Facile Exfoliation and Noncovalent Superacid Functionalization of

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Facile Exfoliation and Noncovalent Superacid Functionalization of Boron Nitride Nanosheets and Their Use for Highly Thermally Conductive and Electrically Insulating Polymer Nanocomposites Takuya Morishita, and Hirotaka Okamoto ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b08404 • Publication Date (Web): 06 Sep 2016 Downloaded from http://pubs.acs.org on September 12, 2016

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ACS Applied Materials & Interfaces

Facile Exfoliation and Noncovalent Superacid Functionalization of Boron Nitride Nanosheets and Their Use for Highly Thermally Conductive and Electrically Insulating Polymer Nanocomposites Takuya Morishita*,†and Hirotaka Okamoto† †

Toyota Central R&D Labs., Inc., Nagakute, Aichi 480-1192, Japan

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]

ACS Paragon Plus Environment

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ABSTRACT: There is an increasing demand for highly thermally conductive and electrically insulating polymer materials for next-generation electronic devices, power systems, and communication equipment. Boron nitride nanosheets (BNNSs) are insulating materials with extremely high thermal conductivity. However, BNNSs suffer from the lack of facile and lowcost methods for producing large volumes of BNNSs, and extremely low through-plane thermal conductivities of BNNS/polymer composites as compared to the in-plane thermal conductivities. Herein, highly soluble, noncovalently functionalized boron nitride nanosheets (NF-BNNSs) with chlorosulfonic acid (CSA) were prepared by extremely facile and low-cost direct exfoliation of hexagonal boron nitrides (h-BNs), and acted as excellent nanofillers for dramatically improving both in- and through-plane thermal conductivities of insulating polymers. CSA is a cheap and versatile superacid with a large production volume. CSA showed strong physical adsorption on h-BN surfaces, giving few-layered NF-BNNSs in high yields (up to ~25%). The crystallinity of the NF-BNNS was perfectly maintained even after CSA treatment. The physical adsorption of CSAs imparted high solubility for BNNSs in various organic solvents, yielding NF-BNNS uniformly dispersed-thermoplastic polymer composite films through a simple wet-process using pre-dispersed NF-BNNS solutions. Random dispersion of NF-BNNSs in thermoplastic polymer films dramatically enhanced both the in- and through-plane thermal conductivities (>10 W m–1 K–1). The through-plane thermal conductivity of the NF-BNNS/polybutylene terephthalate (PBT) composite films was much greater (up to 11.0 W m–1 K–1) than those previously reported for BNNS/thermoplastic polymer composites (≤2.6 W m–1 K–1). These results are also due to an increase of interactions between the BNNS and polymer matrices, caused by physical adsorption of CSAs on BNNS surfaces. Moreover, the volume resistivity of the NF-BNNS/PBT composite films was significantly improved compared with pristine PBT. The NF-BNNS/polymer


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composites are very promising as highly thermally conductive and electrically insulating materials in various applications.

KEYWORDS. boron nitrides, nanosheets, noncovalent functionalization, nanocomposites, exfoliation, superacids, thermal conductivity, electrical insulation

1. INTRODUCTION There is an increasing demand for electrically insulating polymer materials having high thermal conductivity.1–5 Such materials are required to improve heat-dissipation and enable significant downsizing while maintaining high performance for next-generation electronic devices, power systems, and communication equipment applications. Anisotropic nanocarbons including carbon nanotubes (CNTs)2,6,7 and graphenes6–10 exhibit extremely high thermal conductivity, and the incorporation of these isolated nanocarbons into polymers yields highly thermally conductive nanocomposites.2,7,8 However, these nanocarbons are highly electrically conductive and minor additions of these materials into polymers thus led to the high electrical conductivity of the composites.2,10 Meanwhile, anisotropic boron nitride (BN) nanomaterials such as BN nanotubes11 and BN nanosheets (BNNSs)12 are insulating materials with extremely high thermal conductivity. BNNSs are a structural analogue of graphene that has attracted extensive attention as thermally conductive nanofillers for insulating polymers.3,4,12–16 However, BNNSs suffer from two main issues as thermally conductive nanofillers. The first one deals with much lower through-plane thermal conductivities of BNNS/polymer composites as compared to the in-plane parameter (Table S1).3,4,12–15 BNNSs have 2D structures and they are easily oriented along the plane in polymer matrices12, resulting in low through-plane thermal conductivities of


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BNNS/polymer composites. The high in-plane thermal conductivity of BNNS/polymer composites is effective for spreading heat along the plane of abovementioned applications such as electronic devices (e.g. integrated circuits, electrical insulators). However, in most cases of these applications, increasing the through-plane thermal conductivity is more important than the in-plane thermal conductivity for the thermal management of them.5 Therefore, increasing both in- and through-plane thermal conductivities of BNNS/polymer composites is strongly required for these applications. In addition, there is increasing interest in nanofiller-reinforced thermoplastic polymers because thermoplastic polymers are much more useful from the viewpoint of an increased impact resistance, the ability to reform and reshape the part after consolidation, and high recycling potential of the composites than thermoset polymers like epoxies. However, the through-plane thermal conductivities of BNNS/thermoplastic polymer (e.g. poly(methyl methacrylate) (PMMA), polystyrene, polyamide-6) composites were extremely low (1.1–2.6 W m–1 K–1)3,15,16 compared with those of BNNS/thermoset polymer (e.g. epoxy polymer) composites4,12,17,18 (Table S1). In order to improve both in- and through-plane thermal conductivities of BNNS/thermoplastic polymers, BNNSs are required to be more randomly dispersed in the polymer matrices and to increase their affinity with the matrices. In this sense, covalent functionalization of BNNS surfaces and edges is effective to improve their dispersibility in solvents and matrices.4,14,15,19,20 at the expense of generating defects on BNNSs that usually decrease the thermal conductivity of the BNNSs. Therefore, as in the case of nanocarbons,7,21,22 noncovalent functionalization of BNNSs is expected to improve dispersibility without damaging the structure of these materials. The second issue is the lack of scalable methods for producing large volumes of BNNSs for bulk applications including nanocomposites. With the aim to solve these issues, liquid-phase methods for exfoliation of bulk hexagonal BNs (h-BNs) using


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carefully selected solvents have been developed.23–31 However, these methods result in low concentrations (0.025–0.3 mg mL–1) and yields (0.1%–15%) of dispersed BNNSs,23–28 or need to have many (six to nine) extraction cycles,29 careful removal of volatile, toxic liquid,30 or expensive liquids (e.g. ionic liquids (ILs).31 Moreover, these wet exfoliation methods typically involve vigorous tip-type sonication,23 long time bath sonication (≥20 h),24,30 or long time heat treatment (1–6 days).28,29 Strong or extended sonication damages BN frameworks due to the mechanical forces and largely decreases the lateral sizes,12,25,29 leading to degraded performance. Therefore, a facile, highly efficient exfoliation method using a low-cost, high-production volume solvent is required. In this sense, chlorosulfonic acid (CSA, HSO3Cl) is a cheap and versatile superacid with a large production volume (>300,000 tons year–1).32,33 A superacid is defined as an acid having greater acidity than pure H2SO4 (Hammett acidity function (H0) of −11.9).34 CSA is known to exfoliate graphite to graphenes through the formation of electrostatically stabilized graphenes via protonation.33 h-BN presents stronger interlayer interactions than graphites owing to the partly ionic B–N bonds and its inert surface. However, the superacid protons are expected to interact with the N atoms of h-BN surfaces, leading to electrostatically stabilized exfoliation. In this study, h-BNs were exfoliated and noncovalently functionalized by physical adsorption of CSA on BN surfaces under sonication. The obtained noncovalently functionalized BNNSs (NFBNNSs) exhibited high crystallinity and solvent solubility, yielding NF-BNNS/thermoplastic polymer composites with outstanding in- and through-plane thermal conductivities and high electrical insulation.

