Citric acid-assisted hydrothermal synthesis of a self-modified Bi2SiO5/Bi12SiO20 heterojunction for efficient photocatalytic degradation of aqueous pollutants

Article abstract of DOI:10.1039/c7cy02272g

A self-modified Bi₂SiO₅/Bi₁₂SiO₂₀ heterojunction was synthesized for the first time via a citric acid-assisted hydrothermal method. It was found that citric acid (CA) played two roles in the formation of the Bi₂SiO₅/Bi₁₂SiO₂₀ heterojunction: enhancement of the specific surface area due to the release of many volatiles from the decomposition of CA and tuning the weight ratio of Bi₂SiO₅ and Bi₁₂SiO₂₀ in the heterojunction by forming different complexes with Bi³⁺. Thus, a self-modified Bi₂SiO₅/Bi₁₂SiO₂₀ heterojunction prepared at a CA dosage of 11.25 mmol presented an I_27.9:I_29.2 value of 1.52 and a BET specific surface area of 32.6 m² g^-1, 5.7 times that without addition of CA. It showed the highest first-order apparent reaction constant of 0.1694 min^-1 in the photocatalytic degradation of acid orange 7 (AO7), which is 1.8, 53.0, and 2.2 times that of Bi₂SiO₅, Bi₁₂SiO₂₀, and TiO₂ (P25), respectively. The capture experiments show that the main active species that degraded AO7 are not O₂^- and OH but h+vb. During the AO7 degradation, four intermediates, sulfonic acid, oxalic acid, benzenesulfonic acid, and 4-aminobenzenesulfonic acid, were identified by capillary electrophoresis/capacitively coupled contactless conductivity detection. Finally, other pollutants, rhodamine B, p-chlorophenol, and tetracycline, were also found to be efficiently removed by the Bi₂SiO₅/Bi₁₂SiO₂₀-mediated photocatalysis system.

Full text of DOI:10.1039/c7cy02272g

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Citric acid-assisted hydrothermal synthesis of a self-modified
Bi2SiO5/Bi12SiO20 heterojunction for efficient photocatalytic
degradation of aqueous pollutants
Received 00th January 20xx,
Accepted 00th January 20xx
W. Q. Li,a Z. H. Wena, S. H. Tiana, b, L. J. Shanga, and Y. Xionga, b
DOI: 10.1039/x0xx00000x
A self-modified Bi2SiO5/Bi12SiO20 heterojunction was synthesized for the first time via a citric acid-assisted hydrothermal
method. It was found that citric acid (CA) played two roles in the formation of Bi2SiO5/Bi12SiO20 heterojunction：
enhancement of the specific surface area due to the release of many volatiles from the decomposition of CA and tuning
the weight ratio of Bi2SiO5 and Bi12SiO20 in the heterojunction by forming different complex with Bi3+. Thus, a self-modified
Bi2SiO5/Bi12SiO20 heterojunction prepared at the CA dosage of 11.25 mmol presented a I27.9:I29.2 value of 1.52 and a BET
specific surface of 32.6 m2/g, 5.7 times that without addition of CA. It showed the highest first-order apparent reaction
constant of 0.1694 min-1 in the photocatalytic degradation of acid orange 7 (AO7), which is 1.8, 53.0, and 2.2 times that of
Bi2SiO5, Bi12SiO20, and TiO2 (P25), respectively. The capture experiments show that the main active species that degraded
AO7 are not O2·- and ·OH, but ꢀꢁꢃꢂ. During the AO7 degradation, four intermediates of sulfonic acid, oxalic acid,
benzenesulfonic acid, and 4-aminobenzene sulfonic acid were identified by capillary electrophoresis/capacitively coupled
contactless conductivity detector. Finally, the other pollutants of Rhodamine B, p-chlorophenol, and tetracycline were also
www.rsc.org/
found
to
be
efficiently
removed
by
the
Bi2SiO5/Bi12SiO20-mediated
photocatalysis
system.
BiVO4,12 Bi2WO6,13 Bi2MoO614 and Bi2O2CO315 are an attractive group
of non-TiO2 photocatalysts due to their excellent photo-activity
arising from their fresh-construction and well-dispersed valence
bands by the hybridization of Bi 6s and O 2p orbits.16 Bismuth
silicate (Bi2SiO5) is one of the newly found Aurivillius oxides and
composed of SiO32− pyroxene file layers inserted between (Bi2O2)2+
layers.17 Since it was first reported by I. Koiwa in 1996,18
nanostructured Bi2SiO5 has attracted great interests because of its
1. Introduction
Aqueous recalcitrant organic pollutants (ROPs), such as dyes,
antibiotics, and chlorophenols, have attracted increasing attention
due to their nature of persistence, biomagnification and harmful
impacts on both humans and the environment.1 How to treat these
pollutants with green, high efficiency and low-cost has become the
focus of attention. Heterogeneous photocatalysis has been
considered as a promising advanced oxidation technology for water
decontamination due to its mild reaction conditions and no
secondary pollutant.2 The key issue for this technology is to choose
the photocatalysts. Up to now, a large amount of photocatalysts
have been developed, such as titanium oxide, bismuth oxide, zinc
oxide, tungstic oxide, cerium oxide, carbon nitride, and their
modified oxides.3, 4 Among those catalysts, TiO2 is the most widely
investigated one. However, it suffers from low light utilization and
poor quantum efficiency,5 which hinders its practical application.
relatively good dielectric properties, piezoelectric and nonlinear
20
optical effects.19,
Recently, Bi2SiO5 has been found to display
interesting photocatalytic activities in environmental purification
application.21-23 However, to put the application into practice, the
photodegradation ability and the spectral response range of Bi2SiO5
need to be further improved because of the poor charge separation
and migration to surface reaction sites like the other Bi-based
compounds.
