Visible-light-driven SnO2/TiO2 nanotube nanocomposite for textile effluent degradation

Article abstract of DOI:10.1039/c4ra13434f

Nanocomposite of SnO₂/TiO₂ nanotube (NT) was prepared by a simple two-step procedure, viz., hydrothermal preparation of TiO₂ NT followed by chemical precipitation of SnO₂ nanoparticle sensitized TiO₂ NT. Transmission electron microscopy analysis revealed that the as-synthesized TiO₂ NTs exhibit a diameter of 20-30 nm and a length of 1-2 μm with a wall thickness of 2-5 nm. The particle size of SnO₂ nanoparticles in the composite was estimated to be 4-6 nm. Both the TiO₂ NT and SnO₂/TiO₂ NT composite were considered for the photocatalytic degradation of textile dye effluent under UV and sunlight irradiation. The SnO₂/TiO₂ NT composite showed improved degradation efficiency. Photoluminescence spectra of SnO₂/TiO₂ NT composites showed a decrease in intensity when compared to the TiO₂ NT-revealing that the rate of recombination is reduced in the SnO₂/TiO₂ NT composite, which is responsible for larger photocatalytic activity. Further, an effluent degradation efficiency of 85% was obtained for the SnO₂/TiO₂ NT composite under sunlight irradiation and this can be considered as one of the best values achieved so far. This journal is

Full text of DOI:10.1039/c4ra13434f

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PAPER
Visible-light-driven SnO /TiO nanotube
2
2
nanocomposite for textile effluent degradation
Cite this: RSC Adv., 2015, 5, 20424
a
a
b
c
c
K. Rajkumar, P. Vairaselvi, P. Saravanan, V. T. P. Vinod, Miroslav Cˇ ernık
´
and R. T. Rajendra Kumar*d
Nanocomposite of SnO
hydrothermal preparation of TiO
TiO NT. Transmission electron microscopy analysis revealed that the as-synthesized TiO
diameter of 20–30 nm and a length of 1–2 mm with a wall thickness of 2–5 nm. The particle size of
SnO nanoparticles in the composite was estimated to be 4–6 nm. Both the TiO NT and SnO /TiO NT
composite were considered for the photocatalytic degradation of textile dye effluent under UV and
sunlight irradiation. The SnO /TiO NT composite showed improved degradation efficiency.
Photoluminescence spectra of SnO /TiO NT composites showed a decrease in intensity when
compared to the TiO NT – revealing that the rate of recombination is reduced in the SnO /TiO NT
composite, which is responsible for larger photocatalytic activity. Further, an effluent degradation
efficiency of 85% was obtained for the SnO /TiO NT composite under sunlight irradiation and this can
be considered as one of the best values achieved so far.
2
/TiO
2
nanotube (NT) was prepared by a simple two-step procedure, viz.,
NT followed by chemical precipitation of SnO nanoparticle sensitized
NTs exhibit a
2
2
2
2
2
2
2
2
2
2
2
2
Received 29th October 2014
Accepted 11th February 2015
2
2
2
DOI: 10.1039/c4ra13434f
2
2
www.rsc.org/advances
and easy availability. Although several advances have been
made, the high photogenerated electron–hole pairs hinders its
Environmental pollution caused by toxic pollutants from potential applicability in industries. This is because, as the
various industries such as textiles, dye, leather and other recombination of electron–hole pairs in TiO is responsible for
The other main drawback of TiO
In the recent past, there are photocatalyst in practical application is the lack of visible light
1
. Introduction
2
1,3,17
domestic wastes has become a serious concern and topic for lower quantum yield.
environmental remediation.
2
1–3
numerous reports on the photodegradation of organic dyes with utilization because of large bandgap. The reduction in recom-
semiconductor metal oxides of different morphology using bination of charge carriers and the catalytic activity under
4
,5
6,7
8
9
10–12
ZnO, CeO
2
,
WO
3
, CuO and Fe
2
O
3
.
Recently, graphene visible light can be improved by sensitizing the nano TiO
have also been metals and lesser bandgap semiconductors through adopting a
All these works mainly focus on the degradation of strategy of bandgap engineering. The lower bandgap semi-
2
with
composites with nanometal oxides such as TiO
reported.
2
13,14
selected single toxic organic dye under UV or visible light. But in conductor or metal particles act as sink/electron trap to the
practical situation, the effluent will be a mixture of many photogenerated charge carriers, which in turn aid in reducing
3,20–24
organic toxic wastes. Degradation of such industrial effluents the recombination rate of TiO
under sunlight is therefore a challenging aspect, which is the
need of an hour.
photocatalyst.
