Synthesis, characterization, and design of a photocatalyst based on BiOBr nanoplates and tin porphyrin with enhanced visible light photocatalytic activity

Article abstract of DOI:10.1007/s11164-019-03943-9

The bismuth oxybromide (BiOBr) photocatalyst was first synthesized via a simple co-precipitation method. To improve the visible light photocatalytic activity of the prepared photocatalyst, it was then functionalized with the tin porphyrin (SnTCPP) photosensitizer to produce the modified photocatalyst BiOBr/SnTCPP. The modified photocatalyst produced was characterized by the X-ray powder diffraction, field emission scanning electron microscopy, UV–visible diffusive reflectance spectrometry, Fourier transform infrared spectrometry, Raman spectroscopy, and photoluminescence spectrometry analytical techniques. Furthermore, the photocatalytic activity of BiOBr/SnTCPP was evaluated for degradation of methyl orange (MO), Rhodamine B (RhB), and 2, 4-dichlorophenol (DCP). The photocatalytic results obtained showed that BiOBr/SnTCPP was able to destroy 78%, 90%, and 80% of MO, RhB, and DCP, respectively, under the visible light irradiation. The photocatalytic performance of BiOBr/SnTCPP was found to be much higher than that of BiOBr. In addition, investigating the photocatalytic mechanism by using some scavengers for photodegradation of the above-mentioned pollutants showed that the OH and O₂ ^·? radicals were the main species involved in the photocatalytic process.

Full text of DOI:10.1007/s11164-019-03943-9

Research on Chemical Intermediates
https://doi.org/10.1007/s11164-019-03943-9
Synthesis, characterization, and design of a photocatalyst
based on BiOBr nanoplates and tin porphyrin
with enhanced visible light photocatalytic activity
Marzieh Yaghoubi‑berijani1 · Bahram Bahramian1 · Solmaz Zargari2
Received: 29 May 2019 / Accepted: 27 July 2019
© Springer Nature B.V. 2019
Abstract
The bismuth oxybromide (BiOBr) photocatalyst was frst synthesized via a simple
co-precipitation method. To improve the visible light photocatalytic activity of the
prepared photocatalyst, it was then functionalized with the tin porphyrin (SnTCPP)
photosensitizer to produce the modifed photocatalyst BiOBr/SnTCPP. The modi-
fed photocatalyst produced was characterized by the X-ray powder difraction,
feld emission scanning electron microscopy, UV–visible difusive refectance spec-
trometry, Fourier transform infrared spectrometry, Raman spectroscopy, and pho-
toluminescence spectrometry analytical techniques. Furthermore, the photocatalytic
activity of BiOBr/SnTCPP was evaluated for degradation of methyl orange (MO),
Rhodamine B (RhB), and 2, 4-dichlorophenol (DCP). The photocatalytic results
obtained showed that BiOBr/SnTCPP was able to destroy 78%, 90%, and 80% of
MO, RhB, and DCP, respectively, under the visible light irradiation. The photo-
catalytic performance of BiOBr/SnTCPP was found to be much higher than that of
BiOBr. In addition, investigating the photocatalytic mechanism by using some scav-
engers for photodegradation of the above-mentioned pollutants showed that the OH
and O·2− radicals were the main species involved in the photocatalytic process.
Keywords Bismuth oxyhalide · Tin porphyrin · Photodegradation · Visible light
* Bahram Bahramian
bahramian.b@shahroodut.ac.ir
1
Faculty of Chemistry, Shahrood University of Technology, Shahrood, Iran
2
Abadgaran Chemical Technology Development Research Group, Safadasht Industrial Complex,
Safadasht, Shahriar, Iran
Vol.:(0123456789)
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M. Yaghoubi-berijani et al.
Introduction
With the progress of societies, the environmental issues, especially the presence
of toxic organic materials in wastewaters, have received a considerable attention
[1]. Water pollution is a major problem that has been increased drastically, caus-
ing global threats to the human beings and other living organisms. The pollut-
ants are released fuently as wastes from the pharmaceutical, chemical, and other
industries, leading to a decrease in the quality of water [2].
