Size control of semimetal bismuth nanoparticles and the UV - Visible and ER absorption spectra

Article abstract of DOI:10.1021/jp046423v

We introduced a simple chemical method to synthesize semimetal bismuth nanoparticles in N,N-dimethylformamide (DMF) by reducing Bi³⁺ with sodium borohydride (NaBH₄) in the presence of poly-(vinylpyrroldone) (PVP) at room temperature. The size and dispersibility of Bi nanoparticles can be easily controlled by changing the synthetic conditions such as the molar ratio of PVP to BiCl₃ and the concentration of BiCl₃. The UV - visible absorption spectra of Bi nanoparticles of different diameters are systematically studied. The surface plasmon peaks broaden with the increasing molar ratio of PVP to BiCl₃ as the size of bismuth nanoparticles decreases. Infrared (IR) spectra of the complexes with different molar ratios of PVP/BiCl₃ show a strong interaction between the carboxyl oxygen (C=O) of PVP and Bi³⁺ ion and a weak interaction between the carboxyl oxygen (C=O) of PVP and the Bi atom in nanoparticles. This indicates that PVP serves as an effective capping ligand, which prevents the nanoparticles from aggregation. ? 2005 American Chemical Society.

Full text of DOI:10.1021/jp046423v

J. Phys. Chem. B 2005, 109, 7067-7072
7067
Size Control of Semimetal Bismuth Nanoparticles and the UV-Visible and IR Absorption
Spectra
Y. W. Wang, Byung Hee Hong, and Kwang S. Kim*
National CreatiVe Research InitiatiVe Center for Superfunctional Materials, Department of Chemistry, DiVision
of Molecular and Life Sciences, Pohang UniVersity of Science and Technology, San 31, Hyojadong, Namgu,
Pohang 790-784, Korea
ReceiVed: August 9, 2004; In Final Form: December 7, 2004
We introduced a simple chemical method to synthesize semimetal bismuth nanoparticles in N,N-
dimethylformamide (DMF) by reducing Bi3+ with sodium borohydride (NaBH4) in the presence of poly-
(vinylpyrroldone) (PVP) at room temperature. The size and dispersibility of Bi nanoparticles can be easily
controlled by changing the synthetic conditions such as the molar ratio of PVP to BiCl3 and the concentration
of BiCl3. The UV-visible absorption spectra of Bi nanoparticles of different diameters are systematically
studied. The surface plasmon peaks broaden with the increasing molar ratio of PVP to BiCl3 as the size of
bismuth nanoparticles decreases. Infrared (IR) spectra of the complexes with different molar ratios of PVP/
BiCl3 show a strong interaction between the carboxyl oxygen (CdO) of PVP and Bi3+ ion and a weak
interaction between the carboxyl oxygen (CdO) of PVP and the Bi atom in nanoparticles. This indicates that
PVP serves as an effective capping ligand, which prevents the nanoparticles from aggregation.
1. Introduction
recently, using a high-temperature organic solution reduction
method, highly crystalline and single domain Bi nanoparticles
were synthesized and self-assembled.10 Buhro et al. also reported
the growth of nearly monodispersed Bi, Sn, and In nanoparticles
on 1.5 nm Au seeds.11 However, there is no effective method
to control the size and dispersibility of bismuth nanoparticles.
Therefore, the dependence of the surface plasmon bands on the
size of bismuth nanoparticles has not been successfully observed
yet.
In this paper, we introduce a new simple chemical method
to synthesize semimetal Bi nanoparticles in N,N-dimethylform-
amide (DMF) solution by reduction of Bi3+ using sodium
borohydride (NaBH4) in the presence of poly(vinylpyrroldone)
(PVP). By systematically changing the synthetic conditions such
as the molar ratio of PVP to BiCl3 and the concentration of
BiCl3, one can easily prepare the high crystalline Bi nanopar-
ticles in narrow size distribution, high dispersibility and single
phase purity. More importantly, the diameter of bismuth
nanoparticles can be easily controlled from 6 to 13 nm. In
particular, we find the gradual broadening of the surface plasmon
peak, as the nanoparticle size decreases. We also report infrared
spectra (IR) of the pure PVP, the complexes with different molar
ratios of PVP/BiCl3, and the Bi nanoparticles capped with PVP
molecules. This provides the information on the possible
coordination and reaction process for PVP and Bi3+ ions (Bi
atom). On the basis of these results, we propose the protective
mechanism of PVP for the synthesis bismuth nanoparticles.
