Synthesis and characterization of bismuth single-crystalline nanowires and nanospheres

Article abstract of DOI:10.1021/ic049129q

A facile solution-phase process has been demonstrated for the selective preparation of single-crystalline bismuth nanowires and nanospheres by reducing sodium bismuthate with ethylene glycol in the presence of poly(vinyl pyrrolidone) (PVP) or acetone. Bismuth nanobelts and Bi/Bi₂O ₃ nanocables could be also obtained by changing some reaction parameters. Various techniques such as XRD, EDXA, SEM, TEM, HRTEM, and FT-IR have been used to investigate the physical characteristics of these low-dimensional bismuth nanostructures.

Full text of DOI:10.1021/ic049129q

Inorg. Chem. 2004, 43, 7552−7556
Synthesis and Characterization of Bismuth Single-Crystalline Nanowires
and Nanospheres
Junwei Wang, Xun Wang, Qing Peng, and Yadong Li*
Department of Chemistry and the Key Laboratory of Atomic & Molecular Nanosciences
(Ministry of Education, China), Tsinghua UniVersity, Beijing, 100084, P. R. China, and
National Center for Nanoscience and Nanotechnology, Beijing, 100084, P. R. China
Received July 4, 2004
A facile solution-phase process has been demonstrated for the selective preparation of single-crystalline bismuth
nanowires and nanospheres by reducing sodium bismuthate with ethylene glycol in the presence of poly(vinyl
pyrrolidone) (PVP) or acetone. Bismuth nanobelts and Bi/Bi2O3 nanocables could be also obtained by changing
some reaction parameters. Various techniques such as XRD, EDXA, SEM, TEM, HRTEM, and FT-IR have been
used to investigate the physical characteristics of these low-dimensional bismuth nanostructures.
Introduction
nm).5 Bismuth nanowire is also an attractive material for
thermoelectric applications.6
One-dimensional (1D) nanostructures (such as wires, rods,
and tubes) have attracted extensive interest in recent years
due to their potential use as interconnects and nanoscale
electronic, optoelectronic, and sensing devices.1 Considerable
effort has been devoted to the bulk synthesis of semiconduc-
tor and metal nanowires.2,3 Bismuth, as a semimetal with a
very small band gap, provides a very attractive model system
for studying low-dimensional physical phenomena due to its
highly anisotropic Fermi surface, low carrier densities, small
carrier effective masses, and long carrier mean free path.4
Theoretical model and experiments results indicate a semi-
metal-semiconductor transition in bismuth nanowires when
the wire diameter is decreased to a certain value (about 50
However, few methods have been reported for the
preparation of bismuth nanowires until now. An effect
method has been employed to producing Bi nanowires with
diameters in the range 7-200 nm by pressure injection of
Bi liquid melt, or by infiltration and condensation of Bi vapor
into the nanochannels of an anodic alumina template.7
Electrochemical deposition has also been used to fabricate
polycrystalline Bi nanowires, by plating materials of interest
into a template of nanometer-sized pores created by nuclear
particle track etching in polycarbonate membranes.3a,8 So far,
bismuth nanowires with diameters ranging from 30 to 200
nm were extruded from the surfaces of freshly grown
composite thin films consisting of Bi and chrome-nitride
owing to the high compressive stress in these films.9
Recently, a hydrothermal process has been demonstrated by
* To whom correspondence should be addressed. E-mail: ydli@
tsinghua.edu.cn.
(1) Xia, Y.; Yang, P.; Sun, Y.; Wu, Y.; Mayers, B.; Gates, B.; Yin, Y.;
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Choi, H. J.; Yang, P. Nature 2003, 422, 599. (b) Greene, L. E.; Law,
M.; Goldberger, J.; Kim, F.; Johnson, J. C.; Zhang, Y.; Saykally, R.
J.; Yang, P. Angew. Chem., Int. Ed. 2003, 42, 3031. (c) Duan, X. F.;
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7552 Inorganic Chemistry, Vol. 43, No. 23, 2004
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© 2004 American Chemical Society
Published on Web 10/21/2004

