Shape control of single crystalline bismuth nanostructures

Article abstract of DOI:10.1021/jp063474e

A synthesis approach for shape control of single crystalline Bi nanostructures has been developed. By controlling the molar ratio of PVP and Bi in a polyol process, Bi nanocubes with an edge length of ～60-80 nm, triangular nanoplates with an edge length of 200-500 nm, and nanospheres with an average diameter of 75 nm have been successfully synthesized. In the same synthetic process, Bi nanobelts with lengths of up to 80 μm and widths of up to 0.6 μm were synthesized in large quantities by introducing a trace amount of Fe³⁺ species into the reaction system. These single crystalline nanostructure Bi materials are expected to find potential applications in a variety of areas including high efficiency thermoelectric devices.

Full text of DOI:10.1021/jp063474e

25702
J. Phys. Chem. B 2006, 110, 25702-25706
Shape Control of Single Crystalline Bismuth Nanostructures
Wen Zhong Wang,* Bed Poudel, Yi Ma, and Z. F. Ren*
Department of Physics, Boston College, Chestnut Hill, Massachusetts 02467
ReceiVed: June 5, 2006; In Final Form: October 15, 2006
A synthesis approach for shape control of single crystalline Bi nanostructures has been developed. By controlling
the molar ratio of PVP and Bi in a polyol process, Bi nanocubes with an edge length of ∼60-80 nm, triangular
nanoplates with an edge length of 200-500 nm, and nanospheres with an average diameter of 75 nm have
been successfully synthesized. In the same synthetic process, Bi nanobelts with lengths of up to 80 µm and
widths of up to 0.6 µm were synthesized in large quantities by introducing a trace amount of Fe3+ species
into the reaction system. These single crystalline nanostructure Bi materials are expected to find potential
applications in a variety of areas including high efficiency thermoelectric devices.
Introduction
Aldrich) in the presence of the capping polymer poly(vinyl
pyrrolidone) (PVP, MW 1 300 000) (Aldrich) or a trace amount
of Fe3+ (FeCl3, 99%, Aldrich) at 200 °C in a Parr reactor (125
mL, model 4750, Parr Company, Moline, IL). The experiments
suggested that the morphologies of the product are strongly
dependent on the molar ratio of PVP and Bi, and the trace
Bismuth (Bi), as a semimetal with a rhombohedral structure
and a very small band gap, has attracted extensive interest
recently as a potential good room temperature thermoelectric
material due to its low effective mass, highly anisotropic Fermi
surface, and potential to induce a semimetal-semiconductor
transition with decreasing crystallite size.1 Theoretical studies
predicted that nanostructured Bi should have a significantly
enhanced thermoelectric figure of merit,2 which have stimulated
great efforts to synthesize Bi nanostructured materials.3-5 Most
of the previous work has been focused on synthesis of one-
dimensional Bi nanowires using rigid inorganic or polymer
templates.6,7 Recently, there are a few reports on the synthesis
of Bi nanoparticles by a variety of methods, such as inverse
micelles,8 radiolytic reduction in aqueous solution,9 high tem-
perature organic solution reduction,10 and solution-based reduc-
tion.11,12 However, the morphology of the Bi nanoparticles
prepared by these methods are usually spherical. Up to now,
there is no report on the control of the shape of Bi nanocrystals.
Generally, the shape of a nanostructure may enable tailoring
the electronic, optical, magnetic, and catalytic properties of a
functional material. A great deal of effort has therefore been
devoted to the synthesis of nanostructures with well-controlled
shapes.13 Shape-controlled preparation of a number of metal
and alloy nanoparticles have been achieved, such as Co, Ag,
Au, Pt, Pd, Rh, Ir, and FePt, by reduction of salt compounds or
thermal decomposition of organometallic precursors in the
presence of surfactants, polymers, biomoleculars, and coordinat-
ing ligands.14 Here we demonstrate that the shape and size of
nanoscale Bi could be controlled by changing the ratio of the
concentration of the capping polymer material to the concentra-
tion of the Bi cations in the polyol process15-18 or by introducing
a trace amount of Fe3+ species to the reaction system.
