Flower-like Ag2O/Bi2MoO6 p-n heterojunction with enhanced photocatalytic activity under visible light irradiation

Article abstract of DOI:10.1016/j.molcata.2016.08.009

Ag₂O/Bi₂MoO₆ composed of flower-like Bi₂MoO₆ microspheres (diameter: 1–3 μm) and well dispersed Ag₂O nanoparticles (size: ～10 nm) was prepared via solvothermal deposition-precipitation. The catalytic performance of Ag₂O/Bi₂MoO₆ and other samples in photocatalytic degradation of Rhodamine B (RhB) and methyl orange (MO) dye under visible light irradiation (λ > 420 nm) was studied. Ag₂O/Bi₂MoO₆ p-n heterojunction exhibited higher activity than Bi₂MoO₆ or Ag₂O with the same weight of the corresponding component in Ag₂O/Bi₂MoO₆. It was even more active than a mixture of Bi₂MoO₆ and Ag₂O. The high photocatalytic activity of Ag₂O/Bi₂MoO₆ was due to the efficient separation of photogenerated electron-hole pairs. The photogenerated holes (h⁺) and superoxide radical anions ([Formula presented]₂^?) were found to be the primary reactive species for the degradation of RhB dye in aqueous solution.

Full text of DOI:10.1016/j.molcata.2016.08.009

Accepted Manuscript
Title: Flower-like Ag2O/Bi2MoO6 p-n heterojunction with
enhanced photocatalytic activity under visible light irradiation
Author: Junlei Zhang Huan Liu Zhen Ma
PII:
S1381-1169(16)30331-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.molcata.2016.08.009
MOLCAA 9996
To appear in:
Journal of Molecular Catalysis A: Chemical
Received date:
Revised date:
Accepted date:
28-6-2016
9-8-2016
10-8-2016
Please cite this article as: Junlei Zhang, Huan Liu, Zhen Ma, Flower-
like Ag2O/Bi2MoO6 p-n heterojunction with enhanced photocatalytic activity
under visible light irradiation, Journal of Molecular Catalysis A: Chemical
http://dx.doi.org/10.1016/j.molcata.2016.08.009
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Highlights:
 Synthesis of Ag2O/Bi2MoO6 p-n heterojunction via solvothermal deposition-precipitation
 Ag2O/Bi2MoO6 consists of flower-like Bi2MoO6 microspheres and Ag2O nanoparticles
 High photoactivities for the degradation of organic pollutants under visible light irradiation
1

Graphical Abstract
2

Manuscript revised for publication in Journal of Molecular catalysis A: Chemical
Flower-like Ag2O/Bi2MoO6 p-n heterojunction with enhanced photocatalytic
activity under visible light irradiation
Junlei Zhang, Huan Liu, Zhen Ma*
Shanghai Key Laboratory of Atmospheric Particle Pollution and Prevention (LAP3), Department
of Environmental Science and Engineering, Fudan University, Shanghai, 200433, PR China
* Corresponding author. E-mail: zhenma@fudan.edu.cn
Abstract: Ag2O/Bi2MoO6 composed of flower-like Bi2MoO6 microspheres (diameter: 1–3 μm)
and well dispersed Ag2O nanoparticles (size: ~10 nm) was prepared via solvothermal deposition-
precipitation. The catalytic performance of Ag2O/Bi2MoO6 and other samples in photocatalytic
degradation of Rhodamine B (RhB) and methyl orange (MO) dye under visible light irradiation
(λ > 420 nm) was studied. Ag2O/Bi2MoO6 p-n heterojunction exhibited higher activity than
*Corresponding author. E-mail address: zhenma@fudan.edu.cn
3

