Method for preparing carbon nanotube with assistance of carbon dioxide

Abstract

The invention provides a method for preparing a carbon nanotube with assistance of carbon dioxide, which is characterized in that methane is used as a carbon source, carbon dioxide is used as auxiliary gas, and the carbon nanotube is prepared under the action of a catalyst. Carbon deposition generated by using methane as carbon source gas is reduced by using carbon dioxide, and the quality and performance of single-wall, double-wall or three-wall carbon nanotubes are improved, so that application requirements are met.

Description

Technical Field

[0001] The invention belongs to the technical field of carbon nanotube preparation, and in particular relates to a method for preparing carbon nanotubes with the assistance of carbon dioxide. Background Art

[0002] Carbon nanotubes can be considered as a tubular nanomaterial formed by curling graphene. Due to its unique one-dimensional structure, carbon nanotubes are widely studied in the fields of energy storage, conductive materials, electronic devices, optical devices, biomedicine, composite materials, etc.

[0003] The main methods for preparing carbon nanotubes include chemical vapor deposition, arc discharge, laser ablation, etc. Among them, chemical vapor deposition is widely used due to its simple operation, low cost, scalability and continuous production. In chemical vapor deposition, commonly used carbon sources include hydrocarbons, such as C 1 -C 8 Contains various alkanes, alkenes and aromatics, alcohols, carbon monoxide, etc. In addition to the main carbon source, different additives are usually added during the reaction, such as water vapor, hydrogen, hydrogen sulfide, carbon dioxide, etc., which can also etch amorphous carbon, maintain the activity of metal catalyst particles, and improve the selectivity of semiconductor single-walled carbon nanotubes in the product.

[0004] Methane is one of the commonly used carbon sources, but it usually cracks too violently during the reaction, resulting in carbon deposits. In previous studies, researchers have introduced a large amount of hydrogen or increased the gas flow rate to reduce the residence time to inhibit methane cracking and reduce the formation of carbon deposits, but these methods will undoubtedly cause gas source waste, increase production costs and low production efficiency. Therefore, how to inhibit the formation of carbon deposits and avoid catalyst deactivation while improving the methane cracking efficiency, while keeping the production cost as low as possible, is a problem that needs to be solved in practical industrial applications.

[0005] On the other hand, by using different growth methods during chemical vapor deposition, samples in different states can be obtained, such as carbon nanotube films, powders, and arrays. The states of the samples grown by the three methods are shown in Fig. 9 shown.

[0006] (1) Carbon nanotube film is a quasi-two-dimensional assembly formed by carbon nanotubes arranged on a two-dimensional plane. It has a network of rich pores, adjustable light transmittance, excellent flexibility and good conductivity. Therefore, transparent conductive films and flexible electronic devices based on single-walled carbon nanotubes have broad application prospects; (ACS Nano 2022, 16(1), 1063-1071.)

[0007] (2) The carbon nanotube powder sample is obtained by direct growth of carbon nanotubes on the surface of the powder catalyst. The carbon nanotubes are entangled and wrapped with each other, and generally need to be purified and dispersed. The biggest advantage of this method is that it is suitable for mass production of carbon nanotubes and is the main method for industrial production; (Industrial & Engineering Chemistry Research 2007, 46 (4), 997-1012.)

[0008] (3) Carbon nanotube arrays are grown directly on a flat substrate surface, and have the characteristics of high degree of alignment, clean surface, and difficulty in forming tube bundles. They do not need to undergo complex processing such as purification and dispersion, thus avoiding damage to the carbon nanotube structure or introduction of impurities. Therefore, single-walled carbon nanotube arrays can be directly used for the construction and integration of field effect transistors. (Accounts of chemical research 2014, 47(8), 2273-2281.) Summary of the invention

[0009] To solve the above problems, the present invention provides a method for preparing carbon nanotubes with the aid of carbon dioxide, wherein methane is used as a carbon source and carbon dioxide is used as an auxiliary gas, and single-walled, double-walled or triple-walled carbon nanotubes are prepared under the action of a catalyst. Carbon dioxide is used to reduce carbon deposits generated by methane as a carbon source gas, thereby improving the quality of single-walled, double-walled or triple-walled carbon nanotubes.

