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Nano Today (2010) 5, 106—116
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/nanotoday
REVIEW
Morphologically controlled synthesis of Cu2O
nanocrystals and their properties
Chun-Hong Kuo, Michael H. Huang ∗
Department of Chemistry, National Tsing Hua University, Hsinchu 30013, Taiwan
Received 21 December 2009; received in revised form 27 January 2010; accepted 8 February 2010
Available online 4 March 2010
KEYWORDS
Cuprous oxide;
Nanocrystals;
Morphology control;
Hollow;
Core—shell
heterostructures;
Electrodeposition;
Surface properties;
Photodegradation
Summary The ability to prepare inorganic nanocrystals with well-defined morphologies and
sharp faces should facilitate the examination of their facet-dependent surface, catalytic, electrical, and other properties. In this review we cover different synthetic methods for the growth
of Cu2 O nanocrystals with morphological control. Cu2 O nanocrystals with cubic, cuboctahedral,
truncated octahedral, octahedral, and multipod structures have been prepared mainly by wet
chemical, electrodeposition, and solvothermal synthesis methods. Methods used for the formation of hollow Cu2 O nanocubes, octahedra, and truncated rhombic dodecahedra are also
presented. Morphology of Cu2 O nanocrystals can be expanded with the use of gold nanocrystal
cores to guide the overgrowth of Cu2 O shells. Surface properties of Cu2 O nano- and microcrystals
with sharp faces have been examined in a few studies. The {1 1 1} faces were found to interact
well with negatively charged molecules, while the {1 0 0} faces are less sensitive to molecular
charges. Preferential adsorption of sodium dodecyl sulfate molecules on the {1 1 1} faces of
Cu2 O crystals has been demonstrated via plane-selective deposition of gold nanoparticles on
only the {1 0 0} faces. It is expected that the development of improved synthetic methods for
Cu2 O nanocrystals and more knowledge of their facet-dependent properties should lead to their
applications in photoactivated energy conversion and catalysis.
© 2010 Elsevier Ltd. All rights reserved.
Cuprous oxide (Cu2 O) is a p-type semiconductor with a direct
band gap of 2.17 eV [1]. It has a unique cuprite structure
(a body-centered cubic packing of oxygen atoms with copper atoms occupying one-half of the tetrahedral sites). In
recent years, there is a growing interest to synthesize Cu2 O
nanostructures not only for the development of synthetic
strategies, but also for the examination of their sensing,
∗
Corresponding author. Tel.: +886 3 5718472; fax: +886 3 5711082.
E-mail address: [email protected] (M.H. Huang).
catalytic, electrical, and surface properties. Cu2 O nanostructures have been demonstrated to possess properties
useful for applications in gas sensing [1,2], CO oxidation [3],
photocatalysis [4—8], photochemical evolution of H2 from
water [9], photocurrent generation [10,11], and organic synthesis [12,13]. The electrical properties of individual Cu2 O
nanowires synthesized under hydrothermal conditions in
the presence of poly(2,5-dimethoxyanaline) have also been
examined [14]. A solar cell consisted of vertically oriented
n-type ZnO nanowires, surrounded by a film constructed
from p-type Cu2 O nanoparticles has recently been fabri-
1748-0132/$ — see front matter © 2010 Elsevier Ltd. All rights reserved.
doi:10.1016/j.nantod.2010.02.001
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Morphologically controlled synthesis of Cu2 O nanocrystals and their properties
Table 1
107
Synthetic strategies for Cu2 O nanomaterials and the resulting product morphologies.
Applied methods
Crystalline solid
Hollow
structure
Core—shell heterostructure
Ref.
Wet chemical reduction
Sphere Cube
Octahedron RD* Rod,
Wire Multipod
Sphere Box
Octahedron RD*
Tube
Cube
Octahedron
Icosahedron
Column Plate
[1,4—7,11,16—35]
Electrodeposition
Sputtering
Sphere Cube
Octahedron RD* Rod,
Wire Multipod Film
Sphere Rod Multipod
Wire
Sphere
High pressure treatment
Sphere
Irradiation
Octahedron Cube
Solvothermal synthesis
* RD:
[10,36—49]
Cube
Sphere
[8,14,50—57]
[58]
[59]
Cube
[60,61]
rhombic dodecahedron.
cated [15]. Cu2 O nanocrystals are relatively easy to make,
safe, and low in preparation cost because of abundant copper ion sources and low energy consumption. They also can
form a wide variety of morphologies. These attributes make
Cu2 O an important metal oxide for the investigation of their
properties and applications in catalysis and optoelectronics.
