Method of Manufacturing Carbon Nanohorns and the Carbon Nanohorns thus Produced

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims benefit, under 35 U.S.C. § 119(e), of U.S. provisional application Serial No. 63/199,837, filed on January 28, 2021. All documents above are incorporated herein in their entirety by reference.

FIELD OF THE INVENTION

[0001] The present invention relates to a method of manufacturing carbon nanohorns. More specifically, the present invention is concerned with a method of manufacturing carbon nanohorns using a carbon precursor and promotor gas injected in a thermal plasma torch under a set of operation parameters allowing the nucleation and growth of the desired carbon nanohorns. The present invention also relates to the carbon nanohorns produced, which are characterized by their small sizes compared to conventional carbon nanohorns.

BACKGROUND OF THE INVENTION

[0002] As shown in Fig. 1, carbon nanohorns (CNHs) are carbon allotropes with a structure close to carbon nanotubes (CNTs) except that they are closed on one end by a conical cap with an apex angle ranging from ~ 20° to - 113° due to the presence of five to one pentagons respectively. Carbon nanohorns are produced as 3D aggregates. The so-called ‘dahlia-like’ nanohorns are aggregates in which individual nanohorns are arranged in the flower-like pattern shown in Fig. 1a. In prior art 'dahlia-like' nanohorns, the diameter of the individual nanohorns is typically between 2 nm for the top and 5 nm for the base and their length is about 40-50 nm thus yielding 'dahlia-like' nanohorns aggregates with average diameter of about 80-100 nm - see Fig. 1b and 1c.

[0003] The CNHs hollow structure (like carbon nanotubes), high specific surface and inertness make them a good candidate for drug-delivery or nanomarkers; methane or hydrogen storage; supercapacitors; drug medical application (poor toxicity and good biocompatibility), and also, for composite matrix reinforcement. Pentagon-heptagon defects present on the CNHs walls improve their surface reactivity for functionalization or oxidation, expanding further the range of properties required for specific applications. One of the most obvious advantage of CNHs over carbon nanotubes is that their synthesis does not require a metallic catalyst. Such catalyst is typically used to produce carbon nanotubes and undesirably remains as an impurity in the nanotubes. This is a considerable advantage for medical applications as no post-synthesis treatment is required to remove any trace of metals or residues.

[0004] CNHs were first synthesized by Pr. Harris in 1994 and then highlighted by microscopy by S. lijima and his colleagues in 1999 by CO2 laser ablation. Following this discovery, other methods were developed to produce CNHs, such as direct current (DC) arc discharge and direct vaporization of graphite. All these methods produce CNHs on an industrial scale. However, these methods require solid carbon sources such as graphite with a purity of more than 99%, a high energy source to vaporize carbon, and also, a high quenching rate to form the nanostructures in an inert gas such as argon, helium or nitrogen. Typically, 5-15% of by-products are synthesized with CNHs such as amorphous carbon, nanoflakes or graphitic nanocapsules (GNCs), depending on the synthesis parameters.

SUMMARY OF THE INVENTION

[0005] In accordance with the present invention, there is provided:

1 . A method of manufacturing carbon nanohorns, the method comprising the steps of: a) providing a carbon precursor and a promotor gas b) providing a plasma reactor comprising a thermal plasma torch generating a plasma, c) feeding the carbon precursor and the promotor gas into a section of the plasma that is at a temperature allowing decomposition of the carbon precursor, thereby decomposing the carbon precursor into reactive species, d) allowing said reactive species to migrate downstream in the plasma reactor, there causing nucleation and growth of the carbon nanohorns, and e) allowing the carbon nanohorns to migrate further downstream in the plasma reactor into a non-reactive zone of the plasma reactor, wherein the promotor gas is nitrogen, hydrogen, helium, argon, a molecular gas consisting of a combination of nitrogen and hydrogen atoms, or a mixture thereof, wherein, during step c) to e), the pressure in the reactor is between about 66 and about 94 kPa; wherein the carbon precursor and the promotor gas are fed into the plasma in a promotor atom : carbon atom ratio of about 4.6 to about 5.4, and wherein the carbon precursor is fed into the plasma at a carbon feeding rate of from about 0.068 mol/min to about 0.16 mol/min.

2. A method of manufacturing carbon nanohorns, the method comprising the steps of: a) providing a carbon precursor and a promotor gas b) providing a plasma reactor comprising a thermal plasma torch generating a plasma, c) feeding the carbon precursor and the promotor gas into a section of the plasma that is at a temperature allowing decomposition of the carbon precursor, thereby decomposing the carbon precursor into reactive species, d) allowing said reactive species to migrate downstream in the plasma reactor, there causing nucleation and growth of the carbon nanohorns, and e) allowing the carbon nanohorns to migrate further downstream in the plasma reactor into a non-reactive zone of the plasma reactor, wherein the promotor gas is nitrogen, hydrogen, helium, argon, a molecular gas consisting of a combination of nitrogen and hydrogen atoms, or a mixture thereof, wherein the carbon precursor and the promotor gas are fed into the plasma in a promotor atom: carbon atom ratio of about 4.6 to about 5.4, wherein, step c) to e) are carried out under operation parameter A and operation parameter B when using the following reference reactor and reference plasma torch: wherein the reference plasma torch installed in the reference reactor, wherein the reference reactor is cooled, cylindrical, and has an inner diameter of 150 mm, wherein the reference plasma torch is a radio frequency inductively coupled plasma (RF ICP) torch comprising: a cooled ceramic confinement tube with an internal diameter of 50 mm surrounded by an induction coil of 5 turns connected to a 3 MHz radio frequency power supply with a 60 kW plate power operated at 15 kW, and a cooled probe feeding the carbon precursor and the promotor gas into the plasma is inserted through the torch head along a central longitudinal axis of the confinement tube, wherein the reference plasma torch uses argon at 3 slpm as a central gas to generate the plasma, and wherein the walls of the confinement tube of the reference plasma torch are protected using argon at 67 slpm as a sheath gas, or under operation parameters equivalent to operation parameters A and B when using a reactor different from the reference reactor and/or a plasma torch different from the reference plasma torch, and wherein operation parameter A and operation parameter B are: operation parameter A: the pressure in the reactor is between about 66 and about 94 kPa; and operation parameter B: the carbon precursor is fed into the plasma at a carbon feeding rate of about 0.068 mol of C/min to about 0.16 mol of C/min. The method of embodiment 2, wherein during step c) to e), the pressure in the reactor is between about 66 and about 94 kPa. The method of embodiment 2 or 3, wherein the carbon precursor is fed into the plasma at a carbon feeding rate of about 0.068 mol of C/min to about 0.16 mol of C/min. The method of any one of embodiments 1 to 4, wherein the promotor gas is nitrogen, hydrogen, helium, or argon, preferably nitrogen or helium, and more preferably nitrogen. The method of any one of embodiments 1 to 5, wherein, during step c), the pressure in the reactor is: about 66 kPa or more, about 69 kPa or more, about 72 kPa or more, about 75 kPa or more, about 77 kPa or more, about 80 kPa or more, and/or about 94 kPa or less, preferably about 91 kPa or less, more preferably about 88 kPa or less, and most preferably about 85 kPa or less. The method of embodiment 6, wherein, during step c), the pressure in the reactor is about 83 kPa, preferably about 82.7 kPa. The method of any one of embodiments 1 to 7, wherein the promotor atom: carbon atom ratio is from about 4.8 to about 5.2, preferably of about 5.0. The method of any one of embodiments 1 to 8, wherein the carbon precursor is fed into the plasma at a feeding rate of from about 0.071 moles of C/min to about 0.14 mol of C/min, preferably from about 0.071 mol of C/min to about 0.10 mol of C/min, and more preferably at a carbon feeding rate of 0.071 mol of C/min. 10. The method of any one of embodiments 1 to 9, wherein the carbon precursor and/or the promotor gas are fed coaxially into the plasma torch.

11. The method of any one of embodiments 1 to 10, further comprising using argon, preferably at about 3 slpm, as a central gas.

12. The method of any one of embodiments 1 to 11, further comprising using argon, preferably at about 67 slpm, as a sheath gas.

13. The method of any one of embodiments 1 to 12, wherein the carbon precursor can comprise carbon in any of its forms, a hydrocarbon, an oxygenated hydrocarbon, a nitrogenated hydrocarbon, or a mixture thereof.

14. The method of embodiment 13, wherein the carbon precursor is a hydrocarbon, preferably CH4 or C2H2, and most preferably CH4.

15. The method of any one of embodiments 1 to 14, wherein the carbon precursor is in gaseous form.

16. The method of any one of embodiments 1 to 15, wherein the thermal plasma torch is an inductively coupled plasma (ICP) torch, preferably a radio frequency inductively coupled plasma (RF ICP) torch.

17. The method of any one of embodiments 1 to 16, wherein the reactor comprises a refractory insert tube, preferably made of graphite, located at the top of the reactor just below the plasma torch to modify the thermal properties of the plasma jet or the reacting zone.

18. The method of any one of embodiments 1 to 17, wherein, after the carbon nanohorns have reached a non-reactive zone of the plasma reactor at step e), the carbon nanohorns are allowed to settle.

19. The method of any one of embodiments 1 to 17, wherein, after the carbon nanohorns have reached a non-reactive zone of the plasma reactor at step e), the carbon nanohorns are directed toward an auxiliary chamber.

20. The method of embodiment 19, wherein the carbon nanohorns are allowed to settle in the auxiliary chamber.

21. The method of embodiment 19 or 20, wherein the auxiliary chamber is equipped with a solid/gas separation equipment for collecting the carbon nanohorns.

