Patent Application
20090090886
Monodisperse inorganic mesoscopic structures with well-defined size, shape, chemical composition, and crystallinity have attracted extensive synthetic attention because of their novel morphology-dependent properties and their relevant applications including but not limited to biosensing, catalysis, optics, and data storage. (Alivisatos, A. P. Science 1996, 271, 933.; Ahmadi et al, Science 1996, 272, 1924; Cui et al., Science 2001, 291, 851; Sun et al., Science 2002, 298, 2176; Sun et al., Science 2000, 287, 1989; Bruchez et al., Science 1998, 281, 2013; Wang et al., Science 2001, 293, 1455; Xia et al., Adv. Mater. 2003, 15, 353.)
In particular, the generation of nanostructured magnetic materials with controllable shape and size in large quantities is of significant importance due to the potential applications, ranging from ferrofluids, advanced magnetic materials, catalysts, colored pigments, high-density magnetic recording media, to medical diagnostic equipment. (Hyeon, T. Chem. Commun. 2003, 927.) Much of the existing research though has focused on the synthesis and morphological organization of phase-pure nanosized building blocks such as nanoparticles. The fabrication of nanosized composites, such as core-shell, coaxial cable, as well as one and two-dimensional structures, has not been as investigated as comprehensively. (Yang et al., J. Am. Chem. Soc. 2005, 127, 270; Park et al, Angew. Chem. Int. Ed. 2005, 44, 2; Liu et al., J. Am. Chem. Soc. 2005, 127, 6; Kim et al., Nano Lett. 2005, 5, 1987; Zhang et al., J. Am. Chem. Soc. 2005, 127, 13492; Tzitzios et al., Adv. Mater. 2005, 17, 2188; Zeng et al., S, Nano Lett. 2004, 4, 187; Teng et al., J. Am. Chem. Soc. 2003, 125, 14559.)
Among magnetic materials, the Fe/Fe3O4 composite system has specifically attracted considerable attention due to its favorable magnetoelectric and transport (including high conductivity) properties. (Yang et al., J. Phys. D: Appl. Phys. 2005, 38, 1215; Zeng et al., Nature 2002, 420, 395; Bonetti et al., J. Appl. Phys. 2001, 89, 1806; Ding et al., Scr. Mater. 1996, 35, 1307.) Moreover, this composite has been shown to produce a novel and active heterogeneous Fenton system, important in the oxidation of organic contaminants. (Moura et al., Chemosphere 2005, 60, 1118.)
The synthetic routes associated with the formation of these composites follow a “bottom up” strategy, wherein growth of the resulting structure occurs through assembly of constituent molecular species. Such routes do not allow for predictive control of the size and shape of the resulting materials.
Additionally, considerable efforts have been expended in the generation of nanoscale structures of hematite, using a variety of techniques such as chemical precipitation, sol-gel techniques, hydrothermal approaches, forced hydrolysis, and solid-state reaction, to name a representative few. (Wang et al., Chem. Lett. 2005, 34, 184; Matijevic, E. Chem. Mater. 1993, 5, 412; Ocana et al., Adv. Mater. 1995, 7, 212; Wong et al., J. Phys. Chem. B 2001, 105, 599; Frandsen et al., Phys. Rev. Lett. 2005, 94, 027202; Jin et al., Adv. Mater. 2004, 16, 48; Woo et al., Adv. Mater. 2003, 15, 1761; Dong et al., J. Mater. Chem. 2002, 12, 1676; Sugimoto et al., Colloids Surf. A 1998, 134, 265; Jing et al., Wu, S. Mater. Lett. 2005, 59, 804; Wang et al., J. Mater. Chem. 2004, 14, 905; Jones et al., CrystEngComm 2003, 5, 159; Raming et al., J. Colloid Interface Sci. 2002, 249, 346; Chen et al., J. Phys. Chem. B 2002, 106, 8539; Hansen et al., Phys. Rev. B 2000, 62, 1124; Zboril et al., Hyp. Interact. 2002, 139/140, 597; Xu et al., J. Appl. Phys. 2002, 91, 4611; Zysler et al., J. Magn. Magn. Mater. 2001, 224, 39; Shen et al., Chem. Lett. 2004, 33, 1128; Fu et al.)
