Simple thermal evaporation route to synthesize Zn and Cd metal nanowires

Article abstract of DOI:10.1016/j.cplett.2005.11.054

Metallic Zn and Cd nanowires were produced by a simple thermal evaporation process using their respective sulfide powders as the precursors. Metallic nanowires were found to be of pure phase without the presence of any sulfide or oxide phases. These nanowires were single crystalline. Diameters of both the Zn and Cd nanowires varied within 100-150 nm and their lengths were approximately a few micrometers. Crystal structure, phase and chemical purity of the nanowires were characterized by XRD and EDAX. The morphology and crystallinity of the products were revealed by SEM and TEM studies.

Full text of DOI:10.1016/j.cplett.2005.11.054

Chemical Physics Letters 419 (2006) 174–178
www.elsevier.com/locate/cplett
Simple thermal evaporation route to synthesize Zn and Cd metal
nanowires
Soumitra Kar, Tandra Ghoshal, Subhadra Chaudhuri *
Department of Materials Science, Indian Association for the Cultivation of Science, 2A and B Raja SC Mullik Road, Jadavpur,
Kolkata 700 032, West Bengal, India
Received 11 November 2005
Available online 15 December 2005
Abstract
Metallic Zn and Cd nanowires were produced by a simple thermal evaporation process using their respective sulfide powders as the
precursors. Metallic nanowires were found to be of pure phase without the presence of any sulfide or oxide phases. These nanowires were
single crystalline. Diameters of both the Zn and Cd nanowires varied within 100–150 nm and their lengths were approximately a few
micrometers. Crystal structure, phase and chemical purity of the nanowires were characterized by XRD and EDAX. The morphology
and crystallinity of the products were revealed by SEM and TEM studies.
Ó 2005 Elsevier B.V. All rights reserved.
1
. Introduction
reported. Out of these metal nanowires Zn and Cd metals
are of much importance for 1-D nanosystems to study.
Metallic Zn is an important conventional type-I supercon-
ductor and Zn nanowires also reported to have the super-
conducting properties [15]. Cd is another important
material having applications in nickel–cadmium batteries,
in the controlled components of nuclear reactors and in
red-yellow pigments [16]. Zn nanowires were synthesized
by electrodepositing Zn into the pores of different tem-
plates [15], carbothermal reaction of ZnO powder in NH3
atmosphere [17] and Zn nanobelts by carbothermal reduc-
tion of ZnS [18]. Zn–ZnO core-shell nanobelts were also
reported [19]. But there are very few reports on the synthe-
sis of Cd 1-D nanstructures. A complex ion irradiation
method has been utilized to fabricate Cd nanowires [16].
The yield of the Cd nanowires by this ion irradiation
approach was not satisfactory. Recently, Vivekchand
et al. [20] have developed a nebulized spray pyrolysis
approach to synthesize different metal nanowires, such as
Zn, Cd, Co and Pb.
Nanostructures find unique applications in electronics,
optoelectronics and catalysis due to their high surface to
volume ratio, enhanced material characteristics due to
quantum confinement effects and high fraction of chemi-
cally similar surface sites. Since the first discovery of car-
bon nanotubes, one-dimensional (1-D) nanomaterials
have attracted extensive interest because of their funda-
mental physical, chemical, optical, electrical and magnetic
properties, which are distinct from their bulk counterparts,
and also due to their potential applications in nanoscale
devices [1–8]. Through confinement of the metals to the
quasi one-dimension, a variety of changes in their charac-
teristics are induced. Thus to fully study the fundamental
physical and chemical properties of the quasi 1-D metal
systems, the synthesis of 1-D metal system like nanowires
are very important. A variety of 1-D metal nanostructures,
such as Au [9,10], Ag [11,12] Cu [13], Fe [14], etc. have been
*
In this Letter, we report the synthesis of Zn and Cd
nanowires by simple thermal evaporation of their respec-
tive sulfide powders.
Corresponding author. Fax: +91 033 2473 2805.
E-mail addresses: kar_mitra@yahoo.co.in (S. Kar), mssc2@mahendra.
iacs.res.in (S. Chaudhuri).
0
009-2614/$ - see front matter Ó 2005 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2005.11.054

