One-pot synthesis of Bi3O(PO4)2(OH) embedded on rod-like BiPO4 for efficient adsorption and visible-light photocatalytic reduction of aqueous Cr(VI)

Article abstract of DOI:10.1016/j.jallcom.2021.160518

Photocatalytic reduction of Cr(VI) by visible light remains highly challenging. In this work, the composite of Bi₃O(PO₄)₂(OH) embedded on rod-like BiPO₄ synthesized by one-pot precipitation method. Changing the synthetic conditions have significant roles for controllable morphologies of BiPO₄/Bi₃O(PO₄)₂(OH) heterostructures as well as the photocatalytic efficiency. Under optimal synthetic conditions, BiPO₄/Bi₃O(PO₄)₂(OH) heterostructures showed the highest visible light photocatalytic efficiency, which was nearly 44.5 times that of pure BiPO₄. The BiPO₄/Bi₃O(PO₄)₂(OH) heterostructures exhibited highly effective visible-light photocatalytic reduction of Cr(VI) by the self-photosensitization of adsorbed Cr(VI) combined with the Z-scheme heterojunction charge transfer mechanism under initial pH without adding any sacrificial agent. The report could present a significant approach towards Cr(VI) photoreduction from water through the novel BiPO₄/Bi₃O(PO₄)₂(OH) photocatalyst. It will provide an strategy for environmental wastewater treatment, making full use of solar energy. Moreover, it can contribute to future efforts designing heterostructured semiconductor photocatalysts.

Full text of DOI:10.1016/j.jallcom.2021.160518

Journal of Alloys and Compounds 881 (2021) 160518
Contents lists available at ScienceDirect
Journal of Alloys and Compounds
journal homepage: www.elsevier.com/locate/jalcom
One-pot synthesis of Bi3O(PO4)2(OH) embedded on rod-like BiPO4 for
efficient adsorption and visible-light photocatalytic reduction of aqueous
Cr(VI)
Di Na, Liang Jiang⁎, Mi Li, Tao Lei, Qian Liu, Jiao He, Yongjuan Chen, Wei Wang, Jiaqiang Wang⁎
School of Chemical Sciences & Technology, National Center for International Research on Photoelectric and Energy Materials, School of Materials and Energy,
Yunnan University, Kunming 650091, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalytic reduction of Cr(VI) by visible light remains highly challenging. In this work, the composite of
Bi3O(PO4)2(OH) embedded on rod-like BiPO4 synthesized by one-pot precipitation method. Changing the
synthetic conditions have significant roles for controllable morphologies of BiPO4/Bi3O(PO4)2(OH) hetero-
structures as well as the photocatalytic efficiency. Under optimal synthetic conditions, BiPO4/Bi3O
(PO4)2(OH) heterostructures showed the highest visible light photocatalytic efficiency, which was nearly
44.5 times that of pure BiPO4. The BiPO4/Bi3O(PO4)2(OH) heterostructures exhibited highly effective visible-
light photocatalytic reduction of Cr(VI) by the self-photosensitization of adsorbed Cr(VI) combined with the
Z-scheme heterojunction charge transfer mechanism under initial pH without adding any sacrificial agent.
The report could present a significant approach towards Cr(VI) photoreduction from water through the
novel BiPO4/Bi3O(PO4)2(OH) photocatalyst. It will provide an strategy for environmental wastewater
treatment, making full use of solar energy. Moreover, it can contribute to future efforts designing hetero-
structured semiconductor photocatalysts.
Received 16 February 2021
Received in revised form 19 May 2021
Accepted 21 May 2021
Available online 25 May 2021
Keywords:
BiPO4/Bi3O(PO4)2(OH)
Heterostructures
Visible light
Cr(VI) photoreduction
© 2021 Elsevier B.V. All rights reserved.
