Hierarchical self-assembly of graphene-bridged on AgIO3/BiVO4: An efficient heterogeneous photocatalyst with enhanced photodegradation of organic pollutant under visible light

Article abstract of DOI:10.1016/j.jallcom.2020.154820

A series of hierarchical AgIO₃/BiVO₄-GO photocatalysts was synthesized using a solvothermal-precipitation method. The composition, structure, morphology, optical and photoelectrochemical performance of as-obtained photocatalysts were systematically characterized by XPS, XRD, TEM, SEM, EDS, PL, UV–Vis DRS, PT, and EIS, respectively. The photocatalytic activity was evaluated by the degradation of tetracycline hydrochloride (TC) and Rhodamine B (RhB) under visible light irradiation. Experimental results exhibited the photocatalytic degradation activity of AgIO₃/BiVO₄ was significantly improved compared with pure AgIO₃ and BiVO₄. This may be due to the heterojunction formation of AgIO₃ and BiVO₄ and the introduction of GO, which accelerates the separation and migration of electron-hole pairs and reduce recombination. The h⁺ and ?O₂^? was the key active species in the photocatalytic degradation process according to the trapping experiment. Furthermore, a potential photocatalytic mechanism was predicted based on experimental results.

Full text of DOI:10.1016/j.jallcom.2020.154820

Journal of Alloys and Compounds 831 (2020) 154820
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: http://www.elsevier.com/locate/jalcom
Hierarchical self-assembly of graphene-bridged on AgIO3/BiVO4: An
efficient heterogeneous photocatalyst with enhanced
photodegradation of organic pollutant under visible light
Yushan Si , Yongyang Chen , Yiwen Fu , Xinyu Zhang , Fenfang Zuo , Tongtong Zhang ,
Qishe Yan *
a College of Chemistry, Zhengzhou University, No. 100, Science Avenue, Zhengzhou City, Henan Province, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A
series of hierarchical AgIO3/BiVO4-GO photocatalysts was synthesized using a solvothermal-
Received 23 November 2019
Received in revised form
27 February 2020
Accepted 16 March 2020
Available online 18 March 2020
precipitation method. The composition, structure, morphology, optical and photoelectrochemical per-
formance of as-obtained photocatalysts were systematically characterized by XPS, XRD, TEM, SEM, EDS,
PL, UVeVis DRS, PT, and EIS, respectively. The photocatalytic activity was evaluated by the degradation of
tetracycline hydrochloride (TC) and Rhodamine B (RhB) under visible light irradiation. Experimental
results exhibited the photocatalytic degradation activity of AgIO3/BiVO4 was significantly improved
compared with pure AgIO3 and BiVO4. This may be due to the heterojunction formation of AgIO3 and
BiVO4 and the introduction of GO, which accelerates the separation and migration of electron-hole pairs
and reduce recombination. The hþ and ꢀO2ꢁ was the key active species in the photocatalytic degradation
process according to the trapping experiment. Furthermore, a potential photocatalytic mechanism was
predicted based on experimental results.
Keywords:
Graphene-bridged AgIO3/BiVO4
Rhodamine B
Tetracycline hydrochloride
Photocatalysis
© 2020 Elsevier B.V. All rights reserved.
1. Introduction
Photocatalytic technology is more environmentally friendly,
economical, non-toxic, and can effectively degrade pollutants in
Nowadays, environmental pollution is becoming increasingly
serious, especially water pollution, which has done serious harm to
the ecosystem [1e3]. Antibiotics and dyes, which are the typical
organic pollutants in water, have attracted extensive attention.
