrystals is along the c axis. The α″-Fe₁₆N₂@C nanocones agglomerate to form a fan-like microstructure, in which the thin ends of nanocones direct to its center, due to the magnetostatic energy. The lengths of α″-Fe₁₆N₂@C nanocones are ~200 nm and the diameters vary from ~10 nm on one end to ~40 nm on the other end. Carbon shells with a thickness of 2–3 nm protect α″-Fe₁₆N₂ nanocones from oxidation in air atmosphere. The α″-Fe₁₆N₂@C nanocones synthesized at 433 K show a room-temperature saturation magnetization of 82.6 emu/g and a coercive force of 320 Oe.

Strategies to enhance the electrochemical performances of Pt-based intermetallic catalysts

The need for improving the energy conversion efficiency of proton exchange membrane fuel cells (PEMFCs) has motivated the development of advanced electrocatalysts with desirable activity and durability. Pt-Based intermetallic compounds, featuring atomically ordered structures, have long been considered to be very promising alternatives to widely employed Pt and Pt alloy (solid solutions) catalysts. To facilitate the practical application of Pt-based intermetallics in PEMFCs, effective strategies have been developed to further improve their catalytic activity and durability over the last decade. This feature article overviews the recent advances on the strategies for enhancing the electrochemical performances of Pt-based intermetallic catalysts, which include size control, surface engineering, and composition tuning. Thermodynamic and kinetic perspectives on the formation of the intermetallic phases are summarized to better design the synthesis conditions and realize the size control. After this, the size-control approaches (e.g. coating protection, matrix protection) are illustrated and discussed. We highlight the positive effect of surface engineering and discuss the recently developed methods for surface engineering. Finally, we discuss the thermodynamic feasibility of composition tuning and recent work based on composition-tunable intermetallic electrocatalysts.

Chemical Synthesis of Magnetic Nanoparticles for Permanent Magnet Applications

Permanent magnets are a class of critical materials for information storage, energy storage, and other magneto-electronic applications. Compared with conventional bulk magnets, magnetic nanoparticles (MNPs) show unique size-dependent magnetic properties, which make it possible to control and optimize their magnetic performance for specific applications. The synthesis of MNPs has been intensively explored in recent years. Among different methods developed thus far, chemical synthesis based on solution-phase reactions has attracted much attention owing to its potential to achieve the desired size, morphology, structure, and magnetic controls. This Minireview focuses on the recent chemical syntheses of strongly ferromagnetic MNPs (H_c >10 kOe) of rare-earth metals and FePt intermetallic alloys. It further discusses the potential of enhancing the magnetic performance of MNP composites by assembly of hard and soft MNPs into exchange-coupled nanocomposites. High-performance nanocomposites are key to fabricating super-strong permanent magnets for magnetic, electronic, and energy applications.

Enhanced magnetic properties and thermal stability of highly ordered ε-Fe3N1+x (-0.12 ≤ x ≤ -0.01) nanoparticles

ε-Iron nitrides with the general formula ε-Fe3N1+x (-0.40 < x < 0.48) have been widely studied due to their interesting magnetism. However, the phase diagram of the Fe-N binary system indicates the absence of monophasic ε-Fe3N1+x (x < 0) compounds that are stable below their synthetic temperatures. Here, ε-Fe3N1+x (-0.12 ≤ x ≤ -0.01) nanoparticles with excellent thermal stability and magnetic properties were synthesized by a simple chemical solution method. The ε-Fe3N1+x nanoparticles with space group P6322 have excellent oxidation resistance due to a carbon shell with a thickness of 2-3 nm. NPD refinements suggest that the ε-Fe3N1+x nanoparticles possess a highly ordered arrangement of N atoms and their magnetic moments align parallel to the c axis. The Curie temperature (TC) and room temperature saturation magnetization (MS) increase with decreasing N content, which results in record-high TC (632 K) and MS (169.2 emu g-1) at x = -0.12, much higher than the magnetic properties of the corresponding bulk materials. The significant enhancements in the intrinsic magnetic properties and thermal stability of ε-Fe3N1+x are ascribed to chemically engineering the stoichiometry and N occupancy from the disordered to the ordered site.

Facile liquid-assisted one-step sintering synthesis of superfine L10-FePt nanoparticles

A liquid-assisted one-step sintering method was proposed for the synthesis of L1₀-FePt superfine nanoparticles. The liquid assisted Fe and Pt precursors were homogeneously deposited on NaCl media, which facilitated the nucleation rates, obviously reduced the particle size and promoted the orderly transformation. Through optimizing the sintering parameters, superfine L1₀-FePt nanoparticles (about 7 nm, TEM) with coercivity as high as 2.29 T were obtained. This research highlights the feasibility of synthesizing L1₀-FePt nanoparticles with superfine sizes and ultra-high coercivity.

A liquid-assisted one-step sintering method was proposed for the synthesis of L1₀-FePt superfine nanoparticles.

