Facile synthesis of a directZ-scheme BiOCl-phosphotungstic acid heterojunction for the improved photodegradation of tetracycline

Article abstract of DOI:10.1039/d0ra02396e

The fabrication of aZ-scheme heterojunction photocatalyst can effectively modulate the electron transfer and separation of photoinduced charge carriers to enhance photocatalytic performance. Here, we demonstrate a directZ-scheme BiOCl-phosphotungstic acid (BiOCl-HPW) heterojunction, fabricatedviaa one-step hydrothermal synthesis, as a highly active and stable photocatalyst for the photodegradation of tetracycline. BiOCl-HPWs result in a dramatic improvement in visible-light utilization and photogenerated e^-and h⁺separation, as well as a decrease in the electrical resistance by the addition of HPW. Notably, the BiOCl-HPW heterojunction with optimized phosphotungstic acid (HPW) content achieved an outstanding photodegradation rate of tetracycline (0.0195 min^-1). A reasonableZ-scheme photocatalytic mechanism based on the analysis of band structure and monitoring of active radicals is proposed. This study highlights the potential implications of the BiOCl-HPW heterojunctions in the photodegradation of other toxic chemicals and photocatalytic studies.

Full text of DOI:10.1039/d0ra02396e

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Facile synthesis of a direct Z-scheme BiOCl–
phosphotungstic acid heterojunction for the
Cite this: RSC Adv., 2020, 10, 17369
improved photodegradation of tetracycline†
a
a
b
*
*
Haijuan Tong, Bingfang Shi and Shulin Zhao
The fabrication of a Z-scheme heterojunction photocatalyst can effectively modulate the electron transfer
and separation of photoinduced charge carriers to enhance photocatalytic performance. Here, we
demonstrate a direct Z-scheme BiOCl–phosphotungstic acid (BiOCl–HPW) heterojunction, fabricated
via
a
one-step hydrothermal synthesis, as
a
highly active and stable photocatalyst for the
dramatic improvement in visible-light
photodegradation of tetracycline. BiOCl–HPWs result in
a
utilization and photogenerated eꢀ and h+ separation, as well as a decrease in the electrical resistance by
the addition of HPW. Notably, the BiOCl–HPW heterojunction with optimized phosphotungstic acid
(HPW) content achieved an outstanding photodegradation rate of tetracycline (0.0195 minꢀ1). A
reasonable Z-scheme photocatalytic mechanism based on the analysis of band structure and monitoring
of active radicals is proposed. This study highlights the potential implications of the BiOCl–HPW
heterojunctions in the photodegradation of other toxic chemicals and photocatalytic studies.
Received 15th March 2020
Accepted 20th April 2020
DOI: 10.1039/d0ra02396e
rsc.li/rsc-advances
In this regard, bismuth oxyhalides (BiOX, X ¼ F, Cl, Br and I)
1. Introduction
have been applied as promising photocatalysts because of their
low toxicity, high thermal strength, and excellent electronic
properties.10 BiOX exhibits larger photodegradation rate under
visible-light irradiation, mainly attributed to the narrow
bandgap resulting from O 2p and Bi 6s2 hybridized valence
bands.11–13 Consequently, a series of BiOX with high photo-
catalytic activities have been synthesized and successfully
applied in the degradation of organic dyes.14–16 Although the
BiOX mentioned above demonstrates stronger photocatalytic
performance under visible light, their practical photocatalytic
prociency is limited by poor optical absorption capacity and
photogenerated charge carrier recombination.11 To improve the
photocatalytic performance of BiOX, various synthesis strate-
gies based on introducing defects,17 element doping,18 and
constructing heterojunctions19 have been explored. Of these
proposed approaches, fabricating BiOX-based Z-scheme heter-
ojunction has proven to be one of the easiest techniques to
prepare advanced photocatalysts because of the feasible and
effective separation of the isolated photogenerated eꢀ and h+,
thus effectively reducing the recombination efficiency of the
charged carriers. In recent years, researchers have prepared
different BiOX – based Z-scheme heterojunctions such as N-
GQDs/BiOCl,20 CdS/BiOCl,21 BiOCl/g-C3N4,22 and Bi2O3/BiOCl.23
Although these approaches for the synthesis of BiOCl-based
heterojunctions have been successful and have somewhat
effectively improved the photocatalytic performance compared
with one-component catalysts, undesirable features still exist.
Specically, the electron transfer is still restricted because of the
electrical resistance between the contact interfaces of the two
Antibiotics le in the environment have become a worrying
modern society problem because of their wide use as growth
promoters and antibacterial drugs.1 Tetracycline (TC), as
a typical antibiotic, causes serious problems for human health
and the ecosystem due to its potential negative impact on
human beings and the ecological environment.2,3 Excluding
tetracycline from polluted waters has become imperative.4
Photodegradation has recently emerged as a promising tech-
nique for its green, efficient, and effective elimination of toxic
chemicals.5 Traditional semiconductor photocatalysts such as
TiO2 and ZnO have attracted increasing attention for the pho-
todegradation of organic pollutants in water because of their
nontoxicity, chemical stability, low cost, and wide availability.6–8
However, their practical application for photocatalytic water
purication is hindered by their intrinsic wide bandgaps and
low degradation efficiencies.9 Therefore, the development of
photocatalysts with excellent photocatalytic activity that can
utilize the maximum amount of solar light is the focus of
environmental science.
