Boosted photocatalytic performance of uniform hetero-nanostructures of Bi2WO6/CdS and Bi2WO6/ZnS for aerobic selective alcohol oxidation

Article abstract of DOI:10.1016/j.jphotochem.2018.11.013

A two-dimensional hierarchical structural photocatalyst based on Bi₂WO₆ was obtained via a hydrothermal method and modified using ZnS and CdS and characterized by physico-chemical techniques XRD, FTIR, SEM, EDAX, and DRS. Their photo

Full text of DOI:10.1016/j.jphotochem.2018.11.013

JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)173–181
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Journal of Photochemistry & Photobiology A: Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Boosted photocatalytic performance of uniform hetero-nanostructures of
Bi2WO6/CdS and Bi2WO6/ZnS for aerobic selective alcohol oxidation
Elham Safaei, Sajjad Mohebbi⁎
Department of Chemistry, University of Kurdistan, Sanandaj, P.O. Box 66179-415, Iran
A R T I C L E I N F O
A B S T R A C T
Keywords:
A two-dimensional hierarchical structural photocatalyst based on Bi2WO6 was obtained via a hydrothermal
method and modified using ZnS and CdS and characterized by physico-chemical techniques XRD, FTIR, SEM,
EDAX, and DRS. Their photocatalytic performance toward aerobic oxidation of primary alcohols was performed
under incident visible light. In optimal condition, the photocatalytic conversion by modified Bi2WO6 was
boosted from 25 to 68% in comparison to bare Bi2WO6 while in all cases the selectivities were as high as 99%.
This such high photocatalytic activity was confirmed by PEC and photoluminescence (PL) spectroscopy studies
which are related to the improvement of charge recombination and potential energy levels of photogenerated
electrons-holes pairs. A plausible mechanism was suggested using a series of scavengers during photocatalytic
oxidation reactions. Also, this heterogeneous photocatalyst was reused and recovered several times without loss
photocatalytic activity. So, these series of catalysts were synthesized using the green and facial method in water
solvent with significant advantages of high photocatalytic reactivity in presence O2 as oxidant upon incident
visible light.
Nanocomposite
Alcohol oxidation
Mechanism
Photocatalyst conversion
1. Introduction
[17]. Bi2WO6 is a well-known candidate as visible-light-active photo-
catalyst which is formed by accumulation of layers through corner-
The selective oxidation of alcohols to aldehydes is one of the fun-
damental chemical conversions in industrial applications and organic
synthesis [1–3]. Many types of oxidation agents have applied in this
conventional processes, such as permanganate [4], dichromate [5],
chromium(VI) oxide [6], ruthenium(VIII) oxide [7], hypochlorite [8]
and manganese(IV) oxide [9]. However, these methods require the
stoichiometric amounts of the hazardous or toxic reagents that are
unavoidable subject of significant accumulation of by-product [10,11].
During the past decades, heterogeneous photocatalysis has been con-
sidered as the most efficient, environmental friendly and an economical
method for organic compounds conversions or transformations [12].
Hence, semiconductor-based photocatalysis has dramatically
pointed as progressive catalyst agents because of activation either
under natural sunlight or artificial light as the energy sources [13,14].
Aurivillius oxides with general formula Bi2An-1BnO3n+3(A = Sr, Ca, Pb,
Ba, K, Na, and B = Ti, Nb, Mo, Ta, Fe, W) are a series of semiconductors
which have attracted much attention due to exclusive physical and
chemical properties [15]. Additionally, these semiconductors have a
layer structure with great potential to be used in broad applications
[16]. Bismuth tungstate, Bi2WO6, should be the best photocatalyst
candidates as an easy take material in the Aurivillius oxides family
sharing WO6 octahedral sheets and bismuth oxide sheets [18,19]. Many
studies have demonstrated that Bi2WO6 able to decomposition of or-
ganic pollutants, water oxidation [20–22], CO2 conversion to renew-
able hydrocarbons upon visible light irradiation [23] and aerobic oxi-
dation of benzylic alcohols [24].