Bi2MoO6 or Ag2O with the same weight of the corresponding component in Ag2O/Bi2MoO6. It
was even more active than a mixture of Bi2MoO6 and Ag2O. The high photocatalytic activity of
Ag2O/Bi2MoO6 was due to the efficient separation of photogenerated electron-hole pairs. The
+
−
photogenerated holes (h ) and superoxide radical anions (˙O2 ) were found to be the primary
reactive species for the degradation of RhB dye in aqueous solution.
Key words: Ag2O/Bi2MoO6; p-n Heterojunction; Visible light; Photocatalysis
1. Introduction
Photocatalysis can remedy environmental contamination by degrading organic and
inorganic pollutants in water and air efficiently [1-4]. The kernel of photocatalysis research is the
development of new photocatalysts with interesting morphologies, tailored structures, high
activity, and good recyclability [5-11]. The benchmark TiO2 catalyst can only absorb UV ray that
only accounts for ~4% of the solar spectrum, resulting in poor utilization of solar energy [12, 13].
Thus, the development of visible-light-driven photocatalysts is urgently required.
Bi2MoO6 (band gap: 2.5–2.8 eV) is active for environmental decontamination and water
splitting under visible light irradiation because the bismuth oxide layer with the fluorite Bi-O
structure and perovskite layer with corner-shared octahedral Mo-O structure can facilitate
electron conductivity [14-17]. However, the low quantum yield, inefficient charge separation and
4

transportation, insufficient active sites, and poor selectivity to the desired reaction still call for
the modification/improvement of Bi2MoO6. Several strategies (e.g., suitable textural design [18-
25], doping [26-31], and forming semiconductor heterojunctions by combining Bi2MoO6 with
other semiconductors [32-39]) have been adopted to enhance the photocatalytic performance.
Among them, the construction of a semiconductor heterojunction has drawn much attention due
to its effectiveness [40]. There are many reports on Bi2MoO6-containing n-n heterojunctions
such as Bi3.64Mo0.36O6.55/Bi2MoO6 [41], Bi2MoO6/BiVO4 [42], Bi2MoO6/TiO2 [32],
Bi2O2CO3/Bi2MoO6 [33], Bi2MoO6-RGO [43], g-C3N4/Bi2MoO6 [34], MoS2/Bi2MoO6 [37],
Ag/AgCl-Bi2MoO6 [38], Bi2MoO6/MO (M = Cu, Co3/4, Ni) [39], and Bi2S3/Bi2MoO6 [44].
Compared with n-n heterojunctions, p-n heterojunctions can transmit photogenerated holes
more efficiently and facilitate the separation of electron-hole pairs, thus improving the
photocatalytic activity. However, there are only a few reports on the construction of Bi2MoO6-
based p-n heterojunctions such as BiOX/Bi2MoO6 (X = Cl, Br, I) [45-47].
As a p-type semiconductor, Ag2O (band gap: ~1.3 eV) can be used directly as a
photocatalyst [48, 49] or coupled with some n-type semiconductors to make p-n heterojunction
photocatalysts such as Ag2O/TiO2 [50-52], Ag2O/Bi2O2CO3 [53], Ag2O/BiVO4 [54], Ag2O/g-
C3N4 [55], and Ag2O/ZnO [56]. However, to the best of our knowledge, Ag2O/Bi2MoO6 p-n
heterojunction photocatalysts have not been reported.
5

