Characterization of nanocrystalline bismuth telluride (Bi 2Te3) synthesized by a novel approach through aqueous precursor method

Article abstract of DOI:10.1111/j.1551-2916.2005.00784.x

Bi₂Te₃-based alloys are currently best-known, technologically important thermoelectric materials near room temperature. In this paper, nanocrystalline Bi₂Te₃ was synthesized by an aqueous solution technique based on the reaction between the aqueous solution of Bi-ethylenediamine tetraaceticacid (EDTA), TeO-EDTA, and NaBH₄ at room temperature. NaBH₄ was used as a reducing agent. TeO-EDTA was prepared from TeO₂ after satisfactory purification. The sample purity was examined by selected area energy dispersive X-ray analysis. The products were characterized by X-ray diffraction and high-resolution transmission electron microscopy by which the particle morphologies and size were studied. The particle size ranges from 60 to 90 nm.

Full text of DOI:10.1111/j.1551-2916.2005.00784.x

J. Am. Ceram. Soc., 89 [2] 534–537 (2006)
DOI: 10.1111/j.1551-2916.2005.00784.x
r 2005 The American Ceramic Society
ournal
J
Characterization of Nanocrystalline Bismuth Telluride (Bi Te )
2
3
Synthesized by a Novel Approach Through Aqueous Precursor Method
w,z
Debasis Dhak and Panchanan Pramanik
Department of Chemistry, Indian Institute of Technology, Kharagpur 721302, West Bengal, India
Bi Te -based alloys are currently best-known, technologically
2
within the pores of alumina template membranes. All these
process are to increase the Carnot efficiency, which is dependent
on the improvement of the figure of merit, ZT of the thermo-
electric material. Both theoretical and experimental results indi-
cate that the ZT value of certain nanoscale materials increases
3
important thermoelectric materials near room temperature. In
this paper, nanocrystalline Bi Te was synthesized by an aque-
2
3
ous solution technique based on the reaction between the aque-
ous solution of Bi-ethylenediamine tetraaceticacid (EDTA),
TeO-EDTA, and NaBH at room temperature. NaBH was
2
5,26
several times compared with that of their bulk materials.
4
4
used as a reducing agent. TeO-EDTA was prepared from TeO2
after satisfactory purification. The sample purity was examined
by selected area energy dispersive X-ray analysis. The products
were characterized by X-ray diffraction and high-resolution
transmission electron microscopy by which the particle morph-
ologies and size were studied. The particle size ranges from 60 to
Therefore, the ZT value can be tailored through controlling the
size and shape of nanoscale thermoelectric materials, which will
have potential application in thermoelectric devices. New syn-
thetic method for nanoscale bismuth telluride will be useful for
2
6
both basic research and technological applications.
In this report we present a low-energy cost and low-temper-
ature route to get ultrafine Bi Te by aqueous precursor method
9
0 nm.
2
3
using commercial tellurium metal after purification.
I. Introduction
HE bismuth telluride (band gap: E 5 0.15 eV)-based alloys
are known as the best currently available thermo-electric
II. Experimental Procedure
g
T
Materials used are Te metal (Hindustan Copper, Bihar, India,
1–5
material for application near room temperature. They have
many important applications, for example, used as thermo-piles
and thermal sensors, thermoelectric cooler for laser diodes, fiber
98 at.%), HNO
Mumbai, India), ethylenediamine tetraaceticacid (EDTA)
(Merck), 30% NH solution (Merck), and NaBH (Merck).
3 2 3
(Merck, India), KOH (Merck), Bi O (Merck,
3
4
6
–10
optic, microelectrothermal systems, and so on.
technological importance of Bi Te , a lot of processes have been
Because of
41
(1) Purification and Preparation of Stock Solution of Te