Hematite is thought to be catalytic in the oxidation of chlorinated pollutants in groundwater and is found in the clay fraction of tropical and sub-tropical soils, giving them their pink bright red hue. Bacteria in surface waters are known to catalyze the oxidation of magnetite (Fe3O4) to hematite. (Brown et al., Geochimica et Cosmochimica Acta 1997, 61, 3341.) Because of its high stability, relatively low cost, and n-type semiconducting properties with a small bandgap (2.1 eV), α-Fe2O3 has been associated with applications ranging from gas sensing, catalysis, solar energy conversion, to pigmentation. (Chen et al., Adv. Mater. 2005, 17, 582; Gondal et al., Chem. Phys. Lett. 2004, 385, 111; Ohmori et al., Phys. Chem. Chem. Phys. 2000, 2, 3519.)
The methods of making nanoscale structures of hematite (e.g., the aforementioned solution-phase approaches) involve the use of organometallic precursors, surfactants, and solvents in either potentially hazardous or rather complicated protocols. Thus, there is a need to develop an environmentally friendly and efficient methodology to synthesize iron oxides.
Additionally, the shape of a nanoparticle, which determines the exposed crystallographic surface (and its corresponding surface energy) enclosing the particle, have a dramatic effect on its properties. As examples, the relative intensities of X-ray diffraction peaks, the positions of bands in optical spectra, and the magnitude of sublimation energies of a wide variety of materials, including Au and Ag2S, are intrinsically coupled with particle morphology (such as icosahedra, cubes, and tetrahedra). With hematite in particular, changes in microhardness, electrical conductivity (i.e. mobility enhancement), as well as in superparamagnetic blocking behavior are strongly associated with its physical and structural characteristics. (Zysler et al., J. Magn. Magn. Mater. 2001, 224, 39; Stevenson et al., J. Eur. Ceram. Soc. 2002, 22, 1137; Miller et al., Thin Solid Films 2004, 466, 307.)
As another relevant manifestation of the significance of shape for magnetic nanoparticles, shape anisotropy and crystalline anisotropy have a profound influence on their intrinsic magnetic properties (such as coercivity). (Hyeon, T. Chem. Commun. 2003, 927.) In fact, the magnetic anisotropy (i.e. higher coercivity) present in rod-shaped magnetic particles, which by contrast is not observed in symmetrically-shaped spheres or cubes, has been exploited in the use of these acicular particles for commercial magnetic recording media.
However, the prior art methods of making nanoscale iron oxide structures do not allow for the predictive control of size and shape. Thus, there is a need to develop an efficient methodology by which to synthesize iron oxide nanostructures with reproducible dimensions.
In one aspect, the present invention provides single-crystalline hematite rhombohedron wherein the rhombohedron is at least about 90%, preferably 95%, free of defects and/or dislocations. The single-crystalline hematite rhombohedron is at least 95% free amorphous materials and/or impurities. Typically, the rhombohedra have an aspect ratio of about 0.5 to about 5. The single-crystalline hematite rhombohedron can be isolated or part of a plurality of substantially monodisperse single crystalline hematite rhombohedra.
In another aspect, the present invention provides a method of making single-crystalline hematite rhombohedra. The method comprises (a) mixing Fe2O3 nanopowder and a salt to form a precursor mixture, wherein the ratio of the Fe2O3 nanopowder to the salt is about 0.5:40 to about 10:40; and (b) heating said precursor mixture at about 500 to about 1000° C. to provide single-crystalline hematite rhombohedra. The rhombohedra typically have an average aspect ratio of about 0.5 to about 2. Preferably, the method further comprising sonicating the precursor mixture before heating.
In another aspect, the present invention provides a method of making single-crystalline hematite rhombohedra. The method comprises (a) mixing Fe2O3 nanopowder and a salt to form a precursor mixture; (b) adding surfactant to the precursor mixture; and (c) heating said precursor mixture at about 500 to about 1000° C. to provide single-crystalline hematite rhombohedra. As the relative amount of the Fe2O3 nanopowder to surfactant increases, the average aspect ratio of the rhombohedra increases nonlinearly.
Preferably, the ratio of Fe2O3 nanopowder:salt:surfactant is about 0.5:40:6 to about 10:40:6. The ratio of about 0.5:40:6 to about 1.5:40:6 yields rhombohedra with an average aspect ratio of about 1.5 to about 1.9. The ratio of about 1.5:40:6 to about 2.4:40:6 yields rhombohedra with an average aspect ratio of about 1.9 to about 2.4. The ratio of about 2.4:40:6 to about 4.5:40:6 yields rhombohedra with an average aspect ratio of about 2.4 to about 3.3. The ratio of about 4.5:40:6 to about 10:40:6 yields rhombohedra with an average aspect ratio of about 3.3 to about 5.