aKey Laboratory of Regional Ecological Environment Analysis and Pollution Control of
West Guangxi, College of Chemistry and Environmental Engineering, Baise University,
Baise 533000, China. E-mail: shibingfang@126.com
bState Key Laboratory for the Chemistry and Molecular Engineering of Medicinal
Resources, Guangxi Normal University, Guilin, 541004, China. E-mail:
zhaoshulin001@163.com
† Electronic supplementary information (ESI) available. See DOI:
10.1039/d0ra02396e
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semiconductors.24 Noble metals (Ag, Au, or Pt) and carbon containing Bi(NO3)3$5H2O (0.97 g), polyvinylpyrrolidone (0.40
quantum dots have been employed as decorated accessories in g), and 25 mL of ethylene glycol was sonicated for 10 min.
heterojunctions to effectively reduce the electric resistance Subsequently, 0.01 g HPW and 10 mL of saturated KCl solution
because of their excellent electrical conductivity.25–28 However, were slowly added into the above mixture with stirring at room
the high temperature and lengthy fabrication procedures temperature for 10 min, and then the obtained solution was
required as well as the high costs are drawbacks. Theoretically, transferred into a 50 mL Teon-lined stainless steel autoclave
electron mediators are prone to accelerate electron transfer. and hydrothermally heated at 160 ꢁC for 3 h under vigorous
Hence, the introduction of cocatalysts on the BiOX surface that stirring. Aer being naturally cooled, the mixture was centri-
act as mediators and enable easier electronic migration may fuged and washed several times with deionized water. Finally,
ꢁ
suppress the recombination of photogenerated carriers, which the resulting solid powder was dried at 60 C for 10 h and was
is a new challenge.
denoted as BiOCl–HPW-1. According to this method, BiOCl–
As an electron mediator, phosphotungstic acid (HPW) can HPW-2, BiOCl–HPW-3, and BiOCl–HPW-4 were synthesized
promote electron transfer between cocatalysts due to its well- with different HPW content (0.02, 0.03, and 0.04 g), respectively.
dened Keggin structure.29 Additionally, HPW can catalyze
For comparison, a hydrothermal method was used to
many organic synthesis reactions such as oxidation, esterica- prepare BiOCl. The preparation of BiOCl was the same as the
tion, and C–N bond formation because of its strong acidity and above method except for the use of HPW.
oxidative capacity.30–32 Inspired by the excellent properties of
HPW, researchers have invested massive effort on the prepara-
tion of HPW based heterojunctions, such as copolymer–HPW,33
2.3 Characterization of BiOCl–HPWs
choline–HPW,34 and polyimide–HPW.9 These new successful UV-Vis absorption spectra were recorded on a Cary 60 UV-vis
approaches have shown that the synthesized HPW based het- spectrophotometer (Agilent Technologies) at room tempera-
erojunctions have slightly enhanced electron–hole interactions ture. Fourier transform infrared spectroscopy (FTIR) was per-
and photocatalytic activity compared with pure HPW. However, formed using
a
PerkinElmer FTIR spectrophotometer
they inevitably require a rigorous, complex synthesis process, (PerkinElmer, USA) from 400 to 4000 cmꢀ1. X-ray diffraction
including many steps under N2 gas ow,33 complex process and (XRD) patterns were recorded with monochromatic Cu Ka
time consuming (slowly dropped and approximately 24 h),34 and radiation (40 kV, 30 mA). Transmission electron microscopy
high synthesis temperature (325 C for 4 h),9 respectively.
(TEM) was conducted on a FEI Tecnai F-20 eld emission
ꢁ
Herein, a one-step hydrothermal approach for synthesizing HRTEM (Hillsboro, OR, USA) at a voltage of 200 kV. X-ray
BiOCl–phosphotungstic acid (BiOCl–HPW) heterojunctions photoelectron spectroscopy (XPS) was performed on a Kratos
using Bi(NO3)3$5H2O and HPW as precursors is proposed. The XSAM 800 photoelectron spectrometer (Manchester, UK). Pho-
prepared BiOCl–HPW heterojunctions exhibited good stability. toluminescence (PL) were performed on a FL7000 (Hitachi,
Moreover, the photocatalytic activity of BiOCl–HPWs was supe- Japan). N2 adsorption desorption isotherms was conducted on
rior to those of HPW and as-prepared BiOCl. As expected, 92.2% Belsorp-max (Belsorp-max II, Japan). The electrochemical
tetracycline was removed by the BiOCl–HPW-3 aer 120 min impedance spectroscopy (EIS) were obtained on an electro-
simulated solar light irradiation. We also propose a reasonable Z- chemical workstation (Chenhua, CHI-660D) with a three-
scheme photocatalytic mechanism based on the analysis of band electrode system where Ag/AgCl electrode as the reference, Pt
structure and monitoring of active radicals. Our results can offer wire as the counter and material-coated glassy carbon (GC) as
new opportunities for the development and in-depth under- the working electrode in 0.5 M Na2SO4 aqueous solution. The
standing of noble direct Z-scheme heterojunction photocatalysts UV-vis diffuse reectance spectra (DRS) were carried on Hitachi
for efficient photodegradation of organic pollutants.
U-3010 spectrophotometer with BaSO4 as a reference. Bruker
ESR A300 spectrometer was employed to record the electron
spin resonance (ESR) signals under simulated light irradiation.
Total organic carbon (TOC) test was conducted on a TOC-VCPH
analyzer (Shimadzu, Japan).
