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High-yield ionic liquid-promoted synthesis of boron nitride nanosheets by direct exfoliation Received 00th January 20xx, Accepted 00th January 20xx

a

a

a

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Takuya Morishita,* Hirotaka Okamoto, Yoshihide Katagiri, Mitsumasa Matsushita, and Kenzo a Fukumori

DOI: 10.1039/x0xx00000x www.rsc.org/

Boron nitride nanosheets (BNNSs) with micron-sized edges in high yields were prepared by direct exfoliation of bulk hexagonal boron nitrides using ionic liquids (ILs). The ILs strongly attached onto BNNS surfaces, and dramatically enhanced the exfoliation, giving –1 highly concentrated BNNS dispersions (~1.9 mg mL ) and yields reaching ~50%. Boron nitride nanosheets (BNNSs) are known as white graphene because of their two-dimensional nanostructure and unique physical properties.1–5 BNNSs show almost equally excellent thermal conductivity1 and mechanical properties1 compared with graphene.6,7 Moreover, they exhibit unique properties that are absent in graphene, such as high electrical insulation,1 high thermal stability,2 high-temperature oxidation-resistance,4 and chemical inertness.2 These properties have led to a growing interest in BNNSs for several promising applications such as nanocomposites,1,8 dielectric substrates,3,8 sensors,1,9 catalysts,9 and high heat- or oxidation-resistant coatings.4 BNNSs have been isolated by micromechanical cleavage10 of bulk h-BN. However, this technique is inefficient and unscalable. Therefore, a mass producing method is required to enable their use for bulk applications. Large-area BNNSs have been synthesized by chemical vapor deposition on copper substrate.11,12 However, this scalable synthesis usually leads to poor crystallization and requires the transfer of BNNSs onto an appropriate substrate or solvent. More facile and scalable wetchemical approaches8,9,13–18 have generated BNNSs from h-BNs through exfoliation and stabilization using carefully selected solvents such as dimethylformamide (DMF)8 and isopropyl alcohol (IPA)9, dispersants,13 or Lewis bases14. These methods typically involve strong and/or extended sonication,8,9,13,16 or long time heat treatment.14,18 However, the concentration and yields of the obtained BNNSs remain low (yield: 0.1%8–20% (after six to nine extraction cycles)14). Moreover, strong and/or extended sonication damage the BN framework, leading to small lateral sizes (a few tens of nm to ~1 μm),16 defects, and, thus degraded properties.

19,20

Unlike organic solvents, ionic liquids (ILs) benefit from unique properties such as nonvolatility, versatile solubility, and high thermal and electrochemical stability. Because of these properties, these molten salts have been proposed as environmentally friendly alternative to volatile organic solvents and candidate materials for 20 numerous energy-related applications. They have also been used to exfoliate layered materials to yield stable dispersions of 2D 21 nanosheets such as graphenes and bismuth telluride 22 nanosheets. Specifically, their surface energies were roughly –2 19 estimated to ca. 63–83 mJ m from their surface tensions, which –2 lay about 30 mJ m below the surface energies in the case of 9,23 liquids, closely matching the graphite surface energy (~70–80 mJ –2 23 m ) and allowing their use to assist the exfoliation of graphites. The high-yield production of BNNSs by exfoliation of h-BN layers remains a considerable challenge compared with graphite exfoliation, because these layers experience strong interlayer interactions stemming from the partly ionic character of the B–N bond8 and present their chemically inert surfaces. Exfoliation techniques using oxidation6 or alkali metal intercalation,7 which are useful for graphites, are not applicable to inert h-BNs. Alternatively, ILs are expected to solve this issue and stabilize the resulting BNNSs because their surface energies (63–83 mJ m–2) approximate those of h-BNs (44–66 mJ m–2).24 In addition, interactions may occur between ILs and partly ionic BN surfaces. In this study, h-BNs were readily exfoliated into single- to few-layered BNNSs with micronsized edges by physical adsorption of ILs on BN surfaces under weak sonication. Fig. 1a shows a schematic of the exfoliation of h-BN into BNNSs by physical adsorption of ILs on h-BN surfaces. In this approach, h-BNs and ILs were mixed under mild bath sonication for 8 h to give uniform BN dispersions. Subsequent centrifugation (3,000 rpm, 20 min) of the dispersion produced a stable supernatant solution of BNNSs (Fig. 1b). This centrifugation step effectively removes insoluble and/or unexfoliated large particles from various nanomaterial dispersions.9,25,26 Various ILs were explored to improve the exfoliation and its yield (Fig. S1). For example, h-BNs were well dispersed in 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([bmim][Tf2N]) under bath sonication and subsequently centrifuged to generate stable light

