Carbon nanotube purification method and purification device and carbon nanotube assembly produced by them

Abstract

To provide a carbon nanotube purification method and purification device in which safer iodine vapor is used in place of chlorine gas and bromine gas and a carbon nanotube assembly produced by them.SOLUTION: A carbon nanotube is heated to 600 to 1400°C in an inert gas atmosphere and is brought into contact with iodine vapor to remove metal as metal iodide from the carbon nanotube. A carbon nanotube assembly includes a carbon nanotube and a hollow carbon particle from which catalyst metal particles are removed by doping with iodine while a carbon shell in a closed state is retained.SELECTED DRAWING: Figure 3

Description

[Technical Field]

[0001] The present invention relates to a method and apparatus for purifying carbon nanotubes, and to an aggregate of carbon nanotubes produced by the method and apparatus. [Background technology]

[0002] Carbon nanotubes have been put to practical use as an electrode conductive material in lithium-ion batteries (LIBs), and are now indispensable. However, catalytic metal particles such as Fe, Co, and Ni used in the synthesis of carbon nanotubes become contaminated in the carbon nanotubes, and their removal is essential for battery applications.

[0003] Because many of the catalyst metal particles present as impurities are covered with carbon shells such as graphite carbon or amorphous carbon, the conventional purification method involves repeating a process of oxidizing the carbon shells to remove all or part of them and a process of dissolving the metal in acid. However, this process poses many challenges, including damage to the carbon nanotubes and the fact that the carbon nanotubes are immersed in a solution during the acid treatment, which causes the carbon nanotubes to aggregate densely due to the surface tension of the solution during drying after purification, making them difficult to redisperse.

[0004] In recent years, a method for purifying carbon nanotubes has been proposed in which catalytic metal-containing carbon nanotubes containing catalytic metal particles are heated in a dry state and brought into contact with chlorine gas (Cl2 gas) to remove the catalytic metal as a metal chloride (e.g., Patent Document 1 and Non-Patent Document 1).

[0005] The present inventors have also developed a method for purifying carbon nanotubes using bromine (Br2) gas, which is liquid at room temperature and atmospheric pressure (25°C, 1 atm) (see, for example, Patent Document 2). This method involves heating catalytic metal-containing carbon nanotubes containing catalytic metal particles in a dry state and bringing them into contact with bromine gas, thereby removing the catalytic metal particles as metal bromides.

[0006] Furthermore, the present inventors have also developed a method for purifying carbon nanotubes using FeCl3 vapor (for example, Patent Document 3 and Non-Patent Document 2). [Prior art documents] [Non-patent literature]

[0007] [Non-Patent Document 1] G. Mercier, et al., New Journal of Chemistry, 2013, 37, 790-795. [Non-patent document 2] H. Tanaka, et al., Carbon, 2023, 212, 118171. [Patent documents]

[0008] [Patent Document 1] Patent No. 5424481 [Patent Document 2] Patent No. 7044372 [Patent Document 3] Patent No. 7278539 Summary of the Invention [Problem to be solved by the invention]

[0009] In the method of purifying carbon nanotubes using chlorine gas disclosed in Patent Document 1 and Non-Patent Document 1, there were challenges to the practical application of mass production facilities, particularly in terms of the safety of the facilities and operations, because chlorine gas is highly toxic, which makes the facilities complicated and increases costs, and there is a risk of serious accidents if chlorine gas leaks.

[0010] In the method of purifying carbon nanotubes using bromine gas disclosed in Patent Document 2, bromine is a liquid at room temperature and pressure, making it easier to deal with a leak than chlorine, which is a gas at room temperature and pressure. However, bromine has a high vapor pressure of 30.4 kPa even at room temperature (25°C), and there is a risk that the vapor will diffuse in the event of a leak, so a safer purification method has been desired.

[0011] In the method of purifying carbon nanotubes using FeCl3 vapor disclosed in Patent Document 3 and Non-Patent Document 2, FeCl3 has a very low vapor pressure at room temperature and is safe, but when applied to carbon nanotubes that do not contain Fe, there is a problem of trace amounts of Fe being mixed in.