1. Introduction
stability, non-toxic and highly crystalline [8]. It is shown that BiPO4
exhibits much higher photocatalytic efficiency compared to com-
Hexavalent chromium (Cr(VI)) is highly toxic to the ecosystem
and living organisms due to its teratogenicity, mutagenicity and
carcinogenicity [1,2]. Various techniques have been developed to
remove Cr(VI) effectively such as electrocoagulation [3], ion ex-
change [4], adsorption [5] and chemical reduction [6]. While, these
methods almost suffer from the drawbacks like generation of sludge,
high cost and incomplete precipitation [7]. The photocatalytic re-
duction of Cr(VI) to Cr(III) is considered as a promising technique
due to its mild reaction conditions, inexpensiveness and high effi-
ciency [7].
TiO2 is one of widely used photocatalyst for photocatalytic re-
duction of Cr(VI). Nevertheless, the low electron transfer rate of TiO2
limits its performance in the photocatalytic reduction process [4].
Bismuth phosphate (BiPO4), a new kind of inorganic non-metal salt
of oxy-acid photocatalyst, is currently eliciting much attention
owing to its outstanding properties including high photochemical
mercial TiO2 on organic degradation as BiPO4 has lower re-
combination rate [9]. Since BiPO4 can be excited by only UV light,
constructing heterostructure such as BiPO4/Bi2S3 [10] and BiPO4-
Polyaniline [11] is an effective way to increase the visible-light re-
sponse of BiPO4. Interestingly, the photoreduction rate for BiPO4/
Bi2S3 synthesized in diethylene glycol was found to be 20.3 times
larger than that for pure BiPO4 under visible light [10]. However, the
acidic environment and sacrificial agent were employed in the re-
action. In contrast to BiPO4, the connection pattern of Bi and O in
Bi3O(PO4)2(OH) [Petitjeanite (BOHP)] has been changed by the in-
troduction of hydroxyl groups [12,13], which is beneficial to improve
adsorption and photocatalytic efficiency of photocatalysts [14–16].
To the best of our knowledge, there is only one report in the pho-
tocatalytic efficiency of BOHP for degradation of perfluorooctanoic
acid under ultraviolet irradiation [17]. So far, the photocatalytic re-
duction of BOHP has not been explored, let alone the photocatalytic
properties of the composite of BiPO4 and BOHP, BiPO4/BOHP.
In this work, BiPO4/BOHP, with heterogeneous structures were
prepared via one-pot precipitation method. The adsorption and
⁎
Corresponding authors.
E-mail addresses: jiangliang@ynu.edu.cn (L. Jiang), jqwang@ynu.edu.cn (J. Wang).
https://doi.org/10.1016/j.jallcom.2021.160518
0925-8388/© 2021 Elsevier B.V. All rights reserved.

D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
Scheme 1. Schematic illustration of the possible mechanism of BiPO4/BOHP heterostructures.
visible light photocatalytic reduction of Cr(VI) were simultaneously
achieved in BiPO4/BOHP heterojunction.
(Zetasizer Nano ZS90, Britain). The spectra of EIS Nyquist were in-
vestigated using an electrochemical workstation (PGSTAT 302N,
Switzerland).
2. Experimental section
2.3. Photocatalytic activity
2.1. Photocatalyst preparation
The photocatalytic activities of the as-synthesized samples
where studies through photocatalytic reduction was carried out in a
quartz tube photoreactor (Nanjing Xujiang, Ltd.) and an inner 350 W
Xe lamp with a filter (λ ≥ 420 nm), utilized for the purpose of al-
lowing only visible-light radiation. The temperature was maintained
at 10 2 °C via circulating cooling water. Photocatalytic reduction of
Cr(VI) aqueous solution (50 mL, 10 mg L−1) was performed with
50 mg of the powdered catalyst suspended in a quartzose test tube.
Before irradiation, the suspensions were stirred for 90 min in the
dark to reach the adsorption-desorption equilibrium. The solutions
were stirred during the reaction process with a magnetic stirrer.
During illumination, about 5 mL of suspension was filtered to se-
parate the catalyst. The Cr(VI) content was determined at 540 nm by
a spectrophotometer (UV722).
All chemicals were of analytical grade and used without further
purification. Firstly, 1.00 g Bi(NO3)3.5H2O, 0.20 g (NH4)2HPO4, 0.55 g
urea, 0.80 g D-Sorbitol and 3.00 mL HNO3 were added into 95 °C
deionized water under vigorous stirring and the white precipitation
was observed. The reaction mixture was heated at 95 °C for 36 h.
