PROCESS FOR PREPARING SINGLE WALL CARBON NANOTUBES OF PRE-DEFINED CHIRALITY

Abstract

The present invention relates to a process for preparing single wall carbon nanotubes (SWCNT) having a diameter d.sub.SWCNT, which comprises (i) providing a precursor element which comprises a segment S.sub.SWCNT of the single wall carbon nanotube, the segment S.sub.SWCNT being made of at least one ring formed by ortho-fused benzene rings, and having a first end E1 which is open and a second end E2 which is opposite to the first end E1, (ii) growing the precursor element by vapour phase reaction with a carbon-source compound on the surface of a metal-containing catalyst, wherein the precursor element is in contact with the surface of the metal-containing catalyst via the open end E1 of the segment S.sub.SWCNT, and the metal-containing catalyst is in the form of particles having an average diameter d.sub.cat satisfying the following relation: d.sub.cat>2d.sub.SWCNT or in the form of a continuous film.

Claims

1.-17. (canceled)

18. A process for preparing single wall carbon nanotubes (SWCNT) having a diameter d.sub.SWCNT, which comprises (i) providing a precursor element which comprises a segment S.sub.SWCNT of the single wall carbon nanotube, the segment S.sub.SWCNT being made of at least one ring formed by ortho-fused benzene rings, and having a first end E1 which is open and a second end E2 which is opposite to the first end E1, the precursor element optionally further comprising a cap which is attached to the second end E2 of the segment S.sub.SWCNT, (ii) growing the precursor element by vapour phase reaction with a carbon-source compound on the surface of a metal-containing catalyst, wherein the precursor element is in contact with the surface of the metal-containing catalyst via the open end E1 of the segment S.sub.SWCNT, and the metal-containing catalyst is in the form of particles having an average diameter d.sub.cat satisfying the following relation: d.sub.cat>2d.sub.SWCNT or in the form of a continuous film.

19. The process according to claim 18, wherein the precursor element is prepared from a polycyclic aromatic compound.

20. The process according to claim 18, wherein the segment S.sub.SWCNT is made of up to 10 rings, each ring being formed by ortho-fused benzene rings.

21. The process according to claim 18, wherein the precursor element is made of the segment S.sub.SWCNT and the cap being attached to the second end E2 of the segment S.sub.SWCNT.

22. The process according to claim 19, wherein the precursor element is prepared from the polycyclic aromatic compound by a surface-catalyzed intramolecular cyclisation.

23. The process according to claim 22, wherein the surface-catalyzed intramolecular cyclisation is carried out on the surface of a metal-containing catalyst.

24. The process according to claim 23, wherein the metal-containing catalyst of step (i) is in the form of particles having an average diameter d.sub.6 satisfying the following relation: d.sub.cat>2d.sub.SWCNT or in the form of a continuous film.

25. The process according to claim 23, wherein the particles of the metal-containing catalyst in step (i) have an average particle size of at least 5 nm.

26. The process according to claim 18, wherein the precursor element is prepared at a temperature T.sub.1 of from 100 C. to 1000 C.

27. The process according to claim 18, wherein the carbon-source compound of step (ii) is selected from an alkane, an alkene, an alkyne, an alcohol, an aromatic compound, carbon monoxide, a nitrogen-containing organic compounds, a boron-containing organic compound, or any mixture thereof.

28. The process according claim 18, wherein the metal-containing catalyst of step (ii) comprises a metal selected from the group consisting of Pd, Pt, Ru, Ir, Rh, Au, Ag, Fe, Co, Cu, Ni, and mixtures or alloys thereof.

29. The process according to claim 18, wherein the particles of the metal-containing catalyst in step (ii) have an average particle size of at least 5 nm.

30. The process according to claim 23, wherein the precursor element is grown by vapour phase reaction with the carbon-source compound on the surface of the metal-containing catalyst of step (i).

31. The process according to claim 18, wherein step (ii) is carried out at a temperature T2 of 700 C. or less.

32. A single wall carbon nanotube, obtained by the process according to claim 18.

33. A polyaromatic compound having one of the following formulas (I) to (XXIII): ##STR00032## ##STR00033## wherein R is phenyl (i.e. C.sub.6H.sub.5); ##STR00034## ##STR00035## ##STR00036## ##STR00037## ##STR00038##

34. A method comprising preparing a single wall carbon nanotube from the polyaromatic compound of claim 33.

35. The process according to claim 23, wherein the metal is selected from the group consisting of Pd, Pt, Ru, Ir, Rh, Au, Ag, Fe, Co, Cu, Ni, and mixtures or alloys thereof.

36. The process according to claim 19, wherein the precursor element is prepared from the polycyclic aromatic compound by cyclodehydrogenation, cyclodehalogenation, or Bergman cyclization.

