Trace Amount CoFe2O4 Anchored on a TiO2 Photocatalyst Efficiently Catalyzing O2 Reduction and Phenol Oxidation

Article abstract of DOI:10.1021/acs.langmuir.9b00291

Semiconducting TiO₂ is the most studied photocatalyst for organic oxidation by O₂. To accelerate the reaction, a cocatalyst for O₂ reduction or for organic oxidation is often used, but the bifunctional one is rare. Herein we report a spinel CoFe₂O₄ (CF) efficiently catalyzing O₂ reduction and phenol oxidation on TiO₂ in aqueous suspensions at pH 3-11. The composite materials (CF/TiO₂) were made by depositing 0-5 wt % CF onto TiO₂ through a hydrothermal method. Solid characterization showed that CF nanoparticles (5 nm) homogeneously distributed in CF/TiO₂, whereas the TiO₂ phase remained unchanged in crystal structure and crystallite size. For phenol oxidation under UV light, CF was nearly not active, but 0.01 wt % CF/TiO₂ was more active than TiO₂, by approximately a factor of 3.6. Such trend in activity among the catalysts was also observed from the photocatalytic reduction of O₂ to H₂O₂, from the electrochemical reduction of O₂, and from the photoelectrochemical oxidation of phenol and H₂O. An open-circuit potential and photoluminescence measurement suggest that there is an interfacial electron transfer from TiO₂ to CF, followed by O₂ reduction. Accordingly, a possible mechanism is proposed, involving CF catalysis for O₂ reduction and phenol oxidation. Then the mutual promotion between electron and hole transfer results in great enhancement in the efficiency of charge separation and hence in the rate of chemical reaction. Because spinel compounds have rich composition and unique structures, they are worthy of being further investigated as cocatalysts of a semiconductor photocatalysis.

Full text of DOI:10.1021/acs.langmuir.9b00291

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Interfaces: Adsorption, Reactions, Films, Forces, Measurement Techniques, Charge
Transfer, Electrochemistry, Electrocatalysis, Energy Production and Storage
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Trace Amount CoFeOAnchored on a TiO Photocatalyst
2
Efficiently Catalyzing O Reduction and Phenol Oxidation
Min Chen, and Yiming Xu
Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.9b00291 • Publication Date (Web): 26 Jun 2019
Downloaded from http://pubs.acs.org on June 27, 2019
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Trace Amount CoFe2O4 Anchored on a TiO2 Photocatalyst Ef-
ficiently Catalyzing O2 Reduction and Phenol Oxidation
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Min Chen, and Yiming Xu*
State Key Laboratory of Silicon Materials and Department of Chemistry, Zhejiang University, Hangzhou 310027,
China.
Keywords: Photocatalysis; O2 reduction; Organic oxidation; TiO2; CoFe2O4; Semiconductor; Spinel.
ABSTRACT: Semiconducting TiO2 is the most studied photocatalyst for organic oxidation by O2. To accelerate the reac-
tion, a co-catalyst for O2 reduction or for organic oxidation is often used, but the bifunctional one is rare. Herein we re-
port a spinel CoFe2O4 (CF) efficiently catalyzing O2 reduction and phenol oxidation on TiO2 in aqueous suspensions at
pHs 311. The composite materials (CF/TiO2) were made by depositing 05 wt% CF onto TiO2 through a hydrothermal
method. Solid characterization showed that CF nanoparticles (5 nm) homogenously distributed in CF/TiO2, whereas TiO2
phase remained unchanged in crystal structure and crystallite size. For phenol oxidation under UV light, CF was nearly
not active, but 0.01 wt % CF/TiO2 was more active than TiO2, by approximately a factor of 3.6. Such trend in activity
among the catalysts was also observed from the photocatalytic reduction of O2 to H2O2, and from the electrochemical
reduction of O2, and from the photoelectrochemical oxidation of phenol and H2O, respectively. An open circuit potential
and photoluminescence measurement suggest that there is an interfacial electron transfer from TiO2 to CF, followed by
O2 reduction. Accordingly, a possible mechanism is proposed, involving CF catalysis for O2 reduction and phenol oxida-
tion, respectively. Then the mutual promotion between electron and hole transfer results into great enhancement in the
efficiency of charge separation, and hence in the rate of chemical reaction. Since spinel compounds have rich composition
and unique structures, they are worthy of being further investigated as co-catalysts of a semiconductor photocatalysis.
One of the strategies is modification of TiO2 surface
■ INTRODUCTION
with a co-catalyst. For instance, a Fe2O3 cluster deposited
Titanium dioxide is the most studied photocatalyst for
energy and environmental application, due to its low cost,
good activity, and high stability.1,2 The primary process
occurring over the irradiated TiO2 is generation of elec-
trons (ecb) and holes (hvb+) in the conduction and valence
bands, respectively. Then these charge carriers recombine
to heat, or migrate onto the surface to react with sorbents.
on TiO2 can mediate the electron transfer to O2 through a
FeIII/II recycle, and hence accelerate phenol oxidation.11,12
A
cobalt phosphate (CoPi) deposited on TiO2 can mediate
the hole transfer to water,13 and phenol,14 through a CoIV/III
recycle. But Fe2O3/TiO2 is only 1.7 times more active than
TiO2, whereas CoPi/TiO2 is deactivated in absence of CoPi
repairer and electron remover. Furthermore, previous
catalysts rarely possess catalytic activity for both O2 re-
duction and organic oxidation. Herein we report a bifunc-
tional performance of CoFe2O4 in TiO2 photocatalysis for
phenol oxidation in aqueous solutions at pHs 311.
For example, ecb of anatase TiO2 can reduce O2 to O2
,
+
whereas the counterpart hvb can oxidize various organics
to CO2 and other products. There are many factors influ-
encing the efficiency of TiO2 photocatalysis. Among them,
the physical property of TiO2 is the determining one.3,4 In
general, the apparent photocatalytic activity of TiO2 great-
ly changes with its crystal structure, surface area, defect
sites, and so on. But the intrinsic photocatalytic activity of
TiO2 exponentially increases with its synthesis tempera-
ture, regardless of the solid structures.5-9 A high intrinsic
photocatalytic activity means a large number of ecb and
hvb+, which have migrated onto and reached the oxide
surface. Due to fast charge recombination and slow sur-
face reaction, however, the quantum efficiency of TiO2
photocatalysis is still not high enough to enable practical
application.10 Therefore, how to speed up the interfacial
charge transfer is the central issue in TiO2 photocatalysis.
