# Highly Ordered Boron Nitride Nanotube Arrays  with Controllable Texture from Ammonia Borane by Template-Aided Vapor-Phase Pyrolysis

Authors: Yuting Wang; Yasunori Yamamoto; Hajime Kiyono; Shiro Shimada
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

An efficient approach for the preparation of good-quality boron nitride nanotubes (BNNTs) is developed. BNNTs with specific texture were prepared from ammonia borane (BH3NH3) by vapor-phase pyrolysis with the aid of a template in two independent temperature-controlled furnaces. Two kinds of BNNTs, 200–300 nm wide ×60 μm long and 70–80 nm wide ×40 μm long, were produced after removal of the templates. The as-produced BNNTs were heated at different temperatures in the range of 1300–1700C∘ in NH3. FT-IR and XPS results confirmed the formation of BN from BH3NH3. Ordered arrays of BNNTs without cracks on the surface were seen using microstructural observations. The diameter and length of the BNNTs are controlled using templates with different pore sizes and thickness. The wall thickness of the nanotubes was increased by increasing the number of deposition cycles. The crystallinity of the BNNTs was improved by heating at a high temperature (1700C∘) in NH3.


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## Body

## 1. Introduction

Recently, one-dimensional (1D) nanostructured materials,
such as nanotubes, nanowires, and nanobelts, have attracted much attention on
account of their novel chemical and physical properties. Boron nitride
nanotubes (BNNTs), which are structurally similar to carbon nanotubes (CNTs),
display several outstanding properties for technological applications,
including gas absorption, field emission measurement, and thermal conductivity [1–7]. BNNTs are considered to be
more suitable than carbon nanotubes for several applications since BNNTs
demonstrate high oxidation resistance up to 800°C in air, in contrast to CNTs that are readily oxidized
at around 400°C.
Furthermore, the electronic properties of BNNTs, independent of the diameter
and chirality, allow for novel applications in nanoelectronics and
optoelectronics.There have been many studies on CNTs, whereas there have been relatively fewer reports on
the preparation and applications of BNNTs, probably because the synthesis of
BNNTs with high quality as well as the control of texture (i.e., diameter,
length, and inner space) is more difficult and expensive compared to CNTs. Thus
far, BNNTs have been synthesized using arc discharge [8], substitution reaction with
CNTs [9], ball milling of BN powder [10], thermal annealing of a mixed
powder of boron and hexagonal boron nitride (h-BN) [11], and chemical vapor
deposition (CVD) using a mixture of B and MgO vapor [5]. However, almost all of these
methods require complicated and expensive apparatus or severe preparation
conditions. In particular, all methods have difficulty with the control of the
texture of nanotubes and the assembly of individual nanotubes into highly
ordered arrays. To the best of our knowledge, no previous approach to the
synthesis of BNNTs that can fulfill all these requirements has been reported.To produce ordered arrays of nanotubes with controlled
diameter and length, template-aided synthesis has been suggestedas one of the major approaches. A
porous alumina anodic (PAA) membrane has been used as a suitable template for
the synthesis of 1D nanostructures because of its tunable pore dimensions,
narrow pore size distribution, and good mechanical and thermal stability [12]. Although PAA membranes are
commercially available, it is of interest to use homemade ones with different
textures (e.g., uniform length and pore diameter) to produce different kinds of
nanotubes.Several starting materials have been reported for the
synthesis of BN materials, including borazine, polyvinylpentaborane, or
dibromoboranedimethyl sulphide, which is known to be a good precursor to form
BN upon ammonia thermolysis. In our previous study, BNNTs with highly ordered
arrays were successfully prepared by wetting the PAA template with borazine (B3N3H6)
[13]. However, both ends of the
BNNTs were sealed by a thin BN film formed by the deposition of the liquid
polymer precursor on the template surface, causing difficulties with further
adjusting functionality and applications of the BNNTs. In addition, it was also
difficult to control the wall thickness or inner diameter of the nanotubes by
the wetting approach.In order to overcome
these problems in our previous study, it was recommended to use a gaseous
organometallic precursor of BN for the formation of BNNTs in the PAA template.
In this respect, ammonia borane (BH3NH3) is considered to
be a suitable precursor for the preparation of BNNTs through vapor-phase
pyrolysis since BH3NH3 is a stable crystalline solid with
less toxicity without C and O and can be easily transformed into the gas phase
by moderate temperature heating [14, 15]. In addition, no catalyst is needed
to synthesize BN using BH3NH3. This study reports the
successful preparation of BNNTs with high quality in highly ordered arrays and
controllable texture from BH3NH3 using
template-aided vapor-phase pyrolysis.