22. The method of any one of embodiments 1 to 21, further comprising the step f) of collecting the carbon nanohorns, from the plasma reactor or the auxiliary chamber if such a chamber is used. 23. The method of any one of embodiments 1 to 22, further comprising the step g) of removing or separating spatially graphitic nanocapsules from the carbon nanohorns.

24. The method of embodiment 23, wherein step g) comprises:

I. dispersing the carbon nanohorns as collected from the reactor in a liquid to obtain a solid/liquid mixture, ii. centrifuging the mixture to obtain a solid residue containing the graphitic nanocapsules and a supernatant containing the carbon nanohorns, ill. separating the supernatant from the solid residue, and iv. isolating the carbon nanohorns from the supernatant.

25. The method of embodiment 24, wherein the liquid is organic, preferably ethanol.

26. The method of embodiment 24 or 25, wherein step iv) is carried out by removing the liquid from the supernatant, preferably by evaporating the liquid from the supernatant.

27. The method of any one of claims 1 to 26, which is free from a step of using a quench gas.

28. The method of any one of embodiments 1 to 26, which is free from a step of using a catalyst, such as a metal catalyst, for example Fe, Ni, Co, Y2O3, and/or CeO2-containing metal catalysts.

29. Carbon nanohorns aggregated together into aggregates having an average diameter between about 15 nm and about 50 nm, preferably of about 25 or alternatively preferably of about 45nm.

30. Carbon nanohorns of embodiment 29, wherein said aggregates are arranged into superstructures that are disorderly or have a toroid-like (donut) shape.

31 . Carbon nanohorns of embodiment 30, wherein the superstructures with a toroid-like shape have a diameter between about 45 nm and about 105 nm (preferably an average diameter of 66 about nm) with a hole having a diameter between about 11 nm and about 35 nm (preferably an average diameter of 20 about nm).

32. Carbon nanohorns of any one of embodiments 29 to 31, wherein the aggregates are disorderly and spherical or spheroidal shaped aggregates and/or conventional ‘dahlia-like’ aggregates.

33. Carbon nanohorns of any one of embodiments 29 to 32, being free of nitrogen. 34. Carbon nanohorns of any one of embodiments 29 to 33 being produced by the method of any one of embodiments 1 to 28.

35. Use of the carbon nanohorns any one of embodiments 29 to 34 for drug-delivery, as optical or radio nanomarkers, for methane storage, in lubricants; in anodes or cathodes in batteries; in supercapacitors; for energy or electronic or mechanical devices, for drug medical application, or composite matrix reinforcement.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] In the appended drawings:

Fig. 1 A is a cross-sectional scheme of ‘dahlia-like’ carbon nanohorns (left) with a close-up on the tip of an individual carbon nanohorn (right).

Fig. 1B is a microscopic image of numerous 'dahlia-like' carbon nanohorns.

Fig. 1C is a microscopic image of a single 'dahlia-like' carbon nanohorns.

Fig. 2A is a schematic of the inductively coupled thermal plasma reaction set-up.

Fig. 2B is a schematic of the inductively coupled thermal plasma reaction set-up with a refractory insert tube inside the main reactor.

Fig. 3A is a TEM image of the nanostructures synthesized at 46.7 kPa.

Fig. 3B is a TEM image of the nanostructures synthesized at 53.3 kPa.

Fig. 3C is a TEM image of the nanostructures synthesized at 66.7 kPa.

Fig. 3D is a TEM image of the nanostructures synthesized at 78.7 kPa.

Fig. 3E is a TEM image of the nanostructures synthesized at 82.7 kPa.

Fig. 3F is a TEM image of the nanostructures synthesized at 94.7 kPa.

Fig. 4A shows the Raman spectra of the nanostructures synthesized at different pressures.

Fig. 4B shows the intensity ratio of the G and D bands observed in the Raman spectra of the nanostructures synthesized at different pressures.

Fig. 5A is a TEM image of the nanostructures synthesized with a F^CF ratio equal to 0.

Fig. 5B is a TEM image of the nanostructures synthesized with a F^CFU ratio equal to 0.3.

Fig. 5C is a TEM image of the nanostructures synthesized with a F^CFU ratio equal to 0.5.

Fig. 5F is a TEM image of the nanostructures synthesized with a F^CFU ratio equal to 0.7. Fig. 5E is a TEM image of the nanostructures synthesized with a FtCFU ratio equal to 1.0.

Fig. 5F is a TEM image of the nanostructures synthesized with a FtCFU ratio equal to 1.5.

Fig. 6A shows the Raman spectra of the nanostructures synthesized with different F^CFU ratio.

Fig. 6B shows the intensity ratio of the G and D bands observed in the Raman spectra of the nanostructures synthesized at different F^CFU ratios.

Fig. 7A is a TEM image of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa for a F^CFU ratio of 0.3.

Fig. 7B is a TEM image of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa for a F^CFU ratio of 0.5.

Fig. 7C is a TEM image of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa for a F^CFU ratio of 0.8.

Figure 7D is an overview of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa for a F^CFU ratio equal to 0.5.

Fig. 8A shows the Raman spectra of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa with different F^CFU ratio.

Fig. 8B shows the intensity ratio of the G and D bands observed in the Raman spectra of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa at different H2:CH4 ratios.

Fig. 9 is a processing diagram of carbon nanostructures in function of pressures for F^CFU = 0.7 (top row); F^CFU ratio for FH2 + FCH4 = 3.4 slpm (middle row); and FH2 + FCH4 = 3.4 slpm at 82.7 kPa (bottom row).

Fig. 10 shows a purification pathway by centrifugation in ethanol.

Fig. 11 A is a TEM image of the nanostructures synthesized with a N2:CH4 ratio of 0.3.

Fig. 11 B is a TEM image of the nanostructures synthesized with a N2:CH4 ratio of 0.5.

Fig. 11C is a TEM image of the nanostructures synthesized with a N2:CH4 ratio of 0.7.

Fig. 11 D is a TEM image of the nanostructures synthesized with a N2:CH4 ratio of 1 .

Fig. 12A shows the Raman spectra of the nanostructures synthesized with different N2:CH4 ratios.

Fig. 12B shows the intensity ratio of the G and D bands observed in the Raman spectra of the nanostructures synthesized at different N2:CH4 ratios.

Fig. 13 shows a high-resolution TEM image of carbon nanohorns synthesized with a N2:CH4 ratio of 0.5 at 82.7 kPa Fig. 14 shows XPS spectrum of samples synthesized with nitrogen at different N2:CH4 ratios and global flow rates.

Fig. 15 is a TEM image of carbon nanohorns synthesized with a He:CH4 ratio of 0.5 with a FH _S + FCH4 = 3.4 slpm at

82.7 kPa.

Fig. 16 is a TEM image of carbon nanohorns synthesized with a AnCFU ratio of 0.5 with a F _Ar + FCH4 = 3.4 slpm at

82.7 kPa.

Fig. 17A shows C2 molecular temperatures measured by optical emission spectroscopy (OES) in the plasma jet for a ratio X:CH4 = 0.5, with X = argon, helium, hydrogen and nitrogen.

Fig. 17B shows C2 molecular density by OES measured in the plasma jet for a ratio X:CH4 = 0.5, with X = argon, helium, hydrogen and nitrogen.

Fig. 18A is a TEM image of the nanostructures synthesized at a global flow rate of 1.7 slpm at 82.7 kPa for a N2:CH4 ratio of 0.3.

Fig. 18B is a TEM image of the nanostructures synthesized at a global flow rate of 1.7 slpm at 82.7 kPa for a N2:CH4 ratio of 0.5.

Fig. 18C is a TEM image of the nanostructures synthesized at a global flow rate of 1.7 slpm at 82.7 kPa for a N2:CH4 ratio of 0.7.

Fig. 19A shows the Raman spectra of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa with different N2:CH4 ratios.

Fig. 19B shows the intensity ratio of the G and D bands observed in the Raman spectra of the nanostructures synthesized at a global flow rate of 1 .7 slpm at 82.7 kPa at different N2:CH4 ratios.

Fig. 20A is a TEM image of the nanostructures synthesized with the addition of a graphite liner inside the main reactor at

82.7 kPa with FtCFU ratio of 0.5

Fig. 20B is a TEM image of the nanostructures synthesized with the addition of a graphite liner inside the main reactor at

82.7 kPa with N2:CH4 ratio of 0.5

Fig. 21 A is a TEM image of the nanostructures synthesized with a F ^F ratio of 0.5.

Fig. 21 B is a TEM image of the nanostructures synthesized with a F ^F ratio of 0.7.

Fig. 21C is a TEM image showing buds-like carbon nanohorns produced along with the short nanohorns synthesized in Example 4.

Fig. 21 D is a TEM image showing buds-like structures and graphitic structures.

Fig. 22A shows the Raman spectra of the nanostructures synthesized with different F ^F ratios. Fig. 22B shows the intensity ratio of the G and D bands observed in the Raman spectra of the nanostructures synthesized at different F ^F ratios.

Fig. 23 is a TEM image of the nanohorns sold by NEC Corporation®, Japan.

Fig. 24 is a TEM image of the Single Wall Carbon Nanohorns (SWCNH) sold by Carbonium®, Italy.

Fig. 25 shows the Raman spectra of nanohorns according to an embodiment of the invention, NEC nanohorns, and Carbonium nanohorns alone (bottom spectra) and with encapsulated dye before (middle spectra) and after (top spectra) washing.

Fig. 26 is a graphical abstract summarizing the tests and results reported in Examples 1-2 above.

