Simple synthesis of ultra-long Ag2Te nanowires through solvothermal co-reduction method

Article abstract of DOI:10.1016/j.jssc.2010.07.020

Ultra-long single crystal β-Ag₂Te nanowires with the diameter of about 300 nm were fabricated through a solvothermal route in ethylene glycol (EG) system without any template. The long single crystal wires were curves, with high purity, well-crystallized, and dislocation-free and characterized by using X-ray powder diffraction (XRD), Differential scanning calorimetry (DSC) analysis, X-ray photoelectron spectroscope (XPS), field emission scanning electron microscopy (FE-SEM), transmission electron microscopy (TEM) and high-resolution transmission microscopy (HRTEM). The detailed topotactic transformation process from particles into single crystal wires was studied. Furthermore, the electrical conductivity and Seebeck coefficient have been systematically studied between 300 and 600 K.

Full text of DOI:10.1016/j.jssc.2010.07.020

Journal of Solid State Chemistry 183 (2010) 2382–2388
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Simple synthesis of ultra-long Ag2Te nanowires through solvothermal
co-reduction method
Feng Xiao, Gang Chen n, Qun Wang, Lin Wang, Jian Pei, Nan Zhou
Department of Applied Chemistry, Harbin Institute of Technology, Harbin 150001, People’s Republic of China
a r t i c l e i n f o
a b s t r a c t
Article history:
Ultra-long single crystal
b-Ag2Te nanowires with the diameter of about 300 nm were fabricated
Received 11 March 2010
Received in revised form
27 May 2010
through a solvothermal route in ethylene glycol (EG) system without any template. The long single
crystal wires were curves, with high purity, well-crystallized, and dislocation-free and characterized by
using X-ray powder diffraction (XRD), Differential scanning calorimetry (DSC) analysis, X-ray
photoelectron spectroscope (XPS), field emission scanning electron microscopy (FE-SEM), transmission
electron microscopy (TEM) and high-resolution transmission microscopy (HRTEM). The detailed
topotactic transformation process from particles into single crystal wires was studied. Furthermore, the
electrical conductivity and Seebeck coefficient have been systematically studied between 300 and
600 K.
Accepted 12 July 2010
Available online 17 July 2010
Keywords:
Silver telluride
Solvothermal
Nanowires
Crown Copyright & 2010 Published by Elsevier Inc. All rights reserved.
Electrical transport property
1. Introduction
electrical, optical, and magnetic properties different from those of
corresponding polycrystalline powders. A number of methods for
the synthesis of Ag2Te with nanostructures have been explored. For
example, Ag2Te nanowires were obtained by cathodic electrolysis
from dimethyl sulfoxide (DMSO) solution containing AgNO3 and
TeCl4 in porous anodic alumina membranes [20]. Ag2Te nanorods
were prepared by the reaction between AgCl and elemental Te in
the mixed solvent of ethylenediamine and hydrazine hydrate [23].
A room-temperature solution-phase route for the preparation of
one-dimensional silver telluride nanowires (Ag2Te NWs) has been
reported [24]. Silver telluride nanotubes have been prepared by the
solvothermal process without a template or a surfactant [25].
Template-engaged method in which the Te nanorods were used as
template reagents has been developed for the synthesis of the
Ag2Te nanorods [26]. However, most objects of these syntheses
have to use templates which served as a physical scaffold and hard
Metal tellurides have drawn considerable attention because of
their unique physical properties and potential applications in
fabricating sensors for detection of low energy gamma-ray
detection [1], low-dimensional phase-change nanomaterials for
information storage [2,3], thermoelectric materials [4,5], field
sensing devices, etc. [6]. Among these tellurides, silver tellurides
are a series of compounds with variable stoichiometries, and the
four typical stable silver telluride phases are AgTe [7,8], Ag2Te [9],
Ag5Te3 [10], and Ag7Te4 [11]. These silver telluride materials may
find applications in manufacture of magnetic field measurements
[12–15], memory devices [16], and optics materials [17]. Recently,
silver telluride (Ag2Te) has received great attentions due to its
interesting and useful characteristics [18]. The low-temperature
phase of monoclinic silver telluride is a kind of semiconductor with
a narrow band gap, high carrier mobility, and low lattice thermal
conductivity, whereas its high-temperature phase gives rise to
superionic conductivity because Ag+ cations can easily move
through the cubic sublattice formed with tellurium anions [19,20].
