A general solvothermal route to the synthesis of CoTe, Ag2Te/Ag, and CdTe nanostructures with varied morphologies

Article abstract of DOI:10.1002/ejic.200900960

A general and facile surfactant-assisted solvothermal route was developed, for the synthesis of CoTe, Ag₂Te/Ag, and CdTe nanostructures using the corresponding metal salt, Na₂TeO₃, ascorbic acid, and polyvinyl pyrrolidone (PVP) or cetyltrimethylammonium bromide (CTAB) in mixed solvents of ethanolamine and water. It was also found that the morphology of the product varied dramatically when using different surfactants such as PVP and CTAB. A formation mechanism, of telluride nanostructures was proposed. The products were characterized, by X-ray powder diffraction (XRD), transmission electron microscopy (TEM), thermogravimetry (TG), and differential scanning calorimetric (DSC) analysis.

Full text of DOI:10.1002/ejic.200900960

FULL PAPER
DOI: 10.1002/ejic.200900960
A General Solvothermal Route to the Synthesis of CoTe, Ag2Te/Ag, and CdTe
Nanostructures with Varied Morphologies
Ling Jiang[a] and Ying-Jie Zhu*[a]
Keywords: Cobalt / Silver / Nanostructures / Surfactants / Tellurium
A general and facile surfactant-assisted solvothermal route
was developed for the synthesis of CoTe, Ag2Te/Ag, and
CdTe nanostructures using the corresponding metal salt,
Na2TeO3, ascorbic acid, and polyvinyl pyrrolidone (PVP) or
cetyltrimethylammonium bromide (CTAB) in mixed solvents
of ethanolamine and water. It was also found that the mor-
phology of the product varied dramatically when using dif-
ferent surfactants such as PVP and CTAB. A formation
mechanism of telluride nanostructures was proposed. The
products were characterized by X-ray powder diffraction
(XRD), transmission electron microscopy (TEM), thermogra-
vimetry (TG), and differential scanning calorimetric (DSC)
analysis.
plications in the field of photonic crystals, light-emitting
diodes (LEDs), solar cells, and bioimaging.[13–18] The CdTe
nanostructured materials exhibit outstanding optical and
electrical properties,[19–27] and reducing the dimensionality
and size of CdTe to a nanoscale range may be a successful
strategy to gain further enhancements in both optical and
electrical properties.
Although many 1D nanostructures of metal tellurides
have been synthesized, up to now, studies of cobalt telluride
nanostructures have been little reported. Xie et al.[28] pre-
pared CoTe2 nanorods by the reaction of CoCl2·6H2O and
tellurium in ethylenediamine by a solvothermal method.
Peng et al.[12] reported the synthesis of CoTe and NiTe
nanowires through a reduction method by using N2H4·H2O
as the reducing agent. Fan et al.[29] synthesized CoTe nano-
tubes through a Te-templated route. Among these fabricat-
ing methods, the two-step Te template-directed synthesis
was one of the most popular strategies for the fabrication
of 1D telluride nanostructures.[5,30–32] Furthermore,
N2H4·H2O was a common reducing agent for the synthesis
of tellurides.[4–6,12,25,30–33] It is necessary to develop a gene-
ral and one-step route for the synthesis of nanostructured
tellurides.
Introduction
Recently, considerable attention has been paid to the syn-
thesis of one-dimensional (1D) semiconducting nanostruc-
tures such as nanorods, nanowires, and nanotubes because
of their unique and interesting properties derived from their
dimensionality-dependent anisotropy and potential applica-
tions in electronics and optoelectronics.[1] Metal tellurides
are being actively investigated because of their distinctive
properties and applications. For example, silver telluride
(Ag2Te) has great technological prospects in magnetic-field
measurements, microelectronics, and structural studies.[2–7]
The crystal phase of Ag2Te undergoes a phase transforma-
tion from the low-temperature monoclinic structure (β-
Ag2Te) to a high-temperature face-centered cubic structure
(α-Ag2Te) at about 150 °C.[8] β-Ag2Te is known as a narrow
band gap semiconductor with high electron mobility and
low lattice thermal conductivity, whereas α-Ag2Te shows
superionic conductivity because Ag+ cations can move
freely, which enhances the conductivity.[9,10] Recently, it was
reported that binary nanocrystal superlattices of PbTe and
Ag2Te enhanced the conductivity of the composite thin
films.[11] Recently, Batabyal et al.[7] reported a facile route
for the synthesis of axial-junction nanowires of Ag2Te/Ag
composite, which was proven to be a potential material for
memristors. CoTe, as a typical magnetic semiconductor, has
attracted great interest for its distinctive electrical transport
properties.[12] As an important wide band gap II–VI semi-
conductor, CdTe continues to draw increasing attention in
both fundamental studies and for its various promising ap-
In this paper, we report a simple one-step solvothermal
synthesis of nanostructures of three different kinds of metal
tellurides (CoTe, Ag2Te/Ag, and CdTe) by using only one
general method. It is found that the morphology of the
products can be adjusted by using different surfactants such
as polyvinyl pyrrolidone (PVP) and cetyltrimethylammo-
nium bromide (CTAB).
