Fabrication of Ag/AgBr/Ag3VO4 composites with high visible light photocatalytic performance

Article abstract of DOI:10.1039/c8ra10538c

Herein, Ag/AgBr/Ag₃VO₄ composites were synthesized by a simple continuous precipitation method. The obtained composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse reflectance spectroscopy and photoluminescence spectroscopy (PL). Photocatalytic performance of the composites was assessed by the degradation of methyl orange (MO) and tetracycline hydrochloride (TC) under visible light, and the effects of different nominal mass ratios of AgBr and Ag₃VO₄ on the photocatalytic activity were investigated. The results showed that after 20 min of visible light irradiation (λ > 420 nm), the removal rate of MO in the presence of a 5:1 sample reached 98.6%. The EIS and photocurrent results demonstrated that the enhancement of the visible light photocatalytic activity was attributed to the efficient electron-hole pair separation. In addition, the scavenging reactions conducted via the addition of different scavengers confirmed that h⁺ and ·O^2- were the main active species in the reaction. The present study offers potential for the degradation of contaminants.

Full text of DOI:10.1039/c8ra10538c

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Fabrication of Ag/AgBr/Ag3VO4 composites with
high visible light photocatalytic performance
Cite this: RSC Adv., 2019, 9, 5100
ab
Wenxue Li,a Qianlin Chen,
Xianyu Leia and Shang Gonga
*
Herein, Ag/AgBr/Ag3VO4 composites were synthesized by a simple continuous precipitation method. The
obtained composites were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM),
energy dispersive spectrometry (EDS), X-ray photoelectron spectroscopy (XPS), UV-vis diffuse
reflectance spectroscopy and photoluminescence spectroscopy (PL). Photocatalytic performance of the
composites was assessed by the degradation of methyl orange (MO) and tetracycline hydrochloride (TC)
under visible light, and the effects of different nominal mass ratios of AgBr and Ag3VO4 on the
photocatalytic activity were investigated. The results showed that after 20 min of visible light irradiation
(l > 420 nm), the removal rate of MO in the presence of a 5 : 1 sample reached 98.6%. The EIS and
photocurrent results demonstrated that the enhancement of the visible light photocatalytic activity was
attributed to the efficient electron–hole pair separation. In addition, the scavenging reactions conducted
via the addition of different scavengers confirmed that h+ and $O2ꢀ were the main active species in the
reaction. The present study offers potential for the degradation of contaminants.
Received 24th December 2018
Accepted 24th January 2019
DOI: 10.1039/c8ra10538c
rsc.li/rsc-advances
silver-based photocatalysts, including Ag2O,17 Ag3PO4,18
1. Introduction
Ag2CO3,19 AgX (X ¼ Cl, Br, I),20 and Ag6Si2O7,21,22 with excellent
properties in the degradation of organic pollutants.
With the gradual development of industry and technology,
serious environmental pollution and the scarcity of clean
energy have become the most urgent problems. Various forms
of organic wastewater are discharged into the environment
every year; this has caused serious pollution to the environ-
ment. Moreover, the compound structure of organic pollut-
ants is quite stable in water, difficult to decompose, and
resistant to biodegradation.1–4 Some advanced oxidation
processes, such as the Fenton oxidation,5 photocatalytic
oxidation6 and photo-electro-catalytic technology,7,8 have been
developed to solve the problem of wastewater pollution. Pho-
tocatalytic oxidation has the advantages of mild reaction
conditions, no secondary contamination, and low energy
consumption in the reaction process and thus has become one
of the best green environmental protection technologies to be
developed.9–11 To date, the most commonly used photocatalyst
is TiO2 that has excellent chemical stability and is abundant,
non-toxic, and low-cost.12,13 However, TiO2 is a photocatalyst
with a wide band gap (3.2 eV), and its low solar energy utili-
zation and quantum yield limit its practical application.14–16
Researchers have tried to develop a variety of photocatalytic
materials with effective visible light responses, which has
already been a key problem in the eld of photocatalysis. In
the past few years, many studies have been conducted on
AgBr is a traditional photosensitive material whose proper-
ties of light decomposition greatly limit its application as
a photocatalyst.23,24 Studies have shown that AgBr can reduce its
surface component Ag+ to Ag0 under illumination to form Ag/
AgBr, and the photocatalytic activity of Ag/AgBr is improved
because Ag0 exhibits a high efficiency plasmon resonance effect
(SPR) in the visible region.25 Since the electron–hole pairs of
AgBr rapidly combine, AgBr is generally mixed with other
materials to improve its photocatalytic performance. Chen
et al.26 synthesized a AgBr@rGO composite by coating AgBr on
graphene nanosheets via a precipitation method; this increased
the separation rate of the photogenerated charges. Upon further
loading AgBr onto the graphene hydrogel to form a 3D network
structure with good adsorption performance, bisphenol A
rapidly degraded under the synergistic effect of adsorption and
photocatalytic degradation. Lin et al.27 studied the decoration of
nano Ag@AgBr particles on ower-like Bi2WO6 using an oil-in-
water self-assembly method to form Ag@AgBr/Bi2WO6
composites, whose degradation rate for MB was 1.29 times that
of Ag@AgBr. In summary, it can be clearly concluded that the
AgBr-based composite materials show better photocatalytic
activity in the degradation of organic pollutants.
