Growth morphologies, phase formation, optical & biological responses of
nanostructures of CuO and their application as cooling fluid in high energy
density devices
Kajal Kumar Dey,
ac
Ashutosh Kumar,
b
Rishi Shanker,
b
Alok Dhawan,
b
Meher Wan,
c
Raja Ram Yadav
c
and
Avanish Kumar Srivastava*
a
Received 10th September 2011, Accepted 21st October 2011
DOI: 10.1039/c1ra00710f
Different nanoscale objects of CuO have been synthesized by a simple chemical route where the
Cu(OH)
2
nanostructures were first synthesized by the alkaline hydrolysis of Cu(NO
3
)
2
?3H
2
O using
NaOH as a base and the synthesized precipitate was subsequently annealed at a temperature of
130 uC. The alkaline content (pH) of the solutions during the hydrolysis process was varied to tailor
the morphologies and dimensions of the nanostructures, consequently a series of fascinatingly shaped
nanostructures, e.g. seeds, ellipsoidal, rods and leaves were obtained. Topographical characteristics
along with the mechanism behind the structural variation have been rationalized by XRD, FTIR,
SEM and HRTEM investigations. Optical performance of these samples provided simultaneous
emission in the visible bands of blue, green, yellow and red, which were correlated to the size, shape
and structural defects of these nano-scaled objects. The toxicity of these nanostructured materials
were also put into perspective and it was found that the leaf shaped particles were the most toxic
among the various shapes of nano-CuO. Finally the synthesized particles, when applied as nanofluids
(water medium) showed their ability to enhance the thermal conductivity of water to a noticeable
degree (above 40%) at high temperatures, even at very small concentrations, bespeaking their
applicability in cooling fluids.
Introduction
The world of nanomaterials has become an exciting challenge for
physicists, chemists and material scientists. During the last two
decades, a vast amount of knowledge on the synthesis and
properties of various nanoparticles and nanocomposites has
been gathered, with new insights and discoveries emerging on an
almost daily basis.
1–9
Now that the expectations regarding the
feasibility of the potential of these nanoparticles as 21st century
functional materials are at an all time high, the technological
limitations of the existing micro-devices are becoming apparent,
underlining the importance of scaling down the conventional
technologies by at least an order of magnitude, and nanopar-
ticles (NPs) are perfectly suited building blocks for that. The
outstanding physicochemical characteristics of these nanoparti-
cles can be ascribed to their miniaturized size (surface area and
size distribution), chemical composition (purity, crystallinity,
electronic properties, etc.), surface structure (surface reactivity,
surface groups, inorganic or organic coatings, etc.), solubility,
shape and aggregation. Although impressive from a physico-
chemical viewpoint, the novel properties of nanomaterials raise
concerns about adverse effects and potential toxicity on the
biological environment, which at the cellular level includes
structural arrangements that resemble nanomaterials in terms of
their functionality.
10–12
Transition metal oxide nanomaterials are functional materials
that have pioneered advanced applications in diverse fields
because of their unique features in terms of their optical,
magnetic and electrical properties as well as their hardness,
thermal stability and chemical resistance. Amongst the
transition metal oxide nanomaterials which still exude great
interest within the industrial and scientific fraternity, Cupric
Oxide (CuO; known as tenorite in its mineral form) has long
caught the imagination of researchers due to its typical structural
lineaments and broad range of existing and potential applica-
tions. CuO is a unique monoxide amongst the 3d transition
metal monoxides in that it has a square planar coordination of
the Cu atom to the neighboring oxygen atoms and a monoclinic
crystal structure, unlike the oxides of other 3d metals which
predominantly have cubic rock salt crystal structures with an
octahedral coordination. With a narrow band gap varying
between 1.2–1.8 eV CuO has uses as a p-type semiconductor,
13,14
a
Electron and Ion Microscopy, Materials and Chemical Metrology,
National Physical Laboratory, Council of Scientific and Industrial
Research, Dr K. S. Krishnan Road, New Delhi, 110012, India.
E-mail: aks@nplindia.org
b
Nanomaterial Toxicology group, Indian Institute of Toxicology Research,
Council of Scientific and Industrial Research, M.G. Road, Lucknow,
226001, India
c
Department of Physics, University of Allahabad, Allahabad, 211002, India
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a heterogeneous catalyst,
15–17
in hazardous gas sensing,
13,18,19
as
a crucial component in high temperature superconductors,
20,21
fabricating solar cells
22,23
and has a potentially huge application
as an electrode material in Li-ion batteries.
