Mesoscale organization of Cu7S4 nanowires: Formation of novel sheath-like nanotube array

Article abstract of DOI:10.1016/j.cplett.2006.12.002

Highly ordered and uniform array of Cu₇S₄ sheath-like nanotubes has been successfully prepared via a sacrificial template approach. These nanotubes have diameter of 250-600 nm and length of several micrometers with close tips. The wall of the nanotube is actually woven out of many well-distributed Cu₇S₄ nanowires with the diameter of 20-30 nm and the wall thickness of the nanotube is estimated to be about 60-70 nm. This novel hierarchical sheath-like nanotube array may find applications as mesoscale reactors or as other nanodevices.

Full text of DOI:10.1016/j.cplett.2006.12.002

Chemical Physics Letters 434 (2007) 256–259
www.elsevier.com/locate/cplett
Mesoscale organization of Cu7S4 nanowires: Formation of novel
sheath-like nanotube array
*
Weixin Zhang , Jun Xu, Zeheng Yang, Shaixia Ding
School of Chemical Engineering, Hefei University of Technology, Hefei, Anhui 230009, China
Received 12 October 2006; in final form 23 November 2006
Available online 6 December 2006
Abstract
Highly ordered and uniform array of Cu7S4 sheath-like nanotubes has been successfully prepared via a sacrificial template approach.
These nanotubes have diameter of 250–600 nm and length of several micrometers with close tips. The wall of the nanotube is actually
woven out of many well-distributed Cu7S4 nanowires with the diameter of 20–30 nm and the wall thickness of the nanotube is estimated
to be about 60–70 nm. This novel hierarchical sheath-like nanotube array may find applications as mesoscale reactors or as other
nanodevices.
ꢀ 2006 Elsevier B.V. All rights reserved.
1. Introduction
hedral silver phosphate crystals templates [17]. Co3O4 nano-
boxes constructed from compactly assembled cuboid
nanocrystals have been prepared by a surfactant–templated
synthesis [18]. Because of the unique applications of one-
dimensional (1D) nanostructures in mesoscopic physics
and fabrication of nanosensors, switches, nanolasers, and
transistors [19–22], the controllable synthesis of one-dimen-
sional (1D) nanostructures has drawn considerable interest
[16,23,24]. However, it still remains a challenge to synthesize
these one-dimensional hierarchical nanostructures in nano-
science and nanotechnology.
Copper sulfide (in different stoichiometries) thin film has
found numerous technological applications in achievement
of solar cells, in photothermal conversion of solar energy,
as selective radiation filters on architectural windows and
as electroconductive coatings deposited on organic poly-
mers [25–28]. The controlled synthesis of copper sulfide
has attracted much attention [29–32], and considerable
progress has been made in recent years in the synthesis of
copper sulfide hollow nanostructures [14,33,34]. For exam-
ple, Yitai Qian group has reported a single-step synthesis of
CuS hollow spheres via a template interface reaction route.
Xuefeng Qian et al. have synthesized high symmetric 18-
facet polyhedron nanocrystals of Cu7S4 with a hollow
nanocage structure. Well-defined non-spherical copper
Materials with hierarchical structure are omnipresent in
nature and some of them exhibit extraordinary, partly
unparalleled material performances [1,2]. Hierarchy is now
emerging as a promising opportunity for making exciting
new materials in chemistry. Over the past several years,
the high-order organization of meso-, micro- and nano-
structured building blocks into complex architectures with
hierarchy has attracted significant interest in material syn-
thesis and device fabrications [3–7]. In general, self-assem-
bly [8–11] and the use of templates [12–14] are two
fundamental strategies for synthesizing hierarchical nano-
structures. For example, ZnO hollow spheres assembled
from single crystalline ZnO nanorods or nanoplatelets have
been synthesized through the Kirkendall-type diffusion [15].
Tetragonal PbWO4 microcrystals with special hierarchical
structures evolved from the nanopolyhedrons by the ori-
ented attachment process accompanying Ostwald ripening
have been successfully synthesized [16]. Double-walled sil-
ver rhombododecahedral cages built from different building
units have been fabricated by reducing the rhombododeca-
*
Corresponding author. Fax: +86 551 2901450.
E-mail address: wxzhang@hfut.edu.cn (W. Zhang).
0009-2614/$ - see front matter ꢀ 2006 Elsevier B.V. All rights reserved.
doi:10.1016/j.cplett.2006.12.002