S. Kar et al. / Chemical Physics Letters 419 (2006) 174–178
175
2
. Experimental
2
.1. Sample preparation
Details of the experimental set up were described else-
where [21]. In short; a conventional quartz tube furnace
was used for the synthesis of Zn nanowires. Analytical
grade ZnS powder was loaded into a quartz boat, which
was placed near the closed end of a quartz tube. After evac-
uating the quartz tube, Ar gas was flown through it during
3
the entire deposition period with a flow rate of 200 cm /
min. The quartz tube was then inserted into the preheated
20
30
40
50
60
70
80
a
2θ (degree)
tube furnace with temperature fixed at 1225 °C and after
4
5 min of deposition, it was taken out of the furnace to
allow rapid cooling to room temperature. A white foam-
like products consisting of single crystal ZnS nanoribbons
were found at a distance of ꢀ6 cm from the source (temper-
ature ꢀ1050 °C) attached to the inner wall of the quartz
tube over a region of ꢀ1 cm. In addition, a gray product
of Zn nanowires was found on the inner wall of the quart
tube close to the vacuum couplings. The temperature of
this region was ꢀ350 °C. The gray products were also
deposited on the cleaned Si substrates placed at that posi-
tion. ZnS nanoribbons produced by this technique was
reported elsewhere [22]. Here, we report the synthesis and
characterization of the Zn nanowires in the low tempera-
ture region. For the synthesis of Cd nanowires, ZnS pow-
der was replaced by the CdS powder and temperature of
the source region was reduced to 950 °C. CdS powder used
as the source have been synthesized following the route
described elsewhere [23]. The gray products were collected
from the same region.
Zn
Zn
S
Zn
Si
0
2
4
6
8
10
Energy (keV)
b
Fig. 1. (a) XRD pattern of the Zn nanowires. (b) EDAX spectrum of the
nanowires.
formation of any ZnS phase or any crystalline phases of
S, hence the peak due to S could be attributed to the pres-
ence of some amorphous sulfur in the product. The reason
behind the presence of sulfur will be discussed in the later
section.
2
.2. Characterization
The products (Zn and Cd nanowires) were characterized
For morphological characterization, the as deposited Si
wafer was directly transferred to the SEM chamber without
disturbing the original nature of the samples. Fig. 2a shows
the SEM image of the products revealing the formation of
large quantity of Zn nanowires. Further observations at
higher magnification revealed the formation of some rib-
bon- or belt-like quasi one-dimensional nanomaterials as
shown in the inset of Fig. 2a. The nanowires were 100–
150 nm in diameters and few micrometers in length. Widths
of the nanoribbons were ꢀ200 nm and their thickness
ꢀ75 nm. TEM image shown in Fig. 2b indicates the pres-
ence of nanoribbons along with the nanowires. The high
contrasts of the images were attributed to the large diame-
ter or thickness of the Zn nanostructures. The HRTEM
image of one of a Zn nanowire is shown in Fig. 2c. The
image reveals the single crystalline nature of the nanowires.
HRTEM analysis of the nanowires did not show the evi-
dence of any oxide layer surrounding it as was observed
by some of the previous workers [15,17,19]. The measured
using X-ray diffractometer (XRD, Seifert 3000P) with Cu
Ka radiation and the compositional analysis was done by
energy dispersive analysis of X-ray (EDAX, Kevex, Delta
Class I). Microstructures of the products were obtained
by scanning electron microscopy (SEM, Hitachi S-2300)
and transmission electron microscopy (TEM, JEM 2010).
3
. Results and discussion
Crystal structure and phase of the gray products were
determined from the XRD pattern as shown in Fig. 1a.
All the diffraction peaks were indexed to the hexagonal
phase of Zn (JCPDS Card No. 04-0830). The XRD pattern
does not show the presence of any ZnS or ZnO phases. The
chemical purity of the products was tested by the EDAX
measurements. The EDAX spectrum of the product shows
the presence of Zn as the predominant component. It also
shows the presence of S and Si as minor components. The
signal of Si came from the exposed Si substrates. The com-
positional analysis revealed the presence of Zn and S in
˚
spacing of the lattice fringes was ꢀ2.49 A corresponding to
the (002) planes of wurtzite Zn. This (002) plane was also
the growth direction of the Zn nanowires. The fast Fourier
9
5:5 atomic ratio. As the XRD pattern did not reveal the