[a] State Key Laboratory of High Performance Ceramics and
Superfine Microstructures,
Results and Discussion
Shanghai Institute of Ceramics, Chinese Academy of Sciences,
Shanghai 200050, P. R. China
To distinguish the as-prepared samples, the samples with
different morphologies are labeled as A1, A2, and A3 for
Fax: +86-21-52413122
E-mail: y.j.zhu@mail.sic.ac.cn
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Solvothermal Route to CoTe, Ag2Te/Ag, and CdTe
CoTe; B1 and B2 for Ag2Te; and C1 and C2 for CdTe, as and 20 nm in diameter. The SAED pattern (Figure 3, j)
shown in Figure 1. Figure 2 shows XRD patterns of three shows clear diffraction rings of CoTe crystals, thereby indi-
as-synthesized CoTe samples A1, A2, and A3 with varied cating the polycrystalline nature of the nanorod. The mor-
morphologies. Samples A1, A2, and A3 were prepared by phology of the synthesized product (sample A3) in the pres-
using no surfactant, PVP, and CTAB, respectively. The dif- ence of CTAB instead of PVP is displayed in Figure 3 (k–
fraction peaks of the XRD patterns can be indexed to a o), from which CoTe hierarchical 1D nanostructures can be
single phase of CoTe with a hexagonal structure (JCPDS observed, and each hierarchical 1D nanostructure is com-
no. 65-8986), thereby indicating that hexagonal CoTe forms posed of a nanotube on which nanorods are grown. The
independently of the use of the surfactant. TEM micro- magnified TEM image of a single hierarchical 1D nano-
graphs of as-prepared CoTe nanostructures with different structure (Figure 3, l) shows that the nanotube is about
morphologies are shown in Figure 3. Figure 3 (a–e) shows 80 nm in diameter, whereas the nanorods grown on the
TEM micrographs of sample A1 obtained without using nanotube show diameters of less than 10 nm. The SAED
any surfactant, from which one can see that sample A1 con- pattern in Figure 3 (o) shows clear diffraction rings of CoTe
sists of CoTe nanotubes with lengths of up to several mi- crystals, thereby indicating the polycrystalline nature of the
crometers. Parts c and d of Figure 3 exhibit typical individ- hierarchical 1D nanostructure.
ual nanotubes that clearly present a tubular structure with
a diameter of around 60 nm. The selected-area electron dif-
fraction (SAED) pattern (Figure 3, e) indicates that the
CoTe nanotube has a polycrystalline structure. To investi-
gate the influence of the surfactant on the morphology of
the product, we selected PVP and CTAB as morphology-
controlling agents, and the presence of PVP or CTAB has
an obvious influence on the morphology, as shown in Fig-
ure 3 (f–j) (sample A2, using PVP) and Figure 3 (k–o, sam-
ple A3, using CTAB). Part f of Figure 3 displays a typical
panoramic TEM image of the CoTe sample synthesized in
the presence of PVP, thereby indicating that sample A2 is
composed of monodisperse uniform solid nanorods with
Figure 2. XRD patterns of CoTe 1D nanostructures (samples A1,
diameters of about 20 nm and lengths of several hundred
A2, and A3) prepared under different conditions. Sample A1: with-
out using any surfactant; sample A2: in the presence of PVP; sam-
nanometers. The magnified TEM image in Figure 3 (h)
shows individual CoTe nanorods of about 600 nm in length ple A3: in the presence of CTAB.
