Efficient purification method for single-walled carbon nanotubes

Abstract

The invention discloses a high-efficiency purification method of a single-walled carbon nanotube, which belongs to the technical field of nano material preparation and treatment, and comprises the following steps: pre-oxidizing an original single-walled carbon nanotube in an acidic potassium permanganate solution, and carrying out microwave liquid-phase selective heating treatment to promote a carbon shell wrapping a metal catalyst to break, so as to obtain the high-efficiency purification method of the single-walled carbon nanotube. Then, the exposed metal core is effectively dissolved through mixed acid pickling, and finally, the high-purity single-walled carbon nanotube is obtained through washing and drying. According to the method, through the synergistic effect of pre-oxidation and microwave heating, carbon-coated iron and other stubborn impurities are efficiently and deeply removed, meanwhile, the structural integrity of the carbon tubes is protected to the maximum extent, the problems that impurities are difficult to remove and the carbon tubes are prone to being damaged in a traditional acid pickling method are solved, and the method is suitable for industrial production. And the defects of harsh conditions and poor controllability of a high-temperature oxidation method are overcome, so that the final product has high purity, low structural defects and good dispersity, and the application requirements of the fields of high-end electronics, composite materials, energy storage and the like on high-performance carbon nanotube materials are met.

Description

Technical Field

[0001] This invention belongs to the field of nanomaterial preparation and processing technology, specifically a method for efficient purification of single-walled carbon nanotubes. Background Technology

[0002] Single-walled carbon nanotubes (SUVs) have shown great application potential in high-tech fields such as energy, electronics, and composite materials due to their unique electrical, thermal, and mechanical properties. However, mainstream preparation processes, such as chemical vapor deposition, arc discharge, and laser evaporation, all require the use of metal catalysts such as iron, cobalt, and nickel. This inevitably results in a large number of metal impurities and their carbonaceous inclusions remaining in the final product. These impurities, especially the metal nanoparticles encapsulated in a dense carbon layer, severely degrade the performance of SUVs, constituting the primary technical obstacle to their practical application.

[0003] Currently, the mainstream purification methods for removing these impurities all have significant drawbacks. Traditional acid washing methods are difficult to effectively dissolve the internal metal core due to the obstruction of the carbon shell, and strong acids can easily damage the carbon nanotube structure. High-temperature gas-phase methods (such as chlorine treatment) have some effect on carbon-coated iron, but the conditions are harsh, the safety risks are high, and it may lead to carbon nanotube sintering. Surfactant methods have limited ability to remove tightly wrapped impurities, and the subsequent processing steps are cumbersome. The limitations of these methods make the development of efficient and low-damage purification processes an urgent need for the industry.

[0004] To overcome the aforementioned bottlenecks, the industry has introduced pre-oxidation treatment to break down the carbon shell that encapsulates impurities. However, existing pre-oxidation technologies are also not entirely satisfactory: air pre-oxidation is difficult to precisely control the degree of oxidation, which can easily lead to selective damage to the single-walled carbon nanotubes, especially small-diameter tubes; liquid-phase oxidation (such as using hydrogen peroxide or nitric acid) has problems such as low efficiency, long processing time, and limited destructive power to the carbon shell.

[0005] Therefore, how to achieve a pre-oxidation method that can efficiently and selectively destroy the impurity carbon shell while maximally protecting the single-walled carbon nanotube body is the core challenge currently facing purification technology. Summary of the Invention

[0006] The purpose of this invention is to provide a highly efficient purification method for single-walled carbon nanotubes. Through the synergistic effect of pre-oxidation and microwave heating, it achieves efficient and deep removal of stubborn impurities such as "carbon-coated iron" while maximizing the protection of the structural integrity of the carbon nanotubes. This method solves the problems of traditional acid washing methods, which are difficult to remove impurities and easily damage carbon nanotubes, and overcomes the shortcomings of high-temperature oxidation methods, such as harsh conditions and poor controllability. The final product has high purity, low structural defects, and good dispersibility, meeting the application requirements of high-performance carbon nanotube materials in high-end electronics, composite materials, and energy storage fields.

[0007] The objective of this invention can be achieved through the following technical solutions: A method for efficient purification of single-walled carbon nanotubes, characterized by comprising the following steps: Step 1: Disperse the original single-walled carbon nanotube raw material in deionized water to form a uniform suspension, add mixed acid to adjust the system to an acidic environment, add potassium permanganate, and dissolve by stirring at room temperature to obtain a pre-oxidation reaction solution.

