Low temperature synthesis of Cu2O crystals: Shape evolution and growth mechanism

Article abstract of DOI:10.1021/cg900437x

An interesting shape evolution of Cu₂O crystals, that is, from cubes, truncated octahedra, octahedra, and finally to nanospheres was first realized in high yield by reducing the copper-citrate complex solution with glucose. X-ray powder diffraction (XRD), field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron microscopy (H RTEM) techniques were employed to characterize the samples. We elucidate the important parameters (including poly (vinyl pyrrolidone) (PVP) concentration, reaction time, and reaction temperature) responsible for the shape-controlled synthesis of Cu₂O crystals. The possible formation mechanism for the products with various architectures is presented, which is mainly based on the variation of the ratio (R) of the growth rates along the ( 100) and (111) direction. In addition, the effect of the low supersaturation on the formation of star-shaped samples with six symmetric branches is also taken into account. This polymer-mediated method should be readily extended to the controlled synthesis of other metal oxides and the proposed growth model could also be used to explain and direct the growth of crystals with a cubic structure

Full text of DOI:10.1021/cg900437x

DOI: 10.1021/cg900437x
Low Temperature Synthesis of Cu O Crystals: Shape Evolution and
2
Growth Mechanism
2
010, Vol. 10
99–108
†
†
,†
‡
†
Yongming Sui, Wuyou Fu, Haibin Yang,* Yi Zeng, Yanyan Zhang, Qiang Zhao,
†
†
†
†
†
Yangen Li, Xiaoming Zhou, Yan Leng, Minghui Li, and Guangtian Zou
†
†
National Laboratory of Superhard Materials, Jilin University, Changchun, 130012, PR China and
State Key Laboratory on Integrated Optoelectronics, College of Electronic Science and Engineering,
Jilin University, Changchun 130012, PR China
‡
Received April 20, 2009; Revised Manuscript Received November 8, 2009
ABSTRACT: An interesting shape evolution of Cu O crystals, that is, from cubes, truncated octahedra, octahedra, and finally
2
to nanospheres was first realized in high yield by reducing the copper-citrate complex solution with glucose. X-ray powder
diffraction (XRD), field-emission scanning electron microscopy (FESEM), and high-resolution transmission electron micro-
scopy (HRTEM) techniques were employed to characterize the samples. We elucidate the important parameters (including poly
(vinyl pyrrolidone) (PVP) concentration, reaction time, and reaction temperature) responsible for the shape-controlled
synthesis of Cu O crystals. The possible formation mechanism for the products with various architectures is presented, which
2
is mainly based on the variation of the ratio (R) of the growth rates along the Æ100æ and Æ111æ direction. In addition, the effect of
the low supersaturation on the formation of star-shaped samples with six symmetric branches is also taken into account. This
polymer-mediated method should be readily extended to the controlled synthesis of other metal oxides and the proposed growth
model could also be used to explain and direct the growth of crystals with a cubic structure.
1
. Introduction
morphologies: Huang et al. have reported the high yield
growth of monodispersed Cu O nanocubes with approximate
average sizes of 40, 65, 100, 230, and 420 nm using a seed-
mediated synthesis approach in aqueous solution. Mono-
dispersed nano- and microcubes were prepared by Murphy et
al. from the reduction of Cu(II) salts in the presence of
2
In recent years, developing ways of tailoring the structure of
materials on specific morphologies has been one of the
important goals of material scientists. The shape and size of
inorganic materials are well-known to have great effects on
19
1
their widely varied properties. To date, much effort has been
devoted to synthesize novel nano- and microstructured ma-
terials with various shapes, such as low-dimensional struc-
poly(ethylene) glycol (PEG, M =600) and cetyltrimethylam-
w
2
monium chloride (CTAB) as protecting agents. Octahedral
0
2
3
4
5
6
tures (e.g., rods, wires, belts, tubes, cubes ) and
Cu O crystals with a tunable edge length were synthesized by
2
2
reducing copper hydroxide with hydrazine, while nanoocta-
1
7
hierarchical structures (e.g., dendrites, branches, urchins,
8
9
1
0
hedral Cu O crystals with controllable sizes of ∼100 nm have
2
networks ), for their specific properties and corresponding
potential applications. If we could understand the growth
mechanism and the shape-guiding process, it could be possible
to program the system to yield the final crystals with desired
been prepared via γ irradiation in Triton X-100 microemul-
sions as well as via slow oxidation of Cu under ambient
2
conditions. In addition, electrodeposition methods are com-
2
1
1
monly used to synthesize Cu O crystals with various morpho-
2
shape and crystallinity.