Generally, Ag2Te bulk material was prepared by the solid
reaction between elemental Ag and Te at elevated temperature in
evacuated tubes [21], or by the aqueous reaction of metal–salt
solutions with toxic gaseous H2Te [22]. Recently, low-dimensional
nanostructures, including nanowires, nanorods, and nanosheets,
are of importance because they exhibit promising mechanical,
to obtain long wires (420 mm in length). Herein, we have
developed a general strategy to grow large scale, ultra-long Ag2Te
nanowires through solvothermal co-reduction method without any
template. Moreover, the formation mechanism of the nanostruc-
tured silver telluride has been elucidated on the basis of the
experimental observations.
2. Experimental section
2.1. Synthesis
n Corresponding author. Fax: +86 451 86413753.
Ultra-long Ag2Te nanowires were prepared from AgNO3
(99.99%) and Na2TeO3 (99.98%) by a solvothermal co-reduction
E-mail address: gchen@hit.edu.cn (G. Chen).
0022-4596/$ - see front matter Crown Copyright & 2010 Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.jssc.2010.07.020

F. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 2382–2388
2383
method. In
a
typical experiment, the aqueous solutions of
thermal source to produce the temperature gradient; the
temperature and voltage signals were collected by a commercial
data acquisition system (Keithley 2700, Keithley Instruments Inc.,
America).
0.2208 g AgNO3 (1.3 mmol) and 0.2215 g Na2TeO3 (1 mmol) were
prepared by adding 5 mL deionized water. 1 g (2.68 mmol) dis-
odium salt of ethylenediaminetetraacetic acid (Na2(EDTA) ꢀ 2H2O)
was added into the solution of AgNO3. After a vigorous stirring of
5 min, they were transferred into Teflon lined stainless steel
autoclave of capacity 40 mL in sequence. Then 10 mL 1 M NaOH
and 5 mL ethylene glycol (EG) were added immediately, which
result in a few brownish-black spots floating on the surface of the
mixed solution and the white milky liquid could be obtained by
vigorous stirring. The final volume of the solution mixture was
adjusted to 30 mL with deionized water and the autoclave was
heated at 513 K. After 24 h, the vessel was taken out and allowed
to cool to room temperature naturally, and the black cloudy
precipitate found inside the autoclave was separated by centri-
fugation, washed with deionized water and ethanol several times,
3. Results and discussion
3.1. Structure, formation, and morphology
The crystallinity of as-prepared samples was analyzed using X-
ray diffraction (XRD) as shown in Fig. 1. It reveals that the product
is composed of monoclinic Ag2Te, which is consistent with those
of bulk monoclinic Ag2Te [space group: P2n/c (no. 13)] (JCPDS 34-
˚
˚
0142). The calculated lattice constants a¼8.1586 A, b¼8.9339 A,
˚
and c¼8.0100 A were in agreement with the standard literature.
and dried in
a vacuum at 323 K for 10 h. Moreover, the
No other diffraction peaks were detected such as Ag or Te
crystalline phases. From the results of XRD, it can be concluded
that high purity of Ag2Te was obtained by the present synthetic
method. In addition, Brownish-black Ag2O which resulted from
unstable intermediate product AgOH can be obtained in alkaline
solution [27]. On the other hand, there was a strong complexing
action between silver (I) ions and EDTA. So the black spots may be
Ag2O phase and the brownish-black precipitate disappeared
because of the strong complexing action between silver (I) ions
and EDTA.
To understand the chemical situation of elements in the
product, the XPS test was performed, and the results are shown in
Fig. 2. It can be found that a little oxides and carbon are present on
the surface of the product. The oxides may result from the surface
oxidation reaction of the Ag2Te nanowires with O2 [26]. The
binding energies obtained from the XPS spectrum are 367.7 and
experimental conditions with different reaction time were also
investigated in order to understand the growth mechanism of the
ultra-long single crystal Ag2Te wires. Table 1 summarizes these
typical conditions and the corresponding morphologies and
phases evolution of the products.
2.2. Characterization
X-ray diffraction data were collected at ambient temperature
from 51 to 901 with
diffractometer working with CuK
a
step of 0.021 on Rigaku D/max
radiation. Differential scanning
a
calorimetry (DSC) analysis was carried out up to a temperature of
500 K, using a SETARAM DSC-141 under a stream of nitrogen
and a heating rate of 10 K/min. The XPS spectra were collected on
an American Electronics physical PHI5700ESCA system, X-ray
photoelectron spectroscope using AlKa radiation (1486.6 eV). The
1000
800
600
400
200
0
source was operated at 12.5 kV and the anode power was 250 W.
