Results 1 - 10 of 14
Results 1 - 10 of 14. Search took: 0.021 seconds
|Sort by: date | relevance|
[en] A direct probe of superparamagnetism was used to determine the complete anisotropy energy distribution of Co nanoparticle films. The films were composed of self-assembled lattices of uniform Co nanoparticles of 3 or 5nm in diameter, and a variable temperature scanning-SQUID microscope was used to measure temperature-induced spontaneous magnetic noise in the samples. Accurate measurements of anisotropy energy distributions of small volume samples will be critical to magnetic optimization of nanoparticle devices and media
[en] We have studied the magnetic relaxation of monodispersed 4 nm cubic ε-cobalt nanocrystals in both randomly oriented and pre-aligned assemblies. The blocking temperature TB, for the closely packed Co nanocrystal assemblies, is 30% higher than that of the highly diluted and well-dispersed Co nanocrystal-organic composites. This increase is attributed to the strong magnetic dipole interaction induced from the close packing of the nanocrystals. It is found that the frequency-dependent susceptibility data, obtained from the diluted samples, can be fitted to the half-circle Argand Diagrams, indicating a single barrier (or very narrow energy distribution) of the nanocrystals. This agrees well with the physical observation from TEM that the nanocrystals are monodispersed. The long time magnetic relaxation measurements reveal that energy barrier distribution in a pre-aligned nanocrystal assembly is significantly different from that in a randomly oriented one
[en] Self-assembled 4 nm FePt nanoparticle arrays were treated with rapid thermal annealing (RTA) process. The phase transformation from the chemically disordered face-centered-cubic structure of the as-synthesized FePt nanoparticles to the chemically ordered face-centered-tetragonal structure was realized by RTA, with both the annealing temperature and time being greatly reduced, as compared to conventional annealing. The onset of chemical ordering occurred at the annealing condition of around 400 deg. C for only 5 s. Temperature-dependent coercivity measurements revealed strong thermal effects of the nanoparticle assemblies. Curie temperatures (Tc) of the annealed assemblies were derived from the temperature-dependent hysteresis measurements. Tc of the annealed assemblies increases with increasing annealing temperature, but is about 200-300 K lower than that of bulk FePt (750 K). The drastic reduction of Tc may be attributed to size effect and partial chemical ordering of FePt nanoparticles
[en] Highlights: • The fundamentals of the synthesis of monodisperse metal nanoparticles (MNPs) by organic solution phase reactions are outlined. • The common strategies applied to tailor MNP size, shape and structure (core/shell and intermetallic) are summarized. • The representative applications of MNPs in catalyzing oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, methanol oxidation reaction, formic acid reaction and oxygen evolution reaction are highlighted. Metal nanoparticles (MNPs) are essential catalyst components in electrochemical energy conversion and storage devices, including fuel cells, Li-air batteries and water-splitting systems. Syntheses of monodisperse MNPs with controlled sizes, shapes and structures is key to fully harvesting their catalytic capabilities. This review first outlines the fundamentals of the synthesis of monodisperse MNPs by organic solution phase reactions. It then summaries common strategies applied to tailor MNP size, shape and structure. The review further highlights recent advances of using MNPs as efficient catalysts to catalyze some representative reactions that related to energy conversions, including oxygen reduction reaction, hydrogen evolution reaction, CO2 reduction reaction, methanol/formic acid oxidation reaction, and oxygen evolution reaction.