[0008] Step 2: The pre-oxidized reaction solution is subjected to microwave heating, followed by acid washing, filtration, washing and drying to obtain purified single-walled carbon nanotubes.

[0009] Furthermore, the metal content of the original single-walled carbon nanotube raw material is 25%-75%.

[0010] Furthermore, the mass ratio of the original single-walled carbon nanotube raw material to potassium permanganate is 0.1-2:1.

[0011] Furthermore, the mass ratio of the original single-walled carbon nanotube raw material to potassium permanganate is 0.5-1:1.

[0012] Furthermore, the mixed acid is composed of sulfuric acid and phosphoric acid mixed in a volume ratio of 3:0-1.

[0013] Furthermore, the pH value of an acidic environment is 1-4.

[0014] Furthermore, the conditions for microwave heating treatment are: temperature 70-90℃, microwave power 100-1000W, and treatment time 1-12h.

[0015] Furthermore, the microwave power is 300-600W.

[0016] Furthermore, the pickling solution is composed of hydrochloric acid, nitric acid, and hydrofluoric acid in a volume ratio of 1-5:1:0-1.

[0017] Furthermore, the pickling conditions are a temperature of 80-90℃ and a pickling time of 12-96 hours.

[0018] The beneficial effects of this invention are: 1. This invention achieves efficient and targeted removal of impurities in encapsulated metal catalysts such as "carbon-coated iron" by constructing a three-step synergistic purification mechanism centered on acidic potassium permanganate pre-oxidation of the carbon shell, microwave localized thermal rupture, and deep dissolution of the metal core by mixed acid. This fundamentally solves the key technical problems of traditional acid washing methods, which are difficult to access the internal metal due to the dense barrier of the carbon shell, and high-temperature oxidation methods, which are prone to damaging the carbon nanotube lattice structure due to harsh conditions. This method significantly improves the purity of single-walled carbon nanotubes while maximizing the protection of their intrinsic structural integrity, providing a reliable and mild technical path for the large-scale preparation of high-purity, low-defect carbon nanotube materials.

[0019] 2. The acidic potassium permanganate pre-oxidation treatment used in this invention, with its selective oxidation ability on amorphous carbon and defective carbon under mild conditions, can preferentially etch the carbon shell encapsulating the metal, weaken its structural density, create a kinetic advantage for subsequent impurity dissolution, and the process causes minimal damage to the perfect graphite region of the carbon nanotube itself, reducing the risk of tube wall damage caused by traditional strong acid or high temperature pretreatment from the source.

[0020] 3. The microwave-assisted liquid phase heating-acid washing synergistic process introduced in this invention utilizes the selective heating effect of microwaves on polar media and metal impurities to achieve local rapid heating and micro-area reaction enhancement on the basis of carbon shell pre-oxidation. This significantly accelerates carbon shell rupture and promotes the exposure and mass transfer process of metal cores, enabling subsequent mixed acid washing to achieve near-complete dissolution of metal impurities at lower temperatures and in a shorter time. Thus, while efficiently removing impurities, it avoids the cumulative thermal damage to the carbon nanotube structure caused by prolonged high-temperature treatment, ensuring that the product has excellent crystallinity and structural stability. Attached Figure Description

[0021] Figure 1 This is a TEM image of the sample from Example 2 of the present invention.

[0022] Figure 2 This is a TEM image of the sample from Comparative Example 1 of this invention.

[0023] Figure 3 The image shows the Raman spectrum test results of the sample in Example 2 of this invention.

[0024] Figure 4 The image shows the Raman spectrum test results of the sample in Comparative Example 1 of this invention.

[0025] Figure 5 The image shows the Raman spectrum test results of the sample in Comparative Example 2 of this invention. Detailed Implementation

[0026] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0027] Example 1: A method for efficient purification of single-walled carbon nanotubes S1: Weigh 1g of raw single-walled carbon nanotube material (the content of metal impurities was determined to be about 40% by thermogravimetric analysis), place it in 500mL of deionized water, and sonicate it at 300W for 30min to fully disperse it and form a uniform suspension. Add 25mL of a mixed acid composed of sulfuric acid and phosphoric acid (volume ratio of 3:1) to the suspension, stir and mix well, adjust the pH of the system to 1, and then add 0.1g of potassium permanganate. Continue stirring at room temperature until it is completely dissolved to obtain the pre-oxidation reaction solution.

