Ag2S sensitized mesoporous Bi2WO6 architectures with enhanced visible light photocatalytic activity and recycling properties

Article abstract of DOI:10.1039/c5ra05117g

To harvest solar energy more efficiently, novel Ag₂S/Bi₂WO₆ heterojunctions were synthesized by a hydrothermal route. This novel photocatalyst was synthesized by impregnating Ag₂S into a Bi₂WO₆ semiconductor by a hydrothermal route without any surfactants or templates. The as prepared structures were characterized by multiple techniques such as X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Brunauer-Emmet-Teller (BET) analysis, scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy dispersive X-ray spectrometry (EDS), UV-vis diffuse reflection spectroscopy (DRS) and photoluminescence (PL). The characterization results suggest mesoporous hierarchical spherical structures with a high surface area and improved photo response in the visible spectrum. Compared to bare Bi₂WO₆, Ag₂S/Bi₂WO₆ exhibited much higher photocatalytic activity towards the degradation of dye Rhodamine B (RhB). Although silver based catalysts are easily eroded by photogenerated holes, the Ag₂S/Bi₂WO₆ photocatalyst was found to be highly stable in the cyclic experiments. Based on the results of BET, Pl and DRS analysis, two possible reasons have been proposed for the enhanced visible light activity and stability of this novel photocatalyst: (1) broadening of the photoabsorption range and (2) efficient separation of photoinduced charge carriers which does not allow the photoexcited electrons to accumulate on the conduction band of Ag₂S and hence prevents the photocorrosion.

Full text of DOI:10.1039/c5ra05117g

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Ag2S sensitized mesoporous Bi2WO6 architectures with enhanced visible light
photocatalytic activity and recycle properties.
Owais Mehraj†, Bilal M. Pirzada†, Niyaz A. Mir††, Saima Sultana†, Suhail Sabir†*,
†Department of Chemistry, Aligarh Muslim University, Aligarh – 202002, India
††Solid State and Structural Chemistry Unit, Indian Institute of Science, Bangalore-560012,
India
*Corresponding Author: Dr. Suhail Sabir, Department Of Chemistry, Aligarh Muslim
University, Aligarh, India, 202002.
Email: sabirsuhail09@gmail.com
Phone: +91ꢀ571ꢀ2700920ꢀextꢀ3366

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Abstract
To harvest the solar energy more efficiently, novel Ag2S/Bi2WO6 heterojunctions were
synthesized by a hydrothermal route. This novel photocatalyst was synthesized by
impregnating Ag2S into Bi2WO6 semiconductor by hydrothermal route without any
surfactants or templates. The as prepared structures were characterized by multiple
techniques such as Xꢀray diffraction (XRD), Xꢀray photoelectron spectroscopy (XPS),
Brunauer–Emmet–Teller (BET) analysis, scanning electron microscopy (SEM), transmission
electron microscopy (TEM), energy dispersive Xꢀray spectrometry (EDS), UVꢀvis diffuse
reflection spectroscopy (DRS) and photoluminescence (PL). The characterization results
suggest the mesoporous hierarchical spherical structures with high surface area and improved
photo response in the visible spectrum. Compared to the bare Bi2WO6, the Ag2S/Bi2WO6
exhibited much higher photocatalytic activity towards the degradation of Rhodamine B
(RhB). Although silver based catalysts are easily eroded by photogenerated holes, the
Ag2S/Bi2WO6 photocatalyst was found to be highly stable in the cyclic experiments. Based
on the results of BET, Pl and DRS analysis, two possible reasons have been proposed for the
enhanced visible light activity and stability of this novel photocatalyst: (1) broadening of
photoabsorption range and (2) efficient separation of photoinduced charge carriers which
does not allow the photoexcited electrons to accumulate on conduction band of Ag2S and
hence prevents the photocorrosion.
Keywords: Ag2S/Bi2WO6 heterojunctions; Photoresponse; Mesoporous; Rhodamine B;
visible light photocatalysis.

