Preparation method of few-walled carbon nanotubes

Abstract

The invention provides a preparation method of a few-walled carbon nanotube, which is characterized in that a mixed gas of methane and carbon dioxide grows in the presence of a catalyst to obtain the product few-walled carbon nanotube with a tube diameter of less than 10nm and narrow wall number distribution. The carbon dioxide can etch generated amorphous carbon and activate the catalyst to participate in growth of the few-walled carbon nanotubes, so that the conversion rate of methane is increased, and the yield of the few-walled carbon nanotubes is greatly increased, thereby meeting the application requirements of the few-walled carbon nanotubes with narrow wall number distribution.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon nanotubes, and particularly relates to a method for preparing few-walled carbon nanotubes. Background Art

[0002] Carbon nanotubes are one-dimensional carbon nanomaterials, and their unique physical properties such as electrical, mechanical, and optical properties have attracted extensive attention, making them have been applied in the fields of nanoelectronics, sensing, energy, imaging, etc. According to the number of graphite layers forming the carbon nanotubes, that is, the number of walls of the carbon nanotubes, carbon nanotubes can be divided into single-walled carbon nanotubes (the number of walls is 1) and multi-walled carbon nanotubes (the number of walls is more than 1).

[0003] Among the many excellent properties of carbon nanotubes, the electrical conductivity is relatively prominent. Ideal defect-free single-walled carbon nanotubes have both electron mobility and hole mobility higher than 100000 cm 2 / (V·s), and the current-carrying capacity is dozens to hundreds of times that of single-crystalline silicon, and is stronger than common conductive metals; its one-dimensional structure and excellent structural characteristics of strength and toughness enable carbon nanotubes to form effective lap joints and conductive networks on the surfaces of various materials. Therefore, carbon nanotubes have the potential to become excellent conductive materials.

[0004] However, to realize the application of carbon nanotubes as conductive materials, it is necessary to reasonably design the structure of the carbon nanotubes used. When single-walled carbon nanotubes are used as conductive materials, they will inevitably be restricted by the following two aspects: First, due to the different band gaps of different single-walled carbon nanotube materials, only one-third of the single-walled carbon nanotube structures can make them have metallic conductivity, while the remaining semiconducting single-walled carbon nanotubes have a small density of states near the Fermi level, thus affecting the overall conductivity; Second, the structural strength of single-walled carbon nanotubes is relatively fragile compared to multi-walled carbon nanotubes. If chemical modification is carried out or the microenvironment stress is relatively large, its conductive structure may be damaged, affecting its macroscopic conductive performance.

[0005] For large-diameter multi-walled carbon nanotubes, although they are usually metallic and there is no problem of semiconducting nanotubes reducing conductivity, they also have their own limitations. First, the conductive structure integrity of the usually synthesized large-diameter (more than 10 nm) multi-walled carbon nanotubes is not good, and there are more defects in the structure, making the probability of electron scattering large, which instead reduces the conductivity; Second, the large-diameter multi-walled carbon nanotubes are more rigid and lack the flexibility of single-walled carbon nanotubes, and their contact number with other materials such as electrodes may be less, and the formed conductive network is not complete enough, also resulting in a reduction in macroscopic conductivity; Third, the dispersibility of large-diameter multi-walled carbon nanotubes is not good, and it is difficult to uniformly form industrial products such as conductive slurries and conductive masterbatches. Therefore, simply using single-walled carbon nanotubes and large-diameter multi-walled carbon nanotubes as conductive materials both have deficiencies.

[0006] Among multi-walled carbon nanotubes, there is a special type of few-walled carbon nanotubes. Their number of walls is 2 - 6, and the diameter is relatively small (2 - 8 nm). These few-walled carbon nanotubes are usually metallic. Compared with single-walled carbon nanotubes, their mobility and dispersibility are not reduced much, and their structure also has a certain flexibility and stronger stability, combining some advantages of single-walled carbon nanotubes and large-diameter multi-walled carbon nanotubes, and may become a good choice for applications in conductive materials.

[0007] However, the preparation and synthesis of few-walled carbon nanotubes are still restricted by many factors. When using supported catalysts for preparation, there may be problems such as low yield and large carbon emissions. When using methods such as floating catalyst chemical vapor deposition (FCCVD) for preparation, the content of metal particles in the product is relatively high, which is not conducive to subsequent applications.

