Carbon nanotube preparation method with carbon source and plasma timing cooperation and carbon nanotube

Abstract

The application relates to the technical field of nanomaterial preparation, and particularly discloses a carbon nanotube preparation method based on carbon source and plasma time sequence cooperation and a carbon nanotube. In the gas phase deposition process, carbon source gas is input in a pulse mode, and a pulse type plasma is synchronously applied, wherein the carbon source input pulse and the plasma output pulse are time-differently matched, the carbon source supply window and the plasma action window form a non-fully-synchronized time sequence coupling relationship, the carbon source activation, transportation, deposition and structure reconstruction process are controlled in the time dimension, and meanwhile, the carbon source gas is introduced from a position close to the plasma action area to maintain the pulse opening and closing characteristics. The method can improve the growth selectivity of the carbon nanotube, reduce the defect density, and is suitable for controllable preparation of single-wall carbon nanotubes. The single-wall carbon nanotube with a semiconductor content greater than or equal to 83% can be prepared with high selectivity, and the product has the advantages of few defects, high purity and high yield.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation, specifically relating to a vapor deposition method for preparing carbon nanotubes, and more particularly to a method for controlling the growth process of carbon nanotubes by timing misalignment of carbon source pulse input and plasma pulse output, and single-walled carbon nanotubes prepared by this method. Background Technology

[0002] Single-walled carbon nanotubes (SUVs) exhibit exceptional electrical, optical, and mechanical properties due to their structure-dependent one-dimensional quantum confinement effect, making them a promising candidate for next-generation nanoelectronic devices and high-performance composite materials. However, in typical fabrication processes, metallic and semiconductor SUVs coexist in a statistical ratio (approximately 1:2), and their electrical properties differ significantly. This severely limits their direct application in fields such as integrated circuits, which require uniform electrical characteristics.

[0003] Chemical vapor deposition (CVD) is currently the most scalable preparation technique. While catalyst design (e.g., using bimetallic systems like Co-Mo or Fe-Ru) can control carbon nanotube diameter to some extent, its selectivity for conductivity types is limited, and the semiconductor content is typically difficult to consistently exceed 70%. Plasma-enhanced CVD can effectively lower the growth temperature and improve carbon source decomposition efficiency; however, the high-energy particle stream generated by conventional continuous-wave plasma can easily cause irreversible etching damage to the grown carbon nanotubes, introducing numerous defects and potentially disrupting the local equilibrium environment crucial for chiral selective growth. This defect is also evident in Comparative Example 2.

[0004] Therefore, developing a method that can selectively prepare high-quality, low-defect semiconductor single-walled carbon nanotubes with high yield and controllable and reproducible process is a technical bottleneck that urgently needs to be overcome in this field. Summary of the Invention

[0005] The technical problem this invention aims to solve is to overcome the shortcomings of existing technologies, such as the difficulty in effectively coordinating carbon source supply and plasma interaction, the susceptibility of continuous plasma to introducing defects, and the insufficient spatial layout of the reactor to support time-series control. This invention provides a chemical vapor deposition method for preparing carbon nanotubes. This method achieves precise control over the growth behavior of carbon nanotubes through near-zone gas guiding layout within the reactor and pulse timing coordination. By combining precise pre-activation of the catalyst with low-damage pulsed plasma assistance, a synergistically controlled chemical vapor deposition environment is created. In the preferred embodiment, the Co-Mo / MgO pre-activated catalyst further enhances the semiconductor selectivity and product quality.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: A method for preparing carbon nanotubes by sequentially coordinating a carbon source with plasma, characterized by comprising the following steps: The catalyst is placed in the growth zone of a tubular reactor. During the vapor deposition process, carbon source gas is introduced into the reactor in a pulsed manner, and pulsed plasma is applied so that the plasma diffuses downstream with the carrier gas from its origin and maintains its activity to form a plasma interaction zone. Among them, the carbon source input pulse and the plasma output pulse are controlled by the same timing controller under the same clock reference. The two are staggered in time, so that the carbon source supply window and the plasma action window are not completely synchronized in time coupling relationship, so as to regulate the carbon source activation, transport, deposition and structural reconstruction process. Furthermore, the carbon source gas is introduced into the reactor from the upstream boundary of the plasma interaction zone or from the side wall adjacent to the plasma interaction zone, with its introduction direction perpendicular to the reactor axis or inclined along the gas flow direction; the distance between the carbon source gas introduction point and the growth zone is controlled within 10 mm, and the straight-line distance between the carbon source gas introduction point and the plasma excitation source is not greater than 50 mm, so that the carbon source gas maintains a recognizable pulse on / off characteristic before reaching the growth interaction zone.

