Visible light-driven photocatalytic degradation of organic pollutants by a novel Ag3VO4/Ag2CO3 p–n heterojunction photocatalyst: Mechanistic insight and degradation pathways

Article abstract of DOI:10.1016/j.jallcom.2020.155211

In the field of photocatalysis, the construction of a heterojunction system with efficient charge separation at the interface and charge transfer to increase the photocatalyst performance has gained considerable attention. In this study, the Ag₃VO₄/Ag₂CO₃ p–n heterojunction is first synthesized using a simple co-precipitation method. The composite photocatalyst with a p-n heterojunction has a strong internal electric field, and its strong driving force can effectively solve the problem of low separation and migration efficiency of photogenerated electron-hole pairs. The optimized Ag₃VO₄/Ag₂CO₃ composite can effectively degrade organic pollutants (rhodamine b (RhB), methylene blue (MB), levofloxacin (LVF), and tetracycline). More specifically, the Ag₃VO₄/Ag₂CO₃ photocatalyst with a 1:2 mass ratio (VC-12) can remove 97.8percent and 82percent of RhB and LVF within 30 and 60 min, respectively. The LVF degradation rate by VC-12 under visible light irradiation is more than 12.8 and 21.51 times higher than those of pure Ag₃VO₄ and Ag₂CO₃, respectively. The excellent photocatalytic activity of the Ag₃VO₄/Ag₂CO₃ hybrid system is mainly attributed to the internal electric field that forms in the Ag₃VO₄/Ag₂CO₃ p–n heterojunction system, the photogenerated electron hole pairs that separate and facilely migrate, and the specific surface area of VC-12 that is larger than that of the monomer. In addition, the degradation efficiency of VC-12 did not decline significantly after four cycles. In this study, the photocatalytic mechanism for Ag₃VO₄/Ag₂CO₃ photocatalysts is explored in detail based on the energy band analysis results, trapping experiment results, and electron spin resonance spectra. Finally, the LVF degradation products are analyzed by liquid chromatography–mass spectrometry, and the potential LVF degradation pathway is identified. The experiments performed in this research therefore lead to new motivation for the design and synthesis of highly efficient and widely applicable photocatalysts for environmental purification.

Full text of DOI:10.1016/j.jallcom.2020.155211

Journal Pre-proof
Visible light-driven photocatalytic degradation of organic pollutants by a novel
Ag VO /Ag CO p–n heterojunction photocatalyst: Mechanistic insight and
3
4
2
3
degradation pathways
Haibo Sun, Pufeng Qin, Zhibin Wu, Chanjuan Liao, Jiayin Guo, Shuai Luo, Youzheng
Chai
PII:
S0925-8388(20)31574-7
DOI:
https://doi.org/10.1016/j.jallcom.2020.155211
JALCOM 155211
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 31 January 2020
Revised Date: 4 April 2020
Accepted Date: 14 April 2020
Please cite this article as: H. Sun, P. Qin, Z. Wu, C. Liao, J. Guo, S. Luo, Y. Chai, Visible light-driven
photocatalytic degradation of organic pollutants by a novel Ag VO /Ag CO p–n heterojunction
3
4
2
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photocatalyst: Mechanistic insight and degradation pathways, Journal of Alloys and Compounds (2020),
doi: https://doi.org/10.1016/j.jallcom.2020.155211.
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Credit Author Statement
Haibo Sun: Conceptualization, Validation, Investigation, Writing - Original Draft
Pufeng Qin: Supervision, Project administration, Funding acquisition
Zhibin Wu: Writing - Review & Editing, Resources, Funding acquisition
Chanjuan Liao: Supervision, Funding acquisition
Jiayin Guo: Supervision, Validation, Writing - Review & Editing
Shuai Luo: Investigation, Data Curation
Youzheng Chai: Validation, Data Curation

Visible light-driven photocatalytic degradation of
organic pollutants by a novel Ag3VO4/Ag2CO3 p–n
heterojunction photocatalyst: Mechanistic insight and
degradation pathways
a, b, c
Haibo Sun a, b, c, Pufeng Qin
, Zhibin Wu a, b, c , Chanjuan Liao a, Jiayin Guo d,
Shuai Luo a, Youzheng Chai a
a College of Resources and Environment, Hunan Agricultural University, Changsha,
410128, P.R. China
b Hunan Engineering & Technology Research Center for Irrigation Water Purification,
Changsha, China
c Key Laboratory for Rural Ecosystem Health in the Dongting Lake Area of Hunan
Province, Changsha 410128, P.R. China
d College of Environmental Science and Engineering, Hunan University, Changsha
410082, PR China
Corresponding authors. Address: College of Resources and Environment, Hunan Agricultural
University, Changsha, 410128, PR China. Tell: +86 731 84617803; fax: +86 731 84617803. E–
mail address: qinpufeng@126.com (P. F. Qin), wzbaaa11@163.com (Z.B. Wu)

