Efficient catalytic conversion of ammonia borane to borazine and its use for hexagonal boron nitride (white graphene)

Article abstract of DOI:10.1039/c2ta00758d

Nickel nanoparticles (NiNPs) prepared in tetraglyme (TG) efficiently catalyzed the conversion of ammonia borane (AB, NH₃BH₃) to borazine (B₃N₃H₆). Under the optimized conditions, 3 mol% of the NiNPs were introduced into a 1.5 M AB solution in TG and held at 80 °C for 6 h under a dynamic vacuum that was maintained at 30 torr. Borazine was isolated through a series of -45 °C, -78 °C, and -196 °C traps to give (-78 °C trap) pure borazine in 53% yield. The borazine produced was then utilized as a molecular precursor for high quality h-BN (white graphene) and large area h-BN sheets were prepared by applying low pressure chemical vapor deposition (LPCVD). Ultra-thin (single to few layers) h-BN was synthesized on Ni foil at the optimal ratio between borazine and NH₃, and the number of layers was tuned by varying the NH₃ partial pressure. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c2ta00758d

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Materials Chemistry A
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Efficient catalytic conversion of ammonia borane to
borazine and its use for hexagonal boron nitride
Cite this: J. Mater. Chem. A, 2013, 1,
1976
(white graphene)†
b
Sung-Kwan Kim,‡a Hyunjin Cho,‡bd Myung Jong Kim, Hee-Jun Lee,a
*
Jin-hyung Park,a Young-Boo Lee,c Hwan Chul Kim,d Chang Won Yoon,e
e
a
*
Suk Woo Nam and Sang Ook Kang
Nickel nanoparticles (NiNPs) prepared in tetraglyme (TG) efficiently catalyzed the conversion of ammonia
borane (AB, NH3BH3) to borazine (B3N3H6). Under the optimized conditions, 3 mol% of the NiNPs were
introduced into a 1.5 M AB solution in TG and held at 80 ꢀC for 6 h under a dynamic vacuum that was
maintained at 30 torr. Borazine was isolated through a series of ꢁ45 ꢀC, ꢁ78 ꢀC, and ꢁ196 ꢀC traps to
give (ꢁ78 ꢀC trap) pure borazine in 53% yield. The borazine produced was then utilized as a molecular
precursor for high quality h-BN (white graphene) and large area h-BN sheets were prepared by applying
low pressure chemical vapor deposition (LPCVD). Ultra-thin (single to few layers) h-BN was synthesized
on Ni foil at the optimal ratio between borazine and NH3, and the number of layers was tuned by
varying the NH3 partial pressure.
Received 19th October 2012
Accepted 30th November 2012
DOI: 10.1039/c2ta00758d
www.rsc.org/MaterialsA
sintering process appears to be challenging. Therefore, as an
alternate approach, the precursor route has been suggested.8 In
Introduction
this context, borazine (B3N3H6)9 and ammonia borane (AB,
NH3$BH3)10 were reported to be promising precursors for h-
BN.11 Most of all, the AB molecule is regarded as a promising
candidate to fulll the requirements for a high-density, air
stable, and easy-to-handle BN source material. With its high BN
content of 80.4 wt% and the release of (BHNH)n from AB with a
BN content of 92.5 wt% at 150 ꢀC, it possesses outstanding
qualities for this application. In this respect, one important
obstacle to using pure AB is its exothermic dehydropolymeri-
zation (e.g. polyaminoborane or cross-linked BN) generating
hydrogen, borazine and aminoborane at 60–180 ꢀC, which
would limit its use in the chemical vapor deposition process for
h-BN.12
Boron nitride has many important applications particularly in
high temperature structural and functional materials.1 Among
the various boron nitride structures, hexagonal boron nitride
nanosheets (h-BN, also known as white graphene) have attrac-
ted much attention since the discovery of graphene in 2004.2
Although h-BN shows similarly excellent mechanical and
thermal properties, its electrical properties are quite different
from those of graphene;3 while graphene is an electrical
conductor, h-BN is an insulator with a direct band-gap of ꢂ6 eV.
