A continuous method for preparing ultrafine single-wall carbon nanotubes

Abstract

The application discloses a continuous preparation method of superfine single-wall carbon nanotubes, which comprises the following steps: heating and sublimating a metal catalyst precursor under the protection of inert gas, mixing gaseous steam obtained by sublimation with sulfur-containing gas, and then cooling and recrystallizing to obtain metal catalyst recrystallized particles modified by sulfur element; adding the obtained recrystallized particles into an alcohol solvent to realize ultrasonic dispersion, and thus a catalyst dispersion liquid is obtained; atomizing the catalyst dispersion liquid and then spraying into a single-wall carbon nanotube grower, filtering through a rectifier screen, and then slowly and uniformly decomposing through a quartz inner tube; sending the decomposition product into a nucleation zone with a temperature higher than that of the decomposition zone to realize nucleation growth, and finally, superfine single-wall carbon nanotubes with a tube diameter less than 2nm are obtained. Through the synergistic effect of sublimation recrystallization particle size control and in-situ modification of sulfur-containing gas, the application can inhibit the over ripening of catalyst particles in a high-temperature zone, and realize the continuous preparation of superfine single-wall carbon nanotubes with uniform tube diameter and less than 2nm.

Description

Technical Field

[0001] This invention relates to the field of single-walled carbon nanotube growth technology, and specifically to a continuous preparation method for ultrafine single-walled carbon nanotubes. Background Technology

[0002] Single-walled carbon nanotubes possess excellent mechanical, electrical, and thermal properties, as well as a large specific surface area, making them promising candidates for applications in transparent conductive films, flexible electronic devices, lithium-ion batteries, and high-performance fibers. Compared to conventional single-walled carbon nanotubes, ultrafine single-walled carbon nanotubes with smaller diameters typically exhibit higher specific surface areas, higher electronic conductivity, and stronger interfacial interactions, demonstrating their potential for constructing high-capacity energy storage electrodes and high-performance conductive networks.

[0003] The structural characteristics of single-walled carbon nanotubes, such as tube diameter, wall number, crystallinity, length, and chirality, are closely related to the size, composition, carbon dissolution state, and nucleation kinetics of the catalyst particles in the initial growth stage. Especially for the preparation of ultrafine single-walled carbon nanotubes, it is necessary to form active catalyst particles with small size, narrow particle size distribution, and resistance to sintering and agglomeration at high temperatures in the early stages of carbon nanotube growth. If the catalyst particle size is too large or the distribution is too wide, large-diameter single-walled, double-walled, or multi-walled carbon nanotubes are easily generated, which may also lead to an increase in amorphous carbon byproducts and a decrease in product uniformity. Existing methods for preparing carbon nanotubes mainly include arc discharge, laser evaporation, and chemical vapor deposition. Among them, floating catalyst chemical vapor deposition (FC-CVD) is particularly suitable for continuous preparation of carbon nanotubes. It can introduce catalyst precursors, carbon sources, and promoters into a high-temperature reactor to generate catalyst particles and grow carbon nanotubes in situ in the gas phase.

[0004] To obtain high-purity single-walled carbon nanotubes with small diameters, existing research and processes typically employ methods such as reducing catalyst precursor concentration or metal feed amount, adjusting reaction parameters like reaction temperature, carbon source concentration, and hydrogen concentration, and introducing sulfur, alkali metals, or other co-catalytic components into the main catalyst (e.g., iron, cobalt, or nickel) to regulate catalyst activity, carbon source cracking behavior, and carbon nanotube nucleation processes, thereby reducing the probability of catalyst particle collisions and agglomeration and preventing the formation of large-diameter carbon nanotubes. FC-CVD technology has disclosed a scheme to improve the purity and preparation efficiency of single-walled carbon nanotubes by adding alkali metal compounds in the presence of an iron-containing catalyst; however, these methods still have shortcomings. While simply reducing the catalyst precursor concentration can reduce the average size of catalyst particles to some extent, it often simultaneously reduces the effective number of nuclei per unit volume of the reaction system, leading to a decrease in carbon nanotube yield and making it difficult to simultaneously achieve ultrafine nanotubes and continuous, efficient preparation. Simply adjusting the furnace temperature, carbon source flow rate, or hydrogen flow rate can only alter the overall reaction environment, making it difficult to independently and precisely control each stage of catalyst precursor evaporation, decomposition, metal atom aggregation, droplet or particle formation, and carbon nanotube nucleation. For systems using organometallic compounds such as ferrocene as catalyst precursors, the metal species formed after high-temperature decomposition are prone to sintering and agglomeration due to excessively high local concentrations, sudden temperature changes, uneven residence time, or gas-phase collisions, resulting in larger catalyst particles.

[0005] Furthermore, in conventional single-temperature zones or processes where the precursor and carbon source enter the high-temperature reaction zone simultaneously, precursor decomposition, catalyst nucleation, and carbon source cracking may occur in the same space or within a similar timeframe. On the one hand, the unstable ultrafine catalyst particles are easily deactivated by being coated with excess carbon species; on the other hand, the formed metal particles may continue to aggregate and grow, thus widening the catalyst particle size distribution, which is detrimental to the selective formation of ultrafine single-walled carbon nanotubes. Existing FC-CVD technologies also indicate that in high-temperature reactors, metals generated from precursor decomposition may deposit, and carbon buildup is prone to occur on the reactor walls. These phenomena reduce the effective catalyst utilization rate and adversely affect continuous and stable operation.

