JP2005535550A - Carbon nanotube tip opening method and application - Google Patents

Abstract

The present invention relates to an efficient and undamaged method for manipulating carbon nanotubes. This process consists of carrying out two oxidation steps. The first stage is carried out in the liquid phase in concentrated acid and the second stage is carried out in the gas phase. In the present invention, the carbon nanotube is preferably a multi-walled carbon nanotube. In the present invention, the concentrated acid is nitric acid, and it is preferably used in excess. In the present invention, it is preferable to use 1 g of carbon nanotubes for 0.5 to 2 liters of concentrated nitric acid of 60 to 75% by weight, particularly 1 liter of concentrated nitric acid having a concentration of about 68 to 70% by weight.

Description

  The present invention generally relates to post-treatment of carbon nanotubes and uses of carbon nanotubes. The present invention is particularly directed to a method of opening the tips of carbon nanotubes, especially multi-walled carbon nanotubes.

  Most synthesis methods produce carbon nanotubes with closed ends, which can, for example, introduce impurities generated by the reaction medium into the central channel of the carbon nanotubes. This occurs particularly during the catalyst synthesis of carbon nanotubes. Moreover, if the carbon nanotubes are open from the beginning, they may be closed again during high temperature post-treatment.

  The merit of obtaining an open carbon nanotube is that a conductive nanowire for nanoelectronic technology can be manufactured by first filling a central conduit with a number of conductive species (metal, conductive polymer, etc.). Filled carbon nanotubes are also finding increasingly great advantages for catalyst applications and energy storage. Furthermore, hollow carbon nanotubes may prove to be an excellent gas storage for storing hydrogen, natural gas, and the like.

  It is now well known that the presence of phase defects is necessary to close the graphite surface at the end of carbon nanotubes. According to Euler's law, six pentagonal shapes are required to close each end of the carbon nanotube. Of course, these distorted regions are the most effective points in the addition reaction, especially in the double bond that binds a set of pentagonal shapes.

  Proposed methods for opening nanotubes include chemical oxidation with a strong oxidizer in the liquid phase (nitric acid, sulfuric acid or a mixture of these two acids, potassium permanganate, etc.) at temperatures between 500 ° C and 700 ° C. There are gas phase reactions under air flow, and very recently there are impact grinding or sonication, especially for cutting and shortening nanotubes.

  Oxidation in air or oxygen is not sufficiently selective. Damage to the outer graphite surface is often severe because these processes lose a large amount of material and the reaction cannot be adequately controlled.

Another recommended work consists of using CO ₂ at 850 ° C., but at such temperatures close to the general use conditions for activating carbon materials, the yield of open nanotubes is very low. Mass loss is significant and the outer layer of graphite is significantly damaged.

  When carbon nanotubes are dispersed in an oxidizer solution, the oxidation becomes much more homogeneous. For example, carbon nanotubes obtained by decomposing acetylene at 600 ° C. with cobalt particles supported on zeolite often contain carbon impurities, and their ends are closed. In that case, corrosion is performed with potassium permanganate, and these impurities are partially removed by oxidation, and at the same time, a part of the end of the carbon nanotube is opened.

  Again, however, the results regarding efficiency and selectivity have been found to be very poor.

  The present inventor has confirmed that the above-mentioned inconvenience can be eliminated by performing two separate oxidation steps on the nanotube under predetermined conditions.

  An object of the present invention is to provide a method for opening carbon nanotubes quickly and effectively while maintaining the form and quality and reducing the loss.

  Accordingly, the method for opening carbon nanotubes according to the present invention is characterized in that it includes two oxidation steps, ie, a first liquid phase oxidation step and a second vapor phase oxidation step in concentrated acid.

  Open-wall nanotubes are obtained directly by the liquid phase oxidation step. Furthermore, it offers the advantage of being able to access most of the metal impurities remaining at the closed end, for example after synthesis carried out in the presence of a catalyst.

  Irregular carbon generated during the liquid phase oxidation reaction is removed during the second gas phase step.

