Synthesis, Characterization, and Photocatalytic Application of Type-II CdS/Bi2W2O9 Heterojunction Nanomaterials towards Aerobic Oxidation of Amines to Imines

Article abstract of DOI:10.1002/ejic.201800221

A series of new type-II CdS/Bi₂W₂O₉ heterojunction nanomaterials is prepared by a two stage process. Initially, phase-pure Bi₂W₂O₉, with orthorhombic crystalline structure, is prepared by a facile combustion-synthesis route. The combustion-synthesized Bi₂W₂O₉ is subsequently modified by CdS nanoparticles using a hydrothermal route. The CdS/Bi₂W₂O₉ heterojunctions are characterized using XRD, XPS, FTIR spectroscopy, UV/Vis DRS, PL, and FESEM and HRTEM studies. The occurrence of ultrafine CdS nanoparticles, with diameters of 8–15 nm, well-dispersed over BWO plates, is inferred from microscopic characterization studies. The two crystalline phases exhibit microscopic close contact across grain boundaries, facilitating the transfer of excitons. The heterojunction materials exhibit improved visible-light absorption, enhanced charge-carrier separation, and suitable band-alignment characteristics of a type-II heterojunction. The CdS/Bi₂W₂O₉ heterojunctions are evaluated as visible-light-active photocatalysts for the aerobic oxidation of amines to imines. Structurally and functionally diverse amine molecules are oxidized to the corresponding imines, with excellent selectivity, in a short span of time.

Full text of DOI:10.1002/ejic.201800221

A Journal of
Accepted Article
Title: Synthesis, characterization and photocatalytic application of type
II CdS/Bi2W2O9 heterojunction nanomaterials towards aerobic
oxidation of amines to imines
Authors: Yagna Prakash Bhoi and Brajagopal Mishra
This manuscript has been accepted after peer review and appears as an
Accepted Article online prior to editing, proofing, and formal publication
of the final Version of Record (VoR). This work is currently citable by
using the Digital Object Identifier (DOI) given below. The VoR will be
published online in Early View as soon as possible and may be different
to this Accepted Article as a result of editing. Readers should obtain
the VoR from the journal website shown below when it is published
to ensure accuracy of information. The authors are responsible for the
content of this Accepted Article.
To be cited as: Eur. J. Inorg. Chem. 10.1002/ejic.201800221
Link to VoR: http://dx.doi.org/10.1002/ejic.201800221

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
Synthesis, characterization and photocatalytic application of type II CdS/Bi2W2O9
heterojunction nanomaterials towards aerobic oxidation of amines to imines
Yagna Prakash Bhoi and Braja G. Mishra*
Department of Chemistry, National Institute of Technology, Rourkela-769008, Odisha, India.
*Email: brajam@nitrkl.ac.in
Homepage: http://www.nitrkl.ac.in/CY/~brajam
Abstract: A series of novel type-II CdS/Bi2W2O9 heterojunction nanomaterials were
prepared by a two stage process. Initially, phase pure Bi2W2O9 with orthorhombic crystalline
structure was prepared by a facile combustion synthesis route. The combustion synthesized
Bi2W2O9 was subsequently modified by CdS nanoparticles using a hydrothermal route. The
CdS/Bi2W2O9 heterojunctions were characterized using XRD, XPS, FTIR, UV-Vis-DRS, PL,
FESEM and HRTEM study. The occurrence of ultrafine CdS nanoparticles with diameter
between 8-15 nm well dispersed over BWO plates was inferred from microscopic
characterization study. The two crystalline phases exhibited microscopic close contact across
grain boundaries facilitating transfer of excitons. The heterojunction materials exhibited
improved visible light absorption, enhanced charge carrier separation and suitable band
alignment characteristic of a type-II heterojunction. The CdS/Bi2W2O9 heterojunctions were
evaluated as visible light active photocatalyst for aerobic oxidation of amines to imines.
Structurally and functionally diverse amine molecules were oxidized to the corresponding
imines with excellent selectivity in a short span of time.
1
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
Introduction
Organic transformations especially oxidation, reduction and coupling reactions are of
paramount importance in organic synthesis involving value added and fine chemicals. Many
fine chemical synthesis process utilize homogeneous corrosive acid-base catalyst,
stoichiometric reagents and harmful chemical oxidants at elevated temperature to achieve
high yield of the product. However, processes involving these conventional methods are
associated with problems such as catalyst handling, product purification, corrosion and
generation of significant quantity of by-products.[1, 2] The heterogeneous photocatalytic route
is a promising alternative to the conventional method of fine chemical synthesis. The
photocatalytic route is more sustainable, greener and economic towards fine chemical
synthesis, as it makes use of renewable energy source, discourage the use of stoichiometric
hazardous reagents, avoid solvent loss and uses atmospheric air as oxidant.[1-4] The use of
H2O as reaction media is another step to makes the system more eco-friendly.[3, 5] The
•
generation of reactive OH radicals by the photolysis of H2O over photocatalyst surface may
lead the generation of non-selective products in organic synthesis. However, Palmisano and
Bahnemann groups successfully conducted the partial oxidation of alcohol to aldehyde with
excellent selectivity over TiO2 and Bi2WO6 photocatalyst surface.[6-8] Recently, many
improved heterogeneous photocatalytic routes have been developed for selective oxidation
of alcohol, reduction nitro compounds, olefin–imine addition, radical coupling reaction,
Suzuki coupling reaction, synthesis of polymer and fine chemicals.[6-14] The oxidation of
amines to imines is one of the important reactions since imines are used as building blocks
for many pharmaceuticals and organic dyes molecules.[15-17] The conventional oxidation of
amine involves the use of costly noble metal catalysts at elevated temperature. The adverse
environmental impact of oxidants, solvent wastage and unwanted by-product formation is a
concern in the conventional amine oxidation processes.[15, 17] Till date, very few photocatalyst
such as TiO2, Nb2O5, WS2, BiVO4, BiOX, g-C3N4, AgI/AgVO3, Au-cluster, ZnIn2S4, CuWO4,
and organic polymers have been studied for the oxidation of amines.[15-28] Some of these
reported photocatalytic protocols require longer time, employ photocatalyst with poor
response to visible light and require molecular O2 to carry out the oxidation reaction. Keeping
in mind the advantages and disadvantages of the reported photocatalytic protocols, there is a
lot of scope to develop highly efficient novel photocatalytic system which can accomplish
this oxidation reaction in aerobic condition under visible light irradiation.