Tetracycline hydrochloride (TC) is one of the antibiotics, which is
used continuously in livestock, poultry breeding, and People’s Daily
life, and also shows the state of “continuous existence” [3]. The
“persistent” antibiotics in the environment not only selectively
damage or even kill some environmental microorganisms but also
induce the generation of some resistant bacteria, which leads to
special ecotoxicological effects. Rhodamine B (RhB) is a synthetic
dye with a bright pink color. Mainly used in fireworks, colored
glass, and other industries. The accumulated dye pollutants in the
environment could produce aromatic amine intermediates under
the action of microorganisms, which have strong mutagenesis,
teratogenic, carcinogenic, and potential environmental risks [4].
water [5e7] compared with traditional wastewater treatment,
methods such as the biological method, chemical oxidation
method, supercritical treatment technology, etc.,. However, many
semiconductor photocatalysts only respond in the ultraviolet light
region, occupying approximately 4% of sunlight [8,9]. Therefore, it is
essential to explore an effective photocatalyst response to visible
light.
Bismuth-based semiconductor catalysts have caused extensive
attention owing to their good absorption of visible light, rich con-
tent and high catalytic activity, including BiOX (X ¼ Cl, I, Br)
[5,10,11], Bi2WO6 [12], Bi2MoO6 [13], BiVO4 [14,15], Bi2O3 [16],
Bi2O2CO3 [17], etc. Among them, as a green catalyst, BiVO4 has a
narrow bandgap energy (~2.4 eV) with a good photocatalytic
property under visible-light irradiation, which is considered as a
widely used material to photo-catalytic technology to degrade
organic pollutants. However, its poor separation efficiency and high
recombination rate of the current carrier limits the application.
It has been proved as an effective method to construct hetero-
junction photocatalyst to inhibiting the recombination of electron-
hole pairs, such as BiVO4/WO3 [18,19], Ag3PO4/BiVO4 [20], AgI/
* Corresponding author.
E-mail address: qisheyanzzu@163.com (Q. Yan).
https://doi.org/10.1016/j.jallcom.2020.154820
0925-8388/© 2020 Elsevier B.V. All rights reserved.

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Y. Si et al. / Journal of Alloys and Compounds 831 (2020) 154820
BiVO4 [21,22], BiVO4/g-C3N4 [23,24] and so on. Recently, nonme-
tallic oxy-acid salts are considered as promising photocatalytic
materials with good photocatalytic performance for removing
organic pollutants, such as a variety of iodate semiconductor pho-
tocatalyst including BiOIO3 [25], Y(IO3)3 [26], Bi(IO3)3 [27], Ln(IO3)3
[28]. The presence of orphaned electrons in (IOꢁ3 ) promotes the
formation of polar and asymmetric structures, inhibiting the
recombination of carriers and improving the catalytic activity
[25,29e31]. AgIO3, a layered iodate semiconductor photocatalytic
materials with a wide bandgap (~3.1 eV) [32], which was proved to
have a certain degradation effect for organic pollutants. However, it
is relatively weak to absorb visible light. Constructing AgIO3 based
heterojunction photocatalysts may be effective for enhancing the
response to visible light, such as Bi7O9I3/AgI/AgIO3 [29], AgIO3/g-
C3N4 [33], AgIO3/WO3 [34], AgeAg3VO4/AgIO3 [35] Ag/AgX (X ¼ I,
Br, Cl)/AgIO3 [31,36,37]. The bandgap of AgIO3 matches well with
BiVO4 and the AgIO3/BiVO4 photocatalyst material has been rarely
studied. AgIO3/BiVO4 semiconductor heterojunction photocatalyst
can effectively inhibit the recombination of photogenic carriers
[38].
Graphene oxide has been widely used to compound with other
photocatalysts owing to its outstanding photoelectric performance,
efficient carrier migration rate and good chemical stability [39e41].
GO is a single-layer, two-dimensional, hexagonal honeycomb
structure composed of carbon atoms with copious carboxyl and
hydroxyl groups [42]. After combined with GO, the degradation
effect for organic pollutants has been significantly improved,
mainly because of the efficient carrier separation and migration
efficiency of GO, effectively decreasing the recombination of car-
riers, playing an important role of an electron receiver and an
electron migration bridge [40,42,43].
