Facile synthesis of Bi/BiOCl composite with selective photocatalytic properties

Article abstract of DOI:10.1016/j.jallcom.2015.06.113

This paper presents a novel and facile method to fabricate Bi/BiOCl composites with dominant (001) facets in situ via a microwave reduction route. Different characterization techniques, including X-ray diffraction (XRD), field-emission scanning electron microscopy (FE-SEM), transmission scanning electron microscopy (TEM), UV-vis diffuse reflectance spectrometry (DRS), X-ray photoelectron spectroscopy (XPS), electron spin resonance spectroscopy (ESR), cathodoluminescence spectrum (CL), and lifetime, have been employed to investigate the structure, optical and electrical properties of the Bi/BiOCl composites. The experimental results show that the introduction of Bi particles can efficiently enhance the photocatalytic performance of BiOCl for the degradation of several dyes under ultraviolet (UV) light irradiation, especially for negative charged methyl orange (MO). Unlike the UV photocatalytic performance, such Bi/BiOCl composite shows higher degradation efficiency towards rhodamine B (RhB) than MO and methylene blue (MB) under visible light irradiation. This special photocatalytic performance can be ascribed to the synergistic effect between oxygen vacancies and Bi particles. This work provides new insights about the photodegradation mechanisms of MO, MB and RhB under UV and visible light irradiation, which would be helpful to guide the selection of an appropriate catalyst for other pollutants.

Full text of DOI:10.1016/j.jallcom.2015.06.113

Accepted Manuscript
Facile synthesis of Bi/BiOCl composite with selective photocatalytic properties
Dongling Chen, Min Zhang, Qiuju Lu, Junfang Chen, Bitao Liu, Zhaofeng Wang
PII:
S0925-8388(15)30225-5
10.1016/j.jallcom.2015.06.113
JALCOM 34466
DOI:
Reference:
To appear in: Journal of Alloys and Compounds
Received Date: 23 March 2015
Revised Date: 29 May 2015
Accepted Date: 15 June 2015
Please cite this article as: D. Chen, M. Zhang, Q. Lu, J. Chen, B. Liu, Z. Wang, Facile synthesis of Bi/
BiOCl composite with selective photocatalytic properties, Journal of Alloys and Compounds (2015), doi:
10.1016/j.jallcom.2015.06.113.
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ACCEPTED MANUSCRIPT

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Facile synthesis of Bi/BiOCl composite with selective
photocatalytic properties
Dongling Chen1, Min Zhang1, Qiuju Lu1, Junfang Chen1, Bitao Liu1*, Zhaofeng Wang2*
1 Research institute for new material technology, Department of Research Center for
Materials Interdisciplinary Science, Chongqing University of Arts and Science,
Chongqing 402160, PR. China
2 Department of Chemical & Biomolecular Engineering and Polymer Program, Institute
of Materials Science, University of Connecticut, Storrs, Connecticut 06269, United States
*Corresponding authors
Email: liubitao007@163.com (B. Liu); zhaofeng.wang@uconn.edu (Z. Wang)
Tel.: +86-023-49891752

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Abstract:
This paper presents a novel and facile method to fabricate Bi/BiOCl composites with
dominant (001) facets in situ via a microwave reduction route. Different characterization
techniques, including X-ray diffraction (XRD), field-emission scanning electron
microscopy (FE-SEM), transmission scanning electron microscopy (TEM), UV-vis
diffuse reflectance spectrometry (DRS), X-ray photoelectron spectroscopy (XPS),
electron spin resonance spectroscopy (ESR), cathodoluminescence spectrum (CL), and
lifetime, have been employed to investigate the structure, optical and electrical properties
of the Bi/BiOCl composites. The experimental results show that the introduction of Bi
particles can efficiently enhance the photocatalytic performance of BiOCl for the
degradation of several dyes under ultraviolet (UV) light irradiation, especially for
negative charged methyl orange (MO). Unlike the UV photocatalytic performance, such
Bi/BiOCl composite shows higher degradation efficiency towards rhodamine B (RhB)
than MO and methylene blue (MB) under visible light irradiation. This special
photocatalytic performance can be ascribed to the synergistic effect between oxygen
vacancies and Bi particles. This work provides new insights about the photodegradation
mechanisms of MO, MB and RhB under UV and visible light irradiation, which would be
helpful to guide the selection of an appropriate catalyst for other pollutants.
Key words：Photocatalyst; Composites; Semiconductors; BiOCl

