Accepted Manuscript
Microstructure and photocatalytic performance of BiVO4 prepared by hydrothermal
method
Yi Lin, Change Lu, Chengyang Wei
PII:
S0925-8388(18)34618-8
DOI:
https://doi.org/10.1016/j.jallcom.2018.12.071
Reference:
JALCOM 48712
To appear in:
Journal of Alloys and Compounds
Received Date: 22 May 2018
Revised Date:
26 November 2018
Accepted Date: 5 December 2018
Please cite this article as: Y. Lin, C. Lu, C. Wei, Microstructure and photocatalytic performance of
BiVO4 prepared by hydrothermal method, Journal of Alloys and Compounds (2019), doi: https://
doi.org/10.1016/j.jallcom.2018.12.071.
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to
our customers we are providing this early version of the manuscript. The manuscript will undergo
copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please
note that during the production process errors may be discovered which could affect the content, and all
legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
Microstructure and photocatalytic performance of BiVO4 prepared by hydrothermal method
Yi Lin1, 2*, Change Lu3, Chengyang Wei4
1
School of Materials Science and Engineering, Sichuan University of Science & Engineering, Zigong
643000, PR China
Analytical and Testing Center, Sichuan University of Science & Engineering, Zigong 643000, PR
RI
PT
2
China
3
College of Chemistry and Environmental Engineering, Sichuan University of Science & Engineering,
Zigong 643000, PR China
Advanced metal materials research laboratory, Guangdong Zhaoqing Institute of Quality Inspection &
SC
4
Metrology, Zhaoqing 526000, PR China
M
AN
U
Abstract
Monoclinic BiVO4 photocatalyst was hydrothermally synthesized at different pH conditions without
surfactant. The microstructure and photocatalytic behavior of as‒prepared samples were characterized
by X−ray diffraction, scanning electron microscope, transmission electron microscopy, UV−vis diffuse
reflectance spectra, electrochemical workstation and dye degradation experiment. The results show that
TE
D
the pH value had a significant influence on the microstructure and photocatalytic performance of
BiVO4. In acid environment, heavy precipitation of tetragonal BiVO4 took place in a short time at room
temperature, whereas only amorphous powders formed as the pH value was equal or higher than 7. The
EP
monoclinic BiVO4 gradually nucleated and grew at the early stage of synthesis procedure at 180°C, and
finally became the unique crystalline phase. The relationship between microstructure and
AC
C
photocatalytic activity the photocatalyst was well discussed. It is observed that the BiVO4 prepared in
the neutral condition (pH 7) exhibited desirable photocatalytic degradation efficiency toward
Rhodamine B solution, mainly due to the efficient separation of photogenerated electron‒hole pairs and
enhanced visual light absorption during degradation process, which were ascribed to the
comprehensive function of high exposed {010} crystal facets, close packed structure, small particle
size, and special floating characteristic.
Key words
BiVO4; photocatalyst; microstructure; photocatalytic activity; photogenerated electron‒hole pairs
1. Introduction
With rapid industrializing and population explosion, depletion of fossil resources and environment
1
ACCEPTED MANUSCRIPT
pollution have become two of challenges for sustainable development of society in the 21st century. As
one green technology, the semiconductor photocatalysis exhibits great potential in organic pollutant
degradation [1-3], air decontamination [4-7] and hydrogen generation [8-10], providing an avenue for
environmental purification and solar energy utilization. To date, TiO2 is still the most popular
RI
PT
photocatalyst for wastewater treatment and hydrogen production due to its strong oxidation power,
chemical stability, non‒toxicity, and inexpensiveness [11-13]. However, it only responds to ultraviolet
light (accounts for 4% of solar spectrum) stimulation because of its wide band gap of 3.2 eV that
greatly impedes its further practical application [12, 14, 15]. Therefore, the development of efficient
demands of environment and clean renewable energy.
SC
visible light (occupies 43% sunlight) responsive photocatalyst is extremely essential to fulfill the
M
AN
U
Recently, Bismuth vanadate (BiVO4) has been recognized as one of promising photocatalysts and
attracted great attention deriving from its excellent visible light photocatalytic activity in pollutant
decomposition and water splitting [16-18]. There are three major crystal structures of BiVO4 in nature:
monoclinic scheelite, tetragonal zircon and tetragonal scheelite. Among these polymorphs, monoclinic
BiVO4 has the best photon harvesting property due to the relatively narrow band gap (2.4 eV), whereas
TE
D
tetragonal BiVO4 with a 2.9 eV band gap mainly responds to ultraviolet light stimulation [19]. The
BiVO4 can be prepared by different routings, such as microwave–assisted synthesis [20, 21],
ultrasonic–assisted process [22], electrospun [23, 24], multistep ion exchange approach [25],
EP
hydrothermal method [26, 27] and metal–organic decomposition [28]. Basing on these methods, BiVO4
with star, tube, flower, sheet, sphere, leaf, fiber shape have been fabricated.