Cu O is a typical p-type direct band gap semiconductor
logies. Choi et al. have demonstrated a methodological
approach for utilizing the preferential adsorption of surfac-
tant during the electrodeposition process to obtain truncated
2
with a band gap of 2.17 eV and has potential applications in
14
12
13
solar energy conversion, electrode materials, sensors,
1
5
23
and catalysts. It has also been found that high-intensity
photon-excitation can give rise to the coherent propagation of
Cu O excitons through Cu O solid owing to the large exciton
binding energy of 150 meV. Furthermore, its potential
application in catalysts was demonstrated by the discovery
that Cu O could act as a stable photocatalyst for the photo-
octahedral Cu
O crystals. Xu et al. have also reported that
2
truncated octahedral Cu O crystals can be obtainedby adding
2
a small amount of hydrophilic RTILs in the aqueous deposi-
24
tion solution.
Herein, we report a facile solution-phase route for the mass
synthesis of Cu O crystals with different morphologies in the
2
2
2
1
6
2
chemical decomposition of water into O and H under visible
2
presence of poly (vinyl pyrrolidone) (PVP) at relatively mild
temperature of 80 ꢀC. In our system, the morphological
evolution from cubes to truncated octahedra, octahedra,
and nanospheres has been investigated by using different
amounts of PVP, which may be attributed to the selective
interaction of PVP with {111} facets during the reaction. In
2
1
light irradiation. Cu O nanocrystals with cubic and octa-
7
2
hedral geometries are the most interesting, which is because
other more complex particle structures can be derived from
these simple structural forms, and their well-defined surfaces
provide unique opportunities for the examination of their
1
8
facet-specific properties. Recently, various methods have
been reported for the production of Cu O with varied
addition, the formation of star-shaped Cu O with six sym-
2
metric branches is closely related to the low supersaturation,
and the effect of the reaction temperature on the final
morphologies and sizes of Cu O crystals is also explored in
2
*Corresponding author: Tel.: þ86-431-85168763; fax: þ86-431-
5168763; e-mail: yanghb@jlu.edu.cn.
2
8
detail. Finally, optical absorption spectra of these Cu O
2
r
2
009 American Chemical Society
Published on Web 11/30/2009
pubs.acs.org/crystal

1
00 Crystal Growth & Design, Vol. 10, No. 1, 2010
Sui et al.
copper or cupric oxide. The strong and sharp peaks indicate
that the obtained Cu O crystals are highly crystalline. Mean-
2
while, we observe that the intensity ratio between the (111)
and (200) diffraction peaks is lower than the ratio of those
peaks in the standard powder pattern (1.4 versus 2.7) for
cubes, whereas the value (3.05) for octahedra is higher. These
results indicate that cubes are abundant in {100} facets,
whereas {111} facets should dominate octahedra. It has been
known that the facets with a slower growth rate will be
exposed more on the crystal surface and consequently exhibit
relatively stronger diffraction intensity in the corresponding
2
5
XRD pattern. Therefore, it can be concluded that the
relatively slower growth rate for cubes and octahedra of
Cu O are {100} and {111} facets, respectively. Furthermore,
2
such variations of the intensity ratio suggest anisotropic
growth of these planes during the growing processes. These
results are further confirmed by the following discussions.
3
.2. Morphologies and Growth Mechanism of Cu O. Gene-
2
rally, the growth process of crystals is a kinetically and
thermodynamically controlled process that can form diffe-
rent shapes with some degree of shape tunability through
changes in the reaction parameters. In our system, we focus
on the effects of PVP concentration, reaction temperature,
Figure 1. XRD patterns of the Cu
octahedra (b), as well as the standard data for Cu
5-0667).
2
O cubes (a) and the Cu
2
O
2
O (JCPDS
26
0
crystals are obtained to see how the particle shape affects their
absorption characteristics.
and reaction time on the final shapes of Cu O crystals.
