WO2008127396A2

WO2008127396A2 - A solution synthesis of carbon nanotube/metal-containing nanoparticle conjugated assemblies

Info

Publication number: WO2008127396A2
Application number: PCT/US2007/083414
Authority: WO
Inventors: Liwei Chen
Original assignee: Ohio University
Priority date: 2006-11-02
Filing date: 2007-11-02
Publication date: 2008-10-23
Also published as: WO2008127396A3

Abstract

Provided herein are single wall nanotube/metal-containing nanoparticle (SWNT/MCNP) assemblies and methods for making the SWNT/MCNP assemblies in various medium. The MCNP can be a metal nanoparticle, a semiconductor nanoparticle, or a metal-oxide nanoparticles. The methods provided herein comprise the use of at least one polymer surfactant to disperse SWNTs in a medium such as water, organic solvents, polymer matrices and sol-gel matrices. The dispersed SWNTs are mixed with metal ions and incubated and then reacted with various reagents to produce SWNT/MCNP assemblies. Suitable polymer surfactants are naturally occurring materials such as DNA, proteins, peptides, and synthetic polymers. A preferred synthetic polymer surfactant is an alternating styrene and maleic acid (PSMA) copolymer.

Description

TITLE OF THE INVENTION

A SOLUTION SYNTHESIS OF CARBON NANOTUBE/METAL-CONT AINING NANOPARTICLE CONJUGATED ASSEMBLIES

RELATED APPLICATIONS DATA

[0001] This application claims the benefit of United States provisional patent application serial no. 60/856,202 entitled A SOLUTION SYNTHESIS OF CARBON NANOTUBE - METAL NANOPARTICLE CONJUGATES filed November 2, 2006, the entire disclosure and the references listed therein which are fully incorporated herein by reference. This application is related to United States provisional patent application serial no. 60/795,045 entitled PROCESS FOR CARBON NANOTUBE DISPERSION USING SYNTHETIC POLYMER SURFACTANT filed April 26, 2006, now PCT application no. PCT/US2007/10037 entitled SYNTHETIC POLYMER SURFACTANTS FOR DISPERSING CARBON NANOTUBES AND METHODS OF USING THE SAME filed April 26, 2007, the entire disclosure and the references listed therein which are fully incorporated herein by reference.

BACKGROUND

[0002] Conjugated systems of nanomaterials are desired because they have the potential to exhibit collective properties drastically different from what would be expected from a simple combination of the individual components. For example, in conjugated systems composed of more than one type of metal nanoparticles, a coupling occurs between the plasmonic modes for the different types of metal nanoparticles at the nanoparticle interfaces that can lead to interesting physical phenomena such as an enhancement, by several orders of magnitude, of electromagnetic field strengths exhibited in the optical frequency range.

[0003] Carbon nanotubes have extraordinary mechanical, thermal, and electrical properties due to their unique all-carbon structure. A single wall carbon nanotube (SWNT) can be conceptualized as a one-atom-thick layer of graphite called graphene wrapped into a seamless cylinder. The particular properties and unique structures associated with SWNTs is a reason why there is the strong interest in synthesizing SWNT/Metal nanoparticle assemblies. In such systems the coupling between the plasmonic modes of metal nanoparticles and the dipole moments or plasmons in single wall carbon nanotube may possibly be utilized for light harvesting. Futhermore, since carbon nanotubes have large surface areas and high electrical conductivities, they are ideal support substrates for depositing catalytic metal nanoparticles like Pt and Pd. Pt and Pd nanoparticles have great potential for use in electrochemical cell and fuel cell applications. Metal nanoparticles might also be attached to carbon nanotubes in a way that allows the carbon nanotubes to serve as a growth template for producing fused metal nanowires, which could then be used for hydrogen storage or in chemical and biological sensing applications.

[0004] Several approaches have been tried in attempting to prepare carbon nanotube/metal nanoparticle assemblies, including spontaneous reduction, electrochemical reduction, substrate enhanced electrochemical reduction' and nanotube decoration on chemically oxidized carbon nanotube side walls. However, the initial challenge any approach faces is the inherently poor solubility of carbon nanotubes in solution. Carbon nanotube samples, which typically contain a variety of conformational species, are produced as an agglomerated mixture. Agglomeration is due to strong van der Waals and hydrophobic interactions occurring between individual carbon nanotubes in aqueous environments. Thus, before any solution-phase process to assemble carbon nanotubes into conjugated systems with other nanoparticle materials can occur, the agglomerated carbon nanotubes need to first be dispersed. Attempts to produce individually dispersed SWNT-metal nanoparticle assemblies have been unsuccessful and literature reports of SWNT assemblies have been limited to assemblies made with metal nanoparticles using multi-walled CNT, or large bundles of SWNTs, or surface attached SWNTs.

[0005] SWNTs can be dispersed into aqueous solutions by chemically functionalizing the sidewalls of the individual SWNT to make them hydrophilic. Alternatively, SWNTs may be dispersed into aqueous solutions using polymer surfactants to solubilize the SWNTs by incorporating them into micelle-type structures. A natural polymer that has been used for this purpose is single stranded (ss) DNA. It is believed that DNA forms a micelle with the SWNTs by wrapping around the SWNTs such that the aromatic bases of the DNA strand are directed to the SWNT 's hydrophobic exterior surface, while the hydrophilic, charged DNA backbone is directed towards the aqueous solution. SUMMARY

[0006] Provided herein are methods for producing assemblies of single-walled carbon nanotubes (SWNT) and metal-containing nanoparticles (MCNP) in a medium, herein referred to as a SWNT/MCNP assembly. The methods involve dispersing SWNTs in a medium in the presence of one or more polymer surfactants, adding metal ions to the SWNTs dispersed in the medium to form a solution, incubating the solution, and reacting the metal ions with an effective amount of a reagent to form a SWNT/MCNP assembly. Methods are also provided herein for producing SWNT/MCNP assemblies in various types of media. In some embodiments the medium is an aqueous solvent, or an organic solvent, or mixtures thereof. In other embodiments the medium is a matrix, or a mixture of a matrix with an aqueous or organic solvent. For example, the matrix may be but is not limited to a polymer matrix or a sol-gel matrix. The polymer surfactants may be chosen from materials that are naturally occurring polymers such as DNA, proteins, and peptides or may be chosen from synthetic polymers.