[0006] Chinese patent CN110980691B discloses a method for the mass production of high-purity single-walled carbon nanotubes with controllable diameter. This method uses hydrogen as a carrier gas, transition metal as a catalyst, and sulfur as a growth promoter, achieving mass production of single-walled carbon nanotubes by adjusting the thermodynamic and kinetic conditions of the reaction system. Chinese patent CN109437157B discloses a floating catalyst chemical vapor deposition method for single-walled carbon nanotubes. This method introduces trace amounts of water vapor into the reaction system to improve catalyst efficiency, achieving efficient growth of high-purity, high-crystallinity single-walled carbon nanotubes. However, both of these methods affect the carbon nanotube diameter by controlling the thermodynamic and kinetic parameters in the reaction system. The catalyst particles have a wide particle size distribution before entering the furnace, and are prone to agglomeration and growth after entering the high-temperature zone, resulting in larger carbon nanotube diameters. This makes it difficult to stably prepare ultrafine single-walled carbon nanotubes with diameters below 2 nm, and may even lead to the formation of multi-walled structures when the catalyst particle size is too large.

[0007] Therefore, the technical problem that this invention aims to solve is how to effectively control the formation and nucleation stages of the catalyst during continuous preparation, so that the catalyst precursor can form catalyst nucleation units with smaller particle size, more uniform distribution and higher activity under predetermined conditions, and grow the nucleation units into ultrafine single-walled carbon nanotubes under suitable carbon source supply conditions. Summary of the Invention

[0008] To address the technical problems of existing floating catalyst chemical vapor deposition (FC-CVD) methods, such as catalyst particle agglomeration and growth in high-temperature regions, resulting in large-diameter single-walled carbon nanotubes (typically greater than 3 nm), making it difficult to stably prepare ultrafine single-walled carbon nanotubes with diameters below 2 nm, and even generating multi-walled structures when the catalyst particle size is too large, this invention proposes a preparation method. This method improves the selectivity of ultrafine single-walled carbon nanotubes while minimizing catalyst particle sintering and agglomeration, amorphous carbon generation, and reactor wall deposition, thus maintaining the stability and high efficiency of the continuous preparation process.

[0009] A continuous preparation method for ultrafine single-walled carbon nanotubes employs a floating catalyst chemical vapor deposition method, comprising the following steps: heating and sublimating a metal catalyst precursor under inert gas protection, thoroughly mixing the resulting gaseous vapor with a sulfur-containing gas, and then cooling and recrystallizing to obtain sulfur-modified recrystallized metal catalyst particles with a particle size not exceeding 8µm; adding the obtained catalyst recrystallized particles to an alcohol solvent and ultrasonically dispersing them to obtain a catalyst dispersion; introducing hydrogen gas into a single-walled carbon nanotube grower to transport the catalyst dispersion into the grower, filtering it through a rectifier screen, and then slowly and uniformly decomposing it through a quartz inner tube; and sending the resulting decomposition product into a nucleation zone at a temperature higher than that of the decomposition zone for nucleation and growth to obtain ultrafine single-walled carbon nanotubes with a diameter less than 2nm.

[0010] Furthermore, the single-walled carbon nanotube grower includes a reactor tube, which comprises a quartz inner tube and a first inner cavity with a closed cavity; the quartz inner tube is coaxially disposed at the front end of the first inner cavity, and a radial gap is formed between the outer wall of the quartz inner tube and the inner wall of the first inner cavity; the axial length of the quartz inner tube is 1 / 2 of the axial length of the first inner cavity; a rectifier screen is provided at one end of the quartz inner tube, and the rectifier screen is connected to the input end of the first inner cavity; the temperature of the inner cavity of the quartz inner tube is lower than the temperature of the first inner cavity, which is used to provide a slow and uniform decomposition space for the catalyst; preferably, the aperture of the rectifier screen does not exceed 500 nm. The single-walled carbon nanotube grower also includes a heating mechanism and a conveying mechanism; the output end of the conveying mechanism is connected to the first inner cavity for conveying materials; the heating mechanism is used to control the temperature of the first inner cavity of the reactor tube; the conveying mechanism includes a liquid injection pump and an atomizing unit.

[0011] This invention employs a heating-mixing-cooling process to sublimate the metal catalyst precursor before mixing it with sulfur-containing gas and recrystallizing it. This process strictly controls the catalyst particle size to within 8µm before it enters the furnace, fundamentally reducing the initial size of the catalyst particles. The sulfur-containing gas is mixed in situ with the catalyst during the sublimation gaseous vapor stage, allowing sulfur to uniformly adhere to the surface of the recrystallized particles and inhibiting excessive ripening of the catalyst particles in the high-temperature nucleation zone. The uniform decomposition zone, composed of a rectifier screen and a quartz inner tube, further filters large particle agglomerates, ensuring that the catalyst enters the nucleation zone in the form of ultrafine nanoparticles. The partitioned temperature gradient design between the decomposition and nucleation zones allows the catalyst to slowly decompose in the lower temperature zone and then rapidly nucleate in the higher temperature nucleation zone, reducing the excessive residence of catalyst particles in the high-temperature zone. This enables the continuous preparation of ultrafine single-walled carbon nanotubes with uniform diameters of less than 2nm.