2. Experimental section
2.1 Chemicals and reagents
Tetracycline (TC), phosphotungstic acid (HPW), poly-
vinylpyrrolidone, isopropanol (IPA), ammonium oxalate (AO), p-
benzoquinone (BQ), ethylene glycol, Bi(NO3)3$5H2O, tereph-
thalic acid (TA), and KCl were purchased from Aladdin Chem-
istry Co. Ltd. (Shanghai, China). All chemical solvents and
reagents were of analytical grade and were used without addi-
tional purication. Deionized water was puried by a Milli-Q
plus 185 equip from Millipore (Bedford, MA).
2.4 Photocatalytic activity of BiOCl–HPWs
The photocatalytic activities of BiOCl–HPW were evaluated by
the decomposition of tetracycline (TC) and under simulated
solar light irradiation. 10 mg of BiOCl–HPWs was dispersed in
50 mL TC with an initial concentration of 10 mg Lꢀ1 under
magnetic stirring in the dark for 60 min. Aer achieving the
adsorption–desorption equilibrium, photodegradation of TC
was conducted under light irradiation using a PLS-SXE300UV Xe
lamp (15 A, 300 W) as a light source. During tetracycline pho-
2.2 Synthesis of BiOCl–HPWs
The BiOCl–HPWs were prepared through a one-step hydro- todegradation, the reaction solutions were withdrawn at regular
thermal approach. In a typical synthesis, the reaction solution intervals for analysis using a UV-vis spectrophotometer in the
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range of 300–700 nm. All measurements were conducted in similar to those of BiOCl, demonstrating the basic structure of
triplicate.
In order to clarify the photocatalytic mechanism, free radical
BiOCl in BiOCl–HPWs.
To conrm the introduction of HPW, the Fourier transform
scavenge experiment was performed. Prior to addition of the infrared spectroscopy (FTIR) spectra of as-prepared BiOCl and
photocatalyst, 1.0 mM p-benzoquinone (BQ), AO, and iso- BiOCl–HPWs are presented in Fig. S1 (ESI†). For the as-prepared
propanol (IPA) were added into the TC solution to investigate BiOCl, the peak at approximately 527 cmꢀ1 was assigned to the
the effects of hydroxyl radicals (cOH), superoxide radicals Bi–O stretching vibrations of BiOCl (Fig. S1a, ESI†). Distinct
(cO2ꢀ), and holes (h+) on TC photodegradation by BiOCl-HPWs, peaks at approximately 1650 cmꢀ1 and 3400 cmꢀ1 were
respectively. All experiments were conducted according to the observed which were attributed to the stretching vibration
photocatalytic activity experiment.
modes of –OH aer the absorption of water molecules.36 For
BiOCl–HPWs, peaks at 817 cmꢀ1 and 980 cmꢀ1 could be
observed compared with the FT-IR spectra of as-prepared BiOCl
(Fig. S1b, ESI†). These peaks are attributed to the typical
vibration of W–Oc–W and W–O,35 indicating the existence of
HPW in BiOCl-HPWs.
To further verify the fabrication of BiOCl–HPWs, XPS proles
of BiOCl–HPWs were analyzed. Fig. 1b shows the existence of Cl,
O, Bi, and W atoms in BiOCl–HPWs. However, it is difficult to
observe the peaks of P in the XPS proles possibly because of
the low amount of P in HPW. This observation agrees well with
previously reported results.35 The high-resolution Bi 4f spec-
trum (inset of Fig. 1b) shows peaks at 159.1 eV and 164.4 eV
corresponding to Bi 4f7/2 and Bi 4f5/2 of Bi3+, respectively.37 In
Fig. S2 (ESI†), two peaks located at 197.9 eV and 199.1 eV are
consistent with the Cl 2p3/2 and Cl 2p1/2 region belonging to Cl
2p peaks.36 Fig. 1c shows that two signals at 531.5 eV and
533.0 eV can be observed, which are attributed to Bi–O in BiOCl
and W–O in the Keggin structure of HPW, respectively,38,39
which agreed well with the FT-IR results. Fig. 1d indicates that
the XPS peaks of W 4f5/2 and W 4f7/2 in BiOCl–HPWs appear at
2.5 Recyclability of BiOCl–HPWs
BiOCl–HPW-3 was employed to evaluate the recyclability of
BiOCl–HPWs by photodegradation of TC under simulated solar
light irradiation. Ten milligrams of BiOCl–HPW-3 and
10 mg Lꢀ1 of TC underwent similar photocatalytic degradation
processes, following which the used BiOCl–HPW-3 was
collected by centrifugation; then, BiOCl–HPW-3 was washed
ꢁ
and dried at 80 C for 10 h for recyclability tests.
3. Results and discussion
3.1 Structure of BiOCl–HPWs
X-ray diffraction (XRD) and X-ray photoelectron spectroscopy
(XPS) proles were performed to investigate the structure of
BiOCl–HPWs. As presented in Fig. 1a, all photocatalysts exhibit
good crystallinity. For the as-prepared BiOCl, the diffraction
peaks at 2q ¼ 11.95, 25.85, 32.80, 33.50, 36.50, 41.50, 46.64,
49.90, and 54.09 could be readily attributed to the (001), (101),
(110), (102), (003), (112), (200), (113), and (211) crystal faces of
BiOCl, respectively, (JCPDS Card No. 06-2049). The diffraction
peaks of HPW resulting from the amorphous state of HPW
cannot be observed, which is in accordance with previously
reported results.35 All diffraction peaks of BiOCl–HPWs were
Fig. 1 XRD patterns (a) of as-synthetised BiOCl and BiOCl–HPWs. XPS
survey spectrum (b, inset: Bi 4f XPS spectrum), O 1s XPS spectrum (c) Fig. 2 The images of TEM (a), SEM (b, inset: HRTEM of BiOCl–HPW-3)
and W 4f XPS spectrum (d) of the as-prepared BiOCl–HPWs.
and EDX elemental mappings (c–h) of BiOCl–HPW-3.
