Efficient purification of carbon nanotubes by the

2022-08-09
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Abstract: carbon nanotubes were pretreated with alkali. Because alkali is a dispersant, it can strengthen the reaction between carbon nanoparticles and oxidants, and separate carbon nanotubes from other forms of carbon. Compared with the air oxidation method adopted by Ajayan, this method has the advantages of high purification efficiency and less burning loss of carbon nanotubes

key words: carbon nanotubes; Purification

since Iijima first discovered carbon nanotubes from the cathode scar of C60 by using high-resolution electron microscopy (HRTEM) in 1991 [1], because of its structural particularity (the radial size is in the order of nanometers and the axial size is in the order of microns), it shows a typical one-dimensional quantum material, which has high machinery. China wants to complete the new green building of 1billion square meters, strength and extraordinary magnetoresistance and thermal conductivity []. This makes physicists and chemists want to give full research in experiments. At the same time, it may also be used in future molecular electronic devices or nano electronic devices, and it can also provide chemists with the smallest test tube for nano chemical reactions

at present, there are many methods to synthesize carbon nanotubes, but the macro quantity synthesis mostly adopts the arc method. Because the carbon nanotubes generated by the arc method are often mixed with a large number of impurities - carbon nanoparticles, the testing of the properties of carbon nanotubes has been greatly hindered due to the existence of these impurities

ajayan et al. [4] proposed a method to purify carbon nanotubes. They can obtain pure carbon nanotubes when they burn the crude carbon nanotubes to 1%. Using this method, the burning rate of carbon nanotubes is large and the purification efficiency is low. The author improved this method, reduced the burning rate to 80%, and greatly improved the purification efficiency

1 experimental part

1.1 preparation of samples

carbon nanotubes were prepared by arc method. In the atmosphere with helium pressure of 0.067 MPa, a spectral pure carbon rod with a diameter of 8 mm is used as the anode, and a DC power supply is added between the anode and cathode electrodes: the voltage is 18 V and the current is 100 a. The discharge between the two electrodes produces plasma, the anode graphite rod is gradually consumed, and the sediment on the surface of the large graphite cathode gradually bulges the cylinder, which can be clearly divided into a loose core and a hard gray shell. Take out the powder in the hard shell, which contains a large amount of carbon nanotubes

1.2 pretreatment of samples

take 2 g of the above powder and put it into a flask, and then add 50 ml, 2 mol/l NaOH solution. Install the reflux condensing device and heat the reflux for 2h. After cooling to room temperature, filter the resulting mixture to obtain a black fine powder. Wash the black powder with deionized water, and check whether it is clean with pH test paper (the same as the pH value of deionized water). Transfer the washed powder into a watch glass and dry it at 373 K for 3 h

1.3 purification of carbon nanotubes

take 1g of pretreated carbon nanotubes in a pre weighed crucible, put them into a tubular furnace and calcine them at 873 K for 1.5 h. at this time, the weight loss rate can reach 70%. Take out the crucible, cool it to room temperature, and then soak the calcined carbon nanotubes in 100 ml of 10% dilute sulfuric acid for 2 hours (or heat it to slightly boiling state for 10 minutes). Then filter the acid treated carbon nanotubes, wash the products with deionized water and dry them at 373 K to obtain pure carbon nanotubes with a yield of 20%

2 results and discussion

2.1 effect of alkali treatment

in the purification process, we used NaOH solution to pretreat carbon nanotubes. Alkali mainly reacts with organic functional groups in crude carbon nanotubes to dissolve organic functional groups. We know that carbon black will be oxidized slowly in the air, and a large number of organic functional groups such as carboxyl, phenolic hydroxyl, carbonyl will be produced on its surface [5]. These organic functional groups will polymerize with each other, so that carbon nanotubes and carbon nanoparticles will aggregate into larger particles, the surface area will be reduced, and the activity of its reaction with oxidants will be reduced. After alkali treatment, these organic groups react with alkali and no longer exist. The generated substances have certain surface activity, which effectively separates carbon nanotubes and carbon nanoparticles, and increases the surface area. In the purification process, carbon nanoparticles are easier to react with oxidants, thereby reducing the burning rate of carbon nanotubes. In order to investigate the effect of alkali treatment, we used scanning electron microscope to analyze the samples treated with and without alkali treatment. The SEM photos obtained are shown in Figure 1