The Aurivillius oxide Bi2W2O9 is yet to be explored as photocatalyst for the oxidation
of amines. The Bi2W2O9 is the second member of cation deficient Aurivillius family with a
2
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
layered structure. Watanabe et al. in the year 1987 first reported the synthesis of
orthorhombic Bi2W2O9 using high temperature solid state route.[29] Cheng et. al. synthesized
rare earth substituted Bi2W2O9 by solid state synthesis and studied their optical, electronic
properties.[30 16] Recently, Bi2W2O9 has been synthesized by co-precipitation, Pechini method
and a facile combustion synthesis route.[31-33] The Bi2W2O9 material has been studied as a
potential candidate for luminescence, microwave dielectric, gas sensing and photocatalytic
applications.[32-41] However, very few reports are available on the photocatalytic application
of Bi2W2O9 primarily due to its UV absorption feature, wide band gap and poor charge
carrier separation properties.[32, 33, 38-41] Due to the high positive valence band potential (3.30
eV), the Bi2W2O9 is an ideal photocatalyst to carry out oxidation reactions.[33] In order to
improve the visible light absorption and charge carriers separation properties of a
photocatalyst, formation of heterojunctions with a foreign semiconducting material is an
important strategy. The close contact between the two semiconductors minimizes the
recombination rate of excitons which results in enhanced photocatalytic efficiency. Among
various types of heterojunctions, the type II heterojunction is most efficient towards the
separation of photogenerated electron and holes.[42] With an aim to develop novel and
efficient photocatalytic system for selective organic transformation, in this work we have
prepared type II CdS/Bi2W2O9 heterojunction photocatalyst and studied their application for
selective oxidation of amines to imines. CdS is one of the well explored narrow band gap
photocatalyst with band gap energy of 2.42 eV. CdS has been successfully used as visible
light active photocatalyst for oxidation of dichloro acetic acid, organic transformations and
water splitting reaction.[9, 11, 43, 44] Because of its narrow band gap, CdS has been coupled
with many wide band gap semiconductors such as TiO2, Bi2MoO6, g-C3N4, Bi2PO4 and SrS
to prepare highly efficient visible light active photocatalytic systems.[45-49] The CdS based
coupled semiconductor systems have been evaluated as photocatalyst for H2 evolution and
degradation of organic pollutants under visible light.
Results and discussion
Characterization of CdSBWO heterojunction
The XRD patterns of CdSBWO heterojunction materials together with pure Bi2W2O9
and CdS are presented in Figure 1. Pure Bi2W2O9 material exhibits intense and sharp
diffraction peaks at d-spacing values 3.95, 3.79, 3.45, 3.22, 2.98, 2.96, 2.75, 2.71, 2.42, 1.91,
1.65, 1.61 and 1.55 Å (Figure 1a). These peaks correspond to diffraction from the (006),
(111), (113), (114), (115), (008), (116), (020), (120), (220), (134), (228) and (229)
3
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
crystallographic planes of orthorhombic bismuth tungstate (JCPDS No- 33-0221, space
group- Pbn21). No impurity peaks corresponding to any analogous phase could be detected
from XRD indicating the high phase purity of the synthesized material.
# = CdS
@ = Bi2W2O9
e
d
@
@
@
@
@
@
@
@
@
@
@
@
#
@
#
#
#
c
b
a
20
25
30
35
40
45
50
55
60
2 (degrees)
Figure 1. X-ray diffraction patterns of (a) Bi2W2O9; (b) CdS5BWO; (c) CdS10BWO; (d)
CdS20BWO; and (e) CdS materials.
For pure CdS material sharp and intense diffraction peaks are observed at d-spacing
values of 3.58, 3.35, 3.16, 2.45, 2.07, 1.89, 1.76 and 1.67 Å. These peaks correspond to the
diffraction from the (100), (002), (101), (102), (110), (103), (112) and (004) crystallographic
planes of hexagonal cadmium sulphide phase (JCPDS No- 41-1049, space group- P63mc)
(Figure 1e). The CdS5BWO heterojunction display characteristic diffraction peaks of BWO
phase only (Figure 1b). However, for CdS10BWO and CdS20BWO heterojunctions material
low intense but distinguishable diffraction peaks for CdS phase was observed along with
BWO phase (Figure 1c & 1d). XRD results clearly indicate the presence of both the Bi2W2O9
and CdS crystalline phases in the CdSBWO heterojunction materials.
The oxidation state and the chemical environment of the constituent elements in the
heterojunction material was probed using XPS study. The XPS spectra of CdS20BWO
heterojunction is presented in Figure 2. Figure 2a represents the survey spectrum which
confirmed the presence of Bi, W, Cd, O and S along with advantageous carbon in the sample.
4
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
In the high resolution spectral region for Bi, a doublet is observed at 158.8 eV and 164 eV,
respectively (Figure 2b). These peaks are assigned to photoelectron emission from Bi4f7/2 and
Bi4f5/2 electronic states of Bi in +3 oxidation state.[33, 38, 50, 51] In this spectral region, a
noticeable low intense broad peak at 162.4 eV is assigned to the S2p state. [43, 48]
a
Bi4f7/2
Bi4f
b
Bi4f5/2
S2p
0
200
400
600
800
1000
W4f
156
158
160
162
164
166
168
Energy (eV)
Energy (eV)
Cd3d
d
c
Cd3d5/2
W4f7/2
Cd3d3/2
W4f5/2
402
404
406
408
410
412
414
416
33
34
35
36
Energy (eV)
37
38
39
Energy (eV)
e
S2s
O1s
f
526
528
530
532
534
536
222
224
226
228
230
Energy (eV)
Energy (eV)
Figure 2. XPS spectra of CdS20BWO heterojunction.
