Promotion of phenol photodegradation based on novel self-assembled magnetic bismuth oxyiodide core-shell microspheres

Article abstract of DOI:10.1039/c7ra06116a

A novel visible-light driven "plates-over-nanospheres" (PONs) photocatalyst with BiOI nanoplates grown on the surface of Fe₃O₄ nanospheres was prepared using a facile method. By utilizing the self-assembly of the BiOI nanoplates, this environmentally friendly method challenges the classic synthesis of core-shell heterojunctions which always includes tedious synthesis steps as well as linker shells. With the introduction of magnetic cores, the PONs promoted the favorable degradation of phenol under visible-light irradiation. The enhanced visible-light photocatalytic performance of the PONs was ascribed to the unique synergistic effect between the magnetic cores and BiOI shells, resulting in the improved separation and inhibited recombination of photogenerated carriers. Moreover, the PONs maintained nearly constant photodegradation rates even after five cycles of photocatalytic experiments, without obvious catalyst deactivation. All in all, our work paves a novel way for the further design of heterojunction photocatalysts with fine recyclability and enhanced photocatalytic activity.

Full text of DOI:10.1039/c7ra06116a

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Promotion of phenol photodegradation based on
novel self-assembled magnetic bismuth oxyiodide
core–shell microspheres†
Cite this: RSC Adv., 2017, 7, 36653
*
Danlin Qian, Suting Zhong, Suyun Wang, Yaru Lai, Nan Yang and Wei Jiang
A novel visible-light driven “plates-over-nanospheres” (PONs) photocatalyst with BiOI nanoplates grown on
the surface of Fe3O4 nanospheres was prepared using a facile method. By utilizing the self-assembly of the
BiOI nanoplates, this environmentally friendly method challenges the classic synthesis of core–shell
heterojunctions which always includes tedious synthesis steps as well as linker shells. With the
introduction of magnetic cores, the PONs promoted the favorable degradation of phenol under visible-
light irradiation. The enhanced visible-light photocatalytic performance of the PONs was ascribed to the
unique synergistic effect between the magnetic cores and BiOI shells, resulting in the improved
separation and inhibited recombination of photogenerated carriers. Moreover, the PONs maintained
nearly constant photodegradation rates even after five cycles of photocatalytic experiments, without
obvious catalyst deactivation. All in all, our work paves a novel way for the further design of
heterojunction photocatalysts with fine recyclability and enhanced photocatalytic activity.
Received 1st June 2017
Accepted 10th July 2017
DOI: 10.1039/c7ra06116a
rsc.li/rsc-advances
BiOX (X ¼ I, Br, Cl),12–17 Bi2WO6,18,19 BiVO4,20–24 BiO2CO3,25
Bi2MoO6,26,27 etc. Among these, BiOX is generally accepted to be
1. Introduction
a promising photocatalytic material because of its excellent
photocarrier mobility, owing to its layered morphology of
interlacing [Bi2O2]2+ slabs and halogen atoms. In the family of
bismuth oxyhalides, BiOI is proven to be the most promising
photocatalyst candidate for wastewater treatment, having the
narrowest band gap (ca. 1.6–1.9 eV) and the strongest visible-
light absorption.28–30 However, the suspended particulate cata-
lysts are difficult to recover, which limits the functional life-time
of BiOI for photocatalysis in practical applications. Moreover,
the loss of the photocatalyst during the photocatalytic reaction
and separation process may even cause secondary pollution.
Given this situation, it is of crucial signicance to construct
heterogeneous systems and overcome the above recycling
drawback inherent to single phase photocatalysts.
Attempts have been made to introduce magnetic-functional
components into photocatalytic material.31,32 As a commonly
used magnetic material in industrial manufacture, Fe3O4 is
similar to a conductor due to its narrow band gap (0.1 eV).33 So
far, many studies have proved that Fe3O4 could be used as
a favorable co-catalyst to transfer photogenerated electron–hole
pairs, and that it could even offer benecial synergistic effects
with the photocatalyst, rendering it of signicance for photo-
catalytic purposes.34,35 However, the synthesis of special core–
shell orientated composites is always complicated and costly,
due to the introduction of some linker layers (e.g. silica, poly-
mers) as well as the required tedious intermediate steps.36
Additionally, the saturation magnetization (Ms) of composite
materials can be weakened by the existence of linker layers.
As a promising environmental remediation technology, photo-
catalysts have attracted considerable attention since they were
rst investigated about 40 years ago.1 Up to now, there has been
a lot of research into the purication of wastewater, especially
the removal of toxic organic contaminants.2–4 Despite signi-
cant efforts to date, the semiconductor photocatalysis tech-
nology is still insufficient to satisfy the requirements for
industrially oriented applications due to two intrinsic short-
comings of the conventional photocatalytic systems. One is the
wide band gaps, for example light absorption by TiO2 is limited
to ultraviolet light (about 4% of the solar spectrum), making it
possess only moderate photocatalytic performance.5–8 The other
shortcoming is the complexity of catalyst separation and
recovery. Once nishing their mission in the liquid phase,
powder-like photocatalysts are always difficult to separate and
recover. Therefore, research into the construction of high effi-
ciency and recyclable visible-light driven photocatalysts is the
focus of much endeavor under the increasing environmental
pressure.9–11
The past ten years have witnessed an increasing interest in
bismuth-based oxides, and this research is mainly focused on
National Special Superne Powder Engineering Research Center of China, School of
Chemical Engineering, Nanjing University of Science and Technology, Nanjing
210094, China. E-mail: superne_jw@126.com; Fax: +86 25 84315042; Tel: +86 25
84315042
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra06116a
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ꢁ
Herein, a novel and facile synthesis of BiOI nanoplates were stirred constantly at 80 C for 3 h. Finally, the resulting
stacked on Fe3O4 nanospheres (PONs) was employed without precipitates were collected, washed thoroughly with ethanol
the use of any linker shell. It is worth noting that in this study, and deionized water three times, and then dried at 40 ꢁC in
phenol could be degraded within 5 h using this photocatalyst, a vacuum oven. The formation procedure of the PONs is illus-
and PONs showed good recyclability. Additionally, the forma- trated in Fig. 1.
tion process of the composites, as well as their corresponding
optical properties, electrochemical properties and photo- 2.3 Characterization
catalytic mechanism were investigated in detail. The rational
design is a proof of concept for integrating functional magnetic
material with photocatalysts and even provides an alternative
for the destruction of phenol.
