Low-defect single-walled carbon nanotubes and their preparation method by low-pressure chemical vapor deposition

Abstract

This invention discloses a low-pressure chemical vapor deposition method for preparing low-defect single-walled carbon nanotubes, belonging to the field of nanomaterial preparation technology. This method addresses the problems of existing chemical vapor deposition methods for preparing single-walled carbon nanotubes, such as numerous structural defects, abundant amorphous carbon byproducts, and damage to the nanotube walls caused by strong acid purification. Through synergistic improvements in catalyst design and growth process, the method achieves controllable preparation of high-quality single-walled carbon nanotubes. This invention significantly reduces the defect level of single-walled carbon nanotubes without requiring deep purification with strong acids. The Raman spectrum of the prepared product is [I...]. G / I D The ratio is not less than 30, and the diameter distribution is concentrated in the range of 2-5 nm. The method of this invention is process-controllable and has good repeatability, making it suitable for the preparation of low-defect single-walled carbon nanotubes. It has broad application prospects in the fields of conductive composite materials, field-effect transistors, and high-performance sensors.

Description

Technical Field

[0001] This invention relates to the field of carbon nanomaterial preparation technology, specifically to a chemical vapor deposition method for preparing low-defect single-walled carbon nanotubes through the synergistic regulation of solid-phase catalyst coating, low-pressure reaction atmosphere, and trace oxygen. Background Art Single-walled carbon nanotubes (SUVs) have broad application prospects in electronic devices, composite materials, and energy storage devices due to their unique one-dimensional nanostructure and excellent mechanical, electrical, and thermal properties. Chemical vapor deposition (CVD) is currently the mainstream technology for the large-scale preparation of SUVs, offering advantages such as simple equipment, controllable cost, and high product yield.

[0002] However, there are still significant technical bottlenecks in the existing CVD method for preparing single-walled carbon nanotubes: First, the crystallinity of the product is insufficient, and the arrangement of the six-membered carbon rings is locally distorted, making it difficult to fully utilize the intrinsic properties of the material; second, defects such as vacancies and doped atoms are easily formed in the carbon nanotube walls, affecting electron transport efficiency and structural stability; third, amorphous carbon and multi-walled carbon nanotubes are easily generated during the reaction, which not only reduces the purity of the product but also makes subsequent separation and purification extremely difficult; fourth, some processes rely on strong acid purification to remove metal residues and amorphous carbon, but strong acid treatment may introduce tube wall defects, reduce the structural integrity of single-walled carbon nanotubes, and increase post-processing costs and environmental pressure.

[0003] The core reasons for the above problems include: uneven dispersion of catalyst active sites, which are prone to migration and aggregation at high temperatures, leading to an imbalance in the carbon source decomposition rate; a wide reaction temperature range, resulting in a mismatch between carbon atom deposition and migration rates; and carbon source supersaturation in the reaction atmosphere, leading to a mismatch between carbon atom supply and ordered deposition rates, easily resulting in disordered deposition and the formation of amorphous carbon and other impurities. Therefore, there is an urgent need to develop an improved CVD preparation method that can precisely control the reaction process and optimize the carbon nanotube growth environment to achieve the efficient preparation of single-walled carbon nanotubes with high crystallinity, low defects, and low amorphous carbon content. Summary of the Invention

[0004] The purpose of this invention is to overcome the shortcomings of existing CVD methods for preparing single-walled carbon nanotubes and to provide an improved method for preparing single-walled carbon nanotubes with low defects and low amorphous carbon content. This invention achieves ordered deposition and growth of carbon atoms by optimizing the catalyst system, employing a coating loading method, establishing a low-pressure reaction environment for precise control of reaction parameters, and improving the reaction atmosphere. This allows for the acquisition of high-quality single-walled carbon nanotubes without relying on intensive post-treatment purification steps (such as strong acid treatment).

