Method for producing single-walled carbon nanotube

Abstract

To provide a method for producing a carbon nanotube by which a single-walled carbon nanotube having good crystallinity can be further easily obtained by using a liquid phase method.SOLUTION: The method for producing single-walled carbon nanotubes according to the present invention comprises a synthesis step in which the temperature of a substrate 50, on which particles 52 formed from Co (cobalt) are supported on the surface side, is raised in ethanol 32B to synthesize single-walled carbon nanotubes C from the particles 52.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to a method for producing single-walled carbon nanotubes. [Background technology]

[0002] Patent Document 1 discloses a method for producing hollow multi-walled carbon nanotubes by forming a thin film of either Fe (iron), Co (cobalt), or Ni (nickel) on the surface of a Si (silicon) substrate, immersing the substrate in an organic liquid such as methanol, and then heating the substrate. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Japanese Patent Application Laid-Open No. 2003-12312 Summary of the Invention [Problem to be solved by the invention]

[0004] The production method disclosed in Patent Document 1 is a so-called liquid phase method. The liquid phase method is convenient because it requires a simple apparatus and can produce carbon nanotubes at atmospheric pressure. However, it is difficult to produce single-walled carbon nanotubes using the liquid phase method, and a technology for producing single-walled carbon nanotubes using a simple liquid phase method is desired.

[0005] The present invention has been made in view of the above-mentioned conventional situation, and an object of the present invention is to provide a method for producing single-walled carbon nanotubes that can easily obtain single-walled carbon nanotubes with good crystallinity using a liquid phase method. [Means for solving the problem]

[0006] The method for producing single-walled carbon nanotubes of the present invention comprises the steps of: The method includes a synthesis step of synthesizing single-walled carbon nanotubes from particles formed of Co (cobalt) by increasing the temperature of a substrate carrying particles on the surface side in an organic liquid.

[0007] According to the present invention, single-walled carbon nanotubes having good crystallinity equivalent to that of known vapor phase growth methods can be obtained by a liquid phase method, which has previously been considered difficult to produce single-walled carbon nanotubes. [Brief explanation of the drawings]

[0008] [Figure 1] 1 is a schematic diagram showing the procedure of a method for producing single-walled carbon nanotubes in Example 1. FIG. [Figure 2] 1 is a schematic diagram showing a synthesis apparatus used in the method for producing single-walled carbon nanotubes in Example 1. FIG. [Figure 3] 1 is a graph showing the relationship between the number of discharges of a pulse arc plasma gun and the count number of the 2p peak of Co in a photoelectron spectrum. [Figure 4] 1 is a graph showing the G / D ratio and G / Si ratio in the Raman spectrum of each sample produced by changing the substrate temperature in the synthesis process to five different temperatures: 670°C, 700°C, 730°C, 750°C, and 780°C. [Figure 5] This is a graph showing the G / D ratio and G / Si ratio in the Raman spectrum of each sample produced by changing the time for which the substrate was maintained at a predetermined temperature in the synthesis process to five different times: 1 minute, 5 minutes, 10 minutes, 15 minutes, and 20 minutes. [Figure 6] This graph shows Raman spectra obtained by carrying out a synthesis process while maintaining the substrate temperature at 700°C for 10 minutes using samples prepared by varying the number of discharges from the pulse arc plasma gun in the particle formation process (4, 6, 8, and 16 times). [Figure 7] 10 is a graph showing the G / D ratio and G / Si ratio in the Raman spectrum of four types of samples produced by changing the number of discharges to four, 6, 8, and 16 times. [Figure 8]These are images of the surface of a substrate on which single-walled carbon nanotubes were produced by discharging six times and maintaining the substrate temperature at 700°C for 10 minutes. (A) is an SEM image, and (B) is a TEM image. DETAILED DESCRIPTION OF THE INVENTION

[0009] A preferred embodiment of the present invention will now be described.

[0010] In the method for producing single-walled carbon nanotubes, the temperature of the substrate in the synthesis step can be 670° C. or more and 730° C. or less, in which case single-walled carbon nanotubes can be produced satisfactorily.

[0011] In the method for producing single-walled carbon nanotubes, the time required for carrying out the synthesis step may be 5 minutes or more and 15 minutes or less, in which case single-walled carbon nanotubes can be reliably produced.

[0012] Next, a first embodiment of the method for producing single-walled carbon nanotubes according to the present invention will be described with reference to the drawings.

