Large-scale fabrication of boron nitride nanohorn

Article abstract of DOI:10.1063/1.2009056

Boron nitride nanohorns (BNNHs) are synthesized in large scale. Their morphology and structure were investigated by scanning electron microscopy and transmission electron microscopy. The hollow conical structure and particular aggregation behavior are rev

Full text of DOI:10.1063/1.2009056

Large-scale fabrication of boron nitride nanohorn
Chunyi Zhi, Yoshio Bando, Chengchun Tang, Dmitri Golberg, Rongguo Xie, and Takashi Sekiguchi
Citation: Applied Physics Letters 87, 063107 (2005); doi: 10.1063/1.2009056
View online: http://dx.doi.org/10.1063/1.2009056
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APPLIED PHYSICS LETTERS 87, 063107 ͑2005͒
Large-scale fabrication of boron nitride nanohorn
Chunyi Zhi,a͒ Yoshio Bando, Chengchun Tang, and Dmitri Golberg
Advanced Materials Laboratory, National Institute for Materials Science (NIMS), Namiki 1-1, Tsukuba,
Ibaraki 305-0044, Japan
Rongguo Xie and Takashi Sekiguchi
Nanomaterials Laboratory, National Institute for Materials Science. Namiki1-1, Tsukuba, Ibaraki 305-004,
Japan
͑Received 25 April 2005; accepted 22 June 2005; published online 1 August 2005͒
Boron nitride nanohorns ͑BNNHs͒ are synthesized in large scale. Their morphology and structure
were investigated by scanning electron microscopy and transmission electron microscopy. The
hollow conical structure and particular aggregation behavior are revealed. Cathodoluminescence
measurement is performed and ultraviolet light emission is observed, which indicates the potential
applications of BNNHs in optical devices. © 2005 American Institute of Physics.
͓DOI: 10.1063/1.2009056͔
Since the electronic properties of boron nitride nanotube
͑BNNT͒ have been predicted to be independent of various
morphological and/or geometrical factors,1 Boron nitride
͑BN͒ nanostructures continuously attract significant interest.
In fact, BN nanomaterials have excellent mechanical
properties,2 a high resistance to oxidation,3 and chemical sta-
bility, which makes them highly valuable in electronic de-
vices as far as their usage at elevated temperatures or in
hazardous environments is concerned. Following the suc-
cessful synthesis of BNNTs,4,5 the search for related BN
nanostructures has been initiated.6–8 A cone-shaped structure
is of special interest since the deviations from a flat graphitic
surface are accompanied by the appearance of topological
defects located at its apex.9,10 Recently, BN conical nano-
tubes have been found in a material containing large quanti-
ties of BNNTs.11 Their structures were explained based on
the model of orderly stacked 240° disclinations ͑a disclina-
tion angle is defined as the angle of the sector removed from
a flat sheet to form the cone͒, which is the smallest cone
geometry ensuring the presence of B–N bonds only.12 The
conical nanotubes have a solid core and are made of stacked
BN nanocones or formed by a continuous BN sheet wrapped
up in a helical fashion. However, this novel nanostructure
was only a byproduct of the BNNT synthesis and its yield
was marginally low, which make it is impossible to investi-
gate the properties of this nanostrucuture.
It is noteworthy that the nanohorns have been discovered
in graphitic carbon.13 Keeping in mind the unique structural
similarities between C and layered BN, it is natural to expect
the existence of such nanostructure in BN. However, to date,
reports about such structure are overlooked in the literature.
Here, we report large-scale synthesis of an intrigue BN nano-
structure: BN nanohorns ͑BNNHs͒. The BNNHs exhibit spe-
cific aggregation behavior and possess a hollow conical
structure. Their morphology and structure were investigated
by scanning electron microscopy ͑SEM͒ and transmission
electron microscopy ͑TEM͒. The synthesis in large scale
makes it possible to measure the properties of this kind of
nanostructure. Cathodoluminescence ͑CL͒ measurement is
performed and ultraviolet light emission is observed, which
indicates the potential applications of BNNHs in optical de-
vices.
An induction furnace was used to synthesize BNNHs. In
a typical run, a mixture of MgO and boron powder was
heated in a BN crucible to 1700 °C. The heating yields a Mg
vapor and B2O2,5 which are transported by an Ar gas and
meet with ammonia. During heating, a white-colored smoke
was observed coming out of a crucible and adhering to a
quartz tube. After synthesis over 2 h, approximately 30 mg
of a white-colored product were collected from the inner
surface of a quartz tube.
Figure 1͑a͒ presents the SEM image of our product. It
can be seen that the product contains numerous hollow coni-
cal structures, resembling horns. The diameters of their bot-
tom parts are typically 100–200 nm; the length varies from
500 nm to 1 ␮m. An encapsulated particle can be observed
in the bottom part of each BNNH, which is proven to be
MgO by energy dispersive spectroscopy during TEM. The
BNNHs were sonicated in alcohol for 30 min before being
mounted onto a clean silicon wafer and a carbon-film-coated
copper grid for detailed SEM and TEM investigations. After
dispersion, the specific horn aggregation was revealed by
SEM, as shown in Fig. 1͑b͒. The tips of BNNHs prefer to
stick together; this assembles the BNNHs into a flowerlike
nanostructure. During TEM, the aggregation behavior was
displayed more clearly, as shown in Fig. 1͑c͒. The flowerlike
structure is fairly tough and stable under various operations.
For example, more than 80% of BNNHs are still assembling
into the flowerlike morphology even after 30 min of sonica-