Nanocubes, triangular nanoplates, nanospheres, and nanobelts
were synthesized using such approach and reported here.
amount of Fe3+
.
Field emission scanning electron microscopy (SEM, JEOL
6340F) and transmission electron microscopy (TEM, JEOL
2010F), high-resolution TEM (HRTEM), selected area electron
diffraction (SAED), and X-ray diffraction (XRD) were used to
characterize the Bi nanostructures.
Results and Discussions
When the molar ratio of PVP and Bi was kept at 1.6, single
crystalline Bi nanocubes were obtained. Figures 1a and 1b show
the SEM images of the as-prepared Bi nanocubes with different
magnifications. The low-magnification image (Figure 1a) clearly
indicates the abundance and narrow size distribution with an
edge length of ∼60-80 nm. The high magnification image
(Figure 1b) demonstrates the good cubic shape with smooth
surfaces. Figure 1c shows the typical 2-theta XRD pattern of
the as-prepared Bi nanocubes. All diffraction peaks were indexed
to the rhombohedral Bi structure (JCPDS No. 05-0519, R3hm).
Figure 2 shows the TEM and HRTEM images of the Bi
nanocubes. The low magnification TEM image (Figure 2a)
demonstrates that the Bi nanocrystals have fairly uniform size
with cubic structure of an average edge length of ∼70 nm,
consistent with the results of SEM observations as shown in
Figure 1a. Figure 2b shows the TEM image of an individual Bi
nanocrystal, indicating a cubic shape. Figure 2c shows the TEM
image of a single nanocube used for the studies of HRTEM
and SAED (inset in Figure 2c) obtained by aligning the electron
beam perpendicular to one of the faces of the cube. The square
diffraction patterns were indexed based on a rhombohedral cell
with a lattice parameter of a ) 4.546 and c ) 11.860 Å,
indicating the single-crystal nature. Figure 2d shows the
HRTEM image taken from the marked area (area I in Figure
2c). The clear two-dimensional (2D) lattice fringes demonstrate
that the cube is highly crystallized. The spacing of 0.32 nm
corresponds to the (012) planes of Bi, indicating that one of
the cubic faces is (012), stabilized by the PVP:Bi molar ratio
Experimental Section
Bi nanocrystals with controlled shape and size were prepared
by a polyol process. The primary reaction involved the reduction
of NaBiO3 (99.6%, Aldrich) with ethylene glycol (EG, 99.8%,
* Corresponding author. E-mail: renzh@bc.edu; wangwm@bc.edu.
10.1021/jp063474e CCC: $33.50 © 2006 American Chemical Society
Published on Web 12/05/2006

Single Crystalline Bismuth Nanostructures
J. Phys. Chem. B, Vol. 110, No. 51, 2006 25703
Figure 1. (a) Low- and (b) high-magnification SEM images; the scale
bars represent 100 nm. (c) XRD pattern of the as-prepared Bi
nanocubes.
of 1.6. Figures 2e and 2f are the HRTEM images taken along
the edge (area II in Figure 2c) and around the corner (area III
in Figure 2c), respectively. Both 2e and 2f show the Bi crystals
are free of oxide layer on the surface, an indication of good
stability of the cubes in air.
Figure 2. (a) Typical TEM images of the as-prepared Bi nanocubes;
the scale bar represents 50 nm. (b) TEM image of an individual Bi
nanocrystal showing good cubic shape; the scale bar represents 50 nm.
(c) TEM image and the corresponding SAED pattern of a single cube;
the scale bar represents 20 nm. (d-f) HRTEM images taken from the
marked areas I, II, and III in Figure 2c, respectively, the scale bars
represent 2 nm.
