One-pot synthesis of micro/nano structured β-Bi2O3 with tunable morphology for highly efficient photocatalytic degradation of methylparaben under visible-light irradiation

Article abstract of DOI:10.1039/c5ra03200h

β-Bi₂O₃ micro/nanostructures with tunable morphologies were synthesized via a one-pot solvothermal-calcining route, and their photocatalytic activity toward degrading methylparaben (MeP, a widely used preservative with estrogenic activity) was evaluated under visible-light (λ ≥ 420 nm) irradiation. The formation process of β-Bi₂O₃ catalysts can be described as reduction of Bi³⁺ through a solvothermal reaction, followed by oxidization of metal Bi via calcination in air. During this process, the organic reductants (single or a mixture of ethylene glycol, d-fructose, and ascorbic acid) play important roles in determining the final morphologies and structures of the materials. Photocatalytic tests reveal that MeP can be effectively degraded and mineralized by using synthetic β-Bi₂O₃ catalysts, and the reaction rate constant of an optimum sample is more than 25 and 160 times faster than a commercial Bi₂O₃ and synthetic N-TiO₂, respectively. The superior photocatalytic activity of the optimum product is ascribed to its pure beta phase with a narrower band gap, good absorption of visible light, more efficient separation of electrons and holes, relatively higher BET specific surface area, and three-dimensional architectures, which favor more surface active sites and easier mass and photoinduced charge transportations. In addition, the main reactive oxygen species and possible degradation intermediates were detected, and the results suggest that photogenerated holes and superoxide radicals are the predominant species in the photochemical oxidation process.

Full text of DOI:10.1039/c5ra03200h

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Page 1 of 32
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One-pot synthesis of micro/nano structured β-Bi O with tunable
2
3
morphology for highly efficient photocatalytic degradation of
methylparaben under visible-light irradiation
*
Xin Xiao , Ruiping Hu, Shunheng Tu, Chunxia Zheng, Huan Zhong, Xiaoxi Zuo, Junmin Nan
*
School of Chemistry and Environment, South China Normal University; Guangzhou Key
Laboratory of Materials for Energy Conversion and Storage, Guangzhou 510006, PR China
Corresponding author: Xin Xiao
Tel.: +86-20-39310255
Fax: +86-20-39310187
E-mail: xiaox@scnu.edu.cn (X. Xiao); jmnan@scnu.edu.cn (J. Nan)
1

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ABSTRACT
2 3
β-Bi O micro/nanostructures with tunable morphologies were synthesized via an
one-pot solvothermal-calcining route, and their photocatalytic activity toward degrading
methylparaben (MeP, a widely used preservative with estrogenic activity) was evaluated under
2 3
visible-light (λ ≥ 420 nm) irradiation. The formation process of β-Bi O catalysts can be
3
+
described as reduction of Bi through a solvothermal reaction, followed by oxidization of
metal Bi via calcination in air. During this process, the organic reductants (single or mixture of
ethylene glycol, D-fructose, and ascorbic acid) play important roles in determining the final
morphologies and structures of the materials. Photocatalytic tests reveal that MeP can be
effectively degraded and mineralized by using synthetic β-Bi
rate constant of an optimum sample is more than 25 and 160 times faster than a commercial
Bi and synthetic N-TiO , respectively. The superior photocatalytic activity of the optimum
2 3
O catalysts, and the reaction
2
O
3
2
product is ascribed to its pure beta phase with narrower band gap, good absorption of visible
light, more efficient separation of electrons and holes, relatively higher BET specific surface
area, and three-dimensional architectures, which favor more surface active sites and easier
mass and photoinduced charges transportation. In addition, the main reactive oxygen species
and possible degradation intermediates were detected, and the results suggest that
photogenerated holes and superoxide radicals are the predominant species in the photochemical
oxidation process.
2 3
Keywords: Photocatalysis; Visible light; β-Bi O ; Methylparaben; Photodegradation
mechanism.
2

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1. Introduction
Parabens, alkyl and aryl esters of para-hydroxybenzoic acid, are a group of preservatives
widely used in cosmetics, pharmaceuticals, foodstuffs, and other health and personal care
1
products. Human population is easily exposed to parabens from different sources since these
chemicals have been used in over 13,200 formulations developed by various industries,
2
resulting in an approximate global consumption of 8,000 tons annually. In recent years, the
commercial use of parabens has continually growing, in consequence their residues are
3, 4
frequently detected in wastewater, surface water, groundwater, and drinking water, and even
5
in human serum samples. Unfortunately, recent studies have shown that parabens are
6
, 7
endocrine-disrupting compounds (EDCs) with estrogenic activity,
and a possible
8
relationship between breast cancer and the application of parabens is speculated. As a result,
parabens have been classified as emerging environmental contaminants by the U.S.
9
Environmental Protection Agency. Since conventional wastewater treatment plants (WWTPs)
10
cannot fully remove these antimicrobial preservatives, it is essential to develop effective
methods to eliminate these compounds from parabens wastewater.
To date, several methods have been reported in literatures for the treatment of parabens
1
wastewater, including physical adsorption, biodegradation, photodegradation, photolysis
1
12
13
1
4
1
/UV), Fenton-like reactions, ozone oxidation, electrochemical
15
of hydrogen peroxide (H
2
O
2
2
processing, and photocatalytic degradation.
9, 16, 17
Among them, the heterogeneous
photocatalytic process seems to be an effective treatment technique due to it has high
degradation and mineralization efficiency, low toxicity, low cost, and ability to perform under
18
ambient conditions. However, although the photocatalytic decomposition of parabens using
3