However, the photocatalytic properties of nanomaterials has strong
correlation with their morphologies [25–27]. Accordingly, the mor-
phology and structures of Bi2WO6 nano-structures have a fundamental
role in its photocatalytic activity. Besides, many Bi2WO6 structures with
different morphologies have been synthesized with the porous thin film
[28], mesoporous nanoplates [29], flowerlike nanostructure [30] and
hierarchical microspheres [31,32]. So, many efforts have been focused
on controlling the shapes and structures of nanomaterials [33]. In case,
many synthesis approach methods such as ultrasonic spray pyrolysis
[34], superhydrophilicity-assisted [35], microwave-assisted sol-
vothermal [36], solid-state reaction [37] and particularly the hydro-
thermal [33,38,39] were applied for preparation of Bi2WO6 semi-
conductor with different morphologies. However, the Bi2WO6
nanostructures play a principle role as a photocatalyst in the visible
region. Two major challenges of Bi2WO6 encourage scientists to im-
prove the photocatalytic activity of this nanomaterial. [36,40]. The first
⁎ Corresponding author.
E-mail address: smohebbi@uok.ac.ir (S. Mohebbi).
https://doi.org/10.1016/j.jphotochem.2018.11.013
Received 27 August 2018; Received in revised form 30 October 2018; Accepted 7 November 2018
Availableonline12November2018
1010-6030/©2018ElsevierB.V.Allrightsreserved.

E. Safaei, S. Mohebbi
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)173–181
Scheme 1. Illustration process formation of Bi2WO6/MS nanocomposite.
reason is the rapid charge recombination of photogenerated electron-
hole pairs, which significantly decreases the photocatalytic activity of
Bi2WO6 [41–44]. Moreover, increasing the charge separation efficiency
of semiconductors as photocatalyst led to enhancement of the photo-
catalytic activity [45,46]. The next challenge related to its wide band
gap, which strongly impedes the absorption of visible light to the wa-
velength longer than 450 nm [47]. Several strategies have been em-
ployed to improve these constraints. For example, a combination of the
Bi2WO6 and C60 increases the charge separation by photoinduced
electron transfer. This system was used for the degradation of organic
dye in the presence of visible light [41]. As another example, a com-
posite of Bi2WO6 and reduced graphene oxide was obtained using the
one-step hydrothermal method and used for catalytic degradation of
rhodamine B, methylene blue, and phenol. The reduced graphene oxide
was effectively improved the charge separation in the composite
[48,49]. Besides, the coupling of Bi2WO6 with other low band gap
semiconductors, could be facilitated the charge separation and im-
proved remarkably the photocatalytic performance [50,51]. In this
case, excited electrons by incident visible light, would transfer from the
conductive band of semiconductor with narrow band gap to the broad
conduction band of Bi2WO6. So, the mixing of metal sulfides and
Bi2WO6 is an efficient route for overcoming the drawbacks of Bi2WO6
and increasing of photocatalytic activity [47].
0.997 mmol (0.329 g) of Na2WO4.2H2O were dissolved in 25 ml DI
water separately and was stirred for 15 min. Then two solution was
mixed together and was stirred under vigorous stirring for 45 min. The
resulting slurries were transferred to a 50 ml Teflon-lined autoclave and
were impounded in 200 °C for 18 h. After reaction in hydrothermal
condition, resulting solution was cooled down to room temperature. A
white precipitate was collected by centrifugation at 35,000 rpm, wa-
shed with deionized water and ethanol five times, and dried at 50 °C in
a vacuum oven for 12 h.
2.1.2. Preparations of Bi2WO6/CdS and Bi2WO6/ZnS heterostructures
A functionalizing agent is required for the growth of CdS and ZnS
onto hierarchical Bi2WO6. Herein, 4-mercaptosuccinic acid (MSA) was
used as the functionalizing agent. In this procedure, 100 mg hier-
archical Bi2WO6 was dispersed in 30 ml of deionized water using the
ultrasonic bath for 15 min at 20 °C. After that, resulting suspension was
stirred for 4 h at room temperature to complete the surface functiona-
lization of the Bi2WO6 surfaces to achieve pale blue suspension. Then,
0.7 mmol (0.2 g) of Cd(CH3COO)2.4H2O was added to the pale blue
suspension and was stirred vigorously under magnetic stirring. After
2 h, 0.3 mmol (0.124 g) of Na2S.9H2O was added slowly to the mixture
and stirred for 1 h at room temperature. The yellow precipitation was
collected by centrifugation, washed with deionized water and ethanol
and also dried at 50 °C in the oven for 12 h. The Bi2WO6/ZnS nanos-
tructure was obtained by the same process, while Zn(NO3)3.6H2O was
used as a source of Zn(II) in the third stage and all of the condition was
kept constant (Scheme 1).