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37.4 and 35.3 eV, respectively, which are characteristic of the small particles distributed on the nanosheets (Fig. 2b), further
W6+ state.40 These above results conrm the successful fabri- demonstrating that HPW was decorated on the surface of BiOCl
cation of BiOCl–HPWs. As shown in Fig. S3 (ESI†), compared to form heterojunctions.
with the as-prepared BiOCl, a small shi in peaks of Bi and Cl to
the lower energy region indicates the coordination of BiOCl
3.3 Photocatalytic activity
with HPW by chemical bonding in BiOCl–HPWs.41 This
Based on the photocatalytic activity of BiOCl, whether the
suggests HPW underwent self-assembly on the BiOCl surface
prepared BiOCl–HPWs also possessing the photocatalytic
through the hydrothermal method to formed BiOCl–HPW
activity on the photocatalytic degradation of TC under the
heterojunction.
simulated solar light was analyzed. Fig. 3a shows that the
photocatalytic degradation capacity of BiOCl–HPWs is much
higher than that of HPW and as-prepared BiOCl. This suggests
3.2 Morphology of BiOCl–HPWs
that the introduced HPW has an important role in enhancing
The morphologies of BiOCl–HPWs were characterized through
the photocatalytic degradation capacity of BiOCl–HPWs. To
SEM and TEM. As displayed in Fig. 2a, the BiOCl–HPWs are
sheet-like structures, which are similar to the structure of BiOCl
(Fig. S4a, ESI†). The as-prepared BiOCl–HPWs nanosheets have
large and smooth surfaces, which are advantageous for the
investigate the photocatalytic activity of as-prepared BiOCl,
HPWs, and BiOCl–HPWs, the corresponding photocatalytic
degradation kinetics were analyzed. Fig. S5 (ESI†) shows that
the experimental data t the rst-order kinetic model with good
attachment of HPW. The TEM images (Fig. 2b) further
linearity. As shown in Fig. 3b, the apparent rate constant (k) of
conrmed the sheet-like structure of BiOCl–HPWs. High reso-
BiOCl–HPW-3 (k ¼ 0.0195 minꢀ1) was approximately 5.3 and
lution TEM (HRTEM) imaging (inset of Fig. 2b) showed the
13.0 times that of as-prepared BiOCl (k ¼ 0.0037 minꢀ1) and
prepared BiOCl–HPWs displayed a highly crystalline structure
HPW (k ¼ 0.0015 minꢀ1), respectively. These results imply HPW
with a 0.27 nm lattice parameter, which was ascribed to the
has crucial roles in promoting the photocatalytic degradation
(110) plane of the BiOCl nanosheets.36 The lattice spaces of
HPW on the surfaces of BiOCl nanosheets could not be
observed, which match the XRD results of BiOCl–HPWs.
efficiency of TC by BiOCl–HPWs. To further investigate the
photocatalytic activity of BiOCl–HPWs prepared using different
HPW contents, the k of BiOCl–HPWs were compared. As can be
However, the mapping images of P and W conrm the
seen in Fig. 3b, the k of the respective BiOCl–HPWs increased
elemental composition and distribution in BiOCl–HPWs (Fig. 2f
with the incorporation of HPW. However, the k of BiOCl–HPWs
and h). Fig. 2c–h shows the same shape of the mapping images
of Bi, P, and W, further suggesting the successful construction
decreased when HPW content became 0.04 g. The k of
BiOCl–HPW-3, as a result, was higher when compared with that
of BiOCl–HPW heterojunctions. Furthermore, we observed
of BiOCl–HPW-1 (k ¼ 0.0084 minꢀ1), BiOCl–HPW-2 (k ¼
0.0106 minꢀ1) and BiOCl–HPW-4 (k ¼ 0.0133 minꢀ1), which
further conrms that HPW has crucial roles in promoting the
photocatalytic performance of BiOCl–HPWs. Based on the above
observations, BiOCl–HPW-3 was chosen to investigate the possible
reason for the differences in photodegradation efficiencies of the
photocatalysts in the subsequent TC degradation experiments.
Fig. 4 UV-vis absorbance spectra (a) and PL spectra (b) of BiOCl, HPW
Fig. 3 The photocatalytic efficiency (a) and the apparent rate constant and BiOCl–HPW-3. Electrochemical impedance Nyquist plots (c) and
(b) of BiOCl, HPW and BiOCl–HPW-3. The photo-catalyst concen- photocurrent response (d) of BiOCl, HPW and BiOCl–HPW-3 heter-
tration was 0.2 mg mLꢀ1 and TC concentration was 10 mg Lꢀ1
.
ojunction electrodes.
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To reveal the reasons for the enhanced photocatalytic
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degradation efficiency of TC by BiOCl–HPWs, the optical
absorption properties of HPWs, as-prepared BiOCl, and
BiOCl–HPWs were investigated by UV-vis diffuse reectance
spectra. As shown in Fig. 4a, the UV-vis spectra of BiOCl–HPWs
were a combination of the spectra of HPW and as-prepared
BiOCl. It is obvious that the absorption capacity of BiOCl is
much lower than that of HPW. Additionally, an increasing red
shi of BiOCl–HPWs to the longer wavelength region with
increased HPW content was observed, suggesting that the
modication of BiOCl with HPW greatly improves its absorption
range and optical absorption capacity. Consequently, more eꢀ
and h+ pairs were easily produced aer more light was adsor-
bed, thus resulting in the improved photocatalytic perfor-
mances of BiOCl–HPWs.