24,25
Recently CuO
NPs have gained significant attention for their utility in one of
the most exciting new breeds for nanoparticle applications,
thermal conductivity enhancers in nanofluids. These fluids are
engineered by uniformly dispersing nanosized particles in a fluid
and are widely tipped to be the next generation coolants and
working fluids for innovative applications in industry such as
energy, bio and pharmaceutical.
26–29
The most significant challenges remaining in developing its
potential are: i) the facile fabrication of desired structure shapes
where the characteristics are more definitive and thus more
suitable for property studies, or monitoring. ii) Correlating
several shapes in terms of their utility with respect to developed
applications and properties in order to determine the best
corresponding shape for a particular application and iii)
evaluation of these synthesized materials in terms of general
safety and health concerns; keeping in mind the growing
concerns regarding the toxic potential of the materials at the
nanolevel.
Although CuO nanoparticles of multiple shapes and morphol-
ogies have previously been synthesized in various physical
methods such as mechanical milling
30
or simply heating an
elemental copper substrate at 400–700 uC,
31
solution phase
chemical synthesis has arguably been the most effective way of
synthesizing these nanoparticles with its cost effectiveness,
control over morphology and excellent yield. The solution
strategies employed so far include using hydrothermal method to
give urchin like core-shell structures,
19,32
electrodeposition
followed by self-catalytic growth using copper(
II) salt as
electrolyte producing nanofibers,
33
a one step solid state reaction
using a surfactant
34
and the sol–gel method.
35
It is a well known
fact that copper(
II) oxide can be conventionally obtained by the
thermal decomposition of copper salts in the solid state. For
instance, nitrates, and sometimes hydroxysalts, can be decom-
posed to yield CuO.
36
However the solid state decomposition
technique rarely gives the desired mastery over the shapes and
grain sizes of the nanoparticles, unlike the simple solution phase
decomposition technique. Decomposition of cupric hydroxide to
produce CuO has been one such example. The biggest advantage
of this method has been that the synthesized CuO nanoparticles
follow the same morphological characteristics as its parent
Cu(OH)
2
in nearly all the cases, so control over the cupric
hydroxide nanostructures is likely to give sufficient control over
the CuO nanostructures. This method has been adopted by
previous researchers where a cupric salt has been hydrolyzed in
an alkaline medium by the use of NaOH or KOH to yield
Cu(OH)
2
, which has subsequently been thermally decomposed to
yield CuO nanoparticles.
37,38
However, a detailed study of the
effect of the reaction conditions such as pH and the reagents on
the resultant morphological and structural features of the
corresponding particles is still called for.
Herein, CuO nanostructure has been synthesized by a simple
template free wet chemical solution method, the alkaline
hydrolysis of a copper(
II) salt followed by the thermal decom-
position of the precipitated cupric base. We have varied the
alkali concentration (pH) during hydrolysis and have studied its
effect on the size and shape of the corresponding nanostructured
CuO particles and have rationalized the probable mechanism for
the changes.Besides performing the usual morphological char-
acterizations, we have observed the optical properties of these
CuO nanoparticles. Although in the past, the magnetic and
electrical properties of CuO has been thoroughly investi-
gated,
1,32,39,40
its optical properties, especially the existence of
different emission states remains sparsely studied
41,42
and only
vaguely explored. In fact there is only a handful of literature
available reporting the characteristic emission bands of CuO. Here
we have made an attempt to see into the photoluminescence
properties of CuO and suggest a rational explanation for the bands
observed in conjunction with the varied morphologies and
dimensions of the nanoscale CuO. In addition, we have evaluated
these samples in terms of their toxicity with respect to mammalian
(Chinese hamster ovary; CHO) cells and bacterial cells
(Escherichia coli; E. coli) and determined the most bio-adverse of
the various shapes of CuO that we have obtained. The prepared
CuO nanoparticles at a very low concentration were used to
generate water based nanofluids. The obtained results indicate a
very impressive performance of the nanofluids in terms of their
thermal conductivity increment. Both the toxicity response and
nanofluid performance were subjected to evaluation with respect
to the shape of the nanoparticles, which to the best of our
knowledge, has not been attempted before for oxide nanoparticles.
