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CuO nanostructures: Synthesis, characterization,
growth mechanisms, fundamental properties,
and applications
Qiaobao Zhang a, Kaili Zhang a,, Daguo Xu a, Guangcheng Yang b,
Hui Huang b,Fude Nie b, Chenmin Liu c, Shihe Yang d
a Department of Mechanical and Biomedical Engineering, City
University of Hong Kong, 83 Tat Chee Avenue, Kowloon, Hong Kongb
Institute of Chemical Materials, China Academy of Engineering
Physics, Mianyang 621900, Chinac Nano and Advanced Materials
Institute, Hong Kong University of Science and Technology, Clear
Water Bay, Kowloon, Hong Kongd Department of Chemistry, William
Mong Institute of Nano Science and Technology, Hong Kong University
of Science and
Technology, Clear Water Bay, Kowloon, Hong Kong
a r t i c l e i n f o
Article history:
Received 21 May 2013Receivedin revised form 27 September
2013Accepted 29 September 2013Available online 5 October 2013
a b s t r a c t
Nanoscale metal oxide materials have been attracting much
atten-tion because of their unique size- and
dimensionality-dependentphysical and chemical properties as well as
promising applicationsas key components in micro/nanoscale devices.
Cupric oxide (CuO)nanostructures are of particular interest because
of their interest-ing properties and promising applications in
batteries, supercapac-itors, solar cells, gas sensors, bio sensors,
nanofluid, catalysis,photodetectors, energetic materials, field
emissions, superhydro-phobic surfaces, and removal of arsenic and
organic pollutantsfrom waste water. This article presents a
comprehensive reviewof recent synthetic methods along with
associated synthesis mech-anisms, characterization, fundamental
properties, and promisingapplications of CuO nanostructures. The
review begins with adescription of the most common synthetic
strategies, characteriza-tion, and associated synthesis mechanisms
of CuO nanostructures.Then, it introduces the fundamental
properties of CuO nanostruc-tures, and the potential of these
nanostructures as building blocksfor future micro/nanoscale devices
is discussed. Recent develop-ments in the applications of various
CuO nanostructures are alsoreviewed. Finally, several perspectives
in terms of future researchon CuO nanostructures are
highlighted.
2013 Elsevier Ltd. All rights reserved.
0079-6425/$ - see front matter 2013 Elsevier Ltd. All rights
reserved.http://dx.doi.org/10.1016/j.pmatsci.2013.09.003
Corresponding author.
E-mail
addresses:[email protected],[email protected](K.
Zhang).
Progress in Materials Science 60 (2014) 208337
Contents lists available at ScienceDirect
Progress in Materials Science
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m
/ l o c a t e / p m a t s c i
http://dx.doi.org/10.1016/j.pmatsci.2013.09.003mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.pmatsci.2013.09.003http://www.sciencedirect.com/science/journal/00796425http://www.elsevier.com/locate/pmatscihttp://www.elsevier.com/locate/pmatscihttp://www.sciencedirect.com/science/journal/00796425http://dx.doi.org/10.1016/j.pmatsci.2013.09.003mailto:[email protected]:[email protected]://dx.doi.org/10.1016/j.pmatsci.2013.09.003http://crossmark.crossref.org/dialog/?doi=10.1016/j.pmatsci.2013.09.003&domain=pdf
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Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 2092. Synthesis of CuO nanostructures.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 212
2.1. Solution-based methods . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 212
2.1.1. Hydrothermal synthetic method . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2122.1.2.
Solution-based chemical precipitation methods. . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 219
2.2. Solid-state thermal conversion of precursors . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2272.3. Electrochemical method . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . 2312.4. Thermal oxidation method . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . 2322.5. Other synthetic methods . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 236
3. Growth mechanisms . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 2363.1. Oriented attachment . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 2363.2. Ostwald ripening process . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . 2423.3. Scroll of
Cu(OH)2nanosheets . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . 2443.4.
Stress and grain-boundary diffusion . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2453.5.
Stress-induced cracking . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
248
4. Fundamental properties. . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 2514.1. Crystal structures and phase transition .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 2514.2. Electronic band structure . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . 2534.3. Optical properties . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . 2594.4. Electrical
conductivity. . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2624.5.
Photoelectrochemical (PEC) properties . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2664.6.
Magnetic properties . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 269
5. Applications . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . 2705.1. Application in LIBs . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . 2705.2. Application in
supercapacitors . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . 2745.3.
Application in sensors. . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. 279
5.3.1. Application in gas sensors . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
279
5.3.2. Application in enzyme-free glucose electrochemical
sensors . . . . . . . . . . . . . . . . . . . . 2885.4. Application
in solar cells. . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2905.5.
Application in photodetectors . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
2925.6. Application in catalysis. . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . 2985.7. Application in photocatalysis . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . 3015.8. Application in enhancement of thermal
conductivity of nanofluid . . . . . . . . . . . . . . . . . . . . .
. . 3075.9. Application in nEMs . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 3085.10. Application in field emission displays
(FEDs) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . 3115.11. Application in superhydrophobic surfaces.
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . 3125.12. Application in removal of arsenic (As) and
organic pollutants from waste water . . . . . . . . . 3165.13.
Toxicity of CuO nanoparticles. . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
318
6. Conclusion and outlook . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . 320Acknowledgments . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . 322References . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
322
1. Introduction
Nanostructured transition metal oxides (MOs), a particular class
of nanomaterials, are the indisput-able prerequisite for the
development of various novel functional and smart materials. These
transi-tion MO nanocrystals have been attracting much attention not
only for fundamental scientificresearch, but also for various
practical applications because of their unique physical and
chemical
properties [128]. These physical and chemical properties are
strongly dependent on the sizes, shapes,compositions, and
structures of the nanocrystals. Interesting phenomena such as
remarkable increasein surface-to-volume ratio, significant change
in surface energy, and quantum confinement effectsoccur when
transition MOs are reduced to nanoscale dimension [7,20,21]. These
phenomena result
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in a variety of new physical and chemical properties that are
not feasible for materials with bulkdimensionality. Therefore, the
manipulation of well-controlled synthesis and fabrication of
nanostruc-tured transition MOs with different sizes, shapes,
chemical compositions, and structures is crucial inthe advancement
in nanoscience and nanotechnology. Consequently, various
nanostructured transi-tion MOs have been synthesized by diverse
chemical, physicochemical, and physical
strategies[17,9,1417,20,21,25,28]. Compared with their micro or
bulk counterparts, nanostructured transitionMOs exhibit unique
structural characteristics and size confinement effects as well as
novel properties.These properties contribute to the potential of
transition MOs as candidates for both theoreticalstudies and
practical applications in micro/nanodevices.
Cupric oxide (CuO) has been a hot topic among the studies on
transition MOs because of its inter-esting properties as a p-type
semiconductor with a narrow band gap (1.2 eV in bulk) and as the
basisof several high-temperature superconductors and giant magneto
resistance materials [25,2935]. CuOnanostructures with large
surface areas and potential size effects possess superior physical
andchemical properties that remarkably differ from those of their
micro or bulk counterparts. These nano-structures have been
extensively investigated because of their promising applications in
various fields.CuO nanostructures are also considered aselectrode
materials for the next-generation rechargeable
lithium-ion batteries (LIBs) because of their high theoretical
capacity, safety, and environmentalfriendliness[36]. They are also
promising materials for the fabrication of solar cells because of
theirhigh solar absorbance, low thermal emittance, relatively good
electrical properties, and high carrierconcentration[37].
Furthermore, CuO nanostructures are extensively used in various
other applica-tions, including gas sensors [38], bio-sensors [39],
nanofluid [40], photodetectors [41], energetic mate-rials
(EMs)[42], field emissions[43], supercapacitors[44], removal of
inorganic pollutants [45,46],photocatalysis[47], and magnetic
storage media[48]. Recent studies have demonstrated that nano-scale
CuO can be used to prepare various organicinorganic nanocomposites
with high thermal con-ductivity, high electrical conductivity, high
mechanical strength, high-temperature durability, and soon
[32,33,49,50]. Moreover, the nanoscale CuO is an effective catalyst
for CO and NO oxidation as wellasin the oxidation of volatile
organic chemicals such as methanol[5153]. In addition, some
reports
have demonstrated the excellent activities of nanoscale CuO as
catalyst in the CN coupling andCS cross-coupling of thiols with
iodobenzene reactions[51,54,55]. The superhydrophobic propertiesof
CuO nanostructures render these materials as promising candidates
in Lotus effect self-cleaningcoatings (anti-biofouling), surface
protection, textiles, water movement, microfluidics, and
oilwaterseparation [56]. Thus, nanoscale CuO with different shapes
and dimensions, such as zero-dimensional(0D) nanoparticles,
one-dimensional (1D) nanotubes, 1D nanowires/rods, two-dimensional
(2D)nanoplates, 2D nanolayers, and several complex
three-dimensional (3D) nanoflowers, spherical-like,and urchin-like
nanostructures have been synthesized using numerous methodologies.
More interest-ing applications of CuO nanostructures are being
explored.
Cuprous oxide (Cu2O), another important copper (Cu)-based oxide,
is also one of the first knownp-type semiconductor materials [57].
