Solvothermal synthesis of octahedral and magnetic CoFe2O4-reduced graphene oxide hybrids and their photo-Fenton-like behavior under visible-light irradiation

Article abstract of DOI:10.1039/d1ra03103a

We report a facile solvothermal synthesis of novel octahedral CoFe₂O₄-reduced graphene oxide (RGO) hybrid and pure CoFe₂O₄that were used as heterogeneous photo-Fenton catalysts for the degradation of organic dyes in water. We investigated the structures, morphologies and catalytic activity of both the CoFe₂O₄nanoparticles and CoFe₂O₄-RGO hybrids. The morphology of CoFe₂O₄nanoparticles displays size-dependent shapes changing from granular (or sheet) to octahedral shapes with the introduction of RGO. Compared with bare CoFe₂O₄, the octahedral CoFe₂O₄-RGO hybrids serve as novel bifunctional materials displaying higher saturation magnetization values and excellent heterogeneous activation of H₂O₂at nearly neutral pH. The high saturation magnetization (41.98 emu g^?1) of CoFe₂O₄-RGO hybrids aids their separation from the reaction mixture. In addition, the remarkable enhancement in the photo-Fenton activity of the CoFe₂O₄-RGO hybrids under visible light irradiation was attributed to the graphene/CoFe₂O₄heterojunction, which aided the separation of excited electrons and holes. Furthermore, the CoFe₂O₄-RGO hybrids exhibited better removal efficiency for cationic methylene blue (MB) dye than for anionic methyl orange (MO) dye. Meanwhile, the CoFe₂O₄-RGO hybrids displayed acceptable photocatalytic stability, and we proposed an activation mechanism of H₂O₂by the octahedral CoFe₂O₄-RGO hybrids.

Full text of DOI:10.1039/d1ra03103a

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Solvothermal synthesis of octahedral and magnetic
CoFe O –reduced graphene oxide hybrids and
2
4
Cite this: RSC Adv., 2021, 11, 22250
their photo-Fenton-like behavior under visible-
light irradiation†
Liling Hu, Meng Li, Lihong Cheng, Bofan Jiang and Jianping Ai *a
ab
a
a
a
We report a facile solvothermal synthesis of novel octahedral CoFe
hybrid and pure CoFe that were used as heterogeneous photo-Fenton catalysts for the degradation
of organic dyes in water. We investigated the structures, morphologies and catalytic activity of both the
CoFe nanoparticles and CoFe –RGO hybrids. The morphology of CoFe nanoparticles displays
size-dependent shapes changing from granular (or sheet) to octahedral shapes with the introduction of
RGO. Compared with bare CoFe , the octahedral CoFe –RGO hybrids serve as novel bifunctional
materials displaying higher saturation magnetization values and excellent heterogeneous activation of
2 4
O –reduced graphene oxide (RGO)
2 4
O
O
2 4
O
2 4
2 4
O
2
O
4
2 4
O
ꢀ1
H
2 2 2 4
O at nearly neutral pH. The high saturation magnetization (41.98 emu g ) of CoFe O –RGO hybrids
aids their separation from the reaction mixture. In addition, the remarkable enhancement in the photo-
Fenton activity of the CoFe –RGO hybrids under visible light irradiation was attributed to the
graphene/CoFe heterojunction, which aided the separation of excited electrons and holes.
Furthermore, the CoFe –RGO hybrids exhibited better removal efficiency for cationic methylene blue
MB) dye than for anionic methyl orange (MO) dye. Meanwhile, the CoFe –RGO hybrids displayed
acceptable photocatalytic stability, and we proposed an activation mechanism of H by the octahedral
CoFe –RGO hybrids.
2 4
O
2
O
4
Received 21st April 2021
Accepted 11th June 2021
2 4
O
(
2 4
O
DOI: 10.1039/d1ra03103a
2 2
O
rsc.li/rsc-advances
2 4
O
9
during the whole purication process. Furthermore, the
magnetic NPs can enhance the functionalities of graphene-
1
. Introduction
It is well-known that the inappropriate disposal of organic based hybrids; therefore, the potential of magnetic NPs-
1
,2
effluents from industries raises environmental concerns.
graphene hybrids is huge, and they can be widely applied in
10
Among many methods used for the treatment of industrial catalysis and water purication. In view of these, several
effluents, graphene-based adsorption is a highly efficient and studies have been conducted to investigate the adsorption and
less time-consuming method owing to its exceptionally large photocatalytic performance of graphene-based hybrids con-
surface area, open porous structure, and excellent chemical taining magnetic NPs. Yao et al. fabricated magnetic Fe -
3
stability. However, the separation and recycling of graphene
11
3–5
O @graphene hybrids. The methylene blue (MB) and Congo red
4
used for purication of industrial wastewater remain a chal- (CR) absorption capacities of these hybrids good and separation
6
lenge, and this limits its further application.
of the hybrids aer absorption was easy. Fu et al. synthesized
set of graphene-based nanocomposite photocatalysts
–graphene (M ¼ Zn, Ni, Mn, Co) hybrids.
Nowadays, the integration of magnetic particle technology to
solve water pollution problems has attracted the interest of including MFe O
2 4
many researchers. Firstly, the dispersion of magnetic nano- These photocatalysts show high visible-light-responsive photo-
particles (NPs) on graphene sheets enables the development of degradation efficiency towards dyes. They were also stable and
materials with promising separability and reusability. This is the photocatalyst can be conveniently separated by the use of an
a
7,8
12–15
because the graphene-based hybrids containing magnetic NPs external magnetic eld.
do not introduce foreign pollutants into the wastewater system
Among these magnetic NPs, cobalt ferrite (CoFe O ) nano-
2 4
crystals with a spinel structure have attracted a great deal of
attention for environmental remediation owing to their
outstanding properties, such as their size- and shape-
a
Jiangxi Key Laboratory of Surface Engineering, Jiangxi Science & Technology Normal
University, Nanchang 330013, P. R. China. E-mail: ai861027@163.com
b
Dean of Students Office, Jiangxi Science & Technology Normal University, Nanchang dependent magnetic behavior, and their relatively lower
1
6–18
3
†
1
30013, P. R. China
bandgap that perfectly matches with the solar spectrum.