[0012] Preferably, the first inner cavity has an axial length of 200 cm and an inner diameter of 30 cm; the quartz inner tube has an axial length of 100 cm and an inner diameter of 15 cm. The temperature difference between the inner cavity of the quartz inner tube and the first inner cavity is 200°C. This size of quartz inner tube provides sufficient space for slow and uniform decomposition of the catalyst, ensuring that the catalyst particles complete full decomposition before entering the high-temperature nucleation zone.

[0013] Preferably, the sublimation temperature of the metal catalyst precursor is 150℃~250℃. Within this temperature range, the metal catalyst precursor can be fully sublimated, and the recrystallized particle size can be controlled within 8µm, meeting the particle size requirements for the subsequent preparation of ultrafine single-walled carbon nanotubes.

[0014] Preferably, the metal catalyst precursor is selected from at least one of ferrocene and nickel dicene. Both ferrocene and nickel dicene have good sublimation characteristics and can be sublimated at relatively low temperatures under inert gas protection. The recrystallized particles after sublimation have uniform particle size, making them suitable for the continuous preparation of ultrafine single-walled carbon nanotubes.

[0015] Preferably, the sulfur-containing gas is selected from at least one of H2S and CS2. Both H2S and CS2 can be fully mixed with the metal catalyst during the sublimation gaseous stage, allowing sulfur to be uniformly attached to the surface of recrystallized particles in an in-situ modification manner. Compared with adding sulfur-containing compounds after the solution stage, in-situ sulfur-containing gas modification is more effective in suppressing the high-temperature ripening of catalyst particles.

[0016] Preferably, the inert gas is selected from at least one of argon and nitrogen. The inert gas effectively prevents the metallocene catalyst precursor from undergoing oxidative decomposition during heating and sublimation, ensuring the purity of the sublimation vapor.

[0017] Preferably, the alcohol solvent is selected from at least one of methanol, ethanol, and ethylene glycol. Alcohol solvents have good dispersibility for recrystallized catalyst particles; after ultrasonic dispersion, the particles can be dispersed to the nanoscale, facilitating subsequent atomization and spraying into the grower.

[0018] Preferably, the ultrasonic dispersion temperature is 40℃~60℃, the ultrasonic power is 100~200W, and the ultrasonic time is 1h. Under these conditions, the recrystallized catalyst particles can be fully dispersed in the alcohol solvent, and the particle size in the catalyst dispersion can reach more than ten nanometers, with good dispersion uniformity.

[0019] Preferably, the growth temperature of the ultrafine single-walled carbon nanotubes is 1100℃~1300℃, the hydrogen flow rate is 10~20L / min, and the atomization feed flow rate of the catalyst dispersion is 2~3ml / min.

[0020] The present invention also provides an ultrafine single-walled carbon nanotube prepared by the above-described continuous preparation method, wherein the diameter of the ultrafine single-walled carbon nanotube is less than 2 nm.

[0021] Compared with the prior art, the present invention has achieved the following beneficial effects: 1. This invention uses a heating-mixing-cooling process to sublimate the metal catalyst precursor, mix it with sulfur-containing gas, and then recrystallize it. This process strictly controls the particle size of the catalyst before it enters the furnace to within 8µm (the average particle size of the sublimation and recrystallization at 150℃ is 1.12µm, and the average particle size of DLS in the solution is about 13nm). Compared with the traditional method of direct dispersion (DLS particle size is about 238nm), this invention significantly reduces the initial size of the catalyst particles and inhibits the agglomeration and growth of catalyst particles in the high-temperature region from the source. This results in ultrafine single-walled carbon nanotubes with a diameter of less than 2nm and an average diameter of 1.28nm, while the carbon nanotubes obtained by the traditional method have a diameter of more than 10nm and have formed a multi-walled structure.

[0022] 2. Sulfur-containing gases (H2S, CS2) are mixed in situ with the catalyst during the sublimation gaseous vapor stage, allowing sulfur to uniformly adhere to the surface of the recrystallized particles. This forms a metal sulfide modification layer in the high-temperature nucleation region, effectively reducing the surface energy of the catalyst particles, inhibiting the particle ripening process, and stabilizing the particle size of the ultrafine catalytic active particles. Compared to the method of adding sulfur-containing compounds such as thiophene after the solution stage (average carbon nanotube diameter 3.49 nm), the carbon nanotube diameter obtained by the in-situ sulfur-containing gas modification method is significantly smaller, proving that the sulfur-modified metal catalyst process of this invention has made an important technical contribution to the control of ultrafine tube diameter.

