Facile fabrication of Bi2GeO5/Ag@Ag3PO4 for efficient photocatalytic RhB degradation

Article abstract of DOI:10.1016/j.jssc.2021.122309

In this study, Bi₂GeO₅/Ag₃PO₄ nano-composites were synthesized by simple two-step approach. Then, Bi₂GeO₅/Ag@Ag₃PO₄ Z-scheme heterojunction was assembled by subsequent photocatalytic processes. It was characterized using XRD, SEM, TEM, HR-TEM, EDX, SAED, XPS, PL, UV–Vis DRS, photoelectrochemical and photodegradation experiments. Results illustrated that the catalytic activity of Bi₂GeO₅/Ag₃PO₄ nano-composites is remarkably superior to those of Bi₂GeO₅ and Ag₃PO₄. The influence of Bi₂GeO₅ amount on the performance of the composites was also studied. Results showed that 0.1Bi₂GeO₅/Ag₃PO₄ composite exhibited the best photocatalytic efficiency for rhodamine B (RhB) degradation, and gave rise to a 96% degradation of RhB after 30 ?min visible-light irradiation. In the degradation of RhB, the apparent rate constant of 0.1Bi₂GeO₅/Ag₃PO₄ is the largest, which is 0.06836min^?1. After 4 cycles, RhB degradation by 0.1Bi₂GeO₅/Ag₃PO₄ still maintained 84%, equivalent to 1.7-fold higher than that of Ag₃PO₄. The trapping experiments revealed that holes (h⁺) and superoxide anions (O- 2·) were the primary species responsible for the decomposition of RhB in 0.1Bi₂GeO₅/Ag₃PO₄. Furthermore, the mechanism of improving photocatalytic activity was proposed relied on the experiments and characterization results. The formation of Bi₂GeO₅/Ag@Ag₃PO₄ Z-scheme heterojunction by photocatalytic processes dramatically increased its photocatalytic activity and stability.

Full text of DOI:10.1016/j.jssc.2021.122309

Journal of Solid State Chemistry 301 (2021) 122309
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Facile fabrication of Bi2GeO5/Ag@Ag3PO4 for efficient photocatalytic
RhB degradation
,
,**
Dengdeng Liu a, Pengyu Zhu b, Li Yin c, Xinyu Zhang a, Kaijin Zhu c *, Junhua Tan c, Riya Jin a
a Environmental and Safety Engineering Institute, North University of China, Taiyuan, Shanxi, 030051, China
b School of Environmental and Biological Engineering, Nanjing University of Science &Technology, Nanjing, Jingsu, 210094, China
c Department of Materials Engineering, Taiyuan Institute of Technology, Taiyuan, Shanxi, 030051, China
A R T I C L E I N F O
A B S T R A C T ：
Keywords:
Z-Scheme heterojunction
Photocatalysis
Rhodamine B
Nanocomposites
In this study, Bi2GeO5/Ag3PO4 nano-composites were synthesized by simple two-step approach. Then, Bi2GeO5/
Ag@Ag3PO4 Z-scheme heterojunction was assembled by subsequent photocatalytic processes. It was character-
ized using XRD, SEM, TEM, HR-TEM, EDX, SAED, XPS, PL, UV–Vis DRS, photoelectrochemical and photo-
degradation experiments. Results illustrated that the catalytic activity of Bi2GeO5/Ag3PO4 nano-composites is
remarkably superior to those of Bi2GeO5 and Ag3PO4. The influence of Bi2GeO5 amount on the performance of the
composites was also studied. Results showed that 0.1Bi2GeO5/Ag3PO4 composite exhibited the best photo-
catalytic efficiency for rhodamine B (RhB) degradation, and gave rise to a 96% degradation of RhB after 30 min
visible-light irradiation. In the degradation of RhB, the apparent rate constant of 0.1Bi2GeO5/Ag3PO4 is the
largest, which is 0.06836minꢀ1. After 4 cycles, RhB degradation by 0.1Bi2GeO5/Ag3PO4 still maintained 84%,
equivalent to 1.7-fold higher than that of Ag3PO4. The trapping experiments revealed that holes (hþ) and su-
peroxide anions (O- 2⋅) were the primary species responsible for the decomposition of RhB in 0.1Bi2GeO5/
Ag3PO4. Furthermore, the mechanism of improving photocatalytic activity was proposed relied on the experi-
ments and characterization results. The formation of Bi2GeO5/Ag@Ag3PO4 Z-scheme heterojunction by photo-
catalytic processes dramatically increased its photocatalytic activity and stability.
1. Introduction
Common heterojunction ways contain P-N heterojunction [15], surface
heterojunction
[16],
Z-scheme
heterojunction
[17],
and
With the rapid development of catalytic chemistry, it has been
applied in many fields [1–3]. In particular, semiconductor photocatalysis
has widely been applied in solar energy and environmental remediation.