The phase structure information of the samples was obtained
by X-ray diffraction (XRD) on a D8 Advance diffractometer
(Bruker, Germany) with Cu Ka radiation (l ¼ 0.154 nm). The
microstructure and morphology of the samples were investi-
gated by S-4800 scanning electron microscopy (SEM, Hitachi,
Japan) and high resolution transmission electron microscopy
(HRTEM, Tecnai G2 F30 S-TWIN). UV-vis diffuse reectance
spectroscopy (DRS) of the samples was performed with a UV-vis
2. Experimental
2.1 Materials
All chemicals were of analytical grade, commercially available
and used without purication. Bismuth nitrate pentahydrate
(Bi(NO3)3$5H2O), ferric chloride (FeCl3$6H2O) and potassium
iodide (KI) were purchased from Sinopharm Chemical Reagent
Co. Ltd. Other chemicals were all obtained from Shanghai Ling
Feng Chemical Reagent Co. Ltd.
spectrophotometer (UV-2700, Shimadzu). For the DRS sample
preparation, the powders were made as dry-pressed lms with
BaSO4 as the reectance standard. Surface chemical composi-
tions and states were obtained from X-ray photoelectron spec-
troscopy (XPS, K-Alpha, Thermo VG). The magnetic properties
were characterized by a vibrating sample magnetometer (VSM,
Lakeshore-735) at room temperature. Fourier transform
infrared (FT-IR) spectra of the samples were measured on an FT-
IR spectrometer (Nicolet 6700). The photoluminescence (PL)
spectra of the samples were characterized using a uorescence
spectrophotometer (Shimadzu RF6000, Japan). N2 absorption/
desorption isotherms were calculated by Micromeritics ASAP
2020 according to the Brunauer–Emmett–Teller (BET) method.
2.2 Synthesis of PONs
PONs were synthesized via a rst-step solvothermal process and
a second-step chemical precipitation treatment. During the
rst-step solvothermal process, porous magnetite Fe3O4 nano-
spheres were prepared according to the reported strategy.37 For
the preparation of PONs, Bi(NO3)3$5H2O (2 mmol) was next
dissolved in ethylene glycol (EG) solution and ethanol solution,
and the volume ratio of EG to ethanol was controlled at 1/2. To
ensure all materials were dissolved completely, the mixture was
stirred for 30 min at least. Then certain stoichiometric amounts
of Fe3O4 powders were added into the solution under ultra-
sonication to give homogeneous dispersion systems. The
samples are denoted as PONs(x/y), in which x/y is the molar ratio
of Fe/Bi. Aer that, KI (2 mmol) solution was added dropwise
into the above mixtures. Subsequently, 1.5 M NH3$H2O was
added to adjust the pH value to ꢀ7.0 and then the suspensions
2.4 Photocatalytic evaluation
The photocatalytic performances of the as-prepared samples
were monitored by degradation of phenol and methyl orange
(MO) under visible-light irradiation (500 W, l > 420 nm) at
ambient temperature. All photocatalytic tests were performed
using the same initial conditions: the photocatalyst (50 mg) was
added into 50 mL of organic reaction solution (20 mg Lꢂ1). Prior
to photoreaction, the suspension solution was continuously
stirred in the dark to guarantee an absorption–desorption
equilibrium between the degrading pollutants and the photo-
catalysts. Throughout the reaction, at appropriate intervals,
about 3 mL of the reaction suspension was sampled and
magnetically separated to remove the solid. Active-species
trapping experiments were performed with the introduction of
diverse quenchers. The concentrations of supernatant liquids
were measured by monitoring the characteristic absorption
bands (269 nm for phenol, 465 nm for MO, respectively), with
an UV-vis spectrophotometer (Agilent Technologies Cary 100).
2.5 Photoelectrochemical measurements
Typically, the aqueous dispersions of the as-prepared samples
were dip coated on indium-tin oxide glasses (ITO), yielding
electrodes with catalyst loading areas of 1 cm2. The photocur-
rent vs. time curves were measured by an electrochemical work
station (CHI760E, Shanghai, China) in a standard 3-electrode
system with the ITO electrode as the working electrode, a Pt wire
as the counter electrode and an Ag/AgCl electrode as the
Fig. 1 Schematic illustration of the formation procedure for PONs.
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reference electrode under a constant bias of 1.23 V vs. RHE. The
power intensity of the incident light was adjusted to 100 mW
cmꢂ2. Electrochemical impedance spectroscopy (EIS) was
carried out in the frequency range of 10ꢂ2 to 105 Hz with an
amplitude of 5 mV at the open circuit potential. Mott–Schottky
analysis was employed with an amplitude of 5 mV vs. the Ag/
AgCl reference electrode.
