The present invention relates to a method for efficiently separating metallic single-walled carbon nanotubes and semiconducting single-walled carbon nanotubes from single-walled carbon nanotubes (hereinafter, CNTs) containing both of them.
A carbon nanotube (CNT) is a tubular material with a diameter of several nm to several tens of nm made by rolling a graphene sheet (a layer of six-membered carbon rings) into a cylinder, which is drawing attention as a superior nanomaterial that has thermal and chemical stability, mechanical strength, electron conductivity, thermal conductivity and spectral characteristics that extend to the near-infrared region.
Furthermore, CNTs include single-walled CNTs (hereinafter, SWNT) having a single layer of the above-described graphene sheet, double-walled CNTs (hereinafter, DWNTs) having two layers of the above-described graphene sheets, and a multi-walled CNT (hereinafter, MWNTs) having two or more layers of the above-described graphene sheets. In particular, SWNTs are attracting attention for its remarkable quantum effect.
SWNTs can be classified into armchair types, zigzag types and chiral types according to their difference in chirality (helicity), and their electric characteristics (band gap, electron level, etc.) are known to vary depending on the chiral angle along with a change in the structure such as the diameter. It is known that the armchair-type carbon nanotubes have metallic electric characteristics, while carbon nanotubes with other chiral angles may have semiconducting electric characteristics. The band gap of the SWNTs having such semiconducting electric characteristics (hereinafter, “semiconducting SWNTs”) vary depending on chirality. Utilizing such physical properties, semiconducting SWNTs are expected as materials for a high-performance transistor, an ultrashort pulse generator, an optical switch and the like. On the other hand, single-walled carbon nanotubes having metallic electric characteristics (hereinafter, “metallic SWNTs”) are expected as a replacement for a transparent conductive material that uses a rare metal so as to be applied to a transparent electrode for a liquid crystal display or a solar cell panel.
Now, SWNTs can be synthesized by various methods including a laser vaporization method, an arc discharge method and a chemical vapor deposition method (CVD method). Under the existing conditions, however, the metallic SWNTs and the semiconducting SWNTs are obtained only in a form of a mixture using any of these synthesis methods.
Therefore, development of a technique for separating semiconducting SWNTs and metallic SWNTs has been encouraged.
Conventional techniques, however, are associated with problems, such as requirement of multiple steps and poor yield of the SWNTs. These problems are major obstacles to practical (industrial) use. Conventional techniques also have problems, such as difficulty in the removal of the dispersant used for separation, and short length of the separated SWNTs. These problems cause increase in the resistivity upon the above-described application of the metallic SWNTs, while these problems cause deterioration in the transistor performance upon application of the semiconducting SWNTs.
One specific example of the above-mentioned conventional techniques is a method in which CNTs dispersed with a surfactant are subjected to dielectrophoretic between microelectrodes (Non-patent Document 1). There is also a method in which a solution of SWNTs dispersed with a soluble flavin derivative is prepared, to which a surfactant is added to give flavin derivative-dispersed SWNTs having specific chirality and surfactant-dispersed SWNTs having specific chirality and then the surfactant-dispersed SWNTs are removed by a salting-out method for separation (Non-patent Document 2).
Moreover, there are also a method in which a mixture of semiconducting SWNTs and metallic SWNTs is dispersed in a liquid so as to allow the metallic SWNTs to selectively bind to particles and then the metallic SWNTs bound to the particles are removed, thereby separating the semiconducting SWNTs (Patent Document 1), a method in which pH or ionic strength of a solution of SWNTs dispersed with a surfactant is adjusted so as to cause protonations at varying levels depending on the types of the SWNTs, which is subjected to an electric field so as to separate the metallic and semiconducting types (Patent Document 9), and a method, in which SWNTs dispersed with nucleic acid molecules are separated by ion-exchange chromatography (Patent Document 5).
Furthermore, there is a method in which SWNTs dispersed with a surfactant is separated into metallic SWNTs and semiconducting SWNTs by density-gradient ultracentrifugation (Non-patent Document 3).