[0010] The object of the present invention is to provide a method for preparing carbon nanotubes with the aid of carbon dioxide. The method uses methane as a carbon source and carbon dioxide as an auxiliary gas, and heats and reacts in the presence of a catalyst to prepare carbon nanotubes.

[0011] The carbon nanotube has 1-3 walls, and is preferably a single-walled or double-walled carbon nanotube, more preferably a single-walled carbon nanotube.

[0012] The catalyst is a metal catalyst, selected from one or more of Fe, Co, Ni, Cu, Cr, Mn, Ti, V, Mo, Ru, Pt and Au, preferably selected from one or more of the metal catalysts containing Fe, Co, Ni, Cu, Mn, Ti and Mo, more preferably one or more of Fe, Co, Ni and Mo.

[0013] The present invention also provides a carbon nanotube prepared according to the method for preparing carbon nanotubes with the assistance of carbon dioxide, wherein the carbon nanotube has 1-3 walls and is preferably a single-walled or double-walled carbon nanotube, more preferably a single-walled carbon nanotube.

[0014] The present invention has the following beneficial effects:

[0015] (1) When methane is used as the carbon source gas in the present invention, carbon dioxide is introduced simultaneously, which effectively improves the cracking efficiency of the carbon source gas while reducing the generation of carbon deposits and avoiding catalyst deactivation.

[0016] (2) Compared with the existing process, the method of introducing a large amount of hydrogen or increasing the methane flow rate is adopted, and the use of carbon dioxide can ensure the production efficiency while reducing the production cost.

[0017] (3) The carbon nanotubes prepared in the present invention are single-walled, double-walled or triple-walled carbon nanotubes with high purity and good quality. The preparation process is simple and easy to control, and carbon nanotubes with a wall number of 1-3 can be stably prepared.

[0018] (4) The method of using methane as the carbon source and carbon dioxide as the regulating gas in the present invention can be widely applied to various growth methods of preparing single-walled carbon nanotubes by chemical vapor deposition, including substrate surface growth, bulk powder growth, and thin film growth, and has universal applicability. BRIEF DESCRIPTION OF THE DRAWINGS

[0019] Figure 1 shows a SEM image of the carbon nanotube array I in Example 1 of the present invention;

[0020] Figure 2 The 532nm laser-excited Raman spectrum of the carbon nanotube array I in Example 1 of the present invention is shown;

[0021] Figure 3 The transmission electron microscope image and the distribution of the number of tube walls of the carbon nanotube powder II in Example 2 of the present invention are shown;

[0022] Figure 4 The 532 nm laser excited Raman spectra of the carbon nanotube powder II in Example 2 of the present invention and the carbon nanotube powder IV in Comparative Example 2 are shown;

[0023] Figure 5 The transmission electron microscope image of the single-walled carbon nanotube III powder of Example 3 of the present invention is shown;

[0024] Figure 6 The scanning electron microscope images of the single-walled carbon nanotube arrays obtained when adding different flow rates of carbon dioxide in Example 4 of the present invention and Comparative Example 1 are shown;

[0025] Figure 7 The 532nm laser excited Raman spectrum of the single-walled carbon nanotube powder V in Comparative Example 3 of the present invention is shown;

[0026] Figure 8 The weight loss curves obtained by thermogravimetric testing after the catalysts of Example 2 and Comparative Example 3 of the present invention grow carbon nanotubes are shown;

[0027] Fig. 9The transmission electron microscope image, powder scanning electron microscope image and array scanning electron microscope image of the carbon nanotube film prepared by chemical vapor deposition in the background technology are shown. DETAILED DESCRIPTION

[0028] The present invention is described in detail below through specific implementation modes, and the characteristics and advantages of the present invention will become clearer and more specific with these descriptions.

