Alkali-treatment synthesis of bismuth vanadium oxide photocatalysts with different morphologies

Article abstract of DOI:10.1016/j.jssc.2020.121296

The bismuth vanadium oxide photocatalysts with different phases and morphologies were facilely synthesized by the alkali-treatment synthesis method. BiVO₄ polyhedrons as the raw materials were firstly prepared by a facile hydrothermal method. With the change of the concentrations of NaOH, the surfaces of the raw BiVO₄ polyhedrons became rough and then other phases appeared, such as Bi₂O₃ regular tetrahedrons, Bi₇VO₁₃ sheets and rods, and Bi₂₅VO₄₀ triangle-blocks and cuboids. When the concentration of NaOH was 0.1 ?M, the obtained samples were the composites of the polyhedral BiVO₄ and regular tetrahedral Bi₂O₃, which exhibited the best photocatalytic activity in the degradation of several organic pollutants. From the exploration of the degradation mechanism, it showed that the transfer rate of photogenerated electron–hole pairs on the composites of BiVO₄ and Bi₂O₃ was promoted. The h⁺, ?OH and O₂?^- played key effect in the degradation process. Furthermore, the bismuth vanadium oxide photocatalysts also showed antibacterial activities towards E. coli.

Full text of DOI:10.1016/j.jssc.2020.121296

Journal of Solid State Chemistry 286 (2020) 121296
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Alkali-treatment synthesis of bismuth vanadium oxide photocatalysts with
different morphologies
,
Wenjuan Li a *, Defen Kong a, Tingjiang Yan a, Meng Shi b, Desheng Kong a, Yuanyuan Feng a,
Zhihong Jing a, Jinmao You a
a Shandong Province Key Laboratory of Life-Organic Analysis, Qufu Normal University, Qufu, 273165, PR China
b Shandong Provincial Key Laboratory of Laser Polarization and Information Technology, Qufu Normal University, Qufu, 273165, PR China
A R T I C L E I N F O
A B S T R A C T
Keywords:
Alkali treatment
Phases
Morphologies
Photocatalysis
Antibacterial activity
The bismuth vanadium oxide photocatalysts with different phases and morphologies were facilely synthesized by
the alkali-treatment synthesis method. BiVO4 polyhedrons as the raw materials were firstly prepared by a facile
hydrothermal method. With the change of the concentrations of NaOH, the surfaces of the raw BiVO4 polyhedrons
became rough and then other phases appeared, such as Bi2O3 regular tetrahedrons, Bi7VO13 sheets and rods, and
Bi25VO40 triangle-blocks and cuboids. When the concentration of NaOH was 0.1 M, the obtained samples were the
composites of the polyhedral BiVO4 and regular tetrahedral Bi2O3, which exhibited the best photocatalytic ac-
tivity in the degradation of several organic pollutants. From the exploration of the degradation mechanism, it
showed that the transfer rate of photogenerated electron–hole pairs on the composites of BiVO4 and Bi2O3 was
promoted. The hþ, ꢀOH and O2ꢀ- played key effect in the degradation process. Furthermore, the bismuth vana-
dium oxide photocatalysts also showed antibacterial activities towards E. coli.
1. Introduction
rice-like shape BiVO4 were synthesized by low-heating solid-state
chemical reaction method [19]. These studies found that the perfor-
Water pollution of the world had been a major threat to the sur-
vival of human security, human health, and economic and social sus-
tainable development [1–3]. A wide range of traditional approaches
have been widely used in the water pollutant treatment, including
adsorption technology [4], ion-exchange method [5], advanced
oxidation processes [6], biological treatment [7] and membrane sep-
aration technology [8]. Photocatalytic technology as a highly effective
method was well applied to deal with water pollution by using sun-
light. In generally, photocatalysts are diverse, such as TiO2 [9,10],
Ag3PO4 [11], BiFeO3 [12], Ca3Bi8O15 [13], BiOClxBr1-x [14] and
BiVO4. BiVO4 as a visible-light response photocatalyst has been stud-
ied by many researchers. Among these, several researchers have
focused on the preparation of BiVO4 samples with various morphol-
ogies via some special prepared approaches. For example, the
cuboid-like BiVO4 microsphere with high photocatalytic performance
was prepared by a simple co-precipitation method [15]; a coral-like
BiVO4 photocatalyst was synthesized by a facile calcination method
[16]; the needle-like or dendritic-like BiVO4 samples were prepared by
a low-temperature hydrothermal method [17,18]; the peanut-like and
mances of BiVO4 with different morphologies showed some photo-
catalytic activities towards pollutants. Previously, our team used a
facile hydrothermal method to prepare polyhedral BiVO4 [20]. And it
also owned a certain photocatalytic activity towards dyes. But the low
specific surface area of the polyhedral BiVO4 inhibited the increase of
the activity. Therefore, in order to further advance the activities of
BiVO4, we originally wanted to use the alkaline treatment to etch its
surface and increase the specific surface area. But with the intensive
study, we had an accidental discovery that different phases and mor-
phologies of bismuth vanadium oxide photocatalysts can be controlled
and adjusted by changing the amount of NaOH in the synthesis pro-
cess. Several phases with the general formula Bi2nV2O3nþ5 (n ¼ 2, 3.5,
4, and 6) have been reported using stoichiometric mixtures of Bi2O3
and V2O5, such as Bi2VO5.5, Bi4V2O11, Bi28V8O62, Bi12V2O23 and
Bi46V8O89 [21]. However, the series of bismuth vanadium oxides (such
as Bi7VO13 and Bi25VO40) synthesized from BiVO4 in-situ have not
been studied before. In addition, only a few studies reported about the
bactericidal action of bismuth vanadium oxide [22–24]. For example,
C. Adan research group reported that the co-doping of Er3þand Y3þ
ꢀ
* Corresponding author.
