FeOCl/POM Heterojunctions with Excellent Fenton Catalytic Performance via Different Mechanisms

Article abstract of DOI:10.1021/acs.inorgchem.8b02329

To enhance the Fenton catalytic performance in a neutral solution under indoor sunlight, a novel FeOCl/polyoxometalate (POM) (FeOCl/POM-W and FeOCl/POM-Mo) composite was successfully synthesized for the first time, which shows significantly improved Fenton catalytic activity and stability for phenol degradation compared with FeOCl. Furthermore, the degradation constants (k) of FeOCl/POM-Mo (0.08 min^-1) and FeOCl/POM-W (0.06 min^-1) are a factor of 4 and 3 times greater than that of FeOCl (0.02 min^-1), respectively. The enhanced catalytic activity is attributed to the formation of FeOCl/POM heterojunctions, which results in efficient separation of photoinduced electron-hole pairs and electron transfer from POM to FeOCl. Density functional theory calculations indicate a strong interface interaction of Fe-O-Mo and Fe-O-W in the FeOCl/POM heterojunctions. A Z-scheme mechanism for FeOCl/POM-Mo and a double-transfer mechanism for FeOCl/POM-W are proposed for the enhanced catalytic performance. This study sheds new light on the design and fabrication of high-performance photo-Fenton catalysts to overcome the environmental crisis.

Full text of DOI:10.1021/acs.inorgchem.8b02329

Article
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
pubs.acs.org/IC
FeOCl/POM Heterojunctions with Excellent Fenton Catalytic
Performance via Different Mechanisms
,
†
†
†
†
,†
†
Jian Zhang,* Mingyu Zhan, Lulu Zheng, Chen Zhang, Guodong Liu,* Jingquan Sha,
‡
§
Shaojie Liu, and Shuo Tian
†
Department of Chemistry and Chemical Engineering, Jining University, Qufu 273100, P. R. China
Department of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China
Animal Husbandry and Veterinary Bureau of Jinan, Jinan 250002, P. R. China
‡
§
*
S Supporting Information
ABSTRACT: To enhance the Fenton catalytic performance in a neutral solution
under indoor sunlight, a novel FeOCl/polyoxometalate (POM) (FeOCl/POM-W and
FeOCl/POM-Mo) composite was successfully synthesized for the first time, which
shows significantly improved Fenton catalytic activity and stability for phenol
degradation compared with FeOCl. Furthermore, the degradation constants (k) of
−
1
−1
FeOCl/POM-Mo (0.08 min ) and FeOCl/POM-W (0.06 min ) are a factor of 4
−
1
and 3 times greater than that of FeOCl (0.02 min ), respectively. The enhanced
catalytic activity is attributed to the formation of FeOCl/POM heterojunctions, which
results in efficient separation of photoinduced electron−hole pairs and electron
transfer from POM to FeOCl. Density functional theory calculations indicate a strong
interface interaction of Fe−O−Mo and Fe−O−W in the FeOCl/POM hetero-
junctions. A Z-scheme mechanism for FeOCl/POM-Mo and a double-transfer
mechanism for FeOCl/POM-W are proposed for the enhanced catalytic performance.
This study sheds new light on the design and fabrication of high-performance photo-
Fenton catalysts to overcome the environmental crisis.
1
. INTRODUCTION
heterogeneous Fenton catalysts in an acidic solution by Han’s
15−17
group.
In this aspect, two kinds of FeOCl-based photo-
Environmental and energy crises have become two major
challenges in recent years, which hinder the sustainable
development of human society. To overcome the serious
environmental crisis, advanced oxidation processes have
emerged as a powerful solution because they are highly
efficient, environmentally friendly, and have no secondary
pollution effect. Among the technologies, Fenton reactions
have received considerable attention. Owing to the main
shortcoming of iron sludge produced in homogeneous Fenton
Fenton catalysts, namely, two-dimensional FeOCl and FeOCl/
iron hydroxide nanosheets, have been reported by our group,
which show outstanding catalytic performance in a neutral
18,19
solution under visible light.
However, the narrow band gap
of FeOCl (1.90 eV) causes a low separation efficiency of the
electron−hole pairs. Furthermore, it is very difficult to achieve
enhanced catalytic performance with a single material
component. One possible solution is to form a heterojunction
between FeOCl and other semiconductors to increase the
separation efficiency of electron−hole pairs. Polyoxometalates
(POMs), well-defined metal−oxygen clusters with desirable
reversibility in multielectron redox reactions, have been proven
II
processes (Fe −H O ), great efforts have been focused on
2
2
III
1−3
heterogeneous Fenton reactions (Fe −H O ).
However,
2
2
III
II
because the reduction rate of Fe to Fe in the heterogeneous
Fenton reaction is sluggish, this is the first issue that we should
address. Because of the major disadvantage of the strong
acidic solution (pH = 3−4) used in Fenton reactions,
recent developments have highlighted the photoinduced
20−26
4
to be a new kind of photocatalyst.
However, the strong
5
−12
adsorption of organic pollutants on the POM surface limits its
27,28
photocatalytic performance.
Therefore, the construction of
13,14
a FeOCl/POM structure, especially a Z-scheme photocatalytic
system, will be propitious to improve the catalytic performance
of FeOCl and POM.
On the basis of the above-mentioned consideration, we
present a possible solution to these challenges by forming
heterojunctions of FeOCl/POM for the first time. Our
heterogeneous Fenton reaction in a neutral solution.
However, the catalytic performance in the heterogeneous
photo-Fenton reaction is limited within the ultraviolet and
visible (UV−vis) regions. Therefore, the development of
heterogeneous photo-Fenton catalysts with the accelerated
III
II
reduction rate of Fe to Fe in a neutral solution under natural
light is an enormous challenge.
FeOCl- and FeOCl-based composites (FeOCl/SBA-15 and
Received: August 17, 2018
FeOCl/SiO ) have shown excellent catalytic performance as
2
©
XXXX American Chemical Society
A
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
assumption is confirmed by a series of catalytic activities and
active species-trapping measurements. As predicted, the
FeOCl/POM heterojunctions show excellent indoor-sunlight-
driven photo-Fenton catalytic performance in a neutral
solution. The bigger surface area and surface potential of
FeOCl/POM than that of FeOCl contribute to increased
catalytic activity. Two different mechanisms are proposed for
the enhanced catalytic activity, namely, the Z-scheme
mechanism for FeOCl/POM-Mo and the double-transfer
mechanism for FeOCl/POM-W, resulting in an efficient
separation of photoinduced electron−hole pairs and a rapid
electron transfer from POM to FeOCl.
(generalized gradient approximation with the Perdew, Burke, and
34,35
Ernzerhof functional).
The self-consistent convergence accuracy,
the convergence criterion, and maximum displacement were set at 1 ×
−
5
−2
−3
10
eV, 3 × 10 eV/Å, and 1 × 10 Å, respectively. In addition, the
plane-wave cutoff energy was 500 eV. The Mulliken bond population
and charge density were calculated to study the surface state of
FeOCl/POM.
3. RESULTS AND DISCUSSION
3.1. Structure Characterization of FeOCl/POMs. To
determine the crystallographic structure of the prepared
sample, XRD measurements were carried out (Figure 1a),
2
. EXPERIMENTAL SECTION
2
.1. Preparation. All the chemicals were of analytical grade and
purchased from Sinopharm Chemical Reagent Co., Ltd.