The binding energy (BE) was calibrated with the C 1s peak. The
morphology of particles was studied by field emission scanning
electron microscopy (FE-SEM, FEI QUANTA 200F). Transmission
electron micrographs were obtained by employing an FEI Tecnai
G2 S-Twin transmission electron microscope, using an accelerat-
ing voltage of 300 kV. The powder specimens were prepared in
air. Rectangular shape specimens of about 15.7 ꢁ 4.6 ꢁ 0.7 mm3
were obtained under a pressure of 15 MPa. The measurements of
the electrical conductivity and Seebeck coefficient were carried
out using
a laboratory-designed apparatus under an argon
atmosphere. The electrical conductivity of the samples were
measured by four probe method with the increase in temperature
of about 2.5 1C/min. For the measurement of Seebeck coefficient,
thermo-electromotive force measured as a function of tempera-
ture gives a straight line and its slope is Seebeck coefficient. The
detailed procedure for the measurement of Seebeck coefficient is
described below. The sample with a pair of Pt/Rh thermocouples
attached to two ends (bar-shaped sample) was heated to a certain
temperature, and then one of the ends was heated by an extra
20
25
30
35
40
45
50
55
60
2ꢀ (degree)
Fig. 1. The XRD pattern for Ag2Te sample prepared at 513 K for the mol ratio of Ag/
Te¼1.30.
Table 1
Summary of the synthesis of typical products under various experimental parameters.
Sample
T (K)
t (h)
Phase
Morphology
Sample1
Sample2
Sample3
Sample4
Sample5
Sample6
Sample7
513
513
513
513
513
513
513
2
6
Ag and Ag2Te
Straight nanotubes and particles
Straight nanotubes and particles
Straight nanotubes, bent wires, and particles
Straight nanotubes, bent wires, and particles
Straight nanotubes, bent wires, and particles
Bent wires and particles
Ag, Te, and Ag2Te
Ag2Te, Ag, and Te
Ag2Te, Te, and Ag
Ag2Te, Te, and Ag
Main Ag2Te
8
12
16
20
24
Bent wires
Pure b-Ag2Te

2384
F. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 2382–2388
10000
12000
10000
8000
6000
4000
2000
0
Te3d
8000
Ag3d
O1s
Ag3d5/2
6000
Ag3d3/2
TeMNN
AgMNN
4000
2000
0
C1s
Te4d
0
200
400
600
800 1000 1200 1400
365
370
375
380
385
Binding Energy (eV)
Binding Energy (eV)
14000
12000
Te3d5/2
10000
8000
6000
4000
2000
0
Te3d3/2
oxides
oxides
570
575
580
585
590
595
Binding Energy (eV)
Fig. 2. XPS spectrum of the as-prepared pure Ag2Te sample: (a) the survey spectrum, (b) narrow spectrum for Ag 3d, and (c) narrow spectrum for Te 3d.
573.3 eV for Ag 3d5/2 and Te 3d5/2 in Fig. 2a, respectively, which
means that the product is composed of Ag (I+) and Te (IIꢂ). The
inset in Fig. 3d) further substantiates that the Ag2Te nanowire is a
monoclinic phase.
peaks at 367.7 and 373.7 eV correspond to Ag 3d5/2 and Ag 3d3/2
,
A series of experiments were carried out to study the influence
of the ratio of AgNO3 to Na2TeO3 and Na2(EDTA) ꢀ 2H2O as well as
NaOH on the synthesis of products. It can be seen in Fig. 4a–d that
the phases in the samples prepared at 513 K for different ratios of
AgNO3 to Na2TeO3 are composed of monoclinic phase Ag2Te
(JCPDS: 34-0142) and Ag (JCPDS: 04-0783) or Te (JCPDS:
65-3370). As the ratio of AgNO3 to Na2TeO3 decreases, the
amount of the Ag phase decreases gradually, then Te phase
appears (Fig. 4d). Here another point to be noted is that pure
Ag2Te shown in Fig. 1 is for the ratio 1.30. It means that the ratio
of 1.30 for AgNO3 to Na2TeO3 is required for the preparation of
pure long wire Ag2Te under solvothermal synthesis at 513 K. In
addition, Na2(EDTA) ꢀ 2H2O and NaOH played a key role in this
solvothermal co-reduction process. It can be seen in Fig. 4e and f
that the phases Ag7Te4 (JCPDS: 18-1187) and Ag (JCPDS: 04-0783)
appeared in the samples prepared at 513 K and Ag/Te¼1.30 for
24 h in the absence of NaOH and Na2(EDTA) ꢀ 2H2O, respectively.