However, Cu2O and CuO have striking contrasting colors, crystal
structures, and physical properties[58]. Cu2O is a reddish
p-type semiconductor of both ionic andcovalent nature with cubic
structure (space group, O4h pn3m) that exhibits various excitonic
levels.By contrast, CuO has an iron-dark color with a more complex
monoclinic tenorite crystallographicstructure (space group, C2/c)
and displays promising antiferromagnetic ordering [58,59]. Cu2O
isexpected to have an essentially full Cu 3d shell with a direct
forbidden band gap of 2.17 eV in bulk,which can only absorb light
up to the visible region. CuO has an open 3d shell with a direct
bandgap (1.2 eV in bulk) of charge-transfer type, which can absorb
light up to the near infraredregion[59,60]. Recent reports have
demonstrated that CuO has higher conductivity than Cu2O but with
lowercarrier mobility[61].
Although these two Cu-based oxides have contrasting properties,
both oxides are of considerableinterest in photovoltaics, gas
sensors, CO oxidation catalysts, various heterogeneous catalysts,
and
LIBs, because of their low band-gap energy, high optical
absorption, high catalytic activity, nontoxicnature, and low-cost
[30,31,62,63]. In recent years, the size- and morphology-controlled
synthesisand application of Cu2O and CuO have been intensively
investigated[25,2831]. However, CuO ismore stable than Cu2O because
Cu(II) ions are much more stable in ambience, which makes it
moreimportant in practical applications. Furthermore, the
synthesis, properties, and applications of various
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Cu2O nanostructures have been extensively reviewed[28,31,6466].
Therefore, the recent advance-ment in Cu2O will not be covered in
this article to avoid overlapping reviews.
Additionally, compared with other MO nanostructures, such as
TiO2[7,9], ZnO[14], WO3[21], andSnO2 [17], CuO nanostructures have
more interesting magnetic and superhydrophobic
properties.Additionally, these nanostructures demonstrate unique
applications in heterogeneous catalysis inthe complete conversion
of hydrocarbons into carbon dioxide, enhancement of thermal
conductivityof nanofluid, nanoenergetic materials (nEMs), and
superhydrophobic surfaces. CuO nanostructures asanode materials for
LIBs have not been paid as much attention as SnO2 [17,67]and TiO2
[67,68].However, the simplicity of preparation, scalability,
non-toxicity, abundance, and low-cost of CuOnanostructures is
expected to increase the application of these nanomaterials as
anode materialsfor LIBs. MOs, including SnO2, ZnO, TiO2along with
their various sub-stoichiometric forms[38], arewidely considered
for gas sensor applications. Thus, the study of CuO for gas sensors
is expectedto increase rapidly because of the easy synthesis of
high-quality and single-crystalline CuOnanostructures.
However, only few reports have described the synthesis
strategies adopted for CuO nanostructuresalong with the
introduction of their related applications [25,29,31]. Furthermore,
most of these review
papers only focused on the 1D CuO nanostructures[25,30,31]. No
review for the systematic introduc-tion of the recent progresses of
various CuO nanostructures has been published. This article will
beginwith a systemic discussion on the synthesis of different CuO
nanostructures. For each synthetic meth-od, critical comments will
be provided based on our knowledge and related research experience.
Next,the associated synthesis mechanisms for controlling the size,
morphology, and structure of CuO nano-structures will be addressed.
The fundamental properties of CuO nanostructures will also be
intro-duced. The promising applications of 0D CuO nanoparticles, 1D
CuO nanotubes, 1D nanowires/rods,2D CuO nanostructures, and several
complex 3D CuO nanostructures along with perspectives in termsof
future research on CuO nanostructures will be highlighted. This
review aims to provide a criticaldiscussion of the synthesis of CuO
nanostructures. The potential of CuO nanostructures as
functionalcomponents for fabrication of micro/nanodevices are also
evaluated and highlighted. In particular, we
focus on the fundamental properties and various nanostructured
forms of CuO that have been re-ported in the literature to date and
summarize the various synthetic strategies. Promising
selections
Fig. 1. Schematic diagram of a typical hydrothermal synthesis
for CuO nanostructures.
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and interesting applications are presented, and finally some
perspectives on the future research anddevelopment of CuO
nanostructures are provided.
2. Synthesis of CuO nanostructures
The development of synthetic methods has been widely accepted as
an area of fundamental impor-tance to the understanding and
application of nanoscale materials. It allows scientists to
modulate dif-ferent parameters such as morphology, particle size,
size distributions, and composition. Numerousmethods have been
recently developed to synthesize various CuO nanostructures with
diverse mor-phologies, sizes, and dimensions using various chemical
and physical strategies. In this review, wepresent the most common
synthetic strategies and associated mechanisms for tuning the
morphology,size, and structure of the CuO nanostructures along with
the studies of the effects of these parameterson the chemical and
physical properties of the synthesized nanostructures.
2.1. Solution-based methods
Solution-based synthetic methods are very common and effective
ways to prepare various MOnanostructures with good control of
shape, composition, and reproducibility. They usually have
rela-tively low reaction temperature and are flexible and suitable
for large-scale production. Moreover, thesynthesis parameters can
be rationally tailored throughout the entire process, which is
beneficial formore precise control of compositions, sizes, and
dimensions of the resulting materials[12,20,22,30,6972]. Among a
variety of solution-based synthetic methods, hydrothermal and
chem-ical precipitation techniques have been widely used to
synthesize CuO nanostructures.
2.1.1. Hydrothermal synthetic method
Hydrothermal synthetic method, in which the reactions are
conducted in water in a pressurized
sealed container and reaction temperature over the critical
point of a solution, has been widely usedto generate different
nanomaterials because its reaction system is simple/green, with
convenient post-treatment [1,12,69]. Furthermore, the hydrothermal
method exhibits the following advantages: (i)numerous inorganic
salts can be well dissolved in water, allowing a very flexible
adjustment of thesource of the metal ions depending on the
requirements; (ii) water is low-cost, non-toxic, and envi-ronment
friendly; (iii) small coordinating molecules can be easily applied
to modulate the growthof the final nanocrystals; and (iv) the
strong polarity of water may be favorable to the oriented growthof
nanocrystals[69]. A schematic diagram of a typical hydrothermal
synthesis for CuO nanostructuresis shown inFig. 1.
Fig. 2. Temperature contour diagram of a T-type micro mixer[75].
Copyright 2011 Elsevier.
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2.1.1.1. Synthesis of CuO nanoparticles. The hydrothermal
synthesis of CuO nanoparticles is generallybased on a two-step
process. First, cupric hydroxide [Cu(OH)2] particles are formed by
the reactionof a cupric salt precursor with a basic solution, such
as sodium hydroxide (NaOH) or ammoniumhydroxide. The
Cu(OH)2particles are then thermally dehydrated in an autoclave at
fixed temperaturesto obtain the final CuO nanoparticles. With the
hydrothermal techniques, experimental parameterssuch as cupric
concentration, pH, growth time, or growth temperature determine the
final dimension,size, and quality of the CuO nanoparticles.
Neupane et al. [73] synthesized flake-like CuO nanoparticles by
simply controlling the precipitationreaction temperature between
the copper nitrate trihydrate [Cu(NO3)23H2O] and NaOH under
ahydrothermal process. The results showed as-prepared samples with
sizes of 37 nm, regularflake-like morphology, and uniform size
distribution. A series of controlled experiments confirmedthat the
temperature and the partial pressure inside the autoclave changed
the morphology and phaseof CuO nanoparticles during the
hydrothermal process. To study the effect of temperature on the
mor-phology and phase control, Neupane et al. fixed the reaction
time to 2 h by only adjusting the temper-ature in the reaction
system. When the reaction temperature was fixed at 100 C, impure
phases ofCu(OH)2and Cu2O appeared. Increasing the reaction
temperature to 300 C resulted in the formation
of pure metallic Cu. The uniformly dispersed pure CuO
nanoparticles were obtained at an optimumtemperature of 200 C.
These results indicated that the temperature and the partial
pressure insidethe autoclave are essential in controlling the
morphology and phase of CuO nanoparticles duringthe hydrothermal
process.
Chakraborty et al.[74]synthesized CuO nanoparticles by the
hydrothermal route using two differ-ent organometallic and
inorganic precursors of copper acetylacetonate [Cu(C5H7)2:Cu(AA)2]
andCu(NO3)23H2O. The resulting flower-like CuO nanoparticles are
both in single phase. However, thesynthesized nanoparticles from
the two different precursors had different the vibrational
properties.The appearance of the Raman peak at 218 cm1 was observed
only in the CuO nanostructures synthe-sized using Cu(AA)2as the
precursor, which was attributed to a certain concentration of the
precursorresulting in particular defect states which induced the
structural changes in the synthesized product.
Sue et al. [75] useda T-type micro mixer (Fig. 2) at 673 K and
30 MPato synthesize the CuO, nickel oxide(NiO), and iron oxide
(Fe2O3) nanoparticles by the hydrothermal method. The effects of
the variousexperimental and sample parameters such as residence
time, metal species on conversion, crystalstructure, particles
size, and mass loss were investigated. The time variation of
conversion, averageparticle size, and coefficient of variation of
the particles showed diverse behavior depending on the me-tal
species, that is, differences in MO solubilities where Fe2O3<
NiO < CuO. For CuO nanoparticles,
Fig. 3. Transmission electron microscope (TEM) images and SAED
patterns of CuO nanorods prepared at (a) room temperatureand (b)
100 C[79]. Copyright 2004 American Chemical Society.
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nucleation did not occur at 0.002 s, but it remarkably proceeded
to 0.157 s with increasing conversion.The average particle size of
CuO was found to increase quickly from 23.7 nm to 28.7 nm during
theearly reaction stage (60.257 s), and then it gradually increased
to 34.3 nm by increasing the residencetime to 2 s.