Ever since Konta et al.28 have conducted a pioneering study
on Ag3VO4 and found that Ag3VO4 can split H2O into O2 and H2,
the narrow band gap monoclinic scheelite Ag3VO4 (ꢁ2 eV) has
attracted wide attention of researchers. Due to its special band
structure, the electron–hole recombination rate of pure Ag3VO4
aSchool of Chemistry and Chemical Engineering, Guizhou University, Guiyang,
550025, China. E-mail: cql1018@163.com
bInstitute of Advanced Technology, Guizhou University, Guiyang, 550025, China
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is higher.29 Many researchers have further modied its photo- of AgBr to Ag3VO4 were 3 : 1, 5 : 1, 7 : 1, and 9 : 1, and the cor-
catalytic properties by combining it with a large number of responding samples were denoted as S1, S2, S3, S4. In brief,
materials such as WO3,30 BiVO4,31 ZnO,32 g-C3N4,33 and BiOBr.34 In pure Ag/AgBr was prepared by precipitation of 3 mmol AgNO3
general, the construction of a heterojunction photocatalyst is with 3 mmol KBr followed by photoreduction for 20 minutes.
a method that can effectively improve the performance of a single Ag3VO4 was obtained by precipitation of 6 mmol AgNO3 with
photocatalyst. Zhu et al.35 have reported the initial combination 2 mmol Na3VO4.
of Ag3VO4 and AgBr materials; however, investigation of the ideal
ratio of each material is still worth further exploration; in addi-
tion, the preparation of a composite photocatalyst of the system
by a continuous precipitation method and the development of
the composite Ag/AgBr/Ag3VO4 for the degradation of MO and TC
have not been reported to date.
In this study, a series of Ag/AgBr/Ag3VO4 photocatalysts
with different AgBr masses were synthesized by a simple
continuous precipitation method. The visible light catalytic
performance of the photocatalyst in the degradation of MO
and TC was investigated. The effects of different active
substances during the degradation of MO were also studied by
introducing various scavengers into the photocatalytic reac-
tion system. The separation efficiency of electron–hole pairs
has been discussed by investigating the electrochemical
properties, and the possible mechanism of the photocatalytic
processes has been proposed.
2.3. Evaluation of the photocatalytic performance
The photocatalytic reactions occurred in a 200 mL reactor at
room temperature, and a 300 W Xe lamp (CEL-HXF300) equip-
ped with a 420 nm cut-off lter was used as the light source for
the reaction process. Typically, 50 mg photocatalyst was added
to 100 mL of MO (10 mg Lꢀ1) solution (or 50 mg photocatalyst
was added to 100 mL of TC (10 mg Lꢀ1) solution), and the
mixture was stirred in the dark for 30 min to ensure an
adsorption–desorption balance. Then, the light source was
turned on for photocatalysis experiments, and timing was
initiated. During the reaction, quantitative samples were ob-
tained at given time intervals under light irradiation. The
sample was centrifuged at 10 000 rpm for 5 min to separate the
catalyst from the solution. Analysis of the absorbance of the
supernatant at each of the abovementioned intervals was per-
formed using a UV-vis spectrophotometer. The role of various
active species in the photocatalytic system was investigated by
adding different active species capture agents, including 0.2 g
ammonium oxalate (AO), 3 mL isopropanol (IPA) and 8 mg
benzoquinone (BQ), to the solution.
2. Experimental
2.1. Chemicals
Silver nitrate (AgNO3) was purchased from Sinopharm Chemical
Reagent Co., Ltd (Shanghai, China). Potassium bromide (KBr)
was purchased from Chengdu Jinshan Chemical Reagent Co.,
Ltd. Sodium vanadate (Na3VO4) was purchased from Cool
Chemistry (Beijing, China). Methyl orange (MO) was purchased
from Chengdu Jinshan Chemical Reagent Co., Ltd. Tetracycline
hydrochloride (TC) was purchased from Aladdin (Shanghai,
China). All reagents were of analytical grade and used without
further purication.
