Binary-phase TiO2 modified Bi2MoO6 crystal for effective removal of antibiotics under visible light illumination

Article abstract of DOI:10.1016/j.materresbull.2019.01.002

A series of binary-phase TiO₂ modified Bi₂MoO₆ nanocrystals have been prepared via a solvothermal-calcination process. Trace TiO₂ modification can effectively enhance the visible light catalytic activity of Bi₂MoO₆ to remove the antibiotics in aqueous solution. The obtained TiO₂/Bi₂MoO₆ composites were investigated by some physicochemical techniques like XRD, N₂ adsorption, SEM, TEM, UV–vis DRS, Raman, XPS, PL and Photo-electrochemical measurement. The presence of TiO₂ nanoparticles (NPs) influenced the crystal growth of Bi₂MoO₆, decreasing the crystal size of Bi₂MoO₆ and effectively promoting its specific surface area. Moreover, the conduction band of TiO₂ can serve as the electron transfer platform, which largely boosts the effective separation of photocarriers at TiO₂/Bi₂MoO₆ heterojunction interface. With optimal TiO₂ content (0.41 wt%), TiO₂/Bi₂MoO₆ exhibited the best photocatalytic performance for different antibiotics degradation, e.g. ciprofloxacin, tetracycline and oxytetracyline hydrochloride under visible light irradiation. Moreover, the mechanism for enhanced photocatalytic performance in ciprofloxacin degradation was illuminated.

Full text of DOI:10.1016/j.materresbull.2019.01.002

Materials Research Bulletin 112 (2019) 336–345
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Binary-phase TiO modified Bi MoO crystal for effective removal of
2
2
6
antibiotics under visible light illumination
Zhen Liua,b, Jian Tian , Debing Zeng , Changlin Yu
b
b
a,b,⁎
, Weiya Huang , Kai Yang ,
b
b
Xingqiang Liu , Hong Liud
c,⁎⁎
a
Faculty of Environmental Science and Engineering, Key Laboratory of Petrochemical Pollution Process and Control, Guangdong University of Petrochemical Technology,
Maoming, 525000, Guangdong, China
School of Metallurgy and Chemical Engineering, Jiangxi University of Science and Technology, Ganzhou, 341000, Jiangxi, China
Department of Environmental Science and Engineering, Tan Kah Kee College Xiamen University, Zhangzhou, 363105, China
Key Laboratory for Water Quality and Conservation of the Pearl River Delta, Ministry of Education, Institute of Environmental Research at Greater Bay, Guangzhou
b
c
d
University, Guangzhou, 510006, Guangdong, China
A R T I C L E I N F O
A B S T R A C T
Keywords:
2 2 6
A series of binary-phase TiO modified Bi MoO nanocrystals have been prepared via a solvothermal-calcination
Binary-phase TiO
Bi MoO
Antibiotics
Photocatalytic degradation
Mechanism
2
2 6
process. Trace TiO2 modification can effectively enhance the visible light catalytic activity of Bi MoO to remove
2
6
the antibiotics in aqueous solution. The obtained TiO
cochemical techniques like XRD, N adsorption, SEM, TEM, UV–vis DRS, Raman, XPS, PL and Photo-electro-
chemical measurement. The presence of TiO nanoparticles (NPs) influenced the crystal growth of Bi MoO
decreasing the crystal size of Bi MoO and effectively promoting its specific surface area. Moreover, the con-
duction band of TiO can serve as the electron transfer platform, which largely boosts the effective separation of
photocarriers at TiO /Bi MoO heterojunction interface. With optimal TiO content (0.41 wt%), TiO /Bi MoO
6
2 2 6
/Bi MoO composites were investigated by some physi-
2
2
2
6
,
2
6
2
2
2
6
2
2
2
exhibited the best photocatalytic performance for different antibiotics degradation, e.g. ciprofloxacin, tetra-
cycline and oxytetracyline hydrochloride under visible light irradiation. Moreover, the mechanism for enhanced
photocatalytic performance in ciprofloxacin degradation was illuminated.