Abstract
In the field of photocatalysis, the construction of a heterojunction system with
efficient charge separation at the interface and charge transfer to increase the
photocatalyst performance has gained considerable attention. In this study, the
Ag3VO4/Ag2CO3 p–n heterojunction is first synthesized using a simple
co-precipitation method. The composite photocatalyst with a p-n heterojunction has a
strong internal electric field, and its strong driving force can effectively solve the
problem of low separation and migration efficiency of photogenerated electron-hole
pairs. The optimized Ag3VO4/Ag2CO3 composite can effectively degrade organic
pollutants (rhodamine b (RhB), methylene blue (MB), levofloxacin (LVF), and
tetracycline). More specifically, the Ag3VO4/Ag2CO3 photocatalyst with a 1:2 mass
ratio (VC-12) can remove 97.8% and 82% of RhB and LVF within 30 and 60 min,
respectively. The LVF degradation rate by VC-12 under visible light irradiation is
more than 12.8 and 21.51 times higher than those of pure Ag3VO4 and Ag2CO3,
respectively. The excellent photocatalytic activity of the Ag3VO4/Ag2CO3 hybrid
system is mainly attributed to the internal electric field that forms in the
Ag3VO4/Ag2CO3 p–n heterojunction system, the photogenerated electron hole pairs
that separate and facilely migrate, and the specific surface area of VC-12 that is larger
than that of the monomer. In addition, the degradation efficiency of VC-12 did not
decline significantly after four cycles. In this study, the photocatalytic mechanism for
Ag3VO4/Ag2CO3 photocatalysts is explored in detail based on the energy band
analysis results, trapping experiment results, and electron spin resonance spectra.

Finally, the LVF degradation products are analyzed by liquid chromatography–mass
spectrometry, and the potential LVF degradation pathway is identified. The
experiments performed in this research therefore lead to new motivation for the
design and synthesis of highly efficient and widely applicable photocatalysts for
environmental purification.
Keywords: Photocatalysis; Ag3VO4/Ag2CO3; p–n heterojunction; Degradation
pathway; Visible light.

1. Introduction
The rapid development of the world economy and technology has inevitably
caused severe environmental pollution [1-3]. For example, antibiotics and printing
reagents, which are extensively used, have severely threatened the aquatic
environment because of their weak biodegradability and strong toxicity to aquatic
organisms and humans [4-7]. Accordingly, to treat organic contaminants in
wastewater, various processing techniques, including adsorption [8], microwave
catalysis [9], the electro-Fenton process [10], and photocatalysis [11], have been
devised. Adsorption technology affords advantages, such as low energy consumption
and excellent treatment effects; however, the complex process involved in its
preparation hinders its further development [12]. As an advanced oxidation
technology, microwave catalysis suffers from a high energy utilization rate [13]. In
addition, the technique has insufficient catalytic performance [14]. The electro-Fenton
process, as a type of environmental purification technology, exhibits an excellent
treatment effect. Although it currently has certain practical applications, its
prohibitive cost continues to restrict its widespread application [15]. To resolve the
abovementioned problems, the development of low-cost and efficient water treatment
technologies is deemed a feasible approach. In this regard, photocatalytic oxidation
based on the direct utilization of solar energy for the complete mineralization of
refractory organic contaminants in aquatic environments is considered a promising
method because it is clean, nontoxic, energy saving, and highly efficient [16-19].
Among the minerals, titanium dioxide (TiO2) has received considerable attention from

researchers because of its excellent stability and superior ultraviolet (UV)
photocatalytic performance [20]. The wide band gap (Eg, ~3.2 eV) of TiO2, however,
makes it difficult to be driven by visible light, thereby hindering the practical use of
TiO2 in water purification [21, 22].
To improve the visible light utilization and carrier mobility of photocatalysts, the
development of visible light-driven photocatalysts has become a new goal [23, 24].
Thus far, many new photocatalytic materials have emerged, including metal oxides
[25, 26], metal sulfides [27-29], and carbon nitrides [30, 31]. The photocatalytic
activity and visible light utilization of these materials have improved significantly
over those of TiO2. In recent years, Ag-based semiconductor materials have been
considered predominant candidates for photodegrading organic pollutants because of
their moderate band gap. For example, AgX (Cl, Br, and I) [32-34], Ag3PO4 [35],
Ag2CO3 [36], Ag3VO4 [19], and Ag2MoO4 [37] have exhibited considerable
effectiveness in the removal of organic compounds. Konta et al. first reported the use
of silver vanadate (Ag3VO4) in the field of visible light-driven photocatalysis because
of its narrow band gap (Eg, ~2.1 eV) [38]. Furthermore, the Ag 4d and O 2p orbitals
of Ag3VO4 are hybridized to form a dispersed valence band (VB), and the conduction
band (CB) formed by V 3d contributes to charge transfer [39-41]. Nevertheless, the
low quantum efficiency and high recombination rate of the electron hole pair limit the
generation of active species for the degradation of organics. To ameliorate the
abovementioned deficiencies, many Ag3VO4-based composite photocatalytic
materials have been produced (e.g., Ag3VO4/TiO2 [42], g-C3N4/Ag3VO4 [43],