[0007] Also provided are SWNT/MCNP assemblies prepared by the methods described herein. The SWNT/MCNP assemblies may be prepared wherein the MCNPs are metal nanoparticles, though in other embodiments, the SWNT/MCNP assemblies may be prepared wherein the MCNPs are semiconductor nanoparticles or metal-oxide nanoparticles. Also provided is a SWNT/MCNP assembly comprising a SWNT, at least one polymer surfactant, and at least one MCNPs. The MCNP in the SWNT/MCNP assembly may be metal nanoparticles, semiconductor nanoparticles or metal-oxide nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] FIGURE 1 shows various carbon nanotube structures.

[0009] FIGURE 2 is a schematic representation of the synthetic polymeric surfactants described herein.

[0010] FIGURE 3 shows two representative embodiments of polymeric backbones of synthetic polymeric surfactants described herein.

[0011] FIGURE 4(A) is an AFM image of a SWNT complex ed with PSMA with free

PSMA in the background. [0012] FIGURE 4(B) is an AFM image of a SWNT/Pd metal nanoparticle assembly.

[0013] FIGURE 4(C) is an AFM image of a SWTMT/Pt metal nanoparticle assembly.

[0014] FIGURE 4(D) is an AFM image of a mixture containing Pd metal clusters and

SWNTs complexed with PSMA. The mixture was formed without incubation.

[0015] FIGURE 5(A) is an AFM image of S WNT/ Au metal nanoparticle assembly.

[0016] FIGURE 5(B) is an AFM image of S WNT/Cu metal nanoparticle assembly.

[0017] FIGURE 5(C) is an AFM image of SWNT/Fe metal nanoparticle assembly.

[0018] FIGURE 6(A) is a TEM image of a SWNT/Pd metal nanoparticle assembly.

[0019] FIGURE 6(B) is a TEM image of a S WNT/Pt metal nanoparticle assembly.

[0020] FIGURE 7 is an AFM image of Pt particles obtained reduction of Pt(terpy)²⁺ in the absence of SWNTs complexed with PSMA.

DETAILED DESCRIPTION GENERAL

[0021] The present invention will now be described with occasional reference to the specific embodiments of the invention. However, this invention may be embodied in different forms and should not be construed as limited to the embodiments set forth herein. Rather, the embodiments are provided so that the disclosure will be thorough and complete and will fully convey the scope of the invention to those skilled in the art.

[0022] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used in the description of the invention herein is for describing particular embodiments only and is not intended to be limiting of the invention. As used in the description of the invention and the appended claims, the singular foπns "a," "an," and "the" are intended to include the plural foπns as well, unless the context clearly indicates otherwise. All publications, patent applications, patents, and other references mentioned herein are incorporated by reference in their entirety. [0023] Unless otherwise indicated, all numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions and so forth as used in the specification and claims are to be understood as being modified in all instances by the term "about." Accordingly, unless otherwise indicated the numerical properties set forth in the following specification and claims are approximations that may vary depending on the desired properties sought to be obtained in embodiments of the present invention. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as possible. However, any numerical values inherently contain certain errors necessarily resulting from error found in their respective measurements.

[0024] As used herein, the terms "disperse" and "dispersion" means to provide a homogeneous and stable mixture. In some embodiments, these dispersions remain stable to precipitation at room temperature for at least one month, but particular examples can have even greater stability and for such cases these conditions will be specified. As used herein, species that are chemically compatible are species that have similar molecular structures or at least one similar chemical property, (for example polarity, hydrophobicity or hydrophilicity) chemical structures or chemical properties in common.

[0025] As used herein, the term radical reversible addition-fragmentation chain transfer ("RAFT") polymerization is a method for synthesizing living radical polymers. RAFT polymerization uses an agent (such as a dithioester, dithiocarbamate, trithiocarbonate, and xanthate) to mediate the polymerization via a reversible chain-transfer process. RAFT polymerization produces polymers with low polydispersity and high functionality and is suitable for use with a variety of monomers and for producing complex architectures such as block, star, graft, comb, and brush (co)polymers. The procedures, reaction conditions and techniques of RAFT polymerization are well known in the art.

SWNTs

[0026] When SWNTs are made, many varieties of SWNTs may be simultaneously produced, each variety possibly having distinct properties. SWNTs can be produced that have metallic-type or semiconducting-type properties, and SWNTs can be produced that have different chiral vector conformations. When a graphene sheet wraps up to form a SWNT, it wraps in the direction of a chiral vector represented by a pair of indices (n, m). The indices (n, m) are the chiral vector coordinates along the two axes of the plane constituting the 2- dimensional honeycomb lattice of a graphene sheet. Certain chiral vector values are associated with specific conformations. For example, when m = 0 the SWNT is called a "zigzag" conformation, when n = m the SWNT is called an "armchair" conformation, and for all other values of (n, m) the SWNTs are called a "chiral" conformation. Each distinct variety of SWNT (whether conformational or electronic variety) are useful for the methods and assemblies described herein, whether or not the distinct varieties are purified and isolated from one another. The procedures, reaction conditions and techniques associated with synthesizing SWNTs are well known in the art.

POLYMER SURFACTANTS

[0027] The polymer surfactants may be selected from materials that are naturally occurring polymers such as DNA, proteins, peptides, and from synthetic polymers.

Naturally Occurring

[0028] When polymer surfactants are selected from naturally occurring materials such as DNA, protein, and peptides, they can be obtained from a variety of commercial sources, or alternatively, DNA, peptides and protein polymers can be custom made by various suppliers. These materials can be used directly, at the concentrations and in the solutions as they are supplied, or they can be further isolated, derivatized, or otherwise manipulated in order to tailor their solubility in a variety of aqueous solutions, pH ranges, and organic solvents using techniques that are well known in the art, which have been previously reported.

