# Silver DopedTiO2  Nanostructure Composite Photocatalyst Film Synthesized by

Authors: Mojtaba Nasr-Esfahani; Mohammad Hossein Habibi
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

New composite films (P25SGF-MC-Ag, MPC500SGF-MC-Ag, and ANPSGF-MC-Ag) have been synthesized by a modified sol-gel method using different particle sizes ofTiO2 powder and silver addition. Nanostructure TiO2/Ag composite thin films were prepared by a sol-gel spin and dip coating technique. while, by introducing methyl cellulose (MC)
porous, TiO2/Ag films were obtained after calcining at a temperature of 500°C. The as-prepared TiO2 and TiO2/Ag films were characterized by X-ray diffractometry, and scanning electron microscopy to reveal the structural and morphological differences. In addition, the photocatalytic properties of these films were investigated by degrading methyl orange (MO) under UV irradiation. After 500°C calcination, the microstructure of MC-TiO2 film without Ag addition exhibited a microstructure, while significant sintering effect was noticed with Ag additions and the films exhibited a porous microstructure. Nanostructure anatase-phase TiO2 can be observed with respect to the sharpening of XRD diffraction peaks. The photodegradation of porous TiO2 deposited with 5×10−4 mol Ag exhibited the best photocatalytic efficiency, where 69% methyl orange can be decomposed after UV exposure for 1 hour.


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## Body

## 1. Introduction

Titanium dioxide has attracted significant
attention of researchers because of many interesting physical properties that make
it suitable for a variety of applications, and TiO2 has high
corrosion resistance, chemical stability, and an excellent optical transparency
in the visible and near infrared regions as well as high refractive index that
makes it useful for antireflection coatings in optical devices. Efforts are
devoted to the development of efficient water and air purification technologies,
based on TiO2 photocatalysis. Such a treatment typically reduces
toxic organic compounds to nontoxic inorganic compounds, such as carbon dioxide,
water, ammonium or nitrates, and chloride ions. A number of techniques, such as
spray pyrolysis [1], sol-gel method [2], sputtering [3, 4],
solvothermal
method [5], pulsed laser deposition [6], atomic layer deposition [7], chemical
vapour deposition (CVD) [8–12] and
photoassisted CVD [13] have been used to deposit TiO2 thin films. Each
of the techniques for the TiO2 thin film preparation has its own
advantages and disadvantages, and it remains unclear at present as to which of
these will eventually prove to be the most cost/quality effective. The
photocatalytic activity depends strongly on the surface redox potential and the
lifetime of the photogenerated electron-hole pairs. Titanium dioxide exists in
three different crystalline phases: anatase, rutile, and brookite. Anatase was
found to be more active photocatalytically than rutile. This enhanced
photoactivity is attributed to the larger bandgap existing in this crystalline
phase, which generally leads to an increase of both the surface redox potential
and the charge carrier lifetime. Due to the large surface area, thin and
composite coatings consisting of anatase TiO2 nanoparticles show
high photocatalytic efficiency. The photocatalytic activity of TiO2 coatings do not depend only on the phase, but also on the crystallite size and
porosity [14]. Sol-gel derived titanium dioxide composites have been also developed
and investigated for the purpose of producing thin and composite films and
self-supported photocatalysts. The resulting photocatalysts exhibit relatively
high surface area and enhanced mechanical stability and integrity [15].It is well known that the initiation step of
the photocatalytic process consists in the generation of electron-hole pairs upon
irradiation of the material with a photon having energy at least equal to that
of the bandgap of the photocatalyst. The electron-hole pairs formed
can either recombine in the bulk or travel up to the surface, where they can
participate in chemical reaction involving species adsorbed on the external
titanol groups. The only drawback of TiO2 is that its band gap lies
in the near-UV of the electromagnetic spectrum: 3.2 eV (388 nm) and 3.0 eV (410 nm) for anatase and rutile, respectively. As a consequence, only UV light is
able to create electron-hole pairs and to
initiate the photocatalytic processes. It is, therefore, evident that any
modification of the TiO2-based photocatalysts, resulting in a
lowering of its bandgap [16], is representing a breakthrough in the field. This
is the reason why so many scientific works have appeared during recent years.
