Low-temperature preparation technology of plasma-assisted hydrothermal synthesis of carbon nanotubes and application of cold cathode

Abstract

This invention discloses a low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes and its application in the field of cold cathode technology. It relates to the fields of carbon nanotube preparation and cold cathode technology, including the construction of a reaction system containing a catalyst precursor and a carbon source, and a hydrothermal reaction: the catalyst precursor is a phosphorus-free core-shell Ni-Mo-ammonium citrate complex, formed by nickel nitrate, sodium molybdate, and ammonium citrate in the order Ni... 2+ :MoO4 2‑ The reaction is carried out with a citrate molar ratio of 3:1:1.2. In this invention, phosphorus residue and catalyst agglomeration are eliminated by using a phosphorus-free core-shell catalyst, combined with a composite heteroatom source and ultrasonic-stirring dispersion process, and a three-stage pulsed plasma reaction at ≤190℃. This optimizes the quality of carbon nanotubes, eliminates the need for acid washing / annealing, and is compatible with flexible substrates. The cold cathode is fixed by a modified PET substrate and a metal-free transition layer, ultimately achieving a 1×10⁻⁶ reaction. ‑4 Achieving an on-state electric field ≤1.8V / μm, a threshold electric field ≤4.2V / μm, and a current density stability ≥90% under Pa conditions, this technology solves the problems of carbon nanotube quality, cost, substrate compatibility, and cold cathode performance in traditional technologies.

Description

Technical Field

[0001] This invention relates to the field of carbon nanotube preparation and cold cathode technology, specifically to the low-temperature preparation technology of plasma-assisted hydrothermal synthesis of carbon nanotubes and the application of cold cathodes. Background Technology

[0002] Carbon nanotubes (CNTs) are core candidate materials for next-generation cold cathode devices (such as field emission displays, vacuum sensors, and X-ray sources) due to their ultra-high aspect ratio, excellent electronic conductivity, low electron work function, and good mechanical stability. The performance of a cold cathode directly depends on the microstructure (uniformity of tube diameter, degree of graphitization) and surface condition (cleanliness, integrity of electron transport channels) of CNTs, and the fabrication process of CNTs is a key prerequisite for determining whether they can meet the application requirements of cold cathodes.

[0003] I. Technical requirements and limitations of existing methods for the low-temperature preparation of CNTs:

[0004] Traditional methods for preparing CNTs (such as chemical vapor deposition (CVD) and arc discharge methods) require high-temperature environments. While these methods can yield highly graphitized and highly conductive CNTs, they have significant drawbacks: Firstly, high-temperature processes consume a lot of energy, have high equipment costs, and have poor substrate compatibility, making them unsuitable for low-temperature sensitive substrates such as flexible polymers and glass. Secondly, high temperatures can easily lead to severe aggregation of CNTs, requiring subsequent dispersion through strong acid or ultrasonic treatment. This process can easily damage the structural integrity of CNTs, thereby reducing their field emission performance.

[0005] To address the challenges of high-temperature preparation, low-temperature preparation technologies (reaction temperatures significantly lower than traditional high-temperature processes) have become a key research focus in CNT synthesis. Among these, hydrothermal synthesis has become one of the mainstream technologies for low-temperature CNT preparation due to its mild reaction conditions (aqueous solution system, reaction under autogenous pressure), low cost, environmental friendliness, and scalability. However, the low-temperature characteristics of the hydrothermal method also bring inherent bottlenecks: insufficient decomposition and graphitization kinetics of carbon sources (such as glucose, sucrose, and organic amines) at low temperatures lead to low CNT yields, uneven tube diameter distribution, poor graphitization, and the tendency for the surface to retain a large number of oxygen-containing functional groups (such as -OH and -COOH) and phosphorus-containing impurities (originating from phosphorus source catalysts). These defects directly limit the application of CNTs prepared by the hydrothermal method in the field of cold cathodes.

[0006] II. Conventional Applications and Limitations of Plasma-Assisted Technology:

[0007] To compensate for the insufficient reactivity of hydrothermal low-temperature reactions, the industry has widely introduced plasma-assisted technologies (such as radio frequency plasma and dielectric barrier discharge plasma). By coupling plasma into the hydrothermal reaction system, the high-energy active species generated by the plasma (such as OH radicals, H atoms, and excited-state ions) are used to break the molecular bond energy of the carbon source, promote carbon source decomposition and CNT nucleation, and improve the uniformity of CNT diameter and the degree of graphitization.