  Advantageously, the carbon nanotubes are multi-walled carbon nanotubes.

  In particular, the concentrated acid is nitric acid.

  Preferably, concentrated nitric acid is used in excess.

Thus, by using 1 g of carbon nanotubes in 0.5 to 2 liters of concentrated HNO ₃ , in particular 60 to 75% by weight of concentrated HNO ₃ , especially in a concentration of about 68 to 70% by weight of nitric acid, The result of going is obtained.

  According to a special embodiment of the invention, this oxidation step is carried out by refluxing under stirring conditions.

  Advantageously, the heated reflux lasts for 30-50 minutes, in particular for about 35 minutes.

  For purification, an additional low temperature gas phase oxidation step is performed.

  In particular, in this step, the irregular carbon structure generated by the opening of the carbon nanotube end during the opening step by liquid phase oxidation can be removed by gentle oxidation (Oxidation management).

Advantageously, certain embodiments of this step are treated for about 1-2 hours, particularly at 500-600 ° C., especially under CO ₂ , and especially 500-550 ° C., especially 525 ° C., for 1 hour-1 hour 40 minutes. Consists of.

  More particularly, the process according to the invention is carried out with a linear velocity of the carbon dioxide of 40-100 cm / min, in particular 50-70 cm / min, especially about 60 cm / min.

  Advantageously, the method according to the invention comprises an intermediate step between the first stage liquid phase oxidation step and the second stage gas phase oxidation step, in particular filtering and washing open carbon nanotubes with distilled water. . The method according to the present invention can also include an additional step of hydrochloric acid treatment, if necessary, to remove metal particles that are initially trapped in the central conduit and released upon opening of the nanotubes.

  The yield of open nanotubes obtained by implementing the above configuration combining a liquid phase reaction and a subsequent gas phase reaction is at least 90%, there is no degradation of the nanotube surface, and the purity is higher than 97%. Stay on.

  The effectiveness of the present invention will become more apparent upon reading the following detailed examples with reference to the accompanying drawings.

The method of the present invention optimizes multi-walled carbon nanotubes synthesized by acetylene decomposition at 600 ° C. in a solid solution of CO _x Mg _(1-x) O.

  In the first step, carbon nanotubes are dispersed in concentrated nitric acid and subjected to reflux oxidation (130 ° C.) for 35 minutes with continuous stirring (1 g of nanotubes in 1 liter of 69 wt% acid). The mixture is then filtered and the solid is washed with distilled water until the pH of the filtrate is neutral. This first oxidation step opens the tube.

It is then slowly oxidized at a low temperature using a CO ₂ stream. This reaction is carried out based on the Boudouard reaction (C + CO ₂ → 2CO (ΔH = + 159 kJ / mol)).

The carbon nanotube powder is placed in a quartz crucible (Creuset en quartz) equipped with a disk made of porous sintered glass capable of introducing an upward flow of CO _{2 at} a temperature of 525 ° C. and a linear velocity of 60 cm / min.

  Let react for 60-100 minutes. Irregular carbon nanostructures generated during the first oxidation reaction are selectively oxidized.

  The cumulative mass loss is less than 50%.

  A scanning electron microscope (Hitachi S 4200) can be used to estimate the sample volume of the nanotubes (FIG. 1).

  Observation of a 200 kV transmission electron microscope (Philips CM20) shows the effectiveness of this method with respect to the opening of the nanotube tip (FIGS. 2 and 3). For this observation, the sample is sonicated in absolute ethanol and dropped on a copper grid covered with a carbon film.

The porous structure of carbon nanotubes is characterized by nitrogen adsorption at 77 ° K (Micrometrics, ASAP 2000). The sample is degassed at 350 ° C. (10 ⁻⁶ mbar) for 12 hours before the adsorption experiment.

  After opening, another heat treatment can be performed under nitrogen at a high temperature of 1600-2800 ° C. for several hours to graphitize the wall aromatic compound layer and vaporize the metallic Co.