Various interesting cuprous oxide nanostructures such as
cubes, cuboctahedra, octahedra, multipods, nanowires, and
hollow structures have been synthesized. They are mostly
prepared by wet chemical reduction [1,4—7,11,16—35],
electrodeposition [10,36—49], and solvothermal synthesis
methods [8,14,50—57]. In a few studies, Cu2 O nanostructures have been generated by the use of sputtering [58],
high pressure treatment [59], and irradiation techniques
[60,61]. Table 1 gives a summary of some of the synthetic
strategies used for growing Cu2 O nanostructures and the
resulting product morphologies. In this review, we focus
our discussion on the synthesis of Cu2 O nanostructures
with well-defined morphologies. Nano- and microcrystals
with sharp facets provide surfaces for the examination of
their facet-dependent properties with greater distinction
and certainty. Their preparation should represent an important direction for nanomaterials research. We begin by
introducing some methods for the synthesis of regular polyhedra such as cubes, cuboctahedra, truncated octahedra,
octahedra, hexapods, and other complex multipod structures. These nanostructures possess well-defined facets and
sharper edges, and are ideal for facet-dependent property studies. Hollow Cu2 O nanostructures are presented
next. Cu2 O hollow structures are usually synthesized directly
without the use of templates or through the dissolution
of the pre-formed solid Cu2 O nanocrystals. For this reason, they can also possess well-defined morphologies such
as cubic, octahedral, and other complex but symmetrical
shapes. An interesting development in the synthesis of Cu2 O
nanostructures is the use of metal nanoparticle cores to
direct the growth of Cu2 O shells with morphological control.
The metal—Cu2 O core—shell heterostructures may show
enhanced catalytic and electrical properties. Their preparation will be briefly described. Lastly we will illustrate the
importance of being able to prepare Cu2 O crystals with well-
defined facets by showing how the different crystal faces
can display remarkably different surface and photocatalytic
properties. Remarks on some future research directions for
Cu2 O nanocrystals will be given.
Synthesis of Cu2 O nanocrystals with regular
polyhedral structures
A number of studies have described the synthesis of Cu2 O
nanocubes. Murphy et al. have prepared Cu2 O nanocubes
with edge lengths of approximately 450 nm by mixing a
solution of CuSO4 , cetyltrimethylammonium bromide (CTAB)
surfactant, sodium ascorbate and NaOH at a reaction temperature of 55 ◦ C for 15 min [16]. Xia et al. heated a solution
of ethylene glycol at 140 ◦ C, and added NaCl, Cu(NO3 )2 , and
poly(vinyl pyrrolidone) (PVP) to synthesize Cu2 O nanocubes
with an average edge length of 410 nm [18]. Chloride ions
were found to play a pivotal role in the formation of
nanocubes; polycrystalline spheres were generated in the
absence of chloride ions. Growth of monodisperse Cu2 O
nanocubes over a wide size range has generally not been
demonstrated. The Huang group used a seed-mediated synthesis approach in aqueous solution to grow monodisperse
Cu2 O nanocubes with approximate average sizes of 40, 65,
100, 230, and 420 nm in high yield [6]. Cu2 O seed particle solution was added to a growth solution containing
CuSO4 , sodium dodecyl sulfate (SDS) surfactant, sodium
ascorbate, and NaOH to grow into larger nanocubes. These
Cu2 O nanocubes were in turn used as the seeds to make even
larger nanocubes by repeating the same growth process.
Nanocubes with slight edge and corner truncations were
exclusively formed after reaction for 2 h at room temperature. Fig. 1 gives a scheme of the experimental procedure
and some representative SEM and TEM images of the Cu2 O
nanocubes produced.
Reports primarily on the synthesis of octahedral Cu2 O
nanocrystals are also available. Wang et al. mixed aqueous solutions of CuCl2 , NH3 solution, and NaOH to form
blue Cu(OH)2 precipitate, and added N2 H4 to make Cu2 O
nanocrystals [7]. By keeping a molar ratio of NH3 to CuCl2 at
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108
C.-H. Kuo, M.H. Huang
Figure 1 Schematic illustration of the procedure used to grow Cu2 O nanocubes of different sizes by a seed-mediated synthesis
method. The procedure used to prepare samples C—F is the same as that for sample B. SEM images of the Cu2 O nanocube products
observed for samples A—F are also shown. Nanocubes were formed in samples B—F. SA refers to sodium ascorbate (reproduced with
permission from [6], copyright ©2007 WILEY-VCH).
7:1, and varying the molar ratio of NaOH to CuCl2 from 2:1 to
4:1 and 8:1, octahedral Cu2 O nanocrystals with respective
sizes of around 140 to 400 and 600 nm can be synthesized.
Small octahedral Cu2 O nanocrystals with sizes of 100 nm or
less have been prepared by irradiating an aqueous solution
of Cu(NO3 )2 in microemulsions of Triton X-100, n-hexanol,
and cyclohexane with a 60 Co ␥-ray source [60]. This method
is limited to the availability of the ␥-ray radiation.