Recently, spinel compounds as catalysts for the reduc-
tion and evolution of O2 have received great attention.15
For example, CoFe2O4 (devoted as CF) has catalytic activi-
ty not only for a Ru(II) dye sensitized oxidation of H2O to
O2, but also for the electrochemical reduction of O2 to
H2O2 or H2O.16 In structure, CF is a partially inverse spinel,
where Co2+ and Fe3+ occupy both the tetrahedral (A) and
octahedral (B) sites. Due to presence of an unpaired elec-
tron, CF also has ferrimagnetism. Therefore, CF has been
widely used as a support of TiO2.17-26 The composite mate-
rials (devoted as CF/TiO2) was prepared by hydrolysis of a
Ti(IV) precursor in the presence of CF,17,18,24-26 or by for-
mation of CF in the presence of TiO2,21-23 and/or by co-
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formation of CF and TiO2 from a mixed solution.19,20 Since
CF content was high (1178 wt%),18,19 these materials were
magnetic, and separated easily from aqueous suspensions
with an external magnet. Under UV and visible light,
CF/TiO2 is active for organic degradation, but a contro-
versial result has been reported. On one hand, CF/TiO2
was more active than TiO2 for methyl blue and rhodamine
B degradation.17,23,25,26 On the other hand, CF/TiO2 was less
active than TiO2 for methyl orange and rhodamine B deg-
radation.18,19 Such discrepancy is mostly due to the dye
adsorption and sensitization that changes with the model
dye and catalyst used. In aqueous solution, the adsorbed
organic dyes on TiO2 can degrade even under visible light,
where dye is the light absorber, and TiO2 is a conducting
mediator for O2 reduction. 21,27 In other words, the role of
CF in TiO2 photocatalysis is yet to be explored.
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In this work, the effect of CF on the photocatalytic reac-
tions of TiO2 has been examined. To avoid the influence
of organic photolysis and adsorption, phenol was selected
as a model substrate. In aqueous solution, phenol photol-
ysis and its dark adsorption on TiO2 are both negligible.
Then the observed difference between TiO2 and CF/TiO2
in the rate of phenol oxidation is surely due to CF effect
on the photocatalytic reaction. Samples were prepared by
depositing 05 wt % CF onto a commercially available
TiO2, via a hydrothermal method. Solid was characterized
with several techniques. For phenol oxidation under UV
light, CF was nearly not active, but the samples, contain-
ing 0.0010.10 and 15 wt % CF, were more and less active
than TiO2, respectively. To understand the role of CF, sev-
eral experiments were carried out, including the catalyst
stability test, the photocatalytic reduction of O2 to H2O2,
the electrochemical reduction of O2, the photoelectro-
chemical oxidation of water and phenol, and the meas-
urement of open circuit potential and photoluminescence.
Furthermore, a possible mechanism responsible for the
different effect of CF among samples is discussed.
200
300
400
500
600
700
Wavelength  (nm)
Figure 1. (A) XRD patterns of (a) TiO2, (b) 5% CF/TiO2, and
(c) CF. (B) Absorption spectra of CF and 05% CF/TiO2.
Figure 1B shows the solid absorption spectra. In the
spectrum of TiO2, there was an intensive absorption band
at wavelengths shorter than 400 nm, due to the band-to-
band transition of TiO2. Through a Tauc plot, the band
gap energy (Eg) for TiO2 was estimated to be 3.12 eV (Fig-
ure S2), well matching that for anatase TiO2.3 In the spec-
trum of CF, there was a broad band from 200 nm to 800
nm. The estimated Eg for CF was 1.33 eV, similar to those
reported.29-31 In the spectrum of CF/TiO2, there was also a
visible light absorption band, which increased with the
increase of CF content. Meanwhile, the sample changed
its color from white to yellow to brown (Figure S3).
■ RESULTS AND DISCUSSION
Solid structure was examined by X-ray diffraction (Figure
1A). The diffractions of TiO2 were strong, and matched
those for anatase TiO2 (PDF#04-0477). The house-made
CF showed a XRD pattern in good agreement with that for
cubic CoFe2O4 (PDF#02-1045). But the peak intensity of
CF was much weaker than that of TiO2. This is indicative
of CF being poorly crystallized under the conditions used.
The CF/TiO2 samples were nearly the same as TiO2, either
in the diffraction position or peak width of anatase (Fig-
ure S1). As CF content was higher than 1 wt%, however,
the diffractions due to CF became visible. These observa-
tions indicate that after CF loading, TiO2 phase remains
intact in terms of the crystal structure and crystallite size,
while CF in CF/TiO2 is mostly in an amorphous form.
Figure 2 shows the solid images of transmission elec-
tron microscopy (TEM). TiO2 particles were flake-shaped,
with a length of ca. 3040 nm. CF particles were sphere-
like, with a diameter of ca. 5 nm. But most of CF particles
seriously agglomerated, due to magnetism and small size.
In 5% CF/TiO2, there were small particles (CF) present on
large particles (TiO2). In the high resolution TEM image,
there were two interlayer distances at 0.35 and 0.21 nm,
corresponding to the (1 0 1) facet of anatase TiO2
(PDF#04-0477), and the (4 0 0) facet of CoFe2O4 (PDF#02-
1045), respectively. An element mapping with 5% CF/TiO2
showed that Fe and Co species homogenously distributed
in the sample (Figure S4). These observations indicate
that CoFe2O4 has been successfully deposited onto TiO2
via a simple hydrothermal method.
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content. With each sample, moreover, the measured sur-
face area was always lower than the sum of the surface
area calculated from individual CF and TiO2, respectively.
These observations indicate that the micro- and meso-
pores of TiO2 are occupied or blocked by CF nanoparticles,
together with construction of a large porous network.
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Figure 2. TEM images of (A) TiO2, (B) CF, and (C) 5%
CF/TiO2, and (D) HRTEM image of 5% CF/TiO2.
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The chemical state of CF was examined by X-ray photo-
electron spectroscopy (XPS), and the results are shown in
Figure S5. In the spectrum of Co 2p, the peak at 780.20 eV
(a satellite at 786.30 eV) is assigned to Co 2p3/2, while the
peak at 796.20 eV (a satellite at 804.00 eV) is assigned to
Co 2p1/2. These peaks correspond to Co2+ in CoFe2O4.32,33
Moreover, a small peak at 782.90 eV corresponds to Co3+.
In the spectrum of Fe 2p, there were five peaks, character-
istics of Fe3+. The peaks at 710.30 eV and 713.00 eV (a satel-
lite at 718.50 eV), and 723.50 eV (a satellite at 726.22 eV)
are assigned to Fe 2p3/2 and Fe 2p1/2, respectively.34 The
measured valences of Co and Fe elements in CF are in
agreement with the formula of CoFe2O4. Moreover, the
cation distribution of CF was examined by Mӧssbauer
spectroscopy at room temperature,35-37 and the results are
shown in Figure S6 and Table S1. There were 25.44 and
74.56% of Fe ions occupying the tetrahedral (tet) and oc-
tahedral (oct) sites of CF, respectively. By using a litera-
ture model,38 Co ions in A and B sites were calculated to
be 16.85 and 83.15%, respectively. Then CF is assigned as
(Co0.17Fe0.51)tet[Co0.83Fe1.49]octO4. Note that after CF was cal-
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(Co0.26Fe0.75)tet[Co0.70Fe1.25]octO4. That is, the CoFe2O4 sam-
ple obtained in this work is partially inverse spinel.