In the present work, Ag2O/Bi2MoO6 prepared via solvothermal deposition-precipitation was
found to be very active in the photodegradation of Rhodamine B (RhB) and methyl orange (MO)
under visible light irradiation. A possible mechanism for the high activity of Ag2O/Bi2MoO6 was
proposed.
2. Experimental
2.1. Materials
Bi(NO3)3·5H2O, Na2MoO4·2H2O, absolute ethanol, ethylene glycol, AgNO3, NaOH,
Rhodamine B, and methyl orange were purchased from Sinopharm Chemical Reagent. All
chemicals were of analytical grade and used as received.
2.2. Preparation of Bi2MoO6
Bi2MoO6 was prepared solvothermally [44]. Typically, 2 mmol Bi(NO3)3·5H2O and 1 mmol
Na2MoO4·2H2O were ultrasonically dissolved in 10 mL ethylene glycol, respectively. These two
solutions were mixed, 60 mL absolute ethanol was dropped into the mixture slowly, and the
mixture was stirred for 30 min. The clear solution was transferred to a 100 mL autoclave, sealed,
and treated at 160 °C for 12 h. The autoclave was cooled to room temperature naturally. The
solid sample was collected by filtration, washed thoroughly with water-ethanol for three times,
and dried at 60 °C for 24 h.
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2.3. Preparation of Ag2O/Bi2MoO6
Bi2MoO6 powders (0.61 g) were dispersed in 100 mL deionized water with ultrasonication.
Subsequently, 1 mL AgNO3 solution (0.1 mol/L) was dropped into the suspension. The mixture
was kept at 60 oC (using a water bath) and stirred for 30 min to reach complete uptake of Ag+ on
Bi2MoO6. Subsequently, 10 mL aqueous NaOH (0.1 mol/L) was dropped slowly into the mixture,
and the suspension was stirred at 60 °C for 1 h. The mixture was cooled to room temperature
naturally, and the solid sample collected by filtration was washed with water-ethanol for three
times, and dried at 60 °C for 24 h. For comparison, Ag2O was prepared via a similar process
without the involvement of Bi2MoO6.
2.4. Characterization
X-ray diffraction (XRD) measurements were recorded on MSAL XD2 X-ray diffractometer
1
using CuKα radiation at 40 kV and 30 mA with a scanning speed of 8° min− . The oxidation
states of elements of the sample were analyzed by using X-ray photoelectron spectroscopy (Axis
Ultra Dld, Kratos). Scanning electron microscope (SEM) experiments were conducted on a
Shimadz SUPERSCAN SSX-550 field emission scanning electron microscope. Transmission
electron microscope (TEM) experiments were performed by a JEOL JEM-2100F high-resolution
transmission electron microscope. N2 adsorption-desorption isotherms at liquid N2 temperature
were obtained on a Micrometrics ASAP 2020 M+C adsorption apparatus. Optical diffuse
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reflectance spectra were recorded on a UV-VIS-NIR scanning spectrophotometer (Lambda 35,
Perkin-Elmer) using an integrating sphere accessory.
2.5. Evaluation of photocatalytic performance
Photocatalytic performance in the degradation of organic dye RhB and MO was
investigated under visible light irradiation (λ > 420 nm) at room temperature (Fig. S1). In a
typical procedure, a suitable amount of photocatalyst was suspended in 50 mL of RhB (10 mg/L,
initial pH = 6.21) or MO (5.75 mg/L, initial pH = 7.37) solution in a 100 mL beaker. The beaker
was placed in a water bath placed on top of an electromagnetic stirrer. The solution was stirred
(854 r/min) for 30 min in the dark before the Xe lamp (300 W, HSX-F300, Beijing NBeT
Technology Co., Ltd.) coupled with a UV-cutoff filter (420 nm) was turned on. The distance
between the Xe lamp and the beaker mouth was 15 cm. The photocatalytic tests were conducted
in a 100 mL beaker with a cold bath, and a portion of suspension (~4 mL each) was sampled in
certain intervals, centrifuged to separate the catalyst and the liquid, and the clear liquid was then
monitored by a UV-5200PC spectrometer.
Total organic carbon (TOC) analysis was carried out by adding 100 mg Ag2O/Bi2MoO6 into
100 mL RhB aqueous solution (80 mg/L). Prior to irradiation, the suspension was magnetically
stirred in the dark for 60 min. During visible light irradiation, 10 mL suspension was collected at
8

given intervals and filtered by the membrane pore size of 0.45 μm to remove the photocatalyst,
and then was detected by a Shimadzu TOC-VCPH total organic carbon analyzer.
Experiments involving radicals capture were conducted by adding 1 mmol ammonium
oxalate (AO), 1 mmol isopropanol (IPA), or 0.02 mmol benzoquinone (BQ) into RhB (5 mg/L,
50 mL) before the standard photocatalytic tests.
3. Results and discussion
3.1. Preparation and characterization of catalysts
Fig. 1 XRD patterns of as-prepared Bi2MoO6, Ag2O, Ag2O/Bi2MoO6 and standard XRD patterns
of Bi2MoO6 (JCPDS 21-0102) and Ag2O (JCPDS 43-0997).
9

