Novel visible-light-driven BiFeO3-Bi2WO6 nanocomposites toward degradation of dyes

Article abstract of DOI:10.1016/j.jphotochem.2017.09.034

The different mole ratios of novel visible-light-driven BiFeO₃-Bi₂WO₆ nanocomposites were synthesized by a two-step method, i.e. precipitation followed with hydrothermal process. Advanced characterization techniques such a

Full text of DOI:10.1016/j.jphotochem.2017.09.034

Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Novel visible-light-driven BiFeO -Bi WO nanocomposites toward
3
2
6
degradation of dyes
a
b
c
Saranyoo Chaiwichian , Khatcharin Wetchakun , Wiyong Kangwansupamonkon ,
a,
Natda Wetchakun *
a
Department of Physics and Materials Science, Faculty of Science, Chiang Mai University, Chiang Mai 50200, Thailand
Program of Physics, Faculty of Science, Ubon Ratchathani Rajabhat University, Ubon Ratchathani 34000, Thailand
National Nanotechnology Center,Thailand Science Park, Phahonyothin Road, Klong 1, Klong Luang, Pathumthani 12120, Thailand
b
c
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 7 April 2017
3 2 6
The different mole ratios of novel visible-light-driven BiFeO -Bi WO nanocomposites were synthesized
Received in revised form 8 September 2017
Accepted 11 September 2017
Available online 18 September 2017
by a two-step method, i.e. precipitation followed with hydrothermal process. Advanced characterization
techniques such as XRD, TEM, UV–vis DRS, BET-specific surface area, PL, and XPS were used to investigate
the physical and optical properties of the as-prepared samples. The photocatalytic activities of pure
BiFeO
rhodamine B (RhB) under visible light irradiation were investigated. The results were compared with
BiFeO -Bi WO nanocomposites. The highest photocatalytic activity was the 1:1 mol ratio of BiFeO
Bi WO nanocomposite. The increase of photocatalytic activity of BiFeO -Bi WO nanocomposite was
ascribed to the charge transfer through the interface of BiFeO and Bi WO catalysts. This effect helps to
reduce the recombination of electron-hole pairs. The combination of the synergistic effect of strong
visible absorption edge, the amount of BiFeO and Bi WO , and hydroxyl radicals as the active species was
established to enhance photocatalytic efficiency. Based on our investigation, the possible photocatalytic
mechanism of BiFeO -Bi WO nanocomposite was proposed.
3 2 6
and pure Bi WO catalysts for studying the photodegradation of methylene blue (MB) and
Keywords:
BiFeO -Bi WO
3 2 6
Degradation
Dyes
Nanocomposites
Photocatalysis
3
2
6
3
-
2
6
3
2
6
3
2
6
3
2
6
3
2
6
©
2017 Elsevier B.V. All rights reserved.
1. Introduction
Therefore, the designing novel photocatalyst is necessary for
improving photocatalytic efficiency. The development approach of
Photocatalytictechnology has been extended to the area of
2 6
Bi WO by forming composite semiconductor systems was
sustainable treatment technology for the purpose of water
purification [1–3]. A heterogeneous photocatalysis process has
been shown to be effective for treating various types of organic
pollutants by the generation of hydroxyl radicals. These radicals
can degrade organic compounds to less toxic substances, i.e.
carbon dioxide, water, and mineral acids [4,5]. It is well know that
metal oxide materials are an efficient photocatalyst for degrada-
tion of a various organic pollutants under an irradiation of light. For
selected to improve the limitations of charge carrier separation
efficiency. The outcome of photocatalytic performance for
modifying Bi
Photocatalytic activities relating to physicochemical properties
of the modified Bi WO have been reported in previous reports
[13–15]. In 2012, Xu et al. proposed a modification of Bi WO by
adding Fe to form the composite structure by hydrothermal
2 6
WO gave a promising improvement [10–12].
2
6
2
6
3 4
O
method. The enhancement of photocatalytic performance of the
nanocomposite is up to 98.6% for 150 min of degradation time [16].
2 6
example, orthorhombic bismuth tungstate (Bi WO ) is one of the
most promising visible-light-driven photocatalysts for water and
A two-step preparation of CeO
2
2 6
-Bi WO nanocomposite is a
wastewater remediation, especially removing organic toxics [6–9].
precipitation method followed by
a
hydrothermal method,
Although, Bi
2
WO
6
can be used as a photocatalyst, it exhibits low
introduced by Issarapanacheewin [17]. This synthesis process
yields the high photocatalytic activity of 75.94% for RhB degrada-
tion within 75 min under simulated solar light irradiation. Bismuth
electron transport and high rate of charge recombination.
ferrite (BiFeO
ductor, demonstrated the photocatalytic destructions of organic
pollutant. It perfectly matches with Bi WO for photocatalyst
3
), which is the most studied metal oxide semicon-
*
Corresponding author.
