Fabrication of Z-scheme MoO3/Bi2O4 heterojunction photocatalyst with enhanced photocatalytic performance under visible light irradiation

Article abstract of DOI:10.1016/S1872-2067(19)63391-7

Constructing Z-scheme heterojunction to improve the separation efficiency of photogenerated carriers of photocatalysts has gained extensive attention. In this work, we fabricated a novel Z-scheme MoO₃/Bi₂O₄ heterojunction photocatalyst by a hydrothermal method. XPS analysis results indicated that strong interaction between MoO₃ and Bi₂O₄ is generated, which contributes to charge transfer and separation of the photogenerated carriers. This was confirmed by photoluminescence (PL) and electrochemical impedance spectroscopy (EIS) tests. The photocatalytic performance of the as-synthesized photocatalysts was evaluated by degrading rhodamine B (RhB) in aqueous solution under visible light irradiation, showing that 15% MoO₃/Bi₂O₄ (15-MB) composite exhibited the highest photocatalytic activity, which is 2 times higher than that of Bi₂O₄. Besides, the heterojunction photocatalyst can keep good photocatalytic activity and stability after five recycles. Trapping experiments demonstrated that the dominant active radicals in photocatalytic reactions are superoxide radical (?O₂^?) and holes (h⁺), indicating that the 15-MB composite is a Z-scheme photocatalyst. Finally, the mechanism of the Z-scheme MoO₃/Bi₂O₄ composite for photo-degrading RhB in aqueous solution is proposed. This work provides a promising strategy for designing Bi-based Z-scheme heterojunction photocatalysts for highly efficient removal of environmental pollutants.

Full text of DOI:10.1016/S1872-2067(19)63391-7

Chinese Journal of Catalysis 41 (2020) 161–169
a v a i l a b l e a t w w w . s c i e n c e d i r e c t . c o m
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / c h n j c
Article (Special Issue on Photocatalytic H
2
Production and CO Reduction)
2
Fabrication of Z-scheme MoO
3
/Bi
2
O heterojunction photocatalyst
4
with enhanced photocatalytic performance under visible light
irradiation
a
a
a
a
a,b,
Tiangui Jiang , Kai Wang , Ting Guo , Xiaoyong Wu , Gaoke Zhang *
a
Hubei Key Laboratory of Mineral Resources Processing and Environment, School of Resources and Environmental Engineering, Wuhan University of
Technology, Wuhan 430070, Hubei, China
State Key Laboratory of Silicate Materials for Architectures, Wuhan University of Technology, Wuhan 430070, Hubei, China
b
A R T I C L E I N F O
A B S T R A C T
Article history:
Constructing Z-scheme heterojunction to improve the separation efficiency of photogenerated car-
riers of photocatalysts has gained extensive attention. In this work, we fabricated a novel Z-scheme
Received 9 April 2019
Accepted 1 May 2019
Published 5 January 2020
MoO /Bi O
3 2 4
heterojunction photocatalyst by a hydrothermal method. XPS analysis results indicated
and Bi is generated, which contributes to charge transfer
that strong interaction between MoO
3
O
2 4
and separation of the photogenerated carriers. This was confirmed by photoluminescence (PL) and
electrochemical impedance spectroscopy (EIS) tests. The photocatalytic performance of the
as-synthesized photocatalysts was evaluated by degrading rhodamine B (RhB) in aqueous solution
Keywords:
MoO
3
under visible light irradiation, showing that 15% MoO
highest photocatalytic activity, which is 2 times higher than that of Bi
3
/Bi
2
O
4
(15-MB) composite exhibited the
. Besides, the heterojunc-
Bi O
2 4
O
2 4
Z-scheme
tion photocatalyst can keep good photocatalytic activity and stability after five recycles. Trapping
experiments demonstrated that the dominant active radicals in photocatalytic reactions are super-
Heterojunction
Visible-light
Degradation

