KR20220128856A - Method for purifying crude BNNTs and composition for purified BNNTs - Google Patents

Abstract

The present invention relates to a method for purifying boron nitride nanotubes capable of easily separating BNNTs and other impurities by inducing a π-π bond between polyfluorene and BNNTs in a polyfluorene solution. The method for purifying boron nitride nanotubes according to the present invention comprises the step of: preparing the polyfluorene solution; and forming a polyfluorene-BNNT composite by mixing a BNNT compound containing boron nitride nanotubes (BNNT) in the polyfluorene solution.

Description

Boron nitride nanotube purification method and boron nitride nanotube purification composition {Method for purifying crude BNNTs and composition for purified BNNTs}

The present invention relates to a boron nitride nanotube purification method and a boron nitride nanotube purification composition, and more particularly, by inducing a π-π bond between polyfluorene and BNNT in a polyfluorene solution to easily remove BNNT and other impurities It relates to a boron nitride nanotube purification method and a boron nitride nanotube purification composition that can be separated.

Boron nitride nanotubes (BNNTs) are a new material that has recently been in the spotlight due to its excellent mechanical and chemical properties. In particular, the high heat resistance and high oxidation stability of BNNTs promote research into various fields of application.

The synthesis of BNNT uses arc discharge method, ball milling, chemical vapor deposition, laser ablation, plasma stream, etc. Most of the methods suitable for mass synthesis are polycrystalline BN (boron nitride) or hexagonal BN (hBN) as a precursor. is using BNNTs synthesized through this method contain a large amount of polycrystalline BN and hBN, so there is a disadvantage that the purity is generally low. In particular, it is known that it is very difficult to obtain pure BNNT through stagnation because hBN has very similar chemical properties to BNNT.

The purification method of BNNT is largely divided into separation and purification in a solid phase and separation and purification in a solution phase. As a method of separation and purification on a solid phase, a method of removing impurities contained in BNNT in the form of chloride by passing high-temperature chlorine gas (Cl ₂ ) through the synthesized BNNT (Non-Patent Document 1 - Simard et al., Chemistry of Materials) , 2020, 32, 3911), a method of selectively decomposing impurities by high-temperature treatment in a state in which water vapor is injected into the synthesized BNNT (see Non-Patent Document 2 - Pasquali et al., Chemistry of Materials, 2019, 31, 1520) ) have been proposed. This solid-phase separation and purification method has the advantage that it is easy to obtain high-purity BNNTs in large quantities, but has a disadvantage in that the yield is very low due to the destruction of BNNTs due to the high-temperature reaction.

As a method for separation and purification in a solution phase, a method of dispersing the synthesized BNNTs with a surfactant and then extracting the dispersed BNNTs through centrifugation (Non-Patent Document 3 - Marti et al., Nanoscale Advances, 2019, 1, 1096) refer), a method of dispersing the synthesized BNNT in an organic solvent and then removing impurities through continuous filtration accompanied by ultrasonic dispersion (see Non-Patent Document 4 - Alston et al., Nanoscale Advances, 2019, 1, 1693), etc. has been proposed However, the solution-phase separation and purification method as described above has the advantage of being based on a very economical solution-phase process from an industrial point of view, but has a disadvantage in that the purification efficiency of BNNT is significantly lower than that of the solid-phase separation and purification method.

US Patent Publication No. US 2020-0231439 (July 23, 2020) International Patent Publication No. WO2020-010458 (2020.01.16) US Patent Publication No. US 2019-0292052 (2019. 09. 26)

Simard et al., Chemistry of Materials, 2020, 32, 3911 Pasquali et al., Chemistry of Materials, 2019, 31, 1520 Marti et al., Nanoscale Advances, 2019, 1, 1096 Alston et al., Nanoscale Advances, 2019, 1, 1693

The present invention has been devised to solve the above problems, and by inducing π-π bonding between polyfluorene and BNNT in a polyfluorene solution, boron nitride nanotube purification capable of easily separating BNNT and other impurities An object of the present invention is to provide a method and a boron nitride nanotube purification composition.

A boron nitride nanotube purification method according to the present invention for achieving the above object comprises the steps of preparing a polyfluorene solution; and forming a polyfluorene-BNNT composite by mixing a BNNT composite including boron nitride nanotubes (BNNT) in a polyfluorene solution.

By mixing the polyfluorene solution and the BNNT compound, a π-π bond between the fluorene group of polyfluorene and the B-N hexagonal ring of BNNT is induced, so that polyfluorene spirally wraps the outer wall of the BNNT. -BNNT complex is formed.

