WO2009097669A1

WO2009097669A1 - Process for synthetising nanostructured hybrid systems: carbon nanotubes-metal nanoparticles

Info

Publication number: WO2009097669A1
Application number: PCT/BR2009/000013
Authority: WO
Inventors: Luiz Orlando Ladeira; Rodrigo Gribel Lacerda; André Santarosa FERLAUTO; Eudes LORENÇON; Sergio De Oliveira; Edelma Eleto Da Silva
Original assignee: Universidade Federal De Minas Gerais-Ufmg
Priority date: 2008-01-15
Filing date: 2009-01-15
Publication date: 2009-08-13
Also published as: BRPI0800605B1; BRPI0800605A2

Abstract

The present invention relates to a process for the decoration of the external surfaces of carbon nanotubes with metal nanoparticles, resulting in nanostructured hybrid systems formed by carbon nanotubes and metal nanoparticles.

Description

"Synthesis Process of Hybrid Nanostructured Systems: Carbon Nanotubes-Metallic Nanoparticles"

The present invention relates to a process of decorating the outer surfaces of carbon nanotubes with metallic nanoparticles resulting in hybrid nanostructured systems formed by carbon nanotubes and metallic nanoparticles.

In recent decades, scientific and technological advances such as the recent discovery of carbon nanotubes and fullerenes as well as the development of nanoscopic matter observation systems have allowed the rapid development of nanoscale science and technology, known as nanotechnology.

Carbon nanotubes are fibrillary and tubular structures consisting of carbon-carbon sp ² hybridization bonds with a diameter ranging from 0.7 - 80 nm in diameter and length from 10 to 80,000 nm. Carbon nanotubes due to this large aspect ratio and their exceptional mechanical and electronic structural properties have become of great importance to science [Popov, Materials Science & Engineering R-Reports, 2004, 43, 61].

Carbon nanotubes can be synthesized in two forms, namely single-walled carbon nanotubes (NTCPS), formed by a single layer of carbon atoms, and multi-walled carbon nanotubes (NTCMP), consisting of several concentric carbon tubes. . In particular, its properties such as chemical inertia, high aspect ratio and high specific area place this material in a strategic position for the development of nanotechnology mesoscopic systems in which interfacial interactions are preponderant. Hybrid systems that combine carbon nanotubes and other nanostructures such as additional molecular bonds to their wall or miscellaneous nanoparticles allow the creation of new nanoscopic systems with definite function and diverse applications in the areas of new materials and their use in devices such as: hydrogen storage [Zuttel et al., International Journal of Hydrogen Energy, 2002, 27, 203] as catalytic supports [Dicks, Journal of Power Sources, 2006, 156, 128], in the creation of new electronic devices such as single electron transistors, molecular diodes, memory elements.

[Ishibashi et al., Microelectronic Engineering, 2003, 67, 749; Roth et al., Current Opinion in Solid State & Materials Science, 1998, 3, 209; Meunier et al., Physical Review Letters, 2007, 98, 56401].

The process of anchoring nanoparticles to a substrate is called decoration. The decoration of carbon nanotubes with metal nanoparticles (NTC / NPM) is of great interest as it generates a new class of nanoparticle / nanotube hybrid nanomaterials that receive various applications [Sun et al., Journal of Colloid and Interface Science, 2006, 304, 323].

NTC / NPM hybrid systems can be used for heterogeneous catalysis in various chemical, petrochemical and pharmaceutical industry procedures as well as energy applications such as electrodes and catalytic reforming of methanol in PEM (Proton Exchange Membrane) fuel cells [Liang et al., Carbon, 2005, 43, 3144], high performance batteries [Qiu et al., New Journal of Chemistry, 2004, 28, 1056], supercapacitors [Kim et al., Journal of Materials Chemistry, 2005, 15 , 4914], photovoltaic power generation [Camacho et al., Jom, 2007, 59, 39] and nanobiotechnology [Wang et al., Electrochemistry Communications, 2003, 5, 800].

