High-purity and low-damage single-walled carbon nanotube purification method

Abstract

The invention discloses a high-purity and low-damage single-walled carbon nanotube purification method which comprises the following steps: carrying out mechanical shearing treatment on a single-walled carbon nanotube to disperse the single-walled carbon nanotube and open a tube bundle structure to obtain a sheared single-walled carbon nanotube; carrying out pre-oxidation on the sheared single-walled carbon nanotube in a weak oxidizing atmosphere to obtain a pre-oxidized single-walled carbon nanotube; carrying out microwave treatment on the pre-oxidized single-walled carbon nanotube or the acidified single-walled carbon nanotube in an inert atmosphere to obtain a single-walled carbon nanotube subjected to microwave treatment; and acidizing the single-walled carbon nanotube subjected to microwave treatment to obtain the acidized single-walled carbon nanotube. The method is simple in process, mild in condition, easy to amplify and suitable for industrial production. The purity of the obtained single-walled carbon nanotube is higher than 99%, the raman spectrum IG / ID ratio is larger than 150 and higher than that before purification, and the single-walled carbon nanotube is high in structural integrity and suitable for the fields of high-performance composite materials, electronic devices, energy storage and the like.

Description

Technical Field

[0001] This invention relates to the field of single-walled carbon nanotube purification technology, specifically to a method for the synergistic purification of single-walled carbon nanotubes using weak oxidation pretreatment, microwave treatment, and acid washing. Background Technology

[0002] Single-walled carbon nanotubes (SWCNTs) have broad application prospects in composite materials, nanoelectronic devices, and energy storage due to their excellent mechanical, electrical, and thermal properties. Currently, chemical vapor deposition (CVD) is the main method for large-scale preparation of SWCNTs, but its products often contain impurities such as amorphous carbon, graphite sheets, and metal catalysts, which seriously affect their performance and applications.

[0003] Existing purification methods often employ strong oxidizing acids (such as concentrated nitric acid and concentrated sulfuric acid), which, while effectively removing metallic impurities, easily damage the spline structure of carbon nanotubes. 2 Bonding structures introduce structural defects, leading to a significant decrease in their electrical and mechanical properties. On the other hand, high-temperature heat treatment processes, such as vacuum Joule heating at 1600℃, can reduce the catalyst content in single-walled carbon nanotube films by 27 times (from 17% to below 1%) in a short time (e.g., 2 minutes), but this process requires extremely high temperatures, resulting in high energy consumption and costs. Furthermore, traditional purification processes generally suffer from cumbersome steps, harsh reaction conditions, and low product yields, making them unsuitable for large-scale production. Therefore, developing a highly efficient, low-structural-damage, and easily scalable single-walled carbon nanotube purification process has significant scientific and industrial value.

[0004] Taking typical enterprise technical solutions as examples, such as the solvent pretreatment combined with low-temperature oxidation method disclosed in patent document CN119528125A, although it improves the removal efficiency of metal impurities, it is still difficult to completely avoid metal residues due to the limited penetration ability of acid into the dense graphite layer. Another patent document, CN115403034A, uses a high-temperature oxidation combined with acid treatment method, which can achieve high purity, but its high-temperature oxidation method damages the structure of carbon nanotubes to some extent, failing to guarantee high yield and crystallinity. At the international level, leading companies such as OCSiAl have achieved large-scale annual production capacity, but the crystallinity index (Raman spectroscopy I) of their obtained carbon nanotubes... G / I D Even with a value of less than 50, it is still difficult to meet the needs of high-performance application scenarios. Summary of the Invention

[0005] To address the aforementioned technical problems and shortcomings in the field, this invention provides a high-purity, low-damage purification method for single-walled carbon nanotubes. This method is simple, mild, and suitable for large-scale production, achieving high-purity purification while preserving the structural integrity of the carbon nanotubes to the greatest extent possible.