After that, it was aged at room temperature for 5 days and the pH
was controlled between 10.5 and 11.0 by using 30% NH3·H2O. The
products washed thoroughly with deionized water and finally dried
(40 °C, overnight) in vacuum. The final chemical composition and
crystal structure of BiPO4/BOHP heterostructures can be adjusted by
tuning the weight ratio of P/Bi, the amount of nitric acid and reaction
duration, which were described in the Supporting Information.
For comparison, two other pure samples were prepared using a
similar method, except under the following conditions: (1) pure
BiPO4 was prepared without using urea and NH3·H2O; (2) pure BOHP
was prepared without using urea and HNO3.
3. Results and discussion
3.1. Formation mechanism
2.2. Characterization
Generally, BiPO4 is synthesized through the hydrothermal
method or precipitation method under strong acid [8]. BOHP is only
synthesized by the hydrothermal method under aqueous alkali
conditions [17]. Unfortunately, it was failed to prepare BiPO4/BOHP
composite by using conventional hydrothermal method via auto-
clave. To the best of our knowledge, this is the first report on the
synthesis of BiPO4/BOHP composite. The new procedure as a com-
bination of three different approaches: The first approach is the
BiPO4 was directly prepared using a precipitation method [18]. The
approach of the synthesis of BiPO4 using a precipitation method
creates the possibility of BiPO4/BOHP composite without using
conventional hydrothermal method. The second method is that urea
can be used to control the morphology of hydroxyapatite in past
study [19]. The method provides a powerful support for controlling
the morphology of BOHP. The third method is that D-sorbitol as a
structure-directing agent can control the morphology of sample for
improving the performance of sample [20]. The above reports sug-
gested a good possibility to the new procedure. Scheme 1 shows the
X-ray diffraction (XRD) patterns were recorded on a Rigaku
TTRAX III spectrometer with Cu Kα radiation (10° min−1 from 10° to
80°). X-ray photoelectron spectroscopy (XPS) measurements were
carried out on a PHI5500ESCA (Thermo Fisher Scientific Inc.,
Waltham, MA, USA) at 200 W Mg Kα radiation and the spectra were
calibrated by the C 1s signal at 284.8 eV. Scanning electron micro-
graphs (SEM) were taken on a FEI Quanta 200 ESEM scanning
electron microscope (FEI, Hillsboro, OR, USA) operated at an accel-
erating voltage of 10 kV. High-resolution transmission electron mi-
croscopy (HRTEM) was taken on a JEM-2100 (Japan Electron Optics
Laboratory CO., Ltd., Tokyo, Japan) with
a 200 kV. UV-Vis
Spectrophotometer, Shimadzu UV-2401PC (Shimadzu Corp., Japan)
used to identify the absorption spectra for each sample with BaSO4
being the background. The photoluminescence (PL) spectra were
detected at room temperature using a F-380 (GangdongSci. & Tech.,
China) with Xe lamp source. Zeta potentials of catalyst particles
suspended in water were measured using Zetasizer Nano ZSP
2

D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
production of BiPO4/BOHP heterostructures, BiPO4/BOHP hetero-
structures were synthesized by the synergetic effect of nitric acid,
urea and NH3·H2O using D-Sorbitol as a structure-directing agent.
Firstly, Bi(NO3)3·5H2O and (NH4)2HPO4 formed into a hollow and
irregular structure, and the materials became solid with the addition
of HNO3. Due to the addition of urea, a dense layer of BOHP was
formed on the BiPO4 surface. The decomposed urea continually ad-
justed the pH of the solution and BOHP continued to grow. Finally,
30% NH3·H2O was added to adjust the pH during aging, and the
material became uniform. Moreover, what differentiates our work
from others is that water was used as solvent instead of using any
organic reagents.
