WO2004052781A1

WO2004052781A1 - Purification of nanotubes

Info

Publication number: WO2004052781A1
Application number: PCT/GB2003/005409
Authority: WO
Inventors: Andrei Nicolaevich Khlobystov; Martin Robert Austwick; George Andrew Davidson Briggs
Original assignee: Isis Innovation Limited
Priority date: 2002-12-12
Filing date: 2003-12-12
Publication date: 2004-06-24
Also published as: EP1583716A1; AU2003290249A1; GB0229033D0

Abstract

A method of purifying an impure sample of carbon nanotubes is described comprising a method of purifying an impure sample of carbon nanotubes comprising annealing them in air or another oxygen-containing gas which, apart form 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.

Description

PURIFICATION OF NANOTUBES

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.

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. Commercially available 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. Before the SWNTs can be used in such applications it is therefore necessary to first purify this raw soot mixture. While 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. While these methods are effective at removing amorphous and graphitic carbon impurities, the removal of metallic nanoparticles represents a greater challenge as their carbon coating protects the underlying metallic core from acid attack. The use of oxidative acids such as nitric acid also results in damage to the SWNTs and produces matted nanotube aggregates. This makes re-dispersion to obtain discrete nanotubes at a later stage very difficult.

It is very important to remove the metallic nanoparticles, as they can possess magnetic properties that would render the SWNTs useless for electronic-based applications. Attempts have been made to remove the carbon-coated metallic nanoparticles by subjecting the SWNT mixture to high temperatures and/or contact with concentrated acids for prolonged periods of time. However, these have proven ineffective in removing the graphene-encapsulated metal particles and often result in the damage and destruction of the SWNTs themselves.

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.

According to the present invention there is provided 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. It will of course be appreciated that 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.

Since any moisture present significantly inhibits oxidation, it is desirable that the atmosphere should not be damp. Atmospheric air is normally satisfactory although it can be dried if necessary.

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.

It will be appreciated that if a medium will oxidise graphene at 330°C at atmospheric pressure it will generally be capable of oxidising it at a lower temperature under increased pressure. Likewise a medium which is not capable of oxidising graphene at 330 °C at atmospheric pressure may be able to do so if the pressure is increased sufficiently. This latter condition is to be considered as equivalent for the purposes of the present invention. Nitric acid, though starts to oxidise graphene at below 100°C.

While 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. In the case that microwave treatment is used, 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. More preferably, 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. at 240W for 30 minutes, followed by a high power treatment of from, say, 750 to 850W, e.g. 800 W, for a shorter period, e.g. 5 to 7 minutes. This two-stage treatment allows for the effective elimination of the metallic impurities without exposing the sample to high power microwaves for such a prolonged period that significant damage to the SWNTs occurs. Essentially it is believed that the low power burns off impurities while high power is needed to burn off multiple layers. Accordingly increased impurities requires increased low power treatment. At high power it has been found that the localised heating at the metal tends to spread causing damage to the nanotubes. It will be appreciated that although this has been described as a "two-stage" treatment, it can be carried out in more than two stages. Essentially what is required is a stage at a lower power and subsequently a shorter stage at a higher power.

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. By placing the sample in the magnetic field of the induction heater, it is believed that eddy currents are induced within the electrically conductive components of the sample, such as within the metallic nanoparticles. As in the case where microwaves are used, it is believed that the localized heating which results leads to the destruction of the graphene coating.