[0029] The invention provides a method for preparing carbon nanotubes with the aid of carbon dioxide. The method uses methane as a carbon source and carbon dioxide as an auxiliary gas, and heats and reacts in the presence of a catalyst to prepare the carbon nanotubes.

[0030] The carbon nanotube has 1-3 walls, and is preferably a single-walled or double-walled carbon nanotube, more preferably a single-walled carbon nanotube.

[0031] The catalyst is a metal catalyst, selected from one or more of Fe, Co, Ni, Cu, Cr, Mn, Ti, V, Mo, Ru, Pt and Au, preferably selected from one or more of the metal catalysts containing Fe, Co, Ni, Cu, Mn, Ti and Mo, more preferably one or more of Fe, Co, Ni and Mo.

[0032] The catalyst is obtained by optionally calcining and reducing a catalyst precursor. The catalyst precursor is a metal compound vapor, supported on a powder carrier or supported on a flat substrate.

[0033] In one embodiment of the present invention, the catalyst precursor is a metal compound vapor, preferably an organic transition metal compound vapor of Fe, such as ferrocene vapor. The metal compound vapor is carried into the reactor by a protective gas as a carrier gas, wherein the concentration of the metal compound is 0.0001% to 1%, preferably 0.0005% to 0.1%, and more preferably 0.001% to 0.01%.

[0034] The volume ratio of methane to carbon dioxide is (0.8-2.0):1, preferably (1.0-1.8):1, and more preferably (1.2-1.6):1.

[0035] The heating temperature is 1000-1400°C, preferably 1050-1300°C, and more preferably 1100-1200°C.

[0036] In the reactor, the gas pressure is normal pressure.

[0037] The growth time is 1 to 60 min, preferably 2 to 30 min, and more preferably 2 to 10 min.

[0038] In another embodiment of the present invention, the catalyst precursor is supported on a flat substrate. The flat substrate carrier is selected from silicon wafers, silicon wafers with oxide layers on the surface, AT-cut quartz, BT-cut quartz, SC-cut quartz, IT-cut quartz, ST-cut quartz, LST-cut quartz, a-plane sapphire, c-plane sapphire, r-plane sapphire, and mica sheets. The catalyst precursor solution is arranged in strips on a flat substrate by a tape template method, and the concentration of the catalyst precursor solution is 1 to 10 mmol / L, preferably 1 to 5 mmol / L.

[0039] The volume ratio of methane to carbon dioxide is (10-800):1, preferably (50-600):1, and more preferably (80-400):1.

[0040] The flat substrate loaded with the catalyst precursor is placed in a reactor, and high-temperature calcination and reduction are performed in sequence to obtain the flat substrate loaded with the catalyst.

[0041] The calcination is carried out in an air atmosphere at a temperature of 300-600°C, preferably 350-550°C, and more preferably 400-500°C.

[0042] The reduction is carried out using a reducing gas, the reducing gas is selected from one or more of hydrogen, ammonia, hydrazine, phosphine, carbon monoxide, hydrogen sulfide and sulfur dioxide, preferably one or more of hydrogen, hydrazine and carbon monoxide, more preferably hydrogen. The reduction temperature is 550-1000° C., preferably 600-950° C., more preferably 700-900° C., and the reduction method is constant temperature reduction or programmed temperature reduction.

[0043] After reduction to obtain a flat substrate loaded with a catalyst, a mixed gas of methane and carbon dioxide is introduced under heating conditions to grow carbon nanotubes.

[0044] The heating temperature is 700-1200°C, preferably 800-1100°C, and more preferably 900-1000°C.

[0045] The flow rate of the methane and carbon dioxide mixed gas is 10 to 400 sccm, preferably 20 to 200 sccm, and more preferably 30 to 110 sccm.

[0046] After the mixed gas of methane and carbon dioxide is introduced, the pressure of the reactor is normal pressure.

[0047] The growth time is 1 to 60 min, preferably 5 to 40 min, and more preferably 10 to 20 min.