We used a facile one-step calcination method to prepare the
FeOCl/POM heterojunction. In a typical process, 0.018 g of POM-
Mo (phosphomolybdic acid, H [P(Mo O ) ]) was added to 2 mL of
3
3
10 4
FeCl solution (0.25 g/mL) with continuous stirring for 30 min at
3
room temperature. Then, the solution was dried at 40 °C and then
placed in an alumina crucible, which was calcined at 230 °C for 1 h in
a N atmosphere. The product was washed with acetone three times
2
to remove the unreacted ferric chloride and then dried at 90 °C in a
vacuum oven for 12 h. In addition, the preparation of FeOCl/POM-
W was the same as the above method except for POM-W
(
phosphotungstic acid, H O PW ).
3 40 12
2.2. Characterization. X-ray diffraction (XRD) technique was
carried out to measure the phase structure of the prepared samples on
a Rigaku D/Max 2200-PC diffractometer. Transmission electron
microscopy (TEM) (JEM-100CXII) and scanning electron micros-
copy (SEM) (field emission-SEM ZEISS system) were adopted to
descript the morphology. A N2 adsorption technique at 77 K
(
QUADRASORB SI) was used to measure the surface area. Energy-
dispersive X-ray spectrometry (EDXS) was performed at 15 kV.
Fourier transform infrared (FTIR) spectra (Nicolet 5DX-FTIR
spectrometer), a UV−vis spectrophotometer (LAMBDA-35, Perki-
nElmer), and a fluorescence spectrophotometer (Cary Eclipse) were
adopted to characterize the structure. The composite structures were
characterized by X-ray photoelectron spectra (PerkinElmer PHI-5300
ESCA). The content of elements was studied by inductively coupled
plasma-mass spectrometry (Nu AttoM). An F-4600 fluorescence
spectrophotometer (Hitachi) was used to study the photolumines-
cence (PL) spectra. The surface potential was measured by a zeta-
potential meter (Delsa Nano C). Electrochemical workstation
Figure 1. (a) XRD patterns of FeOCl, FeOCl/POM-W, and FeOCl/
POM-Mo, (b) TEM image of FeOCl/POM-Mo, and corresponding
EDXS elemental mapping images of (c) total, (d) Fe, (e) O, (f) Cl,
(g) P, and (h) Mo elements.
which show that the diffraction peaks of FeOCl agree with the
pure phase of FeOCl (orthorhombic lattice, JCPDS, no. 24-
1005). The main diffraction peaks of FeOCl/POM can also be
assigned to the FeOCl phase. The results from the TEM and
SEM images of FeOCl/POM reveal that both FeOCl/POM-W
and FeOCl/POM-Mo are plates piled with nanosheets
(
CHI760E) was used to study the electrochemical impedance
spectroscopy (EIS) spectra, Mott−Schottky plots, and photocurrent.
Modified fluorine-doped tin oxide, Pt wire, and saturated calomel
electrode (SCE) were used as the working electrode, counter
electrode, and reference electrode, respectively.
2.3. Catalysis Measurements. The test method for Fenton
catalytic degradation of phenol by FeOCl/POM was the same as in
(
Figures 1b and S1a,b). The smaller thickness and weaker
18,19,29
the reported literatures.
In a typical experiment, a 100 mL
crystallinity of FeOCl/POM compared with FeOCl indicate
that POM prevents the crystal growth of FeOCl (Figures S2
and 1a), suggesting the intimate contact between FeOCl and
POM. Furthermore, the EDXS measurements of FeOCl/
POMs show that all the essential elements including Fe, O, Cl,
P, W, and Mo are detected. The corresponding EDXS images
of FeOCl/POM reveal the uniform distribution of each
element in the FeOCl/POM photocatalyst (Figures 1c−h and
S1c−h). The above results indicate that the heterojunction
provides enough contact between FeOCl and POMs, which
facilitates the charge carrier transfer.
suspension of FeOCl/POM (0.05 g) and phenol (1 mg) was stirred in
darkness for 60 min. Then, 0.5 mL of H O aqueous solution (30 wt
2
2
%
) was added in the suspension under sunlight irradiation. In the
catalytic process, 5 mL of the mixture was taken out every 10 min to
record the UV−vis absorption spectra.
2
.4. Active Species Measurements. Benzoquinone (BQ, 1
mM) and iso-propyl alcohol (IPA, 1 mM) were used to investigate
•
−
•
superoxide radical ( O ) and hydroxyl radicals ( OH) in the photo-
2
30,31
32,33
Fenton catalytic process.
According to the literature,
a
fluorescence technique was used to study the decomposition of
H O on FeOCl/POM by adding terephthalic acid in the reaction
2
2
system.
XPS measurements were carried out to verify the chemical
state of FeOCl/POM (Figure 2). The peaks of binding energy
centered at 711.44 and 725.32 eV correspond to Fe 2p3/2 and
2
.5. Calculation Methods. To study the catalytic mechanism,
density functional theory (DFT) calculations were adopted in the
CASTEP program by using the exchange−correlation function
B
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
cm− is attributed to the Fe−O vibration (Figure 3a,b). All the
1
−
1
−1
IR absorption bands at 810 cm (Mo−O −Mo), 850 cm
c
−
1
−1
(
Mo−O −Mo), 980 cm (Mo−O), and 1080 cm (P−O)
can be ascribed to the characteristic bands of POM-Mo
b
37,38
(
Figure 3a).
The IR absorption bands of POM-W lie at
−
1
8
20, 880, 1010, and 1100 cm , which are attributed to the
W−O −W, W−O −W, W−O, and P−O vibration (Figure
c
b
3
9,40
3
b).
To further study the interaction between FeOCl and
POMs, the FTIR spectra of pure POMs and the complexes
(
FeCl /POM-Mo and FeCl /POM-W) before calcination
3
3
were also recorded (Figure S3b,c), which confirm the retention
of POM structures in the synthesis of FeOCl/POM. Moreover,
the peak shift of POMs in FeOCl/POMs compared with pure
POMs indicates the interaction between POMs and FeOCl.
Therefore, the FTIR spectra of FeOCl/POMs further suggest
the successful combination of FeOCl and POMs. Furthermore,
we recorded the UV−vis diffusive reflectance spectra (Figure
3
c), which show that the optical absorptions of FeOCl/POMs
23−27
are much better than that of FeOCl.
Moreover, FeOCl/
POM-Mo shows better optical absorption than FeOCl/POM-
41,42
W.
As is well-known, the number of active sites in the catalyst
and the adsorption amount of organic pollutants on the
catalyst depend on its surface area. Therefore, the surface areas
of FeOCl/POMs were measured (Figure 4). The results show
that the surface areas of FeOCl/POM-W and FeOCl/POM-
Figure 2. XPS spectrums of (a) Fe 2p and (b) Mo 3d in FeOCl/
POM-Mo, (c) Fe 2p, and (d) W 4f in FeOCl/POM-W.
Fe 2p1/2, respectively, suggesting that the Fe element in FeOCl
2
III
Mo are 15.6 and 16.0 m /g (Figure 4b,c), respectively, which
presents as the Fe state (Figure S3a). The peaks of Fe 2p3/2
are approximately a factor of 6 greater than that of FeOCl (2.5
and Fe 2p1/2 shift to 710.85 and 724.47 eV in FeOCl/POM-
Mo (Figure 2a) and then decrease to 710.54 and 724.03 eV in
FeOCl/POM-W (Figure 2c), suggesting the partial reduction
2
m /g, Figure 4a). The increased surface areas of FeOCl/POMs
indicate that the composite structure can provide more active
sites and higher adsorption capacity for organic pollutants than
FeOCl, which will improve the catalytic activity of FeOCl/
POMs.
