WO2012056184A2

WO2012056184A2 - Method of purifying carbon nanotubes

Info

Publication number: WO2012056184A2
Application number: PCT/FR2011/052519
Authority: WO
Inventors: Guillaume Mercier; Jean-François MARECHE; Sébastien CAHEN; Claire Herold; Brigitte Vigolo
Original assignee: Centre National De La Recherche Scientifique - Cnrs; Universite De Nancy 1 Henri Poincare; Institut National Polytechnique De Lorraine
Priority date: 2010-10-28
Filing date: 2011-10-27
Publication date: 2012-05-03
Also published as: WO2012056184A3; FR2966815B1; FR2966815A1

Abstract

The present invention relates to a process for purifying carbon nanotubes and applications thereof, especially for the manufacture of composites, of electronic components and of biomaterials. The process of the invention is characterized in that it comprises a step of treating a sample of carbon nanotubes in the presence of dihalogen gas and/or an interhalogen gas at 200-2000°C, and in the absence of H₂O and HCl, without a separate oxidation step and without prior exfoliation of the carbon-based multilayer shells in the sample. The present invention also relates to a device for purifying carbon nanotubes comprising: - a system for introducing a dihalogen gas and/or an interhalogen gas, - a thermal compartment in which temperatures of 200-2000°C may be used, and a system that makes it possible to control the diffusion of the dihalogen and/or interhalogen gas into the carbon-based layers of the samples of nanotubes to be purified, selected from a system for purifying the dihalogen and/or interhalogen gas, a system for controlling the concentration of the dihalogen and/or interhalogen gas, a system for controlling the pressure of the dihalogen and/or interhalogen gas, a system for controlling the flow rate of the dihalogen and/or interhalogen gas, or any combination of two or more of the aforementioned systems.

Description

METHOD OF PURIFYING CARBON NANOTUBES

DESCRIPTION

Technical Field The present invention relates to a process for purifying carbon nanotubes and its applications, in particular for the manufacture of composites, electronic components and biomaterials.

The present invention also relates to a device for purifying carbon nanotubes. In the description below, references in brackets ([]) refer to the list of references after the examples.

State of the art

Carbon nanotubes ("NTCs") are self-assembled nanostructures composed of rolled graphene sheets (Ijima, Nature, 1991, 354, 56-58 [ref 1]). Nanostructures are called single-walled carbon nanotubes (SWNTs) if they are composed of a single cylindrical tube (lijima et al., Nature 1993, 363, 603-605 [ref 2]). and Bethune et al., Nature 1993, 363, 605-607 [ref. 3]). NTCs comprising two or more concentric tubes are called double-walled carbon nanotubes (DWNTs) and multi-walled carbon nanotubes (MWNTs), respectively. Since their discovery in 1991, experimental and theoretical studies on carbon nanotubes, especially single-wall nanotubes (SWNTs), have shown that they have physical properties superior to those of many materials. Their geometry is first and foremost remarkable. Their diameter is of the order of one nanometer (about 0.4 to 3 nm), while their length can reach ten millimeters. Nanotubes are also very good conductors of electrons and heat. They can thus be considered as molecular size threads. Their incorporation into electronic devices is one of the major challenges in this area. From a mechanical point of view, the NTCs have both excellent stiffness (measured by the Young's modulus), comparable to that of steel, while being extremely light. In addition, they have excellent electrical and thermal conductivity properties that allow to consider using them as additives to transfer these properties to various materials, including macromolecular, such as polyamides, polycarbonates, polyesters, polystyrene, polyethylether ketones and polyethylene imine. Thus, nanotubes have mechanical properties that, coupled with their lightness, make them ideal objects to achieve high performance nano composites.

For example, carbon nanotubes are commonly used in composite materials (Schaffer, MSP, Windle, AH, "Manufacturing and Characterization of Carbon Nanotube / Polyvinyl alcohol Composites", Adv Mater., 1 1, pp 937- 941 (1999) [ref. 4]), hydrogen fuel cells (Ye, Y., Ahn, C.C., Witham, C., Fultz, B., Liu, J., Rinzler, AG, Colbert, D. , Smith, KA, Smalley, RE, "Hydrogen Absorption and Cohesive Energy of Single-Walled Carbon Nanotubes", Phys.

74, pp 307-2309 (1999) [ref 5]; Liu, C, Fan, YY, Liu, M., Cong, HT, Cheng, HM, Dresselhaus, MS, "Hydrogen Storage In Single-Walled Carbon Nanotubes At Room Temperature", Science, 286, pp 1,127-1,129 ( 1999) [ref 6]; Kong, J., Chapline, MG, Dai, H., "Functionalized Carbon Nanotubes For Molecular Hydrogen Sensors", Adv. Mater. 13, 1384-1386 (2001) [ref 7]), supercapacitors (Aldissi, M., Schmitz, B., Lazaro, E., Bhamidipati, M. Dixon, B., "Conducting Polymers In Ultracapacitor Applications", 56 ^th Annu. Conf.-Soc., Plast Eng., (Vol 2), pp 1 197-1201 (1998) [ref 8], An, KH, Kim, WS, Park, YS, Moon, J.-M .; Bae, DJ, Lim, S.C., Lee, YS, Lee, YH, "Electrochemical Properties of High-Power Supercapacitors Using Single-Walled Carbon Nanotube Electrodes", Adv Funct, Mater 1 1, pp 387-392 (2001). [ref 9]), catalysis (Yu, R., Chen, L., Liu, Q., Lin, J., Tan, K.-L., Ng, S.C., Chan, HSO, Xu, G. -Q., Hor, TSA "Platinum Deposition On Carbon Nanotubes Via Chemical Modification", Chem Mater 10, pp 718-722 (1998) [ref 10], Planeix, JM, Coustel, N. Coq, B .; Brotons, V. Kumbhar, PS, Dutartre, R. Geneste, P. Bernier, P. Ajayan, PM, "Application of Carbon Nanotubes as Supports in Heterogeneous Catalysis," J. Am., Chem. pp 7935-7936 (1994) [ref 1 1]) and n-sized electronic components or systems anometric (Tans, SJ, Verschueren, ARM, Dekker, C, "Room-Temperature Transistor Based On Single Carbon Nanotube", Nature 393, pp 49-52 (1998) [ref 12]; Bachtold, A .; Hadley, P .; Nakanishi, T .; Dekker, C, "Logic Circuits With Carbon Nanotube Transistors". Science 294 pp 1317-1320 (2001) [ref 13]). However very few devices based on carbon nanotubes have emerged so far. Indeed, one of the major obstacles to their use remains the presence of impurities in the samples.

The methods of production of carbon nanotubes have largely developed in recent years. The samples thus produced, however, remain heterogeneous both in terms of the presence of impurities (metal catalyst residues and carbonaceous particles other than nanotubes) than surface heterogeneities (defects, functions). These heterogeneities are a hindrance to the development of devices and new high-performance materials for applications. Synthetic methods producing nanotubes in 'large quantities', excluding all processes leading to nanotubes (oriented or not) on substrates, can be grouped into two categories: "low temperature" (CVD) methods and "high temperature" methods (laser ablation, electric arc).

For the first category, the carbon input is provided by a gaseous mixture containing molecules of the alkane, alkene, alcohol type, etc., brought to a temperature between 500 and 800 ° C. The preparation of carbon nanotubes by decomposition of hydrocarbons or oxygen compounds in the presence of supported transition metals has been reported in the literature. The most studied route is the catalytic decomposition of methane mainly on iron-type catalyst (Muradov et al., Int.J. Hydrogen Energy, 1993, 18: 2112-15 [ref 14]), or nickel (Ishihara et al. Chem Lett, 1995, 2: 93-94 [ref 15]). For the second, the nanotubes are produced from graphite which is sublimated at temperatures up to 6000 ° C. The arcing method uses two graphite electrodes between which an electric arc is established, the anode is consumed to form a plasma whose temperature can reach 6000 ° C (Ijima, [ref 1]); Ebbeson, TW, Ajayan, PM "Large Scale Synthesis Of Carbon Nanotubes", Nature 358, pp 220-222 (1992) [ref 16]). Laser ablation involves exposing a graphite target to laser radiation, high energy, pulsed or continuous. Graphite is either vaporized or expelled into small fragments of a few atoms (Saito et al., Chem Phys Lett, 1995, 236: 419-426 [ref 17]); Thess, A .; Lee, R .; Nikolaev, P .; Dai, H .; Petit, P .; Robert, J .; Xu, C; Lee, YH; Kim, SG; Rinzler, AG; Colbert, DT; Scuseria, GE; Tomanek, D .; Fischer, JE; _Smalley, RE, "Crystalline Ropes Of Metallic Carbon Nanotubes". Science 273, pp 483-487 (1996) [ref 18]). In both cases (low and high temperature methods), the preparation of single-walled nanotubes requires the presence of metal catalysts. In addition, nanotubes from synthetic methods performed at high temperature have the advantage of having a narrow diameter distribution and few defects on their walls. Therefore, the development of purification procedures for this type of samples, particularly those synthesized by electric arc, is of major interest. In this case, the catalysts used are most often a nickel-yttrium mixture in a 4 to 1 atomic ratio. Although efforts have been made to intensify the production of these materials, all currently known synthetic methods result in the presence of large amounts of impurities in the product. For example, carbon-coated metal residues typically represent 20-30% by weight of carbon nanotubes produced by the HiPco method ("High-Pressure CO Conversion") (Nikolaev et al., Chemical Physics Letters, 1999, 313, 91 -97 [ref 19]), and about 60% of carbonaceous material (ie, non-nanotube) is formed in the electric arc method. Both metal and carbon impurities can compromise the performance of carbon nanotubes in many applications.

Most current methods for purifying carbon nanotubes are based on at least one of the following two steps: (1) removing the carbon layer that encapsulates the metal catalysts by oxidation with O2, CO2 or water vapor at temperatures of 300 to 800 ° C (e.g., Chiang et al., J. Phys Chem B 2001, 105, 115-

1 161 [ref 20]; Chiang et al., J. Phys. Chem. B 2001, 105, 8297-8301 [ref 21]), or by chemical wet oxidation with oxidants, including HNO3, H2O2 or KMnO _4, in which the sonication is frequently used (e.g., Liu et al Science, 1998, 280, 1253-1256 [ref 22]), and (2) using centrifugation or filtration for separate carbon nanotubes from soluble impurities (e.g., Bandow et al., J. Phys Chem B, 1997, 101, 8839-8842 [ref 23]).

Most often, the purification procedures generally proposed follow at least three steps. The first consists of an oxidation of the sample in order to weaken the protective carbon shells of metals to make the latter more accessible to chemical treatments. The oxidation is generally carried out under a gaseous flow containing oxygen, for example air, at temperatures below 500 ° C. If this treatment makes it possible to oxidize the less protected metal particles, it also leads to an attack on the walls of the nanotubes. Thus oxygen functions can be grafted onto the nanotubes, which creates defects in their structure. The second step is to remove the metal oxides that can be solubilized for example in acidic solutions. A high temperature heat treatment (1,100-1600 ° C) is then necessary to eliminate the functions introduced during steps 1 and 2 and restore the crystallinity of the nanotubes.

Although these methods provide better samples, they have a number of limitations. They are highly sample consuming: on average 90% of the sample is lost at the end of the three stages (see Cho et al [ref 48]). These multi-step procedures are also relatively cumbersome to implement. The experimental conditions used at each stage must be optimized to improve the final yield. As a result, the number of experimental parameters is high, which makes it difficult to optimize the procedure. Moreover, they contain in all cases an initial oxidation step which inexorably attacks the walls of the nanotubes. So we lose some of the nanotubes and we introduce unwanted functions on the walls of the remaining part. These functions play the role of defects in the structure of the nanotubes. The final high temperature treatment is not totally effective in eliminating these functions. Even though the oxidation of metals occurs, it only leads to their partial elimination. Indeed, certain functions protected by the multi-layer hulls remain inaccessible to the treatments.

Therefore, an efficient purification process on an industrial scale to remove these impurities is essential, since most applications of carbon nanotubes require highly purified carbon nanotubes.

There is therefore a real need for a process for purifying carbon nanotubes to overcome these defects, disadvantages and obstacles of the prior art, given the fact that many applications based on carbon nanotubes require high purity NTCs. .

Description of the invention

The present invention is specifically intended to meet this need by providing a method of purifying carbon nanotubes comprising a nanotube treatment step in the presence of dihalogen gas and / or interhalogen.

By "dihalogen gas" is meant herein a gas of general formula X ₂ wherein X is F, Cl, Br or I.

By "interhalogen gas" is meant herein a gas of the general formula XY _n wherein X and Y each represent a different halogen atom, n represents 1, 3, 5 or 7 and X is the least electronegative of both halogens. It may be, for example, diatomic interhalogen gas (such as CIF, BrCl, ICI or IBr, for example), tetraatomic (such as CIF ₃ or BrF ₃ for example), hexaatomic (such as CIF ₅ , BrF ₅ or IF ₅ for example), or octaatomic (such as IF ₇ ).

An important aspect of the process of the invention is that it does not require a prior oxidation step such as those used in the processes of the prior art. Another important aspect of the process of the invention is that it does not require a separate oxidation step such as those used in the prior art processes, i.e. a step where the nanotubes are treated with an oxidizing agent such as O2, CO2 and / or H ₂ O, independently of the treatment with dihalogenic gas and / or interhalogen. Typically, in the processes of the prior art, this oxidation step aims at (i) converting metallic catalytic impurities into metal oxides or metal hydroxides, and / or (ii) removing amorphous carbon which can be present in the carbon nanotube sample. Typically, in the processes of the prior art, this oxidation step is carried out by treating the nanotubes in an oxidizing atmosphere, preferably comprising dioxygen or carbon dioxide, preferably at a temperature> 100 ° C., for example from 200 to 500 ° C.

Thus, in one aspect, the method for purifying carbon nanotubes according to the invention comprises a step of treating the nanotubes in the presence of dihalogen and / or interhalogen gas, without prior oxidation step, in particular aimed at (i) converting metallic catalytic impurities to metal oxides or metal hydroxides, and / or (ii) removing the amorphous carbon that may be present in the nanotube sample by producing carbon oxides.

According to another aspect, the process for purifying carbon nanotubes according to the invention comprises a step of treating the nanotubes in the presence of dihalogen and / or interhalogen gas, without a separate oxidation step, in particular aimed at (i) converting catalytic impurities to metal character of metal oxides or metal hydroxides; and / or (ii) removing the amorphous carbon that may be present in the nanotube sample by producing carbon oxides.

Whatever the embodiment, in the nanotube treatment step, the temperature will depend on the catalytic impurity to be extracted (nature of the metal or metals) and the dihalogen and / or interhalogen gas used. For example, the temperature may be between -20 ° C and 2000 ° C, for example between 0 and 2000 ° C, for example between 100 and 2000 ° C, for example between 200 and 2000 ° C, for example between 300 and 2000 ° C, for example between 700 and 2000 ° C, for example between 800 and 2000 ° C, for example between 850 and 2000 ° C, for example between 900 and 2000 ° C, for example between 950 and 2000 ° C, by example between 1000 and 2000 ° C, for example between 1050 and 2000 ° C. The lower temperature limit may be lower than -20 ° C, depending on the catalytic impurities to be extracted and the dihalogen and / or interhalogen gas used. In this case, the treatment conditions may be adapted to favor the purification process. For example, the treatment time of the nanotubes in the presence of dihalogen and / or interhalogen gas may be longer. Similarly, the upper temperature limit that may be used may be greater than 2000 ° C. This depends in particular on the limits of the thermal compartment (e.g., oven) used.

