Journal of Surface Engineered Materials and Advanced Technology, 2012, 2, 278-283
http://dx.doi.org/10.4236/jsemat.2012.24042 Published Online October 2012 (http://www.SciRP.org/journal/jsemat)
Evolution of Morphology of Nano-Scale CuO Grown on
Copper Metal Sheets in 5 wt% NaCl Solution of Spray Fog
Environment
Hao-Long Chen, Tsung-Hsun Chiang, Ming-Cheng Wu
Department of Electronic Engineering, Kao Yuan University, Kaohsiung City, Chinese Taipei.
Received July 1st, 2012; revised August 8th, 2012; accepted August 18th, 2012
ABSTRACT
Nano-scale copper oxide with various morphologies is synthesized via the thermal oxide method and growth in a 5 wt%
NaCl solution of spray fog environment. The nano-scale copper oxide is grown on copper metal sheets via the thermal
oxide method at 650˚C for 60 minutes. Nano-scale copper oxide grains and nanowires are induced on copper metal
sheets then placed in 5 wt% NaCl solution of salt spray fog environment. Significant changes in particle size and mor-
phology are observed with increasing salt spray fog treatement time. The morphology of nano-scale copper oxide varies
from nanograins to nanowires, Ctahedron, and icositetrahedron. The morphologies and structures of the obtained
nano-scale copper oxide are investigated by scanning electron microscopy and energy-dispersive spectroscopy. Possible
growth mechanisms are discussed.
Keywords: Nano; CuO; Morphology; NaCl Solution; Spray Fog
1. Introduction
Nanoscale one-dimensional (1D) and two-dimensional
(2D) materials such as nanowires, nanorods, nanoribbons,
and nanosheets have attracted a lot of interest due to their
unique physical properties and potential applications in
nanodevices.
Copper oxide (CuO) is a well known p-type semicon-
ductor and a photovoltaic material. It has a narrow band
gap (1.2 eV). CuO is desirable due to its availability and
abundance of the starting materials, non-toxicity, and low
production cost [1]. Consequently, it is widely used in
applications such as catalysis [2], sensors [3], solar cells
[4,5], and field-emission emitter materials [6]. However,
the chemical and physical properties of CuO strictly de-
pend on its size and morphology [7]. Many studies have
synthesized various CuO nanostructures. A number of
methods have been used to synthesize and control the
size and morphology of nano-scale CuO, such as thermal
evaporation [1], solution synthesis [7], thermal oxidation
[8], sol-gel [9], hydrothermal method [10], electrospin-
ning [11] and microwave hydrothermal synthesis [12].
These methods have been used to synthesize CuO
nanomaterials with various morphologies, such as nano-
particles, 1D nanowires, nanorods, and nanobelts; 2D
nanosheets, nanoleaves, and nanowiskers; and three-dim-
ensional peachstone-like, boat-like, and ellipsoid-like na-
nostructures [10]. The morphology of nano-scale CuO is
related to growth environment conditions, in addition to
processing methods.
In this work, nano-scale CuO with various morpholo-
gies is synthesized via the thermal oxide method and
growth in a 5 wt% NaCl solution of spray fog environ-
ment. The morphology of nano-scale CuO varied from
nanograins to nanosheets and nanoneedles with increase-
ing salt spray fog treatement time. Possible growth
mechanisms are discussed.
2. Experimental Procedure
CuO with various morphologies was synthesized via the
thermal oxide method and growth in a 5 wt% NaCl solu-
tion of spray fog environment.
Copper (99.99 wt% Cu) substrate samples with di-
mensions of 10 mm (width) × 10 mm (length) × 1 mm
(thickness) were prepared according to the following
procedure: the substrate was polished with various grind-
ing papers (400, 600, 800, 1200, 2000, and 4000 grade)
and then ultrasonically cleaned with acetone, ethanol,
and de-ionized water for 10 minutes sequentially to re-
move the impurities and native oxides on its surface. The
samples were then dried by blowing with high-pressure
nitrogen gas.
Thermal oxidation of the Cu sheets was carried out in