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9
, 16, 17, 19
20-22
TiO
2
and ZnO
photocatalysts has been an active field in recent years, to the best
of our knowledge, the photocatalytic degradation of parabens under visible-light irradiation
has not been explored, and the mechanism governing the visible-light-induced photocatalytic
degradation of parabens still remains unknown.
2 3
As the simplest Bi-based oxide, Bi O has captured increased attention due to its
excellent properties, including a wide energy gap range (2-3.96 eV), high refractive index,
23, 24
high dielectric permittivity, marked photoconductivity and photoluminescence.
it has been widely employed in gas sensors for NO and NO detection, optoelectronics
devices, optical coatings, and ceramic glass manufacturing. Bi O has five polymorphic
As a result,
2
2
4
2
3
25
forms: monoclinic α, tetragonal β, body-centered cubic γ, cubic δ, and triclinic ω. Previous
theoretical studies have revealed that in Bi , the top of valence bands (VBs) is mainly
comprised of O 2p and Bi 6s orbitals, while the bottom of its conduction bands (CBs) is
2
O
3
26-29
dominantly constructed by Bi 6p orbital.
2 3
However, although the CBs of Bi O
polymorphs are similar, their structures of VBs are various, and the VB top of β-Bi
2
O
3
shifts
2
to a lower potential, resulting in its smaller band gap. This diversity of Bi O polymorphs in
6
2 3
band structures could result from their different crystal and electronic structures, in which the
Bi-O bond interaction plays a key role. For β-Bi O , the maximal binding is observed for two
2
3
nearest O atoms instead of strong Bi-O interactions, resulting a stronger hybridization of the
O 2p and Bi 6s, 6p orbitals in its VBs, thus the wide band of the O 2p hybridized states is split
3
into subbands to gain a narrower band gap. In addition, the band structures analysis also
0
shown that β-Bi exhibits a more dispersive band structures which is suitable for the
2
O
3
transfer of the photoinduced electrons and holes, and the tunnels in its distinctive crystal
4

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structure can further provide the channels for the photogenerated electrons and holes transfer
2
7
to prevent the excessive recombination of them.
2 3
Consequently, the β-Bi O has been
recommended that as a promising visible-light-driven photocatalyst thanks to its relatively
small band gap, strong absorption of light over the UV-visible range, better separation of
26
photogenerated carriers, high oxidation power, and non-toxic nature.
Until now, the general route for synthesizing β-Bi
decomposition of bismuth-oxide compounds, such as bismuth subcarbonate, bismuth
2
3
O has been the thermal
3
1
3
2
33
oxalates, [Bi38
O
45(OMc)24(DMSO)
9 2 2 2 3 3 1.12
]·2DMSO·7H O, and (H O)0.75Bi (CH COO)(NO ) .
3
4
Recently, we have found that nano β-Bi O can be easily prepared by direct calcination of
2
3
35
nanometallic bismuth at a relatively low temperature in air. It is well known that the
photocatalytic activities of photocatalysts are greatly influenced by their structures and
morphologies. For example, three-dimensional (3D) micro/nanoarchitectures made-up from
nanosized building blocks may provide advantages, such as high surface area,
anti-aggregation ability, and abundant transport paths for the degradation reaction, resulting in
36
their enhanced photocatalytic performances. Therefore, our previous finding may disclose a
new way to obtain β-Bi with desired structures and morphologies by just controlling the
properties of the metallic bismuth precursors, which offers an opportunity to improve the
photocatalytic efficiency of nano β-Bi for efficient contaminants removal.
In this study, micro/nanostructured β-Bi O photocatalysts with tunable morphologies
2 3
O
2
O
3
2
3
were synthesized through a facile one-pot solvothermal-calcining technique, and
methylparaben (MeP), one of the most commonly used parabens, was chosen as the target
pollutant. The morphology, structure, photoabsorption, nitrogen adsorption properties,
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photocurrent responses, and formation process of as-synthesized β-Bi
2
O
3
samples were
systematically characterized and analyzed. Then, the photocatalytic activity and
mineralization efficiency of the as-prepared samples toward the photocatalytic degradation of
MeP under visible-light irradiation were evaluated. In addition, the degradation intermediates
of MeP and photogenerated reactive species in the system were identified, and a possible
photocatalytic degradation mechanism was proposed based on the experimental results.
2. Experimental
2.1. Materials and methods
Methylparaben (purity: 99.99%) and nitroblue tetrazolium (NBT) were purchased from
3 3 2
J&K Chemical Ltd. Bismuth nitrate pentahydrate (Bi(NO ) ·5H O) was obtained from Tianjin
Kermel Chemical Reagent Co., Ltd. Ethylene glycol (EG) was provided by Chinasun Specialty
Products Co., Ltd. D-fructose was purchased from Shanghai Bio Science & Technology Co.,
Ltd. Ascorbic acid (AA) was supplied by Sinopharm Chemical Reagent Co., Ltd. A
commercially available β-Bi
chemicals were of analytical grade and used without further purification.
In a typical synthesis procedure, 0.728 g Bi(NO ·5H O was dissolved completely in 35
2 3
O was purchased from Aladdin Reagent Co., Ltd. All other
3
)
3
2
mL of EG and maintained stirring at room temperature. Then, 0.540 g D-fructose and 0.100 g
AA were sequentially added to the solution. The mixture was then transferred into a
Teflon-lined stainless-steel autoclave and heated at 180 °C for 4 h in an oven. After completing
the solvothermal reaction, the precipitates were harvested by centrifugation, washed with
distilled water and ethanol for three times to remove any ionic residue, and dried in an oven at
6