2.6 cO2ꢂ and cOH active species trapping and quantication
experiments
For detecting the active species during photocatalytic reaction,
superoxide radicals (cO2ꢂ), hydroxyl radicals (cOH), and holes
(h+) were investigated by adding 1.0 mM benzoquinone (BQ,
a quencher of cO2ꢂ), 1 mM methanol (a quencher of cOH) or
1
mM disodium ethylene-diaminetetraacetate (EDTA-2Na,
a qꢂuencher of h+), respectively.38 The quantitative amount of
cO2 and cOH generated were inspected by nitroblue tetrazo-
lium (NBT) transformation and terephthalic acid photo-
luminescence (TA-PL) probing, respectively.39 The production of
cO2ꢂ was quantitatively analyzed by recording the concentration
of NBT since NBT can react with cO2ꢂ and displays a maximum
absorbance at 260 nm. Terephthalic acid (TA) can react with
cOH to transform 2-hydroxyterephthalic acid (TA-OH), which is
a highly uorescent product. By monitoring the PL intensity of
TA-OH (excitation at 315 nm), the production of cOH was
quantitatively analyzed.
Fig. 2 (a) XRD patterns of PONs(1/3), Fe3O4 and BiOI; (b) FT-IR spectra
3. Results and discussion
of PONs(1/3), Fe3O4 and BiOI.
3.1 Characterization of structure and properties of PONs
The phase composition of the as-prepared PONs(1/3), Fe3O4 and
BiOI samples are exhibited in Fig. 2a. All of the diffraction peaks
in the Fe3O4 and BiOI samples could be perfectly indexed to the
cubic Fe3O4 phase (JCPDS no: 19-0629) and the tetragonal BiOI
phase (JCPDS no: 73-2062), indicative of the production of pure
BiOI and Fe3O4. As shown in Fig. 2a, XRD peaks of tetragonal
BiOI could be clearly observed in the PONs, and the diffraction
peak at 35.4ꢁ corresponded to the (311) crystalline plane of
Fe3O4, conrming the co-existence of Fe3O4 and BiOI in the
composite. Besides, no miscellaneous peaks were observed in
the sample, indicating the high purity of the composite.
The chemical composition and chemical bonding of the as-
prepared samples were studied by the FT-IR investigation. As
shown in Fig. 2b, the main absorption band of PONs(1/3) located
at approximately 484 cmꢂ1 could be attributed to the symmet-
rical A2u-type vibration mode of the Bi–O bond.40 The FT-IR
spectra of both pure BiOI and PONs(1/3) contain characteristic
absorption bands at 3411 cmꢂ1 and 1620 cmꢂ1, which were
assigned to the stretching vibration and bending vibration of
surface O–H respectively. The peak at 1382 cmꢂ1 was assigned
to the O–H deformations of the C–OH groups. Additionally, the
slight absorptions at 2943 cmꢂ1 and 2875 cmꢂ1 could be
attributed to C–H groups. These observed peaks could be
derived from EG adsorbed on the surface of the photocatalyst in
the synthetic process. All of the characteristic peaks of BiOI are
retained in the PONs(1/3), indicating that the addition of Fe3O4
had no effect on the chemical structure of BiOI in the
composites. Moreover, the spectrum of PONs(1/3) exhibits
a band at 574 cmꢂ1, as a result of the stretching vibration of Fe–
O bonds in the Fe3O4. The above results indicate that the
PONs(1/3) are composed of bismuth oxyiodides and Fe3O4. No
impurity absorption peak was detected, which is in accordance
with the XRD result.
To observe the morphology of PONs and reveal the formation
process of the core–shell structure in detail, time-dependent
experiments were carried out and the products were investi-
gated by SEM. The composites with different reaction times of
0 h, 1 h, 2 h and 3 h in the water-bath are denoted as PONs-0,
PONs-1, PONs-2 and PONs-3, respectively. As shown in
Fig. 3a1, the Fe3O4 nanospheres had an average diameter of
400 nm and a relatively rough surface. Sheet-shaped structures
of BiOI were randomly distributed around the Fe3O4 magnetic
core. Aer a reaction time of 1 h (Fig. 3b1 and b2), underde-
veloped hierarchical architectures existed with some individu-
ally dispersed nanoplates, indicating that the oriented
attachment was still in progress. When the reaction time was
prolonged to 2 h (Fig. 3c1 and c2), separate nanoplates faded
away and large scale cabbage-shaped hierarchical architectures
were formed. As the reaction went further, the BiOI nanoplates
were regularly assembled over the magnetic cores, and fully
developed. 3D owerlike PONs with diameters of 1 mm were
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Fig. 4 (a–c) HRTEM images of PONs; (d) SAED pattern of PONs; (e)
EDX spectrum of PONs.
Table 1 Results of specific surface areas, pore volumes and average
pore diameters for PONs(1/3), EBiOI, HBiOI and Fe3O4/BiOI
Average pore
diameter (nm)
Samples
SBET (m2 gꢂ1
)
Vpore (cm3 gꢂ1
)
Fig. 3 SEM images (a1, a2) of PONs-0, (b1, b2) of PONs-1, (c1, c2) of
PONs-2, (d1, d2, e) of PONs-3, and (f) of the Fe3O4/BiOI composite.
PONs(1/3)
EBiOI
HBiOI
46.60
8.84
2.77
2.90
0.149
0.033
0.011
0.012
10.891
8.388
3.560
4.357
Fe3O4/BiOI
eventually formed aer the 3 h water-bath reaction (Fig. 3d1 and
d2). Although very few microspheres were not fully encapsu-
lated (Fig. 3e), their presence proves the existence of the core–
shell structure intuitively. Additionally, the good interface
between BiOI and Fe3O4 suggests that the nanoplates were
compatible with the magnetic core. In the contrast experiment,
the direct reaction of Bi(NO3)3$5H2O, KI and Fe3O4 in aqueous
solution (Fe3O4/BiOI) resulted in Fe3O4 nanospheres randomly
loaded on irregular large plates of BiOI with no distinct trend to
form core–shell hierarchical nanostructures (Fig. 3f).
