Novel synthesis of magnetic, porous C/ZnFe2O4 photocatalyst with enhanced activity under visible light based on the Fenton-like reaction

Article abstract of DOI:10.1039/c7dt01528c

Magnetic visible-light-driven photocatalyst, porous C/ZnFe₂O₄ (denoted as C/ZFO-CE) was fabricated via a CO₂-mediated ethanol route. CO₂-mediated ethanol route largely mitigated the solvent strength and facilita

Full text of DOI:10.1039/c7dt01528c

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Efficient extraction of sulfate from water using
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Novel synthesis of magnetic porous C/ZnFe O photocatalyst with
2
4
enhanced activity under visible light based on the Fenton-like reaction
1
1
2
1,2
Fangxiao Wang , Yanglin Chen , Rongshu Zhu , Jianmin Sun *
1
. State Key Laboratory of Urban Water Resource and Environment, MIIT Key
Laboratory of Critical Materials Technology for New Energy Conversion and Storage,
School of Chemistry and Chemical Engineering, Harbin Institute of Technology, Harbin
1
2
50090, China
. Harbin Institute of Technology (Shenzhen), Shenzhen Key Laboratory of Organic
Pollution Prevention and Control, Shenzhen 518055, China
*
Corresponding author: Jianmin Sun sunjm@hit.edu.cn
Abstract: Magnetic visible-light-driven photocatalyst porous C/ZnFe
2
4
O (denoted as
C/ZFO-CE) was fabricated via CO
2
mediated ethanol route. CO mediated ethanol route
2
largely mitigated the solvent strength, facilitated the homogenous deposition of
ZnFe
(ZFO) through the coordination of metallic cation with CO 2- and HCO -
2
O
4
3
3
anions, which were hydrolyzed from CO
2
and H O, thus additional precipitant was
2
-, CO
2-
and NO -
in the system acted as templates for
avoided. Meanwhile, the HCO
3
3
3
1

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the formation of porous C avoiding additional organic mesoporous templates, thus
reducing the synthesis cost. For the degradation of RhB and phenol, the C/ZFO-CE
system in the presence of minute H
2
2
O exhibited remarkably improved catalytic
performance compared with the systems of H
2
2
O , ZFO, C/ZFO-CE, C/ZFO-E
(
C/ZnFe
2
O
4
synthesized in pure ethanol) and C/ZFO-E in the presence of minute H
2
2
O .
And 2.0 mL of H
2
O
2
(30%) combined with C/ZFO-CE obtained the maxima
degradation efficiencies for RhB at 100% within 60 min and 91% for phenol within 120
min. The high efficiencies for pollutants degradation over C/ZFO-CE catalyst in the
presence of minute H
2
2
O was possibly attributed to the strong harvest to visible light,
the improved separation efficiency of the photoinduced charges and the overall •OH
production by “photo-Fenton” process. The existence of •OH during photodegradation
process was evidenced by PL-TA (photoluminescence-terephthalic acid) technique, ESR
spectra and trapping experiments of active species using different scavengers.
Furthermore, a possible reaction mechanism involvement of Fenton-like reaction for the
photodegradation of pollutants was proposed based on the experimental results.
Keywords: CO
2
mediated ethanol synthesis route; magnetic; porous structured
C/ZnFe ; Fenton-like; visible-light catalysis
2
O
4
2

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Introduction
With the development of industry, the increasing environmental problems such as
water pollution have attracted considerable attentions. In recent years, photocatalysis
technique has been widely used to degrade wastewater due to its low cost, simplicity
1
and high efficiency. However, the subsequent separation treatment of the photocatalyst
from water was complicated, time-consuming and high-cost in practical applications.2,3
Furthermore, if the photocatalyst could not be separated easily and thoroughly,
secondary pollution might be produced by the remaining photocatalyst powder.
Therefore, research on the exploitation and application of magnetic especially
visible-light-driven photocatalyst is of significance.
ZnFe
2
4
O (ZFO), one type of the magnetic semiconductor material, has been widely
4
5,6
applied in the conversion of solar energy, photocatalysis
and photochemical
7
hydrogen production from water due to its unique properties including visible-light
response, good photochemical stability and low cost. Besides, it was reported that
magnetic ZFO particles also possessed intrinsic peroxidase-like activity, which could
react with H
2
2
O to produce •OH as the highly reactive oxidizing species to degrade
organic pollutants.8,9 However, ZFO itself could not be directly used as an efficient
photocatalyst, which stems from its high recombination rates of photoinduced
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charges.10,11 Thus, it is imperative to improve the efficiency of photoinduced charge
separation in ZnFe
2
4
O in order to enhance the photocatalytic performance.
It has been reported that carbon modified ZFO can accelerate electron transfer from
the inside of photocatalyst to the interface by taking advantage of carbon’s unique
electron transport ability.12,13 Hou et al. developed a ZFO multi-porous microbricks
/
graphene hybrid photocatalyst, which exhibited high visible-light catalytic activity for
12
degrading p-chlorophenol. However, the BET specific surface area of the obtained
2
-1
ZnFe
2
O -MM/RGO hybrid measured at 26.35 m g was unsatisfactory. Lu et al.
4
synthesized magnetic ZFO/graphene composite with the excellent photocatalytic
degradation rate for organic dyes in the presence of H
2
O
2
under visible-light
1
3
irradiation. Whereas, the graphene sheets in the magnetic ZFO/graphene composite
exist as aggregated and crumpled silk waves-like carbon sheets due to Van der Waals’
force. Recently, attentions have also been paid to porous carbon-based
visible-light-driven photocatalysts such as porous carbon/TiO
,14,15 mesoporous
2
1
6
carbon/g-C
3
N
4
(mC/g-C
3
N ) thanks to its specific properties of porous carbon such as
4
large pore size, high surface area, and periodically arranged monodispersed
1
7
mesopores. However, in general porous C should be prepared in advance by template
method, which resulted in the complex synthesis procedure and expensive synthesis
4

