Selected-control solvothermal synthesis of nanoscale hollow spheres and single-crystal tubes of PbTe

Article abstract of DOI:10.1002/ejic.200400401

For the first time, hollow PbTe nanospheres with diameters of 80-140 nm have been prepared in an ether/glycerol microemulsion system and PbTe single-crystalline nanotubes with diameters of 60-80 nm have been synthesised in a homogeneous ethanol/glycerol system. They can be facilely controlled via a solvothermal process without any additional catalysts and templates. Two different template mechanisms are discussed herein. The Raman and far infrared spectra of the samples have been investigated at room temperature. Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004.

Full text of DOI:10.1002/ejic.200400401

FULL PAPER
Selected-Control Solvothermal Synthesis of Nanoscale Hollow Spheres and
Single-Crystal Tubes of PbTe
Guifu Zou,[a,b] Zhaopin Liu, Debao Wang, Changlong Jiang, and Yitai Qian*
[b]
[b]
[b]
[a,b]
Keywords: Nanostructures / Semiconductors / Nanotubes
For the first time, hollow PbTe nanospheres with diameters
and templates. Two different template mechanisms are dis-
of 80−140 nm have been prepared in an ether/glycerol micro- cussed herein. The Raman and far infrared spectra of the
emulsion system and PbTe single-crystalline nanotubes with
diameters of 60−80 nm have been synthesised in a homogen-
eous ethanol/glycerol system. They can be facilely controlled
via a solvothermal process without any additional catalysts
samples have been investigated at room temperature.
( Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim,
Germany, 2004)
Introduction
the final product. Therefore, it is always a challenge for the
tubular materials to be prepared by a simple wet chemical
route.
In recent years, one of the important goals of material
scientists has been to tailor the structures of materials to
obtain special nanomorphologies because the properties of
nanomaterials are strongly dependent on the their sizes and
PbTe has been intensively studied for more than forty
years because it has interesting physical properties which
can be applied in many fields such as IR detection, photo-
[
14]
[
1Ϫ5]
conductivity and thermoelectric materials.
PbTe nanoc-
shapes.
Among specific morphologies, hollow nanos-
[
15]
rystals have been prepared by various methods.
To the
cale spheres and tubes have drawn more and more atten-
[
6,7]
best of our knowledge, however, there are no reports re-
garding hollow PbTe nanospheres and nanotubes. We have
successfully synthesised hollow PbTe spheres and nano-
tubes by a wet chemical method, simply using ether and
ethanol as the reaction solvents, respectively. In the solvo-
thermal process, neither physical templates nor catalysts are
required. In addition, two different template mechanisms
are discussed.
tion.
Three-dimensional hollow spheres are interesting
in the fields of medicine, material science and catalysis due
to their hollow structures which make them particularly ap-
plicable for drug delivery, medical imaging, protein and en-
zyme transplantation, encapsulation of products (cosmetics,
inks and dyes) and contaminated waste removal.[8] So far,
various hollow spheres (SiC, ZnS, CdS, Ag S and polymers)
2
have been synthesised by different methods.[
9Ϫ12]
Mean-
while, one-dimensional nanotubes have also attracted atten-
tion because of their unique optical, electronic and mechan-
ical properties resulting in promising applications in electri-
cal and optoelectronic nanodevices. Many tubular struc-
tures have been fabricated for compounds with layered
Results and Discussion
The typical XRD patterns of the products are shown in
Figure 1. All the peaks can be indexed as face-centred cubic
structures such as C, MoS and BN with the assistance of
2
catalysts. Moreover, there have been many tubular materials
without layered structures produced by physical template
[
13]
methods. However, the introduction of templates or cata-
lysts to the reaction system means a much more compli-
cated process involving the preparation of catalysts or the
selection of templates and may introduce impurities into
[
a]
Structure Research Laboratory, University of Science and
Technology of China,
Hefei, Anhui 230026, PR China
Fax: (internat.) ϩ 86-551-3607402
E-mail: ytqian@ustc.edu.cn
[
b]
Department of Chemistry, University of Science and
Technology of China,
Hefei, Anhui 230026, PR China
Figure 1. XRD patterns of (a) hollow spheres and (b) tubular PbTe
Eur. J. Inorg. Chem. 2004, 4521Ϫ4524
DOI: 10.1002/ejic.200400401
 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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G. Zou, Z. Liu, D. Wang, C. Jiang, Y. Qian
FULL PAPER
PbTe (JCPDS, 38-1435). No peaks of any other impurities
are detected, indicating the high purity of the products. In
addition, the strong and sharp reflection peaks suggest that
the products are highly crystalline.