In view of HPW regarding as an electron mediator, it was
speculated that the separation of photogenerated carriers was
promoted aer the introduction of HPW. To conrm the above
hypothesis, the photoluminescence (PL) spectra of BiOCl–HPW-
3 were analyzed. Fig. 4b shows that the PL spectrum of BiOCl–
HPW-3 agrees with that of the as-prepared BiOCl, which may be
attributed to the defective emission of BiOCl. This observation
is similar to a previously reported result.36 Under an excitation
of 410 nm, the PL intensity of BiOCl–HPW-3 was lower than that
of HPW and as-prepared BiOCl, which illustrates the separation
of photogenerated eꢀ and h+ pairs was promoted aer the
introduction of HPW.42 Therefore, it suggests that more photon
participate in the photocatalytic reaction due to the decrease in
recombination rate of photogenerated electron-hole pairs,
resulting in an improved photocatalytic performance. However,
as excess HPW was added, new photogenerated eꢀ and h+
recombination centers were formed on the surface.9 Accord-
ingly, the lower k (Fig. 1b) of BiOCl–HPW-4, compared with
BiOCl–HPW-3, with the highest HPW content can also be
explained reasonably.
To gain further insights into the promoted separation of
photogenerated eꢀ and h+ pairs of BiOCl–HPWs, the EIS
Nyquist plots of HPW, as-prepared BiOCl, and BiOCl–HPW-3
were compared. Fig. 4c shows that the radii of the micro
semicircle of BiOCl–HPW-3 are much smaller than those of as-
prepared BiOCl and HPW, which indicates that the charge
transfer resistance of BiOCl–HPW-3 is lowest.41 The lower elec-
trical resistance can effectively facilitate the surface and inter-
layer electron transfer of BiOCl–HPWs, thus limiting the
recombination of the charge carriers. Analogous, photocurrent
response (Fig. 4d) of samples obviously display BiOCl–HPW-3
composites have a superior photocurrent response property
than HPW and BiOCl, furtherly illustrate BiOCl–HPW-3 heter-
ojunction facilitate the charge migration.
Fig. 5 VB-XPS patterns of BiOCl (a) and HPW (b). The plots of (ahn)2 vs.
photon energy (hn) for the band gap energies of as-prepared BiOCl,
HPW and BiOCl–HPWs (c).
BiOCl–HPW-3 decreased, while its photocatalytic activity was
promoted, which suggests that the surface area has less impact
on the photocatalytic activity of BiOCl–HPWs.
All of the above results clearly demonstrate that the
enhanced photocatalytic activity of BiOCl–HPW-3 originates
from the effective visible-light utilization, efficient photo-
generated eꢀ and h+ separation, and lower electrical resistance
aer the introduction of HPW.
3.4 Band structure
The photocatalytic activity is affected not only by the separation
efficiency of photogenerated carriers but also by the band
structure of the photocatalyst. Fig. 5a shows that the calculated
valence band (VB) of BiOCl in heterojunction was 2.35 eV, which
is higher than that reported for BiOCl (1.8 eV).43 Fig. 5b shows
the VB of HPW is at 3.35 eV, which is more than that reported by
Liu (3.28 eV).44 Theoretically, the oxidation ability of the pho-
togenerated holes is promoted when the VB lies at a lower
position. Accordingly, the photogenerated holes in the VB of
BiOCl and HPW possess stronger oxidation ability. Besides, the
oxidation ability of the photogenerated holes in the VB of HPW
is higher than that of BiOCl in BiOCl–HPWs. As expected, the
bandgap energy of BiOCl–HPWs (3.32 eV) is lower than that of
BiOCl (3.55 eV) (Fig. 5c), resulting in the enhancement of the
ability of harvesting visible light.
3.5 The performance for the removal of TC
It is well known that the removal of TC includes two processes:
As is well-known, the activity of a catalyst depends on its the TC are rst adsorbed on the surface of catalyst, followed by
surface area. Therefore, the surface areas of as-prepared BiOCl photocatalytic degradation. As shown in Fig. 3a, the removal of
and BiOCl–HPW-3 were measured (Fig. S6, ESI†). Results show TC in the presence of different catalysts displayed similar trends
that the BET surface areas of as-prepared BiOCl and BiOCl– during both adsorption and photocatalytic degradation. During
HPW-3 are 20.1 and 18.2 m2 gꢀ1, respectively. The surface areas both adsorption, the best adsorption capacity and the adsorp-
did decrease aer the successful combination of BiOCl and tion ability of BiOCl–HPW-3 was observed aer achieving the
HPW possibly because of the partially blocked pores by HPW.33 adsorption–desorption equilibrium (Fig. S7, ESI†), which
Indeed, aer the introduction of HPW, the surface areas of effectively not only strengthens the contact between TC and
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real water samples maintain a relatively high efficiency, indi-
cating that the reliability and feasibility of BiOCl–HPW-3 in the
photocatalytic degradation of TC in complex samples.
For further evaluating the practicality of photocatalysts,
recyclability and stability of BiOCl–HPW-3 were performed.