Figure 1. Illustration of the sample preparation and sample numbers of the as-synthesized CoTe, Ag2Te/Ag, and CdTe nanostructures.
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L. Jiang, Y.-J. Zhu
FULL PAPER
Figure 3. TEM micrographs of as-prepared CoTe 1D nanostructures prepared under different conditions: (a–e) without using any surfac-
tant (sample A1); (f–j) in the presence of PVP (sample A2); (k–o) in the presence of CTAB (sample A3).
In addition to CoTe 1D nanostructures, Ag2Te/Ag nano-
composites as well as CdTe nanostructures with different
morphologies have also been successfully synthesized using
this method in the presence of PVP or CTAB. Figure 4
shows the XRD patterns of two as-prepared Ag2Te/Ag
samples with different morphologies. The peaks of the
XRD patterns can be confidently indexed to monoclinic
Ag2Te (JCPDS no. 34-0142) and cubic Ag (JCPDS no. 04-
0783). The TEM images of Ag2Te/Ag nanostructures are
shown in Figure 5. It is found that the morphologies of the
Ag2Te/Ag samples vary dramatically using different surfac-
tant, as shown in Figure 5 (a–d). Sample B1 prepared by
Figure 5. TEM micrographs of as-prepared Ag2Te/Ag nanostruc-
tures synthesized in the presence of (a and b) PVP (sample B1) and
(c and d) CTAB (sample B2).
using PVP consists of nanocrystals with diameters of about
10 nm, whereas nanowires with diameters around 20 nm are
obtained in the presence of CTAB (sample B2).
centers at about 150 °C in the DSC curve and the TG curve
shows hardly any mass loss in the temperature range inves-
tigated, thereby indicating that only a structural phase tran-
sition from the monoclinic phase to the cubic phase occurs,
and this result is in good agreement with the result reported
by Li et al. and Batabyal and Vittal.[6,7]
Figure 4. XRD patterns of Ag2Te/Ag nanostrutures synthesized in
the presence of PVP (sample B1) and CTAB (sample B2).
To examine the crystal phase transition of the as-pre-
pared Ag2Te samples from a low-temperature monoclinic
structure (β-Ag2Te) to a high-temperature face-centered cu-
bic structure (α-Ag2Te), the DSC and TG measurements of
Figure 6. DSC–TG curves of Ag2Te/Ag nanostructures (sample B1)
prepared in the presence of PVP.
The XRD patterns of as-prepared CdTe samples are
sample B1 were taken, and the results are shown in Fig- shown in Figure 7. The diffraction peaks can be indexed to
ure 6, from which one can see that an endothermic peak single-phase CdTe with a cubic structure (JCPDS no. 65-
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Solvothermal Route to CoTe, Ag2Te/Ag, and CdTe
0880). The morphology of the obtained CdTe sample pre- soluble Na2TeO3 dissociated into TeO3 and Na+ ions in
pared in the absence of any surfactant consists of irregular water, and then reacted with Mn+ (Mn+ = Co2+, Ag+, or
nanocrystal aggregates (Figure 8, a and b, sample C1), and Cd2+) to form CoTeO3, Ag2TeO3, or CdTeO3. When eth-
cobblestone-like structures assembled by small nanocrystals anolamine (C2H7NO) was added, CoTeO3, Ag2TeO3, or
2–
with sizes of around 20 nm are obtained by using PVP as CdTeO3 was dissolved by the formation of the Mn+
–
the morphology-controlling agent (Figure 8, c–g, sample C2H7NO complex, and simultaneously, the dissociative
2–
C2). The SAED pattern shown in part g of Figure 8 indi- TeO3 formed again. Then, H+ ions that resulted from the
2–
cates that the cobblestone-like structures are polycrystalline. ionization of ascorbic acid reacted with TeO3 to generate
TeO2, which was not dissolved in water at room tempera-
ture.[34] When the reaction mixture was heated to a certain
temperature, the previously formed TeO2 was gradually dis-
solved in the solution. After the temperature reached
200 °C, TeO2 had completely dissolved in the solution and
subsequent reactions took place in the mixture. It is known
that ascorbic acid is a weak reducing agent.[35–37] The ascor-
bic acid would slowly reduce Te4+ to Te in the solution.[34]
At the same time, at such a high temperature, Mn+ could
also be reduced to M in the presence of ethanolamine.