Experimental
Chemicals
The chemicals Cu(NO
3
)
2
?3H
2
O (cupric nitrate trihydrate, 99.5%,
Alfa Aesar) and NaOH (sodium hydroxide), were analytical grade
and used without any further purification. Luria Bertani (LB)
broth and 3-(4, 5-dimethylthiazol-2-yl)-2, 5-diphenyl tetrazolium
bromide (MTT) dye were purchased from Hi-Media Pvt. Ltd.
(Mumbai, India). Phosphate buffered saline (PBS; Ca
2+
,Mg
2+
free), F-12 medium, trypsin–EDTA, fetal bovine serum (FBS),
antibiotic and antimycotic solution (10 000 U ml
21
penicillin,
10 mg ml
21
streptomycin 25 mgml
21
amphotericin B) were
purchased from Life Technologies (India) Pvt. Ltd., (New Delhi,
India). Propidium iodide was purchased from Sigma chemical Co.
Ltd. (St. Louis, MO, USA). Cell culture plastic wares were
obtained from Tarsons Products Pvt. Ltd. (Kolkata, India).
Synthesis of the nanoparticles
Hydrolysis of cupric nitrate salt. In a typical synthesis
procedure, 1 g of Cu(NO
3
)
2
?3H
2
O was dissolved in 100 ml of
distilled water. It was followed by the drop-wise addition of
NaOH solution (y1 M) with constant stirring till the solution
attained the desired pH. A deep blue precipitate appeared. The
appearance of this precipitate indicates the probable formation
of Cu(OH)
2
. One interesting observation to note here is that as
the pH was increased, the colour of the precipitate changed from
greenish blue to dark blue. The precipitate was subsequently
collected, filtered and washed repeatedly in de-ionized water to
remove anions such as nitrate (NO
3
2
).
Transformation of the intermediate precipitate into CuO
nanostructures. The obtained precipitate was heated at a
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temperature of 130 uC for 10 h in a micro oven. The bluish
precipitate of Cu(OH)
2
gradually turned black indicating the
possible formation of CuO particles. The black coloured solid
powder was collected and used for subsequent characterization.
Sample characterization
The crystallographic phase identification and the purity of the
obtained powder was carried out by X-ray diffraction (XRD,
Bruker AXS D8 Advance X-ray Diffractometer) using mono-
chromatized Cu-Ka radiation (l = 1.54059 A
˚
) and scanning in
2h range from 20 to 80u. The FT-IR spectra were recorded with a
single beam Perkin Elmer instrument (Spectrum BX-500) FT-IR
Model spectrophotometer. The morphological identification and
sizes of the synthesized samples were based on the scanning
electron microscopy images recorded on a Zeiss EVO MA-10
SEM equipped with an energy dispersive spectrometer
(OXFORD INCA ENERGY 250), which assisted in the
elemental analysis of the sample. Microstructural characteriza-
tion at high magnifications and reciprocal space analysis were
performed using a high resolution transmission electron micro-
scope (HR-TEM: FEI Tecnai G2 F30 STWIN at 300 keV). The
UV-Vis spectra of the samples were recorded by a JASCO
UV/VIS/NIR SPECTROPHOTOMETER (model V-670) and
the luminescence characteristics were investigated by photolu-
minescence spectroscopy using a Perkin–Elmer LS–55 lumines-
cence spectrophotometer (Xe source).
Toxicity measurements
Sample preparation and characterization. Different CuO
nanoparticles (NPs) were suspended in F12 medium and probe
sonicated (Sonics Vibra cell, Sonics & Material Inc., New Town,
CT, USA) to make stock suspension (100 mgml
21
). This was
characterized by dynamic light scattering (DLS) using a
Zetasizer Nano-ZS equipped with a 4.0 mW, 633 nm laser
(Model ZEN3600, Malvern instruments Ltd., Malvern, UK).
Cell culture and exposure. Chinese Hamster Ovary cell line
(CHO) was obtained from National Centre for Cell Sciences,
Pune, India, and cultured in F12 medium supplemented with
10% FBS, 0.2% sodium bicarbonate and 10 ml L
21
antibiotic
and antimycotic solution at 37 uC under a humidified atmo-
sphere of 5% CO
2
/95% air. CHO cells were exposed to CuO NP
suspensions for 3 and 6 h at concentrations of 1, 5, 10, 20, 30, 50
and 100 mgml
21
for cytotoxicity assays.