Detailed structural information about the PONs(1/3) was
further obtained by HRTEM, selected area electron diffraction
(SAED) and electron dispersive spectroscopy (EDX). Fig. 4a
shows a typical HRTEM image of a BiOI shell, which reveals the
highly crystalline nature of the composites. As shown in Fig. 4b
and c, the spacings of the adjacent lattice fringes were ca. 0.30
and 0.28 nm, which correspond to the interplanar spacings of
the (102) plane and (110) plane of tetragonal BiOI, respectively.
Fig. 4d shows that in the SAED patterns of the composite,
electron diffraction spots could be indexed to the (102) and
(110) planes of BiOI, indicating a good agreement with the
HRTEM observations. EDX analysis (Fig. 4e) reveals the exis-
tence of C, Bi, O, I and Fe on the PONs(1/3) composites. These
results further corroborate the successful synthesis of PONs and
are consistent with the observations from XRD and SEM.
To gain more insight into the specic surface areas and the
porous structures of the samples, BET was conducted. In Table
1, the corresponding BET surface areas of HBiOI and Fe3O4/
BiOI were as low as 2.77 m2 gꢂ1 and 2.90 m2 gꢂ1 respectively.
The isotherm of the as-prepared Fe3O4/BiOI and HBiOI are close
to Type III without a hysteresis loop at high relative pressure,
suggesting the compact structure of HBiOI and Fe3O4/BiOI. It
can be speculated that Fe3O4 does not play a role in increasing
the BET surface area in the Fe3O4/BiOI, where Fe3O4 nano-
spheres are randomly loaded on irregular large plates of BiOI.
Conversely, the BET surface area of PONs(1/3) was measured to
be 46.60 m2 gꢂ1, which is signicantly higher than that of
pristine EBiOI (BiOI prepared in an alcohol solvent solution
system) with a BET surface area of 8.84 m2 gꢂ1, conrming the
supporting role of Fe3O4 cores. Moreover, the isotherm of the
as-prepared PONs(1/1), and EBiOI are close to Type IV with
a distinct H3 hysteresis loop at a high relative pressure between
0.6 and 1.0.41 The behavior shows the predominance of meso-
pores in the microspheres. As shown in Fig. 5b (inset), the pore
volume and size of PONs(1/1) are 0.149 cm3 gꢂ1 and 10.891 nm,
respectively, which are slightly larger than the corresponding
measurements for other samples. The slit-like pores are
generally formed by the aggregation of plate-like particles,
which is consistent with the self-assembled nanoplate-like
morphology observed in the SEM results.42 Generally, a larger
specic surface area and pore volume correspond to more
surface active sites exposed and available to the reaction,43
which in turn results in better adsorption, desorption and
diffusion of reactants. Therefore, the large specic surface area
as well as pore volume might be responsible for the high pho-
tocatalytic activity of the PONs(1/3)
.
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Fig. 6 XPS spectra of the as-prepared PONs(1/3) catalyst: (a) survey
spectrum of the sample; (b) Bi 4f; (c) I 3d; (d) Fe 2p; and (e) O 1s.
holes.46 For efficient photocatalytic activity, a wide spectrum of
optical absorption is necessary. To further explore how the
heterojunction inuences the light absorption ability of PONs,
we studied the optical properties of the as-prepared products by
DRS. As shown in Fig. 7a, pure BiOI exhibited light absorption
in the visible range and showed only one absorption edge at
about 670 nm, whereas the light harvesting range of PONs(1/3)
was slightly blue-shied reaching to ꢀ650 nm. As shown in
Fig. 5 (a) N2 adsorption–desorption isotherm distribution curves for
as-prepared PONs(1/1), EBiOI, Fe3O4/BiOI and HBiOI. (b) Enlarged view
of PONs(1/3) and pore size distribution of PONs(1/3) (inset).
The chemical composition and surface chemical states of the
PONs(1/3) were further investigated by XPS. The peak positions
were calibrated by referencing the C 1s line to 284.50 eV. As
shown in Fig. 6a, the typical XPS survey spectrum indicated the
existence of Bi, O, I, and C at the surface of PONs(1/3) and the
carbon peak could be attributed to the surface adventitious
carbon. Two symmetrical peaks at 159.13 and 164.48 eV in the
spectrum of Bi 4f were respectively assigned to Bi 4f7/2 and Bi
4f5/2, indicating the existence of Bi3+ (Fig. 6b). The peaks at
619.78 and 631.28 eV in the spectrum of I 3d were respectively
assigned to the inner electrons of I 3d5/2 and 3d3/2 (Fig. 6c).13
Notably, Fe 2p peaks were not found in the high-resolution
spectrum, which was attributed to the fact that the technique
probes only the surface of a micrometer sphere and the
magnetic core could not be detected.44 This phenomenon
indirectly proves that the Fe3O4 nanospheres were covered
completely by BiOI shells. Meanwhile, the high-resolution XPS
spectrum of O 1s (Fig. 6e) could be deconvoluted into two peaks
at binding energies of 530.6 and 531.8 eV, which respectively
correspond to Bi–O bonds of BiOI,13 and O–H bonds of the
surface-adsorbed water.45
Fig. 7 (a) UV-vis diffuse reflectance spectra and (b) corresponding
colors of pure Fe3O4, BiOI and PONs(1/3) samples. Plots showing the
band gaps of (c) BiOI and (d) PONs(1/3) composites.