Figure 2c presents the fine spectra of W in the 4f region. The binding energy value of
35.2 eV and 37.4 eV with a splitting factor of 2.2 eV can be attributed to W4f7/2 and W4f5/2
electronic states of W in +6 oxidation state.[33, 38, 51] A symmetrical doublet observed at
binding energy values of 405.1 eV and 411.8 eV with a spin orbit splitting value of 6.7 eV is
due to the Cd3d5/2 and Cd3d3/2 electronic states of Cd in +2 oxidation state (Figure 2d).[43, 48]
Figure 2e represents the high resolution XPS spectra for O in 1s region. A broad asymmetric
peak is observed which is fitted to two peaks. These two peaks correspond to the presence of
oxygen in two different electronic environments. The peaks at binding energy value of 529.7
eV attribute the oxygen in the (W2O7)2- Perovskite layers while the binding energy value of
531.1 eV is due to the interlayer oxygen associated with the interlayer (Bi2O2)2+ species.[33, 38]
In the high resolution XPS spectra in the S 2s region, a broad peak with maxima at 525.7 eV
is observed (Figure 2f). This broad peak confirmed the presence of sulphide ions (S2-).[33, 50,
51]
The morphology of the BWO and CdSBWO heterojunction materials is studied by
FESEM and HRTEM analysis. The FESEM images of BWO and CdSBWO materials are
5
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
presented in Figure 3. The BWO material contains micron size plates with L x B x H in the
range of 0.40-2.30 μm, 0.31-1.68 μm and 0.08-0.42 μm, respectively (Figure 3a). The
CdSBWO heterojunction materials contain uniformly distributed spherical CdS nanoparticles
grafted over the BWO plates (Figure 3b-3d). With increase in the CdS content in the
heterojunction materials the particle density of anchored CdS species increases. Moreover,
local aggregation of the CdS particles is observed at higher content.
(a)
(b)
(c)
(d)
Figure 3. FESEM images of (a) Bi2W2O9; (b) CdS5BWO; (c) CdS10BWO; and (d)
CdS20BWO heterojunction materials.
The size and shape of CdS nanoparticle is further investigated using HRTEM analysis.
Figure 4 represents the TEM images of CdS20BWO heterojunction material. The presence of
ultrafine near spherical CdS nanoparticles with size in the range of 8-15 nm is noticed for the
CdS20BWO material (Figure 4a & 4b). The CdS nanoparticles are in an agglomerated state
and are attached to each other through the grain boundary (Figure 4a). The high
magnification TEM image of CdS20BWO material revealed the formation of heterojunction
between the two phases (Figure 4c). The calculated interplanar distance matches with the
(113) crystallographic plane of Bi2W2O9 and (002), (101) planes of CdS species (Figure 4c).
6
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
This observation gives another evidence for the presence of crystalline phases of CdS and
Bi2W2O9 in the CdSBWO heterojunction sample. The elemental mapping and EDX spectrum
of the heterojunction material is presented in Figure 4d-4j. Homogeneous distribution of Bi,
W, O, Cd and S elements throughout the specimen sample is observed from the elemental
mapping study (Figure 4d-4i). The presence CdS and Bi2W2O9 in required stoichiometric
quantity is observed from the EDX study of CdSBWO heterojunction material (Figure 4j).
The crystallite size of the CdS phase in the CdSBWO heterojunction materials is calculated
by Fourier line profile analysis of XRD peaks by Warren and Averbach method using
BRAEDTH software.[52] The Fourier analysis is performed by using (101) and (110) peaks of
the CdS phase. The distribution function (PV)~Fourier length (L) and Size coefficient (AS) ~
L plots for pure CdS, CdS10BWO and CdS20BWO materials are presented in Figure S1.
The average crystallite size calculated from the initial slope of AS~L plots (Figure S1I) are
presented in Table 1. The heterojunction materials display average crystallite size in the
range of 7-12 nm. The Fourier analysis data is found to be complimentary to the crystallite
size observed from HRTEM study.
(a)
(b)
(c)
(d)
(e)
(f)
7
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
(g)
(h)
(i)
cps/eV
O-KA
S-KA
W-LA
Bi-LA
7
6
5
4
3
2
W
W
Bi
Cd
W
1
Bi
S
W
Bi
Bi
Cd
Cd
S
O
W
W
Bi
Cd
0
2
4
6
8
keV
10
12
14
(j)
Figure 4. (a-c) TEM images; (d-i) EDS mapping; and (j) EDX spectrum of CdS10BWO
heterojunction material.
The optical property of the CdSBWO heterojunction material is studied by UV-Vis
DRS and PL techniques. Figure 5I depicts the UV-Vis DRS spectra of Pure BWO and CdS
together with CdSBWO heterojunction materials. Pure BWO shows strong absorption in the
UV region, which extend up to visible region. Similarly, pure CdS shows optical absorption
in the visible region with an absorption edge near 530 nm. The stiff absorption feature
observed for both BWO and CdS materials is due to band edge transition rather than the
transition from the impurity level. Upon modification with CdS, for all the CdSBWO
heterojunction materials the absorption edge shifted to the lower energy region with
improved visible light absorption feature. The calculated band gap values for the pure BWO
and CdS are 2.95 and 2.36 eV respectively (Figure S2). For the CdS and BWO materials the
observed band gap values are consistent with the literature reported values.[32, 33, 38, 40, 48]
Table 1. Specific surface area and crystallite size of CdS phase in CdSBWO heterojunction
materials.
Specific Surface
area (m2/g)
Crystallite size of CdS
phase (nm)[a]
Sl. No
Catalyst
1
2
3
Bi2W2O9
CdS
14.3
32.5
24.8
---
14.5  1.0
---
CdS5BWO
4
5
CdS10BWO
CdS20BWO
28.6
31.7
8.5  1.5
10.5 1.6
[a] Calculated from the Fourier line profile analysis of the XRD peaks
8
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
I
II
d
CdS
CdS20BWO
c
CdS10BWO
CdS5BWO
b
a
BWO
200
300
400
500
600
700
800
1200
1000
800
600
400
Wavenumber (cm-1)
Wavelength (nm)
III
a
b
c
d
425
450
475
500
Wavelength (nm)
Figure 5. UV-Vis DRS (Panel I); FTIR (Panel II); and PL spectra (Panel III) of (a) Bi2W2O9;
(b) CdS5BWO; (c) CdS10BWO; and (d) CdS20BWO heterojunction materials.
