Effect of drying temperatures on structural performance and photocatalytic activity of BiOCl synthesized by a soft chemical method

Article abstract of DOI:10.1016/j.jssc.2016.04.039

A group of BiOCl photocatalysts with different drying temperatures were prepared by a soft chemical method. The effects of drying temperatures on the crystalline phase, morphology, surface area and optical property of as-prepared samples were investigated in detail by XRD, SEM, N₂ absorption-desorption and DRS. Moreover, their photocatalytic activities on the degradation of rhodamine B were evaluated under visible light irradiation. It was found that the sample dried at 120 °C had the best photocatalytic activity, which was mainly attributed to the highest exposing proportion of {001} facets correspond to BiOCl, largest BET and minimum bandgap. The degradation mechanism was explored that superoxide radicals were mainly contributed to the degradation of chromophore, however, holes and hydroxyl were mainly contributed to the photo degradation. Moreover, holes and hydroxyl dominated the degradation of RhB.

Full text of DOI:10.1016/j.jssc.2016.04.039

Journal of Solid State Chemistry 239 (2016) 259–264
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Effect of drying temperatures on structural performance and
photocatalytic activity of BiOCl synthesized by a soft chemical method
a
b
Lu Guang a, , Wang Hui , Zou Xuejun
n
a College of Chemistry, Chemical Engineering and Environmental Engineering, Liaoning Shihua University, Fushun, Liaoning 113001, China
b Department of Environmental Science and Technology, Dalian Nationalities University, Dalian 116600, China
a r t i c l e i n f o
a b s t r a c t
Article history:
A group of BiOCl photocatalysts with different drying temperatures were prepared by a soft chemical
method. The effects of drying temperatures on the crystalline phase, morphology, surface area and op-
tical property of as-prepared samples were investigated in detail by XRD, SEM, N2 absorption–desorption
and DRS. Moreover, their photocatalytic activities on the degradation of rhodamine B were evaluated
under visible light irradiation. It was found that the sample dried at 120 °C had the best photocatalytic
activity, which was mainly attributed to the highest exposing proportion of {001} facets correspond to
BiOCl, largest BET and minimum bandgap. The degradation mechanism was explored that superoxide
radicals were mainly contributed to the degradation of chromophore, however, holes and hydroxyl were
mainly contributed to the photo degradation. Moreover, holes and hydroxyl dominated the degradation
of RhB.
Received 29 January 2016
Received in revised form
3 April 2016
Accepted 28 April 2016
Available online 3 May 2016
Keyword:
Visible light
Rhodamine B
Nanosheet
Drying temperatures
& 2016 Elsevier Inc. All rights reserved.
1. Introduction
of BiOCl micro/nanostructures with various morphologies. Among
them, the soft chemical method is of great research interest for its
Organic dyes are used extensively to color products in the
textile industry, such as nylon, wool, cotton, and silk, as well as for
coloring oil, fats, waxes, varnish, and plastics [1]. Lots of these dyes
are non-toxic, while they can be converted to toxic complexes
with some heavy metal ions in wastewater. Therefore, it is ne-
cessary to treat dye effluents before they discharge into the re-
ceiving water [2,3]. Up to now, photocatalytic degradation is
considered as one of effective and convenient method for re-
moving of organic dyes [4,5]. Nano-scaled TiO2 is mostly studied
photocatalytic material. However, it can only be activated by ul-
traviolet light (less than 5% of the solar spectrum) [6]. In order to
utilize the visible light (accounting for 43% energy in solar spec-
trum), intensive efforts have been made to develop visible-light-
responsive photocatalysts [7–9].
Recently, bismuth oxychloride (BiOCl) has been attracted many
attentions owing to its optical properties and applications as cat-
alysts, ferroelectric materials, and pigments [10–13]. They can also
be used as excellent photocatalysts to decompose organic com-
pounds for purifying textile dye from polluted wastewater.
Therefore, some methods have been developed for the preparation
low-cost and energy-efficiency. For example, Wang et al. [14]
prepared xBiOI-(1ꢀx)BiOCl (x¼0.0, 0.2, 0.4, 0.6, 0.8, 1.0) com-
pounds in which the Bi(NO3)3 ꢁ 5H2O were added slowly into a
aqueous solution containing stoichiometric amounts of KCl and KI.