Ag3VO4/Bi2O2CO3 [44], and Ag3VO4/WO3 [45]). The heterojunction (type II) formed
by Ag3VO4-based composites enables the spatial separation of photogenerated
carriers and effectively suppresses charge recombination. Furthermore, matching
different band structures can also expand the light response range.
As a typical n-type semiconductor, Ag3VO4 participates in a p–n heterojunction
that has a stronger internal electric field than a type heterojunction [46]. When a
p-type semiconductor and an n-type Ag3VO4 are combined to form a p–n
heterojunction, a built-in electric field with a strong charge-driven capacity can be
formed. This is because of the large flat band potential difference between the two
semiconductors, which is beneficial for the rapid separation and transfer of
photogenerated carriers [47]. The construction of a Ag3VO4-based p–n heterojunction
is therefore an effective means to improve its photocatalytic ability.
As a typical p-type semiconductor, Ag2CO3, which has a moderate band gap (Eg,
~2.3 eV), has been widely reported to exhibit excellent photocatalytic performance. In
view of this, Ag2CO3 composites (e.g., Ag2CO3/TiO2 [48] and Ag2CO3/g-C3N4 [49])
are presently undergoing extensive investigation. Certain research works related to
Ag2CO3-based p–n heterojunctions have also been reported. With the support of a
strong internal electric field, the WO3/Ag2CO3 p–n heterojunction has exhibited
superior performance in terms of optical characteristics. The photocatalytic
performance of the WO3/Ag2CO3 composite has also significantly improved because
of its excellent optical properties [50]. Based on the above research and matching
band structure, the construction of the Ag3VO4/Ag2CO3 p–n heterojunction is

considered promising.
Herein, the Ag3VO4/Ag2CO3 p–n heterojunction is first synthesized using a
simple co-precipitation method. The photocatalysts are characterized by X-ray
photoelectron spectroscopy (XPS), scanning electron microscopy (SEM),
transmission electron microscopy (TEM), X-ray diffraction (XRD), polarized light,
and ultraviolet visible diffuse reflectance spectroscopy (UV-Vis DRS). The
photocatalytic performance of the as-prepared samples is evaluated by degrading
rhodamine b (RhB), methylene blue (MB), levofloxacin hydrochloride (LVF), and
tetracycline (TC) under visible light (λ> 420ꢀnm). The stability of Ag3VO4/Ag2CO3
composites is verified by four stability cycle experiments. Furthermore, the
mechanism of Ag3VO4/Ag2CO3 composites is investigated through quenching
experiments and electron spin resonance (ESR). Finally, the LVF reaction
intermediate is analyzed by liquid chromatography–mass spectrometry (LC–MS), and
the LVF degradation pathway is identified.
2. Experiment
2.1. Materials
Silver nitrate (AgNO3, ≥99.8%), sodium bicarbonate (NaHCO3, ≥99.5%), sodium
sulfate (Na2SO4, ≥99%) 1,4-benzoquinone (BQ, ≥98%) and isopropanol (IPA, ≥99.7%)
were purchased from Sinopharm Chemical Regent Co., Ltd. (Shanghai, China).
Sodium vanadate (Na3VO4, ≥99%) was purchased from Macklin Biochemical Co.,
Ltd. (Shanghai, China). All chemicals were analytically pure and did not require
further purification. The ultrapure water required for the experiment with a resistivity

of 18.25 MΩ·cm.
2.2. Synthesis of Ag3VO4/Ag2CO3 composites
A one-step coprecipitation method was employed to synthesize Ag3VO4/Ag2CO3
photocatalysts: 1.5902 g of AgNO3 was dissolved in 20 mL of deionized water and
stirred for 30 min to form solution A; 0.1839 g of Na3VO4 and 0.2672 g of NaHCO3
were dissolved in 20 mL of deionized water and stirred for 30 min to form solution B.
Solution B was thereafter gradually dropped into solution A to form a suspension that
was rapidly stirred under dark conditions for 10 h. The precipitate was collected by
centrifugation, washed several times with deionized water, and oven-dried at 60 °C
for 12 h. Composite photocatalysts of Ag3VO4/Ag2CO3 with different mass ratios
were produced by varying the Na3VO4–NaHCO3 weight ratio. The samples were
denoted as VC-11, VC-12, and VC-13 to indicate Ag3VO4 and Ag2CO3 weight ratios
of 1:1, 1:2, and 1:3, respectively. Pristine Ag3VO4 and Ag2CO3 were also prepared
using the same method without NaHCO3 and Na3VO4 additions, respectively.
2.3. Characterization
The XRD pattern was obtained using an XRD-6100 advanced diffractometer
based on a Cu-Kα beam (Shimadzu, Japan). The morphology of the photocatalytic
material was observed using a FEI Nova NanoSEM 230 field emission scanning
electron microscope (FEI, USA) and a Tecnai G2 F20/F30 transmission electron
microscope (FEI, USA). The surface chemical composition obtained by Thermo
ESCALAB 250XI and the electron binding energy of the material were characterized
by XPS with Al-Kα radiation (Thermo USA). The UV-vis diffuse reflectance of the

sample (200–800 nm) was recorded using a Hitachi UH4150 spectrophotometer
equipped with an integrating sphere. Photoluminescence data were obtained by means
of a Hitachi-F2500 fluorescence spectrophotometer (Hitachi, Japan). The specific
surface area and pore size distribution were recorded using a Gemini VII 2390
specific surface and pore size distribution analyzer (Micromeritics, USA). The Mott–
Schottky curve and photocurrent results were characterized using CHI760E (Chenhua,
China), in which a 0.5M Na2SO4 solution was used as the electrolyte. The working
electrode was fluorine-doped tin oxide coated with photocatalytic material, and the
reference and counter electrodes were platinum and saturated Ag/AgCl, respectively.
The electron spin resonance (ESR) signal was measured using a JES FA200 electronic
resonance spectrometer (JEOL, Japan) at room temperature under visible light (λ >
420 nm). The total organic carbon (TOC) was tested by a TOC analyzer (Vario TOC;
Elementar, Germany).
2.4. Photocatalytic degradation tests
The photocatalytic performance of Ag3VO4/Ag2CO3 composites at room
temperature was determined by measuring the degradation of RhB in an aqueous
solution under a 500 W xenon lamp (Changzhou Hongming Technology Co., Ltd.,
China) equipped with a UV cut-off wavelength filter (λꢀ≥ꢀ420ꢀnm). Moreover, the
illumination intensity on the reaction suspension surface was 90ꢀ±ꢀ5ꢀmW/cm2. In the
test, 50 mg of the photocatalyst was first uniformly dispersed in 100 mL of a 10
mg·L−1 RhB solution and stirred for 1 h in the dark to achieve the adsorption–
desorption dynamic equilibrium. Thereafter, during the illumination process, a certain

