PbTe nanostructures: Microwave-assisted synthesis by using lead Schiff-base precursor, characterization and formation mechanism

Article abstract of DOI:10.1016/j.crci.2013.03.017

Pure cubic phase lead telluride (PbTe) nanostructures have been produced by using a Schiff-base complex as a precursor in the presence of microwave irradiation. The Schiff base used as ligand was derived from salicylaldehyde and ethylenediamine. The Schiff-base complex was marked as [Pb(salen)]. In addition, the effect of the irradiation time and the type of reducing agent on the morphology and purity of the final products was investigated. The as-synthesized PbTe nanostructures were characterized extensively by techniques like X-ray powder diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). The microwave formation mechanism of the PbTe nanostructures was studied by XRD patterns of the products. Although it was found that both ionic and atomic mechanisms could take place for the preparation of PbTe, the main steps were according to the atomic reaction process, which could occur between elemental Pb and Te.

Full text of DOI:10.1016/j.crci.2013.03.017

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Full paper/M e´ moire
PbTe nanostructures: Microwave-assisted synthesis by using lead
Schiff-base precursor, characterization and formation mechanism
a
b,c,
a
Shahla Ahmadian-Fard-Fini , Masoud Salavati-Niasari *, Azam Monfared ,
Fatemeh Mohandes
b
a
Department of Chemistry, Payame Noor University, P. O. Box. 19395-3697 Tehran, Iran
b
Department of Inorganic Chemistry, Faculty of Chemistry, University of Kashan, P. O. Box. 87317-51167 Kashan, Iran
c
Institute of Nano Science and Nano Technology, University of Kashan, P. O. Box. 87317-51167 Kashan, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
Pure cubic phase lead telluride (PbTe) nanostructures have been produced by using a
Schiff-base complex as a precursor in the presence of microwave irradiation. The Schiff
base used as ligand was derived from salicylaldehyde and ethylenediamine. The Schiff-
base complex was marked as [Pb(salen)]. In addition, the effect of the irradiation time and
the type of reducing agent on the morphology and purity of the final products was
investigated. The as-synthesized PbTe nanostructures were characterized extensively by
techniques like X-ray powder diffraction (XRD), transmission electron microscopy (TEM),
and scanning electron microscopy (SEM). The microwave formation mechanism of the
PbTe nanostructures was studied by XRD patterns of the products. Although it was found
that both ionic and atomic mechanisms could take place for the preparation of PbTe, the
main steps were according to the atomic reaction process, which could occur between
elemental Pb and Te.
Received 9 January 2013
Accepted after revision 28 March 2013
Available online xxx
Keywords:
Nanostructures
Semiconductor
Chalcogenide
Nanoparticle
Lead telluride
ß 2013 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
1
. Introduction
In the past decades, nano-sized semiconductors have
(46 nm) [2–6]. Up to now, various methods have been
explored to synthesize lead telluride nanostructures, such
as room-temperature synthesis process in an alkaline
aqueous solution [7,8], electrochemical deposition without
any template [9], sonochemical [10,11], pulsed laser
fragmentation of PbTe microstructures [12], and alkaline
reducing chemical route [13].
In this investigation, PbTe nanostructures have been
fabricated by a microwave-assisted method. The use of
microwave energy to heat chemical reactions has attracted
a considerable amount of attention due to its many
advantages, like application in chemistry, material
sciences, nanotechnology, and biochemical processes
[14–20]. There are many advantages in the microwave-
assisted approaches, such as very short reaction times, fast
heat transformation, avoidance of local overheating, and
controllable reaction temperature. Although Palchik et al.
attracted significant attention due to their size-tunable
optical properties, which result from three-dimensional
quantum confinement [1]. Therefore, it is essential to
produce and study materials existing in the limit of strong-
quantum confinement. Compared with many traditional
II–VI and III–V semiconductors, the IV–VI lead chalcogen-
ide nanostructures have smaller band-gaps and larger Bohr
radius. Lead telluride (PbTe) as
a member of the
chalcogenide family has been widely applied in many
fields, such as infrared detectors, photo-resistances, lasers,
and thermoelectric materials, because of its small band-
gap (0.31 eV at 300 K) and larger Bohr excitation radius
[21], Cao et al. [22], and Kerner et al. [23] have synthesized
*
Corresponding author.
