The novel two step synthesis of CuO/ZnO and CuO/CdO nanocatalysts for enhancement of catalytic activity

Article abstract of DOI:10.1016/j.molstruc.2020.128772

In the present investigation, Copper oxide (CuO)/Zinc oxide (ZnO) and CuO/cadmium oxide (CdO) nanocomposites (NCs) were fabricated by a simple two-step process involving co-precipitation technique and hydrothermal approach for the first time. The Powder X-ray Diffraction (PXRD) pattern demonstrates the existence of monoclinic, hexagonal and cubic phase structure of CuO/ZnO and CuO/CdO NCs. The absorption spectrum of the material was obtained via UV–Visible Spectroscopy and the band-gap value is calculated using Tauc's relation as 1.7 eV, 2.8 eV and 2.75 eV respectively for pure, CuO/ZnO and CuO/CdO NCs. The FESEM images gave the spherical and stone-like morphologies in the prepared CuO/ZnO and CuO/CdO NCs. At room temperature, CuO/ZnO and CuO/CdO NCs act as superior catalyst for the reduction of Rhodamine B (RhB) and Malachite Green (MG) dyes and degradation efficiency attained to be 77.26%, 73.15% for RhB and 79.82%, 74.69 for MG, respectively.

Full text of DOI:10.1016/j.molstruc.2020.128772

Journal of Molecular Structure 1221 (2020) 128772
Contents lists available at ScienceDirect
Journal of Molecular Structure
journal homepage: http://www.elsevier.com/locate/molstruc
The novel two step synthesis of CuO/ZnO and CuO/CdO nanocatalysts
for enhancement of catalytic activity
,
*
A. Sankaran a, K. Kumaraguru b
a Department of Marine Engineering, Mohamed Sathak Engineering College, Kilakarai, 623806, India
b Department of Petrochemical Technology, University College of Engineering, BIT Campus, Anna University, Tiruchirappalli, 620024, India
a r t i c l e i n f o
a b s t r a c t
Article history:
In the present investigation, Copper oxide (CuO)/Zinc oxide (ZnO) and CuO/cadmium oxide (CdO)
nanocomposites (NCs) were fabricated by a simple two-step process involving co-precipitation technique
and hydrothermal approach for the first time. The Powder X-ray Diffraction (PXRD) pattern demonstrates
the existence of monoclinic, hexagonal and cubic phase structure of CuO/ZnO and CuO/CdO NCs. The
absorption spectrum of the material was obtained via UVeVisible Spectroscopy and the band-gap value
is calculated using Tauc’s relation as 1.7 eV, 2.8 eV and 2.75 eV respectively for pure, CuO/ZnO and CuO/
CdO NCs. The FESEM images gave the spherical and stone-like morphologies in the prepared CuO/ZnO
and CuO/CdO NCs. At room temperature, CuO/ZnO and CuO/CdO NCs act as superior catalyst for the
reduction of Rhodamine B (RhB) and Malachite Green (MG) dyes and degradation efficiency attained to
be 77.26%, 73.15% for RhB and 79.82%, 74.69 for MG, respectively.
Received 30 January 2020
Received in revised form
19 June 2020
Accepted 25 June 2020
Available online 1 July 2020
Keywords:
Nanocomposites
Two-step process
Structural
© 2020 Elsevier B.V. All rights reserved.
Optical studies
Catalytic activity
1. Introduction
[4e6]. On the other hand, the n-type semiconductor of zinc oxide
(ZnO) has a wide bandgap (3.37 eV) with high excitonic binding
In the current decade, wastewater treatment has more attention
due to the increase in the absorption of toxic pollutants like organic
dyes, pharmaceuticals, microbes, etc. In the waste release [1].
Hence, nowadays various technologies are existing in wastewater
treatment for the removal of inorganic and organic dyes. To over-
come this problem, nanotechnology is an emerging application to
solve the wastewater treatment due to their large surface area to
volume ratio of nanomaterials [2]. Among nanotechnology-based
process for removing organic pollutants, the catalytic process is
one of the most efficient, cost-effective and environment-friendly
[3]. Generally, semiconductor catalysts (TiO2, ZnO, CuO, ZnS, CdO,
Ta2O5, WO3, and CdS, etc) act as the promising material for
wastewater treatment and degradation of organic and inorganic
pollutants in water [3]. Among these, copper oxide (CuO) acts as the
important semiconductor material owing to its unique properties
like p-type and narrow bandgap. By these unique properties, CuO
material used as numerous applications such as solar cell, photo-
catalysis, catalysis, magnetic storage media, gas sensors, PN junc-
tion diode, varistors, detectors and lithium-ion battery applications
energy (~60 meV). Due to this property, ZnO material has been used
as various applications like biosensors, gas sensors, nanogenerators
and varistors [7,8]. In addition, Cadmium oxide (CdO) is an n-type
semiconductor material and it has a wide range of applications such
as solar cells, phototransistors, electroplating baths, chemical sen-
sors, liquid crystal displays, transparent electrodes, IR detectors and
gas sensors [9]. The semiconductor metal oxides were synthesized
via various approaches like sol-gel method, auto combustion
method, sonochemical method, wet chemical method, co-
precipitation method and thermal decomposition method, etc
[10]. Among them, the co-precipitation method is one of the most
flourishing techniques for synthesizing the materials and this
method is an efficient pre-concentration technique for tracing
heavy metal ions. In addition, the shape and size of the particles can
be controlled by varying pH medium [1,3]. As well as, various types
of nanostructures were developed by a hydrothermal approach
owing to its narrow crystallite size distribution and low aggregation
level [7].
In a modern trend, new composite materials were fabricated by
various researchers due to support of innovative direction for
research and applications [11]. Hence, CuO-based nanocomposites
were recently developed in current research for catalytic degra-
dation of organic dyes such as Methylene blue, Malachite green,
* Corresponding author.
E-mail address: kumaraguruautt@gmail.com (K. Kumaraguru).
https://doi.org/10.1016/j.molstruc.2020.128772
0022-2860/© 2020 Elsevier B.V. All rights reserved.