The BWO and CdS materials exhibit a specific surface area of 14.3 and 32.3 m2/g (Table 1).
The heterojunction material exhibits surface area in the range of 24-32 m2/g. The FTIR
spectra of CdSBWO heterojunction materials along with pure BWO material are presented in
Figure 5II. Pure BWO possesses broad IR bands at 437, 561, 745, 851 and 952 cm-1 (Figure
5IIa). The bands at 437, 561 cm-1 corresponds to the Bi-O stretching vibrations whereas the
745, 851, 952 cm-1 bands are assigned to the symmetric and asymmetric W-O stretching
vibrations for WO6 octahedron.[31, 33] All the CdSBWO heterojunction display new set of IR
bands at 635, 1150 and 1215 cm-1 along with the characteristic vibrational features of BWO
(Figure 5IIb-d). These bands are due to the characteristic vibration for CdS component in the
9
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
CdSBWO heterojunction materials.[31] The FTIR study reveals the presence of CdS and BWO
components in the CdSBWO heterojunction materials. The charge separation property of the
CdSBWO heterojunction materials is studied by using PL technique and photocurrent
measurement. The PL spectra of CdSBWO heterojunction materials at an excitation
wavelength of 320 nm are presented in Figure 5III. Pure BWO material exhibits PL emission
bands at 465, 480 and 490 nm respectively (Figure 5IIIa). The emission bands observed at
465 nm is due to the band gap transition whereas the PL bands at 480 and 490 nm are due to
presence of defects and vacancies in the BWO material.[33, 40] The intensities of these PL
bands decrease significantly after modification with CdS. This observation indicates efficient
charge carrier separation in the CdSBWO heterojunction materials (Figure 5IIIb- 5IIId). The
charge carrier separation property is further studied by taking the photocurrent measurement
of the BWO and CdS20BWO materials. The charge transport ability of the heterojunction
material is investigated by transient photocurrent measurement (Figure 6). The enhancement
in photocurrent of CdS20BWO material over BWO material indicates an improvement in the
charge carrier separation ability of the heterojunction material (Figure 6I). The current (I) vs
voltage (V) measurement for the BWO and CdS20BWO heterojunction material is presented
in Fig. 6II. The higher slope observed in I~V plot for CdS20BWO heterojunction material
indicate the higher mobility of the photo excited electrons (Figure 6II). The enhanced
photocurrent and improved charge transport efficiency of the CdS20BWO material can be
ascribed to the formation of heterojunction between the CdS and BWO component which
allow the facile migration of charge carriers across the grain boundary. The UV-Vis DRS and
PL study along with the calculated band gap values of CdSBWO heterojunction materials
indicate that these materials can be used as photocatalyst under visible light.
10
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
0.08
0.06
0.04
0.02
0.00
-0.02
-0.04
-0.06
-0.08
OFF
I
II
0.0130
0.0125
0.0120
0.0115
0.0110
b
b
a
a
ON
0.0105
0
30
60
90
120
-1.0
-0.5
0.0
0.5
1.0
Time (sec)
Voltage (mV)
Figure 6. (I) Transient photocurrent; and (II) current (I) vs voltage (V) measurement; plots
for (a) BWO; and (b) CdS20BWO heterojunction materials.
Selective aerobic oxidation of benzylamine catalysed by CdSBWO heterostructure
The photocatalytic activity of the CdSBWO heterojunction materials is evaluated for
the selective oxidation of benzylamine under visible light irradiation using air as oxidant
(Scheme 1).
Scheme 1. Selective photocatalytic oxidation of benzylamine catalysed by CdSBWO
heterostructure photocatalyst
As a model reaction, a reaction mixture containing 1 mmol of benzylamine and 25 mg
of CdSBWO catalyst in 5 ml CH3CN solvent is stirred under dark condition for 30 min to
achieve adsorption-desorption equilibrium. The reaction mixture was exposed to visible light
under constant stirring. The progress of the reaction is monitored by gas chromatography. In
absence of catalyst and under dark condition the formation of N-Benzylidenebenzylamine
could not be detected which confirmed the photocatalytic nature of the oxidation reaction.
The photocatalytic efficiency of the CdSBWO heterojunction photocatalyst together with
CdS and BWO material is presented in Figure 7. Although both pure CdS and BWO
11
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
materials are active towards the oxidation reaction, the percentage conversion is quite low
(22.5 and 40.2 % for BWO and CdS material, respectively after 12 h).
100
80
60
40
20
0
100
80
60
40
20
0
CdS
BWO
CdS5BWO
CdS10BWO
CdS20BWO
I
II
0
2
4
6
8
10
12
5
10
15
20
25
30
35
Time (h)
Catalyst weight (mg)
Figure 7. Photocatalytic efficiency of CdSBWO heterojunction (Panel I) and catalyst weight
variation of CdS20BWO heterojunction (Panel II) for the photocatalytic selective oxidation
of benzylamine.
In contrast to this observation, all the CdSBWO heterojunction materials exhibited higher
photocatalytic efficiency with good % conversion and selectivity towards N-
Benzylidenebenzylamine (Figure 7I). Among the heterojunction materials, CdS20BWO
exhibited highest photocatalytic efficiency achieving 89.3 % conversion after 8 h of reaction.
Since, the surface area of CdSBWO heterojunctions increase with CdS content, the role of
surface area on the photocatalytic activity is evaluated by preparing CdS20BWO material by
precipitation (P) and reflux (R) methods. The CdS20BWO-P and CdS20BWO-R materials
show surface area of 20.5 and 24.3 m2/g, respectively (ESI file). These materials after 8 h of
reaction time exhibited 83.5 and 84.7 % conversion of benzyl amine. This observation
indicates that the CdS content in the heterojunction is the primary factor responsible for
higher photocatalytic activity. The presence of CdS improves the visible light absorption and
charge carrier separation properties of BWO which are vital for the photocatalytic activity.