An exhaustive analysis of the different approaches used to dope TiO2 is beyond scope of this contribution and only a selection of cases will be
summarized below. (i) One case is doping TiO2 with various
transitions metals such as Au, Ag, Pt, Cr, Nb, V, Mn, and Fe [17, 18]. These
systems show an enhanced photoactivity in the visible with an efficiency
depending highly on the preparation method. However, they are characterized by
thermal instability and by a critical control of the cluster dimension and
distribution [19]. (ii) Another case is represented by TiO2 doped with
nonmetals atoms such as N, S, F, C, I, Br, and Cl [20, 21]. Also in this case,
the photocatalytic activity depends on the content of nonmetal atoms and on the
method of preparation. (iii) A third case is a dye-sensitized TiO2 obtained by anchoring a dye on the surface of the photocatalyst [22]. Various
dyes (catechol, porphyrins, phthalocyanines, etc.) have been employed as
sensitizers, but most of them are toxic and, more important, easily undergo a
self-degradation process, that makes them unsuitable for durable applications
in photocatalysis. (iv) As a fourth case TiO2 doped with silver
particles must be mentioned. Electrons in the valence band of TiO2 are excited to the conduction band, with formation of holes in the valence
band. At the same time electrons of Ag nanoparticles, when irradiated with the
light of their plasmon resonance wavelength, are excited and transformed to
adsorbed oxygen with formation of O2−. The Ag nanoparticles are
consequently oxidized by O2− to colorless Ag+ ions [23, 24].
In presence of TiO2 these Ag+ ions are reduced by the
excited electrons and Ag nanoparticles are reformed. A limiting factor of the
photocatalytic reaction is, therefore, recombination of the electron and hole
prior to the superoxide activation step [25]. Silver has been shown to have a
beneficial influence on the photoactivity of nanocrystalline semiconductor
photocatalysts [26–28]. The combination of semiconductor substrate and metal
cluster has been reported to give improved photocatalytic activity by trapping
the photoinduced charge carriers, thereby improving the charge transfer
processes [29–32].In our previous work [33], we described a simple and repeatable approach to achieve useful materials for
photocatalytical application by direct mixing of dispersed nanopowder titanium
dioxide in sol-gel solution with prepolymerized material (polymer). The polymer
was introduced into the sol-gel precursors in order to prevent particle
aggregation, adjust the viscosity of sol, increase the strength of the unfired materials,
and prevent film crack formation. In this case, it is important to avoid phase
separation throughout the sol-gel reaction by a careful choice of solvents and
optimized material processing conditions. Methyl cellulose (MC) is one of the
ideal prepolymerized materials due to its solubility in water, which is
suitable for use in sol-gel processing. Another advantage is that MC belongs to
nonionic cellulose ether, which is substantially free of substances that can
induce crystallization of silica or titania, and so forth during any of the
coating formation and densification process steps.For the reasons briefly mentioned above, we
became interested in modifying the light absorbing property and photocatalytic activity
of nanostructure composite TiO2 films covering glass slide. Coming now
to this study, it can be divided into two parts. In the first part, we
illustrate a new strategy to synthesize a silver-doped titanium dioxide
characterized by the presence of large surface area. In the second part, we
demonstrate that the layer consists of anatase, which is efficient to remove
the adsorbed molecular species from the environment via photocatalytic
reaction. In this study, we have successfully synthesized three types of
composite films (P25SGF-MC-Ag, MPC500SGF-MC-Ag, and ANPSGF-MC-Ag) by a modified
sol-gel method using different particle size of TiO2 powder. Methyl
cellulose (MC) was added as a template to the sol for stress reduction which improved
the amount of crystalline material immobilized on the support and the grain
size of the films calcined at 500∘C. The as-deposited TiO2/Ag films
were characterized by X-ray diffractometry, scanning electron microscopy,
differential scanning calorimetry, and thermal gravimetric analysis. In
addition, photocatalytic properties of these films were evaluated by degrading methyl
orange under UV irradiation. The purpose of this study is to obtain insights on
how the incorporation of Ag into the modified titania composite films affects
the photocatalytic activity of such macroporous films, so as to design high
performance TiO2 composite films for water treatment.