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1. Introduction
The growing environment concerns and energy demand are the main driving forces for the
sustained fundamental and applied research in the area of semiconductor photocatalysis for
organic pollutant degradation and hydrogen generation from water splitting using solar
light.1ꢀ3 Semiconductor photocatalysis involves the photoinduced generation of electron –hole
pairs, therefore an ideal photocatalyst should have extended spectral responsive range and
low recombination rate of photogenerated charge carriers.4 Coupling of two photocatalysts to
form heterojunction may provide a chance to develop a new photocatalyst with enhanced
separation of photoinduced charge carriers and extended photoabsorption range.5,6
2ꢀ
Bi2WO6 is one of the simple Aurivillius Oxide with perovskiteꢀlike slabs of WO4 and
2+ 7,8
Bi2O2
.
Owing to its nonꢀtoxicity and strong oxidizing power, Bi2WO6 has been used as
one of the promising and excellent photocatalyst for the decomposition of organic pollutants
under visible light.9ꢀ12 To improve the photocatalytic activity of bare Bi2WO6, many different
morphologies of Bi2WO6 with different hierarchical architecture and surface area have been
reported. These include the development of nanoplates,13 porous thin films,14 3D nest like
mesoporous Bi2WO6 architectures15 and flower sphere like complex structures.16,17 However
there are still two important factors which limits the photocatalytic efficiency and thus
hinders the practical application of bare Bi2WO6 (1) The absorption edge of pure Bi2WO6 is
ca, 450 nm18,19 which overlaps a small part of solar spectrum leading to the unsatisfactory
photoresponse range (2) The recombination rate of photoinduced charge carriers is large and

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therefore leads to the low quantum efficiency because of the short lifetimes of the electron
hole pairs.20,21
Recent studies have shown that coupling of Bi2WO6 with other semiconductors improves the
photocatalytic performance of Bi2WO6 to a substantial extent by promoting the effective
separation of photoinduced charge carriers and broadening the visible light responsive range,
For eg. αꢀFe2O3/Bi2WO6,22 Bi2S3/Bi2WO6,4 Bi2O3 decorated Bi2WO6 and TiO2 modified
flower like Bi2WO6,23 Bi2WO6/BiOBr,24 WO3–Bi2WO625 and CuPc sensitized Bi2WO6.26
As an important chalcogenide Ag2S with a narrow band gap of 1.1 eV has been investigated
extensively for the numerous applications such as photovoltaic device construction,27,28
photocatalytic decomposition of organic pollutants29,30 and photocatalytic H2 production.31
Moreover Ag2S has high absorption coefficient31ꢀ36 and besides Ag2S is free from toxic heavy
elements like Pb and Cd and thus possess negligible toxicity compared to other narrow band
gap materials.37 Owing to the high chemical stability, narrow band gap, high absorption
coefficient and excellent optical limiting properties of Ag2S, we choose to couple it with
Bi2WO6 to form a heterojunction with enhanced visible light photocatalytic activity and high
stability.
Herein for the first time we report novel Ag2S/Bi2WO6 heterojunction photocatalyst to
overcome the drawbacks of low photocatalytic efficiency of Bi2WO6 brought by narrow
photoresponsive range and high rate of recombination of photoinduced charge carriers .The
photocatalytic activity of the as prepared photocatalysts was evaluated by decomposing the
dye Rhodamine B. The effect of Ag/Bi weight ratio on the photocatalytic activity was also
studied. The stability of the catalyst which is a major concern for catalysts containing silver
was also investigated.
2. Experimental

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2.1. Synthesis of Ag2S-Bi2WO6 photocatalyst
All the reagents were of analytical purity, purchased from SigmaꢀAldrich India and used
without further purification. The hierarchical structures of Ag2S/Bi2WO6 were synthesized by
a twoꢀstep process. In the first step the mesoporous Bi2WO6 microspheres were fabricated by
a simple hydrothermal process via following procedure: 1 mmol of Na2WO4.2H2O was
dissolved in 15 ml of ethylene glycol under constant stirring. Then 2 mmol of Bi
(NO3)3.5H2O was added into the solution and the mixture was stirred for 20 min. Thereafter
absolute ethanol (25 ml) was added into the solution and the suspension was transferred into
a 50 ml Teflonꢀlined stainless steel autoclave and heated at 180 oC for 24h. The autoclave was
subsequently cooled to room temperature and the products were collected by filtration. The as
obtained products were washed with deꢀionized water and absolute ethanol for several times
and then dried at 80 oC for 12h.
In the second step, impregnation of Bi2WO6 surface with Ag2S was carried out by in-situ
growth of Ag2S on Bi2WO6 substrate at room temperature. In a typical experimental
procedure, 0.6 g of Bi2WO6 microspheres and different amounts of AgNO3 were dispersed in
50 ml of absolute ethanol and ultrasonicated for 30 min. After ultrasonication, appropriate
amount of Na2S was added to the suspension with continuous stirring. The obtained
suspension was vigorously stirred for 8 h. The precipitates were collected and washed with
o
deꢀionized water and absolute ethanol for several times and then dried at 80 C for 12h. The
different weight ratios of Ag2S to Bi2WO6 samples were prepared and noted as 3%
Ag2S/Bi2WO6, 5% Ag2S/Bi2WO6, 7% Ag2S/Bi2WO6 and 9% Ag2S/Bi2WO6. For comparison
pure Ag2S was also synthesized in absence of Bi2WO6.
2.2. Characterization