Liquid-solid heterogeneous photocatalysis [4] often follows two steps
[5]: i) reaction adsorption equilibrium [6] and ii) photocatalytic surface
reaction [7]. However, traditional photocatalysts [8]usually possess poor
activity because of the low utilization of solar energy [9] and rapid
recombination of photo-generated electron-hole pairs [10], thereby
limiting the development of photocatalytic technology [11]. To improve
the utilization rate of light and effectively separate the photogenerated
electron-hole pairs, several strategies have been attempted. These
include doping [12], metal loading [13], heterojunction [14], and others.
Heterojunction is an efficient strategy to prepare photocatalysts due to its
feasibility and effectiveness for separating electron-hole pairs in space.
semiconductor-graphene heterojunction [18]. In particular, Z-scheme
heterojunction boosts the spatial electron–hole separation [19–21] and
optimizes the reduction and oxidation processes occurring in semi-
conductors at higher reduction and oxidation potentials [22,23].
Ag3PO4 is widely employed in photolysis with strong photocatalytic
degradation ability under visible-light irradiation. The quantum effi-
ciency of Ag3PO4 reaches 90%, much higher than values obtained by
other metal oxides [24]. However, Ag3PO4 could easily decompose into
Ag0 and PO34-during photocatalysis due to its photo corrosion features.
Also, the presence of Ag0 precipitates on the catalyst surface inhibits its
photocatalytic activity. Hence, reusing Ag3PO4 during catalysis processes
remains very challenging [25,26].
In addition, Bi2GeO5 is a stable n-type semiconductor material with
strong redox ability, suitable for use as dielectric, ferroelectric, and
* Corresponding author.
** Corresponding author.
E-mail addresses: Liudengd14@163.com (D. Liu), 1056114199@qq.com (P. Zhu), 1012241097@qq.com (L. Yin), 15364918764@163.com (X. Zhang),
zkj621104@163.com (K. Zhu), jrya@nuc.edu.cn (R. Jin).
https://doi.org/10.1016/j.jssc.2021.122309
Received 8 April 2021; Received in revised form 28 May 2021; Accepted 30 May 2021
Available online 6 June 2021
0022-4596/© 2021 Elsevier Inc. All rights reserved.

D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
photocatalyst for oxygen evolution [27]. However, the wide bandgap of
Bi2GeO5 (3.35 eV) limits its photocatalysis under visible light [28].
Therefore, the combination of Ag3PO4 with Bi2GeO5 might promote the
performance of light absorption and scattering and significantly reduce
the composite efficiency of photogenerated carriers.
Rhodamine B (RhB) is a common pollutant released in textile
wastewater with stable chemical properties. Hence, traditional degra-
dation approaches, such as microbial processes, adsorption, ozonation,
and flocculation are insufficient to dispose of RhB [29]. Photocatalysis is
high-efficiency environmental remediation method used for degradation
of organic dyes into non-toxic compounds [30–32], thereby worth trying
for the decomposition of RhB.
In this study, Bi2GeO5/Ag3PO4 nano-composites were first prepared
by a simple two-step method and Bi2GeO5/Ag@Ag3PO4 Z-scheme het-
erojunction was then assembled by subsequent photocatalytic processes.
The photocatalytic performances and stability of the as-obtained cata-
lysts toward the degradation of RhB under visible light were evaluated.
Besides, the possible Z-Scheme charge transfer mode was proposed.
different contents of Bi2GeO5 followed by drying for 10 h at 60 ꢁC. The
as-obtained samples were named as 0.1Bi2GeO5/Ag3PO4, 0.2Bi2GeO5/
Ag3PO4, and 0.3Bi2GeO5/Ag3PO4 (0.1, 0.2 and 0.3 refer to the added
quality of Bi2GeO5). Ag3PO4 was also prepared under the same above
conditions in absence of Bi2GeO5. The synthesis procedure for the
preparation of Bi2GeO5/Ag3PO4 is shown in Fig. 1.
2.3. Characterization
The X-ray diffraction (XRD) patterns were collected on a Rigaku D/
max-2500 X-ray diffractometer at 35 kV, ranging from 10 to 80ꢁ. The
JSM-6510 scanning electron microscope (SEM) equipped with an energy
dispersive X-ray spectra (EDX) and the JSM-2100 high-resolution trans-
mission electron microscope (HRTEM) were used to study the samples
morphologies. X-ray photoelectron spectroscopy (XPS) data were
collected with an ESCALAB 250 X-ray photoelectron spectrometer.
UV–visible diffuse reflectance spectra (DRS) data were collected using a
UV-2550 with wavelength between 200 and 850 nm. The BET specific
surface area was characterized on a 3H-2000PS1 surface area analyzer.
Photoluminescence (PL) spectra property was performed on a Shimadzu
RF-5301 fluorescence spectrophotometer with excitation wavelength of
400 nm. The linear sweep voltammogram (LSV) curves, photocurrents
and electrochemical impedance spectroscopy (EIS) were performed with
a Chenhua CHI650E electrochemical workstation.
2. Experimental
2.1. Reagents
Germanium dioxide (GeO2), iminobisetnanol (NH(CH2CH2OH)2),
bismuth nitrate (Bi(NO3)3⋅5H2O), silver nitrate (AgNO3), disodium
hydrogen phosphate (Na2HPO4⋅12H2O), p-benzoquinone (C6H4O2),
triethanolamine (N(CH2CH2OH)3), isopropanol absolute ethanol
((CH3)2CHOH), and RhB were purchased from Sinopharm Chemical
Reagent Co. All chemical reagents are analytical grade and ultrapure
water was used throughout the study.
