N-doped, S-doped TiO2 nanocatalysts: Synthesis, characterization and photocatalytic activity in the presence of sunlight

Article abstract of DOI:10.1039/c4ra08858a

N doped and S doped nano TiO₂ catalysts were synthesized by a sol-gel process followed by hydrothermal treatment at low temperature and tested for catalytic activity by natural sunlight photocatalytic degradation of a toxic chemical warfare agent. It is observed that sulfate groups were anchored on the surface of titania upon doping, and also create active surface oxygen vacancies, both of which are responsible for sunlight absorption and the promotion of electrons to the conduction band. The formation of a superoxide radical (O₂^-) and hydroxyl radicals may be mainly responsible for the photodegradation of sulfur mustard under sunlight.

Full text of DOI:10.1039/c4ra08858a

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N-doped, S-doped TiO nanocatalysts: synthesis,
2
characterization and photocatalytic activity in the
Cite this: RSC Adv., 2015, 5, 37096
presence of sunlight†
a
a
b
P. V. R. K. Ramacharyulu,* Dipak Bapurao Nimbalkar, J. Praveen Kumar,
b
a
G. K. Prasad* and Shyue-Chu Ke*
2
N doped and S doped nano TiO catalysts were synthesized by a sol–gel process followed by hydrothermal
treatment at low temperature and tested for catalytic activity by natural sunlight photocatalytic degradation
of a toxic chemical warfare agent. It is observed that sulfate groups were anchored on the surface of titania
upon doping, and also create active surface oxygen vacancies, both of which are responsible for sunlight
Received 18th August 2014
Accepted 13th April 2015
absorption and the promotion of electrons to the conduction band. The formation of a superoxide
DOI: 10.1039/c4ra08858a
ꢀ
radical (O
2
c ) and hydroxyl radicals may be mainly responsible for the photodegradation of sulfur
www.rsc.org/advances
mustard under sunlight.
large electronegativity differences between S, N, and O atoms
were found to assist the formation of TiO1ꢀx or TiO1ꢀx
Recently there has been growing interest in visible light active structure which led to the shi of threshold wave length to
1
. Introduction
S
x
N
x
6,7
photocatalyts and their applications in environmental remedi- visible light region. Asahi et al. reported theoretical calcula-
ation and also for the preparation of organic compounds that tions of the band structure of nitrogen doped TiO and
2
are difficult to prepare by alternative methods. The visible light studied photocatalytic degradation of acetaldehyde and
8
active photocatalyst effectively utilizes sunlight and promises methylene blue with irradiation of visible light. They found
widespread applications related to degradation of toxic chem- that nitrogen atoms substituted the lattice oxygen sites and
icals with low energy consumption. TiO
nontoxic, and highly effective photocatalyst, has been widely Similarly, several other groups investigated the photocatalytic
used for degradation of toxic chemicals in light irradiation. properties of nitrogen doped TiO powders and thin lms
2
, an economical, narrowed the band gap due to mixing of N2p and O2p states.
2
9–13
However, TiO , because of its large band gap value 3.2 eV, can prepared by different methods. Hashimoto et al. reported the
2
only be activated by 3–4% of UV light fraction of the whole photocatalytic decomposition of 2-propanol. They found that
radiant sunlight. In order to use sunlight effectively, TiO
2
needs decomposition of propanol decreased with increased N dopant
with metal cations or content on TiO when irradiated with visible light. This was
main group elements was observed to induce extension of the attributed to an increase of oxygen vacancies, which promoted
1
to be modied further. Doping of TiO
2
2
2,3
absorption spectrum into visible light region. Although tran- the recombination of electrons and holes with increased dopant
6
sition metals exhibited the desired red shi of absorption concentration.
maxima, many of them were also found to act as recombination
centers.
Umebayashi et al. synthesized S doped TiO
2
, and used it for
4
the studies on photocatalytic degradation of methylene blue
14–16
It was reported that narrowing of band gap of TiO
achieved by using non-metal species instead of metal ions. was doped as an anion and replaced the lattice oxygen in TiO
Sulfur or nitrogen substitutional doping on TiO
was found to On the other hand, Ohno et al. found that S atoms were incor-
be effective for reducing the band gap values. The S or N atoms porated as cations and replaced Ti ions in the S doped TiO
occupy the vacant sites of either titanium ion or oxygen in the It is interesting that S atoms unlike other non metal dopants
2
could be under visible light irradiation.
