US20100143234A1

US20100143234A1 - Methods of preparing and purifying carbon nanotubes, carbon nanotubes, and an element using the same

Info

Publication number: US20100143234A1
Application number: US12/628,718
Authority: US
Inventors: Hisashi Kajiura; Yongming Li; Hongliang Zhang; Yu Wang; Yunqi Liu; Dacheng Wei; Liping Huang
Original assignee: Sony Corp
Current assignee: Sony Corp
Priority date: 2008-12-04
Filing date: 2009-12-01
Publication date: 2010-06-10
Also published as: CN101746745A; JP2010132543A

Abstract

A method of preparing carbon nanotubes (CNT), a method of purifying carbon nanotubes, carbon nanotubes, and an element using said carbon nanotubes are provided. The method includes preparing carbon nanotubes by arc-discharge and employs a coordination chemistry process to remove a catalyst and/or optional promoter used in arc-discharge.

Description

CROSS REFERENCES TO RELATED APPLICATIONS

The present application claims priority to Chinese Patent Application No. CN 200810178857.7 filed in the Chinese Patent Office on Dec. 4, 2008, the entire content of which is hereby incorporated by reference.

BACKGROUND

The present application relates to a method of preparing carbon nanotubes (CNT), a method of purifying carbon nanotubes, carbon nanotubes obtained by said methods, and an element using said carbon nanotubes. More specifically, the present application relates to a method for preparing carbon nanotubes by an arc-discharge method, a method of purifying carbon nanotubes, carbon nanotubes obtained by said methods, and an element using said carbon nanotubes. The preparation method or the purification method of the present application employs a coordination chemistry process to remove the catalyst and/or optional promoter used in an arc-discharge method.
As one-dimension carbon nanomaterials, carbon nanotubes (CNT) have attracted increasing attention for their superior electrical, mechanical and chemical properties. Further study on nanomaterials brings great potential application of carbon nanotubes in a wide range of fields such as electron source for field emission, nano field effect transistor, hydrogen storage materials and high-strength fiber, and the like.
Carbon nanotubes can be classified as single-walled carbon nanotubes (SWNT) and multi-walled carbon nanotubes (MWNT) according to the number of the layers of the carbon atoms to form the wall. Multi-walled carbon nanotubes may be also considered as multi-layers by encasing several single-walled carbon nanotubes with different diameters. In practical research and application, single-walled carbon nanotubes and multi-walled carbon nanotubes with fewer layers are important due to their unique electrical, thermal, mechanical and chemical properties.
Conventional methods for preparing carbon nanotubes include an arc-discharge method, chemical vapor deposition (CVD), laser evaporation, and the like. To date, the arc-discharge method is one of the most efficient techniques for large-scale production of high quality carbon nanotubes.
Nevertheless, impurities normally are formed during the preparation of carbon nanotubes by an arc-discharge method, such as graphite particles, amorphous carbon, carbon nanoparticles in other forms and metal catalyst particles. The mixture of said impurities with carbon nanotubes greatly hampers the further study and application of carbon nanotubes. Therefore, various physical and chemical processes are applied to purify the primary product of carbon nanotubes as-prepared so as to obtain carbon nanotubes with higher purity. The commonly used purification processes include liquid phase oxidation and gas phase oxidation. For example, K. Tohji et al. disclosed hydrothermal treatment in J. Phys. Chem. 1997, 101, 1974. Z. Shi et al. developed gaseous oxidation, see Z. Shi et al. Solid State Commun. 1999, 112, 35. E. Mizoguti et al, reported catalytic oxidation in Chem. Phys. Lett. 2000, 321, 297. Additionally, many researches have been made on nitric acid reflux method, for example, see J. L. Zimmerman et al. Chem. Mater. 2000, 12, 1361. Furthermore, the purification based on an initial selective oxidation to remove amorphous carbon, followed by a reflux in concentrated nitric acid has been found effective in removing metals from the reaction products (see K. Tohji et al. Nature, 1996, 383, 679).
Said purification processes are known in the art, by which the impurities can be removed to achieve the purification based on the fact that carbon nanotubes are more stable and difficult to be oxidized than the impurities such as amorphous carbon, metal catalyst particles, and the like. According to the different oxidizing atmospheres, gas phase oxidation can be classified into oxygen (or air)-oxidation, carbon dioxide-oxidation and the like. The commonly used liquid oxidants in liquid phase oxidation include potassium permanganate, nitric acid solution or potassium dichromate, and the like. In addition, physical processes such as centrifugation and micro-filtration can also be used to separate carbon nanotubes. The above processes can be used singly or in a combination of one or more process. For example, gas phase oxidation such as air-oxidation can be used to remove impurities like amorphous carbon that are easy to remove from carbon nanotubes; liquid phase oxidation such as nitric acid oxidation can be used to remove the impurities like metal catalyst particles that are difficult to remove from carbon nanotubes. Meanwhile, the purified carbon nanotubes can also be obtained by employing the centrifugation process in combination.
Nevertheless, the main challenge is to purify the prepared carbon nanotubes without damaging the carbon nanotubes. As known, liquid phase oxidation, such as a purification method by refluxing in nitric acid, can induce the damage in sidewalls of the nanotubes. Even if gas phase oxidation is employed, carbon nanotubes are also damaged since the oxidation temperature used is high, usually about 470° C.
In addition, when a catalyst such as Y—Ni alloy are employed in the arc discharge process, said catalyst present inside the reaction product or on its surface is difficult to remove by a mineral acid reflux method. Although most of the catalyst particles can be removed after a long time reflux, carbon nanotubes are damaged synchronously. As a result, the quality of carbon nanotubes, especially in the electrical conductivity, is degraded.
Furthermore, strong acid reflux induces the following defects in the carbon nanotubes as prepared:
five- or seven-membered rings in the carbon framework, instead of the normal six-membered ring, leads to a bend in the tube;
Sp³-hybridized defects (R=H and OH);
carbon framework damaged by oxidative conditions, which leaves a hole lined with —COOH groups; and
open end of the carbon nanotubes, terminated with —COOH groups.
The above defects are shown in FIG. 1.
As a consequence, it is essential to find methods to prepare and purify carbon nanotubes without damaging the carbon nanotubes

