Method for separating carbon nanotubes using modified cellulose

Abstract

The present invention relates to the use of modified cellulose (e.g., nitrated cellulose) for separating carbon nanotubes (CNTs). A raw mixture of CNTs of different structures or chiralities can be separated into fractions based on their selective permeability through a separation column packed with nitrated cellulose. The present invention is particularly useful for separating semiconducting and metallic CNTs.

Description

[Technical Field]

[0001] The present invention relates to a method for separating carbon nanotubes using nitrated cellulose. [Background technology]

[0002] The widespread application of carbon nanotubes (CNTs), regardless of the production method, is hindered by the fact that raw CNT products are mixtures of nanotubes with different structures and electrical properties. Some CNTs exhibit metallic properties, while others exhibit semiconducting behavior. While certain types of CNTs are required for certain applications, it is desirable to have them in a pure state, free of other types of CNTs. Thus, metallic CNTs (M-CNTs) have the potential to be used as alternative conductors to metals, such as wires and flexible conductive films, due to their high electrical conductivity. Meanwhile, semiconducting CNTs (S-CNTs) have the potential to be used in transistors and quantum dots, due to their band gap. Furthermore, single-chirality S-CNTs, when extracted, are even more attractive because they can be used as quantum dots, with distinct and unique emission spectra, including very strong and narrow emission bands. Therefore, efficient methods for separating raw CNT mixtures are highly desirable.

[0003] Currently, the three most commonly used methods for separating CNTs are density gradient ultracentrifugation (DGUC), aqueous two-phase extraction (ATPE), and selective gel permeation (SGP).

[0004] DGUC utilizes the difference in buoyant density of surfactant-assembled CNTs with different structures. During centrifugation, the surfactant-wrapped CNTs are precipitated by passing them through a high-density liquid, such as iodixanol, with a predesigned density gradient (www.doi.org / 10.1038 / nnano.2006.52). This method is suitable for research purposes because it can obtain two or more fractions of CNTs, including a single chirality fraction. However, this method cannot be scaled up and is very expensive for industrial use. First, it is limited by the capacity of the ultracentrifuge tubes / bottles. Second, the use of an ultracentrifuge requires high energy consumption. Third, it is very difficult to extract the separated CNTs from the density gradient solution without sacrificing them.

[0005] ATPE is based on the selective extraction of specific CNTs from a blend of two partially miscible polymers, such as dextran and polyethylene glycol. When the blend of two polymers is allowed to settle or separated into two phases by centrifugation, the CNTs present in the mixture are attracted to the two distinct phases based on their structure (Nano Lett., 2015, 15, 1642-1646). While this method is efficient and scalable, it has a significant drawback: it is extremely difficult to extract CNTs from the polymer solution.

[0006] SGP is based on the selective permeation of surfactant-wrapped CNTs of different chiralities through columns filled with gels of several polysaccharides, such as agarose or dextran. This method has been described in a series of papers (Nano Lett., 2009, 9, 1497–1500; Nature Commun. 2011, DOI: 10.1038 / ncomms1313; Nano Lett., 2013, 13, 1996–2003). The gels used for separation are commercially available, such as those sold under the trade names "Sepharose" and "Sephacryl." These products cost more than $1,000 / L. While this method can be easily scaled up, the high cost of the gels is a major drawback. Another drawback is the relatively low flow rate of CNT dispersions through columns packed with these gels.

[0007] A new method for separating M-CNTs from S-CNTs is needed, and it must be scalable to industrial levels and cost-effective. [Prior art documents] [Non-patent literature]

[0008] [Non-Patent Document 1] Nano Lett.,2015,15,1642-1646 [Non-patent document 2] Nano Lett.,2009,9,1497-1500 [Non-patent document 3] Nature Commun.2011,DOI:10.1038 / ncomms1313 [Non-patent document 4] Nano Lett.,2013,13,1996-2003 [Brief explanation of the drawings]