amount of water sample was extracted through a 0.45 μm filter membrane at regular
intervals. The RhB residual concentration in the obtained solution was recorded by a
UV-Vis spectrophotometer (UV-4802S, UNICO Instruments Co., Ltd., China).
Furthermore, the degradation performance of MB TC and LVF was investigated using
a similar method with an initial concentration of 10 mg·L−1.
3. Results and discussion
3.1. Characterization
The crystal structures of Ag3VO4, Ag2CO3, and Ag3VO4/Ag2CO3 composites
were analyzed based on the XRD patterns. As shown in Fig. 1, all the XRD peaks of
pure Ag3VO4 monomer sample fully correspond to the diffraction peak of Ag3VO4
monoclinic phase (JCPDS card no. 43-0542), whereas the other impurities have no
XRD peaks, proving that the prepared Ag3VO4 has a pure monoclinic structure.
Furthermore, the XRD diffraction peak of the Ag2CO3 monomer sample is consistent
with the monoclinic phase of Ag2CO3 in the JCPDS card (JCPDS card no. 26-0339).
After combining Ag3VO4 and Ag2CO3, the diffraction peaks of VC-12 samples
simultaneously exhibited Ag3VO4 and Ag2CO3 diffraction peaks, confirming the
existence of both Ag3VO4 and Ag2CO3. Moreover, no characteristic peaks of other
impurities were detected, indicating that the synthesized composite is a high purity
phase. The crystal size of Ag3VO4, Ag2CO3, and Ag3VO4/Ag2CO3 complexes was
calculated by Scherrer's equation, D = Kλ/βcosθ, where D is the crystal size (nm), K
is Scherrer’s constant (usually set to 0.89), θ is the Bragg diffraction angle (°), and β
is the half peak width (rad) of the diffraction angle. According to the main diffraction

peaks, the average crystallite sizes of VC-11, VC-12, and VC-13 of Ag3VO4 and
Ag3VO4 were 7.75, 3.35, 4.59, and 5.41 nm, respectively. The average crystallite
sizes of VC-11, VC-12, and VC-13 of Ag2CO3 and Ag2CO3 were 6.34, 4.87, 5.14, and
6.27 nm, respectively. This indicates that the crystal size of Ag3VO4/Ag2CO3
composites decreased after Ag3VO4 and Ag2CO3 were combined.
XPS was used to further explore the surface element compositions of the
Ag3VO4, Ag2CO3, and VC-12 samples. The full survey spectra (Fig. 2(a)) indicate
that the composite consists of Ag, V, C, and O. The high resolution XPS spectra of
Ag 3d, V 2p, C 1s, and O ls were further analyzed. As shown in Fig. 2(b), the Ag 3d
high resolution spectra show characteristic binding energy values of 368.2 and 374.2
eV, which are related to the binding energies of Ag 3d 5/2 and Ag 3d 3/2 [51, 52],
respectively. The peaks appearing at 516.7 and 523.7 eV in the high resolution XPS
spectra of V (Fig. 2(c)) correspond to V 2p 3/2 and V 2p 1/2, respectively; these are
consistent with the characteristic peaks of V5+ [19]. The C 1s XPS peak of the
Ag2CO3 monomer and VC-12 composite photocatalyst can be further divided into
three peaks, as shown in Fig. 2(e). The binding energies at 284.8 and 286.2 eV for
Ag2CO3 as well as those at 284.6 and 286.5 eV for VC-12 were attributed to
adventitious carbon, whereas those at 288.6 and 288.9 eV were attributed to carbon in
CO32− [53-55]. More significantly, the binding energies of Ag, V, and C of VC-12
changed slightly (< 0.4 eV) compared to those of Ag3VO4 and Ag2CO3. These
differences were caused by the interface coupling interaction of VC-12 composite that
affects the electron energy distribution of Ag3VO4 and Ag2CO3 [56]. Fig. 2(d)