Fig. 6c shows that the catalytic stability of BiOCl–HPW-3 does
not signicantly decrease aer four cycling runs. Besides, all
diffraction peaks in the XRD patterns of BiOCl–HPW-3 before
and aer four recycles were well preserved (Fig. 6d). These
results illustrate that BiOCl–HPW-3 has good stability.
All observations further indicate the great potential appli-
cations of BiOCl–HPW-3 in wastewater purication.
3.6 Possible photocatalytic mechanism
Generally, photocatalysis is controlled by cO2ꢀ, cOH, and h+
radicals. As shown in Fig. 7, no signals of cO2 and cOH
Fig. 6 TOC removal of TC in the presence of BiOCl, HPW and BiOCl–
HPW-3 within 120 min illumination (a), removal of TC in the presence
of BiOCl–HPW-3 in real wastewater (b), the recyclability test (c) and
XRD patterns (d) of BiOCl–HPW-3 (photocatalyst concentration was
0.2 mg mLꢀ1 and TC concentration was 10 mg Lꢀ1).
ꢀ
observed illustrated that no photodegradation occurred in the
dark. As expected, four characteristic peaks of DMPO-cOH were
detected aer 5 min of simulated solar light irradiation, indi-
cating that cOH is produced during the photocatalytic degra-
dation process. On the other hand, the uorescence intensity of
peak at 424 nm (characteristic peak of TA-OH) gradually
increases with the increase of reaction time (Fig. S8, ESI†),
which further conrmed the result of ESR signals of the DMPO-
BiOCl–HPW-3 but also increased the utilization rate of the
active species.45 Consequently, the best adsorption of BiOCl–
HPW-3 facilitates subsequent photocatalytic degradation. As for
the process of photocatalytic degradation, the TC degradation
efficiency by BiOCl and HPW is relatively low with only 40.9%
and 27.0%, respectively. However, the removal efficiency of TC
by BiOCl–HPWs is remarkably higher than that of BiOCl and
HPW. The highest TC degradation efficiency of BiOCl–HPW-3
reaches 92.2% aer 120 min simulated solar light irradiation
(Fig. 3a), which is similar to the result of Chen46 and much
higher than that in the reported literature.47,48
To evaluate TC mineralization by BiOCl, HPW, and BiOCl–
HPW-3, TOC measurements were conducted, and the results are
shown in Fig. 6a. As shown in Fig. 6a, only 21.1% and 6.1% TOC
were removed in the presence of BiOCl and HPW aer illumi-
nation for 120 min, respectively. Meanwhile, the TOC removal
efficiency of TC by BiOCl–HPW-3 is higher than that of BiOCl
and HPW, indicating that the efficiency of photocatalytic
performance of BiOCl is greatly enhanced aer combination
with HPW. However, the mineralization rate of BiOCl–HPW-3,
compared with its photodegradation efficiency, is much lower,
which is resulted from the production of intermediate materials
during photoreaction.46
For evaluating the practical application of BiOCl–HPW-3 in
complex samples, BiOCl–HPW-3 was applied for the photo-
catalytic degradation of TC in tap water, river water and lake
water, respectively. Three different types of wastewater were
spiked with TC standard solutions with concentrations of
10 mg Lꢀ1. As shown in Fig. 6b, the degradation efficiency of TC
over BiOCl–HPW-3 in tap water, river water and lake water were
82.9%, 77.6% and 68.1%, respectively. A decrease in photo-
catalytic degradation efficiency of TC in three water samples
was observed, which is attributed to the competition between
TC and the interfering components in water on active sites and
active species. Exciting, the photocatalytic degradation of TC in
ꢀ
cOH. Similarly, six characteristic signals of DMPO-cO2 were
clearly observed under the same conditions. In addition, the
intensity of BiOCl–HPW-3 triggering DMPO-cOH and DMPO-
ꢀ
cO2 was all higher than that of BiOCl triggering DMPO-cOH
and DMPO-cO2ꢀ, further conrming the superior photo-
degradation activity of BiOCl–HPWs to that of BiOCl.
To identify the key active species responsible for the degra-
dation, the scavenger regents (BQ for cO2ꢀ, AO for h+, and IPA
for cOH) were analyzed. As indicated in Fig. S9 (ESI†), photo-
degradation is greatly suppressed by addition of IPA, illus-
trating cOH was the most active species for the degradation of
TC. Meanwhile, h+ has the lowest effect on TC degradation,
indicating h+ may tend to generate more cOH not to degrade TC.
From Fig. 7a and b, the intensity of cOH signals was stronger
than that of cO2ꢀ, indicating that the content of cOH was higher
than that of cO2ꢀ in the photodegradation reaction and cOH was
of a critical species, which is consistent with the result of the
scavenger regent experiments.
Based on the above experimental results of band gap energy,
the main reactive species, and the position of valence band and
ꢀ
Fig. 7 ESR signals of the DMPO-cOH (a) and DMPO-cO2 (b) for
BiOCl–HPW-3 in the dark and under 300 W Xe lamp irradiation.
17374 | RSC Adv., 2020, 10, 17369–17376
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ꢀ
band structures as well as cOH and cO2 mediated degradation
suggest the photodegradation by BiOCl–HPW following a direct Z-
scheme-dictated charge carrier transformation mechanism,
which enhanced the photocatalytic activity of the BiOCl–HPWs.
The photocatalytic activity of BiOCl–HPWs only slightly changed
even aer they were reused up to the fourth cycle. Consequently,
BiOCl–HPWs as a Z-scheme heterojunction photocatalyst showed
great potential for photocatalytic applications.