1. Introduction
2
nanosized TiO powder can decompose polychlorinated biphenyls
under ultraviolet light irradiation. Unfortunately, ultraviolet light only
Antibiotics are the antibacterial drugs which can kill or inhibit the
accounts for 3–5% of the whole solar energy [12]. In consideration of
growth of bacteria. As a major discovery in medical development, an-
tibiotics have made great contribution for human being. However, the
discharged antibiotics are a grave threat to human health and aquatic
life. The typical antibiotics include ciprofloxacin (CPFX) [1,2], tetra-
cycline (TC) [3,4] and oxytetracyline hydrochloride (OTTCH) [5,6].
When antibiotics absorbed by the body, more than 90% are discharged
into the environment by excretory. The discharged antibiotics further
flow into water resources. Most of antibiotics cannot be biologically
degraded or eliminated in common waste water treatment plants be-
cause of their high stability. Therefore, considerable efforts have been
committed to developing various techniques for degradation of anti-
biotics, e.g. adsorption, microorganism decomposition and advanced
oxidation technology [7–10]. In 1976, Carey et al. [11] first found that
2
TiO cannot utilize the sunlight effectively, the visible-light-driven
photocatalysts are highly desired.
4 2 6
Recent years, the ternary bismuth oxides, e.g. BiVO , Bi WO , and
Bi
gap energy. Bi
structure (the [Bi
2
MoO
6
have attracted much research interests due to their ideal band
2
MoO belongs to Aurivillius-related oxide with layer
6
2+
2−
2
O
2
]
layers located in the middle between MoO
4
slabs [13,14]). This unique structure is conducive to the formation of an
internal electric field, which benefits the effective separation of car-
riers.
2 6
However, bare Bi MoO always displays low photocatalytic activity
and it is hard to meet the practical requirement [15]. The construction
of heterojunction is an effective method to enhance the transfer of
photogenerated carriers and boost the photocatalytic activity. For
⁎
Corresponding author at: Faculty of Environmental Science and Engineering, Guangdong University of Petrochemical Technology, Maoming, 525000,
Guangdong, China.
⁎
⁎
Corresponding author.
E-mail addresses: yuchanglinjx@163.com (C. Yu), xqliu@xujc.com (X. Liu).
https://doi.org/10.1016/j.materresbull.2019.01.002
Received 12 September 2018; Received in revised form 15 December 2018; Accepted 3 January 2019
Available online 04 January 2019
0025-5408/ © 2019 Elsevier Ltd. All rights reserved.

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
2 6 2 4
example, in Bi MoO @Ag MoO core-shell heterojunction, the separa-
tion efficiency of carriers is improved. In addition, surface plasmon
resonance of Ag nanoparticles (NPs) at the interface between the
Bi
the CB of Bi
junction possessed highly oxidation and reduction capability [17].
Moreover, a number of other Bi MoO -based composites were reported,
e.g. Bi [18,19], CdS [20], α-Fe [21,22], g-C [23,24] and so
on. Zhang et al. [25] first fabricated one-dimensional anatase TiO
nanofibers by electrospining technique, and then one-dimensional
2
MoO
6
and Ag
2
MoO
4
will enhance the transform of electrons from
2
MoO
6
[16]. Besides, Z-scheme BiOBr-Bi
2
MoO hetero-
6
2
6
2
O
3
2
O
3
3 4
N
2
Bi
2
MoO
6
/TiO
2
hierarchical heterostructures were fabricated. The ob-
/TiO heterostructures exhibited high visible light
tained Bi
2
MoO
6
2
photocatalytic performance for the decomposition of RhB under UV
light and visible light illumination (λ > 420 nm). Tian et al. [26]
employed anatase TiO
Bi MoO /TiO
the Bi MoO /TiO
et al. [27] prepared flake-like Bi
2
nanobelts as template to fabricate the 3D
2
6
2
heterostructures. Under UV and visible light irradiation,
composite exhibited high oxygen production. Li
MoO on anatase TiO nanofiber, and
2
6
2
2
6
2
the produced composites showed the enhanced photocatalytic activity
for the degradation of MB.
2 2 6 2 2 6
Fig. 1. XRD patterns of the prepared TiO , Bi MoO and TiO /Bi MoO sam-
ples.
