Template-free synthesis of single-crystalline cadmium nanotubes

Article abstract of DOI:10.1039/b312708g

By pyrolysis of CdS powder in inert gas, single-crystalline cadmium nanotubes growing in the form of a heap, were produced for the first time in high yield.

Full text of DOI:10.1039/b312708g

Template-free synthesis of single-crystalline cadmium nanotubes
Pingàn Hu, Yunqi Liu,* Lei Fu, Lingchao Cao and Daoben Zhu*
Center for Molecular Science, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, P. R.
China. E-mail: liuyq@ iccas.ac.cn; Fax: +86-10-62559373
Received (in Cambridge, UK) 13th October 2003, Accepted 17th December 2003
First published as an Advance Article on the web 4th February 2004
By pyrolysis of CdS powder in inert gas, single-crystalline
cadmium nanotubes growing in the form of a heap, were
produced for the first time in high yield.
No characteristic peaks from impurities, such as CdS and S, are
detected, which indicates that the products are highly pure.
A field emission scanning electron microscopy image (SEM,
Hitachi S-4300F) given in Fig. 2a shows the products grown on a
Si wafer. The wire-like materials are twisted around each other to
form many irregular piles. The energy-dispersion X-ray spectro-
scope (EDS, INCA300) attached to the SEM analysis reveals that
the products are composed of elemental cadmium (Fig. 2b); the Si
signal comes from the Si substrate. As shown in Fig. 2c, which is
an SEM image of an individual wire heap, the wire-like materials
have a length up to tens of microns and a variable diameter along
their length. The diameter range is usually 30–200 nm. These
products seem to be like the intestines of animals, which is fully
illuminated in the high magnification SEM image in Fig. 2d.
Formation of such a structure is due to the physical properties of
elemental Cd. Cadmium with a low melting point of 321 °C is a
very soft metal and is easily cut with a knife. Therefore, it is
1
Since the discovery of carbon nanotubes in 1991, tubular one-
dimensional (1-D) nanomaterials have attracted intense interest
from scientists in various fields such as chemistry, physics, material
science and so on, because the nanotubes exhibit unique physical
and chemical properties and have wide potential applications in
nanoelectronics, chemical sensors, batteries and other fields.2
Various nanotubes made from layer compounds, for example,
1
3
2
graphite, boron nitride, metal disulfides (MX , M = W, Mo, Ti,
4
Nb, Ta, Re, Bi; X = S, Se), have been studied extensively; In
addition, nanotubes constructed from non-layer compounds can be
obtained under appropriate reaction conditions. Thus far, there are
many reports on the developments of new nanotubes of metal
5
oxides (e.g. TiO
2
, Ga
2
O
3
, In
2
O
3
, ZnO, and rare earth oxides),
6
7
metals (Ni, Fe, Co, Au, Te and Bi), sulfides (PbS, CdS and CdSe),
8
9
some semiconductor compounds (GaN, InP ) and other materi-
als. Recently, much attention has been focused on the synthesis
and study of metal nanotubes which show different physical
properties in terms of surface plasmon, transport and magnetic
behavior from their corresponding bulk materials. In addition to
Te and Bi6e nanotubes, which are prepared without using any
10
1
1
12
6a
6d
template, other metallic nanotubes reported so far are synthesized
6
a
6b
6c
through a template route. For example, Ni, Co, Fe and Au
nanotubes are fabricated from a nanoporous template of nanopore
alumina or polycarbonate membranes. Very recently, Xia et al.13
developed a general approach that constructs metal nanotubes as
well as other nanostructures with hollow interiors by means of a
replacement reaction between the surface of a nanoscale template
(e.g. Ag nanowire) and an appropriate salt precursor such as
HAuCl , PdCl . In a typical procedure, the desired metal is coated
4
2
on the surface of a 1-D nanomaterial to form the core–sheath
structure by the corresponding metal salts reacting with the
template; subsequently, the template is removed by selectively
etching in an appropriate solution and 1-D nanostructures with
hollow interiors are generated. By adopting this method, highly
Fig. 1 A typical XRD pattern of cadmium nanotubes.
1
3
crystalline metallic nanotubes of Au, Pd, and Pt are obtained
using Ag nanowires as a sacrificial template. However this method
can not be used to produce nanotubes made from active metals such
as Zn, Cd etc. In this communication, we present a template-free
pyrolysis route to produce single crystalline cadmium nanotubes.
The Cd nanotube was grown in a standard CVD apparatus
consisting of a tube-like high temperature furnace with a quartz
tube (length: 120 cm; diameter: 2.5 cm) and an independent
temperature controller. A quartz boat with 0.5 g CdS powder was
placed at the center of the furnace. Before increasing temperature,
2
O in the reaction system was eliminated completely by flushing
2
1
with Ar gas at 100 ml min for 2 h. Then the reaction was carried
out at 700–1000 °C for 4 h. After the reaction, wool-like silvery
white products were found in the region of 50–300 °C.
The products were structurally characterized by X-ray diffrac-
tion (XRD, Rigaku Dmax2000, CuKa), Their XRD pattern is
shown in Fig. 1 with all diffraction peaks indexed to the known
wurtzite structure phase of cadmium (JCPDS card No. 5–067). The
two peaks are indexed as (002) and (004) of the Cd wurtzite
structure, revealing that the nanotubes grew in the (001) direction.
Fig. 2 SEM images of Cd nanotubes: (a) low magnification; (b) EDS which
indicates the tube is composed of elemental cadmium; (c) an individual
nanotube heap; (d) high magnification.
5
56
C h e m . C o m m u n . , 2 0 0 4 , 5 5 6 – 5 5 7
T h i s j o u r n a l i s © T h e R o y a l S o c i e t y o f C h e m i s t r y 2 0 0 4