They observed that sulphur
.
2
2
17–19
.
2
2
ꢀ
4+
titania lattice. Subsequently, the mixing of S3p states with val- could exist in more than one oxidation state such as S , S or
6
+
5
ance band was found to contribute to band gap narrowing. The
S
2
on TiO depending on the conditions of synthesis or the type
of sulfur precursors. Irrespective of sulfur oxidation state, the
1
8,20
photocatalytic activity was found to be enhanced.
a
Department of Physics, National Dong Hwa University, Hualien 974-01, Taiwan.
0
Sulfur mustard (C H SCl , 2,2 -dichloro diethyl sulde, HD)
4
8
2
E-mail: ramamsc2007@gmail.com; ke@mail.ndhu.edu.tw
is a vesicant, acts as a blister agent, alkylates DNA, and causes
b
Defence Research & Development Establishment, Jhansi Road, Gwalior 474 002, MP,
fatality to cells. Degradation of HD is a difficult process from the
India. E-mail: gkprasad2001@gmail.com
21
mechanism point of view. Its photochemical decomposition
†
Electronic supplementary information available. See DOI: 10.1039/c5ra08858a
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in the presence of visible light suggests the possibility of the use anatase phase (JCPDS 21-1272). CT nanocatalyst also exhibited
of sunlight for degrading HD. anatase phase and the respective XRD data was already
Recently, Prasad and co-workers have studied photocatalytic reported.
degradation of sulfur mustard using various catalysts like TiO , The crystallite sizes of synthesized nanocatalyst materials
is were calculated by Scherrer equation. Crystallite size of NT and
2
22a–d
ZnO, MnO
2
, V-TiO
2
.
Each has its own disadvantages TiO
2
visible inactive, it absorbs only UVA light in solar spectrum. ZnO HT nanocatalysts varied in between 6 to 10 nm. ST nanocatalyst
absorbs some of the visible light but it takes more time for showed a crystallite size of 4.8 nm. CT nanocatalyst showed
degradation. V-TiO
2
absorbs both UVA & visible light in solar crystallite size of 35 nm. XRD data shows similar peak patterns
4
+
spectrum but metal ions can act as recombination centres. for ST, HT, and NT nanocatalysts. The S substituted for Ti
However, Cojocaru et al. studied photocatalytic degradation of hinders the growth of TiO , therefore, the particle size of S-
2
CWA like soman, VX, and yperite on activated carbon supported doped TiO2 photocatalysts get smaller with the increasing
23
with N doped and undoped TiO2 of large particle size.
However, there are no reports available in literature on photo-
catalytic degradation of HD using S or N doped nano TiO
presence of sunlight. The present paper reports the synthesis of shape that have been embedded in the agglomerates with sizes
N doped TiO (NT), S doped TiO (ST) nanocatalysts by sol–gel varying around 4.8 to 10 nm.
method followed by hydrothermal treatment. This synthesis is XPS spectra of ST nanocatalyst are shown in Fig. 3(a). ST
eco-friendly as use of toxic precursors are avoided. The nanocatalyst shows a peak at 167 eV (S2p) depicting the pres-
sulfur doping.
TEM images of HT, ST, and NT nanocatalysts are presented
2
in the in Fig. 2(a–c). TEM images indicate the particles of spherical
2
2
4
+
synthesized materials were characterized by using X-ray ence of S ion in its bulk. In addition to this, a shoulder peak at
diffraction (XRD), transmission electron microscopy (TEM), X- 168 eV (S2p) and a broad peak at 169 eV (S2p) indicate the
6
+
ray photoelectron spectroscopy (XPS), nitrogen adsorption presence of S ion in the same and the data is consistent with
1
8
24
(
BET), UV-vis absorbance spectroscopy, and electron para- previously reported data. G. Yang et al. demonstrated that
2
ꢀ
magnetic resonance spectroscopy (EPR) techniques. The pho- there are two possible coordination models between SO
4
and
2 2
tocatalytic degradation reactions of HD were studied in sunlight TiO as shown in ESI.† The formation of cationic S-doped TiO
using gas chromatography (GC) and the resulting products were could create a charge imbalance in the lattice of catalyst, and
analysed using gas chromatography mass spectrometry (GC- the extra positive charge is probably neutralized by the
MS). The obtained data on doped materials was compared hydroxide ions.
with that of undoped (HT) and commercial TiO2 (CT) nano-
catalysts of anatase phase.