E-mail address: liwenjuan2801@163.com (W. Li).
https://doi.org/10.1016/j.jssc.2020.121296
Received 25 June 2019; Received in revised form 15 February 2020; Accepted 1 March 2020
Available online 4 March 2020
0022-4596/© 2020 Elsevier Inc. All rights reserved.

W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
into BiVO4 exhibited enhanced photocatalytic activity for the inacti-
vation of Escherichia coli (E. coli) under simulated solar light irradi-
ation [23]. InVO4/BiVO4 materials exhibited the most excellent
coupling adsorption and photocatalytic performance toward the
deterioration of bacteria and microalga under visible light irradiation
[24]. However, there were no reports about the antibacterial activities
2.3. Tests of photocatalytic properties
The photocatalytic processes towards the removal of organic pollut-
ants in the liquid were as follows. The visible-light source was a 500 W
halogen lamp (Philips Electronics) positioned beside a cylindrical reac-
tion vessel with a plane side. Two cutoff filters (420 and 800 nm) were
placed before the vessel to ensure that irradiation of the degradation
system occurred only by visible-light wavelengths. The photo energy
density was 17 mW/cm2. The overall system was cooled by wind and
water to maintain the room temperature. A 0.08 g portion of photo-
catalysts was added into 80 ml rhodamine B (RhB, 10ꢃ5 M) solution in a
100 ml Pyrex glass vessel, respectively. Prior to irradiation, the suspen-
sions were magnetically stirred in the dark for 1 h to ensure the equi-
librium of the working solution. After irradiation, at given time intervals,
3 ml suspensions were sampled and centrifuged to remove the catalysts.
The degraded solution was analyzed using UV–vis spectrophotometer
and the maximum absorption peak was monitored. We also tested the
removal rates of other organic pollutants in the photodegradation pro-
cess, such as hydroxyl azobenzene (HAB, 5 ppm), methylene blue (MB, 5
ppm) and methyl orange (MO, 5 ppm). The experimental processes were
the same as the degradation of RhB.
of other phases of bismuth vanadium oxides, such as Bi2O3, Bi7VO13
,
Bi25VO40 and their composites. Therefore, we decided to continue
using the special method to synthesize a series of bismuth vanadium
oxide samples with different crystalline phases and various morphol-
ogies and further study their photocatalytic and antibacterial activ-
ities. We hope this method would be helpful to synthesize novel
photocatalysts with good catalytic performance and these materials
have potential applications in the catalytic and biological fields in the
future.
2. Experimental methods
2.1. Preparation of samples
All chemical reagents used in this experiment were of analytical pu-
rity, and without further purification. Deionized water was used
throughout this study.
The photocatalytic activities of the catalysts were also investigated for
inactivation of E. coli under visible light irradiation. Prior to each inac-
tivation experiment, all glass apparatuses were sterilized at 120 ꢁC for 30
min with autoclave. Typically, the E. coli cells suspensions were obtained
from the treated E. coli strain. The E. coli strain was cultivated in a Luria-
Bertani (LB) medium at 37 ꢁC for 12 h. After that, the E. coli cells sus-
pensions were preliminarily achieved. A 0.05 g portion of photocatalysts
First, we used a facile hydrothermal method to prepare BiVO4 as the
raw materials [20]. Then, the as-prepared BiVO4 materials (1 mmol)
were dealt with NaOH (50 ml) with different concentrations (0.03
M/0.05 M/0.1 M/0.15 M/0.2 M/0.4 M/1 M) in a Teflon-lined stainless
steel autoclave with a capacity of 100 ml. After being stirred for 1 h at
room temperature, the sealed reactor was heated to 180 ꢁC and main-
tained at this temperature for 12 h. The final products were filtered and
washed with deionized water and ethanol several times. Finally, after
being dried at 80 ꢁC for several hours, the bismuth vanadium oxide
samples were obtained and denoted. As a comparison, Bi(NO3)3⋅5H2O (1
mmol) instead of the as-prepared BiVO4 as raw materials were used to
synthesize the Bi2O3 samples, and the other synthetic steps were the
same as those described above.
and the E. coli cells suspensions (500 μl) were added into 49.50 ml 0.9%
NaCl solution in a 100 ml Pyrex glass vessel, respectively. Prior to irra-
diation, the suspensions were magnetically stirred in the dark for 1 h to
ensure the equilibrium of the working suspensions. After irradiation, at
given time intervals, 4 ml suspensions were sampled and centrifuged to
remove the catalysts. Ultimately, 100 μl of the degraded solutions was
immediately coated on the LB agar plates to culture the bacteria at 37 ꢁC
for 12 h, after which the number of bacterial colonies were observed.