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To examine the phase purity and crystal structure of as prepared samples, XRD analysis was
carried out using an PANalytical, XPert PRO powder Xꢀray diffractometer with CuKα
radiation (λ= 1.5418 A°) having a tube voltage of 40kV and current of 30 mA at room
temperature with the scan range 2θ = 15 to 80° and step size 0.02666°. Xꢀray photoelectron
spectroscopy (XPS) was used to analyse the surface properties of samples using MK II
photoelectron spectrometer having AlꢀK(alpha) (1486.6 eV) as the Xꢀray source. The particle
morphology and microstructure of the samples was investigated by FESEM using CarlꢀZeiss
fieldꢀemission scanning electron microscope (FESEM) equipped with EDS attachment and
transmission electron microscopy (TEM) using JEOL, JEM 2100F instrument. UVꢀvis DRS
spectra of samples in the region of 300ꢀ800 nm was recorded by UVꢀvis NIR spectrometer
(Perkin Elmer) equipped with an integrating sphere assembly. The surface area and pore size
distribution of the as prepared samples was characterized by N2 adsorptionꢀ desorption
isotherm using Quantachrome Instruments Autosorb 1C. Adsorption of samples was done at
o
77 K and the samples were degased at 150 C for 3 h before analysis. PL spectra was
recorded on flourospectrometer.
2.3. Photocatalytic activity
To evaluate the photocatalytic activity of the samples, RhB and phenol aqueous solution was
degraded under visible light irradiation. A 500 W, tungsten halogen lamp was used as the
light source. Immersion well photoreactor made of pyrex glass was used to perform the
experiments. Refrigerated circulating liquid bath was used to keep the temperature of the
reaction constant at 20 ± 0.3 0C. The experiments were performed as follows. 0.18 g of the as
prepared
catalyst was added into 180 mL RhB aqueous solution. Prior to the illumination, the
suspension was magnetically stirred for at least 30 min in the dark to attain adsorptionꢀ
desorption equilibrium between dye and catalyst. 5ml suspensions were sampled at 5 min

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time intervals and centrifuged to remove the catalyst particles. The concentration of catalyst
free dye solution was analysed at λmax (554 nm) using Shimadzu UV–vis 1601, UVꢀvis
spectrophotometer.
To determine the effect of scavengers on photocatalytic activity, appropriate quantity of
scavenger species were introduced in the reaction system in a manner similar to
photocatalytic experiment.
Photoluminescence (PL) technique with Terepthalic acid (TA) as a probe molecule was used
for the detection of •OH radicals. The experimental procedure was referred to previous
studies.5
3. Results and discussions
3.1. XRD analysis
XRD analysis was used to determine the purity, phase structure and crystallinity of the as
prepared samples. Fig. 1 presents the XRD pattern of pristine Bi2WO6 and Ag2S loaded
Bi2WO6. As can be seen from the Fig. 1a, the diffraction peaks can be categorised into a set
of 2θ at 28.3o, 32.8o, 47.1o and 55.9o corresponding to the indices (131), (200), (202) and
(331) which indicates a perfectly orthorhombic structure for pure Bi2WO6.38 Compared with
diffraction pattern of bare Bi2WO6, no significant diffraction peaks of any other phase or
impurity was observed on Ag2S/Bi2WO6 composites indicating that the characteristic peaks
associated with Ag2S are not obviously detected (Fig. 1bꢀe). This may be due to the limited
amount of Ag2S (3% ꢀ 9%) used, small size of Ag2S nanoparticles and high dispersion of
Ag2S on the surface of Bi2WO6 spherical architectures. More importantly the absence of
individual Ag2S aggregates in SEM images of as prepared samples indicate high dispersion
of Ag2S in composites. Fig. 1f displays the characteristic XRD pattern of pure Ag2S sample.
All peaks of pure Ag2S can be assigned to monoclinic Ag2S crystal structure.39 The average