illustrates that the oxygen of VC-12 composite is composed of four forms. The peaks
at 529.7 and 531.0 eV correspond to the oxygen in the Ag3VO4 and Ag2CO3 lattices,
respectively. The peaks at 531.2 and 532.6 eV were attributed to the water molecule
and adsorbed oxygen on the sample surface, respectively [57-59]. The XPS analysis
thus further proves the coexistence of Ag3VO4 and Ag2CO3.
The morphologies of Ag3VO4, Ag2CO3, and VC-12 were characterized by SEM
and TEM. Fig. 3(a) shows that the prepared Ag3VO4 consists of nanoparticles with an
average particle diameter of 0.5–3 μm. Fig. 3(b) shows that pure Ag2CO3 is mainly
rod-shaped with an average size of 1×3 μm. For the VC-12 samples, it is evident that
Ag3VO4 nanoparticles and rod-shaped Ag2CO3 particles closely adhere to each other
(Fig. 3(c)); Ag2CO3 nanorods are indicated by red circles. Fig. 3(d) illustrates the
element mapping of the VC-12 composite photocatalyst, indicating that VC-12
contains four elements: silver, vanadium, carbon, and oxygen. Moreover, in the
rod-like region of the sample, the vanadium content is significantly lower and the
carbon content is higher than those in the peripheral region. These results are further
confirmed by the TEM images. It is evident that Ag3VO4 consists of nanoparticles
(Fig. 4(a)) and that Ag2CO3 has a cubic rod-like morphology (Fig. 4(b)). Fig. 4(c)
clearly reveals that Ag3VO4 nanoparticles are tightly bound to rod-shaped Ag2CO3
particles. Fig. 4(d) shows the high resolution TEM image of VC-12. The (220) and
(031) crystal planes of Ag3VO4 and Ag2CO3 are 0.207 and 0.264 nm, respectively.
Consistent with the XRD results, the distance between the two lattices also proves the
coexistence of Ag3VO4 and Ag2CO3. These results therefore consistently demonstrate

that the Ag3VO4/Ag2CO3 composite was successfully synthesized through one step
precipitation.
To determine the specific surface area and pore size distribution of the prepared
samples, the nitrogen adsorption–desorption isotherm was measured. As shown in Fig.
5, Ag3VO4, Ag2CO3, and VC-12 display the characteristics of a type isotherm with
an H3 hysteresis loop, consistent with a mesoporous structure [60, 61]. The calculated
specific surface area and pore size obtained by the Brunauer–Emmet–Teller model
and Barrett–Joyner–Halenda method are summarized in Table 1. It was found that the
specific surface area of VC-12 composite sample is 3.28 m2∙g−1, which is significantly
larger than those of Ag3VO4 (0.68 m2∙g−1) and Ag2CO3 (0.38 m2∙g−1). The increase in
the specific surface area can be attributed to the decrease in particle size and the
increase in Ag3VO4 dispersion during the co-precipitation process. Further, the
average pore diameters of Ag3VO4, Ag2CO3, and VC-12 are 4.60, 19.99, and 11.22
nm, respectively. It is well known that a large specific surface area can provide more
reactive sites on the surface exposed to organic molecules [62]. The larger specific
surface area of VC-12 therefore contributes to the improvement of its photocatalytic
activity.
Light absorption capacity is an important indicator of photocatalyst performance.
As shown in Fig. 6(a), the optical absorption properties of Ag3VO4, Ag2CO3, and
VC-12 were characterized by UV-vis DRS. The absorption edge of single phase
Ag3VO4 was observed at 540 nm, and that of Ag2CO3 was observed at 430 nm. Based
on comparison, it was found that the light absorption intensity of VC-12

heterojunction over the entire visible light region increased because of the formation
of p–n heterojunction [46]. This allows the compound to absorb more solar energy,
thereby improving the photocatalytic activity. Generally, the band gap (Eg) of a
semiconductor photocatalyst can be calculated according to the following Kubelk–
Munk formula:
αhν = Α(hν − ꢀꢁ)ꢂ/ꢃ
(1)
where α, h, ν, Eg, and A are the absorption coefficient, Planck constant, light
frequency, band gap, and constant, respectively. The value of n is determined by the
optical transition form of the photocatalytic semiconductor (n = 1 for the direct
transition semiconductor; n = 4 for the indirect transition semiconductor). Typical
direct and indirect bandgap semiconductors are Ag3VO4 and Ag2CO3, respectively
[63, 64]. The n values of Ag3VO4 and Ag2CO3 are 1 and 4, respectively. The
semiconductor band gap can be obtained by plotting the linear portions of (αhν)2 or
(αhν)1/2 and hv. As shown in Fig. 6(b), the band gaps of Ag3VO4 and Ag2CO3 are 2.05
and 2.32 eV, respectively.
To measure the migration and recombination rates of photogenerated
electron-hole pairs, photoluminescence (PL) spectroscopy was employed. It is
generally assumed that a weak PL intensity represents a low carrier recombination
rate [65]. Fig. 7 shows the PL spectra of Ag3VO4, Ag2CO3, VC-11, VC-12, and
VC-13 at an excitation wavelength of 420 nm. The pure-phase Ag2CO3 and Ag3VO4
have high PL intensities, indicating that photogenerated carriers easily recombine and
release intense fluorescence. The PL strength of VC-1X (X=1, 2, 3) composites,

however, is considerably reduced, proving that the p–n heterojunction contributes to
the swift separation and migration of electron-hole pairs of composite materials.
To further investigate the behavior of photogenerated carriers of prepared
materials, the transient photocurrent intensities of Ag3VO4, Ag2CO3, and VC-12 were
recorded under visible light (λ≥420 nm) irradiation in a 0.5 M Na2SO4 electrolyte
solution. As shown in Fig. 8(a), the Ag3VO4 and Ag2CO3 monomers exhibited low
photocurrent strengths because of higher carrier recombination rates. In contrast, the
VC-12 sample exhibited a strong photocurrent intensity, indicating that the p–n
heterojunction can effectively promote the migration of photoexcited electron-hole
pairs. The electrochemical impedance of the three samples is shown in Fig. 8(b). The
radius of the Nyquist curve of VC-12 is significantly smaller than those of Ag3VO4
and Ag2CO3. Moreover, the equivalent circuit diagram corresponding to the Nyquist
diagram is composed of three parts: solution resistance (R1), charge transfer
resistance (R2), and double layer capacitor (CPE1). It should be noted that the
solution resistance (R1) is 17.43–18.96 Ω; the resistance values of Ag2CO3 (R2),
Ag3VO4 (R2), and VC-12 (R2) are 2494, 758.6, and 448.3 Ω, respectively. These
results indicate that the formation of a p–n heterojunction between Ag3VO4 and
Ag2CO3 can aid in the separation and migration of photogenerated electron-hole pairs.
The VC-12 sample therefore has a smaller charge transfer resistance, indicating a
stronger charge transport performance [66].
The Mott–Schottky test was employed to characterize the semiconductor type and
flat band potential (Efb) of the prepared material. As shown in Fig. 8(c and d),