[0006] Specifically, this invention employs a four-step synergistic process—mechanical shearing pretreatment, weak oxidant pretreatment, gradient microwave treatment, and low-concentration acid washing—to efficiently remove amorphous carbon and metallic catalyst impurities from single-walled carbon nanotubes under mild conditions, while maximizing the preservation of their carbon framework structural integrity. This invention features a simple process, mild conditions, and ease of scale-up, making it suitable for industrial production. The resulting single-walled carbon nanotubes exhibit a purity exceeding 99% and Raman spectroscopy of I0. G / I D With a ratio greater than 150 and higher than before purification, it exhibits strong structural integrity and is suitable for applications such as high-performance composite materials, electronic devices, and energy storage.

[0007] The specific technical solution of this invention is described below: A method for purifying high-purity, low-damage single-walled carbon nanotubes, comprising the following steps: S1, mechanically shear the single-walled carbon nanotubes to disperse them and open up the tube bundle structure to obtain sheared single-walled carbon nanotubes. S2, the sheared single-walled carbon nanotubes are pre-oxidized in a weak oxidizing atmosphere to obtain pre-oxidized single-walled carbon nanotubes; the weak oxidizing atmosphere is an atmosphere containing one or more of oxygen, carbon dioxide, water vapor, ozone, and nitrous oxide, and the oxygen content in the weak oxidizing atmosphere is below 5 vol% (e.g., 3 vol%, including 0 vol%, i.e., no oxygen); the pre-oxidation temperature is 350-450℃, e.g., 370℃, 380℃, 400℃, etc. S3, pre-oxidized or acidified single-walled carbon nanotubes are subjected to microwave treatment in an inert atmosphere to obtain microwave-treated single-walled carbon nanotubes; the microwave power of the microwave treatment is 300-1000 W, for example 700 W. S4. The microwaved single-walled carbon nanotubes are acidified to obtain acidified single-walled carbon nanotubes.

[0008] Step S1 is a mechanical shearing pretreatment, in which a certain amount of single-walled carbon nanotubes are placed in a shearing machine for crushing. This can open up the carbon nanotube bundles, increase the specific surface area, and promote subsequent reactions.

[0009] In some preferred embodiments, in step S1, the initial purity of the single-walled carbon nanotubes is not less than 80%.

[0010] In some preferred embodiments, in step S1, the rotational speed of the mechanical shearing process is 1000-5000 rpm, such as 1500 rpm, 2000 rpm, 3000 rpm, 4500 rpm, etc.

[0011] In some preferred embodiments, the mechanical shearing process in step S1 takes 10-60 minutes, such as 15 minutes, 30 minutes, 40 minutes, 50 minutes, etc.

[0012] Step S2 is pre-oxidation, in which the broken single-walled carbon nanotubes are placed in an oxidation furnace for pre-oxidation at a certain time and temperature. Under low-temperature conditions, a weak oxidizing atmosphere is used to selectively oxidize the amorphous carbon layer that encapsulates the metal catalyst, exposing the metal particles, preserving the carbon nanotube framework to the maximum extent, and preventing over-oxidation.

[0013] In some preferred embodiments, in step S2, the weakly oxidizing atmosphere includes an inert gas.

[0014] In this invention, inert gas and inert atmosphere refer to gases and atmospheres that do not participate in the reaction, such as nitrogen and argon.

[0015] In some preferred embodiments, in step S2, the weakly oxidizing atmosphere is a flowing atmosphere. The inert gas can be used as a carrier gas for the flowing atmosphere. More preferably, the flow rate of the weakly oxidizing atmosphere is 500-2000 sccm, such as 800 sccm, 1000 sccm, 1200 sccm, 1500 sccm, etc.

[0016] In some preferred embodiments, the pre-oxidation time in step S2 is 2-6 h, such as 3 h, 4.5 h, 5 h, etc.

[0017] Step S3 involves microwave treatment, where the pre-oxidized single-walled carbon nanotubes are treated with low-power microwaves in an inert atmosphere such as argon. Metal particles (Fe) are excellent microwave absorbers, rapidly heating up in a microwave field to create localized hot spots, while single-walled carbon nanotubes themselves exhibit relatively low heating efficiency under microwave conditions. The rapidly heating metal particles expand, breaking through the carbon layer from the inside. This localized and controllable damage is mainly limited to the carbon layer region encasing the metal and to inherent defects in the carbon nanotubes. The main structure of the carbon nanotubes (length, graphitized lattice) is well preserved.

