Quaternary semiconductor Cu2ZnSnS4 loaded with MoS2 as a co-catalyst for enhanced photo-catalytic activity

Article abstract of DOI:10.1039/c5ra03401a

Quaternary Cu₂ZnSnS₄ (CZTS) loaded with 1% MoS₂ shows excellent photo-catalytic activity for water oxidation, leading to efficient H₂ generation (AQY 22.67%), as well as in the degradation of an organic pollutant. The photo-catalysts were characterized using powder X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, UV-Vis absorbance and fluorescence spectroscopy. Taking Rhodamine B as a model system, the apparent rate constant for CZTS-MoS₂ (i.e. k_app ～ 0.066 min^-1) is nearly two-fold higher in comparison to CZTS (k_app ～ 0.032 min^-1). Various scavenger tests have been performed to establish the role of O₂^?- and ^?OH scavengers in the photo-degradation of a pollutant.

Full text of DOI:10.1039/c5ra03401a

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Quaternary semiconductor Cu ZnSnS loaded with
2
4
MoS as co-catalyst for enhanced photo-catalytic
2
Cite this: DOI: 10.1039/x0xx00000x
activity
Received 00th January 2012,
Accepted 00th January 2012
a
a
b
b
Gaurangi Gogoi , Sonia Arora , Natarajan Vinothkumar , Mahuya De
a
DOI: 10.1039/x0xx00000x
and Mohammad Qureshi*
www.rsc.org/
2 4 2
Quaternary Cu ZnSnS (CZTS) loaded with 1 %MoS shows an excellent photo-catalytic activity for
water oxidation leading to efficient H2 generation (AQY 22.67%) as well as in degrading organic
pollutant. The photo catalysts were characterized by powder X-ray diffraction, Field Emission Scanning
Electron Microscopy, Transmission Electron Microscopy, UV-Vis absorbance and fluorescence
spectroscopy. Taking Rhodamine B as a model system, the apparent rate constant for CZTS-MoS
2
(i.e.
app ~ 0.032 min ). Various scavenger
2
and •OH scavengers in the photo-degradation of
-
1
-1
k
app ~ 0.066 min ) is nearly two fold higher in comparison to CZTS (k
•
-
tests have been performed to establish the role of O
the pollutant.
cost of In, Ga has led to the search of alternative photoꢀcatalytic
systems from earth abundant elements such as Copper, Zinc
Introduction
1
9
The growing energy and environmental demand has led etc. Cu ZnSnS which is nonꢀtoxic and its constituents are
2
4
intensive research thrust on developing highly efficient, stable available plentiful (Cu: 50 ppm, Zn: 75 ppm, Sn: 2.2 ppm, S:
2
0
and low cost solar absorber materials. One of the primary goals 260 ppm) is of interest as Cu ZnSnS (CZTS) has a nearꢀ
2
4
is to produce high calorific hydrogen fuel from the abundant optimum direct band gap energy of 1.4ꢀ1.6eV and a large
4
ꢀ1
water available on the planet and to degrade the industrial dyes, absorption coefficient (>10 cm ). Being environment friendly
which are carcinogenic and cause unwanted mutations in and of natural abundance, CZTS is finding its place in replacing
1
marine life cycles. An extensive effort for the development of highꢀefficient materials e.g. Cu(InGa)Se . Although, more
2
novel combination of materials to replace traditional emphasis is on fabricating photovoltaic solar cell devices of
2
,3
4,5
6
21ꢀ24
semiconductors such as TiO2,
ZnO,
CdS etc. are CZTS,
yet its photoꢀcatalytic activity for waste water
2
5
26
underway. Over the last few decades several sulphideꢀbased treatment,
polynary alloy semiconductors have been explored and they oxidation for hydrogen generation are also being explored .
proved as ideal photoꢀabsorbers because of their tunable The photoꢀcatalytic efficiency of semiconductor materials is
electronic and optical properties.
materials, polynary alloy semiconductor nanoꢀmaterials are Au, Pt, which act as coꢀcatalyst, or making composite with
able to produce new properties as they could inherent the material having large specific surface area and good electrical
photoꢀelectrochemical devices
and water
2
7
7
ꢀ9
Compared to binary further enhanced by coupling them with either noble metals e.g.