## 2. Experimental

Standard ethanol solution (M =  46.07 g/mol, purity≈99.8%) was purchased from
Fluka Chemical. Hydrochloric
acid (M= 36.5 g/mol, purity  ≈35.5%) and silver nitrate (99%) were supplied
from Merck.
Titanium tetra isopropoxide (Ti(iOPr)4 or TTIP)
(Sigma-Aldrich, 97%), the precursor, was used without further
purification. Three types of commercial TiO2 powders, namely, Aldrich
TiO2 powder (ANP), Degussa P-25 (P25), and Millennium PC-500 (MPC500) were used as the filler in
the nanostructure composite films. Specifications of the photocatalysts filler
according to the manufacturer's data are given in Table 1. Methyl cellulose
(low substitution) was used as an organic binder material [24]. Methyl orange (4-[[4-dimethylamino]phenyl]azo)
benzenesulfonic acid sodium salt) analytical grade [34]
was chosen as a simple model of a series of common azo dyes largely used
in the industry. Methyl orange is a well-known acid-base indicator, orange in
basic medium, and red in acidic medium. The preparation steps for the composite
films derived from TiO2 powder are described in detail elsewhere [35].
The sol was prepared by adding 5 mL TTIP to a 50 mL beaker containing a mixture
of 10 mL ethanol and 1.8 mL HCl 35.5% that had been mixed for five minutes. Methyl
cellulose 2 wt.% solution was prepared using MC and double-distilled water. These
two solutions (titanium precursor and MC solution) were added dropwise and
stirred overnight at room temperature. Precalcinated TiO2 nanopowders (ANP,
P25, MPC500), Degussa P25 (P25), and Millennium
PC-500 (MPC500) were used as filler mixed with the sol (5% of sol).
TiO2 powder was dispersed in the sol and the prepared mixtures were
deposited on the microscope glass slide (75 mm × 25 mm ×  1 mm) by homemade spin coating.
The dried films were heated in a muffle furnace to 500∘C at a heating rate
5∘C min−1 and maintained at this temperature for 60 minutes and
cooled at room temperature with similar rate. Three different composite (P25SGF-MC, MPC500SGF-MC, and ANPSGF-MC) films were dipped
into the silver nitrate solution with various molar concentrations (5×10−6, 10−5,  5×10−5,
10−4, 5×10−4, 5×10−3, 10−3,
10−2 M) for three seconds, washed with distilled water, dried at
nitrogen atmosphere and no color change was observed. The silver-doped
composite films (P25SGF-MC-Ag,
MPC500SGF-MC-Ag, and ANPSGF-MC-Ag) were irradiated with two UV-C lamps (8 W) for
2 hours in air and color change was observed. These composite films were
used for photocatalytic degradation; otherwise the photocatalysts were stored
in the dark to avoid preactivation by room light or sunlight.Table 1
Properties of TiO2 nanopowders.
PhotocatalystCompositionSpecific BET surface area
(m2g−1)Primary particle size (nm)Aldrich powder TiO2 (ANP)≥97% anatase190–29015Millennium PC-500 (MPC500)≥97% anatase>2505–10Degussa P25 (P25)70% anatase~5020Film morphology was characterized by an environmental scanning
electron microscope (ESEM, JEOL JSM-5300LV) with accelerating 10 kV. The phase
composition of photocatalyst was studied by the powder and plate XRD technique.