Fabrication of a heterojunction by coupling semiconductors with
matchable energy levels is an outstanding method to enhance the
photocatalytic activity of the semiconductor via significantly
facilitating the separation of photo-generated carries.24-27 It has
been proved that the heterojunctions of Bi2SiO5 with
Bismuth is a stable heavy element free of toxicity. Bismuth-based
compounds, such as Bi2O3,6 BiOX (X = Cl, Br, I),7 BiOmXn (X = Cl, I),8-11
28
,
31
Bi4O5Br2/Bi24O31Br10
AgI,29 Bi2S3,30 and C3N4 showed much
higher photocatalytic activities than the single semiconductor.
However, these heterojunctions introduce the foreign elements
except Bi, Si, and O, which result in the complexed synthesis
process.
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Bi12SiO20 is a conventional photorefractive, electro-optic material mL Teflon hydrothermal reactor and reacted under different
with good photoconductivity.32 It is composed of Bi, Si, O, with the hydrothermal condition. After the solution was naturally cooled to
same composition of Bi2SiO5, which offers an opportunity of self- room temperature, the products were filtered, then washed with
modification during the synthesis of Bi2SiO5/Bi12SiO20, namely ethanol and water three times, respectively. The products were
tuning the molar ratio of Bi2SiO5 and Bi12SiO20 in the heterojunction collected by centrifuge and dried at 105 oC overnight. Then the
photocatalyst. On one hand, the matchable lattice of Bi2SiO5 and powder was calcined at 550 oC for 2h in a muffle oven. The
Bi12SiO20 favors the formation of heterojunction, and thereof obtained sample was kept in desiccator for further use.
improve the charge separation efficiency. On the other hand, the
For comparison, pure Bi12SiO20 was prepared according to the
literature:32 2 mmol of Bi(NO3)3·5H2O and 0.17 mmol of Na2SiO3
were added into 30 mL NaOH solution with a concentration of 3M.
The mixture was stirred in a 70 ℃ water bath for 3h. The products
were filtered, then washed with ethanol and water three times,
respectively. The products were collected by centrifuge and dried at
105 oC overnight. All the above samples were named as BSO1-
BSO13 depending on the synthesis conditions (Table 1).
good photoconductivity of Bi12SiO20 can promote the migration of
charge to the surface reaction sites, and may also narrower the
band gap of the catalyst. Thus, coupling Bi12SiO20 with Bi2SiO5 to
form a heterojunction may probably further enhance the catalytic
activity of Bi2SiO5. To the best of our knowledge, a Bi2SiO5/ Bi12SiO20
heterojunction
synthesized
by
citric
acid-assisted
hydrothermal method and its self-modification by tuning the CA
dosage has not been reported previously.
2.3. Characterizations and analytical methods
In this work, we developed a citric acid-assisted hydrothermal
method for controllable synthesis of a novel Bi2SiO5/ Bi12SiO20
heterojunction. The molar ratio of Bi2SiO5 and Bi12SiO20 in the
heterojunction can be tuned by altering the amount of citric acid
(CA) and/or controlling the hydrothermal treatment condition. As
an initial work, the investigation focused on three issues: (1) to
synthesize the self-modified Bi2SiO5/ Bi12SiO20 heterojunction by
tuning the addition of CA and the hydrothermal treatment
condition; (2) to explore the tuning role and mechanism of CA in the
The XRD patterns of the prepared powder were obtained on a
D/max 2200 vpc diffractometer (Rigaku Corporation, Japan) with
the Cu Kα radiation at 30 kV and 30 mA. The specific BET surface
area was analyzed with Autosorb-iQ (Quantachrome Instruments,
USA) by nitrogen adsorption at 77 K. The SEM was performed on
gold-coated samples using
a JSM-6330F-mode field emission
scanning electron microscope (JEOL, Japan). The transmission
electron microscopic (TEM) measurement was conducted on
a FEI Tecnai G2 Spirit (FEI, Netherlands) operated at 300 KV. The
UV-vis DRS spectrum of catalysts was collected on UV-PC3101PC
(Shimadzu, Japan). The degradation intermediates of AO7 were
identified by capillary electrophoresis/capacitively coupled
contactless conductivity detector.33
characteristics
of
the
self-modified
Bi2SiO5/
Bi12SiO20
heterojunction; (3) to reveal the photocatalytic mechanism and
detect the degraded intermediates. Acid orange 7 (AO7) is a widely-
used anionic dye, which is herein used as a model contaminant to
probe the photocatalytic performance and mechanism of Bi2SiO5/
Bi12SiO20. In addition, a typical cationic Rhodamine B dye and the
other emerging pollutants of p-chlorophenol (a neutral and small
molecule) and tetracycline (a neutral and large molecule) were also
studied. The aim of these investigations is to reveal the self-
modified method of Bi2SiO5/ Bi12SiO20 heterojunction and disclosure
the potential of Bi2SiO5/ Bi12SiO20 in the photocatalysis for
wastewater treatment.