+
oxide radical (•O
2
) and holes (h ), indicating that the 15-MB composite is a Z-scheme photocatalyst.
/Bi composite for photo-degrading RhB in aqueous
Finally, the mechanism of the Z-scheme MoO
3
O
2 4
solution is proposed. This work provides a promising strategy for designing Bi-based Z-scheme
heterojunction photocatalysts for highly efficient removal of environmental pollutants.
©
2020, Dalian Institute of Chemical Physics, Chinese Academy of Sciences.
Published by Elsevier B.V. All rights reserved.
1
.
Introduction
[5,6]. Even though the concentration of organic dyes in the wa-
ter is low, the water quality has been affected. The organic dyes
in waste water are difficult to be degraded completely by tradi-
tional technologies, which poses a grave threat to human health
and their production activities. Therefore, it is highly desirable
to explore novel technologies for degradation of dye waste
water. Advanced oxidation processes have been developed and
applicated for organic pollutant degradation [7,8]. Especially,
semiconductor photocatalytic technology can remove and de-
Organic pollutants are harmful to human health, such as
dyes and volatile organic compounds (VOCs), which come from
leather, textile, printing industries, and construction and deco-
ration materials [1–4]. Especially, organic dyes containing var-
ious refractory organic compounds were discharged into natu-
ral water without any treatment because of the waste in the
production process, which affects the safety of water quality
*
Corresponding author. Tel: +86-27-87887445; Fax: +86-27-87887445; E-mail: gkzhang@whut.edu.cn
This work was supported by the Natural Science Foundation of Hubei Province (2016CFA078), and the National Natural Science Foundation of China
51472194).
DOI: S1872-2067(19)63391-7 | http://www.sciencedirect.com/science/journal/18722067 | Chin. J. Catal., Vol. 41, No. 1, January 2020
(