2.4. Characterization of the as-prepared samples
To discriminate the phase composition and purity of the
materials, X-ray diffraction (XRD) analysis of the samples was
conducted using the D8 ADVANCE diffractometer with Cu Ka
radiation. The morphology and constituent elements of the
composites were investigated by scanning electron microscopy
(S-4800, accelerating voltage 200 kV) and energy dispersive
spectroscopy (EDS) analysis, respectively. The elemental
composition and the chemical state of the composites were
obtained by X-ray photoelectron spectroscopy (ThermoFisher K-
Alpha). UV-vis diffuse reectance spectra (DRS) were obtained
by a spectrophotometer (UV2700, Shimadzu, Japan) in the
wavelength range of 200–800 nm. Photoluminescence (PL)
spectra of the samples were obtained by the Cary Eclipse uo-
rescence spectrophotometer (Agilent Technologies). Electro-
chemical impedance spectroscopy (EIS) was conducted using an
electrochemical workstation (CHI760E Shanghai). The Pt elec-
trode, the calomel electrode and the sample electrode played
the role of a counter electrode, a reference electrode and
a working electrode, respectively, and were placed in a 0.1 M
Na2SO4 electrolyte for measuring the photocurrent.
2.2. Synthesis of the Ag/AgBr/Ag3VO4 photocatalyst
A series of Ag/AgBr/Ag3VO4 composites were prepared through
a continuous precipitation route. In a typical synthetic proce-
dure, a certain amount of AgNO3 (7.6, 6.7, 6.3, 6.1 mmol) was
added to 50 mL of deionized water followed by stirring to obtain
a uniform clear solution, labeled as solution A. Then, 5.3 mmol
of KBr was dissolved in 20 mL of deionized water and marked as
solution B. A certain amount of Na3VO4 (0.76, 0.46, 0.33, 0.25
mmol) was added to another 20 mL of deionized water and
labeled as solution C. The solution B was gradually added
dropwise to the solution A via a dropping funnel under vigorous
stirring. Aer 30 min, Ag+–AgBr was obtained. Then, solution C
was continuously added to the above mentioned mixture in the
same manner, and magnetic stirring was further continued for
another 4 h to obtain the AgBr/Ag3VO4 composites. Aer this,
3. Results and discussion
the mixture solution was reduced under visible light for 20 The Ag/AgBr/Ag3VO4 composite was prepared by a simple
minutes to produce a portion of Ag0. The nal product was continuous precipitation method. As shown in Fig. 1, the
obtained by ltration, rinsed three times with deionized water synthetic route is described as follows: at rst, KBr was added
and placed in an oven at 60 ꢂC for 24 h. The nominal mass ratios dropwise to the AgNO3 solution, and Brꢀ in the solution reacted
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Fig. 1 Schematic of the synthesis route of the Ag/AgBr/Ag3VO4 composites.
with Ag+ to form AgBr. Then, Na3VO4 was added to the solution,
3.2. XPS analysis
3ꢀ
and Ag+ that had been immobilized further reacted with VO4
To acquire the elemental composition of the composite and
the chemical states of the measured elements, sample S2 was
selected for X-ray photoelectron spectroscopy (XPS). As shown
in Fig. 3, the high-resolution spectrum is composed of Ag 3d,
Br 3d, V 2p and O 1s with no other impurities. Moreover, two
peaks with the binding energies of 368.25 eV and 374.22 eV
were observed in the Ag 3d spectra (Fig. 3b), which were
consistent with Ag 3d5/2 and Ag 3d3/2 of Ag+ for AgBr and
Ag3VO4, respectively, and further conrmed that the Ag
species mainly existed in the form of Ag+. Moreover, there were
two weak peaks at the binding energies of 369.16 eV and
to form Ag3VO4 and covered the surface of AgBr. Finally, Ag/
AgBr/Ag3VO4 was generated aer illumination.
3.1. XRD analysis
The XRD spectra of Ag/AgBr, Ag3VO4 and Ag/AgBr/Ag3VO4
composites are shown in Fig. 2. The peaks of Ag/AgBr and
Ag3VO4 well-matched with those of cubic AgBr (JCPD no. 06-
0438) and monoclinic Ag3VO4 (JCPDS no. 43-0542), respectively.
For AgBr, the sharp diffraction peaks at 30.96ꢂ, 44.35ꢂ and
55.04ꢂ corresponded perfectly to the crystal planes of (2 0 0), (2 2
0) and (2 2 2). This shows that its crystallinity is good and there
are no other impurities. For pure Ag3VO4, the diffraction peaks
at 30.86ꢂ, 32.33ꢂ and 35.07ꢂ corresponded to the crystal planes
of (ꢀ1 2 1), (1 2 1) and (3 0 1); this indicated that the prepared
Ag3VO4 was pure and had no second phase. A fairly weak
intensity peak can also be observed in the XRD pattern of Ag/
AgBr/Ag3VO4; since the intensity of the peak of Ag3VO4 in the
composite is much weaker than that of AgBr and their peak
positions are relatively close at around 31ꢂ, the peak of Ag3VO4
may be covered by the peak of AgBr.36 As the AgBr content
increases, the peak of Ag3VO4 becomes weaker until it is barely
visible. The results of XRD suggest that the composites have
been successfully prepared by a continuous precipitation
method. In addition, the reason why the characteristic diffrac-
375.02 eV, which corresponded to Ag 3d5/2 and Ag 3d3/2
,
indicating that Ag also existed in the form of metallic Ag0.37
The reason for the occurrence of this phenomenon is that
under light, Ag+ is partially reduced to Ag0 and attached to the
surface of the composite photocatalyst. Moreover, two distinct
peaks at the binding energies of 68.85 eV and 69.6 eV were
observed in the Br 3d spectra (Fig. 3c), which were attributed to
Br 3d5/2 and Br 3d3/2 and indicated that Br existed as Brꢀ.38 The
peaks of V 2p1/2 and V 2p3/2 at 516.85 eV and 524.35 eV
conrmed that V existed as V5+ (ref. 39) (Fig. 3d). The peak of O
1s (Fig. 3e) at 530.08 eV corresponded to hydroxyls (–OH), and
the other peak at 531.87 eV conrmed the existence of oxygen
in Ag/AgBr/Ag3VO4. All these results further showed that the
composites consisted of Ag/AgBr and Ag3VO4.