Bi Single-Crystalline Nanowires and Nanospheres
Figure 2. EDXA spectrum of a single bismuth nanowire.
Figure 1. XRD pattern of as-synthesized bismuth nanowires.
us, for preparing single-crystalline Bi nanotubes.10 A solvo-
thermal process was also used to synthesize Bi nanowires
from bismuth nitrite in ethylene diamene; however, this
produced nanowires were with polycrystalline domain struc-
tures and low aspect ratios.11 Here, we develop a template-
free, solution-phase method for the production of single-
crystalline Bi nanowires in large scale by reducing sodium
bismuthate with ethylene glycol in the presence of poly(vinyl
pyrrolidone) (PVP). This low-temperature synthetic route
requires no physical templates or catalysts, and could be
scaled up to produce bismuth nanowires without post-
synthesis treatment for selective removal of templates or
catalysts.
Experimental Section
Synthesis. In a typical process, 0.15 g of analytical grade sodium
bismuthate (NaBiO32H2O) and 35 mL of ethylene glycol or glycerol
were put into a Teflon-lined stainless steel autoclave with a capacity
of 50 mL at room temperature. The solution was then stirred
vigorously and bubbled with a flow of pure argon gas for 30 min,
before the autoclave was sealed and maintained at 180-200 °C
for over 24 h. In different cases, PVP (MW ≈ 1 300 000), acetone,
or water was introduced to control the morphology of the product.
After the reaction was completed, the resulting black solid product
was collected by filtration, washed with large amount of absolute
ethanol and acetone to remove all of impurities, and then dried at
50 °C for 1 h.
Figure 3. (a) SEM and (b, c) TEM images of bismuth nanowires that
were synthesized by the polyol process in the presence of PVP. (d) A
HRTEM image of an individual Bi nanowire showing good ordering of
the lattice planes. (e) The corresponding electron diffraction pattern along
the [0001] zone axis.
Characterization. The X-ray diffraction (XRD) pattern was
recorded on a powder sample using a Bruker advance-D8 diffrac-
tometer with Cu KR radiation. SEM measurements were carried
out with a LEO-1530 microscope, and TEM characterization was
performed with a Hitachi-800 microscope. HRTEM and electron
diffraction studies were obtained with a JEOL-2010 microscope.
FT-IR spectrum was recorded on a Nicolet model 759 Fourier
powders, and all the diffraction peaks could be readily
indexed to the rhombohedral phase of bismuth. The lattice
constants calculated from the XRD pattern were a ) 4.54
Å and c ) 11.85 Å, which were very close to the reported
data (JCPDS 05-0519).
Thorough elemental composition analysis of bismuth
nanowires was performed by energy-disperse X-ray analysis
(EDXA) and revealed that the nanowires contained only
bismuth. Figure 2 shows a typical EDXA spectrum recorded
on an individual Bi nanowire, whose peaks are assigned to
bismuth only (the Cu peaks arise from the copper grid).
Morphologies and Structures. The morphology of the
powders was examined by scanning electron microscopy
(SEM) and transmission electron microscopy (TEM). Figure
3a shows a typical SEM image of bismuth prepared in
ethylene glycol with PVP, revealing high-yield growth of
transform infrared spectrometer at wavenumbers 500-4000 cm-1
.
Results and Discussion
Composition of the Products. On the basis of the above
solution-phase process, pure Bi could be obtained. Figure 1
shows an X-ray diffraction (XRD) pattern of as-synthesized
(10) (a) Li, Y. D.; Wang, J. W.; Deng, Z.; Wu, Y.; Sun, X.; Yu, D.; Yang,
P. J. Am. Chem. Soc. 2001, 123, 9904. (b) Wang, J. W.; Li, Y. D.
AdV. Mater. 2003, 15, 445.
(11) Gao, Y.; Niu, H.; Zeng, C.; Chen, Q. Chem. Phys. Lett. 2003, 367,
141.
Inorganic Chemistry, Vol. 43, No. 23, 2004 7553