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Tiangui Jiang et al. / Chinese Journal of Catalysis 41 (2020) 161–169
grade organic pollutants by catalytic reaction driven by solar
energy, which has been considered as a promising technology
for environmental purification because of the advantages of its
highly efficiency and environmental friendliness [9,10]. The
phology, and surface chemical properties of the as-synthesized
composites were studied by a series of characterizations. Sub-
sequently, the photocatalytic activity of the composites was
evaluated by using RhB as the target pollutant. Chemical trap-
ping experiments were conducted to determine the participa-
tion of active radicals in photocatalytic reactions. Finally, the
possible Z-scheme photocatalytic mechanism was proposed
and discussed in detail.
common photocatalysts, such as TiO and ZnO, which can be
2
excited by UV-light to generate electron-hole pairs and finally
involved in the redox reaction, have been studied for solving
problems of environmental pollution and energy shortage
[11–13]. However, the visible-light excited photocatalysts can
utilize solar energy more efficiently, which have wider applica-
tion prospect. Therefore, it is desirable to explore novel visi-
ble-light excited photocatalysts.
2. Experimental
2.1. Photocatalyst preparation
In recent years, many visible light responsive Bi-based pho-
tocatalysts with highly hole mobility and fantastic optical
properties have been reported. Typical Bi-based photocatalysts
Preparation of MoO
3
is similar to the pioneer’s reported
O was added to the crucible
work [49]. 2 g of (NH Mo
)
4 6
7
O
24·4H
2
such as BiO2–x [14–16], Bi
21–29] have been developed and applied for degradation of
organic pollutants, hydrogen generation, CO reduction, and
NO removal. However, single-phase photocatalyst usually suf-
2
WO
6
[17–20], and BiOX (X = Cl, Br, I)
and heated for 4 h in a muffle furnace at 500 °C (the rate of
heating was set as 5 °C/min). When the muffle furnace was
cooled, the obtained sample was washed four times with water
and dried at 70 °C for 12 h.
[
2
x
fers from quick recombination of charge carriers, affecting their
photocatalytic efficiency seriously [30,31]. Especially, fabrica-
tion of hybrid semiconductor photocatalysts with staggered
band alignments has been identified as one of the most prom-
ising ways for boosting their photocatalytic performance, bene-
fiting from the faster interfacial charge transfer [32–34]. Re-
cently, constructing Z-scheme photocatalysts to widen the
range of light absorption and increase the redox ability of pho-
tocatalysts has drawn much attention [35–37]. Wang et al. [38]
MoO
thermal process. Typically, 1.58 g of NaBiO
into 60 mL ultrapure water, and then a certain amount of MoO
was dispersed in NaBiO solution and stirred for 1 h. After that,
the mixture was transferred into a PPL-lined stainless auto-
clave and heated at 160 °C for 6 h. After the autoclave was
cooled naturally, the precipitation was washed with ultrapure
water and ethanol five times, and dried at 70 °C for 6 h. A series
3
/Bi
2
O
4
photocatalysts were synthesized by a hydro-
3
·2H O was added
2
3
3
of MoO
amount of MoO
3
/Bi
2
O
4
composites were prepared by changing the
. According to the MoO and Bi molar ratios
have successfully synthesized Bi
TaO /g-C N
3 7 3 4
Z-scheme com-
3
3
O
2 4
posites, and this Z-scheme photocatalyst shows higher visible
light catalytic activity for degrading antibiotics. Nie et al. [39]
of 0.05:1, 0.15:1, 0.2:1, and 0.3:1, these samples were named
5-MB, 15-MB, 20-MB, and 30-MB, respectively. Besides, pure
have fabricated Z-scheme g-C
tained composites showed higher photocatalytic performance
for CO reduction, benefiting from the high separation efficien-
3
N
4
/ZnO composites, and the ob-
Bi
MoO
2
O
4
was synthesized by the same method without adding
3
.
2
cy of charge. Therefore, developing novel Z-scheme photocata-
lysts for environment remediation is promising.
2.2. Characterization
Bi
narrow band gap and wide absorption region. Wang et al. [40]
synthesized Bi for application in bacterial inactivation and
decomposition of organic pollutants. To enhance catalytic per-
2
O
4
is a novel visible light responsive photocatalyst with
Chemical compositions of the photocatalysts were analyzed
by X-ray diffraction (XRD) on a D/MAX-RB diffractometer.
Scanning electron microscopy (SEM, JSM-5610LV) and trans-
mission electron microscopy (TEM, JEM 2100 F) were applied
to examine the morphology and microstructure of the samples.
X-ray photoelectron spectroscopy (XPS, VG Multilab2000) was
used to analyze the surface properties of obtained samples, and
the binding energies of the elements in the samples were cali-
brated by 284.6 eV of C 1s. The optical properties of the sam-
ples were tested with an UV-Vis spectrophotometer (UV-3600
plus). Photoluminescence (PL) measurements were performed
on a fluorescence spectrophotometer (RF-5301), and the exci-
tation wavelength was 312 nm.
O
2 4
formance of single phase Bi
2
O
O
4
photocatalyst, Xia et al. [41]
heterojunction photocatalysts,
fabricated Z-scheme C /Bi
N
3 4
2 4
which have better and stable photocatalytic performance. Be-
sides, many studies on Bi have also been reported [42–46].
MoO is a noticeable material for environmental treatment due
to its chemical stability and non-toxicity. MoO is a promising
O
2 4
3
3
material to construct heterostructures with other semiconduc-
tors because of its energetically electrical properties [47]. He et
al. [48] synthesized Z-scheme MoO
3
/C
3
N composites by a
4
mixed calcination method and evaluated their photocatalytic
activities by degrading methyl orange (MO), and the obtained
composites exhibited higher visible-light photocatalytic per-
2.3. Photoelectrochemical measurements
formance than C
3
N
4
. Therefore, it is meaningful to fabricate a
The photoelectrochemical properties of the as-obtained
samples were analyzed on an electrochemical workstation
novel MoO /Bi
3
2 4
O
heterojunction photocatalyst for highly effi-
cient photocatalysis.
(CHI660E) using 0.5 mol/L Na
2
SO aqueous solution as the
4
Herein, we successfully synthesized MoO
composites by a hydrothermal method. The structure, mor-
3
/Bi
2
O
4
Z-scheme
electrolyte. The working electrodes were made as follows. A 10
mg sample was suspended in 1 mL mixed solution (prepared