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Journal Name (Fig. 2d) indicates that 95% of BNNSs consist of one to six layers and the average number of layers () is ~4.4. Fig. 2e shows that the average lateral size () was ~1.2 µm (Fig. 2e). Some [bmim][Tf2N] residues were observed on the BNNS surfaces (Fig. 2a, 2b), although they were gradually decomposed by the HRTEM electron beam. The selected area electron diffraction (SAED) pattern (Fig. 2c) revealed that the exfoliated BNNS with [bmim][Tf2N] consisted of highcrystallinity hexagonal structures. The HRTEM images showed that [bmim][Tf2N] was physically adsorbed on the BNNS surfaces, preventing BNNSs from restacking and improving solubility.

(a)

1) bath sonication 2) centrifugation

IL h-BN









⊕ ⊖









(d)0.35

BNNS h-BN (>300 layers)

IL: IL: ⊕

(e)0.20

= 4.4

0.30 ⊖

= 1.21

0.15

0.25

Probability





0.20 0.15 0.10

0.10 0.05

0.05 0.00

0.00 0

Fig. 1. (a) Schematic of IL-mediated exfoliation of h-BN into BNNSs. (b) Photographs of [bmim][Tf2N], BNNS/[bmim][Tf2N] supernatant, [bmim][PF6], and BNNS/[bmim][PF6] supernatant.

The BNNS/[bmim][Tf2N] supernatants were filtered, washed thoroughly with acetone to remove free [bmim][Tf2N]. The light brown BNNS powders were obtained after drying under vacuum at 80 °C for 12 h. On the contrary, BNNS/[bmim][PF6] and BNNS/[bmim][BF4] supernatants produced white powders. Fig. 2 shows high-resolution transmission electron microscopy (HRTEM) images of the obtained BNNSs. Previous TEM studies have suggested that the distinguishable edges of the exfoliated BNNSs make it possible to count the BNNS layers.8,10 This edge-counting method revealed that h-BNs were highly exfoliated into BNNSs using [bmim][Tf2N]. Fig. 2a and 2b show HRTEM images of singleand triple-layered BNNSs, respectively. The thickness histogram

5 10 15 Number of layers, N

20

0.0

0.5 1.0 1.5 2.0 Length, L (mm)

2.5

Fig. 2. (a) HRTEM images of a BNNS monolayer/[bmim][Tf2N]. (b) HRTEM images of a BNNS triple layer/[bmim][Tf2N]. (c) Electron diffraction pattern of the area marked by the white dotted circle in (a). (d) Thickness and (e) length histograms by HRTEM (40 flakes were measured).

The physical adsorption of [bmim][Tf2N] was further evaluated by X-ray photoelectron spectroscopy (XPS) and (thermogravimetric analysis) TGA measurements. The XPS N1s spectrum for BNNS/[bmim][Tf2N] displayed binding energies at 398.0 and 402.0 eV (Fig. 3a), while that of h-BN showed a peak at 398.1 eV. The N1s peak for [bmim] cation of [bmim][Tf2N] has previously been 21 detected at ca. 402 eV. Therefore, the N1s binding energy at 402.0 eV (Fig. 3a) can be attributed to [bmim] cation of [bmim][Tf2N] on the BNNS surface. The observed binding energy of B1s spectrum of BNNS/[bmim][Tf2N] was 190.3 eV (Fig. 3b), and that of h-BN was 190.4 eV. The N1s and B1s peaks for BNNSs are known to be 1,11,17 essentially the same as those for h-BN. The N1s and B1s spectra of BNNS/[bmim][Tf2N] did not show any additional intense

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orange BNNS/[bmim][Tf2N] supernatants (Fig. 1b). BNNS/[1-ethyl-3methylimidazolium (emim)][Tf2N], BNNS/[emim][trifluoromethanesulfonate (TfO)], and BNNS/[bmim][TfO] supernatants also appeared light orange (Fig. S2). In contrast, BNNS/[bmim][hexafluorophosphate (PF6)] (Fig. 1b) and BNNS/[bmim][tetrafluoroborate (BF4)] supernatants (Fig. S2) showed white colors. These color differences are considered to originate from the existence of sulfonate groups in the IL counter anions because only sulfonate-bearing ILs produced light orange colored supernatants, and sulfonate-bearing ILs without BNNSs became very light yellow after 8h bath sonication at room temperature (Fig. S3). It has been reported that ILs darkened from colorless to amber due to some decomposition during sonication at 85 °C.27 These indicate that some sulfonate-bearing ILs may decompose to generate light yellow color under sonication even at room temperature, and once ILs attached onto BNNS surfaces via interactions such as cation-mediated interactions, some of their sulfonate-based anions might be more unstable and the decomposition may be accelerated.

