WO2015195650A1

WO2015195650A1 - Direct transformation of bulk materials to nanoparticles

Info

Publication number: WO2015195650A1
Application number: PCT/US2015/036010
Authority: WO
Inventors: Christopther SORENSEN; Stefan H. Bossmann; Xiao-min LIN; Emily J. Mclaurin; Jessica CHANGSTROM; Pablo Guimera COLL; Jeffrey Alan POWELL; Hongwang Wang; Asanka Sajini YAPA; Raghavender SIRAMDAS
Original assignee: Kansas State University Research Foundation; Uchicago Argonne, Llc
Priority date: 2014-06-16
Filing date: 2015-06-16
Publication date: 2015-12-23

Abstract

The creation of a nanoparticle solution from bulk materials in a one-step process is disclosed. Particles of the bulk material, typically in granular or powder or pellet form, having a particle size of on the order of 1 m or greater, are introduced into a specially-selected solvent and ligand solution and heated thereby directly producing nanoparticles having essentially the same chemical make-up of the bulk material.

Description

DIRECT TRANSFORMATION OF BULK MATERIALS TO NANOPARTICLES

RELATED APPLICATION

The present application claims the benefit of U.S. Provisional Patent Application No. 62/012,765, filed June 16, 2014, which is incorporated by reference herein in its entirety. STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with Government support under Contract No. DE-AC02- 06CH 11357 between the United States Government and UChicago Argonne, LLC representing Argonne National Laboratory. The United States Government has certain rights in this invention.

BACKGROUND OF THE INVENTION

Field of the Invention

The present invention is generally directed toward a top-down approach to the creation of a nanoparticle solution directly from bulk materials. Particularly, this process is one of direct transformation of a bulk material into nanoparticles of that material using a one-step process. Particles of the bulk material, typically on the order of 1 μιη or greater, are introduced into a specially-selected solvent and ligand solution and directly transformed into nanoparticles of the material.

Description of the Prior Art

The past decade and a half has seen the emergence of chemistries that allow for the synthesis of nanoparticles of a wide variety of compositions with controlled, uniform sizes and shapes. Such nanoparticles have great promise in a variety of applications, ranging from catalysts, optical and electronic devices, magnetic storage media, chemical/biological sensors, drug delivery, and photovoltaic applications. Despite the great promises, there are currently very few large-scale applications of such nanoparticle materials. This is because there is a significant gap between typical lab-scale synthesis (ca. milligram) and industrial scale production of nanoparticles (ca. gram scale, kilogram scale, and beyond). Many of the chemistries that yield nanoparticles start with precursor reagents such as the salts or organometallic complexes of the metals that one desires to compose the nanoparticles. Although there are exceptions, these exceptions are energy intensive, as are most processes by which the precursor chemicals are made. Advantageous scale up applications would desirably be characterized by low energy usage and the creation of relatively few intermediates between the raw materials and the final nanoparticle product. Thus a critical roadblock for industrial scale-up is that most syntheses that yield useful nanoparticles require chemical precursors that are far removed in cost, energy and time from the base constituent elements that will make up the nanoparticle. This bottom-up approach is completely acceptable for lab-scale experimentation where the product is new science, but too expensive in many ways when the product is the nanoparticles themselves.

With increasing demand of nanoparticles in many industrial applications, it becomes important to develop feasible scale up production of nanoparticles. However, there are several critical roadblocks that have prevented large commercial scale production of nanoparticles. Most chemical syntheses are based upon kinetically controlling the reaction heating profile, in order to separate the particle nucleation and growth stages and thus allow a uniform growth of nanoparticles. These approaches are problematic when the reaction volume is scaled up to industrial size, as the heating profile of the reaction media becomes highly non-uniform. Furthermore, expensive high boiling point organic solvents and precursors/surfactants are typically used. Most physical process based techniques to create nanoparticles also suffer from the drawback of high material cost and energy waste.

SUMMARY OF THE INVENTION

According to one embodiment of the present invention there is provided a method of producing a plurality of nanoparticles from a solid bulk material. The solid bulk material is dispersed within a solvent medium to form a mixture. The solvent medium comprises a solvent that is capable of forming a colloidal suspension with the nanoparticles and a ligand that is capable of dissociating a portion of the bulk material without changing the chemical make up of the material. The mixture is heated for a predetermined period of time sufficient to dissociate the plurality of nanoparticles from the bulk material through the action of the ligand. According to another embodiment of the present invention there is provided a method of producing a plurality of nanoparticles from a solid bulk material. The solid bulk material is dispersed within a solvent medium to form a mixture. The solvent medium comprises a solvent that is capable of forming a colloidal suspension with the nanoparticles and a ligand that is capable of dissociating a portion of the bulk material. The mixture is heated under an oxygen-containing atmosphere for a predetermined period of time sufficient to dissociate the portion of the solid bulk material therefrom through the action of the ligand and form the plurality of nanoparticles. The plurality of nanoparticles have the same chemical make-up as the solid bulk material.

According to another embodiment of the present invention a nanoparticle dispersion comprising a plurality of nanoparticles dispersed in a solvent medium is produced by any of the methods described herein.