2.4. Photocatalytic experiments
The photocatalytic degradation experiments were conducted in a
quartz reactor under a 100 W high pressure Hg lamp. For each
experiment, 0.05 g catalyst powder and 50 mL of wastewater
containing dye (AO7 or RhB), para- chlorophenol, or tetracycline
with a certain concentration were added into the reactor. Unless
specified, the concentration of AO7, p-Chlorophenol, RhB, and TC is
set at 20, 5.0, 20.0, and 10.0 mg/L, respectively and the degradation
was performed at the natural solution pH without control during
the reaction. Before the light irradiation, the suspensions were
magnetically stirred in the dark for 30 min to reach the absorption–
desorption equilibrium. Then the light was turned on and the air
was bubbled into the reaction mixture at the flow rate of 100
mL/min. At certain time intervals, 2 mL of the mixed suspension
were extracted and centrifuged to remove the photocatalyst. The
residue concentration of AO7 or RhB was measured by a UV-vis
spectrophotometer (Xinmao UV-7502PC) at the wavelength of 484
nm and 552 nm, respectively. The concentration of TC and p-
chlorophenol was detected by LC-15C HPLC (Shimadzu, Japan)
equipped with a UV detector at 358nm and 280 nm, respectively.
The mobile phase used methanol-water solution (70% v/v
methanol) and the flow rate was 1 mL min−1. The pH of mobile
phase was adjusted to 2.0 by phosphate acid when measuring TC.
2. Experimental
2.1. Materials
Bismuth nitrate pentahydrate and sodium silicate were supplied by
Tianjin Kemule Chemical Reagent Co., Ltd, China. Citric acid
monohydrate, nitric acid, and sodium hydroxide were purchased
from Guangzhou Chemical Reagent Co., Ltd, China. Acid orange 7
(AO7), Rhodamine
B (RhB), p-chlorophenol, and tetracycline
hydrochloride (TC) were obtained from Aladdin Chemistry Co., Ltd,
China. All reagents used in the experiment were of analytical
reagent grade and used without further purification.
2.2. Preparation of catalysts
After dissolving a certain amount of citric acid in 60 mL of deionized
water, 3.75 mmol of Bi(NO3)3·5H2O was added into the solution and
kept stirring for 1h. Then 2.5 mmol of Na2SiO3 was added and
stirred for 30 min. The above suspension was transferred into a 100
2 | Catal. Sci. Technol., 2017, 00, 1-3 | 2
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/wB values. It can be seen from Fig. 1 that the strongest peak for
Bi12SiO20 is at 2θ of 27.9 o and 29.2o for Bi2SiO5. Therefore, I27.9:I29.2
was used to evaluate the weight ratio of Bi12SiO20 and Bi2SiO5 in the
heterojunction. The higher I27.9:I29.2 means the higher weight ratio
of Bi12SiO20 and Bi2SiO5. When the CA dosage is 3.75, 7.50, 11.25,
3. Results and discussion
3.1. Formation of Bi2SiO5/ Bi12SiO20 heterojunction: Role of citric
acid
15.00 mmol，the obtained BSO3, BSO4, BSO5, BSO6 showed the
Herein citric acid (CA) was used to chelate bismuth ions and form a
homogeneous solution. In the preparation of sol, the complexation
of metal ion and citric acid as well as the polymerization of citric
acid are two key steps of the reaction. After the hydrothermal
treatment process, the dried powder is calcined in air to form the
final BSO. To clarify the tuning role of CA in the formation of
bismuth silicate, different dosage of CA was added into the reaction
I27.9:I29.2 values of 0.61, 3.00, 1.52, 2.86 (Table 1). In addition, the
SBET of BSO4, BSO5, BSO6 is 25.3, 32.6, and 39.9 m2/g, respectively,
which increased with the increase of CA dosage. That is because the
CA acted as a pore forming agent in the following calcination
treatment due to the release of many volatiles derived from the
decomposition of CA.35 Based on the above analysis, the
composition and SBET of the BSO, and the weight ratio of Bi2SiO5 and
Bi12SiO20 in the heterojunction can be tuned by varying the CA
dosage. Basically, increasing CA dosage increase the weight ratio of
Bi12SiO20 and stopped the formation of Bi2O3. This can be explained
by the chelating role of CA. The Bi3+ ions can be chelated with the –
COOH group of citric acid for one-tooth and two-tooth complexing.
o
system but fixing the hydrothermal temperature at 180 C and the
time at 6 h as shown in Table 1. Fig. 1a displays the XRD patterns of
BSO1-BSO6 synthesized with different CA dosage. For BSO1 without
addition of CA, the diffraction peaks at 2θ of 23.8°, 29.2°, 32.6°,
33.6°, 35.3°, 49.6°, 56.2°, 57.4°, and 59.0° correspond to the (310),
(311), (020), (002), (600), (602), (331), (313), and (820) plane of
Bi2SiO5 (JCPSC36-0287) while those of 2θ at 27.9°，30.2°，31.7°,
and 32.6° are attributed to the (201), (211), (002), and (220) plane
of Bi2O3 (JCPS65-1209). Therefore, BSO1 might be a heterojunction
of Bi2SiO5/Bi2O3, and its SBET is only 5.7 m2/g. Without CA to
distribute Bi3+ in the reaction system, Bi3+ easily segregated in
calcination process and resulted in the emergence of Bi2O3.
Increasing the CA to 1.88 mmol means the molar ratio of -COOH in
CA to Bi3+ is 3:2. Only 50% Bi3+ in the reaction mixture can be
chelated by CA. Under this condition, pure Bi2SiO5 was obtained
since chelation of CA made Bi3+ more uniformly distributed in the
reaction system, and the SBET significantly increased to 45.3 m2/g.