AC
C
It is well known that the photocatalytic activity of photocatalyst strongly depends on the particle
morphology, particle size, crystalline, grain orientation, crystal structure, specific surface area,
electronic structure and so on. Hence, great efforts have been devoted to explore the relationship
among preparation technology, microstructure and photocatalytic performance of BiVO4, and many
remarkable achievements have been obtained. For example, Yu et al. pointed out that the migration of
photogenerated holes in monoclinic BiVO4 could be accelerated by greater overlap between the Bi 6s
and O 2p orbitals resulting from shorter V–O bond length [29]. Li et al. observed that photoinduced
electrons and holes separated between {010} and {110} facets of BiVO4 stemming from the difference
of energy levels of these facets [30]. Sun et al. showed that BiVO4 could split water into H2 and O2
simultaneously without any cocatalyst as the size of catalyst decreasing from nanoscale to quantum
2
ACCEPTED MANUSCRIPT
level by a quantum confinement effect [31]. Kim et al. synthesized (004)‒BiVO4 photoanode via a seed
layer approach, and demonstrated that the photogenerated charges tended to accumulate in (040) crystal
facets, which was beneficial to suppress the charge carrier recombination at the solid / liquid interface,
leading to the increase of water oxidation kinetics of photoanode [32]. However, the practical solar
RI
PT
light conversion efficiency of BiVO4 is greatly limited by its inherent drawbacks (e.g., fast charge
recombination, short–lived charge and poor electron transport), and the relevance between
microstructural evolution and photocatalytic behavior changing during synthetic procedure still
remains puzzles. Thus, the further investigation, which focuses on the influence of preparation process
SC
on the microstructural evolution behavior, and corresponding charge separation, transport and transfer
in BiVO4, is necessary to quantify the correlation between microstructure and photocatalytic
M
AN
U
performance, realize precise control of photocatalyst synthesis and enhance the quantum efficiency.
In this paper, monoclinic BiVO4 photocatalyst was synthesized using hydrothermal method without
any surfactant, and the corresponding photocatalytic performance was examined by the degradation of
Rhodamine B (RhB) under visual light irradiation. With the help of a thorough investigation of
photocatalyst′s morphology, crystal orientation, particle size, specific surface area and electronic
TE
D
structure, the influence of pH value on the microstructure as well as photocatalytic behavior of BiVO4
was discussed. Moreover, the relationship between microstructure and the photocatalytic behavior was
investigated.
EP
2. Experimental
2.1 Photocatalyst synthesis
AC
C
The BiVO4 samples were synthesized by hydrothermal process. Typically, Bi(NO3)3·5H2O (5 mmol)
and NH4VO3 (5 mmol) were dissolved in 30 mL of 0.2 M nitric acid solution and 30 mL distilled water,
respectively. After stirring at room temperature for 1 h, the above solutions were mixed to form a stable
orange solution. The pH value of mixture was adjusted to appropriate value with ammonia solution
under stirring, and then the yellow suspension formed. Subsequently, the suspension was transferred
into Teflon-lined autoclave and treated at 180 °C for 24 h. Finally, the vivid yellow powders were
obtained by filtration, washing, and drying.
2.2 Characterization
The crystal structures of samples were detected by D2 PHASER diffractometer (Bruker) equipped
with Cu Kα radiation. The X‒ray diffraction (XRD) patterns were recorded in the 2θ range of 10−70°
3
ACCEPTED MANUSCRIPT
with a scan rate of 0.1 °/s, and the crystal sizes were calculated by Total Pattern Analysis Solutions
(TOPAS) program (Bruker). The microstructural features of samples were characterized by Quanta 200
scanning electron microscope (SEM) and FEI‒Tecnai F20 transmission electron microscope (TEM).
The specific surface areas of samples were measured according to the Brunauer–Emmett–Teller (BET)
RI
PT
method using Micromeritics TriStar II instrument at 77 K after a pretreatment at 473 K for 6 h.
The UV‒vis diffuse reflectance spectra (DRS) were obtained on UH4150 spectrophotometer
(HITACHI) in the wavelength range of 200‒800 nm using BaSO4 as reference. Raman spectra were
measured by DXR micro‒Raman spectrometer (Thermo Fisher Scientific) equipped with a diode laser
SC
of excitation of 780 nm. And the spectra were recorded at a laser power of 2 mW in the wavenumber
range of 200–3000 cm-1. The photoluminescence (PL) spectra of the samples were tested by the LS55
M
AN
U
fluorescence spectrometer (Perkin Elmer) with an excitation wavelength of 500 nm.
Electrochemical test was carried out at Parstat 2273 Electrochemical workstation (Princeton Applied
Research) in a three‒electrode configuration with platinum foil as counter electrode, saturated calomel
electrode (SCE) as the reference, and BiVO4 modified carbon paste electrode with a working surface of
1.76 mm2 as the working electrode. Electrochemical impedance spectroscopic (EIS) measurements
TE
D
were implemented in a frequency range from 100 kHz to 10 mHz with amplitude of 10 mV. All
electrochemical measurements were performed in 100 mL of 0.1 M Na2SO4 solution.
2.3 Evaluation of photocatalytic activity
EP
Photocatalytic activity of the sample was evaluated by the photocatalytic degradation of RhB under
visible light irradiation. A 400W metal halide lamp was used as light source with a 420 nm cutoff filter,
AC
C
and the lamp was placed at a distance of 50 mm from the reactor. In a typical procedure, the as‒
prepared photocatalyst (0.02 g) was added into RhB solution (100 mL, 5 mg/L) under magnetic stirring
in a reactor assembled with a cooling water circulator to keep the whole reaction system maintaining at
25 °C. Before illumination, the suspension was stirred for 1 h in the dark to attain adsorption–
desorption equilibrium and then exposed to visible light to start the photocatalytic reaction. At given
time interval, 3 mL solution was abstracted and centrifuged to remove the photocatalyst powders. The
concentration change of RhB during photocatalytic degradation process was measured by detecting the
absorbance at 553 nm using a Lambda 35 UV–vis spectrophotometer (Perkin Elmer).
3. Results and Discussion
3.1 Characterization of photocatalyst
4
ACCEPTED MANUSCRIPT
The XRD patterns of the BiVO4 photocatalysts prepared at different pH values are shown in Fig. 1a.