Several chemical synthesis routes of hybrid NTC / NPM systems have been developed and mostly consisting of a well-dispersed pretreated carbon nanotube solution containing some functionalization for its dispersion in aqueous medium. This dispersed solution is then mixed with an ionic solution of the metal to be reduced and then a reducing agent is added to this mixture to reduce and form nanoparticles on the surface of the carbon nanotubes. Some works describe specific molecular functionalizations in the carbon nanotube wall in order to strongly bond these nanoparticles to the NTC wall. Below we present several literature references that show the state of the art in processes and decoration routes of carbon nanostructures with metallic nanoparticles.

As an example, Lordi and coauthors treat NTCPS with diluted nitric acid to create oxygenated functionalizations on nanotube walls. The surface mainly has carboxyl groups that can act as anchors in the formation and deposition of Pt. Ethylene Glycol nanoparticles which is used as a reducing agent. The final material contains about 10% platinum nanoparticles with a diameter in the range of 1-2 nm. [Lordi et al., Chem. Mater, 200, 13, 733].

Alternatively, nanoparticles may be preformed and covalently connected to nanotubes through organic fragments. Chen and colleagues report that first oxidation of NTCPS occurs and subsequently reaction with aliphatic aminothiols that lead to the formation of thiol-terminated nanotubes, which act as connections for the deposition of Au (gold) nanoparticles. [Chen et al., Science, 1998, 282, 95].

On the reduction of alkali metals, it is known that carbon nanotubes can form polyelectrolyte salts that are soluble in polar organic solvents without the need for functionalization or sonification processes, thus forming a thermodynamically stable solution of isolated nanotubes.

Dissolution consists of chemically reducing nanotubes with alkali metals, leading to the formation of polyelectrolyte salts that can carry a charge every 10 carbon atoms. The reducing metal, when oxidized, acts as a counter ion compensating for the negative charges acquired by the tube.

According to Penicaud et al. [Penicaud et al., Journal of the American Chemical Society, 2005, 127, 8], nanotube reduction can be performed in a system containing a mixture of carbon nanotubes (NTC) and metallic sodium in tetrahydrofuran (THF) in the presence of organic molecules such as naphthalene, anthraquinone and fluorenone under inert atmosphere. The donation of alkaline metal electrons to the nanotube is through the organic molecules, because they have the property of being directly reduced. by the metal, resulting in an anionic radical that cyclically yields electrons to the nanotube. When the nanotube is saturated with charges the organic molecules become permanently charged. This last step can be easily checked by varying the color of the solution that presents the characteristics of the absorption spectrum of the anionic radical.

Petit et al. [Petit et al., Chemical Physics Letters, 1999, 305, 370], show that nanotubes in the form of polyelectrolyte salts have a much higher reduction potential than the reduction potential of unreduced dispersed nanotubes, where the organic molecule It is able to fill the conduction bands of nanotubes with electrons until their reduction potential equals the potential of the organic anionic radical.

Pekker et al. [Pekker et al., Journal of Physical Chemistry B, 2001, 105, 7938], show that carbon nanotubes can form soluble polyelectrolyte salts through their direct reaction with liquid ammonia in the presence of metallic lithium. In this system ammonia is capable of producing solvated electrons, which in turn fill the unoccupied electronic levels of nanotubes until chemical equilibrium is reached. This reference also shows that it is possible to use excess free electrons in nanotubes to promote functionalizations such as hydrogenation and alkylation.

In the classical processes of decorating nanoparticles with metallic nanoparticles described in the literature they use, in the process of reducing ionic species to their elemental form, chemical reducing agents such as: NaBH ₄ (sodium borohydride), N ₂ H ₄ (hydrazine) acid ascorbic, tannic or citrus.

In addition to the references cited, there are some patents that use different processes on the same subject.