2
3.2.1. Effects of PVP Concentration. Figure 2 shows both
FESEM and TEM images of Cu O cubes obtained at 80 ꢀC
2
2
. Experimental Section
for 2 h using 0.5 mM PVP. It can be clearly seen that these
particles are well-defined cubes with sharp edges and uni-
form shape. Through the magnified FESEM image of Cu O
Synthesis. All the reagents were purchased from Beijing Chemical
Reagent Co. Ltd. and used as received. In a typical procedure, an
aqueous solution was first prepared by mixing 17 mL of water, 1 mL
2
cubes, we can observe that it is mainly composed of six {100}
planes and the average edge length is measured to be about
800 nm (Figure 2a). Interestingly, we are able to observe
similar-sized cubes with slightly truncated corner. These
w
of 0.68 M copper sulfate, and PVP (0.3 g; K-30; M =30 000) in a
round-bottomed glass flask. The mixture was stirred with a mag-
netic blender for about 15-20 min, and then 1 mL of 0.74 M sodium
citrate and 1.2 M anhydrous sodium carbonate mixture solution
was added in a dropwise manner into the above solution. A dark
blue solution soon appeared, whereas no precipitate was observed.
After about 10 min, 1 mL of 1.4 M glucose solution was slowly
dropped into it. The solution was kept in a water bath at a
temperature of 80 ꢀC for 2 h, and then cooled to room temperature
naturally. The brick red precipitate was filtered off, washed several
times with distilled water and absolute alcohol, and finally dried in a
vacuum at 60 ꢀC for 8 h. Further experiments were also conducted
under different conditions, using procedures similar to those pre-
sented above (The detailed experimental conditions are shown in
Figure S1, Supporting Information).
Cu O truncated octahedra can be obtained upon further
2
growth of these cubes (Figure 2b). Further investigation by
TEM shows that the cubes form the majority of the products,
and a small portion of truncated cubes is also commonly
observed in the products (Figure 2c). The fringes in a typical
HRTEM image (Figure 1d) are separated by ∼0.21 nm, in
good agreement with the (100) lattice spacing of Cu O. The
2
corresponding FFT pattern (Figure 2d, inset) of a cube
exhibits a cubic pattern along the Æ100æ zone axis.
When the PVP concentration is 1.5 mM and other experi-
mental conditions are kept the same, the morphology of
products is recoded by FESEM and TEM with different
magnifications as shown in Figure 3. Figure 3a shows a
panoramic view of the as-prepared products, from which
Characterization. X-ray powder diffraction (XRD) analysis was
conducted on a Rigaku D/max-2500 X-ray diffractometer with Cu
˚
KR radiation (λ = 1.5418 A). Field-emission scanning electron
microscopic (FESEM) images were performed on a JEOL JEM-
6
700F microscope operating at 5 kV. Transmission electron micro-
regular Cu O truncated octahedra with narrow size distribu-
2
scopic (TEM) images, selected area electron diffraction (SAED)
patterns, and high-resolution transmission electron microscopic
tion are demonstrated. The mean edge length is measured to
be about 300-500 nm. Through the magnified FESEM
image of truncated octahedron (Figure 3b), we can observe
that it is mainly composed of eight {111} planes and six {100}
planes, and the overall structure shares 24 identical edges in a
mecon way. Furthermore, the products are likely to align
along both the Æ100æ and the Æ111æ directions normal to the
substrate, and a mixture of squares and hexagons appears in
the TEM images (Figure 3c,d). The corresponding SAED
pattern reveals that they are single-crystal structure
(Figure 3d, inset).
(
HRTEM) images were obtained on a JEOL JEM-2000EX micro-
scope with accelerating voltage of 200 kV and a JEOL JEM-3010
microscopy operated at 200 kV, respectively. UV-vis absorption
spectra were recorded using a spectrophotometer (Shimadzu,
3
100 UV-vis-NIR).
3
. Results and Discussion
3.1. Phase Formation. The composition and phase purity
of the products were first examined by XRD, and the results
reveal that pure Cu O is obtained in all samples. Figure 1
2
displays the representative XRD patterns of the cubes
(Figure 1a) and octahedra (Figure 1b) as well as the standard
card (JCPDS No. 05-0667), indicating that all the diffraction
peaks are readily indexed to cubic phase Cu O (space group:
2
Pn3m, a = 0.4294 nm) with no impurity, such as metallic
Increasing the PVP concentration to 2.5 mM in a separate
reaction batch leads to the truncation of the octahedral
vertexes as seen in Figure 4. In the ideal (truncated) octa-
hedra structure, the surface is covered with both {100} and
{111} faces. The (truncated) octahedra are observed with a

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 101
2
Figure 2. FESEM images (a, b), TEM image (c), HRTEM image and FFT pattern (d) of as-prepared Cu O cubes.