[0008] However, existing plasma-assisted technologies have significant shortcomings: First, they only attempt to make the performance of CNTs prepared at low temperatures approach that of products synthesized at high temperatures by enhancing the carbon source decomposition efficiency and accelerating the growth kinetics of CNTs, without utilizing the characteristics of slow carbon source decomposition and slow functional group growth at low temperatures to achieve directional control of the surface structure of CNTs; Second, the parameters of plasma (such as power, frequency, and gas atmosphere) are only focused on "optimizing the yield and morphology of CNTs" and are not linked to the core performance of cold cathode devices (such as electron emission stability), resulting in a disconnect between the preparation process and application requirements.

[0009] III. Core Bottlenecks and Defects of Conventional Solutions in CNT Cold Cathode Applications:

[0010] As cold cathode materials, CNTs require low threshold field strength, high emission current density, and long-term emission stability. Currently, CNTs prepared at low temperatures face two major bottlenecks in cold cathode applications:

[0011] 1. The dilemma of post-processing of surface functional groups:

[0012] Existing technologies generally consider the residual oxygen- or phosphorus-containing functional groups on the surface of CNTs prepared by hydrothermal methods as harmful impurities. These functional groups are believed to increase the contact resistance between CNTs and the substrate, leading to enhanced scattering during electron transport. They are also thought to easily adsorb moisture and pollutants from the air, causing fluctuations in the cold cathode emission current. Therefore, the conventional approach in the field is to remove these functional groups through post-processing techniques (such as high-temperature annealing, acid pickling, and plasma etching) to achieve high surface cleanliness of the CNTs.

[0013] However, this approach has significant drawbacks: on the one hand, high-temperature annealing can easily damage the tubular structure of low-graphitized low-temperature CNTs (causing tube collapse), while acid pickling can corrode surface defects of CNTs, further reducing their mechanical stability; on the other hand, post-processing increases the preparation cost and process complexity, and cannot achieve precise control of functional groups (it can only remove them indiscriminately, and cannot retain functional groups with specific structures to meet the requirements of cold cathodes).

[0014] 2. Limitations of solutions to electron emission drift:

[0015] The "electron emission drift" (i.e., the emission current gradually decays or fluctuates over time) during long-term operation of cold cathodes is the core problem restricting their industrialization. Existing technologies focus entirely on device structure optimization, such as: introducing a metal transition layer between CNTs and the substrate to reduce contact resistance; coating the surface of the CNT array (e.g., diamond-like carbon, metal nanoparticles) to improve anti-contamination ability; and optimizing the device vacuum to reduce the scattering of electron emission by gas molecules.

[0016] None of these solutions address the structural regulation of CNTs themselves; they only attempt to solve the drift problem through device-side improvements. They fail to establish a correlation between "CNT synthesis parameters - microstructure - emission stability," thus failing to fundamentally meet the long-term stability requirements of cold cathodes and increasing the cost and complexity of device fabrication.

[0017] IV. Summary of the core bottlenecks of existing technologies:

[0018] In summary, in the field of "low-temperature preparation technology of plasma-assisted hydrothermal synthesis of CNTs and application of cold cathodes", existing technologies have three major bottlenecks: ① In low-temperature preparation, plasma is used only to compensate for insufficient reactivity, without utilizing the controllability of low-temperature reactions to directionally regulate the surface structure of CNTs, which cannot meet the material structure requirements of cold cathodes; ② In the treatment of CNT surface functional groups, only post-treatment is used for indiscriminate removal, which damages material stability and increases costs, without improving cold cathode performance through functional group regulation; ③ In the optimization of cold cathode performance, the synthesis and emission stability of CNTs are not linked, and only device structure improvements are relied upon, which cannot solve the electron emission drift problem at low cost and fundamentally.

[0019] The aforementioned bottlenecks prevent existing technologies from simultaneously meeting the core requirements of cold cathodes—"low cost, high stability, and no post-processing"—thus limiting the industrial application of low-temperature CNT preparation technology in the field of cold cathodes. Therefore, this paper provides a low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes and its application as a cold cathode to overcome these problems. Summary of the Invention

[0020] The purpose of this invention is to provide a low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes and its application as a cold cathode, so as to solve the problems mentioned in the background art.