Sewage generated from the textile industry contains a large amount of azo
dyes. Such colored dye sewages can threat the surrounding ecosystems. The tox-
icity and potentially carcinogenic nature of these materials represent an increas-
ing danger to life [3].
In order to solve such problems, the photodegradation of organic pollutants in
nature through photocatalysis has received a considerable attention over the last
few decades. Semiconducting photocatalysis has attracted an increasing attention
as a promising environmentally friendly technology for wastewater remediation.
Out of the diferent narrow band gap energy semiconductors, bismuth oxyhalides,
BiOXs (X = Cl, Br, I), have shown striking photocatalytic activities under the vis-
ible light irradiation, which also has very interesting physical properties like a
wide band gap energy [1, 4, 5].
BiOXs are a kind of newly exploited and efcient photocatalysts; they have
become promising photocatalysts due to their special layered structure. Their
uniquely layered structure is believed to improve the separation of photoinduced
electron–hole pairs [6], and their light response can be tuned from the UV light to
the visible light range, thus enhancing their photocatalytic performance. Further-
more, BiOXs are non-toxic and eco-friendly photocatalysts with low costs [7–10].
Therefore, BiOXs have attracted an increasing attention in practical industrial
applications, especially the degradation of organic pollutants [11, 12], CO2 con-
version and Cr(VI) reduction [13] and in the fabrication of electrochemical cells
[14].
BiOXs (BiOCl, BiOBr, and BiOI with the band gap energies of 3.2 eV, ~ 2.8 eV,
and ~ 1.7 eV, respectively) [4, 15] are important semiconductors with an adjusta-
ble light responsive property by the halide ion. Presently, the unique performance
of BiOXs in the photocatalytic degradation of organic compounds under both the
UV and visible light irradiations has been given a great importance [16–18]. The
photocatalytic performance of BiOXs strongly depends on their composition and
microstructure [19, 20].
BiOXs, as Bi-based semiconductors, are of great importance for their electri-
cal and optical properties and have exhibited excellent and promising applications
as photocatalysts as well as sensors. All BiOX compounds are crystallized in a
tetragonal matlockite structure [15, 21].
The main aim of this work was to ameliorate the photocatalytic activity of the
synthesized BiOBr nanoplates in the visible light region. Various approaches
have been proposed in order to activate the photocatalysts with a wide band gap
energy in the visible light region such as the semiconductors doped with other
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Synthesis, characterization, and design of a photocatalyst…
compounds like some metals and also sensitizing BiOBr with active compounds
in the visible light region. Porphyrins and metalloporphyrins are among the
strongest compounds used as a strong absorbent of the visible light. Porphyrins
are attractive building blocks of organic macrocyclic compounds. Their 2D 18
π-electron aromatic macrocyclic compounds possess an excellent stability and
unique optical, electronic, catalytic, and biochemical properties, and have been
used in variant felds such as sensors, solar cells, biology, and catalysis [22–24].
‘Pure’ porphyrins are not the best candidates as a photocatalyst because of their
poor conductivity and high probability of electron–hole recombination [25]. In
addition, these compounds are substantially stable in the presence of visible light.
Porphyrins, due to their strong absorption of the visible light, are widely used in
dye-sensitized solar cells. Sensitization of BiOXs by the porphyrin dyes is also
considered as an efective approach to improve the visible spectral absorption and
photocatalytic activity of BiOXs under the visible light irradiation.
In this work, we synthesized the BiOBr and BiOBr/SnTCPP photocatalysts.
Firstly, the BiOBr nanoplates were prepared via a simple co-precipitation method
and were then sensitized by the porphyrin dye SnTCPP. In order to fx the porphyrin
dye on the surface of BiOBr via a strong chemical bonding, the synthesized modi-
fed photocatalyst BiOBr/SnTCPP was thoroughly characterized and its photocata-
lytic activity for degradation of the dyes such as methyl orange (MO), 2,4-dichlo-
rophenol (DCP), and Rhodamine B (RhB) under an LED irradiation source was
studied. In addition, the mechanism for the photocatalytic process was investigated
and described.