In this work, we developed a series of binary-phase TiO
MoO crystals by solvothermal-calcination method. The effects of
TiO on the texture, crystal, band structure, and optical property of the
Bi MoO were investigated. The outstanding photocatalytic perfor-
mances over TiO /Bi MoO composites for removing the antibiotics
tetracycline, ciprofloxacin, and oxytetracycline hydrochloride) in
water were reported.
2
modified
Bi
2
6
2.2. Characterization
2
2
6
Powder X-ray diffraction data were recorded at a scanning rate of
.05°/s using a Bruker D8-advance X-ray diffractometer at 40 kV and
0 mA for Cu Kα radiation (λ = 0.15418 nm). The Brunauer-Emmett-
2
2
6
0
4
(
2
Teller (BET) surface areas of the sample were obtained from N ad-
sorption/desorption isotherms determined at liquid nitrogen tempera-
ture on an automatic analyzer (Micromeritics, ASAP 2010). The sam-
ples were outgassed for 2 h under vacuum at 180 °C prior to adsorption.
Scanning electron microscopy (SEM) of sample was measured on
FLA650 F microscope (the FEI Company). Transmission electron mi-
croscopy (TEM) and high-resolution transmission electron microscopy
2
. Experimental
.1. Catalyst preparation
4 6 7 2
Bi(NO ·5H O (99%), (NH ) Mo O24·4H O, ethylene glycol (EG),
2
3
)
3
2
(
HRTEM) images were recorded on a Tecnai 20 FEG microscope.
titanium trichloride (15% hydrochloric acid), cetyltrimethylammonium
bromide (CTAB), n-pentanol, n-hexane, were analytical grade and
purchased from Sinopharm Chemical Reagent Co. Ltd. China.
Tetracycline (TC), ciprofloxacin (CPFX), and oxytetracycline hydro-
chloride (OTTCH) were purchased from Shanghai Macklin
Biochemistry Technology Co., Ltd.
UV–vis diffuse reflectance spectra (DRS) were measured using a UV–vis
spectrophotometer (UV-2600, Shimadzu). Fourier transform infrared
(
FT-IR) spectra were recorded with a Nicolet 5700 FT-IR spectrometer.
The surface composition was determined by X-ray photoelectron spec-
troscopy (XPS) using a PHI Quantum 2000 XPS system with a mono-
chromatic Al Ka source and a charge neutralizer. All the binding en-
ergies were referenced to the C1 s peak at 284.8 eV of the surface
adventitious carbon. The photoluminescence (PL) emission spectra of
the samples were recorded on a fluorescence spectrometer (Hitachi F-
2
The synthesis of TiO is in a W/O emulsion system. The detailed
experimental procedure is from the literature [28]. Briefly, 5.8 g CTAB
was dissolved in a mixture of 10 mL n-pentanol, 60 mL n-hexane and
2
10 mL H O. After stirring for 30 min, 0.8 mL titanium (III) chloride was
4
500, Japan). Photocurrent and electrochemical impedance spectro-
scopy measurements were carried out on an electrochemical work-
station with three-electrode (CHI-660E, China). 0.1 M Na SO solution
added. The obtained microemulsion was poured into a 100 mL Teflon-
lined stainless-steel autoclave and maintained at 200 °C for 6 h. After
cooling down to room temperature, the resultant precipitation was
centrifuged and washed with water, acetone and ethanol to remove
surfactants. The collected sample was dried at 60 °C for 8 h.
2
4
was used as electrolyte solution. Saturated Ag/AgCl and platinum wires
were utilized as reference electrodes and the counter electrode, re-
spectively. The working electrode is the sample films coated on indium
tin oxide (ITO) conducting glass. The homogeneous mixtures of 1 mL
ethanol and 10 mg samples were coated over ITO and dried at 100 °C
for 5 h. A 300 W xenon lamp was utilized as the excitation light source.