[0012] The present invention has been made in consideration of these circumstances, and its objective is to provide a method and apparatus for purifying carbon nanotubes using safer iodine vapor instead of chlorine gas or bromine gas, as well as an aggregate of carbon nanotubes produced by the method and apparatus. [Means for solving the problem]

[0013] To solve this problem, the present invention provides a method for purifying carbon nanotubes, which comprises heating carbon nanotubes to 600 to 1400°C in an inert gas atmosphere and contacting them with iodine vapor to remove metals from the carbon nanotubes as metal iodides.

[0014] In the purification method according to the present invention, the carbon nanotubes may be catalytic metal-containing carbon nanotubes synthesized using a catalytic metal.

[0015] In the purification method according to the present invention, the metals to be removed may be Fe, Co, Ni, Mg, and Al.

[0016] The present invention also provides a carbon nanotube purification device comprising a reactor into which carbon nanotubes are supplied, a heating means for heating the carbon nanotubes to 600 to 1400°C in an inert gas atmosphere, and an iodine vapor supply means for supplying iodine vapor, wherein the heated carbon nanotubes are brought into contact with the iodine vapor in the reactor to remove metals from the carbon nanotubes as metal iodides.

[0017] In the production apparatus according to the present invention, the carbon nanotubes may be catalytic metal-containing carbon nanotubes synthesized using a catalytic metal.

[0018] In the manufacturing apparatus according to the present invention, the metals to be removed may be Fe, Co, Ni, Mg, and Al.

[0019] The present invention also provides an aggregate of iodine-doped carbon nanotubes, the aggregate comprising at least one of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state, and hollow carbon particles from which the catalytic metal particles have been removed while leaving the carbon shells in a hollow state, wherein the ratio of the total number of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state and the total number of hollow carbon particles from which catalytic metal particles have been removed while leaving the carbon shells in a hollow state, to the total number of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state and the total number of hollow carbon particles from which catalytic metal particles have been removed while leaving the carbon shells in a hollow state, to the total number of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state and the total number of metal particles from which catalytic metal particles have been encapsulated in the carbon shells, is 50% or more and 100% or less. [Effects of the Invention]

[0020] According to the present invention, it is possible to provide a method and apparatus for purifying carbon nanotubes using iodine vapor, which is safer than chlorine gas or bromine gas.

[0021] Furthermore, according to the present invention, a purification process can be performed that increases the removal rate of catalytic metal particles without destroying the carbon shells, and an aggregate of iodine-doped carbon nanotubes can be provided in which the surface area of ​​the carbon shells of the hollow carbon particles is added to increase the overall surface area compared to the surface area of ​​the carbon nanotubes alone, allowing the surface of the carbon shells to be effectively utilized. [Brief explanation of the drawings]

[0022] [Figure 1] FIG. 2 is a schematic conceptual diagram showing an aggregate of carbon nanotubes before purification treatment. [Figure 2] FIG. 2 is a schematic conceptual diagram showing an aggregate of carbon nanotubes after purification treatment. [Figure 3] 1 is a schematic diagram showing a preferred embodiment of a purification device according to the present invention. [Figure 4] FIG. 2 is an explanatory diagram showing an example of a purification treatment process. [Figure 5] 1 is a graph showing the change in the Fe content in the carbon nanotubes of Example 1. FIG. [Figure 6] 1 is a graph showing the Raman spectrum of the carbon nanotubes of Example 1. FIG. [Figure 7] FIG. 2 is a TEM image photograph of an aggregate of carbon nanotubes before purification treatment in Example 1. [Figure 8] FIG. 2 is a TEM image photograph of an aggregate of carbon nanotubes after purification treatment in Example 1. [Figure 9] 10 is a graph showing the change in the Fe content in the carbon nanotubes of Example 2. FIG. [Figure 10] 1 is a graph showing the Raman spectrum of the carbon nanotubes of Example 2. FIG. [Figure 11] 10 is a graph showing the change in the content of Ni and Fe in the carbon nanotubes of Example 3. FIG. [Figure 12] 10 is a graph showing the Raman spectrum of the carbon nanotubes of Example 3. FIG. [Figure 13] 10 is a graph showing the change in the Fe and Mg contents in the carbon nanotubes of Example 4. FIG. [Figure 14] 10 is a graph showing the changes in the Al and Co contents in the carbon nanotubes of Example 5. FIG. [Figure 15] FIG. 10 is an explanatory diagram showing a processing step of a comparative example. [Figure 16] FIG. 10 is an explanatory diagram showing the purification process of Example 7. DETAILED DESCRIPTION OF THE INVENTION

[0023] Hereinafter, preferred embodiments of the carbon nanotube purification method and purification apparatus of the present invention will be described with reference to the drawings and examples.