Conflicts of interest
There are no conicts to declare.
Acknowledgements
Fig. 8 Schematic diagram of direct Z-scheme pathway for charge The nancial support from the National Natural Science
carrier separation.
Foundation of China (No. 21665001), and BAGUI Scholar
Program.
conduction band, the possible photocatalytic mechanism of TC
photo-degradation by BiOCl–HPW heterojunctions was dis-
References
cussed. First,
a conventional type-II heterojunction was
¨
1 K. Kummerer, Chemosphere, 2009, 75, 417–434.
proposed to explain the reaction mechanism of the BiOCl–
HPW. If the transfer path of the photogenerated electrons and
holes of BiOCl–HPW followed the type-II, then, under light
illumination, the photogenerated eꢀ on the conduction band
(CB) of BiOCl (CB ¼ ꢀ1.2 eV) would transfer to and concen-
trated on the CB of HPWs (CB ¼ 0.24 eV), as shown in Fig. 8
(le). However, the concentrated photogenerated electrons (eꢀ)
on the CB of HPWs could not reduce the absorpted O2 to yield
2 C. I. Brinzila, N. Monteiro, M. J. Pacheco, L. Ciriaco,
I. Siminiceanu and A. Lopes, Environ. Sci. Pollut. Res. Int.,
2014, 21, 8457–8465.
3 H. Shi, X. Xiao, L. Zeng, Q. Zhang, J. Nan and L. Wang, J.
Nanosci. Nanotechnol., 2014, 14, 6934–6940.
4 S. Sharma, V. Dutta, P. Singh, P. Raizada, A. Rahmani-Sani,
A. Hosseini-Bandegharaei and V. K. Thakur, J. Cleaner
Prod., 2019, 228, 755–769.
ꢀ
ꢀ
cO2 radicals due to the more positive potential than O2/cO2
5 D. Yue, J. Lei, Y. Peng, J. Li and X. Du, Fuel, 2018, 226, 148–
155.
6 S. Chan, T. Wu, J. Juan and C. Teh, J. Chem. Technol.
Biotechnol., 2011, 86, 1130–1158.
7 H. Sun, S. Liu, S. Liu and S. Wang, Appl. Catal., B, 2014, 146,
162–168.
8 J. Zhang, S. Wageh, A. Al-Ghamdi and J. Yu, Appl. Catal., B,
2016, 192, 101–107.
9 P. Meng, H. Heng, Y. Sun, J. Huang, J. Yang and X. Liu, Appl.
Catal., B, 2018, 226, 487–498.
10 S. Garg, M. Yadav, A. Chandra and K. Hernadi, J. Nanosci.
Nanotechnol., 2019, 19, 280–294.
11 S. Cao, P. Zhou and J. Yu, Chin. J. Catal., 2014, 35, 989–1007.
12 Y. Yang, C. Zhang, D. Huang, G. Zeng, J. Huang, C. Lai,
C. Zhou, W. Wang, H. Guo, W. Xue and R. Deng, Appl.
Catal., B, 2019, 245, 87–99.
(ꢀ0.33 eV), which was inconsistent with the results of radical
species trapping. These indicated that the photocatalytic
mechanism of BiOCl–HPW could not be described by the
charge transfer of type-II heterojunction, which is contrary to
the results of the active species trapping experiments and ESR
spectra. Therefore, direct Z-scheme mechanism is indirectly
supported by the above result. As illustrated in Fig. 8 (right),
under simulated solar light irradiation, eꢀ remain in the CB of
BiOCl and h+ in the VB of HPW. The VB potential of HPW (3.35
eV) is lower than the H2O/cOH position (2.72 eV vs. NHE), and
thus, h+ oxidize H2O to generate cOH for the oxidization of TC,
which is in agreement with the above experimental results.
Meanwhile, ꢀthe remaining eꢀ in the CB of BiOCl reduced O2 to
produce cO2 for further oxidization of TC.
13 K. Zhang, C. Liu, F. Huang, C. Zheng and W. Wang, Appl.
Catal., B, 2006, 68, 125–129.
4. Conclusions
In summary, we have prepared a direct Z-scheme heterojunction 14 R. S. Yuan, C. Lin, B. C. Wu and X. Z. Fu, Eur. J. Inorg. Chem.,
photocatalyst BiOCl–HPWs by a facile hydrothermal synthesis 2009, 24, 3537–3540.
method. Results demonstrate that BiOCl–HPWs prepared using 15 H. Feng, Z. Xu, L. Wang, Y. Yu, D. R. G. Mitchell, D. Cui,
0.03 g HPW displayed superior photocatalytic ability for RhB and
TC degradation under simulated solar light irradiation. The
X. Xu, J. Shi, T. Sannomia, Y. Du, W. Hao and S. Dou, ACS
Appl. Mater. Interfaces, 2015, 7, 27592–27596.
excellent photocatalytic ability was probably caused by the effec- 16 L. Ye, L. Zan, L. Tian, T. Peng and J. Zhang, Chem. Commun.,
tive visible-light utilization, efficient photogenerated eꢀ and h+
2011, 47, 6951–6953.
separation, as well as lower electrical resistance aer the intro- 17 R. Jitta, R. Gundeboina, N. Veldurthi, R. Guje and V. Muga, J.
duction of HPW with the Keggin unit. The well-matched energy
Chem. Technol. Biotechnol., 2015, 24, 1937–1948.