E-mail address: salavati@kashanu.ac.ir (M. Salavati-Niasari).
PbTe nanostructures in the presence of microwave
1
631-0748/$ – see front matter ß 2013 Acad e´ mie des sciences. Published by Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.crci.2013.03.017
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
using lead Schiff-base precursor, characterization and formation mechanism. C. R. Chimie (2013), http://dx.doi.org/
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irradiation by using lead acetate as Pb source, we applied a
Schiff-base complex as a lead precursor.
several times, and finally dried at 50 8C in vacuum. The H
2
1
1
ligand was analyzed by H NMR and FT-IR. H NMR
(400 MHz, CDCl ) data for the H salen are as follows:
13.20 (s, 2H, OH); 8.57 (s, 2H, –CH5N–); 6.85–7.38 (m, 8H,
aromatic); 4.00 (s, 4H, –CH –N5C).
A typical procedure for the synthesis of [Pb(salen)]
complex is as follows: 2 mmol of Pb(NO were dissolved
in 50 mL of methanol. A stoichiometric amount of the
salen dissolved in an equal volume of methanol and
2 mL of thriethylamine (Et N) were added dropwise into
Today, coordination compounds have been widely used
as precursor for the preparation of various nanostructures
to control shape and size distribution [24–30]. Although
lead oleate as a coordination compound has been applied
to the synthesis of PbTe nanocrystals via an injection
solution-phase route [31–33], [Pb(salen)] was used as a
precursor to fabricate PbTe nanostructures for the first
time. In addition, Te powder and propylene glycol (PG)
were utilized as a Te source and a solvent, respectively. To
go further into the study, the effect of irradiation time and
type of reducing agent on the morphology and purity of
PbTe was investigated.
3
2
d:
2
3 2
)
H
2
3
the above solution under magnetic stirring. The as-
obtained yellow precipitate was filtered, washed several
times with methanol and distilled water to remove
impurities, and then dried in vacuum at 50 8C for 12 h.
The as-produced lead precursor was characterized by FT-
2
. Experimental
IR. The formation reactions of the H
2
salen ligand and the
[Pb(salen)] complex were shown in Scheme 1.
2.1. Materials and physical measurements
2.3. Synthesis of pure cubic phase lead telluride
Pb(NO
lamine, Te powder, propylene glycol, hydrazine hydrate
O), and KBH were purchased from Merck (pro-
analysis) and used without further purification. Fourier
transform infrared spectra were recorded using KBr pellets
on an FT-IR spectrometer (Magna-IR, 550 Nicolet) in the
3
)
2
, salicylaldehyde, ethylenediamine, thriethy-
nanostructures
(N
2
H
4
ꢀH
2
4
In a typical process, 0.5 g of NaOH was dissolved in
50 mL of distilled water. Then, 0.001 mol of Te powder and
3 mL of hydrazine hydrate (100%) were added into the
above solution under gentle heating by magnetic stirring.
Then, 0.001 mol of the [Pb(salen)] complex dissolved in
30 mL of propylene glycol (PG) was added dropwise into
the solution. The reaction flask was held at the center of a
microwave system. The exposure time was done at 5, 30,
120 min. After cooling the flask to room temperature, the
gray powder was collected by filtering, washed with
distilled water and methanol, and finally dried at 50 8C in
vacuum. For the sake of comparison, several experiments
ꢁ1
4
00–4000 cm
patterns were collected with a Philips diffractometer using
X’PertPro and the monochromatized Cu K radiation
= 1.54 A). The microscopic morphology of the products
range. Powder X-ray diffraction (XRD)
a
˚
(l
was visualized by a LEO 1455VP scanning electron
microscope (SEM). Energy dispersive X-ray spectrometry
(EDS) analysis was done by XL30, Philips microscope.