For the examination of facet-dependent properties, it
is desirable to synthesize Cu2 O nanocrystals with systematic shape evolution from cubic to octahedral structures
by a single method. Nanocrystals prepared under simi-
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Morphologically controlled synthesis of Cu2 O nanocrystals and their properties
109
Figure 2 Schematic illustration of the procedure used to grow Cu2 O crystals with various shapes. The corresponding SEM images
of the samples (a—h) are shown in Figure 3.
Figure 3 SEM images of the Cu2 O nanocrystals with various morphologies: (a) cubes, (b) truncated cubes, (c) cuboctahedra, (d)
type I truncated octahedra, (e) type II truncated octahedra, (f) octahedra, (g) short hexapods, and (h) extended hexapods. Scale
bar = 1 ␮m (reproduced with permission from [5], copyright ©2009 American Chemical Society).
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110
lar solution conditions are good for comparison studies.
The Huang group has developed a facile aqueous colloidal
solution approach for the synthesis of monodisperse Cu2 O
truncated nanocubes, cuboctahedra, truncated octahedra,
and octahedra at room temperature [4]. Water, CuCl2 solution, SDS surfactant, NH2 OH·HCl reductant, and NaOH were
added in the sequence listed, and the mixture was aged
for 2 h to obtain the products. The systematic variation in
the product morphology can be controlled by changing the
amount of reductant added. Examination of the intermediate Cu2 O products formed after aging the final solutions
for just 5 min showed that nanocrystals with rough surfaces
but overall cubic, cuboctahedral, and octahedral morphologies and sizes approaching the final nanocrystal dimensions
have already appeared, indicating a rapid crystal growth has
occurred. These intermediate structures seem to be formed
through the aggregation of smaller crystals of 50—100 nm
in diameter, which were in turn produced via aggregation of 10—20 nm Cu2 O seed particles. Ripening and surface
reconstruction processes take place in the next 2 h to form
the final nanocrystal structures with distinct shapes. It is
presumed that the amount of NH2 OH·HCl introduced may
influence the growth rate along the [1 0 0] direction relative to that of the [1 1 1] direction, or the value of R, and
results in this morphology evolution. For example, when R is
1.15 and 1.73, truncated octahedral and perfect octahedral
particles are produced [62]. The truncated cubic, cuboctahedral, truncated octahedral, and octahedral nanocrystals
have sizes of approximately 300, 380, 220, and 160 nm,
respectively. The particle size can increase to 400—600 nm
by increasing the volume of NaOH added to the reaction
mixture. However, the surfaces of these nanocrystals are
not very sharp. They have smooth edges and not so perfectly flat faces. In another study, cubic to octahedral and
then to hexapod structures with sharp faces can be systematically synthesized by changing the sequence of the
introduction of the reagents listed above [5]. Fig. 2 provides a schematic illustration of the synthetic procedure
used. Here NaOH was the second reagent added. The idea
is to have Cu(OH)2 and Cu(OH)4 2− species present and wellmixed in the solution before the introduction of NH2 OH·HCl
reductant, since the crystal growth rate is fast and the initial
crystal morphology determines the final product structure.
By simply varying the amount of NH2 OH·HCl added from 0.15
to 0.95 mL, cubic-, truncated cubic-, cuboctahedral-, truncated octahedral-, octahedral-, and short hexapod-shaped
Cu2 O nanocrystals with sharp faces were synthesized. Fig. 3
shows the SEM images of the Cu2 O nanocrystals synthesized
with various morphologies. These crystals have sizes in the
range of 450—600 nm. The extended hexapods have longer
branches and they have grown to about 1 ␮m in size. Clear
transition in the relative intensities of the (1 1 1) and the
(2 0 0) reflection peaks in their XRD patterns was observed.
Gao et al. have also synthesized Cu2 O crystals with shape
evolution [21]. A mixture of CuSO4 aqueous solution, oleic
acid, and ethanol was heated to 100 ◦ C. NaOH and D-(+)glucose, a reducing agent, were subsequently added with
stirring for 60 min. By varying the amount of oleic acid
added, cubic, octahedral, edge-truncated octahedral, and
rhombic dodecahedral Cu2 O microcrystals were formed. The
shape transition is particularly interesting because it shows
that rhombic dodecahedral crystals with only {1 1 0} faces
C.-H. Kuo, M.H. Huang
Figure 4 SEM image of octahedral Cu2 O nanocages obtained
at an aging time of 3 h (reproduced with permission from [33],
copyright ©2005 WILEY-VCH).
exposed can be obtained through the edge truncation of
octahedra. An edge-truncated octahedron is bounded by
both {1 1 1} and {1 1 0} surfaces.