E-mail address: natda_we@yahoo.com (N. Wetchakun).
2
6
http://dx.doi.org/10.1016/j.jphotochem.2017.09.034
010-6030/© 2017 Elsevier B.V. All rights reserved.
1

184
S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
The BiFeO
prepared by following these steps. First, the predetermined
amount of BiFeO powders was subsequently added to the mixture
solution with 1:2 molar ratio of bismuth nitrate pentahydrate to
sodium tungstate dihydrate (Na WO 2H O). Second, the bismuth
3 2 6
-Bi WO nanocomposite with 1:1 mol ratio was
3
2
4
ꢀ
2
nitrate pentahydrate and sodium tungstate dehydrate precursors
were dissolved in nitric acid (2.5 M) and deionized water,
respectively. Then, 6 M of ammonium hydroxide was slowly
dropped into the above-suspended solution. The mixture solution
ꢁ
was treated at a temperature of 200 C for 24 h in the hydrothermal
autoclave. Next, the solid powders were continuously washed and
collected by centrifugation. The resultant precipitate powder was
ꢁ
dried at 70 C overnight. The nominal mole ratios of 0.2:0.8, 0.4:0.6,
0
.6:0.4, 0.8:0.2 of BiFeO
in the same way as described above. The pure Bi
catalyst was also synthesized by hydrothermal method without
adding BiFeO for comparison.
3
:Bi
2
WO
6
were calculated and synthesized
2
WO photo-
6
3
2
.2. Characterization
3 2 6 3 2 6
Fig. 1. XRD patterns of pure BiFeO , pure Bi WO , and BiFeO -Bi WO with varying
mole ratios.
The crystal structure identification of all samples were
characterized by X-ray diffraction technique (XRD, Philips X'Pert
MPD) with Cu K irradiation ( = 1.5418 nm) in the 2 ranging from
0 to 80 . The pure and composite particles were studied under
JEOL JSM-2010 transmission electron microscopy operating at
00 kV. UV–vis diffuse reflectance spectroscopy was employed to
a
l
u
ꢁ
ꢁ
1
2
measure the optical absorption properties of the samples. The
diffuse-refleactance spectra of the dry powders were measured by
a PerkinElmer Lambda 1050 UV–vis spectrophotometer equipped
with an integrating sphere using MgO as a reference. The
Brunauer-Emmett-Teller (BET) surface area was determined by
composites because it exhibits visible-light-induced photocatalyst
with the band gap of 2.2 eV and good chemical stability [18–22].
Surprisingly, the physicochemical properties and photocatalytic
activities of BiFeO
reported. Therefore, the study of the visible-light-driven BiFeO
Bi WO composite is essential to demonstrate an integration of
semiconductor material into photocatalysis technology.
In this report, five different mole ratios (0.2:0.8, 0.4:0.6, 0.5:0.5,
.6:0.4, 0.8:0.2) of BiFeO :Bi WO were synthesized by the
3 2 6
-Bi WO nanocomposites have not been
ꢁ
3
-
nitrogen adsorption-desorption isotherm measurements at 80 C
(Autosorb 1 MP, Quantachrome). Surface element composition and
2
6
oxidation states of the as-prepared samples were characterized
DLD
with X-ray photoelectron spectroscopy (XPS, AXIS ULTRA
,
0
3
2
6
Kratos analytical, Manchester UK), instrument equipped with X-
combination of precipitation and hydrothermal methods. Photo-
catalytic performances were evaluated by photodegradation of
methylene blue and rhodamine B under visible light irradiation.
The effects of the physicochemical properties on photocatalytic
activities were investigated through various techniques, including
X-ray diffraction (XRD), transmission electron microscopy (TEM),
Brunauer-Emmett-Teller (BET)-specific surface area, UV–vis dif-
fuse reflectance spectroscopy (UV–vis DRS), X-ray photoelectron
spectroscopy (XPS), and photoluminescence (PL). The enhance-
ment of photocatalytic activity was discussed.
ray hybrid mode 700 ꢂ 300 nm spot areas with a monochromatic
Al K radiation at 1.4 keV. To study the electron-hole separation in
a
the BiFeO -Bi WO composite compared to pure BiFeO and pure
3
2
6
3
Bi WO photocatalysts, photoluminescence (PL) technique was
2
6
used to measure the evolution of spectrum by a AvaSpec-2048TEC-
USB2-2 spectrophotometer excited by LED (Oceans optics, LLS-
345) as a light source with a wavelength of 345 nm at room
temperature.