In this context, we herein report the synthesis of SnO
NT heterostructures with enhanced solar light degradation of
2
/TiO
2 2
Semiconductor photocatalysts have received a great deal of textile dye effluent, for the rst time to our knowledge. Various
interest for environmental remediation. Among the various techniques for 1-D nanostructure synthesis including template
2
5
26
semiconductor materials, TiO has been regarded as the most assisted growth, electric eld assisted growth and laser
promising photocatalyst
2
15–19
27
owing to its high photocatalytic induced template assisted growth have been reported. In this
2
activity, physical and chemical stability, non toxicity, low cost work, TiO NTs was prepared by simple cost effective hydro-
thermal method and then sensitized with SnO nanoparticles in
2
the form of SnO /TiO composites, in order to reduce the
2
2
a
Advanced Materials and Devices Laboratory (AMDL), Department of Physics,
recombination rate and to increase the photocatalytic degra-
dation efficiency. The results of present study demonstrated an
enhanced photodegradation efficiency could be obtained for
Bharathiar University, Coimbatore-641046, India
b
Defence Metallurgical Research Laboratory, Hyderabad-500058, India
c
Institute for Nanomaterials, Advanced Technologies and Innovation, Department of
2 2
Natural Sciences, Technical University of Liberec, Liberec 1, Studentsk a´ 1402/2, 461 the SnO /TiO NT heterostructure of the probe dye, methylene
1
7 Czech Republic
blue (MB) and the textile dye effluent under UV light, as well as
natural sunlight. The synthesis strategy such as those adoped in
d
Department of Nanoscience and Technology, Bharathiar University, Coimbatore-
41046, India. E-mail: rtrkumar@buc.edu.in
6
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this study for producing SnO
2
/TiO
2
NT heterostructure nano- was used as a source. Philips 130 W mercury vapor lamp and
composites might be a cost-effective procedure for industrial 100 W sodium vapor lamp were used as irradiation sources.
organic pollutant remediation.
Degradation experiments were performed under sunlight in the
month of July and August between 9 am and 4 pm. For effluent
degradation experiments, the textile dye effluent was collected
from a dying industry located at Tirupur, Tamil Nadu, India.
The collected effluent was a mixture of several industrial dyes
2
NTs was as and the concentration was about 400 ppm. The effluent was
2. Experimental
2
.1 Preparation of TiO nanotubes
2
The method employed for the synthesis of TiO
similar to those described by Kasuga et al. In a typical diluted with normal water and the concentration was brought
28
synthesis, 2 g of TiO powder was dispersed in 10 M of 75 mL down to 40 ppm. 50 mg of catalyst was mixed to 40 mL of
2
NaOH solution and stirred for 20 min. The contents were 40 ppm effluent and the pH of the solution was modied by
transferred into a Teon-lined stainless steel autoclave and adding a few drops of dilute HCl or few drops of dilute
ꢀ
placed in an hot air oven at 130 C for 12 h. The autoclave was NH OH. The degradation efficiency was calculated using the
3
naturally cooled and the resultant precipitate was washed equation:
several times with 0.1 M HCl solution and then with distilled
water. The precipitates were nally dried at 80 C for overnight.
C
0
ꢁ C
ꢀ
X ¼
ꢂ 100%
C
where X is the degradation efficiency, C is the initial solution
concentration of the dye and C is the solution concentration of
the dye aer degradation.
0
2
.2 Preparation of SnO
For the preparation of SnO
obtained from the above procedure was dispersed in 200 mL of
distilled water. Simultaneously, SnCl $2H O solution with
varying concentrations (5 and 10% of SnO
Further, 0.3 mL of HCl was added to the SnCl
2
/TiO
2
NT composites
2
/TiO
2
NT composite, 1 g of TiO
2
NTs
2
2
3
. Results and discussion
2
) was prepared.
solution for its
2
3.1 Structure and morphology
complete dissolution. The as-prepared SnCl2 solution was
Fig. 1 shows the XRD pattern of as-prepared TiO
diffraction peaks are consistent with the body centered tetrag-
onal structure of bulk TiO2 (JCPDS card no.: 89-4921). The
2
NT and the
mixed with the solution containing TiO NTs, which resulted in
2
a pale yellow coloured precipitate. The end products so
ꢀ
obtained was centrifuged, washed and dried at 80 C for
observed diffraction peaks at 24.7, 37.82, 48.08, 53.96, 62.7 and
29
overnight.
ꢀ
6
8.7 are corresponding to the planes of (1 0 2), (0 0 4), (2 0 0),
(
1 0 5), (2 0 4) and (1 1 6) of anatase phase of TiO2. The intense
2
.3 Characterization of SnO /TiO NT composites
2
2
ꢀ
ꢀ
peaks observed at 24.2 and 48.32 in Fig. 1c are attributed to
X-ray diffraction (XRD) patterns of TiO NT and SnO /TiO NT
composites were recorded with X-ray diffractometer using a
2
2
2
the anatase phase of TiO ; while the prominent peak identied
2
ꢀ
at 28.15 marked by * corresponds to the SnO
2
. The high
˚
ꢀ
wavelength of 1.5406 A by Bruker D8 advanced instrument.
intensity peak at 48.08 is broad, which shows the nanosize of
the as-prepared samples. Further, the crystallite size was
Both the surface morphology and composition of the samples
were studied by a 300 kV JEOL 3010 transmission electron
microscope (TEM). To investigate the light absorption proper-
ties of photocatalysts, UV-vis diffuse reectance studies were
carried out in the wavelength range of 200–800 nm using UV-Vis
spectrometer (JASCO-650). Photoluminescence (PL) spectra were
recorded with Horiba Jobin Yuon Fluoromax-4 Spectrouorom-
eter using an excitation wavelength of 300 nm. Time resolved PL
spectrum was recorded by an IBH time correlated single photon
counting (TCSPC) system and the decay prole was deconvo-
luted using IBH data station soware V2.1.
