Patent Application
20160280545
The present invention generally relates to the field of carbon nanomaterials.
Carbon nanomaterials, such as graphene and carbon nanotubes, have attracted immense attention in a wide range of promising potential applications.
In many cases, such as nanocomposite materials, electronic inks, display devices, drug delivery and biosensors, the graphene must be individually dispersed in solvents or matrices by a scalable method. However, graphene itself has extremely low solubilities in common solvents, and therefore functionalization is crucial to avoid restacking and enable processing.
Functionalization by the production of graphene oxide by acid exfoliation is popular but this method damages the intrinsic structure and degrades the properties of the graphene (Bai et al. Adv. Mater., 2011, 23, 1089). Alternative milder wet-chemical approaches generate graphenes from graphite by exploiting exfoliation and stabilisation using carefully selected surfactants or solvents (Khan et al., Small, 2010, 6, 864). However such graphenes typically involve extended sonication which leads to the formation of structural defects and reduced flake size.
An approach to retain the bonded network and the lateral dimensions of graphenes involves the formation of electrostatically-stabilised dispersions by protonation in superacids or reduction and dissolution in polar aprotic solvents. For example, graphite is intercalated with liquid potassium-ammonia followed by dissolution in tetrahydrofuran (THF) as well as the dissolution of potassium-based graphite intercalation compounds (GICs) in N-methyl pyrrolidone (Valles et al., J. Am. Chem. Soc., 2008, 130, 15802). The resulting solutions contain individually solvated graphenes (graphenides) and are stable as long as air is excluded.
Covalent functionalization of these graphenides has been achieved through reaction with a suitable electrophile (Englert et al., Nat. Chem., 2011, 3, 279; Englert et al., Chem. Comm. 2012, 48, 5025). In these reactions, an excess of Na/K was used for reducing the graphite. However, low grafting ratio and low solubility of the functionalised products was observed.
Single walled carbon nanotubes (SWNTs) have shown excellent potential in electronic, mechanical, and other functional applications. Semiconducting SWNTs are of particular relevance in the field of nano-electronics, for example in the form of thin film transistors (TFTs) and molecule sensors; networks of metallic tubes are widely considered as transparent conducting films (TCFs) for displays, touch screens, and solar cells.
Functionalization of the sidewalls of SWNTs is challenging due to their relatively poor reactivity and dispersibility. Synthesized carbon nanotubes also routinely contain impurities both in the form of non-nanotube carbon (amorphous and graphitic carbon, and short defective nanotubes) and residual catalytic particles, often contained within graphitic shells and carbon nanotube caps. Although functionalization has been achieved, many processes require ultrasonication of the carbon nanotubes during the functionalization process, which may damage the nanotubes. Exfoliating bundles of SWNTs presents similar challenges to the case of graphene, albeit with different geometry.
There is therefore a need for improved methods for purifying carbon nanomaterials and preparing functionalised carbon nanomaterials.
It has been determined that in a process involving reduction of a carbon nanomaterial, followed by covalent functionalization thereof, control of the absolute alkali metal concentration in the reduction of a carbon nanomaterial results in an improved process.
Accordingly, in a first aspect, the present invention provides a method of preparing a covalently functionalised carbon nanomaterial, comprising the steps of
The use of N,N-dimethylacetamide allows for the use of a single solvent in the preparation of functionalised carbon nanomaterial and allows for significant improvements in yield.
Preferably, the carbon material comprises a carbon nanotube.
Preferably, the concentration of alkali metal [M] in step (i) is between 0.003 mol/L and 0.05 mol/L.
Preferably, the ratio of carbon material to alkali metal (C/M) in solution in step (i) is at least 2:1.
In a second aspect, the present invention provides a method of preparing a covalently functionalised carbon nanomaterial, comprising the steps of
Optimisation of the concentration of alkali metal in step (i) as well as the ratio of the carbon material to alkali metal allows for enhanced exfoliation and improved grafting ratio of the carbon nanomaterial.
Preferably the reducing agent comprises an alkali metal and a charge transfer agent, which may be, for example, a compound comprising one or more aromatic rings. Preferably, the charge transfer agent comprises naphthalene.
Preferably, the carbon material comprises graphite, graphene, graphene nanoribbons or carbon nanotubes.
Preferably, the covalently functionalised carbon nanomaterial is soluble in solvent S.
Preferably, the solvent S is a coordinating solvent. Preferably, the coordinating solvent comprises a cyclic ether, THF, 1,4-dioxane or a crown ether.
Preferably, wherein the carbon material comprises graphite, graphene, or graphene nanoribbons, the concentration of alkali metal in step (i) is between 0.003 mol/L and 0.015 mol/L. Preferably, the concentration of alkali metal in step (i) is between 0.006 mol/L and 0.012 mol/L. Preferably, the concentration of alkali metal in step (i) is between 0.007 mol/L and 0.011 mol/L. More preferably, the concentration of alkali metal in step (i) is about 0.009 mol/L.