2
Figure 3. FESEM images (a, b) and TEM image (c, d) of as-prepared Cu O truncated octahedra. Inset in (d) is a SAED pattern.

1
02 Crystal Growth & Design, Vol. 10, No. 1, 2010
Sui et al.
2
Figure 4. FESEM images (a-c) and TEM image (d) of as-prepared Cu O (truncated) octahedra.
side length from 600 to 800 nm and small square facets at the
vertices. Obviously, the area ratio of {111} and {100} faces of
the (truncated) octahedra increases because the growth
along the (100) plane is quicker than along the (111) plane.
Further investigation shows that a cube with truncation
appears in the TEM image (Figure 4d).
as-synthesized products at 80 ꢀC (Figure 5). The difference
lies in the fact that nanosized pits are formed on the surfaces
and edges. Presumably, the trace oxygen dissolved in solu-
2
þ
tion may gradually oxidize Cu O into Cu , leading to the
2
formation of pits. A similar phenomenon has also been
observed in corrosion-based synthesis of single-crystal Pd
2
8
Fully developed Cu O octahedra are observed when the
2
nanoboxes. When the temperature is increased to 90 ꢀC
(Figure 7b), many ill-defined aggregates are observed in
PVP concentration is increased to 4.5 mM. The as-obtained
Cu O crystals possess perfect octahedral morphology with
2
addition to the Cu O truncated octahedra due to the high
2
sharp corners and well-defined edges. The octahedral edge
lengths is in the range of 1-1.2 μm, increased from that of the
truncated octahedral (Figure 5a,b). Corresponding TEM
analysis shows that these octahedral particles project diffe-
rent two-dimensional shapes onto a plane, such as square
and rhombus, due to the projection of the octahedra on the
nucleation rate. With a further increase of the reaction
temperature to 100 ꢀC, no Cu O truncated octahedra are
2
formed; only equiaxial particles with an average size of about
1 μm are formed (Figure 7c). On the basis of our XRD
results, a small amount of metallic copper appears, indicat-
ing that there is a temperature limit to obtain pure Cu O
2
2
7
TEM grid at a different orientation (Figure 5c,d). The
SAED pattern indicates that this octahedral is a single crystal
of Cu O with a [110]-oriented edge lying on the substrate by
(Figure S2, Supporting Information). The reaction tempera-
ture affects not only the reaction but also the nucleation and
2
9
growth rates of particles. So, increasing the reaction tem-
perature enhances the reaction and increases the diffusion
rate, or nucleation and growth rates. Hence, ill-defined
aggregates and equiaxial particles are more easily formed.
These results show that the reaction temperature plays a key
2
accident (Figure 5d, inset).
When more PVP (9 mM) was used in the reaction,
loose spherical particles with rough surfaces were obtained
(Figure 6a,b). The particles are monodispersed with a dia-
meter of 300-400 nm, and the annular SAED pattern
role in the formation of Cu O crystals.
2
3.2.3. Effects of Reaction Time. For a complete view of the
formation process, we consider the growth process of Cu O
indicates that the Cu O nanospheres are polycrystalline
(Figure 6c,d).
2
2
3
reaction temperature on the formation of Cu O particles
.2.2. Effects of Reaction Temperature. The influence of
octahedra when using 4.5 mM PVP at 80 ꢀC. Time-depen-
dent morphology evolution experiments are performed by
intercepting intermediate products in different reaction
stages of 0.5, 4, 8, 12 h. The Cu O crystals including
2
octahedra (Figure 8a,b), star-shaped (Figure 8c,d), and
star-shaped with six symmetric branches (Figure 8e,f) are
2
were also investigated, at 70, 90, and 100 ꢀC for 2 h using
2
the FESEM image (Figure 7a) shows that the Cu O trun-
.5 mM PVP. When the temperature is decreased to 70 ꢀC,
2
cated octahedra have a similar size distribution to that of the

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 103
Figure 5. FESEM images (a, b) and TEM images (c, d) of as-prepared Cu
2
O octahedra. Inset in (d) is a SAED pattern.
