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Title

Alcoholysis-assisted Exfoliation of Boron Nitride Nanosheets fromHexagonal Boron NitrideAuthors

Witita Laosamathikul, Toshiki Sawada, Takeshi Serizawa

CitationTransactions of the Materials Research Society of Japan, Vol. 42,
Issue 5, pp. 135-138

Pub. date2017, 10

Powered by T2R2 (Science Tokyo Research Repository)

Trans. Mat. Res. Soc. Japan42[5] 135-138 (2017)

Alcoholysis-Assisted Exfoliation of Boron Nitride Nanosheets from Hexagonal
Boron Nitride
Witita Laosamathikul, Toshiki Sawada and Takeshi Serizawa*
Department of Chemical Science and Engineering, Tokyo Institute of Technology,
2-12-1 Ookayama, Meguro, Tokyo 152-8550, Japan* Corresponding author: serizawa@polymer.titech.ac.jp

Boron nitride nanosheets (BNNSs) are attractive 2-dimensional nanomaterials with excellent
thermal conductivity, mechanical properties, and thermal stability. It is therefore significant todevelop new simple methods to produce BNNSs. In this study, BNNSs with nanometer
thicknesses and diameters of several hundreds of nanometers were produced byalcoholysis-assisted exfoliation from hexagonal boron nitride (h-BN) through sonication of h-BNin primary alcohols such as butanol and hexanol. Atomic force microscopy images of the BNNSs
revealed that the morphologies were dependent on alcohol species and preparation methods.
Infrared absorption spectra of the BNNSs suggested that alcoholysis of B–N bonds in primaryalcohols during sonication processes was essential for BNNS production.
Key words: boron nitride nanosheet, hexagonal boron nitride, alcoholysis, exfoliation

production of BNNSs was also analyzed. The
morphologies and chemical structures of BNNSs werecharacterized by atomic force microscopy (AFM) andFourier-transform
infrared(FT-IR)
absorption
spectroscopy, respectively.

1. INTRODUCTION
Boron nitride (BN) has an analogous structure ofcarbon. Three different structures are known for BN;
layered (so-called hexagonal BN (h-BN)), cubic, andtubular structures,1 which are analogs of graphite,diamond, and carbon nanotubes, respectively. BN has
unique physicochemical properties such as excellent
thermal conductivity, mechanical stiffness, and thermal
stability.2 Therefore, BN was potentially used forvarious applications such as transistors, energy storages,thermal conductors, and topological insulators.3
Recently, boron nitride nanosheets (BNNSs) with asingle or few layers of h-BN have received muchattention
asrelatively
new2-dimensional
nanomaterials.4 BNNSs can be produced bysonication-assisted exfoliation from h-BN in suitable
solvents,5 chemical blowing of precursor polymers,6,7
plasma etching of BN nanotubes (BNNTs),7,8 andmicrowave plasma chemical vapor deposition ofprecursor gases on silicon substrates.9 However, it is still
significant to develop new simple preparation methodsof BNNSs.
We have previously demonstrated the chemical
peeling of BNNTs through sonication-assisted
alcoholysis of BNNTs in primary alcohols underambient conditions, followed by partial production ofBN nanoribbons.10 Morphological analysis and DFT
calculation suggested that the reactions more readily
occurred in alcohols with longer alkyl chains. We
hypothesized from this previous study that the reactions
could be extended to h-BN to simply produce BNNSs.
In this study, BNNSs were produced byalcoholysis-assisted exfoliation from h-BN throughsonication of h-BN in primary alcohols (Fig. 1). Butanol
and hexanol, which showed great reactivities withBNNTs,10 were used for reactants with h-BN as well assolvents for the obtained BNNSs. The effect ofpre-treatment of h-BN by mechanical grinding on theFig. 1 Schematic representation of this study.
2. EXPERIMENTAL SECTION
2.1 Preparation of BNNSs from h-BN
Two methods to exfoliate BNNSs from h-BN wereperformed. For Method 1, 3 mL of butanol (nacalaitesque) or hexanol (Sigma-Aldrich) was added to 10 mgof h-BN (Sigma-Aldrich, ~1 µm in size) in a glass tube,and then the mixtures were sonicated in a Branson
Advanced Sonifier (Model 250 AA, output power at 30W) with a cooling system using tap water(approximately 15 °C) for 3 h, followed bycentrifugation at 2000 rpm for 25 min to remove
remained h-BN. After that, the products in supernatants
were characterized. For Method 2, 0.1 mL of butanol orhexanol was added to 10 mg of h-BN in a mortar, andthen the mixtures were manually ground for 30 min atambient temperature. It is noted that alcohols werere-added several times into the mixtures during grinding
to make the volume of alcohols constant as much aspossible. After that, the volume of alcohols was adjusted
to 3 mL, and then the mixtures were further ground for 5min. Finally, the mixtures were sonicated andcentrifuged similarly to Method 1, followed by135

