Facile synthesis of double cone-shaped Ag4V2O7/BiVO4 nanocomposites with enhanced visible light photocatalytic activity for environmental purification

Article abstract of DOI:10.1016/j.jphotochem.2016.12.035

Ag₄V₂O₇/BiVO₄ photocatalysts with double cone-shaped nanostructure were successfully synthesized by a facile sodium polyphosphate-assisted hydrothermal method. The results demonstrate that coupling Ag₄V₂O₇ with BiVO₄ can promote the separation of photoinduced charge carriers and enhance the photon absorption efficiency. Experimental results indicate that Ag₄V₂O₇/BiVO₄ composites exhibit the enhanced photocatalystic activity for degradation of methylene blue (MB) and oxidation of NO in high concentrate (1600?ppb) compared to the pure BiVO₄ under visible light irradiation (λ?>?420?nm). The composite with 0.08 mol% Ag₄V₂O₇ has the highest photocatalytic activity. MB degradation rate can reach 98.48% in 1?h and NO oxidation rate can reach 52.83% in 0.5?h on 0.08-Ag₄V₂O₇/BiVO₄, which are about 2.90 and 3.11 times higher than that of pure BiVO₄ respectively. The excellent activity can be attributed to the efficient charge transfer between Ag₄V₂O₇ and BiVO₄, and active species h⁺ and [rad]O₂^? play important roles during MB degradation and NO oxidation. In addition, this composite exhibits favorable stability during the cycling experiment, suggesting it may be a promising visible light active photocatalyst for environmental applications.

Full text of DOI:10.1016/j.jphotochem.2016.12.035

Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Invited feature article
Facile synthesis of double cone-shaped Ag4V2O7/BiVO4
nanocomposites with enhanced visible light photocatalytic activity for
environmental purification
b,
Yang Hua, Jun Fana, Chenchen Pua, Hua Lia, Enzhou Liua, , Xiaoyun Hu
*
*
a
School of Chemical Engineering, Northwest University, Xi’an 710069, China
b
Department of Physics, Northwest University, Xi’an 710069, China
A R T I C L E I N F O
A B S T R A C T
Article history:
Ag4V2O7/BiVO4 photocatalysts with double cone-shaped nanostructure were successfully synthesized by
a facile sodium polyphosphate-assisted hydrothermal method. The results demonstrate that coupling
Ag4V2O7 with BiVO4 can promote the separation of photoinduced charge carriers and enhance the photon
absorption efficiency. Experimental results indicate that Ag4V2O7/BiVO4 composites exhibit the
enhanced photocatalystic activity for degradation of methylene blue (MB) and oxidation of NO in
Received 18 October 2016
Received in revised form 11 December 2016
Accepted 25 December 2016
Available online 27 January 2017
Keywords:
BiVO4
Ag4V2O7
Double cone-shaped nanostructure
NO oxidation
Visible light photocatalyst
high concentrate (1600 ppb) compared to the pure BiVO4 under visible light irradiation (l> 420 nm). The
composite with 0.08 mol% Ag4V2O7 has the highest photocatalytic activity. MB degradation rate can reach
98.48% in 1 h and NO oxidation rate can reach 52.83% in 0.5 h on 0.08-Ag4V2O7/BiVO4, which are about
2.90 and 3.11 times higher than that of pure BiVO4 respectively. The excellent activity can be attributed to
the efficient charge transfer between Ag4V2O7 and BiVO4, and active species h+ and ꢀO2ꢁ play important
roles during MB degradation and NO oxidation. In addition, this composite exhibits favorable stability
during the cycling experiment, suggesting it may be a promising visible light active photocatalyst for
environmental applications.
© 2017 Elsevier B.V. All rights reserved.
1. Introduction
non-toxic, narrow band gap, good dispersibility, resistance to
corrosion, and higher sunlight utilization [22–27]. There are three
During the past decades, the application of semiconductor-
based photocatalysis in hydrogen production from water splitting
[1–4], degradation of organic pollutants in wastewater, removal of
NOx in air [5–10], and CO2 reduction to obtain high valuable
hydrocarbon fuels [11–15] has attracted considerable attention.
Among various semiconductors investigated, TiO2 has received far
more attention in the photocatalysis field due to its low cost, low
toxicity and high chemical stability [16–19]. However, the
relatively wide band gap (3.2 eV for anatase, 3.0 eV for rutile) of
TiO2 limits its practical applications, which makes TiO2 only
respond to the ultraviolet (UV) light (about 4% of the solar energy)
[20,21]. Therefore, great efforts have been devoted to develop the
novel photocatalyst with highly visible light activity.
crystal structures of BiVO4 according to the previous study,
including monoclinic scheelite structure (ms-BiVO4), tetragonal
scheelite structure (ts-BiVO4), and tetragonal zircon structure (tz-
BiVO4) [28,29]. Among these crystalline phases, ms-BiVO4 exhibits
much higher catalytic activity under visible light irradiation
compared to ts-BiVO4 and tz-BiVO4 due to the effective hybridiza-
tion of Bi 6s with O 2p to form the valence band with a narrower
band gap (Eg = 2.4 eV) [30]. Recently, photocatalytic properties of
BiVO4-based materials have been extensively explored.
It is well-known that the photocatalytic performance of BiVO4
is strongly related to its morphology. For instance, Tan et al.
prepared hierarchical structures of BiVO4 via the microwave
hydrothermal method. It is found that the BiVO4 with different
crystal phases and morphologies can be prepared by varying the
pH values of the precursors, and the irregular rodlike ms-BiVO4
obtained at pH 7.81 exhibited the best visible-light photocatalytic
activity [31]. Ma et al. investigated ms-BiVO4 samples prepared by
a hydrothermal method. The results indicated that the morphology
of the BiVO4 greatly changed with the increase of ethylenediamine
Bismuth vanadate (BiVO4) has garnered considerable attention
as a promising photocatalyst due to its excellent properties, such as
* Corresponding authors.
