Boron-carbon nanotubes from the pyrolysis of C 2 H 2 -B 2 H 6 mixtures

Article abstract of DOI:10.1016/S0009-2614(98)01398-0

Boron-doped carbon nanotubes have been prepared by the pyrolysis of acetylene-diborane mixtures in a stream of helium and hydrogen. The nanotubes so obtained have compositions around C₃₅B. The presence of boron favours nanotube formation and the boron content does not vary with the depth. Considering that the nitrogen-doped nanotubes, reported recently (Chem. Phys. Lett. 287 (1997) 671) have the average composition C₃₅N, it may be possible to use a combination of these p-type and n-type nanotubes for certain applications.

Full text of DOI:10.1016/S0009-2614(98)01398-0

5 February 1999
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Chemical Physics Letters 300 1999 473–477
Boron–carbon nanotubes from the pyrolysis of C2 H2–B2 H6
mixtures
a
a,b
a
c
B.C. Satishkumar , A. Govindaraj , K.R. Harikumar , Jin-Ping Zhang ,
c
a,b,)
A.K. Cheetham , C.N.R. Rao
a
Solid State and Structural Chemistry Unit, CSIR Centre of Excellence in Chemistry, Indian Institute of Science, Bangalore 560 012, India
b
Jawaharlal Nehru Centre for AdÕanced Scientific Research, Jakkur P.O., Bangalore 560 064, India
c
Materials Research Laboratory, UniÕersity of California, Santa Barbara, CA 93106, USA
Received 16 November 1998; in final form 3 December 1998
Abstract
Boron-doped carbon nanotubes have been prepared by the pyrolysis of acetylene–diborane mixtures in a stream of
helium and hydrogen. The nanotubes so obtained have compositions around C35B. The presence of boron favours nanotube
formation and the boron content does not vary with the depth. Considering that the nitrogen-doped nanotubes, reported
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Ž
.
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recently Chem. Phys. Lett. 287 1997 671 have the average composition C35N, it may be possible to use a combination of
these p-type and n-type nanotubes for certain applications. q 1999 Elsevier Science B.V. All rights reserved.
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1. Introduction
deposition 4 . We have attempted to prepare the
boron-doped carbon nanotubes by carrying out the
pyrolysis of appropriate precursors in an atmosphere
of helium and hydrogen. The study has shown that
boron–carbon nanotubes can be prepared starting
with mixtures of acetylene and diborane.
One of the important advances in nanotube re-
search has been the synthesis of carbon nanotubes
containing other elements. Thus, nitrogen-doped car-
w
x
bon nanotubes have been recently prepared 1,2 and
boron–carbon–nitrogen nanotubes have also been
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characterized 1,3 . Since, nitrogen-substitution in
carbon nanotubes gives rise to n-type doping, it
becomes important to prepare boron-doped carbon
nanotubes, corresponding to p-type doping. In this
context, it is noteworthy that boron carbides have
been prepared in film form by chemical vapour
2. Experimental
B2 H6 was first prepared by the addition of boron
trifloride diethyl etherate to sodium borohydride in
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tetraglyme 5 . We later changed to a better proce-
dure, wherein B2 H6 was prepared by the addition of
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iodine to NaBH4 in tetraglyme 6 . We have em-
ployed this procedure for most of our studies. The
experimental set-up for the synthesis of the B–C
nanotubes by the pyrolysis of acetylene–B2 H6 mix-
)
Corresponding author. Fax: q91 80 331 1310; e-mail:
cnrrao@sscu.iisc.emet.in
0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved.
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PII: S0009-2614 98 01398-0

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B.C. Satishkumar et al.rChemical Physics Letters 300 1999 473–477
Fig. 1. Pyrolysis apparatus employed for the synthesis of boron–carbon nanotubes.
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tures is shown in Fig. 1. Helium is used as the carrier
gas and B2 H6 passes through the quartz reaction
tube along with C2 H2 and H2. The flow rates of the
gases were controlled by UNIT mass flow con-
gravimetric studies TGA were carried out using a
Mettler Toledo TG-850 instrument.
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.
trollers. Co nanoparticles 100 mg of ;50 nm in
diameter, produced by the polyol process, were
placed in a quartz boat kept inside the reaction tube.
The temperature of the furnace was ramped slowly
up to 10008C, under flowing hydrogen. Carbona-
ceous products resulting from the pyrolysis were
examined with a Leica S440i scanning electron mi-
3. Results and discussion
TEM images of the nanotubes obtained by the
pyrolysis of acetylene and diborane over Co
nanoparticles at 10008C are shown in Fig. 2. The
image in Fig. 2a shows the nanotubes obtained using
B2 H6 prepared by the addition of BF3–etherate to
NaBH4 in tetraglyme, by maintaining the flow rate
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.
croscope SEM . The samples were dispersed in
carbon tetrachloride and deposited onto holey carbon
copper grids for transmission electron microscope
TEM observations with a JEOL JEM-3010 system.
The B–C nanotubes were characterized by X-ray
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of acetylene at 20 sccm standard cubic centimeter
.
per minute and a total flow rate of gases at 500
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sccm. Fig. 2 are the TEM images of nanotubes
obtained with B2 H6 prepared by iodine addition to
NaBH4 in tetraglyme. An acetylene flow rate of 40
sccm and total flow rate of 500 sccm were employed
in case of the nanotubes in seen in Fig. 2b. A flow
rate of 20 sccm for acetylene with a total flow rate of
250 sccm were used for the nanotubes shown in Fig.
2c; in Fig. 2d, we show an SEM image of these
nanotubes. The nanotubes have diameters in the
range of 30–60 nm and were ;10 micron in length.
They generally contain the nested cone-shaped
graphene sheets in the tubular region and the metal
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photoelectron spectroscopy XPS using a VG scien-
tific ESCALAB V spectrometer. Electron energy
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loss spectroscopy EELS measurements were car-
ried out with a Gatan imaging filter system attached
to a JEOL 2010 microscope and fitted with a CCD
camera. The beam size was 10–150 nm, depending
on the diameter of the nanotube and the dispersion
was 0.3–0.5 eV. X-ray diffraction XRD patterns
were recorded on the as obtained deposits using a
SEIFERT XRD 3000TT diffractometer. Thermo-
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B.C. Satishkumar et al.rChemical Physics Letters 300 1999 473–477
475
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Fig. 2. TEM images of boron–carbon nanotubes prepared a from the pyrolysis of C2 H2 and B2 H6 from BF3–etherate addition to NaBH4
.
Ž .
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in tetraglyme at a C2 H2 flow rate of 20 sccm and a total flow rate of 500 sccm, b from C2 H2 and B2 H6 from iodine addition to NaBH4
.
Ž .
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in tetraglyme at a C2 H2 flow rate of 40 sccm and a total flow rate of 500 sccm, c from C2 H2 and B2 H6 from iodine addition to NaBH4
.
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in tetraglyme at a C2 H2 flow rate of 20 sccm and a total flow rate of 250 sccm and d an SEM image corresponding to the nanotubes in
Fig. 2c.
Fig. 3. High resolution electron microscope image of nanotubes shown in Fig. 2c showing graphitic layers in the nested cone-shaped region.

