Thiourea assisted hydrothermal synthesis of ZnS/CdS/Ag2S nanocatalysts for photocatalytic degradation of Congo red under direct sunlight illumination

Article abstract of DOI:10.1039/c5ra16242d

Solar light active ternary ZnS/CdS/Ag₂S nanocatalysts have been synthesized via a hydrothermal process. The crystal structure, optical, morphological and surface area of the catalyst are characterized by XRD, UV DRS, PL, FE-SEM and BET measurements. The ternary catalyst has a decreased band gap energy value compared to that of the ZnS semiconductor. The catalyst was scrutinized for its catalytic performance under direct sunlight towards the degradation of a textile dye-Congo red at different time intervals. The results confirm that the Ag₂S incorporated ternary ZnS/CdS/Ag₂S nanocatalyst has good photocatalytic performance under direct sunlight when compared to binary and mono semiconductor systems.

Full text of DOI:10.1039/c5ra16242d

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Thiourea assisted hydrothermal synthesis of ZnS/CdS/Ag2S nanocatalysts for
photocatalytic degradation of congo red under direct sunlight illumination
*
Kalyanaraman Kalpana and Vaithilingam Selvaraj
*
Nanotech Research Lab, Department of Chemistry, University College of Engineering
Villupuram, (A Constituent College of Anna University, Chennai), Kakuppam, Villupuram,
605 103, Tamil nadu, India.
Abstract
Solar light active ternary ZnS/CdS/Ag S nanocatalysts have been synthesized via hydrothermal
2
process. The crystal structure, optical, morphological and surface area of the catalyst are
characterized by XRD, UV DRS, PL, FEꢀSEM and BET measurements. The ternary catalyst has
a decreased band gap energy value than that of the ZnS semiconductor. The catalyst was
scrutinized for its catalytic performance under direct sunlight towards the degradation of textile
dye – congo red at different time intervals. The results confirm that the Ag S incorporated
2
ternary ZnS/CdS/Ag S nanocatalyst has good photocatalytic performance under direct sunlight
2
when compared to binary and mono semiconductor systems.
Key words: ZnS/CdS/Ag S catalyst, ternary photocatalyst, congo red, direct sunlight
2
*Corresponding author: V. Selvaraj, Nanotech Research Lab, Department of Chemistry,
University College of Engineering Villupuram, (A Constituent College of Anna University,
Chennai), Kakuppam, Villupuramꢀ605 103, Tamil Nadu, India.
Email: rajselva_77@yahoo.co.in, vselva@aucev.edu.in, vaithilingamselvaraj@gmail.com
(V.Selvaraj), Phone No. +91 4146 224500

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1
. Introduction
The textile, dye, paint and paper industries play an important role in the national
economy and these industries produces large quantities of wastewater with harmful pollutants
that are nonꢀbiodegradable, which has grown as a main environmental concern. The dyes
consumed by textile industries have been discharged directly into the water resources without
1
any preventive measures . Heterogeneous photocatalysis is considered as a cost effective
2
alternate technology for the purification of environmental pollutant . Yet, the higher
recombination rate of photogenerated electrons and holes in semiconductors decreased the
photocatalytic efficiency and it reduces the direct implementation of photocatalytic techniques
for the degradation of pollutants present in the air and water. Thus, the emerging research is
focused on the lowering of recombination of electronꢀhole pairs, which could enhance the
catalytic efficiency of semiconductor photocatalyst. In order to improve the photocatalytic
3
efficiency, several approaches have been adopted such as doping of noble metals and coupling
4
of semiconductor materials . Semiconductor nanomaterials have brought great interest for the
photocatalytic degradation of organic dyes because of their size dependent optical, catalytic,
surface to volume ratio and electrical properties.
5
Titanium dioxide (TiO ) discovered by Fujishima and Honda since 1972 , which was
2
proposed to serve as an effective photocatalyst under ultra violet irradiation. The literature report
stated that the ZnS nanoparticles exhibit better photocatalytic activities than that of the TiO2
6
nanoparticles .This is because ZnS material is a direct wide band gap semiconductor, which has
higher ability to create photogenerated charge carriers than that of TiO and this property can be
2
beneficial for the improvement of photocatalytic activity. Semiconductor nanocomposites have
been extensively studied due to their excellent physicoꢀchemical and optical characteristics.