2
Figure 6. FESEM images (a, b), TEM images (c, d), and corresponding SAED pattern (d, inset) of as-prepared Cu O nanospheres.

1
04 Crystal Growth & Design, Vol. 10, No. 1, 2010
Sui et al.
Figure 7. FESEM images of the samples synthesized for 2 h using 2.5 mM PVP at different reaction temperatures: (a) 70; (b) 90; and (c) 100 ꢀC.
3
1
obtained. Figure 8 indicates clearly the evolution process of
the Cu O crystals by varying the reaction time.
the planes. As the reaction proceeds, the polyhedral parti-
cles grow along the different directions with different growth
rates due to their different surface energies.
2
When the reaction is carried out for 30 min, it can be seen
that all of the obtained particles take on an octahedral-like
morphology (Figure 8a,b). Most of the particles are uniform
in size with an average edge length of about 500 nm. There
are also some small nanooctahedra in the products. When
the reaction proceeds for 4 h, certain amounts of well-defined
star-shaped geometries with symmetrical horns begin to
appear (Figure 8c,d). The particle size determined from
the distance between two neighboring vertices is about
A baseline for crystal morphology is the equilibrium shape
that results from minimizing the anisotropic surface free
energy of a crystal under the constraint of constant volume.
As is well-known, the equilibrium shape that corresponds to
a given γ-plot (γ is the anisotropic interfacial free energy per
unit area) is given by the Wulff construction, according to
which the shape is an inner convex hull bounded by planes
3
2
(Wulff planes). For the cubic phase, a sequence of γ{111}
<
2
-2.5 μm. However, if the reaction time is further increased
γ{100}< γ{110} can be easily deduced from the distances
33
to 8 h, the particle size remains unchanged, while the
morphology evolves toward star-shaped particles with six
symmetric branches (Figure 8e,f). After a reaction time of
1
destroyed, as shown in Figure 8g,h.
between these three faces and the central Wulff’s point.
From a crystallographic point of view, Cu O is a cuprites
2
1
þ
2
structure, and the amount of the Cu is twice that of the O -
tetrahedral. The structure can be described as a cubic close
packing of copper atoms. The {111} and {100} surfaces in the
2 h, it is clearly found that some star-shaped particles are
3
citrate complex can be reduced by glucose to form Cu O
.3. Growth Mechanism. It is well-known that the copper-
Cu O crystal lattice are different in the surface atom struc-
2
2
tures. The {111} is a nonpolar surface, while {100} is a polar
3
4,36
precipitates, which are widely used in the analytical deter-
surface.
{111} and {100} faces can be easily maintained in the final
According to Gibbs-Wulff’s theorem, both the
3
mination of saccharides. Regular polyhedral Cu O parti-
0
2
cles are synthesized by the solution-phase reaction between
the copper-citrate complex and glucose at the appro-
priate temperature of 80 ꢀC. The chemical reaction is as
follows:
appearance. Generally, growth habits of dipolar Cu O crys-
2
tal are dependent on not only their polar characteristic, but
also the supersaturation of the solute. Mcfadyen and
Matijevi found that the shape of Cu O particle changed
2
-
CuðcitrateÞ ðaqÞ þ C5H11O5-CHO ðaqÞ f Cu2OðsÞ
from polyhedron to needle when the solute concentrations
3
were rather low. Chen et al. also reported that the shape of
5
þ C5H11O5COOH ðaqÞ
At the beginning of the reaction, the spherical Cu O
the Cu O particle changed from polyhedron to six-pod-like
2
whisker when the supersaturation of the solution was de-
2
3
creased to a relatively low value. These results suggest that
6
particle nucleates. The surface of the Cu O particle contains
2
high index crystallography planes, through which the parti-
cles connect to each other to decrease the surface energy of
crystal growth along the polar directions (the Æ001æ direction
for Cu O) is enhanced at relatively lower supersaturation.
