The formation of a direct Z-scheme Bi2O3/MoO3 composite nanocatalyst with improved photocatalytic activity under visible light

Article abstract of DOI:10.1016/j.cplett.2018.06.006

A direct Z-scheme photocatalyst Bi₂O₃/MoO₃ was prepared via hydrothermal method and mixing-calcination methods. The as-obtained Bi₂O₃/MoO₃ composites exhibit higher photocatalytic activities than pure Bi₂O₃ and MoO₃ for degradation of methylene blue (MB) under visible light irradiation. The increased photocatalytic activity is mainly attributed to the efficient separation of photogenerated charge carriers through a Z-scheme system between Bi₂O₃ and MoO_3. Furthermore, the results of energy band positions, active species trapping experiments and photoluminescence spectroscopy (PL) provide direct evidence of an efficient Z-scheme as the main mechanism for degradation of MB under visible light irradiation over Bi₂O₃/MoO₃ composites.

Full text of DOI:10.1016/j.cplett.2018.06.006

Accepted Manuscript
Research paper
The Formation of a Direct Z-Scheme Bi2O3/MoO3 Composite Nanocatalyst with
Improved Photocatalytic Activity under Visible Light
Zeshu Hu, Jiabin Zhou, Ying Zhang, Wei Liu, Jun Zhou, Weiquan Cai
PII:
S0009-2614(18)30479-2
DOI:
https://doi.org/10.1016/j.cplett.2018.06.006
CPLETT 35705
Reference:
Chemical Physics Letters
To appear in:
Received Date:
Accepted Date:
3 May 2018
4 June 2018
Please cite this article as: Z. Hu, J. Zhou, Y. Zhang, W. Liu, J. Zhou, W. Cai, The Formation of a Direct Z-Scheme
Bi2O3/MoO3 Composite Nanocatalyst with Improved Photocatalytic Activity under Visible Light, Chemical Physics
Letters (2018), doi: https://doi.org/10.1016/j.cplett.2018.06.006
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The Formation of a Direct Z-Scheme Bi2O3/MoO3 Composite
Nanocatalyst with Improved Photocatalytic Activity under
Visible Light
Zeshu Hu a, b, Jiabin Zhou a, b, *, Ying Zhang a, Wei Liu a, Jun Zhou a, Weiquan Cai c, *
a School of Resources and Environmental Engineering, Wuhan University of
Technology, Wuhan 430070, P. R. China
b School of Chemistry and Chemical Engineering, Southwest Petroleum University,
Chengdu 610500, China
c School of Chemistry and Chemical Engineering, Guangzhou University, Guangzhou
510006, PR China
*Corresponding author. Tel: +86-28-83037310
E-mail: jbzhou@swpu.edu.cn (J. Zhou); cccaiwq@gzhu.edu.cn (W. Cai)
1

Abstract
A direct Z-scheme photocatalyst Bi2O3/MoO3 was prepared via hydrothermal
method and mixing-calcination methods. The as-obtained Bi2O3/MoO3 composites
exhibit higher photocatalytic activities than pure Bi2O3 and MoO3 for degradation of
methylene blue (MB) under visible light irradiation. The increased photocatalytic
activity is mainly attributed to the efficient separation of photogenerated charge
carriers through a Z-scheme system between Bi2O3 and MoO3. Furthermore, the
results of energy band positions, active species trapping experiments and
photoluminescence spectroscopy(PL) provide direct evidence of an efficient
Z-scheme as the main mechanism for degradation of MB under visible light
irradiation over Bi2O3/MoO3 composites.
Keywords: Z-scheme; nanorods; Bi2O3/MoO3; photodegradation; visible light
2