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60 °C. Finally, the product obtained was calcined in air at 290 °C for 1 h in a muffle furnace.
The sample synthesized with D-fructose and AA was denoted as S1, that synthesized with AA
alone was denoted as S2, and that synthesized with D-fructose alone was denoted as S3, while
the sample synthesized without the addition of D-fructose and AA was denoted as S4.
For comparison, a nitrogen-doped TiO
2
sample was obtained by a solvothermal method
37
used ethylenediamine as the nitrogen source according to the method of Yang et al. .
2.2. Catalyst characterization
The crystalline structures and phase compositions of the as-prepared samples were
determined by powder X-ray diffraction (XRD) on a Bruker D8 Advance X-ray diffractometer
Bruker AXS, Germany) with Cu Kα radiation. The surface morphologies of the samples were
(
studied using a field-emission scanning electron microscope (FE-SEM, S-4800, Hitachi).
UV-Vis diffuse reflection spectra (DRS) were recorded on a UV-Vis spectrophotometer
(UV-3010, Hitachi, Japan) applying an integrating sphere with BaSO as a reference, and were
4
converted to absorption spectra by the Kubelka-Munk method. The specific surface areas and
average pore diameter of the samples were evaluated using nitrogen adsorption-desorption
isotherms according to the Brunauer-Emmett-Teller method (BET, ASAP 2020, Micromeritics,
USA). A desorption isotherm was applied to calculate the pore size distribution using the
Barrett-Joyner-Halenda (BJH) method. Photocurrent measurements were performed on a CHI
660C electrochemical station (Chenhua Instruments Co. Shanghai, China) in a standard
three-electrode configuration containing the as-prepared samples as the working electrodes, a
platinum plate as the counter electrode, and a commercial Ag/AgCl electrode as the reference
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-
1
2 4
electrode, with 0.5 mol L Na SO aqueous solution as the electrolyte. The working
electrodes were fabricated by dispersing a certain amount of samples in ethylcellulose-ethanol
solution, following deposited the suspension onto the surface of fluorine-doped transparent
conductive oxide (FTO) glass with a defined area controlled by plastic insulation, and then
tried in an oven at 60 °C to yield film electrodes. A 300 W xenon lamp (PLS-SXE300/300UV,
Beijing Trusttech Co. Ltd., China) assembled with a UV light cut-off filter (λ > 420 nm) was
used as the visible-light source. The Mott–Schottky experiments were conducted to evaluate
the band structure of as-synthesized samples with an electrochemistry workstation (Princeton
PARSTAT 2273, USA) using a working electrode (an appropriate amount of the sample
suspension on a conductive indium tin oxide glass), a platinum plate as counter electrode, and
-1
2 4
an Ag/AgCl electrode as reference electrode. A 0.5 mol L Na SO aqueous solution was used
as the electrolyte. The potential range was -0.8 to +0.6 V at a constant frequency of 1000 Hz.
Electron paramagnetic resonance (EPR) spectrometry (JES FA-200, JEOL, Japan) was used to
-
record signals of ⋅OH and ·O
2
by the reaction with 5,5-dimethyl-1-pyrroline-N-oxide
(DMPO). The settings for EPR spectrometer were center field of 323 mT, sweep width of 5
mT, microwave frequency of 9055 MHz, power of 0.998 mW, modulation frequency of 100
kHz.
2.3. Photocatalytic activity measurements
Photocatalytic activity tests were conducted in a photochemical reactor (XPA-VII,
Xujiang, Nanjing, China) equipped with a 1000 W Xe lamp together with a 420 nm cut-off
filter as the visible-light source. The experiments were performed as follow: 50 mg of
8