Crystalline materials exhibit a variety of interesting optical,
electronic, and magnetic features when the physical size of the
crystalline approaches to the nanometer scale.1 Bismuth is a
semimetal with a rhombohedral structure. It has a small energy
overlap between the conduction and valence bands, high carrier
motilities, highly anisotropic Femi surface, and small effective
mass. As the crystalline size decreases, the induced semimetal-
semiconductor transition would be possible.2 This size induced
semimetal to semiconductor transition and the related quantum
confinement effects are potentially useful for optical and
electrooptical device applications. Recent work also has sug-
gested that Bi materials of reduced dimensions may exhibit
enhanced thermoelectric properties at room temperature.3
Most of previous works have focused on either one-
dimensional wires fabricated by injecting liquid Bi into the
nanochannels of porous anodic alumina membrane or by
electrodepositing Bi into templates4 or on two-dimensional films
grown on substrate by using molecular beam epitaxy (MBE).5
Usually, Bi nanoparticles exhibit quantum confinement effects
as their size becomes smaller than the size of electron’s
wavelength in its bulk solid. Further decrease in size of the
nanoparticles causes a semimetal to semiconductor transition.
Nevertheless, few studies have been directed toward the Bi
nanoparticles so far. Previously, Henglein et al. used the
radiolysis reduction of Bi(III) in aqueous solutions to obtain
sols of Bi nanoparticles (mean diameter: 20 nm). The absorption
spectrum of this colloidal Bi has a weak peak at 253 nm.6
Recently, Bi nanoparticles were chemically synthesized from
the reduction of Bi salts in reverse micelles (water-in-oil inverse
microemulsions).7-9 In these different experiments, the promi-
nent surface plasmon absorption feature was observed at
different wavelengths (268 and 280 nm), respectively.7-9 More
2. Experimental Section
2.1. Chemicals. BiCl3 was used as precursors of Bi nano-
particles. Sodium borohydride (NaBH4) was used as a reducing
agent. N,N-Dimethylformamide (DMF, 99.9+%, water<0.005%)
and poly(vinylpyrroldone) (PVP, Mw ) 55 000) were used as
solvent and stabilizer, respectively. All of these high grade
chemicals were purchased from Aldrich Chemical Company and
used without further purification.
* Corresponding author: Tel: +82-54-279-2110, Fax: +82-54-279-8137
Email: kim@postech.ac.kr.
10.1021/jp046423v CCC: $30.25 © 2005 American Chemical Society
Published on Web 03/30/2005

7068 J. Phys. Chem. B, Vol. 109, No. 15, 2005
Wang et al.
2.2. Preparation Procedure of PVP-Protected Bi Nano-
particles. The detailed synthetic procedure of Bi nanoparticles
is as follows: 31.5 mg (0.1 mmol) BiCl3 and the designed
amount of PVP were dissolved in 10 mL of DMF, which was
stirred and degassed using nitrogen for 15 min. The amount of
PVP was altered from 0.555 to 55.5 mg (0.005 to 0.5 mmol as
a monomeric unit). In addition, 0.3 mL of 1.0 M NaBH4 aqueous
solution was injected into 10 mL of DMF and also degassed
using nitrogen for 15 min. Then, DMF solution containing
NaBH4 was added into the mixture solution of PVP and BiCl3
with constant stirring under flowing nitrogen. Within a few
minutes, the colorless solution darkened to black. Stirring was
continued for another 5 min after addition. Bismuth nanopar-
ticles were precipitated by adding acetone to the resulting black
solution and collected by centrifugation under nitrogen atmo-
sphere. The precipitate was then redispersed into DMF for
further characterization.