Transmission electron microscope (TEM) images were
obtained on a JEM-2100 with an accelerating voltage of
4
were carried out with KBH instead of hydrazine hydrate
6
0–200 kV equipped with a high-resolution CCD Camera.
under the same conditions. In this method, the microwave
oven worked in the following cycling mode: on for 10 s, off
for 10 s. To investigate the effect of microwave power on
the morphology of PbTe, two experiments were carried
out at 600 W and 900 W. The preparation conditions for
the synthesis of PbTe nanostructures were illustrated in
Table 1.
1
The H NMR spectrum of the Schiff base N,N-bis(salicy-
lidene)-ethylenediamine, H salen, was recorded with a
Bruker (400 MHz) instrument in CDCl The optical
properties of the PbTe nanoparticles were studied with a
PerkinElmer spectrophotometer.
2
3
.
2.2. Synthesis of the H
2
salen ligand and the [Pb(salen)]
complex
3. Results and discussion
A typical procedure for the synthesis of H
2
salen ligand
3.1. X-ray powder diffraction studies
is as follows: a solution containing 2 mol of ethylenedia-
mine in 50 mL of methanol was added dropwise into a
solution involving 4 mol of salicylaldehyde in 50 mL of
methanol. The mixture was refluxed under magnetic
stirring for 3 h to produce a yellow precipitate. The yellow
In order to study the effect of irradiation time on the
purity of PbTe nanostructures, the microwave power was
kept constant at 750 W and the irradiation time was
changed from 30 min to 120 min. Fig. 1a shows the XRD
pattern of the product prepared by using hydrazine
2
H salen precipitate was filtered and washed with methanol
2
Scheme 1. Chemical reactions for the synthesis of the H salen and [Pb(salen)].
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
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Table 1
Preparation conditions of PbTe nanostructures.
Sample no
Lead source
Microwave
power (W)
Irradiation
time (min)
Reducing agent
Particle size (nm)
1
2
3
4
5
6
7
8
9
[Pb(salen)]
[Pb(salen)]
[Pb(salen)]
[Pb(salen)]
[Pb(salen)]
[Pb(salen)]
[Pb(salen)]
750
750
750
750
750
750
750
750
600
900
750
30
60
N
N
N
N
2
2
2
2
H
H
H
H
4
4
4
4
ꢀH
ꢀH
ꢀH
ꢀH
2
2
2
2
O
O
O
O
ꢂ25
55–60
90
120–150
120
60
12–15
KBH
KBH
KBH
KBH
4
15–18
90
4
4
4
8–10
120
120
60
8–12
Pb(NO
3
)
2
ꢂ15
[Pb(salen)]
[Pb(salen)]
N
2
N
2
N
2
H
4
ꢀH
2
O
O
90–92
1
1
0
1
60
ꢀH
2
O
95–100
Pb(NO
3
)
2
60
H
4
ꢀH
2
Agglomerated particles
PbTe: pure cubic phase lead telluride.
hydrate as a reductant after irradiation for 30 min (sample
). The XRD pattern of sample 1 indicated the formation of
cubic phase PbTe (space group Fm 3¯ m JCPDS No. 77-0246).
In Fig. 1a, the peaks marked with ‘‘*’’ correspond to the Te
powder, indicating the presence of untreated elemental Te
with the other reagents. The XRD pattern of the product
synthesized after microwave irradiation for 120 min
(sample 4) is displayed in Fig. 1b. All of the diffraction
peaks in Fig. 1b can be indexed to pure cubic PbTe, with cell
1
˚
constants a = b = c = 6.4610 A (JCPDS card No. 77-0246).
