Synthesis of nanocrystalline bismuth in reverse micelles [1]

Full text of DOI:10.1021/ja001118i

7114
J. Am. Chem. Soc. 2000, 122, 7114-7115
Communications to the Editor
Despite the enormous potential of this material for thermo-
electric applications, little work has been directed toward the
solution synthesis of nanocrystalline bismuth clusters. To the best
of our knowledge, only one report exists of such material being
prepared as a stable colloid,12 and these clusters were prepared
at very low concentrations in an aqueous polymer, making the
isolation of significant quantities of such material difficult. In
addition, the size of the nanocrystals was determined to be 8 or
12 nm, sizes which are large when compared to the smallest
diameters of noble metal nanoclusters.5 Very recently, the
synthesis of nanocrystalline bismuth using an in situ polymeri-
zation process was reported;13 however, the resulting particles
were even larger (20 nm) and were also suspended in a polymer
matrix. Although this polymer coating protects the particles from
oxidation, it once again makes the further manipulation and
characterization of the product difficult. The focus of our effort
has been the isolation of macroscopic quantities of bismuth
nanocrystals with diameters below 10 nm that could be obtained
in a more easily manipulated form.
A variety of chemical methods have been reported in the
literature for the preparation of noble metal nanoparticles, such
as sonochemical reduction,14 reduction in the presence of capping
agents,15 and reduction in inverse micelles.16 We have recently
examined a variety of reactions, in an attempt to prepare
nanocrystalline bismuth clusters,17 and have found difficulty in
identifying a suitable capping agent. Traditional choices such as
alkanethiols and TOPO have failed to provide proper control over
both the nucleation and growth in our experiments with bismuth.
Under proper conditions, however, the inverse micelle method
has accomplished this.
The bismuth nanoclusters are prepared through reduction of
an aqueous bismuth salt inside of AOT (dioctyl sulfosuccinate,
sodium salt) reverse micelles. Briefly, AOT is dissolved in
isooctane, and an appropriate amount of H2O containing dissolved
BiOClO4 is added, forming the reverse micelles. A second micelle
solution containing NaBH4 dissolved in the aqueous phase is
prepared at an identical w value (where w ) [H2O]/[AOT]), and
the two solutions are combined under Ar. Within minutes, the
clear and colorless solution darkens to a deep brown color.
Prolonged stirring (on the order of hours) at room temperature
results in the gradual precipitation of elemental bismuth. After
several minutes, the solvent is removed in vacuo, and the resulting
solid dried under vacuum and redispersed in toluene. The mixture
is centrifuged to remove the insoluble solids, leaving a dark
solution that contains the bismuth nanoclusters. The excess
surfactant is removed by addition of MeOH, which causes
precipitation of the AOT-capped bismuth particles and allows
Synthesis of Nanocrystalline Bismuth in Reverse
Micelles
Edward E. Foos,* Rhonda M. Stroud,† Alan D. Berry,
Arthur W. Snow,‡ and J. Paul Armistead§
Chemistry DiVision, Code 6174
NaVal Research Laboratory, Washington, D.C. 20375
ReceiVed March 30, 2000
Over the past decade, there has been a dramatic increase of
interest in both the preparation and properties of nanocrystalline
materials. This interest has been fueled by the unique properties
that such materials possess when compared to bulk phases,1 as
well as the potential they hold for such varied applications as
electronics,2 catalysis,3 and biological labeling.4 The majority of
the work in this field has focused on transition metal and
semiconductor particles, with particular emphasis on gold5 and
II-VI compounds such as CdSe.6 Although the formation of
colloidal silicon7 and germanium8 nanoclusters has been studied,
little work has been directed toward examining the preparation
and fundamental properties of nanocrystalline main-group metals.
