WO2005085131A2

WO2005085131A2 - Separation of carbon nanotubes

Info

Publication number: WO2005085131A2
Application number: PCT/GB2005/000822
Authority: WO
Inventors: Robin John Nicholas; Joseph Gregory Wiltshire; Lain-Jong Li
Original assignee: Isis Innovation Limited
Priority date: 2004-03-02
Filing date: 2005-03-02
Publication date: 2005-09-15
Also published as: GB0404713D0; WO2005085131A3

Abstract

A method of sorting single-walled carbon nanotubes according to their physical structure by use of an inhomogeneous magnetic field. A mixture of single-walled carbon nanotubes in an aqueous surfactant suspension is positioned in a gradient magnetic field. Those nanotubes having metallic properties, and thus being paramagnetic, are attracted to regions of higher field while those having semiconducting properties, and thus being diamagnetic, are repelled to regions of lower field. Within the semiconducting nanotubes, a separation is also achieved according to their diameter upon which depends their diamagnetic susceptibility. The distribution can be adjusted by selecting the density of liquid used to suspend the nanotubes, for example by using heavy water so that the nanotubes move to a position where the net forces of gravity, buoyancy and the magnetic field, are in balance. A method of separating impurities, such as metallic catalyst particles, is also described in which a magnetic field is applied to a suspension of carbon nanotubes and the metallic particles are attracted towards the poles of the magnet allowing them to be removed.

Description

SEPA ATION OF CARBON NANOTUBES

The present invention relates to a method for separating or sorting carbon nanotubes, in particular according to their different electronic properties and different sizes, and to a method of separating impurities from carbon nanotubes.

Carbon nanotubes, which are pure carbon fullerene-related structures consisting of a sheet of carbon atoms formed into a tube and closed at each end, were discovered in the early 1990s. In the accompanying drawings Figure 1 shows schematically a single-walled carbon nanotube as illustrated in Berkeley University's "Introduction to Singled- Walled Nanotubes" which is basically a rolled-up sheet of graphite, known as graphene. Different li-oking structures of the carbon atoms are possible, characterised by different lattice vectors, for example there are "armchair" types, "zig-zag" types and "chiral" types as illustrated in Rice University's "Carbon Nanotubes and their Practical Uses" and shown in Figures 2A, B and C. Single- walled carbon nanotubes have an average diameter of 1.2 to 1.4 nanometres and an average density of 1.36 g/cm³, and can be up to 1 to 1.5 microns long.

Single-walled carbon nanotubes are of interest because of both their physical and electronic properties. They are very strong, 10 to 100 times stronger than steel per unit weight and have a good thermal conductivity. The electronic properties of the carbon nanotubes depend on their structure. "Armchair" tubes are metallic, the other structures are semiconducting. Semiconducting nanotubes have a band gap of 0.4 to 1.5 eV, the band gap being proportional to the reciprocal of the diameter of the nanotube. These band gaps are expected to make them suitable for use in optoelectronic applications. To date a major commercial use of carbon nanotubes has been in the electrodes in compact batteries, where their use increases battery life.

Single- walled carbon nanotubes are currently made in one of two ways. One way involves the burning of methane in an arc discharge, which produces soot, which contains single-walled carbon nanotubes. Another method is the laser-vaporisation of graphite in an argon atmosphere. However, both methods produce random mixtures of different types of nanotube, namely different dimensions and different structures. Approximately two-thirds of typical commercially-available mixture is semiconducting and approximately one third metallic. The metallic and semiconducting nanotubes have different uses, and within the semi-conducting nanotubes the diameter affects the band gap and thus this will affect the use (for example the useful wavelength in an opto-electronics application). It would therefore be useful to be able to separate out or sort the nanotubes according to their electronic properties and size.

In the production of single-walled carbon nanotubes it is necessary to include a catalyst material, typically FeCO₅ or YNi mixtures. However the metal catalyst becomes a contaminant in the final product. Currently techniques such as acid washing is used to dissolve the metal catalyst, but this can damage the SWNTs themselves. Thus another problem with currently available SWCNs is contamination with such impurities.

The present invention provides a method of sorting carbon nanotubes by the step of applying to a mixture of nanotubes an inhomogeneous magnetic field. In more detail it can provide a method of sorting carbon nanotubes comprising: forming a mixture of carbon nanotubes in which the carbon nanotubes are individually mobile, applying to the formed mixture an inhomogeneous magnetic field to separate the carbon nanotubes in the formed mixture in dependence upon at least one of the structure and size of the carbon nanotubes, and collecting the carbon nanotubes from at least one selected location therein.