2.4 Evaluation of photocatalytic performances of TiO
2
NT
and SnO /TiO NT composites
2
2
The photocatalytic activity of the as-prepared samples was
investigated by irradiating them under both UV light and
sunlight using the following procedure. 30 mg of the catalyst
was mixed with 30 mL of 30 ppm MB and the contents were
stirred under dark for 10 min for sustaining adsorption–
desorption equilibrium. At constant intervals of time, 2 mL
solution was drawn and quantitative determination of MB was
performed by measuring the intensity of absorption peak using
2 2 2
Fig. 1 XRD patterns of (a) TiO NT, (b) 5% SnO /TiO NT and (c) 10%
UV-vis spectrophotometer. A UV lamp of wavelength 364 nm SnO
2
/TiO NT composites.
2
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calculated using Scherrer's formula for the high intensity peak 3.2 Optical properties
ꢀ
at 24.7 . The estimated grain size was found to be 3.13, 5.2,
3
.2.1 Photoluminescence analysis. PL spectra is oen used
1
0.6 nm for TiO NT, 5% SnO /TiO NT and 10% SnO /TiO NT,
respectively. The crystallite size of SnO nanoparticles, in the
case of 10% SnO
characteristic peak located at 28.15 .
Fig. 2a–c show the TEM images of hydrothermally synthe-
2 2 2 2 2
sized TiO NT, 5% SnO /TiO NT composite and 10% SnO /TiO
NT composites, respectively. It can be noticed that the as-
synthesised TiO exhibit nanotube morphology with diameter
2 2 2 2 2
to analyse the efficiency of charge carrier trapping and transfer
behaviour of the photo induced electron and holes.
intensity of the PL spectra reects the degree of the radioactive
recombination since the number of photon ux in to the
samples was same in all the samples. Fig. 3 shows the PL
2
30,31
The
2
/TiO
2
NT was calculated to be 3.6 nm for its
ꢀ
18
spectra of hydrothermally synthesized TiO2 NTs and TiO2/
SnO composites with the excitation wavelength of 300 nm.
2
2
The PL spectra of samples exhibits two emission peaks, one at
ꢃ375 nm and another between 450–500 nm. The emission
peak at ꢃ375 nm is due to free excitonic band to band emis-
sion of TiO2. The peak in the range of 450–500 nm is due to the
oxygen vacancies. The PL spectrum of hydrothermally
synthesized TiO
SnO /TiO NT composite. The PL intensity of 10% SnO /TiO
2
NT is lesser than the PL intensity of TiO , the PL intensity of
of 10–15 nm and length of 100–200 nm. In Fig. 2b, the tubular
structure of the TiO is clearly seen along with a small amount
2
of SnO
The TEM image of the 10% SnO
in Fig. 2c, apparently reveals the presence of numerous,
spherical SnO nanoparticles of size 3–5 nm along the nanotube
structures. The SnO nanoparticles decorated over TiO nano-
tubes for 5% SnO /TiO NT. For 10% SnO /TiO sample, SnO
2
nanoparticles with diameter in the range of 5–10 nm.
2
/TiO NT composite shown
2
2
is found with high intensity compared to the
2
2
2
2
2
2
2
2
2
2
2
2
5
% SnO /TiO NT is very less compared to the 10% SnO /TiO
NT and TiO
2
2
2
2
nanoparticles completely covered the TiO nanotube. The esti-
mated SnO particle size from the TEM analysis is in agreement
with the XRD results. The TEM analysis also reveal that the
2 2
increasing proportion of SnO particles with increase in SnO
concentration. The inserts in Fig. 2 show the selected area
electron diffraction (SAED) pattern of the respective sample,
which clearly demonstrates the polycrystalline nature of the
samples. The energy dispersive X-ray spectroscopy (EDS) was
performed for composition analysis and the atomic percentage
of each elements are tabulated in Table 1. The estimated Sn
contents are in consistent with the starting composition. For
2
2
NT.
2
3.2.2 Photoluminescence life time measurement. In order
to nd the life time of the excited charge carriers, time corre-
lated single photon counting (TCSPC) measurement was done.
