Microbeads and Hollow Microcapsules Obtained by Self-Assembly of Pickering Magneto-Responsive Cellulose Nanocrystals

Cellulose microbeads can be used as immobilization supports. We report on the design and preparation of magneto-responsive cellulose microbeads and microcapsules by self-assembled shells of cellulose nanocrystals (CNC) carrying magnetic CoFe₂O₄ nanoparticles, that is, a mixture of isotropic and anisotropic nanomaterials. The magnetic CNCs formed a structured layer, a mesh, consisting of CNCs and magnetic particles bound together on the surface of distinct droplets of hexadecane and styrene dispersed in water. Because of the presence of CNCs the highly crystalline mesh was targeted to provide an improved barrier property of the microbead shell compared to neat polymer shells, while the magnetic particles provided the magnetic response. In situ polymerization of the styrene phase led to the formation of solid microbeads (∼8 μm diameter) consisting of polystyrene (PS) cores encapsulated in the magnetic CNC shells (shell-to-core mass ratio of 4:96). The obtained solid microbeads were ferromagnetic (saturation magnetization of ∼60 emu per gram of the magnetic phase). The magnetic functionality enables easy separation of substances immobilized on the beads. Such a functionality was tested in removal of a dye from water. The microbeads were further utilized to synthesize hollow microcapsules by solubilization of the PS core. The CNC-based, magneto-responsive solid microbeads and hollow microcapsules were characterized by electron microscopy (morphology), X-ray diffraction (phase composition), and magnetometry (magnetic properties). Such hybrid systems can be used in the design of materials and devices for application in colloidal stabilization, concentration, separation, and delivery, among others

Low Lattice Thermal Conductivity in a Wider Temperature Range for Biphasic-Quaternary (Ti,V)CoSb Half-Heusler Alloys

Intrinsically high lattice thermal conductivity has remained a major bottleneck for achieving a high thermoelectric figure of merit (zT) in state-of-the-art ternary half-Heusler (HH) alloys. In this work, we report a stable n-type biphasic-quaternary (Ti,V)CoSb HH alloy with a low lattice thermal conductivity κL ≈ 2 W m–1 K–1 within a wide temperature range (300–873 K), which is comparable to the reported nanostructured HH alloys. A solid-state transformation driven by spinodal decomposition upon annealing is observed in Ti0.5V0.5CoSb HH alloy, which remarkably enhances phonon scattering, while electrical properties correlate well with the altering electronic band structure and valence electron count (VEC). A maximum zT ≈ 0.4 (±0.05) at 873 K was attained by substantial lowering of κL and synergistic enhancement of the power factor. We perform first-principles density functional theory calculations to investigate the structure, stability, electronic structure, and transport properties of the synthesized alloy, which rationalize the reduction in the lattice thermal conductivity to the increase in anharmonicity due to the alloying. This study upholds the new possibilities of finding biphasic-quaternary HH compositions with intrinsically reduced κL for prospective thermoelectric applications

Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe₂P Polymorphs

Iron phosphide (Fe2P) crystallizes in its own hexagonal crystal structure type (h-Fe2P). As found in meteorites, orthorhombic polymorph (o-Fe2P) was originally reported as a high-temperature and high-pressure phase. Recently, o-Fe2P was described as being stable at ambient pressure, yet no synthetic methods were developed for single-crystal growth or single-phase bulk powder synthesis. Here, we report a successful method for growing o-Fe2P single crystals and synthesizing phase-pure polycrystalline samples using tin-flux. In situ powder X-ray diffraction studies showed that the phase transition from o-Fe2P to h-Fe2P occurs at about 873 K, and below that temperature, the formation of the o-Fe2P phase is favored thermodynamically rather than kinetically. Systematic comparison of transport, magnetic, and electrocatalytic properties of both h-Fe2P and o-Fe2P phases showed a substantial impact of the crystal structure on properties. The orthorhombic structural distortion resulted in considerable changes in magnetic properties, with the o-Fe2P phase exhibiting a 60% lower Fe magnetic moment and a substantially higher ferromagnetic Curie temperature than h-Fe2P. Electrochemical measurements toward the hydrogen evolution reaction in acidic media showed that the o-Fe2P phase requires an 80 mV lower overpotential than the h-Fe2P phase to generate a current density of −10 mA/cm2, and their electronic structures suggest that the higher density of states at the Fermi energy is the origin of superior catalytic activity in o-Fe2P

Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe₂P Polymorphs

Iron phosphide (Fe2P) crystallizes in its own hexagonal crystal structure type (h-Fe2P). As found in meteorites, orthorhombic polymorph (o-Fe2P) was originally reported as a high-temperature and high-pressure phase. Recently, o-Fe2P was described as being stable at ambient pressure, yet no synthetic methods were developed for single-crystal growth or single-phase bulk powder synthesis. Here, we report a successful method for growing o-Fe2P single crystals and synthesizing phase-pure polycrystalline samples using tin-flux. In situ powder X-ray diffraction studies showed that the phase transition from o-Fe2P to h-Fe2P occurs at about 873 K, and below that temperature, the formation of the o-Fe2P phase is favored thermodynamically rather than kinetically. Systematic comparison of transport, magnetic, and electrocatalytic properties of both h-Fe2P and o-Fe2P phases showed a substantial impact of the crystal structure on properties. The orthorhombic structural distortion resulted in considerable changes in magnetic properties, with the o-Fe2P phase exhibiting a 60% lower Fe magnetic moment and a substantially higher ferromagnetic Curie temperature than h-Fe2P. Electrochemical measurements toward the hydrogen evolution reaction in acidic media showed that the o-Fe2P phase requires an 80 mV lower overpotential than the h-Fe2P phase to generate a current density of −10 mA/cm2, and their electronic structures suggest that the higher density of states at the Fermi energy is the origin of superior catalytic activity in o-Fe2P

Enhancing Properties with Distortion: A Comparative Study of Two Iron Phosphide Fe₂P Polymorphs

Iron phosphide (Fe2P) crystallizes in its own hexagonal crystal structure type (h-Fe2P). As found in meteorites, orthorhombic polymorph (o-Fe2P) was originally reported as a high-temperature and high-pressure phase. Recently, o-Fe2P was described as being stable at ambient pressure, yet no synthetic methods were developed for single-crystal growth or single-phase bulk powder synthesis. Here, we report a successful method for growing o-Fe2P single crystals and synthesizing phase-pure polycrystalline samples using tin-flux. In situ powder X-ray diffraction studies showed that the phase transition from o-Fe2P to h-Fe2P occurs at about 873 K, and below that temperature, the formation of the o-Fe2P phase is favored thermodynamically rather than kinetically. Systematic comparison of transport, magnetic, and electrocatalytic properties of both h-Fe2P and o-Fe2P phases showed a substantial impact of the crystal structure on properties. The orthorhombic structural distortion resulted in considerable changes in magnetic properties, with the o-Fe2P phase exhibiting a 60% lower Fe magnetic moment and a substantially higher ferromagnetic Curie temperature than h-Fe2P. Electrochemical measurements toward the hydrogen evolution reaction in acidic media showed that the o-Fe2P phase requires an 80 mV lower overpotential than the h-Fe2P phase to generate a current density of −10 mA/cm2, and their electronic structures suggest that the higher density of states at the Fermi energy is the origin of superior catalytic activity in o-Fe2P

From Chromonic Self-Assembly to Hollow Carbon Nanofibers: Efficient Materials in Supercapacitor and Vapor-Sensing Applications

Carbon nanofibers (CNFs) with high surface area (820 m²/g) have been successfully prepared by a nanocasting approach using silica nanofibers obtained from chromonic liquid crystals as a template. CNFs with randomly oriented graphitic layers show outstanding electrochemical supercapacitance performance, exhibiting a specific capacitance of 327 F/g at a scan rate of 5 mV/s with a long life-cycling capability. Approximately 95% capacitance retention is observed after 1000 charge–discharge cycles. Furthermore, about 80% of capacitance is retained at higher scan rates (up to 500 mV/s) and current densities (from 1 to 10 A/g). The high capacitance of CNFs comes from their porous structure, high pore volume, and electrolyte-accessible high surface area. CNFs with ordered graphitic layers were also obtained upon heat treatment at high temperatures (>1500 °C). Although it is expected that these graphitic CNFs have increased electrical conductivity, in the present case, they exhibited lower capacitance values due to a loss in surface area during thermal treatment. High-surface-area CNFs can be used in sensing applications; in particular, they showed selective differential adsorption of volatile organic compounds such as pyridine and toluene. This behavior is attributed to the free diffusion of these volatile aromatic molecules into the pores of CNFs accompanied by interactions with <i>sp</i>² carbon structures and other chemical groups on the surface of the fibers

Design and Synthesis of Highly Active Al–Ni–P Foam Electrode for Hydrogen Evolution Reaction

An effective method to boost the electrocatalytic activity of nickel phosphides in H₂ evolution reaction is reported. The method took advantage of density functional theory calculations that allowed the design of a highly active material based on the combination of d-metal with p-metal within a phosphide structure. Furthermore, the principle is proven experimentally through successful synthesis of self-supported ternary Al–Ni–P foam electrocatalyst by alloying of Ni and Al followed by the gas transport phosphorization reaction. As a cathode for H₂ evolution reaction in acidic electrolyte, Al–Ni–P significantly outperforms pure Ni–P, and it has an exchange current density of 0.6 mA/cm² and a Tafel slope of 65 mV/decade