In another aspect, the present invention provides a crystalline rhombohedral nanocomposite of Fe and Fe3O4. Preferably, the relative amount Fe3O4 to Fe is about 25:75. The crystalline rhombohedral nanocomposite can be isolated or part of a plurality of substantially monodisperse rhombohedral nanocomposites. In one embodiment, the plurality of substantially monodisperse crystalline rhombohedral nanocomposites have an average aspect ratio of about 0.5 to about 5.
In another aspect, the invention provides a method of making substantially monodisperse nanocrystalline rhombohedral composites of Fe and Fe3O4. The method comprises (a) providing a hematite precursor; and (b) heating the hematite precursor at about 200 to about 500° C. in the presence of a reductive gas to make nanocrystalline composites of Fe and Fe3O4.
In one embodiment, the hematite precursor is a substantially monodisperse single crystalline hematite rhombohedra made by a method comprising (a) mixing Fe2O3 nanopowder and a salt to form a precursor mixture, wherein the ratio of the Fe2O3 nanopowder to the salt is about 0.5:40 to about 10:40; and (b) heating said precursor mixture at about 500 to about 1000° C. to provide single crystalline hematite rhombohedra.
In another embodiment, the hematite precursor is single crystalline hematite rhombohedra made by a method comprising (a) mixing Fe2O3 nanopowder and a salt to form a precursor mixture, (b) adding surfactant to the precursor mixture; (c) heating said precursor mixture at about 500 to about 1000° C. to provide single-crystalline hematite rhombohedra, wherein as the relative amount of the Fe2O3 nanopowder increases, the average aspect ratio of the rhombohedra increases nonlinearly.
The advantages of the present methods for the synthesis of iron oxide nanostructures include their environmental friendliness, simplicity, relative non-toxicity, facility of use, generalizable and versatility. Because of these advantages, the methods are suitable for the large-scale preparation of important iron oxides. Additionally, the methods of the present invention allow for predictive and reproducible control over the size, shape and crystallinity of the resulting structures.
The present invention relates to nanostructures, in particular to hematite rhombohedra and iron/magnetite nanocomposites, and methods of making same. Single Crystalline Nanoscale Hematite Rhombohedra In one aspect, the present invention provides single crystalline hematite rhombohedra (i.e., α-Fe2O3). These rhombohedra are submicron-sized or nanoscale-sized. The hematite rhombohedra have substantially corundum structure.
The nanoscale hematite rhombohedra of the invention are crystalline and solid. Preferably, the nanostructures are at least 90% free, more preferably at least 95% free, most preferably at least 99% free, and optimally virtually completely free of defects and/or dislocations. As defined in this specification, defects are irregularities in the crystal lattice (i.e., intrinsic defects). Some examples of defects include a non-alignment of crystallites, an orientational disorder (e.g., of molecules or ions), vacant sites with the migrated atom at the surface (Schottky defect), vacant sites with an interstitial atom (Frenkel defects), point defects, grain boundary defects, and non-stoichiometry of the crystal. An example of a dislocation is a line defect in a crystal lattice.
Additionally, the nanoscale hematite rhombohedra of the invention are preferably at least 90% free, more preferably at least 95% free, most preferably at least 99% free, and optimally virtually completely free of amorphous materials and/or impurities. Examples of amorphous materials include organic surfactant molecular groups, such as bis(2-ethylhexyl)sulphosuccinate, undecylic acid, sodium dodecyl sulfate (SDS), Triton X-100, decylamine, or double-hydrophilic block copolymers, which are present on the surfaces of prior art nanostructures. Examples of impurities include an element different from the elements of the crystalline structure and a vacancy.
The crystallinity and purity of the hematite rhombohedra can be examined using powder XRD measurements. As can be seen in
A single-crystalline hematite rhombohedron of the present invention typically has an aspect ratio of about 0.5 to about 5, more typically from about 1.0 to about 4, and most typically about 2 to about 3.
A single-crystalline hematite rhombohedron of the present invention typically has a width of about 100 nm to about 1000 nm, more typically from about 250 to about 800, and most typically about 400 to about 600.
A single-crystalline hematite rhombohedron of the present invention typically has length of about 200 nm to about 2500 nm, more typically from about 500 to about 2000, and most typically about 1000 to about 1500.
The present invention also provides a plurality of substantially monodisperse single crystalline hematite rhombohedra. “Substantially monodisperse” means at least 90%, more preferably at least 95%, most preferably at least 99%, and optimally virtually all, of the rhombohedra in the plurality has an aspect ratio which is within at least about 1.0 standard deviation of the mean aspect ratio.