The polyfluorene-BNNT composite is individually dispersed in the polyfluorene solution, and impurities contained in the BNNT composite are precipitated in the polyfluorene solution.

The polyfluorene is PFO (poly(9,9-dioctylfluorenyl-2,7-diyl) and PFO-BPy(poly(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6- [2,2-bipyridine])) any one or a combination thereof The PFO-BPy has a property of binding to BNNT having a relatively large diameter compared to PFO.

Separating the polyfluorene-BNNT composite individually dispersed in the polyfluorene solution; further comprising, the polyfluorene-BNNT composite individually dispersed in the polyfluorene solution using a centrifugal separation process or a filtration process can be separated.

The BNNT compound is a compound prepared by a known BNNT synthesis method, and the BNNT compound includes at least one of polycrystalline BN and hBN in addition to BNNT.

The boron nitride nanotube purification composition according to the present invention is characterized in that the content of BNNT is 98wt% or more.

BNNT of the purified BNNT composition is in the form of a polyfluorene-BNNT composite in which polyfluorene and BNNT are combined, and the polyfluorene-BNNT composite is in a form in which polyfluorene surrounds the outer wall of the BNNT in a spiral shape. .

The boron nitride nanotube purification method and the boron nitride nanotube purification composition according to the present invention have the following effects.

The polyfluorene-BNNT complex is formed through the binding reaction of polyfluorene and BNNT in the polyfluorene solution, and the polyfluorene-BNNT complex is individually dispersed in the solution. of BNNTs can be separated and purified.

1 is a flowchart for explaining a boron nitride nanotube purification method according to an embodiment of the present invention.
Figures 2a and 2b is a schematic diagram for a boron nitride nanotube purification method according to an embodiment of the present invention.
Figures 3a and 3b is a photograph showing the dispersion state of the BNNT according to the presence or absence of polyfluorene.
4 is a thermogravimetric analysis result of polyfluorene-BNNT composite and pure polyfluorene.
5a and 5b are XPS analysis results for the material contained in the supernatant of the solution.
6a to 6d are SEM and TEM images of each of the BNNT composites and materials contained in the supernatant of the solution.
7 is an AFM analysis result of the material contained in the supernatant of the solution.
8 is a result showing the BNNT length distribution in the material contained in the supernatant.
9 is a result showing the diameter distribution of BNNTs to which PFO-BPy and PFO are respectively bound.
10a to 10d are the results of measuring the absorption and emission spectra of each of PFO and PFO-BPy, and a complex of PFO and BNNT, and a complex of PFO-BPy and BNNT.

The present invention proposes a BNNT purification technology capable of obtaining high-purity BNNTs by purifying impurities such as polycrystalline BN and hBN contained in a BNNT compound.

As mentioned in the previous 'technology behind the invention', BNNTs can be synthesized through various methods. Alternatively, these impurities may be purified through a solution phase purification process, but there is a problem in yield or purification efficiency.

The present invention proposes a BNNT purification technology that can improve the purification efficiency and yield of BNNTs based on a solution phase purification process.

The present applicant applies the CNT purification process using a polyfluorene solution, which is one of the CNT purification technologies, to the purification of BNNTs, focusing on the fact that BNNTs have a crystal structure similar to that of carbon nanotubes (CNTs). BNNT and CNT have different physical properties, but BNNT and CNT have very similar crystal structures in that the crystal structure of BNNT is a form in which carbon (C) is replaced by boron (B) and nitrogen (N) in the crystal structure of CNT. have

As CNTs have a wide range of applications, many studies have been conducted on purification methods for CNT compounds. For example, CNT surface modification using covalent/non-covalent functionalization, CNT wrapping using peptides and polymers, and CNT separation using surfactants are used for CNT purification. As one of these CNT purification methods, there is a method of using a polyfluorene solution capable of separating and purifying semiconducting CNTs (especially SWCNTs) by controlling the structural chirality of CNTs.

Unlike CNTs, BNNTs do not have semiconductor properties, and thus, it is not required to control the structural asymmetry (chirality) like CNTs. , and the possibility of purifying BNNTs of polyfluorene was verified through experiments, and based on this, a technology for purifying BNNTs using polyfluorene is presented.