These diffraction peaks can be well indexed to monoclinic BiVO4 according to Powder Diffraction File
from the International Centre for Diffraction Data (PDF No. 01−074−4894, space group I2/b, a =
5.1956 Å, b = 5.0935 Å, c = 11.7044 Å, γ = 90.383°), and no peaks for other phases are observed.
RI
PT
There patterns manifest that the pure monoclinic BiVO4 can be obtained in a wide pH range by
hydrothermal method. In order to evaluate the correlation between pH value and crystalline orientation,
the intensity ratio of diffraction peaks (Ihkl / I121) was taken as a reference index and the calculated
values are shown in Fig. 1b. The diffraction peaks ratios for (040) / (121) at pH 0.5 and 7 were higher
SC
than 0.8, whereas the value sharply decreased below 0.3 when the BiVO4 was synthesized at pH 2 or
12. It can be deduced that pH value plays an importance role in controlling the growth of {010} crystal
M
AN
U
facets during hydrothermal procedure, and the extreme acidic (pH 0.5) and neutral environments are
beneficial to their formation. However, the (002) / (121) and (011) / (121) intensity ratios changed
faintly with the increase of pH value, which manifests that there is no apparent dependence of the
growth kinetics of (002) and (011) crystal facets on the acidity and alkalinity of solution. Moreover, the
crystalline sizes were estimated by TOPAS and the results were 200.5, 127.1, 108.7, and 98.9 nm for
TE
D
the samples prepared at pH 0.5, 2, 7 and 12, respectively, suggesting that the monoclinic BiVO4 with a
smaller size tends to form at a higher pH value.
(b)
AC
C
EP
(a)
Fig. 1. XRD patterns of BiVO4 samples prepared at different pH conditions (a), and corresponding
evolution of diffraction peak intensity ratios of (040) / (121) , (002) / (121) and (011) / (121) with the
increasing pH value.
The morphology of as‒prepared BiVO4 was characterized by SEM, as displayed in Fig. 2. BiVO4
hollow spheres with diameter of about 100 µm were obtained at pH 0.5 (Fig. 2a), which were
composed of numerous polyhedrons (ca. 2–4 µm in size). When a small amount ammonium hydroxide
5
ACCEPTED MANUSCRIPT
was added to the reaction solution, decahedron crystals with smooth surfaces and well‒defined edges
were fabricated at pH 2, and the length of edges was in the range of 0.5–2 µm (Fig. 2b). These products
were consistance with the reported BiVO4 decahedron, which mainly exposed (010) and (110) crystal
facets [33]. While the pH value was as high as 7, the BiVO4 exhibited coralline–shaped structure, and
RI
PT
the average size was about 3 µm (Fig. 2c). As the pH value further increased to 12, needle–like
particles were generated with length of 10–20 µm (Fig. 2d). In consist with the SEM observation, the
low‒magnification TEM image presents that the BiVO4 prepared at pH 7 exhibits coral morphology
(Fig 2e). And the HRTEM image clearly shows the lattice spacing of 0.26 nm, corresponding to the
SC
(200) crystalline planes of monoclinic BiVO4 (Fig. 2f). Evidently, the BiVO4 particles compounded at
different pH values exhibited distinct morphology, which implies that the pH value has a significant
M
AN
U
influence on the nucleation and growth behavior of the photocatalyst. As shown in Fig. S1, tetragonal
BiVO4 particles could precipitate in the acid reaction solution at a fast rate at room temperature, that is
to say, the precursors evolved into BiVO4 particles in an instant. However, only amorphous precursors
were obtained when the pH value was equal or greater than 7. The morphology of BiVO4 particles and
precursors before heat treatment are displayed in Fig. S2. These aforementioned phenomena reveal that
TE
D
the nucleation kinetics barrier of BiVO4 in acid solution was lower than in neutral or alkaline solution,
which induced the heavy precipitation of BiVO4. In addition, the tetragonal BiVO4 tended to form at
the early stage of the hydrothermal procedures no matter what the pH value was, whereas it absolutely
EP
disappeared after thermal flux treatment for 24 h. Thus, we deduce that the precipitation of tetragonal
BiVO4 competed with that of monoclinic BiVO4 during the synthesis process, and the nucleation and
AC
C
growth of monoclinic BiVO4 became the dominating reaction at the later period. Furthermore, the
interfacial energy for various crystallographic planes of monoclinic BiVO4 might be different and
would vary with the pH value of the solution. The grain facet with high interfacial energy grew faster
than other facets did or tended to assemble with facets of other grains to low down the free energy of
system. Hence, monocline BiVO4 powders prepared under different pH conditions exhibited distinct
morphology and size.
The BET specific surface area of photocatalyst was estimated by nitrogen adsorption method. From
nitrogen adsorption–desorption isotherm in Fig. 3, the BET surface area of samples prepared at pH 0.5,
2, 7 and 12 were estimated to be 3.05, 1.10, 3.07 and 1.25 m2/g, respectively. The corresponding pore
size distribution of the product, which was calculated from the adsorption branch of the nitrogen
6
ACCEPTED MANUSCRIPT
isotherm by the Barrett–Joyner–Halenda method, is shown as inset in Fig. 3. It is obvious that
dispersive pore distribution was observed in all the samples and the pore diameter covered a relatively
wide range from several to hundreds of nanometers. The cumulative pore volume of samples treated at
different pH conditions followed the order: pH 7 > pH 0.5 > pH 12 > pH 2. In general, the magnitude
RI
PT
of specific surface area of photocatalyst is mainly controlled by the particle size. And the higher surface
area is usually related to the smaller particle size. Meanwhile, the aggregation of these tiny particles
will induce the generation of additional micropores, of which the amount may increase with the
prepared at pH 7 were highest.