XPS spectra of NT nanocatalyst are shown in Fig. 3(b). NT
nanocatalyst shows peaks at 398.3 (N1s), 399.7 (N1s), and 401.3
eV (N1s) indicating the presence of O–Ti–N, N–O–Ti, and O–N–
Ti–O structures in the bulk of same. It also shows peaks at 459.6
(Ti 2p3/2), 464.9 (Ti 2p1/2), 531, 532 eV (O1s), 284.6 eV (C1s) in the
same (Fig. 3(b)) and the data is consistent with previously
2. Results and discussions
2.1. Characterization of the ST, NT, HT nanocatalysts
8,9
reported data. Where, HT nanocatalyst shows peaks 459.6,
Fig. 1 shows the XRD patterns of ST, NT, and HT nanocatalysts.
ꢁ
XRD patterns of all the nanocatalysts exhibit peaks at 25.2 ,
ꢁ ꢁ ꢁ ꢁ
3
7.8 , 48.0 , 53.8 , and 62.7 2q values which are characteristics
of (1 0 1), (0 0 4), (2 0 0), (1 0 5), and (2 1 5) indices. The data
conrmed the formation of ST, HT, and NT nanocatalysts with
Fig. 1 XRD patterns of NT, ST, and HT nanocatalysts.
Fig. 2 TEM images of (a) NT, (b) ST, and (c) HT nanocatalysts.
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have controlled the nucleation and growth of titania particles
and facilitated the formation of some new mesopores which
increased the surface area and mesopore volume and the same
can be seen in Table 1 and Fig. 4. Compared with undoped TiO2,
the particle size of the S-doped TiO samples is much smaller,
2
resulting in larger specic surface area. Small particle size can
shorten the route for an electron migration from the interior of
+
TiO
2
to surface, which can reduce the recombination of h and
ꢀ
2ꢀ
e . Moreover, the larger the surface area, the more SO
4
it has.
2ꢀ
The SO4 adsorbed on the surface of TiO can trap photo-
2
ꢀ
induced electrons (e ), improving the quantum yield.
Fig. 5 shows the UV-vis spectra of the undoped, N-doped and
S-doped TiO
shoulder from 400 nm to the visible light region are observed
for the N and S-doped TiO . These results reveal that the
nitrogen and sulfur dopants are on the surface of TiO but not
. It might have happened due to formation
2
powders. Very minute shis of the absorbance
2
2
in the lattice of TiO
of N atoms could have occupied interstitial sites in oxygen
vacancies in N-TiO . In S-TiO either S or S ions (SO4
2
ꢀ
4
+
6+
2ꢀ
)
2
2
could have anchored on TiO . To understand the dopant
2
ꢁ
localised state, the samples were annealed at 450 C in Ar
atmosphere, noticable shis in the visible region were observed
2 2
Fig. 3 (a) XPS spectra of ST (S-TiO ). (b) XPS spectra of NT (N-TiO ).
(ESI†). Dopants exists as localised states above the valence band
thus altering its crystal and electronic structures.
Electron paramagnetic resonance spectra were recorded
under full-light illumination at 77 K. EPR signals assigned
4
64.9, 531, 532, 284.6 eV depicting Ti2p3/2, Ti 2p1/2, O1s, C1s
17–19
respectively.
The unmarked peaks in the spectra belong to Ti
3+
photogenerated electrons trapped in anatase lattice Ti (gt
¼
2s, OKLL & Ti LMM respectively as reported in the literature.
ꢀ
1
2
.990 and g ¼ 1.960) and hole on the anatase surface Oc (g ¼
k
1
Fig. 4 shows the nitrogen adsorption data of HT, ST, NT and
.016, g
2
¼ 2.012 and g ¼ 2.003) can be seen in Fig. 6. Excited
3
CT nanocatalysts. All the nanocatalysts exhibit type IV adsorp-
tion isotherm typical of mesoporous materials with H3 type
hysteresis. Isotherms also indicate the presence of slit type of
pores formed due to aggregation of TiO
doped or undoped. When compared to HT (230 m g ), ST
electrons get trapped by conduction band defect site to produce
3+
Ti signal and the remained hole in the valence band which
2 2
can react with O , H O and hydroxide ion to from hydroxyl
2
nanocatalysts either
25
radical.
