Morphological evolution, growth mechanism, and magneto-transport properties of silver telluride one-dimensional nanostructures
Nanoscale Research Letters
Morphological evolution, growth mechanism, and magneto-transport properties of silver telluride one-dimensional nanostructures
GaoMin Li 0
XiaoBing Tang 0
ShaoMin Zhou 0
Ning Li 0
XianYou Yuan 1
0 Key Laboratory for Special Functional Materials of Ministry of Education, Henan University , Kaifeng 475004 , People's Republic of China
1 Department of Biology and Chemistry, Hunan University of Science and Engineering , Yongzhou, Hunan 425100 , People's Republic of China
Single crystalline one-dimensional (1D) nanostructures of silver telluride (Ag2Te) with well-controlled shapes and sizes were synthesized via the hydrothermal reduction of sodium tellurite (Na2TeO3) in a mixed solution. The morphological evolution of various 1D nanostructures was mainly determined by properly controlling the nucleation and growth process of Ag2Te in different reaction times. Based on the transmission electron microscopy and scanning electron microscopy studies, the formation mechanism for these 1D nanostructures was rationally interpreted. In addition, the current-voltage (I-V) characteristics as a function of magnetic field of the highly single crystal Ag2Te nanowires were systematically measured. From the investigation of I-V characteristics, we have observed a rapid change of the current in low magnetic field, which can be used as the magnetic field sensor. The magneto-resistance behavior of the Ag2Te nanowires with monoclinic structure was also investigated. Comparing to the bulk and thin film materials, we found that there is generally a larger change in R (T) as the sample size is reduced, which indicates that the size of the sample has a certain impact on magneto-transport properties. Simultaneously, some possible reasons resulting in the observed large positive magneto-resistance behavior are discussed.
Silver telluride; One-dimensional nanostructures; Morphological evolution; Growth mechanism; Magneto-transport properties
During the past few decades, a shape-controlled synthesis
of semiconducting crystals with well-defined morphologies,
such as belts, wires, rods, tubes, spheres, sheets, combs,
and cubes, has attracted considerable attention due to their
novel properties and applications in many fields [
Among these nanostructures, one-dimensional (1D)
nanostructures have increasingly become the subject of
intensive research due to their potential applications in a variety
of novel devices [
]. The most prominent example is
certainly the carbon nanotubes [
]. Not only that,
considerable efforts have been spent on the synthesis of
nanobelts, nanowires (NWs), and other 1D nanostructures.
Especially, with the miniaturization of devices in the future,
searching for interconnects remains a challenge to future
nanoelectronics. Therefore, it is essential to investigate 1D
nanomaterials which can be applied in the nanoscale field.
As one typical example of the silver chalcogenides,
Ag2Te has attracted increasing attention due to its much
more technological prospects [
]. As reported,
Ag2Te can transfer its structural phase from the
lowtemperature monoclinic structure (β-Ag2Te) to the
hightemperature face-centered cubic structure (α-Ag2Te) at
about 145°C [
]. Low-temperature β-Ag2Te is a
narrow band gap semiconductor with high electron
mobility and low lattice thermal conductivity , which is
desirable for its high figure of merit for thermoelectric
applications. In the α-Ag2Te phase, silver cations can
move freely, which enhance the conductivity, leading to
superionic conductivity [
]. More recently, it has been
reported that Ag2Te is a new topological insulator with an
anisotropic single Dirac cone due to a distorted
antifluorite structure [
], leading to new applications in
nanoelectronics and spintronics. It is also known that a
huge large positive magneto-resistance (MR) has been
observed in the case of silver telluride bulk samples [
thin films [
]. However, to the best of our knowledge, the
MR behavior of Ag2Te nanostructured materials is rarely
reported. Here, we systematically investigate the current–
voltage (I-V) characteristics under different magnetic fields
and the extraordinary MR behavior of Ag2Te nanowires.
