Advances in Ma terials Physics and Chemistry, 2012, 2, 40-44
doi:10.4236/ampc.2012.24B012 Published Online December 2012 (http ://www.SciRP.org/journal/ampc)
Photocatalytic of T iO2-SiO2 Thin Films Co-Doped with Fe3+
and Thio-Urea in the Degradation of Formaldehyde
by Indoor and Out door Visible Li gh t s
Charuwan Kaewtip, Kamolporn Accanit, Nat-a-nong Chaowai, Kanokpun Areerat,
Pasuree Reanjaruan, Virote Boonumnauyvitaya
Ch emical Engineering Department, King Mongkut’s University of Technology Thonburi, Bangkok, Thailand
Received 2012
ABSTRACT
In this work the photocatalytic activity of TiO2-SiO2 thin films co-doped with Fe3+ and thio-urea in the degradation of the gaseous
formaldehyde was investigaged by indoor and outdoor visible lights. The films were synthesized by Peroxo Titanic Acid (PTA) me-
thod. The physicochemical properties of prepared samples were characterized using SEM and UV-vis absorption spectroscopy. It
was found that the average film thickness of all coated samples was about 394 ± 5 nm. The band gap energy of un-doped and
co-dop ed ph oto catalysts was 3 .0 8 and 2.8 8 eV , resp ecti vel y. The p ho tocat alytic exp eri men tal resu lts showed that the co-doped TiO2-
SiO2 thin film yield higher photocatalytic efficiency. Under the outdoor light (sunlight in the shade condition) irradiation, with the
initial concentrations of formaldehyde of 1000, 3000 and 5000 ppmV, the efficiencies of formaldehyde degradation were 94.7%,
89.5% and 85.1%, respectively. Under the indoor light (the fluorescent lamp) irradiation, with the same formaldehyde initial concen-
trations, the photocatal ytic act iviti es w er e 87.4%, 85.3% and 81.5%, respectivel y.
Keywords: Perxo Titanic Acid; Titanium Dioxide; Sunlight in the Shade; Formaldehyde
1. Introduction
Formaldehyde is a toxic volatile organic compound (VOCs),
which causes cancer and is harmful to health when uptake into
human bodies. Therefore, purification of ambient air form this
toxic gas is essential for improving indoor air quality and hu-
man being’s health [1]. Titanium dioxide (TiO2) is a promising
tool for environmental purification due to its specific optical
and electronic properties, low cost, chemical stability and
non-toxicity [2-4]. However, the need of an ultraviolet (UV)
excitation which accounts for only a small fraction of the total
solar energy (∼5%) hinders its utility for limited applications
[5] .
Many attempts have been made to enhance the utilization of
solar energy by doping the base photocatalyst with co-dopants
elements such as C and N [6], Fe3+ and C [7], and Fe3+ and N
[8,9]. Ohno et al. reported that the effect of adsorbing Fe3+ on
the N- or S- doped TiO2 and found that the photocatalytic effi-
ciency under visible light region was about twice as high as
without Fe3+ doping [10]. One element can extend the response
of TiO2 to visible light, the other can act as electron and hole
traps. Then TiO2 can respond to visible light and present a high
catalytic activity [11].
In ou r previ ous work [ 12], TiO2-SiO2 thin films with co- do-
pants of Fe3+ and N,S yielded high efficiency in formaldehyde
degradation. However, the study was limited in small lab scale
using fluorescent lamp as the light source. The objective of this
work is to confirm the practical use of these films by studying
the efficiency of formaldehyde degradation using co-doped
TiO2-SiO2 thin films in a large cubic glass chamber under the
outdoor light compared with the indoor fluorescent light.
2. Experimental
2.1. Catalysts Preparation
All reagents were of analytical grade and used without further
purification. The co-doped TiO2-SiO2 thin films were prepared
using the peroxo titanic acid (PTA) approach combined with
the sol–gel method. The procedures for co-doped PTA sol (so-
lution A) preparing were as follows: First, 4.3 g of Titanyl sul-
fate (TiOSO4) was added to 150 cm3 deionized water. While
under vigorous stirring, 26 cm3 of NH4OH (3 mol/dm3) was
added to the solution. Next, the white precip itates were filtered
and sufficiently washed four to six times with distilled water to
remove residues of NH4+ and SO42− ions, then dispersed ho-
mogeneously in 112.5 ml of distilled water. The resulting sol
was peptized in 25 cm3 of hydrogen peroxide (30%), and then
stirred for 15 min. The obtained orange transparent sol was kept
under reflux at 100°C. Before adding the co-dopants, the PTA
sol was kept at room temperature for 24 h. Dopants added into
the TiO2 based on the mass of TiO2 1.64 g in PTA sol. Finally,
CSN2H4 of 0.125 wt.% and Fe(NO3)3 of 1.0 wt.% were added
into the PTA sol.