The added advantage of non-contact electromagnetic heating is that it further separates the more useful semi-conducting nanotubes from nanotubes possessing metallic conductivity. It is impossible to separate the two types of nanotubes by traditional chemical methods. However, there is a structural difference that significantly affects their electronic properties. Nanotubes with a higher electro- conductance (metallic) 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. Typically in the starting material the ratio of the conducting to non- conducting SWNTs is about 1 :2. By means of the process of the present invention this ratio can be reduced generally to at most 1 :4, normally at most 1 :6 and especially at most 1 : 10 or even 1 :20. It will be appreciated that it is not possible to establish visually which tubes are semiconductors and which are metallic. If the ratio is improved the chances of selecting the desired SWNT is increased. In a preferred embodiment, the present invention provides a multi step method for purifying carbon nanotubes comprising the following steps:

(1) Anneal the raw nanotube sample in air

(2) Heat the resulting sample in the gas phase using a form of non-contact electromagnetic heating (3) Reflux the sample in acid, and

(4) Anneal the sample in air

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. For example, 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. In 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. Preferably 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. When more dilute acids are used a longer treatment time is required. These 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.

In general 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. However we have found that certain non-polar low molecular weight solvents are particularly effective. According to another aspect of the present invention it has been found that particularly good dispersions can be obtained by using 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. While many 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. The use of 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. Nevertheless, as indicated, 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 π-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. With 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. For example, the nanotubes can be immersed in _ScCO₂ under pressure, e.g. 150bar at 30- 50°C, for, say, 15 minutes and then quickly depressurised to atmospheric pressure and the procedure repeated, say, 5-10 times. This may be the equivalent of "wash and rinse" cycles. It has also been found that the use of scCO₂ generally results in reduced bundling of the SWNTs. This may be due to intercalated solvent in the bundles rapidly expanding after depressurization.

It is to be appreciated that this dispersion applies to nanotubes regardless of their state of purity or origin. Accordingly, 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.

After step (3) it is normal to carry out a separation step to remove nanotubes/solids from acid by simple filtration and drying. Finally, in 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. Typically 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%. While it can be carried out at about 400 to 450°C for, say, 2 to 4 hours, e.g. at 420°C for 3 hours it is generally preferable to use higher temperatures, for example up to 550°C, typically from 460° to 520°C and preferably form 480 to 510°C e.g. about 500°C. It is desirable to control the amount of oxygen which is available either without an air flow as indicated or by, for example, flowing over the sample an inert gas, such as nitrogen doped with a controlled amount of oxygen.

Typically, the overall yield of nanotubes (SWNTs) 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. Typically the resulting nanotubes are at least lμm long, for example about 2μm long.

Once purified, the nanotubes can be filled with fullerene, for example C₆₀, C₇₀ or C₈₂, including Ce@C_g2 fullerenes. Typically fullerene is added to the suspension in, especially, carbon disulphide or sc- CO₂ 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 - CO₂ they can nevertheless be suspended in it and can enter the nanotubes.

EXAMPLES As-received CarboLex single-walled carbon nanotubes (330mg) were placed in an alumna crucible and annealed in air at 310-315 °C for lhr 50min with occasional stirring with a stainless steel spatula (weight loss 90mg). The air- annealed nanotubes were treated with microwaves at 240W for 27min with occasional stirring and at 800W for 5min (weight loss 80mg). Then the sample of nanotubes was dispersed in 37% HCl and the resulting suspension was refluxed for 30h. The product was filtered on a PTFE membrane filter (pore size 0.2-0.5A-m), throughly washed with deionised water and MeOH. The product was dried in vacuum (weight loss 115mg). Then the dry powder was annealed in air at 420 °C for 2h with occasional stirring (weight loss 14mg; further annealing at 420 °C does not cause any detectable weight loss). Nanotubes after the high temperature treatment are matted. The purified nanotubes were re-dispersed with 10ml of CS₂ 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%.

This is illustrated in the accompanying drawings in which 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 CS₂.

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.

Figure 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.

It can be seen from the result of Raman spectroscopy shown in Figure 6 that 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. Thus there are at least five different diameters present in the raw SWNTs. However, the purified SWNTs contain only two diameters corresponding to 13.6°A and 14.9°A. These diameters are ideal for assembling "peapod" structures. It is clear that the nanotubes sidewalls remain substantially intact during purification. Thus 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.

Claims

1. Method of purifying an impure sample of carbon nanotubes comprising annealing them in air or another oxygen-containing gas which, apart form 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.