date, several studies have been conducted using CoFe
To
as
Electronic supplementary information (ESI) available. See DOI:
0.1039/d1ra03103a
2 4
O
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1
9,20
heterogeneous catalysts for activating hydrogen peroxide
peroxymonosulfate (PMS).
or
2
. Experimental
21–23
When heterogeneous PMS is
2
.1. Chemicals and reagents
activated for the degradation of dyes, the reaction mechanism is
represented by the following equations:
19,23–26
Cobaltous nitrate hexahydrate (Co(NO ) $6H O, 99%), ferric
3
2
2
3 3 2
nitrate nonahydrate (Fe(NO ) $9H O, 99.9%), isopropanol (IPA,
+
CBꢀ
CoFe
2
O
4
+ hv / CoFe
2
O
4
(hVB + e
)
(1)
(2)
(3)
(4)
AR), methylene blue (MB, AR), and methyl orange (MO, AR) were
obtained from Sinopharm chemical reagent Co., Ltd (Shanghai,
China). Hydrogen peroxide (H O , 30%), ethanol, and sodium
ꢀ
ꢀ
ꢀ
ꢀ
CoFe O (e
2
) + HSO5 / CoFe O + OH + SO4c
4
CB
2
4
2
2
hydroxide (NaOH, AR) were purchased from Xilong Chemical
Co., Ltd (Shantou, China). A graphene suspension (thickness,
+
ꢀ
CoFe
2
O
4
(hVB ) + OH / CoFe
2
O
4
+ cOH
ꢀ1
2
+
5ꢀ
3+
ꢀ
ꢀ
ꢁ0.8 nm; 1 mg mL ) was supplied by XFNANO Materials Tech
Co + HSO / Co + SO
4
c
+ OH
Co. Ltd (Nanjing, China). Deionized water was supplied by
3
+
ꢀ
2+
Fe + hv + OH / Fe + cOH
(5) a Milli-Q ultraltration system (Billerica, USA).
2
+
5ꢀ
3+
ꢀ
ꢀ
+ OH
Fe + HSO / Fe + SO
4
c
(6)
2.2. Synthesis of materials
ꢀ
The reduced graphene oxide loading in the CoFe O –RGO
2
4
Dye + SO4c (and cOH) / intermediates / CO + H O (7)
2
2
hybrids was adjusted by changing the initial volume (x) of gra-
phene suspension. And the sample was denoted as CoFe O –
Interestingly, CoFe
2
O
4
possesses obscure Co leaching prop-
2 4
RGO(x) (where x ¼ 35 and 70 mL). As a typical procedure, 35 mL
of the graphene suspension was dissolved in 265 mL of ethanol
while it constantly stirred using a magnetic stirrer, and it was
then sonicated for 30 min. Secondly, 4.3654 g of cobaltous
nitrate hexahydrate and 12.1164 g of ferric nitrate nonahydrate
were directly added to the above graphene solution while it was
erties owning to its strong Fe–Co linkages. Moreover, Fe helps to
enrich the hydroxyl group content on the surface of Fe–Co
catalysts, and CoFe O are magnetically recyclable owing to the
2 4
unique ferromagnetic properties. A number of studies have
shown that when immobilization of CoFe O nanoparticles
2
4
onto graphene sheets, the Fenton-like catalysis efficiency of the
2
+
20
vigorously stirred for 30 min. The concentration of Co and
resultant hybrid catalyst improved remarkably. Moitra et al.
used CoFe –reduced graphene oxide (RGO) nanocomposites
for heterogeneously activating The CoFe –RGO
exhibited good visible-light-responsive photocatalytic proper-
3
+
Fe in the resulting solution (marked as solution A) was
2 4
O
ꢀ
1
ꢀ1
0
.05 mol L and 0.1 mol L , respectively. Subsequently, a 4 M
H
2
O
2
.
2 4
O
NaOH solution was added dropwise to a uniform solution A
until a pH of ꢁ13 was attained, and this resulted in complete
precipitation of the metal cations. The resultant mixture was
then transferred into four Teon-lined stainless-steel autoclaves
9
27
ties for the degradation of various synthetic dyes. Lu and He
et al. have studied the effects of RGO contents, initial solution
pH, H O concentration and operation parameters (e.g. dosage
2
2
(
volume: 100 mL, degree of lling: ꢁ80%), and heated in an
and dye concentration) on the catalytic activity. Their results
showed that the incorporation of the graphene signicantly
ꢂ
oven at 180 C for 20 h. The sample was allowed to naturally cool
down to room temperature. The products were centrifuged and
directly collected, then they were severally washed with deion-
enhanced the photocatalytic performance of the CoFe
which the graphene acted as a charge carrier to capture the
delocalized electrons. In addition, the CoFe –graphene
2 4
O in
ꢂ
ized water and ethanol and dried at 60 C for 24 h. For
2 4
O
2 4
comparison, pure CoFe O NPs were also fabricated following
a similar solvothermal process without the introduction of
graphene.
hybrids exhibited good reusability and were easily separated
from the reaction mixture. To the best of our knowledge, the
effect of particle size and shape of CoFe O on catalytic activity
2
4
and magnetic properties of CoFe O /graphene nanocomposites,
2
4
has been rarely studied. Moreover, there are few reports on the 2.3. Characterization
relationship between the adsorption behavior and surface
The X-ray diffraction (XRD) diffractograms were obtained using
a D/Max-2550 diffractometer with Cu Ka radiation source (l ¼
charge of CoFe
very few reports about the underlying growth mechanism of
octahedral CoFe on graphene sheets are available.