1. Introduction
The semiconductor photocatalyst has been widely studied for elimination of
organic pollutant, water splitting, and carbon dioxide reduction under the natural
diffused sunlight because it holds great promise to address energy and environmental
problem in an efficient, low-cost and environmentally friendly way[1,2].
Molybdenum trioxide (MoO3), as a wide band gap semiconductor with a band gap of
2.92 eV, is one of the most noticeable star materials due to its chemical stability,
nontoxicity and abundant in source, what’s more, its excellent dye adsorption capacity
is significant for photocatalysis process[3]. However, like many pure materials, such
as TiO2 [4-6], ZnO [7-9], NiO [10-12], low quantum yield, the high recombination
rate of photogenerated e−/h+ pairs and low visible light absorption have limited the
practical application of MoO3 in photocatalyst. In order to solve the above problems,
many effective methods were used to suppress the rapid recombination of
photogenerated charge and improve the photocatalytic ability of MoO3, such as
coupling with other band structure matching semiconductors [13], constructing
heterojunctions[14], and Z-scheme systems[15]. It is worth noting that semiconductor
photocatalysts by building a Z-scheme structure between two semiconductors with
matching energy levels can help to enhance photoinduced charge separation and
strengthen redox ability. In recent years, a large number of Z-scheme photocatalysts,
such as g-C3N4/MoO3[16], Ag2CrO4/MoO3 [17] and AgBr/MoO3 [3], have been
reported.
Bismuth oxide (Bi2O3) is a suitable photocatalyst with a band of 2.80eV owing to
3

its higher oxidation power of photogenerated holes, non-toxicity, and can absorb
visible light hence improving the utilization of sunlight[18,19]. However,
recombination of photoelectric charge pairs limits the photocatalytic activity of Bi2O3
under visible light [20]. Hence, Bi2O3 is often combined with other materials to
improve photocatalytic activity, such as Bi2O3/NiO [21-24], Bi2O3/g-C3N4 [25],
Bi2O3/ZnO [26-28]. Considering the valence band and conduction band position of
Bi2O3 and MoO3, it is appropriate to establish a direct Z-scheme system between
Bi2O3 and MoO3. With the introduction of Bi2O3, the fast recombination of
photogenerated charge and low visible light absorption can be solved and in result the
photocatalytic of MoO3 can be improved. To our knowledge, this is the first time to
fabricate the direct Z-scheme photocatalyst Bi2O3/MoO3 for the purpose of improving
the photocatalytic ability of MoO3 under visible light. Herein, in this paper, the
photocatalytic activity of Bi2O3/MoO3 composites with different Bi2O3 weight ratios
under visible light was evaluated by using methylene blue（MB）as the target pollutant
for degradation. In addition, the possible mechanism for improving the photocatalytic
activity of Bi2O3/MoO3 composites was proposed.
2. Experimental
2.1. Materials and methods
1 mmol of Bi (NO3)3·5H2O was dissolved in 35 ml of ethylene glycol (EG), 0.2
mmol of urea was dissolved in 5 ml of distilled water and the two solutions were
mixed in 10 ml of polyethylene glycol (PEG-200). The solution was added to a sealed
100 ml Teflon lined stainless steel autoclave and treated at 160 ° C for 6 hours. It is
4

separated by centrifugation and washed three times with deionized water and ethanol,
respectively. Then dried at 80 ° C for 9h, the product was heated in a muffle furnace at
5 ° C / min to 400 ° C for 3 hours.
5 mmol of Na2MoO4·2H2O was dissolved in 50 ml distilled water. Then, 7.5 ml of
concentrated nitric acid was added dropwise to the above solution and stirred at room
temperature for 30 minutes. The mixed solution was then transferred to a 100 ml
Teflon-lined stainless steel autoclave and then reacted at 180 ° C for 20 hours. It was
separated by centrifugation and washed three times with deionized water and ethanol,
respectively, and dried at 80 ° C for 9h.
The Bi2O3 and MoO3 were mixed at a mass ratio of 1: 2, and then the mixed samples
were heated in a muffle furnace at 5 °C/min to 400 °C for 3 hours and recorded as
Bi2O3/MoO3-0.5. Samples with 10%，30%，80% and 100% of Bi2O3 were recorded by
Bi2O3/MoO3-0.1,Bi2O3/MoO3-0.3,Bi2O3/ MoO3-0.8 and Bi2O3 / MoO3-1, respectively.
2.2. Catalyst characterization
X-ray diffraction (XRD) measurements of the samples were determined using
rotation anode high power X-ray diffractormeter with a RU-200B/D/MAX-RB (Japan)
with Cu-Kα radiation. Morphology and structure were examined using a field
emission scanning electron microscope (SEM, Zeiss Ultra Plus, Zeiss, Germany) and
a field emission high resolution transmission electron microscopy (TEM, JEM-2100F,
Electronics, Japan). X-ray photoelectron spectroscope (XPS) were recorded using
X-ray photoelectron spectrometer (ESCALAB 250Xi, USA). Specific surface areas
were measured using the automatic surface area and porosity analyzer (BET, ASAP
5