[0024] In this embodiment, the carbon nanotube aggregate 1 before the purification treatment, which is the target of the purification treatment, can be a catalyst metal-containing carbon nanotube synthesized using a catalyst metal. As shown in FIG. 1, the carbon nanotube aggregate 1 before the purification treatment includes at least either carbon nanotubes 2 or metal particles 3. The carbon nanotube aggregate 1 before the purification treatment includes catalyst metal particles 4 derived from the raw materials used in synthesizing the carbon nanotubes as impurities. The carbon nanotube aggregate 1 before the purification treatment can be synthesized by a flame synthesis method, an eDIPS method, an arc discharge method, a chemical vapor deposition (CVD) method, or the like, but is not particularly limited to these production methods.

[0025] As shown in FIG. 1, most of the catalytic metal particles 4 are surrounded by a carbon shell 5. The catalytic metal particles 4 are encapsulated in the carbon shell 5 at the end of the carbon nanotube 2. The metal particles 3 have a carbon shell 5, and the catalytic metal particles 4 are encapsulated in the carbon shell 5. Some of the metal particles 3 exist independently, while others are attached to the carbon nanotube 2.

[0026] The carbon shell 5 may be a single layer or multiple layers.

[0027] Some carbon nanotubes 2 exist independently, while others exist as entangled or bundled carbon nanotubes 2. Furthermore, the carbon nanotube aggregate 1 before the purification process may contain catalytic metal particles (not shown) that are not covered with carbon shells 5, or carbon sources or catalyst sources (not shown) that were not decomposed during the synthesis of the carbon nanotubes.

[0028] In the purification method and apparatus according to the present invention, a carbon nanotube aggregate 1 before purification is heated to 600 to 1400°C in an inert gas atmosphere and brought into contact with iodine vapor, thereby removing catalytic metal particles 4 as metal iodide from the carbon nanotube aggregate 1 before purification. This treatment is called iodine treatment. Iodine is a solid at room temperature and pressure, and has a low vapor pressure at room temperature of 0.04 kPa at 25°C, making it safer to handle than chlorine gas or bromine gas. Furthermore, iodine has a sufficiently high vapor pressure of 19.0 kPa at 127°C, making it easy to supply as vapor.

[0029] FIG. 2 shows a schematic conceptual diagram of an aggregate 11 of carbon nanotubes after the purification process according to the present invention. The aggregate 11 of carbon nanotubes is doped with iodine and includes at least one of carbon nanotubes 21 and hollow carbon particles 31. According to the purification process according to the present invention, the carbon nanotubes 21 are produced by removing the catalytic metal particles 4 while leaving the terminal carbon shells 5 in a closed state. The hollow carbon particles 31 have a hollow, closed shape, as the catalytic metal particles 4 have been removed while leaving the carbon shells 5.

[0030] The catalytic metal particles 4 removed by the purification treatment of the present invention are, for example, Fe, Co, Ni, Mg, and Al. The catalytic metal particles 4 are removed as metal iodide from the aggregate 1 of carbon nanotubes before the purification treatment.

[0031] In the carbon nanotube aggregate 11 after the purification process according to the present invention, the ratio of the total number of carbon nanotubes 21 and hollow carbon particles 31 to the total number of carbon nanotubes 2, metal particles 3, carbon nanotubes 21, and hollow carbon particles 31 is 50% or more and 100% or less. If it is 50% or more, the effect of effectively utilizing the surface of the carbon shell 5 is increased.

[0032] For example, the higher the refining temperature, the higher the above ratio. The refining temperature is preferably 600 to 1400°C. In the examples described below, the refining temperature is set to a predetermined value due to the limitations of the experimental equipment and the need to file a patent application early, but the refining temperature is not particularly limited as long as it is within the temperature range.