[0005] To achieve the above-mentioned objectives, the present invention adopts the following technical solution: A low-pressure chemical vapor deposition method for preparing low-defect single-walled carbon nanotubes includes the following steps: S1, loading a composite oxide catalyst containing Fe, Co, Al and Mg as a coating onto the surface of a heat-resistant substrate and placing it in the catalytic growth zone of a low-pressure chemical vapor deposition reactor with a thermal field partition; wherein the molar ratio of the metal elements Fe, Co, Al and Mg is 1:(0.3~0.5):(5~8):(0.8~1.2); the thermal field partition includes a preheating zone, a catalytic growth zone and a cooling zone along the direction of the reaction gas flow; S2. The catalyst is reduced and activated in a mixed atmosphere of inert gas and reducing gas; S3. Under a total pressure of 10~20 kPa, a reaction gas containing methane, ethylene, inert carrier gas, and oxygen is introduced into the reactor, causing methane and ethylene to decompose and grow single-walled carbon nanotubes under the action of the catalyst; wherein, the temperature of the preheating zone is 500~600℃, the temperature of the catalytic growth zone is 950~1100℃, and the temperature of the cooling zone is 300~400℃; the volume ratio of methane to ethylene is 2.5:1~4:1, the volume ratio of the mixed carbon source of methane and ethylene to the inert carrier gas is 1:10~1:15, and the volume ratio of oxygen to the mixed carbon source is 0.001~0.003; S4. Stop the introduction of methane, ethylene and oxygen, and after cooling under a protective atmosphere, treat the obtained carbon nanotube product with inert gas plasma to obtain low-defect single-walled carbon nanotubes.

[0006] This invention employs a composite oxide catalyst containing Fe, Co, Al, and Mg. Fe and Co are the main catalytically active metal components. The inventors discovered that within the specific molar ratio range mentioned above, the addition of Co can effectively regulate the electronic structure and carbon solubility of the Fe-based catalyst. The synergistic effect of the two metals promotes the selective decomposition of the carbon source, improves the nucleation efficiency of single-walled carbon nanotubes, and reduces the formation of amorphous carbon. The Al and Mg oxide components play a role in dispersing and stabilizing active sites in the catalyst. In particular, the introduction of Mg oxide can act as a physical spacer, synergistically inhibiting the migration and aggregation of Fe-Co active particles under high-temperature CVD conditions, thereby maintaining high catalytic activity and selectivity, ensuring the consistency of single-walled carbon nanotube nucleation and growth, and resulting in a more uniform product nanotube diameter distribution.

[0007] The composite oxide catalyst containing Fe, Co, Al, and Mg can be prepared by a liquid-phase complexation-thermal treatment method. This method includes, but is not limited to, sol-gel methods, complexation combustion methods, homogeneous precipitation-thermal treatment methods, and impregnation-complexation-thermal treatment methods. A common feature of these methods is that the precursors of Fe, Co, Al, and Mg are uniformly mixed at the molecular or near-molecular scale in the liquid phase, followed by drying and calcination to form a composite oxide structure, thereby improving the dispersion uniformity of the active components. In a preferred embodiment, the precursors are ferric nitrate, cobalt nitrate, aluminum nitrate, and magnesium nitrate, and the complexing agent is citric acid. However, the invention is not limited to this; any precursor and preparation method that can form a composite catalyst containing Fe, Co, Al, and Mg after thermal treatment and form Fe-Co active sites through reduction under CVD conditions can be used in this invention.

[0008] In the technical solution of this invention, the catalyst is loaded onto the surface of a heat-resistant substrate in the form of a coating. This arrangement facilitates sufficient contact between the catalyst and the reactant gas and improves the thermal stability of the catalyst. The heat-resistant substrate can be a heat-resistant oxide material such as quartz, alumina, or silicon carbide, or a metal or ceramic material capable of withstanding CVD growth temperatures. In a more preferred embodiment, the heat-resistant substrate is a quartz substrate, particularly an annealed quartz substrate. Annealing helps remove adsorbed impurities from the substrate surface and improves the adhesion and dispersion of the catalyst coating on the substrate surface.