2
5
1
0
28
properties from their parent binary materials. For example, conductivity, namely Graphene oxide. MoS being a layered
several Cuꢀchalcopyrite pꢀtype semiconductors namely material with a large surface area,
2
2
9
is of interest as an
1
1
12
13,14
15,17
Cu(In,Ga)Se ,
CuGa S ,
CuGa Se ,
CuInS2,
alternative coꢀcatalyst to the high cost noble metals. MoS has
2
1
3
5
3
5
2
6
17
18
30ꢀ33
Cu(In,Ga)S2, (CuIn) Zn2(1ꢀx)S2 CuGaSe2, owing to their shown high efficiencies when coupled with TiO , CdS etc.
x
2
high absorption coefficients, tunable band gap values (1.0–2.4 which are studied for their photoꢀcatalytic and solar water
eV) and suitable band alignment for water reduction, are used oxidation activity. Owing to their outstanding properties, we
for H production from water. However, the scarcity and higher have studied CZTS and MoS as possible candidates to replace
2
2
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noble and other expensive elements for their photoꢀcatalytic The crystal structure of as prepared samples were characterized
activity using CZTS as a photoꢀcatalyst with MoS as a coꢀ by powder Xꢀray diffraction (XRD) measurements using
2
catalyst. The water oxidation efficiency of CZTSꢀMoS was SEIFERT 3003ꢀTT XRD machine operated at 40kV/30mA,
2
observed to be nearly 20% higher than CZTS owing to a lesser (excitation wavelength CuK ) . The scan range was 10°ꢀ80°
α
recombination rate of photoꢀgenerated electrons and holes as with
evident from the photoꢀluminescence quenching in the CZTSꢀ compositional homogeneity of the photoꢀcatalysts were studied
MoS2 composite. The photoꢀcatalytic degradation of using Zeiss Sigma field emission scanning electron
a step size of 0.03°/sec. Morphology and the
a
a
representative dye viz. Rhodamine B (RhB) was studied for the microscope (FEꢀSEM) operating voltage of 1−5kV and energy
photoꢀcatalytic activity of CZTSꢀMoS2 composite; which dispersive xꢀray spectroscopy (EDX) at an operating voltage of
shows a higher activity compared to CZTS alone as evident 20kV respectively. UV–Visible diffuse reflectance spectra
from the apparent rate constant for CZTSꢀMoS , which is (DRS) of the samples were recorded on a Jasco Vꢀ650
2
ꢀ
1
ꢀ1
nearly double (0.066 min ) than that of CZTS (0.032 min ). spectrophotometer with an integrating sphere of 150 mm in the
Various scavenger tests established that hydroxyl radical ( OH) range of 200 to 800 nm. The absorbance of the RhB solution
.
plays a major role as the reactive intermediate species was measured in Perkin Elmer Lamda 25 UVꢀVisible
participating in dye degradation.
spectrometer. The transmission electron microscopy (TEM)
analysis was carried out using a JEOL JEM 2100 microscope at
200kV operating voltage. BET surface area was analysed by
Experimental
nitrogen (N ) adsorption at liquid N2 temperature with a
BeckmanꢀCoulter SA 3100 nitrogen adsorption apparatus. All
2
Synthesis of Cu ZnSnS -MoS composite
2
4
2
All reagents were of analytical grade and used without the samples were degassed at 150°C for 2h prior to the N
further purification. The synthesis of Cu ZnSnS (CZTS) was adsorption measurements.
2
2
4
2
0
adopted from the reference. In a typical procedure, copper(II)
acetate monohydrate (0.04M, 0.119g) (Merck), zinc(II) nitrate
Photo-catalytic dye degradation experiment
hexahydrate (0.02M, 0.089g) (Himedia), and tin(II) chloride Photoꢀcatalytic dye degradation experiments for CZTS and
dihydrate (0.02M, 0.067g) (Rankem) were dissolved in 15ml of CZTSꢀMoS composites were performed separately in an
2
a solvent mixture of ethylenediamine (EN) (Merck) and immersion well photochemical reactor. The doubleꢀwalled
deionized water (volume ratio 1:9), wherein EN acts as a immersion well permits water circulation and it houses a MVL2
chelating agent and stabilizer. 15ml of 0.16M (0.182g) solution 125W low pressure mercury vapour lamp as the light source to
of thiourea, SC(NH2)2 (Merck) was added dropꢀwise to the initiate the photoꢀcatalytic reaction (SI). In a typical photoꢀ
above solution. Precursors’ solution was then transferred into a catalytic degradation experiment, 100mL of aqueous
ꢀ
5
5
0ml Teflonꢀlined stainless steel autoclave, sealed and heated at RhodamineꢀB dye (RhB) (10 M) and 0.05g of the photoꢀ
1
80ºC for 24h. Liu et al. have proposed that this process catalyst sample were loaded into the reactor. The reaction
2+
+
involves, the reduction of Cu ions to Cu and oxidation of mixture was stirred overnight in the dark at room temperature
2
+
4+
20
Sn ions to Sn through the oxidation–reduction reactions.