The X-ray diffraction patterns were obtained on a D8 Advanced Bruker X-ray
diffractometer using Cu Kα radiation at an angle of 2θ from 15 to 60°. The scan
speed was 1°/min. The strongest peaks of TiO2 corresponding to
anatase (1 0 1) and rutile (1 1 0) were selected to evaluate the crystallinity
of the samples. The mean crystallite size L was determined from the broadening
β of the most intense line, for each polymorph, in the X-ray diffraction pattern,
based on the Scherer equation:(1)L=kλβcos⁡θ,where λ is the radiation wavelength, k=0.90, and θ is the Bragg
angle [36].Evaluation of the adhesion and bonding strength
between the coating and glass substrate was made by using the scratch test
technique.The photocatalytic activity of films was evaluated by degrading
methyl orange solution. The distance between the UV light source and deposited
films was set as 10 cm. Sol-gel derived films were prepared on microscopy glass
slides. To evaluate the catalytic activity, the photodegradation of methyl
orange solution was chosen as a simple model of a series of common azo dyes
largely used in the industry. One TiO2-Ag/glass with one time spin coating (75 mm  × 25 mm  × 1 mm) was
used as photocatalyst and irradiated with two 8 W UV-A (λ = 365 nm) at a
distance of 5 cm from the top of the solutions. 20 mL of the dye solutions with
initial concentration of 5 ppm at pH = 4.5 was used. The solution was stirred in
the dark for 90 minutes before irradiation to reach an equilibrated adsorption.
Therefore, this time has been selected for the initial period to UV-irradiation
to make sure that the initial degradation initiates at the equilibrium of the
adsorption. The color removal of the dye solution was followed by measuring the
absorbance value at λ = 460 nm, using double beam UV-visible spectrophotometer,
(Varian Cary 500 Scan) initially calibrated, according to Beer-Lambert's law.


## 3. Results and Discussion

Sol-gel derived composite film [25] can be easily employed as
a fast and nonenergy consuming procedure to mass production of titania composite
films (of 10 μm in thickness) with good uniformity and reproducible properties.
After dipping of the TiO2 composite films in the silver nitrate
solution and subsequent UV irradiation at 254 nm for 2 hours, the photocatalyst
appears dark brown. Increasing the Ag+ concentration of dipping
solution produces darker films at a gradient rate. Macroscopic observation of
the film suggests that silver particles are widely spread on the surface
without the formation of islands and that the resulting surface modified photocatalysts
possess a solid structure on the glass substrate with more than satisfactory
scratch resistance and adherence.An XRD pattern of sol-gel derived TiO2 thin film
heat treated at 500°C for 1 hour shows that anatase was formed without
contamination by other polymorphs: brookite or rutile
(Figure 1). Nonheated
thin film had an amorphous (and/or an ultra-fine crystalline) structure with a
very broad peak at about 2θ=25° (which is identified as the most intensive peak
(1 0 1) for the anatase TiO2). Diffraction peaks of anatase were
observed for the coatings heat treated at temperatures higher than 300°C.
Diffraction peaks of rutile were not found for the gel-derived deposited
coatings heated up to 500°C.Figure 1
XRD patterns of TiO2 sol-gel derived thin film by one time spin coating
with heat treated at different temperatures.The XRD patterns of different composite sol-gel samples (P25SGF-MC,
ANPSGF-MC, and MPC500SGF-MC) before and after Ag coating are shown in
Figure2.
As shown in Figure 2(a), for P25SGF-MC anatase/rutile ratio remains almost
constant, as compared with the filler phase composition, after heating up to 500°C.
This is because of the relatively high percentage of TiO2 amount
(the filler) compared to the amount of TiO2 which is derived from
the sol. For ANPSGF-MC and MPC500SGF-MC nanostructure composite films, which
are prepared by ANP and MPC500 nanopowder, since ANP and MPC500 contain 100%
anatase, diffraction peaks of rutile were not found (Figures 2(b)
and 2(c)).(a) XRD patterns of (a1) sol-gel derived composite film from
Degussa P25 TiO2 with MC heat treated for 1 hour at 500°C
(P25SGF-MC) and (a2) after silver deposition (P25SGF-MC-Ag).
(b) X-ray diffraction patterns of (b1) sol-gel derived composite
film from Mellenium PC 500 TiO2 with MC heat treated for 1 hour at
500°C (MPC500SGF-MC) and (b2) after silver deposition (MPC500SGF-MC-Ag).
