preparation of zns spherical nanoparticles

Journal of Applied Chemical Research, 7, 4, 63-70 (2013)
Journal of
Applied
Chemical
Research
www.jacr.kiau.ac.ir
Development of a Mild Hydrothermal Method toward
Preparation of ZnS Spherical Nanoparticles
Leila Vafayi
1
, Soodabe Gharibe
1
, Shahrara Afshar
2

1
Department of Science, Islamic Azad University, Firoozkooh Branch, Iran.
2
Department of Chemistry, Iran University of Science and Technology, Tehran, Iran.
Received 13 Jul. 2013; Final version received 11 Aug.2013
Abstract
In this work, ZnS nanostructures of various morphologies have been synthesized via a mild
hydrothermal method. The effects of surfactants such as sodium dodecyl sulfate (SDS)
(anionic) and cetyl-trimethylammonium bromide (CTAB) (cationic), the type of source of
Zn and S on the morphology of ZnS were studied. The products were characterized by XRD,
SEM, EDX, TEM analysis and UV-vis spectroscopy. The XRD patterns showed that the
ZnS samples have a zinc blende structure. The results showed that the ZnS samples are in
spherical, sheet, nano and submicrorod forms.
Keywords: ZnS, Surfactant, Nanosphere, Hydrothermal, TEM.
*Corresponding author: Leila Vafayi, Department of Science, Firoozkooh Branch, Islamic Azad University, Firoozkooh, Iran. Email:
Tel: +98 21 7644 3869, Fax: +98 21 7644 2868.

Introduction
Semiconductors are a class of materials
dened primarily by their electronic properties.
During the past two decades, Nanocrystalline
semiconductor particles have attracted
considerable because of their novel properties,
such as large surface-to-volume ratio and the
three dimensional connement of the electrons
[1-5].
In metals and other conductors, the conduction
and valence bands overlap, without a signicant
energy barrier for promoting electrons from the
valence to the conduction band. In insulators,
there is a large energy barrier for promoting
electrons from the valence to the conduction
band, essentially eliminating conduction. In
semiconductors, however, the energy barrier for
conduction is intermediate between conductors
and insulators. Typically, the band gaps (Eg)
for metals, semiconductors, and insulators are
less than 0.1 eV, between 0.5 and 3.5 eV, and
greater than 4 eV, respectively.
L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013)
64
II-VI semiconductor nanocrystals attract much
attention because of their size dependent photo-
and electro- luminescence properties and
promising applications in optoelectronics [6-
9]. Among the family of II–VI semiconductors,
zinc sulde semiconductor is an important

member of this family because of their
favorable electronic and optical properties for
optoelectronic applications. ZnS can have two
different crystal structures (zinc blende and
wurtzite); both have the same band gap at 340 nm
(3.66 eV) and the direct band structure. ZnS has
been used widely as an important phosphor for
photoluminescence (PL), electroluminescence
(EL) and cathodoluminescence (CL) devices
due to its better chemical stability compared
to other chalcogenides such as ZnSe. In
optoelectronics, it nds use as light emitting
diode, reector, dielectric lter and window
material [10-13].
Nanocrystalline ZnS can be prepared by various
methods such as sonochemical preparation
[14], coevaporation [15], wet chemical process
[16], sol-gel [17] solid state [18], micro-wave
irradiation [19], ultrasonic irradiation [20] or
synthesis under high-gravity environment [21].
The properties and applications of nano
structured semiconductors are strongly
dependant on their crystal phase, size,
composition and shape. Therefore synthesizing
of highly tuned nanocrystals has been a
challenging topic.
Here in, we describe a mild hydrothermal
method for the synthesis of spherical, sheet,
nano and submicrorod ZnS structures. Effects
of the zinc and sulfur sources and surfactant on