2.3. Photocatalytic experiments
2
. Experimental
The photocatalytic performances of the prepared samples were
examined toward the photodegradation of methylene blue (MB)
ꢃ5
2.1. Sample preparation
and rhodamine B (RhB) with a concentration of 2 ꢂ10 M at room
temperature. 0.1 g of the photocatalyst was suspended in 100 ml of
MB and RhB. Before irradiation by 50 W halogen lamp (Essential
To obtain novel BiFeO
3 2 6
-Bi WO nanocomposites, firstly, the
BiFeO
bismuth nitrate pentahydrate (Bi(NO
nonahydrate (Fe(NO 9H O). Bismuth nitrate pentahydrate was
dissolved in 50 ml of nitric acid (2.5 M), and then combined with a
solution of iron nitrate nonahydrate in 50 ml of deionized water.
Both solutions were mixed and stirred with a magnetic stirrer for
3
particles was prepared by using the 1:1 molar ratio of
MR, Philips (Thailand)) with
a filter glass to remove UV
3
)
3
ꢀ
5H O) and iron nitrate
2
components ( < 400 nm), the mixture was magnetically stirred
l
3
)
3
ꢀ
2
for 60 min in the dark to establish an adsorption-desorption
equilibrium of dyes on the catalyst surface. Then, the reaction
system was exposed to visible light for 75 min 5 ml of the
suspension was collected every 15 min, then the particles were
removed by centrifugation at 5000 rpm for 5 min. The concen-
trations of MB/RhB were evaluated by measuring the absorbance of
solutions at the wavelength of 664/554, respectively, with UV–vis
spectrophotometer. The percentage of dye degradation efficiency
was calculated as follows Eq. (1) [12]:
30 min at room temperature. 6 M of sodium hydroxide (NaOH) was
then added to this mixture under vigorous stirring until its pH
equals to 10. Then, it was kept at room temperature for 24 h. A
formed brown precipitate was collected by centrifugation at
ꢁ
7
000 rpm for 10 min. These precipitates were further dried at 80 C
for 24 h. Finally, the as-received precipitate BiFeO
calcined for 1 h at 600 C to remove hydroxide ions and residual
solvents.
3
powder was
ꢁ
%degradation = (C
0
ꢃ C)/C
0
ꢂ100
(1)

S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
185
Fig. 2. TEM images and lattice fringes of (a,d) BiFeO
3
, (b,e) Bi
2
WO
6
and (c,f) BiFeO
3
-Bi
2
WO
6
samples.
ꢄ
where C
0
is the initial concentration of dye, C is the concentration
2
.4. Analysis of hydroxyl radical (OH )
of dye at irradiation time. The total organic carbon (TOC)
concentration was measured using a Multi N/C 2100 TOC (Analytik
Jena AG Corporation) for determining the mineralization of the
RhB and MB solutions.
ꢄ
The generation of hydroxyl radicals (OH ) on the surface of
BiFeO -Bi WO nanocomposite was investigated by photolumi-
3
2
6
nescence (PL) technique. Briefly, the samples containing 0.1 g was
mixed with 100 ml of 5 ꢂ10 M of terephthalic acid (TA) solution
ꢃ4

186
S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
3
. Results and discussion
3
.1. Physical and optical properties
3
Fig. 1 shows the X-ray diffraction (XRD) patterns of BiFeO ,
Bi
Bi
2
WO
WO
6
, and BiFeO
and BiFeO
3
-Bi
2 6
WO samples. The diffraction peaks of
2
6
3
were indexed as orthorhombic phase [cell
constants: a = 5.457 Å, b = 5.436 Å, and c = 16.427 Å; JCPDS No.73-
126] and rhombohedral structure [cell constants: a = b = c = 3.962
Å; JCPDS No.74-2016], respectively. The characteristic peaks of
1
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
3
BiFeO locating at 2u= 22.42 , 31.08 , 32.11 , 38.99 , 39.53 and
4
ꢁ
5.79 were assigned to (010), (110), (ꢃ110), (111), (ꢃ111) and
(
2
020), respectively. The characteristic peak of Bi
2
WO
6
at
ꢁ
u
= 28.33 was observed, corresponding to the (113) plane. No
other diffraction peaks were observed in the XRD patterns of the
BiFeO -Bi WO samples. This suggests that the formation of
impurity phase by chemical reactions between BiFeO and Bi WO
was not allowed. The characteristic diffraction peaks of BiFeO
Bi WO nanocomposite display the combination of cubic BiFeO
and orthorhombic Bi WO
of small amount BiFeO , i.e. 0.2BiFeO
was quite similar to pure Bi WO . In contrast, for the large amount
of BiFeO in BiFeO -Bi WO nanocomposite, is diffraction pattern
resembles to pure BiFeO . Therefore, the characteristic peaks of
BiFeO -Bi WO composite were influenced by the crystallinity, and
3
2
6
3
2
6
3
-
2
6
3
2
6
crystalline phases. The diffraction peak
:0.8Bi WO nanocomposite,
3
3
2
6
2
6
3
3
2
6
3
3
2
6
the majority component.