[0033] After the purification process of the present invention, the carbon nanotube aggregate 11 has a significantly reduced metal content in the catalytic metal particles 4 in the carbon nanotubes compared to the carbon nanotube aggregate 1 before the purification process. Furthermore, by purifying the carbon nanotubes so as to remove the catalytic metal particles 4 while leaving the carbon shells 5 in a closed state, the surface area of ​​the carbon shells 5 as well as the surface area of ​​the carbon nanotubes 21 can be effectively utilized, which has the effect of improving the performance of applied devices that utilize surface area. Applications that utilize surface area include electric double layer capacitors, various gas and ion adsorbents, and gas and biosensor materials. For example, in the case of electric double layer capacitors, the capacity can be increased.

[0034] Figure 3 shows a preferred embodiment of a purification apparatus 100 according to the present invention. Figure 3 shows a simplified purification apparatus used in the experiments of the examples of the present invention as an example of an apparatus having the function of removing catalytic metal particles 4 from an aggregate 1 of carbon nanotubes before purification treatment by the purification method of the present invention, and various modifications are possible when applied to mass production equipment, etc. The configuration is not limited to that shown in Figure 3 as long as it is possible to heat the aggregate 1 of carbon nanotubes before purification treatment and bring iodine vapor into contact with the aggregate 1 of carbon nanotubes before purification treatment.

[0035] An aggregate 1 of carbon nanotubes before purification, which is the target of purification treatment and which has been synthesized using, for example, a catalytic metal, is supplied into the reactor 101. The aggregate 1 of carbon nanotubes before purification treatment may be placed in the reactor 101 in advance, or may be configured to be supplied into the reactor 101 from outside the reactor 101.

[0036] The refining apparatus 100 includes a heating means 102 that heats the aggregate 1 of carbon nanotubes before the refining treatment to 600 to 1400°C in an inert gas atmosphere. The heating means 102 is not particularly limited as long as it can heat the aggregate 1 of carbon nanotubes before the refining treatment, and can be, for example, an electric furnace.

[0037] The purification apparatus 100 includes an iodine vapor supply means 103 that supplies iodine vapor. In FIG. 3 , the iodine vapor supply means 103 places solid iodine 104 in the reactor 101 and heats and vaporizes the solid iodine 104 using a heating means 105. The heating means 104 is not particularly limited as long as it can heat the solid iodine 104, and can be, for example, an electric heating wire. Alternatively, the iodine vapor supply means 103 may be configured so that iodine vapor is supplied into the reactor 101 from outside the reactor 101.

[0038] In Fig. 3, heat-resistant wool 106 is provided above and below solid iodine 104. This not only fixes solid iodine 104 in a predetermined position, but also straightens the flow of iodine vapor to prevent the iodine vapor from leaking out of reactor 101, thereby suppressing thermal convection. This straightening restricts the flow of gas within reactor 101, allowing iodine vapor to efficiently contact aggregates 1 of carbon nanotubes before purification treatment, thereby improving the efficiency of the purification treatment.

[0039] The purification device 100 may further include a water-cooled pipe 107 below the carbon nanotube aggregate 1 before the purification process. The water-cooled pipe 107 allows the metal iodide removed as vapor to be efficiently collected, facilitating post-processing of the metal iodide and preventing the metal iodide from being mixed into the carbon nanotube aggregate 1 after the production process.

[0040] An inert gas 108, such as argon gas, is supplied from above the reactor 101, and the vaporized iodine vapor is brought into contact with the pre-purification carbon nanotube aggregate 1 in the inert gas atmosphere. As a result, the catalytic metal particles 4 are removed as metal iodide from the pre-purification carbon nanotube aggregate 1, and an iodine-doped aggregate 11 of carbon nanotubes is produced, which includes at least one of carbon nanotubes 21 from which the catalytic metal particles 4 have been removed while leaving the terminal carbon shells 5 in a closed state, and hollow carbon particles 31 having a hollow, closed shape from which the catalytic metal particles 4 have been removed while leaving the carbon shells 5.