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Further, ZnS material has good ability to generate charge carriers, which are essential for the
photodegradation of dye under UV irradiation. However, the fast recombination of photoexcited
charge carriers sternly limits the photocatalytic efficiency of ZnS nanomaterials. ZnS catalyst has
a wide band gap energy, which restricts its capability to harvest the solar energy. ZnS catalyst
may become a promising photocatalyst for dye degradation application, when its visible light
response can be enhanced. Therefore, huge efforts have been offered to narrow the band gap of
ZnS semiconductor and to reduce the recombination rate of photogenerated electronꢀhole
carriers. To make ZnS semiconductor material as a solar active catalyst, it can be combined with
different narrow band gap semiconductors, which extends its photocatalytic response to the
7
, 8
visibleꢀlight region
. Otherwise, it might stimulate the separation of photogenerated charge
carriers when ZnS semiconductor was combined with other semiconductors like CdS and Ag S.
2
Both ZnS and CdS are belonging to direct band gap semiconductors with an energy value of 3.77
9
10
eV and 2.42 eV , respectively. They have potential applications in the field of lightꢀemitting
diodes, optoelectronic devices, photocatalysis, the phosphors in thinꢀfilms, electroꢀluminescent
1
1ꢀ13
devices and solar cells
. However, the continuous research demonstrated the development of
1
4
15
16
AgꢀCdS , PANIꢀCdS , graphene with metal sulfides (MoS , Ag S, CdS and CuS), SnS ꢀ
2
2
2
1
7
18
MgFe O /graphene oxide , graphene based materials , graphitic carbon nitrideꢀmetal
2
4
1
9
20
21
oxide/metal chalcogenides , solar light driven photocatalysts and grapheneꢀCdS for
photocatalysis and hydrogen generation applications. ZnS/CdS nanocatalyst has been obtained
through coꢀprecipitation method and its photocatalytic activity of various dyes has been reported
2
2
.
Further, Ag S has received great attention due to its optical and electronic applications such as
2
2
3
photoconductors, IR detectors, superionic conductors , thermoꢀelectric material and photoꢀ
sensitizer for photographic purposes. Ag S semiconductor has direct and narrow band gap
2

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2
4
energy of 1.0 eV , which can acts as an effective solar light harvesting semiconductor for
25
photocatalytic applications . With these views in mind, the present study is focused on the
synthesis of the fresh ternary nanocatalyst system by combining the silver sulphide (Ag S) with
2
ZnS/CdS material, which is believed as an efficient photocatalytic material in order to harvest
the sunlight. Further, an attempt has been made to synthesis the ternary nanocatalyst of
ZnS/CdS/Ag S through one pot method using thiourea as a sulphur source. To the best of our
2
knowledge, there is no report available for the synthesis of ZnS/CdS/Ag S towards
2
photocatalytic degradation of congo red (CR) dye under direct sunlight illuminations. The results
confirm that the ZnS/CdS/Ag S catalyst shows good photocatalytic applications compared with
2
other ratios under direct sunlight illuminations.
2
. Experimental details
.1 Materials
2
Analytical grade cadmium acetate dihydrate, zinc acetate dihydrate, silver nitrate and
thiourea were purchased from MERCK, India and are used as received.
2
.2 Synthesis of ZnS/CdS/Ag S nanoparticles
2
ZnS/CdS/Ag S were synthesized by adding equal molar ratios of Cd(CH COO) .2H O,
2
3
2
2
Zn(CH COO) 2H O, AgNO and two fold molar ratio of thiourea as sulphur source. The metal
3
2.
2
3
salts like Cd(CH COO) .2H O (1 mM), Zn(CH COO) .2H O (1 mM) and AgNO (1 mM) were
3
2
2
3
2
2
3
dissolved in water and the resultant mixture was continuously stirred for 30 minutes in order to
get a clear solution. To the above mixture, 6 mM thiourea dissolved in water was added slowly
and the pH of the reaction mixture was adjusted to 12 by dropping NH ꢀH O mixture (1:1, V/V).
3
2
°
The resulting reaction mixture was heated at 100ꢀ110 C in an autoclave for 8 hours. The product