The magneto-resistance can be strongly affected by the
details of the Fermi surface geometry and character of
electron–electron (e-e) interactions [
] and therefore
gives valuable insight into the physics dominating the
conductivity. Furthermore, Ag2Te with nontrivial MR can
provide great opportunities in magnetic sensor and
It was reported that Ag2Te tended to form 1D
nanostructures. For instance, the rod-like structure of Ag2Te
was synthesized by the method based on the
templateengaged synthesis in which the Te nanorods were used as
template reagents [
]. Ag2Te nanotubes have been
synthesized hydrothermally when sodium tellurite (Na2TeO3)
and silver nitrate (AgNO3) in hydrazine/ammonia mixture
were autoclaved at 393 K [
]. Ag2Te NWs were obtained
by cathodic electrolysis in dimethyl sulfoxide solutions
containing AgNO3 and TeCl4 using porous anodic alumina
membrane as the template [
]. Recently, Ag2Te NWs
were synthesized by a composite hydroxide-mediated
method, where AgNO3 and Te powder were heated at
498 K in a Teflon vessel containing ethylenediamine and
hydrazine hydrate [
]. Samal and Pradeep [
developed a room-temperature solution-phase route for the
preparation of 1D Ag2Te NWs. In addition, our research
group has more recently reported the synthesis and
electrical properties of individual Ag2Te NWs via a
hydrothermal process [
]. Herein, on this basis, we demonstrate a
simple hydrothermal method for the synthesis of Ag2Te
1D nanostructures by employing ammonia acting as a
complexing reagent and pH regulator hydrazine hydrate
(N2H4 · H2O) acting as a reducing reagent. Very
interestingly, we discovered the morphological evolution during
the formation of 1D NWs. The morphological evolution
for the 1D nanostructures is considered as the desired
agent for understanding the growth mechanism and
formation kinetics of crystals [
]. Therefore, we
believe that this discoveryof the formation of 1D Ag2Te
nanostructures could promote further studies and
The materials used include Na2TeO3, AgNO3, aqueous
hydrazine solution (80%) (N2H4 · H2O), and ammonia
(25%) (NH3 · H2O). All of the reagents used in the
experiment were directly used without further purification.
The preparation of Ag2Te nanostructures involved a
hydrothermal process as our previous works [
]. In a
typical experiment, 0.5 mmol of Na2TeO3 and 1.0 mmol
of AgNO3 were dissolved in 15 mL of deionized water.
After stirring for minutes, 0.40 mL of N2H4 · H2O (80%)
and 0.40 mL of NH3 · H2O (25%) were dropped in the
solution. A mixed solution was obtained and then
transferred into a 25-mL Teflon-lined stainless steel
autoclave, followed by heating at 160°C for a period of time
in an electric oven. After heating, the autoclave was
cooled down naturally to room temperature. After the
hydrothermal treatment, the precipitate was collected
and rinsed with distilled water and ethanol and then
dried in air for further characterization. After a serious
treatment, the as-synthesized sample was obtained for
The size and morphology of the as-synthesized Ag2Te
nanostructures were characterized using scanning
electron microscopy (SEM) (JEOL JSM5600LV, Akishima-shi,
Japan), equipped with X-ray energy dispersive analysis
spectrum (EDS). The crystalline structure and chemical
composition were characterized by transmission electron
microscopy (TEM) and high-resolution TEM (HRTEM)
and selected area electron diffraction (SAED) (JEOL 2010,
operated at an accelerating voltage of 200 kV). X-ray
photoelectric spectrum (XPS) (Kratos AXIS Ultra, Kratos
Analytical, Ltd., Manchester, UK) and X-ray diffraction
(XRD) (X’pert MRD-Philips, Holland). Thermogravimetric
and scalable differential thermal analysis (TG-SDTA) was
carried out at a heating rate of 10°C min−1 in N2 gas at a
flowing rate of 50 mL min−1 using a TGA/SDTA851e
system. The room-temperature Raman spectra of the Ag2Te
NWs were recorded with a micro-Raman spectrometer
(Renishaw 1000, Wotton-under-Edge, UK) equipped with
a CCD detector and an Ar+ laser with a 514.5-nm
excitation line (diameter of laser spot, 3 μm) and 4.2 mW of
power. The MR of these device measurements were
carried out at room temperature using a Quantum Design
9 T physical property measurement system (PPMS) with a
rotational sample holder.
Results and discussion
The morphology evolution of hydrothermal treatment of
Ag2Te samples under different reaction times at 160°C
is displayed in Figure 1. From Figure 1a, we clearly see
that the Ag2Te sample exists in the form of a particle
before heating. After 3 h of reaction time, some narrow
and thin nanobelt structures (Figure 1b) begin to appear.
When heated for 6 h, the sample further curls and grows
into nanobelt regularly as obviously observed in Figure 1c.
In addition, The EDS of the as-synthesized Ag2Te
nanobelts is shown in Figure 1d. According to the
quantification of the EDS peaks, the atomic ratio of Ag to Te is
43:22, close to the stoichiometry of Ag2Te, which
confirmed a stoichiometric composition of the Ag2Te
products. The XRD spectra of the Ag2Te products under
Figure 4 The morphology evolution sequence and schematic diagrams of the formation of Ag2Te nanowires and nanostructures.
