International Journal of Advanced Chemical Science and Applications (IJACSA)
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Sensitization of Carbon Doped Tin (IV) Oxide Nanoparticles by
Chlorophyll and Its Application in Photocatalytic Degradation of
Toluidine Blue
1
Meenakshi Singh Solanki, 2Rakshit Ameta, and 3SurbhiBenjamin
1,2,3
Department of Chemistry, PAHER University, Udaipur, 313003, Rajasthan, India
Email: [email protected], [email protected], [email protected]
[Received: 27th July 2015; Revised: 5th August 2015;
Accepted: 10th August 2015]
Abstract- Sensitized C/SnO2 nanoparticles were
prepared
by
precipitation
method
using
Combretumindicum plant leaves ethanolic extract.
The prepared sample was characterized by XRD,
FT-IR and TEM. The average crystalline size of
SnO2 nanoparticles was found to be 60.36 nm. The
photocatalytic activity of sensitized C/SnO2
nanoparticles was evaluated by treating aqueous
solution of toluidine blue dye. The optimum
conditions were obtained by varying pH of the
solution, concentration of dye, amount of
semiconductor, and intensity of light. The rate of dye
degradation was observed to be about 68% in 45
min. The reaction follows pseudo-first order kinetics
as confirmed by kinetics parameters.
Index Terms−Carbon doped SnO2, chlorophyll,
photocatalytic degradation, photosensitizer, toluidine
blue.
I. INTRODUCTION
The industries based on dye stuff have become an
essential need of developing society because these
provide products of great variety. Effluents of the dye
industries which contain organic and inorganic
impurities are discharged into water bodies without any
further treatment. This results in coloured, toxic and
contaminated water bodies leading to reduction in
sunlight penetration deep into the water and finally
diminishing photosynthesis and as a result, flora and
fauna are destroyed. This problem of contaminated
water can be effectively solved by photocatalytic
process that removes impurities even in range of ppb
and do not produce harmful side products [1].
The main problem is wide band gap of the
semiconductors. Therefore, it is important to synthesize
narrow band gap semiconductor that absorbs longer
wavelengths of the solar spectrum. It is very important
to utilize the energy of sun light for environmental
remediation such as degradation and decontamination of
organic pollutant. Many techniques have been attempted
to overcome the shortcomings like wide band gap,
colourless metal oxide, high recombination rate, etc.,
which restrict the usage of any photocatalyst.
Many non-metals such as carbon, nitrogen and sulfur
have been found to improve efficiency of a
photocatalyst, which influences the light absorption,
photoreactivity and morphology of semiconductor
photocatalysts [2]. Non-metals as dopant affect the
electronic structure and also results in red shift in the
adsorption spectra i.e. shifting wavelength from
ultraviolet into the visible light region and thus,
improving the utilization percentage of visible light[3].
Another method of improving activity of a photocatalyst
in broad range of spectrum is sensitization. Rochkind et
al. reported that sensitizer does not simply mean the
use of various dyes or pigments but a bare or undoped
metal oxide semiconductor may also exhibit selfsensitization
mechanism.
Narrow
band
gap
semiconductor coupled with wide band semiconductor
also serves as sensitizer[4]. Sensitizers are organic or
inorganic compounds, which are adsorbed on the surface
of semiconductor by chemisorption or physisorption and
help in improving excitation process. The sensitizers can
extend the range of excitation energies of the
photocatalyst into visible region. The process of coating
any sensitizer on the surface of semiconductor is known
as sensitization.
O’Regan and Gratzel used ruthenium complex as a
sensitizer on nanocrystalline TiO2 film [5]. Since then,
the sensitization of a semiconductor has been vastly
used in solar cells [6],[7], elimination of volatile
components [8], production of hydrogen by water
splitting [9], elimination of various organic pollutants
such as dyes [10], and phenol [11] etc. In this field, a
variety of chemical substances have been used like eosin
Y, aniline blue, bromophenol blue, alcian blue, methyl
orange, crystal violet, fast green, and carbolfuchsin[12],
ruthenium complex [13], anthocyanin, β-carotene,
chlorophyll, and curcumin [14]. Sensitizers basically
belong to the families of organic dyes, metal complexes,
and natural dye sensitizers.
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One of the major disadvanges of using organic dyes for
sensitization of the semiconductors is the slow
decomposition of organic molecule during progess of
photocatalytic degradation. Organic dyes spontaneously
undergo redox reactions. Natural dyes are proved to be
better alternative as senitizers because they are widely
and easily available, non-toxic, enironmental friendly,
safe, complelely biodegradable [15]. Natural sensitizers
like anthocyanin, carotenoides, chlorophyll and
flavonoids may be extracted from various parts of plants
such as leaves, fruits, and flowers [16]. Butea
monosperma[17], Amaranthuscaudatus, Bougainvillea
spectabilis,
Delonixregia,
Nerium
oleander,
Spathodea companulata[18], Nerium oleander and
Hibiscus[19] have been used as sensitizers. Yan et al.
have extracted natural pigment anthocyanin extracted
from red radish and sensitized titania and this was used
for treatment of waste water under visible-light
exposure. It showed good activity for degradation of
some dyes and other organic pollutants like phenol
under visible light illumination because of the decrease
in band gap and red shift of absorption [20].
