Synthesis of BiOF/TiO2 Heterostructures and Their Enhanced Visible-Light Photocatalytic Activity

Article abstract of DOI:10.1002/ejic.201900970

Novel BiOF/TiO₂ heterostructures with controlled amounts of BiOF and TiO₂ have been successfully synthesized via a simple solid-state sintering method. The structural, morphological and optical properties were analyzed using several characterization techniques. A shift of the TiO₂ absorption edge toward the visible region was observed. The optimal molar ratio of BiOF/TiO₂ was found to be (1:3) with a photodegradation percentage of 91.2 % and it was 7 and 5 times more efficient than that of pure BiOF or TiO₂ nanopowder, respectively. BiOF/TiO₂ (1:3) heterojunction has showed high photodegradation stability for the Rh B photodegradation after three cycles. In the proposed photocatalytic mechanism, TiO₂ absorbed the visible light and carried out the photocatalytic reaction owing to the decrease of its band gap value due to the formation of oxygen vacancies, while BiOF played an important role in the charge separation process due to the intimate contact of the BiOF/TiO₂ heterojunction.

Full text of DOI:10.1002/ejic.201900970

A Journal of
Accepted Article
Title: Synthesis of BiOF/TiO2 heterostructures and their enhanced
visible-light photocatalytic activity
Authors: Maryline NASR, Wenshi Huang, Carla Bittencourt, Dandan
Cui, Ying Sun, Lei Wang, Nathalie Gaillard Caperaa, Yuping
Ning, Ping Song, Pierre Bonnet, and Cong Wang
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To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201900970
Link to VoR: http://dx.doi.org/10.1002/ejic.201900970

10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
Synthesis of BiOF/TiO2 heterostructures and their enhanced
visible-light photocatalytic activity
Maryline Nasr,[a] Wenshi Huang,[b] Carla Bittencourt,[c] Dandan Cui,[b] Ying Sun,[b] Lei Wang,[b] Nathalie
Gaillard Caperaa,[a] Yuping Ning,[b] Ping Song,[b] Pierre Bonnet,*[a] Cong Wang*[b]
enhance the TiO2 photocatalytic properties. Several studies
have been devoted to design a heterojunction based on TiO2
material such as g-C3N4/TiO2, Ag-AgBr/TiO2, BiOX (X=Cl, Br and
I)/TiO2…4, 9-12 It was found that the close interfacial contact of the
heterojunction promotes the transfer of photoinduced carriers
from the TiO2 surface to the other component which increase the
life time of the charge carriers and consequently enhance the
photocatalytic activity of TiO2.4, 11 In addition, it was reported that
the design of these heterostructures increase the active surface
area of TiO2 and reduce its band gap energy which will improve
its photocatalytic activity under visible light.9 Much attention has
been devoted recently to bismuth oxyhalides BiOX (X=Cl, Br, F
and I) due to their high chemical stability and favorable
photocatalytic performance under UV irradiation.13 In particular,
bismuth oxyfluoride (BiOF) exhibits a wide direct band gap
estimated between 3.6 and 4.2 eV. The BiOF atomic structure is
Abstract: Novel BiOF/TiO2 heterostructures with controlled amounts
of BiOF and TiO2 have been successfully synthesized via a simple
solid-state sintering method. The structural, morphological and
optical properties were analyzed using several characterization
techniques. A shift of the TiO2 absorption edge toward the visible
region was observed. The optimal molar ratio of BiOF/TiO2 was
found to be (1:3) with a photodegradation percentage of 91.2% and
it was 7 and 5 times more efficient than that of pure BiOF or TiO2
nanopowder, respectively. BiOF/TiO2 (1:3) heterojunction has
showed high photodegradation stability for the Rh
B
photodegradation after three cycles. In the proposed photocatalytic
mechanism, TiO2 absorbed the visible light and carried out the
photocatalytic reaction owing to the decrease of its band gap value
due to the formation of oxygen vacancies, while BiOF played an
important role in the charge separation process due to the intimate
contact of the BiOF/TiO2 heterojunction.
formed by layers of (Bi2O2F2).13 Recently, some studies have
14, 15
been reported on BiOF material
but these studies remain
not sufficient and not developed as for other BiOX compounds.16
To the best of our knowledge no studies have been reported on
the bismuth oxyfluoride/titanium oxide (BiOF/TiO2) photocatalyst.
Therefore, in the present work we will report for the first time on
the synthesis of BiOF/TiO2 heterostructure photocayalst with the
aim to enhance the photocatalytic performance of TiO2 under
visible light. Herein, BiOF/TiO2 composites with different
amounts of BiOF and TiO2 were successfully synthesized via
simple solid state sintering method. The structural,
morphological and optical properties of the as-synthesized
BiOF/TiO2 heterostructures were analyzed using different
characterization techniques. The main objectives of this work
are to study the effect of BiOF on the photocatalytic properties of
TiO2, to report the optimum molar ratio BiOF/TiO2 and to
understand the photocinduced mechanism. The performance of
the synthesized materials for the photodegradation of rhodamine
B (Rh B) and methylene blue (MB) have been evaluated under
visible light irradiation.