[0007] Preferably, as a quantifiable indicator, "maintaining identifiable pulse-on-off characteristics" means that the fluctuation amplitude of the carbon source gas concentration upon reaching the growth region is not less than 50% of its peak value. Specifically, during the reactor structural design phase, a gas concentration detection device can be temporarily installed in the predetermined growth region to measure the change in carbon source gas concentration over time within this region. The detection device can be an external rapid gas analyzer connected after sampling, such as an online mass spectrometer, a rapid infrared gas analyzer, a spectroscopic detection device, or other equivalent time-resolved concentration detection methods. By recording the peak and trough values ​​of the carbon source gas concentration during steady-state operation and calculating the ratio of its fluctuation amplitude to the peak value, it can be determined whether the carbon source gas maintains identifiable pulse-on-off characteristics upon reaching the growth region. When the fluctuation amplitude is not less than 50% of the peak value, it is considered that the carbon source gas still maintains identifiable pulse-on-off characteristics before reaching the growth region.

[0008] In this invention, "temporally misaligned coordination" refers to the fact that the carbon source input pulse and the plasma output pulse are not completely synchronized, but rather have an adjustable offset relationship in relative timing to form a time-divisional interaction window suitable for carbon nanotube growth. This offset relationship can be achieved through phase difference or other equivalent timing control methods, and its specific manifestation can vary depending on reaction conditions, equipment parameters, and target products. That is, the input pulse of the carbon source gas and the output pulse of the plasma can form any of the following temporal relationships: continuous, intermittent, or partially overlapping.

[0009] By introducing pulsed plasma instead of continuous plasma during the growth stage, the pulsed mode provides a periodic "energy injection (promoting fragmentation and diffusion) - relaxation growth (ordered assembly)" window. This leverages the advantages of plasma-activated reactants while minimizing continuous bombardment damage to the growth front and the carbon nanotube body. By synchronously controlling the timing (phase difference) of the plasma pulse and the carbon source inlet pulse, the energy supply and carbon atom supply are optimally matched in time, thereby kinetically guiding the carbon atoms to align in a specific chiral manner and achieving highly selective growth of semiconductor-type carbon nanotubes.

[0010] In this invention, the carbon source gas is preferably introduced close to the plasma interaction region. This is because, at high pulse frequencies, if the carbon source is introduced from a distant location, the carbon source pulse may gradually lose its distinct on / off characteristics during transport due to diffusion and mixing, thereby weakening or even disrupting the timing coordination between the carbon source supply window and the plasma interaction window. Therefore, by introducing the carbon source from a location close to the plasma interaction region, it is beneficial to maintain the carbon source pulse characteristics before reaching the growth region, thus ensuring that the timing coupling control required by this invention can be effectively achieved.

[0011] Preferably, the output pulse of the pulsed plasma has the same frequency and duty cycle as the input pulse of the carbon source gas, and the temporal misalignment is adjusted by the phase difference.

[0012] Preferably, the reactor further includes a carrier gas supply assembly, a heating unit connected to the growth zone, a downstream exhaust gas channel, and a pressure control unit for maintaining growth pressure from atmospheric pressure to 10 kPa.