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cost of the catalyst. Due to the advantages of porous C, as expected the coexistence of
porous carbon and ZFO might be an efficient photocatalyst candidate. And to the best of
our knowledge, there are no reports on the one-pot synthesis of porous carbon/ZFO
composite for contaminant degradation.
Nowadays, CO -mediated approach to fabricate nanomaterials has exhibited
2
considerably fascinating results due to its numerous advantages over conventional
1
8
methods. The lower surface tension and viscosity together with the higher diffusivity
for CO
-mediated medium not only reduce the solvent strength19-21, but also provide the
enhanced mass transfer for the facilitation of the reactants infiltration.22,23 Herein,
2
magnetic porous structured C/ZnFe
2
O
4
(C/ZFO-CE) photocatalyst with
high-performance degradation of RhB and phenol in the presence of 2.0 mL of 30%
H
2
2
O under visible-light irradiation were facilely fabricated in one-step through
CO
2
-mediated route directly from C, Zn and Fe precursors of glucose, Zn(NO
3
)
2
·6H O
2
and Fe(NO
3
)
3
·9H
2
O. The roles of CO in the synthesis of C/ZFO-CE were illustrated
2
thoroughly. In addition, the existence of •OH during photodegradation process was
evidenced by PL-TA (photoluminescence-terephthalic acid) technique, ESR spectra and
trapping experiments of active species using different scavengers. Furthermore, a
possible mechanism involvement of Fenton-like reaction for the photodegradation of
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pollutants was proposed based on the experimental results.
Experimental
Porous C/ZnFe
2
4
O photocatalyst preparation
The typical synthesis procedure for porous C/ZnFe
2
O
4
fabricated in CO
2
-mediated
system (marked as C/ZFO-CE) is as follows: 0.22 g of glucose was first dispersed in 20
mL ethanol solution, followed by adding 0.20 g Zn(NO
3
)
2
•6H O and 0.80 g
2
Fe(NO
3
)
3
•9H O into the above solution with magnetic stirring for 10 min. The resultant
2
solution was then transferred into 50 mL Teflon-lined stainless-steel high pressure
autoclave, which subsequently was placed into an oil bath at 150 °C, and then CO was
2
pumped up to 10 MPa under vigorous stirring. After 8 h, the autoclave was cooled to
room temperature naturally and depressurized slowly, the resultant solid products were
centrifuged, washed and dried at 80 °C. Finally, the as-collected samples were calcined
at 450 °C for 3 h under N
2
atmosphere. To elucidate the role of CO , the reference
2
composite labeled as C/ZFO-E was also prepared under the same procedure in pure
ethanol solvent only without CO
2
presence. And for comparison, bare ZFO was
prepared according to the literature.13
6

