Hierarchical h-, m- and n-BiPO4 microspheres: Facile synthesis and application in the photocatalytic decomposition of refractory phenols and benzene

Article abstract of DOI:10.1039/c7ra06560d

BiPO₄ microspheres with three kinds of crystal phase (h-BiPO₄, m-BiPO₄ and n-BiPO₄) were prepared by a facile hydrothermal-calcination process. The crystalline phase, microstructures and photoelectrochemical properties were studied by various techniques. The results indicated that h-BiPO₄ could transform into m-BiPO₄ at 600 °C. Interestingly, h-BiPO₄ accompanied with a little bit of n-BiPO₄ would transform into n-BiPO₄. After UV irradiation for 2 h, conversion of 94.6% and a mineralization rate of 76.8% could be achieved on n-BiPO₄ for photocatalytic degradation of bisphenol A, which is almost 1.33 times as high as that of m-BiPO₄ and 3.14 times that of h-BiPO₄. Moreover, the n-BiPO₄ also displayed the highest photocatalytic activity for degradation of other phenols (phenol and p-chlorophenol) in water and gaseous benzene. The enhancement of activity could be attributed to the improvement of the photogenerated electron-hole separation efficiency, which would be conducive to the formation of active species (hydroxyl radicals (OH) and superoxide radicals (O₂^-)). The results suggest that n-BiPO₄ shows potential applications as an efficient and stable photocatalyst for the photocatalytic decomposition of persistent organic pollutants in water and in the gas phase.

Full text of DOI:10.1039/c7ra06560d

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PAPER
Hierarchical h-, m- and n-BiPO4 microspheres:
facile synthesis and application in the
photocatalytic decomposition of refractory
phenols and benzene†
Cite this: RSC Adv., 2017, 7, 36705
a
bc
Tong Xu,b Yunjian Wangb and Sugang Meng
*
Xiaoxin Tian,
BiPO4 microspheres with three kinds of crystal phase (h-BiPO4, m-BiPO4 and n-BiPO4) were prepared by
facile hydrothermal-calcination process. The crystalline phase, microstructures and
a
photoelectrochemical properties were studied by various techniques. The results indicated that h-BiPO4
could transform into m-BiPO4 at 600 ꢀC. Interestingly, h-BiPO4 accompanied with a little bit of n-BiPO4
would transform into n-BiPO4. After UV irradiation for 2 h, conversion of 94.6% and a mineralization rate
of 76.8% could be achieved on n-BiPO4 for photocatalytic degradation of bisphenol A, which is almost
1.33 times as high as that of m-BiPO4 and 3.14 times that of h-BiPO4. Moreover, the n-BiPO4 also
displayed the highest photocatalytic activity for degradation of other phenols (phenol and p-
chlorophenol) in water and gaseous benzene. The enhancement of activity could be attributed to the
improvement of the photogenerated electron–hole separation efficiency, which would be conducive to
the formation of active species (hydroxyl radicals (cOH) and superoxide radicals (cO2ꢁ)). The results
suggest that n-BiPO4 shows potential applications as an efficient and stable photocatalyst for the
photocatalytic decomposition of persistent organic pollutants in water and in the gas phase.