Experimental
Materials
The reagents such as bismuth-(III) nitrate, potassium bromide, sodium acetate, nitric
acid, pyrrole, propanoic acid, 4-carboxybenzaldehyde, pyridine, tin (II) chloride
(SnCl2·2H2O), N, N-dimethylformamide (DMF), and ethanol were supplied from the
Merck Company and used without further purifcation. Deionized water was used to
prepare all the solutions.
Instrumentation
The crystalline phase structure of the samples was determined by a D8-Advance
Bruker using CuKα radiation (λ=0.15406 nm) at room temperature. The struc-
tural morphology of the samples was observed by a feld emission scanning elec-
tron microscope, Zeiss SIGMA|VP (Germany). The UV–visible difusive refec-
tance spectra (DRS) were taken using a Shimadzu MPC-2200 spectrophotometer.
The Fourier transform infrared (FT-IR) spectra of the samples were taken using a
Rayleigh spectrophotometer (China). The UV–visible spectra were obtained on a
double-beam PerkinElmer spectrophotometer (America, lambda 360). The Raman
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M. Yaghoubi-berijani et al.
spectra were obtained on a uRaman-532-Ci Raman spectrometer from the Avantes
Company (Netherlands). The photoluminescence spectra of the powdered samples
were taken using a Shimadzu RF-6000 spectrofuorophotometer at an excitation
wavelength of 200 nm. The lamp used in this work was a 5-W white LED (VF:
15–18 V, IF: 300 MA, TC: 600–8500 K) to provide visible light irradiation.
Synthesis of bismuth oxybromide (BiOBr)
In a typical experiment, in order to prepare the BiOBr sample, 0.01 mol Bi(NO3)3
powder was frst dissolved in 15 ml HNO3 (solution A). Solution B contained
0.01 mol KBr and 0.02 mol CH3COONa in 100 ml distilled water. Solution A was
added rapidly to solution B under vigorous stirring. The resulting white mixture was
vigorously stirred for 12 h at room temperature for completion of the reaction. The
resulting product was collected by centrifugation, washed for several times with
water, and dried in an oven at 120 °C [26].
Synthesis of TCPP and SnTCPP
Tetrakis-(4-carboxyphenyl) porphyrin (TCPP) was synthesized according to the
way they suggested in a paper [27]. For the synthesis of its tin complex, TCPP
was dissolved in 100 mL pyridine, and the resulting solution was refuxed with the
SnCl2·2H2O salt at 120 °C. The product obtained was washed with water and dried
in an oven at 60 °C.
Photosensitization of BiOBr with SnTCPP
To photosensitize the prepared BiOBr nanoplates with SnTCPP, a mixture of BiOBr
and SnTCPP (with the diferent weight ratios) was refuxed in 40 mL DMF at
150 °C. The synthesized product obtained was washed with DMF and water and was
subsequently dried in an oven at 60 °C [27].
Photodegradation process
To examine the photocatalytic activity of the prepared photocatalysts BiOBr and
BiOBr/SnTCPP, the photodegradation of the colored contaminants MO (pH=3),
DCP, and RhB (Table 1) with a concentration of 10 mg L−1 was investigated in the
presence of the photocatalysts with a concentration of 0.6 g L−1 under the visible
light irradiation; a 5-W white LED lamp was used as the visible light source. A mix-
ture of the organic pollutants and a photocatalyst was stirred in the dark for 30 min
to achieve an adsorption–desorption equilibrium between the organic pollutants and
the photocatalyst. All the photocatalytic experiments were carried out under the
same conditions at room temperature. At defned intervals (1 h), ca. 3 mL of the
mixture was centrifuged, and then, the spectral absorption changes for MO, DCP,
and RhB were measured at 506 nm, 284 nm, and 552 nm, respectively, to monitor
the photodegradation processes.