the morphology and size of ZnS nanostructures
have been investigated.
Experimental
All reagents were analytical grade and
purchased from Merck Company. Reagents
were used without any further purication.
In order to obtain zinc sulphide particles with
various morphologies the reaction factors
such as: Zinc sources, sulfur sources and
type of the surfactant have been investigated.
The synthesis conditions are reported in the
following sections.
It is noteworthy the Zn to S molar ratio has
been set to be 1 in all experiments.
Preparation of zinc sulde by keeping the type
of the zinc source
In this section, zinc chloride as the zinc source
and thiourea, thioacetamide, sodium sulde
as the sulfur source were employed in the
reactions.
In three different experiments, a solution of 3
mmol ZnCl
2
in 20 mL deionized distilled water
was added to a solution of 3 mmol of sulfur
salt in 20 mL deionized distilled water under
stirring. The mixture was transferred into an
autoclave, which was lled with distilled water
up to 70% of the total volume. The autoclave
was sealed and kept at 120 °C for 5 h. After

L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013)
65
cooling the system to room temperature, the
product was separated by centrifugation,
washed with absolute ethanol and deionized
water for several times, and then dried under
vacuum at 70 °C for 10 h.
Preparation of zinc sulde by keeping the type
of the sulfur source
In this experimental section, sodium sulde as
the sulfur source and zinc nitrate, zinc acetate as
the zinc source were employed in the previous
section reactions in the same condition.
Preparation of zinc sulde nanospheres using
SDS and CTAB surfactants
In a typical synthesis, a solution of 3 mmol
ZnCl2 in 20 mL deionized distilled water was
added to a solution of surfactants with ZnCl
2
to
SDS or CTAB ratio equal to 1 prepared in 20
mL of deionized water under stirring. Then, 3
mol of Na
2
S in 20 ml deionized distilled water
was added to the above mixture under vigorous
stirring. The mixture was transferred into an
autoclave, sealed and kept at 120 °C for 5 h.
After cooling the system to room temperature,
the product was separated by centrifugation,

washed with absolute ethanol and deionized
water for several times, and then dried under
vacuum at 70 °C for 10 h.
Characterization
The crystal phase and particle size of the
synthesized products, after purication, were
characterized by X-ray diffraction (XRD) using
FK60-04 with Cu Kα radiation (λ= 1.54 Å) in
2q ranges from 5° to 70°, and with instrumental
setting of 35 kV and 20 mA. The morphology
of the nanostructures was observed by emission
scanning electron microscopy (SEM, Philips-
XLφ30) and transmission electron microscopy
(TEM, Philips-CM120). UV-vis spectra of the
ZnS nanosphere were recorded by MPC-2200,
UV2550 UV-vis spectrophotometer in the
wavelength range of 200 to 800 nm.
Results and discussion
The XRD patterns of prepared ZnS samples
are shown in Figure 1. All peaks can be well
indexed to cubic zinc blend with lattice
constant of a=b=c=5.406 Å (JCPDS, No.05-
0566). No other crystalline phase was found in
the XRD patterns, indicating the high purity of
the products.
L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013)
66
Figure 1. XRD patterns of the prepared ZnS samples using: (a) ZnCl
2
and Na

2
S , (b) ZnCl
2
and
thioacetamide, (c) Zn(Ac)
2
and Na
2
S, (d) Zn(NO3)
2
and Na
2
S, (e) ZnCl
2
and thiourea, (f) ZnCl
2
and
Na
2
S with SDS surfactant, (g) ZnCl
2
and Na
2
S with CTAB surfactant, as Zn and S sources, respectively.
The morphology of the products was observed
by SEM and TEM images. The SEM images of
the synthesized products are shown in Figure
2 (a-g). These images clearly demonstrate that
the products are in different morphologies.
Figure 2 (a-b, e) are related to the prepared

samples using ZnCl
2
as of the zinc source
and Na
2
S, thioacetamide and thiourea as
of the sulfur sources, respectively. These
images clearly indicate that the products
are in different morphologies. Since the
experimental condition is the same for all of
them; the only difference between the samples
is the source of the sulfur. Figure 2 (a, c and d)
are related to the prepared samples using Na
2
S
as of the sulfur source and ZnCl
2
, Zn(Ac)
2
and
Zn(NO
3
)
2
as of the zinc sources, respectively.
These images clearly indicate that the products
are in different morphologies. Also, the only
difference between these samples is the source
of the Zinc in the same of the experimental
condition. The results clearly indicate that

