Nanoscale zinc antimonides: Synthesis and phase stability

Article abstract of DOI:10.1021/ic051808t

Highly crystalline single-phase nanoparticles of the important thermoelectric materials Zn₄Sb₃ and ZnSb were prepared from solvochemically activated powders of elemental zinc and elemental antimony. Low-temperature reactions with rea

Full text of DOI:10.1021/ic051808t

Inorg. Chem. 2006, 45, 1693−1697
Nanoscale Zinc Antimonides: Synthesis and Phase Stability
Sabine Schlecht,* Christoph Erk, and Maekele Yosef
Freie UniVersit a¨ t Berlin, Institut f u¨ r Chemie und Biochemie, 14195 Berlin, Germany
Received October 19, 2005
4 3
Highly crystalline single-phase nanoparticles of the important thermoelectric materials Zn Sb and ZnSb were prepared
from solvochemically activated powders of elemental zinc and elemental antimony. Low-temperature reactions with
reaction temperatures of 275 300 C were applied using an excess of elemental zinc. The nanoscale thermoelectrics
obtained were characterized by X-ray powder diffraction, transmission electron microscopy, and thermal analysis.
nc-Zn Sb showed particle sizes of 50 70 nm, whereas particle sizes of 15 20 nm were observed for nc-ZnSb.
Calorimetric investigations showed an increased heat capacity, C , for nc-Zn Sb with respect to the bulk material
which could be reduced to the bulk value by annealing nc-Zn Sb at 190 C. Interestingly, nc-Zn Sb showed
C in an open system, indicating that Zn Sb is metastable
−
°
4
3
−
−
p
4
3
4
3
°
4
3
exothermic decomposition into zinc-poorer ZnSn at 196
in nanocrystalline form at room temperature.
°
4
3
Introduction
vacancies seem to be the predominant features leading to
very efficient phonon scattering and thus to a very low
The conversion of heat into electrical energy and vice versa
represents one of the future key challenges in technology.
Thus, the improvement of high-performance thermoelectric
materials for solid-state refrigerators or for the generation
of electrical energy in space, for example, is of major
importance.1,2 Among the best thermoelectric materials
known to date are the zinc antimonides Zn
which are especially effective between 150 and 400 °C. At
a temperature of 400 °C, Zn Sb has the highest figure of
3
,4
thermal conductivity. This structural motif has also been
7
found for the more zinc-rich ú-Zn3-xSb
2
phase, but it is not
present in ZnSb which consequently shows a much higher
3
4 3
thermal conductivity than Zn Sb . The important role of
interstitials as “rattlers” in the voids of a structure has been
pointed out in the “phonon-glass-electron-crystal” model by
4 3
Sb and ZnSb
8
Slack.
One major general concept for lowering the thermal
conductivity is the application of low-dimension materials
such as nanoscale particles or wires that provide effective
phonon scattering at a large number of grain boundaries.9
Prominent examples of this concept are the nanoscale
4
3
3
merit, ZT ≈ 1.3, of all known binary thermoelectrics. The
figure of merit of a thermoelectric material is defined by
2
ZT ) (R σ/κ)T, where σ is the electrical conductivity, κ is
the thermal conductivity, and R is the Seebeck coefficient
1
0
superlattice structures of thin films of Bi
2
Te
3 2 3
/Sb Te , PbTe/
of the respective material. The thermoelectrical power factor,
11a
11b
2
Pb1-xEu Te, or PbSeTe/PbTe and nanocomposite phases
such as AgPb SbTe2+m with very high figures of merit.
m
x
R σ, of Zn
4
Sb
3
is not especially high, and the exceptional
1
2
figure of merit of this compound stems from a particularly
-
3
-1
-1
low thermal conductivity of ∼6 × 10 W cm
K
for
(4) Cargnoni, F.; Nishibori, E.; Rabiller, P.; Bertini, L.; Snyder, G. J.;
Christensen, M.; Gatti, C.; Iversen, B. B. Chem.sEur. J. 2004, 10,
this zinc antimonide in the temperature range of 200-400
3861.