Irradiation time (min)
Figure 3. (A) Photocatalytic degradation of phenol, (B) the
formation of hydroquinone, and (C) the production of H2O2,
measured in aqueous phase of (a) TiO2, (b) 0.01% CF/TiO2, (c)
CF, (d) CoO/TiO2, and (e) Fe2O3/TiO2.
The porosity and surface area of solid was measured by
N2 adsorption, and the results are shown in Figure S7 and
Table S2. All samples showed a hypothesis loop in the
isotherms of N2 adsorptiondesorption. This is indicative
of mesopores present in those samples. In a comparison
with pure CF, pure TiO2 had a smaller surface area, but a
larger pore volume and average pore size. As CF content
in CF/TiO2 increased, the surface area and pore volume
decreased initially, and then increased. However, the av-
erage pore sizeregularly decreased with the increase of CF
Figure 3A shows the results of phenol degradation on
different catalysts in aqueous solution. At first glance,
phenol degradation on CF was very slow. Notably the re-
action on 0.01% CF/TiO2 was much faster than that on
TiO2. To verify the effect of CF, control experiments were
performed. In the dark, phenol adsorption on solid was a
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little (around 2%). Under UV light without catalyst, the
change of phenol concentration with time was negligible.
Then the observed change of phenol concentration with
time is surely due to a reaction initiated by photocatalysis.
Moreover, CoO/TiO2 and Fe2O3/TiO2 were only a little
more active than TiO2. These observations indicate that
the activity of CF/TiO2 much higher than that of TiO2
mostly originates from CF, rather than CoO and Fe2O3.
from O2 reduction to H2O2, is in good agreement with
that, observed from phenol oxidation to HQ.
kobs/Asp (105 g min1 m2
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To quantitatively describe the CF effect, the time profile
of phenol degradation fit the pseudo first-order rate equa-
tion. The resulting rate constants (kobs) from CF, TiO2, and
CF/TiO2 were 0.19, 2.24, and 8.10, respectively, in a unit of
103 min1. Accordingly, CF is 11.8 times less active than
TiO2, whereas CF/TiO2 is 3.6 times more active than TiO2.
These observations indicate that trace CF (0.01 wt.%) can
greatly improve the photocatalytic activity of TiO2.
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CF/TiO2
(B)
During phenol degradation, hydroquinone (HQ) was
formed as the major intermediate, and the result is shown
in Figure 3B. As the irradiation time increased, HQ con-
centration in aqueous phase increased toward a limit.
Among samples, the rate of HQ formation became larger
in the order of 0.01% CF/TiO2 > TiO2 > CF. This trend in
the rate of intermediate production is in agreement with
that in the rate of phenol degradation. In all cases, how-
ever, the amount of HQ formed was much lower than that
of phenol consumed. For example, at 120 min, the mole
ratio of HQ produced to phenol disappeared was 24, 17,
and 6% for CF/TiO2, TiO2, and CF, respectively. These
observations indicate that HQ also degrades in situ.
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8.0
9.0
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Initial pH of the suspensions
Figure 4. Apparent rate constants of phenol degradation, (A)
over 05% CF/TiO2, measured at initial pH7.0, and (B) over
TiO2 and 0.01% CF/TiO2, obtained at different initial pHs.
During phenol degradation, H2O2 was also detectable,
and the results are shown in Figure 3C. As the irradiation
time increased, H2O2 concentration in aqueous phase in-
creased. This is because the electron reduction of O2 and
the hole of phenol oxidation over TiO2 occur at the same
time.39,40 At given time, interestingly, H2O2 concentration
formed from 0.01% CF/TiO2 was approximately 3 times
more than that produced from TiO2. However, H2O2 in
aqueous solution can adsorb and decompose on TiO2.41,42
Then the real amount of H2O2 produced from catalyst
should be larger than that measured in aqueous phase.
Since 0.01% CF/TiO2 is more active than TiO2, it is nec-
essary to examine the effect of CF content. To do this, the
samples containing 05 % CoFe2O4 were prepared under
similar conditions. Figure 4A shows the results of phenol
degradation over those samples in aqueous suspensions.
As CF content in CF/TiO2 increased, the rate of phenol
degradation increased initially, and then decreased. A
maximum reaction rate was observed from 0.01% CF/TiO2.
Since the catalysts in aqueous suspensions may have dif-
ferent surface area, the rate constant of phenol degrada-
tion (kobs) was normalized tentatively with the solid sur-
face area (Asp), measured by N2 gas (Table S2). However,
the change of kobs/Asp with CF content well resembles that
of kobs (Figure 4A). Then the observed difference in activi-
ty among the samples due to the different surface area is
less likely. Since CF strongly absorbs UV light (Figure 1B),
the decreased rate of phenol degradation at high CF load-
ing is ascribed to excess CF, that reduces the number of
photons reaching TiO2, and hence slows down the TiO2-
photocatalyzed reaction. These observations indicate that
only trace amount CF is beneficial to TiO2 photocatalysis.
This result is quite different from those reported in the
literature.17-26 Previous samples containing 1178 % CF
To assess the fate of H2O2, separate experiment with 10
mM H2O2 was performed, and the result is shown in Fig-
ure S8. In the dark, there was approximately 5% of H2O2
adsorbed on TiO2 or 0.01% CF/TiO2. This is due to for-
mation of a peroxide complex on the surface Ti(VI) sites.37
Under UV light, H2O2 concentration in aqueous phase
decreased with time. This is because H2O2 decomposes
over the irradiated TiO2, through a reductive and oxida-
tive pathway.39-42 However, the photocatalytic decomposi-
tion of H2O2 on 0.01% CF/TiO2 was approximately 1.23
times faster than that on TiO2. In combination with the
result of H2O2 formation, it follows that CF/TiO2 is surely
more active than TiO2 for the photocatalytic reduction of
O2. This trend in activity between two catalysts, obtained
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have ferrimagnetism, and show an activity higher or lower
only 2.5% of total phenol degraded (1.91 mM). Moreover,
the possible change in chemical state was examined with
5% CF/TiO2. After illumination with UV light for 6 h, the
sample showed negligible change in the XPS spectra of Co
2p and Fe 2p, respectively (Figure S5). These observations
indicate that CF is not only stable, but also helps degrada-
tion of both phenol and HQ over the irradiated TiO2.
than that of TiO2 for dye degradation under UV-vis light.
The present 0.0010.1% CF samples are more active than
TiO2, but the samples containing 15% CF are less active
than TiO2. Moreover, pure CF prepared and used in this
study also has ferromagnetism, but 0.01% CF/TiO2 is near-
ly not ferromagnetic, as does pure TiO2 (Figure S9).