in order to achieve adsorption–desorption equilibrium among
After cooling, the powders were centrifuged, washed several the photoꢀcatalyst, dye, dissolved oxygen and atmospheric
times with deionized water and absolute ethanol and dried at oxygen. During irradiation, the suspension was kept stirring in
6
0ºC overnight.
MoS catalysts were synthesized by a hydrothermal was collected from the photoꢀreactor every ten minutes up to
order to achieve a homogeneous solution. 2mL of the sample
2
4
3
method
involving ammonium heptamolybdate (0.276g) one hour. UVꢀVis absorption spectra of the collected samples
(
Sigmaꢀ Aldrich) and thiourea (0.36g) (Merck), dissolved in were recorded over the range 200–800nm after removing the
3
0mL distilled water. A surfactant cetyltrimethylammonium solid catalyst particles by centrifugation. The concentration of
bromide (CTAB, 0.036g) (Rankem), was added into the above the aqueous RhB was determined from the absorbance value
solution. Resultant mixture was added to a 50ml Teflonꢀlined monitored at 552nm.
stainless steel autoclave and kept at 180ºC for 24h. After
Detection of reactive intermediates
cooling, catalyst was separated, washed with distilled water and
absolute ethanol several times to remove the residual water and To determine the reactive intermediates involved in dye
waterꢀsoluble impurities. Finally, the resulting product was degradation, photoꢀcatalytic measurements were performed in
dried at 50°C for 8h.
For the preparation of CZTSꢀMoS composite, 1wt.% of dye degradation experiments. In our experiments,
presence of scavengers in the same way as mentioned in the
2
MoS was added into the above precursor solution of CZTS and ethylenediaminetetraaceticacid (EDTA, 1mM), FeSO (1mM)
2
4
•
transferred to a Teflon lined stainless steel autoclave, sealed were employed as hole (h+) and OH scavengers respectively.
and heated at 180ºC for 24h. Resulting powder, after cooling, Nitrogen gas is purged to remove the dissolved oxygen, which
•
−
35ꢀ37
was washed with water and ethanol several times and dried at can serve as a test for O
scavenger.
2
6
0°C overnight.
Photo-catalytic water oxidation to evolve hydrogen
Characterization
2
| J. Name., 2012, 00, 1-3
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03
R
4
T
0
I
1
C
A
LE
Figure 1: Powder X-ray diffraction patterns of Cu
MoS composite. Cu ZnSnS and the composite show tetragonal kesterite
structure whereas MoS shows diffraction pattern corresponding to hexagonal
2
ZnSnS
4
, MoS
2
and Cu
2 4
ZnSnS -
Photoꢀcatalytic water oxidation experiment to estimate
hydrogen evolution, for CZTS and CZTSꢀMoS2 were
2
2
4
2
performed separately in a 100mL borosilicate roundꢀbottom structure
flask sealed with a silicone rubber septum, placed in a vessel
containing a water inlet and an outlet to maintain the Asꢀprepared CZTS was found to be crystallizing in tetragonal
temperature of photoꢀreactor. The photoꢀcatalytic reaction was kesterite phase (JCPDS card no. 26ꢀ0575) as identified by
o
o
o
o
carried out at room temperature and atmospheric pressure. A reflections at 2
θ = 28.5 (112), 33.3 (200), 47.4 (220), 56.1
5
o
o
00W tungsten halogen lamp (Halonix, India) was used as a (312), 69.2 (008) and 76.5 (332). No impurity peaks
light source to initiate the photoꢀcatalytic reaction. The photoꢀ corresponding to ZnS or Cu SnS phases were observed as
2
3
catalyst sample was placed 15cm away from the lamp. In a evident from Raman spectra (SI). Average crystallite size was
typical photoꢀcatalytic experiment, 0.2 g of sample was calculated using DebyeꢀScherrer equation from the full width at
ꢀ
1
dispersed in 25mL of water containing Na SO (0.25molL ) half maximum (FWHM) of the reflection peak (112) and found
2
3
ꢀ
1
and Na2S (0.35molL ) as sacrificial reagents. Solution to be
̴ 4 nm for CZTS and 3.72 nm for CZTSꢀMoS composite.