In this paper, the preparation of uniform Bi2WO6, Bi2WO6/CdS, and
Bi2WO6/ZnS nanocomposite through a facile and safe method in aqu-
eous solution was reported. The photocatalytic performance of Bi2WO6
/metal sulfide composite and bare Bi2WO6 was compared via the
photocatalytic oxidation of primary aromatic alcohol under visible light
irradiation. Also, a plausible mechanism was proposed using several
chemical traps and scavengers to understand the photocatalytic
pathway.
2.2. Measurement
Energy-dispersive X-ray spectroscopy (EDAX, TSCAN) was used for
the elemental analysis of nanomaterial. Powder X-ray diffraction (XRD)
measurements of the samples were recorded on a Holland-Philips X-ray
powder diffractometer using Cu Kα radiation (λ = 0.1542 nm) with
scattering angles of (2θ) of 5-80◦, operating at 40 keV, and a cathode
current of 20 mA. The morphology of the nanostructures was taken on a
field emission scanning electron microscope (FESEM, TSCAN, and S-
300000). FT-IR spectra were recorded on a Bruker VERTEX 80 v model
using the KBr disk method. UV-Vis spectroscopy (UV–vis) analyses were
taken using a Varian Cary 50 UV–Vis spectrophotometer. Brunauer-
Emmett-Teller (BET) parameters were measured via nitrogen adsorp-
tion-desorption isotherms using a Tristar II 3020 Micromeritics ad-
sorption instrument at 77 K to determine the surface area and pore size
distribution. Spectra were recorded in the ange of 350–800 nm. Steady
state diffuse reflectance spectroscopy (DRS) was recorded using an
Avaspec-2048-TEC spectrometer at room temperature. To investigate
the activity of the catalyst photocatalytic reactions was carried out
2. Experimental
2.1. Materials and methods
Bi(NO3)3.5H2O,
Na2WO4.2H2O,
Cd(CH3COO)2.4H2O,
Zn
(NO3)3•6H2O and 4-mercapto succinic acid (MSA), Na2S.9H2O were
obtained from Aldrich. Deionized (DI) water and absolute ethanol were
used. Alcohols as substrates, including benzyl alcohol, 3-chlorobenzyl
alcohol, 4-chlorobenzyl alcohol, 4-nitrobenzyl alcohol, 2-benzyl al-
cohol, and 4-fluorobenzyl alcohol were prepared from commercial
sources.
2.1.1. Hydrothermal synthesis of hierarchical Bi2WO6
The hierarchical Bi2WO6 were synthesized through a modified
method. Typically,
A 1.9 mmol (0.97 g) of Bi(NO3)3.5H2O and
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JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)173–181
under the irradiation of 5 W visible lamp at 550 nm.
2.3. Photocatalytic activity
In a general process, the photocatalytic activity of nanostructures
was investigated by oxidation of aromatic alcohol. In typically,
0.1 mmol of alcohol was added to 1.5 ml of solvent in Pyrex glass cell
and stirred for 5 min. Then, 10 mg of photocatalyst was dispersed in
solution and magnetically stirred in the dark for 15 min to reach an
equilibrium between adsorption-desorption. Photocatalytic reactions
were performed by irradiation with an LED lamp (5 W). To control the
temperature during the oxidation reaction, the water circulation was
applied to cooling of the reaction mixture. During the photocatalytic
process, the reactor was exposed to air to ensure that enough oxygen is
provided for the reaction. The progress of the reaction was monitored
using the TLC by periodic sampling. After completion of the reaction,
the catalyst was separated from the suspension by centrifuging
(3500 rpm) and washed with solvent to separate all of the residual
materials.