Description

EXAMPLES

Characterization Methods

[0121] STM measurements were performed using low temperature Scanning Tunnelling Microscope in constant current mode and sample temperature of 77 K. The system is held in a separate UHV chamber with a base pressure of 110.sup.11 mbar. Raman spectra were recorded in a Bruker Senterra instrument with a spectral resolution of 4 cm.sup.1 using a 532 and 782 nm laser with a power of 20 mW. He Ions Scanning Microscopy measurements were performed in a Carl Zeiss Orion Plus instrument with a beam energy of 30 keV and a beam current 0.4 pA.

[0122] Preparation of Polycyclic Aromatic Compounds

[0123] Polycyclic aromatic compounds of Formulas (I) and (II) were prepared.

[0124] The compound of formula (I) was obtained by multistep organic synthesis according to synthetic route shown in FIG. 2. 2-Bromo-6-methylbenzophenanthrene (1) was obtained by standard Wittig Olefination of 2-acetonaphtone to the respective stilbene and subsequent Mallory-photocyclization using the Katz-improvement. Suzuki coupling of 1 with 2-biphenylboronic acid gave 2. Benzylic bromination with N-bromosuccinimide and following treatment with sodium cyanide in DMSO gave compound 4. The cyano-compound was then hydrolyzed to the corresponding acetic acid and converted to 6 in two steps. First reaction with thionylchloride gave the acid chloride which was then used in a Friedel-Crafts acylation to achieve ring closure. The last synthetic step, the conversion to compound of Formula (I) (referred to as 7 in FIG. 2), was carried out by aldol cyclotrimerization using either TiCl.sub.4 or Brnsted acid conditions. Both cases lead to the desired compound in satisfying yield.

[0125] The compound of formula (II) was obtained by multistep organic synthesis according to synthetic route shown in FIG. 3. 2-methyl-triphenylene was prepared by Suzuki coupling of biphenyl-2-boronic acid and 4-bromotoluene resulting in 8 followed by Mallory-photocyclization using the Katz-improvement. Benzylic bromination with N-bromosuccinimide of 9 gave 2-bromomethyl-triphenylene which was subsequently converted to the corresponding Wittig salt 11 or phosphonate 12. Although stilbene 13 could not be obtained by Wittig reaction of 11 and acetonaphtone, the Homer-Emmons-Wadsworth reaction using 12 and 13 was successful. The following Mallory-photocyclization resulted in formation of the desired compound 14. The corresponding acetic acid 17 was then obtained by benzylic bromination of 14 with N-bromosuccinimide, a following treatment with potassium cyanide in the presence of tetrabutylammoniumbromide in CH.sub.2Cl.sub.2 and finally hydrolysis. 17 was then converted to the acid chloride by treatment with thionylchloride and subsequently Friedel-Crafts acylation was used to achieve ring closure and ketone 18 was obtained. For the trimerization of 18, reaction with TiCl.sub.4 in o-DCB at 180 C. was carried out. The compound of Formula (II) (referred to as 19 in FIG. 3) was obtained.

[0126] Preparation of a Precursor Element for (6,6) Single Wall Carbon Nanotubes by Intramolecular Cyclization of Polycyclic Aromatic Compounds

[0127] The polycyclic aromatic compound of Formula (I) and Formula (II), respectively, was evaporated in UHV from a Knudsen-cell type evaporator at a rate of 0.5 /min on a previously cleaned Pt(111) surface at RT.

[0128] The Pt single crystal acquired from Surface Preparation Lab (SPL) was used as a cyclodehydrogenation catalyst. Surface was cleaned by standard sputtering with Ar ions with an energy of 1 KeV, first at room temperature and then at 1100K, followed by a last flash annealing at 1370 K without ion bombardment.

[0129] A post-annealing at about 200 C. was carried out. Further annealing at about 500 C. induces the complete surface-catalyzed cyclodehydrogenation of the molecules, thereby forming the desired precursor element for the (6,6) SWCNT to be prepared.

[0130] The precursor element is made of a SWCNT segment and a SWCNT cap which is attached to one end of the SWCNT segment, whereas the other end of the SWCNT segment remains open. The SWCNT segment is made of a segment ring (or rather two or more rings) formed by ortho-fused benzene rings. The structure of the precusor element is shown in FIG. 1. The SWCNT segment is represented by the grey shaded part, whereas the cap closing one end of the SWCNT segment is represented by the part without shading.

[0131] FIG. 4 shows a STM image of the polycyclic aromatic molecules deposited on the Pt surface at RT FIG. 4(a) and after Annealing at 500 C. FIG. 4(b). As the Pt single crystal and its surface are by far larger than the dimensions of the deposited molecules, the Pt surface reflects the situation of a more or less continuous and flat film on which the polycyclic aromatic molecules are applied.

[0132] FIG. 5 shows the line profiles of a polycyclic aromatic molecule prior to cyclodehydrogenation (solid line) and the precursor element obtained after cyclodehydrogenation (dotted line). As can be seen from FIG. 5, the height increases from about 0.2 nm (height of the more or less flat aromatic compound) to about 0.45 nm which is the height of the bowl-like precursor element.