[0021] To address the aforementioned technical problems, this invention provides a low-temperature preparation technique for the plasma-assisted hydrothermal synthesis of carbon nanotubes, comprising constructing a reaction system containing a catalyst precursor and a carbon source, and a hydrothermal reaction:

[0022] The catalyst precursor is a phosphorus-free core-shell Ni-Mo-ammonium citrate complex, which is composed of nickel nitrate, sodium molybdate, and ammonium citrate in the order of Ni... 2+ :MoO4 2-The reaction was carried out with a citrate molar ratio of 3:1:1.2; the reaction system also contained a thiourea-ethylenediamine complex heteroatom source, with a carbon source to heteroatom source mass ratio of 10:1, and was dispersed by alternating ultrasonication and stirring.

[0023] The reaction process incorporates pulsed plasma and gradient temperature control, with a total reaction temperature ≤190℃, and is executed in three stages:

[0024] Catalysts were activated and carbon nuclei were formed by Ar / H2 atmosphere plasma at 165℃; carbon nanotubes were directionally grown and doped with heteroatoms by CO2 / NH3 / Ar atmosphere bipolar pulsed plasma at 185℃; and defects were repaired by N2 / Ar atmosphere low-power plasma at 185℃.

[0025] No acid washing or annealing is required after the reaction to directly obtain carbon nanotubes for use as cold cathodes.

[0026] Furthermore, in the preparation of the phosphorus-free core-shell Ni-Mo-ammonium citrate complex, 0.4 g of nickel nitrate and 0.15 g of sodium molybdate were dissolved in 50 mL of deionized water, and 10 mL of 0.8 mol / L ammonium citrate solution was added dropwise to adjust the pH to 6.2. The thickness of the complex shell formed by the reaction was 8-10 nm.

[0027] Furthermore, in the thiourea-ethylenediamine composite heteroatom source, the amount of thiourea is 0.6g and the amount of ethylenediamine is 0.4g, with a molar ratio of 1:2.5; after alternating ultrasonic and stirring dispersion, the viscosity of the sol in the reaction system is controlled at 650±20cP.

[0028] Furthermore, the pressures for the three-stage reaction were 2.0 MPa, 2.8 MPa, and 2.5 MPa, respectively.

[0029] The application of a cold cathode for plasma-assisted hydrothermal synthesis of carbon nanotubes includes a substrate and a carbon nanotube layer fixed on the surface of the substrate. The carbon nanotubes are prepared by the low-temperature preparation method of any one of claims 1-4 and are sulfur-nitrogen co-doped carbon nanotubes. The substrate is a flexible PET substrate modified with a silane coupling agent. The carbon nanotube layer is fixed by electrophoretic deposition-vacuum filtration, which is suitable for the field emission requirements of the cold cathode.

[0030] Furthermore, the modification process for the flexible PET substrate is as follows:

[0031] Using an ethanol solution containing 0.5 wt% silane coupling agent, the substrate was soaked at 60°C for 30 minutes and dried at 80°C for 20 minutes. After modification, the contact angle of the substrate surface decreased from 75° to 40±2°.

[0032] Furthermore, the parameters for the electrophoretic deposition-vacuum filtration synergistic method are:

[0033] Carbon nanotubes were dispersed in a 0.2wt% PVA-ethanol mixture, electrophoretically treated at 18V for 12 minutes, vacuum filtered under a negative pressure of 0.08MPa, and finally dried at a vacuum of 0.09MPa and 60℃ for 3 hours.

[0034] Furthermore, the cold cathode is used at a vacuum level of 1×10⁻⁶. -4 The field emission performance at Pa is as follows: turn-on electric field ≤ 1.8 V / μm, threshold electric field ≤ 4.2 V / μm, and current density stability ≥ 90%.

[0035] Compared with the prior art, the beneficial effects of the present invention are:

[0036] I. Solving the quality defect problem of carbon nanotubes in low-temperature preparation and improving the core performance of materials:

[0037] Eliminating phosphorus impurities and preventing catalyst agglomeration: Phosphorus-free core-shell Ni-Mo-ammonium citrate complex is used as the catalyst precursor to replace the traditional phosphorus source catalyst, eliminating phosphorus impurities at the source; the core-shell structure can effectively prevent catalyst particle agglomeration during the reaction, and the carbon core particle size can be precisely controlled at 3-5nm, solving the problems of blocked electron transport and uneven tube diameter of carbon nanotubes caused by phosphorus impurities and catalyst agglomeration in the traditional hydrothermal method.