2
3
developed to prepare bismuth telluride. Most of the processes
use metallurgical methods like BMA, Bridgman methods
Tellurium metal was boiled with excess of concentrated HNO
3
1
1
12,13
for half an hour. After the completion of reaction, a white lump
of tellurium oxide was formed. Boiling with excess nitric acid,
the impurities (mainly iron and aluminum) were leached out as
water-soluble nitrates. The white lumps of tellurium oxides were
ground and dissolved in a 20% ammonia solution and filtered.
The filtrate was re-precipitated with concentrated HNO , and
3
then washed with double distilled water to wash out the impu-
rities. The lumps of oxides were ground to powders and dried at
for bulk preparation, and require elevated temperature and a
relatively long duration under special protection against oxida-
tion, and it is difficult to tailor their electrical and thermal trans-
port properties independently. This is an important point to
note that thermoelectric properties are greatly correlated with
chemical homogeneity, shape, and size of nanoscale thermoe-
lectric materials.
A simple approach is the coprecipitation of bismuth and
telluride as oxides from aqueous solution, and the coprecipitated
1
501C under vacuum. The powder was examined for impurities
using selected area energy dispersive X-ray (SEDX-ray) analysis.
This process was continued until the purity level reached to 99.9
at.%. Pure TeO was then dissolved in ammonical EDTA (ED-
2
TAH ) solution after warming on water bath for an hour. The
4
strength of the prepared solution was determined by chemical
assay through heating the known volume of solution at 5001C
2 3
oxides are converted directly to fine Bi Te powder through hy-
1
4
drogen reduction. Groshens et al. have reported a different
15
approach to the synthesis of Bi Te by means of reactions con-
2
3
1
4
ducted in hexane at 2301C. Ritter has reported a room-
temperature process of coprecipitation of bismuth telluride
precursor (Bi
2
O
3
3TeO ꢀ xH
2
O) in aqueous media, then its hy-
for 2 h and weighing the residual TeO . The strength of the so-
2
lution obtained was 0.02M.
drogen reduction to polycrystalline Bi Te . This group has also
2
3
found an unusual metal-organo complex method to synthesize
have
1
.
6
17
Yu et al. and Deng et al.
18–20
finely powdered Bi Te
2
3
31
2) Preparation of Solution of Bi
(
reported a low-temperature solvothermal method to synthesize
Bi Te . Electro-deposition has also been employed to synthesize
Bismuth oxide was dissolved in dilute HNO
3
solution and then
2
3
2
1,22
23
Sapp et al. and Foos et al.
24
complexed with EDTA solution in alkaline pH 11. The strength
of the solution was determined from the weight of oxide and
final volume of solution. The strength of the prepared solution
was 0.25M.
Bi Te
2
3
at room temperature.
have synthesized bismuth telluride nanowires by electro-deposi-
tion method in conjunction with template synthesis techniques
P. Paranthaman—contributing editor
(
3) Preparation of Nanoparticles of Bismuth Telluride
Required amount of TeO-EDTA complex solution (0.02M) was
taken in a 500 mL beaker, maintained the pH at 11 with the help
of KOH solution (30%). Proportionate amount of bismuth
EDTA complex solution (0.25M) was poured with the simulta-
Manuscript No. 20677. Received June 17, 2005; approved September 14, 2005.
This work was financially supported by Council of Scientific and Industrial Research,
Ind iwa .
Author to whom correspondence should be addressed. e-mail: pramanik@
chem.iitkgp.ernet.in
INFORMM, University Sains Malaysia, Pulau Pinang, Malaysia.
neous addition of KBH
4
aqueous solution (30%) resulting a
Te nanoparti-
z
vigorous reaction yielding the black colored Bi
2
3
5
34