E-mail addresses: liuenzhou@nwu.edu.cn (E. Liu), hxy3275@nwu.edu.cn (X. Hu).
http://dx.doi.org/10.1016/j.jphotochem.2016.12.035
1010-6030/© 2017 Elsevier B.V. All rights reserved.

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
173
tetraacetic acid (EDTA) amount. The obtained BiVO4 with hollow
2.1. The preparation of Ag4V2O7/BiVO4
polygon morphology showed higher discoloration rate of MB
under simulated sunlight, 90.84% of MB could be degraded within
5 h [32]. Liu et al. discovered that the sheet-like BiVO4 sample with
highly exposed (010) facets could be obtained with assistance of
glycerol, which exhibited the best photocatalytic performance for
MB degradation [33]. Based on the above discussion, it can be
concluded that morphology and microstructure have great effects
on photocatalytic activity of BiVO4. Nevertheless, the reported
BiVO4 with rod-like [31], fusiform-like [32] and sheet-like [33]
morphology showed low degradation rate under visible light
irradiation, which usually takes about 3–5 h to completely discolor
dyestuff. Therefore, the investigation on morphology of BiVO4
would be beneficial to exploration of highly active BiVO4-based
photocatalysts.
The activity of pristine ms-BiVO4 is still low owing to the rapid
recombination rate of electron-hole pairs and weak surface
adsorption properties, which significantly limit its practical
application. Previous studies had shown that composite photo-
catalysts could enlarge the spectral responsive range and promote
the separation of photoinduced charge carriers, and thus could
remarkably improve the photocatalytic activity of an individual
semiconductor [34–38]. As a result, several BiVO4-based compo-
sites have been fabricated by coupling BiVO4 with other semi-
conductors. For instance, Chang et al. described that the p-Co3O4/
n-BiVO4 heterojunction can effectively suppress the excessive
formation of recombination centers at interface and improve the
surface reaction kinetics simultaneously. This composite achieved
the highest photocurrent for water oxidation [39]. Wetchakun
et al. prepared BiVO4/CeO2 nanocomposites by using precipitation
method and hydrothermal techniques. The results clearly show
that BiVO4/CeO2 nanocomposite with mol ratio of 0.6:0.4 exhibited
the highest photocatalytic activity in dye wastewater treatment
[40]. Li et al. synthesized the novel BiVO4/FeVO4 heterojunction
photocatalysts by one-step hydrothermal method. The composites
showed higher photocatalytic efficiency for the photodegradation
activity of MNZ (metronidazole) under visible light irradiation,
which was much higher than individual BiVO4 or FeVO4 [41]. In
addition, other BiVO4-based composites such as BiVO4/TiO2 [42],
V2O5/BiVO4 [43], SnO2/BiVO4 [44], CuO/BiVO4 [45], WO3/BiVO4
[46] and SiO2/BiVO4 [47] have also been reported. These studies
show that the enhanced photoactivity results from a coupling
effect between the two components in the composites, which can
increase the electron transfer and inhibits the recombination of
photo-generated charge carriers [44,48]. However, to the best of
our knowledge, Ag4V2O7/BiVO4 photocatalyst has not been
reported, and photocatalytic removal of high concentration NO
in air using Ag4V2O7/BiVO4 composites under visible light
irradiation has never been investigated.
The double cone-shaped Ag4V2O7/BiVO4 samples were pre-
pared via hydrothermal process, the details of which are as follows:
Bi(NO3)3
NH4VO3 was dissolved in 20 mL of NaOH (solution B) respectively,
the molar ratio of Bi(NO3)3 5H2O and NH4VO3 is 1:1. Then a certain
ꢂ5H2O was dissolved in 20 mL of HNO3 (solution A) and
ꢂ
amount of surfactant sodium polyphosphate was added to solution
B under stirring. The two solutions were magnetically stirred for
30 min at room temperature to obtain transparent solutions, then
solution B was added dropwise into solution A under magnetic
stirring to obtain a yellow homogeneous suspension. Subsequent-
ly, certain amount of AgNO3 was added into the mixture. After
further stirring for 2 h, the pH of the solution was adjusted to 7 by
adding of NaOH solution, which was transferred into a 100 mL
Teflon-lined stainless steel vessel, followed by heating at 180 ꢃC for
4 h. After cooled to room temperature naturally, the yellow
precipitate was centrifuged, washed three times separately with
distilled water and ethanol to remove the impurities. Finally, the
obtained samples were dried in an oven at 60 ꢃC for 24 h. Briefly,
the sample was denoted as 0.02-Ag4V2O7/BiVO4, 0.04-Ag4V2O7/
BiVO4, 0.06-Ag4V2O7/BiVO4, 0.08-Ag4V2O7/BiVO4, 0.10-Ag4V2O7/
BiVO4, 0.02, 0.04, 0.06, 0.08 and 0.10 represent the molar ration of
AgNO3 to Bi(NO3)3 5H2O in the starting materials, respectively. For
ꢂ
comparison, pure double cone-shaped BiVO4 was prepared
without using AgNO3, and bulk-shaped BiVO4 (BS-BiVO4) was
prepared in the absence of sodium polyphosphate and AgNO3. The
synthesis process is illustrated in Fig. S1 of Supplementary
material.