Recent theoretical studies suggest that bismuth materials of
reduced dimensions may exhibit enhanced thermoelectric proper-
ties at room temperature.9 Quantum confinement has already been
exploited to increase the thermoelectric figure of merit, ZT, for
PbTe10 quantum well superlattices and an even larger thermo-
electric effect might be achieved with bismuth under such
dimensionally restricted conditions.9a The focus thus far has been
on making these measurements on bismuth nanowires,11 which
possess diameters of 13-110 nm and lengths on the order of 10
µm.
† Code 6371.
‡ Code 6123.
§ Code 6126.
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(2) (a) Narayan, K. S.; Manoj, A. G.; Nanda, J.; Sarma, D. D. Appl. Phys.
Lett. 1999, 74, 871. (b) Schlamp, M. C.; Peng, X.; Alivisatos, A. P. J. Appl.
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Colloid Polym. Sci. 1995, 273, 101.
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Klingelho¨fer, S.; Heitz, W.; Greiner, A.; Oestreich, S.; Fo¨rster, S.; Antonietti,
M. J. Am. Chem. Soc. 1997, 119, 10116.
(4) (a) Bruchez, M.; Moronne, M.; Gin, P.; Weiss, S.; Alivisatos, A. P.
Science 1998, 281, 2013. (b) Chan, W. C. W.; Nie, S. Science 1998, 281,
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(5) For examples of work with Au, see the following and the references
therein: (a) Rao, C. N. R.; Kulkarni, G. U.; Thomas, P. J.; Edwards, P. P.
Chem. Soc. ReV. 2000, 29, 27. (b) Loweth, C. J.; Caldwell, W. B.; Peng, X.;
Alivisatos, A. P.; Schultz, P. G. Angew. Chem., Int. Ed. 1999, 38, 1808. (c)
Hostetler, M. J.; Murray, R. W. Curr. Opin. Colloid Interface. Sci. 1997, 2,
42.
(6) For examples of work with CdSe, see the following and the references
therein: Green, M.; O’Brien, P. J. Chem. Soc., Chem. Commun. 1999, 2235.
(7) (a) Yang, C. S.; Bley, R. A.; Kauzlarich, S. M.; Lee, H. W. H.; Delgado,
G. R. J. Am. Chem. Soc. 1999, 121, 5191. (b) Bley, R. A.; Kauzlarich, S. M.
J. Am. Chem. Soc. 1996, 118, 12461.
(8) (a) Taylor, B. R.; Kauzlarich, S. M.; Delgado, G. R.; Lee, H. W. H.
Chem. Mater. 1999, 11, 2493. (b) Taylor, B. R.; Kauzlarich, S. M.; Lee, H.
W. H.; Delgado, G. R. Chem. Mater. 1998, 10, 22.
(12) Gutierrez, M.; Henglein, A. J. Phys. Chem. 1996, 100, 7656.
(13) Fang, J.; Stokes, K. L.; Wiemann, J.; Zhou, W. Mater. Lett. 2000, 42,
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(14) (a) Suslick, K. S.; Fang, M.; Hyeon, T. J. Am. Chem. Soc. 1996, 118,
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(15) (a) Yee, C.; Scotti, M.; Ulman, A.; White, H.; Rafailovich, M.;
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Phys. 1999, 85, 4325. (c) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.;
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10.1021/ja001118i CCC: $19.00 © 2000 American Chemical Society
Published on Web 07/08/2000

Communications to the Editor
J. Am. Chem. Soc., Vol. 122, No. 29, 2000 7115
Figure 1. XRD patterns of (a) soluble nanocrystalline Bi (w ) 2
reaction); (b) insoluble nanocrystalline Bi; (c) insoluble nanocrystalline
Bi after annealing (200-260 °C).
them to be isolated as a waxy solid that can be dissolved in a
variety of nonpolar solvents, forming dark brown solutions. These
solutions are stable for several months at ambient conditions, with
no special precautions necessary to protect them from oxygen or
light. Using this procedure, it is possible to isolate up to 50 mg
of nanocrystalline bismuth powder from a single 2.00 mmol scale
reaction.