Introduction
In recent years, the serious water pollution was being addressed
by the low cost and most efficient green technology,
“photocatalysis”, which play an important role in solar energy
conversion and degradation of organic pollutants.1, 2 Among the
numerous semiconductor photocatalysts, titanium dioxide (TiO2)
is the most used owing to its high chemical stability, low cost
and nontoxicity.3 Nevertheless, TiO2 has a wide band gap
energy (3.2eV), so it can only be excited by ultraviolet light with
a wavelength < 388 nm which is equivalent to less than 7% of
solar light.4 However, the solar spectrum mostly consists of
visible light (44%). Thus, the use of solar light as irradiation
source for TiO2 photocatalyst seems to be quite difficult. In
addition, the high rate of the charge recombination in TiO2 limits
its photocatalytic activity efficiency.5 Therefore, several
strategies such as metal/non-metal doping,6, coupling with
7
other semiconductor and heterostructure junctions have been
investigated to overcome these limitations and enhance the
photocatalytic activity of TiO2 under visible light.8 Among these
strategies, the heterostructure design is a promising method to
Results and Discussion
Figure 1-A shows XRD patterns of the different molar ratios of
BiOF/TiO2. The diffraction peaks were indexed with (*) for BiOF
crystalline phase and (O) for TiO2 anatase phase. All the
identified peaks can be assigned to the tetragonal structure of
BiOF and anatase phase of TiO2. The sharp XRD diffraction
peaks can be assigned to the (001), (101), (002), (110), (102),
(003), (112), (200), (103), (211), (212) and (104) crystal planes
of BiOF which correspond to 2θ = 14.28°, 27.90°, 28.85°,
33.87°，37.68°，43.76°, 45.10°, 48.68° 50.45°, 56.89°, 62.03°
and 64.95° respectively.17 The XRD diffraction peaks for pure
anatase phase of TiO2 were detected at 2θ = 26.35°，48.15°,
54.05° and 55.28° corresponding to the (101), (200), (105) and
(211) crystal planes, respectively.18 Furthermore, no peaks
related to metallic bismuth (Bi) or other crystalline phases and
impurities were detected, confirming the heterostructure’s
formation of the BiOF and TiO2 pure phases.17 This result will
be confirmed later by Raman spectroscopy. In addition, it was
[a]
[b]
[c]
Dr. M. Nasr, Dr. P. Bonnet, Mrs. Gaillard Caperaa
Institute of Chemistry of Clermont-Ferrand, Université Clermont
Auvergne, Campus des Cezeaux, batiment chimie 5, 24 Avenue
Blaise Pascal, 63178-AUBIERE Cedex, France
Email:pierre.m.bonnet@uca.fr
Mr. W. Huang, Mr. D. Cui, Mr. Y. Sun, Mr. L. Wang, Mr. Y. Ning,
Mr. P. Song, Dr. C. Wang
Center for Condensed Matter and Material Physics, Department of
Physics, Beihang University, Beijing 100191, P.R.China
Email:congwang@buaa.edu.cn
Mrs. C. Bittencourt
Chimie des Interactions Plasma-Surface (ChIPS), University of
Mons, Place du Parc 22, 7000 Mons, Belgium
Supporting information for this article is given via a link at the end of
the document
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10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
noted that the peak intensity corresponding to the (101) plane of
TiO2 anatase phase is increasing progressively with the
increasing of TiO2 amounts in BiOF/TiO2 samples, while the
peak intensity of BiOF is decreasing (Figure 1-A (d), (e) and (f)).
This increasing could be attributed to the grains sintering during
the heat treatment under argon. In fact the dilution of TiO2 by
BiOF causes a decreasing of TiO2 crystallite size and vice
versa.20 The successful formation of BiOF/TiO2 composites was
further confirmed by Raman spectroscopy as shown in Figure 1-
B. For all the BiOF/TiO2 prepared samples 2 peaks were
detected in Raman spectra, a first peak located at 147 cm-1 and
corresponding to the Eg vibrational mode of TiO2 anatase phase
and a second peak located at 168 cm-1 and can be assigned to
the A1g vibrational mode of BiOF.5, 17 As shown in Figure 1-B, the
main TiO2 peak was shifted from the initial position with the
increasing of TiO2 amount in the BiOF/TiO2 composites. This
significant shift varied between 2 and 6 cm-1 and could be
attributed to the defects and oxygen vacancies’ formation in the
TiO2 lattice.21 In addition, it could be observed that with the
increasing of TiO2 amounts the intensity of TiO2 Raman’s peaks
increased while the intensity of BiOF Raman’s peaks decreased
(Figure 1-B, (a) and (c) to (f)). Moreover, no peak related to
metallic bismuth (Bi) at ~ 96 cm-1 was detected by Raman, which
confirmed that no or only few oxygen vacancies’ and defects
were formed in BiOF lattice.17 This result is in a good agreement
with the XRD analysis and confirming the heterostructure’s
formation of the two pure phases of BiOF and TiO2.
Table 1. Grain size of BiOF and TiO2 in BiOF/TiO2 heterostructures.
Molar
ratio
(5:1)
(3:1)
(1:1)
(1:3)
(1:5)
(1:7)
BiOF
TiO2
45
19
45
19
45
28
52
29
48
30
48
30
Figure 1. (A) XRD patterns and (B) Raman spectra of BiOF/TiO2 with different
molar ratios (a) 5:1 (black line), (b) 3:1 (red line), (c) 1:1 (blue line), (d) 1:3
(green line), (e) 1:5 (pink line) and (f)1:7 (brown line). (A) BiOF and TiO2
crystalline phase are indexed with (*) and (O), respectively. (B) For the sample
BiOF/TiO2 with a molar ratio of (3:1) an increase of the TiO2 peak intensity
(indexed with *) was noticed despite the small amount of TiO2 in the composite,
this result can be explained by the fact that the laser beam has been focused
on an area rich in TiO2 nanoparticles.
Figure 2: Scanning electron microscope images of (A) BiOF plates, (B) TiO2
nanoparticles, (C) BiOF/TiO2 (1:3) heterostructure and (D) EDX data showing
the composition BiOF/TiO2 (1:3) heterostructure, the peak corresponding to Al
element in the EDX data is due to the sample holder which is composed from
aluminum material.
Furthermore, the average crystal size of the BiOF/TiO2
composites with different molar ratio of BiOF and TiO2 was
calculated using Scherrer-Debye equation:19
D = kλ/βcosθ
(Equation 2)
where D is the average crystal size (nm), k is the shape factor
constant (0.9), λ is the X-ray wavelength (0.154 nm), β is the full
width at half maximum of the peak intensity (FWHM) and θ is the
Bragg angle (degrees). The calculated results presented in
Table 1 show a high grain fineness number of BiOF for all the
samples as we can notice an increasing of TiO2 average crystal
size with the decreasing of the molar ratio of BiOF/TiO2 (Table 1).