2.2. Characterization techniques
The morphologies were observed by a scanning electron
microscopy (SEM, JSM-6390A) equipped with an energy-disper-
sive x-ray (EDS) analysis. The crystalline phases of the samples
were identified by X-ray diffractometer (Shimadzu, XRD-6000, Cu
Ka radiation). UV–vis diffuse reflectance spectra were recorded on
a Shimadzu UV-3600 spectrophotometer with an integrating
sphere, and BaSO4 was used as a reference. X-ray photoelectron
spectroscope was performed to examine the surface properties
and composition (XPS, Kratos AXIS NOVA spectrometer). Photo-
luminescence (PL) spectra were obtained using a florescence
spectrophotometer (Hitachi F-7000). The products of the photo-
catalytic oxidation of NO were analyzed by ion chromatography
with ECD detector (DIONEX ICS-2100, AS18 as chromatographic
column, 23 mmol Lꢁ1 of KOH solution as mobile phase).
2.3. Photocurrent-time measurement
In this work, we have successfully prepared a series of double
cone-shaped Ag4V2O7/BiVO4 composites by a sodium polyphos-
phate-assisted hydrothermal method with an expectation to
obtain a promising visible light driven catalyst. The as-obtained
composite has been explored for the degradation of MB and
oxidation of NO in high concentrate under visible light irradiation
Photocurrent-time measurements were performed on an
electro-chemical analyzer (CHI660E, CHI Shanghai, Inc.) with a
standard three electrode cell at room temperature. The prepared
sample, saturated calomel electrode (SCE), and a Pt electrode were
used as the working electrode, the reference electrode and the
counter electrode, respectively. A 300 W Xe-lamp served as a light
source, 0.1 M Na2SO4 aqueous solution was used as the electrolyte.
The working electrodes were prepared as follows: Fluorine doped
tin oxide (FTO) glass pieces (2 cm ꢄ 3 cm) were cleaned succes-
sively by acetone and deionized water, and then dried in air. 5 mg of
prepared sample was mixed with N-methyl-2-pyrrolidone to make
slurry. Then the mixture was ultrasonicated for 1 h to obtain a
suspension, which was coated onto the FTO glass substrate. The
electrolyte was dried at 50 ꢃC for 6 h to obtain working electrode
with a similar film thickness [49].
(l> 420 nm). The composite shows superior photocatalytic activity
as compared with pure material. Moreover, the possible photo-
catalytic mechanism of the double cone-shaped Ag4V2O7/BiVO4
composites was also discussed.
2. Experimental section
All chemicals were of analytical reagent grade and used without
further purification. Distilled water was used throughout.

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2.4. Photocatalytic degradation of MB
as visible light source. The gas mixture with 1600 ppb NO was fed
into the reactor with a total flow rate of 300 mL minꢁ1, which was
obtained by diluting NO in a compressed gas cylinder with 20 ppm
N2 balance. After the system was stable for 20 min, when the
concentration of outlet NO is same with that of inlet gas, the lamp
was turned on. The concentration of NO and NO2 was continuously
analyzed by electrochemical NO analyzer (Shenzhen Jishunan
Technology CO. LTD., JSA9-NO) and NO2 analyzer (JSA9-NO2) at the
outlet of the reaction system [6,50–52]. The conversion efficiency
is defined as followings:
Photocatalytic degradation of MB was carried out to evaluate
the performance of the samples. A 300 W Xe lamp with a visible
light filter (>420 nm, 62.3 mW/cm2) was used as light source and
the distance between the lamp and samples was about 10 cm. In a
typical photocatalytic experiment, 50 mg of as-prepared photo-
catalyst was dispersed into 50 mL of 5 mg/L MB aqueous solution
with constant magnetic stirring. The solution was stirred in the
dark for 30 min prior to irradiation for establishing adsorption and
desorption equilibrium. 3 mL of solution were taken out and
centrifuged to remove the particles every 20 min, the concentra-
tion of MB was analyzed according to the absorption intensity at
664 nm with a Shimadzu UV-3600 spectrophotometer.
NO conversion (%) = (C0 ꢁ C)/C0 ꢄ100%
Where C is the outlet concentration of NO after reaction for time t,
and C0 represents the inlet concentration after achieving the
adsorption–desorption equilibrium.
2.5. Photocatalytic removal of NO
2.6. Active species analysis
The photocatalytic oxidation of NO was performed in a
continuous flow reactor at room temperature. A schematic of
the photocatalytic system is shown in the Supplementary material
(Fig. S2). Briefly, the photocatalytic oxidation of NO experiments
was controlled at the following condition: 0.2 g of as-prepared
photocatalyst was dispersed in 20 mL of ethyl alcohol under
ultrasonic condition for 20 min, and then the suspension was well
dispersed into a dish with a diameter of 6.5 cm and dried at 60 ꢃC,
which was put into the center of a quartz reactor with inner
volume of 380 cm3. A 300 W Xe lamp with a 420 nm filter was used
Different radical scavengers, ethylenediamine tetraacetic acid
(EDTA, scavenger of h+), benzoquinone (BQ, scavenger of ꢀO2ꢁ),
Isopropanol (IPA, scavenger of ꢀOH) was added to the reaction
system under similar conditions. During photocatalytic degrada-
tion of MB, EDTA (0.1460 g, 10 mmol Lꢁ1), BQ (0.0054 g, 1 mmol
Lꢁ1), IPA (3 mL) was added to the reaction system, respectively. In
the process of sample preparation for photocatalytic oxidation of
NO, 0.2 g of photocatalyst was dispersed in 20 mL of ethyl alcohol
Fig. 1. SEM images of (a) pure BiVO4; (b) 0.02-Ag4V2O7/BiVO4; (c–d) 0.04-Ag4V2O7/BiVO4; (e) 0.06-Ag4V2O7/BiVO4; (f) 0.08-Ag4V2O7/BiVO4; (g) 0.10-Ag4V2O7/BiVO4; (h) BS-
BiVO4; (i) EDS pattern of 0.08-Ag4V2O7/BiVO4 photocatalyst.