2020M, USA). UV–visible diffuse reflectance spectra (DRS) were measured by
UV/VIS/NIR Spectrometer (Lambda 750S, PerkinElmer, USA). Photoluminescence
(PL) spectra of the samples were obtained using a fluorescence spectrophotometer
(F-380, Shanghai, China).
2.3. Photocatalytic activity test
Photocatalytic activities of Bi2O3/MoO3 samples were evaluated through
degradation of methylene blue (MB) in aqueous solution under visible light irradiation
at room temperature. A 500w xenon lamp（CEl-HXUV300,CELAULIGHT）with
wavelength range of 420-800 nm was used as a visible light source. The specific
experimental process was as follows: 100 mg of photocatalyst was added to a glass
beaker containing 200 ml of 10 mg / L MB. Before light irradiation, the solution was
stirred in the dark for 30 minutes to achieve photocatalyst adsorption-desorption
equilibrium. In the light process, the reaction solution was taken 3 ml each 30minutes.
After illumination, the samples taken from the reaction suspension were centrifuged
and tested its absorption at 664 nm using a spectrophotometer (UAmini-1240,
SHIMADZU) to obtain the MB concentration. Degradation ratio of MB solution can
be calculated by B=(C0-Ct)/C0×100%, where C0, Ct represent the concentration of MB
solution before illumination and illumination time t, respectively. What' more, the
tests of active species trapping were carried out under the identical procedure
mentioned above except for adding 10 mmol ethylenediaminetetraacetic acid (EDTA,
a quencher of h+), 10 mmol Isopropanol (PIA, a quencher of ·OH) or 100ml/min N2 (a
−
quencher of ·O2 ), respectively. In order to test the stability of the photocatalyst, four
6

cycles of experiments were performed under the same conditions. After each
experiment, the photocatalyst was separated and washed from the solution and the
next experiment was carried out after drying at 80 °C.
3. Results and discussion
3.1. SEM analysis
Fig.1a-c showed the morphologies of the as-synthesized MoO3, Bi2O3 and Bi2O（0.5）
3
/MoO3 composite samples. The SEM image in Fig.1(a) shows that cuboid nanorods of
MoO3 with length in the range of 2.7 um to 4.0um and width of about 220 nm and
height of about 156 nm. The nanorods have a smooth surface with no impurities,
indicating that the material synthesis scheme has reached a superior condition. This is
consistent with the XRD results. In Fig.1(b), it can be seen that irregularly shaped
nanoparticles of Bi2O3 with a diameter of about 240 nm. The rapid nucleation results
in the formation of agglomerated nanoparticles and irregular shape[29]. From the
SEM image of Bi2O3(0.5)/MoO3 composite in Fig.1c, it can be seen that Bi2O3
nanoparticles are firmly attached to the surface of MoO3 during the solid phase
stirring[17]. It indicated that the intimate contact of Bi2O3 nanoparticles and MoO3
nanorods are successfully composited. Moreover, a smooth sample surface results in a
low specific surface area. From special surface areas of MoO3 and Bi2O3(0.5)/MoO3
in Table.1, it can be clearly seen the changes of specific surface area for MoO3
samples with the addition of Bi2O3, and the specific surface area of Bi2O（0.5）/MoO3
3
is smaller than that of MoO3.
3.2. XRD analysis
7