Thus with the present invention a magnetic field gradient is used to separate the different types of nanotube, and a desired type of nanotube can be collected from different locations where the values of the magnetic field gradient are different. The nanotubes may be made individually mobile by suspending them in a liquid. Though other ways, such as sonication, vibration or gaseous or aerosol suspension could be used.

Preferably the mixture contains single-walled carbon nanotubes.

Preferably the carbon nanotubes are in micelle form in the suspension, and this may be achieved by foπning an aqueous suspension using a surfactant, preferably with ultrasonic treatment of the suspension. This allows the nanotubes to untangle and separate from each other.

Metallic nanotubes tend to be paramagnetic and semiconducting nanotubes diamagnetic. This means that the metallic, paramagnetic, nanotubes are attracted to a region of higher magnetic field, while the semiconducting, diamagnetic, nanotubes are repelled to a region of lower magnetic field. Furthermore, the magnetic susceptibility depends on the diameter.

The density of the liquid in which the nanotubes are suspended may be selected to be similar to that of the nanotubes to assist the sorting process. Carbon nanotubes have a density between that of water and heavy water (D₂O) and so by using heavy water to adjust the density of the suspension liquid it is possible to control the buoyancy of the nanotubes, i.e. to make them rise, be suspended or sink. When the magnetic field gradient is applied the nanotubes will respond to the combination of buoyancy, gravitational and magnetic forces, and can be suspended at a point in the suspension/field where these forces balance or forced to separate to opposite ends. Because the magnetic force on the nanotubes depends on their magnetic susceptibility, which in turn depends on their structure, a structure-dependent distribution is formed in the suspension. Thus by controlling the buoyancy and the magnetic force on the nanotubes (by virtue of adjusting the magnetic field) it is possible to control this structure-dependent distribution. For example, by arranging for the carbon nanotubes to have a slight tendency to sink, and arranging for the magnetic field to increase downwardly, metallic nanotubes will be drawn downwards. Semiconducting carbon nanotubes will feel an upwards magnetic force, and those having a lower susceptibility will tend to sink lower in the solution before the upwards magnetic force is sufficient to stop them sinking. Thus the nanotubes of different structure and size are separated and a particular type and size of nanotubes can be collected by drawing off from a selected location in the suspension. Alternatively, the nanotubes can be arranged to float, and/or the magnetic field can be arranged to increase upwardly depending on the desired orientation of the size and structure - dependent distribution.

The suspension may be contained in a fractionating column allowing selective collection of the sorted carbon nanotubes at different levels in the suspension. The magnetic field may be moved relative to the suspension to sort the carbon nanotubes in the suspension and/or the suspension may flow along a flow path through the inhomogeneous magnetic field, with collection occurring at a selected point in the flow path.

In the above arrangements the magnetic field gradient was arranged substantially vertically, so that the nanotubes responded to the combined effects of the magnetic field and buoyancy. However, it is possible to arrange for the magnetic field gradient to be in a different direction, for example horizontally, in which case the metallic nanotubes would be directed, say, to the left and the semi-conducting nanotubes, say, to the right. Thus the suspension containing the mixture of nanotubes could be arranged to flow substantially vertically through a region with the magnetic field gradient arranged horizontally, and the two types of nanotube would then separate to opposite sides of the flow path. They could be collected by collection plates or drawn off from the flow.

In yet another variation centrifugal force can be used in combination with the magnetic force by arranging for the suspension to be in a centrifuge.

The method thus allows the separation of metallic from semiconducting nanotubes and also the separation of nanotubes depending on their physical structure and in particular their diameter.

Another aspect of the invention provides a method of separating impurities from carbon nanotubes comprising applying to the carbon nanotubes a magnetic field. The carbon nanotubes may be formed into a mixture in which they are individually mobile, and the magnetic field may be applied to the mixture. The mixture may be a suspension, for example a liquid suspension, as described above.

This aspect of the invention provides a particularly effective way of removing impurities, such as iron catalyst particles, from a sample of carbon nanotubes, without the use of damaging chemicals.

