CN117776163A - Purification method of single-walled carbon nanotube - Google Patents
Purification method of single-walled carbon nanotube Download PDFInfo
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Abstract
The invention discloses a purification method of single-walled carbon nanotubes, which comprises the following steps: carrying out first medium-temperature oxidation on the single-walled carbon nanotube to be purified at 200-500 ℃; wherein, the single-walled carbon nanotube to be purified contains metal impurity particles coated by graphite inside; carrying out high-temperature reduction on the oxidized product at 600-1200 ℃; and (3) oxidizing the product obtained by high-temperature reduction again at the temperature of 200-500 ℃ and then performing first acid leaching. The first medium-temperature oxidation provides a reaction raw material, namely metal oxide, for high-temperature reduction, the metal oxide and the graphite coating layer undergo a reduction reaction to etch away the graphite coating layer, so that impurity particles of a metal simple substance coated by the metal oxide are exposed, the metal simple substance is further oxidized by re-oxidation, and finally the metal and the metal oxide are completely reacted by first acid leaching, thereby achieving the purpose of removing large-particle metal impurities with thicker graphite coating layers in the single-wall carbon nanotubes.
Description
Technical Field
The invention relates to the technical field of carbon nano materials, in particular to a purification method of single-walled carbon nanotubes.
Background
The single-wall carbon nanotube is a one-dimensional nano material formed by curling a layer of graphite sheet, and has extremely excellent electrical properties, super-strong toughness, strength and other mechanical properties. Since the last century, the catalyst has a wide application prospect in the fields of new energy storage, catalysts, nano devices and the like.
It is generally believed that the growth mechanism of single-walled carbon nanotubes is to grow tubular structures by carbon atoms that precipitate from the interior of the nano-metal particles and form solid carbon caps on their surfaces. The growth mechanism of the single-walled carbon nanotubes can be known, and the prepared single-walled carbon nanotubes necessarily contain metal particles. When the single-wall carbon nano tube is used as a conductive agent of a lithium ion battery, metal impurities such as iron, nickel and the like contained in the single-wall carbon nano tube are easy to cause micro short circuit in the battery, so that potential safety hazards are caused. Thus, the single-walled carbon nanotubes produced must be purified to remove metal impurities, thereby facilitating downstream applications.
The existence forms of metal impurities in the single-walled carbon nanotubes are mainly two types: firstly, metal particles smaller than 2nm exist in the carbon nano tube, and impurities existing in the form of the metal particles occupy less amount; secondly, the graphite carbon coats the core-shell structure of the metal particles, and the metal impurities of the structure occupy a larger proportion. At present, products prepared by domestic single-wall carbon nanotube production enterprises contain a core-shell structure with more graphite coated metal particles, the particle size of the core-shell structure is between 10 and 50nm, and the particle size is larger. For purification of single-walled carbon nanotubes, the two-step chemical method of conventional air oxidation or liquid phase oxidation followed by acid leaching is mainly used in the industry at present, but because the graphite coating layer of metal particles in domestic single-walled carbon nanotubes is thicker (the graphite coating layer of metal particles is mostly more than 5 nm), the gaseous or liquid oxidant is difficult to destroy or penetrate through the graphite coating layer to oxidize internal metal in the conventional oxidation process, and the acid liquor is difficult to penetrate through the graphite coating layer to etch and consume metal in the subsequent acid leaching process.
In view of this, the present invention has been made.
Disclosure of Invention
The present invention is directed to a method for purifying single-walled carbon nanotubes, which is capable of improving the above-mentioned problems.
The invention is realized in the following way:
the invention provides a purification method of single-walled carbon nanotubes, which comprises the following steps: carrying out first medium-temperature oxidation on the single-walled carbon nanotube to be purified at the temperature of 200-500 ℃; wherein the single-walled carbon nanotubes to be purified contain metal impurity particles coated with graphite inside. And (3) carrying out high-temperature reduction on the product obtained by oxidation at 600-1200 ℃. And (3) oxidizing the product obtained by high-temperature reduction again at the temperature of 200-500 ℃, and performing first acid leaching by acid liquor.
Optionally, the single-walled carbon nanotubes to be purified are single-walled carbon nanotubes which are not purified after direct production, and before the first intermediate-temperature oxidation, the single-walled carbon nanotubes to be purified are subjected to the pre-intermediate-temperature oxidation at 200-500 ℃ and the pre-acid leaching by acid liquor.
Optionally, the oxidation modes of pre-intermediate temperature oxidation, first intermediate temperature oxidation and reoxidation are all that air is introduced into a tube furnace for oxidation.
