US20070068343A1

US20070068343A1 - Synthesis of shape-specific transition metal nanoparticles

Info

Publication number: US20070068343A1
Application number: US11/170,319
Authority: US
Inventors: Charles Lukehart; Jason Michel
Original assignee: Individual
Current assignee: Individual
Priority date: 2005-06-30
Filing date: 2005-06-30
Publication date: 2007-03-29

Abstract

A method of producing colloidal, shaped nanoparticles of a transition metal or alloys thereof, comprising reducing ions of at least one transition metal in an aqueous solution thereof in the presence of at least one compound, which, at its isoelectric point, is a zwitterion and/or at least one small, non-polymeric anion and the nanoparticles produced thereby.

Description

FIELD OF THE INVENTION

The present invention relates generally to the production of colloidal particles. More specifically, the present invention relates to the production of metal nanoparticles of various shapes and sizes for use in applications, for example but not limited to, catalysis.

BACKGROUND OF THE INVENTION

There is a vast electrochemical literature and technology base associated with the development of PEM (polymer-electrolyte membrane) or DMFC (direct methanol fuel cell) electrocatalysts. There currently exist strategies for preparing carbon-supported Pt—Ru or other Pt-M alloy nanocomposite materials as anode electrocatalysts for DMFCs or as CO— tolerant PEM electrocatalysts. Although the above described synthesis strategies afford Pt—Ru/carbon nanocomposites showing significant improvement in DMFC performance, dramatic (5- to 20-fold) enhancement of fuel cell performance was not achieved via this approach. Recent advances in electrocatalysis and surface science suggest that dramatic enhancement of fuel cell electrocatalysis might be achieved by designing Pt or Pt-alloy nanocrystals of controlled shape, thereby effecting electrochemical transformations on specific metal facets. Wieckowski and coworkers [Langmuir, 1998, 14, 1967-1970] have reported that Pt(111) faces doped with appropriate amounts of Ru metal oxidize methanol ca. 10 times faster than an accepted industrial Pt—Ru electrocatalyst. On Pt surfaces, this group has also reported that the rate of methanol electrooxidation on Pt(111), Pt(110), and Pt(100) surfaces shows a dependence on both the atomic structure of the Pt metal facet as well as the anion present in solution (perchlorate, sulfate, or phosphate). In related work, other coworkers have reported that oxygen reduction activity on Pt surfaces decreases in the order Pt(111)>Pt(110)>Pt(100) in basic solution. These reactivity differences are attributed to differential adsorption of hydroxide ion on Pt facets having different atomic structures [N. M. Markovic, H. A. Gasteiger, and P. N. Ross, J. Phys. Chem., 1996, 100, 6715-6721.
Catalysis is a reaction process that occurs on the surface of a solid, and more specifically, catalysis is a reaction involving a solid's surface atoms. The relationship of the number and type of bonding of surface atoms to catalysis reactivity is such that catalysis becomes more active on a given surface when more atoms become exposed to that surface. Increasing the number of exposed atoms on the surface of a solid is typically accomplished by decreasing the relative size of the particles comprising the solid. Once a particle reaches a size between 1×10⁻⁹m and 1×10⁻⁶m, the surface area of the particle compared to the particle's volume becomes quite large. Particles possessing these characteristic dimensions can be prepared as colloidal particles which possess unique physical properties, such as not precipitating out of solution due to aggregation, among others. The smaller of these colloidal particles, (i.e., those with dimensions of approximately 1×10⁻⁹m), are known as nanoparticles.
Nanoparticles are of particular interest because of their use as catalysts, photocatalysts, adsorbents and sensors, and ferrofluids, and because of their applications in optical, electronic, and magnetic devices. Since catalytic reactivity depends on the size and the shape of the colloidal particles used in an application, the synthesis of well-controlled shapes and sizes of colloidal particles, and particularly nanoparticles, due to their increased reactivity, can be critical.
The standard method for preparing nanoparticles of various metals is described in Rampino, et al, J. Am. Chem. Soc., 1942, 63, 2745 and Henglein, J. Phys. Chem., 1995, 99, 14129, whereby a solution of a metal salt and water is prepared in a reaction vessel, to which a capping material, such as sodium polyacrylate, sodium monoacrylate, etc., is added. Argon (Ar) gas is then bubbled through the solution for several minutes. The metal ions are then reduced by bubbling hydrogen (H₂) gas at a high flow rate through the solution in order to saturate the solution. The reaction vessel is then sealed, with the solution left to sit for approximately 12 hours. Subsequent absorption spectrum analysis reveals the formation of colloidal metal nanoparticles of a particular shape distribution, including combinations of cubic, tetrahedra, polyhedra and irregular-prismatic particles.
By utilizing the same method of synthesis, including using the same capping material, the same salt, the same temperature and the same solvent, but by changing the ratio of the concentration of the capping material to that of the metal ions, different sizes, shapes and shape distributions of metal nanoparticles are produced.
El-Sayed (U.S. Pat. No. 6,090,858; J. Phys. Chem. B 1998, 102, 3316-3320) discloses an improvement on this standard method; however, the improved method still requires the presence of a polymeric capping agent in order to achieve a successful result. Thus, the state of the prior art at present requires that, for the production of shaped nanoparticles of platinum, for example, it is necessary to employ polymeric capping agents during the reduction process and in many cases, the use of “aged” solutions of platinum salts and the purging of the solution with an inert gas (argon) before commencing reduction.
It has been found, however, that the use of polymeric capping agents inhibits the catalytic sites on the thus produced nanoparticles surfaces. Moreover, the prior art methods are applicable only for very small-scale production of nanoparticles.
It is an object of the present invention to provide novel shaped nanoparticles of transition metals having greater catalytic activity than those of the prior art and novel methods for their production.