BRIEF DESCRIPTION OF THE FIGURES

Figure 1 is a TEM image of Ag nanoparticles produced from a mixture of 6.6mM of PVP40 in ethylene glycol with Ag powder at a temperature of 120°C after a time of 24h under an air atmosphere;

Fig. 2 depicts TEM images of Ag nanoparticles after repeated transformation processing with DDT in DBE at 300°C;

Fig. 3a is a TEM image of cadmium sulfide nanoparticle precursors produced from cadmium shot and sulfur powder in a microwave reactor with octadecylamine in decane at 300°C for 27 minutes;

Fig. 3b is a TEM image of cadmium sulfide nanoparticles following digestive ripening of the CdS precursor particles in decane with octadecylamine at 175°C for 60 minutes;

Fig. 3c is a graph of absorption spectra of the cadmium sulfide nanoparticles in toluene before and after digestive ripening;

Figs. 4 a-f show TEM images of Ag nanoparticles produced via direct

transformation carried out at optimum reaction conditions; (a) Ag (ΙΟμιη, 5.0 mg) + acetonitrile (4.5 ml) 100 °C, Ag/ligand ratio 1 : 15, 60 min.; (b) Ag (ΙΟμιη, 5.0 mg) + acetonitrile (4.5 ml) 100 °C, Ag/ligand ratio 1 :5, 60 min.; (c) Ag (ΙΟμιη, 5.0 mg) + acetonitrile (4.5 ml) 150 °C, Ag/ligand ratio 1 :5, 30 min.; (d) Ag (ΙΟμιη, 5.0 mg) + acetonitrile (4.5 ml) 100 °C, Ag/ligand ratio 1 :5, 30 min.; (e) Ag (ΙΟμιη, 5.0 mg) + acetonitrile (4.5 ml) 75 °C, Ag/ligand ratio 1 :5, 60 min.; (f) Ag (ΙΟμιη, 5.0 mg) + acetonitrile (4.5 ml) 100 °C, Ag/ligand ratio 1 :25, 60 min; and

Fig. 5 is a TEM of indium sulfide nanoparticles produced from indium wire and sulfur powder with dodecanethiol in mineral spirits at 160°C for 10 minutes.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention is directed toward a process whereby nanoparticles may be produced through direct transformation of a solid bulk material. Unlike prior methods, the nanoparticles retain essentially the same form as the bulk material from which they are produced. For example, prior methods of dissolution of metals in their elemental forms often resulted in the formation of dispersions of metal ions. In contrast, the present invention transforms the bulk material directly to the nanoparticle form.

In certain embodiments, the starting bulk materials have particle sizes that are multiples of the resulting nanoparticles. In particular, the bulk materials may comprise particles or granules or pellets of a material with an average particle size or average particle diameter of greater than 1 μιη, greater than 5 μιη, greater than 10 μιη, greater than 100 μιη, greater than 1 mm, or greater than 5 mm. In other embodiments of the present invention, the bulk material may comprise particles or granules of a material having an average particle size or average particle diameter of between about 1 μιη to about 10 mm, between about 2 μιη to about 1 mm, between about 5 μιη to about 500 μιη, or between about 10 μιη to about 100 μιη. In other embodiments, it is preferable for the bulk material to have an average particle size or average particle diameter that is on the lower end of this spectrum so as to provide maximum surface area so that the bulk transformation process may proceed as rapidly as possible. In these embodiments, the bulk material may have an average particle size or average particle diameter of between about 1 μιη to about 10 μιη, between about 1.5 μιη to about 5 μιη, or between about 2 μιη to about 3 μιη. In other embodiments, it is preferable for the bulk material to have an average particle size or average particle diameter that is on the upper end of this spectrum so as to provide small surface area so that the ratio of dissolving ligand or etching agent to material surface area be large so that the transformation process may proceed as rapidly as possible. In these embodiments, the bulk material may have an average particle size or average particle diameter of between about 500 μιη to about 10 mm.

In certain embodiments, the bulk material comprises an elemental metal (i.e., in it's non-ionic, uncompounded form) or metal-containing compound. However, it is within the scope of the present invention for solid non-metals, non-metal compounds, and combinations of metals with other metals or non-metals to be used as well. In particular embodiments of the present invention, the bulk materials comprise an alkali metal, an alkaline earth metal or a transition metal or a basic metal, either in elemental form or compounded with other elements, especially non-metals. Specifically, the compounded materials may comprise metallic oxides and sulfides. In other embodiments, the bulk material may comprise a lanthanide or an actinide element, in elemental form or compounded. It is also within the scope of the present invention for the bulk material to comprise alloys of two or more metallic species.

In certain embodiments of the present invention, the solid bulk materials comprise a member selected from the group consisting of elemental metals, elemental non-metals, and compounds comprising a metal or a non-metal. In particular embodiments, the solid bulk material comprises an elemental transition metal that is selected from the group consisting of Ag, Fe, Au, Co, Cu and Ni. In other embodiments of the present invention, the solid bulk material comprises a metal containing compound, such as a metal oxide or a metal sulfide, or an alloy of two or more metallic species. In particular embodiments, the solid bulk material comprises a member selected from the group consisting of FeCo, CdS, CdTe, FeAu, In₂S₃, Fe₃04, Fe₂0₃, and SmCo.

The bulk material may be initially processed according to conventional means, such as ball milling, in order to reduce its initial particle size to the micron size range. The bulk material is then added to a solvent material that is capable of forming a stable colloidal suspension with the to-be-formed nanoparticles. The solvent selected for a particular application will depend at least in part upon the nature of the particular bulk material undergoing direct transformation. Given the broad range of bulk materials that may undergo this process, the choice of solvent is also quite broad. In certain embodiments, the solvent can be a polar or non-polar, organic or inorganic solvent or mixture thereof. In one particular embodiment the solvent can be a polar inorganic solvent such as water. Exemplary organic solvents include toluene, ethylene glycol, t-butyltoluene, mineral spirits, dimethylformamide (DMF), and dibenzylether (DBE).