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The higher activity of 0.01% CF/TiO2 than that of TiO2
was observed in the wide range of initial pH from 3 to 11,
which is shown in Figure 4B. As the initial pH increased,
the rate constant of phenol degradation increased initially,
and then decreased. A maximum rate was observed at
approximately pH 7.0 from 0.01% CF/TiO2 and at pH 9.0
from TiO2. These observations imply that CF/TiO2 is sta-
ble in a weakly acidic and basic aqueous solution. This is
important to water treatment by TiO2 photocatalysis.
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200
300
500
Reaction time (min)
0
25
50
75
100
125
150
175
Time (s)
Figure 5. (A) Stability test of 0.01% CF/TiO2 for phenol deg-
radation. (B) The corresponding formation of hydroquinone.
Figure 6. LSV curves of the film electrodes of (a) TiO2, (b)
0.01% CF/TiO2, and (c) CF, measured in 0.5 M NaClO4. (A)
Under O2 (solid lines) or N2 (dotted lines) in the dark. (B)
Under N2 and UV light in presence of 0.43 mM phenol. (C)
The corresponding current recorded at 0.90 V vs. NHE.
To illustrate the catalyst stability, a repeated experi-
ment was performed, and the results are shown in Figure
5A. During five cycles, the photocatalytic performance of
0.01 %CF/TiO2 was excellent. From the first run to the
fifth, the rate constant of phenol degradation decreased
only by a factor of 1.33. The decreased reaction rate is
mostly due to the accumulated intermediates in suspen-
sions, competing with phenol for reactive species. In fact,
HQ concentration in aqueous phase increased with time,
and then decreased after reaching a maximum (Figure 5B).
After the last run, total HQ accumulated (48 μM) was
To understand the role of CF in TiO2 photocatalysis, the
reduction and oxidation reactions were examined, inde-
pendently, by a linear sweep voltammetry (LSV). Figure
6A shows the result of O2 reduction over TiO2, CF, and
0.01% CF/TiO2 film electrodes. As the bias swept negative-
ly, the dark current of each electrode increased. But the
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electrode current obtained under air was much larger
than that measured under N2. This is indicative of O2 re-
duction being the dominant process under air.
tion,45 the constant of OCP decay with time was estimated,
which was 0.0061, and 0.011 s1 for TiO2 and CF/TiO2, re-
spectively (Figure S15). That is, the electron transfer from
TiO2 to an acceptor is increased by a factor of 1.8, on the
deposition of 0.01% CF. Since proton reduction was negli-
gible (Figure 6A), it is possible that the “photogenerated”
electrons of TiO2 are transferred into CF (Figure 2D).
Among the electrodes, however, the onset potential
(Eon), and dark current of O2 reduction were different. The
Eon value became more positive in the order of CF >
CF/TiO2 > TiO2, whereas the electrode current also be-
came larger in the order of CF > CF/TiO2 > TiO2. As CF
content in CF/TiO2 increased, the dark current of CF/TiO2
increased, together with a positive shift in Eon (Figure S10).
By using a literature method,43 Eon was estimated, which
was 0.023, 0.011, 0.037, 0.047, and 0.249 V vs. NHE, re-
spectively, for the samples containing 0, 0.001, 0.01, 0.1,
and 100% CF. A more positive Eon and a larger current
correspond to an easier of reduction of O2. These observa-
tions indicate that trace CF (0.01%) is also capable of effi-
ciently catalyzing the electrochemical reduction of O2 on
a TiO2 electrode, as does CF on a FTO or glassy carbon
electrode.16 Note that when a graphite rod was used as
counter electrode (Figure S11), and/or infrared compensa-
tion was considered (Figure S12, no significant change was
observed in LSV or Eon trend among samples (Table S3).
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0.4
Light on
(A)
Light off
0.2
(b)
(a)
0.0
-0.2
-0.4
0
300
600
900
1200
1500
Time (s)
75
(B)
60
45
30
15
0
(a)
Figure 6B shows the results of phenol oxidation in 0.5
M NaClO4, measured under N2 and UV light. As the bias
swept positively, the electrode current increase initially,
and then increased toward a limit. Interestingly, the onset
potential and limit photocurrent of phenol oxidation be-
came more negative and larger, respectively, in the order
of CF/TiO2 > TiO2 > CF. Similar observations were also
obtained from water oxidation (Figure S13). But the onset
potential of water oxidation was slightly more positive
than that of phenol oxidation. That is, under the present
conditions, phenol oxidation is easier than water oxida-
tion. Moreover, at a fixed potential, the electrode photo-
current remained stable during four light-on and light-off
cycles (Figure 6C). Note that the photocurrent for each
catalyst was the average from four parallel electrodes
(Figure S14), and therefore the present result is reliable.
These observations indicate that trace CF (0.01%) is also
capable of efficiently catalyzing the hole oxidation of phe-
nol, as observed from a dye sensitized water oxidation.16
(b)
(c)
350
400
450
500
550
600
Wavelength (nm)
5
4
3
2
1
0
(a)
(C)
(b)
(c)
1.62 V
1.5
0.16 V
0.0
-0.5
0.5
1.0
2.0
Potential (V vs. NHE)
The fate of the photogenerated electrons was also ex-
amined with an open circuit potential (OCP) method.
Figure 7A shows the time profiles of OCP for TiO2, and
0.01% CF/TiO2 film electrodes, measured in 0.5 M NaClO4
under N2. Once the electrode was illuminated, the elec-
trode potential dropped, due to formation of the photoe-
lectrons. When the potential was stable, the light was
blocked off. Immediately, the electrode potentials quickly
raised, due to fast recombination of electrons and holes in
nsμs domain.44 Then OCP decayed slowly with time, due
to the electron scavenging by an acceptor present in elec-
trolyte solution. However, CF/TiO2 showed a decay of
OCP faster than that of TiO2. By using a literature equa-
Figure 7. (A) OCP curves measured in 0.5 M NaClO4 under
N2. (B) Photoluminesce spectra of powders under air. (C)
MS plots measured at 1 kHz in a 0.5 M NaClO4 solution at
pH 5. Samples were (a) TiO2, (b) 0.01% CF/TiO2, and (c) CF.
To further examine the CF effect, the solid photolumi-
nescence (PL) spectrum was recorded, and the results are
shown in Figure 7B. After TiO2 was excited with 325 nm
light, there were several emission peaks, centered at 393,
449, 467, 480, and 492 nm, respectively. The peak at 393
nm (3.16 eV) well matches the band gap of anatase TiO2
(Figure 1B), while other peaks at 449492 nm (2.522.76
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
These observations imply that in CF/TiO2, the amorphous
CF is more efficient than the crystallized CF, in participa-
tion of the TiO2-photocatalyzed reaction. In other words,
CF acts as a molecular cocatalyst of TiO2 photocatalysis,
instead of a semiconductor coupling with TiO2.
eV) are assigned to ecb recombination with the trapped
+

hVB within band gap, and/or to ecb reaction with the ad-
sorbed O2 on the oxide surface.46 Comparatively, the emis-
sions of CF were rather weak, while the PL spectrum of
0.01% CF/TiO2 was similar to that of TiO2. But at given
wavelength, the peak intensity of CF/TiO2 was lower than
that of TiO2. In combination with OCP, it follows that the
photogenerated electrons of TiO2 are transferred into CF,
followed by O2 reduction. As a result, the emissions are
reduced, either at 393 nm, or at 449492 nm. On the oth-
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er hand, it is also possible that the trapped hVB+ of TiO2
transfers into CF, suppressing its recombination with ecb

.