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-1
as-synthesized photocatalyst was added into a 50 mL of MeP solution (10 mg L ). Prior to
irradiation, the suspension was stirred for 1 h in the dark to ensure the adsorption–desorption
equilibrium between photocatalyst powders and MeP. During the photocatalytic process, about
3
mL of the suspension was collected at a specified time; subsequently, the solids were
separated from the solution using a 0.45-ꢀm nitrocellulose filter, and the filtrate was then
analyzed by recording the absorbance at the characteristic band of 255 nm using a UV-Vis
spectroscopy (UV-1800, Shimadzu, Japan) to determine the MeP concentrations. The NBT,
which exhibits a maximum absorption at 259 nm and can be specifically reduced by
superoxide radicals to produce insoluble purple formazan, was chosen as a molecular probe to
-
2
clarify the ·O
generating from as-prepared photocatalysts under visible-light irradiation.
Experiments were carried out using a procedure same as photocatalytic reactions but simply
-1
replaced MeP by 25 mg L NBT solution. The total organic carbon (TOC) content was
determined using an automatic total organic carbon analyzer (TOC-V, Shimadzu, Japan).
2.4. Analysis of the photogeneration intermediates
After the photocatalytic reaction for certain time, sample solutions were collected,
separated, and extracted three times with an ethyl acetate/n-hexane mixture (2:1, v/v).
Afterwards, the organic layers were combined and concentrated to approximately 1 mL under a
gentle stream of high-purity nitrogen. The intermediates were investigated by gas
chromatography-mass spectrometry (GC-MS, DSQ/Trace GC Ultra, Thermo Fisher Scientific,
Dreieich, Germany) on a DB 5-MS column (30 m × 0.25 mm). The GC column temperature
−
was programmed from 50 °C to 250 °C at 5 °C min and maintained at 250 °C for 5 min. The
1
9

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injector and transfer line temperatures were 230 °C. Helium gas was used as the carrier gas at a
−
constant flow rate of 1.2 mL min . The ion MS source temperature was 250 °C, and ionization
1
was performed by electron ionization at 70 eV.
2.5. Calculation of frontier electron density
Quantum calculations using Gaussian 09 were carried out to get the frontier electronic
density (FED) of MeP. Hartree-Fock (HF) theory was used for the full optimization of
geometries, and B3LYP/6-311++G(d,p) was used as the basis set of moderate size and
accuracy. All optimizations were performed without any symmetry restrictions using the
default convergence criteria in the programs and were followed by harmonic frequency
analyses to ensure that the optimized conformation was the true global minimum.
3. Results and discussion
3.1. Characterization of catalysts
Fig. 1. XRD patterns of the as-synthesized samples (A) before and (B) after calcination.
The as-synthesized samples before and after calcination were characterized by XRD to
1
0

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explore their chemical compositions and phase structures. As shown in Fig. 1, all diffraction
peaks of the samples before calcination can be unambiguously indexed as rhombohedral
bismuth (JCPDS No. 85-1329, Fig. 1A), while the samples were found to be tetragonal Bi
2 3
O
(JCPDS No. 78-1793) after calcination at 290 °C (Fig. 1B). Moreover, it can be seen that the
crystallinity of sample S4 before the heat treatment was poor, and some impurity peaks
appeared after the calcination, which was recognized as the existence of a small amount of
monoclinic α-Bi
the sample S4 at a slightly higher temperature (Fig. S1, Electronic Supplementary Information
ESI)). The results of XRD reveal that under the current synthesis route, metallic Bi were
2 3
O (JCPDS No. 71-2274). The analysis can be further verified by calcining
(
2 3
obtained by the solvothermal reaction, then it could subsequently be converted to β-Bi O
after thermal treatment in air.
Fig. 2. SEM images of the as-synthesized samples: (A)-(D) samples S1-S4. The insets show a
high-magnification SEM image of each corresponding sample.
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The morphologies and surface structures of samples S1-S4 were investigated by SEM. As
shown in Fig. 2, the samples overall exhibited agglomerated, irregular sphere-like
morphologies with different grain sizes. Sample S3, S1, and S2 showed apparent diameters of
approximately 1 ꢀm, 5 ꢀm, and 10 ꢀm, respectively, whereas sample S4 showed a greater
diameter of approximately 20 ꢀm. In addition, the samples were observed to be very different
in microstructures. Sample S1 (Fig. 2A) exhibits numerous nanosheets with a thickness of less
than 20 nm, which connect to one another and fabricate three-dimensional microparticles.
Sample S2 (Fig. 2B) consists of numerous nanoparticles aggregated to form spherical
structures. Sample S3 (Fig. 2C) has a large number of individual microspheres with a smooth
surface. And sample S4 (Fig. 2D) also presents a three-dimensional microsphere-like
morphology; however, these microspheres are composed of many sub-micron sheets with a
2 3
thickness about 200 nm. The results suggest that β-Bi O with an adjustable morphology can
be synthesized by an one-pot process with introduce of different additives. It is well known
that there is a close relationship between the size, shape, microstructure, and properties of
materials. Therefore, catalysts with adjustable microstructures are expected to be different
levels of photocatalytic activity.
1
2

