Fabrication of hierarchical BiOI/Bi2MoO6 heterojunction for degradation of bisphenol A and dye under visible light irradiation

Article abstract of DOI:10.1016/j.jallcom.2015.02.064

A novel hierarchical BiOI/Bi₂MoO₆ composites are prepared by a facile precipitation-deposition method. The prepared photocatalyst was characterized by X-ray diffraction (XRD), transmission electron microscope (TEM), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), and X-ray photoelectron spectrometer (XPS). The resulting BiOI/Bi₂MoO₆ composites exhibit excellent photocatalytic activity and stability towards the degradation of methylene blue (MB) and bisphenol A (BPA) in aqueous solution under visible light irradiation. The optimal composite with 25% BiOI content shows the highest photocatalytic activity for MB degradation. The enhanced photocatalytic activity is mainly attributed to the formation of BiOI/Bi₂MoO₆ heterojunction that can facilitate the separation and transfer of the photo-generated charge carriers. The roles of active species in the photocatalytic process are discussed by using different types of active species scavengers. Meanwhile, combined with the photoluminescence (PL) and electrochemical impedance spectroscopy (EIS), the degradation mechanism of the photocatalysts is proposed. It is hoped that the work could provide valuable information on the design of specific structure materials with more excellent properties and set the foundation for the further industrial application.

Full text of DOI:10.1016/j.jallcom.2015.02.064

Journal of Alloys and Compounds 634 (2015) 223–231
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
Fabrication of hierarchical BiOI/Bi2MoO6 heterojunction for degradation
of bisphenol A and dye under visible light irradiation
b,
Tao Yan a,b, Meng Sun b, Hongye Liu c, Tingting Wu b, Xiaojie Liu b, Qing Yan b, Wenguo Xu a, , Bin Du
⇑
⇑
a School of Chemistry, Beijing Institute of Technology, Beijing 100081, PR China
b School of Resources and Environment, University of Jinan, Jinan 250022, PR China
c School of Civil Engineering and Architecture, University of Jinan, Jinan 250022, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
A novel hierarchical BiOI/Bi2MoO6 composites are prepared by a facile precipitation–deposition method.
The prepared photocatalyst was characterized by X-ray diffraction (XRD), transmission electron micro-
scope (TEM), scanning electron microscopy (SEM), diffuse reflectance spectroscopy (DRS), and X-ray pho-
toelectron spectrometer (XPS). The resulting BiOI/Bi2MoO6 composites exhibit excellent photocatalytic
activity and stability towards the degradation of methylene blue (MB) and bisphenol A (BPA) in aqueous
solution under visible light irradiation. The optimal composite with 25% BiOI content shows the highest
photocatalytic activity for MB degradation. The enhanced photocatalytic activity is mainly attributed to
the formation of BiOI/Bi2MoO6 heterojunction that can facilitate the separation and transfer of the photo-
generated charge carriers. The roles of active species in the photocatalytic process are discussed by using
different types of active species scavengers. Meanwhile, combined with the photoluminescence (PL) and
electrochemical impedance spectroscopy (EIS), the degradation mechanism of the photocatalysts is pro-
posed. It is hoped that the work could provide valuable information on the design of specific structure
materials with more excellent properties and set the foundation for the further industrial application.
Ó 2015 Elsevier B.V. All rights reserved.
Received 27 November 2014
Received in revised form 30 January 2015
Accepted 7 February 2015
Available online 16 February 2015
Keywords:
BiOI/Bi2MoO6
Visible light
Photocatalysis
Heterojunction
BPA
1. Introduction
[18–20], BiVO4 [21,22], Bi7O9I3 [23], BiNbO4 [24], and Bi2MoO6
[25] have recently aroused great interest in the scientific commu-
During the past decades, heterogeneous photocatalysis has
been considered as promising process for environmental
nity due to their intriguing electronic structures and excellent
potentiality to be visible-light-active photocatalysts.
a
purification [1–3]. Extensive efforts have been devoted to the
development of highly efficient photocatalysts [4,5]. In general,
the efficient photocatalysts are usually wide-gap semiconductors,
and only utilize ultraviolet light which just takes a low percent
of the solar light [6,7]. From the viewpoint of solar energy
utilization, the development of visible light driven photocatalyst
is indispensable [8–10]. Therefore, high effectiveness of visible
light conversion driven photocatalysts has gained much attention
currently [11,12], and they have been considered as one of the
most promising alternatives [13,14].
