Fabrication of antimony telluride nanoparticles using a brief chemical synthetic process under atmospheric conditions

Article abstract of DOI:10.1016/j.jallcom.2010.10.002

Antimony telluride (Sb₂Te₃) nanoparticles for thermoelectric applications were successfully prepared via a water-based chemical reaction under atmospheric conditions. In this process, we tried to prepare the nanostructured compound by employing both a complexing agent (l-tartaric acid) and a reducing agent (NaBH₄) to stabilize the Sb precursor (SbCl₃) in water and to favor the reaction with Te. It was observed that various products of Te, Sb₂O₃, and Sb ₂Te₃ were individually or simultaneously generated depending on the amount of the complexing and reducing agents used. In order to obtain solely a rhombohedral Sb₂Te₃ compound, the aging time of the reaction needed to be adjusted.

Full text of DOI:10.1016/j.jallcom.2010.10.002

Journal of Alloys and Compounds 509 (2011) 609–613
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Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jallcom
Fabrication of antimony telluride nanoparticles using a brief chemical synthetic
process under atmospheric conditions
Cham Kima,b, Dong Hwan Kima, Yoon Soo Hana, Jong Shik Chungb, Hoyoung Kima,∗
a Daegu Gyeongbuk Institute of Science and Technology (DGIST), 711-623 Hosan-dong, Dalseo-gu, Daegu 704-230, Republic of Korea
b Department of Chemical Engineering, Pohang University of Science and Technology (POSTECH), San 31 Hyoja-dong, Pohang 790-784, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Antimony telluride (Sb2Te3) nanoparticles for thermoelectric applications were successfully prepared via
a water-based chemical reaction under atmospheric conditions. In this process, we tried to prepare the
nanostructured compound by employing both a complexing agent (l-tartaric acid) and a reducing agent
(NaBH4) to stabilize the Sb precursor (SbCl3) in water and to favor the reaction with Te. It was observed
that various products of Te, Sb2O3, and Sb2Te3 were individually or simultaneously generated depending
on the amount of the complexing and reducing agents used. In order to obtain solely a rhombohedral
Sb2Te3 compound, the aging time of the reaction needed to be adjusted.
Received 22 July 2010
Received in revised form
28 September 2010
Accepted 4 October 2010
Available online 25 October 2010
Keywords:
© 2010 Elsevier B.V. All rights reserved.
Sb2Te3 nanoparticles
Thermoelectric
Chemical synthesis
l-Tartaric acid
NaBH4
1. Introduction
due to complicated production processes and high costs. There-
fore, some groups are focusing on the approach using nanobulk
Thermoelectric (TE) generation has been intensively researched
due to its attractive applications such as waste heat-to-electricity
conversion and solid-state cooling [1–7]. In this field, one of the
main topics has always been how to improve the performance of
TE materials to increase the efficiency of TE devices. The com-
prehensive performance of such materials is evaluated via the
dimensionless figure of merit ZT(=˛2ꢀT/ꢁ), where ˛ is the Seebeck
coefficient, ꢀ is the electrical conductivity, T is the absolute tem-
perature, and ꢁ is the thermal conductivity [5–10]. Therefore, an
excellent TE material should have both a high ꢀ and a low ꢁ, which
are characteristics indicative of a so-called phonon-glass/electron-
crystal (PGEC) [1,7,8–15].
With meteoric development in nanotechnology, many groups
have been trying to build low-dimensional structures to use as
PGEC materials. Reports have shown that ZT can be enhanced in
the structures of quantum dots (QD) and superlattice (SL) thin films
due to both the increase in the power factor (˛2ꢀ) and the decrease
in ꢁ, which result from quantum confinement and the phonon scat-
tering effect, respectively [13–21]. Relatively high ZT values were
often exhibited in these QDs or SL structures, but it has generally
been known that commercial use of these substances is difficult
structures such as nanoparticles [22–24], nanotubes [25–27], and
nanowires [28–30]. Not only do these nanostructures still have the
phonon scattering effect, which reduces ꢁ, but they also can be
prepared using cheaper processes.