Fig. 1 shows the XRD patterns of as-prepared Ag2O/Bi2MoO6, Bi2MoO6, and Ag2O. All
peaks from Bi2MoO6 can be indexed to orthorhombic Bi2MoO6 (JCPDS 21-0102), and the peaks
for Ag2O can be assigned to cubic Ag2O (JCPDS 43-0997). The XRD pattern of Ag2O/Bi2MoO6
is similar to that of Bi2MoO6; four obvious peaks at 28.3°, 32.6°, 46.7°, and 55.6° can be
assigned to the (131), (002), (202), and (133) planes of orthorhombic Bi2MoO6. No characteristic
peaks peculiar to Ag2O are observed, due to the low Ag content (theoretical value: 1.84 wt.%).
10

Fig. 2 Survey XPS spectrum of Ag2O/Bi2MoO6 (A) and high-resolution XPS spectra of Bi 4f (B),
Mo 3d (C), O 1s (D), and Ag 3d (E) from Ag2O/Bi2MoO6. The element mass fraction from
Ag2O/Bi2MoO6 (F).
11

Fig. 3 SEM images of Bi2MoO6 (A) and Ag2O/Bi2MoO6 (B-D).
The morphologies of Ag2O/Bi2MoO6, Bi2MoO6, and Ag2O were studied by SEM. Bi2MoO6
is composed of many flower-like microspheres (diameter: 1–3 μm, Fig. 3A) built from
nanosheets (thickness: ~13 nm, Fig. S2). Ag2O/Bi2MoO6 consists of Bi2MoO6 microspheres
(diameters: 1–3 μm, Fig. 3B) built from nanosheets (thickness: ~13 nm) and Ag2O nanoparticles
(size: ~12 nm, Fig. 3C and 3D). There are many nanopores in nanosheets as well as hierarchical
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macro-pores among microspheres (Fig. 3B-D). On the other hand, the as-synthesized Ag2O is
composed of plenty of nanoparticles with a diameter of ∼12 nm (Fig. S3).
Fig. 4 TEM (A-B) and HRTEM (C) images of the as-prepared Ag2O/Bi2MoO6.
The TEM images confirm that Ag2O/Bi2MoO6 displays a flower-like microspherical
morphology with diameter of ~1.2 μm (Fig. 4A), and Ag2O nanoparticles (size: ~10 nm) are
uniformly distributed on Bi2MoO6 nanosheets (Fig. 4B). There are three sets of lattice fringes
13

with different distances (0.335, 0.316, and 0.323 nm, Fig. 4C). The crystal plane with 0.335 nm
d-spacing agrees with the (110) lattice plane of cubic Ag2O. Two lattice fringes with 0.316 and
0.323 nm d-spacings agree with the (131) and (140) crystal planes of orthorhombic Bi2MoO6.
The HAADF-STEM and elemental mapping results indicate that Ag, Bi, Mo, and O
distribute homogeneously on Ag2O/Bi2MoO6 (Fig. S4A-S4E). Furthermore, the EDX pattern
(Fig. S4F) reveals that, besides the Cu signal from the Cu grid, there are only Bi, Mo, O, and Ag
elements, and the Ag content is 1.73 wt.%, close to the theoretical weight (1.84 wt.%).
14

Fig. 5 Nitrogen adsorption-desorption isotherms of Bi2MoO6 (A) and Ag2O/Bi2MoO6 (B).
The N2 adsorption-desorption isotherms of Bi2MoO6 and Ag2O/Bi2MoO6 were measured
(Fig. 5). The BET surface areas of Bi2MoO6 and Ag2O/Bi2MoO6 are 55.2 and 44.9 m²/g,
15

respectively. The pore size distributions calculated from the adsorption branches reveal the
existence of nanopores (diameter: ~15.4 nm) in both Bi2MoO6 and Ag2O/Bi2MoO6.
Fig. 6 UV-Vis diffuse reflectance spectra of pure Bi2MoO6, pure Ag2O and Ag2O/Bi2MoO6.
The absorption spectra were recorded (Fig. 6). Bi2MoO6 exhibits strong absorption from the
UV to visible-light region with an absorption edge around 470 nm (band gap: 2.64 eV),
consistent with the existing results [42, 44, 46, 61]. For comparison, Ag2O exhibits a broad
absorption in the whole UV-Vis region even up to infrared and the narrow band gap is 1.3 eV,
consistent with previous reports [55, 56, 62-64]. Interestingly, the introduction of Ag2O broadens
and increases the light absorption of Bi2MoO6, and the wavelength threshold of Ag2O/Bi2MoO6
is estimated to be ~575 nm, corresponding to a band gap of ~2.15 eV.
16