Electrodeposition is another important method used to
make Cu2 O crystals with systematic crystal morphology control. Choi et al. have demonstrated that pre-grown cubic
Cu2 O crystals formed by electrodeposition can be transformed into edge- and corner-truncated cubes and then
octahedra after the second electrodeposition process in a
Cu(NO3 )2 solution containing (NH4 )2 SO4 [36]. The temporary
appearance of {1 1 0} faces was attributed to their relative stability of planes in this medium being in the order of
{1 0 0} < {1 1 0} < {1 1 1} faces. Conversely, pre-grown octahedra can be transformed into edge- and corner-truncated
octahedra and then cubes over time by carrying out a
second electrodeposition process in a Cu(NO3 )2 solution
containing SDS and NaCl. The relative stability of {1 0 0},
{1 1 1}, and {1 1 0} faces in this medium is in the order of
{1 1 1} < {1 1 0} < {1 0 0}. In another study, they have shown
that Cu2 O crystals with multiple symmetrical branches and
facets can be grown by adjusting the applied potential
or current density in electrocrystallization [37]. A wide
variety of morphologies can be obtained by controlling
the deposition conditions and varying the duration of the
electrodeposition process. Branched crystals can eventually
transform into cubes or octahedra by a filling-in process.
The Cu2 O crystals grown by the electrochemical method are
several micrometers in size. It is not clear if this approach is
still effective in growing Cu2 O nanocrystals with a high level
of morphology control.
Complex but symmetrical Cu2 O multipods have also been
prepared by the solvothermal synthesis approach. By varying
the relative volumes of Cu(NO3 )2 solution, water—ethanol
solvent mixture, and formic acid added to a Teflon-lined
stainless steel autoclave and heating the autoclave in an
electric oven at 150—220 ◦ C for 1.25—5 h, Zeng et al.
obtained hexapods, octapods, and dodecapods with various
branch morphologies [54]. The branches were respectively
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Morphologically controlled synthesis of Cu2 O nanocrystals and their properties
111
Figure 5 SEM and TEM images of the truncated rhombic dodecahedral Cu2 O nanoparticles: (a—d) type-I nanoframes, (e—h)
nanocages, and (i—l) type-II nanoframes. The magnified images clearly show the hollow structures of the nanoparticles. Two different
viewing directions for each type of the hollow nanoparticles are also shown (reproduced with permission from [29], copyright ©2008
American Chemical Society).
grown along the 1 0 0, 1 1 1, and 1 1 0 directions of a
cube. In many structures, the branches have well-defined
facets. These multipods, however, are also several micrometers in size. It remains challenging to prepare Cu2 O
multipods with sizes of less than one micrometer.
Synthesis of hollow Cu2 O nanostructures
Hollow Cu2 O nanostructures have generally been formed
without the use of hard templates. Some reports have
described the formation of hollow Cu2 O spheres and multishelled spheres [31,55]. In this review, the formation of
hollow Cu2 O nanostructures with more well-defined facets is
discussed. Qi et al. synthesized octahedral Cu2 O nanocages
via the catalytic reduction of a basic copper tartrate
complex solution with glucose followed by an oxygenengaged catalytic oxidation process [33]. A stock solution
of CuSO4 ·H2 O, potassium sodium tartrate tetrahydrate, and
KOH was first prepared. PdCl2 and glucose solutions were
successively introduced into the stock solution and water
was added with stirring. The mixture was stirred and then
aged at 75 ◦ C under static condition for 3 h. Fig. 4 gives an
SEM image of the octahedral Cu2 O nanocages produced. The
nanocages appear to be relatively flat and are bounded by
{1 1 1} surfaces. TEM images of the particles also reveal their
hollow interior. Interestingly, the nanocages usually have
holes on the apexes. These holes were considered to be sites
for oxidative etching. Although Pd nanoparticles generated
from the reduction of PdCl2 by glucose was believed to play
a role in the formation of Cu2 O nanocages, the presence of
Pd nanoparticles was not confirmed. Notably, the octahedral nanocages are gradually etched into fragments at an
aging time of 6 h. After 10 h of aging, the Cu2 O particles are
completely dissolved.
Hollow Cu2 O nanocubes have also been prepared by the
solvothermal synthesis method. Jiao et al. made hollow
Cu2 O nanocubes by first heating a mixture of CuCl2 ·2H2 O,
NaOH, and poly(ethylene glycol) 200 (PEG-200) in a Teflonlined autoclave at 180 ◦ C for 6 h in an oven [50]. After cooling
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112
C.-H. Kuo, M.H. Huang
Figure 6 SEM and TEM images of the Au-Cu2 O core—shell nanocrystals. (a, b) Cuboctahedral heterostructures made from octahedral Au nanoparticle cores. (c, d) Truncated stellated icosahedra formed from highly faceted Au nanoparticle cores. The slight
structural variation observed may be related to the morphological variety of the gold nanoparticles used. (e, f) Cu2 O shells with
pentagonal prism shape from 5-fold-twinned Au nanorods as templates. Barlike protrusions can be seen along the side faces of the
pentagonal prism heterostructure. (g, h) Thick truncated triangular Cu2 O plates formed from triangular and truncated triangular
Au plates as templates (reproduced with permission from [30], copyright ©2009 American Chemical Society).