2
containing the photoꢀcatalyst was degassed for 30min and There is no significant change observed in the crystallite size
irradiated with UV–Vis light while stirring, to ensure uniform which is attributed to the interaction and adhering of CZTS to
3
9
exposure of the suspension throughout the process. The the surface of MoS sheets.
2
produced gas was analysed by gas chromatography (Nucon
765), using a thermal conductivity detector (TCD), after the broadening of diffraction peaks. The PXRD pattern
passing through moisture and oxygen traps (SI). In the absence matched well with the hexagonal structure (JCPDS card no. 37ꢀ
The asꢀprepared MoS was weakly crystalline, as seen from
2
5
o
o
o
o
of either light or the photoꢀcatalyst, no appreciable amount of 1592) with reflections at 2
hydrogen was detected, suggesting that CZTS and CZTSꢀMoS2 corresponding to (002), (100), (103) and (110) hkl planes
acted as the photoꢀcatalysts in hydrogen production.
respectively.
θ = 16.7 , 33.4 , 43.5 and 57.4
The apparent quantum yield (AQY) of the photoꢀcatalyst was The composite CZTSꢀMoS showed the XRD pattern of CZTS
2
measured under the same reaction condition and it was only, while no distinct peak corresponding to MoS2 was
3
8
calculated using the following equation:
observed. The absence of MoS peaks could be attributed to the
2
weak crystalline nature of MoS compared to CZTS as well as
2
the low content of MoS i.e. ̴1% in the CZTSꢀMoS composite.
2
2
Numberꢀofꢀreactedꢀelectrons
Numberꢀofꢀincidentꢀphotons
ꢀ
AQY ꢁ
ꢀxꢀ100%
However, presence of MoS has been established using the
2
microscopic techniques such as EDX and TEM.
2
x Number of hydrogen molecules evolved in 1 hour
ꢁ
ꢀx 100 M% orphological studies
Numberꢀof incident photons in 1 hour
Field Emission Scanning Electron Microscopy and Transmission
Electron Microscopy
Results and discussion
The morphological features of CZTS and its composite with
MoS2 are studied by field emission scanning electron
microscopy and shown in Figure 2 (2A and 2B corresponds to
CZTS and 2C, 2D are of CZTSꢀ MoS2).
Powder X-ray diffraction
The powder Xꢀray diffraction patterns of asꢀprepared CZTS,
MoS and the composite CZTSꢀMoS are shown in Figure 1.
2
2
(112)
CZTS MoS2
(220)
(312)
(200)
(
008) (332)
(112)
CZTS
(220)
(312)
(200)
(332)
(008)
(100)
(002)
MoS2
(103)
(110)
10
20
30
40
50
60
70
80
2θ (degree)
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Figure 2: Field Emission scanning electron microscopic features; 2A and 2B show
nearly spherical and similar size particle distribution of Cu ZnSnS , 2C and 2D
show more aggregated particles of Cu ZnSnS -MoS
show a broad absorption feature in UV, visible range. Inset to
Figure 5 shows Tauc’s plot to calculate the optical band gap of
the material. The Tauc’s equation is described as
2
4
2
4
2
1/n
(
αhν) = C x (hν ꢀ Eg)
Asꢀprepared CZTS showed nearly spherical particles of same
size as seen in Figure 2A and 2B. The average particle size was
observed to be in the range of ~200 nm, whereas, the composite
showed bigger particles of irregular shape probably due to more
Where α is the absorption coefficient of the semiconductor
at a certain value of wavelength λ, h is the Plank’s constant, C
is the proportionality constant, ν is the frequency of light, E is
g
the band gap energy and n = ½ for direct transition mode
materials respectively. The absorption coefficient is estimated
from the equation,
CZTS particles getting accumulated into the MoS layers. To
2
ascertain the distribution of MoS , the elemental mapping of
2
composite was recorded by energy dispersive Xꢀray analysis
and observed that Molybdenum and Sulphur were
homogeneously distributed throughout the sample as seen in
Figure 3(AꢀF).