Evolution of Morphology of Nano-Scale CuO Grown on Copper Metal Sheets in 5 wt%
NaCl Solution of Spray Fog Environment
279
a resistance-heated tube furnace in a pure oxygen at-
mosphere at a temperature of 650˚C for 60 minutes.
Nano-scale CuO grains were induced on the copper
metal sheets and then placed in 5 wt% NaCl solution of
spray fog environment. The treatment times of the sam-
ples in the spray fog environment were 1, 7, 14, 21, and
28 days, respectively. The temperature of the spray fog
environment was kept in 45˚C .
The surface morphology of the samples was observed
using a scanning electron microscope (SEM, Hitachi
S3000N, Japan) operated at 30 kV. The chemical com-
position of the nanomaterials was confirmed by energy-
dispersive X-ray analysis (EDX, Horiba, Japan). The
phases and structure of the nanomaterials were identified
by grazing-incidence X-ray diffraction (GIXRD) using a
Rigaku D/MAX 2500 multipurpose X-ray thin-film dif-
fractometer with monochromatic high-intensity CuK

radiation (
 = 0.15418 nm).
3. Results and Discussion
3.1. Phase and Structure Analysis
The X-ray diffraction patterns of polycrystalline Cu metal
sheets prepared by polishing and thermal oxidation are
shown in Figure 1. Figure 1(a) shows the diffraction
peaks of an as-polished Cu metal sheet, which are assigned
to the (111), (200) and (220), planes of face-centered cubic
(fcc) Cu. This result is in agreement with the standard data
from JCPDS card No. 85-1326. The XRD patterns of CuO
samples on Cu metal sheets prepared via thermal oxidation
are shown in Figure 1(b). The XRD peaks can be indexed
as monoclinic structured CuO (JCPDS card No. 80-1917),
cubic structured cuprous oxide (Cu2O) (JCPDS card No.
78-2076), and fcc Cu metal from the substrate (JCPDS
card No. 85-1326). The diffraction peaks of the mono-
clinic structured CuO are assigned to the (110), (–111),
(111), (20-2), (11-3), (31-3), (400), and (22-3) planes. The
peaks of cubic structured Cu2O are assigned to the (111)
and (200) planes.
Figure 2 shows the XRD patterns of nano-scale CuO
prepared via the thermal oxide method and then placed in
a 5 wt% NaCl solution of spray fog environment for 0
(only thermal oxidation), 1, and 28 days, respectively.
The diffraction pattern of the copper(II) chloride (CuCl2)
appeared for the sample prepared via thermal oxidation
and then placed in a 5 wt% NaCl solution of spray fog
environment for 1 day, in addition to the original produc-
tion peaks of CuO, Cu2O, and Cu metal substrate (Figure
2(b)). The diffraction peak of the monoclinic structured
CuCl2 is assigned to the (–405) planes (JCPDS card No.
79-1635). The peaks of CuCl2 increased with increasing
salt spray fog treatment time. Figure 2(c) shows that the
diffraction planes of CuCl2 were (002), (111), (112),
(–313), and (–221) for the sample treated using 5 wt%
Figure 1. XRD patterns of polycrystalline Cu metal sheets
prepared by (a) polishing and (b) thermal oxidation.
Figure 2. XRD patterns of nano-scale copper oxide pre-
pared by thermal oxide method followed by exposure to 5
wt% NaCl solution of spray fog environment for (a) 0 (only
thermal oxidation); (b) 1; and (c) 28 days.
NaCl solution of spray fog for 28 days.
The XRD results indicate that the main phases of the
nano-scale CuO grown on pure Cu metal substrates after
thermal oxidation were CuO and Cu2O. CuCl2 was in-
duced on pure Cu metal substrates when the samples
were placed in 5 wt% NaCl solution of spray fog envi-
ronment.
3.2. Morphology Observation
Figure 3 shows the surface morphology of polycrystal-
line Cu metal sheets prepared by polishing. The surface
of the Cu metal sheets did not have any surface chemical
products.
SEM micrographs of nano-scale CuO and nanowires

