Applied Surface Science 392 (2017) 1078–1087

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Applied Surface Science

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Building of CoFe2 /CoFe2 O4 /MgO architectures: Structure, magnetism

and surface functionalized by TiO2
M. Wang, Y.Q. Ma ∗ , X. Sun, B.Q. Geng, M.Z. Wu, G.H. Zheng, Z.X. Dai
Anhui Key Laboratory of Information Materials and Devices, School of Physics and Materials Science, Anhui University, Hefei 230039, People’s Republic of
China

a r t i c l e i n f o a b s t r a c t

Article history: Well-dispersed uniform CoFe2 O4 nanoparticles were prepared and then coated by MgO through thermal
Received 6 July 2016 decomposition of a metal–organic salt in organic solvent. Then CoFe2 O4 /MgO were reduced in a H2 /N2
Received in revised form mixture gas and subsequently oxidized in an ambient atmosphere in order to build CoFe2 /CoFe2 O4 /MgO
14 September 2016
architectures with high magnetization, good chemical stability and dispersivity, which are useful in some
Accepted 17 September 2016
practical applications. MgO can be dissolved by the HCl solution. The surfaces of CoFe2 O4 , CoFe2 /MgO,
Available online 28 September 2016
CoFe2 and CoFe2 /CoFe2 O4 magnetic particles were functionalized by TiO2 to prepare the magnetically
separable photocatalysts. The rattle-type particles were obtained without the assistance of template and
Keywords:
CoFe2 /CoFe2 O4 /MgO architecture
etchant. The photocatalytic activity of these photocatalysts in degradation of methylene blue and the
Surface functionalization magnetic separability were investigated: The nanosheet-shaped TiO2 and rattle-type particles exhibited
TiO2 good photocatalytic performance; The highest degradation efficiency reaches 93% for the CoFe2 /TiO2
Rattle-type nanostructures sample which has the highest magnetization value of 42 emu/g, beneficial for the recovery of catalyst
Photocatalyst after degradation.
© 2016 Elsevier B.V. All rights reserved.

1. Introduction However, the commercial TiO2 photocatalyst usually has a small

particle size; Small size is favorable in photocatalytic reaction
Magnetic nanoparticles have been extensively investigated because of the high surface area but it is not favorable from the
because they are very popular advanced materials. To function- viewpoint of industry since it needs further costly step to recover
alize their surface with a large variety of functional groups has the photocatalyst after treatment [13]. This issue promotes the
greatly expanded their applications in biomedicine [1,2], homo- investigations on composites of magnetic nanoparticles and TiO2
geneous catalysis [3–5], wastewater processing and many other which can be effectively separated and collected in the environ-
technological fields [6,7]. Practical applications require magnetic ment of the magnetic field. Magnetic nanoparticles in previous
nanoparticles characteristic of not only the suitable size but also reports are usually focused on the spinel ferrite family, such as
the desirable magnetization, dispersivity and chemical stability [8]. Fe3 O4 [14–17], CoFe2 O4 [18,19] and NiFe2 O4 [20,21]. The direct
In regard to the photocatalytic application, titanium dioxide coating of TiO2 on the Fe3 O4 core would decrease the photocatalytic
(TiO2 ) is known to be a promising material for its photocatalytic activity because the generated electrons and holes in TiO2 particles
ability to the removal of organic pollutants [9,10], solar-fuel-based were transferred to the Fe3 O4 core rather than to the TiO2 surface
devices [11] and efficient light-driven oxidation of alcohols [12]. [9,22,23]. To prevent carriers transfer to the Fe3 O4 core, the Fe3 O4
The photocatalytic mechanism of TiO2 can be briefly summarized core was first coated by insulating SiO2 [13,23–25] or polymer [26]
as follows. Illumination of an aqueous TiO2 suspension with irra- and subsequently coated with TiO2 . The coating of insulator and
diation energy (h) greater than the band gap energy (Eg ) of TiO2 TiO2 on the magnetic core enhances the stability and photoactivity
(Eg = 3.2 eV for anatase TiO2 ) generates holes h+ and electrons e− , of photocatalysts but severely reduces the magnetization to a very
i.e. TiO2 + h → e− + h+ . The produced electrons and holes generates small value [13,19,24,27], resulting in the weak magnetic separa-
redox environment which is able to degrade pollutants present in bility. Therefore, to completely remove the catalyst particles from
aqueous medium. the treated water to prevent them from acting as contaminants
themselves, the catalyst particles require the strong magnetic core.
In the present work, first the well-dispersed uniform CoFe2 O4
∗ Corresponding author. nanoparticles were synthesized, and then they were reduced
E-mail addresses: yqma@ahu.edu.cn, yqma@issp.ac.cn (Y.Q. Ma). in a H2 /N2 gas mixture to prepare the CoFe2 alloy, a strong