## 2. Experimental Details

### 2.1. Materials

Ammonia borane was
synthesized from sodium borohydride (NaBH4) and ammonium sulfamate
(NH4NH2SO3), as described by Suenram and Thorne [16]. The composition of this product using H1-NMR
was found to be 97 wt% BH3NH3 and ≈3 wt% (BH2NH2)x(x<3).Two types of PAA
templates were used in this study. One was a commercially available alumina
anodic membrane 60μm thick with a nominal pore diameter of 100 nm (Anodisc 13,
Whatman Ltd.). The other was a homemade membrane, about 40 μm thick with pore
diameters ≤80 nm, which was formed based on the method reported by Thompson
(Figure 1) [12]. Before being used, the PAA templates were
cleaned in an ultrasonic bath using distilled water, ethanol, and acetone, and
successively dried at 50°C in air.Figure 1
SEM image of the homemade PAA membrane.


### 2.2. Preparation of BNNTs

The experimental apparatus used is shown in Figure2(a). Two independent temperature-controlled furnaces (I and II) were set in a
glove box filled with N2. One-side-closed silica glass tube functioned
as the reaction chamber in the furnaces. An amount of 0.3 g of BH3NH3 was placed in
the glass tube positioned in furnace (II), while the alumina template was
attached to the outlet of the silica glass tube in furnace (I).
However, furnace (I) was first heated to 550°C,
and then furnace (II) was heated from room temperature to 250°C at
a heating rate of 10°C min−1, and maintained for 1 hour at this
temperature. After furnace (II) was cooled down naturally, furnace (I)
was kept at 550°C for another 24 hours. Consequently, amorphous BNNTs
were formed in the PAA template. To obtain BNNTs
with different wall thicknesses, the deposition step was repeated using the
same template. The template filled with the BNNTs was then moved out from the
glove box and heated in a horizontal tube furnace using a two-cycle heat
treatment program, as shown in Figure 2(b). Also, BN heated at high temperature
(>1300°C) is chemically inert in a strong base solution.
Crystallized BN obtained by the 1st cycle heat treatment at 1300°C
was not dissolved by the NaOH solution. The template with the BNNTs was then
heated to 1300°C at a rate of 10°C min−1 and then kept for 1 hour in NH3 flowing at 100 mL min−1 (1st cycle) and washed with ethanol for 30
minutes in an ultrasonic bath. The template with BNNTs was etched in a 40 wt%
NaOH solution to dissolve the template. The BNNTs free of alumina were washed
several times using dilute water and ethanol, and then successively dried at 100°C
in air. As shown by the 2nd cycle of the heat treatment, alumina-free BNNTs
were placed in a BN boat and heated from room temperature to 1400°C
at 10°C min−1. They were maintained at this
temperature for 1 hour, followed by further heating at 1500, 1600, and 1700°C
for 1 hour in NH3 gas (100 mL min−1).Experimental apparatus (a) and the two-cycle heat treatment program (b) used in the study.
(a)(b)


### 2.3. Characterization

The texture and
morphology of the BN nanotubes obtained after heat treatment at the final
temperature of 1300, 1500, 1600, and 1700°C
were observed using scanning electron microscopy (SEM; JEOL JSM 6500F) and high-resolution transmission
electron microscopy (HRTEM; JEM2000) with selected area electron diffraction (SAED)
at an accelerating voltage of 200 kV and energy dispersive spectroscopy (EDS).
Fourier transform infrared spectroscopy (FT-IR; Thermo Nicolet AVATAR 320)
measurements were carried out to determine the formation of BN.