All the samples were analyzed by the different characterization
techniques; however the analyses of BiOF/TiO2 (1:3) are given
as an example. The morphology of the samples was
investigated by scanning electron microscope and to confirm the
formation of BiOF/TiO2 composites heterojunction. The average
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10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
diameters and the thickness were measured using image
analysis software (Image J1.29X). Figure 2-(A) shows a single
phase of BiOF which is plate-like with a smooth surface, the
thickness and the average diameter of these plates are around
15 ± 1 nm and 250 ± 10, respectively. TiO2 nanoparticles are
shown in Figure 2-(B), a highly interconnected network with
randomly oriented nanoparticles is clearly observed. The
average diameter of TiO2 nanoparticles was measured from
Figure 2-(B) and it was found to be around 116 ± 5 nm.
BiOF/TiO2 sample with the molar ratio of 1:3 is shown in Figure
2-(C). The deposition of TiO2 nanoparticles on the entire surface
of BiOF nanoplates revealed the intimate contact between the
two materials (BiOF and TiO2) as well the formation of BiOF/TiO2
heterojunction. The presence of different elements Bi, O, F and
Ti was detected from the energy dispersive X-ray spectrum
(EDX) for the BiOF/TiO2 (1:3) as shown in Figure 2-(D), no other
elements were detected in the spectrum confirming that there is
no impurity or contamination in the prepared sample. It is worth
to note that the peak corresponding to Al element in the EDX
data is due to the sample holder which is composed from
aluminum material Elemental mapping images for BiOF/TiO2
(1:3) composite are shown in Figure S1 (supporting information).
We can clearly observe that Ti, Bi, O and F elements are
distributed homogeneously over the entire area of the BiOF/TiO2
sample. The XPS analysis confirms that the sample is mainly
composed of Bi, O, F, Ti (Figure 3-A). The presence of carbon in
the spectrum can be associated with adventitious carbon
contamination from ambient exposure. A close inspection of the
Ti 2p XPS and the Bi 4f core level spectra (Figure 3-B and C)
nanoparticles have rather rounded and slightly elongated
shapes with sizes around 20 nm. The TEM image of BiOF/TiO2
(1:3) composite in Figure 4-(B) confirms the good crystalline
structure of BiOF and TiO2 which is in a good agreement with
XRD results presented above. As shown in Figure 4-(C and D),
the d spaces of TiO2 and BiOF were found to be 0.320 nm and
0.410 nm, corresponding to the (110) plane of anatase phase of
TiO2 and the (111) plane of tetragonal phase of BiOF,
respectively.29, 30 In addition, from Figure 4-B we can clearly see
the intimate contact between the two crystalline structures of
BiOF and TiO2 nanomaterials confirming the formation of
heterojunction between BiOF nanoplates and TiO2 nanoparticles.
As conclusion, this close interfacial contact of the heterojunction
will probably promote the transfer of the charge carriers from
TiO2 to BiOF surface which will reduce the charge recombination
and therefore enhance the photocatalytic activity as it will be
confirmed later in the photocatalytic section.31
indicates that the Ti and Bi atoms have mainly the oxidation
23
state +4 and +3, respectively.22,
The Ti 2p peak was
reproduced using a Shirley background and 2 doublets, a high
intensity one with the component 2p3/2 at 458.5eV and a low-
intensity doublet shifted to lower binding energy by 2.3 eV,
respectively. They can be associated to photoelectrons emitted
from Ti atoms with oxidation state +4 and +3 respectively
(Figure 3-B). As shown in Figure 3-C, two main peaks appeared
Figure 3. XPS spectra of (A) BiOF/TiO2 (1:3), (B) Ti 2p peaks of TiO2, (C) Bi
4f peaks of BiOF and (D) O 1s peaks of TiO2 and BiOF.
at 159.1 and 164.5 eV and correspond to Bi 4f7/2 and Bi 4f5/2
,
respectively. These peaks are assigned to the characteristic
peaks of Bi3+ in BiOF. Moreover, two weak peaks at 157 and
162.2 eV assigned to Bi0 were observed demonstrating that only
a small amount of Bi3+ was reduced to Bi0 (metallic) in
BiOF.24The oxygen 1s XPS spectrum presented in Figure3-D
shows two components, the first one at 530.4 eV and the
second one at higher binding energy, they have been associated
to photoelectrons emitted from oxygen atoms in TiO225 and in Bi-
O bonding,26 respectively. Due to the small amount of vacancies,
it is difficult to identify a single component that can be
associated to photoemission from oxygen atoms near vacancies.
It is reported that for TiO2 this component would be centered in
the range of 531.6-532.0 eV,27, 28 thus being overlapped by the
component associated to Bi-O. At the same time, as presented
above, no peaks related to the metallic Bi were detected by XRD
and Raman analyses which confirm the small amount of
Bi0/oxygen vacancies in BiOF. Surface area of BiOF/TiO2 (1:3)
nanocomposite was calculated using the Brunauer–Emmet–
Teller method (BET) and it was found to be 53.60 m2/g which is
a large surface area and could offer more active adsorption sites
and consequently could enhance the BiOF/TiO2 photocatalytic
activity.
Transmission electron microscopy was furthermore performed
on the as-synthesized TiO2 and BiOF/TiO2 photocatalysts to
confirm the formation of BiOF/TiO2 heterojunction. The granular
aspect and the uniform particle size of BiOF/TiO2 (1:3)
nanocomposite are clearly observed in Figure 4-(A). The
Figure 4. TEM images of (A-B) BiOF/TiO2 (1:3), (C) partial enlarged image of
TiO2 region in (B) and (D) partial enlarged image of BiOF region in (B).
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10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
The successful formation of pure BiOF/TiO2 composite
heterojunctions with appropriated interfaces was confirmed by
XRD, Raman, SEM, TEM, EDX and XPS analyzes. To study the
influence of BiOF on the optical properties of TiO2 nanoparticles
the UV-Visible diffuse reflectance was recorded for TiO2, BiOF
and BiOF/TiO2 (1:3) powders as shown in Figure 5-A. The Tauc
plot method was used to determine the semiconductor optical
band gap using the following formula:32
(αhv)1/2 ~ (hv-Eg)
(Equation 3)
where α, hv and Eg represent the absorption coefficient, photon
energy and band gap, respectively. The band gap values (Eg)
correspond to the intersection of the linear part of the curve with
hν-axis as shown in Figure 5-B. The band gap and the
absorbance edge for the pure TiO2 and pure BiOF powders were
found to be (3.18 eV; 390 nm) and (3.67 eV; 338 nm),
respectively, which are close to the values reported in the
Figure 5: (A) UV-Visible diffuse reflectance spectra, (B) and (C) Visible light
excited photocurrent of TiO2 (red line), BiOF (green line) and BiOF/TiO2 (1:3)
(black line).