Tiangui Jiang et al. / Chinese Journal of Catalysis 41 (2020) 161–169
163
with 1 mL Nafion dispersion and 24 mL ethyl alcohol), and then
the mixed solution was sonicated for 1 h until forming uniform
solution. The above solution was slowly dropped onto the ITO
coexistence of MoO
3
and Bi
2
O
4
. However, this pattern is similar
to that of pure Bi
2
O
4
due to the low content of MoO in the
3
composite. It is obvious that no characteristic peaks of other
impurities existed in the as-obtained photocatalysts, indicating
that the synthesized samples are pure.
glass and dried in an ambient environment. Bi
2 4
O -ITO,
MoO -ITO, or Bi /MoO -ITO were used as working electrode,
3
O
2 4
3
and platinum wire and Ag/AgCl electrode was used as counter
electrode and reference electrode, respectively.
The morphologies of the as-obtained Bi
15-MB composite were observed by SEM and TEM. SEM images
of the representative photocatalysts are shown in Fig. 2. Pure
2
O
4
, MoO , and
3
2.4. Photocatalytic experiments
Bi
MoO
2
O
4
is made up of massive nanorods (Fig. 2a and 2b). For
, it is grain-like morphology with a glazed surface (Fig. 2c
3
RhB was chosen to evaluate photocatalytic performance of
and 2d), which is consistent with previous study [49]. As for the
MoO /Bi composite, large numbers of Bi nanorods are
coated on the surface of MoO (Fig. 2e and 2f). This structure
allows the surface between Bi and MoO with large contact
area, which is beneficial to the transfer of photogenerated car-
ries. The HRTEM image of the Bi is exhibited in Fig. 3a, and
the lattice fringes of 0.295 nm correspond to the (400) planes
of Bi [43]. Fig. 3b is the HAADF-STEM image of the 15-MB
the obtained MoO /Bi O
3 2 4
composites. Typically, 50 mg photo-
3
O
2 4
O
2 4
catalysts were added in RhB solution (10 mg/L, 100 mL). The
suspensions were continuously stirred for 30 min in the dark to
establish adsorption equilibrium. The 100 W LED lamps with
the wavelength of 420 nm as the visible light source. During
visible-light irradiation, 5 mL of the suspensions were with-
drawn at different regular time intervals of 10 min from the
photocatalytic system, then centrifuged to obtain liquid super-
natant, and analyzed on a UV-visible spectrophotometer
3
2
O
4
3
2 4
O
2 4
O
sample. The EDS analysis are showed in Fig. 3c–e, proving that
15-MB composite is only composed of Bi, Mo, and O elements,
these elements uniformly distributed on the surface of 15-MB
(
UV-1100) at its characteristic peak of 554 nm. The calculated
degradation is expressed by C
centration of RhB at each irradiated time (t) and after adsorp-
tion/desorption equilibrium, respectively.
t
/C , where C and C are the con-
0
t
0
sample. These results indicate that the structures of MoO
Bi are not changed after synthesis, and the interactions be-
tween MoO and Bi may lead to different chemical states
and optical properties.
The surface chemical states of Bi
3
and
O
2 4
3
O
2 4
In the cycle experiments of degrading RhB solution, the used
15-MB photocatalyst was washed with ultrapure water and
O
2 4
, MoO , and 15-MB
3
ethanol and then dried at oven for next photocatalytic reaction.
composite were investigated by XPS analysis. The survey spec-
tra in Fig. S1 show the existence of Bi and O elements in the
3.
Results and discussion
Bi
MoO
2
O
4
catalyst and the presence of Mo and O elements in the
sample, while the 15-MB composite consists of Bi, O, and
3
The XRD patterns of pure MoO , Bi O , and MoO /Bi O
3 2 4 3 2 4
Mo elements without other elements detected. Fig. 4 presents
composites were shown in Fig. 1. The main peaks of pure MoO
3
are at 12.8°, 23.4°, 25.7°, 25.8°, 27.3°, and 29.7°, relating to the
020), (110), (040), (120), (021), and (130) planes, respective-
ly. This pattern is consistent with the standard JCPDS
No.05-0508 well. The XRD pattern of Bi corresponds to
Bi (JCPDS No. 50-0864). The characteristic diffraction peaks
at 26.8° and 29.5° are attributed to (111) and (31-1) planes of
(
O
2 4
O
2 4
2 4
Bi O . The XRD pattern of the 20-MB composite indicates the
Fig. 1. XRD patterns of as-synthesized samples.
Fig. 2. SEM images of (a, b) Bi
2
O
4
, (c, d) MoO , and (e, f) 15-MB samples.
3