SUMMARY

Carbon nanotubes can be prepared and purified by a method of the present application without damaging the carbon nanotubes, especially without damaging the sidewalls of the carbon nanotubes according to an embodiment.
In the first aspect, the present application provides a method for preparing carbon nanotubes, the method includes:
producing carbon nanotubes by an arc-discharge method in the presence of a catalyst and an optional promoter;
coordinating the metal elements present in the catalyst and/or the optional promoter with a substance capable of forming a complex with said metal elements to produce a complex; and
removing the complex.
In one embodiment, a promoter is employed during art-discharge. Furthermore, the preferred promoter is FeS.
In another embodiment, the catalyst is selected from the group consisting of lanthanum metal oxides, transition metals, the mixture of nickel and a rare earth element, and mixtures thereof. The catalyst is preferably selected from the group consisting of Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, and Ce—Ni alloy.
In another embodiment coordinating the metal elements includes:
converting the metal elements present in the catalyst and/or the optional promoter into ions; and
coordinating the ions with the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter to produce a complex.
In an embodiment, converting the metal element includes:
oxidizing the catalyst and/or the optional promoter to produce the oxides thereof.
In an embodiment, the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter is selected from aminopolycarboxylic acids.
In an embodiment, the aminopolycarboxylic acid is selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA), and triethylenetetraaminehexaacetic acid (TTHA).
In an embodiment, the aminopolycarboxylic acid is triethylenetetraaminehexaacetic acid (TTHA).
Furthermore, the complex produced from aminopolycarboxylic acid is converted into a salt form to facilitate removing the complex in an embodiment.
In one embodiment converting the metal elements includes:
reacting the oxides with an acid to produce ions of the metal elements present in the catalyst and/or the optional promoter.
In another embodiment, the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter is preferably selected from the group consisting of tetrahydrofuran, trialkyl phosphine, ε-caprolactone, ε-caprolactam, dimethyl formamide, and dimethyl sulfoxide.
Furthermore, in one embodiment, the complex is preferably selected from {M[(NC)₂CC(OCH₂CH₂OH)C(CN)₂]₂(4,4′-bpy)(H₂O)₂}, Dinuclear [{M′(phen)₂}₂V₄O₁₂]C₆H₁₂O.H₂O and [Ni(L)(H₂O)₃]2H₂O, wherein M is selected from Ni, Fe and Co; M′ is selected from Ni and Co; bpy is bipyridine; phen is phenyl; L is (2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid.
In another embodiment the catalyst and/or the optional promoter are oxidized with an oxygen containing gas.
It is desirable that the oxidation time and the oxidation temperature of the oxygen containing gas are sufficient to convert the catalyst and/or the optional promoter into oxides. More preferably, the oxygen containing gas is air. The oxidation temperature is further preferably about 80° C. to about 300° C., and the oxidation time is preferably about 1 hour to about 20 hours.
In one embodiment, the method further comprises a centrifugation step after removing the complex. The centrifugation step is preferably carried out at a speed of about 5000 rpm to about 30000 rpm for about 1 hour to about 20 hours.
In an embodiment, the carbon nanotubes are single-walled carbon nanotubes.
In another embodiment, the present application provides a method for purifying carbon nanotubes produced by an arc-discharge method in the presence of a catalyst and an optional promoter, and the method includes:
coordinating the metal elements present in the catalyst and/or the optional promoter with a substance capable of forming a complex with said metal elements to produce a complex; and
removing the complex.
In one embodiment, a promoter is employed in the arc-discharge method. The preferred promoter is FeS.
In another embodiment, the catalyst is selected from lanthanum metal oxides, transition metals, the mixture of nickel and a rare earth element, and the mixtures thereof. The catalyst is preferably selected from the group consisting of Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, and Ce—Ni alloy.
In another embodiment, coordination of the metal elements include:
converting the metal elements present in the catalyst and/or the optional promoter into ions; and
coordinating the ions with the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter to produce a complex.
It is preferred that converting the metal elements includes:
oxidizing the catalyst and/or the optional promoter to produce the oxides thereof.
It is further preferred that the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter is selected from aminopolycarboxylic acids.
In an embodiment the aminopolycarboxylic acid is selected from ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohe-xane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraaminehexaacetic acid (TTHA).
In an embodiment, the aminopolycarboxylic acid is triethylenetetraaminehexaacetic acid (TTHA).
In an embodiment, the complex produced from aminopolycarboxylic acid is converted into a salt form to facilitate removing the complex.
In another embodiment converting the metal elements include:
reacting said oxides with an acid to produce ions of the metal elements present in the catalyst and/or the optional promoter.
In one embodiment, the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter is preferably selected from tetrahydrofuran, trialkyl phosphine, ε-caprolactone, ε-caprolactam, dimethyl formamide, and dimethyl sulfoxide.
Furthermore, in another embodiment, the complex is preferably selected from {M[(NC)₂CC(OCH₂CH₂OH)C(CN)₂]₂(4,4′-bpy)(H₂O)₂}, Dinuclear [{M′(phen)₂}2V4O12]C₆H₁₂O.H₂O and [Ni(L)(H₂O)₃]2H₂O, wherein M is selected from Ni, Fe, and Co; M′ is selected from Ni and Co; bpy is bipyridine; phen is phenyl; and L is (2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid.
In an embodiment, the catalyst and/or the optional promoter is oxidized with an oxygen containing gas.
It is desirable that the oxidation time and the oxidation temperature of the oxygen containing gas are sufficient to convert the catalyst and/or the optional promoter into oxides. More preferably, the oxygen containing gas is air. The oxidation temperature is further preferably about 80° C. to about 300° C., and the oxidation time is preferably about 1 hour to about 20 hours.
In one embodiment, the method further comprises a centrifugation step after removing the complex. The centrifugation step is preferably carried out at a speed of about 5000 rpm to about 30000 rpm for about 1 hour to about 20 hours.
In an embodiment, the carbon nanotubes are single-walled carbon nanotubes.
In another embodiment, the present application provides carbon nanotubes prepared by a method according to embodiments previously discussed and further discussed below in greater detail. The carbon nanotubes are not damaged in sidewalls compared with carbon nanotubes prepared by a conventional methods.
In a further embodiment, the present application provides an element of carbon nanotubes, preferably carbon nanotubes that are single-walled carbon nanotubes.
Additional features and advantages are described herein, and will be apparent from the following Detailed Description and the figures.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 is a schematic drawing of carbon nanotubes purified by acid reflux.

FIG. 2 is a schematic drawing of an arc furnace for preparing carbon nanotubes according to an embodiment.

FIG. 3 is a schematic drawing of the purification of carbon nanotubes by using CYDTA.

FIG. 4 shows Raman spectra of carbon nanotubes purified by using EDTA and CYDTA, and also shows that of P3 carbon nanotubes (commercially available from Carbon Solutions Inc., and its purity is higher than 85%).

FIG. 5 is XPS spectrum of carbon nanotubes purified by using EDTA.

FIG. 6 a is the SEM image of pristine carbon nanotubes; FIG. 6 b is the SEM image of carbon nanotubes after the purification by TTHA.

FIG. 7 shows TEM images of carbon nanotubes after the purification by TTHA, wherein the difference between FIG. 7 a and FIG. 7 b is just magnification.

FIG. 8 shows Raman spectra of carbon nanotubes before and after the purification by TTHA.

FIG. 9 shows XPS spectra of carbon nanotubes purified by using TTHA and by conventional acid treatment, respectively.

FIG. 10 shows a carbon nanotube film made from carbon nanotubes purified with TTHA.

FIG. 11 shows sheet resistances of the carbon nanotube film made in Film Fabrication Example 1 and the film made in Comparative Example 1.

FIG. 12( a) shows a vapor generator used in the present application; FIG. 12( b) shows the cross section of glass casing in the vapor generator of (a).