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Characterizations
The crystallite structure of the sample was characterized by powder Bruker D8 Advance
X-ray diffraction (XRD, Cu Kα radiation, λ=1.5418 Å). Elemental analyses (EA) for
carbon contents were performed on Vario Microcube CHN analyzer. X-ray
photoelectron spectroscopy (XPS) measurement was carried out on Phi Quantera
spectrometer using Al Kα X-ray as the excitation source. Nitrogen
adsorption/desorption isotherms were measured on ASAP 2020 volumetric analyzer at
7
7 K, the surface area was calculated using BET method and total pore volume was
determined from the amount of nitrogen adsorbed at P/P ca. 0.99. Transmission
0
electron microscopy (TEM) experiments were performed on Tecnai G2 Spirit electron
microscope with an acceleration voltage of 120 kV. UV-vis DRS was recorded on
Perkin Elmer Lambda 750. The magnetization curve was conducted on Quantum
Design MPMS-7 SQUID magnetometer at 300 K under varying magnetic field.
Fluorescence spectra were obtained by Perkin Elmer LS55 spectrometer. Lifetime of the
charge carriers were recorded by time resolved photoluminescence spectroscopy
(TR-PL, LifeSpec II, Edinburgh Instruments, UK). The ESR signal was obtained on a
Bruker model A200 spectrometer. The total organic carbon (TOC) content of the
samples was analyzed by a total organic carbon analyzer (muli N/C 3100). After the
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photocatalytic reaction, the catalyst was filtered and the solution was detected by Perkin
Elmer 8300 ICP-OES analysis.
Visible-light catalytic reaction test
Photocatalytic activity of porous C/ZFO-CE was evaluated by the degradation of
RhB under visible-light irradiation, moreover, in order to eliminate the
photosensitization effect of dye, and colorless organic pollutant of phenol was also
investigated. A 300 W Xe lamp equipped with a 420 nm cut-off filter was used as a light
source. In all photocatalytic degradation experiments, 100 mg of catalyst was added into
-1
1
00 mL of 5 mg L pollutant (RhB or phenol) solution. Before starting the illumination,
each reaction mixture was stirred vigorously for 60 min in dark in order to reach the
adsorption-desorption equilibrium between the dye and the catalyst. Then, the
suspension was exposed to visible-light irradiation. At a given time interval of
irradiation, ~5 mL samples were collected, magnetically separated to remove the
catalyst and then monitored by UV-vis spectroscopy at 552 nm for RhB degradation. In
the case of phenol degradation, the photodegradation process was monitored at 270 nm.
For comparison, the photocatalytic activities of ZFO and C/ZFO-E were investigated.
Meanwhile, the photocatalytic activities for the systems of C/ZFO in the presence of
8

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H
2
O
2
(labeled as C/ZFO-E+H
2
O
2
and C/ZFO-CE+H
2
2
O ) were also conducted, wherein
2
.0 mL 30% H
2
2
O was added to the reaction mixtures after the adsorption-desorption
equilibrium. Additionally, the effects including the amounts of H
2
2
O and visible light
(with or without) were investigated in detail. The resuability of C/ZFO-CE was also
examined by photocatalytic degradation of RhB for up to five times. After each run, the
catalyst was separated from solution by an external magnetic field and then reused
directly after thorough wash with ethanol and distilled water.
Results and discussion
The structure and property characterizations of C/ZnFe
2
4
O catalyst
The crystalline nature and composition of the as-synthesized ZFO and C/ZFO
samples were characterized by XRD as depicted in Fig. 1. For ZFO, C/ZFO-E and
C/ZFO-CE samples, the peaks at 2θ values of 29.9, 35.2, 42.8, 53.1, 56.6, 62.2 and
7
3.5° could be ascribed to (220), (311), (400), (422), (511), (440) and (533) crystal
planes of ZnFe
2
O
4
(JCPDS No. 82-1049).12,13 However, no clear diffraction peaks
ascribed to carbon were observed in both C/ZFO composites, which might be caused
1
6
from the lower carbon contents or high dispersity. However, the amounts of C in both
porous C/ZFO samples were measured by CHN analyzer listed in Table 1, the carbon
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content for C/ZFO-CE was 5.19 wt% higher than that of 4.26 wt% in the C/ZFO-E
sample.
c
b
a
10
20
30
40
50
60
70
80
2
Theta (degree)
Fig. 1. XRD patterns for (a) ZFO, (b) C/ZFO-E and (c) C/ZFO-CE
Further investigation of the elemental composition and chemical states of porous
C/ZFO-CE was performed by XPS analysis. The survey XPS spectrum of C/ZFO-CE in
Fig. 2A confirmed the existence of Zn, Fe, C and O elements. In an attempt to better
understand the composites in terms of amounts and types of different functional groups,
deconvolutions of Fe 2p, Zn 2p, C 1s and O 1s for high resolution XPS spectra were
carried out. As shown in Fig. 2B, the peaks at 724.4 eV and 710.8 eV were attributed to
Fe 2p1/2 and Fe 2p3/2. For Fe 2p3/2, four peaks at 710.3, 711.4, 712.4, and 713.8 eV were
fitted for the spectrum,24,25 the peak of 710.3 eV could be ascribed to the existence of
Fe(II) oxide, and the other three peaks were attributed to Fe(III) oxide.26 It was
calculated that the ratio of Fe(II) to Fe(III) on the surface of C/ZFO-CE was at 0.61. In
Fig. 2C, two distinct peaks at 1021.4 eV and 1044.6 eV were respectively attributed to
1
0