34
literature.33,
As shown in Figure 5-A, BiOF/TiO2 (1:3)
heterostructure exhibits stronger absorption intensity in the
visible light range due to the creation of oxygen vacancies
during the heat treatment, therefore, a stronger photocatalytic
activity in the visible range, compared with TiO2 and BiOF.
Moreover, a red-shift of the absorption edge (410 nm) was
observed compared with the pure BiOF and pure TiO2. This red-
shift resulted in a decrease of the band gap value from 3.18 eV
to 3.02 eV and this decrease can be attributed to the oxygen
vacancies and defects’ mainly formed in the TiO2 lattice as
confirmed by the structural analyzes presented above.35
According to the literature, the formation of oxygen vacancies
during the synthesis of materials can be explained by the heat
treatment under argon atmosphere at 350°C which introduces
surface defects in the form of oxygen vacancies in TiO2 structure.
It is worth noting that our work is focused on the material
preparation and not on the investigation of the pollutant
degradation mechanism. According to the literature, the
photodegradation pathway of pollutants is based on the
excitation of the semiconductor which will lead to the formation
of electron-hole pairs that participate in the redox reactions to
degrade or to reduce the adsorbed molecules such as organic
pollutants (Rh B, MB…).2 The photodegradation process could
be described by the following equations:40
Semiconductor + hv → h+ + e-
H2O + h+ (valence band) → HO• + H+
h+ (valence band) + pollutant → pollutant+
HO• + pollutant → H2O + CO2
(Equation 4)
(Equation 5)
(Equation 6)
(Equation 7)
(Equation 8)
36, 37
This is confirmed by electron spin resonance spectroscopy
analysis (ESR) as shown in Figure S2 (supporting information).
A small amount of oxygen vacancies can also be presented in
BiOF lattice as already shown by XPS. Therefore, the decrease
of the band gap value of BiOF/TiO2 (1:3) heterojunction
compared with pure TiO2 will possibly lead to an increase of the
photocatalytic efficiency under visible-light irradiation.38
Photocurrent characterization was conducted on a CHI650D
electrochemical workstation with a standard three-electrode
system. A platinum wire acted as counter electrode, and a
standard saturated calomel electrode (SCE) in saturated KCl
acted as reference electrode, 0.1M Na2SO4 was used as
electrolyte. The prepared sample was coated on the fluorine
doped tin oxide glass, which was used as the working electrode.
As shown in Figure 5-C, all the electrodes were prompt in
generating photocurrent with a reproducible response to on/off
cycles. As results, the BiOF/TiO2 (1:3) electrode exhibits a
higher photocurrent density (~1.10-6 µA/cm-2) compared with the
pure BiOF (~1.10-7 µA/cm-2) or TiO2 (~5.10-7 µA/cm-2) indicating
the efficient photoinduced charge separation in BiOF/TiO2 (1:3)
heterojunction. Therefore, this heterojunction will probably
presents an enhanced photocatalytic activity compared with
pure TiO2 or BiOF.39
h+ + e-
→ heat
The photocatalytic activity of the as-prepared samples has been
evaluated under visible light using Rh B as a model pollutant
(Figure 6-A). After 90 minutes of light irradiation, the
photodegradation percentage of Rh B was up to 73.9%, 84.9% ,
91.2% , 89.1% , 86.9% and 70.7% for BiOF/TiO2 samples with a
molar ratio of (1:7), (1:5), (1:3), (1:1), (3:1) and (5:1),
respectively. As clearly seen, the BiOF/TiO2 (1:3) exhibits the
highest photocatalytic activity among the other heterostructures.
As shown in Figure 6-B, the blank test (without adding a
photocatalyst) indicates that Rh B is stable and difficult to be
photodegraded under visible light during 90 min. For comparison
with the BiOF/TiO2 (1:3) heterojunction, the photocatalytic
activity of pure TiO2, pure BiOF and a simple mixture of BiOF
and TiO2 (1:3) has been evaluated under the same experimental
conditions. After illumination of 90 min, the percentage of Rh B
removal reaches 40.2%, 28.5%, 32.5% and 91.2% for TiO2,
BiOF, simple mixture of BiOF and TiO2 (1:3) and BiOF/TiO2 (1:3)
heterojunction, respectively (Figure 6-B). As results, the
BiOF/TiO2 heterojunctions with different molar ratios presents a
greater photocatalytic activity compared with pure TiO2 and pure
BiOF nanomaterials. The enhancement of the photocatalytic
efficiency under visible light is attributed to the following
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10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
reasons: (I) the close interfacial contact of the heterojunction,
which promotes the transfer of charge between BiOF and TiO2
surface and reduce the charge recombination, thus more
electrons will be available to take part in the photodegradation
process,41, 42 (II) the decrease of the BiOF/TiO2 band gap due to
the defects and oxygen vacancies’ formation in TiO2 and BiOF
lattices,17, 35 and (III) a good matching of BiOF and TiO2 energy
levels due to the oxygen vacancies’ states.43 Moreover, the (1:3)
represents the optimal ratio for BiOF/TiO2 with the highest
photocatalytic activity (91.2%). Therefore, the use of an
excessive amount of TiO2 or BiOF will cause particle’s
aggregation which will reduce the light absorption on the surface
and promotes the charge’s recombination by increasing the
collision between the photogenerated electrons and holes.44, 45
Thus, the use of an excessive amount of TiO2 or BiOF will lead
to a decrease of the photocatalytic efficiency as it was confirmed
in the present work (Figure 6 A-B). Since TiO2 nanoparticles are
on the surface of BiOF plates, the amount of TiO2 should be
predominant, but not in excess, in order to absorb the maximum
of light and carried out the photocatalytic reaction. At the same
time the amount of BiOF should be sufficient in order to be in an
intimate contact with TiO2 and to ensure the charges’ separation
and hence, increase the photocatalytic efficiency. Therefore, the
BiOF/TiO2 (1:3) composition presents the optimal ratio of BiOF
and TiO2 for the photocatalytic activity under visible light.
