Hydrothermal synthesis of Ca3Bi8O15 rods and their visible light photocatalytic properties

Article abstract of DOI:10.1016/j.materresbull.2013.12.007

High efficient visible light Ca₃Bi₈O₁₅ photocatalysts were synthesized by a hydrothermal method. Characterized by X-ray diffractometer, transmission electron microscopy, and the UV-vis diffuse reflectance spectroscopy, the results showed that the novel Ca ₃Bi₈O₁₅ rods can utilize the sunlight efficiently with the small band-gap. Using methyl orange (MO) as a model organic pollutant, the photocatalysts exhibited good photocatalytic activity, with the photodegradation conversion ratio of MO being up to 90% after 2 h of visible light (420 nm < λ < 800 nm) irradiation. Furthermore, they also showed good photocatalytic activities in the degradation of rhodamine B and p-chlorophenol. Through the investigation of the degraded mechanism, the main active species played important roles in the degradation process were holes, O₂·^- and OH.

Full text of DOI:10.1016/j.materresbull.2013.12.007

Materials Research Bulletin 51 (2014) 69–73
Contents lists available at ScienceDirect
Materials Research Bulletin
journal homepage: www.elsevier.com/locate/matresbu
Hydrothermal synthesis of Ca3Bi8O15 rods and their visible light
photocatalytic properties
*
Wenjuan Li, Desheng Kong, Xiaoli Cui, Dandan Du, Tingjiang Yan, Jinmao You
Shandong Province Key Laboratory of Life-Organic Analysis, Qufu Normal University, Qufu 273165, PR China
A R T I C L E I N F O
A B S T R A C T
Article history:
High efficient visible light Ca3Bi8O15 photocatalysts were synthesized by a hydrothermal method.
Characterized by X-ray diffractometer, transmission electron microscopy, and the UV–vis diffuse
reflectance spectroscopy, the results showed that the novel Ca3Bi8O15 rods can utilize the sunlight
efficiently with the small band-gap. Using methyl orange (MO) as a model organic pollutant, the
photocatalysts exhibited good photocatalytic activity, with the photodegradation conversion ratio of MO
Received 1 June 2013
Received in revised form 2 November 2013
Accepted 2 December 2013
Available online 8 December 2013
being up to 90% after 2 h of visible light (420 nm <
l < 800 nm) irradiation. Furthermore, they also
Keywords:
showed good photocatalytic activities in the degradation of rhodamine B and p-chlorophenol. Through
the investigation of the degraded mechanism, the main active species played important roles in the
degradation process were holes, O2ÁÀ and ꢀOH.
A. Inorganic compounds
B. Chemical synthesis
D. Catalytic properties
ß 2013 Elsevier Ltd. All rights reserved.
1. Introduction
researching photocatalysts were BiVO4 [11], Bi2WO6 [12], Bi2MoO6
[13], etc. However, the reports referred to Ca3Bi8O15 photocatalysts
Photocatalytic oxidation technology using solar energy has
been applied to a variety of problems of environmental interest in
addition to water and air purification and considered as an
ultimate approach to solve both energy and environmental
problems [1]. The semiconductor TiO2 has proven to be an
excellent photocatalyst material in these fields [2–4]. However, it
can only absorb a very small ultraviolet part (3–4%) of the solar
light because of its wide band-gap (3.2 eV for anatase). So the
extensive applications of titanium dioxide as an efficient photo-
catalyst are constrained. Many modification methods such as
metal ion doping, composite semiconductors and metal layer
modification have been used to extend the light absorption of the
catalyst into the visible light region but have little effect [4–7].
Some traditional visible-light photocatalysts were either unstable
upon illumination with light (e.g. CdS, CdSe) [8] or exhibited low
activity (e.g. WO3, Fe2O3) [9]. How to efficiently exploit visible light
active photocatalysts for degradation of environment pollutants
with high quantum efficiency and high stability is the top goal at
present even in future years.
were few in which they were synthesized by high-temperature
solid-state method and their activities were lower [10,14].
Therefore, aimed at the development of the photocatalysts with
high activity that are sensitive to visible light, we reported a new
chemical approach to synthesize Ca3Bi8O15 rods by a hydrothermal
process using stable, inorganic salts CaSO4 and Na2BiO3 as the
reactants without any templates. And their photocatalytic activi-
ties in the degradation of organic pollutants such as methyl orange
(MO), p-chlorophenol (4-CP) and rhodamine B (RhB) under visible
light irradiation have been investigated in detail. The mechanism
of the degradation processes was also explored.
2. Experimental
2.1. One step preparation of Ca3Bi8O15 photocatalysts
First, an appropriate amount of analytical grade CaSO4 (0.041 g)
and NaBiO3Á2H2O (0.474 g) according to a certain ratio were
dissolved in deionized water (50 ml) to form a mixture solution.
The pH value was adjusted by 2 M HNO3 or NaOH. After stirring for
0.5 h, the solution was transferred to the Teflon liner with 100 ml
capacity. The Teflon liner was then sealed in the stainless steel
autoclave and maintained at a certain temperature for different
hours. After the autoclave cooling naturally to ambient tempera-
ture, the resultant products were washed with water/ethanol for
several times and then dried at 80 8C for 12 h. All hydrothermal
treatments were made in the absence of any dispersion or capping
organic agent.
Some Bi-base photocatalysts owning the small band gap have
attracted much attention due to their strong absorption in the
visible light region. They have been broadly applied in the
photocatalytic water splitting into hydrogen and the degradation
of organic pollutants [10–13]. Among them, the most widely
*
Corresponding author. Tel.: +86 537 4458301; fax: +86 537 4458301.
E-mail address: jmyou6304@163.com (J. You).
0025-5408/$ – see front matter ß 2013 Elsevier Ltd. All rights reserved.
http://dx.doi.org/10.1016/j.materresbull.2013.12.007

