Method for purifying carbon nanotubes through ammonium chloride assisted heat treatment

Abstract

The invention relates to the technical field of carbon nanotube purification, in particular to a method for purifying a carbon nanotube through ammonium chloride assisted heat treatment. The method comprises the following steps: firstly, carrying out low-temperature air burning treatment on a carbon nanotube film, removing amorphous carbon, and oxidizing metal catalyst particles into a metal oxide; carrying out heat treatment on the air-burnt carbon nanotubes and excessive ammonium chloride powder in an inert atmosphere, and heating ammonium chloride to decompose into hydrogen chloride and ammonia gas which react with metal oxide particles in the carbon nanotubes to generate corresponding metal chlorides; and carrying out high-temperature heat treatment in an inert atmosphere or vacuum to further remove the metal chloride. Liquid phase treatment processes such as acid pickling and water washing are not involved, the macroscopic morphology and the microstructure of the carbon nanotube film are kept, and the impurity content of the metal catalyst in the purified carbon nanotube film is reduced to be lower than 1wt.%.

Description

Technical Field

[0001] The invention relates to the technical field of carbon nanotube purification, in particular to a method for purifying carbon nanotubes through ammonium chloride-assisted heat treatment. Background Art

[0002] Carbon nanotubes (CNTs) can be regarded as quasi-one-dimensional hollow tubes formed by curling and closing a single or multiple layers of graphene along a certain direction. According to the number of layers of the tube wall, they can be divided into single-walled carbon nanotubes (SWCNTs), double-walled carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes (MWCNTs). Carbon nanotubes have high conductivity, high strength, high specific surface area and stable physical and chemical properties. They have broad application prospects in transistor devices, sensors, aerospace, energy storage, catalysis and other fields (Reference 1: Rao R, Pint C L, Islam AE, et al. Carbon Nanotubes and related nanomaterials: critical advances and challenges for synthesis toward mainstream commercial applications [J]. ACS Nano, 2018, 12 (12): 11756-84.). Single-walled carbon nanotube films have the characteristics of good flexibility and self-support, and have great application potential. However, the carbon nanotubes usually prepared contain impurities, including catalyst particles, amorphous carbon, etc. The presence of these impurities will affect the performance of carbon nanotubes, so the carbon nanotubes need to be purified.

[0003] Currently, the most commonly used purification method is liquid-phase acid treatment (Reference 2: Eatemadi A, Daraee H, Karimkhanloo H, et al. Carbon nanotubes: properties, synthesis, purification, and medical applications [J]. Nanoscale Research Letters, 2014, 9.). Although liquid-phase treatment technology has made significant progress in purifying powdered carbon nanotubes, it is not suitable for carbon nanotube thin films. This is because the liquid-phase treatment process severely damages the macromorphology and microstructure of the carbon nanotube film (film pore structure, flatness, roughness, specific surface area, etc.), thereby affecting the performance of the carbon nanotube film. In addition, the waste acid treatment is costly and easily pollutes the environment. The process of washing the carbon nanotubes to neutrality produces a large amount of wastewater. Another method for removing metal impurities is high-temperature purification. However, this method requires very high temperatures, consumes a lot of energy, and significantly damages the structure of single-walled carbon nanotubes (Reference 3: Yudasaka M, Ichihashi T, Kasuya D, et al. Structure changes of single-wall carbon nanotubes and single-wall carbon nanohorns caused by heat treatment [J]. Carbon, 2003, 41 (6): 1273-80.). Gas-phase purification of carbon nanotubes using gases such as chlorine (Cl2) or chloroform (CHCl3) can better maintain the structure of carbon nanotubes, but these gases are toxic or easily decompose into toxic gases, which is highly dangerous (Reference 4: Goak JC, Lim CJ, Hyun Y, et al. Efficient gas-phase purification using chloroform for metal-free multi-walled carbon nanotubes [J]. Carbon, 2019, 148: 258-66.).

[0004] Therefore, the main problem currently faced is: how to purify the single-walled carbon nanotube film without destroying its flexibility, structural integrity and uniformity, while reducing pollutant emissions and operating energy consumption. Summary of the Invention

[0005] The purpose of the present invention is to provide a safe, non-destructive, and efficient method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment. By combining low-temperature air oxidation with ammonium chloride heat treatment and vacuum / inert atmosphere heat treatment processes, a high-purity carbon nanotube film is obtained while maintaining the macromorphology and microstructure of the carbon nanotube film. This realizes the purification of ultra-thin carbon nanotube films for the first time, and a high-purity carbon nanotube film with unlimited thickness and unlimited size is obtained.