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1. Introduction:
The decontamination of waste air and water through photocatalytic reactions, e.g.,
semiconductor photocatalysis, has drawn much attention during recent years1-3.
Photocatalysis has been considered as one of the most promising approaches to solve the
pollution problems. Bismuth-based oxides are suggested to be good candidates because
4-7
of their chemical and thermal stability, nontoxicity, and photocatalytic activity
.
Among them, bismuth oxychloride (BiOCl) attracts particular interest, which exhibits
higher photocatalytic activity than that of TiO2 in UV light region. The excellent
photocatalytic performance of BiOCl could be attributed to its characteristic layered
structure, optical property, as well as higher stability 8, 9. Since the recombination rate of
electron and hole in semiconductor photocatalysts is crucial to the photocatalytic
efficiency, it is highly desirable to develop an efficient way to improve the separation of
the photo-induced electron–hole pairs in BiOCl10-12
.
It is well known that molecular oxygen is green and low-cost oxidant, and
photocatalysis will activate molecular oxygen when electrons are generated on the
surface of semiconductor photocatalysts. The surface would be the key point, which not
only produces reactive oxygen species (ROS) like ·OH, ·O2− and H2O2, but also inhibits
the recombination of photo-induced electron/hole pairs 13-19. For example, it is reported
that O2 would adsorb onto the oxygen vacancies of TiO2 surfaces and be further
2−
converted into ·O2− or O2 , which act as a recombination center of photo-induced
electron/hole pairs and therefore enhance its photocatalytic activity 13, 17, 19-21. It is
interesting and shows an inspiration to modify BiOCl.
Actually, the layered structure feature of BiOCl endows itself internal static electric

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fields that can promote the effective separation of the photo-induced electron–hole pairs
22, 23. Moreover, density functional theory (DFT) calculations by Zhou et al. showed that
oxygen vacancies were stable and energetically favorable within BiOCl surface 24. Aimed
25
at this, some experimental work has also been carried out. For example, Ye et al.
reported that black BiOCl was prepared from white BiOCl by UV light irradiation; Yan
et al. 26 synthesized oxygen vacancies contained black BiOCl by Fe reduction; Hu et al. 27
prepared oxygen vacancies contained Bi/BiOCl composites by KBH4 reduction. All of
the above work showed enhanced photocatalytic activities. However, for an ideal catalyst,
it should be multifunctional under various environmental conditions. To date,
investigations on various organic molecule dyes have not been carried out.
Herein, we prepared oxygen vacancies contained Bi/BiOCl under microwave irradiation,
and examined its adsorption and photocatalytic performance for the degradation of dyes
with different molecular structures (MO, MB and RhB). The results provide a clearly
understanding of the degradation mechanisms of oxygen vacancies contained BiOCl,
which will be useful for the development of catalysts in water treatment facilities and
environmental remediation projects.
2. Experimental section
2.1 Preparation of BiOCl
Typically, 1.2148 g Bi(NO3)·5H2O and 1.1378 g mannitol were dissolved in 20 mL
deionized water, after which 10 mL saturated NaCl solution were added. The mixture was
o
then transferred to a 50 mL Teflon-lined autoclave, and maintained at 150 C for 4 h.
After cooling down, the product was washed and dried.

