Classifications

• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y30/00—Nanotechnology for materials or surface science, e.g. nanocomposites
• • B—PERFORMING OPERATIONS; TRANSPORTING
  • B82—NANOTECHNOLOGY
  • B82Y—SPECIFIC USES OR APPLICATIONS OF NANOSTRUCTURES; MEASUREMENT OR ANALYSIS OF NANOSTRUCTURES; MANUFACTURE OR TREATMENT OF NANOSTRUCTURES
  • B82Y40/00—Manufacture or treatment of nanostructures
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B32/00—Carbon; Compounds thereof
  • C01B32/15—Nano-sized carbon materials
  • C01B32/158—Carbon nanotubes
  • C01B32/168—After-treatment
  • C01B32/17—Purification
• • C—CHEMISTRY; METALLURGY
  • C01—INORGANIC CHEMISTRY
  • C01B—NON-METALLIC ELEMENTS; COMPOUNDS THEREOF; METALLOIDS OR COMPOUNDS THEREOF NOT COVERED BY SUBCLASS C01C
  • C01B2202/00—Structure or properties of carbon nanotubes
  • C01B2202/02—Single-walled nanotubes

Definitions

• This invention relates to a method for the purification of carbon nanotubes. Since the discovery of carbon nanotubes and their unique chemical, physical and electronic properties, a lot of research concerned with finding possible applications for them has been carried out. Much of this research has been concentrated on single walled nanotubes (SWNTs). A SWNT can be viewed as a tube of a single graphite layer, which is capped at both ends; the caps are generally removed on purification. Possible applications for SWNTs include their use in supercapacitors, high strength composite materials and quantum computers (where a fullerene may be inserted in the tube). High precision technology of this type demands the highest possible purity of nanotube.
• SWNTs single walled nanotubes
• Nanotubes such as SWNTs are produced by the ablation of a carbon source, for example by the formation of an electric arc between graphite electrodes, in the presence of a molten catalyst such as an iron, nickel or cobalt based catalyst, optionally with an yttrium cocatalyst.
• the raw soot produced by these methods contains not only SWNTs but also amorphous and graphitic carbon impurities and condensed metal catalyst particles, typically of nano-sized dimensions, and which are themselves coated in layers of carbon such as graphene layers.
• Graphene is, as used herein, a single 2D layer made of hexagonal carbon rings fused by their edges. The coating on the metal particle is typically from 3 to over 50 graphene layers thick.
• SWNTs claim to be in the order of 60 to 70% pure; however microscopic investigations suggests that their purity may in fact be significantly less (semi-quantitative TEM analysis suggests that the purity of CarboLex nanotubes is around 20-30%).
• the SWNTs themselves may be of either metallic or semi-conducting nature.
• the scope of possible uses for metallic SWNTs is much narrower than that for semiconducting SWNTs and it is the semi-conducting SWNTs that have the greater potential in the quantum computer field of technology.
• purification processes such as extraction and chromatography are extremely successful for purifying molecular carbon species such as fullerene, for multiwalled nanotubes (MWNTs) and SWNTs these processes are not effective.
• Purification of SWNTs is currently accomplished by various forms of oxidation and acid treatment, for example by reflux in nitric acid or by heating in air at high temperature.
• the present invention relates to a method of purifying carbon nanotubes that overcomes the abovementioned difficulties.
• the method enables one to remove carbon-coated metallic nanoparticle impurities and further purifies the sample by removing nanotubes with metallic conductivity from the sample.
• the invention is particularly relevant for the purification of SWNTs.
• a method of purifying carbon nanotubes comprising annealing them in air or an oxygen-containing gas which, apart from oxygen, is inert to carbon nanotubes under the conditions used and then heating them in a medium which oxidises graphene only at a temperature of at least 330°C at atmospheric pressure or a medium equivalent thereto using a form of non-contact electromagnetic heating.
• the annealing step is important for removing primarily amorphous carbon from the sample.