(a, b, c) Morphology evolution sequence of the formation of Ag2Te nanowires. (d) The schematic diagrams of the formation of Ag2Te
nanostructures: nanobelt, nanotube, and nanowire.
various growth times (3, 6, and 12 h reaction time) are
shown in Additional file 1: Figure A1.
The morphology and structure of the Ag2Te nanotubes
were examined with SEM and TEM. The SEM image
(Figure 2a) of the Ag2Te nanotubes shows that the
product obviously presents tubular structures which have been
rolled into tubes or half-pipes. As can be seen from the
image, the nanotubes have lengths of several microns and
outer diameters of 100 to 230 nm. Figure 2b is a TEM
image of a single Ag2Te nanotube. The TEM image
further provides that the product is tubular with an
approximately 80 nm of tube wall in thickness. In addition, we
can obviously see that the outer diameter of the tube is
approximately 200 nm. The high-quality crystal structure of
Ag2Te nanotubes is demonstrated in a HRTEM image
shown in Figure 2c, where abruptness at an atomic level
can be confirmed and no defects are observed. The lattice
spacing between the atomic planes was determined to be
0.56 nm in accordance with the distance between layers,
indexed to the monoclinic Ag2Te phase.
Correspondingly, the fast Fourier transform (FFT) pattern (inset in
Figure 2c) shows obvious single crystalline nature and
can be easily indexed to the cubic structure. The
corresponding SAED pattern in Figure 2d can be indexed to
the crystal of Ag2Te, which further provides strong
evidence for confirming single crystalline growth in the
fine monoclinic crystal structure.
The morphology and structure of the Ag2Te nanowires
were examined with SEM in Figure 3a. Numerous long
straight nanowires are formed, and all of the nanowires
are demonstrated with the relatively uniform diameter
about 200 nm and a typical length of tens of micrometers.
A detailed investigation was performed using
highmagnification SEM (HRSEM)/HRTEM/TEM. Figure 3b
shows a typical high-magnification SEM image of the
single Ag2Te nanowire with diameters about 150 nm and
lengths ranging from 8 to 10 μm. A typical HRTEM image
(Figure 3c) taken from a small square in Figure 3b
demonstrates clear lattice fringes with an interplanar spacing of
0.65 nm. Moreover, a representative SAED (upper right
inset in Figure 3c, taken from a small square in Figure 3b,
too) further substantiates that the Ag2Te nanowire has a
single crystalline structure with a monoclinic phase.
Further, according to the quantification of XPS peaks shown
in Additional file 2: Figure A2, the molar ratio of Ag to
Te is 2.08:1.00, which is close to the stoichiometry of
Ag2Te. To further ascertain the chemical compositions
of the nanowires, the as-prepared products were
examined by TG-SDTA and Raman scattering spectroscopy
in Additional file 3: Figure A3 and Additional file 4:
Figure A4, respectively.
To further obtain a complete view of the Ag2Te
ultralong and straight NW formation process and its growth
mechanism, the detailed time-dependent evolution of
the morphology was evaluated by SEM (Figure 4a,b,c).
As shown in Figure 4a, when the hydrothermal reaction
proceeded for 3 h, the products are mainly composed of
Ag2Te nanobelts or half-nanotubes. If the reaction time
is increased to 12 h, these Ag2Te nanobelts further curled
up along the axis, became half-tubes, and finally grew into
nanotubes (Figure 4b). When the reaction time was
increased to 24 h, the Ag2Te nanotubes grew into NWs with
a diameter of about 100 to 200 nm and a typical length of
tens of micrometers eventually. Based on the above
experimental observations, a plausible formation mechanism
of the Ag2Te ultra-long NWs is proposed (Figure 4d). We
believe that the formation process of the ultra-straight and
long Ag2Te NWs could be rationally expressed into three
sequential steps: (1) the formation of Ag2Te nanobelts
and the existence of half-tube structures at an early stage,
(2) the nanobelts further curled up along the axis, became
half-tubes, and finally grew into nanotubes via the
Figure 5 I-V characteristics of the Ag2Te nanowires at room
temperature and normalized magneto-resistance for Ag2Te
nanowires. (a) I-V characteristics of the Ag2Te nanowires at room
temperature under a series of magnetic field, B = 1, 3, 5, and 7 T;
(b) the normalized magneto-resistance Δρ (T, H) / ρ (T, H) for Ag2Te
nanowires as a function of magnetic field H at a series of
temperatures T = 5, 10, 20, 40, 80, 160, and 300 K.