2.2.2. Preparation of AgIO3/BiVO4
AgIO3/BiVO4 composites were prepared by precipitation
method. As follows: 0.15
g as-synthesized BV powder was
dispersed into 30 mL of ultra-pure water with stirring (solution A).
Then, 10 mL AgNO3 solution (0.018 g) was added to solution A drop
by drop with stirring 30 min in the dark (solution B). The last, 10 mL
KIO3 solution (0.0227 g) was added into solution B slowly. The
suspension was stirred for 2 h under the dark. The 20% AgIO3/BiVO4
was obtained after centrifuged, washed with distilled water and
absolute ethanol three times repeatedly, and dried at 60 ꢂC for 12 h,
referred to as 20% AB. The AgIO3/BiVO4 composites containing
different amounts of AgIO3 (weight ratios, AgIO3/BiVO4, 10 wt%,
30 wt%) were also prepared following the same method, which was
marked as 10% AB, 30% AB. For contrast, the pure AgIO3 was syn-
thesized without BiVO4 keeping the conditions unchanged, which
were marked as AIO.
2.2.3. Preparation of AgIO3/BiVO4-GO
In our previous work, the GO synthesized by an improved
Hummer’s method [44]. The AgIO3/BiVO4-GO composite was pre-
pared by the following procedure: 0.12 mL (10 mg/L) as-prepared
GO was dispersed into 60 mL ultra-pure water and ultrasound
treatment for 2 h to get GO exfoliated. Then, 0.12 g 20% AB power
was added to the above solution, followed by stirring at 60 ꢂC for
6 h thermostatic water bath. 20% AgIO3/BiVO4-1% GO was obtained,
which was centrifuged, washed with distilled water and absolute
ethanol for three times repeatedly, and frozen drying for 48 h. The
mass ratios of GO to 20% AB were 0.5, 1, 1.5 and 2 wt %, which
denoted as AB-0.5% G, AB-1% G, AB-1.5% G and AB-2% G.
2.3. Characterization
In this paper, a series of hierarchical AgIO3/BiVO4-GO was syn-
thesized via a mild and simple solvothermal-precipitation method,
and have been methodically characterized. TC and RhB was selected
to assess the catalytic activity under visible-light irradiation. The
experiment consequent displays that AgIO3/BiVO4-GO possessed
the best photocatalytic efficiency. The trapping experiments have
been done and the possible degradation mechanism of photo-
catalytic degradation had also been illustrated.
The X-ray diffraction (XRD) data were determined with Bruker
D8 diffractometer (X’Pert PRO, PANalytical, Holland) equipped with
a Cu K
a
X-ray source (
l
¼ 0.15406 Å) and performed at a 40 mA and
40 kV in the 2-theta range from 5 to 90ꢂ. The chemical composi-
tions were investigated by X-ray photoemission spectroscopy (XPS)
produced by American Thermo company with a model of escalab
250Xi. The monochromatic AlKa target was used as the excitation
source, and the C1s of binding energy 184.8 was used as the
reference to correct other elements. The morphologies and
component elements were obtained by scanning electron micro-
scopy (SEM) produced by Germany Zeiss company with a model of
Sigma 500 and transmission electron microscope (TEM) produced
by USA FEI company with a model of TECNAIG2F20eS-TWIN. The
UVeVis DRS were recorded by using a spectrophotometer (Cary-
5000), using BaSO4 as the reference. Photoluminescence (PL)
spectra were measured on the F-4600 Fluorescence Spectropho-
tometer with an excitation wavelength at 325 nm. The photo-
electrochemical characterization of the as-prepared samples was
investigated by an electrochemical workstation with a conven-
tional three-electrode (CHI660E, Chenhua, China). The working
electrode is a thin film electrode made from the sample. Ag/AgCl
electrode was used as the reference electrode. Pt sheet was used as
the reverse electrode.0.1 m Na2SO4 (pH ¼ 7) was used as the
electrolyte. The light source was a 300 W Xenon lamp with a
400 nm cutoff filter.