DETAILED DESCRIPTION

The present application will be described below in detail with reference to the drawings according to an embodiment.
In an embodiment, the present application provides a method for preparing carbon nanotubes, the method includes:
producing carbon nanotubes by an arc-discharge method in the presence of a catalyst and an optional promoter;
coordinating the metal elements present in the catalyst and/or the optional promoter with a substance capable of forming a complex with said metal elements to produce a complex; and
removing the complex.
Each step will be described in detail as follows.
(1) Arc-Discharge Process
An arc-discharge method is one of the earliest techniques for preparing carbon nanotubes. There is no specific restriction on the arc-discharge method utilized to prepare carbon nanotubes according to the present application. Conventional arc-discharge methods may be used to prepare application carbon nanotubes in the present application. The device, conditions and materials in the arc-discharge process will be described briefly hereafter.
FIG. 2 shows a drawing of an arc furnace 100 for preparing carbon nanotubes. Said arc furnace includes a vacuum chamber 160, a cathode connection 110, a cathode 120, an anode 130, an anode connection 140 and a linear motion feedthrough 150. Cathode 120 is normally a graphite rod with large diameter (e.g. about 13 mm), or a metal electrode such as copper. Anode 130 is a graphite rod with small diameter (e.g. about 6 mm).
In one embodiment, the anode graphite rod used for anode 130 is prepared as follows. A hole is drilled in the center of the anode graphite rod, in which filled with an anode mixture by uniformly mixing graphite powders and powders of catalyst and optional promoter, and then being compacted so as to form an anode 130 to generate arc. Alternatively, said anode 130 can be formed as an anode graphite rod by mixing the catalyst and the optional promoter with graphite to obtain the anode mixture and then molding the mixture.
Before the arc discharging, the vacuum chamber 160 is vacuumed, and then filled with protective inert gas (e.g. Helium or argon gas), hydrogen gas, nitrogen gas or the mixture thereof. When connected to the power supply, an arc can be stably generated between anode 130 and cathode 120 by adjusting the distance (the distance is generally held as a predetermined constant value, e.g. about 1-5 mm) therebetween using a linear motion feedthrough 150. Cathode 120 and anode 130 will not be connected at first so that no initial current is generated, and then anode 120 was gradually moved towards cathode 130 until an arc is generated. During the arc discharging, the high-speed plasma flow was generated between anode 130 and cathode 120 so that the surfaces of cathode 120 and anode 130 reached very high temperature, e.g. about 3000° C. and 5000° C. or more, respectively and anode 130 rapidly evaporates as carbon clusters and is gradually consumed. In high temperature region between cathode 120 and anode 130, said carbon clusters evaporated from anode 130 can form carbon nanotubes and they fulfill the whole vacuum chamber to deposit on the wall of the vacuum chamber 160 and/or cathode 120. Normally, anode is consumed out for merely 10 min to complete arc-discharge, and then the vacuum chamber is cooled.
After reaction and fully cooling, the following substances may be collected in the vacuum chamber 160: cloth-like soots which adhere on the wall of vacuum chamber 160; web-like soots which hang between the chamber wall and cathode; deposits which adhere on one end of the cathode; and collar-like soots surrounding the deposit. The as-prepared carbon nanotubes normally are bound with each other by van der Waals force, and arranged in a hexagonal crystal structure. Carbon nanotubes, especially single-walled carbon nanotubes mainly exist in three parts: cloth-like soots, web-like soots and collar-like soots. Among them, the purity of carbon nanotubes, especially single-walled carbon nanotubes in the web-like soot is the highest, the purity of those in the cloth-like soot is the lowest, and the purity of those in the collar-like soot is between them. Many impurities, such as amorphous carbon and metal catalyst particles, may be present together with the carbon nanotubes. The impurities can be removed by subsequent purification process, which will be specifically set forth hereafter.
In the preparation of carbon nanotubes by an arc-discharge method according to an embodiment, it is required to use catalyst. Catalyst exert important effect on the growth of carbon nanotubes, especially single-walled carbon nanotubes. The catalyst used in the present application can be transition metals or the oxides of lanthanum metals. In addition, catalyst can be the mixtures of nickel and a rare earth element such as Y, Ce, Er, Tb, Ho, La, Nd, Gd, Dy or the mixture thereof. In one embodiment, catalyst are preferably selected from Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, and Ce—Ni alloy.
In the method of preparing carbon nanotubes according to an embodiment, it is optional to use a promoter, which also exerts important effect on the growth of carbon nanotubes, especially single-walled carbon nanotubes, and in particular, on improving the purity and controlling the diameter distribution of carbon nanotubes. Therefore, it is preferred that FeS is used as the promoter in the present application.
In an embodiment, the catalyst and the promoter can be used at any ratio, provide that the ratio used will not negatively affect the properties such as the growth, purity and diameter distribution of carbon nanotubes.
Normally, the weight ratio of catalyst and promoter (catalyst/promoter) is in a range from 1:1 to 20:1, preferably in a range from 5:1 to 15:1, and more preferably 10:1. However, other weight ratios outside the above range are also useful if necessary.
During the preparation of carbon nanotubes, it is also required to use carbon source. Preferred carbon source is graphite. In an embodiment, the catalyst and carbon source can be used at any ratio, providing that said ratio will not negatively affect the properties such as the growth, purity and diameter distribution of carbon nanotubes. In one embodiment, the mole ratio of said carbon source and catalyst is in a range from 1:1 to 50:1, preferably in a range from 5:1 to 30:1 and more preferably 15:1.
In an embodiment, the carbon source is graphite, the catalyst is Y—Ni alloy, and the promoter is FeS.
Protective gas normally is applied during arc discharging, such as inert gas (e.g. Helium, argon gas, or the mixture thereof), hydrogen gas, nitrogen gas or the mixture thereof, and the like. Helium is a conventional protective gas. If the hydrogen gas is applied, its pressure may be lower than that of Helium gas. Since hydrogen gas has greater heat conductivity, and can form a C—H bond with carbon and etch the amorphous carbon, carbon nanotubes with higher purity can be produced. The pressure of protective gas may be about 6.67 kPa to 203, preferably about 13.3 kPa to 160 and more preferably about 66.7 kPa to 120 such as about 80.0 kPa to 93.3.
In order to achieve the arc discharge between the anode and the cathode, the current is generally about 30-200 amperes (A), preferably about 70-120 A, e.g. about 100 A. If the current is too low, the stable arc cannot be achieved, whereas if the current is too high, the impurities such as amorphous carbon and metal particles will increase and render the subsequent purification process difficult. The direct voltage used is about 20-40V, e.g. about 30V. Since carbon nanotubes may be integrated with other by-products such as amorphous carbon and metal particles by sintering, they are difficult to be separated and purified. Thus, the water-cooling is normally used to reduce the temperature of the cathode of graphite so as to prepare carbon nanotubes with perfect structure and higher purity. For example, the cathode of graphite can be fixed on the copper base cooled with water to reduce its temperature. In addition, metals with superior heat conductivity to dissipate heat such as copper (Cu) can be used as cathode to facilitate the formation of carbon nanotubes. During the arc-discharge process, the temperature controlling apparatus can be additionally used to control the temperature of vacuum chamber 160 to further avoid the increase of impurities such as amorphous carbon and the like due to the low temperature.
Furthermore, although in the furnace 100 of FIG. 2, the arc is generated between the opposite end sides of cathode and anode, the cathode and anode can be placed in the same side to form a certain angle, then the discharge between the anode and cathode is performed in a way of point to point, and the resultants formed as sheets adhere to the wall of vacuum chamber 160 and somewhere else. The yield of carbon nanotubes can increase thereby.
After the arc-discharge process, web-like soot is usually collected for the following purification process since the purity of carbon nanotubes, especially single-walled carbon nanotubes in the web-like soot is the highest.
(2) The Purification Process—Forming a Complex and Removing the Complex
As previously described, oxidation at a high temperature or oxidation in a strong acid (reflux) is generally employed to remove impurities in carbon nanotubes, especially to remove the residual catalyst and/or optional promoter. But said methods damage the carbon nanotubes, especially in the sidewalls of the carbon nanotubes.
In order to eliminate the damage caused by oxidation at a high temperature or oxidation in a strong acid in carbon nanotubes, the inventor has found a method based on coordination chemistry to remove the residual catalyst and/or optional promoter in carbon nanotubes without damaging the carbon nanotubes through in-depth researches.
Compared with the conventional purification process, the purification process based on coordination chemistry is gentle and does not damage the sidewalls of the carbon nanotubes. Hence, the quality of carbon nanotubes as prepared according to the present application is superior to that of conventional methods, particularly in the electric conductivity.
The inventor has found that a substance capable of forming a complex with metal elements present in the catalyst and/or the optional promoter to produce a complex can be employed to coordinate said metal elements to facilitate removing the catalyst and/or the optional promoter.
As used herein, the term “a substance capable of forming a complex with metal elements present in the catalyst and/or the optional promoter to produce a complex” refers to a substance capable of forming a complex with metal elements present in the catalyst or the promoter to produce a complex, or a substance capable of forming a complex with metal elements present in the catalyst and the promoter to produce a complex. As previously described, the catalyst used in the present application may be transition metals or lanthanum metal oxides. In addition, the catalyst may also be the mixture of the metal nickel (Ni) and rare earth elements such as Y, Ce, Er, Tb, Ho, La, Nd, Gd, Dy or a mixture thereof. Therefore, a substance capable of forming a complex with said transition metal elements and rare earth metal elements to produce a complex should be preferably selected as “a substance capable of forming a complex with metal elements present in the catalyst and/or the optional promoter to produce a complex” to facilitate removing the catalyst and/or the optional promoter in the prepared carbon nanotubes.
Since the catalyst in an arc discharge are typically selected from Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, and Ce—Ni alloy, a substance capable of coordination with Y, Ni, Fe, Co, Rh, Pt or Ce is preferably selected as “a substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter”.
When a promoter is employed in the arc discharge to produce carbon nanotubes, the amount of the promoter is negligible compared with that of the catalyst. Likewise, the amount of the promoter in the prepared carbon nanotubes is also negligible compared with that of the catalyst. Hence, it is suitable that only the metal elements present in the catalyst is taken into account for selecting the substances to form a complex.
Even so, it is more preferred that both the metal elements present in the catalyst and the metal elements present in the promoter are taken into account. A substance capable of forming complexes with all the metal elements present in the catalyst and the promoter is further preferred so that all the residual catalyst and promoter can be removed by just a substance.
Although some substances can coordinate with elementary metals (0 valent metals) to form a complex, it is difficult to directly form a complex between substances and metals employed in an arc discharge process since the metals are present as alloys in the arc discharge process. Therefore, it is preferred that the metal elements present in the catalyst and/or the optional promoter are converted into ions for the coordination.
Various methods of converting the metal elements into ions can be used without any specific restriction. For example, strong acid oxidation can be used to convert the metal elements into ions. However, In order to minimize the damage in the prepared carbon nanotubes, metal ions can be obtained by suitable acids treatment after suitable oxidation of the metal elements into metal oxides. Since the reaction condition of the oxidation and the acid treatment is gentler than that of conventional purification methods, the quality of the resulting carbon nanotubes is not significantly degraded.
In an embodiment, since “a substance capable of forming a complex with metal elements present in the catalyst and/or the optional promoter to produce a complex” is an acid such as an aminopolycarboxylic acid which can not only convert the metal oxides into metal ions but also coordinate with the metal ions to form complexes. No other acid is required to achieve the metal ions.
In an embodiment, in order to improve their solubility in a solvent (typically in water) to facilitate removing the complexes, the formed complexes may be converted into other suitable forms such as salt forms. Said conversion improves the solubility of complexes and facilitates separating insoluble carbon nanotubes from complexes with the residual catalyst and/or optional promoter minimized by a method such as filtration. Any filtration medium, such as Polytetrafluoroethylene filter membrane, can be used in the above filtration.