[0048] When the catalyst precursor is loaded on the powder carrier, the volume ratio of methane to carbon dioxide is (0.1-14): 1, preferably (0.5-12): 1, and more preferably (1-10): 1. The powder carrier loaded with the catalyst precursor is placed in a reactor, and high-temperature calcination and reduction are performed in sequence to obtain a powder carrier loaded with the catalyst.

[0049] The catalyst precursor is supported on a powder carrier. The powder carrier is selected from one or more metal oxides, preferably one or more selected from magnesium oxide, aluminum oxide, silicon oxide, zirconium oxide, cerium oxide and lanthanum oxide, more preferably magnesium oxide and aluminum oxide. The molar ratio of the catalyst metal precursor to the powder carrier is 0.01 to 1, preferably 0.05 to 0.5, more preferably 0.1 to 0.3.

[0050] The reduction is carried out using a reducing gas, which is selected from one or more of hydrogen, ammonia, hydrazine, phosphine, carbon monoxide, hydrogen sulfide and sulfur dioxide, preferably one or more of hydrogen, hydrazine and carbon monoxide, more preferably hydrogen. The reduction temperature is 550-850°C, preferably 600-800°C, more preferably 650-750°C.

[0051] After reduction to obtain a powder carrier loaded with a catalyst, a mixed gas of methane and carbon dioxide is introduced under heating conditions to grow carbon nanotubes.

[0052] The heating temperature is 700-1200°C, preferably 800-1100°C, and more preferably 900-1000°C.

[0053] The flow rate of the methane and carbon dioxide mixed gas is 10 to 500 sccm, preferably 50 to 300 sccm, and more preferably 100 to 200 sccm. After the methane and carbon dioxide mixed gas is introduced, the pressure of the reactor is normal pressure.

[0054] The growth time is 1 to 60 min, preferably 5 to 40 min, and more preferably 10 to 20 min.

[0055] Example

[0056] Example 1

[0057] Precursor arrangement: Prepare 2mM ferric chloride ethanol solution as catalyst precursor solution. Use tape template method to arrange catalyst precursor in strips on clean ST-cut quartz substrate (substrate pre-annealed at high temperature for 0.5 to 8h). The arrangement process is as follows: stick 3M tape to the surface of quartz substrate; use a blade to make parallel scratches on the tape in a direction perpendicular to the quartz lattice; drop the catalyst precursor solution on one side of the scratch, the solution will spread along the gap of the tape, that is, loaded on the quartz substrate in the form of strips; after the ethanol solvent evaporates, peel off the tape.

[0058] Growth of carbon nanotube arrays: Place the quartz substrate loaded with catalyst precursor into a one-inch quartz tube and push it to the center of the tube furnace heating zone. Anneal at 450°C in air for 5 minutes. Raise the temperature to 700°C under argon protection, introduce 200 sccm of hydrogen and program the temperature to 900°C for reduction within 10 minutes. Introduce 100 sccm of methane and 1 sccm of carbon dioxide (denoted as +CO 2 1%), grown at 900°C for 20 min. After the growth was completed, the mixture was cooled to room temperature under argon protection and taken out to obtain carbon nanotube array I.

[0059] The scanning electron microscope (SEM) image of carbon nanotube array I is shown in Figure 1 As shown, the array density is about 8 to 10 / μm, the surface of the carbon nanotubes is clean, and no amorphous carbon coating is observed under a scanning electron microscope.

[0060] Raman spectroscopy test was performed on carbon nanotube array I, with a laser wavelength of 532 nm. Figure 2 As shown. It can be clearly seen that at 125-250cm -1 There is an excitation signal, which is the vibration mode (RBM) of the carbon atoms of the single-walled carbon nanotube along the diameter direction of the carbon nanotube, which is considered to be the "fingerprint" of the single-walled carbon nanotube. According to the RBM peak position (ω RBM ) corresponds to the diameter (d cnt ) formula ω RBM =217.8 / d cnt +15.7, the diameter of the carbon nanotubes is calculated to be in the range of 1.04 to 1.99 nm. Considering the relationship between the diameter of the carbon nanotubes and the tube wall, the existence of a large number of single-walled carbon nanotubes can be confirmed.