BiPO4/BOHP heterostructures. As far as we know, this is the first
report on the synthesis of BiPO4/BOHP heterostructures by using
facile one-pot precipitation method without using autoclave. The P/
Bi weight ratio induced transformation from pure BOHP to BiPO4/
BOHP, the BiPO4 diffraction peaks can be indexed to a hexagonal
phase (JCPDS, no.15–7066), and finally production of monoclinic
phase pure BiPO4 (JCPDS, no.80–0209) when P/Bi weight ratio was
increased over 20% (Fig. 2b). 95 °C was found a suitable temperature
for the synthesis of BiPO4/BOHP. Moreover, compared with pre-
viously reported synthesis strategies, adding urea helps the control
of the local pH and morphology [19], which finally led to the
synthesis of BOHP. However, the detailed synthesis mechanism is
not completely clear, and further research is needed.
The effect of the amount of nitric acid and urea, D-sorbitol and
reaction time on the BiPO4/BOHP ratio are summarized. Obviously,
the amount of adding nitric acid and D-sorbitol are the key factors to
control the morphology of BiPO4. Though BiPO4/BOHP composite
was prepared even in the absence of D-sorbitol, BiPO4/BOHP mor-
phology was not uniform, and the average length was found to in-
crease from 3 µm to more than 10 µm (Fig. S1), while the BOHP
nanoparticles were maintained its size. Other reaction conditions
remain unchanged, by increasing the amount of nitric acid, hollow
rods BiPO4 was changed to solid rods, and finally to spherical BiPO4
(Fig. 1). As observed from Fig. 2a, the diffraction peaks of BOHP at 2θ
values of 27.4°, 28.0°, 28.4°, 30.2° are observed, which correspond to
the lattice planes of (−220), (−221), (0−21), (−310) of Triclinic BOHP
(JCPDS, no.46-1477), respectively. Meanwhile, with increasing in the
amount of nitric acid, the intensity of BOHP lattice planes gradually
increased. With a further increase in the amount of nitric acid to
5.00 mL, all the diffraction peaks were decreased. Furthermore,
without adding urea, only BiPO4 other than BOHP could be obtained
before adding NH3·H2O (Fig. S2a, c). And after adding NH3·H2O, the
two-phase particulate morphology was not uniform and numerous
irregular structures were observed (Fig. S2b, c). In addition, other
reaction conditions remain unchanged, with increasing in the
amount of urea, all the diffraction peaks gradually increased. With a
further increase in the amount of urea to 0.5 g, all the diffraction
peaks were decreased (Fig. S2d). Consequently, urea not only helps
to the growth of BOHP, but also can control the morphology of BOHP.
With the increase of reaction time, BOHP nanoparticles grew on the
surface of BiPO4 rods. The diameter increased from 30 nm to 90 nm,
and further increase in the reaction time resulted in the formation of
non-regular agglomerates (Fig. S3). Meanwhile, with increasing in
the reaction time, the BOHP diffraction peaks gradually increased,
and reaction time to 48 h, the BOHP diffraction peaks were de-
creased (Fig. S4).
3.2. Characterization
The crystalline phase of as-prepared samples was analyzed by
XRD measurement. As shown in Fig. 3, the pure hexagonal phase
BiPO4 (JCPDS, no.15-0766) was formed without using urea and
NH3·H2O, and the triclinic BOHP (JCPDS, no.46-1477) was formed
without using urea and HNO3. BiPO4/BOHP heterostructures were
synthesized by the synergetic effect of nitric acid, urea and NH3·H2O.
The hexagonal phase BiPO4 (JCPDS, no.15-0766) and triclinic BOHP
(JCPDS, no.46-1477) presence in the BiPO4/BOHP heterostructures,
and the weight percentage of BiPO4 and BOHP were 76.3% and 23.7%,
respectively. No other possible phases were detected.
The BET analysis data of the as-prepared samples are listed in
Table 1 and S5. The N2 adsorption/desorption isotherms of the BiPO4,
BOHP and BiPO4/BOHP heterostructure are similar. The nitrogen
adsorption-desorption isotherms for the samples were all type IV
(Fig. S5). In addition, the textural properties of all as-prepared cat-
alysts were summarized in Table 1. A larger BET surface area, pore
volume and pore size are observed for BiPO4/BOHP heterostructure
in relation to the pure BiPO4 and BOHP. The increased BET surface
area may cause by widening of existing pore volume and creating
new pores after the formation of composite heterostructure [36]. A
larger BET surface area would provide more active sites in the re-
action, resulting in the possibility of the larger adsorption capacity
than pure BiPO4 and BOHP [35].