Fig.1(d) exhibited the X-ray diffraction (XRD) patterns of all as-obtained samples.
It can be found that the main diffraction peaks of pure MoO3 located at 2θ=12.8°,
25.7°, 39.0°, 58.8°and 67.6°, which can be perfectly indexed to (020), (040), (060),
(081) and (0100) planes of orthorhombic phase of MoO3 according to the standard
card (JCPDS 35-0609) [16,17,30]. No other peaks arise in the XRD patterns, thereby
indicating that these samples are pure and crystalline[31]. For the XRD patterns of
MoO3 and Bi2O3/MoO3, two weak diffraction peaks at 27.4°,28.0° are found after
Bi2O3 introducing, which correspond to the (120), (012) planes of monoclinic phase
of Bi2O3 (JCPDS 41-1449)[27,32]. As Bi2O3 concentration increases from10 wt% to
100 wt%, the diffraction peaks of Bi2O3 are gradually intensified, whereas the peaks
of MoO3 are weakened. Remarkably, it can be seen that the pattern of
Bi2O3/MoO3-0.1 shows only the (120) peak of Bi2O3 with a weak peak, this is because
the peaks of Bi2O3 with lower mass ratio in composite are difficult to be detected by
XRD analysis. The XRD results can illustrate that Bi2O3 nanoparticles successfully
combined with MoO3 nanorods.
3.3. TEM analysis
In order to better understand the intimate contact structure between Bi2O3
nanoparticles and MoO3 nanorods, the TEM images of the Bi2O3 (0.5)/MoO3
composite were shown in Fig.2(a) and (b). In Fig.2(a), the dark particles can be
assigned to Bi2O3 nanoparticles, which are dispersed on the surface of MoO3 nanorods.
This is consistent with the SEM results. Fig.2(b) shows the two types of lattice fringes
have a spacing of 0.526 nm and 0.204nm, respectively, which are assigned to
8

interplanar spacing of (120) of Bi2O3(JCPDS 41-1449) and（040）of MoO3(JCPDS
35-0609), respectively. The results are in accordance with the XRD patterns in
Fig.1(d). It is clearly seen that Bi2O3 nanoparticles and MoO3 nanorods are closely
bound together with no gaps, suggesting that Bi2O3 particles are successfully coupled
on the surface of MoO3 nanorods. Based on above analyses, it can be concluded that
there exists interaction between Bi2O3 and MoO3 interfaces, which is favorable for the
transport of photoexcited carriers [25]. The formed interaction is essential for the
photolytic reaction[16].
3.4. XPS analysis
XPS was carried out to investigate the presence of elements and their valence states
in Bi2O3/MoO3 composite. The XPS results of Bi2O3/MoO3 are illustrated in Fig.3a–d.
All peak positions were calibrated by C1s peak at 284.6 eV[30,33]. The Survey
spectrum in Fig.3(a) shows the presence of Mo, O and Bi in Bi2O3/MoO3 composite.
The C1s peaks of Bi2O3/MoO3 are ascribed to adventitious hydrocarbon from XPS
instrument itself[28]. High resolution spectra of Mo of Bi2O3/MoO3 was shown in
Fig.3(b).Two bands at 233 and 235.7 eV, ascribed to Mo 3d5/2 and Mo3d3/2 binding
energies can be observed, which is consistent with the characteristic peaks of
Mo6+[30,34].Fig.3(c) shows two peaks of the Bi of Bi2O3/MoO3 at 159.5 and 164.6
eV correspond to Bi4f7/2 and Bi4f5/2, respectively. The result shows the existence of
Bi3+[28,33]. As displayed in Fig.3(d), the only peak of O1s at 530.6 eV is attributed to
lattice oxygen of Bi2O3 and MoO3.
3.5. UV-Vis DRS analysis
9