For example, for the catalytic impurities based on iron (eg, single-walled carbon nanotubes synthesized by the HiPCo method, acronym for "High Pressure carbon monoxide"), a temperature around 200 to 300 ° C may for example be used. Of course, the treatment of carbon nanotubes containing iron-based catalytic impurities (such as HiPCo nanotubes) can be carried out at higher temperatures, for example in the temperature ranges given above. For example, the temperature may be between 200 and 2000 ° C, for example between 300 and 2000 ° C. According to one embodiment, the step of treating the nanotubes is carried out at high temperature, for example at a temperature> 100 ° C. The temperature may be, for example between 200 and 2000 ° C, for example between 300 and 2000 ° C, for example between 700 and 2000 ° C, for example between 800 and 2000 ° C, for example between 850 and 2000 ° C, for example between 900 and 2000 ° C, for example between 950 and 2000 ° C, for example between 1000 and 2000 ° C, for example between 1050 and 2000 ° C. As discussed above, the upper temperature limit that may be used may be greater than 2000 ° C. This depends in particular on the limits of the thermal compartment (eg, oven) used.

According to one embodiment, for the process according to the invention, the treatment temperature of the nanotubes can be> 850 ° C. and can be up to the upper limit of temperature accessible by the thermal compartment in which the carbon nanotubes are contained. to purify. Thus, in one embodiment, in the nanotube treatment step, the temperature can be between 850 ° C <T≤2000 ° C, preferably 900 ° C <T <2000 ° C, for example 900 ° C <T <1800 ° C, for example 900 ° C <T <1700 ° C, for example 900 ° C <T <1600 ° C, for example 900 ° C <T <1500 ° C, for example 900 ° C <T <1400 ° C, for example 900 ° C <T <1300 ° C, for example 900 ° C <T <1200 ° C, for example 900 ° C <T <1100 ° C.

The process eliminates catalytic impurities polluting carbon nanotube samples in a single chemical treatment. Carbon nanotube samples contain non-carbon impurities that inhibit the use of nanotubes for many applications. Their presence is indeed not desirable because they can induce significant changes or even mask the chemical and physical properties of nanotubes. It is for example reported that these particles can catalyze certain parasitic reactions. Moreover, these catalytic impurities have themselves chemical and physical properties different from those of nanotubes. For example, the signal of the metals contained in the catalytic impurities can very strongly disturb the response of the carbon nanotubes to an electrical or magnetic stress. By "catalytic impurity" is meant herein the non-carbon impurities contained in carbon nanotubes from metal catalysts that can be used in the synthesis of nanotubes, including SWNTs. The catalytic impurities targeted by the process according to the invention are any impurity originating from metal catalysts that can be used in the synthesis of carbon nanotubes, in particular SWNTs. For example, they may be catalytic metal-based impurities, such as transition metals, lanthanides or alkaline earths. By way of example, without being limiting, mention may be made of chromium (Cr), molybdenum (Mo), tungsten (W), iron (Fe), cobalt (Co), nickel (Ni) , ruthenium (Ru), rhodium (Rh), palladium (Pd), osmium (Os), iridium (Ir), platinum (Pt), vanadium (V), yt Y), magnesium (Mg), titanium (Ti), gold (Au), zirconium (Zr), lanthanides such as lanthanum (La) or holmium (Ho), and their alloys, or a mixture of these. Most often, it is catalytic impurities based on nickel, yttrium, iron, tungsten, molybdenum, palladium or cobalt, or a mixture thereof. The catalytic impurities may be in the form of pure metals, metal oxides, metal carbides, metal nitrates or other compounds containing the metal element. Preferably, the treatment of the nanotubes is not preceded by a step of exfoliation of the multilayer shells empty or filled with catalytic impurities in the sample (that is to say in the sample of carbon nanotubes), in particular carbonaceous multilayer shells. Typically, in the state of the art, this pre-exfoliation step of the sample aims to remove at least a portion of the shells multilayer carbon impurities contained in the carbon nanotube samples, in order to make available to the treatment metal impurities previously encapsulated in these multilayer shells. In the state of the art, this exfoliation was considered necessary to allow the elimination of the metal impurities encapsulated by the multilayer hulls of the carbonaceous impurities of the samples of carbon nanotubes. Generally, such an exfoliation is carried out by intercalation of an element such as potassium between the carbonaceous layers. Removal of exfoliated carbon species is generally achieved by chemical transformation methods (eg, wet oxidation and dissolution, filtration (eg, cross-flow or microfiltration methods), and chromatographic methods (eg, exclusion chromatography) The major disadvantage of these methods is that they lead to the destruction or loss of a part of the sample. Exfoliation is not necessary or even unnecessary.

Thus, according to any one of the embodiments, the process for purifying carbon nanotubes according to the invention may comprise a step of treating a sample of nanotubes in the presence of dihalogen and / or interhalogen gas, without exfoliation. preliminary multilayer hulls empty or filled with catalytic impurities in the sample, including carbonaceous multilayer hulls. For example, the process may comprise a step of treating the nanotubes in the presence of dihalogen and / or interhalogen gas, between -20 ° C. and 2000 ° C., for example between 0 and 2000 ° C., for example between 100 and 2000. ° C, for example between 200 and 2000 ° C, for example between 300 and 2000 ° C, for example between 700 and 2000 ° C, for example between 800 and 2000 ° C, for example between 850 and 2000 ° C, for example between 900 and 2000 ° C, for example between 950 and 2000 ° C, for example between 1000 and 2000 ° C, for example between 1050 and 2000 ° C, without prior exfoliation of the multilayer shells empty or filled with catalytic impurities in the sample, in particular carbonaceous multilayer shells.

According to any one of the embodiments, the nanotube treatment step can be carried out at high temperature, and the method can comprise a nanotube treatment step in the presence of dihalogen and / or interhalogen gas, for example a temperature> 100 ° C. For example, the temperature may be between 200 and 2000 ° C, for example between 300 and 2000 ° C, for example between 700 and 2000 ° C, for example between 800 and 2000 ° C, for example between 850 and 2000 ° C, for example between 900 and 2000 ° C, for example between 950 and 2000 ° C, for example between 1000 and 2000 ° C, for example between 1050 and 2000 ° C. As discussed above, the upper temperature limit that may be used may be greater than 2000 ° C. This depends in particular on the limits of the thermal compartment (eg, oven) used. For example, the nanotubes treatment step may be performed at 200 ^o C≤T <2000 ° C, 300 ° C <T <2000 ° C, preferably 850 ° C <T <2000 ° C, preferably 900 ^o C≤T <2000 ° C, without prior exfoliation of the multilayer shells empty or filled with catalytic impurities in the sample, especially carbonaceous multilayer shells.

According to any one of the embodiments, the nanotubes can be processed in a continuous stream of dihalogenated and / or interhalogenated gas or in a static system.

According to any one of the preceding embodiments, the dihalogen gas may be F ₂ , Cl ₂ , Br ₂ or. For example, it may be CI2.

The dihalogen gas can come from a commercial source, for example in the form of bottles of F ₂ or CI ₂ gas. For bromine, the Br ₂ vapors can be obtained from liquid bromine which is commercially available, optionally by moderately heating for increase the flow of Br ₂ gas according to the desired flow rate. Similarly, the vapors can be obtained from solid diiodide which is available on the market and sublimates spontaneously at 20-25 ° C. Optionally, it can be moderately heated to increase the flow of gas at the desired rate.

Alternatively, the dihalogen gas can be prepared "in situ" in the nanotube purification device. Any method compatible with a continuous flow device comprising a thermal compartment (e.g., furnace) for the treatment of nanotubes can be used. For example, the following methods may be mentioned:

Difluor:

Electrolysis at 100 ° C of a molten electrolytic bath composed of an anhydrous liquid mixture of potassium fluoride (KF) and hydrogen fluoride (HF). Dichlore: a. Addition of a concentrated aqueous solution of HCl to KMnO ₄ crystals (see Example 1). b. Electrolysis of NaCl

For example, it may be an electrolysis on platinum anode and nickel cathode, molten sodium chloride. It is also possible to mention the techniques of the chlorine industry (ie, the industry that produces chlorine (C) and sodium hydroxide (NaOH) or potassium hydroxide (KOH), by electrolysis of a salt solution) . The main techniques used to produce chlor-alkali are electrolysis in mercury, diaphragm or membrane cells, which mainly use sodium chloride (NaCl) as a feedstock or, to a lesser extent, chloride. of potassium to produce potassium hydroxide. For example, we can use the electrolysis of sodium chloride dissolved in water, for example on graphite anode and an iron cathode, or on graphite anode and a mercury cathode. vs. Addition of a concentrated aqueous solution of hydrochloric acid on bleach.

Since the purity of the dihalogen or interhalogen gas used in the process of the present invention can influence its activity and therefore the effectiveness of the treatment, it is sought to optimize that of chlorine as follows. Indeed, in certain embodiments where the temperature ranges used can be relatively high (e.g., around 1000 ° C), it is preferable to avoid especially trace water and / or HCI. This is the reason why, as is described in the examples, advantageously, care is taken to place on the flow of the chlorine before it reaches the sample, purification systems, in particular a solution of saturated NaCl for remove any residues of HCI.

In some embodiments, chlorine may be formed by oxidation of HCl with potassium permanganate. As for water, it can be removed by trapping in a CaC column and by the action of concentrated sulfuric acid (36N). Furthermore, the carrier gas (for example N ₂ ) is advantageously ultrapure (for example Alphagaz 2 bottle of N ₂ which contains less than 0.1 ppm of O 2, less than 0.1 ppm of CO, less than 0.1 ppm less than 0.5 ppm H ₂ O and less than 0.1 ppm H ₂ ).

Dibrome a. By action of chlorine gas on a bromide For example, a container of KBr (or NaBr or NH Br) may be placed in a vessel, and dichlor gas gas generated elsewhere is circulated therein. Liquid and vapor dibrom is obtained according to: a. 2 KBr (s) + Cl ₂ (g) → 2 KCl (s) + Br ₂ (l) b. From a bromide and manganese dioxide

For example, a stoichiometric mixture of KBr (or of NaBr) and of MnO 2 can be produced in a two-necked flask equipped with a preferably non-isobaric dropping funnel containing sulfuric acid, a column head with a temperature measurement, and a straight refrigerant with a bent elongate and a collector flask cooled by ice bath (if an isobaric bulb is used, the bromine formed escapes through the isobaric tube before going to the top of the column).

The reaction is carried out according to:

2 KBr (s) + 3 H ₂ SO ₄ (l) + MnO ₂ (s) → MnSO ₄ (aq) + 2 KHSO ₄ (aq) + 2H ₂ O (1) + Br ₂ (l)

The flask may be moderately heated to distill the bromine vapors.

Diiode:

Action of an oxidant, for example hydrogen peroxide, on iodide ions H ₂ 0 ₂ (aq) + 2 r (aq) + 2H ₃ 0 ⁺ (aq) = (aq) + 4H ₂ O ( I)

The interhalogen gas can come from a commercial source. Alternatively, it can be prepared "in situ" in the nanotube purification device by any method available to those skilled in the art. Interhalogens are obtained by reaction between two different halogens. CIF for example can be obtained by reaction activated thermally from CI2 with F ₂ . Table 1 below summarizes the various known interhalogens:

Table 1

The reader may refer for example to the work of Greenwood, Norman N .; Earnshaw, A. (1997), Chemistry of the Elements (2nd ed.), Oxford: Butterworth-Heinemann (ref [47]).

The carbon nanotubes targeted by the present process may be of the single-wall or multi-wall type.

The term "carbon nanotubes" herein has the conventional meaning given to this term in the art, namely a self-assembled nanostructure composed of one or more sheets of graphene wound in a cylinder (s), depending on whether it is single-walled or multi-walled nanotubes. Thus, the nanotubes are in the form of concentric hollow tubes, possibly closed at their ends by carbon pentagons characteristic of fullerenes. The multi-walled nanotubes may be in the form of graphene planes arranged in concentric cylinders ("Russian doll" model), or in the form of a single plane of graphene wound on itself, as a sheet of paper (model "parchment"). The carbon nanotubes may be in the form of bundles or bundles, or in individualized form for example according to the method of Penicaud et al. (see FR 2 864 454 [ref 49], WO 2005/073127 [ref 50], and references 51 (Pénicaud et al.) and 52 (Vigolo et al.)), without however including nanostructures of microfiber type or carbon nanofibres which, unlike carbon nanotubes, do not have a central channel (the carbon microfibers or nanofibers are typically hollow or non-hollow shapes with a diameter generally of> 50 nm whose arrangement of the graphene planes is not cylindrical long distance).

Preferably, the nanotubes are single-walled carbon nanotubes. It is known that single-wall carbon nanotubes have properties superior to those of multiwall carbon nanotubes, which is one of the reasons why they are ideal candidates for their use in many applications. Indeed, the quasi-one-dimensional nature of single-walled nanotubes, lost when the nanotubes have several walls, is particularly at the origin of the superiority of their properties (mechanical and conductivity for example) compared to those of the latter. Single-walled carbon nanotubes therefore have a greater potential for application than multi-walled carbon nanotubes, placing them at the forefront of their interest.

By way of example, the single-wall carbon nanotubes that can be used in the present process may have diameters of 0.4 to 2-3 nm.

By way of example, the multi-walled carbon nanotubes that can be used in the present process may have diameters ranging from 2 to 300 nm or more. By way of example, the carbon nanotubes that can be used in the present process can have a length ranging from 10 nm to 1 millimeter (mm), 1 centimeter (cm), 3 cm, 5 cm or more than 5 cm.

By way of example, their length / diameter ratio may advantageously be greater than 10 and most often greater than 1000.

By way of example, their specific surface area is for example between 20 and 1000 m ² / g and their apparent density may especially be between 0.5 and 1.5 g / cm ³ and more preferably between 1, 2 and 1. , 4 g / cm ³ .

By "bulk density" is meant herein the ratio of the mass of a set composition heterogeneous set of separate substances, possibly stored separately, by the total volume occupied by these substances. For carbon nanotubes according to the invention, the bulk density represents the ratio of the mass of a sample of carbon nanotubes by the total volume occupied by the sample.

By way of example, the multi-walled carbon nanotubes may for example comprise from 5 to 100 sheets and more preferably from 7 to 20 sheets.

The present process is applicable to any source of commercially available NTCs, in particular by any method using one or more metal catalyst (s). An example of crude carbon nanotubes is especially commercially available from Carbolex companies Nanocyl (under the trade name NANOCYL ™ NC 7000), Nanolab, CheapTubes, CNI, or Arkema (under the trade name Graphistrength® ^® C100).