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A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
Methyl orange and 4-nitrophenol [12e19]. In this connection,
Jayapriya et al. fabricated MgO rice/ZnO nanocomposite from bract
extract of Musa paradisiaca for catalytic activity. Where, 96%, 93%
and 91% degradation efficiencies are achieved from the organic
dyes of MO, O-Nip and MB [20]. Kavita Sahu et al. [21] developed
CuO/Cu2O nanowire for catalytic reduction of 4-nitrophenol within
4 min due to their large surface area leading to enhanced adsorp-
tion and enhanced charge carrier separation. Nasrollahzadeh et al.
reported Cu/Al2O3 nanoparticles were developed by leaf extract of
Commersonia Bartramia for catalytic reduction of Methylene blue
and Congo red in aqueous medium [22].
prepared NaBH4solution (1 ꢁ 10ꢂ2 M) was added to the above
mixture solution at room temperature. During the reaction, the
reduction of dyes was observed from the intensity of wavelength
was linearly decreased with respect to time intervals (30 s for RhB
and
2
min for MG), which were recorded by UVeVis
spectrophotometer.
2.4. Analytical techniques
The PXRD pattern of synthesized pure CuO, CuO/ZnO and CuO/
CdO NCs were analyzed by the Pro Penalty CAL (Cu-Ka radiation
The Rhodamine B (RhB) dye is a cationic xanthene class dye and
it has been used as textile and food industries. The Malachite green
(MG) is widely used as a coloring agent in silk, jute, wool, paper for
textile and leather industries and applied in aquaculture as a
parasiticide and antibacterial agent. However, MG is toxic and
carcinogenic. The wastewater containing MG causes environmental
pollution and threats to human health. Thus, it is significant to
eliminate MG from wastewater before its discharge [23]. Hence,
catalytic activity has been investigated by Malachite green (MG)
and Rhodamine B (RB) organic dyes from the developed catalyst of
CuO/ZnO and CuO/CdO NCs via a two-step approach. The CuO NPs
were prepared by the co-precipitation approach and subsequently,
CuO/ZnO and CuO/CdO NCs were developed from the hydrothermal
method.
(1.5406 Å)) instrument model. The UVeVisible absorption spectra
of developed samples were taken from the JASCO V-670 spectro-
photometer. From the Zeiss Gemini Ultra-55 instrument model,
FESEM and EDAX color mapping analysis were identified.
3. Results and discussion
3.1. Powder X-ray diffraction (PXRD) analysis
The PXRD pattern of (a) pure CuO, (b) CuO/ZnO and (c) CuO/CdO
NCs were shown in Fig. 1. The major diffraction peaks are obtained
at the 2q
values of 32.65ꢀ, 35.64ꢀ, 38.81ꢀ, 48.80ꢀ, 53.63ꢀ, 58.43ꢀ,
61.61ꢀ, 66.31ꢀ, 68.15ꢀ, 72.58ꢀ and 75.17ꢀ respectively for the planes
of (110), (002), (111), (ꢂ202), (020), (202), (ꢂ113), (ꢂ311), (220),
(311) and (ꢂ222), which are clearly reflect the monoclinic phase of