2.3. Characterization. The disappearance of Bi3+ ions and
the formation of Bi nanoparticles were confirmed by UV-
visible spectra with a HP8453 diode arrays spectrophotometer
equipped with a thermostatic multiple sample holder. The
bismuth nanoparticle solutions were diluted 30 times with water
before taking spectra. Deionized water was used as the reference.
The Bi nanoparticles were characterized using energy-dispersed
X-ray spectrometry (EDS) as well as high-resolution transition
electron microscope (HRTEM JEOL-2010F) operating at 200
kV. To deposit the nanoparticles, the carbon-coated copper grids
were dipped directly into colloidal solution, which was previ-
ously diluted with ethanol to avoid the dissolution of the polymer
layer on the grids by DMF and to decrease the concentration
of PVP for better contrast.
Figure 1. UV-visible spectra of BiCl3 aqueous solution, PVP + BiCl3
aqueous solution and PVP-Bi nanoparticle DMF solution formed with
3+
n
PVP/nBi ) 0.075 and 10 mM BiCl3.
of electrons in response to optical excitation. The Bi clusters
with diameters below 10 nm did not show any surface plasmon
absorption band,9 while the larger nanoparticles show different
absorption peaks at 268 and 280 nm in two different experi-
ments.7,8 According to calculation,13 the first absorption band
of 10 nm Bi nanoparticles should appear around 270-280 nm.
IR study was carried out on a Nicolet model 759 Fourier
transform infrared spectrometer at wavenumbers 500-4000
cm-1 with a resolution of 1 cm-1. The KBr pellets for IR spectra
were made by mixing KBr powder with the pure PVP, the
complexes and the bismuth nanoparticles capped with PVP
molecules. The complexes with different molar ratios of PVP/
BiCl3 were prepared by dissolving the corresponding compo-
nents in methanol. The solution was then cast on a Teflon glass
plate and dried in an N2 environment. The PVP-stabilized
bismuth nanoparticles were precipitated by adding acetone to
the resulting black solution, and collected by centrifugation
under nitrogen atmosphere. The obtained precipitates were
washed with a large amount of ethanol, and then dried in an N2
environment for IR measurement.
3+
In the present case (nPVP/nBi ) 0.075, 10 mM BiCl3 solution),
the prominent absorption feature at 281 nm is almost certainly
the vestige of the plasmon peak characteristics of large bismuth
nanoparticles, which is consistent with refs 8 and 10. Therefore,
the disappearance of peak at 268 nm and the appearance of a
new peak at 281 nm in Figure 1 indicate that the Bi3+ions are
reduced completely and the bismuth nanoparticles are formed.
The TEM image in Figure 2a further shows the morphology
and the size distribution of the as-prepared Bi nanoparticles.
This micrograph reveals that the Bi nanoparticles formed in the
presence of PVP are spherical. The size of these nanoparticles
is in the range of 13 nm in diameter with a broad size
distribution. The selected area electron diffraction (SAED)
pattern shown in the inset of Figure 2a indicates that the
rhombohedral Bi nanoparticles are highly crystalline, which
corresponds to the diffraction indices (003), (012), (110), and
(013), respectively, according to JCPDS card No. 05-0519.14
We have further used EDS to address the composition of
nanoparticles. The EDS analysis (Figure 2b) demonstrates that
the as-prepared nanoparticles contain only Bi element, and no
other element is found. The carbon and copper peaks are
generated from the carbon-coated copper grid.
3. Results and Discussion
3.1. Formation of Bi Nanoparticles. The formation process
3+
of PVP-stabilized Bi (PVP-Bi) nanoparticles (nPVP/nBi
)
0.075, molar ratio between the monomeric unit of PVP and
BiCl3) is described as a representative. Figure 1 shows the UV-
visible spectra of PVP, BiCl3 + PVP aqueous solution, and as-
prepared bismuth nanoparticle solution diluted with water. Two
absorption peaks at 214 and 267 nm are associated with the
presence of PVP and Bi3+, respectively. When the addition of
NaBH4 into the mixed BiCl3/PVP solution is complete, the peak
at 267 nm disappears and a new peak at 281 nm appears. The
high optical density at long wavelength is ascribed to the
presence of organic solvent (DMF) in the bismuth nanoparticle
solutions, which can also be found in the UV-absorption spectra
of other metal nanoparticle solutions.12 The surface plasmon
resonance is a characteristic of metal nanoparticles embedded
in a dielectric host and is attributed to the collective oscillation
3.2. Change of the Absorption Spectra Depending on the
Metallic Cluster Size. The absorption spectra of colloidal metal
particles depending on the particle size have been reported. If
the particle dimensions are smaller than the mean free path of
the conduction electrons, collisions of these electrons with the
particle surface are noticed.15 This lowers the effective mean
free path.