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2
7
the spectra of Zn 2p3/2 and Zn 2p1/2. As depicted in Fig. 2D, C 1s spectrum was
divided into three peaks at 284.6, 286.1, and 288.9 eV, which respectively corresponded
to C-C, C-O and C=C(C=O) bonds.28-30 Typically, the binding energy at 286.1 eV
suggested that carbon was partially doped into the interstitial position of ZFO lattice and
2
8
formed C-O bond. Hence, the C in the porous C/ZFO-CE existed in the forms of
composed C and doped C. The O 1s spectrum displayed in Fig. 2E was split into three
peaks at 530.1, 531.0, and 533.5 eV, which were ascribed to the lattice oxygen,
chemisorbed oxygen and the band of O-C.31
Survey
A
Fe 2p
B
Zn 2p3/2
Fe 2p3/2
710.8 eV
Fe 2p1/2
724.4 eV
Zn 2p1/2
Fe 2p3/2 Fe 2p3/2
710.3 eV
711.4 eV
713.8 eV
O 1s
C 1s
712.4 eV
0
200
400
600
800
1000
1200
705
710
715
720
725
730
Binding Energy (eV)
Binding Energy (eV)
Zn 2p
C 1s
D
C
Zn 2p
3/2
284.6 eV
1021.4 eV
Zn 2p1/2
044.6 eV
1
286.1 eV
288.9 eV
1015 1020 1025 1030 1035 1040 1045 1050
280
284
288
292
296
300
Binding Energy (eV)
Binding Energy (eV)
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O 1s
E
530.1 eV
531.0 eV
533.5 eV
525
530
535
540
Binding Energy (eV)
Fig. 2. XPS spectra of (A) survey spectrum, (B) Fe 2p, (C) Zn 2p, (D) C 1s and (E) O 1s
of C/ZFO-CE composite
TEM analysis is a direct technique for obtaining more detailed structural and
morphological information on the prepared sample. For the C/ZFO-CE composite (Fig.
3
A and B), ZFO particles were small sizes at 10-20 nm, and the disordered worm-like
porous C layer were clearly observed in C/ZFO-CE composite. Moreover, the lattice
5
spacing of 0.254 nm for the (311) plane clearly verified the structure of ZFO. For
comparison, the TEM image of C/ZFO-E composite in Fig. 3C showed that the large
sized and aggregated ZFO particles were formed. Combined with the results above, It
was deduced that the porous C/ZFO-CE were facilely one-step synthesized in
CO
2
-mediated ethanol route. CO mediated ethanol system largely mitigated the solvent
2
strength, facilitated the homogenous deposition of ZFO through the coordination of
2
-
and HCO -
metallic cation with CO
3
3
anions, which were hydrolyzed from CO
2
and
in
O, thus additional precipitant was avoided. Meanwhile, the HCO -
, CO
2-
and NO -
H
2
3
3
3
1
2

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the system acted as templates for the formation of porous C avoiding additional organic
3
2
mesoporous templates, thus reducing the synthesis cost. This new synthetic method in
CO -mediated ethanol media is simple, cost-effective and green, also expands the
2
fabrication for the porous C-based metal oxides with wide applications.
B
Fig. 3. TEM images for (A)-(B) C/ZFO-CE and (C) C/ZFO-E composites
Specific surface area is an important characteristic of a catalyst candidate, hence,
adsorption-desorption isotherms were conducted to investigate the surface area and
N
2
pore structure of the as-obtained samples in Fig. 4A. In the case of ZFO, a small
2
-1
hysteresis loop was observed and the BET surface area was 13.5 m g . Both the
1
3

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isotherms of C/ZFO-E and C/ZFO-CE composites belonged to type IV curves with type
3
3
H1 hysteresis loops, which are typical characteristic of mesoporous materials, and the
2
-1
2
-1
BET surface areas were remarkably improved to 47.4 m g and 60.3 m g due to the
existence of porous C. Correspondingly, the pore size distribution displayed in Fig. 4B
further verified the narrow mesoporous distribution with average pore size at 10.3 nm in
the C/ZFO-CE composite. The C/ZFO-E composite exhibited a broad pore distribution
between 10 and 30 nm. Moreover, the BET surface areas and pore volumes of various
samples are summarized in Table 1, the higher specific surface area for C/ZFO-CE
would facilitate the composite enhancing the adsorption capability to pollutants and
provide more catalytically active centers. Additionally, the composed porous carbon
could also provide spacious paths for the charges transfer and mass transfer of reactants
during the photocatalytic reaction, and thus the C/ZFO-CE composite is anticipated to
manifest the enhanced photodegradation efficiency.
2
00
60
20
1.0
.8
.6
.4
.2
0.0
A
B
0
1
c
0
1
0
8
0
c
0
4
0
b
a
b
a
0
0.0
0.2
0.4
0.6
0.8
1.0
0
10
20
30
40
50
Relative Pressure (P/P0)
Pore Size (nm)
Fig. 4. (A) N adsorption-desorption isotherms and (B) pore size distributions for (a)
2
1
4