Preferably, wherein the carbon material comprises a carbon nanotube, the concentration of alkali metal in step (i) is between 0.015 mol/L and 0.05 mol/L. More preferably, the concentration of alkali metal in step (i) is between 0.020 mol/L and 0.035 mol/L. More preferably, the concentration of alkali metal in step (i) is between 0.025 mol/L and 0.030 mol/L.
Preferably, wherein the carbon material comprises a carbon nanotube or a pre-exfoliated graphene, the solvent S comprises an amide. More preferably, the amide is N,N-dimethylacetamide (DMAC).
In any aspects of the invention and in any preferred embodiment, the method may further comprise a step (i)(a) comprising sonicating the reduced carbon material solution prior to step (ii).
In all aspects of the invention, preferably, the functionalising reagent is an electrophile. Preferably, the electrophile is a compound comprising a moiety R, wherein R is an organic moiety. Preferably, R comprises an aliphatic, heteroaliphatic, aromatic, heteroaromatic, carbonyl, epoxy, disulphide or peroxide moiety.
Preferably, the electrophile is a compound comprising a moiety R—X, wherein R is an organic moiety and X is a leaving group.
Preferably, R comprises an aliphatic, heteroaliphatic, aromatic, heteroaromatic moiety and X is a halide. In some embodiments, the functionalising reagent is R—X, wherein R is C1-30 aliphatic (e.g. C10-24 alkyl) and X is a halide.
In all aspects of the invention, preferably, the alkali metal M comprises lithium, sodium, potassium or an alloy thereof.
Preferably, the ratio of carbon material to alkali metal (C/M) in solution in step (i) is at least 2:1. More preferably, the C/M ratio is at least 5:1. More preferably, the C/M ratio is at least 10:1.
Preferably, prior to step (i), the carbon material is subjected to purification, the purification comprising the steps of:
Preferably, the solvent in the reducing solution is an amide solvent, for example DMAc.
In a third aspect, the invention is directed to a method of purifying carbon nanomaterial, the method comprising the steps of
Preferably, the carbon material comprises carbon nanotubes.
Preferably, the purification comprises a step of determining the concentration of alkali metal required to dissolve impurities without dissolving a desired fraction of carbon material prior to contacting the carbon material with a reducing solution.
Preferably, the solvent in the reducing solution is an amide solvent, for example DMAc.
In a fourth aspect, the present invention relates to a functionalised carbon nanomaterial produced by the method according to the first or second aspect.
Preferably, the carbon material comprises graphite, graphene, graphene nanoribbons or carbon nanotubes.
In a fifth aspect, the present invention relates to a method as substantially described herein with reference to or as illustrated in any one or more of the examples or accompanying figures.
In a sixth aspect, the present invention relates to a functionalised carbon nanomaterial as substantially described herein with reference to or as illustrated in any one or more of the examples or accompanying figures.
The invention may be put into practice in various ways and a number of specific embodiments will be described by way of example to illustrate the invention with reference to the accompanying figures, in which:
The present invention is defined in the accompanying claims.
In a first aspect, the invention relates to a method of preparing functionalized carbon materials, comprising the steps of
In a second aspect, the invention relates to a method of preparing a covalently functionalised carbon nanomaterial, comprising the steps of
Carbon material may be carbon nanomaterial. Carbon material according to the present invention, may comprise graphite, graphene, or carbon nanotubes. Graphene carbon material may comprise graphene sheets or graphene nanoribbons. Carbon nanotubes, according to the present invention, include, but are not limited to, single-wall carbon nanotubes (SWNTs), double-wall carbon nanotubes (DWNTs), multi-wall carbon nanotubes (MWNTs), small diameter carbon nanotubes, and combinations thereof. Nanomaterials are materials with at least one external dimension in the size range from about 1 to 100 nm.
The nanotube may be any type of nanotube, that is, it may be any hollow tubular structure having at least one dimension measuring on the nanometer scale. For example, the nanotube may have a smallest inner diameter measuring between about 0.5 nm to about 50 nm, such as about 0.5 nm to about 20 nm, for example between about 0.7 nm to about 10 nm, e.g. between about 0.8 nm to about 2 nm. Small diameter carbon nanotubes are defined herein as carbon nanotubes having diameters of at most about 3 nm, regardless of the number of walls. The nanotube may be of any length. For example, the nanotube may have a length between about 5 nm to about 500 μm.
The solvent S is aprotic, for example an ether, an amide or an amine solvent, or a mixture thereof. The ether may comprise alkyl or cycloalkyl ethers. Exemplary ethers include tetrahydrofuran (THF), dioxane, diethyl ether, diisopropyl ether, di-n-butyl ether, di-sec-butyl ether, methyl t-butyl ether, 1,2-dimethoxyethane, 1,2-dimethoxypropane, 1,3-dimethoxypropane, 1,2-diethoxyethane, 1,2-diethoxypropane, 12-crown-4 ether, 15-crown-5 ether, 18-crown-6 ether or combinations thereof. Amine solvents may be used and may comprise tertiary amines. Useful amines may comprise tertiary alkyl or cycloalkyl amines. Exemplary amines include tertiary amine including n-methyl piperidine, n-methyl morpholine, N,N,N′,N′-tetramethyl-1,2-diaminoethane, or combinations thereof. Amide solvents may be used. Exemplary amides include dimethylformamide, N-methyl-2-pyrrolidone, N,N-dimethylacetamide. Amide solvents should preferably be inert towards the alkali metal. The solvent should be stable in the presence of both the change transfer agent and the charged carbon material (formed by treatment with a reducing agent). A preferable amide solvent includes N-N-dimethylacetamide.