[0013] Preferably, the pressure control unit includes a back pressure valve, a throttle valve, a vacuum pump, or a combination thereof.

[0014] Preferably, the heating unit is a single-temperature zone or multi-temperature zone tube furnace, used to maintain the plasma interaction zone and the growth zone at a set temperature respectively.

[0015] Preferably, the plasma excitation source is one of a radio frequency electrode, a microwave waveguide, or a dielectric barrier discharge electrode.

[0016] This invention posits that the transport time, flow field broadening, and diffusion mixing degree of the carbon source gas in the reactor are jointly influenced by carrier gas flow rate, temperature, gas composition, pressure, and reactor geometry. For a given reactor structure and process conditions, there exists a more suitable range of carbon source input pulse frequencies, enabling the carbon source gas to maintain the expected pulse on / off characteristics and achieve the necessary timing coordination with the plasma output pulses during transport to the plasma interaction region and / or growth region.

[0017] Preferably, the frequency of the carbon source input pulse is preset according to the carrier gas flow rate, reaction temperature, and gas composition.

[0018] Specifically, to ensure the stable implementation of this invention, the frequency of the carbon source input pulse is not fixed but is preset according to specific process conditions. Those skilled in the art will understand that the transport time, flow field broadening, and diffusion mixing degree of the carbon source gas in the reactor are influenced by both variable parameters (such as carrier gas flow rate, temperature, gas composition, and pressure) and inherent reactor parameters (such as reactor pipe diameter, distance from the carbon source inlet point to the plasma interaction zone, distance from the carbon source inlet point to the growth zone, and the relative arrangement of functional zones within the reactor).

[0019] Therefore, for a given reactor structure and target process conditions (e.g., carrier gas flow rate 200 sccm, reaction temperature 700℃, C2H4:CH4 = 1:3), a preferred carbon source input pulse frequency range can be determined through simple preliminary experiments. For example, while keeping other conditions constant, product quality (e.g., Raman spectral IG / ID value and semiconductor content) can be tested at frequencies of 1 kHz, 2 kHz, 5 kHz, and 10 kHz, and the frequency with the best overall effect can be selected as the set frequency. Examples 1-5 below use this method, selecting specific frequency values ​​of 5 kHz, 2 kHz, and 8 kHz under their respective specific process conditions, all achieving good technical results. When encountering different process conditions, those skilled in the art can determine suitable frequencies through similar preliminary experiments without creative effort, referring to the teachings of this specification.

[0020] Preferably, the phase difference is 90° to 180°.

[0021] Preferably, the frequency is 1 to 10 kHz; the duty cycle of the carbon source input pulse is 10% to 50%.

[0022] Preferably, the plasma is pulsed low-temperature plasma, and continuous wave plasma is not used as the plasma input method for the growth stage.

[0023] Preferably, the reactor is a vertically or horizontally arranged tubular reactor, which is divided into a plasma generation zone, a plasma interaction zone and a growth zone in sequence along the airflow direction.

[0024] Preferably, the plasma generation zone is located upstream or at the top of the reactor and is connected to a pulsed plasma power supply; the growth zone is located downstream or inside the plasma interaction zone and is used to place the catalyst substrate or a support for loading the catalyst.

[0025] Preferably, the catalyst is any one of a supported metal catalyst, a bimetallic catalyst, a single metal catalyst, or a gas-phase suspension catalyst.

[0026] Preferably, the carbon source gas is a mixture of ethylene and methane, with a volume ratio of C2H4:CH4 = 1:(2-4).

[0027] Preferably, the growth pressure of the vapor deposition process is atmospheric pressure to 10 kPa.

[0028] Preferably, the catalyst is prepared by: supporting a Co-Mo bimetallic catalyst on a porous MgO support, and pretreating it at 400-500°C for 1-2 hours in an H2 / Ar mixed atmosphere with a volume fraction of 10-30%. By pre-activating the Co-Mo / MgO catalyst under specific reduction conditions, the size, crystal phase, and surface chemical state of the catalyst nanoparticles are precisely controlled, forming active sites conducive to the nucleation of semiconductor-type carbon nanotubes.