The ultraviolet–visible diffuse reflectance spectroscopy (UV–Vis DRS) of the
as-obtained pure MoO3, Bi2O3 and Bi2O3 (wt.%)/MoO3 composites were shown in
Fig.4(a ) and (b). It is clear that the pure MoO3 has absorption in the visible region
with an absorption edge of around 425nm [35], and the pure Bi2O3 sample shows
absorption wavelengths from the UV to the visible range up to 450 nm[25,36]. In
compared with the pure MoO3, the absorption wavelength region of the
Bi2O3(0.5)/MoO3 sample exhibits significant red shift. This may be attributed to the
interaction between MoO3 and Bi2O3 interfaces. Enhanced UV-visible absorption
indicates that the as-obtained Bi2O3 / MoO3 composites can be used as visible
light-driven photocatalysts.
UV-Vis DRS also can get the information about the band gap energy of a
semiconductor. The band gap energy (Eg) was estimated from the formula [37]:
Eg = 1240/λ
(1)
where λ is the intercept of a straight line of the absorption edge against wavelength.
By calculation, the band gap energy of MoO3 and Bi2O3 are approximately 2.92 and
2.80 eV, respectively. Besides, the valance band (VB) potential and the conduction
band (CB) potential of MoO3 and Bi2O3 are also calculated by the following equation
[17,25]:
EVB = X + 0.5Eg - Ee
(2)
(3)
ECB = EVB – Eg
where EVB is the valence band potential; ECB is the conduction band potential; X is
the absolute electronegativity of the semiconductor; Ee is the energy of free
10

electrons on the hydrogen scale (Ee = 4.5 eV). According to previous literatures
reported, the values of X for Bi2O3 and MoO3 are 6.23 eV[25] and 6.40 eV[17],
respectively. It can be calculated from the above equations that the EVB of Bi2O3 and
MoO3 are 3.13 and 3.36eV, respectively, and the ECB of Bi2O3 and MoO3 are 0.33
and 0.44 eV, respectively.
3.6. Photocatalytic activity and stability
The photocatalytic activities of the pure MoO3, Bi2O3 and Bi2O3(wt.%)/MoO3
composites for the MB degradation under the visible light were evaluated at room
temperature. As displayed in Fig.5(a), there is no obvious self-degradation of MB in
absence of photocatalyst, indicating that the MB is stable under visible light
irradiation. It can be observed that the MB solution is decolorized approximately 86.6%
(Adsorption of 32.3% and the photodegradation of 54.3%) after 120 min under visible
light over the Bi2O3(0.5)/MoO3 photocatalyst, indicating the excellent photocatalytic
activity of Bi2O3(0.5)/MoO3 photocatalyst. In order to highlight the excellent
degradation ability of Bi2O3(0.5)/MoO3 photocatalyst, pure MoO3 and Bi2O3 also
were studied for the photodegradation ability of MB under visible light irradiation.
The pure MoO3 and Bi2O3 show the weaker photocatalytic performance than Bi2O3
(0.5) /MoO3, which remove approximately 70.2% (Adsorption of 59.2% and the
photodegradation of 11.0%) and 22.1% (Adsorption of 1.5% and the
photodegradation of 21.95%) of MB in 120 min, respectively. Meanwhile, the
removal rate of MB are about 70.4%, 76.0%, 64.2% and 44.22% after 120 min over
Bi2O3(0.1)/MoO3, Bi2O3(0.3)/MoO3, Bi2O3(0.8)/MoO3 and Bi2O3(1.0)/MoO3,
11