2
ꢀ1
The trapped electrons and holes is as given below.
2
ꢀ1
nanocatalyst exhibited slightly greater surface area (290 m g
)
+
ꢀ
which can be ascribed to the presence of relatively more
number of mesopores in ST nanocatalyst. S atom was found to
h + OH / cOH
(1)
(2)
(3)
(4)
2
2 2
cOH / H O
+
ꢀ
2
+
H O + h / O + 2H
2
2
2ꢀ
+
O
+ h / O
2
As the samples were prepared at low temperature and less
percentage of N and S doping sources in TiO , Nc–hole signal
2
was not observed in N-TiO . This infers that N and sulfur were
2
on the surface but not inside the lattice.
2.2. Photocatalytic degradation of HD
Photocatalytic activity of ST nanocatalyst was examined by
monitoring the degradation of HD in the presence sunlight
irradiation. There was no obvious change of concentration of
HD under the sole treatment of sunlight even aer 6–8 h of
irradiation. It is evident that, photodegradation of HD under
the sole treatment of sunlight is not a signicant process as it
could not absorb sunlight above 290 nm. Further, there was no
signicant loss of HD due to evaporation or degradation owing
Fig.
4 Nitrogen adsorption isotherms of HT, ST, NT, and CT
nanocatalysts.
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Table 1 Nitrogen adsorption data of HT, ST, NT, and CT nanocatalysts
2
ꢀ1
ꢀ1
ꢀ1
Type of nanocatalyst
BET surface area (m g
)
Total pore volume (mL g
)
Meso pore volume (mL g
)
HT
ST
NT
CT
230.4
290.3
243.3
75.3
0.50
0.54
0.54
0.29
0.07
0.09
0.08
0.09
showed much higher photocatalytic activities under sunlight
than HT nanocatalyst. Under sunlight irradiation, ST nano-
catalyst, NT nanocatalyst, and undoped nano TiO
2
degraded
100% of HD in 120 min, 180 min, 240 min respectively. CT
nanocatalyst took 420 min for 100% degradation of HD as
observed from Fig. 7. Irrespective of catalyst, similar products
were observed on doped (NT, ST) and undoped (HT, CT)
nanocatalysts. On the whole, higher sunlight assisted photo-
catalytic degradation activity of S or N doped TiO nanocatalyst
2
could be attributed to the synergetic effects of strong absorption
in the visible light region due to red shi of adsorption edge;
and enhanced separation of charge carriers. Size of crystallites,
2
large surface area of the S doped nano TiO also facilitated this
sunlight assisted photocatalytic degradation of HD. Relatively
higher activity of ST nanocatalyst can be attributed to the
highest absorption efficiency. The sulfur dopant was found to
2 2 2
Fig. 5 UV-vis absorbance of TiO , N-TiO and S-TiO nanocatalysts.
4
+
6+
2ꢀ
exist as either S or S (SO4 ) and substituted titanium ion as
observed from XPS data. The dopant sites seemed to have
facilitated enhanced trapping of electrons. The size difference
of nitrogen and oxygen atoms is very less and hence the
nitrogen dopants seemed to have occupied oxygen interstitial
sites. This could also be a reason for enhanced activity of ST
catalyst relative to NT nanocatalyst. ST and NT nanocatalysts
rd
were observed to be deactivated aer 3 use due to poisoning of
nanocatalysts through formation of alkoxy species, H SO and
2
4
2 2
Fig. 6 EPR spectra of (A) N-TiO (NT) and (B) S-TiO (ST) recorded at
77 K.
to heat produced by sunlight. In the absence of light and in the
presence of catalyst, 40 to 50% of HD was observed to be
degraded in 8 h on all the nanocatalysts. The degradation
reactions were found to be very slow, incomplete, and led only
to hydrolysis products like hemisulfur mustard and thiodiglycol
as per GC-MS data. The degradation of HD in the absence of
sunlight could be ascribed to available surface area, surface
hydroxyl groups, Lewis acid sites, and the moisture present on
the surface (1–3% w/w) of the catalyst.