On the basis of the above structure analysis, the successful
combination of POM and FeOCl provides a large contact area
between them and is beneficial to the separation of electron−
hole pairs. Therefore, it is expected that FeOCl/POMs would
boast the photo-Fenton catalytic activity in a neutral solution
under sunlight.
III
of Fe in the FeOCl/POM heterojunctions. POM, serving as a
reducing agent, can transfer electrons to FeOCl, which is
beneficial not only for the separation of electron−hole pairs
but also for electron accumulation in FeOCl. The binding
energies of Fe 2p in POM-W are smaller than those in POM-
Mo, owing to the stronger reducing ability of POM-W than
3
6
that of POM-Mo. Figure 2b indicates that the XPS peaks of
Mo 3d5/2 and Mo 3d3/2 in FeOCl/POM-Mo appear at 232.32
and 235.45 eV, respectively, which are characteristic of the
6
+
37,38
3.2. Enhanced Photo-Fenton Catalytic Performance.
Mo state.
The W 4f XPS spectrum of FeOCl/POM-W
demonstrates two peaks of binding energies at 35.68 eV for W
To verify the above hypothesis, a natural solution under indoor
sunlight for phenol degradation was adopted in our experi-
ments. The photo-Fenton catalytic activities of FeOCl,
FeOCl/POM-Mo, and FeOCl/POM-W were first studied
(Figure 5a). The degradation rates reach 51% over FeOCl,
94% over FeOCl/POM-W, and 100% over FeOCl/POM-Mo
after 40 min (Figure 5a), which suggests that POMs effectively
4
f7/2 and 37.81 eV for W 4f5/2 (Figure 2d), which reveal that
6
+
the W element in FeOCl/POM-W presents as the W
state.
3
9,40
The XPS results further prove the successful
composite of FeOCl and POMs.
To present the detailed structure of FeOCl/POM, FTIR
spectra were also recorded (Figure 3a,b). The peak at 530
Figure 3. FTIR spectrum of (a) FeOCl/POM-Mo and (b) FeOCl/POM-W, and (c) UV−vis diffusive reflectance spectra of FeOCl and FeOCl/
POMs.
C
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 4. N adsorption−desorption isotherms of (a) FeOCl, (b) FeOCl/POM-W, and (c) FeOCl/POM-Mo.
2
verify this point, we further measured the catalytic activities of
POMs, which show that POMs are inactive on phenol
2
7,28
degradation.
Therefore, excessive POMs in FeOCl/
POMs will decrease the catalytic activity.
In addition, cycling degradation experiments were also
carried out to study the catalytic stability of FeOCl/POMs
(
Figure S5b,c), which show that there is no decrease in the
catalytic activity of FeOCl/POMs after four cycling runs,
suggesting the excellent stability of FeOCl/POMs. To further
study the structural stability, we also measured the phase
structures and morphologies of FeOCl/POMs after the
catalytic reaction (Figures S5d and S6). The morphologies of
FeOCl/POMs are well preserved (Figure S6), and the XRD
patterns remain unchanged (Figure S5d). In addition, the
FTIR spectrum and EDXS measurement of FeOCl/POMs
after the catalytic reaction were also measured as seen in Figure
S7. The FTIR spectrum of FeOCl/POMs after the catalytic
reaction indicates that the catalyst still maintains the composite
structure of FeOCl and POMs. The EDXS measurements of
FeOCl/POMs in Figure S5e,f show that all the essential
elements (Fe, O, Cl, P, Mo, and W) are detected. In
conclusion, the structure of FeOCl/POM after the photo-
Fenton reaction is not destroyed, indicating the excellent
structure stability. Simultaneously, the pH was 5.2 in the
FeOCl system after the catalytic reaction, whereas the pH
values were 6.1 and 6.3 in FeOCl/POM-W and FeOCl/POM-
Mo systems, respectively. Compared with most of the Fenton
catalysts used in strong acidic solutions (pH = 3 or 4), FeOCl/
POMs show excellent catalytic performance in a near-neutral
solution. To further certify the effective protection of POMs
for FeOCl, we further performed Fe leaching. The leaching
amounts of Fe ions in FeOCl/POM-W and FeOCl/POM-Mo
are 1.82 and 1.68 ppm, respectively, whereas it is 3.01 ppm in
FeOCl. The leaching amounts of Fe ions in FeOCl/POMs are
small and will not cause secondary pollution to water.
Moreover, FeOCl/POM-Mo shows stronger protection than
FeOCl/POM-W. Thus, we can draw the conclusion that
POMs do improve the structural stability of FeOCl.
Figure 5. (a) Catalytic activity and (b) corresponding kinetic plots
over FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo.
enhance the catalytic activity of FeOCl. To study the effect of
sunlight on the photo-Fenton catalytic activity, the degradation
activities over FeOCl, FeOCl/POM-Mo, and FeOCl/POM-W
in darkness were also measured (Figure S4), which indicate
that the degradation rates over FeOCl, FeOCl/POM-Mo, and
FeOCl/POM-W are 32, 84, and 89%, respectively. The
degradation activity under darkness is lower than that under
sunlight, suggesting that sunlight promotes the Fenton catalytic
activity. The corresponding kinetic plots of FeOCl, FeOCl/
POM-Mo, and FeOCl/POM-W are depicted in Figure 5b,
which show a good linearity, confirming the role of the first-
order kinetic model in the photo-Fenton catalytic reaction.
The degradation constants (k) of FeOCl/POM-Mo (0.08
−
1
−1
min ) and FeOCl/POM-W (0.06 min ) are a factor of 4
−
1
times and 3 times greater than that of FeOCl (0.02 min )
Table 1), respectively. Furthermore, the catalytic activity of
(
FeOCl/POM-Mo is higher than that of FeOCl/POM-W.
Table 1. Rate Constants (k) and Regression Coefficients
2
(
R ) of Photo-Fenton Catalytic Activities over FeOCl,
FeOCl/POM-W, and FeOCl/POM-Mo
k (min−1
R2
photocatalyst
FeOCl
FeOCl/POM-W
FeOCl/POM-Mo
)
0.02
0.06
0.08
0.99
0.98
0.96
3.3. Mechanism for Enhanced Catalytic Performance.
To study the effect of surface area on the catalysis, the catalytic
activities of FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo
with the same total surface area were also measured (Figure
6a). To ensure the same total surface area, the required weights
of FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo are 0.32,
0.051 and 0.05 g, respectively. The improved catalytic activity
of FeOCl (from 51 to 69%, Figures 4a and 6a) indicates that
the surface area does contribute to the enhanced catalytic
activity. Because the surface areas of FeOCl/POM-Mo and
To optimize the conditions, the catalytic activities of
FeOCl/POMs with different contents of POMs were measured
(
Figure S5a), which show that the optimized contents of
POM-Mo and POM-W in FeOCl/POMs are 6.9 and 7.3 wt %,
respectively. Hence, FeOCl plays a major role in the photo-
Fenton catalysis, whereas POMs play a secondary role. To
D
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
•
indicate that the OH radical is the reactive oxygen species in
FeOCl and FeOCl/POMs. On the other hand, it also reveals
•
that the production of OH radicals over FeOCl-POMs is
larger than that over FeOCl. Meanwhile, FeOCl/POM-Mo
•
produces more OH radicals than FeOCl/POM-W because of
the higher suppressed degradation degree of FeOCl/POM-
Mo. To prove this viewpoint, the decomposition of H O was
2
2
measured (Figure S8). The higher decomposition rate of H O
2
2
•
reveals the higher production of OH radicals. The results
reveal that the H O conversion rates over FeOCl/POMs are
2
2
obviously higher than that over FeOCl, suggesting the
•
contribution of POMs on increased production of OH
Figure 6. (a) Catalytic activities with the same total surface area and
radicals. Meanwhile, it also indicates the rapid electron transfer
(
b) adsorption of phenol over FeOCl, FeOCl/POM-Mo, and FeOCl/
POM-W under dark conditions.