[0009] It should be noted that the active sites for catalytic methane and ethylene cracking and single-walled carbon nanotube nucleation and growth in this invention mainly originate from the Fe-Co active components formed after reduction and activation. The heat-resistant substrate mainly serves as a support and dispersion medium for the catalyst coating and is not itself the main source of catalytic activity for carbon source cracking.

[0010] The core of this invention lies in the synergistic effect of three process conditions: low pressure, thermal field zoning, and a specific ratio of composite carbon source / micro-oxygen. These three conditions are interrelated and together achieve highly efficient control over the carbon atom deposition process.

[0011] Low-pressure environment (10-20 kPa): By reducing the total reaction pressure to 10-20 kPa, the partial pressure of the carbon source gas can be reduced, kinetically slowing down its thermal decomposition rate and the supersaturation of carbon atoms on the catalyst surface. This allows carbon atoms sufficient time to diffuse, migrate, and arrange themselves into thermodynamically stable, defect-free sp atoms on the catalyst surface. 2 The six-membered ring network effectively suppresses the formation of amorphous carbon due to excessively rapid carbon atom accumulation. The low-pressure environment in this invention can be achieved in a tubular furnace or other types of fixed-bed reactors using conventional equipment such as vacuum pumps and back pressure valves.

[0012] Thermal zone division: The preheating zone (500-600℃), growth zone (950-1100℃), and cooling zone (300-400℃) set along the reaction gas flow direction are another crucial condition for achieving high-quality single-walled carbon nanotube growth. The preheating zone allows highly reactive carbon sources such as ethylene to begin partial activation, but prevents their violent decomposition before entering the main reaction zone. The growth zone provides optimal catalytic cracking and ordered growth temperatures. The cooling zone rapidly cools the product after it leaves the growth zone, shortening its residence time in the high-temperature carbon source atmosphere, thereby reducing secondary deposition and disordered carbon adhesion. This zoned temperature control method can be achieved using a tube furnace with multiple independently controlled temperature sections, or through other reaction devices capable of establishing a stable temperature gradient.

[0013] Synergistic effect of composite carbon source and micro-oxygen: One of the key features of this invention is the use of a mixed carbon source with a methane to ethylene volume ratio between 2.5:1 and 4:1. Ethylene has high reactivity and can efficiently provide carbon atoms for rapid nucleation, solving the problem of low nucleation efficiency when using methane alone; while the slow decomposition of methane provides a continuous and stable carbon source supply for subsequent ordered growth, avoiding the risk of disordered carbon deposition due to excessively high local carbon concentration when using ethylene alone. The synergistic effect of the two carbon sources achieves a kinetic balance between nucleation and growth.

[0014] In this system, the introduction of extremely small amounts of oxygen (O2 / mixed carbon source volume ratio = 0.001-0.003) is another key point. Within this extremely low concentration window, the role of oxygen is not large-scale oxidation and etching, but rather as an "in-situ cleaner," preferentially reacting with the more reactive and weakly bound amorphous carbon, thus helping to maintain the cleanliness of the catalyst's active sites and its catalytic activity. Simultaneously, because the oxygen content is strictly controlled, the risk of over-oxidative etching of the already stable tubular single-walled carbon nanotube walls can be reduced.

[0015] The method of oxygen introduction is crucial. Since it is difficult to precisely and stably control the flow of trace amounts of pure oxygen, in the preferred industrial implementation, an oxygen-containing inert premixed gas (such as argon with an oxygen concentration of 100-5000 ppm) is introduced. By adjusting the flow rate of the premixed gas, the volume ratio of oxygen to the mixed carbon source of methane and ethylene is precisely maintained within the range of 0.001-0.003 to ensure the stability of the oxygen-carbon ratio and the growth environment throughout the growth process.

[0016] Post-processing and collection After growth is complete, the carbon source and oxygen supply are stopped, and the reactor is cooled to room temperature under a protective atmosphere. Subsequently, the obtained carbon nanotube product is subjected to inert gas plasma treatment, preferably argon plasma.