2. Experimental
2.1. Chemical materials
Silver nitrate (AgNO3), ammonium metavanadate (NH4VO3),
bismuth nitrate pentahydrate (Bi(NO3)3$5H2O), ethylene glycol
(EG), and potassium iodate (KIO3) was purchased from Tianjin
Kermel Chemical Reagents Factory. All reagents were of analytical
grade and were directly analyzed without further purification.
2.2. Preparation of photocatalysts
2.2.1. Preparation of BiVO4
The sphere-like BiVO4 was obtained by a solvothermal method.
Firstly, 0.97 g Bi(NO3)3$5H2O and 0.234 g NH4VO3 were dispersed
into 50 mL mixed solution of H2O: CH3COOH: C2H5OH ¼ 3:1:1
(volume ratio) and 20 mL NaOH (5 M) solution, respectively, with
stirring to form a clear solution. Secondly, Add NH4VO3 solution
drop-wise to Bi(NO3)3$5H2O solution under vigorous stirring, then
adjusted the pH to 5 with acetic acid. After the 30 min of aging, The
solution was transferred to a stainless steel autoclave lined with
100 mL and heated at 80 ꢂC for 3 h. Finally, the precipitate was
centrifuged and washed with distilled water and absolute ethanol
three times repeatedly followed by drying at 60 ꢂC for 12 h. A
sample can be obtained, which were named as BV.
2.4. Photocatalytic properties
The photocatalytic properties of as-synthesized catalysts were
evaluated by degrading RhB and TC. under a 300 W 100 mW/cm Xe
lamp with a 400 nm cutoff filter. Briefly, 30 mg prepared sample
was suspended into TC (100 mL, 20 mg/L) and RhB (50 mL, 10 mg/L)
solution, respectively. The suspension was continuously stirred

Y. Si et al. / Journal of Alloys and Compounds 831 (2020) 154820
3
Fig. 1. (a, b) XRD patterns of as-synthesized photocatalysts.
under the darkness for 30 min before the photocatalytic reaction to
3. Results and discussion
achieve an adsorption-desorption equilibrium. Turn on the light,
about 3 mL of the solution was taken and filtered every 10 min with
The phases, purity and crystalline phases of AIO, BV, AB, and AB-
G compounds were surveyed by XRD, and the obtained results with
the region of 10e70ꢂ were shown in Fig.1. From Fig.1a, all the peaks
a 0.22
mm filter. Finally, the concentration changes of RhB and TC
were measured by an ultraviolet spectrophotometer (UV-2450).
Fig. 2. High resolution XPS spectra of (a) Bi 4f, (b) Ag 3d, (c) O 1s, (d) V 2p, (e) I 3d, (f) C 1s.

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Y. Si et al. / Journal of Alloys and Compounds 831 (2020) 154820
Fig. 3. SEM images of (a) BV, (b) AIO, (ced) 20% AB, (eef) AB-1% G.
in the patterns of AIO and BV were consistent with orthorhombic
AgIO3 (JCPDF No. 71-1928) and tetragonal BiVO4 (JCPDS No. 75-
2481). AB compounds coincided well with standard tetragonal
BiVO4 (JCPDS No. 75-2481) and orthorhombic AgIO3 (JCPDF No. 71-
originated from AIO and BV, respectively [3,13,38,47,48]. Oxygen in
the system could catch photo-excited electrons, further inhibiting
the recombination of electron-hole pairs, effectively improving
photocatalytic activity [38]. The I 3d spectrum in Fig. 2d included
two characteristic peaks at 635.4 and 623.9 eV, attributed to I 3d3/2
and I 3d5/2 of I5þ [29,35,36]. The XPS signal has a slight shift
compared to individual AIO and BV, implying the formation of AIO/
BV heterojunction. Fig. 2f displayed the C 1s spectra of AB-1% G, the
peaks in which could be decomposed into three characteristic
peaks with binding energies of 288.7, 286.6 and 284.9 eV, respec-
tively, attributed to the carboxyl (OeC]O), epoxy/hydroxyls (CeO)
and sp2 bonded carbon (CeC) [3,13,49,50].