February 2006
Characterization of Nanocrystalline Bi Te3
2
535
cles suspended into the solution. After the completion of the
reaction, the suspended nanoparticles were collected by using a
centrifuge and washed with double distilled water followed by
0.000006
0.000005
0.000004
0.000003
0.000002
(2 0 5)
2 3
99.99% ethyl alcohol. Finally, the washed Bi Te nanoparticles
were dried in vacuum at temperature 701C.
Powder X-ray diffraction (XRD) was taken to characterize
the samples using a Philips PW 1710 X-ray diffractometer
(
1 0 10)
(
Eindhoven, the Netherlands) with CuK radiation (l 5 1.5406
˚
A), equipped with a vertical goniometer. The XRD spectrum of
Si crystal was used as a standard to calibrate the scanning an-
gles. The step size of the scan was 0.0412y, with a scanning speed
of 0.0212y/s. The particle morphology, size, and further struc-
tural and elemental analyses were performed by high-resolution
transmission electron microscopy (HRTEM) in a JEOL 2010
microscope (Tokyo, Japan) with an ultra-high resolution pole-
(1 1 0)
(
0 1 5)
piece using a LaB filament operated at 200 kV, selected area
6
electron diffraction (SAED), and SEDX in a JEOL JSM-5800.
0.05
0.10
0.15
0.20
0.25
0.30
sin θ/λ)2
(
2
2
Fig. 2. (b
i
cos y/l) versus (sin y/l) for Bi
Te
2 3
whose y/2y scans are
shown in Fig. 1.
III. Results and Discussion
The composition of the as-prepared product was determined by
XRD (Fig. 1). All peaks in the patterns correspond to the re-
TEM images of the samples taken onto a 300 mesh copper TEM
grids coated with 50 nm carbon films after dispersing into ac-
etone revealed that the products are mostly deformed spherical
as well as some rod-like nanoparticle as shown in Fig. 3. The
particle size was calculated with the help of UTHSCSA image
tool (The University of Texas Health Science Center, San
Antonio, TX) considering 50 particles. After statistic analysis
the average particle size was found to be 8072 nm which is in
close agreement obtained from XRD analysis. The dependency
of the size of the nanocrystalline on the salt concentration has
not been studied completely. This aspect is on the way of our
investigation. Dilute solutions (r0.2M) of the salts do not find
any significant change in their crystallite size on further dilution.
The corresponding selected area diffraction (SAD) pattern in
Fig. 4(d) shows a symmetrically dotted pattern, implying that
these nanoparticles are well crystallized single crystals of Bi Te ,
ꢁ
actions of rhombohedral phase R 3m, Bi Te
2
3
, with cell con-
˚
stants a 5 b 5 4.38 A and c 5 30.50 A, which are in agreement
˚
˚
with the reported values a 5 4.385 and c 5 30.48 A (JCPDS 15-
0863). Information on crystallite size (D) and the effective strain
(Z) was obtained from the full-width at half-maximum
(
FWHMs) of the diffraction peaks. After applying the correc-
tion for instrumental broadening with respect to standard sili-
con, the FWHMs can be expressed by the following equation
assuming particle size and strain profiles are of Gaussian type:
Ob
2
p
2
s
2
ðbi Þ ¼ ðb Þ þ ðb Þ
(1)
or
2
2
2
2
3
ðb cos y=lÞ ¼ ð1=DÞ þ ðZ sin y=lÞ
(2)
i
which is consistent with the XRD measurements (Fig. 1). An
HRTEM image of top-view of two attached particles is shown
in Fig. 4(a), which is taken from the selected area shown in Fig.
p
where b is the integral of FWHM; b the FWHM considering
s
the particle size effect; b the FWHM because of strain; y the
3. Figure 4(b) is the HRTEM image of the area indicated by the
half incident angle of the X-ray, and l the wave length of the
target material.
bracket in Fig. 4(a). The details of lattice fringe indicate that
there are very similar orientations in the attached particle, and
only slightly angular mis-orientations are present in two at-
tached particles. From the micrograph we have determined the
2
2
Figure 2 shows the plot of (b cos y/l) versus (sin y/l) . From
i
the slope of the best fitted line and the intercept at x 5 0, the
crystallite size (D) and the effective strain (Z) were found to be
65.5 nm and 0.003675. Essentially the sample shows the absence
of any significant strain.
˚
lattice spacing in the particle to be 3.2 A, which corresponds to
(015) plane of rhombohedral phase of this material. The white
line in Fig. 4(b), which is drawn parallel to the Bi Te (015) lat-
2
3
tice fringes on the upper rod-like particle, is slightly inclined to
the white line, which is also parallel to the (015) fringes on the
lower particle. This slight misorientation suggests that defects
are introduced into the particles during the process. The side-
2
0
30
40
θ (deg.)
Fig. 1. X-ray diffraction patterns of as prepared Bi
50
60
70
2
Fig. 3. Bright field transmission electron microscopy image of Bi
nanoparticle.
2 3
Te
2
3
Te powder.