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was centrifuged, washed several times with distilled water and absolute ethanol and then dried at
°
7
0 C. For comparison, bare ZnS, ZnS/CdS and ZnS/Ag S were synthesized by following the
2
similar procedure as mentioned above from their respective precursors. For ternary system,
1
1
0.25
1
1
0.50
1
1
0.75
ZnS /CdS /Ag S , ZnS /CdS /Ag S
and ZnS /CdS /Ag S
catalysts were prepared by
2
2
2
varying ratios of AgNO using the above mentioned procedure.
3
2.3 Characterization Techniques
Powder Xꢀray diffraction (XRD) studies were carried out using Rikagu SMART Labꢀ9 kW
model. The peak positions and the crystalline phase in the XRD spectra were identified by
comparing with standard JCPDS files. The diffuse reflectance spectra (DRS) and UVꢀvisible
spectral measurements were explored by using Jasco Vꢀ650 spectrophotometer. The
photoluminescence spectra were analyzed using a Jasco spectrofluorometer Model FP ꢀ 8300.
The surface morphology of the sample was determined by Field emission scanning electron
microscope (FEꢀSEM) and EDX analysis using the Hitachi SU6600 Model. The specific surface
area of the sample was determined by nitrogen adsorption at 77 K on the basis of the Brunauer–
Emmett–Teller (BET) equation using a Micromeritics ASAP 2020 V3.00 H instrument. The
chemical state of the synthesized samples were recorded using Omicron Nanotechnology,
ꢀ
10
GmbH, (Germany) XM 1000 XPS system at a base pressure of 10 mbar at room
temperature in an air with a monochromatic MgKα (hν = 1253.6 eV) source and a charge
neutralizer. The identification of byproducts obtained after 30 min of direct sunlight irradiation
were characterized by GC (PerkinElmer Clarus 580 GC) to ascertain the reaction mechanism
related with CR degradation. The decomposed dye end products were extracted with
dichloromethane after centrifuge process for the removal of ZnS/CdS/Ag S. 1 ꢁL of the sample
2

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was injected into the GC analyzer for chromatogram measurements. The FTꢀIR spectra were
analyzed using the infra red spectrometer of JASCO with model FT/IRꢀ4200 type A
system.
2.4 Photodegradation experiments
The photocatalytic degradation experiments were performed on sunlight under similar
conditions of sunny days between 11 a.m. to 2 p.m. The solar intensity is 1250×100±100 Lux
and it was nearly constant throughout the experiments. The photocatalytic reactions were carried
out in glass tubes of 50 mL capacity. In this work, CR dye was employed as a model
environmental pollutant. The aqueous suspensions are magnetically stirred in the dark for 30 min
to attain adsorptionꢀdesorption equilibrium between the dye and ZnS/CdS/Ag S catalyst. The
2
irradiation experiments were carried out in an open air conditions. 50 mL of aqueous dye
solution (12 ppm) with 50 mg of catalyst was continuously aerated by a pump for complete
mixing of reaction mixture. The progress of photocatalytic degradation of CR dye was monitored
by measuring its absorption peak with the help of UV–visible spectrophotometer. At a specific
time interval, 2ꢀ3 mL of reaction mixture was withdrawn and the catalyst was removed through
the centrifugation process. The characteristic absorbance peak was measured for CR dye using
UVꢀvisible spectrophotometer and the decrease in peak intensity indicates the degradation of the
CR dye solution.
3
. Results and discussion
The preliminary studies were carried out for the degradation of CR dye on
1
1
0.25
1
1
0.50
1
1
0.75
1
1
1
ZnS /CdS /Ag S
,
ZnS /CdS /Ag S
,
ZnS /CdS /Ag S
and ZnS /CdS /Ag S
2
2
2
2
1
1/
catalysts. The results corroborate that the ZnS /CdS Ag S catalyst has an effective
2