In the present invention, the mechanism for the separation and purification of BNNTs by polyfluorene is described as follows. In a state in which the BNNT compound is mixed in the polyfluorene solution, the fluorene group provided in the polyfluorene forms a π-π bond with the B-N hexagonal ring provided on the outer wall of the BNNT, and the hydrocarbon group of a long chain structure connected to the fluorene group is poly This leads to individual dispersion of BNNTs in the fluorene solution. On the other hand, polycrystalline BN or hBN present in the BNNT composite has a property that it does not combine with the helical structure, which is a thermodynamically stable structure of polyfluorene, as it forms a planar structure. Therefore, only BNNTs react with polyfluorene, and the polyfluorenes are bound in a spiral fashion along the outer wall of the BNNTs, and the BNNTs bound with polyfluorenes are individually dispersed in the solution. In contrast, polycrystalline BN or hBN exists in aggregated form and precipitates in solution.

As such, the polyfluorene-bound BNNT, that is, the polyfluorene-BNNT complex, forms individually and evenly dispersed in the solution, and polycrystalline BN or hBN, which is an impurity of the BNNT compound, forms a precipitated form in the solution, It becomes possible to separate and purify high-purity BNNTs.

In addition, the present invention proposes a purification technique capable of separating and selecting BNNTs according to the diameter of BNNTs in purifying BNNTs. This can be implemented using the selectivity of the polyfluorene-based polymer. The present invention is a polyfluorene-based polymer with PFO (polyfluorene, poly(9,9-dioctylfluorenyl-2,7-diyl) and PFO-BPy(poly(9,9-dioctylfluorenyl-2,7-diyl)-alt) -co-(6,6-[2,2-bipyridine])) was used, and through experiments, it was confirmed that PFO-BPy was bound to BNNTs having a relatively larger diameter than PFO, which was found in PFO-BPy and PFO. This is due to the difference in stiffness and folding symmetry between them.

In the above-mentioned bar, polyfluorene means any one of PFO and PFO-BPy or including these. In addition, the BNNT composite refers to a composite prepared through various known BNNT synthesis methods, and the BNNT composite includes polycrystalline BN or hBN in addition to BNNT as described above.

Hereinafter, a boron nitride nanotube purification method according to an embodiment of the present invention will be described in detail with reference to the drawings.

1, a polyfluorene solution is prepared by mixing polyfluorene with a solvent (S101). The polyfluorene is PFO (poly(9,9-dioctylfluorenyl-2,7-diyl), PFO-BPy(poly(9,9-dioctylfluorenyl-2,7-diyl)-alt-co-(6,6- [2,2-bipyridine])) any one or a combination thereof Toluene may be used as the solvent in one embodiment.

Then, the BNNT compound is mixed with the polyfluorene solution (S102). The BNNT compound is a compound prepared by a known BNNT synthesis process, and may include polycrystalline BN, hBN, etc. in addition to BNNT.

When the BNNT compound is mixed with the polyfluorene solution, polyfluorene-BNNT complex is formed by the binding reaction of polyfluorene and BNNT, and polycrystalline BN or hBN is precipitated.

Polyfluorene, that is, PFO or PFO-BPy is bonded to the BNNT in a manner that wraps the outer wall of the BNNT in a spiral form through the π-π bond between the fluorene group and the B-N hexagonal ring (see FIGS. 2a and 2b ). Specifically, the fluorene group provided in polyfluorene and the B-N hexagonal ring provided on the outer wall of the BNNT form a π-π bond. In addition, the hydrocarbon group having a long chain structure connected to the fluorene group leads to the individual dispersion of the BNNT bonded to the polyfluorene, that is, the polyfluorene-BNNT complex while wrapping the outer wall of the BNNT in a spiral form. That is, polyfluorene binds selectively with BNNTs, leading to individual dispersion of BNNTs in solution.

While BNNTs are individually and evenly dispersed in solution by bonding with polyfluorene, polycrystalline BN or hBN contained in the BNNT composite forms a planar crystal structure, so it is not bonded to polyfluorene, which is a helical structure, and forms an agglomerated form. precipitated in solution. Therefore, polyfluorene-BNNT composites individually dispersed in solution form a physically separated state from polycrystalline BN or hBN precipitated in solution.

In this way, when the polyfluorene-BNNT composite is physically separated from the polyfluorene-BNNT composite and impurities such as polycrystalline BN or hBN in the solution (S103), nitridation according to an embodiment of the present invention The boron nanotube purification method is completed.

As a method of separating the polyfluorene-BNNT complex from the solution, a method such as centrifugation or filtration may be used. As the polyfluorene-BNNT complex forms an individually dispersed state and impurities such as polycrystalline BN or hBN form a precipitated state, the polyfluorene-BNNT complex can be easily prepared by using physical methods such as centrifugation and filtration. can be separated.