2 4
In the present study, we synthesized octahedral CoFe O –
2 4
O /graphene nanocomposites. Furthermore,
0
.15406 nm). The Raman spectra were obtained using
2 4
O
a HORIBA JY LabRAM HR evolution microspectrometer at an
excitation wavelength of 532 nm. The Fourier transform
infrared spectroscopy (FT-IR) spectra were recorded using
a Spectrum Two FT-IR spectrophotometer with samples pressed
in KBr. The exact amount of RGO in the as-obtained products
was measured using a thermal gravimetric analyzer (TGA/DSC
RGO hybrids via interface engineering. These hybrids served as
excellent heterogeneous catalysts for activating H O at nearly
2
2
2 4
neutral pH. In addition, the catalytic performance of CoFe O –
RGO hybrids was evaluated for the degradation of diverse dyes,
i.e. MB and methyl orange (MO), under visible-light irradiation.
Also, we have presented the underlying photo-Fenton-like
behavior and have investigated the remarkable effect of
particle size and shape of CoFe O NPs in CoFe O –RGO
2 4 2 4
hybrids to promote the Fenton chemistry and tuning of the
magnetic property.
3+, Mettler-toledo, Switzerland). The UV-visible diffuse reec-
tance spectra (DRS) were obtained using a UV-vis spectropho-
tometer (UV-2550, Shimadzu, Japan). The surface morphologies
of products were observed using a scanning electron micros-
copy (GeminiSEM 500, Zeiss, Germany). The X-ray photoelec-
tron spectroscopy (XPS) was carried out using a ThermoFisher
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28
Scientic ESCALAB250Xi photoelectron spectrometer using 1086). Moreover, the observed diffraction peaks were sharp,
with Al X-ray source. All binding energy values were calibrated implying that the as-prepared CoFe O particles were highly
2
4
using the carbon (C 1s) peak observed at 284.0 eV. The specic crystalline. Peaks attributed to impurities were not observed on
surface area and the pore-size distribution were measured using the XRD diffractogram, indicating that the CoFe O samples
2
4
Autosorb-iQ2 (Quantachrome, USA) based on the nitrogen were highly pure. In addition, the characteristic peak that was
adsorption–desorption isotherms. The hysteresis loops were observed at 2q ¼ 26.5 corresponding to RGO (002) was not
2 4
determined using a vibrating sample magnetometer (PPMS detected in the XRD diffractogram of the CoFe O –RGO
29
DynaCool Cryogen-free System 9 T, Quantum Design, USA). The hybrids. This may be ascribed to the proper separation and
zeta potential measurements were conducted on a zeta poten- decoration of the RGO sheets by excess CoFe nanoclusters,
2 4
O
tial analyzer JS94K2 (Shanghai, China).
resulting in a low diffraction intensity of graphene in the XRD
diffractogram of the CoFe O –RGO hybrids.
2
4
2
.4. Catalytic test procedure
Fig. 1(b) shows the Raman spectra of bare CoFe
CoFe –RGO hybrids. Three major Raman scattering peaks
can be observed at about 312, 468, and 687 cm , which are
attributed to the specic characteristics (A1g, E , and 3T2g) of
–RGO hybrids and pristine CoFe
2 4
O and the
2 4
O
The photocatalytic activities of the as-obtained samples were
measured using the photocatalytic degradation of water-soluble
dyes (MO or MB) under visible light irradiation together with
ꢀ
1
g
30
CoFe
2
O
4
ferrite. CoFe
2
O
4
2 4
O
2 2
H O . A 500 W xenon lamp (cut-off wavelength 420 nm) was
ꢀ
1
show similar peaks in the range of 250 to 750 cm . In addition,
used as the visible-light source in the photo-Fenton reaction.
The distance between the surface of the reaction solution and
the visible-light source was approximately 11 cm. The experi-
ment was conducted as follows: 60 mg of the photocatalyst was
the CoFe O –RGO hybrids exhibit two Raman peaks at
2
4
ꢀ
1
approximately 1347 (D band) and 1589 cm (G band), where
1
7,31
the D and G bands correspond to the RGO.
indicate that graphene has been incorporated into the
CoFe –RGO hybrids.
The hybridization of the CoFe
veried by observing the FT-IR spectra (shown in Fig. 1(c)). For
These results
ꢀ
1
dispersed into 60 mL of the 10 mg mL dye aqueous solution
MO or MB) while it was constantly stirred. Before irradiation,
2 4
O
(
2 4
O NPs with RGO was further
the solution was stirred using a magnetic stirrer in a dark room
for 1.5 h to ensure the establishment of an adsorption/
desorption equilibrium. Next, a certain amount of 30% H O
2 2
was introduced into the above resultant mixture, which was
then exposed to the visible-light source while the mixture was
continuously stirred. The concentrations of MO and MB in the
reacting mixtures were monitored by measuring the character-
istic absorbance at wavelengths of 464 and 664 nm, respectively.
The removal rate (rev (%)) of the dyes (MO and MB) was
determined using eqn (8):
ꢀ
1
2 4
the CoFe O sample, the peaks at approximately 3446 cm and
ꢀ
1
1
628 cm were respectively ascribed to the stretching and
bending vibrations of O–H in the presence of water molecules
2
9,32
absorbed on the surface.
In addition, tetrahedral Co–O
ꢀ1
bonds were observed at approximately 593 cm , and the
octahedral Fe–O stretching vibrations were observed at
ꢀ
1
16,21
approximately 458 cm
CoFe , some new peaks were observed in the CoFe
hybrids. The band at approximate 1630 cm may correspond to
.
When compared with the bare
2
O
4
2 4
O
–RGO
ꢀ
1
33
the C]C skeletal vibration of graphitic domains of RGO.
Furthermore, the peaks observed at 1398 and 1594 cm can be
Removal rate (%) ¼ (A
0
ꢀ A
t
)/A
0
ꢃ 100%
ꢀ
1
¼
(C ꢀ C )/C ꢃ 100%
(8)
0
t
0
attributed to the deformation vibrations of the tertiary C–OH
where A
t
is the absorbance of dye solution at t time, A
0
is the groups and the unoxidized carbon backbone on the surface of
34–36
initial absorbance of dye solution, C
0
is the initial concentration RGO, respectively.