ST nanocatalyst showed greater activity towards the photo- Fig. 7 HD degradation reactions on CT, HT, NT, and ST nanocatalysts
in presence of sunlight and in the absence of light; (a) NT + sunlight (b)
NT + dark (c) ST + dark (d) ST + sunlight (e) HT + sunlight (f) HT + dark
catalytic degradation of HD when compared with NT or HT or
CT nanocatalysts. It could be seen that NT and ST nanocatalysts
(g) CT + dark (h) CT + sunlight.
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ꢀ
1
HCl on the surface of catalyst. Initially they exhibited 98, 100% cm indicates the formation of –COOH group typical of acetic
of HD degradation efficiency, then decreased to 95.0, 92.1% acid formed during photocatalytic decontamination of HD on
when it was reused aer washing with acetonitrile. However, it above said catalysts.
rd
decreased to 85, 81.6% aer 3 use and then to 65, 60% aer
5
th use. On the other hand, ꢂ95% of HD degradation was
3
. Experimental
observed continuously for h time when the titania was
thoroughly washed with 30% hydrogen peroxide solution fol-
3.1. Materials
lowed by washing with copious amount of water, ethanol and Thiourea, urea, titanium tetraisopropoxide (TTIP), bis-
dichloromethane. It could be ascribed to regeneration of active (trimethylsilyl)triuoro acetamide were procured from Acros
sites poisoned due to formation of surface bound alkoxy species organics, UK. Dichloromethane, ethanol, ethyl acetate,
ꢀ
1
ꢀ1
ꢀ1
(
1100 cm ) as well as sulfonate species (1126 cm , 1200 cm
)
hydrogen peroxide and acetonitrile were obtained from E.
and the conclusion is supported by IR data as previously Merck India Ltd. HD of 99% purity was obtained from synthetic
22
reported.
chemistry division of our establishment (this is a very toxic
Photocatalytic degradation of HD resulted in the formation agent; hence these experiments should be done under the
of several products like HD sulfoxide, HD disulde, chloro ethyl guidance of trained personnel equipped with individual
vinyl sulde, chloro ethyl vinyl sulfoxide, bis(2-hydroxy ethyl protective equipment only). Commercial nano TiO
sulphide) and other products like CH CHO, CO and H
O. GC- procured from Alfa Aeser, UK.
MS data was found to be similar to that which was already
2
(CT) was
3
2
2
22
reported.
3.2. Preparation of nanocatalysts
TiO nanocatalysts were prepared by sol–gel hydrolysis of tita-
2
nium(IV) isopropoxide (TTIP) followed by hydrothermal treat-
ment. TTIP was dissolved in anhydrous ethanol and the
solution was added dropwise to the mixture of distilled water
and ethanol under vigorous stirring at room temperature.
Obtained gel was transferred into a Teon lined autoclave and
2
.3. Mechanism of HD photocatalytic degradation
Sunlight is composed of 4–5% of UV light and 46% of visible
light. It is apparent from data that, both UV and visible light
fractions present in the sunlight contribute to photocatalytic
26
degradation of HD. On non-metal doped nano TiO
2
(ST or NT),
ꢁ
heated at 120 C for 10 h. The resulted samples were centrifuged
photogenerated holes and electrons are generated and quickly
trapped on the surface defect sites on full light illumination.
The trapped holes react with the HD molecule and degrade it
directly. Superoxide anion radicals further react with moisture
and form the hydroxyl radicals. These hydroxyl radicals react
with HD and degrade it to the corresponding reaction products
mentioned above. Formation of hydroxyl radicals on the surface
of ST, NT, HT and CT in the presence of sunlight was probed by
ꢁ
washed with water and ethanol and dried at 100 C for 10 h and

2
nally labelled as undoped TiO nanocatalyst (HT). N or S
dopants were introduced by adding appropriate amounts of
urea or thiourea into deionised water prior to the hydrolysis of
TTIP. Obtained samples were labelled as NT (N/Ti ¼ 0.1) or ST
(S/Ti ¼ 0.1) nanocatalyst.
3.3. Characterization of photocatalysts
2
the reactions with DMSO. S or N doped TiO nanoparticles were
XRD data was recorded on X'Pert Pro Diffractometer of Pan-
alytical, Netherlands make using Cu Ka radiation. TEM
measurements were done on Tecnai transmission electron
microscope of FEI make. XPS data was recorded on KRATOS
AXIS 165 instrument. Nitrogen adsorption measurements were
done on ASAP 2020 surface area analyser of Micrometrics, USA.
