CoFe2O4/CdS nanocomposite: Preparation, characterisation, and application in sonocatalytic degradation of organic dye pollutants

Article abstract of DOI:10.1016/S1872-2067(16)62473-7

A magnetic CoFe₂O₄/CdS nanocomposite was prepared via one-step hydrothermal decomposition of cadmium diethanoldithiocarbamate complex on the surface of CoFe₂O₄ nanoparticles at a low temperature of 200 °C. The nanocomposite was characterised by X-ray diffraction (XRD), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy, energy-dispersive X-ray spectroscopy (EDX), UV-visible spectroscopy, transmission electron microscopy (TEM), N₂ gas sorption analysis, X-ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry. The FT-IR, XRD, EDX and XPS results confirmed the formation of the CoFe₂O₄/CdS nanocomposite. Based on the TEM analysis, the CoFe₂O₄/CdS nanocomposite constituted nearly uniform, sphere-like nanoparticles of ～20 nm in size. The optical absorption spectrum of the CoFe₂O₄/CdS nanocomposite displayed a band gap of 2.21 eV, which made it a suitable candidate for application in sono/photocatalytic degradation of organic pollutants. Accordingly, the sonocatalytic activity of the CoFe₂O₄/CdS nanocomposite was evaluated towards the H₂O₂-assisted degradation of methylene blue, rhodamine B, and methyl orange under ultrasonic irradiation. The nanocomposite displayed excellent sonocatalytic activity towards the degradation of all dyes examined—the dyes were completely decomposed within 5–9 min. Furthermore, a comparison study revealed that the CoFe₂O₄/CdS nanocomposite is a more efficient sonocatalyst than pure CdS; thus, adopting the nanocomposite approach is an excellent means to improve the sonoactivity of CdS. Moreover, the magnetic properties displayed by the CoFe₂O₄/CdS nanocomposite allow easy retrieval of the catalyst from the reaction mixture for subsequent uses.

Full text of DOI:10.1016/S1872-2067(16)62473-7

Chinese Journal of Catalysis 37 (2016) 1487–1495
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
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 / c h n j c
Article
CoFe2O4/CdS nanocomposite: Preparation, characterisation, and
application in sonocatalytic degradation of organic dye pollutants
Saeed Farhadi*, Firouzeh Siadatnasab
Department of Chemistry, Lorestan University, Khoramabad 68135‐465, Iran
A R T I C L E I N F O
A B S T R A C T
Article history:
A magnetic CoFe2O4/CdS nanocomposite was prepared via one‐step hydrothermal decomposition
of cadmium diethanoldithiocarbamate complex on the surface of CoFe2O4 nanoparticles at a low
temperature of 200 °C. The nanocomposite was characterised by X‐ray diffraction (XRD), Fourier
transform infrared spectroscopy (FT‐IR), scanning electron microscopy, energy‐dispersive X‐ray
spectroscopy (EDX), UV‐visible spectroscopy, transmission electron microscopy (TEM), N2 gas
sorption analysis, X‐ray photoelectron spectroscopy (XPS), and vibrating sample magnetometry.
The FT‐IR, XRD, EDX and XPS results confirmed the formation of the CoFe2O4/CdS nanocomposite.
Based on the TEM analysis, the CoFe2O4/CdS nanocomposite constituted nearly uniform,
sphere‐like nanoparticles of ~20 nm in size. The optical absorption spectrum of the CoFe2O4/CdS
nanocomposite displayed a band gap of 2.21 eV, which made it a suitable candidate for application
in sono/photocatalytic degradation of organic pollutants. Accordingly, the sonocatalytic activity of
the CoFe2O4/CdS nanocomposite was evaluated towards the H2O2‐assisted degradation of meth‐
ylene blue, rhodamine B, and methyl orange under ultrasonic irradiation. The nanocomposite dis‐
played excellent sonocatalytic activity towards the degradation of all dyes examined—the dyes
were completely decomposed within 5–9 min. Furthermore, a comparison study revealed that the
CoFe2O4/CdS nanocomposite is a more efficient sonocatalyst than pure CdS; thus, adopting the
nanocomposite approach is an excellent means to improve the sonoactivity of CdS. Moreover, the
magnetic properties displayed by the CoFe2O4/CdS nanocomposite allow easy retrieval of the cata‐
lyst from the reaction mixture for subsequent uses.