W. Zhang et al. / Chemical Physics Letters 434 (2007) 256–259
257
sulfide mesocages using shape-controlled Cu2O crystals as
sacrificial templates have been prepared by Dongsheng
Xu group. Inspired by these studies on the formation of
spherical or non-spherical copper sulfide mesocages by
template-assistant methods, we successfully synthesize
highly-ordered sheath-like nanotube array constructed
from Cu7S4 nanowires by using Cu(OH)2 nanorod array
as sacrificial template. To the best of our knowledge, this
novel hierarchical Cu7S4 nanostructure has not been
reported yet.
tern of the as-prepared Cu(OH)2 nanorods on the copper
substrate. All the other diffraction peaks can be indexed
to orthorhombic Cu(OH)2 (JCPDF 13-0420) except only
two peaks (marked with ꢀ) which come from the copper
substrate. The XRD pattern of the copper sulfide sample
scraped from the copper substrate is presented in Fig. 1b,
which is consistent with the standard spectrum of mono-
clinic Cu7S4 not only in position, but also in relative inten-
sity (JCPDF 23-0958).
Fig. 2 shows the morphologies of the Cu7S4 sample as
well as the precursor Cu(OH)2 nanorod array. Fig. 2a pre-
sents the FE-SEM images of Cu(OH)2 nanorod array with
high order and uniformity, grown on the copper foil sur-
face, indicating that the Cu(OH)2 nanorods with smooth
surface have diameters of 250–600 nm and lengths of about
several micrometers. By the reaction of these Cu(OH)2
nanorods with aqueous Na2S solution for 10 min, followed
by dissolving the Cu(OH)2 cores in ammonia solution,
Cu7S4 sheath-like nanotube array with high order and uni-
formity similar to those of the Cu(OH)2 templates is
obtained. The FE-SEM image of the as-prepared Cu7S4
nanostructures is presented in Fig. 2b, which shows that
these one-dimensional nanostructures have diameter of
250–600 nm and length of several micrometers with close
tips. To examine that these close-tip nanostructures have
hollow interiors, the Cu7S4 array is scraped slightly to cut
off some of the close tips. Fig. 2c shows the image of some
broken structures of the Cu7S4 array. It can be seen that
these rod-like nanostructures have hollow interiors, and
actually exist in sheath-like nanotube array. Fig. 2d shows
a typical FE-SEM image of a broken Cu7S4 sheath-like
nanotube, revealing that the wall of the nanotube is not
smooth; and in fact, it is assembled from well-distributed
Cu7S4 nanowires. These nanowires with diameters of 20–
30 nm and lengths of several microns are woven compactly
to form the novel hierarchical nanostructure. The wall
thickness of the tube is estimated to be about 60–70 nm.
Fig. 2e confirms that all the Cu7S4 sheath-like nanotubes
are woven out of nanowires under the reported conditions.
The morphology and interior structure of the sheath-like
tubes are further investigated by TEM and HRTEM. The
2. Experimental
The synthesis of Cu(OH)2 nanorod arrays was referred
to our previous report [35]. A typical procedure was per-
formed as follows. An aqueous solution was prepared in a
60 mL glass bottle by mixing 2.0 mL of NaOH solution
(10 M), 1.0 mL of ammonia solution (25%), 1.0 mL of
(NH4)2S2O8 solution (1.0 M), and 10.0 mL of distilled
water. A piece of copper foil (15 · 15 · 0.25 mm3, 99.99%)
which had been ultrasonically cleaned in acetone and then
in deionized water was immersed in the solution at room
temperature. After about 45 min, the copper foil covered
with a blue film was taken out of the solution, rinsed with
distilled water and absolute ethanol, and dried in air.
Cu(OH)2/Cu7S4 core/sheath nanostructures were first
obtained by immersing the Cu(OH)2 nanorod array precur-
sor grown on the copper foil in a 50 mL mixed aqueous
solution composed of 0.01 M Na2S and 0.0025 M NaOH
at room temperature for 10 min in a glass bottle. The
obtained Cu(OH)2/Cu7S4 core/sheath nanorod array was
then immersed in an ammonia solution (15%) and kept
for 8 h to remove the inner Cu(OH)2 core. The resultant
product was then washed with distilled water and absolute
ethanol, and dried in air.
The as-prepared samples were characterized by X-ray
powder diffraction (XRD) in a Rigaku D/max-cB X-ray dif-
˚
fractometer with a Cu Ka radiation source (k = 1.5418 A)
operated at 40 kV and 80 mA. Field-emission scanning elec-
tron microscopy (FE-SEM) measurement was taken by
JEOL-7500B scanning electron microscope. Transmission
electron microscopic (TEM) images and high-resolution
transmission electron microscopic (HRTEM) images were
taken with Hitachi H-800 and JEOL-2010 transmission
electron microscope performed at an accelerating voltage
of 200 kV, respectively. The photoluminescence (PL) mea-
surements were recorded on a Hitachi F-4500 fluorescence
spectrophotometer at room temperature (kex = 370 nm,
Ex slit = 5 nm, Em slit = 5 nm).
–
Cu
b
a
3. Results and discussion
The as-prepared products have been characterized by X-
ray diffraction (XRD) analysis, field-emission scanning
electron microscopy (FE-SEM), transmission electron
microscopy (TEM) and high-resolution transmission elec-
tron microscopy (HRTEM). Fig. 1a shows the XRD pat-
20
30
40
50
60
2θ (degrees)
Fig. 1. XRD patterns: (a) Cu(OH)2 nanorods on copper substrate and (b)
Cu7S4 sample scraped from copper substrates.