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
175
with EDTA (0.01 g) or BQ (0.01 g), and then the suspension was well
dispersed into a dish with a diameter of 6.5 cm and dried at 60 ꢃC. It
is different to adding EDTA and BQ, 3 mL of IPA was dropwise added
into the dried photocatalyst in the dish. Other conditions were the
same as that in the 2.3 and 2.4 parts.
purity and good crystallinity. The signals at 18.66ꢃ, 18.98ꢃ, 28.82ꢃ,
30.54ꢃ, 34.49ꢃ, 35.22ꢃ, 39.78ꢃ, 42.46ꢃ, 46.71ꢃ, 47.30ꢃ, 53.31ꢃ, 57.91ꢃ
and 58.53ꢃ can be respectively indexed as (110), (011), (121), (040),
(200), (002), (211), (051), (240), (042), (161), (170) and (321) planes
of monoclinic BiVO4 structure respectively. Weak diffraction peaks
at 31.93ꢃ and 32.92ꢃ (marked with star) are detected in the XRD
pattern of the composites in Fig. 2(c–f) after adding AgNO3, which
are corresponding to (224) and (040) facets of Ag4V2O7 (JCPDS 77-
0097), respectively. In addition, it can be seen that the diffraction of
a series of as prepared Ag4V2O7/BiVO4 photocatalyst was similar to
that of BiVO4 sample. It is possibly due to the high crystallinity and
content of the BiVO4 in the samples, it has the dominant peaks in
the XRD patterns of the composite samples [41]. As the molar
3. Results and discussions
3.1. Structure and morphology
3.1.1. SEM and EDS analysis
The morphology and size of the samples were characterized by
SEM. Pure BiVO4 displays a double cone-shaped morphology,
which possesses smooth surface with approximately 10
m
m in
ration AgNO3 to Bi(NO3)3 5H2O increases from 2% to 10%, the
ꢂ
length (Fig. 1a). The SEM images of the Ag4V2O7/BiVO4 are shown
in Fig. 1b–g. It is observed that the surface of BiVO4 is covered by
small particle and a rough surface was formed after adding AgNO3
during preparation process. The increase of AgNO3 resulted in the
formation of more Ag4V2O7 nanoparticles on BiVO4. The detailed
morphologies of the 0.04-Ag4V2O7/BiVO4 are shown in Fig. 1c and
d. The overall view of Ag4V2O7/BiVO4 samples are double cone-
shaped from the front with length of 6–8 mm (Fig. 1c), which is
consist of irregularly nanorod observed from the side of the surface
(Fig. 1d). However, when the molar ration of AgNO3 to Bi
diffraction peaks of orthorhombic Ag4V2O7 are gradually intensi-
fied. Nevertheless, the signals corresponding to the Ag4V2O7
species were not observed in 0.02-Ag4V2O7/BiVO4 (Fig. 2b), which
may be resulted from the low concentration. We also tried to
prepare Ag4V2O7 by the same method without adding Bi
(NO3)3 5H2O, the SEM, XRD and UV–vis diffuse reflectance spectra
ꢂ
are shown in Figs. S3–S5. However, the sample obtained in our
work is Ag3VO4 (JCPDS 43-0542) based on Fig. S4. It is hard to
obtain pure Ag4V2O7 (JCPDS 77-0097) using our method.
(NO3)3
ꢂ
5H2O increased to 0.1, the shape of the composite began
3.1.3. XPS analysis
to separate from each other, which was broken from the middle
(Fig. 1g). Further investigation revealed that the formation of the
above double cone-like structures depended on the existence of
surfactant sodium polyphosphate, which plays an important role
in the structure formation process. For comparison, Fig. 1h shows
the SEM image of the BiVO4 without surfactant sodium poly-
phosphate, it has bulk-shaped (marked as BS-BiVO4). The
composition of the Ag4V2O7/BiVO4 photocatalysts was further
studied by energy-dispersive spectroscopy (EDS). As shown in
Fig.1i, which further verifies that Bi, V, Ag, and O as major elements
in the Ag4V2O7/BiVO4 composites.
The electronic state and composition of the double cone-shaped
Ag4V2O7/BiVO4 samples were further evaluated by XPS. The XPS
spectra demonstrated that the characteristic peaks of the Bi, V, O,
Ag and C elements were detected obviously (Fig. 3a). The binding
peak of C 1s at 284.61 eV originates from carbon in the instrument.
In the XPS spectra, all peaks of the Bi, V, O, and Ag elements were
calibrated according to the deviation between the C 1s peak and
the standard binding energy of C 1s at 284.8 eV [53]. Fig. 3b shows
the high resolution XPS spectrum of Bi 4f. Two peaks were detected
with binding energy of 159 and 164.39 eV, corresponding to Bi 4f7/2
and Bi 4f5/2, respectively, ascribed to Bi3+ in BiVO4 [48]. In Fig. 3c,
the peaks located at 516.65 and 524.19 eV are ascribed to V 2p1/2
and V 2p3/2 of V5+ species, respectively. The O 1s XPS spectrum is
shown in Fig. 3d, two peaks at 530.18 and 532.38 eV are observed.
The former is ascribed to the lattice oxide species, while the later is
from the adsorbed oxygen species [54,55]. As illustrated in Fig. 3e,
two typical peaks of Ag 3d can be observed, which corresponding
to Ag 3d5/2 and 3d3/2, respectively. Particularly, Ag 3d peaks can be
further divided into four peaks by the XPS peak fitting program.