(a)
TE
D
M
AN
U
(b)
SC
decreasing of particle size. Thus both the specific surface area and the pore volume of tiniest powders
(d)
AC
C
EP
(c)
(e)
(f)
Fig. 2. SEM images of BiVO4 powders produced at pH 0.5 (a), 2 (b), 7 (c), 12 (d); TEM images of
7
ACCEPTED MANUSCRIPT
BiVO4 powders prepared at pH 7condition (e) and corresponding HRTEM image (f) (circular region in
SC
RI
PT
(e)).
Fig. 3. Nitrogen adsorption–desorption isotherms and pore size distributions (inset) of BiVO4 powders
M
AN
U
produced at different pH conditions.
Raman spectra of BiVO4 powders synthesized at different pH values are shown in Fig. 4. Raman
bands around 210, 327, 366, 640, 710 and 826 cm-1 were observed for all the samples, which are the
classical vibrational bands of BiVO4 [34, 35]. The band around 327 and 366 cm-1 were attributed to the
asymmetric and symmetric deformation modes of the VO43- tetrahedron, respectively. The asymmetric
stretching vibration of the shorter V‒O bond was determined by the band at 640 cm-1. The Raman
TE
D
bands at 710 and 826 cm-1 were ascribed to the stretching modes of two different types of V‒O bonds.
It should be noted that the V‒O stretching mode at approximately 826 cm-1 shifted to higher
frequencies when the pH value increased from 0.5 to 7, and it did not change any more as the pH value
EP
higher than 7. According to the dependency between V‒O bond length and Raman stretching
frequencies, the V‒O bond length can be estimated via empirical formula proposed by Wachs [36]. And
AC
C
the calculated values were 1.6962, 1.6956, 1.6947 and 1.6947 Å for the samples synthesized at pH 0.5,
2, 7 and 12, respectively. This result indicates that shorter V‒O bond length tends to form under neutral
or alkaline reaction condition, which implies a closer packed VO43- tetragonal structure, leading to a
higher level of distortion in VO43- tetrahedron.
8
RI
PT
ACCEPTED MANUSCRIPT
Fig. 4. Raman spectra of BiVO4 synthesized at different pH values.
SC
The UV‒vis DRS of as‒prepared samples are shown in Fig. 5a. All the materials exhibited strong
absorption in the visible light region, revealing the possibility of photocatalytic response of these
M
AN
U
samples under visible light irradiation. This good visible light absorption characteristic of BiVO4 is
mainly attributed to the electron transition from hybrid Bi 6s ‒ O 2p valence band to the V 3d
conduction band [37]. In addition, remarkable blue shift of absorption edges was observed for these
samples as pH value increased. Based on the relationship between absorbance and photon energy, the
band gap energy of semiconductor can be estimated by the equation αhυ=A(hυ-Eg)1/2 [34]. Where α
TE
D
stands for the absorption coefficient, hυ describes the incident photon energy, A is a constant and Eg
represents the band gap of material. Thus, the band gaps were calculated to be 2.34, 2.40, 2.47 and 2.41
eV for the samples synthesized at pH 0.5, 2, 7 and 12, respectively, as shown in Fig. 5b. It is apparent
that BiVO4 prepared under lower pH condition is more beneficial to obtain better visual light
EP
absorption performance, and the increasing of pH value brings about the band gap enlargement,
suggesting that the electronic structure of BiVO4 varies with the changing of solution pH value during
AC
C
the hydrothermal process.
(a)
(b)
Fig. 5. UV–vis diffuse reflectance spectra of the BiVO4 samples produced at different pH values (a)
9
ACCEPTED MANUSCRIPT
and the corresponding plot of (αhv)2 vs. hv and band gap energy of as‒prepared BiVO4 (b).
3.2 Photocatalytic behavior
The photocatalytic activities of the samples produced at different pH conditions were evaluated by
the degradation of RhB under visible light irradiation (λ > 420 nm). The temporal evolution of UV‒vis
RI
PT
absorption spectra during the photodegradation of RhB over BiVO4 samples are shown in Fig. S3.
From these spectra, it can be found that all the absorption peaks of RhB aqueous solution initial at 553
nm gradually decreased and shifted to shorter wavelength with prolongation of the irradiation time,
which was due to removal of the N–ethyl group from RhB molecules, revealing that the chromophoric
SC
structure of the dye was stepwise destroyed. Fig. 6a shows the photocatalytic degradation efficiency of
RhB solution in the presence of BiVO4 powders. It seems that the concentration of RhB solution over
M
AN
U
these photocatalysts undertook a tiny change during 1 h of magnetic stirring in the darkness, implying
that physics adsorption has marginal effect on the change of dye concentration. Moreover, the blank
experimental test confirms that the self‒photolysis of RhB is almost negligible. However, the
degradation rate of RhB was varied with the BiVO4 powders. Under visible light irradiation for 1 h, the
degradation percentages of RhB were 35, 72, 87 and 70%, respectively, when using the samples
TE
D
prepared at pH 0.5, 2, 7 and 12 as photocatalysts. This finding shows that the photodegradation
efficiency of BiVO4 produced at pH 7 was significantly higher than those of the others, demonstrating
that the neutral environment is suitable to synthesize BiVO4 with desirable photocatalytic activity in
EP
the hydrothermal process.