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Synthesis, characterization, and design of a photocatalyst…
Table 1 Physicochemical properties of the understudied organic pollutants
Organic pollutant
Chemical formula
Chemical structure
Molar
mass
(g mol−1
)
MO
C14H14N3NaO3S
C28H31ClN2O3
327.33
479.02
RhB
DCP
C6H4Cl2O
163.00
Results and discussion
Structural and morphological analyses
FT‑IR analysis
The FT-IR spectra shown in Fig. 1 confrm the chemical structures of BiOBr
and BiOBr/SnTCPP. In these spectra, the vibration peak for the metal–oxygen
bond (Bi–O) appears at about 511 cm−1, and the same peak can be observed in
the BiOBr/SnTCPP photocatalyst, which indicates the existence of BiOBr, as
expected [28, 29].
Figure 1a, b shows the structure and FT-IR spectra for SnTCPP–Cl2.
In the FT-IR spectra for SnTCPP–Cl2 shown in Fig. 1b, a strong C=O stretching
band could be observed at around 1703 cm−1. Two weak C–O peaks appeared at
1402 and 1276 cm−1, which could be attributed to the anti-symmetric and symmet-
ric stretches of the –COO− functional group present in SnTCPP–Cl2. The peak at
1606 cm−1 can be assigned to C=C stretching of the benzenoid rings. Also, the peak
at 565 cm−1 can be assigned to the Sn–Cl bond, confrming the presence of the axial
Cl ligands in SnTCPP–Cl2.
After the SnTCPP molecules were fxed on the surface of BiOBr (Fig. 1b), a
strong C=O stretching band could be observed around at 1700 cm−1. Also, the peak
at 1602 cm−1 could be assigned to the C=C stretching of the benzenoid rings. In
the spectrum for the BiOBr/SnTCPP photocatalyst, the defned peak for the C=O
group at 1700 cm−1 shows that SnTCPP is located on the surface of the nanoplates.
Also, in the BiOBr/SnTCPP spectrum, the characteristic peak for the Sn–Cl bond
was removed, which confrmed the connection of SnTCPP through the tin atom on
the surface of the BiOBr nanoplates.
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M. Yaghoubi-berijani et al.
Fig. 1 a Structure of as-prepared SnTCPP–Cl2 and b FT-IR spectrum for the samples
UV–visible difuse refectance spectra
Figure 2 shows the UV–visible difuse refection spectra for the BiOBr nanoplates.
As it is evident in this fgure, the spectrum for pure BiOBr powder clearly shows an
absorption edge at 440 nm.
The band gap energy of a semiconductor is important in determining its photocat-
alytic activity and thus worth investigating. The relationship between the absorbance
and the incident photon energy hυ can be described as:
(
)
1∕2
Ahꢀ = C hꢀ − Eg
where h, A, Eg, and υ represent the Planck constant, absorption coefcient, band
gap energy, and incident light frequency, respectively, and C denotes a constant.
The band gap energy for the synthesized photocatalysts could be estimated from a
plot depicting (Ahυ)2 versus hυ (Fig. 2c). The intercept of the tangent to the X-axis
gives rise to a good approximation of the band gap energies for BiOBr and BiOBr/
SnTCPP; the band gap energy for the BiOBr nanoplates was estimated to be almost
2.8 eV. The SnTCPP spectrum exhibited a Soret band at 400 nm and two Q bands in
the 558 and 602 nm (Fig. 2a).
As shown in Fig. 2b, BiOBr/SnTCPP exhibits a visible light absorption in the
range of 400–700 nm. Consequently, BiOBr/SnTCPP exhibited a broader absorption
range in the solar spectrum than BiOBr. The BiOBr/SnTCPP spectrum exhibited a
1 3

Synthesis, characterization, and design of a photocatalyst…
Fig. 2 UV–visible difusive refectance spectra for a SnTCPP and b, c BiOBr and BiOBr/SnTCPP pho-
tocatalysts
Soret band at 434 nm and two Q bands in the range of 566–606 nm. The presence
of these characteristic peaks for porphyrin in the difusive refectance spectrum for
BiOBr/SnTCPP is an evidence for the existence of SnTCPP in BiOBr/SnTCPP. Fur-
thermore, the absorbance of BiOBr/SnTCPP in the visible light region was increased
relative to BiOBr, which might be due to the high porphyrin ability in absorbing
the visible light. These results indicate the photosensitizing process, with porphyrin
forming an efcient approach to increase the BiOBr photocatalytic activity under the
visible light irradiation [27].