when the anions of the metal salts are a
multidentate, the products are not formed in
the shape of spherical.
The SEM images of the synthesized products
using ZnCl
2
as source of Zn and Na
2
S as of
the sulfur with SDS and CTAB surfactants are
shown in Figure 2 (f and g), respectively.
The SEM observations show that the samples
prepared with surfactants are less aggregated.
This can be due to coating of inorganic
core by the surfactant, which prevents the
nanoparticles to aggregate. It is proved that the
prevention of aggregation of nanoparticles in
the presence of the surfactant is more effective
when the surfactant has a long and branched
chain structure [22, 23].
Figure 1. XRD patterns of the prepared ZnS samples using: (a) ZnCl
2
and Na
2
S , (b) ZnCl
2
and
thioacetamide, (c) Zn(Ac)
2
and Na

2
S, (d) Zn(NO
3
)
2
and Na
2
S, (e) ZnCl
2
and thiourea, (f) ZnCl
2
and Na
2
S
with SDS surfactant, (g) ZnCl
2
and Na
2
S with CTAB surfactant, as Zn and S sources, respectively.
L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013)
67
Figure 2. SEM images of ZnS samples prepared using: (a) ZnCl
2
and Na
2
S , (b) ZnCl
2
and
thioacetamide, (c) Zn(Ac)
2

and Na
2
S, (d) Zn(NO3)
2
and Na
2
S, (e) ZnCl
2
and thiourea, (f) ZnCl
2
and
Na
2
S with SDS surfactant, (g) ZnCl
2
and Na
2
S with CTAB surfactant, as Zn and S sources, respectively.
L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013)
68
The SEM images show that the ZnS prepared
with CTAB surfactant is less aggregated
because of its longer and branched chain
structure. The energy dispersive X-ray
(EDAX) analyses of prepared samples conrm
that the Zn and S elemental ratio is 1:1 for all
ZnS samples (Figure 3).
Figure 4 shows the TEM image of the prepared
ZnS nanoparticles using CTAB surfactant. It
is clear that ZnS nanoparticles are spherical

with average diameter of 40 nm.
The optical properties of the semiconductor
nanomaterials depend on the size and the shape
of the particles. To investigate the optical
properties of the as-prepared ZnS samples,
the UV–vis absorption spectra were recorded,
as shown in Figure 5. The optical absorption
spectrum can be used to estimate the bandgap
of the semiconductors. The bandgap for the
prepared ZnS samples was calculated to be
3.76 (330nm) and 3.81 eV (323nm), using
ZnCl
2
and Na
2
S as starting materials without
and with CTAB surfactant, respectively. The
L. Vafayi et al., J. Appl. Chem. Res., 7, 4, 63-70 (2013)
69
absorption edge shows a blue shift to higher
energies as compared with the bulk ZnS
(3.66 eV). This is due to the fact that smaller
particles have larger band gaps and absorb at
shorter wavelengths [24, 25].
Investigation of the spectra reveals that the
synthesized ZnS using CTAB surfactant
(Figure 5b) has a sharper absorption edge.
This is in agreement with the SEM images and
indicates that the synthesized particles can be
considered to be monodispersed.

Conclusion
ZnS nanostructures have been synthesized
using mild hydrothermal method. The
results show that by changing of the sources
of zinc and sulfur from multidentate to
monodentate, the obtained pure ZnS
nanocrystals show a tendency towards ZnS
nanospherical structures. Also, it is concluded
that the ZnS nanospheres prepared using
CTAB surfactant are spherical and can be
considered as monodispersed particles. The
nanoparticles with monodispersed nanosphere
characteristics are appropriate templates
for hollow nanostructures. Therefore, these
hollow nanostructures via removal of ZnS
attract the attention of many researchers in
applying them in drug delivery and catalysis.
Acknowledgments
This article was extracted from the project
entitled: “Synthesis and characterization of
ZnS@SiO
2
core-shell nanocomposite for drug
delivery application”. Financial support for
this project was provided by Islamic Azad
University Firoozkooh branch.
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