The peaks at 367.8 and 373.8 eV originate from the Ag+ ion, and the
peaks at 368.7 and 374.7 eV are assigned to metallic Ag00 species,
respectively [56,57]. Thus, the XPS results further confirm that the
double cone-shaped Ag4V2O7/BiVO4 nanocomposites can be
fabricated via present method.
3.1.2. XRD analysis
Phase structures of the as-prepared samples were characterized
by XRD. As shown in Fig. 2. All the diffraction peaks can be well
indexed to monoclinic scheelite type BiVO4 (JCPDS 14-0688), and
the peaks of impurities Bi2O3 and V2O5 are not observed, indicating
that BiVO4 obtained by above hydrothermal method with high
3.2. Optical properties
3.2.1. UV–vis diffuse reflectance spectra
Fig. 4 shows the typical UV–vis diffuse reflectance spectra of
pure BiVO4 and a series of as-prepared Ag4V2O7/BiVO4 samples.
Apparently, all the samples exhibit excellent visible light absorp-
tion ability, which is attributed to the band–band electron
transition of monoclinic BiVO4 confirmed by XRD results
(Fig. 2). As depicted in Fig. 4a, pure BiVO4 displays significantly
absorption from ultraviolet to 527 nm region. After the addition of
AgNO3 aqueous solution, the absorption edges of the samples
extended to 560 nm. In addition, the absorption intensity of
Ag4V2O7/BiVO4 samples increases with the increase of AgNO3
concentration in the whole region.
The band gap energies (Eg) of BiVO4 and Ag4V2O7/BiVO4
samples are calculated by the following formula:
Fig. 2. XRD patterns of (a) pure BiVO4; (b) 0.02-Ag4V2O7/BiVO4; (c) 0.04-Ag4V2O7/
BiVO4; (d) 0.06-Ag4V2O7/BiVO4;(e) 0.08-Ag4V2O7/BiVO4; (f) 0.10-Ag4V2O7/BiVO4.

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Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
Fig. 3. XPS spectra of 0.08-Ag4V2O7/BiVO4 sample: (a) overall XPS spectra; (b) Bi 4f spectrum; (c) V 2p spectrum; (d) O 1s spectrum; (e) Ag 3d spectrum.
(
a
h
n
) = A (h
n
ꢁ Eg)n
(1)
n = 2 for an indirect transition) [58,59]. For BiVO4, n equals 0.5 for
direct band gap materials. According to Eq. (1), the band gaps
Where a, n, h, Eg and A are the absorption coefficients, the light
2
values for the samples can be estimated from a plot of (
versus energy (h
axis will give a good approximation of the band gap energies for
ahn)
frequency, Planck’s constant, band gap and a constant, respectively.
Among these parameters, n is determined by the type of optical
transition of a semiconductor (n = 0.5 for a direct transition and
n) as presented in Fig. 4b. The straight line to the x

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
177
Fig. 4. (a) UV–vis diffuse reflectance spectra of Ag4V2O7/BiVO4 nanocomposites with different mole ratios; (b) plots of (
ah
n
)
2 versus (h
n
) for samples with different mole
ratios.
excitation wavelength of 328 nm. It was clearly observed that
the emission spectra of samples are similar, but the intensity is
different. Ag4V2O7/BiVO4 sample exhibits much lower intensity
than that of pure BiVO4. Possible explanation is that the
combination of BiVO4 and Ag4V2O7 could promote the separation
of the photogenerated electron-hole pairs and enhance the
interfacial charge transfer efficiency, leading to the Ag4V2O7/BiVO4
sample with enhanced photocatalytic performance under visible
light irradiation [60]. Photocurrent measurements were also
carried out for BiVO4 and 0.08-Ag4V2O7/BiVO4 composites via
several switch-on and switch-off cycles to investigate the photo-
induced charges separation efficiency. The measurements were
carried out at 0.5 V vs SCE. As shown in Fig. 5b, an instantaneous
rise in the photocurrent when the light source is turned on, and
then a prompt recovery to the original value once the illumination
is turned off. The changes of both “on” and “off” currents are nearly
vertical, which indicates that charge transport in the as-prepared
sample occurs very quickly [61,62]. The 0.08-Ag4V2O7/BiVO4
sample exhibits a higher photocurrent in comparison with pure
BiVO4, implying that the introduction of Ag4V2O7 is helpful in
suppressing the recombination of electrons and holes, which is
Table 1
Band gap values of as-synthesized samples.
Sample
Band gap/eV
Pure BiVO4
2.40
2.41
2.39
2.38
2.37
2.36
0.02-Ag4V2O7/BiVO4
0.04-Ag4V2O7/BiVO4
0.06-Ag4V2O7/BiVO4
0.08-Ag4V2O7/BiVO4
0.10-Ag4V2O7/BiVO4
samples, the estimated Eg values of all samples are listed in Table 1,
ranged from 2.41 eV to 2.36 eV, which is consistent with previous
reports. Compared to pure double cone-shaped BiVO4 samples, a
week redshift of band gap happens after the addition of AgNO3, the
lower band gap energy indicates that the Ag4V2O7/BiVO4 samples
can harvest more visible light and generate more electron–hole
pairs, which could lead to higher photocatalytic performance.
3.2.2. Photoluminescence spectra and photocurrent measurements
Fig. 5a shows the comparison of PL spectra of the pure BiVO4
and double cone-shaped Ag4V2O7/BiVO4 samples under the
Fig. 5. (a) PL spectra of as-synthesized samples under the excitation wavelength of 328 nm; (b) transient photocurrent response of BiVO4 and 0.08-Ag4V2O7/BiVO4 under
visible light irradiation.