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ZFO, (b) C/ZFO-E and (c) C/ZFO-CE composites
Table 1. Textural parameters of various samples
2
-1
3
-1
a
Samples
C/ZFO-CE
C/ZFO-E
ZFO
S
BET (m g )
V (cm g )
D (nm) Carbon content (wt%)
60.3
0.21
10.3
21.5
23.7
5.19
4.26
---
47.4
0.19
13.5
0.06
a: Measured by CHN analyzer
It is well known that the optical property of a photocatalyst is one of the key factors
in determining its photocatalytic activity. The DRS in Fig. 5A suggested that ZFO,
C/ZFO-E and C/ZFO-CE composites showed strong photo harvests in the visible-light
region. The absorption edge for ZFO exhibited at about 645 nm, similar to that of
C/ZFO-E, but the visible-light absorption of the latter was enhanced due to the presence
of composed carbon. Especially, the absorption edge for C/ZFO-CE red shifted slightly
to 660 nm and showed distinct absorption behavior during 660-800 nm, the reason for
the above phenomenon was ascribed to the existence of doped and composed porous C
in the C/ZFO-CE composite. Moreover, the doped C might reduced the band-gap
energies (E
g
) of ZFO. Hence, the band-gap energies (E
g
) of ZFO, C/ZFO-E and
)n/2,34 where α, h, ν,
C/ZFO-CE were estimated by the empirical formula: αhν=A(hν-E
g
1
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g
E and A is respectively the absorption coefficient, Planck’s constant, the incident light
frequency, band gap energy and a constant, and n depends on the characteristics of the
type of optical transition in semiconductor, herein n is 1 for the direct transition of
1
2
ZnFe
2
O
4
. Thus the band-gap energies (E ) of the samples could be estimated from
g
plots of (αhν)2/n vs. photon energy (hν), and the intercept of the tangent to x axis gives a
good approximation of the band-gap energy shown in Fig. 5B. The estimated Eg values
were respectively about 1.91, 1.91 and 1.79 eV for ZFO, C/ZFO-E and C/ZFO-CE, and
the narrowed bandgap over C/ZFO-CE composites were ascribed to the carbon doping.
And the photoluminescence emission (PL) spectra of nanostructured semiconductor
materials are relative to the transfer behavior of the photoinduced electrons and holes, it
is well recognized that the higher PL intensity means more charge carriers participation
in the recombination. For PL spectrum of ZFO (Fig. 5C), the intensity of prominent
emission peak was the highest and centered at about 650 nm, suggesting the highest
recombination rates of the photogenerated carriers in all the three samples. And the
emission intensity of C/ZFO-E was lower than ZFO because of the C presence in the
former sample. Obviously, the PL peak intensity for porous C/ZFO-CE composite
greatly reduced due to the existed porous carbon and doped C in the form of O-C bond,
which corporately favored the separation rates of photogenerated electrons and holes.
1
6

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To further investigate the lifetime of the photogenerated carriers, the time-resolved PL
decay curves of ZFO, C/ZFO-E and C/ZFO-CE are obtained in Fig. 5D. As shown in
Table 2, the τ
1
and τ values for the three samples were calculated via double
2
exponential decay fitting.35 The fast decay process (τ
) is usually attributed to the
1
nonradiative relaxation process relevant to the defects of the oxide, and the slow decay
range (τ
2
) comes from the radiative process related to the recombination of the
photogenerated holes and electrons.36 It is clear that τ
for ZFO, C/ZFO-E and
2
C/ZFO-CE are 9.11, 9.64 and 10.56 ns, obviously implying that efficient separation of
photogenerated carriers is achieved in C/ZFO-CE composite.
A
B
a
c
b
c
b
a
300
400
500
600
700
800
1.6
2.0
2.4
2.8
3.2
3.6
4.0
Wavelength (nm)
hv (eV)
1
1
000
00
D
C
1
1
0
c
0
.1
000
00
1
a
1
1
1
0
b
a
b
c
0
.1
000
100
1
10
1
0.1
550
600
650
700
750
20
40
60
80
100
Time (ns)
Wavelength (nm)
2
Fig. 5. (A) UV-vis diffuse reflectance spectra, (B) the corresponding plots of (αhν) vs.
1
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hν, (C) PL spectra and (D) time-resolved PL decay curves for (a) ZFO, (b) C/ZFO-E and
c) C/ZFO-CE composites
Table 2. Parameters of fluorescence lifetimes for various samples
(
Samples
C/ZFO-CE
C/ZFO-E
ZFO
τ
1
(ns)
A
1
(%)
τ
2
(ns)
A (%)
2
1.18
1.09
1.02
60.41
65.04
45.93
10.56
9.64
9.11
39.59
34.96
54.07
The magnetic behavior of the obtained materials was conducted using a vibrating
sample magnetometer (VSM). Fig. 6 showed the room-temperature magnetic hysteresis
loops of ZFO, C/ZFO-E and C/ZFO-CE. All the magnetic hysteresis loops were S-like
curves, indicating that there were almost no remaining magnetizations when the external
magnetic field was removed.37 The saturation magnetization of C/ZFO-CE and
-1
-1
C/ZFO-E composites were 43.9 emu g and 51.2 emu g , lower than that of bare ZFO
-1
at 66.9 emu g , possibly due to the C coating on ZFO. The saturation magnetization of
C/ZFO-CE reduced slightly compared with C/ZFO-E possibly resulting from the
different carbon contents shown in Table 1. The high saturation magnetizations were
favorable for the easy separation of C/ZFO photocatalyst from the solution after the
reaction by external magnetic field, which was very attractive to the practical
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8