U.S. Patent No. 6,987,302 to Yingjian Chen; Xiaozhong Dang, entitled "Method of Decorating NTC with Magnetic Nanoparticles" NTC are treated with H2SO4 / HNO3 solution to promote carboxylation on their walls, and subsequent treatment with cationic polymers resulting in a composite that is able to attract by interactions. negatively charged colloidal electrostatic nanoparticles. In this method no Direct adhesion of nanoparticles to NTC occurs, thus limiting applications, as in electrochemical catalysis. Moreover, the method indicated in this technology is time consuming, which reduces its industrial applications.

Another patent is US Patent No. 7250188 to Jean Pol Dodelet; et. al. whose invention is based on a decoration process in which a silane solution of the metal salt to be decorated is prepared and subsequently an electrode containing carbon nanotubes is immersed in the silane solution in which the metal cations are reduced over the walls of the NTC by applying a potential difference between the NTC containing electrode and a counter electrode. In the method proposed in the invention the first step is more complex and time consuming and restricted to few metals, which also limits its use on an industrial scale.

The technology disclosed in US Patent No. 6975063 to Dongsheng Mao et. al. It is based on the electrolytic reduction of a metal on a carbon nanotube electrode, promoting the total metallization of carbon nanotubes in buffered aqueous solutions (with fixed pH). This method leads to the formation of very thick metal films that completely cover the NTC, resulting in little metal utilization.

Given the technologies already developed on the subject, the electrochemical reduction capacity of such compounds is sufficient only to reduce, to the elemental form, metals with higher reduction potentials, such as: Au, Ag, Pt and Pd. These reducers inevitably generate large quantities of non-nanotube bonded metal nanoparticles, resulting in a waste of material and little control over the process stoichiometry. Another problem with presently developed decoration processes is the need to obtain well-dispersed suspensions of carbon nanotubes. For well-dispersed NTC solutions, most of the methods used consist of complex and time-consuming processes of functionalization and often long periods of sonification, which makes the process very slow and more costly.

The present invention has great advantages over the existing technological framework on the subject, since the method here The proposed approach describes a rapid process of decoration of NTC with a wide range of binary and ternary metallic NP or alloys and nanoparticles are generated in situ by reducing their ions directly by the NTC, ensuring good adhesion. Also no initial steps of nanotube functionalization are required.

The process described in this patent, by transferring electrical charges to the NTC, promotes an electrostatic repulsion between them greatly facilitating their dispersion in liquid medium. This solves two problems simultaneously: dispersion and decoration of the NTC.

Therefore, to solve the problems presented, the present process proposes the use of carbon nanostructure itself as a reducing agent, whose electrochemical potential in thermodynamic equilibrium is equal to that of Na (sodium) in metallic form. Thus, such carbon nanostructures become very strong reducing agents thus serving to reduce numerous metal cations in addition to Au, Ag, Pt and Pd cations. In addition, the reduction of metal ions will be induced directly on the surface of the NTC, with no free nanoparticles forming in solution. Another advantage is that the new technique does not require any prior NTC functionalization process since the reduced form nanotubes are formed by well dispersed and isolated polyelectrolyte salts.

Thus, the present invention proposes a process using carbon nanotubes themselves in the form of polyelectrolyte salts as reducing agents of metal salt solutions for the formation of metal nanoparticles and alloys of metal nanoparticles on their outer surface. Thus, an in situ process of reducing metal ions of various metals on the outer surface of carbon nanotubes is described by an electronic transfer process between a donor material such as: Na (sodium), K (potassium) or Li (lithium) in unrestricted elemental form and dispersed or non-functionalized NTCPS or NTCPM carbon nanotubes in aprotic polar organic solvents. This charge transfer is mediated by molecular compounds with high anionic radical formation capacity. The process of decoration of nanostructured materials with metal nanoparticles occurs through the process of chemical reduction of the metal nanoparticle ions present in liquid medium in solid nanoparticles attached to the surface of the solid support, which are carbon nanostructures (NEC) such as: carbon nanotubes single or multiple walls, functionalized or not dispersed in liquid medium.