3.2. Photocatalytic performances under visible light irradiation
The photocatalytic degradation of Rhodamine B (RhB) under visible light irradiation (λ >
420 nm) was studied (Fig. 7). Prior to irradiation, the mixture containing RhB solution and a
catalyst was kept in the dark for 30 min to ensure the adsorption-desorption equilibrium. Fig. 7A
shows the temporal evolution of the absorption spectra when using Ag2O/Bi2MoO6 (30 mg) as
the photocatalyst. A rapid decrease of RhB absorption at 553 nm is observed, and the absorption
band shifts to shorter wavelengths. The color of RhB solution gradually changes from red to pale
yellow as the reaction proceeds. The RhB photodegradation efficiency reaches 95% after 60 min
of reaction (Fig. 7B). For comparison, the degradation of RhB is extremely slow without
photocatalyst even under visible light irradiation. The RhB photodegradation efficiencies
achieved by using P25 (a benchmark photocatalyst-TiO2, 30 mg), Bi2MoO6 (29.4 mg), Ag2O (0.6
mg), and a mixture of Bi2MoO6 (29.4 mg) and Ag2O (0.6 mg) are 23.6%, 45%, 30.7%, and 42%,
respectively, after 60 min of reaction.
The degradation data were analyzed with the pseudo-first-order model [44, 65]: -ln(C/C0) =
Kt, where C is the concentration of RhB at time t, C0 is the original concentration of RhB, and K
is the reaction rate constant (Fig. S5). The K values of Ag2O/Bi2MoO6 (30 mg), P25 (30 mg),
17

Bi2MoO6 (29.4 mg), Ag2O (0.6 mg), and a mixture of Bi2MoO6 (29.4 mg) and Ag2O (0.6 mg)
are 0.047, 0.00438, 0.00982, 0.00589, and 0.00878 min-1, respectively.
Fig. 7 (A) Absorption spectra of RhB with reaction time in the presence of Ag2O/Bi2MoO6 (30
mg); (B) the adsorption and degradation curves of RhB without photocatalyst and in the presence
18

of different catalysts under visible light irradiation: P25 (30 mg), Bi2MoO6 (29.4 mg), Ag2O (0.6
mg), 29.4 mg Bi2MoO6 + 0.6 mg Ag2O, or Ag2O/Bi2MoO6 (30 mg).
The photocatalytic degradation of MO dye under visible light irradiation (λ > 420 nm) was
studied (Fig. 8). Using Ag2O/Bi2MoO6 (30 mg) as the photocatalyst, the characteristic absorption
peak of MO at 463 nm diminishes quickly as the reaction proceeds, and the color of MO dye
solution gradually changes from yellow to colorless. The photodegradation efficiency reaches
90% after 180 min of reaction. For comparison, the degradation of MO is extremely slow
without photocatalyst under visible light irradiation. When using P25 (30 mg), Bi2MoO6 (29.4
mg), Ag2O (0.6 mg), or a mixture of 29.4 mg Bi2MoO6 and 0.6 mg Ag2O, the photodegradation
efficiencies are 17.7%, 31.1%, 34.3%, and 32.1%, respectively, after 180 min of reaction.
The pseudo-first-order model was used to investigate the reaction kinetics of MO
degradation (Fig. S6). The K values of Ag2O/Bi2MoO6 (30 mg), P25 (30 mg), Bi2MoO6 (29.4
mg), Ag2O (0.6 mg), and a mixture of Bi2MoO6 (29.4 mg) and Ag2O (0.6 mg) are 0.01158,
0.00093, 0.00186, 0.00243, and 0.00192 min-1, respectively, indicating that Ag2O/Bi2MoO6 p-n
heterojunction possesses enhanced photocatalytic activity.
19