176
S. Kar et al. / Chemical Physics Letters 419 (2006) 174–178
Fig. 2. (a) SEM image of the Zn nanowires along with the inset showing the closer view of a Zn nanoribbon. (b) TEM image showing the presence of the
Zn nanoribbons and nanowires. (c) HRTEM image of a single Zn nanowire along with the inset showing the FFT pattern. (d) TEM image of a nanowire
tip showing the presence of several Zn nanoparticles surrounding the Zn nanowire.
transformation (FFT) of the HRTEM image shown in the
inset of Fig. 2c also confirmed the growth direction. The
TEM image shown in Fig. 2d reveals the presence of few
nanoparticles on the surface of a single nanowire. The pres-
ence of the nanoparticles and the growth mechanism of the
Zn nanowires and nanoribbons could be understood on the
basis of the following discussions.
We believed, that the growth of Zn nanostructures was
governed by the vapor–solid (VS) process [24]. At high
temperature, ZnS evaporates to form molecular ZnS spe-
cies. Also, the high temperature helps some of these ZnS
molecules to dissociate to their atomic constituents. Ar
gas carried these molecular and atomic vapor species and
at a region having temperature ꢀ1050 °C, the ZnS vapor
species condensed and ZnS nanoribbons were formed as
discussed in our earlier Letter [22]. As the temperature of
this region was well above the melting point of Zn
dinal growth. When the experimental set up was suddenly
taken out of the furnace to allow rapid cooling, the newly
arrived vapor species could not travel to the growth front
due to the decrease in their mobility at the lower tempera-
tures resulting in some island-like nanoparticles towards
the end of the nanowires as revealed by the TEM image
in Fig. 2d. Formation of few Zn nanoribbons could be
attributed to the fact that nanoribbons with defined faceted
surfaces are thermodynamically more stable structure than
that of the cylindrical nanowires, because of the low sur-
face energy of the facets. So during the rapid growth, the
system tried to achieve lowest possible energy states as
quickly as possible resulting in the formation of some rib-
bon-like nanostructures. The flowing Ar gas carried out the
elementary sulfur species but some of these sulfur species
might have been trapped within the Zn nanostructures as
revealed by the EDAX spectrum. The presence of the sul-
fur vapor might have prevented the oxidation, hence for-
mation of the ZnO shell over the Zn nanowires, as some
leakage oxygen were always present in the system.
Fig. 3a shows the XRD pattern of the gray products
deposited on the Si substrate with CdS as the source,
revealing the formation of metallic Cd. All the XRD peaks
were indexed to the hexagonal phase of Cd (JCPDS Card
No. 05-0674). The XRD pattern does not reveal the pres-
ence of any impurity phases like CdS or CdO. The EDAX
spectrum of the product revealed the presence of the Cd as
the predominant component. Just like the Zn nanowires
some sulfur species were also detected in the EDAX spec-
trum of the Cd nanowires.
(
410 °C), Zn did not get condensed at the position. Instead
the lighter atomic species were further carried away by the
flowing Ar gas to a region having temperature less than the
melting point of Zn (410 °C), where the Zn species con-
densed to form Zn nanoclusters. These nanoclusters were
energetically favored sites to absorb the more upcoming
vapor species and the continuous feeding of the source
materials leads to the one-dimensional growth. The newly
arrived Zn species can stick to the side as well as the rough
growth fronts, but the high mobility of the Zn atoms pre-
vents them from staying on the surface instead they moved
to the rough growth front which being energetically more
favored site absorbed these species to continue the longitu-

S. Kar et al. / Chemical Physics Letters 419 (2006) 174–178
177
Fig. 4a shows the SEM image of the products indicating
their wire-like morphology. Diameters of the Cd nanowires
varied within 100–150 nm and their lengths were of the
order of few micrometers. The growth mechanism is similar
to that of Zn nanowires as discussed above. The closer
observations did not indicate the presence of any ribbon-
like nanoforms. This could be due to the fact that Cd nano-
wires were produced at much lower temperature than Zn
nanowires. Fig. 4b shows the TEM image of a Cd nano-
wire. Fig. 4c reveals the presence of some pearl necklace
type Cd nanowires. This was due to the fact that as the
deposition system was suddenly taken out of the furnace
the lowering of the mobility of the Cd species allows them
to stay on the side surface of the nanowires resulting in the
formation of Cd nanoparticle clusters surrounding the
nanowires. This was further confirmed by the TEM image
shown in Fig. 4d. The low temperature prevents the grain
agglomeration of the CdS nanoparticles; instead they
formed nanoparticle assemblies at certain interval giving
the nanowire a pearl necklace-like morphology.
2
0
30
40
50
60
70
80
a
2θ (degree)
Cd
Cd
Cd
4. Conclusions
S
Cd
In conclusion, we have synthesized Zn and Cd metal
nanowires by a simple thermal evaporation route. ZnS
and CdS powders were used as the precursor, which at high
temperature decompose to give the elementary Zn and Cd
vapor species. These elementary species condensed at a
favorable temperature region to produce one-dimensional
nanostructures via vapor–solid technique.
0
2
4
6
8
b
Energy (keV)
Fig. 3. (a) XRD pattern of the Cd nanowires and (b) EDAX spectrum of
the Cd nanowires.
Fig. 4. (a) SEM image of the Cd nanowires (b) TEM image of Cd nanowire (c) SEM image of the Cd nanowires showing few pearl necklace-like structures
and (d) TEM image of a pearl necklace type Cd nanowire.

178
S. Kar et al. / Chemical Physics Letters 419 (2006) 174–178
Acknowledgment
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One of the authors (T.G.) expresses her sincere gratitude
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