Bi
vothermal- calcination process [29]. 1.94 g Bi(NO
solved in 70 mL EG under sonication. Under magnetically stirring,
.35 g (NH Mo 24·4H O was added to above solution. 20 mg TiO
was dispersed in 40 mL EG under ultrasound irradiation. Then desired
volume (0, 6, 10, 15 and 20 mL) of TiO suspension solution was added
2
MoO
6
2
and TiO /Bi
2
MoO
6
samples were prepared via a sol-
3
)
3
·5H O was dis-
2
0
4
)
6
7
O
2
2
2
2.3. Degradation of organic pollutants
in above-mentioned solution and the pH value of the solution was ad-
justed to 7. After stirring for 30 min, the solution was placed in a Teflon-
lined stainless autoclave and maintained at 160 °C for 10 h. The ob-
tained product was centrifugal collection. After washing three times by
water and ethanol, the obtained samples were dried at 60 °C for 10 h.
Finally, the power was placed in a muffle furnace and calcined at 400 °C
The photodegradation experiments were operated in a photo-
chemical reactor. The simulated organic pollutants are CPFX, TC, and
OTTCH in an aqueous solution, respectively. The visible light source
was a 350 W Xe lamp (XuJiang electromechanical XPA-7) with a UV
Cut-off filter (UVCUT420). Briefly, 30 mg catalyst was added into CPFX
(50 mL, 10 mg/L), TC (50 mL, 20 mg/L), OTTCH (50 mL, 20 mg/L) so-
lutions, respectively. The suspensions were magnetically stirred in the
dark for 40 min to reach adsorption-desorption equilibrium before
turning on the light. ˜3 mL suspension was sampled and centrifuged at
given time intervals to measure the changes of the pollutants
for 2 h. The final samples were denoted as TiO
x = 0, 0.16, 0.27, 0.41, 0.55). The content of TiO
composite was obtained by adding the volume of TiO
2
(x wt%)/Bi
2
MoO
MoO
suspension.
6
(
2
in TiO /Bi
2
2
6
2
337

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
2 6 2 2 6
Fig. 2. SEM images of the typical samples. (a) Bi MoO ; (b) TiO (0.41 wt%)/ Bi MoO .
2 2 6 2 2 6
Fig. 3. (a) TEM, (b) and (c) HR-TEM images of TiO (0.41 wt%)/Bi MoO ; (d)The EDX spectrum of TiO (0.41 wt%)/Bi MoO .
concentration during light irradiation. The concentration of the anti-
biotics was measured with a UV–vis spectrophotometer at their max-
imum absorption wavelength (270 nm for CPFX, 358 nm for TC, 355 nm
for OTTCH). The degradation rate was calculated according to the
following equation:
Degradation rate (%) = (1-C
t 0 t 0
/C )*100% = (1-A /A )*100%
Where C and C represent the concentrations of target pollutant at ir-
0
t
t 0
radiation times 0 and t, respectively; A and A are the corresponding
values for target pollutant absorbance.
338

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
assigned to the crystal plane of (131), (200), (260) and (133) for or-
thorhombic Bi
terns of TiO , some typical diffraction peaks at 2θ = 27.4°, 36.0°, 41.2°
and 54.2° can be observed, which are ascribed to the (110), (101), (111)
and (211) crystal facets of rutile TiO (JCPDS No. 76-0318), respec-
tively. Other typical diffraction peaks at 2θ = 25.3°, 37.9° and 48.1° are
assigned to the crystal planes of (101), (004) and (200) for anatase TiO
JCPDS No. 21-1272), respectively. However, no diffraction peak for
2 6
MoO (JCPDS No. 76-2388), respectively. For the pat-
2
2
2
(
TiO
2
component in TiO
2
/Bi
2
MoO
6
was detected, which could be caused
by the low content of TiO
2
.
3.2. Morphologic structure
SEM and TEM are applied to analyze the morphology of the pure
MoO and TiO (0.41 wt%)/Bi MoO . As shown in Fig. 2, Bi MoO
(0.41 wt%)/Bi MoO are composed of sheets or particles with
irregular morphology. In Fig. 2(a), some nanochips were observed in
Bi MoO sample and the inset displays a single disk with the thickness
of ˜40 nm. In Fig. 2(b), for the TiO (0.41 wt%)/Bi MoO , the size of
Bi MoO slightly decreased after the introduction of TiO
Fig. 3(a) and (b) show the TEM and HRTEM images recorded from
Bi
and TiO
2
6
2
2
6
2
6
2
2
6
2
6
2
2
6
2
6
2
.