These newly produced atomic Te and M were highly reac-
Figure 7. XRD patterns of CdTe nanostructures synthesized with-
tive, and they reacted with each other to form CoTe, Ag2Te,
and CdTe. As a result, after 24 h of reaction, nucleation,
and crystal growth, the formation of CoTe, Ag2Te, and
CdTe nanostructures finally reached completion. The
chemical reactions involved in this process are shown as
follows (for example, Co2+) and a scheme for the formation
of CoTe 1D nanostructures with different morphologies is
shown in Scheme 1.
out any surfactant (sample C1) and in the presence of PVP (sample
C2).
Figure 8. TEM images of as-prepared CdTe nanostructures synthe-
sized without any surfactant (sample C1) (a and b) and in the pres-
ence of PVP (sample C2) (c–g).
Scheme 1. Illustration of the formation of CoTe 1D nanostructures
with different morphologies.
The solid–solution–solid transformation has been exten-
sively used to explain the formation of 1D nanostructured
Some reports in the literature have described the effects
tellurides in the solution phase.[5,12,23,30,31,34] On the basis of the surfactants on the formation of metal tellurides. For
of the experimental results, we propose a possible solid– example, Gong et al.[25] reported a CTAB-assisted route for
solution–solid reaction mechanism for the formation of the synthesis of CdTe nanorods; they discussed the role of
CoTe, Ag2Te, and CdTe nanostructures. In the early stage, CTAB and proposed that one possible function of CTAB
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L. Jiang, Y.-J. Zhu
FULL PAPER
tron microscope. The TG and DSC curves were taken with a heat-
ing rate of 10 °Cmin–1 in flowing air with a STA 409/PC simulta-
neous thermal analyzer (Netzsch, Germany).
was to generate large numbers of rodlike micelles in aque-
ous solution, which acted as a soft template for the forma-
tion of 1D nanostructures as well as stabilized 1D nano-
structures. Tai et al.[31] also discussed the effects of PVP on
the morphologies of PbTe nanowires. PVP played an im-
portant role in the transformation process from the trigonal
tellurium (t-Te) nanowires to the pearl-necklace-shaped
PbTe nanowires. Based on both our experimental results
and the reports in the literature, we propose that in the for-
mation process of these metal telluride nanostructures, the
surfactant functioned as the morphology-controlling agent.
PVP or CTAB molecules could generate micelles in solu-
tion, which acted as the soft template for the formation of
metal telluride nanostructures as well as the stabilizer for
these nanostructures.
Acknowledgments
Financial support from the National Natural Science Foundation
of China (50772124, 50821004), the Program of Shanghai Subject
Chief Scientist (07XD14031), Science and Technology Commission
of Shanghai (0852nm05800), and the Opening Project of State Key
Laboratory of High Performance Ceramics and Superfine Micro-
structures (SKL200901SIC) is gratefully acknowledged.
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Conclusion
In summary, we have developed a general and facile sur-
factant-assisted solvothermal route for the synthesis of
CoTe, Ag2Te/Ag, and CdTe nanostructures by using the
corresponding metal salt, Na2TeO3, ascorbic acid, and
polyvinyl pyrrolidone (PVP) or cetyltrimethylammonium
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Experimental Section
Synthesis of CoTe, Ag2Te/Ag, and CdTe Nanostructures: All chemi-
cals were purchased and used as received without any further puri-
fication. In a typical procedure for the preparation of CoTe, Ag2Te/
Ag, and CdTe nanostructures, polyvinyl pyrrolidone (PVP; 0.5 g)
or cetyltrimethylammonium bromide (CTAB; 0.5 g) was dissolved
in deionized water (30 mL). Co(NO3)2·6H2O (0.29 g), AgNO3
(0.34 g), or CdCl2·2.5H2O (0.228 g) was added into the above solu-
tion under magnetic stirring at room temperature. Then Na2TeO3
(0.22 g) was added while being stirred for about 10 min, and ascor-
bic acid (2 g) and ethanolamine (40 mL) were added. Next, the
mixture was stirred for 20 min to form a well-dispersed suspension
and was put into a Teflon-lined stainless steel autoclave of 100 mL
capacity. The autoclave was sealed and maintained at 200 °C for
24 h. After the heating process, the autoclave was taken out and
cooled to room temperature in air naturally. The black product
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Received: September 27, 2009
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