[0013] Example 1 The method for producing single-walled carbon nanotubes in Example 1 first performs a cleaning step of cleaning the substrate 50. The substrate 50 is made of Si and has a flat plate shape (see FIG. 1(A)). The substrate 50 has a predetermined resistance value and generates heat when a current is passed through it. The outer shape of the substrate 50 is a rectangle measuring approximately 5 mm x 30 mm. The surface of the substrate 50 is the upper surface in FIG. 1(A).

[0014] In the cleaning process, the substrate 50 is immersed in acetone, cleaned for 5 minutes using an ultrasonic cleaner, and dried. Next, the substrate 50 is immersed in methanol, cleaned for 5 minutes using an ultrasonic cleaner, and dried. After that, the substrate 50 is immersed in pure water, cleaned for 5 minutes using an ultrasonic cleaner, and dried. This removes foreign matter such as dust adhering to the surface of the substrate 50 from the surface of the substrate 50. In this manner, the cleaning process is carried out.

[0015] Next, after the cleaning process, an oxide film forming process is carried out to form an SiO2 film 51 on the surface of the substrate 50 (see FIG. 1(B)). In the oxide film forming process, the substrate 50 is placed and held in an oxidation furnace heated to a predetermined temperature for a predetermined time. As a result, an SiO2 film 51 is formed on the surface of the substrate 50.

[0016] Next, after the oxide film formation process, a particle formation process is carried out to form particulate Co (cobalt) on the surface of the SiO2 film 51 (see FIG. 1(C)). Specifically, a pulse arc plasma gun (APD-S manufactured by Advance Riko) is used to deposit particulate Co, which acts as a catalyst, on the surface of the SiO2 film 51 formed on the surface of the substrate 50, to form particles 52. The conditions for depositing Co are the discharge voltage, the number of discharges, and the discharge interval, and each can be adjusted to a desired value.

[0017] The pulse arc plasma gun emits a plurality of plasmatized granular Co particles with each discharge. The emitted granular Co particles are carried on the surface of SiO2 film 51 as a plurality of particles 52. When the pulse arc plasma gun emits another discharge, the granular Co particles collide with and integrate with the particles 52 previously carried on the surface of SiO2 film 51, increasing the particle size of the particles 52.

[0018] Figure 3 shows the relationship between the number of discharges of the pulsed arc plasma gun and the count of the Co 2p peak. The discharge interval was 1 / 3 second. The discharge voltage of the pulsed arc plasma gun was 80 V. As shown in Figure 3, the count of the Co 2p peak increases as the number of discharges increases. Furthermore, the average thickness of the deposited particles 52 after 6 discharges is approximately 0.2 nm. After 12 discharges, the average thickness is approximately 0.4 nm. After 18 discharges, the average thickness is approximately 0.8 nm. The results shown in Figure 3 indicate that increasing the number of discharges of the pulsed arc plasma gun increases the amount of deposited particles 52. The diameter of the particles 52 ranges from approximately 1 nm to 5 nm.

[0019] In this way, the particle forming step is carried out, and particles 52 made of particulate Co are formed on the surface of the SiO 2 film 51 .

[0020] Next, after the particle formation process is performed, a synthesis process is performed in which single-walled carbon nanotubes C are synthesized on the surface of the substrate 50 (see FIG. 1(D)). In the synthesis process, a synthesis apparatus 30 shown in FIG. 2 is used. The synthesis apparatus 30 has a water tank 31, a reaction tank 32, a cooling unit 33, and a temperature detection unit 34. The water tank 31 is a container with an open top. Water 31A and ice 31B are placed in the water tank 31. The temperature of the water 31A is, for example, 10°C.

[0021] The reaction vessel 32 is formed with a plurality of tubes 32A that extend upward and have open upper ends. The reaction vessel 32 contains ethanol 32B, an organic liquid. A substrate 50 carrying particles 52 made of Co is immersed in the ethanol 32B. Electrode plates 53 made of Cu (copper) are attached to opposite edges of the substrate 50. Electric wires 54 are connected to each electrode plate 53. Each electric wire 54 is led out via a plug 32C that closes the tube 32A. Each end of the led out electric wire 54 is connected to, for example, a known DC power supply 55. The reaction vessel 32 is immersed in water 31A so that each tube 32A is not submerged.

[0022] For example, a Liebig cooling pipe is used for the cooling unit 33. The cooling unit 33 is configured so that water flows in through an inlet 33A and flows out from an outlet 33B. For example, cooled tap water is used as this water.