3.3. SEM analysis
tion peak of Ag0, which is expected to occur, is not observed in The morphologies and composition of Ag3VO4, Ag/AgBr, and
the spectrum may be that the content of Ag0 is low.
Ag/AgBr/Ag3VO4 were investigated by SEM and EDS, respectively
Fig. 2 (a) XRD patterns of Ag/AgBr and Ag/AgBr/Ag3VO4 composites and (b) pure Ag3VO4.
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Fig. 3 (a) Survey XPS spectra of the S2 sample; (b, c, d and e) high-resolution XPS data of Ag 3d, Br 3d, V 2p, and O 1s for S2, respectively.
(Fig. 4). As shown in Fig. 4a, ne particles of Ag/AgBr having an and Ag3VO4 will enhance the transfer and separation of pho-
average particle diameter of more than 1 mm can be observed. togenerated charges;40 one of the advantages of a Ag/AgBr/
Fig. 4b shows the prepared Ag3VO4 with nanometer-sized Ag3VO4 composite is that it has more active sites, and the
particles. Both Ag/AgBr and Ag3VO4 prepared by the contin- photogenerated electron–hole pairs can be rapidly transferred;
uous precipitation method are irregular small particles, and the this is very benecial to the degradation of MO by the photo-
particle size of Ag3VO4 is much smaller than that of Ag/AgBr. catalyst.41 The EDS spectra of Ag/AgBr and S2 are shown in
The composite Ag/AgBr/Ag3VO4 also exhibited irregular parti- Fig. 4e and f, respectively. Pure AgBr consisted of Ag and Br
cles, and the surface of the large particles was occupied by many elements, whereas the Ag/AgBr/Ag3VO4 composite material was
small particles (Fig. 4c). The abundant content of AgBr in the conrmed to contain Ag, Br, V and O elements. Based on the
composite material made a signicant impact on the formation results of the abovementioned XRD, SEM and EDS data, the
of its nal morphology, and the particle size of the composite preparation of the Ag/AgBr/Ag3VO4 composite was successfully
was similar to that of AgBr. Moreover, the composite exhibits conrmed.
good dispersibility (Fig. 4d), and the close contact between AgBr
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Fig. 4 SEM images of (a) pure Ag/AgBr, (b) pure Ag3VO4, and (c and d) S2 sample; (e and f) EDS spectra of pure Ag/AgBr and S2 sample,
respectively.
equation (ahv) ¼ A(hv ꢀ Eg)n/2, where Eg, v, h, A and a are the band
3.4. The optical property analysis
gap energy of the semiconductor, light frequency, Planck's
The optical properties of the prepared samples were investigated
via a UV-vis spectrophotometer. The light absorption character-
istic of the composite material can be analyzed from Fig. 5. All
constant, a constant value and the absorption coefficient,
respectively.19,34 The relationship between (ahv)2 and hv was ob-
tained as shown in Fig. 5b, and the Eg value was acquired by
estimating the intercept of the tangent of the curve. In this study,
the band gap values of Ag/AgBr, Ag3VO4, and sample S2 were
samples exhibited absorbance in the visible region, and AgBr
caused a signicant absorption band in the sample curve due to
the surface plasmon resonance effect (SPR) of Ag0, which was
calculated to be 2.43 eV, 2.01 eV, and 2.64 eV, respectively, which
derived from the collective oscillation of free electrons.27,42 With
were consistent with the previously reported values.31,43,44
a change in the AgBr content in the composite, the edge absorp-
tion band value of the composite appears to increase in a regular
way. As the absorption edge value of the sample becomes larger,
the response of the sample to visible light is enhanced. The band
gap width of the composite materials was obtained by the
3.5. Charge separation analysis
In general, PL spectra can be used to detect the separation and
recombination rates of photogenerated charges. A lower PL
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Fig. 5 (a) UV-vis diffuse reflectance spectra of Ag/AgBr, Ag3VO4 and Ag/AgBr/Ag3VO4 composites; (b) Tauc plot of Ag/AgBr, Ag3VO4 and S2.
intensity means that the photogenerated electron–hole pairs the fact that the electron–hole transfer efficiency between
have high separation efficiency and thus produce high photo- interfaces becomes higher aer the combination of AgBr and
catalytic activity.45 The photoluminescence spectra of Ag3VO4, Ag3VO4 into a composite material.