The nanotubes can be in individualized form, or in agglomerated form in bundles or bundles. When the treatment of the nanotubes according to the present process is carried out at a temperature higher than the ambient temperature, that is to say> 25 ° C ± 5 ° C, any heating mode compatible with the process can be used. For example, it may be an oven, a filament resistor, an infrared lamp or a laser.

Similarly, when the nanotube treatment according to the present process is carried out at a temperature below room temperature, ie <25 ° C ± 5 ° C, any method of refrigeration compatible with the process may to be used. For example, it may be an electric refrigeration mode (such as a refrigerator or freezer), or a chemical mode (for example, an acetone / dry ice bath or liquid nitrogen).

According to any one of the preceding embodiments, the treatment of nanotubes according to the present method can be carried out using a conventional heating device such as an oven, for example for 1 minute to 96 hours, for example from 30 minutes to 96 hours, preferably from 1 hour to 72 hours, preferably from 2 hours to 48 hours.

The carbon nanotube treatment step can be carried out at a constant temperature throughout the duration of the treatment. For example, if it is an oven, it can be pre-heated to the desired temperature before the nanotube treatment step.

Alternatively, the carbon nanotube treatment step can be performed with a variable temperature profile during the treatment period, comprising at least one tray at the desired processing temperature. For example, the temperature profile may include a temperature ramp, followed by a plateau at the processing temperature. The temperature profile may also include a temperature ramp down to the recovery temperature nanotubes once the processing step is complete (for example, the temperature can be reduced to 25 ° C ± 5 ° C).

According to any one of the preceding embodiments, the treatment of nanotubes according to the present method can be carried out using a pulsed laser, continuously or quasi-continuously, for example by a series of pulses. Each pulse may be less than 1 second, or even less than 1 nanosecond, or less than 1 picosecond. In the series of puises, the puises can be of the same duration, or of different duration.

It may be a gas laser (for example a CO2 laser sealed or pulsed TEA ("Transversely Excited Atmospheric Pressure CO2 Laser")), a chemical laser, a fiber laser, steam metallic, solid or semiconductor, of a diode-pumped laser (for example a DPSS (Diodes-Pumped Solid Laser)) or lamp-pumped laser, a UV laser, a red or near-infrared laser , or any type of laser that can operate in pulsed mode, continuous or quasi-continuous, capable of generating heat.

Those skilled in the art will be able to select the type of laser, the operating mode (pulsed, continuous or quasi-continuous) and the intensity, the power and the wavelength of the irradiation, depending on the heating conditions. desired.

The process according to the invention is not limited in terms of dihalogen and / or interhalogen gas pressure. It can be implemented both under-pressure, over-pressure and atmospheric pressure. Advantageously, the process is carried out at atmospheric pressure or overpressure in order to promote the diffusion of the dihalogenated and / or inter-halogenated gas into the carbonaceous layers of the nanotubes, and the access to the catalytic impurities not being on the surface carbon species present in the samples of carbon nanotubes (ie, catalytic impurities coated with carbon). According to any one In previous embodiments, the treatment of the nanotubes according to the present process can be carried out at a pressure of 1000 Pa to 1 .10 ⁶ Pa. Advantageously, the treatment of the nanotubes according to the present process can be carried out at a pressure of 101 325 Pa. (1 atm) to 1 .10 ⁶ Pa. Preferably, the treatment of nanotubes according to the present process can be carried out at a pressure of 101 325 Pa (1 atm).

It is known that the presence of too much water (H ₂ O), carbon dioxide (CO2) and / or oxygen (O2) may result in the oxidation of carbon nanotubes and damage. Consequently, according to one embodiment, water, carbon dioxide and oxygen are preferably excluded from the gaseous atmosphere in the contact zone between the dihalogen and / or interhalogen gas and the nanotubes. Usually, the use of a gaseous atmosphere within the purification device comprising less than 5% by weight, more preferably less than 1% by weight of oxidizing compound (or mixture of oxidizing compounds) will be sufficient. For example, a gaseous atmosphere comprising less than 5% by weight, more preferably less than 1% by weight of water, carbon dioxide and / or dioxygen may be used (the% by weight refers to the sum of compounds oxidants (eg, H ₂ O, O ₂ and / or CO2) present in the gaseous atmosphere). Preferably, the amount of water, carbon dioxide and oxygen will be less than 0.1% by weight. Advantageously, the carbon nanotube treatment step can be carried out in the absence of oxidizing compounds such as H ₂ O, O ₂ and / or CO 2. Advantageously, the carbon nanotube treatment step can be carried out in the absence of water.

According to any one of the preceding embodiments, the dihalogen and / or interhalogen gas may be previously dried. For example, the water content of the dihalogen and / or interhalogen gas may be between 0 and 10% vol, for example between 0 and 8% vol, by example between 0 and 5% vol, for example between 0 and 3% vol, for example between 0 and 1% vol.

According to any one of the preceding embodiments, the dihalogenated and / or interhalogenated gas may be pretreated so as to reduce or eliminate traces of oxidizing compounds, such as O 2, CO 2 and / or H ₂ O. For example , the content of O2, CO2 and / or H ₂ O of the dihalogen and / or interhalogen gas may be between 0 and 10% vol, for example between 0 and 8% vol, for example between 0 and 5% vol, for example between 0 and 3% vol, for example between 0 and 1% vol. According to any one of the preceding embodiments, the dihalogen and / or interhalogen gas may be pretreated so as to reduce or even eliminate traces of hydrogen halide. This can be the case for Hl, which breaks down into l ₂ and H ₂ . For example, when the presence of H ₂ in the system is not desired (for example for safety reasons), the Hl content of the dihalogen and / or interhalogen gas can be between 0 and 1%. It can for example be decomposed in a solution of acidified potassium permanganate. He is then solubilized. Ions are oxidized to form the ₂ and H ⁺ reacts with MnO ₄ ^"ions to form Mn ²⁺ and water. For hydrogen halides other than Hl, the presence of hydrogen halides in the dihalogen and / or inter-halogen is a priori not troublesome because these are stable, however, their presence may be undesirable, especially at high temperatures.Thus, advantageously, the carbon nanotube treatment step can be performed in the absence of hydrogen halides In particular, advantageously, the carbon nanotube treatment step can be carried out in the absence of HI, HBr, HCl and / or HF Advantageously, the treatment step carbon nanotubes can be produced in the absence of HI Advantageously, the carbon nanotube treatment step can be carried out in the absence of HCl. Advantageously, the carbon nanotube treatment step can be carried out in the absence of water and HCl.

According to one variant, the process for purifying carbon nanotubes according to the invention may comprise a step of treatment of the nanotubes in the presence of dihalogen gas and / or an interhalogen gas at 200-2000 ° C., and in the absence of H ₂ O and HCl, without a separate step of oxidation and without prior exfoliation of the carbonaceous multilayer shells in the sample (that is to say in the sample of carbon nanotubes). All the embodiments described above (in particular concerning the carbon nanotubes, the temperature range applied, the dihalogenated and / or interhalogenated gas (including its nature, its mode of introduction (eg, continuous or static flow), its origin (eg, commercial or in situ preparation), its flow rate, its concentration, and its purity (eg, O ₂ , CO ₂ and / or H ₂ O content) apply mutatis mutandis to this variant, and so are not repeated here.

According to any one of the preceding embodiments, the dihalogen and / or interhalogen gas may be entrained by another gas. Any gas that does not damage carbon nanotubes and / or is compatible with the present process can act as a flushing gas. Preferably, the flushing gas is an inert gas such as nitrogen, helium, neon, argon, krypton, xenon, or mixtures of two or more of these gases. Preferably, the flushing gas is nitrogen or argon, or a mixture thereof.

By "inert gas" is meant herein a gas or a gaseous mixture which does not react with carbon nanotubes, dihalogen and / or interhalogen. For example, the process is conducted under gaseous atmosphere free of oxygen. In particular, the process can be carried out under an argon or nitrogen atmosphere. According to any one of the preceding embodiments, the volume concentration of the dihalogen gas and / or the interhalogen can be between 0.1% vol and 100% vol, for example between 10 and 100% vol, by example between 30 and 100% vol, for example between 50 and 100% vol, for example between 70 and 100% vol, for example between 80 and 100% vol.

In the previous embodiments, the flow rate of the dihalogen gas and / or the interhalogen can be between 0 (static) and 100 l / min or more. In a dynamic configuration (gas flow), the maximum flow rate of the dihalogen gas and / or interhalogen will be determined so as to avoid the entrainment of nanotubes in the gas stream.

According to a variant of the invention, the process may be adapted to also eliminate, or reduce (ie, decrease the amount), the carbonaceous impurities present in the samples of carbon nanotubes. Surprisingly, the inventors have discovered that this could be accomplished by introducing an oxidizing compound in gaseous form, such as oxygen, together with the dihalogen gas and / or inter-halogen, preferably in concentration ranging between 10 ^"5 Advantageously, the content of oxidizing compound, such as dioxygen, is in the minority relative to the content of dihalogen and / or interhalogen gas, and may be, for example, between 10 ^-3 % vol and 15% by volume. % vol, for example between 10 ^"2 % vol and 10% vol, for example between 10 ^{" 1} % vol and 5% vol, for example between 0.5% vol and 3% vol, for example between 0.5% vol and 1, 5% vol.

Thus, according to any one of the preceding embodiments, it may be advantageous to introduce with the dihalogen gas and / or the interhalogen a certain amount of at least one gas capable of reducing (ie, decreasing the amount of ), or eliminate the carbon impurities contained in the sample of carbon nanotubes. For example, it may be an oxidizing compound (or mixture of oxidizing compounds) such as oxygen, carbon dioxide and / or water vapor. Thus, according to any one of the preceding embodiments, the treatment of the nanotubes may be carried out additionally in the presence of an oxidizing compound (or mixture of oxidizing compounds) such as dioxygen, carbon dioxide and / or steam. 'water. In one embodiment, the volume concentration of oxidizing compound (or mixture of oxidizing compounds) can be between 10 ^"5 % vol and 20% vol, for example between 10 ^{" 3} % vol and 15% vol, for example between 10 ^{~ 2} % vol and 10% vol, for example between 10 ^"1 % vol and 5% vol, for example between 0.5% vol and 3% vol, for example between 0.5% vol and 1, 5% vol The concentration of oxidizing compound used may be adjusted according to the treatment temperature of the carbon nanotubes in the presence of dihalogen and / or interhalogen gas. The higher the treatment temperature, the greater the oxidation of the nanotubes by the compound This oxidation of the nanotubes is of course to be avoided, the concentration of oxidizing compound will therefore be adjusted so as to limit the oxidation of the carbon nanotubes.The concentration in volume of oxidizing compound (or mixture of oxidizing compounds) used will so much lower than the temperature of The treatment of the nanotubes may preferably be carried out in the presence of oxygen, for example in a volume concentration of between 10 ^-5 % vol and 20% vol, for example between 10 ^-3 % vol and 15%. flight, for example between 10 ^"2 % vol and 10% vol, for example between 10 ^{" 1} % vol and 5% vol, for example between 0.5% vol and 3% vol, for example between 0.5% vol and 1, 5% vol. Advantageously, the content of oxidizing compound, such as dioxygen, is minor in relation to the dihalogen and / or interhalogen gas content.

According to any one of the preceding embodiments, the dihalogen gas may be Cl 2. As mentioned above, the oxidation treatments, whether they are carried out in the liquid phase (typically under reflux in HNO3, etc.) or in the gas phase in the presence of O2 or other oxidizing agent, are widely used in the field of CNTs [ ref 34, 55]. In particular, they play an important role in purification procedures because they aim to eliminate by combustion less stable carbon phases than CNTs, namely amorphous or poorly organized impurities. Thus, the methods of purification of conventional NTCs of the prior art, include an independent step of oxidation of NTCs samples most often in the gas phase using oxygen as an oxidizing species, that is to say a step where the nanotubes are treated with an oxidizing agent such as O2, CO2 and / or H ₂ O, independently of any other treatment.

A major disadvantage of these processes is the problem of controlling the selectivity of the combustion towards the carbon phases other than the NTCs. Indeed, the similarities of chemical nature and organization between the NTCs and the carbon impurities present in the samples make it difficult to develop experimental conditions (eg, gas mixture, temperature, duration, etc.) making it possible to obtain combustion selectivity preserving NTCs. Moreover, the temperature range in which CNT combustion occurs inevitably depends on the gaseous mixture used and the treatments undergone before oxidation [ref 56, 57]. From 300 ° C, NTCs can suffer structural damage due to the introduction of defects by the grafting of chemical groups on their surface in particular. When the processing conditions are more aggressive, or the higher temperatures, the removal of NTCs by combustion inevitably occurs. The combustion temperatures of SWNTs commonly observed are generally around 400 ° C and can in some cases reach 600 ° C or 700 ° C [ref 20, 58, 59]. It is therefore particularly difficult to develop experimental conditions for the oxidation treatments in order to direct the selectivity of the combustion towards the carbon phases other than the CNTs. So far, efforts directed in this direction have remained inconclusive and elusive. For example, the purification of SWNTs in the presence of Cl 2 and water as an oxidizing agent has been unsuccessfully tested (see Zimmerman et al., [Ref 60]). In particular, the experimental conditions used, ie temperatures below 500 ° C., do not make it possible to obtain purified nanotubes. In addition, the treatments carried out by Zimmerman et al. under pure dichlor were ineffective under the conditions used for the purification of NTCs [ref 60].

The present invention provides a solution to this problem by providing a method for purifying NTCs involving a nanotube treatment carried out in the presence of a dihalogen or interhalogen gas, and an oxidizing compound (or mixture of oxidizing compounds) such as oxygen, carbon dioxide and / or water vapor. This variant of the process of the invention is illustrated in Examples 7 and 12. Thus, quite unexpectedly, the treatment of the nanotubes in an oxidizing atmosphere according to the process of the invention, in particular at temperatures well above those reported to date in the literature, has led to an efficient purification of carbon nanotubes, with two major advantages: (i) the one-step removal of catalytic impurities and carbonaceous impurities, (ii) by preserving the integrity of the nanotubes (see Examples 7 and 12 where temperatures of the order of 1000 ° C. and 1% vol of O 2 in a mixture

CI2 / N2 (90% vol / 10% vol) were used). This result was unexpected in view of the work of the prior art.

According to any one of the preceding embodiments, it may be advantageous to wash the purified nanotubes with an organic or inorganic solvent and / or with an acid treatment in order to eliminate the residual metal halides formed during the step of treating the nanotubes in the presence of dihalogen and / or interhalogen gas. This washing and / or acid treatment can be carried out with an aqueous solution of an inorganic or organic acid, for example an aqueous solution of HCl, nitric acid or sulfuric acid, hydrofluoric acid or other. They can be used pure or diluted in water at concentrations of preferably 6 to 2 mol / L. The washing and / or acid treatment conditions may be adapted to optimize the solubilization of the residual metal halides in the sample of carbon nanotubes, while avoiding the oxidation of the nanotubes.