Ag3VO4 and Ag2CO3 were evaluated as n-type and p-type semiconductors based on
the slope of the tangent to the Mott–Schottky curve, respectively. The p–n
heterojunction formation was verified by measuring the Mott–Schottky curve of the
Ag3VO4/Ag2CO3 sample. As shown in Fig. 8(e), the characteristic feature of the p–n
heterojunction was explicitly observed from the Mott–Schottky curve of the VC-12
composite in which an inverted ‘V-shaped’ curve was observed [67]. This indicates
that two different electronic actions (n-type and p-type) are exhibited by the electrode.
The Efb values of Ag3VO4 and Ag2CO3 are measured to be −0.16 and 1.59 eV (vs.
Ag/AgCl) from the Mott–Schottky curve, respectively. Moreover, the Efb (vs.
Ag/AgCl) of Ag3VO4 and Ag2CO3 can be converted to Efb (vs. NHE) by the formula
Efb (vs. NHE) = Efb (vs. Ag/AgCl) + 0.197 [68]. The Efb (vs. NHE) values of Ag3VO4
and Ag2CO3 are 0.037 and 1.787 eV, respectively. Generally, the conduction band
(CB) of the n-type semiconductor is 0.1 eV lower than Efb, and the valence band (VB)
of the p-type semiconductor is 0.1 eV higher than Efb [69]. The CB and VB of
Ag3VO4 and Ag2CO3 were 0.063 and 1.887 eV, respectively. The band gaps of
Ag3VO4 and Ag2CO3 were 2.05 and 2.32 eV, respectively. The corresponding VB and
CB were obtained by [70]
ꢀꢄꢅ = ꢀꢆꢅ + ꢀꢁ
(2)
where Eg is the band gap, EVB is the valence band edge potential, and ECB is the
conduction band edge potential. The VB and CB of Ag3VO4 and Ag2CO3 were
calculated to be 1.987 and −0.433 eV, respectively.

3.2 Photocatalytic activity and stability analysis
The photocatalytic oxidation performance of the Ag3VO4/Ag2CO3 composites was
investigated by degrading two types of pollutants, specifically dyes (RhB and
methylene MB) and antibiotics (levofloxacin hydrochloride (LVF) and tetracycline
(TC)), under visible light (λ> 420 nm). As shown in Fig. 9(a), when the reaction
system reaches the adsorption–desorption equilibrium after a 1-h dark reaction, the
RhB degradation efficiency is represented by C/C0, where C corresponds to the RhB
concentration at time t and C0 corresponds to the concentration of RhB at the
adsorption–desorption equilibrium. The adsorption and desorption equilibrium
diagrams are shown in Fig. S1. The RhB removal rates by pure Ag3VO4 and Ag2CO3
were 76.8% and 71.7% within 30 min, respectively. Compared with the monomer
materials, the Ag3VO4/Ag2CO3 composites with mass ratios of 1:1, 1:2, and 1:3
(VC-11, VC-12, and VC-13, respectively) had significantly improved RhB
degradation rates within 30 min. It is clear that as the Ag2CO3 proportion increased,
the degradation efficiency first increased and thereafter decreased. Initially, as the
proportion of Ag2CO3 increased, the contact between the two photocatalysts became
more sufficient, and the charge transport distance was quite short [71]. Therefore, the
photocatalytic effect of Ag3VO4/Ag2CO3 composites improved. With the overloading
of Ag2CO3, the heterojunction structure was partially destroyed, resulting in a
decrease in the photocatalytic effect [72]. Among the composites, VC-11 and VC-13
exhibited RhB removal rates of 94.6% and 92.3%, respectively; VC-12 had the
highest RhB degradation rate (97.8%) after 30 min of visible light irradiation. Fig. 9(b)

shows the degradation rate of LVF on the photocatalytic material with different
components. It is clearly observed that 39.1% and 8.2% of LVF were degraded within
60ꢀmin in the presence of pristine Ag3VO4 and Ag2CO3, respectively. The degradation
effect of composites with different proportions on the LVF, however, is consistent
with their effect on RhB. Substantial photocatalytic activity in LVF degradation was
exhibited by VC-11, VC-12, and VC-13; within 60ꢀmin, LVF was naturally degraded
by 77.1%, 82%, and 78.9%, respectively.
It is well known that the degradation of RhB and LVF follows the Langmuir–
Hinshelwood first-order kinetic model [66, 73]. The simulated first-order kinetic
formula is ln(C0/C) = kt, where C0 represents the RhB and LVF concentrations at the
adsorption–desorption equilibrium, C represents the RhB and LVF concentrations at
time t, k is the reaction rate constant, and t is time. As shown in Fig. 9(d and f), the
first-order kinetic fit curves of ln(C0/C) vs. time are plotted to show that the reaction
rate constants (k) of Ag3VO4 and Ag2CO3 monomers are (0.0482 and 0.0085 min−1)
and (0.0359 and 0.0015 min−1), respectively. It should be noted that the heterojunction
formed between Ag3VO4 and Ag2CO3 promotes the effective separation and
migration of photogenerated electron-hole pairs. The reaction rate constants (k) of
VC-12 sample to RhB and LVF are 0.1398 and 0.0264 min−1, which are 2.9 and 12.8
times that of pure Ag3VO4 and 3.9 and 21.5 times that of Ag2CO3 monomers,
respectively.
To investigate the universality of VC-12 for the degradation of organic pollutants,
two other pollutants (i.e., MB and TC) were selected as test objects in this experiment.