First of all, the pH value played an important role during this
reaction, which influenced on the purity of samples. Ag7Te4 phase
would appear without NaOH added, which implied that NaOH
respectively (Fig. 4b), and those at 573.3 and 583.5 eV correspond
to Te 3d5/2 and Te 3d3/2, respectively (Fig. 4c). In addition, two
small peaks are observed at 577.1 and 587.5 eV, which can be
attributed to Te(IV) oxide. According to the quantification of XPS
peaks, the molar ratio of Ag to Te is 2.05:1.00.
Morphological features of the samples were investigated by
FE-SEM as shown in Fig. 3. The low-magnification FE-SEM image
(Fig. 3a) reveals that the typical products consist of a large
quantity of wire-like structures with the lengths in the range of
several tens to several hundred micrometers. The high-
magnification FE-SEM image (Fig. 3b) shows that the diameter
of wire-like structures is 300ꢂ500 nm. To the best of our
knowledge, the synthesis of ultra-long silver telluride nanowires
have never previously been reported.
In order to further understand the morphology and micro-
structure of the wire-like Ag2Te, a detailed investigation was
performed using high-resolution TEM (HRTEM). Fig. 3c shows the
low-magnification TEM image of a single ultra-long Ag2Te wire
with diameters between 200 and 400 nm, lengths ranging from 8
to 10
mm. In addition, from the TEM iamge, we found that the
could influence the reaction greatly. A branch reaction for
ultra-long silver telluride wires can be destroyed by ultrasound
during the TEM sample preparation. Fig. 3d shows a typical
HRTEM image of a single nanowire and reveals the single-
crystalline nature of the nanowires, and free of dislocation. The
interplanar spacing of 0.372 and 0.685 nm are consistent with the
(1 1 0) and (1 0 0) plane of Ag2Te. Moreover, the FFT pattern (left
elemental Te would occur in an alkaline environment to form
both positive ions (Te occurs as +4 valence in TeO23ꢂ) and negative
ions (Te2ꢂ) simultaneously [28]. The disproportionation of a little
Te in alkaline environment may be essential for the purity of
samples. There would be a little surplus Te to produce Ag7Te4
phase without the disproportionation. On the other hand, EAg+/Ag

F. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 2382–2388
2385
Fig. 3. SEM images of the as-prepared Ag2Te samples synthesized at 513 K for 24 h with 1.30 (a) and (b) for the ratio of AgNO3 to Na2TeO3, (c) TEM image of the Ag2Te wire.
(d) HRTEM image recorded on the edge of this wire. Inset: FFT image of the HRTEM image (left).
Ag2Te nanowires. Na2(EDTA) ꢀ 2H2O could not only stabilize Ag+
Ag7Te4
Ag
Te
(f)
at the beginning stage, but also release Ag+ gradually when main
reaction consumes Ag phase. In aqueous solution, the strong
complexing action between silver (I) ions and Na2(EDTA) ꢀ 2H2O
resulted in the formation of Ag-EDTA complexes. The formation of
the complexes could reduce the concentration of free Ag+ in the
solution, and slow the reaction rate, which is favorable for the
growth of Ag2Te nanowires. While if no Na2(EDTA) ꢀ 2H2O was
added, Ag+ would be quickly reduced by EG at 240 1C, which
resulted in Ag impurity in samples [30].
(e)
(d)
(c)
It is known that silver telluride exhibits a structural phase
transition from low-temperature monoclinic phase (
a-Ag2Te) to
cubic phase ( -Ag2Te) [18]. DTA, TG, and DSC analyses have been
b
(b)
(a)
carried out to determine transformation temperature of our
sample and to apply to subsequent test of electrical conductivity
and Seebeck coefficient, and the results are shown in Fig. 5a. A
distinct endothermic peak can be seen at 421 K in Fig. 5a, which
20
25
30
35
40
45
50
55
60
65
70
corresponds to the phase transformation temperature (
a-Ag2Te-
2ꢀ (degree)
b-Ag2Te) [18]. According to the DSC plot in Fig. 5b, the latent heat
of the phase transition is 29.34 J gꢂ1. This value is much different
from that reported by Li (3.93 J gꢂ1) [31] but close to the
calculated results of the thin film (25.75 J gꢂ1) and the bulk
(23.12 J gꢂ1) of Ag2Te [32,33].