Fig. 4 shows the PL decay curves recorded at 375 nm with the
the 5% SnO
for the 10% SnO
is no apparent SnO
patterns of 5% SnO
2 2
/TiO NT composite, the Sn content is 5.24%; while
2
/TiO
2
NT composite it is 8.43%. Though there
2
diffraction peak observed in the XRD
2
2
/TiO NT, the EDS analysis conrmed the
presence of Sn content in this sample.
2 2
Fig. 3 PL spectra of hydrothermally synthesized TiO NT and SnO /
TiO NT composites.
2
2
Fig. 2 TEM images of (a) hydrothermally synthesized TiO NT, (b) 5%
2 2 2 2
SnO /TiO NT composite and (c) 10% SnO /TiO NT composite. Insets
show their respective SAED patterns.
Table 1 Atomic percentages of the samples as revealed from EDS
analysis
Atomic percentage
Sample
Ti
O
Sn
TiO
5
1
2
NT
% SnO
0% SnO
15.31
13.38
19.44
84.68
81.36
72.11
—
5.24
2
/TiO
2
NT
NT
8.43 Fig. 4 PL Decay curves recorded at 375 nm with the excitation of
66 nm.
2
/TiO
2
2
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excitation of 266 nm laser source. The emission decay prole 3.3 Photocatalytic activity of TiO NT and SnO /TiO NT
2
2
2
was tted using triexponential function and the average life composites against methylene blue under UV irradiation
time value was calculated using the standard equation. From
the decay curves it can be found that the decay time increases
Decomposition of MB was studied under UV irradiation for TiO
NT, 5% SnO /TiO NT and 10% SnO /TiO NT. These three
2 2 2 2
2
with increase in the percentage of SnO2. The life time of TiO
NT, 5% SnO /TiO NT and 10% SnO /TiO NT was 29 ps, 30 ps
and 37 ps, respectively. The life time of 10% SnO /TiO NT is
higher compared to the 5% SnO /TiO NT and TiO NT.
.2.3 Optical bandgap analysis. Fig. 5 shows the represen-
2
photocatalysts were subjected to degradation of MB for a xed
time (30 min) and their overall degradation percentages were
determined. In Fig. 7 we show the degradation percentage as a
function of Sn content used in the SnO /TiO NT composite. It
2
2
2
2
2
2
2
2
2
2
2
3
can be noticed that the pure TiO2 NT has less degradation
efficiency compared to the composite samples. The degradation
efficiency is maximum for 5% SnO /TiO NT composites. The
2 2
degradation efficiency decreases as the amount of Sn content is
tative plot of Kubelka Munk function vs. energy. The bandgap of
the as-synthesized TiO2 NT and 10% SnO /TiO NT and
2
2
composite were estimated from the UV-vis diffuse reectance
spectra. The bandgap values are found to be 3.22, 3.19 and
increased to 10%. It is inferred from this experiment that the
3
.14 eV for TiO
composites, respectively, which indicate the minimal variation
in the bandgap occurred upon adding SnO
.2.4 Raman analysis. Fig. 6 shows the Raman spectra of
TiO2 NT and composite SnO /TiO NT before and aer dye
2 2 2 2 2
NT, 5% SnO /TiO NT and 10% SnO /TiO NT
5
% SnO
2
/TiO
2
NT composite has higher degradation efficiency
NT and 10% SnO /TiO NT composite. Accord-
than the TiO
2
2
2
2
.
ingly, TiO NT and 5% SnO /TiO NT composites were consid-
2
2
2
3
ered for further detailed dye degradation experiments.
Fig. 8 shows the degradation plots of TiO NT and SnO /TiO
2
2
2
2
2
degradation. Raman active modes for anatase TiO could be
2
NT composite. From the graph, the MB solution irradiated
under UV light has showed no signicant decrease in the
degradation. On the other hand, the degradation of MB by
ꢁ1
detected at 139, 194, 395, 514 and 634 cm for the photo-
catalyst before dye degradation. There was no signature for the
SnO
2
vibration modes in the composites. The Raman spectra
SnO
ciency than the pure TiO
the photocatalytic activity of 5% SnO /TiO NT composite is
2
/TiO
2
NT composite shows improved degradation effi-
taken aer dye degradation showed a shi in the vibration
2
NT. As expected from the PL spectra,
ꢁ1
modes to 144, 197, 392, 511 and 633 cm associated with a
peak broadening. This may be attributed to the increased
2
2
higher as compared to the TiO NT. The rate of degradation is
2
surface strain of TiO caused by the adsorption of dye
2
found to be more for the 5% SnO /TiO NT composite.
2
2
32
molecules.
Fig. 7 Degradation percentage of MB as a function of Sn content.
2 2 2
Fig. 5 Diffuse reflectance spectra of TiO NT and SnO /TiO NT
composites.
Fig. 6 Raman spectra of TiO
2
NT and composite SnO
2
/TiO
2
NT before Fig.
2 2 2
8 Photocatalytic activity of the TiO NT, TiO NT/SnO
and after dye degradation.
composite for the degradation of MB under UV irradiation.