difficult for the cadmium wires to form a straight structure with a
uniform diameter.
into S
of the Cd changes into CdS. In addition, Cd gas has a much higher
partial pressure than S gas; for example, the partial pressures of Cd
and S at 900 °C over CdS are 214.6 Pa and 108.0 Pa respectively.
Therefore, Cd gas has a higher diffusion velocity. As a result, some
Cd can be separated from sulfur and deposited at the low
temperature ( < 300 °C) region in the form of tubes. The formation
n
which deposits and does not react with Cd, therefore not all
17
The products are further characterized by transmission electron
microscopy (TEM: Hitachi H-800, JEOL-2010F). Fig. 3a shows
the general morphology of the products. The clear contrast
observed along the length of the cadmium wire-like materials
suggests a tubular structure (the outer part is darker than the inner
part). These nanotubes are entangled with each other. An individual
straight cadmium nanotube with a uniform diameter of 100 nm is
shown in Fig. 3b. A very obvious hollow interior is observed. Some
fracture pieces, which are also made of elemental Cd by EDS
analysis, absorbed to the surface of the nanotube. These Cd pieces
originated from the rupture of Cd nanotubes by ultrasound during
the procedure of sample preparation. A Y-structure was observed
by TEM (Fig. 3c), which is similar to the branched carbon
2
1
6
2
2 2
mechanism of BN, WS and MoS nanotubes is by rolling layer
4
structures into tubes; the soft template (e.g. surfactants) or solid
template (e.g. CNTs, porous alumina) confined growth of non-layer
compound nanotubes such as CdS nanotubes,7b Ni nanotubes is
also easily understood. These two mechanisms can not account for
the tubular formation of cadmium, since cadmium is just a
hexagonal material, and no template was used in the growth
process. Therefore, subsequent efforts will be made to study the
growth mechanism.
6a
1
4
nanotubes reported in the literature.
These Cd nanotubes are sensitive to beam irradiation during the
TEM examinations, which is expected due to their low melting
points (321 °C). Fig. 3d shows a high resolution TEM image of an
individual Cd nanotube. It is observed that after several seconds of
intensive electron beam irradiation, the Cd nanotube is partly
cracked and has a tendency to transform into a polycrystalline
nanowire. But still there is an obvious preferential crystal direction
along the length, which indicates that the Cd nanotubes are
characteristically single crystalline. This beam sensitivity makes
electron diffraction analysis difficult. Nonetheless, our XRD, SEM,
TEM, and composition analysis have unambiguously demonstrated
that the tubular structures in our sample are metallic Cd nano-
tubes.
In conclusion, the metallic cadmium was obtained for the first
time by a simple pyrolysis route, and no template was applied
during the growth process. These metallic nanotubes provide a new
system for studying the peculiar electronic transformation of 1-D
nanomaterials. We expect that this route can be used to prepare Zn
nanotubes.
The research is funded by the Major State Basic Research
Development Program, National Natural Science Foundation of
China (NNSFC), and Chinese Academy of Sciences.
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nanotubes could be described as follows:
2
CdS ? 2Cd (g) + S
During reaction, Cd and S gas flow with the Ar carrier gas to the
low temperature region, and some S gas reacts with Cd to form
CdS again, which deposits on the quartz tube. At the lower
temperature region ( < 400 °C), the S has a tendency to polymerize
2
(g)
(1)
1
1
2
2
17 L. Q. Zhang, B. W. Shen, J. Z. Yun, X. Z. Cao and Y. Y. Lv, Series on
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2
C h e m . C o m m u n . , 2 0 0 4 , 5 5 6 – 5 5 7
557