3
°
C. A significant body of work was published recently to
(5) Nyl e´ n, J.; Andersson, M.; Lidin, S.; H a¨ ussermann, U. J. Am. Chem.
Soc. 2004, 126, 16306.
3
-6
elucidate the structural origins of this very low value.
Disordered interstitial zinc atoms and the corresponding
(6) Mozharivskyj, Y.; Pecharsky, A. O.; Bud’ko, S.; Miller, G. J. Chem.
Mater. 2004, 16, 1580.
(
7) Bostr o¨ m, M.; Lidin, S. J. Alloys Compd. 2004, 376, 49.
*
To whom correspondence should be addressed. E-mail: schlecht@
(8) Slack, G. A. In CRC Handbook of Thermoelectrics; Rowe, D. M.,
Ed.; CRC Press: Boca Raton, FL, 1995; p 407.
(9) (a) Casimir, H. B. G. Physica 1938, 5, 495. (b) Hicks, L. D.;
Dresselhaus, M. S. Phys. ReV. B. 1993, 47, 12727. (c) Hicks, L. D.;
Dresselhaus, M. S. Phys. ReV. B. 1993, 47, 16631.
(10) (a) Venkatasubramanian, R.; Colpitts, T.; O’Quinn, B.; Liu, S.; El-
Masry, N.; Lamvik, M. Appl. Phys. Lett. 1999, 75, 1104. (b)
Venkatasubramanian, R.; Siivola, E.; Colpitts, T.; O’Quinn, B. Nature
2001, 413, 597.
chemie.fu-berlin.de. Fax: +49-(0)30-838-53310.
(
(
(
1) Nolas, G. S.; Sharp, J.; Goldsmid, H. J. Thermoelectrics: Basic
Principles and New Materials DeVelopments; Springer: Berlin, 2001.
2) (a) DiSalvo, F. J. Science 1999, 285, 703. (b) Tritt, T. M. Science
1999, 283, 804.
3) (a) Snyder, G. J.; Christensen, M.; Nishibori, E.; Caillat, T.; Iversen,
B. B. Nature Mater. 2004, 3, 458. (b) Caillat, T.; Fleurial, J. P.;
Borshchevsky, A. J. Phys. Chem. Solids 1997, 58, 1119.
1
0.1021/ic051808t CCC: $33.50
© 2006 American Chemical Society
Inorganic Chemistry, Vol. 45, No. 4, 2006 1693
Published on Web 01/10/2006

Schlecht et al.
Here, we present an extension of the concept of nanoscale
thermoelectrics on the zinc antimonides Zn Sb and ZnSb.
Because of the intrinsically metastable nature of nanoscale
matter, a low-temperature synthetic route to ZnSb and Zn
Sb had to be applied. A simple and versatile approach to
4
3
4
-
3
nanoscale binary intermetallic compounds is the reaction of
the two required elements in activated (i.e., finely dispersed)
13
form leading to nanoparticles of the binary compound. The
small particle size of the reactands ensures minimum
diffusion paths, low activation barriers, and low reaction
temperatures.
Figure 1. TEM overview micrograph of nanoparticles of Zn4Sb3 (left)
and an HRTEM image of a single nanoparticle showing {122} lattice fringes
(right).
Results and Discussion
4 3
The synthesis of nanoparticles of Zn Sb and ZnSb was
achieved through the reaction of activated zinc with activated
antimony in the temperature region of 275-300 °C. The
reactands were produced by reduction of solutions of ZnCl
and SbCl in tetrahydrofuran with a solution of lithium
triethylboronhydride Li[Et BH] in tetrahydrofuran. The exact
2
3
3
reaction conditions are given in eqs 1 and 2.