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Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
attributed to the efficient charge transfer between Ag4V2O7 and
BiVO4. This result is consistent with PL analyses.
in 60 min, while the 0.02-Ag4V2O7/BiVO4, 0.04-Ag4V2O7/BiVO4,
0.06-Ag4V2O7/BiVO4 and 0.10-Ag4V2O7/BiVO4 composites remove
88.69%, 93.68%, 98.04% and 96.59% in the same time period,
respectively.
3.3. Photocatalytic activity
To further investigate the photocatalytic performance, we
estimated the reaction kinetics of the MB degradation. Fig. 6c
shows the photodegradation kinetics of MB over the samples. It
demonstrates the relationship between ln (C0/C) and time by fitting
the data in Fig. 6b. It is well known that when the pollutant is
within the millimolar concentration range, photocatalytic oxida-
tion of organic pollutants follows first-order kinetics, and the value
of the rate constant k can be calculated by the following formula:
3.3.1. Photocatalytic degradation of MB
The photocatalytic performances of the samples were evaluated
by degradation of MB under visible light irradiation. The
characteristic absorption of MB at about 664 nm was selected to
monitoring the photocatalytic degradation process(Fig. 6a). As
depicted in Fig. 6b, most of the composites exhibit better
photocatalytic activity than pure BiVO4, only 9.65% of the dye
was degraded for 60 min without using any photocatalyst. The
equilibrium adsorption quantity of Ag4V2O7/BiVO4 samples is
higher than that of the pure BiVO4, which should be attributed to
the rough surface of the composite photocatalysts. It is known that
adsorption is the first step in photocatalytic process and the
excellent adsorption property will benefit the photocatalytic
performance [63]. For BS-BiVO4, 29.58% of MB is decreased after
60 min, but it reaches 79.41% after 60 min by pure double cone-
shaped BiVO4. So, this new structure could enhance the photo-
catalytic activities of BiVO4. Furthermore, Ag4V2O7/BiVO4 displays
remarkably higher photocatalytic activities than that of pure
BiVO4. The 0.08-Ag4V2O7/BiVO4 sample exhibits the highest
photocatalytic activity compared to the other tested photo-
catalysts, 98.48% of MB can be decreased on 0.08-Ag4V2O7/BiVO4
ln(C0/C) = kt
(2)
Where C0 and C are the dye concentrations in solution at time 0 and
t, respectively, and k is the apparent first-order rate constant [64].
As shown in Fig. 6c, the reaction rate constant k increases at first
and then decreases with the increase of the Ag4V2O7 concentra-
tion. It can be seen that the constants k for the MB degradation over
0.02-Ag4V2O7/BiVO4, 0.04-Ag4V2O7/BiVO4, 0.06-Ag4V2O7/BiVO4,
0.08-Ag4V2O7/BiVO4 and 0.10-Ag4V2O7/BiVO4 are about 1.37,
1.92, 2.79, 2.90 and 2.15 times higher than that of pure BiVO4,
respectively. The constant k is the largest when the mole ratio of
Ag4V2O7 content is 0.08, indicating that 0.08-Ag4V2O7/BiVO4
sample gives the highest rate constant for MB dye degradation
under visible light irradiation. It is well-known that the
Fig. 6. (a) Absorption spectra of solution of MB in the presence of 0.08-Ag4V2O7/BiVO4 under visible light irradiation at different times; (b) comparison of photocatalytic
degradation efficiency of MB under visible light on all photocatalysts; (c) Plot of the rate constant k for degradation of MB over different samples under visible light irradiation.

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
179
photocatalytic performance of BiVO4 is strongly related to its
morphology and microstructure, the nanoparticles keep original
double cone shape for 0.02-Ag4V2O7/BiVO4, 0.04-Ag4V2O7/BiVO4,
0.06-Ag4V2O7/BiVO4 and 0.08-Ag4V2O7/BiVO4, while for 0.1 or
higher Ag4V2O7/BiVO4, the original shape were broken, as shown in
Fig. 1g, which could be the reason that the activity of 0.1-Ag4V2O7/
BiVO4 decreases after introducing more Ag4V2O7. Besides, the
active sites of BiVO4 might be covered by more Ag4V2O7 particles.
In view of practical applications, the stability and recyclability
of the catalysts are also indispensable for photocatalysts. The
lifetime of catalysts was also evaluated by using 0.08-Ag4V2O7/
BiVO4 sample to react with MB. The degradation rate of MB can be
well remained after four recycles photocatalytic experiment under
visible light illumination. It can be seen that no significant loss in
photocatalytic activity is observed (Fig. 7a). The XRD pattern and
SEM image of photocatalyst (Fig. 7b and c) reveal that the structure
and the phase of the photocatalyst remains intact. Above result
shows that the Ag4V2O7/BiVO4 photocatalyst can work as a stable
and efficient visible light photocatalyst.
shown in Fig. 8a, NO conversion efficiency increases gradually with
irradiation time over all the photocatalysts and then reaches a
steady state concentration [65]. The possible reasons are as
follows: at the beginning of the photocatalytic reaction, lots of NO
is adsorbed on the surface of the catalyst with strong adsorption
ability until equilibrium achieved. Then NO are oxidized to NO2
along with the prolonging of reaction time along, then NO2 adsorbs
on the surface of the samples and occupies active sites. Therefore,
the conversion efficiency of NO decreased until approached a
steady state [10,50]. Pure double cone-shaped BiVO4 exhibits a
relatively low NO conversion rate (16.98%). The photocatalytic
oxidation ability of NO greatly improved with the further increase
Ag4V2O7 content. The NO conversion for 0.02-Ag4V2O7/BiVO4,
0.04-Ag4V2O7/BiVO4, 0.06-Ag4V2O7/BiVO4, 0.08-Ag4V2O7/BiVO4
and 0.10-Ag4V2O7/BiVO4 composites at 30 min is 19.81%, 32.64%,
41.04%, 52.83% and 46.56% in the same time period, respectively.