According to any one of the preceding embodiments, it may be advantageous to treat the purified nanotubes with an acidic solution in order to rid them of any residual mineral and / or carbonaceous impurities resulting from their preparation process, for example amorphous carbon. , graphite, fullerenes or ceramic support residues (catalytic support) that may be present in carbon nanotube samples. Thus, according to any one of the preceding embodiments, the method according to the invention may further comprise a step of washing the nanotubes by acid treatment. This acid treatment may be carried out with an aqueous solution of an inorganic or organic acid, for example an aqueous solution of HCl, nitric acid or sulfuric acid, hydrofluoric acid or the like. These can be used pure or diluted in water at concentrations of preferably 6 to 2 mol / L. Preferably, the nanotubes are treated with an aqueous solution of an organic or inorganic acid at a concentration sufficiently high to attack the amorphous carbon in a practical period of time, but not high enough for the carbon nanotube material to be attacked. to a significant degree. In the washing operation and / or acid treatment, the weight ratio of the NTCs to the mineral acid may especially be between 1: 2 and 1: 100 or less, limits included.

The washing operation and / or acid treatment may also be carried out at a temperature ranging from -10 to 300 ^° C, preferably from 20 to 25 ° C, for example for a period of 1 min to 48 hours. This operation may advantageously be followed by rinsing steps with water and / or with an aqueous basic solution to return to a neutral pH, and drying of the purified NTCs. The present invention also relates to a device for purifying carbon nanotubes to implement the method according to the present invention. In one embodiment, the device comprises: a system for introducing a dihalogenic gas and / or interhalogen gas, and a thermal compartment in which temperatures of -20 to 2000 ° C can be implemented.

Advantageously, the device further comprises a system for controlling the diffusion of dihalogen and / or inter-halogen gas into the carbonaceous layers of the samples of nanotubes to be purified. It may be for example a dihalogen and / or interhalogen gas purification system, a dihalogen and / or interhalogen gas concentration control system, a pressure control system of the dihalogen and / or interhalogen gas, a dihalogen and / or interhalogen gas flow control system, or any combination of two or more of the aforesaid systems. Indeed, an important aspect of the process according to the present invention is to promote the diffusion of dihalogen and / or inter-halogen gas into the carbonaceous layers of the carbonaceous impurities and / or nanotubes. This good diffusion allows the dihalogenated and / or interhalogenated gas to reach the catalytic impurities which carbonaceous impurities and / or nanotubes are not on the surface, and a separate additional oxidation step is avoided to remove the carbonaceous impurities and to give access to the catalytic impurities. The methods of the prior art do not seem to deal with the problem of the diffusion of dihalogen and / or inter-halogen gas into carbon in order to eliminate catalytic impurities from carbon nanotubes. EP 2 135 842 mentions the problem of eliminating catalytic impurities buried in the carbonaceous layers, without however linking it to a limited diffusion of Cl 2 in the carbonaceous layers. Like all the processes of the prior art, EP 2 135 842 attempts to solve this problem by proposing a separate oxidation step (before and / or after treatment with IC 2). As discussed previously, this oxidation step is undesirable. Indeed, in addition to the fact that it is an additional step (and thus hinders the efficiency of the purification process as a whole), the oxidation itself is not very selective, and leads to the combustion partial nanotubes (the efficiency of the purification process is thus negatively affected).

The method and the device according to the invention allow on the contrary to overcome this additional oxidation step, promoting the diffusion of dihalogen / interhalogen in carbon.

The diffusion of dihalogen and / or interhalogen gas can be controlled by various parameters. For example, the purity, concentration, pressure, and / or flow rate of dihalogen and / or interhalogen gas. Thus, the diffusion of dihalogen and / or interhalogen gas can be promoted by ensuring that the dihalogen and / or interhalogen gas has:

- good purity, - a relatively high concentration,

- high pressure, and / or

- low flow.

Those skilled in the art will know how to adjust and combine these different parameters, taking inspiration from the teaching given in this document, in particular as regards the ranges of values of these various parameters that can be used, to obtain the desired diffusion, for the implementation of the method according to the invention.

All the previously described embodiments relating to the dihalogen and / or interhalogen gas (including its nature, its mode of introduction (eg, continuous or static flow), its origin (eg, commercial or in situ preparation), its flow rate, concentration, and purity (eg, O ₂ , CO ₂ and / or H ₂ O content) apply mutatis mutandis to the device, and are therefore not repeated here.

Thus, according to any of the embodiments relating to the device, the dihalogen and / or inter-halogen gas introduction system can be adapted to allow the introduction of said gas into the device, preferably in a controlled manner, from preferably under continuous flow. Thus, the dihalogen and / or interhalogen gas introduction system may comprise a valve for controlling the gas flow introduced into the device. The introduction system may be in the form of a gas cylinder (for example F ₂ or CI ₂ gas), which can be connected to the device by a suitable sealed connection. The introduction system may be in the form of a container, connected to the device by a suitable sealed connection, possibly equipped with a valve for regulating the rate of introduction of the gas into the device, the container may contain bromine liquid or solid diiod. The container may optionally be provided with a heating system, for moderately heating the liquid dibrome or the solid diode to increase the flow of Br ₂ or l ₂ gas according to the desired flow rate.

Alternatively, the dihalogen and / or interhalogen gas introduction system may be adapted to allow the preparation of dihalogen and / or interhalogen gas "in situ" just prior to its introduction into the nanotube purification device. Thus, in one embodiment, the dihalogen and / or interhalogen gas introduction system may comprise a device for producing said dihalogen and / or interhalogen gas. The reader will be able to refer to the discussion of gas preparation methods F ₂ , CI ₂ , Br ₂ and I ₂ detailed above. The system for introducing dihalogenic gas and / or interhalogen may include any suitable system for implementing these methods of preparation. For example, when the dihalogen gas is chlorine, the dihalogen gas introduction system may comprise a device as illustrated in FIG. 2, namely a flask containing potassium permanganate crystals surmounted by a glass bulb. addition containing a concentrated aqueous solution of HCl, for example a 12N solution of HCl. Alternatively, when a chlorine industry technique is used to produce chlorine, the dihalogen gas feed system may include an electrolyzer. For example, it may be a membrane electrolyser or diaphragm without asbestos.

In a preferred embodiment, the introduction system is a chlorine introduction system. In a preferred embodiment, the dihalogen gas feed system comprises a chlorine generating device comprising a vessel containing KMnO ₄ connected to a device for adding an aqueous solution of HCl, for example a concentrated aqueous solution. of HCI like HCI 12N. In the same way, all the embodiments described above concerning the treatment temperature apply mutatis mutandis to the device, and are therefore not repeated here. Thus, according to one embodiment, the present invention covers a device for purifying carbon nanotubes comprising:

a system for introducing a dihalogen and / or interhalogen gas, and

a thermal compartment in which temperatures for example between 200 and 2000 ° C, for example between 300 and 2000 ° C, for example between 700 and 2000 ° C, for example between 800 and 2000 ° C, for example between 850 and 2000 ° C, for example between 900 and 2000 ° C, for example between 950 and 2000 ° C, for example between 1000 and 2000 ° C, for example between 1050 and 2000 ° C can be implemented. Advantageously, the aforementioned device further comprises a system for controlling the diffusion of dihalogen and / or inter-halogen gas into the carbonaceous layers of the nanotubes to be purified. It may be, for example, a dihalogen and / or interhalogen gas purification system, a dihalogen and / or interhalogen gas concentration control system, a system for controlling the pressure of the dihalogen and / or interhalogen gas, a dihalogen and / or interhalogen gas flow control system, or any combination of two or more of the aforesaid systems.

According to one embodiment, the present invention covers a device for purifying carbon nanotubes comprising: a system for introducing a dihalogenic gas and / or interhalogen, and

a thermal compartment in which temperatures 200 <T≤2000 ° C, 300 <T≤2000 ° C, preferably 850 ° C <T <2000 ° C, preferably 900 ° C <T <2000 ° C, for example 900 ° C <T <1800 ° C, for example 900 ° C <T <1700 ° C, for example 900 ° C <T <1600 ° C, for example 900 ° C <T <1500 ° C, for example 900 ° C <T <1400 ° C, for example 900 ° C <T <1300 ° C, for example 900 ° C <T <1200 ° C, for example 900 ° C≤T <1100 ° C may be used. Advantageously, the aforementioned device further comprises a system for controlling the diffusion of dihalogen and / or inter-halogen gas into the carbonaceous layers of the nanotubes to be purified. This may be, for example, a dihalogen and / or interhalogen gas purification system, a dihalogen and / or interhalogen gas concentration control system, a control system of the dihalogen and / or interhalogen gas pressure, a dihalogen and / or interhalogen gas flow control system, or any combination of two or more of the aforesaid systems.

According to any of the previous embodiments, the device may further comprise a compartment for purifying the dihalogen gas and / or interhalogen. For example, the purification compartment may comprise a system adapted to reduce or even eliminate traces of oxidizing compounds, such as O 2, CO 2 and / or H ₂ O. For example, the purification compartment may comprise a system for trapping the water, such as a column containing a desiccant such as CaC, and / or a container containing a concentrated inorganic acid, such as sulfuric acid 36N, in which the dihalogen and / or interhalogen gas is passed before it is introduced into the thermal compartment containing the nanotubes. The purification compartment may further comprise a system adapted to reduce or even eliminate traces of hydrogen halide (HF, HCl, HBr, H1). For example, the purification compartment may comprise a container containing a concentrated solution of NaCl, NaBr, or other alkaline halide solution. in which the dihalogenated and / or interhalogenated gas is passed before it is introduced into the thermal compartment containing the nanotubes.

According to any one of the preceding embodiments, the device may furthermore comprise a compartment for treating the gaseous effluent at the outlet of the thermal compartment. For example, the treatment compartment may comprise a system adapted to reduce or even avoid any backscattering of moisture in the nanotube purification device, in particular in the thermal compartment. For example, the treatment compartment may comprise a system for trapping water, such as a column containing a desiccant such as CaC. The treatment compartment may further comprise a system adapted to decompose the dihalogen or interhalogen gas. For example, when the gas is chlorine, the treatment compartment may comprise a container containing a concentrated solution of sodium hydroxide (NaOH), potassium hydroxide (KOH) or calcium hydroxide (Ca (OH) ₂ ) by example, in which the effluent gas is passed to the outlet of the thermal compartment, to decompose the IC ₂ chloride and sodium hypochlorite. For the decomposition of other alkali or alkaline earth hydroxides such as Br ₂ , a concentrated solution of NaOH, KOH or Ca (OH) ₂ for example may also be used.

According to any of the previous embodiments, the device may further comprise a gas introduction compartment and / or carrier for driving the dihalogen or interhalogen gas in the purification device. Any gas that does not damage carbon nanotubes and / or is compatible with the present process can act as a flushing gas. Preferably, the flushing gas is an inert gas such as nitrogen, helium, neon, argon, krypton, xenon, or mixtures of two or more of these gases. Preferably, the flushing gas is nitrogen or argon, or a mixture thereof. So, in a mode of embodiment, the device may further comprise a compartment for introducing an inert carrier gas, such as nitrogen or argon. In one embodiment, the carrier gas introduction compartment ("carrier gas" in English) can take the form of a valve, in particular to control the flow of the carrier gas.

According to any of the previous embodiments, the device may further comprise guard systems placed in the mounting of the device to prevent contact between the different reagents used in case of depression or overpressure. According to any one of the preceding embodiments, the device may further comprise a compartment for introducing at least one oxidizing gas capable of reducing or even eliminating the carbonaceous impurities contained in the sample of carbon nanotubes. For example, it may be dioxygen, carbon dioxide and / or water vapor. Advantageously, the oxidizing gas may be dioxygen. Of course, this compartment can be adapted to allow the introduction of the oxidizing gas in a controlled manner, for example via a valve and / or flow meter to control the rate of introduction of the oxidizing gas into the device. Preferably, the device may further comprise a dioxygen introduction compartment. This compartment can also be equipped with a valve and / or a flow meter for regulating / controlling the flow of oxygen gas. Advantageously, the flow rate of oxygen may be between 0.05 and 10.00 mL / min, preferably between 0.10 and 8.00 mL / min, preferably between 0.30 and 5 mL / min.

The present invention also relates to carbon nanotubes that can be obtained according to a method as described above. In the present, we will denote these nanotubes "purified carbon nanotubes". The purified carbon nanotubes according to the invention can in particular be used for the reinforcement of polymeric matrices; for the manufacture of packaging materials for electronic components (for example for electromagnetic shielding and / or antistatic dissipation), such as housings for mobile telephones, computers, electronic devices on motor vehicles, rail or air ; for the manufacture of inks for the electrical connection between two electronic components; or for the manufacture of medical instruments, fuel lines (gasoline or diesel), adhesive materials, antistatic coatings, thermistors, or electroluminescent diode electrodes, photovoltaic cells or supercapacitors. The purified carbon nanotubes according to the invention can also be used as biomaterials (ie, materials for medical use), as mechanical reinforcements, drug vectors or for the manufacture of electronic or microelectronic components. In one embodiment, the electronic or microelectronic component is a capacitor or a transistor.

The present invention therefore also relates to the use of purified carbon nanotubes as defined above for the aforementioned purposes.

Thus, in one embodiment, the present invention relates to the use of so-called "purified" carbon nanotubes obtainable by a process according to the invention for the preparation of composite materials. According to one embodiment, the composite material is obtained by a process comprising a step of mixing the purified carbon nanotubes and a solution of polymer or a mixture of polymers.

According to one embodiment, the composite material is obtained by a process comprising a step of polymerization in situ of a monomer or a mixture of monomers in a suspension of purified carbon nanotubes. Such polymers and their method of manufacture are described for example in Matyjaszewski, K.; Eds. ; Advances in Controlled / Living Radical Polymerization, (American Chemical Society 2003) [ref 24]; Hsieh, HL; Quirk, RP; Eds. ; Anionic Polymerization Principles and Practical Applications, (Marcel Dekker 1996) [ref 25]; Matyjaszewski, K .; Davies, T. P .; Eds .; Handbook of Radical Polymerization, (Wiley-Interscience 2002) [ref 26] or Fontaine, L.; Initiation to Macromolecular Chemistry and Physico-Chemistry (French Group for Studies and Applications of Polymers Volume 12 (Chapter 3)) [ref 27].

According to the invention, the polymer may be any polymer that makes it possible to implement the present invention. It can be chosen for example from the group comprising polystyrene; polyolefins, for example polyethylene, polypropylene, poly (alpha-olefin) s, polyisobutene and polypropylene; polyethers; polyesters; polyamides; polyacrylamides; polyacrylates; polysilanes; polysiloxanes.

According to the invention, the polymer may be a linear block copolymer or a random copolymer. Those skilled in the art will be able to identify suitable operating conditions and the polymer (s) to be used to produce a composite material having the required properties. In particular, those skilled in the art can draw inspiration from the methods described in FR 2 870 251 [ref 28] and / or WO 2006/136715 [ref 29] which describe the preparation of composite materials from carbon nanotubes and polymers or polymer blends. Those skilled in the art will be able to adapt the methods described in these documents to carry out the preparation of composite materials from purified carbon nanotubes obtainable by the process of the present application. The polymer (s) can be selected in order to optimize the carbon nanotube / polymer surface interactions, and to allow better dispersion of the purified carbon nanotubes in the polymer matrix. Such materials may be used, for example, in paint or solvent formulations, in coatings, or as additives or antistatic materials. By "block copolymer" herein is meant a block polymer comprising more than one species of monomer. In a block copolymer, identical monomers are grouped together. Such polymers and their method of manufacture are described for example in Matyjaszewski, K.; Eds. ; Advances in Controlled / Living Radical Polymerization, (American Chemical Society 2003) [ref 24] or Hsieh, HL; Quirk, RP; Eds. ; Anionic Polymerization Principles and Practical Applications, (Marcel Dekker 1996) [ref 25].