258
W. Zhang et al. / Chemical Physics Letters 434 (2007) 256–259
Fig. 2. FE-SEM images: (a) Cu(OH)2 nanorod array, inset: magnified image of a single Cu(OH)2 nanorod, (b) Cu7S4 sheath-like nanotube array, (c)
Cu7S4 sheath-like nanotubes, (d) high-magnification FE-SEM image of the surface morphology of the Cu7S4 sheath-like nanotube and (e) low-
magnification FE-SEM image of the surface morphology of the Cu7S4 sheath-like nanotubes.
TEM image of the as-prepared Cu7S4 is shown in Fig. 3a.
Consistent with the FE-SEM observation, the Cu7S4 sam-
ple has hollow interiors. Across the tubes, the intensity pro-
file shows a clear variation, and the edge surfaces shows
darker contrast, suggesting a hollow structure. The
HRTEM image of a single nanowire in Fig. 3b shows a lat-
tice fringe with a constant spacing of 0.187 nm, corre-
sponding to that of the (886) plane of Cu7S4. The inset
SAED pattern of the nanotube displays several sets of elec-
tron diffraction spots which may result from different crys-
talline Cu7S4 nanowires of the sheath-like nanotube. One
set of the electron diffraction spots (marked with o) can
interior morphology. When the Cu(OH)2 nanorod array
is immersed in the Na2S solution, S2ꢂ will replace the
OHꢂ in Cu(OH)2 and Cu(OH)2 Cu7S4 core/sheath struc-
tures are formed. After the unreacted Cu(OH)2 nanorod
core is removed in ammonia solution, Cu7S4 sheath-like
nanotube can be obtained.
The formation of Cu7S4 nanowires is considered as
results of the conversion of crystal structure from ortho-
rhombic Cu(OH)2 to monoclinic Cu7S4, which is different
from the previous reports that the monoclinic crystal struc-
ture of Cu7S4 does not have significant effects on its final
morphology when cubic Cu2O precursor templates are
used [33,34]. In the present work, when Cu(OH)2 is con-
verted into Cu7S4, the monoclinic Cu7S4 crystals have the
intrinsic nature of forming flexible nanowires under the
experimental conditions, and the Cu(OH)2 nanorods
impose a spatial confinement and play the templating role
in formation of the sheath-like nanotubes, which are woven
out of Cu7S4 nanowires. As we know, the functionality of
nanotubes does not only stem from their dimensions and
the type of wall materials, but also from the hierarchical
structure of the nanotube walls. Due to the surface perme-
ability and large surface area of the walls, these novel
ꢀ
be well indexed to the monoclinic Cu7S4 along the ½182ꢁ
zone axis.
The strategy for the fabrication of Cu7S4 sheath-like
nanotube array is illustrated in Scheme 1. Cu(OH)2 has a
relatively larger Ksp value ð10ꢂ20Þ compared to that of cop-
per sulfide ð10ꢂ48Þ. This implies that the Cu(OH)2 nanorods
can act as both reactants to synthesize more stable chalcog-
enides and templates to obtain structures with a hollow
Cu(OH)2/Cu7S4
core/sheath
nanorods
Cu7S4
sheath-like
nanotubes
Cu(OH)2
nanorods
Na2S
solution
ammonia
solution
copper substrate
Fig. 3. (a) TEM image (b) High-resolution TEM image of a single
nanowire of the Cu7S4 sheath-like nanotube, inset: the SAED pattern of a
single nanowire on the Cu7S4 nanotube.
Scheme 1. Schematic illustration for the formation of Cu7S4 sheath-like
nanotube array.

W. Zhang et al. / Chemical Physics Letters 434 (2007) 256–259
259
ers Program of the Ministry of Education of China and the
Scientific Research Foundation for the Returned Overseas
Chinese Scholars supported by the State Education
Ministry.
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This project was financially supported by the National
Natural Science Foundation of China (NSFC Grant No.
20301005 and No. 20576024), the Excellent Young Teach-