Further increasing the CA to 3.75 mmol results in the fact that the
molar ratio of -COOH in CA to Bi3+ is 3:1, which indicates that all the
bismuth ions have the opportunity to be chelated by CA. It can be
seen from BSO3 XRD pattern that except the peaks of Bi2SiO5, a
series of new diffraction peaks at 2θ of 24.9°, 27.9°, 30.6°, 33.1°,
35.5°, 37.7°, 39.8°, 41.8°, 45.7°, 49.3°, 52.7°, 54.4°, 62.2°, and 79.2°
appear. Those peaks correspond to the (220), (310), (222), (321),
(400), (330), (422), (332), (510), (521), (530), (600), (611), (631),
(222), (653) plane of Bi12SiO20 (JCPSC37-0485). The results
demonstrated that a new heterojunction of Bi2SiO5/Bi12SiO20 might
form in BSO3. When further increasing the CA to 7.5, 11.25, and
even 15.0 mmol, the corresponding BSO4, BSO5, and BSO6 exhibit
nearly the same diffraction with BSO3.
Although BSO3-BSO6 are all pure Bi2SiO5/Bi12SiO20, the peaks
strength ratio of Bi2SiO5 and Bi12SiO20 changed greatly, which
implying different ratio of Bi2SiO5 and Bi12SiO20 in the BSO3-BSO6
samples. Their weight ratio can be estimated according to Eq.(1).34
ꢄꢅ
ꢄꢆ
ꢆꢉꢅ
ꢇ ꢈ
(1)
ꢈꢅꢉꢆ
Where wA and wB presents the mass of compound A and B in the
mixture, the ratio of KB to KA is constant for a definite two
compounds system, IA and IB is the strongest peak intensity of
compound A and B, respectively. The IA/ IB values can be calculated
based on the XRD patterns by Jade software. Since KB/KA is a
Fig. 1 XRD patterns of the synthesized powders (a) with various CA
dosage at the hydrothermal treatment temperature of 180 oC for
6h, (b) under various hydrothermal temperature for 6 h with the CA
dosage of 11.25 mmol, (c) under various hydrothermal time at the
hydrothermal temperature of 180 oC with the CA dosage of 11.25
mmol.
constant for Bi2SiO5/Bi12SiO20, the IA/ IB values are in direct
proportion to the wA / wB values and can indicate the level of wA
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Table 1. Physicochemical properties of BSO prepared under different conditions.
•
CA
Phase
SBET
Ta
tb
RRc
Sample
name
d
I27.9:I29.2
kap
（
mmol
(m2/g)
（℃）
（
h）
（
%）
）
5.7
BSO1
BSO2
BSO3
BSO4
BSO5
BSO6
BSO7
BSO8
BSO9
BSO10
BSO11
BSO12
0.00
1.88
180
180
180
180
180
180
160
200
180
180
180
180
6
6
Bi2SiO5/Bi2O3
Bi2SiO5
-
27.71
55.12
68.51
70.74
80.10
75.43
47.90
61.63
40.28
75.91
76.98
51.85
0.0332
0.0825
0.1159
0.1249
0.1694
0.1399
0.0661
0.098
45.3
38.1
25.3
32.6
39.9
21.1
19.8
10.3
39.4
41.8
17.9
-
3.75
6
Bi2SiO5/Bi12SiO20
Bi2SiO5/Bi12SiO20
Bi2SiO5/Bi12SiO20
Bi2SiO5/Bi12SiO20
Bi2SiO5/Bi12SiO20/Bi2O3
Bi2SiO5/Bi12SiO20
Bi2O3
0.61
3.0
1.52
2.86
-
7.50
6
11.25
15.00
11.25
11.25
11.25
11.25
11.25
11.25
6
6
6
6
6.21
-
2
0.0534
0.1493
0.1573
0.078
4
Bi2SiO5/Bi12SiO20
Bi2SiO5/Bi12SiO20
Bi2SiO5/Bi12SiO20/Bi2O3
0.98
3.86
-
8
10
Note: a is the hydrothermal treatment temperature,
b is the hydrothermal treatment temperature,
c is the removal efficiency of AO7 within the reaction time of 10 min,
d is the apparent first order reaction constant within 10 min.
When adding an excess of citric acid, the one-tooth complexing
Hydrothermal treatment temperature and time are also
prevails, whose bond strength is weaker than two-tooth complexing important for the synthesis and crystallization of BSO. The results
o
and thus easier to break down under a moderate condition. are shown in Table 1 and Fig. 1b and c. BSO7 synthesized at 160 C
Therefore, in the following hydrothermal treatment and calcination for 6h and BSO12 at 180 oC for 10h displayed a Bi2SiO5/Bi12SiO20
processes, more Bi3+ can be released in advance due to the weak heterojunction phase but with some impurity of Bi2O3. Enhancing
complexing. This results in the formation of more Bi12SiO20, which the temperature to 200 oC (BSO8) or keeping the treatment time for
needed more Bi3+ during the synthesis. Thus, two roles of CA in the 4h (BSO10), 6h (BSO5) and 8h (BSO11) gave a pure phase of
characteristics of BSO are revealed: enhancement of the specific Bi2SiO5/Bi12SiO20. However, the high I27.9:I29.2 value of 6.21 indicates
surface area due to the release of many volatiles from the that much more Bi12SiO20 contained in BSO8 than BSO5 synthesized
decomposition of CA and tuning the phase composition of BSO and at 180 oC. Moreover, I27.9:I29.2 values follow the order of BSO10 <
the weight ratio of Bi2SiO5 and Bi12SiO20 in the heterojunction by BSO5< BSO11<BSO8, indicating that Bi12SiO20 increased with the
forming different complex with Bi3+
. It is believed that the increase of hydrothermal treatment time and temperature. When
characteristics of BSO will finally affect its photocatalytic activity.