The corresponding photocatalytic degradation kinetics of RhB over these samples was investigated,
AC
C
and the results are shown in Fig. 6b. Clearly, the photodegradation reaction of BiVO4 photocatalysts
can be well described by a pseudo−first−order pattern using the equation ln(C0/C) = kt, where C0 and C
are the concentrations of RhB solution at times 0 and t, k is reaction rate constant. And the k values of
four photocatalysts were estimated to be 0.8, 2.1, 2.9 and 1.8 h-1, respectively. Evidently, the k value of
BiVO4 produced at pH 7 was 3.6, 1.4 and 1.6 times higher than that of sample obtained at pH 0.5, 2
and 12, respectively, which manifests that photocatalytic activity of BiVO4 prepared by hydrothermal
method exhibits a strong dependence on the pH value of the reaction environment, and the preferable
photodegradation performance toward RhB can be obtained under neutral condition.
10
ACCEPTED MANUSCRIPT
(a)
RI
PT
(b)
Fig. 6. Degradation rate of RhB as a function of irradiation time (a) and corresponding first−order
SC
kinetic plots of samples prepared at pH 0.5, 2, 7 and 12, respectively.
The photocatalytic performance of monocline BiVO4 was also evaluated by the degradation of
M
AN
U
phenol solution with the addition of H2O2 (an electron scavenger), and the results are shown in Fig. S4.
Obviously, after visible light irradiation for 3 h, the degradation efficiency of phenol was only 4% over
the BiVO4 sample prepared at pH 7, whereas it sharply increased in the presence of H2O2 (Fig. S4). The
enhanced photocatalytic activity can be ascribed to the trapping of photoinduced electron by the
electron scavenger that effectively suppressed the recombination of electron‒hole pairs. Due to the
TE
D
synergistic effect between the photocatalyst and the electron scavenger, all the degradation rate of
phenol over the BiVO4 samples exceeded 40% within 3 h of reaction, and the degradation efficiency
decreased in the order of BiVO4 (pH 7) > BiVO4 (pH 2) > BiVO4 (pH 12) > BiVO4 (pH 0.5). However,
the concentration of phenol almost unchanged with only H2O2 under visible light.
EP
In order to verify the photocatalytic efficiency of the as‒prepared samples, EIS and PL emission
spectra were applied to investigate the efficiency of charge carrier transformation and separation in the
AC
C
photocatalysts. The EIS Nyquist plots of four BiVO4 samples are shown in Fig. 7a. Herein, Randles–
Ershler equivalent circuit model is used to stimulate the solid–liquid interface, where Rs is electrolyte
resistance, Rct is the charge transfer resistance across the interface, and CPE is a constant phase
element for semiconductor and electrolyte interface. The impedance parameters of the fitting are
presented in Table S1. It is found that the diameter of semicircle and the fitted value of Rct of BiVO4
produced at pH 7 were lower than those of others. Rct usually relates to the photocatalytic kinetics and
the smaller value represents more efficient charge carrier transfer and faster redox reaction rate.
Therefore, it can be deduced that the sample synthesized at neutral condition possessed preferable
charge transfer efficiency and photocatalytic activity. Fig. 7b presents the PL spectra of as–synthesized
11
ACCEPTED MANUSCRIPT
BiVO4 with an excitation wavelength of 500 nm. All the samples had a wide emission peak around 547
nm, deriving from the recombination of the electron formed in the V 3d band and hole in the O 2p band
[37]. And the order of PL intensity of the photocatalysts can be described as follow: BiVO4 (pH 7) <
BiVO4 (pH 2) < BiVO4 (pH 12) < BiVO4 (pH 0.5). The greatest PL intensity of BiVO4 sample
RI
PT
prepared at pH 0.5 signifies that the electrons and holes in that sample were easy to recombine,
whereas the PL intensity of sample obtained at pH 7 was lowest, indicating the high efficiency in
photogenerated electron–hole separation. Based on the above results, we can conclude that the pH
value has a great influence on the charge carrier separation, transport and transfer in the monocline
SC
BiVO4, and the photogenerated electron‒hole pairs in the sample produced under neutral condition are
intensively suppressed that enhances the charge carrier separation as well as prolongs the lifetime of
M
AN
U
the charge carrier, resulting in the efficient photocatalytic decomposition toward RhB.
(b)
TE
D
(a)
Fig. 7. (a) Electrochemical impedance spectroscopy of BiVO4 samples measured at 0.7 V (vs. SCE) in
EP
0.1 m Na2SO4 solution, and the inset shows an equivalent electrical circuit; (b) The room temperature
PL spectra of BiVO4 samples prepared at different pH conditions.
AC
C
To ascertain the contributions of reactive species in the photocatalytic degradation of RhB, the
trapping experiments were carried out, and the ethylene diamine tetraacetic acid (EDTA), isopropanol
(IPA) and benzoquinone (BQ) were used as scavengers to quench photogenerated hole h+, hydroxyl
free radical •OH and superoxide radical anion •O2-, respectively. The degradation efficiency of RhB
over BiVO4 samples with different scavengers is shown in Fig. 8. With regard to BiVO4 (pH 7), the
degradation rate of RhB without any scavengers after illuminated for 1 h was 87%. The addition of
EDTA induced obvious decrease of photocatalytic activity, and the degradation efficiency was also
quenched evidently when adding the IPA as a scavenger, illustrating that h+ and •OH are crucial active
species in the photocatalytic degradation process. However, a tiny decrease in the degradation rate of
12
ACCEPTED MANUSCRIPT
RhB is observed after the addition of BQ, indicating that •O2- has a minor effect on the photocatalytic
reaction, which may due to the conduction band edge potential of monocline BiVO4 is not sufficiently
high to reduce the dissolved oxygen. Meanwhile, similar changes of degradation activity were found
during trapping experiments with other photocatalysts, illustrating that the pH value has minor
RI
PT
influence on the active species of photocatalytic degradation process of RhB over the monocline
M
AN
U
SC
BiVO4.