The solvent S may be a coordinating solvent. The coordinating solvent is aprotic, and is substantially free of protic contaminants such as moisture, alcohols, protic amines, hydroperoxides, or other reactive species including carbonyl compounds such as acids, ketones, aldehydes, esters. The coordinating solvent comprises an ether, an amine or a combination thereof. In one embodiment, the coordinating solvent is a cyclic ether. Preferably, the cyclic ether comprises THF or a crown ether.
The reducing agent may further comprise a charge transfer agent. The charge transfer agent is an agent which supports electride formation. Charge transfer agents may comprise aromatics. Examples of such charge transfer agents are naphthalene, anthracene, phenanthrene, 4,4′-di-tert-butylbiphenyl, azulene or combinations thereof. Preferably, the charge transfer agent is naphthalene.
The alkali metal comprises lithium, sodium, potassium or an alloy thereof. The total alkali metal in step (i) is as defined above. It will be apparent that a mixture of one or more alkali metal may be present.
The ratio of carbon material to alkali metal (C/M) refers to the ratio of carbon material (mol) to alkali metal (mol).
In some embodiments, wherein the carbon material comprises graphite, graphene, or graphene nanoribbons, the concentration of alkali metal in step (i) is between 0.003 mol/L and 0.015 mol/L. The concentration of alkali metal in step (i) may be between 0.006 mol/L and 0.012 mol/L. The concentration of alkali metal in step (i) may be between 0.007 mol/L and 0.011 mol/L. The concentration of alkali metal in step (i) may be about 0.009 mol/L.
In some embodiments, wherein the carbon material comprises a carbon nanotube, the concentration of alkali metal in step (i) may be between 0.015 mol/L and 0.05 mol/L. The concentration of alkali metal in step (i) may be between 0.020 mol/L and 0.035 mol/L. The concentration of alkali metal in step (i) may be between 0.025 mol/L and 0.030 mol/L.
In one embodiment, the functionalised carbon nanomaterial has a grafting ratio GR of at least 0.7.
Step (ii) of the method comprises treating the reduced carbon nanomaterial solution with a functionalizing reagent to form a covalently functionalized carbon nanomaterial. The above-described reductive functionalization method allows various functional groups (e.g. alkyl-, aryl-, allyl-, and benzyl-) to be covalently linked to the carbon material. The functional group may be a monomer. Functionalization is covalent functionalization, involving covalent bonding of the functional group to the carbon material.
The functionalizing reagent may comprise an aliphatic, heteroaliphatic, aromatic, heteroaromatic, carbonyl, epoxy, disulfide or peroxide moiety. The aliphatic, heteroaliphatic, aromatic, heteroaromatic, carbonyl, epoxy moieties may be optionally substituted with aliphatic, heteroaliphatic, aromatic, heteroaromatic, carbonyl, epoxy, disulfide or peroxide moieties. The carbonyl moiety may comprise an ester, amide, carbonate, aldehyde, or acyl moiety. The ester moiety may be saturated or unsaturated.
The term “aliphatic”, as used herein, means a substituted or unsubstituted straight-chain, branched, or cyclic hydrocarbon, which is completely saturated or which contains one or more units of unsaturation, but which is not aromatic. For example, suitable aliphatic groups include substituted or unsubstituted linear, branched or cyclic alkyl, alkenyl, or alkynyl groups and hybrids thereof, such as (cycloalkyl)alkyl, (cycloalkenyl)alkyl or (cycloalkyl)-alkenyl. In certain embodiments, a straight chain or branched chain alkyl has about 30 or fewer carbon atoms in its backbone (e.g., C1-C30 for straight chain, C3-C30 for branched chain), and alternatively, about 24 or fewer. Likewise, cycloalkyls have from about 3 to about 10 carbon atoms in their ring structure, and alternatively about 5, 6 or 7 carbons in the ring structure. Exemplary alkyl groups include methyl, ethyl, n-propyl, isopropyl, n-butyl, sec-butyl, isobutyl, tert-butyl, cyclopropyl, and cyclobutyl, dodecyl, eicosyl. The term aliphatic may also refer to an alkyl group that is substituted with at least one halogen. Exemplary haloalkyl groups include —CH2F, —CHF2, —CF3, —CH2CF3, —CF2CF3, and the like. The term “hydroxyalkyl” refers to an alkyl group that is substituted with at least one hydroxyl group. Exemplary hydroxyl alkyl groups include —CH2OH, —CH2CH2OH, —C(H)(OH)C(OH)H2, and the like. The term aliphatic may also refer to polyether groups. Exemplary polyether groups include poylethyleneglycol. The term aliphatic may also refer to an alkyl group substituted with an aryl group. The term aliphatic may also refer to an alkyl group substituted with a heteroaryl group. In one embodiment, the aliphatic group comprises C4H9, C12H25, or C20H41.