SUMMARY OF THE INVENTION

The above and other objects are realized by the present invention, one embodiment of which relates to a method of producing colloidal, shaped nanoparticles of a transition metal or alloys thereof, said method comprising reducing ions of at least one transition metal in an aqueous solution thereof in the presence of at least one compound which at its isoelectric point is a zwitterion and/or at least one small, non-polymeric anion.
Another embodiment of the invention relates to the novel shaped nanoparticles produced by the improved method of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1-3 are TEM micrographs of Pt nanoparticles prepared according to the process of the invention.
FIG. 4 is a histogram of particle sizes of Pt metal nanoparticles prepared according to the process of the invention.
FIG. 5 is a powder XRD scan (Cu Ka radiation) of Pt metal nanoparticles prepared according to the process of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention is primarily predicated on the discovery that novel, shaped transitional metal nanocrystals having unprecedented catalytic activity are formed by reduction of the transition metal salts in aqueous solution at basic pH, the solutions containing appropriate small-molecule zwitterions and/or anions. K₂PtI₆or H₂Pt(OH)₆are examples of sources of Pt ion in the examples that follow. Pt ions are typically reduced to metal using hydrogen gas; however, those skilled in the art will be aware that any conventional reducing agent may be employed in the practice of the invention, such as, e.g., hydrazine, any of the borohydrides and the like. However, these reducing agents will not afford the same shape distributions.
Unexpectedly, it has been found that the same results may be obtained in the absence of such zwitterions, provided that small, non-polymeric anions are present in the solution. Indeed, successful results are also obtained when both the capping agent and/or anion are present.
Without wishing to be bound by any theory or mechanism, it is believed that either the zwitterion or the non-polymeric anion functions during the reaction as a capping and/or dispersing agent, protecting the particles as they are formed from agglomeration, and the other functions as a shape director for the nanoparticles as they are formed during the reduction.
Zwitterions that may be employed in the practice of the invention include virtually any zwitterions (a zwitterions being defined as a bipolar ion at its isoelectric point), such as amino acids (α-heterocarboxylate) structures and include glycine, sarcosine (N-methyl glycine), praline, serine, dimethyl glycine, betaine, β-alanine (a β-heterocarboxylate structure), phosphonoacetic acid, N,N,N′,N′-ethylenediaminetetramethylenephosphonic acid or pyroglutamic acid.
Small non-polymeric anions that may be employed in the practice of the invention, either alone or in the co-presence of the above-described zwitterions include hydroxide, a halide (iodide), triflate or one derived from phosphorous acid. It will be understood by those skilled in the art that the foregoing list is merely exemplary and that any anion or small non-polymeric molecule that does not deleteriously affect the method may be employed.
Predominantly Pt metal nanocubes are formed by H₂reduction of H₂Pt(OH)₆in the presence of dimethyl glycine surfactant (see FIG. 1). Hydrogen reduction of K₂PtI₆in the presence of dimethyl glycine surfactant gives predominantly Pt metal nano-tetrahedra (see FIG. 2—note shading). Samples of 0.1 gram mass of both of these Pt nanoparticles have been prepared. Pt metal nano-tetrahedra are also obtained when betaine is present as surfactant (see FIG. 3). A representative histogram of Pt nano-tetrahedra, shown in FIG. 4, reveals an average particle size of 5 (2) nm. A representative powder XRD scan of a different sample of Pt nano-tetrahedra is shown in FIG. 5. Analysis of XRD peak widths using Scherrer's equation gives a volume-weighted average particle size of ca. 5.2 nm.