Based on the results described in Figure 7I, the CdS20BWO material is chosen for further
study.
The catalyst weight is optimized by varying the catalyst amount between 5-35 mg in
the reaction mixture. It is observed that, 25 mg of CdS20BWO catalyst is sufficient to carry
out the oxidation of 1 mmol of the reactant with excellent conversion and selectivity (Figure
12
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
7II). Further increase in catalyst amount does not have any significant impact on amine
oxidation. The effect of solvent on benzylamine oxidation is studied by using different media
in presence of CdS20BWO photocatalyst (Table 2).
Table 2. Photocatalytic Oxidation of benzylaminea
Sl. No
Solvent
Oxidant Time (h)
Conversion[b]
(%)
Selectivity[b]
(%)
1
2
CH3CN
CH3OH
C2H5OH
H2O
Air
Air
12
12
12
12
12
12
12
12
12
12
24
8
92.5
> 99
88.4
85.8
26.5
84.7
82.4
56.4
53.6
81.2
83.5
---
> 99
> 99
45
3
Air
4
Air
5
CHCl3
Air
> 99
> 99
> 99
> 99
> 99
> 99
---
6
CH2Cl2
Hexane
Tolune
DMF
Air
7
Air
8
Air
9
Air
10
11
12
13
14
15
16
DMSO
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
CH3CN
Air
N2
O2
93.6
90.2
94.5
92.3
92.7
> 99
> 99
> 99
> 99
> 99
O3
6
H2O2
Oxone
NaClO
6
6
6
[a]Reaction conditions: benzylamine (1 mmol), solvent (5 mL), catalyst 25 mg
[b](CdS20BWO), Determined from GC analysis
The catalytic activity is significantly higher in polar solvent in comparison to the
nonpolar solvent. The higher conversion in polar organic solvent can be ascribed to better
solubility of the reactant molecules and their easy accessibility to the catalyst surface. Highest
conversion of benzyl amine is obtained using acetonitrile as solvent. Hence further studies are
performed using acetonitrile as solvent. In order to study the effect of different oxidant on the
oxidation of benzylamine, the oxidation reaction is carryout in presence of 3 molar excess of
different oxidant. It is observed that in presence of different oxidants, the oxidation rate is
expedited and > 90 % of conversion is achieved within 6 h (Table 2). When the reaction is
conducted under N2 atmosphere no benzylamine conversion is noticed even after 24 h of
13
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
reaction. This observation suggests that under aerobic condition the molecular O2 present in
dissolved state acts as oxidant in oxidizing the benzylamine. Although better conversion are
achieved using different oxidant within a short span of time, further studies are carried out in
presence of atmospheric air as oxidant in order to make the processes sustainable and
greener.
After optimizing various reaction parameters, the versatility the CdS20BWO catalyzed
oxidation protocol is tested by using different amines (Table 3).
Table 3. Selective Oxidation of various amines catalysed by CdS20BWO heterojunctiona
Entry
1
Reactant
Product
Conversionb
(%)
Selectivityb
(%)
N
H
81.6
> 90
N
CH3
CH3
2
3
66.4
85
N
N
H
NH2
86.4
88.6
> 90
> 90
N
N
N
N
N
H
N
H
N
H
NH2
4
N
N
N
O
N
O
O
NH2
5
6
N
90.3
89.4
> 90
85
N
N
N
NH2
N
NH2
+
c
7
8
18.5
14.3
70
68
H3C
NH2
H3C
N
N
CH3
H3C
H3C
CH3
NH2
[a]Reaction conditions: amine (1 mmol), solvent (5 mL), catalyst 25 mg (CdS20BWO),reaction time
12 h, [b]Determined from GC analysis
The CdS20BWO photocatalyst is highly active for selective oxidation of both the
primary as well as secondary amines to the corresponding imines. However, the oxidation
14
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
rate for secondary amines is relatively sluggish along with by-products formation (Table 3,
Entry 1 & 2). Excellent conversion is also observed for the oxidation of heterocyclic amines
with very good selectivity towards imines (> 90 %) (Table 3, Entry 3, 4 & 5). In case of
aliphatic primary amine the conversion is quite low with poor selectivity to corresponding
imines (Table 3, Entry 7 & 8). The oxidation protocol is also extended to mixed system by
taking 0.5 mmol each of benzylamine and aniline (Table 3, Entry 6). Formation of both N-
benzylidene(phenyl)methanamine and N-benzylidenebenzenamine is noticed for this mixed
amine system. However, the CdS20BWO catalysed photocatalytic protocol is more selective
towards formation of N-benzylidene(phenyl)methanamine (Selectivity 85 %). The results
described in Table 3 suggest that the CdS20BWO catalysed photocatalytic protocol is
applicable for the oxidation of a variety of amines under visible light irradiation.
In order to understand the mechanism of photocatalytic oxidation of benzylamine,
control experiments are performed in presence of equimolar quantity of different radical
scavengers (Figure S3). In presence of hole scavenger (ammonium oxalate), superoxide
radical scavenger (benzoquinone) and electron scavenger (AgNO3), the oxidation rate of
benzylamine amine decrease drastically. However, presence of hydroxyl radical scavenger (t-
BuOH and isopropanol) only marginally affects the oxidation reaction (Figure S3). This
observation rules out the active role of hydroxyl radical during the oxidation of benzylamine.
-
Hence from scavenger experiments, the h+, e- and O2 are identified as the primary active

species responsible for the selective aerobic oxidation of benzylamine.
Based on the scavenger experiment, a plausible mechanism for the amine oxidation is
proposed over the CdSBWO photocatalyst surface (Figure 8). Upon irradiation, the electrons
get excited to the conduction band (CB) leaving behind the holes in the valence band (VB).