After the reaction was completed, the resulting solid product was
collected by filtration, washed several time and then dried at 80 °C
for 12 h. Zhang et al. [15] synthesized BiOCl powders using Bi
(NO3)3 ꢁ 5H2O, KCl and ethylene glycol as reactants. In addition,
the drying temperature was 50 °C. Chang et al. [16] prepared
BiOCl samples where HCl aqueous solution was added into
NaBiO3 ꢁ 2H2O solution. After the precipitation was complete, the
product BiOCl was centrifuged, washed and then dried at 100 °C
for 12 h. Based on these studies, it is found that the drying tem-
peratures seem to play a crucial role in shaping morphology of
BiOCl crystal and achieving consequent photocatalytic activity.
However, to the best of our knowledge, there has not been a
systematical study on the relationship among drying tempera-
tures, morphology of BiOCl crystal and consequent photocatalytic
activity so far.
In this study, an attempt has been made to systematically in-
vestigate the influence of drying temperatures on structural per-
formance and photocatalytic activity of BiOCl samples synthesized
by a soft chemical method. The results indicated that the drying
n
Corresponding author.
E-mail addresses: lu_g@163.com, luguang20121101@126.com (L. Guang).
http://dx.doi.org/10.1016/j.jssc.2016.04.039
0022-4596/& 2016 Elsevier Inc. All rights reserved.

260
L. Guang et al. / Journal of Solid State Chemistry 239 (2016) 259–264
temperatures can affect the crystalline form, morphology, BET, the
energy gap, and photocatalytic degradation efficiency.
2. Experiment
2.1. Sample preparation
All the samples were synthesized by a soft chemical method. Bi
(NO3)3 ꢁ 5H2O was added slowly into an hydrochloric acid solution
containing stoichiometric amounts of HCl with the Bi/Cl molar
ratio of 1. Subsequently, NH4OH was added in drops until the value
of pH was 6. After being stirred for 30 min, the resulting solid
product was collected by filtration, washed several times with
ethanol and deionized water thoroughly. The samples were ob-
tained by being dried in an air oven at different temperatures (40,
70, 100, 120, 140, 160 °C) before further characterizations.
Fig. 1. XRD patterns of different samples. ( BiOCl; BiCl3; Bi3O4Cl).
2.2. Characterization
(2
θ
¼12.11) becomes stronger with increase in drying tempera-
X-ray powder diffraction (XRD) measurements of all the sam-
ture, suggesting the higher exposing proportion of {001} facets. In
addition, the samples dried at 140 and 160 °C can be identified as
mixed phase of BiCl3 and Bi3O4Cl (JCPDS Card no. 36-0760), and
the Bi3O4Cl contents in these composites increase with increase-
ment of drying temperature. These results indicate that crystalline
phase of BiOCl are destroyed when drying temperatures are over
120 °C, and exposing proportion of {001} facets correspond to
BiOCl is highest as drying temperatures is 120 °C.
ples were performed in the reflection mode (Cu K
¼1.5406 Å) on a XRD-6000 diffractometer with a scanning rate
α
radiation,
λ
of 2° minꢀ1. Scanning electron microscopy (SEM) measurements
were performed using a Hitachi S-4300 scanning electron field
emission microscope operating at 15 kV. The nitrogen adsorption
and desorption isotherms at 77 K were measured using a Micro-
metrics ASAP 2020 V3.00 H system after the sample was degassed
in a vacuum at 120 °C for 400 min UV–vis diffuse reflectance
spectra (DRS) were obtained with the JASCO 570 spectro-
photometer by using BaSO4 as a reference and were converted
from reflection to absorbance by the Kubelka-Munk method.