2.4. Photocatalytic activity
The photocatalytic degradation experiments were performed in a
JOYN-GHX-DC photochemical reactor. A 1000W xenon lamp was used as
a visible light source. The prepared photocatalyst (50 mg) was first
weighed and dispersed in a test tube containing 50 mL of 10 mg/L RhB.
The test tube was placed in a sealed photochemical reactor to prevent
exposure to light. The suspension was sufficiently stirred in the darkness
for 20 min to make sure the adsorption-desorption equilibrium. The light
source was then turned on and 4 mL sample was taken out every 5 min.
During reaction, the temperature of the solution was controlled to 10 ꢁC
by a circulating cooling water system. The degradation rate of RhB via
photocatalytic degradation was calculated using Beer Lambert's law (Eq.
(1)). The photocatalytic degradation of RhB was characterized using ki-
netics, according to Eq. (2).
2.2. Catalysts preparation
2.2.1. Bi2GeO5
Bi2GeO5 was prepared by first mixing 0.26g GeO2 and 2.5g
Bi(NO3)3⋅5H2O in 40 mL solvent (10 mL ultrapure water and 30 mL
iminobisetnanol) under constant stirring for 1 h. The mixture was then
placed in a 50 mL Teflon-lined stainless steel autoclave and left to react at
170 ꢁC for 24 h. After cooling down, the precipitate was centrifuged and
product was washed with ultrapure water and absolute ethanol for four
times. The precipitate was finally collected and dried for 10 h at 60 ꢁC to
yield a white compound Bi2GeO5.
∁t = ∁0 ¼ At=A0
(1)
(2)
2.2.2. Bi2GeO5/Ag3PO4 composite
Kt ¼ ꢀ InðCt = C0Þ
To prepared Bi2GeO5/Ag3PO4 composite, 0.612g AgNO3 was dis-
solved into 30 mL ultrapure water. Then, Bi2GeO5 (0.1g, 0.2g, 0.3g) was
added into the above solution by magnetic stirring for 30min, followed
by ultrasonic treatment for 30min to obtain uniformly dispersed
Bi2GeO5. Next, 30 mL solution containing 0.43g Na2HPO4⋅12H2O was
slowly dropped into the above mixture and stirred at room temperature
in darkness for 120min. The precipitates were washed with ultrapure
water and absolute ethanol for four times to yield Bi2GeO5/Ag3PO4 with
where A0 and At represent the initial absorbance and absorbance at time t
after illumination, respectively. C0 and Ct are the corresponding original
concentration and t time concentration of the solution, respectively.
The photocatalysts were cycled three times to examine their cycling
stabilities: after each RhB degradation reaction, the samples are collected
after several washing and drying. Then repeat the experiment three
times.
Fig. 1. Synthesis procedure for the preparation of Bi2GeO5/Ag3PO4.
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D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Fig. 2. a XRD patterns of Ag3PO4, Bi2GeO5, 0.1Bi2GeO5/Ag3PO4, 0.2Bi2GeO5/Ag3PO4, and 0.3Bi2GeO5/Ag3PO4; b XRD patterns of fresh 0.1Bi2GeO5/Ag3PO4, after 1
and after 4 cycles.
The free radical capture experiments were performed to analyze the
active substances present in degrading pollutants. During free radical
capture experiments, 50 mg catalyst with IPA (0.1 mL) or TEOA (0.2487
mL) or BQ (0.1802 g) was dispersed in a test tube containing 50 mL of 10
mg/L RhB before visible light irradiation. To this end, isopropanol (IPA)
was employed as a hydroxyl radical (OH⋅) capture agent, triethanolamine
(TEOA) as a hole (hþ) capture agent, and p-benzoquinone (BQ) as a su-
peroxide anion (O- 2⋅) capture agent.
determined as 0.309 and 0.3002 nm and corresponded to (311) and
(200) crystal planes of Bi2GeO5 and Ag3PO4, respectively. The SAED
results of 0.1Bi2GeO5/Ag3PO4 are displayed in Fig. 3g. The bright
diffraction rings of 0.1Bi2GeO5/Ag3PO4 indicated its polycrystalline
features. The lattice spacings of the two crystal planes were determined
as 0.309 and 0.3002 nm, respectively (Fig. 3g). These results were
consistent with those of HRTEM.
In EDX diagram of 0.1Bi2GeO5/Ag3PO4 (Fig. 3h), the elements Bi, Ge,
O, Ag, and P were detected. The apparent diffraction peak of Cu was
generated by Cu mesh used to prepare the samples for TEM testing. The
SEM image and corresponding element mappings of Bi2GeO5/Ag3PO4 are
gathered in Fig. 4. The elements Bi, Ge, O, Ag and P looked homoge-
neously dispersed on 0.1Bi2GeO5/Ag3PO4 surface. In summary, Bi2GeO5
was well-dispersed and supported on Ag3PO4 crystal surface as granular,
forming relatively uniform spherical nanocomposites.