Received 12th June 2017
Accepted 20th July 2017
DOI: 10.1039/c7ra06560d
rsc.li/rsc-advances
under UV light irradiation. Moreover, the photocatalytic activity
of BiPO4 could be enhanced remarkably by decreasing the grain
size,14 building phase junction,15 exposing high-energy facet16 or
1. Introduction
As one of the most promising technologies for environmental
remediation (wastewater and exhaust gas), photocatalysis has
been intensively studied because of its environmental merits
and the capability to be driven by solar light.1–4 Since Fujishima5
and Carey6 found that H2 could be generated and organic
pollutants could be degraded over TiO2 by a photocatalytic
process, TiO2 has been deemed as a candidate due to its non-
toxicity, stability and high activity.1 However, the shortcomings
of rapid recombination of photogenerated carriers and low
quantum efficiency hinder its extensive application in envi-
ronmental remediation.7–9 To solve the problem, exploitation of
new photocatalysts with excellent activity is one of the devel-
oping directions of photocatalysis.10–12 In numerous photo-
catalysts, BiPO4 has drawn much great attention in recent
years.13–24 For example, Zhu's group13 reported that the photo-
catalytic activity of BiPO4 is superior to P25 (TiO2, Degussa)
coupling with other semiconductors.17 It is not only efficient for
decoloration of dyes, but also diverse in crystal phase (hexagon
(h-BiPO4) and monoclinic phase with space group P21/m (m-
BiPO4) and P21/n (n-BiPO4)).18–20 What's more, Zhu's group has
demonstrated that n-BiPO4 displayed higher activity than m-
BiPO4 and h-BiPO4 due to the most distorted PO4 tetrahedron.20
Therefore, it is a promising photocatalyst and has much
potential for environmental remediation in air and water by the
direct absorption of light.21 However, the present studies mainly
focus on the degradation of dyes (methylene blue,13–17 methyl
orange22 and rhodamine B17,23), while dyes are not suitable used
to test the photocatalytic activity of photocatalyst because dyes
have the dye-sensitized effect and an additional and more
substantial problem.4 On the other hand, the colorless, highly
toxic and refractory environmental endocrine disruptors and
gaseous benzene were less reported.24 And the photocatalytic
activity is not high enough, for example, less than 20% of
benzene was degraded on BiPO4 photocatalyst under UV light
irradiation for 12 h.24a Therefore, fabrication of BiPO4 with
excellent photocatalytic activity on degradation of environ-
mental endocrine disruptors and gaseous benzene are signi-
cant foci on BiPO4 photocatalyst. It should be noted that the
reason why the UV-light driven BiPO4 photocatalyst (the band-
aCollege of Chemistry and Chemical Engineering, Xinxiang University, Xinxiang
453003, Henan, China
bCollege of Chemistry and Materials Science, Huaibei Normal University, Huaibei
235000, Anhui, China. E-mail: mengsugang@126.com; Fax: +86-561-3090518; Tel:
+86-561-3802235
cAnhui Key Laboratory of Energetic Materials, Huaibei 235000, Anhui, China
† Electronic supplementary information (ESI) available. See DOI:
10.1039/c7ra06560d
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gap energies are in the range of 3.8–4.6 eV) was chosen in our microwave (rated output of microwave is 800 W), denoted as S-
work is not only because what we have introduced on the above MH. All the obtained BiPO4 samples (S-OH and S-MH) were
but also because UV-light driven photocatalysis has potential washed several times with distilled water and fully dried at 80 ꢀC
use in the actual water industry.3 This is primarily due to the in oven. For further analysis, the prepared products S-MH and
ꢀ
exorbitant occupied area that would result in replacing UV lamp S-OH were calcined in a muffle furnace at 600 C for 2 h. The
reactors with solar irradiation. For instance, how much solar obtained samples were called S-MHT and S-OHT, respectively.
collection area is required to replace one 1000 W mercury lamp?
2.6 m2 of solar exposure would be needed to achieve the treat-
2.2. Characterization
ment power of one lamp. If for industrialization, this area would
X-ray powder diffraction (XRD, Bruker D8 Advance, Cu Ka
quickly add up due to the energy input requirements.3
radiation) was used to characterize the crystalline and phase
Herein, BiPO4 with three different crystal phases (h-BiPO4,
constitutions of the samples. The morphology was analyzed by
m-BiPO4 and n-BiPO4) and similar morphology (spherically
scanning electron microscopy (SEM, FEL Noval NanoSEM 230).