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W. Li et al. / Materials Research Bulletin 51 (2014) 69–73
2.2. Characterization of photocatalysts
Transmission electron microscopy (TEM) images were collected
by using a JEOL JEM 2010F microscope working at 200 kV and
equipped with an energy-dispersive X-ray analyzer (Phoenix). X-
ray diffraction (XRD) patterns were recorded on a Bruker D8
Advance X-ray diffractometer with CuK
a radiation. The UV–vis
spectra of various liquid samples and diffuse reflectance spectra
(DRS) were performed on Beijing Puxi TU1801 UV–vis spectro-
photometer and Varian Cary 500 UV–vis spectrophotometer with
an integrating sphere attachment ranging from 200 to 800 nm,
respectively. X-ray photoelectron spectroscopy (XPS) analysis was
conducted on an ESCALAB 250 photoelectron spectroscopy
(Thermo Fisher Scientific Inc.) at 3.0 Â 10À10 mbar with mono-
chromatic AlKa Radiation (E = 1486.2 eV). The generation of
Fig. 1. XRD patterns of the samples prepared at different pH values.
hydroxyl radicals was investigated by the method of photolumi-
nescence technique with terephthalic acid (PL-TA) [15]. The
photoluminescence spectra were surveyed by Hitachi F4600
spectrometer. Photoelectrochemical measurements were per-
formed in an H-type two-compartment, three-electrode cell, with
a fine porous glass frit between the two compartments and with a
3.5-cm diameter quartz window for light illumination. The
potential and current of the photoelectrode were controlled/
recorded by an EG&G Princeton Applied Research Potentiostatat/
Galvanostat (PAR model 283) through PowerSuite software. The
catalyst was deposited as a film form on a 1 cm  1 cm indium-tin-
oxide conducting glass served as working electrode, the saturated
calomel electrode (SCE) as the reference electrode, and Pt as the
counter electrode. The electrolyte was 0.1 M Na2SO4 aqueous
solution.
the working solution. After irradiation, at given time intervals, 3 ml
aliquots were sampled and centrifuged to remove the catalysts.
The degraded solution was analyzed using a Beijing Puxi TU1801
UV–vis spectrophotometer and the maximum absorption peak was
monitored. After degradation, the catalysts were centrifuged from
the suspensions and dried for the following compared experi-
ments.
3. Results and discussion
3.1. Physicochemical properties of Ca3Bi8O15
Fig. 1 showed XRD patterns of the samples prepared at different
pH values under the condition: T = 120 8C, t = 24 h. It indicated that
as the pH = 3, the sample was Ca3Bi8O15, in line with the standard
spectra (JCPDS, card no. 48-0214). When the pH changed, Bi2O4
(JCPDS50-0864) appeared in the products. For example, when
pH = 9 or 13, the as-prepared samples were the mixture of
Ca3Bi8O15 and Bi2O4. Therefore, the appropriate synthesized pH
value was 3.
Fig. 2(a) indicates that Ca3Bi8O15 sample was composed of a
large quantity of rods with diameter about 200 nm. The high-
resolution TEM (HRTEM) in Fig. 2(b) shows the lattice fringes of
d = 0.31 nm allowed for identification of the (0 0 3) planes for
crystallographic spacing of Ca3Bi8O15. The selected-area electron
diffraction (SAED) pattern taken along a randomly selected single
rod indicated that these rods were single crystalline.
2.3. Tests of photocatalytic activity
The visible-light source was a 500 W halogen lamp (Philips
Electronics) positioned beside a cylindrical reaction vessel with a
plane side. Two cutoff filters (420 and 800 nm) were placed before
the vessel to ensure that irradiation of the degradation system
occurred only by visible-light wavelengths. The photo energy
density was 230
m
W/cm2. The overall system was cooled by wind
and water to maintain the room temperature. A 0.04 g portion of
catalysts was added to 80 ml MO (6.1 Â 10À5 M), 4-CP
(3 Â 10À4 M) and RhB (3 Â 10À4 M) solution in a 100 ml Pyrex
glass vessel, respectively. Prior to irradiation, the suspensions were
magnetically stirred in the dark for 1 h to ensure the equilibrium of
Fig. 2. (a) Low-magnification TEM, (b) high-magnification TEM, high-resolution TEM image and selected-area electron diffraction pattern of the Ca3Bi8O15 rods.