The surface chemical states of Bi2GeO5/Ag3PO4 were studied by XPS
in Fig. 5. The full-spectrum of fresh Bi2GeO5/Ag3PO4 composite is pre-
sented in Fig. 5a. A total of six elements (Bi, Ge, O, Ag, P and C, where C is
the substrate) were recorded, indicating the coexistence of both Ag3PO4
and Bi2GeO5. For Ag 3d XPS peaks of 0.1 Bi2GeO5/Ag3PO4 (Fig. 5b), the
Ag 3d spectra of fresh 0.1Bi2GeO5/Ag3PO4 at 367.75 eV and 373.8 eV
corresponded to Ag 3d5/2 and Ag 3d3/2, respectively. Hence, Ag mainly
existed as Agþ in 0.1Bi2GeO5/Ag3PO4 photocatalyst [33]. After the first
cycle, Ag 3d5/2 and Ag 3d3/2 displayed peaks at 368.1 eV and 374.1 eV,
respectively. From Fig. 5b, the Ag 3d5/2 and Ag 3d3/2 can be divided into
four different peaks, which are 368 eV, 368.5 eV, 374 eV and 374.6 eV.
The peak at 368.5 eV and 374.6 eV were assigned to Ag0, and the peaks at
368 eV and 374 eV were attributed to Agþ [34]. Therefore, small
amounts of Agþ converted into metal Ag during photocatalysis, consis-
tent with the XRD results. The XPS analysis of P 2p is shown in Fig. 5c,
and it represents P5þ in the PO43þ structure at 131.7eV. After single
photocatalysis, the P 2p XPS spectral intensity further decreased, con-
firming the formation of the silver element [26]. In XPS peaks of O 1s
(Fig. 5d), the O 1s XPS spectrum of fresh 0.1Bi2GeO5/Ag3PO4 was
composed of three multi-state peaks attributed to 530.62 eV, 531.27 eV,
and 532.2 eV. The peaks at 530.62 eV and 531.27 eV corresponded to
Ag3PO4 and Bi2GeO5 lattices in the material, respectively. The peak at
532.2 eV confirmed the adsorption of H2O or OHꢀ on the surface [35].
The Bi 4f XPS analyses of fresh 0.1Bi2GeO5/Ag3PO4 are provided in
Fig. 5e. Bi 4f was well-fitted with two main peaks at 159.53 eV and
164.85 eV, corresponding to Bi 4f7/2 and Bi 4f5/2, respectively. Hence, Bi
was present in the oxidation state of Bi3þ. The Ge 3d XPS peak of fresh
0.1Bi2GeO5/Ag3PO4 is displayed in Fig. 5f. The peak corresponding to
29.5 eV belonged to [GeO3]2-(Ge4þ) [10]. Thus, Bi2GeO5/Ag3PO4
transformed into Bi2GeO5/Ag@Ag3PO4 during photocatalysis.
3. Results and discussion
3.1. Catalyst characterization
The XRD spectra of the as-synthesized Bi2GeO5, Ag3PO4, 0.1Bi2GeO5/
Ag3PO4, 0.2Bi2GeO5/Ag3PO4, and 0.3Bi2GeO5/Ag3PO4 catalysts are
shown in Fig. 2a. For Bi2GeO5, characteristic diffraction peaks were
observed at 11.3ꢁ, 23.8ꢁ, 28.8ꢁ, 33.3ꢁ, 36.96ꢁ, 47.3ꢁ and 58.2ꢁ, consistent
with JCPDS No. 78-1334 standards of (200), (111), (311), (002), (511),
(022) and (802) crystal planes, respectively. According to Ag3PO4
(JCPDS No. 70-0702), the characteristic diffraction peaks of Bi2GeO5/
Ag3PO4 catalysts at 20.9ꢁ, 29.7ꢁ, 33.3ꢁ, 36.6ꢁ, 47.9ꢁ, 52.8ꢁ, 55.1ꢁ, 57.4ꢁ,
61.7ꢁ and 72ꢁ corresponded to (110), (200), (210), (211), (310), (222),
(320), (321), (400) and (421) crystal planes of cubic crystal of Ag3PO4,
respectively. Also, the peaks belonging to Bi2GeO5 in XRD profile of
Bi2GeO5/Ag3PO4 became increasingly obvious as Bi2GeO5 amount rose.
The crystal sizes of Bi2GeO5 and Ag3PO4 were calculated by using Debye
Scherer's Equation in Jade software. The crystal size of Bi2GeO5 and
Ag3PO4 are about 230 nm and 643 nm, respectively.
The XRD spectrum of 0.1Bi2GeO5/Ag3PO4 is displayed in Fig. 2b.
Changes were noticed after 1 and 4 cycles. The diffraction peaks of the
photocatalysts at 38.2ꢁ, 64.6ꢁ and 77.6ꢁcorresponded to Ag characteristic
peaks of (111), (220), and (311) crystal planes (JCPDS No. 87-0718),
respectively. The crystal structures of Bi2GeO5 and Ag3PO4 were also
preserved.