Fig. 2. X-ray powder diffraction patterns of the CdS and ZnS nanoparticles.
1691. Additionally, according to this pattern in Fig. 2 main peaks of
CdS was situated at 2θ˚ = 26.5, 27.6, 43.9, 48, 53 and 70.5 respectively
corresponding to (111), (200), (220), (311), (222) and (400) which this
plants are related to highly crystalline cubic phase with reference file
with JCPD = 01-0647.
3. Result and discussion
As mentioned in the experimental section, MSA was used as a
functionalizing agent for the formation of CdS and ZnS on Bi2WO6
surface. In addition, MSA as a functionalizing agent plays the main role
to adsorb alcohol on heterostructures nanocatalyst due to two carboxyl
groups. According to FTIR analyses (S1), pure MSA shows important
peaks in 2912, 2650., 2559, 1699 and 1313 cm−1, which can be as-
signed to the OeH (νO–H), CeH (νC–H), SeH (νS-H), C]O (νC=O) and
C–O (νC–O) stretching bands, respectively. After adding MSA to the
suspension of Bi2WO6 the peaks corresponding to νS–H disappears that
result of this reaction was indicated in FTIR in figure S2a. In this case,
MSA was bonded to Bi3+ on the surface of Bi2WO6 via sulfhydryl group
and SeH stretching band disappears. When cadmium acetate was
added to the mixture, Cd2+ (or Zn2+) coordinated to some carbonyl
groups. The expanded peaks of Bi2WO6, CdS/Bi2WO6, and ZnS/Bi2WO6
(inset in S2) was indicated two series significant stretching band in
1721.56, 1551.79 cm−1 for CdS/Bi2WO6 and 1721.03, 1618.20 cm−1
for ZnS/Bi2WO6 while these peaks are not visible in FTIR of Bi2WO6.
Typically, the stretching band in 1721.56 Cm−1 related to un-
coordinated carbonyl and 1551.79 cm−1 related to coordinated car-
bonyl to Cd2+ in CdS/Bi2WO6. Therefore, MSA molecule can be con-
sidered to be a “linker” for the formation of CdS and ZnS on the surface
of hierarchical Bi2WO6 as well as an absorbent to absorb alcohol on
photocatalyst surface. Also, FTIR of as-nanostructures in S2a, S2b and
S2c show main absorption bands at 400-800 cm−1, which are attributed
to BieO, WeO stretching and WeOeW bridging stretching modes and
the two-week absorption bands at 3446.73 and 1628.13 cm−1 was as-
signed to the stretching and bending vibrations of the adsorbed water
molecules.
3.1. Crystalline, morphology and purity
The purity, crystallinity, and phase of nanostructures were de-
termined using XRD pattern at 2θ = 10–80. The XRD pattern of Bi2WO6
was displayed in Fig. 1. As shown in Fig. 1, the B2WO6 displays a highly
crystalline orthorhombic phase and lattice parameters includes
a = 0.545 nm, b = 1.643 nm, c = 0.543 nm and α=β=γ = 90˚ (JCPDS
card No. 39-0256). The diffraction peaks correspond to (131), (060),
(171), (311), (123), (282), (440) and (193) with a maximal intensity
which in accordance with the orthorhombic phase of Bi2WO6. The XRD
pattern of heterostructures nanocomposite was demonstrated in Fig. 1.