The term “heteroraliphatic” or “heteroalkyl” refers to aliphatic groups that include at least one heteroatom. In certain instances, a heteroaliphatic group contains 1, 2, 3, or 4 heteroatoms, N, O, S or P.
The term “aromatic” refers to a carbocyclic aromatic group. Representative aromatic groups include phenyl, naphthyl, anthracenyl, and the like. Unless specified otherwise, the aromatic ring may be substituted at one or more ring positions with, for example, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, carboxylic acid, —C(O)alkyl, —CO2alkyl, carbonyl, carboxyl, alkylthio, sulfonyl, sulfonamido, sulfonamide, ketone, aldehyde, ester, heterocyclyl, aryl or heteroaryl moieties, —CF3, —CN, or the like. The term “aryl” also includes polycyclic aromatic ring systems having two or more carbocyclic rings in which two or more carbons are common to two adjoining rings (the rings are “fused rings”) wherein all of the fused rings are aromatic rings, e.g., in a naphthyl group.
The term “heteroromatic” or “heteroaryl” refers to aromatic groups that include at least one ring heteroatom. In certain instances, a heteroaromatic group contains 1, 2, 3, or 4 ring heteroatoms, N, O, S or P. Representative examples of heteroaryl groups include pyrrolyl, furanyl, thiophenyl, imidazolyl, oxazolyl, thiazolyl, triazolyl, pyrazolyl, pyridinyl, pyrazinyl, pyridazinyl and pyrimidinyl, and the like. Unless specified otherwise, the heteroaryl ring may be substituted at one or more ring positions with, for example, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl, cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido, carboxylic acid, —C(O)alkyl, —CO2alkyl, carbonyl, carboxyl, alkylthio, sulfonyl, sulfonamido, sulfonamide, ketone, aldehyde, ester, heterocyclyl, aryl or heteroaryl moieties, —CF3, —CN, or the like. The term “heteroaryl” also includes polycyclic aromatic ring systems having two or more rings in which two or more carbons are common to two adjoining rings (the rings are “fused rings”) wherein all of the fused rings are heteroaromatic, e.g., in a naphthyridinyl group.
In some embodiments, the functionalizing reagent comprises a compound comprising a moiety R—X, wherein R is an organic moiety and X is a leaving group. X may comprise a halide, sulfoxide, or tosylate. R may comprise aliphatic, heteroaliphatic, aromatic or heteroaromatic moiety, or combinations thereof.
In some embodiments, the functionalizing reagent may comprise an alkyl halide, alkyl sulphoxide. In other embodiments, the aromatic moiety may comprise an aralkyl halide. In some embodiments, the acyl moiety may be an acyl halide, acyl tosylate, or acid anhydride.
The halide may be selected from the group consisting of —F, —Cl, —Br, —I.
In some embodiments, the functionalizing reagent, the charged carbon nanomaterial and the resulting functionalized carbon nanomaterial are soluble in solvent S.
In some embodiments, the ratio of carbon material to alkali metal (C/M) in solution in step (i) may be at least 2:1. More preferably, the moles of carbon to moles of alkali metal (C/M) ratio may be at least 5:1. More preferably, the C/M ratio may be at least 10:1. In some embodiments, the upper range of the C/M ratio may be 100:1, 80:1, 60:1 or 45:1. In some embodiments, for graphene, the C/M ratio may be between 22:1 and 32:1.
In some embodiments, the method may further comprise a step (i)(a) comprising sonicating the reduced carbon material solution prior to step (ii), i.e. prior to the treatment with a functionalizing reagent. The sonication step may be a mild sonication, for example the sonication may be bath sonication, preferably for up to ten minutes, i.e. approximately 5 minutes.
In some embodiments, further (subsequent) functionalization of the functionalized carbon nanomaterial is possible. For example, in another embodiment of the present invention, the method allows for the polymerization of monomers from an initially grafted functionalization group or by direct anionic polymerization from the charged carbon material. In such embodiments, the method allows for the in situ polymerization of monomeric material via reductive pathway to form polymer chains attached to the carbon material. Various monomeric species or combination of species can be used.
For the purposes of this invention, a monomer may be selected from a monomer which is accessible by free radical or anionic polymerization, such as a (meth)acrylate monomer or a vinyl monomer, a polymer, a fluorescent dye, a coupling agent, a surfactant, a free radical tag/trap (such as nitroxides, organic halides and especially organic iodides for example 1-iodododecane) or a free radical initiator (such as azo compounds, persulfates and organic peroxides). The vinyl monomer is preferably one or more selected from the group comprising ethylene, propylene, methyl methacrylate, styrene, (3,5,5-trimethylcyclohex-2-enylidene)malononitrile, 1,1-dichloroethylene, 1-(3-sulfopropyl)-2-vinylpyridinium hydroxide, 1-vinyl-2-pyrrolidinone, vinylnaphthalene 2-isopropenyl-2-oxazoline, 2-vinyl-1,3-dioxolane, vinylnaphthalene, vinylpyridine, 4-vinyl-1-cyclohexene 1,2-epoxide, 4-vinyl-1-cyclohexene, vinylanthracene, vinylcarbazole, divinyl sulfone, ethyl vinyl sulfide, N-ethyl-2-vinylcarbazole, N-methyl-N-vinylacetamide, N-vinylformamide, N-vinylphthalimide, trichlorovinylsilane, vinyl bromide, vinyl chloride, vinylcyclohexane, vinylcyclopentane, vinylphosphonic acid, vinylsulfonic acid, vinyltrimethylsilane, cis-1,3-dichloropropene, vinyl acetate, acrylic acid, acrylonitrile, (dimethylamino)ethylmethacrylate, lauryl methacrylate, 2-(methylthio)ethyl methacrylate, trimethylsilyl methacrylate, 2-hydroxyethyl methacrylate, hydroxy propyl methylacrylate, acrylamide, oleic acid, glycidyl methacrylate (GMA) and maleic anhydride.