Similar to graphene, its potential application covers a broad
area, such as gate dielectric layers,4 thermal management,5 light
emitters,6 and transparent and protective coatings.7 However,
preparation of h-BN through the conventional high temperature
On the other hand, borazine has an advantage in that it
represents a 1 : 1 stoichiometric ratio of boron and nitrogen
atoms constituting a single precursor and has a high enough
vapor pressure. From the recent work on the formation of
borazine from AB in weakly polar solvent diglyme, Autrey et al.
reported possible ꢁtraces of DADB (diammoniate of diborane,
aDepartment of Advanced Materials Chemistry, Korea University, Sejong, Chungnam
339-700, South Korea. E-mail: sangok@korea.ac.kr; Fax: +82 44-867-5396; Tel: +82
44-860-1334
bSo Innovative Materials Research Center, Korea Institute of Science and Technology,
Eunha-ri San 101, Bongdong-eup, Wanju-gun, Jeollabuk-do 565-905, South Korea.
E-mail: myung@kist.re.kr; Fax: +82 63-219-8249; Tel: +82 63-219-8135
cKorea Basic Science Institute, 643-18 Keumam-dong, Dukjin-gu, Jeonju, 561-180,
South Korea
+
ꢀ
[BH2(NH3)2] [BH4] ) in a 2 M solution at 50 C which acceler-
ated the formation of borazine.13 In highly concentrated AB–
triglyme solutions, the kinetics of this borazine formation
process was increased by a higher concentration of DADB and
BCDB (B-(cyclodiborazanyl)aminoborohydride, cyc[NH2BH2–
NH2BH]–NH2BH3).14
dDepartment of Organic Materials and Fiber Engineering, Chonbuk National
University, Jeonju, 561-756, South Korea
eFuel Cell Center, Korea Institute of Science and Technology, Hawolgok-dong 39-1,
Seongbuk-gu, Seoul 136-791, South Korea
Sneddon and Wideman investigated the production of bor-
azine from AB in tetraglyme (TG) solution with the aim of
developing procedures that were both efficient and reliable and
† Electronic supplementary information (ESI) available: Experimental details of
TEM images of NiNPs and NMR data of the borazine. See DOI: 10.1039/c2ta00758d
‡ These authors contributed equally.
1976 | J. Mater. Chem. A, 2013, 1, 1976–1981
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could be readily accomplished with normal laboratory equip-
ment.15 With small modications, the basic Callery batch
procedure can be adapted for laboratory scale preparations, and
they prepared 10–20 g of borazine over a period of 3–4 h in 67%
yield at 140–160 ꢀC (Scheme 1).
Scheme 3 Preparation of borazine from AB by metal nanoparticles.
In spite of the simplicity of the thermal reaction conditions
applied, special care should be taken especially for the large
scale synthesis due to its high reaction temperature of 140–160
ꢀC. Consequently, it is desirable to nd a suitable catalyst for
the reaction to reduce the reaction temperature and to bring the
reaction to completion in a shorter period of time.16
Scheme 4 Synthesis of h-BN nanosheets from a borazine precursor using low
pressure chemical vapor deposition (LPCVD).
In this regard, thus far, several efficient catalyst systems have
been reported involving rhodium,17 iridium,18 platinum,19
palladium,20 ruthenium,21 and nickel.22 However, Manners et al.
reported that the isolation of borazine from the reaction
mixture proved to be difficult; pure borazine was isolated in
only ca. 10% yield with the major products being non-volatile,
oligomeric species.17a If the synthetic conditions are improved,
the metal catalysis may be advantageous, which is signicant as
borazine has been shown to be a useful precursor for BN
materials.
As described in a previous report, we found that TG was an
efficient growth source for the synthesis of Pd nanoparticles
(NPs).23 This PdNP catalyst was an efficient catalyst for the
synthesis of borazine from AB. However, it has been noted that
PdNPs were too active in converting AB to borazine and they
even activated the produced borazine to polyborazylene which
lowered the overall borazine yield. Therefore, more selective
and yet economical catalysts are highly sought aer. In this
study, by adopting the same solvent-mediated nanoparticle
synthesis, the related NiNPs were also conveniently prepared
from TG solution. The respective nanocatalyst was generally
produced by the thermal reaction in TG solvent without
surfactants and reductants (Scheme 2). The nanocatalyst could
easily be re-dispersed in TG which was used for the synthesis of
borazine. These highly reactive and convenient systems are
attractive due partly to their heterogeneous nature, which
facilitates the separation of the product from the catalyst, and
perhaps more importantly due to their relatively low cost and
ease of synthesis.
chemical vapor deposition (LPCVD) (Scheme 4). The h-BN
formed on the Ni catalyst foil showed a highly crystalline
structure with minimal impurities.