Figure 2. TEM of soluble Bi particles. (a) w ) 1; (b) w ) 2; (c) w )
3; (d) lattice image of a particle from the w ) 3 sample, imaged along
a [241] zone. The parallel lines indicate 0.328 nm, (102)-type spacings.
standard deviation, approximately 50 particles counted) is: w )
1, 3.2 ( 0.35 nm; w ) 2, 6.9 ( 2.2 nm; w ) 3, 8.0 ( 2.7 nm.
High-resolution lattice images of each of the samples exhibit
fringes (d ) 0.395, 0.328 nm) that index to the (003) and (102)
spacings of rhombohedral bismuth, respectively. Figure 2d shows
a particle from the w ) 3 sample imaged along a [241] zone.
The composition of the particles was verified by energy-dispersive
X-ray analysis, which showed only the presence of C and Cu
(from the sample grid) and Bi; no oxygen peak was observed.
A maximum is seen in the UV/vis spectra of these clusters at
approximately 196 nm, with no apparent change in the spectra
between the w ) 1, 2, and 3 samples. Gutierrez and Henglein
have reported the only UV/vis data in the 200-700 nm region
for colloidal bismuth,12 as well as a calculated spectrum in the
same region based on Mie theory. They report a maximum at
253 nm, and while our results do not show this absorbance, they
are in good agreement with the theoretical spectrum above 200
nm and with a similarly calculated spectrum for colloidal bismuth
reported elswhere.18
In summary, using the inverse micelle technique we have
prepared nanocrystalline bismuth clusters exhibiting domain sizes
of less than 10 nm, which fall well below the size range that has
been obtained using other synthesis methods reported to date.
The clusters can be isolated as stable colloidal material or, through
repeated washing, as an air-stable powder. Further experiments
into the potentially interesting properties of this material are
currently underway.
As the soluble material is washed with MeOH, the capping
AOT groups are gradually removed and an insoluble black powder
is isolated, a process which has been observed by FTIR. The initial
(soluble) material shows several strong absorbances that cor-
respond well with pure AOT. As this material is gradually washed
from the surface, the peaks diminish in intensity. After ap-
proximately 10 washings, it appears that a small amount of AOT
is still left on the surface and is responsible for protecting the
particles from rapid oxidation. Results of elemental analyses (EA)
on these samples are consistent with the infrared spectra. The
soluble material contained 52.96% Bi and a significant amount
of C (23.15%) and H (3.60%), along with several other elements
(Na, 2.87%; Cl, 0.95%). The insoluble material washed 10 times
with MeOH contained 95.35% Bi and considerably reduced
quantities of C (4.00%) and H (0.59%). Thermogravimetric
analysis (TGA) of the soluble material indicates a significant
weight drop beginning about 300 °C, which is attributed to loss
of the AOT. Powder X-ray diffraction (XRD) analysis of the black
powder left on the pan after cooling to room temperature under
nitrogen confirms the presence of elemental bismuth.
Powder XRD analysis of both the soluble and insoluble material
shows a single, well-defined and extremely broad peak corre-
sponding to the (012) reflection of rhombohedral bismuth, as seen
in Figure 1a and b. Estimation of the approximate average particle
size using the Scherrer equation leads to a calculated value of 2
nm, slightly smaller than observed by TEM (vide infra). Annealing
the insoluble powder under Ar (at 200-260 °C) results in a
sharpening of this main peak, as well as the appearance of peaks
assigned to the (104) and (110) reflections, consistent with the
presence of elemental bismuth as shown in Figure 1c.
Acknowledgment. We thank the Office of Naval Research and the
Naval Research Laboratory for financial support of this work, which was
performed while E.E.F. held a National Research Council Research
Associateship.
Supporting Information Available: Full details of the synthesis as
well as FTIR, TGA, and UV/vis data (PDF). This material is available
free of charge via the Internet at http://pubs.acs.org.
To further characterize the size and crystal structure of the
particles, the soluble products have also been examined using
TEM. Bright-field images of the particles are shown in Figure
2a-c. The average particle size for each sample (mean ( 1
JA001118I