These results indicated that the optimum irradiation time
to produce pure PbTe was 120 min at 750 W. Besides
4
hydrazine hydrate, the experiment was done by KBH as a
reducing agent. Fig. 1c shows the XRD pattern of the
product produced at 750 W after irradiation for 120 min in
the presence of KBH4 (sample 7). All of the diffraction
peaks in Fig. 1c can be attributed to pure cubic PbTe (JCPDS
card No. 77-0246). Therefore, by using hydrazine hydrate
4
and KBH after irradiation for 120 min at 750 W, pure PbTe
nanostructures have been obtained. To investigate the
effect of lead precursor on the formation of PbTe
nanostructures, one experiment was performed by using
Pb(NO
pattern of the product synthesized by Pb(NO
presence of KBH as a reductant after irradiation for
20 min at 750 W (sample 8) is shown on Fig. 1d. The XRD
pattern of sample 8 indicated the formation of PbTe along
with Na (BO (OH)). In the XRD pattern of sample 8
synthesized with Pb(NO (Fig. 1d), the presence of
3
)
2
salt instead of the [Pb(salen)] complex. The XRD
3 2
)
in the
4
1
2
2
3 2
)
elemental untreated Te was observed. According to the
XRD results, it was found that pure PbTe could be obtained
by using the [Pb(salen)] complex as a precursor after
irradiation for 120 min at 750 W. The crystalline size of the
as-synthesized PbTe (Dc) was calculated from the major
diffraction peak in the XRD pattern (200) using the Debye–
Scherrer formula (Equation 1) [34].
D
c
¼ K
l=b
cosu
(1)
where K is a constant (0.9);
l
is the X-ray wavelength used
˚
in XRD (1.5418 A);
u is the Bragg angle; b is the pure
diffraction broadening of a peak at half-height, that is the
broadening due to the crystalline dimensions. The crystal
diameters of samples 1, 4, 7, and 8 calculated by the
Debye–Scherrer formula were about 25, 12, 10, 15 nm,
respectively. The XRD results indicated that by using the
[Pb(salen)] complex as a precursor at 750 W in the
presence of N O and KBH as a reducing agent, pure
2
H
4
ꢀH
2
4
PbTe nanostructures could be produced after microwave
irradiation for 120 min.
Fig. 1. X-ray powder diffraction patterns of (a) sample 1, (b) sample 4, (c)
sample 7, and (d) sample 8.
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3.2. Scanning electron microscopy and transmission electron
particle size of sample 2 are irregular. The morphology of
microscopy images
sample 2 is composed of particle-like and rod-like micro/
nanostructures. By increasing the irradiation time from
60 min to 90 min (sample 3), only the presence of micro-
sized particles was observed (Fig. 2b). When the irradiation
time was 120 min (sample 4), PbTe nanoparticles with
grain size in the 12–15-nm range were produced (Fig. 2c).
In addition, the effect of irradiation time on the morphol-
To study the effect of irradiation time on the
morphology of the products, SEM images of samples 2–4
produced by N
2
H
4
ꢀH
2
O as a reducing agent were taken and
presented in Figs. 2a–c, respectively. Fig. 2a shows a SEM
image of the product synthesized at 750 W after irradiation
for 60 min (sample 2). As shown in Fig. 2a, morphology and
4
ogy of PbTe synthesized by KBH as a reductant was
investigated by SEM images. Fig. 3a shows SEM image of
Fig. 2. Scanning electron microscopy images of (a) sample 2, (b) sample 3,
Fig. 3. Scanning electron microscopy images of (a) sample 5, (b) sample 6,
and (c) sample 4.
and (c) sample 7.
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Fig. 4. (a–c) Transmission electron microscopy images and (d) SAED pattern of PbTe nanostructures obtained from sample 4. The rings are characteristic of
well-crystallized polycrystals.
the product generated by KBH
4
after irradiation for 60 min
When microwave irradiation was 120 min in the same
conditions, PbTe nanoparticles with a particle size about
8–12 nm were formed (Fig. 3c). To further investigate the
morphological details, TEM images were taken and shown
in Fig. 4. Panels a, b, and c of Fig. 4 are TEM images of
(sample 5). In Fig. 3a, it can be seen that the morphology of
sample 5 is that of particle-like structures, with a particle
size between 15 and 18 nm. Fig. 3b presents the SEM image
of the product fabricated by KBH
4
after microwave
irradiation for 90 min (sample 6). According to the SEM
image displayed in Fig. 3b, the grain size of PbTe
nanoparticles from sample 6 is between 8 and 10 nm.