And the optimal sample is 0.08-Ag4V2O7/BiVO4, its photocatalytic
oxidation rate is nearly 3.11 times higher than that of pure sample.
The significant improvement of NO conversion shows that the
combination of Ag4V2O7 with BiVO4 can significantly enhance the
photocatalytic activity of BiVO4.
3.3.2. Photocatalytic removal of NO
To evaluate their potential ability for air purification, the double
cone-shaped Ag4V2O7/BiVO4 composites were employed to NO in
gas phase. Fig. 8a shows the conversion of NO with the time of
irradiation under visible light irradiation over different samples. As
To further investigate the photocatalytic performance of the
samples, first order rate constants were calculated based on the
experimental data. The results are shown in Fig. 8b. It can be clearly
seen that the constants k for oxidation of NO over 0.02-Ag4V2O7/
Fig. 7. (a) Cycling runs for the photocatalytic degradation of MB over 0.08-Ag4V2O7/BiVO4 sample under visible light irradiation; (b) XRD patterns of used 0.08-Ag4V2O7/
BiVO4; (c) SEM image of used 0.08-Ag4V2O7/BiVO4.

180
Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
irradiation. Fig. 8c shows after 3 successive NO degradation under
visible light illumination, the photocatalytic activities of the 0.08-
Ag4V2O7/BiVO4 is obviously stable, no significant loss in photo-
catalytic activity is observed.
A possible process of photocatalytic removal of NO is as follows
using ꢀOH as oxidizing agent. It is reported that the photo-
generated reactive radicals generated on the surface of thꢁe
ꢁ
photocatalyst was able to oxidize NO to NO2 and NO3
[66–68]. At the end of reaction, the catalyst was washed with
10 mL of water, and then it was centrifuged to obtain the
supernatant, HNO3 (0.6937 ppm) was detected in the supernatant
ꢁ
by ion chromatography (IC), suggesting that NO3 exists in the
solution. Besides, NO2ꢁ was not detected by IC. These results could
be attributed to the rate of Reactions (4) and (5) is very quickly that
of Reaction (3). NO2 and HNO2 are intermediate products. Tꢁhey
may be oxide to HNO3 immediately after generation, so NO2 is
hard to detect. Therefore, HNO3 is the main products during the
photocatalytic oxidation of NO over Ag4V2O7/BiVO4, it is consistent
with the previous report [69].
NO + ꢀOH ! HNO2
(3)
(4)
(5)
HNO2 + ꢀOH ! NO2 + H2O
NO2 + ꢀOH ! HNO3
3.3.3. Active species analysis
In general, several reactive intermediate species such as h+, ꢀO2
and ꢀOH may be involved in the photocatalytic decompositions of
dyes and NO. To further insight on the degradation mechanism, the
role of the reactive species responsible for degradation of MB and
oxidation of NO was determined using a series of scavengers [70].
Fig. 9 shows plot of the rate constant for the degradation reaction
in presence of the selected scavengers. The rate constant for
degradation of MB (Fig. 9a) without using any scavenger
ꢁ
(0.05945 minꢁ1
) , , and
decreases to 0.0045 minꢁ1 0.032 minꢁ1
0.0421 minꢁ1 in the presence of ethylenediamine tetraacetic acid
(EDTA, scavenger of h+), benzoquinone (BQ, scavenger of ꢀO2ꢁ),
Isopropanol (IPA, scavenger of ꢀOH), respectively. The rate constant
for oxidation of NO (Fig. 9b) without using any scavenger
) , , and
(0.0439 minꢁ1 decreases to 0.0114 minꢁ1 0.0205 minꢁ1
0.0419 minꢁ1, respectively. It was apparent that all these scav-
engers could partially suppress the reaction. The decrease of the
degradation rate constant in presence of EDTA is much higher than
those of other scavengers. So, the role of the reactive species is as
the following sequence: holes > superoxide radicals > hydroxyl
radicals. Therefore, it is concluded that holes play a key role
during the photocatalytic decompositions of MB and oxidation of
NO [71].
As we all known, BiVO4 and Ag4V2O7 are both visible light
photocatalysts. The composites exhibit the enhanced photo-
catalystic activity for degradation of methylene blue (MB) dye
molecules and oxidation of NO under visible light irradiation. In
addition, we also synthesized BiVO4/TiO2 composite as shown in
Fig. S6. The photoactivity of BiVO4/TiO2 is very poor than that of
Ag4V2O7/BiVO4 under visible light irradiation (data no shown),
because TiO2 only respond to UV light, the efficiency of photo-
catalytic activity is limited by its wide bandgap (3.2 eV). Besides,
BiVO4/TiO2 has also been reported in recent years. Natda
Wetchakun et al. synthesized BiVO4/TiO2 nanocomposites. The
mole ratio of BiVO4/TiO2 at 0.5:0.5 provided the highest photo-
catalytic activity, the highest MB degradation of 84% was obtained
within 120 min of simulated solar light irradiation [34]. In this
Fig. 8. (a) Variations of NO conversion efficiency with the time of irradiation using
different samples under visible light irradiation; (b) reaction rate constant (minꢁ1
)
of the as-prepared photocatalysts. (c) cycling runs for the photocatalytic oxidation
of NO over 0.08-Ag4V2O7/BiVO4 under visible light irradiation.