[0029] Preferably, the active components of the catalyst are Co and Mo, with a molar ratio of Co:Mo = 1:(0.2-0.5).

[0030] Preferably, the porous MgO support has a pore size of 5-20 nm and a specific surface area greater than 200 m². 2 / g.

[0031] Secondly, the present invention provides single-walled carbon nanotubes prepared by the above method.

[0032] Preferably, the content of semiconductor carbon nanotubes in the single-walled carbon nanotube is greater than or equal to 83%.

[0033] Compared with the prior art, the present invention has at least the following beneficial effects: By staggering the timing of the carbon source input pulse and the plasma output pulse, a coordinated carbon source supply window and plasma interaction window can be formed in the same reaction process, which is conducive to the dynamic balance between carbon source decomposition, active species transport and orderly deposition.

[0034] Using pulsed plasma instead of continuous plasma helps reduce the continuous bombardment of high-energy particles on the growth front and the formed carbon nanostructures, thereby reducing defect generation.

[0035] This control approach is essentially a process timing regulation method that does not depend on a specific catalyst system. Therefore, it has good universality and can be applied to various processes for preparing carbon nanomaterials by vapor deposition.

[0036] In a preferred embodiment, the present invention can selectively prepare semiconducting single-walled carbon nanotubes, and the resulting product has a high semiconducting content, low defect density, and high yield. Examples 1-5 show that the semiconducting content can reach 83% to 87%, while the selectivity and quality of the continuous plasma comparative sample are significantly reduced. Attached Figure Description

[0037] To more clearly illustrate the technical solutions of the embodiments of this application, the accompanying drawings used in the embodiments will be briefly introduced below. It should be understood that the following drawings only show some embodiments of this application and should not be regarded as a limitation of the scope. For those skilled in the art, other related drawings can be obtained based on these drawings without creative effort.

[0038] Figure 1 Transmission electron microscopy (TEM) image of the single-walled carbon nanotubes prepared in Example 1; Figure 2 The Raman spectrum of the single-walled carbon nanotubes prepared in Example 1 is shown below. Figure 3 Thermogravimetric analysis (TGA) diagram of the single-walled carbon nanotubes prepared in Example 1; Figure 4 This is a schematic diagram of the reactor structure and pulse control used in the example; Figure 5 This is a schematic diagram of the timing of the carbon source input pulse and the plasma output pulse. Detailed Implementation

[0039] The following specific embodiments illustrate the implementation of the present invention. Those skilled in the art can easily understand other advantages and effects of the present invention from the content disclosed in this specification. Obviously, the described embodiments are only some embodiments of the present invention, not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention. Conventional substitutions or equivalent changes made by those skilled in the art to the reactor structure, catalyst type, support material, carbon source type, pulse parameters, and plasma excitation method without departing from the concept of the present invention should all fall within the scope of protection of the present invention.

[0040] The present invention will be further illustrated below with reference to specific embodiments, but the embodiments do not limit the present invention in any way. Unless otherwise specified, the reagents, methods, and equipment used in the present invention are conventional reagents, methods, and equipment in this technical field.

[0041] The present invention will be described in detail below with reference to examples and comparative examples. In the following examples, the total catalyst metal loading was 10 wt% (calculated as Co+Mo), and the carbon nanotube yield was the yield per unit mass of catalyst per unit time. The semiconductor content was calculated by UV-Vis-NIR absorption spectroscopy analysis. The carbon nanotube mass was expressed as the ratio of the intensity of the G peak to the D peak in the Raman spectrum (Ig). G / I D The higher the ratio, the fewer the defects.