[0006] The technical solution of the present invention is:

[0007] A method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment comprises the following steps: firstly, the carbon nanotubes are subjected to low-temperature air calcination to remove amorphous carbon and simultaneously oxidize metal catalyst particles into metal oxides; then, the air-calcined carbon nanotubes are heat-treated together with excess ammonium chloride powder in an inert atmosphere; the ammonium chloride is thermally decomposed into hydrogen chloride and ammonia, which react with metal oxide particles in the carbon nanotubes to form corresponding metal chlorides; and finally, high-temperature heat treatment is performed in an inert atmosphere or in a vacuum to further remove the metal chlorides, thereby obtaining high-purity carbon nanotubes.

[0008] In the method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment, the carbon nanotubes are one or more of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes, and the forms include but are not limited to thin films, powders, or aerogels.

[0009] In the method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment, the metal catalyst includes but is not limited to iron (Fe), cobalt (Co) or nickel (Ni).

[0010] The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment includes low-temperature air burning treatment: transferring the carbon nanotubes to a tubular furnace and heating them in an air atmosphere at a heat treatment temperature of 400-500° C. for 1-2 hours.

[0011] The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment comprises placing the air-burned carbon nanotubes and ammonium chloride in a mass ratio of 1:10 to 1:500 in a quartz boat, placing the quartz boat in a quartz tube with an open end, and sealing the open end of the quartz tube with ceramic fiber wool; transferring the quartz tube into a tube furnace with the open end of the quartz tube facing the air inlet end of the tube, and performing heat treatment in an inert atmosphere at a heat treatment temperature of 400 to 900° C. and a heat treatment time of 1 to 3 hours.

[0012] The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment is as follows: the carbon nanotubes after the ammonium chloride heat treatment are transferred to a tubular furnace and subjected to high-temperature heat treatment in an inert atmosphere or vacuum: the heat treatment temperature in the inert atmosphere is 1050-1100°C, the heat treatment temperature in the vacuum is 800-1100°C, and the heat treatment time is 0.5-2h.

[0013] In the method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment, the inert atmosphere is an argon (Ar) or nitrogen (N2) atmosphere, and the vacuum is a gas pressure lower than 100 Pa.

[0014] The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment has a purity of the purified carbon nanotubes higher than 99 wt %, with the optimum value being 99.9 wt %.

[0015] In the method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment, the size and thickness of the carbon nanotube film are not limited, and the preferred range of the thickness of the carbon nanotube film is 20 nm to 1000 μm.

[0016] The design idea of ​​the present invention is:

[0017] The present invention adopts a gas phase purification method to remove amorphous carbon in carbon nanotubes through low-temperature air calcination treatment, and simultaneously oxidize metal catalyst particles into metal oxides. The air-calcined carbon nanotubes are heat-treated together with ammonium chloride powder in an inert atmosphere, and the high-boiling-point metal oxides are converted into low-boiling-point metal chlorides using hydrogen chloride gas generated by thermal decomposition of ammonium chloride. The metal chlorides are gasified and separated from the carbon nanotubes by high-temperature heat treatment in an inert atmosphere or in a vacuum, thereby obtaining a high-purity carbon nanotube film while maintaining the macroscopic morphology and microstructure of the carbon nanotube film.

[0018] The advantages and beneficial effects of the present invention are:

[0019] 1. The present invention converts high-boiling-point metal elements and oxides into low-boiling-point metal chlorides. The heat treatment temperature is lower than the reported high-temperature heat treatment temperature, thereby avoiding damage to the flexibility, structural integrity and uniformity of the carbon nanotube film and reducing energy consumption.

[0020] 2. The present invention has a wide range of applications and can be used to purify carbon nanotube films containing impurities of different metal catalysts. The film thickness and size are not limited. It is also applicable to carbon nanotube samples in the form of powder, aerogel, etc.

[0021] 3. The present invention does not involve liquid-phase treatment processes such as pickling and water washing, thus preserving the macromorphology and microstructure of the carbon nanotube film. After purification, the content of metal catalyst impurities in the carbon nanotube film is reduced to less than 1 wt.%. The purified carbon nanotube film of the present invention has a high purity, reaching up to 99.9 wt.%.