## 2.1. Materials

Ammonia borane was
synthesized from sodium borohydride (NaBH4) and ammonium sulfamate
(NH4NH2SO3), as described by Suenram and Thorne [16]. The composition of this product using H1-NMR
was found to be 97 wt% BH3NH3 and ≈3 wt% (BH2NH2)x(x<3).Two types of PAA
templates were used in this study. One was a commercially available alumina
anodic membrane 60μm thick with a nominal pore diameter of 100 nm (Anodisc 13,
Whatman Ltd.). The other was a homemade membrane, about 40 μm thick with pore
diameters ≤80 nm, which was formed based on the method reported by Thompson
(Figure 1) [12]. Before being used, the PAA templates were
cleaned in an ultrasonic bath using distilled water, ethanol, and acetone, and
successively dried at 50°C in air.Figure 1
SEM image of the homemade PAA membrane.


## 2.2. Preparation of BNNTs

The experimental apparatus used is shown in Figure2(a). Two independent temperature-controlled furnaces (I and II) were set in a
glove box filled with N2. One-side-closed silica glass tube functioned
as the reaction chamber in the furnaces. An amount of 0.3 g of BH3NH3 was placed in
the glass tube positioned in furnace (II), while the alumina template was
attached to the outlet of the silica glass tube in furnace (I).
However, furnace (I) was first heated to 550°C,
and then furnace (II) was heated from room temperature to 250°C at
a heating rate of 10°C min−1, and maintained for 1 hour at this
temperature. After furnace (II) was cooled down naturally, furnace (I)
was kept at 550°C for another 24 hours. Consequently, amorphous BNNTs
were formed in the PAA template. To obtain BNNTs
with different wall thicknesses, the deposition step was repeated using the
same template. The template filled with the BNNTs was then moved out from the
glove box and heated in a horizontal tube furnace using a two-cycle heat
treatment program, as shown in Figure 2(b). Also, BN heated at high temperature
(>1300°C) is chemically inert in a strong base solution.
Crystallized BN obtained by the 1st cycle heat treatment at 1300°C
was not dissolved by the NaOH solution. The template with the BNNTs was then
heated to 1300°C at a rate of 10°C min−1 and then kept for 1 hour in NH3 flowing at 100 mL min−1 (1st cycle) and washed with ethanol for 30
minutes in an ultrasonic bath. The template with BNNTs was etched in a 40 wt%
NaOH solution to dissolve the template. The BNNTs free of alumina were washed
several times using dilute water and ethanol, and then successively dried at 100°C
in air. As shown by the 2nd cycle of the heat treatment, alumina-free BNNTs
were placed in a BN boat and heated from room temperature to 1400°C
at 10°C min−1. They were maintained at this
temperature for 1 hour, followed by further heating at 1500, 1600, and 1700°C
for 1 hour in NH3 gas (100 mL min−1).Experimental apparatus (a) and the two-cycle heat treatment program (b) used in the study.
(a)(b)


## 2.3. Characterization

The texture and
morphology of the BN nanotubes obtained after heat treatment at the final
temperature of 1300, 1500, 1600, and 1700°C
were observed using scanning electron microscopy (SEM; JEOL JSM 6500F) and high-resolution transmission
electron microscopy (HRTEM; JEM2000) with selected area electron diffraction (SAED)
at an accelerating voltage of 200 kV and energy dispersive spectroscopy (EDS).
Fourier transform infrared spectroscopy (FT-IR; Thermo Nicolet AVATAR 320)
measurements were carried out to determine the formation of BN.


