Photocatalytic study of BiOCl for degradation of organic pollutants under UV irradiation

Article abstract of DOI:10.1016/j.jphotochem.2010.07.026

BiOCl exhibited high photocatalytic activities for the degradation of rhodamine B, methyl orange and phenol. Surface chloride ions were adverse to the BiOCl photocatalysis and dissociated from BiOCl via reaction with photogenerated holes and electrons under UV irradiation. Conduction band electrons of BiOCl directly reduced either chlorine radical or the azo-bond of MO during the photocatalytic process. Hydroxyl radical was the main oxidative species in the BiOCl photocatalysis, whose generation can be accelerated via enhancing the conduction band electron consumption by MO. After the photocatalytic reaction, the dissolved chloride ion would spontaneously recombine back to the BiOCl photocatalyst, hence qualifies BiOCl as a practical high-activity photocatalyst with long lifetime.

Full text of DOI:10.1016/j.jphotochem.2010.07.026

Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 76–80
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic study of BiOCl for degradation of organic pollutants under UV
irradiation
∗
Feng Chen , Hongqi Liu, Segomotso Bagwasi, Xingxing Shen, Jinlong Zhang
Key Lab for Advanced Materials and Institute of Fine Chemicals, East China University of Science and Technology, 130 Meilong Road, Shanghai 200237, PR China
a r t i c l e i n f o
a b s t r a c t
Article history:
BiOCl exhibited high photocatalytic activities for the degradation of rhodamine B, methyl orange and
phenol. Surface chloride ions were adverse to the BiOCl photocatalysis and dissociated from BiOCl via
reaction with photogenerated holes and electrons under UV irradiation. Conduction band electrons of
BiOCl directly reduced either chlorine radical or the azo-bond of MO during the photocatalytic process.
Hydroxyl radical was the main oxidative species in the BiOCl photocatalysis, whose generation can be
accelerated via enhancing the conduction band electron consumption by MO. After the photocatalytic
reaction, the dissolved chloride ion would spontaneously recombine back to the BiOCl photocatalyst,
hence qualifies BiOCl as a practical high-activity photocatalyst with long lifetime.
Received 15 March 2010
Received in revised form 24 June 2010
Accepted 22 July 2010
Available online 30 July 2010
Keywords:
BiOCl
Chlorides
Chlorine radical
Photocatalysis
Photostability
©
2010 Elsevier B.V. All rights reserved.
rides such as ferric chloride ([FeCl]2+) also produce Cl atom, namely,
Cl radical [8]. Thus, undesired organic halides would possibly be
1
. Introduction
•
Semiconductor photocatalysis has attracted increasing atten-
produced as the products of Cl radicals attack on organics.
tion as a potential environmental technology for wastewater
remediation [1–5]. The most utilized photocatalysts are metal
oxides such as TiO2 [1,2], while halides such as AgCl and AgBr
have been found to exhibit higher activities than metal oxides for
the degradation of dyes [3–5]. The main drawback for the metal
halide photocatalyst is their poor photostability, as metal halide
bond is possibly cleaved under irradiation [4]. Recently, Huang and
Yu reported highly enhanced photostability of silver halides com-
Like halides, oxyhalides are also indirect band-gap semicon-
ductors, which have comparatively slow recombination rate for
photogenerated electrons and holes. Consequently, they usually
exhibit high activity in photocatalytic applications. As an oxy-
halide photocatalyst, BiOCl has a similar valence band (VB) as that
of AgCl, which is mainly composed of Cl3p orbital. BiOCl showed
a significant photocatalytic reactivity for organic degradation in
either gas or aqueous solution [9,10], which is much higher than
the commercial P25 TiO2 photocatalyst. However, if BiOCl photo-
catalytic reaction generates undesirable organic chlorides and/or
BiOCl exhibits poor photostability, BiOCl photocatalysis would
not be employed in practical use as an environmental-unfriendly
method. Hence, an investigation into the photocatalytic mecha-
nism of BiOCl was carried out in this work. The degradation of
methyl orange (MO), rhodamine B (RhB) and phenol were traced.
Active oxidative species as well as possible chloride-containing by-
products were detected with the fluorescence spectroscopy, while
chloride ions were measured with ion chromatography.
0
bined with a small amount of Ag [3,5], however, the worry on the
lifetime of silver halides still remained, as the photolytic reaction
0
of AgX to Ag and X in aqueous solutions is an irreversible process.
2
Photocatalysis mechanism of metal oxide semiconductor has
been well investigated, in which hydroxyl radical and superoxide
radical from the photogenerated hole and electron are confirmed
as the main active species responsible for the degradation of
organic compounds in aqueous solution [6,7]. However, in the
halide/oxyhalide photocatalysts, the valence band is composed of p
orbital of halides instead of O2p. Therefore, there is a high possibil-
•
ity that a halide radical instead of OH radical is the active species
responsible for the degradation under UV irradiation. A well-known
phenomenon is the photo-decomposition of AgCl, which results
in Ag0 and Cl atom [3]. In addition, the photolysis of other chlo-
2. Experimental
2.1. Preparation of BiOCl photocatalyst
BiOCl photocatalyst was prepared according to the literature
procedure [11] except CTAC instead of KCl was adopted as a chlo-
ride source [12]. 1.96 g Bi(NO ) ·5H O and 1.31 g cetyltrimethyl
∗ Corresponding author. Tel.: +86 21 64252062; fax: +86 21 64252062.
E-mail address: fengchen@ecust.edu.cn (F. Chen).
3
3
2
1
010-6030/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.jphotochem.2010.07.026