Wang et al.
nanowire materials with lengths up to several micrometers.
Figure 3b shows a typical low-magnification TEM image in
which Bi nanowires with diameters in the range 10-200 nm
were displayed. Figure 3c shows the TEM image of several
nanowires, indicating the straightness of these nanowires.
High-resolution TEM reveals high-quality single-crystalline
Bi nanowires prepared by this polyol process. Figure 3d
shows a high-magnification TEM image of a single-crystal-
line Bi nanowire. The spacing of 0.22 nm between adjacent
lattice planes corresponds to the distance between two (110)
crystal planes (at 60 to the wire axis), indicating 100 as
the growth direction for the Bi nanowires. The electron
diffraction (ED) pattern (Figure 3e) is identified to be along
the [0001] diffraction zone axis, which is in good agreement
with the hexagonal structure of bismuth. The oxide layer on
the wire surface can be observed in HRTEM images, as
shown in Figure 3d, because bismuth could be easily
oxidized.7b The newly prepared bismuth nanowires could be
preserved in some organic solvent such as hexane in order
to prevent the oxidation.
The polyol process has been, recently, exploited to
synthesize silver nanowires by reducing silver nitrate with
ethylene glycol in the presence of PVP.12 In the current
process, PVP was found to play an important role in the
production of Bi nanowires. Although Bi nanowires could
be also obtained in the absence of PVP, the product contained
a mixture of nanowires and nanospheres in this case. When
PVP was introduced into ethylene glycol, the nanospheres
would be hardly seen in the product, and the range of the
diameters of as-produced nanowires narrowed. That is to say,
PVP could strongly enhance the growth rate of a special
crystallographic facet of seeds (for example, [100] for Bi),
and the anisotropic growth of Bi was achieved with the help
of PVP.
Figure 4. SEM (a, b) and TEM (c, d) images of Bi nanospheres
synthesized in mixed solvents of ethylene glycol and acetone at ratios of
1:1, 2:1, 1:2, 1:4, respectively.
shape control of nanostructures. In the current process, the
temperature was another factor affecting the growth of Bi
nanowires or spheres. The temperature should not be less
than 180 °C; otherwise, Bi nanowires or spheres could hardly
be formed.
In order to further understand the nature of PVP influenc-
ing the formation of Bi nanowires, acetone was selected for
manipulating the crystal growth of Bi in contrast. Figure 4
shows typical SEM and TEM images of Bi nanostructures
synthesized in mixed solvents of ethylene glycol and acetone
at different ratios, which indicate monodisperse and highly
crystalline Bi nanospheres with different diameters. When
the volume ratio of acetone and ethylene glycol reached 1,
Bi nanoshperes dominated in the final product. The control-
lable diameters of Bi nanospheres could be also achieved
by simply changing the ratios between these two solvents.
Apparently, the “dilute” role of acetone leading to isotropic
growth of Bi crystals is very different with that of PVP in
our synthesis. As a result, Bi nanowires and nanoshperes
can be selectively prepared by adjusting the additive
components in the system. Because the solution-based
reaction is quite a complex process, the systematic work is
worth deeply investigating. Various reaction conditions, such
as temperature, monomer concentration, and capping mol-
ecules, can be used to monitor and manipulate the kinetics
of crystals growth.14,15 We verified here that the adsorbent
activity on the surface of crystal seeds could influence the
Other than Bi nanowires and nanospheres, we have also
obtained some novel 1D nanostructures, such as Bi nanobelts
and Bi/Bi2O3 nanocables, by changing some reaction pa-
rameters. If ethylene glycol was replaced by glycerol, the
product would contain quite a few Bi nanobelts and nano-
spheres. Figure 5a shows a typical SEM image of Bi products
obtained in glycerol, which contains Bi nanobelts and
nanosphers. Figure 5b,c shows TEM images of structural
uniform and single crystalline Bi nanobelts with a typical
thickness in the range 20-50 nm and width 0.8-1.2 µm.
The ED pattern (Figure 5b, inset) recorded on a single
nanobelt is similar with that of Bi nanowires, which confirms
the Bi structure of nanobelts.
Because bismuth is oxidizable, Bi/Bi2O3 nanocables could
be obtained through a controllable oxidizing process in the
presence of a trace of water. Figure 6 shows typical TEM
images of Bi/Bi2O3 nanocables prepared in mixed solvents
(13) (a) Toneguzzo, P.; Viau, G.; Acher, O.; Fie´vet-Vincent, F.; Fie´vet, F.
AdV. Mater. 1998, 10, 1032. (b) Fie´vet, F.; Lagier, J. P.; Figlarz, M.
MRS Bull. 1989, 14 (12), 29.
(14) Roosen, A. R.; Carter, W. C. Physica A 1998, 261, 232.
(15) (a) Peng, X.; Manna, L.; Yang, W. D.; Wickham, J.; Scher, E.;
Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (b) Peng, X.
AdV. Mater. 2003, 15, 459.
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Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736.
7554 Inorganic Chemistry, Vol. 43, No. 23, 2004