[0009] [Figure 1]Examples of modified cellulose are shown below, where the hydroxyl groups are modified by oxidation, esterification, etherification, urethanization, amidation, and non-covalent modifications. [Figure 2] An example of modified cellulose is shown below: the hydroxyl groups are modified by nitration. [Figure 3] Micrographs of raw cotton and modified cotton. (a) is an optical microscope image of raw cotton, (b) is an optical microscope image of modified cotton, (c) is a low-magnification SEM image of modified cotton, (d) is a high-magnification SEM image of raw cotton, and (e) is a high-magnification SEM image of modified cotton. [Figure 4] The optical absorbance spectra of the two separated fractions are compared to the spectrum of the pristine unseparated CNTs. The spectra have been normalized to have the same absorbance at 870 nm. [Figure 5] These are Raman spectra acquired from the two fractions of CNTs and the reference sample, pristine CNTs. The spectrum of M-CNT is shown in blue, and the spectrum of S-CNT is shown in red. The spectrum of pristine CNTs is shown in black. (a,b) were acquired using a 532 nm excitation laser. (c,d) were acquired using a 633 nm excitation laser. (a) and (c) show the radial breathing mode (RBM) band, while (b) and (d) show the tangential mode band (commonly called the G band and D band). [Figure 6] 1 is a UV-Vis spectrum of the three collected fractions described in Example 4. DETAILED DESCRIPTION OF THE INVENTION

[0010] The present invention relates to the use of modified cellulose (MC) for separating carbon nanotubes (CNTs). The inventors have found that by using modified cellulose, particularly nitrated cellulose, a raw mixture of CNTs of different structures or chiral angles (chiralities) can be separated into fractions based on their selective permeability through a separation column packed with modified cellulose (MC).

[0011] "Modified cellulose" refers to cellulose in which the hydroxyl groups of the cellulose backbone have been chemically modified. Each cellulose unit contains primary and secondary hydroxyl groups, and the C6-hydroxyl group (-CH2OH) is more reactive than the other C2 and C3 hydroxyl groups.

[0012] Figure 1 shows some examples of modified celluloses, where the hydroxyl groups are modified by oxidation, esterification, etherification, urethanization, amidation, or non-covalent modifications.

[0013] Figure 2 shows another example of modified cellulose, a nitrated cellulose in which hydrogen atoms in the hydroxyl groups of cellulose have been modified to contain nitro (-NO2) functional groups. While Figure 2 shows all hydrogen atoms of the hydroxyls at the C2, C3, and C6 positions replaced with nitro groups, it is preferred that the nitrated cellulose useful in the present invention is not fully nitrated at all available positions. Nitrated cellulose useful in the present invention generally contains about 1-15% by weight of nitrogen, preferably about 2-12% or about 3-11% by weight of nitrogen, and more preferably about 5-9% by weight, or about 5-7% by weight, or about 6-7% by weight of nitrogen. A nitrogen content of about 6-7% by weight in modified cellulose is considered to be a nitrated cellulose. 24 N 36 O 16 It corresponds to the molecular formula (NO3)4.

[0014] Nitration of cellulose can be achieved by immersing the cellulose in nitric acid or a solution containing nitric acid, such as a mixture of nitric acid and sulfuric acid. The desired level of nitration in the modified cellulose (MC) can be achieved by controlling basic reaction parameters such as the HNO3 / H2SO4 ratio, acid / cellulose ratio, reaction temperature, and reaction time.

[0015] As used herein, "about" means ±10% of the stated value.

[0016] The present invention relates to a method for separating CNTs from a mixture containing CNTs of different structures or different chiral angles (chiralities) based on the selective permeability of CNTs to MCs. The method includes the steps of (a) dispersing a mixture of CNTs in a dispersion solution to form a CNT dispersion, (b) passing the CNT dispersion through a column packed with modified cellulose, such as nitrated cellulose, (c) eluting a first type of CNTs with a first aqueous solution containing a first surfactant, and (d) eluting a second type of CNTs with a second aqueous solution containing a second surfactant. To elute CNTs of different structures, the first and second aqueous solutions are different. In one embodiment, the first and second surfactants are different. In another embodiment, the concentrations of the first and second surfactants are different. Generally, the second aqueous solution has a stronger ability to elute bound CNTs from the modified cellulose than the first aqueous solution. If necessary, an additional elution step can be performed between steps (c) and (d) using a mixture of the first and second surfactants in different ratios, thus obtaining a more structurally homogeneous CNT fraction.