Alcoholysis-Assisted Exfoliation of Boron Nitride Nanosheets from Hexagonal Boron Nitride

BuOH

HeOH

h/ nm

h/ nm

50500 nm500 nm

80

70

70

60

60

50

50

Number(b)80

40

40

30

30202010100(c)500 2 4 6 8 10 12 14 16 18 20 22Height/ nm

00 2 4 6 8 10 12 14 16 18 20 2250403030

Number40

2020101000Diameter/ nm

Height/ nm

030
60

9012 0
15018 021 024 0
27030 033 0

3. RESULTS AND DISCUSSION
The colorless and transparent supernatants that
contained the products were obtained under all
experimental conditions. The percent yields of theproducts in supernatants were estimated to beapproximately 30% under all experimental conditions,suggesting that alcohol species and preparation methods
did not affect the total amounts of the products
exfoliated from h-BN.
Fig. 2 shows the morphological analyses of theproducts obtained by Method 1, in which the mechanicalgrinding was not performed before sonication.
Significantly, round-shaped nanosheet-like objects wereobserved from the AFM images, irrespective of alcoholspecies, suggesting that the exfoliation of BNNSs fromh-BN were successfully demonstrated by sonication
treatment of h-BN in primary alcohols. The thickness
and its distribution of the products obtained using
hexanol were smaller than those obtained using butanol.
The mean thickness for hexanol was estimated to beapproximately 3 nm, which corresponded to a 6-layer
structure of h-BN.11 Furthermore, the distribution ofdiameters for hexanol was also smaller than that for
butanol. The mean diameter for hexanol was estimatedto be approximately 130 nm. It was therefore suggested
that hexanol was a more effective reactant to produce
BNNSs with more homogeneous morphologies byMethod 1. Our previous study demonstrated thatsonication-assisted alcoholysis of B–N bonds more
readily occurred in primary alcohols with longer alkyl
chains.10 A possible reason for the greater reactivities isderived from the greater electron densities of hydroxyl
groups for primary alcohols with longer alkyl chains due
to hyperconjugation of alkyl chains. Therefore, thegreater reactivity of hexanol might promote theproduction of morphologically homogeneous BNNSs
from h-BN.

3020100Number

2.2 Characterization of BNNSs
For AFM observations, adequate amounts ofsupernatants were spin-cast on a freshly cleaved mica
substrate at 600 rpm, followed by drying at ambient
temperature. Images were obtained with an SPM-9600
microscope (Shimadzu) in a tapping mode using
standard silicon cantilevers under ambient conditions.
The thicknesses of BNNSs were estimated fromcross-sectional images. The diameters of BNNSs wereestimated by averaging the long- and short-axes ofBNNSs.
For FT-IR absorption spectra, adequate amounts ofsupernatants were cast on a gold-coated poly(ethyleneterephthalate) film. The substrate was set on FT/IR-4100(JASCO) with attenuated total reflection attachments.
Spectra were obtained with the cumulative number of
100 under ambient conditions.

(a)

Numbercharacterization of the products in supernatants. To
estimate the percent yields of the products insupernatants, the product amounts were estimated bysubtracting the amounts of precipitates after
centrifugation from the initial amounts of h-BN. Alcohol
solvents for the precipitates were dried in an oven at110 °C for 24 h. The percent yields were estimated fromexperiments in triplicate.

030
60

90
12 0
15018 021 024 027 030 0
330

136

Diameter/ nmFig. 2 Morphological analyses of the products obtainedin butanol (left) and hexanol (right) by Method 1. (a)AFM images, (b) histograms for thickness, and (c)histograms for diameters.
In order to evaluate the effect of h-BN pre-treatment
on the production of BNNSs, mechanical grinding ofh-BN was performed before sonication as Method 2. Fig.
3 shows the photos of the mixtures composed of h-BN
and alcohols with or without grinding. With grinding,the greater amounts of h-BN were dispersed in alcohols
even after standing for 3 h, suggesting that the size ofh-BN became small through grinding. Fig. 4 shows theAFM images and morphological analyses of theproducts. The thickness of the products obtained in bothalcohols tended to decrease, while the diameter tended
to increase, compared with the products obtained byMethod 1 (see Fig. 2). It is noted that the distributions of+(a)

BuOH

Grinding
HeOH

BuOH


HeOH

(b)

Fig. 3 Photos of the mixtures composed of h-BN andalcohols before and after grinding. Photos were taken (a)just after grinding and (b) after standing for 3 h.