In addition, Outokesh et al.[76]reported the hydrothermal
synthesis of CuO nanoparticles undernear-critical and supercritical
conditions. During synthesis, three targets of the sample
parameters,namely, yield of the reaction, size of the
nanoparticles, and purity of the products, were optimizedby the
authors through a series of controlled experiments. Results show
that the optimization ofthe three parameters was obtained under the
following reaction conditions: T= 500 C, time = 2
h,[Cu(NO3)2]=0.1moldm
3, and at pH 3. The appropriate mechanisms of the formation of
CuOnanoparticles were proposed by the authors as follows. First,
nanoparticles are suggested to bein the liquid phase similar to
Cu(OH)2. Second, in the presence of HNO3 under relatively high
temperature, some of the initially formed Cu(OH)2or CuO are
transformed to Cu2(OH)3NO3. Finally,Cu(OH)2and Cu2(OH)3NO3are
decomposed to CuO.
2.1.1.2. Synthesis of CuO 1D nanostructures. Hydrothermal method
can also synthesize CuO 1D nano-structures including CuO nanorods,
nanotubes, and nanoneedles. Cao et al.[77] reported the synthesisof
Cu, Cu2O, and CuO nanotubes as well as nanorods. In a typical
process, a 0.8 mM CuCl2and 3 MNaOH were dissolved to prepare CuOH24
solution. Then, a surfactant cetyltrimethyl ammonium bro-mide
(CTAB) was added to the solution under vigorous stirring at 50 C
for 30 min to ensure completedissolution of CTAB. After adding 0.8
mM glucose, the solution was transferred into a stainless
steelautoclave and kept at room temperature for 1 h. Finally, after
the CuO nanotubes were collected, they
were washed by ethanol and water, centrifuged, and dried. The
concentration of CuOH24 was justadjusted to 15 mM to synthesize the
CuO nanorods. Cheng[78]recently synthesized CuO nanorodsin a large
scale using the same method and proved that the concentration of
surfactant CTAB criticallyinfluences the morphology of CuO
nanorods.
Gao et al. [79] reported that the temperature in the
hydrothermal treatment during synthesisremarkably influences the
crystalline structures and morphology of CuO nanorods. Similarly,
Cu(OH)2was prepared from NaOH and CuCl2 and dispersed in NaOH
solution. Then, the mixture was trans-ferred into a
Polytetrafluoroethylene (PTFE) container in an autoclave and kept
at room temperatureat 48 h. The temperature was then increased to
100 C for another 48 h. After washing and drying, thefinal products
were collected. The fine CuO nanorods prepared at room temperature
have higher as-pect ratio and smaller diameters compared with the
bulk CuO nanorods prepared at 100 C(Fig. 3).
However, the SAED images show that the fine CuO nanorods are
polycrystalline, in contrast to themonocrystalline bulk CuO
nanorods.
Shrestha et al.[80]used the following chemical combinations to
synthesize CuO nanorods by thehydrothermal method: (1) Cu(NO3)2,
lactic acid, and NaOH; (2) CuSO4, sodium lactate, and NaOH; and(3)
Cu(NO3)2and NaOH. The chemical reagents were mixed and stirred, and
then transferred into an
Fig. 4. Scanning electron microscope (SEM) images of CuO
nanorods (a), (b), and (c) synthesized with chemical
combinations(1), (2), and (3), respectively[80]. Copyright 2010
American Chemical Society.
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autoclave for 24 h at 140 C. The as-synthesized CuO nanorods
have similar morphologies (Fig. 4).However, unlike the other two
cases, the CuO nanorods synthesized from combination (1)
assembledin a spherical structure without separation of individual
nanorods (Fig. 4a) The CuO nanorodsachieved from combination (2)
separated, but still tended to aggregate. Interestingly, some of
theCuO nanorods obtained from combination (3) show rectangular
cross-sections (Fig. 4c). Althoughthe chemical combinations were
different, the shapes of the CuO nanorods did not remarkably
change.
However, when the concentration of NaOH, aging period, and
temperature were altered, the resultingproducts varied from
plate-like structure to octahedral structures instead of
nanorods.Dar et al.[81]reported a hydrothermal method of
synthesizing CuO nanoneedles by the reaction
between Cu(NO3)23H2O and NaOH under continuous stirring followed
by heating from 120 C to180 C for 20 h to 60 h. The CuO nanoneedles
had very sharp tips and large-diameter bottoms. In
Fig. 5. (a) Schematic representation of the synthesis of CuO
using alcohols, EG, and nonionic polymeric surfactants
[83].Copyright 2011 Elsevier. (b) The schematic growth of CuO
nanostructures with diverse morphologies [47].Copyright
2012Elsevier.
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addition, the nanoneedles were grown by surfactant-free
approach. Yang et al. [82] recentlyintroduced microwave-assisted
hydrothermal method for the synthesis of CuO nanorods.
Similarly,CuSO45H2O, polyethylene glycol 400 (PEG-400), and urea
were dissolved and heated to 80 C in anultrasonic bath. NaOH was
added to the solution and aged for 5 min under the same condition.
Then,
the mixture was transferred to an autoclave, sealed, and treated
in the microwave digestion system at120 C for just 20 min to obtain
the final CuO nanorods. The size of the as-synthesized CuO
nanorodswas smaller than those from normal hydrothermal method.
During the microwave-assistedhydrothermal stage, CuO nanorod
formation just required 20 min.
Fig. 6. Typical SEM images of (a) butterfly-like[90](Copyright
2011 Indian Academy of Sciences), (b) gear wheel
bundles[105](Copyright 2007 American Association of Nanoscience and
Technology), (c) flower-like assemblies [32] (Copyright
2007American Chemical Society), (d) dendrite-like [101](Copyright
2007 Elsevier), (e) nanobat-like[108](Copyright 2009 Elsevier),(f)
layered hexagonal discs [112] (Copyright 2011 Institute of
Physics), (g) honeycomb-like[32](Copyright 2007 AmericanChemical
Society), (h) self-assembled leaf-like[99](Copyright 2010
Springer), (i) hierarchical peachstone-like[109](Copyright2010
Elsevier), (j) shrimp-like[47](Copyright 2006 American Chemical
Society), (k) sheaf-like[111](Copyright 2009 Elsevier),and (l)
urchin-like microspheres[104](Copyright 2009 Elsevier).
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Table 1
Summary of CuO nanostructures obtained hydrothermally and the
various synthesis conditions [PEG, sodium dodecyl sulfate
(SDS), CTAB, EG, sodium dodecylbenzenesulfonate (SDBS),
dodecylsulfate (DS), hexamethylenediamine (HMDA), poly-sodium
4-styrenesulfonate (PSS)].
Morphology Size (nm) Starting materials Additives
Temperature
(C)
Duration
(h)Flower-like, boat-
like, plate-like,and ellipsoid-like[85]
Various Cu(NO3)23H2O PEG 100180 0.52NH3H2O
Honeycomb-like[32]
90100 nm in diameter, tens ofmicrometers long
Cu foil Na2WO4,Na2MoO4,SDS
160 24
Nanoplatelets[9698] 400 nm to 2 mm in width and 1 mm toseveral
micrometers long
CuxS 150, 200, 230 1.5NaOH
1 lm in width Cu foil H2O2 150 12120300 nm in width and 480700
nmlong
Cu(DS)2 120 12NaOH
Leaf-like[99] 0.51.5 lm in length, 80110 nm long Cu(OH)2 Urea
150 612Momordica-like
[100]Less than 100 nm long and 3050 nm inwidth
CuSO45H2O 180 15NH3H2O
Dendrite-like[101] About 100 nm in width and severalmicrometers
in length
CuSO45H2O EG 200 20NaOH
Branch-like andflake-like[102]
50200 nm width and 300400 nmlength for flake-like
nanostructure;hundreds of nanometers long anddiameters of 20100 nm
for branch-likenanostructures
CuSO45H2O Sodiumcitrate
160 12
Spherical-like[103] 500 nm in diameter Cu(CH3COO)2 Urea 121
0.67Hierarchical hollow
microspheres [87]3.5lm in diameter Cu(CH3COO)2H2O 120 24
Urchin-like[104] 3 lm in diameter CuCl22H2O EG 100 12
Gear wheel andclew like[105]
10 lm in diameter for gear wheelnanostructure
Cu(NO3)23H2O PEG 150180 10, 21
Flower-like[91,106]
2.53 lm long Cu threads K2Cr2O7,H2SO4
140 12
6 lm in diameter CuCl22H2O CTAB 150 12Hierarchical
flower-like[107]510 lm in diameter CuSO45H2O H2O2 120 6
NaOHNanobat-like[108] 70 nm in diameter, 170 nm long
Cu(NO3)23H2O Urea 100150 615Hierarchical
butterfly-like[90]6 lm long and 24 lm in width CuCl22H2O SDBS
80150 115
Nanobundles [89] 2030 nm in diameter and 350500 nmlong
CuCl22H2O SDBS 130 1824
Peachstone-like
[109]
4 lm long and 3 lm in width CuCl22H2O [Omim]TA 100 24
Hollow micro/nanostructures[110]
1.53 lm in diameter CuSO4 Tyrosine 130 4NH3H2O
Shuttle-like[88] 200300 nm in width and severalmicrometers
long
Cu(CH3COO)2H2O CTAB 120 12
Sheaf-like[111] 2 lm long Cu(NO3)23H2O Urea 120 8Layered
hexagonal
discs[112]34lm long and 1.52.0 lm in width Cu(NO3)23H2O HMDA 130
310
NH3H2OHierarchical
dandelion-likemicrospheres [113]
36lm in size Cu(NO3)23H2O EG 130 16NH3H2ONaOH
Twinned-hemisphere-like
[114]
22.5 lm in size Cu(CH3COO)2H2O PSS 180 2NaF
Hierarchicaldendrite-like[115]
69lm in size Cu(NO3)23H2O Urea 160 12
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2.1.1.3. Synthesis of CuO 2D/3D nanostructures. The simplicity
of the hydrothermal method facilitateditsperformance at low
temperatures and in a large scale. Additionally, it can be used for
the production ofcomplex nanostructured CuO with diverse
morphologies and sizes. In most cases, the hydrothermalsynthesis of
CuO nanostructures starts with the formation of intermediate
compound Cu(OH)2precip-itationor other intermediate
phases,whicharepreparedfrom cupric salt/cupricacids/Cu foil
precursorsin alkaline media. Then, thesolutionis kept at an
elevated temperature in an autoclave for a certainper-iod, allowing
the decomposition of Cu(OH)2 or other intermediate phases into the
final product of CuO.Therefore, the manipulation of well-controlled
synthesis of CuO nanostructures with various shapes ispossible by
choosing different solutions andby adjusting the concentrations of
precursors. To obtain di-verse morphologies and dimensions of CuO
nanostructures, several surfactant and structure-directingagents
are normally added into the precursor solution. Systematic studies
of the experimental param-eters reveal that the Cu source, reaction
temperature, reaction time, and surfactant along with the pHvalue
of the precursor solution influence the morphology, growth, size,
and dimensions of the resultingCuO nanostructures (Fig.