[0008] Therefore, how to synthesize few-walled carbon nanotubes with green, high yield and high quality is still an urgent problem to be solved before few-walled carbon nanotubes can be applied in practice. Summary of the Invention

[0009] To solve the above problems, the present invention provides a method for preparing few-walled carbon nanotubes. Using a mixed gas of methane and carbon dioxide as the carbon source gas, under the action of a metal catalyst, by controlling process conditions, carbon nanotubes with a diameter less than 10 nm and a wall number of 2 - 9 can reach more than 80%. Carbon dioxide can etch the generated amorphous carbon and activate the catalyst. As the second carbon source, it participates in the growth of few-walled carbon nanotubes, improves the conversion rate of methane, and significantly increases the yield of few-walled carbon nanotubes to meet the application requirements for few-walled carbon nanotubes with a narrow wall number distribution.

[0010] The purpose of the present invention is to provide a method for preparing few-walled carbon nanotubes. The method uses a mixed gas of methane and carbon dioxide to grow the product few-walled carbon nanotubes in the presence of a catalyst.

[0011] Among the product few-walled carbon nanotubes, more than 80% of the carbon nanotubes have a wall number of 2 - 9, preferably more than 90% of the carbon nanotubes have a wall number of 2 - 9; more than 50% of the carbon nanotubes have a wall number of 2 - 6, preferably more than 90% of the carbon nanotubes have a wall number of 2 - 6; more than 30% of the carbon nanotubes have a wall number of 2 - 4, preferably more than 80% of the carbon nanotubes have a wall number of 2 - 4.

[0012] The average diameter of the product few-walled carbon nanotubes is less than 10 nm, preferably 2 - 8 nm.

[0013] In the Raman spectrum of the product few-walled carbon nanotubes, the relative intensity ratio of the G peak to the D peak is greater than 4, preferably greater than 7, and more preferably greater than 9.

[0014] The catalyst includes a metal catalyst, wherein the metal element is selected from one or more of vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), tungsten (W), rhenium (Re), and platinum (Pt), preferably one or more of iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), and tungsten (W), more preferably one or more of iron (Fe), cobalt (Co), and molybdenum (Mo), such as Fe and Mo, Co and Mo.

[0015] The volume ratio of the methane to the carbon dioxide is (1 - 1000):10, preferably (1 - 50):1, more preferably (2 - 20):1.

[0016] The object of the present invention also lies in providing few-walled carbon nanotubes prepared by the preparation method of the few-walled carbon nanotubes.

[0017] The present invention has the following beneficial effects:

[0018] (1) In the present invention, carbon dioxide serves both as a regulating gas to etch the generated amorphous carbon and activate the catalyst, and as a second carbon source to participate in the growth of the few-walled carbon nanotubes, improving the conversion rate of methane and the yield of the few-walled carbon nanotubes.

[0019] (2) The method provided in the present invention can obtain a few-walled carbon nanotube product with a narrower wall number distribution, optimize the conductive structure of the carbon nanotubes, reduce defects, improve conductivity, have flexibility, and broaden the application fields.

[0020] (3) The few-walled carbon nanotube product prepared by the method provided in the present invention has higher purity, better quality, low cost, and can reduce carbon emissions in the synthesis, which is beneficial to the green synthesis of high-quality few-walled carbon nanotubes and is suitable for large-scale synthesis. Description of the Drawings

[0021] Figure 1 Shows the Raman spectrum of the crude product Ⅰ of the few-walled carbon nanotubes prepared in Example 1 of the present invention.

[0022] Figure 2 Shows the Raman spectrum of the crude product Ⅱ of the few-walled carbon nanotubes prepared in Example 2 of the present invention.

[0023] Figure 3 Shows the Raman spectrum of the crude product Ⅲ of the few-walled carbon nanotubes prepared in Example 3 of the present invention.

[0024] Figure 4 Shows the Raman spectrum of the crude product Ⅳ of the few-walled carbon nanotubes prepared in Example 4 of the present invention.

[0025] Figure 5Shows the Raman spectrum of the crude multi-walled carbon nanotubes V prepared in Example 5 of the present invention.

[0026] Figure 6 Shows the Raman spectrum of the crude multi-walled carbon nanotubes prepared in Comparative Example 1 of the present invention.