Fig. 4. The XRD patterns for Ag2Te samples for (a) Ag/Te¼1.70, (b) Ag/Te¼1.58,
(c) Ag/Te¼1.44, (d) Ag/Te¼1.00, and Ag/Te¼1.30 for (e) no NaOH and (f) no
Na2(EDTA) ꢀ 2H2O.
was so high [29] to produce Ag phase easily in a short time that it
could not react with Te phase completely which was produced
relatively slowly. Consequently, pH value could adjust EAg+/Ag to a
proper value in order to obtain a pure phase. Similarly, Batabyal
and Vittal [16] has approved that the pH value influenced the
Ag+ reduction. Then, the role of Na2(EDTA) ꢀ 2H2O was also
investigated. Sample S2 which contained both Ag phase and
Ag2Te phase was obtained when no Na2(EDTA) ꢀ 2H2O was added.
Na2(EDTA) ꢀ 2H2O played a critical role in the formation of the
3.2. Growth mechanism of Ag2Te nanowires
To understand the growth mechanism of Ag2Te nanowires
prepared in an solvothermal system, the silver tellurium nanos-
tructures at various stages of the growth process were character-
ized using FE-SEM. Fig. 6 shows the images of the samples that
were taken from the reaction mixture after an aging time of (a)
2 h, (b) 4 h, (c) 6 h, (d) 8 h, (e) 12 h, (f) 16 h, (g) 18 h, (h) 20 h, and

2386
F. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 2382–2388
tubes, which is similar with the report of Purkayastha et al. [36]
0
-2
for the preparation of single-crystal lead telluride nanorods. It
should be mentioned that there were some bent Ag2Te nanowires
as indicated in Fig. 6b–e because of the mechanical stress resulted
from the change of atom radius and band length in the conversion
of Te to Ag2Te [26,37]. In addition, Na2(EDTA) ꢀ 2H2O could not
only stabilize Ag+ at the beginning stage, but also releases Ag+
gradually when main reaction consume Ag phase, which was a
very important factor to obtain pure samples.
6.0
5.8
5.6
5.4
5.2
5.0
4.8
4.6
4.4
4.2
4.0
DTA
-4
-6
Finally, Ostwald Ripening of Ag2Te nanowires: As soon as
Ag2Te crystallites existed, there was a balance between Ag2Te
nanowires and smaller Ag2Te crystallites. As indicated in Fig. 6f–i,
Ag2Te wires grown at the expense of the smaller ones [38] and it
can be seen there is only wire-like morphology for the sample
prepared for 24 h. This mechanism is totally different from the
work of Qin et al. [25] and Zuo et al. [26] for the preparation of
silver telluride. Furthermore, XRD and TEM results have proved
that Ag2Te phase existed in samples prepared for 8 and 16 h.
Experimental studies with X-ray diffraction for the samples
that were taken from the reaction mixture after an aging time of
(a) 4 h, (b) 8 h (c) 16 h, and (d) 24 h at 513 K have also provided
evidence to support this proposed mechanism. In this process, the
ethylene glycol could serve as both solvent and reducing reagent
[34]. The XRD investigation shown in Fig. 7 indicates that all of the
sample prepared in the present work can be indexed to the
monoclinic Ag2Te phase. Almost only Te phase and Ag phase could
be obtained by reaction for 4 h, which can be seen in Fig. 7a. In
particular, the intensity of Te crystallographic phase in the
products is indeed increased gradually with the reaction time
from 8 to 16 h in Fig. 7b and c which indicates that preferential
orientation of Te nanotubes occurred [34]. It is easy to understand
that the relative amount of Ag2Te, Ag, and Te phase would change
gradually which depended on the reaction time. Finally, pure and
ultra-long Ag2Te wires could be obtained as reaction time went to
24 h in Fig. 7d. On the basis of the experimental results, the whole
process for the formation of Ag2Te wires can be schematically
illustrated in Fig. 8. In the early stage, the intermediate products
indicate the coexistence of Ag particles and Te particles. As the
reaction continued, it can be noted that the particles gradually
disappear with the dramatical increase in the production of Te
nanotubes. For the transformation process from Te nanotubes to
Ag2Te wires during 8–24 h, we can understand this interesting
phenomenon by topotactic transformation.