THF
ZnCl + 2Li[Et BH] reflux, 2 h8
2
3
Zn* + H + 2BEt + 2LiCl (1)
2
3
Figure 2. TEM overview micrograph of nanoparticles of ZnSb (left) and
an HRTEM image of a single nanoparticle showing {021} lattice fringes
(right).
THF
SbCl + 3Li[Et BH] rt, 30 min8
3
3
Sb* + 1.5H + 3BEt + 3LiCl (2)
2
3
1
5 h. Overview TEM micrographs and high-resolution TEM
images of as-obtained nanoparticles of Zn Sb and ZnSb,
after the elemental zinc was removed, were taken to
investigate particle sizes and crystallinity. In the case of Zn
Sb , particles without facets of 50-70 nm in size were
The metal powders obtained were washed repeatedly with
dry tetrahydrofuran to remove residual lithium salts and were
subsequently dried under vacuum. The activated elements
were placed in a Schlenk ampule and heated in an electrical
furnace under argon. The reactions were conducted with an
excess of zinc metal which was removed by a short treatment
with dilute acetic acid after the reaction was completed. For
4
3
4
-
3
produced (Figure 1), whereas the ZnSb particles were only
about 15-20 nm in size and were also rather featureless
(Figure 2). HRTEM images of nanoparticles of both phases
show the good crystallinity of the nanoscale material (Figures
1 and 2). This is confirmed by the X-ray powder diffraction
patterns of Zn
could be indexed on the basis of bulk orthorhombic ZnSb
(space group Pbca) and rhombohedral Zn Sb (space group
R 3h c). As the reflections of the rather big particles of Zn
Sb show only marginal broadening, an estimation of the
4 3
the preparation of the Zn Sb nanoparticles, an ampule filled
with the activated elements in a molar ratio between 2.5:1
and 3:1 (Zn/Sb) was heated to 300 °C at a rate of 15 K/min
and immediately cooled to 275 °C after this temperature was
reached. The reaction temperature was held at 275 °C for
4 3
Sb and ZnSb (Figure 3). The XRD patterns
4
3
6
1
5 h; then the reaction mixture was allowed to cool to room
4
-
temperature. The product was collected and stirred in 2%
acetic acid at room temperature for 5-10 min, depending
on the overall amount of material. The lowering of the
reaction temperature from 300 to 275 °C turned out to be
3
1
4
grain size according to Scherrer’s equation was only done
for ZnSb. A particle size of 18 ( 2 nm was obtained, which
is in good agreement with the geometrical particle size found
on the TEM images.
crucial for the synthesis of nanocrystalline Zn
the reaction temperature is held at more than 285 °C, the
nanocrystalline Zn Sb decomposes into nanocrystalline
4 3
Sb . When
4 3
The thermal lability of the Zn Sb particles during the
process of preparation is striking and does not correspond
to the thermal stability of this phase in the bulk according
to the phase diagram where a decomposition temperature of
4
3
ZnSb with loss of elemental zinc. Hence, nanoparticles of
ZnSb are obtained when the reaction is held at 300 °C for
1
5
4
92 °C is denoted; however, a certain tendency to lose
elemental zinc at a fairly low temperature had already been
observed for bulk Zn Sb earlier. In these experiments, bulk
Zn Sb was heated under vacuum and significant decomposi-
(
11) (a) Harman, T. C.; Spears, D. L.; Manfra, M. J. J. Electron. Mater.
996, 25, 1121. (b) Harman, T. C.; Taylor, P. J.; Walsh, M. P.;
LaForge, B. E. Science 2002, 297, 2229.
12) (a) Hsu, K. F.; Loo, S.; Guo, F.; Chen, W.; Dyck, J. S.; Uher, C.;
Hogan, T.; Polychroniadis, E. K.; Kanatzidis, M. G. Science 2004,
1
4
3
(
4
3
6
tion into ZnSb and Zn started around 290 °C. We heated
3
03, 818. (b) Quarez, E.; Hsu, K. F.; Pcionek, R.; Frangis, N.;
Polychroniadis, E. K.; Kanatzidis, M. G. J. Am. Chem. Soc. 2005,
27, 9177.