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2.2 Preparation of Bi/BiOCl composites
For the Bi/BiOCl composites, 1 g sample was first dispersed in 100 mL deionized water.
o
Then, the mixture was treated at 140 C for 5-180 min by reflux under microwave
irradiation in a microwave reactor (WF-3000, PreeKem Scientific Instruments Co., Ltd.,
China) operating at a power of 900W. The obtained sample was finally washed and dried.
The samples were numbered by its microwave treated time, e.g., sample treated with
microwave for 5 min was labeled as 5 MT-BiOCl.
2.3 Characterization
The morphology and microstructure of the samples were observed by field emission
scanning electron microscope (FESEM, Hitachi, SU-8020), transmission electron
micros-copy (TEM), highly resolution transmission electron micros-copy (HRTEM),
selected area electron diffraction (SAED). Crystal phase of the obtained materials were
determined by powder X-ray diffraction (XRD, Rigaku D/MAX-2400 X-ray diffract
meter with Ni-filtered Cu-Ka irradiation λ=1.54056 Å). UV-vis diffuse reflectance
spectra were recorded on a Hitachi UV-3900 spectrophotometer with an integrating
sphere, and BaSO4 was used as the reference. X-ray photoelectron spectroscopy (XPS,
PHI-5702, Physical Electronics) was performed using a monochromated Al Kα irradiation.
Peak deconvolution and quantification of elements were accomplished using Origin 8.0.
Low-voltage CL spectra were obtained using a modified Mp-Micro-S instrument.
Fluorescence lifetimes were measured on a Perkin-Elmer LS 50B spectrofluorimeter by
the time-correlated single photon counting method.
2.4 Catalyst activity test
The liquid-phase photo-degradation of different dyes (MO, MB and RhB) was carried

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out in a quartz tube under the irradiation of UV light and visible light, respectively. In a
typical process for degradation of dyes under the UV irradiation, 0.2 g catalyst was
suspended in 500 mL of 20 ppm dyes solution. Before irradiation, the suspensions were
stirred in the dark for 0.5 h to ensure the establishment of adsorption desorption
equilibrium. Under ambient conditions and stirring, the quartz tube was exposed to the
UV irradiation produced by a 500 W Xe arc lamp equipped with a band-pass light filter
(365 ± 15 nm). 3 mL sample solution was taken at a certain time interval during the
experiment and centrifuged to remove the catalyst completely. The solution was analyzed
on a Varian UV–vis spectrophotometer (Cary-50, Varian Co.). The percentage of
degradation is reported as C/C0. Here, C is the absorption of dyes solution at each
irradiated time interval of the main peak of the adsorption spectrum, while C0 is the
absorption of the initial concentration when the adsorption desorption equilibrium is
reached. For the visible light photocatalytic degradation of dyes, a UV cut off filter (λ>
420 nm) was used while the other experimental conditions are the same as that of the
above-mentioned degradation of dyes under the UV light irradiation.
3. Results and discussions
SEM images of pure BiOCl and the microwave treated BiOCl (MT-BiOCl) powders are
shown in Fig. 1. It is clearly observed that these products are composed of a large amount
of well-defined square-like nanosheets, which are 100–200 nm in width and 20–25 nm in
thickness. As suggested in previous researches 22-25, the indices of crystal face of the
square facets should be (001). For the MT-BiOCl composites (Fig. 1c, d), the
morphology is almost same as the pure BiOCl. Obviously, the microwave treatment of
BiOCl did not significantly affect their morphologies in relatively macro perspective. The

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detailed microstructure is shown in Fig. 2.
As shown in the TEM/HRTEM images of Fig. 2, it is also found that the BiOCl
nanoplates are 100–200 nm in width and 20-25 nm in thickness, which is in good
consistent with the SEM observations. For the pure BiOCl, the surface is smooth and
clean (as shown in Fig. 2c), which exhibits clear lattice fringes of the side face of a single
BiOCl nanosheet with d-spacing of 0.734 nm (Fig. 2e), corresponding to the (001) lattice
plane. This suggests that the BiOCl nanosheets are well-crystallized and possess a high
order of crystallinity. The SAED pattern also confirms the single-crystal feature of the
free standing crystal and its unique pattern of diffraction spots could be readily indexed to
(110) and (200) planes, indicating that the preponderant growth direction of the obtained
samples is [001] orientation. For the MT-BiOCl, it is found that there are some particles
adhered on the surface as shown in Fig. 2d and f. As the lattice fringes with d-spacing of
0.328 nm revealed by the HRTEM corresponds to the (012) lattice plane of Bi, these
particles are confirmed to be Bi, which agrees well with previous reports 26, 27
.
Figure 3 shows typical XRD patterns of the different samples. The diffraction peaks of
pure BiOCl are in good accordance with the standard card (JCPDS 06–0249) without
appearing any impurity peaks, suggesting it is of tetragonal structure. It can be seen that
the diffraction peak intensity of the (001) plane in these spectra is obviously stronger than
that of the (110) plane. It indicates that the crystal exhibits special anisotropic growth
along the [001] direction, which agrees well with the TEM results. However, after BiOCl
undergoing the microwave treatment (Fig. 3b-g), the (001) peak is greatly decreased
along with the increased microwave irradiation time, suggesting there is a change on the
microstructure.