Experimental section
Materials
All experiments were conducted under a nitrogen atmosphere
using Schlenk techniques or in an HE-493 dry box (Vacuum
Atmosphere Co., Hawthorne, CA, USA). Nickel acetate tetrahy-
drate (Ni(OAc)2$4H2O) was purchased from Aldrich. TG was
purchased from Alfa Aesar. AB was prepared as described in the
literature.24 Solvents were dried over Na (TG) below 100 ꢀC and
CaH2 (methanol) and distilled under nitrogen. Deuterated
solvents were dried through trap-to-trap distillation from CaH2
(CDCl3) and Na (C6D6) and deoxygenated using three freeze–
pump–thaw cycles.
Synthesis of NiNPs
NiNPs were synthesized from Ni(OAc)2$4H2O (1). 1 (0.2 g, 0.3
mmol) was added to 20 mL of TG and held at 250 ꢀC for 3 h. The
color of the solution changed from green to black. The product
was separated by centrifugation. The resulting residue was
washed with methanol (50 mL) twice, yielding the NiNPs.
Synthesis of NiNPs deposited on Vulcan XC-72R (NiNPs/VC)
NiNPs/VC were synthesized from Ni(OAc)2$4H2O (1) and Vulcan
XC-72R (VC). 1 (0.2 g, 0.3 mmol) was added to 20 mL of TG
The potential for TG mediated Ni nanoparticles to act as
readily available heterogeneous catalysts provided us with a
strong incitement to investigate their use in the preparation of
borazine (Scheme 3). From the borazine synthesized from AB by
Ni nanocatalysts, h-BN was synthesized by low pressure
ꢀ
containing VC (0.17 g) and held at 250 C for 3 h. The product
was separated by centrifugation. The resulting residue was
washed with methanol (50 mL) twice, yielding 10 wt% NiNPs/
VC.
Preparation of borazine from AB using NiNP catalysts
Powdered AB (23.0 g, 747 mmol) dissolved in TG (500 mL) was
added to a 2 L, three-necked, round-bottom ask tted with a
thermometer, a reux condenser, and a solid addition funnel
lled with NiNPs (1.31 g, 22.4 mmol, 3 mol%). The exit of the
Scheme 1 Synthesis of borazine from AB.
ꢀ
ꢀ
condenser was connected through a series of ꢁ45 C, ꢁ78 C,
ꢀ
and ꢁ196 C traps. The system was evacuated and lled with
nitrogen gas several times. The NiNPs were added to the ask
ꢀ
and the reaction mixture was gradually warmed to 80 C over
Scheme 2 TG mediated synthesis of Ni nanoparticles from Ni acetate.
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the course of 30 min and held at this temperature for 6 h under
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a dynamic vacuum that was maintained at 30 torr by the BN nanosheets, respectively. The NBED measurements were
continuous removal of the evolved hydrogen and borazine performed using a JEM-2200FS microscope operated at 200 kV
through the vacuum line. Following the reaction, the borazine with a 5 mm condenser aperture, which gives rise to a semi-
ꢀ
that had been retained in the ꢁ78 C trap was further puried convergence angle of under 0.1 mrad.
by a trap-to-trap distillation to give 10.6 g (131.9 mmol, yield
53.0%). Aer completion of the reaction and isolation of bor-
Results and discussion
azine, the 11B NMR spectrum of the remains of the reaction
In this study, rst, we investigated the potential utility of TG as a
growth source for producing NiNPs for organonickel complexes.
