Thermal behavior of antimony nanowire arrays embedded in anodic aluminum oxide template

Article abstract of DOI:10.1007/s10973-005-7017-9

Highly oriented single crystal antimony nanowire arrays have been synthesized within anodic aluminum oxide (AAO) template by pulsed electrodeposition. Thermal behavior and oxidation analysis of the antimony nanowires have been investigated by means of thermogravimetry and differential scanning calorimetry in Ar and air atmosphere, respectively. Compared to bulk antimony, the antimony nanowires exhibit a lower sublimation temperature at 496.4°C. Evident oxidation of the Sb nanowires occurs at 429.8°C in air atmosphere and α-Sb₂O₄ nanowires have been obtained as the oxidation product. The results indicate that the sublimation and the oxidation of the antimony nanowires in the AAO template is a slow multi-step process. The present results are of relevance when processing antimony nanowries for thermoelectric applications at high temperatures.

Full text of DOI:10.1007/s10973-005-7017-9

Journal of Thermal Analysis and Calorimetry, Vol. 89 (2007) 2, 493–497
THERMAL BEHAVIOR OF ANTIMONY NANOWIRE ARRAYS
EMBEDDED IN ANODIC ALUMINUM OXIDE TEMPLATE
X. Zhang1, Y. Ding2, Y. Zhang1, Y. Hao1, G. Meng1* and L. Zhang1
1Key Laboratory of Materials Physics, and Anhui Key Laboratory of Nanomaterials and Nanostructures, Institute of Solid State
Physics, Chinese Academy of Sciences, Hefei 230031 P. R. China
2Hefei National Laboratory for Physical Science at Microscale, Structure Research Laboratory, University of Science and
Technology of China, Hefei 230026 P. R. China
Highly oriented single crystal antimony nanowire arrays have been synthesized within anodic aluminum oxide (AAO) template by
pulsed electrodeposition. Thermal behavior and oxidation analysis of the antimony nanowires have been investigated by means of
thermogravimetry and differential scanning calorimetry in Ar and air atmosphere, respectively. Compared to bulk antimony, the an-
timony nanowires exhibit a lower sublimation temperature at 496.4°C. Evident oxidation of the Sb nanowires occurs at 429.8°C in
air atmosphere and a-Sb2O4 nanowires have been obtained as the oxidation product. The results indicate that the sublimation and
the oxidation of the antimony nanowires in the AAO template is a slow multi-step process. The present results are of relevance
when processing antimony nanowires for thermoelectric applications at high temperatures.
Keywords: antimony nanowires, TEM, thermal analysis, thermoelectric material
Introduction
materials, such as Bi1–xSbx [1, 4], CoSb3 [5], b-Zn4Sb3
[6–8], and Sb2Te3 [9]. Although the electronic proper-
ties of one-dimensional nanostructured materials
have been intensively studied, there are very few re-
ports about thermal properties of these materials,
which are important for both basic research and possi-
ble applications [10–13].
In this paper, we have synthesized highly ori-
ented single crystalline Sb nanowire arrays within the
nanochannels of anodic aluminum oxide (AAO) tem-
plate using pulsed electrodeposition, and investigated
their thermal properties including sublimation, oxida-
tion, and enthalpy change of the Sb nanowire arrays.
Microstructures of the Sb nanowires and their oxida-
tion product have been investigated by X-ray diffrac-
tion (XRD) and transmission electron microscopy
(TEM) analysis. The present work has evaluated the
thermal stability of the Sb nanowire arrays embedded
in the AAO template and may provide some interest-
ing data for exploring potential applications of the Sb
nanowires on high temperature thermoelectric
nanodevices.
Thermoelectric materials, which convert energy be-
tween heat and electricity directly, have been always
fascinating because their great potential for refrigera-
tion and power generation applications. Figure-of-
merit (ZT), given by ZT=S2sT/k, represents the rela-
tive magnitudes of thermal and electrical cross-effect
transport in materials: larger ZT signifies efficient
conversion of thermal to electric power. Where the
factors are, the Seebeck coefficient (S), the electrical
conductivity (s) and thermal conductivity (k), respec-
tively [1]. Low dimensional nanostructures give rise
to new opportunities for developing large ZT thermo-
electric materials owing to decreased lattice thermal
conductivity, improved carrier mobility, and/or a high
density of state just above Fermi level [2].
Antimony (Sb) is a semimetal with a narrow en-
ergy overlap of 180 meV between the L-point of con-
duction band and T-point of valence band at 4.2 K,
long mean free path of a few micrometers, larger
de Broglie wavelength of 40 nm [3]. If Sb can be syn-
thesized as nanowires, enhanced density of states due
to quantum confinement effects will elevate S without
decreasing s, strong ballistic effects may be observed
due to long electron mean free path, boundary scatter-
ing on the nanowire wall will reduce the value of k
more than that of s. Therefore, Sb nanowires are ex-
pected to play an important role for high efficiency
thermoelectric applications. In addition, many
antimonides are also widely used as thermoelectric
Experimental
Synthesis of Sb nanowires embedded in the
nanochannels of AAO template
AAO templates were prepared by using a two-step an-
odization process as described previously [14]. After the
*
Author for correspondence: gwmeng@issp.ac.cn
1388–6150/$20.00
Akadémiai Kiadó, Budapest, Hungary
Springer, Dordrecht, The Netherlands
© 2007 Akadémiai Kiadó, Budapest