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photocatalytic activity among other catalysts. Hence, this sample was characterized and used
for further experiments.
3
.1 XRD Analysis
Fig. 1 shows the combined Xꢀray diffraction pattern of ZnS, ZnS/CdS and ZnS/CdS/Ag2S
o
o
nanocatalysts. ZnS semiconductor exhibits the diffraction peaks located at 25.09 , 26.6 ,
o
o
o
2
8.0 , 44.0 and 52.12 are corresponding to crystallographic planes of (100), (002), (101),
2
6,
(110) and (112), which are indexed to the hexagonal structure (JCPDS card no. 36ꢀ1450)
2
7
.
From the XRD pattern of ZnS/CdS (Fig. 1b), it was noticed that the diffraction peaks have
2
8
shown a shift in peak position that are corresponding to hexagonal phase and was found to
be good agreement with literature value. In addition to that the monoclinic Ag S peaks
2
2
9, 30
(
JCPDF 14ꢀ0072) were present in the ZnS/CdS/Ag S catalyst (Fig. 1(c))
. The data
2
obtained from the mono ZnS, binary ZnS/CdS and ternary ZnS/CdS/Ag S catalysts were
2
found to be good agreement with the literature values of binary systems of ZnS/CdS and
2
8, 29, 30
ZnS/Ag S nanocatalysts
.
2
Fig. 1 XRD patterns of (a) ZnS, (b) ZnS/CdS and (c) ZnS/CdS/Ag S.
2

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3
.2 UV- visible DRS
UVꢀvisible DRS were recorded to investigate the optical properties of the samples. Fig. 2
depicts the DRS spectra of bare ZnS and various ratios of ZnS/CdS/Ag S semiconductors.
2
The band gap energy values were calculated by using following formula according to the
3
1
previous report .
E = 1240/λ (eV)
bg
Eq. (1)
3
2
λ is the wavelength in nanometer and E is the photon energy.
bg
1
1
0.25
1
1
0.50
The estimated band gap energy values for bare ZnS, ZnS /CdS /Ag S , ZnS /CdS /Ag S
,
2
2
1
1
0.75
1
1
1
ZnS /CdS /Ag S
and ZnS /CdS /Ag S catalysts are 3.79, 3.02, 2.95, 2.77 and 2.73 eV,
2
2
respectively. The red shift in the absorption edge was clearly observed from the reflectance
spectrum of ZnS/CdS/Ag S nanocatalysts when compared to ZnS material. The decreased band
2
gap energy infers that the desired catalytic activity of the photocatalyst can be achieved through
3
3, 34
the coupling of semiconductors
, which will be expected to enhance the number of photoꢀ
generations of electronꢀhole pairs under direct sunlight exposure.

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Fig. 2 UVꢀ visible diffuse reflectance spectra of ZnS catalyst.
1
1
0.25
1
1
0.50
Fig. 3 UVꢀvisible diffuse reflectance spectra of (a) ZnS /CdS /Ag S , (b) ZnS /CdS /Ag S
,
2
2
1
1
0.75
1
1
1
(
c) ZnS /CdS /Ag S and (d) ZnS /CdS /Ag S catalysts.
2
2
3
.3 PL studies
The photoluminescence spectra of bare ZnS, ZnS/CdS, ZnS/Ag S and various ratios of
2
ZnS/CdS/Ag S nanocatalysts were presented in the Figure 4. They have two emission bands,
2
which are mainly located at 410 and 450 nm the weak emission bands at 410 is ascribed to the
35, 13
sulfur vacancies or defects in the sample surface
. The reduction in PL intensity peak was
observed in the case of ZnS/CdS/Ag S catalyst when compared to bare ZnS, which reveals that
2
the suppression of photogenerated electronꢀhole pairs is achieved by ternary system. Further, the
Fig. 4(i) shows the photoluminescence spectra of ZnS, ZnS/CdS, ZnS/Ag S and ZnS/CdS/Ag S
2
2
nanocatalysts for comparative studies. The photoluminescence studies confirm that the resultant
ternary material has lower PL emission peak when compared to mono and binary semiconductor

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0
system, which supports the suppression of electronꢀhole pair recombination and reduction in the
defects of the sample surface. Hence, the ternary catalyst leads to higher photocatalytic activity
than that of the mono and binary catalyst.
Fig. 4 PL spectra of (i) ZnS (a), ZnS/CdS (b), ZnS/Ag S (c) and ZnS/CdS/Ag S (d) and (ii)
2
2
1
1
0.25
1
1
0.50
1
1
0.75
1
1
1
ZnS /CdS /Ag S (a), ZnS /CdS /Ag S (b), ZnS /CdS /Ag S (c) and ZnS /CdS /Ag S (d)
2
2
2
2
nanocatalysts.
3.4 FE-SEM and EDX analysis
Figure 5 shows the FEꢀSEM images of ZnS/CdS/Ag S catalyst at various magnifications.
2
From the SEM images, it was noticed that the particles appear as spherical, distorted spherical
and hexagonal shape (Fig. 5 (a & b)). Figure 5 (c) shows the existence of rod like structure. The
EDX spectrum confirms the presence of elements such as Zn, Cd, Ag and S in the
ZnS/CdS/Ag S catalyst (Fig. 5 (d)). Further, the EDX analysis has been carried out to determine
2
the actual ratio of the prepared ZnS/CdS/Ag S catalyst. Based on EDX measurements, the
2
atomic percentages of Zn, Cd, Ag and S were 20.46%, 21.71%, 21.96% and 34.87%,