α = 1/t x ln (I /I ) = 1/t x A x log e
t
0
1
.1
1.0
.9
0.8
.7
0.6
The interaction of CZTS and MoS was further evaluated by
2
transmission electron microscopy as shown in Figure 4. Figure
0
1
00
90
80
4
A shows a nearly spherical composite particle and inset to
MoS2
Figure 4A shows the SAED pattern of CZTSꢀMoS2
corresponding to tetragonal phase. Figure 4B and 4C shows the
high resolution TEM image wherein a lattice spacing of 0.61nm
CZTS
CZTS MoS2
0
7
6
5
0
0
0
corresponding to MoS (100) plane and interꢀplanar distance of
2
0
.32nm of CZTS (112) plane have been observed. This
0.5
0.4
0.3
observation substantiates successful impregnation of MoS on
2
1.62 eV
CZTS.
1.5
1.6
1.7
1.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
hν
200
300
400
500
600
700
800
Wavelength (nm)
Figure 5 shows UV-Vis spectra of Cu ZnSnS , MoS and Cu ZnSnS -MoS depicting
2 4 2
2
4
2
a broad absorption from UV and Visible range of the spectrum. The inset shows
the Tauc’s plot to evaluate the optical band gap values for the three compounds.
Where A, t, It and I0 represent the absorbance,
thickness of the photoꢀcatalyst, intensity of transmitted light
and intensity of incident light respectively. Tauc’s plot shown
2
in the inset to Figure 5 is plotted between (αhν) (on yꢀaxis) and
photon energy (hν) (on xꢀaxis). The band gap energies were
Figure 3 shows the compositional distribution of (A) Cu
2
ZnSnS
4
- MoS
2
and the estimated by extrapolating lines and the band gap values for
corresponding EDX elemental mapping of (B) S, (C) Mo, (D) Cu, (E) Zn and (F) Sn
in the scan area of Figure 3 (A). The elemental mapping shows a homogeneous
distribution of all the elements throughout the sample area.
CZTS, MoS and CZTSꢀMoS are 1.56 eV, 1.51 eV and 1.62
eV respectively.
2
2
BET surface area analysis
Figure
6 shows N2 adsorptionꢀdesorption isotherm and
corresponding pore size distribution curve (inset) for CZTS and
CZTSꢀMoS . According to the BrunauerꢀDemingꢀDemingꢀ
2
Teller (BDDT) classification, the majority of physisorption
isotherms can be grouped into six types. The shapes of
hysteresis loops are often used to identify the specific pore
2
6
structure. Both the samples display a typical typeꢀIV
isotherms and type H3 hysteresis loops. Type IV isotherms are
generally shown by mesoporous industrial adsorbents. The
Figure 4: (A) shows the TEM image of Cu
shown in the inset of (A). (B) High resolution transmission electron microscopy
HRTEM) image of Cu ZnSnS -MoS (C) HRTEM showing lattice patterns of
Cu ZnSnS and MoS . Inset of (C) shows the FFT of the HRTEM image.
2
ZnSnS
4
-MoS
2
and its SAED pattern is
40
observed BET surface area of CZTS and CZTSꢀMoS are 20.10
2
(
2
4
2
2
2
m /g and 17.17 m /g respectively. The Barrett–Joyner–Halenda
BJH) pore size distribution curve (inset) indicates the high
2
4
2
(
degree of uniformity of pores in the range of ~3 nm for CZTS
UV-Vis Diffuse Reflectance spectra
and ~7 nm for CZTSꢀMoS . Although the surface area for both
2
samples are comparable, yet larger pore diameter in CZTSꢀ
UVꢀVis absorbance spectra of asꢀprepared CZTS, MoS and
their composite is shown in Figure 5 wherein all the samples
2
MoS suggests more effective active sites leading to enhanced
2
photoꢀcatalytic efficiency.
4
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reductive electrons efficiently separated in the CB. At the same
time, the reductive electrons are transferred from the CB of
CZTS to the CB of MoS2 because of the close proximity
alignment of the conduction bands of the two, thereby leading
to the reduction in the rate of electron–hole pair recombination
hence enhanced charge separation. The pool of reductive
5
4
3
2
1
0
0
0
0
0
0
4
0
CZTS
CZTS MoS2
30
2
1
0
0
electrons thus generated in the CB of MoS2 promotes the
+
0
reduction of H ions, thereby producing H gas. Thus, improved
2
photoꢀcatalytic activity of CZTSꢀMoS2 is attributed to the
efficient separation and lesser recombination of photoꢀ
generated electron–hole pairs in the composite.
1
0
20
30
40
Pore diameter (nm)
0.0
0.2
0.4
0.6
0.8
1.0
Relative Pressure (P /P )
s
0
2 4 2 4 2
Figure 6 shows the BET surface area analysis of Cu ZnSnS and Cu ZnSnS -MoS .