To understand the photocatalytic mechanism of a sem-
iconductor (SC), one needs to know the conduction band
edge potential (ECB), and the valence band edge potential
(EVB). But such data for CoFe2O4 are not found in the liter-
ature. Figure 7C shows the MottSchottky (MS) plots of
three film electrodes in 0.5 M NaClO4 at pH 5.0. First, the
plot slope was positive for TiO2, but negative for CF. It
means that TiO2 and CF are n-type and p-type SC, respec-
tively.45 Second, the plot intercept with the potential axis
corresponds to the flat potential (Efb). In general, Efb is
close to ECB for n-type SC, and to EVB for p-type SC. From
Eg = EVB  ECB, ECB of CF was calculated to be 0.46 V, more
positive than that of TiO2. The MS plot of 0.01% CF/TiO2
was similar to that of TiO2. But Ef of 0.01% CF/TiO2 was
approximately 50 mV more negative than that of TiO2.
Scheme 1. Possible mechanism for the enhanced photocata-
lytic activity of CF/TiO2, where small balls represent CF.
Accordingly, a possible role of CF in TiO2 photocatalysis
is proposed in Scheme 1. First, the photogenerated elec-
trons and holes of TiO2 are captured by the Fe3+ and Co2+
species of CF, respectively. Then, the reduced CF (Fe2+
species) is oxidized by O2, and the oxidized CF (Co3+ spe-
cies) is reduced by phenol. As a result, CF is regenerated.
These processes resemble those of Fe2O3 and CoPi depos-
ited on TiO2, respectively.11-14 However, the photocatalytic
reactions are different from the (photo)electrochemical
reactions. The former occurs on the same particles in an
air-exposed aqueous suspension, whereas the latter occurs
on different electrodes in a N2-purged electrolyte under
an external potential. Then one may worry about the fast
In aqueous solution at pH 0, the literature values of ECB
and EVB for anatase TiO2 are 0.12 and 3.08 V vs. normal
hydrogen electrode (NHE), respectively.47,48 Then ECB and
EVB of CF are estimated to be 0.34 and 1.67 V vs. NHE, re-
spectively. In thermodynamics, the holes of both TiO2 and
CF are capable of oxidizing phenol (1.44 V vs. NHE) or
H2O (1.23 V vs. NHE).49,50 But the electron reduction of O2
to HO2• (0.05 V vs. NHE) is allowed on TiO2,46 but not on
CF. This is one of the reasons why CF is much less active
than TiO2 for the photocatalytic degradation of phenol in
aqueous suspensions (Figure 3A). Recall that CF and

+
recombination of ecb (Fe2+) and hvb (Co3+) on TiO2, mak-
ing CF in null cycle. This possibly is argued as follows.
First, the electron reduction of O2 and the hole oxidation
of phenol on the irradiated TiO2 can occur simultaneously.
Second, the rate of phenol degradation is proportional to
the crystallite size of TiO2.7 Third, CoFe2O4 has a unique
structure, where both Fe and Co ions occupy the tetra-
and octahedral sites (Figure S6). Forth, a XPS study shows
that in CoFe2O4, there are also Fe2+ and Co3+ species,
peacefully coexisting together without redox reaction.16,33
Then the reduction and oxidation of CF can occur on the
different particles of TiO2, and/or at different sites of CF.
Therefore, the proposed role of CF as a bifunctional cocat-
alyst in TiO2 photocatalysis is highly plausible.
o
CF/TiO2 used in photocatalysis were prepared at 160 C,
whereas TiO2 is industrially made at a temperature lower
than 450 oC.7 Then the low activity of CF is also due to its
poor crystallinity, as compared with TiO2 (Figure 1A).
To verify the crystallinty effect, the solid was further
calcined at 500 oC for 2 h. After that, all solids had an en-
hanced XRD diffraction (Figure S16). Meanwhile, the rates
of phenol degradation on TiO2 and CF were increased by
factors of 1.3 and 1.6, respectively (Figure S17). These ob-
servations confirm that the reaction rate does increase
with the crystallinity of SC (TiO2 or CF).5-9 After thermal
treatment, however, the rate of phenol degradation on
0.01% CF/TiO2 decreased by a factor of 1.6. The changes in
the reaction rate on the thermal treatment are not due to
changes in the solid surface area (Table S4 and Figure S17).
In the suspensions, both TiO2 and CF are the light ab-
sorbing species. But the observed reaction on CF/TiO2 is
predominantly initiated by TiO2. This is inferred from the
fact that CF is nearly not active and that 0.01% CF/TiO2 is
more active than TiO2. Furthermore, in TiO2 photocataly-
sis, the electrons and holes are photogenerated and con-
sumed in pairs. Any action that accelerates the electron
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transfer would promote the hole transfer, and vice visa. In
orded on a Shimadzu UV-2600 with BaSO4 as a reference.
the present case, CF acts as a co-catalyst for both electron
and hole transfer. As a result, the efficiency of charge sep-
aration is greatly improved, and CF/TiO2 is much more
active TiO2 for the photocatalytic degradation of phenol.
Reflectance (R) was transferred into Kubelka‒Munk ab-
sorbance, FR = (1‒R)2/(2R). X-ray photoelectron spectros-
copy (XPS) was made on an ESCA Lab 220i-XL. The spec-
trum was calibrated with C 1s at 284.6 eV. Adsorption
isotherms of N2 on solid were measured at 77 K on a Mi-
cromeritics ASAP2020. The solid surface area was calcu-
lated using BrunauerEmmettTeller (BET) equation.
Photoluminescence (PL) spectrum was recorded in air at
room temperature, on a Shimadzu F-2500 spectropho-
tometer. Scanning electron microscope (SEM) measure-
ment was performed on a Hitachi SU-8010. Transmission
electron microscopy (TEM) and high-resolution transmis-
sion electron microscopy (HRTEM) images were recorded
on a JEM-2100F field-emission instrument. 57Fe Mӧssbauer
spectroscopy was recorded on a spectrometer operating in
constant acceleration mode at 300K. The source used was
a 57Co (Rh), and the isomer shift was referred to α-Fe.