Synthetic

Synthetic Polymers - Backbone

[0029] The synthetic polymers described herein have a backbone comprised of a plurality of repeat units, where each repeat unit is a structure derived from one or more monomers capable of undergoing polymerization to form the repeat unit structure. The plurality of repeat units may be selected to have chemical compatibility with the media of choice. Optionally, a side chain moiety may be attached to one or more of the repeat units in the polymer backbone. In some embodiments, these side chain moieties are selected to have chemical compatibility with the SWNTs. Optionally, in some embodiments, a spacer group may also be present attached in-between the side chain moiety and a repeat unit of the polymer backbone. Synthetic polymers have a basic structure as represented by FIGURE 1.

[0030] In certain embodiments, polymer backbones are contemplated where at least one monomer used to produce the repeating unit is either styrene or vinyl ether. Other polymer backbones are contemplated where at least two monomers are used to produce the repeat unit and wherein one of the monomers is either styrene or vinyl ether and the other monomer is selected from the group consisting of acrylic acid, acrylic anhydride, acrylamide, acrylate, methacrylate, methacrylamide, methyl -methacrylate, fumaric acid, fumaric anhydride, maleic acid, and maleic anhydride, and mixtures thereof. Exemplary embodiments of polymer backbones may also include a styrene/maleic acid (anhydride) alternating polymers; vinyl ether/maleic acid (anhydride) alternating copolymers, in which vinyl ether is vinyl methyl, ethyl, propyl, isopropyl, or butyl ether and etc; other copolymers containing maleic acid (anhydride), fumaric acid, and fumaric esters in the backbone; acrylic polymers, which are either homo- or co-polymers made from pure or mixture of acrylic acid, acrylic anhydride, acrylates, acrylamides, methacrylates, and methacrylamides. A preferred polymer backbone is an alternating copolymer of styrene and maleic acid (PSMA).

[0031] Alternatively, in certain embodiments synthetic polymers described herein may have a cationic polymer backbone structure and may be capable of carrying a cationic charge. In this regard, the polymer backbone may contain amine or amide-type functionalities or mixtures thereof as part of the repeating unit. Examples of amine or amide- type functionalized polymers include polymers having the repeat unit structures shown in FIGURE 2. Synthetic polymers having a cationic polymer backbone structure may be water- soluble and may manifest a Lower Critical Solution Temperature (LCST). The LCST is the temperature at which a polymer dissolved in aqueous solution undergoes a phase transition, going from one phase (a homogeneous solution) to a two-phase system (a polymer rich phase and a water rich phase). Polymers that change from a one to two phase system as the temperature increases are characterized as having inverse solubility and are called temperature sensitive polymers. As used in some embodiments, temperature sensitive polymers undergo solubilization quickly and exhibit highly dispersive properties in cold water but remain relatively inert in wanner water. Preferred are synthetic polymers having an LCST in the temperature range from about 25 to about 35 ⁰C. Useful temperature sensitive polymers include polymers synthesized from n-alklyacrylamide-based monomers, as for example isoproplymethacrylamide, diethylacrylamide, proplyacrylamide, ethylproplyacrylamide, n- and tert-butlyacrylamide and ethoxyethylacrylamide. Other useful temperature sensitive polymers include poly(lysine), poly(N-isopropylacrylamide), poly(N- (3-ethoxypropyl)acryl-amide), or derivatives thereof.

[0032] It is contemplated that temperature sensitive polymers having a cationic polymer backbone structure that is based on monomers of the type described herein may not need to be further derivatized with side group moieties in order to be useful. However, the temperature sensitive polymers may be further converted if it is so desired by attaching any of the various side group moieties described herein onto the backbone structures of the temperature sensitive polymers using conventional chemical synthesis techniques.

[0033] Any suitable polymerization method may be used to make the backbone part of the synthetic polymers contemplated and claimed herein. One useful method is radical reversible addition-fragmentation chain transfer (RAFT) polymerization. Polymers having repeat units derived from at least two monomers capable of undergoing RAFT polymerization can be made when the two monomers undergo RAFT polymerization to form the repeat units. In one example, RAFT polymerization synthesis is carried out to provide a copolymer having alternating styrene and maleic acid monomers in the backbone (PSMA). SCHEME 1 shows the synthesis of a copolymer of styrene and maleic acid (PSMA) obtained by reacting styrene (PS) monomers with maleic acid (MA) monomers.

SCHEME 1

[0034] SCHEME 1 shows how PSMA may be further derivatized with side chain moieties to produce additional embodiments of polymer surfactants. Additional embodiments of polymer surfactants can be synthesized by subjecting PSMA to further functionalization with, for example, an amino-containing side chain moiety. Shown in SCHEME 1 is a particular embodiment where PSMA is derivatized with the side chain moiety amino-pyrene. Functionalization of PSMA in this manner involves a condensation reaction between the electrophilic maleic acid anhydride groups present on PSMA and the nucleophilic amine groups present on amino-pyrene.

Synthetic Polymers - Side Chain Moieties

[0035] As previously stated, SCHEME 1 exemplifies a particular embodiment of how the synthetic polymers herein contemplated may be derivatized to contain a side chain moiety. Furthermore, side chain substitution may take place anywhere along the polymer backbone such that the side chain moiety is attached to a repeat unit located anywhere in the polymer chain, including for example at the end of a polymer chain. In embodiments where attachment of a side chain moiety occurs at this position, in the terminal position of the polymer chain, this constitutes an end-cap to the polymer chain. Side chain moieties are optional, and one skilled in the art will recognize that the amount of side chain substitution required to disperse SWNT in a media may depend on the nature of the polymer backbone and the nature of the media. For example, when the polymer backbone is PSMA and the media is water, the benzene-like structures (arising from the styrene monomers used in the polymer backbone) are sufficiently compatible with SWNTs that underivatized PSMA polymer alone is capable of dispersing SWNTs into an aqueous solution even though no SWNT-compatible side chains moieties are incorporated into the polymer structure.