2.2. Characterization of samples
3. Results and discussion
The crystalline phases of the as-prepared samples were determined by
a MiniFlex-600 X-ray diffractometer (XRD) at 30 kV and 10 mA with Cu
3.1. Physicochemical properties
Kα
radiation at a scanning rate of 10ꢁ/min. Scanning electron microscope
Fig. 1 showed the XRD patterns of the bismuth vanadium oxide
samples synthesized by NaOH aqueous solution with different
(SEM) images were obtained on a JEOL JSM 6700F instrument with an
accelerating voltage of 5 kV. The obtained samples were also charac-
terized by Sigma 500VP field emission scanning electron microscope
(FESEM) and energy dispersive spectroscope (EDS). Transmission elec-
tron microscope (TEM) images were collected by a JEM-2100PLUS in-
strument operated at an accelerating voltage of 200 kV. The UV–Vis
spectra of various liquid samples and diffuse reflectance spectra (DRS) of
the photocatalysts were performed on Shanghai Youke UV756CRT
UV–Vis spectrophotometer and Varian Cary 500 UV–Vis spectropho-
tometer with an integrating sphere attachment ranging from 200 to 800
nm, respectively. X-ray photoelectron spectra (XPS) were conducted on
an ESCALAB 250 photoelectron spectroscopy (Thermo Fisher Scientific
Inc.) at 3.0 ꢂ 10ꢃ10 mbar with monochromatic AlK
α Radiation. Photo-
electrochemical measurements were performed using a CHI-660D elec-
trochemical workstation (CH Instruments, USA) in a conventional three-
electrode cell, filled with 0.1 M Na2SO4 electrolyte. The catalyst was
deposited as a film form on a 1 cm ꢂ 1 cm indium–tin–oxide conducting
glass served as working electrode. The saturated calomel electrode (SCE)
was used as the reference electrode, and the Pt plate was used as the
counter electrode. The photoluminescence (PL) spectra were obtained by
using a F-4600 Fluorescence spectrophotometer with an excitation
wavelength of 325 nm. The as-prepared samples and E. coli cells were
observed by LSM 880 NLO two-photon and confocal laser scanning
microscopy.
Fig. 1. XRD patterns of the bismuth vanadium oxide samples synthesized by
NaOH aqueous solution with different concentrations.
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W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
concentrations. As can be seen from Fig. 1, all the diffraction peaks of the
BiVO4 samples (prepared as the raw materials) can be readily indexed to
pure monoclinic crystals of BiVO4 (JCPDS No. 14–0688) with charac-
teristic diffraction peaks at 18.9ꢁ, 28.9ꢁ and 30.5ꢁ. And a set of diffraction
peaks of the Bi2O3 samples (prepared for comparison) were assigned to
crystalline Bi2O3 (JCPDS No. 50–1088) with characteristic diffraction
peaks at 27.5ꢁ, 30.2ꢁ and 32.8ꢁ. After alkali etching in-situ, the bismuth
vanadium oxide samples appeared with different crystal phases and
morphologies. When the NaOH concentration was 0.03 or 0.05 M, the
surfaces were slowly etched and became rough (as shown in the
following). And their XRD patterns were still indexed to monoclinic
BiVO4. When the NaOH concentration was 0.1 M, besides monoclinic
BiVO4 in the samples, there was another phase - Bi2O3 appeared. The
samples were the composites of BiVO4 and Bi2O3. As NaOH concentra-
tion was increased to 0.15 M or 0.2 M, another new crystal phase -
Bi7VO13 showed and the XRD patterns can be readily indexed to a
monoclinic crystalline Bi7VO13 (JCPDS No. 44–0322) with characteristic
diffraction peaks at 28.0ꢁ and 32.9ꢁ. When NaOH concentration was 0.4
M or 1 M, the XRD patterns of the new samples can be well indexed to a
phase of Bi25VO40 (JCPDS No. 46–0419) with characteristic diffraction
peaks at 27.5ꢁ and 32.7ꢁ. Therefore, with alkali etching or treatment in-
situ, the pure BiVO4 samples can be transformed into other phases such as
Bi2O3, Bi7VO13 and Bi25VO40.
The SEM images of the bismuth vanadium oxide samples synthesized
by NaOH aqueous solution with different concentrations were shown in
Fig. 2. It can be seen that, after the alkali treatment in-situ, the mor-
phologies of the obtained samples had a great change. When the NaOH
concentration was 0.03 or 0.05 M, the pure monoclinic BiVO4 samples
were etched in-situ on the (040) facets of the polyhedrons. The smooth
surfaces were slowly etched and became rough. In other words, the
NaOH can etch the preferential facets of BiVO4 crystal and change the
surface morphology of the samples. When the concentration of NaOH
was 0.1 M, the particular Bi2O3/BiVO4 samples were obtained containing
polyhedral BiVO4 with rough surfaces and regular tetrahedral Bi2O3.
Fig. 2. SEM images of the bismuth vanadium oxide samples synthesized by NaOH aqueous solution with different concentrations.