Received 16 April 2016
Accepted 14 May 2016
Published 5 September 2016
Keywords:
CoFe2O4/CdS nanocomposite
Cadmium dithiocarbamate
Hydrothermal decomposition
Sonocatalyst
Dye degradation
© 2016, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1. Introduction
have been developed for the treatment of organic dye
wastewater [4–9]. However, most of the above‐listed methods
Organic dye wastewater has become a major source of wa‐
ter pollution. Furthermore, exposure to discharged organic
dyes is known to present potential health risks to humans. Ac‐
cordingly, this issue has attracted considerable attention, and
extensive and intensive research studies have been conducted
to address it [1–3]. Specifically, different treatment technolo‐
gies, including biological methods, adsorption, electrochemical
processes, membrane technologies, and oxidation processes,
have one or more disadvantages such as high cost, low recycla‐
bility, complex operations, and complicated designs. Therefore,
exploring an alternative, simple, cost‐effective, and green ap‐
proach to addressing organic dye pollution is important.
Photocatalytic degradation is regarded as a promising
technology for organic dye treatment in wastewater owing to
its high oxidation capabilities and environmental friendliness
[10,11]. Among various semiconductor photocatalysts, cad‐
* Corresponding author. Tel: +98‐6633120611; Fax: +98‐6633120618; E‐mail: sfarhadi1348@yahoo.com
DOI: 10.1016/S1872‐2067(16)62473‐7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 37, No. 9, September 2016

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mium sulfide (CdS), with a band gap of 2.4 eV, is one of the
most important semiconductors and has been extensively
studied in diverse applications, particularly in visible light‐
driven photocatalysis over the past decades [12–19]. However,
rapid recombination of the excited electron‐hole pairs consid‐
erably limits the photocatalytic efficiency of CdS [13]. Further‐
more, when employed in photocatalytic reactions in aqueous
media, CdS is susceptible to photocorrosion and oxidation by
the photogenerated holes [12,14–17]. To overcome these
problems, several approaches have been developed including
combining CdS with other semiconductors [14,18,20], noble
metals [21], conductive polymers [22], or carbon nanomateri‐
als [23,24]. In recent years, the combination of CdS with mag‐
netic nanomaterials, such as Fe3O4, α‐Fe2O3, and ZnFe2O4, has
proven to successfully increase the catalytic activity, photocor‐
rosion resistance, recovery, and reuse of CdS catalyst [25–31].
However, dye wastewater is typically non‐transparent and
highly concentrated. Consequently, light penetration is limited
to only several millimetres, thereby affecting the efficacy of
semiconductor nanocomposites to achieve complete photo‐
catalytic degradation of such wastewater. To meet the
fast‐developing water treatment requirements, there is a great
need to devise innovative technologies and materials for effi‐
cient removal of pollutants from non‐transparent wastewater.
As a potential candidate, semiconductor‐based sonocatalysis
can overcome the above‐mentioned disadvantages of the pho‐
tocatalytic technology because ultrasonic irradiation has a
strong penetration ability in all types of water media [32–38]. It
is well known that the sonocatalytic efficiency depends highly
on the type of catalyst. Therefore, the development of novel
sonocatalysts is important to further understand the sonocata‐
lytic mechanism and promote sonocatalysis‐based applications.
In the present study, a CoFe2O4/CdS nanocomposite was
prepared, for the first time, by a facile, one‐step hydrothermal
were purchased from Merck Chemical Company.
X‐ray diffraction (XRD) patterns were recorded on a Rigaku
D‐max C III X‐ray diffractometer using Ni‐filtered Cu Kα radia‐
tion (λ = 0.15406 nm) for the phase determination of the de‐
composed samples. Infrared spectra were obtained using a
Schimadzu Fourier transform infrared (FT‐IR) 160 spectro‐
photometer using KBr pellets. The morphology and elemental
analysis of the CoFe2O4/CdS nanocomposite were conducted by
scanning electron microscopy‐energy‐dispersive X‐ray spec‐
troscopy (SEM‐EDX) on a Mira3 Tescan scanning electron mi‐
croscope fitted with an energy‐dispersive X‐ray unit. The size of
the nanocomposite particles was determined by transmission
electron microscopy (TEM) on an EM10C microscope operating
at an accelerating voltage of 100 kV. Optical absorption spectra
of the nanocatalysts and dye solutions were recorded on a Cary
100 Conc Varian UV‐visible spectrophotometer in the wave‐
length range of 200–800 nm. For the UV‐visible measurements,
a homogeneous suspension of the nanocatalyst in ethanol was
prepared by sonication for 25 min. The Brunauer‐Emmett‐
Teller (BET) surface area of the nanocomposite was deter‐
mined by N2 adsorption measurements at –196 °C using a Nova
2000. X‐ray photoelectron spectroscopy (XPS; Bestec, Germa‐
ny) measurements were performed to study the chemical
states of the elements. Vibrating sample magnetometry (Me‐
ghnatis Daghigh Kavir Co., Iran) was employed to measure the
magnetic parameters of the nanocatalysts at room tempera‐
ture. An ultrasonic apparatus, operating at 37 kHz (Sonic 6MX;
output acoustic power = 100 W), was used for the degradation
of the dye solutions.
