Preparation and photocatalytic properties of hybrid core-shell reusable CoFe2O4-ZnO nanospheres

Article abstract of DOI:10.1016/j.jmmm.2012.02.009

Magnetically separable and reusable core-shell CoFe₂O ₄-ZnO photocatalyst nanospheres were prepared by the hydrothermal synthesis technique using glucose derived carbon nanospheres as the template. The morphology and the phase of cor

Full text of DOI:10.1016/j.jmmm.2012.02.009

Journal of Magnetism and Magnetic Materials 324 (2012) 2597–2601
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Journal of Magnetism and Magnetic Materials
journal homepage: www.elsevier.com/locate/jmmm
Preparation and photocatalytic properties of hybrid core–shell reusable
CoFe2O4–ZnO nanospheres
A. Wilson a, S.R. Mishra a,n, R. Gupta b, K. Ghosh b
a Department of Physics, The University of Memphis, Memphis, TN 38152, USA
b Department of Physics, Materials Science, and Astronomy, Missouri State University, Springfield, MO, USA
a r t i c l e i n f o
a b s t r a c t
Article history:
Magnetically separable and reusable core–shell CoFe2O4–ZnO photocatalyst nanospheres were pre-
pared by the hydrothermal synthesis technique using glucose derived carbon nanospheres as the
template. The morphology and the phase of core–shell hybrid structure of CoFe2O4–ZnO were assessed
via TEM, SEM and XRD. The magnetic composite showed high UV photocatalytic activity for the
degradation of methylene blue in water. The photocatalytic activity was found to be ZnO shell thickness
dependent. Thicker ZnO shells lead to higher rate of photocatalytic activity. Hybrid nanospheres
recovered using an external magnetic field demonstrated good repeatability of photocatalytic activity.
These results promise the reusability of the hybrid nanospheres for photocatalytic activity.
& 2012 Elsevier B.V. All rights reserved.
Received 26 January 2012
Available online 22 February 2012
Keywords:
Magnetic core–shell hollow nanosphere
Composite nanosphere
Hybrid nanosphere
Photocatalysis
1. Introduction
quantum efficiency and higher PCA than those of TiO2 have been
reported [14,15]. However, the PCA of ZnO should be further
Large industrial wastes, mainly aqueous toxic dyes such as Azo
dyes, from textile industries are routinely poured out in water as
waste [1–3]. Nanoparticles (NPs) loaded slurry reactors may be
used as devices for waste-water treatment because of their high
specific surface area and good dispersion [4,5]. However, these
slurry reactors are inefficient because of difficulty in separating
NPs from treated water. To overcome this difficulty various
strategies including NPs beads, thin films, NPs loaded porous
substrate etc, have been used [6–9]. However, the activity of
nanophotocatalyst is reduced considerably due to immobilization
techniques. To overcome these problems, recently the use of
magnetic cores–shell photocatalyst particles was proposed,
where photocatalyst particles can be collected using an external
magnetic field [10,11]. Generally these NPs use Fe3O4 as a
magnetic core, which can easily oxidize to paramagnetic Fe2O3
upon heat treatment while coating it with a photocatalyst layer
such as TiO2 [12,13]. Thus, it is inherently difficult to produce
photocatalyst-coated particles of high photocatalytic activity
(PCA) without loss of magnetic property by an iron oxide phase
transition.
enhanced from the viewpoint of practical use and commerce.
Therefore, various methods have been developed to reduce the
eꢀ/hþ recombination rate of ZnO in the photocatalytic processes
[16,17]. In particular, it has been demonstrated that the PCA of
ZnO is strongly dependent on its crystallite size, specific surface
area, morphologies, and textures [18]. Thus with the aim of
synthesizing reusable hybrid photocatalyst structures, here in
we report the synthesis of CoFe2O4–ZnO core–shell hollow nano-
spheres. The hollow mesoporous nanospheres of composite are
preferred because of their low density, high surface area,and good
surface permeability as well as hollow textures [19,20]. Further-
more, agglomeration of NPs is often found to be deleterious to its
potential applications [21]. Only the primary particles that are
near the surface region of the secondary particles are active while
the inner part remains inactive [22]. Core–shell type CoFe2O4–
ZnO [23] and ZnFe2O4–TiO2 [24], and Fe3O4–ZnO [25] NPs have
been reported earlier. However, as explained above, the efficacy of
these NPs is severely affected by the agglomeration effect.
Mesoporous structures are reported to show high gas responses
[26], rapid gas responding kinetics [27], and high PCA [28] which
attribute to their high dispersion, surface area and well defined
porous architecture.
Although TiO2 is universally considered to be the most
important photocatalyst, ZnO, an n-type semiconductor, is also
a suitable alternative to TiO2 due to its similar bandgap energy
(3.2 eV) and its lower cost. Moreover, in certain cases, larger
Template-assisted synthesis has been used for the synthesis of
well dispersed hollow micro- or nanospheres. Generally these
templates are of different compositions, such as silica spherical
particles [29–31] or polystyrene (PS) spherical particles [32–34].
Recently the use of glucose derived monodisperse carbonaceous
microspheres as a sacrificial template has been presented [35,36].
n Corresponding author.
E-mail address: srmishra@memphis.edu (S.R. Mishra).
0304-8853/$ - see front matter & 2012 Elsevier B.V. All rights reserved.
doi:10.1016/j.jmmm.2012.02.009