[0027] Figure 7 Shows the SEM image of the crude multi-walled carbon nanotubes I prepared in Example 1 of the present invention.

[0028] Figure 8 Shows the SEM image of the crude multi-walled carbon nanotubes II prepared in Example 2 of the present invention.

[0029] Figure 9 Shows the SEM image of the crude multi-walled carbon nanotubes III prepared in Example 3 of the present invention.

[0030] Figure 10 Shows the SEM image of the crude multi-walled carbon nanotubes IV prepared in Example 4 of the present invention.

[0031] Figure 11 Shows the SEM image of the crude multi-walled carbon nanotubes V prepared in Example 5 of the present invention.

[0032] Figure 12 Shows the SEM image of the crude multi-walled carbon nanotubes prepared in Comparative Example 1 of the present invention.

[0033] Figure 13 Shows the TEM image of the crude multi-walled carbon nanotubes I prepared in Example 1 of the present invention.

[0034] Figure 14 Shows the TEM image of the crude multi-walled carbon nanotubes II prepared in Example 2 of the present invention.

[0035] Figure 15 Shows the TEM image of the crude multi-walled carbon nanotubes III prepared in Example 3 of the present invention.

[0036] Figure 16 Shows the TEM image of the crude multi-walled carbon nanotubes IV prepared in Example 4 of the present invention.

[0037] Figure 17 Shows the TEM photograph of the crude multi-walled carbon nanotubes V prepared in Example 5 of the present invention.

[0038] Figure 18 Perform a wall number statistics on the multi-walled carbon nanotubes appearing in the TEM photograph of the product powder obtained in Example 1.

[0039] Figure 19 Perform a wall number statistics on the multi-walled carbon nanotubes appearing in the TEM photograph of the product powder obtained in Example 2.

[0040] Figure 20 Count the number of walls of the few-walled carbon nanotubes that appear in the TEM image of the product powder obtained in Example 3.

[0041] Figure 21 Count the number of walls of the few-walled carbon nanotubes that appear in the TEM image of the product powder obtained in Example 4.

[0042] Figure 22 Count the number of walls of the few-walled carbon nanotubes that appear in the TEM image of the product powder obtained in Example 5. Detailed implementation manners

[0043] The present invention will be described in detail below through specific implementation manners, and the features and advantages of the present invention will become clearer and more definite along with these descriptions.

[0044] The present invention provides a method for preparing few-walled carbon nanotubes. The method uses a mixed gas of methane and carbon dioxide and grows the product few-walled carbon nanotubes in the presence of a catalyst.

[0045] In the product few-walled carbon nanotubes, the number of walls of more than 80% of the carbon nanotubes is 2 - 9, preferably the number of walls of more than 90% of the carbon nanotubes is 2 - 9; the number of walls of more than 50% of the carbon nanotubes is 2 - 6, preferably the number of walls of more than 90% of the carbon nanotubes is 2 - 6; the number of walls of more than 30% of the carbon nanotubes is 2 - 4, preferably the number of walls of more than 80% of the carbon nanotubes is 2 - 4.

[0046] The average tube diameter of the product few-walled carbon nanotubes is less than 10 nm, preferably 2 - 8 nm.

[0047] In the Raman spectrum of the product few-walled carbon nanotubes, the relative intensity ratio of the G peak to the D peak is greater than 4, preferably greater than 7, and more preferably greater than 9.

[0048] The catalyst includes a metal catalyst, wherein the metal element is selected from one or more of vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), tungsten (W), rhenium (Re), and platinum (Pt), preferably one or more of iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), and tungsten (W), and more preferably one or more of iron (Fe), cobalt (Co), and molybdenum (Mo), such as Fe and Mo, Co and Mo.

[0049] In the catalyst, the molar ratio of Fe to Mo is 1:(0.05 - 3.6), preferably 1:(0.05 - 3.0), more preferably 1:(0.1 - 2.4); the molar ratio of Co to Mo is 1:(0.05 - 3.6), preferably 1:(0.05 - 3.0), more preferably 1:(0.1 - 2.4). In the catalyst with the preferred composition, the lower the molar ratio of Fe (or Co) to Mo, the higher the average number of walls of the few-walled carbon nanotubes, the narrower the wall number distribution, and the more stable the control of product performance; the higher the molar ratio of Fe (or Co) to Mo, the higher the yield of the few-walled carbon nanotubes.