III
from POMs to FeOCl to accelerate the reduction of Fe to
II
Fe . In addition, the higher H O conversion rate over FeOCl/
2
2
POM-Mo than that over FeOCl/POM-W suggests the more
efficient reduction of Fe to Fe .
FeOCl/POM-W are very close, the catalytic activity over
FeOCl/POM-W exhibits unobvious improvement.
III
II
In contrast to IPA, the introduction of BQ has a small effect
on the photo-Fenton catalytic activity of FeOCl (from 51 to
Another contribution of POMs is to increase the adsorption
capacity for organic pollutants. Because both the life and
transport distance of active species in the Fenton reaction are
•
−
4
7%), indicating that the O2 radicals have smaller
•
4
3,44
contribution than the OH radicals (Figure 7a,b). However,
the suppressed catalytic activities of FeOCl-POMs (82% for
FeOCl/POM-Mo and 84% for FeOCl/POM-W) are larger
short,
the adsorption of organic pollutants on the catalyst
surface can effectively decrease the transport distance and
45
increase the utilization rate of the active species. The
adsorption of phenol on the FeOCl surface under dark
circumstances is weak (9%), whereas that on FeOCl/POMs is
much stronger than that on FeOCl (Figure 6b). According to
the zeta potential measurements, the zeta potential of FeOCl is
•
−
than that of FeOCl, which indicates that the amount of O
2
radicals over FeOCl-POMs increases compared with FeOCl.
48,49
•
−
According to the literature,
the amount of O radicals
2
III
II
•
−
can reflect the reduction of Fe to Fe . The O radicals in
2
the Fenton reaction mainly come from two aspects: the
−30.46 mV, which however increases to −54.72 mV for
III
reduction of O and the reaction of Fe with H O . To study
2
2
2
FeOCl/POM-W and −54.13 mV for FeOCl/POM-Mo. The
increased surface electronegativity of FeOCl/POMs can
effectively enhance the adsorption for phenol. In addition,
the increased electronegativity of FeOCl/POMs can also
increase the repulsion between nanoparticles to promote the
dispersion of the suspension, which reduces the agglomeration
of nanoparticles and increases the adsorption of pollutants.
Because the generated reactive oxygen species are mainly
•
−
the source of the O radicals, the band edge positions of the
2
valence band (VB) and conduction band (CB) potentials of
FeOCl, POM-W, and POM-Mo (Figure S9) were analyzed,
which show that the direct optical band gaps (E ) of FeOCl,
g
POM-W, and POM-Mo are 1.85, 2.98, and 2.34 eV,
37,50,51
respectively (Figures 3b and S8a,b).
The flat band
potentials (E ) of FeOCl, POM-W, and POM-Mo are
fb
•
•
−
2,3
calculated to be −0.54, −0.62, and −0.05 eV (vs SCE, pH =
OH and O in the heterogeneous Fenton reactions, BQ
2
50
7
) (Figure S10a−c), respectively. Moreover, the CB
•
−
•
and IPA were applied as scavengers for the O2 and OH
radicals (Figure 7). Using scavenger as a probe molecule to
potentials of FeOCl, POM-W, and POM-Mo are calculated
to be 0.11, 0.03, and 0.60 eV [vs normal hydrogen electrode
(
NHE), pH = 0], respectively, which are all more positive than
•
−
the potential of O / O (−0.046 V vs NHE). Furthermore, to
study the effect of dissolved oxygen on the reduction of Fe to
Fe , a trapping experiment of O radicals was performed
2
2
III
II
•
−
2
under deaerated conditions (using N sparging), which was
2
compared to that under air conditions. It reveals that the
degradation rate hardly changed over FeOCl and FeOCl/
•
−
POMs. Therefore, the generated O radicals are from the
2
III
reaction of Fe with H O . In addition, the corresponding VB
2
2
potentials of FeOCl, POM-W, and POM-Mo are 1.96, 3.01,
and 2.94 eV (vs NHE, pH = 0), respectively. On the one hand,
the bigger suppressed degradation degree over FeOCl/POMs
•
Figure 7. Trapping experiments of active species (a) OH radical and
•
(
b) O radical over FeOCl, FeOCl/POM-W, and FeOCl/POM-Mo
2
III
II
under indoor sunlight irradiation.
than FeOCl suggests the accelerated reduction of Fe to Fe .
On the other hand, the separation efficiency of electron−hole
pairs over FeOCl/POM-Mo is higher than that over FeOCl/
POM-W. The contribution of POMs in FeOCl/POMs lies on
the rapid photogenerated electron transfer from POMs to
detect the oxidative species in the catalytic system has been
46,47
testified to be an effective method.
If the radical plays an
important role in the catalysis, the degradation rate will
decrease greatly in the presence of the corresponding
scavenger. The suppressed degradation degrees by the OH
and O radicals can reflect their content in the catalytic
III
II
FeOCl to promote the reduction of Fe to Fe .
•
•
On the basis of the above analysis, the OH radical is the
primary reactive oxygen species, whereas the O radical plays
•
−
•
−
2
2
process. The catalytic activity of FeOCl is suppressed to 45
from 51% by the introduction of IPA, which is lower than that
of FeOCl/POMs (Figure 7a). On the one hand, the results
a secondary role in the photo-Fenton reaction. Therefore, the
oxidation mechanism of FeOCl/POMs proceeds through the
attack of OH radical to target organic compounds. POMs
•
E
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Figure 8. (a) PL spectra, (b) electrochemical impedance spectra (Nyquist plot), and (c) photocurrents of FeOCl, FeOCl/POM-W, and FeOCl/
POM-Mo.
promote the separation of photogenerated electron−hole pairs
To further analyze the interface between FeOCl and POMs,
DFT calculations were carried out (Figure 9), and the results
III
II 41
and the reduction of Fe to Fe , which subsequently reacts
•
with H O to generate OH radicals on the FeOCl/POMs
2
2
surface.
To further verify the viewpoint that FeOCl/POMs have
higher charge separation efficiency than FeOCl in the photo-
Fenton catalysis, we further carried out PL spectroscopy
measurements (Figure 8a), which shows that the PL emission
intensities of FeOCl/POMs are obviously weaker than that of
FeOCl, indicating the higher separation efficiency of FeOCl/
5
1
POMs. Therefore, the interfacial contact between FeOCl and
POMs effectively increases the separation efficiency. Moreover,
the weaker PL emission intensity of FeOCl/POM-Mo than
that of FeOCl/POM-W indicates a higher separation efficiency
of FeOCl/POM-Mo, which is in accordance with the results in
the trapping experiments of active species (Figure 7).