[0023] 3. This invention incorporates a quartz inner tube and a rectifier screen with a pore size not exceeding 500 nm within a single-walled carbon nanotube grower. The rectifier screen filters out large particle agglomerates, while the quartz inner tube provides a uniform and slow heating and decomposition space. This allows the catalyst precursor to undergo a relatively mild and controllable volatilization, decomposition, and primary nucleation process, preventing excessively high temperatures after the catalyst enters the furnace, which could lead to rapid and violent decomposition of the catalyst precursor and excessively high local concentrations of metal atoms or primary particles. This inhibits collisions, coalescence, sintering, and abnormal growth between catalyst particles. The catalyst enters the nucleation zone uniformly in the form of ultrafine nanoparticles. The temperature in the decomposition zone is lower than that in the nucleation zone, further reducing the excessive residence time of catalyst particles in the high-temperature zone and inhibiting collisions and agglomeration between particles. Ultimately, this achieves continuous and stable preparation of ultrafine single-walled carbon nanotubes with a yield of 2–3 g / h, exhibiting high carbon nanotube formation, complete structure, and good diameter uniformity.

[0024] 4. This invention regulates and links the catalyst particle formation process with the carbon nanotube nucleation and growth process, enabling catalyst particles with smaller particle sizes and narrower particle size distributions to participate in the growth of single-walled carbon nanotubes. This is beneficial for increasing the proportion of single-walled carbon nanotubes formed and reducing the probability of the formation of double-walled carbon nanotubes, multi-walled carbon nanotubes, and amorphous carbon byproducts. Because catalyst particle size growth is inhibited, this invention is advantageous for obtaining ultrafine single-walled carbon nanotubes with smaller average diameters and more concentrated diameter distributions. Simultaneously, the more uniform catalyst active sites help improve the structural consistency and quality stability of different batches of products.

[0025] In summary, this invention reduces the particle size and distribution width of the precursor catalyst by pretreatment with sublimation and recrystallization of the metal catalyst, particle refinement and dissolution, and atomization dispersion, thereby reducing the risk of large particle formation in the subsequent nucleation process. It provides a method for effectively controlling the formation and nucleation stages of the catalyst during continuous preparation, enabling the catalyst precursor to form smaller, more uniformly distributed, and highly active catalyst nucleation units under predetermined conditions, and allowing these nucleation units to grow into ultrafine single-walled carbon nanotubes under suitable carbon source supply conditions. This method should improve the selectivity of ultrafine single-walled carbon nanotubes while minimizing catalyst particle sintering and agglomeration, amorphous carbon formation, and reactor wall deposition, while maintaining the stability and high efficiency of the continuous preparation process. Attached Figure Description

[0026] Figure 1 This is a schematic diagram of a catalyst precursor heating-mixing-cooling device.

[0027] Figure 2 This is a schematic diagram of a single-walled carbon nanotube growth apparatus, including the reactor tube and the conveying mechanism.

[0028] Figure 3 This is a flowchart of the growth process of ultrafine single-walled carbon nanotubes.

[0029] Figure 4 The results are from the DLS test of the catalyst solution prepared in Example 1.

[0030] Figure 5 This is the Raman RBM peak spectrum of single-walled carbon nanotubes prepared in Example 1 of the present invention.

[0031] Figure 6 This is a TEM image of single-walled carbon nanotubes prepared in Example 1 of the present invention.

[0032] Figure 7 The results are from the DLS test of the catalyst solution in Comparative Example 1.

[0033] Figure 8 This is the TEM image of the single-walled carbon nanotubes prepared in Comparative Example 1.

[0034] Labeling explanations: 1. Reactor tube; 2. Conveying mechanism; 3. Collecting device; 101. First inner cavity; 102. Quartz inner tube; 103. Rectifying screen. Detailed Implementation

[0035] The technical solution of the present invention is illustrated below through specific examples. It should be understood that the one or more method steps mentioned in the present invention do not preclude the existence of other method steps before or after the combined steps, or the insertion of other method steps between these explicitly mentioned steps; it should also be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the present invention. Furthermore, unless otherwise stated, the numbering of each method step is merely a convenient tool for identifying each method step, and not for limiting the order of the method steps or defining the scope of the present invention. Changes or adjustments to their relative relationships, without substantially altering the technical content, should also be considered within the scope of the present invention.

[0036] The present invention will now be described in further detail with reference to the accompanying drawings and embodiments. Unless otherwise specified, all raw materials used in the following embodiments are commercially available. Unless otherwise specified, the technical means used in the embodiments are conventional means well known to those skilled in the art. Ferrocene, nickel dicene, ethanol, methanol, and ethylene glycol are all analytical grade reagents. H2S gas and CS2 are both industrial grade pure products. Argon and nitrogen are both high-purity (purity ≥99.999%).

[0037] The particle size of the recrystallized metal catalyst was measured using a dry method with a Zhenli Optics-LT3600 Plus laser particle size analyzer. The particle size of the catalyst dispersion was measured using dynamic light scattering (DLS) laser particle size analysis with a Malvern Zetasizer Nano ZS particle size analyzer at 25℃. Raman spectroscopy was performed using a 532nm laser excitation, and the particle size was calculated using the tube diameter formula: dt≈248 / ω. RBM (nm) Calculate the diameter of the carbon nanotube. TEM transmission electron microscopy was performed using a JEOL JEM-2100F transmission electron microscope with an accelerating voltage of 200kV.