The proposed new process involves, in a first step, the electronic transfer of solid Na (sodium) to carbon nanotubes, dispersed in liquid medium through a polar aprotic solvent, such as tetrahydrofuran. Electron transfer by means of some organic compound with a large amount of conjugated double bonds, which have the ability to form anionic radicals such as naphthalene, benzophenone, anthraquinone or unconstrained conjugated organic polymers, used for conducting electrons of the alkali metal to the carbon nanotube (NTC) thus creates a CNT ^{n "} polyelectrolyte salt (negatively charged carbon nanotubes) under an inert atmosphere. In a second step, excess electrons from the carbon nanotube are donated to the metal ions present in solution leading to its reduction in the elemental form on the NTC walls This process of decorating carbon nanostructures with metallic nanoparticles is a two-step chemical process, the first one being slower and dependent on the concentration of the molecular species responsible. by the electrical charge transfer between Na (sodium) and NTC The decoration process ends when this system reaches the electrochemical equilibrium, that is, when the NTCs are in their initial nonionic form.

The detailed description of the process of decorating carbon nanotubes with metallic nanoparticles to their outer surface, object of the present invention, will be made according to the following steps:

1- Preparation of a carbon nanotube mixture in a polar aprotic solvent

At this early stage, carbon nanotubes are dispersed in an anhydrous polar aprotic solvent such as tetrahydrofuran (THF), not restricted. Carbon nanotubes may be single or multi-walled (NTCPSs or NTCMPs) functionalized or not with other molecular compounds, at varying mixing ratios depending on the type of NTC and other molecular groups attached or not to its outer wall.

2-Addition of molecular organic species capable of forming anionic radicals to the NTC mixture.

A certain amount of naphthalene is added to the above mixture and stirred until complete solubilization. Naphthalene is an organic molecule capable of reacting with alkali metal to form an anionic species which has the alkali metal cation as a counterion. There are other substances that play the same role and can be used at this stage in place of naphthalene such as: anthraquinone or benzophenone, not restricted. The amount of naphthalene present in the mixture is only limited by its solubility.

3-Addition of a finely divided alkali metal.

An amount of finely divided metallic sodium is added to the mixture from the previous step. Metallic sodium can also be replaced by other alkaline metals such as lithium and unrestricted potassium. The amount of sodium required is on the order of milligrams per gram of nanotube (0.1-1000 mg / g NTC). Using a larger quantity does not change the end result of the process.

The mixture is then placed in an inert atmosphere and constantly stirred until a well dispersed solution of carbon nanotubes in the form of polyelectrolytes is obtained. The formation of polyelectrolytes is usually accompanied by a change in coloration of this solution due to the permanent formation of anionic radicals. In addition, the formed anionic radicals interact with carbon nanotubes by solvation process, further increasing the colloidal dispersion of the NTC and avoiding their agglomeration. In this step this solution is called solution A.

4-Preparation of the metal precursor solution.

A suitable amount of salt containing the metal ion of interest is dissolved in an unreacted polar anhydrous solvent such as THF, not restricted. This solution is prepared under ultrasonification or mechanical stirring until completely dissolved. When decorating carbon nanotubes with binary or ternary alloys, the precursor compounds of these elements are simultaneously dissolved in this non-reactive anhydrous polar solvent in the desired ratio for the formation of the corresponding alloy or for the formation of two or more types of metal nanoparticles. . The most commonly used metal precursor compounds are chlorides, sulphates, oxalates and organometallic compounds of metals such as Fe, Ni, Co, Cu, Zn, Cd, Sn, Rh, Ru, Pd, HAuCU, H ₂ PtCL _6l AgNO ₃ , unrestricted, and their binary or ternary solutions. The solution prepared in this step is called solution B.

5- Reduction and decoration of carbon nanotubes from metal precursors

Solution B is now slowly mixed with solution A without oxygen, i.e. in an inert atmosphere, by injection through the rubber stopper of the vial containing solution A under vigorous stirring. Because the reaction is very fast, long stirring times are unnecessary and do not alter the outcome of the final product. At this stage, the metal ions receive electrons from carbon nanotubes, thus being reduced into metal nanoparticles on the outer walls of the NTC. This final mixture is called solution C.