2
ꢃ1
The specific surface area of BiFeO
3
and Bi
2
WO
6
were 8.44 m g
adsorption. The
6
nanocomposites decrease
2
ꢃ1
and 44 m g based on the BET method using N
specific surface areas of BiFeO -Bi WO
in the following order: 0.2BiFeO -0.8Bi
BiFeO -0.6Bi WO (35.09 m g ) > 0.5BiFeO
2
3
2
2
ꢃ1
3
2
WO
6
(36.92 m g ) > 0.4
-0.5Bi WO
(30.70 m g ) > 0.8BiFeO
2 6
.2Bi WO (21.69 m g ). Typically, photocatalytic performance
2
ꢃ1
3
2
6
3
2
6
2
ꢃ1
2 ꢃ1
(
0
30.06 m g ) > 0.6BiFeO
3
-0.4Bi
2
WO
6
3
-
2
ꢃ1
was determined by many factors such as surface area, adsorption
capacity, optical absorption, carrier recombination rate, and etc
[23,24]. A larger surface area normally corresponds to smaller
particle size and high adsorption capacity, resulting to an
enhancement of photocatalytic activity. However, the surface area
of Bi
nanocomposite, whereas the photocatalytic performance of
Bi WO lowers than BiFeO -Bi WO nanocomposite (the result
was shown in section of 3.2 Photocatalytic activity studies).
Therefore, the influence of a specific surface area of the BiFeO
2 6 3 2 6
WO nanocomposite is higher than that of BiFeO -Bi WO
2
6
3
2
6
3
-
Bi
2
WO
The size, morphology and lattice fringe of pure BiFeO
WO , and BiFeO -Bi WO with mole ratio 1:1 was observed by
6
nanocomposite to photocatalytic activity is negligible.
3
, pure
Bi
2
6
3
2
6
transmission electron microscopy (TEM) as shown in Fig. 2. The
pure BiFeO crystallite has rectangular-like shape with the width of
5–50 nm and the length of 50–100 nm (Fig. 2a). In Fig. 2b, the
morphology of Bi WO reveals plate-like shape with the size of
approximately 50–100 nm. The TEM images of BiFeO -Bi WO
nanocomposite consisted of rectangular-like and plate-like of
BiFeO and Bi WO , respectively as shown in Fig. 2c. The lattice
fringe images of pure BiFeO and pure Bi WO indicate that the
3
2
2
6
3
2
6
3
2
6
3
2
6
spacings of the lattice are 0.318 nm and 0.472 nm as depicted in
Fig. 2d and e, respectively. These interlayer gaps are consistent
with the spacings of the corresponding (110) and (202) planes,
Fig. 3. UV–vis diffuse reflectance spectra of pure BiFeO
.5BiFeO - 0.5Bi WO photocatalysts. The inset figure shows the band gap of pure
BiFeO , pure Bi WO , and 0.5BiFeO -0.5Bi WO photocatalysts.
3 2 6
, pure Bi WO , and
0
3
2
6
3
2
6
3
2
6
respectively. The interface between BiFeO
lines can also be observed from the image of BiFeO
nanocomposite in Fig. 2f.
and Bi
WO
crystal-
-Bi WO
6
3
2
6
3
2
ꢃ3
and 2 ꢂ10 M of a diluted sodium hydroxide (NaOH) aqueous
solution. Under visible light irradiation, the fluorescence produc-
tion of 2-hydroxyterephthalic acid increased from the reactions of
terephthalic acid and hydroxyl radicals. The suspensions were
The Kubelka-Munk (KM) model has been used to determine the
absorption value through
following Eq. (2) [24]:
a
reflectance (R
1
)
measurement
ꢄ
collected every 30 min to evaluate the generated TA-OH , and
2
F(R
1
) = (1 ꢃ (R
1
)) /2R
1
(2)
analyzed by fluorescence spectroscopy at 425 nm.