Witita Laosamathikul et al. Trans. Mat. Res. Soc. Japan

BuOH

5h/h/ nm
nm001500µm
nm
nm

500nm
500nm
nm

40404030303020202010Height/ nm

0050403030

Number40

200 2 4 6 8 10 12 14 16 18 20 22Height/ nm

010 0
20030 0
40050 060 070080 0
900

0Diameter/ nm
Diameter/ nm(b) HeOH by Method 1(c) BuOH by Method 2(d) HeOH by Method 2(e) Original h-BN

3500

3000

2500

2000

1500

Wavenumber/ cm-1

1000

500

Fig. 5 FT-IR absorption spectra of the products obtainedin (a) butanol and (b) hexanol by Method 1, the products
obtained in (c) butanol and (d) hexanol by Method 2,and (e) original h-BN.

201010ν(C-H)

4000

0010 0
20030 040 050 0600
70080 0
900

Number(c)50

10100 2 4 6 8 10 12 14 16 18 20 22

ν(B-O-C)
ν(C-O) δ(B-N-B)
ν(B-O)ν(B-N)
δ(O-B-O)

(a) BuOH by Method 1

Number50
50

Number(b)50

0round-shape, directional unzipping for the production ofBNNRs from BNNTs10 appeared to hardly proceed forh-BN. There must be many alkyl groups derived fromprimary alcohols at least at the edge of BNNSs. Moredetailed structures of BNNSs will be analyzed in thenear future.

HeOH

Absorbance (a.u.)

30
1020
5
1000h/h/ nm
nm

(a)

137

42[5] 135-138 (2017)

Diameter/ nmFig. 4 Morphological analyses of the products obtainedin butanol (left) and hexanol (right) by Method 2. (a)AFM images, (b) histograms for thickness, and (c)histograms for diameters.
thickness and diameter for hexanol became large,compared with the products obtained by Method 1. The
mean thickness and diameter for butanol were estimatedto be approximately 4 nm and 260 nm, respectively. It
was therefore found that mechanical pre-treatment ofh-BN has potentials for controlling the morphologies of
BNNSs exfoliated by sonication processes.
The chemical structures of the products werecharacterized by FT-IR absorption spectroscopy. Fig. 5
shows the spectra for all the products. In all spectra, twoabsorption peaks were observed at around 1260 and 723
cm-1, which can be assigned to B–N stretching vibration
and B–N–B bending vibration bands of h-BN,
respectively.12 Absorption peaks were also observed ataround 1720, 1105, 1021, and 510 cm-1. These peaks canbe assigned to B–O stretching, C–O stretching, B–O–C
stretching, and O–B–O bending vibration bands,respectively.11,13-16 Absorptions in the region of2920-2850 cm-1 can be assigned to C–H stretchingvibration bands. These peaks were similarly observed
for the chemical peeling of BNNTs throughsonication-assisted alcoholysis of BNNTs in primaryalcohols.10 All these observations strongly suggested
that the production of BNNSs was originated fromalcoholysis of B–N bonds of h-BN in primary alcohols
during sonication processes. It was therefore found thatsonication-assisted alcoholysis proceeded similarly for
h-BN with a flake structure, which was different from
BNNTs. However, since the obtained BNNSs had

4. CONCLUSION
Sonication of h-BN, which was pre-treated with orwithout mechanical grinding, in butanol and hexanol
exfoliated BNNSs with nanometer thicknesses anddiameters of several hundreds of nanometers. The
percent yields of BNNSs were estimated to beapproximately 30%, which were irrespective of alcohol
species and preparation methods. AFM revealed that themorphologies of BNNSs were dependent on alcohol
species and preparation methods. FT-IR absorption
spectroscopy showed the representative peaks assigned
to the chemical bonds for alcoholysis of B–N bonds. The
present sonication-assisted alcoholysis of h-BN will bepotentially useful for simple production of functional
BNNSs.
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(Received July 26, 2017; Accepted September 3, 2017;
Published Online October 1, 2017)