2. EXPERIMENTAL SECTION


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2.1. Materials. Hexagonal boron nitrides (h-BNs), UHP-S1 (Showa Denko K. K., Japan, BN purity: 99.8wt%, mean particle size: ~0.8 µm, mean lateral size of platelet: ~0.4 µm), HP-P1 (Mizushima ferroalloy Co., Ltd., Japan, BN purity: 98.3wt%, mean particle size: ~1.4 µm, mean lateral size of platelet: ~0.5 µm), UHP-1K (Showa Denko K. K., Japan, BN purity: 99.9wt%, mean particle size: ~24 µm, mean lateral size of platelet: ~4.0 µm) were used and dried under vacuum at 80 °C for 12 h before use. Mean lateral size of platelet of each h-BN was estimated as average value by scanning electron microscopy (N = 20). Chlorosulfonic acid (CSA) (99%) and 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP) (≥99%) were purchased from Sigma-Aldrich Corporation, USA. Poly(methyl methacrylate) (PMMA; grade G) was purchased from Kuraray Co., Ltd., Japan. Polybutylene terephthalate (PBT; DURANEX-2002) was obtained from WinTech Polymer Ltd., Japan. 2.2. Characterization. High-resolution transmission electron microscopy (HRTEM) images of noncovalent functionalized boron nitride nanosheet (NF-BNNS) samples were obtained with an FEI Titan 80-300 and a JEM-2100F instrument operating at 200 kV. HRTEM samples were prepared by dropping each NF-BNNS/isopropanol (IPA) dispersion onto microgrids. Morphologies of NF-BNNS/PMMA composites were observed using a Hitachi SU-3500 scanning electron microscope (SEM). For SEM measurements, frozen fracture surfaces of the films (2.5 mm × 30 mm; 0.3 mm thickness) of NF-BNNS/PMMA composites were prepared. The frozen fracture surfaces of the films were coated with Pt under pure argon by sputtering (Hitachi E-1045 ion sputter). X-ray photoelectron spectroscopy (XPS) data were collected using an ULVAC PHI Quantera SXM spectrometer with a monochromated Al–Kα (1486.6 eV). Experimental energy shifts were corrected relative to the C1s band at 284.6 eV (C–C bond). XPS spectra were collected for a DIW-washed and vacuum-dried NF-BNNS powder and pristine h-


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BN. Thermogravimetric analysis (TGA) measurements were performed using a Rigaku-Thermo plus TG8120 instrument. Samples were held at 100 °C for 30 min under a N2 flow of 500 mL min−1 and heated to 800 °C at 10 °C min−1. Raman spectra of NF-BNNSs were collected using a JASCO NRS-3300 Raman spectrometer with a 532 nm laser excitation. X-ray powder diffraction (XRD) patterns were recorded at a scan rate of 50° min−1 with the Cu–Kα (1.542 Å) line using a Rigaku Ultima IV multipurpose XRD system. Powder samples were loaded onto XRD glass plates. Differential scanning calorimetry (DSC) measurements were performed using a calorimeter (TA Instruments Q1000) with a heating rate of 5 °C min−1. The thermal conductivity (κ) (W K–1 m–1) was calculated from the thermal diffusivity (α) (m2 s–1) by the equation: κ =αρc, where ρ is the density (kg m–3) and c is the specific heat capacity (J K–1 kg–1) of PMMA-based composites. The density of each composite was measured with a specific gravity bottle. The specific heat capacity was measured using DSC. The thorough-plane thermal diffusivity of each composite film (10 mm × 10 mm; 0.3 mm thickness) was measured at room temperature by the laser flash technique using a laser flash apparatus (NETZSCH LFA447 Nanoflash). The in-plane thermal diffusivity of the in-plane direction for each composite film (10 mm × 10 mm; 0.3 mm thickness) was measured at room temperature with a thermo-wave analyzer TA3 (Bethel Co., Ltd., Japan) under periodic laser heating. The average of three measurements was regarded as thermal diffusivity. Volume resistivity of each composite film (50 mm × 50 mm; 0.3 mm thickness) was measured with a high-resistance meter (AGILENT 4339B, Agilent Technologies) equipped with a 16008B resistivity cell with main electrode size of 26 mm. The operating load was 5 kgf, and a potential of 1,000 V was applied to the film. The volume resistivity was measured after a potential of 1,000 V was applied to the film for 20 seconds. The average of four measurements was regarded as volume resistivity. The solubility of BNNSs was evaluated by


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estimating the concentration of soluble BNNS in IPA. NF-BNNSs were dispersed in IPA (initial BNNS concentration: 1 mg mL–1) by bath sonication (20 min). Soluble NF-BNNS supernatants were obtained after settling the NF-BNNS-IPA dispersions for 20 h. Then the supernatants were filtered through a preweighted 0.1 µm membrane filter, and the NF-BNNSs were obtained after drying under vacuum at 80 °C for 12 h. The concentration of BNNS redispersed in IPA was calculated using the weight of BNNS minus that of CSA. 2.3. Typical procedure for the preparation of NF-BNNSs. 20 mL of CSA was added into a flask containing dried h-BN (100 mg) under N2 atmosphere, and the mixture was subjected to bath sonication using an ultrasonic cleaner (Branson B-220, 125 watts) for 8 h. The resulting dispersion was carefully added dropwise to 2 L of deionized water (DIW) under vigorous stirring, giving BNNS/DIW dispersion. BNNS/DIW supernatant was collected after allowing the initial BNNS/DIW dispersion to stand for 24 h. The supernatant was subjected to vacuum filtration through a preweighted 0.1 µm membrane filter, washed with DIW till the pH of the filtrate turned to ~7. NF-BNNS was obtained after drying under vacuum at 80 °C for 12 h. The NFBNNSs were measured by TGA and the amount of BNNS in the NF-BNNS was estimated using the resulting weight loss corresponding to CSA. The yields were calculated using y = 100 × MBNNS /Mh-BN, where MBNNS is the weight of BNNS estimated from the weight of the obtained NF-BNNS and the TGA weight loss corresponding to CSA, and Mh-BN is the weight of the h-BN. The total yields of BNNS including an additional extraction were evaluated as follows. The BN precipitates obtained after settling the BNNS dispersion for 24 h were redispersed in 1 L of DIW under bath sonication (60 min). After settling the dispersion for 24 h, the supernatant was collected. Additional NF-BNNSs were obtained after filtration, washing and drying under vacuum.