Furthermore, EIS was also measured (Figure 8b), which
reveals that the arc radius of FeOCl is much larger than that of
FeOCl/POMs, suggesting that FeOCl/POMs exhibit a faster
charge transfer and a more efficient separation of the
Figure 9. Total charge densities of (a) FeOCl/POM-Mo and (b)
FeOCl/POM-W interface. The isosurface level is 1.5.
52
photogenerated charge carriers than FeOCl. In addition,
the interfacial charge transfer and separation of the photo-
generated charge carriers over FeOCl/POM-Mo is more
efficient than those over FeOCl/POM-W (Figure 8b).
Combining the PL spectra and EIS results, the combination
of FeOCl and POMs effectively improves the charge transfer
and separation of the photogenerated electron−hole pair. To
further investigate the electronic interaction between FeOCl
and POMs, the photocurrent transient response measurements
for FeOCl, FeOCl/POM-Mo, and FeOCl/POM-W were
employed. Figure 8c illustrates the current−time (I−t)
characteristics in three different electrodes upon several light
ON/OFF cycles, which show that the photocurrent value
rapidly decreases as soon as the light turns off and comes back
to a constant value when the light is on again. Compared with
FeOCl, the combination of POMs and FeOCl effectively
increases the photocurrent value, indicating an increase in the
photoinduced carrier transport rate and the photoinduced
indicate that the interfaces are connected by Mo−O−Fe and
55−58
W−O−Fe.
The bond lengths of Fe−O at the interface of
FeOCl/POMs (1.78 Å in FeOCl/POM-Mo and 1.77 Å in
FeOCl/POM-W) are smaller than that of FeOCl (2.03 Å,
Figure S11), which indicates the stronger interaction of Fe−O
in FeOCl/POMs than that in FeOCl, suggesting enhanced
structural stability at the interface between FeOCl and POMs
59,60
(
Figure 9a,b).
To understand the charge redistribution at
the interface, the Mulliken bond population was calculated,
which shows that the electropositivity of Fe in FeOCl/POMs
(1.04 in FeOCl/POM-Mo and 1.19 in FeOCl/POM-W) is
larger than that of Fe in FeOCl (0.75, Figure S10), suggesting
61,62
a strong interfacial interaction.
In addition, the increased
electropositivity of Fe in FeOCl/POM-W compared with that
of Fe in FeOCl/POM-Mo reveals the stronger reduction in
POM-W than in POM-Mo, which is consistent with the XPS
results (Figure 2). Furthermore, the total charge densities in
FeOCl/POMs demonstrate that the charges accumulate at the
53,54
63,64
electron−hole pair separation.
FeOCl/POM-Mo displays
interface of Fe−O−Mo and Fe−O−W.
the best photoswitching performance among the three samples.
Therefore, the remarkably increased photocurrent density of
FeOCl/POM-Mo can be ascribed to the fastest electron
transport from the VB to the CB and the highest separation
efficiency of the photogenerated electron−hole pairs among
the three samples. In addition, these results are good in accord
with the photocatalytic activity measurements of the as-
prepared samples (Figures 5 and 6).
On the basis of the above discussions, a synergistic
mechanism for the enhanced photo-Fenton catalytic perform-
ance of FeOCl/POMs is illustrated (Figure 10). Both FeOCl
and POMs can yield photogenerated electron and hole pairs
under sunlight. The CB potential of POM-W (0.03 eV) is
more negative than that of FeOCl (0.11 eV), which leads to
the rapid electron transfer from POM-W to FeOCl, resulting in
the electron accumulation in the CB of FeOCl. This behavior
F
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
nism of FeOCl/POM-W are proposed for the enhanced photo-
Fenton catalytic performance. The study has made great strides
toward enhancing our understanding of the novel photo-
Fenton catalyst in the applications of environmental crisis.
ASSOCIATED CONTENT
Supporting Information
■
*
S
The Supporting Information is available free of charge on the
ACS Publications website at DOI: 10.1021/acs.inorg-
chem.8b02329.
TEM, SEM, EDXS, and elemental mapping images of
FeOCl/POMs; XPS of FeOCl; catalytic activity under
darkness; cycling degradation experiments; XRD pat-
terns and TEM after reaction; UV−vis diffusive
reflectance spectra; Mott−Schottky plots; and total
charge densities of FeOCl (PDF)
Figure 10. Schematic illustration of the mechanism for the photo-
Fenton catalytic performance of FeOCl/POMs.
III
II
effectively accelerates the reduction of Fe to Fe , which
•
subsequently reacts with H O to generate OH radicals. The
AUTHOR INFORMATION
Corresponding Authors
E-mail: sduchemzhang@163.com (J.Z.).
E-mail: liugd615@163.com (G.L.).
2
2
■
•
VB of POM-W (3.01 eV) is more positive than the OH/H O
2
potential (ca. 2.27 V), which is beneficial for driving the
*
*
•
oxidation process of H O to form OH radicals. In addition,
2
+
the h in the VB of POM-W can also directly oxidize the
ORCID
degradation of phenol. Therefore, the enhanced photo-Fenton
catalytic performance of FeOCl/POM-W is attributed to the
double-transfer mechanism. If the double-transfer mechanism
is adopted for FeOCl/POM-Mo, the photogenerated electrons
in the CB of FeOCl could quickly transfer to POM-Mo
because of the more negative CB of FeOCl (0.11 eV) than that
of POM-Mo (0.60 eV), which leads to the inadequacy of
electrons in the CB of FeOCl. However, both productions of
Jian Zhang: 0000-0003-2411-3037
Jingquan Sha: 0000-0002-5925-9565
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work is supported by the Natural Science Foundation of
Shandong Province (ZR2016BB21), the Science and Tech-
nology plan project of Shandong Colleges and Universities
J17KB063), and Talent Team Culturing Plan for Leading
Disciplines of University in Shandong Province.
•
•
−
OH radicals and O2 radicals over FeOCl/POM-Mo are
larger than that over FeOCl/POM-W (Figures 7 and S7),
which contradicts the hypothesis of the double-transfer
mechanism. Therefore, a Z-scheme mechanism for FeOCl/
POM-Mo is adopted under our experimental conditions. As
both the CB and VB positions of POM-Mo are lower than that
of FeOCl, the electrons in the CB of POM-Mo tend to transfer
and recombine with the holes in the VB of FeOCl. As such, the
holes left behind in the VB of POM-Mo (2.94 eV) can directly
oxidize the degradation of phenol. Hence, the contributions of
POM-W and POM-Mo for the enhanced photo-Fenton
catalytic performance of FeOCl/POMs are mainly manifested
by the accumulation of electrons in FeOCl and the reduced
recombination of electron−hole pairs in FeOCl. Because the
CB potentials of FeOCl and POM-W are close, the separation
efficiency of the electron−hole pairs of FeOCl/POM-W are
lower than that of FeOCl/POM-Mo, which has been proven
by the PL spectra and EIS spectra (Figure 8). Therefore,
FeOCl/POM-Mo shows higher photo-Fenton catalytic activity
than FeOCl/POM-W.
(
REFERENCES
■
(
1) Clarizia, L.; Russo, D.; Di Somma, I.; Marotta, R.; Andreozzi, R.
Homogeneous Photo-Fenton Processes at Near Neutral pH: A
Review. Appl. Catal., B 2017, 209, 358−371.