hierarchical structure) were controllably synthesized by a facile
Transmission electron microscopy (TEM) and high-resolution
hydrothermal-calcination method. Various characterization
transmission electron microscopy (HRTEM) images were
techniques were carried out on the as-prepared BiPO4 to obtain
taken on a JEOL-2100 electron microscope operating at an
the information of crystal phase, micro-structure and optical
accelerating voltage of 200 kV. The UV-vis diffuse reection
properties. Bisphenol A (BPA) was selected as a model pollutant,
spectroscopy (DRS) patterns were measured by a Hitachi UV-365
because it is a representative material among environmental
spectrophotometer equipped with an integrating sphere, and
endocrine disruptors, ubiquitous in natural water, colorless,
BaSO4 as a reference sample coated standard pattern. The
hard to be decomposed and has attracted much attention in
photoluminescence (PL) spectra were recorded on a JASCO FP-
recent years.25–27 For example, A. Omoike reported Heliscus
6500 with an excitation wavelength of 256 nm. The photo-
lugdunensis, an aquatic fungus, could degrade over 70% of BPA
electrochemical analysis was performed in a conventional
aer 12 days (microorganism method).27c Zhu's group reported
three-electrode cell (CHI-660E, Chenhua Instruments Co.,
that the maximum adsorption capacity of graphene for decon-
Shanghai, China), using a Pt wire and an Ag/AgCl electrode as
tamination of BPA could reach up to 128 mg gꢁ1, whereas the
the counter electrode and reference electrode, respectively. The
adsorption capacity of graphene is sensitive to pH and
working electrode was prepared on indium-tin oxide (ITO) glass
temperature of the solution (adsorption method).27d For pho-
which was covered by 0.5 cm ꢂ 0.5 cm samples. The electrolyte
tocatalytic method, although some photocatalysts such as Pd/
was 0.1 M KCl aqueous solution containing 0.01 M K3[Fe(CN)6]–
mpg-C3N4 (ref. 27a) and Ag3PO4 (ref. 27b) exhibited high activity
K4[Fe(CN)6].
with 93.9% and 100% of BPA removal from the solution within
180 min and 10 min, respectively. The removal efficiency of
noble metal-free photocatalysts is still low. For instance, only
2.3. Photocatalytic tests
6% and 6.7% of BPA was degraded over g-C3N4 and TiO2 aer The photocatalytic activities of the synthesized samples were
irradiation for 180 min, respectively.27a Therefore, it calls for the tested by the degradation of phenol, p-chlorophenol (4-CP) and
development of more economical, powerful and durable pho- bisphenol A (BPA) aqueous solution and degradation of gaseous
tocatalyst for the removal of BPA. Up to now, there is scarce benzene (C6H6). The photocatalytic reaction system has been
report about BPA degradation over BiPO4 photocatalyst. The reported in our previous work.12 The degradation of phenols
results showed that n-BiPO4 photocatalyst displayed the highest was operated in a tubular reactor, which was surrounded by
activity and separation efficiency of photogenerated electron– a water cooling jacket. A low pressure mercury UV light (H-
hole pairs among these three BiPO4 photocatalysts. Moreover, shaped, 16 W, a monochromatic emission at 254 nm) was
the mechanism of inuence on the activity of BiPO4 photo- assembled at the center of the tubular reactor. 0.1 g sample was
catalyst was also discussed. This work attempts to explore the put into 150 mL phenol, p-chlorophenol or bisphenol A aqueous
potential application of BiPO4 for photocatalytic degradation of solution (20 mg Lꢁ1) and then magnetically stirred in dark for
refractory phenols (BPA, including phenol and 4-chlorophenol 30 min to establish an adsorption–desorption equilibrium.
(4-CP)) and decomposition of benzene in the gas phase.
Aer that, a 4 mL solution was sampled at various illumination
time intervals and analyzed through a UV-vis spectrophotom-
eter (Persee TU-1950). Photocatalytic degradation of benzene
was carried out in a xed-bed tubular quartz reactor (250 mm ꢂ
4 mm), in which 0.1 g sample mixed evenly with 1 g 50–70 mesh
2. Experimental
2.1. Synthesis
All the chemicals used in this work were analytical grade high purity quartz sands was loaded, surrounded by four UV-
reagents without any further purication. In a typical synthesis, 254 nm lamps (TUV 6W/G6 T5, Philips) and operated in a single-
1.03 g polyphosphoric acid and 0.97 g bismuth nitrate were pass mode. Benzene vapor with a constant concentration of
successively dissolved in 20 mL deionized water. Then, the 250 ppm was supplied by a gas cylinder (Dalian Da'te gas Co.
resulting solution dealt with two different heating ways to get China) and fed to the sample a total ow rate of 30 mL minꢁ1
.