5)[25,47,83,84].
The various morphologies of the CuO nanostructures achieved
using the hydrothermal technique isshown in Fig. 6. The obtained
CuO nanostructures exhibit diverse optical, electrical, and
catalytic prop-
erties[8587].The hydrothermally synthesized CuO nanostructures
fabricated with and without additives exhibit
evidently different morphologies and properties. However, the
exact mechanism for the formation ofthese architectures by the
addition of inorganic or organic materials has not been fully
understood. Interms of organic additives, several reports have
suggested that certain ions in the additives are ad-sorbed on the
CuO surface, which alters the growth mechanism[78,8890]. Other
studies have dem-onstrated that additives can act as a template for
the formation of CuO nanostructures with differentmorphologies
[83,91]. The addition of basic media such as NaOH, urea, NH3H2O,
and (CH2)6N4 leads tothe formation of intermediate compound
Cu(OH)2that is transformed into CuO under heat treatmentby the
oriented attachment growth mechanism as detailed in Section3. The
final morphology of CuOwas determined by the internal
crystallographic structure of Cu(OH)2. Cudennec et al.
[92]showed
that the transformation of Cu(OH)2into CuO is a reconstructive
transformation involving a dissolutionreaction followed by the
precipitation of CuO according to the following scheme:
CuOH22s2OHaq ! CuOH
24aq $ CuOs 2OH
aq H2O. Moreover, the application of microwave power
during the hydrothermal process to synthesize CuO nanostructure
has emerged as an important topicin the scientific community
because of its low energy consumption, rapid heating process, and
fastkinetics of crystallization[93]. The use of hydrothermal
microwave to synthesize CuO nanostructureleads to small diameter at
short annealing time with high yield compared with the
conventionalhydrothermal process[94,95].
In summary, various CuO nanostructures, including CuO
nanoparticles, 1D CuO nanowires/rods/tubes, 2D, and several complex
3D CuO nanostructures, have been hydrothermally synthesized
byadding various reactants to the cupric salt/cupric acids/Cu foil
precursor solution in the presence of
some capping agents. Moreover, morphologies of the synthesized
CuO nanostructures can be con-trolled by selecting certain types of
structure-directing and dispersing modifying agents. A brief
sum-mary of the obtained CuO nanostructures that are hydrothermally
synthesized with or withoutdifferent additives is shown inTable
1.
The possible growth scheme for the formation of
rectangular-shaped nanobat-like CuO nanostruc-tures through a
typical hydrothermal synthetic process is shown in Fig. 7[108]. The
reagents areCu(NO3)23H2O and urea, where urea acts as pH buffer to
control the supply of OH
ions. Duringthe initial stage, water reacts with urea to form
ammonia and CO2. In addition, ammonia further reactswith water to
produce ammonium and OH ions. The reaction slowly generates solid
building unitsinto solution (b). The concentration of solid
building units in the solution increases continuouslyand nucleation
starts (c) after a crucial super-saturation level is reached. The
crystallographic struc-
tures of the nuclei and seeds greatly affect the final shapes of
the nanocrystals (d). The variation inthe CuO nuclei is caused by
the increasing pH of the reaction solution (c). The small CuO
crystalincreases and forms a bigger crystal with increasing
reaction time. O and Cu2+ are stacked alternatelyalong with the
specific directions to form alternating planes, which causes the
anisotropic growth ofthe CuO crystal. The different growth rates
are due to the crystallographic faces, where the growth rate
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is sequenced as [01 0]length> [100]breadth> [001]height.
The slowest growing (010) planes dominate thecrystal into the
typical rectangular-shaped structure (e). The formed nanobat-like
structure thenforms a circular fashion via self-assembly to build
hollow microspheres. This phenomenon was causedby the formation of
several unbalanced charge centers as a result of the
dissimilarities in the surfacecharges. These charge centers attract
the nanobats in the solution through rotating adjacent structure(e)
to share identical orientation. Finally, sphere-like CuO
microsphere assemblies are formed withincreasing reaction time.
2.1.2. Solution-based chemical precipitation methods
Chemical precipitation synthesis is similar to the hydrothermal
method with a reaction also occur-ring in the solution, but the
chemical reactions can be conducted in an open container with a
relativelylow reaction temperature (normally below 100 C). This
process can be simply defined as the chemicalreaction between the
precursors to produce monomers that subsequently aggregate into
final result-ing materials [69]. A schematic drawing of the
solution-based chemical precipitation to producenanoscale CuO is
shown inFig. 8.
2.1.2.1. Synthesis of CuO nanoparticles. Cupric salt (normally
nitrate or sulfate) and alkaline compounds(normally NaOH) are often
used in the synthesis of CuO nanoparticles. A typical process of
chemicalprecipitation is as follows [116]: Cu(NO3)2 solution (300
mL 0.02 M) is prepared by dissolvingCu(NO3)23H2O in deionized
water. The solution is placed into a round-bottom flask equipped
with
a refluxing device. The Cu(NO3)2solution is kept at an
appropriate temperature (6100 C) with vigor-ous stirring. Then, 0.5
g solid NaOH (platelet) is rapidly added into the solution,
resulting in the pro-duction of a large amount of blue or black
precipitates and the crystallization temperature ismaintained for
10 min. Next, the precipitates are heated at 100 C for another 10
min. After completereactions, the resulting products are
centrifuged, washed with water and ethanol for several times,
Fig. 7. Schematic illustration of the growth scheme for the
formation of rectangular-shaped nanobat-like CuO nanostructuresby
the hydrothermal synthesis method[108]. Copyright 2009
Elsevier.
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and then dried in air at room temperature. Zhu et al.
[116]synthesized CuO nanoparticles by thisroute. The average
diameter of the as-prepared particles is 10 nm.
The proportion of reactive compounds and reactive temperature
markedly affects the size and mor-phology of the synthesized CuO
nanoparticles. Using a similar method and three sets of recipes,
Zhouet al.[52]obtained three types of CuO products with distinct
sizes and morphologies: nanoparticles,nanobelts, and nanoplatelets
(Fig. 9). The recipes were (a) Cu(NO3)23H2O + NaOH, (b)
Cu(OAc)2H2O+ NaOH, and (c) Cu(NO3)23H2O + N a2CO3.
The synthesized CuO nanoparticles using ordinary chemical
precipitation methods usually have acommon problem, that is, the
achieved nanoparticles are apt to agglomerate. To solve this
limitation,chemical precipitation methods have been extensively
investigated to improve the separation of thenanoparticles through
the application of some external energy such as ultrasonic or high
pressure. Thesonochemical method is used to apply ultrasonic
cavitation during the synthesis procedure. Kumaret
al.[48]synthesized well-separated 26 nm CuO nanoparticles by
irradiating the reactive solutionwith a high-intensity ultrasonic
horn under 1.5 atm of argon at room temperature for 3 h.
However,this method requires expensive apparatus and excessive
organic solvent as well as severe reactionconditions. In addition
to the sonochemical synthesis of dispersed CuO nanoparticles, Zhu
et al.[117]developed a simple quick-precipitation procedure to
prepare highly dispersed CuO nanoparti-cles with the size of about
6 nm in aqueous solution. The authors noted that the tendency of
CuOnanoparticles to aggregate during preparation may have been
caused by the low nucleation andgrowth rates of CuO particles at
mild reaction condition. A large amount of well-dispersed CuO
nano-particles were obtained by the rapid addition of the NaOH
solid to the mixture of an aqueous solutionof Cu(CH3COO)2 and a
glacial acetic acid at 100 C. This result indicates that higher
temperatures cause
Fig. 8. Schematic of a typical chemical precipitation synthetic
process for CuO nanostructures.
