Synthesis and Size-Selective Precipitation of Monodisperse Nonstoichiometric M_<i>x</i>Fe_3–<i>x</i>O₄ (M = Mn, Co) Nanocrystals and Their DC and AC Magnetic Properties

Spinel ferrite nanocrystals (NCs) have shown great promise for a wide variety of electromagnetic and medical applications. In this work, the AC magnetic properties of nonstoichiometric manganese and cobalt ferrites (M_<i>x</i>Fe_3–<i>x</i>O₄, M = Mn, Co) NCs are systematically studied as a function of composition. Samples of very similar average size and shape, but different Mn to Fe and Co to Fe ratios are prepared to study the effect of composition. Conventional syntheses are combined with a size-selective precipitation method using oleic acid as an antisolvent yielding nearly monodisperse samples. DC and AC magnetic measurements shows that introducing Co to the ferrite crystal increases the blocking temperatures and magnetic anisotropies of the nanocrystals with corresponding shifts in AC magnetic susceptibilities, while manganese ferrites are relatively insensitive to the variation in compositions as size and shape dominate over crystal anisotropy. The systematic AC-magnetic characterizations of superparamagnetic Mn_<i>x</i>Fe_3–<i>x</i>O₄ and Co_<i>x</i>Fe_3–<i>x</i>O₄ NCs raise the importance of controlling chemical composition of ferrite NCs for AC magnetic applications

X‑ray Mapping of Nanoparticle Superlattice Thin Films

We combine grazing-incidence and transmission small-angle X-ray diffraction with electron microscopy studies to characterize the structure of nanoparticle films with long-range order. Transmission diffraction is used to collect in-plane diffraction data from single grains and locally aligned nanoparticle superlattice films. Systematic mapping of samples can be achieved by translating the sample in front of the X-ray beam with a spot size selected to be on the order of superlattice grain features. This allows a statistical determination of superlattice grain size and size distribution over much larger areas than typically accessible with electron microscopy. Transmission X-ray measurements enables spatial mapping of the grain size, orientation, uniformity, strain, or crystal projections and polymorphs. We expand this methodology to binary nanoparticle superlattice and nanorod superlattice films. This study provides a framework for characterization of nanoparticle superlattices over large areas which complements or expands microstructure information from real-space imaging

Efficient Removal of Organic Ligands from Supported Nanocrystals by Fast Thermal Annealing Enables Catalytic Studies on Well-Defined Active Phases

A simple yet efficient method to remove organic ligands from supported nanocrystals is reported for activating uniform catalysts prepared by colloidal synthesis procedures. The method relies on a fast thermal treatment in which ligands are quickly removed in air, before sintering can cause changes in the size and shape of the supported nanocrystals. A short treatment at high temperatures is found to be sufficient for activating the systems for catalytic reactions. We show that this method is widely applicable to nanostructures of different sizes, shapes, and compositions. Being rapid and effective, this procedure allows the production of monodisperse heterogeneous catalysts for studying a variety of structure–activity relationships. We show here results on methane steam reforming, where the particle size controls the CO/CO₂ ratio on alumina-supported Pd, demonstrating the potential applications of the method in catalysis

Characterization of Shape and Monodispersity of Anisotropic Nanocrystals through Atomistic X‑ray Scattering Simulation

Nanocrystals with anisotropic shape and high uniformity are now commonly produced as a result of significant advances in synthetic control. In most cases, the morphology of such materials is characterized only by electron microscopy, which makes the extraction of statistical information laborious and subject to bias. In this work, we describe how X-ray scattering patterns in conjunction with Debye formula simulations can be used to provide accurate atomisitic models for ensembles of anisotropic nanocrystals to complement and extend microscopic studies. Methods of sample preparation and measurement conditions are also discussed to provide appropriate experimental data. The scripts written to implement the Debye function are provided as a tool to allow researchers to obtain atomisitic models of nanocrystals

Monodisperse Core/Shell Ni/FePt Nanoparticles and Their Conversion to Ni/Pt to Catalyze Oxygen Reduction

We report a size-controllable synthesis of monodisperse core/shell Ni/FePt nanoparticles (NPs) via a seed-mediated growth and their subsequent conversion to Ni/Pt NPs. Preventing surface oxidation of the Ni seeds is essential for the growth of uniform FePt shells. These Ni/FePt NPs have a thin (≈1 nm) FePt shell and can be converted to Ni/Pt by acetic acid wash to yield active catalysts for oxygen reduction reaction (ORR). Tuning the core size allows the optimization of their electrocatalytic activity. The specific activity and mass activity of 4.2/0.8 nm core/shell Ni/FePt after acetic acid wash reach 1.95 mA/cm² and 490 mA/mg_Pt at 0.9 V (vs reversible hydrogen electrode), which are much higher than those of benchmark commercial Pt catalyst (0.34 mA/cm² and 92 mA/mg_Pt at 0.9 V). Our studies provide a robust approach to monodisperse core/shell NPs with nonprecious metal core, making it possible to develop advanced NP catalysts with ultralow Pt content for ORR and many other heterogeneous reactions

Synthesis and X‑ray Characterization of Cobalt Phosphide (Co₂P) Nanorods for the Oxygen Reduction Reaction