the treatment time is shortened to 2h, only pure phase of Bi2O3 was
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obtained. Hence, it can be concluded that the composition of the assigned to the 4f 5/2 and 4f 7/2 of Bi3+ in three catalysts,
BSO and the weight ratio of Bi2SiO5 and Bi12SiO20 in the respectively. From Fig. 4d, the spectra for O 1s are split into two
heterojunction also depend greatly on the hydrothermal treatment peaks. The peak at about 530 eV is characteristic of lattice oxygen
temperature and time. Increasing the treatment temperature and atoms in the three catalysts and the other at about 532 eV is
lengthening the treatment time prohibits the formation of Bi2O3 but attributed to the external hydroxyl groups or the water molecules
promotes the formation of Bi12SiO20. That is because Bi12SiO20 is a adsorbed on the surface.37-40 Although the three catalysts displayed
more stable phase than Bi2SiO5, and Bi2O3 is the most easily formed similar peaks, the location of the corresponding peaks is different
phase compared to Bi12SiO20 and Bi2SiO5.36
from each other. Compared with the peaks of Bi2SiO5 and Bi12SiO20,
those of Bi2SiO5/Bi12SiO20 shifted obviously, revealing the existence
of interactions between Bi2SiO5 and Bi12SiO20 in the heterojunction.
To better understand the physical-chemical properties of the
Bi2SiO5/Bi12SiO20 heterojunction, BSO5 (Bi2SiO5/Bi12SiO20
) was
selected as a model BSO to be further characterized by SEM, TEM,
and DRS. As shown in Fig. 2a, BSO5 (Bi2SiO5/Bi12SiO20) displayed a
coralline morphology constituted of irregularly shaped particles
mainly with the size of 200-500 nm. Fig. 2b shows a high resolution
TEM (HRTEM) image of BSO5. In the BSO5 nanosheet, the lattice
spacing of 0.357 nm corresponds to (220) planes of Bi12SiO20, and
that of 0.371 nm corresponds to (310) plane of Bi2SiO5. The HRTEM
results confirm that a heterojunction of Bi2SiO5/Bi12SiO20 were
successfully formed in BSO5. The optical properties of BSO5
(Bi2SiO5/Bi12SiO20), Bi2SiO5, and Bi12SiO20 were measured by UV–Vis
DRS. As shown in Fig. 2c, Bi2SiO5 could only absorb UV light with the
wavelength less than ~385 nm while the adsorption wavelength of
Bi12SiO20 red shifted to the visible light region. In contrast, BSO5
(Bi2SiO5/Bi12SiO20) sample present a wide photo-absorption in the
range of 200–500 nm, which also confirms the existence of Bi12SiO20
Fig. 2d displays the plot of (αhν)1/2 vs. hν, from which the band gap
(Eg) of each sample can be estimated. The Eg of Bi2SiO5 and Bi12SiO20
are 3.27 and 2.49 eV, respectively, consistent with the reported
results. 21, 25, 30
.
Fig. 3 XPS spectra of the Bi2SiO5, Bi12SiO20, and Bi2SiO5/Bi12SiO20
(BSO5) heterojunction (a) Bi 4f, (b) Si 2p.
3.2. Catalytic performance of BSO
Effect of CA on the degradation kinetics were investigated, and the
results are shown in Fig. 4a. The degradation of AO7 within 10 min
was fitted with the pseudo first-order kinetics (Fig. 4d). The kinetics
may be expressed as Eq. (2):
Fig.
2 (a) SEM image of Bi2SiO5/Bi12SiO20, (b) HRTEM of
Bi2SiO5/Bi12SiO20, (c) UV–Vis diffuse reflectance spectra and (d) plots
of (αhν)1/2 versus photon energy (hν). Herein Bi2SiO5/Bi12SiO20 is
BSO5.
ln ꢊꢊ ꢇ ꢍꢎꢏ
ꢐ
(2)
ꢋ
ꢌ
where kap is the apparent reaction constant and C0 and Ct are the
initial concentration and the concentration at time t, of AO7,
respectively.
Fig. 3 presents the Bi 4f and O 1s XPS spectra of the Bi2SiO5,
Bi12SiO20, and Bi2SiO5/Bi12SiO20 (BSO5) heterojunction. Bi 4f spectra
(Fig. 3a) shows two strong peaks at about 159 eV and 164 eV
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The AO7 removal by adsorption in all cases are less than 18.5% fitted with the pseudo first-order kinetics (Fig. 4 e and f). The kap
regardless of the catalyst type. However, the CA dosage used in the were listed in Table 1. The hydrothermal treatment temperature
preparation of BSO significantly affected the degradation of AO7. and time had a great effect on catalytic activity of BSO. The best
BSO1 prepared without the addition of CA showed the lowest BSO5 was hydrothermally treated at 180 oC for 6h. Raising the
catalytic activity probably due to the low SBET. BSO3-BSO6 resulted temperature to 200 oC or decreasing it to 160 oC deteriorate the
in high degradation efficiency of about 90% within the reaction time photocatalytic degradation. As discussed above, BSO7 (160 oC)
of 30 min and the kap values are in the range of 0.1159-0.1694 min- contained another compound of Bi2O3 except Bi2SiO5 and Bi12SiO20
,
1. It is expected that BSO3-BSO6 showed high catalytic activity due which may result in too many defects and therefore increase the
to the large SBET and the heterojunction structure. In general, the recombination centres of the photo-generated electron and hole
large SBET can afford more catalytic active sites and the pairs. In contrast, BSO7 (200 oC) only contained Bi2SiO5 and Bi12SiO20
heterojunction can promote the separation of photo-generated like BSO5. However, the I27.9:I29.2 of BSO7 reached as high as 6.21,
electrons and holes.35 However, it was unexpected that the catalytic indicating the existence of too many Bi12SiO20 in the heterojunction,
activity of BSO5 is higher than that of BSO3 although BSO5 had a which is also not helpful. Similarly, lengthening the hydrothermal
smaller SBET. The unexpected result obviously contradicted to the time or shortening it introduced the impurity of Bi2O3 or changed
general catalytic principle. As discussed above, the higher I27.9:I29.2 of the weight ratio of Bi2SiO5 and Bi12SiO20, which all decreased the
BSO means the higher weight ratio of the Bi12SiO20 and Bi2SiO5 in catalytic activity of BSO. Based on the above comparison and
the heterojunction. Since the I27.9:I29.2 values of the four BSO analysis, it can be concluded that BSO with the I27.9:I29.2 values of
differentiate, it is deduced that the weight ratio of BSO 0.98~3.86 and high SBET may be inclined to exhibit good
heterojunction may explain the discrepancy of catalytic activity. photocatalytic activity. Hence, the optimal CA dosage,
Since BSO5 presented the highest catalytic activity, the I27.9:I29.2 hydrothermal treatment temperature and time in the synthesis of
value of 1.52 in BSO5 might indicate the proper weight ratio of BSO can be de determined as 11.25 mmol, 180 oC, and 6h.