Fig. 9. TEM images of CuO (a) nanoparticles, (b) nanobelts, and
(c) nanoplatelets [52]. Copyright 2008 Institute of Physics.
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higher reaction rates, resulting in large amounts of nuclei to
form in a short period and the inhibitionof the aggregation of
crystals. Consequently, well-dispersed CuO nanoparticles with small
sizes wereachieved at relatively high reaction temperature.
However, Mahapatra et al.[118]introduced a wetchemical method to
synthesize ultrafine dispersed CuO nanoparticles. The authors
claimed that thesize of as-prepared CuO nanoparticles can be
controlled by simply changing the concentration ofthe basic Cu
precursor. In addition, hot-solution decomposition and
microemulsion process, whichhave been widely applied to synthesize
well-dispersed quantum dots, may be applied in the prepara-tion of
dispersed CuO nanomaterials with various shapes and sizes. Other
synthetic technologies suchas by introducing sonic waves or
microwaves in the synthetic process can also be tried to obtain
well-dispersed CuO nanoparticles in the future[119].
2.1.2.2. Synthesis of CuO 1D nanostructures. Wang et
al.[120]reported a simple wet-chemical methodfor preparing CuO
nanorods. In a typical process, a non-ionic surfactant,
polyethylene glycol (PEG; Mw20,000), and CuCl22H2O are dissolved in
water. After stirring for 15 min, NaOH is added into the solu-tion
to generate Cu(OH)2precipitate. The Cu(OH)2is placed into a steam
trace for 30 min and is trans-formed into CuO precipitate, followed
by washing, filtering, and drying. The as-synthesized CuO
nanorods have monoclinic structure up to 400 nm long with a
diameter ranging from 5 nm to15 nm. Lu et al.[121]synthesized CuO
nanowires by dehydration of the precursor of Cu(OH)2nano-wires.
First, KOH solution was added dropwise into CuSO4solution under
rigorous stirring, followedby the dropwise addition of ammonia
solution. Then, the Cu(OH)2 nanostructures were heated at120 C for
2 h and at 180 C for another 3 h. The resulting CuO nanowires have
structures similar tothose of the precursor Cu(OH)2nanowires.
Ethiraj and Kang [122] introduced an organic molecule thi-oglycerol
(TG) as a stabilizer in the synthesis of CuO nanowires. TG was
first added in a copper acetatesolution under stirring. Then, the
NaOH solution was added dropwise into the mixture, followed
byimmediate addition of water under continuous stirring for a few
minutes. The precipitate was col-lected by centrifuging, washing
and overnight drying. By comparing the samples with and withoutthe
use of TG, the authors found that a small amount of TG led to
well-dispersed CuO nanowires,
otherwise the CuO nanowires would assemble into a flower-like
structure. Zhang et al. [44]reportedthe preparation of CuO
nanobelts by adding ammonia to a Cu(NO3)2solution directly under
constantstirring for 15 min. After the formation of a blue
precipitate, the mixture was then sealed and heatedat 60 C for 4 h.
The product was collected by repeated washing and centrifugation.
The widths andlengths of the as-obtained CuO nanobelts were 510 nm
and 13 lm, respectively. Moreover, thethickness of the CuO
nanobelts was estimated to be 25 nm. Interestingly, Yangs group
[123]synthe-sized vertically aligned CuO nanorod arrays on a Cu
substrate. In a typical synthesis, a Cu foil reactedwith NaOH under
the presence of some oxidants or surfactants. Then the foil was
removed, washed,and dried. The CuO nanorod arrays were found to
uniformly cover the surface of the Cu foil.
Remarkably, by using PEG 200 as the capping agent, ultralong CuO
nanowire bundles with lengthsranging from tens to hundreds of
micrometers as shown inFig. 10a were selectively synthesized on
a
large scale at room temperature by a facile solution-phase
method by Li et al. [124]. Transmissionelectron microscope (TEM)
characterizations (Fig. 10b) demonstrated that the obtained CuO
nanowirebundles are polycrystalline. Moreover, a series of
controlled experiments performed by the authorsrevealed that the
presence of PEG 200 and the concentration of OH affected the
morphology andphase control of CuO nanostructures during the
reaction process. No CuO nanowire bundles can beobtained, but CuO
nanoleaves (NLs), without PEG 200 (Fig. 10c). These NLs were single
crystals andgrew along the [111] crystal plane with diameters
ranging from 200 nm to 500 nm (Fig. 10d). TheCuO nanowire bundles
could only be synthesized when the molar ratio of OH/Cu2+ was
higher thanfour, indicating that PEG200 and the concentration of OH
are essential in the formation of CuO nano-wire bundles.
Wang et al.[125]synthesized large-scale ultralong CuO nanowires
with an average diameter of
8 nm and lengths of up to several tens of micrometers by a
facile room temperature solution-phasechemical route without any
capping agent, as depicted in Fig. 11. Cu(OH)2 nanowires were first
formedand subsequently served as template to direct the formation
of CuO nanowires. Compared with thecommercial CuO powders, the
obtained ultralong CuO nanowires exhibit enhanced
photocatalyticactivity for RhB degradation and the potential for
applications in LIBs and catalysis.
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2.1.2.3. Synthesis of CuO 2D/3D Nanostructures. The
solution-based chemical precipitation method canalso be used to
synthesize more complex 2D/3D CuO nanostructures. A brief summary
of the obtainedCuO nanostructures prepared by the solution-based
chemical precipitation method with differentadditives are listed in
Table 2. Some examples of CuO nanostructures with various
morphologiesachieved by this technique are shown inFigs. 13 and 14.
In the whole synthetic process, two keypoints, namely, nucleation
and growth, dominate the formation of different CuO
nanoarchitectures.An appropriate precursor, a rational condition
for the reaction system together with additional ligandsto adjust
the surface energies will significantly influence the nucleation
and growth of the CuO
Fig. 10. (a) SEM image of the CuO nanowire bundles. (b) TEM
image and the corresponding SAED pattern taken in a zone rich
innanowires. (c) SEM image and TEM image (inset). (d)
High-resolution (HRTEM) image of one CuO NL and the
corresponding
SAED pattern, indicating growing along the
111direction[124].Copyright 2010 Elsevier.
Fig. 11. (a) TEM image of CuO nanowires. (b) A histogram of the
CuO nanowires, revealing the diameter distribution[125].Copyright
2012 Royal Society of Chemistry.
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nanostructures. Thus, to achieve the controlled synthesis of CuO
nanoarchitectures by solution-basedchemical precipitation, various
experimental parameters including the type of precursors,
reactiontemperature, reactant concentration, and surfactant can be
manipulated to control the morphologyand structure of the obtained
CuO nanocrystals. Moreover, recent reports have determined that
thepH value together with concentration of OH ions can
significantly affect the nucleation and growth
behavior (for example, the number of nuclei and concentrations
of growth units) of the CuO crystals,resulting in the formation of
the diverse morphologies of CuO nanostructures as shown in Fig. 12.
Asimilar phenomenon is also observed in controlling the
hydrothermal synthesis of CuOnanostructures.
In addition, the various additives used in the preparation
process have distinct functions. Severalstudies have suggested that
ionic surfactants, such as SDBS and CTAB, can be absorbed onto the
sur-faces of CuO nanocrystals[130,131]. This effect can reduce the
interfacial tension between the crys-tallizing phase and the
surrounding solutions, which strongly affects the growth rate
andorientation of the crystals, resulting in the formation of CuO
nanostructures with diverse morpholo-gies. Non-ionic surfactants,
polyvinylpyrrolidone (PVP) and PEG were thought to serve as
templatesfor the formation of CuO nanostructures. However, the
mechanism responsible for the growth of
CuO nanostructures by solution-based chemical precipitation
method remains unclear. Someresearchers have used oriented
attachment as the possible statistical growth mechanism as
detailedin Section3[128,138].
The schematic growth mechanism for the formation of
flower-shaped CuO nanostructures througha typical solution-based
chemical precipitation process is shown in Fig. 15[33]. When Cu
nitrate is
Table 2
Summary of CuO nanostructures obtained by solution-based
chemical precipitation methods and their synthesis conditions
(PEG,
CTAB, SDBS, DS, HMDA, and PVP).