■ CONCLUSIONS
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In this work, a novel photocatalyst of TiO2 has been suc-
cessfully deposited with trace CF (0.01 %) through a sim-
ple hydrothermal method. This material is much more
active than parent TiO2 for the photocatalytic oxidation of
phenol in an air-saturated aqueous suspension. Impres-
sively, the composite photocatalyst is not only stable, but
also operative in both a weakly acidic and alkaline aque-
ous solution. An independent study of electrochemistry
clearly indicates that trace CF can efficiently catalyze O2
reduction and phenol oxidation, respectively. With the
aid of an OCP and PL, it is proposed that CF acts as a bi-
functional co-catalyst for both O2 reduction and organic
oxidation in TiO2 photocatalysis. There are many spinel
compounds with different compositions and unique prop-
erties.15 The present work of CoFe2O4 effect would help
further study of spinel compounds as co-catalysts of a
semiconductor photocatalysis for water splitting, organic
oxidation, and environmental remediation.
Photocatalysis. Reactions were carried out on a XPA-7
photochemical reactions instrument (Xujiang Nanjing).
Reactor was a quartz glass tube (50 mL), thermostated at
o
25 C. Light source was a high pressure mercury lamp
(300W), attached with a 365 nm cut-off filter, whose in-
tensity reaching the reactor surface was 4.5 mW/cm2. Un-
less stated otherwise, experiments were performed under
fixed conditions (1.00g/L catalyst, 0.43mM phenol, and
pH 7). A suspension containing necessary components
was stirred in the dark for 1 h, and then irradiated with
UV light. At given intervals, 2 mL of sample was with-
drawn, and filtered through a 0.22 μm membrane. The
filtrate was analyzed by HPLC (high-performance liquid
chromatography) on a Dionex P680 (Apollo C18 reverse
column, and 50% CH3OH as an eluent). H2O2 was ana-
lyzed using the literature method.28 Briefly, 2.7 mL of the
filtrate was mixed with 0.3 mL buffer solution (0.5 M
NaH2PO4/Na2HPO4), 25 μL DPD, and 25 μL peroxidase in
order. The solution absorbance at 553 nm was recorded on
an Agilent 8453 UV−visible spectrophotometer.
■ EXPERIMENTAL SECTION
Material. Anatase TiO2, peroxidase, and N,N-diethyl-
1,4-phenylenediamine (DPD) were purchased from Sigma-
Aldrich. FeCl3∙6H2O, Co(NO3)2∙6H2O, and others in ana-
lytic grade were purchased from Shanghai Chemicals. A
Milli-Q ultrapure water was used, and solution pH was
adjusted with a dilution solution of NaOH or HClO4.
Synthesis. Pure CF was synthesized using a literature
method.16 Briefly, KOH solution (2.24 g, 50 mL) was slow-
ly added into a mixed solution of iron salt (1.53 g, 15 mL),
and cobalt salt (0.62 g, 15 mL). Then the suspension was
transferred into a Teflon-lined autoclave, and heated at
o
160 C for 6 h. After the autoclave cooled down, the solid
Stability Test. In the first run, a suspension containing
2.00 g/L 0.01% CF/TiO2 and 0.22 mM phenol at pH 7.0
was stirred in the dark for 1 h, and then irradiated for 90
min. Sample was analyzed as the above. In the second run,
a certain amount of the non-irradiated catalyst suspen-
sion (2 g/L) and phenol stock solution (10.75 mM) was
injected into the above irradiated suspension, as so to
restore previous conditions. After stirring in the dark for
30 min, new suspension was irradiated with light for 90
min. The above procedure was repeated five times.
was collected, and washed several times with water and
ethanol, and dried at 50 oC overnight.
The above method was used to prepare 05% CF/TiO2.
Typically, 0.5 g of TiO2 was suspended in KOH solution
KOH (2.24 g, 60 mL). Then 58 μL of Fe3+ solution (2 g/L),
31 μL of Co2+ solution(2 g/L), and 20 mL H2O were mixed,
and slowly added into the TiO2 suspension. After that, the
suspension was treated as described above. In this sample,
the content of CF was calculated to be 0.01% in weight.
Moreover, by mimicking 0.01% CF/TiO2, two reference
samples of CoO/TiO2 and Fe2O3/TiO2 were also prepared,
without the addition of Fe3+ and Co2+, respectively.
(Photo)electrochemical Measurement. A working
electrode was fabricated by doctor blade method. First, a
fluorinated tin oxide (FTO) substrate (1214 Ω/sq, and 2.2
mm thick), purchased from Pilkington Glass, was cleaned
with ethanol and water, and dried in N2. Second, the FTO
substrate was coated with a gel containing 0.5 wt % cata-
Characterizations. Powder X-ray diffraction (XRD)
pattern was recorded on an Ultima IV X-ray diffractome-
ter (Rigaku, Japan). Diffuse reflectance spectra were rec-
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lysts and 3.9 wt % PVA, and then sintered at 500 oC for 2 h.
Photocatalytic Activity for Organic Degradation in Water. ACS
Catal. 2014, 4, 32733280.
Third, the film glass was cut into several parts. Each part
had an exposed area of 1 × 1 cm2. Measurements were per-
formed in 0.5M NaClO4 (pH 7) at 10 mV/s, using a plati-
num gauze as counter electrode, and an Ag/AgCl elec-
trode as reference electrode. Instrument was a CHI660E
Electrochemical Station (Chenghua, Shanghai), attached
with a 500 W Xenon lamp and a 320 nm cut-off filter.
(7) Cong, S.; Xu, Y. Explaining the High Photocatalytic Activity of
a Mixed Phase TiO2: A Combined Effect of O2 and Crystallinity. J.
Phys. Chem. C 2011, 115, 2116121168.
(8) Luo, B.; Li, Z.; Xu, Y. The Positive Effect of Anatase and Rutile
on the Brookite-Photocatalyzed Degradation of Phenol. RSC
Advances 2015, 5, 105999106004.
(9) Li, Z.; Liu, R.; Xu, Y. Larger Effect of Sintering Temperature
Than Particle Size on the Photocatalytic Activity of Anatase TiO2.
J. Phys. Chem. C 2013, 117, 2436024367.
(10) Emeline, A. V.; Zhang, X.; Jin, M.; Murakami, T.; Fujishima, A.
Application of a "Black Body" Like Reactor forMeasurements of
Quantum Yields of Photochemical Reactions in Heterogeneous
Systems. J. Phys. Chem. B 2006, 110, 74097413.
(11) Sun, Q.; Leng, W.; Li, Z.; Xu, Y. Effect of Surface Fe2O3
Clusters on the Photocatalytic Activity of TiO2 for Phenol
Degradation in Water. J. Hazard. Mater. 2012, 229, 224232.
(12) Wan, L.; Sheng, J.; Chen, H.; Xu, Y. Different Recycle Behavior
of Cu2+ and Fe3+ Ions for Phenol Photodegradation over TiO2 and
WO3. J. Hazard. Mater. 2013, 262, 114120.