-8
TG
-10
320 340 360 380 400 420 440 460 480
Temperature (K)
-20
-22
-24
-26
-28
-30
-32
-34
-36
340 360 380 400 420 440 460 480
Temperature (K)
Fig. 5. (a) DTA-TG analysis and (b) DSC analysis of the Ag2Te samples heated with
10 K/min up to 493 K.
(i) 24 h at 513 K. These images clearly show the evolution of silver
tellurium nanostructures from particles into ultra-long wire-like
morphology over time at 513 K. For the transformation of the
nanowires, topotactic transformation mechanisms can be
considered. Based on the experimental results, the formation
process of ultra-long Ag2Te wires is proposed as follows:
3.3. The electrical conductivity and Seebeck coefficient of Ag2Te
nanowires
First of all, Ag, Te, Ag2Te nucleation: Fig. 6a shows that the
samples were comprised of numerous smaller crystallites of
about 100–500 nm and we can believe that in this step, Ag phase,
highly anisotropic Te phase and a little amount of Ag2Te phase
proceeded through nucleation and coexisted. To the best of our
knowledge, Ag core and Te core could be easily nucleated and can
be grown in EG solvents at appropriate temperature [34]. In
addition, Na2(EDTA) ꢀ 2H2O and NaOH could reduce the concen-
tration of free Ag+ and EAg+/Ag effectively, which played a crisis
role in this reduction condition.
Then, Te nanotubes and Ag2Te wires Growth: In this step,
highly anisotropic Te nanotubes can be easily and directly grown
along c-axis from Te seeds without the use of any physical
templates [34,35]. But how Te tubes transform to Ag2Te wires?
Consulting the literature, two possible mechanisms can be
considered. The one is that active Ag atom in supersaturated
solution adsorbed on the surface of Te nanotubes inner and outer,
and then rearranged, reaction. Similarly, the copper and silver
chalcogenides have been prepared by this topotactic transforma-
tion mechanism [32–35]. Another one is the split of Ag2Te tubes
and half-tubes formed by inward diffusion of Ag on surface of Te
The thermoelectric properties of the bulk and film silver
telluride have been studied by many researchers. From these
studies, they have observed abrupt changes of the Hall coefficient,
the Seebeck coefficient, and the electrical conductivity at 140 1C, a
consequence of the structural phase transition. Fujikane et al.
[39,40] have studied the thermoelectric properties of silver
telluride in bulk form over a wide temperature range from 4 to
450 K. Around 150 1C, Das et al. [41] have observed the maximum
Seebeck coefficient of the thin film to be ꢂ110
thickness of 58 nm. Gnanadurai et al. [32] have also found the
maximum Seebeck coefficient of the thin film to be 130
V Kꢂ1 in
m
V Kꢂ1 in the
m
the optimal thickness of 41 nm during the cooling process.
The temperature-dependent Seebeck coefficient of as-prepared
wire-like Ag2Te (Fig. 9a) also demonstrates the structural phase
transition. The two standard error bars stands for the maximum
and minimum values include the uncertainties in measuring
electrical conductivity as well as Seebeck coefficient. The
maximum value observed for the Seebeck coefficient (S)

F. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 2382–2388
2387
Fig. 6. FE-SEM images of the Ag2Te samples hydrothermal treated at 513 K for (a) 2 h, (b) 4 h, (c) 6 h, (d) 8 h, (e) 12 h, (f) 16 h, (g) 18 h, (h) 20 h, and (i) 24 h.
temperature range of 300–673 K. The electrical conductivity
increases with increasing temperature below 373 K, which is
consistent with the property of semiconductor’s electrical
24h
16h
8h
Ag
Te
conductivity. From 373 to 423 K, the
s(T) curve greatly
4h
(d)
(c)
decreases with increase in temperature, which indicated the
structure of Ag2Te changes greatly as results of phase transition of
b-Ag2Te to a-Ag2Te. Then s(T) curve slowly increases with
increase in temperature above 423 K. Because the electronic
conductivity is strongly dependent on Seebeck coefficient (S) [42],
it is easy to explain the abrupt decrease of the value of the
electrical conductivity in the temperature range between
about 373 and 423 K. The maximum value observed for the
(b)
(a)
electrical conductivity
s
was 30 S cmꢂ1, which was much higher
than reported value (0.7 S cmꢂ1) for wire-like Ag2Te [18]. This is
possibly due to interconnection in the NW network and the length
(420 mm) of the single crystal NWs are higher than the mean free
path of electron in Ag2Te far away [43,44]. The interconnection in
the NW network can reduce the interface electron scattering
while ultra-long single crystal nanowires morphology can provide
25
30
35
2ꢀ (degree)
40
a
good 1-D transporting channels. Further investigation is
Fig. 7. The XRD patterns of the Ag2Te samples treated hydrothermally for (a) 4 h,
currently undergoing to verify this.