Probability

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(a)

(b)

N1s

Table 1 Properties of BNNS/IL complexes obtained using various ILs. ILa

B1s

h-BN

h-BN Intensity (a. u.)

Intensity (a. u.)

hydrogen bonds between C-2 protons of imidazolium cations and BNNS surface N atoms. Increasing the FRs enhanced the BNNS concentrations and consequently the yields (Table 1). A comparison of [emim][Tf2N], [bmim][Tf2N], [1-hexyl-3-methylimidazolium (hmim)][Tf2N] and [1-butylpyridinium (bpy)][Tf2N] shows that the [bmim] cation increases FRs to a greater extent than other cations (Table 1). However, when combined with [TfO] and [BF4] as anions, [bmim] cations gave much smaller FRs compared with the case of using [Tf2N] anion. These results show that anions more strongly affected the FRs and concentrations than cations. Specifically, ILs with [PF6] anions gave much higher FRs and BNNS concentrations than those with [Tf2N] anions (Table 1). A clear anion effect on the FRs and concentrations was observed with the order: [PF6] > [Tf2N] > [BF4], [TfO]. This trend seems to be related to the order of anion basicity (nucleophilicity) and hydrogen-bond ability ([PF6] < [Tf2N] < 29 29 [BF4], TfO]). [PF6] has the least basicity among these anions, and therefore, [PF6] anions screen [bmim] cations to a lower extent compared with other anions, resulting in stronger cation–π interactions of BNNS surfaces with [bmim][PF6] than with other ILs.

BNNS/[bmim][Tf2N]

BNNS/[bmim][Tf2N]

409 407 405 403 401 399 397 395 393 199 197 195 193 191 189 187 185 Binding energy (eV) Binding energy (eV)

(c)

(d)

h-BN

100

BNNS/[bmim][Tf2N] Intensity (a. u.)

80 60 40

N1s BNNS/[bmim][Tf2N] after TGA

[bmim][Tf2N] 100

200

300

400 500 600 Temperature (℃ ℃)

700

800

weight loss (wt%)g

conc. (mg ml−1)

yield (%)j

yield (two cycles) (%)k

[emim][Tf2N]

37c

4.7

0.049

0.17

3.3

4.3

[bmim][Tf2N]

33c

9.7

0.107

0.51

10.0

15.2

[hmim][Tf2N]

32c

6.9

0.074

0.30

5.9



[bpy][Tf2N]

33d

7.6

0.082

0.34

6.8

9.0

[emim][TfO]

39c

2.0

0.020

0.10

1.9

2.3

[bmim][TfO]

35c

2.1

0.021

0.13

2.5



[emim][BF4]

53e

1.0

0.010

0.08

1.6



[bmim][BF4]

47c

2.2

0.022

0.11

2.1

2.6

[bmim][PF6]

49c

18.8

0.231

1.86

35.9

49.6

[hmim][PF6]

43c

12.0

0.136

0.67

13.4



IPAb

21f





0.04

0.7

i

BNNS/[bmim][Tf2N]

20 0

FRh

surface tension (mJ m−2)

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

Fig. 3. (a) N1s and (b) B1s XPS spectra of BNNS/[bmim][Tf2N] and h-BN. (c) TGA diagrams of BNNS/[bmim][Tf2N], h-BN, and [bmim][Tf2N]. (d) N1s XPS spectra of BNNS/[bmim][Tf2N] after TGA and BNNS/[bmim][Tf2N].

Table 1 shows FR values for BNNS/ILs as well as BNNS concentrations and yields. All ILs used in Table 1 gave much higher concentration dispersions and yields than IPA, which has previously 7 been reported effective for the exfoliation of h-BN. This probably stems from strong physical adsorption of ILs onto BN surfaces 28 through cation-π interactions between IL cations and BNNS surface (Figure 1a). In addition, these interactions may involve

a

b

– c

All chemical structures are shown in Fig. S1. Used instead of IL. Measured by the Du Noüy ring (DNR) method at 298 K.19 dMeasured by the maximum bubble pressure method at 298 K.19 eMeasured by DNR method at 293 K.19 f Measured by DNR method at 298 K.30 gTGA weight loss of the ILs. hIL/BNNS mass ratio. iAverage BNNS concentration in the supernatant calculated using the weight of BNNS minus that of IL. jAverage yield of BNNS after one cycle. k Average total yield of BNNS including an additional extraction (brief sonication and centrifugation) cycle.