By "random copolymer" is meant herein a polymer in which the different monomers are mixed according to the reactivity and concentration thereof. Such polymers and their methods of manufacture are described for example in Matyjaszewski, K .; Davies, T. P .; Eds .; Handbook of Radical Polymerization, (Wiley-Interscience 2002) [ref 26] or Fontaine, L.; Initiation to Macromolecular Chemistry and Physico-Chemistry (French Group for Studies and Applications of Polymers Volume 12 (Chapter 3)) [ref 27].

According to the invention, when it is a block copolymer, it may be for example a diblock copolymer synthesized for example by controlled radical polymerization or by living anionic polymerization or by living cationic polymerization or a copolymer statistic synthesized by controlled radical polymerization or uncontrolled radical polymerization.

Controlled radical polymerization (CRP) is a method of choice for preparing well-defined polymers and copolymers with flexible molecular weights and low polymolecularity indices. of the techniques useful in the present invention are described for example in Matyjaszewski, K .; Davies, T. P .; Eds .; Handbook of Radical Polymerization, (Wiley-Interscience 2002) [ref 26].

By "living polymerization" is meant a polymerization in which there are no termination reactions, no transfer reactions, and where the polymer chains continue to grow as long as there are monomer molecules to be added to the chains. According to the invention, the living polymerization can be cationic or anionic. Such methods are described for example in Matyjaszewski, K.; Eds. ; Cationic Polymerizations Mechanisms, Synthesis, and Applications, (Marcel Dekker 1996) [ref. 30] or Hsieh, H. L.; Quirk, R.P .; Eds. ; Anionic Polymerization Principles and Practical Applications, (Marcel Dekker 1996) [ref 25].

The monomers can be introduced in full during the polymerization step. They can also be introduced separately or in a mixture, continuously or discontinuously. A monomer supplement may also be introduced at the end of the polymerization in order to obtain the desired polymer composition.

The adjuvants that may be incorporated during conventional polymerization processes can be used according to the method of the invention. Thus, initiators, chain transfer agents, catalysts, antioxidants and lubricants known to those skilled in the art can be used.

As one skilled in the art can see from the present description, one of the main advantages of the present invention is the simplicity of implementation of the process, as well as its ability to provide purified carbon nanotubes more efficiently than the processes of the prior art. In particular, the process according to the invention makes it possible to dispense with the methods of the prior art which require a prior or separate oxidation step using an oxygen-containing compound (eg, O 2, CO 2 , H ₂ O) for oxidizing metal impurities to metal oxides or metal hydroxides to remove them. The major disadvantage of this type of method is that the oxidation step is not selective: the carbon nanotubes are also oxidized, and thus damaged and even destroyed, at the end of these processes. Conversely, the process according to the invention does not require this prior or separate oxidation step, which is problematic.

In the same way, the method of the invention makes it possible to avoid a prior step of exfoliation of the carbon shells empty or filled with metallic impurities in a sample of carbon nanotubes, in particular carbonaceous multilayer shells. This prior exfoliation step was recommended in the state of the art in order to expose the metal particles before eliminating them.

Thus, the process according to the invention is very selective, whatever the variant used (treatment of nanotubes with a dihalogenic or interhalogen gas, in the presence or absence of an oxidizing compound (or mixture of oxidizing compounds) such as oxygen, carbon dioxide and / or water vapor): it makes it possible to eliminate catalytic impurities, and possibly carbonaceous impurities (in its variant using an oxidizing atmosphere), in a single step, avoiding the attack chemical nanotubes. The method according to the invention is therefore little consumer of sample. By way of comparison, conventional methods (ie, involving an oxidation step) typically consume more than 90% of the sample [ref 48]. As demonstrated in Example 8, the process according to the invention makes it possible to reduce to about 20% by weight (including the mass of catalysts removed) of sample consumption in the Experimental conditions of Example 8. This is a remarkable improvement.

Although the method of the invention can be enhanced by a washing step and / or acid treatment, preferably in a liquid medium, this step is not crucial to obtain nanotubes of adequate purity for industrial applications typical of nanotubes .

The only step of treatment with a dihalogen and / or an interhalogen, optionally in the presence of an oxidizing compound (or mixture of oxidizing compounds), is sufficient to effectively purify the nanotubes with a remarkably improved yield compared to the art. previous, and with an acceptable degree of purity.

In addition, the possibility offered by the process according to the invention to overcome this step of washing / treatment in a liquid medium (a step which the conventional methods of the prior art have recourse to) makes it possible to preserve the state ventilated sample. After treatment in a liquid medium, the liquid in which the samples have been treated must be removed so that the solid can be recovered. Most often this is done by filtration or simple heating. It is also possible to remove the solvent from the sample by lyophilization (see Maugey et al [ref 53]), but this type of method is relatively heavy to implement and is expensive. A simple filtration or drying step may seem innocuous to the sample; however, it is responsible for a densification of the sample in which the nanotubes strongly aggregate irreversibly. Manipulation and the realization of subsequent chemical treatments for their dispersion or functionalization proves for example very difficult to achieve on such samples because the accessibility of the nanotube walls to the reagents is greatly reduced. Thus, the preservation of the aerated state of Nanotube samples are important because it fully conditions their "processability" for future uses.

A major advantage of the process according to the invention is that it preserves the structural integrity of the nanotubes: the nanotubes are not etched. The advantageous properties of the nanotubes are therefore fully preserved.

In summary, the process according to the invention proposes a new route for effectively removing catalytic impurities from carbon nanotube samples. The process according to the invention also makes it possible to remove amorphous or slightly crystalline carbonaceous impurities, in particular in embodiments in which the dihalogen or interhalogen gas is used in the presence of an oxidizing compound (or mixture of oxidizing compounds). ) such as oxygen, carbon dioxide and / or water vapor. It differs from commonly used procedures and has many advantages. In particular, it avoids the chemical etching of nanotubes, is therefore of low sample consumption, and thus allows purification with better yields than conventional methods.

Other advantages may still appear to those skilled in the art on reading the examples below, illustrated by the appended figures, given for illustrative purposes.

Brief description of the figures

Figure 1 schematically illustrates the principle of one embodiment of the invention, namely the spontaneous elimination of metallic impurities of the sample by treatment of carbon nanotubes with a dihalogen gas (X ₂ ) to form metal halides within the multi-layered carbon shells. Under the action of a high temperature, the metal halides sublimate, thus eliminating metal impurities from carbon nanotubes.

- Figure 2 schematically illustrates an embodiment of the invention. This is an assembly used to carry out the heat treatment under a continuous stream of dichlor for a sample of carbon nanotubes, for example SWNTs. - Figure 3 shows a typical image obtained by transmission electron microscopy (TEM) of a sample of SWNTs after electric arc synthesis. The SWNTs are clustered in bundles of ten to several hundred nanotubes. Perpendicular to the axis of the nanotubes, these beams form a triangular network. The latter nevertheless has arrangement defects which can be eliminated by a high temperature heat treatment. In addition, at the output of the synthesis reactor, in the raw state, the walls of the nanotubes have surface functions due to the low synthesis conditions. The samples also contain impurities in a significant amount. These impurities are in the form of amorphous carbon and more or less graphitized particles, carbon shells multi-layer protecting the catalytic impurities. Because of these multi-layer protections, metal impurities are very difficult to remove from the samples. Their presence is however not desirable because they can induce significant modifications or even mask the chemical and physical properties of the nanotubes. It is for example reported that these particles can catalyze certain undesired reactions during chemical treatments. Moreover, the electrical conductivity properties or the magnetic properties of the sample can be strongly disturbed by these non-carbonaceous impurities. FIG. 4 represents an image obtained by TEM of a sample of SWNTs after synthesis by electric arc, in which the impurities are identified.

FIG. 5 represents images obtained by TEM of samples of SWNTs after dichlore heat treatment according to one embodiment of the present invention, (treatment at 1100 ° C. under a flow of 90% vol chlorine in N ₂ as gas carrier (flow rate: 100mL / nnin) for 2 hours).

FIG. 6 represents images obtained by TEM of samples of SWNTs after thermal treatment under chlorine in the presence of approximately 1% vol of oxygen according to one embodiment of the present invention (treatment at 1100 ° C. under a flow of chlorine / O2 90% vol in N ₂ as carrier gas (flow rate: 100mL / nnin) for 2 hours).

FIG. 7 represents TEM images of a sample of carbon nanotubes after treatment at 850 ° C. under a flow of chlorine at 90 ° C.

% vol in N ₂ as a carrier gas (flow rate: 100 ml / min) for 2 hours. Zone 1: hulls emptied after treatment; zone 2: bundles of SWNTs.

FIG. 8 represents a curve Loss of mass = f (Temperature) obtained after characterization by ATG of a purified sample of SWNTs by treatment in a flow of IC 2 according to the invention. (See Example 3) (treatment at 1050 ° C. under a stream of chlorine at 90% vol. In N ₂ as a carrier gas (flow rate: 100 ml / ml) for 2 hours).

FIG. 9 represents representative TEM images of samples of SWNTs purified according to the process of the invention after treatment at 800 ° C. under a flow of chlorine at 90% vol. In N ₂ as a carrier gas (flow rate: 100 ml / min). Figure 9A: for 2 hours. Figure 9B: for 4 hours. FIG. 10 represents TEM images representative of samples of SWNTs purified according to the process of the invention after treatment at 1100 ° C. under a stream of 90% vol chlorine in N ₂ as a carrier gas (flow rate: 100 ml / min) during 2 hours. Figure 10A: Low magnification. Figure 10B: High magnification.

FIG. 11 represents representative TEM images of samples of SWNTs purified according to the process of the invention after treatment at 1050 ° C. or 1100 ° C. under a flow of 90% vol chlorine in N ₂ as carrier gas (flow rate). 100mL / min) for 2 hours. Figure 1 1A: 1050 ° C low magnification. Figure 1 1 B: 1050 ° C high magnification. Figure 1 1 C: 1100 ° C low magnification. Figure 1 1 D: 1100 ° C high magnification.

FIG. 12 represents a table listing the quantification by thermogravimetric analysis of the concentration of metallic impurities in a sample of SWNTs after purification by treatment in a flow of IC 2 before and after washing the purified SWNTs in a solution of 4N HCl at room temperature for 15 hours.

13 is a graphical representation of the results of FIG. 12. Concentration of impurities of metal oxides (M _x O _y ) obtained after ATG of a sample of SWNTs containing, before treatment under flow of dichlor, 1 1, 6 mass% of M _x O _y . After purification treatment under the conditions mentioned in the figure and before the washing step, the mass concentration in M _× O _y is between 5% and 7.5%. After the washing step in 4N HCl solution at room temperature for 15 hours, the concentration of M _x O _y is around 3.5%.

14 represents 3 typical RAMAN spectra of samples of SWNTs before and after purification treatment under IC 2 at 950 ° C. for 2 hours and after washing in 4N HCI solution at room temperature for 15 hours. In the wavenumber range 100-200 cm ^-1 , the so-called RBM band for 'Radial Breathing Mode' is little modified by the different treatments and indicates the presence of SWNTs for the 3 samples. C = C bonds of SWNTs are typically visible around 1600 cm ^-1 . The Raman intensity of all the spectra is normalized to that of the G band. The intensity of the D or ID band with respect to the intensity of the G or IG band decreases after the purification treatment under CI 2 and is little modified after the washing step. This shows that the washing as described does not damage the structure of the SWNTs.

FIG. 15 represents a table comparing the ratios of the intensities of the band D and of the band G noted I _D / IG for a sample of SWNTs before purification, after the heat treatment under flow of chlorine and after washing. Band D is sensitive to the introduction of defects in the structure of the CNTs, the increase of the ratio I _D / IG would mean that the treatment undergone induces damage to the walls of the SWNTs. Its decrease observed in the table of Figure 15 indicates that the purification treatment improves the quality of the structure of purified SWNTs and that the washing does not introduce defects on the walls of the SWNTs.

FIG. 16 represents photographs of different parts of the assembly produced for Example 16 to carry out a purification treatment P _C II according to EP 2 135 842. A and B: Injection of HCI using a shoot -Syring through a capillary directly put in contact with the potassium permanganate placed in a reaction flask. C: In-situ measurement of the pressure in the assembly during the treatment. D: Formation of a solid deposit at the furnace outlet due to the condensation of the metal chlorides formed during the treatment. 17 represents the ATG curves for the raw ARC SWNT sample (black), the sample after a purification treatment P _C i (light gray) and after a P _C it> is treatment (dark gray) obtained at the same time. Comparative Example 10 - Figure 18 shows the TG curves for the ARC sample

Crude SWNT (black), the sample after a purification treatment P _C i (dark gray) and after a treatment P _C it (is light gray) obtained in Comparative Example 10.

FIG. 19 represents a representative image of a sample after the Pcibis treatment of Comparative Example 10.

FIG. 20 shows typical MET images of the HIPCO SWNT sample before (A) and after (B) the purification process P _{C 1} .

FIG. 21 represents a focus of the observations made by MET on the metallic impurities of the HIPCO sample

SWNT before (A, B) and after (C, D) purification Ρ _α carried out at 950 ° C for 2h.

FIG. 22 represents an ATG analysis of the sample HIPCO SWNT before (black) and after (gray) the purification treatment P _C i carried out at 950 ° C. for 2 hours. Above: mass loss as a function of temperature; bottom: representation of the derivative of mass loss as a function of temperature.

FIG. 23 represents the concentration of metal iron and iron carbide catalyst determined by ATG and magnetic measurements before and after purification by thermal treatment under dichlor flow (950 ° C., 2 h, 90% Cl ₂ / N ₂ , 100 ml) / min, pressure: 1 atm).

- Figure 24 shows the yields obtained by a conventional purification method (P _0, or "oxygen-Based Process") and a method of punishing according to the invention (P _C i, or "clo ne-based process") on the two types of samples of nanotubes studied, "yields calculated only from the variation of the catalyst concentration, c ' that is: ((catalyst concentration in the raw sample) - (catalyst concentration in the sample after purification) / (catalyst concentration in the raw sample)).

- Figure 25 shows typical Raman spectra for HIPCO SWNT samples before and after purification under chlorine flow (950 ° C, for 2h). The Raman spectrum obtained directly after the treatment (before washing) and that corresponding to the sample which has also been washed with HCl are shown. The irradiation wavelength is 514 nm.

Figure 26 shows an ATG analysis of the ARC SWNT sample before (black) and after the purification treatment; dark gray curve: treatment without the addition of oxygen; light gray curve: treatment with addition of oxygen.

