Fabrication of upended taper-shaped cuprous thiocyanate arrays on a copper surface at room temperature

Article abstract of DOI:10.1021/jp061274x

A new strategy has been well designed to form upended taper-shaped cuprous thiocyanate (hereafter abbreviated as CuCNS) arrays on a copper substrate with use of a simple solution-phase method at room temperature. This method consists of a liquid-solid reaction between a solution of thiocyanate ammonium and the copper substrate itself in the assistance of formamide. Novel CuCNS arrays are approximately perpendicular to copper substrate surfaces. Every single crystal shows an upended taper-like morphology (i.e., the tip end points into the surface of copper substrate and the other big end of the taper exposes out, like a dart thrusting into the copper substrate). On the basis of structure and chemical bond analysis, CuCNS crystals tend to grow along the c-axis, which is essential for the formation of CuCNS arrays on a copper substrate. This approach also provides a facile strategy to produce different patterns on different copper substrates, which may be applicable to the synthesis of other inorganic materials with various potential applications.

Full text of DOI:10.1021/jp061274x

11232
J. Phys. Chem. B 2006, 110, 11232-11236
Fabrication of Upended Taper-Shaped Cuprous Thiocyanate Arrays on a Copper Surface
at Room Temperature
Jiasheng Xu and Dongfeng Xue*
State Key Laboratory of Fine Chemicals, Department of Materials Science and Chemical Engineering, School
of Chemical Engineering, Dalian UniVersity of Technology, 158 Zhongshan Road, Dalian 116012, P. R. China
ReceiVed: February 28, 2006; In Final Form: April 15, 2006
A new strategy has been well designed to form upended taper-shaped cuprous thiocyanate (hereafter abbreviated
as CuCNS) arrays on a copper substrate with use of a simple solution-phase method at room temperature.
This method consists of a liquid-solid reaction between a solution of thiocyanate ammonium and the copper
substrate itself in the assistance of formamide. Novel CuCNS arrays are approximately perpendicular to copper
substrate surfaces. Every single crystal shows an upended taper-like morphology (i.e., the tip end points into
the surface of copper substrate and the other big end of the taper exposes out, like a dart thrusting into the
copper substrate). On the basis of structure and chemical bond analysis, CuCNS crystals tend to grow along
the c-axis, which is essential for the formation of CuCNS arrays on a copper substrate. This approach also
provides a facile strategy to produce different patterns on different copper substrates, which may be applicable
to the synthesis of other inorganic materials with various potential applications.
Introduction
very high efficiencies.9,10 CuCNS has been used as a material
for the synthesis of polymer electrolytes,11 and has been reported
to act as a catalyst for the synthesis of organic compounds.12
CuCNS usually has been prepared through several different
methods, such as the electrodeposition technique,13 microcrystal
growth technology,14 the alumina template process,15 and
solution reaction.16 Herein, a new strategy has been successfully
designed by employing a liquid-solid reaction between an
aqueous solution of NH4CNS and a copper substrate at room
temperature. The copper substrates are used not only as a source
of copper but also as a support for CuCNS crystals. The
controlled reactions allow the large-scale, template-free, cost-
effective synthesis of CuCNS with an ordered, uniform, and
stable structure. Especially, CuCNS crystals grow directly on
the conducting copper substrate with good electrical contact,
which is of crucial importance for the development of a device
with high performance.