2) Giannakis, S.; Lopez, M. I. P.; Spuhler, D.; Perez, J. A. S.; Ibanez,
́ ́ ́
̃
(
P. F.; Pulgarin, C. Solar Disinfection Is an Augmentable, In Situ-
Generated Photo-Fenton Reaction-Part 2: A Review of the
Mechanisms and the Fundamental Aspects of the Process. Appl.
Catal., B 2016, 199, 199−223.
(
3) Giannakis, S.; Lopez, M. I. P.; Spuhler, D.; Perez, J. A. S.; Ibanez,
́ ́ ́
̃
P. F.; Pulgarin, C. Solar Disinfection Is an Augmentable, In Situ-
Generated Photo-Fenton Reaction-Part 2: A Review of the
Applications for Drinking Water and Wastewater Disinfection. Appl.
Catal., B 2016, 198, 431−446.
(
4) Zhong, Y.; Yu, L.; Chen, Z.-F.; He, H.; Ye, F.; Cheng, G.; Zhang,
Q. Microwave-Assisted Synthesis of Fe O Nanocrystals with
3
4
Predominantly Exposed Facets and Their Heterogeneous UVA/
Fenton Catalytic Activity. ACS Appl. Mater. Interfaces 2017, 9,
29203−29212.
4
. CONCLUSIONS
In summary, using a facile one-step calcination method, the
novel FeOCl/POMs composite was successfully synthesized.
The contributions of POMs in FeOCl/POMs are mainly
reflected in increased optical absorption, surface area, and
surface potential. On the basis of the above-mentioned aspects,
FeOCl/POMs show greatly enhanced photocatalytic activities
and structure stability for phenol degradation than FeOCl. The
DFT calculations reveal the strong interface interaction by Fe−
O−Mo and Fe−O−W in FeOCl/POMs. The Z-scheme
mechanism of FeOCl/POM-Mo and double-transfer mecha-
(
5) Do, Q. C.; Kim, D.-G.; Ko, S.-O. Nonsacrificial Template
Synthesis of Magnetic-Based Yolk-Shell Nanostructures for the
Removal of Acetaminophen in Fenton-like Systems. ACS Appl.
Mater. Interfaces 2017, 9, 28508−28518.
(6) Nguyen, X. S.; Zhang, G.; Yang, X. Mesocrystalline Zn-Doped
Fe O Hollow Submicrospheres: Formation Mechanism and
3
4
Enhanced Photo-Fenton Catalytic Performance. ACS Appl. Mater.
Interfaces 2017, 9, 8900−8909.
(7) Qiu, B.; Li, Q.; Shen, B.; Xing, M.; Zhang, J. Stober-like Method
̈
to Synthesize Ultradispersed Fe O Nanoparticles on Graphene with
3
4
G
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Excellent Photo-Fenton Reaction and High-Performance Lithium
(26) Kong, X.-J.; Lin, Z.; Zhang, Z.-M.; Zhang, T.; Lin, W.
Hierarchical Integration of Photosensitizing Metal-Organic Frame-
works and Nickel-Containing Polyoxometalates for Efficient Visible-
Light-Driven Hydrogen Evolution. Angew. Chem., Int. Ed. 2016, 55,
6411−6416.
(27) Liu, Y.; Luo, F.; Liu, S.; Liu, S.; Lai, X.; Li, X.; Lu, Y.; Li, Y.; Hu,
C.; Shi, Z.; Zheng, Z. Aminated Graphene Oxide Impregnated with
Photocatalytic Polyoxometalate for Efficient Adsorption of Dye
Pollutants and Its Facile and Complete Photoregeneration. Small
2017, 13, 1603174.
Storage. Appl. Catal., B 2016, 183, 216−223.
(
8) Zubir, N. A.; Yacou, C.; Motuzas, J.; Zhang, X.; Zhao, X. S.; da
Costa, J. C. D. The Sacrificial Role of Graphene Oxide in Stabilizing a
Fenton-like Catalyst GO-Fe O . Chem. Commun. 2015, 51, 9291−
3
4
9
(
293.
9) Zhu, T.; Ong, W. L.; Zhu, L.; Ho, G. W. TiO2 Fibers Supported
β-FeOOH Nanostructures as Efficient Visible Light Photocatalyst and
Room Temperature Sensor. Sci. Rep. 2015, 5, 10601.
(
10) Qiu, B.; Deng, Y.; Du, M.; Xing, M.; Zhang, J. Ultradispersed
Cobalt Ferrite Nanoparticles Assembled in Graphene Aerogel for
Continuous Photo-Fenton Reaction and Enhanced Lithium Storage
Performance. Sci. Rep. 2016, 6, 29099.
(28) Yan, A.-X.; Yao, S.; Li, Y.-G.; Zhang, Z.-M.; Lu, Y.; Chen, W.-
L.; Wang, E.-B. Incorporating Polyoxometalates Into a Porous MOF
Greatly Improves Its Selective Adsorption of Cationic Dyes. Chem.
Eur. J. 2014, 20, 6927−6933.
(
11) Shi, W.; Du, D.; Shen, B.; Cui, C.; Lu, L.; Wang, L.; Zhang, J.
Synthesis of Yolk-Shell Structured Fe O @void@CdS Nanoparticles:
(29) Zhang, J.; Wu, S.; Bi, W.; Zhao, X.; Liu, G. Z-scheme Fe O -
3
4
2
3
A General and Effective Structure Design for Photo-Fenton Reaction.
doped Cu O as An Efficient Photo-Fenton-like Catalyst for
2
ACS Appl. Mater. Interfaces 2016, 8, 20831−20838.
Degradation of Phenol. Mater. Lett. 2019, 234, 13−16.
(
12) Wang, Y.; Zhao, H.; Zhao, G. Iron-Copper Bimetallic
(30) Liu, Y.; Jin, W.; Zhao, Y.; Zhang, G.; Zhang, W. Enhanced
Catalytic Degradation of Methylene Blue by α-Fe O /Graphene
Nanoparticles Embedded within Ordered Mesoporous Carbon as
Effective and Stable Heterogeneous Fenton Catalyst for the
Degradation of Organic Contaminants. Appl. Catal., B 2015, 164,
2
3
Oxide Via Heterogeneous Photo-Fenton Reactions. Appl. Catal., B
2017, 206, 642−652.
(31) Hou, G.; Li, Y.; An, W.; Gao, S.; Zhang, W.; Cui, W.
Fabrication and Photocatalytic Activity of Floating Type Ag PO /
3
(
96−406.
13) Gao, C.; Chen, S.; Quan, X.; Yu, H.; Zhang, Y. Enhanced
3
4
Fenton-like Catalysis by Iron-Based Metal Organic Frameworks for
ZnFe O /FACs Photocatalyst. Mater. Res. Bull. 2017, 94, 263−271.
2
4
Degradation of Organic Pollutants. J. Catal. 2017, 356, 125−132.
(32) Yu, J.; Xiang, Q.; Zhou, M. Preparation, Characterization and
Visible-Light-Driven Photocatalytic Activity of Fe-doped Titania
Nanorods and First-Principles Study for Electronic Structures. Appl.
Catal., B 2009, 90, 595−602.
(
14) Liu, Y.; Liu, X.; Zhao, Y.; Dionysiou, D. D. Aligned α-FeOOH
Nanorods Anchored on a Graphene Oxide-Carbon Nanotubes
Aerogel Can Serve as An Effective Fenton-like Oxidation Catalyst.