According to this pattern, Bi2WO6 /CdS and Bi2WO6 /ZnS have cubic
phase. The diffraction peaks correspond to (111), (200), (220), (311),
(222), and (400) are agreement whit cubic phase of CdS/ Bi2WO6 and
also (111), (200), (220), (311), (222), (400), (311) and (420) are re-
lated to cubic phase of ZnS/ Bi2WO6 with JCPDS card No. 21-0829 and
77–2100, respectively. Moreover, The XRD pattern at 2θ = 10–80 was
provided for two metal sulphides. The diagram of XRD in Fig. 2 shows
five important peaks of ZnS at 2θ˚ = 28.6, 47.6, 64.85, 56.5, 68.5 and
77 respectively and represent which plane value can be indexed at
(111), (200), (220), (311), (400) and (331). These planes and phase of
the ZnS nanoparticle are agreement with reference file with JCPD = 65-
The morphology and composition of nanocatalysts were docu-
mented using FESEM images. Fig. 3a shows a number of the uniform
spheroid structure of Bi2WO6 in the low magnification, with an average
diameter of 2 to 3 μm are clearly observed. Other morphologies cannot
be observed in the FESEM images, which demonstrate the high yield
growth of the hierarchical structure. High magnification SEM images
were investigated in Fig. 3b to prove the detailed structural information
of nanostructures. As Fig. 3b shows, the spherical architectures are
constructed from some 2D nanosheet-like, in which the thickness of
about 30 nm (inset image). Nanosheets are oriented in perpendicular
directions successively to form the hierarchical morphology of Bi2WO6;
also many concaves can be observed in this morphology.
Fig. 4a-d) shows SEM images of modified Bi2WO6 hierarchical by
CdS/ZnS; low magnification SEM images demonstrate that the modified
Bi2WO6 has retained its uniform spherical morphology. The high
magnification SEM images (Fig. 4(b) and (d)) show that the thickness of
Fig. 1. X-ray powder diffraction patterns of the Bi2WO6 nano-sphere (a), CdS/
Bi2WO6 nanocomposite (b) and ZnS/ Bi2WO6 nanocomposite (c).
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JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)173–181
Fig. 3. Low -magnification (a) and high-magnification (b) FESEM images of the Bi2WO6 hierarchical structures.
sheets increases up to 40 nm in diameter. Indeed, the nanostructure of
CdS intercalated between layers of Bi2WO6 hierarchical structures.
According to Fig. 4(b) and (d), after the intercalation of CdS and ZnS,
the surface of Bi2WO6 hierarchical became roughened and massive.
Beside, intercalation of CdS and ZnS nanoparticle between layers of
hierarchical structures lead to deepening of concave. This observation is
proved that the CdS and ZnS should be deposited on the surface of
Bi2WO6 nanostructures. Hence, The EDAX analyses were confirmed
purity and the relative atomic ratio of the element in bare and modified
Bi2WO6 (Figure. S3 (a–c)).
Also, the specific surface area was measured from BET isotherms,
and the pore size distribution diagram was obtained using BJH ap-
proach. The diagrams in the Fig. 1(a–c) represent the BET surface area
plot, which referred to Bi2WO6, Bi2WO6/CdS and Bi2WO6/ZnS nano
photocatalyst, respectively. The both specific surface area (SSA) and
total pore volume were calculated and displayed. In Addition, all the
extracted results from those diagrams were summarized in Table 1.
Consequently, the measurement indicate that the specific surface area
of pure Bi2WO6, Bi2WO6/CdS and Bi2WO6/Zn were about 26.68, 26.72
and 26.70 m2 g−1, respectively. Actually, the morphology has not
Fig. 4. Low -magnification and high-magnification FESEM images of the Bi2WO6 /CdS (a, b) and Bi2WO6/ZnS (c, d).
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JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)173–181
Table 1
The BET surface area, total pore volume and average pore size Bi2WO6,
Bi2WO6/ CdS and Bi2WO6/ ZnS nanostructure.
Sample
BET surface area
(m2/g)
Average pore size (nm)
Bi2WO6
Bi2WO6/ CdS
Bi2WO6/ ZnS
26.68
26.72
26.70
12.46
16.45
16.04
changed in this photocatalyst series therefore the BET surface area has
not remarkable change.
Evaluation of optical absorption property and electronic state of
Bi2WO6, Bi2WO6 /CdS and Bi2WO6 /ZnS nanocomposite were studied
by diffuse reflectance spectroscopy (DRS) and is shown in Fig. 5. DRS of
the Bi2WO6 has a steep absorption edge in the visible range about
450 nm, with a band gap energy of about 2.85 eV (inset image). Fig. 5b
shows DRS of the Bi2WO6/CdS that reveals the extended absorption
edge to 503.29 nm in the visible light region. However, in comparing
with bare Bi2WO6, the absorption edge of Bi2WO6/CdS shows a red shift
which related to a band-gap energy of about 2.27 eV (inset image). In
Fig. 5c the DRS of Bi2WO6 /ZnS has been compared to Bi2WO6. The
spectrum demonstrates the onset adsorption of Bi2WO6 /ZnS nano-
composite about 453 nm that has a little change in comparison to bare
Bi2WO6. The band gap for the Bi2WO6 /ZnS was evaluated to be about
2.80 eV from the onset of the absorption edge (inset of Fig. 5c).