For example, when aminopolycarboxylic acids are used as substances, it is preferred that the complexes formed by the aminopolycarboxylic acid are converted into the salt forms. A basic solution such as NaOH and KOH may be added to adjust the pH into basic and convert the complexes into the salt forms.
When aminopolycarboxylic acids are employed in the method of the first aspect of the present application, there is no any restriction on the aminopolycarboxylic acids. For example, ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA), or triethylenetetraaminehexaacetic acid (TTHA) may be used.
When aminopolycarboxylic acids are used for coordination with Y to form a complex, the preferred aminopolycarboxylic acids are ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraaminehexaacetic acid (TTHA). The structures are as follows:
The structures of the complexes formed by Y and EDTA, CYDTA, DTPA or TTHA are as follows:
The structure of the complex formed by Y and EDTA
The structure of the complex formed by Y and CYDTA
The structure of the complex formed by Y and DTPA
The structure of the complex formed by Y and TTHA
In addition, TTHA can also be coordinated with Ni to form the following complex:
Among them, triethylenetetraaminehexaacetic acid (TTHA) is most preferred since the purity and the transparence of the carbon nanotubes purified with TTHA is superior to that of the carbon nanotubes purified with other aminopolycarboxylic acids.
FIG. 3 is a schematic drawing of the purification of carbon nanotubes by using CYDTA. It can be seen from FIG. 3 that CYDTA is coordinated with Y present in the carbon nanotubes to form a complex and separated from the carbon nanotubes.
In addition, suitable substances that can be coordinated with Y to form complexes include, but are not limited to, tetrahydrofuran, trialkyl phosphine, ε-caprolactone, ε-caprolactam, dimethyl formamide, and dimethyl sulfoxide. The substances that can be coordinated with Y to form complexes and the complexes are described in Shashank Mishra “Anhydrous scandium, yttrium, lanthanide and actinide halide complexes with neutral oxygen and nitrogen donor substances” Coordination Chemistry Review, 2008, 252, 1996-2025, which is incorporated herein by reference. All the substances that can be coordinated with Y to form complexes listed in that document may be used to remove the catalyst such as Y in the Y—Ni alloys.
One exemplary example of the complexes of Ni, Co or Fe according to the present application is {M[(NC)₂CC(OCH₂CH₂OH)C(CN)₂]₂(4,4′-bpy)(H₂O)₂}, wherein M is selected from Ni, Fe and Co; bpy is bipyridine, see Inorganica Chemica Acta, 2008, 361, 3856-3862. The compounds and substances listed in the document to form said complexes are suitable for the present application. Another exemplary example of the complexes of Ni, Co or Fe according to the present application is Dinuclear [{M′(phen)₂}₂V₄O₁₂]C₆H₁₂O.H₂O, wherein M′ is selected from Ni and Co; phen is phenyl, see Inorganica Chemica Acta, 2008, 361, 3681-3689. The compounds and substances listed in the document to form said complexes are also suitable for the present application.
Complexes of Ni suitable for the present application also include [Ni(L)(H₂O)₃]2H₂O, wherein L is (2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid; Ni[P(Ph₂)—N(H)—CH₂Py]₄, wherein Ph is phenyl, Py is pyridine. As for suitable substances and compounds to form complexes, please refer to Inorganica Chemica Acta, 2008, 361, 3723-3729; Journal of Organometallic Chemistry, 2008, 693, 2171-2176; and Inorganic Chemistry Communications, 2008, 11, 1023-1026 herein incorporated by reference.
As for suitable substances and compounds to form complexes, please refer to in Inorganica Chimica Acta, 2008, 361, 3926-3930; Coordination Chemistry Reviews, 2002, 229, 27-35; Coordination Chemistry Reviews, 2002, 232, 151-171; and Coordination Chemistry Reviews, 2002, 234, 273-287 herein incorporated by reference.
Some non-limiting examples of suitable the substances that can be coordinated with Fe, Co or Ni to form complexes are described in Coordination Chemistry Reviews, 1974, 12, 151-184; Coordination Chemistry Reviews, 1973, 11, 343-402; and Coordination Chemistry Reviews, 1967, 2, 173-193.
All the above documents are incorporated herein by reference.
The substances can be provided in forms such as alloy, elementary substance and compound, of the catalyst and/or the optional promoter and the species of the metal elements present.
Furthermore, the coordination step is carried out in a manner according to the specific catalyst and/or optional promoter. For example, it is dependent on the specific catalyst and/or optional promoter whether the catalyst and/or optional promoter need to be converted into ions. When oxidation is employed to convert the catalyst and/or optional promoter into ions, it further needs to be determined whether an acid other than the substances is necessary for the conversion.
As previously described, in order to convert metal elements present in the catalyst and/or optional promoter into ions, the metal elements may be converted into oxides by moderate oxidation at first. It is preferred that an oxygen-containing gas such as air is employed to oxidize the catalyst and/or optional promoter. The oxidation condition of said oxygen-containing gas is gentler than that of conventional purification by gas phase oxidation. Generally, there is no specific restriction on the oxidation time and the oxidation temperature for the oxygen-containing gas to oxidize the catalyst and/or optional promoter, provided that the oxidation time and the oxidation temperature are sufficient to convert the catalyst and/or the optional promoter into oxides. The selected oxidation temperature is typically about 80° C. to about 300° C., more preferably about 100° C. to about 200° C., most preferably about 150° C. to about 200° C. The oxidation time can vary according to the selected oxidation temperature. The selected oxidation time is typically about 1 hour to about 20 hours, more preferably about 5 hours to about 15 hours, most preferably about 8 hours to about 10 hours. It can be seen that the oxidation employed in the preparation of carbon nanotubes according to the present invention is at low temperature compared with the high temperature oxidation employed in conventional methods (typically at about 470° C.). Therefore, the low temperature oxidation does not damage the carbon nanotubes.
The process for preparing carbon nanotubes can further include centrifugation to remove the residual amorphous carbon incorporated in the carbon nanotubes after removing the complex, thereby so as to further improve the purity of the carbon nanotubes. Although any speed may be used for the centrifugation step, a higher centrifugation speed is preferred. For example, the centrifugation step may be carried out at a speed of about 5000 rpm to about 30000 rpm, preferably about 10000 rpm to about 20000 rpm. The selected time for centrifugation actually depends on the selected speed for centrifugation, and the centrifugation step is carried out typically for about 1 hour to about 20 hours, preferably about 2 hours to about 10 hours, for example, 3 hours.
The preferred carbon nanotubes are single-walled carbon nanotubes in an embodiment, including metallic single-walled carbon nanotubes (M-SWNT), semiconductor single-walled carbon nanotubes (S-SWNT) and the combinations thereof.
In another embodiment, the present application provides a process for purifying carbon nanotubes, wherein the carbon nanotubes are synthesized using an arc-discharge method in the presence of a catalyst and optional promoter. The process includes:
coordinating metal elements present in the catalyst and/or the optional promoter with a substance capable of forming a complex with said metal elements to produce a complex; and
removing the complex.
According to an embodiment, carbon nanotubes produced by an arc discharge method can be purified, so as to improve the purity of the carbon nanotubes and properties thereof.
There is no specific restriction on carbon nanotubes, provided that the carbon nanotubes are produced by an arc discharge method.
In general, the catalyst used is a transition metal, the oxide of lanthanum metal, or the mixture thereof. In addition, the catalyst can be the mixture of nickel and a rare earth element such as Y, Ce, Er, Tb, Ho, La, Nd, Gd, Dy or mixture thereof. In one embodiment, the catalyst is preferably selected from Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, or Ce—Ni alloy.
A promoter such as FeS is typically employed in the arc discharge process.
The residual catalyst and promoter can be efficiently removed from the carbon nanotubes produced by an arc discharge method by the purification process according to an embodiment without damaging the quality of the carbon nanotubes, particularly in the electric property.
According to an embodiment, “a substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter” is employed to coordinate with the metal elements to form complexes which are removed for purifying carbon nanotubes.
In an embodiment, the substance capable of forming a complex with said transition metal elements and rare earth metal elements should be selected as “the substance capable of forming a complex with the metals present in the catalyst and/or the optional promoter” to facilitate removing the catalyst from the carbon nanotubes.
Since the catalyst in an arc discharge are typically selected from Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, and Ce—Ni alloy, a substance capable of coordination with Y, Ni, Fe, Co, Rh, Pt or Ce is selected as “a substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter”.
When a promoter is employed in the arc discharge to produce carbon nanotubes, the amount of the promoter is negligible compared with that of the catalyst. Likewise, the amount of the promoter in the prepared carbon nanotubes is also negligible compared with that of the catalyst. Hence, it is suitable that only the metal elements present in the catalyst is taken into account for selecting the substances to form a complex.
Even so, it is more preferred that both the metal elements present in the catalyst and the metal elements present in the promoter are taken into account. A substance capable of forming complexes with all the metal elements present in the catalyst and the promoter is further preferred so that all the residual catalyst and promoter can be removed by just a substance.
Although some substances can coordinate with elementary metals (0 valent metals) to form a complex, it is difficult to directly form a complex between substances and metals employed in an arc discharge process since the metals are present as alloys in the arc discharge process. Therefore, it is preferred that the metal elements present in the catalyst and/or the optional promoter are converted into ions for the coordination according to an embodiment.
Various methods for converting the metal elements into ions can be used without any specific restriction. For example, strong acid oxidation can be used to convert the metal elements into ions. However, in order to minimize the damage in the prepared carbon nanotubes, metal ions can be obtained by suitable acids treatment after suitable oxidation of the metal elements into metal oxides. Since the reaction condition of the oxidation and the acid treatment is gentler than that of conventional purification methods, the quality of the resulting carbon nanotubes is not significantly degraded.
In an embodiment, since “a substance capable of forming a complex with metal elements present in the catalyst and/or the optional promoter to produce a complex” is an acid such as an aminopolycarboxylic acid which can not only convert the metal oxides into metal ions but also coordinate with the metal ions to form complexes, no other acid is required to achieve the metal ions.
In an embodiment, in order to improve their solubility in a solvent (typically in water) to facilitate removing the complexes, the formed complexes may be converted into other suitable forms such as salt forms. The conversion improves the solubility of complexes and facilitates separating insoluble carbon nanotubes from complexes with the residual catalyst and/or optional promoter minimized by a method such as filtration. Any filtration medium, such as Polytetrafluoroethylene filter membrane, can be used in the above filtration.
For example, when aminopolycarboxylic acids are used as substances, it is preferred that the complexes formed by the aminopolycarboxylic acid are converted into the salt forms. A basic solution such as NaOH and KOH may be added to adjust the pH into basic and convert the complexes into the salt forms.
When aminopolycarboxylic acids are employed, there is no specific restriction on the aminopolycarboxylic acids. For example, ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA), or triethylenetetraaminehexaacetic acid (TTHA) may be used, according to an embodiment.
When aminopolycarboxylic acids are used for coordination with Y to form a complex, the preferred aminopolycarboxylic acids are ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraaminehexaacetic acid (TTHA), according to an embodiment. The structures are as follows:
The structures of the complexes formed by Y and EDTA, CYDTA, DTPA or TTHA are as follows:
The structure of the complex formed by Y and EDTA
The structure of the complex formed by Y and CYDTA
The structure of the complex formed by Y and DTPA
The structure of the complex formed by Y and TTHA.
In addition, TTHA can also be coordinated with Ni to form a complex:
Among them, triethylenetetraaminehexaacetic acid (TTHA) is most preferred since the purity and the transparence of the carbon nanotubes purified with TTHA is superior to that of the carbon nanotubes purified with other aminopolycarboxylic acids, according to an embodiment.
In addition, suitable substances that can be coordinated with Y to form complexes include, but are not limited to, tetrahydrofuran, trialkyl phosphine, ε-caprolactone, ε-caprolactam, dimethyl formamide, and dimethyl sulfoxide. The substances that can be coordinated with Y to form complexes and the complexes are described in Shashank Mishra “Anhydrous scandium, yttrium, lanthanide and actinide halide complexes with neutral oxygen and nitrogen donor substances” Coordination Chemistry Review, 2008, 252, 1996-2025, which is incorporated herein by reference. All the substances that can be coordinated with Y to form complexes listed in aforesaid document may be used to remove the catalyst such as Y in the Y—Ni alloys.