The SEM, TEM, HRTEM, SAED, and EDX images of Ag3PO4, Bi2GeO5,
and 0.1Bi2GeO5/Ag3PO4 catalysts are provided in Fig. 3. Note that Fig. 3a
and b represent SEM images of pure Bi2GeO5 sample with microflowers.
The morphology mainly contained layered microflowers (diameters
0.2–0.5
hexahedral structure of Ag3PO4 crystals showed particle sizes ranging
from 0.5 to 1 m (Fig. 3c). The SEM image of 0.1Bi2GeO5/Ag3PO4 after
μm) agglomerated by two-dimensional nanosheets. The regular
μ
composite formation is displayed in Fig. 3d. Bi2GeO5 particles were
deposited on Ag3PO4 surface to yield relatively uniform spherical
nanocomposites. In TEM image of 0.1Bi2GeO5/Ag3PO4 (Fig. 3e), Bi2GeO5
nanoparticles were attached to Ag3PO4 surface. In HRTEM images of
0.1Bi2GeO5/Ag3PO4 (Fig. 3f), the spacings of lattice fringe were
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D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Fig. 3. a Low and b high-magnification SEM images of Bi2GeO5 micoflowers; c SEM images of Ag3PO4; d SEM images of 0.1Bi2GeO5/Ag3PO4; e TEM image of
0.1Bi2GeO5/Ag3PO4; f HRTEM image of 0.1Bi2GeO5/Ag3PO4; g SAED pattern of 0.1Bi2GeO5/Ag3PO4 and h corresponding EDX pattern of 0.1Bi2GeO5/Ag3PO4.
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D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Fig. 4. EDX elemental mapping of 0.1Bi2GeO5/Ag3PO4.
The DRS of Ag3PO4, Bi2GeO5, 0.1Bi2GeO5/Ag3PO4, 0.2Bi2GeO5/
The large surface area can provide abundant active sites, which is
beneficial to the improvement of photocatalytic activity. Fig. 7 showed
the N2 adsorption-desorption isotherms and the corresponding pore size
distribution plots for Bi2GeO5, Ag3PO4 and 0.1Bi2GeO5/Ag3PO4. The
isotherm of three products indicate a representative Langmuir type IV
isotherm with an obvious hysteresis loop [39]. The surface area of
Bi2GeO5, Ag3PO4 and 0.1Bi2GeO5/Ag3PO4 were about 9.59 m2/g, 7.93
m2/g and 17.98 m2/g, respectively. For 0.1Bi2GeO5/Ag3PO4, a large
specific surface area can provide abundant active sites, which is condu-
cive to the attachment of organic pollutants and the generation of active
radicals [40–42]. The inset of Fig. 7 shows the barrett-Joyner-Halenda
(BJH) aperture distribution of the sample. The pore sizes of
0.1Bi2GeO5/Ag3PO4 is mainly less than 10 nm, which is beneficial to the
enhanced photocatalytic activity [6].
Ag3PO4, and 0.3Bi2GeO5/Ag3PO4 catalysts are gathered in Fig. 6a. Pure
Bi2GeO5 showed strong absorption peak at 300 nm with absorption
boundary of about 380 nm, belonging to the ultraviolet range. Ag3PO4
was more widely absorbed in the visible region with absorption bound-
ary of about 530 nm. The support of Bi2GeO5 on Ag3PO4 enlarged the
optical absorption range and increased the optical absorption intensity
over the whole wavelength range. The optical absorption effect of
0.1Bi2GeO5/Ag3PO4 improved, indicating synergy between Ag3PO4 and
Bi2GeO5. However, excess addition of Ag3PO4 was not conducive to light
absorption performance.
The band gaps of Ag3PO4 and Bi2GeO5 can be calculated according to
the Kubelka-Munk formula shown by Eq. (3) [36]:
n=2
α
hv ¼ Aðhv ꢀ EgÞ
(3)
3.2. Photocatalytic properties and mechanism
where
α
is the absorption coefficient, h represents the Planck's constant, v
is light frequency, Eg represents the bandgap energy, and A is a constant.
For Bi2GeO5/Ag3PO4 composite, the value of n was 1 since materials with
indirect bandgaps possess n ¼ 4 and semiconductors with direct
UV–visible full-wavelength scanning was utilized to analyze the
changes in RhB levels during the photocatalytic degradation by
0.1Bi2GeO5/Ag3PO4 catalyst. In Fig. 8a, the characteristic peak intensity
of RhB at 553 nm gradually weakened as reaction time was prolonged.
After irradiation for 30min, the peak intensity at 553 nm gradually
decreased, suggesting the destruction of the chromophore group of RhB.
Fig. 8b shows the degradation of RhB by Ag3PO4, Bi2GeO5, 0.1Bi2GeO5/
Ag3PO4, 0.2Bi2GeO5/Ag3PO4, and 0.3Bi2GeO5/Ag3PO4. The degradation
rates of Ag3PO4, Bi2GeO5, 0.2Bi2GeO5/Ag3PO4 and 0.3Bi2GeO5/Ag3PO4
after 30 min irradiation were estimated to 82%, 7%, 91% and 85%,
respectively. Among catalysts, the worst effect was recorded with pure
Bi2GeO5.