loss of photocatalyst during the centrifugation process.47The
improvement of photocatalyst recovery at the end of degradation
process may be improved, in particular by adding a small
amount of magnetic material such as Fe3O448 or by depositing it
on a support.49
In order to examine the photocatalytic capacity of the
nanomaterial, the photodegradation activity was also analyzed
using another dye, methylene blue (MB), and under the same
experimental conditions used for Rh B. The BiOF/TiO2 sample
with the optimal ratio (1:3) has been employed as photocatalyst.
The results in Figure 7 indicated that MB was stable and difficult
to be photodegraded under visible light in the absence of
photocatalyst. The BiOF/TiO2 heterojunction was able to
photodegrade MB up to 60% after 90 minutes compare to 43.7%
and 27.1% for pure TiO2 and BiOF, respectively. Therefore, the
BiOF/TiO2 (1:3) heterojunction exhibits an enhanced
photocatalytic activity and stability compared with pure TiO2,
moreover this composite is able to photodegrade multiple dyes
under visible light.
The kinetics of the photodegradation’ curves were analyzed and
the obtained results were presented in Figure 6-C. The
photodegradation reactions followed a Langmuir– Hinshelwood
first order kinetics model, as described by equation (9):46
ln (C0/C)=kKt = ka t
(Equation 9)
where C0 corresponds to the initial concentration of Rh B
solutions , C is the concentration of Rh B solution at time t, t is
the irradiation time, k is the reaction rate constant (mg (L min)-1),
K is Rh B adsorption coefficient (mg L-1) and ka corresponds to
the apparent first-order rate constant (min-1). The obtained ka
values from the linear dependence between ln(C0/C) and time
are reported in Figure 6-C. The apparent rate constant value of
all BiOF/TiO2 heterojunctions is higher than that of pure BiOF
and TiO2 powder. Moreover, compared with other
heterojunctions, BiOF/TiO2 with the optimal ratio of (1:3) showed
the highest photocatalytic activity under visible light, which is
represented by the highest ka value. The rate constant exhibits a
maximum of 0.0273 min−1 for BiOF/TiO2 (1:3), which is around 7
and 5 times higher than that of BiOF and TiO2, respectively.
Therefore, the heterojunction design between BiOF and TiO2
greatly improves the photocatalytic activity under visible light. To
investigate the potential reusability of these materials for the
photocatalytic decomposition of Rh B under visible light, the
photodegradation test has been repeated for three cycles under
the same conditions and using BiOF/TiO2 with the optimal ratio
(1:3) as photocatalyst. After each cycle, the photcatalyst was
separated from the solution by centrifugation to be used in the
next cycle. It is worth noting that even after several
photocatalytic cycles, the characteristic diffractions peaks of
BiOF/TiO2 (1:3) remains unchanged compared with the XRD
pattern of BiOF/TiO2 (1:3) before photocatalysis (Figure S3,
supporting information). As shown in Figure 6-D, BiOF/TiO2 (1:3)
heterostructure shows a high stability for the photodegradation
of Rh B with a percentage of 91.2%, 90.5% and 87.3% for the
first, second and third cycle, respectively. A slight decrease of
the photocatalytic efficiency was noted and could be due to the
Figure 6: (A-B) Photocatalytic degradation of Rh B solution in the presence of
TiO2, BiOF and BiOF/TiO2 with different molar ratios under visible light
irradiation, (C) Apparent rate constants for the photodecomposition of RhB
over BiOF, TiO2, BiOF/TiO2 heterostructures (D) Photocatalytic stability of
BiOF/TiO2 (1:3) in three repeated cycles for Rh B photocatalytic degradation
under visible light irradiation.
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10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
Figure 7: Photocatalytic degradation of methylene blue without photocatalyst
(black line) and in the presence of photocatalyst: BiOF (blue line), TiO2 (green
line), BiOF/TiO2 (1:3) (red line) under visible light irradiation.
Figure 8: Proposed mechanism for the photogenerated charge separation and
migration process in BiOF/TiO2 heterostructured under visible-light irradiation.
BiOF could be excited and participates to the photodegradation process
(indexed by dashed arrows) only under UV light irradiation.
In order to investigate the photo-induced carriers transfer route,
the relative conduction band (CB) and valence band (VB)
potentials of BiOF and TiO2 were calculated using the following
formulas:4
Conclusions
In summary, the novel BiOF/TiO2 heterostructure with different
molar ratio of BiOF and TiO2 were successfully elaborated via
simple solid state sintering method followed by a heat treatment
at 350oC under argon atmosphere. The good crystalline phase
of BiOF and TiO2 and the formation of BiOF/TiO2
heterostructures were confirmed by Raman and XRD analyzes.
The chemical composition and the intimate contact between the
two phases of BiOF and TiO2 were revealed by XPS, EDX, SEM
and TEM characterizations. The red shift of the absorbance
edge and the decrease of the TiO2 band gap value from 3.18 to
3.02 eV for the BiOF/TiO2 (1:3) sample were confirmed by
diffuse reflectance analysis. All the synthesized BiOF/TiO2
heterostructures have presented an enhanced photocatalytic
activity for the photodegradation of Rh B under visible light
compared with pure TiO2 or BiOF. The optimal molar ratio of
(BiOF/TiO2) was found to be (1:3) with a photodegradation
percentage of 91.2% and it was 7 and 5 times more efficient
than that of BiOF and TiO2, respectively. In addition, the
photocatalytic repeatability tests confirmed the long-term stability
of BiOF/TiO2 (1:3) heterojunction for Rh B photodegradation
after three cycles. The optimal ratio (1:3) of BiOF and TiO2
results from a good matching between TiO2 and BiOF amounts
where TiO2 is predominant and absorbs the maximum of light
and at the same time the BiOF amount is sufficient to be in an
intimate contact with TiO2 and to ensure the charges’ separation
and hence, increase the photocatalytic efficiency under visible
light. The enhanced visible-light photocatalytic activity of
BiOF/TiO2 heterostructure was attributed to the following
reasons: (I) the red-shift of the absorption edge due to the
formation of oxygen vacancies mainly in TiO2 lattice, (II) the
good matching of BiOF and TiO2 energy levels due to the
oxygen vacancies’ states and (III) the close interfacial contact
between BiOF and TiO2 nanomaterials, which reduces the
charge’s recombination and hence, increase the photocatalytic
activity of BiOF/TiO2 heterostructure under visible light. In
conclusion, the BiOF/TiO2 heterostructure is able to enhance the
efficiency of the photocatalytic process in a simple manner and
could be used as photocatalyst in the photodegradation of
different molecules.