1928) altogether. The main peaks at
2
q
¼
18.8ꢂ, 28.85ꢂ,
30.48ꢂ,34.83ꢂ, 39.88ꢂ, 46.95ꢂ, 53.17ꢂ, and 58.83ꢂ were indexed as
(101), (112), (004), (200), (211), (204), (116) and (132) crystal planes
of BV, respectively. Peaks at 2q
of 11.66ꢂ, 28.109ꢂ, 34.08ꢂ, 38.8ꢂ,
43.79ꢂ, 51.54ꢂ were indexed as (020), (041), (231), (061), (232),
(271) crystal planes of AgIO3, respectively. The characteristic peaks
of AIO gradually enhance with the increasing AIO ratio in AB
composites. No other miscellaneous peak was found in the XRD
patterns of the AB compound, suggesting as-prepared materials
were successfully synthesized with high purity and crystallinity. It
could be seen in Fig. 1b that the presence of GO did not obviously
change the composition or crystallinity of the original AB material.
The peaks of GO could not be found possibly due to its low content.
The composition of as-prepared samples was further analyzed
by XPS. The high-resolution spectra of the element in AB-1% G
composite was shown in Fig. 2a-f. Peaks in Bi 4f spectrum (Fig. 2a)
at 164.7 (Bi 4f5/2) and 159.4 eV (Bi 4f7/2) proved the presence of Bi3þ,
which could be attributed to BV [21,45]. As shown in Fig. 2b, the
strong peaks located at 373.9 eV and 367.9 eV were ascribed to Ag
3d3/2and Ag 3d5/2 of Agþ in AIO [46]. The O 1s spectra in Fig. 2c
could be divided to three peaks at 533.3, 532.0, and 530.4 eV,
corresponding to C]O (OC), the adsorption oxygen (OA) on surface
of AB-1% G by adsorption of (-OH) and the lattice oxygen (OL)
The morphology structures of BV, AIO, AB, AB-G were examined
by SEM, TEM. It can be seen that pure BV (Fig. 3a) has a regular
spherical shape with size 0.5e1 mm, while AIO is a series of irregular
slices stacked on top of each other (Fig. 3b). The SEM photographs
of the 20% AB were shown in Fig. 3c-d, both types of morphologies
could be found. The stratiform morphology of GO can be observed
clearly in Fig. 3e-f. The intimate contact of AB with graphene is
beneficial for the electron transfer, leading to improved photo-
catalytic activity. The EDS elemental mapping of the AB-G com-
posite was used to research the elements dispersion. Fig. S1 (a-g)
show Bi (Fig. S1 b), V (Fig. S1 c), C (Fig. S1 d), Ag (Fig. S1 e) and I
(Fig. S1 f), O (Fig. S1 g) were uniformly distributed within the
composites, explaining the AIO, BV and GO coexist in the AB-1% G
composite. The TEM images of the synthesized materials were
shown in Fig. 4, where detailed morphological information can be

Y. Si et al. / Journal of Alloys and Compounds 831 (2020) 154820
5
Fig. 4. TEM images: BV (a), AIO (b), 20% AB (c), AB-1% G (f) and HRTEM images of AB-1% G composite (e).
obtained. Contrasting Fig. 4 (a-d, f), it can be seen obviously BV is
uniformly sphere shapes, AIO is the irregular lamellar, which was in
good agreement with the SEM results. The close contact of AB with
GO would be beneficial for the electron transfer. In addition, the
HR-TEM images of the AB-1% G compound were exhibited in Fig. 4e
There are lattice spacing of 0.308 nm and 0.318 nm, which can be
attributed to the crystal plane of BV (121) and AIO (041). The results
are consistent with the (121) crystal plane and the (041) crystal
plane of m-BV (JCPDS No. 14-0688) and AIO (JCPDS No. 71-1298) in
the XRD standard card.