[0017] In step (a), the CNT mixture is dispersed in a dispersion solution to form a CNT dispersion. As used herein, "dispersion" refers to the formation of a stable suspension of CNTs. A stable suspension is one in which, upon standing, no visible carbon powder, particles, flakes, or loose agglomerates precipitate out of the solution or settle to the bottom of the mixture. Typically, to disperse the CNT mixture, the mixture is mechanically agitated by one or more standard methods, which may include stirring, sonication, microfluidization, wet milling, etc. In one embodiment, the dispersion solution includes one or more surfactants to loosen the CNTs. Preferably, the one or more surfactants are anionic surfactants. For example, the one or more anionic surfactants are cholate, deoxycholate, sulfate, sulfonate, phosphate, phosphonate, carboxylate, and any combination thereof. Preferred anionic surfactants for dispersing CNTs are dodecyl sulfate, sodium cholate, sodium deoxycholate, sodium dodecylbenzenecholate, or any combination thereof.

[0018] If necessary, between steps (a) and (b), the CNT mixture may be centrifuged to remove undispersed CNT bundles or particles, and the supernatant CNT dispersion may be used in the following steps (b) to (d).

[0019] In step (b), the CNT dispersion is loaded onto the top of a column packed with modified cellulose and allowed to pass through the column. The CNT dispersion can be used directly as a loading solution and loaded directly onto the column. Alternatively, the CNT dispersion can be mixed with another aqueous solution to prepare a loading solution before loading onto the column. The other aqueous solution may optionally contain an anionic surfactant.

[0020] Step (b) can be varied by premixing the CNT dispersion with modified cellulose to allow adsorption of CNT and MC, and then packing the CNT / MC mixture into a column.

[0021] In step (c), the first type of CNTs are eluted with a first aqueous solution containing a first surfactant. In one embodiment, the CNT dispersion in (a) contains the same first aqueous solution as in step (c), so the first type of CNTs do not bind to or adsorb to the MC in step (b) and flow through the column in the void volume. In another embodiment, the CNT dispersion in (a) does not contain the same first aqueous solution, and the first type of CNTs bind to or adsorb to the MC in step (b) and are eluted with the first aqueous solution in step (c).

[0022] In step (d), the second type of CNTs is eluted with a second aqueous solution containing a second surfactant. The first and second surfactants are different so that the first and second types of CNTs are eluted separately. Preferably, the second aqueous solution is stronger than the first aqueous solution in its ability to elute the remaining CNTs bound to the nitrated cellulose. In a preferred embodiment, the first type of CNTs are semiconducting CNTs (S-CNTs) and the second type of CNTs are metallic CNTs (M-CNTs). This method separates the S-CNTs and M-CNTs by eluting them with different first and second surfactants.

[0023] In another embodiment, the first type of CNTs are M-CNTs and the second type of CNTs are S-CNTs, and the method separates the M-CNTs and S-CNTs by eluting them with different first and second surfactants.

[0024] In one embodiment, the cellulose is natural cotton. In another embodiment, the cellulose is any textile made from natural cotton, either woven or nonwoven. In another embodiment, the cellulose is natural linen or any woven or nonwoven fabric made from natural linen. In another embodiment, the cellulose is any other natural fiber material.

[0025] In one embodiment, the CNTs in the CNT mixture are single-walled CNTs, double-walled CNTs, or multi-walled CNTs. Preferred CNTs are single-walled CNTs.

[0026] In one embodiment, the first and second surfactants are anionic surfactants independently selected from the group consisting of cholate, deoxycholate, sulfate, sulfonate, phosphate, phosphonate, carboxylate, and any combination thereof. For example, the first and second surfactants are independently selected from the group consisting of, but not limited to, sodium dodecyl sulfate, sodium cholate, sodium deoxycholate, sodium dodecylbenzenecholate, and any combination thereof.