  After the oxidation treatment, the tube diameter decreases slightly and the open area ratio is greater than 90% (see FIG. 2: arrows indicate open tubes). Sample quality is not compromised by the aperture treatment and the nanotube content is greater than 97%.

  Observation of the interference fringe mode of the 002 lattice sample with a TEM (transmission electron microscope) shows that the wall is not damaged (FIG. 3).

The carbon nanotubes used are terribly complicated. The nitrogen adsorption isotherm at 77K is type IV, which is characteristic of expandable mesoporous solids (FIG. 4). The BET area is 220 m ² / g, the mesoporous volume is very large (about 1 cm ³ / g) and the BJH diameter is about 15 nm, which is determined by the meniscuses of the nanotubes. ). After opening the edge according to the present invention, the mesoporous volume increases to about 1.6 cm ³ / g. At this time, since the BET area is about 300 m ² / g, it becomes clear that these nanotubes are suitable for storing energy or gas.

The above method is applied to nanotubes having an outer diameter of about 7 to 25 mm, but can also be applied to nanotubes of larger diameters by adjusting the nitric acid or CO ₂ treatment time.

  Of course, this method can also be used with carbon nanotubes other than those obtained by the catalytic method.

  In order to open carbon nanotubes having very high crystallinity, particularly nanotubes synthesized by vaporization of graphite, it is necessary to make the reaction time longer.

  The method according to the present invention is effective in opening carbon nanotubes. In particular, the method according to the invention is applied to the openings of multi-walled carbon nanotubes.

  In particular, the method according to the invention is applied to multi-walled carbon nanotubes with an outer diameter of 7 to 25 nm.

In particular, the multi-walled carbon nanotube to which the method according to the present invention is applied is preferably obtained by subjecting a solid solution of CO _x Mg _(1-x) O to acetylene decomposition at 600 ° C.

  All carbon nanotubes treated and opened in this way are economical, industrial, especially when used for the production of conductive nanowires, for energy storage, for gas storage or filtration, and / or for the production of catalyst supports. The major advantages are great.

Is an image photograph by the scanning electron microscope of the carbon nanotubes after _HNO 3 + CO ₂ processing according to the present invention (SEM). Is an image photograph by a transmission electron microscope of the carbon nanotubes after _HNO 3 + CO ₂ processing according to the present invention (TEM). It is a TEM photograph of the open end of a carbon nanotube after processing by the method of the present invention (interference fringe mode of CO ₂ lattice). It is a graph which shows the nitrogen adsorption-desorption isotherm in 77K of the carbon nanotube before implementation (solid line) and after implementation (broken line) of the method by this invention.

Claims (9)

  A method for opening carbon nanotubes, comprising a two-stage oxidation step, a first-stage liquid-phase oxidation step and a second-stage gas-phase oxidation step in concentrated acid.   The method according to claim 1, wherein the carbon nanotube is a multi-walled carbon nanotube.   3. A process according to claim 2, characterized in that the concentrated acid is nitric acid, preferably used in excess.   4. The method according to claim 2, wherein 1 g of carbon nanotubes is used for 0.5 to 2 liters of concentrated nitric acid of 60 to 75% by weight, particularly 1 liter of concentrated nitric acid having a concentration of about 68 to 70% by weight. The method described.   The method according to any one of claims 2 to 4, wherein the mixture is heated to reflux under stirring conditions.   The method according to any one of claims 1 to 5, wherein the second-stage gas phase oxidation is low-temperature oxidation of the carbon nanotubes with carbon dioxide.   The method according to claim 6, wherein the carbon nanotubes are treated with the carbon dioxide at 500 to 600 ° C. for 1 to 2 hours, particularly at 500 to 550 ° C. for 1 hour to 1 hour and 40 minutes.   The method of claim 1, further comprising an intermediate step of filtering and washing the open carbon nanotubes with distilled water between the first-stage liquid phase oxidation step and the second-stage gas phase oxidation step. The method according to any one of 7 above.   Use of carbon nanotubes obtained by carrying out the method according to any one of claims 1 to 8, for energy storage, for gas storage or filtration, and / or for the production of catalyst supports.

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