The optical properties of AIO, BV, and GO were studied by
UVeVis DRS, which were very critical influencing factors in
impacting the photocatalytic properties. As shown in Fig. 5a, pure
BV, and pure AIO possessed the absorption edge located at
approximately 575 and 380 nm. The absorption edges of 20% AB
(570 nm) had a blue-shifted slightly relative to pure BV, while had a
red-shifted greatly contrasted with AIO. After GO attended, the AB-
G compound showed a further red-shift, which indicates that the
synergetic effect between AIO, BV and GO enhancing the utilization
efficiency of sunlight. The corresponding energy gap of AIO and BV
can be reckoned through following the formula:
While AIO is an indirect bandgap: n ¼ 2, the Eg of AIO is about
3.13 eV.
The recombination of the electron-hole pairs is one of the
nonnegligible elements affecting the catalytic efficiency of semi-
conductors, which can be studied by PL spectroscopy. The PL
emission spectra of AIO, BV, 20% AB, and AB-1% G excited at 325 nm
were presented in Fig. 6. The 20% AB composite displayed obviously
weaker photoluminescence signals contrast with pure BV and AIO,
possibly because the presence of AIO providing more electron
capture sites, inhibiting the recombination of charges. The fact that
the PL emission signal of AB-1% G is much lower than AIO, BV and
20% AB, may be attributed to the introduction of GO. GO as a
transport channel for electrons, accelerates the separation of
charge carrier and the transfer of electrons. Furthermore, GO can
also be used as an electron receptor.
The separation and interface charge transfer efficiency were
characterized by the EIS and PT test. The photocurrent responses of
AIO, BV, 20% AB and AB-1% G composites were shown in Fig. 7a.
Under the same operating conditions, the separation efficiency of
photogenerated electron-hole pairs is enhanced with the increase
of photocurrent intensity. Compared to AIO and BV, the 20% AB has
a higher photocurrent response, which indicates that the formation
of AB heterojunction improves the efficiency of electron transfer at
the interface. In addition, the AB-1% G shows the greatest photo-
current response, indicating charge separation and transfer effi-
ð
a
hvÞ1=n ¼ Aðhv ꢁ EgÞ
(1)
where a, A, n, h, and Eg on behalf of the absorption coefficient, a
ciency of AB-1%
Furthermore, the EIS of AIO, BV, 20% AB and AB-1% G composites
were shown in Fig. 7b. In general, a smaller radius of the arc
G is further enhanced after deposing GO.
constant, the light frequency, the Planck’s constant, and the
bandgap energy, respectively. For BV, a direct bandgap semi-
conductor with n ¼ 1/2, the Eg of BV is estimated to be 2.45 eV.

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Y. Si et al. / Journal of Alloys and Compounds 831 (2020) 154820
Fig. 5. (a) UVeVis spectrum of BV, AIO, 20% AB, and AB-G composites and (b) the bandgap of AIO and BV.
corresponding to a smaller charge-transfer resistance and a higher
transfer rate of charge. The relative arc sizes of as-synthesized
of rate constants (k) were shown in Fig. 8c. The k values of AIO, BV,
20% AB and AB-1% G were 0.00444, 0.00836, 0.07081 and 0.12157
minꢁ1, respectively. The AB-1% G is 27-fold higher than AIO and
14.5-fold higher than BV. The photocatalytic activity of TC was
further researched over the as-synthesized compounds and shut
out the photosensitization of RhB. From Fig. 8d, it can be seen the
catalytic activity of AB-1% G for degrading TC was significantly
improved, which further indicates the photocatalytic activity was
enhanced through the formation of AB-1% G heterojunction.
The influence of dye concentration was also investigated by
photodegraded different initial concentrations (5, 10, 15, 20 mg/L)
of 50 mL RhB solution over 30 mg AB-1% G. It can be seen from
Fig. 9, with the increasing RhB concentrations, the degradation
efficiency had an obvious decrease. The phenomenon mainly
attributable to the high initial concentration of RhB, reducing the
light transmittance, making it more difficult for photons to shift to
the surface of the catalyst. Besides the higher pollutant concen-
tration, the more intermediates would be produced. Intermediates
would compete for the finite active sites on the surface of the
catalyst with the RhB. Thus, in practical wastewater treatment,
dilution before the treatment and cyclic treatment is necessary.