Figure 27 illustrates a representation of the derivative of the mass loss as a function of temperature for the ARC SWNT sample before (black) and after the purification treatment; dark gray curve: treatment without the addition of oxygen; light gray curve: treatment with addition of oxygen, Example 12.

FIG. 28 represents representative images obtained by TEM at different magnifications of the ARC SWNT sample before (a and b) and after purification without addition of oxygen (c and d) and with oxygen in the dichlor (e and f).

- Figure 29 shows mass losses recorded for the ARC SWNT sample before (black) and after the purification treatment performed with different flow rates of oxygen. - Figure 30 shows a representation of the derivative of the loss of mass as a function of temperature for the ARC SWNT sample before (black) and after the purification treatment carried out with different flow rates of oxygen. FIG. 31 represents typical images obtained by TEM of the ARC SWNT sample for the 3 dioxygen flow rates used in Example 12.2 (D1: 0.45 mL / min, D2: 2.00 mL / min, D3: 4.10 mL / min).

EXAMPLES

Unless otherwise indicated, all experiments were performed with the experimental setup described in Figure 2.

Introduction. The most common synthetic methods can be grouped into two categories: the so-called 'low temperature' or CVD methods for Chemical Vapor Deposition and 'high temperature' methods (such as laser ablation or electric arc). For the first category, the carbon input is provided by a gaseous mixture containing alkane, alkene or alcohol type molecules or complexes such as iron pentacarbonyl for the HiPco method [62] heated to a temperature between 500 and 800. ° C. For the second category, CNTs are produced from graphite which is sublimated at temperatures up to 3000 ° C [63]. In both cases, the preparation of SWNTs requires the presence of metal catalysts which then constitute impurities. Moreover, even if the samples of nanotubes from high temperature methods contain more carbonaceous impurities, the SWNTs obtained have fewer structural defects and a distribution more narrow in diameter than the SWNTs obtained by CVD. Thus, the two types of samples have different characteristics and therefore each have an interest in being studied. Unless otherwise indicated, the SWNTs used in Examples 1 to

10 are SWNTs synthesized by electric arc. The synthesis of the electric arc type samples was carried out in the laboratory. [63]

EXAMPLE 1 Purification of SWNTs under a continuous flow of chlorine As illustrated in FIG. 2, the process according to the invention can be implemented in a device comprising a chlorine production zone, a chlorine purification zone, a chlorine purification zone or a chlorine purification zone. chlorination and a neutralization zone of the gaseous effluent, the assembly for purifying carbon nanotubes by treatment with dichlore in continuous flow. In this installation, the carbon nanotubes are arranged in a silica nacelle, itself placed in a silica tube in the center of a horizontal tubular furnace (Figure 2). They are gradually heated to a given temperature under a stream of chlorine gas, and then maintained at this temperature for a selected period. They are then cooled, still under current of chlorine.

The gaseous chlorine is generated by the reaction between crystals of potassium permanganate (KMnO ₄ ) and concentrated hydrochloric acid (HCl) (Figure 2). The flow of chlorine is regulated by the acid flow. The use of a carrier gas (N ₂ ) makes it possible to establish a controlled gas flow. The gaseous mixture passes into a saturated solution of sodium chloride (NaCl), which makes it possible to remove all traces of hydrochloric acid.

It is then carefully dehydrated by passing successively on a column containing calcium chloride (CaC) and then in a vial containing concentrated sulfuric acid (H ₂ SO ₄ 36 N). The chlorine thus dehydrated then passes into the oven (700 - 1200 ° C.) in order to react with the sample. At the outlet of the oven, a column containing calcium chloride makes it possible to avoid any backscattering of moisture. The excess chlorine passes at the end of the assembly into a soda solution in order to be decomposed into chloride and sodium hypochlorite. Keeping flasks are systematically placed in the assembly in order to avoid any contact between the different reagents used in this protocol in case of depression or overpressure. The advantage of such an assembly lies in benefiting from a continuous flow of chlorine since it is directly generated during the treatment of the sample. This dynamic geometry also allows a faster diffusion of metal chlorides formed during the reaction.

The preparation of the assembly consists of the connection of the different elements and then the filling of the vials.

200 ml of saturated solution of sodium chloride (NaCl) are prepared by adding until saturation of the solid NaCl to 200 ml of demineralized water and introduced into the dedicated flask; 200 ml of concentrated sulfuric acid (H ₂ SO ₄ 36 N), used as received, are introduced into the dedicated flask. 30 g of sodium hydroxide (NaOH) are added to 170 g of demineralized water and solubilized with magnetic stirring, and then this solution of 10% NaOH is introduced into the dedicated flask. Columns filled with CaC are connected to the rest of the assembly. For a typical treatment, 250 g of potassium permanganate (KMnO ₄ ) are placed in the flask and 400 ml of hydrochloric acid (HCl, 12N) are introduced into the ampoule. The flow of chlorine is regulated by the flow rate of HCI itself controlled by a tap. The flow rate is kept constant throughout the treatment and set at 100 mL / min. The concentration of chlorine in N ₂ is 90% vol. A carrier gas (N ₂ , alphagaz 2) is used which makes it possible to establish a controlled gaseous flow and to modify the concentration of chlorine.

For a typical experiment, about 200 mg of a sample of SWNTs is placed in a silica nacelle. This nacelle is placed in the center of a tubular furnace and heated with a rise in temperature of 10 ° C / min to the chosen temperature (800-1 100 ° C) for several hours (2h-4h). Then let the temperature drop naturally. Up to a temperature in the region of 300 ° C., the flow conditions of the chlorine / N ₂ mixture remain unchanged. From 300 ° C to room temperature, the HCI valve is closed to no longer produce chlorine. The sample is left under a flow of N ₂ to remove the chlorine from the assembly before it is opened for the recovery of the sample.

Samples of SWNTs were purified at 800 ° C, 950 ° C, 1050 ° C or 1100 ° C, for 2 or 4 hours (see Example 5).

Example 2 Characterization of Metallic Impurities by MET

Transmission electron microscopy (TEM) is a technique of choice for evaluating the efficiency of the purification method used because it allows a direct visualization of the presence or absence of metal particles.

Characterization by MET has several advantages, namely:

• Its excellent resolution in real space can reach the atomic scale (can go up to 0.8 Angstrom), with magnifications varying from one thousand to one million. The possibility of combining the information obtained in real space in image mode with the information obtained in the Fourier (or reciprocal) space in diffraction mode. For crystalline samples, another way of using is to visualize the diffraction pattern of the sample. However, it should be noted that the MET allows only qualitative and in no case quantitative studies: in fact, the observed area measures a few tens of micrometers maximum and can not be generalized to the entire sample. To overcome this drawback, the number of shots has been multiplied and only a few have been selected and are considered the most representative.

The metallic particles, which come from the catalysts necessary to obtain SWNTs, observed by MET, are in the form of dark points (because they have a richer electronic structure than that of carbon) and are often covered with several carbon layers. They are therefore easily recognizable. Amorphous carbon is often arranged in blocks and forms greyish masses that can extend over several hundred nanometers. The bundles of nanotubes resemble more or less thick wires depending on the number of SWNTs they contain. The quality of their wall guarantees the low number of defects of synthesized SWNTs. Carbonated multihulls are often empty spheres but they can also be filled by the catalytic metals which are thus protected, which greatly complicates their elimination.

To prepare the samples before placing them in the microscope, the carbon nanotubes were added to an ethanol solution which was placed in an ultrasonic bath to be dispersed. Then, a drop of this liquid is deposited on a grid which itself is placed on a sample holder introduced into the MET.

Figures 5, 6 and 7 are representative TEM images of the samples after the different treatments described herein. The most striking difference with the starting sample is the virtual disappearance of particles that appear dark on these brightfield images: the catalysts.

A detailed observation of the samples for treatment conditions deviating from the optimal conditions makes it possible to demonstrate the presence of empty multi-layered carbon shells (FIG. 7). Before the treatment, the latter were filled with catalytic impurities, the chlorine vapors were able to diffuse through the carbon layers and react with the metals present to form metal chlorides. Under the effect of heat, the highly volatile chlorides could sublimate and be removed from the sample leaving the shells empty (part 1 of the image, Figure 7). Moreover, we can notice that the quality of the walls of the nanotubes is preserved; chlorine does not attack the structure of nanotubes. In addition, the comparison of the images 5 and 6 shows that when oxygen is added to the mixture CI2 / N2, the carbonaceous particles have practically disappeared (FIG. 6) whereas they remain present in a significant quantity when a mixture CI2 / N2 (Figure 5) is used, that is to say without addition of O2.

Example 3 Characterization of Metal Impurities by Thermogravimetric Analysis: ATG

Thermogravimetric analysis is a quantitative method that allows the analysis of a macroscopic sample quantity (approximately 10 mg).

It consists (after a preliminary vacuum carried out on the measuring chamber) in heating in dry air from the ambient temperature to the temperature of 1000 ° C according to an identical ramp for all the samples studied (3 ° C / min). The mass loss of the sample is measured as a function of the temperature in the enclosure. Heating results in the combustion elimination of all carbonaceous particles and also leads to the oxidation of catalytic particles. Thus, at the end of the ATG measurement, a certain mass is recovered which corresponds only to the oxidized catalytic residues, that is to say to the nickel NiO and yttrium Y 2 O 3 oxides. Since no trace of carbon remains, smaller mass quantities measured by ATG are indicative of a higher purity sample.

A crucible that will contain the sample is tared, then a few milligrams of the sample to be studied are placed inside and the crucible is introduced into the oven. After purging the chamber to remove ambient air, dry commercial air is introduced and the temperature control program is started, raising the temperature from 20 ° C to 1000 ° C with a ramp of 3 ° C / min. The mass loss curve = f (T) as shown in Figure 8 is obtained.

The loss of mass has several waves depending on the stability of the carbon species present in the sample. We can see a significant mass loss around 500 ° C and another less marked around 750 ° C. Beyond this, the sample contains only the metal oxides. Subsequently, we will focus only on the final mass loss (at 1000 ° C for example). For the example given in Figure 8 the remaining mass is 4.70%.

Example 4 Characterization of SWNTs Samples by RAMAN Spectroscopy

Raman spectroscopy is an effective analytical technique for studying the vibration properties of NTCs that are directly related to their structural and electronic properties. Thanks to their quasi-dimensional structure, the response of the NTCs obtained by Raman spectroscopy is resonant, which has the advantage of amplifying their signal strongly compared to that of the impurities present in the medium and therefore to directly probe the behavior of NTCs in samples that are far from pure.

In the range 70-1800 cm ^"1 , several characteristic bands are to report:

- The RBM band ("Radial Breathing Mode") in the range 100-200 cm ^"1 is the so-called breathing band of the SWNTs.It corresponds, in fact, to the radial deformation of their diameter.Its position is also inversely proportional to the diameter of the resonant SWNTs - Band D (around 1340 cm ^"1 ) represents the band of defects. Its intensity will increase if the number of defects in the structure of SWNTs increases.

The band G corresponds to the vibrations of the carbon-carbon bonds of the nanotube. It is characterized by two components. The band G ^" (low frequency component around 1560 cm ^-1 ) corresponds to the vibrations along the circumferential axis of the tube. The band G ⁺ (high frequency component around 1595 cm ^-1 ) corresponds to vibrations along the longitudinal axis of the tube.

The ratio between the intensity of the D and G bands provides an indication of the number of defects in the NTCs after a chemical treatment for example. For sample preparation for Raman analyzes, the samples are first dispersed in ethanol using a low power ultrasonic bath for 5 min.

A drop of this solution is then deposited on the surface of a glass slide and dried at room temperature. Several (average 3) Raman spectra are recorded for each sample on different areas to ensure some statistics. The sample is illuminated, via an optical microscope (objective x50), with a light beam of wavelength λ = 514.5 nm (green) from an Argon laser, whose output power is maintained at the value of 15 mW. The power of the incident laser beam is then focused on a small area of the sample, the beam diameter being approximately 5 μιτι for the objective x50. The samples being very absorbent, the incident power is decreased by optical filters chosen according to the samples and objectives used in order to overcome any changes in the spectra (or nanotubes themselves) that may be caused by a heating up too much.

The Raman signal from the sample is collected by the objective of the microscope which was used to focus the incident light (backscattering geometry) and directed to the input of the spectrometer; the scattered light is finally directed, at the output of the spectrometer, to a CCD camera cooled to -70 ° C. by a Peltier effect device.

For the analysis of the data, in order to bring the baseline back to zero in order to be able to compare the spectra between them, the gross spectrum is subtracted from a constant value. The recordings being made at room temperature in a non-air-conditioned room, reference spectra are systematically and regularly recorded, to check for any drifts in frequency. The recorded spectra have thus been corrected by taking the line at 520.70 cm ^-1 of silicon as a reference, and the ratio of the D and G bands denoted ID / IG is determined from the heights of the D and G ⁺ peaks. represents a table comparing the intensity ratios of the D-band on the intensity of the G band noted I _D / IG for a sample of SWNTs before purification, after heat treatment under flow of dichlor and after washing. / IG indicates that the purification treatment improves the quality of the structure of SWNTs after the purification treatment and that the washing does not introduce defects in the structure of the purified SWNTs.

Example 5: Results Obtained at Different Treatment Times and Temperatures

Samples of SWNTs were purified according to Example 1 at 800 ° C, 1050 ° C or 1100 ° C, for 2 or 4 hours.

The samples thus purified were analyzed by MET (see FIGS. 9 to 11).

By analyzing the images of Figures 9A and 9B, it is noted that there are no significant differences both in terms of the amount of impurities such as catalytic metals, as from the point of view of that of the nanotubes. The samples were analyzed by ATG. The results are given in Table 2 below:

Table 2

The results obtained by ATG show that increasing the treatment time from 2 hours to 4 hours reduces the remaining catalyst mass by about 1%.

Increasing the duration of treatment at this temperature improves the removal of catalytic impurities. We can therefore think that Chlorides formed take time to diffuse through the protective shells. It can be assumed, however, that increasing the duration of treatment favors their elimination because they can leave the sample. As illustrated in Figure 10, the sample obtained after a treatment carried out according to Example 1 at 1100 ° C for 2 hours has a remarkable purity. To verify the reproducibility of these results, we conducted two experiments for a period of two hours at 1050 ° C and 1100 ° C (see Figure 1 1). As confirmed by the images of FIG. 11 obtained by MET, the quality of the sample is globally improved. In fact, the concentration of nanotubes is significantly increased for these experiments at high temperatures (1050 ° C., 1100 ° C.) compared to the experiments carried out at 800 ° C. and even at 950 ° C. In fact, the carbonaceous impurities present in a rather large quantity at the end of the treatment at 800 ° C. (see FIG. 9) were strongly removed from the samples treated at 1050 ° C. and 1100 ° C. In addition, the amount of catalytic particles also appears to decrease as the temperature is increased. These results were confirmed by ATG analysis. The results of the experiments carried out at 1050 ° C. and 1100 ° C. in comparison with those obtained at lower temperatures (800 ° C. and 950 ° C.) are reported in the table of FIG. 12.