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applications.
80
a
b
40
c
0
-40
-80
-15
-10
-5
0
5
10
15
H (KOe)
Fig. 6. Room-temperature magnetization hysteresis loops of (a) ZFO, (b) C/ZFO-E and
c) C/ZFO-CE composites
(
Photocatalytic activity and stability
It was reported that slight H
2
2
O addition played a significant role in promoting
the pollutant photodegradation due to the peroxidase-like activity of ZnFe
2
4
O . Thus, the
effect of 30% H
2
2
O amount on photocatalytic activity for RhB degradation over
C/ZFO-CE was investigated and summarized in Fig. 7. The rate constant (k) increased
with H
2
2
O amount up to 2.0 mL, then decreased with further addition to 2.5 and 3.0 mL.
The increase of the degradation activity in the presence of H
2
2
O was attributed to the
formation of more •OH resulted from the activation of H
2
O
2
with ZnFe
2
4
O . The slight
decrease of RhB removal efficiency at >2.0 mL was possibly due to the quenching of
•
OH by competing reactions with H
2
O
2
and •OOH at higher amounts of H
2
O
2
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9

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38
(
•OH+H
2
O
2
→H
2
O+•OOH and •OH+•OOH→H
2
O + O
2
). Hence, the amount of H
2
O
2
in the following experiments was confirmed as 2 mL.
0.06
0.05
0.04
0.03
0.02
0.5
1.0
1.5
2.0
2.5
3.0
3
0% H O (mL)
2 2
Fig. 7. Effect of H
2
2
O amounts on the degradation of RhB under visible light over
-1
C/ZFO-CE. Reaction conditions: 25 °C, visible light, 100 mL 5 mg L RhB, 100 mg
catalyst.
The photocatalytic activities of different systems under visible-light irradiation
were carried out for degradation of RhB at room temperature. Fig. 8A displayed the
adsorption in dark and the photodegradation process at different irradiation intervals for
RhB. In the absence of pristine H
2
2
O or pure ZFO, only 11% and 13% of RhB were
degraded within 90 min. Additionally, 38% and 55% of RhB were degraded over
C/ZFO-E and C/ZFO-CE within 90 min irradiation, the improved catalytic activities
were ascribed to the synergistic effects between C and ZFO, which led to the efficient
separation of photogenerated carriers and contributed to the enhanced activity. However,
the improved catalytic activity of C/ZFO-CE over C/ZFO-E was resulted from the even
2
0

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mixing of carbon and small-sized ZFO in the C/ZFO-CE composite, which were
favorably synthesized by the high dispersity of the CO medium. Fascinatingly, the
2
degradation rates of RhB molecules were significantly enhanced when 2 mL H
2
2
O was
added to C/ZFO-E and C/ZFO-CE systems. Almost 100% of RhB were degraded over
C/ZFO-E and C/ZFO-CE in the presence of 2 mL of 30% H
2
2
O within 75 and 60 min.
The remarkable enhancements of activities were attributed to the “photo-Fenton”
process. In “photo-Fenton” process, the visible light irradiation to the solution including
3
+
3+
2+
Fe and H
2
O improved the overall •OH production by photo-reduction of Fe to Fe
2
3
+
2+
+
(
Fe + hν + H
2
O → Fe + •OH + H ) and H
2
O
2
photolysis (H
2
O
2
+ hν → 2•OH), and
correspondingly the overall •OH generation increased the rate of degradation of organic
39
pollutants. Fig. 8B illustrated the variations in the optical absorption spectra of the
RhB dye with irradiation time in the C/ZFO-CE+H
2
2
O system. The obviously reduced
absorption intensity indicated that RhB degraded completely after 60 min irradiation.
The blue shift of λmax implied that RhB was N-deethylated and the conjugated structure
was gradually destructed.40 Moreover, the photocatalytic data afforded linear
relationships by fitting with pseudo-first-order kinetics model, as shown in Fig. 8C.
Notably as exhibited in Fig. 8D, the C/ZFO-CE+H
2
2
O system exhibited the highest rate
constant at 4.84 × 10-2 min , approximately 1.29 times larger than that of
-1
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-2
-1
C/ZFO-E+H
2
O
2
(3.75×10 min ) and 6.47 times larger than that of C/ZFO-CE (7.47
10-3 min ). Moreover, the changes in the TOC of products reflected the
mineralization degree of an organic molecule during the testing period. Fig. 8E
-1
×
exhibited the TOC removal rate of C/ZFO-E, C/ZFO-CE, C/ZFO-E+H
2
O
2
and
C/ZFO-CE+H
2
O
2
after 60 min irradiation. The corresponding mineralization
efficiencies to RhB were 4.76%, 10.93%, 45.61% and 76.37%. Combined with the
results of TOC and UV-vis absorption spectra, some small molecule organic compounds
such as succinic acid were detected by GC-MS during the photodegradation of RhB.
According to the previous reports, 41,42 the possible degradation pathways of RhB were
shown in Fig. 9. The photodegradation of RhB includes four main processes:
N-deethylation, chromophore cleavage, opening-ring and mineralization. During the
photocatalytic oxidation process, various active intermediate species including •O
-,
2
+
-
•
OH, h , e were formed, which were involved in destruction process of RhB structure.
Their oxidation or reduction activities made RhB N-deethylated and destruct the
conjugated structure. With the prolonged time of light irradiation, the effects of active
intermediate species were enhanced, resulting in further opening-ring and
mineralization of less organic matters. For comparison, the experiments without light
irradiation were carried out (Fig. 8F). The degradation efficiencies for RhB in the
2
2