[0027] In one embodiment, the dispersion solution for dispersing the CNTs is an aqueous solution containing a surfactant, such as an anionic surfactant selected from the group consisting of cholate, deoxycholate, sulfate, sulfonate, phosphate, phosphonate, carboxylate, and any combination thereof. For example, the anionic surfactant is sodium dodecyl sulfate, sodium cholate, sodium deoxycholate, sodium dodecylbenzenecholate, or any combination thereof.

[0028] Separation of CNTs occurs when a CNT dispersion in solution is passed through a column packed with MC and successively eluted with solutions of specific surfactants. For example, S-CNTs can be eluted in the first fraction with a solution of sodium dodecyl sulfate (SDS), and M-CNTs can be eluted in the second fraction with sodium deoxycholate (SDC).

[0029] Furthermore, an additional elution step can be performed using different concentrations or mixture ratios of the same surfactants to enable separation of CNTs with a narrow range of selected chiralities.

[0030] Different pH and ionic strengths in the dispersion and elution solutions can be used to optimize CNT separation.

[0031] In the present invention, different permutations of surfactants and surfactant concentrations can be optimized. In one embodiment, the initial dispersion solution contains 0.4-5 wt. % or 0.5-2 wt. % SDC to disperse the CNTs. The dispersion solution is then adjusted to a mixture of 0.2-2 wt. % SDS and 0.02-0.3 wt. % SDC before loading onto an MC-packed column. The S-CNTs are eluted with a first aqueous solution containing 0.5-5 wt. % or 1-2 wt. % SDS (e.g., 1 wt. % SDS). The M-CNTs are then eluted with a second aqueous solution containing 0.5-5 wt. % or 1-3 wt. % SDC (e.g., 2 wt. % SDC). In this embodiment, an optional elution step may be added after the first elution step and before the second elution step. The optional elution step uses a combination of SDS and SDC surfactants, with each surfactant concentration lower than the concentration in the first or second aqueous solution. This additional step can elute M-CNTs with different chiralities.

[0032] In another embodiment, the initial dispersion solution does not contain SDC and contains 0.5 to 5 wt % or 0.5 to 2 wt % SDS (e.g., 1 wt % SDS). First, the M-CNT is eluted with a first aqueous solution containing 0.5 to 5 wt % or 1 to 2 wt % SDS (e.g., 1 wt % SDS). Next, the S-CNT is eluted with 0.5 to 5 wt % or 1 to 2 wt % SDC (e.g., 1 wt % SDC). In this embodiment, since the initial dispersion solution does not contain SDC, the elution order is reversed compared to the above-mentioned embodiment in which the initial dispersion solution contains both SDS and SDC. That is, in this embodiment, the M-CNT is eluted first and the S-CNT is eluted second, whereas in the above-mentioned embodiment, the S-CNT is eluted first and then the M-CNT is eluted.

[0033] The method is robust, environmentally friendly, cost-effective, and suitable for large-scale production. MC is inexpensive, and after separation, the packed column can be reused for at least 20 cycles for CNT separation.

[0034] The method is useful for separating CNTs from commercially available mass-produced CNTs, such as the single-walled CNTs sold under the trade name TUBALL™ by OCSiAl of Luxembourg.

[0035] The following examples further illustrate the present invention. These examples are intended to be merely illustrative of the present invention and are not to be construed as limiting. [Example]

[0036] Example 1: Preparation of modified cellulose (MC) A nitration mixture was prepared by blending 200 mL of nitric acid (65%) and 600 mL of sulfuric acid (95-96%). The mixture was cooled to 22°C. Absorbent cotton (30.0 g, purchased from a local pharmacy) was immersed in the acid mixture and manually stirred with a glass rod. The reaction temperature was maintained at 22-23°C. After 6 minutes, the cotton was removed from the acid and placed on a Buchner funnel to drain the acid from the cotton. The cotton was then transferred to a glass container and filled with 3 L of cold water to quench the reaction. Next, the cotton was placed back on the Buchner funnel, and the wash water was drained from the cotton. This wash cycle was repeated several times until the wash water was neutral based on pH indicator paper. After the final wash, the cotton was squeezed by hand to remove as much water as possible and placed on a shelf to dry at ambient conditions. The yield of the reaction was 36.18 g, i.e., the weight gain due to nitration was 6.18 g. This procedure resulted in modified cellulose (MC) in the form of modified absorbent cotton.