respectively. And the adsorption rate of MB are about 52.05%, 45.6%, 27.7% and
19.3% of removal rate, respectively. The above results show that the adsorption
capacity of Bi2O3/MoO3 decreases with increasing amount of Bi2O3, suggesting that
the specific surface area of Bi2O3/MoO3 decreases with the increase of Bi2O3. The
result is consistent with the Table.1. More notably, MoO3 has a strong adsorption
capacity for MB but its specific surface area is very small. It can be stated that the
strong adsorption is mainly attributed to the efficient and characteristic absorption of
MB by MoO3 [3,34].What’s more, the degradation ability of all Bi2O3/MoO3
composites is higher than that of MoO3, and it indicted that addition of Bi2O3 can
indeed improve the photocatalytic activity of MoO3. In order to the photocatalytic
practical applications, the stability of Bi2O3(0.5)/MoO3 were tested in MB degradation
under visible light irradiation for four recycled measurements. The results were
presented in Fig.5(b), and all the four cycles show 86.6% to 84.0% degradation after
120 min. The efficiency of photodegradation of MB under visible light does not
significantly reduce within four recycles, indicating that the Bi2O3(0.5)/MoO3
composite has a good photocatalytic stability.
3.7. Photoluminescence spectra analysis
In order to provide evidence of photogenerated electron-hole pair transfer and
separation efficiency before and after coupling of MoO3 with Bi2O3, the
photoluminescence (PL) spectra of pure MoO3 and Bi2O3（wt.%）/ MoO3 composites
with an excitation wavelength of 325 nm [29] were studied. As shown in Fig.6(a), it
can be seen that all as prepared samples exhibit two peaks around 440 nm and 470 nm,
12

respectively, which is ascribed to the band gap recombination of electron-hole pairs
[17]. The PL emission intensity of pure MoO3 is highest in all samples. However, all
Bi2O3 / MoO3 composites exhibit the lower PL emission intensity than pure MoO3
with the introduction of Bi2O3. The results confirm that the formation of interaction
between Bi2O3 and MoO3 can effectively inhibit the electron-hole recombination rate
and improve the photodegradation ability of the Bi2O3 /MoO3 composite
photocatalyst.
3.8. Detection of active species
In order to clarify which active specie plays the important role in photocatalytic
oxidation of Bi2O3(0.5)/MoO3 composite photocatalyst, the active species trapping
experiments were carried out. As can be seen in Fig.6(b), under visible light
irradiation, the photodegradation of MB is obviously suppressed after the addition of
EDTA, while addition of IPA in the MB solution has little effect on the photocatalytic
activity of Bi2O3(0.5)/MoO3 composite photocatalyst. In addition, the effect of
addition of N2 in MB solution is lower than the effect of addition of EDTA and higher
−
than the effect of addition of IPA. The results illustrate that h+ and ·O2 are the major
active species to the enhancement photoactivity oxidation of MB solution on
−
Bi2O3(0.5)/MoO3 composite photocatalyst, which h+ is active than·O2 , and the action
of ·OH is the least. The formation of holes can be through the light irradiation of the
−
Bi2O3(0.5)/MoO3 composite material and the formation of ·O2 can be through the
electron reduction of O2(Reactions4-6).According to the results, the photocatalytic
ability of Bi2O3(0.5)/MoO3 composite photocatalyst in MB solution which was added
13

EDTA or N2 is limited proves that the decrease of surface charge transfer[36].
Bi2O3/MoO3 +hv → h++ e-
(4)
(5)
-
e- + O2 → ·O2
-
h+,·O2 + MB → degradation productions (6)
3.9. Possible photocatalytic mechanism
Based on above results, the excellent photocatalytic activity of MoO3 is associated
with the enhanced separation efficiency of electron-hole pairs resulting from the
addition of Bi2O3[16]. Therefore, there may be two photocatalytic mechanisms to
explain the separation of electron-hole pairs in Bi2O3/MoO3 systems. The conduction
band (CB) and valence band (VB) potentials of the Bi2O3 semiconductor are 3.13 and
0.33 eV, respectively. The CB and VB for MoO3 are 3.36and 0.44eV, respectively.
Bi2O3 and MoO3 both can be excited to generate the electron-hole pairs under visible
light irradiation. If the Bi2O3(0.5)/MoO3 composite follows the traditional mechanism
of heterojunction[11,38] in Fig.7(a), the photogenerated holes will transfer from the
VB of MoO3 to the VB of Bi2O3 to directly oxidize MB to degradation productions, at
the same time, photogenerated electrons will transfer from the CB of Bi2O3 to the CB
-
of MoO3 to reduce O2 to ·O2 , because the CB and VB potentials Bi2O3 are both more
negative than those of MoO3.However, due to the CB potential of MoO3 is lower than
the CB of Bi O meanwhile, CB potential of them both are lower than the standard
，
3
2
-
redox potential of O2/·O2 (−0.33 eV versus NHE)[4,7] , the electron transfer from the
-
CB of Bi2O3 to that of MoO3 will lead to the significant decrease in ·O2
-
concentration[16]. This is not consistent with the result that ·O2 is one of the major
14

