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Enhancement of the photocatalytic activity of TiO2 film via surface modification of the substrate
Applied Catalysis A: General 226 (2002) 199–211
Enhancement of the photocatalytic activity of TiO2 film via surface modification of the substrate Chun-Guey Wu∗ , Liang-Feng Tzeng, Yen-Ting Kuo, Chung Hsien Shu Department of Chemistry, National Central University, Chung-Li 32054, Taiwan, ROC Received 1 May 2001; received in revised form 23 August 2001; accepted 5 September 2001
Abstract A new type of TiO2 film is prepared by insertion of a bifunctional silane agent bearing sulfonate group in between TiO2 and the substrate. The alkoxyl silane is bound to the surface of the substrate via molecular self-absorption and the terminal sulfonate group is used to grasp the TiO2 particles to form stable film. Modifications of the TiO2 catalysts, such as metal deposition and calcination, were carried out simply with the films on substrates. The activity of this new type TiO2 film is tested by catalytic photodegrading of salicylic acid in water. It was found that when the particle size of the TiO2 is smaller than 200 nm, the catalytic activities of the TiO2 films deposited on sulfonate-grafted glass are better than those of the films deposited on unmodified glass. Increasing the crystallinity of TiO2 particles, depositing the precious metals (such as Au or Pd) on TiO2 , or aging the TiO2 film will enhance the catalytic activity of TiO2 . The enhancement is better for the film deposited on sulfonate-grafted substrates. In spite of calcining the TiO2 film decreased its catalytic activity due to the particle aggregation, the decrease is also less significant for TiO2 films deposited on sulfonate-grafted substrates. The sulfonate group can be regarded as a dispersion agent for TiO2 film deposition and calcination. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium oxide; Sulfonate; Catalyst film; Photodegrading; Organic waste
1. Introduction Titanium oxide (TiO2 ) is a well-known pigment; its high refractive index (2.57 for anatase and 2.74 for rutile at 550 nm) and excellent transmittance to the visible light make it useful in antireflective coatings [1], dielectric layers [2], and optical filters [3]. TiO2 is also known as a useful photosensitive material, such as for photoanodes [4] and photocatalysts [5]. In fact, TiO2 is the most investigated photocatalyst for the degradation of pollutant chemicals. The advantages of using TiO2 in the photocatalytic decomposition of organic pol∗ Corresponding author. Tel.: +11-886-3-4227151x5903; fax: +11-886-3-4227664. E-mail address: [email protected] (C.-G. Wu).
lutants are based on its remarkable activity, low cost, chemical and radiation stability; no strong oxidizing agents, such as O3 or H2 O2 , are required [6]; and also its non-toxic properties are important. Many studies have reported about the relations between crystallographic structure and surface properties and the effect of these properties on the catalytic properties [7–9] of TiO2 . Nevertheless, the immobilization of TiO2 on various supports remains one of the prerequisites to obtain an effective catalyst, because it is a technological requirement to avoid the separation/filtration step. Different supports and different immobilization techniques have been investigated, such as immobilization of TiO2 powder in a polymer matrix [10], sand [11], Vycor glass [12], clay [13], zeolites [14,15] or electrophoretic deposition over conducting glass
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[16,17], stainless steel [18,19], Ti–Al alloy, titanium foil or tin oxide-coated glass [20]. Another route is the coating of the support by in situ catalyst generation via a sol–gel technique [21–25]. However, in spite of so many efforts, it is still unclear which methods and supports are the most convenient in terms of mechanical stability and photocatalytic activity. On the other hand, titanium oxide could be readily attached to a glass surface by allowing the TiO2 to be illuminated as a stationary phase with water passing over the catalyst. Thus open system continuous operation is possible [26]. It was reported [27] that TiO2 film coated on metal substrate was a catalyst film for photon decomposition of formic acid in water. Fujishima et al. [28]. proposed a system using a film of TiO2 coated on the inside of window glass for air purification of buildings. Fu et al. [29] coated the reactor wall with TiO2 gel to study the photon decomposition of benzene