In addition, there is a method, in which SWNT-containing gel obtained by soaking SWNTs dispersed with a surfactant into gel is used to separate the metallic SWNTs and the semiconducting SWNTs by a physical separating procedure (Patent Documents 6-8, and Non-patent Documents 4 and 5).
These methods proceed in two stages, namely, a step of dispersing SWNTs with a dispersant and a step of separating the SWNTs, requiring a multiple-stage process, which is difficult to be applied to industrial use. Moreover, since high-power ultrasonic irradiation and ultracentrifugation are employed in the first step, there are problems of poor yield of the SWNTs and short length of the separated SWNTs.
As other conventional methods, for example, there is a method in which semiconducting SWNTs are selectively burned with hydrogen peroxide (Non-patent Document 6). There are also a method in which SWNTs are treated with a nitronium-ion-containing solution and then subjected to filtration and heat treatment to remove the metallic SWNTs being contained in the SWNTs, thereby obtaining the semiconducting SWNTs (Patent Document 2), a method in which sulfuric acid and nitric acid are used (Patent Document 3), a method in which an electric field is applied so as to selectively move and separate the SWNTs, thereby obtaining the semiconducting SWNTs in the narrowed electric conductivity range (Patent Document 4).
Although these methods allow dispersion and separation to take place in a single step, they have problems in that only either the semiconducting SWNTs or the metallic SWNTs can be obtained, in that the recovery rate of the SWNTs is poor, and in that the length of the separated SWNTs is short, leading to defects.
As other conventional methods, there are, for example, methods in which a polyfluorene derivative (Non-patent Documents 7-10), polyalkylcarbazole (Non-patent Document 11) or polyalkylthiophene (Non-patent Document 12) is used to selectively disperse the semiconducting SWNTs in an organic solvent. These methods include a single working step and do not require ultracentrifugation upon separation. However, there is a problem of poor yield of the dispersed semiconducting SWNTs. In addition, since the dispersant is a polymer, there is a problem of significant difficulty in removal thereof after the separation due to strong adsorption with the SWNTs.
On the other hand, as methods for removing a dispersant after the separation, for example, there are a method in which an oligomer of a fluorene derivative is synthesized to disperse the SWNTs (Non-patent Document 13), a method in which the structure of the polymer is altered by photoreaction to reduce the adsorption power to the SWNTs (Non-patent Document 14), and a method in which a foldamer is used to alter the solvent condition so as to reduce the adsorption power to the SWNTs (Non-patent Document 15). These methods, however, have problems in that they do not allow selective dispersibility between the semiconducting SWNTs and the metallic SWNTs, and in that the yield of the dispersed semiconducting SWNTs is poor.
The above-described conventional techniques have problems in that they require a multiple-stage process and in that the yield of SWNTs is poor, where these problems are major obstacles to industrial use. Moreover, conventional techniques also have problems in that removal of the dispersant used for separation is difficult and in that the length of the separated SWNTs is short. These problems cause increase in the resistivity upon the above-described application of the metallic SWNTs while these problems cause deterioration in the transistor performance upon application of the semiconducting SWNTs.
Hence, the problem that the present invention intends to solve is to provide a novel method for efficiently separating metallic SWNTs and semiconducting SWNTs from SWNTs, which can solve the above-described problems.
The present inventors have gone through keen examination to solve the above-described problem. As a result, they found that SWNTs can be separated into metallic SWNTs and semiconducting SWNTs by selectively dispersing (solubilizing) the semiconducting SWNTs with a low-molecular-weight compound. They also found that the low-molecular-weight compound can be removed from the SWNTs by washing with a solvent and that the SWNTs can be redispersed with other surfactant or the like. Consequently, the present invention was completed.
A “low-molecular-weight compound” as used herein refers to a low-molecular-weight compound having an alkyl chain moiety for exhibiting solubility in a solvent and an aromatic-ring-containing moiety for interacting with SWNTs. For example, a flavin derivative soluble in an organic solvent is preferable. Specific steps include, for example, but not limited to, adding a flavin derivative and SWNTs in an organic solvent, dispersing the SWNTs by ultrasonic irradiation and subjecting this dispersion solution to centrifugation, thereby obtaining the supernatant (solution fraction) thereof as a solution having the dispersed semiconducting SWNTs. Meanwhile, the metallic SWNTs can be obtained as a precipitate (solid fraction) containing the same.