In some embodiments, prior to step (i), the carbon material is subjected to purification, the purification comprising the steps of:
In a third aspect, the invention is directed to a method of purifying carbon nanomaterial, the method comprising the steps of
The carbon nanomaterial may be carbon nanotubes.
The alkali metal M may be provided in an amount sufficient to solubilise impurities without solubilising the desired carbon material. Impurities may comprise amorphous carbon, graphitic carbon, short defective nanotubes, and residual catalytic particles. The impurities may require less charge to solubilise than a desired fraction of carbon material. Accordingly, the alkali metal M may be provided in an amount insufficient to dissolve a desired fraction of the carbon material. The alkali metal may be provided in an amount such that the C/M ratio is at least 10:1, 30:1, 40:1, 50:1, 60:1, 70:1, 80:1, 100:1.
The amount of M and desired fraction may be determined by a fractionation process. For example, the carbon material may be exposed to different concentrations of alkali metal. The amount of dissolved impurities in the supernatant may be assessed, for example by UV vis spectroscopy. The concentration of alkali metal is determined at which impurities are dissolved without dissolving a desired fraction of carbon material. This fractionation process may involve the steps of:
The solvent for purification may be N,N-dimethylacetamide.
The contacting of the carbon nanomaterial with a reducing solution may take place without stirring.
The purification may further comprise a step of quenching the carbon material following removal of the supernatant. Quench occurs to remove residual charge on the carbon material. The purified carbon material may be quenched with dry oxygen.
Following purification, and optionally following quenching, the carbon material may be subjected to the functionalization method of the first or second aspect of the present invention.
A functionalized carbon material is soluble in a solvent under conditions used in the claimed method. The functionalized carbon material may preferably be dissolved in solvent, optionally by sonication at room temperature or by application of stirring, sonication or other methods.
The following examples of the invention are provided to aid understanding of the invention but should be not taken to limit the scope of the invention.
Graphite (Graphexel natural crystalline flake graphite, grade: 2369, Graphexel Ltd., UK) was obtained from the manufacturer and used without any further purification. Elicarb single walled carbon nanotubes (PR929, batch 108511/g) were supplied by Thomas Swan Ltd and were dried under vacuum (˜10−2 mbar) at 300° C. for 1 h and 16 h at room temperature before use.
1-Chlorododecane (>97%), 1-bromododecane (>98%) and 1-chloroeicosane (>96%) were purchased from Tokyo Chemical Industry Co., Ltd. 1-Bromobutane (99%), 1-bromoeicosane (98%), 1-iodododecane (98%) and anhydrous THF were obtained from Sigma-Aldrich. 1-Chloroeicosane and 1-bromoeicosane were dried at room temperature for at least 4 days under vacuum before using in the glove box. 1-Chlorododecane, 1-bromobutane, 1-bromododecane, 1-iodododecane, DMSO and THF were degassed via a freeze-pump-thaw method and dried over 20% wt, molecular sieves 4 Å. Sodium (99.95%, ingot, No. 262714) and naphthalene (99%) were purchased from Sigma-Aldrich. Naphthalene was dried under vacuum in the presence of P2O5.
All work is carried out in a nitrogen glovebox. If not supplied anhydrous, solids are dried under vacuum in the presence of P2O5 whilst liquids are degassed via freeze-pump-thaw and dried with molecular sieves.
Thermogravimetric analysis (TGA) was performed using a Perkin Elmer Pyris 1 TGA under perfect N2 atmosphere (samples were held at 100° C. for 90 min at the N2 flow rate=60 ml min−1, ramped 10° C. min−1 to 800° C. (N2 flow rate=60 ml min−1). FT-IR spectra were measured using a Perkin Elmer Spectrum 100 with universal ATR sampling accessary. X-ray powder diffraction (XRD) was recorded at a scan rate of 0.108°/s with the Cu Kα (1.542 Å) line using a PANalytical X'Pert PRO diffractometer. UV-vis-NIR absorption spectra were measured using a Perkin Elmer Lambda 950 UV/Vis spectrometer. Sonication was performed using an ultrasonic cleaner (USC300T, 80 W). Raman spectra were measured using an ISA Jobin Yvon SPEX Raman spectrometer equipped with a 532 nm excitation laser source. Typical tapping-mode atomic force microscopy (AFM) measurements were taken using Bruker MultiMode 8 AFM. Samples for AFM images were prepared by drop-casting dilute graphene-dispersed chloroform solutions on silica substrates.