Photocurrent measurements were carried out for investigation of
photostability and photosensitivity of the as-prepared Bi2WO6, Bi2WO3
/CdS and Bi2WO6 /ZnS photoelectrochemical cell at a constant poten-
tial as well as compare the rate of charge recombination was done using
photoluminescence (PL) in the visible light region (500–600 nm).
Fig. 6. Transient photocurrent response for the (a) CdS/Bi2WO6, (b) ZnS/
Bi2WO6, and (c) Bi2WO6.
3.2. Photocurrent response under visible-light irradiation
As already mentioned, electrons and holes in semiconductors are
generated under visible light irradiation and increase separation effi-
ciency of photogenerated electrons which holes play a principal role in
enhancing the photocatalytic power. In this regard, photocurrent re-
sponse of photocatalysts was carried out in the present of visible-light
irradiation[52]. Fig. 6 indicates the I-t curves of the photocatalyst in
presence of visible light for 200 s. According to result, Bi2WO6/ZnS and
Bi2WO6/CdS have high photocurrent density in comparison to Bi2WO6
which this result proves the increases separation efficiency of photo-
generated electrons and holes in modified Bi2WO6 with both ZnS and
CdS. Hence, photocatalytic activity remarkably increases with enhan-
cing in photocurrent which this due to increasing the lifetime of pho-
togenerated e− and h+[53].
Fig. 5. DRS spectrum of the obtained (a) Bi2WO6 (b) Bi2WO6 /CdS nanocomposite and (c) Bi2WO6/ZnS nanocomposite.
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Table 2
The photocatalytic oxidation of benzyl alcohol in the presence of visible light.
Entry Catalyst Amount of catalyst (mg) Selectivity% Isolated yield %
1
2
3
4
5
6
7
8
Bi2WO6
5
5
5
8
8
8
10
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
> 99
15
25
58
20
26
64
25
30
66
26
34
68
0
Bi2WO6 /ZnS
Bi2WO6 /CdS
Bi2WO6
Bi2WO6 /ZnS
Bi2WO6 /CdS
Bi2WO6
Bi2WO6 /ZnS 10
Bi2WO6 /CdS 10
Bi2WO6
9
10
11
12
13(b)
14(c)
15
Bi2WO6 /ZnS 15
Bi2WO6 /CdS 15
No catalyst
Fig. 7. PL spectra of sample (a) Bi2WO6, (b) ZnS/ Bi2WO6, and (c) CdS/
Bi2WO6.
0
15
CdS/Bi2WO6
0
(a)Reaction conditions: substrate (0.1 mmol); MeCN (1.5 ml); time (180 min);
temperature (30 °C); amount of catalyst (10 mg) and visible light irradiation.
(b)Reaction conditions: substrate (0.1 mmol); MeCN (1.5 ml); time (240 min);
temperature (30 °C); amount of catalyst (0) and visible light irradiation.
(c)Reaction conditions: substrate (0.1 mmol); MeCN (1.5 ml); time (240 min);
temperature (30 °C); CdS/Bi2WO6 (1.5 mg) and dark.
3.3. Compare the rate of charge recombination using photoluminescence
(PL) spectroscopy
PL spectroscopy was performed at room temperature in air. It has
been proven that reducing the charge recombination rate of the photo-
generated electron and hole led to increasing the catalytic activity of
the semiconductor. In this current, PL spectrum can be indicated charge
recombination rate of e− and h+. Hence, according to the result in
Fig. 7, PL emission intensities of modified Bi2WO6 are weakened in
comparison with Bi2WO6 which display decreasing charge recombina-
tion rate of electron/hole pairs in Bi2WO6/ZnS and Bi2WO6/CdS. It was
observed that Bi2WO6/CdS sample have lower PL intensity in com-
parison with Bi2WO6/ZnS because of increases separation efficiency of
photogenerated electrons-holes pairs. Fundamentally, it is expected
that both increases of separation efficiency and decreases of charge
recombination rate improve the photocatalytic performance of mod-
ified Bi2WO6.