and ethylene in air. Pichat et al. [30] showed that TiO2 coated on glass mesh can be used for purification/deodorization of indoor air. It seems that TiO2 catalyst in the film form has the advantages of not only easy fabrication/separation and good catalytic activity but also suitability for some special practical applications. Nevertheless, the film thickness of the semiconductors is though to be an important factor affecting the photocatalytic activity, particularly when the catalyst is exposed to back face illumination. It was reported [31] that an optimal thickness of TiO2 exists for back face illumination. Cui et al. [32] also found that thicker TiO2 film has higher catalytic activity but the activity saturated at the thickness of 0.6 m. Therefore, a strategy for preparing thick TiO2 film with good stable and high activity is necessary. Furthermore, the catalytic activity of TiO2 film decreases when it is coated on glass substrate due to the contamination of the catalyst with the Na+ ions in the glass. This result indicated that the interface between catalyst film and substrate also plays an important role in the catalytic performance of the TiO2 . The use of functionalized self-absorbed monolayers (SAMs) as organic templates for the controlled deposition of the TiO2 films has been reported [33]. Here, we report a new synthetic strategy to prepare the homogeneous, well-adhered TiO2 film without the contamination from the substrate. The novel synthetic strategy is based on the insertion of a SAM of bifunctional alkoxylsilane bearing a sulfonate group (–SO3 H) in between the substrate
and TiO2 film. The alkoxylsilane group reacted with the hydroxyl group on the substrate to form a SAM. The terminal sulfonate groups were then used to fix the TiO2 particles to the substrate. In this article, the detailed preparation of new type TiO2 catalyst film is reported; its catalytic activity in the photodecomposition of salicylic acid in water was also tested.
2. Experimental 2.1. Chemicals TiCl4 , Ti(O´ı-Pr)4 , Degussa P-25 (particle size 440 nm, shown by X-ray diffraction analysis to be mostlyof the anatase form), salicylic acid, 3-mercapto-propyltrimethoxysilane, HCl (aq), CH3 COOH (aq), methyl alcohol, H2 SO4 (aq), H2 O2 (aq), HNO3 (aq), and potassium bromide were purchased from commercial resources and used without further treatment. Glass slides (trademark) were obtained from a local manufacturer and silicon wafers ((1 0 0) oriented, p-type) were obtained from Topsil Co. 2.2. The cleaning procedure for glass and silicon substrates The glass and silicon substrates were cut into 2 cm× 2 cm slides, then dipped in a mixture (7/3 in volume) of concentrated H2 SO4 and 30% H2 O2 at 90 ◦ C for 30 min (caution: piraha solution is a very dangerous solution with high oxidation power). The acid treated substrates were then washed with copious amounts of water and, dried with nitrogen gas. The cleaned glass and silicon substrates were used right after cleaning or stored in methanol prior to use. 2.3. The surface modification of glass and silicon substrates In a glove box, the cleaned glass and silicon substrates were dipped in 7 mM HSC3 H6 Si(OMe)3 / MeOH solution for 14 h, then washed with MeOH thoroughly, and dried with nitrogen gas at room temperature. The terminal –SH groups on the substrate were converted to –SO3 H by dipping the silane modified substrate in the mixture of acetic acid and hydrogen peroxide (30 ml CH3 COOH and 6 ml,
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30% H2 O2 ) for 30 min at 80 ◦ C. The oxidant-treated substrate was then washed extensively with distilled water and dried with a stream of nitrogen gas.
that the film was coated on both side of the substrate, We can simplify Eq. (1) as Eq. (2)
2.4. Preparation of TiO2 stock solution
The weight of TiO2 film was calculated from Eq. (3)
TiCl4 , Ti(O´ı-Pr)4 , and Degussa P-25 were used as Ti sources for preparing the TiO2 stock solution. • Using TiCl4 as a Ti source: 1.89 g of TiCl4 was dissolved in 20 ml 6 M HCl (aq), then heated at 80 ◦ C for 2 h. • Using Ti(O´ı-Pr)4 as a Ti source: a proper amount of Ti(O´ı-Pr)4 was mixed well with iso-proponal, then added slowly into 0.15 M HNO3 (aq). The mixture was stirred at room temperature for 1-day, kept at room temperature for 2 days without disturbance, and then heated at 80 ◦ C for 10 h. The mole ratio of Ti:´ı-PrOH:H2 O is 1:3:100–400. • Using Degussa P-25 as a Ti source: a proper amount of Degussa P-25 powder was dispersed in water to form TiO2 stock solution, P-25.