[0050] Preferably, the catalyst further includes a powder carrier. The powder carrier is selected from one or more of magnesium oxide, aluminum oxide, silicon dioxide, scandium oxide, yttrium oxide, lanthanum oxide, titanium dioxide, zirconium dioxide, cerium dioxide, aluminum nitride, silicon nitride, and gallium nitride, preferably one or more of magnesium oxide, aluminum oxide, and silicon dioxide.

[0051] The molar mass ratio of the metal catalyst to the powder carrier is (0.5 - 4.5) mol:1400 g, preferably (0.8 - 3.5) mol:1400 g, more preferably (1.1 - 2.5) mol:1400 g.

[0052] In the present invention, the preparation method of the catalyst is not specifically limited, and any method capable of preparing the catalyst can be used. For example, a metal compound solution or colloid is used and loaded on the powder carrier by one of the sol-gel method, co-precipitation method, combustion method, high-temperature solid-phase synthesis method, self-propagating high-temperature synthesis method, freeze-drying method, and spray-drying method.

[0053] Preferably, the catalyst is obtained by dispersing or dissolving a metal compound and a carrier compound in a solvent, removing the solvent, calcining to obtain a catalyst precursor, and then reducing to obtain the catalyst loaded on the powder carrier.

[0054] The calcination is carried out in an air atmosphere at 250 - 550 °C, preferably 300 - 500 °C. Optionally, combustion is initiated before calcination.

[0055] The reduction is carried out under a reducing atmosphere and heating conditions. The reducing gas is selected from one or more of hydrogen, ammonia, hydrazine, phosphine, carbon monoxide, methane, hydrogen sulfide, and sulfur dioxide, preferably hydrogen. The reduction temperature is 400 - 1000 °C, preferably 500 - 800 °C.

[0056] The volume ratio of methane to carbon dioxide is (1 - 1000):10, preferably (1 - 50):1, more preferably (2 - 20):1.

[0057] If the volume ratio of methane to carbon dioxide is too small, the etching effect of carbon dioxide is obvious, and the few-walled carbon nanotubes formed by etching will be etched, resulting in a decrease in yield and a decline in the quality of carbon nanotubes. If the volume ratio of methane to carbon dioxide is too large, the cracking of methane is not fully regulated, and the amorphous carbon generated during the cracking process cannot be fully etched by carbon dioxide, resulting in a slight decrease in the yield, and the product will be mixed with amorphous carbon. Within the preferred range, the promoting effect of carbon dioxide on the cracking of methane and the etching effect on amorphous carbon are sufficient, so that high-quality few-walled carbon nanotubes with a high yield and less amorphous carbon can be obtained.

[0058] Within the preferred range, when the volume ratio of methane to carbon dioxide decreases, the average number of walls of the few-walled carbon nanotubes will increase slightly. The reason is that when the concentration of carbon dioxide increases, it gradually exerts an etching effect on the few-walled carbon nanotubes. This etching effect preferentially occurs on the carbon nanotubes with a smaller diameter, a larger curvature, and fewer walls, resulting in an increase in the average number of walls of the obtained few-walled carbon nanotube product.

[0059] In the present invention, methane is used as the main carbon source, and few-walled carbon nanotubes are produced by decomposition on the surface of the metal catalyst; carbon dioxide is used both as a regulating gas to etch the generated amorphous carbon and activate the catalyst, and as a second carbon source to participate in the growth of few-walled carbon nanotubes, improving the conversion rate of methane and the yield of few-walled carbon nanotubes.

[0060] The participation of carbon dioxide reduces the formation of amorphous carbon and increases the yield of few-walled carbon nanotubes; in addition, both carbon dioxide and methane are greenhouse gases, and the use of carbon dioxide and methane for synthesis helps to reduce the greenhouse gas emissions of synthesizing few-walled carbon nanotubes by this method, and few-walled carbon nanotubes can be prepared in a green and high-yield manner.

[0061] The growth temperature is 500 - 1200 °C, preferably 700 - 1100 °C, more preferably 800 - 1000 °C. If the temperature is too high, the cracking of methane will intensify and the regulation will be more difficult, and a large amount of amorphous carbon will be generated on the reactants and the hot-wall reactor. If the temperature is too low, the cracking of methane will be insufficient, resulting in a decrease in yield; the graphitization degree of the product will decrease, and the quality and conductivity of the product few-walled carbon nanotubes will also decline.