(c) X-ray diffraction patterns of (c1) sol-gel derived composite
film from Aldrich nanopowder TiO2 with MC heat treated for 1 hour at
500°C (ANPSGF-MC) and (c2) after silver deposition (ANPSGF-MC-Ag).
(a)(b)(c)It can be found that films exhibited the TiO2 composite films calcined at
500°C show relative sharp peaks indicating the coalescence
of nanocrystalline anatase-phase TiO2. According to the
Figures 2(a),
2(b), and 2(c), the intensity of the anatase and rutile main peaks in the
nanostructure composite films, both before coating or after coating by Ag, are
very similar. It is noticed that no additional XRD peaks corresponding to
Ag additions can be revealed. This may be attributed to the well dispersion of
nanocrystalline Ag particles in the TiO2 matrix, the overlapping of
Ag XRD peak or silver oxide particles are not crystallized on TiO2 surface. On the contrary, well-organized crystal structure from titania particles
is observed for the all composite films.As shown in Table2, for the sample sol-gel dried gel
modified with MC, the crystallite size of anatase phase, which was used as a
measure of TiO2 crystallinity, increases considerably with calcinations
temperature, from about 0.5 nm at room temperature to 13.4 nm at 500°C. However,
for P25 and ANP, the initial anatase crystallite size increased only slightly
versus temperature, from 15 nm for anatase at room temperature to more than 16.3 nm for anatase, in ANP (at 500°C) and more than 19.2 nm for anatase, in P25
powder (at 500°C). It is suggested that the growth process of nanocrystalline anatase
is mainly because of the sintering of the single crystals within the
agglomerates, and finally the original agglomerate transforms to a larger
single crystal [37]. However, as shown in Table 2, For MPC500 nanopowder is
observed that the initial anatase crystallite size increased more significantly
versus temperature, comparing other nanopowder TiO2.Table 2
Physical and chemical characteristics of different
TiO2 photocatalysts.
MaterialHeat-treatment   conditionsParticle size   (nm)(a)Scratch adhesion   (g/mm2)Sol-gel derived TiO2——150modified with MC500∘C, 1 h13.41500P25—18.8~0500∘C, 1 h19.2~0ANP—15~0500∘C, 1 h16.3~0MPC500—6.6~0500∘C, 1 h16.7~0ANPSGF-MC—14.99500∘C, 1 h16.3200MPC500SGF-MC—6.610500∘C, 1 h18.1350P25SGF-MC—19.210500∘C, 1 h18.6200(a) Using Scherrer equation: D=0.9λ/(Bcosθ), where λ=0.15418 nm.For the heat-treated sol-gel derived composite film, the
crystallite size is mainly determined by the particle size of the calcined
filler powder and remains approximately constant up to 500ºC. The average
size of the grains in TiO2 films prepared by P25, ANP, and MPC500 in
the sol was approximately 18.6, 16.3, and 18.1 nm, respectively. The grain size
of composite films, prepared with ANP which is 16.3 nm, is only a little larger than the average
crystal size of ANP particles (15 nm), which demonstrates that ANP particles in
the sol under these conditions cannot form much larger grains. The grain size
of composite film prepared by MPC500 particles in the sol is much larger than
the average crystal size of MPC500 particles (5–10 nm), which indicates
that MPC500 particles in the sol can aggregate to form larger grains. It is
worth to mention that the composite film with MPC500 has more scratch adhesion than other one which is confirmed
by XRD and SEM images.Because the decomposition temperature of MC as template is
below 400ºC, porous TiO2 composite films can be
prepared by calcining at 500∘C to remove the MC microspheres. However, conventional SEM
(Figure 3(a)) can only reveal a smooth surface of TiO2 films made from the precursor
sol containing no nanopowder heat treated for 1 hour at 500°C and no
individual TiO2 grain can be observed.
Figure 3(b) shows the
high-magnification field-emission ESEM micrograph ANPSGF-MC and Figure 3(c) shows
MPC500SGF-MC after calcining where nanocrystalline TiO2 particles
can be observed. As shown in Figures 3(b) and 3(c), the diameter of the
calcined TiO2 nanoparticles was small (less than 20 nm).Scanning electron micrographs of the surface of TiO2 films made
from the precursor sol containing (a) no nanopowder, (b) ANPSGF-MC-Ag, and (c)
MPC500SGF-MC-Ag. All the samples are heat treated for 1 hour at 500°C.