ZHANG et al.
second anodization, the aluminum substrate at the bot-
tom was removed in a saturated SnCl4 solution. Then
the alumina barrier layer was dissolved in 6 mass%
phosphoric acid solution at 30°C for 1.5 h to get the
through-pore AAO template with the pore diameter of
about 50 nm. A layer of Au with about 200 nm in thick-
ness was sputtered onto the bottom side of the AAO
template to serve as working electrode. The Sb plating
solution consists of 0.02 M SbCl3, 0.1 M C6H8O7×H2O
and 0.05 M K3C6H5O7×H2O. The initial pH was adjusted
to 2.0 by adding appropriate amount of 5 M H2SO4 solu-
tion. The pulsed electrodeposition was performed at
12°C, to get single crystal Sb nanowire arrays [15].
Fig. 1 XRD pattern of the synthesized nanowire arrays
Thermal analysis
Thermogravimetry (TG) and differential scanning
calorimetry (DSC) analyses of the samples were car-
ried out on a Shimadzu TGA-50H thermogravimetry
and a DSC-50 Shimadzu differential scanning calo-
rimetry. For TG analysis, records were obtained with
a heating rate of 10°C min–1 under dynamic air and Ar
from 100 to 1000°C in an open alumina crucible, re-
spectively. The sample mass was about 5 mg. Mean-
while AAO template sputtered with Au film of
3.587 mg was analyzed in Ar atmosphere in the same
temperature range as reference. For DSC analysis,
samples about 1.5 mg were weighed onto an open
platinum crucible. Samples were also analyzed in Ar
and air atmosphere respectively, with a heating rate of
10°C min–1 from 100 to 650°C.
Fig. 2 a – a typical TEM image of Sb nanowires, b – SAED
pattern from one of the nanowires in (a), c – high reso-
lution TEM image of the nanowire
is shown in Fig. 1. It can be seen that there is a very
sharp diffraction peak at 2q=41.98° corresponding
to (110) crystal plane of Sb, and the other diffraction
peaks are very weak, indicating that the Sb nanowires
deposited in the template grow perpendicularly
to (110) plane.
Typical TEM image the nanowires (Fig. 2a) in-
dicates that the diameter of synthesized nanowires is
about 50 nm. Figure 2b shows the selected area elec-
tron diffraction (SAED) pattern of a single Sb
nanowire, the diffraction spots of (100), (110)
and (010) correspond to single crystal hexagonal
packed structure of antimony. As shown in Fig. 2c,
lattice-resolved HRTEM image of Sb (102) plane re-
veals high crystalline quality of the Sb nanowires.
Structural characterization
The as-electrodeposited products and the residue of
DSC analysis in air atmosphere were characterized by
a rotating anode X-ray powder diffraction (XRD,
D/MAX-Ra) with CuKa radiation (l=1.5418 ).
Microstructures of these nanowires were character-
ized using transmission electron microscopy (TEM,
JEM 200CX) and high-resolution TEM (HRTEM,
JEOL 2010). For TEM analysis, the specimens were
prepared by dissolving the AAO template with a
5 mass% NaOH solution, dispersing the nanowires in
ethanol by ultrasonic cleaning. Then drops of the so-
lution were dripped onto copper grids coated with
holey carbon film.
Thermal analysis
Figure 3 shows the TG curve of the gold sputtered
AAO template. The curve was obtained in Ar atmo-
sphere, showing three obvious mass loss steps in the
curve. Because there is no apparent mass loss for the
AAO template in this temperature range, this mass
loss can be assigned to the release of the thin gold
film. The melting point of the gold film shifts to a
lower temperature at about 844.2°C in comparison
with that of bulk gold [16]. The main mass loss in the
temperature range 844.2 and 909.1°C caused by the
evaporation of the molten gold is about 3.5%. And the
melting rate of the gold particles increases with
further elevating temperature.
Results and discussion
The characterization of the as-electrodeposited
nanowire arrays
An X-ray diffraction pattern of the Sb nanowire ar-
rays embedded in the nanochannels of AAO template
494
J. Therm. Anal. Cal., 89, 2007