Morphology Starting Materials Additives Temperature(C)
Duration(h)
Nanowires, rectangles, and seed-, belt- and sheet-like[132]
Cu(NO3)23H2O 80 0.5NaOH
Urchin-like[51] Cu(NO3)23H2O Urea 100 6Flower-shaped[33]
Cu(NO3)23H2O HMTA 100 3
NaOHBelt-, bamboo leaf- and shuttle-like[130] CuCl22H2O PEG,
CTAB 75 0.5
NaOHLeaflet-like nanosheet[133] CuSO45H2O 25 2
KOH, NH3H2OFlower-like hierarchical assemblies[134] Cu(NO3)23H2O
25, 100 1272
NaOHCarambola-like[135] Cu(NO3)23H2O HMTA 95 3
NaOH
Shuttle- and flower-like[136] Cu(NO3)23H2O HMTA, CTAB 100
3NaOH
Nanoribbons and nanorings[137] CuCl22H2O SDBS 120 0.5NaOH
NLs[138] CuCl22H2O 35 40NH3H2O
Hierarchical nanochains[139] CuCl22H2O PVP,(NH4)2SO4
100 12NaOH
Hierarchical nanosheets[140] Cu(CH3COO)2H2O 30 72NH3H2O
Oval nanosheets and nanoellipsoid[128] Cu(CH3COO)2H2O 65
24NH3H2O
Flower-like microspheres[141] Cu power, NaOH (NH4)2S2O8 25
20Lenticular-like, pseudo-like and elliptical[84] Cu(CH3COO),
NaOH
25 24
Spindle-like and plate-like[142] Cu(NO3)2, NaOH 80
0.5Flower-like hierarchical nanostructures[143] Cu(NO3)23H2O 80
4
NH3H2O
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mixed only with HMTA (a), a turbid solution containing the
building units (b) is obtained. After theturbid solution is
refluxed at 100 C for >1 h, Cu(OH)2 nuclei are formed from the
origination ofOH ions by the hydrolyzation of HMTA, and then
nuclear aggregation occurs (c). With increasingreaction time, the
Cu(OH)2 is converted into small CuO crystals. Moreover, the CuO
crystals are ar-ranged with time and finally form petal-like
structures (d). Interestingly, if the Cu(NO3)2is mixed withboth
HMTA and NaOH, the solution immediately changes to blue color
because of the instant forma-tion of Cu(OH)2nuclei (e) caused by
the fast formation of OH
ions from NaOH. The Cu(OH)2nuclei aretransformed into CuO
through a simple chemical reaction of Cu(OH)
2? CuO+H
2O. With increasing
reaction time, the initially formed CuO nuclei are assembled,
forming individual petals (f) and finallyflower-like morphologies
(g).
As mentioned in Section 2.1, the solution-based synthesis of CuO
nanostructures is commonly per-formed in solutions containing
Cu(II) salts, and the obtained final products are normally
free-standing.In addition, this simple method can also be used to
synthesize CuO nanostructures directly on the sur-face of Cu
substrates. Generally, Cu substrates are washed in an HCl solution
for a few minutes andsubsequently rinsed with deionized water and
absolute ethanol to remove the surface impuritiesalong with oxide
layers. Then, the treated Cu substrates are immersed into alkaline
solutions withor without additives of oxidative reagents (e.g.,
(NH4)2S2O8 or K2S2O8) at certain temperature for afixed
period[129,144147]. The reaction system without additives of
oxidative reagents commonlyrequires a longer time to yield the
final CuO nanostructures on the Cu surface[146,147].
In 2003, Yang et al.[129]synthesized sheet- and whisker-like CuO
nanostructures on a Cu sub-strate surface by a simple liquidsolid
reaction under alkaline and oxidative conditions at room
tem-perature. They demonstrated an evolution of the film structures
as a function of the solutiontreatment time and the concentration
of NaOH aqueous solution from the fibers of Cu(OH)2 onto
Fig. 12. Schematic illustration of the growth of CuO
nanostructures under different pH conditions and the corresponding
SEMimages[127]. Copyright 2010 Elsevier.
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the scrolls of Cu(OH)2onto the sheets or whiskers of CuO. The
formation of Cu(OH)2and CuO nano-structures on Cu surfaces involved
inorganic polymerization (polycondensation) reactions. The
ob-tained CuO nanostructures are phase-pure single crystallites.
This method for the preparation of Cucompound films with ultrafine
structures is advantageous because of its simplicity, high yield,
andmild reaction conditions. It can also produce single crystal
nanomaterials in array form, which offersan attractive and
convenient path to the large-scale engineering of ordered inorganic
nanostructureson metal electrodes.
Large-area flower-like 3D CuO nanostructures were also
successfully synthesized on a Cu surfaceby a template-free solution
route under alkaline and oxidative conditions at 70 C[145]. The
forma-tion of flower-like nanostructures was found to strongly
depend on the concentration of oxidantK2S2O8. Liu et
al.[146]reported the fabrication of CuO hierarchical nanostructures
on Cu substratesby the oxidation of Cu in alkaline conditions at 60
C without the addition of oxidative reagents. They
Fig. 13. Typical SEM images of (a) urchin-like
nanostructure[51](Copyright 2009 American Chemical Society), (b)
hierarchicalnanochains[139](Copyright 2011 Elsevier), (c)
Nanosheets[128](Copyright 2006 American Chemical Society), (d)
flower-like[32](Copyright 2008 American Chemical Society), (e)
flower-like assembles [134](Copyright 2007 Elsevier), (f)
nanoellipsoid[128] (Copyright 2006 American Chemical Society), (g)
spindle-like [142](Copyright 2012 Royal Society of Chemistry),
(h)microplates [84] (Copyright 2012 Royal Society of Chemistry),
(i) flower-like microspheres [141], (Copyright 2012 Royal Societyof
Chemistry), (j) lenticular-like[84](Copyright 2012 Royal Society of
Chemistry), (k) Pseudo-like[84](Copyright 2012 RoyalSociety of
Chemistry), and (l) elliptical-like nanostructure[84](Copyright
2012 Royal Society of Chemistry).
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demonstrated that CuO flower-like structures composed of
hierarchical 2D nanosheets and sphericalarchitectures constructed
by ultrathin nanowalls could be selectively generated by
controlling thealkaline reactant. When NaOH was used, 3D CuO
flower-like structures on a Cu foil was obtained
Fig. 14. Typical TEM images of (a) sheet-like[136](Copyright
2009 Springer), (b) flower-like[136](Copyright 2009 Springer),(c)
NLs[138](Copyright 2007 American Chemical Society), (d)
spindle-like[142](Copyright 2012 Royal Society of Chemistry),(e)
lenticular-like[84](Copyright 2012 Royal Society of Chemistry), and
(f) elliptical-like nanostructures[84](Copyright 2012Royal Society
of Chemistry).
Fig. 15. Schematic growth mechanism for the formation of
flower-shaped CuO nanostructures [33].Copyright 2008
AmericanChemical Society.
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(Fig. 16a). Whereas NaOH was replaced by NH3H2O, CuO with 3D
spherical architectures was formedon the Cu foil as illustrated
inFig. 16b. In addition, by reducing the concentration of the
alkaline solu-tion, well-defined 2D nanosheets and nanowall arrays
can be fabricated on a large scale on the Cu foilaccordingly. A new
hierarchical CuO microcabbage architecture[147]consisting of
densely packednanoplates and nanoribbons was also directly
fabricated on Cu foils via a similar synthetic processat room
temperature for 6d. In addition, CuO nanostructures such as
nanoneedles, nanoflowers, and
stacking of flake-like structures on Cu foils can also be
obtained by Cu oxidation under alkaline con-dition by a simple wet
chemical route[144].
2.2. Solid-state thermal conversion of precursors
Various CuO nanostructures can be generated through the thermal
conversion of the precursors,and the morphological features of the
precursors can be well-preserved in the final products giventhat
heat treatment is appropriately performed. Cu(OH)2 and basic Cu
salts have become favorableprecursor candidates to exploit the
interesting morphologies of CuO because of their unique
andwell-known layered structure [148,149]. The process usually
starts with the synthesis of the Cuprecursors via the reaction of
cupric salt (normally nitrate or chloride) with alkaline
compounds
(normally NaOH). The obtained cupric precursors are then
centrifuged and washed with distilledwater and absolute ethanol.
Finally, these cupric precursors are calcined in solid state to
obtain thefinal CuO nanostructures. Moreover, the corresponding
Cu(OH)2particles are formed and precipitatedin H2O by adding a
basic solution (usually NaOH) to the obtained cupric precursors
solution. Theresulting nitrate or chloride salts are then washed
away, and the corresponding Cu(OH)2 particlesare thermally
dehydrated after filtration and washed to obtain the final CuO
nanostructures.
This method is similar to the solution-based chemical
precipitation method, but the thermal dehy-dration of cupric
precursors, such as Cu2(OH)2CO3, Cu2Cl(OH)3, CuC2O4, and Cu(OH)2is
in solid statewith relatively higher treatment temperature.
Additionally, the morphological features of the cupricprecursors
can be well-preserved in the final CuO nanostructures. The
mechanism of shape-reservedtransformation from Cu(OH)2 nanowires to
CuO nanowires as illustrated inFig. 17clearly demon-
strates that the morphology can be properly reserved during the
transformation from Cu(OH) 2 toCuO because of the topotactic
transformation in the dehydration process[151].
In addition, a schematic diagram of the metamorphosis of CuO as
given by Dey et al. [152]illus-trates the formation process (Fig.
18). This diagram presents a simple depiction of the
transformationthat occurs at the atomic level for the first
time.
Fig. 16. SEM images of (a) 3D CuO flower-like structures on Cu
foil and (b) 3D spherical CuO architectures on Cu foil
[146].Copyright Royal Society of Chemistry 2006.
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However, synthesis parameters such as alkaline content (pH) of
the solutions during hydrolysisplay a key role in the formation of
various morphologies and dimensions of the Cu(OH)2nanostruc-tures,
leading to a series of fascinatingly shaped nanostructures of final
CuO by subsequent heattreatment.