[0017] Unlike conventional purification methods that rely on strong oxidizing chemical reagents (such as nitric acid and sulfuric acid), this low-power (50-80W) and short-time (5-10min) physical treatment method can selectively remove disordered carbon with weak bonding to the carbon nanotube surface. More importantly, it avoids the irreversible damage to the single-walled carbon nanotube wall structure and the introduction of new defects (such as oxygen-containing functional groups) caused by strong acid treatment. Therefore, in this invention, this plasma treatment step is an important post-processing method for obtaining "low-defect" single-walled carbon nanotubes and distinguishing them from products purified by strong acid.

[0018] Preferably, the composite oxide catalyst containing Fe, Co, Al and Mg is prepared by the following steps: mixing iron salt, cobalt salt, aluminum salt and magnesium salt with citric acid in deionized water to form a uniform precursor, drying and then calcining at 500-600℃ for 2-3 hours.

[0019] Preferably, the heat-resistant substrate is an oxide heat-resistant substrate. More preferably, the heat-resistant substrate is a quartz substrate, an alumina substrate, or a silicon carbide substrate. Most preferably, the heat-resistant substrate is an annealed quartz substrate.

[0020] Preferably, the inert gas is argon, and the reducing gas is hydrogen; the volume fraction of hydrogen in the total amount of inert gas and reducing gas during the reduction and activation process is 5-8%.

[0021] Preferably, in step S4, the inert gas plasma is argon plasma, with a processing power of 50-80W and a processing time of 5-10min.

[0022] Furthermore, this invention also proposes a low-defect single-walled carbon nanotube prepared using the method described above. The resulting low-defect single-walled carbon nanotube is not subjected to strong acid purification treatment with nitric acid, sulfuric acid, hydrochloric acid, or a mixture thereof. The ratio of the G peak intensity to the D peak intensity in its Raman spectrum is I. G / I D The amorphous carbon content is not less than 30, and the amorphous carbon content measured by thermogravimetric analysis is not higher than 3%. According to high-resolution transmission electron microscopy, the number of single-walled carbon nanotubes accounts for not less than 90%, and the diameter is mainly distributed in the range of 2~5 nm.

[0023] In addition, the present invention also proposes the application of the above-mentioned low-defect single-walled carbon nanotubes in conductive composite materials, transparent conductive films, field-effect transistors, sensors, electromagnetic shielding materials or energy storage devices.

[0024] Compared with the prior art, the present invention has at least the following beneficial effects: 1. The catalyst exhibits a stable structure and uniformly dispersed active sites. A composite oxide catalyst containing Fe, Co, Al, and Mg is employed and loaded onto a heat-resistant substrate as a coating. After reduction activation, the Fe-Co bimetallic catalyst provides active sites for catalytic carbon source pyrolysis and carbon nanotube nucleation and growth; the Al and Mg oxide components effectively inhibit the migration and aggregation of active particles at high temperatures through physical separation. This ensures high catalyst activity and long lifespan, which is beneficial for obtaining single-walled carbon nanotubes with uniform diameter distribution.

[0025] 2. Low pressure, mixed carbon source, and trace oxygen synergistically suppress amorphous carbon. The 10-20 kPa low-pressure environment reduces the supersaturation of the carbon source, providing kinetic conditions for the ordered deposition of carbon atoms; the specific ratio of methane and ethylene balances the nucleation rate and growth stability; the presence of trace oxygen (O2 / mixed carbon source = 0.001-0.003) preferentially etches amorphous carbon, keeping the catalyst active sites clean. The synergistic effect of these three factors significantly suppresses the formation of amorphous carbon at its source, thus allowing the product to achieve a very low defect level (I0.003) without requiring deep purification with strong acids. G / I D ≥30) and amorphous carbon content (≤3%).

[0026] 3. Inert gas plasma post-treatment replaces strong acid purification. A gentle inert gas plasma treatment removes residual disordered carbon, avoiding the damage to the carbon nanotube wall structure and the introduction of new defects caused by strong acid treatment. Combined with native growth control, this ensures that the resulting product is a high-quality single-walled carbon nanotube with a low-defect native surface.