[0041] The content of each element remaining in the carbon nanotube can be measured by analyzing the composition using, for example, an energy dispersive X-ray analyzer (EDX) attached to a scanning electron microscope (SEM).

[0042] The crystallinity of carbon nanotubes can be analyzed by, for example, laser micro-Raman spectroscopy. -1 The peak that appears around 1350 cm is called the G-band and is due to the in-plane stretching vibration of the carbon atoms in the six-membered ring structure. -1 The peak that appears around is called the D-band, and is likely to appear when there are defects in the six-membered ring structure. The relative crystallinity of carbon nanotubes is determined by the peak intensity ratio of the G-band to the D-band, I G / I D The higher the G / D ratio, the more crystalline the CNT. -1 The peak that appears around this point is called RBM (Radial Breathing Mode), which is unique to single-walled carbon nanotubes and is a mode in which the tube vibrates in the diameter direction. [Example]

[0043] All documents mentioned herein are incorporated by reference in their entirety. The examples described herein illustrate embodiments of the present invention and should not be construed as limiting the scope of the invention.

[0044] In Examples 1 to 5, carbon nanotubes synthesized by different methods were purified using the method and apparatus for purifying carbon nanotubes according to the present invention. The configuration of the purification apparatus, the purification process, the evaluation method, and the evaluation results are described below.

[0045] (Configuration of refining equipment) For the purification process, a purification apparatus 100 shown in Fig. 3 was used. A quartz glass tube was used as the reactor 101. An electric furnace provided on the outer periphery of the reactor 101 was used as the heating means 102 for heating the aggregate 1 of carbon nanotubes before the purification process. An electric heating wire provided on the outer periphery of the reactor 101 was used as the heating means 105 for heating the solid iodine 104. Silica wool was used as the heat-resistant wool 106. Argon gas was used as the inert gas 108.

[0046] (Refining process) The purification process will be described with reference to FIG. 4. 50 mg of carbon nanotube aggregate 1 before purification and 2.0 g of iodine solid 104 were placed in reactor 101. First, reactor 101 was evacuated. Then, the atmosphere was replaced with argon gas at normal pressure for 3 minutes. The process of evacuating and replacing with argon gas was repeated three times. Next, reactor 101 was heated to 1000°C over 10 minutes using an electric furnace. After the temperature increase, the heating wire was heated to 180°C. Then, reactor 101 was maintained at 1000°C under normal pressure, and iodine treatment was performed for 60 minutes. After iodine treatment, 10 minutes of vacuum annealing and replacement with argon gas were repeated three times. Finally, the electric furnace and heating wire were cooled to room temperature over 20 minutes.

[0047] (Evaluation method) The content of each element in the carbon nanotubes was measured (Examples 1 to 5) and laser microscopic Raman spectroscopy (Examples 1 to 3) was carried out by the following methods.

[0048] <Content measurement> The content of each element in the carbon nanotubes was measured using an inductively coupled plasma optical emission spectroscopy (ICP-OES) (model: Agilent 5100, manufactured by Agilent Technologies). First, approximately 10 mg of carbon nanotubes were weighed, placed in a platinum crucible, and heated to 1000 °C in a muffle furnace (model: PF312, manufactured by Yamato Scientific Co., Ltd.) to combust the carbon nanotubes. The residue was then completely dissolved with hydrochloric acid while being heated on a hot plate. This solution was diluted to 100 mL with pure water to prepare a sample solution, which was then weighed. A calibration curve showing the relationship between concentration and spectral intensity was created using a solution with a known metal concentration. The spectral intensity of the sample solution was measured, and the metal concentration in the sample solution was calculated using the calibration curve equation. Finally, the amount of metal in the carbon nanotubes was calculated from the calculated metal concentration, the sample solution, and the weight of the carbon nanotubes. Three measurements were performed per sample, and the average was used for quantification.

[0049] <Laser microscopic Raman spectroscopy> A carbon nanotube powder sample was placed in a laser micro-Raman spectrometer (model number: HR-800, manufactured by Horiba, Ltd.) and subjected to laser micro-Raman spectroscopy analysis using a laser wavelength of 488 nm. An aggregate of carbon nanotubes was placed on a silicon substrate, and 2-3 drops of ethanol were placed on top of it. The ethanol was then dried on a hot plate, and the carbon nanotubes were attached to the silicon substrate. Measurements were performed at 5 or 10 points per sample.