The stability of the as-prepared photocatalysts is another key
factor of practical application. The used catalyst was collected
through centrifuging and washing with ultrapure water after each
run to remove the residual RhB. In order to reduce the loss of cat-
alysts, we only took samples after irradiation for 30 min each time
under the same experimental conditions with a photocatalytic ac-
tivity test. As shown in Fig. 10a, the results of the cycle test clearly
showed good photocatalytic activity of the AB-1% G compound
with a degradation rate of 73% after four cycles. Moreover, the used
AB-1% G compound was tested by XPS and XRD. Compared to the
used sample with the fresh, the XRD pattern was a little conflict. As
photocatalysts electrodes successively: AB-1%
G
<
20%
AB < BV < AIO, suggesting that charge-transfer rate of AB-1% G
nanocomposite is the highest, which is beneficial for enhancing
photocatalytic activity.
The photocatalytic properties of AIO, BV, 20% AB and AB-1% G
composites were evaluated by degrading RhB and TC. Contrast with
pure AIO and BV, the AB compounds exhibit excellent catalytic
activity and the 20% AB exhibits the optimal outcome (Fig. 8a). The
photodegradation rate of RhB over pure AIO, BV and 20% AB were
21.3%, 47.6%, and 100% within 60 min, respectively. As shown in
Fig. 8b, compared with 20% AB (88.9%), the efficiency of photo-
degradation of RhB by the AB-G compound is further improved. The
AB-1% G showed the best photocatalytic activity, and the degra-
dation efficiency reached about 97.5% within 30 min. Fig.S2 (a, b)
exhibits the corresponding UVeVis absorption spectra of RhB by
adding 20% AB and AB-1% G from 200 to 800 nm. The decreasing
characteristic absorption peak of RhB (ca. 550 nm) reveals that the
concentration of RhB is decreasing gradually with the increase of
irradiation time
ꢁlnðCt = C0Þ ¼ kt
(2)
where C0 and Ct are the concentration at time 0 and t, t is irradiation
time (min) and k is the rate constant (minꢁ1). The obtained values
shown in Fig. 10c, a weak peak appeared at 2
q
¼ 38.12ꢂ in the used
AB-1% G compound, which could be ascribed to planes of Ag0 (111)
(JCPDS No. 89-3722). Fig. 10d shows the amplified partial XRD
patterns, and there is a peak at 38.12ꢂ, suggesting Ag0 generated
during the photodegradation procedure. As shown in Fig. 10b, the
Ag3d can be segmented to four peaks. The strong peaks located at
373.9 and 367.9 eV were indexed to Ag 3d3/2 and Ag d5/2 binding
energies of Agþ in AIO, and the weak peaks at 375 and 369 eV were
ascribed to Ag 3d3/2 and Ag 3d5/2 of Ag0 at the same time [3,38,45].
The results showed that after four cycles, the composition barely
changed except for generating the silver.
In order to elucidate the photocatalytic mechanism, confirming
the dominant active species is important. Herein, the scavengers of
Fig. 6. Photoluminescence spectra of the AIO, BV, 20% AB, and AB-1% G composite.

Y. Si et al. / Journal of Alloys and Compounds 831 (2020) 154820
7
Fig. 7. (a)Transient photocurrent responses of AIO, BV, 20% AB and AB-1% G electrodes; (b) EIS Nyquist plots of AIO, BV, 20% AB and AB-1% G electrodes.