Optionally, the air flow rates of the pre-intermediate temperature oxidation, the first intermediate temperature oxidation and the reoxidation are all 0.1L/min-10L/min, the oxidation time of the first two intermediate temperature oxidation is at least 0.2h, preferably 0.2 h-6 h, and the oxidation time of the reoxidation is at least 1h, preferably 1 h-6 h.
Optionally, the pre-acid leaching and the first acid leaching each comprise: placing the acid-leaching matter into the acid liquor, and carrying out ultrasonic oscillation in an ultrasonic oscillator.
Optionally, the ultrasonic oscillation time is 1-24 hours.
Alternatively, the acid solution is a strong acid, preferably hydrochloric acid, sulfuric acid, nitric acid or any combination thereof.
Optionally, the concentration of the acid liquor is 3 mol/L-16 mol/L, and the liquid-solid mass ratio of the acid liquor to the to-be-leached matter is 50-500: 1.
optionally, the purification method further comprises sequentially performing solid-liquid separation, washing and drying after the pre-acid leaching and the first acid leaching respectively.
Optionally, the temperature of the pre-medium temperature oxidation and the first medium temperature oxidation are both 350 ℃ to 400 ℃; and/or, the temperature of reoxidation is 250 ℃ to 300 ℃.
Alternatively, the time for the high temperature reduction is at least 0.5h, preferably 0.5h to 6h.
Alternatively, the high temperature reduction is performed under an inert atmosphere.
Optionally, the purified single-walled carbon nanotubes comprise at least one of the following conditions:
(1) The metal content is below 1 wt%;
(2) I of Raman Spectroscopy G /I D Above 70;
(3) The yield is more than 40 percent.
The invention has the following beneficial effects: the method comprises the steps of carrying out first medium-temperature oxidation and high-temperature reduction on a single-walled carbon nanotube to be purified, wherein the first medium-temperature oxidation provides a reaction raw material, namely metal oxide, for the high-temperature reduction, the metal oxide and a graphite coating layer undergo a reduction reaction to enable the graphite coating layer to be etched and consumed, so that impurity particles of the metal oxide coated metal simple substance are exposed, further oxidizing the metal simple substance and the metal oxide by re-oxidation, and finally completely reacting the metal and the metal oxide by first acid leaching, thereby achieving the purpose of removing large-particle metal impurities with thicker graphite coating layers in the single-walled carbon nanotube.
Drawings
In order to more clearly illustrate the technical solutions of the embodiments of the present invention, the drawings that are needed in the embodiments will be briefly described below, it being understood that the following drawings only illustrate some embodiments of the present invention and therefore should not be considered as limiting the scope, and other related drawings may be obtained according to these drawings without inventive effort for a person skilled in the art.
FIG. 1 is a schematic drawing of the HRTEM micro morphology of the coated metal in a single-walled carbon nanotube according to an embodiment of the present invention;
FIG. 2 is a TGA curve of unpurified single-walled carbon nanotubes of example 1 of the present invention;
FIG. 3 is a TGA-DTG curve of purified single-walled carbon nanotubes obtained in example 1 of the present invention;
FIG. 4 is a Raman spectrum of single-walled carbon nanotubes before and after purification of example 1 of the present invention.
Detailed Description
In order to make the objects, technical solutions and advantages of the embodiments of the present invention more clear, the technical solutions of the embodiments of the present invention will be clearly and completely described below. The specific conditions are not noted in the examples and are carried out according to conventional conditions or conditions recommended by the manufacturer. The reagents or apparatus used were conventional products commercially available without the manufacturer's attention.
The purification method of the single-walled carbon nanotube provided by the invention is specifically described below.
Some embodiments of the present invention provide a method for purifying single-walled carbon nanotubes comprising: carrying out first medium-temperature oxidation on the single-walled carbon nanotube to be purified at the temperature of 200-500 ℃; wherein the single-walled carbon nanotubes to be purified contain metal impurity particles coated with graphite inside. And (3) carrying out high-temperature reduction on the product obtained by oxidation at 600-1200 ℃. And (3) oxidizing the product obtained by high-temperature reduction again at the temperature of 200-500 ℃, and performing first acid leaching by acid liquor.