Intermetallic Nanoparticles: Synthetic Control and Their Enhanced Electrocatalysis

Intermetallic nanoparticles (NPs) described in this Account are a class of metallic alloy NPs within which metal atoms are bonded via strong d-orbital interaction and ordered anisotropically in a specific crystallographic direction. Compared to the common metallic alloy NPs with solid solution structure, intermetallic NPs are generally more stable against chemical oxidation and etching. The strict stoichiometry requirement, well-defined atom binding environment and layered atomic arrangement also make intermetallic NPs an ideal model for understanding their physical and catalytic properties. This account summarizes the synthetic principles and strategies developed to obtain monodisperse intermetallic NPs, especially tetragonal L1₀-NPs. The thermodynamics and kinetics involved in the conversion between disordered and ordered structures are briefly discussed. The synthetic methods are grouped into two slightly different categories: solution-phase synthesis followed by solid state annealing and direct solution-phase synthesis. In the former method, high-surface-area supports are often needed to disperse NPs and to prevent them from aggregation, while in the latter method such supports are not required since the structure conversion temperature is lowered to a level that the conversion can proceed in the solution reaction condition. In any of these two synthetic approaches, various factors influencing intermetallic structure formation should be carefully controlled to ensure more complete structural transition within NPs. Using representative synthetic examples, we highlight the strategies explored to facilitate the formation of intermetallic structure, including the introduction of vacancies/defects within NP structures and the control of atom addition rate/seed-mediated diffusion to lower the energy barrier. These strategies illustrate how the concept of thermodynamics and kinetics can be used to design the synthesis of intermetallic NPs. Additionally, to correlate NP structure and catalysis, we introduce briefly the d-band theory to explain how the electronic, strain and ensemble effects can be used to tune NP catalysis. We focus specifically on Pt-, Pd-, and Au-based L1₀-NPs and demonstrate how these L1₀-NPs could be prepared to show much enhanced catalysis for electrochemical reactions, including oxygen reduction reaction (ORR), hydrogen evolution reaction (HER), formic acid oxidation reaction (FAOR), and thermo-oxidation reaction of CO. Due to the enhanced metal atom stability in the "sandwich"-type structure, the roles of the first-row transition metal atoms in catalysis are better understood to achieve catalysis optimization. This concept can be extended to other alloy NPs, demonstrating great potentials in using intermetallic structures to control NP reduction and oxidation catalysis for important chemical and energy applications.

Eutectic melt crystallization of L10-FePt

Herein, we report the eutectic growth control of ordered L10-FePt, which directs the nucleation, growth and crystallization of FePt sheets in a single-step reaction. The nature of eutectic crystallization at the eutectic point suggests its role as a high-temperature solvent, exhibiting an advantage in the scale-up production of metastable alloys.

Halide Ion-Mediated Synthesis of L10-FePt Nanoparticles with Tunable Magnetic Properties

L1₀-FePt nanoparticles (NPs) have great potential in areas of advanced magnetic and catalytic applications. Here, we present a facile control route for synthesis of hard magnetic L1₀-FePt NPs in which halide ions (Cl^-, Br^-, or I^-) were added to the synthetic process to promote the phase transformation. It is confirmed that the strong ionic binding force between halide ions and Fe³⁺ or Pt²⁺ ions could facilitate the formation of L1₀-FePt phase due to favoring growth of FePt NPs in a more thermodynamically stable way, which enables the formation of an ordered structure. L1₀-FePt NPs with the highest coercivity of 8.64 kOe and saturation magnetization of 64.21 emu/g at room temperature can be directly obtained by controlling the amount of the halide ions. In comparison with conventional solution phase reduction methods, the halide ion-assisted method shows enhanced capability to tune the growth of hard magnetic bimetallic NPs, particularly Pt-based bimetallic NPs.

A general strategy for synthesizing high-coercivity L10-FePt nanoparticles

It is extremely desirable but challenging to develop a facile solution phase synthesis to directly prepare well-dispersed L1₀-FePt nanoparticles (NPs) to meet the requirements of advanced magnets in modern industry and information technology. Here, we report a novel strategy to synthesize hard magnetic L1₀-FePt NPs via controlled co-reduction of Fe(acac)₃ and K₂PtCl₆ in the presence of oleylamine, in which effective control of the magnetic properties and chemical ordering of L1₀-FePt NPs was achieved by tuning the mole ratio of the precursors, reaction time and temperature. The highest coercivity of 10.5 kOe can be obtained for the NPs synthesized at 350 °C for 8 h, which is much higher than the coercivities reported by the previous studies on solution-synthesized FePt NPs without annealing or the third elemental additive. The reported one-pot synthesis of L1₀-FePt NPs may provide an ideal class of building blocks for magnetic energy applications.