[0028] S2: Transfer the pre-oxidized reaction solution to a microwave chemical reactor, set the reaction temperature to 70℃ and the microwave power to 400W, start the reactor, and continue heating under these conditions for 6 hours. After the treatment, allow the reaction system to cool naturally to room temperature, and slowly add 200mL of mixed acid solution (prepared by hydrochloric acid, nitric acid and hydrofluoric acid in a volume ratio of 3:1:0.5) to the system. Place the entire system in an 80℃ constant temperature water bath and stir continuously for 4 hours to fully dissolve the exposed metal impurities. After acid washing, vacuum filter the mixture to separate the solid product. Wash the filter cake repeatedly with a large amount of deionized water until the last filtrate is neutral. Transfer the washed solid product to a vacuum drying oven and dry it at 80℃ for 12 hours to obtain purified single-walled carbon nanotubes.

[0029] Example 2: A highly efficient purification method for single-walled carbon nanotubes, differing from Example 1 in that the mass ratio of potassium permanganate to single-walled carbon nanotubes is increased to 0.3:1, and the microwave treatment temperature is raised to 80°C. The specific steps are as follows: S1: Weigh 1g of raw single-walled carbon nanotube material (the content of metal impurities was determined to be about 40% by thermogravimetric analysis), place it in 500mL of deionized water, and sonicate it at 300W for 30min to fully disperse it and form a uniform suspension. Add 25mL of a mixed acid composed of sulfuric acid and phosphoric acid (volume ratio of 3:1) to the suspension, stir and mix well, adjust the pH of the system to 1, and then add 0.3g of potassium permanganate. Continue stirring at room temperature until it is completely dissolved to obtain the pre-oxidation reaction solution.

[0030] S2: Transfer the pre-oxidized reaction solution to a microwave chemical reactor, set the reaction temperature to 80℃ and the microwave power to 400W, start the reactor, and continue heating under these conditions for 6 hours. After the treatment, allow the reaction system to cool naturally to room temperature, and slowly add 200mL of mixed acid solution (prepared from hydrochloric acid, nitric acid, and hydrofluoric acid in a volume ratio of 3:1:0.5) to the system. Place the entire system in an 80℃ constant temperature water bath and stir continuously for 4 hours to fully dissolve the exposed metal impurities. After acid washing, vacuum filter the mixture to separate the solid product. Wash the filter cake repeatedly with a large amount of deionized water until the last filtrate is neutral. Transfer the washed solid product to a vacuum drying oven and dry it at 80℃ for 12 hours to obtain purified single-walled carbon nanotubes.

[0031] Example 3: This example provides a method for efficient purification of single-walled carbon nanotubes. Based on Example 2, the mass ratio of potassium permanganate to single-walled carbon nanotubes is further increased to 0.5:1, and the microwave treatment temperature is increased to 90°C. The specific steps are the same as in Example 2, but the key parameters are adjusted as follows: 0.5g of potassium permanganate is added, the microwave treatment temperature is 90°C, and the treatment time is 6h.

[0032] Example 4: This example provides a highly efficient purification method for single-walled carbon nanotubes. Based on Example 2, the mass ratio of potassium permanganate to single-walled carbon nanotubes is significantly increased to 1:1, and the microwave treatment time is extended to 45 min. The specific steps are the same as in Example 2, but the key parameters are adjusted as follows: 1 g of potassium permanganate is added, the microwave treatment temperature is 80 °C, and the treatment time is extended to 7.5 h.

[0033] Example 5: This example provides a highly efficient purification method for single-walled carbon nanotubes. Based on Example 2, the mass ratio of potassium permanganate to single-walled carbon nanotubes is significantly increased to 2:1. Other conditions remain the same as in Example 2. The specific steps are the same as in Example 2, but the key parameters are adjusted as follows: 2g of potassium permanganate is added, the microwave treatment temperature is 80℃, and the treatment time is 6h.

[0034] Examples 6-8: This example provides a method for efficient purification of single-walled carbon nanotubes. Single-walled carbon nanotube raw materials with different initial metal impurity contents are processed. The operation steps are the same as in Example 1. The difference is that the mass ratio of potassium permanganate to single-walled carbon nanotubes and the microwave treatment temperature are adjusted according to the metal content of the raw materials. The specific process parameters are listed in Table 1.