sample 4 prepared by N
2
H
4
ꢀH
2
O as a reducing agent after
irradiation for 120 min. It is possible to detect particle-like
and hexahedron PbTe nanostructures in these images. A
closer observation of the morphology of sample 4 (Fig. 4c)
reveals that the particle size of the PbTe nanostructures is
between 10 and 15 nm. These nanostructures are well-
crystallized, as it can be observed from the selected area
electron diffraction pattern displayed in Fig. 4d. After all, it
was concluded that by increasing the irradiation time from
6
0 to 120 min by using KBH
4
instead of H
2
H
4
ꢀH
2
O, the
particle size of the PbTe nanostructures decreased.
The effect of microwave power on the morphology of
PbTe nanostructures prepared by using a microwave-
assisted aqueous solution method has been studied.
Figs. 5a and 5b show SEM images of the products
synthesized at 600 W (sample 9) and 900 W (sample 10)
after irradiation for 60 min, respectively. According to
these images, it was found that the morphology of samples
9
and 10 synthesized at 600 and 900 W is composed of
cubic-like and rod-like nanostructures. In fact, morphology
of PbTe nanostructures shifted to nanocubes and nanorods
with decreasing and increasing microwave power from
750 W.
3.3. FT-IR spectra
Fig. 6 shows FT-IR spectra of the as-synthesized
ꢁ1
samples in the range of 400–4000 cm range. The FT-IR
spectrum of the H salen ligand is shown on Fig. 6a. The
strong peak at 1632 cm
2
ꢁ1
is attributed to
v(C5N) as a
characteristic band for the Schiff-base ligand. Fig. 6b shows
the FT-IR spectrum of the [Pb(salen)] complex. In Fig. 6b,
ꢁ1
the strong band at 1628 cm is assigned to
v(C5N). The
ring skeletal vibrations (C5C) of Schiff-base ligand were
ꢁ
1
observed in the region between 1439 and 1533 cm . The
Fig. 5. Scanning electron microscopy images of the products synthesized
ꢁ
1
at 600 W (a) and 900 W (b).
phenolic C–O stretching vibration is seen at 1277 cm in
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
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were observed in the FT-IR spectrum of the PbTe
nanostructures.
3.4. Energy dispersive X-ray spectrometry patterns
Figs. 7a and 7b show EDS patterns of the PbTe
nanoparticles obtained from samples 4 and 7, respectively.
The EDS patterns in Fig. 7 indicate that the elements in
samples 4 and 7 are Pb and Te only (Si, and C signals are
from the substrate). The EDS results gave a rough atomic
Pb:Te ratio close to 1:1, confirming the purity of the
nanoparticles.
3.5. Optical property of pure cubic phase lead telluride
nanoparticles
The UV–vis spectrum of the PbTe nanoparticles
dispersed in ethanol was recorded using a quartz cell (1-
cm path length). Fig. 8a depicts the UV–vis spectrum of the
PbTe nanoparticles with a particle size between 10 and
15 nm (sample 4). This spectrum shows a strong absorp-
tion peak in the visible region. For the PbTe nanoparticles,
the absorption peak shifted to 694 nm, which is higher
than that of PbTe nanorods synthesized by Sridharan et al.
2
[38]. Fig. 8b shows the plot of (
a
hy
) versus photon energy
(
g
hy) for the PbTe nanoparticles. The band-gap value (E ) of
sample 4 was calculated by extending the linear part of the
curve to zero absorption. The band-gap estimated of the
PbTe nanoparticles was about 1.2 eV. By comparing the
band-gap value of the PbTe nanoparticles with band-gap
values of PbTe microstructures [9] and hierarchical
microcrystals [39], it was found that the band-gap value
of the PbTe nanoparticles synthesized according to this
method has a blue shift. This blue shift can be related to the
small crystallite size of the PbTe nanoparticles (10–15 nm),
which leads to a quantum confinement effect [40]. The
large excitonic Bohr radius of about 46 nm in PbTe allows a
strong-quantum confinement within a large range of
particle sizes.