The theory of the surface plasmon band of metal particles
depending on their size was already derived in the past.16,17 Here,
we summarize it in order to facilitate the discussion of our
experimental results.

Semimetal Bismuth Nanoparticles
J. Phys. Chem. B, Vol. 109, No. 15, 2005 7069
ωd ) Vf/Rbulk (7)
When the particle radius (R) is smaller than the mean free path
in the bulk metal, the conduction electrons are additionally
scattered by the surface, and the mean fee path (Reff) becomes
size dependent,
1/Reff ) 1/R + 1/Rbulk
(8)
The most important parameter affecting ωd is the particle size.
From eqs 2, 7, and 8, it can be seen that a decrease in particle
size leads to an increase in ωd, causing the band to broaden
and the maximum intensity to decrease. The position of the peak
is virtually unaffected by small changes, while a slow shift to
the lower energy occurs for large damping.
3.3. Size Control of Bi Nanoparticles by Varying the
Amount of PVP and the Change in UV)Visible Absorption
Spectra. The protective polymer, PVP, was widely used as a
stabilizer to prevent nanoparticles from aggregation in chemical
preparation of noble metal nanoparticles including gold,18
silver,19 palladium,20 and platinum.21 In this paper, we also used
PVP as a stabilizer to prevent Bi nanoparticles from aggregation,
as we did to make antimony nanoparticles and nanowires.22
When the PVP was absent in our synthetic process, black
precipitate appeared quickly upon adding the reducing agent
solution. However, when very low concentration of PVP was
added into the BiCl3 solution, a black nanoparticle solution was
obtained after the reaction finished. Furthermore, as more PVP
was added, more stable Bi nanoparticle solution was obtained.
It is apparent that the PVP keeps the Bi nanoparticles from
aggregation during the reaction process. To investigate the
protective mechanism of PVP for the synthesis of bismuth
nanoparticles, we recorded the IR spectra of the pure PVP, the
complexes with different molar ratios of PVP/BiCl3, and the
resulting PVP-stabilized bismuth nanoparticles. Parts a-d of
Figure 3 show the IR spectra of PVP (Figure 3a) and the PVP/
BiCl3 complexes with different PVP/BiCl3 molar ratios of 10
(Figure 3b), 5 (Figure 3c), and 1 (Figure 3d), respectively. The
Figure 2. (a) TEM image of the as-prepared Bi nanoparticles (nPVP
/
3+
nBi ) 0.075, 10 mM BiCl3). The inset shows the SAED pattern of
the as-prepared nanoparticles. (b) EDS pattern of the as-prepared Bi
nanoparticles.
In the presence of a dilute colloidal solution containing N
particles per unit volume, the measured attenuation of light
intensity I0 over a path length d cm is given by16
A ) log10I0/Id ) NCextd/2.303
(1)
where Cext is the extinction cross section of a single particle. In
the limit when 2πR < λ (where R is the radius of the particles
and λ is the wavelength of the light in the media), only the
electric dipole term, developed in Mie’s theory, is significant.