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Tiangui Jiang et al. / Chinese Journal of Catalysis 41 (2020) 161–169
is improved. The results of UV-vis diffuse reflectance spectros-
copy analysis indicate that the UV-vis absorption properties of
MoO /Bi composites are impacted, because the intermolec-
ular interaction occurs between Bi and MoO . The band gaps
of Bi and MoO can be estimated by the formula [43] E
240/λ , where E is the band gap energy, and λ is the absorp-
tion threshold wavelength of catalysts. The band gaps of Bi
and MoO are about 1.9 and 2.8 eV, respectively. These results
3
O
2 4
O
2 4
3
O
2 4
3
g
=
1
g
g
g
O
2 4
3
matched well with the previous reports [41,49].
Fig. 3. (a) HRTEM image of the Bi
5-MB sample, and (c–e) elemental EDS-mapping images of the 15-MB
sample.
2
O
4
, (b) HAADF-STEM image of the
The Mott-Schottky plot measurement was conducted to de-
1
termine the energy band structure of Bi
O
2 4
and MoO , and the
3
results are shown in Fig. 5b. Generally, the conduction band of
semiconductor is close to its flat-band potential. The flat-band
the high-resolution spectra of Bi 4f, O 1s, and Mo 3d. The Bi
5/2 (or Bi 4f7/2) spectrum of Bi is shown in Fig. 4a, the
binding energies located at 163.5 and 164.0 eV (or 158.2 and
potentials of Bi
Ag/AgCl, pH = 7), respectively. And they were determined to be
–0.39 and 0.49 V (vs NHE, pH = 7), respectively. The VB posi-
2
O
4
and MoO were at –0.59 and 0.29 V (vs
3
4f
O
2 4
1
1
4
1
58.7 eV) correspond to Bi3+ and Bi5+ [43]. However, for the
5-MB composite, the characteristic peaks of Bi 4f5/2 (or Bi
tions of Bi
1.51 and 3.29 V (vs NHE, pH = 7) according to the empirical
formula EVB = ECB + E [50,51].
The photocatalytic activity of Bi
O
2 4
and MoO
3
were, respectively, determined to be
f
7/2) respectively shift to 163.6 and 164.1 eV (or 158.3 and
58.8 eV), which might be due to the interactions between
and MoO . The asymmetric O1s peaks of 15-MB compo-
g
2 4 3 3 2 4
O , MoO , and MoO /Bi O
Bi
O
2 4
3
heterojunction photocatalysts was evaluated by degrading RhB
solution under visible light (λ = 420 nm). The photocatalytic
degradation experiments were carried out after the adsorption
experiments. The degradation results are presented in Fig. 6a.
As we can see, about 2% RhB was decomposed under visible
site appeared at 529.4 and 530.8 eV (Fig. 4b). The peak at 529.4
eV belongs to the lattice oxygen of Bi–O bonds, and the peak at
5
30.8 eV can be assigned to O2– in molybdenum oxide. As for
the Mo 3d peaks of MoO , the binding energy located at 232.9
3
and 236.1 eV (Fig. 4c), respectively. These two peaks were at-
light irradiation without photocatalyst, and pure MoO photo-
3
tributed to Mo 3d5/2 and Mo 3d3/2 [49], but the Mo 3d peaks of
catalyst had neglected photocatalytic performance for RhB
degradation. After irradiating for 40 min by visible light, about
15-MB were shifted to 232.6 and 235.8 eV. The XPS results
demonstrate that MoO /Bi composites were successfully
3
2
O
4
73% of RhB were eliminated over pure Bi
2
O . By comparison,
4
synthesized, which might lead to distinct optical and electro-
chemical properties.
degradation rate of RhB was determined to be 67%, 99.6%,
92.5%, and 75.2% for 5-MB, 15-MB, 20-MB, and 30-MB compo-
Fig. 5a shows the UV-vis diffuse reflectance spectra of pure
sites, respectively. Obviously, the MoO
its enhanced photocatalytic performance. With increasing the
MoO content, the photocatalytic activity of as-obtained com-
posites increased firstly and then decreased. It is obvious that
the content of MoO in the composites exerted an influence on
3
/Bi
2
O
4
composite exhib-
Bi
absorption wavelengths of Bi
47 and 435 nm. After combining MoO
tion edge red-shifted, and the absorption ability of visible light
2
O , MoO , and MoO /Bi O
4 3 3 2 4
composites. It is obvious that the
and MoO are approximately
with Bi , the absorp-
O
2 4
3
3
6
3
O
2 4
3
Fig. 4. XPS spectra of Bi
2
O
4
, MoO , and 15-MB composite. (a) Bi 4f; (b) O 1s; (c) Mo 3d.
3