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respectively. The above results were found to be close to 1:1:1 atomic ratio of ZnS/CdS/Ag2S
catalyst.
Fig. 5 FEꢀSEM images of ZnS/CdS/Ag S nanocatalyst (aꢀc) and (d) EDX spectrum of
2
ZnS/CdS/Ag S nanocatalyst.
2
3.5 BET surface area measurements
The surface area of the catalyst was determined by using nitrogen adsorptionꢀ desorption
method. The various ratio of ZnS/CdS/Ag S (Fig. 6) catalysts show the type II hysteresis loop of
2
1
1
0.25
nitrogen adsorptionꢀdesorption curve. The BET surface areas of ZnS /CdS /Ag S
,
2
1
1
0.5
1
1
0.75
1
1
1
ZnS /CdS /Ag S , ZnS /CdS /Ag S
and ZnS /CdS /Ag S catalyst are 116.74, 96.5, 89.02
2
2
2
3
and 81.64 cm /g, respectively. The BET surface area of ZnS/CdS/Ag S nanocatalyst (81.64
2

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2
2
2
36
m /g) is higher than the bare ZnS (68. 5 m /g) . The decreased BET surface area of
2
ZnS/CdS/Ag S catalyst is expressed in terms of m /g of the samples, which are related to the
2
quantity or the density of the sample. So, the density of the sample plays an important role for
3
determining the BET surface area of the sample. The density of Ag S (7.23 g/cm ) is higher than
2
3
3
the ZnS (3.98 g/cm ) and CdS (4.82 g/cm ). Hence, the density of the resulting ZnS/CdS/Ag S
2
sample was increased with the addition of Ag S semiconductor into ZnS or ZnS/CdS
2
3
7, 38
nanocatalyst, which results in the reduction of the BET surface area
.
1
1
0.25
Fig. 6 Nitrogen adsorption–desorption isotherm of (a) ZnS /CdS /Ag S , (b)
2
1
1
0.50
1
1
0.75
1
1
1
ZnS /CdS /Ag S , (c) ZnS /CdS /Ag S and (d) ZnS /CdS /Ag S catalysts.
2
2
2
3.6 XPS analysis
The XPS measurements were performed to investigate the chemical state of the prepared
ZnS/CdS/Ag S catalyst. The XPS survey spectrum of ZnS/CdS/Ag S sample was recorded
2
2

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and depicted in Fig. 7. It shows the existence of Zn, Cd, Ag and S in the ZnS/CdS/Ag2S
catalyst. Figure 8 represents the XPS spectra of Zn 2p, Cd 3d, Ag 3d and S 2p, respectively.
Figure 8(a) shows two peaks at 1020.41 eV and 1043.51 eV, which are matching to the
2
+
binding energies of Zn 2p3/2 and 2p1/2, respectively. The obtained values are in good
3
9
agreement with the previous report . The peaks appeared at 405.24 eV and 412.0 eV are
2
+
belonging to the binding energies of Cd 3d5/2 and 3d3/2 and they are good consistent with
4
0
+
the previous results . The characteristic binding energies of Ag 3d5/2 and 3d3/2 are 368.10
+
41
eV and 374.16 eV, which confirms the existence of Ag in the ZnS/CdS/Ag S material .
2
The S2p peak energy was obtained at 162.54 eV, which is in good agreement with the
4
2, 40
previous reports
. The above XPS results conclude the existence of ZnS, CdS and Ag2S
in ZnS/CdS/Ag S catalyst.
2
Fig. 7 The full survey XPS spectrum of ZnS/CdS/Ag S nanocatalyst.
2