Ag/AgBr and S2 are shown in Fig. 6. It can be found that the
intensities of both pure Ag3VO4 and Ag/AgBr in the plot are
3.6. The photocatalytic performance test
higher than those in the curve of the S2 sample at different
wavelengths; this indicates the reduction of photogenerated
electron–hole pair recombination in sample S2.
To explore the ability of the as-prepared composites to degrade
pollutants, the degradation of the common pollutants MO and
TC by the composites was studied by the degradation experi-
ments. As shown in Fig. 8a, the negligible self-photolysis of MO
under visible light demonstrates that MO has stable chemical
properties to light. The adsorption rate in the dark reaction is
also low; this indicates that the main cause of MO degradation
is visible light irradiation rather than surface adsorption of the
photocatalyst. As expected, all the prepared composite photo-
catalysts have higher photocatalytic activity than Ag/AgBr and
Ag3VO4 with the ranking of S2 > S3 > S4 > S1 > Ag/AgBr > Ag3VO4,
proving that AgBr plays an active role in enhancing the photo-
catalytic activity.45 Moreover, as the proportion of AgBr in the
composites increases, the degradation of MO rst increases and
then decreases. Particularly, the removal efficiency of all the
composite photocatalysts could reach over 90% aer 10 min of
illumination. S2 exhibited the most excellent photocatalytic
activity, and 98.6% of MO was decomposed aer 20 min of
irradiation. The ndings demonstrate that the composite pho-
tocatalyst has a shorter degradation time and higher degrada-
tion efficiency than the single photocatalyst. The photocatalytic
activity of S3 and S4 in the plot is also lower than that of S2; this
may be explained by the recombination of photogenerated
electrons and holes that suppresses the photocatalytic effi-
ciency. In addition, for the typical colorless antibiotic TC, S2
exhibited superior photocatalytic activity in the degradation of
TC (Fig. 8b). Its degradation efficiency reached 77% aer 10
minutes. However, aer 4 min, the degradation rate of TC
became very low, possibly reaching equilibrium rather than
complete degradation.39
To further analyze the separation efficiency of photo-
generated electron–hole pairs, the EIS and transient photocur-
rent responses of the photocatalytic samples were investigated.
For the EIS, a small arc radius leads to a low charge transfer
resistance.18 Fig. 7a clearly reveals that the Nyquist plots of S2
are smaller than that of Ag/AgBr; this indicates that the pho-
togenerated electron–hole transfer efficiency of S2 is high. In
the transient photocurrent response of the samples, a change in
the light source causes a sudden change in the photocurrent. A
high transient photocurrent response is generally indicative of
a high separation efficiency of the photogenerated electron–
hole pairs. It can be observed that the photocurrent of S2 is
stronger than that of Ag/AgBr; this denotes that S2 has a faster
electron–hole separation efficiency (Fig. 7b). This may be due to
The pseudo rst-order plots of MO degradation reactions for
different samples are shown in Fig. 8c, which have been ob-
tained by processing the data using a pseudo rst-order model
whose equation is ln(C0/Ct) ¼ kt. C0 is the initial concentration
of MO before the photoreaction, Ct is the concentration of MO
Fig. 6 PL spectra of pure Ag/AgBr, Ag3VO4 and S2 sample.
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Fig. 7 (a) Electrochemical impedance spectra of Ag/AgBr and S2 photocatalysts and (b) transient photocurrent response of Ag/AgBr and S2.
aer the reaction time t, and k is the reaction rate constant. The material signicantly improves the degradation rate of the
degradation rates of MO were calculated to be 0.0026 minꢀ1
0.1118 minꢀ1, and 0.2509 minꢀ1 over the Ag3VO4, Ag/AgBr, and
,
pollutants.
The MO solution was taken at a given reaction time, and the
S2 samples, respectively. The degradation rate of S2 is higher change in its TOC content was measured to account for the
than the degradation rate of a single photocatalyst, which is 2.2 change of the remaining content of MO in the solution. As
and 96.5 times that of Ag/AgBr and Ag3VO4, respectively. This is shown in Fig. 8d, during the photocatalytic degradation of MO,
highly consistent with the results of photocatalytic degradation. the TOC gradually decreased to 31% with time. It showed that
These results more accurately demonstrate that the composite the composite could effectively remove MO, and MO was
mineralized into CO2 and H2O.
Fig. 8 (a) The photocatalytic degradation of MO via different as-synthesized photocatalysts; (b) the photocatalytic degradation of TC via
different as-synthesized photocatalysts; (c) pseudo first-order plot of the photocatalytic degradation of MO via different as-synthesized pho-
tocatalysts; (d) change of TOC removal rate of MO in the presence of the S2 sample.
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Fig. 9 (a) Reusable performance of S2 and (b) XRD data contrast diagram before and after the use of S2.