Therefore, we can infer that the target
of aqueous dye, and C
aqueous dye at t time.
t
represents the concentration of the CoFe
TG–DSC analysis was carried out under air atmosphere to
–RGO
, one obvious weight
2 4
O –RGO hybrids had been synthesized.
2 4
A trapping experiment, which is similar to the photo- conrm the amount of RGO present in the CoFe O
degradation procedure, was conducted to identify the dominant hybrids (Fig. 1(d)–(f)). For pure CoFe
O
2 4
ꢂ
reactive species upon the addition of 0.1 M IPA. Moreover, the loss region was observed at 90–110 C, which was attributed to
photocatalytic stability of CoFe O –RGO hybrids was investi- the desorption of adsorbed water. Meanwhile, the heat
2
4
ꢂ
gated via recycling tests. Aer each cycle, the catalyst was absorption peak at 100 C can be observed on the DSC curve of
separated from the reaction solution using an external magnet, the pure CoFe
and it was washed several times with deionized water and absorption region was observed at 800–900 C, which may be
2
O
4
sample. Furthermore, another obvious heat
ꢂ
ethanol. Then the same volume of fresh MB solution was added ascribed to intrinsic magnetic transformation of CoFe
for the next cycle.
this variation of magnetic structure does not lead to obvious
weight loss. However, two obvious weight loss steps were
2 4
O . But
ꢂ
ꢂ
observed at 90–110 C and 215–460 C for the CoFe O –RGO
2
4
3
. Results and discussion
hybrids, which were ascribed to the loss of adsorbed water and
the burnt-out of RGO, respectively. On the basis of the present
3.1. Characterization and the synthesis mechanism
2
4
2 4
2 4
The XRD diffractograms of the bare CoFe O and CoFe O –RGO TG–DSC analysis results, the RGO content in the CoFe O –
hybrids are shown in Fig. 1(a). The XRD diffractograms of RGO(35) and CoFe
CoFe –RGO hybrids are identical to that of the bare CoFe 3.80 wt%, respectively. Moreover, the UV-vis DRS of pure
which matches well with cubic spinel-type CoFe (JCPDS 22- CoFe and CoFe –RGO are shown in Fig. S1.† The pure
2
O
4
–RGO(70) samples was ꢁ1.97 and
2
O
4
2 4
O ,
2
O
4
2
O
4
2 4
O
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Fig. 1 The crystal structure of the as-synthesized pure CoFe
2
O
4
and the CoFe
2
O
4
–RGO hybrids with different amounts of graphene: (a) XRD
diffractograms, (b) Raman spectra, (c) FTIR spectra, and (d–f) TG–DSC curves.
2 4 2 4
CoFe O and CoFe O –RGO hybrids all exhibit strong absorp- this resulted in the formation of a large amount of void space
tion in the range of 350–800 nm, which may be due to their (Fig. 2(a)).
black color and narrow bandgap, implying that these materials
can act as excellent visible-light-responsive photocatalysts.
Furthermore, the SEM micrographs of CoFe
hybrids contained three distinct morphological features: (i)
2 4
O –RGO
To investigate the morphology and composition of the as- intimate interfacial connections exist between the individual
fabricated products, the eld emission scanning electron CoFe O and the RGO nanosheets, (ii) the CoFe O particles
2
4
2 4
microscope (FESEM) and the elemental mapping of the pure were mainly composed of regular octahedrons and irregular
CoFe and CoFe –RGO hybrids were obtained. Two subunit particles with a broad size distribution. The edge
morphological features were observed on the FESEM of the bare lengths of octahedrons are in the range of 60 to 480 nm, while
CoFe NPs, i.e. granular and sheet-like, and the individual the sizes of the irregular subunit particles are in the range of 10
pure CoFe NPs were withing a size range of 4–98 nm; to 40 nm, (iii) part of the irregular subunit particles were
however, aggregation of some granular particles occurred, and dispersed on the octahedral CoFe and the wrinkled
2
O
4
2 4
O
2 4
O
2 4
O
2 4
O
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Fig. 2 FESEM micrographs of (a) pure CoFe
O
2 4
, CoFe O
2 4
–RGO(35) hybrid ((b) low-magnification, (h) high-magnification), and (i) CoFe
2 4
O –
RGO(70) hybrid. The low-magnification SEM micrograph (c), and the corresponding EDX mapping micrographs of CoFe
d) C, (e) Co, (f) Fe, and (g) O.
2 4
O
–RGO(35) hybrid for
(
graphene planes, the other part of subunit particles agglomer-
ated to form nanoclusters (Fig. 2(b, h, and i)).
To further understand the synergistic effects of the CoFe O –
2 4
RGO hybrid in photo-Fenton behavior, XPS was used to deter-
Compared with the pristine CoFe
2
O
4
, the average particle mine the chemical state of the corresponding transition metal
–RGO hybrids was large than that and oxygen in the samples. From the survey spectrum in
owing to the introduction of graphene sheets. Fig. 3(a), Fe, Co, O, and C were present in the sample, and no
particles in the CoFe other impurity was detected in the CoFe –RGO hybrid. The
RGO hybrids were different from that of the pristine CoFe high-resolution Fe 2p spectra of the pure CoFe and
i.e. it contained octahedrons and irregular subunit particles. CoFe O –RGO(35) are shown in Fig. 3(b) and (c). For the
size of CoFe
2
O
4
in the CoFe
2
O
4
of pure CoFe
2
O
4
Also, the morphologies of the CoFe
2
O
4
2
O
4
–
2 4
O
2
O
4
,
2 4
O
2
4
This indicates that the hierarchical exible graphene sheets CoFe O –RGO(35) sample, the doublets binding energies of Fe
2
4
facilitate the quick growth and Ostwald Ripening of the 2p3/2 at 710.3 eV and Fe 2p
at 723.3 eV are due to the
1/2
3
+
CoFe
mapping micrographs of CoFe
showed that Co, Fe, and O elements were uniformly distributed Fe 2p3/2 at 712.7 eV and Fe 2p1/2 at 724.9 eV are due to the
2
O
4
nanocrystals. In addition, the corresponding EDX contributions from Fe ions in the octahedral sites (A-site) of
2
O
4
–RGO(35) hybrid clearly the spinel-type CoFe , while the doublets binding energies of
2 4
O
3
+
38
in the CoFe
2
O
4
particles, implying the formation of bimetallic contributions from Fe ions in the tetrahedral sites (B-site).