3.2. SEM analysis
The microstructure and morphology of the resulting products
are investigated by SEM, as presented in Fig. 2. At the initial drying
temperature of 40 °C, lots of nanosheets in irregular shape are
agglomerated together. Then, some nanosheets disengage form
congeries and form independent individuals at drying tempera-
ture of 70 °C. As the drying temperatures prolong to 100–120 °C,
the orderly BiOCl nanosheets are formed with the in-plane size of
about 100–300 nm and thickness of about 20–50 nm. Further in-
creasing the drying temperatures to 140–160 °C, the structure of
nanosheets are changed and even collapsed.
2.3. Photocatalytic activity test
The catalytic reaction was carried out to degrade 10 mg/L
rhodamine B (RhB) aqueous solution. Reaction suspensions were
prepared by adding 50 mg photocatalyst powders into 200 mL RhB
aqueous solution under vigorously stirring. Prior to irradiation, the
suspensions were stirred in the dark for 1 h to reach adsorption-
desorption equilibrium. Subsequently, the photocatalytic reaction
was initiated by irradiating the system with a 500 W xenon lamp
with a 420 nm cutoff filter, which was placed under the reaction
cell to completely remove all incoming wavelengths shorter than
420 nm to ensure irradiation with visible light only. At given time
intervals, 4 mL analytical suspension was collected and centrifuged
to remove photocatalyst particles. Then the concentration of the
filtrate was analyzed by the UV–vis spectroscopy (UV-7504/PC),
and the absorbance at 553 nm was monitored.
3.3. BET surface areas and pore structure
Fig. 3 shows the N2 adsorption–desorption isotherm of as-
prepared BiOCl samples. According to the Brunauer-Deming-
Deming-Teller (BDDT) classification [17], the isotherms can be
nearly categorized as type IV with a distinct hysteresis loop in-
dicating a mesoporous structure. The shape of the hysteresis loops
is close to Type H3, suggesting the existence of slit-like pores that
are generally formed by the aggregation of plate-like particles [18].
In addition, few the hysteresis loop is discovered for the BiOCl
sample dried at 160 °C, showing the obtained BiOCl particles ser-
iously cumulate. The BET surface area, pore volume and pore size
of as-prepared BiOCl samples are summarized in Table 1. With an
increase in drying temperatures, the BET surface areas and pore
volumes of as-prepared BiOCl samples increase passing through a
maximum and then decrease at higher dry temperatures. And the
BiOCl sample dried at 120 °C exhibits the highest BET surface area
(8.621 m2/g) and pore volume (0.01846 cm3/g). Moreover, the pore
sizes of as-prepared BiOCl samples decrease with an increase in
drying temperatures. According to Fig. 2, the sizes of BiOCl na-
nosheets become uniform and small with increase in drying
temperatures from 40 °C to 120 °C, resulting in the increase in BET
and pore volumes. However, sintering phenomenon happens with
Different scavenger molecules were introduced to the reaction
medium prior to the addition of the photocatalysts to evaluate the
reactive species formed during the degradation of RhB. The dosage
of these scavengers (NaHCO3, i-PrOH and BQ) was 10 mM.
3. Results and discussion
3.1. XRD analysis
Fig. 1 shows the XRD patterns of the as-synthesized BiOCl
materials dried at different temperatures. All of the peaks of
samples dried at 40, 70 and 100 °C can be reliably indexed as
tetragonal phase of BiOCl (JCPDS Card no. 85-0861). Then the
sample dried at 120 °C can be identified as mixed phase of BiCl3
(JCPDS Card no. 88-2077) and BiOCl. For these samples, the re-
lative diffraction intensity of the {001} peak correspond to BiOCl

L. Guang et al. / Journal of Solid State Chemistry 239 (2016) 259–264
261
Fig. 2. SEM images of the different BiOCl samples dried at (a) 40 °C, (b) 70 °C, (c) 100 °C, (d) 120 °C, (e) 140 °C and (f) 160 °C.