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2.4. Preparation of NF-BNNS/PMMA composite films (BNNS content: 2 wt%). 60.4 mg of NF-BNNS prepared from UHP-1K, containing 60 mg of BNNS and 0.4 mg of CSA, was added into 2.0 g of acetone and bath sonicated for 20 min. The NF-BNNS/acetone solution was mixed with a PMMA/acetone (3.0 g/8.0 g) solution under bath sonication for 30 min. The resulting NFBNNS/PMMA/acetone solution was spread on a glass plate, and acetone was naturally volatilized for at least 24 h. NF-BNNS/PMMA (BNNS content: 2 wt% =~1.1 vol%) composite films with ~0.3 mm thickness were obtained after further drying at 50 °C for 12 h under vacuum to remove the residual acetone. 2.5. Preparation of NF-BNNS/PMMA composite films (BNNS content: 24.5 wt%). 100.7 mg of NF-BNNS prepared from UHP-1K, containing 100 mg of BNNS and 0.7 mg of CSA was added into 300 mg of acetone. The mixture was bath sonicated for 20 min, and subsequently the NF-BNNS/acetone solution was mixed with a PMMA/acetone (307.5 mg/724.9 mg) solution under bath sonication for 30 min. The resulting NF-BNNS/PMMA/acetone solution was spread on a glass plate, and acetone was naturally volatilized for at least 24 h. NF-BNNS/PMMA (BNNS content: 24.5 wt% =~15 vol%) composite films were obtained after further drying under vacuum. 2.6. Preparation of NF-BNNS/PMMA composite films (BNNS content: 80 wt%). 386.7 mg of NF-BNNS prepared from UHP-1K, containing 384 mg of BNNS and 2.7 mg of CSA was added into 1300 mg of acetone. The mixture was bath sonicated for 20 min and subsequently the NF-BNNS/acetone solution was mixed with a PMMA/acetone (93.3 mg/300 mg) solution under bath sonication for 30 min. The resulting NF-BNNS/PMMA/acetone solution was spread on a glass plate, and acetone was naturally volatilized for at least 24 h. NF-BNNS/PMMA (BNNS content: 80 wt% =~68 vol%) composite films were obtained after further drying under vacuum.


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3. RESULTS AND DISCUSSION Figure 1a shows a schematic of the methodology used to exfoliate h-BN into few-layered BNNSs by physical adsorption of CSA on h-BN surfaces. Three types of h-BN with different mean particle size (HP-P1, UHP-1K, and UHP-S1) were used as starting materials. First, HP-P1 (mean particle size: ~1.4 µm, mean lateral size of platelet: ~0.5 µm) was used as h-BN. In this approach, h-BNs were bath sonicated for 8 h in CSA, yielding uniform orange BNNS dispersions (Figure 1b). This color change was considered to originate from the sulfonate groups in the CSA as bath sonication of CSA itself for 8 h yielded very light yellow color dispersions. Other sulfonate-bearing compounds such as sulfonate-bearing ILs and methanesulfonic acid (MSA) produced orange color dispersions of BNNSs after bath sonication31 (Figure S1a). Therefore, some of the sulfonate groups in CSA are considered to be unstable and decompose under sonication. Then, the BNNS/CSA dispersion was gradually dropped into deionized water (DIW) under vigorous stirring, resulting in a BNNS/DIW dispersion. During this process, free CSA reacted violently with water producing gaseous HCl and H2SO4. It is suggested that pressure produced by the gaseous HCl between the layers led to additional exfoliation of the BNNS sheets. The BNNS/DIW dispersion was settled for 24 h, after which a stable supernatant solution of BNNSs was produced. The BNNS/DIW supernatant was filtered and thoroughly washed with DIW. A light brown BNNS powder was obtained after vacuum drying at 80 °C for 12 h (Figure 1b). In this process, instead of settling of the BNNS/DIW dispersion for 24 h, centrifugation at 3,000 rpm for 10 min was also effective for obtaining the BNNS/DIW supernatant with almost the same BNNS yield.


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Figure 1. (a) Schematic of CSA-promoted exfoliation of h-BNs into NF-BNNSs. (b) Photographs of BNNS/CSA dispersion after sonication, NF-BNNS/DIW supernatant, and NFBNNS powder. All the BNNS in (b) were prepared using HP-P1 as a h-BN material. The BNNS sample prepared from HP-P1 was observed by high-resolution transmission electron microscopy (HRTEM, Figure 2a). The number of distinguishable curled edges of the BNNS, as observed by HRTEM images, can provide information about the number of layers.23,29,31 For example, enlargement shown in Figure 2a revealed that the curled edge consisted of five layers. The thickness histogram (Figure 2c) revealed that most of the BNNSs had less than 10 layers, with the average number of layers being approximately 6.0. As shown by the selected area electron diffraction (SAED) pattern (Figure 2b), the exfoliated BNNS showed


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high-crystallinity hexagonal structure patterns, thereby revealing that the high crystallinity was preserved under strong acid conditions using CSA. The average lateral size was relatively small (0.20 µm) (Figure S2) as the lateral size of the original h-BN (HP-P1) was not large (~0.50 µm). Therefore, larger h-BN samples (UHP-1K, mean lateral size: ~4.0 µm) were also used to obtain microsized BNNSs. The obtained BNNS mainly comprised less than 10 layers (Figures 2d, S3 and S4) with an average lateral size of ~1.3 µm. The SAED pattern of the BNNS prepared from UHP-1K showed its high-crystallinity hexagonal structure (Figure S4). X-ray powder diffraction (XRD) patterns also corroborated the high crystallinity of BNNSs as sharp interlayer (002) and (004) peaks were observed (Figure S5). BNNSs exhibited higher intensities for (002) and (004) peaks and significantly lower intensity (I) ratio between (100) and (004) planes (I100/I004) as compared to h-BN (Figure S5). These findings were consistent with previously reported results27,35 and indicated that the (002) crystal faces of BNNS were selectively exposed on the XRD glass plate as compared to h-BN. In addition, these results showed that BNNSs were exfoliated from h-BN along the (002) plane while maintaining the crystalline structure.


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Figure 2. (a) HRTEM images of NF-BNNSs prepared from HP-P1. (b) Electron diffraction pattern of the area marked by the white-dotted circle in (a). (c) Thickness histogram of NFBNNSs prepared from HP-P1 (20 flakes were measured by HRTEM). (d) HRTEM images of NF-BNNSs prepared from UHP-1K. The BNNSs were further characterized by X-ray photoelectron spectroscopy (XPS), Raman measurements, and thermogravimetric analysis (TGA). The XPS N1s spectrum for BNNS prepared from HP-P1 was observed at a binding energy of 398.0 eV (Figure 3a). This binding energy was practically the same than that of h-BN (HP-P1) (398.1 eV). The N–H bonds potentially formed upon protonation of N atoms by CSA on BNNS surfaces and edges are thought to be observed at N1s binding energies ranging from 400 to 402 eV.36 However, these peaks were not detected in the XPS N1s spectrum for BNNS. The binding energy of the B1s


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spectrum of BNNS was 190.5 eV (Figure 3b), which was similar to that of h-BN (190.4 eV). The B1s spectrum of the BNNS (Figure 3b) did not show any additional intense peaks corresponding to covalent bonds between BNNS and CSA, such as B–O bonds (at ~192.5 eV) and B–S bonds (at ~191 eV).37 The XPS B1s and N1s spectra of the BNNS had no spectral broadening compared with those of the h-BN. A survey scan of the BNNS showed S and Cl elements even though these elements were not originally present in h-BN (Table S1). The XPS S2p spectrum for BNNS (Figure 3c), observed at a binding energy of 169.0 eV, which is similar to that of – SO3– bonds of pristine CSA.38 The Cl2p spectrum (centered at approximately 201 eV) showed the existence of S–Cl bonds of CSA38 (Figure 3d). When the initial BNNS/CSA dispersion was gradually dropped into DIW, pure CSA reacted with water to give gaseous HCl and H2SO4. However, the XPS Cl2p spectrum of the BNNS did not show a binding energy of approximately 198 eV to show the presence of HCl.38 This shows that HCl was not adsorbed on the BNNS surfaces and it was easily removed from the BNNS during the process. The signal at a binding energy of 199.5 eV of the h-BN was not related to Cl peaks because the survey scan of the h-BN did not show the presence of Cl element (Table S2). The same signal was also observed in other pristine h-BNs.39 This peak observed at 199.5 eV for h-BN is due to a π bond shake-up satellite of B1s.39 The XPS O1s spectrum for BNNS (Figure S6), observed at a binding energy of 532.2 eV, also showed the existence of –SO3– bond of CSA.38 A binding energy observed at 532.5 eV is ascribed to oxygen of impurities by contamination from the atmosphere.38 From these XPS results, it was confirmed that CSA residuals having –SO3– and S–Cl bonds remained on BNNS surfaces even after washing with DIW. This result suggests that chlorosulfate (SO3Cl) anions attached on BNNS surfaces and were not removed from the BNNS surfaces by washing with water. This may be due to the strong interaction of SO3Cl anions with electron-deficient B atoms