The solvent medium also comprises one or more ligands that are functional to attack the bulk material and remove atoms, molecules, ions, or clusters of atoms or molecules therefrom and place them into solution. As used in the context of a mixture of nanoparticles and solvent, the term "solution" refers to true solutions in which an otherwise solid material has become dissolved in the solvent, or nanoparticle colloids, as it has been discovered that nanoparticle colloids behave similarly to solutions with solvent dependent and thermally reversible solubility. In certain embodiments, care should be taken in the selection of the appropriate ligand as the ligand selected should be functional to dissociate a portion of the bulk material, but not change the chemical make-up of the material. It is understood that the nanoparticles formed may be at least partially coated with the ligand material; however, the underlying nanoparticles retain substantially the same chemical make-up as the bulk material from which they were removed and have not been transformed into new compounds or ions. Exemplary classes of ligands that have been found suitable for use in achieving the transformation of various bulk materials include alkanethiols, alkylamines, carboxylic acids, phosphines, phosphine oxides, aromatic amines, amino acids, amides, and phenols. In particular, exemplary ligands include dodecane thiol, dodecylamine, octylamine, oleic acid, lysine and polyvinyl pyrrolidone (PVP). Table 1, below, describes certain exemplary material/ligand pairings that may be used in accordance with the present invention. It is understood, however, that this listing is exemplary and should not be taken as limiting the scope of the present invention in any way.

Table 1

Co, Ni Alkylamines

In₂S₃ Alkanethiols

Fe₃C"4, Fe₂0₃ Carboxylic acids, Phenols, Alkylamines

SmCo Alkylamines

The mixture of the bulk material, solvent, and ligand are then heated so as to cause the bulk material to undergo either digestive ripening or an oxidation/reduction process thereby resulting in the formation of a nanoparticle solution. In certain embodiments, the nanoparticles are formed via direct dissolution of the solid bulk material. However, in alternate embodiments, heating of the mixture of bulk material, solvent, and ligand occurs under an oxygen-containing atmosphere resulting in oxidation of at least a portion of the atoms making up the outer surface of the bulk material. The ligand operates to dissociate these oxidized materials from the surface of the bulk material. Once dispersed within the solvent, the ligand reduces the oxidized material back into its original form and aggregating into the desired nanoparticles having the same chemical make-up of the original bulk material. The resulting nanoparticles may be protected from further oxidation by the ligand material. In certain embodiments, the oxygen-containing atmosphere comprises from about 15% to about 25% v/v oxygen. Alternatively, the oxygen-containing atmosphere comprises air (which contains approximately 20% v/v oxygen). In certain embodiments, it may be desirable to avoid utilizing a pure oxygen atmosphere (i.e., greater than 95% oxygen, greater than 99% oxygen, or 100% oxygen). Thus, in certain preferred embodiments, the oxygen-containing atmosphere comprises less than 50%, less than 35%, or less than 25% oxygen, with the balance being made up of one or more inert gases such as nitrogen. This oxidation/reduction process is particularly well suited for the transformation of intermediately reactive metal species, such as certain elemental transition metals like silver and copper.

The heating step may occur under various temperature and pressure conditions depending upon the solvent selected. In one embodiment, the mixture can be heated at substantially atmospheric pressure at or near the boiling point of the solvent. However, it has been discovered that in certain embodiments higher temperatures result in an increase in the rate of dissolution and nanoparticle generation. Therefore, in alternate embodiments, higher temperatures can be achieved by conducting the digestive ripening process at pressures above atmospheric, such as in a pressurized reaction vessel. In such embodiments, the digestive ripening may be conducted at any temperature and pressure combination above the liquid- vapor line on the solvent's pressure-temperature phase diagram. However, it is within the scope of the present invention for the digestive ripening to occur at supercritical conditions for the solvent, provided that such conditions do not result in a transformative reaction of the bulk material.

The time under which the bulk digestive ripening or oxidation/reduction process is carried out can vary depending upon the temperature and pressure employed. The bulk transformation process may be carried out for a period of minutes to hours to days, depending upon the chosen conditions.

In certain embodiments, general parameters for the heating step resulting in the formation of a nanoparticle solutions comprise heating the mixture to a temperature of at least 50°C, at least 75°C, at least 100°C, at least 125°C, or at least 150°C. In other embodiments, the heating step comprises heating the mixture to a temperature of no more than 400°C, no more than 350°C, no more than 300°C, or no more than 250°C. In other embodiments, the heating step comprises heating the mixture to a temperature of from about 60°C to about 390°C, from about 80°C to about 330°C, from about 110°C to about 275°C, or from about 160°C to about 225°C.

In certain embodiments, the heating step can be conducted for a period of time of from about 5 minutes to 48 hours, from about 30 minutes to about 24 hours, from about 1 hour to about 12 hours, or from about 2 hours to about 6 hours, depending upon the temperature and pressure employed during the heating step. In certain embodiments, the heating step can be performed at a pressure at or near atmospheric pressure, or at higher pressures (e.g., 5 atm, 20 atm, 50 atm, or 100 atm), approaching the critical point for the solvent.

It was discovered that the molar ratio of the solid bulk material to ligand can have an effect on transformation rates of certain materials. In certain embodiments, it is desirable for the ligand to be present in excess of the solid bulk material. In particular embodiments, the molar ratio of solid bulk material to ligand is at least 1 :5, at least 1 : 10, or at least 1 : 15. In other embodiments, the molar ratio of solid bulk material to ligand is less than 1 :100, less than 1 :75, or less than 1 :50. In still other embodiments, the molar ratio of solid bulk material to ligand is from about 1 :5 to about 1 : 100, from about 1 : 15 to about 1 :75, from about 1 :20 to about 1 :50, or about 1 :25.