Fig. 9(h) shows that under visible light irradiation (λ > 420 nm), the degradation rates
of MB and TC by VC-12 composites were 99.2%, 81.9%, and 75.1%. These are
considerably higher than the degradation efficiencies of Ag3VO4 and Ag2CO3
monomers. These results indicate that the Ag3VO4/Ag2CO3 composite photocatalyst
has excellent versatility in degrading organic pollutants.
From a practical perspective, photocatalytic materials with good stability are
essential. Accordingly, cyclic degradation studies on RhB and LVF were performed
by collecting the samples after the photocatalytic degradation reaction. As shown in
Fig. 10(a), the RhB and LVF degradation rates within 30 and 60 min were 90.3% and
72.8% after four cycles, respectively. This indicates that there is no significant
photocatalytic activity attenuation in used samples after four cycles compared to fresh
samples. Compared with those of the VC-12 composites, the photocorrosion effects of
Ag3VO4 and Ag2CO3 were extremely evident. As shown in Fig. S2(a), the removal
rates of RhB and LFV by Ag3VO4 after four cycles decreased to 12.1 and 6.3%,
respectively. Fig. S2(b) illustrates that the removal rates of RhB and LFV by Ag2CO3
decreased to 4.8 and 1.9%, respectively. This shows that the stability of VC-12
composites was considerably improved compared to that of the monomers. Moreover,
the XRD patterns of the composites before and after the cycle were analyzed to
further evaluate the stability of materials. In Fig. 10(b), the XRD pattern of the
composite material shows a diffraction peak corresponding to the (111) plane of
metallic silver at 38.1° after four cycles. This indicates that the Ag3VO4/Ag2CO3
composite material exhibits a certain stability after repeated use. Furthermore, Fig.

10(c) depicts the high resolution XPS spectra of Ag 3d for used and fresh VC-12
composites. The peaks at 374.4 and 368.4ꢀeV can be ascribed to Ag+, and the
emergence of two peaks at 374.9 and 369 eV are assigned to Ag0 [74, 75]. The
foregoing results are highly consistent with the XRD analysis. It was observed that
although the Ag3VO4/Ag2CO3 material has a weak photocorrosion, it continues to
exhibit good photocatalytic performance and stability after repeated use. This can be
attributed to the surface plasmon resonance caused by the generation of elemental
silver [76].
3.3 Photocatalytic mechanism analysis
-
Reactive substances (e.g., ·O2 , ·OH, and h+) are generally considered to be key
factors in the photocatalytic degradation of organic pollutants. In this study,
1,4-benzoquinon（e BQ), triethanolamine (TEOA), and isopropanol (IPA) were used as
-
capture agents for ·O2 , h+, and ·OH, respectively [77-79]. The main active species in
the degradation of RhB and LVF by VC-12 composites were analyzed. As shown in
Fig. 11(a), when TEOA was included in the photocatalytic degradation experiments
of RhB and LVF, the degradation rate considerably decreased. This indicates that the
direct oxidation of holes is an important step in the degradation process of dyes and
antibiotics via the as-prepared photocatalytic materials. Furthermore, the addition of
BQ has a relatively considerable effect on the degradation rates of RhB and LVF,
−
indicating that ·O2 is also the main oxidizing substance in the degradation process.
The addition of IPA to the reaction system, however, weakly inhibited the degradation
of RhB and LVF. In the degradation process by the VC-12 composite photocatalyst,

−
h+ and ·O2 were the main active substances, and ·OH only performed a certain
subordinate function.
-
To further ensure that active substances (·O2 and ·OH) were produced during the
−
aforementioned degradation process, ·O2 and ·OH capture experiments were
performed with the ESR spin capture technique under visible light irradiation. Fig.
−
11(b) shows the four signal peaks of VC-12 DMPO–·O2 adduct in methanol; no
signal peaks were produced in the dark. The signal peak intensity, however, increased
−
during a (5–10) min period under illuminated conditions. It was confirmed that ·O2
free radicals were generated in VC-12 under illumination. As shown in Fig. 11(c), no
signal characteristic peak of DMPO–·OH was detected under dark conditions. As the
illumination time was extended to 10 min, the signal peak gradually increased,
suggesting the appearance of ·OH in the reaction system.
Based on the experimental results above, the reaction mechanism of VC-12 was
determined through analysis, as shown in Scheme 1. When n-type Ag3VO4 and p-type
Ag2CO3 come into contact, a p–n heterojunction is formed. The Fermi level difference
of the two semiconductors is large; hence, the electrons of n-type Ag3VO4 in the p–n
interface region tend to diffuse to the interface of p-type Ag2CO3 [80]. The foregoing
factors lead to band bending; the Fermi level also reaches equilibrium, and a built-in
electric field at the p–n interface is formed [81]. Under visible light irradiation, the
electrons in the valence bands of Ag3VO4 and Ag2CO3 transit to the conduction band,
resulting in electron-hole pairs. Depending on the effect of the internal electric field,
the electrons on the conduction band of Ag2CO3 transfer to that of Ag3VO4. Moreover,