Up to now, many semiconductor photocatalysts, which are
highly effective and highly visible-light sensitive, have attracted
worldwide attentions because of their applicability to the degrada-
tion of harmful organic contaminants. Among them, the bismuth-
based compounds, such as BiOX (X = Cl, Br, I) [15–17], Bi2WO6
Among these compounds, Bi2MoO6, as an Aurivillius-phase
perovskite, is one of newly developed visible-light-driven photo-
catalysts. Its photocatalytic performance is largely dependent on
its structure, morphology and components [26,27]. In particular,
hierarchical nanostructures have attracted extensive attention
due to their unique interior void, large surface area and surface
permeability [28]. However, the application of Bi2MoO6 is current-
ly limited because of a low overall efficiency, due to rapid electron–
hole recombination, and potentially poor surface chemistry. To
overcome these problems, particular emphasis has been placed
on complex or hierarchical hetero-nano-structures to facilitate
the separation of photo-generated electron–hole pairs, and to
further improve the photocatalytic activity [29–32]. This may
represent a promising and efficient method for further enhance-
ment of photocatalytic performance.
In addition, as another member of the Aurivillius-related oxide
family, BiOI have narrow band gaps and are potential visible-light
photocatalysts [33–36]. It is known that BiOI has a band gap of 1.7–
1.85 eV and can absorb most of the visible light (k < 700 nm).
⇑
Corresponding authors. Tel.: +86 10 68913125; fax: +86 10 68912631 (W. Xu).
Tel.: +86 531 82769235; fax: +86 531 82765969 (B. Du).
E-mail addresses: xuwgujn@163.com (W. Xu), dubin61@gmail.com (B. Du).
http://dx.doi.org/10.1016/j.jallcom.2015.02.064
0925-8388/Ó 2015 Elsevier B.V. All rights reserved.

224
T. Yan et al. / Journal of Alloys and Compounds 634 (2015) 223–231
and a saturated calomel electrode as reference. The sample deposited ITO glass has
been used as the working electrode. The electrolyte used in the experiment was
0.5 M Na2SO4 solution.
However, by itself, BiOI is always poor in photocatalytic activity
[35,36]. In view of the fact that Bi2MoO6 and BiOI are similar in
structure, it is envisaged that a BiOI/Bi2MoO6 composite can have
the merits of Bi2MoO6 and BiOI, consequently showing high
photocatalytic activity under visible light.
2.5. Photocatalytic activity tests
Therefore, in this work, a novel hierarchical BiOI/Bi2MoO6 com-
posite was fabricated by a facile precipitation–deposition method.
The as-prepared composites exhibited an excellent photocatalytic
activity in the visible light irradiation due to good light absorption
capability and excellent charge separation characteristics of the
formed heterojunction between Bi2MoO6 and BiOI. The possible
photocatalytic mechanism of BiOI/Bi2MoO6 heterostructure to
organic pollutant degradation is proposed by adding radical scav-
engers in the photocatalytic system.
Photocatalytic reactions were conducted in a customized reactor with a cool-
ing-water-cycle system, and the reaction temperature of the aqueous solution
was maintained at 25 °C. The visible light photocatalytic activity of BiOI/Bi2MoO6
composites were evaluated by the degradation of MB in aqueous solution using a
300 W xenon lamp (PLS-SXE300C, Beijing Perfect Light Co. Ltd., Beijing) with a cut-
off filter (k > 420 nm) as light source. In each experiment, 100 mg of photocatalyst
was added into 100 mL dye aqueous solution (10 mg/L). Before visible light irra-
diation, the suspension was stirred 60 min for the dye and catalyst to reach adsorp-
tion–desorption equilibrium. During the reaction process under visible light
illumination, 3 mL of suspension was sampled at given time intervals and
centrifuged to remove the catalyst. The resulting clear liquor was analyzed on a
Perkin–Elmer UV–vis spectrophotometer (Model: Lambda 35) to record the concen-
tration changes of dye solutions. The percentage for dye degradation has been
reported as C/C0. C is the absorption intensity for the main spectrum peak of dye
at each irradiated time interval. And C0 is the absorption intensity of the starting
concentration when adsorption–desorption equilibrium was achieved.
2. Experimental
2.1. Chemicals
Bismuth nitrate hydrate (Bi(NO3)3ꢀ5H2O), ammonium molybdate tetrahydrate
((NH4)6Mo7O24ꢀ4H2O), potassium iodide (KI) were purchased from Shanghai Chemi-
cal Plant or Tianjin Chemical Plant. All the chemicals are analytical reagents and
used directly without further purification. Deionized water was used in all
experiments.