[0050] (Evaluation results) The evaluation results of Examples 1 to 5 are explained below.

[0051] Example 1: Purification of TUBALL (registered trademark) single-walled carbon nanotubes TUBALL (registered trademark) single-walled carbon nanotubes were subjected to a purification process. As shown in Figure 5, the content of Fe remaining in the carbon nanotubes after the purification process was 3.81 wt%, a 72% decrease from 13.5 wt% before the purification process. As shown in Figure 6, G / I D No significant decrease was observed in the RBM. It was also confirmed that the purification process did not damage or change the structure of the carbon nanotubes.

[0052] Figures 7 and 8 are transmission electron microscope (TEM) images of carbon nanotube aggregates before and after purification. Before purification, catalytic metal particles remain in the carbon shells, but after purification, as shown by the arrows, the catalytic metal particles are removed without damaging the carbon shells.

[0053] Example 2: Purification of single-walled carbon nanotubes synthesized by the eDIPS method Single-walled carbon nanotubes (eDIPS EC1.5 manufactured by Meijo Nano Carbon Co., Ltd.) synthesized by the eDIPS method were subjected to a purification process. As shown in Figure 9, the Fe content remaining in the carbon nanotubes after the purification process was 0.76 wt%, a 92% decrease from the 9.75 wt% before the purification process. As shown in Figure 10, the Fe content was 0.76 wt%. G / I D No significant decrease was observed in the RBM. It was also confirmed that the purification process did not damage or change the structure of the carbon nanotubes.

[0054] Example 3: Purification of single-walled carbon nanotubes synthesized by arc discharge method Single-walled carbon nanotubes synthesized by arc discharge using Ni and Fe as catalytic metals were subjected to a purification process. As shown in Figure 11, the Fe content remaining in the carbon nanotubes after the purification process was 0.34 wt%, a 39% decrease from 0.56 wt% before the purification process. The Ni content remaining in the carbon nanotubes after the purification process was 15.5 wt%, a 57% decrease from 36.3 wt% before the purification process. As shown in Figure 12, the I G / I D No significant decrease was observed in the RBM. It was also confirmed that the purification process did not damage or change the structure of the carbon nanotubes.

[0055] Example 4: Purification of multi-walled carbon nanotubes synthesized by chemical vapor deposition (CVD) Multi-walled carbon nanotubes synthesized by chemical vapor deposition (CVD) using Fe and Mg as catalytic metals were purified. As shown in Figure 13, the Fe content remaining in the carbon nanotubes after the purification process was 0.05 wt%, a 99% decrease from 4.95 wt% before the purification process. The Mg content remaining in the carbon nanotubes after the purification process was 0.01 wt%, a 99% decrease from 1.21 wt% before the purification process.

[0056] Example 5: Purification of JENOTUBE (registered trademark) carbon nanotubes JENOTUBE (registered trademark) carbon nanotubes were subjected to a purification process. As shown in Figure 14, the Al content remaining in the carbon nanotubes after the purification process was 0.17 wt%, a 62% decrease from the 0.45 wt% before the purification process. The Co content remaining in the carbon nanotubes after the purification process was 0.0045 wt%, a 99.5% decrease from the 0.98 wt% before the purification process.

[0057] (Iodine doping and conductivity change study) In the following Example 6, Comparative Example, and Example 7, single-walled carbon nanotubes (eDIPS EC1.5 manufactured by Meijo Nanocarbon Co., Ltd.) synthesized by the eDIPS method were subjected to various treatments, and iodine doping and changes in conductivity due to iodine treatment were investigated.