the autoclave to room temperature, the top solution was
removed and water was added to this solution with stirring for 30 min. The precipitate collected was found to be
hollow Cu2 O nanocubes with side lengths of 50—90 nm. The
underlying solid in the autoclave, however, was a mixture
of Cu2 O, CuO, and Cu, as determined from its XRD pattern. The nanocubes have truncated corners. Many of the
nanocubes show partially removed or unformed faces. Zeng
et al. have also prepared hollow Cu2 O nanocubes by mixing
Cu(NO3 )·3H2 O, N,N-dimethylformamide (DMF), and a small
amount of water in a Teflon-lined stainless steel autoclave
and heating the mixture at 200 ◦ C for 6.5 h [51]. The hollow nanocubes are about 200 nm in size. TEM images of the
cubes reveal that there is a void space in the center of each
cube. XRD and XPS analyses indicated the initial formation
of CuO crystallites at a reaction time of 1.5 h. Both CuO and
Cu2 O crystallites are present at a reaction time of 2.5 h. The
CuO crystallites have mostly been reduced to Cu2 O nanocrystals after 3.5 h of reaction. The nanocubes were found to be
formed through aggregation of small crystallites via an Ostwald ripening mechanism. Hollowing and recrystallization
take place over the next 2 h of reaction. Metallic Cu component was identified after 7 h of reaction. Results of these
studies show that the preparation of hollow Cu2 O nanostructures with well-defined shapes and pure crystal phase is still
challenging.
Fabrication of hollow Cu2 O nanostructures does not necessarily require heating. The Huang group has developed
a facile procedure for the synthesis of truncated rhombic
dodecahedral Cu2 O nanocages and nanoframes that involves
the preparation of an aqueous solution containing CuCl2 ,
SDS, NH2 OH·HCl, HCl, and NaOH. The procedure used is similar to that employed for the synthesis of cuboctahedral
Cu2 O nanocrystals, but with the addition of HCl [4]. Allowing the Cu2 O nanostructures to grow by aging the reaction
mixture for ∼45 min and 2 h generated type-I nanoframes
and nanocages, respectively. After the formation of the
nanocages, addition of ethanol followed by sonication of the
solution transformed the nanocages into type-II nanoframes.
Fig. 5 displays SEM and TEM images of the Cu2 O nanoframes
and nanocages synthesized. Each truncated rhombic dodecahedral particle contains twelve hexagonal {1 1 0} faces and
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Morphologically controlled synthesis of Cu2 O nanocrystals and their properties
six {1 0 0} faces. The type-I nanoframes are constructed of
hexagonal {1 1 0} skeleton. They are 300—350 nm in diameter. The {1 0 0} faces are formed in the nanocages, so they
have a truncated rhombic dodecahedral morphology. The
nanocages have larger diameters (350—400 nm) and thicker
walls than the type-I nanoframes. The added HCl promoted
the etching process. When ethanol is added and sonication
of the solution is applied, the adsorption of SDS molecules
on the nanocage surfaces likely is temporarily disrupted.
Removal of SDS facilitates the reaction of Cu2 O and HCl to
form HCuCl2 . The faster etching rate on the {1 1 0} faces
than on the {1 0 0} faces transforms the nanocages into
type-II nanoframes with thinner walls and smaller particle
sizes. Type-II nanoframes are 200—250 nm in diameter. The
nanocages are not stable under the acidic solution condition, and become collapsed pieces after aging the solution
for 6 h.
Synthesis of metal—Cu2 O core—shell
heterostructures
Metal—Cu2 O core—shell heterostructures have been synthesized using gold nanoparticles as the cores. Novel
Cu2 O nanostructures may be obtained using this synthetic
strategy. Wang et al. have covered a copper grid with
tetraoctylammonium bromide-stabilized gold nanoparticles
of 3—7 nm in diameter and heated the copper grid in an
oven at 300 ◦ C for 30 min to prepare Au-Cu2 O core—shell heterostructures [63]. The heterostructures exhibit cubic and
octahedral shapes with edge dimensions of 15—45 nm. TEM
characterization shows that the (1 1 1) lattice planes of gold
are parallel to the (1 1 1) lattice planes of Cu2 O, and that the
(2 0 0) lattice planes of gold are parallel to the (2 0 0) lattice
planes of Cu2 O. The gold nanoparticles do not necessarily
reside at the centers of the heterostructures.