3. Results and discussion
3.1. Structural characterization
2.2. Synthesis of Bi2MoO6
Fig. 1 shows the XRD patterns of BiOI, Bi2MoO6, and BiOI/Bi2MoO6
composites with different mass ratios. As we can see from Fig. 1a,
four peaks located at 28.3°, 32.6°, 46.7°, and 55.4° have been
observed, which matched well with the (131), (002), (060), and
(331) crystal planes of Bi2MoO6 (JCPDS no. 84-0787). There is no
trace of any impurity phase under the present resolution, indicat-
ing the high purity of the as-prepared samples. For BiOI, all the
diffraction peaks in Fig. 1h could be indexed to the tetragonal
phase of BiOI (JCPDS no. 73-2062) [37]. For the BiOI/Bi2MoO6 com-
posites (Fig. 1b–g), characteristic peaks for BiOI and Bi2MoO6 are
all both observed. The sharp and intense diffraction peaks of BiOI
and Bi2MoO6 indicated that the samples are well crystallized.
Furthermore, it can be observed that with the increase of BiOI to
Bi2MoO6 mass ratio, the diffraction peaks intensity of Bi2MoO6
significantly reduced, and that of BiOI is accordingly increased. In
addition, no other impurity peaks can be found from the diffraction
patterns, indicating that the composites are only composed of BiOI
and Bi2MoO6.
The Bi2MoO6 samples were synthesized by
a facile solvothermal method.
Typically, 2 mM of Bi(NO3)3ꢀ5H2O was added into 20 mL of ethylene glycol solution
containing dissolved (NH4)6Mo7O24ꢀ4H2O with the equivalent molar ratio. Then,
50 mL ethanol was slowly added into the above solution, followed by ultrasonic
for 20 min to completely disperse. The resulting solution was transferred into a
100 mL Teflon-lined stainless steel autoclave and kept at 160 °C for 12 h.
Subsequently, the autoclave was cooled to room temperature gradually. Finally,
the precipitate was centrifuged, washed with ethanol and deionized water for three
times. The resulting product was dried in a vacuum oven at 60 °C for 6 h.
2.3. Preparation of BiOI/Bi2MoO6 composites
The BiOI/Bi2MoO6 composites with different BiOI contents were prepared by a
precipitation–deposition method. In a typical experiment, different stoichiometric
amounts of Bi(NO3)3ꢀ5H2O and KI were dissolved in 20 mL ethylene glycol to obtain
a clear solution A. The as-prepared Bi2MoO6 was ultrasonically dispersed into 80 mL
deionized water to form a homogeneous solution B. Then, the solution A was added
drop-wise into the solution B under strong magnetic stirring. After being stirred for
60 min at room temperature, the resulting mixtures were heated at 80 °C for 2 h in
an oil bath. Finally, the precipitates were collected, washed thoroughly with deion-
ized water and ethanol, and dried at 60 °C in air. According to this method, the BiOI/
Bi2MoO6 composites with different mass ratios of 10%, 15%, 20%, 25%, and 30% were
synthesized and named as BiOI/BMO-10, BiOI/BMO-15, BiOI/BMO-20, BiOI/BMO-25,
and BiOI/BMO-30, respectively. For comparison, pure BiOI was prepared by
adopting the method in the absence of Bi2MoO6. Pure Bi2MoO6 was named as
BMO. PM-BiOI/BMO-25 is the abbreviation for the BiOI/BMO-25 composite
prepared by physical mixing method with the same composition as BiOI/BMO-25
(25% BiOI and 75% Bi2MoO6 powders physical mixed without any treatment).
The morphologies of BiOI, Bi2MoO6, and BiOI/Bi2MoO6 compos-
ites were investigated by SEM. As shown in Fig. 2a and b, the pure
BiOI was consisted of large number of irregular plates with smooth
surfaces. Fig. 2c and d shows that the Bi2MoO6 exhibited micro-
sphere morphology with rough surfaces, and the microspheres
2.4. Characterization of photocatalysts
(g)
X-ray diffraction (XRD) patterns of the obtained products were recorded on a
Bruker D8 Advance X-ray diffractometer under the conditions of generator volt-
(f)
(e)
(d)
age = 40 kV; generator current = 40 mA; divergence slit = 1.0 mm; Cu
Ka
(k = 1.5406 Å); and polyethylene holder. The morphologies and microstructures of
the samples were examined with a Hitachi S-4800 scanning electron microscopy
(SEM). The transmission electron microscopy (TEM) and high-resolution transmis-
sion electron microscopy (HRTEM) images were measured by a JEOL model JEM
2010 EX instrument at an accelerating voltage of 200 kV. Carbon-coated copper grid
was used as the sample holder. X-ray photoelectron spectroscopy (XPS) measure-
(c)
(b)
(a)
ments were performed on a 2000 XPS system with a monochromatic Al Ka source
and a charge neutralizer. Energy-dispersive X-ray spectra (EDS) were obtained on a
JEOL-2010 at an accelerating voltage of 200 kV. Diffuse reflectance ultraviolet–
visible light spectra (DRS) were measured at room temperature in the range of
200–700 nm on a UV–vis spectrophotometer (Cary 500 Scan Spectrophotometers,
Varian, and USA) equipped with an integrating sphere attachment. The
electrochemical impedance spectroscopy (EIS) was conducted on electrochemical
15
30
45
60
75
2 Theta (degree)
workstation (CHI 760E Chenhua Instrument Company, Shanghai, China) in
a
Fig. 1. XRD patterns of (a) pure Bi2MoO6, (b) BiOI/BMO-10, (c) BiOI/BMO-15, (d)
conventional three-electrode configuration with a Pt wire as the counter electrode
BiOI/BMO-20, (e) BiOI/BMO-25, (f) BiOI/BMO-30, and (g) pure BiOI.