The inventor finds that, because the graphite coating layer of the metal particles in the domestic single-wall carbon nanotube is thicker (the graphite coating layer of the metal particles is mostly more than 5 nm), the transmission electron microscope image is shown as shown in fig. 1, so that the gaseous or liquid oxidant is difficult to destroy or penetrate the graphite layer to oxidize all internal metals in the conventional oxidation process, and acid liquor is difficult to enter the graphite coating layer to etch and consume the metals in the subsequent first acid leaching process. Based on this, the inventors creatively proposed the above-mentioned scheme through a great deal of research and practice. The metal close to the graphite coating layer is oxidized through first medium-temperature oxidation, and at the moment, the graphite coating layer is not broken by the metal oxide yet because of thicker graphite coating layer, so that a three-layer core-shell structure with the outer layer of graphite, the middle layer of metal oxide and the inner layer of metal is formed. The middle layer (metal oxide) of the three-layer core-shell structure is in contact with graphite, reaction raw materials and reaction conditions are provided for high-temperature reduction, and the reaction raw materials and the reaction conditions are subjected to reduction reaction with the graphite coating layer at high temperature to etch and consume the graphite coating layer, so that impurity particles of metal oxide coated metal simple substance are exposed. And then oxidizing again, wherein the metal simple substance can be oxidized well due to no blocking of the graphite coating layer, and finally the metal and the oxide thereof are reacted completely through the first acid leaching, so that metal particles with thicker graphite coating layers in the single-walled carbon nanotube are removed effectively, and the purity of the single-walled carbon nanotube is improved greatly.
It should be noted that the single-walled carbon nanotubes to be purified may be directly produced without any purification treatment, or may be single-walled carbon nanotubes subjected to preliminary purification treatment to remove some easily removed metal particles or impurities.
As a reference, the temperature of the first medium-temperature oxidation may be selected to be 200 ℃, 210 ℃, 220 ℃, 230 ℃, 240 ℃, 250 ℃, 260 ℃, 270 ℃, 280 ℃, 290 ℃, 300 ℃, 310 ℃, 320 ℃, 330 ℃, 340 ℃, 350 ℃, 360 ℃, 370 ℃, 380 ℃, 390 ℃, 400 ℃, 410 ℃, 420 ℃, 430 ℃, 440 ℃, 450 ℃, 460 ℃, 470 ℃, 480 ℃, 490 ℃, 500 ℃, or the like, and the lower the temperature, the longer the corresponding oxidation time is.
In some embodiments, in order to further optimize the oxidation effect of the first medium-temperature oxidation, the temperature of the first medium-temperature oxidation is selected to be 350-400 ℃, that is, the graphite coating layer coated by some metal particles inside the single-walled carbon nanotubes is thicker, so that the oxidation difficulty of the metal particles inside the single-walled carbon nanotubes is increased, the oxidation effect of the metal particles inside the single-walled carbon nanotubes is easily poor when the temperature is lower, and the loss of the carbon nanotubes is easily caused by the oxidation of the carbon nanotubes catalyzed by some metal particles inside the single-walled carbon nanotubes when the temperature is too high.
Further, in order to achieve the purpose of the first medium-temperature oxidation, that is, oxidation of the metal inside the thicker carbon coating layer, the oxidation mode of the first medium-temperature oxidation is that air is introduced into a tube furnace for oxidation. The molecules of the carbon coating are relatively smaller than those of the liquid oxidation by the gas oxidation mode, so that the carbon coating is easier to enter the inside of the carbon coating for partial oxidation. The air subjected to oxidation is illustratively compressed air, and in some other embodiments, the sample may be selected from oxygen and the like.
In some embodiments, the air flow rate of the first medium temperature oxidation is from 0.1L/min to 10L/min, e.g., the flow rate is selected to be 0.1L/min, 0.5L/min, 1L/min, 1.5L/min, 2L/min, 2.5L/min, 3L/min, 3.5L/min, 4L/min, 4.5L/min, 5L/min, 6L/min, 6.5L/min, 7L/min, 7.5L/min, 8L/min, 9L/min, 10L/min, or the like. The oxidation time of the first medium temperature oxidation is at least 0.2h to achieve sufficient oxidation of the internal metal particles, and in general, the greater the air flow, the correspondingly reduced the oxidation time, and in some preferred embodiments, the time of the first medium temperature oxidation may be selected to be from 0.2h to 6h, e.g., may be selected to be 0.2h, 0.5h, 0.8h, 1h, 1.5h, 2h, 2.5h, 3h, 3.5h, 4h, 4.5h, 5h, 5.5h, or 6h, etc.
As a reference, the temperature of the high-temperature reduction may be selected from 600 ℃, 620 ℃, 630 ℃, 650 ℃, 680 ℃, 700 ℃, 720 ℃, 750 ℃, 780 ℃, 790 ℃, 800 ℃, 810 ℃, 820 ℃, 850 ℃, 900 ℃, 920 ℃, 950 ℃, 970 ℃, 1000 ℃, 1050 ℃, 1100 ℃, 1150 ℃, 1200 ℃ or the like. Preferably, the high temperature reduction temperature is 800 ℃ to 1100 ℃. Further, in order to enable sufficient removal of the carbon coating, the time of high temperature reduction is at least 0.5h, in general, the lower the temperature, the longer the time of high temperature reduction. Preferably, the time for the high temperature reduction may be selected to be 0.5 to 6 hours, for example, 0.5 hours, 0.8 hours, 1 hour, 1.5 hours, 2 hours, 2.5 hours, 3 hours, 3.5 hours, 4 hours, 4.5 hours, 5 hours, 5.5 hours, or 6 hours, etc. In addition, the high temperature reduction time is not easy to be too long, and the residual metal oxide is easy to react with the carbon nano tube under the high temperature condition, so that the yield is reduced.