Thus, the present invention is as follows.
(1) A method for separating a metallic SWNT and a semiconducting SWNT from SWNTs, said method comprising the steps of: dispersing the SWNTs in a solution containing a low-molecular-weight compound having an alkyl chain moiety for exhibiting solubility in a solvent and an aromatic-ring-containing moiety for interacting with the SWNTs; and separating said dispersion solution into a solution fraction and a solid fraction.
Here, the above-mentioned low-molecular-weight compound may be any low-molecular-weight compound with chiral selectivity, for example, but not particularly limited to, those containing a flavin derivative, specifically, those containing 10-dodecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (chemical structural formula shown as structural formula (1) below) and/or 10-octadecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione.
Preferably, in the separation method according to (1) above, the solubilized semiconducting SWNTs are contained in the above-described solution fraction, while the metallic SWNTs are contained in the precipitated fraction.
In the separation method according to (1) above, the dispersion is carried out, for example, by stirring, shaking, ball milling or ultrasonic irradiation, while the separation is carried out, for example, by settling, filtration, membrane separation, centrifugation or ultracentrifugation.
The separation method according to (1) above may be, for example, a method that further comprises the steps of: collecting the semiconducting SWNTs from the above-described solution fraction; and/or collecting the metallic SWNTs from the above-described solid fraction.
(2) An agent for separating metallic SWNTs and semiconducting SWNTs, the agent comprising a low-molecular-weight compound having an alkyl chain moiety for exhibiting solubility in a solvent and an aromatic-ring-containing moiety for interacting with the SWNTs.
Here, the above-mentioned low-molecular-weight compound may be any low-molecular-weight compound with chiral selectivity, for example, but not particularly limited to, those containing a flavin derivative, specifically, those containing 10-dodecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (chemical structural formula shown as structural formula (1) below) and/or 10-octadecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione.
According to the present invention, SWNTs separated into semiconducting SWNTs and metallic SWNTs can be obtained in a single working step with an inexpensive equipment. Moreover, SWNTs longer than those obtained by the conventional techniques can be obtained at a high recovery rate. Furthermore, since the dispersant can be removed after the separation, application to a wide range of usage is not restrained by separation.
Hereinafter, the present invention will be described in detail. The scope of the present invention is not restricted to these descriptions, and can appropriately be modified and carried out apart from the following examples without departing from the spirit of the present invention.
The present specification incorporates the entire content of the specification of Japanese Patent Application No. 2013-046851 (filed on Mar. 18, 2013) based on which the present application claims priority. In addition, all of the publications, for example, prior art documents and publications, patent publications and other patent documents, cited herein are incorporated herein by reference.
As already mentioned above, herein, a single-walled carbon nanotube is referred to as a “SWNT”, a semiconducting single-walled carbon nanotube is referred to as a “semiconducting SWNT” and a metallic single-walled carbon nanotube is referred to as a “metallic SWNT”.
As already mentioned above, the present invention is a method for separating a metallic SWNT and a semiconducting SWNT from SWNTs containing a mixture of the metallic SWNTs and the semiconducting SWNT.
Specifically, this separation method comprises the steps of: dispersing SWNTs in a solution containing a low-molecular-weight compound having predetermined physical property and structure; and separating said dispersion solution into a solution fraction and a solid fraction. According to this method, the solubilized semiconducting SWNTs are contained (separated) in the solution fraction while the metallic SWNTs are contained (separated) in the solid fraction. This separation method may also comprise the step of recovering the semiconducting SWNTs from the above-mentioned solution fraction or recovering the metallic SWNTs from the above-mentioned solid fraction.
According to the separation method of the present invention, examples of SWNTs targeted by the above-described separation include those that are synthesized by the HiPCO method, CoMocat method, ACCVD method, arc discharge method, laser ablation method or the like.