2.2. Preparation of the CdS and CoFe2O4 nanoparticles
Diethanoldithiocarbamic acid was prepared from diethano‐
lamine (0.02 mol) and carbon disulphide (0.02 mol) in ethanol
(20 mL) under ice‐cold conditions (5 °C). An aqueous solution
of Cd(NO3)2·4H2O (3.08 g, 0.01 mol) was added to the yellow
dithiocarbamic acid solution under continuous stirring. The
obtained Cd(DEDTC)2 was filtered, washed with alcohol, and
dried.
To prepare the CdS nanoparticles, Cd(DEDTC)2 (0.5 g) pre‐
cursor was introduced into a Teflon‐lined stainless steel auto‐
clave (50 mL), to which distilled water (30 mL) was added. The
autoclave was sealed and maintained at 200 °C for 24 h. Then it
was cooled to room temperature. The as‐formed yellowish
precipitates were filtered and washed with distilled water and
ethanol thrice to eliminate any unreacted precursor or
by‐products.
decomposition
of
cadmium
diethanoldithiocarbamate
(Cd(DEDTC)2) precursor over magnetic CoFe2O4 nanoparticles.
The sonocatalytic activity of the CoFe2O4/CdS nanocomposite
towards the degradation of organic pollutants, such as meth‐
ylene blue (MB), rhodamine B (RhB), and methyl orange (MO),
was studied under ultrasound irradiation. The effect of differ‐
ent parameters on the efficiency of the process i.e., H2O2
amount and catalyst dosage was investigated. Additionally, the
activity of the CoFe2O4/CdS nanocomposite was compared with
that of pure CdS under similar conditions. To our knowledge,
there are no reports on the use of nanocomposites for the rapid
sonodegradation of organic dyes, especially in the presence of
H2O2 as an environmentally friendly oxidising agent.
To prepare the CoFe2O4 nanoparticles, Co(NO3)2·6H2O (0.62
g) and Fe(NO3)3·9H2O (1.71 g) were added to deionised water
(25 mL). The solution was stirred for 1 h at room temperature.
Then, NaOH aqueous solution (1 mol/L) was supplemented to
the suspension to achieve pH = 11 while stirring for 1 h. A hy‐
drothermal treatment at 180 °C for 12 h was subsequently
conducted. The resulting black product was washed with de‐
ionised water and ethanol.
2. Experimental
2.1. Materials and characterisation
Cadmium nitrate tetrahydrate (Cd(NO3)2·4H2O), diethano‐
lamine (C4H11NO2), and carbon disulphide (CS2) were provided
by Sigma‐Aldrich. Iron nitrate nonahydrate (Fe(NO3)3·9H2O),
cobalt
nitrate
hexahydrate
(Co(NO3)2·6H2O),
MO
(C14H14N3NaO3S), MB (C16H18ClN3S), and RhB (C28H31ClN2O3)
2.3. Preparation of the CoFe2O4/CdS nanocomposite

Saeed Farhadi et al. / Chinese Journal of Catalysis 37 (2016) 1487–1495
1489
The synthesised CoFe2O4 nanoparticles (0.16 g) were dis‐
persed in deionised water (30 mL) and ultrasonicated for 1 h.
Then, Cd(DEDTC)2 (0.3 g) was added to the CoFe2O4 suspen‐
sion. The suspension was ultrasonicated for another hour and
then autoclaved in a 50‐mL Teflon‐lined stainless steel auto‐
clave at 200 °C for 24 h under autogenous pressure. The prod‐
uct was washed with water after cooling to room temperature
and then filtered.
c
2.4. Sonocatalytic tests
b
a
The sonocatalytic degradation of the aqueous dye solutions
was examined in the presence of CoFe2O4/CdS catalyst powder
under 37‐kHz ultrasonic irradiation. The reactions were con‐
ducted in an open Pyrex cylindrical glass vessel containing
CoFe2O4/CdS sonocatalyst (0.5 g/L), H2O2 (30 mmol/L), and
the dye solution (50 mL, 25 mg/L). First, the suspensions were
magnetically stirred in the dark for 30 min to achieve adsorp‐
tion‐desorption balance between the dye and sonocatalyst.
