Asian Journal of Chemistry; Vol. 25, Supplementary Issue (2013), S69-S72
Synthesis of Gallium Doped Mesoporous KIT-6 for the Photocatalytic Degradation of Dyes†
N.S. SANJINI and S.VELMATHI*
Organic and Polymer Synthesis Laboratory, Department of Chemistry, National Institute of Technology, Tiruchirappalli-620 015, India
*Corresponding author: Fax: +91 431 2500133; Tel: +91 431 2503640; E-mail: [email protected]
AJC-12794
Gallium doped highly ordered three dimensional la3d mesoporous KIT-6 material was synthesized by simple impregnation procedure
with an aqueous solution of Ga(NO3)3·H2O. Different concentrations of gallium were doped onto KIT-6. The incorporation of gallium on
the surface of KIT-6 was confirmed by various characterization techniques such as X-ray diffraction, BET surface area, Fourier transform
infrared spectroscopy and diffuse reflectance spectroscopy. The XRD confirms the 3D structure of the materials. KIT 6 and the gallium
doped KIT 6 show very high surface area and pore diameter. The UV DRS spectrum of the prepared materials shows absorbance in UVvisible region indicates their potential application in photocatalytic reaction. Photodegradation of methylene blue with Ga-KIT-6 under
visible and ultra violet light were followed by UV visible spectrophotometer. The complete degradation of methylene blue dye was
observed within 135 min and the optimization studies for the photocatalytic degradation of dyes were also systematically done.
Key Words: Gallium, KIT-6, Mesoporous material, Photodegradation, Methylene blue.
INTRODUCTION
The discovery of mesoporous materials exhibits wide
applications in industrial catalytic reaction since they have high
surface area, large pore volume, multidimensional structure
and easy recycling. Incorporation of transition metal ions on
mesoporous silica has become an excellent strategy to generate
catalytic active sites1-3. Semiconductor like titania incorporated
mesoporous silica has extensive application on photocatalysis because of wide band gap (3.2 eV for anatase phase)4-7.
The major proportion of metal oxides can be activated only in
the presence of UV light8-13. Mainly semiconductors (Ga3+,
In3+, Se3+, Sb3+) with d10 electronic configuration shows better
photocatalytic activity because of their conduction and
valence bands are formed by sp hybridization with large
band dispersion which leads to high charge mobility14-18. GaN,
a well-known blue light emitting semiconductor which is
having wide band gap (3.4 eV) is used for photocatalytic
activity19.
Colour is the main contaminant to be present in the waste
water coming from industries. In most of the industries like
food, leather, textile, paper, printing etc, dyes are used to colour
the products. Various methods developed for removal of dye
have been inspected such as physico-chemical methods,
filtration techniques, advanced oxidation process and biological processes20. For all these process further water treatment
and regeneration of adsorbent is needed which will cost more
for these process. So far, numerous heterogeneous catalysts
are used for the photodegradation of dyes. Heterogeneous
photocatalysis is one of the attractive and highly efficient
method to degrade the toxic and non-biodegradable organic
compounds which are pollutant to the environment. Transition metal doped mesoporous silica MCM-41, MCM-48,
SBA-15, etc. was used for degradation of stable dyes21,22. In
this paper we synthesized gallium doped ordered mesoporous
silica KIT-6 by simple impregnation method and examined
the photocatalytic dye degradation property under visible and
UV light.
EXPERIMENTAL
Methylene blue (MB), analytical grade dye was purchased
and used for photo-catalytic degradation studies. Other
chemical reagents such as pluronic acid P123 (MW 5800),
hydrochloric acid (HCl), n-butanol, tetraethylorthosilicate
(TEOS) and gallium nitrate hydrate (Ga(NO3)3·H2O) were
received from Sigma Aldrich chemical company and used
without further purification.
X-ray powder diffraction (XRD) was conducted on a
Philips Xpert diffractometer with CuKα (1.5404 Å) radiation.
Scans were made in the 2θ range 0.5-10º with a scan rate of 2
º/min (low angle diffraction). BET surface area and pore size
distributions were measured by N2 adsorption/desorption using
†International Conference on Nanoscience & Nanotechnology, (ICONN 2013), 18-20 March 2013, SRM University, Kattankulathur, Chennai, India
S70 Sanjini et al.
C0 − C
× 100
C0
where, C0 is the dye concentration at the beginning of illumination; C concentration after photodegradation.