[0062] The growth process lasts for at least 8 minutes, preferably 10 - 30 minutes, more preferably 10 - 15 minutes.

[0063] The few-walled carbon nanotubes are combined with the catalyst and the carrier after the growth is completed. The combined product (crude product) can be obtained as a pure few-walled carbon nanotube powder through steps such as carrier removal, purification, and separation.

[0064] The yield of the crude multi-walled carbon nanotubes is calculated by the following method: (mass of the powder after growth – mass of the powder before reduction) / mass of the powder before reduction × 100%.

[0065] In the said method, the yield of the multi-walled carbon nanotubes is not less than 30%, and can even reach not less than 80%, or even not less than 200%.

[0066] The present invention also provides a product, namely multi-walled carbon nanotubes, prepared according to the preparation method of the multi-walled carbon nanotubes described above.

[0067] In the said product of multi-walled carbon nanotubes, the wall number of more than 80% of the carbon nanotubes is 2 - 9, preferably the wall number of more than 90% of the carbon nanotubes is 2 - 9; the wall number of more than 50% of the carbon nanotubes is 2 - 6, preferably the wall number of more than 90% of the carbon nanotubes is 2 - 6; the wall number of more than 30% of the carbon nanotubes is 2 - 4, preferably the wall number of more than 80% of the carbon nanotubes is 2 - 4.

[0068] The average tube diameter of the said product of multi-walled carbon nanotubes is less than 10 nm, preferably 2 - 8 nm.

[0069] In the Raman spectrum of the said product of multi-walled carbon nanotubes, the relative intensity ratio of the G peak to the D peak is greater than 4, preferably greater than 7, and more preferably greater than 9.

[0070] The preparation method of multi-walled carbon nanotubes provided by the present invention uses a mixed gas of methane and carbon dioxide to grow multi-walled carbon nanotubes on a metal catalyst supported by a powder carrier. Among them, methane is the main carbon source, and carbon dioxide not only plays a regulatory role in etching amorphous carbon and maintaining the activity of the catalyst, but also serves as a second carbon source and participates in the growth of multi-walled carbon nanotubes together. This method can utilize two inexpensive and readily available industrial raw materials, methane and carbon dioxide, which are also greenhouse gases, with low cost and can reduce carbon emissions in synthesis, facilitating the green and large-scale synthesis of high-quality multi-walled carbon nanotubes. The multi-walled carbon nanotube products synthesized by this method have complete structures, controllable wall numbers, and excellent quality, and have broad application prospects.

[0071] Examples

[0072] Example 1

[0073] Hydrated magnesium nitrate [Mg(NO 3 ) 2 ·6H 2 O], hydrated ferric nitrate [Fe(NO 3 ) 3 ·9H 2 O], ammonium heptamolybdate tetrahydrate [(NH 4 ) 6 Mo 7 O 24 ·4H 2O], citric acid (C 6 H 8 O 7 ), glycine (C 2 H 5 NO 2 ) were added to deionized water, and their molar ratio was 350:7:2:7:120. After stirring and dissolving, a catalyst precursor solution was formed. After the solution was carefully evaporated to a viscous state, combustion was initiated on a hot stage at 340 °C, and the finally formed solid product was calcined in air at 550 °C until stable, obtaining the catalyst precursor Ⅰ supported on a powdered MgO carrier. Among them, the molar ratio of the contained Fe element, Mo element and MgO was Fe:Mo:MgO = 1:2:50.

[0074] 39.34 mg of the above catalyst precursor Ⅰ was loaded into a quartz tube and synthesized in a horizontal tube furnace. Under an argon atmosphere, the tube furnace was heated to 800 °C, and hydrogen was introduced at a flow rate of 200 sccm for reduction to obtain a catalyst supported on a powdered carrier (Fe-Mo / MgO). Then, it was heated to 1000 °C under a hydrogen atmosphere, and a mixed gas of methane and carbon dioxide with a volume ratio of 5:1 was introduced at a flow rate of 120 sccm. The gas pressure in the quartz tube was maintained at atmospheric pressure, and carbon nanotubes were grown for 10 minutes. The obtained product was cooled to room temperature under an argon-hydrogen mixed atmosphere. The crude product Ⅰ of multi-walled carbon nanotubes was obtained, and the yield was 212% (mass of crude product Ⅰ of multi-walled carbon nanotubes - mass of catalyst precursor Ⅰ / mass of catalyst precursor Ⅰ).