Then, ultrasonic irradiation was applied to the reaction mix‐
ture. All the tests were conducted at room temperature and
ambient pressure. During the sonocatalytic reaction, sample
aliquots were collected in the determined time intervals, and
the catalyst was separated from the suspension by an external
magnetic field. The residual MB, RhB, and MO concentrations
were determined by UV‐visible spectroscopy at 663, 557, and
462 nm, respectively. The dye conversion was calculated as
follows: (C0 − C)/C0, where C is the concentration of the dye
after irradiation and C0 is the concentration of the dye after
adsorption equilibrium (before irradiation). The effect of oper‐
ational parameters on the sonocatalytic activity of the
CoFe2O4/CdS nanocomposite i.e., the amount of H2O2 (0–40
mmol/L), sonocatalyst dosage (0–0.75 g/L), and initial dye
concentration (5–35 mg/L) was studied.
10
20
30
40
2θ/( º )
50
60
70
80
Fig. 1. XRD patterns of (a) CoFe2O4, (b) CdS and (c) CoFe2O4/CdS.
particles was ~20 nm, as estimated by the Debye‐Scherrer
equation [39]: DXRD = 0.9λ/(βcosθ), where DXRD is the average
crystallite size, λ is the wavelength of Cu Kα radiation, β is the
full‐width at half‐maximum of the chosen diffraction peak, and
θ is the Bragg angle.
Fig. 2(a) shows the FT‐IR spectrum of pure CoFe2O4, which
displayed two principal absorption bands below 1000 cm−1, a
characteristic feature of all ferrites [40]. Fig. 2(b) illustrates the
FT‐IR spectrum of the CdS nanoparticles produced via hydro‐
thermal decomposition of Cd(DEDTC)2 complex at 200 °C. The
bands associated with the complex were no longer apparent,
thus indicating the high purity of the product [41]. Fig. 2(c)
shows the FT‐IR spectrum of the CoFe2O4/CdS nanocomposite,
which displayed two main absorption bands below 1000 cm−1.
These bands could be related to the CoFe2O4 nanoparticles in
the nanocomposite [42].
The morphology of the CoFe2O4/CdS nanocomposite was
examined by SEM, as shown in Fig. 3. The SEM images show
that the CoFe2O4/CdS nanocomposite constituted fine, loosely
aggregated, sphere‐like nanoparticles with high porosity.
Fig. 4 shows TEM images of the CoFe2O4/CdS nanocompo‐
site. As deduced from the TEM analysis, the particle size distri‐
3. Results and discussion
3.1. Characterisation of the sonocatalyst
The XRD patterns of CoFe2O4, CdS, and CoFe2O4/CdS are
presented in Fig. 1. As observed in the XRD pattern of CoFe2O4,
all diffraction peaks could be assigned to spinel‐type CoFe2O4
(JCPDS Card No. 01‐1121). The diffraction pattern of CdS
showed the presence of both hexagonal and cubic phases
(JCPDS Card Nos. 75‐1545 and 10‐0454, respectively). The XRD
pattern of the CoFe2O4/CdS nanocomposite is shown in Fig.
1(c). Comparison of the XRD patterns in Fig. 1(c) and Fig. 1(a),
(b) confirmed that the nanocomposite was composed of
CoFe2O4 and CdS phases. The peaks at 2θ = 24.88°, 26.58°,
28.25°, 43.86°, 47.92°, 51.97°, and 66.97° were assigned to the
(100), (002), (101), (110), (103), (200), and (203) planes of the
hexagonal structure of CdS (JCPDS Card No. 41‐1049). The
peaks at 2θ = 29.96°, 35.46°, 43.86°, 57.05°, and 62.47° were
assigned to the (220), (311), (400), (511), and (440) planes of
the cubic structure of CoFe2O4 (JCPDS Card No. 01‐1121). The
diffraction peaks were broader because of the small size effect
of the nanocomposite. The average size of the nanocomposite
c
b
a
4000 3600 3200 2800 2400 2000 1600 1200 800
Wavenumber (cm-1)
400
Fig. 2. FT‐IR spectra of (a) CoFe2O4, (b) CdS and (c) CoFe2O4/CdS.