The grafting ratio is the molar ratio of grafted moiety against molar ratio of carbon within the raw carbon nanomaterial with both these values taken from the weights calculated from the TGA as described below.
The derivative of percentage weight with respect to temperature of the TGA was taken and smoothed with a Savitzky-Golay filter. The resultant peak(s) allowed the determination of an onset and ending temperatures of the grafted moiety degradation, whilst the plateau of the derivative game the rate of degradation of the grafted species which was presumed to be constant. The percentage weight loss between the two temperatures was calculated and the contribution of thermal degradation (calculated using the plateau rate multiplied by the difference in temperature) was subtracted. This weight loss was attributed to the grafting species minus the leaving group. For graphite, the sample weight minus the grafted weight was attributed to graphite. For CNTs, the remaining weight was normalized to take into account the residual catalyst calculated from the residual weight of an oxidative TGA of the as received material.
69 mg (3 mmol) of sodium and 384.5 mg (3 mmol) of dried naphthalene were added into 30 mL of degassed anhydrous THF in a N2 filled glove box, and stirred for 1 day forming a green Na/naphthalene solution. A pre-made sodium naphthalide THF solution was used to allow for accurate, simple addition of sodium to the carefully dried nanocarbon. Typically, a Schlenk tube including graphite (36 mg, 3 mmol of carbon) together with a magnetic bar was flame-dried, and placed in a glove box. A variable mass of the Na/naphthalene solution (1:1 in THF) was added into the Schlenk tube containing graphite and the concentration of graphite in THF adjusted to 0.1 M (mmol ml−1) by addition of degassed anhydrous THF. The suspension was stirred for 1 day, and alkyl halides (9 mmol, 3 equiv. per sodium) were added to the tube. Then, the reaction was stirred at room temperature for 1 day under N2. After bubbling dry O2 into the solution for 15 min, the solution was stirred for 1 day under dry O2 for oxidation of any remaining charges on the functionalized graphenes. The solution was stirred as ethanol (10 ml) was added slowly followed by water (20 ml). After neutralization using 0.1 N HCl, the functionalized graphenes were extracted into hexane and washed several times with water. The mixture was filtered through a 0.1 mm PTFE membrane filter, washed thoroughly with hexane, THF, ethanol and water. After washing the sample with ethanol and THF again, the product was obtained after drying overnight under vacuum at 80° C.
As shown in
The same optimal C/Na ratio (C/Na=12) is also observed for dodecylated graphenes. At this charge ratio, the (002) diffraction peak of the dodecylated graphene is almost absent (
Graphene that has been pre-exfoliated, for example by treatment in the presence of surfactant under high shear (Coleman et al, Nature Materials, 13, 624-630), can be reduced and dispersed in a one pot method by sonication in solutions of sodium naphthalide in DMAc. In an inert environment, 20 mg of graphene was added to 20 ml of 12.5 mM sodium naphthalide (1:1 Na/C10H8) in DMAc and bath sonicated for 30 min. The solution was then centrifuged at 10,000 g for 30 min to sediment the undissolved fraction, and the solution was pipetted off by hand.
The pre-exfoliated graphene can also be functionalised. For example, in an inert environment, 20 mg of graphene was added to 20 ml of 12.5 mM sodium naphthalide (1:1 Na/C10H8) in DMAc or THF and bath sonicated for 30 min. 1-iodododecane (250 mg) was added to the solution and stirred overnight with a glass stirrer bar. The material was filtered and washed with THF, ethanol and water. The solubility of the functionalized graphene was assessed by UV-vis. The samples were prepared as follows: the functionalised graphene was bath sonicated for 30 min in chloroform at an initial concentration of 0.5 mg/ml and lightly centrifuged (1000 g, 5 min).
The absorbances and concentrations of the reductively dodecylated and as received graphene dispered in chloroform in table 1. Graphene concentrations were calculated from ε600=2460 L g−1 m−1 (Y. Hernandez et al., Nat. Nanotechnol., 2008, 3, 563) with 10 mm pathlength
Lumps of sodium and dried naphthalide powder in one to one molar ratio were stirred overnight with a glass stirrer bar in dried N,N-dimethylacetamide to form a bulk 1 mg Na ml−1 sodium naphthalide solution. Within minutes, a dark green solution (indicative of the formation of a naphthalide radical anion) is formed which is stable for over a month under inert atmosphere. The solution is diluted down to the desired concentration and poured over dried single walled carbon nanotube (SWCNT) powder and stirred overnight. Within 30 minutes, dissolution of the SWCNTs is evident from the solution turning black and after overnight stirring an increase in viscosity is seen (if solutions of >1 mg SWCNT/ml are being created). The solutions were centrifuged at 10,000 g for 30 min in PTFE centrifuge tubes before pipetting off the SWCNT solution.
Dry and degassed dodecyl bromide (3 eq. vs. Na) was added to the reduced SWCNT solution and stirred overnight. The solution was quenched with dry oxygen, filtered under vacuum and washed with THF, water and acetone.