[0022] 4. The method of the present invention is simple, safe and highly reliable. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1Schematic diagram of the apparatus for purifying carbon nanotube films using ammonium chloride-assisted heat treatment. In the figure, 1 is a tubular furnace, 2 is a carbon nanotube film, 3 is a quartz frame, 4 is a quartz boat, 5 is a furnace tube, 6 is a ceramic fiber wool, 7 is ammonium chloride powder, 8 is a quartz tube, 9 is a vacuum pump gas line switch, 10 is a vacuum pump, 11 is a tail gas bottle gas line switch, and 12 is a tail gas bottle.

[0024] Figure 2 : (a) is an optical photograph of the original single-walled carbon nanotube film, and (b) is an optical photograph of the single-walled carbon nanotube film after purification by ammonium chloride-assisted heat treatment.

[0025] Figure 3 Scanning electron micrographs (SEM) and transmission electron micrographs (TEM) of single-walled carbon nanotube films. Scanning electron micrographs (a) and transmission electron micrographs (d) of pristine single-walled carbon nanotube films. Scanning electron micrographs (b) and transmission electron micrographs (e) of single-walled carbon nanotube films purified by ammonium chloride-assisted heat treatment. Scanning electron micrographs (c) and transmission electron micrographs (f) of single-walled carbon nanotube films treated with conventional acid.

[0026] Figure 4 Thermogravimetric curves are shown for a pristine single-walled carbon nanotube film (pristine SWCNT), a single-walled carbon nanotube film heat-treated at 700°C with ammonium chloride (SWCNT-NH₄Cl-700), and a single-walled carbon nanotube film heat-treated at 700°C with ammonium chloride plus at 1100°C (SWCNT-NH₄Cl-700-1100). In the figure, the horizontal axis (Temperature) represents temperature (°C), and the vertical axis (Mass) represents mass percentage (%).

[0027] Figure 5 The Raman spectra of the original single-walled carbon nanotube film (pristine SWCNT), the single-walled carbon nanotube film purified by ammonium chloride assisted heat treatment (SWCNT-NH4Cl), and the single-walled carbon nanotube film treated with acid (SWCNT-acid). In the figure, the horizontal axis Raman Shift is the Raman shift (cm -1 ), the vertical axis Intensity is the relative intensity (au). DETAILED DESCRIPTION

[0028] like Figure 1 As shown, the present invention proposes an ammonium chloride-assisted heat treatment device for purifying carbon nanotube films, which mainly includes: a tubular furnace 1, a carbon nanotube film 2, a quartz frame 3, a quartz boat 4, a furnace tube 5, ceramic fiber wool 6, ammonium chloride powder 7, a quartz tube 8, a vacuum pump gas circuit switch 9, a vacuum pump 10, an exhaust gas bottle gas circuit switch 11, and an exhaust gas bottle 12. The specific structure is as follows:

[0029] One end of the furnace tube 5 passing through the tubular furnace 1 is the air inlet end, and the other end of the furnace tube 5 is divided into two branches through the pipeline. One branch is connected to the vacuum pump 10 through the vacuum pump air path switch 9, and the other branch is connected to the tail gas bottle 12 through the tail gas bottle air path switch 11; the carbon nanotube film 2 is fixed on the quartz frame 3, the quartz frame 3 and the ammonium chloride powder 7 are arranged on the quartz boat 4, and the quartz boat 4 is arranged in a quartz tube 8 with an open end. The quartz tube 8 is arranged in the horizontal inner cavity of the horizontal furnace tube 5 along the horizontal direction. One end of the quartz tube 8 is open toward the air inlet end of the furnace tube 5 and is sealed by ceramic fiber cotton 6. The ceramic fiber cotton 6 is plugged into the two ends of the furnace tube 5.

[0030] In the specific implementation process, the present invention proposes a method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment, which mainly includes the following steps: Figure 1 ):

[0031] (1) Carbon nanotube film 2 ( Figure 2 (a) The quartz frame 3 is fixed to the quartz frame 3, and the quartz frame 3 is placed on the quartz boat 4. The quartz boat 4 is placed in the furnace tube 5 of the tube furnace 1 and heat-treated in an air atmosphere to remove amorphous carbon and oxidize the metal catalyst particles into corresponding metal oxides;

[0032] (2) placing a quartz boat 4 containing the air-fired carbon nanotube film 2 and ammonium chloride powder 7 in a quartz tube 8 with one end open, sealing the one end of the quartz tube 8 with ceramic fiber cotton 6, transferring the quartz tube 8 to the furnace tube 5 of the tubular furnace 1, with one end of the quartz tube 8 opening toward the air inlet end of the furnace tube 5, plugging ceramic fiber cotton 6 at both ends of the furnace tube 5, and performing heat treatment in an argon or nitrogen atmosphere to convert the metal oxide into a metal chloride;

[0033] (3) The carbon nanotube film 2 after the ammonium chloride heat treatment is placed on a quartz boat 4, which is then placed in the furnace tube 5 of a tubular furnace 1 and heated under an inert atmosphere or vacuum to remove metal chlorides and obtain a high-purity carbon nanotube film.