2. Method according to claim 1 wherein the annealing step is carried out in air.

3. Method according to claim 2 wherein the annealing step is carried out for 30 minutes to 5 hours at 275° to 400°C.

4. Method according to any one of claims lto 3 wherein the medium is air.

5. Method according to anyone of claims lto 4 wherein the nanotubes to be purified are single walled nanotubes (SWNTs).

6. Method according to any one of claims 1 to 5 wherein the form of non-contact electromagnetic heating is microwave or induction heating.

7. Method according to claim 6 wherein the non-contact electromagnetic heating is carried out in more than one stage, involving a stage at lower power and subsequently a shorter stage at higher power.

8. Method according to claim 7 wherein the non-contact electromagnetic heating is carried out in two stages, the first stage being carried out at lower power for a longer time, the second stage being carried out at a higher power for a shorter time.

9. Method according to any one of claims 6 to 8 wherein the microwave heating is carried out using a power of from 100 to 1000 W for a treatment time of from 10 minutes to 3 hours.

10. Method according to claim 9 wherein the first stage of microwave heating is carried out at 200 to 300 W for 10 minutes to 1.5 hours and the second stage is carried out at 750 to 850W for 5 to 7 minutes

11. Method according to any one of claims 6 to 8 wherein the induction heating is carried out using a power of from 5 to 25 kW for from 1 to 3 hours.

12. Method according to any one of the preceding claims wherein the annealing step is carried out for 30 minutes to 5 hours at a temperature of 275° to 400°C.

13. Method according to claim 12 wherein the annealing step is carried out for 1 to 2 hours at a temperature of from 250 to 400°C.

14. Method according to any one of the preceding claims which comprises the following steps: (1) Anneal the raw nanotube sample in air

(2) Heat the resulting sample in the gas phase using a form of non-contact electromagnetic heating

(3) Agitate the sample in acid, and

(4) Anneal the sample in air.

15. Method according to claim 14 wherein step (3) is carried out by refluxing in acid.

16. Method according to claim 14 or 15 wherein the acid used in step (3) is a non-oxidative acid.

17. Method according to claim 16 wherein the acid is hydrochloric acid, hydrobromic acid or phosphoric(V) acid.

18. Method according to claim 17 wherein the acid is hydrochloric acid.

19. Method according to any one of claims 14 to 18, wherein the acid used in step (3) has a concentration of 20% or greater.

20. Method according to any one of claims 14 to 19 wherein step (4) is carried out at a temperature from 400° to 550°C.

21. Method according to claim 20 wherein step (4) is carried out at temperature of 480° to 510°C.

22. Method according to claim 20 wherein step (4) is carried out at a temperature from 400° to 500°C for 1 to 5 hours.

23. Method according to any one of the preceding claims wherein the impure sample of carbon nanotubes has been produced by a catalytic arc-discharge method.

24. Method according to claim 23 wherein the catalyst is a Ni/Y or Cr based catalyst.

25. Method according to any one of the preceding claim wherein the purified carbon nanotubes are suspended in a non-polar solvent which has a molecular weight not exceeding 100.

26. Method according to claim 25 wherein the said solvent is carbon disulphide or supercritical carbon dioxide.

27. Method according to claim 1 substantially as hereinbefore described.

28. Purified SWNTs obtainable by a method according to any one of claims 2 to 27 wherein the ratio of metallic SWNTs to semi-conducting SWNTs is at most 1:4.

29. Purified SWNTs according to claim 28 wherein the said ratio is at most 1 :10.

30. A suspension of SWNTs according to claim 29 in a solvent.

31. A suspension according to claim 30 wherein the solvent is non-polar and has a molecular weight not exceeding 100.

32. A suspension according to claim 31 wherein the solvent is carbon disulphide or super critical carbon dioxide.

33. A suspension of SWNTs in a solvent which is non-polar and has a molecular weight not exceeding 100.

34. A suspension according to claim 33 wherein the solvent is carbon disulphide or super critical carbon dioxide.