active species in Bi2O3/MoO3 photocatalytic system.Thus,Bi2O3/MoO3 composite
photocatalyst follows the direct Z-scheme type mechanism in Fig.7(b), under the
visible light irradiation, the photoexcited electrons of Bi2O3 and MoO3 will migrate
from VB to CB and then the fast combination between the photoexcited electrons in
the CB of MoO3 and photoexcited holes in the VB of Bi2O3 will be achieved[15].
Meanwhile, electrons accumulated in the CB of Bi2O3 have more negative potential to
-
reduce the O2 to •O2 [25], and holes retained in the VB of MoO3 will directly oxidize
MB to degradation products. Therefore, the Z-scheme type mechanism of
Bi2O3/MoO3 enables the effective separation of photoexcited electrons and holes, and
it promotes a significant increase in the photocatalytic capability. Furthermore, a
direct Z-scheme mechanism for degradation of MB over Bi2O3/MoO3 composites can
be proved based on the results of energy band positions, active species trapping
experiments and photoluminescence spectroscopy.
4. Conclusions
A direct Z-scheme photocatalyst Bi2O3/MoO3 was successfully prepared via
hydrothermal method and mixing-calcination methods. The prepared Bi2O3/MoO3
composite materials exhibit excellent degradation ability of MB solution than pure
Bi2O3 and MoO3 under visible light irradiation. The rate of MB degradation for the
optimized Bi2O3（0.5）/MoO3 composite is much higher than that of Bi2O3 and MoO3.
Active species trapping experiments and photoluminescence spectroscopy can be used
to prove the formation of the Z-scheme Bi2O3/MoO3 photocatalytic system, which
could enable the effective separation and transfer of photoexcited electrons and holes.
15

The increased photocatalytic ability is owed to the fast combination between the
photoexcited electrons in the CB of MoO3 and photoexcited holes in the VB of
-
Bi2O3. ·O2 and h+ are the major active species in the Bi2O3/MoO3 photocatalytic
reaction system. This study provides a simple way to synthesize high- efficiency
photocatalytic material for water treatment.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (No
. 21277108).
16

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20

Table. 1 The specific surface areas of the Bi2O3, MoO3 and Bi2O3/MoO3 samples
Samples
Bi2O3 ratio（%）
SBET(m2/g)
MoO3
Bi2O3
0
9.2
3.2
7.6
100
50
Bi2O3/MoO3
21

Figure Captions
Fig.1. SEM images of (a) MoO3, (b) Bi2O3, (c) Bi2O3(0.5)/MoO3. (b)XRD pattern of
MoO3, Bi2O3and Bi2O3(wt.%)/MoO3 composites.
Fig.2. (a)TEM image of Bi2O3(0.5)/MoO3 composite and（b）HRTEM image of
Bi2O3(0.5)/MoO3 composite.
Fig. 3. XPS spectra of Bi2O3(0.5)/MoO3: (a) survey, (b) Mo3d, (c) Bi4f, and (d) O 1s.
Fig.4. (a) UV–vis spectra of MoO3，Bi2O3 and Bi2O3(wt.%) /MoO3 composites and (b)
UV–vis spectra of MoO3 and Bi2O3(0.5) /MoO3 composite
Fig.5. (a)Photocatalytic degradation of MB over the as-prepared samples (MoO3,
Bi2O3 and Bi2O3(wt.%)/MoO3 composites) under visible light irradiation.
(b)Photodegradation efficiencies of MB with Bi2O3 (0.5)/MoO3 photocatalyst in
different recycle runs.
Fig.6.(a)Plots of photogenerated active species trapped in the system of
photodegradation of MB byBi2O3(0.5)/MoO3 under visible light irradiation.
(b)Photoluminescence spectra of MoO3 and Bi2O3(wt.%)/MoO3 composites with an
excitation wavelength of 325 nm.
Fig.7. Possible schemes for photogenerated electron–hole pairs separation and
transport at the interface of Bi2O3/MoO3 composite under visible light irradiation:
(a)double charge transfer mechanism and (b) Z-scheme mechanism.
22