2598
A. Wilson et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2597–2601
These sacrificial cores inherit functional groups and have reactive
surfaces, which facilitate the absorption of cationic metal ions and
the precipitation of metal precursors [37,38]. An advantage of
template based method is that the size of the final hollow spheres
can be controlled by the size of the template.
2.4. Synthesis of CoFe2O4–ZnO core–shell hollow spheres
CoFe2O4 nanospheres of amount 0.5 g were added and mixed
into the 200 ml mixture of Zn(OH)2. Then 2.0 ml of PEG 200 was
added to the mixture and was ultrasonicated for 30 min. The
CoFe2O4 and Zn(OH)2 collosol was then placed into an oven at
70 1C, forming a gel. The now formed gel was then calcined in an
oven at 200 1C for 1 h to remove any residual solvent and the
temperature was increased to 400 1C for 2 h where hollow
CoFe2O4–ZnO structures were formed [41,42]. The sample was
left to cool down to RT, was washed with deionized water, and
dried at 60 1C. The shell thickness of ZnO was varied by varying
the initial concentration of Zn precursor at 0.1834, 0.3668, and
0.5502 g. The corresponding samples are designated as CoFe2O4–
ZnO (A), CoFe2O4–ZnO (B), and CoFe2O4–ZnO (C).
The aim of this study is to obtain a magnetically separable
photocatalyst, which is stable, well dispersed, and easy to pre-
pare. In this research, study of template based synthesis of core–
shell CoFe2O4–ZnO hollow nanospheres and its efficacy as a
photocatalyst in the presence of UV light is presented. CoFe2O4
as a magnetic material is used as a catalyst support because of its
high magnetic anisotropy, moderate saturation magnetization,
and high coercivity. Core–shell hybrid mesoporous spheres have
been obtained via a hydrothermal route using glucose derived
carbon nanospheres (CNS) as the template [37]. Magnetic
CoFe2O4 core and oxide ZnO shell were deposited on to the CNS
using a wet-chemical technique [38–40]. The degradation process
of modeled pollutant methylene blue (MB) was carried out using
a mixture of hybrid hollow nanospheres in MB aqueous solution
under UV light exposure. In this paper we discuss the effect of
ZnO shell thickness and effectiveness of recycled nanospheres in
UV degradation of MB.
3. Characterization
The structural and phase analyses of as prepared samples were
performed using a JEOL JEM1200EX II transmission electron
microscope (TEM),
electron microscope (SEM), and X-ray diffraction (XRD) using a
Bruker D8 Advance and Cu K . RT hysteresis loops were obtained
a Philips XL 30 environmental scanning
a
using SQUID (Quantum Design).
2. Preparation of magnetically separable photocatalytic NPs
3.1. Photocatalytic degradation study
The employed process involves coating of CNS as a template
with successive layers of CoFe2O4 and ZnO NPs in two different
steps resulting in the fabrication of C–CoFe2O4–ZnO core–shell
composite particles. Subsequently, calcination of these particles
at elevated temperature results in the fabrication of monodis-
perse, closed and porous CoFe2O4–ZnO hybrid hollow spheres.
PCA of the as prepared samples was assessed using degrada-
tion of MB exposed to UV light. The reaction suspensions contain-
ing MB and CoFe2O4–ZnO photocatalyst were irradiated by a
300 W high-pressure mercury lamp with continuous ultrasonica-
tion of MB mixture. Absorbance measurements using an Ocean
Optics UV–vis spectrometer were recorded in the range of 200–
800 nm. The degradation of MB absorption peak intensity at
298 nm was monitored as a function of UV exposure time. For
the solution, 5 mg of MB was dissolved in 500 ml of deionized
water. Then 50 ml of MB solution as measured into a 50 ml beaker
into which 2.5 mg of photocatalytic CoFe2O4–ZnO nanospheres as
also added. The mixture was ultrasonicated in the dark for 20 min
before the UV light was turned on. A smaller sample solution was
also made to be used as a reference during the UV–vis spectro-
scopy. After completion of each run, the CoFe2O4–ZnO particles
were recovered using an external magnet, dried at 100 1C for 2 h
in air, and were reassessed for there PCA.
2.1. Synthesis of carbon templates
The aqueous solution containing 18.01 g of glucose in 100 ml
was heated at 170 1C for 6 h in a 45 ml Teflon autoclave. After
cooling down the solution to RT, the solution was washed with
deionized water/ethanol several times and then dried at 80 1C for
4 h in the presence of nitrogen [23].
2.2. Synthesis of CoFe2O4 hollow spheres
Synthesis of CoFe2O4 is adapted from Zhang et al. and is
modified for the synthesis of hollow CoFe2O4 nanospheres [23].
As derived 40.2 mg carbon templates was mixed into 20 ml of
ethylene glycol (EG) and ultrasonicated for 30 min. Then 0.5 g of
cobalt nitrate hexahydrate, 0.9287 g of iron chloride hexahydrate,
and 1.127 g of sodium acetate were added into the carbon–EG
mixture and stirred with a magnetic stirrer for 30 min. The
mixture was heated at 200 1C for 12 h in a 45 ml Teflon autoclave.
Later hollow CoFe2O4 nanospheres were extracted from the
solution by the way of magnetic separation. After washing with
water/ethanol, the sample was then dried in an oven at 60 1C
overnight.
4. Results and discussion
Fig. 1 compares the XRD patterns of hybrid nanospheres. The
CoFe2O4 diffraction peaks are matched with ICDD 22-1086 while
ZnO peaks are identified with ICDD 36-1451. The broad peaks of
as prepared CoFe2O4 shows the presence of small crystallite size
in the samples. As CoFe2O4 as later covered with ZnO and heated
at 400 1C, the sharpening of peaks takes place which is indicative
of the crystalline growth of both CoFe2O4 and ZnO. Using
Scherrer’s equation [43] and using the FWHM of the (311)
reflection the average size of the CoFe2O4 crystallites in hollow
spheres is estimated to be ca. 35 nm. The ZnO peak shows
continuous growth in samples A–C indicative of increasing
amount of ZnO in the corresponding samples.
2.3. Synthesis of Zn(OH)2
The morphology characterization of samples was carried out
using TEM and SEM techniques. The TEM images of carbon
template, hollow CoFe2O4, and ZnO coated CoFe2O4 hollow
spheres are shown in Fig. 2(a)–(e) along with corresponding size
distribution histograms. The average particle size calculated from
0.1834 g of zinc acetate (Zn(CH3COO)2) and 0.08 g of
NaOH were dissolved in 100 ml of ethanol (C2H5OH), separately
and both solutions were mixed. This formed a 200 ml Zn(OH)2
collosol.