Bi Single-Crystalline Nanowires and Nanospheres
Figure 7. (a) TEM image of a U-type bismuth nanowire, and (b) ED
patterns taken from different areas along with a single bismuth nanowire.
These images show the formation of bismuth nanowires from nanoparticles.
Figure 5. (a) SEM image of Bi products obtained in glycerol and (b, c)
TEM images of an individual Bi nanobelt. The inset in b shows the
corresponding ED pattern.
Figure 6. TEM images of Bi/Bi2O3 nanocables obtained through the polyol
process in the presence of distilled water (5% volume ratio): (a) magnifica-
tion TEM image of an individual nanocable, (b) TEM image of some Bi/
Bi2O3 nanocables.
of ethylene glycol and distilled water (5% volume ratio) with
PVP. Figure 6a shows a high-magnification TEM image of
an individual nanocable, indicating clearly the crystalline Bi
core and oxide shell. A variety of different 1D nanostructures,
such as nanowires, nanobelts, and nanocables, will provide
an opportunity to study the electronic properties of semimetal
bismuth.
Mechanism for the Formation of Bismuth Nanowires
and Nanospheres. The reproducible process involved the
reduction of sodium bismuthate with ethylene glycol at 180
°C. In this so-called polyol process, ethylene glycol served
as both solvent and reducing agent.13 We selected sodium
bismuthate as another reactant for its strong oxidation
capability (the electric potential for NaBiO3/Bi is above 0.9
V). The chemical reaction can be formulated as
Figure 8. FT-IR spectrum of bismuth nanowires obtained in ethylene
glycol with PVP.
to careful TEM studies, we found that the growth direction
of some bismuth nanowires is not strictly consistent. Figure
7b shows a single bismuth nanowire and ED patterns taken
from different areas along with this wire. These different
ED patterns also reveal the formation of bismuth nanowires
from nanoparticles.
Some recent research has indicated that the capping
organic molecules in the reaction system could modulate the
kinetics of the crystal growth and determine the subsequent
morphology of the product.12,15,16 For the current system,
ethylene glycol or PVP molecules could adsorb onto the
surface of Bi crystals through O-Bi bonding. The chemical
adsorption between these organic molecules and bismuth
nanocrystals could be proved by the FT-IR spectrum. Figure
8 shows a typical FT-IR spectrum of as-prepared bismuth
nanowires in ethylene glycol with PVP. The sample was
washed with absolute ethanol several times and then dried
in a vacuum before used. The peaks could be assigned to
PVP molecules adsorbed on bismuth nanowires,17 but the
CdO peak (1654 cm-1) became weaker, which results from
the interaction of O-Bi. Ethylene glycol or PVP molecules
adsorbed on some surfaces of Bi nanocrystals could signifi-
cantly decrease their growth rates and lead to highly
anisotropic growth. Obviously, the interaction of PVP
HOCH2 - CH2OH f CH3CHO + H2O
5CH3CHO + 2NaBiO3 f
(1)
2CH3COONa + 3CH3COOH + 2Bi + H2O (2)
The formation of Bi nanowires or nanospheres is believed
to be based on the Ostwald ripening process.14 At first, Bi
nanoparticles were formed in the solution according to eqs
1 and 2. Some of these nanoparticles were able to grow into
large crystals, which could serve as the seeds to grow into
nanowires or nanospheres as the small ones were continu-
ously dissolved. Figure 7a shows a U-type bismuth nanowire
produced through this polyol process, which indicates that
nanoparticles continuously grow into nanowires. According
(16) Lee, S. M.; Cho, S. N.; Cheon, J. AdV. Mater. 2003, 15, 441.
(17) http://www.chemexpert.com/.
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Wang et al.
molecules with the facets of Bi crystals is stronger than that
of ethylene glycol. So, in the presence of PVP, a large
amount of Bi nanowires in the product were obtained. As
for Bi nanospheres, acetone should decrease the interaction
between ethylene glycol and Bi nanoparticles, so the isotropic
growth of Bi was subsequently realized.
kinetically controlling the growth of bismuth nanowires.
These nanowires could serve as templates to prepare some
useful nanocomposites (for instance, Bi/SiO2 nanocables).
Bismuth and composite nanowires should exhibit excellent
electronic and thermoelectric properties, and the measurement
of these is currently underway.
Conclusion
Acknowledgment. This work was supported by NSFC
(50372030, 20025102, 20151001), the Foundation for the
Author of National Excellent Doctoral Dissertation of P. R.
China, and the state key project of fundamental research for
nanomaterials and nanostructures (2003CB716901).
In summary, we have demonstrated a solution-phase
approach to the large-scale synthesis of single-crystalline
nanowires and monodisperse nanospheres of bismuth by
reducing sodium bismuthate with ethylene glycol in the
presence of PVP or acetone. The capping molecules in the
reaction system have been found to play a key role in
IC049129Q
7556 Inorganic Chemistry, Vol. 43, No. 23, 2004