Large-area Sb2Te3 nanowire arrays

Article abstract of DOI:10.1021/jp046100z

High-density large-area nanowire arrays of thermoelectric material Sb ₂Te₃ have been successfully prepared using electrochemical deposition into the channels of the porous anodic alumina membrane. The morphologies, structure, and composition of the as-prepared Sb₂Te₃ nanowires have been characterized using field-emission scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Individual Sb₂Te₃ nanowires are single crystalline and continuous with uniform diameters (a??50 nm) throughout the entire length. The atomic ratio of Sb to Te is very close to the 2:3 stoichiometry.

Full text of DOI:10.1021/jp046100z

1430
J. Phys. Chem. B 2005, 109, 1430-1432
Large-Area Sb2Te3 Nanowire Arrays
Chuangui Jin,†,‡,§ Genqiang Zhang,†,‡ Tian Qian,†,‡ Xiaoguang Li,*,†,‡ and Zhen Yao|
Hefei National Laboratory for Physical Science at Microscale, Department of Materials Science and
Engineering, UniVersity of Science and Technology of China, Hefei 230026, China, International Center for
Materials Physics, Academia Sinica, Shenyang 110015, China, College of Chemical Engineering and
EnVironment, Anhui UniVersity of Technology, Maanshan 243002, China, and Department of Physics,
UniVersity of Texas at Austin, Texas 78712
ReceiVed: August 29, 2004; In Final Form: NoVember 2, 2004
High-density large-area nanowire arrays of thermoelectric material Sb2Te3 have been successfully prepared
using electrochemical deposition into the channels of the porous anodic alumina membrane. The morphologies,
structure, and composition of the as-prepared Sb2Te3 nanowires have been characterized using field-emission
scanning electron microscopy, transmission electron microscopy, high-resolution transmission electron
microscopy, X-ray diffraction, and X-ray photoelectron spectroscopy. Individual Sb2Te3 nanowires are single
crystalline and continuous with uniform diameters (∼50 nm) throughout the entire length. The atomic ratio
of Sb to Te is very close to the 2:3 stoichiometry.
Introduction
the antimony telluride nanowire arrays has not been reported
in the literature yet. In this paper, large-area, high filling rate,
There is a growing interest in nanostructured thermoelectric
(TE) materials because of the promise that the quantum
confinement effect will increase the efficiencies of these
materials as compared with their bulk counterparts.1 This
stimulates study in the preparation of nanowires of TE materials
because nanowires are the most highly confined materials that
still retain electrical connectivity. A single nanowire cannot
transport enough current for TE applications. Therefore, the
fabrication of high-density, large-area nanowire arrays is of
particular importance. Many kinds of TE material nanowire
arrays, such as Bi2Te3, Bi1-xSbx, Bi2-xSbxTe3, and Bi2Te3-ySey,2-5
have been prepared. Our previous work6 has shown that Bi2-
Te3 nanowire arrays with high filling rate, aspect ratio, and large
area can be directly deposited by electrochemical reduction of
and highly ordered antimony telluride arrays have been suc-
cessfully prepared in PAAM templates using a simple electro-
chemical deposition technique under suitable conditions at room
temperature.
Experimental Procedures
The homemade PAAM templates were prepared using a two-
step anodization process.18,19 A layer of Au film was sputtered
onto one side of the through-hole PAAM template to serve as
the working electrode in a two-electrode electrochemical cell,
and a graphite plate was used as the counter electrode. The
antimony telluride nanowires were electrochemically deposited
at a constant current density of 0.5 mA/cm2 for 2 h in a glass
cell at room temperature, which was strictly controlled by a
potentiostat/galvanostat (HDV-7C); the volume of the electro-
bath was about 100 mL. The electrolyte solution consisted of
+
HTeO2 and Bi3+ from an acidic aqueous solution using
uniformly sized, parallel, and high aspect ratio porous anodic
alumina membrane (PAAM) as templates.7-10
0.075 M HTeO2 and 0.05 M SbO+, and the pH of it is equal
+
Antimony telluride belongs to layered semiconductors with
tetradymite structure. This compound and its doped derivatives
are considered to be the best candidates for near room-
temperature TE applications.11 Commonly, Sb2Te3 is obtained
by chemical precipitation from an aqueous solution Sb3+ with
H2Te12 or from sintering the elements.13 Thin films are also
obtained by means of sputtering,14 metallorganic chemical vapor
deposition (MOCVD),15 flash evaporation,16 and electrodepo-
sition onto an indium tin oxide (ITO) substrate.17 Because the
Sb3+ is easy to hydrolyze and precipitate, it is difficult to obtain
a relatively high concentration of Sb3+, electrochemical deposi-
tion of compounds of Sb3+ is very difficult in aqueous solution,
and the electrodeposition of the Sb2Te3 nanowires is quite
challenging. To the best of our knowledge, the fabrication of
to 1. If the pH value of the electrolyte solution is too high, it
will cause HTeO2+ and SbO+ to hydrolyze and precipitate, and
if it is too low, it will erode the PAAM template. The electrolyte
solution is prepared as follows: to dissolve the SbCl3 and avoid
Sb3+ hydrolysis and precipitation, citric acid and potassium
citrate were added to form an Sb-citric complex in one beaker
+
by being heated, and the HTeO2 came from the dissolved Te
in 5 M HNO3 by being heated in another beaker. Then, two
beakers were mixed together, and distilled water and 5 M HNO3
were added to reach the final volume and pH in the volumetric
flask.
The templates gradually turn black during electrodeposition.
When electrodeposition occurs on the surface of the templates,
the potential suddenly increases because of larger area surface
electrodeposition. After electrodeposition, the black antimony
telluride nanowire arrays/PAAM template samples were ob-
tained, rinsed with distilled water and absolute ethanol, and then
dried in air at room temperature for further analysis.
The morphologies and structure of the Sb2Te3 nanowires were
studied using X-ray diffraction (XRD, MXPAHF) with CuKR
* To whom correspondence should be addressed. Tel: +86-551-3603408;
fax: +86-551-3603408; e-mail: lixg@ustc.edu.cn
† University of Science and Technology of China.
‡ International Center for Materials Physics.
§ Anhui University of Technology.
| University of Texas at Austin.
10.1021/jp046100z CCC: $30.25 © 2005 American Chemical Society
Published on Web 01/08/2005