spectra (DRS), as shown in Fig. 4(a). It is found that all the samples
exhibit strong photo absorption in the UV region and different
strengths of absorption in the visible light region. The BiOCl
samples dried at 40, 70, 100, 120, 140 and 160 °C have intense
absorption edges at about 424, 424, 432, 445, 442 and 442 nm,
respectively. The Fig. 4(b) shows the plot of the (αh ) vs. hν for
ν 1/2
different BiOCl samples. The band gap energies (Eg) of as-prepared
samples are listed in Table 1. At drying temperatures of 40 and
70 °C, an Eg value of 2.86 eV is obtained and corresponds to the
value of crystalline BiOCl. Above 100 °C, the Eg values decrease
with drying temperature until about 120 °C. This continuous de-
crease from 100 to 120 °C is related to the presence of the amor-
phous or crystalline BiCl3 present above BiOCl, which is consistent
with the XRD results presented above. Above 140 °C, the Eg value
is relatively constant and close to the value found for reference
compounds consisting of Bi3O4Cl and BiCl3. Thus, the Eg value
depends on drying temperature and is related to the crystalline
structure of as-prepared photocatalysts. In addition, it is clear that
all these samples are responsive to visible light, implying that they
are able to work with visible light.
Fig. 3. N2 absorption–desorption isotherms of the as-synthesized BiOCl samples.
Table 1
Characteristics of the synthesized BiOCl particles.
Dry temperatures
(°C)
BET (m2/ Vp (cm3/g) Dp (nm) Eg (eV) Composition
g)
3.5. Photocatalytic activity
The photocatalytic activities of the prepared BiOCl photo-
catalysts are evaluated by the degradation of RhB solution under
visible light irradiation. The photo degradation rate of RhB is
shown in Fig. 5. The degradation of RhB in the absence of photo-
catalyst is less than 3% within 18 min, which could be neglected.
About 71%, 94%, 99%, 100%, 100% and 67% of RhB is degraded
within 18 min by the BiOCl samples dried at 40, 70, 100, 120, 140
and 160 °C, respectively. It is evident that the BiOCl sample dried
at 120 °C exhibits the highest photocatalytic activity. Three reasons
may account for this result. First, it is generally accepted that the
catalytic process is mainly related to the adsorption and deso-
rption of molecules on the surface of the catalyst [19–20]. Ac-
cording to N2 adsorption results, it is found that the BET of the
BiOCl nanosheets dried at 120 °C is calculated to be 8.621 m2/g,
40
70
100
120
140
160
0.1817
4.001
6.704
8.621
5.433
1.329
0.00265
0.01018
0.01447
0.01846
0.01820
0.01191
583.6
14.47
8.855
4.416
4.410
3.584
2.86
2.86
2.81
2.76
2.77
2.77
BiOCl
BiOCl
BiOCl
BiOCl, BiCl3
Bi3O4Cl, BiCl3
Bi3O4Cl, BiCl3
further increase in drying temperature, leading to the decrease in
BET and pore volumes.
3.4. DRS results
The optical properties of the BiOCl samples dried at different
temperatures are measured using UV–vis diffuse reflectance

262
L. Guang et al. / Journal of Solid State Chemistry 239 (2016) 259–264
Fig. 4. DRS spectra (a) and the plot of the (αhν)1/2 vs. hν (b) of as-prepared BiOCl samples.
100 °C, it exhibits the better visible-light photodegradation per-
formance. In addition, although the sample dried at 140 °C pos-
sesses the similar band gap than the sample dried at 160 °C, it
exhibits the better RhB degradation.
3.6. Possible degradation mechanism
It is generally accepted that RhB can be degraded via photo-
catalytic oxidation by different species, such as holes (hþ), hy-
ꢂ
droxyl (
ꢂ
OH) and superoxide radicals (O2 ꢀ) [22–24]. Therefore,
the corresponding effective scavengers are added to the reaction
solution, namely, NaHCO3 [25], i-PrOH [26] and BQ [27], respec-
tively. NaHCO3 is employed to trap hþ and
ꢂ
ꢂ
OH, i-PrOH scavenges
OH and BQ scavenges O2 ꢀ. By adding different scavengers to the
ꢂ
reaction solution to remove the corresponding reactive species,
the role of different reactive species in the photocatalytic process
can be evaluated based on changes in the photocatalytic efficiency.