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B.C. Satishkumar et al.rChemical Physics Letters 300 1999 473–477
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Fig. 4. B1s and C1s spectra of the nanotubes corresponding to those shown in Fig. 2, giving an average composition of C35 B. Spectra a
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b and c are after the first etch 20 min and second etch 40 min respectively. Inset shows the
are of the as-prepared nanotubes;
core-level spectra of nanotubes corresponding those shown in Fig. 2b giving an analysis of C40 B.
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particles from which they grow at one end. A high
resolution TEM image of the cone region is shown
in Fig. 3. The growth mechanism of the B–C nan-
otubes appears to be similar to that of the B–C–N
composition of C50 B after correction, C80 B . The
BF3–etherate procedure of preparing B2 H6 is not
satisfactory with the experimental set-up of the type
shown in Fig. 1.
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nanotubes 3 .
In order to confirm the presence of boron in the
nanotubes, it is necessary to carry out EELS analysis
in the electron microscope. The EELS analysis was
carried out on the different batches of nanotubes
shown in Fig. 2, with the composition C30 B from the
Core-level X-ray photoelectron spectra of the nan-
otubes shown in Fig. 2 are shown in Fig. 4. The B1s
feature is found as the prominant peak at 190 eV and
the C1s signal at 285 eV. The shift of B1s signal
towards higher binding energy compared to 188 eV
in pure boron is an indication of boron substitution
in the sp2 carbon network. Based on the total B1s
and C1s intensities, the boron to carbon ratios in the
nanotube samples were calculated by taking the ratio
of the respective integrated peak areas and dividing
them by the photoionization cross-section for the 1s
level. The average composition thus obtained for the
nanotubes depicted in Fig. 2c works out to be C17 B.
After correcting for the B1s intensity at 188 eV due
to impurities, the composition works out to be close
to C30 B. For the nanotubes in Fig. 2b, the core level
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spectra see inset of Fig. 4 yielded a composition of
Ž .
B after correction, C40 B . The B1s and C1s
C30
signals of the nanotubes shown in Fig. 2a gave the
Fig. 5. EELS spectrum of the nanotubes shown in Fig. 2.

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B.C. Satishkumar et al.rChemical Physics Letters 300 1999 473–477
477
core-level spectra. These nanotubes gave characteris-
4. Conclusions
tic edges in EELS corresponding to K-shell ioniza-
tion of boron and carbon respectively. We show a
typical EEL spectrum in Fig. 5. The ionization edges
have sharply defined fine structures due to p ) and
s ) pre-ionizations of boron as well as carbon,
characteristic of sp2 hybridization. This is an indica-
tion of the substitution of boron at the trigonal sites
in the sp2 carbon network. The analysis of EEL
spectra gave compositions to be in the range from
C25 B to C50 B. EELS analysis of the nanotubes
Boron–carbon nanotubes prepared by the pyroly-
sis of acetylene and diborane mixtures have an aver-
age composition of C35 B. Depth profile analysis
shows that the boron content does not vary signifi-
cantly with depth. The magnitude of boron doping in
the B–C nanotubes is comparable to that of nitrogen
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in C–N nanotubes 1 . With both the p-type and and
n-type nanotubes being available, one can conceive
of possible applications. It may however, be neces-
sary to prepare well-characterized B–C and C–N
nanotubes with exactly known dopant concentrations
before such applications become feasible.
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corresponding to Fig. 2a with the composition of
C80 B from core-level spectra showed negligible con-
centration of boron.
Depth-profile experiments were carried out on the
nanotubes corresponding to those in Fig. 2 with the
composition of C30 B. For this purpose, B1s and C1s
core-level spectra were recorded after etching with a
argon ion beam of 4 keV. The core-level spectra
after etching for 20 min and 40 min are shown in
Fig. 4. The spectra reveal negligible variation in the
boron content. It appears that boron plays an impor-
tant role in the formation of these nanotubes.
X-ray diffraction studies of the different B–C
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