Bi12SiO20 and Bi2SiO5. Too much or too less Bi12SiO20 in the
Since BSO5 is the most active heterojunction of Bi2SiO5/Bi12SiO20
among all the BSO samples, its catalytic activity was furthermore
compared with the pure Bi2SiO5 (BSO2) and Bi12SiO20 (Fig. 5a). The
AO7 removal by adsorption in all cases are very small and thus the
difference in adsorption over different catalysts can be ignored
compared to that in photocatalytic degradation. After 30 min light
illumination, the degraded AO7 by Bi2SiO5 and Bi12SiO20 were only
about 79.6±1.0% and 19.6±1.0%, respectively. However, with the
Bi2SiO5/Bi12SiO20 heterojunction as the photocatalyst, 90.9±1.5% of
AO7 was degraded within 30 min. Obviously, the heterojunction of
Bi2SiO5/Bi12SiO20 showed much higher catalytic activity than the
pure Bi2SiO5 and Bi12SiO20, and the classic TiO2 (P25). Within 10 min,
the photocatalytic activity of the Bi2SiO5/Bi12SiO20 heterojunctions
reached the maximum rate constant of 0.1696 min-1, which is 1.8,
53.0, and 2.2 times that of Bi2SiO5, Bi12SiO20, and TiO2 (P25),
respectively (Fig. 5b). Herein the pretty low catalytic activity of
Bi12SiO20 may be correlated with its low SBET of 4.7 m2/g. Since the
SBET Bi2SiO5/Bi12SiO20 (BSO5) is 32.6 m2/g,, lower than that of
heterojunction will deteriorate the catalytic activity of BSO.
Bi2SiO5 (BSO2, 45.3 m2/g),
the high catalytic activity of
Bi2SiO5/Bi12SiO20 proved that the heterojunction played a role in
enhancing the photocatalytic activity. As is known, the
photocatalysts become excited and generate electron–hole pairs
directly under illumination in the photocatalytic process. The
photocatalytic performance mainly depends on the recombination
rate or the lifetime of the photo-generated electron–hole pairs. The
faster recombination occurs, the less time is required for the
chemical reactions. Therefore, PL spectra are utilized for
investigating the recombination rates of the photogenerated
electron–hole pairs.411 The PL spectra of BSO2(Bi2SiO5),
BSO5(Bi2SiO5/Bi12SiO20), and Bi12SiO20 were obtained and the results
are shown in Fig. 5c. A strong emission peak around 400 nm
appears for the as-prepared samples, which could have been
Fig. 4 Photocatalytic degradation of AO7 and its Pseudo first-order
kinetics fitting with BSO synthesized (a, d) with various CA dosage at
the hydrothermal treatment temperature of 180 oC for 6h, (b, e)
under various hydrothermal temperature for 6 h with the CA
dosage of 11.25 mmol, (c, f) under various hydrothermal time at the
hydrothermal temperature of 180 oC with the CA dosage of 11.25
mmol
Effect of hydrothermal treatment temperature and time on the derived from the direct electron–hole recombination of band
degradation kinetics were also investigated. The results are shown transitions. The characteristic emission peak of lowest intensity for
in Fig. 4 b and c. The degradation of AO7 within 10 min was also Bi2SiO5/Bi12SiO20 indicates that the recombination of
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69.1±2.5% to 90.9±1.5% when increasing the catalyst dosage from
250 to 1000 mg/L due to more available catalytic active sites on the
surface of Bi2SiO5/Bi12SiO20. Further raising the catalyst dosage from
1000 to 1500 mg/L did not enhance the AO7 degradation any more.
This is because too many catalyst particles may increase the
scattering and turbidity effects, which result in diminishing the light
penetration into the solution and over the catalyst surface and thus
declining utilization rate of optical energy for photocatalytic
degradation. As shown in Fig. 6b, at the initial solution pH of 3.0,
the AO7 removal reached the highest 96.8±1.8%. Adjusting the
initial solution pH from 3.0 to 9.0 did not obviously affect the
degradation of AO7. However, only 53.1±2.7% AO7 was removed
when the initial solution pH increased to 10.8. That is possibly
ascribed to the lower fermi energy level, which shifted negatively
59 mV when the solution pH decreased one unit. It implies that the
oxidation stability of the holes is stronger under the lower pH.