A. Wilson et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2597–2601
2599
TEM particle size analysis was found to be 390752 nm,
551797 nm, 672761 nm, 726777 nm, and 752777 nm, for
CNS, CoFe2O4, CoFe2O4–ZnO-A, CoFe2O4–ZnO-B, and CoFe2O4–
ZnO-C, respectively. Overall all particles have tight size distribu-
tion. The increase in the particle size is due to the thickening of
the ZnO shell. The ZnO shell thickness for the sample C is found to
be around 100710 nm. Fig. 2(f) is the SEM image of sample C,
which shows broken and unbroken hollow spheres.
The 300 K magnetization curve of composite hollow spheres is
shown in Fig. 3. The Presence of hysteresis loop shows that the
composite hollow spheres are ferromagnetic. A saturation mag-
netization of 62 emu/g for CoFe2O4 hollow spheres and 59, 56,
and 51 emu/g for samples A, B, and C, respectively is achieved at
50,000 Oe. These values are less than that of the bulk cobalt
ferrite (72 emu/g) [44]. The observed lowering of MS in CoFe2O4
may result from the surface effect and the small particle size
effect [34]. Further reduction in MS of CoFe2O4 upon ZnO coating
may also result from interface effects as well as reduction in
overall ferromagnetic component in the composite. This observa-
tion further suggests that the hollow and hierarchical structure
does have an effect on MS. The coercivity of CoFe2O4 hollow
spheres and ZnO coated CoFe2O4 hollow spheres samples A, B, and
C are 75, 241, 143, and 173 Oe, respectively. These values are
much lower than that of the bulk CoFe2O4 (980 Oe) [45]. The
coercivity of ZnO coated CoFe2O4 samples is much higher than
that of CoFe2O4 hollow spheres, indicating the role of interfacial
Fig. 1. X-ray diffraction patterns of CoFe2O4–ZnO composites.
Fig. 3. Room temperature hysteresis loops of CoFe2O4–ZnO composites.
Fig. 2. TEM images and particle size histograms of nanospheres: (a) Carbon (398746 nm), (b) CoFe2O4 (551797 nm), (c) CoFe2O4–ZnO-A (672761 nm), (d) CoFe2O4–
ZnO-B (726777 nm), (e) CoFe2O4–ZnO-C (752777 nm), and (f) SEM image of sample C. The scale bar in SEM image is 2 m.
m