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3
.4 Recyclability of TiO
composites
The recyclability of the photocatalytic performances of the as-
prepared TiO NT and SnO /TiO NT composite was subjected
2 2 2
NT and 5% SnO /TiO NT
2
2
2
to degrade 30 ppm of 30 mL MB for ve repeated cycles and the
results are shown in the Fig. 9. With the above experiments, a
reduction in the photocatalytic degradation efficiency of 4.91%
and 1.31% for the TiO2 NT and SnO /TiO NT composites,
2
2
respectively was obtained. This clearly indicates that the pho-
tostability of SnO /TiO NT composite is superior than the bare
Fig. 11 Photocatalytic activity of the TiO
composite for the degradation of MB under different intensities of
2 2 2
NT, TiO NT/SnO
2
2
2
TiO NT.
mercury vapour lamp irradiation. (a) 2000 lux and (b) 1000 lux.
2 2 2
3.5 Photocatalytic activity of TiO NT and SnO /TiO NT
composites against Methylene Blue under different visible
light irradiation
2 2 2
Fig. 10–12 show the degradation plots of TiO NT and SnO /TiO
NT composite under different irradiation sources (sunlight,
mercury vapour lamp and sodium vapour lamp) with different
intensities. From the above three gures, it is clear that our
samples are active under visible light. Comparing the degra-
dation efficiency under different sources, the samples actively
2 2 2
Fig. 12 Photocatalytic activity of the TiO NT and TiO NT/SnO
composite for the degradation of MB under different intensities of
sodium vapour lamp irradiation. (a) 1000 lux and (b) 500 lux.
degrade the dye under sunlight irradiation. In all the cases, the
samples under greater intensity showed higher degradation
efficiency. Under sodium vapour lamp irradiation the degra-
dation is found to be lesser compared to other sources. In all the
cases, 5% SnO
dation efficiency than the pure TiO
2
/TiO
2
NT composite showed improved degra-
NT.
2
2 2 2
Fig. 9 Recyclability study of TiO NT and SnO /TiO NT composite
against degradation of MB.
3
.6 Photocatalytic activity of TiO NT and SnO /TiO NT
2 2 2
composite against industrial dye effluent
The TiO NT and 5% SnO /TiO NT composite was subjected to
2
2
2
industrial dye effluent degradation. The degradation experi-
ments were initially carried at pH 7 and these experiments
showed that the degradation process was sufficiently poor. For
this reason, the pH of the effluent was modied from 1 (acidic)
to 14 (basic) by adding a few drops of either dilute HCl or
3
NH OH and then the degradation experiments were carried out
under UV light. Fig. 13 shows the photocatalytic degradation of
industrial effluent by TiO NT under UV light at different pH. It
2
can be observed that the effluent in the acidic pH (from 1 to 5)
gets degraded fast compared to the basic pH. In acidic pH, the
photocatalyst surface is charged negatively which adsorbs the
dye on its surface more and degrades it.
For this reason, the pH of the effluent was maintained at 1
for further experiments. The insert of Fig. 14a shows the
absorbance graph of the effluent irradiated to sunlight taken at
different time intervals. The characteristic absorbance peak of
effluent was observed at 579, 388 and 305 nm. During the
photocatalytic degradation process, because of the degradation
of the effluent the peak intensity is found to decrease gradually.
Fig. 10 Photocatalytic activity of the TiO
composite for the degradation of MB under different intensities of
sunlight irradiation (a) 90–120 K lux, (b) 50–70 K lux and (c) 35–50 K
lux.
2 2 2
NT, TiO NT/SnO
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Fig. 15 Schematic diagram demonstrating the mechanism of reactive
oxygen species production in SnO /TiO composite.
2 2
Fig. 13 Photocatalytic degradation of industrial effluent by TiO
2
NT
under UV at different pH.
2 2
the VB of SnO get transferred to TiO and reacts with OH to
form hydroxyl radicals. This is also supported from PL life time
measurements. The life time of composites increases compared
to TiO NT. The superoxide anions and hydroxyl radicals act on
2
19,24,31
the dye molecules and decompose them.
Fig. 16 shows the schematic diagram demonstrating the
possible scheme of the process taking place during photo-
degradation of the dye by TiO
2
NT, 5% SnO
2
/TiO
2
NT and 10%
NT and its
SnO /TiO NT composites. Fig. 16a depicts the TiO
2
2
2
possible scheme of degradation. In this case, because of high
charge carrier recombination, the number of electrons that
Fig. 14 Photocatalytic degradation of industrial effluent by TiO
and 5% SnO /TiO NT composites under (a) UV light and (b) sunlight.
2 2
Inset shows the absorbance of effluent taken at different time intervals.
2
NT contributing the production of reactive oxygen species is less.