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Saeed Farhadi et al. / Chinese Journal of Catalysis 37 (2016) 1487–1495
(a)
(b)
S K
2500
2000
1500
1000
500
CdL
CdL
CoK
FeK
CdK5
CdK4
CdK3
CoK
FeK
S K
CdK2 CdK2
CdK CdK
Fig. 3. SEM images of the CoFe2O4/CdS nanocomposite.
keV
0
0
10
20
30
40
Fig. 5. EDX analysis of the CoFe2O4/CdS nanocomposite.
bution was narrow, ranging from 10 to 30 nm, and the mean
particle diameter was ~20 nm. These results were consistent
with the average particle size calculated by the Debye–Scherer
formula from the XRD pattern.
Additionally, the chemical purity and stoichiometry of the
CoFe2O4/CdS nanocomposite were determined by EDX analy‐
sis. The EDX spectrum in Fig. 5 revealed the presence of Cd, S,
Co, Fe, and O in the CoFe2O4/CdS nanocomposite.
suggests that the as‐synthesised CoFe2O4/CdS nanocomposite
has a suitable Eg for application in the catalytic degradation of
organic pollutants under visible light. The decrease in the band
gap of the CoFe2O4/CdS nanocomposite (2.21 eV) when com‐
pared with CdS nanoparticles (2.37 eV) may be ascribed to the
presence of the CoFe2O4 ferrite.
The magnetic behaviours of CoFe2O4/CdS and CoFe2O4 at
room temperature were also investigated. As seen in Fig. 7, the
nanocomposite displayed a ferromagnetic behaviour. The coer‐
cive force (Hc), saturation magnetisation (Ms), and remanent
magnetisation (Mr) of CoFe2O4/CdS were compared with those
of CoFe2O4 in the inset of Fig. 7. As observed, the Hc, Ms, and Mr
values of CoFe2O4/CdS were smaller than those of CoFe2O4
owing to the presence of non‐magnetic CdS [44].
The porosity and specific surface area were determined us‐
ing the Barrett‐Joyner‐Halenda method and the BET equation,
respectively. The calculated BET surface area of the nanocom‐
posite was 39.329 m²/g [45,46]. The pore size distribution of
the nanocomposite was centred at ~15.5 nm. The mesoporous
structure and relatively high surface area are expected to offer
high catalytic activity.
The UV‐visible absorption spectra of the CdS nanoparticles
and CoFe2O4/CdS nanocomposite are shown in Fig. 6. When
compared with the spectrum of the pure CdS sample, that of the
CoFe2O4/CdS composite displayed spectral features that were
shifted to the visible light region. The following equation is
typically used to calculate the optical absorption near the band
edge of a semiconductor [43]: (αhv)2 = K(hv – Eg), where α, h, v,
Eg, and K are the optical absorption coefficient, Planck constant,
light frequency, band gap energy, and a constant relative to the
material, respectively. The band gap can be estimated by ex‐
trapolating the linear region in the plot of (αhv)2 versus photon
energy (hv). As shown in the inset of Fig. 6, the calculated Eg of
the CoFe2O4/CdS nanocomposite was ~2.21 eV. This value
The chemical composition and electronic structure of the
synthesised CoFe2O4/CdS were analysed by XPS. As observed in
Fig. 8(a), the survey spectrum of CoFe2O4/CdS displayed peaks
2
10
9
1.8
8
7
1.6
1.4
1.2
1
6
5
4
3
2
1
0
d
c
b
1.5
2
2.5
3
3.5
4
0.8
0.6
0.4
0.2
0
hv (eV)
a
300
400
500
600
700
800
Wavelength (nm)
Fig. 6. UV‐Vis spectra of (a) CdS nanoparticles and (b) CoFe2O4/CdS
nanocomposite. The inset shows the (αhv)2–hv curves of (c) CdS nano‐
particles and (d) CoFe2O4/CdS nanocomposite.
Fig. 4. TEM images of the CoFe2O4/CdS nanocomposite.