The morphologies and microstructures were observed by SEM,
TEM and HRTEM. As shown in Fig. 4a and b, pure BiPO4 has a rod-like
morphology. The HRTEM images (Fig. 4c) revealed that the intervals
of 0.442 nm correspond to the (101) crystal planes of hexagonal
phase BiPO4. As shown in Fig. 4d and e, pure BOHP exhibits
1.5–3.0 µm long agminated particles. The HRTEM images (Fig. 4f)
revealed that the intervals of 0.325 nm correspond to the (−220)
crystal planes of triclinic phase BOHP. As shown in Fig. 4g and h, the
heterostructures were identified as BOHP nanoparticle-wrapped
BiPO4 rod-like structure. The enlarged HRTEM image in Fig. 4i in-
dicates that the samples were composed of a bi-component of BiPO4
and BOHP. The lattice distance of 0.349 nm was assigned to the (110)
crystal plane of hexagonal phase BiPO4, while the lattice spacing of
By the addition of both urea and D-sorbitol, the BOHP nano-
particles with an average diameter of about 30 nm embedded on
BiPO4 rods. After reaction 36 h process, BOHP nanoparticles were
converted into BOHP nanorods which were grown on the surface of
BiPO4 rods to form hetero-assemblies. The assembled BOHP na-
norods turned into nanoparticles with an average diameter of
about 90 nm by adding NH3·H2O, and finally production of stable
Fig. 1. SEM images of as-sample V(HNO3) = 0 mL(a), 3.00 mL (b), 5.00 mL (c).
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D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
Fig. 2. XRD patterns of as-prepared samples.
structure (Fig. 5b). A single peak at 133.0 eV was detected in the P 2p
region, corresponding to P(V) in PO43- (Fig. 5c) [21]. In the spectrum
of O 1s (Fig. 5d), thanks to chemisorbed oxygen, the asymmetric
peaks were curve-fitted into two Gaussian feature peaks with a
binding energy of 532.2 eV and 530.3 eV. The former was the surface
O species like -OH, and the latter was crystal lattice oxygen [22].
The UV–vis diffuse reflectance spectra of the as-prepared sam-
ples show that the absorption edges of BiPO4, BOHP and BiPO4/BOHP
at about 305 nm, 315 nm, and 345 nm, respectively (Fig. S7). Corre-
spondingly, the energy band gap of BiPO4 is 3.80 eV, and that of
BOHP is 3.62 eV. In contrast, the energy band gap of BiPO4/BOHP is
3.19 eV (Fig. 6a). This indicated that the synthetic strategies did
narrow the band gap of BiPO4 and BOHP. Thus, the narrower energy
band gap could facilitate charge transferring among hybrid struc-
tures. Fig. 6b shows the photoluminescence spectra of as-prepared
samples at room temperature. The PL of BiPO4, BOHP, and BiPO4/
BOHP were excited at 350 nm and emitted at around 468 nm. The PL
intensity of the BiPO4/BOHP heterostructures were lower than that
of the BiPO4 and BOHP. Thus, loaded BOHP could inhibit the re-
combination of electrons [23]. The electrochemical impedance
spectra (Fig. 6c) shows that photoluminescence BiPO4/BOHP het-
erostructures have smaller electrochemical impedance. Therefore,
low charge transfer resistance of BiPO4/BOHP heterostructures
makes it easier to facilitate the electron transfer. These results in-
dicate that BiPO4/BOHP heterostructures enlarge the range for light
absorption and that the charge carrier migration rate was higher,
thus accelerating the separation of electron-hole pairs. These ob-
servations justify why BiPO4/BOHP can be an efficient potential
photocatalyst candidate.
Fig. 3. XRD patterns of BiPO4, BOHP and BiPO4/BOHP heterostructures.
Table 1
Textural properties of as-prepared samples.