2
Fig. 6. FT-IR spectra of (a) H salen ligand, (b) [Pb(salen)] precursor, PbTe
nanostructures obtained from samples 4 (c) and 7 (d).
the FT-IR spectrum of the Schiff-base ligand [35]. The FT-
IR spectrum of the [Pb(salen)] complex presented on
Fig. 6b showed evidence of the synthesis of the complex,
ꢁ
1
since the bands centered at 1630 cm
(attributed to
ꢁ
1
ꢁ1
v
C5N), 1533 cm
attributed to
the [Pb(salen)] complex. By comparing the frequency
attributed to the (C5N) in the IR spectrum of the H salen
Fig. 6a) and [Pb(salen)] (Fig. 6b), it was found that this
band shifted to lower frequency and appeared at
(attributed to
v
C5C) and 1277 cm
3.6. The formation mechanism of pure cubic phase lead
telluride nanostructures
(
v phenolic C–O) are characteristic peaks of
v
2
In this work, a Schiff-base complex was synthesized,
characterized, and then used as a lead precursor to
fabricate PbTe nanostructures. When the [Pb(salen)]
(
ꢁ
1
2+
1
628 cm in the IR spectrum of the [Pb(salen)], indicating
molecules were dissolved in propylene glycol, Pb ions
2
ꢁ
the coordination of the nitrogen atom in the C5N group to
the Pb ion. Figs. 6c and 6d show the FT-IR spectra of the
PbTe nanoparticles obtained from samples 4 and 7,
respectively. In Fig. 6c, the broad absorption band
capped by salen were released. In fact, the Schiff-base
molecules were applied as surfactants due to their steric
hindrance effect. By adding the solution including the Te
source to the [Pb(salen)] solution, PbTe nuclei were formed
in the presence of microwave irradiation. It was found that
PbTe can be formed by two independent pathways,
involving ionic and atomic processes. In the ionic process,
ꢁ1
centered at 3437 cm and the weak peaks at 1634 and
ꢁ
1
6
79 cm
are attributable to the v(OH) stretching and
bending vibrations, respectively, which indicates the
presence of physisorbed water molecules linked to the
PbTe nanoparticles [36]. The presence of a broad peak at
2
+
2ꢁ
Pb ions react with Te ions to generate PbTe nuclei.
Besides, a directly atomic process can proceed between
ꢁ
1
2+
3
396 cm in Fig. 6d is attributed to the
v
(OH) stretching
elemental Te and Pb. It must be remembered that the Pb
vibration. The observation of some weak bands located at
ions released from the lead precursor can be reduced by
ꢁ
1
1
457, 1135, 1043, 839 and 469 cm could be attributable
to the absorptions of water and CO molecules from the
atmosphere [37]. No characteristic peaks of impurities
N
2
H
4
ꢀH
2 4
O and KBH [41–43]. The formation mechanism of
2
PbTe through both ionic and atomic processes results from
redox reactions. According to the standard redox potential
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
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Fig. 7. Energy dispersive X-ray spectrometry spectra of samples 4 (a) and 7 (b).
2
Fig. 8. (a) UV–vis spectrum and (b) the plot of (
a
hv) versus hv for sample 4.
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
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2
ꢁ
of Te/Te
powder cannot be efficiently reduced to Te
hydrazine hydrate because of the close redox potentials of
(ꢁ1.143 V) and N
2
/N
2
H
4
(ꢁ1.15 V), the Te
pattern shows the presence of elemental Pb in the product.
This result proved that the Pb ions could be reduced by
2
ꢁ
2+
ions by
hydrazine hydrate. Besides elemental Pb, lead oxide, lead
hydroxide and carbonate are seen on Fig. 9a due to the
presence of NaOH in the solution. In this case, there is a
question. Why did not elemental lead exist in the final
products synthesized in the presence of Te powder?