In recent years, there has been considerable effort in the
fabrication of inorganic materials with novel shapes on various
solid substrates, which are essential to the realization of high-
performance devices.1,2 Synthesizing inorganic materials with
unusual shapes could therefore be relevant to the design of new
types of inorganic materials because the crystal shape dictates
the interfacial atomic arrangement of the material. Copper-based
compounds are among the most important materials which have
been widely used in the area of catalysis, ion exchange, proton
conductivity, intercalation chemistry, photochemistry, and chem-
istry materials. Therefore, much research work has been done
in relation to copper-based compounds.3-6 For instance, Cu2O
with various unique architectures has been manipulatively
constructed on Au substrate through an electrochemical crystal-
lization process.3 Complex 3D dendritic nanoarchitectures of
copper hydroxide built up of ultrathin nanosheets have been
synthesized in an aqueous solution of formamide on a copper
foil substrate.4 Quasialigned submicrometer CuO ribbons have
been successfully synthesized on a copper surface through a
simple chemical route involving an oxidation-dehydration
process.5 A series of copper compound materials (including Cu-
(OH)2 fibers and scrolls, CuO sheets and whiskers, and Cu2-
(OH)CO3 rod arrays) have been synthesized on a copper foil
by a liquid-solid reaction under alkaline and oxidative condi-
tions at ambient temperature and pressure.6
Experimental Section
The products were prepared on copper substrates by a liquid-
solid reaction. Formamide (99.8%) was used without further
purification, and a 0.2 M NH4CNS aqueous solution was freshly
prepared. Copper foils (99.9%) with a thickness of 0.12 mm
were ultrasonically washed with deionized water before use. In
a typical procedure, a piece of copper foil (10.0 × 5.0 × 0.12
mm3) was first laid on the bottom of a 20-mL glass vial
containing 1.0 mL of formamide and 1.0 mL of NH4CNS
aqueous solution, then the mixed reaction solution was left still
at room temperature under atmospheric environment for 1-12
days. The copper foil was then taken out, washed with deionized
water and anhydrous ethanol (respectively) several times, and
dried in air at 50 °C for more than 5 h. In some cases, copper
mesh was also used as a copper substrate and formamide was
replaced by an aqueous solution of ammonia (NH3‚H2O, 25%).
The phase and crystallographic structure of the samples were
determined by powder X-ray diffraction (XRD, D/Max 2400,
As a transparent wide-gap p-type semiconductor, cuprous
thiocyanate CuCNS with its excellent chemical and thermal
stability becomes one of the most important functional materials
with various applications.7 For example, CuCNS has been found
to photooxidize water in the presence of sacrificial electron
acceptors more readily than most photocatalytic materials.8 It
is also an excellent candidate material for application in dye-
sensitized solid-state photovoltaic cells due to photographic
cyanine dyes being capable of injecting holes into CuCNS with
* Address correspondence to this author. E-mail: dfxue@chem.dlut.edu.cn.
10.1021/jp061274x CCC: $33.50 © 2006 American Chemical Society
Published on Web 05/20/2006

Upended Taper-Shaped Cuprous Thiocyanate Arrays
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11233
Figure 1. SEM images of cuprous thiocyanate crystals on a copper
foil substrate (experimental conditions: 1.0 mL of NH4CNS (0.2 M)
+ 1.0 mL of formamide + 3.0 mL of H2O, at room temperature for 6
days): (a) panoramic morphologies, scale bar ) 50 µm; (b) a detailed
view in a high magnification, scale bar ) 2 µm.
Rigaku, by a diffractometer equipped with the graphite mono-
chromatized Cu KR radiation). The morphology and size of
these crystals were characterized with a scanning electron
microscope (SEM, JSM-5600LV, JEOL). UV/vis diffuse re-
flectance spectra were recorded on a UV-vis-NIR spectro-
photometer (JASCO, V-550). FT-IR spectra were recorded on
a Fourier transform infrared spectrometry (FT-IR, KBr disk
Figure 2. SEM images of cuprous thiocyanate crystals on a copper
mesh substrate (experimental conditions: 1.0 mL of NH4CNS (0.2 M)
+ 1.0 mL of formamide + 3.0 mL of H2O, at room temperature for 6
days): (a) panoramic morphologies, scale bar ) 100 µm; (b, c) detailed
views at different places of copper mesh substrate, scale bar ) 20 and
10 µm, respectively; (d) side view on a single Cu wire, scale bar ) 5
µm.
method; NEXUS) at wavenumbers in the range 400-4000 cm-1
.