• the treatment is generally carried out in air, typically for no longer than 5 hours, for example 30 minutes or lhour to 5 hours at a temperature of from 275 to 400 °C, e.g. 300 to 375°C.
• the annealing step should be controlled since prolonged heating and/or too high a temperature will result in the SWNTs being destroyed; the higher the temperature used the shorter the time should be i.e. use of, say, 375°C should typically be for 30 minutes to 90 minutes. Conversely, decreasing the temperature requires extension of the annealing time. For example, in a typical sample, 30-40% weight loss of the sample, corresponding to the loss of amorphous carbon, occurs when the sample is annealed in air at around 315°C for 2 hours or at around 350°C for 1 hour. It will be appreciated that if another oxygen-containing gas is used, the conditions may need to be varied consistent with the need to avoid the SWNTs being destroyed. Thus a higher oxygen concentration will generally require a lower temperature and/or a shorter time.
• Suitable forms of non-contact heating include microwave heating and induction heating.
• the advantage of using such forms is that they provide a method of localized heating of the metallic impurities. This leads to the oxidation and break down of the graphene coating thus liberating the metal core. Once exposed, the metal can be eliminated using a conventional acid treatment.
• the medium employed can be liquid it is preferably gaseous, such as air.
• the medium should be oxidising at ambient temperature.
• Microwaves will generally interact with any conducting material present in the sample causing an electric current to flow through the metal particles. This causes the metal particles, and subsequently the carbon coating surrounding them, to heat up.
• the most effective power range is generally from 100 to 1000W, typically from 200 to 850W and typical treatment time is fromlO minutes to 3 hours, typically from 0.5 to 3 hours; naturally the total power supplied will be dependent on the mass to be treated.
• the nanotube mixture is first treated at a low power, typically of from 200 to 300W for from, say, 10 minutes to 1.5 hours, e.g.
• a longer treatment time is required in the case that induction heating is used, typically from to 3 hours, e.g. 1.5 to 2 hours for example with a 5 cm diameter coil or about 1 hour for a 3.5 cm coil.
• Coil diameter and configuration significantly affects treatment time; a coil diameter of 0.1 to 10 cm, especially 0.5 to 5 cm, is typical.
• the current typically varies 1 to 150, for example 50 to 100 kW for high power or, for lower power, from 5 to 25 kW, for example about 15 kW.
• the frequency of AC in the heater is generally from 5kHz to 1.2 MHz. Generally speaking the higher the frequency the less the depth of penetration of the magnetic field into the sample.
• a similar procedure involving a relatively long period at low power followed by a relatively short period at high power may be advantageous.
• Nanotubes with a higher electro- conductance are more strongly coupled to an electromagnetic field than those with a lower conductance (semi-conducting). This results in the selective burning of the metallic nanotubes leaving the semi-conducting nanotubes behind intact.
• the ratio of the conducting to non- conducting SWNTs is about 1 :2.
• the present invention provides a multi step method for purifying carbon nanotubes comprising the following steps:
• the sample to be purified can be a raw sample containing nanotubes that has been produced by a standard technique with the use of a metal catalyst.
• the sample may have been synthesized by arc-discharge using a Ni/Y or Cr based catalyst, such as commercially available CarboLex SWNTs.
• Step (1) has been discussed above.
• step (2) the sample of reduced amorphous carbon content is then heated in the gas phase using a form of non- contact electromagnetic heating such as microwave or induction heating as described above. This results in the oxidation and removal of graphene layers which coat the metallic nanoparticles (and any amorphous carbon which remains).
• the sample is then heated under reflux or otherwise agitated in acid (step (3)) in order to remove the metallic impurities.
• the sample is typically treated until a weight loss of, say, 20 to 30%, for example approximately 25% has occurred.
• Typical reflux time is 5 to 15 hours, e.g. approximately 10 hours, however the exact treatment time will depend on the nature and concentration of the acid used.
• the choice of acid is important.