Large-Area Sb2Te3 Nanowire Arrays
J. Phys. Chem. B, Vol. 109, No. 4, 2005 1431
Figure 3. TEM image of Sb2Te3 nanowires.
Figure 1. XRD pattern of Sb2Te3 nanowires.
Figure 4. TEM and HRTEM images of the same single Sb2Te3
nanowire; the inset is the corresponding ED pattern.
the perfect hexagonal cells of the template, and that the surface
of the nanowires is smooth. The beautiful flower formations,
as shown in Figure 2a,b, probably result from the action of
capillary forces that gather the vertical nanofibers into bundles,
but supercritical drying can be used to avoid the detrimental
action of capillary forces. Figure 2d is a typical FE-SEM surface
image of the sample with an eroding time of 10 min. It is clear
that the exposed length of the Sb2Te3 nanowires increases with
an increase in the eroding time.
A representative TEM image of the Sb2Te3 nanowires is
shown in Figure 3. It is clear that the diameters are uniform
and the length of the nanowires is up to tens of micrometers.
Because the electrodeposition follows a bottom-up mechanism,
namely, the Sb2Te3 nucleates at the bottom of the pores and
grows along the pores to the top, the length of the nanowires is
the same as the thickness of the template used.
HRTEM images provide further insight into the structures
of the Sb2Te3 nanowires. Figure 4 shows a typical HRTEM
image of a single Sb2Te3 nanowire and the corresponding ED
pattern. The diameter of the nanowire is highly uniform and
appears rather homogeneous. The nanowires are free of observ-
able defects, and ED patterns are bright spots, suggesting single
crystallinity of the Sb2Te3 nanowires. The diffraction spots of
the (112h0), (1h21h0), and (2110) correspond to lattice planes of
the single-crystal hexagonal packed structure of Sb2Te3. Cal-
culation results indicate that the incident beam is parrllel to the
[0001] direction. The lattice-resolved image of the representative
nanowire further reveals that the nanowires are structurally
uniform and single crystalline, with an interplanar spacing of
0.212 nm corresponding to the {112h0} distance of hexagonal
Sb2Te3.
Figure 2. SEM images of Sb2Te3 nanowire array after the template is
partially etched. Panels a and b are top views after 5 min of etching.
Panels c and d are side and top views after 10 min of etching,
respectively.
radiation (λ ) 1.5406 Å) in the range of 10° e 2θ e 120°;
field-emission scanning electron microscopy (FE-SEM, JEOL,
JSM-6700F); transmission electron microscopy (TEM, Hitachi-
800); and high-resolution transmission electron microscopy
(HRTEM, JEOL-2010). The chemical composition of the
nanowires was determined by X-ray photoelectron spectroscopy
(XPS, VGESCALAB MK-II).
Results and Discussion
The XRD pattern of the as-prepared sample is shown in
Figure 1. All the peaks can be indexed to the hexagonal Sb2-
Te3 structure with cell constants a ) 0.426 nm and c ) 3.05
nm (JCPDS No. 15-874). The intensity for (112h0) is much
higher than those for the other peaks, indicating that the Sb2-
Te3 nanowires have preferential orientation along the [112h0]
direction, which will be further confirmed by HRTEM results.
Figure 2a-d shows the FE-SEM morphologies of the Sb2-
Te3 nanowires in the PAAM template. Figure 2a,b is different
magnification FE-SEM micrographs of the Sb2Te3 nanowires
after the PAAM template has been etched in 1 M NaOH for 5
min. From Figure 2a, a high nanopore filling efficiency (100%)
and high homogeneity over a large area can be observed. Figure
2b,c is a surface and a cross-sectional FE-SEM image of the
nanowire arrays after the template has been etched for 5 and
10 min, respectively. They clearly display that the Sb2Te3
nanowire arrays are ordered, continuous, dense, and uniform
in diameter and length, that the cylindrical nanowires grow from
The composition of Sb2Te3 is determined by XPS. The XPS
data were collected in the constant analyzer energy mode at 20
eV. MgKR (hν ) 1253.6 eV) radiation was employed as the
excitation source with an anode voltage of 12 kV and an