More particularly, the platinum complex salt that provides the best yields of cubic platinum nanocrystals is potassium hexahydroxyplatinate (IV). In a typical procedure, the precursor concentration is 0.5 mM, the small anion, amino acid, and/or the respective salt (e.g., alkali metal) thereof concentration is also 3.0 mM. In the case where an amino acid or other acid is used, the concentration of KOH is also 3.0 mM in order to neutralize the acid. The pH is adjusted to 10.0 using a few drops of 1 M KOH. The precursor solution is then sealed with a rubber septum in a round-bottom flask that is at least twice the volume of the solution, and hydrogen is bubbled through for approximately 20 min. Neither bubbling the solution with inert gas prior to bubbling with hydrogen nor the presence of ambient light seems to have an effect on the shape distribution of the samples. After bubbling with hydrogen the flask is left overnight.
The next day, the solution becomes a non-turbid colloid with a golden color, indicating the presence of platinum nanocrystals. In a few cases, the solution is colorless and one equivalent of hydrazine may be added to ensure reduction of the platinum ions. In these cases, the reduction proceeds over the course of a few hours, but the condition of the colloid as well as the size and shape distributions of the nanocrystals are unaffected. One exception is that there is the addition of a very small percentage of tetragonal nanocrystals (slightly elongated cubes).
The crystallinity of the particles is confirmed by powder X-ray diffraction and the particle shapes are determined by transmission electron microscopy.
The platinum complex salt that provides the best yields of tetrahedral platinum nanocrystals is potassium hexaiodoplatinate (IV). In a typical procedure, the platinum precursor concentration is 0.1 mM, the small anion, amino acid, and/or the respective salt thereof concentration is 0.6 mM. In the case where an amino acid or other acid is used, the concentration of KOH is also 0.6 mM in order to neutralize the acid. The pH of the solution is adjusted to 10.0 using a few drops of 1 M KOH. At this point, the appropriate amount of solid potassium hexaiodoplatinate (IV) is added with rapid stirring and sonication to ensure rapid dissolution of the solid. The solution is transferred to a flask with at least twice the volume of the solution and sealed with a rubber septum.
The solution begins to turn from a red-violet to amber within 2-3 min. At this point, the solution is bubbled with hydrogen for 20 min. During this time, the solution becomes bright yellow and remains this color until reduced. Once fully reduced, the non-turbid colloid retains the golden hue indicative of platinum nanocrystals. Bubbling with inert gas prior to hydrogen bubbling and exposure to ambient light seem to have no effect on the shape distribution of the resulting nanocrystals.
The crystallinity of the particles is confirmed by powder X-ray diffraction and the particle shapes are determined by transmission electron microscopy.
Cubic and tetrahedral platinum nanocrystals may also be obtained from platinum complex salts merely in the presence of six equivalents of potassium hydroxide.
The method of claim 1 wherein said transition metal or alloy is Pt or Pt—Ru.
Although the invention has been exemplified employing K₂PtI₆or H₂Pt(OH)₆as the source of Pt, it will be understood by those skilled in the art that any platinum salt that is capable of being reduced to platinum nanoparticles may be used in the practice of the invention. Moreover, composites or alloys of two or more transition metals may be prepared by reducing solutions of multiple metal salts according to the invention. For example, Pt/Ru composite nanoparticles may be prepared by including in the solution of one of the above platinum salts, RuI₃, Ru(NO)(OAc)₃or Ru(CO)₁₂.
It will also be appreciated by those skilled in the art that the method of the invention is more amenable to scale-up than the methods described in the art that employ polymeric capping agents.