[0058] Carbon nanotubes were dispersed by the repeated dispersion and separation method reported in the following literature. H. Shirae, DY Kim, K. Hasegawa, T. Takenobu, Y. Ohno, and S. Noda, Carbon 91, 20-29 (2015). http: / / dx.doi.org / 10.1016 / j.carbon.2015.04.033

[0059] Ten mg of carbon nanotubes were added to 30 mL of 0.3 mass% aqueous sodium dodecylbenzenesulfonate (SDBS) solution. After stirring, the mixture was sonicated for 2 minutes at 400 W using a tip-type ultrasonicator (Hielscher UP400S). The mixture was then centrifuged for 10-20 minutes at 4000 rpm using a centrifuge (Kokusan H36α), and the supernatant containing the dispersed carbon nanotubes was separated. The precipitate was redispersed and separated in the same manner, a total of six times. The separated carbon nanotube dispersion was subjected to suction filtration to obtain circular carbon nanotube membranes with a diameter of 36 mm. Three 25 mm x 7 mm membranes were cut from the resulting carbon nanotube membrane and subjected to the following treatments (Example 6, Comparative Example, and Example 7).

[0060] Example 6 The purification process shown in FIG. 4 was carried out using the apparatus shown in FIG.

[0061] <Comparative Example> In the apparatus shown in FIG. 3, a quartz glass tube of the same type as that used for iodine treatment was used as the reactor 101, but without the iodine vapor supply means 103 and heat-resistant wool 106. Treatment was performed according to the treatment steps shown in FIG. 15. First, the reactor 101 was evacuated. Then, the atmosphere was replaced with argon gas at normal pressure for 3 minutes. The process of evacuation and replacement with argon gas was repeated three times. Next, the reactor 101 was heated in an electric furnace and heated to 1000°C over 10 minutes. Then, the reactor 101 was maintained at 1000°C under normal pressure, and treatment was performed for 60 minutes. After treatment, 10-minute vacuum annealing and replacement with argon gas were repeated three times. Finally, the electric furnace was cooled to room temperature over 20 minutes.

[0062] Example 7 Using the apparatus shown in FIG. 3, treatment was performed according to the purification treatment process shown in FIG. 16. First, the reactor 101 was evacuated. Then, the atmosphere was replaced with argon gas at normal pressure for 3 minutes. The process of evacuating and replacing with argon gas was repeated three times. Next, the reactor 101 was heated in an electric furnace and heated to 1000°C over 10 minutes. After the temperature increase, the heating wire was heated to 180°C. Then, the reactor 101 was maintained at 1000°C at normal pressure, and iodine treatment was performed for 60 minutes. After the iodine treatment, the electric furnace and heating wire were cooled to room temperature over 20 minutes.

[0063] Table 1 shows the changes in the physical properties of the carbon nanotube film before and after treatment. The film thickness was measured at three points using a micrometer, and the average value was calculated. Although the film thickness varied depending on the location where the film was cut, there was no change in film thickness before and after treatment. The film mass decreased by about 30% before and after treatment. This is thought to be due to the thermal decomposition of the surfactant SDBS at 1000°C and the desorption of adsorbed water.

[0064] [Table 1]

[0065] The elemental composition of the carbon nanotubes before and after treatment is shown in Table 2. The elemental composition was measured using an energy dispersive X-ray spectrometer (EDS; AMETEK EDAX Genesis, B) attached to a scanning electron microscope (SEM; Hitachi High-Technologies Corporation S-4800). The carbon nanotubes before treatment are the analytical values ​​of the remaining carbon nanotube film after the three cutouts described above. The carbon nanotubes before treatment contained 20.2 mass% Fe, but this significantly decreased to 2.4 mass% in Example 6 and 2.6 mass% in Example 7. On the other hand, the comparative example showed only a slight decrease to 17.0 mass%. It is known that a portion of Fe evaporates upon vacuum annealing at 1000°C, and Fe also slightly decreased in the comparative example.

[0066] In both the pre-treatment and comparative examples, EDS analysis showed no iodine peaks, and automated quantitative analysis yielded values ​​of 0.5-0.6% by mass, but these were noise signals, and essentially no iodine was detected. The carbon nanotubes contained 5.0% by mass of iodine in Example 6 and 7.4% by mass in Example 7, demonstrating that the purification process of the present invention can dope carbon nanotubes with iodine. Because vacuum treatment removes metal iodide and iodine, it is best not to perform vacuum treatment after iodine treatment in order to increase the iodine doping rate.