In the present study, we intended to develop Sb2Te3 nanoparti-
cles via a brief chemical synthetic route for the application to low-
temperature operations (0–250 ◦C). The stabilization and the dis-
solution of precursors were accomplished in water using chemical
additives, and the overall reaction was carried out using different
aging conditions. Although we did not use a toxic organic solvent
and we just conducted the aging process under ambient condition
(without high pressure) unlike other works [31,32], our product
was confirmed to show the single crystalline structure without any
second phases and more homogeneous particle size distribution.
The crystalline structures and the particle size distributions of the
products were characterized, and the influences of various reac-
tion conditions on the results were discussed. Consequently, we
demonstrated that pure Sb2Te3 nanoparticles, which were highly
dispersed and well crystallized, were successfully fabricated via a
brief chemical synthetic route under ambient conditions.
2. Experimental details
2.1. Chemicals
∗
Corresponding author. Tel.: +82 54 770 2936; fax: +82 54 770 2940.
E-mail addresses: hoykim@dgist.ac.kr, kc0207@postech.ac.kr (H. Kim).
Antimony(III) chloride (SbCl3, Aldrich, ACS reagent) and Te powder (Kojundo
chemical, 99.999%, 45 ␮m) were employed as precursors. l-Tartaric acid (TA:
0925-8388/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jallcom.2010.10.002

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C. Kim et al. / Journal of Alloys and Compounds 509 (2011) 609–613
HO2CCH(OH)CH(OH)CO2H, Aldrich, ACS reagent) and sodium borohydride (NaBH4,
Aldrich, ACS reagent) were used to form Sb complex ions and to reduce them with
Te, respectively. All of the chemicals were used without any further purification.
(a)
OH
O
O
HO
2.2. Sample preparation
OH
OH
SbCl3 was mixed with deionized (DI) water, and various amounts of the com-
plexing agent, TA, were added. The resulting white suspension was vigorously
stirred until a homogeneous solution was formed without any precipitation (solu-
tion A). Meanwhile, Te was placed in a two-neck round-bottom flask, and the flask
was purged with N2, after which the reducing agent, NaBH4(aq), was added. This Te
mixture was heated to 120 ◦C with vigorous stirring. When the mixture completely
turned into a transparent purple-colored solution, solution A was added dropwise
with a syringe. After the resulting black compound was refluxed for 6–12 h, it was
filtered and rinsed thoroughly using dry ethanol and DI water. The product was
dried under vacuum at 60 ◦C overnight.
2-
(b)
O
O
O
O
O
O
O
O
O
O
O
O
Sb3+
Sb
Sb
+
O
O
O
O
O
O
O
O
Sb3+
O
O
O
O
2.3. Characterization
Fig. 1. Structural formula of l-tartaric acid (a) and formation mechanism of the anti-
Powder X-ray diffraction (XRD) patterns were collected with a D/MAX-2500
diffractometer (Rigaku) using Cu K˛ radiation and a scintillation counter detec-
tor. Relevant patterns were recorded in the 2ꢂ range of 20–80◦. Field emission
scanning electron micrographs were obtained using an S-4800 FE-SEM (Hitachi)
to observe the relevant morphology. The atomic composition ratios of the products
were measured via inductively coupled plasma spectroscopy (ICP, Flame Modula S,
Spectro).
mony tartrate anion, [Sb2(C4H2O6)2]2−, which stabilizes Sb3+ in an aqueous solution
(b).