Photodegradation yield (%) =
RESULTS AND DISCUSSION
The small angle X-ray pattern of samples KIT-6 and GaKIT-6 in Fig. 1a shows high intensity Brag peaks which
corresponds to (211 ), (220) and (322) reflections. These are
the well distinguished reflections for 3-D cubic structure. So
this indicates the body centered cubic la3d space group of the
prepared samples. After doping Ga3+ substituted in to lattice
of Si4+ sites as well as the interstitial sites creates nonuniform
strain within the doped KIT-6 which reflects in the peak
Intensity (a.u.)
Intensity
(a.u)
KIT-6
Ga-KIT-6
300
200
100
0
2
4
6
8
10
2 Theta (Degree)
2θ
θ (º)
Fig. 1. Low angle XRD pattern of KIT-6 and Ga-KIT-6
broadening in XRD pattern. The doped mesoporous silica once
more maintained cubic structure which proves the efficacy of
the impregnation method.
N2 adsorption/desorption isotherm of Ga-KIT-6 samples
are shown in Fig. 2 is type IV isotherm which is typical for
mesoporous material with narrow pore size distribution. The
surface area of Ga-KIT-6 was 383.9 m2/g are found to be
decreased with those of KIT-6, 705.7 m2/g which confirms
the large amount of distribution of dopant in the pores of
KIT-6 while retaining a high ordered mesostructure.
400
300
5
dV/dD cm 3/g.nm
Adsorption studies: Before starting the photodegradation
experiment, the adsorption study was demonstrated to ensure
the adsorption /desorption equilibrium. Ga-KIT-6 suspended
methylene blue solution (100 mL) was stirred for 0.5 h in the
darkness. The samples before and after adsorption were
examined by UV-visible spectrophotometer with a range of
200 to 800 nm. λmax of methylene blue is 665 nm.
Photodegradation studies: The photodegradation
studies were conducted in visible photo reactor (> 400 nm)
and UV immersion type photo reactor (254 nm and 365 nm).
Visible reactor is a batch reactor with 350 W xenon lamps.
UV reactor consists of a glass vessel made of borosilicate glass,
wherein the solution to be degraded was placed. This glass
housed with a double walled quartz vessel to hold lowpressure mercury lamp. 100 mL of aqueous methylene blue
(10 mg/L) solution with 25 mg of Ga-KIT-6 catalyst was stirred
either in visible (350 W) photo chamber or in UV reactor. The
maximum absorbance wavelength of methylene blue dye
was measured before and after degradation using UV-visible
spectrophotometer. The degradation yield was calculated as
follows,
400
3
General experimental procedure for photodegradation
500
Amount adsorbed (cm /g) 3
Amount
adsorbed cm /g
Micromerritics Gemini V -2380 surface area analyzer. Prior
to the analysis samples were degassed at 300 ºC for 1 h. IR
spectra were recorded by KBr pellet method on a Perkin-Elmer
Spectrum One FTIR spectrometer instrument. UV-visible
diffuse reflectance was measured at room temperature in air
on a SHIMADZU UV-2600 UV-Vis spectrophotometer over
the range from 200 to 800 nm.
Synthesis of Ga-KIT-6: To 4 g of P123, 150 g of distilled
water and 7.9 g of conc. HCl were added and stirred for 2 h at
35 °C. Then 4 g of n-butanol was added and stirred for another
1 h followed by the addition of 9 g of TEOS and stirred for
24 h. The resultant gel was allowed to stand for crystallization
under static hydrothermal condition at 100 ºC for 24 h in a
Teflon parr reactor. The white coloured solid product was
filtered off, washed with warm distilled water several times
and dried at 100 ºC overnight. The as-synthesized solid product
was calcined at 540 ºC in presence of air for 24 h. 0.05 g of
prepared KIT-6 was soaked in different mol/L solution of
Ga(NO3)3·H2O by ultrasonication for 2 h. The material was
filtered, dried at 100 ºC, calcined in a muffle furnace maintained
at 750 ºC for 6 h and named as Ga-KIT-6.
Asian J. Chem.
4
Ga-KIT-6
3
2
1
0
5
200
10
15
20
25
30
35
Pore diameter (nm)
100
0.0
0.2
0.4
0.6
0.8
1.0
Relativepressure
pressure (P/PP/P
) o
Relative
0
Fig. 2. N2 adsorption isotherm of Ga-KIT-6
FTIR spectrum (Fig. 3) of calcined samples shows broad
peak around 3441 cm-1 for silanol group which play an
important role in photo degradation process. The peak at 457,
761 and 1007 cm-1 corresponds to rocking, bending and asymmetric stretching of Si-O-Si bonds respectively and 588 cm-1
for Ga-O-Ga bond vibration. The abundance of surface
hydroxyl groups play an important role in photodegradation
of organic pollutants through their interaction with photo generated holes.