When sufficiently large gold nanocrystal cores with
well-defined facets are used to make Au-Cu2 O core—shell
heterostructures, the gold nanoparticle cores can guide
the growth of Cu2 O shells with morphological and orientation control. The Huang group has recently synthesized
unusual Au-Cu2 O core—shell heterostructures by use of
Au nanoplates, nanorods, octahedra, and highly faceted
nanoparticles as structure-directing cores for the overgrowth of Cu2 O crystals [30]. The heterostructures were
synthesized by simply preparing a mixture of CuCl2 , SDS
surfactant, Au nanocrystals, NaOH, and NH2 OH·HCl aqueous
solution, added in the order listed, and aging the mixture for 2 h. Fig. 6 shows the SEM and TEM images of the
Au-Cu2 O core—shell nanocrystals. Uniform shell growth has
been achieved for the samples. Every composite nanocrystal contains just one Au nanoparticle inside. Octahedral
gold nanocrystals with sizes of 80 to over 100 nm developed into cuboctahedral Au-Cu2 O heterostructures. The
octahedral gold core has an orientation with its six corners aligned perpendicular to the six {1 0 0} faces of the
cuboctahedral Cu2 O shell, strongly suggesting that the shell
growth is precisely guided by the shape of the metal core.
Unusual truncated stellated icosahedra were produced from
the highly faceted gold nanocrystals. Each structure contains 20 protruded triangular {1 1 1} faces. When long gold
nanorods with a pentatwinned structure were employed
113
as the metal cores, the thick Cu2 O shells also develop
into a similar pentagonal prism shape with five side protrusions running along the length of the heterostructures.
Finally, by use of micrometer- and submicrometer-sized triangular, truncated triangular, and hexagonal gold plates as
the templating cores, conformal growth of Cu2 O shells was
observed. Cross-sectional TEM images of the heterostructures reveal that the gold nanocrystals are located at the
centers of the composite nanostructures. The orientation
relationship between the core and the shell can be clearly
identified for the cases with octahedral nanocrystals and
pentatwinned nanorods as the structure-directing cores.
The (1 1 1) planes of Cu2 O were found to grow epitaxially
on the {1 1 1} facets of gold, while the (2 0 0) planes of
Cu2 O can grow over the {2 0 0} facets of gold to form the
interfaces. Thus, despite the significant lattice mismatches
between the different gold surfaces and the lattice planes
of Cu2 O, these metal—semiconductor core—shell structures
can still be prepared. Systematic morphological evolution of
the shell morphology can also be easily achieved by varying
the amount of NH2 OH·HCl reductant added. This synthetic
approach also allows the formation of Cu2 O stellated icosahedra and star columns formed by adjusting the volume of
reductant introduced. TEM analysis shows that each triangular pyramid of the stellated icosahedron is bounded by
three {1 0 0} side faces and a {1 1 1} top face. A star column was found to be bounded by {1 0 0}, {1 1 0}, and {1 1 1}
faces. Examination of the intermediate products collected
after 10 min of reaction surprisingly showed that the overall hollow shell framework structure was constructed first
before completely filling the space between the outer shell
and the core. Nevertheless, the shell was connected to the
core via pre-formed Cu2 O bridges, through which the core
can guide the growth of the shell with a proper orientation.
This unusual hollow-shell-refilled (HSR) growth mechanism
is different from the epitaxial overgrowth of shells on the
surfaces of the core particles normally observed. This study
illustrates the possibility of expanding the rich morphological variety of Cu2 O nanostructures by the use of metal
nanocrystal cores.
Facet-dependent properties of Cu2 O crystals
The ability to make Cu2 O nano- and microcrystals with
sharp faces allows the examination of their facet-dependent
properties with greater confidence. Few studies have interrogated the facet-specific properties of Cu2 O nano- and
microcrystals. This represents another interesting aspect
of research for the exploration of applications of Cu2 O
nanostructures. The Huang group has used the synthesized
Cu2 O nanocrystals with well-defined structures and sharp
faces to investigate their comparative photocatalytic activity [5]. They have previously demonstrated that octahedral
Cu2 O crystals with entirely {1 1 1} faces are photocatalytically more active than truncated cubic crystals with mostly
{1 0 0} facets [4,6]. A crystal model analysis shows that the
(1 0 0) planes contain oxygen atoms as they do in the unit
cell. However, a cut of the unit cell over one of its (1 1 1)
planes reveals the presence of surface Cu atoms with dangling bonds [5]. This simple comparison indicate that the
{1 1 1} faces are higher in surface energy and expected to
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114
C.-H. Kuo, M.H. Huang
Figure 7 A plot of the extent of photodegradation of methyl
orange vs. time for the various Cu2 O nanostructures is shown.