To further assess the performance of the photocatalyst, the
stability of the photocatalyst was explored through cycling
experiments. As shown in Fig. 9a, the catalyst can be subjected
to multiple cycling experiments to show that it has good
stability; however, aer three cycles, the degradation efficiency
is reduced, and only 61.3% of the pollutants are degraded.
There are two possibilities for explaining this phenomenon: one
is that some catalysts are inevitably lost during the experimental
operation, and the second is that the catalyst decomposes a part
of Ag0; this directly leads to a decrease in the photocatalytic
activity. The latter reason could be supported by the XRD
comparison data obtained before and aer the sample was
used. The Ag0 characteristic diffraction peak was clearly
observed in the spectrum of the catalyst recovered aer the
cycling experiment, and the other structures hardly changed
(Fig. 9b).
Fig. 10 Photocatalytic degradation of MO in an aqueous solution via
S2 under different conditions.
3.7. Possible photocatalytic mechanism of the Ag/AgBr/
Ag3VO4 composites
The photocatalysts produce various active substances in the
solution such as hydroxyl radicals ($OH), $O2ꢀ and h+. The
active substances produced can oxidize or reduce the pollutants
into new non-toxic and harmless substances; thus, the purpose
of removing the target pollutants in the solution is achieved.
Therefore, experiments to capture the reactive species have
been conducted to identify the active species that are primarily
catalytically active. Herein, ammonium oxalate (AO), benzo-
quinone (BQ) and isopropanol (IPA) acted as scavengers to
eliminate h+, $O2ꢀ and $OH, respectively.46 As Fig. 10 shows, the
degradation rate of MO was 98.6% before the addition of
a scavenger, and the removal rate of MO aer the addition of AO
and BQ was 62% and 21%, respectively. The phenomenon of
photocatalytic activity was signicantly inhibited; this indicated
that h+ and $O2ꢀ might be the main reactive species involved in
the photocatalytic degradation process, and $O2ꢀ played
a dominant role in the reaction. When IPA was added as
a scavenger, the degradation rate of MO was still as high as 98%,
which was enough to show that $OH was a partly reactive
species in the reaction and had only a weak inuence on the
degradation process.
Based on the previous analysis, a possible mechanism for
the degradation of MO by the composites can be proposed, as
shown in Fig. 11. The band gaps of AgBr and Ag3VO4 estimated
by UV-vis spectroscopy were about 2.43 eV and 2.01 eV,
respectively. Through the Mulliken electronegativity theory, we
computed the position of the semiconductor band, and the
formulas are as follows:
ECB ¼ X ꢀ Ee ꢀ 0.5Eg
EVB ¼ ECB + Eg
(1)
(2)
where ECB and EVB represent the edge potential of the conduc-
tion band (CB) and valence band (VB), respectively, and X stands
for the absolute electronegativity of the semiconductor, which is
derived from the geometric mean of the electronegativity of the
constituent atoms (the X value of AgBr is 4.42,47 and the X value
of Ag3VO4 is 5.64 (ref. 48)). Ee is the energy of free electrons in
the hydrogen scale (about 4.5 eV). Eg is the band gap energy of
the semiconductor. It has been determined by calculation that
the valence band edges of AgBr and Ag3VO4 are located at
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pairs; on the other hand, Ag0 on the photocatalyst surface can
absorb more visible light, thus further improving the photo-
catalytic performance.
4. Conclusions
A series of Ag/AgBr/Ag3VO4 composites were synthesized by
a simple continuous precipitation method. The degradation
rates of the sample S2 for MO and TC were 98.6% and 77%,
which were better than those of the single photocatalysts
Ag3VO4 and Ag/AgBr, indicating that S2 had a better degrada-
tion effect on MO and TC in an aqueous solution. In addition,
the evidence that h+ and $O2ꢀ were the main active substances
in the degradation process of the pollutant MO was obtained by
conducting active group capture experiments. The results of PL,
EIS and photocurrent response show that the composite
exhibits better charge separation efficiency, and the surface
plasmon resonance (SPR) effect of Ag0 together improves the
photocatalytic performance of the composite. As a highly effi-
cient photocatalyst, Ag/AgBr/Ag3VO4 is a promising photo-
catalyst for wastewater purication.
Fig. 11 The possible photocatalytic mechanism of the Ag/AgBr/
Ag3VO4 composites.