CuO and good agreement with the standard reported JCPDS NO:
2. Experimental procedure
05e0661 [24,25]. The intensity peak positioned at 2q
of 31.79ꢀ,
2.1. Materials for synthesizing of CuO NPs, CuO/ZnO and CuO/CdO
NCs
34.28ꢀ, 36.20ꢀ, 47.36ꢀ, 56.51ꢀ and 62.76ꢀ is corresponding to the
crystal planes of (100), (002), (101), (102), (110) and (103),
respectively for hexagonal wurtzite phase structure of ZnO [JCPDS
Card No 36e1451] [26]. The cubic phase structure of CdO has crystal
planes (111), (200), (220), (311) and (222) with respective in-
tensities at 33.02ꢀ, 38.33ꢀ, 55.15ꢀ, 65.83ꢀ and 69.30ꢀ, which are well
coincided with the JCPDS No: 73e2245 [27]. The average crystallite
sizes of the CuO, CuO/ZnO and CuO/CdO NCs were estimated by
following Debye-Scherrer’s formula.
The precursor material of Copper chloride (CuCl2), Zinc acetate
(Zn(CH₃CO₂)₂) and Cadmium acetate (Cd(CH₃CO₂)₂) were used to
develop the CuO NPs, CuO/ZnO and CuO/CdO NCs. The sodium
borohydride (NaBH4) performs as a reducing agent. In overall
experimental studies, Double Distilled (DD) water is used as a
solvent. All the chemicals and organic dyes (Rhodamine B and
Malachite Green) were bought from Merk.
D ¼ K
l=b cosq
2.2. Synthesis procedure of CuO NPs, CuO/ZnO and CuO/CdO NCs
where, ‘D’ and ‘K’ is Debye-Scherrer’s crystallite size and constant,
The pure CuO NPs were developed via a co-precipitation
approach as per our previous report [3]. To prepare CuO/ZnO and
CuO/CdO NCs, each 0.5 gm of prepared CuO NPs was added grad-
ually into every 50 ml of DD water under sonication (15 min).
Subsequently, each 0.05 M of CTAB was added into the above-
prepared solution under constant stirring process. Later that,
0.1 M of Zn(CH₃CO₂)₂ was added into one of the solutions and 0.1 M
of Cd(CH₃CO₂)₂ was mixed with another prepared solution. Then,
the NaOH solution was added with the above prepared both solu-
tions until pH 9 was reached. After that, the above-prepared solu-
tions were changed into 100 ml of autoclave (Teflon-lined stainless
steel) and heated at 140 ꢀC for 12 h. Later, attained samples were
collected and washed thoroughly into ethanol and DD water. After
the washing process, the obtained material was heated at 90 ꢀC and
annealed at 300 ꢀC for 2 h. Finally, CuO/ZnO and CuO/CdO NCs were
collected for further analytical studies.
‘
l
’ represents the incident wavelength of the X ray’s, ‘ ’ and ‘q’
b
denotes the full-width half maximum of the XRD corresponding
2.3. The catalytic activity test for RhB and MG
The organic dyes of Rhodamine B (1 ꢁ 10ꢂ3 M) and Malachite
Green (1 ꢁ 10ꢂ5 M) were taken in 100 ml of DD water for an esti-
mate of the catalytic activity of CuO/ZnO NCs and CuO/CdO NCs.
Then, 10 mg of CuO/ZnO NCs and CuO/CdO NCs was added into RhB
and MG solutions, separately. After that, the required amount of
Fig. 1. PXRDpattern of (a) CuO, (b) CuO/ZnO NCs and CuO/CdONCs.