2

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 105
Figure 8. FESEM images of the samples synthesized at 80 ꢀC using 4.5 mM PVP at different reaction times: (a, b) 30 min; (c, d) 4 h; (e, f) 8 h; and
(
g, h) 12 h.
3
7
When inorganic crystals are formed under equilibrium
condition, their crystal habit is determined by the relative
order of surface energies. The fastest crystal growth will
occur in the direction perpendicular to the face with the

1
06 Crystal Growth & Design, Vol. 10, No. 1, 2010
Sui et al.
Scheme 1. Summary of the Influence of Different Concentration of PVP on the Morphology of the As-Prepared Products
Scheme 2. Schematic Illustration of the Shape Evolution Process of Cu O Crystals as a Function of the Reaction Time
2
highest surface energy. This results in the elimination of
higher-energy surfaces while the lower-energy surfaces in-
crease in area. When organic or inorganic additives are
added during the crystal growth process, the relative order
surfactant PVP could act as not only a stabilizer to prevent
the aggregation of the products but also a shape-controller to
assist the formation of polyhedral Cu O crystals. As a well-
2
known capping or stabilizing agent, PVP molecules with
long chains can be adsorbed to the Cu O particle surfaces via
3
8
of surface energies can be modified. Because of anisotropy
in adsorption stability, these additives adsorb onto a certain
crystallographic plane more strongly than others. This pre-
ferential adsorption lowers the surface energy of the bound
plane and hinders the crystal growth perpendicular to this
plane, resulting in a change in the final morphology. Murphy
et al. have reported that the preferential adsorption of
molecules and ions in solution to different crystal faces
makes the nanoparticles develop into various shapes by
2
physical and chemical bonding. When we add PVP into
reaction system, it is believed that PVP tends to suppress
the growth rate of the {111} planes more than that of the
{100} planes, since it interacts more strongly with the {111}
facets than with the {100} facets. When a smaller amount of
PVP is added to the reaction system, the capping effect of
PVP toward Cu O particles is weaker or insufficient. Thus,
2
particles with cubic shapes similar to that in the absence of
PVP are formed. When the PVP concentration is 1.5 mM,
this interaction strength is greatly enhanced, and it could
efficiently lower the surface energies of {111} facets. The
capping effect of PVP would block the growth on the {111}
facets and facilitate the growth on the {100} facets. The
obviously different changes of the growth rates on the {100}
and {111} facets possibly induce the ratio R to have a value
close to 1, leading to the formation of the truncated octa-
3
9
controlling the growth rates along different crystal axes.
A similar phenomenon has been also observed in the shape
control of noble metal and other semiconductors with well-
defined shapes. For example, as for the synthesis of fcc
structured Pd, PVP molecules are adsorbed onto the {100}
facets more strongly than onto others, leading to the forma-
tion of nanocubes with truncated corners mainly bounded by
six {100} facets. Because PVP is a capping polymer, strong
interaction exists between the oxygen (and/or nitrogen)
hedral Cu O crystals. Further increasing the PVP concen-
2
4
0
atoms of PVP and the {100} facets. As for the synthesis
of cubic structured Cu O, it has been demonstrated that the
tration to 4.5 mM, truncated corners ascribed to {100} facets
begin to disappear gradually, and even perfect octahedral
(R = 1.73) are obtained due to the further increase of the
growth rate on {100} facets relative to the {111} facets.
Finally, spherical particles are generated only when the
PVP concentration is 9 mM, owing to the high coverage of
2
surface energy of the {100} facets of Cu O can be selectively
2
lowered relative to that of the {111} facets in the presence of
chloride ions, which can stabilize {100} facets and thus
4
induce the formation of single-crystal nanocubes. More-
1
3
1
over, as well demonstrated by Wang, the ratio (R) of the
growth rate along the Æ100æ to Æ111æ directions determines the
geometrical shape of a crystal. The shape of a cubic nano-
crystal will evolve from a perfect cube (R = 0.58) to a
cuboctahedra (R=0.87), a truncated octahedra (0.87 < R
PVP on all the planes of Cu O nanocrystals. Presumably, the
2
steric effect of PVP against agglomeration and growth is
4
fulfilled and the Cu O particles are smaller, leading to an
2
2
isotropic growth mode and loose spherical particles. Accord-
ing to this tendency, we can speculate that truncated cubes
(R = 0.7) and other kinds of cuboctahedra (R = 0.87) as
intermediate products can be achieved through the regula-
tion of the PVP concentration in the range of 0.5-1.5 mM.