This journal is © The Royal Society of Chemistry 2020
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View Article Online
RSC Advances
Paper
18 J. Bolobajev, M. Kasle, K. Kreek, M. Kulp, M. Koel and 34 X. Zeng, X. Xiao, J. Chen and H. Wang, Appl. Catal., B, 2019,
A. Goib, Chem. Eng. J., 2019, 357, 120–128. 248, 573–586.
19 H. Che, G. Che, P. Zhou, C. Liu and H. Dong, J. Colloid 35 L. Li, L. Li, T. Sun, X. Yu, L. Long, X. Lei and J. Yan, J. Solid
Interface Sci., 2019, 546, 262–275. State Chem., 2019, 274, 152–161.
20 H. Wang, B. Zhang, F. Zhao and B. Zeng, ACS Appl. Mater. 36 D. Wu, X. Wang, H. Wang, F. Wang, D. Wang, Z. Gao,
Interfaces, 2018, 10, 35281–35288.
X. Wang, F. Xu and K. Jiang, J. Colloid Interface Sci., 2019,
533, 539–547.
37 X. Zhang, X. Wang, L. Wang, W. Wang, L. Long, W. Li and
H. Yu, ACS Appl. Mater. Interfaces, 2014, 6, 7766–7772.
38 F. Gao, D. Zeng, Q. Huang, S. Tian and C. Xie, Phys. Chem.
Chem. Phys., 2012, 14, 10572–10578.
39 J. Liu, S. Xie, Z. Geng, K. Huang, L. Fan, W. Zhou, L. Qiu,
D. Gao, L. Ji, L. Duan, L. Lu, W. Li, S. Bai, Z. Liu, W. Chen,
S. Feng and S. Zhang, Nano Lett., 2016, 16, 6568–6575.
21 W. Lin, X. Yu, Y. Shen, H. Chen, Y. Zhu, Y. Zhang and
H. Meng, J. Nanopart. Res., 2017, 19, 56–67.
22 Y. Bai, P. Wang, J. Liu and X. Liu, RSC Adv., 2014, 4, 19456–
19461.
23 F. Deng, Q. Zhang, L. Yang, X. Luo, A. Wang, S. Luo and
D. Dionysiou, Appl. Catal., B, 2018, 238, 61–69.
24 R. Jiang, G. Lu, Z. Yan, D. Wu, R. Zhou and X. Bao, Chem.
Eng. J., 2019, 374, 79–90.
25 M. Liang, T. Borjigin, Y. Zhang, H. Liu, B. Liu and H. Guo, 40 P. Meng, H. Heng, Y. Sun, J. Huang, J. Yang and X. Liu, Appl.
ACS Appl. Mater. Interfaces, 2018, 10, 34123–34131. Catal., B, 2018, 226, 487–498.
26 W. Zhu, D. Han, L. Niu, T. Wu and H. Guan, Int. J. Hydrogen 41 W. Yang, B. Ma, W. Wang, Y. Wen, D. Zeng and B. Shan,
Energy, 2016, 41, 14713–14720. Phys. Chem. Chem. Phys., 2013, 15, 19387–19394.
27 R. Wang, K. Lu, Z. Tang and Y. Xu, J. Mater. Chem. A, 2017, 5, 42 K. Li, L. Yan, Z. Zeng, S. Luo, X. Luo, X. Liu, H. Guo and
3717–3734. Y. Guo, Appl. Catal., B, 2014, 156, 141–152.
28 T. Bao, L. Song and S. Zhang, Chem. Eng. J., 2018, 351, 189– 43 C. Liu, J. Zhou, J. Su and L. Guo, Appl. Catal., B, 2019, 241,
194. 506–513.
29 A. Hiskia, M. Ecke, A. Troupis, A. Kokorakis, H. Hennig and 44 J. Meng, X. Wang, X. Yang, A. Hu, Y. Guo and Y. Yang, Appl.
E. Papaconstantinou, Environ. Sci. Technol., 2001, 35, 2358–
Catal., B, 2019, 251, 168–180.
2364.
45 X. Li, J. Wang, A. Rykov, V. Sharma, H. Wei, C. Jin, X. Liu,
M. Li, S. Yu, C. Sun and D. Dionysiou, Catal. Sci. Technol.,
2015, 5, 504–514.
30 G. Wang, Y. Shen and X. Wu, Eur. J. Org. Chem., 2008, 25,
4367–4371.
31 A. Jha, A. Garade, S. Mirajkar and C. Rode, Ind. Eng. Chem. 46 S. Chen, D. Huang, G. Zeng, X. Gong, W. Xue, J. Li, Y. Yang,
Res., 2012, 51, 3916–3922.
32 S. Wang and G. Yang, Chem. Rev., 2015, 115, 4893–4962.
C. Zhou, Z. Li, X. Yan, T. Li and Q. Zhang, Chem. Eng. J., 2019,
370, 1087–1100.
33 M. Betiha, H. Hassan, E. El-Sharkawy, A. Al-Sabagh, 47 Y. Liu, J. Kong, J. Yuan, W. Zhao, X. Zhu, C. Sun and J. Xie,
M. Menoufy and H. Abdelmoniem, Appl. Catal., B, 2016,
182, 15–25.
Chem. Eng. J., 2018, 331, 242–254.
48 L. Rimoldi, A. Giordana, G. Cerrato, E. Falletta and
D. Meroni, Catal. Today, 2019, 328, 210–215.
17376 | RSC Adv., 2020, 10, 17369–17376
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