in order to obtain stabilized Sb3+ ions in water. If the appropriate
amount of HCl is added to the precipitate, Sb3+ ions can effectively
be dispersed in
3. Results and discussion
SbCl3 (aq) + H2O(l) ↔ SbOCl (s) + 2HCl (aq)
(1)
Unlike most of the chloride-type metal salts, SbCl3 is easy to
hydrolyze to form antimony(III) oxychloride, SbOCl, in an aqueous
solution (Eq. (1)). Because of the extensive precipitation of SbOCl
in this system, it is impossible to produce the desired nanoparti-
cles without using a complementary treatment. Therefore, some
researchers have used hydrochloric acid (HCl) to dissolve SbCl3
water. This treatment is based on Le Chatelier’s principle; how-
ever, a strong acid could not be used in the present study since
the Te source (Te powder) should easily be oxidized at a low pH,
which would prevent Te from reacting with the Sb3+ ions in the
system. Therefore, we took note of some organic acids, which are
known to play a role in stabilizing metal ions, and we expected
Te
Sb2Te3
(a)
(b)
TA7S
Sb2O3
TA21S
TA14S
TA7S
TA3.5S
20
30
40
50
60
70
80
20
30
40
50
60
70
80
2 theta / deg.
2 theta / deg.
(d)
(c)
TA21S
TA3.5S
20
30
40
50
60
70
80
20
30
40
50
60
70
80
2 theta / deg.
2 theta / deg.
Fig. 2. Effect of the amount of l-tartaric acid on the crystalline structures of the products: comparison of the diffraction patterns of the products (a) and presentation of each
crystalline direction (b–d).

C. Kim et al. / Journal of Alloys and Compounds 509 (2011) 609–613
611
them to generate complexes stabilized with organic ligands, such
as Sb(III)-complexes, in water. Tella and Pokrovski arranged the
stability and structure of Sb(III) with various organic compounds
having O-bearing functional groups (carboxyl or hydroxyl groups)
in aqueous solution [33]. They reported that some poly-functional
carboxylic or hydroxyl-carboxylic acids can form Sb(III)-complexes
over a wide pH range (3 < pH < 9), whereas the hydrolyzed species,
Sb(OH)3, is mainly detected with mono-functional organic com-
pounds, indicating the poor complexing ability of mono-functional
organic compounds. Accordingly, we considered adopting the
poly-functional acids for the present study, and we finally chose
l-tartaric acid (TA) as the complexing agent due to its numerous
functional groups and its high dissociation constant. The chemical
formula of TA and the formation mechanism of antimony tartrate
anion are presented in Fig. 1. As shown in Fig. 1a, TA has two
carboxylic groups and hydroxyl groups (poly-functional O-bearing
organic ligand). An Sb3+ ion is possibly attracted to one carboxylic
group and two hydroxyl groups of TA, resulting in a 1:1 stoichiome-
try for the Sb(III)-TA complex. Then, the remaining carboxylic group
of TA interacts with the Sb(III) of another complex; thus the anti-
mony tartrate anion, [Sb2(C4H2O6)2]2− is formed. This mechanism
was first described by Kamenar et al. in 1969 and has subsequently
been described and applied by many researchers [33–35].
Although the theoretical stoichiometric ratio (TA/Sb(III)) was
known to be 1 for the stabilization, the molar ratio of TA and
SbCl3 actually required could be different since SbCl3 is largely
hydrolyzed in water and large amounts of SbOCl should instantly
form instead of generating free Sb3+. Due to this fact, we first tried
to search for the most appropriate ratio of TA to SbCl3 (TA/SbCl3) in
the present study. TA was gradually added to a mixture of SbCl3
and deionized water until a transparent solution was obtained.
When the ratio was fixed at ca. 3.5, a clear solution was observed.
This solution was referred to as TA3.5S, and TA–Sb(III) complexes
([Sb2(C4H2O6)2]2−) with different TA/SbCl3 ratios were also pre-
pared (TA7S: 7.0, TA14S: 14, TA21S: 21). We reacted each of these
complexes separately with the reduced Te source (HTe−), which
had previously been prepared (Eq. (2)), and we aged these reac-
tion mixtures for 6 h to form the desired product, Sb2Te3 (Eq. (3)).