BiPO4 samples. The rst one is hydrothermal way that the The reactor temperature was kept at 30 ꢃ 1 ꢀC by an air-cooling
ꢀ
beaker was heated to 160 C and kept at this temperature for system and circulating water. Simultaneous determination of
1 h, which was marked with S-OH. The second was heated by the concentrations of residual C6H6 and generated CO2 was
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performed on an online gas chromatograph (Agilent 7820A)
The morphology, particle size and micro-structure of the
with a ame ionization detector (FID) and a thermal conduc- as-prepared samples were investigated by SEM and TEM
tivity detector (TCD), respectively.
(Fig. 2). As shown in Fig. 2a and e, the morphology of S-MH is
the solid sphere with the diameter of 1–3 mm, which is further
composed of tiny nanoparticles. Aer heat treatment of S-MH,
S-MHT is also kept sphere shape, while its surface is rugged
(Fig. 2b and f). This may be caused by sintering of tiny S-MH
nanoparticles under high temperature calcination. HRTEM
images of S-MH (Fig. 2i) and S-MHT (Fig. 2j) display the
resolved lattice spacings of 0.28 nm and 0.32 nm, which cor-
responding to the d-spacing values for (102) plane of h-BiPO4
and (101) plane of m-BiPO4, respectively. It is in good agree-
ment with the XRD results. For S-OH, its morphology is very
interesting. As shown in Fig. 2c and g, the shape of S-OH is like
sea urchin. It shows spherically hierarchical micro-structure,
which is built by some connected nanoparticles, some nano-
rods like needle inserted into the sphere partly and pores.
While for S-OHT (Fig. 2d and h), the sphere is different from S-
OH. It is composed of nanorods not just in the surface, but
some shortly scattered nanorods inside sphere, and some
spheres are broken. From Fig. 2k and l, it can be observed that
the fringe spacings detected at nanorods of S-OH and S-OHT
are about 0.32 nm and 0.47 nm, corresponding to the (200)
and (011) lattice planes of n-BiPO4, respectively. In addition,
the lattice fringes of S-OHT are very clear, indicating high
degree of crystallinity. It is in good agreement with the results
of the XRD analysis. Based on XRD and the morphology
transformation analysis between S-OH and S-OHT, it can be
concluded that the nanoparticles with hexagonal phase in S-
OH would be transformed into nanorods with monoclinic
phase aer calcination at 600 ꢀC. S-OHT is the pure mono-
clinic phase of BiPO4 with P21/n space group, lattice constants
3. Results and discussion
3.1. Characterization
Fig. 1 shows the XRD patterns of S-MH, S-MHT, S-OH, S-OHT
and the standard diffraction peaks of h-BiPO4 (JCPDS 45-
1370), m-BiPO4 (JCPDS 43-0637) and n-BiPO4 (JCPDS 15-0767).
It can be seen that the crystalline phase of S-MH is hexagonal
phase with the main diffraction peaks at about 14.6ꢀ, 20.1ꢀ,
25.5ꢀ, 29.5ꢀ, 31.3ꢀ and 41.8ꢀ, which could be indexed to the
(100), (101), (110), (200), (102) and (211) planes of h-BiPO4,
respectively. No impurity peaks are found, indicating high
purity of the sample S-MH. Aer calcination at high tempera-
ture, the crystalline phase of S-MHT is converted into m-BiPO4,
and all the diffraction peaks of S-MHT are in good agreement
with m-BiPO4. However, the sample S-OH synthesized by
a hydrothermal method is a mixture of h-BiPO4 and n-BiPO4.
The peaks at 14.6ꢀ and 20.1ꢀ belong to (100) and (101) planes of
h-BiPO4 can be observed obviously. It is clear that the XRD
pattern of S-OHT corresponds to n-BiPO4 can be obtained by
calcination of S-OH at 600 ꢀC, and no impurity peaks are found.
It is interesting to nd that the h-BiPO4 (S-MH) is easily trans-
formed into m-BiPO4 (S-MHT). But if it is a mixture of h-BiPO4
and n-BiPO4 (S-OH), it would be converted into n-BiPO4 (S-
OHT). The existence of n-BiPO4 would play the part of seed
crystal. Moreover, the sharp and intense XRD peaks suggest that
the as-prepared samples S-MH, S-MHT and S-OHT are well-
crystallized.