Saeed Farhadi et al. / Chinese Journal of Catalysis 37 (2016) 1487–1495
1491
tants to evaluate the sonocatalytic performance of the
CoFe2O4/CdS nanocomposite in the presence of H2O2. Accord‐
ingly, a series of sonocatalytic experiments were conducted. As
observed in Fig. 9(a), the intensity of the MB absorption peak at
663 nm decreased as the reaction proceeded and disappeared
after 5 min, thus indicating the high sonocatalytic performance
of the CoFe2O4/CdS nanocomposite. The concentration changes
(C/C0) of MB at 663 nm as a function of irradiation time under
different conditions are shown in Fig. 9(b). Curve (I) in Fig. 9(b)
shows that negligible amounts of MB degraded in the presence
of ultrasonic irradiation only. Curves (II)–(VI) in Fig. 9(b) show
that the degradation percentages of MB in the presence of H2O2
only, and CoFe2O4/H2O2, CdS/H2O2, and CoFe2O4/CdS systems
under ultrasonic irradiation and CoFe2O4/CdS/H2O2 system
without ultrasonic irradiation, were 0.05%, 0.05%, 12%, 15%,
and 22%, respectively. Curve (VII) in Fig. 9(b) shows that in the
presence of the CoFe2O4/CdS/H2O2 system, complete degrada‐
tion (100%) of MB was achieved under ultrasonic irradiation
within 5 min. The above results confirmed that the combined
use of ultrasonic irradiation, H2O2, and CoFe2O4/CdSnanocom‐
posite is necessary to achieve fast and complete degradation of
the dyes.
To understand the sonocatalytic degradation kinetic of MB
degradation, the pseudo‐first‐order model [48] was used:
ln(C0/C) = kt, where C0 and C are the dye concentrations before
and after irradiation, respectively, k is the pseudo‐first‐order
rate constant, and t is the reaction time. As shown in Fig. 9(c),
the k value for the degradation of MB was 0.77 min−1.
To evaluate the scope of this method, the sonocatalytic deg‐
radation of RhB and MO was also investigated. As observed
from Fig. 10, complete degradation (100%) of both RhB and
MO was achieved within 5 and 9 min (k = 0.77 min−1 and 0.059
min−1, respectively). The slow degradation of MO can be related
to its high redox potential.
80
60
a
Hc
898.26 25.9 62.78
CoFe2O4/CdS 898.26 12.51 24.9
M
M
r
s
CoFe2O4
40
b
20
0
-20
-40
-60
-80
-10000 -8000 -6000 -4000 -2000
0
2000 4000 6000 8000 10000
H (Oe)
Fig. 7. Room‐temperature magnetization curves of (a) CoFe2O4 and (b)
CoFe2O4/CdS nanocomposite. The inset shows the solution after mag‐
netic separation using an external magnetic.
of Cd, S, Co, Fe, and O. The peaks at 406.5 eV (3d5/2), 413 eV
(3d3/2), and 162.7 eV (S 2p) in the Cd (3d) and S (2p) XPS scans
(Fig. 8(b), (c)) confirmed the presence of Cd2+ and S2− in the
form of CdS. The Co 2p XPS spectrum (Fig. 8(d)) featured a
main peak at 782.1 eV (2p3/2) along with the characteristic
satellite peak at 789.2 eV. The presence of the broad satellite
peak confirmed the +2 oxidation state of Co element. Similarly,
the Fe 2p XPS spectrum (Fig. 8(e)) displayed peaks centred at
713 eV (2p3/2) and 726.5 eV (2p1/2), thereby confirming the +3
oxidation state of Fe element. The O 1s XPS spectrum (Fig. 8(f))
displayed a broad peak at ~532.1 eV, which corresponds to
lattice oxygen in the CoFe2O4 matrix [47].
3.2. Sonocatalytic degradation of dyes over the CoFe2O4/CdS
nanocomposite
MB, RhB, and MO were selected as models of organic pollu‐
Fig. 8. XPS fully scanned spectrum of CoFe2O4/CdS nanocomposite (a); XPS spectra of Cd 3d (b), S 2p (c), Co 2p (d), Fe 2p (e) and O 1s (f).

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Saeed Farhadi et al. / Chinese Journal of Catalysis 37 (2016) 1487–1495
1.95
0 min
0.5 min
1 min
a
1.0
2.5 min
0 min
5 min
1.5 min
2 min
0.8
1.45
0.95
0.45
-0.05
2.5 min
3 min
4 min
5 min
0.6
0.4
MO
0.2
RhB
MB
0.0
0
2
4
6
8
Time (min)
200
400
600
800
Wavelength (nm)
Fig. 10. Comparison of the concentration changes of MB, RhB and MO
(C/C0) as a function of irradiation time over the CoFe2O4/CdS nano‐
composite. Experimental conditions: [dye] = 25 mg/L, [nanocatalyst] =
0.5 g/L, [H2O2] = 30 mmol/L, 25 °C, 9 min.