http://dx.doi.org/10.1016/j.apsusc.2016.09.076
0169-4332/© 2016 Elsevier B.V. All rights reserved.
 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087 1079

Scheme 1. Schematic illustration of the synthesis process of surface functionalized magnetic particles.

magnetic material with the highest Ms value of 235 emu/g [28]. 2.2. Synthesis of CoFe2 O4 /MgO nanocomposites
From the viewpoint of the practical applications, the metals or
alloys are prone to be oxidized or corroded [29], so the CoFe2 CoFe2 O4 /MgO NPs were synthesized by the following proce-
alloy was oxidized to prepare the CoFe2 core/CoFe2 O4 shell archi- dure. Firstly, Mg(acac)2 (98%), benzyl ether (97%), oleic acid (90%)
tectures. Such the architecture possesses the strong magnetism and oleylamine (80–90%) were mixed into a 1000 mL three-necked
due to the CoFe2 core and biocompatibility due to ferrite shell, round-bottom flask by magnetic stirring under a flow of nitro-
expected to be promising materials for various bio-sensing and gen (99.999%). The mixture was heated up to 80 ◦ C and kept for
photocatalytic applications [30,31]. The reduction and oxidation 0.5 h to dissolve the mixture completely. Then, CoFe2 O4 NPs were
reactions were both carried out at high temperatures, inevitably added quickly into the flask within 2 s. The mass ratio of CoFe2 O4
resulting in the conglomeration of magnetic nanoparticles. In order to MgO was 1:1. Subsequently, the mixture was heated to 120 ◦ C
to prohibit the conglomeration, CoFe2 O4 nanoparticles were coated and kept for 0.5 h to ensure absolute ethanol or water was evap-
by the MgO matrix before reduction and oxidation. Subsequently orated. Next, the solution was heated to 298 ◦ C and kept for 1 h
MgO was able to be removed by a dilute acid wash [32]. Next, under the protection of nitrogen gas. Finally, the mixed solution
the magnetic nanoparticles, including the as-prepared CoFe2 O4 , was cooled naturally to room temperature, the precipitate was sep-
CoFe2 /MgO, CoFe2 and the oxidized CoFe2 /MgO with MgO being arated and washed with absolute ethanol several times to obtain
removed, were coated by TiO2 , and the magnetic properties, photo- the CoFe2 O4 /MgO nanocomposites.
catalytic performance and magnetic separability were investigated
on these surface functionalized magnetic nanoparticles. Interest-
ingly some functionalized particles exhibited rattle-type eccentric 2.3. Synthesis of CoFe2 /MgO nanocomposites
nanostructures without the assistance of template, and some parti-
cles exhibited solid concentric structures. The rattle-type structure To get CoFe2 /MgO, the CoFe2 O4 /MgO nanocomposite was
is beneficial for the photocatalytic performance because it pro- reduced in the H2 ambience (500 sccm, 96% N2 + 4% H2 ) for 6 h at
vides additional surface area, more active sites and lower density. 800 ◦ C.
The synthesis of rattle-type structure in previous works generally
involved templates, hazardous precursors, or dangerous etchants
2.4. Synthesis of CoFe2 /CoFe2 O4 /MgO nanocomposites
[9], which were not used in the present work.
The CoFe2 /MgO was annealed at 400 ◦ C and 900 ◦ C for 1 h to
obtain CoFe2 /CoFe2 O4 /MgO nanocomposites which have different
content of CoFe2 O4 . The sample oxidized at 400 ◦ C and 900 ◦ C are
hereafter referred to as CF/CFO1/MgO and CF/CFO2/MgO, respec-
2. Experimental section
tively.