Figure 2 (see a and b) shows the TEM images of the
samples prepared by the process using ether as the solvent.
The strong contrast between the dark edge and pale centre
is evidence of its hollow nature. Figure 2 clearly displays
many hollow PbTe spheres prepared in the products. The
proportion of hollow spheres in the samples was estimated
to be 25Ϫ30% using large numbers of TEM observations.
Their external diameter and wall thickness are ca.
80Ϫ140 nm and 15Ϫ30 nm, respectively. Close observations
indicate that the hollow spheres consist of PbTe nanopart-
icles. The inset of Figure 2 (see b) shows the selected area
electron diffraction (SAED) pattern of the hollow PbTe
spheres. The SAED exhibits diffuse polycrystalline ring pat-
terns and agrees well with the XRD peaks of the products.
It also further confirms that the hollow spheres are con-
structed from highly crystalline PbTe nanoparticles. In ad-
dition, it could be shown that the hollow spheres are sensi-
tive to beam irradiation during TEM examination. This
may be related to their thin and porous walls. A similar
phenomenon can be also found in former reports concern-
[
6,16]
ing hollow spheres.
In addition to the hollow spheres,
some cubes are also present in the samples (Figure 2, c).
Figure 2 (see dϪg) shows tubular PbTe prepared in the
process using ethanol as the solvent. The yield of the nano-
tubes in the sample was about 20%. The external diameter
and wall thickness of the nanotubes are ca. 70 nm and
1
5 nm, respectively. Their lengths are about several hundred Figure 2. TEM images of (a) and (b) hollow PbTe spheres and
(
(
(
c) a few cubes, the inset shows the hollow sphere ED patterns;
d) Tubular PbTe TEM images and (e) their magnification images.
f) HRTEM images of the wall and (g) the open end of the nano-
nanometers. Most nanotubes have closed ends and a few
have open ends. The microstructure of tubular PbTe was
further characterised by HRTEM and SAED. The HRTEM tubes, the inset show the corresponding ED patterns
images looking down onto the wall and open end of the
nanotubes are given in Figure 2 (see fϪh), where the inset
is the corresponding open end ED pattern. The regular to Te by hydrazine. Secondly, the PbTe was formed slowly
spacing of the observed lattice planes is ca. 0.32 nm which through a redox reaction between Pb(C H O ), Te and hy-
3
6
3
is consistent with the separation of the (200) planes of c- drazine. Here, glycerol was used as the complexing reagent
PbTe. It shows that the nanotube is single-crystalline, struc- to form a complex, Pb(C H O ). The complex served to
3
6
3
turally uniform and dislocation free. Moreover, the SAED reduce the speed of reaction (2) because the Pb(C H O )
3
6
3
patterns taken from various regions of an individual nano- sharply decreased the free Pb2 concentration in the solu-
tube are essentially the same. It can also be concluded that tion. It is well-known that a slow reaction rate is favourable
the nanotubes synthesised using the present procedure are for crystallisation as well as separating the growth step from
ϩ
[
19]
single-crystalline in structure. According to the HRTEM the nucleation step. Thus, highly crystalline PbTe was ob-
images and the SAED pattern, it can be concluded that the tained in our experiments and the ED, XRD and HRTEM
nanotubes grow along the [100] direction.
of the products also confirmed this. For the detailed forma-
To investigate the reaction process, a series of experi- tion process of the hollow spheres, we assume the process
ments was carried out. When Pb(NO ) was not added to to be a template-interface route. Because the ether was in-
3
2
the reaction system, only Te nanocrystals formed. On the soluble in the glycerol, the whole system can be described
other hand, Pb was not produced when Na TeO was left as a microemulsion state. The ether could form stable mic-
2
4
out of the reaction system. Moreover, a large amount of roscale oil droplets in glycerol at a moderate temperature.
Te mixed with some PbTe was obtained when the reaction When Te nanoparticles were formed through reaction (1) in
proceeded for a short time and then more PbTe and less an ether/glycerol system, many Te particles accumulated on
Te were produced when the reaction time was continually the surface of the oil droplets and formed a template-in-
increased. Based on the above results and the former re- terface. The reaction then progressed according to Equation
[
15,17,18]
ports,
all the reactions can be described according (2). Finally the shell-core structure formed. Moreover, the
to part A in Scheme 1. Firstly, the Na TO was reduced hollow spheres can be obtained when the ether is removed
2
4
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 2004 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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Eur. J. Inorg. Chem. 2004, 4521Ϫ4524