Figure 1 scanning electron microscope image of cathode deposit

from Figure 1, we can see that the particle dispersion in Figure 1 (b) is better than that in Figure 1 (a). It can be proved that alkali is a good dispersant, which can disperse the carbon nanotubes and carbon nanoparticles gathered together into smaller particles, so that they have a larger specific surface area and increase the chemical reaction activity. We used pretreated and untreated carbon nanotube crude products to calcine at 837 K. The results are shown in Figure 2

Fig. 2 weight loss rate of different active samples

from the results of Fig. 2, we can see that the activity of the samples treated with alkali is increased by about 60% compared with that without alkali treatment. This proves that alkali can effectively separate carbon nanotubes and carbon nanoparticles, and increase the surface area. In the purification process, the more active carbon nanoparticles are easier to preferentially react with oxidants, reducing the burning rate of carbon nanotubes. The purification efficiency was increased to 20%, much higher than that of Ajayan

2.2 pickling effect

after carbon nanotubes are calcined, it is difficult to obtain pure carbon nanotubes without post-treatment. We observed the calcined carbon nanotube samples with transmission electron microscope, and the TEM image is shown in Figure 3

Fig. 3 TEM image of carbon nanotubes without acid pickling

from Fig. 3, we can see that carbon nanotubes are surrounded by a large number of substances, which makes it difficult for us to observe carbon nanotubes. Part of these substances may be the oxidation of amorphous carbon and oxygen in the air. In addition to the formation of CO gas, some carbon atoms are slowly oxidized to form oxygen-containing functional groups with carboxyl, phenolic hydroxyl, carbonyl, ether, hydrogen peroxide, nitroso and lactone structures. These functional groups adhere to the surface of carbon nanotubes through the formation of chemical bonds, and they cannot be separated from carbon nanotubes by ultrasonic oscillation; Another part of these substances may be the fullerene oxide formed. We know that the cap of carbon nanotubes can be seen as composed of fullerene hemispheres. Carbon nanoparticles have a similar structure to the cap of carbon nanotubes, that is, the outer layer of carbon nanoparticles is low-carbon fullerenes composed of fewer carbon atoms. Low carbon fullerenes react with oxygen to form fullerene oxide, a new material that is expected to replace ABS by 2012. In fact, in the experiment, we found that the sample without pickling treatment was reddish brown, and the solution after pickling was red. These characteristics were consistent with fullerene oxide

these fullerene oxides are difficult to dissolve in alkali solution to form hydroxyl compounds, but they are easy to dissolve in acid solution to form corresponding fullerene compounds; At the same time, pickling can also eliminate the metal ions introduced in the synthesis process (in order to obtain a higher yield and a specific shape, metal containing catalysts are often used in the synthesis process), but alkali washing does not have this function

Figure 4 is the TEM image after acid pickling. From Figure 4, we can see that there are basically no carbon nanoparticles and other impurities around carbon nanotubes, only carbon nanotubes. In the calcination process, because carbon nanoparticles are composed of five member rings with more hanging bonds, it preferentially reacts with oxygen to produce the above substances. However, the chemical properties of the carbon nanotube wall composed of six member rings are relatively stable, and the above reactions do not occur. After pickling, the acid reacts with the above reaction products, which are dissolved in the acid and enter the filtrate during the filtration process, so as to realize the purification of carbon nanotubes

Figure 4 TEM image of carbon nanotubes after pickling

Article No. (1999)

author unit: Changsha 410083 Institute of metallurgical physical chemistry and new chemical materials, Central South University of technology

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