A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
3
peaks and Bragg angle [3,26]. The average crystallite size is found to
be 32 nm, 44 nm and 57 nm, respectively for pure CuO, CuO/ZnO
and CuO/CdO NCs. The presence of defects in the developed ma-
ð
a
h
y
Þ2 ¼ A ðh
ꢂ EgÞ
y
where,
a, hy & A is absorption coefficient, photon energy and
terials was established by dislocation density (
determined by the given relation.
d), which was
Constant related to the effective mass of the electrons. The band-
gap energy (Eg) is achieved to be 1.7 eV, 2.8 eV and 2.75 eV
respectively for pure, CuO/ZnO and CuO/CdO NCs (Fig. 3aec). From
the result, the bandgap of the prepared NCs was a good agreement
with the previously reported values [9,28].
.
d
¼ 1 D2
The dislocation density (
d
) of the pure CuO, CuO/ZnO and CuO/
CdO NCs were attained to be 9.76 ꢁ 10ꢂ4 nmꢂ2, 5.16 ꢁ 10ꢂ4 nmꢂ2
and 3.07 ꢁ 10ꢂ4 nmꢂ2, respectively [26]. From the result, the de-
fects of the NCs materials are reduced than pure due to its surface
charge properties.
3.3. Morphological analysis
The FESEM images of CuO/ZnO and CuO/CdO NCs were shown in
Fig. 4(aec) and Fig. 5(aec). The attained particles of the synthesized
CuO/ZnO and CuO/CdO NCs are seemed as spherical and stone-like
morphology. From the observed figure, CuO/ZnO NCs have a uni-
form distribution of the particles and it has a large surface area
compared to CuO/CdO NCs. The catalytic activity has been
enhanced by these types of materials. Fig. 8(aec) reveals that the
TEM analysis of CuO/ZnO NCs. The average particle size ranges
around 50e200 nm approximately from the developed CuO/ZnO
NCs. The EDAX color mapping of CuO/ZnO and CuO/CdO NCs (Fig. 6
and Fig. 7) demonstrates additional support in the statement of
individual atoms are evenly distributed with a combined ratio
throughout the composite matrix of CuO/ZnO and CuO/CdO NCs.
The color mapping clearly reveals only elements of Cu, Cd, Zn, and O
were a presence in the developed composite materials.
3.2. Optical absorption studies
The UVeVis spectrum of pure, CuO/ZnO and CuO/CdO NCs had
been recorded in the visible range (Fig. 2). From the absorption
spectra, band around between 200 nm and 300 nm for CuO and
366 nm, 372 nm respectively for ZnO and CdO. The maximum
absorbance (366 nm and 372 nm) is assigned by the intrinsic band-
gap adsorption of CuO/ZnO and CuO/CdO NCs owing to the electron
transitions from the valence band to conduction band [9]. Ac-
cording to the maximum level band absorption, the band-gap en-
ergy of the pure, CuO/ZnO and CuO/CdO NCs was estimated based
on the given Tauc’s equation,
Fig. 2. UVVisible absorption spectrum of (a) CuO, (b) CuO/ZnO NCs and CuO/CdONCs.