(a)(b)(c)In order to check the adhesion strength of these
photocatalytic films on the glass substrate, scratch test technique is performed on these films. All composite films
calcined at 500°C and the results are provided in Table 2, which demonstrates
that 500°C is the optimum calcination temperature at or above which excellent adhesion
between the nanostructure composite films and the glass substrate can be
obtained. For P25, ANP, and MPC500 TiO2 nanopowder coatings deposited from slurry
(i.e., without the sol binder), the normal load needed to remove the coatings
from the substrate is ~0 g/mm2. In fact, this load is so small that
can be considered as zero, especially when compared to the respective values
for the nanostructure composite films, in the range of 200–350 gr/mm2.The photocatalysis experiments took place in an aqueous
solution of methyl orange to evaluate the composite films activity. The
photocatalytic activity was evaluated as the percentage of pollutant
disappearance. The photodegradation of methyl orange was followed using absorption
atλmax
270 nm and 505 nm during the photodegradation reaction as a
function of irradiation time. The color of methyl orange solution changes from
orange to red and finally become colorless with increasing irradiation time. It
should be pointed out that decomposition of methyl orange itself under UV irradiation
is negligible. Each experimental set was repeated three times. The results were
reproducible within narrow limits (2%) and the mean value was selected.
Detailed preliminary tests were first performed in order to determine minor
silver deposition parameters such as dipping duration and UV illumination time.
The results with different AgNO3 solutions have shown that the
optimum conditions correspond to 3-second dipping and a 120-minute UV illumination,
respectively, leading to silver-modified titania films with the best
photocatalytic effect. It was also confirmed that the crucial parameter that
strongly influences the films photocatalytic activity is the concentration of
the AgNO3 dipping solution.The photocatalytic efficiency (degradation percentage corresponding
to 1-hour illumination) of the silver-doped titania nanostructure composite
films for the MO decomposition are illustrated in Figures4(a),
4(b), and 4(c).
The corresponding performance of the original nonsupported nanostructure
composite titania films is also given for comparison. The porous P25SGF-MC-Ag, MPC500SGF-MC-Ag, and ANPSGF-MC-Ag composite films
exhibited 69%, 42%, and 64% degradation, respectively, after the same exposure
time.Effect of the
Ag loading in the composite films on the photocatalytic activity, (a)
ANPSGF-MC-Ag, (b) MPC500SGF-MC-Ag, and (c) P25SGF-MC-Ag. Conditions of the
photocatalytic experiments: one-time spin coating (75 mm  × 25 mm × 1 mm) was used
as photocatalyst and irradiated with two 8 W UV-A (λ = 365 nm) at a distance of
5 cm from the top of the solutions. 20 mL of the dye solutions with initial
concentration of 5 mg/L at pH  =  4.5.