4. Photocatalytic activity under visible light
The catalytic performance of Bi2WO6, Bi2WO6/CdS, and Bi2WO6/
ZnS were evaluated by selective oxidation of aromatic alcohols to
corresponding aldehydes under visible light in presence of oxygen air at
room temperature. Several parameters were changed to obtain op-
timum condition using 0.1 mmol of alcohol (benzyl alcohol, BA) in
acetonitrile incident by a LED lamp for 2 h. According to the summar-
ized results in Table 2, the best amount of the catalysts is 20 mg. The
controlled experiments were performed in absence of the catalyst and/
or light (Table 2 entry 13 and 14). So the photoreactivity of catalyst was
confirmed in presence of substrate, air, and light. Besides, no detectable
conversion in inert atmosphere confirms the essential role of oxygen is
this photocatalytic oxidation.
Since solvent plays a fundamental role in the absorption of alcohol
or desorption of product from the surface of the catalyst so the effect of
solvent was investigated. Solvents with vary polarity such as acetoni-
trile, dichloromethane, water, and dimethylformamide were utilized
and the results figured in Fig. 8. According to this diagram, the pho-
tocatalytic reaction in presents solvents with more polarity shows
higher yields. The selectivity in all cases was higher than 99%.
Table 3 indicates the yield and selectivity of this photocatalytic
system for oxidation of different derivatives benzyl alcohol in present
Bi2WO6 and Bi2WO6/ZnS or Bi2WO6/CdS under the optimum condi-
tion. However, in all of the oxidation reactions, the Bi2WO6 nano-
particles show the lowest yield in comparison to others, which could be
attributed to the fast charge recombination upon adsorption of visible
light. The modified Bi2WO6 with both ZnS and CdS led to promote the
yield of oxidation reaction under visible light and O2 as the oxidant.
The yield of the photocatalytic reaction on the surface of the Bi2WO6/
CdS was higher then Bi2WO6/ZnS because of considerable decreasing
Fig. 8. Effect of solvent on the oxidation reaction of benzyl alcohol.
the charge recombination in the Bi2WO6/CdS according to the DLS
analysis.
5. The mechanism of the photocatalytic oxidation of BA to BAD
To investigate the mechanism of photocatalytic conversion BA to
BAD, control experiments were performed under an argon atmosphere
and also used several radical scavengers to indicate the primary active
species [54,55]. So, the photo-oxidation reaction was carried out in the
presence of ammonium oxalate (AO) as a photogenerated holes sca-
venger, potassium persulfate (K2S2O8) as electrons scavenger, benzo-
quinone (BQ) as a superoxide radical (O2%−) scavenger and tert-butyl
alcohol (t-BuOH) as scavenger for hydroxyl radicals (OH%) using
Bi2WO6, Bi2WO6/CdS and Bi2WO6/ZnS incident by visible light. Ac-
cording to the diagram in Fig. 9, the photo-oxidation of alcohol in argon
atmosphere show no significant progress which this concept approve
the principal role of O2 in the conversion of benzyl alcohol to benzal-
dehyde. A strong decrease in the oxidation efficiency of alcohol was
recorded in the presence of AO as h+ scavenger usage O2 as an oxidant.
Similarly, K2S2O8 as electrons scavenger shows the inhibitory effect by
decreasing of conversion percent of alcohol to an aldehyde. Moreover,
the addition of BQ to the reaction has a considerable effect on decreases
conversion yield which this test affirm essential role the O2%− active
species in this reactions. In contrast, the reactivity of catalyst doesn't
change by the addition of BuOH, so, hydroxyl radical (OH%) as very
active radical doesn’t play a major role in the oxidation of alcohol as
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JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)173–181
Table 3
Selective oxidation of aromatic alcohols over the Bi2WO6 and modified Bi2WO6 photocatalysts under the visible light irradiation for 180 min.a.