X‑ray difraction (XRD) analysis
The crystallographic structure and phase purity of the prepared photocatalysts were
examined by XRD analysis. Figure 3 shows the XRD patterns for BiOBr and BiOBr/
SnTCPP.
The observed difraction peaks at 2θ=10.76°, 21.8°, 25.08°, 31.56°, 31.92°,
39.2°, 46.16°, 50.52°, and 57.08° correspond to the planes of (001), (002), (001),
(012), (110), (112), (020), (014), and (212), respectively, in the order that can
be readily indexed to the tetragonal phase of BiOBr with the lattice parameters
a=b=3.926 Å and c=8.103 Å [space group: P4/nmm] (corresponding to the
JCPDS File No. 73-2061) [26]. The strong and sharp peaks in the XRD pattern for
BiOBr indicate that it is well crystallized.
Characterizations of BiOBr/SnTCPP are shown in Fig. 3, in which it can be seen
that there is no signifcant change in the XRD pattern for BiOBr/SnTCPP compared
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M. Yaghoubi-berijani et al.
Fig. 3 XRD patterns for (a)
BiOBr and (b) BiOBr/SnTCPP
to that for pure BiOBr. Therefore, BiOBr functionalization with SnTCPP had little
efect on the inter-layer spacing and structure of BiOBr.
Raman spectra
In the Raman spectra for BiOBr (Fig. 4), the strong peaks at 126 cm−1 can be
assigned to the A1g stretching vibration of the Bi–Br bond, the peak at 154 cm−1
can be attributed to the Eg stretching vibration of the Bi–Br bond, and the peak at
370 cm−1 is caused by the vibration of the Bi–O bond [30, 31]. The above-men-
tioned observations could further confrm that the understudied sample was indeed
BiOBr.
In Fig. 4, the Raman spectroscopy of SnTCPP indicates the important role
of vibronic efects. Also show that these terms play a predominant role in induc-
ing hot diference bands corresponding to resonance Raman transitions between
fundamental levels of vibrational modes of diferent symmetry. In the range of
1300–1590 cm−1, the spectra bands are assigned to the C–C bond stretching [32].
The observed Raman band at 1589 cm−1 are vibrational stretching of the phenyl
rings [33].
The Raman spectra for the BiOBr/SnTCPP photocatalyst also show the above-
mentioned peaks with weak intensities, which could be due to the covering of the
Fig. 4 Raman spectra for the SnTCPP, BiOBr, and BiOBr/SnTCPP photocatalyst samples
1 3

Synthesis, characterization, and design of a photocatalyst…
BiOBr peaks with the SnTCPP ones. Also, in the Raman spectra of BiOBr/SnTCPP,
the peaks are shifted to a lower frequency. The peak at 605 cm−1 can be attributed to
SnTCPP, which could be due to the connection of SnTCPP to BiOBr.
Photoluminescence (PL) spectra
It is well known that a photocatalytic activity is usually perfected by the photogen-
erated electrons and holes. Thus, to further explain the separation efciency of
the charge carriers, the PL spectra for BiOBr and BiOBr/SnTCPP were studied to
appraise the segregation efciency of the photoinduced charge carriers [34].
The PL emissions (Fig. 5) of BiOBr and BiOBr/SnTCPP were characterized
using the 200 nm excitation wavelength. The powdered samples were used without
further treatment. As indicated in Fig. 5, the main emission peaks were centered at
435–520 and 625 nm for BiOBr [35]. The emission intensity of pure BiOBr is the
highest; this intensity also elucidates a higher carrier recombination occurring for
BiOBr [36].
The PL intensity of the BiOBr/SnTCPP photocatalyst is low. A lower PL inten-
sity usually indicates a higher segregation efciency of the photoinduced charges
and thus a higher photocatalytic activity [37, 38].