Generally, the rst step of photocatalysis for a semi-
conductor is the conversion of light into energetic electrons and
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Fig. 7b, the color of BiOI was saffron and pure Fe3O4 was deep a smaller semicircle radius in its EIS Nyquist plot than pure
black. The slight blue shi of the absorption edge was in BiOI at high frequency. This suggests that the presence of Fe3O4
accordance with the color change from saffron (BiOI) to saddle can be a benet to the interfacial charge transfer. To further
brown (PONs(1/3)). Nevertheless, PONs(1/3) still showed prom- determine the semiconductor type of the as-prepared product
inent absorbance in the visible-light range. The band gap (Eg) of as well as its at potential, Mott–Schottky analysis was
a crystalline semiconductor follows the equation:47
employed herein. The positive or negative slope of a linear 1/C2
vs. potential curve implies the type of semiconductor. As shown
in Fig. 8c, the linear slope for the PONs(1/3) catalyst is positive,
indicating that it is an n-type semiconductor whose Fermi level
(EF) is close to the conduction band (CB). Moreover, the at-
band potential of the binary PONs(1/3) composite, from the
results at a frequency of 1000 Hz, is determined to be ꢂ0.85 V
(1.23 eV vs. RHE). The above results indicate that an improved
separation and an inhibited recombination of photogenerated
carriers occur on the binary PONs composite, and further
conrm that the improved photocatalytic performance can be
attributed to this unique synergistic effect.
Additionally, PL spectroscopy was applied to elucidate the
separation capacity of the photogenerated charge carriers and
surface defects as shown in Fig. 8d. Generally, a lower intensity
PL peak corresponds to a higher separation efficiency of elec-
tron and hole pairs, thus leading to a higher photocatalytic
activity.48 With emission peaks located at 446 nm, the
ahv ¼ A(hv ꢂ Eg)n/2
(1)
where a, v, A and h are absorbance, light frequency, a constant
value and Planck's constant, respectively. For the indirect
transition semiconductor BiOX, the value of n is 4.17 The band
gap of BiOX could be obtained from the plots of (Abs)1/2 vs. hv;
the intercepts of the tangents to the x-axis (hv) give an approx-
imation of the band gap. Therefore, the Eg value of BiOI was
calculated to be 1.76 eV (Fig. 7c), which is in accordance with
the value reported in the literature.16,47 Notably, as shown in
Fig. 7d, the band gap of the PONs(1/3) composite was scarcely
changed (Eg ¼ 1.78 eV) despite the introduction of optically
inactive Fe3O4, demonstrating that the heterojunction effect
does not have a great inuence on the optical absorption
properties.
To further explore the correlation between synergistic effects
and interfacial charge transfer, we carefully conducted transient
photocurrent response measurements, EIS, and constructed the
Mott–Schottky plots of the as-prepared products. As shown in
Fig. 8a, under each 50 s on/off visible-light irradiation cycle,
photocurrents with rapid and reproducible responses were
generated. In comparison with pure BiOI, the PONs(1/3) elec-
trode exhibited increased photocurrent density as well as more
consistent photocurrent, demonstrating higher efficiency and
lower recombination of photogenerated electron–hole pairs in
the core–shell structures. The radius of the high frequency
semicircle on an EIS spectrum mirrors the interface layer
resistance of the material.14 As shown in Fig. 8b, PONs showed
room temperature PL spectra of the BiOI, PONs(1/1), PONs(1/3)
,
PONs(1/6) and Fe3O4 samples are shown in Fig. 8d. The PL
spectra show that pristine BiOI displays a more intense uo-
rescence emission peak than PONs. It can be speculated that
the lower PL intensity of PONs is related to the good electrical
conductivity of Fe3O4. The interfacial electrons efficiently
transfer from the CB of BiOI to that of Fe3O4, hampering the
recombination of photogenerated charge carriers. Moreover, it
can be noticed that PONs(1/3) shows the lowest PL intensity
when compared with PONs(1/1) and PONs(1/6), indicating that
the ratio of Fe3O4 to BiOI is an important factor in the recom-
bination of photogenerated charge carriers. Furthermore, the
PL spectroscopy result perfectly matches the trend shown in the
photocatalytic activity discussed previously, indicating that the
recombination rate of electrons and holes could be suppressed
greatly by the addition of Fe3O4.
3.2 Photocatalytic activity
To evaluate the photocatalytic efficiency of the as-prepared
samples under visible-light irradiation, phenol with no indi-
rect dye photosensitization was chosen to be a target pollutant.
Fig. 9a shows the absorption spectra of phenol in the presence
of PONs(1/3). The absorbance of phenol at 269 nm decreased
sharply and almost disappeared aer 5 hours, indicating that
the PONs(1/3) possesses satisfactory photolysis efficiency. As
shown in Fig. 9b, the inuence of synthesis parameters such as
the solvent system and the proportion of Fe3O4 were further
investigated. Although BiOI has been considered as an excellent
visible-light driven photocatalyst,15 the BiOI random nanoplates
(prepared in aqueous solution system, and denoted as HBiOI)
displayed poor photocatalytic activity as did the BiOI micro-
spheres (prepared in alcohol solvent solution system and
denoted as EBiOI). The increased surface area and particle size
Fig. 8 (a) Photocurrent responses of BiOI and PONs(1/3) in 0.5 M Na2SO4
aqueous solutions (pH ¼ 5.8) under irradiation with simulated sunlight at
1.23 V vs. RHE; (b) EIS Nyquist plots of pure BiOI and PONs(1/3); (c) flat
potential determination of PONs(1/3) by Mott–Schottky plots; (d) PL
spectra of BiOI, PONs(1/3), PONs(1/1), PONs(1/6) and Fe3O4.
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Fig. 10 (a) Hysteresis loops of Fe3O4 and PONs(1/3) under a magnetic
field at room temperature; (b) photodegradation of phenol over
PONs(1/3) in successive cycles; (c) schematic diagram of sample
recovery under an external magnetic field.