up mechanism [
], (3) with the extended reaction
time, Ag2Te nanotubes continue to grow and grow into
NWs eventually. On the basis of the experimental
results and discussion, and according to previous
], a possible mechanism for the formation of
ultra-straight and long Ag2Te NWs may be explained by
the following reactions:
TeO23‐þN2H4→Te þ N2þH2O
Te þ N2H4þOH‐1→Te2‐þN2þH2O
To investigate the magneto-transport properties of
Ag2Te NWs, PPMS measurements were carried out. I-V
characteristics of the nanowires at room temperature as
a function of magnetic field (B = 1, 3, 5, and 7 T) are
shown in Figure 5a. The black curve is the I-V of the
magnetic field of 1 T. Obviously, the current increases
nonlinearly with the increasing voltage. Without
changing the other experimental conditions, only changing B
to 3 T, the I-V of the Ag2Te sample (red line) displays a
smaller absolute value of the corresponding current and
a larger resistance at the same voltage conditions. When
the magnetic field is adjusted to 5 and 7 T (the blue and
the green line), respectively, the absolute value of the
current continues to decrease at the same voltage
conditions. It is noteworthy that from Figure 5a, we can
clearly see that ΔI from 1 to 3 T is larger than that from
3 to 7 T where the voltage is −4 V. That is to say, the I-V
of Ag2Te sample is more sensitive at low magnetic field.
This phenomenon reveals that the Ag2Te nanowires are
suitable for low magnetic field sensor. In addition, the
magneto-resistance curves under different temperature
conditions are illustrated in Figure 5b. The MR was
calculated as MR = (ρH − ρ0)/ρ0. The MR (Δρ/ρ) increases
when the magnetic field increases gradually. At each
temperature, the curves for the sample look very similar.
But at T = 5 K, MR rises faster slightly than other higher
temperature conditions. As shown in the black curve, the
Δρ/ρ value is centered at 11.79% when the magnetic field
is 4 T at a temperature of 300 K. When the temperature
decreased at 5 K, keeping the same magnetic field of 4 T,
the Δρ/ρ value increased to 38.35% (purple curves). These
results experimentally suggest that the Δρ/ρ of Ag2Te
NWs increased with the temperature decreasing gradually
at the same magnetic field. Here, we also found a novel
phenomenon that the magneto-resistance crosses over
from a linear to a quadratic dependence on H (T) at the
place of 4 T approximately. The Δρ/ρ shows a linear
dependence on the low magnetic field (Figure 5b), but from
the slope, we can notice that Δρ/ρ increases nonlinearly
with increasing temperature at high H(T), which is
different from the previous report [
]. We deduced that this
novel phenomenon was caused by the nanostructure of
Temperature-dependent MR of zero field (R0) and
field (RH) resistivity is shown in Figure 6. The MR was
calculated as MR = (RH − R0) / R0, and the sample
behavior was measured in temperature from 300 to 4 K. It is
noteworthy that the resistivity measured by the magnetic
field of 9 T becomes larger with the increasing magnetic
field, and the field resistivity curve is peaked with a strong
maximum at 66 K exhibited by the red line. Then, the
product exhibits a steep decline of the resistivity with
increasing temperature as illustrated in the figure. In
contrast, no maximum peak was observed in the
temperature-dependence curve of zero field resistivity of
the Ag2Te nanowires, and the decrease of the resistance
with decreasing temperature is pronounced (black line). At
each temperature, the curves for the sample look very
similar to the previous report [
]. However, comparing to
the bulk [
] and thin film materials [
], we found that
there is generally a larger change in R(T) as the sample size
is reduced, which indicates that the size of the sample has
a certain impact on the magneto-transport properties.
While both field resistivity of 9 T and zero shows
semiconductor characteristics at a high temperature region, it
presents that resistivity is almost temperature-independent at
a temperature more than 165 and 115 K, respectively. The
inset shows the relative MR of as-synthesized nanowires.