EXAMPLES

Tetrahedral Platinum Nanocrystals

Example 1

Potassium hexaiodoplatinate (IV) was purchased from Alfa Aesar (Ward Hill, Mass.), N,N-dimethylglycine was purchased from Sigma-Aldrich Co. (St. Louis, Mo.), potassium hydroxide was purchased from EMD Chemicals, Inc. (Gibbstown, N.J.), and distilled water was purchased from Fisher Scientific International, Inc. (Hampton, N.H.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.1 mM K₂PtI₆and 0.6 mM N,N-dimethylglycine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing tetrahedral platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 2

The chemical reagents used in this example are the same as in Example 1 except that N,N-dimethylglycine had been replaced by glycine purchased from Fisher Scientific International, Inc. (Hampton, N.H.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.1 mM K₂PtI₆and 0.6 mM glycine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing tetrahedral platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 3

The chemical reagents used in this example are the same as in Example 1 except that N,N-dimethylglycine had been replaced by N-methylglycine (sarcosine) purchased from Acros Organics (Geel, Belgium).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.1 mM K₂PtI₆and 0.6 mM N-methylglycine (sarcosine). This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing tetrahedral platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 4

The chemical reagents used in this example are the same as in Example 1 except that N,N-dimethylglycine had been replaced by betaine (inner salt) purchased from Acros Organics (Geel, Belgium).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.1 mM K₂PtI₆and 0.6 mM betaine (inner salt). This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing tetrahedral platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 5

The chemical reagents used in this example are the same as in Example 1 except that N,N-dimethylglycine had been replaced by β-alanine purchased from Sigma-Aldrich Co. (St. Louis, Mo.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.1 mM K₂PtI₆and 0.6 mM β-alanine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing tetrahedral platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 6

The chemical reagents used in this example are the same as in Example 1 except that N,N-dimethylglycine had been left out.
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10.77 was then prepared that was 0.1 mM K₂PtI₆. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing tetrahedral platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.
Cubic Platinum Nanocrystal

Example 7

Potassium hexahydroxyplatinate (IV) was purchased from Alfa Aesar (Ward Hill, Mass.), N,N-dimethylglycine was purchased from Sigma-Aldrich Co. (St. Louis, Mo.), potassium hydroxide was purchased from EMD Chemicals, Inc. (Gibbstown, N.J.), and distilled water was purchased from Fisher Scientific International, Inc. (Hampton, N.H.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM N,N-dimethylglycine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals. The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 8

The chemical reagents used in this example are the same as in Example 7 except that N,N-dimethylglycine had been replaced by β-alanine purchased from Acros Organics (Geel, Belgium).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM β-alanine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 9

The chemical reagents used in this example are the same as in Example 7 except that N,N-dimethylglycine had been replaced by betaine (inner salt) purchased from Acros Organics (Geel, Belgium).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM betaine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 10

The chemical reagents used in this example are the same as in Example 7 except that N,N-dimethylglycine had been replaced by glycine purchased from Fisher Scientific International, Inc. (Hampton, N.H.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM glycine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 11

The chemical reagents used in this example are the same as in Example 7 except that N,N-dimethylglycine had been replaced by N-methylglycine (sarcosine) purchased from Acros Organics (Geel, Belgium).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM N-methylglycine. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 12

The chemical reagents used in this example are the same as in Example 7 except that N,N-dimethylglycine had been replaced by phosphorous acid (phosphonic acid) purchased from Alfa Aesar (Ward Hill, Mass.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution at pH 10 was then prepared that was 0.5 mM K₂Pt(OH)₆and 6.0 mM phosphonate. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 13

The chemical reagents used in this example are the same as in Example 7 except that N,N-dimethylglycine had been replaced by potassium sulfate purchased from Fisher Scientific International, Inc. (Hampton, N.H.). Also, anhydrous hydrazine purchased from Sigma-Aldrich Co. (St. Louis, Mo.) was also used and potassium hydroxide had been left out.
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM potassium sulfate. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. At this point, 0.1 mL of a freshly prepared 0.5 M aqueous solution of hydrazine was added via syringe. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 14