2.1. Synthesis of CoFe2 O4 nanoparticles

2.5. Acid wash
CoFe2 O4 nanoparticles (NPs) were synthesized by the thermal
decomposition of Co(acac)2 (acac is acetylacetonate) and Fe(acac)3 In order to remove MgO from CoFe2 /MgO, CF/CFO1/MgO and
in high-boiling-point organic solvent of benzyl ether, oleic acid and CF/CFO2/MgO, 800 mg of magnetic particles and 20 mL diluted
oleylamine. The detailed process was described elsewhere [33]. hydrochloric acid were mixed for 3 min. And then washed with
1080 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087

absolute ethanol several times to obtain the CF/CFO1 and CF/CFO2

nanocomposites.

2.6. The surface functionalization of magnetic nanocomposites

The surface of CoFe2 O4 , CoFe2 /MgO, CoFe2 , CF/CFO1 and

CF/CFO2 magnetic NPS were functionalized by TiO2 . Firstly, mag-
netic NPS was mixed into isopropyl alcohol (IPA, 99.7%, SCR) and
stirred for 20 min, diethylenetriamine (DETA, 99%, Sigma-Aldrich)
was added to the mixture and stirred for 5 min. Secondly, titanium
(IV) isopropoxide (TIP, 97%, Sigma-Aldrich) was added. The reac-
tion solution was then transferred to a 50 mL Teflon-lined stainless
steel autoclave and kept at 200 ◦ C for 24 h. The autoclave cooled
naturally to room temperature. The precipitation was washed thor-
oughly with absolute ethanol several times, and dried at 40 ◦ C. All of
the powders were calcined at 400 ◦ C under the protection of nitro-
gen gas (200 sccm) for 2 h with a heating rate of 6 ◦ C/min in order
to crystallize TiO2 . The crystallization of TiO2 is essential for trans-
Fig. 1. XRD patterns for (a) as-prepared CoFe2 O4 and (b) CoFe2 O4 /TiO2 .
forming the amorphous TiO2 material into the photocatalytically
active anatase phase.
The synthesis route of all samples is shown in Scheme 1. with the concentration of 8 mg/L was mixed with 5 mg mag-
netic catalysts in a vessel. Before the irradiation, the mixed
2.7. Characterization solution was stirred in a dark condition for 60 min until an
adsorption–desorption equilibrium was established. Samples of
The crystal structure of the products was determined by X-ray solution were extracted every 10 min from the reactor and the
diffraction (XRD) using an X-ray diffractometer (XRD; DX-2000 concentration of methylene blue was analyzed by an UV–vis spec-
SSC, Dandong Fangyuan Instrument Company, DanDong, Liaon- trometer (UV-3200S, MAPADA, Shanghai, China) and calculated by
ing, China) with Cu K␣ irradiation (␭ = 1.5406 Å) in the scanning a calibration curve. At the end of photoactivity experiment, the
range 10–80◦ with a step size of 0.04◦ . Transmission electron magnetic catalysts were magnetically collected, and then washed
microscopy (TEM; JEOL JEM-2100, Tokyo, Japan) was used to with absolute ethanol and dried at 40 ◦ C for the use in the next
observe microstructure features of samples dispersed in octane. circle.
Surface morphologies were observed using a scanning electron
microscope (SEM; S-4800, Hitachi, Tokyo, Japan). Magnetic mea- 3. Results and discussion
surements were carried out using a superconducting quantum
interference device PPMS system (SQUID, PPMS EC-II, Quantum 3.1. As-prepared CoFe2 O4 and CoFe2 O4 /TiO2
Design Inc., San Diego, California, USA).The photocatalytic activ-
ity of the prepared samples was evaluated by the degradation of As shown in Fig. 1a, the XRD spectrum of the as-prepared
methylene blue, which was exposed under xenon lamp. Experi- CoFe2 O4 can be indexed to the cubic spinel structure (JCPDS Card
ments were as follows: 100 mL methylene blue aqueous solution No. 22-1086); The diffraction peaks of CoFe2 O4 are marked with