Tiangui Jiang et al. / Chinese Journal of Catalysis 41 (2020) 161–169
165
Fig. 5. (a) UV-vis diffuse reflectance spectra of the as-obtained samples; (b) Mott-Schottky plots of Bi
O
2 4
and MoO .
3
Fig. 6. (a) Photocatalytic degradation curves of RhB solution over different photocatalysts; (b) UV-vis absorption spectra of RhB solution over the
5-MB composite; (c) Degradation percentage of RhB solution by the 15-MB composite in cycle experiments; (d) XRD patterns of the 15-MB compo-
site before and after five cycles of photocatalytic degradation process.
1
degradation activity. A suitable content of MoO
3
can promote
composites,
less than 10% of RhB was adsorbed by different photocatalysts.
To quantify the photocatalytic ability of the as-synthesized
samples, the reaction kinetic was analyzed. As shown in Fig. S3,
the absorption of visible light for MoO /Bi
3
2 4
O
which accelerates the transfer and separation of pho-
to-generated carriers, and finally improves the photocatalytic
the k value of 15-MB is 2 times as that of Bi
2
O , implying the
4
efficiency. However, when MoO
catalytic activity decreased, which is probable that MoO
vided more recombination centers. The 15-MB composite with
moderate MoO content has the best photocatalytic activity
3
was over-loaded, the photo-
best photocatalytic activity of the 15-MB composite. In Fig. 6b,
it is apparent that the UV-vis adsorption peaks of RhB solution
at 554 nm decreased rapidly, indicating that the molecular
structure of RhB was destroyed.
3
pro-
3
among the as-synthesized samples by RhB degrading experi-
ments. In addition, the adsorption experiments of the
as-synthesized photocatalysts toward RhB solution were car-
ried out, and the results are shown in Fig. S2. As we can see,
The stability of photocatalyst is essential for its practical ap-
plication. A cycle experiment of degrading RhB in aqueous so-
lution over the 15-MB heterojunction composite was conduct-
ed, and the results are shown in Fig. 6c. 15-MB still held high

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Tiangui Jiang et al. / Chinese Journal of Catalysis 41 (2020) 161–169
photocatalytic activity after five cycles, and the degradation
ratio of RhB declined a little because of the loss of the catalyst
during the recovery process. The XRD pattern of the 15-MB
sample after recycle is nearly the same as that of the fresh sam-
ple, as can be seen from Fig. 6d. The results confirmed that the
photocatalyst is stable and effective during the process of de-
grading RhB.
To display the charge transfer efficiency and the separation
of photogenerated hole-electron of as-obtained samples, the
EIS and photocurrent tests were performed. Fig. 7a shows the
the 15-MB heterojunction photocatalyst. It is apparent that
15-MB shows the weakest intensity of PL spectra, which indi-
cates that the formation of MoO
3
/Bi
2
O heterojunction contrib-
4
utes to separation of photogenerated electrons and holes. This
result means that 15-MB might have the best photocatalytic
performance compared with pure MoO
3
and Bi O .
2 4
The electrons and holes would be generated in photocata-
lysts during the degradation process, and then migrate to the
surface of photocatalysts to produce •OH (hydroxyl radicals),
–
h+, and •O
radical species, which play a significant role in
2
Nyquist plots of the pure MoO
3
, Bi
2
O
4
, and 15-MB composite
photocatalytic reactions. To investigate the mechanism of the
15-MB composite for photocatalytic degradation of RhB solu-
tion under visible light and evaluate the role of radical species,
trapping experiments were performed. Isopropyl alcohol (IPA),
under dark. In general, the smaller arc radius means a lower
resistance of charge transfer [52,53]. The circle size of
as-obtained samples follows the order: 15-MB composite <
Bi
2
O
4
< MoO
3
. This result suggested that the 15-MB composite
sodium oxalate (Na
spectively used to capture the generated •OH, h , and •O
ing the RhB degradation [60–62]. The results are shown in Fig.
7d. After IPA was introduced, the degradation rates of RhB
2
C
2
O
4
), and benzoquinone (BQ) were re-