Inset depicts Barrett–Joyner–Halenda pore size distribution curves for both the
samples.
Photo-catalytic water oxidation
Photoꢀcatalytic water oxidation experiment was carried out
using CZTS and CZTSꢀMoS as the photoꢀcatalyst and the
2
amount of hydrogen liberated were estimated using standard
procedure. The H evolution rates under visible irradiation are
2
shown in Figure 7. The CZTSꢀMoS2 composite showed a
2 4 2
Scheme 1 Schematic representation of electron transfer of Cu ZnSnS and MoS
greater photoꢀcatalytic activity with the H production rate of in the composite.
2
2
64 µmol/0.2g/h in comparison to bare CZTS (214
µmol/0.2g/h). The calculated AQY of CZTSꢀMoS is 22.67% The decrease in the recombination rate of electrons and holes in
2
which is about 1.23 times higher than that of CZTS i.e. 18.42%. the composite is further substantiated by the quenching of
fluorescence intensity (~ 11%) in the MoS loaded CZTS
2
compared to bare CZTS as seen in Figure 8.
0.8
0.6
0.4
0.2
CZTS
CZTS MoS2
Figure 7: Amount of H
2 2 4 2 4 2
generated from Cu ZnSnS and Cu ZnSnS -MoS catalyst
under light irradiation and their respective AQY (%) values.
650
700
750
800
850
900
The higher photoꢀcatalytic activity is attributed to more active
Wavelength (nm)
sites in CZTSꢀMoS as well as higher charge separation. A
2
Figure 8 shows the photoluminescence spectra of Cu
2 4 2 4
ZnSnS and Cu ZnSnS -
schematic of the band diagram showing energy alignments of
MoS . Emission intensity is quenched in case of the composite suggesting
2
CZTSꢀMoS which facilitates an efficient transfer of electrons favourable charge separation of photo-generated electrons and holes
2
is shown in Scheme 1. Upon light irradiation on CZTSꢀMoS2,
electronꢀhole pair is generated in CZTS in the CB and VB
respectively. The sacrificial electron donor (Na SO ) rapidly
2
3
consumes the oxidative holes in the VB of CZTS, leaving the
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Reusability of CZTS-MoS2
degradation of RhB was achieved in case of CZTSꢀMoS in
2
about 40 minutes whereas it was only ~70% for bare CZTS.
To examine the reusability of the CZTSꢀMoS catalyst, we have
2
carried out three cycles of the photocatalytic experiment of
hydrogen production, with the catalyst. The catalyst retained
optimal activity till three cycles, which shows the efficient
reusability of the catalyst. The result of the repeated cycles is
shown in Figure 9. In a typical experiment 0.2 g of sample was
dispersed in 25mL of water containing Na SO (0.25molLꢀ1)
1
.0
blank
[7a] CZTS
0
1
2
min
0 min
0 min
0.8
30 min
0 min
50 min
0 min
4
2
3
0.6
and Na S (0.35molLꢀ1) as sacrificial reagents. Solution
2
6
containing the photoꢀcatalyst was degassed for 30min and
irradiated with UV–Vis light while stirring, to ensure uniform
exposure of the suspension throughout the process. The
produced gas was analysed by gas chromatography (Nucon
0
.4
0.2
5
765), using a thermal conductivity detector (TCD), after
passing through moisture and oxygen traps. After irradiation of
one hour, the solution was removed and the same experiment
was repeated with fresh solution without any processing of the
catalyst and the amount of gas evolved per 10 minutes was
measured. Similarly, the experiment was repeated for a third
cycle.
0
.0
450
500
550
600
650
Wavelength (nm)
1
0
0
.0
.8
.6
[
7b] CZTS-MoS (1%)
blank
min
10 min
0 min
0 min
0 min
0 min
0 min
2
0
300.00
2
3
4
5
6
250.00
2
00.00
0.4
0.2
st
1
nd
2
rd
150.00
Cycle
Cycle
3 Cycle
0.0
100.00
450
500
550
600
650
Wavelength (nm)
50.00
0
20
40
60
80
100
120
140
160
180
Figure 10(a) shows the UV-Vis absorption spectra of RhB degradation with
Cu ZnSnS catalyst; Figure 10(b) shows the UV-Vis absorption spectra of RhB
degradation with Cu
2
4
Time(min)
2
ZnSnS
4
-MoS
2
catalyst
Figure 9: Amount of H2 generated from Cu2ZnSnS4-MoS2 catalyst after repeated
cycles.