Fig. 9 (a) The time-dependent absorption spectra of phenol in the
presence of PONs(1/3) under visible-light irradiation (l > 420 nm). (b)
Degradation ratios, (c) first-order kinetics data and (d) rate constants
for the photocatalytic removal of phenol by various photocatalysts
under visible-light irradiation.
recycling. To verify the stability of the catalyst during photo-
degradation, a series of successive experiments with MO as the
target pollutant were conducted. As depicted in Fig. 10b, the
rate constants, k, of each cycle were calculated to be 0.0375,
0.0359, 0.0358, 0.0328, and 0.0314, respectively. It is worth
mentioning that there is a 16% difference in the rate constant
values between the rst cycle and the h cycle. To investigate
the reason for the decline in catalytic efficiency and to validate
the stability of the photocatalysts, XRD and thermogravimetric
(TG) investigations were carried out. In Fig. S1,† it can be seen
by comparing the XRD patterns of samples before and aer
reactions that the photocatalytic process had little impact on
the phase composition. However, aer ve cycles of photo-
catalytic reactions, the mass fraction of BiOI had changed from
90% to 80%, as shown in Fig. S2.† It can be speculated that the
catalyst deactivation could be attributed to the small release of
BiOI during the separation and recycling process. In general, as
an effective separation and recycling catalyst, the PONs(1/3)
composite is still very stable and reliable.
of EBiOI might account for the higher photocatalytic activity of
this catalyst as compared to HBiOI,17 although its activity was
still much lower than that of PONs. To quantitatively explore the
degradation rate of phenol, a pseudo-rst order model was
applied to the photocatalytic kinetic study.49 It was found that
the phenol photodegradation exhibits a good linear relation-
ship (Fig. 9c). As shown from Fig. 9d, the rate constants k of
HBiOI, EBiOI, Fe3O4/BiOI(1/3), PONs(1/1), PONs(1/3) and PONs(1/6)
were respectively calculated to be 0.0206, 0.0464, 0.0205, 0.052,
0.4397 and 0.0683 hꢂ1. The optimal photocatalyst, PONs(1/3)
,
could remove 90% phenol within 5 h and possessed the highest
photocatalytic activity as compared to PONs(1/1) and PONs(1/6). It
can be deduced that the proportions of the components affect
the photocatalytic activity of PONs signicantly. However, the
removal of phenol over Fe3O4/BiOI(1/3) (mechanically mixed)
upon visible-light irradiation was as low as 10%, indicating that
the interaction between the BiOI and Fe3O4 also plays an
important role in promoting photocatalytic efficiency.
The magnetic properties of the Fe3O4 nanospheres and PONs
composite were measured on a VSM with the applied eld of ꢂ8
kOe to 8 kOe at room temperature (300 K). As shown in Fig. 10a,
the S-like curves exhibited distinctly symmetrical hysteresis
loops and showed ferromagnetic behavior: the Ms values for
pure Fe3O4 and the PONs(1/3) composite were 69.7 emu gꢂ1 (32
Oe) and 14.1 emu gꢂ1 (30 Oe), respectively. The decrease in
magnetization of the PONs(1/3) composite may be caused by the
coating of the BiOI shell, which reduces the magnetite fraction
in each microsphere.35 Nevertheless, as shown in Fig. 10c, the
well-dispersed PONs(1/3) powders could be separated rapidly
from water within 20 s, demonstrating that the as-prepared
composite possesses a good response towards the external
magnetic eld. The specic magnetic property of the PONs(1/3)
composite provides a possibility for effective separation and
3.3 Proposed mechanism
Generally, there are three possible reaction mechanisms for the
photodegradation of organics by semiconductors, namely (i)
photocatalysis, (ii) photolysis, and (iii) dye photosensitization.41
However, the decomposition of phenol by photolysis is negli-
gible; degradation caused by photolysis under visible light in
a blank test is not observable, as shown in Fig. 11a. Since phenol
is one of the organics with no indirect dye photosensitization, the
effect of dye photosensitization may also be neglected. Therefore,
photocatalysis can be considered as the only possible reaction
mechanism for the degradation of phenol over PONs(1/3)
.
To detect the active species, a series of trapping experiments
during photocatalysis were performed. The radical trapping
agents include BQ (1 mM, a scavenger of cO2ꢂ), EDTA-2Na
(1 mM, a scavenger of h+), and methanol (1 mM, a scavenger
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Fig. 12 Schematic illustration of the proposed photocatalytic degra-
dation mechanism of phenol over PONs(1/3) under visible light.
ꢂ
forming cOH should be much lower than that for cO2 on
Fig. 11 (a) Trapping experiments for detecting the active species over
PONs(1/3); (b) removal efficiency of MO over PONs(1/3) with the addition
of different scavengers; (c) nitroblue tetrazolium (NBT) transforming
efficiencies; (d) hydroxyterephthalic acid (TA-OH) fluorescence
intensities of PONs(1/3), PONs(1/1), PONs(1/6) and BiOI.
account of the fact that the redox potential of BiV/BiIII (1.59 eV)
is more negative than the standard potential of cOH/OHꢂ (2.38
eV).53 Since the limited transformation reaction of h+ + OHꢂ
/
cOH, a fraction of h+ can participate in the oxidization. On the
basis of the above mentioned reason, it is proposed that under
visible-light irradiation, electron–hole pairs are rstly generated
over the BiOI coatings. Then phenol adsorbed on the surface of
the catalyst is oxidized by the activated oxygen cO2ꢂ, h+ and cOH
species to form the corresponding intermediate which is further
oxidized into degradation products as shown in Fig. 12.
of cOH), respectively.38 As shown in Fig. 11b, the photo-
degradation was remarkably prohibited when the BQ radical
scavenger (scavenger of cO2ꢂ) was introduced. Besides, the
degradation efficiency was relatively less affected by EDTA-2Na
(scavenger of h+) or methanol (scavenger of cOH). The abovꢂe
trapping experiment results suggest that photogenerated cO2
play a chief role, while the h+ and cOH play minor roles in the
photocatalytic system.