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Fig. 3. (A) XRD patterns of the sample S1 precursor after solvothermal reaction for 1, 2, 3,
and 4 h, respectively; (B) SEM images of the sample S1 precursor after solvothermal reaction
for 1 h; (C-F) SEM images of the sample S1-S4 precursors after solvothermal reaction for 4 h,
respectively.
A series of time-dependent experiments has been carried out to explore the possible
formation mechanism of the as-synthesized samples with different morphologies, and the
obtained products were studied by XRD and SEM. The XRD patterns of the precursor of
sample S1 obtained after solvothermal reaction for 1, 2, 3, and 4 h are shown in Fig. 3A and
1
3

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Fig. S2 (ESI). The patterns clearly show that the typical diffraction peaks of metallic bismuth
JCPDS No. 85-1329) simultaneously increased with the reaction time. These results imply
(
3
that a reduction reaction occurs during the solvothermal process, through which Bi is
+
0
gradually converted to Bi . It is noteworthy that in addition to the peaks of bismuth, some
extra diffraction peaks with lower intensity can be also observed in Figs. 3A and S2, including
2
8.4, 32.8, and 47.1 degree. Then these peaks can be identified as BiO2-x (JCPDS No.
7-1057), yet the exact formation mechanism of this species needs further exploration. In
4
addition, the products obtained after a reaction for 1 h (Fig. 3B) or 4 h (Fig. 3C) are not much
different in morphology and exhibit a little change before (Figs. 3C-F) and after calcination
(Figs. 2A-D), suggesting that the shape of the final products is largely determined by their
early aggregated form in the solvothermal reaction.
2 3
From above analysis based on the XRD and SEM, the formation of β-Bi O with different
morphologies may be described by a reduction process by solvothermal reaction and oxidation
3
of Bi metal via heat treatment, which similar to that reported in our previous study. During
5
3
this process, EG may act as a complexing agent reacted with Bi to form glycolate
+
38
3 3
complexes to avoid the hydrolysis of Bi(NO ) . Within a solvothermal process, EG (a polyol
39
that can produce acetaldehyde at high temperature ) alone or mixed with D-fructose (a
ketone-bearing sugar that can readily transform into reducing sugar because ketoenol
40
41
tautomerism ) and AA (based on the reducing properties of its enediol group ) act as
reducing agents to obtain metallic bismuth, and the reaction conditions (reductants and
reaction temperature) critically affect the morphologies and structures of the Bi precursors.
2
After subsequent annealing in an oxygen atmosphere, the metallic Bi reacts with O to form
1
4

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bismuth oxides. At a comparatively low calcination temperature (290 °C), a metastable phase
of bismuth oxide (β-Bi ) can be achieved. The low crystallinity (before calcination) and the
2 3
O
impurity XRD peaks (after calcination) of sample S4 may be attributed to the lower reducing
power of EG alone.
Fig. 4. UV-Vis diffuse reflection spectra (DRS) of samples S1-S4. The upper inset shows plots
2
of (αhν) vs. photon energy (hν), and the lower inset shows photographs of the samples.
It is known that the optical absorption of a semiconductor is one of key factors in
determining its photocatalytic activity. The optical absorption of as-prepared samples S1-S4
was measured using diffuse reflectance spectra (DRS) and compared in Fig. 4. It reveals the
absorption edge of sample S1, S2, and S3 are about 542, 544, and 548 nm, respectively.
Nevertheless, sample S4 presented two absorption bands, and the absorption edge of them can
be estimated to be 466 and 541 nm, respectively, which reflects the multi-phase nature of the
1
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sample S4 (Fig. S1). The results is consistent with their light yellow color (inset in Fig.4) and
suggests they could efficiently absorb visible light. The band gap energies (E ) of the samples
g
4
2
were calculated using the equation (Eq. 1):
n/2
α
(hv) = A(hv − Eg )
(1)
g
where α, ν, E and A are the absorption coefficient, light frequency, band gap energy, and a
constant, respectively, and n depends on the characteristics of the transition in semiconductor.
4
3
According to the literature, the value of n is 1 for the direct transition of β-Bi O . Then the
2
3
2
band gap energies of as-synthesized samples could be derived from a plot of (αhν) as a
function of photon energy (hν) (also inset in Fig. 4) to be approximately 2.44, 2.43, and 2.40 eV
2 3
for samples S1-S3, respectively. The band gap energies of β-Bi O found in this work are close
26
to the value obtained in previous study.
Fig. 5. Nitrogen adsorption-desorption isotherms and the corresponding pore size distribution
curve (inset) of the as-synthesized samples S1-S4.
1
6