The sodium naphthalide/DMAc solution can lead to higher concentrations by starting with a higher loading of nanotubes. By starting with 5.5 mg/ml of SWCNTs and charging at 10:1 C/Na, a solution of 5.1 mg/ml was obtained after centrifugation.
A bulk solution of sodium naphthalide (NaNp) in DMAc was prepared by stirring sodium (50 mg) and naphthalene (278 mg) in DMAc (50 ml) using a glass stirrer bar. For high levels of charging at high SWCNT loadings, higher concentration solutions may be necessary. Sodium naphthalide solutions were used within a week of preparation. For a 1 mg ml−1 loading of SWCNTs charged to a C:Na ratio of 10:1, 19.2 ml of 1 mg(Na) ml−1 sodium naphthalide was diluted to 100 ml added to SWCNTs (100 mg). The mixture was stirred with a glass stirrer bar overnight before pipetting into fluorinated ethylene propylene (FEP) centrifuge tubes with PTFE tape sealing the cap thread and centrifuging at 10,000 g for 30 min and solutions were then pipetted off by hand. Concentrations were measured by quenching 10 ml of solution by bubbling with dry oxygen for ˜20 min and filtering over a tared 100 nm pore PTFE membrane and washing with copious ethanol, DI water, and acetone ensuring the sample did not dry out between washings. The sample was then dried at 150° C. for 3 hours and weighed. A membrane put through this procedure using 10 ml of DMAc in lieu of the SWCNT solution returned the tared weight.
Nanotubide solubilising solvents include tertiary/cyclic amides, most commonly N,N-dimethylformamide (DMF), N-menthyl-2-pyrrolidone (NMP), and N-cyclohexyl-2-pyrrolidone (CHP). Sodium naphthalide (NaNp) was selected as the reductant as it can be easily visually identified by a characteristic green colour. When equimolar sodium and naphthalene or presynthesised NaNp crystals were stirred into the common nanotubide solvents at 1 mg (Na) ml−1, the solvents turned yellow/orange, increased in viscosity and the UV-vis spectra of these solutions did not show napthalide's characteristic double peak (762 nm and 827 nm in THF,
Stirring of dried SWCNT powder into a solution of sodium naphthalide in DMAc (NaNp/DMAc) led to rapid dissolution with a black solution forming within minutes. At a set charging ratio, here C/Na=20:1 (
At higher degrees of charge (i.e. lower C/Na ratios), the limiting solubility occurs at higher concentrations (
A solution of reduced SWCNTs (10 mg SWCNT) was diluted with DMAc to the desired concentration. Alkyl halide (1 molar eq. vs. Na) was added to the solution and stirred overnight with a glass stirrer bar before bubbling with dry oxygen for ˜20 min and filtering over a 100 nm pore PTFE membrane and washing with copious ethanol, DI water, acetone ensuring the sample did not dry out between washings.
The reaction between nanotubide and alkyl halide is initiated by reduction of the alkyl halide, so it would be expected that a compound that can more easily form this radical anion transition state would be more easily reduced and facilitate a greater degree of grafting. To demonstrate this, the functionalisation of the 1-halide octanes (X=F, Cl, Br, I) with 20:1 charged NaNp/DMAc reduced SWCNTs was carried out by simple addition of the alkyl halide into the nanotubide solution, stirring overnight, filtering and washing. Grafting was calculated via TGA in N2, and uncharged SWCNTs mixed with alkyl halides returned TGAs with no weight loss identical to the as-received SWCNTs. The increasing polarisability of F<Cl<Br<I dictates the stability of the radical anion formed after reduction by nanotubide and this trend is reflected in the grafting efficiency of the 1-halide octanes with C8H17I leading to the highest degree of grafting (lowest SWCNT/alkyl) while C8H17F gives a very low grafting ratio.
The sterics of the alkyl radical formed from the decomposition of the reduced alkyl halide can also be seen to impact of the effectiveness of the functionalisation reaction; as the reactivity of n-alkyl radicals do not vary significantly any change in grafting ratio when varying alkyl length is attributed primarily to sterics. By grafting a series of linear alkyl bromides (CnH2n+1Br where n=4, 6, 8, 12, 16) to 20:1 charged NaNp/DMAc reduced SWCNTs, a linear decrease in grafting can be seen with increasing alkyl length. As the radius of gyration of linear chains is proportional to the number of bonds in the chain, this linear trend can be expected as a result of volume exclusion on the SWCNT surface.
Even with a highly effective leaving group or a short chain, the grafting ratio remains below 20:1 which would correspond with 100% utilisation of the 20:1 charge used to reduce the system. While some charge will be lost creating radicals which react to form dimers, previous work has demonstrated that the quantity of dimer produced is small, and it is more likely that as the nanotubide loses charge during the reaction and its Fermi level drops, it is eventually too low to reduce the alkyl halide. This process has been demonstrated previously with analogous reduced graphene sheets being unable to reduce certain metal salts unless sufficiently charged (Hodge et al, Faraday Discussions, 2014).