S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
187
equation, n is a coefficient dependent on the type of transition in
semiconductor (n = 2 for allowed direct transitions and n = 1/2 for
allowed indirect transitions). The calculated n for BiFeO
Bi WO is 2. It means that the optical transition is direct allowed
25,26]. The band gaps of BiFeO , Bi WO , BiFeO -Bi WO are
3
and
2
6
[
3
2
6
3
2
6
found to be 2.30 eV, 3.00 eV, and 2.28 eV, respectively (as shown in
the inset of Fig. 3a–c). These band gaps were attributed to enhance
visible light photocatalytic performance [27–29]. The band gap of
3 2 6
BiFeO -Bi WO nanocomposite was not changed after the forma-
tion of the nanocomposite. However, its ability to harvest visible
light energy was improved. This suggests the photocatalytic
3 2
Fig. 4. Band structure of BiFeO and Bi WO6.
performance of BiFeO
the chemical bonding between BiFeO
To study the photophysical properties of BiFeO
the band structures of BiFeO and Bi WO
electron configuration and electron transition as illustrated in
Fig. 4. For the valence band (VB) of BiFeO , the lowest energy bands
3 2 6
-Bi WO nanocomposite might be caused by
3
and Bi
2
WO
6
.
2 6
and Bi WO ,
where F(R
material. F(R
1
) is proportional to the absorption constant of the
) is indicative of the absorbance of the sample at a
3
3
2
6
were described by
1
particular wavelength. Fig. 3 displays the UV–vis diffuse reflec-
tance spectra of all samples with the strong absorption in the
3
are occupied by O 2s states. The upside of VB consists of O 2p states
mixing with Fe 3d and Bi 6p states. The Fe 3d state was strongly
hybridized with O 2p state in the valence band. In the Fe 3d state of
BiFeO , the valence state of Fe ions can be changed to Fe or Fe
3
after activation by photon energy. The oxygen vacancies were
created for charge compensation. Therefore, the band gap of
visible-light region. The absorption edges of BiFeO
BiFeO -Bi WO samples were determined at about 557 nm,
14 nm, and 556 nm, respectively as shown in Fig. 3a–c. The band
gaps (E ) of the obtained samples were estimated using by Tauc
equation as following Eq. (3):
3 2 6
, Bi WO , and
3
2
6
4
3
+
2+
4+
g
(a
hy
)n = A(h
-E
where , E
y
g
)
(3)
BiFeO
oxygen vacancies and valence fluctuations of Fe ions decrease. For
the Bi WO , the band gap between the valence band and the
3
may decrease because electron correlation energy with
a
,
y
g
, and A are the absorption coefficient, the light
frequency, the band gap, and a constant, respectively. In this
2
6
3 2 6 3 2 6 3 2 6
Fig. 5. Comparisons of XPS spectra for (a) Bi 4f, (b) W 4f, (c) Fe 2p and (d) O 1s of BiFeO , Bi WO , 0.5BiFeO -0.5Bi WO composite, and mixed 0.5BiFeO -0.5Bi WO
photocatalysts.

188
S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
level, leading to a reduced the band edge. Thus, an interband
transition in Bi WO was allowed [31]. From Fe XPS spectra
2
6
showed in Fig. 5c, two peaks with binding energies of ꢅ711 eV and
ꢅ724 eV were assigned to Fe 2p3/2 and Fe 2p1/2, respectively. This
3
+
suggests the presence of Fe , agreeing with the previous reported
values [32]. Fig. 5d shows the O 1s core level spectrum. Three main
peaks can be identified. The peaks at about 530 eV, 531 eV, and
5
32 eV correspond to the lattice oxygen of metal oxide lattice, the
oxygen defects in the matrix of metal oxide, and the hydroxyl
groups due to water adsorption [33–36]. The Bi and O peaks of
BiFeO
suggests electronic interactions between BiFeO
coexisting in BiFeO -Bi WO composite rather than a physical
mixture phase of BiFeO and Bi WO [37].