FE‑SEM images
The morphologies of the samples SnTCPP, BiOBr, and BiOBr/SnTCPP were exam-
ined by feld emission scanning electron microscopy (FE-SEM). As it is shown in
Fig. 6a, SEM shows isolated tin porphyrin aggregates with sizes in the nanometer
range.
The BiOBr sample is composed of a large amount of relatively uniform plates.
Furthermore, it can be clearly seen that the surface of the BiOBr nanoplates is fat
and smooth. Some plates aggregate into a fower-like cluster. A unique nanoplate
morphology with an average diameter of approximately 20–50 nm can be clearly
observed (Fig. 6b).
60000
50000
40000
BiOBr
30000
20000
BiOBr/SnTCPP
10000
0
350
400
450
500
550
600
650
Wavelength (nm)
Fig. 5 PL spectra for BiOBr and BiOBr/SnTCPP with an excitation wavelength of 200 nm
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M. Yaghoubi-berijani et al.
Fig. 6 SEM images for a SnTCPP, b BiOBr, c BiOBr/SnTCPP, elemental mapping, d SnTCPP, and e
BiOBr/SnTCPP
Taking into account the evolution of the surface area of BiOBr, the structure and
morphology of the BiOBr nanoplates were discussed. A modifcation in the activity
betterment of BiOBr could increase its photocatalytic activity [39].
As shown in Fig. 6c, the surface of BiOBr nanoplates is attached to the SnTCPP
nanoparticles, and the good contact between the two compounds is thought to pro-
mote the transfer of the photoinduced carriers from SnTCPP to BiOBr. The nano-
surface structure of BiOBr/SnTCPP is similar to that for BiOBr, and it appears to be
stacked by SnTCPP, but the thickness of BiOBr is relatively narrow.
In the BiOBr/SnTCPP photocatalyst, an intercalation between SnTCPP and
BiOBr is formed through the Bi–O–Sn–O–Bi bonding. This bonding is formed
1 3

Synthesis, characterization, and design of a photocatalyst…
Fig. 6 (continued)
through the metal–ligand axial coordination to the surface hydroxyl groups of the
BiOBr nanoplates [40]. To indicate the elemental mapping of the SnTCPP and
BiOBr/SnTCPP photocatalyst, elemental mapping was performed. The elemental
mapping of SnTCPP shows a distribution of the elements of C, O, N, Cl, and Sn
(Fig. 6d).
As indicated in the corresponding elemental mapping of BiOBr/SnTCPP photo-
catalyst (Fig. 6e), Bi, O, Br, C, N, and Sn elements are clearly distributed. On the
basis of the elemental mapping, the presence of element Sn indicates the photosensi-
tization of BiOBr nanoplates with SnTCPP.
Photocatalytic activities of BiOBr and BiOBr/SnTCPP
Photodegradation of MO, DCP, and RhB
To further evaluate the photocatalytic performance of BiOBr and BiOBr/SnTCPP, a
series of experiments were carried out for the photodegradation of the understudied
organic pollutants under the visible light irradiation. Figure 7 shows the photocata-
lytic efciencies of BiOBr and BiOBr/SnTCPP. RhB and MO are the two kinds of
organic azo dyes that are often used as model pollutants to study the photocatalytic
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M. Yaghoubi-berijani et al.
Fig. 7 Photocatalytic degradation efciencies of a BiOBr and b BiOBr/SnTCPP (10:1) (MO), c BiOBr/
SnTCPP with diferent ratios, d, e photocatalytic degradation efciencies of DCP and RhB for BiOBr/
SnTCPP (10:1); kinetics of f MO, g DCP and h RhB using BiOBr and BiOBr/SnTCPP under the visible
light irradiation
▸
performance of the photocatalysts. In this work, RhB, MO, and DCP were chosen
as the target pollutants to trace the photocatalytic activities of BiOBr and BiOBr/
SnTCPP under the visible light irradiation.