[0018] In some preferred embodiments, the microwave treatment time in step S3 is 10-600 s, for example 20 s.

[0019] In some preferred embodiments, step S4, the acidification treatment includes: placing the microwaved single-walled carbon nanotubes in an acid solution and stirring to react, then washing and drying the single-walled carbon nanotubes and controlling the pH of the single-walled carbon nanotubes at 6.5-7.5 (e.g., neutral, i.e., pH=7).

[0020] The acidification process includes low-concentration acid washing and washing and drying. The microwaved single-walled carbon nanotubes are placed in an acid solution and acid-washed with stirring at a certain temperature. The exposed metal catalyst is dissolved using a low-concentration acid solution. The acid-washed single-walled carbon nanotubes are then washed and dried, with the pH controlled at 6.5-7.5 to remove residual acid and impurities.

[0021] In some preferred embodiments, the acid concentration in the acid solution during the acidification treatment is 0.5-2 M, for example 0.75 M, 1 M, etc.

[0022] In some preferred embodiments, the acid in the acid solution during the acidification treatment includes one or more of hydrochloric acid, nitric acid, and sulfuric acid.

[0023] In some preferred embodiments, the temperature of the stirring reaction during the acidification process is 60-90°C, for example, 65°C, 70°C, 80°C, etc.

[0024] In some preferred embodiments, the stirring reaction time in the acidification treatment is 3-6 h, for example 4 h, 5 h, etc.

[0025] In some preferred embodiments, the stirring speed of the stirring reaction in the acidification treatment is 250-350 rpm, for example 300 rpm.

[0026] In some preferred embodiments, the drying process in the acidification treatment includes spray drying.

[0027] In some preferred embodiments, the method for purifying high-purity, low-damage single-walled carbon nanotubes further includes the step of: S5, repeat steps S3 and S4 once or multiple times.

[0028] Repeating the microwave and acidification steps yields higher purity, low-damage single-walled carbon nanotubes.

[0029] Furthermore, the Raman spectrum of the single-walled carbon nanotubes purified by the aforementioned high-purity, low-damage purification method is shown in Figure I. G / I D The ratio is greater than that of the single-walled carbon nanotubes described in step S1.

[0030] Furthermore, the Raman spectrum of the single-walled carbon nanotubes purified by the aforementioned high-purity, low-damage purification method is shown in Figure I. G / I D The ratio is greater than 150.

[0031] Furthermore, the purity of the single-walled carbon nanotubes purified by the high-purity, low-damage single-walled carbon nanotube purification method is higher than 99%.

[0032] Compared with the prior art, the beneficial effects of this invention are as follows: 1. A synergistic strategy of weak oxidation and microwave acid washing is adopted to remove impurities while maximizing the protection of carbon nanotube structure.

[0033] 2. The process is simple and the conditions are mild, which shortens the traditional pre-oxidation and acid washing purification time, improves efficiency, and is suitable for continuous and large-scale production.

[0034] 3. The purified single-walled carbon nanotubes have high purity (>99%) and intact structure (Raman spectroscopy I). G / I D >150), with higher crystallinity compared to before purification (Raman spectrum I). G / I D (Improved after purification), performance is close to the intrinsic state.

[0035] 4. High yield, low cost, and environmentally friendly. Attached Figure Description

[0036] Figure 1 The images shown are transmission electron microscope (TEM) images of single-walled carbon nanotubes before and after purification in Example 1, wherein: (a) is a TEM image of crude single-walled carbon nanotubes; (b) is a TEM image of purified single-walled carbon nanotubes; the scale bar in the images is 50 nm.

[0037] Figure 2 Figure 1 shows the thermal weight loss curves of single-walled carbon nanotubes in air before and after purification in Example 1. (a) Figure 1 shows the thermal weight loss curve of crude single-walled carbon nanotubes; (b) Figure 1 shows the thermal weight loss curve of purified single-walled carbon nanotubes.