6- Separation purification and storage of the final product.

The separation of NTC decorated with metallic nanoparticles (NPMs) can be performed by ultrafiltration or ultra centrifugation processes. For separation of NTCs decorated with NPMs by filtration, vacuum assisted pore size 0.45 µm filters are used. In this way, after the passage of the mixture through the filter, the NTCs are retained in the filter and, by successive passages of ethanol through the filtration membrane, the NTCs are purified by eliminating solvent residues and other undesirable components.

Another more efficient and faster way of purifying NPM decorated NTC is through ultra centrifugation processes. In this process, Solution C is centrifuged at 5000rpm for 5 minutes, which causes NPM-decorated NTCs to precipitate and concentrate to the bottom of the centrifuge vessel. The suspension is then discarded and ethanol is added to the centrifuge vessel. The NTCs are then resuspended and centrifuged again at 5000rpm for 5 minutes. This process is repeated 3 to 5 times and finally the NTC are resuspended in ethanol and transferred to Petri dishes and oven dried at 100 ° C for 4 to 12 hours.

In some decorations it is possible to observe, by X-ray photoelectron spectroscopy (XPS), excitation energies corresponding to naphthalene, which means that the solvent removal process in the filtration or ultracentrifugation purification step was not effective. In this situation, two processes are very efficient for final purification. The first is to wash the NTC by 0.45 µm filter filtration with a solvent in which naphthalene has greater solubility such as hexane. The second process is by heat treatment of the NTC at 200 ° C under high vacuum (10 ⁵ mbar) for 1 hour. The choice of the final purification process depends greatly on the amount of material obtained in the synthesis and the second process is preferable when large amounts of product are obtained after synthesis.

The final product should be stored dry under inert atmosphere or in liquid N ₂ toluene solution, depending on the chemical reactivity of the nanoparticle used in the decoration. In general, the higher the reactivity, the greater the storage control and care should be. Example 1: Deposition of Au (gold) nanoparticles on the surface of single walled carbon nanotubes.

25mg of high purity NTCPS is added in an "X" container containing 25ml of THF. 1mmol naphthalene is added to the stirred vessel until completely dissolved. 1g finely divided metallic sodium is added to the container. The container "X" is placed in an inert atmosphere under constant agitation until complete dispersion of the carbon nanotubes is observed, this event is indicated by the appearance of a greenish-yellow coloration in the mixture. A "Y" solution is prepared by dissolving 5mg HAuCl ₄ in 10 ml THF. Solution "Y" is rapidly added to solution "X" under constant stirring. After 15 minutes, the sodium contained in the mixture is removed and the supernatant is filtered through a porous membrane filter (pores 0.45 µm in diameter), washed four times with ethanol and vacuum dried for 2 hours at 100 ° C to obtain nanotubes. carbon with gold nanoparticles on its surface.

Example 2: Deposition of Cu (copper) nanoparticles on the surface of single walled carbon nanotubes.

25mg of high purity NTCPS is added in an "X" container containing 25ml of THF. 1mmol naphthalene is added to the stirred vessel until completely dissolved. 1g finely divided metallic sodium is added to the container. The container "X" is placed in an inert atmosphere under constant agitation until complete dispersion of the carbon nanotubes is observed. This event is indicated by the appearance of a greenish-yellow color in the mixture. A "Y" solution is prepared by dissolving 5mg CuCl2.2H ₂₀ in 10ml THF. Solution Ύ "is rapidly added to solution" X "under constant stirring. After 15 minutes, the sodium contained in the mixture is removed and the supernatant is filtered through a porous membrane filter (0.45 µm pore diameter), washed four times. with ethanol and vacuum dried for 2 hours at 100 ° C to obtain carbon nanotubes with copper nanoparticles on their surface.

Example 3: Deposition of Rh (rhodium) nanoparticles on the surface of single walled carbon nanotubes.