When the molar ratio of PVP and Bi was reduced to 0.8,
single crystalline Bi triangular nanoplates were obtained together
with cubic and irregular shaped nanoparticles. The percentage
of triangular nanoplates in the sample is about 30%. Figures 3a
and 3b show the low- and high-magnification TEM images of
the triangular nanoplates, respectively. The TEM images indicate
that the lengths of the three edges of each triangular nanoplate
are equal in the range of 200-500 nm. Figure 3c shows the
TEM image of a single triangular nanoplate with the SAED as
the inset obtained by aligning the electron beam perpendicular
to the face of this plate. The hexagonal symmetric diffraction
pattern indicates the single crystalline nature of the nanoplate.
Figure 3d shows the HRTEM image with clear lattice fringes
that confirm the highly crystalline nature. The spacing of 0.222
nm corresponds to the (110) planes of Bi. From the SAED
pattern (inset, Figure 3c), we know that the crystal plane is (002)
that is stabilized by the PVP:Bi molar ratio of 0.8. However, if
the molar ratio of PVP and Bi was further reduced to 0 or
increased to 5, spherical nanoparticles were formed. Figure 3e
shows the TEM image of the product prepared in the absence
of PVP. The product is composed of spherical particles with
wide diameter distribution ranging from 20 to 200 nm. When
the molar ratio is increased to 5, the product is composed of
uniform spheres with an average diameter of 75 nm as shown
in Figure 3f.
wide diameter distribution were obtained in the absence of PVP
(Figure 3e) indicates that some Bi tiny crystals aggregate to
form big particles due to lack of protection from the stabilizing
agent PVP. However, when the molar ratio is increased to 5,
excess PVP molecules could act as stabilizing agents to protect
particles from aggregating to form big particles. On the other
hand, PVP acts as a shape-controlling agent for the formation
of Bi nanocubes and triangular nanoplates. It has been reported
that the capping organic molecules could modulate the kinetics
of the crystal growth and determine the subsequent shape of
the crystals.13c,19-21 For the present case, PVP molecules could
adsorb onto the surfaces of Bi crystals through O-Bi bonding
as reported in previous work.4 PVP molecules adsorbed on some
surfaces of the Bi crystals could significantly decrease their
growth rates and lead to highly anisotropic growth. A decrease
in PVP concentration leads to an ineffectively selective adsorp-
tion of the PVP molecule and its respective counter ions on the
Bi crystal faces, resulting in the formation of various morphol-
ogies, such as triangular nanoplates and cubic and irregular
shape. As for high PVP concentration, excess PVP molecules
could decrease the interaction between ethylene glycol and Bi
nanocrystals, so the isotropic growth of Bi crystals was
subsequently realized resulting in the formation of Bi nano-
spheres with uniform size. Therefore the selective interaction
between PVP and various crystallographic planes of Bi plays
The above experimental results indicate that the molar ratio
of PVP and Bi plays a key role for the shape control of Bi
nanocrystals. We think that PVP plays multiple roles in our
synthetic system. It acts not only as the shape control agent but
also as the stabilizing agent. The fact that only spheres with

25704 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Wang et al.
Figure 4. SEM images of the as- prepared Bi nanobelts at (a) low-
and (b) high-magnification; the scale bars represent 10 and 1 µm,
respectively. (c) XRD pattern of the Bi nanobelts.
the studies of HRTEM and SAED obtained by aligning the
electron beam perpendicular to the face of the belt. The square
diffraction pattern was indexed on the basis of a rhombohedral
cell with a lattice parameter of a ) 4.546 and c ) 11.860 Å,
indicating that the Bi nanobelt is a single crystal. Figure 5d
shows the HRTEM image of the nanobelt shown in Figure 5c.
The clear 2D lattice fringes demonstrate that the belt is highly
crystallized. The spacing of 0.32 nm corresponds to the (012)
planes of Bi, from which we determined that the crystal plane
is (221), stabilized by PVP:Bi ratio of 1.6 and a trace amount
of Fe3+. From TEM image (Figure 5c) and the corresponding
SAED pattern (inset in Figure 5c), the growth direction of the
Bi nanobelt is derived as [11h0].