The invention will be further described by way of example with reference to the accompanying drawings in which:- Figure 1 schematically illustrates a section of a single- walled carbon nanotube; Figures 2(a), (b), (c) illustrate schematically the three main types of carbon nanotubes; Figure 3 illustrates schematically a first embodiment of the invention; Figure 4 illustrates schematically a second embodiment of the invention; Figure 5 illustrates schematically a third embodiment of the invention; Figure 6A-D illustrates schematically the balance of forces on the carbon nanotubes in suspension; Figure 7 illustrates a fourth embodiment of the invention; Figure 8 illustrates a fifth embodiment of the invention; and Figure 9 illustrates a sixth embodiment of the invention.

Figure 3 schematically illustrates a first embodiment of the invention. A suspension 1 of a mixture of single-walled carbon nanotubes contained in a container 3 is placed in an inhomogeneous magnetic field 5. This magnetic field is, in this embodiment, created in a region just above the magnetic coils 8 (permanent magnets may, of course, be used). Thus a magnetic field gradient is created in the suspension, with the higher magnetic field lower down.

Example

The suspension 1 may be prepared by using a proprietary single-walled carbon nanotube product, such as that produced by Carbon Nanotechnologies, synthesised by the HiPCO method which consists of tubes with a broad diameter distribution of 0.8 to 1.3 nanometres. This material is already purified for sale and is specified as being 92 to 94% single-walled carbon nanotubes, with the remainder being approximately 3% residual iron catalyst and 3 to 5% residual amorphous carbon. An aqueous surfactant suspension can be prepared from this material using 50 to 60 mg of carbon material dispersed and stirred in 100 mm of D₂O, with 1 wt % sodium dodecyl sulphate (SDS) as a surfactant. Other suitable surfactants (ionic, cationic or polymeric) may be used. Also an organic rather than an aqueous solution may be prepared. The solution may be treated by ultrasonication at a power of 250 watts for 12 hours to form a colloid solution. This solution may be used as the suspension, or it may be ultra-centrifuged, for example at 26,000 rpm for 4 hours, in order to separate individual nanotubes from nanotube bundles. The enriched solution of individual single-walled nanotubes can be collected from the upper supernatant, and used as the suspension 1.

As indicated different solutions may be used with different densities, which affect the buoyancy and thus the behaviour of the nanotubes. The magnetic field and gradient used depends on the type and degree of separation required. As an example, a B field of 10 to 15 T with a gradient of 10 to 15 T/m may be used.

After the suspension has been placed in the magnetic field gradient, the carbon nanotubes from different parts of the suspension, now separated according to their magnetic properties, can be collected by means of a collector 9 which can be introduced into the suspension 1 to remove material from different selected locations. In the illustrated embodiment semiconducting nanotubes which are diamagnetic will be repelled to the region of lower field (upwards) while the metallic nanotubes, which are paramagnetic, will be attracted downwards to the region of higher field (downwards). In addition, amongst the semiconducting nanotubes, those of higher diamagnetic susceptibility will be repelled more strongly. Thus, assuming that the nanotubes have a tendency to sink in the suspension (because they are more dense than the liquid), semiconducting nanotubes of higher magnetic susceptibility will end up higher in the tube. So amongst the semiconducting nanotubes towards the top of the tube there will be a size distribution. The nanotubes also align with the magnetic field.

By adjusting the density of the liquid used for the suspension 1 it is possible to arrange for nanotubes having different magnetic properties to be suspended in the magnetic field at different levels. A given carbon nanotube will be suspended at the point at which the forces upon it (due to the magnetic field and, its buoyancy and gravity) balance. Thus by altering the density of the liquid, nanotubes can be made to tend to sink or float or to remain suspended in the absence of a magnetic field gradient. The application of the magnetic field gradient then affects the balance of forces in dependence upon the magnetic properties, and thus the structure of the nanotubes. There will then be a distribution through the suspension of nanotubes in accordance with their different structures. Semi-conducting nanotubes will be separated from metallic ones because of their opposite behaviour in the magnetic field, and amongst each type there will be a size distribution because of the magnetic susceptibility depending in diameter. Various possibilities are illustrated in Figures 6A-D.