25mg of high purity NTCPS is added in an "X" container containing 25ml of THF. 1 mmol Naphthalene is added to the vessel under mechanical stirring until complete dissolution. 1g finely divided metallic sodium is added to the container. The container "X" is placed in an inert atmosphere under constant agitation until complete dispersion of the carbon nanotubes is observed. This event is indicated by the appearance of a greenish-yellow color in the mixture. A solution "Y" is prepared by dissolving 5mg RhCl ₃ in 10ml THF. Solution Ύ "is rapidly added to solution" X "under constant stirring. After 15 minutes, the sodium contained in the mixture is removed and the supernatant is filtered through a porous membrane filter (0.45 µm pore diameter), washed four times. with ethanol and vacuum dried for 2 hours at 100 ° C to obtain carbon nanotubes with rhodium nanoparticles on their surface.

Example 4: Deposition of Au (gold) nanoparticles on the surface of multiwall carbon nanotubes.

50mg of high purity NTCMP is added in a "X" container containing 25ml of THF. 1mmol naphthalene is added to the stirred vessel until completely dissolved. 1g finely divided metallic sodium is added to the container. The container "X" is placed in an inert atmosphere under constant agitation until complete dispersion of the carbon nanotubes is observed. This event is indicated by the appearance of a greenish-yellow color in the mixture. A "Y" solution is prepared by dissolving 10mg HAuCU in 10ml THF. Solution "Y" is rapidly added to solution "X" under constant stirring. After 15 minutes, the sodium contained in the mixture is removed and the supernatant is filtered through a porous membrane filter (pores 0.45 µm in diameter), washed four times with ethanol and vacuum dried for 2 hours at 100 ° C to obtain multi-walled carbon nanotubes with gold nanoparticles on their surface.

Claims

1. Method of depositing metallic nanoparticles on the surface of carbon nanotubes, comprising the following steps:

a) Preparation of a mixture of carbon nanotubes in a polar aprotic solvent;

(b) Addition of molecular organic species capable of forming anionic radicals to the NTC mixture;

c) Addition of a finely divided alkali metal;

d) Preparation of the metal precursor solution;

e) Reduction and decoration of carbon nanotubes from metallic precursors under inert atmosphere;

(f) Separation and purification by ultrafiltration or ultracentrifugation and storage of the final product under an inert atmosphere or N ₂ toluene solution.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claim 1, characterized in that step "a" comprises the use of an aprotic polar solvent and anhydrous state.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claims 1 and 2, characterized in that step "b" comprises non-limiting organic species such as naphthalene, anthraquinone or benzophenone.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claims 1 to 3, characterized in that step "c" comprises a finely divided alkali metal such as metallic sodium, metallic potassium or unrestricted lithium metal. .

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claims 1 to 4, characterized in that step "d" e "e" comprises metal precursors such as metal chlorides, sulphates, oxalates and organometallic compounds like Fe, Ni, Co, Cu, Zn, Cd, Sn, Rh, Ru, Pd, HAuCI ₄ , H ₂ PtCL ₆ , AgN0 ₃ , unrestricted, and their binary or ternary solutions.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claims 1 to 5, characterized in that it has single walled carbon nanotubes in the form of polyelectrolyte salts.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to any one of claims 1 to 5, characterized in that it has multiwall carbon nanotubes in the form of polyelectrolyte salts.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claims 1 to 7, characterized in that it has at least one metal salt such as platinum, gold, iridium, silver, palladium, rhodium, ruthenium, cadmium, iron, cobalt, copper, zinc, tin, bismuth, indium, or a mixture of several of these materials.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claims 1 to 8, characterized in that it has a concentration of 0.1-1000 mg / g of nanotube salts in the form of polyelectrolytes in an inert solvent.

Method of depositing metal nanoparticles on the surface of carbon nanotubes according to claims 1 to 9, characterized in that it has percentages of metal salts between 0.01 to 200% relative to the mass of nanotubes in the form of polyelectrolyte salts. added.