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systems of H
2
2
O , ZFO, C/ZFO-E and C/ZFO-CE were extremely low within 90 min.
Only 20% and 29% of RhB were degraded in C/ZFO-E+H
2
O
2
and C/ZFO-CE+H
2
O
2
systems within 90 min, and the lower degradation rate was attributed to the classical
Fenton reaction without the participation of visible light. Thus, the combination of
C/ZFO-CE+H
2
2
O and visible-light irradiation improved the degradation rate.
1
.2
.0
.8
.6
.4
.2
.0
1
.0
.8
.6
.4
.2
.0
400
A
B
1
0
a
b
0
min
15 min
0 min
0
0
3
0
c
d
45 min
60 min
0
0
0
0
0
e
f
0
-60 -40 -20
0
20
40
60
80 100
500
600
700
Irradiation Time (min)
Wavelength (nm)
3
.5
.0
.5
.0
.5
.0
.5
.0
0
.06
C
D
3
f
e
0.05
2
2
0.04
0.03
0.02
1
1
d
c
b
a
0
0.01
0
0.00
0
20
40
60
80
100
a
b
c
d
e
f
Irradiation Time (min)
1
.1
.0
.9
.8
F
E
8
0
1
a
b
60
0
c
d
e
40
0
f
2
0
0.7
0.6
0
c
d
e
f
-60 -40 -20
0
20
40
60
80 100
Time (min)
Fig. 8. (A) Adsorption and photocatalytic activities to RhB, (B) UV-vis absorption
2
3

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spectra of RhB in C/ZFO-CE+H
2
2
O system for different irradiation time, (C) first-order
kinetic plots, (D) rate constants for photodegradation of RhB, (E) TOC removal rates of
RhB, (F) adsorption and degradation of RhB in dark of (a) H
2
2
O , (b) ZFO, (c) C/ZFO-E,
(
d) C/ZFO-CE, (e) C/ZFO-E+H
2
O
2
and (f) C/ZFO-CE+H
2
2
O . Reaction conditions:
-1
2
5 °C, visible light (except Fig. F), 100 mL of 5 mg L RhB, 100 mg catalyst, 2 mL of
3
0% H (except b, c and d).
2
O
2
2
4

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Fig. 9. Possible degradation pathway of RhB
In order to eliminate the photosensitization effect of dye, the photodegradation of
colorless organic pollutant phenol was recorded in Fig. 10A. After visible-light
irradiation for 3 h, only 14% and 16% of phenol were degraded in the H
2
2
O or pure
ZFO system. Over C/ZFO-E and C/ZFO-CE catalysts, the degradation efficiencies were
2
5

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respectively increased to 42% and 58%. Furthermore, 91% and 99% of phenol were
degraded by C/ZFO-E and C/ZFO-CE in the presence of 2 mL 30% H
2
2
O within 2 h.
Fig. 10B illustrated the variations in the optical absorption spectra of phenol with
irradiation time in the C/ZFO-CE+H
2
2
O system. The reduced absorption intensity at
λ=270 nm indicated that phenol mostly degraded after 120 min irradiation, further
confirming the high performance of C/ZFO-CE+H
2
2
O system under visible-light
irradiation. The photocatalytic data afforded linear relationships by fitting with
pseudo-first-order kinetics model and the C/ZFO-CE+H
2
2
O system exhibited the highest
-1
rate constant at 0.0183 min , approximately 1.35 times larger than that of
-1
C/ZFO-E+H
2
O (0.0136 min ) and 4.59 times larger than that of C/ZFO-CE (3.99×
2
-3
-1
1
0 min ). Fig. 10D displayed the TOC removal rates of various systems after 2 h
irradiation. The TOC results indicated that the mineralization efficiencies of C/ZFO-E,
C/ZFO-CE, C/ZFO-E+H
2
O
2
and C/ZFO-CE+H
2
2
O were at 3.83%, 10.67%, 36.98% and
6
3.92%, respectively. The C/ZFO-CE in the presence of H
2
2
O could not only efficiently
degrade phenol but also mineralize to CO under visible-light irradiation.
2
2
6