1.1
1
b
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0
CoFe2O4/CdS+H2O2+Ultrasonic VII
CoFe2O4/CdS+ultrasonic VI
CoFe2O4/CdS+H2O2 V
respectively, within 5 min of ultrasonic irradiation. A higher
dosage of CoFe2O4/CdS nanocomposite in the solution provides
more active reaction sites for the generation of hydroxyl radi‐
cals (•OH), consequently accelerating the degradation rate.
Moreover, increasing the sonocatalyst dosage enables the for‐
mation of additional nuclei for the cavitation phenomenon, thus
CdS + H2O2+ultrasonic IV
CoFe2O4+H2O2+ultrasonic III
H2O2+ultrasonic II
Ultrasonic alone I
•
generating more OH radicals [49]. However, increasing the
CoFe2O4/CdS nanocomposite dosage further from 0.5 to 0.75
g/L did not increase the degradation rate further. In fact, at the
higher dosages, aggregation of the sonocatalyst particles oc‐
curred. This phenomenon leads to a reduction in effective sur‐
face area and amount of surface active sites for the generation
-0.1
-5
35
75
Time (min)
115
155
195
4.5
4
Ultrasonic+CoFe2O4/CdS+H2O2
Ultrasonic+CoFe2O4/CdS
CoFe2O4/CdS+H2O2
c
•
of OH radicals. Moreover, the extent of the cavitation phe‐
3.5
3
nomenon is dependent on the active surface area of the sono‐
catalyst. Thus, in the presence of excess amounts of sonocata‐
lyst, fewer ultrasonic waves circulate in the solution to form
cavitation bubbles [50–52].
Ultrasonic+CdS+H2O2
Ultrasonic+CoFe2O4+H2O2
Ultrasonic+H2O2
2.5
2
Ultrasonic alone
Additionally, the effect of the amount of H2O2 on catalytic ef‐
ficiency was investigated under the provided reaction condi‐
1.5
1
0.5
0
100
80
60
40
20
0
-0.5
-5
35
75
115
Time (min)
155
195
Fig. 9. (a) The temporal evolution of MB absorbance spectra in the
CoFe2O4/CdS nanocomposite under ultrasonic irradiation at different
intervals. The inset photo shows the colour change of dye solution dur‐
ing irradiation. (b) Concentration changes of MB at 663 nm as a func‐
tion of irradiation time. (c) ln(C0/C) versus irradiation time in MB solu‐
tion. Experimental conditions: [MB] = 25 mg/L, [nanocatalyst] = 0.5 g/L
[H2O2] = 30 mmol/L, 25 °C.
Fig. 11 shows the effect of CoFe2O4/CdS dosage on the son‐
ocatalytic degradation of MB. The degradation of MB was negli‐
gible in the absence of the CoFe2O4/CdS nanocomposite. As the
CoFe2O4/CdS dosage was increased to 0.25 and 0.5 g/L, the
sonocatalytic degradation of MB increased to 57% and 100%,
0.00
0.25
0.50
0.75
CoFe2O4/CdS catalyst (g/L)
Fig. 11. Effect of the CoFe2O4/CdS dosage on the sonocatalytic degrada‐
tion of MB. Experimental conditions: [MB] = 25 mg/L, [H2O2] = 30
mmol/L, 25 °C, 5 min.

Saeed Farhadi et al. / Chinese Journal of Catalysis 37 (2016) 1487–1495
1493
100
80
60
40
20
0
100
80
60
40
20
0
1
2
3
0
10
20
30
40
Reaction cycle
Concentration of H2O2 (mmol/L)
Fig. 13. Recyclability of CoFe2O4/CdS nanocomposite. Experimental
conditions: [MB] = 25 mg/L, [nanocatalyst] = 0.5 g/L, [H2O2] = 30
mmol/L, 25 °C.
Fig. 12. Effect of the H2O2 dosage on the sonocatalytic degradation of
MB. Experimental conditions: [dye] = 25 mg/L, [nanocatalyst] = 0.5 g/L,
25 °C, 5 min.
higher sonocatalytic performance [18,22,54,55]. The electrons
in the CB react with H2O2 to produce highly reactive •OH, which
can degrade MB. The generated holes oxidise OH− ions (or H2O2
molecules) adsorbed on the surface of the CdS particles to
produce •OH, which indirectly degrade the organic pollutants in
aqueous solution. Finally, the active species oxidise the dye
molecules to the degradation products (e.g., CO2, H2O) [56–61].