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A. Wilson et al. / Journal of Magnetism and Magnetic Materials 324 (2012) 2597–2601
Fig. 4. UV–vis spectra (
l¼250–350 nm) of the aqueous solutions of MB dye after
UV irradiation for different time periods in the presence of CoFe2O4–ZnO (sample C).
The inset shows full MB UV–vis spectra.
defects and interaction between the NPs composing the hollow
structure [46]. The magnetic signature of CoFe2O4–ZnO composite
is essential in order to retrieve these particles after each MB
degradation cycle using an external magnet.
As an exmaple, Fig. 4 shows the change of absorption spectra
of an MB aqueous solution exposed to UV light for various times
in the presence of the CoFe2O4–ZnO (C) sample. The absorption
peak at
lꢁ298 nm drops gradually with increasing UV exposure
time, for up to 60 min. No new absorption bands appeared in
either the visible or ultraviolet regions, which indicates the
complete photocatalytic decolorization of MB aqueous solution
during the reaction. After each run of 60 min, samples were
recovered using external magnet, dried at 100 1C for 2 h and were
reused for photocatalytic study.
Fig. 5. Photodegradation rate constant as determined using first-order kinetic law
Eq. (1) for varying ZnO shell thicknesses.
5. Conclusion
The photocatalytic decomposition of MB on the surface of ZnO
follows a pseudo-first-order kinetic law, expressed as
A template assisted approach for the fabrication of hybrid
structure of hollow spheres composed of a magnetic core CoFe2O4
and ZnO shell mixed metal oxides has been demonstrated. More-
over, ZnO content as well as shell thickness of these hollow
structures can be manipulated by tuning the employed concentra-
tion of Zn acetate. The study further demonstrates the reusability
of magnetically separable hybrid composite for photocatalysis. The
hybrid structure exhibited a good PCA in cyclic use, suggesting it as
one of the promising candidates for photocatalytic treatment of
wastewater.
ꢀlnðC=CoÞ ¼ kt
ð1Þ
where C and Co are the reactant concentration at time t¼t and
t¼0, respectively, k and t are the pseudo-first-order rate constant
(reaction rate constant) and time, respectively [47]. The relation-
ship between –ln(C/Co) and irradiation time (reaction time) is
shown in Fig. 5. A linear relationship between –ln(C/Co) and
irradiation time is observed. The pseudo-first-order rate constant
k is calculated from the slope of the best fit line.
Fig. 5 shows that the k values are comparatively greater in
CoFe2O4–ZnO (C) sample than others. This indicates that the ZnO
shell thickness is obviously a PCA factor. This is probably due to
several factors. (1) ZnO shell possesing an unusual hierarchicall
structure [20,48,49], which allows more efficient transport of the
reactant molecules to get to the active sites on the framework
walls, (2) as per the mechanistic view presented by Hong et al.
[25], the grain boundary diffusion between CoFe2O4 and ZnO will
directly give rise to lattice defects, which can improve photo-
catalysis of ZnO, (3) increased crystallinity with thicker ZnO shell,
and (4) improved junction potential between p-type CoFe2O4 and
n-type ZnO. After three cycles for the photodegradation of MB, the
catalyst did not exhibit any significant loss of activity, confirming
that CoFe2O4–ZnO hollow spheres were not photobleached.
Acknowledgment
This work was supported by NSF-EPSCoR under TN-SCORE grant.
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