[0036] Synthetic polymers are contemplated containing side chain moieties that may be derived from organic dyes. Suitable organic dyes include structures such as porphyrins, porphyrin derivatives, metal porphyrin complexes, metal porphyrin derivative complexes, rhodamine dyes, fluorescein dyes, any other type of organic dyes, and combinations thereof. Synthetic polymers are also contemplated containing side chain moieties that may be derived from aromatic hydrocarbons. Suitable aromatic hydrocarbons include aromatic hydrocarbons, substituted aromatic hydrocarbons, and derivatives thereof selected from the group consisting of benzene, naphthalene, anthracene, tetracene, acenaphthylene, bezoanthracene, benzopyrene, benzofluoranthene, benzofluoranthene, benzophenanthrene, fluoranthene, fluorine, phenanthrene, acenapthene, pyrene, perylene any other polycyclic aromatic hydrocarbons (PAHs), and mixtures thereof.

[0037] In certain embodiments pyrene is a preferred side chain moiety. The fused aromatic ring structure of pyrene is graphene-like and resembles the molecular structure of SWNTs, which indicates that pyrene may have chemical compatibility with SWNTs and may have a strong tendency to adsorb onto SWNTs. Without intending to be bound by theory, we believe that pyrene may be used as the side chain moiety in the polymer structure, and that the resulting polymer formed may have the capacity to interact with and adhere to SWNTs. Furthermore, we believe that if maleic acid is used as one of the monomers constituting the polymer backbone, residual acid functionalities present on such a polymer may give the polymer water-solubility. Thus, we believe that polymers synthesized and derivatized in the manner shown in SCHEME 1 to have pyrene side chain moieties may have sufficient SWNT compatibility and compatibility with water to mimic the surfactant behavior of ssDNA, in order to disperse SWNTs into aqueous solutions.

Synthetic Polymers - Spacer Group

[0038] hi certain embodiments, a spacer group may also be present and may be attached in-between a side chain moiety and a repeat unit of the polymer backbone. Such spacer groups may have a short or an extended structure. One skilled in the art will understand that the longer a spacer group is, the less sterically hindered any attached side chain moiety will be, meaning the more it will be able to act independent of any influence the polymer backbone may have on the bulk properties in solution. By comparison, polymer compositions having either short spacer groups or no spacer groups may have side chain moieties that are too sterically hindered to exert any properties independent of the polymer backbone, and in these instances the net physical and chemical properties of the polymer may be dominated by the nature of the backbone structure. When the choice over polymer backbone is considered with the choice over both the side chain moiety and the spacer group, one skilled in the art will recognize how the properties of a given polymer system may be tuned to exhibit specifically desired properties.

[0039] Typical examples of spacer groups include but are not limited to the following structures and derivatives thereof: -(CH₂),-,-, where n is from 1-10; -[A(CH₂)_mB]_n-, where A and B are -N(H)- or -N(R)- (where R is an alkyl group) or -S- or -C(O)- or -C(O)O- or - CH(OH)- or -CH(R)- (where R is an alkyl group) and m is from 1-6 and n is from 1-16; and - [C₆H₄J_n-, where n is from 1-2. The longer or more extended the spacer group, the more likely it will be that the side chain moiety that is attached to it will physically extend away from the polymer backbone.

[0040] Particular examples of useful synthetic polymer surfactants are described in co-pending PCT application no. PCT/US2007/10037 of Liwei Chen et al, entitled SYNTHETIC POLYMER SURFACTANTS FOR DISPERSING CARBON NANOTUBES AND METHODS OF USING THE SAME (foπnerly, US provisional patent application no. 60/795,045), which is incorporated herein by reference.

METAL-CONTAINING NANOPARTICLES (MCNPs)

[0041] SWNT/MCNP assemblies may be prepared wherein the MCNPs may be, for example, metal nanoparticles, semiconductor nanoparticles, metal-oxide nanoparticles, or any mixture thereof.

[0042] In the case where MCNPs are metal nanoparticles, these metal nanoparticles may exhibit a variety of quantum mechanical effects, and may have a variety of properties including but not limited to tunable electronic, magnetic, optical and catalytic properties. The metal nanoparticles may be prepared using solution-based and colloidal-based synthesis techniques to reduce salts of the metal cations using a suitable metal ion reducing agent, according to any of the methods that are known to one of ordinary skill in the art, to produce zero-valence state metal nanoparticles. Exemplary metal nanoparticles that may be made into SWNT/MCNP assemblies using the methods described herein include Fe, Ru, Rh, Pt, Pd, Cu, Ag, and Au metal nanoparticles. Exemplary metal nanoparticles useful as catalysts for various reactions include but are not limited to Pt, Pd, Au, Cu, Ni, Ru, Rh, Fe metal nanoparticles.

[0043] In the case where MCNPs are semiconductor nanoparticles, these semiconductor nanoparticles may exhibit a variety of quantum mechanical effects, and may have a variety of properties including but not limited to tunable electronic, optical and catalytic properties. The semiconductor nanoparticles may be prepared using solution-based and colloidal-based synthesis techniques to react salts of the group lib metal cations with any suitable group Via reagent containing at least one group Via element selected from the group S, Se, Te, or mixtures thereof, according to any of the methods that are known to one of ordinary skill in the art, to produce II- VI semiconductor nanoparticles. Exemplary semiconductor nanoparticles that may be made into SWNT/MCNP assemblies using the methods described herein include ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, and HgTe semiconductor nanoparticles.

[0044] In the case where MCNPs are metal-oxide nanoparticles, these metal-oxide nanoparticles may exhibit a variety of quantum mechanical effects, and may have a variety of properties including but not limited to tunable electronic, optical and catalytic properties. The metal -oxide nanoparticles may be prepared using solution-based and colloidal -based synthesis teclmiques to react non-oxide salts of the metal cations with any suitable metal-ion oxidizing agent, according to any of the methods that are known to one of ordinary skill in the art, to produce metal-oxide nanoparticles. hi order to produce metal-oxide nanoparticles, the metal-ion oxidizing agent should be an oxygen-based oxidizing agent. Exemplary semiconductor nanoparticles that may be made into SWNT/MCNP assemblies using the methods described herein include TiO₂, ZrO₂, MnO₂, FeO, Fe₂O₃, CuO, ZnO₂, ZnO, SnO, SnO₂, PbO, Pb₃O₄, and PbO₂ metal-oxide nanoparticles.