Large-Scale Colloidal Synthesis of Chalcogenides for Thermoelectric Applications

A simple and effective preparation of solution-processed chalcogenide thermoelectric materials is described. First, PbTe, PbSe, and SnSe were prepared by gram-scale colloidal synthesis relying on the reaction between metal acetates and diphenyl dichalcogenides in hexadecylamine solvent. The resultant phase-pure chalcogenides consist of highly crystalline and defect-free particles with distinct cubic-, tetrapod-, and rod-like morphologies. The powdered PbTe, PbSe, and SnSe products were subjected to densification by spark plasma sintering (SPS), affording dense pellets of the respective chalcogenides. Scanning electron microscopy shows that the SPS-derived pellets exhibit fine nano-/micro-structures dictated by the original morphology of the key constituting particles, while the powder X-ray diffraction and electron microscopy analyses confirm that the SPS-derived pellets are phase-pure materials, preserving the structure of the colloidal synthesis products. The resultant solution-processed PbTe, PbSe, and SnSe exhibit low thermal conductivity, which might be due to the enhanced phonon scattering developed over fine microstructures. For undoped n-type PbTe and p-type SnSe samples, an expected moderate thermoelectric performance is achieved. In contrast, an outstanding figure-of-merit of 0.73 at 673 K was achieved for undoped n-type PbSe outperforming, the majority of the optimized PbSe-based thermoelectric materials. Overall, our findings facilitate the design of efficient solution-processed chalcogenide thermoelectrics

Large-Scale Synthesis of Colloidal Fe₃O₄ Nanoparticles Exhibiting High Heating Efficiency in Magnetic Hyperthermia

Exceptional magnetic properties of magnetite, Fe₃O₄, nanoparticles make them one of the most intensively studied inorganic nanomaterials for biomedical applications. We report successful gram-scale syntheses, via hydrothermal route or controlled coprecipitation in an automated reactor, of colloidal Fe₃O₄ nanoparticles with sizes of 12.9 ± 5.9, 17.9 ± 4.4, and 19.8 ± 3.2 nm. To investigate structure–property relationships as a function of the synthetic procedure, we used multiple techniques to characterize the structure, phase composition, and magnetic behavior of these nanoparticles. For the iron oxide cores of these nanoparticles, powder X-ray diffraction and electron microscopy both confirm single-phase Fe₃O₄ composition. In addition to the core composition, the magnetic performance of nanoparticles in the 13–20 nm size range can be strongly influenced by the surface properties, which we analyzed by three complementary techniques. Raman scattering and X-ray photoelectron spectroscopy (XPS) measurements indicate overoxidation of nanoparticle surfaces, while transmission electron microscopy (TEM) shows no distinct core–shell structure. Considered together, Raman, XPS, and TEM observations suggest that our nanoparticles have a gradually varying nonstoichiometric Fe₃O_4+δ composition, which could be attributed to the formation of Fe₃O₄–γ-Fe₂O₃ solid solutions at their outermost surface. Detailed analyses by TEM reveal that the hydrothermally produced samples include single-domain nanocrystals coexisting with defective twinned and dimer nanoparticles, which form as a result of oriented-attachment crystal growth. All our nanoparticles exhibit superparamagnetic-like behavior with a characteristic blocking temperature above room temperature. We attribute the estimated saturation magnetization values up to 84.01 ± 0.25 emu/g at 300 K to the relatively large size of the nanoparticles (13–20 nm) coupled with the syntheses under elevated temperature; alternative explanations, such as surface-mediated effects, are not supported by our spectroscopy or microscopy measurements. For these colloids, the heating efficiency in magnetic hyperthermia correlates with their saturation magnetization, making them appealing for therapeutic and other biomedical applications that rely on high-performance nanoparticle-mediated hyperthermia

High-Temperature Magnetism as a Probe for Structural and Compositional Uniformity in Ligand-Capped Magnetite Nanoparticles

To investigate magnetostructural relationships in colloidal magnetite (Fe₃O₄) nanoparticles (NPs) at high temperature (300–900 K), we measured the temperature dependence of magnetization (<i>M</i>) of oleate-capped magnetite NPs ca. 20 nm in size. Magnetometry revealed an unusual irreversible high-temperature dependence of <i>M</i> for these NPs, with dip and loop features observed during heating–cooling cycles. Detailed characterizations of as-synthesized and annealed Fe₃O₄ NPs as well as reference ligand-free Fe₃O₄ NPs indicate that both types of features in <i>M</i>(<i>T</i>) are related to thermal decomposition of the capping ligands. The ligand decomposition upon the initial heating induces a reduction of Fe³⁺ to Fe²⁺ and the associated dip in <i>M</i>, leading to more structurally and compositionally uniform magnetite NPs. Having lost the protective ligands, the NPs continually sinter during subsequent heating cycles, resulting in divergent <i>M</i> curves featuring loops. The increase in <i>M</i> with sintering proceeds not only through elimination of a magnetically dead layer on the particle surface, as a result of a decrease in specific surface area with increasing size, but also through an uncommonly invoked effect resulting from a significant change in Fe³⁺/Fe²⁺ ratio with heat treatment. The interpretation of irreversible features in <i>M</i>(<i>T</i>) indicates that reversible <i>M</i>(<i>T</i>) behavior, conversely, can be expected only for ligand-free, structurally and compositionally uniform magnetite NPs, suggesting a general applicability of high-temperature <i>M</i>(<i>T</i>) measurements as an analytical method for probing the structure and composition of magnetic nanomaterials