Results and Discussion
The surfaces of all these copper substrates were tarnished
(when viewed by the naked eye) after they had been treated in
the mixed solution at room temperature. Further examination
under an electron microscope indicated that the formation of
upended taper-shaped CuCNS crystals is over the entire surface
of these substrates, which covered a large area of the copper
substrate uniformly and compactly. Figure 1a clearly shows a
large-area array of CuCNS grown on a copper foil substrate.
The whole substrate surface was successfully covered with
uniform and dense arrays of upended taper-like CuCNS, which
are approximately perpendicular to the substrate surface. When
copper mesh is used as a copper substrate, CuCNS crystals with
a similar morphology can be obtained on the copper mesh
substrate, as shown in Figure 2. It is clear that every single
crystal shows an upended taper-like morphology (Figure 2d).
Every tip end points into the surface of copper substrate and
the other big end of the taper exposes out, like a dart thrusting
into the copper substrate.
When the concentration of mixed solution is low, sparse
CuCNS crystals can still be obtained on the surface of the copper
mesh substrate with a random arrangement, as shown in Figure
S5 (see the Supporting Information). Note that the crystals are
better aligned on the copper substrate under high concentration
than those under low concentration (comparing Figure 2 to
Figure S5). This indicates that the crystal alignment is caused
by the space-limited growth. Under low concentration, growth
can explore all directions on the copper surface. However, under
high concentration, the growth of tilted crystals is more likely
to be hindered, whereas the upright crystals grow unabated. An
interesting finding (a hollow structure viewed from the top of
CuCNS crystal in some of the CuCNS crystals) appeared when
CuCNS single crystals were further magnified, as shown in
Figure 3. XRD patterns of cuprous thiocyanate crystals on different
copper substrates: (a) on a copper foil substrate (SEM images
correspond to Figure 1) and (b) on a copper mesh substrate (SEM
images correspond to Figure S5, Supporting Information). The standard
diffraction pattern of â-CuCNS (JCPDS card File No. 29-0581) is
shown as a reference, and reflections generated by the Cu substrate
are marked with an asterisk.
Figure S5 (Supporting Information). It is likely that the open
structures were formed during the crystallization of CuCNS in
the assistance of formamide at a low precursor supersaturation
level.
CuCNS exists in two polymorphic forms, R and â, where
the â form is commonly available and more stable.17 â-CuCNS
has a hexagonal crystal structure where layers of CNS ions
separate Cu cation planes and strong Cu-S bonds three-
dimensionally interconnect these layers (as shown in the
crystallographic structure of Figures S1-S4, see the Supporting
Information). XRD patterns of the as-prepared samples are
shown in Figure 3, and all diffraction peaks can be well indexed
to the trigonal-phase â-CuCNS (space group R3m, No. 160),
except those marked with asterisks from the copper substrate.
Compared with the standard diffraction patterns (JCPDS card

11234 J. Phys. Chem. B, Vol. 110, No. 23, 2006
Xu and Xue
Figure 4. SEM images of cuprous thiocyanate arrays on a copper foil substrate prepared at different reaction stages (experimental conditions: 1.0
mL of NH4CNS (0.2 M) + 1.0 mL of formamide + 3.0 mL of H2O): (a) aging at room temperature for 1 day, scale bar ) 50 µm; (b) aging at
room temperature for 2 days, scale bar ) 5 µm; (c) aging at room temperature for 3 days, scale bar ) 5 µm; (d) aging at room temperature for 6
days, scale bar ) 5 µm.