1
0
0
0
0
.0
.8
.6
.4
.2
CZTS
CZTSꢀMoS2
Photo-catalytic dye degradation analysis
The photoꢀcatalytic dye degradation experiments were
performed by suspending 0.05 gm of catalyst in a 100ml
ꢀ
5
solution of Rhodamine B (RhB) (10 M) as a reference dye.
The solution was stirred overnight in dark to achieve an
adsorptionꢀdesorption equilibrium followed by an irradiation to
start photoꢀcatalytic dye degradation. The photoꢀdegradation of
RhB was followed by measuring its concentration at
absorbance value of 552 nm. The absorbance spectra of RhB
was collected at a regular intervals (10 min up to 1 hour) of
degradation experiments, as shown in Figure 10a (catalyst used
0.0
0
10
20
30
40
50
60
Time (min)
Figure 11 shows the C/C
RhB by Cu ZnSnS and Cu ZnSnS -MoS . The degradation is much faster in case of
0
versus irradiation time curves of photo-degradation of
is CZTS) and Figure 10b (catalyst used is CZTSꢀMoS ). The
2
2
4
2
4
2
concentration (C/C ) versus time is plotted as shown in Figure Cu
2 4 2
ZnSnS -MoS .
0
1
1. From Figure 10, it is clearly observed that RhB was
degraded much faster by CZTSꢀMoS than CZTS alone. ~ 97%
2
6
| J. Name., 2012, 00, 1-3
This journal is © The Royal Society of Chemistry 2012

Page 7 of 10
Journal Name
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ARTICLE
DOI: 10.1039/C5RA03401A
2
ꢀ
Figure 12 shows the first order linear transform ln(C/C ) = ꢀ rate of dye degradation. SO4 ions also causes a decrease in
0
•
kappt corresponding to the photoꢀdegradation of RhB by CZTS. percentage degradation as they react with OH asꢀ
2
ꢀ
•
2ꢀ
ꢀ
Here, kapp is the apparent first order reaction rate constant SO4 + OH → SO4 + OH
which represents the rate of the reaction. CZTSꢀMoS2
composite shows higher activity compared to CZTS alone as Specificity of choosing FeSO over other iron salts is that both
4
2+
2ꢀ
•
the apparent rate constant for CZTSꢀMoS is almost twice cation and anion (Fe and SO4 ) can act as the OH
2
ꢀ
1
ꢀ1
(
0.066 min ) than that of CZTS (0.032 min ).
scavengers. The apparent rate constant (kapp) for RhB
degradation in presence of FeSO was less (0.004 min ) than
ꢀ
1
4
Photo-catalytic dye degradation mechanism:
that without the metal ion as can be seen in figure 14A.
Another important intermediate that plays an important role
•
ꢀ
in the degradation process is superoxide radical anion (O )
Time (minutes)
2
0
10
20
30
40
50
60
which is formed by the reduction of dissolved molecular
oxygen in water. In order to prove its role, degradation
1
y = e-0.032x
R² = 0.9812
experiment was carried out under continuous N purging which
2
is used to replace the dissolved oxygen from the system. The
apparent rate constant (kapp) for degradation in deoxygenated
ꢀ
1
0
.1
water was less (0.025 min ) than that in normal oxygenated
ꢀ
1
water (0.066 min ) as can be seen in figure 14B, proving the
importance of dissolved oxygen in generating superoxide
radical, which enhances the kinetics of degradation.
CZTS
y = e-0.066x
R² = 0.9432
CZTS MoS2
0
.01
Further a degradation experiment was performed in the
presence of EDTA which acts as a hole scavenger. The plot for
Figure 12 shows the evolution of apparent rate constant in the presence of
Cu ZnSnS and Cu ZnSnS -MoS
apparent rate constant (k
app) for degradation showed a lower
2
4
2
4
2
ꢀ
1
rate constant (0.012 min )in presence of EDTA as compared
The photoꢀcatalytic degradation of a dye generally takes place to the rate constant in the absence of EDTA (0.066 minꢀ1)
3
5, 36
by the following mechanism.
When a catalyst is exposed to (Figure 14C)
UVꢀVis irradiation, electrons are promoted from the valence
band to the conduction band producing an electron–hole pair.
The e and h can migrate to the catalyst surface, where they controlling the kinetics of dye degradation, it was observed that
Although, each scavenger has its role to play in
ꢀ
+
•
can undergo a redox reaction with other species present on the OH plays more prominent role, evident from the kinetic data.