1.13 eV and 2.15 eV, and the conduction band edges are located
at the positions ꢀ1.3 eV and 0.14 eV, respectively. Under visible
light irradiation, the energy of light is greater than the energy
band gap of AgBr and Ag3VO4. Ag0, AgBr and Ag3VO4 can absorb
visible light photons to form electron–hole pairs. Subsequently,
the electrons will migrate from the surface of Ag0 to the CB of
AgBr and, in turn, eventually to the CB of Ag3VO4, whereas the
holes remain on Ag0. The electrons accumulate to a higher
concentration in the CB of Ag3VO4, and the electrons can be
trapped by molecular oxygen in the solution to form a strong
oxidizing substance, $O2ꢀ, and other reactive oxygen species,
which can oxidize MO and result in high photocatalytic
activity.35 Furthermore, the holes in the VB of Ag3VO4 gradually
migrate to the VB of AgBr and aggregate. A large number of
holes can react with H2O or –OH to produce the $OH radicals,
and excess holes can be combined with Brꢀ to reduce it to Br0
since Br0 is also a kind of active radical species.35,38,49 Finally,
$O2ꢀ, h+, $OH and Br0 react with MO as oxidants to generate
H2O, CO2 and other small molecules, resulting in the degra-
dation of MO. The possible mechanism by which Ag/AgBr/
Ag3VO4 degrades MO can be explained as follows:
Conflicts of interest
The authors declare that they have no conict of interest.
Acknowledgements
The authors gratefully acknowledge the nancial supports
provided by the Guizhou Province High-level Talent Training
Program [Talent[2016]5658].
References
´
1 E. Forgacs, T. Cserhati and G. Oros, Environ. Int., 2004, 30,
953–971.
2 W.-K. Jo and R. J. Tayade, Chin. J. Catal., 2014, 35, 1781–1792.
3 S. Natarajan, H. C. Bajaj and R. J. Tayade, J. Environ. Sci.,
2018, 65, 201–222.
4 H. S. Rai, M. S. Bhattacharyya, J. Singh, T. K. Bansal, P. Vats
and U. C. Banerjee, Crit. Rev. Env. Sci. Technol., 2005, 35, 219–
238.
5 J. Hu, P. Zhang, W. An, L. Liu, Y. Liang and W. Cui, Appl.
Catal., B, 2019, 245, 130–142.
6 L. Liu, M. Yue, J. Lu, J. Hu, Y. Liang and W. Cui, Appl. Surf.
Sci., 2018, 456, 645–656.
7 Y. Zhang, W. Cui, W. An, L. Liu, Y. Liang and Y. Zhu, Appl.
Catal., B, 2018, 221, 36–46.
8 W. Cui, J. He, H. Wang, J. Hu, L. Liu and Y. Liang, Appl.
Catal., B, 2018, 232, 232–245.
9 Y. Yang, W. Zhang, R. Liu, J. Cui and C. Deng, Sep. Purif.
Technol., 2018, 190, 278–287.
ꢀ
Ag/AgBr/Ag3VO4 + hn / Ag/AgBr/Ag3VO4(eCB + hVB+) (3)
ꢀ
Ag3VO4(eꢀ) + O2 / $O2
(4)
(5)
(6)
(7)
(8)
(9)
ꢀ
$O2 + H+ / $OOH
$OOH + H+ + 2eꢀ / $OH + OHꢀ
AgBr(h+) + OHꢀ / $OH
AgBr(h+) + Brꢀ / AgBr + Br0
$Oꢀ2 , h+, $OH, Br0 + MO / small molecules
10 J. Kou, C. Lu, J. Wang, Y. Chen, Z. Xu and R. S. Varma, Chem.
Rev., 2017, 117, 1445–1514.
11 L. V. Bora and R. K. Mewada, Renewable Sustainable Energy
Rev., 2017, 76, 1393–1421.
On the one hand, the composite material of this structure
accelerates the effective separation of photogenerated electrons
and holes and reduces the recombination of electron–hole
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RSC Advances
12 F. K. Mohamad Alosfur, N. J. Ridha, M. H. H. Jumali and 30 M. Yan, Y. Wu, F. Zhu, Y. Hua and W. Shi, Phys. Chem. Chem.
S. Radiman, Nanotechnology, 2018, 29, 145707. Phys., 2016, 18, 3308–3315.
13 M. Hadnadjev-Kostic, T. Vulic and R. Marinkovic-Neducin, 31 M. Yan, Y. Wu, Y. Yan, X. Yan, F. Zhu, Y. Hua and W. Shi, ACS
Adv. Powder Technol., 2014, 25, 1624–1633. Sustainable Chem. Eng., 2015, 4, 757–766.
14 V. Moradi, M. B. G. Jun, A. Blackburn and R. A. Herring, Appl. 32 F. Kiantazh and A. Habibi-Yangjeh, Solid State Sci., 2015, 49,
Surf. Sci., 2018, 427, 791–799. 68–77.
15 X.-N. Wei, H.-L. Wang, X.-K. Wang and W.-F. Jiang, Appl. 33 T. Zhu, Y. Song, H. Ji, Y. Xu, Y. Song, J. Xia, S. Yin, Y. Li,
Surf. Sci., 2017, 412, 357–365. H. Xu, Q. Zhang and H. Li, Chem. Eng. J., 2015, 271, 96–105.
16 C. Mu, Y. Zhang, W. Cui, Y. Liang and Y. Zhu, Appl. Catal., B, 34 J. Zhang and Z. Ma, Molecular Catalysis, 2017, 436, 190–198.