+
has the lowest resistance, which would enhance photocatalytic
performance. Fig. 7b displays the photocurrent response
curves of MoO
2
dur-
3
, Bi
2
O , and 15-MB composite. The 15-MB com-
4
posite exhibits the highest photocurrent intensity, indicating
that the photogenerated hole-electron separated more effi-
ciently [54,55]. Furthermore, PL was used to analyze the exci-
tation and transfer of charge carriers. In general, higher inten-
sity of PL spectra indicates lower separation efficiency of carri-
declined a little. When BQ or Na
2
C
2
O was introduced, 10% and
4
20% of RhB were degraded. The results of trapping experi-

+
ments demonstrate that h and •O
2
play an important role, and
•OH is also involved in the degradation process.
Based on the above results, two possible mechanisms of or-
ers [56–59]. Fig. 7c shows the PL spectra of MoO
3 2 4
, Bi O , and
ganic pollutant degradation over MoO
3
/Bi
2
O heterojunction
4
Fig. 7. (a) EIS spectra of Bi
Photoluminescence spectra of Bi
with different scavengers.
2
O
4
, MoO
, 15-MB, and MoO
3
, and 15-MB photocatalysts; (b) Photocurrent response curves of Bi
photocatalysts; (d) Photocatalytic degradation curves of RhB solution over the 15-MB sample
2
O
4
, MoO , and 15-MB photocatalysts; (c)
3
2
O
4
3

Tiangui Jiang et al. / Chinese Journal of Catalysis 41 (2020) 161–169
167
Fig. 8. Possible photocatalytic degradation mechanism on the 15-MB composite under visible light irradiation. (a) Type-II heterojunction mechanism;
b) Direct Z-scheme mechanism.
(
photocatalyst are predicted and shown in Fig. 8. Upon visible
light irradiation, electrons in valence band (VB) of Bi and
MoO can be activated [41,49], and then transfer to their con-
duction band (CB), the photogenerated holes are left in VB,
respectively. If the separation and recombination process of
photogenerated carriers follows the Type-II photocatalytic
mechanism like Fig. 8a, the photoexcited electrons in CB of
O
2 4
4. Conclusions
A novel MoO /Bi
hydrothermal method and used as a Z-scheme photocatalyst
for highly efficient degradation of RhB in aqueous solution. The
3
3
2
O heterojunction has been fabricated by a
4
degradation rate of the optimized MoO
higher than that of pure MoO and Bi
. Trapping experiments
/Bi O
3 2 4
photocatalyst is
Bi
holes transfer from VB of MoO
because of the more negative band potential of MoO
2
O
4
can easily transfer to CB of MoO
3
, and photogenerated
simultaneously
. However,
3
O
2 4
samples, approxi-
3
to VB of Bi
O
2 4
mately 2 times that of pure Bi
2 4
O

+
3
demonstrated that h , •O
2
, and •OH participate in the degrada-
the CB potential of MoO
tive than the potential of O
3
(0.49 V vs NHE, pH = 7) is more posi-
tion of RhB solution. The PL and photoelectrochemical analysis
revealed a Z-scheme charge transfer path between the MoO
and Bi . This work A the Z-scheme photocatalytic mechanism
of the Bi-based heterojunction system was proposed, which can
help design highly efficient photocatalysts for organic pollu-
tants degradation.

2
/•O
2
(–0.046 V vs NHE, pH = 7)
cannot
. Meanwhile, the holes on VB
3
[63,64], so the photoexcited electrons on the CB of MoO
3
O
2 4

react with oxygen to produce •O
2
of Bi
Bi
of H
2
O
4
could not oxide H O to •OH because the VB potential of
2
2
O
4
(1.51 V vs NHE, pH = 7) is lower than the redox potential
O/•OH (2.27 V vs NHE, pH = 7) [65]. In this case, only
2
holes can oxidize dyes, which is not consistent with the result
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Chin. J. Catal., 2020, 41: 161–169 doi: S1872-2067(19)63391-7
Fabrication of Z-scheme MoO
light irradiation
3
/Bi
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O heterojunction photocatalyst with enhanced photocatalytic performance under visible
4
Tiangui Jiang, Kai Wang, Ting Guo, Xiaoyong Wu, Gaoke Zhang *
Wuhan University of Technology
Novel Z-scheme MoO
3
/Bi
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4
fiting from efficiently charge transfer and photogenerated hole-electron separation.