Figure 7f shows the variation in the MO concentration with the irradiation time
over the BiOBr and BiOBr/SnTCPP photocatalysts. As a comparison, the direct
photolysis of MO was performed under the identical conditions. We observed that
the photolysis of MO was negligible under the visible light irradiation. The degrada-
tion of MO by BiOBr was about 38% (Fig. 7a). The photocatalytic activity of the
prepared samples was evaluated for the photodegradation of MO, and the results
are presented in Fig. 7b. It is shown in Fig. 7b that the photodegradation efciency
of MO for the BiOBr/SnTCPP photocatalysts enhances compared with BiOBr. For
comparison, the BiOBr/SnTCPP photocatalysts with diferent weight ratios (10:0.5,
10:1, 10:2 and 10:5) were designed and the results were calculated 60%, 78%, 70%
and 62.5%, respectively (Fig. 7c). A suitable amount of SnTCPP might increase an
efcient absorption of light in the visible region and help the separation and trans-
portation of induced charge carriers, leading to an improved photocatalytic activity.
The decrease in the photocatalytic performance at a high SnTCPP loading may be
because of the inhibition of the light arriving at the catalyst surface.
The photocatalytic activity of BiOBr/SnTCPP was evaluated for the degradation
of DCP under the visible light irradiation. The experimental results obtained for the
DCP degradation are displayed in Fig. 7d, g. The results shown in Fig. 7g revealed
that around 33% and 80% of DCP were degraded by BiOBr and BiOBr/SnTCPP,
respectively.
Figure 7e, h shows the photocatalytic degradation of RhB on the BiOBr/SnTCPP
photocatalysts. The degradation of RhB by BiOBr and BiOBr/SnTCPP was about
43% and 90%, respectively. Under the visible light irradiation, the BiOBr/SnTCPP
photocatalyst showed a much better photocatalytic activity than BiOBr alone.
Photocatalytic mechanism
In order to investigate the photocatalytic mechanism for the photocatalytic activity
of the BiOBr/SnTCPP photocatalyst, the photodegradation of MO was conducted
using various scavengers like AgNO3 (2 mmol) as an electron scavenger, tert-butyl
alcohol (tert-BuOH, 1 mmol) and methanol (MeOH, 1 mmol) as OH radical scav-
engers, in the absence of O2 (N2 saturation condition) to investigate the superox-
ide anion radical (O·2−) generation and disodium ethylenediaminetetraacetic acid
(Na2EDTA, 2 mmol) as a hole (h+) scavenger. As it is seen in Fig. 8, tert-BuOH,
MeOH, Na2EDTA, and N2 inhibited the photocatalytic activity of BiOBr/SnTCPP.
This could be resulted from the generation of OH and O·2− radicals in the photocata-
lytic process. Observation of the photodegradation of the understudied pollutants
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Synthesis, characterization, and design of a photocatalyst…
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M. Yaghoubi-berijani et al.
1.2
1
No scavenger
AgNO3
MeOH
N2
Tert-BuOH
Na2EDTA
0.8
0.6
0.4
0.2
0
-30
30
90
150
210
Time (min)
Fig. 8 Photocatalytic mechanism of BiOBr/SnTCPP in the presence of diferent scavengers
with BiOBr/SnTCPP indicates that the OH and O·2− radicals are the main species
involved in the photodegradation process.
It attracts one’s attention that the photodegradation performance was improved
with the addition of AgNO3. This phenomenon is very likely to be related to the
accelerated use of e− by addition of the scavengers of AgNO3, which efciently
improve the rate of separation of electrons and holes, resulting in an increased deg-
radation rate [41, 42].
Description of photocatalytic activity
The photocatalytic activity of the semiconducting photocatalysts depends on sev-
eral factors such as their crystallinity, surface structure, surface area, and band gap
energy. The BiOBr nanoplates induce a rapid generation of radical species under the
visible light irradiation. The activity of the BiOBr nanoplates is due to the formation
of a non-faceted morphology comprising a high surface area and a small crystallite
size distribution [43, 44].
The schematic charge transfer and enhanced photoactivity of BiOBr photosensi-
tized with SnTCPP are shown in Scheme 1.