(b) 8 h, (c) 16 h, and (d) 24 h.
m
V Kꢂ1, which was lower than the reported value
4. Conclusions
was ꢂ70
(ꢂ160
m
V Kꢂ1) [18]. It can be observed distinctly that the Seebeck
coefficient greatly decreases at 426 K, implying that sample’s
structure and the carrier concentration changes greatly. Fig. 9b
shows the behavior of the electrical conductivity in the
In summary, ultra-long single crystal b-Ag2Te nanowires have
been successfully synthesized by one-pot solvothermal co-reduc-
tion method at moderate temperatures and ratio of AgNO3 to

2388
F. Xiao et al. / Journal of Solid State Chemistry 183 (2010) 2382–2388
Fig. 8. Schematic representation of the formation mechanism of Ag2Te wires. The diagram displays the variation of the phases and morphologies of products with different
reaction time.
50
45
40
35
30
25
20
15
10
[4] W.J. Xie, X.F. Tang, Y.G. Yan, Q.J. Zhang, T.M. Tritt, J. Appl. Phys. 105 (2009)
113713.
[5] J.P. Heremans, Acta Phys. Pol. A 108 (2005) 609.
[6] M. Karlowatz, M. Kraft, B. Mizaikoff, Anal. Chem. 76 (2004) 2643.
[7] V. Venugopal, S.G. Kulkarni, A.A. Banerjee, G.A.R. Rao, K.N. Roy, D.D. Sood,
J. Nucl. Mater. 238 (1996) 218.
Max
-20
-30
-40
-50
-60
-70
-80
-90
ꢂ
S
Min
Max
[8] L. Bindi, J. Alloys Compd. 473 (2009) 262.
[9] Y. Izumi, S. Miyatani, J. Phys. Soc. Jpn. 35 (1973) 312.
[10] M. Gobec, W. Sitte, J. Alloys Compd. 220 (1995) 152.
[11] C. Jia, B. Zhang, W.F. Liu, C.G. Jin, L.Z. Yao, W.L. Cai, X.G. Li, J. Cryst. Growth 285
(2005) 527.
[12] Y. Sun, M.B. Salamon, M. Lee, T.F. Rosenbaum, Appl. Phys. Lett. 82 (2003)
1440.
[13] H.S. Schnyders, M.L. Saboungi, T.F. Rosenbaum, Appl. Phys. Lett. 76 (2000)
1710.
[14] A. Husmann, J.B. Betts, G.S. Boebinger, A. Migliori, T.F. Rosenbaum, M.L.
Saboungi, Nature 417 (2002) 421.
[15] M. Lee, T.F. Rosenbaum, M.-L. Saboungi, H.S. Schnyders, Phys. Rev. Lett. 88
(2002) 066602.
[16] S.K. Batabyal, J.J. Vittal, Chem. Mater. 20 (2008) 5845.
[17] V.B. Prabhune, V.J. Fulari, Opt. Commun. 282 (2009) 2118.
[18] F.Y. Li, C.G. Hu, Y.F. Xiong, B.Y. Wan, W. Yan, M.C. Zhang, J. Phys. Chem. C 112
(2008) 16130.
Min
300
400
500
600
700
800
Temperature (K)
[19] R. Dalven, R. Gill, J. Appl. Phys. 38 (1967) 753.
[20] R.Z. Chen, D.S. Xu, G.L. Guo, L.L. Gui, J. Mater. Chem. 12 (2002) 2435.
[21] R.J. Coustal, J. Chem. Phys. 38 (1958) 277.
Fig. 9. (a) Temperature dependence of the Seebeck coefficient (b) Electrical
conductivity for pure Ag2Te samples. The two standard error bars include the
uncertainties in measuring Seebeck coefficient and electrical conductivity.
[22] C.J. Warren, R.C. Haushalter, A.B. Bocarsly, J. Alloys Compd. 229 (1995) 175.