–RGO(35) hybrid is nearly
oxides but not the formation of simple mixtures of the two The Fe 2p spectrum of the CoFe
corresponding single-metal oxides. Also, it can be seen that identical to that of the high-resolution Fe 2p spectrum of the
2 4
O
37
the graphene sheets signicantly aggregated owing to surface pristine CoFe O (Fig. 3(b)), except for a negative shi in the
2
4
effect that may result in a low specic surface area of the binding energy (Fig. S2†). Also, the high-resolution Co 2p
CoFe –RGO(35) hybrid. spectrum of CoFe –RGO(35) hybrid is nearly identical to that
2
O
4
2 4
O
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Fig. 3 (a) A survey spectrum of the XPS of CoFe
2
O
4
–RGO(35) hybrid, (b) high-resolution Fe 2p XPS analysis of the pure CoFe
2 4
O and the
2 4 2 4
CoFe O –RGO(35), and high-resolution XPS spectra of (c) Fe 2p, (d) Co 2p, (e) C 1s, and (f) O 1s of CoFe O –RGO(35).
of the pure CoFe
energy. These results indicate that efficient charge transfer tions from Co ions in tetrahedral sites (B-site). Compared with
between the CoFe NPs and graphene sheets occurred owing the Fe 2p spectrum, there are two obvious shake-up satellite
to the intimate interfacial connections, which may be benecial peaks at 785.8 eV and 802.2 eV in the Co 2p spectrum. The
2 4
O , except for a negative shi in the binding 781.2 eV and Co 2p1/2 at 795.7 eV are ascribed to the contribu-
2
+
2 4
O
3
9
for the separation of charge carriers.
presence of these intense satellite signals indicates that a large
2
+
The high-resolution Co 2p spectrum of CoFe O –RGO(35) number of high spin Co ions occupy the octahedral sites (A-
hybrid is similar to its Fe 2p spectrum (Fig. 3(d)); the peaks site) in spinel-type CoFe O .
observed at 779.2 eV (Co 2p3/2) and 794.6 eV (Co 2p1/2) can be magnetic material with a partially inverse spinel structure. The
assigned to the contributions from Co ions in the octahedral deconvoluted C 1s spectra of CoFe
2
4
40,41
Moreover, CoFe O is a ferro-
2 4
2
4
42
2
+
2 4
O –RGO(35), as shown in
sites (A-site), while the doublets binding energies of Co 2p3/2 at Fig. 3(e), reveals the presence of oxygen-containing groups (C–
©
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OH, 284.8 eV) and sp carbons (C]C, 283.9 eV). These results
Paper
2
43
2 4
The fabrication process of the octahedral CoFe O –RGO
are in accordance with the FT-IR and Raman spectroscopy hybrids in this study is depicted in Fig. 5. In our study, the
analyzes. The high-resolution O 1s spectra of CoFe O –RGO(35) growth conditions used to fabricate the octahedral CoFe O /
2
4
2 4
are divided into two individual peaks (Fig. 3(f)); the O 1s peaks RGO hybrids were a high pH (ꢁ13) and a relatively long sol-
at 529.2 and 530.8 eV, which originated from the lattice oxygen vothermal time, which is briey described as follows:
atoms (denoted as Olatt) and surface hydroxyl species or the
2
+
ꢀ / Co(OH)2
44,45
Co + 2OH
(9)
adsorbed oxygen (denoted as Osurf).
In addition, both Olatt
and Osurf play an important role in the radical's generation
process.
3
+
ꢀ
46
Fe + 3OH / Fe(OH)
3
(10)
The specic surface area and pore-size distribution of the as-
prepared samples were determined using the nitrogen sorption
technique. The nitrogen adsorption–desorption isotherms are
shown in Fig. 4(a). All the samples exhibit typical II-type curves
Co(OH)
2
+ Fe(OH)
3
+ graphene oxide
+
EtOH / CoFe
2
O
4
/RGO + H
2
O
(11)
with steep uptakes (P/P
0.40 < P/P < 0.95), indicating the co-existence of non-ordered
mesopores (2–50 nm), and numerous macropores (>50 nm).
The primary CoFe NPs are randomly stacked and this results
0
> 0.95) and distinct hysteresis loops
This indicates that the formation of this unique nanohybrid
can be controlled in two stages: (i) the formation of Co(OH)
and Fe(OH) colloidal particles; it is well known that GO
(
0
2
47
3
2 4
O
possesses negative charge owing to the existence of numerous
oxygen-containing functional groups, such as carboxyl,
in the creation of macropores. The Brunauer–Emmett–Teller
surface areas of CoFe O , CoFe O –RGO(35), and CoFe O –
2
4
2
4
2
4
49
carbonyl, and hydroxyl. In addition, the oxygen-containing
2
ꢀ1
RGO(70) were 71.20, 35.11, and 40.42 m
Furthermore, the CoFe –RGO(35) and CoFe
g
, respectively.