Catalyst
Substrate
Product
Yield %
Selectivity %
Substrate
Product
Yield %
Selectivity %
Bi2WO6
Bi2WO6 /ZnS
Bi2WO6 /CdS
25
30
66
> 99
> 99
> 99
25
34
75
> 99
> 99
> 99
Bi2WO6
Bi2WO6 /ZnS
Bi2WO6 /CdS
25
32
70
> 99
> 99
> 99
36
45
78
> 99
> 99
> 99
Bi2WO6
Bi2WO6 /ZnS
Bi2WO6 /CdS
30
34
75
> 99
> 99
> 99
25
37
66
> 99
> 99
> 99
Bi2WO6
Bi2WO6 /ZnS
Bi2WO6 /CdS
26
34
70
> 99
> 99
> 99
(a)Reaction conditions: substrate (0.1 mmol); MeCN (1.5 ml); time (240 min); temperature (30 °C); amount of catalyst (10 mg) and visible light irradiation.
Fig. 9. Effects of a series of scavengers on the photo-oxidation efficiency of
alcohol using Bi2WO6/CdS.
expected. Because it's hard to generate the hydroxyl radical in the
acetonitrile solvent. Therefore, both of the O2%− and h+ are known as
principal species in photocatalytic reactions under visible light irra-
Scheme 2. Schematic diagram demonstrating the proposed oxidation me-
chanism of alcohols on the surface of Bi2WO6/CdS.
diation. Beside, radicals OH% not only haven’t a positive role in progress
oxidation but also led to the producing multi-product simultaneously
even with low yield, which this causes the non-selectivity of the reac-
the catalyst, the catalyst was reused for several times. Hence, after each
tion.
catalytic reaction, the photocatalyst was collected using a centrifuge
Based on the above arguments, a possible pathway for the photo-
and washed with acetonitrile and dried in an oven at 70 °C. After this
catalytic conversion of alcohols to the corresponding aldehydes has
step, the recovered catalyst was reused in the same photocatalytic re-
been considered in Eqs. (1)–(4). In this case, excited the valence band
action condition for several times. According to Fig. 10, the catalyst was
electron in CdS to the conductive band under visible light irradiation
reactive even after 10 times. However, the yield of the product
will be transferred to the conductive band of Bi2WO6 due to adaptation
bang gap of CdS with Bi2WO6. So, the electron is concentrated on
Bi2WO6 while hole remained in CdS and the rate of charge re-
combination would be decreased dramatically. The produced electron
will be adsorbed by the O2 to generate O2%−. Finally, the pair of O2%−
/
hole will participate in the oxidation reaction according to the fol-
lowing equations an−d scheme+2 .
CdS + hν → CdS (eCB………..hVB
)
(1)
(2)
(3)
(4)
CdS (ec−b) + Bi2WO6 → CdS + Bi2WO6 (eC−B
Bi2WO6 (eC−B) + O2 → Bi2WO6 + O2.−
CdS (hV+B) + BA → BAD + H2O
)
6. Reusability and stability of photocatalyst
Fig. 10. Reusing of recycled Bi2WO6/CdS nanocatalyst in the oxidation reac-
tion of BA.
In this research, to evaluate the total performance and stability of
179

E. Safaei, S. Mohebbi
JournalofPhotochemistry&PhotobiologyA:Chemistry371(2019)173–181
Fig. 11. (a) XRD pattern and (b) SEM image of Bi2WO6/CdS after oxidation reaction of an alcohol.
decreases slightly in each run due to loss of the amount of catalyst
during washing.
online version, at doi:https://doi.org/10.1016/j.jphotochem.2018.11.
013.
Besides, the stability of reused catalyst was investigated by com-
paring SEM image and X-ray pattern, before and after the reaction.
Fig. 11a shows the XRD pattern of reused Bi2WO6/CdS comparison to
fresh catalyst. Results approve that the Bi2WO6/CdS structure is pre-
served after utilization in numerous catalytic cycles. Additionally, the
SEM image of Bi2WO6/CdS (Fig. 11b) shows the stability of morphology
and structure of reused Bi2WO6/CdS comparison to fresh catalyst.
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