DETAILED DESCRIPTION OF THE INVENTION

[0007] Turning now to the invention in more detail, there is provided a method for manufacturing carbon nanohorns (CNHs). The invention also relates to the carbon nanohorns themselves as produced by the method of the invention.

Manufacturing Method

[0008] The method of the invention is an advantageous way of producing CNHs. Using the set-up described in the Examples below (a 50kW RF ICP torch, operated at 15 kW), nanohorns yields of 15-20 g/h were achieved. A higher production rate could easily be achieved simply by using a larger reactor with a plasma torch operated with a higher power and flow (industrial-sized 100, 200, 500, and even 1000 kW RF ICP apparatus are available). Thus, compared to laser or arc-discharge techniques, the present plasma-based method is more easily scalable. Compared to inductionvaporization of a graphite rod, the use of a gaseous precursor in some embodiments of the method of the invention facilitates the implementation of a continuous process. Other advantages will be apparent to the skilled person in view of the description below.

[0009] There is provided a method of manufacturing carbon nanohorns, the method comprising the steps of: a) providing a carbon precursor and a promotor gas, b) providing a plasma reactor comprising a thermal plasma torch generating a plasma, c) feeding the carbon precursor and the promotor gas into a section of the plasma that is at a temperature allowing decomposition of the carbon precursor, thereby decomposing the carbon precursor into reactive species, d) allowing said reactive species to migrate downstream in the plasma reactor, there causing nucleation and growth of the carbon nanohorns, and e) allowing the carbon nanohorns to migrate further downstream in the plasma reactor into a non-reactive zone of the plasma reactor, wherein the promotor gas is nitrogen, hydrogen, helium, argon, a molecular gas consisting of a combination of nitrogen and hydrogen atoms, or a mixture thereof, wherein, during step c) to e), the pressure in the reactor is between about 66 and about 94 kPa; wherein the carbon precursor and the promotor gas are fed into the plasma in a promotor atom : carbon atom ratio of about

4.6 to about 5.4, and wherein the carbon precursor is fed into the plasma at a carbon feeding rate of about 0.068 mol of C/min to about 0.16 mol of C/min.

[0010] There is also provided, a method of manufacturing carbon nanohorns, the method comprising the steps of: a) providing a carbon precursor and a promotor gas b) providing a plasma reactor comprising a thermal plasma torch generating a plasma, c) feeding the carbon precursor and the promotor gas into a section of the plasma that is at a temperature allowing decomposition of the carbon precursor, thereby decomposing the carbon precursor into reactive species, d) allowing said reactive species to migrate downstream in the plasma reactor, there causing nucleation and growth of the carbon nanohorns, and e) allowing the carbon nanohorns to migrate further downstream in the plasma reactor into a non-reactive zone of the plasma reactor, wherein the promotor gas is nitrogen, hydrogen, helium, argon, a molecular gas consisting of a combination of nitrogen and hydrogen atoms, or a mixture thereof, wherein the carbon precursor and the promotor gas are fed into the plasma in a promotor atom : carbon atom ratio of about 4.6 to about 5.4, wherein, step c) to e) are carried out under operation parameter A and operation parameter B when using the following reference reactor and reference plasma torch: wherein the reference plasma torch installed in the reference reactor, wherein the reference reactor is cooled, cylindrical, and has an inner diameter of 150 mm, wherein the reference plasma torch is a radio frequency inductively coupled plasma (RF ICP) torch comprising: a cooled ceramic confinement tube with an internal diameter of 50 mm surrounded by an induction coil of 5 turns connected to a 3 MHz radio frequency power supply with a 60 kW plate power operated at 15 kW, and a cooled probe feeding the carbon precursor and the promotor gas into the plasma, inserted through the torch head along a central longitudinal axis of the confinement tube, wherein the reference plasma torch uses argon at 3 slpm as a central gas to generate the plasma, and wherein the walls of the confinement tube of the reference plasma torch are protected using argon at 67 slpm as a sheath gas, or under operation parameters equivalent to operation parameters A and B when using a reactor different from the reference reactor and/or a plasma torch different from the reference plasma torch, and wherein operation parameter A and operation parameter B are: operation parameter A: the pressure in the reactor is between about 66 and about 94 kPa; and operation parameter B: the carbon precursor is fed into the plasma at a carbon feeding rate of about 0.068 mol of C/min to about 0.16 mol of C/min.

[0011] In the method of the invention, the carbon precursor and the promotor gas are fed into the plasma generated by the thermal plasma torch using central gas (and the sheath gas) as is well known to the skilled person. They are thus brought into a section of the plasma that is at a temperature allowing decomposition of the carbon precursor. As is well- known to the skilled person, this would typically be the center of the induction coil. Indeed, the thermal plasma generated by the thermal plasma torch is typically "flame-shaped” (the flame being more or less elongated) with the plasma temperature being the highest at the plasma core and gradually decreasing as the plasma flows toward the torch exit. Temperatures of up to about 10000 K and even more can be reached in the hottest zone of a plasma generated by a thermal plasma torch.

[0012] The shape and dimension of the plasma jet can be modulated by several parameters, for example: the type of ionizing gas, the ratio between the central gas and sheath gas used, the shape of the reactor, the pressure in the reactor, and the power level. In embodiments, the central gas is argon, preferably at a feed rate of 3 about slpm. In embodiments, the sheath gas is argon, preferably at a feed rate of about 67 slpm.

[0013] Decomposition of the carbon precursors yields various reactive species such as electrons, ions, and radicals. These reactive species will be carried by the flow of species and other forces, such as gravity, electrophoresis, thermophoresis, and magnetic fields, through a temperature gradient toward colder parts of the jet and then colder parts of the reactor, which will allow nucleation and growth of these reactive species into the carbon nanohorns and eventually allow the produced carbon nanohorns to exit the reactive zone of the plasma reactor and migrate into a non-reactive zone of the plasma reactor, typically referred to as a "quench zone”.

[0014] The set of parameters provided herein (use of a carbon precursor, nature of the promotor, pressure, power, promotor atom : carbon atom ratio, and carbon feeding rate) create conditions in the reactor (temperature, temperature gradients, plasma viscosity, plasma density, plasma enthalpy, plasma specific heat, plasma thermal conductivity, nature and concentration of the reactive species, as well as the residence time, etc.) that allow the nucleation and growth of carbon nanohorns.

[0015] In the broadest range of the operation parameters provided herein, the carbon nanohorns may be accompanied by other forms of carbon (e.g., nanoflakes, bud-like nanohorns, embryos of nanohorns, hybrid form of flakes and nanotubules, etc.). In the preferred operation parameter ranges provided (when these preferred operation parameters are combined), a greater proportion of carbon nanohorns are obtained (although other forms of carbon are still present). This is especially true when using the preferred equipment described hereinbelow (PL-50 torch).

[0016] However, when the geometry/dimensions of the reactor and/or the type/power of plasma torch varies, the operation parameters may vary accordingly. Hence, in embodiments, step c) to e) are carried out under operation parameters A and B when using the reference reactor and torch described above or under operation parameters equivalent to operation parameters A and B when using a different reactor and/or a different plasma torch.

[0017] Herein, operation parameters that are "equivalent to operation parameters A and B” (or "equivalent operation parameters”) are operation parameters that create conditions in said different reactor that are similar enough to those in the reference reactor to allow the nucleation and growth of carbon nanohorns.

[0018] It is well within the skills of the person of skill in the art to determine what operation parameters are equivalent to operation parameters A and B when using a different reactor and/or a different plasma torch. For example, the skilled person would know that they might e.g., increase the carbon feed rate when using a more powerful plasma torch.

[0019] Further, the skilled person may also routinely use computational fluid dynamics (CFD) simulation to determine equivalent operation parameters for other reactors and/or torches. The use of such simulations is relatively common in the plasma field where, as described above, many aspects of a reactor/torch can affect the required operation parameters. This method is often used in the literature, for example in the following studies: Proux & al. 1985, Bernardi & al. 2005, Kim & al. 2009 and Kim & al. 2010, which are incorporated herein by reference. For example, the skilled person could use CFD create a concentration profile and/or a temperature gradient in said different reactor that would match those in the reference reactor (when using the reference plasma torch and operating under operation parameters A and B). In embodiments, the conditions (e.g., the concentration profile and/or the temperature gradient) in said different reactor could be within +/- about 15%, preferably +/- about 10%, +/- about 5%, +/- about 1% of the conditions (e.g., the concentration profile and/or the temperature gradient) in the reference reactor.

[0020] As noted above, the promotor gas is argon, nitrogen, hydrogen, helium, a molecular gas consisting of a combination of nitrogen and hydrogen, or a mixture thereof. Non-limiting examples of molecular gases consisting of a combination of nitrogen and hydrogen atoms include NH3 and N2H2. In preferred embodiments, the promotor gas is nitrogen, hydrogen, helium, or argon, preferably nitrogen, hydrogen, or helium, and more preferably nitrogen or helium. These last two promoters produce less by-product - such graphitic nanocomposites - than hydrogen and argon. Indeed, the Examples below show that products prepared with hydrogen contain in average about 40% nanocapsules, while those prepared with nitrogen and helium comprise in average about 14% and 11% nanocapsules, respectively. In most preferred embodiments, the promotor gas is nitrogen.

[0021] As noted above, during step c) to e), when using the reactor/torch described above or others, the pressure in the reactor is between about 66 and about 94 kPa. In embodiments:

I. the pressure is about 66 kPa or more, about 69 kPa or more, about 72 kPa or more, about 75 kPa or more, about 77 kPa or more, about 80 kPa or more, and/or

II. the pressure is about 94 kPa or less, preferably about 91 kPa or less, more preferably about 88 kPa or less, and most preferably about 85 kPa or less.