1
(14) Guinier, A. X-ray Diffraction in Crystals, Imperfect Crystals and
Amorphous Bodies; Dover: New York, 1994; pp 121-130.
(15) Massalski, T. E., Ed. Binary Alloy Phase Diagrams; ASM Interna-
tional: Materials Park, OH, 1990.
(
13) B o¨ nnemann, H.; Brijoux, W. Catalytically Active Metal Powders. In
ActiVe Metals; F u¨ rstner, A., Ed.; VCH: Weinheim, Germany, 1996;
pp 339-379.
1694 Inorganic Chemistry, Vol. 45, No. 4, 2006

Nanoscale Zinc Antimonides
Figure 4. DSC curves in the temperature range of 50-250 °C for bulk-
Zn4Sb3, as-prepared nc-Zn4Sb3, and annealed nc-Zn4Sb3. Both nc-Zn4Sb3
samples show an exothermic decomposition event with an onset temperature
of 196 °C (the different shape of the curve for annealed nc-Zn4Sb3 results
from the use of a different set of pans).
Figure 3. Experimental and calculated X-ray powder diffraction patterns
of nc-ZnSb (top) and nc-Zn4Sb3 (bottom).
Figure 5. X-ray powder diffraction pattern of nc-Zn4Sb3 before and after
the decomposition step at 196 °C. Dominant reflections of the ZnSb phase
formed in the decomposition are marked with an asterisk.
4 3
samples of bulk Zn Sb in an argon atmosphere at ambient
pressure and observed a continuous slow decomposition
starting at 300 °C. This decomposition process seems to be
much more favorable for nanoscale Zn
4
Sb
3
, and we observed
degree of annealing was achieved by this procedure, the onset
temperature for the decomposition remained unchanged at
196 °C (Figure 4), and no improvement of the thermal
stability was observed.
decomposition starting at 280 °C under the preparation
conditions (i.e., in the presence of an excess of zinc metal
under normal pressure (eq 3)).
Several possible reasons for the thermal instability of
nanocrystalline Zn Sb can be imagined. The decomposition
4 3
could originate from a strong increase of the vapor pressure
of zinc in the nanoscale material or from a significant
2
80 °C
nc-Zn Sb + excess Zn
8 3nc-ZnSb + excess Zn (3a)
4
3
200 °C
nc-Zn Sb3
8 3nc-ZnSb + Zn
(3b)
4
increase of the surface energy of nc-Zn
bulk material that is much more pronounced for Zn
4
Sb
3
compared to the
Sb
Aiming at a further elucidation of these decomposition
phenomena, we conducted calorimetric measurements with
4
3
nanoparticles than for nanoscale ZnSb. Another factor of
thermal instability could be a significantly higher uptake of
thermal energy in the case of the particles. Thus, we
nanocrystalline Zn
Sb
4 3
in the absence of additional zinc. The
Sb to 250 °C revealed an exothermic
heating of pure nc-Zn
4
3
event with an onset temperature of 196 °C (Figure 4). An
XRD pattern of this sample was recorded after the DSC
measured the heat capacities, C
pure nc-Zn Sb (named as-prepared nc-Zn
lowing), and pure nc-Zn Sb which was previously annealed
at 190 °C for 1 h in the temperature range of 50-90 °C.