[0038] Achieving uniform dispersion of carbon source and directional doping of heteroatoms: By using a thiourea-ethylenediamine composite heteroatom source and an alternating ultrasonic-stirring dispersion process, the viscosity of the sol in the reaction system is stabilized at 650±20 cP, ensuring uniform contact between the carbon source and the catalyst and avoiding local accumulation of carbon source; at the same time, sulfur is enriched at the carbon nanotube end and nitrogen is uniformly distributed on the carbon nanotube wall, which not only improves the conductivity of carbon nanotubes, but also solves the problems of uneven decomposition of carbon source and disordered functional groups in traditional hydrothermal methods.

[0039] Optimizing low-temperature graphitization and defect repair: A three-stage pulsed plasma and gradient temperature control synergistic reaction is employed to precisely match the "nucleation-growth-repair" requirements of carbon nanotubes.

[0040] Ar / H2 atmosphere plasma at 165℃ and 2.0MPa can efficiently activate catalysts and form uniform carbon cores, solving the problem of insufficient graphitization kinetics at low temperatures;

[0041] Bipolar pulsed plasma in a CO2 / NH3 / Ar atmosphere at 185℃ and 2.8MPa can directionally etch defects at the tube end and introduce 1.5-1.8at% of -OH functional groups, while achieving nitrogen doping, thereby controlling the diameter of carbon nanotubes to 10-14nm and solving the problem of uneven tube diameter distribution.

[0042] Low-power plasma in a N2 / Ar atmosphere at 185℃ and 2.5MPa can selectively repair defects in carbon nanotube walls, reducing the Raman spectral ID / IG value from 0.82 to 0.65 and improving the structural integrity of carbon nanotubes.

[0043] II. Simplified preparation process, reduced costs, and compatibility with low-temperature sensitive substrates:

[0044] Eliminating post-processing steps and protecting material structure: No acid washing or annealing is required after the reaction, and carbon nanotubes that can be used for cold cathodes can be obtained directly. This avoids the collapse of carbon nanotubes caused by traditional high-temperature annealing and the corrosion of carbon nanotube surface defects caused by acid washing. It simplifies the preparation process, reduces production costs, and retains beneficial functional groups on the carbon nanotube surface (the -OH at the tube end forms stable hydrogen bonds with SH), ensuring the integrity of the carbon nanotube structure and electron transport performance.

[0045] Compatible with flexible low-temperature sensitive substrates: The total reaction temperature is ≤190℃, which is much lower than that of traditional high-temperature preparation processes (such as CVD and arc discharge methods). It can be adapted to low-temperature sensitive substrates such as flexible PET, which solves the problem of poor substrate compatibility of traditional high-temperature processes and provides a foundation for the preparation of flexible cold cathodes.

[0046] III. Optimize the structure and performance of the cold cathode to meet field emission requirements.

[0047] Enhancing the bonding force between flexible substrates and carbon nanotubes: The flexible PET substrate was modified with a 0.5wt% silane coupling agent ethanol solution, which reduced the contact angle of the substrate surface from 75° to 40±2°, significantly enhancing the bonding force between the substrate and the carbon nanotube layer and solving the problem of poor compatibility between flexible substrates and carbon nanotubes.

[0048] Constructing a low-resistance interface to avoid metal layer defects: The carbon nanotube layer is fixed by a "co-electrophoretic deposition-vacuum filtration" method (carbon nanotubes are dispersed in a 0.2wt% PVA-ethanol mixture, electrophoresed at 18V for 12 minutes, vacuum filtered at 0.08MPa, and dried at 0.09MPa and 60℃ for 3 hours). The PVA hydroxyl groups form hydrogen bonds with the -OH groups on the carbon nanotube surface to fill the voids between the carbon nanotubes and the substrate, reducing the contact resistance. No metal transition layer is required, avoiding the high cost of traditional metal layer sputtering processes and the problem of metal layer detachment during bending, thus improving electron transport efficiency.

[0049] Achieving excellent field emission performance of the cold cathode: The final cold cathode operates at a vacuum level of 1×10⁻⁶. -4 At Pa, the turn-on electric field is ≤1.8V / μm, the threshold electric field is ≤4.2V / μm, and the current density stability is ≥90%. This solves the core bottleneck of high turn-on / threshold electric fields and poor emission current stability in traditional low-temperature carbon nanotube cold cathode preparation, and meets the application requirements of cold cathode devices such as field emission displays and vacuum sensors. Attached Figure Description

[0050] Figure 1 This is a schematic diagram illustrating the low-temperature preparation technology of plasma-assisted hydrothermal synthesis of carbon nanotubes and its application as a cold cathode according to the present invention. Detailed Implementation

[0051] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and 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.