THERMAL BEHAVIOR OF ANTIMONY NANOWIRE ARRAYS
Fig. 3 TG curve for the AAO template sputtered with a
golden film
Fig. 5 TG and DSC curves of the Sb nanowire arrays in air at-
mosphere
sublimes. Finally, the sublimation of the middle Sb
nanowires occurs. The enthalpy changes of three
stages are about 3.61, 11.12 and 3.38 J g–1, respec-
tively. Due to the restriction of the instrument mea-
surement range, the whole enthalpy change in the
sublimation reaction cannot be obtained.
As shown in Fig. 5, the TG curve of the Sb
nanowires obtained in air environment shows multi-
ple stages. The first stage is a 1.9% mass loss between
156.9 and 465.5°C, which may be due to the release
of water, in consistent with the analysis in Ar condi-
tion. Different from the curve in Ar atmosphere, there
are two stages in the temperature range from 465.5 to
592.1°C for the curve obtained in air atmosphere.
This phenomenon can be explained as follows.
Firstly, the oxidization of surface antimony leads to
mass gain, whereas the sublimation of Sb nanowires
causes the mass loss, the net effect of the above two
processes produces these two stages. Subsequently,
there is an obvious mass gain about 2.9% from 592.1
to 858.1°C, which is mainly due to the oxidation of Sb
in the AAO template. The broad mass gain tempera-
ture range may mean that the oxidation of the Sb
nanowires is a slow process. Similar to the curve in Ar
atmosphere, there also exists a mass loss stage of the
gold film for the curve in air atmosphere. Following
this, there is an obvious mass gain till 957.8°C.
The percentage of the mass loss of Au in air is smaller
than that in Ar, which may be due to the partial
oxidation of gold in air.
As shown in Fig. 5, there is an obvious exother-
mic peak in the DSC curve for the Sb nanowire arrays
in air atmosphere. The onset oxidation temperature of
the Sb nanowire arrays embedded in the AAO tem-
plate is about 429.8°C. The broad strong exothermic
peak means that there is a slow oxidation process for
the Sb nanowire arrays in air atmosphere. Similar to
the sublimation of the Sb nanowires in Ar atmo-
sphere, the oxidation of the Sb nanowires can be fitted
into three stages. The lower temperature step is re-
Fig. 4 TG and DSC curves of the Sb nanowire arrays in Ar at-
mosphere
Figure 4 exhibits the TG curve of the Sb nano-
wire arrays embedded in the AAO template in Ar at-
mosphere, two obvious mass loss steps can be ob-
served here. The first mass loss is 3.2% located in be-
tween 528.5 and 642.6°C, which is associated with
the sublimation of Sb [17]. The second mass loss is
3.8% located in the temperature range from 861.0 to
914.4°C, which may be due to the evaporation of the
molten gold film used as electrode during electro-
deposition of the nanowires. This result is in consis-
tence with TG result of the template sputtered with
gold layer but without Sb nanowires.
DSC curve (Fig. 4) of the same sample displays
multiple weak endothermic peaks. The endothermic
peaks from 194.4 to 408.0°C are caused by the release
of water, coming from the hydroxyl group that is orig-
inated from anodization process [18]. The onset subli-
mation temperature of the Sb nanowires embedded in
AAO template is about 496.4°C, indicating that the
sublimation temperature of the Sb nanowires is
shifted to a lower temperature compared with that of
bulk Sb [17]. The broad endothermic peak from 496.4
to 644.2°C, which are split into three peaks, can be as-
signed to the sublimation of antimony in three stages.
The first stage of lower temperature is related to the
sublimation of Sb at the grain boundaries of the Sb
nanowires [19]. Following this, more and more Sb
J. Therm. Anal. Cal., 89, 2007
495