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2
N adsorption-desorption isotherms were conducted to investigate the surface area and
pore structures of as-obtained samples S1-S4. As shown in Fig.5, all the isotherms are belong
to type IV with an H3 hysteresis loop as defined by IUPAC, which is characteristic of
4
mesoporous materials. The BET surface areas of the samples S1-S4 were then estimated
4
2
-1
2
using N isotherms to be 5.18, 4.01, 3.66, and 3.88 m g , and the corresponding pore size
distributions were calculated using the BJH method to be 49.2, 46.9, 45.9, and 43.6 nm (inset
in Fig. 5), respectively. The mesoporous structure with wide pore size distribution may arise
from the inter-nanosheet/particle spaces within the samples. The relatively small specific
surface area may be ascribed to the particles aggregation that occurred in these samples after
the calcination.
Fig. 6. Photocurrent transient responses of the as-synthesized samples S1-S4.
It is generally believed that the photocatalytic activity of a catalyst depends on its ability
to generate and separate photo-induced charge carriers. Fig. 6 showed the transient
photocurrent responses via on-off cycles of each sample electrode under visible-light
1
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irradiation, which may be directly correlated with the separation and recombination efficiency
of photogenerated carriers. It can be found that the sample S1 exhibited the highest
photoelectric current response among these samples, which indicates its higher ability of
charge generate and separation. The better separation and transfer of photogenerated charge
carriers of sample S1 may be associated with its high pure β-phase and unique 3D architecture
composed of numerous ultrathin 2D nanosheets which allow for faster electron transport.
3.2. Photocatalytic degradation of methylparaben
Fig. 7. (A) Photocatalytic degradation kinetics of MeP over the as-synthesized samples S1-S4,
commercial Bi (C-Bi ), N-doped TiO (N-TiO ), and a blank sample (without catalyst)
under visible-light irradiation. (B) Linear plots of ln(C /C ) versus degradation time.
2
O
3
2
O
3
2
2
0
t
The photocatalytic activities of the as-fabricated samples S1-S4 were evaluated through
the degradation of MeP solution and compared with that of direct photolysis (Blank), a
N-doped TiO
2
, and a commercially available β-Bi
2
O
3
sample under visible-light irradiation.
vs.
Fig. 7A shows the degradation efficiencies of the MeP solution by the spots of C/C
0
1
8

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irradiation time, where C is the concentration of the remaining of MeP, while C
0
is the initial
concentration of MeP solution after adsorption equilibrium. It is obvious that MeP was hardly
degraded by direct visible-light irradiation (Blank). When submitted to identical irradiation
conditions for 60 min, the N-doped TiO sample showed only 4.2% degradation and the
2
commercial β-Bi
S1-S4, on the other hand, exhibited much higher degradation efficiencies of 99.9%, 99.6%,
8.9%, and 48.2%, respectively, demonstrating that the as-synthesized samples S1-S3
2 3
O sample presented a degradation of 28.2%. The as-synthesized samples
9
possessed excellent visible-light-induced photocatalytic performances. And among them,
sample S1 revealed the best activity. To estimate the influence of samples’ surface area on the
MeP removal. A profiles of MeP elimination by combination of photodegradation and
dark-adsorption was presented in Fig. S3 (ESI). The results showed that the adsorption capacity
of the samples consistent with the trend of their corresponding specific surface area, and the
sample S1 has a maximum adsorption of 19% before photodegradation, which is benefit its
photocatalytic performance.
Furthermore, the kinetics of MeP photodegradation over the as-synthesized samples,
2 3 2
commercial β-Bi O , and N-TiO can be further investigated by fitting the experimental data to
45
the following pseudo-first-order kinetics equation (Eqn. 2):
Ct
−
ln( ) = k t
(2)
app
C0
where kapp (min ) is the apparent reaction rate constant determined by plotting ln (C
reaction time (t), t is the reaction time, C and C are the initial concentration and the residual
-1
0 t
/C ) vs. the
0
t
concentration of MeP, respectively. As shown in Fig. 7B, all the degradation processes were
fitted well by the pseudo-first-order rate model (Eqn.2) with correlation coefficients higher than
1
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0
.99. The removal rate constant (kapp values) of MeP degradation over the samples S1-S4,
C-Bi , and N-TiO were calculated to be 0.126, 0.060, 0.034, 0.011, 0.005, and 0.00078
min , respectively. Namely, the reaction rate constant of MeP on sample S1 is 2.1, 3.7, 11.5,
5.2, and 161.5 times compared to samples S2, S3, S4, C-Bi O , and N-TiO , respectively.
2
O
3
2
-1
2
2
3
2
As is well known, the photocatalytic efficiency of a catalyst is governed by various
factors, such as composition and phase structure, optical absorption properties (band gap
energy), the oxidation potential of photogenerated holes, the separation efficiency of
photogenerated electrons and holes pairs, surface area, and so on. Compared with other
samples, sample S1 had a high pure
β phase, good photoabsorption from UV to visible-light,
fast photogenerated charge separation and interfacial charge transfer, higher specific surface
area with mesoporous feature, and a unique three-dimensional structure composed of
numerous ultrathin 2D nanosheets, resulting in its superior photocatalytic performance for the
degradation of MeP under visible-light irradiation. Then, the sample S1 was used as the
optimum catalyst in the following experiments.
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0