In general, the high-temperature reduction reaction is performed under an inert atmosphere, for example, under an atmosphere of nitrogen or a rare gas (argon or the like) to avoid the influence of other oxidizing gases on the reduction reaction. In addition, the cooling process after reduction is performed under an inert atmosphere, which is because the temperature of the reduced product is relatively high during the cooling process, and the presence of the oxidizing gas can cause reoxidation of the reduced product, resulting in mass loss of the carbon nanotubes.
Further, in some embodiments, the high-temperature reduction is performed in a tube furnace, specifically, the product after the first medium-temperature oxidation is placed in the tube furnace, a certain inert gas is continuously introduced until the air is exhausted, and then the temperature is raised again to perform the high-temperature reduction reaction.
As a reference, the temperature of the reoxidation may be selected to be 200 ℃, 210 ℃, 220 ℃, 230 ℃, 240 ℃, 250 ℃, 260 ℃, 270 ℃, 280 ℃, 290 ℃, 300 ℃, 310 ℃, 320 ℃, 330 ℃, 340 ℃, 350 ℃, 360 ℃, 370 ℃, 380 ℃, 390 ℃, 400 ℃, 410 ℃, 420 ℃, 430 ℃, 440 ℃, 450 ℃, 460 ℃, 470 ℃, 480 ℃, 490 ℃, 500 ℃, or the like, and the lower the temperature is, the longer the corresponding oxidation time is.
In some embodiments, the reoxidation temperature is 250 ℃ to 300 ℃ in order to achieve a better reoxidation effect and to maintain a better yield of single-walled carbon nanotubes. In this case, the re-oxidation is actually a low-temperature oxidation with respect to the first intermediate-temperature oxidation, because the graphite coating layer on the surface of the metal impurity particles is removed by the first intermediate-temperature oxidation and high-temperature reduction, and the difficulty in oxidizing the metal element is greatly reduced, while the oxidation of the metal element is easily performed at too high a temperature (for example, 300 ℃ or higher), and the yield is easily lowered by catalyzing the oxidation of the single-walled carbon nanotube with the metal element. Further, in order to make reoxidation sufficient, the reoxidation time is at least 1h, and since reoxidation is a low-temperature oxidation at a temperature lower than that of the first intermediate-temperature oxidation, the oxidation time is also relatively long, and preferably the reoxidation time is 1 to 6h, for example, 1h, 2h, 3h, 4h, 5h, 6h, or the like.
Further, after the thicker carbon coating layer is removed and the metal is fully oxidized, the product is subjected to first acid leaching through acid liquor, so that the metal and the oxide thereof can be well removed. The specific operation of the first acid leaching is as follows: placing the acid-leaching matter into acid liquor, and performing ultrasonic oscillation in an ultrasonic oscillator to realize full reaction and dissolution of metal and oxide thereof. The ultrasonic oscillation time is 1 h-24 h, such as 1h, 2h, 3h, 4h, 5h, 6h, 7h, 8h, 9h, 10h, 11h, 12h, 13h, 14h, 15h, 16h, 17h, 18h, 19h, 20h, 21h, 22h, 23h or 24h, etc.
In particular, the acid solution is a strong acid, including but not limited to hydrochloric acid, sulfuric acid, or nitric acid. In order to achieve better reaction corrosion effect, the concentration of the acid solution can be selected to be 3mol/L to 16mol/L, for example, 3mol/L, 4mol/L, 5mol/L, 6mol/L, 7mol/L, 8mol/L, 9mol/L, 10mol/L, 11mol/L, 12mol/L, 13mol/L, 14mol/L, 15mol/L or 16mol/L and the like, and the liquid-solid mass ratio of the acid solution to the to-be-leached matter is 50 to 500:1, for example 50: 1. 100: 1. 150: 1. 200: 1. 250: 1. 300: 1. 350: 1. 400: 1. 450:1 or 500:1, etc.
In general, the first acid leaching is followed by solid-liquid separation, washing and drying, and the solid-liquid separation is usually carried out by filtration.