5
36
Journal of the American Ceramic Society—Dhak and Pramanik
Vol. 89, No. 2
Fig. 4. (a, b) High-resolution transmission electron microscopy (HRTEM) image of top view; (c) HRTEM image of side view; and (d) the corre-
sponding electron diffraction pattern taken from the interval of particles; and (e) HRTEM image of the center between contiguous particles.
view is shown in Fig. 4(c), in which two particles are attached
very closely.
All the lattice fringes in two continuous particles have almost
no apparent boundary is present. It implies that the rod-particles
are composed of consecutive particles. Figure 4(e) shows the
attachment of the rod-particle is imperfect, which involve dis-
locations in the interface.
the same spacing and direction where the singular fringe spacing is
˚
9.8 A, which is nearly consistent with the (003) reflection plane
spacing of the rhombohedral Bi Te phase. The (003) direction
The exact mechanism has not been studied intensively. As
known, tellurium, transition metals, and some donor solvents
2
3
2
7,28
was parallel to the long axis of the rods, indicating that the [003]
direction is a common growth direction for the Bi Te particle.
The SAED patterns of two adjoining particles and the joint in rods
Fig. 4(d)) are similar and confirm that it is a large single crystal.
can form organometallic complexes.
Bi Te may be presented as follows:
Reactions:
The formation of
2
3
2
3
(
boil for 3 h
ammonia
SEDX analysis of sheet-rods showed that the atomic com-
position ratio of Bi/Te is always about 2:3, which is in good
agreement with that of the stoichiometric composition of
Te ðmetalÞ þ HNO3 ꢁ! TeO2ðpptÞ ꢁ! ðNH4Þ TeO3
2
HNO3
ꢁ! TeO2ðpptÞ
2 3
Bi Te . In addition, neither N nor C signals was detected in
the SEDX spectrum, which means there exist no solvent or
EDTA ligand in the sheet-rod crystals.
TeO2 þ EDTA4ꢁ þ NH3 ! Ammonium salt of EDTA
4þ
complex of Te ðNH4Þ ½TeO EDTAꢂ
The structural details of the joint sheets shown in Figs. 4(c)
and (e) show the lattice resolved HRTEM images taken near the
edge and center between two continuous particles. Fig. 4(c)
shows that the lattice planes in the joint of sheets have almost
the same spacing and direction with the adjoining particles, and
2
1
ꢁ
3ðNH4Þ ½TeO EDTAꢂ þ 2½BiðEDTAÞꢂ þ NaBH4
2
pꢁH!¼11 nano Bi2 Te3:

February 2006
Characterization of Nanocrystalline Bi Te3
2
537
3
1
4
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41
Bi0.5Sb1.5Te
3
Te
Bi
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. The pH more than 11 is not beneficial because at that
5
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41
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10
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1
2ꢁ KBH4
½
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multinuclear complexes. Then the chains of crystallite seeds
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sheet-rod-like products and thus control the particle size in
nanometric range.
11
2
3 0.25
2
3 0.75
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14
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16
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IV. Conclusion
2 3
E
1
8
In summary, the present study demonstrates a new approach for
the synthesis of Bi Te nanoparticles. One of the advantages of
the method is that the synthesized Bi
2
3
2
3
19
Y. Deng, C. W. Nan, G. D. Wei, L. Guo, and Y. Lin, ‘‘Organic Assisted
Growth of Bismuth Telluride Nanocrystals,’’ Chem. Phys. Lett., 374, 410–5
2 3
Te nanoparticles are
formed directly at room temperature using easily available re-
agents EDTA and NaBH . EDTA plays a key role in this
(
2003).
4ꢁ
20
2 3
Y. Deng, C. W. Nan, and L. Guo, ‘‘A Novel Approach to Bi Te Nanorods by
4
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2
3
21
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2
3
Acknowledgment
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the HRTEM work.
24
25
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