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Fig. 8. TOC removal efficiency of MeP using the as-synthesized
visible-light irradiation.
2 3
β-Bi O (sample S1) under
The removal of total organic carbon (TOC) was chosen as a mineralization index for the
decomposition of MeP in the presence of the sample S1 as catalyst during the photocatalytic
process. The time dependency of the TOC concentration is shown in Fig. 8. It can be seen that
about 66.8% TOC was decomposed to inorganic compounds after visible-light irradiation for
6
0 min, indicating that mineralization reaction is very fast. In addition, compared with the
TOC degradation of ~35% from the same initial concentration of MeP for an identical
9
as a photocatalyst and UV light as a light source, the TOC
irradiation time using nano-TiO
2
removal in current work demonstrates that MeP can be effectively mineralized using
as photocatalyst under visible light irradiation.
2 3
β-Bi O
To examination the stability and reusability of the as-synthesized sample, the sample S1
was reused for three times (once a day for three days) under the same conditions, and compared
with the commercial
β-Bi
2
3
O . As displayed in Fig.S4 (ESI), the results showed that the
recyclability of commercial
β
2 3
-Bi O sample is very poor, which may be ascribed to its structural
46
instability during photocatalytic process, while the sample S1 demonstration a good stability
during three cycles, which is important for its practical application.
3.3. Identification of intermediates and photocatalytic mechanism
2
1

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2 3
Fig. 9. GC-MS chromatograms of MeP degradation over an as-synthesized β-Bi O sample
(sample S1) under visible-light irradiation.
To explore the degradation intermediates of MeP, during the photocatalytic process, the
reaction solution was separated, extracted, concentrated, and then examined with GC/MS
technique. As shown in Fig. 9, all components with retention times (RT) less than 12 min, MeP
were eluted at a RT of 9.23 min and disappeared rapidly after the photoreaction, which is in
accordance with the UV-Vis spectroscopy data. During the degradation process, four
intermediates including tartaric acid (RT=3.81), phenol (RT=4.71), 6-hydroxyhexan-2-one
(RT=5.18), and 4-methylpentanoic acid (RT=5.98) were identified. Previous reports on the
degradation of MeP using advanced oxidation processes (AOPs) have suggested that the
degradation pathways proceed mainly through the hydroxylation of MeP due to the presence of
1
, 2, 9, 15
reactive hydroxyl radicals (⋅OH).
2 3
However, in this visible-light-induced β-Bi O
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photodegradation system, no hydroxylated compounds were identified, which implies it has a
different mechanism of photocatalytic oxidation of MeP.
2 3
Fig. 10. (A) Photodegradation of MeP over an as-synthesized β-Bi O sample in the presence
3
of different scavengers: no scavenger, isopropanol, sodium oxalate, KBrO , TEMPOL, and
-
2
N
2
. (B) DMPO spin-trapping EPR spectra for DMPO-⋅OH and DMPO-·O
under visible-light
irradiation with a β-Bi O sample.
2
3
To investigate the role of the major reactive species on the photodegradation reaction, the
surface radicals and holes trapping experiments were performed by adding individual
scavengers to the photodegradation system with the method similar to the former
photocatalytic activity experiments. Among them, isopropanol (IPA) was used to scavenge
+
OH, sodium oxalate was used to scavenge photogenerated h , KBrO
•
3
was used to scavenge
-
photogenerated e , 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy (TEMPOL) was used to
-
47
scavenge •O
2
, and N
2
was used to remove oxygen that dissolved in the solution.
As
illustrated in Fig. 10A, the photocatalytic degradation efficiency of MeP has no obvious
decrease after adding isopropanol as a sacrificial donor. However, recognizable inhibitions of
3
the photocatalytic degradation were occurred when performed in the presence of KBrO ,
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2
TEMPOL, sodium oxalate, or N , suggesting the significance of superoxide anion radicals and
photogenerated holes in this process. This conjecture may be further confirmed by the EPR
-
method (Fig. 10B), which reveals strong DMPO-·O
irradiation.
2
-adduct signals after visible-light
It is generally believed that the conduction band (CB) minimum of Bi
2
O
3
polymorphs are
In order to confirm this inference, the flatband potentials of as-prepared
samples S1 and S2 were determined from Mott-Schottky measurements. As shown in Fig. S5
ESI), the flatband potentials of sample S1 and S2 are estimated to be -0.28 V and -0.26 V vs.
2
9, 48
close to NHE.
(
Ag/AgCl (-0.09 V and -0.07 V vs. NHE), respectively. Thus, the low conduction band
-
potential of
β
-Bi
2
O
3
seems cannot reaction of O
2
to produce ·O
2
(-0.33 V vs. NHE). However,
due to the valence band potential of
β
-Bi
2
O
3
is about 2.3 V vs. NHE according to the above
analysis. Under λ ≥ 420 nm (E ≤ 2.95 eV) light irradiation, some of electrons can be excited
to be more negative potentials to reaction with oxygen to generate superoxide radicals. To
further confirm this hypothesis, the sample S1 calcined at 290 °C (
α-Bi ) were used to photodegrade NBT to determine the amount of ·O
can be observed from the Fig. S5 (ESI) that after 120 min of visible-light irradiation, about
β-Bi O ) and 500 °C
2 3
-
49
(
2
O
3
2
generating. As
2
2 % of NBT was degraded over sample S1 (calcined at 290 °C), confirming the generation of
-
photoelectrons and ·O
2
. For comparison, the NBT hardly be degraded by α-Bi
2 3
O sample
(
calcined at 500 °C) after identical process, which is related to its more positive valence band
potential (~ 2.8 V vs. NHE).
Consequently, the photocatalytic degradation of MeP using the as-prepared
photocatalyst under visible-light may be dominated by directed holes and superoxide anion
β
2 3
-Bi O
2
4