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On the basis of the above results, we tested the photocatalytic activity of these samples
for the photodegradation of MO, MB and RhB under UV and visible light irradiation. It
can be seen that all samples show a quick degradation rate under UV light irradiation. It
degrades MO faster than MB and RhB. For the 30 MT-BiOCl, it can degrade almost all
the MO in 6 min, faster than that of 76% and 68% for MB and RhB. Interestingly, for
MB and RhB, the dark absorption properties are enhanced with the microwave treatment
time increased. For the 30 MT-BiOCl sample, it can absorb 30% of MB and 20% of RhB
in 30 min, which is much higher than the 7% of MB and 8% of RhB for the untreated
sample. However, there is almost no change for the dark absorption properties of MO by
increasing the treat time. Such phenomena could be attributed to the electrostatic
interactions between the surface of catalyst and the dye with different structures as shown
in Fig. 5. It is well accepted that RhB and MB molecules would form positively charged
cations in aqueous solution and thus could be adsorbed on the surface of BiOCl via
electrostatic interactions. In contrast, MO molecules would form anions in solution,
preventing the above absorption behavior. With the increase of the microwave irradiation
time, more Bi particles will be generated, and the surface of BiOCl becomes more
negatively charged. As a result, the absorption of positively charged RhB and MB
molecules would be enhanced with the microwave irradiation time increased.
When the dyes are tested under visible light irradiation, the degradation properties are
significant different. It only degrades 23% of MO and 5% of MB in 200 min, both of
which are much slower than 100% of RhB. It is obviously found that the MT-BiOCl
exhibits better photocatalyst properties than the untreated sample. In addition, the
MT-BiOCl shows a very good photocatalytic activity toward RhB under visible light

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irradiation, implying that the dye degradation occurs via a dye sensitized mechanism 28, 29
In addition to the different molecular structures, the changed surface microstructure
should also be responsible for this phenomenon. To better understand the relationship
between surface microstructure and photocatalytic activity, more effective detection
methods were used.
.
XPS is a powerful tool to identify the surface element states 30-32. XPS spectra were
recorded to confirm the elemental composition of the BiOCl and MT-BiOCl as shown in
Fig. 6. The high-resolution XPS spectra of Bi 4f are shown in Fig. 6a. Two peaks with
binding energies of 158.8 eV and 164.2 eV can be observed in the spectrum of the BiOCl
nanosheets, which can be ascribed to the binding energies of Bi3+ 4f7/2 and Bi3+ 4f5/2,
respectively. The result suggests that there are only triple charged Bi ions in the untreated
BiOCl. However, for the MT-BiOCl, it can be seen that two additional peaks with
relative low intensity appear at 162.4 eV and 157.0 eV, which can be ascribed to metallic
Bi 26, 27. Therefore, after a microwave treatment, the sample contains both BiOCl and
metallic Bi.
The XPS spectrum of O 1s is presented in Fig. 6b, which displays two peaks at 530.1
eV and 532.9 eV, respectively. It is confirmed that low-BE peak at 530.1 eV could be
assigned to the normal O in the crystal structure of BiOCl, while the high-BE peak at
532.9 eV could be attributed to the oxygen vacancies 33, 34. This result is consistent with
24, 25
the reported work, which would be beneficial to the photocatalytic activity
.
Therefore, these oxygen vacancies play roles on gathering positive charges on the surface,
leading to the preferential absorption of the negative dye molecules (MO), which well
explains the photocatalytic results under UV light irradiation.