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crystallite size of the samples was calculated by using Scherer formula given below5 and
results are listed in table 1.
ꢁꢂ
ꢀ =
(1)
ꢈ
ꢃꢄꢅꢆꢇ
where D is taken as crystallite size, k is a constant, λ is Xꢀray wavelength, β is the full width
at half maximum (FWHM) measured in radians on the 2θ scale, θ is the Bragg angle for
diffraction peaks. From the results of crystallite size displayed in table. 1, it is clear that there
is negligible change in the crystallite size of pure Bi2WO6 and Ag2S loaded Bi2WO6 samples.
3.2. XPX analysis
To further investigate the composition and oxidation states of as prepared samples, XPS
analysis of 7% Ag2S/Bi2WO6 heterostructure (Fig. 2) was carried out. Fig. 2a displays the
overall XPX spectra for the 7% Ag2S/Bi2WO6 in which peaks for Bi, W, O, Ag and S could
be detected. The high resolution XPS spectra of Bi4f, W4f, O2S, S2p and Ag3d is shown in
Fig. 2bꢀf. The Bi4f peaks (Fig. 2b) at 157.8 and 163.10 eV are attributed to the binding
energies of Bi4f7/2 and Bi4f5/2 respectively which corresponds to previous results.40 The W4f
peaks at 34.00 and 36.1 eV are ascribed to binding energies of 4f7/2 and 4f5/2 (Fig. 2c).The
splitting energy of 2.1 eV for 4f doublet of W indicates the +6 oxidation state of W in
composite catalyst.41 The O1S peak (Fig. 2d) at 528.8 eV is in good agreement with previous
results.40 The high resolution Ag3d XPS spectrum is displayed in Fig. 2e. The two peaks
corresponding to Ag+ at approximately 366.50 and 372.60 eV are attributed to the Ag3d5/2
and Ag3d3/2 respectively.42 Since in previous results the peaks at 368.3 and 374.2 eV are
attributed to metallic Ag whereas the peaks at 367.8 and 373.8 eV are attributed to Ag+, 43,44 it
implies that peaks obtained in our XPS results can be ascribed to Ag+ only suggesting that no
metallic Ag0 is formed. Fig. 2f displays the S2P XPS spectrum. The peak at around 163 eV
can be attributed to S2P1/2 indicating the presence of S2ꢀ which further suggests the formation

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of Ag2S.45 From these results it can be confirmed that Ag2S/Bi2WO6 composites have been
successfully synthesized.
3.3. SEM, EDS and TEM analysis
To investigate the surface morphology of as prepared samples SEM characterization was
used. Fig. 3 presents the FESEM images of pure Bi2WO6 and 7% Ag2S/Bi2WO6 composite.
As clear from the Fig. 3a, the FESEM image of bare Bi2WO6 consist of abundant
hierarchically structured architectures of Bi2WO6 microspheres. The higher magnification
(Fig. 3b) reveals that Bi2WO6 architectures are built from numerous nanoplates aligned to the
spherical surface forming a microsphere. Compared with bare Bi2WO6 the Ag2S loading
affects the surface morphology of Bi2WO6 in composite (Fig. 3c) to some extent. As can be
seen clearly from the higher magnification FESEM image of 7% Ag2S/Bi2WO6 composite
(Fig. 3d), it is somewhat different from spherical structure showing that Ag2S has combined
well with Bi2WO6 to form a heterostructure. EDX spectrum of Bi2WO6 and 7%
Ag2S/Bi2WO6 was collected to confirm the presence of Ag2S in hybrids. The EDS spectrum
in Fig. 3e shows clearly that bare Bi2WO6 is composed of Bi, W and O elements while as
Ag2S/Bi2WO6 (Fig. 3f) composite is composed of Bi, W, O, Ag and S elements indicating the
formation of Ag2S/Bi2WO6 composite. To further confirm the high dispersion of Ag2S in
composite, EDX elemental mapping of 7 % Ag2S/Bi2WO6 composite was performed as
shown in Fig. 4. It can be seen from the Fig.4, that Ag (Fig. 4b) and S (Fig. 4e) are highly
dispersed in 7% Ag2S/Bi2WO6 hybrid.
TEM analysis was further used to investigate the detailed structure as shown in Fig. 5. It can
be seen that Bi2WO6 structures are composed of nanoplates with a thickness around 10ꢀ20 nm
(Fig. 5a). Fig. 5b shows that Bi2WO6 microspheres are mesoporous in nature which helps the

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Ag2S particles to bind in this composite system. The TEM image for 7% Ag2S /Bi2WO6 (Fig.
5eꢀf) shows subtle particles of Ag2S grown on the surface of Bi2WO6 microsphere. The
presence of lattice fringes (Fig. 5c) confirms the crystalline nature of Bi2WO6. Furthermore
the spacing of lattice fringes was found to be 0.313 nm which corresponds to the 131 planes
of orthorhombic Bi2WO6.38 The SAED (Selected Area Diffraction Pattern) demonstrates the
presence of clear diffraction spots (Fig. 5d) which further confirms the crystalline structure of
Bi2WO6. All these above results are in good agreement with the results of XRD and XPS
analyses and strongly demonstrate the formation of Ag2S/Bi2WO6 heterojunction.
3.4. N2 Adsorption-Desorption
Fig. 6 shows the nitrogen adsorption isotherm and pore size distribution of pure Bi2WO6 (Fig.
6a) and 7% Ag2S/Bi2WO6 (Fig. 6b). As can be seen from the figure, the shape of both the
isotherms seem to be nearly of type IV isotherm with hysteresis loop at higher relative
pressure which is usually associated with capillary condensation in mesopores.46,47 The type
H3 hysteresis loop at higher relative pressure according to IUPAC classification reflect the
presence of slit like pores indicating the presence of mesopores in the region of 2ꢀ10 nm
which is further confirmed by the BJH pore size distribution diagram in the inset of Fig. 6.
The BET surface area of bare Bi2WO6 and 7% Ag2S/Bi2WO6 are 77.6 m2gꢀ1 and 66 m2gꢀ1
respectively indicating that the loading of Ag2S leads to the decrease in surface area to some
extent which is probably because Ag2S particles are embedded in the pores of Bi2WO6.48 The
presence of mesoporous structure with large surface area provides more active sites and
transport paths for the decomposition of contaminants in photocatalytic reaction.
3.5. Optical properties
Fig. 7 displays the UVꢀvis diffuse reflectance (DRS) spectra of bare Bi2WO6 and
Ag2S/Bi2WO6 heterostructures. As shown in Fig.7a the bare Bi2WO6 has an absorption edge