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Fig. 8 XPS analysis of ZnS/CdS/Ag S nanocatalyst showing the binding energy spectrum of (a)
2
Zn 2p peak, (b) Cd 3d peak, (c) Ag 3d peak and (d) S 2p peak.
3.7 Photocatalytic degradation of dyes
Photocatalytic activity of ZnS/CdS/Ag S nanocatalyst was verified with the synthetic CR
2
dye solution as a model test pollutant under direct sunlight illumination. Fig. 9 displays the
absorption spectra of CR dye solution in the presence of ZnS/CdS/Ag S material as a solar
2
active photocatalyst under the exposure of natural sunlight for different time durations
towards photocatalytic dye degradation process. CR dye shows a characteristic absorption
peak at 495 nm and its peak intensity gradually decreased without the appearance of a new

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absorption peak during the photocatalytic process. Moreover, the intensity of absorption
peaks declines more smoothly and it undergoes complete photodegradation of CR within 120
min under direct sunlight exposures.
Fig. 9 The optical changes in the UVꢀvisible spectra of CR dye under natural sunlight
illumination in the presence of ZnS/CdS/Ag S catalyst and their respective optical image of CR
2
dye solution during photocatalytic process are inserted as an inset image.
The plot of the percentage of photodegradation against time was presented in Fig. 10 for
various photocatalyst under dark and direct sunlight illuminations. Compared to others ratios, the
ternary system (ZnS/CdS/Ag S) of 1:1:1 ratio exhibits improved photocatalytic activity. The
2
photocatalytic experiments were also carried out with ZnS/CdS/Ag S catalyst under dark
2
condition and it was noticed that the photodegradation efficiency of the catalyst was decreased.
In addition, the dye degradation tests were performed without the catalyst and the result indicates
that the only an insignificant amount of dye gets degraded in the absence of a catalyst. Hence,
these studies inferred that the existence of both sunlight and photocatalyst are the essential

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factors in photodegradation process. The present investigation proves that the ZnS/CdS/Ag2S
catalyst has a higher photocatalytic performance under direct sunlight when compared to that of
mono and binary semiconductor systems (Fig. 10). Thus, the above result confirms that the
ZnS/CdS/Ag S material will work as an effective catalyst for the decomposition of dye pollutant
2
present in the wastewater.
Fig. 10 Photodegradability of CR dye a) without catalyst, b) dark, c) ZnS, d) ZnS/CdS, e)
1
1
0.25
1
1
0.5
1
1
75
ZnS/Ag S, f) CdS/Ag S, g) ZnS /CdS /Ag S , h) ZnS /CdS /Ag S ,i) ZnS /CdS /Ag S and
2
2
2
2
2
1
1
1
j) ZnS /CdS /Ag S catalysts under direct sunlight exposure.
2
Under direct sunlight illuminations, both Ag S and CdS due to their narrower band gap
2
1
0, 24
+
ꢀ
(1.0 and 2.42 eV)
can be excited to eject the electrons in order to create the h and e pairs,
which are essential for the decomposition dye pollutant. CdS has lower energy of highly
occupied orbital compared to Ag S semiconductor. However, the band gap energy of CdS is
2
higher than Ag S. Hence, the expected ejection of electron will be more in the case of CdS
2

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comparable to Ag S. But, all the range of photons was available in natural solar light and hence
2
the ejected electrons will be more in the case Ag S when compared to CdS semiconductor (Fig.
2
11). Further, present result expected to conclude the overlapping of orbital's since the metal
atoms were arranged in the atomic level and observed reduced band gap energy. So, the electron
in the valence band of CdS was expected to move to conduction band of Ag S as well as
2
4
3, 44
electrons in valence band of Ag S to conduction band of CdS semiconductor
and vice versa.
2
Further, the electron in the valence band of CdS and Ag S semiconductor were excited to the
2
conduction band of ZnS semiconductor. The migration of photoꢀgenerated charge carriers
through the heterojunction interfaces will greatly suppress the recombination of electron–hole
pairs and it favors the separation of photogenerated charge carriers on ZnS/CdS/Ag S catalyst.
2
So, it can actively participate in the photocatalytic reactions to decompose the organic pollutant
and hence their photocatalytic properties were improved. The electrons ejected from the catalyst
•−
will react with the adsorbed O molecule on the surface of the nanocatalyst to produce O2 and
2
H O , which will favor to degrade CR dye. Further, the holes created in the VB of
2
2
ZnS/CdS/Ag S react with H O molecule to produce reactive hydroxyl species (•OH), which
2
2
further enhance the oxidation ability of the catalyst to degrade CR dye. In short, the adsorbed CR
•−
dye molecules will react with O2 and •OH radicals generated on the catalyst in order to form
mineralization products such as acids, CO , etc.
2