T. Yan et al. / Journal of Alloys and Compounds 634 (2015) 223–231
225
Fig. 2. SEM images of (a and b) pure BiOI, (c and d) pure Bi2MoO6, and (e and f) BiOI/BMO-25 composites.
were composed of nanosheets with several nanometers in
thickness. The cross-linking of nanosheets resulted in the
formation of microspheres, which could provide more active points
for the adsorption of organic pollutants and help to enhance the
photocatalytic activity of the pollutants decomposition. As for the
BiOI/BMO-25 composites (Fig. 2e and f), it can be observed that
the Bi2MoO6 microsphere were coated tightly with the BiOI
nanosheets, which presents irregularly shaped flaky aggregates
on the surfaces of Bi2MoO6 microsphere. The intimate interaction
of BiOI and Bi2MoO6 could enhances the electron transfer and
separation capacity during the photocatalysis process.
Fig. 3a and b shows the TEM and HRTEM images of BiOI/BMO-
25 composite. It can be seen that two sets of different lattice
images are observed with d spaces of 0.28 and 0.36 nm corre-
sponding to the (110) and (101) planes of tetragonal BiOI [38].
Another set of clear fringes with an interplaner spacing of
0.38 nm corresponds to the (111) lattice plane of Bi2MoO6. In such
a way, the BiOI and Bi2MoO6 would contact extensively meanwhile
the heterojunction interface between BiOI and Bi2MoO6 might be
efficiently generated. Furthermore, the chemical composition of
BiOI/BMO-25 composite was examined by EDX analysis (Fig. 3c).
It can be seen that only Bi, O, I and Mo elements existed in the sam-
ple, demonstrating that BiOI/BMO-25 composite was composed of
both BiOI and Bi2MoO6.
3.2. XPS analysis
XPS was used to investigate the chemical states of Bi, Mo, O and
I in the BiOI/BMO-25 composite. The binding energies obtained in
the XPS analysis were corrected for specimen charging by referenc-
ing the C 1s line to 284.6 eV. The survey XPS spectra (Fig. 4a) pro-
vided Bi, Mo and O peaks for BiOI, Bi2MoO6, and BiOI/BMO-25
composite, which was consistent with chemical composition of
the photocatalysts. Fig. 4b shows high resolution Mo 3d spectra
of the samples. The peaks corresponding to Mo 3d (Mo 3d5/2 and
Mo 3d3/2) in the spectra of BiOI/BMO-25 lightly shift (about
0.3 eV) toward higher binding energies as compared to pure
Bi2MoO6. Fig. 4c shows high resolution Bi 4f spectra of the samples.

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T. Yan et al. / Journal of Alloys and Compounds 634 (2015) 223–231
Cu
Elem Weight % Atomic %
Cu K
O K
I L
Bi L
Mo K
01.40
42.30
00.80
49.80
05.80
08.20
62.60
01.10
22.40
05.70
Bi
Mo
Bi
O
I
Mo
Bi
15
I
I
0
5
10
20
25
30
Energy (keV)
Fig. 3. TEM and HRTEM images of BiOI/BMO-25 composites and the corresponding EDX spectrum.