Mitochondrial activity. Mitochondrial succinate dehydrogen-
ase activity was assessed using the MTT assay according to a
modified method of Mosmann
43
described earlier by Shukla
et al.
44
Briefly, cells were treated with different NPs for 3 to 6 h
and MTT dye (0.5 mg ml
21
) was added after the treatment
period. After incubation, the reaction mixture was carefully
removed and formazan crystals were solubilized in 200 ml
dimethyl sulfoxide (DMSO). The interference of nanoparticles
was obviated by centrifuging the plates, transferring the super-
natant into a new plate and then measuring the absorbance at
550 nm in a SYNERGY-HT multiwell plate reader, Bio-Tek
(USA) using KC4 software. The quantification of the cell
viability in terms of metabolically active cells was calculated
using the formula below.
%Viability = (Mean Absorbance of Sample/Mean Absorbance of
Control) 6 100
Escherichia coli (E. coli) culture and exposure. The E. coli (K12
sub-strain DH10B) was procured from MTCC, Chandigarh,
India and cultured in 5 ml Luria Bertani (LB) broth at 37 uC for
12–16 h in an environmental shaker incubator at 180 rpm. One
ml of overnight grown culture was re-inoculated in 100 ml LB
broth and allowed to grow up to early log phase at an OD
600
of
0.2–0.3 (1 6 10
9
CFU ml
21
). 5 6 10
9
cells (concentrated 5 ml
culture) were treated with different NP concentrations ranging
from 1–100 mgml
21
in PBS at 37 uC for 60 min.
Bacterial cytotoxicity assay. The toxicity assessment of the NP
treated bacterial culture was carried out according to the
protocol described by Jung et al.
45
Treated E. coli culture was
washed twice with PBS and incubated with propidium iodide
(PI) for 15 min at room temperature. The red fluorescence
emitted from PI was collected using a 650 nm ¡ 13 nm band
pass filter. The proportions of live and dead cell were determined
and analyzed using BD FACSCanto II and FACSDiva
TM
software (Ver. 6.1.2).
Thermal conductivity measurements
CuO nanofluids were prepared by dispersing the as synthesized
CuO nanoparticles in distilled water at a volume percentage of
0.1%. The thermal conductivity of the corresponding nanofluids
was measured using hot disc thermal constant analyzer (model
TPS-500).
Results and discussion
Microstructural features and phase formation
To evaluate the effect of pH on the morphological evolution of
the CuO nanoparticles, we selectively prepared a series of
samples using five different pH values of 7.25, 8.5, 10.0, 11.25
and 12.0 under parallel experimental conditions. The intermedi-
ate bluish precipitate, obtained immediately after the hydrolysis
of the cupric salt, was characterized by XRD and SEM. The
XRD patterns (Fig. 1) reveal a noteworthy sequence of events in
terms of the component phases present in the intermediate. It
was found that at lower pH values (7.25 and 8.5), the bluish
precipitate consisted of mixed phase of materials, copper(
II)
hydroxy nitrate {Cu
2
(OH)
3
NO
3
} and copper(II) hydroxide
{Cu(OH)
2
}, most of the peaks were characteristic of the hydroxy
salt. The visible diffraction intensities corresponding to
Cu
2
(OH)
3
NO
3
agree well with the peaks of monoclinic crystal
structure of Cu
2
(OH)
3
NO
3
(JCPDS No.15-0014) with unit cell
parameters a = 5.605 A
˚
, b = 6.087 A
˚
, c = 6.929 A
˚
and b = 94.48u.