The photocatalytic degradation rate of 0.1Bi2GeO5/Ag3PO4 after 30
min irradiation reached 96%, where most RhB was degraded. Therefore,
Ag3PO4 obviously improved the photocatalytic performances of Bi2GeO5
photocatalyst. Recently, Tian [43], prepared core-shell Ag2S/Ag3PO4
composites via an in-situ anion-exchange reaction which degraded 100%
methyl orange after 120 min light irradiation, while the
0.1Bi2GeO5/Ag3PO4 exhibited the 96% photocatalytic degradation rate
of rhodamine B (RhB) after 30 min irradiation. Liu [44] synthesized
Ag3PO4/TiO2 photocatalyst via two-step method, and the degradation
rate of RhB was only 80% after 4 cycles. However, the degradation sta-
bility of 0.1Bi2GeO5/Ag3PO4 can keeping 84% after recycling four times.
Photocatalytic reaction includes mass transfer of reactant to catalyst
surface, adsorption, surface reaction, desorption and mass transfer of
bandgaps have n ¼ 1. The variations of (
α
hv)2 as a function of bandgap
energy (hv) of Ag3PO4, Bi2GeO5, 0.1Bi2GeO5/Ag3PO4, 0.2Bi2GeO5/
Ag3PO4, and 0.3Bi2GeO5/Ag3PO4 are provided in Fig. 6b. The bandgaps
of Ag3PO4, Bi2GeO5 and 0.1Bi2GeO5/Ag3PO4 were determined as 2.45
eV, 3.35 eV and 2.25 eV, respectively. The narrow gap was conducive to
the photocatalytic performance of Bi2GeO5/Ag3PO4 composite in the
visible region [37].
To reveal the transfer of photogenerated electrons of Bi2GeO5/
Ag3PO4 during photocatalysis, the VB (valence band) and CB (conduction
band) of Bi2GeO5 and Ag3PO4 were predicted by Eqs. (4) and (5) [38]:
ECB ¼ X ꢀ Ee ꢀ 0:5Eg
EVB ¼ ECB þ Eg
(4)
(5)
where X is absolute electronegativity, Ee represents the free electron
energy at hydrogen electrode (approximately 4.5 eV), Eg is the bandgap
energy, and ECB and EVB are the CB and VB potentials, respectively. The X
and EVB values of Bi2GeO5 were recorded as 6.29 eV and 3.47 eV,
respectively. The ECB value of Bi2GeO5 was estimated to 0.12 eV. For X
value of Ag3PO4 (6.17 eV), the EVB and ECB values were determined as
2.9 eV and 0.45 eV, respectively.
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D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Fig. 5. a XPS survey scans of fresh 0.1Bi2GeO5/Ag3PO4; b Ag 3d XPS spectra of fresh 0.1Bi2GeO5/Ag3PO4 and after 1 cycle; c P 2p XPS spectra of fresh 0.1Bi2GeO5/
Ag3PO4 and after 1 cycle; d O 1s XPS spectra of fresh 0.1Bi2GeO5/Ag3PO4; e Bi 4f XPS spectra of fresh 0.1Bi2GeO5/Ag3PO4 and f Ge 3d XPS spectra of fresh
0.1Bi2GeO5/Ag3PO4.
reaction products [45–47]. Previous studies have shown that adsorption
is an important factor affecting photocatalysis, and the presence of
adsorption can greatly increase the rate of photocatalytic reaction [48].
In this study, Bi2GeO5/Ag3PO4 photocatalyst was used for isothermal
adsorption of Rhodamine B molecule, and the adsorption behavior was
found to conform to Langmuir isothermal adsorption model, as shown in
Fig. 7. On this basis, the kinetic equation of photocatalytic degradation of
organic matter is used and the effect of intermediate products is ignored.
The kinetic equation of photocatalytic reaction based on adsorption is
further derived. The decomposition of RhB by Bi2GeO5/Ag3PO4 followed
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D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Fig. 6. a UV–Vis DRS results of Ag3PO4, Bi2GeO5, 0.1Bi2GeO5/Ag3PO4, 0.2Bi2GeO5/Ag3PO4, and 0.3Bi2GeO5/Ag3PO4; b Plots of (α
hv)2 as a function of energy (hv)
for bandgap energies of Ag3PO4, Bi2GeO5, 0.1Bi2GeO5/Ag3PO4, 0.2Bi2GeO5/Ag3PO4, and 0.3Bi2GeO5/Ag3PO4.
The photoelectrochemical characterization and photoluminescence
(PL) spectra are effective methods to evaluate the capture efficiency of
electron-hole species, as well as the migration and transfer of electrons.