[0023] For example, a known radiation thermometer is used as the temperature detection unit 34. For example, the temperature detection unit 34 is disposed below the water tank 31 and out of contact with the water tank 31. The temperature detection unit 34 detects heat radiated from the underside of the substrate 50. An electric wire 34B connected to the temperature detection unit 34 is connected to, for example, a display device (not shown), and the display device is configured to be able to display a temperature corresponding to the heat detected by the temperature detection unit 34.

[0024] The reaction vessel 32 is configured to receive N2 (nitrogen) via a stopper 32C and a pipe 32A to which a cooling unit 33 is not attached. The N2 (nitrogen) supplied via the pipe 32A is released to the outside via the cooling unit 33. For example, the flow rate of N2 into the reaction vessel 32 is 100 sccm. The synthesis apparatus 30 is configured in this manner. The N2 that flows into the reaction vessel 32 is discharged to the outside via the cooling unit 33 and the pipe 32A to which the cooling unit 33 is attached.

[0025] A synthesis step using ethanol 32B as a carbon source is carried out using synthesis apparatus 30. When no DC voltage is applied to substrate 50 from DC power supply 55, the temperature of ethanol 32B and substrate 50 immersed in ethanol 32B is approximately the same as the temperature of water 31A.

[0026] A DC voltage is applied from a DC power supply 55 to the substrate 50 immersed in ethanol 32B. This causes a DC current to flow through the substrate 50, which generates heat and raises its temperature from room temperature to a predetermined temperature. It takes approximately 10 minutes for the substrate 50 to rise from room temperature to the predetermined temperature. The temperature of the substrate 50 increases as the current flow increases. The substrate 50 is then maintained at the predetermined temperature for a predetermined time. At this time, the ethanol 32B is boiled by the substrate 50 and turns into vapor, which is cooled and liquefied in the cooling unit 33. This makes it difficult for the ethanol 32B to leak to the outside.

[0027] In this way, by raising the temperature of the substrate 50 to a predetermined temperature in the ethanol 32B, single-walled carbon nanotubes C are synthesized so as to extend from each particle 52 (see FIG. 1(D)). After a predetermined time has elapsed, the application of the DC voltage to the substrate 50 is stopped, the temperatures of the ethanol 32B and the substrate 50 are lowered to room temperature, and the substrate 50 on which a plurality of single-walled carbon nanotubes C have been synthesized is removed from the reaction vessel 32.

[0028] [Consideration of substrate temperature during synthesis process] Figure 4 shows the G / D ratio and G / Si ratio in the Raman spectra obtained when irradiating light with a wavelength of 532 nm for five samples produced using the manufacturing method of Example 1 by varying the temperature of the substrate 50 in the synthesis step between five different temperatures: 670°C, 700°C, 730°C, 750°C, and 780°C. For each sample, the substrate 50 was held at the specified temperature for 10 minutes, and the pulse arc plasma gun discharged four times. The G / D ratio is shown in the solid line graph, with the scale on the left. The G / Si ratio is shown in the dotted line graph, with the scale on the right.

[0029] Here, the G / D ratio is an index showing the crystallinity of the single-walled carbon nanotubes C, and is the value obtained by dividing the peak value of the G-band spectrum by the peak value of the D-band spectrum. The peak of the D-band spectrum becomes larger as the number of point defects and the like generated in the single-walled carbon nanotubes C increases. In other words, as the peak value of the D-band spectrum increases, the G / D ratio decreases, indicating that the crystallinity of the single-walled carbon nanotubes C decreases. Furthermore, as the peak value of the D-band spectrum decreases, the G / D ratio increases, indicating that the crystallinity of the single-walled carbon nanotubes C improves. The G / Si ratio is an index that gives a rough indication of the growth amount of the single-walled carbon nanotubes C, and is expressed as the peak value of 1590 cm in the Raman spectrum. -1 Nearby G band and 520cm -1 This is the intensity ratio to the phonon peak of the nearby substrate 50.

[0030] Focusing on the G / D ratio in Figure 4, it was found that the G / D ratio was highest (approximately 15) when the temperature of the substrate 50 was 700°C, the next highest (approximately 14) when the temperature of the substrate 50 was 670°C, the next highest (approximately 10) when the temperature of the substrate 50 was 730°C, the next highest (approximately 4) when the temperature of the substrate 50 was 750°C, and the lowest (approximately 1) when the temperature of the substrate 50 was 780°C.