Fig. 16b depicts the 5% SnO /TiO NT and its possible scheme
2
2
of dye degradation. Herein, the SnO
adhere onto the surface of the nanotube, during photo excita-
tion the electrons are promoted to the CB in both TiO and
are immediately transferred to
and thereby, reducing the recombination. Also the holes,
2
nanoparticles are found to
Fig. 14 a and b show the degradation plot of the industrial
effluent under UV and sunlight irradiation. From the plots, it
can be seen that for 5% SnO /TiO NT composite, the degra-
2 2
2
SnO
SnO
2
2
. The electrons in the TiO
2
which are generated in the SnO are transferred to the VB of
dation efficiency of the effluent is higher under both UV and
2
TiO and thereby involved in the production of OH radicals.
sunlight irradiation.
2
Fig. 16c depicts the 10% SnO /TiO NT and its possible scheme
2
2
of dye degradation. In this case, more number of SnO
2
3
.7 Mechanism of photodegradation
The schematic diagram explaining the mechanism of produc-
tion of reactive oxygen species in the SnO /TiO composite is
shown in Fig. 15. The conduction band (CB) edges of TiO and
SnO
are positioned at ꢁ0.34 V and +0.007 V relative to the
normal hydrogen electrode (NHE) at pH 7. The valance band
2
2
2
2
(
VB) edge of SnO
2
is at +3.67 V which is much less than that of
23
TiO
2
which is at +2.87 V.
The observed difference in the CB edges between the SnO2
and TiO2 makes the photogenerated electrons on the TiO2
surface to easily transfer to the CB of SnO
charge separation rate of the photogenerated charge carriers,
while the photogenerated holes accumulate on TiO due to their
2
. This improves the
2
higher VB level. The decrease in intensity of the PL signal in the
composite is attributed to the enhanced charge separation of
the photogenerated electron–hole pairs. Thus, the charges are
separated effectively.
The electrons in the TiO once excited to the CB gets easily
2
transfered to the CB of SnO
2
. These electrons react with the
Fig. 16 Schematic diagram demonstrating the degradation mecha-
NT, (b) 5% SnO /TiO NT and (c) 10% SnO /TiO NT.
environment and convert O
2
to superoxide anions. The holes in nism in (a) TiO
2
2
2
2
2
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Table 2 Degradation percentages of dye obtained by various photocatalysts
Concentration Volume/catalyst Degradation
ꢁ1
Photocatalyst
Target dye
Irradiation source
(mg L
)
(mL) : (mg)
(%) : (time)
References
33
Ag/TiO
TiO –SiO
AgBr/BiWO
BiWO /TiO
Au/TiO NT
TiO –WO
2
/BCD
MB
AO
MB
400 W UV lamp
10
15
50 : 10
— : 50
250 : 200
100 : 100
50 : 20
50 : 50
20 : 20
20 : 20
5 : 2
100 : 75 min
100 : 120 min 34
2
2
125 W HP Hg lamp ꢁ365 nm
6
500 W Xe arc lamp with UV lter 10
100 : 60 min
100 : 60 min
100 : 3 h
95 : 68 min
100 : 24 min
74 : 24 min
95 : 2.5 h
70 : 5 h
59 : 5 h
85 : 60 min
100 : 2.5 h
>90 : 0.6 h
35
36
37
38
3
6
2
RhB
MO
RhB
MO
MO
RhB
RhB
RhB
RhB
RhB
RhB
RhB
MB
160 W HP Hg lamp
500 W HP Xe lamp
Sunlight
20 W UV lamp
300 W Xe arc lamp
Sunlight
15
40
10
20
20
95
10
5
2
2
3
2
2
Ag
Ag
2
O/TiO
O/TiO
2
3
Anatase TiO
Heat treated anatase TiO
C-doped TiO
Pt/N–TiO NT
N–TiO NT
Graphene/TiO
2
39
40
41
42
43
44
2
Sunlight
50 : 50
2
500 W tungsten halogen lamp
1000 W Xe lamp
300 W Xe lamp
500 W Xe lamp
800 W xe lamp
30 W UV lamp
20 W UV lamp
100 : 100
150 : 200
100 : 100
50 : 15
200 : 100
100 : 50
40 : 30
2
10
3
2
2
NT
10
10
15
30
ꢃ40
TiO
2
NT
98.9 : 60 min 45
SnO
SnO
SnO
2
2
2
/TiO
/TiO
/TiO
2
2
2
100 : 20 min
100 : 45 min
100 : 6 h
46
NT
NT
MB
Present work
Present work
Textile effluent Sunlight
40 : 30
nanoparticles are sensitized on the surface of TiO2 NTs. reduced, which in turn enhances photocatalytic activity for the
Although the life time of 10% SnO /TiO NT is slightly higher as 5% SnO /TiO . The results presented in this study indicate that
2
2
2
2
compared to the 5% SnO
irradiated light may not be exposed to the TiO
nanotube is covered by the SnO nanoparticles thus there is a strated with good degradation efficiency in acidic pH. The SnO
decrease in photocatalytic degradation efficiency. For this TiO NT photocatalyst such as those produced in this study,
2
/TiO
2
NT, during photo excitation the the nanotube morphology of TiO
2
offers larger photocatalytic
2
NT since the reaction. The effluent degradation was successfully demon-
2
2
/
2
reason, the total numbers of photo excited charge carriers might be an candidate material for effluent degradation.
produced are very low in TiO NT. Furthermore, the holes that
2
are generated may not nd suitable path to produce OH radi-
cals, as the TiO2 NTs are capped by more number of SnO2
Acknowledgements
nanoparticle. This may be the reason for the observed low
degradation efficiency in the 10% SnO /TiO NT.