–RGO(70)
groups on the GO sheets may serve as heterogeneous nucle-
2
O
4
2
O
4
50
ation sites. When Co(NO ) $6H O and Fe(NO ) $9H O are
3
2
2
3 3
2
exhibit classical type H3 hysteresis loops, arising from the
capillary condensation and slit-shaped type pore structure of
added into the GO suspension, the positively charged metal
2+
3+
ions (Co and Fe ) would anchor onto the surface of the GO
48
the RGO nanosheets. In addition, the specic surface area of
sheets via electrostatic attraction. Aer increasing the pH of the
graphene oxide (GO) raw material was estimated to be as high as
ꢀ
mixed solution by the introduction of sufficient OH , the
2
ꢀ1
5
56.38 m g (Fig. S3†). Because the excessive CoFe O nano-
2+
3+
2
4
alkaline conditions promotes the precipitation of Co and Fe
clusters were anchored on the graphene nanosheets, the
ions in ethyl alcohol and this result in the formation of nucle-
specic surface area of CoFe O –RGO hybrid was much lower
2
4
ation precursors, i.e. Co(OH) and Fe(OH) colloidal particles
2
3
than that of the pure CoFe
2
O
4
and GO raw material. The large
51,52
(
eqn (1) and (2)).
and the conning effects of graphene; during the solvothermal
treatment, the Co(OH) and Fe(OH) , colloidal particles gradu-
ally combine to form CoFe nuclei. Meanwhile, the GO sheets
(ii) in situ growth of the CoFe O crystals
2 4
specic surface area of pure CoFe
to absorb dyes in wastewater. Moreover, the pore-size distribu-
tion shown in Fig. 4(b) indicates that the pure CoFe has
2 4
O should enhance its ability
2
3
2 4
O
2 4
O
a broad pore size distribution in the range of 5–20 nm, while the
central peak was displayed at approximately 8.6 nm. The
CoFe O –RGO hybrid exhibited a bimodal pore size distribution
53
were reduced during the solvothermal reactions (eqn (3)).
2 4
To reduce the surface energy, aggregation of many CoFe O
2
4
50,54
nanocrystals further occurred.
It is well known that the
owing to the abundant mesoporous channels formed upon the
incorporation of layered graphene sheets. For example, a sharp
pore size distribution at ꢁ3.6 nm and 6.9 nm were observed for
CoFe O nanocrystals with a broad size distribution may result
2
4
38
from the quick nucleation and growth. The CoFe O nano-
2
4
crystals which are tightly anchored onto the graphene sheets
2 4
CoFe O –RGO(70).
Fig. 4 (a) Nitrogen adsorption–desorption isotherm curves and (b) the corresponding Barrett–Joyner–Halenda pore size distribution of the as-
prepared CoFe and CoFe –RGO.
2
O
4
2 4
O
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2 4
Fig. 5 Schematic of the fabrication process of the octahedral CoFe O –RGO hybrids.
grew quickly owing to the abundant growth units near the obtained. However, further experimental evidence is required to
surface of the RGO sheets. Then the octahedral CoFe O parti- verify the present observations.
2
4
cles, which are formed under high autogenous pressure at
ꢂ
1
80 C, are enclosed by the {111} surfaces to achieve the lowest
3
.2. Catalytic activity
tightly anchors itself onto graphene sheets, the growth of the Control experiments were conducted at near-neutral pH
CoFe nanocrystals in the mixture solution was slower and it conditions (adjusted with phosphate buffer solution) to
formed irregular CoFe nanoclusters. Consequently, the compare the degradation efficiency of MB dye by the as-
CoFe O particles within the CoFe O –RGO hybrids exhibited prepared sample and CoFe O –RGO/H O at various condi-
2 4
Gibbs free energy. Compared with CoFe O nanocrystals which
2 4
O
2 4
O
2
4
2
4
2
4
2 2
a more obvious difference in morphology, i.e. octahedrons and tions. MB dye was selected as the targeted pollutant owing to its
irregular subunit particles, and the CoFe O –RGO hybrids were stability under the visible light. Fig. 6(a) and (b) shows the
2
4
removal efficiency of MB in the presence of either the catalysts
Fig. 6 Photocatalytic degradation of MB by the as-prepared samples alone (a), or the catalysts in conjunction with H
2
O
2
(b) under visible-light
ꢀ1
ꢀ1
irradiation. Unless otherwise stated, the reaction conditions are: [MB] ¼ 10 mg L , 60 mL MB solution, [catalyst] ¼ 1 mg mL , visible light
ꢂ
irradiation (l > 420 nm, 500 W), T ¼ 25 C, initial pH ¼ 6.4. (c) Effect of solution pH on the zeta potentials of CoFe
2 4 2 4
O and CoFe O –RGO hybrids.
: effect of hydrogen peroxide dose (reaction conditions: [MB] ¼ 10 mg L , 60 mL MB solution,
ꢀ
1
(
[
d) MB degradation using CoFe
2
O
4
–RGO/H
O
2 2
ꢀ1
catalyst] ¼ 1 mg mL ).
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i.e. CoFe and CoFe
Paper
56,57
(
2
O
4
2
O
4
–RGO) alone or the catalysts together graphene.
However, with the simultaneous introduction of
with 1.0 mL 30 wt% H O under visible-light irradiation. The the as-prepared catalysts and the H O co-catalyst, the photo-
2
2
2 2
adsorption process was conducted by dispersing the products in degradation of MB was fast and efficient owing to the reaction
58
the MB solution in the dark for 1.5 h and the photodegradation of photogenerated carriers and H O . The order of the pho-
2
2
step began at t ¼ 0. Clearly, the adsorption ability of bare todegradation efficiency is CoFe
2
O
4
–RGO(35) > CoFe
2
O
4
–
CoFe
2
O
4
was much higher than that of the CoFe
2
O
4
–RGO RGO(70) > CoFe
2
O
4
> H
–RGO(35) exhibited the highest MB photo-
sample and the MB molecules having an acceptable molecular degradation efficiency (ꢁ43%) in 3.5 h among all the samples,
size. Interestingly, for bare CoFe
, the MB molecules were and the total MB removal efficiency (ꢁ73%) was achieved when
easily re-released into an aqueous solution from its surface aer the CoFe O –RGO(35)/H O system was used (Fig. 6(b)).