Herein, we present a facile TG-mediated synthesis of NiNPs in
the absence of extra reducing agents or a capping agent. Among
the various nickel precursors, Ni(OAc)2$4H2O (1) showed the
best TG solubility and therefore was used for the synthesis of
mixture
showed
B-(cyclodiborazanyl)aminoborohydride
(BCDB), as indicated by the peaks at ꢁ5.0, ꢁ10.9, and ꢁ24.0
ppm, cyclotriborazane (CTB) at ꢁ10.0 ppm, and polyborazylene
at 25–30 ppm.14
h-BN growth procedure
ꢀ
the NPs. At 250 C, 1 was quantitatively converted to NiNPs in
Hexagonal boron nitride nanosheets (h-BN) called white gra-
phene were synthesized using low pressure chemical vapor
deposition (LPCVD). The nickel thin lm (25 mm thick, Alfa
Aesar) was used as the metal catalyst substrate and the Ni lm
was treated by chemical mechanical polishing. Then, the metal
catalyst substrate was placed at the middle position in a small
quartz tube in the CVD system. The furnace of the CVD system
was heated up to 1000 ꢀC at a rate of 10 ꢀC minꢁ1 under a
hydrogen atmosphere with a ow rate of 9 sccm with a pressure
of 118 mtorr. Before synthesizing h-BN, the nickel lm was
annealed at 1000 ꢀC for 30 min under the same conditions when
the CVD furnace was heated in order to make the nickel grains
grow and obtain a smooth surface. Aer annealing, the ow of
hydrogen gas into the quartz tube in the CVD system was
stopped. Subsequently, h-BN was synthesized for 10 minutes
using borazine and ammonia under high vacuum conditions by
operating a turbo pump. The operating pressure was approxi-
mately 1–5 mtorr. Then, 1–12 sccm of borazine and 1–10 sccm
of ammonia were introduced simultaneously in order to form
hexagonal BN structures. Aer synthesizing h-BN, the ow of
borazine and ammonia gas into the quartz tube in the CVD
system was stopped. Subsequently, the furnace of the CVD
system was cooled down to room temperature under a hydrogen
atmosphere with a ow rate of 9 sccm.
the presence of TG.
The thermal decomposition of 1 has been the subject of
several previous studies.25 On linear heating, 1 rst releases
water at ca. 120 ꢀC. On continued heating, the subsequent
decomposition of 1 at ca. 340 ꢀC leads directly to the formation
of either NiO or Ni.26 It was also reported that the evolution of
some CH3COOH occurs and, on continued heating, anion
breakdown occurs yielding CO2, CO, acetic anhydride and
acetone.27
In Fig. 1(a), the TEM analysis of the NiNPs prepared in TG at
250 ꢀC showed spherical 5 nm size nanoparticles. On the other
hand, agglomerates in the size range of 100–200 nm coexisted
with the smaller 5 nm size NiNPs (Fig. S1†). This is explained by
the weak binding of TG on the surface of the NiNPs.
The diffraction peaks at 2q values of 39.1ꢀ, 41.5ꢀ, 44.5ꢀ, 58.5ꢀ,
71.1ꢀ, and 78.0ꢀ in Fig. 1(b) are assigned to the (010), (002),
(011), (012), (110), and (103) planes of hcp Ni (JCPDS no. 45-
1027). On the other hand, small peaks are observed at 52.0ꢀ and
76.4ꢀ which are assigned to the (200) and (220) planes of fcc Ni
(JCPDS no. 04-0850), respectively. It should be noted that the
main peak (111) of fcc Ni overlaps with that of the hcp Ni (011).28
These screened NiNPs were found to promote the formation
of borazine relative to the background reaction, as shown in
Fig. 2(a).29 The reaction of AB was found to proceed vigorously at
low temperature (80 ꢀC) when the NiNPs were introduced. It
took 6 h to complete the reaction at 80 ꢀC with the NiNPs.
Borazine was obtained in 53% yield aer purication, which is a
substantial improvement over the yield obtained when the
Transfer
Once the synthesis was completed, h-BN was transferred onto
the various substrates and TEM grids for analysis with the
graphene transfer method. PMMA (polymethyl methacrylate)
was spun on the h-BN/nickel foil at 4200 rpm for 40 seconds,
followed by etching the nickel foil with nickel etchant (FeCl3).
The lm of PMMA/h-BN was washed three times with deionized
water for 20 minutes each. Then, the PMMA/h-BN lm was
transferred onto a 300 nm SiO2/Si substrate and nally PMMA
was removed using acetone.