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The EPR technique is an effective and common method to detect defects in materials.
Here, the EPR technique was applied to detect oxygen vacancies. Fig. 7 shows the EPR
spectra of pure BiOCl and MT-BiOCl. There is no EPR signal from the pure BiOCl,
which means there are few oxygen vacancies, hard to be detected by EPR 24, 25. On the
contrary, a remarkable EPR signal from MT-BiOCl is observed, and the peak at g = 2.001
is the characteristic signal of oxygen vacancies 35, 36. It indicates that the microwave
treatment could largely modify the surface microstructure and induce the formation of
oxygen vacancies like the UV irradiation in previous work 25.
UV-vis spectroscopy has been proved to be an effective optical characterization method
for understanding the electronic structure in semiconductors. As shown in Fig. 8, the
optical properties of pure BiOCl and MT-BiOCl were measured by UV-vis diffuse
reflectance spectroscopy. It reveals that the absorbance of BiOCl in the visible region is
negligible. Unlike pure BiOCl, the MT-BiOCl exhibits a continuous absorption band in
the range of 400–700 nm, which is in accordance with the color change of the samples.
Interestingly, the MT-BiOCl shows stronger absorption intensity in the UV region than
that of pure BiOCl, demonstrating its higher photocatalytic activity under UV light
irradiation. The band gaps of BiOCl and MT-BiOCl estimated by the Kubelka–Munk
function are approximately 3.32 eV and 3.24 eV, respectively. The band gap narrowing
of the BiOCl semiconductor is ascribed to the modification of surface microstructure by
Bi particles and oxygen vacancies, following the color change from white to black as
shown in Fig. 8.
For a deep investigation, the CL spectra were obtained. As shown in Fig. 9, the CL
spectrum of pure BiOCl exhibits an asymmetric distribution, and the intensity is much

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higher than that of the MT-BiOCl. Using Gaussian decomposition, the spectrum can be
divided into two main peaks at 385 and 475 nm, respectively. Obviously, the peak at 385
nm (~3.22eV) is the bandgap emission, while the emission at 475 nm should be ascribed
to the surface oxygen vacancies which can be easily formed in the high energy (001)
facets 24, 25. This is consistent with the results in previous work 24, 25
.
However, when BiOCl is treated by microwave (MT-BiOCl), the CL spectrum is very
different. First of all, the emission intensity is decreased to 38.9% of the pure BiOCl;
secondly, the emissions of the bandgap and oxygen vacancies shift to 414 and 485 nm,
respectively; thirdly, a new peak appears at 586 nm. The emission peak shift of bandgap
and oxygen vacancies could be attributed to the Schottky barrier between BiOCl and Bi
particles, while the new CL peak at 586 nm should be originated from the Bi particles.
The energy barrier would make the conduction band of BiOCl at the junction area
bending to a lower energy level, and the band gap is slightly decreased. Consequently,
the CL spectra would shift to a lower energy. This implies these Bi particles would play
an important role in photocatalysis.
The recombination of electron–hole pairs is also characterized by the lifetime of
37
carriers, which can indirectly imply the electrons transfer in semiconductors . It can be
reflected by the decays of PL transition centered on 480 nm excited at 350 nm as shown
in Fig. 10. The PL lifetime calculated by the exponential analysis is also presented. The
PL lifetimes of BiOCl and MT-BiOCl are determined to be ca. 1.1 and 2.04 ns,
respectively. The longer PL lifetime of MT-BiOCl suggests its lower electron–hole
recombination rate, which implies the more efficient trap ability of the oxygen vacancies
states.