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Fig. 11 Systematic view for the separation of electronꢀhole pairs on ZnS/CdS/Ag S catalyst
2
under direct sunlight illuminations.
Further, the photodecomposition of dye molecules may also possible to follow the dye
sensitized photodegradation mechanism along with heteroꢀjunction interfaces transitions
mechanism. According to photosensitized mechanism, the CR dye molecules are get adsorbed on
the surface of the ZnS/CdS/Ag S catalyst and they work as a solar light harvester. Hence, the
2
ꢀ
solar light excites the CR dye molecules and the excited molecules directly inject the excited e
into the conduction band (CB) of the photocatalyst. As a result, the electron gets transfer to the
surface of the catalyst (CB) and thus it increases the rate of oxidation of dyes. So, the photo
excited electrons present on the surface of the catalyst were transfer to the adsorbed oxygen.
Therefore, the electrons in the surface react with the oxygen molecules at ambient conditions in

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4
5, 46
order to generate O2 and the photosensitized dye is converted into cationic dye radicals
.
Hence, the cationic dye radicals react with superoxide radicals and they were degraded by the
4
7
photosensitized mechanism, which are in good agreement with the literature data .
In this study, additionally dye degradation experiments were also carried out in the
presence of appropriate quenchers like 1,4ꢀbenzoquinone (BQ) which is a superoxide radical
•
(
O ) and isopropanol as OH radical scavengers to confirm the generation of oxidative and
2
4
8
reductive intermediate species during the solar light illumination . Table 1 shows the effect of
isopropanol and 1,4ꢀbenzoquinone concentrations (0.1 M and 1 M) for the photodegradation of
CR dye in presence of ZnS/CdS/Ag S catalyst. From the results, it has been noticed that the
2
addition of isopropanol and 1,4ꢀbenzoquinone inhibit the transformation of CR dye molecules. In
addition to that, it was observed that the decrease in degradation efficiency occurs with
increasing the concentration of isopropanol and 1,4ꢀbenzoquinone. The inhibitory effects of
isopropanol and 1,4ꢀbenzoquinone on the photodegradation of CR are significant. This is
•
•
because isopropanol is an OH radical scavenger, which could react with OH radical and
4
9
•
quenches the photocatalytic reactions . Hence, the present investigation concludes that OH
radicals were generated during the dye degradation experiments, which could also support the
dye degradation mechanism. Further, the inhibition effect of 1,4 benzoquinone authenticates the
4
8, 50
generation of O radicals during dye degradation process
. The above experimental results
2
conclude that the superoxide and hydroxyl radicals are the important active intermediate species

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0
generated during dye degradation process are responsible for the photodegradation of CR dye
molecules.
Table 1 Effect of isopropanol and 1,4 benzoquinone on the photodegradation efficiency of the
CR dye (30 min).
Quencher
% of degradation
ZnS/CdS/Ag2S
65.01
No quencher
0
1
0
1
.1 M isopropanol
55.63
M isopropanol
42.74
.1 M 1,4 benzoquinone
M 1,4 benzoquinone
59.82
47.57
The analysis of CR dye decomposition has been performed using different analytical
techniques. In this regard, the GC and FTꢀIR studies have been performed to support the
decomposition of congo red (CR) dye. The GC spectra of CR dye after treatment with
ZnS/CdS/Ag S nanocatalyst showed two peaks at a retention time of 2.3 and 3.5 min (Fig. 12),
2
while the control dye showed a peak at a retention time of 1.5 min. The disappearance of the
control dye peak and appearance of two new peaks in the treated samples suggests the
transformation of CR dye into two different byproducts. Further, the intensities of these two
peaks are decreasing with respect to the time, which concludes the complete decomposition of
CR dye molecules and are good agreement with the FTꢀIR results.