ꢀ
+
surface. This prevents the recombination of the e and h
Hence we propose that hydroxyl radical could be the
+
generated in the first step. h (vb) can react easily with surface determining species in controlling the kinetics of the dye
•
−
bound H O to produce OH radicals, whereas, e (cb) can react degradation.
2
•
ꢀ
with O to produce superoxide radical anion of oxygen, O .
OH radicals can combine to form H O . The H O can further
2
2
•
2
2
2
2
1
.0
.8
•
ꢀ
•
react with O2 to produce OH, which is responsible for the
degradation of the organic dye.
0
ꢀ
+
Catalyst + hυ → e +h
Step 1
Step 2
Step 3
Step 4
Step 5
Step 6
cb
vb
ꢀ
•ꢀ
e
h
•
+ O → O
2
0.6
0.4
0.2
0.0
cb
2
+
•
+
+ H O → OH + H
vb
2
•
OH + OH → H O
2
2
•
ꢀ
•
ꢀ
2+
H O + O → OH + OH + O
2
Fe
2
2
2
EDTA
N2
•
Dye + OH → CO + H O
2
2
•
It is seen that hydroxyl radical ( OH) and superoxide anion
radical (O ) are mainly responsible for the degradation of the
CZTSꢀMoS
2
•
ꢀ
2
0
10
20
30
40
50
60
Time (min)
dye. Therefore in order to study the mechanism of dye
degradation, various scavenger tests were performed wherein
radical or hole trapping agents were added into the dye catalyst
system and the rate of degradation was observed.
Figure 13 shows the concentration versus time for RhB degradation when
different hole or hydroxyl radical scavengers were added along with the catalyst,
.
suggesting that OH is the main intermediate reactive species promoting the dye
degradation, as degradation is much slower in the presence of FeSO
hydroxyl radical scavenger).
4
(the
2+
•
2+
Fe salt was used as a OH trapping agent. Fe undergoes the
•
following reaction with OH present in the solution.
2
+
–
3+
•
OH + Fe → OH + Fe
8
ꢀ1 ꢀ1
This reaction has a very high rate constant (k = 3.5x10 M s ).
2
+
•
–
In the presence of Fe , OH is converted to OH , decreasing
•
the OH concentration in the solution, leading to a decreased
This journal is © The Royal Society of Chemistry 2012
J. Name., 2012, 00, 1-3 | 7

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ARTICLE
5RA0340
DOI: 10.1039Jo/Curnal Na1Ame
Acknowledgements
Time (minutes)
30
0
1
10
20
40
50
60
MQ thanks CSIR Grant
No: CSIR/01 (2704)/12/ EMRꢀII
India for financial grant. SA thanks DST India for Women
Scientist fellowship (WOSꢀA) Grant No: SR/WOS – A/CSꢀ33/
y = e-0.004x
R² = 0.8025
A
2
013 (G): C/503/IFD/2014ꢀ15. The infrastructure and
instrumentation help from IIT Guwahati, CIF – IIT Guwahati
are highly appreciated.
0
.1
y = e-0.066x
R² = 0.9432
FeSO
CZTS MoS
4
References
2
0
.01
1
Notes and references
Materials Science Laboratory, Department of Chemistry, Indian Institute
a
Time (minutes)
30
of Technology Guwahati, India ꢀ 781039. *mq@iitg.ernet.in
0
10
20
40
50
60
b
Department of Chemical Engineering, Indian Institute of Technology
y = e-0.025x
R² = 0.9175
Guwahati, India ꢀ 781039
B
†
Electronic Supplementary Information (ESI) available: [Details of the
experimental setup for photo catalytic hydrogen generation, Dye
degradation setup, power spectrum distribution for the 500W tungsten
halogen lamp are given . See DOI: 10.1039/b000000x/
0
.1
y = e-0.066x
R² = 0.9432
N2 purging
CZTS MoS2
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.01
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2
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4
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ꢀ
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8
| J. Name., 2012, 00, 1-3
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DOI: 10.1039/C5RA03401A
Graphical Abstract:
Quaternary semiconductor Cu
2
ZnSnS
4
loaded with MoS as co-
2
catalyst for enhanced photo-catalytic activity
a
a
b
b
Gaurangi Gogoi , Sonia Arora , Natarajan Vinothkumar ,Mahuya De and Mohammad
Qureshi*a