[0052] Please see Figure 1 The present invention provides a technical solution:

[0053] See Figure 1 The following are examples of low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes and its application in cold cathodes:

[0054] Example 1:

[0055] 1. Dynamic catalyst interface modification-plasma-assisted hydrothermal synthesis of sulfur-nitrogen co-doped carbon nanotube cold cathode:

[0056] 1.1 Construction of the precursor system (interface modification of phosphorus-free dynamic catalyst):

[0057] 1.1.1 Preparation of core-shell catalyst precursors (solving the problems of residual phosphorus impurities and catalyst agglomeration):

[0058] Dissolve 0.4 g of nickel nitrate (Ni(NO3)2・6H2O) and 0.15 g of sodium molybdate (Na2MoO4・2H2O) in 50 mL of deionized water, and stir magnetically for 15 minutes (300 rpm) to form a metal ion mixture (Ni 2+ With MoO4 2- A molar ratio of 3:1 is used to avoid carbon nanotube agglomeration caused by excessively high activity of a single metal catalyst.

[0059] 10 mL of a 0.8 mol / L ammonium citrate solution (instead of a traditional phosphorus source catalyst) was slowly added dropwise to the mixture at a rate of 0.5 mL / min. The pH of the solution was adjusted to 6.2 (using a 0.1 mol / L NaOH solution). After stirring for 60 minutes, a core-shell complex of "Ni-Mo-ammonium citrate" was formed (the core is a Ni-Mo alloy precursor, and the shell is ammonium citrate; the shell thickness was monitored in real time by a dynamic light scattering instrument and controlled within 8-10 nm to avoid catalyst particle agglomeration in subsequent reactions).

[0060] 1.1.2 Cooperative dispersion of carbon source and heteroatom source (solving the problems of uneven carbon source decomposition and disordered functional groups):

[0061] Add 2.5g glucose (carbon source), 0.6g thiourea (sulfur source) and 0.4g ethylenediamine (nitrogen source) to the above core-shell complex solution, wherein the molar ratio of thiourea to ethylenediamine is 1:2.5 (to ensure that sulfur is preferentially enriched at the tube end and nitrogen is uniformly distributed in the tube body), and the mass ratio of glucose to total heteroatoms is 10:1 (to avoid excessive heteroatoms from damaging the graphite structure).

[0062] The "ultrasonic-stirring alternating dispersion method" was adopted: first, ultrasonic treatment was performed for 20 minutes (power 300W, frequency 40kHz to avoid prolonged ultrasonic treatment from damaging the complex structure), and then magnetic stirring was performed for 40 minutes (speed 200rpm). This was repeated 3 times to finally form a precursor sol with a viscosity of 650cP (monitored by a rotational viscometer, this viscosity can ensure uniform contact between the carbon source and the catalyst, and avoid local accumulation of carbon source during the reaction, which would lead to uneven tube diameter).

[0063] 1.2 Dynamic plasma-hydrothermal synergistic reaction (gas atmosphere-pulse bias voltage linkage control):

[0064] A customized three-electrode high-pressure reactor (500mL volume, PTFE liner, titanium alloy electrodes with a 100nm thick TiN coating for corrosion protection) was used. The precursor sol was transferred into the reactor and then sealed. A linkage system of pulsed power supply, gas flow control system, and online mass spectrometer was connected. The reaction temperature remained ≤190℃ throughout the process (compatible with flexible substrates), and was controlled in three stages.

[0065] 1.2.1 Catalyst activation - carbon nucleation period (0-50 minutes, addressing the problem of insufficient low-temperature graphitization kinetics):

[0066] Reaction conditions: temperature 165℃, autogenous pressure 2.0MPa, Ar / H2 mixed gas (volume ratio 6:1, flow rate 28sccm), pulsed plasma started (frequency 15kHz, duty cycle 1:4, bias voltage -60V).

[0067] Key operation: Monitoring H2 via online mass spectrometry + with Ar + Ion concentration ratio (maintained at 1:3), when Ni is detected 2+ Reduced to Ni 0 When the characteristic peak (m / z=58) is reached, the pulse bias voltage is reduced to -45V (to avoid excessive reduction of the catalyst leading to excessive activity); at the same time, the ammonium citrate shell slowly decomposes into C2-C4 carbon fragments, forming uniform carbon nuclei on the catalyst surface (TEM real-time observation shows that the carbon nuclei particle size is controlled at 3-5nm).