According to the separation method of the present invention, an example of the low-molecular-weight compound used as the dispersant includes a low-molecular-weight compound having an alkyl chain moiety for exhibiting solubility in the solvent and an aromatic-ring-containing moiety for interacting with the SWNTs. This low-molecular-weight compound may be any low-molecular-weight compound having chiral selectivity, for example, but not particularly limited to, a flavin derivative, particularly preferably a flavin derivative soluble in an organic solvent. Specifically, this flavin derivative is preferably, for example, but not limited to, 10-dodecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (FC12 or dmC12) or the like represented by the following Structural formula (1). For example, a moiety of an alkyl group expressed as —C12H25 in the following structural formula may have variation in the lengths of the alkyl group within a range that allows solubility in the solvent. Specifically, preferable examples includes those with an alkyl group expressed as —CmH2m+1 (wherein, m is preferably an integer of 5-25, and more preferably an integer of 10-20). In particular, a preferable example includes a flavin derivative wherein “m” mentioned above is 18, namely, 10-octadecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (dmC18).
Here, the “methyl groups (—CH3)” existing at positions 7 and 8 of the flavin derivative represented by Structural formula (1) above (including those having different lengths of alkyl groups “—CmH2m+1” as described above) are considered to be important in that they can cause the CH-π interaction with the SWNTs targeted for separation (namely, attracting force acting between the hydrogen bound to carbon and the π electron system) and thus enhance solubility of the SWNTs (in particular, the semiconducting SWNTs).
The “imide hydrogen (—NH—)” existing at position 3 of the flavin derivative is also considered to be important in that it is involved in the dimerization between the flavin derivatives used via hydrogen bonding, as a result of which more flavin derivatives can interact with (adsorb to) the SWNTs (in particular, the semiconducting SWNTs). This can also be understood from the fact that a flavin derivative with said imide hydrogen has shorter average migration length on the semiconducting SWNT, in other words, the interaction with (adsorptive property to) the semiconducting SWNTs is greater, as compared to a flavin derivative without said imide hydrogen as will be shown in Example 6 below and
In addition, the above-described flavin derivative that interacts with (adsorbs to) the SWNTs has significant difference in the average migration length on the SWNTs, namely interaction with (adsorptive property to) the SWNTs, depending on whether the target of the interaction is semiconducting SWNTs or metallic SWNTs (average migration length: metallic SWNTs>semiconducting SWNTs) as will be shown in Example 7 below and
The solvent used with the separation method of the present invention may be any known organic solvent, for example, but not particularly limited to, benzene, toluene, xylene, ethylbenzene and the like, chlorobenzene, dichlorobenzene, chloromethylbenzene, bromobenzene and the like, naphthalene derivatives and the like, hexane, cyclohexane, THF, DMF and the like.
The procedure used upon dispersion (preparation of a dispersion solution) after the addition of a low-molecular-weight compound as the dispersant and SWNTs as the target of separation to the above-described solvent is not particularly limited and may be, for example, a procedure such as stirring, shaking, ball milling, ultrasonic irradiation (bath-type, probe-type, cup-type) or the like.
When this dispersion is carried out, for example, by ultrasonic irradiation, it is preferably carried out at a temperature condition of 5-80° C. (more preferably, 10-40° C.) for 5-720 minutes (more preferably, 10-180 minutes) although it is not particularly limited thereto.
The procedure for separating the dispersion solution into a solution fraction and a solid fraction after the dispersion described above, is not particularly limited, and may be, for example, a procedure such as settling, filtration, membrane separation, centrifugation, ultracentrifugation or the like.
The procedure for collecting the semiconducting SWNTs from the solution fraction after the separation, is not particularly limited, and may preferably be, for example, a procedure in which the solvent is removed by natural drying, with an evaporator or the like, or a procedure in which the semiconducting SWNTs are once aggregated by heating the solution fraction or dropping a good solvent for the dispersant and then subjected to filtration or membrane separation. In addition, the procedure for removing the dispersant is not particularly limited and may preferably be, for example, recrystallization (precipitation using change in the solubility by cooling), washing, sublimation, burning or the like.