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Based on the above investigations, a schematic illustration is proposed as shown in Fig.
11. When irradiated by UV light, electrons will be excited from valance band to
conduction band of BiOCl. Because of the newly formed oxygen vacancies, most of the
excited electrons will be trapped by the oxygen vacancies states. Due to the potential
differences among conduction band, oxygen vacancies states and the adhered Bi particles,
the excited electrons would be transferred to Bi particles. The oxygen vacancies would
make this transformation more easily as process ① exhibited. This would activate the
catalyst and the hole in the valance band can participate in oxidizing reaction.
38
Additionally, Bi particles also would act as a semiconductor . The electron band of the
Bi particles is at about -0.28 eV as calculated by the work function of metallic bismuth of
27
4.22 eV, which is more positive than -1.1 eV for the conduction band of BiOCl
.
Therefore, the space charge layer at the Bi-BiOCl junctions could accelerate the electrons
transfer from BiOCl to Bi particles, efficiently decreasing the recombination rate and thus
enhance the photocatalytic performance 27.
For the visible light irradiation, the dye degradation is achieved via a dye sensitized
mechanism. The excited dye can transfer electrons to the electron band of Bi particles or
other semiconductors to generate reactive oxygen species (ROSs) as process ②. The dye
acts as both a sensitizer and a pollutant 39-41. In the dye-sensitized mechanism, a good
charge transfer followed by separation is an important step. The LUMO of dyes and the
CB of the catalyst is another key factors for the higher charge transfer. A good interfacial
wave function mixing (or a well-aligned energy level) promotes the charge transfer rate.
Obviously, this decomposed process depends on the structures of the organic dyes. As

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reported, an efficient N-dealkylation (preceded by radical cation formation) was observed
with RhB, but not MB 42. The injection of an electron from excited dye and its transfer to
a mild oxidant proceeds at separate positions on the semiconductor surface, and thus
-
RhB+·-econd recombination is effectively prevented 42. Additionally, for the dyes can
hardly excited by visible light, such as MO and MB, their degradation should due to the
oxidation of the actively species as process ① exhibited. As a result, dyes with different
structures show various decomposition efficiency under visible light irradiation as shown
in Fig. 4.
4. Conclusion
This work presents a novel and facile method to fabricate Bi/BiOCl composites. The
structure, optical and electrical properties of the Bi/BiOCl composite were investigated in
detail. The results show that the properties of the samples, including their optical and
electrical properties, can be tuned by the modification of Bi particles. It is found that the
Bi particles can efficiently enhance the photocatalytic performance of BiOCl for the
degradation of different dyes under UV light irradiation. It can degrade almost all the MO
in 6 min, faster than that of MB and RhB. This should be ascribed to the co-work
between the oxygen vacancies and Bi particles, which can efficiently separate the
electron-hole recombination via the oxygen vacancies states. For the photocatalytic
performance under visible light irradiation, it can degrade much more RhB than MO and
MB. Compared with the 100% of RhB, it only degrades 23% of MO and 5% of MB in
200 min, due to the molecular structures. The results provide a new insight on the
photodegradation mechanism of different dyes under UV and visible light irradiation,
which will be helpful to select appropriate catalysts for pollutants.

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Acknowledgement
This study is supported by Chongqing Natural Science Foundation
(cstc2013jcyjA20023), Scientific and Technological Research Program of Chongqing
Municipal Education Commission (KJ1401111), Foundation of Chongqing University of
Arts and Sciences (Z2013CJ01 and R2013CJ05).
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Figure caption
Figure 1. SEM micrographs of pure BiOCl (a, b) and MT-BiOCl (c, d).
Figure 2. TEM/HRTEM images of pure BiOCl (a, c, e) and MT- BiOCl (b, d, f).
Figure 3. Typical XRD patterns pure BiOCl (a) and seriouse MT- BiOCl (b-f).
Figure 4. Liquid-phase photocatalytic degradation of MO (a, b), MB(c, d) and Rhb (e, f)
under UV light and visible light irradiation.
Figure 5. The molecular structures of the three different dyes.

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Figure 6. High-re solution XPS spectra of Bi 4f (a) and O 1s (b).
Figure 7. EPR spectra and sample image of pure BiOCl (a), and 30 MT- BiOCl (b).
Figure 8. UV-vis diffuses reflectance spectra of BiOCl (a) and 30 MT-BiOCl (b).
Figure 9. CL spectra of different samples (black line: pure BiOCl, red line: 30 MT-BiOCl,
inset: normalized CL spectra).
Figure 10. Lifetime of different samples (black line: pure BiOCl, red line: 30
MT-BiOCl).
Figure 11. A proposed schematic illustrations showing the reaction mechanism for
photocatalytic degradation of organic pollutants.

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