Core-shell magnetic nanoparticles and nanocomposite materials formed therefrom

Abstract

Monodisperse core-shell types of metal, metal oxide, and alloy nanoparticles with variable core diameters and shell thicknesses, and controlled synthesis methods for producing such nanoparticles are provided. Nanocomposite materials fabricated from nanoparticles, and methods for producing such nanocomposite materials are also provided.

Description

BACKGROUND OF THE INVENTION [0001] 1. Field of Invention [0002] The present invention is directed to core-shell types of metal, metal oxide and alloy nanoparticles with variable core diameters and shell thicknesses, and controlled synthesis methods for producing such nanoparticles. The present invention is also directed to nanocomposite materials fabricated from nanoparticles, such as the nanoparticles provided by this invention, and methods for producing such nanocomposite materials. [0003] 2. Description of Related Art [0004] Nanoparticle based materials have been under investigation because of their novel catalytic, electronic, magnetic and optical properties. Among the various materials, transition metal nanoalloys containing core-shell types of nanoparticles are particularly interesting. The magnetic and chemical properties of these nanoparticles have significantly enhanced functions that do not exist in single-component compounds combined with unique properties that exist only...

AI Technical Summary

Benefits of technology

[0109] One advantage of the present invention is that the synthesis method allows for the controlled formation of core-shell nanoparticles designed to have varying core-diameter and shell-thickness. The structure and composition of the nanoparticles can be modulated by the amount of reactants, in particular, the molar amount of metal salt and organometallic compound. The nanoparticle structure and composition can also be controlled by other reaction parameters such as the reaction time and temperature, and the stabilizers and surfactants included in the reaction. Core-shell nanoparticles of various structures can then serve as precursors to an assortment of metal alloy nanoparticles with the desired chemical and magnetic properties.

[0110]FIG. 9 shows PXRD spectra of exemplary Fe / Pt alloys produced using exemplary core-shell nanoparticles according to this invention. Among the exemplary Fe / Pt alloys, diffraction patterns indexing to planes indicative of cubic platinum (Fm3m) are obained. Diffraction peaks that match with the characteristic peaks belonging to planes of a face-centered cubic phase structure of Fe3Pt and diffraction peaks that match with the characteristic peaks belonging to planes of a face-centered cubic phase structure of FePt3 are shown in FIG. 9.

[0111] Conversion of closely packed synthetic core-shell nanoparticles can lead to the formation of thin films of FePt alloy. One of the unique aspects of using core-shell based nanoparticles for nanocomposite films lies in the fact that the particles are single-source precursors and therefore, could potentially eliminate the needs for homogenous mixing of two or more types of particles. A further unique aspect is that the required chemical compositions and nanostructures can be pre-modulated prior to particle assembly by using core-shell nanoparticles with appropriate core-diameter and shell thickness. The core-shell nanoparticles can be assembled into thin films using controlled deposition methods, such as Langmuir-Blodgett (LB) and Langmuir-Schaffer techniques. In particular, LB enables fine control of thickness, particle density and homogeneity of monolayer, and ease of multilayer deposition.

[0112] The LB technique is suitable for producing large areas of polydisperse and monodisperse nanoparticles. In various exemplary embodiments, LB films of γ-Fe2O3 nanoparticles can be made using a KSV 3000 Langmuir trough in a Class 10000 clean room. The surface pressure can be monitored using a Wilhepy plate method. Nanoparticles can be suspended in hexane (e.g., 800-1000 μL) and spread at the air-water (Barnstead Nanopure II, 16.7 MΩ) interface using a microsyringe. After the hexane evaporates (e.g., <5 min), compression of the film can be performed at a rate of 10 mN / (m min). LB films can be prepared at surface pressures ranging from about 30 to about 65 mN / m and lifted onto solid substrates such as silicon wafer or poly(dimethlysiloxane) stamp (PDMS) at a rate of about 5 mm / min. Monolayer deposition can be performed with substrates submerged in water prior to spreading.

[0113]FIG. 10 shows an isotherm of surface pressure vs. area (π-A) for monodisperse nanoparticles of γ-Fe2O3 and TEM images of LB films at four different compression pressures. At low surface pressure (about 15 mN / m), nanoparticles are packed in patches connected randomly with large spaces. In each individual patch, nanoparticles are highly ordered and form hexagonal close-packed (hcp) structure. This ordered structure is very different from those LB films made of polydisperse particles and most likely is due to the monodispersity of the nanoparticles. This Langmuir film can be compressed into close-packed domains with diminishing void spaces at increasing surface pressure. Very high compressing pressure (about 65 mN / m) can be achieved for the γ-Fe2O3 monolayer. Complete coverage of the substrate by magnetic nanoparticles can be readily achieved with double layered LB films. It is feasible that Pt@Fe2O3 core-shell nanoparticles can be compressed at equally high pressure since the surface properties of the particles are similar to those of monodisperse nanoparticles of γ-Fe2O3.

[0114] The magnetic properties of exemplary FePt alloy nanostructures on silicon wafers can be examined using a Quantum Design AC and DC Superconducting Quantum Interface Device (AC / DC SQUID; model: MPMS XL) magnetometer. FIG. 11 shows the M-H curves of exemplary FePt alloys at (a) 5 K and (b) 300 K. The M-H measurement of such exemplary materials shows a characteristic hysteresis loop for ferromagnetic materials. The coercivity at low (5 K) and high (300 K) temperatures are 9.1 and 7.0 kOe, respectively.

Problems solved by technology

It continues to be a challenge in the area of solid state nanomaterials to precisely control the chemical composition, structure, size and shape of nanoparticles.

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