Sample
BET (m2/g)
Pore volume (10−3 cm3/g)
Pore size (nm)
BiPO4
BOHP
2.7
4.0
3.8
7.3
5.7
7.2
3.3. Visible light photocatalytic activity
BiPO4/BOHP
19.3
63.3
13.1
Photocatalytic efficiency of all as-prepared photocatalysts were
evaluated by reducing Cr(VI) both under UV and visible light irra-
diation without using any hole scavenger or sacrificial electron
donor (Fig. 7). The initial pH of 10 mg·L−1 Cr(VI) water solution was
5.6. Controlled experiments suggest that the photoreduction effi-
ciency was negligible without catalyst addition (blank) both under
UV and visible light irradiation (λ ≥ 420 nm). Before light irradiation,
the solution was stirred for 90 min in the dark to reach adsorption
equilibrium. The adsorption rates of BiPO4, BOHP, and BiPO4/BOHP
were 0%, 3.4%, and 52.1%, respectively. As expected, BiPO4/BOHP
heterostructures displayed a significant photoreduction efficiency as
ultraviolet photocatalyst under UV light (Fig. S8). For comparison, no
meaningful activity for adsorption and photocatalytic reduction of Cr
(VI) was detected in the presence of pure BiPO4 or BOHP under
visible light irradiation (Fig. 7a). By contrast, BiPO4/BOHP hetero-
structures exhibited significant photocatalytic efficiency owing to
self-photosensitization via the efficient adsorption of Cr(VI) [24,25].
Thus, the adsorption and visible light photocatalytic reduction of Cr
0.325 nm was corresponding to (−220) crystal plane of triclinic
phase BOHP. These results signify that the relatively small BOHP
nanoparticles spread uniformly on the BiPO4 surface, indicating that
BiPO4 was essentially interacting with BOHP nanoparticles and thus
creating tight heterostructure. Fig. S6 shows the energy dispersive x-
ray spectrometry (EDS) mapping of BiPO4/BOHP heterostructure. The
mapping results demonstrate the co-existence of Bi, P and O ele-
ments. The lower concentration of Bi and O at some parts in the rods
is attributed to the uncovered BiPO4 surface [34]. The composition of
internal and external regions was verified by mapping and HRTEM
image.
The chemical composition and states analysis of the as-prepared
samples was carried out by XPS measurement (Fig. 5). The survey
scan spectrum (Fig. 5a) reveals the presence of Bi, P, and O elements.
The peaks at 164.7 eV and 159.4 eV were assigned to the Bi 4f7/2 and
Bi 4f5/2, respectively, suggesting the Bi(III) state in the crystal
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D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
Fig. 4. SEM images of as-prepared samples: (a) BiPO4, (d) BOHP and (g) BiPO4/BOHP heterostructures; TEM and HRTEM images of as-prepared samples: (b–c) BiPO4, (e–f) BOHP
and (h–i) BiPO4/BOHP heterostructures.
(VI) were simultaneously performed in the reaction system. After
120 min of visible light irradiation, the photoreduction rate of Cr(VI)
was about 100.0%. As shown in Fig. 7b, XPS analysis was used to
confirm the Cr oxidation state on the surface of the BiPO4/BOHP
heterostructures sample. The presence of Cr(III) was confirmed by
the observation of two bands emerging at 576.40 eV (Cr 2p3/2) and
585.81 eV (Cr 2p1/2). This indicates the complete reduction of Cr(VI)
to Cr(III) over the catalyst surface [26]. The deposited Cr(III) species
could be removed via simple washing of the sample with deionized
water.
The photocatalytic reduction efficiency of the samples obtained
under different preparing conditions were also evaluated under
visible light irradiation (Fig. 8). Fig. 8a shows that the P/Bi ratio has
strong effect on the photoreduction efficiency. An increase of P/Bi
ratio from 0.05 to 0.20 resulted in a significant increase of the
photoreduction rate of Cr(VI) to 100%. While further increase of P/Bi
ratio caused a decrease of the photoreduction rate. Similarly, the
amount of nitric acid and urea had big impact on the photoreduction
rate (Fig. 8b, and c). As previously mentioned, these preparation
factors affected the size of BiPO4/BOHP composite. Interestingly, as
the size of BOHP increased to about 90 nm, the BiPO4/BOHP com-
posite exhibited the highest photoreduction rate (Fig. 8d). Under
optimal synthetic conditions, which is P/Bi ratio, amount of nitric
acid, urea were 0.20, 3.00 mL, 0.50 g, respectively. BiPO4/BOHP
showed the highest visible light photocatalytic efficiency, which was
nearly 44.5 times that of pure BiPO4.