One exemplary example of the complexes of Ni, Co or Fe according to an embodiment is {M[(NC)₂CC(OCH₂CH₂OH)C(CN)₂]₂(4,4′-bpy)(H₂O)₂}, wherein M is selected from Ni, Fe and Co; bpy is bipyridine; see Inorganica Chemica Acta, 2008, 361, 3856-3862. The compounds and substances listed in the document to form the complexes are suitable for the present application. Another exemplary example of the complexes of Ni, Co or Fe according to the present application is Dinuclear [{M′(phen)₂}₂V₄O₁₂]C₆H₁₂O.H₂O, wherein M′ is selected from Ni and Co; phen is phenyl, see Inorganica Chemica Acta, 2008, 361, 3681-3689. The compounds and substances listed in the document to form said complexes are also suitable for the present application.
Complexes of Ni suitable for the present application also include [Ni(L)(H₂O)₃]2H₂O, wherein L is (2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid; Ni[P(Ph₂)—N(H)—CH₂Py]₄, wherein Ph is phenyl, Py is pyridine. Other suitable substances and compounds to form complexes provided, for example in, Inorganica Chemica Acta, 2008, 361, 3723-3729; Journal of Organometallic Chemistry, 2008, 693, 2171-2176; and Inorganic Chemistry Communications, 2008, 11, 1023-1026. Inorganica Chimica Acta, 2008, 361, 3926-3930; Coordination Chemistry Reviews, 2002, 229, 27-35; Coordination Chemistry Reviews, 2002, 232, 151-171; and Coordination Chemistry Reviews, 2002, 234, 273-287.
Non-limiting examples of suitable substances that can be coordinated with Fe, Co or Ni to form complexes are described in, for example, Coordination Chemistry Reviews, 1974, 12, 151-184; Coordination Chemistry Reviews, 1973, 11, 343-402; and Coordination Chemistry Reviews, 1967, 2, 173-193.
All the above documents are incorporated herein by reference.
The substances can be selected according to the forms (such as alloy, elementary substance and compound) of the catalyst and/or the optional promoter and the species of the metal elements present.
Furthermore, the coordination step is carried out in a manner according to the specific catalyst and/or optional promoter. For example, it depends on the specific catalyst and/or optional promoter whether the catalyst and/or optional promoter need to be converted into ions. When oxidation is employed to convert the catalyst and/or optional promoter into ions, it further needs to be determined whether an acid other than said the substances is necessary for the conversion.
As previously described, in order to convert metal elements present in the catalyst and/or optional promoter into ions, the metal elements may be firstly converted into oxides by moderate oxidation. It is preferred that an oxygen-containing gas such as air is employed to oxidize the catalyst and/or optional promoter. The oxidation condition of said oxygen-containing gas is gentler than that of conventional purification by gas phase oxidation. Generally, there is no specific restriction on the oxidation time and the oxidation temperature for the oxygen-containing gas to oxidize the catalyst and/or optional promoter, provided that the oxidation time and the oxidation temperature are sufficient to convert the catalyst and/or the optional promoter into oxides. The selected oxidation temperature is typically about 80° C. to about 300° C., more preferably about 100° C. to about 200° C., most preferably about 150° C. to about 200° C. The oxidation time can vary according to the selected oxidation temperature. The selected oxidation time is typically about 1 hour to about 20 hours, more preferably about 5 hours to about 15 hours, most preferably about 8 hours to about 10 hours. It can be seen that the oxidation employed in the purification of carbon nanotubes according to the present invention is at low temperature compared with the high temperature oxidation employed in conventional methods. Therefore, said low temperature oxidation does not damage the carbon nanotubes.
The process for purifying carbon nanotubes can further include centrifugation to remove the residual amorphous carbon incorporated in the carbon nanotubes after removing the complex, thereby so as to further improve the purity of the carbon nanotubes. Although any speed may be used for the centrifugation step, a higher centrifugation speed is preferred. For example, the centrifugation step may be carried out at a speed of about 5000 rpm to about 30000 rpm, preferably about 10000 rpm to about 20000 rpm. The selected time for centrifugation actually depends on the selected speed for centrifugation, and the centrifugation step is carried out typically for about 1 hour to about 20 hours, preferably about 2 hours to about 10 hours, for example, 3 hours.
In a further embodiment, the present application provides carbon nanotubes as prepared and purified according to an embodiment described in the present application.
According to the number of the layers of the carbon atoms forming the wall, the carbon nanotubes may include single-walled carbon nanotubes, multi-walled carbon nanotubes and the combinations thereof. According to their electrical property, the carbon nanotubes may include metallic carbon nanotubes, semiconductor carbon nanotubes and the combination thereof. However, the preferred carbon nanotubes are single-walled carbon nanotubes, including metallic single-walled carbon nanotubes (M-SWNT), semiconductor single-walled carbon nanotubes (S-SWNT) and the combinations thereof according to an embodiment.
As described before, the process of preparing carbon nanotubes and the process of purifying carbon nanotubes do not damage the sidewalls of the carbon nanotubes. Furthermore, the properties, particularly electric conductivity of the carbon nanotubes are not degraded.
As showed in FIG. 1, typically known carbon nanotubes are defective due to the limitation of preparation and purification conditions. However, carbon nanotubes according to an embodiment are smooth in the sidewalls and not damaged, as shown in FIG. 7. Furthermore, as shown in FIG. 8 and FIG. 9, the purity of carbon nanotubes of the present application is desirably high, and the quality is desirably good.
In another embodiment, the present application provides an element of carbon nanotubes, including the carbon nanotubes as described in the present application.
The elements of carbon nanotubes include but not limited to conductive film of carbon nanotubes, field emission source, transistor, conductive wire, electrode material (such as transparent, porous or gas diffusion electrode material) nano-electro-mechanic system (NEMS), spin conduction device, nano cantilever, quantum computing device, lighting emitting diode, solar cell, surface-conduction electron-emitter display, filter (such as high frequency filter or optical filter), drug delivery system, thermal conductive material, nano nozzle, energy storage system (such as hydrogen storage material), space elevator, fuel cell, sensor (such as gas, glucose or ion sensor), or catalyst carrier.
The following examples are provided to illustrate the elements of carbon nanotubes, whereas the present application is not limited to those examples.
1. Conductive Film of Carbon Nanotubes
Carbon nanotubes combine strength and flexibility and are excellent candidates for flexible electronic components. In particular, flexible, transparent, conductive thin films made of CNTs have attracted much attention, partly because of the applications in electroluminescent, photoconductor and photovoltaic devices.
Although the optically transparent and highly conductive indium tin oxide (ITO) has enjoyed widespread use in optoelectronic applications, the inherent brittleness of ITO severely limits film flexibility. The properties of CNT thin films are suitable to replace ITO. For instance, CNT films can be repeatedly bent without fracture. The thin films with low sheet resistance are also transparent in the visible and infrared range. Furthermore, both the low cost and tunable electronic properties offer additional advantages for CNT thin films.
Conductive films of carbon nanotubes can be fabricated according to the present application as follows:
Typically, 10 mg of carbon nanotubes are dispersed in 200 ml of 1 wt. % aqueous octyl-phenol-ethoxylate (denoted Triton X-100) solution for 20 min in an ultrasonic bath. The dispersion is filtered out with a mixed cellulose ester (MCE) filter membrane (Millipore, 0.2 μm pore), and the resulting carbon nanotube film is formed on the membrane in a vacuum filtration apparatus (Millipore). Substantially all of the Triton X-100 on the obverse of the carbon nanotube film is dialyzed against a Tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) buffer (50 mM, PH 7.5) for two days. The Tris-HCl buffer is subsequently washed away with purified water, then the carbon nanotube films are transferred onto a quartz substrate. After drying the sample for 1 h at 90° C., the filter membrane is removed by using acetone vapor. Finally, the carbon nanotube films are dried in a vacuum at 100° C. for 1 h.
The method for preparing the carbon nanotube films, particularly employing acetone vapor to remove the filter membrane, described in Chinese Patent Application No. 200810005631.7 filed on Feb. 14, 2008, which is hereby incorporated by reference.
For example, the vapor generator shown in FIG. 12 may be used. FIG. 12( a) shows a vapor generator used in the present invention. FIG. 12( b) provides a schematic cross-section view of glass casing of vapor generator shown in FIG. 12( a).
The vapor generator includes:
a glass casing with a condensing device, having a porous support station in the inner of the casing for disposing a sample, an inlet of a condensing media located on the lower portion of the casing and an outlet of the condensing media located on the upper portion of the casing, and the height of the station is nearly the same as that of the inlet of the condensing media;
a vessel, such as a round-bottomed flask, to hold solvent (such as acetone);
a heating device, such as a temperature-adjustable heating jacket, to heat the solvent; and
an optional mixing device, such as a magnetic stirrer.
The porous support station is made of glass, for example. There is no specific restriction on the pore size of said station, provided that a sufficient amount of vapors can flow through the station and a sample can be supported by the station. The size of the station is determined by the inner diameter of the glass casing.
2. Carbon Nanotubes Field Effect Transistor
Single carbon nanotube and a bundle thereof can be used to construct the basic element of nano-electro-element, that is, carbon nanotubes field effect transistor (CNT FET). Carbon nanotubes in the as-prepared products normally exist in bundles other than separately. That is, several, even hundreds of carbon nanotubes are combined parallel in the same axial orientation, to form carbon nanotubes bundles with diameters varying from several to decades of nanometers. However, in order to apply said CNT FET to nano-electro-elements, single or small size carbon nanotubes should be firstly separated from the carbon nanotubes bundles.
The diameters of carbon nanotubes can be the same with each other in bundles, and arrange in a close packed form so that the bundles themselves exhibit crystallization in some extent. Normally, the separation of the carbon nanotubes bundle or carbon nanotubes in bundles from each other is achieved by dispersing the powder of carbon nanotubes into the organic solution, and then by carrying out the ultrasonic treatment for a long time. The effect depends on the type of solutions, the time of ultrasonic treatment and so on. The commonly used solutions include alcohol, isopropanol, acetone, carbon tetrachloride, dichloroethane, dimethyl formamide (DMF) and the like.
3. Transistor—Nano-Electro-Dynatron
There are two kinds of nano-electro-dynatrons at present: single electron transistor (SET) and carbon nanotubes dynatron. The latter is also called field effect transistor (FET), including carbon nanotubes between source and drain electrodes, and the transportation of electron (or hole) via carbon nanotubes is controlled by gate voltage.
One typical FET preparation process is as follow: as above-mentioned, the primary products of the carbon nanotubes normally are bundles which are winded together. Firstly, they are fully separated by ultrasonic treatment in organic solution (e.g. alcohol), and then the liquid is dropped onto the chips of silicon with SiO₂surface. Numerous metal electrodes are prepared on the chips of silicon by traditional photoetching, metal evaporation, or screen printing. Then, an atomic force microscope (AFM) is used to detect whether single carbon nanotube or the bundle thereof connecting two electrodes is present. Those two electrodes will be used as source and drain electrodes of as-prepared FET. The typical distance between the two electrodes is about 100 nm, for example, varying from 0.1 micron to 1 micron. Another electrode under SiO₂layer or doped silicon substrate is used as a gate electrode of FET, to control the current through carbon nanotubes by applying the gate voltage, and thereby the FET thus prepared is a bottom gate FET. Of course, a top gate FET can also be prepared as follows: firstly, carbon nanotubes or bundles thereof are prepared on the substrate to connect source and drain electrodes, and then the gate insulating layer is deposited thereon, and then the gate electrode is prepared on the insulating layer above the carbon nanotubes or bundles thereof by screen printing. Alternatively, single carbon nanotube or the bundle thereof can be firstly sputtered to the substrate in a given orientation, and then the electrodes are deposited on two ends of said carbon nanotubes or bundle thereof by electron beam. However, the process might break the carbon nanotubes between two electrodes.
The relation between transmission result and gate voltage (I-V property) is detected at room temperature. In the detection, the linear conductivity of metal carbon nanotubes are not or weakly affected by the gate voltage, whereas semiconductor carbon nanotubes show strong dependence on gate voltage.