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around 450 nm, where as weak and broadened peaks of Ag2S (Fig. 7b) are similar to the
reported UVꢀvis spectra of pure Ag2S.48,49 With increase in Ag2S loading the absorption edge
of heterostructure samples is shifted to longer wavelengths from 450ꢀ490 nm.
The optical band gaps (insets of Fig. 7a and 7b) of Bi2WO6 and Ag2S were calculated using
the equation reported by Butler.50
hv
Since α is proportional to Kubelka ꢀ Munk Function F(R), the equation becomes
hv (3)
F(R) = (Ahv ꢀ Eg)n/2
X
α = (Ahv ꢀ Eg)n/2
(2)
X
Where, h is the planks constant, v is the frequency, A is the proportionality constant and Eg is
the band gap energy. The value of n is determined by the type of transition (n = 1 for direct
and n = 4 for indirect transition). The n for Bi2WO6 has been reported by some researchers as
1 (direct transition),51,52 and by some other researchers as 4 (indirect transition).53,54 In our
case we plotted (F(R)
X
hv) 1/2 versus hv for Bi2WO6 (inset of Fig. 7a) and (F(R)
X
hv)2 versus
hv for Ag2S (inset of Fig. 7b). The band gaps for Bi2WO6 and Ag2S were estimated as 2.74
eV and 1.1 eV respectively.
3.6. Photocatalytic properties
To assess the photocatalytic activity of as prepared samples, RhB was selected as a probe
molecule. The characteristic absorption band of RhB at 554 nm was employed to determine
the process of degradation. Fig. 8 displays the photocatalytic performances of all
Ag2S/Bi2WO6 heterojunctions with different Ag2S contents over degradation of RhB (Ct/C0)
under visible light irradiation. As shown in Fig. 8b, the blank experiment (without
photocatalyst) shows that the concentration of RhB remains unchanged even after 50 min of
visible light irradiation indicating the significance of photocatalyst. The adsorption of RhB

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dye by 7% Ag2S/Bi2WO6 was also checked for 50 min and it was found that the adsorption
in dark is insignificant. As is shown the pure mesoporous Bi2WO6 exhibit photocatalytic
efficiency of 56.5 % in 50 min. The better performance of pure mesoporous Bi2WO6
photocatalyst was attributed to the high surface area and mesoporous structure of Bi2WO6.
The high surface area and mesoporous structure not only provide more active sites and
transport paths in photocatalytic reaction but also improves the contact of photocatalyst with
organic pollutants. When Ag2S and Bi2WO6 are combined to construct a heterostructure, the
photocatalytic activity of the composites is dramatically enhanced even though the Ag2S
content is very low. The photocatalytic activity of all Ag2S/Bi2WO6 heterostructures with
Ag2S weight percentage of 3 %, 5%, 7% and 9% is higher than the bare Bi2WO6 and are
listed in table 1. The best photocatalytic performance was obtained for 7 % Ag2S/Bi2WO6 for
which almost 85% of RhB was degraded in 50 min. The reason for enhanced visible light
activity is the formation of heterojunction which results into the generation of natural energy
bias at heterojunction with subsequent transfer of photoinduced charge carriers and thereby
impeding the electron hole recombination, which is the main factor responsible for low
visible light activity of Bi2WO6. With further increase in Ag2S content to 9%, the
photocatalytic activity decreases but is still higher than bare Bi2WO6. Therefore the optimal
content of Ag2S in Ag2S/Bi2WO6 composite is 7 %. The optimum content of AgBr in the
heterojunction can be related to two factors. (1) For efficient electron–hole separation the
space charge region potential must be certain.38 When Ag2S content was above 7% (optimal
value) the space charge region might become narrow and lead to the recombination of
photoinduced charge carriers. (2) The loading of Ag2S leads to decrease in surface area as
discussed in section 3.4. When Ag2S content was above optimal value the surface area might
decrease substantially leading to decrease in photocatalytic activity. Fig. 8a displays the
absorption spectral changes of the photocatalytic degradation of RhB aqueous solution over