Limited work has been done using natural pigment as a
photosensitzer for waste water treatment. In the present
work, crystalline nanoparticles of sensitized carbon
doped SnO2 were synthesized by precipitation method.
The nanoparticles of semiconductor lead to increase in
the surface/volume ratio and thus, reduction in particle
size [21]. This property has found application in
photocatalysis because nanosized particles reduce the
probability of undesired recombination of electron-hole
pairs before they reach to the surface of the
semiconductor by decreasing the pathway from the place
of generation of the electron-hole pair to the
semiconductor surface [22].
when white precipitates were observed. 1.0 N NaOH (as
a precipitation agent) was added drop wise into the
stirred solution until the pH was maintained at 8.0. Then
solution was again stirred for 30 min. and kept
overnight. The white precipitate of SnC2O4 was
obtained.
The transparent supernatant of SnC2O4 solution was
decanted. 50 g dextrose glucose was added as a carbon
precursor to this wet precipitate and under magnetic
stirrer for 1 hour. The precipitate was filtered and
washed 6-7 times using distilled water for complete
removal of chloride ions. Then precipitate was dried in
oven at 70˚C-80˚C till it gets completely dried. Finally,
dried compound was calcined in a muffle furnace at
800˚C for 1 hour to obtain carbon doped SnO2. The
calcined powder was ground vigorously for 20 min. with
an agate mortar and pestle.
100 mL ethanolic extract was mixed with carbon doped
SnO2 and was stirred using a magnetic stirrer for 5
hours. Sensitized SnO2 was filtered using Whatmann
filter paper No. 1 and washed with flowing distilled
water to remove any component of extract. Sensitized
photocatalyst was dried in an oven and powdered with
an agate mortar and pestle. The C/SnO2/Chlorophyll
sample was kept in dark for further use. Fig. 1 shows the
step by step process.
II. EXPERIMENTAL
A. Preparation of Semiconductor
i) Extraction of Natural Sensitizer
Extraction of pigment was done by a simple method.
Fresh leaves of plant Combretumindicum (Madhumalti)
was blended using electric blender, then 90% ethanol
was added and blending was continued. Extract was
filtered using Whatmann filter paper No. 1 and was
collected in dark bottle and kept overnight. The prepared
natural sensitizer solution was kept at ambient
temperature, without exposure to direct sunlight. The
chlorophyll (pigment) extracted was used in sensitizing
the C-SnO2 nanoparticles.
ii) Preparation of Sensitized Carbon Doped SnO2
Nanoparticles
Fig. 1: Preparation of semiconductor
B. Charaterization
500 mL solution of tin (II) chloride (0.08 M) and 500
The XRD patterns of the samples were obtained by XmL solution of oxalic acid dihydrate (0.16 M) were
ray diffraction (XRD, PanlyticalX’pert Pro model). The
prepared separately. Oxalic acid dehydrate solution was
Cukα radiations wavelength (λ) was 0.154060 n,
added slowly into aqueous solution of tin (II) chloride
operated at 40 mA and 40 kV and secondary
with continuous stirring using a magnetic stirrer at room
monochromator in the 2θ range from 20˚ to 80˚. The
temperature till a clear and homogeneous solution was
average crystalline size (D) of the prepared
obtained. Solution was continuously stirred for 30 min.
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International Journal of Advanced Chemical Science and Applications (IJACSA)
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photocatalyst was calculated from XRD peaks by using
(1) Debye-Scherrer’s equation:
1
1 + log A
0.8
(1)
When D is an average particles size in nanometer, k is a
correction factor taken as 0.94 (for spherical shape), λ is
a wavelength of Cu target-Kα X-ray radiation (0.1541
nm), β is a broadening of diffraction line measured at
half of its maximum intensity (FWHM in radian), and θ
is a Bragg’s diffraction angle (in degree).
The infrared spectra were recorded using FT-IR
spectrophotometer (FT-IR, Perkin-Elmer Spectrum RX1
Spectrophotometer in KBr pellets, in range 4400 cm-1 to
450 cm-1 with scanning rate of 1 cm-1 min-1.
C. Photocatalytic Experiments
The photocatalytic activity of the sample was examined
by photodegradation of water soluble chemical
contaminant dyestuff, toluidine blue. The stock solution
1.0 × 10-3 M was prepared in double distilled water and
diluted as required to prepare working solutions. Visible
light irradiation was done from a 200 W tungsten lamp.