In most preferred embodiments, the pressure is about 83 kPa, and more preferably about 82.7 kPa.

[0022] Also as noted above, the carbon precursor and the promotor gas are fed into the plasma in a promotor atom: carbon atom ratio of from about 4.6 to about 5.4. In preferred embodiments, the promotor atom: carbon atom ratio is from about 4.8 to about 5.2, and more preferably of about 5.0. The promotor atom: carbon atom ratio is calculated using the number of atoms of the promotor provided in the reactor and the number of carbon atoms provided in the reactor in a given amount of time. For example, if H2 and CH4 are used at flow rate ratio of hydrogen to methane (F^CFU) of 0.5; 5 atoms of hydrogen and provided for each atom of carbon, thus yielding a promotor atom: carbon atom ratio of 5.0.

[0023] As noted above, the carbon precursor is fed into the plasma at a carbon feeding rate of from about 0.068 mol of C/min to about 0.16 mol of C/min. In preferred embodiments, the carbon precursor is fed into the plasma at a carbon feeding rate of from about 0.071 mol of C/min to about 0.14 mol of C/min, more preferably from about 0.071 mol of C/min to about 0.10 mol of C/min, and most preferably at a carbon feeding rate of 0.071 mol of C/min.

[0024] Interestingly, is it shown in the Examples below that, when nitrogen is used as a promotor gas, for a given promotor atom: carbon atom ratio, the carbon nanohorns yield surprisingly increases with decreasing carbon feeding rate.

[0025] The carbon precursor and the promotor gas can be fed coaxially or radially into the plasma torch. When a reactant is fed coaxially, it is fed in a direction that is more or less parallel to the longitudinal axis of the plasma jet (i.e., the axis going from the base to tip of the jet). When a reactant is fed radially, it is fed in a direction that is more or less normal to the longitudinal axis of the plasma jet. In both cases, the reactants are fed close toward the base of the plasma jet. In preferred embodiments, the carbon precursor and/or the promotor gas are fed coaxially to favour a uniform decomposition of the precursor.

[0026] The carbon precursor can be in powder form, in liquid form, in gaseous form, or can be any mixture of these forms. In preferred embodiments, the carbon precursor is in gaseous form. Liquid forms of carbon precursors include, for example, solutions, suspensions, slurries etc., in liquids such as organic solvents.

[0027] The carbon precursor can be any source of carbon, which is free of elements deleterious to the reaction. The carbon precursor can comprise carbon in any of its forms, hydrocarbons, oxygenated or nitrogenated hydrocarbon, or a mixture thereof. In preferred embodiments, the carbon precursor is a hydrocarbon.

[0028] Non-limiting examples of carbon precursors that are in powder form include ashes, carbon nanoparticles/nanospheres, carbon nanoflakes and carbon black.

[0029] For clarity, hydrocarbons (C _x H _y ) are defined as an organic compound consisting entirely of hydrogen and carbon atoms. Preferred hydrocarbons include CH4, C2H6 and C2H2, and most preferably CH4.

[0030] Oxygenated hydrocarbons (C _x H _y O _z ) or nitrogenated hydrocarbons (C _x H _y N _z ) are hydrocarbons that contain one or more atoms of oxygen or nitrogen, respectively.

[0031] Any suitable thermal plasma torch can be used in the method of the invention, including those generating plasma by direct current (DC) (including non-transferred and transferred type DC plasma torches), alternating current (AC), radio-frequency (RF), microwave discharge, dielectric-barrier discharge and other discharges, such as capaciti vely- or inductively-coupled RF discharge.

[0032] In preferred embodiments, the thermal plasma torch is an inductively coupled plasma (ICP) torch, which is a type of plasma source in which the energy is supplied by electric currents produced by electromagnetic induction, that is, by time-varying magnetic fields. There are three types of ICP geometries: planar (in which the electrode is a length of flat metal wound like a spiral or coil), cylindrical (in which the electrode is like a helical spring) and half-toroidal (in which the electrode is a toroidal solenoid cut along its main diameter to two equal halves).

[0033] In more preferred embodiments, the thermal plasma torch is a radio frequency inductively coupled plasma (RF ICP) torch. In such torches, radio frequency AC currents in a coil generate an oscillating magnetic field that couples to a partially ionized gas flowing through the coil (the discharge cavity) generating thereby a stable plasma discharge.

[0034] In most preferred embodiments, the plasma torch is a PL-50 torch from Tekna Plasma System® (the number 50 referring to the internal diameter in mm of its ceramic confinement tube) equipped with a 60 kW plate power but preferably operated at 15 kW powered by a 3 MHz Lepel RF power supply. For more information on this torch including its structure and its plasma temperature profile, see Dolbec et al. NSTI-Nanotech 2008, vol.1 , 672-675, incorporated herein by reference.

[0035] As noted above, the thermal plasma torch is housed into a reactor. Any type of reactor equipped with a plasma torch can be used in the method of the invention. Such reactors are typically cylindrical, but other configuration can be contemplated such as conical reactors. Typically, the walls of the reactor are cooled to prevent premature damage. This has also the advantage to attract the produced nanohorns (via thermophoresis). As noted above, the plasma reactor can be connected to an auxiliary chamber.

[0036] In embodiments, the reactor comprises a refractory insert tube, preferably made of graphite, located at the top of the reactor just below the plasma torch. Such tube allows modifying the thermal properties of the plasma jet or the reacting zone. The examples below show that the use of such a tube promoted growth of the carbon nanohorns, by decreasing the gradient of temperature in the plasma jet

[0037] In embodiments, after the produced carbon nanohorns have reached a non-reactive zone of the plasma reactor at step e), they can be allowed to settle (e.g., on the reactor walls, at the bottom of the reactor, etc.) where they can be collected, or they can be directed toward an auxiliary chamber (e.g., using a vacuum) for easier collection. The produced carbon nanohorns can be allowed to settle in the auxiliary chamber or the auxiliary chamber may be equipped with any desired solid/gas separation equipment (for example filters) separating the nanohorns from their carrier gases (namely, the gas mixture exiting the plasma after the reaction).

[0038] In embodiments, the method of the invention further comprises the step f) of collecting the carbon nanohorns, e.g. from the plasma reactor or from the auxiliary chamber in which filters capture the carbon species, if such a chamber is used.

Optional Steps for Eliminating the Graphitic Nanocapsules

[0039] The carbon nanohorns produced by the method of the invention may comprise graphitic nanocapsules as an impurity. Various methods can be used, together or separately, to eliminate these graphitic nanocapsules. These methods are described below.

Method for removing the graphitic nanocapsules

[0040] In embodiments, the method of the invention further comprises the step g) of removing the graphitic nanocapsules from the carbon nanohorns after their manufacture.

[0041] In embodiments, step g) comprises: i. dispersing the carbon nanohorns as collected from the reactor in a liquid to obtain a solid/liquid mixture, ii. centrifuging the mixture to obtain a solid residue containing the graphitic nanocapsules and a supernatant containing the carbon nanohorns, ill. separating the supernatant from the solid residue, and iv. isolating the carbon nanohorns from the supernatant.

[0042] The liquid can be any liquid in which the carbon nanohorns are not soluble, but in which the nanohorns can be dispersed in a stable manner. Since the carbon nanohorns as produced by the above method are hydrophobic, organic non solvents are preferred. In embodiments, the liquid is ethanol.

[0043] Step iv) of isolating can be carried out, for example, by removing the liquid from the supernatant. In embodiments, the liquid is evaporated from the supernatant yielding the carbon nanohorns.

Steps/Components Used in the Prior Art that are not Necessary in the Method of the Invention

[0044] In other nanostructures plasma synthesis methods, a quench gas is often used to decrease abruptly and dramatically the temperature in the plasma reactor. This can be done after the nanostructures have exited the plasma or beforehand (the quench gas, in effect, abruptly "cutting” the plasma jet). In the method of the invention, this is not required. Thus, in embodiments, the method of the invention does not include a step of using a quench gas.

[0045] Similarly, other nanostructures plasma synthesis methods used various catalysts, which are advantageously not needed in the method in the invention. Therefore, the method of the invention does not include the use of catalysts, such as metal catalysts, e.g. Fe, Ni, Co, Y2O3, and/or CeO2-containing catalysts.

Carbon Nanohorns Produced by the Method of the Invention

[0046] The carbon nanohorns of the invention (as produced by the above method) are aggregated together into disorderly and roughly spherical or spheroidal aggregates and/or arranged into conventional ‘dahlia-like’ aggregates. Both types of aggregates can be present simultaneously or separately in the produced nanohorns.

[0047] These aggregates (both types) have an average diameter between about 15 nm and about 50 nm, preferably of about 25 nm (when produced using H2 as a promotor) or preferably of about 45 nm (when produced using N2 as a promotor). This is in contrast with 'dahlia-like' nanohorns aggregates of the prior art, which are typically 80-100 nm in diameter. See, for example, the TEM images of the conventional nanohorns aggregates used in Example 5, which have diameters of 89 nm and 94 nm.

[0048] In embodiments, these aggregates (both types) are themselves arranged into superstructures. These superstructures can be disorderly or have a toroid-like (donut) shape - see for example Fig. 5g and 14b. Typically, the toroid superstructures have a diameter between about 45 nm and about 105 nm (preferably an average diameter of about 66 nm) with a hole having a diameter between about 11 nm and about 35 nm (preferably an average diameter of about 20 nm).