[0038] This invention employs a heating-mixing-cooling process to first sublimate the metal catalyst precursor, then mix it with a sulfur-containing gas and recrystallize it. This process can be, but is not limited to, other methods. Figure 1The heating-mixing-cooling apparatus shown is composed of three interconnected sections: a heating chamber, a mixing chamber, and a cooling chamber. Specifically, the output of the heating chamber is connected to the input of the mixing chamber, and the output of the mixing chamber is connected to the input of the cooling chamber. The mixing chamber has an independent gas supply channel for conveying sulfur-containing gas. Temperature control units (not shown in the figure) are configured in both the heating and mixing chambers, and a circulating cooling unit is configured in the cooling chamber. The specific process is as follows: a solid metal catalyst precursor is placed in the heating chamber and heated to sublimate under inert gas protection. The resulting gaseous vapor enters the mixing chamber and is thoroughly mixed with the sulfur-containing gas introduced through the independent gas supply channel. The mixed gas then enters the cooling chamber for cooling and recrystallization, yielding sulfur-modified recrystallized metal catalyst particles. The particle size range of the metal catalyst recrystallized particles prepared by this invention does not exceed 8 µm.

[0039] like Figure 2 As shown, the single-walled carbon nanotube grower used in this invention includes a reactor tube 1, a heating mechanism (not shown in the figure), and a conveying mechanism 2. The reactor tube 1 includes a first inner cavity 101 with a closed cavity and a quartz inner tube 102 coaxially disposed at the front end of the first inner cavity 101. A radial gap is formed between the outer wall of the quartz inner tube 102 and the inner wall of the first inner cavity 101, and the axial length of the quartz inner tube 102 is 1 / 2 of the axial length of the first inner cavity 101. A rectifier screen 103 is provided at one end of the quartz inner tube 102, which is arranged on one side of the input end of the first inner cavity 101 to filter large particle agglomerates in the atomized catalyst solution. A collection device 3 is provided at the outlet of the first inner cavity 101. The conveying mechanism 2 includes a liquid injection pump and an atomization unit. The output end of the injection pump is connected to the atomization unit, and the output end of the atomization unit is connected to the input end of the quartz inner tube 102.

[0040] The temperature inside the quartz inner tube 102 is lower than the temperature inside the first inner tube 101, which provides a slow and uniform decomposition space for the catalyst, thus forming a decomposition zone for the catalyst within the reactor tube 1. Figure 2 (Area A, as indicated by the symbol), the other end of the quartz inner tube 102 leads into the nucleation zone ( Figure 2 (Region B, in Chinese characters); The temperature of the nucleation zone is higher than that of the decomposition zone where the quartz inner tube 102 is located, which allows the particles after the catalyst dispersion is atomized to undergo slow decomposition in the decomposition zone and then achieve rapid nucleation and growth in the nucleation zone.

[0041] Under preferred conditions, the first inner cavity 101 has a length of 200 cm and an inner diameter of 30 cm. The rectifier screen 103 has an aperture of no more than 500 nm. The quartz inner tube 102 has a length of 100 cm, an inner diameter of 15 cm, and a wall thickness of 5 mm.

[0042] like Figure 3The diagram illustrates the growth process for preparing ultrafine single-walled carbon nanotubes according to the present invention. This method employs a floating catalyst chemical vapor deposition (CVD) approach, specifically including the following steps: The aforementioned sulfur-modified metal catalyst recrystallized particles are added to an alcohol solvent and ultrasonically dispersed at a set temperature for a set time to obtain a catalyst dispersion. The single-walled carbon nanotube grower is preheated, and once the set temperature is reached, hydrogen gas is introduced. The catalyst dispersion is then atomized via a conveying mechanism 2 and sprayed into the reactor tube 1. After passing through a rectifier screen 103, it is conveyed into the quartz inner tube 102, where it undergoes thorough, slow, and uniform decomposition. The decomposition products enter a higher-temperature nucleation zone, where carbon nanotubes begin to nucleate and grow, ultimately yielding the ultrafine single-walled carbon nanotube product.

[0043] Example 1 Ferrocene powder is subjected to a heating-mixing-cooling process. It is placed in the heating chamber of a heating-mixing-cooling device, with argon gas introduced as a protective gas. The powder is heated to 150°C to sublimate the ferrocene. The resulting gaseous vapor enters the mixing chamber and is thoroughly mixed with H2S gas supplied through an independent gas supply channel. The mass of H2S gas is 1 / 2 to 1 / 3 of the mass of ferrocene.

[0044] After mixing, the gaseous vapor enters a cooling chamber for recrystallization, yielding sulfur-modified recrystallized metal catalyst particles. A dry method was used, employing a Zhenli Optics-LT3600 Plus laser particle size analyzer to measure the particle size of the recrystallized metal catalyst particles. Air was used as the dispersion medium, without adding any liquid dispersant; the powder was tested directly. The average particle size of the recrystallized metal catalyst particles was found to be 1.12 µm, meeting the particle size control requirements before furnace entry.

[0045] The recrystallized metal catalyst particles were added to an ethanol solvent at a mass-to-volume ratio of 0.005–0.02 g / mL. The mixture was ultrasonically dispersed at 60°C with 200 W for 1 h to obtain a catalyst dispersion. 2 mL of the catalyst dispersion was subjected to DLS laser particle size analysis, and the results are as follows: Figure 4 As shown, the particle size in the solution is mainly distributed between 10 and 20 nm, with an average particle size of 13.25 nm.