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W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
However, when the concentration of NaOH exceeded 0.1 M, the phases
and morphologies of the products were diverse. For example, when
NaOH concentrations were 0.15 and 0.2 M, the morphologies of Bi7VO13
were sheets and rods, respectively. When NaOH concentrations were 0.4
and 1 M, the morphologies of the obtained Bi25VO40 were triangle-blocks
and cuboids, respectively. Compared with these samples, the
morphology of pure Bi2O3 (prepared for comparison) was a flowered
structure assembled by triangular flakes. The results showed that upon
NaOH etching and treatment, the polyhedral BiVO4 can be transformed
into other bismuth vanadium oxide photocatalysts with different phases
and morphologies (BiVO4 polyhedrons with rough surfaces, Bi2O3 reg-
ular tetrahedrons, Bi7VO13 sheets and rods, and Bi25VO40 triangle-blocks
and cuboids).
To further determine the compositions of different bismuth vanadium
oxide samples, we also used EDS test and TEM to testify the above results.
The analysis results of EDS (Table 1) showed that the atomic ratios of
bismuth and vanadium elements in different samples were largely in
accord with the compositions of the corresponding BiVO4, Bi2O3,
Bi7VO13 and Bi25VO40 phases. When the NaOH concentrations were 0.1,
0.15 and 0.2 M, the samples were the composites of two phases. The
results were in line with the XRD conclusions. The sample which was
prepared at 0.1 M NaOH was further characterized by EDS mapping
observations and TEM images (shown in Fig. 3). In terms of the EDS
mapping observations (shown in Fig. 3a–e), it can be seen that the
composites were composed of Bi, V and O elements. Combined with the
results of their atomic ratios (Table 1), we can infer that the polyhedrons
with rough surfaces were BiVO4 and the regular tetrahedrons were Bi2O3.
The results of TEM (Fig. 3f) also confirmed this conclusion. The fringes of
d ¼ 0.47 and 0.36 nm belonging to BiVO4 and Bi2O3 respectively can be
also found in it.
The surface compositions and chemical states of elements of Bi2O3/
BiVO4 sample were also analyzed by XPS (shown in Fig. 4) technique. As
shown in XPS survey spectrum (Fig. 4a), only the characteristic peaks of
Bi, V, O and C elements were detected on the surface of the sample. The
binding energies of Bi 4f7/2 and Bi 4f5/2 which can be detected at 158.9
and 164.1 eV respectively were typical for Bi3þ cations (Fig. 4b) [25].
The peaks located at 517.0 and 524.6 eV can be assigned to the V 2p3/2
and V 2p1/2 respectively, suggesting the existence of the V5þ oxidation
state (Fig. 4c) [26]. In Fig. 4d, the asymmetrical O 1s signals were fitted
into three bands at 530.0, 531.6, and 534.4 eV. The peaks at 530.0 and
531.6 eV were attributed to O2ꢃ at the lattice sites and oxygen-vacancy
regions, respectively, and the band at 534.4 eV was attributed to sur-
face adsorbed oxygen species as hydroxyl group or Oꢃ defect oxide [27].
Simply put, all the results of EDS, TEM and XPS confirmed the presence of
Bi, V and O elements in the composite.
photocatalysts with different phases and morphologies were investigated
and assumed in Scheme 1. As a conventional mineral reagent, the pres-
ence of appropriate amount of NaOH in the hydrothermal process can
remarkably influence the reaction rate. With lower amount of NaOH, the
(040) facets of polyhedral BiVO4 were etched slowly and the surfaces of
the samples became rough. The low concentrations of NaOH did not lead
to the appearance of new phases. When the concentration of NaOH
continued to be increased, some BiVO4 samples were etched and sepa-
rated into Bi3þ positive ions and VO3ꢃ anions (Eq. (1)). VOꢃ3 anions were
dissolved into the solution and removed. The regular tetrahedral Bi2O3
was formed by reacting Bi3þ with –OH ions. With a great deal of –OH ions,
BiVO4 polyhedrons with rough surfaces were completely etched, and the
obtained small particles grew into Bi7VO13 sheets or rods (Eq. (2)). When
the amount of –OH ions was still increased, triangle-block and cuboid
Bi25VO40 were synthesized (Eq. (3)). Herein, the structural rearrange-
ment during crystallization was mainly influenced by NaOH concentra-
tions. In analogy with organic surfactants, the selective adsorption of –OH
onto the surfaces of the new phases would lead to an adjustment of the
growth rate ratio of the crystalline facets [28,29]. Moreover, the solu-
bility of BiVO4 particles increased with the increasing NaOH concen-
tration, which led to fast growth and the subsequent extrinsic regular
tetrahedrons, sheets and rods, triangle-blocks and cuboids shape [30].