In certain embodiments, the bulk material can be subjected to multiple successive transformation processes without significant loss in efficiency. That is, a mixture of the bulk material, solvent, and ligand can be prepared to cause the material to undergo the transformation process for a predetermined period of time. Subsequently at least a portion of the liquid medium containing the solvated nanoparticles can then be removed, and fresh solvent and ligand added to the remaining bulk material particles and further transformation processing conducted. This process may be repeated a plurality of times. It is also within the scope of the present invention, depending upon the transformation processing conditions employed, to employ a continuous-type process for the removal of liquid medium and the addition of fresh solvent and ligand.

As noted previously, the nanoparticles produced by the transformation process are essentially of the same chemical make up as the bulk material. For example, when the bulk material comprises a metal in elemental form, the resulting nanoparticles are also of the same elemental form and have not been modified into ions or compounds of the metal. In other embodiments, the bulk material may comprise multiple components such as metal alloys or compounds, such as Ag and Au, or Cd and S. In these embodiments, the present invention provides methods of dissolving these multicomponent materials to form nanoparticle alloys or compounds. It is understood, though, that the nanoparticles may be complexed with the ligands. The resulting nanoparticle solution generally comprises a plurality of polydisperse particles, as opposed to traditional digestive ripening processes that involve the generation of monodisperse colloidal dispersions. The polydisperse nanoparticles may have particle sizes ranging from a few nanometers to a few hundred nanometers. In certain embodiments, the nanoparticles produced have an average particle diameter of from about 1 nm to about 500 nm, from about 2 nm to about 200 nm, from about 4 nm to about 100 nm, or from about 5 nm to about 50 nm. In other embodiments, the majority (i.e., greater than 50%) of the nanoparticles dissolved in the solvent as a result of the transformation process have particle sizes of less than 500 nm, less than 250 nm, less than 100 nm, less than 50 nm, or less than 25 nm. The transformation process may also result in the formation of nanoparticle concentrations greater than 1 mg/rnL of solvent, between about 1 mg/mL to about 5 mg/mL of solvent, or between about 2 mg/mL to about 4 mg/mL of solvent. Rates of nanoparticle creation are broadly dependent upon the temperature at which the transformation process is conducted. However, in certain embodiments, rates of transformation of greater than 1 mg/h, or between about 1 mg/h and about 5 mg/h, or between about 2 mg/h and 4 mg/h per 10 mL reaction volume can be achieved.

Until now "dissolution" implied the coming apart of a solid material to its molecular or atomic/ionic components in the presence of a solvent to form a solution of these components. For example, silver is soluble in aqueous nitric acid to form silver nitrate that dissolves to Ag⁺ and NO3^". In the transformation process of the present invention the solid is subjected to a hot mixture of a solvent and a surface active ligand and the solid materials dissolves, either directly or indirectly, not to molecular or atomic/ionic components, but to nanoparticles of the initial solid material. The solution of nanoparticles is a true solution with solvent dependent and thermally reversible solubility. As an example, silver, either powder or shot with mean sizes of microns to millimeters, can be mixed with an ethylene glycol solution of PVP at low temperature, 100-120°C, under an ambient atmosphere of air to yield a yellow solution of poly disperse Ag nanoparticles with mean size ca. 50 nm. This dissolution is a physical process that dissolves metals and metal oxides to nanoparticles and eliminates the need for a multi-step series of chemical reactions, chemical precursors, and careful attention to reaction conditions.

Two industries that would potentially benefit from using the present invention for a scale-up effort in nanoparticle production are: 1) alternative energy source/conservation; and 2) biomedical applications. These are two key areas that will form the backbone of high-tech manufacturing industries in the US for years to come. However, the present invention should be applicable to nearly any material that future research might discover to be important. Below, current industrially significant target materials are discussed.

Silver. Silver has both alternative energy and biomedical applications and is easy to handle in the laboratory. Among all the materials used in nanotechnology-based commercial products, silver is the most widely used. Silver nanoparticles have been used as antimicrobial agents for medical devices. The high electrical and thermal conductivities of sliver make silver nanoparticles popular in electronics as conductive fillers in conductive adhesives and thermal interfacial materials. It is also an important industrial catalyst for oxidation of ethylene to ethylene oxide, propylene to propylene oxide and methanol to formaldehyde. Along with gold nanoparticles, silver nanoparticles exhibit surface plasmon resonance absorption in the visible spectrum, which is also tunable with particle size and shape. This allows applications of silver nanoparticles as an important photocatalyst in plasmon-enhanced photocatalytic water splitting, reduction of CO2 with H2O to form hydrocarbon fuels, and degradation of organic molecules.

Metal Chalcogenides. The cadmium chalcogenides have applications in alternative energy and biological imaging and sensing. Future large-scale electronics, including displays and photovoltaic (PV) devices require functional components that are not only high performance, but also can be mass produced with low cost. Ideally, the processing of these materials needs to be compatible with the polymer processing temperature, so electronic devices can be manufactured on flexible polymer substrate. Semiconductor nanoparticles, especially those made of chalcogenides, have already demonstrated great potential in this area. Nanoparticles can be made either n-type or p-type through chemical doping during synthesis, and can be deposited using high throughput printing process on substrate with well controlled crystal structures. CdSe nanocrystals capped with [Cd2Se₃ ^2~] were deposited and annealed at 350- 400^°C exhibited field-effect mobilities of > 30 cm² V^-1 s^_1, the highest reported values for solution-processed semiconductor nanocrystal films. Semiconductor nanoparticles have also been studied as light absorbers in PV cells and have the potential to replace crystalline and amorphous silicon, which the processing temperature is not compatible with the polymer substrate. Semiconductor-polymer composites can be made at low processing temperatures and can be easily integrated into polymer-based flexible electronics platforms. CdTe-based PV cells have exhibited efficiencies > 20%.