According to the atomic formation mechanism of PbTe,
elementalPb and Te could react witheachother toformPbTe.
During the formation process of PbTe, Te powder
quickly dissolved into strong alkaline hydrazine hydrate,
2
+
2 4
Te and N H . On the other hand, Pb ions can be easily
0
2+
reduced by hydrazine hydrate (E Pb /Pb = ꢁ0.126 V). To
study the formation mechanism of the PbTe nanostructures,
four blank tests were done under microwave irradiation at
7
50 W for 1 h in PG as a solvent. One experiment (blank test
No. 1) was carried out with the aid of [Pb(salen)], hydrazine
hydrate and NaOH without using Te powder. The XRD
pattern of the blank test No. 1 is shown on Fig. 9a. The XRD
Fig. 9. X-ray powder diffraction patterns of blank tests No. 1 (a) and No. 2 (b).
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
using lead Schiff-base precursor, characterization and formation mechanism. C. R. Chimie (2013), http://dx.doi.org/
1
0.1016/j.crci.2013.03.017

G Model
CRAS2C-3742; No. of Pages 11
S. Ahmadian-Fard-Fini et al. / C. R. Chimie xxx (2013) xxx–xxx
9
whereas the color of the solution quickly turned purple
black. To study the effect of the alkaline solution on this
method, one experiment was done in the absence of NaOH.
Fig. 9b shows the XRD pattern of the product obtained
without using NaOH (blank test No. 2). As shown in Fig. 9b,
the XRD pattern of the product indicates the formation of
cubic phase PbTe (space group Fm 3¯ m, JCPDS No. 77-0246)
along with hexagonal-phase Te (JCPDS No. 79–0736).
Based on this result, we evidenced the presence of Te
powder in the solution prepared without NaOH. In fact, the
addition of NaOH accelerated the dissolution process of the
Te powder. The disproportional reaction of Te can produce
2
ꢁ 2ꢁ
TeO
hydrate or KBH
3
ions, which can reduce to Tea+1 ions by hydrazine
2
ꢁ
ions with purple black color
4
. Tea+1
2
ꢁ
consist of Te and Te ions [41]. To prove the reducing
effect of hydrazine hydrate for Te, blank test No. 3 was
done in the presence of Te powder, hydrazine hydrate and
NaOH, without using lead precursor. The presence of NaH
as a by-product in the XRD pattern of the blank test No. 3
(Fig. 10a) indicated that hydrazine hydrate molecules
Fig. 10. X-ray powder diffraction patterns of blank tests No. 3 (a) and No. 4 (b).
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
using lead Schiff-base precursor, characterization and formation mechanism. C. R. Chimie (2013), http://dx.doi.org/
1
0.1016/j.crci.2013.03.017

G Model
CRAS2C-3742; No. of Pages 11
1
0
S. Ahmadian-Fard-Fini et al. / C. R. Chimie xxx (2013) xxx–xxx
Table 2
Preparation conditions and corresponding products of blank tests Nos. 1–4.
Blank test no.
[Pb(salen)]
Te powder
2
N H
4
.H
2
O
NaOH
Products
Figure no.