[0038] Figure 3 The figures are Raman spectra of single-walled carbon nanotubes before and after purification in Example 1, where: (a) is the Raman spectrum of crude single-walled carbon nanotubes, and (b) is the Raman spectrum of purified single-walled carbon nanotubes. Detailed Implementation

[0039] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be understood that these embodiments are for illustrative purposes only and are not intended to limit the scope of the invention. Operating methods not specifically specified in the following embodiments are generally performed under conventional conditions or as recommended by the manufacturer.

[0040] In the following examples, the initial purity of the crude single-walled carbon nanotubes was no less than 80%.

[0041] Example 1: Shearing and crushing: 2g of crude single-walled carbon nanotubes prepared by floating catalytic vapor deposition were placed in a shearing machine and sheared at 3000 rpm for 30 minutes. After drying, single-walled carbon nanotube powder was obtained, which is the pretreated sample.

[0042] Pre-oxidation treatment: The pretreated sample was placed in an oxidation furnace, and the oxidation temperature was set to 380℃. Under nitrogen protection, the temperature was raised to 380℃, and then a mixed gas with a CO2:H2O volume ratio of 1:2 was introduced (the mixed gas composition consisted of a total CO2 and H2O flow rate of 500 sccm and a nitrogen flow rate of 500 sccm), and oxidation was carried out for 5 hours. After the reaction was completed, the temperature was lowered under a nitrogen atmosphere. When the temperature dropped to room temperature, the sample was removed to obtain pre-oxidized single-walled carbon nanotubes.

[0043] Microwave treatment: The pre-oxidized (or acidified) single-walled carbon nanotubes are placed in a microwave oven, and after repeated vacuuming, argon gas is introduced. The microwave power is set to 700W and the time is 20s to obtain microwaved single-walled carbon nanotubes.

[0044] Acidification treatment: The microwaved single-walled carbon nanotubes were placed in a 1M hydrochloric acid solution and heated and stirred at 300 rpm for 5 hours at 70℃. After the reaction was completed, the single-walled carbon nanotubes were filtered, washed, spray-dried, and the pH of the single-walled carbon nanotubes was controlled at 6.5-7.5.

[0045] The microwave treatment and acidification treatment were repeated once to obtain purified single-walled carbon nanotubes.

[0046] TEM characterization of the initial sample (crude single-walled carbon nanotubes) and the purified single-walled carbon nanotubes showed that the purified single-walled carbon nanotubes had no obvious metal particle residue, the carbon nanotube structure was intact, and the crystal lattice was not destroyed. (See [link to documentation]). Figure 1 The crude and purified single-walled carbon nanotubes were characterized by Raman spectroscopy and purity analysis. The purified single-walled carbon nanotubes achieved a purity of 99.25%, and the Raman spectrum was [I-value missing]. G / I D =219, see also Figure 2 , Figure 3 .

[0047] Example 2: Shearing and crushing: Take 2g of crude single-walled carbon nanotubes, shear at 1500 rpm for 50 minutes, and dry to obtain the sheared sample.

[0048] Pre-oxidation treatment: The sheared sample was placed in a tube furnace and heated to 350°C under nitrogen protection. Then, a nitrogen mixture containing 3% O2 ​​(total flow rate 1200 sccm) was introduced for oxidation for 3 hours. After the reaction, the sample was cooled to room temperature under nitrogen protection to obtain the pre-oxidized sample.

[0049] Microwave treatment: The pre-oxidized sample (or acid-treated single-walled carbon nanotubes) is placed in a microwave reactor under an argon atmosphere, with the microwave power set to 300 W and the treatment time set to 600 s to obtain the microwave-treated sample.

[0050] Acidification treatment: The microwave-treated sample was added to a 1 M nitric acid solution and stirred at 80°C for 4 hours at a stirring speed of 300 rpm. After the reaction was completed, the sample was filtered, washed with deionized water until neutral, and spray-dried.

[0051] Repeat the microwave treatment and acidification treatment once to obtain purified single-walled carbon nanotubes.

[0052] The Raman spectrum of the purified single-walled carbon nanotubes was tested. G / I D The ratio is 158, and the purity is 99.18%.

[0053] Example 3: Shearing and fragmentation: Take 2g of crude single-walled carbon nanotubes, shear at 4500 rpm for 15 minutes, and dry to obtain the sheared sample.