The chemical reagents used in this example are the same as in Example 13 except that N,N-dimethylglycine had been replaced by potassium trifluoromethane-sulfonate (potassium triflate) purchased from Sigma-Aldrich Co. (St. Louis, Mo.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM potassium triflate. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. At this point, 0.1 mL of a freshly prepared 0.5 M aqueous solution of hydrazine was added via syringe. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 15

The chemical reagents used in this example are the same as in Example 13 except that N,N-dimethylglycine had been replaced by potassium phosphate, tribasic, purchased from Fisher Scientific International, Inc. (Hampton, N.H.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM potassium phosphate. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. At this point, 0.1 mL of a freshly prepared 0.5 M aqueous solution of hydrazine was added via syringe. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 16

The chemical reagents used in this example are the same as in Example 13 except that N,N-dimethylglycine had been replaced by potassium chloride purchased from Fisher Scientific International, Inc. (Hampton, N.H.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM potassium chloride. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. At this point, 0.1 mL of a freshly prepared 0.5 M aqueous solution of hydrazine was added via syringe. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 17

Example 18

The chemical reagents used in this example are the same as in Example 13 except that N,N-dimethylglycine had been replaced by potassium bromide purchased from Fisher Scientific International, Inc. (Hampton, N.H.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM potassium bromide. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. At this point, 0.1 mL of a freshly prepared 0.5 M aqueous solution of hydrazine was added via syringe. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 19

The chemical reagents used in this example are the same as in Example 13 except that N,N-dimethylglycine had been replaced by potassium iodide purchased from Sigma-Aldrich (St. Louis, Mo.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 100 mL aqueous solution was then prepared that was 0.5 mM K₂Pt(OH)₆and 3.0 mM potassium iodide. This solution was sealed in a 250 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. At this point, 0.1 mL of a freshly prepared 0.5 M aqueous solution of hydrazine was added via syringe. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.
Mutltipodal Platinum Nanoparticles

Example 20

Potassium tetrachloroplatinate (II) and potassium iodide were both purchased from Sigma-Aldrich Co. (St. Louis, Mo.). Glycine and distilled water were purchased from Fisher Scientific International, Inc. (Hampton, N.H.). Potassium hydroxide was purchased from EMD Chemicals, Inc. (Gibbstown, N.J.).
Potassium tetrachloroplatinate (II), 0.042 g, was dissolved in 3 mL of water. Potassium iodide, 0.067 g, was subsequently added while stirring. After about 10 minutes, the solution turned from orange to deep violet. At this point, 0.03 g glycine was added while stirring. Four drops of 1 M NaOH were added which caused the solution to turn from deep violet to bright yellow. The solution was diluted to 100 mL and the pH was adjusted to 3.5 by dropwise addition of 1 M HCl. The entire solution was sealed in a 250 mL round bottom flask and degassed by the freeze-pump-thaw method three times. After the final thaw, the solution was brought to room temperature and hydrogen was bubbled through it for 15 minutes. Completion of the reaction was indicated by a dark turbidity throughout the solution, at which point it contained multipodal platinum nanoparticles. These nanoparticles possessed two, three, or four branches from a central core.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.
Mixtures of Cubic and Tetrahedral Nanoparticles

Example 21

The chemical reagents used in this example are the same as in Example 20. Potassium tetrachloroplatinate (II), 0.042 g, was dissolved in 3 mL of water. Potassium iodide, 0.067 g, was subsequently added while stirring. After about 10 minutes, the solution turned from orange to deep violet. At this point, 0.03 g glycine was added while stirring. Four drops of 1 M NaOH were added which caused the solution to turn from deep violet to bright yellow. The entire solution was sealed in a 250 mL round bottom flask and degassed by the freeze-pump-thaw method three times. After the final thaw, the solution was brought to room temperature and hydrogen was bubbled through it for 15 minutes. Completion of the reaction was indicated by a dark golden-black hue, at which point it contained cubic and tetrahedral nanoparticles.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 22

The chemical reagents used in this example are the same as in Example 20, except glycine was replaced by N-methylglycine (sarcosine) purchased from Acros Organics (Geel, Belgium).
Potassium tetrachloroplatinate (II), 0.042 g, was dissolved in 3 mL of water. Potassium iodide, 0.067 g, was subsequently added while stirring. After about 10 minutes, the solution turned from orange to deep violet. At this point, 0.036 g N-methylglycine was added while stirring. Four drops of 1 M NaOH were added which caused the solution to turn from deep violet to bright yellow. The entire solution was sealed in a 250 mL round bottom flask and degassed by the freeze-pump-thaw method three times. After the final thaw, the solution was brought to room temperature and hydrogen was bubbled through it for 15 minutes.
Completion of the reaction was indicated by a dark golden-black hue, at which point it contained cubic and tetrahedral nanoparticles.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.