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A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
Fig. 3. Tauc’s plot of (a) CuO, (b) CuO/ZnO NCs and CuO/CdONCs.
Fig. 4. Different magnification FESEM images of (aec) CuO/ZnO NCs.

A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
5
Fig. 5. Different magnification FESEM images of (aec) CuO/CdO NCs.
Fig. 6. EDAX color mapping images of (aed) CuO/ZnO NCs.
3.4. Catalytic activity of RhB and MG for CuO/ZnO NCs and CuO/
CdO NCs
The reduction rate of RhB has no change by the absence of catalyst
which suggesting that it is very difficult for the reduction of RhB
without any catalyst. Therefore, the reduction rate of the RhB was
increased in addition of CuO/ZnO and CuO/CdO nanocatalyst with
NaBH4 for degradation of RhB and the strong absorption intensity
peak located at 554 nm rapidly decreases along with deep pink
color solution change to colorless solution completely in 180 s due
The catalytic performance of RhB has been investigated by the
prepared catalyst of CuO/ZnO and CuO/CdO NCs. In the catalytic
activity, NaBH4 acts as a reducing agent which is monitored by
UVeVisible absorption spectra at different time intervals (Fig. 9).

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A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
Fig. 7. EDAXcolor mapping images of (aed) CuO/CdO NCs.
Fig. 8. Different magnification TEM images of (aec) CuO/ZnO NCs.

A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
7
Fig. 9. Time-dependent UVeVis absorption spectra for the catalytic reduction of MG (a) CuO/ZnO NCs and (b) CuO/CdO NCs.
to conversion of RhB (Pink color) into Leuco-RhB (colorless). The
prepared catalyst of CuO/ZnO and CuO/CdO NCs were performance
as an electron relay and supporting electron transfer from ions
(donor) into RhB dye (acceptor). During the catalysis process,
nucleophilic ions donate the electrons to the prepared catalyst,
whereas electrophilic ions (RhB) capture the electrons from the
catalyst [29,30].
and MG results were compared with previous reports which are
shown in Table 1.
Fig. 10 shows the reduction absorption spectra of MG with
NaBH4 by (a) CuO/ZnO and (b) CuO/CdO NCs. From the spectra, the
high-intensity peak at 615 nm of MG was linearly decreased within
12 min due to the conversion of MG (green color) into leucoma-
lachite green (LMG) (colorless). Here, CuO/ZnO and CuO/CdO NCs
act as an important electron transfer mediator because it allows the
electrons from NaBH4 and donates to MG by acting as redox cata-
lyst, which in turn termed as electron relay effect [31,32]. The
catalytic behavior for the reduction of MG and RhB dyes are
dependent on the efficient separation of charge carriers and surface
area of the catalyst [29,30]. Further, heterojunction nanostructures
of SiO2/Ag2O/TiO2 have superior catalytic activity by the reduction
of 4-NP to 4-AP owing to the heterojunction structure, which im-
proves charge separation [33]. Sahu et al. developed CuO/Cu2O
hybrid nanowires for the reduction of 4-NP to 4-AP due to their
large surface area and separation of charge carrier [20]. The RhB
Table 1
Comparison of catalytic activity CuO/ZnO and CuO/CdO NPs with some reported
catalyst in the degradation of RhB and MG.
Catalyst
Dyes Time (sec) Rate constant(k)
Ref.
CuO/rGO
Ni(OH)2
ZnO/MgO
4-NP 27 min
13.951 minꢂ1
[34]
[30]
[19]
RhB
MO
MB
13 min
14 min
12 min
10 min
14 min
9 min
1.3 ꢁ 10ꢂ2 minꢂ1
8.39 ꢁ 10ꢂ3 sꢂ1
4.43 ꢁ 10ꢂ3 sꢂ1
Au/CeO2eTiO2
RhB
MG
223.9 ꢁ 10ꢂ3 minꢂ1 [35]
233.2 ꢁ 10ꢂ3 minꢂ1
PET2-APTES-Ag/Cu MG
CuO/TiO2/ZnO
0.145 minꢂ1
[36]
[37]
4-NP 360 s
8.13 ꢁ 10ꢂ2 sꢂ1
1.351 ꢁ 10ꢂ2 sꢂ1
1.262 ꢁ 10ꢂ2 sꢂ1
7.23 ꢁ 10ꢂ3 sꢂ1
1.29 ꢁ 10ꢂ2 sꢂ1
6.30 ꢁ 10ꢂ3 sꢂ1
1.13 ꢁ 10ꢂ2 sꢂ1
MB
CV
210 s
135 s
180 s
180 s
180 s
180 s
CuO/ZnO
CuO/CdO
RhB
MG
RhB
MG
Present work
Present work
Fig. 10. Time-dependent UVeVis absorption spectra for the catalytic reduction of RhB (a) CuO/ZnO NCs and (b) CuO/CdO NCs.