The Cd chalcogenides have toxicity, hence at first thought scale-up seems dangerous. However, given the high promise of these materials, these possible deleterious properties have been successfully controlled in the solar industry. For example, with glass/glass encapsulation Cd chalcogenides are effectively sequestered from the environment and extreme events such as house fires. CdTe modules are a good way to sequester Cd from the mine tailings of Zn and Cu mines, which is the source of all Cd today. Life cycle analysis (from mining to recycling at end of life) shows that emissions of CO2, Hg and Cd when using CdTe solar modules are the lowest of any solar technology. Furthermore, Cd chalcogenides companies set up prefunded escrow accounts to pay recycling even if the company fails.

Fe and FeesCo35. Soft magnetic materials have important energy conservation and biomedical applications in electrical power grids because of their high saturation magnetization and permeability. To implement a smart grid technology to revolutionize current US electricity distribution system, it is important to develop new soft magnetic materials for inductive motor and power transformers. Among various soft magnets, Fe6₅Co35 alloy exhibits exceptional magnetic properties. Addition of a third element, such as vanadium, into the alloy could also change the ductility, allowing it to be processed into different shapes for various applications. The grain size of the nanocrystalline alloy is much smaller than the exchange coherence length, causing the magnetic anisotropy to be averaged over many grains, resulting in even lower coercivity and higher permeability than corresponding bulk materials. These properties make them ideal for constructing future inductive motors. Another application of soft magnetic nanoparticles is to construct permanent magnets based upon the exchange spring magnet concept. The idea is to couple a hard magnetic material with a soft magnetic material to form a composite that has high saturation magnetization and large coercivity. Theoretically, such a magnet requires_the size of the soft phase be less than twice the domain wall thickness of the hard phase, or ~10 nm. Thus precisely controlling the particle size of the soft magnetic core is critical to realize this type of new magnets.

For biomedical imaging applications, soft magnetic nanoparticles can be used as a contrast agent for magnetic resonance imaging (MRI). Typical magnetic nanoparticle MRI probes consist of a magnetic core surrounded by a polymer shell. Typically, these are iron oxide based nanoparticles. However, switching from Fe₃04 to Fe(0)-containing nanoparticles, such as Fe/Fe₃04, could enable an increase in the Ti-relaxation time of the surrounding ¾ spins, leading to positive MRI contrasts, whereas the conventional Fe₃04 nanoparticles cause a decrease in T2 and, therefore, negative MRI contrasts. Also, the higher the core magnetic moment, the better the signal contrast. Zero valent Fe, FeCo nanoparticles are among the best candidates for these applications. The problem with the zero-valent magnetic nanoparticles is the likelihood of oxidation by air and water. However, by engineering the oxide surface during synthesis, it might be possible to control the permeation of air/water, thus creating stable particles.

Gold. Gold has both alternative energy source (see silver, above) and biomedical applications and is easy to deal with in the laboratory. Gold nanoparticles, especially nanorods, are used for laser-powered plasmonic hyperthermia against cancer and bacterial infections. The medicinal community is in dire need of having defined Au nanoparticles in technical quantities available to study its biochemical properties. During recent years, the safety of using gold in patients has been questioned due to the fact that Au° has a tendency to biocorrode in the presence of glutathione and to migrate to the bones. Aurimune, produced by Cytlmmune Sciences, consisting of protein tumor necrosis factor (TNF, a previously discontinued chemotherapeutic) bound to pegylated 27nm Au NPs, has completed phase I and II clinical trials. Although a significantly enhanced delivery of NP- bound TNF has been observed, further studies have been delayed because defined Au NPs in large quantities are not (yet) available. In this context it is noteworthy that toxicity studies of Au NPs in rodents have been clouded by the non-availability of clearly defined monodisperse nanomaterials.

Fe-Au core-shell or alloy nanoparticles. Metal nanoparticles (Au, Ag, Fe, Fe/Au, Fe/Pt) have biomedical applications as vehicles for drug delivery, because their size is ideal for making use of extravasation. There is a growing community of researchers exploring in vitro and in vivo nanoplatforms for sensing enzyme activities. We anticipate that this research will soon translate into a broad range of application. For instance, AuroShell (Nanospectra Biosciences) consisting of a Au 10 nm nanoshell around an 145 nm silicon core nanoparticle, has been tested in a phase I clinical trials for the plasmonic hyperthermia treatment of head and neck cancers. Magnetic nanoparticles have medicinal applications, such as MRI, biosensors and collectors for various cells types. They are also versatile catalysts for Fischer-Tropsch, Haber-Bosch-type processes, and CO2 sequestering/reduction.

Samarium-cobalt alloy particles. The rare-earth elements have many high-tech applications, including high performance permanent magnets, which are key components for advanced motor vehicles and wind turbines. These alternative energy applications will place a huge demand for rare-earth permanent magnets, to the point where demand could quickly exceed the current production pace. The most commonly used bulk magnets are based on either SmCo or NdFeB. The SmCo based magnet is better suited for advanced vehicles because it has better elevated temperature, thermal stability, and corrosion resistance properties than NdFeB; SmCo shortcoming is the relatively low magnetization. These issues can be mitigated through careful engineering of the nanostructure and composition of magnets, for instance, through the exchange-spring mechanism described above. Thus in a composite material, both the overall magnetization and anisotropy can be greatly enhanced. This would significant reduce the rare-earth demand. The challenge, however, is to make magnetic hard phase in a nanometer length scale while maintaining the magnetic hard axis alignment. A top-down approach based on ball milling of bulk materials into nanoparticle form alone has been pursued for several decades, but so far has failed to produce the desired permanent magnets, largely because ball milling alone cannot produce well controlled nanoparticle sizes and shapes. Chemical syntheses of both hard and soft magnets are experimentally challenging, especially for hard phase SmCo. This is because Sm has a high reduction potential. Its reactivity is close to sodium metal. Alloying of nanoparticles through a low temperature reduction process directly has also proven to be difficult. Furthermore, Sm is very reactive towards oxygen, especially in the nanometer regime.