• the acid is both concentrated, typically a concentration of at least 20%, and a non-oxidative acid including inorganic acids such as hydrochloric, hydrobromic or phosphoric(V) acid as well as strong organic acids such as formic acid and trifluoroacetic acids. More preferably hydrochloric acid is used, such as 37% hydrochloric acid.
• non-oxidative acids are generally capable of dissolving metals and metal oxides without damaging the nanotubes.
• the use of non- oxidative acids is generally preferred as the use of oxidative acids, such as nitric acid and sulphuric acid tends to result in nanotubes which are damaged and which aggregate into interlocking mats, felts and clumps that are very difficult or impossible to re-disperse after purification is complete.
• the use of non-oxidative acids in the present invention allows the formation of a suspension of discrete nanotubes in a solvent.
• the product of the purification is a bundle of, say, 40 to 50 nanotubes; these are generally aligned so are not in the form of a mat. Nevertheless there is a need to obtain the nanotubes in as discrete a form as possible.
• Typical solvents which can be used include dimethylformamide, aqueous surfactant solutions, for example of sodium lauryl sulphate and Triton X-1-00, as well as aromatic solvents, especially those having a high affinity for fullerenes, such as 1, 2 - dichlorobenzene.
• aqueous surfactant solutions for example of sodium lauryl sulphate and Triton X-1-00
• aromatic solvents especially those having a high affinity for fullerenes, such as 1, 2 - dichlorobenzene.
• certain non-polar low molecular weight solvents are particularly effective.
• a non-polar low molecular weight solvent i.e. a solvent which possesses substantially no dipole moment and has a molecular weight no exceeding 100, especially not exceeding 80.
• a non-polar low molecular weight solvent i.e. a solvent which possesses substantially no dipole moment and has a molecular weight no exceeding 100, especially not exceeding 80.
• inert gases such as argon, neon and nitrogen satisfy these requirements, for them to be used as liquid solvents either very high pressures have to be used or very low temperatures have to be used, although the pressures needed for argon are generally acceptable.
• very high pressures does, of course, require special equipment while the use of very low temperatures has the effect that they do not have sufficient energy for them to perform a solvating action.
• the molecular weights should be low so that the molecules can disperse amongst the nanotubes. It is also desirable that the molecule is substantially flat, for a similar reason. By this it is meant that they are generally aromatic molecules with planar ⁇ -systems (they usually have a high affinity for sp2- carbon species) which do not possess bulky lateral groups such as alkyl groups (except perhaps methyl groups).
• Preferred solvents which can be used include carbon disulphide and carbon dioxide which can be in the form of super critical carbon dioxide at a temperature of about 50 °C. It is envisaged that liquid carbon dioxide can be added at this temperature. Dropping the temperature to ambient temperature is unlikely to result in significant agglomeration.
• Carbon dioxide has a similar geometry to carbon disulphide but is smaller and under supercritical conditions when it possesses extremely low surface tension it can penetrate through very small channels. Furthermore they leave no residue when the pressure is relieved.
• carbon disulphide in particular, it is desirable to aid dispersion in the liquid by subjecting the mixture to an ultrasound treatment, typically for a few minutes. While it is possible to subject the SWNTs to supercritical carbon dioxide at, say, room temperature to 50°C for a prolonged period, for example 2 to 5 hours, it has been found that generally better results can be obtained by subjecting the nanotubes to short bursts of the liquid solvent.
• the nanotubes can be immersed in Sc CO 2 under pressure, e.g.
• the present invention also provides a dispersion of nanotubes in a non-polar solvent having a molecular weight not exceeding 100. Agitation in this, and the other steps, is desirable to improve dispersion. For this step sonication is preferably performed. For steps which are not carried out in the liquid phase, tumbling the sample in a tube will generally suffice.