Since the band gap of both the BWO and CdS components are < 3 eV, both the components
of the heterojunction is capable to generate excitons under visible light irradiation. The
valance band (VB) and conduction band (CB) potentials of the pure CdS and BWO materials
are calculated theoretically using the formula in equation 1.[33, 50, 51]
EVB = X + 0.5Eg– Ee and ECB = EVB–Eg
(1)
EVB and ECB correspond to the valence and conduction band edge potential (eV vs NHE). X is
the electronegativity of the semiconductor. Whereas, Ee and Eg are the energy of free electron
(4.5 eV) on hydrogen scale and band gap of the photocatalyst. The X values for CdS and
Bi2W2O9 photocatalyst are 5.05 and 6.3 eV, respectively.[33, 46] The CB of pure CdS and
BWO occur at -0.62 eV and 0.325 eV respectively, whereas the VB are placed at 1.74 eV and
3.275 eV. Upon material contact between the two semiconductor phases, there is realignment
15
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
in band positions due to Fermi level equilibration. The band realignment facilitates transfer
of electrons from the CB of CdS to CB of BWO. The movement of holes takes place in
reverse direction in VB resulting in formation of a type II heterojunction. The VB of both
CdS and BWO are placed at a sufficiently positive potential to oxidize the benzylamine.[24]
The VB holes oxidise the benzylamine to the radical cationic species I. On the other hand, the
-
-


trapping of CB electron by O2 leads to the formation of O2 radical. The O2 radical react
with the benzylamine radical cation to give benzylideneamine (II) by release of H2O2. A
second molecule of benzylamine reacts with benzylideneamine to give the final oxidized
product N-benzylidene(phenyl)methanamine with the release of H2O and NH3. The proposed
mechanism is in line with the earlier reports for photocatalytic oxidation of amines.[16, 20-23]
The practical applicability of the CdS20BWO heterojunction photocatalyst has been
evaluated by checking its activity for several consecutive cycles. After each photocatalytic
cycle, the catalyst particle are collected and washed with ethanol followed by drying at 110
oC for 12 h. The CdS20BWO heterojunction material shows excellent photocatalytic
efficiency for the selective oxidation of benzylamine without any appreciable loss in activity
for five consecutive cycles (Figure S4). This observation suggests that the CdSBWO
heterojunction photocatalyst systems are stable and recyclable for the oxidation of amine.
Figure 8. Possible mechanism for the oxidation of benzylamine over CdSBWO
heterojunction photocatalyst surface.
16
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
Conclusions
In this study, we have developed a novel type-II CdS/Bi2W2O9 heterojunction
photocatalytic system for efficient and selective oxidation of amines to imines under aerobic
condition and visible light irradiation. Bi2W2O9 with excellent phase purity is synthesized by
employing combustion synthesis method in a short period of time. The deposition of CdS
nanoparticles over BWO plates leads to formation of heterojunction system with improved
visible light absorption and charge carrier separation. The CdS nanoparticles are of 8-15 nm
in size and are well dispersed over the BWO plates. The CdS/Bi2W2O9 heterojunction
materials display excellent photocatalytic activity towards selective aerobic oxidation of
amines to imines. A variety of primary, secondary and heterocyclic amines are oxidized to
the corresponding imines using the CdSBWO material as photocatalyst. The photocatalytic
protocol developed in this work is advantageous in terms of use of reneawable energy source,
preclusion of toxic oxidants, short reaction time, high selectivity and recyclable
photocatalyst.
Experimental section
Materials: All the chemicals were of analytical grade and used directly without purification.
Bismuth nitrate (Bi(NO3).5H2O), cadmium nitrate (Cd(NO3)2.4H2O), ammonium tungstate
((NH4)10W12O41), urea (N2H4CO), thiourea (N2H4CS) and amines were procured from
Himedia India Ltd and Avra synthesis India Ltd.
Synthesis of Bi2W2O9 (BWO): The Bi2W2O9 nanomaterial was synthesized by solution
combustion method by taking ammonium tungstate, bismuth nitrate as precursor salts and
urea as fuel.[19] The fuel to oxidizer ratio was maintained at 1.[53] In a typical synthesis,
required amount of Bi(NO3)3.3H2O, (NH4)10W12O41 and urea were mixed well and made a
paste by drop wise addition of distilled water. The mixture was transferred to a furnace pre-
o
heated at 400 C. The mixture instantaneously got ignited producing large quantities of
gaseous product and a foamy solid. The combustion mixture was maintained at 400 oC for 30
o
minutes which was subsequently cooled, grinded to fine powder and calcined at 800 C for 6
h to obtain the Bi2W2O9 material.
Synthesis of CdS/Bi2W2O9 heterojunction material (CdSBWO): The CdS/Bi2W2O9
heterojunction materials were synthesis by hydrothermal method by depositing CdS
nanoparticles over the BWO surface. For CdS synthesis, Cd(NO3)2.4H2O was taken as salt
precursor and thiourea as sulphur source. In a typical synthesis, required amount of cadmium
nitrate and thiourea were dissolved in 50 ml H2O, to which 1 g of BWO powder was added
17
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
and stirred for 30 min. The aqueous suspension was then transfer to a Teflon lined autoclave
o
of 100 ml capacity. The hydrothermal treatment was carried at 150 C for 12 h under
autogeneous pressure. After completion of reaction, the reaction mixture was allowed to cool
and the solid materials were collected by centrifugation, washed each with water and ethanol.
The solid residue was dried at 110oC for 12 h to get the CdSBWO material. Using this
procedure, CdS/Bi2W2O9 materials containing 5, 10 and 20 wt % of CdS were prepared. The
CdS/Bi2W2O9 heterojunction materials are referred as CdSyBWO in the subsequent text,
where ‘y’ represents the wt % of CdS present in the heterojunction.
Characterization techniques: The XRD patterns of the CdSyBWO materials were recorded
using a Rigaku, Ultima-IV multipurpose X-ray diffraction system using Ni filtered CuK (
= 1.5418 Å) radiation at a scan rate of 2o per min. The UV-visible diffuse reflectance spectra
of the heterojunction materials were recorded using a Jasco V-650 spectrometer fitted with
BaSO4 coated integration sphere. The FTIR spectra (as KBr pellets) were recorded using a
Perkin–Elmer infrared spectrometer with a resolution of 1 cm-1. The X-ray photoelectron
spectra (XPS) of CdS20BWO material was recorded using SPECS make (Germany)
spectrophotometer with 150 mm hemispherical analyzer at band pass energy of 12 eV.