As shown in the first column ("mass remaining before washing"), the increase in temperature improves the removal of metal impurities due to the catalysts. Indeed, their concentration (in mass) is around 7% for the treatments at 800 and 950 ° C and it decreases to values close to 5% after the treatments at 1050 and 1100 ° C. Example 6 Purification of SWNTs under a continuous stream of dichlor, followed by washing with an aqueous acid solution

The carbon nanotubes are purified according to Example 1. After cooling, the nanotubes were washed with a hydrochloric acid solution of 4N concentration at room temperature, for about 15 hours in order to solubilize the metal chlorides that would have remained trapped in the sample. The pH is then brought back to neutrality with successive washes. Typically 3 washes are required for pure water. After filtration, the sample is dried at 50 ° C for 24 to 48 hours before being analyzed.

The results obtained are given in FIG. 12 (second column "mass remaining after washing") and show that this washing step, for all the conditions used, leads to a decrease in the amount of catalysts. A real influence of the washing step in hydrochloric acid is observed to improve the elimination of the catalysts. Indeed, for all the experiments carried out, the remaining mass obtained after ATG is significantly lower after washing. It is even approximately 2 times lower for the experiments carried out at 800 ° C. and 950 ° C. For treatments performed at higher temperatures, however, its effect is less pronounced (see also Figure 13).

Example 7 Purification of SWNTs under a continuous flow of chlorine in the presence of oxygen The device illustrated in FIG. 2 is used.

About 200 mg of a sample of SWNTs is placed in a silica nacelle. This nacelle is placed in the center of a tubular oven and heated with a rise in temperature of 10 ° C / min at the chosen temperature (800-1100 ° C) under a stream of chlorine gas containing about 1% vol oxygen for several hours (2h-4h). The flow of oxygen from a bottle is introduced into the gaseous mixture (Cl 2 / N 2) via a flow meter at a rate of 1 ml / min throughout the treatment under chlorine. Then let the temperature drop naturally. The chlorine is generated as in Example 1. Oxygen is introduced through a valve provided for this purpose, from a commercial gas cylinder connected to the device.

Up to a temperature in the region of 300 ° C., the flow conditions of the chlorine / O ₂ / N ₂ mixture remain unchanged. From 300 ° C to room temperature, the HCI valve and the oxygen valve are closed so as not to produce chlorine and no longer introduce oxygen into the system. The sample is left under an N ₂ stream to remove the chlorine and oxygen from the assembly before it is opened for sample recovery.

A TEM analysis shows that this sample purified in the presence of a small amount of oxygen is of better quality in terms of content of carbonaceous impurities than the samples prepared according to Example 1, that is to say without adding oxygen. In particular, the content of carbonaceous impurities in the sample purified according to Example 7 is much lower than that obtained with the procedure of Example 1. The content of carbonaceous impurities in a sample synthesized by electric arc is typically 40% vol (see Figure 3). An analysis from the images obtained by TEM (for example FIG. 6) makes it possible to estimate the volume concentration remaining after the procedure of Example 7 at less than 2% vol (FIG. 6D).

Example 8: Demonstration of the Low Consumption of SWNTs Samples with the Purification Process According to the Invention A simple weighing of the sample before and after each treatment makes it possible to follow the loss of mass due to the treatment undergone.

The consumption of samples of SWNTs purified according to the method of the invention, mentioned in Examples 1 to 7 above, and in Figures 5 to 7 was measured. In all cases, it is less than 20% by weight.

Comparative Example 8A: Consumption of SWNTs Samples with a Traditional Purification Method of the Prior Art

Cho et al. [ref 48] discuss the problem of the yield of multi-step purification methods including oxidation steps, exfoliation, micro-filtration, acid treatment ... Most of the procedures developed do not allow to achieve yields greater than 50% and these yields are often around 10% and can be lower than 1% for the highest purities. A conventional three-step purification procedure (ie, (i) thermal oxidation under dry air, (ii) acidic treatment with 6N hydrochloric acid, and (iii) high temperature vacuum treatment) was carried out on samples of SWNTs (see Vigolo et al [ref.54]). Using optimized experimental conditions, the mass losses of the sample were 60, 90 and 15% mass. after steps (i) (oxidation), (ii) (acid treatment) and (iii) (heat treatment), respectively. Which gives a total consumption of the sample greater than 95% mass.

EXAMPLE 9 Demonstration of the Aerated Aspect of SWNTs Samples with the Purification Process According to the Invention

The loss of accessibility of the solvent or reagent to the walls of the nanotubes because of excessive densification due to the filtration of the sample after a treatment carried out in a liquid medium can be demonstrated by comparing the behavior of two purified SWNTs samples: a sample which has undergone a purification procedure comprising a treatment in a liquid medium and another sample which has undergone the purification method described right here.

It is possible to effectively and spontaneously disperse SWNTs directly from the synthesis in a polar solvent (for example dimethyl sulphoxide or DMSO) after they have been reduced by an alkali metal in the liquid phase (see Penicaud et al. [ref 51]) or gaseous (see Vigolo et al [ref 52]). The accessibility of the metal atoms to the walls of the SWNTs completely conditions the success of the dispersion since if the alkali metal can not diffuse within the material, the reduction reaction will not be able to occur and the dispersion will not take place . For samples of SWNTs purified by a conventional method involving a liquid treatment step (e.g. reflux in an acidic solution for several hours), the nanotubes do not disperse after the alkali metal reduction treatment; they remain aggregated and precipitate in the solvent. On the other hand, by using the purification method according to the present invention, the behavior of the purified SWNTs is similar to that of SWNTs directly derived from the synthesis. They disperse easily. This validates the advantage of the present purification method, a good dispersion of the nanotubes being difficult to obtain with other purification treatments. This advantage conferred by the process according to the present invention offers the possibility of manipulating and applying chemical treatments (reduction, functionalization, etc.) to SWNTs having been purified. Comparative Example 9A: appearance of the sample of SWNTs having undergone a conventional purification method of the prior art

The evolution of the apparent density is determined by measuring the volume occupied by three different samples of known mass, (i) A sample at the outlet of the reactor having undergone no treatment named ap_SWNTs (for 'as-produced SWNTs'). (ii) a sample of SWNTs having undergone a three-step purification procedure according to reference [54], namely step 1: oxidation under dry air at 350 ° C. for 90 min; step 2: reflux in HCI (6N) for 24 hours; step 3: vacuum annealing at 1100 ° C. This sample is noted cp_SWNTs (for 'common purified SWNTs'). (iii) a sample having undergone the procedure according to Example 1, named hp_SWNTs (for 'halogen-purified SWNTs'). The calculation of the apparent density is carried out as follows. A known volume is filled with a known known mass of the sample by allowing it to occupy naturally this volume, that is to say without applying any compacting action of the powder. The comparison of the apparent densities of the carbon phase of the samples ap_SWNTs, cp_SWNTs and hp_SWNTs requires taking into account the quantity of catalysts present; this is all the more important as the catalyst concentration is different for each sample. The apparent densities obtained are presented in Table 3 below:

Table 3 apparent density sample (g / cm ³ ) ap_SWNTs 0.058 cp_SWNTs 0, 182 hp_SWNTs 0.086 Thus, for the sample having undergone the purification procedure according to Example 1, the apparent density is only slightly increased compared to that of a sample which has not undergone any treatment, whereas it is more than 3 times upper for a sample of SWNTs having undergone a standard purification procedure according to reference [54].

Comparative Example 10 Results obtained with purified SWNTs according to a process of the prior art

The document EP 2 135 842 describes a method for purifying carbon nanotubes comprising a step of treating the nanotubes in the presence of a halogen gas under reduced pressure. The method also comprises a step of oxidation of the nanotubes in the presence of oxygen, before or after the halogen gas treatment step (cf.

[0024]). Indeed, according to the document EP 2 135 842, the Cl ₂ treatment makes it possible to eliminate the catalytic impurities on the surface of the carbonaceous impurities and / or the nanotubes (ie, not covered with carbon), which requires a step separated oxidation to remove carbonaceous impurities and give access to catalytic impurities. The method according to the present invention makes it possible to overcome this additional oxidation step by promoting the diffusion of the dihalogen / interhalogen in the carbon.

In order to highlight this aspect, a comparative test was performed by purifying SWNTs according to the method of EP 2 135 842 using only the chlorine treatment step. The chlorine treatment was carried out under conditions very close to those described in EP 2 135 842, namely the conditions described in paragraph [0075] of this document. The assembly used is shown in Figure 16. This allows to perfectly control the pressure and the concentration of chlorine in the assembly. In this example, the comparative purification method (according to EP 2 135 842) is called Pci _b is, while the purification method according to the present invention (as illustrated in the preceding examples) is designated Pci.

Experimental part. The comparative purification purification under chlorine (Pci _b is) was performed by subjecting a 100 mg sample of electrically prepared SWNTs (ie the same type of SWNTs as those used in Examples 1 to 9). ) at a stream of chlorine under reduced pressure (flow rate: 300 ml / min, 20% of CI ₂ in N ₂ , pressure: 90 torr) at a temperature of 950 ° C. for 2 h. This choice of duration and temperature made it possible to directly compare the results obtained under the conditions of Pcibis with the conditions used in Pci as described in the preceding examples (flow rate: 100 ml / min, 90% of CI ₂ in N ₂ , pressure: 1 atm). The sample of carbon nanotubes used here is an arc-synthesized sample named: ARC SWNT.

The amount of chlorine used during the treatment for Pci _b is significantly lower than that used for the method according to the invention, P i _C (lower and higher flow concentration). However, it is much greater than the amount of chlorine needed to chlorinate all the catalysts present in the sample. Experimentally, it has been found that indeed the formation of metal chlorides takes place since they are visible as a deposit at the furnace outlet (Figure 16D). The HCI injection rate (whose reaction with KMnO ₄ makes it possible to continuously produce the chlorine) is controlled by means of a syringe pump through a capillary (FIGS. 16A and 16B). The injection rate is set so that the concentration of chlorine is 20% in the gas mixture CI2 / N2. The stream of nitrous oxide (ultrapure type Alphagaz 2: O2 <0.1 ppm, CO <0.1 ppm, CO ₂ <0.1 ppm, H ₂ O <0.5 ppm and H ₂ <0.1 ppm) from a cylinder is regulated by a valve and is fixed so that the gas flow rate is 300 mL / min and the pressure is maintained at 90 torr (or 0.12 bar) (Figure 16C). The depression in the system is provided by a water pump at the output of assembly.

Results and discussion.

- Thermogravimetric analysis

The catalyst concentration after the Pcibis treatment was determined by ATG in dry air (temperature ramp: 3 ° C / min, air flow:

20mL / min). Figure 17 shows the loss of mass recorded as a function of temperature for the ARC SWNT sample before treatment, after treatment according to P _C i and according to Pcibis.

The shape of the curves for the three samples is different. While for the raw sample and the sample after treatment P _C i, there are two mass loss domains (one towards 500 ° C and another at higher temperature, around 750 ° C for the raw sample and between 650 and 850 ° C for the sample after Pci), for the sample after the Pcibis treatment, only one mass loss persists around 730 ° C (see also Figure 18). This means that in the sample after PCibis, only one carbon phase is present after the treatment.

Regarding the effectiveness of treatment, Table 4 below summarizes the results:

Table 4

Sample method

Pci Pcibis

crude purification

Concentration

8.33 2.98 6.86

mass in

Table 4 compiles the catalyst concentrations obtained before and after the purification process P _C i and Pcib. The relevant yields to discuss the effectiveness of the treatments used are also given. It can be seen that the removal efficiency of metallic impurities (R _m ) is significantly lower using the Pcibis method than with the Pci method since it is decreased from 64 to 18%.

An important aspect of any purification process developed for the samples of carbon nanotubes concerns in particular its selectivity of elimination of the catalytic species without strongly attacking the carbon phases (and therefore the nanotubes). Knowing the loss of total mass of the sample, lost because of manipulations for the execution of the whole process and chemical reactions (elimination of catalysts and attack of the carbon phases) generated by the action of the chemicals used for the process treatments, the lost mass in carbon phases (P _c ) which is not directly related to the decrease in the concentration of catalysts is easily determined (Table 4). For the Pcibis treatment, a significant part of the sample, about 99% by weight, is lost during the treatment, which means that a significant combustion of the carbon species has probably occurred. because of a protective effect by chlorine very attenuated because of the reduction of its concentration and its pressure, compared to the method according to the invention P _C i which, it, fully benefits from this protective effect by the chlorine (see Example 12 for the controlled addition of oxygen during treatment with chlorine, which clearly shows this aspect).

A good indication of the preservation of the carbon phases relative to the efficiency of the removal of metallic impurities (R _m ) is given by the ratio between the percentage of the lost carbon phases and the efficiency of the elimination of the catalysts (| R _m -Pc l / P ( _c ) - The report | R _m . Pj / Pc reports the efficiency and selectivity of the purification process used. The greater this ratio, the more efficient the process is and at the same time selective for the removal of the catalysts. This ratio is greater for the treatment P _C i than for the process Pcibis (see Table 4), which shows that the conditions of concentration, flow and pressure of chlorine have a significant role as regards the treatment of chlorine. purification of the samples of carbon nanotubes.

- Observation by Transmission Electron Microscopy (TEM)

The observations by MET confirm the high consumption of the carbon species in the sample since we could not observe carbon nanotubes in the sample after Pcibis (see Figure 19). Graphite platelets can be observed which probably correspond to the carbon species whose removal during air ATG occurs at 730 ° C (Figures 17 and 18). The dark objects correspond to the catalysts still present after the treatment.

Example 11 Purification of HiPco type SWNTs under a continuous flow of dichloride In this example, the purification method according to the invention was tested on HiPco type SWNTs. The results were compared with the same method applied on electric arc SWNTs. In this example, a method for purifying conventional SWNTs according to the prior art, namely a method involving a separate oxidation step, was also compared.

In this example, the purification method according to the invention is designated Pci, while the conventional purification method is designated Po.

Experimental part.

The synthesis of the electric arc type sample was done in the laboratory, it is named ARC SWNT [63].

The sample produced by the HiPco method used was purchased from Nanolntegris Inc. and named HIPCO SWNT.

The conventional purification treatment (Po) contains 3 steps: (i) oxidation in dry air carried out at 350 ° C for 90 min; (ii) a reflux in hydrochloric acid (6N) for 24 hours to solubilize the metal oxides formed during step (i). The sample is then filtered to remove metal ions and the pH is raised to neutrality by successive washings with water. The final step (iii) consists of annealing carried out under secondary vacuum at 1100 ° C. for 1 hour.

In this example, the purification treatment according to the invention (P _C i) consists in subjecting the sample to a flow of chlorine (flow rate: 10OmL / min, 90% in N ₂ , pressure: 1 atm) at a temperature between 800 and 1100 ° C maintained for 2 hours or 4 hours. After this treatment, the sample is put in a solution of HCl (4N) at room temperature for 1 night. he is then filtered and after returning to neutrality by successive washings with water, it is dried at 50 ° C overnight.

Whether for the P ₀ or Pci treatment, the HIPCO SWNT sample was purified at 950 ° C for 2h all other experimental conditions remaining unchanged.

Results and discussion.

- Observation by transmission electron microscopy.