[0027] 4. Significant synergistic effects in the process, resulting in excellent overall product performance. The technical effect of this invention is not due to a single feature, but rather stems from the synergistic effect of a whole set of process parameters, including catalyst coating structure, low-pressure environment, mixed carbon source ratio, trace oxygen concentration, thermal field zoning, and mild plasma post-treatment. Any deviation of any parameter from the aforementioned synergistic window will lead to product degradation in I... G / I D Significant degradation occurs in aspects such as amorphous carbon content, single-wall selectivity, and tube diameter distribution. It is this synergistic effect that enables the present invention to stably obtain low-defect single-walled carbon nanotube products without the need for strong acid purification, simplifying the process, reducing costs, and preserving the original structure of carbon nanotubes to the greatest extent. Attached Figure Description

[0028] The above features and advantages of the present invention will become clearer and more readily understood from the following description of exemplary embodiments thereof in conjunction with the accompanying drawings.

[0029] Figure 1This is a TEM image of the highly crystalline, low-defect single-walled carbon nanotubes prepared in Example 1.

[0030] Figure 2 The image shows the Raman spectrum of the highly crystalline, low-defect single-walled carbon nanotubes prepared in Example 1. Detailed Implementation

[0031] The present invention will be further described below with reference to the accompanying drawings. It should be understood that the following embodiments are for illustrative purposes only and are not intended to limit the scope of protection of the present invention. For those skilled in the art, any equivalent substitutions or modifications made without departing from the concept of the present invention should fall within the scope of protection of the present invention.

[0032] Testing and Characterization Methods 1. Raman spectroscopy test The carbon nanotube samples were analyzed using Raman spectroscopy. The laser wavelength was 532 nm, and the D peak was located at approximately 1350 cm⁻¹. -1 Nearby, G peak is located at approximately 1580 cm. -1 Nearby. In this application, IG / ID is the ratio of the intensity of peak G to the intensity of peak D. It can also be calculated using the fitted peak area ratio, and this should be consistent throughout the application.

[0033] 2. Thermogravimetric analysis (TGA) test The amorphous carbon content in carbon nanotube products was determined using a thermogravimetric analyzer (TGA). The test atmosphere was air, the heating rate was 10 °C / min, and the test temperature range was room temperature to 900 °C. The amorphous carbon content was calculated as the proportion of mass loss corresponding to the low-temperature oxidation weight loss range (typically 300–450 °C) in the TGA / DTG curve to the total sample mass.

[0034] 3. Transmission electron microscopy (TEM) testing The morphology, wall structure, and diameter distribution of carbon nanotube products were observed using high-resolution transmission electron microscopy (HRTEM). At least 200 carbon nanotubes were randomly selected from at least five different fields of view for statistical analysis. The percentage of single-walled carbon nanotubes was calculated using the following formula: Percentage of single-walled carbon nanotubes = (Number of single-walled carbon nanotubes / Total number of carbon nanotubes counted) × 100%. The diameter distribution was obtained by measuring and statistically analyzing the diameter of the carbon nanotubes.

[0035] 4. Definition of Yield In this application, the yield is calculated as the ratio of the mass of the carbon product obtained after growth to the mass of the catalyst supported on the quartz substrate surface, in wt%. The mass of the carbon product is the mass of the sample after cooling under a protective atmosphere and treatment with inert gas plasma, minus the mass of the quartz substrate and residual catalyst.

[0036] 5. Definition of Deep Purification of Strong Acids In this application, strong acid deep purification refers to a process in which carbon nanotube products are soaked, refluxed, or heated for oxidation using nitric acid, sulfuric acid, hydrochloric acid, or mixtures thereof. This process aims to remove residual metal catalysts and amorphous carbon. The low-defect single-walled carbon nanotubes obtained in this invention do not undergo the aforementioned strong acid deep purification treatment.