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of BNNS surfaces. SO3Cl anion is relatively stable in water and it was used as anion for ILs.40 Therefore, SO3Cl anions attached on B atoms of BNNS surfaces are also considered to be relatively stable in water. The Raman spectrum of NF-BNNS (Figure S7) did not show a defect peak at approximately 1050 cm–1.12 The Raman spectra of BNNS were intrinsically similar to those of h-BN, which is consistent with the results observed in few-layered, high-crystallinity BNNS samples.12 Each BNNS sample displayed a broad rise of the baseline as a result of CSAinduced fluorescence. Figure 4a shows TGA results of NF-BNNS and h-BN. h-BN did not show any weight loss from 100 °C to 500 °C. Conversely, BNNS showed weight loss at 150 °C–370 °C, which can be attributed to the CSA residuals immobilized on the BNNS surface, according to the XPS results described above, which show the existence of –SO3– and S–Cl bonds. The total weight loss observed in BNNS was 1.5 wt% (Table 1, 1a). Moreover, the light brown BNNS powders turned white after the TGA (Figure 4b and 4c) as a result of their volatilization from the BNNS surface during the measurements. The SAED patterns, XRD patterns, XPS spectra, and Raman data revealed that NF-BNNS comprised high-crystallinity hexagonal structures without covalent bonds between BNNS and CSA. The protonation on N atoms of the BNNS surface might occur during the dispersion of BNNS in CSA. However, the hexagonal structures were preserved or recovered after the transfer of the BNNS/CSA dispersion into DIW. Meanwhile, as described above, SO3Cl anion of CSA is believed to be remained on the BNNS surface due to its interaction with B atom of the BNNS surfaces, because the XPS spectra of NF-BNNSs showed – SO3– and S–Cl peaks and the TGA curve had a clear weight loss. On the other hand, CSA is known to react with R-OH, R-NH2 and aryl-H.32 It has also been reported that B(OH)3 reacts with CSA generating B(OSO3H)3 and gaseous HCl.41 Hence, if the BNNS edges have a sufficient amount of B-OH or B-NH2 groups to react, there may be a possibility that CSA reacts


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with these groups on the edges (sulfation of B-OH or sulfamation of B-NH2 groups). In this case, B-OSO3H or B-NHSO3H groups are considered to be generated together with removal of gaseous HCl.32 However, for forming B-OH groups on the edges and surface of BNNSs for reacting with modification agents, methods such as solution-phase oxygen radical functionalization of B atoms, and oxidation by heat treatment of h-BNs at 1,000 °C under air were required.20 Actually, the XPS spectra of h-BNs and BNNSs did not show the existence of B-O bonds (at ~192.5 eV)37 and N-H bonds (at ~400–402 eV).36 In addition, if sulfation of B-OH and sulfamation of B-NH2 groups occur, content of Cl element in the BNNS should be much smaller compared with that of S element, because gaseous HCl is generated and can be easily removed from the BNNS during the process. However, the XPS measurement showed the existence of S–Cl bond, and the XPS surface elemental compositions of NF-BNNS showed that the amount of Cl element (1.3 atomic %) is almost the same as that of S element (0.9 atomic %) (Table S2). Therefore, it is considered that these reactions leading to covalent functionalization of the edges did not occur, or they can be negligible in the current case. On the other hand, the hBNs used in this paper have very high BN purity and did not include aryl-H groups, which excludes the possibility of reactions of aryl-H groups with CSA. Moreover, it has been reported that even the treatment of graphite or carbon nanotube by CSA did not functionalize the material.33,42 h-BN is more chemically inert in acids, solvents, and oxidizers than graphite and CNT.43


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(a)

(b)

N1s

B1s

Intensity (a. u.)

Intensity (a. u.)

h-BN

NF-BNNS

409 407 405 403 401 399 397 395 393 Binding energy (eV)

(c)

S2p

h-BN

NF-BNNS

199 197 195 193 191 189 187 185 Binding energy (eV)

(d)

-SO3-

Cl2p -Cl

Intensity (a. u.)

NF-BNNS

Intensity (a. u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NF-BNNS π bond shake-up satellite of B1s peak

h-BN

h-BN

207 205 203 201 199 197 195 193 175 173 171 169 167 165 163 161 159 157 Binding energy (eV) Binding energy (eV) Figure 3. (a) N1s, (b) B1s, (c) S2p, and (d) Cl2p XPS spectra of NF-BNNS and h-BN. HP-P1

was used as the starting material (h-BN).


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Figure 4. (a) TGA diagrams of NF-BNNS and h-BN. Photographs of NF-BNNS (b) before and (c) after TGA measurement (heating rate = 10 °C min−1, N2 atmosphere). HP-P1 was used as the starting material (h-BN). The functionalization ratios (FRs), defined as the mass ratio of CSA adsorbed on BNNS surfaces, were calculated from the TGA weight losses. Table 1 shows FRs, yields, and solubility in IPA for BNNS. MSA and 98% H2SO4 were used for the control experiments using other acids. BNNS prepared using MSA (Table 1, 1f) gave light brown colored powders, and the obtained BNNS mainly consisted of 6–40 layers (Figure S1b) with low yield (~2.1%). The BNNS/MSA powders showed a small TGA weight loss (~0.3 wt%). This weight loss is attributed to the adsorption of MSA on BNNS surfaces, because –SO3– bond of MSA was observed at ~169 eV in the S2p XPS spectrum. However, CSA produced greater FR values as compared to MSA (Table 1) as a result of the significantly stronger interactions of CSA onto BN surfaces with regard to MSA. MSA is not a superacid and its acidity is much weaker than CSA (H0= –7.9 vs. –


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13.8).34 Therefore, acidity plays an important role in increasing the extent of physical adsorption. This implies that protonation on N atoms of BNNS surfaces occurs in CSA. Meanwhile, the use of 98% H2SO4 (H0 = −10.4)44 gave white powders and did not result in the adsorption of H2SO4 on BNNS surfaces (Table 1, 1g), although the acidity of 98% H2SO4 is higher than that of MSA. This indicates that anion species also play an important role, implying that interactions of SO4 anions with BNNS surfaces are weaker than those between CH3–SO3 anions and BNNSs. Cl– SO3 anions interact much strongly with BNNS surfaces than CH3–SO3, and the interactions may be due to anion–π interactions between Cl–SO3 anions and electron-deficient B atoms of BNNS surfaces. It has been reported that electron deficient B atoms of BN surfaces had Lewis acid– base interactions with amino groups of alkyl amine.12,29 Such interactions are noncovalent in nature, but they have strong ionic (electrostatic) characteristics.12 Similarly, Cl–SO3 anions are considered to have strong interactions with electron deficient B atoms of BN surfaces, and the interactions may be stronger than the case of using alkyl amine, because the use of alkyl amine needed long time (4–6 days) to exfoliate h-BNs.29 An increase in the FR resulted in the enhancement of both exfoliation extent (yields) and solubility (Table 1). Unlike graphite,33 spontaneous exfoliation of h-BNs did not occur during this process and bath-sonication was necessary for exfoliating h-BN into BNNSs. This is probably due to stronger interlayer interactions between adjacent BN layers with partly ionic character, and higher chemical resistance of BN surfaces. The crystallinity of the h-BNs used in this study is relatively high. Therefore, the exfoliation mechanism was postulated to involve the formation of small gaps at the h-BN sheet ends by cavitation bubbles under sonication, in addition to CSA adsorption induced by protonation. The propagation of these gaps along the sheet surface by further CSA adsorption and diffusion resulted in the eventual exfoliation of the individual sheet. The use of