[0049] Furthermore, the Examples below show that the carbon nanohorns produced do not contain nitrogen (even when they are produced with nitrogen). This also in contrast with conventional nanohorns, synthesized by other methods using nitrogen, which do contain nitrogen. Thus, in embodiments, the carbon nanohorns of the invention are free of nitrogen.

[0050] The differences between carbon nanohorns of the invention (as produced by the above method) are significant. Indeed, as shown in Example 5, the carbon nanohorns are better than the commercially available nanohorns art for the encapsulation of compounds (and the corresponding amplification of the Raman signal).

[0051] The carbon nanohorns of the invention are expected to find application in all the same fields as conventional nanohorns including drug-delivery or optical or radio nanomarkers; methane or hydrogen storage; lubricant; anode or cathodes in batteries; supercapacitors; for electronic or mechanical devices, for drug medical application, or for composite matrix reinforcement.

Definitions

[0052] The use of the terms "a" and "an" and "the" and similar referents in the context of describing the invention (especially in the context of the following claims) are to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context.

[0053] The terms "comprising, "having, "including, and "containing are to be construed as open-ended terms (i.e., meaning "including, but not limited to") unless otherwise noted. In contrast, the phrase "consisting of” excludes any unspecified element, step, ingredient, or the like. The phrase "consisting essentially of' limits the scope to the specified materials or steps and those that do not materially affect the basic and novel characteristic(s) of the invention.

[0054] Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All subsets of values within the ranges are also incorporated into the specification as if they were individually recited herein.

[0055] All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context.

[0056] The use of any and all examples, or exemplary language (e.g., "such as") provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. [0057] No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention.

[0058] Herein, the term "about ¹ has its ordinary meaning. In embodiments, it may mean plus or minus 10% or plus or minus 5% of the numerical value qualified.

[0059] Unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs.

[0060] Other objects, advantages and features of the present invention will become more apparent upon reading of the following non-restrictive description of specific embodiments thereof, given by way of example only with reference to the accompanying drawings.

DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0061] The present invention is illustrated in further details by the following non-limiting examples.

[0062] In these examples, the nanohorns of the invention are often referred to as "short carbon nanohorns''.

Example 1 - Manufacturing carbon nanohorns using H2 as promotor gas

[0063] We demonstrate hereinbelow a new pathway for carbon nanohorns (CNHs) synthesis by inductively coupled thermal plasma (ICP) from a gaseous carbon precursor (CH4). Nanohorns were formed preferentially by injection of a 2:1 flow rate ratio of CH4 to H2 at 82.7 kPa. Pressure had an impact on the nature of the synthesized carbon nanostructure, with lower pressures favoring carbon nanoflakes formation and higher pressures, up to 80.0 kPa, to CNHs formation. We demonstrated that a promoter gas (such as hydrogen) is essential to the gas-phase reaction. Also, the global concentration of methane and hydrogen inside the plasma jet also has an impact to the reaction. The RF ICP is a potentially continuous process allowing a high production rate, in excess of 20 g/h of CNHs with a plasma torch operated at 15 kW. Graphitic nanocapsules (GNCs) were also produced during the CNHs synthesis but can be easily removed by centrifugation.

Experimental

[0064] Herein below, we propose a novel synthesis pathway to produce CNHs using RF ICP. By injecting CH4 with H2 at a ratio of 2:1, CNHs could be readily produced at a rate of 20 g/h using a 15 kW power. We identified the growth conditions that favor the growth of CNHs by optimizing the ratio of precursors (CH4 with H2) and their respective flow rate and total pressure. We mainly used Transmission Electron Microscopy (TEM) and Raman spectroscopy to evaluate the various structures produced at different ratio CH4:H2, total concentration and pressure. In addition to CNHs, various carbonaceous structures such as graphene nanoflakes, graphitic nanocapsules were produced by changing the growth conditions, which led us to propose a growth model for the CNHs by RF ICP method.

Plasma reactor

[0065] The synthesis set-up used in the present study is illustrated in the Fig. 2A. A reactor comprising a refractory insert tube, used in some of the experiments, is shown in Fig. 2B. The setup was composed of three main parts. First, the plasma torch (PL-50 from Tekna Plasma System Inc. Canada) where reactive gases were mixed coaxially in a ceramic tube and ionized by an induction coil. The torch was driven by a power set to 15 kW with an excitation frequency of 3 MHz. The second part is the main reactor where the reaction occurred, and the third part is an auxiliary chamber with three porous filters connected to a vacuum pump to control the reaction pressure with a butterfly valve. The reactor, the probe and the plasma torch were cooled by water flowing in a double-walled jacket and attracted the nanostructures by thermophoresis. A thermocouple was placed in the middle of the main reactor, to monitor the reaction temperature just below the plasma jet. The pressure, water coolant temperature and reaction temperature were monitored during the reaction. Finally, powder was collected separately on the main reactor (M), auxiliary chamber (Aux) and filters (F). For extended reaction times (> 1 h), a blow-back system injected high-pressure gas flow at regular intervals to unclog the filters.

[0066] Central and sheath gas (argon) were injected inside the ceramic tube (central gas) to create and stabilize the plasma between the ceramic and the quartz tube (sheath gas) to protect this tube from high temperatures. The central gas is injected tangentially into the quartz tube placed in the center of the ceramic tube in the upper part of the torch. A CH4: H2 mixture was injected by the water-cooled probe inserted axially inside the center of the torch. The gas mixtures were ionized by the plasma jet and NPs nucleation occurred along the plasma jet and were quenched.

Parameters

[0067] We studied the effect of (I) pressure, (II) the ratio between the flow rate of hydrogen and methane (H2:CH4) with F _H2 + F _CH4 = 3.4 and 1.7 slpm and (ill) overall gas flow rate (Table 1) on the morphology of the carbon products. All other operating conditions were previously optimized and were fixed to the values summarized by Table 1.

[0068] Once the work pressure chosen was stable and the plasma plate power was settled to 15 kW, sheath gas and central gas were set to the working parameters. The reaction started when CH4 was injected into the probe and ended after 20 min. The reactor was then purged three times to flush all by-product gases and refilled by argon before collecting the produced powder.

Table 1. Plasma reactor operation parameters for the CNHs synthesis without graphite tube

Reactor pressure (kPa) 46.7, 53.3, 66.7, 78.7, 82.7 and 94.7

Ratio between the flow rate of hydrogen and methane (H2:CH4) 0, 0.3, 0.5, 0.7, 1 and 1 .5

Total flow rate of reactive gases (F _H1 + F _CHi ) (Q1) 1 .7 slpm and 3.4 slpm Sheath gas (Q2) 67 slpm Ar

Central gas (Q3) 3 slpm Ar

Plasma plate power 15 kW

Characterization

[0069] Carbon nanostructures were first characterized by TEM (Hitachi, H-7500) at an acceleration voltage of 120 kV. Samples were prepared by sonicating ~1 mg of CNHs materials in ethanol during 10 min in ultrasonic bath. The suspension was then drop casted on a 200-mesh holey carbon-coated Cu TEM grid and left to dry. Care was taken to limit the dose of the electron beam, as amorphization occurred within seconds under normal observation.

[0070] Raman spectroscopy provided additional information about the general structure present in the sample but not on the precise structure of the CNHs. The samples were analyzed by SP2500 Acton Raman spectrometer with a 514 nm laser at 30 mW to evaluate the overall carbon nature. Raman G, D and D' bands present in the spectra were fitted with 3 Lorentzians and the maximum (height) of the fitting curves was used to determine the ratio I _G /I _D .

Results

Effect of gas pressure

[0071] Fig. 3a-f shows the TEM micrographs of carbon nanostructures synthesized at different pressures with the mixture H2:CH4. Symbols F, E, H, G and sC designate nanoflakes, embryos of carbon nanohorns, hybrid forms of tubules and flakes, graphitic nanocapsules, and short carbon nanohorns, respectively.

[0072] At low pressures (46.7 and 53.3 kPa), -100 nm large carbon nanoflakes (F) are mostly observed (Fig. 3a). Nevertheless, Fig. 3b shows a second structure, which appears to be embryos of aggregates of carbon nanohorns (E). Increasing the pressure to 66.7 kPa reveals another carbon nanostructure in the form of a graphitic nanocapsule, indicated by a “G” in Fig. 3c (GNC). These nanocapsules are characterized by faceted cages (diameter of -100 nm). GNCs walls are composed of 20-25 graphene layers. In addition to flakes still present at 66.7 kPa, a hybrid form of tubules and flakes (H) can be observed. At 78.7 kPa (Fig. 3d), structures similar to those observed at 66.7 kPa are observed, but with more embryos of carbon nanohorns (E). At 82.7 kPa (Fig. 3e) short -24 nm long carbon nanohorns are found along with graphitic nanocapsules and some traces of flakes (F) and hybrid (H) structures. Finally, at 94.7 kPa (Fig. 3f), the sample present a mixture of G and sC, the hybrid form and embryos of horns, but the carbon nanohorns themselves are no longer observed. Between 66.7 and 94.7 kPa, graphitic nanocapsules are always synthesized as a by-product.