[0037] Example 1

[0038] 1. Catalyst preparation: Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Al(NO3)3·9H2O and Mg(NO3)2·6H2O were dissolved in deionized water in a molar ratio of 1:0.3:5:0.8. Citric acid was added (the molar ratio of total metal ions to citric acid was 1:1.5). The mixture was stirred at 70°C to form a sol. After drying, the sol was calcined at 550°C for 2.5 h and then ground to obtain catalyst powder with a particle size of about 30 nm.

[0039] 2. Substrate pretreatment: Quartz sheets were ultrasonically cleaned with acetone, ethanol, and deionized water for 18 min each, dried, and then annealed at 850℃ for 1 h under an argon atmosphere. Catalyst powder was dispersed in ethanol and spin-coated onto the surface of the quartz sheets at a loading of 0.7 mg / cm³. 2 Dry and store for later use.

[0040] 3. Atmosphere control: Argon-hydrogen mixed gas (hydrogen volume 6%) is introduced into the horizontal tube furnace for purging for 30 minutes. The temperature is raised to 550℃ in the preheating zone, 1000℃ in the growth zone, and 350℃ in the cooling zone. The temperature is held for 30 minutes to reduce the catalyst.

[0041] 4. Growth process: Introduce a methane-ethylene mixed carbon source (volume ratio 3:1) and argon gas (carbon source to carrier gas volume ratio 1:12), add oxygen to make the O2 / mixed carbon source volume ratio 0.002; control the total gas pressure at 15 kPa, the carbon source flow rate at 8 sccm, the carrier gas flow rate at 100 sccm, and the growth time at 20 min.

[0042] 5. Post-processing: After cooling, the quartz sheet was removed and cleaned with argon plasma (60W power, 8min) to obtain single-walled carbon nanotubes. Characterization was performed, and Raman spectroscopy was used to analyze the product. G / I D The value is 32; the defect density is 3.2 × 10⁻⁶. -3 The number of nanotubes per nm was 2.1%; according to high-resolution transmission electron microscopy, the diameter of single-walled carbon nanotubes was distributed between 3-5 nm, with single-walled carbon nanotubes accounting for 92%, double-walled carbon nanotubes accounting for 3.6%, and multi-walled carbon nanotubes accounting for 4.4%.

[0043] In this embodiment, after the substrate was annealed at 850℃ in a reducing atmosphere, the microstructure and adhesion properties of the catalyst were significantly optimized: the average particle size of the catalyst in the unannealed sample was 4.82±1.16nm, with large-sized agglomerated particles accounting for 28.7 vol%, uneven particle dispersion, severe adhesion and sintering, and free attached catalyst accounting for 11.36 wt%, with a large number of particles coated with amorphous carbon; after annealing, the average particle size of the catalyst decreased to 3.95±0.48nm, with large-sized agglomerated particles accounting for only 3.2 vol%, and the particles were monodisperse and uniformly distributed, without agglomeration or sintering, and the free catalyst accounting for only 0.87 wt%, with amorphous carbon coating completely eliminated. The catalyst and carbon nanotube interface were firmly bonded, and the active sites were fully exposed, greatly improving the preparation quality and structural uniformity of single-walled carbon nanotubes.

[0044] The 10-30 min growth time described in this embodiment is the single-batch reaction time under batch experimental conditions. It should be understood that in industrial production, the growth time can be adjusted according to the reactor size and process conditions. Technically, the growth time corresponds to the effective contact time or residence time between the carbon source gas and the catalyst coating within the catalytic growth zone. This residence time can be controlled by adjusting parameters such as the reaction zone length and gas flow rate. When the catalyst coating is placed in a tubular furnace or other type of fixed-bed reactor, the growth time is the effective reaction time of the carbon source gas flowing across the substrate surface loaded with the catalyst coating.

[0045] Similarly, this invention employs inert gas plasma treatment to remove residual amorphous carbon. This treatment step can be performed offline after the product is removed from the reactor and cooled (as described in the examples), or a separate plasma treatment zone can be set up at the rear end of the CVD reactor to perform continuous or semi-continuous online treatment on the product after it leaves the catalytic growth zone and cools. Online treatment is beneficial for improving production efficiency and process consistency.