(a)(b)(c)As shown in Figure4(a), the results show that by increasing
the concentration of the dipping solution, the photocatalytic efficiency of
ANPSGF-MC increases. A maximum (64%) is observed for ANPSGF-MC-Ag films
immersed in 10−3 M AgNO3 solution. Further increase of the
Ag+ concentration results in a considerable efficiency decrease. For
instance, for ANPSGF-MC films dipped in 10−2 M AgNO3 solution the MO decomposition percentage goes to 58%. More spectacularly, it
falls to lower decomposition percentage for more concentrated AgNO3,
a value lower than that obtained with the nonsupported composite film. It is
suspected that superfluous Ag additions reduced the surface activation sites of
TiO2 and thus the degradation efficiency decreased.As shown in Figures4(b) and 4(c), the similar results are obtained
for other nanostructure composite films (MPC500SGF-MC-Ag and P25SGF-MC-Ag). A maximum
(42%) is observed for the MPC500SGF-MC-Ag films immersed in 10−4 M AgNO3 solution, while for P25SGF-MC-Ag films, maximum degradation percentage (69%) is
observed after immersion in 5×10−4 M AgNO3 solution. It is
interesting to note that the composite film with P25 has more photo efficiency than other one, however
photoefficiency of ANPSGF-MC after coating by Ag have promoted more than other
one comparing to the pure titania films. According to the results, ANPSGF-MC
composite films have more ability to dope with Ag particles comparing other
composite films (P25SGF-MC and MPC500SGF-MC), which is confirmed by difference
in grain size of TiO2 in these composite films.Sol-gel derived porous and nanostructure composite film prepared
by P25 (P25SGF-MC) after coating with Ag exhibited the best photocatalytic
performance. It has been reported that porous TiO2 films [4] or Ag-doped
TiO2 films [13] show an enhanced photocatalytic efficiency. In the
present study, we have demonstrated the improved photocatalytic performance
with both porous structure and Ag additions. Practical applications of these
films are highly potential.Figures5(a), 5(b),
and 5(c) show the photodegradation
results of the TiO2 nanostructure composite films, after deducting
both the self-degradation of methyl orange under UV and physical adsorption. Methyl
orange photodegradation kinetics permitted to directly compare the performance
of the silver-modified photocatalysts to the nonsupported materials. It is
clear that under the experimental conditions used, all photocatalytic curves
follow first-order reaction kinetics. Initial reaction conditions provide realistic
comparison of the photodegradation rates. To a reasonable approximation, the
slope of the fitted lines corresponds to a relative pseudo-first-order rate constant
[28]. Kinetic parameters after 2 hours of illumination for decolorization of MO
are summarized in Table 3.Table 3
The rate constant for degradation of MO for nanostructure
composite films with different mol percentages of silver.
Concentration of AgNO3 dipping solution (M)Rate constant (min−1)ANPSGF-MCMPC500SGF-MCP25SGF-MC00.00810.00520.00985×10−60.00980.00620.01151×10−50.01150.00690.01325×10−50.01280.00810.01581×10−40.01510.0090.01615×10−40.01620.00820.01811×10−30.01730.00640.01635×10−30.01660.00600.01581×10−20.01530.00510.0143(a) Plot of the percent of MO remaining after
photodegradation versus irradiation time in the presence of (a)  ANPSGF-MC
composite film, (b)  ANPSGF-MC-Ag composite film (immersed in
10-3 M AgNO3
solution), and (c)  TiO2 suspension. Conditions of the
photocatalytic experiments: one-time spin coating (75 mm  × 25 mm  × 1 mm) was used
as photocatalyst and irradiated with two 8 W UV-A (λ  =  365 nm) at a distance of
5 cm from the top of the solutions. 20 mL of the dye solutions with initial
concentration of 5 mg/L at pH = 4.5. (b) Plot of the percent of MO remaining after
photodegradation versus irradiation time in the presence of (a)  MPC500SGF-MC
composite film, (b)   MPC500SGF-MC-Ag composite film (immersed in
10-3 M
AgNO3 solution), and (c)    TiO2 suspension. Conditions of the photocatalytic experiments: one-time
spin coating (75 mm × 25 mm × 1 mm) was used as photocatalyst and irradiated
with two 8 W UV-A (λ = 365 nm) at a distance of 5 cm from the top of the
solutions. 20 mL of the dye solutions with initial concentration of 5 mg/L at
pH = 4.5. (c) Plot of the percent of MO remaining after
photodegradation versus irradiation time in the presence of (a)  P25SGF-MC
composite film, (b)   P25SGF-MC-Ag composite film (immersed in 10-3 M AgNO3
solution), and (c)    TiO2 suspension. Conditions of the photocatalytic experiments: one-time
spin coating (75 mm  × 25 mm  × 1 mm) was used as photocatalyst and irradiated
with two 8 W UV-A (λ = 365 nm) at a distance of 5 cm from the top of the
solutions. 20 mL of the dye solutions with initial concentration of 5 mg/L at
pH = 4.5.