4. Conclusions
In summary, we have constructed a novel photocatalyst (PONs)
through a simple and direct precipitation method, which
exhibits substantially enhanced photocatalytic activity as
compared to pure BiOI and Fe3O4/BiOI. Under visible-light
irradiation, PONs possessed prominent photocatalytic activity
toward phenol and could eliminate more than 90% phenol
within 5 h. Such enhanced photoactivity is due to the unique
synergistic effect between the magnetic core and BiOI shell of
the catalyst. Additionally, PONs have satisfactory cycling
stability and superior magnetic recovery performance. This
work not only paves a way for the design of efficient photo-
catalysts with magnetic cores and photocatalyst shells, but also
opens up new possibilities for photocatalysts with favorable
durability and recyclability in practical applications.
To verify the aforementioned results and quantitatively
compare the amounts of radicals generated on the as-prepared
samples, the NBT (detection agent for cO2ꢂ) transformation and
TA-PL probing techniques were applied with 1 h visible-light
irradiation during the photocatalytic reaction.50 As shown in
Fig. 11c, the time-resolved absorption spectra of NBT show that
the transformation percentage of NBT catalysed by the PONs(1/3)
was greater than that for the other samples. Moreover, the NBT
conversion percentage by PONs(1/3) was up to 69%, which
ꢂ
further demonstrates that the cO2 radicals serving as active
species play crucial roles in the degradation process. As shown
in Fig. 11d, the uorescence intensity of TA-OH over PONs(1/3)
was higher than that with other samples, although all the peak
intensities were very weak, indicating that cOH radicals only
play minor roles in the catalytic process.
Accordingly, a possible mechanism for the PONs(1/3) photo-
catalyst can be proposed; the above results clearly suggest that
the degradation of phenol is primarily attributed to cO2ꢂ, h+ and
cOH. As aforementioned, the as-prepared catalyst is an n-type
semiconductor (Mott–Schottky plots), and hence the BiOI
coating proves to be an excellent platform to transfer electrons.
Conflict of interest
There are no conicts of interest to declare.
Acknowledgements
Those electrons consequently reduce the O2 molecules adsor-
This work was nancially supported by the Qing Lan Project, the
Environmental Protection Scientic Research Project of Jiangsu
Province (2016056), the Shanghai Aerospace Science and
Technology Innovation Fund (SAST2015020), and a project
funded by the Priority Academic Program Development of
Jiangsu Higher Education Institutions.
ꢂ
bed on the catalyst surface into superoxide radicals of cO2
.
ꢂ
Jiang et al. reported that cO2 is the most important active
species in the photocatalytic degradation of phenol by BiOI.51
Theoretically, cOH is an extremely strong oxidant with nonse-
lective characteristics.52 It is proposed that the probability of
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28 Y. Huang, H. Li, M. S. Balogun, W. Liu, Y. Tong, X. Lu and
H. Ji, ACS Appl. Mater. Interfaces, 2014, 6, 22920–22927.
29 L. Ye, L. Tian, T. Peng and L. Zan, J. Mater. Chem., 2011, 21,
12479–12484.
30 X. Zhang, L. Zhang, T. Xie and D. Wang, J. Phys. Chem. C,
2009, 113, 7371–7378.
31 M. Shekoeh-Gohari and A. Habibi-Yangjeh, J. Colloid
Interface Sci., 2016, 461, 144–153.
32 S. Xuan, W. Jiang, X. Gong, Y. Hu and Z. Chen, J. Phys. Chem.
C, 2009, 113, 553–558.
33 Z. Zhu, Z. Lu, D. Wang, X. Tang, Y. Yan, W. Shi, Y. Wang,
N. Gao, X. Yao and H. Dong, Appl. Catal., B, 2016, 182,
115–122.
34 S. Kumar, T. Surendar, B. Kumar, A. Baruah and V. Shanker,
J. Phys. Chem. C, 2013, 117, 26135–26143.
35 G. Xi, B. Yue, J. Cao and J. Ye, Chemistry, 2011, 17, 5145–
5154.
36 Y. Wang, S. Li, X. Xing, F. Huang, Y. Shen, A. Xie, X. Wang
and J. Zhang, Chemistry, 2011, 17, 4802–4808.
37 M. Zhu and G. Diao, J. Phys. Chem. C, 2011, 115, 18923–
18934.
38 S. Zhong, W. Jiang, M. Han, G. Liu, N. Zhang and Y. Lu, Appl.
Surf. Sci., 2015, 347, 242–249.
39 H. Huang, Y. He, R. He, Z. Lin, Y. Zhang and S. Wang, Inorg.
Chem., 2014, 53, 8114–8119.
40 S. Y. Chou, W. H. Chung, L. W. Chen, Y. M. Dai, W. Y. Lin,
J. H. Lin and C. C. Chen, RSC Adv., 2016, 6, 82743–82758.
41 Y. H. Lee, Y. M. Dai, J. Y. Fu and C. C. Chen, Mol. Catal.,
2017, 432, 196–209.
42 S.-Y. Chou, C. C. Chen, Y. M. Dai, J. H. Lin and W. W. Lee,
RSC Adv., 2016, 6, 33478–33491.
43 S.-T. Huang, Y.-R. Jiang, S. Y. Chou, Y.-M. Dai and
C.-C. Chen, J. Mol. Catal. A: Chem., 2014, 391, 105–120.
44 X. Zhou, D. Xu, J. Lu and K. Zhang, Chem.–Eur. J., 2015, 266,
163–170.