A single-crystalline hematite rhombohedron of the present invention can be part of a plurality of substantially monodisperse single crystalline hematite rhombohedra, or can be in isolated form.
In another aspect of the present invention, methods of making single crystalline hematite rhombohedra are provided. All the types of rhombohedra described above can be made by these methods.
Typically, the single-crystalline hematite rhombohedra of the present invention can have aspect ratios of about 0.5 to about 5. The resulting aspect ratio can be controlled by the particular reaction conditions and reagents used to form the rhombohedra, as described below. Accordingly, in one embodiment of the present invention, methods of controllably producing single-crystalline hematite rhombohedra of particular aspect ratios are provided.
Single-Crystalline Hematite Rhombohedra with Relatively Low Aspect Ratio
In one embodiment, a method of making single-crystalline hematite rhombohedra having an average aspect ratio of about 0.5 to about 2, more typically of about 1 to about 1.5, is provided. The method comprises mixing Fe2O3 nanopowder and a salt to form a precursor mixture. The Fe2O3 powder can be polydisperse, monodisperse, polycrystalline, single crystalline, amorphous, or combinations thereof.
The ratio of the Fe2O3 nanopowder to the salt is about 0.5:40 to about 10:40, more typically about 1:40 to about 5:40, most typically about 1.5:40 to about 2.5:40.
The precursor mixture is heated at about 500 to about 1000° C., more preferably at about 700 to about 900° C., most preferably at about 800 to about 850° C., to provide single-crystalline hematite rhombohedra.
The salt used can be any salt. Examples of suitable salts include NaCl, KCl, NaOH/KOH, NaNO3, and NaNO3/NaCl.
In a preferred embodiment, prior to the heating step, the precursor mixture is sonicated.
Preferably, the prepared single-crystalline hematite rhombohedra are brought to room temperature and washed. The rhombohedra can be collected by any means known in the art (e.g., centrifugation).
Single-Crystalline Hematite Rhombohedra with Elongated Structures
In another embodiment, methods of making single-crystalline hematite rhombohedra with elongated structures are provided. These rhombohedra have an average aspect ratio of about 1.5 to about 5.0, more typically of about 2.0 to about 3.5.
The method comprises mixing Fe2O3 nanopowder and a salt to form a precursor mixture. The Fe2O3 powder can be polydisperse, monodisperse, polycrystalline, single crystalline or amorphous, or combinations thereof. Surfactant is added to the precursor mixture.
Preferably, the ratio of Fe2O3 nanopowder:salt:surfactant is about 0.5:40:6 to about 10:40:6. The precursor mixture is heated at about 500 to about 1000° C., more preferably at about 700 to about 900° C., most preferably at about 800 to about 850° C., to provide single-crystalline hematite rhombohedra.
Any non-ionic surfactant can be used. Most nonionic surfactants are composed of linear or nonyl-phenol alcohols or fatty acids. The function of surfactants in this class is to reduce surface tension and dispersibility.
Non-ionic surfactants are known in the art. Examples of non-ionic surfactants can be found, for instance, in “Non-ionic Surfactants: Organic Chemistry,” edited by Nico M. van Os, published by Marcel Dekker (1998), and “Non-ionic Surfactants: Chemical Analysis (Surfactant Science Series, Vol. 19)” by John Cross, published by Marcel Dekker (Oct. 1, 1986). Some non-ionic surfactants can be divided into classes depending on the type of hydrophilic group appearing in the surfactants.
Two classes of non-ionic surfactants that comprise poly(ethylene oxide) groups as their hydrophilic groups are alcohol ethoxylates and the alkylphenol ethoxylates. Examples of non-ionic surfactants of these classes include tetraethylene glycol monododecyl ether; polyoxyethylene 23 glycol monododecyl ether; polyethylenoxide-polypropylenoxide (PEO-PPO) block-copolymers (such as the commercially available PEO-PPO-PEO triblockcopolymers, called Synperonics F108 and F127), polyoxyethylene alkylphenols; polyoxyethylene alcohols; polyoxyethylene esters of fatty acids; polyoxyethylene mercaptans; and polyoxyethylene alkylamines.
Another class of non-ionic surfactants is the alkyl polyglycosides. In these molecules, the hydrophilic group is a sugar molecule, such as a polysaccharide, disaccharide, trisaccharide, maltose, etc. Preferably, the polyglycosides have one or two sugar groups in their chains. Examples of non-ionic surfactants of this class include alkyl glucoside and a glucose ester.