[0042] Reactor structure and necessary components for practical implementation To ensure the stable implementation of this invention, the tubular reactor in this embodiment preferably includes a furnace body, a reaction tube, a heating unit, a plasma excitation source, a carbon source introduction assembly, a carrier gas supply assembly, a catalyst support assembly, a tail gas discharge assembly, a pressure control assembly, and a timing control assembly. The furnace body can be a horizontal or vertical tubular furnace, and the reaction tube is preferably a quartz tube, an alumina tube, or a ceramic tube resistant to plasma corrosion. Airtight flanges or sealing joints are preferably provided at both ends of the reaction tube to connect to the upstream inlet main pipe and the downstream tail gas discharge channel, respectively.

[0043] The heating unit is preferably a single-temperature zone or multi-temperature zone resistance heating module, used to provide a stable thermal environment for the plasma generation zone and the growth zone; if necessary, the plasma generation zone and the growth zone can be controlled with different temperature zones to balance plasma stability and catalyst growth temperature. The catalyst support component is preferably a quartz boat, ceramic boat, mesh support, or substrate clamping component, which is fixed in position in the growth zone so that activated carbon species can quickly reach the catalyst surface after formation.

[0044] The carbon source introduction assembly preferably includes a carbon source gas source, a mass flow controller, a pulse control valve, and an introduction tube that extends into the reaction tube.

[0045] The carbon source gas is introduced at the upstream boundary of the plasma interaction zone or at a sidewall adjacent to the plasma interaction zone. Its introduction direction is perpendicular to the reactor axis or inclined along the gas flow direction, so that the carbon source gas enters the plasma interaction zone rapidly upon introduction. The distance between the carbon source gas introduction point and the growth zone is controlled within 10 mm, and the straight-line distance between the carbon source gas introduction point and the plasma excitation source is no greater than 50 mm.

[0046] The plasma excitation source (such as a radio frequency electrode, microwave waveguide, or dielectric barrier discharge electrode) is arranged outside or inside the reactor, opposite to the carbon source inlet point, so that the plasma interaction region spatially overlaps or is adjacent to the carbon source inlet point. The output pulse of the pulsed plasma and the input pulse of the carbon source gas are controlled collaboratively by the same timing controller or programmable logic controller. The timing controller outputs two pulse signals: one to the carbon source pulse control valve to control the intermittent flow of carbon source gas; the other to the pulsed plasma power supply to control the intermittent output of plasma. The two signals have the same frequency and duty cycle, and the required phase difference is achieved by setting a relative delay. By setting a relative delay under the same clock reference, pulse misalignment with a phase difference of 90° to 180° can be achieved.

[0047] Meanwhile, the timing controller is also connected to the carrier gas supply unit to receive or set the carrier gas flow rate parameters. The carrier gas supply unit includes a carrier gas source and a mass flow controller.

[0048] For a tubular reactor with a given pipe diameter and a given carbon source inlet location, after setting the carrier gas flow rate, reaction temperature, and reaction atmosphere composition, a preliminary experiment is conducted to establish the correspondence between the process parameters and the carbon source input pulse frequency. For example, under different carrier gas flow rates and temperatures, the pulse retention degree, timing offset, and product growth effect when the carbon source pulse reaches the plasma interaction zone or growth zone are examined, thereby obtaining a frequency selection table suitable for this reactor structure.

[0049] Furthermore, with a fixed reactor geometry, if an increase in carrier gas flow rate leads to a shortening of the carbon source pulse transport time, a higher or lower target frequency can be set to maintain the effective resolution of the carbon source pulse in the action or growth region. The specific selection depends on the pre-calibrated correspondence of the device. If an increase in reaction temperature or a change in gas composition causes changes in diffusion mixing characteristics, the target frequency can also be adjusted accordingly. In this way, the timing relationship between the carbon source input pulse and the plasma pulse can be maintained under different operating conditions.