[0075] Example 2

[0076] The crude product Ⅱ of multi-walled carbon nanotubes was prepared according to the method of Example 1, with the only difference being that Mg(NO 3 ) 2 ·6H 2 O, Fe(NO 3 ) 3 ·9H 2 O, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, citric acid (C 6 H 8 O 7 ), glycine (C 2 H 5 NO 2 ) had a molar ratio of 350:7:1:7:120, obtaining the catalyst precursor Ⅱ supported on a powdered carrier. Among them, the molar ratio Fe:Mo:MgO = 1:1:50. The yield of the crude product Ⅱ of multi-walled carbon nanotubes was 79.2%.

[0077] Example 3

[0078] The crude multi-walled carbon nanotube product III was prepared according to the method of Example 1, with the only difference being that: Mg(NO 3 ) 2 ·6H 2 O, Fe(NO 3 ) 3 ·9H 2 O, (NH 4 ) 6 Mo 7 O 24 ·4H 2 O, citric acid (C 6 H 8 O 7 ), glycine (C 2 H 5 NO 2 ) had a molar ratio of 350:14:1:7:120, and the catalyst precursor III supported on a powder carrier was obtained. Among them, the molar ratio of Fe:Mo:MgO = 2:1:50. The yield of the crude multi-walled carbon nanotube product III was 93.2%.

[0079] Example 4

[0080] The crude multi-walled carbon nanotube product IV was prepared according to the method of Example 1, with the only difference being that the volume ratio of methane to carbon dioxide was 5:2. The yield of the crude multi-walled carbon nanotube product IV was 218%.

[0081] Example 5

[0082] The crude multi-walled carbon nanotube product V was prepared according to the method of Example 1, with the only difference being that an equimolar amount of Co(NO 3 ) 2 ·6H 2 O was added to replace Fe(NO 3 ) 3 ·9H 2 O, and the catalyst precursor V supported on a powder carrier was obtained. Among them, the molar ratio of Co:Mo:MgO = 1:2:50. The yield of the crude multi-walled carbon nanotube product V was 66.6%.

[0083] Comparative Example

[0084] Comparative Example 1

[0085] The multi-walled carbon nanotubes were prepared according to the method of Example 2, with the only difference being that during the growth of the carbon nanotubes, the mixed gas of methane and carbon dioxide was not introduced, and only pure methane with the same volume as the mixed gas of methane and carbon dioxide was introduced.

[0086] The carbon yield of the obtained crude multi-walled carbon nanotubes was 95.1%. The crude powder was gray instead of black, indicating that the multi-walled carbon nanotubes were impure. During the growth process, a large amount of amorphous carbon was deposited in the quartz tube reactor.

[0087] Experimental Example

[0088] Experimental Example 1

[0089] The product powders obtained in Example 1, Example 2, Example 3, Example 4, Example 5, and Comparative Example 1 were characterized by Raman spectroscopy (Labram ARAMIS type Raman spectroscopy, laser wavelength 532 nm).

[0090] Figure 1 This is the Raman spectrum of the crude multi-walled carbon nanotubes I prepared in Example 1 (results of three sampling measurements). The characteristic peaks G and D belonging to the carbon nanotubes are significant, and there are RBM peaks at some positions. The relative intensity ratio of the G peak to the D peak is 9 - 14.

[0091] Figure 2 This is the Raman spectrum of the crude multi-walled carbon nanotubes II prepared in Example 2 (results of three sampling measurements). The characteristic peaks G and D belonging to the carbon nanotubes are equally significant, and there are RBM peaks at some positions. The intensity ratio of the G peak to the D peak is 9 - 15.

[0092] Figure 3 This is the Raman spectrum of the crude multi-walled carbon nanotubes III prepared in Example 3 (results of three sampling measurements). The characteristic peaks G and D belonging to the carbon nanotubes are equally significant, and there are RBM peaks at some positions. The intensity ratio of the G peak to the D peak is 8 - 15.

[0093] Figure 4 This is the Raman spectrum of the crude multi-walled carbon nanotubes IV prepared in Example 4 (results of three sampling measurements). The characteristic peaks G and D belonging to the carbon nanotubes are equally significant, and there are RBM peaks at some positions. The intensity ratio of the G peak to the D peak is 7 - 13.