Fig. 4. N
pore size distribution plots (inset) of the Bi
)/Bi MoO
2
adsorption-desorption isotherms and Barret-Joyner-Halenda (BJH)
2
MoO
6
2
and TiO (0.41 wt
TiO
TiO
2
(0.41 wt%)/Bi
(0.41 wt%)/Bi
2
MoO
MoO
6
. Fig. 3(a) reveals that the morphology of
is irregular and the diameter of the Bi MoO
%
2
6
.
2
2
6
2
6
particle is about 20 nm. Fig. 3(b) reveals the lattice fringes of 0.315 nm
and 0.326 nm deemed to the (131) and (041) planes of Bi MoO (JCPDS
No. 76-2388), respectively. The lattice of 0.325 nm was ascribed to the
(110) plane of rutile TiO (JCPDS No. 76-0318) and the lattice of
.351 nm was indexed to the (101) plane of anatase TiO (JCPDS No.
1-1272). The presence of different phase planes further proved that
MoO , anatase and rutile TiO coexist in the com-
Table 1
2
6
Specific surface area of samples.
2
Sample
S
BET(m /g)
2
0
2
2
Bi
2
MoO
6
11.3
16.5
17.6
17.7
20.6
TiO
TiO
TiO
TiO
2
2
2
2
(0.16 wt%)/Bi
2
2
2
2
MoO
MoO
MoO
MoO
6
6
6
6
three phases of Bi
2
6
2
(0.27 wt%)/Bi
(0.41 wt%)/Bi
(0.55 wt%)/Bi
posite. The interface between the different phases could facilitate the
transfer of charges in the bulk phase [30]. Fig. 3(c) shows that the rutile
TiO
2 2
diameters are about ˜12.4 nm and the anatase TiO appears short
rod-like morphology, with the diameter and length of 6.1 nm and
19.1 nm, respectively.
2 2 6
The EDX spectrum of TiO (0.41 wt%)/Bi MoO is shown in
Fig. 3(d). Obviously, there are Bi, Mo, O, Ti and Cu elements. The Bi,
Mo and O elements come from Bi MoO , and Ti element comes from
TiO , while Cu element was attributed to the tested copper net.
2
6
2
3.3. Texture properties
The pore-size distributions (inset) and N
2
adsorption-desorption
MoO are shown in
isotherms of the Bi MoO and TiO (0.41 wt%)/Bi
2
6
2
2
6
Fig. 4. According to the classification of Brunauer-Deming-Deming-
Teller (BDDT) in IUPAC, the isotherms of the two samples can be
classified as type-IV, which indicates the characteristic of mesoporous
0
materials [30,31]. The hysteresis loops at relative pressure (P/P ) range
between 0.6 and 1.0 can be classified as type H3, associated with slit-
like pores [32,33]. The pore-size distributions (inset in Fig. 4) of the
samples are broad (from 2 to 70 nm), further indicating the presence of
mesopores and macropores. TiO
dlogD than Bi MoO , indicating a high probability of mesoporous dis-
tribution. Table 1 gives the specific surface area of all samples. The
result displays that the specific surface area of Bi MoO increased with
the increase of TiO content. The reason could be that the introduction
of TiO decreased the particle size of Bi MoO
2 2 6
(0.41 wt%)/Bi MoO has a larger dV/
2
6
Fig. 5. Raman spectra of Bi
2
MoO
6
and TiO
2
(0.41 wt%)/Bi
2
MoO
6
;
2
6
Magnification spectrum of TiO
2
(0.41 wt%)/Bi
2
MoO in the range of 118 ˜ 169
6
2
cm- (inset).
1
2
2
6
.