The binding energies of Bi 4f7/2 and Bi 4f5/2 peaks in Bi2MoO6 are
located at 156.5 and 161.8 eV, and that in BiOI are 157.5 eV and
162.8 eV. After the BiOI deposited on the surface of Bi2MoO6 micro-
sphere, the corresponding peaks of BiOI/BMO-25 shifted to 156.7
and 162.0 eV, respectively. Fig. 4d shows high-resolution I 3d spec-
tra of BiOI and BiOI/BMO-25. The two typical peaks in BiOI/BMO-
25 located at 616.6 eV and 628.1 eV could be ascribed to I 3d5/2
and I 3d3/2, respectively, which also slightly lower than those of
BiOI (617.6 eV and 629.0 eV). Fig. 4e shows the high-resolution
XPS spectra of O1s for BiOI, Bi2MoO6 and BiOI/BMO-25. The peak
of O 1s profile for BiOI/ZGO-30 is asymmetric and divides into
two peaks (shown in Fig. 4e inset). As we can see, the peaks at
527.5 eV and 528.5 eV are characteristic of O2ꢁ in Bi2MoO6 and
BiOI, respectively. In a word, binding energy shifts in the XPS spec-
tra could be ascribed to the strong interaction between BiOI and
Bi2MoO6 which possess different electronegativity, resulting in
the shift of valence electrons and the change of electric screening
of inner shells. The XPS spectra analysis reveals the existence of
electron transfer and chemical bonds between BiOI and Bi2MoO6
in the heterojunctions.
BiOI contents showed different absorption abilities compared with
pure Bi2MoO6. The absorption edges of BiOI/Bi2MoO6 composites
were gradually red shifted when increasing the amount of BiOI
added. However, all the BiOI/Bi2MoO6 composites exhibited good
visible light absorption ability.
3.4. Photocatalytic activity
The photocatalytic performances of the BiOI/Bi2MoO6 compos-
ites are compared with those of Bi2MoO6 and BiOI by photocat-
alytic dye degradation under visible light (k > 420 nm)
irradiation. MB was chosen as a representative model pollutant.
The temporal evolution of MB concentration are displayed in
Fig. 6. As we can see, the photolysis of MB is quite slow under
irradiation without photocatalysts. The photocatalytic activity of
the physical mixture one (PM-BiOI/BMO-25) is appreciably higher
than that of pure BiOI and Bi2MoO6, whereas all of the BiOI/Bi2MoO6
composites exhibited higher photocatalytic activity than the
physical mixture one. It suggests that there should be strong
interaction between BiOI nanoflakes and Bi2MoO6 microspheres
in the precipitation-deposition process, which plays crucial role
in the transfer and separation of photo-generated carriers.
Particularly, owing to the intimate interaction between BiOI and
Bi2MoO6, the Bi2MoO6 microspheres can provide a higher charge
carrier mobility throughout the longitudinal direction, so the
photo-generated electrons can easily transfer to the BiOI
nanoflakes moiety, leading to efficient separation and prolonged
recombination time of electron–hole pairs.
3.3. Optical characterization
The optical absorption properties played a critical role in deter-
mining the photocatalytic performance of photocatalysts. The DRS
spectra of BiOI, Bi2MoO6, and BiOI/Bi2MoO6 composites are shown
in Fig. 5. As we can see, the light absorption abilities of the samples
were different. The absorption edges of as-synthesized BiOI and
Bi2MoO6 are located at about 667 and 477 nm, and the band gaps
(Eg) were calculated to be 1.85 and 2.59 eV, respectively. It also can
be clearly seen that the BiOI/Bi2MoO6 composites with different
Fig. 7a shows the corresponding first-order kinetics plot by the
equation of ꢁln(C/C0) = kt, where C0 and C are the MB concentra-
tions in solution at times 0 and t, respectively, and k is the apparent

T. Yan et al. / Journal of Alloys and Compounds 634 (2015) 223–231
227
(a)
Bi 4f
BiOI
O 1s
BiOI/BMO-25
Mo 3d
Bi 5d
O 1s
Bi 4p
BMO
C 1s
0
200
400
600
800
1000
Energy (eV)
229.7 eV
230.0 eV
157.5 eV
Mo 3d
Bi 4f
(b)
(c)
Bi 4f7/2
Mo 3d5/2
162.8 eV
162.0 eV
161.8 eV
156.7 eV
156.5 eV
Bi 4f5/2
232.9 eV
Mo 3d3/2
233.2 eV
BiOI
BMO
BiOI/BMO-25
BMO
BiOI/BMO-25
225
150
155
160
165
220
230
235
240
Energy (eV)
Energy (eV)
617.6 eV
527.5 eV
I 3d
O 1s
(d)
(e)
O 1s
527.5 eV
I 3d5/2
BiOI/BMO-25
527.6 eV
629.0 eV
528.5 eV
528.5 eV
I 3d3/2
616.6 eV
525 526 527 528 529 530 531
Energy (eV)
628.1 eV
BiOI
BiOI
BiOI/BMO-25
BMO
BiOI/BMO-25
610
615
620
625
630
635
640
520
525
530
535
Energy (eV)
Energy (eV)
Fig. 4. (a) XPS spectra of BiOI, Bi2MoO6, and BiOI/BMO-25 composite, and high-resolution XPS spectra of (b) Bi 4f, (c) Mo 3d, (d) I 3d, and (e) O 1s.