˚
a ¼ 6.752, b ¼ 6.933 and c ¼ 6.468 A, and the spherically
hierarchical micro-structure is composed of some shortly
scattered nanorods. S-MH and S-MHT both exhibit the solid
sphere shape, which is constituted by tiny and sintered
nanoparticles, respectively. While S-MH is the pure hexagonal
phase of BiPO4 with P3121 space group and lattice constants
˚
a ¼ 6.986, b ¼ 6.986 and c ¼ 6.475 A; S-MHT is the pure
monoclinic phase of BiPO4 with P21/m space group and lattice
˚
constants a ¼ 4.883, b ¼ 7.069 and c ¼ 4.704 A.
Fig. 3 shows the UV-vis DRS of S-MH, S-MHT and S-OHT. The
absorption edges of these three samples all occur at about
275 nm. The band gap energy (Eg) is calculated via the Tauc
equation,28–30 (ahn)n ¼ A(hn ꢁ Eg), where A, a, h and n are pro-
portionality constant, absorption coefficient, Planck constant
and light frequency, respectively. The value of the exponent n is
decided from the properties of the transition in a semi-
conductor.21,30 Monoclinic BiPO4 (S-MHT and S-OHT) is an
indirect band semiconductor, the value of n is 0.5. While
hexagonal BiPO4 (S-MH) is direct, the value of n is 2. As shown in
the inset of Fig. 3, the band gaps of S-MH, S-MHT and S-OHT are
about 4.65, 4.54 and 4.49 eV, respectively. The difference among
the band gaps of these three samples may be caused by their
electronic structural difference limited by the crystalline phase,
as also reported by Zhu's group21 and observed in anatase and
brookite titanium dioxide.31
Fig. 1 XRD patterns of the as-synthesized S-MH, S-MHT, S-OH, S-
OHT, together with the standard patterns of h-BiPO4 (JCPDS 45-
1370), m-BiPO4 (JCPDS 43-0637) and n-BiPO4 (JCPDS 15-0767).
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Fig. 2 (a–d) SEM, (e–h) TEM and (i–l) HRTEM images of the as-synthesized (a, e, i) S-MH, (b, f, j) S-MHT, (c, g, k) S-OH and (d, h, l) S-OHT.
Fig. S2.† The results showed that the as-prepared n-BiPO4 could
effectively photocatalytic degrade different concentrations of
BPA (5, 10, 15 and 20 mg Lꢁ1). Additionally, 150 mL BPA with
concentration of 5 mg Lꢁ1 could be eliminated completely over
n-BiPO4 (0.1 g) within irradiation for 30 min. Fig. 4a displays the
degradation proles of BPA with BiPO4 samples. Aer 120 min
of irradiation, 55.5%, 87.9% and 94.6% of BPA are degraded by
the S-MH, S-MHT and S-OHT photocatalysts, respectively. The
outstanding activity of the BiPO4 is further detected by
comparing the apparent reaction rate constants (k). As shown in
Fig. 4b, the degradation of BPA can be described by the pseudo
rst-order reaction kinetics, ln(C/C0) ¼ ꢁkt, where C0 and C are
the initial concentration and the concentration at reaction time
t, respectively. The ks for the S-MH, S-MHT and S-OHT are
estimated to be about 0.42, 0.99 and 1.32 hꢁ1, respectively. The
3.2. Photocatalytic properties
The photocatalytic activities of the as-prepared S-MH, S-MHT
and S-OHT were tested by photocatalytic degradation of BPA,
phenol and 4-CP under UV light irradiation. To investigate the
effect of the amount of photocatalyst on the photodegradation
efficiency of BPA, experiments performed (Fig. S1†) with
different amount of n-BiPO4 exhibited that the photocatalytic
efficiency increases with an increase in n-BiPO4 amount up to
0.15 g, and is then decreased. A similar phenomenon had been
reported by others when studying photocatalytic degradation of
dyes,32 and it can be explained in terms of availability of active
sites on the catalyst surface and the penetration of light into the
suspension.32 To further study the activity of n-BiPO4 for pho-
tocatalytic degradation of low concentration of BPA, the inu-
ence of the BPA concentration was investigated and shown in
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Fig. 3 UV-vis DRS spectra and bandgap energies (the inset) of the as-
synthesized S-MH, S-MHT, S-OH and S-OHT.