The LSV curves of Ag3PO4, Bi2GeO5, fresh 0.1 Bi2GeO5/Ag3PO4 and after
1 cycle are shown in Fig. 10a. The negative shift of oxygen evolution
potential leads to the rapid separation of electron-hole pairs and the
formation of more catalytic oxidation sites [6]. The transient photocur-
rent responses of Ag3PO4, Bi2GeO5, fresh 0.1 Bi2GeO5/Ag3PO4 and after 1
cycle are presented in Fig. 10b. The photocurrents of fresh 0.1
Bi2GeO5/Ag3PO4 is higher than that of Ag3PO4 and Bi2GeO5, and the
photocurrent is further increased after 1 cycle. This result indicates that
fresh 0.1 Bi2GeO5/Ag3PO4 formed a small amount of Ag nanoparticles
during photocatalysis, which further increases the separation efficiency
of electron-hole pairs. The EIS spectra of those samples are shown in
Fig. 10c. 0.1 Bi2GeO5/Ag3PO4 presents the smallest EIS radius after 1
cycle, indicating that the appearance of silver nanoparticles increases the
efficiency of charge separation. The PL spectra of Ag3PO4, Bi2GeO5, and
0.1Bi2GeO5/Ag3PO4 are illustrated in Fig. 10d. The fluorescence in-
tensity of 0.1Bi2GeO5/Ag3PO4 nanocomposite significantly reduced after
combination with Ag3PO4. The intensity of the fluorescence signal re-
flected the probability of photogenic electron-hole pair recombination, in
which low fluorescence intensity indicated low probability of
photoelectron-hole pair recombination and high photocatalytic activity
[49]. Among catalysts, 0.1Bi2GeO5/Ag3PO4 rapidly achieved electron
transfer, beneficial for enhancing the electron/hole separation and
improving the photocatalytic performance.
The effects of different capture factors are illustrated in Fig. 11.
Adding IPA has only a small impact on the degradation of RhB during
photocatalysis. However, the addition of both BQ and TEOA obviously
inhibited the photocatalytic process. Thus, the oxidation process of holes
(hþ) and superoxide anions (O- 2⋅) were the main active species involved
in the photocatalytic degradation process on RhB by 0.1Bi2GeO5/
Ag3PO4.
Based on the aforesaid analysis, a Z-scheme reaction mechanism was
proposed for the Bi2GeO5/Ag3PO4 photocatalytic degradation process.
The valence and conduction band potential levels of Ag3PO4 were esti-
mated to about 2.9 eV and 0.45 eV (vs. NHE), respectively. As a result,
the forbidden bandwidth should be 2.45 eV. The bandgap width of
Bi2GeO5 was 3.35 eV, and both conduction and valence band energy
levels were 0.12 eV and 3.47 eV (vs. NHE), respectively. As shown in
Scheme 1, Ag3PO4 absorbed visible photons to form photoexcited
electron-hole pairs (Eq. (6)). By comparison, Bi2GeO5 cannot be excited.
Using the backlog of photoexcited electrons present in the conduction
band of Ag3PO4, some Ag nanoparticles were produced due to photo-
corrosion of Ag3PO4 photocatalyst (Eq. (7)). In turn, Ag nanoparticles can
Fig. 7. Nitrogen adsorption–desorption isotherms and pore-size distribution
curve (inset) of Bi2GeO5, Ag3PO4 and 0.1Bi2GeO5/Ag3PO4.
the pseudo-first-order kinetic model (Fig. 8c), and Table 1 shows the k
values that were calculated from linear fitting. The reaction rate con-
stants of Ag3PO4, Bi2GeO5, 0.1Bi2GeO5/Ag3PO4, 0.2Bi2GeO5/Ag3PO4
and 0.3Bi2GeO5/Ag3PO4 were determined as 0.03688minꢀ1
,
,
0.00148minꢀ1
, ,
0.06836minꢀ1 0.05385minꢀ1 and 0.03931minꢀ1
respectively. The largest reaction rate constant was obtained with
0.1Bi2GeO5/Ag3PO4 (0.06836minꢀ1). Thus, the combination of Ag3PO4
and Bi2GeO5 enhanced the photocatalytic performances. The combina-
tion of Bi2GeO5 with Ag3PO4 led to the formation of small amounts of Ag
nanoparticles formed during photocatalysis. This yielded Bi2GeO5/A-
g@Ag3PO4 photocatalyst with a Z-shaped heterojunction.
The stability tests of Ag3PO4 and 0.1Bi2GeO5/Ag3PO4 are shown in
Fig. 8d. After 4 cycles, the degradation rate of RhB by Ag3PO4 reached
only 48% while that obtained by 0.1Bi2GeO5/Ag3PO4 kept 84%. Thus,
the composite formed by Ag3PO4 and Bi2GeO5 showed improved stability
performance. SEM images of 0.1Bi2GeO5/Ag3PO4 after 1 and 4 cycles are
gathered in Fig. 9. The product looked basically stable, confirming stable
microstructure consistent with the stability of catalytic degradation
performance after 4 cycles. Meanwhile, some spherical nanocomposites
became connected after use, which may be caused by silver nanoparticles
produced during photocatalysis.