[0035] Comparative Example 1: Using the traditional pickling method, based on Example 2, the wet pre-oxidation in step S1 and the microwave heating treatment in step S2 are completely omitted. No oxidation pretreatment is performed, and only the subsequent pickling and post-treatment steps are retained. The specific steps are as follows: Take 1g of raw single-walled carbon nanotubes (metal content 40%), add 200mL of mixed acid (HCl:HNO3:HF=3:1:0.5), stir at 80℃ for 96h, then filter, wash and dry to obtain purified single-walled carbon nanotubes.

[0036] Comparative Example 2: Based on Example 2, microwave treatment is omitted, and microwave heating in step S2 is replaced with oil bath heating. All other steps and parameters remain the same as in Example 2. The specific steps are as follows: Take 1g of raw single-walled carbon nanotubes, disperse them in 500mL of deionized water, sonicate for 30min, add 25mL of sulfuric acid-phosphoric acid mixture to adjust the pH to 2, add 5g of potassium permanganate, stir to dissolve fully, heat in an oil bath at 80℃ for 30min, cool, add 200mL of mixed acid (HCl:HNO3:HF=3:1:0.5), stir at 60℃ for 48h, then filter, wash and dry to obtain purified single-walled carbon nanotubes.

[0037] The single-walled nanotube samples obtained from Examples 1-8 and Comparative Examples 1-2 were subjected to the following performance tests and analyses to evaluate the structural integrity, purity, and removal effect of metal impurities of the purified single-walled carbon nanotubes.

[0038] Transmission electron microscopy (TEM) analysis: To visually verify the removal effect of metal impurities and evaluate the microstructural integrity of single-walled carbon nanotubes, the micromorphology of the samples was observed using TEM. The specific steps are as follows: A small amount of sample was ultrasonically dispersed in ethanol and dropped onto a copper grid supported by an ultrathin carbon film. After the solvent evaporated, observation was performed at an accelerating voltage of 80-120 kV. The focus was on whether there were any residual metal catalyst particles and their carbonaceous inclusions (such as "carbon-coated iron"), and the morphological integrity and dispersion state of the carbon nanotubes themselves were evaluated at the same time. This analysis can provide direct structural evidence for the effectiveness of the purification method.

[0039] according to Figure 2 (TEM image of Comparative Example 1) shows that a large number of black spherical or near-spherical particles with sizes ranging from tens to hundreds of nanometers are scattered throughout the sample. These particles are attached to the surface of carbon nanotubes or scattered between the tube bundle network, and can be identified as ineffectively removed metal catalyst and its dense carbonaceous encapsulation; however, according to Figure 1(TEM image of sample from Example 2) As can be seen, the sample field of view is mainly composed of interwoven and entangled single-walled tubular bundles. The tubular structure is clear and continuous, and it is distributed in a network pattern. Almost no other structures were observed within the entire observation range. Figure 2 The presence of similar metal or carbon-encapsulated particles.

[0040] The TEM images above clearly demonstrate that, compared to the traditional acid washing method in Comparative Example 1, the method provided by this invention can efficiently destroy and remove the carbon shell encapsulating the metal catalyst, thereby fully exposing and dissolving the internal metal core in the subsequent acid washing step. This provides the most direct microscopic morphological evidence for the high purity data of the examples, fully proving that the present invention has significant advantages in removing "carbon-encased iron" impurities. At the same time, the carbon nanotubes in the TEM images of Example 2 have intact morphology, without breakage or obvious defects, which also confirms that the method can effectively protect the bulk structure of carbon nanotubes while efficiently purifying them.

[0041] Purity testing: The ash content determination method was adopted. First, several 30ml ceramic crucibles were ignited at 900℃ for 4 hours, removed and cooled to room temperature in a desiccator, and weighed. The same conditions were repeated for ignition, cooling and weighing until the weight difference between two weighings was ≤0.0002g, which was recorded as the constant weight m0 of the empty crucible. After constant weight, the crucible was placed on an analytical balance, and 1.0g of the example / comparative sample was weighed after tare (accurate to 0.0001g, recorded as the sample amount m1). The sample crucibles were placed in sequence in a muffle furnace at 900℃ for ignition for 5 hours, removed and cooled to room temperature in a desiccator, and the total weight m2 of (crucible + residue) after ignition was directly weighed. The ash content was calculated according to the following formula: Ash content (%) = (total weight after ignition m2 - crucible weight m0) / sample amount m1 × 100%, and then the sample purity was calculated: Sample purity = 1 - Ash content; The purity results are shown in Table 2.