Anyway, the results show that the Bi2SiO5/Bi12SiO20 heterojunction
can work in a wide pH range of 3.0-9.0, which is important for the
practical wastewater treatment.
Fig. 5 (a) Photocatalytic degradation of AO7 with the catalysts of
Bi2SiO5 (BSO2), Bi12SiO20 and Bi2SiO5/Bi12SiO20 (BSO5), (b) Pseudo-
first-order rate fitting for photocatalytic degradation of AO7 with
various catalysts in 10 min. (c) PL spectra of Bi2SiO5、Bi12SiO20 and
Bi2SiO5/Bi12SiO20 (BSO5).
photogenerated charge carriers is greatly inhibited. The low
recombination rate of electron–hole pairs favors the photo
utilization and thereof the photocatalytic degradation of pollutant.
Fig.6 (a) Effect of Bi2SiO5/Bi12SiO20 (BSO5) dosage on AO7 removal,
(b) Effect of initial solution pH on AO7 removal.
Afterwards, the effect of Bi2SiO5/Bi12SiO20 (BSO5) dosage and the
initial solution pH on the AO7 degradation was studied (Fig. 6). As
shown in Fig. 6a, the AO7 removal within 30 min increased from
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3.3. Exploration of catalytic mechanism and detection of the of the atomic electron affinity and the first ionization energy. Ee is
degraded intermediates.
the energy of free electrons vs. normal hydrogen electrode (NHE)
(Ee≈4.5 eV) and Eg is the band gap energy of the semiconductor. As
discussed above, the band gap energies (Eg) of Bi2SiO5 and Bi12SiO20
are estimated to be 3.27 and 2.49 eV, respectively, based on the
UV-Vis DRS result (Fig. 2d). According to Eq. (3), the EVB of Bi2SiO5
and Bi12SiO20 is 2.30 and 2.72 eV, respectively. Therefore, the
In the heterogeneous photocatalysis, the O2 adsorbed onto the
photocatalyst surface is believed to be able to capture the
photogenerated electrons and transfer to superoxide radical (O2·-).
This capture can thus promote the separation of photogenerated
electron and hole pairs and enhance the degradation efficiency of
pollutants. According to Henry’s law, the amount of O2 adsorbed
onto the photocatalyst surface depends on the concentration of
dissolved O2, which relates to the O2 concentration in the gas under
the atmosphere pressure. The effect of O2 concentration was
investigated for the photocatalytic degradation of AO7 using pure
O2 (100%), air (20% O2) and nitrogen (0% O2), respectively. As
shown in Fig. 7a, the kinetics curves of AO7 degradation using
different gas almost overlapped. The type of inletting gas seems to
have no effect on the Bi2SiO5/Bi12SiO20-mediated photocatalytic
degradation. It demonstrates that the separation of
photogenerated electron and hole pairs do not depend on the
capture by oxygen molecules. Moreover, the main active species in
this reaction is not superoxide radical (O2·-). There must be other
active species which result in the degradation of AO7.
To reveal the active species, three active species scavengers,
including tert-Butanol (TB) and ammonium oxalate (AO), and
benzoquinone (BQ) were added into the reaction system,
respectively. It can be seen from Fig. 7b that the addition of BQ
·-
(O2 scavenger) had nearly no effect on the degradation of AO7,
which proved again the inexistence of O2·-. When TB (·OH
scavenger) was added into the reaction system, the photocatalytic
degradation of AO7 decreased slightly compared to the reaction
without any scavenger. It implies that ·OH is involved but not the
main active species in the Bi2SiO5/Bi12SiO20-mediated photocatalytic
degradation. However, with the presence of AO (ꢀꢁꢃꢂ scavenger, ꢀꢁꢃꢂ
represents the photogenerated holes on the valence band) in the
system, the removal efficiency of AO7 decreased from 90.4±1.5% to
14.7±3.5%. The reaction is almost completely quenched if
considering the adsorption contribution. The result demonstrates
that ꢀꢁꢃꢂ is the main active species in the photocatalytic degradation
of AO7. Since the above oxygen trapping experiment cannot prove
the existence of photogenerated electrons, we added the stronger
oxidant Ag+ as the photogenerated electron trapping agent. The
removal efficiency of AO7 within 5 min has already reached as high
as 85.1±2.0%, 25.5% higher than that without any trapping agent.
That is because the trapping of photogenerated electron is an
advantage for the separation of carried pairs. Thus, more holes are
available to degrade AO7. This result again proves the role of
photogenerated holes
photocatalytic degradation.
in the Bi2SiO5/Bi12SiO20-mediated
The valence band top energy positions (EVB) of Bi2SiO5 and
Bi12SiO20 can be determined by using Eq. (3): 22
EVB=X-Ee+0.5Eg
(3)
Fig. 7 Photocatalytic degradation of AO7 by Bi2SiO5/Bi12SiO20 (a)
under different atmosphere of air, N2 or O2, (b) with different
quenchers
， and (c) Schematic diagram of synergy band of
where X is the absolute electronegativity of the semiconductor,
expressed as the geometric mean of the absolute electronegativity
of the constituent atoms, which is defined as the arithmetic mean
Bi2SiO5/Bi12SiO20. Herein Bi2SiO5/Bi12SiO20 is BSO5.