[0031] Focusing on the G / Si ratio, it was found that the highest (approximately 0.5) was when the temperature of the substrate 50 was 700°C, the next highest (approximately 0.4) when the temperature of the substrate 50 was 670°C, the next highest (approximately 0.3) when the temperature of the substrate 50 was 730°C, the next highest (approximately 0.15) when the temperature of the substrate 50 was 750°C, and the lowest (approximately 0.1) when the temperature of the substrate 50 was 780°C.

[0032] From the results shown in Figure 4, it was found that the crystallinity and growth amount of single-walled carbon nanotubes C were good when the temperature of substrate 50 in the synthesis process was set to 670°C or higher and 730°C or lower. It was also found that the crystallinity and growth amount of single-walled carbon nanotubes C were best when the temperature of substrate 50 in the synthesis process was set to 700°C. In other words, it was found that single-walled carbon nanotubes C could be synthesized best with the temperature of substrate 50 in the synthesis process peaking at 700°C, and that as the temperature of substrate 50 increased (decreased), the degree of crystallinity and growth amount of the synthesized single-walled carbon nanotubes C decreased. It should be noted that the G / D ratio (approximately 15) when the temperature of substrate 50 was 700°C indicates that the crystallinity was equivalent to that of single-walled carbon nanotubes produced by known vapor phase growth methods.

[0033] [Consideration of time in the synthesis process] 5 shows the G / D ratio and G / Si ratio in the Raman spectrum obtained when irradiating light with a wavelength of 532 nm for each sample produced by changing the time for which substrate 50 was maintained at a predetermined temperature in the synthesis step to five different times: 1 minute, 5 minutes, 10 minutes, 15 minutes, and 20 minutes, using the manufacturing method of Example 1. The temperature of substrate 50 in each sample was 700°C.

[0034] Focusing on the G / D ratio shown in Figure 5(A), it was found that the highest G / D ratio (approximately 15) was obtained when the growth time was 10 minutes, the next highest (approximately 3) when it was 15 minutes, the next highest (approximately 2.5) when it was 5 minutes and 20 minutes, and the lowest (approximately 2) when it was 1 minute.

[0035] Focusing on the G / Si ratio shown in Figure 5(B), it was found that the highest G / Si ratio was obtained when the growth time was 10 minutes (approximately 0.5), the next highest when it was 20 minutes (approximately 0.08), the next highest when it was 15 minutes (approximately 0.07), the next highest when it was 5 minutes (approximately 0.02), and the lowest when it was 1 minute (approximately 0.01).

[0036] 5, it was found that by maintaining the substrate 50 at a predetermined temperature for 5 to 15 minutes in the synthesis process, the crystallinity and growth amount of the single-walled carbon nanotubes C were improved, and that by maintaining the substrate 50 at a predetermined temperature for 10 minutes, the crystallinity and growth amount of the single-walled carbon nanotubes C were best. In other words, it was found that as the time for maintaining the substrate 50 at a predetermined temperature became longer (shorter) than 10 minutes, the degree of crystallinity and growth amount of the synthesized single-walled carbon nanotubes C decreased.

[0037] [Study on the number of discharges of a pulsed arc plasma gun] Next, the results of an investigation into the number of discharges of the pulse arc plasma gun will be described. As mentioned above, increasing the number of discharges of the pulse arc plasma gun increases the particle size of the particles 52 in proportion to the number of discharges. For this reason, the effect of the particle size of the particles 52 on the synthesis of the single-walled carbon nanotubes C was investigated. Figure 6 shows Raman spectra obtained when the synthesis process was carried out using samples produced by changing the number of discharges of the pulse arc plasma gun to four types: 4, 6, 8, and 16 times using the manufacturing method of Example 1, and irradiating the samples with light having a wavelength of 532 nm. The temperature of the substrate 50 in each sample was 730°C, and the time for which the substrate 50 was held at the predetermined temperature was 10 minutes. Note that for each sample, -1 From 500cm -1The graph for the range is shown on the left, and -1 From 1800cm -1 The graph for the range is shown on the right.

[0038] As shown in Figure 6(B), in each sample, -1 From 1800cm -1 The G-band spectrum peak is clearly visible in the graph in the range (high wavenumber region). It can also be seen that the G-band spectrum peak tends to become larger as the number of discharges decreases.

[0039] As shown in Figure 6(A), 100 cm -1 From 500cm -1 Graph of the range (low wavenumber region) 200cm -1 300cm from -1 In the vicinity of this point, multiple RBM (radial breathing mode) spectrum peaks appear. In other words, since both the G-band spectrum and the RBM spectrum appear in each sample in Figure 6, it was found that single-walled carbon nanotubes C were grown in each sample.