The light intensity was measured with the help of a
solarimeter (CEL, Model SM 201). A water filter was
used between light source and solution, to cut off the
thermal radiation (eliminate infrared fraction). The pH
of the solution was determined by a digital pH meter
(Systronic Model 335) and pre-standardized solutions of
0.1 N sulfuric acid and 0.1N sodium hydroxide were
used to adjust pH.
Absorbance of reaction mixture was determined using
UV–vis spectrophotometer (Systronics Model 106) at
the maximum absorption wavelength (λmax = 630 nm) of
toluidine blue. Reaction mixture was exposed to visible
light and about 3 mL of reaction mixture was taken out
extracted every 10 min. and transferred to a
poly(methylmethacrylate) (PMMA) cuvette for analysis
of absorbance (A) using UV–vis spectrophotometer. It
was observed that absorbance of the solution decreases
with increase in time of irradiation, which shows that
toluidine blue is degraded. A plot of 1 + log A
(absorbance) versus time was linear. The rate of the
reaction (k) was determined by using equation (2) and
the results are represented in Fig. 2.
0.6
0.4
0.2
45
40
35
30
25
20
15
10
5
0
0
Time (min)
Fig. 2: Typical run
III. RESULTS AND DISCUSSION
A.
Characterization
X-ray diffraction pattern of the C/SnO2/Chlorophyll
sample is shown in Fig. 3. Average particle size of the
crystalline powders was calculated by Deby-Scherrer
equation as about 60.36 nm.
Fig. 4 shows the FT-IR spectrum of C/SnO2/Chlorophyll
nanoparticles in the range 400-4000 cm-1. The formation
of SnO2 was confirmed by characteristic vibration mode
of Sn-O at 580 to 680 cm-1[23]. The peak at about 850
cm-1 corresponds to the O-Sn-O bond stretching mode
[24]. The δ(Sn-OH) vibration is shown by a peak at
about 1115 cm-1 due to tin oxychloride [25]. The H2O
deformation exhibits a peak at about 1640 cm-1and OH
stretching mode was located in the range of 3700-2000
cm-1[26]. The infrared absorption bands of chlorophyll
pigment are shown in Table I [27].
Table 1: FT-IR data for C/SnO2/Chlorophyll
Absorption frequency (cm-1)
Bond
~ 3600-3850
N-H
~ 3400
O-H
~ 2800-2900
C-H
~ 1750 (low intensity)
Ester
~ 1634
C=C
~ 1063
C=N
(2)
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B. Effect of pH on the Photocatalytic Activity
9.5
9.0
8.5
8.0
7.5
7.0
6.5
6.0
7
6
5
4
3
2
1
0
5.5
k × 104 (s-1)
Fig. 3: XRD spectrum of C/SnO2/Chlorophyll
nanoparticles
The pH of the solution affects the rate of degradation of
dye solution. It was in the pH range 5.5-9.5. It was
observed that on increasing pH of the solution, rate of
reaction increases up to 9.5 and rate becomes maximum
(Fig. 6). This behavior may be explained on the basis
that the rate of reaction increases due to increase in the
availability of reducing species O2ˉ•. This reducing
species is formed as the result of reaction between O2
molecule and electron (eˉ) of photocatalyst. Thus, the
rate of the photocatalytic degradation of the dye
increases with increasing pH of the medium as more
O2ˉ• are formed. Beyond pH 9.5, rate of dye degradation
was so fast that it could not be determined by simple
volumetric technique.
pH
Fig. 6: Effect of pH
Fig. 4: FT-IR Spectra for C/SnO2/Chlorophyll
nanoparticles
Structural characterization of the sensitized C/SnO2 was
also done by TEM photograph Fig. 5. It was found that
the precipitation method leads to formation of spherical
nanoparticles. It was observed that the sensitized carbon
nanoparticles appears as slight grey transparent outer
spherical layer and the center of the sphere represents
the Sn of SnO2 nanoparticles as a dark patch.
C. Effect of Dye Concentration
The effect of dye concentration on the photocatalytic
degradation was investigated in the range of 1.50 × 10-5
to 2.40 × 10-5 M. The results are reported in Fig. 7.
At less concentration of dye, less molecules of dye were
present, but as the concentration of the dye was
increased, more dye molecules were available for the
excitation and energy transfer and hence, an increase in
the rate of degradation of the dye was observed. But
after optimum condition (2.10 × 10-5 M), the rate of the
photocatalytic degradation showed a decline on further
increase in the dye concentration, as the dye itself will
start acting as a filter for the incident irradiation. At
higher concentration, it will not permit the desired light
intensity to penetrate the solution and reach to the
surface of the semiconductor and as a result, decrease in
the number of the excited dye molecules finally leads to
a decline in the degradation rate [28]. Increase in dye
concentration may also lead to absorption on the surface
of semiconductor, which reduces active sites. Therefore,
generation of hydroxyl radicals decline and as a
consequence, rate of reaction decreases [29].