[0067] [Table 2]

[0068] Table 3 shows the change in sheet resistance of the carbon nanotube film before and after treatment. The sheet resistance was measured by the four-probe method using a Keithley Instruments 2400 Series Source Meter. The carbon nanotube film before treatment exhibited a sheet resistance of 0.54 to 0.56 Ω / sq. In the comparative example, the sheet resistance was 1.89 Ω / sq, indicating that heating at 1000°C increased the resistance by more than three times. On the other hand, in Example 6, the resistance change was limited to 1.5 times, to 0.82 Ω / sq, and in Example 7, it was limited to 1.2 times, to 0.63 Ω / sq.

[0069] [Table 3]

[0070] From Example 6, Comparative Example, and Example 7, it was found that the purification treatment of the present invention effectively removes Fe, dopes iodine into carbon nanotubes, and suppresses an increase in electrical resistance.

[0071] Although the present invention has been described above based on the embodiments and examples, the present invention can be embodied in various modifications. For example, in this embodiment, the solid iodine 104 is placed above the aggregate 1 of carbon nanotubes before the purification treatment, but the solid iodine 104 may be mixed with the aggregate 1 of carbon nanotubes before the purification treatment and heated together. [Explanation of symbols]

[0072] 1. Aggregates of carbon nanotubes (catalytic metal-containing carbon nanotubes) before purification 2. Carbon nanotubes with catalytic metal particles encapsulated in the carbon shell at the ends 3 Metal particles with catalytic metal particles encapsulated in a carbon shell 4 Catalytic metal particles 5 Carbon shell 11 Aggregates of carbon nanotubes doped with iodine 21 Carbon nanotubes from which catalytic metal particles have been removed, leaving the terminal carbon shells closed. 31 Hollow carbon particles that have a hollow, closed shape in which the catalytic metal particles have been removed while leaving the carbon shell. 100 Purification equipment 101 Reactor 102 Heating means (electric furnace) 103 Iodine vapor supply means 104 Solid Iodine 105 Heating means (heating wire) 108 Inert Gas

Claims

1. A method for purifying carbon nanotubes, comprising heating carbon nanotubes to 1100 to 1400° C. in an inert gas atmosphere and contacting them with iodine vapor to remove metals from the carbon nanotubes as metal iodides.

2. 2. The method for purifying carbon nanotubes according to claim 1, wherein the carbon nanotubes are catalytic metal-containing carbon nanotubes synthesized using a catalytic metal.

3. 2. The method for purifying carbon nanotubes according to claim 1, wherein the metals removed are Fe, Co, Ni, Mg, and Al.

4. a reactor into which carbon nanotubes are supplied; a carbon nanotube heating means for heating the carbon nanotubes to 1100 to 1400°C in an inert gas atmosphere; an iodine vapor supply means for supplying iodine vapor, The carbon nanotube purification apparatus contacts the heated carbon nanotubes with the iodine vapor in the reactor to remove metals from the carbon nanotubes as metal iodides.

5. A carbon nanotube purification apparatus as described in claim 4, wherein the iodine vapor supply means places solid iodine in the reactor, provides heat-resistant wool above and below the solid iodine, and heats and vaporizes the solid iodine using an iodine heating means.

6. 5. The carbon nanotube purification apparatus according to claim 4, wherein the carbon nanotubes are catalytic metal-containing carbon nanotubes synthesized using a catalytic metal.

7. 5. The carbon nanotube purification apparatus according to claim 4, wherein the metals to be removed are Fe, Co, Ni, Mg, and Al.

8. An aggregate of iodine-doped carbon nanotubes, comprising at least one of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state, and hollow carbon particles from which the catalytic metal particles have been removed while leaving the carbon shells in a hollow state, wherein the ratio of the total number of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state and the total number of hollow carbon particles from which catalytic metal particles have been removed while leaving the carbon shells in a hollow state, to the total number of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state and the total number of hollow carbon particles from which catalytic metal particles have been removed while leaving the carbon shells in a hollow state, to the total number of carbon nanotubes from which catalytic metal particles have been removed while leaving the terminal carbon shells in a closed state and the total number of metal particles from which catalytic metal particles have been encapsulated in the carbon shells, is 50% or more and 100% or less.

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