Altering the degree of charge on the nanotubide by varying the C/Na ratio also impacts the degree of grafting. At low charge ratios, there is less charge available for functionalisation and functionalisation is low, however at high levels of charge the degree of grafting also decreases (
Premade sodium naphthalide solution in DMAc was added to SWCNTs as above, but solutions were left unmoved for 48 h (unless stated). The mixture was pipetted into FEP centrifuge tubes with PTFE tape sealing the cap thread and centrifuging at 10,000 g for 30 min and solutions were then pipetted off by hand. Filtrate concentrations were measured by quenching 10 ml of filtrate by bubbling with dry oxygen for ˜20 min and filtering over a tarred 100 nm pore PTFE membrane and washing with copious ethanol, DI water, and acetone ensuring the sample did not dry out between washings. The sample was then dried at 150° C. for 3 hours and weighed. Residual purified nanotubes were exposed to a dry oxygen environment overnight and washed by stirring in ethanol, water and acetone, filtering over 100 nm PTFE membranes between steps. It should be noted that some impurities (e.g. residual catalyst particles) may be smaller than the membrane pore size thus the yields should be treated as a lower bound.
Carbonaceous and residual catalyst particles are substantially smaller than SWCNTs and can dissolve unimpeded once reduced, whilst charged SWCNTs can only enter solution after reptation from the entangled bulk of nanotubes.
During reductive purification with NaNp/DMAc, the system potential was fixed by using a fixed stoichiometry of sodium to SWCNT. The NaNp/DMAc solution was simply poured over the dried SWCNTs and to prevent untangling of the SWCNTs and maximising the kinetic difference in dissolution of impurities and SWCNTs, the solutions were not stirred. Increasing charge can be seen to lead to increasing dissolution, but a lack of stirring led to substantially low levels of dissolution.
At charging levels >30:1 C/Na, under 5% weight of the material is dissolved. The increases in the G/D peak intensity in the purified SWCNT Raman spectra (
In a further example, Thomas Swan Elicarb P929 was purified using the following method. A sample of sodium was taken and weighed and added to an equimolar quantity of naphthalene. Anhydrous N,N-dimethylacetamide (1 ml/mg(Na)) was added and the solution was mixed with a glass stirrer bar for 16 h. 10 ml of this solution was diluted to 100 ml with anhydrous N,N-dimethylacetamide and added to 156.5 mg of dry SWCNT (1/30 molar ratio assuming SWCNT to consist entirely of carbon). The mixture was left undisturbed for 24 h before being pippetted into FEP centrifuge tubes and centrifuges at 10,000 g for 30 min. The supernatant was removed by hand to leave the purified SWCNTs which were quenched by dry oxygen and washed with copious water, ethanol and THF over a PTFE membrane. In this example, a large jump in dissolved fraction is seen between C/Na of 30:1 and 20:1 (
In another example, OCSiAl's Tuball SWCNT were purified using the following method. In inert atmosphere, a premade solution of sodium naphthalide in DMAc (9.6 mg sodium, 53 mg naphthalene, 50 ml DMAc) was added over 50 mg of raw Tuball SWCNT powder and left for 24 hours. The mixture was centrifuged at 10,000 g and the solution was pipetted off by hand. The residual nanotubes were quenched with dry oxygen and washed with ethanol, acetone and water.
Raman D/G peak ratios for different purification methods are set out in
Sodium (5 mg) and naphthalene (28 mg) were added to 5 ml of solvent and stirred with a glass stirrer bar for 24 h. For preprepared sodium naphthalide tests, sodium (5 mg) and naphthalene (56 mg) were added to THF (10 ml) and stirred overnight with a glass stirrer bar. Excess naphthalene was used to ensure no metallic sodium was present. The THF was then distilled off (66° C., 1.005 bar), the crystals were allowed to cool to RT and solvent (5 ml) was added and stirred overnight with a glass stirrer bar.
UV-vis spectra were taken with a Perkin Elmer Lambda 450 with an integration time of 0.5 s in an optical glass cuvette with a pathlength of 4 mm and screw-top lid that with PTFE tape sealing the thread for air sensitive samples. When A >1.0, the sample was diluted 10× and spectra values were multiplied by 10. SEM and EDX were taken using a Leo Gemini 1525 FEGSEM at an accelerating voltage of 10 keV and 20 keV respectively. TEM were taken with a JEOL 2000 with an accelerating voltage of 100 keV. Raman spectra were taken with a ISA Jobin Yvon SPEX Raman exciting with a 25 mW 632 nm laser. TGA measurements were taken with a Perkin Elmer Pyris 1 under N2 at 60 ml min−1 holding for 60 min at 100° C. before increasing the temperature at 10° C. min−1 to 800° C.
It should be appreciated that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications, which can be made without department from the spirit and scope of the invention, fall within the scope of the invention.
| Number | Date | Country | Kind |
|---|---|---|---|
| 1320143.9 | Nov 2013 | GB | national |
| 1320829.3 | Nov 2013 | GB | national |
| Filing Document | Filing Date | Country | Kind |
|---|---|---|---|
| PCT/EP2014/074686 | 11/14/2014 | WO | 00 |