ꢀ
reaction was performed without catalysts at 80 C (19% yield).
The NiNPs, besides increasing both the rate and conversion of
Characterization
h-BN was analyzed with a scanning electron microscopy instru-
ment (S-4700, HITACHI) and a Raman microscopy instrument
(Horiba). A transmission electron microscope instrument (JEM-
2200FS) in conjunction with electron energy loss spectroscopy
(EELS) and nano-beam electron diffraction (NBED) was used in
Fig. 1 (a) TEM image of NiNPs prepared from Ni(OAc)2$4H2O (1). Scale bar ¼ 20
order to analyze the chemical elements and structure of the h- nm. (b) XRD pattern of NiNPs prepared from 1.
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Fig. 3 (a) Conversion of AB to borazine by NiNPs and NiNPs/VC. (b) TON (turnover
number: mol of borazine/mol of Ni) for NiNPs and NiNPs/VC (10 wt% Ni).
Fig. 2 (a) Conversion of AB to borazine by NiNPs and catalyst recycle (inset). (b)
Attraction of NiNPs by a magnetic bar in reaction solution.
performance, with an initial turnover frequency (TOF) of 62 hꢁ1
as compared to 7 hꢁ1 for the metallic catalyst NiNPs, as shown
in Fig. 3(b).
AB to borazine, were found to have a highly benecial effect of
signicantly reducing foaming during borazine synthesis. This
effect resulted from the feature that NiNPs play a role as boiling
chips.
To understand the underlying mechanistic steps responsible
for the formation of borazine by the NiNPs, we coupled the
synthesis experiments with ex situ 11B NMR spectroscopy as
shown in Fig. S2.† The 11B NMR analysis of the reaction
mixtures in TG demonstrated the initial formation of BCDB.
Following the initial growth of BCDB, we observed new reso-
nances due to borazine. Further dehydrogenation showed the
simultaneous decay of BCDB and formation of borazine, as
identied by Sneddon et al.30 and Shaw et al.13b
Our synthesis method of NiNPs is convenient, requiring only
a single chemical reagent. We believe that their broad size
distribution and irregular shape come from the weak interac-
tion between the nickel metals and TG. It has been suggested
that the catalytic activity is dependent on the number of low
coordination number sites available on the surface of the
nanoparticles and irregular NiNPs bear more of these sites than
atter and more faceted ones.31 If these low coordination
number corners and/or edge positions are implicated as active
sites for the activation of AB, then their relatively higher
occurrence in the NiNPs generated in this study could account
for their superior catalytic performance.
An advantageous feature of the use of NiNPs is their recy-
clability in that they can be collected using a magnet aer the
reaction. As can be seen in Fig. 2(b), the dark-colored dispersion
turned transparent aer 5 min, while all of the particles
assembled at the side of the ask next to the magnet. Aer the
catalytic dehydrogenation and separation of the NiNPs, a visible
layer appeared to coat the catalyst. This coating was removed by
simple immersion in methanol and the NiNPs retrieved from
the solvent exhibited close to their original catalytic activity,
which was sustainable up to the h cycle without any
discernible catalytic loss as shown in Fig. 2(a).
With this convenient AB to borazine route in hand, we
pursued precursor routes for the synthesis of h-BN and low
pressure CVD (LPCVD) was adopted for the sake of improving
the controllability. LPCVD has an advantage over APCVD
(atmospheric CVD) in that it provides for a uniform lm
thickness stemming from the surface reaction limited
kinetics.32 Ammonia (NH3) gas was used not only for controlling
the kinetics or achieving amorphous etching, but also for the
purpose of providing extra nitrogen during the growth process
due to the limited solubility of nitrogen in the Ni catalyst.
Though borazine provides boron and nitrogen with a 1 : 1 ratio,
the ability of Ni to dissolve each atom is different. Solubility of
boron in Ni is much higher than that of nitrogen. Considering
h-BN formation occurs at the Ni surface, highly concentrated
nitrogen has better chance to combine with dissolved boron by
diffusion to the surface.33 Also, this tendency was observed in
BN nanotube CVD synthesis with borazine. Boron nitride could
not be synthesized without NH3, which is an extra source of
nitrogen.34
As shown in Fig. 4, single to few-layered h-BN was synthe-
sized at the optimized ratio between borazine and ammonia
(borazine: 1 sccm, NH3: 8 sccm). From the TEM images taken
from the edges of h-BN, the wall structures of h-BN were clearly
identied and the interlayer spacing of h-BN was ꢂ0.34 nm.