Monochromatic Al K radiation of 1486.74 eV was used as X-ray source. Due to
electrostatic charging, binding energy corrections were made using the C1s peak at 284.6 eV
as standard. The FESEM of BWO and CdSyBWO heterostructure were taken using a Nova
NanoSEM microscope model FEI operating at an acceleration voltage of 10 kV. HRTEM
images of the CdS20BWO material was recorded using Tecnai 300 kV using carbon coated
copper grids of 300 mesh size. The photoluminescence (PL) spectra of the heterojunctions
were recorded with a Horiba Scientific Fluoromax-4 spectrofluorometer with an excitation
wavelength of 320 nm. The specific surface area of the samples was determined by BET
method using N2 adsorption/desorption at 77 K using a Quantachorme AUTOSORB 1
instrument.
Photocatalytic oxidation of amines catalysed by CdSyBWO heterojunction under visible
light: The photocatalytic activity of the CdS/BWO heterojunction materials were evaluated
for selective aerial oxidation of amines to imines under visible light irradiation. In a typical
experiment, 1 mmol of benzylamine in 5 ml of acetonitrile was taken in a round bottom flask,
to which 25 mg of CdSyBWO catalyst was added and stirred for 15 min under dark
condition. The reaction mixture was then exposed to a 125 W high pressure mercury vapor
lamp with λ > 420 nm. The percentage conversion at different interval of time was analyzed
using a Nucon gas chromatograph using an EC-Wax capillary column and FID detector.
18
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
After the completion of the reaction, catalyst particle was separated with a membrane filter
and the N-benzylidenebenzylamine was recovered from reaction mixture and purified using
column chromatography. All the molecules synthesized in this work are known compounds
and were identified by comparing its physical and spectral characteristics with the literature
reported values.[54-58]
Supplementary information can be found in a separate document. Tauc’s plots, Radical
scavenger experiment and Reusability study of the CdSBWO heterojunction photocatalyst.
Acknowledgements
We thank DST-WTI, New Delhi, India for financial support through grant no-
DST/TM/WTI/2K15/93.
Keywords: CdS  Bi2W2O9  heterojunction  amine oxidation
References
[1] G. Palmisano, E. Garcıa-Lopez, G. Marcı, V. Loddo, S. Yurdakal, V. Augugliaro, L.
Palmisano Chemical Communications. 2010, 46, 7074-7089.
[2] D. Friedmann, A. Hakki, H. Kim, W. Choi, D. Bahnemann Green Chemistry. 2016, 18,
5391-5411.
[3] G. Palmisano,V. Augugliaro, M. Pagliaro, L. Palmisano Chemical Communications.
2007, 3425-3437.
[4] V. Augugliaro, M. Bellardita, V. Loddo, G. Palmisano, L. Palmisano, S. Yurdakal
Journal of Photochemistry and Photobiology C: Photochemistry Reviews. 2012, 13, 224-
245.
[5] D. Kumar, V. B. Reddy, B. G. Mishra, R. K. Rana, M. N. Nadagouda, R. S. Varma
Tetrahedron. 2007, 63, 3093-3097.
[6] Sedat Yurdakal, Giovanni Palmisano, Vittorio Loddo, Vincenzo Augugliaro, Leonardo
Palmisano Journal of Ammerican Chemical Society. 2008, 130, 1568-1569.
[7] G. Palmisano, M. Addamo, V. Augugliaro, T. Caronna, E. Garcıa-Lopez, V. Loddo, L.
Palmisano Chemical Communications. 2006, 1012–1014.
[8] M. Qamar, R. B. Elsayed, K. R. Alhooshani, M. I. Ahmed, D. W. Bahnemann ACS
Applied Materials & Interfaces. 2015, 7, 1257-1269.
[9] M. Gartner, J. Ballmann, C. Damm, F. W. Heinemann, H. Kisch Photochemical &
Photobiological Sciences. 2007, 6, 159-164.
19
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
[10] H. Kisch Accounts of Chemical Research. 2017, 50, 1002-1010.
[11] T. Mitkina, C. Stanglmair, W. Setzer, M. Gruber, H. Kisch, B. Konig Organic &
Biomolecular Chemistry. 2012, 10, 3556-3561.
[12] A. Hakki, R. Dillert, D. Bahnemann Catalysis Today. 2009, 144, 154-159.
[13] Z. Jiao, Z. Zhai, X. Guo, X. Y. Guo The Journal of Physical Chemistry C. 2015, 119,
3238-3243.
[14] N. Corrigan, S. Shanmugam, J. Xu, C. Boyer Chemical Society Review. 2016, 45, 6165-
6212
[15] Y. Wu, B. Yuan, M. Li, W. H. Zhang, Y. Liu, C. Li Chemical Science. 2015, 6, 1873-
1878.
[16] A. Han, H. Zhang, G. K. Chuah, S. Jaenicke Applied Catalysis B: Environmental. 2017,
219, 269-275.
[17] S. Furukawa, Y. Ohno, T. Shishido, K. Teramura, T. Tanaka ACS Catalysis. 2011, 1,
1150-1153.
[18] Z. Wang, X. Lang Applied Catalysis B: Environmental. 2018, 224, 404-409.
[19] X. Lang, W. Ma, Y. Zhao, C. Chen, H. Ji, J. Zhao Chemistry: A European Journal.
2012, 18, 2624-2631.
[20] X. Lang, H. Ji, C. Chen, W. Ma, J. Zhao Angewandte Chemie International Edition.
2011, 50, 3934-3937.
[21] F. Raza, J. H. Park, H. R. Lee, H. I. Kim, S. J. Jeon, J. H. Kim ACS Catalysis. 2016, 6,
2754-2759.