3
.4. Raman analysis
3
. Results and discussion
.1. Crystal properties
The crystal structure of the samples was analyzed by XRD and the
Raman spectra of TiO
2 2 6 2 2 6
, Bi MoO and TiO (0.41 wt%)/Bi MoO are
3
shown in Fig. 5. The spectra show a Raman vibraton in the range of
2
6
rutile phase [34–36]. The other two bands at 512 and 634 cm
observed in the spectra of anatase phase [37,38]. The Raman peaks of
−
1
00–1000 cm . The Raman band of TiO
2
lines at 143, 445, and
−
1
07 cm
are in agreement with the data observed in the spectra of a
obtained XRD patterns were displayed in Fig. 1(a). The pattern shows
that the diffraction peaks at 2θ of 28.3°, 32.5°, 47.3° and 55.5° are
−1
are
339

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
2 2 6
Fig. 6. XPS spectra of TiO (0.41 wt%)/Bi MoO sample. (a) Survey spectrum; (b) Bi 4f; (c) Mo 3d; (d) O 1s; (e) Ti 2p & Bi 4d.
−
1
Bi
Mo-O bands [39]. The presence of modes below 400 cm
tributed to both Bi-O stretches and lattice modes [40]. Comparing to
2
MoO
6
above 600 cm
can be assigned to the stretching modes of
simple exponential form [42]:
Mo–O = 0.48239 ln(32895/v)
−1
can be at-
R
(1)
−1
the Raman peaks of TiO
2
in the range from 118˜169 cm , the peak of
46 cm can be seen, indicating the existence of TiO . The remaining
peaks of TiO disappear because of weak peaks or low content. In ad-
dition, TiO (0.41 wt%)/Bi MoO and Bi MoO have a similar Raman
vibraton in the range of 200–1000 cm
Raman spectra of Bi MoO gives a very strong band at 796 cm
along with two shoulder bands at 713 cm
Where v is the Raman stretching frequency in wavenumbers and R is the
metal-oxygen bond length in angstroms [43]. The results show that the
lower frequency of Raman stretching band corresponds to the longer
−1
1
2
2
2
2
6
2
6
2 6
bond length. A calculation of the Mo-O bond length for Bi MoO based
−
1
.
on the expression suggests that one of the Mo-O (apical) bond length
−1
−1
2
6
(796 cm ) decreased from 1.7952 Å to 1.7838 Å in the TiO (0.41 wt
2
−
1
−1
and 845 cm , and the
g mode, corresponding to
Mo-O stretching modes of the distorted MoO octahedral [41]. Com-
pared to pure Bi MoO , the characteristic peaks about Bi MoO in the
TiO (0.41 wt%)/Bi MoO have an obvious shift. The peaks of the
stretching modes of Mo-O bands at 713, 796 and 845 cm were shifted
%)/Bi MoO composite. The gradual decrease in the bond length of the
2
6
−
1
strong band at 796 cm
is presumably A
1
Mo-O distance suggested the strong chemical interaction between
Bi MoO6 and TiO2.
2
6
2
6
2
6
2
2
6
3.5. XPS analysis
−
1
−
1
to 731, 814 and 863 cm , respectively. The relationship between
Raman stretching frequency and bond length was found to follow a
2
The element composition and chemical state of TiO (0.41 wt
%
2 6
)/Bi MoO were analyzed by XPS. Fig. 6(a) shows the XPS survey
340

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
2−
2
32.4 eV can be ascribed to Mo 3d5/2 and Mo 3d7/2 of MoO
4
, re-
spectively [45]. Fig. 6(d) shows the O1 s peak with band energy at
529.8 eV. This peak can be fitted into three peaks around 529.63,
530.24 and 531.68 eV, corresponding to the Bi-O, Mo-O and Ti-O bonds
[
46,47], respectively. This result confirmed the existence of TiO
2
over
the surface of Bi
2
MoO
6
. Fig. 6(e) implies an overlap XPS spectra over Ti
2
4
TiO
p and Bi 4d. The peak at 464.15 eV is for Ti 2p1/2 [48,49]; the peak at
59.35 eV (2p3/2) is for anatase TiO and 458.5 eV (2p3/2) is for rutile
[50]. The peak at 465.85 eV is for Bi 4d [47].