Further, if the single-walled carbon nanotube to be purified is an unpurified single-walled carbon nanotube obtained by direct production, that is, the unpurified single-walled carbon nanotube is directly subjected to the first intermediate-temperature oxidation and high-temperature reduction operation in the above embodiment, although the single-walled carbon nanotube with higher purity (the metal content is less than 1%) can also be obtained, because some metal particles which are not coated with graphite or are not completely coated with graphite or are coated with thinner graphite (the graphite coating is less than or equal to 2 nm) exist in the unpurified single-walled carbon nanotube, the metal oxide formed after the first intermediate-temperature oxidation reacts with the single-walled carbon nanotube in the high-temperature reduction process, so that the mass loss of the single-walled carbon nanotube is further increased, and finally the yield of the purified carbon nanotube is extremely low and the industrialization significance is lower.
Thus, for the above reasons, the single-walled carbon nanotubes to be purified in the above embodiments generally refer to single-walled carbon nanotubes from which the easily removable metal particles and less graphite-coated metal particles are removed.
Even if the carbon nanotubes to be purified are single-walled carbon nanotubes that are directly produced without purification, some embodiments of the present invention also disclose a method for purifying single-walled carbon nanotubes, comprising: pre-intermediate temperature oxidation is carried out on the single-walled carbon nano tube to be purified at 200-500 ℃, acid liquor pre-acid leaching is carried out, and then the single-walled carbon nano tube subjected to the pre-acid leaching is subjected to first intermediate temperature oxidation at 200-500 ℃; carrying out high-temperature reduction on the oxidized product at 600-1200 ℃; and (3) oxidizing the product obtained by high-temperature reduction again at the temperature of 200-500 ℃, and performing first acid leaching by acid liquor. The process of the first intermediate-temperature oxidation, high-temperature reduction, reoxidation, first acid leaching and the like is the same as that of the previous embodiment, and will not be described again.
Firstly, through a pre-medium temperature oxidation and pre-acid leaching process, the metal and oxide of a small-particle-size and thin-graphite coating layer contacted with the carbon nano tube are removed, so that on one hand, the quality loss of the carbon nano tube caused by the oxidation of the carbon nano tube catalyzed by metal particles in the secondary oxidation process can be avoided, and on the other hand, the carbon nano tube loss caused by the reduction reaction of the metal oxide contacted with the carbon nano tube and the carbon nano tube in the high-temperature reduction stage can be avoided. Therefore, the yield of the purified carbon nano tube can be improved to more than 40 percent.
Further, the operation processes of the pre-intermediate temperature oxidation and the pre-acid leaching in the above embodiment are referred to the operation processes of the first intermediate temperature oxidation and the first acid leaching in the subsequent stage in the foregoing embodiment, and are not described herein. The specific parameter values may be the same as or different from the subsequent stages, as long as they are within the defined range values.
The features and capabilities of the present invention are described in further detail below in connection with the examples.
The unpurified single-walled carbon nanotubes used in examples 1 to 6 and comparative examples 1 to 3 of the present invention had a metal content of 30.5% (as measured by TG method, see FIG. 2, calculated), raman I G /I D Single-walled carbon nanotubes at 40 (see raman spectrum of fig. 4).
Example 1
The embodiment provides a purification method of a single-walled carbon nanotube, which specifically comprises the following steps:
1) Firstly, placing an unpurified single-walled carbon nanotube in a corundum crucible, introducing compressed air into a tube furnace, performing medium-temperature oxidation, and controlling the flow rate of the compressed air: 0.3L/min, oxidation temperature: the sample is placed in a clean glass beaker after the sample is cooled after the oxidation is completed at 380 ℃ for 2.5 hours.
2) Pouring acid liquor with certain concentration and certain mass into a beaker containing oxidized carbon nano tubes, sealing the beaker by using a preservative film, and then placing the beaker in an ultrasonic oscillator for ultrasonic oscillation. The acid liquor is hydrochloric acid, the concentration is 6mol/L, and the liquid-solid ratio is 150:1, ultrasonic oscillation time is 5 hours. Filtering, washing and drying after the reaction is finished for standby.
3) And (3) continuously placing the dried sample in a tube furnace for continuous air oxidation, wherein the air flow, the oxidation time and the oxidation temperature are unchanged, and cooling to room temperature for standby after the oxidation is finished.
4) And placing the sample subjected to secondary oxidation in a tube furnace, continuously introducing a certain amount of inert gas until air is exhausted, and then heating to perform high-temperature reduction reaction. The reaction temperature is 850 ℃, the reaction time is 2 hours, and the reaction is naturally cooled to a lower temperature in an inert atmosphere after the reaction is finished.
5) And stopping introducing inert gas when the sample subjected to high-temperature reduction is cooled to 250 ℃ in the tubular furnace, introducing a certain amount of compressed air, keeping the flow unchanged, and preserving the temperature for 4 hours at the temperature to strengthen the oxidation of bare metal particles, and cooling to room temperature along with the furnace after the reaction is finished.
6) And repeating the acid leaching process, and then filtering, washing and drying to obtain the high-purity single-walled carbon nanotube.