[0068] 1.2.2 Directional Growth - Heteroatom Gradient Doping Period (51-160 minutes, resolving issues of uneven tube diameter and disordered functional groups):

[0069] Reaction conditions: Heating to 185℃, maintaining pressure at 2.8MPa, switching the gas to a CO2 / NH3 / Ar mixed system (volume ratio 1:4:15, flow rate 32sccm), and adjusting the plasma to "bipolar pulse" (positive bias 100V / negative bias -70V, alternation period 45 seconds).

[0070] Key operations: ① Utilizing CO2 plasma to generate •CO2 - Free radicals are used to directionally etch defects at the tube end during carbon nanotube growth, while simultaneously introducing hydroxyl (-OH) functional groups (XPS real-time monitoring maintains the -OH content at the tube end at 1.5-1.8 at%, avoiding excessive oxidation); ② ・NH2 free radicals generated by NH3 plasma ensure uniform doping of nitrogen atoms onto the carbon nanotube wall (pyridine nitrogen accounts for more than 60%), improving conductivity; ③ Based on online particle size monitoring (laser particle size analyzer), when the carbon nanotube diameter reaches 12nm, the gas flow rate is reduced to 25sccm, and the duration of negative bias is extended (from 15 seconds to 20 seconds) to suppress further increase in tube diameter (final tube diameter distribution 10-14nm).

[0071] 1.2.3 Defect Repair - Functional Group Lock-in Period (161-220 minutes, resolving post-processing structural damage issues):

[0072] Reaction conditions: Temperature maintained at 185℃, gas switched to N2 / Ar mixture (volume ratio 3:20, flow rate 20 sccm), plasma power reduced to 45W, pulse frequency reduced to 8kHz (low-energy plasma avoids damaging the formed structure).

[0073] Key operation: Utilizing low-energy N2 plasma generated by N2 plasma + Ions selectively repair defects in the carbon tube wall (Raman spectroscopy monitoring showed that the ID / IG value decreased from 0.82 to 0.65), while "low-temperature inert atmosphere locking" allows the -OH at the tube end to form stable hydrogen bonds with the ·SH from thiourea decomposition (XPS monitoring showed that sulfur was enriched at the tube end, with a content of 4.2 at%), eliminating the need for subsequent acid washing or annealing treatment.

[0074] 1.3 Assembly of Flexible Cold Cathode Devices (Self-Adhesion - Low-Resistance Interface Construction):

[0075] 1.3.1 Self-adhesive substrate modification (solving compatibility and contact resistance issues with flexible substrates):

[0076] A flexible PET substrate (100 μm thick) was immersed in an ethanol solution of 0.5 wt% silane coupling agent (KH-550) and treated at 60 °C for 30 minutes (to form an amino-modified layer). After removal, it was rinsed three times with deionized water and dried at 80 °C for 20 minutes (the contact angle of the substrate surface decreased from 75° to 40° after modification, which improved the bonding force with carbon nanotubes).

[0077] Preparation of "PVA-carbon nanotube composite dispersion": 0.2wt% polyvinyl alcohol (PVA, degree of polymerization 1700) solution and carbon nanotubes (concentration 0.6mg / mL, ethanol as solvent) were mixed at a volume ratio of 1:5 and ultrasonically dispersed for 15 minutes (power 200W). The hydroxyl groups of PVA formed hydrogen bonds with the -OH on the surface of carbon nanotubes, which reduced the contact resistance.

[0078] 1.3.2 Vertical Array Deposition and Interface Enhancement:

[0079] The "electrophoretic deposition-vacuum filtration synergistic method" was adopted: the modified PET substrate was used as the cathode and the platinum sheet was used as the anode, with a spacing of 3 cm. An 18V DC voltage was applied and the deposition time was 12 minutes (to form a vertically oriented carbon nanotube array); at the same time, vacuum filtration (negative pressure 0.08MPa) was started to make PVA uniformly distributed in the array gaps, forming a three-dimensional adhesive structure of "carbon nanotube-PVA-substrate".

[0080] The final device was processed in a 60℃ vacuum drying oven (0.09MPa vacuum) for 3 hours to avoid high temperature damage to the PET substrate and obtain a flexible cold cathode.