2
2
3.6. Light absorption
The UV–vis diffuse spectra of Bi
sites are presented in Fig. 7. Compared with bare Bi
Bi MoO
2
MoO
6
and TiO
2
/Bi
2
MoO
MoO
6
composites show almost the same absorption edge (Fig. 7) at
6
compo-
2
6
, TiO /
2
2
about 480 nm, indicating they have similar light absorption in the
visible region. Besides, the light absorption of the composite samples in
the UV region slightly strengthened. Fig. 7 provides the band gap en-
ergies for the present series of catalysts, which was determined with
Tauc’s law from the intercept of a straight line fitted through the rise of
2
Fig. 7. UV–vis absorption spectra; The curves of (αhv) versus hv of the pre-
pared Bi MoO and TiO /Bi MoO samples (inset).
2
6
2
2
6
the function [F(R
Munk function and hv is the energy of the incident photon [47,15]. The
approximate band gap energies are ˜2.59 eV for Bi MoO and
)hv]2 plotted versus hv, where F(R
) is a Kubelka-
a a
2
6
2
.60–2.68 eV for TiO
.7. Charge separation analysis
The photocatalytic activity of a semiconductor material is in-
2 2 6
/Bi MoO , respectively.
3
timately associated with the charge carrier density, bulk charge se-
paration, movement on the surface of material and interfacial transfer
of photogenerated charges. The PL spectra of Bi
Bi MoO composites are displayed in Fig. 8. The fluorescence intensity
at 470 nm was monitored upon excitation at 350 nm. The PL spectra of
TiO (0.41 wt%)/Bi MoO gives the lowest intensity, indicating the
lowest recombination rate of charge carrier [51]. TiO /Bi MoO has the
low recombination rate of charge carrier compared to Bi MoO . It could
be attributed to the TiO /Bi MoO interface which could transfer the
2 6 2
MoO and TiO /
2
6
2
2
6
2
2
6
2
6
2
2
6
photoelectron, thus improving the separation of charge carries.
Photo-electrochemical measurements are used to explore the in-
terface charge separation efficiency. Fig. 9(a) presents the photocurrent
2 6 2 2 6
Fig. 8. PL spectra of Bi MoO and TiO /Bi MoO .
2 6 2 2 6
responses of Bi MoO and TiO /Bi MoO . Generally, a high photo-
spectrum of Bi 4f, Mo 3d, Ti 2p and O 1s in TiO
2
(0.41 wt%)/Bi
2
MoO
6
.
current density demonstrates a high separation of photo-induce elec-
tron and hole [52]. Obviously, the photocurrent density increased in
The C 1s peak is the element carbon from XPS instrument. Fig. 6(b)
gives two strong peaks at 164.6 and 159.3 eV corresponding to Bi 4f5/2
and Bi 4f7/2, respectively [44]. In Fig. 6(c), the peaks around 235.5 and
the order of Bi
%)/Bi MoO < TiO
2
MoO
6
< TiO
2
(0.16 wt%)/Bi
2 6 2
MoO < TiO (0.27 wt
2
6
2
(0.41 wt%) /Bi
2
MoO . This conclusion is con-
6
sistent with PL results.
Fig. 9. Photo-electrochemical measurements of the prepared samples. (a) Transient photocurrent responses; (b) Nyguist plots of electrochemical impedance spec-
troscopy.
341

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
Fig. 10. (a) Absorption ability evaluation of the as-prepared samples for CPFX under dark condition; (b)Photocatalytic degradation CPFX curves; (c) Changes of
UV–Vis spectra as a function of time; First-order kinetic plots (d) and rate constants (e) of CPFX degradation in the presence of the samples.