The recyclability and stability are essential for the practical ap-
plication of photocatalyst. In this study, the recycling runs of the
BiPO4/BOHP heterostructure in photocatalytic reduction of Cr(VI)
were evaluated under visible-light irradiation (Fig. 9a). After five
photocatalytic cycles, the photoreduction efficiency of Cr(VI) could
reach 85%. This indicates that the BiPO4/BOHP heterostructure ex-
hibits a high recyclability for the photocatalytic reduction of Cr(VI).
In order to confirm the structural stability of the catalysts, the re-
cycled catalysts were characterized by XRD (Fig. 9b). The results
prove no significant change in BiPO4/BOHP heterostructure after the
photocatalytic reaction process.
3.4. Possible reasons for the enhancement of the visible-light
performance
In order to understand the above observations, the Mott-
Schottky plots of both BiPO4 and BOHP were obtained and show
positive slopes, indicating that both are n-type semiconductors
(Fig. 10a, and b). In this circumstance, the estimated conduction
band positions of BiPO4 and BOHP are converted to −0.48 V and
−0.58 V (vs. NHE, pH = 0). Therefore, the conduction band and va-
lence band positions of BiPO4 are −0.48 V and 3.32 V, respectively.
The conduction band and valence band positions of BOHP are cal-
culated to be −0.58 V and 3.04 V, respectively [27]. The BiPO4/BOHP
heterostructures show high adsorption activity. On the one hand, a
larger BET surface area, pore volume and pore size are observed for
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D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
Fig. 5. XPS spectra of BiPO4/BOHP heterostructures, full survey (a), Bi 4f (b), P 2p (c), O 1s (d).
BiPO4/BOHP heterostructure in relation to the pure BiPO4 and BOHP,
which would provide more active sites in the reaction [35]. On the
other hand, BOHP and BiPO4/BOHP heterostructures surface were
positively charged in water solution by Zeta potential tests (Fig. 10c).
It is beneficial for adsorbing ions with negative charges such as
were jointly promoted the adsorption of Cr(VI). As shown in Fig. 6d,
once the Cr2O72- molecules were adsorbed on the surface of samples,
the absorption edges of samples were red shifted. When the Cr2O7
molecules were directly adsorbed onto BiPO4/BOHP heterostructure,
it showed two absorption bands with the largest absorption edge
located near 421 nm and the second near 527 nm, respectively.
2-
2-
Cr2O7 [28]. More active sites and positive charged on the surface
Fig. 6. (a) Tauc Plots ((F(R)*hυ)0.5 vs hυ) constructed from as-prepared samples; (b) PL emission spectra of the as-prepared samples; (c) EIS Nyquist spectra. (d) UV−Vis diffuse
reflectance spectra of as-prepared samples by adsorbing Cr(VI).
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D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
Fig. 7. (a) Photocatalytic activities of BiPO4, BOHP, and BiPO4/BOHP heterostructures under visible light. (b) Cr 2p high resolution XPS spectra of BiPO4/BOHP heterostructures
after photocatalytic reduction of Cr(VI).
Fig. 8. In the 10 mg·L−1 Cr(VI) solution (pH = 5.6) under visible-light irradiation. Photoreduction efficiency as a function of (a) the weight ratio of P/Bi; (b) the amount of HNO3; (c)
the weight of urea; (d) the size of BOHP.
Fig. 9. (a) Cycling run in the photocatalytic reduction of Cr(VI) under visible light over BiPO4/BOHP heterostructure; (b) XRD pattern of the before and after BiPO4/BOHP
heterostructure.
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D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
Fig. 10. Mott-Schottky plots for (a) BiPO4 and (b) BOHP. (c) Zeta potentials of various catalyst samples suspended in water as a function of pH. (d) Schematic of photocatalytic
reduction of Cr(VI) by BiPO4/BOHP Z-scheme heterostructures under irradiation of visible light.