Fig. 2. (a) The TEM micrograph of as-prepared CoFe2 O4 . (b) The size histogram and Gaussian-fitting curve (solid line). TEM micrographs of CoFe2 O4 /TiO2 with low (c) and
high (d) magnification.
 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087 1081

Fig. 3. (a) M(H) loops (−4 T < H < 4 T) measured at 300 K for the samples CoFe2 O4 and CoFe2 O4 /TiO2 . (b) Variations of methylene blue concentration C/C0 with time for two
runs in the presence of CoFe2 O4 /TiO2 catalyst (inset, linearly fitting of the degradation process according to the pseudo first-order kinetic model).

stars and their indices of crystal faces are also listed in figure, Table 1
The values of adsorption efficiency (Ae ), degradation efficiency (De ), apparent
confirming that the sample is single-phase. After coating TiO2 on
reaction rate constant kapp and saturation magnetization (Ms ) of all magnetic
CoFe2 O4 , the crystallization was carried out in order to produce the photocatalysts.
photocatalytically active anatase phase TiO2 . The crystallization at
600 ◦ C yielded CoFe2 Ti3 O10 while the crystallization at 500 ◦ C gen- Ae (%) De (%) kapp Ms
(10−2 min−1 ) (emu/g)
erated the ␣-Fe2 O3 impurity phase, consistent with the previous
reports [16,19]. The presence of ␣-Fe2 O3 will reduce the magneti- CoFe2 O4 /TiO2 1st run 52 85 1.20 12
2nd run 49 83 1.18
zation due to its antiferromagnetic property, lowering the magnetic
separability. Therefore the crystallization was performed at a rel- CoFe2 /MgO/TiO2 1st run 54 86 1.28 28
atively low temperature of 400 ◦ C. The XRD pattern of the product 2nd run 62 83 1.07

after crystallizing treatment was shown in Fig. 1b. Apart from the CoFe2 /TiO2 1st run 3 93 1.47 42
diffraction peaks of CoFe2 O4 , other ones (marked with solid cir- 2nd run 3 86 1.22
cles) around 2␪ = 25.3◦ and 48.1◦ can be indexed to the anatase TiO2 CF/CFO1/TiO2 1st run 32 76 0.88 23
(JCPDS Card No. 21-1272) [19]. 2nd run 16 53 0.42
The as-prepared CoFe2 O4 sample comprises the well-dispersed CF/CFO2/TiO2 1st run 61 82 1.17 13
nanoparticles, as shown by the TEM micrograph in Fig. 2a. As shown 2nd run 31 74 0.80
in Fig. 2b, the particle size ranges from 11 to 21 nm; the average
particle size, defined as the size corresponding to the peak of the
Gaussian fitting curve, is ∼16 nm. After coating TiO2 on CoFe2 O4
nanoparticles, the typical TEM micrographs of CoFe2 O4 /TiO2 with
low and high magnifications are shown in Fig. 2c and d; CoFe2 O4
nanoparticles are dispersed in the TiO2 nanosheets, and the mor-
phology feature is similar to that observed in Fe3 O4 /TiO2 [2].
Fig. 3a shows the dependences of magnetization (M) on the
applied field (H), viz M(H) loops (−4 T < H < 4 T) of the samples
CoFe2 O4 and CoFe2 O4 /TiO2 measured at 300 K, which are char-
acteristic of ferromagnetic behavior. The values of the saturation
magnetization (Ms ) is 78 emu/g for the as-prepared CoFe2 O4 . After
coating TiO2 on CoFe2 O4 , the Ms value decreases to 12 emu/g.
In order to assess the ability of the CoFe2 O4 /TiO2 particles to be
applied as a recyclable photocatalyst for water treatment, degrada-
tion trials and a recyclability study were performed, and the results
are shown in Fig. 3b. Negative time points reflect dark exposure to
methylene blue to allow adsorption equilibrium to be reached, and Fig. 4. XRD patterns for (a) CoFe2 /MgO, (b) CoFe2 /MgO/TiO2 and (c) CoFe2 /TiO2 .
positive ones denote the concentration variation of methylene blue
under the light irradiation. The adsorption efficiency (Ae ) is calcu-
lated by (C0 − Ce )/C0 , where C0 is the initial concentration and Ce
is the adsorption–desorption equilibrium concentration of methy- The apparent reaction rate constant kapp (min−1 ) of the catal-
lene blue. The adsorption efficiency is 52% and 49% for the first ysis of methylene blue over CoFe2 O4 /TiO2 nanostructures can be
and second circle, respectively. The degradation efficiency (De ) is obtained from ln[Ce /C] = kapp t according to the pseudo first-order
expressed by (Ce − C)/Ce , where C is the concentration of methylene kinetic model [35,36]. The plots of ln[Ce /C] versus time are shown
blue at irradiation time t [34]. After 150 min under light irradiation, in the insets of Fig. 3b for two circles, indicating that the catalysis
the degradation efficiencies reach 85% and 83% for the first and sec- of methylene blue over CoFe2 O4 /TiO2 nanostructures follow the
ond circle, respectively. The Ae and De values decreases slightly for pseudo first-order kinetic model due to the fact that all calculated
the 2nd run, because, on the one hand, absorbed methylene blue on values of ln[Ce /C] are nearly linear with the reaction time. The kapp
TiO2 in the 1st run was not completely washed away, on the other value is 1.20 × 10−2 min−1 and 1.18 × 10−2 min−1 (listed in Table 1)
hand, drying of catalyst after the 1st run may decrease the surface with high correlation coefficients R2 = 0.9967 and R2 = 0.9986 for
area of TiO2 . The Ae and De values are listed in Table 1. the first and second circle, respectively.
1082 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087