## 3. Results and Discussion

FT-IR spectra of BH3NH3 and BNNTs
after heating using a two-step heat treatment at 1400/1700°C in NH3 are shown in Figure 3. Typical peaks
due to stretching of N–H and B–H bonds in BH3NH3 appear at
3280 and 2320 cm−1, respectively, with the N–H deformation at 1560 cm−1
(Figure 3(a)). After the heat treatment at 1400/1700°C, these peaks disappeared
because of dehydrogenation with polymerization and condensation [17]. Peaks corresponding to B–N
stretching were observed at 1380 and 780 cm−1 (Figure 3(b)). On the other hand, XPS spectra of
the BNNTs formed at the final temperature of 1700°C showed N1s and B1s peaks at the
binding energy of 397.65 and 190.15 eV [18], respectively. The B:N ratio
was given to be about 1:1 from the
areas under the peaks. The formation of BN from BH3NH3 by
pyrolysis was confirmed on the basis of the FT-IR and XPS results. Since
neither oxygen nor carbon was seen to exist in BH3NH3, it
is obvious that BNNTs can be prepared without oxygen and carbon impurities.Figure 3
FT-IR spectra of BH3NH3 (A) and BNNTs heated to 1400/1700°C
in NH3 (B).Figure4 shows bundles of template-free BNNTs prepared by
heating at 1300°C (1st
cycle). Highly ordered arrays of BNNT bundles can be observed, with the maximum
length of the BNNTs reaching 60 μm
(Figure 4(a)). As seen in Figure 4(b), straight BNNTs were formed with diameters
ranging from 200 to 300 nm, corresponding to the pore sizes of the PAA template.
Neither cracks nor residual alumina can be seen on the surfaces of the BNNTs. An
enlarged image (Figure 4(c)) of the middle part from Figure 4(a) shows broken BNNTs
which were probably formed during SEM sample preparation. Figure 4(c) confirms the hollow structure of the
BNNTs, as indicated by the arrows. Also, Figure 4(d) shows that BNNTs with open
ends were formed without the formation of films on the top surfaces.Figure 4
SEM images of highly ordered BNNT arrays obtained at 1300°C (1st
cycle) after removal of the PAA template. Cross-section of BNNTs ((a) and (b)),
fragments of broken BNNTs (c), and top view of the BNNT bundles (d).Figure5 shows TEM images of BNNTs produced at 1300°C (1st cycle) in NH3.
Long, straight, and hollow nanotubes about 200 nm wide including Y-shaped ones
were observed (Figure 5(a)). The Y-shaped nanotube should be inherited from
channels of the template. The wall
thickness of the BNNT was about 8 nm (Figure 5(b)). It was found that the wall
thickness of the as-prepared BNNTs had a narrow range from 5 to 10 nm. Because
of the thin walls, the BNNTs have very large inner diameters ranging from 180
to 280 nm, several tens of times larger than those of BNNTs prepared using
other approaches [5, 10, 11]. SAED from the walls of the
nanotubes yielded BN (002), (100), and (110) reflections (inset in Figure 5(b)),
and the strong arcs of (002) revealed the turbostratic structure of the BNNTs.
EDX analysis showed the absence of Al and O elements in the BNNT samples. When
the BNNTs that were formed at 1300°C
were directly heated to temperatures higher than 1500°C, the original morphology of the BNNTs was broken.
Therefore, a two-step heat treatment, shown in Figure 2(b), was applied to
maintain the morphology of the BNNTs with high crystallinity.TEM images of BNNTs formed at 1300°C (1st cycle) in NH3 ((a)
and (b)); the inset in image (b) shows the ED pattern of the nanotube wall.
(a)(b)Figure6 shows high-resolution TEM
images of BNNTs obtained by the one-step heat treatment at 1300°C and the two-step heat treatment
at 1400/1500°C, 1400/1600°C, and 1400/1700°C (2nd
cycle). Compared with the BNNTs prepared using the single-step process at 1300°C (1st cycle) (Figure 6(a)),
the crystalline planes of the BNNTs obtained by the two-step heat treatment tend
to run along the long axis of the nanotubes parallel to the c-axis of h-BN (see
arrows). The BNNTs treated at 1700°C (Figure 6(d)) show regular spacingsof
about 0.34 nm, close to the theoretical value of 0.333 nm of h-BN (002) (Joint Committee on Powder
Diffraction Standards (JCPDS) Card no. 73-2095), which also
confirms the formation of h-BN. The order degree of the stacking layers of BN
was gradually increased by heating from 1300 to 1700°C. The angle of inclination of the crystallites from the
long axis of the nanotube decreased gradually from 1300 to 1700°C. It is to be noted that
the BNNTs obtained at 1700°C show a well-ordered layer structure similar to the BNNTs deposited on graphite