Evolution of Morphology of Nano-Scale CuO Grown on Copper Metal Sheets in 5 wt%
NaCl Solution of Spray Fog Environment
280
Figure 3. Surface morphology of polycrystalline Cu metal
sheet prepared by polishing.
grown via thermal oxidation on Cu metal sheets for 60
minutes at 650˚C are shown in Figure 4(a). The mor-
phologies of nano-scale surface products formed after
thermal oxidation followed by exposure to 5 wt% NaCl
solution of spray fog environment for various durations
significantly change, as shown in Figures 4(b)-(f). Fig-
ure 4(a) shows Ctahedral-shaped grains about 1 μm ag-
gregated on the nanowire after 1 day in a 5 wt% NaCl
solution of spray fog environment. The Ctahedral-shaped
grain size increased with increasing spray fog treatment
time. The Ctahedral-shaped grains are replaced by a six-
horn-shaped icositetrahedron. Each crystal has six apexes,
each of which is connected to four sides to form an ico-
sitetrahedron [13]. Icositetrahedron-shaped crystals can
be observed in the Figure 4(d). When the salt spray fog
treatment time was increased to 28 days, the edges and
corners of the icositetrahedron-shaped crystals become
more extrusive (Figure 4(f)). The crystal size increased
from about 1 μm (1 day) to about 8 μm (14 days) and
then decreased to about 4 μm (28 days).
The chemical composition of the nano-scale oxide
grown on pure Cu metal substrates after thermal oxida-
tion and salt spray fog treatment was confirmed by EDX.
Figure 5 shows the nanowire crystal after thermal oxida-
tion that is CuO. The nano-scale oxide grown in a salt
spray fog environment, which formed Ctahedral-shaped
and icositetrahedron-shaped crystals, was Cu2O. The crys-
tal shape similar to a tetrahedron was CuCl2, as shown in
Figure 6.
3.3. Growth Mechanisms of Nano-Scale Copper
Oxides
There are two major growth models, namely vapor-liq-
uid-solid (VLS) catalyst-assisted growth [8] and vapor-
solid (VS) growth [14], for describing the growth
of 1D nanostructures. Kaur et al. [8] synthesized CuO
nanowires via the thermal oxidation of copper foils and
found that the growth of the nanowires is initiated by the
condensation of the vapor of the material at the tip of the
droplet in the VLS process. The morphology of a 1D
structure has a round droplet, which is usually found near
the tip of the wire. However, the CuO nanowires pre-
pared via thermal oxidation had no droplets at their tips,
indicating that the VLS mechanism is not applicable.
The oxidation process of the Cu metal substrate in-
cludes two steps [15]:
2
4CuO2Cu O
2

 (1)
22
2Cu OO4CuO

 (2)
In this work, the nano-scale Cu grains and nanowires
were induced on copper metal sheets by thermal oxida-
tion. The Cu atoms or ions diffused to the surface and
then reacted with oxygen ions that formed CuO in the
initial stage. When thickness of CuO film increased with
increasing oxidation time, Cu atoms or ions difficult dif-
fusion to reactive with surface O2. Consequently, the
CuO, Cu2O, and Cu phases were obtained in the samples.
The following mechanism has been proposed for the
oxidation and reduction processes of copper in NaCl
solution [16]:

ads
Cu ClCuCle


 (3)
 
ads film
CuCl CuCl (4)
CuCl eCu Cl  

(5)

2 ads
CuCl ClCuCl
2
(6)
2
CuClCu2Cl e (7)
The reactions in Equations (3)-(5) represent the dis-
solving, chelating, and film-forming processes of copper,
respectively. The reactions in Equations (6) and (7) are a
competition of the growth and dissolution of the corro-
sion products on the surface of the specimens.
Nano-scale CuO and Cu2O were grown on the Cu
metal substrate and then placed in 5 wt% NaCl solution of
spray fog environment. The morphologies of nano-scale
CuO variation were dominated by an electrochemical
mechanism. Zhao et al. [13] synthesized Cu2O micro-
crystals with various shapes using an electrochemical
method and found that the Cu2O crystal shapes changed
from Ctahedral to icositetrahedral with increasing solu-
tion PH value. The cations in the solution have little in-
fluence on the morphology of copper, but the anions
have a complex influence. Hu et al. [17] found that Cl
can form in certain local regions of copper. Ctahedron,
nanowire, and prism structures are obtained besides cubic
copper when the concentration of Cl reaches a certain
value. The Cl adsorption on special crystal faces may