[0050] The phase difference is preferably 90° to 180°, the pulse frequency can be 1 to 10 kHz, and the duty cycle can be 10% to 50%. Under these conditions, it is ensured that the carbon source gas does not substantially change to a continuous supply state due to diffusion and mixing during transport, thereby maintaining the pulse on / off characteristics.

[0051] The combination of the above spatial layout and temporal control allows the carbon source gas to enter the plasma interaction zone in a pulsed manner after entering the reactor, and to form a temporal coupling relationship with the plasma output window in the time dimension, which can be connected, spaced, or partially overlapped, thereby realizing the time-division regulation of carbon source activation, active species transport, and carbon nanotube growth.

[0052] Figure 4 Correspondence between elements in the text: 1. Reactor body: tubular reactor, horizontally arranged.

[0053] 2. Plasma generation zone: Within the dashed box, connect the pulsed plasma power supply.

[0054] 3. Plasma interaction region: within the dashed box, adjacent to the carbon source introduction point.

[0055] 4. Carbon source inlet tube: The tip points towards the plasma interaction zone, and the distance between it and the growth zone is ≤10 mm.

[0056] 5. Growth zone: The area where the catalyst substrate is placed.

[0057] 6. Timing controller: Simultaneously controls the carbon source pulse valve and plasma power supply to generate synchronous but phase-adjustable pulse signals.

[0058] 7. Pulse timing diagram: showing the time misalignment (phase difference) between the carbon source pulse and the plasma pulse.

[0059] Example 1

[0060] (1) Catalyst preparation: Cobalt nitrate and ammonium molybdate solution (Co:Mo molar ratio = 1:0.3) was loaded onto porous MgO powder (specific surface area 250 m²) using the equal volume impregnation method. 2 / g, with an average pore size of 10 nm). After drying, it was calcined in air at 550°C for 3 hours to obtain the catalyst precursor.

[0061] (2) Pre-activation: Take 0.2 g of precursor in a quartz boat and place it in a tube furnace. Introduce 200 sccm of H2 / Ar (H2 accounts for 20%), raise the temperature to 450 ℃ at 10 ℃ / min, and keep it at the same temperature for 1.5 hours. Then cool it with the furnace.

[0062] (3) Carbon nanotube growth: The catalyst was pushed to the growth zone at 700 °C. 200 sccm of Ar was introduced, followed by carbon source gas (C2H4:CH4=1:3, total flow rate 60 sccm). Pulsed plasma was simultaneously activated (frequency 5 kHz, duty cycle 30%, power 250 W), and its pulse was set to be 120° phase-delayed relative to the carbon source pulse. The reaction pressure was 2 kPa, and the growth time was 30 minutes.

[0063] Example 2

[0064] Differences from Example 1: Co:Mo molar ratio = 1:0.4, pre-activation temperature 480℃, growth temperature 680℃, plasma frequency 2 kHz, duty cycle 40%.

[0065] Example 3

[0066] Differences from Example 1: Pre-activated H2 concentration 30%, carbon source is pure C2H4 (40 sccm), plasma frequency 8 kHz, duty cycle 20%, phase difference 180°, and growth at atmospheric pressure.

[0067] Example 4

[0068] Difference from Example 1: A pore size of 15 nm and a specific surface area of ​​180 m² were used. 2 / g of MgO support, pre-activated for 2 hours, and grown for 45 minutes.

[0069] Example 5

[0070] The difference from Example 1 is that the catalyst was prepared by a stepwise impregnation method, and the phase difference between the plasma and the carbon source pulse was 90°.

[0071] Comparative Example 1 (without pre-activation) The catalyst precursor was grown directly without pre-activation (other aspects are the same as in Example 1).

[0072] Comparative Example 2 (Continuous Plasma) A continuous wave plasma with a power of 250 W was used instead of pulsed plasma (other aspects are the same as in Example 1).

[0073] Comparative Example 3 (Single Metal Co Catalyst) The catalyst contains only Co and no Mo (other aspects are the same as in Example 1).