Dey et al.[152]reported that different morphologies of CuO
nanostructures have been synthesizedby a simple chemical route, in
which the Cu(OH)2 nanostructures are first synthesized, and
theprecipitate is subsequently annealed at 130 C. A variety of
shapes such as seed-like, ellipsoidal, rods,and leaves of CuO can
be obtained by simply varying the pH value during synthesis (Fig.
19), indicatingthat alkaline content (pH) of the solutions are
essential in the formation of various CuOnanostructures.
Fig. 17. (a) Crystal structure of Cu(OH)2. (b) When the
temperature rises, long CuO and HO bonds break. The structure
ofCu(OH)2changes to a layered Cu(OH)4structure. (c) Projection
along thea-axis for (b), (d), and (e). The dehydration process
intheb, cplane of Cu(OH)2. (f) Crystal structure of CuO[151].
Copyright 2012 Royal Society of Chemistry.
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Fig. 18. Schematic representation of CuO formation at the
molecular level from a crystallographic perspective. The blue
ballsrepresent Cu atoms, and the red balls represent O atoms. (a)
FCC lattice of Cu and (b) monoclinic cell of CuO. (c) Two unit
cells
joined by a common face, showing the stacking of the 1 11
planes[152]. Copyright 2012 Royal Society of Chemistry.
Fig. 19. Diagram of the formation of various shapes of CuO under
different synthesis conditions [152].Copyright 2012 RoyalSociety of
Chemistry.
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With the method mentioned above, CuO with different morphologies
such as nanoribbons[153], hierarchical sphere-like [154],
peanut-shaped superstructures [150], sisal-like [148],
fish-bone-like [155], pillow-shaped [156], perpendicularly
cross-bedded microstructure [157], NCline-assembled bundle-like
[158], butterfly-sheet-like or nanotubes [159], and worm-like
[160]were obtained through thermal conversion of their
corresponding Cu salts. In addition, otherCuO nanostructures,
including CuO nanoribbon arrays, CuO nanotube arrays, nanotube
arrays withspecial nanoplate wall structure, and quasi-aligned
submicrometer CuO ribbons formed on Cu foils,achieved by heating
the corresponding Cu(OH)2 precursors have also been reported
[161163].Chen et al. [164]fabricated novel hierarchically
mesoporous nanosheet-assembled gear-like pillarCuO arrays directly
grown on Cu foil by the vapor-phase corrosion approach and
subsequent heattreatment [164]. The hierarchically
nanosheet-assembled gear-like pillar arrays (HNGPAs) ofCu(OH)2 were
first formed on Cu foil. Hierarchical micro/nanostructure CuO was
obtained by thesolid-state thermal transformation of the formed
Cu(OH)2 arrays on Cu foil. The obtained CuOnanostructures were
found to inherit the intact hierarchical superstructures with
retained radialsymmetry and nanosheets subcomponents from Cu(OH)2
HNGPAs without collapse and aggrega-tion (seeFig. 20).
Novel CuO mesoporous nanosheet cluster arrays that are directly
grown on a Cu substrate via thesame ammonia vapor-phase corrosion
route were also found by Chen et al.[165]. This simple andeffective
fabrication strategy shows promising potential for the preparation
of other nanoarchitec-tured materials for both high-energy and
high-power applications.
The advantages of solid-state thermal conversion of precursors
are simple, easy and safe to use,controllable, allows the
mass-production of CuO nanostructures with unique superstructures,
practi-cal, and promising for various applications ranging from
catalytic reaction to sensing.
Fig. 20. (a) Schematic illustration of the fabrication of CuO
HMNGPAs via a vapor-phase corrosion route; SEM images
ofCu(OH)2HNGPAs and (c) CuO HNGPAs; and (d) schematic of the growth
process of Cu(OH) 2 HNGPAs[164]. Copyright 2012American Chemical
Society.
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2.3. Electrochemical method
Electrochemical method is widely used for the preparation of
nanoporous MOs because of its sim-plicity, low-temperature
operation process, and viability of commercial production. This
method is
also advantageous because the growth orientation, morphology,
and size of the resulting productscan be modestly controlled by
adjusting the deposition parameters (deposition voltage, current
den-sity, temperature, etc.). The setup of an electrochemical
deposition facility is illustrated in Fig. 21.
A typical fabrication process for the electrochemical synthesis
of CuO nanostructures with differentshapes at room temperature was
described by Yuan et al.[166]. In this fabrication, a
two-electrodesystem was used with Cu plates as the anode and
stainless steel plate as the cathode. The electrolytecontained 1 M
NaNO3dissolved in distilled water. Systematic studies of the
experimental parameterssuch as electrolyte, composition of
electrolyte solution, and current density reveal that the
composi-tion of electrolyte solution and the use of current density
during synthesis remarkably influence thesize and morphology of the
resulting CuO nanostructures. When H2OEtOH mix was used as an
elec-trolytic solvent, CuO nanorods with sharp-end morphology and
with 2050 nm in diameter and
200300 nm in length were obtained. The products obtained with
pure distilled water as an electro-lytic solvent are almost uniform
and mono-disperse spindle-like nanoparticles (i.e.,
nanospindles)with 80100 nm in diameter and 200300 nm in length.
However, by simply increasing the currentdensity from 5 mAcm2 to 10
mAcm2 and then to 20 mAcm2, the morphologies of CuO nanostruc-tures
tend to vary from nanospindles to nanorods and then to irregular
nanoplates. These results sug-gest that electrochemical shape- and
size-controlled synthesis of CuO nanostructure could be
easilyrealized by controlling the current density or changing the
electrolytic solvent.
Toboonsung et al.[167]synthesized CuO nanorods and their bundles
on a glass substrate using anelectrochemical dissolution and
deposition process. The deposition time, electrode separation,
andvoltage were found to play key roles in the formation of CuO
nanorods and the ratio of bundles to
Fig. 21. Schematic for typical setup of an electrochemical
deposition facility for synthesizing CuO nanostructures.
Fig. 22. TEM images of the leaf-like CuO mesocrystals under (a)
low and (b) high magnification [171].Copyright 2012 RoyalSociety of
Chemistry.
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nanorods. In addition, Xu et al. [168] synthesized large-scale
CuO honeycombs using two-step electro-chemical deposition and
subsequent heat-treatment. The Cu(OH)2 precursor was first
fabricated bytwo-step electrochemical deposition of a Cu foil in an
aqueous solution of KOH. The CuO honeycombswere then synthesized by
heating the Cu(OH)2precursor at 160 C for 3 h under nitrogen
protection.The applied potential and deposition time were found to
be the most important parameters affectingthe honeycomb-like CuO
growth. The CuO honeycombs comprised well-oriented nanowires with
uni-form average diameters of approximately 100 nm and lengths of
tens of micrometers. In addition tothe nanorods and honeycombs,
flower-shaped CuO nanostructures can also be achieved
electrochem-ically. Lu and Huang [169] recently reported the
synthesis of flower-like CuO microspheres from nano-flakes by
electrochemical anodic dissolution of pure Cu in an NaOH aqueous
solution at roomtemperature. Whisker-shaped CuO nanostructures
electrochemically fabricated from a metallic Cuprecursor, followed
by annealing at 600 C for 30 min in air, have also been recently
reported[170].
Xu et al.[171]reported the synthesis of leaf-like CuO
mesocrystals using the electrochemical pro-cess (Fig. 28), in which
Cu foils were used as the working and counter electrodes. The
electrodes weresubmerged into an aqueous solution of NaNO3. The
distance between the two electrodes was main-tained at 25 mm with
moderate magnetic stirring being applied throughout the process.
The CuO
mesocrystals were electrochemically grown at a constant voltage
of 3 V for 200 s at 70 C. The ob-tained precipitates were finally
harvested from the solution by centrifugation and dried at 70 C.TEM
images (Fig. 22) reveal that the width of the CuO NLs is 50 nm, and
the length is estimatedto be approximately several hundred
nanometers. Each CuO NL comprised numerous small particles,causing
the very rough surfaces of the obtained CuO NLs. When usedas LIB
anode materials, the ob-tained leaf-like CuO mesocrystals showed
high specific capability and good cycle performance becauseof the
novel feature of the CuO mesocrystals.
2.4. Thermal oxidation method
Different methods have been employed to synthesize 1D CuO
nanostructures[25,30,31]. The mostcommonly used method is to
directly heat Cu substrates in air, during which the reaction
between Cu
Fig. 23. SEM images of the CuO nanowires synthesized by directly
heating Cu grids in air at 500 C for (ac) 4 h and (d) 2
h[172].Copyright 2002 American Chemical Society.
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and oxygen (O2) results in the growth of CuO nanowires[31].
Typically, the process of thermal oxida-tion just involves heat
treatment of pure Cu substrates in either ambient air or O
2atmosphere. Themorphology of the grown CuO nanowires depends on
the oxidation temperature, growth time, andgas flow rate.
In 2002, Jiang et al.[172]synthesized CuO nanowires by directly
heating Cu substrate in air. Typ-ically, the Cu substrate was
cleaned using HCl solution to remove the native oxidation layer and
rinsedwith distilled water. After blow-drying with N2gas, the Cu
substrate was placed in a furnace. Then, thesubstrate was heated at
500 C for 4 h and naturally cooled to room temperature. The CuO
nanowireswith large aspect ratio grew on the surface of the Cu grid
or wire (Fig. 23). These nanowires had diam-eters ranging from 30
nm to 100 nm and can be up to 15lm long. The authors also reported
that CuOnanowires only grow within the temperature range from 400 C
to 700 C.