[0046] The single-walled carbon nanotube grower is preheated to 1200℃ and hydrogen gas is introduced at a flow rate of 20L / min. Then, the above catalyst dispersion is atomized through an atomizing nozzle at a flow rate of 2mL / min and sprayed into the quartz inner tube 102 in the furnace. After being filtered through a rectifier screen 103 with a pore size not exceeding 500nm, it undergoes thorough, slow and uniform decomposition.

[0047] The decomposition products enter a higher-temperature nucleation zone, where they undergo nucleation and growth to obtain ultrafine single-walled carbon nanotubes. The carbon nanotube yield is 2–3 g / h.

[0048] Raman spectroscopy was performed on the obtained single-walled carbon nanotubes, and the results are as follows: Figure 5 As shown, the effective peak frequency ω of the radial breathing pattern (RBM) is... RBM All are greater than 124cm -1 The pipe diameter can be calculated using the formula dt≈248 / ω. RBM Calculations showed that the diameter of all carbon nanotubes was less than 2 nm, with an average diameter of 1.58 nm. TEM (transmission electron microscopy) analysis of the obtained carbon nanotubes yielded the following results: Figure 6 As shown, the carbon nanotubes have high formability, complete structure, and diameters of less than 2 nm, confirming that they are single-walled structures.

[0049] Example 2 This embodiment follows the steps of Example 1, only adjusting the sublimation temperature of ferrocene. Specifically, the ferrocene powder was sublimated at 175℃, 200℃, 225℃, and 250℃ to obtain sulfur-modified recrystallized metal catalyst particles. A dry method was used, employing a Zhenli Optics-LT3600 Plus laser particle size analyzer to measure the particle size of the recrystallized metal catalyst particles. Air was used as the dispersion medium, without adding any liquid dispersant; the powder was tested directly. The average particle size of the recrystallized particles is shown in Table 1.

[0050] In this invention, when the sublimation temperature is below 150°C, no obvious sublimation phenomenon or the rate is too slow. If the sublimation temperature exceeds 250°C, the particle size of the recrystallized metal catalyst particles becomes too large, which will affect the particle size in the catalyst dispersion and thus affect the diameter of the single-walled carbon nanotube product. In some preferred embodiments, when the average particle size of the recrystallized metal catalyst particles is less than 8 μm, combined with the preparation method of this invention, ultrafine single-walled carbon nanotubes with a diameter of less than 2 nm can be obtained.

[0051] Table 1. Grain size of ferrocene recrystallization at different sublimation temperatures

[0052] Example 3 Ferrocene powder is subjected to a heating-mixing-cooling process. The heating temperature is 250℃. The gaseous vapor obtained from the sublimation of ferrocene enters the mixing chamber and is thoroughly mixed with the introduced H2S gas. The mass of H2S gas is 1 / 2 to 1 / 3 of the mass of ferrocene.

[0053] After mixing, the gaseous vapor enters the cooling chamber for cooling and recrystallization, resulting in sulfur-modified metal catalyst recrystallized particles. The particle size of the metal catalyst recrystallized particles was measured by dry method using a Zhenli Optics-LT3600 Plus laser particle size analyzer and found to be no more than 8µm.

[0054] The recrystallized product was added to methanol solvent, with a mass-to-volume ratio of metal catalyst recrystallized particles to methanol solvent of 0.005–0.02 g / mL. The mixture was ultrasonically dispersed at 60 °C with an ultrasonic power of 200 W for 1 h to obtain a catalyst dispersion. 2 mL of the catalyst dispersion was subjected to DLS laser particle size analysis. The results showed that the particle size was mainly distributed between 10 and 30 nm, with an average particle size of 19.47 nm.

[0055] The single-walled carbon nanotube grower was preheated to 1300℃ and hydrogen gas was introduced at a rate of 20L / min. Then, the catalyst dispersion was atomized at a flow rate of 2mL / min, filtered through a rectifier screen 103 with a pore size of no more than 500nm, and then entered into the quartz inner tube 102 for thorough, slow and uniform decomposition.

[0056] The decomposition products enter a higher-temperature nucleation region, where they undergo nucleation and growth to obtain ultrafine single-walled carbon nanotubes. Raman spectroscopy and TEM analysis were performed on the obtained carbon nanotubes, revealing an average diameter of 1.47 nm.

[0057] Example 4 Nickel-ceramium powder is subjected to a heating-mixing-cooling process. It is placed in a heating chamber, with argon gas introduced as a protective gas. Heating at 250°C causes the nickel-ceramium to sublimate. The resulting gaseous vapor enters a mixing chamber and is thoroughly mixed with the introduced CS2 gas. The mass of the CS2 gas is 1 / 2 to 1 / 3 of the mass of the nickel-ceramium.

[0058] After mixing, the gaseous vapor was cooled and recrystallized in a cooling chamber to obtain sulfur-modified metal catalyst recrystallized particles. Dry particle size analysis using a Zhenli Optics-LT3600 Plus laser particle size analyzer showed that the average particle size of the recrystallized metal catalyst particles did not exceed 8 µm. The recrystallized product was then added to ethylene glycol solvent at a mass-to-volume ratio of 0.005–0.02 g / mL. The mixture was ultrasonically dispersed at 60 °C with 200 W ultrasonic power for 1 h to obtain a catalyst dispersion. DLS particle size analysis showed that the particle size was mainly distributed between 20 and 40 nm, with an average particle size of 28.24 nm.