EVB = χ - Ee + ꢀꢁ Eg
(Equation 10)
(Equation 11)
ECB = EVB - Eg
Where EVB is the valence band (VB), χ is the absolute
electronegativity and corresponds to the geometric mean of the
absolute electronegativity of the constituents atoms; Ee is the
energy of free electrons on the hydrogen scale ca. 4.5 eV and Eg
corresponds to the band gap energy of the semiconductor. The
band gap energies of BiOF and TiO2 were calculated to be 3.66
and 3.18 eV, respectively. After calculation, the conduction band
bottom and valence band positions (ECB / EVB) of the BiOF and
TiO2 were found to be 0.20/ 3.86 eV and -0.28/2.90 eV,
respectively. It is well known that the key factor of an efficient
photocatalytic activity is the charge separation at the interfaces
of photocatalysts.43 Therefore a proposed mechanism for the
photogenerated charge separation and migration process in
BiOF/TiO2 heterostructured under visible-light irradiation is
shown in Figure 8. The conduction band edge potential of TiO2 (-
0.28 eV) is more negative than that of BiOF (0.20 eV) which
means that the photogenerated electrons on TiO2 surface
migrate easily to BiOF due to the close interfacial contact of the
BiOF/TiO2 heterojunction. Once transported to the surface of
BiOF, these electrons will react with O2 to form O2¯ radicals.
Moreover, the photogenerated holes in TiO2 will oxidize the
organic pollutant to H2O or other inorganic molecules. This
efficient migration process prevents the recombination of charge
and hence improves the photocatalytic activity.50,51 As results,
TiO2 absorbed the visible light and carried out the photocatalytic
reaction owing to the decrease of its band gap value from 3.18
to 3.02 eV due to the oxygen vacancies’ formation, while BiOF
played an important role in the charge separation process due to
the intimate contact of the BiOF/TiO2 heterojunction. Few
oxygen vacancies in BiOF could also participate to the optical
absorption
.
Experimental Section
Materials
Titanium (IV) oxide, anatase nanopowder (99.7%) with a particle size <
25 nm and Hydrofluoric acid solution (40%) were purchased from Sigma
Aldrich. Bismuth (III) oxide (99.9%-Bi) was obtained from Strem
Chamicals. All chemicals were used without any further purification.
Table 2: BiOF/TiO2 nanocomposites prepared with different molar ratios
of BiOF and TiO2.
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10.1002/ejic.201900970
European Journal of Inorganic Chemistry
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2.4. Photocatalytic activity
Sample Name
Molar Ratio of Bi:Ti
Rhodamine B (Rh B) and Methylene Blue (MB) were employed as
organic pollutants’ references to evaluate the photocatalytic activity of
BiOF/TiO2 composites under a 500W Xe lamp with a 420 nm cut off filter.
The experiments were carried out in a homemade reactor with a cooling
water system in order to keep the reaction’s temperature constant at 25 ±
0.2°C. The photodegradation reactions were carried out in separates
beaker containing a suspension of 20 mg of photocatalysts and 40 mL of
MB or Rh B solution (10 mg/L). The suspensions were firstly stirred for 2
hours in dark to reach the adsorption/desorption equilibrium. The
distance between the lamp and the dye solution was maintained at 10 cm.
After light irradiation, every 10 minutes 2 mL of each solution was taken
out and centrifuged at 6000 rpm for 2 minutes in the darkness. The
centrifuged solutions were analyzed with a UV/VIS spectrometer to study
the decrease in dyes’ concentration over time during light irradiation. The
absorbance of the solution was measured at 664 and 554 nm for Rh B
and MB, respectively. The photocatalytic degradation efficiency has been
calculated using equation 1:32
BiOF/TiO2 (5:1)
BiOF/TiO2 (3:1)
BiOF/TiO2 (1:1)
BiOF/TiO2 (1:3)
BiOF/TiO2 (1:5)
BiOF/TiO2 (1:7)
5:1
3:1
1:1
1:3
1:5
1 :7
Synthesis of BiOF/TiO2 nanocomposite heterostructure
BiF3 nanoparticles were synthesized in our laboratory by direct reaction
between Bi2O3 and HF (aq) at 70°C in a Teflon beaker. The precipitate
obtained through HF was removed by evaporation under a hood and as a
final product we get BiF3 powder. BiOF powder was obtained by a solid
state sintering method in air using the synthesized BiF3 nanoparticles
and the commercial Bi2O3 powder with a molar ratio of BiF3/Bi2O3 = 1:1.
Then the mixture was heated at 425°C for 12 hours under argon
atmosphere in order to obtain BiOF nanoparticles. BiOF/TiO2
heterostructures with different molar ratios of BiOF and TiO2 were also
Degradation efficiency (%) = (C0 − C) /C0 × 100
(Equation 1)
Where C0 and C are the initial and final dye’s concentration, respectively.
Acknowledgments
This work was supported by NSFC (No.51572010, 51502010),
Aeronautical Science Foundation of China (2014ZF51067),
State Key Lab of Advance Metals and Materials (2014-ZD03)
and Fundamental Research Funds for the Central Universities
(Grand No. YWF-16-JCTD-B-03). The authors would like to
thank Miss Maimonatou Mar and Mr. Lawrence Frezet’s help.
elaborated via
a solid state sintering method. Therefore, different
amounts of the synthesized BiOF and the commercial TiO2 nanoparticles
were mixed and grinded for 30 minutes. Then, the different mixtures of
BiOF/TiO2 were calcined at 350°C for 5 hours under argon atmosphere.