W. Li et al. / Materials Research Bulletin 51 (2014) 69–73
71
Fig. 3. XPS spectra of Ca3Bi8O15
.
Fig. 4. UV–vis DRS of Ca3Bi8O15
.
From the detection of XPS (Fig. 3), the binding energies of 30,
159, 443, 465, 533, 680 and 810 eV were corresponding to Bi5d,
Bi4f, Bi4d5, Bi4d3, O1s, Bi4p3 and Bi4p1, which were similar to
3.2. Photocatalytic activity of Ca3Bi8O15
those on
b-Bi2O3. It indicated that the valence state of Bi was +3.
The peak at 346 eV belonged to Ca2p. Thus, the as-prepared sample
involved the elements of Ca, Bi, and O.
The photocatalytic activity of Ca3Bi8O15 under visible light
irradiation was evaluated by the degradation of MO, RhB and 4-CP
as the model. As shown in Fig. 5, after 120 min of irradiation under
Fig. 4 shows the UV–vis DRS of Ca3Bi8O15. From this figure, the
as-prepared Ca3Bi8O15 can absorb much visible light and the small
band gap resulted in its absorption moving toward infrared region.
It indicated that this sample could apply the sunlight efficiently
which was favorable for the photocatalytic activity.
visible light (420 nm <
l < 800 nm), the photodegradation con-
version ratio (PCR) of MO in the presence of Ca3Bi8O15 sample was
up to 90%. It was superior to that of TiO2ÀxNx (prepared according
to the Ref. [5]). Furthermore, it also showed good photocatalytic
Fig. 5. Photocatalytic activity in the degradation of MO on (a) Ca3Bi8O15 and (b) TiO2ÀxNx; Photocatalytic activity in the degradation of (c) RhB and (d) 4-CP on Ca3Bi8O15 under
exposure to visible light.