[0034] This method does not involve any liquid phase treatment, maintains the intrinsic macroscopic morphology, microstructure and properties of carbon nanotubes, and when used for the purification of single-walled carbon nanotube films, can maintain the flexibility, structural integrity, porous network structure and large specific surface area of ​​single-walled carbon nanotube films.

[0035] Hereinafter, the present invention will be described in detail with reference to examples and drawings.

[0036] Example 1

[0037] In this embodiment, the method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment includes the following steps:

[0038] (1) A carbon nanotube film (5 cm × 4.5 cm, 200 μm thick, consisting of entangled single-walled carbon nanotube bundles) was prepared. Figure 3 (a) with Figure 3 (d)) Place the catalyst on a quartz boat, push the quartz boat into a tube furnace, and heat it in an air atmosphere: heat it to 450°C at a rate of 10°C / min and keep it there for 2 hours. During this heat treatment, the carbon layer covering the catalyst particles is oxidized into carbon dioxide under the catalytic action of the metal particles, and the metal catalyst particles are simultaneously oxidized into metal oxides.

[0039] (2) Place the air-calcined carbon nanotube film and 5g of ammonium chloride powder on a quartz boat. Place the quartz boat in a quartz tube. Seal one end of the quartz tube with ceramic fiber wool. Transfer the quartz tube to a tube furnace and insert ceramic fiber wool at both ends. Pass 1000sccm of argon gas through the tube for 10 minutes to expel the air. Change the flow rate to 100sccm and heat to 700℃ at a rate of 10℃ / min. Hold for 2 hours and cool naturally to room temperature to convert the metal oxide into metal chloride.

[0040] (3) The ammonium chloride-treated carbon nanotube film was placed on a quartz boat, which was then placed in a tube furnace. Argon gas was introduced at a flow rate of 1000 sccm for 10 minutes to expel the air from the tube. The flow rate was then changed to 100 sccm, and the film was heated to 1100°C at a heating rate of 10°C / min and held at that temperature for 1 hour. The film was then cooled naturally to room temperature, and the metal chloride was removed to obtain a flexible single-walled carbon nanotube film with a purity of 99.8 wt%.

[0041] like Figure 2 As shown in (b), since the entire process does not involve any liquid phase treatment, the purified single-walled carbon nanotube film maintains its flexible and self-supporting structural characteristics. Figure 3 b) and transmission electron microscopy ( Figure 3 e) As can be seen from the photo, the porous network structure of the single-walled carbon nanotube film has not changed significantly, and impurities such as metal catalyst particles and amorphous carbon have been effectively removed.

[0042] like Figure 4 As shown, the purified single-walled carbon nanotube film has a higher centralized oxidation temperature and less residue (0.2 wt %) than the original single-walled carbon nanotube film, indicating that amorphous carbon and metal impurities are effectively removed.

[0043] like Figure 5 As shown in the figure, the G / D ratio of the purified single-walled carbon nanotube film is only slightly reduced compared with the original single-walled carbon nanotube film, indicating that this method has little damage to the structure of the single-walled carbon nanotube film, and the treated single-walled carbon nanotube film still maintains high quality.

[0044] Example 2

[0045] In this embodiment, the method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment includes the following steps:

[0046] (1) Step (1) The method and process of step (1) of Example 1 are adopted, except that the size of the single-walled carbon nanotube film is 8 cm×4 cm, the thickness is 1000 μm, and the heat treatment temperature is 500°C.

[0047] (2) Step (2) The method and process of step (2) of Example 1 are adopted, except that the mass of ammonium chloride is 6 g, and the method of excluding air is to evacuate the air and then pass argon gas to normal pressure.