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CSA improved exfoliation of h-BN and prevented restacking of exfoliated NF-BNNSs in CSA. For achieving these, CSA has to attach on BN surfaces between BN layers (not only on the edges). If only the edges are noncovalently (or potentially covalently) functionalized, the extent of exfoliation of h-BN is considered to be very small, leading to low yield of BNNSs. Therefore, the contribution of noncovalent functionalization of BNNS surfaces with CSA to the total FR of the NF-BNNS is considered to be large. A comparison of h-BN having different lateral sizes (HP-P1, UHP-1K, and UHP-S1) showed that lower lateral size samples (UHP-S1) increase FR and yield as compared to other h-BN (HP-P1, UHP-1K, Table 1). However, the difference in FRs between 1a (NF-BNNS prepared from HP-P1) and 1b (NF-BNNS prepared from UHP-1K) was not so large (FR of 1a: 0.015, FR of 1b: 0.007), although 1a has much more edges than 1b because the lateral size of 1b (~1.3 um) was ~6.5 times larger than that of 1a (~0.2 um). Meanwhile, the difference in FRs between 1c (NF-BNNS prepared from UHP-S1) and 1b (NFBNNS prepared from UHP-1K) was also not large, although the lateral size of 1b (~1.3 um) was much larger than that of 1c (~0.15 um). These results also suggest that the possibility of covalent functionalization of the BNNS edges can be excluded or negligible. If CSAs mainly react with the edges of BNNSs, the FRs of 1a and 1c should be much larger than that of 1b. The yield of BNNS prepared from UHP-S1 was 13.8%. The BN precipitates obtained after settling the BNNS dispersion for 24 h were further washed using DIW under bath sonication (60 min). After settling the dispersion for 24 h, the supernatant was collected and filtrated. This extraction process resulted in additional NF-BNNS after drying, with the total yield reaching 19.0% on the basis of the original UHP-S1 sample (Table 1). In addition, the yield of BNNSs increased with prolonged sonication time (Table 1). Higher yield of BNNSs was obtained after 16 h bath sonication, although the lateral size of the BNNSs slightly decreased. The total yield of BNNS prepared


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from UHP-S1 reached ~25%, at the expense of decreasing the lateral size (from 0.20 µm to 0.15 µm). This yield was higher than those obtained with most of the wet approaches previously reported.23−30 Meanwhile, the total yield of BNNS prepared from UHP-1K was ~15% with relatively large lateral sizes (~1.1 µm). The NF-BNNSs dispersed in IPA with significantly higher concentrations (Table 1, up to 0.75 mg mL–1) (Figure S8) as compared to previously reported results (0.06 mg mL–1 in IPA).24 The physical adsorption of CSA residuals on BNNS surfaces and decrease of the number of BN layers are considered to lead to improved solubility. The physical adsorption of CSA is likely to prevent BNNSs from restacking. NF-BNNS dispersed in other organic solvents such as acetone and dimethyl sulfoxide (DMSO). In addition, the exfoliation method for preparing these highly soluble NF-BNNSs is scalable. The initial hBN concentration in CSA (5 mg mL–1, initial h-BN concentrations in most of previous liquidphase exfoliation methods: 1–3 mg mL–1)24–26,28,30 can be further increased for reducing the quantity of DIW used in the process (see Supporting Information). Table 1 Properties of BNNS/IL complexes obtained using various ILs. code h-BN a Solvent Sonication FRc Yield Yield (two Solubility species time (h) (%)d cycles) (%)e (mg mL−1)f 1a

HP-P1

CSA

8

0.015

9.4

14.0

0.50

1b

UHP-1K

CSA

8

0.007

6.0

8.3

0.31

1c

UHP-S1

CSA

8

0.017

13.8

19.0

0.69

1d

UHP-1K

CSA

16

0.010

10.1

15.4

0.49

1e

UHP-S1

CSA

16

0.025

19.1

24.9

0.75

1f

UHP-1K

MSAb

8

0.003

2.1

2.9

0.11

1g

UHP-1K

H2SO4b

8

0.000

1.0

-

0.08

a

Each lateral size was estimated by SEM: HP-P1 0.5 µm, UHP-1K 4.0 µm, and UHP-S1 0.4 µm. Methanesulfonic acid (MSA) and 98% H2SO4 were used instead of CSA. cCSA (MSA, or H2SO4)/BNNS mass ratio estimated by TGA weight losses. dAverage yield of BNNS after one cycle calculated from the weight of BNNS minus that of CSA or MSA4. eAverage total yield of BNNS including an additional extraction (sonication and settling) cycle. fBNNS concentration in b


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the NF-BNNS/IPA supernatant collected after sonication (20 min) and settling (20 h) of NFBNNS/IPA dispersion (initial BNNS concentration: 1 mg mL−1).

Further, the thermal conductivities of NF-BNNS/thermoplastic polymer composites were evaluated. NF-BNNS prepared from UHP-1K was used for this evaluation as large lateral size BNNSs are known to be more effective for reinforced polymeric composites, largely improving the thermal conductivity.3 First, NF-BNNSs were pre-dispersed in acetone under short bath sonication and subsequently this NF-BNNS solution was mixed with a PMMA/acetone solution with a controlled weight ratio under sonication (30 min). This method produced NF-BNNSdispersed PMMA/acetone solutions (Figure S9). Meanwhile, when NF-BNNS powders were directly added into a PMMA/acetone solution, some NF-BNNS aggregates were formed in the dispersion. The solution was spread on a glass plate, and acetone was naturally volatilized for at least 24 h. NF-BNNS/PMMA films (thickness: 0.3 mm) were obtained after further drying at 50 °C under vacuum (Figure 5a (i)–(iii)). Figure 5a shows photographs of NF-BNNS/PMMA and hBN/PMMA composite films prepared by the same method. h-BN/PMMA composites produced an uneven h-BN dispersion in PMMA (Figure 5a (iv)–(vi)), which was also confirmed by SEM images of frozen fracture surfaces of the films of h-BN/PMMA composites (Figure 6b). In this composite, large h-BN particles did not disperse uniformly in the matrix. This was originated from the low interaction extent between h-BNs and a PMMA matrix. In particular, the composite films containing 80 wt% (= 67.7 vol%) of h-BN were extremely brittle, and films with showing smooth surface and edges were extremely difficult to prepare from this material (Figure 5a (vi)). This h-BN volume fraction was larger than the maximum fraction (63.4 vol%) for random packing of monodispersed hard spheres.45 In contrast, the use of NF-BNNS yielded good surface appearance films uniformly dispersed in a PMMA matrix (Figure 5a (i)–(iii)) compared with h-