3
-Bi
2
WO
6
nanocomposite show a small positive shift, which
3
and Bi WO
2
6
3
2
6
3
2
6
3
.2. Photocatalytic activity studies
Methylene blue (MB) and rhodamine B (RhB) dyes are the most
commonly used in textile and plastic industries. The both dyes
have widely studied as a model of organic pollutants because of
their toxicity and carcinogenicity. In this work, MB and RhB are
selected to evaluate the photocatalytic properties of the BiFeO
3
-
Bi WO nanocomposites. The C/C of MB and RhB versus dark
2
6
0
adsorption time for 60 min of all photocatalyst samples were
shown in Fig. 6a and b. The concentrations of MB and RhB gradually
decreased with time after the catalysts were immersed in dye
solutions. Beyond 60 min of dark adsorption, the small adsorptions
of MB and RhB on all samples surface were observed. It implies that
the dyes adsorbed on the surface of the catalysts until the
equilibrium adsorption-desorption was reached. Fig. 7a and c
display the photocatalytic activities of the prepared samples for
photodegradation of MB and RhB solutions under visible light
irradiation, respectively. The pure BiFeO
photocatalytic activity compared to BiFeO
sites for MB and RhB photodegradation. After 75 min of MB
degradation, the photocatalytic activities of BiFeO -Bi WO nano-
composites observed in the following order: 0.5BiFeO -0.5Bi WO
0.8BiFeO -0.2Bi WO > 0.6BiFeO -0.4Bi WO > 0.4BiFeO
.6Bi WO > 0.2BiFeO -0.8Bi WO as presented in Table 1. The
.5BiFeO -0.5Bi WO nanocomposite obtains the highest photo-
3
and Bi
2
WO
6
show low
3
-Bi WO
2
6
nanocompo-
3
2
6
3
2
6
Fig. 6. Dark adsorption abilities of all samples on the degradation of (a) MB and (b)
RhB for 60 min.
>
3
2
6
3
2
6
3
-
0
0
2
6
3
2
6
3
2
6
catalytic activity equaling to 54% for MB degradation and 36% for
RhB degradation. Although, the Bi WO has the highest specific
surface area but its photocatalytic activity was slightly low
compared to pure BiFeO and BiFeO -Bi WO nanocomposites.
This is possibly because the coexistence of BiFeO and Bi WO
conduction band consists of O 2p + Bi 6 s VB to W 5d high-energy
2
6
CB. The energy level of the W 5d state of Bi
Fe 3d CB of BiFeO . Therefore, the band gap of Bi
BiFeO . The difference of energy level between BiFeO
2
WO
6
is higher than the
WO is larger than
and Bi WO
3
2
6
3
3
2
6
3
3
2
6
3
2
6
could assist the transferring of photogenerated electron-hole pair.
This explanation agrees with the results of their photocatalytic
activities.
leads to the separation of electron and hole pairs, resulting an
enhancement of photocatalytic activity. For RhB degradation, the
photocatalytic activities of BiFeO -Bi WO nanocomposites exhibit
3 2 6
To confirm the chemical bonding between BiFeO
3
and Bi
2
WO
6
,
the same trend as MB degradation. These results indicate that the
photodegradation efficiency for RhB is lower than MB degradation
over all types of photocatalysts. The complicated structure of RhB
may be the cause [38,39]. The pseudo-first order model can be
expressed by the Eq. (4) [40]:
X-ray photoelectron spectroscopy (XPS) is an essential technique
to investigate this phenomenon of the construction of the
composite. In Fig. 5, the typical survey XPS spectrum indicates
that Bi, W, Fe, and O elements were detected for pure BiFeO
Bi WO , BiFeO -Bi WO , and mixed BiFeO -Bi WO (Fig. 5). For
Fig. 5a, the peaks of Bi 4f7/2 and Bi 4f5/2 were addressed at about
3
, pure
2
6
3
2
6
3
2
6
ꢃln(C/C (4)
0
) = kapp
t
3+
1
59 and 164 eV, respectively, which are assigned to Bi in BiFeO
and Bi WO [30]. Also, two distinct peaks were observed at binding
energies of about 160 eV (Bi 4f7/2 and 165 eV (Bi 4f5/2
corresponding to the Bi -OH state. These possibly originate from
water adsorption in humidity air. The W 4f spectrum for Bi WO
and BiFeO -Bi WO
presents the binding energies at ꢅ35 eV (W
7/2) and ꢅ37 eV (W 4f5/2), respectively, corresponding to W . The
peaks at binding energies of ꢅ36 eV and ꢅ38 eV as seen in Fig. 5b
3
where C is the initial concentration of dye, C is the dye
0
2
6
concentration at irradiation time and kapp is the apparent
ꢃ1
)
)
pseudo-first order rate constant (min ). The reaction kinetic rate
3
+
constants of all samples for MB and RhB degradation were
calculated, and shown in Fig. 7b,d and Table 1. The reaction kinetic
rate constants for pure MB can be arranged in an order of
2
6
3
2
6
6
+
4
f
0.5BiFeO
0.4Bi WO
3
-0.5Bi
2
WO
6
> 0.8BiFeO
-0.6Bi WO
3
-0.2Bi
2
WO
6
> 0.6BiFeO
-0.8Bi WO
3
-
2
6
> 0.4BiFeO
3
2
6
> 0.2BiFeO
3
2
6
.