Example 2
The embodiment provides a purification method of a single-walled carbon nanotube, which specifically comprises the following steps:
1) Firstly, placing an unpurified single-walled carbon nanotube in a corundum crucible, introducing compressed air into a tube furnace, performing medium-temperature oxidation, and controlling the flow rate of the compressed air: 0.5L/min, oxidation temperature: and (3) oxidizing for 1h at 450 ℃, and placing the sample in a clean glass beaker after the sample is cooled after the oxidization is completed.
2) Pouring acid liquor with certain concentration and certain mass into a beaker containing oxidized carbon nano tubes, sealing the beaker by using a preservative film, and then placing the beaker in an ultrasonic oscillator for ultrasonic oscillation. The acid liquor is hydrochloric acid, the concentration is 8mol/L, and the liquid-solid ratio is 100:1, ultrasonic oscillation time is 4 hours. Filtering, washing and drying after the reaction is finished for standby.
3) And (3) continuously placing the dried sample in a tube furnace for continuous air oxidation, wherein the air flow, the oxidation time and the oxidation temperature are unchanged, and cooling to room temperature for standby after the oxidation is finished.
4) And placing the sample subjected to secondary oxidation in a tube furnace, continuously introducing a certain amount of inert gas until air is exhausted, and then heating to perform high-temperature reduction reaction. The reaction temperature is 800 ℃, the reaction time is 2.5 hours, and the reaction is naturally cooled to a lower temperature in an inert atmosphere after the reaction is finished.
5) And stopping introducing inert gas when the sample subjected to high-temperature reduction is cooled to 300 ℃ in the tubular furnace, introducing a certain amount of compressed air, keeping the flow unchanged, and preserving the temperature for 3 hours at the temperature to strengthen the oxidation of bare metal particles, and cooling to room temperature along with the furnace after the reaction is finished.
6) And repeating the acid leaching process, and then filtering, washing and drying to obtain the high-purity single-walled carbon nanotube.
Example 3
The embodiment provides a purification method of a single-walled carbon nanotube, which specifically comprises the following steps:
1) Firstly, placing an unpurified single-walled carbon nanotube in a corundum crucible, introducing compressed air into a tube furnace, performing medium-temperature oxidation, and controlling the flow rate of the compressed air: 1L/min, oxidation temperature: and (3) oxidizing for 2 hours at 400 ℃, and placing the sample in a clean glass beaker after the sample is cooled after the oxidization is completed.
2) Pouring acid liquor with certain concentration and certain mass into a beaker containing oxidized carbon nano tubes, sealing the beaker by using a preservative film, and then placing the beaker in an ultrasonic oscillator for ultrasonic oscillation. The acid liquor is sulfuric acid, the concentration is 7mol/L, and the liquid-solid ratio is 200:1, ultrasonic oscillation time is 3h. Filtering, washing and drying after the reaction is finished for standby.
3) And (3) continuously placing the dried sample in a tube furnace for continuous air oxidation, wherein the air flow, the oxidation time and the oxidation temperature are unchanged, and cooling to room temperature for standby after the oxidation is finished.
4) And placing the sample subjected to secondary oxidation in a tube furnace, continuously introducing a certain amount of inert gas until air is exhausted, and then heating to perform high-temperature reduction reaction. The reaction temperature is 900 ℃, the reaction time is 1.5 hours, and the reaction is naturally cooled to a lower temperature in an inert atmosphere after the reaction is finished.
5) And stopping introducing inert gas when the sample subjected to high-temperature reduction is cooled to 350 ℃ in the tubular furnace, introducing a certain amount of compressed air, keeping the flow unchanged, and preserving the temperature for 2 hours at the temperature to strengthen the oxidation of the exposed metal particles, and cooling to room temperature along with the furnace after the reaction is finished.
6) And repeating the acid leaching process, and then filtering, washing and drying to obtain the high-purity single-walled carbon nanotube.
Example 4
The embodiment provides a purification method of a single-walled carbon nanotube, which specifically comprises the following steps:
1) Firstly, placing an unpurified single-walled carbon nanotube in a corundum crucible, introducing compressed air into a tube furnace, performing medium-temperature oxidation, and controlling the flow rate of the compressed air: 0.1L/min, oxidation temperature: the oxidation time is 3.5h at 350 ℃, and after the sample is cooled after the oxidation, the sample is placed in a clean glass beaker.
2) Pouring acid liquor with certain concentration and certain mass into a beaker containing oxidized carbon nano tubes, sealing the beaker by using a preservative film, and then placing the beaker in an ultrasonic oscillator for ultrasonic oscillation. The acid liquid is nitric acid, the concentration is 3mol/L, and the liquid-solid ratio is 200:1, ultrasonic oscillation time is 6h. Filtering, washing and drying after the reaction is finished for standby.