2t = 2.303
Abs α
wt = 2tAd
(2)
(3)
where wt is the weight of TiO2 on the substrate, A the area of the substrate (4 cm2 ) and d the density of TiO2 = 3.9 g/cm3 . 2.6. Modification of TiO2 films TiO2 films on substrates were dipped in 0.01 M AuCl3 /CH3 CN, PdCl2 /CH3 CN, or AgNO3 /H2 O, then exposed to the UV light (30 W, λ > 300 nm) for 24 h. The deposition of metal particles on the TiO2 film was confirmed with the ESCA spectra. Calcination of the TiO2 films was accomplished by heating the films at 400, 500, or 900 ◦ C in air for 2 h. 2.7. Photocatalytic activity of TiO2 films
2.5. Fabrication of titanium oxide catalyst films The sulfonate-grafted glass substrates (or unmodified glass substrates) were dipped in a TiO2 stock solution for 5 min, then pulled up with a speed of 5 cm/min, and finally dried in air. The thickness of the TiO2 films was controlled by the concentration of the TiO2 stock solution and the dip-coating cycles. The homogeneity of the film was tested by the UV–VIS absorption. Only homogenous films (the surface roughness is less than 10% of the film thickness) were used for studying the catalytic activity. The weight of the TiO2 on the substrate was calculated from the absorbance of the UV light at 280 nm using the following equation [34] α = 2.303 × 133 ×
Abs LCM
(1)
where α is the absorption coefficient e12 (∼160,000 cm−1 ), Abs the absorption intensity at 280 nm, L the optical path, C the molar concentration and M is the molecular weight (79.9 g/mol for TiO2 ). Assuming that the TiO2 film under discussion is a nonporous film with average thickness of t (cm) and
The catalytic activities of TiO2 films deposited on glass were investigated using the photodegrading of salicylic acid in water as test reactions. The experiments were carried out by dipping the TiO2 film on glass substrate in a salicylic acid aqueous solution (40 ml, 5×10−4 M) and stirring slightly. The solutions were irradiated with a 30 W high-pressure mercury arc (λ > 300 nm) in air. A blank reaction (salicylic acid solution without TiO2 catalyst) was run in parallel to determine the background decomposition of salicylic acid by UV photons. The decreasing of the salicylic acid after reacting for 24 h was used to calculate the catalytic activities of TiO2 films. The concentration of salicylic acid was monitored via its absorption intensity at 298 nm. The photocatalytic activity of the TiO2 film was evaluated as the amount of salicylic acid decomposed for 24 h irradiation at room temperature per gram of TiO2 used. 2.8. Physicochemical measurements Fourier transform infrared (FTIR) spectra were recorded as catalyst films on Si substrate using a
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Bio-Rad 155 FTIR spectrometer. The attenuated total reflectance (ATR) spectra of silane monolayer on silicon substrates were obtained from Bio-Rad FTS185 FTIR with a linearized MCT detector and a reflection accessory from Specac Analytic Inc. UV–VIS–NIR spectra were obtained using a Varian Cary 5E spectrophotometer in the laboratory atmosphere at room temperature. Static contact angles were measured with a homemade goniometer at room temperature and at ambient humidity. Water was used as a probe liquid. A 2 l water droplet was placed on the substrate with a syringe. The angle was obtained by estimating the tangent to the drop at its intersection with the surface, and three measurements were taken for the reported contact angle readings. X-ray photoelectron spectroscopy studies were carried out on a Perkin-Elmer PHI-590AM ESCA/XPS spectrometer system with a cylindrical mirror electron (CMA) energy analyzer. The X-ray sources were Al K␣ at 600 W. and Mg K␣ at 400 W. X-ray diffraction studies were performed with a Shimadzu XRD-6000 X-ray diffractometer using Cu K␣ radiation at 30 kV and 30 mA. Scanning electron micrographs (SEM) were recorded with a Hitachi S-800 at 15 kV. The samples for SEM imaging were mounted on metal stubs with a piece of conducting tape then coated with a thin layer of gold film to avoid charging. Transmission electron microscopy (TEM) was performed with a JEOL Jem-2000, FXII microscope at 120 kV. Dynamic light scattering measurements of the TiO2 particles were carried out with a Malvern Zetasizer 3000 laser light scattering spectrometer equipped with a He–Ne laser operated at λo = 633 nm.