Fig. 5: TEM Image of C/SnO2/Chlorophyll nanoparticles
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27
semiconductor surface per unit time. The maximum rate
of degradation was observed at 60 mWcm-2 in case of
toluidine blue. After 60 mWcm-2, there was a slight
decrease in the rate of reaction. This may be due to some
side reactions or thermal effects.
7
6
5
4
3
2
1
0
[Toluidine blue] × 105 M
k × 104 (s-1)
8
1.5
1.6
1.7
1.8
1.9
1.0
2.1
2.2
2.3
2.4
k × 104 (s-1)
International Journal of Advanced Chemical Science and Applications (IJACSA)
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6
4
2
0
Fig. 7: Concentration of dye
20 30 40 50 60 70
D. Effect of Photocatalyst Dosage
Intensity of light (mWcm-2)
Fig. 9: Intensity of light
F. Mechanism
The photosensitization of doped semiconductor by
chlorophyll pigment involves four steps. These steps are
excitation of chlorophyll pigment, electron injection into
semiconductor, electron scavenging and mineralization
of dye. The photocatalytic degradation process begins
with electron injection step. In this step, chlorophyll
molecules (Chl.) absorb light of suitable wavelength,
which results into excitation of electron from HOMO to
LUMO and thus, electron-hole formation takes place. In
second step, the excited electron is transferred into the
conduction band of the semiconductor. Now these
electrons will be abstracted by dissolved oxygen in
aqueous solution to form superoxide (O2−•) anion
radical. Sidewise, an electron is transferred from dye
toluidine blue to chlorophyll converting it into its
cationic radical. The superoxide generates •HO2 on
reacting with H+ present in the solution. Hence, both
these agents, O2−• and •HO2, are responsible for
mineralization or photodegradation of dye and/or its
cationic radical.
0.16
0.14
0.12
0.1
0.08
0.06
0.04
7
6
5
4
3
2
1
0
0.02
k × 104 (s-1)
The rate of photodegradation of dye is influenced by
varying amount of semiconductor. According to the L-H
model, the rate of photodegradation increases as the
amount of photocatalyst increases because more active
sites are available for adsorption. But increase in rate
was observed only upto 0.06 g. Above this optimum
condition, there was a slight decrease or almost constant
rate of degradation of dyes was noticed. The reduction
in photodegradation rate may be explained on the fact
that after optimization, an increase in the amount of
semiconductor only increases the thickness of the
semiconductor layer and not the exposed surface area.
This may be also due to the fact that excessive amount
of semiconductor may cause some hindrance and
blocking of the light penetration or deactivation of
activated photocatalyst takes place due to collision with
ground state photocatalyst [30]. The effect of amount of
semiconductor in the range of 0.02 to 0.16 g has been
reported in Fig. 8.
Amount of semiconductor (g)
Fig. 8: Effect of amount of semiconductor
E. Effect of Light Intensity
The effect of light intensity on the photocatalytic
degradation of dye was studied by varying the distance
between the light source and exposed surface area of
semiconductor. The results are reported in Fig.9. The
results showed that photocatalytic degradation of dye
was accelerated as the intensity of light was increased as
any increase in the light intensity will increase the
Figure 8: A tentative mechanism
number of photons striking per unit area of
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Where SC is carbon doped SnO2, Chl. is chlorophyll
pigment and TB is a toluidine blue.
titanium dioxide based dye sensitized solar cells,”
Spectrochim. ActaA Mol. Biomol.Spectrosc.,
vol. 128, pp. 420-426, July 2014.
IV. CONCLUSION
The rate of the photocatalytic degradation of toluidine
blue is increased by using chlorophyll as a
photosensitizer from Combretumindicum plant leaves.
The photodegradation was observed higher in
chlorophyll sensitized carbon doped SnO2 as compared
to pure SnO2. The rate of degradation was observed at
4.23 × 10-4 s-1 and 68.30% degradation of toluidine blue
was achieved in 45 min.
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V. ACKNOWLEDGMENT
The authors express their sincere thanks to Prof. Suresh
C. Ameta, Dean, Faculty of Science, for his guidance
and support. PAHER University, Udaipur for providing
necessary laboratory facilities and also are thankful to
Sophisticated Analytical Instrument Facility (SAIF),
Panjab University, Chandigarh for FT-IR,TEM, and
XRD data.
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ISSN (Print):2347-7601, ISSN (Online): 2347-761X, Volume -3, Issue -3, 2015
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