[0094] Figure 5 This is the Raman spectrum of the crude multi-walled carbon nanotubes V prepared in Example 5 (results of three sampling measurements). The characteristic peaks G and D belonging to the carbon nanotubes are equally significant, and there are RBM peaks at some positions. The intensity ratio of the G peak to the D peak is 8 - 13.

[0095] Figure 6 This is the Raman spectrum of the crude multi-walled carbon nanotubes prepared in Comparative Example 1 (results of three sampling measurements). The characteristic peaks G and D belonging to the carbon nanotubes are equally significant, and there are RBM peaks at some positions. The intensity ratio of the G peak to the D peak is approximately 3 - 5. Compared with the crude multi-walled carbon nanotubes I - V, when carbon dioxide is not added, the quality of the prepared multi-walled carbon nanotubes decreases significantly.

[0096] Experimental Example 2

[0097] The product powders obtained in Example 1, Example 2, Example 3, Example 4, Example 5, and Comparative Example 1 were characterized by field emission scanning electron microscopy (SEM, Hitachi, model S4800). The acceleration voltage was 10 kV, the current was 10 μA, and signals were collected using a combination of upper and lower lenses.

[0098] Figure 7 SEM photograph of the crude multi-walled carbon nanotube I obtained in Example 1. It can be seen from the figure that the surface of the product presents a network structure, with small tube diameters, straight tube walls, and good quality.

[0099] Figure 8 SEM photograph of the crude multi-walled carbon nanotube II obtained in Example 2. It can be seen from the figure that the multi-walled carbon nanotubes present a network structure on the surface of the product, with the same small tube diameters, straight tube walls, and good quality.

[0100] Figure 9 SEM photograph of the crude multi-walled carbon nanotube III obtained in Example 3. It can be seen from the figure that the multi-walled carbon nanotubes present a network structure on the surface of the product, with small tube diameters, straight tube walls, and relatively good quality.

[0101] Figure 10 SEM photograph of the crude multi-walled carbon nanotube IV obtained in Example 4. It can be seen from the figure that the multi-walled carbon nanotubes present a network structure on the surface of the product, with the same small tube diameters, straight tube walls, and good quality.

[0102] Figure 11 SEM photograph of the crude multi-walled carbon nanotube V obtained in Example 5. It can be seen from the figure that the multi-walled carbon nanotubes present a network structure on the surface of the product, with the same small tube diameters, straight tube walls, and good quality.

[0103] Figure 12 SEM photograph of the crude multi-walled carbon nanotube obtained in Comparative Example 1. It can be seen from the figure that most of the carbon nanotubes have large tube diameters, even reaching dozens of nanometers, and are no longer multi-walled carbon nanotubes. Moreover, their tube walls are uneven, and the quality is significantly lower than that of the multi-walled carbon nanotubes obtained in each example.

[0104] Experimental Example 3

[0105] The product powders obtained in Example 1, Example 2, Example 3, Example 4, and Example 5 were ultrasonically dispersed in ethanol and dropped onto a microgrid for characterization by transmission electron microscopy (TEM, FEI Tecnai, model F20). The acceleration voltage was 200 kV, and the beam current was approximately 135 μA.

[0106] Figure 13TEM photograph of the crude multi-walled carbon nanotube product Ⅰ prepared in Example 1. It can be confirmed from the TEM photograph that the carbon nanotubes are multi-walled carbon nanotubes.

[0107] Figure 14 TEM photograph of the crude multi-walled carbon nanotube product Ⅱ prepared in Example 2. It can be confirmed from the TEM photograph that the carbon nanotubes are also multi-walled carbon nanotubes.

[0108] Figure 15 TEM photograph of the crude multi-walled carbon nanotube product Ⅲ prepared in Example 3. It can be confirmed from the TEM photograph that the carbon nanotubes are also multi-walled carbon nanotubes.

[0109] Figure 16 TEM photograph of the crude multi-walled carbon nanotube product Ⅳ prepared in Example 4. It can be confirmed from the TEM photograph that the carbon nanotubes are also multi-walled carbon nanotubes.

[0110] Figure 17 TEM photograph of the crude multi-walled carbon nanotube product Ⅴ prepared in Example 5. It can be confirmed from the TEM photograph that the carbon nanotubes are also multi-walled carbon nanotubes.