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conduction band bottom energy positions (ECB) of Bi2SiO5 and peak 4, which belongs to AO7, decreased along the time and almost
Bi12SiO20 are calculated to be -0.97 and 0.22 eV vs. NHE according to disappeared at the reaction time of 30 min. In addition, four
Eq. (4):
intermediates appeared at the initial reaction stage. It is worthwhile
mentioning that the high intensity of oxalic acid peak at 5 min
indicates the high oxidation ability of Bi2SiO5/Bi12SiO20-mediated
ECB = EVB − Eg
(4)
photocatalysis system because oxalic acid is
a deep-oxidized
Based on the determined-above results of active species and the
energy band positions, possible mechanism for the high
product. Moreover, the formation of a large amount of oxalic acid
implies that a lot of intermediates were further degraded by the
holes although the intermediates still existed at the reaction time of
a
photocatalytic activity of the Bi2SiO5/Bi12SiO20 photocatalyst is
proposed in Fig. 7c. Under the light irradiation, both Bi2SiO5 and
Bi12SiO20 adsorb photons, then the electrons transferred to the ECB
and the holes are left at the EVB. The photogenerated electrons
easily migrate from the ECB of Bi2SiO5 to that of Bi12SiO20 because
the former has a more negative ECB (-0.97 eV) than the latter (0.22
eV). Simultaneously, the holes on Bi12SiO20 are readily transferred
to Bi2SiO5 due to the more positive EVB (2.72 eV) of Bi12SiO20
compared to that of Bi2SiO5 (2.3 eV). Therefore, the photogenerated
electrons and holes are separated spatially and their combinations
are hindered. More long-lived holes accumulate at the EVB of
Bi2SiO5, which directly oxidized the pollutants. Hence, the
30 min. The results demonstrate that the Bi2SiO5/Bi12SiO20
-
mediated photocatalysis can effectively degrade AO7 and its
products.
photocatalytic activity enhanced
a lot by forming such a
heterojunction. However, many literatures reported that
superoxide radical (O2·-) formed after the oxygen adsorbed on the
surface of the catalysts trapped the photogenerated electrons, and
hydroxide radicals (·OH) produced when photogenerated holes
·-
oxided the adsorbed H2O/hydroxyl groups or reduced O2 by a
·-
multistep process.42-45 Then O2 and ·OH as the reactive species
could oxidize the pollutants. It is obviously not the fact in the
Bi2SiO5/Bi12SiO20-mediated photocatalyic degradation as proved in
Fig.
8 Electropherogram of the photocatalytic degradation
·-
intermediates of AO7 with the catalyst of Bi2SiO5/Bi12SiO20 (BSO5) at
different time.
Fig. 7. As is well known, the reduction potential of O2/ O2 is at
about -0.28 eV vs. NHE while the oxidation potential of ·OH/H2O is
at about +2.27 eV vs. NHE at the solution pH of 7.0. According to
the previous discussion, the ECB of Bi12SiO20 is +0.22 eV vs. NHE and
the EvB of Bi2SiO5 is +2.30 eV vs. NHE. That is to say, the ECB of
Bi12SiO20 is possibly not negative enough to reduce O2 and the EvB of
Bi2SiO5 is possibly not positive enough to oxidize the adsorbed
water. That is the possible reason that ꢀꢁꢃꢂ is the main active
3.5 Photocatalytic degradation of other aqueous pollutants
AO7 is a typical anionic dye, which is herein used as a model
contaminant to probe the photocatalytic performance and
mechanism of Bi2SiO5/Bi12SiO20(BSO5). To evaluate the
photocatalytic activity of Bi2SiO5/Bi12SiO20 more comprehensively, a
typical cationic Rhodamine B dye and the other emerging pollutants
of p- chlorophenol (a neutral and small molecule) and tetracycline
(a neutral and large molecule) were also studied. As shown in Fig. 9,
Bi2SiO5/Bi12SiO20 is also active towards the photocatalytic
degradation of the other pollutants although those molecules have
totally different characteristics. Within the reaction time of 30 min,
the removal efficiencies of all the pollutants except TC reached
about 90%. Even the removal efficiency of TC reached 79.0±2.1%.
The result indicates that the Bi2SiO5/Bi12SiO2-mediated
photocatalytic process can efficiently degrade all kinds of pollutants
with different characteristics, which is also important for the
practice wastewater treatment.
species
in the Bi2SiO5/Bi12SiO20-mediated photocatalytic
·-
degradation of AO7, not O2 and ·OH. Dongfang Hou et. al. also
·-
found that the effect of O2 in the Bi12SiO20 system was nearly
negligible. 46
3.4 Identification of intermediates produced during degradation
Due to the high polarity, non-volatility, and thermal instability of
AO7 and most of its degradation products, the widely-used gas
chromatography (GC) and high-performance liquid chromatography
(HPLC) equipped with mass spectrometry (MS) are not appropriate
for the identification of intermediates during AO7 degradation.
According to our previous report, the degradation intermediates of
AO7
could
be
successfully
identified
by
capillary
electrophoresis/capacitively coupled contactless conductivity
detector33. The result is shown in Fig. 7. During the reaction time of
30 min, four intermediates except AO7 were detected, which are
sulfonic acid (intermediate 1), oxalic acid (intermediate 2)
，
benzenesulfonic acid (intermediate 3), and 4-aminobenzene
sulfonic acid (intermediate 5), as shown in Fig. 8. The intensity of
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The research was supported by the Nature Science
Foundations of China (21677180, 21777196), the Nature
Foundations of Guangdong Province (2014A030313218);
Science and Technology Key Projects of Guangdong Province
(2014B020216004, 2015B020237005), Science and Technology
Research Programs of Guangzhou City (201510010083).
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Conflicts of interest
There are no conflicts to declare.
Acknowledgements
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Catalysis Science & Technology
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DOI: 10.1039/C7CY02272G
Table of Contents Entry
The phase composition and the weight ratio of Bi12SiO20 and Bi2SiO5 in the
heterojunction can be tuned by simply adjusting the CA dosage.