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Fig. 12 Chromatograms obtained from GC analysis during the photodegradation of CR dye on
(
a) control CR dye and (b) treated CR dye ZnS/CdS/Ag S catalysts.
2
FTꢀIR analysis of CR dye and its products after decomposition on ZnS/CdS/Ag2S
−
1
nanocatalyst was shown in Fig. 13. The peaks (Fig.13 (a)) appeared at 1322 and 1064 cm are
−
1
corresponding to C–N bond and S=O stretching, respectively. The peak observed at 1588 cm
indicates the presence of azo –N=N‒ double bond stretching vibration, which confirms the
−
1
presence of an azo compounds. The absorption peaks observed at 1445 and 1493 cm were
ascribed to the C–C stretching in aromatic rings (aromatic skeletal vibrations). The FTꢀIR
ꢀ1
spectrum of CR dye showed the characteristic bands of 1226, 1178 and 1064 cm associated to
CꢀH inꢀplane bending vibrations and the occurrence of characteristic peaks of 836, 750 and 703

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ꢀ1
cm corresponds to the out of plane or oop CꢀH bending indicates the hydrogen substitution in
the aromatic ring. In the degradation process of dye on ZnS/CdS/Ag S nanocatalyst, the
2
characteristic changes in the chemical structure of CR dye were observed (Fig. 13(b)). The peaks
−
1
ꢀ1
due to existence of azo group (1588 cm ) and CꢀC aromatic carbon bond (1445 cm ) were
disappeared after ZnS/CdS/Ag S treatment indicating the breakdown of these bonds present in
2
−
1
the CR dye (Fig. 13(b)). Additionally, the peaks from 836 to 703 cm representing the benzene
ring structures of the parent CR dye also disappeared suggesting the cleavage of the dye
molecule.The overall prediction of the FTꢀIR spectra conclude that the CR dye have been
decolorized or degraded into different products obtained after photodegradation process on
ZnS/CdS/Ag S nanocatalyst, which confirms the transformation of the dye into different
2
byproducts.

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Fig. 13 FTꢀIR profile of (a) CR dye and (b) its decolorized products obtained after treatment
with ZnS/CdS/Ag S nanocatalyst.
2
3.8 Recyclability test
To promote the industrialization process, the stability and the recycle efficiency of the
catalyst has to be checked under the dye degradation process. Fig. 14 shows four successive
cycles of photodegradation of CR dye on ZnS, ZnS/CdS, ZnS/Ag S and ternary ZnS/CdS/Ag S
2
2
catalysts. Hence, the stability and reusability of the photocatalyst were determined by undergoing
the dye degradation experiments on the same catalyst for four different cycles towards CR dye
decomposition process under direct sunlight irradiation. In the case of ZnS/CdS/Ag S catalyst, it
2
was observed that degradation efficiency of CR dye remains almost constant for four repeated
cycles when compared to that of other mono and binary systems. The results (Fig. 14 (d))
indicate that there is no remarkable change in the activity of ZnS/CdS/Ag S catalyst was
2
perceived after four cycles.

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Fig. 14 Catalyst reusability tests on (a) ZnS, (b) ZnS/CdS, (c) ZnS/Ag S and (d) ZnS/CdS/Ag S
2
2
catalyst for CR dye degradation under direct sunlight irradiations.
Fig. 15 shows the XRD patterns of ZnS/CdS/Ag S nanocatalyst before and after solar irradiation.
2
The XRD result indicates that no significant changes were occurred in the crystalline nature of
the catalyst after the completion of photodegradation process.

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Fig.15 XRD patterns of ZnS/CdS/Ag S nanocatalyst before and after irradiation.
2
4
. Conclusion
The ternary sulphide system of ZnS/CdS/Ag S nanocatalyst was synthesized through
2
hydrothermal method using thiourea as a sulfur source. The low band gap energy and decreased
PL emission peak of ZnS/CdS/Ag S catalyst concludes that the optical properties of the ternary
2
system gets improved which in turn enhances the photocatalytic efficiency of the catalyst under
direct sunlight. The photocatalytic experiment results of ZnS/CdS/Ag S catalyst also show
2
significantly higher photocatalytic activity for the degradation of CR dye than that of mono and
binary semiconductors. Hence, the novel ternary ZnS/CdS/Ag S nanocatalyst can work as a
2
potent catalyst for waste water treatment process, which will attract the industrialist for
commercialization process.
Acknowledgments

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The authors like to thank DST /Nanomission, New Delhi, India for the financial support
to carry out this work and the establishment of the Nanotech research lab through grant No.
SR/NM/NSꢀ05/2011(G).
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