Fig. 8. Trapping experiments of photocatalytic degradation of RhB over BiVO4 synthesized at different
pH conditions.
TE
D
3.3 Relationship between microstructure and the photocatalytic activity
Generally, photocatalytic activity is dominated by multiple factors such as phase structure, crystal
facet, particle size, band structure and surface area. In this study, the pH adjustment brought about the
EP
variation of microstructure and photodegradation activity of monocline BiVO4. The preferable
photocatalytic activity was attained under neutral condition, which can be ascribed to the following
AC
C
reasons. Firstly, high exposed of {010} crystal facets were obtained. In the monoclinic BiVO4, the
electron mobility is the largest along [010] direction (ca. 8.53×10-4 cm2/Vs), leading to the
accumulation of electron in the {010} facets [38]. Although the hole mobility along [100] direction is
the largest one which has the value of 5.64×10-8 cm2/Vs, the photoinduced holes still tend to transfer to
the relative stable {110} facets at a rate of 8.63×10-12 cm2/Vs [30, 38]. This difference of electron / hole
mobility in crystal orientation not only enhances the spatial separation of photoinduced electron‒hole
pairs, but also makes the {010} / {110} facets become reduction / oxidation sites [17, 30]. Thus more
{010} facets in the photocatalyst is advantage to the photooxidation reaction. However, accumulation
behavior of electron on the {010} is also influenced by the particle size of BiVO4. Upon
13
ACCEPTED MANUSCRIPT
photoexcitation, the photogenerated electron and hole must migrate from bulk of photocatalyst to the
surface to take part in the redox reaction. The smaller particle size, which means shorter diffusion
length of charge carrier, is beneficial to overcome the limitations of BiVO4 such as short carrier
lifetime and low mobility, and promote the separation of electron‒hole pairs. Hence, the photocatalytic
RI
PT
activity of BiVO4 sample with smaller size synthesized at neutral condition was greatly enhanced, and
was significantly higher than that of photocatalyst fabricated at pH 0.5. In addition, the shorter V−O
bond length, which represents the increase of distortion of VO43- tetrahedron due to the lone−pair
electron of Bi3+ [29], was obtained at pH 7. For the band structure of monoclinic BiVO4, the valence
SC
band is constructed by the hybridization of Bi 6s and O 2p orbitals, while the conduction band is
comprised primarily of V 3d orbitals [37]. The extent of overlapping between Bi 6s and O 2p orbitals is
M
AN
U
proportional to the distortion of the VO43- structure, and the greater overlap facilitates the migration of
photogenerated holes. That is to say, shorter V‒O bond length enhanced the separation efficiency of
charge carrier that promoted the increase of photocatalytic activity. It is also worth noting that the
improved charge carrier transfer characteristic of BiVO4 should be the key factor responsible for the
enhanced photocatalytic performance. The efficient surface charge separation at the semiconductor /
TE
D
solution interface suppressed the electron‒hole pair recombination and increased the efficiency of
organic pollutant degradation. Furthermore, the optical absorption property of BiVO4 has an important
role on its photocatalytic activity. As a direct band gap semiconductor, the electrons of BiVO4 can be
EP
stimulated to transfer from valence band to conduction band by incident photons with energy greater
than its band gap energy. With the decrease of band gap energy (Eg > 1.23 eV) or increasing the amount
AC
C
of incident light, more photons will be absorbed thus more photogenerated electron–hole pairs can be
produced leading to the enhancement of photocatalytic activity. During degradation processing, it is
visible that a thick layer of bright yellow film made up of BiVO4 powders prepared at pH 7 condition
floated on the surface of RhB solution (Fig. S5 and S6). This suspension behavior reduced the
transmission loss of incident light (e.g., reflection) in the solution that raised the incident photon energy
per unit surface area of photocatalyst, accelerating the improvement of photocatalytic activity.
4. Conclusions
The monoclinic BiVO4 photocatalyst was fabricated by a surfactant free hydrothermal method. The
results show that the morphology, grain size, surface area, and V‒O bond length have a strong
dependent on the pH value. In the extreme acidic (pH 0.5) and neutral environment, {010} grain facets
14
ACCEPTED MANUSCRIPT
were prone to form during hydrothermal process. As the pH value increased, smaller grain size and
closer stacking of crystal structure were obtained. The coralloid particles synthesized at pH 7 exhibited
preferable photodegradation efficiency towards RhB under visual light irradiation, which was nearly
four times than that of sample produced at pH 0.5. And the enhanced photocatalytic performance can
RI
PT
be ascribed to the suppression of charge recombination and the improved visible light absorption
resulting from synergistic effect of high exposed {010} grain facets, increased level of overlapping
between Bi 6s and O 2p orbitals, shortened charge transfer path and special floating characteristics.
Acknowledgement
SC
This work was supported by the Program of Education Department of Sichuan (Grant No.
17ZB0304); Sichuan University of Science & Engineering Recruitment Program of Experts (Grant No.
M
AN
U
2016RCL38); Opening Project of Key Laboratory of Green Chemistry of Sichuan Institutes of Higher
Education (Grant No. LZJ1703); Guangxi Key Laboratory of Processing for Non‒ferrous Metallic and
Featured Materials (Grant No. GXYSOF1841).
References
[1] D. Xu, B. Cheng, S. Cao, J. Yu, Enhanced photocatalytic activity and stability of Z-scheme
Ag2CrO4-GO composite photocatalysts for organic pollutant degradation, Applied Catalysis B:
TE
D
Environmental, 164 (2015) 380-388.