first-order rate constant. Concretely, pure Bi2MoO6 shows poor
photocatalytic activity with a rate constant of 0.0087 minꢁ1
not increase obviously whereas the excessive BiOI could be acted
as the recombination center of electrons and holes. Thus, the
separation efficiency of electron–hole pairs decreased and further
inhibited the photocatalytic activity. Fig. 7b shows the time-de-
pendent optical absorption spectra of MB over BiOI/BMO-25. As
illustrated, with prolonged visible light irradiation time, the inten-
sities of characteristic absorption peaks of MB at 664 nm decrease
gradually, implying the decrease of MB concentration. It is
observed that no new absorption peaks appear during the degrada-
tion, indicating the structure of MB molecules is destroyed.
For comparison, the degradation of colorless BPA over BiOI,
Bi2MoO6, and BiOI/BMO-25 was also carried out to further
ascertain the visible light photocatalytic activity of BiOI/Bi2MoO6
composite. As shown in Fig. 8a, the BiOI/BMO-25 has an excellent
catalytic activity on the degradation of BPA than that over BiOI and
Bi2MoO6. Fig. 8b displays the temporal evolution of the absorption
.
Although the broader light absorption range, the pure BiOI only
exhibits a low degradation rate of 0.0028 minꢁ1 due to the fast
recombination of photon-generated electron and hole pairs. It is
worth emphasizing that BiOI content greatly influenced the rate
constant k. When the mass ratio of BiOI was 25%, the value of k
is 0.0781 minꢁ1, which reaches the maximum value. These results
could be understood through considering the synergetic effects of
light absorption and heterojunction structure of BiOI/Bi2MoO6
composite. As the BiOI content increased, the visible light absorp-
tion of BiOI/Bi2MoO6 strengthened and generated more electron–
hole pairs, meanwhile more heterojunction interface between BiOI
and Bi2MoO6 was formed, facilitating the transfer and separation of
electron–hole pairs. However, when the BiOI content was above
the optimal value, the efficient heterojunction interface would

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T. Yan et al. / Journal of Alloys and Compounds 634 (2015) 223–231
1.5
1.0
0.5
0.0
3.6. Separation efficiency of photo-generated electron–hole pairs
BiOI
BiOI/BMO-10
BiOI/BMO-15
BiOI/BMO-20
BiOI/BMO-25
BiOI/BMO-30
BMO
3.6.1. Photoluminescence spectra
Photoluminescence (PL) spectra analysis is a powerful tech-
nique to survey the separation efficiency of the photo-generated
electron–hole pairs in a semiconductor. The lower the PL intensity,
the smaller probability the photogenerated electron–hole pairs
recombination. Fig. 10 presents the PL spectra of Bi2MoO6 and
BiOI/BMO-25 with an excitation wavelength of 285 nm. The emis-
sion peak around 460 nm appears for Bi2MoO6, which may be
derived from the direct electron–hole recombination of band tran-
sition. However, the characteristic emission peak nearly disappears
for BiOI/BMO-25, indicating that the recombination of photo-gen-
erated charge carriers was inhibited greatly. The efficient charge
separation could increase the lifetime of charge carriers and
enhance the efficiency of interfacial charge transfer to adsorbed
substrates, and then improve the photocatalytic activity.
200
300
400
500
600
700
800
Wavelength (nm)
3.6.2. Electrochemical impedance spectroscopy
Fig. 5. UV–vis diffuse reflectance spectra of the as-prepared BiOI/Bi2MoO6
composites.
EIS measurement was also used to investigate the charge trans-
fer resistance and the separation efficiency between the photogen-
erated electrons and holes. Fig. 11 shows the EIS Nyquist plots of
Bi2MoO6, BiOI, and BiOI/BMO-25 composite. It can be seen that
the arc radius of BiOI/BMO-25 was smaller than that of Bi2MoO6
or BiOI, which meant a fast interfacial charge-transfer process
and effective separation of photo-generated electron–hole pairs.
This result indicated that the coupling of Bi2MoO6 with BiOI could
remarkably enhance the separation and transfer efficiency of
spectra of BPA solution degraded by BiOI/BMO-25. As we can see,
the intensity of absorption peaks at about 278 nm decreased
gradually and about 95% BPA was degraded after 4 h. Although
the degradation efficiency of BPA was lower than that of MB, the
BiOI/Bi2MoO6 composite still displayed its visible light photocatalytic
activity without the photosensitization effect. It is reasonable to
conclude that MB degradation over BiOI/Bi2MoO6 composite
mainly ascribed to the photocatalysis process of BiOI/Bi2MoO6
composite not the photosensitization effect of MB.