[0054] Pre-oxidation treatment: The sheared sample was heated to 400℃ under nitrogen protection, and then a mixture of water vapor and nitrogen (water vapor flow rate 400 sccm, nitrogen flow rate 800 sccm) was introduced for oxidation for 4.5 hours. Subsequently, the sample was cooled under nitrogen protection to obtain the pre-oxidized sample.

[0055] Microwave treatment: In an argon atmosphere, the pre-oxidized sample (or acidified single-walled carbon nanotubes) was treated with a microwave at a power of 700 W for 10 s to obtain the microwave-treated sample.

[0056] Acidification treatment: The microwave-treated sample was placed in a 1 M sulfuric acid solution and stirred at 65 °C for 6 hours (stirring speed 250 rpm). After that, it was filtered, washed, spray-dried and the pH of the single-walled carbon nanotubes was controlled at 6.5-7.5.

[0057] Repeat the microwave treatment and acidification treatment once to obtain purified single-walled carbon nanotubes.

[0058] The Raman spectrum of the purified single-walled carbon nanotubes was tested. G / I D The ratio is 192, and the purity is 99.42%.

[0059] Example 4: Shearing and crushing: Take 2g of crude single-walled carbon nanotubes, shear at 2000 rpm for 40 minutes, and dry to obtain the sheared sample.

[0060] Pre-oxidation treatment: The sheared sample was heated to 370°C under nitrogen protection, and then a CO2 / nitrogen mixture with a CO2 volume fraction of 20% (total flow rate 1500 sccm) was introduced for oxidation for 6 hours. The sample was then cooled under nitrogen protection to obtain the pre-oxidized sample.

[0061] Microwave treatment: Under an argon atmosphere, microwave power of 1000 W was used to treat the pre-oxidized sample (or acid-treated single-walled carbon nanotubes) for 10 seconds.

[0062] Acidification treatment: The sample was acidified at 90°C for 3 hours (stirring speed 250 rpm) using a mixed acid solution (1 M hydrochloric acid and 0.5 M nitric acid in equal volumes). After that, the sample was filtered, washed, spray-dried and the pH of the single-walled carbon nanotubes was controlled at 6.5-7.5.

[0063] Repeat the microwave treatment and acidification treatment once to obtain purified single-walled carbon nanotubes.

[0064] The Raman spectrum of the purified single-walled carbon nanotubes was tested. G / I D The ratio is 165, and the purity is 99.10%.

[0065] Comparative Example 1: The only difference from Example 1 is that there is no pre-oxidation treatment; the single-walled carbon nanotube powder obtained by shearing and crushing is directly subjected to microwave treatment. All other aspects are the same. The purified product's Raman spectrum was tested. G / I D The ratio was 195, and the purity was 96.2%. TEM showed that a large number of metal particles were still encased in the carbon layer and had not been effectively removed.

[0066] Comparative Example 2: Two g of crude single-walled carbon nanotubes from the same batch as in Example 1 were taken and placed directly into concentrated nitric acid (approximately 8 M) without shearing or pre-oxidation, and refluxed at 120°C for 12 hours. After cooling, the product was filtered, washed, spray-dried, and the pH of the single-walled carbon nanotubes was controlled at 6.5-7.5. The purified product was tested, and the Raman spectrum I was obtained. G / I D The ratio was 52, and although the purity reached 98.5%, the carbon nanotube structure was severely damaged and the length was significantly shortened.

[0067] Comparative Example 3: The shearing, crushing, and pre-oxidation treatments are the same as in Example 1.

[0068] No microwave processing required.

[0069] Acidification treatment: The pre-oxidized single-walled carbon nanotubes were directly acid-washed (under the same conditions as in Example 1) without microwave treatment.

[0070] Repeat the pickling process once.

[0071] The purified product's Raman spectrum I was tested. G / I D The ratio is 120, and the purity is 96.8%. The pickling efficiency is low, and some metals tightly wrapped by the carbon layer cannot be completely exposed and removed.