Various synthesis parameters, including oxidation temperature,
oxidation time, O2flow rate, anddifferent type of Cu substrates,
have been extensively studied to achieve better results from
thermaloxidation. Chen et al.[173]annealed a Cu foil in air using
various temperatures and growth times.Their results showed that the
density and the length of the CuO nanowires increased as the
growthtime was prolonged at 400 C. Instead of directly heating in
ambient air, Kumar et al. [174]synthe-
sized CuO nanowires by thermal annealing Cu foil in an oxygen
atmosphere. The results showed thatthe oxygen flow rate and the
annealing temperature both affected the aspect ratio and density of
CuOnanowires, but annealing time primarily affected only the aspect
ratio. Compared with the direct oxi-dation in air, the presence of
oxygen flow during thermal oxidation can reduce the necessary
growthtime for CuO nanowires to no more than 1 h. Mema et
al.[175]improved the growth density of CuOnanowires by applying
bending stresses on the Cu surface. The Cu foil was initially bent
at a 10 mmradius, followed by typical cleaning, annealing in 200210
Torr oxygen pressure, and cooling. Theupper surface of the Cu foil
was shortened by dL, whereas the bottom surface was elongated by
dL,
Fig. 24. A schematic diagram illustrating the generation of
stresses at the upper and bottom surfaces of the Cu foil
[175].Copyright 2011 Elsevier.
Fig. 25. SEM images of the oxide surface for (a) unbent Cu, (b)
upper surface of the bent Cu, and (c) bottom surface of the
Cu[175]. Copyright 2011 Elsevier.
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causing the compressive and tensile stresses in the upper and
bottom surfaces, respectively (Fig. 24).The results shown inFig.
25indicated that the tensile stress in the bottom surface promoted
thegrowth density of the CuO nanowires. However, the length of the
CuO nanowires is not significantlydependent on tensile stress.
Yuan and Zhou[176]reported that the growth density and length of
the CuO nanowires could beenhanced by increasing the surface
roughness of the Cu substrate. The Cu substrates were first
sand-blasted with different durations to generate different surface
roughness, followed by typical thermaloxidation process under 200
Torr oxygen pressure. Their results proved that surface roughness
signif-icantly increases the length and density of the grown CuO
nanowires. However, the synthesis of CuOnanowires directly on Cu
substrates (foils, plates, or grids) is not suitable for device
integration withthe current semiconductor industry. Therefore,
novel techniques to directly grow CuO nanowires onother substrates
(especially semiconducting silicon) are desired to achieve CuO
nanowire-based func-tional devices. Zhang et al.[177]reported
large-scale and aligned CuO nanowires synthesized onto asilicon
substrate by thermal oxidation of a Cu thin film deposited onto
silicon. Comparative results oftwo Cu thin films deposited by
thermal evaporation and electroplating show that a uniform and
largeamount of CuO nanowires grew on the electroplated thin film
only, which provided higher roughness
and larger surface grain size. Given that the CuO nanowires were
synthesized onto silicon, a basicmaterial for microelectronics and
microsystems, integrating CuO nanowires into silicon-based
micro-systems is more convenient to achieve promising functional
devices. The introduction of high-purityN2and O2gas during
annealing results in more vertically aligned and uniform CuO
nanowires[178].
Furthermore, Zhang et al.[179]presented a novel localized
thermal oxidation method in ambientair instead of heating the
entire Cu foils/films to synthesize the CuO nanowires. The CuO
nanowiresonly grew on the surface of the heated area realized by
localized joule heating ( Fig. 26). This methodis CMOS-compatible
and can potentially integrate CuO nanowires with conventional
microelectronics.
Vertically aligned CuO nanowires by thermal oxidation of a Cu
thin film deposited onto 30 nmCu/Ti film-coated silicon substrates
were fabricated by Cheng and Chen [180]. The length of the
Fig. 26. (a) SEM image of the suspended microheaters, (b) highly
magnified SEM image of the tip in (a), (c) highly magnifiedSEM
image of CuO nanowires in (b), and (d) optical image of one tip
heater being heated [179]. Copyright 2010 Institute ofPhysics.
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CuO nanowires was observed to be tuned from several to tens of
micrometers by tailoring theoxidation temperature and time. The
obtained CuO nanowires were single-crystalline with differentaxial
crystallographic orientations, and the average length of CuO
nanowires produced at eachtemperature followed a parabolic
relationship with the oxidation time (Fig. 27).
Tsai et al. [181] synthesized uniformly aligned single-crystal
CuO nanowires using a Co WP cappinglayer as a nanofilter to
catalyze CuO nanowires on Cu (100 nm)/TaN/Ta/SiO2/Si blanket
substrates bythermal oxidation. Their results suggested that the
obtained CuO nanowires can grow at a relativelyhigher growth rate
than those reported and become longer and denser with increasing
calcinationtime. Park et al.[182]demonstrated that CuO nanowire
growth can be achieved by thermal oxidationof Cu metal deposited on
CuO (20 nm)/SiO2/Si substrate. The CuO nanowires were grown by a
contin-uous supply of both Cu from the Cu films and O2from air. The
growth of CuO nanowires by heating Cufilm deposited on glass
substrates in the air was realized by Hsueh et al.[183]and Chang
and Yang[184](Fig. 28). To avoid cracking of the obtained CuO
nanowires/film during oxidation, a 100-nm-thick CuO film was first
deposited onto the glass substrate to serve as an adhesion layer (
Fig. 28).Moreover, the average length of the CuO nanowires was
determined by the initial Cu film thickness.
In summary, various methodologies have been reported to
synthesize nanostructured CuO. The
hydrothermal synthesis, a wet chemical process with a reaction
occurring in solution, is well knownfor low temperature, simple
equipment, environmentally safe process, and good potential
forhigh-quantity production. The solution-based chemical
precipitation method that utilizes chemicalsolutions is another
promising synthetic route because of its high efficiency,
relatively low-cost,and its advantages in adjusting the size and
morphology of the CuO nanostructures. The electrochem-ical method
for the formation of nanostructured CuO is also of particular
interest because of its manymerits over other methods including
low-temperature, ease of process, and viability of
commercialproduction. Furthermore, with the right selection of pH
level and/or potential, the Cu or CuO phasecan be well controlled.
Thermal oxidation is a simple, efficient, and low-cost method for
synthesizing1D CuO nanostructures, and it is suitable for batch
fabrication and mass production. The achieved 1DCuO nanostructures
are uniform and vertically aligned with low impurity. In addition,
the morphol-
ogy, density, diameter, and length ofthe1D CuO nanostructures
can be easily tailored by adjustingthe synthesis parameters.
Fig. 27. Photographs of as-deposited Cu/Ti thin film and
electrodeposited Cu film and SEM images of nanowires. Photographs
ofthe (a) as-deposited Cu/Ti thin film on a silicon substrate and
the electrodeposited Cu film-coated silicon samples (b) before
and(c) after thermal oxidation. (d) A typical cross-sectional SEM
image of nanowires grown on an oxidized Cu film-coated
siliconsubstrate. (e) A typical high magnification SEM image of an
individual nanowire [180]. Copyright 2012 Springer.
Q. Zhang et al. / Progress in Materials Science 60 (2014) 208337
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2.5. Other synthetic methods
In addition to the methods described in Section 2.4, other
synthetic techniques have also been usedto prepare CuO
nanostructures. These techniques include sonochemical synthesis
[48,50,185189],microwave irradiation synthesis [190196],
template-assisted method [197205], solgel [206],microemulsion
[207210], electrospinning technique [211213], synthesis combined
with somephysical methods (e.g., spray pyrolysis) [214,215],
thermal-based chemical vapor deposition[216],andc-irradiation[217].
For example, nanorods, nanoparticles, and submicrospheres from CuO
havebeen obtained by sonochemical synthesis. Various CuO
nanostructures, which include nanorods, shut-tle-like, flower-like,
plate-like, leaf-like, dandelion-like, and hollow structures,
together with CuOnanoparticles have been synthesized by microwave
irradiation synthesis. CuO nanowires and nanof-ibers have been
achieved by template-assisted method and electro-spinning
technique.
3. Growth mechanisms
The development of nanotechnology has resulted in the
fabrication of CuO nanostructures withvarious morphologies and
sizes using different synthetic methods. However, the growth
mechanismsresponsible for the formation of CuO nanostructures with
various morphologies during syntheses arestill not fully
understood, and extensive studies have been conducted to determine
the growth mech-anisms of different CuO nanostructures. In this
section, we briefly review the most important mech-anisms that are
proposed for the growth of CuO nanostructures.
3.1. Oriented attachment
Oriented attachment is defined as a special kind of crystal
growth in which small crystallites attachto each other through
their suitable crystal planes or facets along the same
crystallographic directions.
In this sense, the final aggregates can be considered as large
single crystals built from the pristine crys-tallite in an
irreversible and highly oriented manner[218,219].
Zhang et al.[220]demonstrated an anisotropic aggregation-based
crystal growth of a few hundredmonoclinic CuO nanoparticles into
uniform ellipsoidal monocrystalline architectures by taking
advan-tage of the oriented attachment (Fig. 29). Stepwise
orientation and aggregation in three dimensionswere observed at
room temperature, caused by the formation of primary CuO
nanoparticles in amother solution via the preferential 1D [001]
or