[0040] 7 shows the G / D ratio and G / Si ratio in the Raman spectra obtained when irradiating light with a wavelength of 532 nm for four samples fabricated using the manufacturing method of Example 1 with four different discharge cycles (4, 6, 8, and 16). The temperature of the substrate 50 for each sample was 700°C, and the substrate 50 was maintained at the predetermined temperature for 10 minutes. The G / D ratio is shown in the solid line graph, with the scale on the left. The G / Si ratio is shown in the dotted line graph, with the scale on the right.

[0041] Focusing on the G / D ratio in Figure 7, it was found that the highest G / D ratio was when the number of discharges was 4 (approximately 15), the next highest when the number of discharges was 6 (approximately 10), the next highest when the number of discharges was 8 (approximately 6), and the lowest when the number of discharges was 16 (approximately 5).

[0042] Focusing on the G / Si ratio, it was found that the highest value (approximately 0.55) was obtained when the number of discharges was four, the next highest (approximately 0.3) when the number of discharges was six, the next highest (approximately 0.2) when the number of discharges was 16, and the lowest value (approximately 0.1) when the number of discharges was eight.

[0043] From the results shown in Figure 7, it was found that the crystallinity and growth amount of the single-walled carbon nanotubes C were improved by setting the number of discharges to six or less, and that the crystallinity and growth amount of the single-walled carbon nanotubes C were best when the number of discharges was four. It is considered that if the number of discharges was three or less, the particle size of the particles 52 would become too small, preventing good growth of the single-walled carbon nanotubes C. Therefore, it is preferable that the number of discharges of the pulse arc plasma gun be four or more and six or less.

[0044] Figure 8 shows an image of the surface of substrate 50 on which single-walled carbon nanotubes C were produced using the manufacturing method of Example 1, with six discharges, a substrate 50 temperature of 700°C, and a time period in which substrate 50 was held at the predetermined temperature of 10 minutes. Figure 8(A) is an SEM image, and Figure 8(B) is a TEM image. As shown in Figure 8, it can be seen that the manufacturing method of Example 1 successfully grows single-walled carbon nanotubes C in an amount comparable to that achieved by vapor phase growth. The diameters of these single-walled carbon nanotubes C are thought to be distributed between approximately 1.0 nm and 1.4 nm.

[0045] Next, the effects of the above embodiment will be described.

[0046] The method for producing single-walled carbon nanotubes includes a synthesis step of elevating the temperature of a substrate 50 carrying particles 52 made of Co (cobalt) on its surface in ethanol 32B, and synthesizing single-walled carbon nanotubes C from the particles 52. According to this configuration, single-walled carbon nanotubes C can be obtained by a liquid phase method, which has conventionally been considered difficult to produce single-walled carbon nanotubes C.

[0047] In the method for producing single-walled carbon nanotubes, the temperature of the substrate 50 in the synthesis step is not less than 670° C. and not more than 730° C. According to this configuration, the single-walled carbon nanotubes C can be produced satisfactorily.

[0048] In the method for producing single-walled carbon nanotubes, the time required for carrying out the synthesis step is 5 minutes or more and 15 minutes or less. With this configuration, the single-walled carbon nanotubes C can be produced reliably.

[0049] The present invention is not limited to the embodiments described above and illustrated in the drawings, and the following embodiments, for example, are also included within the technical scope of the present invention. (1) Unlike Example 1, methanol may be used as the organic liquid. (2) Unlike the first embodiment, silicon carbide or the like may be used as the substrate. (3) Any device capable of depositing Co may be used, not limited to a pulse arc plasma gun. [Explanation of symbols]

[0050] 20...Co (cobalt) 32B...Ethanol (organic liquid) 50...Substrate 52…Particle C...carbon nanotube

Claims

1. A method for producing single-walled carbon nanotubes, comprising a synthesis step of raising the temperature of a substrate carrying particles made of Co (cobalt) on its surface in an organic liquid, and synthesizing single-walled carbon nanotubes from the particles.

2. 2. The method for producing single-walled carbon nanotubes according to claim 1, wherein the temperature of the substrate in the synthesis step is 670°C or higher and 730°C or lower.

3. 3. The method for producing single-walled carbon nanotubes according to claim 1, wherein the synthesis step is carried out for a time period of 5 minutes or more and 15 minutes or less.

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