This low-temperature region was chosen to avoid sintering
and particle growth during the measurements. Whereas nc-
p
, of nc-ZnSb, bulk Zn
4 3
Sb ,
4
3
4
Sb in the fol-
3
experiment which showed that most of the Zn
decomposed during this exothermic transformation (Figure
). When a sample of nc-Zn Sb was subjected to a
4
Sb
3
had
4
3
5
4
3
temperature of 250 °C for 1 h, decomposition almost reached
completion. In an additional DSC experiment, we heated a
ZnSb shows C
bulk value of 3R ) 24.9 J mol
samples show C values higher than the Dulong-Petit limit,
reflecting the “phonon-glass-type” characteristics of this
compound (Figure 6). In comparison to the heat capacities
p
values only slightly higher than the expected
-
1
-1
sample of nc-Zn
4
Sb
3
to 190 °C (e.g., slightly below the
K
4 3
for ZnSb, all Zn Sb
decomposition temperature), held this temperature for 1 h,
cooled the sample down to room temperature, and looked at
its decomposition characteristics again. Although a certain
p
Inorganic Chemistry, Vol. 45, No. 4, 2006 1695

Schlecht et al.
properties also seem to be responsible for the thermal
collapse of this phase on the nanoscale. The interstitial zinc
atoms in the structure of Zn Sb are especially mobile and
4 3
can easily migrate to the particle surface where enhanced
lattice dynamics will facilitate the transformation into ZnSb
under zinc loss. The, generally, further reduced kinetic barrier
for the transformation of Zn
facilitates the decomposition of nc-Zn
4
Sb
3
into ZnSb in small particles
Sb , which is a
4
3
metastable phase at low temperatures when nanocrystalline.
Summary
The use of nanoscale powders of activated zinc and
antimony allowed the low-temperature synthesis of nano-
Figure 6. Heat capacities, Cp, of bulk-Zn4Sb3, as-prepared nc-Zn4Sb3, and
annealed nc-Zn4Sb3 in the temperature range of 50-90 °C.
particles of the binary thermoelectrics ZnSb and Zn
as-obtained products were highly crystalline single-phase
materials. Thermal investigation of nc-ZnSb and nc-Zn Sb
revealed an enhanced heat capacity for nc-Zn Sb which
could be reduced to the bulk value of Zn Sb by annealing
4 3
Sb . The
4 3 p
of the Zn Sb samples, the C values of the nanocrystalline
sample are again higher than the value of the bulk material.
This tendency is generally observed for a number of
nanoscale materials, and the enhancement of the heat
4
3
4
3
4
3
1
6
at 190 °C. A finding of substantial significance for possible
future applications is the thermal decomposition of nanoscale
Zn Sb below 200 °C in an open system.
4 3
capacities can vary from 1-2% (for Ni80
P
20 and selenium)
17
to 48% (for palladium). For the as-prepared nc-Zn Sb , the
4
3
enhancement is about 12% on average in the temperature
range measured here. Interestingly, the heat capacity of nc-
Experimental Section
Zn
annealed at 190 °C for only 1 h (Figure 6). Thus, the
enhancement of the C value of the as-prepared nanoparticles,
4 3
Sb dropped to the bulk value when the sample was
Synthetic Procedures. Tetrahydrofuran was dried over sodium
and freshly distilled before use. Zinc chloride was dried with thionyl
chloride, washed with toluene, and dried under a vacuum before
use. Lithium triethylboronhydride (Aldrich, 1 M in THF) and
antimony(III) chloride were used as obtained. Activated antimony
p
normally attributed to a higher-than-average contribution
from surface lattice vibrations,1
moderate thermal treatment in the case of Zn
7-19
can be eliminated by
Sb . The
4
3
was obtained in a room-temperature reaction of a solution of SbCl
3
annealing seems to heal certain defects and surface states
which were created either during the synthesis or with the
removal of elemental zinc. The decrease in the heat capacity
of the annealed sample did not influence the decomposition
temperature and thermal stability of the material (Figure 4).
Hence, the higher uptake of thermal energy by the nano-
in tetrahydrofuran with 3 equiv of a 1 M solution of lithium
triethylboronhydride for 30 min. Activated zinc was produced by
the reaction of a solution of ZnCl in tetrahydrofuran with 2 equiv
2
of a 1 M solution of lithium triethylboronhydride at 65 °C for 2 h.