F. Chen et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 76–80
77
Fig. 1. (A) XRD pattern and (B and C) SEM images of the as-prepared BiOCl.
ammonium chloride (CTAC) were dissolved in 80 mL ethylene
2.2. Photocatalytic activity measurement
glycol (EG), pH of the solution was then adjusted to 1.0 with
KOH/EG solution (1.0 M). The resulting solution was then kept
at 160 C for 12 h in a 100 mL Teflon-lined stainless autoclave.
In order to avoid the possible interference of organic species,
the obtained powder was irradiated with ultraviolet light for
In a typical photocatalytic experiment, aqueous suspension of
MO (60 mL, 20 mg/L), RhB (60 mL, 20 mg/L) and 0.06 g photocata-
lyst powder were placed in a quartz tube with vigorous agitation.
The pH value of the aqueous solution was adjusted to 5.00 for MO
and 6.86 for RhB with HCl solution. UV irradiation was carried out
using a 300 W high-pressure mercury lamp (Yaming Lighting). The
distance between the light and the center of quartz tube was 20 cm.
◦
2
h after careful washing. The as-prepared powder was BiOCl
microsphere composed of numerous BiOCl nanoplates (Fig. 1,
2
SBET = 8.0 m /g).
Fig. 2. Degradation of (A) RhB (20 mg/L, pH 6.86) and (B) MO (20 mg/L, pH 5.00) with BiOCl under UV irradiation.

7
8
F. Chen et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 76–80
During the reaction, the water-jacketed photochemical reactor was
cooled with water-cooling system to maintain the solution at room
temperature. Prior to irradiation, the suspensions were stirred in
the dark for 0.5 h to allow an adsorption–desorption equilibrium.
At irradiation time intervals, 4.0 mL of the suspensions was col-
lected and then centrifuged to remove the photocatalyst powder.
A Cary 100 UV–vis spectrometer was used to record the change of
concentration of dyes during light irradiation.
2.3. Hydroxyl radical detection
Relative concentration of hydroxyl radical was measured
with
a fluorescent method. Hydroxyl radicals reacted with
3
.0 mM terephthalic acid, and their concentrations were deduced
semi-quantitatively from the fluorescent intensity of hydrox-
yterephthalic acid at 425 nm (excitation wavelength: 312 nm).
Fig. 3. Fluorescent intensity of terephthalic acid (3.0 mM) aqueous solution at
25 nm with BiOCl under UV irradiation (a) in the absence of MO in air atmo-
2.4. Intermediates detection
4
sphere, (b) in the presence of MO in air atmosphere, (c) in the absence of MO in
N2 atmosphere and (d) in the presence of MO in N2 atmosphere.
Reaction intermediates detection was carried out in the degra-
dation of 1.0 mM phenol. After a given irradiation time, samples
of 4.0 mL were withdrawn, and centrifuged to remove the
photocatalyst powder. Then the fluorescence spectra of the cor-
responding intermediates in the supernatant were measured with
a Cary Eclipse spectrophotofluorometer under different excitation
wavelengths. Chloride ion detection was performed on an ion chro-
matography (DIONEX, DX 600).
the N2 bubbling, some other factors should be considered. BiOCl
is an indirect band structure material with a band gap of 3.50 eV.
The potentials of conduction band (CB) and VB for BiOCl are −1.1
and 2.4 eV, while those for anatase TiO are −0.3 and 2.9 eV [14,15],
2
respectively. As we know, electron transfer depends greatly on the
energy difference between the CB and the reduction potential of
the acceptor redox couple. The CB electron in the BiOCl has much
3
. Results and discussion
negative redox potential than that in the TiO . Therefore, besides
2
MO and RhB were selected as the target pollutants to trace the
the reaction of electrons with adsorbed oxygen molecules, it is pos-
sible that CB electron directly reduce adsorbed organics, which led
to the faster degradation of MO under the N2 bubbling.
photocatalytic activity of BiOCl under the UV irradiation. The dyes
were both degraded smoothly under either aerated or deaerated
condition as shown in Fig. 2. Most surprisingly, the degradation of
MO was even enhanced by N2 bubbling. Present knowledge on the
photocatalytic mechanism shows that the surface redox reactions
initiated by photogenerated electrons and holes start the photocat-
alytic degradation of organics. If the surface chemical reactions of
CB electrons are blocked, the VB holes would tend to be consumed
by the accumulated electrons via the electron–hole recombination
inside the BiOCl nanoparticles; hence, the photocatalytic degrada-
tion process would be significantly suppressed. For example, the
To further study the photocatalytic mechanism of BiOCl,
hydroxyl radicals were measured with a fluorescent method. By
reacting with terephthalic acid, hydroxyl radical can be analyzed
semi-quantitatively from the fluorescent intensity of hydrox-
yterephthalic acid at 425 nm [16]. Fig. 3 shows the fluorescent
intensity changes vs irradiation time under various conditions. In
the absence of MO, the generation of ·OH was obviously decreased
by N bubbling. According to the previous literatures in TiO2 pho-
2
tocatalysis [13], it should be due to the cutting off of the cathode
reaction between the CB electron and the oxygen, which favors
the charge recombination. However, in the presence of MO, the
generation of ·OH was even enhanced with the N2 bubbling. The
generation of ·OH was significantly increased by MO either with
or without the N2 bubbling. It is presumed a direct reduction
occurred under the UV irradiation between the CB electron and
photocatalytic activity of TiO is nearly lost in the absence of oxygen
2
[
13].
The cathode reaction of TiO2 photocatalysis is usually regarded
as the reaction of electron with pre-adsorbed oxygen molecules
and thus would be blocked by N2 bubbling. However, since the
degradation rate of MO over BiOCl particles was not reduced by
Fig. 4. UV–vis spectra of methyl orange degradation (A) with and (B) without N2 bubbling.