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7% Ag2S/Bi2WO6. As can be seen from the Fig. 8a, the absorbance of RhB at 554 nm
decreases significantly with increase in reaction time accompanied by a slight shift of
absorption band towards blue region which is attributed to the step wise deꢀethylation process
of RhB into intermediates as reported in previous works.55,56 The apparent pseudo first order
kinetics model given by the equation (4) was applied in experiments to understand the
reaction mechanism.57
ln(C0/Ct) = Kappt
(4)
Kapp is apparent pseudoꢀfirst order reaction constant and its value for bare Bi2WO6 and all
heterojunctions was calculated via the first order linear fit data (Fig. 8c) and are displayed in
table 1. From table I, it is clear that photocatalytic activity of 7%Ag2S/Bi2WO6 is 2.4 times
higher than the bare Bi2WO6 suggesting that Ag2S/Bi2WO6 is an excellent photocatalyst
under visible light.
3.7. Proposed Mechanism
The trapping experiments of active species formed during photocatalytic oxidation process
were carried out to investigate the possible mechanism involved in RhB degradation over 7%
Ag2S/Bi2WO6 heterojunction. Benzoquinone (BQ)58,59 isopropyl alcohol (IPA)58,60 and
ꢀ
ammonium oxalate (AO)61 were used as superoxide (•O2 ), hydroxyl (•OH) and hole (h+)
scavengers respectively. Fig. 9 displays the effect of different scavengers and it can be seen
from the figure that the rate of RhB degradation decreases substantially in presence of AO.
However addition of IPA had no obvious effect on Rate of RhB degradation whereas addition
of BQ slightly decreases the photocatalytic performance of 7%Ag2S/Bi2WO6 compared with
no scavenger under same conditions. These results indicate that h+ are the main active species
•
ꢀ
•
rather than O2 and OH in the RhB degradation over 7%Ag2S/Bi2WO6 system under visible
light irradiation.

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•
To further confirm that OH did not play any role, we carried out the Terepthalic acid
photoluminescence studies. The PL spectral changes observed during irradiation of
Ag2S/Bi2WO6 in a basic terepthalic acid solution (315 excitation wavelength) are displayed in
Fig. 10. As clear from the figure no Pl signals were observed at 425nm throughout the
photocatalytic oxidation process, indicating that no •OH is formed on the surface of catalyst.
To ensure that the reactive species originate from the photocatalytic process of catalysts and
not form the sensitization of dye, we selected a colourless molecule phenol as a probe. Fig.
9b displays the photocatalytic performances of pure Bi2WO6 and 7% Ag2S/Bi2WO6
heterojunctions over degradation of phenol (Ct/C0) under visible light irradiation. As can be
seen from the Fig. 9b the concentration of phenol decreases with increasing reaction time in
presence of both pure Bi2WO6 and 7% Ag2S/Bi2WO6. The concentration (Ct/C0) decreases
sharply in presence of 7% Ag2S/Bi2WO6 under visible light irradiation while as in case of
pure Bi2WO6 the decrease in the concentration (Ct/C0) of phenol is lower than 7%
Ag2S/Bi2WO6. Since phenol cannot absorb visible light, it follows directly that reactive spies
originate from the photocatalytic process of samples under visible light ruling out the
possibility of sensitization of dye. The results obtained are in accordance with the results
obtained for the degradation of RhB.
The BET surface area of bare Bi2WO6 was calculated to be 77.6 m2gꢀ1. The high surface area
and mesoporous structure of Bi2WO6 makes it an excellent visible light photocatalyst but the
surface area of 7% Ag2S/Bi2WO6 was calculated to be only 66 m2gꢀ1 which is obviously less
than the bare Bi2WO6. The Ag2S loading on the surface of Bi2WO6 enhances the visible light
photocatalytic activity of Bi2WO6 substantially although the surface area is decreased.
Therefore it can be assumed that large surface area may be responsible for the high
photocatalytic activity of bare Bi2WO6 but it is not the influence factor for the enhancement
of visible light activity of Ag2S/Bi2WO6 composites up to 7% loading. Therefore to analyse