Here type II truncated octahedra were used. The blank sample did not contain Cu2 O crystals but only the methyl orange
solution. The temperature change of the solution over this time
period is also given (reproduced with permission from [5], copyright ©2009 American Chemical Society).
Figure 8 A plot of the extent of photodegradation of methylene blue vs. time for the Cu2 O cubes and octahedra is shown.
The blank sample did not contain Cu2 O crystals but only
the methylene blue solution. Lower cell temperatures were
recorded because the room temperature was lower here (reproduced with permission from [5], copyright ©2009 American
Chemical Society).
be more catalytically active than the {1 0 0} faces. Furthermore, Cu2 O crystals bounded by the {1 1 1} faces contain
positively charged copper atoms at the surfaces, whereas
those bounded by the {1 0 0} faces such as the cubes are
electrically neutral. This observation suggests that Cu2 O
octahedra should interact more strongly with negatively
charged molecules and photodegradation of these molecules
is more effective. Use of a positively charged molecule can
result in a poor photodecomposition performance. On the
other hand, cubic Cu2 O crystals are less sensitive to the
charge of the adsorbed molecules and are simply not photocatalytically active.
To test the relative photocatalytic activities of the
Cu2 O nanocrystals synthesized, methyl orange, a negatively
charged molecule, was first used for the photodecomposition experiments. A cell containing the particle solution was
constantly stirred and irradiated with light from a 200 W
mercury lamp. UV—vis absorption spectra of the nanocrystal
solutions were taken before and after every 60 min of irradiation. Fig. 7a is a plot of the extent of photodegradation of
methyl orange vs. time for the various Cu2 O nanostructures
used. As expected, octahedra with entirely {1 1 1} facets
are much more photocatalytically active than cubes. The
cubes were practically not effective at photodecomposing
methyl orange. Another notable finding from this series of
experiments is the exceptionally high photocatalytic performance of the extended hexapods. This result suggests
that Cu2 O nanocrystals with more {1 1 1} facets can serve as
more efficient photocatalysts. The presence of more sharp
edges between the {1 1 1} facets in the hexapods may also
enhance their catalytic activity. When methylene blue, a
positively charged molecule, was used for the photodegradation experiments, both the cubes and octahedra did not
cause any photodegradation, as the degree of degradation
after 2 h of irradiation for these crystals was the same as
that of the blank sample (see Fig. 8a). Amazingly, although
the cubes can be dispersed in the methylene blue solution as
usual, the octahedra cannot mix well with the solution. After
the solution was stirred for minutes to over an hour without
irradiation, the octahedra gradually moved to the surface
of the solution or adhered to the inner top wall of the cell.
Extended hexapods showed a similar effect with a significant
amount of the particles moving to the surface of the solution
in minutes. The electrostatic repulsion force is believed to
cause this effect, a result consistent with the crystal model
analysis. The cubes, as predicted, were insensitive to the
molecular charge and can stay in the solution. However,
they were not photocatalytically active. The results clearly
demonstrate the dramatic difference in the catalytic activities of the {1 1 1} and {1 0 0} faces of Cu2 O crystals. Huang
et al. have reported the instability of the {1 0 0} and {1 1 0}
facets of large Cu2 O microcrystals during photocatalysis of
methyl orange [64]. Flakelike structures were produced on
the {1 0 0} and {1 1 0} surfaces of the microcrystals after
1 h of photoirradiation, while the {1 1 1} facets of the same
microcrystals were more resistent to deformation.
Preferential adsorption of additives on specific surfaces
of Cu2 O crystals has been exploited through the planeselective deposition of gold nanoparticles. Choi et al. have
grown cubic, octahedral, and truncated octahedral Cu2 O
microcrystals on indium tin oxide (ITO) substrates electrochemically with and without the introduction of SDS
surfactant to the plating solution [65]. They have previously reported that the preferential adsorption of SDS on
{1 1 1} faces of Cu2 O crystals can be utilized to obtain
octahedral crystal shapes [36,38]. Gold nanoparticles were
placed on these Cu2 O crystals via electrodeposition by using
Cu2 O crystals on ITO as the working electrode. A AuCl3
aqueous solution was used as a plating solution. Fig. 9
shows SEM images of the truncated octahedral Cu2 O crystals
with plane-selective deposition of gold nanoparticles. Gold
nanoparticles have exclusively been deposited on the {1 0 0}
faces of the Cu2 O crystals. The result indicates that the
preferential adsorption of SDS on the {1 1 1} faces can effectively inhibit the nucleation of gold on these planes. This
study also suggests that the {1 1 1} faces of Cu2 O crystals
Author's personal copy
Morphologically controlled synthesis of Cu2 O nanocrystals and their properties
115
Figure 9 SEM images of truncated octahedral Cu2 O crystals showing the selective deposition of gold nanoparticles only on the
{1 0 0} faces of the Cu2 O crystals. SDS molecules adsorb preferentially on the {1 1 1} faces of the Cu2 O crystals (reproduced with
permission from [64], copyright ©2009 American Chemical Society).
are charged surfaces. More research should be conducted
to explore the dramatic facet-dependent surface properties of Cu2 O nanocrystals and utilize the surface property
differences for novel demonstrations and applications.