2 2
O . During the photodegradation
hybrids owing to the large specic surface area of the CoFe
2
O
4
2 4
process, CoFe O
2 4
O
2
4
2 2
light irradiation or on addition of the H O co-catalyst, which is
The adsorption reaction is oen related to the surface charge
consistent with a previous report. The explanation for this of the NPs. The effect of solution pH on zeta potentials of the
observation is that a weak interaction exists between the CoFe and CoFe –RGO hybrids is shown in Fig. 6(c). It can
CoFe NPs and MB dye. In addition, the degradation effi- be seen that zeta potentials of the as-prepared particles
ciency of the MB solution was rather low in the absence of the decreased as the pH increases and that of the CoFe and
photocatalysts. CoFe –RGO hybrids were negatively charged at pH > 4.3. In
Fig. 6(a) shows the total removal efficiency of three samples addition, the estimated zeta potentials observed for CoFe
i.e. CoFe O , CoFe O –RGO(35), and CoFe O –RGO(70)) CoFe O –RGO(35), and CoFe O –RGO(70) were about ꢀ19.1,
2
2
55
2
O
4
2 4
O
2 4
O
2 4
O
2 4
O
2 4
O ,
(
2
4
2
4
2
4
2
4
2 4
without H O co-catalyst for MB dye. The order of the removal ꢀ33.7, and ꢀ42.9 mV at pH ¼ ꢁ6.3, respectively. Therefore, the
2
2
efficiency is CoFe O –RGO(70) > CoFe O –RGO(35) > CoFe O . adsorption equilibrium rate toward the cationic MB dye of the
2
4
2
4
2 4
The elimination of MB dye by only the catalysts was dominated CoFe
by the adsorption process due to the p–p stacking between the pure CoFe
aromatic dye molecules and the hexagonal regions of actions between MB and CoFe
2
O
4
–RGO hybrids was more rapid compared to that of the
. This is because of the strong electrostatic inter-
–RGO hybrids surfaces. Also,
2 4
O
2 4
O
Fig. 7 (a) Effect of IPA and different dyes on the degradation process of “CoFe
2
O
4
–RGO(70) + H
2
O
2
” system. (b) The performance of the
ꢀ1
ꢀ1
CoFe
2
O
4
–RGO(70) hybrids on reuse in the degradation of MB (reaction conditions: [MB] ¼ 10 mg L , [H
2
O
2
] ¼ 0.035 g mL , [catalyst] ¼ 1 mg
ꢀ1
ꢂ
mL , T ¼ 25 C). (c) Magnetization vs. applied magnetic field at room temperature with H up to 14 kOe for the CoFe
2 4 2 4
O and CoFe O –RGO
ꢀ
1
samples. (d) Digital images of the 10 mg L MB solution before adsorption and after adsorption at various time (adsorption reaction conditions:
[
MB] ¼ 10 mg L 1, 4 mL MB solution, [catalyst] ¼ 0.05 g mLꢀ1, T ¼ 25 C).
ꢀ
ꢂ
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desorption of the MB molecules from the surface of the conditions. Aer each cycle, the catalyst was separated from the
CoFe O –RGO hybrids into the aqueous solution was difficult reaction solution using an external magnet, and it was washed
2
4
aer light irradiation or addition of the H O co-catalyst.
several times with deionized water and ethanol. As shown in
Fig. 6(d) depicts the effect of hydrogen peroxide dose on MB Fig. 7(b), the removal efficiencies for MB in the third cycle was
degradation using CoFe –RGO(70) hybrids. We observed that nearly the same as that of the rst two cycles. Although the
the concentration of MB reduced slowly under light irradiation activity of the CoFe –RGO(70) hybrid slightly decreased in the
for the “visible light with H ” system. The introduction of third cycle, the removal efficiencies for MB was still 79.2%. The
weakly improves the photocatalytic effects by increasing slight loss of the photodegradation activity may be due to the
the addition level of H in current experiment, e.g. from 1 mL loss of the catalysts during the recovery process or the weak
to 2 mL and from 2 mL to 3 mL under visible light irradiations decoration of CoFe O and RGO.
2
2
2 4
O
2 4
O
2 2
O
2 2
H O
2 2
O
37,53
All these analyses indicate
for 3.5 h, the degradation rate of MB can be slightly increased to that CoFe O –RGO hybrid is still highly efficient, arising from
2
4
2
4
2
2.2% and 25.14%, respectively. This result indicates that the the graphene/CoFe O heterojunction and the lowest energy
2 4
degradation rate of MB could not be signicantly improved by {111} facets of octahedral CoFe
simply increasing the content of H . Compared with the The magnetic hysteresis loops of the pure CoFe
visible light with H ” system, a faster and more efficient CoFe –RGO hybrids were conducted using a vibrating sample
degradation of MB solution occurred in the “CoFe
–RGO(70) magnetometer (VSM) from ꢀ14 000 to 14 000 Oe at room
” system. Obviously, the introduction of 2 mL H temperature, as shown in Fig. 7(c). All the samples exhibit
2 4
O , when reused.
2
O
2
2 4
O and the
“
2
O
2
2 4
O
2 4
O
+
H
2
O
2
2 2
O
remarkably promotes the photo-Fenton catalytic activity, the ferromagnetic behavior. The saturation magnetization (Ms)
removal rate of MB can be further increased to 84.5%. In value of the pure CoFe O nanocrystals is approximately 31.15
2
4
ꢀ
1
addition, the optimum amount of H O is always present in the emu g , which is lower than that of the CoFe O –RGO hybrids
2
2
2 4
ꢀ
1
Fenton-like reaction system. Previous work have shown that the (41.98–45.14 emu g ). Compared with the bare CoFe
addition of excessive H may result in scavenging of the the enhancement of the M for CoFe –RGO hybrids was
hydroxyl radicals (cOH), thus not only reducing the photo- mainly due to the larger average particle size of the octahedral
2 4
O NPs,
2
O
2
s
2 4
O
61,62
Fenton reaction rate but also causing the wastage of H
2
O
2
CoFe
CoFe
2
O
O
4
4
nanocrystals.
Hence, more efficient separation of
5
9
23
(
eqn (12) and (13)).