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the mechanism of improved photocatalytic activity of Ag2S/Bi2WO6 composites the relative
positions of the conduction band and valence band edges of Ag2S and Bi2WO6 were
investigated for the flow chart of photoinduced charge carriers in a heterojunction. The
relative position of valence band (VB) of Ag2S and Bi2WO6 was evaluated by the empirical
formula in equation (5).62
EVB = X ꢀ Ec + 0.5Eg
(5)
Where X is the electronegativity of semiconductor expressed as geometric mean of absolute
electronegativity of constituent atoms, Ec is the energy of free electrons on hydrogen scale
(4.5 eV) and Eg is the band gap energy. The EVB for Ag2S and Bi2WO6 were calculated to be
1.1 eV and 3.23 eV respectively. The relative position of conduction band (CB) of Ag2S and
Bi2WO6 were estimated from equation (6).
ECB= EVB – Eg
(6)
The ECB for Ag2S and Bi2WO6 were calculated to be 0 eV and 0.49 eV respectively.
Based on the results of BET analysis, DRS, Trapping experiments and TA PL spectra we
presented a schematic diagram (Fig. 11) to illustrate the photocatalytic reaction mechanism
and charge transfer of Ag2S/Bi2WO6 system. We propose that under visible light irradiation
both Ag2S and Bi2WO6 are excited and generate electron hole pairs. Since the CB energy
level of Ag2S and Bi2WO6 is 0 and 0.49 eV (Vs SHE), the single electron reduction of O2 to
ꢀ
63,64
•O2 (ꢀ0.046 vs SHE)
is not possible .When two semiconductors are in contact, the
electrons from less positive conduction band of Ag2S (0 eV) are injected with high efficiency
into more positive conduction band of Bi2WO6 (0.49 ev). On the other hand holes from more
positive valence band of Bi2WO6 (3.23 eV) are transferred to the less positive VB of Ag2S
(1.1 eV). The photoinduced charge carriers are thus effectively separated at the interface of
Ag2S/Bi2WO6. The holes (h+) on the valence band of Ag2S could not react with OH-/H2O to

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•
•
form OH because VB of Ag2S is more negative (1.1 eV) than that of OH/H2O (2.30 eV vs
SHE).38 Hence holes from valence band of Ag2S migrate to the surface of a photocatalyst
where they take part in photocatalytic reaction to mineralize the organic pollutants and
thereby enhancing the photocatalytic activity.
Photoluminescence spectra of hierarchically structured bare Bi2WO6 and 7% Ag2S/Bi2WO6
was carried out to further demonstrate the enhancement of separation of charge carriers. The
lower intensity of PL peak represents the lower probability of electronꢀhole recombination.
Fig.12 presents the PL spectra of bare Bi2WO6 and 7%Ag2S/Bi2WO6 with an excitation
wavelength of 380 nm. From the Fig.12 it is clear that the intensity of PL spectra decreases
significantly in 7% Ag2S/Bi2WO6 compared to the bare Bi2WO6 indicating that the loading of
Ag2S on Bi2WO6 has greatly restrained the recombination of photoinduced charge carriers
between Bi 6S and O 2P to the empty W4d orbitals.22
3.8. Stability of the Catalyst and cycling runs.
The stability of silver based compounds has always been a matter of concern,65 particularly
the metal chalcogenides get easily eroded by the photogenerated holes.66 Therefore cyclic
experiments with 7% Ag2S/Bi2WO6 sample were carried out to determine the stability of a
photocatalyst. Fig. 13 displays the results of RhB degradation by 7%Ag2S/Bi2WO6 after 4
repeated cycles. It can be seen from the Fig. 13 that the degradation of RhB by 7%
Ag2S/Bi2WO6 is maintained at 81.3 % even after 4 repeated cycles indicating the high
stability of photocatalyst. Fig.14 presents the Ag3d XPS spectrum and XRD analysis of used
7% Ag2S/Bi2WO6 catalyst after 4 repeated cycles. It is clear from the Fig. 14a that the Ag3d
XPS spectrum of used catalyst is similar to that of fresh catalyst (Fig. 2e) with no peak
detected for metallic Ag in used catalyst after 4 repeated cycles. The XRD analysis of used
catalyst (Fig. 14b) after 4 repeated cycles also confirms that the structure and phase of the
used catalyst remains intact with no additional peak detected for Ag metal. From these results