The as-synthesized samples were labeled according to their molar ratios
(BiOF/TiO2) and are listed in Table 2. Pure BiOF and TiO2 nanomaterials
were calcined under the same conditions to serve as references in the
photocatalytic section.
Keywords: Heterojunction • Photocatalysts • TiO2 • BiOF •
visible-light photocatalysis •
Chemical and Physical Characterizations
[1]
[2]
[3]
[4]
[5]
Y. Zhang, M. Sivakumar, S. Yang, K. Enever and M.
Ramezanianpour, Desalination. 2018, 428, 116-145.
M. Nasr, C. Eid, R. Habchi, P. Miele and M. Bechelany,
ChemSusChem. 2018, 11, 3023-3047.
M. Nasr, A. A. Chaaya, N. Abboud, M. Bechelany, R. Viter, C. Eid,
A. Khoury and P. Miele, Superlattices Microstruct. 2015, 77, 18-24.
D. Sánchez-Rodríguez, M. G. M. Medrano, H. Remita and V.
Escobar-Barrios, J. Environ. Chem. Eng. 2018, 6, 1601-1612.
M. Thomas and T. S. Natarajan, Photocatalytic Nanomaterials for
Environmental Applications (Eds. R.J. Tayade, V. Gandhi),
Materials Research Foundations, 2018, vol. 27, pp. 48.
R. Yadav, V. Amoli, J. Singh, M. K. Tripathi, P. Bhanja, A. Bhaumik
and A. K. Sinha, J. CO2 Util. 2018, 27, 11-21.
V. Amoli, M. G. Sibi, B. Banerjee, M. Anand, A. Maurya, S. A.
Farooqui, A. Bhaumik and A. K. Sinha, ACS Appl. Mater.
Interfaces. 2014, 7, 810-822.
K. K. Paul and P. Giri, J. Nanosci. Nanotechnol. 2019, 19, 307-331.
R. Hao, G. Wang, H. Tang, L. Sun, C. Xu and D. Han, Appl.
Catal.,B. 2016, 187, 47-58.
Y. Sui, C. Su, X. Yang, J. Hu and X. Lin, J. Mol. Catal. A: Chem.
2015, 410, 226-234.
B. Li, X. Chen, T. Zhang, S. Jiang, G. Zhang, W. Wu and X. Ma,
Appl. Surf. Sci. 2018, 439, 1047-1056.
L. Lu, T. Jiang, W.-s. Jing, G.-w. Zhou and H.-x. Shi, Chem. Lett.
2018, 47, 613-616.
X. Meng and Z. Zhang, Mater. Lett. 2018, 225, 152-156.
C.-C. Chen, J.-Y. Fu, J.-L. Chang, S.-T. Huang, T.-W. Yeh, J.-T.
Hung, P.-H. Huang, F.-Y. Liu and L.-W. Chen, J. Colloid Interface
Sci. 2018, 532, 375-386.
The structure of the samples were characterized by X-ray diffraction
(XRD) using a PANAlytical Xpert-PRO diffractometer (Cu-radiation, λ =
1.5406 Å) and Raman spectroscopy using
a Jobin Yvon T64000
spectrometer (λ = 514.5 nm; 2.41 eV) in a confocal microscopy mode in
air at room temperature. Energy dispersive X-ray photoelectron
spectroscopy analysis (EDX) was taken with a Hitachi S3500. The
surface area was determined from nitrogen adsorption-desorption
isotherms at liquid nitrogen temperature using the Micromeritics ASAP
2010 system (outgassing conditions: 200°C-12 h). X-ray photoelectron
spectroscopy (XPS) was performed using a Versaprobe PHI 5000
spectrometer from Physical Electronics, equipped with a monochromatic
Al Kα X-ray source. The base pressure at the analysis chamber was 10−9
mbar. The sample was mounted on double-sided conductive tape
suitable for vacuum. The X-ray photoelectron spectra were collected at a
take-off angle of 45° with respect to the electron energy analyzer and the
spot size was 200 µm. The overall energy resolution was 0.5 eV. The
analysis of the XPS spectra was implemented using the CASA XPS
software. The morphology and the size distribution of the prepared
nanomaterials were analyzed by scanning electron microscopy (SEM,
Hitachi S3500) and transmission electron microscopy (TEM, Hitachi H-
700). The UV-VIS absorbance spectra were recorded by a U-3010 UV-
VIS spectroscopic spectrophotometer. The photocurrent density was
measured using a CHI650D electrochemical workstation with a standard
three-electrode system.
[6]
[7]
[8]
[9]
[10]
[11]
[12]
[13]
[14]
[15]
D. Ni, W. Sun, L. Xie, Q. Fan, Z. Wang and K. Sun, J. Power
Sources, 2018, 374, 166-174.
This article is protected by copyright. All rights reserved.

10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
[16]
S. Vadivel, B. Paul, D. Maruthamani, M. Kumaravel, T.
Vijayaraghavan, S. Hariganesh and R. Pothu, Mater. Sci. Energ.
Technol, 2019, 2, 112-116.
J. Cheng, L. Frezet, P. Bonnet and C. Wang, Catal. Lett. 2018,
148, 1281-1288.
A. V. Vorontsov and S. V. Tsybulya, Ind. Eng. Chem. Res. 2018,
57, 2526-2536.
U. Holzwarth and N. Gibson, Nat. Nanotechnol. 2011, 6, 534.
M. Nasr, L. Soussan, R. Viter, C. Eid, R. Habchi, P. Miele and M.
Bechelany, New J. Chem. 2018, 42, 1250-1259.
M. Salari, K. Konstantinov and H. K. Liu, J. Mater. Chem. 2011, 21,
5128-5133.
A. J. Gmitter, A. Halajko, P. J. Sideris, S. G. Greenbaum and G. G.
Amatucci, Electrochim Acta, 2013, 88, 735-744.