File No. 29-0581), no characteristic peaks from other phases
or impurities can be observed, indicating a high purity and
crystallinity of the final products. The phase composition is
further supported by FT-IR spectra, as shown in Figure S8
(Supporting Information). The relative intensities of peaks in
Figures 3a and 3b are obviously different. The relative intensities
of (003), (006), (101), and (110) planes provide information
regarding the crystal orientation against the copper substrate,
since the powder diffractometer collects reflections only from
the crystallographic planes parallel to the substrate (which was
used to prepare XRD samples). For example, the XRD pattern
of Figure 3a shows the strong intensities of (003) and (006)
planes compared to Figure 3b, since alomost all upended tape-
shaped CuCNS arrays are approximately perpendicular to the
copper foil substrate surface (SEM images are shown in Figure
1). However, Figure 3b shows the strong intensities of (101)
and (110) planes but weak intensities of (003) and (006) planes.
The observed intensitis of XRD peaks indicate that the elongated
direction of taper-like shaped thiocyanate crystals is along the
c-axis.
Meanwhile, it should be noted that the heterogeneous
nucleation and growth is dominant during the formation of
upended taper-shaped CuCNS crystals, because corresponding
homogeneous nucleation and growth in solution were not
observed. Heterogeneous nucleation on the substrate is promoted
over homogeneous precipitation in solution by controlling the
precursor supersaturation levels and the interfacial energy
between the substrate and the new phase to be formed.2e In the
current synthetic system, without surface modification, the free
energy of crystallization of CuCNS on the copper substrate
interface must be intrinsically low enough to relive the super-
saturation and induce direct film growth. XRD patterns show
that the as-prepared CuCNS crystals grow along the c-axis,
which is mainly due to the intrinsic CuCNS crystal growth habit.
As shown in Figures S1-S4 (Supporting Information), the
â-CuCNS structure is composed of {001} layers of closest-
packed cylinders of CuCNS units. High site symmetry in
â-CuCNS causes the angles to be very regular. The N-C-S
and Cu-N-C sequences are strictly linear; two independent
angles at Cu and S are nearly tetrahedral (109.5°). There are
two types of bonds in the â-CuCNS crystal lattice: type I, C-N
and C-S bonds that are formed before the crystallization, and
type II, Cu-S and Cu-N bonds that are formed during the
crystallization process on the copper surface. Type II bonds have
an essential effect on the crystallization and then determine the
final crystal morphology. Cu-N bonds (1.923 Å) are parallel
to the c-axis symmetrically, and the bond strength is 0.302 vu
(the bond strength is quantitatively calculated by the bond-
valence model,19,20 valence unit is abbreviated as vu). Cu-S
bonds (2.342 Å) interconnect {001} layers, and the bond
strength is 0.238 vu. It is clear that the bond strength along the
c-axis is stronger than that along other directions in the
â-CuCNS crystal lattice. The â-CuCNS crystal therefore prefers
to grow along the c-axis (the growth rate is determined by the
bond number and the bond strength, which formed during the
crystallization).
Generally speaking, the natural oxidation of copper metal
naturally dissolving oxygen in water is very slow due to the
surface oxide layer. However, in the presence of formamide,
this spontaneous oxidation reaction can be accelerated drasti-
cally, which has been widely investigated in previous reports.4,18
In the presence of an aqueous solution of NH4CNS, copper can
form Cu+ and react easily with CNS- to form CuCNS crystals
on the surface of copper substrates. The formation processes of
the arrays prepared at different reaction stages are respectively
studied by SEM measurements, as shown in Figure 4. The
growth procedure can be described as follows: at the beginning
of the reaction, 1D nuclei started and gradually formed into a
large quantity of CuCNS taper-like “roots” on the copper surface
first, which served as the nuclei for the crystal growth. The
growth of CuCNS crystals stemmed from these “roots” and
continued to grow into arrays with reaction proceeding. In this
period a typical dissolution-crystallization state was established
under the current experimental conditions.