Then the extinction cross section Cext can be expressed as
24π2R3ꢀ3/2
ꢀ2
Cext
)
(2)
free CdO stretching band of pure PVP is at 1675 cm-1 23
. The
2
(ꢀ1 + 2ꢀ)2 + ꢀ2
λ
intensity of this band diminishes because the number of free
CdO band decreases, and a new band at 1588 cm-1 appears
with the decreasing molar ratio of PVP/BiCl3.24 This new band
is thought to be associated with the coordination interactions
between bismuth ions and carbonyl oxygen. The shift of the
free CdO band to a lower wavenumber originates from the
loosening of the CdO double bond by Bi3+-coordination. When
the molar ratio of PVP/BiCl3 increases at a constant concentra-
tion of BiCl3, the number of bismuth ions becomes much smaller
than the number of carbonyl oxygens in PVP.25 After reduction
of bismuth ions, the number of bismuth atoms present at close
range decreases, and so the average size of the bismuth
nanoparticles decreases. Furthermore, the IR spectra of the
complex with a PVP/BiCl3 molar ratio of 5 and the resulting
washed PVP-stabilized bismuth nanoparticles are compared in
Figure 4, parts a-c. The disappearances of the bands at 1558
and 1675 cm-1 in the IR spectra of PVP-stabilized bismuth
nanoparticles are attributed to the reduction of bismuth ions and
the removal of free PVP, respectively. A new band at 1659
cm-1 appears which is thought to be associated with the weak
interactions between the reduced bismuth atoms and the carbonyl
oxygen atoms. As Bi3+ binds the O atom in the CdO bond,
this CdO bond weakens, and the frequency red-shifts by a large
amount, while as the reduced Bi atom binds the O atom in the
CdO bond, this CdO bond weakens only slightly, and the
frequency red-shifts by a small amount. Therefore, the protective
where ꢀ is the dielectric permittivity:
ꢀ ) ꢀ1 + iꢀ2
(3)
In the case of many metals, the region of absorption up to the
bulk plasma frequency is dominated by the free electron
behavior, and the dielectric response is well described by the
simple Drude model. According to this theory,17 the real and
imaginary parts of the dielectric function are
ꢀ1 ) ꢀ∞ - ωp /(ω2 + ωd )
(4)
(5)
2
2
ꢀ2 ) ωp ωd/ω(ω2 + ωd )
2
2
where ꢀ∞ is the high-frequency dielectric constant due to
interband and core transitions and ωp is the bulk plasma
frequency
ωp2 ) Ne2/mꢀ0
(6)
in terms of the concentration of free electrons in metal (N), and
the effective mass of the electron (m). ωd is the relaxation or
damping frequency, which is related to the mean free path of
the conduction electrons (Rbulk) and the velocity of electrons at
the Fermi energy (Vf):

7070 J. Phys. Chem. B, Vol. 109, No. 15, 2005
Wang et al.
Figure 3. IR spectra of (a) pure PVP; the complexes with different
molar ratios of PVP to BiCl3: (b) 10, (c) 5, and (d) 1. In all cases, the
concentration of BiCl3 is 10 mM.
Figure 4. IR spectra of the pure PVP (a), the complex with a PVP/
BiCl3 molar ratio of 5 (b), and the resulting bismuth nanoparticles with
a PVP/BiCl3 molar ratio of 5 (c).
mechanism of PVP for the synthesis of bismuth nanoparticles
is summarized as follows. When PVP and BiCl3 are dissolved
in DMF, bismuth ions interact with the carbonyl oxygen of PVP
to form the coordinative bonding. Then bismuth ions are reduced
to bismuth atoms by adding reducing reagents; subsequently,
the nearby bismuth atoms aggregate at close range (increasing
the amount of PVP or decreasing the concentration of BiCl3
will result in decreasing the number of bismuth atoms at close
range, so the average size of the bismuth nanoparticles
decreases). Finally, the PVP adsorbed on the surface of bismuth
nanoparticle prevents them from aggregation.
molar ratio of PVP to BiCl3 increased from 0.075 to 0.15. In
this case, Bi nanoparticles with the average diameter of 11 nm
were obtained. Although the average diameter of these Bi
nanoparticles just slightly decreased than that obtained at low
3+
molar ratio of PVP to BiCl3 (nPVP/nBi ) 0.075, as shown in
Figure 2a), the distribution of diameters and the dispersibility
of nanoparticles are better than those in Figure 2a. Increasing
the molar ratio of PVP to BiCl3 from 0.15 to 1.00, as shown in
Figure 6, parts c and d, the average size of synthesized
nanoparticles decreases to ∼8 nm. Further increasing the molar
ratio from 1.00 to 5.00, the average nanoparticle size decreases
down to ∼6 nm. In addition, it is easy to find that Bi
nanoparticles in Figure 6, parts e and f, are almost isolated and
well dispersed, which indicates that the diameter distribution
and the dispersibility of these Bi nanoparticles are much better
than those synthesized at a low ratio of PVP to BiCl3 shown in
Figures 2a and 6a-d. In conclusion, the molar ratio of PVP to
BiCl3 is intrinsically important to control the size of the Bi
nanoparticles. The Bi nanoparticles with small diameter, high
dispersibility, and narrow diameter distribution can be easily
obtained by increasing the molar ratio of PVP to BiCl3.