According to the crystalline patterns of the
tion rate between TA–Sb(III) and HTe− gradually diminished with
an increasing amount of TA. In other words, when more TA was
used in the reaction, its complexing effect was enhanced, leading
to inhibition of the reactivity of Sb3+. As a result, HTe− was not able
to react with Sb3+, and Te therefore increasingly precipitated in the
system. TA–Sb(III) complexes were expected to remain during this
reaction step but could have been removed from the system via
the washing step. Therefore, the end product only exhibited the
hexagonal phase of Te, as shown in Fig. 2c.
In contrast, we detected three different diffraction patterns
when setting the ratio at 3.5 (TA3.5S). A third crystalline struc-
ture was found in TA3.5S in addition to the rhombohedral Sb2Te3
and the hexagonal Te, which also appeared in TA7S and TA21S,
respectively (Fig. 2a). The structure was identified as the cubic
Sb2O3 (JCPDS card No. 5–534) having the crystalline planes of
(1 0 0), (2 2 2), (4 0 0), (3 3 1), (4 4 0), (6 2 2), (4 4 4), (5 5 1), (7 3 1),
and (6 6 2) at 23.1◦, 27.7◦, 32.1◦, 35.1◦, 46.1◦, 54.6◦, 57.2◦, 59.1◦,
64.1◦, and 74.2◦, respectively (Fig. 2d). In this case, some SbCl3
might have formed TA–Sb(III) complexes, but the remnant was
likely hydrolyzed to form SbOCl due to the insufficient amount of TA
present. It seems that the SbOCl precipitates were further oxidized
during the process, resulting in the formation of Sb2O3. Therefore,
some HTe− ions should have reacted with the TA–Sb(III) complexes
resulting in the formation of Sb2Te3; however, the rest of the HTe−
ions must have re-precipitated as Te particles since they could not
react with the Sb2O3 particles. Consequently, TA3.5S has three dif-
ferent crystalline phases, which are cubic Sb2O3, rhombohedral
Sb2Te3, and hexagonal Te.
ST4
ST6
ST7
(a)
ST8
Te
ST4
ST6
ST7
Sb2Te3
20
25
30
ST8
BH4− + Te → BH3 + HTe−
[Sb2(C4H2O6)2]2− + 3HTe− → Sb2Te3 + 2(C4H2O6)4− + 3H+
(2)
(3)
resulting products (Fig. 2a), it was confirmed that the compo-
nents in solution were largely affected by the TA/SbCl3 ratio.
Most importantly, TA7S showed the single-phase rhombohe-
dral structure of Sb2Te3, having the diffraction lines of (0 1 5),
(0 1 8), (1 0 10), (0 1 11), (1 1 0), (0 0 15), (1 0 13), (1 1 9), (2 0 5),
(0 0 18/1 0 16), (0 1 17), (0 2 10), (1 0 19/1 1 15), (0 1 20/2 0 14),
(1 2 5), (1 1 18/0 2 16), (0 0 24/2 1 10), and (1 1 0) planes at 2ꢂ values
of 28.3◦, 33.1◦, 38.2◦, 41.1◦, 42.5◦, 44.5◦, 45.2◦, 49.8◦, 52.0◦, 54.2◦,
57.0◦, 58.5◦, 63.2◦, 66.1◦, 68.7◦, 72.1◦, 75.3◦, and 77.5◦, respectively,
which exactly coincide with the values of JCPDS card No. 71–0393
(Fig. 2b).
Similarly, unwanted constituents were also detected when the
samples were prepared with higher or lower TA / SbCl3 ratios
than that of TA7S. We found that Te began to come out of the
rhombohedral phase of Sb2Te3 when the ratio was 14:1 (TA14S
in Fig. 2a). If the ratio was increased to 21:1 (TA21S in Fig. 2a), the
rhombohedral phase was completely converted into the hexago-
nal structure of Te (JCPDS card No. 86–2269) with the crystalline
planes of (1 0 0), (1 0 1), (0 1 2), (1 1 0), (1 1 1), (0 0 3), (2 0 0), (0 2 1),
(1 1 2), (2 0 2), (1 1 3), (2 1 0), (2 1 1), (1 0 4), (1 2 2), (3 0 0), (0 3 1),
and (1 1 4) at 23.1◦, 27.7◦, 38.5◦, 40.7◦, 43.6◦, 46.2◦, 47.4◦, 49.9◦,
51.6◦, 57.2◦, 63.3◦, 64.2◦, 66.4◦, 68.2◦, 72.5◦, 74.1◦, 76.0◦, and
77.7◦, respectively (Fig. 2c). These results indicate that the reac-
20
30
40
50
60
70
80
2 theta / deg.