EXAMPLES

The following examples further illustrate the present application according to an embodiment. Unless otherwise indicated, various raw materials and reagents used in the present application are commercially available, or can be prepared in an conventional manner.
The sources of the main raw materials are summarized as follows:
Y—Ni alloy catalyst is purchased from General Research Institute for Nonferrous Metals.
Graphite rod is purchased from Shanghai Carbon Works.
FeS is purchased from Beijing Yili Chemical Co., Ltd.
NaOH and o-dichlorobenzene are purchased from Beijing Chemical Works.
Ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohexane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraaminehexaacetic acid (TTHA) are purchased from Alfa Aesar.
Triton X-100 is purchased from Acros.
Tri(hydroxymethyl)aminomethane is purchased from Acros 99%.
HCl aqueous solution is purchased from Beijing Chemical Works, with a HCl content of 36-38%.
Characterization
As-prepared carbon nanotubes can be characterized as follows:
Raman spectroscopy data are obtained with Renishaw 100 micro-Raman System.
X-ray photoelectron spectroscopy (XPS) data are obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al kα radiation.
Scanning electron microscope data are obtained with JEOL JSM-6700F.
Transmission electron microscope data are obtained with JEOL-2010, 200 kV.
The sheet resistances of the carbon nanotube films are measured by Loresta-EP MCP-T360 with a 4-pin probe, and the transparency of the carbon nanotube films are measured by UV-vis-NIR Spectrophotometer (JASCO V-570).
Raman spectroscopy is one of the useful methods to detect carbon nanotubes, which not only shows the regularity and purity of the sample, but also defines the diameter distribution of carbon nanotubes. The sample can be treated as follows to preclude the effect of carbon nanotubes in bundles imposed on the result of Raman spectroscopy detection: ultrasonic treating for 5 min in ethanol, then dropping the obtained suspension onto the glasses and drying in air.
In the Raman spectra, there are three peaks or regions we are concerned about, the radial breathing modes (RBM) (about 100-300 cm⁻¹), D band (˜1350 cm⁻¹), and G band (˜1570 cm⁻¹) (see M. S. Dresselhaus, et al., Raman Spectroscopy of Carbon Nanotubes in 1997 and 2007, J. Phys. Chem. C, 111(48), 2007, 17887-17893). The RMB peaks are the characteristic peaks of carbon nanotubes, corresponding with the diameters of carbon nanotubes. From the RBM peaks, we can tell the distribution of carbon nanotubes diameters. According to the relation (see Araujo, P. T., et al., Third and fourth optical transitions in semiconducting carbon nanotubes. Phys. Rev. Lett., 98, 2007, 067401.) ω_RBM=A/d_t+B, with A=217.8±0.3 cm⁻¹nm and B=15.7±0.3 cm⁻¹, where ω_RBMrefers to the wave number at the RBM peak in cm⁻¹, and d_trefers to the diameter of carbon nanotubes in nm, we can infer the diameter distribution of the as-prepared carbon nanotubes. The D band and G band are corresponding to amorphous carbon and graphitic carbon, respectively. We can estimate the purity of carbon nanotubes by the intensity ratio of G band and D band (G/D). The larger G/D is, and the more graphitic carbon are, and the less impurities or defects are, so the purity is higher.