1432 J. Phys. Chem. B, Vol. 109, No. 4, 2005
Jin et al.
diffusion, it will cause a lack of ions near the Au electrode or
in the nanochannels of the PAAM templates, resulting in
concentration gradient. To avoid the rapid nucleation, growth,
and concentration gradient, we tried to minimize the rate of the
nucleation and growth by reducing the concentrations of
HTeO2+ and SbO+ in our experiment because the slower growth
rate can increase the crystallinity of the nanowires, filling-rate,
and compositional homogeneity.5 Other factors, such as tem-
perature,22 structural integrity of the template,2 pH value, current
density, or deposition potential, are also important. Only at
optimal conditions can large-area and stoichiometric Sb2Te3
nanowire arrays with a high-filling ratio be fabricated.
Figure 5. XPS of the Sb2Te3 nanowires.
emission current of 20 mA. The binding energies of Te4d and
Sb4d are 41.52 and 33.91 eV, respectively, as seen in Figure 5,
which are in good agreement with those of the Sb2Te3 bulk
material (41.4 and 33.78 eV, respectively).20 The quantification
of the peaks gives a ratio of Sb/Te ) 2.1:2.9, and no other peaks
were observed, demonstrating that stoichiometric Sb2Te3 nano-
wires were formed.
Conclusion
In summary, we have used a simple direct current elec-
trodeposition process to fabricate large-area Sb2Te3 nanowire
arrays using PAAM templates. FE-SEM, TEM, and HRTEM
investigation results show that the Sb2Te3 nanowire arrays are
dense, parallel, and large-area, with 100% of the pores of the
PAAM templates filled. We believe that this simple approach
can be generalized to controllably produce a variety of nanowires
of interesting nanotechnological applications.
The electrodeposition process of the Sb2Te3 nanowires mainly
involves the following four steps:
+
(i) HTeO2 and SbO+ diffuse to the Au electrode surface
and are adsorbed on the surface by the electric field force.
(ii) The adsorbed HTeO2+ and SbO+ get electrons to produce
elemental Te and Sb by the reactions
Acknowledgment. This work was supported by the National
Natural Science Foundation of China.
HTeO2+ + 3H+ + 4e ) Te(s) + 2H2O
SbO+ + 2H+ + 3e ) Sb(s) + 3H2O
(1)
(2)
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