Fig. 5. The TEM micrographs of (a) CoFe2 O4 /MgO, (b) CoFe2 /MgO and (c–e) CoFe2 /MgO/TiO2 . (f–h) SEM micrographs of CoFe2 /MgO/TiO2 . (i) TEM micrograph of CoFe2 /TiO2 .

3.2. CoFe2 /MgO/TiO2 and CoFe2 /TiO2 were coated with TiO2 ; TEM micrograph of CoFe2 /TiO2 in Fig. 5i
shows the solid concentric structure; The XRD profile was shown
In order to obtain the strong magnetic particles, the CoFe2 O4 in Fig. 4c, indicating that on the one hand MgO was not com-
nanoparticles were reduced to CoFe2 , an alloy magnet with high- pletely removed, on the other hand, no obvious peak from TiO2
est magnetization in all binary alloys. Before reduction, CoFe2 O4 was detected. The strong backgrounds in the 2␪ range between
nanoparticles were separated by the MgO matrix for the purpose 20◦ and 40◦ imply that TiO2 is amorphous in CoFe2 /MgO/TiO2 and
of preventing them from conglomeration; The TEM micrograph in CoFe2 /TiO2 .
Fig. 5a shows that CoFe2 O4 nanoparticles were uniformly dispersed The M(H) loops of the samples CoFe2 /MgO, CoFe2 /MgO/TiO2
in the MgO matrix. The sample after reduction is CoFe2 /MgO, and and CoFe2 /TiO2 measured at 300 K are shown in Fig. 6a. All sam-
its XRD pattern exhibits the diffraction peaks from CoFe2 (JCPDS ples show the small Hc values because CoFe2 is a typical soft
Card No. 65-4131) and MgO (JCPDS Card No. 45-0946) phases, magnetic material. In the case of the CoFe2 /MgO sample, Ms is
as shown in Fig. 4a; The reduction reaction was carried out at 99 emu/g. After coating TiO2 on CoFe2 /MgO, the Ms value decreases
800 ◦ C for 6 h, resulting in the conglomeration of nanoparticles into to 28 emu/g, while it is 42 emu/g for the CoFe2 /TiO2 sample, larger
large spherical-like particles with the diameter of several hundred than 28 emu/g, because MgO was partially removed.
nanometers, as shown by the TEM micrograph in Fig. 5b. After coat- For the CoFe2 /MgO/TiO2 and CoFe2 /TiO2 particles, tests were
ing TiO2 on CoFe2 /MgO, the obtained sample CoFe2 /MgO/TiO2 does performed to assess their photocatalytic capabilities to the methy-
not show the diffraction peaks from TiO2 , as shown in Fig. 4b; TEM lene blue, recyclability as well as their ability to be quickly
characterizations confirm that some CoFe2 /MgO particles are indi- magnetically separated. The results of these experiments can be
vidually coated by TiO2 , forming the hollow eccentric [10] (Fig. 5c) seen in Fig. 6b–d. In the case of CoFe2 /MgO/TiO2 , the Ae value is
and solid concentric (Fig. 5d) structures, and some CoFe2 /MgO par- 54% and 62% for the first and second circle, respectively; The De
ticles are agglomerated and then coated by TiO2 , as seen in Fig. 5e; value after 150 min under light irradiation reaches 86% and 83%
SEM characterizations in Fig. 5f and g further confirm the micro- for the first and second circle, respectively. The apparent reaction
sphere shape of the particles and the meso-/macroporous TiO2 rate constant kapp obtained by the fitting of the pseudo first-order
shells; The SEM micrograph (Fig. 5h) with higher magnification kinetic model is 1.28 × 10−2 min−1 and 1.07 × 10−2 min−1 with high
shows the flowerlike porous structure of TiO2 shell is assembled by correlation coefficients R2 = 0.9856 and R2 = 0.9705 for the first and
nanosheet petals, similar to that previously reported in Fe3 O4 /TiO2 second circle, respectively.
[37]. In order to remove MgO in CoFe2 /MgO, some CoFe2 /MgO parti- In the case of CoFe2 /TiO2 , the adsorption efficiency is 3%
cles were dissolved by the HCl solution, and the obtained particles for two circles; The degradation efficiency after 150 min under
 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087 1083

