Method of synthesizing tungsten nanoparticles

The United States Government has rights in this invention pursuant to Department of Energy Contract No. DE-AC04-94AL85000 with Sandia Corporation.

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The present invention relates to method for synthesizing a suspension of monodisperse tungsten nanoparticles. There are many existent metal nanoparticle syntheses. See: [1] Landstrom et al, “Characterization and modeling of tungsten nanoparticles generated by laser-assisted chemical vapor deposition: Journal of Applied Physics 95 (2004) 4408-4414; [2] Lei et al, “Synthesis of tungsten nanoparticles by sonoelectrochemistry”, Ultrasonic Sonochemistry 14 (2007) 81-83; [3] Xiong et al, “Synthesis and characterization of ultrafine tungsten and tungsten oxide nanoparticles by a reverse microemulsion-mediated method”, Chem. Mater. 18 (2006) 2211-2218; [4] Magnusson et al, “Single-crystal tungsten nanoparticles produced by thermal decomposition of tungsten hexacarbonyl”, J. Mater. Res. 15 (2000) 1564-1569; [5] Prasanta et at “Synthesis of tungsten nanoparticles by solvothermal decomposition of tungsten hexacarbonyl”, International Journal of Refractory Metals and Hard Materials 27 (2009) 784-791; and [6] (WO/20071030254) Methods of Producing Tungsten Nanoparticles. All of these, however, are either energy-intensive; are difficult to scale up; cannot produce monodisperse particles; and require the use of surfactants or other stabilizing ligands (which decrease the utility of the synthesized product). For instance, in order to get high loadings, such as 50 weight percent metal nanoparticles into an optically clear polymer without loss of optical properties, the nanoparticles must be sufficiently small and complete nanoparticle dispersion is required. In order to avoid the detrimental agglomeration, the polymer must chemically interact favorably with the metal nanoparticles. If the metal nanoparticle has a surface layer of surfactant, or other stabilizing ligand, then such a favorable interaction is difficult or impossible to achieve.

Tetrahydrofuran (THF) is chemically compatible with many systems, and is easily removed due to its relatively low boiling point. The ability to synthesize metal nanoparticles that are stabilized by only THF could enable production of high loading nanoparticle-polymer composites without sacrificing the original (desired) polymer properties.

We have developed an improved synthesis technique to produce suspended tungsten nanoparticles stabilized by coordinating solvents, that doesn't require the use of surfactants or other stabilizing ligands. The metal atom in tungsten carbonyls is already in the zero oxidation state (fully reduced) and is therefore capable of forming bulk metal-metal chemical bonds if the carbonyl groups were removed. Carbonyl groups are removed by use of a reducing agent that acts on the carbonyl group, rather than the metal; thereby releasing it. Removal of the carbonyl groups in a coordinating solvent can yield highly monodisperse, nanometer size metal particles stabilized only by the coordinating solvent. Coordinating solvents that contain oxygen as ether groups, such as tetrahydrofuran, are particularly suited to stabilize carbonyl forming metals. These syntheses can be performed at room temperature, which is a distinct advantage over metal carbonyl thermal decomposition synthetic methods. The improved synthesis method can also be applied to tungsten halide salts of varied oxidation states, provided sufficient reducing agent is added to fully reduce the metal to its zero valence state.

The accompanying drawings, which are incorporated in and form part of the specification, illustrate various examples of the present invention and, together with the detailed description, serve to explain the principles of the invention.

FIG. 1 TEM micrograph of suspended tungsten nanoparticles made using the method of Example 1.

FIG. 2 Optical absorption spectra of suspended tungsten nanoparticles made using the method of Example 1.

We have developed an improved synthesis technique for producing suspended tungsten nanoparticles that are stabilized by coordinating solvents, without having to use surfactants or other stabilizing ligands. The method can be used at room temperature, is scalable, and the product concentrated by standard means. Since no additives or stabilizing surfactants are required, this method is particularly well suited for producing tungsten nanoparticles for dispersion in polymers. If complete dispersion is achieved due to the size of the nanoparticles, then the optical properties of the polymer may be largely maintained.