The MR amplitude increases from about 50% at room
temperature to more than 250%. The MR also has a strong
maximum at 100 K up to 280% corresponding to the
maximum of the field resistance of 9 T. It was noted [
the classical picture seems incapable of explaining the
silver chalcogenide data. That is why the search of analogies
to other materials can be very helpful in understanding
and explaining the observed phenomena. According to
reports, the peak on the MR temperature curve of the Ag2Te
nanowires suggests that grain boundary transport can play
an important role in the MR effect in these materials [
Through analyzing the crystal structure of the monoclinic
phase of Ag2Te [
], we know that this material can be
considered a natural multilayered compound. Similar large
positive MR was also discovered by Vernbank [
] et al. in
nonmagnetic Cr/Ag/Cr trilayer structure. Nevertheless,
more recently, a band calculation paper [
] by first
principle calculations reported that β-Ag2Te is in fact a
new binary topological insulator with gapless linear
Dirac-type surface states. This raises the possibility that
the observed unusual MR behavior can be understood
from its topological nature and may largely come from
the surface or interface contributions. This scenario is
supported by the fact that experimental samples, doped
with excess Ag, are granular materials [
makes the interface contribution significant. On the
other hand, the highly anisotropic surface states may
cause large fluctuation of mobility, which may also help
to explain the unusual MR behavior . To observe
the unique electronic transport properties arising from
the anisotropic Dirac cone, further experimental and
theoretical studies are needed.
In summary, a series of single crystalline 1D
nanostructures of Ag2Te with well-controlled shapes and sizes
were prepared by a facile one-pot hydrothermal
synthesis approach. On the basis of these results, a rolling-up
growth mechanism of the ultra-straight and long Ag2Te
nanowires has been proposed. The formation of these
1D Ag2Te nanostructures can promote further studies
and potential applications. Moreover, we systematically
investigated the I-V characteristics and unusual MR
behavior of the Ag2Te nanowires with monoclinic structure.
It was found that the I-V of Ag2Te nanowires is more
sensitive at low magnetic field, which reveals that the Ag2Te
nanowires are suitable for low magnetic field sensor. In
addition, the excellent single crystal quality with
monoclinic structure raises the possibility for observing the
unusual MR behavior in the as-prepared nanowires.
Significantly, comparing to the bulk and thin film materials,
we found that there is generally a larger change in R(T) as
the sample size is reduced. This raises the possibility that
the observed unusual MR behavior can be understood
from its topological nature and may largely come from the
surface or interface contributions.
Additional file 1: Figure A1. XRD spectra of the Ag2Te products under
various growth times (3, 6, and 12 h reaction time) The XRD patterns
reveal that these Ag2Te nanostructures have a monoclinic structure.
Additional file 2: Figure A2. (a) XPS survey spectrum of the Ag2Te
nanowires, and HRXPS in the (b) Ag 3d and (c) Te 3d regions. The molar
ratio of silver to tellurium according to the quantification of peaks is
2.08:1.00, close to the stoichiometry of Ag2Te.
Additional file 3: Figure A3. TG-DTA curves of the Ag2Te nanowires.
From the DTA curve, it can be seen that the phase transition during the
heating procedure occurred at 152°C, which confirms structural phase
transition of Ag2Te.
Additional file 4: Figure A4. Raman scattering spectrum of the
asprepared Ag2Te nanowires under different times of exposure. An
interesting Raman scattering enhancement phenomenon has also been
observed during the observation of Raman spectra.
1D: One-dimensional; AgNO3: Silver nitrate; Ag2Te: Silver telluride; EDS: X-ray
energy dispersive analysis spectrum; FFT: Fast fourier transform;
HRSEM: High-magnification SEM; HRTEM: High-resolution TEM; I-V:
Current–voltage; MR: Magneto-resistance; N2H4 · H2O: Hydrazine hydrate;
Na2TeO3: Sodium tellurite; NH3 · H2O: Ammonia; PPMS: Physical property
measurement system; SAED: Selected area electron diffraction;
SEM: Scanning electron microscopy; TEM: Transmission electron microscopy;
TG-SDTA: Thermogravimetric and scalable differential thermal analysis;
XPS: X-ray spectroscopy; XRD: X-ray diffraction.
The authors declare that they have no competing interests.
GML designed and performed the fabrication and characterization
experiments, analyzed the data, and drafted the manuscript. XBT performed
the tests on the samples and helped in the drafting and revision of the
manuscript. SMZ carried out current–voltage and magneto-resistance
characteristics and critically revised the manuscript. NL conceived the study
and helped in performing the experiment. XYY helped in the revision of the
manuscript. All authors read and approved the final manuscript.
This work is financially supported by the National Natural Science
Foundation of China (grant no. 20971036) and Changjiang Scholars and
Innovative Research Team in University, no. PCS IRT1126, and the construct
program of the key discipline in Hunan province (no.2011-76).
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