Fig. 6. (a) M(H) loops measured at 300 K for samples CoFe2 /MgO, CoFe2 /MgO/TiO2 and CoFe2 /TiO2 . Variations of methylene blue concentration C/C0 with time for two runs
in the presence of (b) CoFe2 /MgO/TiO2 (inset, linearly fitting of the degradation process according to the pseudo first-order kinetic model) and (c) CoFe2 /TiO2 catalysts. (d)
Linearly fitting of the degradation process according to the pseudo first-order kinetic model for CoFe2 /TiO2 .

Fig. 7. (a) The mass gain of CoFe2 /MgO in the process of oxidation at different temperatures. (b) XRD patterns for CF/CFO1/MgO, CF/CFO1 and CF/CFO1/TiO2 .

light irradiation reaches 93% and 86% for the first and second CoFe2 was not completely oxidized at 400 ◦ C. After the acid wash,
circle, respectively. The apparent reaction rate constant kapp is the diffraction intensity of MgO in the CF/CFO1 sample weakens
1.47 × 10−2 min−1 and 1.22 × 10−2 min−1 with high correlation obviously. For the TiO2 coated CF/CFO1 sample, the diffraction peak
coefficients R2 = 0.9692 and R2 = 0.9828 for the first and second cir- from TiO2 can be detected in the CF/CFO1/TiO2 sample.
cle, respectively. The Ae , De and kapp values are also listed in Table 1. The TEM micrographs of CF/CFO1/TiO2 and CF/CFO2/TiO2 are
shown in Fig. 8. CF/CFO1/TiO2 comprises of hollow eccentric
3.3. CoFe2 /CoFe2 O4 /TiO2 (Fig. 8a) and solid concentric (Fig. 8b) particles, while almost all
of particles in CF/CFO2/TiO2 exhibit the hollow eccentric struc-
CoFe2 /MgO NPs (0.8 g) was annealed in air at temperatures of ture. The solid concentric structure is similar to that in Fe3 O4 @TiO2
200, 300, 400, 500, 700 and 900 ◦ C to prepare CoFe2 /CoFe2 O4 /MgO. reported by Tan et al. [38]. The hollow eccentric structure is simi-
With increasing the annealing temperature, the mass of CoFe2 /MgO lar to the yolk–shell or rattle-type nanostructure, which has been
increases because CoFe2 receives oxygen during the oxidation intensively studied owing to their large specific area, low den-
reaction, CoFe2 + 2O2 = CoFe2 O4 . As shown in Fig. 7a, the mass sity, and big void space. The void space within the shell provides
gain due to oxidation rapidly increases as the annealing temper- a unique space for the core material in confined catalysis, drug
ature increases above 300 ◦ C, and tends to saturation above 700 ◦ C release, nanoscale reactors, and lithium-ion batteries [9,39–43].
because CoFe2 was completely oxidized. Generally synthesis of rattle-type nanostructures involves the use
The annealed CoFe2 /MgO samples at 400 ◦ C and 900 ◦ C, i.e. of an organic or inorganic template, hazardous precursors, or
CF/CFO1/MgO and CF/CFO2/MgO were dispersed in the HCl solution dangerous etchants [9,39,40]. However in the present work the
to remove MgO, and then coated by TiO2 to prepare CF/CFO1/TiO2 rattle-type nanostructures were prepared in the absence of any
and CF/CFO2/TiO2 . As shown in Fig. 7b, CF/CFO1/MgO exhibits the templates and etchants. The morphology of TiO2 may be deter-
diffractions peaks from CoFe2 , CoFe2 O4 and MgO, indicating that
1084 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087