22

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BN/PMMA films at the same filler loading. An SEM image of frozen fracture surface of the NFBNNS/PMMA film showed uniformly dispersed BNNSs in the matrix (Figure 6a), although some agglomerates, formed during drying (volatilization of acetone) for preparing the film, were also observed. This improved dispersibility is considered to be due to high affinity between NFBNNSs and a PMMA matrix. The existence of SO3Cl anions of CSA on BNNS surfaces is likely to improve the interactions of BNNS surfaces with PMMA. PMMA is known to be well soluble in ILs having anions with sulfonyl groups (e.g. CF3SO3),46 and organic solvents with sulfinyl (S=O) groups (e.g. DMSO). In the case of ILs, their anion species play a more important role in their miscibility in solvents and polymers than their cations.31,46 From the structural similarity, SO3Cl anions on BNNS surfaces are considered to have high affinity with PMMA. Even NFBNNS/PMMA composite films containing 80 wt% of BNNS showed good surface appearance (Figure 5a (iii)). NF-BNNS/PMMA films showed significantly higher through-plane thermal conductivities than h-BN/PMMA films (Figure 5b). Even the use of 80 wt% of h-BNs for hBN/PMMA films (Figure 5a (vi)) resulted in the relatively low thermal conductivity (3.81 W m–1 K–1), which is due to increased thermal resistance by the significant phonon scattering at the interfaces between h-BNs and the PMMA matrix because of the weak h-BN/PMMA affinity. The through-plane thermal conductivity of NF-BNNS/PMMA (BNNS content: 24.5 wt%) films was 0.96 W m–1 K–1, which was much higher than that of h-BN/PMMA films (0.52 W m–1 K–1). This result is owing to uniform dispersion of NF-BNNS with high crystallinity in the PMMA matrix (Figure 6a), increased aspect ratio of NF-BNNS caused by exfoliation, and the improved interactions between NF-BNNSs and the PMMA matrix. Noncovalent functionalization of BNNSs with CSA led to improved exfoliation and dispersibility of BNNSs without forming defects on the surfaces, and enhanced interactions of BNNSs and PMMA. Therefore,


23


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Page 24 of 36

noncovalent functionalization of BNNSs with CSA was effective for improving the thermal conductivity of the composites. The uniform dispersion of NF-BNNS is originated not only from the existence of CSA on BNNS surfaces but also from the film preparation method mixing the pre-dispersed NF-BNNS/acetone solutions with PMMA/acetone solutions. When NF-BNNS powders were directly added to PMMA/acetone solutions, some NF-BNNS agglomerates were still observed in the dispersion even after 50 min bath sonication. The obtained NFBNNS/PMMA (BNNS content: 24.5 wt%) films prepared using this dispersion showed lower through-plane thermal conductivity (0.86 W m–1 K–1) than NF-BNNS/PMMA composites using pre-dispersed NF-BNNS solutions. The viscosity of PMMA/acetone solutions is much higher than acetone. Hence, pre-dispersing NF-BNNSs in acetone was very effective for dispersing them uniformly in PMMA/acetone solutions. On the other hand, the in-plane thermal conductivity for the NF-BNNS/PMMA (BNNS content: 24.5 wt%) films (Figure 5a (ii)) was 1.35 W m–1 K–1. The difference between the through- and in-plane thermal conductivities was not large, which is due to randomly dispersed NF-BNNSs in the PMMA matrix. In particular, the through- and in-plane thermal conductivities of a NF-BNNS/PMMA film containing 80 wt% of BNNS (Figure 5a (iii)) were significantly high (10.2 W m–1 K–1, 14.7 W m–1 K–1, respectively).


24

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(b)

12

Through-plane thermal conductivity (W m-1 K-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(iii)

10

10.22

8 NF-BNNS/PMMA

6 5.26

4

(vi) 3.81

2.91

2

PMMA (i) 0.30 0.22

0 0

(ii) 0.96 h-BN/PMMA (v) 0.52

(iv) 0.25 10 20 30 40 50 60 70 80 Content of BNNS or h-BN （wt%））

Figure 5. (a) Photographs of films (10 mm×10 mm× 0.3 mm) of NF-BNNS/PMMA composites having different BNNS loadings ((i) 2 wt%, (ii) 24.5 wt%, and (iii) 80 wt%), and h-BN/PMMA composites with different h-BN loadings ((iv) 2 wt%, (v) 24.5 wt%, and (vi) 80 wt%). (b) Through-plane thermal conductivities of films of NF-BNNS/PMMA composites, h-BN/PMMA composites and PMMA.


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Figure 6. SEM images of frozen fracture surfaces of the films of (a) NF-BNNS/PMMA composites containing 24.5 wt% of BNNSs, and (b) h-BN/PMMA composites containing 24.5 wt% of h-BNs (UHP-1K). Some BNNSs or h-BNs are surrounded by the white dotted-line circles in each figure. In addition, NF-BNNS/polybutylene terephthalate (PBT) composite films were prepared using a NF-BNNS-dispersed PBT/1,1,1,3,3,3-Hexafluoro-2-propanol (HFIP) solution (Figures S9b). The through- and in-plane thermal conductivities of the NF-BNNS/PBT composite films containing 80 wt% of BNNSs were also extremely high (11.0 W m–1 K–1 and 15.1 W m–1 K–1, respectively) (Table S1, Figure S10). In the NF-BNNS/PBT composites containing 80 wt% of BNNSs, NFBNNSs were randomly oriented in the composite (Figure 7a), and they were firmly embedded in the PBT matrix (Figure 7b), indicating strong interactions between NF-BNNS surfaces and PBT. The through-plane thermal conductivities of the NF-BNNS/thermoplastic polymer composite films were much greater than those previously reported for BNNS/thermoplastic polymer composites (1.1–2.6 W m–1 K–1) (Table S1). Moreover, the volume resistivity after a potential of 1,000 V was applied to the film of NF-BNNS/PBT composites containing 80 wt% of BNNS was


26

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~3.0 × 1016 Ω cm, which was much higher than that of pristine PBT (~3.7 × 1015 Ω cm) (Figure 8). Uniform dispersion of NF-BNNSs in the matrix improved the electrical insulation. The volume resistivity of the h-BN/PBT (80 wt%/20 wt%) composites (~1.2 × 1016 Ω cm) was also higher than that of pristine PBT. However, the value was lower than that of the NF-BNNS/PBT composites, which is probably due to weak interfaces between h-BNs and the PBT matrix and an uneven dispersion of h-BNs in the matrix. From these results, the NF-BNNS/PBT composites were confirmed to have extremely high through-plane thermal conductivity and improved electrical insulation.

Figure 7. (a, b) SEM images of frozen fracture surfaces of the films of NF-BNNS/PBT composites containing 80 wt% of BNNSs. NF-BNNSs were randomly oriented in the composite.


27


1.0E+17

Volume resistivity (Ω cm)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 28 of 36

1.0E+15

1.0E+13

1.0E+11

(a)

(b)

(c)

Figure 8. Volume resistivity of films after a potential of 1,000 V was applied. (a) PBT, (b) hBN/PBT (80 wt%/20 wt%) composites, and (c) NF-BNNS/PBT composites (BNNS content: 80 wt%).

4. CONCLUSIONS Highly soluble NF-BNNSs with high crystallinity were prepared in high yields up to 25% by an extremely facile, low-cost and scalable exfoliation method using CSA. CSA showed strong physical adsorption on BNNS surfaces, which was probably due to interactions between SO3Cl anions of CSA and electron deficient B atoms of BN surfaces, in addition to protonation on N atoms of BN surfaces. The obtained NF-BNNSs with CSA showed high solubility in various organic solvents such as IPA, acetone, and HFIP. The crystallinity of the NF-BNNS was perfectly maintained even after superacid treatment, which was confirmed by SAED patterns, XPS spectra, XRD patterns and Raman spectra. Highly thermally conductive NF-


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BNNS/thermoplastic polymer composite films were easily produced by a simple wet-process using pre-dispersed NF-BNNS solutions. By the use of NF-BNNS and pre-dispersing them in solvents before mixing with polymer solutions, NF-BNNSs were randomly dispersed in the thermoplastic polymer matrices. In general, the through-plane thermal conductivities of BNNS/polymer composites are much lower than the in-plane parameter. However, random dispersion of the NF-BNNSs in the thermoplastic polymer matrices dramatically improved both in- and through-plane thermal conductivities (>10 W m–1 K–1) with good surface appearance. In particular, the through-plane thermal conductivities of our prepared BNNS/PBT composites were much greater (up to 11.0 W m–1 K–1) as compared to state-of-the-art BNNS/thermoplastic polymer composites data (≤2.6 W m–1 K–1). Moreover, the volume resistivity of NF-BNNS/PBT composites was also improved compared with those of h-BN/PBT composites and pristine PBT. The use of NF-BNNSs can be also very effective for enhancing through-plane thermal conductivity of thermoset polymer-based composites with electrical insulation. The NFBNNS/polymer composites are very attractive for the thermal management of various applications such as next-generation electronic devices, power systems, and communication equipment.