METHODS FOR PREPARING SWNT/MCNP ASSEMBLIES

[0045] Method for preparing SWNT/MCNP assemblies in a medium are provided herein. The method comprises dispersing SWNTs in a medium in the presence of at least one polymer surfactant; mixing metal ions with the SWNTs dispersed in the medium; incubating the mixture; and reacting the metal ions with an effective amount of a reagent to form a SWMT/MCNP assembly. Provided are methods for the solution-phase synthesis of SWNT/MCNP assemblies that can be generally applied to a wide variety of metal ions.

[0046] The general method for producing SWMT/MCNP assemblies is illustrated in

SCHEME 2, for forming SWNT/Metal nanoparticles in an aqueous medium. The SWNTs are first dispersed with a polymer surfactant in an aqueous medium such as water. The SWNTs become individually dispersed by forming SWNT-polymer surfactant complexes. Then suitable metal ions are mixed with the SWNT-polymer surfactant complexes in the aqueous medium and this mixture is incubated for a sufficient time before the metal ions are chemically reduced with a metal-ion reducing agent to form metal nanoparticles coordinated and assembled along the outside of the SWNT. SCHEME 2

Metal ions, > 12 hr

[0047] Suitable SWNTs are any of the variety of SWNTs herein described. Suitable polymer surfactants are selected from the group consisting of DNA, proteins, peptides, and synthetic polymers, where the synthetic polymers are any of the synthetic polymers herein described. The step of dispersing SWNTs in the medium with at least one polymer surfactant may take some time to achieve a suitable dispersion of the SWNT in the medium. The length of time it takes to disperse the SWNTs with the polymer surfactant will depend on several factors including but not limited to the nature of the polymer, the polymer backbone, the side chain moieties (if any are present), the nature of the media, the final concentration of SWNTs desired, and whether heat, stirring, sonication or other mechanical assistance is utilized to disperse the SWNTs. One skilled in the art will understand how the length of time it takes to disperse SWNTs may be specific to the type of polymer surfactant used, how this will be affected by the aforementioned factors, and will further understand how reaction conditions may be modified without undue experimentation to suitably disperse the SWNTs. In preferred embodiments, SWNTs can be dispersed in aqueous medium in about 1 hour when mechanical assistance from sonication is utilized. Furthermore, methods are contemplated wherein the step of providing a polymer surfactant involves providing one or more naturally occurring polymers (such as DNA, proteins, peptides), or one or more synthetic or polymers, or a mixture of any of the synthetic polymers herein described. A preferred embodiment of a synthetic polymer is an alternating copolymer of styrene and maleic acid (PSMA). [0048] Suitable metal ions are selected from the group consisting of Ti, Zr, Mn, Fe,

Rd, Rh, Co, Ni, Pt, Pd, Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb and combinations thereof. The metal ions are provided in the form of a salt having solubility in the desired medium, as for example, in one embodiment where the medium is water and suitable metal salts are water- soluble and are dissolved in water to provide an aqueous solution of metal ions. The metal ions can be provided at any suitable concentration appropriate for making the desired assemblies and for use with the methods described herein. Suitable concentrations may be determined by the concentration of SWNTs and the concentration of polymer surfactant used. For example, in a preferred embodiment of the methods described herein, where the polymer surfactant is the copolymer PSMA, a suitable concentration of metal ions is where the molar ratio of metal ions to the carboxylic acid groups present on PSMA in the reaction mixture is generally less than 1: 100. Using this molar ratio of metal ions to the carboxylic acid groups ensures the majority of the metal ions are bound with SWNT-PSMA complexes after being incubated and that there is not a lot of excess and/or free metal ions remaining that would undergo uncontrolled growth to form metal nanoparticles unassociated with any SWNT- PSMA complexes.

[0049] Metal ions are mixed with the SWNT-polymer surfactant complexes and incubated for a period of time before a reagent is added. This incubation period allows the metal ions to associate with the SWNT-polymer surfactant complexes to help template or control the growth of the MCNT once the reagent is added. Controlling the growth of the MCNP facilitates formation of SWNT/MCNP assemblies. The incubation and controlled MCNP growth is illustrated in FIGURES 4(A)-(D). Atomic force microscopy (AFM) is used to analyze samples of various reaction mixtures. FIGURE 4(A) is a sample of SWNTs dispersed in an aqueous medium using PSMA as the polymer surfactant and the image shows the SWNT-PSMA complexes that form. When various metal ion salts are mixed in with these SWNT-PSMA complexes and given a period of time (approximately 12 hours) to incubate with the SWNT-PSMA complexes, adding a reducing agent to this mixture results in controlled production of metal nanoparticles. Metal nanoparticles grow as discrete nodular-like structures interspaced along the length of individual SWNT-PSMA complexes and SWNT/Metal nanoparticle assemblies are produced. FIGURE 4(B) images a SWNT/Pd nanoparticle assembly and FIGURE 4(C) images a SWNT/Pt nanoparticle assembly. Both these assemblies are produced using corresponding metal ion salts, PSMA as the polymeric surfactant to disperse SWNTs, and an incubation period before introducing a reducing agent and both images show the distinctive formation of nodular-like metal nanoparticles along the length of individual SWNT-PSMA complexes.

[0050] In contrast, when metal ions are mixed with SWNT-PSMA complexes and are given no time-period for incubation before a reducing agent is introduced, discrete nodular- like metal nanoparticles growth along the length of individual SWNT-PSMA complexes does not occur and SWNT/MCNP assemblies are not formed. Rather, metal nanoparticle growth in the absence of an incubation period is uncontrolled and indiscriminately produces metal particulates of various sizes that can aggregate further to form metallic chunks unassociated with any SWNT-PSMA complexes. Furthermore, SWNT/MCNP assemblies do not form in this situation. FIGURE 4(D) is sampled from an attempt to produce SWNT/Pt nanoparticle assemblies using identical reaction conditions as used to produce the assemblies shown in FIGURE 4(B) but without allowing any incubation time to pass before introducing the reducing agent. The image provided in FIGURE 4(D) shows that the distinctive formation of nodular-like metal nanoparticles along the length of individual SWNT-PSMA complexes does not occur. Rather, a mixture of SWNT-PSMA complexes and various-sized clusters of Pd metal are obtained.