The diffraction intensities corresponding to Cu(OH)
2
were in
agreement with the peaks of orthorhombic crystal structure
(JCPDS No.035-505) and unit cell parameters a = 2.951 A
˚
, b =
10.592 A
˚
and c = 5.273 A
˚
. The intensity of the Cu
2
(OH)
3
NO
3
peaks reduce as we cross over from the pH 7.25 intermediate to
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the pH 8.5 intermediate, eventually becoming nonexistent in the
higher pH region (10.0 onwards), where the peaks can all be
identified with Cu(OH)
2
diffraction intensities. These observa-
tions tend to suggest that at lower pH the concentration of
2
OH
ions is not sufficient to replace all of the NO
3
2
ions coordinated
to the Copper atom. As a result, we get predominant formation
of Cu
2
(OH)
3
NO
3
where Cu(II) atoms are coordinated to both
2
OH and NO
3
2
in a botallackite type crystal structure. The few
peaks corresponding to Cu(OH)
2
present in the XRD patterns of
the two low pH intermediates suggest the initial transformation
to this species has began. But on further increasing the pH (10.0
onwards) the obtained XRD patterns suggest the formation of
only Cu(OH)
2
and no other species, indicating that at higher pH
the concentration of
2
OH is enough to convert Cu(NO
3
)
2
to
Cu(OH)
2
completely. This is expected because previous works
have shown that Cu
2
(OH)
3
NO
3
can be transformed to Cu(OH)
2
by treating it with a base.
46
Starting from the sample
corresponding to pH 10.0, the peaks for Cu(OH)
2
become
increasingly more intense and narrower with the increase in pH,
suggesting an increase in crystallite size and better crystalline
quality.
The SEM images of these intermediates (Fig. 2) projects the
morphological evolution of these particles at varying pH values.
When the pH is 7.25, the particles formed cannot be categorized
under any particular shape. Round shaped, grain shaped and
some wire like (with diameter in the range of 50–60 nm)
structures are all visible (Fig. 2a). It is worth mentioning that the
XRD of this sample showed the presence of a mixed phase of
Cu(OH)
2
and Cu
2
(OH)
3
NO
3
, so the non-uniformity in the
shapes can be attributed to the presence of two chemically
distinguished components to some extent. The samples synthe-
sized at pH 8.5 show more of a regular shape formation, with
well defined seed-like particles (width of 120–150 nm, length
varying between 350–400 nm) clearly observed (Fig. 2b). When
the pH was raised to 10.0, the particles showed a tendency to
grow longer vertically (600–700 nm) and in the process attained
an ellipsoidal morphology (Fig. 2c). One interesting phenomena
to note here is that at the higher pH region, the particles exist in
bundles or, more precisely, an agglomerated form. The probable
reason behind this agglomeration could well be the hydrogen
bonding due to the high concentrations of
2
OH species present
in the particles. At pH 11.25, some plate shaped particles are
observed (Fig. 2d) and these plates show a tendency to
accumulate, producing flower-like topography of these accumu-
lated nanostructures. There is also the simultaneous presence of
a bunch of small particles (as shown in the encircled area in
Fig. 2d) with an average width of about 50 nm and around
300 nm in length. This gives a possibility that the plates may have
been formed by the smaller particles, as discussed later. At pH
12.0 we see the formation of rod shaped Cu(OH)
2
nanostructures
about 20 nm in diameter and 400–500 nm in length (Fig. 2e) and
following the trend, these particles too show preference for
agglomeration, although to a much lower extent than what was
observed at pH 11.25.
Fig. 3 shows XRD patterns of the CuO particles synthesized
by annealing the blue precipitates at a temperature of 130 uC for
10 h at a stretch. The diffraction peaks for CuO were identified
with respect to the JCPDS file No.45-0937; and crystal phase
could be identified as monoclinic (unit cell parameters of a =
4.6853 A
˚
, b = 3.4257 A
˚
, c = 5.1303 A
˚
and b = 99.549u). From the
XRD patterns it is clearly observed that at the lower pH region
(7.25 and 8.5), along with the peaks for CuO, few peaks of
Cu
2
(OH)
3
NO
3
are also present. It presents the case that part of
the Cu
2
(OH)
3
NO
3
has not been transformed to CuO on heating
the sample. As we increase the pH from 7.25 to 8.5, the
Cu
2
(OH)
3
NO
3
peaks diminish in intensity and finally disappear
at pH 10.0 and above, the samples from pH 10.0 onwards were
compositionally pure. On the other hand, the CuO peaks
continued to grow in intensity and gradually became prominent
Fig. 1 XRD patterns of the bluish products obtained by hydrolyzing
Cu(NO
3
)
2
at different pH values. The pH values are shown in the inset.
The patterns have been shifted for clarity.
Fig. 2 SEM micrographs of the bluish precipitates obtained after
hydrolyzing Cu(NO
3
)
2
at different pH values. Micrographs correspond
to a) pH 7.25, b) pH 8.5, c) pH 10.0, d) pH 11.25 and e) pH 12.0.