ASSOCIATED CONTENT Supporting Information. Preparation of large quantities of NF-BNNSs, preparation of NFBNNS/PBT composite films, preparation of h-BN/PMMA composite films, supplementary tables and figures. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION


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Corresponding Author *Email: [email protected] Notes The authors declare no competing financial interests. ACKNOWLEDGMENT We are grateful to Mitsumasa Matsushita, Yoshihide Katagiri, and Dr. Kenzo Fukumori for helpful discussions. We also thank Naoko Takahashi for XPS measurements, Dr. Yuichi Kato for Raman analysis, and Noritomo Suzuki for performing TEM observations.

REFERENCES (1) Sato, K.; Horibe, H.; Shirai, T.; Hotta, Y.; Nakano, H.; Nagai, H.; Mitsuishi, K.; Watari, K. Thermally Conductive Composite Films of Hexagonal Boron Nitride and Polyimide with Affinity-Enhanced Interfaces. J. Mater. Chem. 2010, 20, 2749–2752. (2) Morishita, T.; Matsushita, M.; Katagiri, Y.; Fukumori, K. A Novel Morphological Model for Carbon Nanotube/Polymer Composites Having High Thermal Conductivity and Electrical Insulation. J. Mater. Chem. 2011, 21, 5610–5614. (3) Wang, X.; Pakdel, A. Zhang, J.; Weng, Q.; Zhai, T.; Zhi, C.; Goberg, D.; Bando, Y. LargeSurface-Area BN Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Dielectric Properties. Nanoscale Res. Lett. 2012, 7, 662. (4) Yu, J.; Mo, H.; Jiang, P. Polymer/Boron Nitride Nanosheet Composite with High Thermal Conductivity and Sufficient Dielectric Strength. Polym. Adv. Technol. 2015, 26, 514–520.


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Page 31 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(5) Uetani, K.; Ata, S.; Tomonoh, S.; Yamada, T.; Yumura, M.; Hata, K. Elastomeric Thermal Interface Materials with High Through-Plane Thermal Conductivity from Carbon Fiber Fillers Vertically Aligned by Electrostatic Flocking. Adv. Mater. 2014, 26, 5857–5862. (6) Balandin, A. A. Thermal Properties of Graphene and Nanostructured Carbon Materials. Nat. Mater. 2011, 10, 569–581. (7) Morishita, T.; Matsushita, M.; Katagiri, Y.; Fukumori, K. Noncovalent Functionalization of Carbon Nanotubes with Maleimide Polymers Applicable to High-Melting Polymer-Based Composites. Carbon 2010, 48, 2308–2316. (8) Song, S. H.; Park, K. H.; Kim, B. H.; Choi, Y. W.; Jun, G. H.; Lee, D. J.; Kong. B. -S.; Paik K. -W.; Jeon, S. Enhanced Thermal Conductivity of Epoxy–Graphene Composites by Using Non-Oxidized Graphene Flakes with Non-Covalent Functionalization. Adv. Mater. 2013, 25, 732–737. (9) Leese, H. S.; Govada, L.; Saridakis, E.; Khurshid, S.; Menzel, R.; Morishita, T.; Clancy, A. J.; White, E. R.; Chayen, N. E.; Shaffer, M. S. P. Reductively PEGylated Carbon Nanomaterials and Their Use to Nucleate 3D Protein Crystals: A Comparison of Dimensionality. Chem. Sci. 2016, 7, 2916–2923. (10) Hu, K.; Kulkarni, D. D.; Choi I.; Tsukruk, V. V. Graphene-Polymer Nanocomposites for Structural and Functional Applications. Prog. Polym. Sci. 2014, 39, 1934–1972. (11) Golberg, D.; Bando, Y.; Huang, Y.; Terao, T.; Mitome, M.; Tang C.; Zhi, C. Boron Nitride Nanotubes and Nanosheets. ACS Nano 2010, 4, 2979–2993. (12) Lin, Y.; Connell, J. W. Advances in 2D Boron Nitride Nanostructures: Nanosheets, Nanoribbons, Nanomeshes, and Hybrids with Graphene. Nanoscale 2012, 4, 6908–6939. (13) Yu, J.; Huang, X.; Wu, C.; Wu, X.; Wang, G.; Jiang, P. Interfacial Modification of Boron Nitride Nanoplatelets for Epoxy Composites with Improved Thermal Properties. Polymer 2012, 53, 471–480.


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(14) Tsai, M. –H.; Tseng, I. –H.; Chiang, J. -C.; Li, J. -J. Flexible Polyimide Films Hybrid with Functionalized Boron Nitride and Graphene Oxide Simultaneously to Improve Thermal Conduction and Dimensional Stability. ACS Appl. Mater. Interfaces 2014, 6, 8639–8645. (15) Huang, X.; Wang, S.; Zhu, M.; Yang, K.; Jiang, P.; Bando, Y.; Golberg D.; Zhi, C. Thermally Conductive, Electrically Insulating and Melt-Processable Polystyrene/Boron Nitride Nanocomposites Prepared by In Situ Reversible Addition Fragmentation Chaintransfer Polymerization. Nanotechnology 2015, 26, 015705. (16) Li, S.; Yang, T.; Zou, H.; Liang, M.; Chen, Y. Enhancement in Thermal Conductivity and Mechanical Properties via Large-Scale Fabrication of Boron Nitride Nanosheets. High Perform. Polym. 2016, doi: 10.1177/0954008316643766. (17) Kim, K.; Kim, M.; Kim, J. Thermal and Mechanical Properties of Epoxy Composites with a Binary Particle Filler System Consisting of Aggregated and Whisker Type Boron Nitride Particles. Compos. Sci. Technol. 2014, 103, 72–77. (18) Wang, X. -B.; Weng, Q.; Wang, X.; Li, X.; Zhang, J.; Liu, F.; Jiang, X. -F.; Guo, H.; Xu, N.; Golberg D.; Bando, Y. Biomass-Directed Synthesis of 20 g High-Quality Boron Nitride Nanosheets for Thermoconductive Polymeric Composites. ACS Nano 2014, 8, 9081–9088. (19) Lin, Y.; Williams, T. V.; Cao, W.; Elsayed-Ali, H. E.; Connell, J. W. Defect Functionalization of Hexagonal Boron Nitride Nanosheets. J. Phys. Chem. C 2010, 114, 17434– 17439. (20) Yu, B.; Xing, W.; Guo, W.; Qiu, S.; Wang, X.; Lo, S.; Hu, Y. Thermal Exfoliation of Hexagonal Boron Nitride for Effective Enhancements on Thermal Stability, Flame Retardancy and Smoke Suppression of Epoxy Resin Nanocomposites via Sol–Gel Process. J. Mater. Chem. A 2016, 4, 7330–7340. (21) Morishita, T.; Matsushita, M.; Katagiri, Y.; Fukumori, K. A. Synthesis and Properties of Macromer-Grafted Polymers for Noncovalent Functionalization of Multiwalled Carbon Nanotubes. Carbon 2009, 47, 2716–2726.