Fig. 8 (A) Absorption spectra of MO with reaction time in the presence of Ag2O/Bi2MoO6; (B)
the adsorption and degradation curves of MO without photocatalyst and in the presence of
different catalysts under visible light irradiation: P25 (30 mg), Bi2MoO6 (29.4 mg), Ag2O (0.6
mg), 29.4 mg Bi2MoO6 + 0.6 mg Ag2O, or Ag2O/Bi2MoO6 (30 mg).
20

The effect of initial concentration of RhB dye on the photocatalytic activity of
Ag2O/Bi2MoO6 was further studied. The photodegradation efficiency (Fig. S7A) and rate (Fig.
S7B) decrease with the increase of initial dye concentration, possibly due to two reasons. One is
the visible light screening effect of the dye itself. When the concentration of the dye is high, a
significant portion of visible light may be absorbed by the dye molecules rather than by
Ag2O/Bi2MoO6, and thus the efficiency of the catalytic reaction is reduced. Another possible
reason may be that the intermediates formed during the photocatalytic process may compete with
the dye molecules for the limited adsorption and catalytic sites on the catalyst particles and thus
inhibit decolorization [66]. Such suppression would be even more pronounced in the presence of
an elevated level of degradation intermediates formed upon an increased initial dye concentration.
Total organic carbon (TOC) value is an important index for the mineralization degree of
organics. Herein, the mineralization of RhB dye was investigated by immersing 100 mg
Ag2O/Bi2MoO6 in 100 mL RhB aqueous solution (80 mg/L) under visible light irradiation, and
the TOC value was recorded during the photocatalytic process (Fig. S8). With the increase of
irradiation time, the TOC concentration continuously decreases, indicating that RhB is steadily
mineralized. After 240 min, the TOC concentration decreases from 69.15 mg/L at 0 min to 30.02
mg/L at 240 min, reaching a high mineralization ratio of 56.6%.
21

The recyclability of Ag2O/Bi2MoO6 during RhB degradation under visible light irradiation
was studied. Fig. 9 shows the photodegradation efficiencies in eight cycles. Interstingly, the
photocatalytic activity is gradually improved as more photocatalytic runs are conducted in the
first four cycles. A similar phenomenon was observed previously [67]. Notably, after
additionally four cycles of photodegradation, Ag2O/Bi2MoO6 does not exhibit loss of activity
(Fig. 9), demonstrating its good recyclability. XRD characterization (Fig. S9A) reveals that
Ag2O/Bi2MoO6 collected after the reaction exhibits the similar diffraction peaks as as-
synthesized Ag2O/Bi2MoO6. In addition, according to XPS data (Fig. S9B), some Ag+ is reduced
to Ag0 after the recycling experiments, implying that the in situ formed Ag-Ag2O/Bi2MoO6 is
probably more active for this reaction.
Fig. 9 Cycling runs in photocatalytic degradation of RhB (5 mg/L, 50 mL) over Ag2O/Bi2MoO6
(30 mg) under visible light irradiation.
22

It is essential to investigate the main active species on the catalyst. Trapping agents, such as
benzoquinone (BQ), ammonium oxalate (AO), AgNO3, and isopropyl alcohol (IPA) have been
-
used to capture superoxide radical anions (˙O2 ), photogenerated holes (h+), photogenerated
electrons (e−), and hydroxyl free radicals (˙OH), respectively [37, 65]. As shown in Fig. 10, the
addition of AgNO3 or IPA has no obvious effect on the photocatalytic activity in the degradation
of RhB, suggesting that e− and ˙OH are not the primary active species. However, the addition of
AO or BQ makes the photogradation efficiency descrease from 96.6% to 17.3% or 25.5% after
60 min of reaction, and the photogradation rate decreases from 0.05281 to 0.00301 and 0.00486
-
min-1, respectively (Fig. S10). Thus, ˙O2 and h+ are the primary active species in the
photodegradation of RhB over Ag2O/Bi2MoO6.
23