2 BiVO4 þ 2 OH → Bi2O3 þ 2 VOꢃ3 þ H2O
(1)
(2)
(3)
–
7 BiVO4 þ 6 OH → Bi7VO13 þ 6 VOꢃ3 þ 3H2O
–
25 BiVO4 þ 24 OH → Bi25VO40 þ 24 VOꢃ3 þ 12H2O
–
The energy band structure feature of a semiconductor is considered
the substantial factor in determining its photocatalytic activity. We chose
three representative samples to test their absorption edges and estimate
their band-gap energies. DRS results of the BiVO4, Bi25VO40 and Bi2O3
samples were showed in Fig. 5. As can be seen, the BiVO4, Bi25VO40 and
Bi2O3 samples had steep absorption edges, respectively. The pure BiVO4
absorbed the light at a wavelength of 510 nm, corresponding to the band-
gap energy of 2.43 eV. For Bi25VO40, its absorption edge extended to
about 600 nm, whereas the absorption edge of Bi2O3 shortened to 438
nm. Therefore, the estimated band-gap energies of the as-obtained
Bi25VO40 and Bi2O3 samples were about 2.07 and 2.83 eV calculated
from the absorption edges, respectively. Then, that is to say, BiVO4,
Bi25VO40 and Bi2O3 samples showed remarkable absorption in the visible
light range and would exhibit the visible-light photocatalytic activity in
theory. From this, we may certainly infer that the other samples (besides
the three samples) also showed visible light absorption, because their
appearance colors were between those of the Bi25VO40 and Bi2O3
samples.
The formation processes of the bismuth vanadium oxide
In general, the greater the specific surface area of the photocatalyst,
the greater the ability of the photocatalyst to adsorb pollutants, thus
promoting its photocatalytic activity. We tested the BET specific surface
areas of the bismuth vanadium oxide samples synthesized by NaOH
aqueous solution with different concentrations, as shown in Table 2. The
Table 1
EDS of the bismuth vanadium oxide samples synthesized by NaOH with different
concentrations.
specific surface area of BiVO4 prepared at 0.05 M NaOH was 4.18 m2 gꢃ1
.
NaOH
concentration
Sample
Morphology
Atomic
Ratio of
Bismuth (%)
Atomic Ratio
of Vanadium
(%)
Crystal
phases
Compared with the original BiVO4 (1.90 m2 ꢃ1), the increase of the
g
specific surface area further verified the formation of the rough surface
after the alkali treatment with low concentration of NaOH. When the
NaOH concentration was increased to a certain value, the polyhedral
BiVO4 samples completely changed into other phases, such as Bi2O3
regular tetrahedrons, Bi7VO13 sheets and rods, and Bi25VO40 triangle-
blocks and cuboids. The appearance of the new phases resulted in a
smaller specific surface area of the samples.
0.03 M
0.1 M
polyhedrons
with rough
surfaces
polyhedrons
with rough
surfaces
regular
tetrahedrons
sheets
21.34
22.65
24.08
BiVO4
BiVO4
Bi2O3
21.63
34.86
33.84
40.96
0
0.15 M
0.2 M
10.2
4.2
Bi7VO13
Bi2O3
Bi7VO13
Bi2O3
/
/
3.2. Photocatalytic properties in the degradation of RhB and the
mechanism
rods
0.4 M
1 M
triangle-blocks
cuboids
24.17
26.86
3.29
3.1
Bi25VO40
Bi25VO40
The photocatalytic activities of the as-prepared bismuth vanadium
oxide samples synthesized by NaOH aqueous solution with different
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W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
Fig. 3. EDS mapping observations (a–e) and TEM images (f) of Bi2O3/BiVO4 samples synthesized by NaOH aqueous solution (0.1 M).
concentrations were evaluated towards the degradation of RhB under
visible light irradiation, and the results are shown in Fig. 6. It can be seen
that before light irradiation, the adsorption properties of all the obtained
samples (after NaOH treatment) decreased comparing with the original
BiVO4 (without NaOH treatment). However, after light irradiation, the
photocatalytic degradation rates of some samples increased comparing
with the original BiVO4. For example, below the NaOH amount of 0.1 M,
the photocatalytic activities of BiVO4 (with rough surfaces) increased
with the increase of the amount of NaOH. Until the NaOH concentration
reached 0.1 M, the prepared sample owned the best photocatalytic ac-
tivity. Then, beyond the NaOH amount of 0.1 M, the photocatalytic
degradation rates of RhB decreased with the increase of the amount of
NaOH. Therefore, from the comparison of the activities, we can infer that
the differences may be due to the different surface morphologies. After
treatment of NaOH with low concentration, the surfaces of the poly-
hedrons were slowly etched into rough surfaces. This may lead to more
surface defects generating on the samples and avail the transfer of the
photo-electrons. When the NaOH concentration increased to a certain
value, the polyhedral BiVO4 samples completely changed into other
phases, such as Bi2O3, Bi7VO13 and Bi25VO40. The surfaces of these
samples became smooth again, restraining the transfer of the photo-
electrons. Then the sample prepared at 0.1 M NaOH (that was the
composite of BiVO4 and Bi2O3) owned the best photocatalytic activity.
This may be due to the special structure and surface morphologies which
would be discussed in the following.
We also investigated the potential applications of the Bi2O3/BiVO4
composite towards the degradation of other organic pollutants. Hydroxyl
azobenzene (HAB), methylene blue (MB) and methyl orange (MO) were
chosen as the degradation objects in the experiments and the results are
showed in Fig. 7. After 3 h visible light irradiation, the removal rate of the
cationic dyes - RhB was 44%, as the same as that of MB. For acid dyes -
MO, the degradation rate was 12% in the presence of the composites.
Meanwhile, the removal rate of the hydroxyl dyes HAB achieved to 68%.