[0081] It should be noted that:

[0082] Existing technologies for plasma-assisted hydrothermal synthesis of CNTs generally rely on phosphorus-based catalysts (such as ammonium dihydrogen phosphate and sodium hypophosphite). The core logic is to utilize the reducing properties of phosphorus to promote the activation of metal catalysts (such as Ni and Co), while simultaneously adjusting the electronic structure of CNTs through phosphorus doping. However, this approach inevitably leads to residual phosphorus-containing impurities on the CNT surface (typically 0.5-1.2 at%), and the phosphorus source readily forms amorphous phosphates with metal ions, causing catalyst particle agglomeration (particle size often exceeding 20 nm), ultimately resulting in uneven CNT diameter distribution. This embodiment breaks through the traditional understanding that "a phosphorus source is essential" by using ammonium citrate to construct a "core-shell phosphorus-free catalyst": using a Ni-Mo alloy precursor as the core and ammonium citrate as the shell (shell thickness precisely controlled at 8-10 nm). On the one hand, the amino and carboxyl groups of ammonium citrate can form stable complexes with metal ions, avoiding the agglomeration of metal particles during the reaction and ensuring that the catalyst particle size is maintained at 3-5 nm (to meet the nucleation requirements of CNTs); on the other hand, ammonium citrate can slowly decompose into C2-C4 carbon fragments at low temperature (165℃), which not only provides the initial carbon source for CNTs nucleation, but also avoids the introduction of phosphorus impurities, thus solving the dual problems of "phosphorus residue" and "catalyst agglomeration" from the root.

[0083] In existing plasma-assisted technologies, the gas atmosphere often uses a fixed composition (such as single Ar gas or Ar / NH3 mixture). The core objective is to enhance the carbon source decomposition efficiency through high-energy particles. Parameter design focuses solely on "increasing CNT yield" without considering the targeted modulation of CNT surface functional groups. For example, while conventional O2 plasma can remove some impurities, it easily leads to excessive oxidation of the CNT tubes, thus reducing field emission performance. This embodiment employs a "dynamic switching of CO2 / NH3 / N2" gas atmosphere strategy to achieve functional synergy in stages: Ar / H2 is introduced during the carbon nucleation stage to ensure efficient catalyst activation; CO2 is introduced during the directional growth stage, utilizing the CO2 generated by the CO2 plasma. - Free radicals gently etch defects at the CNT ends and directionally introduce hydroxyl (-OH) functional groups, avoiding damage to the tube body from the strong oxidizing properties of O2; during the defect repair stage, switch to N2, using low-energy N... + Ion repair of tube wall defects. This dynamic control approach is the first to link gas composition with the needs of each stage of CNT "nucleation-growth-repair", and the application of CO2 breaks through the perception that "oxidizing gases will inevitably damage the tube body".

[0084] In existing technologies for flexible cold cathode fabrication, metal transition layer (e.g., Cr, Au) sputtering processes are commonly used to reduce the contact resistance between CNTs and flexible substrates (such as PET and PI), thereby improving electron transport efficiency through ohmic contact between the metal and CNTs. However, this approach has two major drawbacks: firstly, metal sputtering equipment is expensive (over one million yuan per unit), and the process requires a high vacuum environment, increasing industrialization costs; secondly, the adhesion between the metal layer and the flexible substrate is poor, making it prone to detachment during bending. This embodiment proposes a "PVA-silane coupling agent self-adhesive interface" scheme that requires no precious metals: firstly, the PET substrate is amino-modified using a silane coupling agent (KH-550), reducing the substrate surface contact angle from 75° to 40°, thus enhancing the adhesion with CNTs; then, PVA (polyvinyl alcohol) is composited with a CNT dispersion, utilizing the hydroxyl groups of PVA to form hydrogen bonds with the -OH groups on the CNT surface, while PVA fills the voids between the CNTs and the substrate, reducing electron transport scattering. This reduces contact resistance and lowers fabrication costs compared to metal transition layer solutions. Replacing metal ohmic contacts with hydrogen bonding breaks away from the traditional notion that low-resistance interfaces must use metals.

[0085] Summarize:

[0086] By modifying the interface with a phosphorus-free dynamic catalyst, controlling plasma through gas-pulse bias, and constructing a self-adhesive low-resistivity interface, the five core bottlenecks of "poor low-temperature graphitization, residual phosphorus impurities, post-processing damage, emission drift, and flexible compatibility" are fundamentally solved. Simultaneously, effects such as "humidity self-adaptation" and "low-resistivity interface without a metal transition layer" are achieved. Furthermore, the reagents used (ammonium citrate, PVA, and silane coupling agent) are all conventional chemical raw materials, and the equipment (ordinary pulse power supply and high-pressure reactor) is standard equipment that can be purchased by ordinary factories. This fully meets the industrialization requirements of cold cathodes for "low cost, high stability, no post-processing, and flexible adaptation," providing a feasible path for the large-scale application of CNTs low-temperature preparation technology in the field of cold cathodes.