Fig. 5 Photocatalytic activities of the samples (0.1 g) for (a) phenol and
(b) 4-CP degradation under UV light irradiation.
k of S-OHT is 1.33-fold and 3.14-fold higher than that of S-MHT
and S-MH. As we all known, degradation does not mean
mineralization, because the original organic pollutants would
be degraded to small organic molecules. Therefore, total
organic carbon (TOC) technique was applied to further measure
the mineralization of the solution, and the results are shown in
Fig. 4c. It is clear that S-OHT exhibits the higher capacity
(76.8%) of mineralization for BPA than S-MHT (65.2%) and S-
MH (25.1%). It means that the reactive species produced on
n-BiPO4 photocatalyst not only can degrade BPA, but also can
efficient decompose BPA into CO2. Moreover, the as-prepared S-
OHT is a stable photocatalyst in the process of photocatalytic
degradation of BPA in water. Aer 3 cycles of the photocatalytic
reaction, the photodegradation efficiency of BPA over the used
S-OHT is similar to that over the fresh sample (Fig. 4d). The
similar result was also reported by Zhu's group that n-BiPO4 is
stable during the photocatalytic degradation of methylene
blue.13
To study the broad-spectrum activity of BiPO4 for other
phenols, phenol and 4-chlorophenol (4-CP) were further
selected as probe molecules to investigate the photocatalytic
activities of the as-prepared BiPO4 samples. As shown in Fig. 5,
whether the degradation of phenol or 4-CP over BiPO4 samples
follows pseudo-rst-order kinetics and the corresponding
kinetic constants are showed in Fig. 5. Moreover, the photo-
catalytic activity for phenol and 4-CP degradation decreases in
Fig. 4 (a) Photocatalytic degradation curves of BPA (20 mg Lꢁ1) for different samples (0.1 g) under UV light irradiation; (b) kinetic plots of ln(C/C0)
vs. irradiation time; (c) mineralization rate of BPA by different samples under UV light irradiation for 2 h; (d) cycling runs of S-OHT for BPA
degradation under UV light irradiation.
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the order S-OHT > S-MHT > S-MH. The sequence is consistent conversion remained 67% with more than 675 ppm CO2
with their performance for BPA degradation. In order to further generated, and the average mineralization ratio of benzene was
illustrate the photocatalytic activity of the photocatalysts, the about 55%, suggesting that most of benzene was mineralized to
dark absorption test and the blank test were carried out. As CO2. In the following 3 h, the S-OHT sample displayed no
shown in Fig. S3a,† the dark adsorption test in the absence of obvious inactivation. Moreover, benzene could not be decom-
irradiation but with the catalysts showed that no signicant posed under UV-light irradiation, and the S-OHT sample didn't
change in the substrate concentration is found, and an produce CO2 in the dark reaction. The results proved that the
adsorption–desorption equilibrium had been established aer benzene ring was indeed decomposed by the S-OHT.
stir in dark for 30 min. On the other hand, the blank tests
(Fig. S3b†) showed that 11.7% of BPA, 16.6% of phenol and
24.3% of 4-CP were removed aer irradiation for 2 h due to
photo-induced self-photodegradation. However, compared with
the distinctively photocatalytic activities of n-BiPO4 (94.6% of
absorption, surface area, and the separation efficiency of pho-
BPA, 81.6% of phenol and 100% of 4-CP were eliminated aer
irradiation for 2 h), the presence of both photocatalyst and
illumination is necessary for the effectively photocatalytic
3.3. Mechanism of photocatalysis
In general, there are many factors inuencing the photocatalytic
activity of photocatalyst, such as the phase structure, light
togenerated electron–hole pairs.1 It should be noted that the
light source used in this work is monochromatic light with
wavelength of 254 nm. The light absorption of these three
degradation of refractory phenols.