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in appearance when the pH was increased. Also with the increase
in pH, the CuO peaks became better defined, richer in intensity
and sharper, suggesting an improvement in crystalline quality
and crystallite size. Interestingly, in the lower pH region (7.25,
8.5 & 10.0) the 111 plane of the CuO nanocrystal is the most
intense peak, but at pH 11.25 & 12.0 the 002 peak is the most
intense. It indicates a change in preference for the crystal plane
orientation as the pH increases. In addition to this, in the lower
pH region the intermediate {Cu(OH)
2
} is relatively more
unstable towards decomposition into CuO and the greenish
shade of the intermediate indicates that some parts of the
Cu(OH)
2
have already been transformed into black CuO.
Presence of CuO peaks in the XRD pattern of these inter-
mediates (Fig. 1) confirms it. In fact it was observed that, in the
lower pH region, the entire precipitate would convert into CuO
at room temperature itself within 15 h of the preparation of the
Cu(OH)
2
precipitate. But to maintain uniformity in the process
parameters we employed annealing induced decomposition of
the intermediate for all the pH values.
The presence of Cu
2
(OH)
3
NO
3
in the intermediates at low pH
values was further confirmed by the FT-IR spectra of the CuO
samples (Fig. 4a–e). The peak centered at 1048 cm
21
(present in
the samples with the pH values 7.25, 8.5 and 10.0) arises due to
ONO
2
2
stretching.
47
Although no peak corresponding to the
Cu
2
(OH)
3
NO
3
molecule for the pH 10.0 sample was observed
in the XRD data, FT-IR reveals a very low intensity peak
at 1048 cm
21
indicating an extremely minute amount of
Cu
2
(OH)
3
NO
3
still present in the sample. All the peaks present
in the range of 420 cm
21
to 610 cm
21
and at 1386 cm
21
correspond to the Cu–O stretching mode.
46,48
Curiously, these
peaks show a prominent red shift as we move towards the
samples synthesised at higher pH values, indicating a probable
weakening or elongation of the Cu–O bonds.
Fig. 5 shows the SEM images of the as synthesized CuO
nanoparticles acquired by thermal annealing of the bluish
intermediates, synthesized at different pHs. Like their precur-
sors, these particles also show a regular structural evolution with
the increase in pH. At the low pH 7.25, the image shows the
predominant formation of grain like particles with no definite
shape and size (Fig. 5a). At pH 8.5, the formation of seed-like
particles was observed (with width of these particles in the range
of 70–200 nm and the length 200–550 nm). Although most of the
particles conformed to seed like shapes, as can be viewed in
Fig. 5b,f, the HRTEM images of these particles reveal the
simultaneous presence of some grain like particles (Fig. 6a)
which were undetected in the SEM micrographs. The presence of
these aberrations maybe attributed to the presence of
Cu
2
(OH)
3
NO
3
, as detected by the XRD plots. Fig. 6b shows
the HRTEM image of a single particle, clearly exposing the
roughness of the surface and around the edges, suggesting that
these seeds were formed by the accumulation of other smaller
sized particles. The selected area electron diffraction (SAED)
pattern shown in Fig. 6b (inset) also indicates the presence of
mixed phases {CuO and Cu
2
(OH)
3
NO
3
} and due to the haziness
in these patterns caused by the mixing of the phases all the lines
could not be distinctively identified. Fig. 7 shows the difference
between the SAED pattern of the mixed phase sample and that
of a pure CuO sample. At pH 10.0, although the basic shape of
these particles remains similar, the aspect ratio does not and the
horizontal dimension shows a tendency to decrease (in the range
of 90–175 nm) and the vertical dimension increases (in the range
of 470–800 nm), thus gaining more of a ellipsoidal shape with
Fig. 3 XRD patterns of the CuO nanoparticles synthesized by
annealing the obtained precipitates of Cu(OH)
2
at a temperature of
130 uC for 10 h. Only the peaks corresponding to CuO are designated
with corresponding hkl values in the plot. The corresponding pH values
are shown in the inset. The patterns are shifted for clarity.
Fig. 4 The FTIR spectra of the synthesized CuO nanoparticles
synthesized at different pH values a) pH 7.25, b) pH 8.5, c) pH 10.0,
d) pH 11.25 and e) pH 12.0.
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