at 1200°C from the B–N–O precursor vapor generated at 1700°C by the CVD method [19]. The distribution of plane
crystallites in the BNNTs after heat treatment at 1300, 1500, 1600, and 1700°C is shown in Figure 7(a). The
lengths of the crystallites became longer with increasing heat treatment from
1300 to 1700°C. The average length of the BN crystallites exponentially
increased with temperature (Figure 7(b)). In particular, the mean length of the crystallites
grew to 17 nm when the BNNTs were heated to 1700°C. It is understood that the crystallinity of the
BNNTs can be distinctly improved by heating at high temperatures in NH3.HRTEM images of the BN layers after heating
to 1300 (a), 1500 (b), 1600 (c), and 1700°C (d).
(a)(b)(c)(d)Figure 7
Length distribution of the BN crystallites
(a) and the relationship between average length of the BN crystallites and
temperature (b) in the BNNTs after heating to a final temperature of 1300,
1500, 1600, and 1700°C in
NH3.The BNNTs after heat treatment at 1300–1700°C were washed in distilled water and ethanol to remove
contamination on the surface. When BNNTs heated at 1300°C (1st cycle) were inadequately dried and moved into the
vacuum chamber of the SEM or TEM, some of the BNNTs became flat and were bent,
curling the bundles (Figure 8(a)). It is of interest to observe a flat, twisted
BNNT without cracks (Figure 8(b)).(a) SEM image of flat BNNT containing curled or twisted ones obtained at 1300°C
(1st cycle). (b) TEM image of a flat BNNT without cracks.
(a)(b)In order to increase the thickness of the walls of the
BNNTs, a template was reversed after the first deposition and the B–N–H
containing polymer was subsequently deposited again in such a manner as
mentioned in experimental procedure. As a result, we obtained the wall of BNNTs
doubly thick. The wall thickness was increased from 9 to 20 nm by the double
deposition (Figure9), indicating that the wall thickness can be controlled by
the number of deposition cycles. When the homemade template with a pore diameter
≤80 nm and a thickness of 40 μm
was used, BNNTs with corresponding sizes (i.e., diameter ≤80 nm and length ≈40 μm)
were successfully prepared after heating at 1300°C (1st cycle) (Figure 10(a)). The wall thickness in this
case was less than 10 nm (Figure 10(b)), comparable to that of the BNNTs
fabricated using the commercial template, suggesting that the wall thickness is
determined by the amount of starting BH3NH3 material,
independent of the template size. In addition, flat and bent BNNTs surrounding the
bundles also were observed (see arrows).TEM images of BNNTs by depositing BN once (a) and twice (b).
(a)(b)SEM (a) and TEM (b) images of BNNTs
prepared using the homemade PAA template with a pore diameter of about 80 nm.
(a)(b)The overall reaction for the formation of BNNTs from BH3NH3 can be expressed as follows [14, 16, 20]:(1)BH3NH3→BN+3H2.A mechanism for the formation of BNNTs is schematically shown in Figure11. The formation of
BNNTs from BH3NH3 consists of two steps. First, BH3NH3 is heated to 250°C in furnace (II)
and it decomposes to gaseous H2, BH2NH2, and B3N3H6 [20]. These gaseous precursors BH2NH2 and B3N3H6 are carried upward with H2.
Second, when these gases go through the narrow channels of the template at 550°C in furnace (I), pyrolysis of BH2NH2 and B3N3H6 readily occurs, resulting in deposition of solid, amorphous BN films at the
surfaces of the walls of the channels. As the reaction proceeds, BN films
without cracks on the surface are uniformly thickened to several nanometers. After
heat treatment (≥1300°C), with subsequent removal of the PAA template, highly
ordered BNNTs arrays with a specific texture are produced.Figure 11
A mechanism for the formation of
BNNTs from BH3NH3
with the aid of a template.


## 4. Conclusions

We report a simple and efficient route to the production of
highly ordered BNNT arrays with controllable texture from BH3NH3.
Two kinds of BNNTs, 200–300 nm wide ×60 μm long and 70–80 nm wide ×40 μm
long, were obtained with a wall thickness less than 10 nm. It was found that
the BNNT wall thickness could be increased by repeating the deposition. The crystallinity
of the BNNTs improved by heat treatment at ≥1300°C in NH3, and
BN crystallites grew to 17–44 nm at 1700°C. A mechanism for the
formation of BNNTs from BH3NH3 was proposed including
several steps involving the pyrolysis of BH3NH3, deposition
in the PAA templates, and crystallization of BN.



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*Source: 606283-2009-01-19.xml*