The diffuse reflectance spectrum of Ga-KIT-6 mesoporous
material is given in Fig. 4 which is in agreement for photocatalytic activity. The absorbance starts around 400 nm and
increases sharply around 600 nm. These are corresponds to
Vol. 25, Suppl. Issue (2013)
Synthesis of Gallium Doped Mesoporous KIT-6 for the Photocatalytic Degradation of Dyes S71
2.5
200
KIT-6
Ga-KIT-6
2.0
Absorbance
Absorbance
Transmittance (%) (%)
Transmittance
250
150
100
50
0
4000 3500 3000 2500 2000 1500 1000
-1)
Wavenumber (cm(cm
)
Wavenumber
0 min
30 min
60 min
90 min
120 min
150 min
180 min
1.5
1.0
0.5
0.0
200
500
300
400
500
600
700
800
Wavelength (nm)
Wavelength (nm)
–1
Fig. 5. Methylene blue degradation in the absence of photocatalyst under
visible light
Fig. 3. FTIR spectra of KIT-6 and Ga-KIT-6
Bf ads
0 min
15 min
30 min
45 min
60 min
75 min
90 min
105 min
120 min
135 min
1.5
0.3
Absorbance
Absorbance
Absorbance
Absorbance
0.4
KIT-6
Ga-KIT-6
0.2
0.1
0.0
200
1.0
0.5
0.0
300
400
500
600
700
800
200
300
Wavelength (nm)
Wavelength (nm)
500
600
700
800
Wavelength (nm)
Wavelength
(nm)
Fig. 4. DRS UV-visible spectra of KIT-6 and Ga-KIT-6
Fig. 6. Methylene blue degradation under visible light
100
Photodegradation
rate
Photodegradation rate
(%) (%)
absorption of light caused by excitation of electrons from
valence band to the conduction band of Ga-KIT-6.
Photocatalytic activity: The photocatalytic activities of
prepared samples were estimated by measuring the degradation rate of methylene blue in the presence of UV and visible
light irradiation. The absorbance spectra of methylene blue
solution was recorded each 0.5 h in absence of photocatalyst
under visible and shown in Fig. 5. The identical spectra
suggesting that direct photolysis not occurs. This confirms the
higher stability of methylene blue and inactivity under visible
and UV light. 25 mg of Ga-KIT-6 with methylene blue
solution treated under different conditions. The degradation
of methylene blue solution as shown in Fig. 6 depicts the
photodegradation of methylene blue with Ga-KIT-6 occurred
within 135 min under visible light (350 W) whereas KIT-6
undergoes adsorption saturation after 120 min as shown in
Fig. 7. The gallium distribution on the surface of KIT-6 makes
it possible to degrade the dye properly. The Fig. 8 explained
absolute degradation of methylene blue under UV light
(254 nm and 365 nm) with same amount of Ga-KIT-6. The
degradation was greatly enhanced under UV light irradiation;
excess light energy is given off to generate electron-hole pairs
through band to band transitions compared with visible
light.
400
90
80
KIT-6
Ga-KIT-6
70
60
0
30
60
90
120
150
180
Irradiation time (min)
Irradiation Time (min)
Fig. 7. Methylene blue degradation with KIT-6 and Ga-KIT-6 under visible
light
The substitution of a Ga3+ into Si4+ sites will create oxygen
vacancy, which serves as an electron trap and effectively
enhances the lifetime of the hole, which, in turn, helps in the
oxidative degradation of methylene blue. The dye molecules
are easily adsorbed on the surface of Ga-KIT-6. The incident
photons generate an electron-hole pair at the surface of
Photodegradation rate
(%) (%)
hotodegradation
rate
S72 Sanjini et al.
Asian J. Chem.
100
ACKNOWLEDGEMENTS
90
The authors expressed their sincere thanks to Dr. S. Sundarrajan,
Director, NIT Trichy for his constant support and encouragement, DST- Nanomission (SR/NM/NS-27/2008) and DST-SR/
S1/OC-06/2011 (G) dated 29-06-2011 projects for financial
assistance.
80
UV-365 (nm)
UV-254 (nm)
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Fig. 8. Methylene blue degradation with Ga-KIT-6 under UV light (254
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Conclusion
Highly ordered gallium doped mesoporous KIT-6 silica
is synthesized by simple impregnation method. The 3D cubic
structure of Ga-KIT-6 was confirmed by low angle XRD. The
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