(13) Khnayzer, R. S.; Mara, M. W.; Huang, J.; Shelby, M. L.; Chen,
L. X.; Castellano, F. N. Structure and Activity of Photochemically
Deposited "CoPi" Oxygen Evolving Catalyst on Titania. ACS Catal.
2012, 2, 21502160.
(14) Zhang, X.; Xiong, X.; Wan, L.; Xu, Y. Effect of a Co-Based
Oxygen-Evolving Catalyst on TiO2-Photocatalyzed Organic
Oxidation. Langmuir 2017, 33, 81658173.
(15) Zhao, Q.; Yan, Z.; Chen, C.; Chen, J. Spinels: Controlled
Preparation, Oxygen Reduction/Evolution Reaction Application,
and Beyond. Chem. Rev. 2017, 117, 1012110211.
(16) Indra, A.; Menezes, P. W.; Sahraie, N. R.; Bergmann, A.; Das,
C.; Tallarida, M.; Schmeisser, D.; Strasser, P.; Driess, M.
Unification of Catalytic Water Oxidation and Oxygen Reduction
Reactions: Amorphous Beat Crystalline Cobalt Iron Oxides. J. Am.
Chem. Soc. 2014, 136, 1753017536.
(17) Zuo, X.; Fei, P.; Hu, C.; Su, B.; Lei, Z. Preparation of Magnetic
TiO2/CoFe2O4 Composite Photocatalytic Nanomaterial. Chinese J.
Inorg. Chem. 2009, 25, 12331237.
(18) Mourão, H. A. J. L.; Malagutti, A. R.; Ribeiro, C. Synthesis of
TiO2-Coated CoFe2O4 Photocatalysts Applied to the
Photodegradation of Atrazine and Rhodamine B in Water.
Applied Catal. A 2010, 382, 284292.
(19) Zhang, X.; Sun, D.; Han, Y.; Wang, J. Preparation and Sunlight
Photocatalytic Properties of TiO2-CoFe2O4 Magnetic Composite
Materials. Chinese J. Inorg. Chem. 2011, 27, 13731377.
(20) Li, C.; Wang, J.; Wang, B.; Gong, J.; Lin, Z. A Novel
Magnetically Separable TiO2/CoFe2O4 Nanofiber with High
Photocatalytic Activity under UV–vis Light. Mater. Res. Bull. 2012,
47, 333337.
(21) Sathishkumar, P.; Mangalaraja, R. V.; Anandan, S.;
Ashokkumar, M. CoFe2O4/TiO2 Nanocatalysts for the
Photocatalytic Degradation of Reactive Red 120 in Aqueous
Solutions in the Presence and Absence of Electron Acceptors.
Chem. Eng. J. 2013, 220, 302310.
(22) Sun, J.; Fu, Y.; Xiong, P.; Sun, X.; Xu, B.; Wang, X. A
Magnetically Separable P25/CoFe2O4/Graphene Catalyst with
Enhanced Adsorption Capacity and Visible-Light-Driven
Photocatalytic Activity. RSC Advances 2013, 3, 2249022497.
(23) Haw, C.; Chiu, W.; Abdul Rahman, S.; Khiew, P.; Radiman, S.;
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■ ASSOCIATED CONTENT
Supporting Information. Tables of solid parameters in
Mӧssbauer, N2 adsorption, O2 reduction onset potentials,
XRD patterns, integrated data, Tauc plots, SEM images, pho-
tographs, Mӧssbauer plots, XPS spectra, N2 adsorption iso-
therm, H2O2 decomposition, magnetization curves, dark LSV
curves for IR compensation and carbon rod counter electrode,
LSV curves for water oxidation, four parallel LSV curves for
phenol oxidation, OCP data fitting, XRD patterns and phenol
degradation on calcined samples. This material is available
free of charge via the Internet at http://pubs.acs.org.
■ AUTHOR INFORMATION
Corresponding Author
* Email: xuym@ zju.edu.cn.
Author Contributions
The manuscript was written through contributions of all au-
thors.
Notes
The authors declare no competing financial interest.
■ ACKNOWLEDGMENT
This work was supported by the Funds for Creative Research
Group of NSFC (No. 21621005). We thank Prof. R. Liu, Dr. Q.
Xiong, and group members in instrument use and discussion.
■ REFERENCES
(1) Hoffmann, M. R.; Martin, S. T.; Choi, W. Y.; Bahnemann, D. W.
Environmental Application of Semiconductor Photocatalysis.
Chem. Rev. 1995, 95, 6996.
(2) Kumar, S. G.; Devi, L. G. Review on Modified TiO2
Photocatalysis under UV/Visible Light: Selected Results and
Related Mechanisms on Interfacial Charge Carrier Transfer
Dynamics. J. Phys. Chem. A 2011, 115, 1321113241.
(3) Carp, O.; Huisman, C. L.; Reller, A. Photoinduced Reactivity of
Titanium Dioxide. Prog. Solid State Chem. 2004, 32, 33177.
(4) Ryu, J.; Choi, W. Substrate-Specific Photocatalytic Activities of
TiO2 and Multiactivity Test for Water Treatment Application.
Environ. Sci. Tehcnol. 2008, 42, 294300.
(5) Sun, Q.; Xu, Y. Evaluating Intrinsic Photocatalytic Activities of
Anatase and Rutile TiO2 for Organic Degradation in Water. J. Phys.
Chem. C 2010, 114, 1891118918.
(6) Li, Z.; Cong, S.; Xu, Y. Brookite vs Anatase TiO2 in the
9
ACS Paragon Plus Environment

Langmuir
Page 10 of 11
1
2
3
4
5
6
7
8
9
Abdul Shukor, R.; Hamid, M. A. A.; Ghazali, N. The Design ofNew
Aqueous Suspensions of TiO2, ZnO, and Desert Sand. Environ. Sci.
Technol. 1988, 22, 798806.
Magnetic-Photocatalyst Nanocomposites (CoFe2O4–TiO2) as
Smart Nanomaterials for Recyclable-Photocatalysis Applications.
New J. Chem. 2016, 40, 11241136.
(24) Yang, Z.; Shi, Y.; Wang, B., Photocatalytic Activity of
Magnetically Anatase TiO2 with High Crystallinity and Stability
for Dyes Degradation: Insights into the Dual roles of SiO2
Interlayer between TiO2 and CoFe2O4. Appl. Surf. Sci. 2017, 399,
192199.
(25) Sohail, M.; Xue, H.; Jiao, Q.; Li, H.; Khan, K.; Wang, S.; Feng,
C.; Zhao, Y. Synthesis of Well-Dispersed TiO2/CNTs@CoFe2O4
Nanocomposites and Their Photocatalytic Properties. Mater. Res.
Bull. 2018, 101, 8389.