Conclusions and outlook
Synthesis of Cu2 O nanocrystals with morphological control is
important for their property examinations and applications.
Cu2 O nanocrystals with cubic, cuboctahedra, truncated
octahedral, octahedral, and multipod structures have been
prepared mainly by wet chemical, electrodeposition, and
solvothermal synthesis methods. These nanocrystals, especially ones with sharp faces, should provide distinct surface
planes for the characterization of facet-dependent properties. Hollow Cu2 O nanostructures such as hollow nanocubes,
octahedra, and truncated rhombic dodecahedra have also
been formed directly by wet chemical and solvothermal synthesis approaches without the use of hard templates. To
enrich the morphological variety of Cu2 O nanostructures,
gold nanocrystal-directed growth of Au-Cu2 O core—shell
heterostructures have been employed to produce unusual
Cu2 O stellated icosahedra and star columns.
Surface properties of Cu2 O nano- and microcrystals with
sharp faces have been examined in a few studies. The {1 1 1}
faces contain surface copper atoms with dangling bonds, and
interact more strongly with negatively charged molecules.
Octahedra and hexapods with {1 1 1} exposed facets are catalytically active at photodecomposing negatively charged
molecules such as methyl orange, but are inactive towards to
the photodegradation of positively charged molecules such
as methylene blue. The cubes with only the {1 0 0} faces
do not interact well with charged molecules and are not
photocatalytically active. In another study, the preferential adsorption of SDS surfactant molecules on the {1 1 1}
faces of truncated octahedral Cu2 O microcrystals lead to
the plane-selective deposition of gold nanoparticles on only
the {1 0 0} faces of the microcrystals in an electrochemical
gold particle deposition process.
This review on the morphologically controlled synthesis
of Cu2 O nanocrystals shows that there are still significant challenges towards the growth of relatively small
Cu2 O nanoparticles and multipods with well-defined facets.
Efforts should be placed on the growth of regular polyhedra
with uniform sizes of less than 100 nm. Cu2 O nanocrystals
of different geometric shapes may be used as templates
for the formation of other substances such as copper sulfide nanocrystals maintaining the original morphologies.
The preparation of hollow and sharp-faced Cu2 O nanostructures with cubic, octahedral, and other geometric shapes
is also challenging. Chemical etching and fine control of
experimental conditions may be exploited to improve the
preparation of hollow Cu2 O nanostructures. Electrical properties of individual Cu2 O nanocrystals with different exposed
surfaces is also interesting to study. Structurally welldefined Cu2 O nanocrystals may also be considered for the
fabrication of solar cells and the photoactivated generation
of hydrogen from water. Coupling of Cu2 O nanocrystals to
metal and semiconductor systems through epitaxial growth
and face-selective deposition may bring about enhanced
properties and allow possible applications. Finally, the use
of Cu2 O nanocrystals with distinct surface facets should
be explored for their roles in catalyzing organic reactions.
These are some interesting research directions for Cu2 O
nanocrystals with morphological control.
Acknowledgement
We thank the National Science Council of Taiwan (NSC
98-2113-M-007-005-MY3 and NSC 98-2811-M-007-066) for
support of this work.
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Chun-Hong Kuo received his BS (2002) in
chemistry from National Cheng Kung University and his PhD (2009) in chemistry from
National Tsing Hua University. During his
PhD course under the direction of professor Michael H. Huang, he had conducted
research on the synthesis and characterization of cuprous oxide nanocrystals with
systematic shape evolution. He is currently a
postdoctoral researcher working with professor Michael H. Huang at National Tsing Hua
University.
Michael H. Huang received his BS (1994) in
chemistry from Queens College, City University of New York. He obtained his PhD in
chemistry from University of California at Los
Angeles (1999, with Prof. Jeffrey I. Zink).
He did his postdoctoral research at University of California, Berkeley (1999—2001, with
Prof. Peidong Yang) and University of California, Los Angeles (2001—2002, with Prof.
Jeffrey I. Zink). He joined the Department of
Chemistry at National Tsing Hua University in
August 2002 as an assistant professor, and was promoted to the
rank of associate professor in 2006. His research has been focused
on the synthesis and characterization of metal and semiconductor
nanocrystals with size and shape control. He has published more
than 30 papers in prominent international journals since joining
NTHU. He has received the Young Chemist Award from the Chemical
Society located in Taipei in 2008 and the Outstanding Young Scholar
Award from the Tsing Hua Chemistry Foundation in 2009.