Synthesis of Nanoscale Hollow Spheres and Single-Crystal Tubes of PbTe
FULL PAPER
by washing and drying of the product. The whole process
can be described diagrammatically as in part B of
Scheme 1.
Scheme 1. A) Illustration of the hollow spheres formation process
by a template-interface route B) Illustration of the nanotube forma-
tion process by a self-generated template route C
In order to discuss the formation mechanism of PbTe
nanotubes, some experiments were carried out. Te nano-
tubes could be obtained in the products (the images not
shown here) when Pb(NO ) was not added to the ethanol/
3
2
glycerol reaction system. This phenomenon can also be
found in the literature.[
17,18]
Moreover, more PbTe and less
Te were obtained when the reaction time was continually
increased. In addition, it is worth noting that the diameter
of the Te nanotubes obtained was almost the same as that
of the PbTe nanotubes. Based on these results, we propose a
self-generated template process. Firstly, some Te nanotubes
Figure 3. (a) the Raman and (b) far infrared spectra of the hollow
were produced according to reaction (1) in the ethanol/gly- PbTe nanospheres
cerol homogeneous system and then according to reaction
(2) considerable tubular PbTe formed using the Te nano-
tubes as the templates. Some similar template syntheses
1 cmϪ1. A similar phenomenon has been reported in the
have been reported.[
20Ϫ22]
Accordingly, we are inclined to
6
[
27]
literature.
From Figure 3 (see a and b), it can be also
describe the formation process of the PbTe nanotubes as a
self-generated template route. It can be described according
to part C in Scheme 1. However, the detailed mechanism
needs to be further studied.
shown that the frequency of the mixed plasmon-phonon
mode (at 154 cm ) in the Raman spectrum of the hollow
PbTe nanospheres is almost same as that (at 151cm ) in
Ϫ1
Ϫ1
the infrared spectrum. Additionally, the Raman and far in-
frared spectra (not shown here) of the PbTe nanotubes were
similar to the above Raman and far infrared spectra of the
hollow PbTe nanospheres, respectively.
The room temperature Raman and far infrared spectra
of the hollow PbTe nanospheres are shown in Figure 3.
Peaks at 121, 137 and 154 cm are observed as indicated
Ϫ1
in Figure 3 (a). Among them, the peaks marked with aster-
isks are related to the vibrational modes of a thin TeO
2
[23,24]
layer on the PbTe nanocrystals.
The wide band with
Ϫ1
the peak at ca. 154 cm corresponds to the mixed plas-
mon-phonon modes of the PbTe nanoparticles forming hol-
low nanospheres. It is worth noting that the frequency is
lower than that for bulk PbTe. This can be explained by
Conclusion
In conclusion, hollow nanoscale spheres and single-crys-
the effect of dimensional quantization as the average PbTe tal tubular PbTe have been synthesised via facile solvother-
nanocrystal radius decreases. This phenomenon has also mal processes for the first time. Two different models, tem-
been observed by others.[ Figure 3 (b) shows the far infra- plate-interface and self-generated template routes, have
25]
red spectrum of the hollow PbTe nanospheres. The peaks been proposed to explain the formation mechanism of hol-
Ϫ1
(
marked with asterisks) at 81 and 110 cm are related to low PbTe nanospheres and nanotubes. The present study
[26]
two TeO infrared active modes. The other peaks in part will not only open up great opportunities for novel morpho-
2
b of Figure 3 corresponded to the infrared active modes of logies of PbTe and their applications to be investigated, but
[25]
the hollow PbTe nanospheres. Here, the wide band from might also be extended to the synthesis of hollow spheres
Ϫ1
5
0 to 70 cm is split into two bands appearing at 57 and and nanotubes of other materials.
Eur. J. Inorg. Chem. 2004, 4521Ϫ4524
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G. Zou, Z. Liu, D. Wang, C. Jiang, Y. Qian
FULL PAPER
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Experimental Section
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typical procedure, Pb(NO
(
(
3
)
2
(0.5 mmol) and Na
0.5 mmol) were added to a mixture of ether (18 mL) and glycerol
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2 4 2
TO ·2H O
[7]
[
[
8]
9]
mulsion phase. After this, hydrazine (10 mL) was added to the mic-
roemulsion system and stirring was continued for 10 min. Finally,
the mixture was transferred into a 60 mL Teflon-lined autoclave
and the autoclave was sealed and heated at 160 °C for 20 h. The
autoclave was then allowed to cool to room temperature without
assistance. The grey precipitate was filtered and washed several
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ies and was then dried in vacuo at 60 °C for 4 h. Finally the product
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˚
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croscopy (TEM) using a Hitachi H-80 instrument. The high-resolu-
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ethanol and were ultrasonicated before TEM and HRTEM. The
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Laser MicroRaman Spectrometer with 514.5 nm radiation from a
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[
[
24]
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[
26]
Acknowledgments
The project was supported by the Chinese Natural Science
Foundation. The 973 project of China and the State key project
of fundamental research for nanomaterials and nanostructures are
gratefully acknowledged. The authors gratefully acknowledge in-
formative discussions with Professor Jian Zuo.
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Received May 14, 2004
Early View Article
Published Online October 1, 2004
4524
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Eur. J. Inorg. Chem. 2004, 4521Ϫ4524