[0111] Figure 18 Perform a wall number statistics on the multi-walled carbon nanotubes appearing in the TEM photograph of the product powder obtained in Example 1.

[0112] Figure 19 Perform a wall number statistics on the multi-walled carbon nanotubes appearing in the TEM photograph of the product powder obtained in Example 2.

[0113] Figure 20 Perform a wall number statistics on the multi-walled carbon nanotubes appearing in the TEM photograph of the product powder obtained in Example 3.

[0114] Figure 21 Perform a wall number statistics on the multi-walled carbon nanotubes appearing in the TEM photograph of the product powder obtained in Example 4.

[0115] Figure 22 Perform a wall number statistics on the multi-walled carbon nanotubes appearing in the TEM photograph of the product powder obtained in Example 5.

[0116] The present invention has been described in detail above in conjunction with specific embodiments and / or exemplary examples and the accompanying drawings. However, these descriptions should not be construed 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 can be made to the technical solutions of the present invention and their implementation manners, and these all fall within the scope of the present invention. The protection scope of the present invention is subject to the appended claims.

Claims

1. A method for preparing few-walled carbon nanotubes, characterized in that, the method uses a mixed gas of methane and carbon dioxide to grow the product few-walled carbon nanotubes in the presence of a catalyst, in the product few-walled carbon nanotubes, more than 80% of the carbon nanotubes have 2-9 walls, and the average tube diameter is less than 10 nm.

2. The method according to claim 1, characterized in that, the catalyst includes a metal catalyst, wherein the metal element is selected from one or more of vanadium (V), chromium (Cr), manganese (Mn), iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo), ruthenium (Ru), rhodium (Rh), palladium (Pd), tungsten (W), rhenium (Re) and platinum (Pt), preferably one or more of iron (Fe), cobalt (Co), nickel (Ni), copper (Cu), molybdenum (Mo) and tungsten (W), more preferably one or more of iron (Fe), cobalt (Co) and molybdenum (Mo).

3. The method according to claim 1, characterized in that, the catalyst further includes a powder carrier, selected from one or more of magnesium oxide, aluminum oxide, silicon dioxide, scandium oxide, yttrium oxide, lanthanum oxide, titanium dioxide, zirconium dioxide, cerium dioxide, aluminum nitride, silicon nitride and gallium nitride.

4. The method according to claim 1, characterized in that, the catalyst is Fe and Mo or Co and Mo.

5. The method according to claim 4, characterized in that, in the catalyst, the molar ratio of Fe and Mo is 1:(0.05-3.6), preferably 1:(0.05-3.0), more preferably 1:(0.1-2.4).

6. The method according to claim 4, characterized in that, when using a powder carrier, in the catalyst, the molar ratio of Co and Mo is 1:(0.05-3.6), preferably 1:(0.05-3.0), more preferably 1:(0.1-2.4).

7. The method according to claim 3, characterized in that, the molar mass ratio of the metal catalyst to the powder carrier is (0.5-4.5) mol:1400 g, preferably (0.8-3.5) mol:1400 g, more preferably (1.1-2.5) mol:1400 g.

8. The method according to claim 1, characterized in that, the volume ratio of methane to carbon dioxide is (1-1000):10, preferably (1-50):1, more preferably (2-20):

1.

9. The method according to claim 1, characterized in that, the growth temperature is 500-1200 °C, preferably 700-1100 °C, more preferably 800-1000 °C.

10. A product few-walled carbon nanotube prepared by the method according to claim 1, characterized in that, In the few-walled carbon nanotubes of the product, the wall number of more than 80% of the carbon nanotubes is 2-9, preferably the wall number of more than 90% of the carbon nanotubes is 2-9; the wall number of more than 50% of the carbon nanotubes is 2-6, preferably the wall number of more than 90% of the carbon nanotubes is 2-6; the wall number of more than 30% of the carbon nanotubes is 2-4, preferably the wall number of more than 80% of the carbon nanotubes is 2-4, The average tube diameter of the few-walled carbon nanotubes of the product is less than 10 nm, preferably 2-8 nm, In the Raman spectrum of the few-walled carbon nanotubes of the product, the relative intensity ratio of the G peak to the D peak is greater than 4, preferably greater than 7, more preferably greater than 9.

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