[0037] The MC appeared almost identical to the original cotton. As is evident from Figure 3(a), b, and c), the MC retained its fibrous structure. The only noticeable effect of nitration was the formation of pores, which can be seen in high-magnification SEM images (Figure 3(d) and (e)). The prepared MC (32 g) was introduced into a glass tube and used as a separation column.

[0038] Example 2: Preparation of CNT dispersion The CNT dispersion for separation was prepared as follows: 40 mg of metal catalyst-purified TUBALL™ CNTs were dispersed in a formulation consisting of 40 mL of 1% SDC (sodium deoxycholate) solution in deionized water and sonicated for 3 hours at 30% amplitude using a tip sonicator (Sonic-Vibra 750). Next, 160 mL of 1% SDS solution in deionized water was added to the CNT dispersion in SDC and sonicated under the same conditions for 1 hour. During this step, the CNTs were debundled to a significant extent, yielding individual CNTs and small bundles. After sonication, the solution (containing 0.8% SDS and 0.2% SDC) was centrifuged at 24,000 RPM (60,000 g) and T = 25 °C for 12 hours to precipitate the remaining bundles. The supernatant (SN, the top 80% of the solution in the centrifuge tube) was sampled and loaded onto a MC-packed column for separation.

[0039] Example 3: Separation of CNTs into S-CNTs and M-CNTs A separation column filled with the MC from Example 1 was fixed vertically. Next, 32.8 mL of the supernatant CNT dispersion from Example 2 was poured onto the top of the column. Next, the column was eluted with a 1% SDS aqueous solution, and the CNT-containing solution emerging from the bottom of the column was collected. Elution with 1% SDS was continued until no CNTs were observed in the solution emerging from the column. This collected portion of the CNT solution was designated the first fraction (Fr-1). Next, the column was eluted with a 2% SDC aqueous solution, and the emerging solution was again collected until no CNTs were observed in the solution emerging from the column. In this manner, a second fraction was collected (Fr-2). The first fraction (eluted with 1% SDS) had a distinct red color and contained primarily semiconducting CNTs (S-CNTs). The second fraction (eluted with 2% SDC) had a distinct green color and contained primarily metallic CNTs (M-CNTs). These procedures constitute a complete separation cycle.

[0040] Figure 4 shows the optical absorption spectra in the wavelength range of approximately 400–1100 nm of fraction 1 (S-CNT) and fraction 2 (M-CNT) compared to the spectrum of the original, unseparated CNTs.

[0041] Fraction 1 exhibits absorption in the 870 nm–1100 nm spectral region associated with the S22 transition of semiconducting CNTs (www.doi.org / 10.1103 / PhysRevB.66.045411). Fraction 2 exhibits absorption in the 580 nm–870 nm region associated with the M11 transition of metallic CNTs (ibid.). Fraction 2 exhibits slight but very low absorption in the 870 nm–1100 nm region, suggesting the presence of some S-CNTs in the fraction. However, based on the spectrum, the S-CNT content is very low. Fraction 1 has almost no absorbance in the 580 nm–870 nm region, suggesting that Fraction 1 is almost entirely composed of S-CNTs. Table 1 provides the mass percentages of CNTs in the separated fractions.

[0042] [Table 1]

[0043] Figure 5 shows the Raman spectra obtained from the two fractions of CNTs and from the pristine CNT as a reference sample. The radial breathing mode (RBM) band clearly shows that the spectrum of the pristine CNT is the algebraic sum of the two fractions. When the Raman spectrum is obtained with a 532 nm excitation laser, the G bands of the two fractions are similar (3b). When the Raman spectrum is obtained with a 633 nm excitation laser, the G bands of the two fractions are different (3d). M-CNT has a peak at 1540 cm -1 There is an additional component at approximately 1520 cm -1 The broadening of the G band is a phenomenon unique to metallic CNTs. Thus, the Raman spectra additionally confirmed the metallic and semiconducting properties of the two separated fractions.