2 2
The authors KR and RTR acknowledge the funding granted by
the Department of Atomic Energy-Board of Research in Nuclear
Sciences (DAE-BRNS), Government of India. (Sanction no.2012/
The degradation efficiency of the present system are
compared with the earlier reports and are presented in Table 2.
It should be noted that the earlier studies have used high power
xenon and mercury lamps for the degradation of very low
concentration of dye with more amount of photocatalyst. In
contrast, in this study, we have used a high concentration dye
with less amount of photocatalyst to degrade 40 mL dye within a
duration of 1 h and the degradation of industrial dye effluent
was essentially carried out under sunlight. This apparently
20/34/6/BRNS). The authors KR and RTR also thank Dr P.
Ramamurthy, Director, National Centre for Ultrafast Processes,
Chennai, to conduct the time resolved photoluminescence
study. The authors VTPV and MC acknowledge the funding
granted by the Ministry of Education, Youth and Sports in the
framework of targeted support of ‘National Programme for
Sustainability I’, OPR&DI project (LO1201) and OP VaVpI of the
Centre for Nanomaterials, Advanced Technologies and Inno-
vation – CZ.1.05/2.1.00/01.0005.
2 2
shows that the degradation efficiency of SnO /TiO NT photo-
catalyst is superior than that of those observed in the previous
reports.
Notes and references
1
S. Ghasemi, S. Rahimnejad, S. Rahman Setayesh, S. Rohani
and M. R. Gholami, J. Hazard. Mater., 2009, 172, 1573–1578.
4
. Conclusions
We have successfully demonstrated the degradation of indus-
trial dye effluent under sunlight by means of SnO nano-
particles sensitized TiO NT. The addition of Sn contents with
TiO NT was varied from 0 to 10%. Among the investigated
samples, the 5% SnO /TiO NT composite showed increased
2 B. Merzouk, B. Church, A. Sekki, K. Madani, C. Vial and
M. Barkaoui, Chem. Eng. J., 2009, 149, 207–214.
3 W. Zhou, H. Liu, J. Wang, D. Liu, G. Du and J. Cui, ACS Appl.
Mater. Interfaces, 2010, 2, 2385–2392.
4 C. Tian, Q. Zhang, A. Wu, M. Jiang, Z. Liang, B. Jiang and
H. Fu, Chem. Commun., 2012, 48, 2858–2860.
2
2
2
2
2
degradation efficiency. The PL spectra of the 5% SnO /TiO NT
2
2
composite showed a decrease in intensity when compared to
the TiO NT, revealing that the rate of recombination is
5 K. S. Ranjith, R. Pandian, N. Gomathi, M. Kamruddin and
R. T. Rajendrakumar, Adv. Mater. Res., 2012, 584, 319–323.
2
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Paper
RSC Advances
6
7
J. Saranya, K. S. Ranjith, P. Saravanan, D. Mangalaraj and 26 P. Kumar, M. G. Krishna and A. K. Bhattacharya, Int. J.
R. T. Rajendra Kumar, Mater. Sci. Semicond. Process., 2014,
6, 218–224.
Y. Zhai, S. Zhang and H. Pang, Mater. Lett., 2007, 61, 1863–
866.
D. Chen and J. Ye, Adv. Funct. Mater., 2008, 18, 1922–1928.
Nanosci., 2008, 7, 255–261.
2
27 Z. Liu, Z. Cao, B. Deng, Y. Wang, J. Shao, P. Kumar, C. R. Liu,
B. Wei and G. J. Cheng, Nanoscale, 2014, 6, 5853–5858.
28 T. Kasuga, M. Hiramatsu, A. Hoson, T. Sekino and
K. Niihara, Langmuir, 1998, 14, 3160–3163.
1
8
9
S. Wang, H. Xu, L. Qian, X. Jia, J. Wang, Y. Liu and W. Tang, 29 F. Niu, Y. Jiang and W. Song, Nano Res., 2010, 3, 757–763.
J. Solid State Chem., 2009, 182(5), 1088–1093.
30 Q. Xiao, Z. Si, Z. Yu and G. Qiu, J. Mater. Sci. Eng. B, 2007,
137, 189–194.
31 A. Testino, I. R. Bellobono, V. Buscaglia, C. Canevali,
M. D'Arienzo, S. Polizzi, R. Scotti and F. Morazzoni, J. Am.
Chem. Soc., 2007, 129, 3564–3575.