Appl. Catal., B 2017, 213, 74−86.
(33) Zhou, M.; Zhang, J.; Cheng, B.; Yu, H. Enhancement of Visible-
(
15) Yang, X.-j.; Xu, X.-m.; Xu, J.; Han, Y.-f. Iron Oxychloride
Light Photocatalytic Activity of Mesoporous Au-TiO Nanocompo-
2
(
FeOCl): An Efficient Fenton-like Catalyst for Producing Hydroxyl
Radicals in Degradation of Organic Contaminants. J. Am. Chem. Soc.
013, 135, 16058−16061.
16) Yang, X.-j.; Tian, P.-f.; Zhang, X.-m.; Yu, X.; Wu, T.; Xu, J.;
sites by Surface Plasmon Resonance. Int. J. Photoenergy 2012, 2012,
532843.
(34) Clark, S. J.; Segall, M. D.; Pickard, C. J.; Hasnip, P. J.; Probert,
M. I. J.; Refson, K.; Payne, M. C. First Principles Methods Using
CASTEP. Z. Kristallogr. 2005, 220, 567−570.
(35) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient
Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868.
(36) Zhang, G.; Xu, Y. Polyoxometalate-Mediated Reduction of
Dichromate Under UV Irradiation. Inorg. Chem. Commun. 2005, 8,
520−523.
(37) Zhou, L.; Wang, L.; Zhang, S.; Yan, R.; Diao, Y. Effect of
Vanadyl Species in Keggin-Type Heteropoly Catalysts in Selective
Oxidation of Methacrolein to Methacrylic Acid. J. Catal. 2015, 329,
431−440.
(38) She, J.; Fu, Z.; Li, J.; Zeng, B.; Tang, S.; Wu, W.; Zhao, H.; Yin,
D.; Kirk, S. R. Visible Light-Triggered Vanadium-Substituted
Molybdophosphoric Acids to Catalyze Liquid Phase Oxygenation of
Cyclohexane to KA Oil by Nitrous Oxide. Appl. Catal., B 2016, 182,
392−404.
(39) Niu, S.; Wang, Z.; Zhou, T.; Yu, M.; Yu, M.; Qiu, J. A
Polymetallic Metal-Organic Framework-Derived Strategy toward
Synergistically Multidoped Metal Oxide Electrodes with Ultralong
Cycle Life and High Volumetric Capacity. Adv. Funct. Mater. 2016,
27, 1605332.
2
(
Han, Y.-f. The Generation of Hydroxyl Radicals by Hydrogen
Peroxide Decomposition on FeOCl/SBA-15 Catalysts for Phenol
Degradation. AIChE J. 2014, 61, 166−176.
(
17) Yang, X.-j.; Xu, X.-m.; Xu, X.-c.; Xu, J.; Wang, H.-l.; Semiat, R.;
Han, Y.-f. Modeling and Kinetics Study of Bisphenol A (BPA)
Degradation Over an FeOCl/SiO Fenton-like Catalyst. Catal. Today
2
2
016, 276, 85−96.
(
18) Zhang, J.; Liu, G.; Wang, P.; Liu, S. Facile Synthesis of FeOCl/
Iron Hydroxide Hybrid Nanosheets: Enhanced Catalytic Activity as a
Fenton-like Catalyst. New J. Chem. 2017, 41, 10339−10346.
(
19) Zhang, J.; Jiao, X.-L.; Xia, Y.-G.; Liu, F.-F.; Pang, Y.-P.; Zhao,
X.-F.; Chen, D.-R. Enhanced Catalytic Activity in Liquid-Exfoliated
FeOCl Nanosheets as a Fenton-Like Catalyst. Chem.Eur. J. 2016,
2
2, 9321−9329.
(
20) Boskovic, C. Rare Earth Polyoxometalates. Acc. Chem. Res.
017, 50, 2205−2214.
21) Izzet, G.; Abecassis, B.; Brouri, D.; Piot, M.; Matt, B.; Serapian,
2
(
́
S. A.; Bo, C.; Proust, A. Hierarchical Self-Assembly of Polyox-
ometalate-Based Hybrids Driven by Metal Coordination and
Electrostatic Interactions: From Discrete Supramolecular Species to
Dense Monodisperse Nanoparticles. J. Am. Chem. Soc. 2016, 138,
(40) Kolesov, V. A.; Fuentes-Hernandez, C.; Chou, W.-F.; Aizawa,
N.; Larrain, F. A.; Wang, M.; Perrotta, A.; Choi, S.; Graham, S.;
Bazan, G. C.; Nguyen, T.-Q.; Marder, S. R.; Kippelen, B. Solution-
based Electrical Doping of Semiconducting Polymer Films Over a
Limited Depth. Nat. Mater. 2016, 16, 474−480.
5
093−5099.
(
22) Vila-Nadal, L.; Cronin, L. Design and Synthesis of
̀
Polyoxometalate-Framework Materials from Cluster Precursors. Nat.
Rev. Mater. 2017, 2, 17054.
(
23) Yang, X.; Zhao, H.; Feng, J.; Chen, Y.; Gao, S.; Cao, R. Visible-
(41) He, J.; Sun, H.; Indrawirawan, S.; Duan, X.; Tade, M. O.;
Wang, S. Novel Polyoxometalate@ g-C N Hybrid Photocatalysts for
Light-Driven Selective Oxidation of Alcohols Using a Dye-Sensitized
3
4
TiO -Polyoxometalate Catalyst. J. Catal. 2017, 351, 59−66.
Degradation of Dyes and Phenolics. J. Colloid Interface Sci. 2015, 456,
2
(
24) Shi, H.; Yu, Y.; Zhang, Y.; Feng, X.; Zhao, X.; Tan, H.; Khan, S.
15−21.
U.; Li, Y.; Wang, E. Polyoxometalate/TiO /Ag Composite Nanofibers
(42) Yan, G.; Feng, X.; Xiao, L.; Xi, W.; Tan, H.; Shi, H.; Wang, Y.;
Li, Y. Tuning of the Photocatalytic Performance of g-C N by
2
with Enhanced Photocatalytic Performance Under Visible Light. Appl.
Catal., B 2018, 221, 280−289.
3
4
Polyoxometalates Under Visible Light. Dalton Trans. 2017, 46,
(
25) Li, Y.; Liu, M.; Chen, L. Polyoxometalate Built-in Conjugated
16019−16024.
Microporous Polymers for Visible-Light Heterogeneous Photo-
catalysis. J. Mater. Chem. A 2017, 5, 13757−13762.
(43) Bao, J.; Zhang, X.; Fan, B.; Zhang, J.; Zhou, M.; Yang, W.; Hu,
X.; Wang, H.; Pan, B.; Xie, Y. Ultrathin Spinel-Structured Nanosheets
H
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX

Inorganic Chemistry
Article
Rich in Oxygen Deficiencies for Enhanced Electrocatalytic Water
(60) Chappell, H. F.; Thom, W.; Bowron, D. T.; Faria, N.; Hasnip,
P. J.; Powell, J. J. Structure of Naturally Hydrated Ferrihydrite
Revealed Through Neutron Diffraction and First-Principles Modeling.
Phys. Rev. Mater. 2017, 1, 036002.
Oxidation. Angew. Chem. 2015, 127, 7507−7512.