(b)
2.0
1.5
1.0
0.5
0.0
Sb
Te
ST4
ST6
ST7
ST8
Fig. 3. Gradual conversion of the crystalline structures of the products fabricated
under different concentrations of NaBH4 (a) and their atomic composition ratios (b).

612
C. Kim et al. / Journal of Alloys and Compounds 509 (2011) 609–613
Fig. 4. Overall and enlarged diffraction patterns of the products formed using different reaction times (a and b) and the observed morphology of the products aged for 6 h
(c) and 12 h (d).
Although we have optimized the TA/SbCl3 ratio (TA7S: 7.0) to
obtain single-phase Sb2Te3 nanoparticles, we still doubted whether
the product was homogeneous since its phase could greatly be
affected by other factors. In the present study, we also paid atten-
tion to the amount of a reductant (NaBH4) used. If sufficient NaBH4
is not supplied to the system, Te would not be reduced and would
remain in the final product. The crystalline structures of the sam-
ples prepared with different concentrations of NaBH4 are shown
in Fig. 3a (ST4: 0.08 M, ST6: 0.16 M, ST7: 0.24 M, ST8: 0.32 M). It
was confirmed that the crystalline structure of the sample varied
considerably from hexagonal Te to rhombohedral Sb2Te3 as the
NaBH4 concentration was increased. In the case of ST4, Te would
be left behind without a further reaction with the TA–Sb(III) com-
plex because it was barely reduced due to the lack of NaBH4. Thus,
most of the complex was likely discharged from the system, and
a single-phase hexagonal Te was obtained. These results were in
good agreement with the ICP measurements – ST4 was mainly
composed of Te (Fig. 3b). Meanwhile, the diffraction lines indicated
that rhombohedral Sb2Te3 started to appear in ST6 with relatively
weakened hexagonal Te, and the whole phase was finally converted
into the rhombohedral Sb2Te3 structure in ST7. These results indi-
cate that Te was adequately reduced to HTe− by sufficient NaBH4
in ST7 but not in ST6. In other words, when ST6 was prepared,
both reduced and unreduced Te coexisted due to the insufficient
amount of NaBH4. Obviously, the reduced Te should react with
the TA–Sb(III) complex, resulting in the formation of Sb2Te3, but
the unreduced Te remained in the system in the form of hexago-
nal Te without undergoing further reaction. Therefore, some of the
TA–Sb(III) complex must have been removed during washing step
when ST6 was prepared. Consequently, ST7 exhibited an atomic
ratio (Sb:Te = 0.84:1.5) close to that of the theoretical stoichiome-
try between the two atoms (1:1.5), whereas ST6 still showed the
Sb-deficient ratio (0.38:1.5), as presented in Fig. 3b.
We closely examined the crystalline structure of ST7, since it
was still questionable whether the sample had the single-phase
rhombohedral structure of Sb2Te3 (Fig. 3a, inlet). The (0 1 5) plane
of Sb2Te3 was slightly increased from ST4 to ST8, while both the
(1 0 0) and (1 0 1) planes of Te were weakened. The main peak of
ST7, however, became asymmetric when it was divided at the 2ꢂ
value of 28.3◦, which corresponds to the (0 1 5) plane of Sb2Te3.