[0072] Comparative Example 4: The only difference from Example 1 is that the microwave power was set to 1200W and the time to 600s during microwave treatment; all other settings were the same. The Raman spectrum of the purified product was tested. G / I D The ratio was 95, and the purity was 99.1%. Excessive microwave power caused localized overheating, which not only damaged the carbon layer encasing the metal but also damaged part of the carbon nanotube's main structure.

[0073] Summarize: This invention achieves efficient and low-damage purification of single-walled carbon nanotubes through a four-step synergistic process: shearing, pre-oxidation, microwave treatment, and acid washing. The pre-oxidation step initially opens the carbon layer encapsulating the metal, removing amorphous carbon; microwave treatment fully exposes metal impurities, effectively improving acid washing efficiency; and shearing treatment enhances the dispersion of carbon nanotubes and promotes reaction uniformity. This method maintains high purity while maximizing the preservation of the structural integrity and performance stability of the carbon nanotubes.

[0074] Furthermore, it should be understood that after reading the above description of the present invention, those skilled in the art can make various alterations or modifications to the present invention, and these equivalent forms also fall within the scope defined by the appended claims.

Claims

1. A method for purifying high-purity, low-damage single-walled carbon nanotubes, characterized in that, Including the following steps: S1, mechanically shear the single-walled carbon nanotubes to disperse them and open up the tube bundle structure to obtain sheared single-walled carbon nanotubes. S2, the sheared single-walled carbon nanotubes are pre-oxidized in a weak oxidizing atmosphere to obtain pre-oxidized single-walled carbon nanotubes; the weak oxidizing atmosphere is an atmosphere containing one or more of oxygen, carbon dioxide, water vapor, ozone, and nitrous oxide, and the oxygen content in the weak oxidizing atmosphere is below 5 vol%; the pre-oxidation temperature is 350-450℃. S3, pre-oxidized or acidified single-walled carbon nanotubes are subjected to microwave treatment in an inert atmosphere to obtain microwave-treated single-walled carbon nanotubes; the microwave power of the microwave treatment is 300-1000 W. S4. The microwaved single-walled carbon nanotubes are acidified to obtain acidified single-walled carbon nanotubes.

2. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 1, characterized in that, In step S1: The initial purity of the single-walled carbon nanotubes is not less than 80%. The rotational speed for mechanical shearing is 1000-5000 rpm; The mechanical shearing process takes 10-60 minutes.

3. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 1, characterized in that, In step S2: Weakly oxidizing atmospheres include inert gases; The weakly oxidizing atmosphere is a fluid atmosphere with a flow rate of 500-2000 sccm.

4. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 1, characterized in that, In step S2, the pre-oxidation time is 2-6 hours.

5. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 1, characterized in that, In step S3, the microwave treatment time is 10-600 s.

6. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 1, characterized in that, In step S4, the acidification treatment includes: placing the microwaved single-walled carbon nanotubes in an acid solution and stirring to react, then washing and drying the single-walled carbon nanotubes and controlling the pH of the single-walled carbon nanotubes at 6.5-7.

5.

7. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 6, characterized in that, In the acidification process: The acid concentration in the acid solution is 0.5-2 M; The acid in the acid solution includes one or more of hydrochloric acid, nitric acid, and sulfuric acid. The temperature of the stirring reaction is 60-90℃; The stirring reaction time is 3-6 hours; The stirring speed of the stirring reaction is 250-350 rpm; The drying includes spray drying.

8. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 1, characterized in that, The method for purifying high-purity, low-damage single-walled carbon nanotubes also includes the following steps: S5, repeat steps S3 and S4 once or multiple times.

9. The method for purifying high-purity, low-damage single-walled carbon nanotubes according to claim 1, characterized in that, Raman spectra of single-walled carbon nanotubes purified by the aforementioned high-purity, low-damage purification method. G / I D The ratio is greater than that of the single-walled carbon nanotubes described in step S1; Raman spectra of single-walled carbon nanotubes purified by the aforementioned high-purity, low-damage purification method. G / I D The ratio is greater than 150; The high-purity, low-damage single-walled carbon nanotube purification method described above yields single-walled carbon nanotubes with a purity exceeding 99%.

Images