EXAMPLES

The following examples describe particular embodiments of direct transformation of bulk materials into nanoparticles according to the present invention. It is to be understood, however, that these examples are provided by way of illustration and nothing therein should be taken as a limitation upon the overall scope of the invention.

In these Examples, the materials transformed into nanoparticles are selected from the group consisting of Ag, Au, CdS and FesC CosC mixtures to form the ferrite CoFe₂03. Exemplary solvents that may be used with these bulk materials include toluene, t-butyltoluene, dimethylformamide (DMF), dibenzylether (DBE), ethylene glycol and water (for Ag only). Ligands suitable for use with these selected solvents and bulk materials include dodecylthiol, dodecylamine, octylamine, oleic acid (for the oxides) for the organic solvents, and Lysine, starch and PVP for ethylene glycol and water. In addition, a number of process conditions are capable of being varied so as to affect the transformation process. These conditions include atmospheric boiling under reflux at the boiling points of the solvents, elevated temperatures in a pressurized Parr reactor, and elevated temperatures in a microwave reactor. All these have led to nanoparticles from the bulk materials.

In certain embodiments, it has been discovered transformation rates are faster with higher temperatures. Transformation reaches an equilibrium concentration on the order of minutes to hours (depending on temperature). The supernatant can be removed from the bulk material and replaced with fresh solvent/ligand and the same dissolution occurs again indicating the bulk material remains reactive. Typical nanoparticle concentrations are better than 1 mg/mL and rates of dissolution are better than 1 mg/h in 10 mL reaction volumes. The resulting nanoparticles are polydisperse and can range from a few nm to a few 100 nm.

The direct transformation synthesis of nanoparticle experiments were conducted on the bench top and in reactors. The reactors offer fast and efficient synthesis. In particular the microwave reactor was found well suited for use with the present invention with regard to both synthesis of nanoparticles and scale up.

Example 1

Silver nanoparticles from silver powders (3-5 μιη) or pellets (2-3) mm in PVP (40,000 Daltons) and ethylene glycol.

All of the experiments were done in a glass round bottom flask with a condenser and a heat mantle in open air. A typical synthesis involved the following steps: to an ethylene glycol solution of PVP (20mL, 6mM), 20mg of silver powder, 2-5 μιη, or 2g of silver pellets, 2-3 mm, was added. This mixture was heated by the heat mantle at different temperatures, ranging from 75 °C to 195°C (at which ethylene glycol boils), and a stir plate acted with a stir bar to agitate the solution, continuously. Absorbance of the resulting solution was measured with a UV-Vis spectrometer and the maximum values of absorbance peaks were obtained. The absorbance peak at 410nm shows the amount of spherical Ag NPs in solution for all the different temperatures studied. Even though they all produce nanoparticles, experiments done in the range of 75 to 120°C showed the most prominent peaks where at higher temperatures from 120 to 195°C (ethylene glycol's boiling point) show a much smaller peak. On the other hand, the transformation rate to obtain nanoparticles increases with temperature. Figure 1 shows a TEM of the nanoparticles produced. Other concentration and molecular weight ranges of PVP were explored with similar results. It is theorized that the nanoparticle production followed an oxidation/reduction scheme in which silver atoms located on the surface of the silver powder were first oxidized by the atmospheric oxygen to silver ions. The silver ions were then dissociated from the silver powder and then reduced back into elemental silver (Ag°) by the PVP ligand. It was also discovered that this oxidation/reduction scheme was most successful when employed under an atmosphere comprising oxygen at ambient air concentrations of approximately 20%. Subsequent heating produced other nanoparticle shapes such as triangles and rods. Example 2

Gold and silver nanoparticles from bulk powers in a microwave reactor.

152 mg of gold pellet was added to DBE (3 ml) and DDT (0.3 ml) in a 10 ml microwave vial, and heated at 300°C for 30 min in a microwave reactor (Anton Paar Monowave 300). After cooling to room temperature, the solution was brick red. 500 μΐ of the reaction solution was diluted in 2.5 ml of isopropanol and DLS showed a narrow distribution of particles with 450 nm hydrodynamic diameter. The UV spectrum showed an absorption peak at 500 nm. Silver microparticles (0.01 g, d ~ 2-3 μιη) were combined with dodecanethiol (DDT, 0.11 g) in dibenzylether (DBE, 3 ml) in a 10 ml microwave vial. The vial was sealed and heated in a microwave reactor at 300°C for 30 minutes. After cooling to room temperature using pressurized air, nanoparticles were isolated by centrifugation (5 min at 5000 rpm). Fresh ligand was added and the procedure repeated several times. Figure 2 shows TEMs of the silver nanoparticles produced by each run through the process. Example 3

Cadmium Sulfide Nanoparticles from Cadmium Shot and Sulfur Powder In this example, nanoparticles were synthesized in a microwave reactor (Anton Paar Mono wave 300) using cadmium metal shot and elemental sulfur powder. CdS precursor particles were formed after reaction with octadecylamine in decane at 300°C for 27 minutes (see, Fig. 3a). Nanoparticles were isolated by precipitation with acetone and methanol and resuspended in toluene. Digestive ripening of the CdS precursor particles in decane with octadecylamine at 175°C for 60 minutes yielded nanoparticles as shown in Fig. 3b. Absorption spectra of the particles in toluene before and after digestive ripening are shown in Fig. 3c. The broad absorption tail is likely due to the presence of excess ligands.