Figure 6A illustrates a semiconducting carbon nanotube in a suspension where the suspension liquid is less dense than the carbon nanotube (e.g. less or no heavy water is used). Thus the carbon nanotube has a tendency to sink because the upwards buoyant force is less than the force of gravity. As illustrated the magnetic field increases downwardly and so a magnetic force proportional to B.dB/dx is exerted upwardly on this semiconducting carbon nanotube. Thus the carbon nanotube will move in the suspension to a point at which the upwards magnetic and buoyant forces balance the gravitational force. In Figure 6B the situation is shown for a metallic carbon nanotube with the same magnetic field gradient increasing downwardly. Thus the metallic carbon nanotube will feel a downwards magnetic force which will add to the gravitational force. By using a suspension liquid with a density greater than that of the carbon nanotubes, for example by use of heavy water, the carbon nanotubes will feel a stronger buoyant force than in Figure 6A, and so in this case the metallic carbon nanotubes will have a tendency to float upwards in the absence of a magnetic field, but with the magnetic field gradient applied, they will move to a position where the forces are in balance. Figures 6C and 6D show similar situations with the magnetic field gradient reversed so that it decreases downwardly. In Figure 6C, for a semiconducting carbon nanotube, the magnetic force will tend to pull the nanotube downwardly, adding to gravity, and thus by using a dense suspension liquid, the buoyant force can be increased. Again, therefore, the nanotube will move to a position at which the forces are in balance. Figure 6D illustrates the situation for a metallic carbon nanotube with a magnetic field decreasing downwardly. Using a suspension liquid which is less dense than the carbon nanotubes they will tend to sink, but the magnetic force will, in this case, be upwards for the metallic carbon nanotubes, so they will, again, move to a position where the forces are in balance. Figure 4 schematically illustrates a second embodiment of the invention in which the container 3 of the first embodiment is replaced by a container 3 a having outlets 11 at different levels within it, thus allowing carbon nanotubes of different structures to be drawn off from the suspension after application of the gradient magnetic field.

Figure 5 illustrates a third embodiment of the invention in which the suspension 1 is caused to flow along a flow path through a region of varying magnetic field down a conduit 3b. An outlet 11 is provided for drawing off material at a particular point in the magnetic field. In the illustrated example this may be arranged, for example, so that only metallic nanotubes, which are attracted to the region of higher magnetic field, can reach the region of the outlet 11, semiconducting nanotubes being repelled upwards against the flow.

The effects of using liquids of different, or selectable, densities is discussed above. The suspension may also be formed using magnetic fluids, such as solutions of paramagentic ions such as copper sulphate, in which case the forces on the nanotubes depend on the relative susceptiblity of the nanotubes compared to the magnetic fluid as well as buoyancy.

In the examples above material is collected from the liquid suspension. However, after application of the magnetic field gradient to sort the nanotubes it is also possible to preserve the distribution of carbon nanotubes by causing the liquid to solidify, e.g. by freezing or polymerisation, whereupon the sections of the solid suspension which have the desired nanotubes in it can be removed mechanically.

Figure 7 illustrates a further possible arrangement in which the suspension 1 is caused to flow through a tube 3c placed orthogonally to the magnetic field. In this case the suspension containing the nanotubes has been aerosolised to a sufficient extent that each droplet in the aerosol is expected to contain a single nanotube. As the aerosolised suspension flows through the sample tube 3c, the semi-conducting, diamagnetic nanotubes are diverted towards the left-hand side in the drawing and collect on collection plates 10a. The paramagnetic, metallic nanotubes are diverted to the right and collect on the collection plates 10b. Of course rather than collection plates, the sample tube 3c may be arranged so that the aerosol flows containing the different types of nanotube are diverted along separate flow paths to the left and right.

Figure 8 illustrates a further embodiment in which the balance between magnetic and buoyant forces described above is replaced by a balance between magnetic and centrifugal forces. To this end the suspension 1 is provided in a sample container 3d which spins about an axis inside a magnetic field gradient created between two magnetic poles 7.

Another aspect of the invention utilises a magnetic field to separate impurities, such as residual iron catalyst, from the nanotube material. Figure 9 illustrates an ^ ^" embodiment of the invention in which the suspension of SWNTs is placed in a magnetic field, in this case between two pole pieces 7 whereupon the metallic particles form deposits 12 on the sides of the sample container. It is also found that if the liquid is allowed to evaporate, the catalyst impurities can be left as deposits above the level of the liquid on the sides of the container, thus allowing easy separation of the purified suspension from the impurities. A suspension prepared in the manner described above with reference to embodiment 1 has been purified in this way by placing it in a 5mm diameter glass sample tube positioned between the poles of 1.2T permanent magnet as illustrated in Figure 9.

Claims

1. A method of sorting carbon nanotubes comprising: forming a mixture of carbon nanotubes in which the carbon nanotubes are individually mobile, applying to the formed mixture an inhomogeneous magnetic field to separate the carbon nanotubes in the formed mixture in dependence upon at least one of the structure and size of the carbon nanotubes, and collecting the carbon nanotubes from at least one selected location therein.