Preparation Example 1

In the arc furnace 100 as shown in FIG. 2, the anode 130 is a 100 mm graphite rod with a diameter of 6 mm and the cathode 120 is a graphite rod with a diameter of 8 mm. A hole 4 mm in diameter and 80 mm in length was drilled in one end of anode graphite rod, which was filled with a powder mixture of high purity graphite powders, YNi_4.2alloy powder as metal catalyst and FeS powder as promoter wherein the mole ratio between carbon and the catalyst is 15:1, the weight ratio between the catalyst and the promoter is 10:1. Then the powders filled in the hole were compacted. The cathode was fixed on a water-cooled copper base. The arc furnace 100 was vacuumized to about 3.0 Pa, then the vacuum valve was closed, and Helium gas was filled to reach about 0.07 MPa. After the power supply was connected, the current and the voltage were controlled at about 80-120 A and about 20-25V, respectively, and the distance between two electrodes was maintained about 3 mm by manually adjusting the cathode, so that a stably arc-discharge was provided.
The three parts of the samples were collected: cloth-like soot, which adhere on the wall of chamber, web-like soot, which hang between the wall of the chamber and cathode, and collar-like soot, which adhere on one end of the cathode. Among those three parts, the purity of the web-like soot is highest, that of the cloth-like soot is lowest, and that of the collar-like soot is between them.
The web-like soot is used for the following examples as the pristine sample.

Purification Example 1

Firstly, 10 mg of the pristine samples were annealed at 200° C. using an air flow of 20 ml/min for 10 hours. The samples as annealed were dispersed in deionized water and ultrasonic treatment for 30 min. 0.5M EDTA aqueous solution was prepared and then added into the carbon nanotube dispersion. After the mixture was refluxed in 110° C. for 18 hours, pH value was adjusted to about 8 by using 1M NaOH solution. Subsequently the dispersion was filtrated with 0.5 μm porous polytetrafluoroethylene filter and rinsed by hot water for many times. Then the samples are dispersed in o-dichlorobenzene and centrifuged at 15000 rpm for 3 hours. The supernatant was decanted and collected via filtration with a mixed cellulose ester (MCE) membrane filter.

Purification Example 2

The similar procedure as used in Purification Example 1 was followed, except that 0.5M EDTA aqueous solution was replaced by 0.5M CYDTA aqueous solution.

Purification Example 3

The similar procedure as used in Purification Example 1 was followed, except that 0.5M EDTA aqueous solution was replaced by 0.5M DTPA aqueous solution.

Purification Example 4

The similar procedure as used in Purification Example 1 was followed, except that 0.5M EDTA aqueous solution was replaced by 0.5M TTHA aqueous solution.
Firstly, the samples purified with EDTA and CYDTA were compared respectively with P3 nanotubes (available from Carbon Solutions Inc., obtained with the acid reflux purification, with a purity of higher than 85%). Their Raman spectra are showed in FIG. 4.
It can be seen from FIG. 4 that all three characteristic spectral regions of carbon nanotubes are essentially retained during the process: the RBM (150-250 cm-1), the D band (1330 cm-1), and the G band (1520-1600 cm-1). Furthermore, it can be seen that the G/D ratios from EDTA and CYDAT are larger than the one of P3, which indicates that the purity of carbon nanotubes purified with EDTA and CYDAT is higher than that of P3.
In addition, it also can be seen that the purity of carbon nanotubes purified with CYDAT is higher than the purity of carbon nanotubes purified with EDTA. This may be due to that the complex of CYDAT and Y is more stable than the complex of EDTA and Y.
The carbon nanotubes purified with EDTA were analyzed with XPS, and the result is showed in FIG. 5. It can be seen that the peak of Na is appears in FIG. 5. This indicates that the sodium salt of EDTA remains in the carbon nanotubes after the purification, i.e., EDTA was not entirely eliminated.
Although it is not showed herein, the peak of Na 1 s also appears in the XPS spectrum of carbon nanotubes purified with CYDAT.
It can be seen that the catalyst impurities were not entirely eliminated by CYDAT and EDTA. The possible reason is the poor solubility of the sodium salts of CYDAT and EDTA in water.
The carbon nanotubes purified with TTHA were analyzed with SEM, and the result is showed in FIG. 6 b. The SEM image of pristine carbon nanotubes is also showed in FIG. 6 a.
The carbon nanotubes purified with TTHA were also analyzed with TEM, and the result is showed in FIG. 7, wherein FIG. 7 a differs from FIG. 7 b only in magnification.
By comparing FIG. 6 a with FIG. 6 b, it can be seen that most of impurities are removed after the purification by TTHA with only a little amorphous carbon adhered to the carbon nanotubes. The residual impurities can be further removed by a combination of sonication and ultracentrifugation.
It can be seen from FIG. 7 a and FIG. 7 b that the sidewalls of the resulting carbon nanotubes are very smooth, which indicates that the sidewalls are not damaged during the purification.
FIG. 8 shows Raman spectra of carbon nanotubes before and after the purification by TTHA. By comparing the G/D ratios of the curves, it is evident seen that the purity of carbon nanotubes is significantly increased after the purification by TTHA. It is also seen that the carbon nanotubes after the purification by TTHA are of high quality and purity.
FIG. 9 shows XPS spectra of carbon nanotubes purified by using TTHA and purified by conventional acid treatment (P3). It is obviously seen from FIG. 9 that yttrium was entirely removed by TTHA treatment, while more yttrium remains in the carbon nanotubes after acid reflux treatment.

Film Fabrication Example 1

In this example, the carbon nanotubes purified by THHA were used to fabricate films following a procedure based on the filtration method. The process is described as follows.
10 mg of CNTs were dispersed in 200 ml of 1 wt. % aqueous octyl-phenol-ethoxylate (denoted Triton X-100) solution for 20 min in an ultrasonic bath. The dispersion was filtered out with a mixed cellulose ester (MCE) filter membrane (Millipore, 0.2 μm pore), and the resulting carbon nanotube film was formed on the membrane in a vacuum filtration apparatus (Millipore). Substantially all of the Triton X-100 on the obverse of the carbon nanotube film was dialyzed against a Tris(hydroxymethyl)aminomethane hydrochloride (Tris-HCl) buffer (50 mM, PH 7.5) for two days. The Tric-HCl buffer was subsequently washed away with purified water, then the carbon nanotube films were transferred onto a quartz substrate. After drying the sample for 1 h at 90° C., the filter membrane was removed by using acetone vapor. Finally, the carbon nanotube films were dried in a vacuum at 100° C. for 1 h.