[22] B. Yuan, R. Chong, B. Zhang, J. Li, Y. Liu, C. Li Chemical Communications. 2014, 50,
15593-15596.
[23] F. Su, S. C. Mathew, L. Mohlmann, M. Antonietti, X. Wang, S. Blechert Angewandte
Chemie International Edition. 2011, 50, 657-660.
[24] X. Wang, J. Yang, S. Ma, D. Zhao, J. Dai, D. Zhang Catalysis Science & Technology
2016, 6, 243-253.
[25] ] H. Chen, C. Liu, M. Wang, C. Zhang, N. Luo, Y. Wang, H. Abroshan, G. Li, F. Wang
ACS Catalysis. 2017, 7, 3632-3638.
[26] L. Ye, Z. Li ChemCatChem. 2014, 6, 2540-2543.
[27] A. D. Proctor, S. Panuganti, B. M. Bartlett Chemical Communications. 2018, 54, 1101-
1104.
[28] Z. J. Wang, S. Ghasimi, K. Landfester, K. A. I. Zhang Advanced Materials. 2015, 27,
6265-6270.
20
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
[29] A. Watanabe, M. Goto Journal of the Less-Common Metals. 1978, 61, 265-272.
[30] Y. Y. Li, W. D. Cheng, H. Zhang, C. S. Lin, W. L. Zhang, L. Geng, G. L. Chai, Z. Z.
Luo, Z. Z. He Dalton Transactions. 2011, 40, 7357-7364.
[31] M. Maczka, M. Ptak, L. Kepinski, P.E. Tomaszewski, J. Hanuza Vibrational
Spectroscopy. 2010, 53, 199-203.
[32] S. O. Alfaro, A. M. Cruz Applied Catalysis A: General. 2010, 383, 128-133.
[33] Y. P. Bhoi, C. Behera, D. Majhi, Sk. Md. Equeenuddin, B. G. Mishra New Journal of
Chemistry. 2018, 42, 281-292.
[34] O. M. Bordun, A. T. Stetskiv, T. M. Yaremchuk Journal of Applied Spectroscopy. 2004,
71, 136-138.
[35] O. M. Bordun, A. T. Stetskiv, T. M. Yaremchuk Ukrainian Journal of Physics. 2004, 49,
991-995.
[36] A. Feteiraw, D. C. Sinclair Journal of the American Ceramic Society. 2008, 91, 1338-
1341.
[37] L. Wu, J. Xia, J. Wu, Q. Li Ionics. 2015, 21, 3239-3244.
[38] S. Obregón, M. A. R. Gómez, D. B. H. Uresti Journal of Colloid and Interface Science.
2017, 506, 111-119.
[39] S. O. Alfaro, A. M. Cruz, L. M. T. Martínez, S. W. Lee Catalysis Communications.
2010, 11, 326-330.
[40] J. Tang, J. Ye Journal of Materials Chemistry. 2005, 15, 4246-4251.
[41] A. M. Cruz, S. O. Alfaro, L. M. T. Martínez, I. J. Ramírez Journal of Ceramic
Processing Research. 2008, 9, 490-494.
[42] R. Marschall Advanced Functional Materials. 2014, 24, 2421-2440.
[43] S. Sakthivel, S.U. Geissen, D.W. Bahnemann, V. Murugesan, A. Vogelpohl Journal of
Photochemistry and Photobiology A: Chemistry. 2002, 148, 283-293.
[44] S. Naskar, F. Lübkemann, S. Hamid, A. Freytag, A. Wolf, J. Koch, I. Ivanova, H. Pfnür,
D. Dorfs, D. W. Bahnemann, N. C. Bigall Advanced Functional Materials. 2017, 27,
1604685.
[45] Y. Huo, X. Yang, J. Zhu, H. Li Applied Catalysis B: Environmental. 2011, 106, 69-75.
[46] Y. Feng, X. Yan, C. Liu, Y. Hong, L. Zhu, M. Zhou, W. Shi Applied Surface Science.
2015, 353, 87–94.
[47] J. Fu, B. Chang, Y. Tian, F. Xi, X. Dong Journal of Materials Chemistry A. 2013, 1,
3083-3090.
21
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
[48] D. Chen, Z. Kuang, Q. Zhu, Y. Du, H. Zhu Materials Research Bulletin. 2015, 66, 262-
267.
[49] Z. Khan, T. R. Chetia, M. Qureshi Nanoscale. 2012, 4, 3543-3550.
[50] Y. P. Bhoi, B. G. Mishra Chemical Engineering Journal. 2017, 316, 70-81.
[51] Y. P. Bhoi, S. R. Pradhan, C. Behera, B. G. Mishra RSC Advances. 2016, 6, 35589-
35601.
[52] D. Balzar, H. Ledbetter Journal of Applied Crystallography. 1993, 26, 97-103.
[53] S. R. Jain, K. C. Adiga, V. R. P. Verneker Combustion and Flame. 1981, 40, 71-79.
[54] B. Chen, S. Shang, L. Wang, Y. Zhang, S. Gao Chemical Communication. 2016, 52,
481-484.
[55] Y. Zhi, K. Li, H. Xia, M. Xue, Y. Mua, X. Liu Journal of Materials Chemistry A. 2017,
5, 8697–8704.
[56] T. B. Nguyen, L. Ermolenko, A. Al-Mourabit Green Chemistry. 2013, 15, 2713-2717.
[57] X. Qiu, C. Len, R. Luque, Y Li ChemSusChem. 2014, 7, 1684-1688.
[58] Y. Zhang, F. Lu, R. Huang, H. Zhang, J. Zhao Catalysis Communications. 2016, 81, 10-
13.
22
This article is protected by copyright. All rights reserved.

10.1002/ejic.201800221
European Journal of Inorganic Chemistry
Entry for the Table of contents
A novel CdS/Bi2W2O9 heterojunction photocatalyst is highly efficient towards the aerobic
oxidation of amines to imines under visible light irradiation. The heterojunction system
shows improved photo induced properties, stable photocatalytic activity and excellent
selectivity towards imines.
23
This article is protected by copyright. All rights reserved.