Low temperature fuel cells are clean, effective alternative fuel conversion technology. Oxygen reduction reaction (ORR) at the fuel cell cathode has required Pt as the electrocatalyst for high activity and selectivity of the four-electron reaction pathway. Targeting a less expensive, earth abundant alternative, we have developed the synthesis of cobalt phosphide (Co₂P) nanorods for ORR. Characterization techniques that include total X-ray scattering and extended X-ray absorption fine structure revealed a deviation of the nanorods from bulk crystal structure with a contraction along the <i>b</i> orthorhombic lattice parameter. The carbon supported nanorods have comparable activity but are remarkably more stable than conventional Pt catalysts for the oxygen reduction reaction in alkaline environments

Na₂Ti₃O₇ Nanoplatelets and Nanosheets Derived from a Modified Exfoliation Process for Use as a High-Capacity Sodium-Ion Negative Electrode

The increasing interest in Na-ion batteries (NIBs) can be traced to sodium abundance, its low cost compared to lithium, and its intercalation chemistry being similar to that of lithium. We report that the electrochemical properties of a promising negative electrode material, Na₂Ti₃O₇, are improved by exfoliating its layered structure and forming 2D nanoscale morphologies, nanoplatelets, and nanosheets. Exfoliation of Na₂Ti₃O₇ was carried out by controlling the amount of proton exchange for Na⁺ and then proceeding with the intercalation of larger cations such as methylammonium and propylammonium. An optimized mixture of nanoplatelets and nanosheets exhibited the best electrochemical performance in terms of high capacities in the range of 100–150 mA h g^–1 at high rates with stable cycling over several hundred cycles. These properties far exceed those of the corresponding bulk material, which is characterized by slow charge-storage kinetics and poor long-term stability. The results reported in this study demonstrate that charge-storage processes directed at 2D morphologies of surfaces and few layers of sheets are an exciting direction for improving the energy and power density of electrode materials for NIBs

Bulk Metallic Glass-like Scattering Signal in Small Metallic Nanoparticles

The atomic structure of Ni–Pd nanoparticles has been studied using atomic pair distribution function (PDF) analysis of X-ray total scattering data and with transmission electron microscopy (TEM). Larger nanoparticles have PDFs corresponding to the bulk face-centered cubic packing. However, the smallest nanoparticles have PDFs that strongly resemble those obtained from bulk metallic glasses (BMGs). In fact, by simply scaling the distance axis by the mean metallic radius, the curves may be collapsed onto each other and onto the PDF from a metallic glass sample. In common with a wide range of BMG materials, the intermediate range order may be fit with a damped single-frequency sine wave. When viewed in high-resolution TEM, these nanoparticles exhibit atomic fringes typical of those seen in small metallic clusters with icosahedral or decahedral order. These two seemingly contradictory results are reconciled by calculating the PDFs of models of icosahedra that would be consistent with the fringes seen in TEM. These model PDFs resemble the measured ones when significant atom-position disorder is introduced, drawing together the two diverse fields of metallic nanoparticles and BMGs and supporting the view that BMGs may contain significant icosahedral or decahedral order

Local Structure Evolution and Modes of Charge Storage in Secondary Li–FeS₂ Cells

In the pursuit of high-capacity electrochemical energy storage, a promising domain of research involves conversion reaction schemes, wherein electrode materials are fully transformed during charge and discharge. There are, however, numerous difficulties in realizing theoretical capacity and high rate capability in many conversion schemes. Here we employ <i>operando</i> studies to understand the conversion material FeS₂, focusing on the local structure evolution of this relatively reversible material. X-ray absorption spectroscopy, pair distribution function analysis, and first-principles calculations of intermediate structures shed light on the mechanism of charge storage in the Li–FeS₂ system, with some general principles emerging for charge storage in chalcogenide materials. Focusing on second and later charge/discharge cycles, we find small, disordered domains that locally resemble Fe and Li₂S at the end of the first discharge. Upon charge, this is converted to a Li–Fe–S composition whose local structure reveals tetrahedrally coordinated Fe. With continued charge, this ternary composition displays insertion–extraction behavior at higher potentials and lower Li content. The finding of hybrid modes of charge storage, rather than simple conversion, points to the important role of intermediates that appear to store charge by mechanisms that more closely resemble intercalation

Molybdenum Polysulfide Chalcogels as High-Capacity, Anion-Redox-Driven Electrode Materials for Li-Ion Batteries

Sulfur cathodes in conversion reaction batteries offer high gravimetric capacity but suffer from parasitic polysulfide shuttling. We demonstrate here that transition metal chalcogels of approximate formula MoS_3.4 achieve a high gravimetric capacity close to 600 mAh g^–1 (close to 1000 mAh g^–1 on a sulfur basis) as electrode materials for lithium-ion batteries. Transition metal chalcogels are amorphous and comprise polysulfide chains connected by inorganic linkers. The linkers appear to act as a “glue” in the electrode to prevent polysulfide shuttling. The Mo chalcogels function as electrodes in carbonate- and ether-based electrolytes, which further provides evidence of polysulfide solubility not being a limiting issue. We employ X-ray spectroscopy and <i>operando</i> pair distribution function techniques to elucidate the structural evolution of the electrode. Raman and X-ray photoelectron spectroscopy track the chemical moieties that arise during the anion-redox-driven processes. We find the redox state of Mo remains unchanged across the electrochemical cycling and, correspondingly, the redox is anion-driven