3) And (3) continuously placing the dried sample in a tube furnace for continuous air oxidation, wherein the air flow, the oxidation time and the oxidation temperature are unchanged, and cooling to room temperature for standby after the oxidation is finished.
4) And placing the sample subjected to secondary oxidation in a tube furnace, continuously introducing a certain amount of inert gas until air is exhausted, and then heating to perform high-temperature reduction reaction. The reaction temperature is 750 ℃, the reaction time is 3 hours, and the reaction is naturally cooled to a lower temperature in an inert atmosphere after the reaction is finished.
5) And stopping introducing inert gas when the sample subjected to high-temperature reduction is cooled to 250 ℃ in the tubular furnace, introducing a certain amount of compressed air, keeping the flow unchanged, and preserving the temperature for 5 hours at the temperature to strengthen the oxidation of bare metal particles, and cooling to room temperature along with the furnace after the reaction is finished.
6) And repeating the acid leaching process, and then filtering, washing and drying to obtain the high-purity single-walled carbon nanotube.
Example 5
This example differs from example 1 only in that step 5) the sample after high temperature reduction was cooled in a tube furnace to 450 ℃ for incubation.
Example 6
This example differs from example 1 only in that step 1) and step 2) are not performed, and the unpurified single-walled carbon nanotubes are directly subjected to steps 3) to 6).
Comparative example 1
This comparative example differs from example 1 only in that only steps 1) and 2) are performed.
Comparative example 2
This comparative example differs from example 1 only in that the temperature of the high temperature reduction in step 4) is 1250 ℃.
Comparative example 3
This comparative example differs from example 1 in that step 4) high temperature reduction is followed by direct acid leaching, filtration, washing and drying after acid leaching. The acid concentration and the liquid-solid ratio were the same as in example 1, and the acid leaching temperature was 80 ℃.
The single-walled carbon nanotubes obtained in examples 1 to 6 and comparative examples 1 to 3 were subjected to yield calculation and metal content test, and the specific method for the metal content test was as follows:
1) Thermal gravimetric method: weighing the quality of a sample to be detected: 100mg; atmosphere: compressed air 50ml/min and protective gas 20ml/min; rate of temperature rise: 10 ℃/min; temperature range: 0-1000 ℃. The mass remaining after the carbon is completely oxidized is the mass of metal oxide, and the iron content in the carbon nano tube is calculated by completely oxidizing metal iron into ferric oxide.
2) ICP method: weighing the quality of a sample to be detected: 20mg; the digestion method comprises the following steps: the strong oxidizing acid capable of etching graphite and carbon nano tube and the mixed acid of strong acid capable of dissolving metal or oxide thereof are digested for 30min at 220 ℃; and (5) sampling and detecting after digestion is complete.
The yield and metal content results are shown in Table 1.
TABLE 1
| Group of | Yield (%) | Metallic iron content (wt%) | Raman I G /I D |
| Example 1 | 48.8 | 0.60 | 82 |
| Example 2 | 43.3 | 0.49 | 75 |
| Example 3 | 40.1 | 0.36 | 72 |
| Example 4 | 52.7 | 0.99 | 91 |
| Example 5 | 38.9 | 0.52 | 64 |
| Example 6 | 33.2 | 0.75 | 60 |
| Comparative example 1 | 66.9 | 12.9 | 89 |
| Comparative example 2 | 22.5 | 0.21 | 51 |
| Comparative example 3 | 37.3 | 9.7 | 87 |
FIG. 3 is a TGA-DTG curve of the purified carbon nanotubes of example 1; the index pairs before and after purification in example 1 are shown in Table 2.
The prior researches show that amorphous carbon and a small part of incompletely crystallized short-range ordered graphite impurities in the single-walled carbon nano tube are removed through a plurality of air oxidation processes at the oxidation temperature of 300-500 ℃, wherein the amorphous carbon content can be reduced to below 2%, the impurity defects in the single-walled carbon nano tube are reduced, and the conductivity of the carbon nano tube is further improved. As can be seen from fig. 3, the weight loss of the purified carbon nanotubes in the TGA curve is reduced from 97% to 96% at 200-500 ℃, and the mass loss of 1% is the difference between the oxidized weight loss of amorphous carbon and the oxidized weight gain of the residual very small amount of metal, and the metal content is reduced to below 1%, thus indicating that most of amorphous carbon in the purified sample is removed; the platform at 800-1000 ℃ is the mass of the metal oxide remained after the carbon is completely oxidized, the metal is completely oxidized into ferric oxide, the iron content in the purified carbon nano tube is 0.6%, and the ICP detection result in Table 2 can prove that the metal content in the purified carbon nano tube is less than 1%.