[23] Y. Jiang, Y. Wu, Z.P. Yang, Y. Xie, Y.T. Qian, J. Cryst. Growth 224 (2001) 1.
[24] A.K. Samal, T. Pradeep, J. Phys. Chem. C 113 (2009) 13539.
[25] A.M. Qin, Y.P. Fang, P.F. Tao, J.Y. Zhang, C.Y. Su, Inorg. Chem. 46 (2007) 7403.
[26] P.F. Zuo, S.Y. Zhang, B.K. Jin, Y.P. Tian, J.X. Yang, J. Phys. Chem. C 112 (2008)
14825.
[27] X. Wang, H.F. Wu, Q. Kuang, R.B. Huang, Z.X. Xie, L.S. Zheng, Langmuir 26
(2009) 2774.
[28] X. Ji, B. Zhang, T.M. Tritt, J.W. Kolis, A. Kumbhar, J. Electron. Mater. 36 (2007)
721.
[29] T. Gunter, A. Koberl, Electrochim. Acta 52 (2007) 2716.
[30] J.M. McLellan, A. Siekkinen, J.Y. Chen, Y.N. Xia, Chem. Phys. Lett. 427 (2006)
122.
[31] R. Venkatasubramanian, E. Siivola, T. Colpitts, B. O’Quinn, Nature 413 (2001)
597.
Na2TeO3. The phenomenon of phase transitions of the as-prepared
long wire-like Ag2Te from monoclinic structure (
b-Ag2Te) to
face-centered cubic structure -Ag2Te) around 421 K are
(
a
obviously observed. In our experiment, reaction time and mol
ratio of AgNO3 to Na2TeO3 were found to have more influence on
the morphology of the products. Based on our experimental
results, anisotropic Te nanotubes grow and transform the
nanotubes into single-crystal
Ag salt.
b-Ag2Te wires via reaction with an
[32] P. Gnanadurai, N. Soundararajan, C.E. Sooriamoorthy, Vacuum 67 (2002) 275.
[33] S. Aliev, F.F. Aliev, Z.S. Gasanov, Phys. Solid State 40 (1998) 1540.
[34] B. Mayers, Y.N. Xia, Adv. Mater. 14 (2002) 279.
[35] G.C. Xi, Y.K. Liu, X.Q. Wang, X.Y. Liu, Y.Y. Peng, Y.T. Qian, Cryst. Growth Des. 6
(2006) 2567.
[36] A. Purkayastha, Q.Y. Yan, D.D. Gandhi, H.F. Li, G. Pattanaik, T. Borca-Tasciuc,
N. Ravishankar, G. Ramanath, Chem. Mater. 20 (2008) 4791.
[37] G.D. Moon, S.W. Ko, Y.N. Xia, U. Jeong, ACS Nano 4 (2010) 2307.
[38] W.Z. Ostwald, Phys. Chem. 34 (1900) 495.
Acknowledgment
This work was supported by the National Natural Science
Foundation of China (Project no. 20871036) and the Development
Program for Outstanding Young Teachers in Harbin Institute of
Technology (HITQNJS.2009.001).
[39] M. Fujikane, K. Kurosaki, H. Muta, S. Yamanaka, J. Alloys Compd. 387 (2005)
297.
[40] M. Fujikane, K. Kurosaki, H. Muta, S. Yamanaka, J. Alloys Compd. 393 (2005)
299.
References
[41] V.D. Das, D. Karunakaran, Phys. Rev. B 30 (1984) 2036.
[42] K. Park, K.U. Jang, H.C. Kwon, J.G. Kim, W.S. Cho, J. Alloys Compd. 419 (2006)
213.
[1] T. Gandhi, K.S. Raja, I. Chatterjee, M. Misra, X. Luo, P. Dzurella, Int. J.
Nanotechnol. 5 (2008) 519.
[2] X.H. Sun, B. Yu, G. Ng, M. Meyyappan, J. Phys. Chem. C 111 (2007) 2421.
[3] S. Meister, D.T. Schoen, M.A. Topinka, A.M. Minor, Y. Cui, Nano Lett. 8 (2008)
4562.
[43] Q.Y. Yan, H. Chen, W.W. Zhou, H.H. Hng, F.Y.C. Boey, J. Ma, Chem. Mater. 20
(2008) 6298.
[44] Y.P. Song, A.L. Schmitt, S. Jin, Nano Lett. 7 (2007) 965.