samples is all around 275 nm, as shown in Fig. 3. Moreover, the
The photocatalytic degradation of phenols over S-OHT
BET surface area is all negligible (<1 m2 gꢁ1). Therefore, it may
be due to other factors that inuence the activities of S-MH, S-
MHT and S-OHT. In the previous report, Zhu's group21 found
that the highest photoactivity of n-BiPO4 originates from the
most distorted PO4 tetrahedron, not other factors such as photo
indicates that S-OHT has a great capability to decompose the
benzene ring in the phenols. However, the degradation of
benzene in the gas phase may be different to that in a liquid
phase for the deciency of water, which is crucial for the
elimination of volatile organic compounds. To verify the validity
absorption, BET surface area and the oxidation potential of
of S-OHT on the photocatalytic decomposition of the benzene
ring, the photocatalytic degradation behaviors of gaseous
benzene by BiPO4 samples were carried out with the dry O2
atmosphere under the UV-light irradiation. As shown in Fig. 6,
excluding S-MH, the degradation of benzene can be achieved on
the S-MHT and S-OHT. The initial conversion of benzene on S-
MHT sample was about 80%, with 500 ppm CO2 produced.
carried out in our experiments to investigate the differences of
However, aer 10 h reaction time, the amount of CO2 gradually
dropped to 105 ppm, and the conversion decreased to 13%.
electrons and holes between these samples. As previous
Compared with S-MHT, the conversion of benzene over S-OHT
photoexcited holes. The distorted PO4 tetrahedron facilitates
the separation of photoexcited electron–hole pairs. Therefore,
the photocatalytic activity of BiPO4 is deeply relying on the
separation efficiency of photogenerated electrons and holes. To
further conrm this summing-up, electrochemical impedance
spectroscopy (EIS) and photoluminescence test (PL) were
separation, transfer and recombination of the photoexcited
studies,33,34 the diameters of the EIS plots under light irradia-
tion are supposed to manifest the charge separation and
sample was about 85% with 800 ppm CO2 produced during
the rst half hour. Aer irradiation for 7 h, the benzene
transfer resistance across the solid–liquid junction in the elec-
trode–electrolyte interface region. Besides, the smallest arc
radius indicates most effective separation of the photoexcited
electrons and holes, and vice versa. The typical EIS responses of
BiPO4 with different crystal phase are presented in Fig. 7. The
Fig. 6 (a) The production of CO2 and (b) the conversion and (c)
mineralization efficiencies of C6H6 over prepared S-MH, S-MHT and S- Fig.
7
EIS plots for the as-synthesized samples under UV-light
OHT as a function of irradiation time.
irradiation.
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arc radiuses of prepared BiPO4 samples are different from each to determine the photogenerated radical species cOH, cO2ꢁ and
other. EIS data was analyzed in terms of an equivalent circuit H2O2 over BiPO4 upon the irradiation of light, respectively.36–39
model and the values of charge transfer resistance were about
TA used as the indicator can readily react with the generated
330, 435 and 579 U cmꢁ2 for n-BiPO4, m-BiPO4 and h-BiPO4, cOH producing 2-hydroxyterephthalic acid (HOA), which has
respectively. It indicates that S-OHT with n-BiPO4 phase a good uorescence. The intensity of PL implies the amount of
possesses the highest efficiency of electron–hole separation generated cOH in the reaction system.36,37 As shown in Fig. 9a, it
among these three BiPO4 samples. This is also supported by the is clear that the PL peaks located at about 426 nm for these three
results of PL spectra, which can reect the recombination rate samples are observed, indicating that cOH is produced in the
of photoexcited electros and holes.