[0046] Example 2

[0047] 1. Catalyst preparation: Fe(NO3)3·9H2O, Co(NO3)2·6H2O, Al(NO3)3·9H2O and Mg(NO3)2·6H2O were prepared in a molar ratio of 1:0.5:8:1.2. The remaining steps were the same as in Example 1, and catalyst powder with a size of about 30 nm was obtained.

[0048] 2. Substrate pretreatment: loading rate 1.0 mg / cm³ 2 The remaining steps are the same as in Example 1.

[0049] 3. Atmosphere control: Hydrogen gas integral 8%, growth zone temperature 1050℃, the remaining steps are the same as in Example 1.

[0050] 4. Growth process: The volume ratio of carbon source to carrier gas is 1:15, the volume ratio of O2 to mixed carbon source is 0.003, the total gas pressure is 20 kPa, the growth time is 30 min, and the remaining steps are the same as in Example 1.

[0051] 5. Post-processing: Plasma cleaning power 80W, time 10min, the remaining steps are the same as in Example 1.

[0052] Characterization showed that the crystallinity of the product (I) G / I D The value is 38, and the defect density is 2.8 × 10⁻⁶. -3 The amorphous carbon content is 1.8%. According to high-resolution transmission electron microscopy, the diameter of single-walled carbon nanotubes is distributed between 2-4 nm, with single-walled carbon nanotubes accounting for more than 95%, double-walled carbon nanotubes accounting for 1.2%, and multi-walled carbon nanotubes accounting for 3.8%.

[0053] Comparative Example 1: Growth under normal pressure This comparative example illustrates the effect of low-pressure reaction conditions on reducing disordered carbon deposition and improving the quality of single-walled carbon nanotubes. This comparative example is essentially the same as Example 1, except that the total reaction pressure was adjusted from 15 kPa to 101 kPa, while all other conditions remained unchanged.

[0054] The test results are shown in Table 1:

[0055] At normal pressure, as the partial pressure of the carbon source increases, the degree of carbon supersaturation on the catalyst surface increases, making it easier to form amorphous carbon or multi-walled carbon nanotubes, which manifests as I G / I D The content of amorphous carbon is reduced, while the content of amorphous carbon is increased and the pipe diameter distribution becomes wider.

[0056] Comparative Example 2: No oxygen was introduced. This comparative example illustrates the role of trace amounts of oxygen in inhibiting amorphous carbon deposition.

[0057] This comparative example is basically the same as Example 1, except that oxygen is not introduced into the reaction gas, that is, the volume ratio of O2 / mixed carbon source is 0, and the other conditions remain unchanged.

[0058] The test results are as follows:

[0059] When oxygen is not introduced, the disordered carbon generated during the reaction is difficult to be suppressed or removed in time, and the catalyst surface is more easily covered by amorphous carbon, resulting in a decrease in IG / ID and an increase in amorphous carbon content.

[0060] Comparative Example 3: Excess Oxygen This comparative example illustrates the adverse effects of excessive oxygen content on the growth of single-walled carbon nanotubes and demonstrates the existence of a suitable window for trace amounts of oxygen.

[0061] This comparative example is basically the same as Example 1, except that the volume ratio of O2 / mixed carbon source is increased from 0.002 to 0.01, while the other conditions remain unchanged.

[0062] The test results are as follows:

[0063] When the oxygen content is too high, oxidation no longer primarily manifests as selective inhibition of amorphous carbon, but may instead etch the forming single-walled carbon nanotubes or inhibit catalyst activity, resulting in decreased yield, increased defects, or reduced IG / ID.