tion. This indicates the strong interactions and the formation
of chemical bonds between the adjacent BNNH tips. The
detailed structure of the two particular interacting BNNH
tips was investigated by high-resolution TEM ͑HRTEM͒.
The result is shown in Fig. 1͑d͒. Two tips have independent
BN graphiticlike layers. The structure may be sketched as a
schematic shown in inset B ͑different from that shown in
inset A͒. The observations suggest that the chemical bonding
between tips forms after the formation of tips.
Different types of tips are shown in Fig. 2. Figure 2͑a͒ is
a BNNH with the tubular tip, the diameter of a five-walled
BN tube is approximately 7 nm. Figure 2͑b͒ is a BNNH with
the triangular tip. Some transverse layers are found in the tip
part of BNNHs, while the body of BNNH has a hollow struc-
a͒
Electronic mail: zhi.chunyi@nims.go.jp
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0003-6951/2005/87͑6͒/063107/3/$22.50 87, 063107-1 © 2005 American Institute of Physics
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063107-2
Zhi et al.
Appl. Phys. Lett. 87, 063107 ͑2005͒
FIG. 1. ͑a͒ SEM image of as-grown BNNHs. ͑b͒ SEM
image of flowerlike structures of BNNHs, the inset is an
isolated “flower” formed by two BNNHs. ͑c͒ TEM im-
age of BNNHs, the inset shows the aggregation of two
BNNHs. ͑d͒ HRTEM image of the two interacting tips
of BNNHs.
ture, as shown in Fig. 3. The wall thickness of the nanohorns
is fairly uniform, always being approximately 20 nm, in spite
of the marked difference in the BNNH sizes. The structure of
a BNNH is not as perfect as a BN cone.11 Varieties of dis-
clinations appear in each BNNH. This is supposed to be
induced by a nonlinear increase of the catalyst size during
the growth. Inset A of Fig. 3 is a selected area electron dif-
fraction pattern ͑SAED͒ obtained from the small tip area. A
single disclination is indicated by the two BN002 reflections
separated by an angle, which is caused by the scattering from
the two opposite sides of the horn’s wall parallel to an inci-
dent electron beam. The BN002 reflections become arcs when
a larger tip area is selected, as shown in inset B, which indi-
cates that several disclinations coexist within a single tip.
Most disclination angles appear in the BNNH is in the range
of 20°–40°.
oxidized to a solid MgO ͑the melting point of MgO is ap-
proximately 2800 °C, while the temperature in the growth
area is less than 1700 °C͒ and then loses its catalytic activity.
This results in the short lengths of BNNH. As the ambient
temperature decreases dramatically, when the BNNHs escape
from a crucible with a vapor flow, a Mg drop enlarges due to
the aggregation and the dimensions of a BNNH’s bottom
increase accordingly. The simultaneous growth of a nano-
structure and a catalytic particle leads to the formation of a
horn-shaped structure. The tips of BNNHs are prone to nu-
merous defects and dangling bonds.12 They are apt to bond
with each other to finally form a flowerlike structure.
CL measurements are performed using a field emission
SEM. The spectra obtained from two different sites of the
sample are shown in Fig. 4. Both of the spectra are collected
from several BNNHs, which are dominated by peak at 410
and 436 nm, which corresponds to 3.0 eV and 2.8 eV, respec-
tively. The electronic structure of BNNH has been investi-
gated in some theoretical works.15,16 The highest occupied-
lowest unoccupied molecular orbital gap of B31N31 nanohorn
was given to be 0.8 eV by molecular orbital calculation,15
while a more reasonable band gap at around 3.0 eV is given
by density function theory calculation.16 Since BNNTs have
a constant band gap independent of their geometrical prop-
erties, the variable CL peak position for different BNNHs is
thought to be induced by variable impurities, defects, or B
and N vacancies in each BNNH. The CL peaks of BNNTs
are centered around 3.3 eV,17,18 which is attributed to defects,
or B or/and N vacancies. So, a similar band structure is ex-
pected for BNNHs and BNNTs.
The growth process of the BNNHs determines their re-
sultant morphology and microstructure characteristics. The
formation of BNNH can be explained in the frame of a
vapor-liquid-solid mechanism.14 It is believed that Mg is a
catalyst for the growth of BNNH. The growth is hampered
very soon after the synthesis starts, because Mg is quickly
In summary, BNNHs have been synthesized in large
scale. The yield during a single experimental run may reach
several tens of milligrams. The tips of BNNHs prefer to stick
together to organize BNNHs as a flowerlike structure. A typi-
cal BNNH has a conical hollow structure and several bridg-
ing layers are found in its tip parts. Ultraviolet light emission
is observed when BNNHs are excited by electrons, which
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FIG. 2. ͑a͒ Tubular tip and ͑b͒ triangular tip of a BNNH.
indicated BNNHs may have applications in optical devices.
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063107-3
Zhi et al.
Appl. Phys. Lett. 87, 063107 ͑2005͒
FIG. 4. CL spectra of BNNHs obtained from different sites of the sample.
Both of them are collected from several BNNHs. The spectra are shifted
vertically.
gives the possibility for the prospective usage of BNNHs as
hydrogen ͑or lithium͒ storage medium.
The authors thank Dr Y. Umemura, Dr. M. Mitome, K.
Kurashima, and B. D. Liu for their cooperation and kind
help.
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FIG. 3. TEM image of a single BNNH. The insets A and B are SAED
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