23

Bi O /MoO -1
2 3
（ d）
3
Bi O /MoO -0.8
2 3
3
Bi O /MoO -0.5
2 3
3
Bi O /MoO -0.3
2 3
3
•
Bi O /MoO -0.1
2 3
3
•040
•131
• 020
MoO
3
•0100
•081
♦
120
♦200
Bi O
2 3
♦
012
♦041
10
20
30 40
2Theta/degree
50
60
70
Fig.1. SEM images of (a) MoO3, (b) Bi2O3, (c) Bi2O3(0.5)/MoO3. (d)XRD pattern of
MoO3, Bi2O3and Bi2O3(wt.%)/MoO3 composites
24

Fig.2. (a)TEM image of Bi2O3(0.5)/MoO3 composite and（b）HRTEM image of
Bi2O3(0.5)/MoO3 composite.
25

（
a）
0
200
400
600
800
1000 1200
（
Binding Energy eV
）
Mo3d
5/2
（
b
）
Mo3d
3/2
230
232
234
236
238
Binding Energy(eV)
26

Bi4f
7/2
c
（ ）
Bi4f
5/2
156
158
160
162
164
166
（
Binding Energy eV
）
O1s
（
d）
526 528 530 532 534 536 538 540 542
（
Binding Energy eV
）
Fig.3. XPS spectra of Bi2O3(0.5)/MoO3: (a) survey, (b) Mo3d, (c) Bi4f, and (d) O 1s.
27

1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
a
（ ）
MoO
3
Bi O
2 3
Bi O3/MoO -0.1
2
3
Bi O /MoO -0.3
2 3
3
Bi O /MoO -0.5
2 3
3
Bi O /MoO -0.8
2 3
3
Bi O /MoO -1
2 3
3
300
400
500
600
700
Wavelength(nm)
1.0
0.9
0.8
0.7
0.6
0.5
0.4
0.3
0.2
0.1
0.0
（
b）
Bi O /MoO -0.5
2 3
3
MoO
3
300
400
500
600
700
Wavelength
（ nm）
Fig.4. (a) UV–vis spectra of MoO3,Bi2O3 and Bi2O3(wt.%) /MoO3 composites and (b)
UV–vis spectra of MoO3 and Bi2O3 /MoO3 composite.
28

MoO
3
1.6
1.4
1.2
1.0
0.8
0.6
0.4
Bi O
2
3
Bi O /MoO -0.1
2
3
3
Bi O /MoO -0.3
2
3
3
Bi O /MoO -0.5
2
3
3
Bi O /MoO -0.8
2
3
3
Bi O /MoO -1
2
3
3
（
a）
-30
0
30
60
90
120
Irradiation time min
（ ）
1.4
1.2
1.0
0.8
0.6
0.4
Third
Second
First
Fourth
（
b）
-30
0 30 60 90 120 150 180 210 240 270 300 330 360 390 420 450 480 510 540 570
Time
（ min）
Fig.5. (a)photocatalytic degradation of MB over the as-prepared samples MoO3,
Bi2O3 and Bi2O3(wt.%)/MoO3 composites) under visible light irradiation .(b)
photodegradation efficiencies of MB with Bi2O3 (0.5)/MoO3 photocatalyst in different
recycle runs.
29