The aqueous solution of ammonia has a similar function with
formamide. There have been many reports on the synthesis of

Upended Taper-Shaped Cuprous Thiocyanate Arrays
J. Phys. Chem. B, Vol. 110, No. 23, 2006 11235
Figure 5. SEM images of cuprous thiocyanate crystals with a shape evolution: (a-c) evolution from the hexagon to trigon, scale bar ) 10 µm
(experimental conditions: 1.0 mL of NH4CNS (0.2 M) + 1.0 mL of aqueous solution of ammonia + 3.0 mL of H2O, at room temperature for 3
days).
integrated into devices and may be helpful in the design of new
types of photovoltaic devices and materials.
Conclusions
In summary, a new strategy has been designed to form
upended taper-shaped CuCNS arrays on a copper substrate by
using a solution-phase method at room temperature. Novel
CuCNS arrays are approximately perpendicular to copper
substrate surfaces. Every single crystal shows an upended taper-
like morphology (i.e., the tip end points into the surface of the
copper substrate and the other big end of the taper exposes out,
like a dart thrusting into the copper substrate). The advantages
Figure 6. UV/vis diffuse reflectance spectra of the as-prepared
of our method for the preparation of inorganic materials lie in
its simplicity, large-scale, template-free, mild reaction condi-
tions, and the ability to produce different patterns on different
copper substrates. The versatile fabrication method demonstrated
herein provides not only valuable information for the formation
of CuCNS crystal arrays on a copper surface but also builds a
foundation for the study of directed crystal growth of various
semiconducting crystals on a conducting substrate. In addition,
CuCNS crystals grow directly on the conducting copper
substrate with good electrical contact, which can be easily
integrated into devices. These systems will be useful for a broad
range of applications such as sensors, optoelectronics, and
catalysis.
â-CuCNS samples. The inset shows the energy level structure (VB
refers to the valence band; CB refers to the conduction band).
copper compound materials on a copper foil by a simple liquid-
solid reaction under alkaline (aqueous solution of ammonia)
conditions.6 In the current synthetic condition, different shapes
of CuCNS can be obtained on the copper substrate, as shown
in Figure 5. It may be helpful for us to understand the crystal
growth habit of CuCNS. The shape evolution of CuCNS (i.e.,
from the hexagon to trigon) has been shown in Figure 5, parts
a-c, which correspond to the crystal structure of CuCNS from
the c-axis view (Figure S4, see the Supporting Information). It
is well-known that both intrinsic and extrinsic factors (i.e., the
crystal structure and growth environment) have significant
effects on the final crystal morphology.20 On the basis of
structure and chemical bond analysis, â-CuCNS crystals tend
to grow along the c-axis, which is essential for the formation
of â-CuCNS arrays on the copper surface. The crystal growth
environments also have a significant effect on the final
morphology. The existence of formamide or ammonia is very
important for the formation of â-CuCNS crystals in the current
synthetic condition. The default experiments (in the absence of
formamide or ammonia) have been employed for evaluating the
role of formamide or ammonia in crystal growth. As shown in
Figures S6 and S7 (Supporting Information), only some irregular
particles are formed on the surface of mesh copper substrate.
Therefore, the existence of formamide (or ammonia) plays an
important role during the whole crystallization process.
Acknowledgment. The authors gratefully acknowledge the
financial support of a Program for New Century Excellent
Talents in University (NCET-05-0278), the National Natural
Science Foundation of China (NSFC No. 20471012), a Founda-
tion for the Author of National Excellent Doctoral Dissertation
of P. R. China (FANEDD No. 200322), the Research Fund for
the Doctoral Program of Higher Education (RFDP No.
20040141004), and the Scientific Research Foundation for the
Returned Overseas Chinese Scholars, State Education Ministry.
Supporting Information Available: SEM and FT-IR results
of the reported samples and the crystal structure of CuCNS.
This material is available free of charge via the Internet at http://
pubs.acs.org.
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â-CuCNS possesses a steep absorption edge in the UV region
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intrinsic band gap absorption of â-CuCNS. It is clear that the
as-prepared CuCNS is a wide-band-gap semiconductor and
photosensitive material, and grows on the conducting copper
substrate (good electrical contact) directly, which can be easily
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