(26) Guo, X. H.; Ma, J. Q.; Ge, H. CoFe2O4@TiO2@Au Core-Shell
Structured Microspheres: Synthesis and Photocatalyltic
Properties. Russian J. Phys. Chem. A 2018, 91, 26432650.
(27) Xu, Y.; Langford, C. H. UV- or Visible-Light-Induced
Degradation of X3B on TiO2 Nanoparticles: The Influence of
Adsorption. Langmuir 2001, 17, 897902.
(28) Bader, H.; Sturzenegger, V.; Hoigné, J. Photometric Method
for the Determination of Low Concentrations of Hydrogen
Peroxide by the Peroxidase Catalyzed Oxidation of N,N-Diethyl-
p-Phenylenediamine (DPD). Water Res. 1988, 22, 11091115.
(29) Jing, L.; Xu, Y.; Huang, S.; Xie, M.; He, M.; Xu, H.; Li, H.;
Zhang, Q. Novel Magnetic CoFe2O4/Ag/Ag3VO4 Composites:
Highly Efficient Visible Light Photocatalytic and Antibacterial
Activity. Appl. Catal. B 2016, 199, 1122.
(30) Zeng, Y.; Guo, N.; Song, Y.; Zhao, Y.; Li, H.; Xu, X.; Qiu, J.; Yu,
H. Fabrication of Z-Scheme Magnetic MoS2/CoFe2O4
Nanocomposites with Highly Efficient Photocatalytic Activity. J.
Colloid Interface Sci. 2018, 514, 664674.
(31) Yang, H.; Mao, Y.; Li, M.; Liu, P.; Tong, Y. Electrochemical
Synthesis of CoFe2O4 Porous Nanosheets Visible Light Driven
Photoelectrochemical Applications. New J. Chem. 2013, 37,
29652968.
(32) Aghavnian, T.; Moussy, J. B.; Stanescu, D.; Belkhou, R.;
Jedrecy, N.; Magnan, H.; Ohresser, P.; Arrio, M. A.; Sainctavit, P.;
Barbier, A. Determination of the Cation Site Distribution of the
Spinel in Multiferroic CoFe2O4/BaTiO3 Layers by X-Ray
Photoelectron Spectroscopy. J. Electron. Spectrosc. Relat. Phenom.
2015, 202, 1621.
(33) Zhou, Z.; Zhang, Y.; Wang, Z.; Wei, W.; Tang, W.; Shi, J.;
Xiong, R. Electronic Structure Studies of the Spinel CoFe2O4 by
X-ray Photoelectron Spectroscopy. Appl. Surf. Sci. 2008, 254,
69726975.
(34) Song, G.; Xin, F.; Yin, X. Photocatalytic Reduction of Carbon
Dioxide over ZnFe2O4/TiO2 Nanobelts Heterostructure in
Cyclohexanol. J. Colloid Interface Sci. 2015, 442, 6066.
(35) Uzunova, E. L.; Mitov, I. G.; Klissurski, D. G. Synthesis of
Nano-Dimensional Iron–Cobalt Spinel Mixed Oxides from
Layered-Type Carbonate Hydroxide Precursors. Bull. Chem. Soc.
Jpn. 1997, 70, 19851993.
(36) Lin X.; Duan H.; Wang J. Mӧssbauer spectra and magnetic
properties study of CoFe2O4 nanoparticles prepared by
hydrothermal method. J. Funct. Mater. 2012, 43, 1020-1023.
(37) Cao S.; Wang J. Mӧssbauer Spectra and Magnetic Properties
of CoFe2O4 Nanoparticles. J. Synth. Cryst. 2011, 40, 1000-1005.
(38) Murray, P. J.; late, J. W. L. Cation distribution in the spinels
CoxFe3−xO4. J. Phys. Chem. Solids. 1976, 37, 1041-1042.
(40) Goto, H., Quantitative Analysis of Superoxide Ion and
Hydrogen Peroxide Produced from Molecular Oxygen on
Photoirradiated TiO2 Particles. J. Catal. 2004, 225, 223229.
(41) Sahel, K.; Elsellami, L.; Mirali, I.; Dappozze, F.; Bouhent, M.;
Guillard, C. Hydrogen Peroxide and Photocatalysis. Appl. Catal. B
2016, 188, 106112.
(42) Yi, J.; Bahrini, C.; Schoemaecker, C.; Fittschen, C.; Choi, W.
Photocatalytic Decomposition of H2O2 on Different TiO2 Surfaces
Along with the Concurrent Generation of HO2 Radicals
Monitored Using Cavity Ring Down Spectroscopy. J. Phys. Chem.
C 2012, 116, 1009010097.
(43) Chen, W. F.; Sasaki, K.; Ma, C.; Frenkel, A. I.; Marinkovic, N.;
Muckerman, J. T.; Zhu, Y.; Adzic, R. R. Hydrogen-evolution
catalysts based on non-noble metal nickel-molybdenum nitride
nanosheets. Angew. Chem. Int. Ed. Engl. 2012, 51 (25), 6131-5.
(44) Peiro, A. M.; Colombo, C.; Doyle, G.; Nelson, J.; Mills, A.;
Durrant, J. R. Photochemical reduction of Oxygen Adsorbed to
Nanocrystalline TiO2 Films: A Transient Absorption and Oxygen
Scavenging Study of Different TiO2 Preparations. J. Phys. Chem. B
2006, 110, 2325523263.
(45) Kim, J.; Monllor-Satoca, D.; Choi, W. Simultaneous
Production of Hydrogen with the Degradation of Organic
Pollutants Using TiO2 Photocatalyst Modified with Dual Surface
Components. Energy Environ. Sci. 2012, 5, 7647 7656.
(46) Pallotti, D. K.; Passoni, L.; Maddalena, P.; Di Fonzo, F.;
Lettieri, S. Photoluminescence Mechanisms in Anatase and Rutile
TiO2. J. Phys. Chem. C 2017, 121, 90119021.
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45
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47
48
49
50
51
52
53
54
55
56
57
58
59
60
(47) Kavan, L.; Gratzel, M.; Gilbert, S. E.; Klemenz, C.; Scheel, H.
J. Electrochemical and Photoelectrochemical Investigation of
Single-Crystal Anatase. J. Am. Chem. Soc. 1996, 118, 67166723.
(48) Tachikawa, T.; Fujitsuka, M.; Majima, T. Mechanistic Insight
into the TiO2 Photocatalytic Reactions: Design of New
Photocatalysts. J. Phys. Chem. C 2007, 111, 52595275.
(49) Jinnouchi, R.; Anderson, A. B. Aqueous and Surface Rredox
Potentials from Self-Consistently Determined Gibbs Energies. J.
Phys. Chem. C 2008, 112, 87478750.
(50) Li, C.; Hoffman, M. Z. One-electron Redox Potentials of
Phenols in Aaqueous Solution. J. Phys. Chem. B 1999, 103,
66536656.
(39) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R.
Photocatalytic Production of H2O2 and Organic Peroxides in
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