[0061] Example 2

[0062] Catalyst preparation: Cobalt nitrate, ammonium molybdate, magnesium nitrate and aluminum nitrate are dissolved in water to form a mixed solution, and the molar ratio of each component is 1:0.2:5:2.5, calculated by the molar amount of metal elements. Then, ammonia water as a precipitant is added to form a hydroxide, and the obtained mixed hydroxide is washed and dried to obtain a hydrotalcite catalyst.

[0063] A quartz boat containing 20 mg of hydrotalcite catalyst powder was placed in a tube furnace and heated to 700 °C under argon protection. The argon gas was cut to 200 sccm H 2 , keep warm for 10 minutes, turn off H 2 Then the temperature was raised to 900°C under argon protection, and 100 sccm CH 4 and CO 2 The reactor is at normal pressure, CO 2 The volume fraction is 10%. After 20 minutes, the reaction is stopped and cooled to room temperature to obtain single-walled carbon nanotube II powder. Its transmission electron microscope image and tube wall number distribution are shown in Figure 3 As shown, from Figure 3 It can be seen that the obtained carbon nanotubes are mainly single-walled. The yield of grown carbon nanotubes was measured to be 20%. Figure 8 This is the weight loss curve of the grown product obtained through thermogravimetric testing.

[0064] The Raman spectrum of single-walled carbon nanotube II powder was tested with a laser wavelength of 532nm. Figure 4 shown.

[0065] Example 3

[0066] First, argon gas is continuously introduced into the tubular furnace to exhaust the air inside, and then the tubular furnace is heated to 1100°C. Ferrocene vapor carried by argon is introduced into the tubular furnace, and the volume content of ferrocene is 0.004%. Methane, carbon dioxide, and argon are introduced at the same time. The flow rates of methane, argon, and argon-carrying ferrocene vapor are 2sscm, 532sccm, and 35sccm, respectively. The flow ratio of carbon dioxide to methane is 1.4:1. The reaction lasts for 10 minutes, and the product single-walled carbon nanotube III sample is obtained, and its transmission electron microscope image is shown as follows Figure 5 As shown, from Figure 5 It can be seen that the obtained carbon nanotubes are single-walled.

[0067] Example 4

[0068] A carbon nanotube array was prepared according to the method of Example 1, except that 0.25 sccm, 0.75 sccm and 1.25 sccm of carbon dioxide were added during the growth of the carbon nanotube array.

[0069] The SEM images of the prepared carbon nanotube arrays are shown in Figure 6 As shown,

[0070] 1) Add 0.25 sccm carbon dioxide (denoted as +CO 2 0.25%), the array density was about 1-2 pieces / μm, and the surface carbon was significantly reduced, indicating the effect of carbon dioxide in removing carbon deposits;

[0071] 2) Add 0.75 sccm of carbon dioxide (denoted as +CO 2 0.75%), the array density was about 1-2 pieces / μm, and the surface carbon was significantly reduced, indicating the effect of carbon dioxide in removing carbon deposits;

[0072] 3) Add 1.25 sccm of carbon dioxide (denoted as +CO 2 The array density was about 3-4 arrays / μm when the carbon dioxide content was 1.25%, indicating that excess carbon dioxide showed an inhibitory effect.

[0073] Example 1 and Example 4 together illustrate that adding an appropriate amount of carbon dioxide can significantly increase the yield of single-walled carbon nanotubes, thereby increasing the array density.

[0074] Comparative Example

[0075] Comparative Example 1

[0076] A carbon nanotube array was prepared according to the method of Example 1, except that 0 sccm and 2 sccm of carbon dioxide were added during the growth of the carbon nanotube array.