It is worthwhile to elucidate the growth mechanism of the
nanobelts. As reported in previous works, the organic molecules
could play the roles of a surface modifier and assembling agents
during the formation of nanostructures by oriented attachment
mechanism, because the organic molecules adsorbed on a
crystallographically specific surface can modify oriented at-
tachment mechanism by changing the surface energy and also
preventing contact between the surfaces on which adsorption
has selectively occurred.22-24 Thus, in our current case, the
freshly formed Bi nanocrystals could align and subsequently
form nanobelts in the presence of assembling agent PVP. The
conclusions could be verified by extensive and careful TEM
and HRTEM observations. In TEM examinations, we found that
some belts were composed of fused nanoparticles (Figure 5e),
and further HRTEM studies indicated that these fused nano-
particles had the same growth direction (Figure 5f), revealing
that aligning of orientations might have happened before they
connected to each other. Therefore the Bi nanobelts were
probably formed through the oriented attachment mechanism
foundinthegrowthofotherlow-dimensionalnanostructures.22,25-27
On the basis of the above experimental results and analysis,
we speculate that the formation process of Bi nanobelts could
be as the following: (i) when a small amount of Fe3+ was added
Figure 3. Typical TEM images of the Bi triangular nanoplates at (a)
low- and (b) high-magnification; the scale bars represent 500 and 200
nm respectively. (c) TEM image and the corresponding SAED pattern
of a single nanoplate; the scale bar represents 20 nm. (d) HRTEM image
of the nanoplate of part c; the scale bar represents 2 nm. (e) TEM
image of the as-prepared products without PVP; the scale bar represent
100 nm. (f) TEM images of the as-prepared products with the molar
ratio of PVP and Bi of 5; the scale bar represents 50 nm.
the major role for the formation of Bi nanocubes and triangular
nanoplates, although the exact bonding geometry and the nature
of the selective adsorption of PVP on various crystallographic
planes of Bi are still not clear. The fundamental basis of shape
selectivity for this system has yet to be fully understood.
It was also found that introduction of a trace amount of Fe3+
species to the reaction system greatly changed the shape of the
final Bi nanocrystals. Figure 4 shows the SEM images of the
Bi nanobelts prepared in the presence of Fe3+ species with the
molar ratio of PVP and Bi of 1.6. The low magnification image
(Figure 4a) indicates that large quantities of nanobelts are formed
with lengths up to 80 µm. The high magnification image (Figure
4b) demonstrates that the width of the nanobelts is up to 0.6
µm. Figure 4c shows the typical XRD pattern of the as-prepared
Bi nanobelts. All the diffraction peaks were indexed to the
rhombohedral Bi structure (JCPDS No. 05-0519, R3hm).
Figure 5 shows the typical TEM images of the Bi nanobelts
with different magnifications. The low magnification image
(Figure 5a) indicates that the nanobelts display a broad width
distribution from 50 to 600 nm. High magnification image
(Figure 5b) indicates that the nanobelts are transparent under
the irradiation of electric beam, revealing that the thickness of
the nanobelts is very thin although the width is up to 0.6 µm.
Figure 5c shows the TEM image of a single nanobelt used for

Single Crystalline Bismuth Nanostructures
J. Phys. Chem. B, Vol. 110, No. 51, 2006 25705
TABLE 1: The Relationship Between the Synthesis
Conditions and the Structures of the Crystals
stabilized
molar ratio of PVP/Bi
morphology
planes
0
0.8
sphere
triangular nanoplate (30%) (002)
with cubic and irregular
shape
1.6
5
nanocube
(012), (102),
and (221)
nanosphere
1.6 with the presence of nanobelt
(221)
a trace amount of Fe3+
Conclusions
In summary, Bi nanoscale materials with different shapes have
been successfully synthesized by a polymer-assisted polyol
process. The molar ratio of PVP and Bi played an important
role for the formation of Bi nanocubes, triangular nanoplates,
and spheres. The experiments also indicated that introduction
of a trace amount of Fe3+ species greatly reduced the nucleation
density of Bi and the growth rate, leading to the formation of
single-crystal Bi nanobelts.
Acknowledgment. The work is funded by NSF NIRT
0506830.