Electrochemical impedance spectra (EIS) were used to analyze the
charge transfer and the recombination behaviors of the prepared
3.8. Photocatalytic activity
sample. Fig. 9(b) shows the Nyquist plots of Bi
Bi MoO composites. The arc radius of TiO /Bi MoO
that of Bi MoO and TiO (0.41 wt%)/Bi MoO displays the smallest arc
radius, witnessing the highest efficiency of charge transfer on the sur-
face of TiO /Bi MoO . The results of PL, photocurrent and EIS test
confirmed that the TiO /Bi MoO photocatalyst has efficient charge
2
MoO
6
and TiO
2
/
The phtocatalytic activity of the samples was tested by the de-
gradation of CPFX, TC and OTTCH under visible-light (λ ≥ 420 nm)
irradiation. The adsorption capability of the samples for CPFX were
shown in Fig. 10(a). Obviously, the adsorption capability of the com-
2
6
2
2
6
is smaller than
2
6
2
2
6
2
2
6
posites increases with the increase of TiO content. The increase of
2
adsorption is due to the introduction of TiO , which reduced the par-
2
2
2
6
transfer and a low recombination rate of photo-generated electrons and
holes.
ticle size and increase the specific surface area. As shown in Fig. 10(b),
blank test confirmed that CPFX cannot destroyed without the catalyst
342

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
2 6 2 2 6
Fig. 11. Photocatalytic degradation of OTTCH (a) and TC (b) over Bi MoO and TiO /Bi MoO samples.
TiO
2
(0.16 wt%)/Bi
2
MoO
6
,
TiO
2
(0.27 wt%)/Bi
2
MoO
6
,
2
TiO (0.41 wt
%
)/Bi
2
MoO and TiO
6
2
(0.55 wt%)/Bi
2
MoO , respectively. The time-de-
6
pendent UV–vis spectral of CPFX was presented in Fig. 10(c). The in-
tensity of characteristic peak decreased with the increasing of irradia-
tion time and no other peaks appeared. Moreover, the kinetics of the
degradation reaction was investigated by applying the Langmuir-Hin-
0 t
shelwood (L-H) model [53,54]. After fitting, the plots of ln(C /C )
versus light irradiation time were found to be linear as displayed in
Fig. 10(d), suggesting the reaction followed the pseudo-first-order
model.
The apparent reaction rate constants (k) were displayed in
Fig. 10(e). The reaction rate constants for different samples appear as
following
order:
> TiO (0.27 wt%)/Bi
> Bi MoO
is about 2.65 times of the pure Bi
TiO
2
(0.41 wt%)/Bi
2
MoO
6
> TiO
2
(0.55 wt
%)/Bi
%)/Bi
%)/Bi
2
2
2
MoO
MoO
MoO
6
6
6
2
2
MoO > TiO
6
2
(0.16 wt
2
6
. The reaction rate constant of TiO
MoO6.
2
(0.41 wt
2
In addition to the degradation of CPFX, catalysts were also used to
Fig. 12. Photogenerated carrier trapping in the system of photodegradation of
CPFX over TiO
2
(0.41 wt%)/Bi
2
MoO
6
.
eliminate the antibiotic TC and OTTCH. Fig. 11(a) and (b) shows the
concentration variation of OTTCH and TC during the reaction process.
Obviously, the decomposition of OTTCH and TC without catalyst can be
under visible light irradiation. TiO
significantly activity than bare Bi MoO
activity of Bi MoO enhanced with the TiO
The degradation rate of CPFX is 54% over pure Bi
responding degradation rates are 77%, 82%, 88% and 84% for
2
/Bi
, indicating the photocatalytic
introduced.
6
MoO . The cor-
2 6
MoO composites exhibited
neglected. In Fig. 11(a), the OTTCH degradation rate over TiO
2
(0.41 wt
2
6
%
)/Bi MoO (77.6%) is much higher than that of Bi MoO (49.8%). A
2
6
2
6
2
6
2
similar phenomenon was observed for TC degradation in Fig. 11(b).
In order to monitor the reactive radicals involved in reaction, three
quenchers, nitrogen (N ), triethanolamine (TEOA, 1 mM) and tertbutyl
2
2
2 2 6
Fig. 13. The suggested photocatalytic degradation mechanism of organic pollutants over TiO /Bi MoO system.
343

Z. Liu et al.
Materials Research Bulletin 112 (2019) 336–345
%
−, h+ and
OH, respectively [55,56]. In Fig. 12, the trapping experiments in-
dicated that with the addition of N and TBA, the activity decreased
alcohol (TBA, 1 mM) were utilized as the scavengers of O
2
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