UV-visible absorption was obtained using Shimatzu UV-2500
spectrophotometer equipped with an integrating sphere. X-
band Electron Paramagnetic Resonance (EPR) spectra were
recorded by a Bruker EMX spectrometer. Gas chromatograph of
Nucon Engineers, India make equipped with FID detector, BP5
column (30 m length, 0.5 mm i.d.) was used to monitor degra-
dation of HD. GC-MS system (5975 B) of Agilent, USA make was
used for quantication and identication of reaction products.
mixed with DMSO (1 : 1%, w/w) and exposed to sunlight. Aer
exposure, products were extracted with ethanol, concentrated
and subjected to GC-MS analysis. The GC-MS data indicated the
formation of methane sulfonic acid ethyl ester and methane
sulfonic acid as depicted by m/z values at 124, 109, 97, 79, 65, 45
and 96, 79, 65, 48, 31. Hydroxyl radical reacts with DMSO and
converts it to methane sulfonic acid.
The reactions in absence of light lead to hydrolysis products.
In presence of light, the decomposition of HD over titania
systems leads to oxidation, elimination, rearranged products in
addition to hydrolysis products. The products were consistent
with earlier reports. GC-MS spectra of HD disulde, HD sulf-
oxide, chloro ethyl vinyl sulde, chloro ethyl vinyl sulfoxide were
given in ESI.†
Decontamination of HD is also supported by FT-IR data as
3
.4. Photocatalytic degradation
reported earlier. In the case of HD adsorbed on TiO
2
, a band at
S, bands at around 100 mg of nanocatalyst (ST or NT or HT or CT) was taken in a
220 cm and 1270 cm correspond to CH
vibration of the quartz tube and 100 mL of dichloromethane solution containing
ꢀ
1
2
1
930 cm indicates C–H vibration of CH
2
ꢀ
1
ꢀ1
2
CH –S group which are characteristics of adsorbed HD on nano 2 mL of HD was spiked on it and dichloromethane was allowed
2
ꢀ
1
TiO . Band at 700 cm disappeared due to hydrolysis of HD on to evaporate. Subsequently, quartz tube containing the nano
2
2
TiO without irradiation. With irradiation of sunlight the band photocatalyst was irradiated with natural sunlight. Intensity of
pattern observed to have changed. Weak band at around 1410 light was measured by digital light meter (SLM 110 model) of
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A.W. Sperry Instruments, USA with help of adopters provided.
Value of irradiance of sunlight was determined to be 95 mW
cm . All the experiments were carried out at 35 ꢃ 5 ꢁC.
Remaining HD was extracted aer periodic intervals of time
using acetonitrile. The extracted samples were analyzed with GC
affixed with FID detector to monitor the amount of degraded
7 H. Irie, Y. Watanabe and K. Hashimoto, J. Phys. Chem. B,
2003, 107, 5483.
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ꢀ
2
HD. Later, the solution was concentrated to 1 mL and analyzed 10 T. Lindgren, J. M. Mwabora, E. Avendano, J. Jonsson,
for products by GC-MS. Few degradation products formed were
identied by GC-MS (Model 5975B and 6890N). Reaction
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B, 2003, 107, 5709.
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12 T. Ihara, M. Miyoshi, Y. Iriyama, O. Matsumoto and
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1
3 S. Yin, Q. Zhang, F. Saito and T. Sato, Chem. Lett., 2003, 32,
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4
. Conclusion
3
2
TiO
photocatalytic degradation under sunlight irradiation. This 15 T. Umebayashi, T. Yamaki, S. Tanala and K. Asai, Chem. Lett.,
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364.
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M. Matsumura, Appl. Catal., A, 2004, 265, 115.
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, undoped TiO
2
and commercial nano TiO
2
towards
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2
4
+
6+
ꢀ
products. These interesting properties make these materials as 19 T. Ohno, Water Sci. Technol., 2004, 49, 159.
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2
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Acknowledgements
Authors thank to the Director, DRDE, Gwalior for his permis-
sion to publish this work. They also thank Dr Beer Singh, Head
PD Division, DRDE, Gwalior, Prof. Ramesh Chandra, IITR, Prof.
B. Sreedhar, IICT for providing GC-MS, TEM, XPS.
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P. V. R. K. Ramacharyulu, J. Praveen Kumar, G. K. Prasad,
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