The S iO2 (solution B) sol was prepared via the conden sation
reaction of methyltrimethoxysilane (MTMOS). First, 4.3 cm3 of
MTMOS were hydrolyzed with the mixture of 8.22 cm3 of
methano l, 2 cm3 of H 2O, and 0.5 cm3 of HCl (0.055M) aqueous
solution. After the solutions were vigorously stirred at 50°C fo r

C. KAE WTIP ET AL.
41
60 min, the mixture of 8.22 cm3 of methanol and 3.2 cm3 of
NH4OH (0.856M), which was stirred at 25°C for 60 min, was
added into the hydrolyzed MTMOS solution and stirred for 15
min. Then, solution B was added into solution A under stirring
at room temperature for 15 min. The obtained sol was coated on
a glass plat e (100 x 100 x 3 mm) by spin coating machine, and
then kept for drying for 24 h at room temperature before using.
The obtained photocatalysts were characterized by Scanning
electron microscopy (SEM) to determine the thickness of coted
film on the glass plate. The optical transmission and absorption
spectra of the films were measured using UV-vis spectropho-
tometer (U1900 UV/VIS, Hitachi) with the wave range of 200-
1000 nm.
2.2. Photocatalytic Activity of Formaldehyde
Degradtion
The experiments were conducted in a cubic 9.1 x 104 cm3 glass
chamber where 70 pieces of coated glasses were attached
around the inner wall. The schematic and components of glass
reactor chamber showed in Figure 1. Injection and sampling of
formaldehyde gas in the glass chamber were conducted through
the septum port by means of a syringe. The initial concentra-
tions of formaldehyde gas of 1000, 3000 and 5000 ppmV were
used. The glass reactor chambers were covered with black fa-
bric for shutting out light around 50-60 min until adsorption
equilibrium conditions have been reached. Then the black fa-
bric was removed to start the photocatalytic reaction. The
chambers were placed indoor for fluorescent light and outdoor
for sunlight (in the shade) exposures. The concentration of for-
maldehyde before and after photocatalytic reactions were
measured by a gas chromatography (Shimadzu: GC2014)
equipped with DB-WAX column.
3. Result and Discussion
3.1. Characterization of the Photocatalysts
Cross sectional SEM image as shown in Figure 2 shows a
complete coverage of the substrate surface by the photocata-
lysts film with the average film thickness 3 94 ± 5 nm.
Figure 1. The schematic and components of glass reactor chamber
(1) lid, (2) septum port, (3) fan hole, (4) fan, and (5) hygrometer
and thermometer.
The color of a coated film is opalescent-semitransparent due
to the TiO2 powder dispersed in the film. The corresponding
UV-Vis spectra for coated film are shown in Figure 3. Com-
pared with the un-doped film, the transmittan ce o f the co -doped
film was about 80% in the visible wavelength region and lower
than those of the un-doped film. The difference in transmittance
between th e un-doped and the co-doped films was attributed to
the adsorption of light by dopant. A significant decrease in the
transmittance below 400 nm can be assigned to absorption of
light caused by the excitation of electrons from the valence
band to the co nductio n ba nd of TiO2 [13]
Figure 4 shows the UV-Vis absorption spectra of un-doped
Figure 2. SEM micrographs showing the cross section of the film
thickness on glass substrate.
Wavenumber (n m)
300 400 500 600 700 800 9001000
Transmittance (%)
0
20
40
60
80
100
no doped film
co-doped fi lm
Figure 3. The UV-Vis transmittance spectra of the transparent
films of un-doped and co-doped TiO2-Si O 2 th in fi lms.
Wavenumber (nm)
200 300 400 500 600 700 800 9001000
Absorbance
0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
un-doped film
co-doped film
Figure 4. The UV-Vis absorption spectra of coated TiO2-SiO2 thin
f i lms.

C. KAE WTIP ET AL.
42
and co -doped TiO2-SiO2 thin films. The absorption edge of un-
doped is limited only to ultraviolet light region, whereas the
absorption threshold values of co-doped photocatalyst is ex-
tended up to the visible light range. The energy band gap (Eg)
is determined by the formula [14], Eg = 1239.8/ λ, where λ (nm)
is the wavenumber of the absorption edge in the spectrum. The
energy band gap of un-doped and co-doped TiO2-SiO2 thin
films was 3.08 and 2.88 eV, respectivel y.