Another class of non-ionic surfactants is sorbitan ester surfactants. Examples of non-ionic surfactants of this class include polysorbate 20 (i.e. polyoxyethylene (20) sorbitan monolaurate, sold as Tween 20™); polysorbate 60 (i.e. polyoxyethylene (60) sorbitan monostearate); polysorbate 80 (i.e. polyoxyethylene (20) sorbitan monooleate); and polysorbate 65 (i.e. polyoxyethylene (20) sorbitan tristearate).
Examples of other suitable surfactants include NP-9 surfactant (Aldrich, polyoxyethylene(9)nonylphenyl ether), sodium dodecyl sulfate (SDS), dodecylbenzene sulphonate (SBDS), Triton X-100, X-77 (UAP), Induce (Helena), Activator 90 (UAP), Triton Ag 98 (Rhone-Poulenc) and R-11 (Wilfarm), Pluronic Acid, bis(2-ethylhexyl)sulphosuccinate, undecylic acid, decylamine, and double-hydrophilic block copolymers.
The salt used can be any salt. Examples of suitable salts include NaCl, KCl, NaOH/KOH, NaNO3, and NaNO3/NaCl.
As the relative amount of the Fe2O3 nanopowder is increased, the average aspect ratio of the rhombohedra increases nonlinearly. Examples of how varying the relative amount of the Fe2O3 nanopowder affects the average aspect ratio of the rhombohedra follow. When the ratio of Fe2O3 nanopowder:salt:surfactant is about 0.5:40:6 to about 1.5:40:6, the resulting rhombohedra have an average aspect ratio of about 1.5 to about 1.9. When the ratio of Fe2O3 nanopowder:salt:surfactant is about 1.5:40:6 to about 2.4:40:6, the resulting rhombohedra have an average aspect ratio of about 1.9 to about 2.4. When the ratio of Fe2O3 nanopowder:salt:surfactant is about 2.4:40:6 to about 4.5:40:6, the resulting rhombohedra have an average aspect ratio of about 2.4 to about 3.3. When the ratio of Fe2O3 nanopowder:salt:surfactant is about 4.5:40:6 to about 10:40:6, the resulting rhombohedra have an average aspect ratio of about 3.3 to about 5.
Additionally, for a constant amount of Fe2O3 nanopowder, the ratio of salt:surfactant can be about 100:1 to about 1:1, wherein as the relative amount of surfactant increases, the aspect ratio of the resulting rhombohedra decreases.
In a preferred embodiment, prior to the heating step, the precursor mixture is sonicated.
Preferably, the prepared single-crystalline hematite rhombohedra are brought to room temperature and washed. The rhombohedra can be collected by any means known in the art (e.g., centrifugation).
In one aspect, the present invention provides crystalline rhombohedral nanocomposites of Fe and magnetite (i.e., Fe3O4). A single rhombohedral nanocomposite consists essentially of Fe and Fe3O4.
The Fe and Fe3O4 can be dispersed within a single rhombohedral nanocomposite in domain regions. The Fe domain regions and the Fe3O4 domain regions are adjacent to one another within a rhombohedron. The domain regions are each typically about 1 to about 50 nm, more typically from about 10 to about 20 nm, most typically about 11 to about 15 nm in area.
The nanocomposites of the invention are crystalline and solid. Preferably, the nanocomposites are at least 90% free, more preferably at least 95% free, most preferably at least 99% free, and optimally virtually completely free of defects and/or dislocations. Additionally, the nanocomposites of the invention are preferably at least 90% free, more preferably at least 95% free, most preferably at least 99% free, and optimally virtually completely free of amorphous materials and/or impurities.
The invention also provides a substantially monodisperse plurality (i.e., assembly) of crystalline rhombohedral nanocomposites of Fe and magnetite (i.e., Fe3O4).
“Substantially monodisperse” means at least 90%, more preferably at least 95%, most preferably at least 99%, and optimally virtually all of the nanocomposites of a plurality consists of nanostructures with a rhombohedral shape and consists of Fe and Fe3O4.
In some embodiments, “substantially monodisperse” also means that at least 90%, more preferably at least 95%, most preferably at least 99%, and optimally virtually all of the rhombohedra in the nanocomposite plurality have an average aspect ratio which is within at least 1.0 standard deviation of the mean aspect ratio.