[0074] The following is an analysis of the results of Examples 1-5 and Comparative Examples 1-3:

[0075] Results and discussion Data from Examples 1-5 demonstrate that, under the preferred experimental system employed in this application, by introducing a timing sequence between the carbon source pulse input and plasma pulse output during the vapor deposition process, single-walled carbon nanotubes with a semiconductor content exceeding 80%, fewer defects, and higher yield can be stably prepared. In the original data, the semiconductor content in Examples 1-5 was 83%–87%, and the average Raman spectrum I... G / I D The values ​​of 32 to 43 indicate that the timing control approach of this invention has good stability and repeatability.

[0076] Comparative Example 1 Results: The product contained carbon black, and the carbon nanotubes were short and uneven. G / I DThe concentration dropped sharply to 18, the semiconductor content was only 55%, the TGA ash content was as high as 12 wt%, and the yield was 3.0 g / g-cat·h. This indicates that preactivation is crucial for the formation of a highly active and selective catalyst. It should be noted that these results are used to illustrate the beneficial role of preactivation as a preferred embodiment and do not imply that the timing control concept of the present invention depends solely on this preactivation step.

[0077] Comparative Example 2 Results: Carbon nanotubes showed signs of twisting. G / I D The initial oxidation temperature was reduced to 24°C, the semiconductor content was 73%, the initial oxidation temperature decreased, and the yield was 4.2 g / g-cat·h. This demonstrates that continuous plasma introduces defects and impairs selectivity. Comparative Example 2 shows that, compared to continuous mode, pulsed plasma mode significantly reduces damage to grown carbon nanotubes while providing activation energy, making it a better method for obtaining high Ig content. G / I D Value and high selectivity are key.

[0078] Comparative Example 3 results: When using a single-metal Co catalyst, the tube diameter distribution widens. The semiconductor content is only 48%. This result indicates that in the preferred system used in this application, Mo plays a key regulatory role in chiral selectivity within the bimetallic system. Comparative Example 3 highlights the synergistic effect of the Co-Mo bimetallic system, which is beneficial for further improving the selectivity of semiconductor single-walled carbon nanotubes. This conclusion is also used to illustrate the effect of the preferred catalytic system and does not constitute a limitation on the scope of application of the method of this invention.

[0079] It should be understood that the above embodiments only illustrate the technical solution of the present invention using chemical vapor deposition of single-walled carbon nanotubes as an example, and do not constitute a limitation on the scope of application of the present invention. The key to the present invention does not lie in a specific catalyst system, support type, or single carbon nanomaterial target product itself, but in the staggered timing of carbon source input and plasma interaction during the vapor-phase growth process, thereby controlling the activation, transport, deposition, and structural reconstruction processes of the carbon precursor in the time dimension. Based on the same mechanism, it can be expected that in other processes for preparing carbon nanomaterials using the vapor-phase deposition mechanism, as long as the non-completely synchronous temporal coupling between the carbon source pulse input and the plasma pulse output can be achieved, and the carbon source maintains the corresponding pulse on / off characteristics after entering the reaction zone, the effects of improved growth selectivity, increased structural order, reduced defect density, and / or optimized yield can also be obtained.

[0080] For example, in the vapor deposition preparation of multi-walled carbon nanotubes, double-walled carbon nanotubes, few-walled carbon nanotubes and other one-dimensional carbon nanomaterials, the time-sequential misalignment coordination method described in this invention is also expected to reduce disordered deposition and structural damage caused by continuous high-energy particle bombardment by adjusting the generation and deposition process of highly active carbon species in a time-sequential manner, and improve the graphitization degree, crystal integrity, tube diameter distribution, purity and yield of the material.