References and Notes
(1) (a) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 12727.
(b) Koga, T.; Harman, T. C.; Cronin, S. B.; Dresselhaus, M. S. Phys. ReV.
B. 1999, 60, 14286.
(2) Hicks, L. D.; Dresselhaus, M. S. Phys. ReV. B 1993, 47, 16631.
(b) Sun, X.; Zhang, Z.; Dresselhaus, M. S. Appl. Phys. Lett. 1999, 74, 4005.
(c) Lin, Y.-M.; Sun, X.; Dresselhaus, M. S. Phys. ReV. B 2000, 62, 4610.
(3) Yu, H.; Gibbons, P. C.; Buhro, W. E. J. Mater. Chem. 2004, 14,
595.
(4) Wang, J.; Wang, X.; Peng, Q.; Li, Y. D. Inorg. Chem. 2004, 43,
7552.
(5) Fu, R. L.; Xu, S.; Lu, Y. N.; Zhu, J. J. Cryst. Growth Des. 2005,
5, 1379.
Figure 5. Typical TEM images of the Bi nanobelts at (a) low- and
(b) high-magnification; the scale bars represent 500 and 200 nm,
respectively. (c) TEM image and the corresponding SAED pattern of
a single nanobelt; the scale bar represents 100 nm. (d) HRTEM image
of the nanobelt shown in part c; the scale bar represents 2 nm. (e) A
typical TEM image of an individual belt, clearly indicating that the
belt is composed of fused nanoparticles, the scale bar represents 20
nm. (f) HRTEM image of part e indicating that the fused nanoparticles
have the same growth direction; the scale bar represents 5 nm.
(6) Dresselhaus, M. S.; Zhang, Z.; Sun, X.; Ying, J. Y.; Heremans, J.;
Dresselhaus, G.; Chen, G. Mater. Res. Soc. Symp. Proc. 1999, 545, 215.
(7) Heremans, J. P.; Thrush, C. M.; Morelli, D. T.; Wu, M. C. Phys.
ReV. Lett. 2002, 88, 216801.
(8) Foos, E. E.; Stroud, R. M.; Berry, A. D.; Snow, A. W.; Armistead,
J. P. J. Am. Chem. Soc. 2000, 122, 7114.
(9) Gutierrez, M.; Henglein, A. J. Phys. Chem. 1996, 100, 7656.
(10) Fang, J. Y.; Stokes, K. L.; Zhou, W. L.; Wang, W. D.; Lin, J. Chem.
Commun. 2001, 18, 1872.
(11) Balan, L.; Schneider, R.; Billaud, D.; Fort, Y.; Ghanbaja, J.
Nanotechnology 2004, 15, 940.
(12) Wang, Y. W.; Hong, B. H.; Kim, K. S. J. Phys. Chem. B 2005,
109, 7067.
into the system, the reduction of Bi3+ was significantly slowed
down, resulting in a substantial decrease of the nucleation rate
of the Bi nanoparticles; (ii) the nanoparticles aligned to the same
direction and then connected to each other to form wire-like
structures induced by assembling agent PVP; (iii) with the
prolongation of reaction time and the improvement of crystal-
linity, these nanoparticles further fused and then grew into
single-crystal Bi nanobelts. The role of Fe3+ is to oxidize Bi(0)
to Bi(III) and thus greatly reduce the nucleation density of Bi.
We think that the role of Fe3+ in our case is the same as that in
the cases of platinum nanowires17 or palladium nanostructures.18
The detailed study on the growth mechanism of Bi nanobelt is
still under way. Our experimental results indicate that the Bi
nanostructures with different morphology can be synthesized
by controlling the molar ration of PVP and Bi, or introducing
a trace amount of Fe3+ species in the polyol process. The
relationship between synthesis conditions and the structures of
the crystals is summarized in Table 1.