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it is clear that the Ag2S/Bi2WO6 is a stable photocatalyst under visible light irradiation and
the main reason for the enhanced stability is the transfer of CB electrons from Ag2S to
Bi2WO6 which reduces the chances of erosion of catalyst.
3.9. Conclusion
Mesoporous Bi2WO6 was modified with Ag2S by hydrothermal method without any
surfactant or template. The introduction of Ag2S extended the photoabsorption range of
Bi2WO6 and the composite showed red shift compared to the pure Bi2WO6. The
Ag2S/Bi2WO6 heterostructures showed enhanced photocatalytic activity in the degradation of
RhB under visible light irradiation which is 2.4 times higher than the pure Bi2WO6. The
Ag2S/Bi2WO6 composite even after 4 recycling runs did not show any significant decrease in
photocatalytic activity except for about 4%. The enhanced photocatalytic activity and
stability was attributed to the broadening of photoabsorption range and efficient separation of
the photoinduced electron hole pairs as a result of natural energy bias at heterojunction. This
study inspires a thought of utilizing visible spectrum efficiently and simultaneously presents a
way to promote separation of photo induced electron hole pairs.
Acknowledgements
The authors are highly thankful to Department of Physics AMU Aligarh, USIF AMU Aligarh
and Department of Chemistry AMU Aligarh for providing the instrumentation facility. DST
and UGC is duly acknowledged for financial support in the form of FIST, PURSE and DRS
II research grants .The authors are also thankful to UGC, New Delhi, India for providing
financial assistance. Niyaz A. Mir thanks Department of Science & Technology, India for the
award of Young Scientist grant (SERB/F/1214/2014ꢀ2015).
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Figure Captions
Fig. 1 XRD patterns of (a) the pure Bi2WO6, (b) the 3% Ag2S/Bi2WO6 catalyst, (c) the 5%
Ag2S/Bi2WO6 catalyst, (d) the 7% Ag2S/Bi2WO6 catalyst, (e) the 9% Ag2S/Bi2WO6 catalyst,
and (f) the pure Ag2S.
Fig. 2 XPS spectra of 7% Ag2S/Bi2WO6. (a) Full survey spectra, and the high resolution XPS
spectra of the sample: (b) Bi 4f, (c) W 4f, (d) O 1s, (e) Ag 3d, and (f) S 2p.
Fig. 3 FEꢀSEM images of samples. (a) PureꢀBi2WO6, (b) the high magnification SEM image
of PureꢀBi2WO6, (c) 7% Ag2S/Bi2WO6, (d) the high magnification SEM image of 7%
Ag2S/Bi2WO6, (e) EDS spectrum of PureꢀBi2WO6 sample, and (f) EDS spectrum of 7%
Ag2S/Bi2WO6 sample.
Fig. 4 EDS elemental mapping of 7% Ag2S/Bi2WO6.
Fig. 5 TEM images of samples. (a and b) Pure Bi2WO6, (c and d) SAED pattern of Pure
Bi2WO6, and (e and f) 7% Ag2S/Bi2WO6
Fig. 6 N2 adsorption–desorption isotherm of the samples. (a) Pure Bi2WO6, and (b) 7%
Ag2S/Bi2WO6. The inserts are the pore size distribution of the respective catalysts.
Fig. 7 UV–Vis diffuse reflectance spectra of (a) the pure Bi2WO6, and Ag2S/Bi2WO6
composites (b) the pure Ag2S catalyst. The insets in the (Fig. 7a and 7b) are band gap
energies of the pure Bi2WO6, and Pure Ag2S respectively.

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Fig. 8 (a) UV–Vis spectral changes of the degradation of RhB by the 7% Ag2S/Bi2WO6
catalyst, (b) the degradation efficiency (Ct/C0) of RhB in presence of pure Bi2WO6 and
Ag2S/Bi2WO6 composites with different Ag2S contents
and (c) Rhodamine B
decolourization curves of ln(C0/Ct) versus irradiation time for different catalysts.
Fig. 9 (a) Effect of different scavengers on the degradation of RhB over 7% Ag2S/Bi2WO6
catalyst; BQ = benzoquinone, IA = Isopropyl alcohol, AO = ammonium oxalate. (b) the
degradation efficiency (Ct/C0) of phenol in presence of pure Bi2WO6 and 7 % Ag2S/Bi2WO6
composite.
Fig. 10 •OH trapping PL spectral changes over 7% Ag2S/Bi2WO6 catalyst in a basic
terepthalic acid solution (excitation at 315 nm) with irradiation time.
Fig. 11 Schematic diagram showing the band structure and separation of photoinduced
electron hole pairs at interface of 7% Ag2S/Bi2WO6 catalyst under visible light irradiation.
Fig. 12 PL spectra of samples. (a) PureꢀBi2WO6 and (b) 7% Ag2S/Bi2WO6 (Ex = 380 nm).
Fig. 13 The repeated experiments of photocatalytic degradation of RhB over 7%
Ag2S/Bi2WO6 catalyst.
Fig.14 (a) Ag 3d XPS spectra of used 7% Ag2S/Bi2WO6 catalyst after 4 recycling runs, and
(b) XRD pattern of used 7% Ag2S/Bi2WO6 catalyst after 4 recycling runs.
Table. 1 Pseudoꢀfirstꢀorder rate constants (kapp) calculated from the plots of ln (C0/Ct) versus
irradiation time for decolorization of RhB over different catalysts, % decolorization of RhB
and average crystallite size of Pure Bi2WO6 and Ag2S/Bi2WO6 catalysts with different Ag2S
contents.

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Fig.1

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Fig.2

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Fig.3

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Fig.4

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Fig.5