[0042] Raman spectroscopy analysis: To systematically evaluate the effect of the purification process on the crystal structure of single-walled carbon nanotubes, Raman spectroscopy was used to test the product samples of Examples 1-8 and Comparative Examples 1-2. Raman spectroscopy is an effective technique for characterizing the structural order of carbon nanotubes, and its D peak (approximately 1350 cm⁻¹) is used. -1 ) and G peak (approximately 1580cm) -1 The intensity ratio (ID / IG) of carbon materials directly reflects their structural defects and graphitization degree. A lower ID / IG value indicates a higher degree of graphitization and fewer structural defects, meaning the purification process causes less damage to the crystal structure of the carbon nanotubes. The specific testing method is as follows: The dried sample was placed on a glass slide and tested using a Raman spectrometer equipped with a laser wavelength of 532 nm. The laser power was controlled below 1 mW to avoid thermal damage to the sample. By accurately analyzing the peak position, full width at half maximum (FWHM), and integrated intensity of the D and G peaks in the spectrum, the IG / ID ratio was calculated, thereby quantitatively evaluating the structural quality of the carbon nanotubes.

[0043] Raman spectroscopy test results ( Figures 3-5 The results show that all samples exhibit clear D and G peak characteristics. Key parameters are compared below: Example 2 ( Figure 3 The product exhibits an IG / ID ratio as high as 82, demonstrating extremely low structural defect density and excellent structural integrity. In contrast, the comparative example 1 (using the conventional acid leaching method) shows a much lower IG / ID ratio. Figure 4 ) and Comparative Example 2 ( Figure 5 The IG / ID ratios of the products were 45 and 65, respectively, which were significantly lower than those of Example 2. This data comparison shows that the purification method provided by the present invention can effectively remove impurities while causing less damage to the crystal structure of the single-walled carbon nanotube body, and has a significant protective effect.

[0044] It should be noted that, in this document, terms such as “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0045] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for efficient purification of single-walled carbon nanotubes, characterized in that, Includes the following steps: Step 1: Disperse the raw single-walled carbon nanotubes in deionized water to form a uniform suspension, add a mixed acid to adjust the system to an acidic environment, add potassium permanganate, and dissolve by stirring at room temperature to obtain a pre-oxidation reaction solution; Step 2: The pre-oxidized reaction solution is subjected to microwave heating, followed by acid washing, filtration, washing and drying to obtain purified single-walled carbon nanotubes.

2. The method for efficient purification of single-walled carbon nanotubes according to claim 1, characterized in that, The metal content of the original single-walled carbon nanotube raw material is 25%-75%.

3. The method for efficient purification of single-walled carbon nanotubes according to claim 1, characterized in that, The mass ratio of the original single-walled carbon nanotube raw material to potassium permanganate is 0.1-2:

1.

4. The method for efficient purification of single-walled carbon nanotubes according to claim 3, characterized in that, The mass ratio of the original single-walled carbon nanotube raw material to potassium permanganate is 0.5-1:

1.

5. The method for efficient purification of single-walled carbon nanotubes according to claim 1, characterized in that, The mixed acid is composed of sulfuric acid and phosphoric acid mixed in a volume ratio of 3:0-1.

6. The method for efficient purification of single-walled carbon nanotubes according to claim 1, characterized in that, The pH value of the acidic environment is 1-4.

7. The method for efficient purification of single-walled carbon nanotubes according to claim 1, characterized in that, The conditions for microwave heating treatment are: temperature 70-90℃, microwave power 100-1000W, and treatment time 1-12h.

8. The method for efficient purification of single-walled carbon nanotubes according to claim 7, characterized in that, The microwave power is 300-600W.

9. The method for efficient purification of single-walled carbon nanotubes according to claim 1, characterized in that, The pickling solution is composed of hydrochloric acid, nitric acid and hydrofluoric acid in a volume ratio of 1-5:1:0-1.

10. The method for efficient purification of single-walled carbon nanotubes according to claim 1, characterized in that, The pickling conditions are a temperature of 80-90℃ and a pickling time of 12-96 hours.

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