1
1
1
1
0 S. Bharathi, D. Nataraj, D. Mangalaraj, Y. Masuda, K. Senthil
and K. Yong, J. Phys. D: Appl. Phys., 2010, 43(1), 015501.
1 L. Li, Y. Chu, Y. Liu and L. Dong, J. Phys. Chem. C, 2007,
111(5), 2123–2127.
2 S. K. Apte, S. D. Naik, R. S. Sonawane, B. B. Kale and 32 Q. Xiao, Z. Si, J. Zhang, C. Xiao, Z. Yu and G. Qiu, J. Mater.
J. O. Baeg, J. Am. Chem. Soc., 2007, 90(2), 412–414. Sci., 2007, 42, 9194–9199.
3 K. Gopalakrishnan, M. Joshi Hrushikesh, K. Prashant, 33 N. Attarchi, M. Montazera and T. Toliyat, Appl. Catal., A,
L. S. Panchakarla and C. N. R. Rao, Chem. Phys. Lett., 2011, 2013, 467, 107–116.
11, 304–308.
ˇ´
5
34 P. Sarka, J. Krysa, J. Jirkovsky, V. Prevot and G. Mailhot,
1
1
1
4 K. Zhou, Z. Yihua, Y. Xiaoling, J. Xin and L. Chunzhong, New
J. Chem., 2011, 35, 353–359.
5 P. Liu, W. Cai, M. Fang, Z. Li, H. Zeng, J. Hu, X. Luo and
W. Jing, Nanotechnology, 2009, 20, 285707.
J. Chem. Technol. Biotechnol., 2014, 89, 1129–1135.
35 D. Wang, G. Li, Y. Zhen, L. Yue, G. Xue and F. Fu, J. Mater.
Chem. A, 2014, 2, 11716–11727.
36 L. Zhi, X. Liu, D. Lu, P. Fang and S. Wang, Mater. Lett., 2014,
130, 143–145.
6 S. Indris, R. Amade, P. Heitjans, M. Finger, A. Haeger,
D. Hesse, W. Gr u¨ nert, A. B ¨o rger and K. D. Becker, J. Phys. 37 C. Y. Xu, P. X. Zhang and L. Yan, J. Raman Spectrosc., 2001,
Chem. C, 2005, 109, 23274–23278. 32, 862–865.
7 H. Chor, G. Zayani, M. Saadoun, L. Bousselmi and 38 L. Shanshan, J. Huang, L. Cao, J. Li, H. Ouyang, X. Tao and
B. Bessa ¨ı s, Appl. Surf. Sci., 2010, 256, 2170–2175.
C. Liu, Mater. Sci. Semicond. Process., 2014, 25, 106–111.
8 M. Yang, C. Hume, S. Lee, Y.-H. Son and J.-K. Lee, J. Phys. 39 A. Hosseinnia, M. Keyanpour-Rad and M. Pazouki, World
Chem. C, 2010, 114, 15292–15297. Appl. Sci. J., 2010, 8, 1327–1332.
9 N. Zhang, S. Liu, X. Fu and Y.-J. Xu, J. Phys. Chem. C, 2011, 40 J. Wang, L. Jing, L. Xue, Y. Qu and H. Fu, J. Hazard. Mater.,
15, 9136–9145. 2008, 160, 208–212.
0 E. Formo, E. Lee, D. Campbell and Y. Xia, Nano. Lett., 2008, 41 W. Ren, Z. Ai, F. Jia, L. Zhang, X. Fan and Z. Zou, Appl. Catal.,
, 668–672. B, 2007, 69, 138–144.
1 S.-H. Chien, Y.-C. Liou and M.-C. Kuo, Synth. Met., 2005, 152, 42 L. H. Huang, C. Sun and Y. L. Liu, Appl. Surf. Sci., 2007, 253,
33–336. 7029–7035.
2 W. S. Tung and W. A. Daoud, ACS Appl. Mater. Interfaces, 43 J. Fu, Y. Tian, B. Chang, F. Xi and X. Dong, J. Solid State
009, 1, 2453–2461. Chem., 2013, 199, 280–286.
3 L. Huang, F. Peng, H. Wang, H. Yu and Z. Li, Catal. 44 Z. Qianqian, B. Tang and H. Guoxin, J. Hazard. Mater., 2011,
Commun., 2009, 10, 1839–1843. 198, 78–86.
4 K. Nagaveni, M. S. Hegde and G. Madras, J. Phys. Chem. B, 45 C. Guo, J. Xu, Y. He, Y. Zhang and Y. Wang, Appl. Surf. Sci.,
004, 108, 20204–20212. 2011, 257, 3798–3803.
5 P. Kumar, M. G. Krishna, A. K. Bhatnagar and 46 L.-R. Hou, C.-Z. Yuan and Y. Peng, J. Hazard. Mater., 2007,
A. K. Bhattacharya, Int. J. Nanomanuf., 2008, 2, 477–495. 139, 310–315.
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