Evolution of Morphology of Nano-Scale CuO Grown on Copper Metal Sheets in 5 wt%
NaCl Solution of Spray Fog Environment
281
Figure 4. Surface morphology of polycrystalline Cu metal sheet prepared by thermal oxide method followed by exposure to 5
wt% NaCl solution of spray fog environment for (a) 0 (only thermal oxidation); (b) 1; (c) 7; (d) 14; (e) 21; and (f) 28 days.
4. Conclusions
cause some crystal faces with a faster growth rate to
gradually shrink or even disappear with the growth of
crystals. This reasonably explains why the crystal size
increased from about 1 μm (1 day) to about 8 μm (14
days) and then decreased to about 5 μm (28 days). The
diffraction plane (–405) of CuCl2 disappeared when the
samples were treated using 5 wt% NaCl solution of spray
fog for 28 days.
Nano-scale CuO grains and nanowires were induced on
copper metal sheets and then placed in 5 wt% NaCl
solution of spray fog environment. The following con-
clusions were obtained:
 Nano-scale copper oxide likely developed as CuO
nanograins and CuO nanowires, and then Ctahe-
dral-shaped Cu2O and tetrahedral-shaped CuCl2 ag-

Evolution of Morphology of Nano-Scale CuO Grown on Copper Metal Sheets in 5 wt%
NaCl Solution of Spray Fog Environment
282
CuO
Figure 5. SEM images and EDX spectrum of CuO nano-
wires.
Figure 6. SEM images and EDX spectrum of Cu2O icositet-
rahedron and CuCl2 tetrahedron.
gregated on the nanowires. When the salt spray fog
treatment time was increased, the Ctahedral-shaped
Cu2O and tetrahedral-shaped CuCl2 mixed to form
icositetrahedron-shaped Cu2O crystals.
 CuO nanowires prepared via thermal oxidation re-
garding likely follow the VS mechanism.
 Nano-scale CuO and Cu2O were grown on a Cu metal
substrate and then placed in 5 wt% NaCl solution of
spray fog environment. The morphology of nano-
scale copper oxide was dominated by an electro-
chemical mechanism.
 The Cl adsorption on special crystal faces may cause
some crystal faces with a faster growth rate to gradu-
ally shrink or even disappear with the growth of crys-
tals.
5. Acknowledgements
The authors are grateful to the Ministry of Economic
Affairs (MOEA) Science and Technology Development
Program, Taiwan, Republic of China for supporting of
this research under grant 99-EC-17-A-05-S1-122.
REFERENCES
[1] M. F. Al-Kuhaili, “Characterization of Copper Oxide Thin
Films Deposited by the Thermal Evaporation of Cuprous
Oxide (Cu2O),” Vacuum, Vol. 82, No. 6, 2008, pp. 623-
629. doi:10.1016/j.vacuum.2007.10.004
[2] S. Ohya, S. Kaneco, H. Katsumata, T. Suzuki and K.
Ohta, “Electrochemical Reduction of CO2 in Methanol
with Aid of CuO and Cu2O,” Catalysis Today, Vol. 148,
No. 3-4, 2009, pp. 329-334.
doi:10.1016/j.cattod.2009.07.077
[3] F. Teng, W. Q. Yao, Y. F. Zheng, Y. T. Ma, Y. Teng, T.
G. Xu, S. H. Liang and Y. F. Zhu, “Synthesis of Flower-
Like CuO Nanostructures as a Sensitive Sensor for Ca-
talysis,” Sensor and Actuators B: Chemical, Vol. 134, No.
2, 2008, pp. 761-768. doi:10.1016/j.snb.2008.06.023
[4] S. Anandan, X. G. Wen and S. H. Yang, “Room Temper-
ature Growth of CuO Nanorod Arrays on Copper and
Their Application as a Cathode in Dye-Sensitized Solar
Cells,” Materials Chemistry and Physics, Vol. 93, No. 1,
2005. pp. 35-40. doi:10.1016/j.matchemphys.2005.02.002
[5] R. P. Wijesundera, “Fabrication of the CuO/Cu2O Hetero-
junction Using an Electrodeposition Technique for Solar
Cell Applications,” Semiconductor Science and Technol-
ogy, Vol. 25, No. 4, 2010, Article ID: 045015.
doi:10.1088/0268-1242/25/4/045015
[6] Y. W. Zhu, T. Yu, F. C. Cheong, X. J. Xu, C. T. Lim, V.
B. C. Tan, J. T. L. Thong and C. H. Sow, “Large-Scale
Synthesis and Field Emission Properties of Vertically
Oriented CuO Nanowire Films,” Nanotechnology, Vol.
16, No. 1, 2005, pp. 88-92.
doi:10.1088/0957-4484/16/1/018
[7] F. Bayansal, S. Kahraman, G. Cankaya, H. A. Cetinkara,
H. S. Güder and H. M. Cakmak, “Growth of Homogenous