Figure 20 shows typical TEM images of the samples before and after the purification treatment. First, it is observed that the amount of metal impurities visibly decreased significantly after the treatment. A more detailed observation makes it possible to highlight the presence of empty multilayered carbon shells (Figure 21). Prior to the treatment, the latter were filled with catalytic metals (FIGS. 21A and 21B). Under the effect of heat, the highly volatile chlorides could sublimate and be removed from the sample leaving the shells empty (Figures 21 C and 21 D).

- Thermogravimetric analysis and magnetic measurements

The catalyst concentration was determined by ATG in dry air (temperature ramp: 3 ° C / min, air flow: 20mL / min). In the presence of oxygen in the air, the carbon phases of the sample are removed by combustion. At 1000 ° C., the remaining mass corresponds to the oxidized metal catalysts. Figure 22 shows the mass losses before and after treatment with chlorine flow (950 ° C, 2h, 90% Cl ₂ / N ₂ , 100mL / min, pressure: 1 atm) for the HIPCO SWNT sample. The concentration of metal oxides is reduced from 44.95% to 5.55%. The increase in mass between 185 ° C and 295 ° C prior to purification is due to the oxidation of iron. Table 5 below gives the metal oxide concentrations determined from the ATG measurements in metals in mass and atomic percentages for the samples synthesized by HiPco type CVD (HIPCO SWNT) and by electric arc (ARC SWNT) (for comparison) under the same experimental conditions after purification treatment Ρ _α under chlorine (950 ° C, 2h, 90% Cl ₂ / N ₂ , 100mL / min, pressure: 1 atm). The metal concentrations are thus decreased by a factor greater than 3 for the SWNTs synthesized by electric arc and by a factor greater than 8 for the HiPco type nanotubes. Table 5

ARC SWNT HIPCO SWNT

Metal l ic oxides (wt.%) 15 - 4 47 - 6

Metal (wt.%) 10 - 2.8 32 -> 4

Metal (at.%) 1.8 - 0.5 6.8 - 0.8

Determination of the catalyst concentration was also performed by magnetic measurements. The magnetization measurement in DC mode as a function of the applied magnetic field (between 0 and 7T) and the temperature (between 5 and 300K) is carried out using a SQUID equipment (superconducting quantum interference device (SQUID) by

Quantum Design [64]. Measuring the magnetic moment at 5K which is associated with the presence of iron impurities in the case of HiPco nanotubes or nickel in the case of arc-synthesized nanotubes makes it possible to directly determine the metal impurity content of the samples.

Figure 23 gives the results obtained on the HiPco nanotubes for which the catalytic impurities are in several forms: metallic iron, iron carbide [65] and probably also a small amount of iron oxide from non-ferrous particles. protected by carbon shells. The oxidized form is not expected to have a significant influence on the chlorination mechanism involved in the purification treatment discussed here because under the conditions used (temperature, pressure, etc.) the formation of the chlorides of iron is favored that the iron is in metallic form or oxide. Regarding the carbide form, we can see in Fig. 23 that the purification process used is also effective on iron carbides. Indeed, their concentration is reduced from 20% to 1% after purification.

FIG. 24 gives the yields (R _m ) calculated only from the evolution of the catalyst concentration for the two purification treatments, a conventional treatment based on an oxidation step (P ₀ , previously described) and the treatment. purification under chlorine

Several observations can be made. Firstly, catalyst removal efficiencies are higher with the purification method under chlorine Pci. In addition, there is good agreement between measurements made by ATG and magnetic measurements. It should be borne in mind that these are complementary techniques whose principles differ and which are not sensitive to the same forms of catalysts. The same yield values are not expected, but a good agreement confirms the good yield values obtained. Finally, for the same experimental conditions (950 ° C, 2h), good yields are obtained for both types of sample. This demonstrates the adaptability of the treatment developed according to the invention.

The mass lost carbon phases (P _c) and the removal efficiency of the metal impurities (R _m) and the parameter ratio | Rm-Pc l / P _c, which gives information on both the selectivity and the efficacy of the treatment, are given in Table 6 below:

Table 6 Process of

Sample purification I Rm-Pc I / Pc used (in mass%) (in mass%)

96 ** 58.0 ** 0.7 **

Po

ARC SWNT 41 18.8 1.2

70 1 1.4 5.1

Pc.

HIPCO SWNT 87 2.5 33.8

Table 6 lists the yields relating to the removal of catalysts and the loss of carbon phases after purification for the 2 samples studied: HIPCO SWNT and ARC SWNT. ^** classic purification treatment [ref. 66] comprising 3 steps: (i) oxidation under dry air carried out at 365 ° C for 90min; (ii) reflux in hydrochloric acid (6N) for 24 hours; (iii) vacuum annealing at 1100 ° C for 1 h. For the process under chlorine Pci, the mass losses vary little, only the averages have been reported.

The report | R _m -Pc l / P _c reports the efficiency and selectivity of the purification process used. The greater this ratio, the more efficient the process is and at the same time selective for the removal of the catalysts. This ratio is much greater for the process according to the invention than for a conventional purification process.

- Raman spectroscopy The quality of the HIPCO SWNT nanotube structure after the purification treatment was characterized by Raman spectroscopy. Figure 13 shows typical Raman spectra for HIPCO SWNT samples before and after purification under chlorine flow (950 ° C, for 2h), and after washing with HCl. The RBM modes differ little and the main contributions at 247 and 283 cm- ¹ are preserved, they correspond to diameters of 0.86 and 0.99 nm, as we have observed for the sample synthesized by electric arc, for the SWNTs obtained by the HiPco method, the treatment used not only preserves the quality of the nanotubes but induces an improvement in their structure since the ratios A (D) / A (G ⁺ ) and H (D) / H (G ⁺ ) decreased after the purification treatment (Figure 25 and Table 7) They are very little modified by the final wash, which means that the conditions (ambient temperature, moderate HCl concentration) do not induce nanotube damage. The temperature used near 1000 ° C is probably high enough for some defects to be removed from the sample by an annealing effect.

Table 7. Area and height ratios of the D band on the G ⁺ band calculated for all recorded spectra and averages.

Sample spectrum A (D) / A (G ⁺ ) average H (D) / H (G ⁺ ) average

Ν 0.38 0.17

before

No. 2 0.35 0.37 0.15 0.16 purification

N ° 3 0.38 0.16

Ν 0.17

after 0,13

purification, No. 2 0.28 0.23 0.14 0.14 before washing No. 3 0.24 0.13

Ν 0.22 0.14

after

purification, No. 2 0.29 0.26 0.14 0.135 after washing No. 0.26 0.12 These results obtained by Raman spectroscopy which show that the treatment under dichlor does not attack the structure of the nanotubes and on the contrary induces an elimination of the defects is in agreement with the strong shift observed towards the high temperatures of the combustion of the sample HIPCO SWNT after purification (see Figure 22). HiPco nanotubes contain more structural defects than nanotubes synthesized by electric arc (combustion temperature ~ 500 ° C); this is the reason why, the combustion temperature of the raw sample is located towards 300 ° C. After the purification treatment, it increases by 200 ° C., showing better stability of the nanotubes after the purification treatment.

Example 12 Purification of SWNTs under a continuous flow of dichlor with concomitant addition of oxygen

Experimental part :

The samples of nanotubes used are derived from a laboratory electric arc synthesis (ARC SWNT).

The purification treatment is carried out:

(i) either under a stream of pure chlorine at 1100 ° C. or at 950 ° C. for 2 h, the other experimental parameters being: 90% CI 2 / N 2, 100 mL / min, total pressure: 1 atm.

(ii) With addition of oxygen with a variable flow rate for 1 h in the CI2 / N2 mixture, the other experimental conditions of the treatment are: 1100 ° C or 950 ° C, for 2 h, 90% Cl ₂ / N ₂ , lOOmL / min, total pressure: 1 atm.

1. Results obtained without oxygen flow control (estimated flow: I mL / min) Figure 26 illustrates the ATG curves obtained after the purification treatment with and without oxygen in chlorine.

It is found that the efficiency of catalyst removal is not diminished when oxygen is added during the treatment under chlorine. In the sample of crude carbon nanotubes, the mass concentration of oxidized catalysts is 14% and decreases to 3.2% after purification under pure dichlor and reaches even 2.7% when dioxygen is added to the catalyst. chlorine during treatment.

The representation of TG shows a strong shift in the combustion range of carbon nanotubes towards high temperatures when the treatment is carried out in the presence of oxygen. This means that the stability of the nanotubes has been greatly improved after treatment with oxygen. These results are in agreement with the TEM images of FIGS. 5, 6, 7, 9, 10 and 11 which show no visible degradation of the walls of the nanotubes, see FIG.

2. Results obtained by varying the flow of oxygen in the flow of chlorine.

Flow rates used: D1: 0.45 mL / min, D2: 2.00 mL / min, D3: 4.10 mL / min. Figure 18 shows the mass losses achieved by ATG for the raw sample and after the treatment. purification under chlorine with variable flow rates of oxygen.

As it could be observed in Example 7, we can see that the concentration of catalysts is not affected by the presence of oxygen whatever the flow rate used. The purification treatment under chlorine remains effective. The variations in catalyst concentration observed as well as the calculated yields for the removal of catalytic impurities are reported in Table 8 below: Table 8. Catalyst concentrations obtained before and after the purification treatment with and without oxygen with variable flow rates.

The yields obtained show that the efficiency of the purification process is not decreased but instead increased when increasing the flow of oxygen. Moreover, the quality of the nanotubes is also improved since the mass loss area during their combustion is tightened and all the more offset to the high temperatures when the flow of oxygen during the treatment under chlorine is increased. This shift is greater than 100 ° C when a flow rate of 4.1 mL / min is reached (Figure 30).

The TEM observations of the samples after the treatments carried out do not show, as for the experiment carried out without control of the flow of oxygen at 1100 ° C, a significant increase in the concentration of nanotubes compared with the other carbon species. Nevertheless, for these experiments carried out at 950 ° C. with a variable oxygen flow rate, we do not observe any chemical attack of the nanotubes, their walls remaining intact in accordance with the temperature shifts of the combustion domain observed by ATG. Figure 31 shows typical images of the samples after purification for the different oxygen flow rates used.

The above Examples show, confirm or exacerbate several advantages of the purification treatment according to the invention. The most important are summarized below:

1) The purification treatment (which incidentally is advantageously a single-stage treatment, that is to say not requiring a separate step of oxidation of the nanotubes) is adaptable to different types of samples since:

it is effective in eliminating the catalytic impurities commonly produced in the synthesis of nanotubes (SWNT type HiPco and SWNT arc type), and;

under the experimental conditions used, the dihalogenated and / or inter-halogenated gas (eg, dichlor) is capable of diffusing well through the carbonaceous shells that protect the catalytic impurities, that is to say even in the case where the particles metallic catalytic impurities do not have a surface directly in contact with the dihalogen and / or inter-halogen gas (eg, dichlor).

2) The addition of oxygen during treatment under dihalogen and / or interhalogen gas (eg, dichlor) improves the quality of the nanotubes without reducing the efficiency of the removal of catalytic impurities. A relatively strong protective effect of the dihalogen and / or interhalogen gas (eg, dichlor) for the nanotubes is demonstrated since the temperature range used in the examples (~ 1000 ° C) is much higher than that used for conventional oxidation reactions of carbon nanotube samples. ) A "driving force" (or driving force in English) of the efficiency of the purification process according to the invention is directly related to the reaction power of the dihalogen and / or interhalogen gas (eg, dichlor). The greater the activity of the dihalogen and / or interhalogen gas (eg, dichlor), the more effective the treatment. According to one variant, this quantity can be controlled by the molar fraction (or the concentration) and / or the partial pressure of the dihalogenated and / or inter-halogenated gas (eg, chlorine) in the gaseous mixture. Moreover, without being able to quantify it, it is reasonable to think that the more pure the dihalogen and / or interhalogen gas (eg, dichlor) is pure, the greater its reaction power is important, the impurities being able to induce parasitic reactions. Moreover, the activity of the dihalogenated and / or inter-halogenated gas (eg, chlorine) varies inversely proportional to its flow rate, since a high flow rate limits its residence time in the reactor. The optimization of the experimental conditions to obtain a good reaction power of the dihalogenic gas and / or interhalogen (eg, dichlor) guaranteeing a good efficiency of the purification treatment thus leads to working under dihalogen and / or interhalogen gas (eg , dichlore) having:

- good purity,

- a relatively high concentration,

- high pressure,

- low flow, - a high temperature.

Compared to the experimental conditions used in the processes of the prior art, the experimental conditions of the process according to the invention thus allow a good efficiency of the purification treatment in a single step.

On reading the present application and illustrative examples above, those skilled in the art will recognize that the present process is general and is applicable to the purification of any carbon material whose synthesis involves the use of metal catalysts. .

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Claims

claims

1. Process for the purification of carbon nanotubes comprising a step of treating the nanotubes in the presence of dihalogen gas and / or an interhalogen gas at 200-2000 ° C., and in the absence of H ₂ O and HCl, without a separate oxidation step and without prior exfoliation of the carbonaceous multilayer shells in the sample.

2. Process according to claim 1, wherein the temperature is between 900 and 1100 ° C.

3. Method according to claim 1 or 2, wherein the treatment in the presence of dihalogen gas and / or inter-halogen is carried out in continuous flow or static system dihalogen gas and / or interhalogen.

4. Process according to any one of claims 1 to 3, wherein the treatment in the presence of dihalogen and / or interhalogen gas is carried out for 2 to 4 hours.

5. Method according to any one of claims 1 to 4, wherein the volume concentration of dihalogen gas and / or interhalogen is between 0.1% vol and 100% vol.

6. Method according to any one of claims 1 to 5, wherein the nanotube treatment is carried out further in the presence of oxygen in volume concentration between 10 ^"5 % vol and 20% vol.

The process of any one of claims 1 to 6, wherein the dihalogen gas is Cl 2.

The method of any one of claims 1 to 7, further comprising a step of washing the carbon nanotubes with an organic or inorganic solvent and / or using an acidic treatment.

9. Method according to any one of claims 1 to 8, wherein the carbon nanotubes are single-walled nanotubes.

10. A device for purifying carbon nanotubes comprising: a system for introducing a dihalogenic gas and / or interhalogen, - a thermal compartment in which temperatures of

200-2000 ° C can be implemented, and a system for controlling the diffusion of dihalogenated and / or inter-halogenated gas into the carbonaceous layers of the nanotubes to be purified, selected from a dihalogen gas purification system and / or inter -halogen, a dihalogen and / or inter-halogen gas concentration control system, a dihalogen and / or inter-halogen gas pressure control system, a dihalogen and / or inter-gas flow control system -halogen, or any combination of two or more of the aforesaid systems.

1 1. Device according to claim 10, further comprising a treatment compartment for the gaseous effluent at the outlet of the thermal compartment.

12. Device according to any one of claims 10 to 1 1, further comprising a compartment for introducing an inert carrier gas.

13. The device of any one of claims 10 to 12, further comprising a dioxygen introduction compartment.

14. Device according to any one of claims 10 to 13, wherein the dihalogen gas introduction system comprises a device for producing said dihalogen gas comprising a container containing KMnO ₄ connected to a solution addition device. aqueous HCl, and the dihalogen gas is CI2.