This unique role of PVP for the formation of spherical
structures evolved from polyhedra is frequently reported in
< 1.73), and finally to a perfect octahedron (R=1.73) as R
increases.
As the reaction proceeds, the polyhedral particles grow
along the different directions with different growth rates due
to their different surface energies. In the present system,

Article
Crystal Growth & Design, Vol. 10, No. 1, 2010 107
of six symmetric branch structures on the octahedral. The
total process is depicted in Scheme 2.
3
sorption spectra of Cu O products with different morpho-
.4. Optical Properties. Figure 9 shows the UV-vis ab-
2
logy including cubes, truncated octahedra, (truncated)
octahedra, octahedra. A photograph of cuvettes containing
the four Cu O solutions is provided in the Supporting
2
Information (Figure S4). The absorption spectra for all the
sample are dominated by strong light scattering bands
19
resulting from the relatively large sizes of these crystals.
The light absorption band of these Cu O crystals at 493 nm
2
for samples cubes, truncated octahedra and at 503 nm for
samples (truncated) octahedra, octahedra can be clearly
distinguished from the light scattering bands, and the calcu-
lated band gap energy of these samples are in the range of
2
that of bulk Cu O at 2.17 eV, is attributed to quantum
.46-2.51 eV. This greater band gap value, as compared to
2
1
8
confinement effects.
4
. Conclusion
2
Figure 9. UV-vis absorption spectra of Cu O products with dif-
ferent morphologies: (a) cube, (b) truncated octahedra, (c)
(
In this paper, we present a facile and effective route for the
shape-controlled synthesis of Cu O crystals via reducing the
truncated) octahedra, (d) octahedra.
2
4
3,29
copper-citrate complex solution with glucose and selective
interaction abilities of PVP. A series of morphologies, such as
cubes, truncated octahedra, octahedral, and star-shaped par-
ticles withsixsymmetric branches, are easily obtained through
the delicate manipulation of the PVP concentration and the
reaction time. On the basis of a relatively clear understanding
of shape evolution and corresponding growth mechanism, we
believe thatthispolymer-mediated approach can be appliedto
other shapes and compositions of cubic structure materials.
previous literature.
The whole shape evolution of Cu O
2
crystals from cubes to octahedra and finally nanospheres as a
function of the PVP concentration is schematically illu-
strated in Scheme 1.
Generally, the intermediates obtained at different reaction
intervals can be used to shed light on the growth mechanism
of the crystals. So far, this method is widely used to study the
morphological formation of various kinds of crystals under
different reaction conditions. To investigate the formation
process of stable Cu O octahedral, samples (Figures 5 and
a,b) at different intervals are prepared with other experi-
mental conditions the same. The octahedral with a diameter
of ∼500 nm and some small nanooctahedra are obtained
after a reaction time of 30 min, as shown in Figure 7a,b. This
is consistent with the idea that the big octahedra grows from
smaller particles. Particle edge sharpening occurs concomi-
tantly with particle growth. With the reaction proceeding to
h, the products show uniform octahedra with a narrow size
distribution (Figure 5). At this stage, smaller particles are
dissolved again and larger particles grow more, which is
4
4
2
Acknowledgment. This work was supported by the Natu-
ral Science Foundation of China (Grant Nos. 10874053,
7
5
(
0532020)and the National Basic ResearchProgramofChina
Grant No. 2005CB724404).
Supporting Information Available: Table 1. Different Synthesis
Conditions of Cu O Particles. Figure S2. XRD patterns of the Cu O
2
2
synthesized at 100 ꢀC. Figure S3. FESEM images of the samples
synthesized for 2 h using 2.25 mM PVP at 80 ꢀC. Figure S4. A
photograph of the four solutions showing slight color variation.
This information is available free of charge via the Internet at http://
pubs.acs.org/.
2
4
5
typical of Ostwald ripening. Accordingly, the initially
formed nanooctahedra can further grow into large ones via
Ostwald ripening subsequently. Furthermore, in our system,
it is worth noting that when the reaction time is further
increased to 8 h, the greatly enhanced growth rate on the
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