[0073] Fig. 4a shows a series of Raman spectra in the range from 1200 to 1900 cm ¹ recorded from samples produced at different pressures. Two main peaks are assigned to the G band (1580-1595 cm ¹ ) and the D band (1340- 1355 cm ¹ ). The D band is due to the presence of (i) sp ³ -type of defects, such as punctual defects, edges or lattice disorder and (ii) the presence of pentagons on the cap. For the nanohorns, the D band is mostly assigned to a loss of basal symmetry due to the presence pentagons on the cone region. In the case of CNHs, the D-band shape is characterized by a single Lorentzian, indicating that the cone structure is well-ordered. The G band is the result of E2 _g vibrations of the sp ² hybridized carbon atoms. The G band can present a shoulder peak, which is called the D' band, located above 1600 cm ¹ . The D' band is sometimes observed at 1615 cm ¹ and this feature is known to be induced by defects on the graphene. The D' band is more prominent for experiments at 66.7, 78.7 and 82.7 kPa, where horns grow in the vicinity of nanoflakes or as aggregates. The relative intensity ratio, I _G /I _D , (Fig. 4b) is used to correlate the Raman spectra with each nanostructure and this ratio is estimated using the maxima of the Lorentzian deconvolution peaks (not shown). A decrease of I _G /I _D is indicative of the presence of defects. Indeed, the CNHs I _G /I _D is lower than carbon nanoflakes or graphitic nanocapsules due to the disorder caused by the presence of pentagon-heptagon defects and their aggregation. This is consistent with the TEM observations indicating that embryos of horns stabilize at 66.7 kPa. At higher pressures, short carbon nanohorns dominate in the collected samples.

Effect of gases flow rate ratio (H . CH^ on CNHs morphology

[0074] Hydrogen is known to reduce the electrons density and, more generally, the plasma density resulting in a decrease of dehydrogenation reaction.

[0075] Fig. 5a-f shows TEM micrographs of nanostructures synthesized at 82.7 kPa with different H2:CH4 ratios. The sample synthesized without H2 (Fig. 5a) exhibited spherical aggregates of amorphous structures referred to as 'seed-like' carbon nanohorns. Increasing the H2:CH4 to 0.3 (Fig. 5b) allows the nucleation of embryos of carbon nanohorns (E) and the hybrid form of nanotubules and flakes (H). At a H2:CH4 ratio of 0.5 and 0.7 (Fig. 5c, d, respectively), the nanohorns are better defined and the main structure is comprised of short carbon nanohorns with a few carbon nanoflakes and of the hybrid form H. A ratio of H2:CH4 > 0.7 promotes the formation of carbon nanoflakes and large by-product graphitic nanocapsules (Fig. 5e and f). Graphitic nanocapsules are present for all samples, as described below. We note that aggregates of short CNHs tend to assemble into a toroid-like shape (see Fig. 5c and Fig. 5g, for instance). To our knowledge, this morphology has never been reported before.

[0076] Fig. 5h is a high-resolution TEM image of a carbon nanohorn.

[0077] Fig. 6a shows the Raman spectra for different H2:CH4 ratios and a graph of the I _G /I _D ratio as a function of H2:CH4 ratio (Fig. 6b). The shoulder at 1615 cm ¹ is present in all spectra, but it significantly widens the G band at low H2:CH4 ratios. For a H2:CH4 ratio between 0 and 0.5, the I _G /I _D is comprised between 1 and 1 .8, which is consistent with the dahlia-like or buds-like CNHs. I _G /1 _D increases, however, with the ratio H2:CH4, which is ascribed to the presence of graphene flakes and graphitic nanocapsules (Fig. 5e). Effect of the overall gases flow rate (F _H2 + F _CH )

[0078] Lower gas flow rates injected in the plasma jet translates to longer residence time. Here, we compare the nanostructures obtained at pressure set to 82.7 kPa using the standard F _H2 + F _CH4 = 3.4 slpm with those obtained at FH ₂ + FCH ₄ = 1-7 slpm.

[0079] Fig. 7 shows TEM images of CNHs synthesized using a total flow rate of 1 .7 slpm for different H2:CH4 ratio (0.3, 0.5 and 0.7). The nanostructures formed with a low ratio (0.3, Fig. 7a) are mainly CNHs with some horns embryos (E) and carbon nanoflakes (F). At a ratio of 0.5, CNHs and few flakes are still predominant in the sample, but the embryos are no longer present. Graphitic nanocapsules are present in every condition explored. Finally, a ratio of H2:CH4 = 0.8 produces short carbon nanohorns (Fig. 7c), but they are no longer predominant while nanoflakes, hydride form of flakes and tubules, and also graphitic nanocapsules are prevalent. Note that the CNHs are seen to assemble again in the shapes of a tore and holey structures - see e.g. Fig. 7b.

[0080] Fig. 8 shows Raman spectra (Fig. 8a) and the corresponding graph of the ratios I _G /I _D vs. H2:CH4 (Fig. 8b). I _G /1 _D increases with the H2:CH4 ratio and remains between 1 .5 and 2.8. The G band shoulder is still present and it is more visible for samples grown at a H2:CH4 ratio of 0.3.

Discussion

[0081] Similar to other CNHs manufacturing techniques, the results show that CNHs growth is possible in a specific environment (pressure, temperature, concentration) set by a narrow range of process parameters for the plasma reactor. The qualitative distribution of the carbon nanostructures as a function of pressure and H2:CH4 ratio are represented in Fig. 9. Other carbon allotropes are stabilized just outside this rather narrow window of growth parameters found for the growth of CNHs. Between 53.3 and 96.0 kPa and at H2:CH4 ratios between 0.3 and 1 .5, embryos and hybrids flakes and tubules are produced. This range of conditions overlaps partially with that of the region for CNHs growth. At pressures below ~ 53.3 kPa and H2:CH4 ratios below 0.7 and above 1 .0, carbon nanoflakes are present and dominant in the products. The production of undesirable graphitic nanocapsules starts above 66.7 kPa and remains significant for H2:CH4 ratio above 0.7 and decreases slightly for H2:CH4 ratios near 1 and above. A similar distribution of carbonaceous species is found with total flow rate of 1 .7 slpm, although the prevalence and crystallinity of CNHs is somewhat higher than with a total flow rate of 3.4 slpm. The overall conditions for the nucleation and the growth of carbon nanostructures are therefore influenced by pressures, gases flow rates, plasma plate power and temperature, which is not surprising since they directly determine the residence time of precursors thermodynamic and transport properties of the plasma jet of the plasma jet.

[0082] Methane decomposition is favored by low pressure, high power and low total gas flow rate. Longer residence time in the plasma achieved by a smaller flow rate was also found to be beneficial to the occurrence and crystallinity of CNHs. Indeed, a longer residence time helps the assembly of carbon atoms into graphitic structures. In addition, a smaller flow rate improved the conversion of CH4 into carbon. Preliminary results indicate that the CH4 conversion almost doubles with a flow rate cut by half.

[0083] We have demonstrated a novel method for the synthesis of carbon nanohorns based on inductively coupled plasma in a mixture of methane and promotor gases. CNHs with lengths between 25 nm are produced at a rate of 15-20 g/h in a plasma jet maintained by a 15 kW power at a pressure of 82.7 kPa of hydrogen and methane (H2:CH4 = 0.5) maintained with a low flow rate (F _H2 + F _CH4 = 1.7 slpm). These parameters are necessary for the nucleation and growth of CNHs. Buds structures are produced without H2 and are considered as a precursor to CNHs. The importance of hydrogen in the CNHs nucleation and growth is highlighted for the plasma process. Based on the results above and in Example 2 below, we conclude that H2 act as a promotor rather than a catalyst and plays a role in the growth of CNHs, probably by having an impact (among others) on temperature and temperature gradient of the growth environment. This synthesis is easily scalable and can be made continuous, which is clearly an interesting alternative compared to previous methods based on laser-, arc- or induction-based vaporization of graphite rods. Our method also allows the production of additional carbon allotropes, such as carbon nanoflakes, hybrids, embryos, bud-like CNHs and graphitic nanocapsules depending on the synthesis conditions.

Example 1’ - Purification of the product of Example 1 by centrifugation

[0084] The second pathway to separate residual by-products from the products obtained in Example 1 is to perform a centrifugation of the collected materials in ethanol. This method is easy to implement and is summarized in Fig. 10. The centrifugal force used depends on the GNCs structures (and mass) found in the samples.

[0085] Preliminary results indicate that more than 90% of the graphitic nanocapsules can be removed this way from the CNHs.

Example 2 - Manufacturing carbon nanohorn using different promotor gases (H2, N2, Ar, and He)

[0086] As in the Example 1 above, hydrogen may have two possible roles in the CNHs synthesis: (i) it is a promotor of ionized species or (ii) it is a catalyst of the growth of carbon sp ² materials. To elucidate the role of H2, we replaced hydrogen by nitrogen, helium or argon. This substitution is motivated by similar physical properties between hydrogen and nitrogen or between helium and argon in the plasma, such as electrical conductivity (proportional to the electron number density). However, nitrogen is not known to be a catalyst for CNH formation.

[0087] More specifically, we demonstrate that CNHs can be produced by RF ICP using methane mixed with various promotor gases (argon, hydrogen, helium and nitrogen). The main role of the promotor gas is to modulate the properties of the plasma jet (specific heat, thermal conductivity, viscosity, and C2 local concentration), but it could also participate to catalytic activities promoting nucleation and growth of various nanocarbon species. Using H2 and Ar promoters, the growth generates unstructured CNHs with a large fraction of GNCs (40% vol. for H2 and 22% vol. for Ar). The aspect ratio of the CNHs and their purity are, however, improved using nitrogen or helium. Since no incorporation of N atoms is seen into the CNH structures after growth, we think that the role of the precursor gases is mostly on the plasma properties.