In order to obtain the diffraction pattern from clean h-BN,
the nano-beam electron diffraction (NBED) method was used.
The NBED method has been proven to be a useful tool for the
structural analysis of nanomaterials.35–38 NBED refers to
Moreover, the connement of the NiNPs in ordered carbon
support materials induces high catalytic activity. It is note-
worthy that 10 wt% NiNPs/Vulcan XC-72R (metal loading 0.3
mol%) show similar catalytic activity to that of the NiNPs (metal
loading 3.0 mol%) by themselves (Fig. 3(a)). These results
suggest that NiNPs on a carbon support bear more active sites
than the NiNPs, which nally results in enhanced catalytic Fig. 4 TEM images of (a) single-layered and (b) few-layered h-BN.
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Fig. 5 (a) Nanobeam electron diffraction (NBED) data and (b) EELS data of h-BN.
Fig. 7 TEM images of grown h-BNs at different flow rates of (a) 2 sccm, (b) 4
acquiring a diffraction area of the size of a few nanometers
while maintaining a small convergence angle (<0.1 mrad).
As shown in Fig. 5(a), the grown h-BN has an unambiguous
hexagonal structure with a [0001] orientation. The EELS data in
Fig. 5(b) show sharp boron and nitrogen K edges that are
sccm, (c) 6 sccm, and (d) 8 sccm.
Conclusions
characteristic of the sp2 hybridized atoms of boron nitride, Pure borazine was effectively synthesized by an efficient method
while the carbon K edge resulting from contamination is at the utilizing NiNPs. The lower synthetic temperature proved the
noise level.
catalytic action of the NiNPs. The use of borazine as a stable
h-BN grown on Ni foil could be transferred to various precursor in conjunction with LPCVD provides a high degree of
substrates including Si wafers, glass substrates, and PET using a control for the synthesis of h-BN. Considering the TEM, EELS,
similar method to that reported for graphene.39 The piece of Si NBED, and Raman spectrum, high quality h-BN with a large
wafer, onto which the h-BN was transferred, developed a area was synthesized on Ni foil, and the number of layers could
whitish color indicating a large band gap, as shown in Fig. 6(a). be tuned by varying the NH3 partial pressure.
The Raman spectrum was obtained from the h-BN/SiO2/Si (h-BN
on SiO2/Si wafer) sample. The thickness of the h-BN for Raman
Acknowledgements
analysis was ꢂ2.7 nm measured by AFM. It clearly showed a
peak at 1366.5 cmꢁ1 indicating the E2g mode stretching of
This work was supported by a grant from the Korea Institute of
hexagonal boron nitride.40 No peaks indicating the presence of
BN soot (1322–1350 cmꢁ1) or c-BN (1304 cmꢁ1) were observed,
as shown in Fig. 6(b).
Science and Technology (KIST) Institutional Program and the
Converging Research Center Program funded by the Ministry of
Education, Science and Technology (2012K001429). This work
was also supported by the New & Renewable Energy Technology
Development Program of the Korea Institute of Energy Tech-
nology Evaluation and Planning (KETEP) grant funded by the
Korea government Ministry of Knowledge Economy
(20113030040020). We also acknowledge the Korea Basic
Science Institute (KBSI) for TEM experiments.
Based on the advantage of borazine as a single chemical
precursor, layer by layer control was attempted by varying the
NH3 gas ow rate. While the borazine ow rate was xed at 1
sccm, the NH3 ow rate was regulated at 2, 4, 6, or 8 sccm for
each experiment. As shown in Fig. 7, there was an observed
tendency that the number of layers gradually decreased with
increasing ow rate of NH3. However, this does not mean that
we have absolute layer by layer control over a large area,
because the growth rate of h-BN differs in each grain of the Ni
foil. Absolute control over the number of layers is still an open
issue.
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