Based on the above results, a mechanism of the sonogenerated
electron‐hole separation process in the presence of
CoFe2O4/CdS is proposed and illustrated in Fig. 14. It is known
that the CB of CoFe2O4 is more positive than that of CdS, and its
VB is also positive when compared with that of CdS
[18,54,62,63]. Hence, CoFe2O4 can act as a sink for the sono‐
generated electrons, whereas CdS can act as an acceptor for the
sonogenerated holes in the hybrid sonocatalyst [18,54].
tions, and the results are shown in Fig. 12. As observed, in the
absence of H2O2, only ~15% of MB was degraded. When H2O2
dosage increased from 10 to 30 mmol/L, the degradation effi‐
ciency increased correspondingly from 39% to 100%. This
degradation rate enhancement was attributed to an increase in
•OH as a result of the higher H2O2 dosage. However, increasing
the amount of H2O2 further from 30 to 40 mmol/L did not in‐
crease the degradation efficiency further—in contrast, it re‐
mained rather constant. The excess H2O2 molecules act as a
scavenger of •OH to generate perhydroxyl radicals, which have
a lower oxidation capability (Eq. (1)) [53]:
H2O2 + •OH → H2O + HOO•
(1)
Finally, the recyclability of the sonocatalyst was tested (Fig.
13). No significant loss in activity was observed for up to three
catalytic cycles, thereby indicating that the as‐prepared mag‐
netic sonocatalyst is stable and efficient in the degradation of
organic dyes in wastewater.
To confirm the above‐proposed mechanism, trapping ex‐
periments were conducted to determine the main active oxi‐
dant species. Upon addition of 10 and 20 mmol/L tert‐butyl
It is well known that the sonocatalytic degradation of or‐
ganic pollutants over a sonocatalyst is based on three main
phenomena i.e., sonoluminescence, ‘hot spot’, and oxygen atom
escape [37]. Sonoluminescence occurs when a sound wave has
sufficient intensity and induces gaseous cavities within a liquid
to collapse quickly. These cavities may form pre‐existing bub‐
bles or may be generated through a process known as cavita‐
tion. Sonoluminescence can be stabilised under laboratory
conditions; during this process, a single bubble will periodically
expand and collapse continuously while emitting a burst of
light at each collapse. Therefore, ultrasonic irradiation results
in sonoluminescence and the production of abundant visible
light. Visible light has a relatively wide wavelength range be‐
cause of acoustic cavitations. Both CdS and CoFe2O4 can be ex‐
cited by visible light to produce sonogenerated carriers and
excited holes. The excited‐state electrons in the conduction
band (CB) of CdS can be readily injected into the CB of CoFe2O4.
Simultaneously, the sonogenerated holes in the valence band
(VB) of CoFe2O4 are able to move easily to the VB of CdS. Thus,
the sonogenerated electrons and holes move in opposite direc‐
tions while reducing the recombination probability and en‐
hancing charge separation efficiency, thereby leading to a
•
alcohol (t‐BuOH), as a scavenger of OH, the degradation effi‐
ciency of MB considerably decreased, and only 40% and 30% of
MB was degraded, respectively, after 5 min irradiation. These
results indicate that •OH radicals are the primary oxidant in the
MB
MB
MB
e-e-e-e-
MB
MB
CB
VB
H2O2
.
OH-+ OH
0
1
MB
e-e-e-e-
CB
h+h+h+h+
MB
VB
2
h+h+h+h+
MB
OH-
.
OH
MB
.
MB
+ OH
CO2+H2O +Inorganic compounds
Fig. 14. Schematic drawing the separation of generated electrons and
holes on the interface of the CoFe2O4/CdS nanocomposite under ultra‐
sonic irradiation.

1494
Saeed Farhadi et al. / Chinese Journal of Catalysis 37 (2016) 1487–1495
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A CoFe2O4/CdS nanocomposite (with a particle diameter of
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separation of the nanocomposite from aqueous solutions. Over
the CoFe2O4/CdS nanocomposite, rapid degradation of MB,
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min, respectively, under ultrasound and in the presence of H2O2
•
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Acknowledgments
The authors gratefully acknowledge the Lorestan University
and Iran Nanotechnology Initiative Council (INIC) for their
financial support.
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CoFe2O4/CdS nanocomposite: Preparation, characterisation, and
application in sonocatalytic degradation of organic dye pollu‐
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