Fig. 8. The TEM micrographs with several typical morphology characteristics for CF/CFO1/TiO2 (a and b) and CF/CFO2/TiO2 (c and d).

Fig. 9. M(H) loops measured at 300 K for samples CF/CFO1 and CF/CFO1/TiO2 (a), CF/CFO2 and CF/CFO2/TiO2 (b). Variations of methylene blue concentration C/C0 with time
for two runs in the presence of (c) CF/CFO1/TiO2 and (d) CF/CFO2/TiO2 catalysts (insets, linearly fitting of the degradation process according to the pseudo first-order kinetic
model).

mined by the growth rate and the deposition rate of TiO2 on the For the CF/CFO1/TiO2 and CF/CFO2/TiO2 particles, their pho-
magnetic core, which depend on the core size and the composition tocatalytic capabilities, recyclability as well as their magnetic
of material at the surface of magnetic core. separability are investigated. From the results in Fig. 9c and d, the
The room temperature M(H) loops are shown in Fig. 9 for Ae , De and kapp values can be obtained; They, together with those
CF/CFO1 and CF/CFO1/TiO2 (a), CF/CFO2 and CF/CFO2/TiO2 (b). The for the samples of CoFe2 O4 /TiO2 , CoFe2 /MgO/TiO2 and CoFe2 /TiO2
Ms value is 67 emu/g for CF/CFO1 and 23 emu/g for CF/CFO1/TiO2 ; are listed in Table 1.
38 emu/g for CF/CFO2 and 13 emu/g for CF/CFO2/TiO2 .
 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087 1085

Table 2
The values of degradation efficiency (De ) and apparent reaction rate constant kapp
at different solution temperatures for magnetic photocatalysts CoFe2 O4 /TiO2 and
CoFe2 /MgO/TiO2 .

30 ◦ C 40 ◦ C 50 ◦ C 60 ◦ C 70 ◦ C

CoFe2 O4 /TiO2 De (%) 87 87 80 79 69

kapp 1.38 1.32 1.03 1.03 0.78
(10−2 min−1 )

CoFe2 /MgO/TiO2 De (%) 87 84 70 74 80

kapp 1.30 1.19 0.78 0.87 1.05
(10−2 min−1 )