F. Chen et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 76–80
79
Fig. 6. Concentration variation of (a) phenol and its degradation intermediates, (b)
catechol, (c) hydroquinone and (d) 2-chlorophenol/4-chlorophenol.
Fig. 5. The photocatalytic degradation of MO with BiOCl under UV irradiation in
the (a) absence and (b) presence of 50 mM methanol, and (c) presence of 3.0 mM
terephthalic acid.
oxidative species in BiOCl photocatalysis in aqueous solution. Pho-
togenerated holes should react with the adsorbed water and surface
hydroxyl group to produce hydroxyl radical, which process was
however not inhibited with 50 mM of methanol in the presence of
55.5 M water solvent. Chlorine radical (1.359 eV) has lower oxida-
the adsorbed MO, and consequently promoted the electron–hole
separation. From the UV–vis spectra obtained during the MO degra-
dation(Fig. 4), anewUV absorbance peakat 246 nmappearedunder
•
tive potential than that of OH radical (1.7 eV). It has been reported
•
−
N2 bubbling, and it was not observed in the presence of O . Elec-
that OH radical in aqueous solution would oxidize Cl anion to
2
•
tron reduction of azo dyes has been reported elsewhere [17,18].
The new absorption can be attributed to the electron reduction of
azo band, which resulted in the generation of hydrazine derivative
produce Cl radical, which is adverse to the oxidation of organ-
ics [21]. However, one should notice that the potential of valence
band of BiOCl is 2.4 eV (although it is also composed of 3p orbital
•
[
19].
Therefore, in the presence of O , the reaction of CB electron
of Cl atom), much more positive than that of OH radical. The reac-
tion between photogenerated holes from BiOCl photocatalyst with
2
•
with adsorbed oxygen molecules competes with the direct electron
reduction of azo-bond, which leads to the relatively slow degrada-
tion kinetics of MO. As for the xanthene dye RhB, due to the absence
of oxidative groups, a direct electron reduction can hardly take
place. Therefore, the degradation of RhB was significantly reduced
by N2 bubbling, which is common observation in TiO2 photocatal-
ysis.
adsorbed water or hydroxyl group is favored to produce OH rad-
ical. As shown in Fig. 5, the degradation of MO by BiOCl greatly
decreased in the presence of 3.0 mM terephthalic acid, which acted
•
as a good scavenger for OH radical.
•
However, if Cl radicals were bleached from the BiOCl or the
possible reaction between Cl anions and photogenerated holes
or secondary OH radicals occurred, the risk of the production of
−
•
Several possible primary active oxidative species exists in the
organic chlorides still remains. Hence, the degradation intermedi-
ates of phenol were carefully investigated from fluorescent spectra
to check for the possible chloride-adducts of phenol.
•
•
•
BiOCl photocatalytic reaction: OH radical, hole and Cl radical. OH
radical seemed to be predominant as a great amount of hydroxyl
adducts were detected in the fluorescent spectra. Alcohol was
proved to be an effective hole-trapper according to previous study
In general, phenol and phenolic compounds have strong fluo-
rescence under suitable UV excitation, which is highly sensitive
and selective [22]. Such characteristics can be used to deter-
mine a particular phenolic compound in the aqueous solution. By
choosing proper excitation wavelengths for phenol (ꢀex = 267 nm,
ꢀem = 296 nm), catechol (ꢀex = 286 nm, ꢀem = 324 nm), hydro-
quinone (ꢀex = 303 nm, ꢀem = 333 nm), and 2-chlorophenol/4-
chlorophenol (ꢀex = 232 nm, ꢀem = 346 nm), phenol and its inter-
[
20]. If holes act as a main oxidative species in a photocatalytic
system for the degradation of organics, the addition of alcohol
would decrease the degradation kinetics. However, the photocat-
alytic degradations of MO by BiOCl were not affected by the addition
of 50 mM methanol (Fig. 5). Therefore, in this regard, it can be con-
fidently concluded that photogenerated holes were not the main
−
Fig. 7. (A) Concentration change of dissolved Cl anion in phenol/BiOCl suspension under UV irradiation and (B) the XRD patterns of BiOCl photocatalyst (a) before and (b)
after 15 h UV irradiation.