Evolution of Morphology of Nano-Scale CuO Grown on Copper Metal Sheets in 5 wt%
NaCl Solution of Spray Fog Environment
283
CuO Nano-Structured Thin Films by a Simple Solution
Method,” Journal of Alloys and Compounds, Vol. 509,
No. 5, 2011, pp. 2094-2098.
doi:10.1016/j.jallcom.2010.10.146
[8] M. Kaur, K. P. Muthe, S. K. Despande, S. Choudhury, J.
B. Singh, N. Verma, S. K. Gupta and J. V. Yakhmi,
“Growth and Branching of CuO Nanowires by Thermal
Oxidation of Copper,” Journal of Crystal Growth, Vol.
289, No. 2, 2006, pp. 670-675.
doi:10.1016/j.jcrysgro.2005.11.111
[9] G. N. Rao, Y. D. Yao and J. W. Chen, “Evolution of Size,
Morphology, and Magnetic Properties of CuO Nanoparti-
cles by Thermal Annealing,” Journal of Applied Physics,
Vol. 105, No. 9, 2009, Article ID: 093901.
[10] Z. P. Cheng, J. M. Xu, H. Zhong, X. Z. Chu and J. Song,
“Hydrogen Peroxide-Assisted Hydrothermal Synthesis of
Hierarchical CuO Flower-Like Nanostructures,” Materi-
als Letters, Vol. 65, No. 13, 2011, pp. 2047-2050.
doi:10.1016/j.matlet.2011.04.021
[11] S. W. Choi, J. Y. Park and S. S. Kim, “Growth Behavior
and Sensing Properties of Nanograins in CuO Nanofibers,”
Chemical Engineering Journal, Vol. 172, No. 1, 2011, pp.
550-556. doi:10.1016/j.cej.2011.05.100
[12] A. P. Moura, L. S. Cavalcante, J. C. Sczancoski, D. G.
Stroppa, E. C. Paris and A. J. Ramirez, “Structure and
Growth Mechanism of CuO Plates Obtained by Micro-
wave-Hydrothermal without Surfactants,” Advanced Pow-
der Technology, Vol. 21, No. 2, 2010, pp. 197-202.
doi:10.1016/j.apt.2009.11.007
[13] W. Y. Zhao, W. Y. Fu, H. B. Yang, C. J. Tian, R. X. Ge,
C. J. Wang, Z. L. Liu, Y. Y. Zhang, M. H. Li and Y. X.
Li, “Shape-Controlled Synthesis of Cu2O Microcrystals
by Electrochemical Method,” Applied Surface Science,
Vol. 256, No. 7, 2010, pp. 2269-2275.
doi:10.1016/j.apsusc.2009.10.051
[14] R. S. Wanger and W. C. Ellis, “Vapor-Liquid-Solid Mecha-
nism of Single Crystal Growth,” Applied Physics Letters,
Vol. 4, No. 5, 1964, pp. 89-90. doi:10.1063/1.1753975
[15] C. H. Xu, C. H. Woo and S. Q. Shi, “Formation of CuO
Nanowires on Cu Foil,” Chemical Physics Letters, Vol.
399, No. 1-3, 2004, pp. 62-66.
doi:10.1016/j.cplett.2004.10.005
[16] B. Guo, P. Zhang, Y. P. Jin and S. K. Cheng, “Effects of
Alternating Magnetic Field on the Corrosion Rate and
Corrosion Products of Copper,” Rare Metals, Vol. 27, No.
3, 2008, pp. 324-328.
doi:10.1016/S1001-0521(08)60138-2
[17] M. Y. Hu, K. G. Zhou, C. G. Wang and R. Xu, “Cl− In-
duced Synthesis of Submicron Cubic Copper Particles in
Solution,” Journal of University of Science and Technol-
ogy Beijing, Mineral, Metallurgy, Material, Vol. 15, No.
5, 2008, pp. 659-664.
doi:10.1016/S1005-8850(08)60123-1