ZHANG et al.
lated to the oxidation of Sb at the grain boundaries of
of (002), (200) and (202) lattice planes are character-
istics of the orthorhombic structure of a-Sb2O4.
It should be noted that some inhibited diffraction
spots appear here, i.e. (001) lattice plane. From the
HRTEM image of the same nanowire, the (001) lat-
tice fringe with lattice spacing around 1.2 nm can be
clearly observed, being in consistence with the dif-
fraction pattern.
the Sb nanowire arrays. Secondly, more and more Sb
is oxidized with increasing temperature. Finally, the
oxidation of Sb in the middle of nanowires occurs at
higher temperature [19]. The enthalpy change of each
stage is about 299.25, 97.58 and 61.94 J g–1, respec-
tively. This confirms that the main chemical reaction
of the Sb nanowire arrays in air atmosphere is an oxi-
dization process, while the sublimation of the Sb
nanowire arrays is very weak. Unfortunately, the
whole enthalpy change in the oxidation reaction can-
not be obtained because of the restriction of the in-
strument measurement range.
Conclusions
Systemic thermal analysis of Sb nanowires embedded
in the AAO template has been investigated in Ar and
air atmosphere, respectively. The results indicate that
single crystalline Sb nanowires embedded in the
AAO template possess excellent thermal stability be-
low 400°C. DSC and TG analyses show that obvious
sublimation of the Sb nanowires occurs up to 496.4°C
in Ar atmosphere, while oxidation process dominates
from 429.8°C in air ambience, accompanied with
minute sublimation. After analysis in air atmosphere,
the Sb nanowires with hexagonal structure were oxi-
dized into a-Sb2O4 nanowires with orthorhombic
structure. The results are in good agreement with that
of the thermal analysis. This research will outline the
optimal operation temperature range of the Sb nano-
wire arrays in thermoelectric device.
Structural characterization of the residue after DSC
analysis in air atmosphere
XRD pattern of the residue after DSC analysis in air
atmosphere reveals that the residue is dehydrated
orthorhombic a-Sb2O4 (Fig. 6). The structure is in
C2qv /P na21 space group, and the parameters of the unit
cell are a=4.804 , b=5.424  and c=11.76 , respec-
tively [20].
Typical bright field TEM image of and high-res-
olution TEM image of the nanowires with diffraction
pattern are shown in Fig. 7. Intense diffraction spots
Acknowledgements
This work was financially supported by the National Science
Fund for Distinguished Young Scholars (Grant
No. 50525207), the Natural Science Foundation of China
(Grant No. 10374092), and the National Major Project of Fun-
damental Research: Nanomaterials and Nanostructures (Grant
No. 2005CB623603).
References
1 Y. M. Lin, O. Rabin, S. B. Cronin, J. Y. Ying and
M. S. Dresselhaus, Appl. Phys. Lett., 81 (2002) 2403.
2 L. D. Hicks, T. C. Harman and M. S. Dresselhaus, Appl.
Phys. Lett., 63 (1993) 3230.
Fig. 6 XRD pattern of the residue after measurement in air at-
mosphere
3 L. R. Windmiller, Phys. Rev., 149 (1966) 472.
4 T. Kyratsi, D. Y. Chung and M. G. Kanatzidis, J. Alloys
Compd., 338 (2002) 36.
5 T. Shimozaki, K. Kim, T. Iwata, T. Okino and C.G. Lee,
Mater. Trans., 43 (2002) 2609.
6 J. Nylen, M. Andersson, S. Lidin and U. Haussermann,
J. Am. Chem. Soc., 126 (2004) 16306.
7 Y. Mozharivskyj, A. O. Pecharsky, S. Bud'ko and
G. J. Miller, Chem. Mater., 16 (2004) 1580.
8 L. T. Zhang, M. Tsutsui, K. Ito and M. Yamaguchi, Thin
Solid Film, 443 (2003) 84.
Fig. 7 a – TEM image of the a-Sb2O4 nanowires obtained af-
ter analysis in air atmosphere, b – SAED pattern taken
from the nanowire shown in (a), c – high resolution
TEM image of a single a-Sb2O4 nanowire
496
J. Therm. Anal. Cal., 89, 2007

THERMAL BEHAVIOR OF ANTIMONY NANOWIRE ARRAYS
9 C. G. Jin, G. Q. Zhang, T. Qian, X. G. Li and Z. Yao,
J. Phys. Chem. B, 109 (2005) 1430.
18 O. Jessensky, F. Muller and U. Gosele, Appl. Phys. Lett.,
72 (1998) 1173.
10 J. Heremans, C. M. Thrush, Y. M. Lin, S. B. Cronin and
M. S. Dresselhaus, Phys. Rev. B, 63 (2001) 085406.
11 L. D. Hicks and M. S. Dresselhaus, Phys. Rev. B,
47 (1993) 16631.
19 A. Kolmakov, Y. X. Zhang and M. Moskovits, Nano Lett.,
3 (2003) 1125.
20 Z. L. Zhang, J. Mater. Res., 17 (2002) 1698.
12 D. Y. Li, Y. Y. Wu, P. Kim, L. Shi, P. D. Yang and
A. Majumdar, Appl. Phys. Lett., 83 (2003) 2934.
13 H. Matsushita, E. Hagiwara and A. Katsui, J. Mater. Sci.,
39 (2004) 6299.
Received: March 25, 2005
Accepted: December 20, 2006
OnlineFirst: April 29, 2007
14 H. Masuda and K. Fukuda, Science, 268 (1995) 1466.
15 Y. Zhang, G. H. Li, Y. C.Wu, B. Zhang, W. H. Song and
L. D. Zhang, Adv. Mater., 14 (2002) 1227.
16 P. Buffat and J-P. Borel, Phys. Rev. A, 13 (1976) 2287.
17 B. V. L’vov and A. V. Novichikhin, Thermochim. Acta,
290 (1997) 239.
DOI: 10.1007/s10973-005-7017-9
J. Therm. Anal. Cal., 89, 2007
497