FIG. 4 is a Raman spectrum of the carbon nanotubes before and after purification of example 1. As can be seen from FIG. 4, the D peak in the Raman spectrum of the purified carbon nanotubes is the characteristic peak of amorphous carbon and carbon nanotube defects, the peak intensity is extremely low, I G /I D Up to 82, the carbon nanotubes after purification are shown to have fewer defects and higher single-walled carbon nanotubes. Thus, the purification effect by the composite process of pre-intermediate temperature oxidation, pre-acid leaching, first intermediate temperature oxidation, high temperature reduction, low temperature oxidation and first acid leaching in the embodiment of the invention can be also illustrated to be good.
In summary, according to the embodiment of the invention, the metal particles which are coated in the carbon nanotubes and difficult to remove can be removed by the composite process of the pre-intermediate temperature oxidation, the pre-acid leaching, the first intermediate temperature oxidation, the high temperature reduction, the low temperature oxidation and the first acid leaching, and finally the metal content can be reduced to below 1%. The carbon nano tube with low metal content avoids harmful phenomena such as micro short circuit and the like in the battery generated when the carbon nano tube is used for the lithium ion battery conductive agent, and potential safety hazards are avoided.
The above description is only of the preferred embodiments of the present invention and is not intended to limit the present invention, but various modifications and variations can be made to the present invention by those skilled in the art. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present invention should be included in the protection scope of the present invention.
Claims (10)
1. A method for purifying single-walled carbon nanotubes, comprising:
carrying out first medium-temperature oxidation on the single-walled carbon nanotube to be purified at the temperature of 200-500 ℃; wherein, the single-walled carbon nanotube to be purified contains metal impurity particles coated by graphite inside;
carrying out high-temperature reduction on the oxidized product at 600-1200 ℃;
and (3) oxidizing the product obtained by high-temperature reduction again at the temperature of 200-500 ℃, and performing first acid leaching by acid liquor.
2. The method for purifying single-walled carbon nanotubes according to claim 1, wherein the single-walled carbon nanotubes to be purified are single-walled carbon nanotubes which have not been purified after direct production, and the single-walled carbon nanotubes to be purified are subjected to pre-intermediate-temperature oxidation at 200 ℃ to 500 ℃ and pre-pickling with an acid solution before the first intermediate-temperature oxidation.
3. The method for purifying single-walled carbon nanotubes according to claim 2, wherein the pre-intermediate-temperature oxidation, the first intermediate-temperature oxidation and the reoxidation are performed by introducing air into a tube furnace;
preferably, the air flow rates of the pre-intermediate temperature oxidation, the first intermediate temperature oxidation and the reoxidation are all 0.1L/min-10L/min, the oxidation time of the pre-intermediate temperature oxidation and the first intermediate temperature oxidation is at least 0.2h, preferably 0.2 h-6 h, and the oxidation time of the reoxidation is at least 1h, preferably 1 h-6 h.
4. The method of purifying single-walled carbon nanotubes according to claim 2, wherein the pre-acid leaching and the first acid leaching each comprise: placing the to-be-leached matter into the acid liquor, and carrying out ultrasonic oscillation in an ultrasonic oscillator; preferably, the ultrasonic oscillation time is 1-24 hours.
5. The method for purifying single-walled carbon nanotubes according to claim 2, characterized in that the acid solution is a strong acid, preferably hydrochloric acid, sulfuric acid, nitric acid or any mixture thereof;
and/or the concentration of the acid liquor is 3 mol/L-16 mol/L, and the liquid-solid mass ratio of the acid liquor to the to-be-leached matter is 50-500: 1.
6. the method for purifying a single-walled carbon nanotube according to claim 2, further comprising sequentially performing solid-liquid separation, washing and drying after the pre-acid leaching and the first acid leaching, respectively.
7. The method for purifying single-walled carbon nanotubes according to any one of claims 1 to 6, wherein the temperature of the pre-intermediate temperature oxidation and the first intermediate temperature oxidation are each 350 ℃ to 400 ℃; and/or, the temperature of reoxidation is 250 ℃ to 300 ℃.
8. The method for purifying single-walled carbon nanotubes according to any of claims 1 to 6, characterized in that the time of high temperature reduction is at least 0.5h, preferably 0.5h to 6h.
9. The method for purifying single-walled carbon nanotubes according to any of claims 1 to 6, wherein the high-temperature reduction is performed under an inert atmosphere.
10. The method of purifying single-walled carbon nanotubes according to any of claims 1 to 6, wherein the single-walled carbon nanotubes after purification comprise at least one of the following conditions:
(1) The metal content is below 1 wt%;
(2) I of Raman Spectroscopy G /I D Above 70;
(3) The yield is more than 40 percent.
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