[0051] We contemplate that suitable incubation times may depend upon several factors including but not limited to the nature of the metal ion, the nature, concentration and solubility of the metal ion salt used, the nature of the polymer, the polymer backbone, the side chain moieties (if any are present), the nature of the media, the final concentration of SWNT/MCNP assemblies desired, and whether heat, stirring, sonication or other mechanical assistance is utilized during incubation. One skilled in the art will understand how a sufficient time for incubation may be specific to both the types of metal ions and types of polymer surfactants used, how results will be affected by these factors and will understand how periods of time for incubation may be modified without undue experimentation to achieve suitable results.

[0052] Embodiments are contemplated of methods for dispersing SWNTs using a polymer surfactant to foπn SWNT-polymer surfactant complexes that may further bind metal ions and act as a template to control the growth of metal nanoparticles, in order to produce metal nanoparticles bound to a SWNT-polymer surfactant complex. For example, it is contemplated that when PSMA is used as the polymer surfactant, carbonyl groups present in PSMA can act as capping groups for metal nanoparticles formed in order to prevent the metal nanoparticles from aggregating with each other.

[0053] Reacting the metal ions with an effective amount of a reagent to form a

SWMTYMCNP assembly involves adding enough reagent as may be required in order to have a reaction between the metal ions present in the medium and the added reagent to produce the requisite MCNPs needed to form SWNT/MCNP assemblies. The reagent should be provided in a form that is soluble or miscible with the desired medium, as for example, in one embodiment where the medium is water and suitable reagents are water-soluble and are dissolved in water to provide an aqueous solution of the reagent.

[0054] The reagent is reacted with metal ions in the medium long enough to react with (i.e.: to reduce, to form a product or precipitate with, or to oxidize) metals ions that are bound to the SWNT-polymer surfactant complexes, in order to produce the desired SWNT/MCNP assemblies. How long this is depends on several factors including but not limited to the type of MCNPs that are produced, the concentration of metal-ions present, the final concentration of SWNT/MCNP assemblies desired, and whether heat, stirring, sonication or other thermal or mechanical energy is utilized to aid in reacting the reagent with the metal ions. One skilled in the art will understand how the length of time for reacting the reagent with the metal ions in solution may be tailored to the specific SWNT/MCNP assembly produced, how the production will be affected by the various aforementioned factors and will understand how reaction conditions may be modified without undue experimentation to achieve desired embodiments of the SWNT/MCNP assemblies.

Reagents

[0055] Reagents can be a metal-ion reducing agent, a group Via reagent, or a metal- ion oxidizing agent. Substances that are metal-ion reducing agents have the ability to reduce the metal ions present and are said to be reductive and are known as reducing agents, reductants, or reducers. Put another way, the reductant transfers electrons to the metal ions and is thus oxidized itself, and because it "donates" electrons it is called an electron donor. Substances that are group Via reagents have the ability to form a product or precipitate with any group lib metal ions that are present. Substances that are metal-ion oxidizing agents have the ability to oxidize the metal ions present and are said to be oxidative and are known as oxidizing agents, oxidants or oxidizers. Put another way, the oxidant removes electrons from the metal ions and is thus reduced itself, and because it "accepts" electrons it is called an electron acceptor.

Metal-ion reducing agent

[0056] A material that is a metal-ion reducing agent should primarily react with metal ions present to reduce these cationically charged metal ions to a valence state of zero. Although side reactions with other reagents in the medium, such as SWNTs or polymer surfactants, or reactions with the medium itself may occur, these are kept to a minimum and do not generate substantial amounts of any byproduct. Thus, SWNT/Metal nanoparticle assemblies produced can be used as-is or they can be isolated from the mixtures using standard purification and isolation techniques known in the chemical arts. Common reducing agents include alkali metal hydrides; lithium aluminum hydride (LiAlH₄), hydrogen gas, sodium borohydride (NaBH₄); sulfite compounds; hydrazine; diisobutylaluminum hydride (DIBAH); and oxalic acid (C₂H₂O₄). A preferred metal-ion reducing agent is NaBH₄, which is a suitable metal-ion reducing agent for most metals. However, those skilled in the art will recognize that other reducing agents may be used for other particular reactions.

Group Via reagent

[0057] A group Via reagent should contain a group Via element selected from the group S, Se, Te, or mixtures and should be suitable for reacting with salts of the group lib metal cations to produce II-VI semiconductor nanoparticles. Although in certain embodiments the group Via reagent may also be an oxidizing agent, the group Via reagent should primarily react with any cationically charged group lib metal ions present to form a product or precipitate with any group lib metal ions that are present. The group Via reagent forms a product with or precipitates with any group lib metal ions because a preferential exchange occurs of the group Via element for the negatively charged counter ions present in the group lib metal-ion salt, in order to produce a II-VI semiconductor material. Although side reactions with other reagents in the medium, such as SWNTs or polymer surfactants, or reactions with the medium itself may occur, these are kept to a minimum and do not generate substantial amounts of any byproduct. Thus, SWNT/Semiconductor nanoparticle assemblies produced can be used as-is or they can be isolated from the mixtures using standard purification and isolation techniques known in the chemical arts. Common group Via reagents include ammonium sulfide, hydrogen sulfide, the alkali-hydrogen sulfide salts, and alkali-sulfide salts; hydrogen selenide, the alkali-hydrogen selenide salts, and alkali-selenide salts; hydrogen teluride, the alkali-hydrogen teluride salts, and alkali-teluride salts. A preferred group Via reagent for making ZnS, CdS, or HgS semiconductor nanoparticles is ammonium sulfide. However, those skilled in the art will recognize that other group Via reagent may be used for other particular reactions.

Metal-ion oxidizing agent

[0058] A material that is a metal-ion oxidizing agent should primarily react with metal ions present to either oxidize the cationic metal ions or preferentially replace the negatively charged counter ions present in the metal-ion salt with an oxide species to produce a metal-oxide. Although side reactions with other reagents in the medium, such as SWNTs or polymer surfactants, or reactions with the medium itself may occur, these are kept to a minimum and do not generate substantial amounts of any byproduct. Thus, SWNT/Metal- oxide nanoparticle assemblies produced can be used as-is or they can be isolated from the mixtures using standard purification and isolation techniques known in the chemical arts. In order to produce metal-oxide nanoparticles, the metal-ion oxidizing agent should be an oxygen-based oxidizing agent. Common oxidizing agents include hypochlorite and other hypohalite compounds such as Bleach; chlorite, chlorate, perchlorate, and other analogous halogen compounds; permanganate salts; hexavalent chromium compounds such as chromic and dichromic acids and chromium trioxide; chromate/dichromate compounds; peroxide compounds such as H₂O₂; persulfuric acid; ozone, oxygen gas, osmium tetroxide (OsO₄); nitric acid; and nitrous oxide (N₂O). However, those skilled in the art will recognize that other oxidizing agents may be used for other particular reactions.