45 Z. Ai, W. Ho, S. Lee and L. Zhang, Environ. Sci. Technol., 2009,
43, 4143–4150.
Notes and references
1 A. Fujishima, Nature, 1972, 238, 37–38.
2 J. Tang, Z. Zou and J. Ye, Angew. Chem., Int. Ed., 2004, 43,
4463–4466.
3 S. H. S. Chan, T. Yeong Wu, J. C. Juan and C. Y. Teh, J. Chem.
Technol. Biotechnol., 2011, 86, 1130–1158.
4 C. Hachem, F. Bocquillon, O. Zahraa and M. Bouchy, Dyes
Pigm., 2001, 49, 117–125.
5 L. Ren, Y. Li, J. Hou, X. Zhao and C. Pan, ACS Appl. Mater.
Interfaces, 2014, 6, 1608–1615.
6 Y. C. Pu, G. Wang, K. D. Chang, Y. Ling, Y. K. Lin,
B. C. Fitzmorris, C. M. Liu, X. Lu, Y. Tong, J. Z. Zhang,
Y. J. Hsu and Y. Li, Nano Lett., 2013, 13, 3817–3823.
7 A. W. Xu, Y. Gao and H. Q. Liu, J. Catal., 2002, 207, 151–157.
8 R. Leary and A. Westwood, Carbon, 2011, 49, 741–772.
9 X. Wang, S. Blechert and M. Antonietti, ACS Catal., 2012, 2,
1596–1606.
10 Y. Qu, L. Liao, R. Cheng, Y. Wang, Y. C. Lin, Y. Huang and
X. Duan, Nano Lett., 2010, 10, 1941–1949.
11 Y. Guo, H. Huang, Y. He, N. Tian, T. Zhang, P. K. Chu, Q. An
and Y. Zhang, Nanoscale, 2015, 7, 11702–11711.
12 J. Jiang, X. Zhang, P. Sun and L. Zhang, J. Phys. Chem. C,
2011, 115, 20555–20564.
13 H. Huang, K. Xiao, K. Liu, S. Yu and Y. Zhang, Cryst. Growth
Des., 2016, 16, 221–228.
14 L. Sun, L. Xiang, X. Zhao, C. J. Jia, J. Yang, Z. Jin, X. Cheng
and W. Fan, ACS Catal., 2015, 5, 3540–3551.
15 F. Tian, H. Zhao, Z. Dai, G. Cheng and R. Chen, Ind. Eng.
Chem. Res., 2016, 55, 4969–4978.
16 Y. Wang, K. Deng and L. Zhang, J. Phys. Chem. C, 2011, 115,
14300–14308.
17 X. Xiao and W. D. Zhang, J. Mater. Chem., 2010, 20, 5866.
18 X. Qian, D. Yue, Z. Tian, M. Reng, Y. Zhu, M. Kan, T. Zhang
and Y. Zhao, Appl. Catal., B, 2016, 193, 16–21.
19 Y. Zhang, N. Zhang, Z. R. Tang and Y.-J. Xu, Chem. Sci., 2013,
4, 1820–1824.
46 J. Li, L. Cai, J. Shang, Y. Yu and L. Zhang, Adv. Mater., 2016,
28, 4059–4064.
20 L. Chen, Q. Zhang, R. Huang, S. F. Yin, S. L. Luo and C. T. Au,
Dalton Trans., 2012, 41, 9513–9518.
47 H. Huang, X. Han, X. Li, S. Wang, P. K. Chu and Y. Zhang,
ACS Appl. Mater. Interfaces, 2015, 7, 482–492.
48 W. W. Lee, C. S. Lu, C. W. Chuang, Y. J. Chen, J. Y. Fu,
C. W. Siao and C. C. Chen, RSC Adv., 2015, 5, 23450–23463.
49 K. V. Kumar, K. Porkodi and F. Rocha, Catal. Commun., 2008,
9, 82–84.
21 C. Li, S. Wang, T. Wang, Y. Wei, P. Zhang and J. Gong, Small,
2014, 10, 2783–2790.
22 H. Li, K. Yu, X. Lei, B. Guo, H. Fu and Z. Zhu, J. Phys. Chem. C,
2015, 119, 22681–22689.
23 H. S. Park, K. E. Kweon, H. Ye, E. Paek, G. S. Hwang and
A. J. Bard, J. Phys. Chem. C, 2011, 115, 17870–17879.
24 W. Zhao, Y. Wang, Y. Yang, J. Tang and Y. Yang, Appl. Catal.,
B, 2012, 115–116, 90–99.
50 L. Ye, J. Liu, Z. Jiang, T. Peng and L. Zan, Appl. Catal., B, 2013,
142–143, 1–7.
51 Y. R. Jiang, S. Y. Chou, J.-L. Chang, S. T. Huang, H.-P. Lin and
C. C. Chen, RSC Adv., 2015, 5, 30851–30860.
52 H. L. Chen, W. W. Lee, W. H. Chung, H.-P. Lin, Y. J. Chen,
Y. R. Jiang, W. Y. Lin and C. C. Chen, J. Taiwan Inst. Chem.
Eng., 2014, 45, 1892–1909.
25 P. Madhusudan, J. Ran, J. Zhang, J. Yu and G. Liu, Appl.
Catal., B, 2011, 110, 286–295.
26 J. Tian, P. Hao, N. Wei, H. Cui and H. Liu, ACS Catal., 2015, 5,
4530–4536.
27 C. Yu, Z. Wu, R. Liu, D. D. Dionysiou, K. Yang, C. Wang and
H. Liu, Appl. Catal., B, 2017, 209, 1–11.
53 Y. R. Jiang, H. P. Lin, W. H. Chung, Y. M. Dai, W. Y. Lin and
C. C. Chen, J. Hazard. Mater., 2015, 283, 787–805.
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