Fig. 10 Comparison of the photocatalytic performance of Ag2O/Bi2MoO6 for the degradation of
RhB (5 mg/L, 50 mL) with or without adding AgNO3, ammonium oxalate (AO), isopropyl
alcohol (IPA), or benzoquinone (BQ), under visible light irradiation.
The potentials of the conduction band (CB) and valence band (VB) edges of Bi2MoO6 and
Ag2O were investigated. The band edge positions of Bi2MoO6 (band gap: 2.64 eV) and Ag2O
(band gap: 1.3 eV) were evaluated based on the following empirical equations [68]: EVB = X – E0
+ 0.5Eg (1); ECB = EVB – Eg (2), where EVB is the VB edge potential and X is the
electronegativity of the semiconductor, which is the geometric mean of the electronegativity of
the constituent atoms. The X values for Bi2MoO6 and Ag2O are calculated to be 5.50 [44] and
5.29 eV [53], respectively. E0 is the energy of free electrons on the hydrogen scale (∼4.5 eV) and
Eg is the band gap energy of the semiconductor. According to eqn (1) and (2), the EVB and ECB
24

values of Bi2MoO6 are calculated to be 2.32 and -0.32 eV, and those of Ag2O are determined to
be 1.44 and 0.14 eV, respectively.
On the base of the above results and energy band diagram (Fig. 11), one can conclude that
there are chiefly two factors contributing to the high photocatalytic activity of Ag2O/Bi2MoO6.
On the one hand, the presence of many nano-pores and macro-pores in Ag2O/Bi2MoO6 (Fig. 3B-
D) may serve as transport paths for small molecules, thus facilitating the adsorption and
photocatalytic degradation of RhB/MO dye [44, 65, 69, 70]. On the other hand, the p-n
heterojunction built between p-Ag2O and n-Bi2MoO6 is an effective architecture for highly
efficient charge collection and separation. When Ag2O/Bi2MoO6 is irradiated by visible light, the
photogenerated carriers will break the thermodynamic equilibrium, and generate quasi-Fermi
levels (quasi-static equilibrium) for each semiconductor (Fig. 11). Due to the built-in potential in
Ag2O/Bi2MoO6, photogenerated electrons (e−) in the CB of Ag2O will drift to the CB of
Bi2MoO6, resulting in an upward movement of the quasi-Fermi level of Bi2MoO6, whereas the
drifting of photogenerated holes (h+) leads to a downward movement of the quasi-Fermi level of
Ag2O. The holes can be consumed to directly oxidize RhB/MO dye. Meanwhile, electrons can
-
reduce the chemisorbed O2 to produce the strong oxidizing species ˙O2 that can break down the
chromophores of RhB/MO dye into small molecules, e.g., H2O and CO2 [40, 65, 66, 71-74].
Finally, quasi-equilibrium energy band structure of p-n heterojunction would be formed under
25

visible light irradiation. Such a structure can efficiently generate VB-holes (Bi2MoO6) with
strong oxidation power and CB-electrons (Ag2O) with strong reduction power. Thus, a large
number of the photogenerated holes (electrons) will be concentrated on the Ag2O (Bi2MoO6) and
consumed, further participating in photocatalytic reactions to enhance the photocatalytic activity.
Fig. 11 The proposed possible mechanism for the improvement of photocatalytic activity.
4. Conclusions
Ag2O/Bi2MoO6 p-n heterojunction composed of flower-like Bi2MoO6 microspheres
(diameters: 1~3 μm) and Ag2O nanoparticles (size: ~10 nm) was prepared by solvothermal
deposition-precipitation. Ag2O nanoparticles were uniformly distributed on flower-like Bi2MoO6
microspheres. Ag2O/Bi2MoO6 displayed much higher photocatalytic activity than individual
Bi2MoO6, Ag2O, and a mixture of Bi2MoO6 and Ag2O in the photocatalytic degradation of RhB
26

and MO dye. The high photocatalytic activity of Ag2O/Bi2MoO6 can be attributed to its porous
structures and p-n heterojunctions, promoting the efficient absorption of visible light and
efficient separation of photogenerated electron-hole pairs, respectively. Furthermore, the
recycling experiments demonstrate that Ag2O/Bi2MoO6 possesses stable photocatalytic activity.
As a result, it has great potential as an efficient and stable visible-light-driven photocatalyst for
the purification of wastewater and other pollutants in the future.
Acknowledgements
Zhen Ma acknowledges the financial support by National Natural Science Foundation of
China (Grant No. 21477022).
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