[0133] In FIG. 6, it is shown that different conformational isomers exist for the polycyclic aromatic compound of Formula (II). However, only one of these conformational isomers forms a curved bowl-like element made a SWCNT segment (in this particular case, a (6,6)-SWCNT segment) and a SWCNT cap. It is only this particular bowl-like element having the correct conformation which will subsequently grow in the CVD step. In FIG. 6, the conformational isomer on the very left forms a precursor element (shown from two different perspectives) which consists of a (6,6)-SWCNT segment and a cap attached to one end of the tube segment.

[0134] Growth of the Precursor Elements to Isomerically Pure (6,6) SWCNTs by Vapour Phase Reaction with Carbon-Source Molecules

[0135] The carbon source compound used for growing the precursor elements to the desired (6,6) SWCNTs was ethylene (C.sub.2H.sub.4) and ethanol (C.sub.2H.sub.5OH), respectively. A pressure of 110.sup.7 mbar was maintained in the chamber. The substrates were annealed at 400 C. or 500 C. during 1 h. To have control on the low doses experiments, a pressure of 110.sup.8 mbar was used.

[0136] For the CVD growth step, the precursor elements were left on the surface of the cyclodehydrogenation catalyst already used in step (i). So, the metal-containing catalyst of step (ii) was the same as used in step (i). As already mentioned above, the Pt single crystal catalyst and its surface are by far larger than the dimensions of the deposited molecules and the precursor elements prepared therefrom, which is why the Pt surface reflects the situation of a more or less continuous and flat catalyst film on which the isomerically pure SWCNTs are manufactured.

[0137] FIG. 7 shows the line profiles of the SWCNT precursor element before being reacted with the carbon source compound (solid line), and after having initiated the growth phase by feeding the carbon source compound (dotted line: dose of 1 Langmuir, dashed line: dose of 5 Langmuirs). As demonstrated by FIG. 7, a precursor element which consists of a (6,6)-SWCNT segment and a cap attached to one end of the tube segment will grow to the desired SWCNT by vapour phase reaction

[0138] FIG. 8 shows the scanning He ions microscopy image of a SWCNT of 300 nm in length.

[0139] FIG. 9 shows the SWCNT Raman spectrum and is in conformity with monodisperse isomerically pure (6,6)-SWCNTs. The Raman spectrum presents very well defined bands at the expected positions for a (6,6) SWCNT. The extremely narrow band at 295 cm.sup.1 is associated to the Radial Breathing Mode (RBM) of a (6,6) tube. The RBM is a tangential out-of-plane acoustic mode which frequency depends strongly on the nanotube diameter. It is extremely important to outline that the spectrum does not show any further band within the RBM range (from 200-400 cm.sup.1), not even under illumination with a red laser with =782 nm, therefore supporting the extremely high selectivity of the process. In addition, the Raman spectrum presents the same bands when illuminating much bigger areas by using a lower magnifying objective. The inset of the figure shows the RBM band recorded by illuminating an area of 80 m.sup.2, thus assuring the measurement of a high number of SWCNT and therefore demonstrating the high selectivity of the process. The G band associated to the in-plane optical vibration of the graphene lattice appears as a double peak at 1518 and 1591 cm.sup.1. The significant curvature in small diameter SWCNT causes a shift to lower frequencies in the G peaks, especially for the vibrations associated to transversal (perpendicular to the tube axis) atomic displacements (G.sup.), which appears downshifted with respect to the sp2 graphene lattice vibrations. The splitting observed is consistent with SWCNT of similar diameter and chirality. The additional peaks in the range from 400-1200 cm.sup.1 have been previously observed and used as an evidence for the presence of armchair SWCNT, since no peak in this range is present in semiconducting tubes.

[0140] The extremely cleanliness of the process reported here yields predefined chirality and defect-free SWCNT. A proof of that is the absence of any D band in the Raman spectrum.

[0141] A high resolution STM picture of the SWCNT structure is shown in FIG. 10 which is a further proof for the true single chirality of the SWCNTs prepared in the present invention. In FIG. 10, it can be observed that the SWCNTs present an internal structure consisting in higher contrast lines in the direction of the tube axis. The model of the SWCNT is superimposed over the STM picture to further corroborate that those higher contrast lines are indeed the carbon positions of the graphene structure in a (6,6) SWCNT. The outstanding agreement in both diameter of the tube and periodicity of the graphene lattice demonstrate that the 1-D structures observed are the expected (6,6) SWCNTs.

[0142] Effect of Temperature in Step (ii) on Product Quality and Yield

[0143] FIG. 11 shows the Raman spectra of (6,6)-SWCNTs prepared at a temperature of 500 C. (solid line) and 400 C. (dotted line) in step (ii).

[0144] Both spectra are consistent with isomerically pure (6,6)-SWCNTs. However, by comparing the relative intensities between D/G bands and RBM/G, it can be concluded that the desired pre-defined SWCNTs can be obtained at higher yield when operating at lower temperature.