From the experiments, it proved that Bi2O3/BiVO4 sample had good
degradation effects on RhB, MO, MB and HAB. But why did this sample
prepared at 0.1 M NaOH (that was the composite of BiVO4 and Bi2O3)
own the best photocatalytic activity? To solve the question, we explored
the degradation mechanism of the Bi2O3/BiVO4 composite in the
following section.
Electrochemical test was one of the vital measurements to reveal the
photocatalytic mechanism. The separation of electron–hole pairs was
recognized as the initiator in the photocatalytic degradation process.
Thus, we applied the electrochemical analysis to test the generation of
photoelectrons. Transient photocurrent responses and electrochemical
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W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
Fig. 4. XPS images of Bi2O3/BiVO4 sample.
Scheme 1. The formation processes of the bismuth vanadium oxide photocatalysts.
impedance spectra (EIS) Nynquist plots of the as-synthesized Bi2O3/
BiVO4, BiVO4 and Bi2O3 samples in 0.1 M Na2SO4 aqueous solution
without bias versus Pt counter electrode under visible light irradiation
were shown in Fig. 8a and b. Comparing with original BiVO4 and pure
Bi2O3, the photocurrent intensity of the Bi2O3/BiVO4 sample as the
electrode photocurrent was higher. It suggested that the separation ratio
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W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
Fig. 7. Photocatalytic activities of Bi2O3/BiVO4 composite in the removal of
RhB, HAB, MB and MO.
Fig. 5. DRS of the BiVO4, Bi25VO40 and Bi2O3 samples.
When the electrons generated, they would transfer to the surface of
semiconductors to engage in the oxidation-reduction reactions. In this
process, some other active species with high activities such as superoxide
free radicals (O2ꢀꢃ), OH free radicals (ꢀOH), and holes generated and
would also participate in the reactions. Therefore, we investigated the
effect of the active species by adding some different types of scavengers
in the degradation process of RhB on Bi2O3/BiVO4. The results are shown
in Fig. 9. KI, AgNO3, p-Benzoquinone (PBQ) and tert-butyl alcohol (TBA)
were holes (hþ), electrons (eꢃ), O2ꢀꢃ, and ꢀOH scavengers, respectively.
Comparing with the activity in the absence of these scavengers, the effect
of their addings was obvious. After adding AgNO3 to capture electrons,
the photocatalytic activity of the Bi2O3/BiVO4 sample was enhanced
remarkably, with the degradation rate of RhB being 100% after only 1 h
light irradiation. The presence of TBA also had some effect on the
degradation, making the degradation rate increase. On the contrary, the
existence of KI and PBQ restrained the photocatalytic reaction. Thus, we
can judge that hþ, ꢀOH and O2ꢀꢃ may play a certain effect in the
degradation process of RhB. Which active species can be generated
theoretically? It is related with the energy level structure of the
semiconductor.
Table 2
BET specific surface areas of the bismuth vanadium oxide samples synthesized by
NaOH aqueous solution with different concentrations.
Samples
BiVO4
1.90
0.05 M
4.18
0.1 M
0.51
0.2 M
1.49
1 M
Bi2O3
0.77
SBET(m2⋅gꢃ1
)
0.49
Bi2O3 is a p-type semiconductor with its Fermi level close to the
valence band (VB), whereas BiVO4 is n-type semiconductor and its Fermi
level lies near to the conduction band (CB). When they were combined, a
p-n-type heterojunction would be produced [31–34]. The VB edge of a
semiconductor at the point of zero charge can be calculated by the
empirical equation EVB ¼ X - Ee þ 0.5Eg, where EVB is the valence
band-edge potential, X is the electronegativity of the semiconductor,
expressed as the geometric mean of the absolute electronegativity of the
constituent atoms, Ee is the energy of free electrons on the hydrogen scale
(about 4.5 eV), Eg is the band-gap energy of the semiconductor, and ECB
can be determined by ECB ¼ EVB- Eg. The X values for BiVO4 and Bi2O3 are
ca. 6.04 and 5.95 eV [33]. The band-gap energies of BiVO4 and Bi2O3
were 2.43 and 2.83 eV (from DRS data in Fig. 5), respectively. Given the
equation above, the top of VB and the bottom of CB of BiVO4 and Bi2O3
were calculated to be 2.75 and 0.32 eV and 2.88 and 0.05 eV, respec-
tively. According to the above results, Scheme 2 was proposed to illus-
trate the energy band structures of Bi2O3 and BiVO4 before and after
forming composite and the possible charge-separation processes between
them. As shown in Scheme 2, both BiVO4 and Bi2O3 could be easily
excited by visible light (λ > 420 nm) and induce the generation of
electron and hole pairs. When the two semiconductors contacted with
each other, the band positions shifted. The bands of BiVO4 shifted
downward while the bands of Bi2O3 shifted upward. Until an equilibrium
state was formed, the Fermi levels of both materials aligned. Then
Fig. 6. Photocatalytic activities of the different bismuth vanadium oxide sam-
ples in the degradation of RhB under visible light irradiation.
of electron–hole pairs on the Bi2O3/BiVO4 composite was higher than
BiVO4 and Bi2O3. The prolonged lifetime of the photogenerated charge
carriers on different samples was reflected by the result of EIS. As shown
in Fig. 8b, comparing with bare BiVO4 and Bi2O3, the Bi2O3/BiVO4
sample showed a relatively small semi-circle arc diameter, suggesting
that the Bi2O3/BiVO4 sample owned smaller charge transfer resistance
and fast charge transport efficiency. And in Fig. 8c, the photo-
luminescence intensities of Bi2O3/BiVO4 and BiVO4 samples were far less
than that of Bi2O3, which indicated that Bi2O3/BiVO4 and BiVO4 samples
had lower recombination efficiency of electron–hole pairs. The results of
the photocurrent and PL tests were consistent with the results of the
photocatalytic activity, and revealed a reason for the higher photo-
catalytic activity of the composite.