2
–RGO hybrids is possible. As shown in Fig. 7(d), the
rst bottle (marked with 0 s) is the initial MB (10 mg L
ꢀ
1

)
cOH + H O / H O + cOOH
(12)
(13)
2
2
2
solution without CoFe O –RGO(70) catalyst. The CoFe O –RGO
2
4
2 4
hybrids were uniformly dispersed in MB solution to form a dark
brown suspension (marked with 30 s). However, when an
external magnet was placed close to the bottle, the synthesized
samples which had absorbed MB from the aqueous solution
were rapidly separated from the aqueous media (marked with
0 s). Thus, the CoFe O –RGO catalyst is quite acceptable owing
2 4
to its good separation ability. In addition, the M values of the
samples were lower than that of the bulk counterpart (74 emu
2 2
HOOc + cOH / H O + O
The heterogeneous photo-Fenton mechanism of the
CoFe O –RGO + H O ” system will be discussed in a later
“
2
4
2 2
section of this paper.
To identify the main active species for the degradation of
dyes in the “CoFe –RGO(70) + H ” system, IPA was sepa-
rately added into the system as a scavenger of hydroxyl radicals
6
s
2
O
4
2 2
O
ꢀ1
g
), which may be ascribed to the possible effect of surface
spin canting on the CoFe NPs as well as the weight-fraction
of the RGO. Moreover, the eld-dependent magnetization
60
(
cOH). As shown in Fig. 7(a), the dye removal efficiency of the
CoFe O –RGO(70) for MB was higher than that of MO. In
addition, the adsorption and photodegradation rate of the
CoFe O –RGO(70) hybrid for the anionic dye (MO) was lower
when compared with that of the cationic dye (MB), and this was
attributed to the repulsive forces that exist between MO and the
OH/R–COO groups on the surface of CoFe
hybrid. Furthermore, the original photodegradation rate is
high, i.e. without adding a quenching agent into the reaction
solution, and the degree of MB photodegradation was as high as
2 4
O
2
4
63
curves of the CoFe
negligible remanence (M
coercivity (H
magnetic materials (Table S1†).
2
O
4
and CoFe
2
O
4
–RGO hybrids exhibit non-
¼ 14.92–25.42 emu g ) and high
2
4
ꢀ1
r
c
¼ 4.32–6.41 kOe), indicating that they are hard
ꢀ
–
2
O
4
–RGO(70)
48
3
.3. Activation mechanism
Based on the information above, we propose an activation
mechanism of H O by the CoFe O –RGO hybrids, as presented
5
3.3%. However, the degradation of MB was obviously inhibited
2
2
2 4
by the introduction of 0.1 M IPA, which resulted in a decrease in
the degree of MB photodegradation to 4.4% within 3.5 h. This
result clearly proved that cOH is the dominating reactive species
in Fig. 8. Clearly, the main active sites of CoFe
2
O
4
–RGO hybrids
) in a spinel-type
particles are excited
using a daylight lamp, electron–hole pairs are produced on the
surface of the CoFe particles (eqn (14)). The presence of
highly conductive RGO in the CoFe –RGO hybrids can inhibit
2
+/3+
3+/2+
are the cobalt and iron ions (Co
and Fe
CoFe surface lattice. When the CoFe O
2
O
4
2 4
in the “CoFe
2 4 2 2
O –RGO + H O ” system, and it contributes to the
decolorization of MB.
2 4
O
The recyclability and stability of the catalyst are important
parameters for its commercialization. To evaluate the reus-
ability of our catalytic system, three rounds of the photo-
degradation reaction for MB dye were conducted with the
2 4
O
the recombination of electrons and holes because electrons are
quickly transferred to the RGO sheets due to a percolation
2
0
mechanism (eqn (15)). Then the photogenerated electrons
within the graphene sheets could be captured by H to
2 4
recyclable CoFe O –RGO(70) hybrid under the same reaction
2 2
O
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Paper
CoFe
2
O
4
. We demonstrated that cOH is the dominating reactive
species in the “CoFe O –RGO + H O ” heterogeneous photo-
2
4
2 2
Fenton system, and it is responsible for the decolorization of
MB. In addition, the high catalytic activity of CoFe O –RGO(70)
2
4
2
+/3+
hybrids was attributed to the synergistic effect of redox Co
3
+/2+
and Fe
experiment conducted, we proposed an activation mechanism
of H by the CoFe –RGO hybrids. In view of the excellent
pairs. Based on the results obtained from the
2
O
2
2 4
O
catalytic activity, the outstanding reusability and stability, as
well as the magnetic recyclability of CoFe O –RGO hybrid, it can
2
4
potentially be applied in advanced oxidation processes as
a heterogeneous photo-Fenton catalyst.
Conflicts of interest
Fig. 8 Schematic illustration of an active heterogeneous photo-
Fenton mechanism based on CoFe
O
2 4
–RGO hybrids.
There are no conicts to declare.
Acknowledgements
6
4
generate cOH (eqn (16)). Meanwhile, the photogenerated holes
can participate in the photo-Fenton reaction to yield cOH (eqn
This study was supported by the Science and Technology
Research Project of Jiangxi Education Department in 2019
65
(
17)). Furthermore, the reaction of H
ions generate a Fenton reagent which yields hydroxyl radical
cOH) (eqn (18) and (19)).
Co by Fe is represented by eqn (20). Efficient regeneration of
2 2
O with cobalt and iron
(
(
GJJ190585), the Natural Science Foundation of Jiangxi Province
20202BABL204016), Science and Technology Research Project
19,66
(
The thermodynamic reduction of
3
+
2+
of Jiangxi Education Department (GJJ180608), School Youth Top
Talent Program of JXSTNU (2018QNBJRC005).
2
+
the surface Co may be responsible for the signicant increase
1
9
2 2
in the activity of H O .
+
CBꢀ
)
CoFe
2
O
4
+ hv / CoFe
2
O
4
(hVB + e
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. Conclusions
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2 4
O
2 2
O
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