On the other hand, the procedure for collecting the metallic semiconducting SWNTs from the solid fraction after the separation, is not particularly limited, and may preferably be, for example, a procedure such as filtration, membrane separation, centrifugation, ultracentrifugation or the like.
The present invention can also provide a dispersant that is capable of separating metallic SWNTs and semiconducting SWNTs from SWNTs containing a mixture of the metallic SWNTs and the semiconducting SWNTs.
Specifically, the dispersant contains a low-molecular-weight compound having an alkyl chain moiety for exhibiting solubility in the solvent and an aromatic-ring-containing moiety for interacting with single-walled carbon nanotubes as an active element, where the low-molecular-weight compound is not particularly limited as long as it has chiral selectivity.
Preferable examples of said low-molecular-weight compound include those that contain flavin derivatives. Specifically, those containing 10-dodecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (FC12 or dmC12) represented by Structural formula (1) shown above are more preferable although it is not particularly limited thereto. For example, a moiety of an alkyl group expressed as —C12H25 in the Structural formula (1) above may have variation in the length of the alkyl group within a range that exhibits solubility in the solvent. Specifically, preferable examples includes those with an alkyl group expressed as —CmH2m+1 (wherein, m is preferably an integer of 5-25, and more preferably an integer of 10-20). In particular, a preferable example includes a flavin derivative wherein “m” mentioned above is 18, namely, 10-octadecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (dmC18).
The separating agent of the present invention may appropriately contain components other than the above-described low-molecular-weight compound as the active element, and is not particularly limited.
Hereinafter, the present invention will be described more specifically by means of examples although the present invention should not be limited thereto.
One of flavin derivatives, 10-dodecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (FC12 or dmC12; see Structural formula (1) below), was synthesized.
To toluene, FC12 and SWNTs synthesized by the HiPCO method, one of CVD methods, (which had already been removed of the catalyst with acid) were added to 0.6 mg/mL, and the resultant was subjected to ultrasonic irradiation for 3 hours with a bath-type ultrasonic irradiator (BRANSON5510). Thereafter, the dispersion liquid was centrifuged for 10 minutes under the condition of 10000×G at 25° C. with a cooling centrifuge (himac CF-15R) to collect the supernatant.
The absorption spectrum of the collected supernatant solution was measured. The solid line in
The photoluminescence of the collected supernatant solution was determined.
The collected supernatant solution was filtrated with a membrane filter (PTFE 0.1 μm (Millipore)) and washed with acetone. The Raman spectrum of the paper filter was determined. As a control, SWNTs synthesized by the HiPCO method was dispersed in water, filtrated with a membrane filter (HTTP 0.4 μm (Millipore)) to determine the Raman spectrum of the paper filter (excitation: wavelength 633 nm). The Raman spectra are shown in
In order to examine the distribution of the SWNT lengths, an atomic force microscope (AFM) was used for determination. The collected supernatant solution was spin coated and washed with dichloromethane.