As shown in Fig. 6, about 100% RhB is degraded after 30 min of
irradiation without the addition of scavengers. By adding i-PrOH
and NaHCO3 to the photocatalytic reaction systems, about 73.5%
and 37.2% RhB is degraded, respectively, indicating that the pho-
Fig. 5. Photocatalytic activities of BiOCl samples with different drying tempera-
tures for the degradation of RhB under visible light.
which is the largest in as-prepared samples. Since the BiOCl na-
nosheets dried at 120 °C has the largest specific surface areas, each
can absorb more active species and reactants on its surface. Sec-
ond, another factor could be broadened the band gap energy. The
small band gap energy could be excited to produce active species
easily and then make the photocatalytic process more efficient. As
shown in Table 1, it can be observed that the band gap energy of
BiOCl nanosheets dried at 120 °C is 2.76 eV and is smaller than the
other samples. Third, the exposed crystal facets play a critical role
in determining the photocatalytic reactivity and efficiency, since
the photocatalytic reaction takes place on the surface of the pho-
tocatalyst [21]. The more the specific sites there are in the exposed
facet, the more active the photocatalyst becomes. The {001} facets
possesses more specific sites in BiOCl crystals [13,15], thus high
exposing proportion of {001} facets should be beneficial to the
high photocatalytic efficiency. The BiOCl sample dried at 120 °C
possesses the higher exposing proportion of {001} facets, which
results in the better visible-light photodegradation performance
than the samples dried at 40, 70, 100 °C. Since the crystalline
structure of samples dried at 140 and 160 °C are different from
above four samples (Fig. 1), it is reasonable for these two samples
to have different photocatalytic effects. However, which crystal
face possesses more specific sites for Bi3Cl4O and whose crystal
face possesses more specific sites for Bi3Cl4O and BiOCl are still
open questions to answer. Finally, it is worth noting that these
factors could cooperate with each other to affect the photocatalytic
activity. For example, although the sample dried at 140 °C pos-
sesses the smaller specific surface areas than the sample dried at
togenerated hþ and
ꢂ
OH play important roles in the photo-
catalytic degradation of RhB. In the presence of BQ, degradation
efficiency of RhB is about 92.7%, which indicates that dissolved
ꢂ
oxygen in water can capture electrons and produce O2 ꢀ. These
results suggest that i-PrOH, NaHCO3 and BQ inhibit the photo-
catalytic degradation of RhB, and that
roles in the photocatalytic degradation of RhB.
ꢂ
OH and hþ play important
It is well known that RhB can absorb visible light in the range of
460–600 nm. In addition, the band gap energy of BiOCl nanosheets
dried at 120 °C is 2.76 eV (as shown in Table 1). Therefore, RhB and
BiOCl can be excited by visible light. The schematic diagram of RhB
photodegradation using BiOCl is shown in Fig. 7. Upon illumina-
tion of visible light, electron–hole pairs are generated in BiOCl
nanasheets. The CB potential of BiOCl semiconductor (0.46 eV) is
less positive than the standard reduction potential of oxygen
(ꢀ0.046 eV) [28]. Thus, the electrons of CB on the BiOCl surface
ꢂ
cannot deoxidize O2 into O2 ꢀ. The CB potential of BiOCl semi-
conductor is more positive than lowest unoccupied molecular or-
bital (LUMO) (ꢀ0.6 eV) of RhB [29], but more negative than the
highest occupied molecular orbital (HOMO) of RhB (1.6 eV) [30].
Thus, the electrons of LUMO of RhB are injected into the CB of
BiOCl, when the RhB molecules are irradiated with visible light
and excited to the excited state (RhB*). Then, these electrons are
ꢂ
ꢀ
captured by the O2 in water, producing superoxide radicals O2
,
leading to degradation of chromophore of RhB. The VB potential of
BiOCl semiconductor (3.22 eV) is more positive than the oxidation

L. Guang et al. / Journal of Solid State Chemistry 239 (2016) 259–264
263
activity. This high activity is mainly attributed to photogenerated
holes and hydroxyl.
Acknowledgements
This work was supported by the Key Laboratory of Industrial
Ecology and Environmental Engineering, China Ministry of Edu-
cation (KLIEEE-13-03).
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