photocatalytic reaction process. Moreover, S-OHT displays the
As well known, there are certain intrinsic relationships higher formation efficiency of cOH than S-MHT and S-MH. The
between the PL spectrum and photocatalytic activity of a semi- formation of cOH order is S-OHT > S-MHT > S-MH, which is in
conductor material according to the mechanisms of PL and accordance with the results of the photocatalytic activity and
photocatalysis.35 The lower the PL intensity, the higher the separation of photoexcited carriers tests. Nitroblue tetrazolium,
separation efficiency of photoexcited carriers, and thus the exhibiting an absorption maximum at 259 nm, was used for the
ꢁ
higher the photocatalytic activity.29,35 Fig. 8 presents the PL analysis of cO2ꢁ. NBT can react with the formed cO2 in the
spectra excited at 255 nm wavelength at room temperature. A water solution, and then its absorption peak would be
main emission peak centered at about 301 nm are observed for reduced.38 Thereforeꢁ, NBT method used as a simple approach
all samples. A weaker PL emission intensity than that of S-MH for detection of cO2 has been widely adopted. As shown in
and S-MHT is produced on S-OHT, indicating that the recom- Fig. 9b, the intensity of NBT at about 259 nm decreases grad-
bination of the photoexcited electrons and holes is efficiently ually with the order: S-MH > S-MHT > S-OHT. It indicates that
hampered on S-OHT. The result is in good agreement with the the concentration of NBT in the presence of S-OHT is reduced
ꢁ
EIS. According to the above analysis, the photocatalytic activity greatly. That is more cO2 is produced over S-OHT in the pho-
difference between h-BiPO4, m-BiPO4 and n-BiPO4 is deeply tocatalytic reaction process. Furthermore, the DPD method was
relying on the separation efficiency of photogenerated electrons employed for detecting the existence of H2O2.39 As shown in
and holes. The results further conrm the previous report.21
Fig. 10, in the presence of either S-MH, S-MHT or S-OHT under
It is well known that the photoexcited electrons and holes are UV-light irradiation, two clearly characteristic peaks of existence
the cornerstone of the photocatalysis.1 The efficient separation of H2O2 with absorption maxima at 510 nm and 550 nm are
of photoexcited electrons (eꢁ) and holes (h+) favors the found. Moreover, the production of H2O2 decreases in the order
production of hydroxyl radicals (cOH) and superoxide radicals S-OHT > S-MHT > S-MH. It is in line with the results of TA-PL
(cO2ꢁ), which are the primary and straightforward reactive experiment and the photocatalytic activity test.
species for the photocatalytic degradation of organic pollutants
In order to further understand the roles of the main oxidative
in the water. The photoexcited electrons and holes at the species involved in the decomposition of BPA over BiPO4 pho-
conduction band and valaꢁnce band of BiPO4 can reduce O2 and tocatalyst, the trapping experiments of active species was per-
oxidize OHꢁ to form cO2 ꢁ and cOH, respectively.15 Moreover, formed, using isopropanol (IPA) as cOH scavenger,40
cOH may come from cO2 via chain reactions, and H2O2 is ammonium oxalate (AO) as h+ capturer,41 and benzoquinone
a signicant intermediate species in the process. In order to
conrm the presence of the photoactive radicals in the reaction
system, following experiments were carried out. Terephthalic
acid-photoluminescence (TA-PL), nitroblue tetrazolium (NBT)
and N,N-diethyl-p-phenylenediamine (DPD) methods were used
Fig. 9 (a) The photoluminescence emission spectra of TA solution and
Fig. 8 Photoluminescence spectra of the as-synthesized samples (b) the absorption of NBT solution in the presence of different samples
(excitation wavelength was 256 nm).
under UV light irradiation.
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a simple hydrothermal-calcination method, respectively. The n-
BiPO4 exhibited much higher photocatalytic activity towards
degradation of BPA, phenol, 4-CP and benzene than h-BiPO4
and m-BiPO4. The BPA degradation and mineralization rates
could reach up to 94.6% and 76.8% under UV light irradiation
for 2 h, respectively. And the degradation efficiency is main-
tained at over 94% aer three cycles. The excellent photo-
catalytic activity could be attributed to the distortion of PO4
tetrahedron, which improves the separation efficiency of the
photoexcited electron–hole pairs and produces more oxidative
species. During the photocatalytic process, effect of thꢁe oxida-
tive specie to BPA degradation follows the order of cO2 > h+ >
cOH.
Acknowledgements
Fig. 10 H2O2 was detected by addition of N,N-diethyl-p-phenyl-
enediamine and horseradish peroxidase to S-MH, S-MHT and S-OHT
aqueous solution after UV light irradiation for 20 min, respectively.
This work was supported by the Natural Science Foundation of
Anhui Province (1608085QB37), and the Collaborative Innova-
tion
Center
of
Advanced
Functional
Materials
(XTZX103732016001).
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