7

W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
Fig. 8. Transient photocurrent responses (a), and EIS Nynquist plots (b) of the as-synthesized Bi2O3/BiVO4, BiVO4 and Bi2O3 samples in 0.1 M Na2SO4 aqueous
solution without bias versus Pt counter electrode under visible light irradiation, and photoluminescence (PL) spectra (c) of the as-synthesized Bi2O3/BiVO4, BiVO4 and
Bi2O3 samples (Ex ¼ 325 nm).
move to that of BiVO4; meantime, holes could transfer more easily from
the VB of BiVO4 to Bi2O3 owing to the increasing energy difference of VB
between BiVO4 and Bi2O3. Thus, electron–hole pairs on the composite
were separated efficiently. The proposed mechanism was also deduced
by the electrochemical analysis (Fig. 8). The photocurrent intensity of the
Bi2O3/BiVO4 composite was higher than BiVO4 and Bi2O3, indicating the
separation ratio of electron–hole pairs on the composite was increased.
Furthermore, the VB potentials of Bi2O3 and BiVO4 were more positive
than the standard redox potential of ꢀOH/OHꢃ (2.38 V vs NHE) [35] and
the CB potential of the Bi2O3 was negative than the standard redox po-
tential of O2/O2ꢀ-(-0.33 V vs NHE) [36]. As a result, the photogenerated
holes on Bi2O3 and BiVO4 can oxidize OHꢃ to give ꢀOH, whereas the
photo-electrons generated on Bi2O3 can reduce O2 to give O2ꢀ-. There-
fore, we can infer that hþ, ꢀOH and O2ꢀ- play key effect in the degra-
dation process of RhB.
The reaction surroundings were important to the photocatalytic
properties of the materials. So, surface acidities of the Bi2O3/BiVO4
sample were investigated by measuring the pH values in the H2O and
RhB solution, respectively. As shown in Table 3, initially, the RhB blank
Fig. 9. Photocatalytic activities of the Bi2O3/BiVO4 sample in the presence of
solution was neutral with pH ¼ 7.4. When a portion of Bi2O3/BiVO4
different types of scavengers.
sample was added into the RhB solution, the pH of the reaction system
was about 7.2, as the same as that of the pure BiVO4 reaction system. In
according to the semiconductor theory, in this newly formed band
structure, photo-holes and photo-electrons can be transferred between
Bi2O3 and BiVO4 due to the suitable VB and CB positions for the sepa-
ration of photocharges. The electrons in the CB of Bi2O3 could easily
the following, we tested the zeta potentials of different reaction systems
to verify the reaction surroundings. When pH of the reaction system was
greater than isoelectric point (IEP), the surface of materials would own
negative surface charge and negative zeta potential. When pH was
8

W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
Scheme 2. Energy band structures of Bi2O3 and BiVO4 before and after forming composites and the possible charge-separation processes.
Table 3
PH values of Bi2O3/BiVO4, BiVO4 and Bi2O3 suspensions.
Samples
RhB blank
7.4
Bi2O3/RhB
7.2
BiVO4/RhB
7.2
Bi2O3/BiVO4/RhB
7.2
pH values
smaller than IEP, the surface has a net positive charge and positive zeta
potential [37]. Therefore, surface charge of different samples was usually
determined by the IEP and pH [38]. Zeta potentials of the as-synthesized
Bi2O3/BiVO4, BiVO4 and Bi2O3 samples were shown in Fig. 10. It was
found that the IEPs of the as-synthesized Bi2O3/BiVO4, BiVO4 and Bi2O3
samples were 4.6, 4.1 and 4.3, respectively. The pH values of three
samples suspensions were all greater than the IEP of 4.1–4.6. Therefore,
Bi2O3/BiVO4, BiVO4 and Bi2O3 samples were negatively charged,
respectively. This meant that the surfaces of the three samples had
negative charges and were covered by the anionic groups such as surface
hydroxyl groups (OH). Some reports had revealed the effect of negative
charges on the transfer of electrons. For example, TiO2 covered with
Ti–O-, could facilitate electron transfer to adsorbed oxygen molecules
[39]. The negative charges on the surface can draw the photogenerated
holes to TiO2 surface by electrostatic force, and accelerate the formation
of ꢀOH [40]. Therefore, the negative charges on the surface of Bi2O3/-
BiVO4 can promote the separation of electrons and holes, and force the
formation of the active species.
According to the above results, the reasons why the Bi2O3/BiVO4
composite had the best activity were put forward. The difference in
photocatalytic efficiency could be related to crystal structure, crystal size,
morphology, surface area, and energy band structure [41–43]. By
exploration of several main influencing factors, we can deduce that the
enhancement of the composite sample was clearly not associated to the
magnitude of surface area, but strongly affected by the band-gap energy
and charge separation. The sample had good optical absorption in
UV–Vis region with a lower band-gap energy. And the negative charges
on the surface promoted the transfer of photoelectron-hole pairs.