Activated zinc powder and activated antimony powder were washed
several times with tetrahydrofuran after the initial reaction solution
was decanted. The washed products were dried in a vacuum. The
antimonides were prepared in long Schlenk ampules with a pressure
particles of Zn
position characteristics. In addition to an expected increase
of the zinc vapor pressure for nanoscale particles of Zn Sb
4 3
Sb cannot be correlated with their decom-
valve attached to the top. The preparation of nanoparticles of Zn
4
-
4
3
Sb and ZnSb was conducted with a 3:1 excess of zinc powder.
3
and an increased surface energy, a higher volume enthalpy
content of the nanocrystals caused by subtle structural
differences and defects in nanoscale Zn Sb might account
4 3
for the low-temperature decomposition of these particles
leading to ZnSb.
The key result of the investigations of the thermal
The residual excess zinc was removed after the reaction by a 10
min treatment with a 2% acetic acid solution and subsequent
washing of the remaining zinc antimonide with deionized water
until neutrality was reached.
Calorimetric Measurements. Thermal analyses were carried out
under a dynamic argon atmosphere using a Netzsch Pegasus 404
properties of nanoscale ZnSb and Zn
tion of nc-Zn Sb below 200 °C with substantial conse-
quences for possible applications of nanoscale Zn Sb as a
4 3
Sb is the decomposi-
4 3
DSC calorimeter. Thirty milligrams of ZnSb, Zn Sb , nc-ZnSb, or
4
3
nc-Zn Sb was placed in a double-wall DSC pan consisting of a
4
3
4
3
2 3
Pt/Rh mantle and an Al O inlay, covered with a Pt/Rh lid. The
thermoelectric material. The desired temperature range with
an excellent figure of merit for this compound is between
calorimeter was evacuated twice and filled with argon before the
measurements. The measurements were carried out under a constant
gentle flow of argon at a heating rate of 10 K/min. C
were calibrated on a sample of elemental silver, the heat capacity
of which was given in ref 20. C values are given for an average
atom equivalent of the molar heat capacity, namely, 1/7C
Zn Sb ).
Electron Microscopy. Transmission electron microscopy was
performed using a JEOL JEM 3010 electron microscope (300 kV,
LaB cathode). The samples were finely dispersed in ethanol. One
p
measurements
2
00 and 400 °C, but these temperatures are not tolerated by
nc-Zn Sb when it is heated in an open system. The peculiar
structural features of Zn Sb , namely, the presence of
4
3
p
4
3
p
-
interstitial zinc atoms and an only partially occupied zinc
(
4
3
4
site leading to a significant degree of intrinsic disorder, a
very low thermal conductivity, and superior thermoelectric
6
(
(
(
(
16) Sun, N. X.; Lu, K. Phys. ReV. B 1996, 54, 6058.
drop of the dispersion was placed on a carbon-coated copper grid
17) Rupp, J.; Birringer, R. Phys. ReV. B 1987, 36, 7888.
18) Bai, H. Y.; Luo, J. L.; Jin, D.; Sun, J. R. J. Appl. Phys. 1996, 79, 361.
19) Novotny, V.; Meincke, P. P. M. Phys. ReV. B 1973, 8, 4186.
(20) Moser, H. Physik. Z. 1936, 37, 737.
1696 Inorganic Chemistry, Vol. 45, No. 4, 2006

Nanoscale Zinc Antimonides
and allowed to dry, leaving the crystallites in a random orientation
on the support film.
4 3
providing a sample of bulk Zn Sb . Financial support by
the Deutsche Forschungsgemeinschaft and the Fonds der
Chemischen Industrie is gratefully acknowledged.
Acknowledgment. We thank Prof. Dr. Bernd Harbrecht
and Samuel Freistein (Philipps-Universit a¨ t Marburg) for
IC051808T
Inorganic Chemistry, Vol. 45, No. 4, 2006 1697