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1
.0
1
.2
.0
.8
.6
.4
.2
.0
A
B
1
0.8
0.6
0
3
6
9
min
0 min
0 min
0 min
a
b
0
0
c
d
120 min
0
.4
.2
.0
200
0
0
0
e
f
0
0
-60 -30
0
30 60 90 120 150 180
300
400
500
600
Irradiation Time (min)
Wavelength (nm)
3
.5
8
0
-
4
-1
k =7.33 x10 min
a
C
D
3.0
-4
-1
k =7.78 x10 min
b
-
3
-1
2.5 k =2.57 x10 min
60
40
c
f
-
3
-1
k =3.99 x10 min
2.0
d
-1
ke=0.0136 min
e
1.5
-1
kf=0.0183 min
1
.0
.5
.0
d
c
20
0
b
a
0
0
0
20 40 60 80 100 120 140 160 180 200
c
d
e
f
Irradiation Time (min)
Fig. 10. (A) Adsorption and photocatalytic activities to phenol, (B) UV-vis absorption
spectra of phenol in C/ZFO-CE+H
2
2
O system for different irradiation time, (C)
first-order kinetic plots and rate constants for photodegradation of phenol, (E) TOC
removal rates of phenol of (a) H
2
2
O , (b) ZFO, (c) C/ZFO-E, (d) C/ZFO-CE, (e)
C/ZFO-E+H
2
O
2
and (f) C/ZFO-CE+H
2
2
O . Reaction conditions: 25 °C, visible light, 100
-1
mL of 5 mg L phenol, 100 mg catalyst, 2 mL of 30% H
2
2
O .
It is well known that the practical applications also rely on the resuability of a
catalyst. Thus, the photocatalytic tests for RhB degradation were recycled for
consecutive five runs in the C/ZFO-CE+H
2
2
O system. For the fifth recycle, there was no
obvious reduction in activity (Fig. 11A). Besides, the structure of C/ZFO-CE composite
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7

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was further measured after five runs by XRD (Fig. 11B). The XRD peaks did not
change obviously for the fresh and spent photocatalysts. However, after the
photocatalytic reaction, the catalyst was filtered and the solution was detected by
-1
ICP-OES analysis, and the concentration of Fe in the solution was 0.019 mg L , which
was resulted from the minute leaching of ZnFe
2
4
O . Furthermore, Fe 2p high resolution
XPS spectra of before and after five photocatalytic runs were carried out (Fig. 11C).
Compared with the XPS of the fresh C/ZFO-CE composite, although the ratio of
Fe(II)/Fe(III) on the spent catalyst surface was elevated from 0.61 to 0.74, implying that
4
3
Fe(II) was increased during the photocatalytic process. In the next catalytic run for
RhB degradation, the addition of H
2
2
O would help the oxidation of Fe(II) to Fe(III)
again. Thus the catalytic activity of C/ZFO-CE composite kept almost stable even in the
five runs.
1
.2
.0
.8
.6
.4
.2
.0
A
B
1st
2nd
3rd
4th
5th
1
b
0
0
0
a
0
0
0
60
120
180
240
300
10
20
30
40
50
60
70
80
2
Theta (degree)
Irradiation Time (min)
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8

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Fe 2p Fe 2p3/2
Fe 2p1/2
24.4 eV
C
7
10.8 eV
7
7
11.2 eV
710.3 eV
712.5 eV
b
7
14.0 eV
7
11.4 eV
710.3 eV
a
7
13.8 eV
7
12.4 eV
705
710
715
720
725
730
Binding Energy (eV)
Fig. 11. (A) Recycling experiments of C/ZFO-CE+H
2
2
O system. Reaction conditions:
-1
2
5 °C, visible light, 100 mL of 5 mg L RhB, 100 mg catalyst, 2 mL of 30% H
2
2
O . (B)
XRD patterns and (C) Fe 2p XPS spectra of (a) fresh and (b) spent C/ZFO-CE
composite after five recycles
Photocatalytic mechanism detection
It has been widely accepted that hydroxyl radical •OH, which is generated under
illumination of a photocatalyst, as the main species is responsible for the degradation of
pollutant molecules. Hence the formation of •OH in the system of C/ZFO-CE+H
2
O
2
under visible-light irradiation was studied by PL-TA technique and ESR spectra. Using
terephthalic acid (TA) as probe, the produced •OH was measured by recording the
maximum fluorescence intensity of the formed 2-hydroxyterephthalic acid (TAOH) at
13
about 425 nm with an excitation source of 315 nm. Fig. 12A showed fluorescence
spectra of TAOH formed in the C/ZFO-CE suspension containing 0.1 mL TA and 2.0
2
9