• Step (3) may optionally be preceded by an additional step to disperse the nanotubes in the acid before reflux begins, for example by ultra sound agitation. Sonication generally disperses the nanotubes quickly in the acid solution and is typically carried out for 15 minutes or longer. This step is not absolutely necessary, however, as the nanotubes do eventually get dispersed during reflux.
• step (3) it is normal to carry out a separation step to remove nanotubes/solids from acid by simple filtration and drying.
• step (4) the nanotubes are annealed in air.
• This step is generally carried out at a higher temperature than the pre-annealing step (step (1)) in order to remove graphitic particles and any traces of amorphous carbon that may still remain in the sample. Care must however be taken not to overheat the sample as this will result in damage to the nanotubes.
• the time of annealing and weight loss will therefore depend on the temperature used.
• this step is carried out without an air flow at from 400 to 500°C, generally for 1 to 5 hours and is typically accompanied by a weight loss of approximately 2 to 3%.
• an inert gas such as nitrogen doped with a controlled amount of oxygen.
• the overall yield of nanotubes is 7 to 8%.
• Semi- quantitative analysis indicates that, generally, no amorphous carbon and only a trace amount of graphitic carbon are present.
• the metal content of the purified sample is generally not more than 10%, and especially not more than 1% by weight.
• the purification process generally results in the removal of the caps of the nanotubes and this in consequence causing some shortening of the tubes since fresh material is exposed by the removal.
• the resulting nanotubes are at least l ⁇ m long, for example about 2 ⁇ m long.
• the nanotubes can be filled with fullerene, for example C 60 , C 70 or C 82 , including Ce@C g2 fullerenes.
• fullerene is added to the suspension in, especially, carbon disulphide or sc- CO 2 in a high pressure cell or subjected to vacuum heating in known manner, typically at a vacuum of 10 '6 torr and a temperature of 300° to 500 °C. It is surpri singly been found that although fullerenes are not dissolved by sc - CO 2 they can nevertheless be suspended in it and can enter the nanotubes.
• Nanotubes after the high temperature treatment are matted.
• the purified nanotubes were re-dispersed with 10ml of CS 2 in an ultrasound bath at room temperature over 30min.
• TEM and AFM analyses indicate that the resulting nanotubes are very well dispersed, free of amorphous impurities and contain about 1% (by weight) of metal. Overall yield is 7-8%.
• Figure 1 TEM image of SWNT purified by a non-contact heating method the method of the present invention.
• Figure 2 TEM image of purified nanotubes after dispersing in CS 2 .
• Figure 3 High resolution TEM image of a nanotube filled with fullerene. The diameter of each circle is about 0.7 nm corresponding to the diameter of C60.
• Figure 4 TEM image of raw CarboLex nanotubes and purified nanotubes after annealing at 420° and 500°C in air according to the present invention.
• Figure 5 ESR spectra of raw nanotubes and those purified according to the present invention, normalised for the mass of the samples.
• Figure 6 Raman spectroscopy of raw nanotubes and those purified according to the present invention using microwave gas phase purification.
• FIG 4 illustrates the fact that although a final annealing temperature of 420°C in air in the process of this invention will eliminate most residual graphitic particles a few remain (see lower right hand corner) whereas substantially none of them remain if the annealing takes place at 500°C.
• the electron spin resonance spectroscopy (ESR) shown in Figure 5 demonstrates that the process of the present invention effectively removes metallic impurities. It can be seen that the amount of paramagnetic impurities drastically decreases during purification.
• the D-mode peak does not increase, indicating that the purification method does not damage the nanotubes.
• the raw SWNTs give rise to five peaks which correspond to a radial breathing mode (RBM) corresponding to a particular diameter.
• RBM radial breathing mode
• the purified SWNTs contain only two diameters corresponding to 13.6°A and 14.9°A. These diameters are ideal for assembling "peapod" structures.
• the nanotubes sidewalls remain substantially intact during purification.
• the nanotubes structures and, hence, their electronic properties are not affected by the purification process. This is, of course, very important for the future application of purified SWNT in electronic nanodevices.

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