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A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
Fig. 11. (a) Plots of Ct/C0 versus reaction time (sec), (b) MG Degradation efficiency (%) versus reaction time (sec) and (c) ln (Ct/C0) versus reaction time (sec) for CuO/ZnO NCs and
CuO/CdO NCs.
Fig. 12. (a) Plots of Ct/C0 versus reaction time (sec), (b) RhB Degradation efficiency (%) versus reaction time (sec) and (c) ln (Ct/C0) versus reaction time (sec) for CuO/ZnO NCs and
CuO/CdONCs.
Fig. 13. Possible reduction mechanism of malachite green (MG) into leucomalachite green (LMG) in presence of CuO/ZnO NCs.
3.5. The reaction kinetics of RhB and MG for CuO/ZnO NCs and CuO/
CdO NCs
ZnO and CuO/CdO NCs. The dye degradation efficiency can be
determined by the given relation
Figs. 11a and 12(a) reveals that the Ct/C0 versus reaction time (t)
for the degradation of RhB and MG dye from the catalyst of CuO/

A. Sankaran, K. Kumaraguru / Journal of Molecular Structure 1221 (2020) 128772
9
Fig. 14. (a & b) The recyclability of the catalyst in the reduction of MG and RhB dye solution with NaBH4 at room temperature.
efficiency was rapidly reduced owing to their dye molecules are
extremely adsorbed from the surface area (Fig. 14) [37].
R ¼ ½ðC0eCtÞ =C0ꢃ ꢁ 100 %
where, C0 and Ct are initial and final concentrations with respect to
time ‘t’ and ‘0’ respectively for RhB and MG. The degradation effi-
ciency of RhB and MG for CuO/ZnO and CuO/CdO NCs are plotted
with time and depicts in Figs. 11 (b) and 12 (b). The degradation
efficiency of CuO/ZnO and CuO/CdO NCs are found to be 77.26%,
73.15% for RhB and 79.82%, 74.69 for MG, respectively. From the
observed result, CuO/ZnO NCs catalyst increases the reduction rate
of MG dye compare to CuO/CdO NCs due to their large surface area
of the adsorption of MG anions, which leads to an increase in the
catalytic activity. The rate constant for the reduction of dyes was
estimated by pseudo-first-order kinetics [20]. The kinetic reaction
can be determined from the given equation
4. Conclusion
The CuO/CdO and CuO/ZnO NCs were developed via the two-
step approach. The CuO prepared by the co-precipitation method
and subsequently ZnO and CdO were developed from a hydro-
thermal approach for catalytic activity. From the Powder X-ray
diffraction (PXRD) analysis, crystalline sizes were achieved to be
32 nm, 44 nm and 57 nm respectively for pure CuO, CuO/ZnO and
CuO/CdO NCs. The spherical and stone-like morphology was
attained by the developed composite material and individual atoms
are evenly distributed with a combined ratio throughout the
composite matrix of CuO/ZnO and CuO/CdO NCs, which were pri-
marily confirmed by the EDAX color mapping. In catalytic degra-
dation studies, the degradation efficiency and rate constant of CuO/
ZnO NCs are high at both the dyes due to their large surface area
and separation of charge carrier. In the recyclable test, catalytic
activity loss was observed after five consecutive rounds owing to
their dye molecules are extremely adsorbed from the surface area.
The better catalytic property of the material reveals that it is
applicable to remove the environmental pollutants released from
the industries.
ln ðCt = CoÞ ¼ kt
where Ct and Co are the final and initial concentration of RhB and
MG and ‘k’ denotes the rate constant at the given reaction time (t).
From Figs.11 (c) and 12 (c) demonstrates that the difference of In Ct/
Co against reaction time for the degradation of RhB and MG for CuO/
ZnO and CuO/CdO NCs. The rate constant (k) of CuO/ZnO and CuO/
CdO NCs are determined from the linear slope value and found to
be 7.23 ꢁ 10ꢂ3 sꢂ1 and 6.30 ꢁ 10ꢂ3 sꢂ1 for RhB, 1.29 ꢁ 10ꢂ2 sꢂ1and
1.13 ꢁ 10ꢂ2 sꢂ1 for MG respectively. The (R2) values of CuO/ZnO and
CuO/CdO NCs are calculated to be 0.9823 and 0.9580 for RhB,
0.9746 and 0.9207 for MG, respectively. From the result, the
degradation efficiency and rate constant of CuO/ZnO are high in the
degradation of both dyes. The possible reduction mechanism of
malachite green (MG) into leucomalachite green (LMG) in the
presence of CuO/ZnO NCs were shown in Fig. 13.
CRediT authorship contribution statement
A. Sankaran: Conceptualization, Methodology, Validation,
Writing - review & editing. K. Kumaraguru: Resources, Investiga-
tion, Writing - original draft, Supervision.
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