[0059] Methods are contemplated herein for producing SWNT/MCNP assemblies in aqueous solutions. Methods are also contemplated herein for producing SWNT/MCNP assemblies in other medium. In some embodiments this medium is water-based, or it is an organic solvent, or mixtures thereof. In still other embodiments the medium may be a solid or semi-solid matrix (such as a gel), or it may be a mixture of a matrix with an aqueous solution or organic solvent. The matrix may be for example, but is not limited to a polymer matrix or a sol-gel matrix. Useful polymer matrices and sol-gel matrices are any of the polymer matrices and sol-gel matrices previously known and one skilled in the art will understand how these matrices can be produced. [0060] Since the methods described herein are suitable for producing SWNT/MCNP assemblies in medium that include water, water-based solutions, organic solvents, polymer matrices, sol-gel matrices, and mixtures thereof; polymers that are useful for carrying out these method in the different medium may have side chain moieties that interact with the SWNTs, to aid in dispersion of the SWNTs, while at the same time having a backbone that is soluble or compatible with the chosen medium that the SWNT/MCNP assemblies are to be produced in. Monomers that may be useful for polymer backbones when the medium used is water include any of the various water-soluble structures herein described, and this provides for polymer surfactants having water-solubility. In other embodiments when the medium is a non-aqueous based system such as an organic solvent, or a polymer matrix, or a sol-gel matrix, one skilled in the art will understand how to choose specific monomers to produce a polymer backbone that has suitable chemical compatibility with and solubility within the non-aqueous based system, or polymer matrix, or sol-gel matrix used.

[0061] Methods for producing SWNT/MCNP assemblies described herein are useful for producing SWNT/MCNP assemblies that then may be used in a variety of functions and applications. For example, these SWNT/MCNP assemblies and methods may be used to purify various types of SWNTs from raw products. In some embodiments these SWNT/MCNP assemblies and methods may also be used or may play a role in the preparation of SWNT/MCNP-based electrochemical electrodes or SWNT/MCNP-based films or membranes, in making SWNT/MCNP assemblies that are then further used in solar energy conversion (in the form of photovoltaic cells or photoelectrochemical solar cells) or in solar fuel production or in solar hydrogen production. Likewise, these SWNT/MCNP assemblies and methods may also be used or may play a role in making SWNT/MCNP assemblies that are then used to prepare optoelectronic devices including light emitting diodes, light-emitting transistors, photo-detecting diodes, and transistors.

EXAMPLES

[0062] The examples disclosed herein are for illustrative purposes, only and are not meant to limit the scope of the invention.

PSMA Preparation

[0063] PSMA is synthesized as shown in SCHEME 1. 2.08 g (0.02 mol) of styrene

(PS) and 1.96 g (0.02 mol) maleic anhydride (MA) monomers is polymerized using typical RAFT polymerization techniques, and results in an alternating polystyrene-maleic acid copolymer (PSMA) with 80% yield. The molecular weight of the PSMA is measured using standard Gel Permeation Chromatography (GPC) techniques to be 8,400 daltons with a molecular weight distribution of (M_w/M_n~1.2). This molecular weight distribution is much narrower than that obtained when simple radical copolymerization procedures are used.

[0064] The dispersability of SWNTs in an aqueous medium using PSMA as a polymer surfactant is compared to a control, which is the dispersability of SWNTs in an aqueous medium using (single strand) ssDNA d(GT)₂o. The dispersability of the SWNTs as a function of the polymer surfactant is expressed as the mass ratio of SWNT:polymer surfactant that is obtained, where the higher the ratio is the better the polymer surfactant is capable of dispersing SWNTs. The dispersability of SWNTs in an aqueous medium using ssDNA d(GT)₂₀ was previously determined to be less than 0.4: 1. In comparison, the dispersability of SWNTs in an aqueous medium using PSMA, synthesized according to SCHEME 1, conducted under similar conditions as for the control [ssDNA d(GT)₂₀], is 0.7:1. Overall, the ability of PSMA to disperse SWNTs in an aqueous medium is much greater than for the control [ssDNA d(GT)₂₀].

Preparation of SWNT/MCNP Assemblies

[0065] SWNT/MCNP assemblies are synthesized where the MCNP produced are metal nanoparticles. As shown in SCHEME 2, SWNTs are first individually dispersed in an aqueous medium in the presence of polymer surfactant to make SWNT-polymer surfactant complexes. Metal ions are then mixed with the SWNT-polymer surfactant complexes and this mixture is incubated for a period of time before introducing a metal-ion reducing agent to chemically reduce the metal ions to produce metal nanoparticles. Once the metal-ion reducing agent is added, metal nanoparticles are formed as nodules associated with and located along the SWNT-polymer surfactant complexes. In a typical experiment, using PSMA as the polymer surfactant, 1 mg of PSMA is dissolved in 1 ml of 0.1 M NaOH solution, and ~2 mg of purified HiPCO SWNT powder (Carbon nanotechnology Inc., TX) is added. This mixture is sonicated for an hour at approximately 5 W power and centrifuged for an hour at 15,500 g. Free PSMA, which is unbound to any SWNTs, is observed in the supernatant. Atomic Force Microscopy (AFM) conducted on a sample of the supernatant, and shown in FIGURE 4(A), confirms the presence of the free PMSA. The supernatant is then dialyzed using cellulose ester membrane with 1 million Dalton molecular weight cut-off (Spectrum Labs Inc., Rancho Dominguez, CA) to eliminate the free PSMA. In order to form metal nanoparticles, 10 μl of a 0.5 niM Pt(terpy)Cl₂ aqueous solution is added to 100 μl of the SWNT-PSMA mixture and this is incubated at room temperature for more than 12 h. After this time, 3 μl of a 6 mg/mL NaBH₄ aqueous solution is added and SWNT/MCNP assemblies are obtained.