Meier et al.26 have reported that the plasmon would be
suppressed by the finite size effect, as the size of metal particles
is very small. By decreasing the gold crystallite size, the
absorption spectra show a systematic evolution, specifically, a
broadening of surface plasmon band until the crystallites having
Accordingly, the amount of PVP added to the solution is
expected to affect the growth process for the Bi nanoparticles.
Therefore, the change in size of the Bi nanoparticles will be
investigated by varying the amount of PVP. In the first set of
experiments, the concentration of BiCl3 was kept at 10 mM
and the concentration of PVP was varied by changing the molar
3+
ratios of nPVP/nBi (0.05, 0.10, 0.15, 1.00, 5.00). The UV-
visible absorption spectra of these prepared Bi nanoparticle
solutions with different molar ratios were also studied, along
with the dependence of the surface plasmon absorption band
of bismuth nanoparticles on their size.
Figure 5a shows the UV-visible absorption spectra taken
from the Bi nanoparticle solutions with different molar ratios
between the monomeric unit of PVP and BiCl3. In the case of
a molar ratio of 0.15, a continuous absorption spectrum with a
shoulder at 281 nm was observed. As the amount of PVP
decreases, the absorption peak at 281 nm appears progressively
in the absorption spectra. When the molar ratio decreases to
0.05, the prominent surface plasmon absorption peak of Bi
nanoparticles at 281 nm is obvious. When the molar ratio
increases (from 0.15 to 1.00 and then to 5.00), the shoulder at
281 nm broadens in the absorption spectra, as shown in Figure
5b. Thus, it can be concluded that the surface plasmon
absorption peak of Bi nanoparticles gradually broadens with
increasing molar ratio of PVP to BiCl3, and then the peak
broadens almost completely at a high molar ratio (5.00).
Figure 6 presents the TEM images of the PVP-Bi nanopar-
less than 2.0 nm in effective diameter are unidentifiable.27
A
representative UV-visible spectrum of gold nanoparticles with
diameters below 2 nm shows no significant surface plasmon
band at 520 nm.28 Pileni et al.12 also have proven that a gradual
disappearance in the 570 nm plasmon peak occurs upon
decreasing the size of the copper clusters. It is clear that the
copper particles with a diameter below 4 nm are characterized
by a large broadening of the plasmon resonance. In the above-
mentioned references, the gradual broadening in the surface
plasmon band occurs upon decreasing the size of the metal
nanoparticles. This demonstrates experimentally the theory
summarized in section 3.2 of this paper. In this study, we also
find that the surface plasmon band at the 281 nm gradually
broadens as the size of the bismuth nanoparticles decreases. This
plasmon band widely broadens when the Bi nanoparticle
diameter decrease to 6 nm which is confirmed by TEM images
shown in Figure 6. It can be concluded that the surface plasmon
band of bismuth nanoparticles gradually broadens with the
3+
ticles synthesized with different molar ratios of nPVP/nBi in
DMF. Decreasing the molar ratio results in large Bi nanopar-
ticles with a wide range of size distribution. Parts a and b of
Figure 6 show the low and high magnification TEM images of
a sample synthesized by using the procedure similar to the
standard synthesis shown in Figures 1 and 2, except that the

Semimetal Bismuth Nanoparticles
J. Phys. Chem. B, Vol. 109, No. 15, 2005 7071
Figure 5. UV-visible spectra of PVP-Bi nanoparticles synthesized at various molar ratios of PVP to BiCl3, keeping the concentration of BiCl3
3+
3+
at a constant value of 10 mM. (a) nPVP/nBi ) 0.05, 0.075, 0.10, 0.125 and 0.15; (b) nPVP/nBi ) 0.15, 1.00 and 5.00.