Controlled synthesis and magnetic properties of iron-cobalt-phosphide nanorods

A simple one-step solution-phase synthesis of iron-cobalt-phosphide ((Fe1-xCox)2P) nanorods (NRs) is reported in this paper. Through the control of the amount of Co in the samples, the crystal structure of (Fe1-xCox)2P NRs changes from a pure Fe-rich hexagonal Fe2P type structure to a mixture of Fe-rich hexagonal Fe2P and Co-rich orthorhombic Co2P type structures. These samples show superparamagnetic behavior at room temperature and ferromagnetic properties at 10 K. When the Co composition is 0.09, the (Fe0.91Co0.09)2P sample has the highest coercivity around 5.74 kOe at 10 K. The current route provides a new and general chemical method for tunable preparation of (Fe1-xCox)2P (x < 0.28) NRs, which are significant for the development of new iron- or cobalt-rich permanent magnet materials without rare-earth or noble metals.

Organic Phase Syntheses of Magnetic Nanoparticles and Their Applications

In the past two decades, the synthetic development of magnetic nanoparticles (NPs) has been intensively explored for both fundamental scientific research and technological applications. Different from the bulk magnet, magnetic NPs exhibit unique magnetism, which enables the tuning of their magnetism by systematic nanoscale engineering. In this review, we first briefly discuss the fundamental features of magnetic NPs. We then summarize the synthesis of various magnetic NPs, including magnetic metal, metallic alloy, metal oxide, and multifunctional NPs. We focus on the organic phase syntheses of magnetic NPs with precise control over their sizes, shapes, compositions, and structures. Finally we discuss the applications of various magnetic NPs in sensitive diagnostics and therapeutics, high-density magnetic data recording and energy storage, as well as in highly efficient catalysis.

Oriented-assembly of hollow FePt nanochains with tunable catalytic and magnetic properties

Hollow nanoparticles with large surface areas exhibit a lot of advantages for applications such as catalysis and energy storage. Furthermore, their performance can be manipulated by their deliberate assemblies. Dispersive hollow FePt nanospheres have been assembled into one-dimensional hollow FePt nanochains under the magnetic fields at room temperature. Based on the activation of galvanic replacement at different reaction stages, the size of hollow FePt nanochains can be deliberately manipulated varying from 20 nm to 300 nm, together with the length changing from 200 nm to 10 μm. The competition between movement of paramagnetic Fe(3+) ions and shape recovering due to thermal fluctuations plays a critical effect on the structure of contact area between hollow nanospheres, leading to perforative structures. Compared with commercial Pt/C, well aligned hollow FePt nanochains show greatly enhanced catalytic activities in the methanol oxidation reaction (MOR) due to more favorable mass flow. Magnetic measurements indicate that the magnetic properties including Curie temperature and saturation magnetization can be tuned by the control of the size and shape of hollow nanochains.

Cl-assisted highly efficient synthesis of FePd3 alloys encapsulated in graphite papers: a two stage CVD approach

We propose an advanced two-stage CVD method which allows the synthesis of very thick deposits of planar rolled-like graphite structures filled with FePd₃ alloys as dominant product in the entire reactor.

Direct liquid phase synthesis of ordered L10 FePt colloidal particles with high coercivity using an Au nanoparticle seeding approach

L1₀ ordered FePt nanoparticles that reveal an enhanced coercive field were synthesized following a liquid phase approach using Au nanoparticles as seeds.

Low-Energy Bead-Mill Dispersion of Agglomerated Core-Shell α-Fe/Al₂O₃ and α″-Fe₁₆N₂/Al₂O₃ Ferromagnetic Nanoparticles in Toluene

Magnetic materials such as α″-Fe16N2 and α-Fe, which have the largest magnetic moment as hard and soft magnetic materials, are difficult to produce as single domain magnetic nanoparticles (MNPs) because of quasistable state and high reactivity, respectively. The present work reports dispersion of agglomerated plasma-synthesized core-shell α″-Fe16N2/Al2O3 and α-Fe/Al2O3 in toluene by a new bead-mill with very fine beads to prepare single domain MNPs. As a result, optimization of the experimental conditions (bead size, rotation speed, and dispersion time) enables the break-up of agglomerated particles into primary particles without destroying the particle structure. Slight deviation from the optimum conditions, i.e., lower or higher dispersion energy, gives undispersed or broken particles due to fragile core-shell structure against stress or impact force of beads. The dispersibility of α″-Fe16N2/Al2O3 is more restricted than that of α-Fe/Al2O3, because of the preparation conditions. Especially for α″-Fe16N2/Al2O3, no change on crystallinity (98% α″-Fe16N2) or magnetization saturation after dispersion was observed, showing that this method is appropriate to disperse α″-Fe16N2/Al2O3 MNPs. A different magnetic hysteresis behavior is observed for well-dispersed α″-Fe16N2/Al2O3 MNPs, and the magnetic coercivity of these NPs is constricted when the magnetic field close to zero due to magnetic dipole coupling among dispersed α″-Fe16N2 MNPs.