ꢀOꢁ2 , hþ, and ꢀOH are 1,4-benzoquinone (BQ), ammonium oxalate
ECB ¼ EVB ꢁ Eg
(4)
(AO), and isopropanol (IPA), respectively [3,45,51]. As shown in
Fig. 11, compared with no scavenger, the degradation curve of RhB
where X is the absolute electronegativity of the relevant semi-
conductor, the X value of AIO and BV were 6.64 and 6.04 eV,
respectively. Ee is the energy of free electrons on the hydrogen scale
(~4.5 eV). The Eg of AIO and BV are 3.13 eV and 2.45 eV according to
the analyses of UVeVis DRS. Accordingly, ECB was obtained as 0.575
and 0.315 eV and EVB was 3.75 and 2.765 eV for AIO and BV,
respectively. Based on the above-mentioned analysis conse-
quences, a probable photocatalytic mechanism was predicted and
demonstrated in Fig. 12. The CB of AIO (0.575 eV) is positive than
that of BV (0.31 eV), the photo-generated electrons can shift to the
CB of AIO from the CB of BV. Meanwhile, many of the holes would
scarcely changes by introducing of IPA, explaining ꢀOH is not the
key active component in this photocatalytic degradation progress.
While the degradation curves of RhB decreased significantly with
the adding of AO and BQ, implying the most crucial active species
were ꢀOꢁ2 and hþ.
The conduction band edge (ECB) and value band edge (EVB) of
AIO and BV can be calculated by the following equation.
EVB ¼ X ꢁ Ee þ 0:5Eg
(3)
Fig. 8. Photocatalytic degradation curves of RhB (a,b) and TC (d) under visible-light; The apparent rate constants for RhB degradation (c).

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Fig. 11. Photocatalytic degradation of RhB usingAB-1% G with different quenchers.
Fig. 9. Effect of the RhB concentration.
play an important role in capturing and transferring electrons, and
previous studies have suggested the same role. The excited electron
in the surface of Ag and GO will react with the dissolved oxygen
(O2) to form ꢀOꢁ2 , which participates in the degradation of pollut-
ants. As well as most of hþ in the VB of BV would oxidize the organic
pollutants directly.
stay on the VB of BV. Thus, the recombination of electron-hole is
effectually inhibited, which is beneficial for improving catalytic
activity. Due to wide bandgap (3.13 eV), AIO maybe not excited by
visible-light. However, a small amount of AIO will be reduced to
metal Ag in the process of irradiation. Some of the metal Ag were
excited to form electron-hole pairs, the others have an effect
plasmon resonance (SPR) effect and electron trapping effect
transfer the electrons efficiently and suppress the recombination of
charges [45]. Then, the photoelectrons from the Ag0 surface are
transferred to the CB of AIO. As electronic receivers, GO and Ag0
4. Conclusions
In summary, a series of hierarchical AB-G was prepared using a
simple
and mild
solvothermal-precipitation way.
The
Fig. 10. (a) Cycling experiments for the degradation of RhB over AB-1% G; (b) High-resolution XPS spectra of Ag 3d in the used AB-1% G, (c, d) XRD patterns of the used and fresh AB-
1% G.

Y. Si et al. / Journal of Alloys and Compounds 831 (2020) 154820
9
Fig. 12. Possible photocatalytic processes of AB-1% G.
characterization results showed that AIO, BV and GO co-existed in
Appendix A. Supplementary data
the AB-G compound. The catalytic activity was enhanced, which
may owe to the heterojunction formation of AIO and BV and the
introduction of GO, which accelerated the separation, migration of
electron-holes and reduce recombination. Experiment results
showed that AB-1% G displayed the optimal photocatalytic activity,
and the degradation rates of RhB and TC reached 97.7% and 89.2%
within 30min, respectively. ꢀOꢁ2 , hþ radicals were the key active
components according to the capture experiments. This study
shows that AB-1% G is a very promising photocatalyst driven by
visible-light for organic pollutants remove, which is expected to be
widely used in wastewater purification.
Supplementary data to this article can be found online at
https://doi.org/10.1016/j.jallcom.2020.154820.
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CRediT authorship contribution statement
Yushan Si: Conceptualization, Methodology, Validation, Formal
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