This implies that the Te diffraction line of the (1 0 1) plane still
existed in ST7. Therefore, we tried to prepare the sample with a
higher concentration of NaBH4 (ST8). The entire diffraction pattern
of ST8 was almost same as that of ST7, as indicated in Fig. 3a, but its
main peak was shifted slightly to a higher angle side compared to
that of ST7 – the main peak was located at precisely 28.3◦ (Fig. 3a,
inlet). As a result, the hexagonal Te completely disappeared in ST8;
therefore, we obtained the desired structure: single-phase rhom-
bohedral Sb2Te3. The atomic ratio of ST8 (0.92:1.5) was closer to the
theoretical stoichiometry than that of ST7 but still showed a slight
error (Fig. 3b). Although we further increased the NaBH4 concen-
tration, no considerable change was detected in the atomic ratio;
thus, the error seemed to be caused by a lack of SbCl3, which was
initially supplied, or by inaccuracy of the ICP measurement.
As we already mentioned above, the reaction between TA–Sb(III)
and HTe− was run for only 6 h (aging time) from the moment
when the reaction components were mixed. To verify the effect
of aging time on the product composition, we varied the time from
1 to 12 h. ST8 samples were prepared with various aging times,
and their crystalline patterns are shown in Fig. 4a (ST8-X, X: aging
time). The reaction did not seem to be completed when the samples
were aged for less than 6 h. In the case of ST8-1 h, many diffraction

C. Kim et al. / Journal of Alloys and Compounds 509 (2011) 609–613
613
lines of Te were still observed, especially the lines corresponding
to the Te (1 0 0) and (1 0 1) planes (Fig. 4b). We considered that the
Te-rich mixture, ST8-1 h, was generated because most TA–Sb(III)
complexes could not react with HTe− during such a short aging
time and were likely removed during the washing process. In the
case of ST8-3 h, both the Te planes became remarkably weakened
when compared to those of ST8-1 h since the complete rate of the
reaction between TA–Sb(III) and HTe− should become higher with
increased aging time. The Te phase, however, still appeared in ST8-
3 h because its main peak was somewhat shifted to the direction of
the Te (1 0 1) plane as shown in Fig. 4b. The reaction was considered
to be complete when the samples were aged for more than 6 h since
we did not find any remarkable difference between the main peaks
of ST8-6 h and ST8-12 h (Fig. 4b). ST8-12 h showed the single-phase
rhombohedral structure of Sb2Te3, which was almost the same as
that of ST8-6 h, as presented in Fig. 4a. This indicates that the reac-
tion was likely complete about 6 h after it was initiated. We also
confirmed that the particle size and distribution were not largely
affected by aging time if the reaction was maintained for more than
6 h – both the samples were composed of Sb2Te3 nanoparticles
under 100 nm with similar distributions (Fig. 4c and d). Therefore,
aging times between 6 and 12 h resulted in rhombohedral Sb2Te3
nanoparticles with good distributions.
We have shown that the final phases of products largely
depended on reaction parameters such as chemical additives and
the aging time. The experimental data exhibited in the present
study showed the influence of each parameter on the products.
We used the data to optimize the reaction conditions and finally
obtained the desired product: highly dispersed single-phase Sb2Te3
nanoparticles. Manufacturing sintered bodies is now being inves-
tigated to measure the thermoelectric performance (ZT) of the
nanoparticles and to feasibly apply them to thermoelectric mod-
ules.
funded by the Ministry of Knowledge Economy, Republic of Korea
(No. 2007EID11P050000).
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4. Conclusions
In this study, we tried to prepare Sb2Te3 thermoelectric
nanoparticles using a water-based chemical reaction under atmo-
spheric conditions. In order to obtain only a rhombohedral Sb2Te3,
we varied some of the reaction parameters, such as the amount of
chemical agents (TA and NaBH4) and the aging time. The chem-
ical and physical properties of the products turned out to be
largely affected by the parameters. Finally, we obtained the desired
thermoelectric Sb2Te3 nanoparticles having a single crystalline
structure with the appropriate atomic composition.
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Acknowledgements
This work was supported by the DGIST Basic Research Program
of the Ministry of Education, Science and Technology. It was also
supported by the Energy Efficiency & Resources of the Korea Insti-
tute of Energy Technology Evaluation and Planning (KETEP) grant