Example 4

Transformation of C03O4 and Fe₃04 powders to nanoparticles

Micron sized cubes of C03O4 (0.01 g, d ~ 2 μιη) and iron oxide (0.01 g, d ~ 1 μιη) were combined with oleic acid (OA, 0.3 g) in DBE (3 ml) in a 10 ml microwave vial. The vial was sealed and heated in a microwave reactor at 300°C for 30 minutes. After cooling to room temperature using pressurized air, nanoparticles were isolated by centrifugation (5 min x 5000 rpm).

In additional runs, isolated particles were further dissolved through repetition of the above procedure. Specifically, the isolated nanoparticles, OA (0.3 g), and DBE (3 ml) were combined in a 10 ml vessel. The solution was heated at 300 °C for 30 min in a microwave reactor. Nanoparticles were isolated by centrifugation and additional dissolution runs were performed as needed. UV-Vis absorption spectra of Co₃04/Fe₃04 nanoparticles before dissolution, after one run, two runs, three runs, and four runs are shown in Fig. 4.

Example 5

Iron/cobalt nanoparticles from bulk powders of iron and cobalt.

Bulk powder mixtures of iron and cobalt were mixed in a diphenylether/ oleylamine/hydrazine solution and sealed in a vial. The vial was heated in a microwave reactor at 120 °C for 60 minutes. Nanoparticle Fe/Co colloids were obtained.

Example 6 Ag powder (10 μηι) direct transformation to nanoparticles via microwave reaction

Ag powder (5 mg) was added to acetonitrile (4.5 ml) and P(OEt)₃ in a 10 ml microwave vial and reacted in a microwave reactor. Experiments were carried out to test (i) different temperatures (75°C, 100°C, 150°C), (ii) different reaction times (30 min and 1 hr), and (iii) different Ag to P(OEt)₃ ligand ratios (1 :0, 1 :5, 1 : 15, 1 :25, and 1 :50).

In all cases, the solution turned yellow, and unreacted, gray/silver powder remained after the reaction. Reaction mixtures were transferred into a glass vials with screw caps and centrifuged at 10000 rpm for 2 min. Supernatants were taken to measure UV7VIS absorbance and the spectra showed absorption maxima around 400 nm to indicate Ag nanoparticles.

Optimum reaction parameters were found to be 100°C, lh, and 1 :25 Ag:ligand molar ratio. TEM images showed the presence of 15-20 nm particles (See, Fig. 5a-e). The supernatant corresponding to this optimized reaction was subjected to digestive ripening in microwave reactor at 100 °C for lhour. TEM images established the formation of 2-5 nm particles (Fig. 5f).

Example 7

Indium Sulfide Nanoparticles from Indium Wire and Sulfur Powder

In this example, nanoparticles were synthesized using indium wire and elemental sulfur powder with dodecanethiol in mineral spirits. Precursors were formed after rapid heating to 160°C and aliquots were taken during heating of the reaction mixture at 180°C over an hour. Absorbance and photoluminescence spectra for these aliquots were taken.

Over time, a first excitonic absorption feature grows in near 380 nm and a photoluminescence peak near 675 nm appears. The low intensity and large Stokes shift of the luminescence indicates the peak is likely due to trap states on the surfaces of the nanoparticles. A TEM of the aliquot after 10 minutes of reaction time is shown in Fig. 6 and indicates the presence of nanoparticles.

Claims

We claim:

1. A method of producing a plurality of nanoparticles from a solid bulk material comprising:

dispersing said solid bulk material within a solvent medium to form a mixture, said solvent medium comprising a solvent that is capable of forming a colloidal suspension with said nanoparticles and a ligand that is capable of dissociating a portion of said solid bulk material without changing the chemical make-up of the material;

heating said mixture for a predetermined period of time sufficient to dissociate said plurality of nanoparticles from said bulk material through the action of said ligand.

2. The method according to claim 1, wherein said solid bulk material comprises a member selected from the group consisting of elemental metals, elemental non-metals, and compounds comprising a metal or a non-metal.

3. The method according to claim 2, wherein said solid bulk material comprises an elemental transition metal.

4. The method according to claim 3, wherein said elemental transition metal is selected from the group consisting of Ag, Fe, Au, Co, Cu and Ni.

5. The method according to claim 2, wherein said solid bulk material comprises a metal-containing compound or alloy selected from the group consisting of

FeCo, CdS, CdTe, FeAu, ImSs, Fe₃0₄, Fe₂0₃, and SmCo.

6. The method according to claim 2, wherein said solid bulk material comprises an alkali or alkaline earth metal, in elemental form or compounded with one or more other elements.

7. The method according to claim 2, wherein said solid bulk material comprises a metal oxide or metal sulfide.

8. The method according to claim 2, wherein said solid bulk material comprises a lanthanide or an actinide element, in elemental form or compounded with one or more other elements.

9. The method according to claim 1, wherein said solvent comprises a member selected from the group consisting of polar and non-polar organic and inorganic solvents and mixtures thereof.

10. The method according to claim 9, wherein said solvent comprises a member selected from the group consisting of water, toluene, ethylene glycol, t- butyltoluene, mineral spirits, dimethylformamide (DMF), and dibenzylether (DBE).

11. The method according to claim 1 , wherein said ligand is functional to remove atoms, molecules, ions, or clusters of atoms or molecules from said solid bulk material.