7

D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Fig. 8. a Visible light scanning pattern of RhB degradation by Bi2GeO5/Ag3PO4; b Effects of different catalysts on photocatalytic degradation of RhB under visible
light; c First-order kinetic fitting plots of RhB degradation by different catalysts; d Cycling runs of Ag3PO4 and Bi2GeO5/Ag3PO4 during the degradation of RhB.
absorb visible photons to form photoexcited electron-hole pairs (Eq. (8)),
Table 1
useful for surface plasma resonance (SPR) [11,50]. The photogenerated
Photodegradation rate constants and linear regression coefficients of different
electrons formed in Ag nanoparticles were then transferred to the con-
catalysts obtained according to the formula: ln(C/C0) ¼ kt.
duction band of Bi2GeO5, and can be captured by dissolved oxygen to
k(minꢀ1
)
Regression equation
R2
form superoxide anions (O- 2⋅) (Eq. (9)). The strong oxidation properties
of O- 2⋅ effectively degraded RhB (Eq. (10)). Meanwhile, the positively
charged holes in the lower energy level of Ag nanoparticles were com-
bined with the photogenerated electrons in the conduction band of
Ag3PO4, thereby preventing further corrosion of the photocatalyst. The
0.1 Bi2GeO5/Ag3PO4
0.2 Bi2GeO5/Ag3PO4
0.3 Bi2GeO5/Ag3PO4
Ag3PO4
0.06836
0.05385
0.03931
0.03688
0.00148
-ln(C/C0)¼0.06836xþ0.74009
-ln(C/C0)¼0.05385xþ0.55866
-ln(C/C0)¼0.03931xþ0.42863
-ln(C/C0)¼0.03688xþ0.37419
-ln(C/C0)¼0.00148xþ0.02047
R2¼0.86996
R2¼0.86403
R2¼0.87174
R2¼0.8565
R2¼0.93051
Bi2GeO5
Fig. 9. SEM image of 0.1Bi2GeO5/Ag3PO4 after: a 1 and b 4 cycles of use.
8

D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Fig. 10. a LSV curves, b Transient photocurrent responses, and c EIS Nyquist plots of Ag3PO4, Bi2GeO5, fresh 0.1 Bi2GeO5/Ag3PO4 and after 1 cycles; d Photo-
luminescence spectra of Ag3PO4, Bi2GeO5, and 0.1 Bi2GeO5/Ag3PO4.
holes remaining on the valence band of Ag3PO4 directly oxidized to
decompose RhB (Eq. (11)). The latter was consistent with the active
factor capture results.
Ag3PO4 þ hv → eꢀ þ hþ
Agþ þ eꢀ → Ag
(6)
(7)
Ag þ hv → eꢀ þ hþ
(8)
e
ꢀ þ O2 → Oꢀ2 ⋅
(9)
Oꢀ2 ⋅ þ RhB → CO2 þ H2O
(10)
(11)
h
þ þ RhB → CO2 þ H2O
4. Conclusions
A simple precipitation route was developed to prepare Bi2GeO5/
Ag3PO4 photocatalysts with Z-scheme heterojunctions. Under visible
light irradiation, Bi2GeO5/Ag3PO4 showed superior photocatalytic ac-
tivity and good stability. The photocatalytic degradation rate of
0.1Bi2GeO5/Ag3PO4 reached 96% within 30 min, in which most RhB was
degraded. After 4 cycles, the degradation rate of RhB by 0.1Bi2GeO5/
Ag3PO4 still retained 84%, suggesting the significantly improved stability
of Ag3PO4 and Bi2GeO5 composite when compared to Ag3PO4. The
enhanced catalytic activity was attributed to the formation of Ag particles
during photocatalysis that resulted in Bi2GeO5/Ag@Ag3PO4 photo-
catalyst with a Z-scheme heterojunction. Ag nanoparticles can also
absorb visible photons and form photoexcited electron-hole pairs thanks
Fig. 11. Trapping experiments of active species during photocatalysis.
to surface plasma resonance (SPR), thereby promoting the transfer of
photogenerated electrons to the CB of Bi2GeO5 and participating in the
degradation of target pollutants. Besides, Ag nanoparticles might also
inhibit further corrosion of Ag3PO4 photocatalyst, suggesting the holes
(hþ) and superoxide anions (O- 2⋅) as primary species responsible for the
decomposition of RhB by 0.1Bi2GeO5/Ag3PO4. In summary, the forma-
tion of Bi2GeO5/Ag@Ag3PO4 Z-scheme heterojunction during photo-
catalysis looks a promising method and could be extended to other
catalysts.
9

D. Liu et al.
Journal of Solid State Chemistry 301 (2021) 122309
Scheme 1. Schematic diagram of the photocatalytic mechanism of Bi2GeO5/Ag3PO4.
activity after loading with Ag nanoparticles, Mater. Res. Bull. 47 (2012)
2422–2427.
CRediT authorship contribution statement
[11] H. Guo, C.-G. Niu, D.-W. Huang, N. Tang, C. Liang, L. Zhang, X.-J. Wen, Y. Yang, W.-
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Dengdeng Liu: Conceptualization, Methodology, Software, Data
curation, Writing – original draft. Pengyu Zhu: Visualization, Investi-
gation. Li Yin: Visualization, Investigation. Xinyu Zhang: Writing – re-
view & editing. Kaijin Zhu: Supervision. Junhua Tan: Supervision. Riya
Jin: Supervision.
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Declaration of competing interest
The authors declare that they have no known competing financial
interests or personal relationships that could have appeared to influence
the work reported in this paper.
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11