(a)(b)(c)It is obvious that all nanostructure composite TiO2 films after coating by Ag are more efficient catalysts, comparing to the pure
titania film. This can be explained on the basis of surface characteristics. Complete
pollutant decolorization for P25SGF-MC-Ag was achieved after 120 minutes of illumination
and for P25SGF-MC film after about 240 minutes, respectively. As shown in Figures
4(a) and 4(b) and Table 3, the silver-modified composite film prepared by
MPC500 (MPC500SGF-MC-Ag) showed lower photocatalytic activity than the film
prepared by ANP (ANPSGF-MC-Ag). The impact of surface area on the
photocatalytic activity is clearly demonstrated, where
MPC500SGF-MC-Ag film
shows significantly lower degradation rate than ANPSGF-MC-Ag. As a result,
P25SGF-MC-Ag nanostructure composite films show more photoefficiency than other
composite films, which was as expected due to presence of rutile phase in
Degussa P-25 powder. We encapsulated Degussa P25 TiO2 instead of
preparing the films in order to have a combination of anatase and rutile phases
in the TiO2 films, since the presence of rutile phase enhances the
activity of the anatase phase by serving as an electrons’ sink. In fact, by
encapsulating P25 TiO2 within mesoporous TiO2 film, it
should be possible to optimize the two components of the composite film, for
example, by incorporating more active TiO2 particles [14]. In this
work, it was found that an incorporation of commercial anatase nanoparticles
into the TiO2 thin films, then coating them by Ag provides
advantages in the photodecomposition of TiO2 by eliminating the
filtration of the solutions, therefore, allowing for more efficient processes.To evaluate the catalytic strength changes due to
immobilization of the catalysts and compare the films efficiency to the Degussa
P25 powder, the film weight was measured. The slurry concentration of TiO2 was then adjusted to meet the exact quantity of TiO2 nanostructure
composite immobilized film, in order to safely compare the obtained results.
Thus, a pollutant solution (20 mL) was also photolyzed in the presence of a
suspension (dispersion) of Degussa P25 powder (0.0032 g = 160 ppm) and the
results are represented in Figures 5(a),
5(b), and 5(c). The slope for the
slurry is greater than that of the silver-modified nanostructure composite films,
showing a higher efficiency as a photocatalyst. Complete depolarization of MO
occurs in less than 90 minutes. It is worth mentioning that on composite film
samples, only a part of the catalyst is exposed to the pollutant solution and
it can, therefore, be photocatalytically active. As a result, a decrease in the
overall photocatalytic performance of composite films compared to slurry
solution is expected. This difference can be easily understood, if one
considers that the photocatalytic process is a surface and not a volume or mass
phenomenon. The active photocatalyst is the illuminated TiO2 material,
which can be in contact with the organic pollutant. As Degussa’s
powder surface
area is very high (50 m2g−1), the slurry’s surface is
much higher than the film’s real external surface and the silver synergetic
action cannot promote such a high efficiency. However, the specific surface
area of most of the composite samples, at the heat treatment (500∘C), changes
mostly due to the increase in the surface area of the sol-gel derived TiO2 (titania sol),
as a result of
the decomposition and oxidation of the organic residues (i.e., MC), the
evaporation of the solvent and excess water, and so  forth.


## 4. Conclusion

In the present study, we have prepared nanostructure
composite films developed on microscope glass sildes by a sol-gel spin coating technique
and were modified by silver deposition, characterized and successfully tested
for the photocatalytic degradation of the pollutant methyl orange. TiO2/Ag
films were obtained by introducing MC microspheres and calcining at a
temperature of 500∘C. All the prepared composite films exhibited anatase-phase
TiO2 as determined by XRD. MC-TiO2 films with Ag
additions exhibited porous nanostructure but differed from that of porous TiO2 film.  The photocatalytic properties of
the prepared thin films were evaluated by degrading methyl orange under UV. ANPSGF-MC-Ag, MPC500SGF-MC-Ag,
and P25SGF-MC-Ag composite films exhibited 64%, 42%, and 69% degradation,
respectively. By making the TiO2 film porous and adding Ag element,
the photocatalytic performance of the TiO2-based composite films can
be improved significantly.



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*Source: 628713-2008-04-24.xml*