Example 23

The chemical reagents used in this example are the same as in Example 20 except glycine was replaced by N,N-dimethylglycine purchased from Sigma-Aldrich (St. Louis, Mo.).
Potassium tetrachloroplatinate (II), 0.042 g, was dissolved in 3 mL of water. Potassium iodide, 0.067 g, was subsequently added while stirring. After about 10 minutes, the solution turned from orange to deep violet. At this point, 0.042 g N-methylglycine was added while stirring. Four drops of 1 M NaOH were added which caused the solution to turn from deep violet to bright yellow. The entire solution was sealed in a 250 mL round bottom flask and degassed by the freeze-pump-thaw method three times. After the final thaw, the solution was brought to room temperature and hydrogen was bubbled through it for 15 minutes. Completion of the reaction was indicated by a dark golden-black hue, at which point it contained cubic and tetrahedral nanoparticles.
The shapes of the resulting platinum nanocrystals were verified by transmission electron microscopy (TEM). Their composition as platinum was verified by energy dispersive spectroscopy (EDS) while the sample was in the TEM instrument. Samples were prepared for TEM analysis by placing a drop of the colloidal solution onto a carbon-coated copper grid and allowing it to dry in air.
Cubic or Tetrahedral Platinum Nanocrystals Deposited on Vulcan XC-72R

Example 24

The chemical reagents used in this example are the same as in Example 1 except for the addition of Vulcan XC-72R carbon black purchased from Cabot Corp. (Alpharetta, Ga.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. To produce quantities suitable for handling, five identical batches were prepared by the following method for a total of 5000 mL of colloidal solution. Per batch, a 1000 mL aqueous solution at pH 10 was prepared that was 0.1 mM K₂PtI₆and 0.6 mM N,N-dimethylglycine. This solution was sealed in a 3000 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing tetrahedral platinum nanocrystals.
The total volume of 5000 mL was reduced to 1000 mL by concentrating the nanoparticle solution using three 400 mL Amicon® Stirred Ultra-filtration Cell purchased from Millipore Corp. (Bellerica, Mass.) using regenerated cellulose ultra-filtration membranes with a nominal molecular weight limit (NMWL) of 10,000. This concentrated solution of tetrahedral platinum nanocrystals was then stirred with 0.97 g Vulcan carbon black that had been briefly sonicated in 20 mL ethanol purchased from Aaper Alcohol (Shelbyville, Ky.). While stirring, the pH was lowered to 8 by dropwise addition of 1 M perchloric acid. The solution was allowed to stir for several hours whereupon the pH was lowered to 5 by dropwise addition of 1 M perchloric acid. The solution was allowed to stir for several more hours whereupon the pH was lowered to 3 by dropwise addition of 1 M perchloric acid. The solution was then allowed to stir for several more hours. At this point the stirring was halted and the carbon black was allowed to settle. The solution above the settled carbon was colorless indicating that the tetrahedral platinum nanoparticles had adsorbed to the surface of the carbon.
The carbon was re-suspended by stirring before filtering from the solution using a nylon ultra-filtration membrane with a pore size of 0.45 μm purchased from Millipore Corp. (Bellerica, Mass.). The carbon powder was then washed with 1000 mL of 1 M perchloric acid, with 2000 mL distilled water, and finally with 50 mL ethanol. The powder was then allowed to dry in air.
The coverage of the carbon black material with tetrahedral platinum nanoparticles was confirmed and characterized by TEM. The identity of the nanoparticles as platinum and their crystallinity were verified by powder X-ray diffraction (XRD).