On the other hand, in order to promote individual dispersion of the polyfluorene-BNNT composite, the solution may be irradiated with ultrasonic waves.

Above, the boron nitride nanotube purification method according to an embodiment of the present invention has been described. Hereinafter, the present invention will be described in more detail through experimental examples.

<Experimental Example: Polyfluorene-BNNT Composite Preparation and Characterization>

A solution in which 10 g of polyfluorene was dissolved in 10 ml of toluene was prepared, and 10 g of a multi-walled BNNT composite prepared by a plasma process was mixed with the polyfluorene solution. Then, the solution was irradiated with ultrasound for 10 minutes. Then, after centrifugation at a speed of 15000 rpm for 10 minutes, 95% of the supernatant was further centrifuged for 1 hour. After centrifugation, various characterizations were performed on 95% of the supernatant.

If you look at the state of the solution after ultrasonic irradiation, it can be seen that BNNT is suspended (see PFO-BPy@BNNT in FIG. 3a ). On the other hand, when the BNNT compound is mixed in a toluene solution without polyfluorene, BNNT and polycrystalline BN, hBN It can be confirmed that impurities such as agglomerate and precipitate (see BNNT in FIG. 3a ). In the PFO-BPy@BNNT of FIG. 3a, the material precipitated in the solution is impurities such as polycrystalline BN and hBN, and it was confirmed that the suspension state of the BNNT was stably maintained for at least 1 month (see FIG. 3b).

In addition, the presence of BNNT was confirmed by performing thermogravimetric analysis (TGA) and XPS analysis on the centrifuged supernatant. In general, it is known that polyfluorene, which is a conductive polymer, is easily thermally decomposed at a temperature of less than 600° C. as an organic material. On the other hand, in the case of BNNT, which is a crystal made of an inorganic material, its crystal structure is maintained due to its high thermal stability even at a temperature of 700° C. or higher. Therefore, the presence of BNNTs in the supernatant can be checked using this difference in thermal stability. Specifically, the polyfluorene-BNNT composite (PFO-BPy@BNNT in FIG. 4) obtained in a solid form by drying the supernatant obtained through centrifugation as in FIG. 4 was heated to 800° C. and the mass change was observed. . As a control, the mass change of pure PFO-BPy under the same temperature condition was measured and the difference was compared. Looking at the mass change according to the temperature increase of pure PFO-BPy, it can be confirmed that all of them are thermally decomposed at a temperature of less than 600 °C, as is known. On the other hand, in the case of PFO-BPy@BNNT, it can be confirmed that a mass corresponding to half of the initial mass remains even at a temperature of 600° C. or higher, and it can be confirmed that the corresponding mass is due to the presence of BNNT.

As a complementary analysis method to the TGA analysis, the presence or absence of BNNT was additionally confirmed using XPS. As a result of XPS analysis of PFO-BPy@BNNT obtained in a solid form by drying the supernatant obtained through centrifugation, the presence of boron (B) and nitrogen (N) was confirmed at binding energies around 190 eV and 400 eV, respectively. (See FIGS. 5A and 5B).

AFM, SEM, and TEM analysis were performed to analyze the effect of polyfluorene on individual dispersion of BNNTs. 6a is an SEM photograph of the BNNT composite, FIGS. 6b and 6c are SEM photographs of the material contained in the supernatant, and FIG. 6d is a TEM photograph of the material contained in the supernatant.

Referring to FIG. 6a, the BNNT content in the BNNT composite was found to be about 50%, which is consistent with previously reported content. In addition, it is estimated that the remaining materials except for BNNT in FIG. 6a are a mixture of hBN and α-BN. As a result of performing SEM and TEM analysis on the material contained in the supernatant (see FIGS. 6b, 6c, 6d), it was confirmed that it was composed of high-purity BNNT, and the AFM analysis result showed that the material contained in the supernatant was high-purity BNNT (See FIG. 7) is also confirmed. As a result of SEM analysis of the high-purity BNNTs contained in the supernatant, it was confirmed that the average length of the BNNTs was about 1.5 μm (see FIG. 8 ).

In addition, as a result of analyzing the SEM photographs of FIGS. 6B and 6C, it was confirmed that the content of BNNT was 98% or more, and the remaining materials were impurities such as hBN and α-BN. From these results, it can be seen that high-purity BNNTs can be obtained by the boron nitride nanotube purification method according to the present invention.

On the other hand, it was confirmed through an experiment that the diameter selectivity of the BNNT bonded to the polyfluorene exists depending on the type of polyfluorene. Specifically, it was confirmed that PFO-BPy was bound to BNNTs having a relatively large diameter compared to PFO.