The nanocomposites have hard and soft magnetic phases juxtaposed within one discrete, anisotropic structure. Typically, Fe3O4 is the soft phase and Fe is the hard phase. The relative amount of Fe to Fe3O4 can be from about 1:99 to about 99:1. Typically, the dimension of the soft phase is made to be smaller than that of the hard phase. For example, the amount of Fe3O4 to Fe can be about 25:75 to about 40:60, more typically about 30:70 to about 33:67.
The nanocomposites typically exhibit strong ferromagnetic behavior. Typically, there is some degree of magnetic exchange coupling between the Fe and magnetite domains.
The structural inhomogeneity of the Fe/magnetite nanocomposites has a strong influence on its saturation magnetization (i.e., MS). Depending on the relative amount of Fe, the nanocomposite can exhibit MS of about 50 to about 250 emu/g. The MS increases as the relative amount of Fe in the composite increases. As an example, when the amount of Fe3O4 to Fe is about 33:67, the MS is about 100 emu/g.
The Fe/magnetite nanocomposite possesses a relatively high Verwey transition temperature. Depending on the relative amount of Fe, the nanocomposite can exhibit Verwey transition temperature of about 100 K to about 200 K. The Verwey transition temperature increases as the relative amount of Fe in the composite increases. As an example, when the amount of Fe3O4 to Fe is about 33:67, the Verwey transition temperature is about 135 K.
The Fe/magnetite nanocomposites possess a relatively high coercivity. Coercivity is the intensity of the applied magnetic field required to reduce the magnetization of that material to zero after the magnetization of the sample has been driven to saturation. The nanocomposite can exhibit coercivity of about 100 to about 500 Oe. As an example, when the amount of Fe3O4 to Fe is about 33:67, the coercivity is about 250 Oe.
In another aspect of the present invention, methods of making crystalline rhombohedral nanocomposites of Fe and Fe3O4 are provided. All the types of nanocomposites described above can be made by these methods.
In the methods, hematite (i.e., hematite precursor) is transformed into Fe/Fe3O4 nanocomposites. Any type of hematite can be used in these methods, including, for example, polydisperse or monodisperse hematite. If substantially monodisperse hematite is used, then substantially monodisperse nanocomposites are produced.
The methods comprise heating hematite at about 200 to about 500° C., more preferably at about 250 to about 450° C., most preferably at about 300 to about 400° C., in the presence of a reductive gas to provide Fe/Fe3O4 nanocomposites. The reductive gas can be, for example, hydrogen gas by itself, or a gasesous mixture comprising hydrogen. A gaseous mixture can be, for example, hydrogen gas in the presence of an inert gas (e.g., nitrogen gas or any of the noble gases). For example, the gasesous mixture can be about 1% H2 to about 10% H2 in an inert gas, e.g., about 5% H2 in N2.
The relative amount of Fe in the composite increases as the duration and/or temperature of the reduction reaction is increased. If the reduction reaction is run long enough, then there will be virtually only a Fe phase.
As the hematite is reduced, the reddish hue of hematite changes to the deep black of the resulting iron/magnetite composite.
In a preferred embodiment, the hematite is single crystalline hematite in the shape of rhombohedra (i.e., α-Fe2O3). In a highly preferred embodiment, the single crystalline hematite rhombohedra are substantially monodisperse. Methods of making substantially monodisperse single crystalline hematite rhombohedra are described above. The shape, size and aspect ratio of the starting hematite rhombohedra directly transforms into the shape, size and aspect ratio of the resulting nanocomposite.
The composites have many practical applications. For example, the relatively high coercivity indicates the applicability of these nanoscale composite magnetic structures as low-cost hard magnetic materials. The magnetically hard/soft phases can be used to tune magnetic properties by controlling the transformation conditions so as to obtain desired saturation magnetizations. The control of Verwey transition has applications associated with metallic and insulating properties.
The Examples demonstrate the generation of distinctive structural polymorphs of hematite iron oxide from relatively polydisperse, commercially available starting precursor materials. Subsequently, these single-crystalline α-Fe2O3 rhombohedral structures were transformed into their magnetic nanocrystalline composite counterparts, Fe/Fe3O4. These samples were characterized by a number of techniques, including scanning electron microscopy (SEM), transmission electron microscopy (TEM), high-resolution TEM (HRTEM), energy-dispersive X-ray spectroscopy (EDS), selected area electron diffraction (SAED), X-ray diffraction (XRD), and superconducting quantum interference device (SQUID) measurements.