1
2
3
4
Yes
Yes
No
No
Yes
Yes
Yes
Yes
Yes
No
Pb, PbO, PbCO
Te, PbTe
3
, Pb(OH)
2
9a
Yes
Yes
Yes
9b
Yes
Yes
Te, NaH
10a
10b
H
2
salen
2 3
Te, Na CO
could release hydride ions to reduce Te powder. In this
work, [Pb(salen)] was applied as a lead precursor. The
-
2
-
2ꢁ
3
3
Te þ 6OH ! 2Te þ TeO þ 3H
2
O
2ꢁ
ꢁ
2ꢁ
TeO þ N
2
H
4
þ 4OH ! Te
þ N
2
þ 4H
2
O
2+
3
aþ1
[
Pb(salen)] precursor could release Pb ions after dissol-
2ꢁ
2-
Te
! Te þ Te
aþ1
ving in propylene glycol. To find what happened to the N,N-
bis(salicylidene)-ethylenediamine ligand during the
microwave treatment, the experiment was carried out
ꢁ
2þ
N
2
H
4
þ 4OH þ 2Pb ! 2Pb þ N
2
þ 4H
2
O
2þ
2-
Pb þ Te ! PbTe
Pb þ Te ! PbTe
with H
2
salen, N
2
H
4
ꢀH
2
O, NaOH, Te powder under micro-
wave irradiation. This experiment’s product was marked as
blank test No. 4. The XRD pattern of blank test No. 4 can be
seen in Fig. 10b. According to this pattern, it was found that
When KBH4 was used instead of N H ꢀH O, the main
2
4
2
reactions were as follows:
the H
gases like H
2
salen ligand decomposed to carbonate and other
O and CO under microwave irradiation. The
-
2
-
2ꢁ
3
2
2
3
Te þ 6OH ! 2Te þ TeO þ 3H
2
O
2
3
ꢁ
-
ꢁ
XRD results of the blank tests were illustrated in Table 2.
Based on the XRD results and experimental observations,
the main steps for the formation of the PbTe nanostruc-
TeO þ BH ! Te þ H
2
BO þ H
2
4
3
ꢁ
ꢁ
2þ
ꢁ
2
BH þ 6OH þ 3Pb ! 3Pb þ 2H
2
BO þ 5H
2
4
3
2þ
2ꢁ
Pb þ Te ! PbTe
Pb þ Te ! PbTe
tures with the aid of N
2
H
4
ꢀH
2
O are as follows:
Therefore, both atomic and ionic processes could take
place in the formation mechanism and growth of the PbTe
nanostructures.
3.7. Influence of lead precursor on the morphology of pure
cubic phase lead telluride
To investigate the effect of the lead precursor on the
3 2
morphology of PbTe, Pb(NO ) was used instead of the
Schiff-base complex in the same conditions. Although in
this experiment the preparation conditions (sample 11)
were alike those used to synthesize sample 2 in Table 1,
3 2
Pb(NO ) was used instead of the [Pb(salen)] complex. SEM
images of sample 11 with different magnifications are
given in Figs. 11a and 11b. Based on the SEM images
presented in Fig. 11, it can be seen that PbTe nanoparticles
with high agglomeration have been obtained by using
3 2
Pb(NO ) . The particles are highly agglomerated, and it is
difficult to measure the individual particle size. By
comparing the SEM images of samples 2 (Fig. 2a) and 11
(
Fig. 11), it was found that the Schiff-base complex could
protect the PbTe nanoparticles from agglomeration due to
its steric hindrance effect. In fact, the Schiff-base molecules
could act as surfactant in the solution.
4. Conclusion
In summary, PbTe nanostructures have been synthe-
sized with the aid of a Schiff-base complex, Te powder and
a reducing agent through a microwave-assisted route. The
effects of N
2
H
4
ꢀH
2 4
O and KBH (as reducing agents), of the
microwave power and of the irradiation time on the
morphology and purity of the PbTe nanostructures were
studied. According to SEM micrographs, it was found that
Fig. 11. (a, b) scanning electron microscopy images of the product
synthesized with Pb(NO
)
3 2
.
4
KBH as a reducing agent plays a key role in controlling the
Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
using lead Schiff-base precursor, characterization and formation mechanism. C. R. Chimie (2013), http://dx.doi.org/
1
0.1016/j.crci.2013.03.017

G Model
CRAS2C-3742; No. of Pages 11
S. Ahmadian-Fard-Fini et al. / C. R. Chimie xxx (2013) xxx–xxx
11
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pure PbTe nanostructures was 120 min. The XRD results of
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Please cite this article in press as: Ahmadian-Fard-Fini S, et al. PbTe nanostructures: Microwave-assisted synthesis by
using lead Schiff-base precursor, characterization and formation mechanism. C. R. Chimie (2013), http://dx.doi.org/
1
0.1016/j.crci.2013.03.017