[2] Y. Gao, S. Li, Y. Li, L. Yao, H. Zhang, Accelerated photocatalytic degradation of organic pollutant
over metal-organic framework MIL-53(Fe) under visible LED light mediated by persulfate, Applied
Catalysis B: Environmental, 202 (2017) 165-174.
[3] Z. Tong, D. Yang, T. Xiao, Y. Tian, Z. Jiang, Biomimetic fabrication of g-C3N4/TiO2 nanosheets
EP
with enhanced photocatalytic activity toward organic pollutant degradation, Chemical Engineering
Journal, 260 (2015) 117-125.
[4] E. Poulakis, C. Philippopoulos, Photocatalytic treatment of automotive exhaust emissions,
AC
C
Chemical Engineering Journal, 309 (2017) 178-186.
[5] T. Nakajima, Y. Tamaki, K. Ueno, E. Kato, T. Nishikawa, K. Ohkubo, Y. Yamazaki, T. Morimoto, O.
Ishitani, Photocatalytic Reduction of Low Concentration of CO2, Journal of the American Chemical
Society, 138 (2016) 13818-13821.
[6] B.I. Stefanov, G.A. Niklasson, C.G. Granqvist, L. Österlund, Gas-phase photocatalytic activity of
sputter-deposited anatase TiO2 films: Effect of〈001〉preferential orientation, surface temperature and
humidity, Journal of Catalysis, 335 (2016) 187-196.
[7] Y. Ye, Z. Zang, T. Zhou, F. Dong, S. Lu, X. Tang, W. Wei, Y. Zhang, Theoretical and experimental
investigation of highly photocatalytic performance of CuInZnS nanoporous structure for removing the
NO gas, J. Catal., 357 (2018) 100-107.
[8] J. Chen, X.-J. Wu, L. Yin, B. Li, X. Hong, Z. Fan, B. Chen, C. Xue, H. Zhang, One-pot Synthesis of
CdS Nanocrystals Hybridized with Single-Layer Transition-Metal Dichalcogenide Nanosheets for
Efficient Photocatalytic Hydrogen Evolution, Angewandte Chemie International Edition, 54 (2015)
15
ACCEPTED MANUSCRIPT
1210-1214.
[9] Q. Han, B. Wang, J. Gao, Z. Cheng, Y. Zhao, Z. Zhang, L. Qu, Atomically Thin Mesoporous
Nanomesh of Graphitic C3N4 for High-Efficiency Photocatalytic Hydrogen Evolution, ACS Nano, 10
(2016) 2745-2751.
[10] J.Y. Zheng, G. Song, C.W. Kim, Y.S. Kang, Fabrication of (001)-oriented monoclinic WO3 films
on FTO substrates, Nanoscale, 5 (2013) 5279-5282.
[11] X. Cheng, H. Liu, Q. Chen, J. Li, P. Wang, Preparation of graphene film decorated TiO2 nano-tube
RI
PT
array photoelectrode and its enhanced visible light photocatalytic mechanism, Carbon, 66 (2014)
450-458.
[12] S.-Y. Lee, S.-J. Park, TiO2 photocatalyst for water treatment applications, Journal of Industrial and
Engineering Chemistry, 19 (2013) 1761-1769.
[13] T.-K. Van , C.K. Nguyen , Y.S. Kang, Axis-Oriented, Continuous Anatase Titania Films with
SC
Exposed Reactive {100} Facets, Chemistry – A European Journal, 19 (2013) 9376-9380.
[14] H. Xu, S. Ouyang, L. Liu, P. Reunchan, N. Umezawa, J. Ye, Recent advances in TiO2-based
photocatalysis, J Mater Chem A, 2 (2014) 12642.
[15] Z. Haider, Y.S. Kang, Facile Preparation of Hierarchical TiO2 Nano Structures: Growth
M
AN
U
Mechanism and Enhanced Photocatalytic H2 Production from Water Splitting Using Methanol as a
Sacrificial Reagent, Acs Appl Mater Inter, 6 (2014) 10342-10352.
[16] J. Zhu, F. Fan, R. Chen, H. An, Z. Feng, C. Li, Direct Imaging of Highly Anisotropic
Photogenerated Charge Separations on Different Facets of a Single BiVO4 Photocatalyst, Angewandte
Chemie International Edition, 54 (2015) 9111-9114.
[17] H.L. Tan, X. Wen, R. Amal, Y.H. Ng, BiVO4 {010} and {110} Relative Exposure Extent:
Governing Factor of Surface Charge Population and Photocatalytic Activity, The Journal of Physical
TE
D
Chemistry Letters, 7 (2016) 1400-1405.
[18] T.W. Kim, K.-S. Choi, Nanoporous BiVO₄ Photoanodes with Dual-Layer Oxygen
Evolution Catalysts for Solar Water Splitting, Science, 343 (2014) 990-994.
[19] T. Saison, N. Chemin, C. Chanéac, O. Durupthy, L. Mariey, F. Maugé, V. Brezová, J.-P. Jolivet,
New Insights Into BiVO4 Properties as Visible Light Photocatalyst, The Journal of Physical Chemistry
EP
C, 119 (2015) 12967-12977.
[20] G. Tan, L. Zhang, H. Ren, S. Wei, J. Huang, A. Xia, Effects of pH on the Hierarchical Structures
and Photocatalytic Performance of BiVO4 Powders Prepared via the Microwave Hydrothermal Method,
AC
C
ACS applied materials & interfaces, 5 (2013) 5186-5193.
[21] W. Shi, Y. Yan, X. Yan, Microwave-assisted synthesis of nano-scale BiVO4 photocatalysts and
their excellent visible-light-driven photocatalytic activity for the degradation of ciprofloxacin,
Chemical Engineering Journal, 215-216 (2013) 740-746.