[0048] (3) Step (3) The method and process of step (3) of Example 1 are adopted, except that the method for excluding air is to evacuate the air and then pass argon to normal pressure, heat to 1000°C and then evacuate the air, keep the temperature under vacuum for 30 minutes, and then pass argon to normal pressure to obtain a flexible single-walled carbon nanotube film with a purity of 99.3wt%.

[0049] Example 3

[0050] In this embodiment, the method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment includes the following steps:

[0051] (1) Step (1) The method and process of step (1) of Example 1 are adopted, except that the single-walled carbon nanotube film has a size of 4 cm×3 cm and a thickness of 100 nm, is fixed on a quartz frame, and the heat treatment time is 1.5 h.

[0052] (2) Step (2) The method and process of step (2) of Example 1 are adopted, except that the mass of ammonium chloride is 8 g, the gas introduced is nitrogen, and the heat treatment temperature is 600°C.

[0053] (3) Step (3) The method and process of step (3) of Example 1 were adopted, except that the gas introduced was nitrogen and the holding time was 2 hours, to obtain a flexible single-walled carbon nanotube film with a purity of 99.9 wt%.

[0054] Example 4

[0055] In this embodiment, the method for purifying carbon nanotube powder by ammonium chloride-assisted heat treatment includes the following steps:

[0056] (1) Step (1) The method and process of step (1) of Example 1 are adopted, except that multi-walled carbon nanotube powder is selected, the powder weight is 200 mg, and the metal catalyst is nickel particles.

[0057] (2) Step (2) adopts the method and process of step (2) of Example 1.

[0058] (3) Step (3) The method and process of step (3) of Example 1 were used to obtain multi-walled carbon nanotube powder with a purity of 99.9 wt%.

[0059] Example 5

[0060] In this embodiment, the method for purifying carbon nanotube powder by ammonium chloride-assisted heat treatment includes the following steps:

[0061] (1) Step (1) The method and process of step (1) of Example 1 are adopted, except that multi-walled carbon nanotube aerogel is selected, the aerogel weight is 100 mg, and the metal catalyst is cobalt particles.

[0062] (2) Step (2) adopts the method and process of step (2) of Example 1.

[0063] (3) Step (3) The method and process of step (3) of Example 1 were used to obtain a multi-walled carbon nanotube aerogel with a purity of 99.7 wt%.

[0064] Comparative Example 1

[0065] In this comparative example, the effects of liquid-phase acid treatment on the structure and properties of single-walled carbon nanotube films were compared and studied. The specific steps are as follows:

[0066] (1) Step (1) is exactly the same as step (1) in Example 1.

[0067] (2) The calcined single-walled carbon nanotube film was fixed with a polytetrafluoroethylene frame and immersed in nitric acid with a molar concentration of 2M for 24 hours.

[0068] (3) The acid-treated single-walled carbon nanotube film was taken out and washed several times with deionized water until the pH of the washing solution reached 7.

[0069] (4) freeze-drying the washed single-walled carbon nanotube film to obtain a single-walled carbon nanotube film with a purity of 99.9 wt %.

[0070] From the scanning electron microscope ( Figure 3 (c)) and transmission electron microscopy ( Figure 3 As can be seen from the photo (f), although the liquid-phase acid treatment method can also effectively remove impurities such as metal catalyst particles, it causes significant damage to the structure of the single-walled carbon nanotube film. The loose and porous network structure undergoes significant changes, the film becomes dense, and the tube bundles become significantly larger.

[0071] like Figure 5 As shown in the figure, the G / D ratio of the single-walled carbon nanotube film after liquid-phase acid treatment is significantly lower than that of the original single-walled carbon nanotube film, indicating that this method has some damage to the structure of the single-walled carbon nanotubes and the quality has been reduced.

[0072] This comparative example shows that acid treatment significantly damages both the macroscopic and microscopic structures of the single-walled carbon nanotube film, which affects the performance of the single-walled carbon nanotube film and is not conducive to the application of the single-walled carbon nanotube film.

[0073] Comparative Example 2

[0074] In this comparative example, the third vacuum / inert atmosphere heat treatment step is omitted, and the specific steps are as follows:

[0075] (1) Step (1) is exactly the same as step (1) in Example 1.

[0076] (2) Step (2) is exactly the same as step (2) in the embodiment.