30 mL of the collected supernatant solution was cooled in a freezer (−5° C.) to precipitate and remove excessive FC12. Toluene was evaporated with an evaporator to precipitate the SWNTs on the side surface of the sample tube. These SWNTs were washed with 500 mL of acetone for 13 times. 10 mL of 1 wt % aqueous sodium cholate solution was added to that sample tube, which was subjected to ultrasonic irradiation using a bath-type ultrasonic irradiator for 3 hours and using a probe-type ultrasonic irradiator for 30 minutes while cooling in a water bath. The resulting solution was centrifuged once using an ultracentrifuge under the conditions of 120000×G and 25° C. to collect the supernatant. The absorption spectrum of the supernatant was determined. The result is represented by the solid line in
The centrifugal acceleration condition for the liquid of FC12 and SWNTs dispersed in toluene in Example 1 was altered to be 100×G, 500×G, 1000×G and 3000×G to determine the absorption spectra of the collected supernatants. The absorption spectra are shown in
To o-xylene, FC12 and HiPCO SWNTs (SWNTs synthesized by the HiPCO method) (which had already been removed of the catalyst) were added, and the resultant was subjected to ultrasonic irradiation for 3 hours using a bath-type ultrasonic irradiator (BRANSON5510). Thereafter, the dispersion liquid was centrifuged with a cooling centrifuge (himac CF-15R) for 10 minutes under the condition of 10000×G at 25° C. to collect the supernatant. The absorption spectrum of the collected supernatant solution was measured (light path length: 1 cm). The absorption spectrum is shown in
To p-xylene, FC12 and HiPCO SWNTs (which had already been removed of the catalyst) were added, and the resultant was subjected to ultrasonic irradiation for 3 hours using a bath-type ultrasonic irradiator (BRANSON5510). Thereafter, the dispersion liquid was centrifuged with a cooling centrifuge (himac CF-15R) for 10 minutes under the condition of 10000×G at 25° C. to collect the supernatant. The absorption spectrum of the collected supernatant solution was measured (light path length: 1 cm). The absorption spectrum is shown in
To o-dichlorobenzene, FC12 and HiPCO SWNTs (which had already been removed of the catalyst) were added, and the resultant was subjected to ultrasonic irradiation for 3 times with a bath-type ultrasonic irradiator (BRANSON5510). Thereafter, the dispersion liquid was centrifuged with a cooling centrifuge (himac CF-15R) for 10 minutes under the condition of 10000×G at 25° C. to collect the supernatant. The absorption spectrum of the collected supernatant solution was measured (light path length: 1 cm). The absorption spectrum is shown in
Interactions with (adsorptive properties to) semiconducting SWNTs were compared between the presence and the absence of imide hydrogen (—NH—) at position 3 of flavin derivatives (FC12 or dmC12), namely, the presence and the absence of hydrogen bonding between the flavin derivatives (the presence or the absence of dimerization capacity) by measuring the average migration lengths of the flavin derivatives on the semiconducting SWNTs according to the following measurement and experiment conditions. Specifically, the MD (Molecular Dynamics) was carried out following structural optimization with the molecular mechanics calculation (MM). The results are shown in
<Measurement and Experiment Conditions>
As can be appreciated from the results shown in
Interactions (adsorptive properties) of flavin derivatives (FC12 or dmC12) (specifically, dimers formed between flavin derivatives) with the semiconducting SWNTs and the metallic SWNTs were compared by measuring the average migration lengths of the flavin derivatives on the respective SWNTs according to the following measurement and experiment conditions. Specifically, the MD (Molecular Dynamics) was carried out following structural optimization with the molecular mechanics calculation (MM). The results are shown in
<Measurement and Experiment Conditions>
As can be appreciated from the results shown in
Following the procedure and the method of Example 1, an absorption spectrum and a photoluminescence spectrum of SWNTs dispersed in toluene with one of flavin derivatives, 10-octadecyl-7,8-dimethyl-10H-benzo[g]pteridine-2,4-dione (dmC18) were determined in the same manner as the absorption spectrum and the photoluminescence spectrum of the SWNTs dispersed in toluene with dmC12 (FC12) (see
As a result, similar to the case using dmC12 (FC12), the semiconducting SWNTs were confirmed to be selectively solubilized among the SWNTs when dmC18 was used.
Since the present invention is capable of obtaining SWNTs that have been separated into semiconducting SWNTs and metallic SWNTs in a single working step with an inexpensive equipment, the present invention is extremely beneficial in terms of usability. The present invention is also capable of obtaining SWNTs with longer lengths at a high recovery rate as compared to conventional techniques. Furthermore, according to the present invention, since the dispersant can be removed after separating the semiconducting SWNTs and the metallic SWNTs, application to a wide range of usage is not restrained by separation. Therefore, the present invention is also extremely beneficial in terms of practical use.
Number | Date | Country | Kind |
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2013-046851 | Mar 2013 | JP | national |
Filing Document | Filing Date | Country | Kind |
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PCT/JP2014/056214 | 3/10/2014 | WO | 00 |