32

Page 33 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(22) Morishita, T.; Matsushita, M.; Katagiri, Y.; Fukumori, K. A. Effects of the Composition and Molecular Weight of Maleimide Polymers on the Dispersibility of Carbon Nanotubes in Chloroform. Carbon 2011, 49, 5185–5195. (23) Zhi, C.; Bando, Y.; Tang, C.; Kuwahara, H.; Golberg, D. Large-Scale Fabrication of Boron Nitride Nanosheets and Their Utilization in Polymeric Composites with Improved Thermal and Mechanical Properties. Adv. Mater. 2009, 21, 2889–2893. (24) Coleman, J. N.; Lotya, M.; O’Neill1, A.; Bergin, S. D.; King, P. J.; Khan, U.; Young, K.; Gaucher, A.; De, S.; Smith, R. J.; Shvets, I. V.; Arora, S. K.; Stanton, G.; Kim, H.; Lee, K.; Kim, G. T.; Duesberg, G. S.; Hallam, T.; Boland, J. J.; Wang, J. J.; Donegan, J. F.; Grunlan, J. C.; Moriarty, G.; Shmeliov, A.; Nicholls, R. J.; Perkins, J. M.; Grieveson, E. M.; Theuwissen, K.; McComb, D. W.; Nellist, P. D.; Nicolosi, V. Two-Dimensional Nanosheets Produced by Liquid Exfoliation of Layered Materials. Science 2011, 331, 568–571. (25) Wang, Y.; Shi, Z.; Yin, J. Boron Nitride Nanosheets: Large-Scale Exfoliation in Methanesulfonic Acid and Their Composites with Polybenzimidazole. J. Mater. Chem. 2011, 21, 11371–11377. (26) Zhou, K. G.; Mao, N. N.; Wang, H. X.; Peng, Y.; Zhang, H. L. A Mixed-Solvent Strategy for Efficient Exfoliation of Inorganic Graphene Analogues. Angew. Chem. Int. Ed. 2011, 50, 10839–10842. (27) Du, M.; Wu, Y.; Hao, X. A Facile Chemical Exfoliation Method to Obtain Large Size Boron Nitride Nanosheets. CrystEngComm 2013, 15, 1782–1786. (28) Du, M.; Li, X.; Wang, A.; Wu, Y.; Hao, X.; Zhao, M. One-Step Exfoliation and Fluorination of Boron Nitride Nanosheets and a Study of Their Magnetic Properties. Angew. Chem. Int. Ed. 2014, 53, 3645–3649. (29) Lin, Y.; Williams, T. V.; Connell, J. W. Soluble, Exfoliated Hexagonal Boron Nitride Nanosheets. J. Phys. Chem. Lett. 2010, 1, 277–283.


33


1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 36

(30) Sun, W.; Meng, Y.; Fu, Q.; Wang, F.; Wang, G.; Gao, W.; Huang, X.; Lu, F. High-Yield Production of Boron Nitride Nanosheets and Its Uses as a Catalyst Support for Hydrogenation of Nitroaromatics. ACS Appl. Mater. Interfaces 2016, 8, 9881–9888. (31) Morishita, T.; Okamoto, H.; Katagiri, Y.; Matsushita, M.; Fukumori, K. A High-Yield Ionic Liquid-Promoted Synthesis of Boron Nitride Nanosheets by Direct Exfoliation. Chem. Commun. 2015, 51, 12068–12071. (32) Cremlyn, R. J. W. Chlorosulfonic Acid: A Versatile Reagent, The Royal Society of Chemistry, 2002. (33) Behabtu, N.; Lomeda, J. R.; Green, M. J.; Higginbotham, A. L.; Sinitskii, A.; Kosynkin, D. V.; Tsentalovich, D.; Parra-Vasquez, A. N. G.; Schmidt, J.; Kesselman, E.; Cohen, Y.; Talmon, Y.; Tour J. M.; Pasquali, M. Spontaneous High-Concentration Dispersions and Liquid Crystals of Graphene. Nat. Nanotechnol. 2010, 5, 406–411. (34) Hiyama, T. Organofluorine Compounds: Chemistry and Applications. Springer Science & Business Media, 2000, p. 5. (35) Xue, Y.; Liu, Q.; He, G.; Xu, K.; Jiang, L.; Hu, X.; Hu, J. Excellent Electrical Conductivity of the Exfoliated and Fluorinated Hexagonal Boron Nitride Nanosheets. Nanoscale Res. Lett. 2013, 8, 49. (36) Grzyb, B.; Gryglewicz, S.; Śliwak, A.; Diez, N.; Machnikowski, J.; Gryglewicz, G. Guanidine, Amitrole and Imidazole as Nitrogen Dopants for the Synthesis of N-Graphenes. RSC Adv. 2016, 6, 15782–15787. (37) Bhimanapati, G. R.; Kozuch, D.; Robinson, J. A. Large-scale synthesis and functionalization of hexagonal boron nitride nanosheets, Nanoscale 2014, 6, 11671–11675. (38) Wren, A. G.; Phillips, R. W.; Tolentino, L. U. Surface Reactions of Chlorine Molecules and Atoms with Water and Sulfuric Acid at Low Temperatures, J. Colloid Interface Sci. 1979, 70, 544–557.


34

Page 35 of 36

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(39) Kidambi, P. R.; Blume, R.; Kling, J.; Wagner, J. B.; Baehtz, C.; Weatherup, R. S.; Schloeg, R.; Bayer B. C.; Hofmann, S. In Situ Observations during Chemical Vapor Deposition of Hexagonal Boron Nitride on Polycrystalline Copper, Chem. Mater. 2014, 26, 6380–6392. (40) Okano, T.; Qiao, K.; Bao, Q.; Tomida, D.; Hagiwara, H.; Yokoyama, C. Dehydration of Fructose to 5-Hydroxymethylfurfural (HMF) in an Aqueous Acetonitrile Biphasic System in the Presence of Acidic Ionic Liquids. Appl. Catal. A-Gen. 2013, 451, 1–5. (41) Kiasat, A. R.; Fallah-Mehrjardi, M. B(HSO4)3: A Novel and Efficient Solid Acid Catalyst for the Regioselective Conversion of Epoxides to Thiocyanohydrins under Solvent-Free Conditions. J. Braz. Chem. Soc. 2008, 19, 1595–1599. (42) Ramesh, S.; Ericson, L. M.; Davis, V. A.; Saini, R. K.; Kittrell, C.; Pasquali, M.; Billups, W. E.; Adams, W. W.; Hauge, R. H.; Smalley, R. E. Dissolution of Pristine Single Walled Carbon Nanotubes in Superacids by Direct Protonation. J. Phys. Chem. B 2004, 108, 8794–8798. (43) Iacopi, F.; Boeckl, J. J.; Jagadish, C. 2D Materials, Academic Press, Elsevier Science, 2016, p. 103. (44) Johnson, C. D.; Katritzky, A. R.; Shapiro, S. A. Temperature Variation of the H0 Acidity Function in Aqueous Sulfuric Acid Solution. J. Am. Chem. Soc. 1969, 91, 6654–6662. (45) Song, C.; Wang, P.; Makse, H. A. A Phase Diagram for Jammed Matter, Nature 2008, 453, 629–632. (46) Ueno, K.; Fukai, T.; Nagatsuka, T.; Yasuda, T.; Watanabe, M. Solubility of Poly (methyl methacrylate) in Ionic Liquids in Relation to Solvent Parameters. Langmuir 2014, 30, 3228– 3235.


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TOC GRAPHICS


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Facile Exfoliation and Noncovalent Superacid Functionalization of

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