[0064] As can be clearly seen from the comparison of Example 1 and the comparative examples, the technical effect of the present invention is not generated by a single technical feature, but rather stems from the synergistic effect of a whole set of factors, including the Fe-Co / Al2O3-MgO composite catalyst, the 10-20 kPa low-pressure environment, the methane / ethylene composite carbon source ratio of 2.5:1-4:1, the trace O2 / carbon source ratio of 0.001-0.003, the thermal field partitioning, and the mild inert plasma post-treatment. Deviations of any parameter from this combined window will lead to a significant deterioration in one or more properties of the product, such as crystallinity (IG / ID), amorphous carbon content, and tube diameter distribution. It is this synergistic effect that enables the present invention to stably obtain high-quality single-walled carbon nanotube products without relying on high-intensity post-treatment purification steps (such as strong acid treatment), thereby simplifying the process, reducing costs, and maintaining the original structure of the carbon nanotubes.

Claims

1. A method for preparing low-defect single-walled carbon nanotubes by low-pressure chemical vapor deposition, characterized in that, Includes the following steps: S1. A composite oxide catalyst containing Fe, Co, Al, and Mg is loaded onto the surface of a heat-resistant substrate in the form of a coating and placed in the catalytic growth zone of a low-pressure chemical vapor deposition reactor with a thermal field partition; wherein the molar ratio of the metal elements Fe, Co, Al, and Mg is 1:(0.3~0.5):(5~8):(0.8~1.2); the thermal field partition includes a preheating zone, a growth zone, and a cooling zone along the direction of the reaction gas flow. S2. The catalyst is reduced and activated in a mixed atmosphere of inert gas and reducing gas; S3. Under a total pressure of 10~20 kPa, a reaction gas containing methane, ethylene, inert carrier gas, and oxygen is introduced into the reactor, causing methane and ethylene to decompose and grow single-walled carbon nanotubes under the action of the catalyst; wherein, the temperature of the preheating zone is 500~600℃, the temperature of the catalytic growth zone is 950~1100℃, and the temperature of the cooling zone is 300~400℃; the volume ratio of methane to ethylene is 2.5:1~4:1, the volume ratio of the mixed carbon source of methane and ethylene to the inert carrier gas is 1:10~1:15, and the volume ratio of oxygen to the mixed carbon source is 0.001~0.003; S4. Stop the introduction of methane, ethylene and oxygen, and after cooling under a protective atmosphere, the resulting carbon nanotube product is subjected to inert gas plasma treatment to obtain low-defect single-walled carbon nanotubes.

2. The method according to claim 1, characterized in that, The composite oxide catalyst containing Fe, Co, Al and Mg is prepared by the following steps: soluble salts of Fe, Co, Al and Mg are mixed with an organic complexing agent in a solvent to form a uniform precursor, which is then dried and calcined at 500-600℃ for 2-3 hours.

3. The method according to claim 2, characterized in that, The soluble salt is a nitrate, the organic complexing agent is citric acid, and the solvent is deionized water.

4. The method according to claim 1, characterized in that, The heat-resistant substrate is an oxide heat-resistant substrate.

5. The method according to claim 4, characterized in that, The heat-resistant substrate is a quartz substrate.

6. The method according to claim 5, characterized in that, The quartz substrate is annealed at 800-900°C under an inert atmosphere before the catalyst is loaded.

7. The method according to claim 1, characterized in that, In step S3, the oxygen is introduced as an oxygen-containing inert premixed gas, and the volume ratio of oxygen to the mixed carbon source is maintained at 0.001~0.003 by adjusting the flow rate of the premixed gas.

8. The method according to claim 1, characterized in that, The inert gas is one or more of argon, nitrogen, and helium, and the reducing gas is hydrogen; the volume fraction of hydrogen in the total amount of inert gas and reducing gas during the reduction and activation process is 5-8%.

9. The method according to claim 1, characterized in that, In step S4, the inert gas plasma is argon plasma, with a processing power of 50-80W and a processing time of 5-10min.

10. A low-defect single-walled carbon nanotube, characterized in that, The single-walled carbon nanotubes prepared by the method of any one of claims 1 to 9 do not contain oxygen-containing functional group defects introduced by nitric acid, sulfuric acid, hydrochloric acid or mixed acid treatment, and have a Raman spectrum I G / I D The content of amorphous carbon is not higher than 3%, the number of single-walled carbon nanotubes accounts for not less than 90%, and the diameter distribution is 2-5 nm.

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