[0059] The single-walled carbon nanotube grower was preheated to 1300℃, and hydrogen gas was introduced at a flow rate of 15 L / min. The catalyst dispersion was then atomized at a flow rate of 2 mL / min and passed through a rectifier screen 103 with a pore size not exceeding 500 nm into the quartz inner tube 102 for thorough, slow, and uniform decomposition. The decomposition products then entered a higher-temperature nucleation zone for nucleation growth, yielding the carbon nanotube product.

[0060] The obtained carbon nanotubes were subjected to Raman spectroscopy and TEM testing, and the average diameter of the carbon nanotubes was 1.91 nm. This example verifies the applicability of nickel-cadmium and CS2 as catalyst precursors and sulfur-containing gases.

[0061] Comparative Example 1 Ferrocene powder (commercially available, dry powder particle size range of 30~40 micrometers) was added directly to ethanol solvent without undergoing a heating-mixing-cooling process for sublimation and recrystallization, and ultrasonically dispersed at 60°C with an ultrasonic power of 200W for 1 hour to obtain a catalyst dispersion.

[0062] 2 ml of the catalyst dispersion was subjected to DLS laser particle size analysis, and the results are as follows: Figure 7 As shown in the figure, the particle size is mainly distributed between 200 and 300 nm, with an average particle size of 238.3 nm. This is much larger than the 13.25 nm particle size after sublimation and recrystallization in Example 1. Comparing Example 1 and Example 2, it can be seen that sublimation and recrystallization through a heating-mixing-cooling process can remove impurities while obtaining particles with uniform size and smaller dimensions.

[0063] Remove the rectifier screen 103 and the quartz inner tube 102 from the single-walled carbon nanotube grower. Preheat the single-walled carbon nanotube grower to 1300℃ and introduce hydrogen gas at a flow rate of 10L / min. Then, directly spray the above-mentioned catalyst dispersion into the furnace at a flow rate of 3mL / min to grow carbon nanotubes and obtain the carbon nanotube product.

[0064] The obtained carbon nanotubes were subjected to TEM transmission electron microscopy, and the results are as follows: Figure 8 As shown. From Figure 8 As can be seen, the carbon nanotubes have high shape density and complete structure, but the diameter of each tube is greater than 10 nm, forming a multi-walled structure. A comparison between Example 1 and Comparative Example 1 shows that the metal catalyst particles without sublimation recrystallization treatment have a larger particle size; and after removing the rectifier screen 103 and the quartz inner tube 102, the metal catalyst particles cannot be effectively filtered and uniformly dispersed before entering the high-temperature zone, resulting in a large agglomeration and growth of the metal catalyst particles in the high-temperature zone. The resulting carbon nanotubes all have a diameter greater than 10 nm and form a multi-walled structure. This result demonstrates that the particle size control step of sublimation recrystallization, and the uniform decomposition step achieved by the combined structure of the rectifier screen 103 and the quartz inner tube 102, are indispensable for the preparation of ultrafine single-walled carbon nanotubes.

[0065] Comparative Example 2 Ferrocene powder is placed in the heating chamber of a heating-mixing-cooling device to start the heating-mixing-cooling process. Argon gas is introduced as a protective gas, and the ferrocene is sublimated at 150°C. The gaseous vapor obtained from the sublimation does not mix with the sulfur-containing gas and directly enters the cooling chamber for cooling and recrystallization to obtain metal catalyst recrystallized particles (without sulfur element modification).

[0066] The recrystallized product was added to an ethanol solvent, along with 0.5% (by weight) of recrystallized thiophene. The mixture was then ultrasonically dispersed at 60°C with an ultrasonic power of 200W for 1 hour to obtain a catalyst dispersion. 2 mL of the catalyst dispersion was subjected to DLS laser particle size analysis. The particle size was mainly distributed between 30 and 50 nm, with an average particle size of 41.33 nm.

[0067] The single-walled carbon nanotube grower was preheated to 1200℃ and hydrogen gas was introduced at a flow rate of 20L / min. Then, the catalyst dispersion was atomized at a flow rate of 2mL / min and sprayed into the quartz inner tube 102 for thorough, slow and uniform decomposition. The decomposition products entered the nucleation zone for nucleation and growth, and carbon nanotube products were obtained.

[0068] Raman spectroscopy and TEM tests were performed on the obtained carbon nanotubes. The diameters of all carbon nanotubes were greater than 3 nm, with an average diameter of 3.49 nm. A comparison between Example 1 and Comparative Example 2 shows that in the scheme of adding thiophene after the solution stage (non-in-situ sublimation mixing modification), the average particle size of the catalyst dispersion DLS was 41.33 nm, and the average diameter of the obtained carbon nanotubes was 3.49 nm. In contrast, in Example 1, using sulfur-containing gas H2S in-situ mixed modification with the catalyst during the sublimation gaseous stage, the average particle size of the catalyst dispersion DLS was only 13.25 nm, and the average diameter of the obtained carbon nanotubes was 1.08 nm. These results indicate that the in-situ sublimation mixing modification mechanism with sulfur-containing gas, compared to adding sulfur-containing compounds after the solution stage, can more effectively ensure that sulfur elements are uniformly attached to the surface of the recrystallized particles, thereby significantly inhibiting catalyst particle ripening in the high-temperature nucleation region and achieving ultra-fine tube diameter control.