[22] I. Khan, S. Ali, M. Mansha, A. Qurashi, Sonochemical assisted hydrothermal synthesis of
pseudo-flower shaped Bismuth vanadate (BiVO4) and their solar-driven water splitting application,
Ultrasonics Sonochemistry, 36 (2017) 386-392.
[23] H. Liu, H. Hou, F. Gao, X. Yao, W. Yang, Tailored Fabrication of Thoroughly Mesoporous BiVO4
Nanofibers and Their Visible-Light Photocatalytic Activities, ACS applied materials & interfaces, 8
(2016) 1929-1936.
[24] C. Lv, J. Sun, G. Chen, Y. Zhou, D. Li, Z. Wang, B. Zhao, Organic salt induced electrospinning
gradient effect: Achievement of BiVO4 nanotubes with promoted photocatalytic performance, Applied
Catalysis B: Environmental, 208 (2017) 14-21.
16
ACCEPTED MANUSCRIPT
[25] C. Liu, J. Su, J. Zhou, L. Guo, A Multistep Ion Exchange Approach for Fabrication of Porous
BiVO4 Nanorod Arrays on Transparent Conductive Substrate, ACS Sustainable Chemistry &
Engineering, 4 (2016) 4492-4497.
[26] U.M. García-Pérez, A. Martínez-de la Cruz, S. Sepúlveda-Guzmán, J. Peral, Low-temperature
synthesis of BiVO4 powders by Pluronic-assisted hydrothermal method: Effect of the surfactant and
temperature on the morphology and structural control, Ceramics International, 40 (2014) 4631-4638.
[27] M. Hojamberdiev, G. Zhu, Z.C. Kadirova, J. Han, J. Liang, J. Zhou, X. Wei, P. Liu,
RI
PT
Morphology-controlled growth of BiVO4 crystals by hydrothermal method assisted with ethylene
glycol and ethylenediamine and their photocatalytic activity, Materials Chemistry and Physics, 165
(2015) 188-195.
[28] S. Ho-Kimura, S.J.A. Moniz, A.D. Handoko, J. Tang, Enhanced photoelectrochemical water
splitting by nanostructured BiVO4-TiO2 composite electrodes, Journal of Materials Chemistry A, 2
SC
(2014) 3948-3953.
[29] J. Yu, A. Kudo, Effects of Structural Variation on the Photocatalytic Performance of
Hydrothermally Synthesized BiVO4, Advanced Functional Materials, 16 (2006) 2163-2169.
[30] R. Li, F. Zhang, D. Wang, J. Yang, M. Li, J. Zhu, X. Zhou, H. Han, C. Li, Spatial separation of
M
AN
U
photogenerated electrons and holes among {010} and {110} crystal facets of BiVO4, Nat Commun, 4
(2013) 1432.
[31] S. Sun, W. Wang, D. Li, L. Zhang, D. Jiang, Solar Light Driven Pure Water Splitting on Quantum
Sized BiVO4 without any Cocatalyst, ACS Catalysis, 4 (2014) 3498-3503.
[32] C.W. Kim, Y.S. Son, M.J. Kang, D.Y. Kim, Y.S. Kang, (040)-Crystal Facet Engineering of BiVO4
Plate Photoanodes for Solar Fuel Production, Adv Energy Mater, 6 (2016) 1501754.
[33] W. Zhao, Y. Liu, Z. Wei, S. Yang, H. He, C. Sun, Fabrication of a novel p–n heterojunction
TE
D
photocatalyst n-BiVO4@p-MoS2 with core–shell structure and its excellent visible-light photocatalytic
reduction and oxidation activities, Applied Catalysis B: Environmental, 185 (2016) 242-252.
[34] H. Li, Y. Sun, B. Cai, S. Gan, D. Han, L. Niu, T. Wu, Hierarchically Z-scheme photocatalyst of
Ag@AgCl decorated on BiVO4 (040) with enhancing photoelectrochemical and photocatalytic
performance, Applied Catalysis B: Environmental, 170-171 (2015) 206-214.
EP
[35] S.M. Thalluri, C. Martinez Suarez, M. Hussain, S. Hernandez, A. Virga, G. Saracco, N. Russo,
Evaluation of the Parameters Affecting the Visible-Light-Induced Photocatalytic Activity of
Monoclinic BiVO4 for Water Oxidation, Industrial & Engineering Chemistry Research, 52 (2013)
AC
C
17414-17418.
[36] F.D. Hardcastle, I.E. Wachs, Determination of vanadium-oxygen bond distances and bond orders
by Raman spectroscopy, The Journal of Physical Chemistry, 95 (1991) 5031-5041.
[37] J.K. Cooper, S. Gul, F.M. Toma, L. Chen, P.-A. Glans, J. Guo, J.W. Ager, J. Yano, I.D. Sharp,
Electronic Structure of Monoclinic BiVO4, Chemistry of Materials, 26 (2014) 5365-5373.
[38] T. Liu, X. Zhou, M. Dupuis, C. Li, The nature of photogenerated charge separation among
different crystal facets of BiVO4 studied by density functional theory, PCCP, 17 (2015) 23503-23510.
17
ACCEPTED MANUSCRIPT
Highlights
The pH value influenced the microstructure and photocatalytic activity of BiVO4.
The precipitation of BiVO4 can be enhanced in the acid reaction solution.
The precipitation of tetragonal BiVO4 competed with that of monoclinic BiVO4.
BiVO4 fabricated at pH 7 obtained more desirable photocatalytic performance.
AC
C
EP
TE
D
M
AN
U
SC
RI
PT