General. Specifically, commercial iron(III) oxide or Fe2O3 (Aldrich, polydisperse nanopowder), NP-9 surfactant (Aldrich, polyoxyethylene(9)nonylphenyl ether), and NaCl (Mallinckrodt, sodium chloride) were used as supplied. The choice of the surfactant was governed by its prior versatility in the preparation of elongated structures of metal oxides, its relative non-toxicity, and comparative facility of use. (Park et al., J. Mater. Chem. 2005, 15, 2099.) Stoichiometric amounts of Fe2O3, NaCl, and NP-9 were mixed (in molar ratios of 1:40:6, 2:40:6, 3:40:6, and 6:40:6, respectively, for the generation of varying structural motifs of hematite), thoroughly ground in an agate mortar, and subsequently sonicated.
In a typical synthesis of submicron-sized, single-crystalline α-Fe2O3 rhombohedra, 0.5 and 20 mmol of Fe2O3 and NaCl, respectively, were mixed thoroughly in an agate mortar. For elongated α-Fe2O3 structures, 1, 1.5, and 3 mmol of Fe2O3 along with 20 mmol of NaCl, respectively, were meticulously mixed, after which 2 ml of NP-9 was subsequently added. The resulting mixture was ground for at least 30 min prior to sonication for an additional 5 min. Identical procedures were employed for samples containing different molar ratios of initial precursors. The resulting mixture was then placed in a ceramic crucible, inserted into a quartz tube, heated at a ramp rate of 5° C. per min up to an annealing temperature at 820° C. for 3.5 h, and cooled thereafter to room temperature. As-prepared material was subsequently washed several times with distilled water, collected by centrifugation, and dried at 120° C. in a drying oven.
Hematite rhombohedra were converted to their magnetic analogues (i.e. composites of Fe/Fe3O4) through a reduction reaction in a flowing gaseous mixture. Briefly, the as-prepared hematite product was heated in a tube furnace at 360° C. for 5 h under a continuous flow of 5% H2 in N2. After the gas flow was stopped, the resulting product was subsequently heated to 240° C. for 2 h, cooled to room temperature, and then collected without further treatment.
X-Ray diffraction. Crystallographic information of as-prepared samples was obtained on a Scintag diffractometer, operating in the Bragg configuration using Cu Kα radiation (k=1.54 Å). Powder X-ray diffraction (XRD) samples were prepared by grinding products thoroughly in ethanol using a mortar and pestle, followed by loading onto glass slides, and subsequent drying in air. Diffraction patterns were collected from 10 to 80° at a scanning rate of 2°/min with a step size of 0.02°. Parameters used for slit widths and accelerating voltage were identical for all samples.
Electron Microscopy. The particle size and morphology of the resulting Fe2O3 as well as Fe/Fe3O4 products were initially characterized using a field emission scanning electron microscopy (SEM, Leo 1550) operated at accelerating voltages of 15 kV and equipped with energy dispersion X-ray spectroscopy (EDS) capabilities. Specifically, samples were deposited onto a Si wafer, which were then attached to the surface of SEM brass stubs using a copper tape. These samples were then conductively coated with gold by sputtering them for 10-20 seconds to minimize charging effects under SEM imaging conditions.
Specimens for transmission electron microscopy (TEM) and high resolution TEM (HRTEM) were obtained by drying sample droplets from an ethanolic dispersion onto a 300 mesh Cu grid coated with a lacey carbon film. Low magnification TEM images were taken at an accelerating voltage of 120 kV on a Philip CM12 instrument. High-resolution images were obtained on a JEOL 2010F HRTEM at an accelerating voltage of 200 kV. This instrument was equipped with an Oxford INCA EDS system with the potential of performing selected area electron diffraction (SAED) to further characterize individual iron oxide nanostructures.
SQUID. Magnetization measurements were obtained using an MPMS magnetometer. Powder samples of as-prepared products were pressed lightly, then loaded into a gel cap, and covered with silica wool. This was held within a uniform drinking straw, which was attached to the sample rod of the MPMS apparatus. Signals generated by measurements of an empty sample holder demonstrated that the holder assembly contributes <1% to the overall magnetic signal.