Therefore, the Bi2O3/BiVO4 sample when excited could produce more
holes and electrons under visible light and prolong the charge separation.
More oxidant species such as holes, ꢀOH and O2ꢀ- were generated on the
composites and participated in the degradation of the organic com-
pounds. Then the activity was increased.
Fig. 10. Zeta potentials of the as-synthesized Bi2O3/BiVO4, BiVO4 and
Bi2O3 samples.
3.3. Antibacterial activities
Furthermore, the bismuth vanadium oxide photocatalysts with
different phases and morphologies also showed antibacterial activities
towards E. coli. The number of bacterial colonies was used to evaluate the
photocatalytic antimicrobial performance of the samples under visible
irradiation. The representative photographs of the E. coli colonies before
and after treatment by the photocatalysts were presented in Fig. 11.
Before light irradiation, in the absence of photocatalysts, the bacterial
grew and proliferated fast. After visible light irradiation of 3 h, we can see
that under the action of different photocatalysts, the bacterial growth and
proliferation were largely restrained. From the comparison, the original
BiVO4 and Bi2O3/BiVO4 samples showed the best antibacterial activity
towards E. coli.
The morphological structures of the E. coli bacterial strains on the
Bi2O3/BiVO4 sample were also tested by the two-photon and confocal
laser scanning microscopy. From the images, E. coli cells retained their
intact morphologies and well preserved cell walls and membranes before
and after visible light irradiation for 3 h as shown in Fig. 12a and b. After
adding the photocatalysts in the dark, E. coli cells still maintained their
9

W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
Fig. 11. Antibacterial activities of the bismuth vanadium oxide samples towards the E. coli solutions under visible light irradiation.
Fig. 12. Two-photon and confocal laser scanning microscopy images of (a, b) the original E. coli solution before and after visible light irradiation for 3 h, and (c, d) the
E. coli solution treated with Bi2O3/BiVO4 sample in dark for 1 h, and under the visible light irradiation for 3 h (blue color representing for Bi2O3/BiVO4 sample, green
color representing for E. coli). (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)
morphologies as shown in Fig. 12c. Then after visible light irradiation for
3 h, E. coli cells were completely destroyed and adsorbed on the Bi2O3/
BiVO4 surfaces, resulting in the release of their cytoplasmic contents into
their surrounding environment (green color representing for E. coli,
shown in Fig. 12d). Thus, the Bi2O3/BiVO4 sample can restrain the
growth of the E. coli and meanwhile damage them. Therefore, the
10

W. Li et al.
Journal of Solid State Chemistry 286 (2020) 121296
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4. Conclusions
We used a facile hydrothermal method to prepare BiVO4 polyhedrons
as the raw materials. Then, the as-prepared BiVO4 was dealt with NaOH
with different concentrations. Different phases and morphologies of
bismuth vanadium oxide photocatalysts can be controlled and adjusted
by changing the amount of NaOH in the synthesis process. After treat-
ment of low NaOH concentration (0.03 or 0.05 M), the smooth surfaces of
the BiVO4 polyhedrons were slowly etched in-situ on the preferential
(040) facets of the polyhedrons and then became rough. When the con-
centration of NaOH was 0.1 M, the particular Bi2O3/BiVO4 sample was
obtained containing polyhedral BiVO4 with rough surfaces and regular
tetrahedral Bi2O3. When the concentration of NaOH exceeded 0.1 M, the
phases and morphologies of samples were diverse. The polyhedral BiVO4
samples completely changed into other phases, such as Bi2O3 regular
tetrahedrons, Bi7VO13 sheets and rods, and Bi25VO40 triangle-blocks and
cuboids. The formation processes of different samples from BiVO4 was
investigated and assumed. The sample prepared at 0.1 M NaOH (that was
the composite of BiVO4 and Bi2O3) owned the best photocatalytic ac-
tivity. It showed good degradation effects on several organic pollutants
such as RhB, MO, MB and HAB. From the exploration of the degradation
mechanism on Bi2O3/BiVO4 composite, it showed that the transfer rate of
photogenerated electron–hole pairs between BiVO4 and Bi2O3 was pro-
moted. The negative charges on the surface of Bi2O3/BiVO4 accelerated
the formation of active species. Then the activity was increased.
Furthermore, the bismuth vanadium oxide photocatalysts also showed
antibacterial activities towards E. coli. It is expected to have potential
applications in the biological field in the future.
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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CRediT authorship contribution statement
Wenjuan Li: Conceptualization, Methodology, Visualization, Writing
- review & editing. Defen Kong: Methodology, Writing - review &
editing, Data curation, Writing - original draft. Tingjiang Yan: Meth-
odology, Validation. Meng Shi: Methodology, Software. Desheng Kong:
Formal analysis. Yuanyuan Feng: Supervision. Zhihong Jing: Supervi-
sion. Jinmao You: Supervision.
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This work is financially supported by the National Natural Science
Foundation of China (21303094, and 21872081), Natural Science
Foundation of Shandong Province (2016ZRB01220), and Scientific
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