BiVO4, 0.04-Ag4V2O7/BiVO4, 0.06-Ag4V2O7/BiVO4, 0.08-Ag4V2O7/
BiVO4 and 0.10-Ag4V2O7/BiVO4 catalysts are about 1.40, 1.95, 2.85,
4.43 and 3.32 times higher than that of pure BiVO4 (0.0099 minꢁ1
)
respectively, indicating that 0.08-Ag4V2O7/BiVO4 sample gives the
highest rate constant for NO removal under visible light

Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
181
Fig. 9. (a) MB degradation rate constants over 0.08-Ag4V2O7/BiVO4 in presence of various scavengers; (b) NO oxidation rate constants over 0.08-Ag4V2O7/BiVO4 in presence of
various scavengers.
work, the 0.08-Ag4V2O7/BiVO4 sample exhibited 98.48% MB
decomposition in 60 min.
irradiation (VLI) was proposed, and the applicable reactions are
given as follows:
BiVO4 + VLI ! BiVO4 (h+ + eꢁ)
(9a)
3.4. Theoretical calculation
Furthermore, the photocatalytic property of the photocatalyst is
associated with its band gap structure. In order to clarify the
enhanced photocatalytic mechanism over the composite photo-
Ag4V2O7 + VLI ! Ag4V2O7 (h+ + eꢁ)
h+ + eꢁ ! Heat
(9b)
catalyst, it is necessary to find out the conduction band edge (ECB
)
(10)
and valence band edge (EVB) of the catalysts. The ECB and EVB of a
semiconductor are calculated according to the following empirical
formula:
Ag4V2O7 (eꢁ) + BiVO4 ! Ag4V2O7 + BiVO4 (injected eꢁ)
BiVO4 (h+) + Ag4V2O7 ! BiVO4 + Ag4V2O7 (injected h+)
Ag + VLI ! Ag (SPR)
(11a)
(11b)
(12)
EVB = X + 0.5Eg ꢁ Ee
(6)
(7)
(8)
ECB = EVB ꢁ Eg
X = [x(A)ax(B)bx(C)c]1/(a+b+c)
Where EVB is the VB edge potentials, ECB is the CB edge potential, Eg
is the band gap energy of the semiconductor, is the
Ag (eꢁ) + O2 ! Ag+ + ꢀO2
(13a)
(13b)
ꢁ
X
electronegativity of the semiconductor (a, b, c are the atomic
number of compounds), Ee is the energy of free electrons vs.
hydrogen (4.5 eV) [72]. The band gap energy of BiVO4 and Ag4V2O7
is 2.40 eV and 2.50 eV, respectively. According to the X and Eg
values, the ECB value of BiVO4 was calculated to be 0.45 eV, and the
EVB value of BiVO4 was estimated to be 2.86 eV. The VB holes of
BiVO4 own a large oxidation power for the oxidation of organic
pollutants owing to the high potential. The ECB and the EVB of
Ag4V2O7 were determined to be ꢁ0.03 and 2.46 eV, respectively (as
shown in Table 2).
Ag4V2O7 (h+) + H2O ! ꢀOH + H+ + Ag4V2O7
3.5. Photocatalytic mechanism
Based on the experimental results, a possible pathway for the
photocatalytic degradation of MB and the removal of NO in the
presence of Ag4V2O7/BiVO4 composite under visible light
Table 2
The VB and CB edge positions of BiVO4 and Ag4V2O7 estimated by empirical formula.
Semiconductor
X/eV
Eg/eV
EVB/eV
ECB/eV
BiVO4
Ag4V2O7
6.16
5.72
2.40
2.50
2.86
2.46
0.45
ꢁ0.03
Scheme 1. Possible photocatalytic mechanism for degradation of MB and oxidation
of NO over Ag4V2O7/BiVO4 under visible light irradiation.

182
Y. Hu et al. / Journal of Photochemistry and Photobiology A: Chemistry 337 (2017) 172–183
Foundation (2016M600809), the Specialized Research Fund for
the Doctoral Program of Higher Education of China
(20136101110009), the Shaanxi Provincial Research Foundation
for Basic Research (2015JM5159), and the Scientific Research
Staring Foundation of Northwest University (PR12216).
h+ + ꢀOH + ꢀO2ꢁ + MB/NO ! products
(14)
In accordance with Reactions (9a)–(14), a possible mechanism
is proposed and illustrated in Scheme 1. When the Ag4V2O7/BiVO4
photocatalyst was subjected to visible light irradiation, both BiVO4
and Ag4V2O7 can be excited and produce photogenerated electron–
hole pairs (Formulas (9a) and (9b)). Under normal situation, most
of electron-hole pairs recombine rapidly (Formula (10)), making
pure BiVO4 with low photocatalytic activity. Considering the
relative positions of the CB and VB edges, the conduction band
potential of Ag4V2O7 were more negative than that of BiVO4, the
photo-induced electrons could migrate from Ag4V2O7 to the BiVO4
surface (Formula (11a)). At the same time, h+ produced by BiVO4
transfer to the valence band of Ag4V2O7 (Formula (11b)), which
avoid recombination between the excited electrons and holes.
Thus, BiVO4 and Ag4V2O7 have the suitable conduction and valence
band levels for promoting charge separation at the composite
photocatalysts interfaces [73]. In addition, Ag nanoparticles are
also excited due to the localized SPR and generate electron–hole
pairs under visible light irradiation (Formula (12)). The photo-
generated electrons in Ag nanoparticles and those transferred from
BiVO4 could be trapped by O2 to form ꢀO2ꢁ reactive oxygen species,
which further reacts with MB/NO (Formula (13a)) [74,75].
However, the CB of BiVO4 (0.456 eV) shows a poor reduction
power and are not enough to reduce O2 to ꢀO2ꢁ radicals because the
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the
online
version,
at
http://dx.doi.org/10.1016/j.
jphotochem.2016.12.035.
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