Referring to FIG. 9 , it can be confirmed that PFO-BPy is bound to BNNTs having a relatively larger diameter than that of PFO, and in detail, the average diameter of BNNTs to which PFO-BPy is bound is about 5.8 nm, and PFO-bound BNNTs. It was confirmed that the average diameter of was about 4.7 nm. Based on these experimental results, it can be seen that it is possible to separate and purify high-purity BNNTs having a specific diameter.

10a to 10d, the absorption and emission spectral tendencies of each of PFO and PFO-BPy, and a complex of PFO and BNNT (PFO@BNNT) and a complex of PFO-BPy and BNNT (PFO-BPy@BNNT) can be checked Pure PFO and PFO-BPy contain fluorene, a directional functional group, and thus show strong visible light absorption in the vicinity of 350 nm to 400 nm. Comparing the corresponding absorption curves with the absorption curves of PFO@BNNT and PFO-BPy@BNNT, it can be seen that the maximum absorbance wavelength shifts toward the red wavelength in both cases of PFO and PFO-BPy (Red Shift, Red-Shift) . This is due to the change in energy level due to the interaction between the fluorene functional group of PFO or PFO-BPy and the aromatic hexagonal ring on the surface of the BNNT by forming a π-π bond. .

Additionally, it is possible to infer what kind of structural change the corresponding conductive polymers have upon interaction through emission spectrum trend analysis. PFO and PFO-BPy are near-planar polymer chains, characterized by an axial rotation of about 18 degrees between each monomer, and thus have a wide helical structure. This structure is characterized by three prominent emission wavelengths in the emission spectrum, and the relative magnitude of the emission wavelengths is determined by the degree of rotation. Analyzing the emission spectrum of FIG. 10b, it can be seen that the ratio of the emission wavelength intensity of PFO-BPy@BNNT is significantly changed compared to that of PFO-BPy in a pure state. This means that the axis rotation of the PFO-BPy chain increases in the process of finding the most thermodynamically stable structure during the interaction between PFO-BPy and BNNT, and thus the formation of a narrow helical structure can be confirmed. In the case of PFO@BNNT, the relative size change of emission wavelengths is not large compared to that of PFO, which means that the increase in axis rotation during the interaction of the PFO chains is not large, and a relatively wide helical structure is maintained.

Claims (10)

preparing a polyfluorene solution; and
A method for purifying boron nitride nanotubes, comprising: mixing a BNNT composite including boron nitride nanotubes (BNNT) in a polyfluorene solution to form a polyfluorene-BNNT composite.
According to claim 1, By mixing the polyfluorene solution and the BNNT compound, a π-π bond between the fluorene group of polyfluorene and the BN hexagonal ring of BNNT is induced,
A method for purifying boron nitride nanotubes, characterized in that a polyfluorene-BNNT composite is formed in which polyfluorene surrounds the outer wall of the BNNT in a spiral shape.
The boron nitride nanotube according to claim 1, wherein the polyfluorene-BNNT composite is individually dispersed in the polyfluorene solution, and impurities contained in the BNNT composite are precipitated in the polyfluorene solution. purification method.
According to claim 1, wherein the polyfluorene is PFO (poly (9,9-dioctylfluorenyl-2,7-diyl) and PFO-BPy (poly (9,9-dioctylfluorenyl-2,7-diyl) -alt-co -(6,6-[2,2-bipyridine])) boron nitride nanotube purification method, characterized in that any one or a combination thereof.
[Claim 5] The method of claim 4, wherein the PFO-BPy is combined with BNNT having a relatively larger diameter than that of PFO.
The method according to claim 1, further comprising the step of separating the polyfluorene-BNNT composite individually dispersed in the polyfluorene solution.
The method according to claim 6, wherein the polyfluorene-BNNT composite individually dispersed in the polyfluorene solution is separated using a centrifugation process or a filtration process.
The method according to claim 1, wherein the BNNT compound is a compound prepared by a known BNNT synthesis method, and the BNNT compound contains at least one of polycrystalline BN and hBN in addition to BNNT.
In the purified BNNT composition, the boron nitride nanotube purification composition, characterized in that the content of BNNT is 98wt% or more.
10. The method of claim 9, wherein the BNNT of the purified BNNT composition,
Boron nitride nanotube tablet composition, characterized in that polyfluorene and BNNT are combined in the form of a polyfluorene-BNNT composite, wherein the polyfluorene-BNNT composite is in a form in which polyfluorene surrounds the outer wall of the BNNT in a spiral shape .

Images