3+
Figure 7. (a) TEM image of PVP-Bi nanoparticles (nPVP/nBi ) 0.075,
1 mM BiCl3); (b) HRTEM image of the isolated Bi nanoparticles shown
in part a.
ticles synthesized with 1 mM BiCl3 and 0.075 mM PVP. It can
be seen that the Bi nanoparticles with very small diameter of
about 6.5 nm are near regular spherical, but the dispersibility
is not good because of the low concentration of PVP. The
HRTEM image of two isolated nanoparticles is shown in Figure
7b, which provides further insight into the structure of Bi
nanoparticles. Dislocations and stacking faults are rarely
observed in the HRTEM image. The spacing of the lattice
fringes is found to be about 0.32 nm, which indexes to the (012)
spacing of rhombohedral Bi. Compared with the product shown
in Figure 2a and Figure 5, we can conclude that the decrease in
concentration of BiCl3 helps the formation of smaller Bi
nanoparticles. When the concentration of BiCl3 decreases with
a constant molar ratio of PVP/BiCl3, the number of bismuth
ions that can interact with the carbonyl oxygen of a molecule
of PVP decreases. After reduction of bismuth ions, the number
of bismuth ions present at close range decreases, so the average
size of the bismuth nanoparticles decreases.
Figure 6. TEM images of PVP-Bi nanoparticles synthesized at various
molar ratios of PVP to BiCl3, keeping the concentration of BiCl3 at a
constant value of 10 mM. Low and high magnification of the
3+
3+
synthesized nanoparticles [(a, b) nPVP/nBi ) 0.15; (c, d) nPVP/nBi
)
3+
1.00; (e, f) nPVP/nBi ) 5.00).
decreasing nanoparticle size. According to the above analysis,
the absence of plasmon band of bismuth clusters in ref 9 is due
to the small size of the clusters.
3.4. Size Control of Bi Nanoparticles by Varying the
Concentration of BiCl3. The concentration of BiCl3 was also
found to play an important role in determining the size of Bi
nanoparticles. Figure 7a shows the TEM image of Bi nanopar-
Figure 8 shows the UV-visible absorption spectra taken from
the Bi nanoparticle solutions at different BiCl3 concentrations,
while keeping the molar ratio of PVP to BiCl3 at a constant
value of 0.075. In the case of a BiCl3 concentration of 10 mM,
an obvious surface plasmon absorption peak at 281 nm is

7072 J. Phys. Chem. B, Vol. 109, No. 15, 2005
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Figure 8. UV-visible spectrum of the as-prepared Bi nanoparticles
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ratio at a constant value of 0.075.
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the prominent surface plasmon absorption peak broadens, which
is due to the decreased size of synthesized Bi nanoparticles.
This result is similar to that shown in Figure 5.
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4. Conclusion
In this paper, we introduced the method to control the size
of Bi nanoparticles in the range 6-13 nm by adjusting the molar
ratio of PVP to BiCl3 or the concentration of BiCl3. Well-
dispersed small Bi nanoparticles (about 6 nm) with a narrow
size distribution were easily synthesized through this method
3+
at nPVP/nBi ) 5.00 and 10 mM BiCl3. It was found that the
surface plasmon absorption peak in UV-visible spectra broad-
ened, due to the decreased size of synthesized Bi nanoparticles.
The IR study of the pure PVP, the complexes with different
molar ratios of PVP/BiCl3, and the PVP-stabilized bismuth
nanoparticles demonstrates the protective mechanism of PVP
for the synthesis of bismuth nanoparticles.
Compared to Bi nanoparticles synthesized by using other
methods, the present nanoparticles show high crystallinity, small
diameter, narrow size distribution, high dispersibility, and single
phase purity. This synthetic method provides a convenient route
for the formation of highly crystalline Bi nanoparticles, which
can serve as the basis for further study of quantum-confinement
phenomena of small Bi nanoparticles.
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Acknowledgment. This work was supported by BK21 and
KOSEF/CRI.
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References and Notes
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