The metal atom in tungsten carbonyls is already in the zero oxidation state (fully reduced) and is therefore capable of forming bulk metal-metal chemical bonds if the carbonyl groups were removed. Carbonyl groups are removed by use of a reducing agent that acts on the carbonyl group, rather than the metal; thereby releasing it. Removal of the carbonyl groups in a coordinating solvent can yield nanometer-size metal particles stabilized only by the coordinating solvent. Particles formed using this method can be in the range of 1-20 nanometers. Coordinating solvents that contain oxygen as ether groups, such as tetrahydrofuran, are particularly well suited to stabilize carbonyl forming metals. The term “coordinating solvents” is defined as being “a solvent that has electrons capable of forming chemical bonds.”

The coordinating solvent can comprise solvents that contain oxygen as ether groups. Alternatively, the coordinating solvent can comprise tetrahydrofuran. Alternatively, the coordinating solvent can comprise a sufficient amount of tetrahydrofuran to stabilize and suspend the formed tungsten nanoparticles. Alternatively, the molar concentration of tetrahydrofuran can be greater than or equal to 4 times the molar concentration of tungsten. Alternatively, the molar concentration of tetrahydrofuran can be greater than or equal to 6 times the molar concentration of tungsten. Alternatively, the molar concentration of tetrahydrofuran can be equal to 6 times the molar concentration of tungsten.

The size distribution of the particles formed using these methods can also be highly monodisperse. We define “highly monodisperse” as being “a set of particles having a Mean Diameter, and a full-width, half-maximum width of the size distribution being less than or equal to +/−10% of the Mean Diameter.” For example, if the mean particle size is 5 nanometers, then a highly monodisperse size distribution would have a full-width, half-maximum width that is less than or equal to +/−0.5 nanometers.

These syntheses can be performed at room temperature, which is a distinct advantage over metal carbonyl thermal decomposition synthetic methods. The improved synthesis method can also be applied to tungsten halide salts of varied oxidation states, provided sufficient reducing agent is added to fully reduce the metal to its zero valence state. The general size range of tungsten particles synthesized using this method is from 5 to 20 nm.

The general method of synthesizing a suspension of tungsten nanoscale particles, according to the present invention, comprises mixing the following reagents (dispersed in suitable organic solvents) to the listed final concentrations, in an oxygen and moisture free environment, at room temperature, preferably with stirring:

0.0001-0.1 M tungsten precursor; 0.0001-0.1M reducing agent; and a sufficient molar concentration of a coordinating solvent.

The tungsten precursor can be tungsten hexacarbonyl, or a tungsten halide (such as tungsten hexachloride), as long as sufficient amount of reducing agent is added to fully reduce the tungsten in the halides to zero valence.

The reducing agent can be lithium borohydride, lithium aluminum hydride, or any other suitable reducing agent, as is well-known in the art.

A preferable solvent is pure tetrahydrofuran (THF). As a rule-of-thumb, the molar concentration of the solvent is considered sufficient when it is greater than the molar concentration of carbonyl (when the tungsten precursor is tungsten hexacarbonyl). While other solvents and solvent mixtures may be used (such as methanol or toluene), not all enable suspended tungsten nanoparticle formation. We define “suspended tungsten nanoparticle formation” as meaning that more than 50% of the tungsten precursor (e.g., tungsten hexacarbonyl) is converted into product (e.g., 1-20 nm size tungsten particles). The term “suspended” is related to the range of particle size (1-20 nm), because, in general, particles greater than the maximum size of 20 nm will settle out/precipitate in the solution. Hence, particles smaller than 20 nm remain suspended in solution, and are non-precipitating and non-aggregating.

The post-synthesis solution can be concentrated by solvent removal using standard methods (e.g., evaporative concentration). No added stabilizing agents, ligands, or surfactants are required.

General Method

A stock solution of tungsten hexacarbonyl (THC, W(CO) 6 ) in a coordinating organic solvent, such as methanol or tetrahydrofuran (THF), is prepared. This solution is either reacted neat; diluted to lower concentration with the same solvent; or can be diluted to lower concentration in a non-polar solvent, such as toluene. THC concentrations for reaction are typically 0.1-0.001 molar. To this mixture, a sufficient amount of a reducing agent, such as lithium borohydride (LiBH 4 ) or lithium aluminum hydride (LiAlH 4 ), is added from a concentrated stock solution in THF, to a final molar concentration of 1 to 10 times the molar amount of THC. All work is performed in an air and moisture free inert atmosphere; and addition of the reducing agent solution is preferably done rapidly with vigorous stirring.