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radicals oxidation rather than reaction with ·OH. This phenomenon may be explained by the
5
standard redox potential of Bi /Bi (E = 1.59 V at pH 0) is more negative than that of
+
3+
0
-
OH/OH (E = 1.99 V at pH 0). Therefore, the photogenerated holes on the surface of catalysts
0
⋅
50
could not oxidize H
2
O to yield ⋅OH.
Fig. 11. The molecular structure of MeP with atoms labeled.
Table 1. The frontier electron density and atom point charge of MeP
2
2
2
Atom label
2FED HOMO
FED HOMO+FED LUMO
C
C
C
C
C
C
1
2
3
4
5
6
0.314
0.133
0.087
0.339
0.246
0.102
0.093
0.190
0.077
0.012
0.015
0.695
1.149
0.703
0.881
2.190
1.454
0.063
0.316
0.158
0.056
0.063
O
11
C
13
O
O
14
15
C
16
2
5

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A theoretical calculation of the frontier electron density (FED) of MeP (Fig. 11) was
employed to promote understand the photodegradation intermediates. According to frontier
orbital theory, electrophilic, nucleophilic, and radical reactions may occur at different atomic
2
of a molecular: the positions with higher values of FED HOMO+FED
2
are more
LUMO
2
51
susceptible to a radical attack, while positions with higher value of 2FED HOMO are more
52
easily subject to an electron extraction. The calculated results are summarized in Table 1,
2
which clearly suggests that the highest 2FED HOMO value of MeP occurs at C
4
. Consequently,
C
4
may be the first site from oxidized by photogenerated holes. The formed phenols can be
further oxidized, resulting in the breakdown of aromatic structures subsequently production of
carboxylate acid and evolution of CO . Eventually, the MeP is mineralized into inorganic
molecules through the photodegradation process.
2
4. Conclusions
In this work, we have synthesized micro/nano
β
-Bi O materials with tunable morphology
2 3
via a facile one-pot solvothermal-calcining process using EG, AA, and D-fructose as mild
organic reducing agent. XRD and SEM observation of the samples before and after the
2 3
calcination indicated that the β-Bi O were developed by reduction of Bi ions to form metallic
bismuth precursors with different morphologies by a solvothermal reaction. Thereinto, EG
acts as a solvent and complexing agent, EG, AA, and D-fructose serve as reducing agents as
well as structure-directing agents, and followed by oxidization of metal Bi after the
2 2 3
calcination in air, in which O serves as an oxidizing agent. The as-synthesized β-Bi O
samples exhibited effective photoabsorption from UV to visible-light regions and high
2
6

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photogenerated charge separation ability, resulting in their excellent photocatalytic properties
in the degradation of MeP under visible-light irradiation. Besides, photogenerated holes and
superoxide radicals were confirmed to be the governing reactive oxygen species in the
photodegradation and mineralization of MeP.
Acknowledgments
This work was financially supported by the National Natural Science Foundation of China
(No. 21477040), the Natural Science Foundation of Guangdong Province of China (No.
S2012040007074), and the Scientific Research Foundation of Graduate School of South
China Normal University (2014ssxm29)
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