2017, 212, 41–49.
35 Q. Zhu, W.-S. Wang, L. Lin, G.-Q. Gao, H.-L. Guo, H. Du and
17 G. Wang, X. Ma, B. Huang, H. Cheng, Z. Wang, J. Zhan,
A.-W. Xu, J. Phys. Chem. C, 2013, 117, 5894–5900.
X. Qin, X. Zhang and Y. Dai, J. Mater. Chem., 2012, 22, 36 S. Huang, Y. Xu, Z. Chen, M. Xie, H. Xu, M. He, H. Li and
21189–21194. Q. Zhang, RSC Adv., 2015, 5, 71035–71045.
18 L. Liu, L. Ding, Y. Liu, W. An, S. Lin, Y. Liang and W. Cui, 37 J. Zhang and Z. Ma, J. Colloid Interface Sci., 2018, 524, 16–24.
Appl. Catal., B, 2017, 201, 92–104.
19 O. Mehraj, N. A. Mir, B. M. Pirzada, S. Sabir and M. Muneer,
J. Mol. Catal. A: Chem., 2014, 395, 16–24.
38 Z. Wang, J. Zhang, J. Lv, K. Dai and C. Liang, Appl. Surf. Sci.,
2017, 396, 791–798.
39 L. Jing, Y. Xu, C. Qin, J. Liu, S. Huang, M. He, H. Xu and
H. Li, Mater. Res. Bull., 2017, 95, 607–615.
20 P. Wang, B. Huang, Q. Zhang, X. Zhang, X. Qin, Y. Dai,
J. Zhan, J. Yu, H. Liu and Z. Lou, Chem.–Eur. J., 2010, 16, 40 M. Gaumet, A. Vargas, R. Gurny and F. Delie, Eur. J. Pharm.
10042–10047. Biopharm., 2008, 69, 1–9.
21 Z. Lou, B. Huang, Z. Wang, X. Ma, R. Zhang, X. Zhang, 41 R. Cao, H. Yang, X. Deng, S. Zhang and X. Xu, Sci. Rep., 2017,
X. Qin, Y. Dai and M.-H. Whangbo, Chem. Mater., 2014, 26,
3873–3875.
22 J. Liu, W. Wu, Q. Tian, Z. Dai, Z. Wu, X. Xiao and C. Jiang,
Dalton Trans., 2016, 45, 12745–12755.
23 Y. Zhao, L. Kuai and B. Geng, Catal. Sci. Technol., 2012, 2,
1269–1274.
24 A. Abulizi, K. Kadeer, L. Zhou, Y. Tursun and T. Dilinuer, J.
Taiwan Inst. Chem. Eng., 2018, 88, 243–251.
25 P. Wang, B. Huang, X. Zhang, X. Qin, H. Jin, Y. Dai, Z. Wang,
J. Wei, J. Zhan, S. Wang, J. Wang and M. H. Whangbo,
Chem.–Eur. J., 2009, 15, 1821–1824.
26 F. Chen, W. An, L. Liu, Y. Liang and W. Cui, Appl. Catal., B,
2017, 217, 65–80.
27 S. Lin, L. Liu, J. Hu, Y. Liang and W. Cui, Appl. Surf. Sci.,
2015, 324, 20–29.
7, 15001.
42 B. Wiley, Y. Sun, B. Mayers and Y. Xia, Chem. Eur. J., 2005, 11,
454–463.
43 Y. Song, S. Xue, G. Wang, J. Jin, Q. Liang, Z. Li and S. Xu, J.
Phys. Chem. Solids, 2018, 121, 329–338.
44 T. Li, Y. He, H. Lin, J. Cai, L. Dong, X. Wang, M. Luo, L. Zhao,
X. Yi and W. Weng, Appl. Catal., B, 2013, 138–139, 95–103.
45 S. Zhang, H. Gao, X. Liu, Y. Huang, X. Xu, N. S. Alharbi,
T. Hayat and J. Li, ACS Appl. Mater. Interfaces, 2016, 8,
35138–35149.
46 J. Zhang and Z. Ma, J. Taiwan Inst. Chem. Eng., 2018, 88, 177–
185.
47 K. Dai, D. Li, L. Lu, Q. Liu, C. Liang, J. Lv and G. Zhu, Appl.
Surf. Sci., 2014, 314, 864–871.
48 S. Wang, D. Li, C. Sun, S. Yang, Y. Guan and H. He, Appl.
Catal., B, 2014, 144, 885–892.
49 Y. Wan, C. Liang, Y. Xia, W. Huang and Z. Li, Appl. Surf. Sci.,
2017, 396, 48–57.
28 R. Konta, H. Kato, H. Kobayashi and A. Kudo, Phys. Chem.
Chem. Phys., 2003, 5, 3061–3065.
29 Y. Xu, L. Jing, X. Chen, H. Ji, H. Xu, H. Li, H. Li and Q. Zhang,
RSC Adv., 2016, 6, 3600–3607.
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