8
0
F. Chen et al. / Journal of Photochemistry and Photobiology A: Chemistry 215 (2010) 76–80
mediates were independently detected from the reaction solution.
Fig. 6 shows the concentration variations of phenol and its degra-
dation intermediates. Hydroxyl adducts of phenol such as catechol
and hydroquinone were significantly produced, which confirmed
•
a OH radical-attack mechanism. Chloride-adducts of phenol were
not observed throughout the degradation process of phenol, which
partially relieved our worry on the generation of organic chlorides.
The cleavage of chloride anion from the BiOCl occurred under
UV irradiation (Fig. 7). The concentration of dissolved chloride
anion obeyed first-order kinetics and had a final concentration
2
of 0.216 mM (with a UV light intensity of 1.10 mW/cm ), which
is similar to that ever observed with AgCl. Fortunately, the dis-
solved chloride anion tends to react with the chloride anion-lack
powder BiOCl (OH)x to form BiOCl, as the pKsp of BiOCl is 30.75
1
−x
−
−31
2
(
[BiO][Cl ] = 1.8 × 10
M ). The concentration of dissolved chlo-
ride anion was reduced very fast to a very low level in 90 min.
The dissolution and recovery of the chloride anion can be repeated
many times by turning on and off the UV lamp (Fig. 7A). It was
observed that BiOCl photocatalyst can be used repeatedly for organ-
ics degradation without any significant changes on its chemical
composition. Although the layered structure of BiOCl was a little
bit distorted after long time UV irradiation (15 h, Fig. 7B), the lat-
tice structure inside the [BiO] layer was kept almost unchanged.
Because of the application of chlorine disinfectant in the water
Fig. 8. Degradation of MO (20 mg/L) under UV irradiation with (a) BiOCl, (b) surface
Cl abundant BiOCl and (c) surface Bi abundant BiOCl.
−
3+
whose generation can be accelerated via enhancing the CB electron
consumption by azo dyes such as MO. During the photocatalytic
reaction, surface chloride ion captured the photogenerated hole to
produce chlorine radical. Immediately, the chlorine radical recom-
bined with the CB electron instead of reacting with organics, which
resulted in dissolution of chloride ions. After the photocatalytic
reaction, the dissolved chloride ion spontaneously recombines back
to the BiOCl photocatalyst, which ensured BiOCl as a practical pho-
tocatalyst with long lifetime.
−
plant, it is reported that the concentration of Cl in the tap water of
−
China is varied from 0.14 to 1.4 mM, e.g., [Cl ] in the tap water
of Shanghai is ca. 1.4 mM. Further, the municipal effluents and
−
the industrial wastewater generally contain a concentration of Cl
higher than several hundreds mg/L. Therefore, although the recov-
ery of BiOCl was observed to proceed in batch reactor in this work,
the application of BiOCl for the flow reactor seems applicable to tap
water, the municipal effluents and the industrial wastewater with-
out chloride dissolving. However, as for natural water which has
Acknowledgements
This work was supported by the National Science Foundation
of China (20777015), National Basic Research Program of China
−
a low [Cl ] level (0.02 mM to more than 4.0 mM [23–25]), further
(2010CB732306, 2007CB613301) and the Fundamental Research
−
modification of BiOCl material is need to avoid the Cl escaping
Funds for the Central Universities.
from the photocatalyst for the potential application in the flow
reactor.
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[
[
tion of RhB, MO and phenol. BiOCl exhibited strong reduction ability
due to the CB potential of −1.1 eV; therefore, a direct reduction of
azo-bond was observed in the photodegradation of MO. Hydroxyl
radical was the main oxidative species in the BiOCl photocatalysis,