(a)
3.5. Regeneration and reusability
The regeneration, reuse, and stability are three major factors of
a photocatalyst to be used in the practical applications. Based on
this purpose, the BiOI/BMO-25 photocatalyst was regenerated by
centrifugation after degradation reaction and washed with water
3 times to eliminate contaminants on surface. Then the catalyst
was dried at 80 °C for 12 h and reused for another cycle under
the same conditions. The photocatalytic activity was found to be
nearly the same up to four cycles as a shown in Fig. 9, which
determined the stability of the photocatalysts.
1.0
0.8
0.6
(b)
λ
= 664 nm
0 min
10 min
20 min
30 min
40 min
50 min
60 min
0.4
0.2
0.0
0.6
No Cat.
BiOI
BMO
PM-BiOI/BMO-25
BiOI/BMO-10
BiOI/BMO-15
BiOI/BMO-20
BiOI/BMO-25
BiOI/BMO-30
0.4
0.2
0.0
300
400
500
600
700
0
20
40
60
Wavelength (nm)
Time (min)
Fig. 7. (a) Photocatalytic degradation rate constant k of MB as a function of BiOI
content under visible light irradiation. (b) Time-dependent optical absorption
spectra of MB over BiOI/BMO-25 composite.
Fig. 6. The photocatalytic activities of BiOI, Bi2MoO6, BiOI/Bi2MoO6 composites, and
PM-BiOI/BMO-25 for MB degradation under visible light irradiation.

T. Yan et al. / Journal of Alloys and Compounds 634 (2015) 223–231
229
1.0
0.8
0.6
0.4
0.2
0.0
Bi2MoO6
(a)
BiOI/BMO-25
Bi2MoO6
BiOI
BiOI/BMO-25
375
400
425
450
475
500
525
0
50
100
150
200
250
Wavelength (nm)
Time (min)
Fig. 10. The photoluminescence spectra of Bi2MoO6 and BiOI/BMO-25.
(b)
λ
= 278 nm
0 min
20 min
40 min
40
BiOI
Bi2MoO6
220 min
240 min
260 min
30
BiOI/BMO-25
20
10
0
240
260
280
300
320
Wavelength (nm)
Fig. 8. (a) Photodegradation activities of BPA (10 mg/L) over BiOI, Bi2MoO6 and
BiOI/BMO-25 and (b) UV–vis spectral changes of BPA over BiOI/BMO-25 composite
under visible light irradiation.
20
40
60
80
100
Z' (ohm)
1.0
0.8
0.6
0.4
0.2
0.0
Fig. 11. EIS profiles of Bi2MoO6, BiOI, and BiOI/BMO-25 composite under visible
light irradiation.
Therefore, different types of active species scavengers are added in
catalyst system. Fig. 12 shows MB degradation curves over BiOI/
BMO-25 under conditions such as air-equilibrated, N2-bubbled,
adding benzoquinone (BQ), adding ammonium oxalate (AO), and
adding isopropyl alcohol (IPA), respectively. As we can see, the
addition of BQ (1.0 mg) provokes a large degree of inhibition of
Å
MB degradation, which confirms the indispensable role of Oꢁ2 in
the degradation process. To further verify the decisive role of mole-
cular oxygen, N2 was bubbled into the suspension at the rate of
40 mL/min to ensure that the reaction was operated without O2.
As clearly shown, the degradation rate is obviously decreased,
0
50
100
150
200
250
Time (min)
Å
and undoubtedly demonstrating the important role of Oꢁ2 evolved
Fig. 9. Recycling test on the BiOI/BMO-25 composite for degradation of MB under
visible light irradiation.
by dissolved O2 in the reaction system. On the contrary, the photo-
catalytic activity could be greatly suppressed by the addition of a
scavenger for holes (1.0 mM AO). This result suggests that the pho-
to-generated holes are one of the main oxidative species in MB
photo-generated electron–hole pairs, which contributing to the
enhancement of photocatalytic activity.
Å
degradation. Furthermore, the addition of a scavenger of OH radi-
cals (1.0 mM IPA) only causes a small change in the photo-degra-
dation of MB, indicating that the MB oxidation is driven by the
Å
contribution of OH radicals to a lesser extent. It is concluded that
3.7. Detection of active species
the degradation of MB over BiOI/BMO-25 photocatalyst is driven
mainly by the participation of ÅOꢁ2 radicals and holes, and to a lesser
extent by the contribution of OH radicals.
It is important to detect the main oxidative species in the
photocatalytic process for revealing the photocatalytic mechanism.