[0044] Example 4: Separation of CNTs into S-CNTs and M-CNTs In this example, a CNT dispersion was prepared as described in Example 2, except that CNTs from different TUBALL production batches were used. The different batches had slightly different CNT contents, i.e., CNTs with different structures (chiralities), which is evident from the optical absorption spectra (Figure 6).

[0045] A separation column packed with the MC of Example 1 was fixed vertically. Then, 30 mL of a CNT dispersion (containing 0.8% SDS and 0.2% SDC) was poured onto the top of the column. Next, the column was eluted with a 1% SDS aqueous solution, and the CNT-containing solution emerging from the bottom of the column was collected. Elution with 1% SDS was continued until no CNTs were observed in the solution emerging from the column. The collected portion of this CNT solution was designated the first fraction (Fr-1). Next, the column was eluted with an aqueous solution containing a combination of SDC and SDS (a solution prepared by mixing 6 parts by volume of 1% SDC and 4 parts by volume of 1% SDS, i.e., a solution volume ratio of 60% / 40%). The collected portion of this CNT solution was designated the second fraction (Fr-2). Finally, the column was eluted with a 1% SDC solution to obtain the third fraction (Fr-3). These procedures constitute a complete separation cycle.

[0046] The optical absorption spectra for the three collected fractions are shown in Figure 6, compared with the spectrum of the original CNTs. The spectra of all three fractions were different from each other, suggesting that each fraction consisted of different types of CNTs. The first fraction (eluted with 1% SDS) had a reddish hue and was enriched in semiconducting CNTs (S-CNTs). The second fraction (eluted with a 6 / 4 mixture of SDC / SDS) had a distinct bluish-green color and contained primarily metallic CNTs (M-CNTs). The third fraction was greenish-gray and consisted mostly of M-CNTs and some S-CNTs. Table 2 provides the mass percentage of CNTs in the separated fractions.

[0047] [Table 2]

[0048] The foregoing describes preferred embodiments of the present invention and it will be understood that modifications can be made without departing from the scope of the invention as set forth in the claims.

Claims

1. 1. A method for separating metallic CNTs (M-CNTs) from semiconducting CNTs (S-CNTs) in a mixture of carbon nanotubes (CNTs), comprising: dispersing the mixture of CNTs in a dispersion solution comprising sodium deoxycholate (SDC) and sodium dodecyl sulfate (SDS) to form a CNT dispersion; passing the CNT dispersion through a column packed with nitrated cellulose; Eluting the S-CNT with a first aqueous solution containing SDS; Eluting the M-CNT with a second aqueous solution containing SDC. and, in that order, The method according to claim 1, wherein the nitrogen content in the nitrated cellulose is 1 to 15% by weight.

2. 1. A method for separating metallic CNTs (M-CNTs) from semiconducting CNTs (S-CNTs) in a mixture of carbon nanotubes (CNTs), comprising: dispersing the mixture of CNTs in a dispersion solution containing sodium dodecyl sulfate (SDS) but not sodium deoxycholate (SDC) to form a CNT dispersion; passing the CNT dispersion through a column packed with nitrated cellulose; Eluting the M-CNT with a first aqueous solution containing SDS; Eluting the S-CNT with a second aqueous solution containing SDC. and, in that order, The method according to claim 1, wherein the nitrogen content in the nitrated cellulose is 1 to 15% by weight.

3. 3. The method of claim 1 or 2, wherein the cellulose is cotton.

4. The method according to claim 1 or 2, wherein the nitrogen content in the nitrated cellulose is 2 to 12% by weight.

5. 3. The method according to claim 1, wherein the nitrogen content in the nitrated cellulose is 5 to 9% by weight.

6. The method of claim 1 or 2, wherein the M-CNT and the S-CNT are single-walled CNTs.

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