The
could be ascribed to the W6 ion from W reduction into W . This
+
6+
5+
reaction kinetic rate constants for pure RhB have the same ordered
as MB degradation.
was because the oxygen vacancies in Bi WO can raise the Fermi
2
6

S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
189
0
Fig. 7. (a,c) Photocatalytic activities relationship between C/C versus degradation time and (b,d) kinetic plot for linear fitting of data obtained from pseudo-first order
3 2 6 3 2 6
reaction for the photodegradation of MB and RhB under visible light irradiation over pure BiFeO , pure Bi WO , and BiFeO -Bi WO nanocomposites with different mole ratios
of 0.2:0.8, 0.4:0.6,0.5:0.5, 0.6:0.4, 0.8:0.2.
Table 1
2
Percentage of degradation, pseudo first order rate constant (kapp), and coefficient of determination (R ) for all the synthesized samples promoted dyes degradation under
visible light irradiation.
Samples
MB
RhB
ꢃ1
app (min
R2
ꢃ1
R2
%degradation
k
)
%degradation
kapp (min
)
ꢃ3
ꢃ4
photolysis
8%
1.10 ꢂ10
2.76 ꢂ10
3.59 ꢂ10
4.36 ꢂ10
5.89 ꢂ10
0.9964
0.9957
0.9966
0.9814
0.9848
0.9969
0.9863
0.9917
2%
2.4 ꢂ10
0.9861
0.9902
0.9869
0.9952
0.9973
0.9885
0.9987
0.9924
ꢃ3
ꢃ3
ꢃ3
ꢃ3
ꢃ3
ꢃ3
ꢃ3
BiFeO
Bi WO
3
18%
23%
26%
33%
54%
41%
45%
12%
10%
20%
26%
36%
30%
31%
1.86 ꢂ10
1.45 ꢂ10
2
6
0
0
0
0
0
.2BiFeO
.4BiFeO
.5BiFeO
.6BiFeO
.8BiFeO
3
3
3
3
3
-0.8Bi
-0.6Bi
-0.5Bi
-0.4Bi
-0.2Bi
2
2
2
2
2
WO
WO
WO
WO
WO
6
6
6
6
6
2.89 ꢂ10
ꢃ3
4.01 ꢂ10
6.39 ꢂ10
4.69 ꢂ10
5.46 ꢂ10
ꢃ3
ꢃ3
ꢃ3
ꢃ3
10.83 ꢂ10
ꢃ3
ꢃ3
7.78 ꢂ 10
8.55 ꢂ10
In order to investigate the photocatalytic process, the relative
conduction band (CB) and valence band (CB) potentials of the
semiconductors can be calculated by following Eq. (5):
The valence band (VB) and conduction band (CB) of BiFeO
WO were evaluated to be 2.58 eV, 0.28 eV and 3.36 eV, 0.36 eV,
respectively. According to our experimental results, the possible
pathways of degradation of MB/RhB toward BiFeO -Bi WO
nanocomposite under visible light illumination were illustrated
in Scheme 1. In this condition, BiFeO -Bi WO nanocomposites
were excited and electron-hole (e /h ) pairs were generated. The
migration of electrons at the CB of BiFeO to the CB of Bi WO is
assisted by the difference of conduction band potential of BiFeO
3
and
Bi
2
6
C
3
2
6
E
VB
=
x
-E + 0.5E
g
(5)
where
is 5.93 eV for BiFeO
energy of free electrons on the hydrogen scale (ꢅ4.5 eV). E
band gap energy of the semiconductor. The band gap energies of
BiFeO and Bi WO derived from UV–vis diffuse reflectance spectra
x
is the absolute electronegativity of the semiconductor (
x
3
2
6
C
ꢃ
+
3
[41] and 6.36 eV for Bi
2
WO
6
[42]). E is the
is the
g
3
2
6
3
+
and Bi
2
WO
6
. On the other hand, holes at VB (h VB) of Bi
2 6
WO can
3
2
6
were 2.30 eV and 3.00 eV, respectively. The conduction band
potential (CB) was determined by the following Eq. (6):
also be injected into VB of BiFeO
effectively enhance the separation of electrons and holes,
3
. This holes injection could
accelerating the photocatalysis process. The transferred holes on
to the valence band level of BiFeO can be captured with the OH
3
ECB = EVB ꢃ E
g
(6)
ꢃ

190
S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
3 2 6
Scheme 1. Schematic representation of band energy levels of BiFeO -Bi WO nanocomposites under visible light irradiation.