12. The method according to claim 1, wherein said ligand comprises a member selected from the group consisting of alkanethiols, alkylamines, carboxylic acids, phosphines, phosphine oxides, aromatic amines, amino acids, amides, and phenols.

13. The method according to claim 12, wherein said ligand comprises a member selected from the group consisting of dodecane thiol, dodecylamine, octylamine, oleic acid, lysine and polyvinyl pyrrolidone (PVP).

14. The method according to claim 1, wherein at least a portion of said plurality of nanoparticles formed through the action of said ligand may be come at least partially coated with said ligand.

15. The method according to claim 1, wherein said solid bulk material undergoes processing, prior to being dispersed within said solvent medium, to reduce the size thereof.

16. The method according to claim 15, wherein said processing comprises reducing the size of said solid bulk material using a ball mill.

17. The method according to claim 1, wherein said bulk material is in the form of a particle having an average particle size of from about 1 μιη to about 10 mm.

18. The method according to claim 1 , wherein said heating step is carried out for a period of between about 5 minutes to about 48 hours.

19. The method according to claim 1, wherein said heating step occurs at substantially atmospheric pressure.

20. The method according to claim 1, wherein said heating step occurs at pressures above atmospheric pressure.

21. The method according to claim 1, wherein during said heating step said solvent is heated to a temperature that is above room temperature and at or below the boiling point for the solvent.

22. The method according to claim 1, wherein said heating step occurs under supercritical conditions for the solvent.

23. The method according to claim 1, wherein following the dissociation of said plurality of nanoparticles from said solid bulk material through the action of said ligand, a quantity of fresh ligand is added to said solvent medium and said heating step is repeated.

24. The method according to claim 1, wherein following the dissociation of said plurality of nanoparticles from said bulk material through the action of said ligand, at least a portion of said solvent and ligand are removed from said solvent medium, and fresh solvent and fresh ligand are added to said solvent medium containing said nanoparticles and/or said solid bulk material.

25. The method according to claim 24, wherein the addition of fresh solvent and fresh ligand occurs while heating said solvent medium.

26. The method according to claim 1, wherein said nanoparticles produced from said solid bulk material through the action of said ligand are polydisperse.

27. The method according to claim 1, wherein greater than 50% of said nanoparticles produced from said solid bulk material through the action of said ligand have particle sizes of less than 500 nm.

28. The method according to claim 1, wherein said nanoparticles produced from said solid bulk material through the action of said ligand have an average particle size of between about 1 to about 500 nm.

29. The method according to claim 1, wherein said solvent medium, following the dissociation of said plurality of nanoparticles from said solid bulk material through the action of said ligand, comprises a nanoparticle concentration of greater than 1 mg/mL of solvent.

30. The method according to claim 1 , wherein said nanoparticles are produced at a rate of greater than 1 mg/h per 10 mL of said mixture.

31. A method of producing a plurality of nanoparticles from a solid bulk material comprising: dispersing said solid bulk material within a solvent medium to form a mixture, said solvent medium comprising a solvent that is capable of forming a colloidal suspension with said nanoparticles and a ligand that is capable of dissociating a portion of said bulk material;

heating said mixture under an oxygen-containing atmosphere for a predetermined period of time sufficient to dissociate said portion of said solid bulk material therefrom through the action of said ligand and form said plurality of nanoparticles, said plurality of nanoparticles having the same chemical make-up as said solid bulk material.

32. The method according to claim 31 , wherein said portion of said solid bulk material comprises an outer portion of said solid bulk material oxidizes in the presence of said oxygen-containing atmosphere and becomes dissociated therefrom through the action of said ligand.

33. The method according to claim 32, wherein said ligand reduces said oxidized and dissociated outer portion of said solid bulk material.

34. The method according to claim 31, wherein said plurality of nanoparticles are at least partially coated with said ligand.

35. The method according to claim 31 , wherein said solid bulk material comprises an elemental metal.

36. The method according to claim 31 , wherein said oxygen-containing atmosphere comprises air.

37. The method according to claim 31 , wherein said oxygen-containing atmosphere comprises from about 15% to about 25% v/v oxygen.

38. The method according to claim 31, wherein said solvent comprises a member selected from the group consisting of polar and non-polar organic and inorganic solvents and mixtures thereof.

39. The method according to claim 31, wherein said bulk material undergoes processing, prior to being dispersed within said solvent medium, to reduce the size thereof to a particle size of from about 1 μιη to about 10 mm.

40. The method according to claim 31, wherein following the dissociation of said portion of said solid bulk material through the action of said ligand, a quantity of fresh ligand is added to said solvent medium and said heating step is repeated.

41. The method according to claim 31, wherein following the dissociation of said portion of said solid bulk material through the action of said ligand, at least a portion of said solvent and ligand are removed from said solvent medium, and fresh solvent and fresh ligand are added to said solvent medium containing said nanoparticles and/or said solid bulk material.

42. The method according to claim 41, wherein the addition of fresh solvent and fresh ligand occurs while heating said solvent medium.

43. The method according to claim 31, wherein said nanoparticles produced from said solid bulk material have an average particle size of between about 1 to about 500 nm.

44. The method according to claim 31, wherein said solvent medium, following the dissociation of said plurality of nanoparticles from said solid bulk material, comprises a nanoparticle concentration of greater than 1 mg/mL of solvent.

45. The method according to claim 1 , wherein said nanoparticles are produced at a rate of greater than 1 mg/h per 10 mL of said mixture.

46. A nanoparticle dispersion comprising a plurality of nanoparticles a solvent medium produced by the method of any of claims 1-45.