[en] Monodispersed cobalt nanoparticles (NPs) with controllable size (8-14 nm) have been synthesized using thermal decomposition of dicobaltoctacarbonyl in organic solvent. The as-synthesized high magnetic moment (125 emu/g) Co NPs are dispersible in various organic solvents, and can be easily transferred into aqueous phase by surface modification using phospholipids. However, the modified hydrophilic Co NPs are not stable as they are quickly oxidized, agglomerated in buffer. Co NPs are stabilized by coating the MFe2O4 (M=Fe, Mn) ferrite shell. Core/shell structured bimagnetic Co/MFe2O4 nanocomposites are prepared with tunable shell thickness (1-5 nm). The Co/MFe2O4 nanocomposites retain the high magnetic moment density from the Co core, while gaining chemical and magnetic stability from the ferrite shell. Compared to Co NPs, the nanocomposites show much enhanced stability in buffer solution at elevated temperatures, making them promising for biomedical applications. - Graphical abstract: The 10 nm/3 nm Co/MFe2O4 (M=Fe, Mn) bimagnetic core/shell nanocomposites are synthesized from the surface coating of ferrite shell over 10 nm Co nanoparticle seeds. The nanocomposites show much enhanced chemical and magnetic stability in solid state, organic solution and aqueous phase, and are promising for biomedical applications
[en] Self-assembly of FePt and Fe3O4 nanoparticles of different sizes led to various FePt-Fe3O4 nanocomposites. Annealing the composite under reducing atmosphere at 650 and 700 deg. C induced magnetically hard FePt phase and magnetically soft Fe3Pt phase. The FePt and Fe3Pt phases were either linked by a common interface or coexisted within one grain as domains with sizes <10 nm. This ensures the effective exchange coupling of magnetically hard and soft phases. High-resolution transmission electron microscopy studies provide detailed structural characterization for the FePt based nanocomposites
[en] We demonstrate a magneto-optic technique to measure Brownian relaxation of magnetic nanoparticles suspended in liquids. We used AC susceptibility data as a function of frequency of the applied AC magnetic field to verify that the results agree with those obtained via a conventional inductive detection technique. However, compared with a commercial AC susceptometer using the conventional detection scheme, our magneto-optic setup is able to detect a density of nanoparticles at least three orders smaller. This technique has the potential of being used as a sensor for magnetic nanoparticles such as in local temperature, viscoelasticity, or molecular-binding measurements
[en] Highlights: • Monodisperse Cu nanoparticles are synthesized and assembled on pyridinic-N rich graphene (p-NG). • 7 nm Cu NPs assembled on p-NG are active and selective for electrochemical CO2 reduction to ethylene. • Standalone p-NG shows much higher activity than graphitic-N rich graphene and graphene oxide for CO2 reduction to formate. Monodisperse Cu nanoparticles (NPs) assembled on a pyridinic-N rich graphene (p-NG) support show a Cu NP mass- and size-dependent catalysis for the selective electrochemical reduction of CO2 to ethylene (C2H4). For the 7 nm Cu NPs assembled on the p-NG with the p-NG/Cu mass ratio of 1:1, the C2H4 formation Faradaic efficiency and hydrocarbon selectivity reach 19% and 79% respectively at −0.9 V (vs reversible hydrogen electrode). The p-NG itself can catalyze the CO2 reduction to formate, but in the composite p-NG-Cu structure, the pyridinic-N functions as a CO2 and proton absorber, facilitating hydrogenation and carbon–carbon coupling reactions on Cu for the formation of C2H4. The work demonstrates a new strategy to improve Cu NP catalytic activity and selectivity for the electrochemical reduction of CO2 for sustainable chemistry and energy applications.
[en] We report exchange bias (EB) effect in the Au-Fe_3O_4 composite nanoparticle system, where one or more Fe_3O_4 nanoparticles are attached to an Au seed particle forming ‘dimer’ and ‘cluster’ morphologies, with the clusters showing much stronger EB in comparison with the dimers. The EB effect develops due to the presence of stress at the Au-Fe_3O_4 interface which leads to the generation of highly disordered, anisotropic surface spins in the Fe_3O_4 particle. The EB effect is lost with the removal of the interfacial stress. Our atomistic Monte Carlo studies are in excellent agreement with the experimental results. These results show a new path towards tuning EB in nanostructures, namely controllably creating interfacial stress, and opens up the possibility of tuning the anisotropic properties of biocompatible nanoparticles via a controllable exchange coupling mechanism. (paper)
[en] We report a general chemical approach to synthesize strongly ferromagnetic rare-earth metal (REM) based SmCo and SmFeN nanoparticles (NPs) with ultra-large coercivity. The synthesis started with the preparation of hexagonal CoO+Sm2O3 (denoted as SmCo-O) multipods via decomposition of Sm(acac)3 and Co(acac)3 in oleylamine. These multipods were further reduced with Ca at 850 C to form SmCo5 NPs with sizes tunable from 50 to 200 nm. The 200 nm SmCo5 NPs were dispersed in ethanol, and magnetically aligned in polyethylene glycol (PEG) matrix, yielding a PEG-SmCo5 NP composite with the room temperature coercivity (Hc) of 49.2 kOe, the largest Hc among all ferromagnetic NPs ever reported, and saturated magnetic moment (Ms) of 88.7 emu g-1, the highest value reported for SmCo5 NPs. The method was extended to synthesize other ferromagnetic NPs of Sm2Co17, and, for the first time, of Sm2Fe17N3 NPs with Hc over 15 kOe and Ms reaching 127.9 emu g-1. These REM based NPs are important magnetic building blocks for fabrication of high-performance permanent magnets, flexible magnets, and printable magnetic inks for energy and sensing applications. (copyright 2019 Wiley-VCH Verlag GmbH and Co. KGaA, Weinheim)