C. Bittencourt, M. Rutar, P. Umek, A. Mrzel, K. Vozel, D. Arčon, K.
Henzler, P. Krüger and P. Guttmann, RSC Adv. 2015, 5, 23350-
23356.
J. Han, G. Li, L. Qiang, X. Zhai and C. Jiang, J. Mater. Sci. : Mater.
Electron. 2019, 30, 5995-6006.
U. Diebold and T. Madey, Surf. Sci. Spectra, 1996, 4, 227-231.
H.-P. Jiao, X. Yu, Z.-Q. Liu, P.-Y. Kuang and Y.-M. Zhang, Rsc
Adv. 2015, 5, 16239-16249.
A. Ghobadi, T. G. Ulusoy, R. Garifullin, M. O. Guler and A. K.
Okyay, Sci. Rep. 2016, 6, 30587.
X. Zhang and Z. Chen, RSC Adv. 2015, 5, 9482-9488.
W. Su, J. Wang, Y. Huang, W. Wang, L. Wu, X. Wang and P. Liu,
Scr. Mater. 2010, 62, 345-348.
Y. G. Guo, Y. S. Hu, W. Sigle and J. Maier, Adv. Mater. 2007, 19,
2087-2091.
J. Pan, Z. Dong, B. Wang, Z. Jiang, C. Zhao, J. Wang, C. Song, Y.
Zheng and C. Li, Appl. Catal. B: Environ. 2019, 242, 92-99.
M. Nasr, R. Viter, C. Eid, R. Habchi, P. Miele and M. Bechelany,
Surf. Coat. Technol. 2018, 343, 24-29.
M. Canpolat, C. Kürkçü, Ç. Yamçıçıer and Z. Merdan, Solid State
Commun. 2019, 288, 33-37.
P. K. Sharma, M. A. L. Cortes, J. W. Hamilton, Y. Han, J. A. Byrne
and M. Nolan, Catal. Today, 2019, 321, 9-17.
[35]
[36]
[37]
[38]
[39]
[40]
[41]
[42]
[43]
[44]
Y. Yang, L. C. Yin, Y. Gong, P. Niu, J. Q. Wang, L. Gu, X. Chen, G.
Liu, L. Wang and H. M. Cheng, Adv. Mater. 2018, 30, 1704479.
H. Albetran, B. O'Connor and I. Low, Mater. Des. 2016, 92, 480-
485.
X.-j. Wang, Y. Zhao, F.-t. Li, L.-j. Dou, Y.-p. Li, J. Zhao and Y.-j.
Hao, Sci. Rep. 2016, 6, 24918.
H. Wei, W. A. McMaster, J. Z. Tan, D. Chen and R. A. Caruso, J.
Mater. Chem. A, 2018, 6, 7236-7245.
H. Yu, X. Quan, S. Chen and H. Zhao, J. Phys. Chem. C, 2007,
111, 12987-12991.
Y. Zhang, J. Zhou, W. Cai, J. Zhou and Z. Li, Applied Surface
Science, 2018, 430, 549-560.
L. Sun, L. Xiang, X. Zhao, C.-J. Jia, J. Yang, Z. Jin, X. Cheng and
W. Fan, ACS Catal. 2015, 5, 3540-3551.
L. Sun, X. Zhao, C.-J. Jia, Y. Zhou, X. Cheng, P. Li, L. Liu and W.
Fan, J. Mater. Chem. 2012, 22, 23428-23438.
M. Yang, Q. Yang, J. Zhong, S. Huang, J. Li, J. Song and C. Burda,
Appl. Surf. Sci. 2017, 416, 666-671.
K. Kočí, K. Matějů, L. Obalová, S. Krejčíková, Z. Lacný, D. Plachá,
L. Čapek, A. Hospodková and O. Šolcová, Appl. Catal. B: Environ.
2010, 96, 239-244.
A.-W. Xu, Y. Gao and H.-Q. Liu, J. Catal. 2002, 207, 151-157.
D. Ollis, Appl. Catal. B: Environ. 2019, 242, 431-440.
M. Nasr, R. Viter, C. Eid, F. Warmont, R. Habchi, P. Miele and M.
Bechelany, RSC Adv. 2016, 6, 103692-103699.
X. Zhao, C. Yi, M. Zhang, C. Ma, C. Li and Y. Yan, J. Nanosci.
Nanotechnol. 2018, 18, 4675-4683.
M. Tasbihi, K. Kočí, M. Edelmannová, I. Troppova, M. Reli and R.
Schomaecker, J. Photochem. Photobiol. A: Chem. 2018, 366, 72-
80.
B. Bukowski and N. A. Deskins, Phys. Chem. Chem. Phys. 2015,
17, 29734-29746.
B. Bhanvase, T. Shende and S. Sonawane, Environ. Technol. Rev.
2017, 6, 1-14.
[17]
[18]
[19]
[20]
[21]
[22]
[23]
[24]
[25]
[26]
[45]
[46]
[47]
[27]
[28]
[29]
[48]
[49]
[30]
[31]
[32]
[33]
[34]
[50]
[51]
This article is protected by copyright. All rights reserved.

10.1002/ejic.201900970
European Journal of Inorganic Chemistry
FULL PAPER
FULL PAPER
The novel BiOF/TiO2 heterostructure
with different molar ratio of BiOF and
TiO2 were successfully elaborated
via simple solid state sintering
method. The optimal molar ratio of
BiOF/TiO2 was found to be (1:3) with
a photodegradation percentage of
91.2% and it was 7 and 5 times more
efficient than that of pure BiOF or
TiO2 nanopowder, respectively.
BiOF/TiO2 heterostructure
Maryline Nasr, Wenshi Huang, Carla
Bittencourt, Dandan Cui, Ying Sun, Lei
Wang, Nathalie Gaillard Caperaa,
Yuping Ning, Ping Song, Pierre
Bonnet,* and Cong Wang*
Page No. – Page No.
Synthesis of BiOF/TiO2
heterostructures and their enhanced
visible-light photocatalytic activity
FULL PAPER
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