[0069] Table 2. Main process parameters and carbon nanotube diameters for each embodiment and comparative example.

[0070] The above-described technical solution provides a continuous preparation method for ultrafine single-walled carbon nanotubes based on nucleation control. This method allows for comprehensive adjustment of the size, concentration, and spatial distribution of catalyst particles, reducing the likelihood of catalyst particle agglomeration and growth, and avoiding over-maturation during the nucleation stage. Ultimately, it yields ultrafine single-walled carbon nanotubes with a diameter of less than 2 nm. Furthermore, this method improves the selectivity of ultrafine single-walled carbon nanotube formation, structural uniformity, and the stability of the preparation process.

[0071] It is understood that the metal catalyst precursor is not limited to ferrocene and nickel ferrocene, but can also be other metallocene compounds with good sublimation properties, as long as they can be sublimated at an appropriate temperature under inert gas protection and recrystallized after modification with sulfur-containing gas with a particle size not exceeding 8µm.

[0072] It is understood that the sulfur-containing gas is not limited to H2S and CS2, but may also be other sulfur-containing gases that can be fully mixed with the metallocene catalyst in the gaseous vapor stage and allow sulfur elements to adhere to the surface of the recrystallized particles.

[0073] It is understood that the alcohol solvent is not limited to ethanol, methanol, and ethylene glycol, but may also be other alcohol solvents that have good dispersibility for recrystallization catalyst particles. The inert gas is not limited to argon, but may also be nitrogen or other inert gases.

[0074] Obviously, the present invention is not limited to the specific embodiments described above. Various modifications or substitutions that can be made by those skilled in the art based on the technical solution of the present invention without creative effort are all within the protection scope of the present invention.

Claims

1. A continuous preparation method for ultrafine single-walled carbon nanotubes, employing a floating catalyst chemical vapor deposition method, characterized in that, Includes the following steps: (1) The metal catalyst precursor is heated and sublimated under the protection of an inert gas. The gaseous vapor obtained from the sublimation is fully mixed with the sulfur-containing gas and then cooled and recrystallized to obtain sulfur-modified metal catalyst recrystallized particles. The particle size of the metal catalyst recrystallized particles does not exceed 8µm. The metal catalyst precursor is selected from at least one of ferrocene and nickel ferrocene. The sulfur-containing gas is selected from at least one of H2S and CS2. (2) The recrystallized metal catalyst particles from step (1) are added to an alcohol solvent and ultrasonically dispersed to obtain a catalyst dispersion; the alcohol solvent is selected from at least one of methanol, ethanol, and ethylene glycol. (3) Hydrogen gas is introduced into the single-walled carbon nanotube grower to transport the catalyst dispersion to the single-walled carbon nanotube grower for atomization, sieving, decomposition, nucleation and growth to obtain ultrafine single-walled carbon nanotubes. The single-walled carbon nanotube grower includes a reactor tube, which comprises a quartz inner tube and a first inner cavity with a closed air cavity. The quartz inner tube is coaxially disposed at the front end of the first inner cavity, and a radial gap is formed between the outer wall of the quartz inner tube and the inner wall of the first inner cavity. The axial length of the quartz inner tube is 1 / 2 of the axial length of the first inner cavity. A rectifier screen is provided at one end of the quartz inner tube and is arranged on the input side of the first inner cavity. The temperature of the inner cavity of the quartz inner tube is lower than that of the first inner cavity, which is used to provide a slow and uniform decomposition space for the catalyst. The aperture of the rectifier screen does not exceed 500 nm.

2. The continuous preparation method according to claim 1, characterized in that, The first inner cavity has an axial length of 200cm and an inner diameter of 30cm; the quartz inner tube has an axial length of 100cm and an inner diameter of 15cm.

3. The continuous preparation method according to claim 1, characterized in that, The temperature of the inner cavity of the quartz inner tube differs from the temperature of the first inner cavity by 200°C.

4. The continuous preparation method according to claim 1, characterized in that, In step (1), the temperature for heating and sublimation is 150℃~250℃.

5. The continuous preparation method according to claim 1, characterized in that, In step (2), the temperature of the ultrasonic dispersion is 40℃~60℃ and the ultrasonic power is 100~200W.

6. The continuous preparation method according to claim 1, characterized in that, In step (3), the growth temperature of the ultrafine single-walled carbon nanotubes is 1100℃~1300℃.

7. The continuous preparation method according to claim 1, characterized in that, The atomization feed flow rate of the catalyst dispersion in step (3) is 2-3 ml / min.

8. The continuous preparation method according to claim 1, characterized in that, The single-walled carbon nanotube grower also includes a heating mechanism and a conveying mechanism; the output end of the conveying mechanism is connected to the first inner cavity and is used to convey materials; the heating mechanism is used to control the temperature of the first inner cavity of the reactor tube; the conveying mechanism includes a liquid injection pump and an atomizing unit.

9. The continuous preparation method according to any one of claims 1 to 8 yields ultrafine single-walled carbon nanotubes with a diameter of less than 2 nm.

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