[0081] Furthermore, in thermochemical vapor deposition, plasma-enhanced chemical vapor deposition, remote plasma-enhanced chemical vapor deposition, microwave plasma-assisted deposition, and other preparation processes involving the coupling of carbon source pyrolysis and surface deposition, the timing-dislocation coordination method described in this invention can be introduced as an independent process control measure. Even if there are differences in catalyst composition, substrate type, reaction pressure, carbon source type, plasma excitation method, and pulse parameters in specific processes, as long as a non-completely synchronous coupling relationship can be formed between the carbon source supply window and the plasma action window, the same technical inspiration and beneficial effects as this invention can usually be obtained.

[0082] In some embodiments, the carbon source input pulse and the plasma output pulse can be adjusted by the phase difference; in other embodiments, the timing misalignment can also be achieved by controlling the pulse start time, duration, duty cycle, or other equivalent time parameters. Therefore, the focus of this invention is on the timing coordination mechanism between the carbon source input and the plasma interaction, rather than being limited to a specific device parameter or a single pulse waveform.

[0083] In summary, this invention, through the ingenious combination of pulsed plasma-assisted synthesis, successfully solves the problem of highly selective preparation of semiconductor single-walled carbon nanotubes, providing an ideal material preparation solution for related applications.

Claims

1. A method for preparing carbon nanotubes by sequentially coordinating a carbon source with plasma, characterized in that, Includes the following steps: The catalyst is placed in the growth zone of a tubular reactor. During the vapor deposition process, carbon source gas is introduced into the reactor in a pulsed manner, and pulsed plasma is applied so that the plasma diffuses downstream with the carrier gas from its origin and maintains its activity to form a plasma interaction zone. Among them, the carbon source input pulse and the plasma output pulse are controlled by the same timing controller under the same clock reference. The two are staggered in time, so that the carbon source supply window and the plasma action window are not completely synchronized in time coupling relationship, so as to regulate the carbon source activation, transport, deposition and structural reconstruction process. Furthermore, the carbon source gas is introduced into the reactor from the upstream boundary of the plasma interaction zone or from the side wall adjacent to the plasma interaction zone, with its introduction direction perpendicular to the reactor axis or inclined along the gas flow direction; the distance between the carbon source gas introduction point and the growth zone is controlled within 10 mm, and the straight-line distance between the carbon source gas introduction point and the plasma excitation source is not greater than 50 mm, so that the carbon source gas maintains a recognizable pulse on / off characteristic before reaching the growth interaction zone.

2. The preparation method according to claim 1, characterized in that, The output pulse of the pulsed plasma has the same frequency and duty cycle as the input pulse of the carbon source gas, and the temporal misalignment is adjusted by the phase difference.

3. The preparation method according to claim 2, characterized in that, The phase difference is 90° to 180°.

4. The preparation method according to claim 3, characterized in that, The frequency is 1 to 10 kHz; the duty cycle of the carbon source input pulse is 10% to 50%.

5. The preparation method according to claim 4, characterized in that, The frequency of the carbon source input pulse is preset according to the carrier gas flow rate, reaction temperature, and gas composition.

6. The preparation method according to claim 1, characterized in that, The growth region is located downstream or inside the plasma interaction region and is used to place the catalyst substrate or the support for the catalyst.

7. The preparation method according to claim 1, characterized in that, The identifiable pulse on / off characteristic is that the fluctuation amplitude of the carbon source gas concentration when it reaches the growth zone is not less than 50% of its peak value.

8. The preparation method according to claim 1, characterized in that, The carbon source gas is a mixture of ethylene and methane, with a volume ratio of C2H4:CH4 = 1:(2-4).

9. The preparation method according to claim 1, characterized in that, The catalyst is prepared by loading a Co-Mo bimetallic catalyst onto a porous MgO support and pretreating it at 400-500°C for 1-2 hours in an H2 / Ar mixed atmosphere with a volume fraction of 10-30%.

10. A carbon nanotube, characterized in that, The carbon nanotubes prepared by the method of claim 9 are single-walled carbon nanotubes, and the content of their semiconductor carbon nanotubes is greater than or equal to 83%.

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