(13) (a) Wang, Z. L.; Ahmad, T. S.; El-Sayed, M. A. Surf. Sci. 1997,
380, 302. (b) Wang, Z. L. J. Phys. Chem. B 2000, 104, 1153. (c) Milliron,
D. J.; Hughes, S. M.; Cui, Y.; Manna, L.; Li, J.; Wang, L. W.; Alivisatos,
A. P. Nature 2004, 430, 190. (d) Peng, X.; Manna, L.; Yang, W.; Wickham,
J.; Scher, E.; Kadavanich, A.; Alivisatos, A. P. Nature 2000, 404, 59. (e)
Kim, F.; Connor, S.; Sond, H.; Kuykendall, T.; Yang, P. Angew. Chem.,
Int. Ed. 2004, 43, 3673. (f) Tapan, T. K.; Murphy, C. J. J. Am. Chem. Soc.
2004, 126, 8648. (g) Chen, S.; Wang, Z.; Ballato, J.; Foulger, S. H.; Carroll,
D. L. J. Am. Chem. Soc. 2003, 125, 16186. (h) Pileni, M. Nat. Mater. 2003,
2, 145.
(14) (a) Sun, S.; Murray, C. B.; Weller, D.; Folks, L.; Moser, A. Science
2000, 287, 1989. (b) Chen, S.; Wang, Z. L.; Ballato, J.; Foulger, S. H.;
Carroll, D. L. J. Am. Chem. Soc. 2003, 125, 16186. (c) Caswell, K. K.;
Wilson, J. N.; Bunz, U. H. F.; Murphy, C. J. J. Am. Chem. Soc. 2003, 125,
13914. (d) Kim, F.; Connor, S.; Song, H.; Kuykendall, T.; Yang, P. Angew.
Chem., Int. Ed. 2004, 43, 3673. (e) Narayanan, R.; El-Sayed, M. A. J. Phys.
Chem. B 2004, 108, 5726. (f) Hao, E.; Bailey, R. C.; Schatz, G. C.; Hupp,
J. T.; Li, S. Nano Lett. 2004, 4, 327. (g) Yin, Y,; Rioux, R. M.; Erdonmez,
C. K.; Hughes, S.; Somorjai, G. A.; Alivisatos, A. P. Science 2004, 304,
711.
(15) Fievet, F.; Lagier, J. P.; Figlarz, M. MRS Bulletin 1989, December,
29.
(16) Wang, Y. L.; Xia, Y. N. Nano Lett 2004, 4, 2047.

25706 J. Phys. Chem. B, Vol. 110, No. 51, 2006
Wang et al.
(17) Cheng, J.; Herricks, T.; Geissler, M.; Xia, Y. N. J. Am. Chem. Soc.
2004, 126, 10854.
(23) Polleux J.; Pinna, N.; Antonietti, M.; Niederberger, M. AdV. Mater.
2004, 16, 436.
(18) Xiong, Y.; McLellan, J. M.; Chen, J.; Yin, Y.; Li, Z. Y.; Xia, Y.
N. J. Am. Chem. Soc. 2005, 127, 17118.
(24) J.; Pinna, N.; Pinna, N.; Antonietti, M.; Hess, C.; Wild, U.; Schlo¨gl,
R.; Niederberger, M. Chem. Eur. J. 2005, 11, 3541.
(25) Oskam, G.; Nellore, A.; Penn, R. L.; Searson, P. C. J. Phys. Chem.
B 2003, 107, 1734.
(26) Oskam, G.; Hu, Z.; Penn, R. L.; Pesika, N.; Searson, P. C. Phys.
ReV. E 2002, 66, 11403.
(19) (a) Sun, Y.; Xia, Y. AdV. Mater. 2002, 14, 833. (b) Sun, Y.; Yin,
Y.; Mayers, B. T.; Herricks, T.; Xia, Y. Chem. Mater. 2002, 14, 4736.
(20) Peng, X. AdV. Mater. 2003, 15, 459.
(21) Lee, S. M.; Cho, S. N.; Cheon, J. AdV. Mater. 2003, 15, 441.
(22) Penn, R. L.; Banfield, J. F. Geochim. Cosmochim. Acta 1999, 63,
1549
(27) Alivisatas, A. P. Science 2000, 289, 736.