2. A method according to claim 1 wherein the step of forming the mixture in which the carbon nanotubes are individually mobile comprises forming a liquid aerosol or gaseous suspension of the mixture of carbon nanotubes.

3. A method according to claim 2 wherein the carbon nanotubes are in micelle form in the suspension.

4. A method according to claim 2 or 3 wherein the suspension is one of an aqueous and an organic suspension.

5. A method according to claim 2, 3 or 4 wherein the suspension comprises a surfactant.

6. A method according to claim 2, 3, 4 or 5 further comprising the step of ultrasonically treating the suspension prior to applying the inhomogeneous magnetic field.

7. A method according to any one of claims 2 to 6 wherein the mixture of carbon nanotubes is suspended in a liquid having a density similar to that of the carbon nanotubes.

8. A method according to claim 7 wherein the density ofthe liquid is in the range 0.7g/cm³ to 1.4g/cm³.

9. A method according to claim 7 or 8 further comprising the step of selecting the density ofthe liquid in relation to the gradient ofthe inhomogeneous magnetic field to control the distribution ofthe carbon nanotubes.

10. A method according to any one of claims 2 to 9 wherein the suspension is contained in a fractionating column allowing selective collection ofthe sorted carbon nanotubes in the suspension.

11. A method according to any one of the preceding claims further comprising ^'the step of adjusting the gradient ofthe inhomogeneous magnetic field to control the - « . distribution ofthe carbon nanotubes.

12. A method according to any one ofthe preceding claims wherein the inhomogeneous magnetic field is moved relative to the formed mixture to sort the carbon nanotubes in the suspension.

13. A method according to any one of the preceding claims wherein the formed mixture flows along a flow path through the inhomogeneous magnetic field and sorted carbon nanotubes are collected from at least one selected point in the flow path.

14. A method according to any one ofthe preceding claims wherein the mixture of carbon nanotubes comprises single-walled carbon nanotubes.

15. A method according to any one ofthe preceding claims wherein the mixture of carbon nanotubes comprises carbon nanotubes of different physical structures, and the inhomogeneous magnetic field is applied to the mixture to separate the carbon nanotubes according to their different physical structures, and the step of collecting the carbon nanotubes from at least one selected location comprises collecting separately carbon nanotubes from a location selected according to a desired physical structure.

16. A method according to any one ofthe preceding claims wherein the mixture of carbon nanotubes comprises both metallic and semiconducting single- walled carbon nanotubes, and the inhomogeneous magnetic field is applied to the mixture to separate the metallic from the semiconducting single-walled carbon nanotubes, and the step of collecting the carbon nanotubes from at least one selected location comprises collecting separately at least one of a) the separated metallic carbon nanotubes, and b) the semiconducting carbon nanotubes.

17. A method according to any one of the preceding claims wherein the mixture of carbon nanotubes comprises carbon nanotubes of different diameters, and the inhomogeneous magnetic field is applied to the mixture to separate the carbon nanotubes according to their diameter, and the step of collecting the carbon nanotubes from at least one selected location comprises collecting separately carbon nanotubes from a location selected according to a desired carbon nanotube diameter.

18. A method of sorting carbon nanotubes, the method being substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

19. Apparatus for performing the method of any one of the preceding claims and comprising a container for containing the mixture of carbon nanotubes, a magnetic flux source for applying an inhomogeneous magnetic field to the mixture, and a collector for collecting the desired carbon nanotubes from said selected location.

20. A method of separating impurities from carbon nanotubes comprising applying to the carbon nanotubes a magnetic field to separate the impurities from the carbon nanotubes.

21. A method according to claim 20 further comprising the step of forming a mixture of carbon nanotubes in which the carbon nanotubes are individually mobile and applying the magnetic field to the mixture.

22. A method according to claim 21 wherein the step of forming the mixture in which the carbon nanotubes are individually mobile comprises forming a liquid, aerosol or gaseous suspension ofthe mixture of carbon nanotubes.

23. A method according to claim 22 wherein the carbon nanotubes are in micelle form in the suspension.

24. A method according to claim 22 or 23 wherein the suspension is an aqueous suspension.

25. A method according to claim 22, 23 or 24 wherein the suspension comprises a surfactant.

26. A method according to claim 22, 23, 24 or 25 further comprising the step of ultrasonically treating the suspension prior to applying the magnetic field.