[0066] Assemblies of SWNT with various other metal nanoparticles (Pd, Au, Cu, and

Fe) are prepared using the same method as described above, substituting the volumes and concentrations of reagents as set forth in TABLE 1 as appropriate. Shown in FIGURES 4(B) and 4(C) are AFM images of SWNT assemblies containing Pd and Pt metal nanoparticles. Shown in FIGURES 5(A)-(C) are AFM images of SWNT assemblies containing Au, Cu and Fe metal nanoparticles. The suspensions of the SWNT/Metal nanoparticle assemblies are stable in the aqueous medium at room temperature for weeks without precipitation or aggregation.

TABLE 1 Experimental Conditions for SWNT/Metal nanoparticles

[0067] Atomic force microscopy (AFM) and transmission electron microscopy

(TEM) techniques are used to characterize the SWNT/Metal nanoparticle assemblies. FIGURE 4 shows images of spin-cast samples on freshly cleaved mica obtained with MFP 3D AFM (Asylum Research, Santa Barbara, CA) in ambient. Comparing FIGURES 4(B) and 4(C) with the SWNT-PSMA complexes in FIGURE 4(A), it is clear that metal nanoparticles of about 3-6 nm in diameter form as nodules along the SWNT-PSMA complexes after chemical reduction. The spacing between these particles is about 25-100 nm. TEM micrographs similarly confirm these results. FIGURE 6(A) is a TEM image of the same SWNTYPd nanoparticle assembly shown in FIGURE 4(B), while FIGURE 6(B) is a TEM image of the same SWNTVPt nanoparticle assembly shown in FIGURE 4(C)

[0068] As comparative examples, three control experiments were carried out. First, testing whether metal nanoparticle formation occurs due to the addition of a metal-ion reducing agent or due to a reaction between SWNTs with the metal ions, a solution containing SWNT-PSMA complex was mixed with Pt(terpy)²⁺ in the absence of any metal ion reducing agent. Although previous reports indicated that a spontaneous redox reaction would occur between SWNTs and noble metal ions, which would lead to formation of metal nanoparticles, no discernable amount of Pt nanoparticle formation is observed in this control experiment. Second, to examine the influence a SWNT-PMSA complex exhibits on the growth of metal nanoparticles, the reduction of a Pt metal ion containing species Pt(terpy)²⁺ with NaBH₄, in the absence of any SWNT-PSMA complex is carried out. The results of this control experiment yield large clumps of solid metallic Pt. The AFM image of these solid metallic clumps of Pt that are obtained is shown in FIGURE 7. Third, to examine the influence incubation exhibits on the growth of metal nanoparticles, an experiment is done where NaBH₄ is immediately added to a mixture containing SWNT-PSMA and metal ions before any time of incubation has passed. The AFM image provided in FIGURE 4(D) shows that large clusters of particles are foπned when attempts to produce SWNT/Metal nanoparticle assemblies in this way are made, that is when no time is provided for the metal ions to incubate with the SWNT-PMSA complexes. The result of these control experiments indicates that providing a time for incubation, aids in controlling the growth and association of metal nanoparticles so that SWNT/Metal nanoparticle assemblies may be prepared.

Claims

CLAIMS:

1. A method for preparing a single-walled carbon nanotube (SWNT)/metal-containing nanoparticle (MCNP) assembly in a medium, comprising:

a. dispersing SWNTs in the medium in the presence of at least one polymer surfactant;

b. mixing metal ions with the SWNTs dispersed in the medium;

c. incubating the mixture; and

d. reacting the metal ions with an effective amount of a reagent to form a SWMT/MCNP assembly.

2. The method of claim 1 wherein the polymer surfactant is selected from the group consisting of DNA, proteins, peptides, and synthetic polymers.

3. The method of claim 2 wherein the polymer surfactant is an alternating copolymer of styrene and maleic acid (PSMA).

4. The method of claim 1 wherein the metal ions are selected from the group consisting of Ti, Zr, Mn, Fe, Ru, Rh, Co, Ni, Pt, Pd, Cu, Ag, Au, Zn, Cd, Hg, Sn, Pb, and combinations thereof.

5. The method of claim 4, wherein the MCNP is selected from a metal nanoparticle, a semiconductor nanoparticle, or a metal-oxide nanoparticle.

6. The method of claim 5, wherein the reagent is selected from a metal-ion reducing agent, a group Via reagent, or a metal-ion oxidizing agent.

7. The method of claim 6, wherein the metal-ion reducing agent is NaBH₄.

8. The method of claim 6, wherein the group Via reagent is ammonium sulfide.

9. The method of claim 1, wherein the medium is selected from the group consisting of water, an organic solvent, and mixtures thereof.

10. The S WNT/MCNP assembly prepared by the method of claim 1.

11. A S WNT/MCNP assembly comprising a SWNT, at least one polymer surfactant, and at least one MCNP.

12. The SWNT/MCNP assembly of claim 11 wherein the polymer surfactant is a synthetic polymer.

13. The SWNT/MCNP assembly of claim 12 wherein the synthetic polymer is an alternating copolymer of styrene and nialeic acid (PSMA).

14. The SWNT/MCNP assembly of claim 12 wherein the MCNP is selected from a metal nanoparticle, a semiconductor nanoparticle, or a metal-oxide nanoparticle.

15. The SWNT/MCNP assembly of claim 14, wherein the metal nanoparticle is a Fe, Ru, Rh, Pt, Pd, Cu, Ag, or Au nanoparticle.

16. The SWNT/MCNP assembly of claim 14, wherein the semiconductor nanoparticle is a ZnO, ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, HgS, HgSe, or HgTe nanoparticle.

17. The SWNT/MCNP assembly of claim 14, wherein the metal-oxide nanoparticle is a TiO₂, ZrO₂, MnO₂, FeO, Fe₂O₃, CuO, ZnO₂, SnO, SnO₂, PbO, Pb₃O₄, or PbO₂ nanoparticle.