Comparative Example 1

The similar procedure as used in Film Fabrication Example 1 was followed, except that the carbon nanotubes purified by THHA were replaced by the carbon nanotubes (P3) purified by the conventional nitric acid reflux.
FIG. 10 shows a carbon nanotube film made from carbon nanotubes purified with TTHA. The film was placed on a quartz substrate with the word ICCAS written on. It can be seen from FIG. 10 that the word ICCAS can be clearly observed through the carbon nanotube film (about 70 nm), which indicates the desirable transparency of the film.
FIG. 11 compares sheet resistances of the carbon nanotube film made in Film Fabrication Example 1 and the film made in Comparative Example 1. It can be seen that sheet resistance of the film made in Film Fabrication Example 1 drops significantly compared with Comparative Example 1.
Since the damage of sidewalls in the carbon nanotubes is avoided, the carbon nanotubes prepared or purified according to the present application have superior properties and can be widely used in optoelectronic applications.
It should be appreciated that additional process procedures can be utilized and may include drying, washing and the like, as long as there is no adverse impacts on the effects of the present application.
“Optional’ and “optionally” as used herein mean that the subsequently described event or circumstance (such as treatment step) may or may not occur, and that the description includes instances where the event occurs and instances where it does not.
All the references cited are incorporated by reference into the present description to the extent applicable.
It should be understood that various changes and modifications to the presently preferred embodiments described herein will be apparent to those skilled in the art. Such changes and modifications can be made without departing from the spirit and scope of the present subject matter and without diminishing its intended advantages. It is therefore intended that such changes and modifications be covered by the appended claims.

Claims

1. A method for preparing carbon nanotubes, the method comprising:

producing carbon nanotubes by an arc-discharge method in presence of a catalyst and optionally a promoter;

coordinating the metal elements present in the catalyst and/or the optional promoter with a substance capable of forming a complex with the metal elements to produce a complex; and

removing the complex.

2. The method of claim 1, wherein the promoter is employed.

3. The method of claim 2, wherein the promoter is FeS.

4. The method of claim 1, wherein the catalyst is selected from the group consisting of lanthanum metal oxide, transition metal, the mixture of nickel and a rare earth element, and mixtures thereof.

5. The method of claim 1, wherein the catalyst is selected from the group consisting of Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, and Ce—Ni alloy.

6. The method of claim 1, wherein coordinating the metal elements includes:

converting the metal elements present in the catalyst and/or the optional promoter into ions; and

coordinating the ions with the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter to produce a complex.

7. The method of claim 6, wherein converting the metal elements includes:

oxidizing the catalyst and/or the optional promoter to produce the oxides thereof.

8. The method of claim 7, wherein corresponding metal ions are obtained from the oxide by using the substance capable of forming a complex with the metal elements present in the catalyst and/or optionally the promoter, and are coordinated with the substance to produce a complex.

9. The method of claim 8, wherein the substance capable of forming a complex with the metal elements present in the catalyst and/or optionally promoter is selected from aminopolycarboxylic acids.

10. The method of claim 9, wherein the aminopolycarboxylic acid is selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohe-xane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA), and triethylenetetraaminehexaacetic acid (TTHA).

11. The method of claim 9, wherein the aminopolycarboxylic acid is triethylenetetraaminehexaacetic acid (TTHA).

12. The method of claim 1, wherein removing the complex includes converting the complex into a salt form and removing the complex in the salt form.

13. The method of claim 7, wherein converting the metal elements further comprises:

reacting the oxides with an acid to produce ions of the metal elements present in the catalyst and/or the optional promoter.

14. The method of claim 6, wherein the substance capable of forming a complex with the metal elements present in the catalyst and/or optionally the promoter is selected from the group consisting of tetrahydrofuran, trialkyl phosphine, ε-caprolactone, ε-caprolactam, dimethyl formamide, and dimethyl sulfoxide.

15. The method of claim 6, wherein the complex is selected from the group consisting of {M[(NC)₂CC(OCH₂CH₂OH)C(CN)₂]₂(4,4′-bpy)(H₂O)₂}, Dinuclear [{M′(phen)₂}₂V₄O₁₂]C₆H₁₂O.H₂O and [Ni(L)(H₂O)₃]2H₂O, wherein M is selected from Ni, Fe and Co; M′ is selected from Ni and Co; bpy is bipyridine; phen is phenyl; L is (2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid.

16. The method of claim 7, wherein oxidizing the catalyst comprises oxidizing the catalyst and/or optionally the promoter with an oxygen containing gas.

17. The method of claim 16, wherein the oxidation time and the oxidation temperature of the oxygen containing gas are sufficient to convert the catalyst and/or the optional promoter into oxides.

18. The method of claim 16, wherein the oxygen containing gas is air.

19. The method of claim 17, wherein the oxidation temperature is about 80° C. to about 300° C.

20. The method of claim 17, wherein the oxidation time is about 1 hour to about 20 hours.

21. The method of claim 1, further comprising centrifugation after removing the complex.

22. The method of claim 21, wherein centrifugation is carried out at a speed of about 5000 rpm to about 30000 rpm for about 1 hour to about 20 hours.

23. The method according to claim 1, wherein the carbon nanotubes are single-walled carbon nanotubes.

24. A method for purifying carbon nanotubes produced by an arc-discharge method in the presence of a catalyst and optionally a promoter, the method comprising:

removing the complex.

25. The method of claim 24, wherein coordinating the metal elements includes:

26. The method of claim 25, wherein converting the metal elements includes: oxidizing the catalyst and/or the optional promoter to produce the oxides thereof.

27. The method of claim 26, wherein corresponding metal ions are obtained from the oxide by using the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter, and are coordinated with the substance to produce a complex.

28. The method of claim 27, wherein the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter is selected from aminopolycarboxylic acids.

29. The method of claim 28, wherein said aminopolycarboxylic acid is selected from the group consisting of ethylenediaminetetraacetic acid (EDTA), trans-1,2-diaminocyclohe-xane-N,N,N′,N′-tetracetic acid hydrate (CYDTA), diethylenetriaminepentaacetic acid (DTPA) and triethylenetetraaminehexaacetic acid (TTHA).

30. The method of claim 28, wherein the aminopolycarboxylic acid is triethylenetetraaminehexaacetic acid (TTHA).

31. The method of claim 24, wherein removing the complex comprises converting the complex into a salt form and removing the complex in the salt form.

32. The method of claim 26, wherein oxidizing the catalyst and/or optional promoter comprises:

33. The method of claim 24, wherein said catalyst is selected from Y—Ni alloy, Fe—Ni alloy, Fe—Co alloy, Co—Ni alloy, Rh—Pt alloy, and Ce—Ni alloy.

34. The method of claim 25, wherein the substance capable of forming a complex with the metal elements present in the catalyst and/or the optional promoter is selected from the group consisting of tetrahydrofuran, trialkyl phosphine, ε-caprolactone, ε-caprolactam, dimethyl formamide, and dimethyl sulfoxide.

35. The method of claim 25, wherein the complex is selected from M[(NC)₂CC(OCH₂CH₂OH)C(CN)₂]₂(4,4′-bpy)(H₂O)₂}, Dinuclear [{M′(phen)₂}₂V₄O₁₂]C₆H₁₂O.H₂O and [Ni(L)(H₂O)₃]2H₂O, wherein M is selected from Ni, Fe and Co; M′ is selected from Ni and Co; bpy is bipyridine; phen is phenyl; L is (2-methoxycarbonylmethylimino-5-methyl-thiazol-3-yl)-acetic acid.

36. The method of claim 26, wherein the catalyst and/or the optional promoter are oxidized with an oxygen containing gas.

37. The method of claim 36, wherein an oxidation time and an oxidation temperature of the oxygen containing gas are sufficient to convert the catalyst and/or the optional promoter into oxides.

38. The method of claim 36, wherein the oxygen containing gas is air.

39. The method of claim 37, wherein the oxidation temperature is about 80° C. to about 300° C.

40. The method of claim 37, wherein the oxidation time is about 1 hour to about 20 hours.

41. The method of claim 24, further comprising a centrifugation step after removing the complex.

42. The method of claim 41, wherein the centrifugation step is carried out at a speed of about 5000 rpm to about 30000 rpm for about 1 hour to about 20 hours.

43. The method according to claim 24, wherein the carbon nanotubes are single-walled carbon nanotubes.

44. A carbon nanotube material comprising carbon nanotubes produced by arc-discharge in presence of a catalyst and optionally a promoter, wherein metal elements present in the catalyst and/or the optional promoter are coordinated with a substance capable of forming a complex with the metal elements to produce a complex, and wherein the complex is removed.

45. An element of carbon nanotubes comprising a carbon nanotube material including carbon nanotubes produced by arc-discharge in presence of a catalyst and optionally a promoter, wherein metal elements present in the catalyst and/or the optional promoter are coordinated with a substance capable of forming a complex with the metal elements to produce a complex, and wherein the complex is removed.

46. The element of carbon nanotubes of claim 45, wherein the element of carbon nanotubes is selected from the group consisting of conductive film of carbon nanotubes, field emission source, transistor, conductive wire, nano-electro-mechanic system, spin conduction device, nano cantilever, quantum computing device, lighting emitting diode, solar cell, surface-conduction electron-emitter display, filter, drug delivery system, thermal conductive material, nano nozzle, energy storage system, space elevator, fuel cell, sensor, and catalyst support material.