[0077] like Figure 4 As shown, the weight of the SWCNT film, which had only been treated with ammonium chloride at 700°C, began to decrease at 500°C, corresponding to the oxidation of ferrous chloride to low-boiling-point ferric chloride. This indicates that further heat treatment is required to remove the metal chloride after the ammonium chloride treatment. ICP (inductively coupled plasma) analysis revealed an Fe content of 0.38 wt%, lower than the original SWCNTs (2.60 wt%), but higher than the SWCNTs treated with ammonium chloride at 700°C and then at 1100°C (0.11 wt%). This indicates that some Fe was removed as low-boiling-point ferric chloride during the 700°C ammonium chloride treatment, but some Fe remained as higher-boiling-point ferrous chloride, requiring further removal.

[0078] Comparing Example 1 and Comparative Example 2, this comparative example illustrates that after the ammonium chloride heat treatment, the metal oxides in the carbon nanotubes are converted into corresponding metal chlorides. For Fe, low-boiling-point ferric chloride and high-boiling-point ferrous chloride are generated. For other metal impurities, the boiling points of their corresponding metal chlorides are mostly higher than 900°C. Therefore, further heat treatment is required at a higher temperature in an inert atmosphere or under vacuum to remove the metal chlorides.

[0079] The results of the examples and comparative examples show that the present invention has developed a simple gas-phase purification method that maintains the flexibility, structural integrity, porous network structure and large specific surface area of ​​the single-walled carbon nanotube film, while effectively removing impurities such as metal catalysts and amorphous carbon. The purified single-walled carbon nanotube film has a higher centralized oxidation temperature and an extremely low catalyst impurity content.

[0080] Although the present invention has been described in detail above using general descriptions and specific embodiments, it will be apparent to those skilled in the art that modifications and improvements may be made thereto. Therefore, such modifications and improvements, without departing from the spirit of the present invention, are intended to be within the scope of the present invention.

Claims

1. A method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment, characterized in that: First, the carbon nanotubes are subjected to low-temperature air calcination to remove amorphous carbon and oxidize the metal catalyst particles into metal oxides. The air-calcined carbon nanotubes are then heat-treated together with excess ammonium chloride powder in an inert atmosphere. The ammonium chloride is decomposed into hydrogen chloride and ammonia by heat, and reacts with the metal oxide particles in the carbon nanotubes to form the corresponding metal chloride. The metal chloride is then further removed by high-temperature heat treatment in an inert atmosphere or vacuum, ultimately obtaining high-purity carbon nanotubes.

2. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 1, characterized in that: The carbon nanotubes are one or more of single-walled carbon nanotubes, double-walled carbon nanotubes, and multi-walled carbon nanotubes, and the forms include but are not limited to thin films, powders, or aerogels.

3. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 1, characterized in that: The metal catalyst includes, but is not limited to, iron (Fe), cobalt (Co), or nickel (Ni).

4. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 1, characterized in that: The low-temperature air-firing treatment is as follows: the carbon nanotubes are transferred to a tube furnace and heated in an air atmosphere at a heat treatment temperature of 400 to 500° C. for 1 to 2 hours.

5. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 4, characterized in that: The air-burned carbon nanotubes and ammonium chloride are placed together in a quartz boat at a mass ratio of 1:10 to 1:

500. The quartz boat is then placed in a quartz tube with an open end, and the open end of the quartz tube is sealed with ceramic fiber wool. The quartz tube is transferred to the furnace tube of a tube furnace with one end of the quartz tube facing the air inlet end of the furnace tube. Heat treatment is performed under an inert atmosphere at a heat treatment temperature of 400 to 900°C and a heat treatment time of 1 to 3 hours.

6. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 5, characterized in that: The carbon nanotubes after heat treatment with ammonium chloride are transferred to a tube furnace and subjected to high-temperature heat treatment in an inert atmosphere or vacuum: the heat treatment temperature in an inert atmosphere is 1050-1100°C, the heat treatment temperature in a vacuum is 800-1100°C, and the heat treatment time is 0.5-2h.

7. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 1, 5 or 6, characterized in that: The inert atmosphere is an argon (Ar) or nitrogen (N2) atmosphere, and the vacuum atmosphere is an atmosphere with a pressure lower than 100 Pa.

8. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 1 or 6, characterized in that: The purity of the purified carbon nanotubes is higher than 99 wt %, and the optimal value is 99.9 wt %.

9. The method for purifying carbon nanotubes by ammonium chloride-assisted heat treatment according to claim 1, 2 or 6, characterized in that: The size and thickness of the carbon nanotube film are not limited. The preferred thickness range of the carbon nanotube film is 20 nm to 1000 μm.

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