In particular, [bmim][PF6] showed the highest FR, leading to the –1 highest BNNS concentration (1.86±0.27 mg mL ) and yield (35.9±5.3%). The BNNS/[bmim][PF6] predominantly showed less

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peaks corresponding to covalent bonds between BNNS and [bmim][Tf2N]. In addition, a survey scan of the BNNS/[bmim][Tf2N] showed S and F elements despite the absence of these elements in h-BN (Table S1). TGA results (Fig. 3c) showed that [bmim][Tf2N] decomposed almost completely before reaching 500 °C, although hBN did not show any weight loss from 100 to 800 °C. Therefore, the weight loss observed from approximately 200 to 700 °C for BNNS/[bmim][Tf2N] was attributed to the BNNS-immobilized ILs. The light brown BNNS/[bmim][Tf2N] powders turned white or light gray after the TGA measurements (Fig. S7). This stems from the volatilization of [bmim][Tf2N] during these measurements. The N1s spectrum for BNNS/[bmim][Tf2N] after the TGA measurement displayed a binding energy only at 398.0 eV, and the peak for [bmim] cation at 402.0 eV disappeared (Fig. 3d). In addition, the XPS survey scan of the BNNS/[bmim][Tf2N] after the TGA measurement revealed that the S and F elements of [bmim][Tf2N] almost disappeared (Table S1). These results show that [bmim][Tf2N] was removed from the BNNS surface during the measurement. Moreover, the BNNSs after the TGA measurement consisted of predominantly less than ten layers (Fig. S8). These results show that BNNSs without ILs were easily obtained by heat treatment of BNNS/ILs. In addition, the functionalization ratios (FRs) of ILs, defined as the mass ratio of the BNNS-adsorbed ILs, were estimated using the TGA weight losses (Fig. 3c, Fig. S9).

Residual weight (wt%)

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than ten layers (Fig. S10, S11). The length histogram (Fig. S10) showed that 87.5% of the sheets had micron-sized edges (≥1 µm). Their SAED patterns (Fig. S11) revealed that the hexagonal structures were perfectly maintained. The estimated surface energy –2 – of [bmim][PF6] (~79 mJ m ) exceeded that of h-BN (44–66 mJ m 2 24 ). Therefore, the cation–π interactions between [bmim][PF6] and BNNS surface N atoms play a more important role in the BNNS exfoliation than the matching of their surface energies. Anion–π interactions may also occur between [PF6] anions and BNNS surface B atoms but their role appears negligible relative to that of the cation–π interactions in this case because of the least basicity of [PF6]. The BN precipitates obtained after centrifugation of the BNNS/[bmim][PF6] dispersion were further washed with −1 [bmim][PF6] (the initial BN precipitate concentration: ~5 mg mL ) under a short bath sonication (30 min), and subsequently centrifuged to dissolve additional BNNSs containing mainly less than 10 layers (Fig. S12). This additional cycle gave a total yield of ~49.6% on the basis of the original h-BN sample (Table 1). This highly efficient exfoliation only required weak sonication and mild centrifugation, and therefore can be achieved with BNNS amounts reaching tens of grams at the laboratory scale (Fig. S13). In addition, the IL filtrate collected from BNNS dispersions could be recycled to generate another batch of BNNSs. Moreover, the BNNS/ILs dispersed well not only in ILs but also in various organic solvents such as DMF, N-methyl-2-pyrrolidone (NMP), IPA, ethanol, and acetone (Fig. S14, S15). BNNS/ILs dissolved well in good solvents for the IL used. The combination of BNNSs and ILs can also lead to very interesting applications. In summary, highly soluble BNNSs were prepared in high yields by direct exfoliation of h-BNs using ILs under weak sonication. In particular, [bmim][PF6] afforded a highly stable BNNS suspension at –1 high concentration (~1.9 mg mL ) in high yield (up to ~50%). To the best of our knowledge, this BNNS concentration and yield are the highest obtained for BNNS production. This mild, very facile, and versatile liquid-phase process is very attractive for the large-scale production of BNNSs for various applications. We are grateful to Dr. Hisato Takeuchi and Mitsuru Nakano for helpful discussions and Naoko Takahashi for XPS measurements.

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A high-yield ionic liquid-promoted synthesis of boron nitride nanosheets by direct exfoliation.

Boron nitride nanosheets (BNNSs) with micron-sized edges were prepared in high yields by direct exfoliation of bulk hexagonal boron nitrides using ion...
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