[0077] 1) When 0 sccm of carbon dioxide is added, the array density is about 1-2 / μm, and there is a large amount of flaky amorphous carbon on the surface;

[0078] 2) Add 2sccm of carbon dioxide (denoted as +CO 2 2%), the carbon nanotubes no longer grow. Its SEM image is as follows Figure 6 shown.

[0079] Example 1 and Comparative Example 1 together illustrate that adding an appropriate amount of carbon dioxide can significantly increase the yield of single-walled carbon nanotubes, remove carbon deposits, and thus increase the array density.

[0080] Comparative Example 2

[0081] Carbon nanotubes IV were obtained according to the method of Example 2, except that 0 sccm of carbon dioxide was added.

[0082] The Raman spectrum of the prepared carbon nanotube IV is shown in the figure below: Figure 4 As shown, the D peak intensity is significantly increased in the absence of carbon dioxide, indicating an increase in carbon deposition.

[0083] Example 2 and Comparative Example 2 together illustrate that adding carbon dioxide can effectively reduce carbon deposits and improve the quality of single-walled carbon nanotubes.

[0084] Comparative Example 3

[0085] Single-walled carbon nanotubes V were obtained according to the method of Example 2, except that: when preparing the catalyst, magnesium nitrate and cerium nitrate were first dissolved in water to form a mixed solution, and then dried and calcined to obtain a magnesium oxide-cerium oxide composite carrier. Cobalt nitrate and nickel nitrate were then loaded onto the magnesium oxide-cerium oxide composite carrier by an impregnation method, dried, and calcined at 200°C in an air atmosphere for about 10 minutes to obtain a catalyst precursor; when growing carbon nanotubes, a quartz boat containing the catalyst precursor powder was placed in a tube furnace, heated to 600°C under argon protection, argon was cut to 200sccm hydrogen, and the temperature was kept for 30 minutes, the hydrogen was turned off, and then heated to 900°C under argon protection, 20sccm methane and 20sccm carbon dioxide were introduced at 900°C to grow single-walled carbon nanotubes IV. Figure 7 The 532nm excitation Raman spectrum of carbon nanotube V. The yield of grown carbon nanotubes was measured to be 10%. Figure 8 This is the weight loss curve of the grown product obtained through thermogravimetric testing.

[0086] The present invention is described in detail above in conjunction with specific embodiments and / or exemplary examples and drawings, but these descriptions cannot be understood as limiting the present invention. Those skilled in the art understand that, without departing from the spirit and scope of the present invention, various equivalent substitutions, modifications or improvements may be made to the technical solution of the present invention and its implementation methods, all of which fall within the scope of the present invention. The scope of protection of the present invention shall be subject to the attached claims.

Claims

1. A method for preparing carbon nanotubes with the aid of carbon dioxide, wherein methane is used as a carbon source and carbon dioxide is used as an auxiliary gas, and a heating reaction is performed in the presence of a catalyst to prepare carbon nanotubes. The carbon nanotube has 1-3 walls, and is preferably a single-walled or double-walled carbon nanotube, more preferably a single-walled carbon nanotube.

2. The method according to claim 1, It is characterized in that The catalyst is a metal catalyst, selected from one or more of Fe, Co, Ni, Cu, Cr, Mn, Ti, V, Mo, Ru, Pt and Au, preferably selected from one or more of metal catalysts containing Fe, Co, Ni, Cu, Mn, Ti and Mo.

3. The method according to claim 1, It is characterized in that The catalyst is obtained by optionally calcining and reducing a catalyst precursor, wherein the catalyst precursor is a metal compound vapor, supported on a powder carrier or supported on a flat substrate. The metal compound vapor is preferably Fe organic transition metal compound vapor, such as ferrocene vapor, and the metal compound vapor is carried into the reactor by using a protective gas as a carrier gas, wherein the metal compound concentration is 0.0001% to 1%, preferably 0.0005% to 0.1%, and more preferably 0.001% to 0.01%. The powder carrier is selected from one or more metal oxides, preferably one or more selected from magnesium oxide, aluminum oxide, silicon oxide, zirconium oxide, cerium oxide and lanthanum oxide, more preferably magnesium oxide and aluminum oxide. The flat substrate carrier is selected from silicon wafers, silicon wafers with oxide layers on the surface, AT-cut quartz, BT-cut quartz, SC-cut quartz, IT-cut quartz, ST-cut quartz, LST-cut quartz, a-plane sapphire, c-plane sapphire, r-plane sapphire, and mica sheets. The catalyst precursor solution is arranged in strips on the flat substrate by a tape template method.