BiPO4/BOHP heterostructure display red-shifted absorption edges
into the visible spectral region. Thus, it was reasonably speculated
that the adsorbed Cr2O7 coupling with wider band gap of BiPO4/
without using autoclave. The P/Bi weight ratio induced the synthesis
of BiPO4/BOHP composite and crystalline phase transformation.
Nitric acid and D-sorbitol are the key factors to control the mor-
phology of BiPO4, and urea and reaction time affected the growth of
BOHP. The BiPO4/BOHP heterostructure as ultraviolet photocatalyst
exhibited highly effective visible-light photocatalytic reduction of Cr
(VI) by the self-photosensitization of adsorbed Cr(VI) combined with
the Z-scheme heterojunction charge transfer mechanism. The BiPO4/
BOHP heterostructures with a P/Bi weight ratio of 1:5 exhibited the
highest visible light photocatalytic efficiency, which was 44.5 times
that of pure BiPO4. To the best of author knowledge the results are
the first evidence on the feasibility of sensitization of wide-bandgap
BiPO4/BOHP by self-photosensitization of adsorbed Cr(VI). The re-
port of the findings presented here will invigorate efforts in the
research of BiPO4/BOHP for efficient environmental remediation or
solar energy conversion applications. Moreover, it can contribute to
future efforts designing heterostructured semiconductor photo-
catalysts.
2-
BOHP heterostructures may produce significantly enhanced effects
to the photocatalytic efficiency as well as visible light response
through a self-sensitive mechanism [29–31]. In addition, both SEM
and Zeta potential analysis indicated the formation of BiPO4/BOHP Z-
scheme heterostructure [32,33]. Accordingly, a possible mechanism
for the photoreduction Cr(VI) over BiPO4/BOHP heterostructure is
described as follows (Fig. 10d) though mechanism details for the
formation this Z-scheme heterostructure and its highly efficient
visible-light photocatalytic efficiency still far from understanding.
2-
The possible explanation is that the adsorbed Cr2O7 significantly
improved the absorption of BiPO4/BOHP heterostructure in the
visible light region confirmed by UV–vis spectra (Fig. 6d). The po-
sitively charged BOHP nanoparticle-wrapped the negatively charged
BiPO4 rod-like structure, and the BiPO4/BOHP heterostructure shows
positive charge. This is beneficial for the adsorption of Cr2O72-. Under
visible light irradiation, photoinduced electron-hole pairs was gen-
erated in BiPO4 and BOHP. These photoinduced holes tend to remain
in the valence band of BiPO4, whereas the electrons from the con-
duction band of BiPO4 were responsible for combining with the
holes from the valence band of BOHP at the contact interface.
Afterward, the electrons were excited and transported to the cor-
responding conduction bands, resulting in an efficient space se-
paration of photoinduced charge carriers. Therefore, the reduction of
Cr(VI) to Cr(III) was achieved due to electrons at conduction bands of
BOHP. Hence, BiPO4/BOHP heterojunction exhibited much higher
adsorption of Cr(VI) and photocatalytic performance than BiPO4
and BOHP.
CRediT authorship contribution statement
Jiaqiang Wang, Liang Jiang and Di Na conceived and designed
the experiments. Mi Li, Tao Lei and Qian Liu performed the ex-
periments. Jiao He, Yongjuan Chen and Wei Wang analyzed the
data. Di Na, Liang Jiang and Jiaqiang Wang analyzed the data and
contributed in writing the manuscript. All authors provided critical
feedback and helped shape the research, analysis and manuscript.
Declaration of Competing Interest
4. Conclusion
All the work has been done by these authors. The experimental
work has been independently carried out in our group and the paper
was also independently wrote by these authors. Therefore, the au-
thors declare no competing financial interest or conflict of interest.
In summary, BOHP nanoparticle-wrapped BiPO4 rod-like struc-
ture were successfully synthesized by one-pot precipitation method
8

D. Na, L. Jiang, M. Li et al.
Journal of Alloys and Compounds 881 (2021) 160518
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