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3
4
增强可见光催化活性的Z型MoO /Bi O 复合光催化剂的制备
3
2
4
a
a
a
a
a,b,*
江天贵 , 王 楷 , 郭 婷 , 吴晓勇 , 张高科
a
武汉理工大学资源与环境工程学院矿物资源加工与环境湖北省重点实验室, 湖北武汉430070
b
武汉理工大学硅酸盐建筑材料国家重点实验室, 湖北武汉430070
摘要: 全球工业化进程的加快使人们饱受环境污染问题的困扰. 半导体光催化技术作为一种高效、绿色、有潜力的新技术,
在环境净化方面有着广阔的应用前景. Bi O 是近年来新开发出的一种铋基光催化剂, 在环境净化方面已有一些研究. 但
2
4
是, 单体光催化剂通常存在光响应范围窄、光生载流子复合率高等问题, 这些不足限制了Bi
过适当的改性来拓宽其光响应范围和提高其载流子的分离效率, 从而提高其光催化活性. 构建Z型异质结被认为是提高光
催化剂光生载流子分离效率并进一步提高光催化活性的有效方法. MoO 是一种宽禁带的n型半导体, 具有独特的能带结
构、光学特性和表面效应, 是一种非常有前景的半导体光催化剂. 虽然MoO 材料的光生载流子复合率高, 带隙(2.7–3.2 eV)
与其他合适的半导体配位形成复合材料后能够有效提高其光生载流子的分离效率,
2 4
O 的进一步应用. 因此, 需要通
3
3
大, 不利于其参与光催化反应, 但MoO
从而提高其光催化活性.
3
本研究采用简单的水热法制备了一种新型Z型MoO
密结合在一起. X射线光电子能谱分析表明, MoO 和Bi
的分离. 光致发光光谱、电阻抗和光电流测试也证明了MoO /Bi O 复合光催化剂的光生载流子分离效率更高, 形成了更强
3
/Bi
2 4 3 2 4
O 复合光催化剂, SEM和TEM分析结果表明, MoO 和Bi O 紧
3
2 4
O 之间存在很强的界面相互作用, 这有助于电荷转移和光生载流子
3
2
4
的光电流. 通过在可见光下降解RhB溶液评价了所合成光催化剂的光催化性能. 15% MoO
3
/Bi
2
O
4
(15-MB)复合光催化剂表
的2倍. 此外,
5-MB复合光催化剂在经过五次循环降解RhB溶液后仍保持良好的光催化活性和稳定性, 表明MoO /Bi O 复合光催化剂
现出了最佳的可见光催化活性, 在40 min内对10 mg/L RhB溶液的降解率达到了99.6%, 其降解速率是Bi
2
O
4
1
3
2
4
–
+
具有较强的应用潜力. 通过自由基捕获实验确定了光催化反应中主要的活性自由基为•O
2
和h . 通过莫特-肖特基测试和
/Bi 复合光催化剂在可见光
带隙计算得到MoO
3
和Bi
2
O
4
的价带和导带位置. 最后, 根据实验和分析结果提出了Z型MoO
3
2 4
O
下降解RhB溶液的机理. 本研究为设计铋基Z型异质结光催化剂用于高效去除环境污染物提供了一种有前景的策略.
关键词: 氧化钼; 四氧化二铋; Z型; 异质结; 可见光; 降解
收稿日期: 2019-04-09. 接受日期: 2019-05-01. 出版日期: 2020-01-05.
*
通讯联系人. 电话: (027)87887445; 传真: (027)87887445; 电子信箱: gkzhang@whut.edu.cn
基金来源: 湖北省自然科学基金(2016CFA078); 国家自然科学基金(51472194).
本文的电子版全文由Elsevier出版社在ScienceDirect上出版(http://www.sciencedirect.com/science/journal/18722067).