[0088] The results show also that production rate increases when decreasing the global flow rate (GFR) defined as follow F _x + F _CHi with (X = H2, N2, Ar or He), which can be explained by a better conversion of methane to solid carbon in the nucleation and growth zone within the plasma jet. The BET pore size decreases with the GFR, which indicates that CNHs are larger. In addition, the volume of nitrogen adsorbed is larger than for CNHs in Utsumi & al. 2005 produced by CO2 laser ablation, even if the CNHs obtained here are twice as small.

[0089] In a nutshell, CNHs nucleation and growth are favoured by a low concentration of methane leading to a low density of C2 species in the nucleation zone. The role of the promotor is mainly to modulate the properties of the plasma jet rather than participating to the reaction mechanism. Contrary to the conclusions in Zhang & al. 2021, our work hereinbelow show that it is possible to synthesized CNHs with hydrogen.

[0090] Our results also indicate that hydrogen or argon improves the decomposition and dehydrogenation rates of methane. In contrast, helium hinders theses decomposition reactions of methane, probably due to its high enthalpy to form He* and favors the presence of C2 in the hot zones where nucleation and growth of CNHs rather than GNCs is taking place. The addition, nitrogen precursor has a similar impact on CNHs growth and this is probably due to a high dissociation energy. Atomic nitrogen readily reacts with carbon or hydrogen species to form additional gaseous molecules and it seems that these species do not take part of the growth chemistry. That is, the absence of nitrogen in the carbon nanopowder confirms that N2 does not incorporate into the CNHs as C-N bonds, nor that it catalyzes the CNHs growth.

Experimental

[0091] In the Example 1, we propose a novel method to produce CNHs using RF ICP by injecting CH4 with H2 at a ratio of 2:1 . We identified the growth conditions that favor the growth of CNHs by optimizing the ratio of precursors (CH4 with H2) and their respective flow rate and total pressure.

[0092] Here, to elucidate the role of H2, we replaced hydrogen by nitrogen, helium or argon. We mainly used Transmission Electron Microscopy (TEM) and Raman spectroscopy to evaluate the various structures produced at different CH4:X ratio (X= H2, N2, He or Ar), total concentration. Secondary characterizations are produced X-ray Photoelectron spectroscopy (XPS) and Optical Emission Spectroscopy (OES).

[0093] X-ray photoelectron spectroscopy (XPS) is performed with the VG Escalab 220i XL. Spectra were acquired with the Mg Ka radiation (hv = 1253.6eV) and a pressure of 1.10-10 mbar. All spectra were analyzed and quantified with Casa XPS.

[0094] Measurements of the plasma were acquired with the IsoPlane SCT 320 spectrograph from Princeton Instruments connected to a PIXI S:256E CCD camera. The light was collected through seven optical fibers spaced vertically from each other with a distance of 1 .5 cm, and with a scan every 0.2 cm (left and right) giving a map of 20x90 cm. The OES measurement was performed with an IsoPlane SCT-320 spectrometer (Princeton Instruments) connected to a PIXIS: 256E CCD camera. The 1800 g/mm grating allowed a resolution of 0.037 nm and covered a spectral range of 38 nm. The fibers were moved horizontally during the experiment to map the plasma The temperature and density measurements were based on C2 species acquired in the wavelength range from 410 nm to 667 nm. All data were processed by a home-made code.

[0095]

Plasma reactor

[0096] The reactor used in this second study is described in the Example 1 . The promotor H2 is replaced successively by nitrogen, helium and argon before to be mixed with methane. Those precursors are then injected inside the water- cooled probe inserted axially inside the center of the torch. The gas mixtures were ionized by the plasma jet and NPs nucleation occurred along the plasma jet and were quenched.

Parameters

[0097] We studied the effect of promotor by replacing (I) hydrogen by nitrogen first and (II) replacing hydrogen by helium or argon through the study of the ratio between the flow rate of promotor X (X= H2, N2, He or Ar) and methane (X:CH4) with F _x + F _CHI = 3.4 slpm (Table 2) and (ill) overall gas flow rate (GFR) F _x + F _CHI = 3.4 and 1.7 slpm for the mixture N2:CH4 (Table 3) on the morphology of the carbon products. All other operating conditions were previously optimized and were fixed to the values summarized by Table 2 and Table 3.

Once the work pressure chosen was stable and the plasma plate power was settled to 15 kW, sheath gas and central gas were set to the working parameters. The reaction started when CH4 was injected into the probe and ended after 20 min. The reactor was then purged three times to flush all by-product gases and refilled by argon before collecting the produced powder.

[0098] Table 2 summarizes the parameters studied. Nitrogen, helium or argon was injected in the probe in replacement of hydrogen.

Table 2. Parameters of the plasma reactor during the operation for CNHs syntheses mixing CH4 with promotor such as hydrogen, nitrogen, helium or argon Reactor pressure (kPa) 82.7 kPa 82.7-86.7 kPa

Ratio between the flow rate of promotor and methane (X: CH4)

0.5 0.3, 0.5, 0.7

(X = H ₂ , N ₂ , He or Ar)

CH4 (Q1) 2.27 slpm 3.7, 2, 2 slpm

Sheath gas (Q2) 67 slpm Ar 67 slpm Ar

Central gas (Q3) 3 slpm Ar 3 slpm Ar

X flow rate (Q4) 1.13 slpm 1.1, 1, 1.4 slpm

Plasma plate power 15 kW 15 kW

[0099] Table 3 summarizes the parameters studied. Nitrogen was injected in the probe in replacement of hydrogen.

Table 3. Parameters of the plasma reactor during the operation for CNHs syntheses using CH4 and N2 precursors

Reactor pressure (kPa) 82.7 kPa 82.7-86.7 kPa

Ratio between the flow rate of promotor and methane (N ₂ :CH4) 0.5 0.5

CH4 (Q1) 2.27 slpm 1.13, 2.27 slpm

Sheath gas (Q2) 67 slpm Ar 67 slpm Ar

Central gas (Q3) 3 slpm Ar 3 slpm Ar

N ₂ flow rate (Q4) 1.13 slpm 0.6, 1.13 slpm

Plasma plate power 15 kW 15 kW

Results

Effect of the N^Ch ratio on CNHs morphology

[00100] Fig. 11 a-d shows TEM micrographs of carbon nanostructures synthesized at 82.7 kPa with the mixture N2:CH4 with ratio between 0.3 and 1. The samples synthesized with a ratio N2:CH4 of 0.3 and 0.5 (Fig. 11a and b) show short

10 carbon nanohorns but the nanostructure seems more defined at 0.5. A ratio of N2:CH4 s 0.7 promotes the formation of carbon nanoflakes, hybrid form of flakes and horns with embryos (Fig. 11c and d). Symbols F, E, H and sC represent respectively nanoflakes, embryos of carbon nanohorns, hybrid form of flakes and tubules and short carbon nanohorns. [00101] At low nitrogen concentrations (N2:CH4 ratio = 0.3), a hybrid form of flakes and tubules (H) is formed along with embryos of carbon nanohorns (E). Increasing the nitrogen concentration (N2:CH4 ratio = 0.5) produces short carbon nanohorns (sC) instead of NCCs embryos. For N2:CH4 = 0.7, there is a mixture of short NCCs, embryos and bud-like carbon nanohorns. Finally, for N2:CH4 = 1 , carbon nanohorns are mainly replaced by carbon nanoflakes mixed with fewer CNHs embryos. As in the Example, 1 , graphitic nanocapsules are present in every sample and for all the conditions.

[00102] The Raman spectra shown in Fig. 12a for the different conditions explored are similar between each other, and indicates the existence of highly disordered sp ² carbon typical of the carbonaceous species observed by TEM. The I _G /I _D ratio (Fig. 12b) is -1 .3 for N2:CH4 = 0.5, which corresponds to the sample with the highest content of CNHs. A I _G /ID ratio increases to ~1 .2, 1 .6 and 2.3 when the N2:CH4 changes to 0.3, 0.7 and 1 , respectively.

[00103] Nitrogen was thus successfully used as a promotor for the synthesis of CNHs by thermal plasma. Even if H2 is not introduced as a reactant, the decomposition of the hydrocarbon precursor is observed and the plasma conditions in the presence of methane induce the formation of CNHs. Here, we observe that the use of a promotor gas, such as N2 or H2, is essential to the formation of CNHs.

[00104] The CHNs aggregates produced with N2 are larger than those obtained using H2 (diameter of about 45 nm, in average, Fig. 13).

[00105] Carbon nanohorns are only made of carbon. X-ray photoelectron spectroscopy (XPS) shown in Fig. 14 that these carbon nanostructures, when synthesized with nitrogen, mainly contain carbon and some trace of oxygen due to exposure to ambient air. The quantitative analysis of the XPS peak intensity reveals an amount of 0.19 % atomic of nitrogen, which is within the error of the method. The carbon content in the sample exceeds 97% atomic.

Effect of promoter (He or Ar) on CNHs morphology

[00106] Fig. 15 shows TEM micrographs of CNHs synthesized at 82.7 kPa with He:CH4 of 0.5. Short carbon nanohorns are produced with hybrid forms of flakes and horns.

[00107] Fig. 16 shows TEM micrographs of CNHs synthesized at 82.7 kPa with Ar:CH4 of 0.5. Embryos and short carbon nanohorns were produced with argon.

[00108] Fig. 17a and b show respectively C2 temperature mapping (Fig. 17a) and C2 density mapping (Fig. 17b) obtained by Emission optical spectroscopy of the plasma jet at the probe exit with different promotor (Ar, He, H2, N2).