−
they react with dissolved oxygen in the aqueous solution to form ·O2 , which directly
oxidizes organic contaminants or reacts with H+ adsorbed on the surface of VC-12 to
form ·OH. The holes in the VB of Ag3VO4 and Ag2CO3 can also be involved in the
degradation process of organic pollutants. The mechanism for the degradation of
organic pollutants by VC-12 photocatalyst is thus described as follows.
ꢇꢈꢉ ꢊꢋꢌ + ℎꢍ → ꢇꢈꢉ ꢊꢋꢌ (ℎꢎ/eꢏ)
(3)
(4)
(5)
(6)
(7)
ꢇꢈꢃ ꢐꢋꢉ + ℎꢍ → ꢇꢈꢃ ꢐꢋꢉ (ℎꢎ/eꢏ)
ꢏ
ꢑꢏ ꢇꢈꢃ ꢐꢋꢉ → ꢑ ꢇꢈꢉ ꢊꢋꢌ
(
)
(
)
)
ꢎ
ℎꢎ ꢇꢈꢉ ꢊꢋꢌ → ℎ ꢇꢈꢃ ꢐꢋꢉ
(
)
(
ꢑꢏ(ꢇꢈꢉ ꢊꢋꢌ) + ꢋ₂ →· ꢋꢃꢏ
· ꢋꢃꢏ ꢇꢈꢉ ꢊꢋꢌ − + ꢒ → · ꢋꢒ
ꢎ
(
)
( )
8
ꢏ
ℎꢎ ꢇꢈꢃ ꢐꢋꢉ +· ꢋꢒ/· ꢋꢃ (ꢇꢈꢉ ꢊꢋꢌ) + ꢓꢔꢕꢕꢖꢗꢘꢙꢗꢚ → ꢛꢑꢈꢜꢘꢛꢑꢛ ꢓꢜꢔꢛꢖꢝꢗꢚ (9)
(
)
3.4 Possible degradation path of LVF
Numerous researches on the degradation pathway of RhB have been reported;
accordingly, these are not be discussed here [82-84]. The degradation of LVF was
therefore chosen to illustrate the photocatalytic process of VC-12. To further analyze
the possible degradation pathway of LVF, LC–MS detection was employed to identify
the intermediate structure that may exist in the degradation process of LVF by VC-12;
the specific MS spectrum is shown in Fig. S3. As shown in Fig. S3(a), the [M + H]+
ion peak shown at m/z = 362.15 was attributed to LVF molecules, indicating that the
molecular structure of the LVF is not destroyed in the absence of illumination. The
degradation of LVF mainly includes demethylation, decarboxylation, defluorination,

and piperazine ring and quinoline ring destruction. The first possible pathway is the
replacement of F by OH groups because the attack of hydroxyl radicals forms P1 [85].
Thereafter, the carboxyl group is completely oxidized, and the piperazine ring
undergoes oxidative ring-opening to form P4. The superoxide radical attacks the N
atom in the piperazinyl part to form P7 on successive attacks. Another possible
pathway is that the piperazine ring is demethylated by the attack of non-selective
hydroxyl and superoxide radicals to form P2 and P3, thereby leading to the generation
of P5 and P6, respectively [86]. Subsequently, P5 and P6 piperazine rings are
removed, and the quinoline ring is decarboxylated to form P8 and P9 [87]; P10 is
formed by oxidizing the OH groups and F atoms of P7, P8, and P9. The quinoline ring
and morpholine ring of P10 are further oxidized and finally completely mineralized.
The possible LVF degradation path is shown in Fig. 12.
4. Conclusions
In summary, Ag3VO4/Ag2CO3 p–n heterojunctions were prepared using a simple
co-precipitation method. Among the as-prepared composites, the Ag3VO4/Ag2CO3
composite with a ratio of 1:2 exhibited the best degradation performance and
excellent stability. The synthesized VC-12 p–n heterojunction system exhibited
excellent degradation efficiency and remarkable mineralization ability (λ>420 nm).
These exceptional photocatalytic properties were attributed to the following
synergistic effects: (a) The formation of p–n heterojunctions effectively promoted the
separation and migration efficiency of photoexcited electron-hole pairs. (b) Compared
with the monomer, the larger specific surface area of the composite photocatalysts

provided a larger reaction area. Moreover, a possible LVF degradation pathway was
proposed by analyzing the degradation intermediates. This explains the improvement
of TOC removal with the oxidation of piperazine ring and quinoline ring. The
composites of Ag3VO4/Ag2CO3 therefore have excellent application prospects in the
field of environmental functional materials.
Acknowledgements
The authors gratefully acknowledge the financial support provided by the
National Key R&D Program of China (2016YFC0403001), the National Natural
Science Foundation of China (No. 51808215), the integration and demonstration of
cadmium excess irrigation water purification technology (special project of Ministry
of agriculture and Ministry of finance 20160606), Provincial Natural Science
Foundation of Hunan (No. 2019JJ50253，2018JJ3244)

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