The purity and crystallinity of as-prepared hematite (α-Fe2O3) structures were examined using powder XRD measurements (
A typical TEM image of α-Fe2O3 rhombohedra, generated from the current molten salt method with a 1:40 molar ratio of Fe2O3 to NaCl, is shown in
To obtain further insight into the formation of different shapes of the α-Fe2O3 structures, the morphologies of α-Fe2O3 structures generated from the precursors, derived from 2:40:6, 3:40:6, and 6:40:6 (Fe2O3:NaCl:NP-9) molar ratios, are shown in
To analyze the role of surfactant in this reaction, α-Fe2O3 products were prepared employing identical, as-reported experimental procedures in the absence of any surfactant. The morphologies of the resultant products generated from mixtures of (a) 3:40 and (b) 6:40 molar ratios of Fe2O3 to NaCl precursors, respectively, are shown in
The observations described above confirm the significant role of the surfactant combined with other experimental parameters, such as the molar ratios of precursors and the addition of salt, to collectively yield single-crystalline α-Fe2O3 products with predictive control of size and shape. The presence of salt is expected to decrease the overall reaction temperature. (Wiley et al., Science 1992, 255, 1093.) The liquid-like phase of the molten flux is expected to increase the mobility of its constituent components.
The morphologies of the resulting Fe/Fe3O4 composites generated from the reduction reaction are shown in
In order to further confirm the composition of the as-transformed magnetic materials, HRTEM was performed on an individual Fe/Fe3O4 composite shown in
The higher magnification image of a typical Fe/Fe3O4 composite reveals that the surface of the rhombohedron is composed of multiple nanostructures with mean sizes of 11±3 nm (
Thus, monodisperse nanocrystalline rhombohedral composites of Fe and Fe3O4 magnetic materials have been obtained employing a reduction reaction, in a flowing gas mixture of H2 and N2, of single-crystalline, submicron-sized α-Fe2O3 rhombohedral precursors. This synthesis is significant in that nanocomposites with hard and soft magnetic phases juxtaposed within one discrete, anisotropic structure were created. In turn, the precursor hematite rhombohedra of reproducible shape were successfully prepared using a facile, large-scale molten-salt reaction. Rhombohedra represent a high-surface-area, anisotropic formulation of an industrially important material (iron oxide) which is an active component of gas sensors, photocatalysts, and other types of catalytic materials.
Accordingly, the present invention demonstrates the generation of monodisperse assemblies of rhombohedral nanocrystalline composites of Fe and Fe3O4 formed from a reordering of the microstructure of single-crystalline hematite rhombohedra precursors which occurred as a product of the reduction reaction in a gaseous mixture of 5% H2 in N2. A significance of the work is that a nanocomposite with hard and soft magnetic phases juxtaposed within one discrete, anisotropic structure was created.
Also, demonstrated is the ability to prepare monodisperse hematite products with controlled size, shape, and monodispersity, starting from relatively inexpensive, commercially available polydisperse, polycrystalline or amorphous precursors. Single-crystalline, monodisperse nanoscale α-Fe2O3 rhombohedra, as well as elongated motifs of these materials, are in fact an excellent model system to demonstrate this synthetic principle. This methodology allows for control over size, shape, and chemical composition of as-prepared products using a simple, versatile, one-step, environmentally-friendly, and large-scale solid-state chemical reaction in the presence of NaCl and a nonionic surfactant.
Additionally, rhombohedra represent a high-surface-area, anisotropic formulation of an industrially important material (iron oxide) which is an active component of gas sensors, photocatalysts, and other types of catalytic materials. Moreover, α-Fe2O3 hematite rhombohedra present themselves as a practical, low-cost chemical precursor material to the subsequent synthesis of magnetite. The development of a facile and economically viable synthetic strategy for the synthesis of hydrophilic, biocompatible magnetic particles (including magnetite) benefits their technical use in biomedical fields, such as biomolecular separations, targeted drug delivery, tags for sensing and imaging, antitumor therapy, as well as magnetic resonance imaging. Nanostructured magnetic materials of the present invention also have applications as ferrofluids, catalysts, colored pigments, and high-density magnetic recording media.
As relevant manifestation of the significance of shape, for magnetic nanoparticles in particular such as the Fe/Fe3O4 composites of the present invention, shape anisotropy and crystalline anisotropy are expected to have a profound influence on their intrinsic magnetic properties (such as coercivity). In fact, the magnetic anisotropy (i.e. higher coercivity) present in rod-shaped magnetic particles, which by contrast is not observed in symmetrically-shaped spheres or cubes, has been exploited in the use of these acicular particles for commercial magnetic recording media.
This invention was made with Government support from the U.S. Department of Energy under contract number DE-AC02-98CH10886 for facility support, the National Science Foundation under CAREER award Grant No. DMR-0348239, and the donors of the American Chemical Society Petroleum Research Fund. The Government has certain rights in this invention.