(
44) He, J.; Yang, X.; Men, B.; Wang, D. Interfacial Mechanisms of
Heterogeneous Fenton Reactions Catalyzed by Iron-Based Materials:
A review. J. Environ. Sci. 2016, 39, 97−109.
(61) Gutsev, G. L.; Belay, K. G.; Gutsev, L. G.; Ramachandran, B.
R.; Jena, P. Effect of Hydrogenation on The Structure and Magnetic
Properties of An Iron Oxide Cluster. Phys. Chem. Chem. Phys. 2018,
20, 4546−4553.
(
45) Li, X.; Wang, J.; Rykov, A. I.; Sharma, V. K.; Wei, H.; Jin, C.;
Liu, X.; Li, M.; Yu, S.; Sun, C.; Dionysiou, D. D. Prussian Blue/TiO2
Nanocomposites as a Heterogeneous Photo-Fenton Catalyst for
Degradation of Organic Pollutants in Water. Catal. Sci. Technol. 2015,
(62) Liu, T.; Xue, L.; Guo, X.; Zheng, C.-G. DFT study of mercury
5
(
, 504−514.
adsorption on α-Fe O surface: Role of Oxygen. Fuel 2014, 115, 179−
2 3
46) Zheng, Y.; Yu, Z.; Ou, H.; Asiri, A. M.; Chen, Y.; Wang, X.
185.
Black Phosphorus and Polymeric Carbon Nitride Heterostructure for
Photoinduced Molecular Oxygen Activation. Adv. Funct. Mater. 2018,
(63) Hong, J.; Hu, Z.; Probert, M.; Li, K..; Lv, D.; Yang, X.; Gu, L..;
Mao, N.; Feng, Q.; Xie, L.; Zhang, J.; Wu, D.; Zhang, Z.; Jin, C.; Ji,
W..; Zhang, X.; Yuan, J.; Zhang, Z. Exploring Atomic Defects in
Molybdenum Disulphide Monolayers. Nat. Commun. 2015, 6, 6293.
(64) Sun, Z.; Liao, T.; Dou, Y.; Hwang, S. M.; Park, M.-S.; Jiang, L.;
Kim, J. H.; Dou, S. X. Generalized Self-assembly of Scalable Two-
Dimensional Transition Metal Oxide Nanosheets. Nat. Commun.
2
(
8, 1705407.
47) Lei, X.-W.; Yue, C.-Y.; Wei, J.-C.; Li, R.-Q.; Mi, F.-Q.; Li, Y.;
Gao, L.; Liu, Q.-X. Novel 3D Semiconducting Open-Frameworks
based on Cuprous Bromides with Visible Light Driven Photocatalytic
Properties. Chem.Eur. J. 2017, 23, 14547−14553.
2014, 5, 3813.
(
48) Shi, J.; Ai, Z.; Zhang, L. Fe@ Fe O Core-Shell Nanowires
2 3
Enhanced Fenton Oxidation by Accelerating the Fe (III)/Fe (II)
Cycles. Water Res. 2014, 59, 145−153.
(
49) Niu, H.; Zheng, Y.; Wang, S.; Zhao, L.; Yang, S.; Cai, Y.
Continuous Generation of Hydroxyl Radicals for Highly Efficient
Elimination of Chlorophenols And Phenols Catalyzed by Heteroge-
neous Fenton-like Catalysts Yolk/Shell Pd@Fe O @Metal Organic
3
4
Frameworks. J. Hazard. Mater. 2018, 346, 174−183.
(
50) Chu, S.; Wang, Y.; Guo, Y.; Feng, J.; Wang, C.; Luo, W.; Fan,
X.; Zou, Z. Band Structure Engineering of Carbon Nitride: In Search
of a Polymer Photocatalyst with High Photooxidation Property. ACS
Catal. 2013, 3, 912−919.
(
51) Simon, T.; Bouchonville, N.; Berr, M. J.; Vaneski, A.; Adrovic,
blinger, M.; Susha, A. S.; Rogach, A.
kel, F.; Stolarczyk, J. K.; Feldmann, J. Redox Shuttle Mechanism
́
A.; Volbers, D.; Wyrwich, R.; Do
L.; Jac
̈
̈
Enhances Photocatalytic H Generation on Ni-Decorated CdS
2
Nanorods. Nat. Mater. 2014, 13, 1013−1018.
(
52) Zhou, X.; Haublein, V.; Liu, N.; Nguyen, N. T.; Zolnhofer, E.
̈
M.; Tsuchiya, H.; Killian, M. S.; Meyer, K.; Frey, L.; Schmuki, P. TiO2
Nanotubes: Nitrogen-Ion Implantation at Low Dose Provides Noble-
Metal-Free Photocatalytic H -Evolution Activity. Angew. Chem. 2016,
2
1
(
28, 3827−3831.
53) Lv, Y.; Liu, Y.; Zhu, Y.; Zhu, Y. Surface Oxygen Vacancy
Induced Photocatalytic Performance Enhancement of a BiPO4
Nanorod. J. Mater. Chem. A 2014, 2, 1174−1182.
(
54) Zheng, L.; Han, S.; Liu, H.; Yu, P.; Fang, X. Hierarchical MoS
2
Nanosheet@TiO2 Nanotube Array Composites with Enhanced
Photocatalytic and Photocurrent Performances. Small 2016, 12,
1
(
527−1536.
55) Maestre, J. M.; Lopez, X.; Bo, C.; Poblet, J.-M.; Casan
N. Electronic and Magnetic Properties of α-Keggin Anions: A DFT
̃
-Pastor,
n‑
III
IV
V
III
II
Study of [XM O ] , (M = W, Mo; X = Al , Si , P , Fe , Co ,
Co ) and [SiM VO ] (M = Mo and W). J. Am. Chem. Soc. 2001,
12
40
III
m‑
11
40
1
(
23, 3749−3758.
56) Kumar, D.; Derat, E.; Khenkin, A. M.; Neumann, R.; Shaik, S.
The High-Valent Iron-Oxo Species of Polyoxometalate, if It Can Be
Made, Will Be a Highly Potent Catalyst for C-H Hydroxylation and
Double-Bond Epoxidation. J. Am. Chem. Soc. 2005, 127, 17712−
1
(
7718.
57) Vila,
Mbomekalle,
̀
N.; Aparicio, P. A.; Sec
I. M. Electrochemical Behavior of α1/α2-[Fe(H O)-
heresse, F.; Poblet, J. M.; Lopez, X.;
́ ́
́
2
7
‑
P W O ] Isomers in Solution: Experimental and DFT Studies.
2
17 61
Inorg. Chem. 2012, 51, 6129−6138.
(
58) de Visser, S. P.; Kumar, D.; Neumann, R.; Shaik, S. Computer-
Generated High-Valent Iron-Oxo and Manganese-Oxo Species with
Polyoxometalate Ligands: How do they Compare with the Iron-Oxo
Active Species of Heme Enzymes? Angew. Chem. Int. Ed. 2004, 43,
5
(
661−5665.
59) Qiu, X. Y.; Huang, H. W.; Gao, Y. D. Effects of Surface
Properties of (010), (001) And (100) of MnWO And FeWO On
4
4
Absorption of Collector. Appl. Surf. Sci. 2016, 367, 354−361.
I
DOI: 10.1021/acs.inorgchem.8b02329
Inorg. Chem. XXXX, XXX, XXX−XXX