ꢄ
ꢄ
and H
oxidant species for degradation of the organic compounds to form
CO , H O and degradation products. The CB edge potential of
BiFeO (0.28 V) is more negative than that of Bi WO (0.36 V).
Therefore, the photogenerated electrons from conduction band of
BiFeO can transfer to the conduction band of Bi WO . However,
electrons at CB of BiFeO and Bi WO cannot reduce O to
) because the conduction band levels
2
O, and finally form hydroxyl radicals (OH ). The OH is an
presented in Fig. 8. The mineralizations of MB and RhB after
120 min of photodegradationwere 13.81% and 19.40%, respectively.
2
2
3 2 6
These results suggest that the BiFeO -Bi WO is able to mineralize
3
2
6
organic pollutants into CO and H O.
2
2
3
2
6
3
ꢄꢃ
2
6
2
superoxide ion radical (O
of BiFeO (0.28 V) and Bi
2
3
2
WO
6
(0.36 V) are more positive than the
potential for the single electron reduction of oxygen (e + O
ꢃ
2
!
ꢄꢃ
0
ꢄꢃ
O
2
, E (O
SHE) [43]. In our results, the pure Bi
photocatalytic performance compared to the BiFeO
composite. The enhancement of photocatalytic activity of
BiFeO -Bi WO nanocomposite can be explained by the promotion
2 2
of the multi-electron reduction of O O ,
2
/O
2
)) of ꢃ0.28 V vs. standard hydrogen electrode
WO indeed exhibited a low
-Bi WO
(
2
6
3
2
6
3
2
6
ꢃ
+
2
(O
2
+ 2e + 2H ! H
0
ꢃ
+
0
E (O
.23 V) vs. SHE [44,45]. The two-electron process is the most
probable mechanism for the enhanced photocatalytic reaction
using BiFeO -Bi WO nanocomposites. This is because the two-
2
/H
2
O
2
) of 0.68 V or O
2
+ 4e + 4H ! 2H
2 2 2
O, E (O /H O) of
1
3
2
6
electron process is the only possible process that can generate
hydrogen peroxide as an oxidizing agent.
The measurement of total organic carbon (TOC) was conducted
to study the mineralization of MB and RhB. The results were
Fig. 8. Removal the TOC during the degradation process of (a) methylene blue and
Fig. 9. Effect of different scavengers on the photocatalytic degradation of MB and
RhB over BiFeO , Bi WO , and BiFeO -Bi WO under visible light irradiation.
(
b) rhodamine B over BiFeO
3
-Bi
2
WO
6
composite under visible light irradiation.
3
2
6
3
2
6

S. Chaiwichian et al. / Journal of Photochemistry and Photobiology A: Chemistry 349 (2017) 183–192
191
ꢄ
hydroxyl radicals (OH ) on the surface of BiFeO
3
-Bi
2
WO
6
nano-
composite were probed via the reaction of terephthalic acid (TA)
ꢄ
with OH , and then forming a fluorescent 2-hydroxy terephthalate
ꢄ
ꢄ
(
TA-OH ). The fluorescent of the TA-OH can be detected at a
wavelength of 425 nm as presented in Fig. 10b. The intensity of
fluorescence spectra steadily increased with an increase of
irradiation time, indicating an improvement of photocatalytic
activity.
4
. Conclusions
BiFeO
precipitation with hydrothermal methods showed higher activity
than pure BiFeO and pure Bi WO . This enhancement of photo-
3 2 6
-Bi WO nanocomposites prepared by the coupling of
3
2
6
catalytic degradation can be attributed to the synergic effects of
various factors, such as the crystallinity, separation of photo-
generated electron-hole pairs between BiFeO and Bi WO , and
3 2 6
strong absorption in the visible light region. Importantly, the
hydroxyl radical was the main active species in photocatalysis
process for MB and RhB degradation. On the basis of our study, the
3 2 6
BiFeO -Bi WO nanocomposite is expected to be a novel composite
system for degrading toxic dyes.
Acknowledgements
The authors gratefully acknowledge the financial support from
the Thailand Graduate Institute of Science and Technology
Development Agency (TGIST) and CMU Mid-Career Research
Fellowship Program, Chiang Mai University.
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