3.2. Photocatalytic Activity
The formaldehyde degradation activity of the prepared photo-
catalytic films was deter mined and shown in Figure 5. In dark
condition, the formaldehyde adsorbed onto the all films within
30 min and approached equilibrium after 50-60 min. Under
ligh t i rr adia ti on, in bo th cas es o f in d oo r and ou t door con d iti ons ,
all co-doped samples, demonstrate higher photocatalytic effi-
ciencies t han that of the un-doped photocatalysts. This could be
Reaction Time (min)
050100 150 200 250 300
Formaldehy de concentration (ppmV)
0
1000
2000
3000
4000
5000
6000
control
un-doped
co-doped
Dark Sunlight in the shade
c2.
Reaction Time (min)
050100 150 200 250
Formaldehy de concentration (ppmV)
0
1000
2000
3000
4000
5000
6000
control
un-doped
co-doped
Dark Floresent irradiation
c1.
Reaction Time (min)
020406080100 120 140 160
Formaldehy de concentration (ppmV)
0
1000
2000
3000
4000
5000
control
un-doped
co-doped
Dark Sunlight in the shade
b2.
Reaction Time (min)
020406080100 120 140 160 180
Formaldehy de concentration (ppmV)
0
1000
2000
3000
4000
5000
control
un-doped
co-doped
Dark Floresent irradiation b1.
Reaction Time (min)
010 20 30 40 50 60 70 80
Formaldehy de concentration (ppmV)
0
200
400
600
800
1000
1200
1400
1600
control
un-doped
co-doped
Dark Sunlight in the shade
a2.
Recation Time (min)
020406080100 120 140 160180
Formaldehy de concentration (ppmV)
0
200
400
600
800
1000
1200
1400
1600
control
un-doped
co-doped
Dark Floresent irradiation
a1.
Figure 5. Photocatalytic decomposition profiles of gaseous formaldehyde by control (no photocatalyst) and photocatalysts chambers with
different initial formaldehyde concentration a) 1,000 ppmV, b) 3,000 ppmV and c) 5,000 ppmV and different light source of irradiation.

C. KAE WTIP ET AL.
43
Fig ure 6. The photocatalytic efficiencies of the co-doped TiO2-Si O 2
thin films with different of initial formaldehyde concentration.
attributed to the effect of the synergistic role of co-dopants in
narrowing TiO2 band gap.
It is generally accepted that a dopant level can form above
the valence band for the substitutional nitrogen, and below the
conduction band for Fe3+ doping, both of which cou ld decrease
the band gap of TiO2 and improve the photocatalytic activity in
the visible light region [15]. On the other hand, the co-doping
of nitrogen and Fe3+ ion inhibits the recombination of the pho-
togener ated electr on and ho le [16]. The effect o f ligh t s ou rce on
the formaldeh yde degrad atio n efficienc y is illustrated i n Figure
6. It is clearly observed in the figure that the formaldehyde
degradation efficiency of sun light in shade is higher than that
of fluorescent irradiation due tothe higer light intensity [17].
4. Conclusion
The co-doped TiO2-SiO2 thin films were synthesized by using
PTA sol as the TiO2 source. The prepared films showed the
average thickness of 394 ± 5 nm. The band gap energy of un-
doped and co-doped photocatalysts was 3.08 and 2.88 eV, re-
spectivel y. The co -doping of Fe3+ and N,S ion into TiO2 photo-
catalysts showed the highest photocatalytic activity of formal-
dehyde degradation. Under the sunlight in the shade condition,
with the initial concentrations of formaldehyde of 1000, 3000
and 5000 ppmV, the efficiencies of formaldehyde degradation
were 94.7%, 89.5% and 85.1%, respectively. For the fluores-
cent irradiation, with the same formaldehyde initial concentra-
tions, the photocatalytic activities were 87.4%, 85.3% and
81.5%, respectively. Both iron ions and nitrogen species could
lead to a narrowing of the band gap of TiO2. In addition, the
co-doping of nitrogen and Fe3+ ion inhibits the recombination of
the photogenerated electrons and holes.
5. Acknowledgements
The authors gratefully acknowledge the financial support from
the Ro yal Gold en Jubi lee p rogram and S eni or Research Sch olar
Grant from Thailand Research Fund (TRF) and National Re-
search University Project of Thailand from the Office of the
Higher Education Commission, Thailand.
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