In the photocatalyst BiOBr/SnTCPP, due to the presence of two axial chlorine
ligands in SnTCPP·Cl2, the bond between BiOBr and SnTCPP is established through
the interaction between the chlorine ligands in SnTCPP·Cl2 and the OH groups pre-
sent on the surface of BiOBr. The presence of this connection as well as the cen-
tral metal atom is very important in the transfer of electrons from porphyrin to the
BiOBr semiconductor. The electron transfer has a signifcant efect on the photodeg-
radation of pollutants. In this photocatalyst, the electrons excited in porphyrin are
transmitted through the axial connections to BiOBr, and SnTCPP acts as a channel
in this electron transfer process. The presence of axial ligand connections is very
efective in the process of electron transfer from the metal atom in the center of the
porphyrin ring to BiOBr.
1 3

Synthesis, characterization, and design of a photocatalyst…
Scheme 1 Schematic illustration of charge transfer in the photocatalytic degradation of BiOBr/SnTCPP
[36, 45]
Photocatalyst stability and reuse
The important characteristics of an efcient photocatalyst are its easy recovery,
resistance to light, and long lifetime. After completion of the photodegradation of
MO, the BiOBr/SnTCPP photocatalyst was separated, washed with acetone and
water, and dried in an oven. The FT-IR and UV–visible difusive refectance spec-
tra for BiOBr/SnTCPP confrm the presence of SnTCPP on the BiOBr/SnTCPP
photocatalyst (Fig. 9). As shown in Fig. 10, the photocatalyst was then collected
and reused to assess its performance in a new cycle of photodegradation of MO.
In the second, third, and fourth tests, the rate of MO degradation exhibited a low
decrease as a result of a smaller quantity of the recovered photocatalyst. It could
be seen that even after 4 cycles, more than 72% MO was photodegraded.
Fig. 9 a FT-IR spectra for BiOBr/SnTCPP after the reaction and b difusive refectance spectra for
BiOBr/SnTCPP (a) before and (b) after MO degradation
1 3

M. Yaghoubi-berijani et al.
1.2
1
Reuse 1
Reuse 3
Reuse 2
Reuse 4
0.8
0.6
0.4
0.2
0
-30
20
70
120
170
220
Time (min)
Fig. 10 Recycling tests for photodegrading MO using the BiOBr/SnTCPP photocatalyst
Conclusions
In order to improve the visible light efciency of the prepared BiOBr photocatalyst for
its photocatalytic activity, it was functionalized with the tin complex of porphyrin pho-
tosensitizer (SnTCPP). The new photocatalyst BiOBr/SnTCPP was successfully pre-
pared through a simple method.
The as-prepared BiOBr/SnTCPP photocatalyst showed a much better visible light
absorption in the photocatalytic degradation of RhB, MO, and DCP. BiOBr/SnTCPP
exhibited an excellent photocatalytic activity and a higher stability than BiOBr. The
better photocatalytic performance of BiOBr/SnTCPP was strongly afected by its crys-
tallite size and morphology. A better photocatalytic performance was achieved for
BiOBr/SnTCPP with a nanoplate morphology. With a low-intensity lamp (5 W), the
conversion rate for degradation of the understudied colored dyes was about 60–80%
after 4 h of irradiation. As expected, the new photocatalyst BiOBr/SnTCPP not only
had higher photodegradation efciency than pure BiOBr but also had an excellent pho-
tostability for photodegradation under the visible light irradiation. Also, the sensitiza-
tion of BiOBr with SnTCPP was increased by about 50% in photodegradation com-
pared to BiOBr (38%). This is because the energy level of the excited SnTCPP and the
CB of BiOBr is properly overlapped, which led to a decrease in the recombination of
the electron–hole pair, and thus promoted the photodegradation efciency. The above-
mentioned results clearly stated that MO was degraded by the active hydroxyl species
and superoxide anion radicals. The results obtained indicate that the BiOBr nanoplates
can be further studied as a promising visible light active support for SnTCPP in the
photoassisted solar energy conversion applications.
Acknowledgements The authors wish to thank the Vice-President’s Ofce for Research Afairs in the
University of Shahrood for supporting this work.
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