Example 25

The chemical reagents used in this example are the same as in Example 7 except for the addition of Vulcan XC-72R carbon black purchased from Cabot Corp. (Alpharetta, Ga.).
The water used in this experiment was degassed by bubbling argon through it for about 15 minutes. A 1000 mL aqueous solution at pH 10 was prepared that was 0.5 mM K₂PtI₆and 3.0 mM N,N-dimethylglycine. This solution was sealed in a 3000 mL round-bottom flask with a rubber septum and hydrogen gas was then bubbled through it for several minutes. Completion of the reaction was indicated by the solution attaining a golden hue, at which point it had become a colloid containing cubic platinum nanocrystals.
This 1000 mL solution of cubic platinum nanocrystals was then stirred with 0.97 g Vulcan carbon black that had been briefly sonicated in 20 mL ethanol purchased from Aaper Alcohol (Shelbyville, Ky.). While stirring, the pH was lowered to 8 by dropwise addition of 1 M perchloric acid. The solution was allowed to stir for several hours whereupon the pH was lowered to 5 by dropwise addition of 1 M perchloric acid. The solution was allowed to stir for several more hours whereupon the pH was lowered to 3 by dropwise addition of 1 M perchloric acid. The solution was then allowed to stir for several more hours. At this point the stirring was halted and the carbon black was allowed to settle. The solution above the settled carbon was colorless indicating that the cubic platinum nanoparticles had adsorbed to the surface of the carbon.
The carbon was re-suspended by stirring before filtering from the solution using a nylon ultra-filtration membrane with a pore size of 0.45 μm purchased from Millipore Corp. (Bellerica, Mass.). The carbon powder was then washed with 1000 mL of 1 M perchloric acid, with 2000 mL distilled water, and finally with 50 mL ethanol. The powder was then allowed to dry in air.
The coverage of the carbon black material with cubic platinum nanoparticles was confirmed and characterized by TEM. The identity of the nanoparticles as platinum and their crystallinity were verified by powder X-ray diffraction (XRD).

Claims

1. A method of producing colloidal, shaped nanoparticles of a transition metal or alloys thereof, said method comprising reducing ions of at least one transition metal in an aqueous solution thereof in the presence of at least one compound which, at its isoelectric point, is a zwitterion and/or at least one small, non-polymeric anion.

2. The method of claim 1 wherein said transition metal or alloy is Pt or Pt—Ru.

3. The method of claim 2 wherein said transition metal ions are Pt ions and are supplied in said aqueous solution by K₂PtI₆or H₂Pt(OH)₆and said nanoparticles comprise Pt.

4. The method of claim 3 wherein said transition metal ions also include Ru ions, supplied in said aqueous solution by RuI₃, Ru(NO)(OAc)₃or Ru(CO)₁₂and said nanoparticles comprise a Pt—Ru alloy.

5. The method of claim 1 wherein said reduction is carried out in the presence of a zwitterion.

6. The method of claim 5 wherein said zwitterion is an organic acid or an organic acid containing one or more heteroatomic functional groups.

7. The method of claim 6 wherein said organic acid is an amino acid.

8. The method of claim 7 wherein said organic acid is glycine, sarcosine, N,N-dimethylglycine, betaine or β-alanine, proline, serine, phosphonoacetic acid, N,N,N′,N′-ethylenediaminetetramethylenephosphonic acid or pyroglutamic acid.

9. The method of claim 1 wherein said reduction is carried out in the presence of a small, non-polymeric anion.

10. The method of claim 9 wherein said small anion is hydroxide, a halide, triflate or is derived from phosphorous acid

11. The method of claim 10 wherein said halide is iodide.

12. The method of claim 1 wherein said reduction is carried out in the presence of both a zwitterions and a small, non-polymeric anion.

13. The method of claim 1 wherein said transition metal ions are reduced with hydrogen, hydrazine, or borohydride reducing agent.

14. The method of claim 1 wherein the pH of said aqueous solution during said reduction is greater than 7.0

15. The method of claim 14 wherein said pH is about 10.

16. The method of claim 1 wherein said nanoparticles are cubic, tetrahedral, cubo-octrahedral, octrahedral, tetragonal, podal or a mixture thereof.

17. The method of claim 1, including the isolation of said nanoparticles.

18. The method of claim 17 wherein said shaped nanoparticles are isolated on a carbon surface.

19. The method of claim 18 wherein said carbon is particulate carbon.

20. The method of claim 17 wherein ultrafiltration is employed in the isolation of said nanoparticles.

21. The nanoparticles produced by the method of claim 1.

22. The product produced by the method of claim 18.

23. The product produced by the method of claim 19.