Å

230
T. Yan et al. / Journal of Alloys and Compounds 634 (2015) 223–231
is the geometric mean of the electronegativity of the constituent
1.0
0.8
0.6
0.4
0.2
0.0
atoms (the electronegativity of an atom is the arithmetic mean of
the atomic electron affinity and the first ionization energy), Ee is
the energy of free electrons on the hydrogen scale (ꢂ4.5 eV). The
X values for Bi2MoO6 and BiOI were calculated to be 5.08 eV and
5.94 eV [39,40], respectively. Accordingly, the VB and CB of Bi2MoO6
were calculated to be +1.88 and ꢁ0.72 eV (vs. NHE), respectively.
Similarly, the VB and CB of BiOI were estimated to be +2.42 and
+0.57 eV (vs. NHE), respectively.
N2 bubbled
According to the previous reports, the energies of the photo-
generated carriers are approximately equal to that of the energy
bands they stay in [41]. After the migration of the photo-generated
charges, the E (h+) (BiOI, +2.42 V) is negative than E (ÅOH/H2O)
(+2.68 V). As a result, photo-induced holes cannot oxidize the
AO added
BQ added
IPA added
Air-equilibrated
Å
0
10
20
30
40
50
60
adsorbed H2O molecules to OH radicals in BiOI. Nevertheless, the
E (eꢁ) (BiOI, +0.57 V) is more positive than E (O2/ÅOꢁ2 ) (+0.13 V),
Time (min)
Å
which cannot produce Oꢁ2 radicals via the reduction of dissolved
O2 by CB electrons in BiOI, but the E (eꢁ) of Bi2MoO6 is lower than
E (O2/ÅOꢁ2 ), so the ÅOꢁ2 radicals can still be produced. Notably, we all
know that the ÅOH radicals could be generated not only from
photo-generated holes but also from photo-generated electrons
[42]. Therefore, the photo-degradation efficiency of MB just slight-
ly decreased in the presence of IPA (see Fig. 12), perhaps because a
small amount of ÅOH radicals produced by photo-generated
Fig. 12. Effects of different active species scavengers on the degradation of MB over
BiOI/BMO-25 under visible light irradiation.
3.8. The proposed photocatalytic mechanism
On the basis of the above results, a proposed mechanism is
discussed to explain the enhancement of the photocatalytic
properties of BiOI/Bi2MoO6 hierarchical structure (Scheme 1). The
BiOI/Bi2MoO6 photocatalyst is believed to exhibit cooperative or
synergic effects between BiOI nanosheets and Bi2MoO6 micro-
spheres. Upon irradiation by light with k > 420 nm, a high-energy
photon excites an electron from the valence band (VB) to the
conduction band (CB) of BiOI and Bi2MoO6. The photo-generated
electrons in Bi2MoO6 can move freely toward the surface of the
BiOI while the holes can transfer to the VB of Bi2MoO6 convenient-
ly. As a result, the photo-generated electrons and holes are
efficiently separated between BiOI and Bi2MoO6, thus improves
the photocatalytic activity.
Å
electrons via Oꢁ2 route.
4. Conclusions
In summary, this work provided a facile method for the prepa-
ration of hierarchical BiOI/Bi2MoO6 composites. The formation of
BiOI/Bi2MoO6 heterojunction was confirmed by XRD, SEM, TEM,
EDX, XPS, and HRTEM. The BiOI/BMO-25 composite exhibits the
highest efficiency towards MB degradation under visible light
irradiation. The photocatalytic performance enhancement mainly
attributed to the synergistic effect between Bi2MoO6 and BiOI.
The possible photocatalytic mechanism is also proposed. This
study demonstrates that the BiOI/Bi2MoO6 composites may be a
promising efficient visible light driven photocatalyst for the
treatment of organic pollutants.
The band edge position calculations for Bi2MoO6 and BiOI from
the absolute electronegativity follow the empirical formulas [39]:
EVB ¼ X ꢁ Ee þ 0:5Eg
ECB ¼ EVB ꢁ Eg
Acknowledgments
where EVB is the valence band (VB) edge potentials, ECB is the con-
duction band (CB) edge potential, Eg is the band gap energy of the
semiconductor, X is the electronegativity of the semiconductor that
This work was financially supported by the National Natural
Science Foundation of China (Nos. 21175057, 21375047, and
21377046), the Natural Science Foundation of Shandong Province
(No. ZR2014BL017), the Science and Technology Development Plan
of Shandong Province (No. 2014GSF120004), Scientific Research
Reward Fund for Excellent Young and Middle-Aged Scientists of
Shandong Province (No. BS2012HZ001), and Scientific Research
Foundation for Doctors of University of Jinan (Nos. XBS1037 and
XKY1321).
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