4. The method according to claim 1, It is characterized in that The volume ratio of methane to carbon dioxide is (0.5-1000):

1.

5. The method according to claim 4, It is characterized in that When the catalyst precursor is a metal compound vapor, The volume ratio of methane to carbon dioxide is (0.8-2.0):1, preferably (1.0-1.8):1, and more preferably (1.2-1.6):

1. The heating temperature is 1000-1400°C, preferably 1050-1300°C, more preferably 1100-1200°C. In the reactor, the gas pressure is normal pressure. The growth time is 1 to 60 min, preferably 2 to 30 min, and more preferably 2 to 10 min.

6. The method according to claim 4, It is characterized in that When the catalyst precursor is loaded on a powder carrier, The volume ratio of methane to carbon dioxide is (4-14):1, preferably (6-12):1, more preferably (8-10):1, The powder carrier loaded with the catalyst precursor is placed in a reactor, and high-temperature calcination and reduction are performed in sequence to obtain a powder carrier loaded with the catalyst. The reduction is carried out using reducing gas at a reduction temperature of 550-850°C, preferably 600-800°C, more preferably 650-750°C. After reduction to obtain a powder carrier loaded with a catalyst, a mixed gas of methane and carbon dioxide is introduced under heating conditions to grow carbon nanotubes. The heating temperature is 700-1200°C, preferably 800-1100°C, more preferably 900-1000°C, The flow rate of the methane and carbon dioxide mixed gas is 10 to 500 sccm, preferably 50 to 300 sccm, more preferably 100 to 200 sccm. After the mixed gas of methane and carbon dioxide is introduced, the pressure of the reactor is normal pressure. The growth time is 1 to 60 min, preferably 5 to 40 min, and more preferably 10 to 20 min.

7. The method according to claim 4, It is characterized in that When the catalyst precursor is supported on a flat substrate, The volume ratio of methane to carbon dioxide is (10-800):1, preferably (50-600):1, more preferably (80-400):1, The flat substrate loaded with the catalyst precursor is placed in a reactor, and high-temperature calcination and reduction are performed in sequence to obtain a flat substrate loaded with the catalyst. The calcination is carried out in an air atmosphere at a temperature of 300-600°C, preferably 350-550°C, more preferably 400-500°C. The reduction is performed using reducing gas at a reduction temperature of 550-1000°C, preferably 600-950°C, and more preferably 700-900°C.

8. The method according to claim 7, It is characterized in that When the catalyst precursor is supported on a flat substrate, After reduction to obtain a flat substrate loaded with the catalyst, a mixed gas of methane and carbon dioxide is introduced under heating conditions to grow carbon nanotubes. The heating temperature is 700-1200°C, preferably 800-1100°C, more preferably 900-1000°C, The flow rate of the methane and carbon dioxide mixed gas is 10 to 400 sccm, preferably 20 to 200 sccm, and more preferably 30 to 110 sccm.

9. The method according to claim 7, It is characterized in that When the catalyst precursor is supported on a flat substrate, After the mixed gas of methane and carbon dioxide is introduced, the pressure of the reactor is normal pressure. The growth time is 1 to 60 min, preferably 5 to 40 min, and more preferably 10 to 20 min.

10. A carbon nanotube prepared by the method for preparing carbon nanotubes with assistance of carbon dioxide according to claim 1, It is characterized in that The carbon nanotube has 1-3 walls, and is preferably a single-walled or double-walled carbon nanotube, more preferably a single-walled carbon nanotube.

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