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W. Li et al. / Materials Research Bulletin 51 (2014) 69–73
run. After 120 min of irradiation, the PCR of MO in the presence of
Ca3Bi8O15 was still up to 60%. Furthermore, the recycled sample
after degradation of MO was also investigated and compared with
the fresh one by XRD and XPS. Fig. 7a shows the XRD patterns of the
samples before and after degradation. It indicated that the patterns
had no obvious change. From the detection of XPS (Fig. 7b), the
binding energies of the elements before and after degradation are
nearly the same in the full survey spectra. It indicated that the
Ca3Bi8O15 sample possessed good chemical and mechanical
stability.
3.3. Mechanism
The transfer of electrons in the photocatalytic process is very
important, which would inhibit the recombination of electrons and
holes and thus increase the activity. The separation of electrons
and holes is always recognized to be the initial step in the
photodegradation mechanism. Therefore, we first applied the
electrochemical analysis to test the generation of photo-electrons.
The photo-current line shown in Fig. 8 was tested in the 0.1 M
Na2SO4 solution. At the appropriate electric potential with À0.1 V
(chosen by the polarization curve, from Fig. 8b), the current
generated and then reduced with the light on or off at a certain
intervals. This result showed the generation of electrons in the
photocatalytic process.
Fig. 6. The degradation rate of cycling runs for the photodegradation of MO in the
presence of Ca3Bi8O15 under visible light irradiation.
activity in the degradation of RhB and 4-CP. After 200 min of
irradiation, the PCR of RhB achieved to 60%. As for 4-CP, the PCR
was 55% after 120 min of irradiation. Therefore, the Ca3Bi8O15
photocatalyst exhibited
a good activity toward the organic
pollutants under visible light irradiation.
We also tested the stability of Ca3Bi8O15 sample through the
detection of the degradation rate of cycling runs for the
photodegradation of MO. In Fig. 6, although the conversion ratios
of the second and third cycling runs for the photodegradation are
decreased, the sample still remains a certain activity after the third
When the electrons were generated, separated with holes, they
would transfer to the surface of the photocatalysts to engage in the
oxidation–reduction reaction. In this process, some other active
species with high activities generated and would also participate in
Fig. 7. The comparison of XRD and XPS of Ca3Bi8O15 before and after degradation.
Fig. 8. (a) Photo-current line and (b) polarization curve of Ca3Bi8O15 under visible light irradiation.

W. Li et al. / Materials Research Bulletin 51 (2014) 69–73
73
Fig. 9. The effect of different types of active species scavengers on the degradation of MO over Ca3Bi8O15
.
Fig. 10. (a) OH-trapping PL spectra of suspensions containing Ca3Bi8O15 and TA, (b) changes of fluorescence intensity of the peak at
l = 426 nm with irradiation time.
the reaction. Therefore, the factors affecting the photocatalytic
activity are related not only to the structure of materials, but also
to the active species in the photodegradation process. We
investigated the effect of the active species by adding some
different types of scavengers in the degradation process of MO on
Ca3Bi8O15. The results were shown in Fig. 9. Ammonium oxalate
(AO) and ethylene diamine tetraacetic acid (EDTA) are holes
scavengers; benzoquinone (BQ) is O2ÁÀscavenger; tert-butyl
alcohol (TBA) is ꢀOH scavenger. Compared with the activity in
the absence of these scavengers, the effect of their adding was
obvious. When N2 was bubbled in the degradation system, the
activities under visible light irradiation and obtained good
stability. Through the exploration of the mechanism, it indicated
that the main active species played in the degradation process
were holes, O2ÁÀand ꢀOH, and then the electrons. The further
research about its mechanism should be proceeded.
Acknowledgments
This work is financially supported by the National Natural
Science Foundation of China (21303094, 21275089 and
21103193), Natural Foundation of Shandong Province
(ZR2010EM026), and Scientific Research Foundation of Qufu
Normal University (XJ201213).
–
photocatalytic activity reduced which testified the effect of O2Á .
The addition of AgNO3, electron scavengers, had a little effect on
the process and it showed the effect of electron was relatively
small. In a word, the main active species played in the degradation
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