and O2 −• + methylene blue → CO2 + H2 O at the interface of TiO2 and

CoFe2 .
In the cases of CF/CFO1/TiO2 and CF/CFO2/TiO2 , the
CF/CFO2/TiO2 sample has the higher photocatalytic performance,
most likely due to more rattle-type particles in CF/CFO2/TiO2 .
As to the CF/CFO1/TiO2 sample, its degradation efficiency and
apparent reaction rate constant are lowest among all samples,
and the possible reason is suggested as follows: CF/CFO1 contains
CoFe2 , CoFe2 O4 and MgO as seen from the XRD results in Fig. 7b,
Scheme 2. Schematic illustration of the photocatalytic degradation process of and therefore the interface between CF/CFO1 and TiO2 becomes
methylene blue for the CoFe2 /TiO2 catalyst. complex, as shown in Scheme 1, which could induce the surface
states in the band gap of TiO2 , and some surface states may act
as the recombination centers, reducing the lifetime of photo-
CoFe2 O4 /TiO2 exhibited high adsorption and degradation effi- generated carriers and consequently weakening the photocatalytic
ciencies as well as the apparent reaction rate constant, as a result performance. Furthermore, it can be seen in Table 1 that the
of the nanosheet morphology of TiO2 , because nanosheet has the samples which contains the CoFe2 component has the larger Ms
thin thickness and high surface-to-volume ratio [44]. values, beneficial for the recovery of catalysts after photocatalytic
For CoFe2 /MgO/TiO2 and CoFe2 /TiO2 , the former has much degradation. This can be confirmed by the photos in Fig. 10:
larger adsorption efficiency than the latter, attributable to the After the photocatalytic trials, aqueous solution containing the
nanosheet-shaped TiO2 and the hollow rattle-type nanostructure particles was transferred to a vial and placed next to a magnet.
in CoFe2 /MgO/TiO2 as seen in Fig. 5. Specially, the hollow rattle- Complete separation of the CoFe2 /TiO2 particles (Ms = 42 emu/g)
type structure provides the catalyst particles with an additional from solution is achieved within 1 min after the vial was placed
surface area, giving the catalyst excellent adsorption properties adjacent to the magnet (Fig. 10a), while it shows the residual
[9]. It can be noticed from Table 1 that the degradation efficiency CF/CFO2/TiO2 particles (Ms = 13 emu/g) after 10 min (Fig. 10b).
of CoFe2 /TiO2 reaches 93%, the highest value among all samples;
The possible reason for this can be understood according to the 3.4. Temperature effect on the photocatalytic performance
Scheme 2. The light irradiation generates electrons e− and holes
h+ in TiO2 ; Electrons are captured by CoFe2 , while the holes are For organic substances degradation, the optimal tempera-
transferred to the exterior surface of TiO2 , which inhibits the ture generally lies between 20 and 80 ◦ C [45,46]. Temperature
recombination of photo-generated electrons and holes; The radical effect on the photocatalytic performance was carried out for the
O2 −• is generated via the reduction of adsorbed O2 on the sur- CoFe2O4/TiO2 and CoFe2/MgO/TiO2 at the solution temperatures
face of catalysts by electrons, i.e. e− + O2 → O2 −• , and the radical of 30, 40, 50, 60 and 70 ◦ C, and the results are shown in Fig. 11a
of OH• is produced by the reaction of holes and water (or hydroxyl and b. The De and kapp values are listed in Table 2. With increasing
ions), i.e. h+ + H2 O → OH• + H+ [36]. The radicals O2 −• and OH• are temperature from 30 ◦ C, the De and kapp values tend to decrease
responsible for the methylene blue degradation due to the reactions for CoFe2O4/TiO2, while they first decrease as temperature
of OH• + methylene blue → CO2 + H2 O at the outer surface of TiO2 increases to 50 ◦ C and then increase. The curves of lnkapp versus

Fig. 10. Photos of magnetic separation of (a) CoFe2 /TiO2 and (b) CF/CFO2/TiO2 catalysts.
1086 M. Wang et al. / Applied Surface Science 392 (2017) 1078–1087

Fig. 11. Variations of methylene blue concentration C/Ce with time measured at different solution temperatures for magnetic photocatalysts CoFe2 O4 /TiO2 (a) and
CoFe2 /MgO/TiO2 (b); the curves of lnkapp versus 1000/solution temperature T for CoFe2 O4 /TiO2 (c) and CoFe2 /MgO/TiO2 (d).

1000/solution temperature T (Arrhenius plots) [47] are shown in netic field, may be potential applications in other fields, such as the
Fig. 11c for CoFe2O4/TiO2 and in Fig. 11d for CoFe2/MgO/TiO2, cell isolation and targeting drug.
which do not show the Arrhenius dependence as observed in the
photodetoxification of salicylic acid [48]. The temperature effect in Conflict of interest
the present work may be attributable to the synergetic effects of
adsorption, desorption and surface migration [45,46]. The authors declare no competing financial interest.

4. Conclusions Acknowledgment

The goal of this work is to build magnetic nanoparticles This work was supported by the National Natural Science Foun-
with high magnetization, good chemical stability and dispersivity, dation of China (Grant Nos. 11174004 and 51471001).
expanding their practical applications. The CoFe2 alloy was cho-
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