[00109] Sample synthesized by mixing nitrogen and methane contains mainly carbon nanohorns (-93%) with -7% of GNCs, as estimated by analyzing multiple TEM micrographs of the sample. GNCs are co-produced, but can be easily removed by centrifugation as demonstrated in the Example T. The co-precursors, such as nitrogen, hydrogen, helium or argon influence mainly properties of the plasma jet, especially the temperature gradient and the C2 density, as shown in in Fig. 17a and b. These parameters change the conditions of nucleation and growth of carbon nanohorns inside the plasma reactor.

Effect of gases flow rate ratio (N^CEE) on CNHs morphology

[00110] Fig. 18 shows TEM images of CNHs synthesized using a total flow rate of 1.7 slpm for different N2:CH4 ratio (0.3, 0.5 and 0.7). The nanostructures synthesized with a low ratio (0.3, Fig. 18a) are mainly short carbon nanohorns with hybrid forms of flakes and horns while at a ratio N2:CH4 of 0.5, CNHs are mainly formed (Fig. 18b). At higher ratio (0.7, Fig. 18b) short carbon nanohorns are formed with hybrid forms and a large quantity of graphitic nanocapsules.

[00111] Fig. 19 shows Raman spectra (Fig. 19a) and the corresponding graph of the ratios I _G /I _D vs. N2:CH4 (Fig. 19b). I _G /I _D increases with the N2:CH4 ratio and remains between 1 and 1.5.

Discussion

[00112] The promotor has a predominant role on the properties of the plasma jet as demonstrated by Fig. 17a-b. Nitrogen and helium confine the C2 density near the exit of the probe that promotes nucleation and growth of CNHs, while argon and hydrogen extend the C2 density through the plasma jet increasing the production of graphitic nanocapsules. A quantification of GNCs using a thresholding method of a dozen of TEM micrographs for each sample reveals that mixing methane with a promotor such as hydrogen, argon, nitrogen or helium respectively produces respectively 40%, 22%, 14% and 11% of GNCs.

[00113] Previous work using CO2 laser and arc-discharge processes has shown that hydrogen is not essential for the synthesis of CNHs. Our results demonstrate that the addition of hydrogen to the methane precursor promotes the formation of crystalline nanohorns. Without a promotor, carbon nanostructures, which are often referred to as ‘buds' or 'seed' and considered as a non-crystalline form of CNHs, are produced (Fig. 5a). A promotor (H2, N2, He or Ar) induce a specific environment (by gradient of temperature, density and electronic species) inside the plasma jet, that favours the nucleation and growth of different carbon nanostructures.

Example 3 - Nanohorns Synthesis Using a Refractory insert tube

[00114] As noted above, Fig. 2B shows a schematic set-up of the addition of a refractory insert tube made in graphite inside the top of the main reactor.

[00115] A refractory insert tube is added inside the main reactor to promote the CNHs growth by decreasing the gradient of temperature in the plasma jet. All other operating conditions were previously optimized and were fixed to the values summarized by Table 4.

Table 4. Plasma reactor operation parameters for the CNHs synthesis with graphite tube

Reactor pressure (kPa) 82.7 Liner temperature (°C) 80.0

Methane (Q1) 1.13 slpm

Promotor (Q2) 0.6 slpm (H2 or N2)

Sheath gas (Q3) 67 slpm Ar

Central gas (Q4) 3 slpm Ar

Plasma plate power 15 kW

[00116] Before starting the reaction, the refractory insert tube made in graphite should be pre-heated following parameters detailed in the Table 5 for 15 minutes.

Table 5. Plasma reactor parameters for the pre-heated refractory insert tube

Reactor pressure (kPa) 66.7

Probe gas (Q1) 12 slpm Ar

Sheath gas (Q2) 90 slpm Ar + 8 slpm H2

Central gas (Q3) 35 slpm Ar

Plasma plate power 15 kW

[00117] Fig20-b show TEM micrographs of CNHs synthesized with a graphite liner pre-heated at 600°C and a mixture of CH4:H2 (Fig. 20a) and CH4:N2 (Fig. 20b) following parameters detailed in the Table 5. The refractory insert tube also allows the production of CNHs by decomposing a stream of methane with either hydrogen or nitrogen. Fig. 20a.

Example 4 - Nanohorns Synthesis Using C2H2 instead of CH4 [00118] Carbon nanohorns (NCCs) were synthesized using acetylene (C2H2) instead of methane (CH ₄ ). The parameters were the same as those used NCCs production with methane and hydrogen as reported in Example 1 . Table 6 summarizes the parameters used.

Table 6. Plasma reactor operation parameters for CNHs synthesis from C2H2 and H2 precursors

Ref CH ₄ Experiment Exp1 Exp2

(Example 1)

Reactor pressure (kPa) 82.7 77.3 550

Ratio between the flow rate of hydrogen and methane (H2:C2H2) 0.5 0.5 0.7 Total flow rate of reactive gases (F _W2 (Q1) 3.4 slpm 2.5 slpm 2 slpm

Sheath gas (Q2) 67 slpm Ar 59 slpm Ar 67 slpm Ar

Central gas (Q3) 3 slpm Ar 14 slpm Ar 3 slpm Ar

Plasma plate power 15 kW 20 kW 15 kW

[00119] The differences between the experimental parameters in these experiments compared to those used in Example 1 (Table 1) are due to the higher reactivity of acetylene versus methane. Namely, we had to increase the flow of the central gas (argon) relative to the total flow in Exp1 to ensure enough dilution of acetylene and a better stabilization of the plasma jet.

[00120] TEM micrographs from samples produced in the conditions listed in Table 6 for Exp 1 (Fig. 21a) and 2 (Fig. 21b) exhibit short carbon nanohorns aggregates with diameter between 30 and 40 nm. Along with NCCs, bud-like carbon nanohorns are also produced in about the same proportion (Fig. 21c). Furthermore, the graphitic nanocapsules are always present as by-products, but with walls having a lower crystalline character (Fig. 21d) than graphitic nanocapsules produced in Example 1, using CH4.

[00121] The corresponding Raman spectra are shown in Fig. 22a. The Raman signatures with I _G /I _D < 1 (Fig. 22b) indicate that the sample is made of disordered graphitic carbons, typical of carbon nanohorns, bud-like NCCs and distorted graphitic nanocapsules. For the conditions explored in Table 2, I _G /I _D ranges from 0.8 and 0.9. This value is slightly below 1-1.5, and therefore consistent with reported I _G /I _D values for carbon nanohorns.

[00122] This Example has shown how short carbon nanohorns can be produced by thermal plasma with a mixture of acetylene and hydrogen precursors. Bud-like NCCs and distorted graphitic nanocapsules were also found as byproducts. unlike the example using CH4 precursor, there was no trace of carbon nanoflakes in the collected samples. According to the TEM analyses, the NCCs produced with C2H2 in the plasma reactor are almost twice as long as those produced using H2 and CH4 mixture. The reason for this is still under investigation.

Example 5 - Improving Encapsulation

[00123] The present nanohorns were characterized using an encapsulation procedure with organic dyes. As is well- known from the prior art, the encapsulation of an organic dye in a nanostructure like carbon nanotubes and nanohorns increases the Raman signal of this compound. We have tested the CNHs structures produced in the plasma reactor and found after encapsulating an organic dye with a better loading than conventional nanohorns.

[00124] The nanohorns of the invention used for this experiment were manufactured as described in Example 1 with a pressure of 82.7 kPa, a global flow rate of 1 .7 slpm and H2:CH4 ratio of 0.5. The conventional (commercial) nanohorns were:

• NEC: The nanohorns sold by NEC Corporation®, Japan as described in "Carbon Nanohorns'' Tech Sheet, by NEC Corporation, dated February 2017, 2 pages. These nanohorns feature Dalhia-like nanohorns aggregates of about 94 nm in diameter and bud-like nanohorns of about 73 nm in diameter (see Fig 23).

• Carbonium: The Single Wall Carbon Nanohorns (SWCNH) sold by Carbonium®, Italy, which are described in the product brochure entitled "Single Wall Carbon NanoHorns/Graphene Nanostars published by Carbonium si. These nanohorns feature Dalhia-like nanohorns aggregates of about 89 nm in diameter and bud-like nanohorns of about 58 nm in diameter (see Fig 24).

[00125] The encapsulated dye was o-sexithiophene (6T) and this dye served as a reference because of many available results from the literature. The experiment involved exposing the nanohorns to a solution of the dye in N,N- dimethylformamide (DMF) and then filtering and washing with DMF to remove the excess of o-sexithiophene located outside the nanohorns. This procedure ensured that the measured Raman signal comes only from the encapsulated (therefore protected) molecules. The Raman spectra of these nanohorns is shown in Fig. 25, middle spectra. We can see that both the nanohorns of this invention and the NEC nanohorns provide a stronger Raman signal than that of the Carbonium nanohorns.

[00126] The nanohorns were subsequently exposed to piranha solution (F SO^F O?) 5:1 for 5 minutes. The piranha solution eliminated all traces of organic molecules located outside the CNHs. If a dye molecule had not been encapsulated in a stable manner inside a nanohorn, it would have been eliminated by the piranha solution. The Raman spectra of these nanohorns is shown in Fig. 25, top spectra. As can be seen, the Raman signal of the nanohorns of the invention is about twice as intense as the signal for the NEC and Carbonium nanohorns. This early result shows that the nanohorns of the invention are better adapted than conventional nanohorns for encapsulating dyes for Raman detection.

[00127] Fig. 26 is a figure summarizing the tests and results reported in Examples 1-2 above.

[00128] The scope of the claims should not be limited by the preferred embodiments set forth in these examples, but should be given the broadest interpretation consistent with the description as a whole.

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