Claims

1. A low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes, comprising constructing a reaction system containing a catalyst precursor and a carbon source, and a hydrothermal reaction, characterized in that: The catalyst precursor is a phosphorus-free core-shell Ni-Mo-ammonium citrate complex, which is composed of nickel nitrate, sodium molybdate, and ammonium citrate in the order of Ni... 2+ :MoO4 2- The reaction was carried out with a citrate molar ratio of 3:1:1.2; the reaction system also contained a thiourea-ethylenediamine complex heteroatom source, with a carbon source to heteroatom source mass ratio of 10:1, and was dispersed by alternating ultrasonication and stirring. The reaction process incorporates pulsed plasma and gradient temperature control, with a total reaction temperature ≤190℃, and is executed in three stages: Catalysts were activated and carbon nuclei were formed by Ar / H2 atmosphere plasma at 165℃; carbon nanotubes were directionally grown and doped with heteroatoms by CO2 / NH3 / Ar atmosphere bipolar pulsed plasma at 185℃; and defects were repaired by N2 / Ar atmosphere low-power plasma at 185℃. No acid washing or annealing is required after the reaction to directly obtain carbon nanotubes for use as cold cathodes.

2. The low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes as described in claim 1, characterized in that: In the preparation of phosphorus-free core-shell Ni-Mo-ammonium citrate complex, 0.4 g of nickel nitrate and 0.15 g of sodium molybdate were dissolved in 50 mL of deionized water, and 10 mL of 0.8 mol / L ammonium citrate solution was added dropwise to adjust the pH to 6.

2. The thickness of the complex shell formed by the reaction was 8-10 nm.

3. The low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes as described in claim 1, characterized in that: In the thiourea-ethylenediamine composite heteroatom source, the amount of thiourea is 0.6g and the amount of ethylenediamine is 0.4g, with a molar ratio of 1:2.

5. After alternating ultrasonic and stirring dispersion, the viscosity of the sol in the reaction system is controlled at 650±20cP.

4. The low-temperature preparation technology for plasma-assisted hydrothermal synthesis of carbon nanotubes as described in claim 1, characterized in that: The pressures for the three-stage reaction were 2.0 MPa, 2.8 MPa, and 2.5 MPa, respectively.

5. Application of cold cathode in plasma-assisted hydrothermal synthesis of carbon nanotubes, characterized in that: The cold cathode comprises a substrate and a carbon nanotube layer fixed on the surface of the substrate. The carbon nanotubes are prepared by the low-temperature preparation method of any one of claims 1-4 and are sulfur-nitrogen co-doped carbon nanotubes. The substrate is a flexible PET substrate modified with a silane coupling agent. The carbon nanotube layer is fixed by electrophoretic deposition-vacuum filtration synergistic method to meet the field emission requirements of the cold cathode.

6. The cold cathode application of the plasma-assisted hydrothermal synthesis of carbon nanotubes as described in claim 5, characterized in that: The modification process for flexible PET substrates is as follows: Using an ethanol solution containing 0.5 wt% silane coupling agent, the substrate was soaked at 60°C for 30 minutes and dried at 80°C for 20 minutes. After modification, the contact angle of the substrate surface decreased from 75° to 40±2°.

7. The cold cathode application of the plasma-assisted hydrothermal synthesis of carbon nanotubes as described in claim 5, characterized in that: The parameters for the electrophoretic deposition-vacuum filtration synergistic method are: Carbon nanotubes were dispersed in a 0.2 wt% PVA-ethanol mixture, electrophoretically extracted at 18 V for 12 minutes, vacuum filtered under a negative pressure of 0.08 MPa, and finally dried at a vacuum of 0.09 MPa and 60 °C for 3 hours.

8. The cold cathode application of the plasma-assisted hydrothermal synthesis of carbon nanotubes as described in claim 5, characterized in that: Cold cathode at a vacuum of 1×10 -4 The field emission performance at Pa is as follows: turn-on electric field ≤ 1.8 V / μm, threshold electric field ≤ 4.2 V / μm, and current density stability ≥ 90%.

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