3. Results and discussion 3.1. Surface modification of the glass and silicon substrates
alkoxylsilyl (or trichlorosilyl) group with the hydrated surface of the substrate [35]. The terminal –SH groups were further oxidized to –SO3 H with an oxidant in acidic solution. The terminal sulfonate group provides both a high degree of local (surface) acidity and net negative charge, thus promoting the hydrolysis and/or surface attachment of the solvated titanium-containing species. Sulfonate groups dissociate even in strong acid (6 M HCl) environment to –SO3 − and these sites are then available to react with the cationic complexes of Ti, promoting the attachment of the Ti complexes onto the SAM. The SAM also presents a more uniform and complete array of such sites, leading to adherent films with uniform thickness. The formation of a SAM thiol-silane on the substrate was confirmed with the contact angle, XPS, and ATR-IR data. After treating with the silane compound, the surface contact angle of the silicon substrate increased from 18 to 30◦ . This result was consistent with the deposition of organic molecules on an inorganic surface [36]. The terminal thiol group is quite hydrophilic; therefore, after deposition of the SAM, the contact angle of the substrate increases only 12◦ . XPS spectrum of the thiol-silane modified surface revealed an S2p peak at 167–169 eV, which is close to the binding energy of sulfur in a thiol compound. ATR-IR spectra of thiol-silane modified silicon wafer showed two well-defined broad peaks at 2874 and 2948 cm−1 . These two peaks are similar to the absorption peaks of bulk thiol-silane molecules, as shown in Fig. 1. The absorption intensity of the peaks is reasonable for the absorption of an organic monolayer. The surface –SH groups were further oxidized to –SO3 H with a H2 O2 /CH3 COOH solution. The formation of –SO3 H groups on the surface of the substrate was also confirmed with the ESCA spectra, which showed a peak at 169.5 eV. This peak is reasonably assigned to the binding energy of the S2p in a sulfonate group [37]. 3.2. Preparation of TiO2 stock solution
To attach the –SO3 H group on a solid substrate, we grafted the surface of the substrate with a silane compound bearing a thiol group. This reaction was carried out by absorbing the SHC3 H6 Si(OMe)3 on the surface of the substrate via molecular self-absorption. The chemical attachment of the self-absorption monolayer (SAM) of silane on glass substrate is based on the siloxane network formed by the reaction of the tri-
The Ti sources used in our experiments are TiCl4 , Ti(O´ı-Pr)4 and commercial Degussa P-25 (P-25). TiO2 films obtained from stock solution made from TiCl4 showed very low catalytic activity compared to those made from Ti(O´ı-Pr)4 and P-25. Therefore, the following discussions are focused on the TiO2 made from Ti(O´ı-Pr)4 , and P-25 is used for comparison. The
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Fig. 1. ATR-IR spectra of (a) bulk thiol-silane. (b) thiol-silane monolayer.
particle size of TiO2 sol obtained from Ti(O´ı-Pr)4 depends on the experimental conditions. In general, big TiO2 particles were obtained at lower acid concentration, higher water content and temperature. TiO2 with particle sizes in the range of 80–2000 nm was prepared from different reaction conditions. However, a homogeneous clear sol solution exists only when the particle size is smaller than 200 nm. White precipitate was formed when the average particle size is greater than 200 nm. X-ray powder diffraction pattern, Fig. 2, of TiO2 powder prepared from Ti(O´ı-Pr)4 at 80 ◦ C showed predominantly the anatase phase. The crystalline domain size is close to the size of the TiO2 sol in stock solution (measured with a Dynamic Light Scattering Spectrometer) when the particle size is smaller than 150 nm. In fact, exact control of the size-homogeneity and crystallinity of TiO2 via reaction conditions is not easy; therefore, the comparisons of the catalytic activity were based on the same batch of TiO2 stock solution. 3.3. The deposition and modification of TiO2 films on glass substrates The thickness of TiO2 films deposited on sulfonategrafted and unmodified substrates, obtained from one
dip-coating cycle, is similar. However, TiO2 films deposited on sulfonate-grafted substrates have a better adhesion and are more reproducible in film thickness for every dipping cycle. Under SEM (Fig. 3), TiO2 film deposited on sulfonate-grafted glass is smoother than that deposited on unmodified glass. The surface roughness of TiO films deposited on unmodified and sulfonate-grafted quartz (estimated from the atomic force microscope (AFM) data) are 27 and 20 nm, respectively. Nevertheless, both of the films are rather compact and homogeneous, therefore, the absorbance of the film can be used to calculate the average thickness and the weight of the TiO2 film on the glass substrates. The thickness of TiO2 film depends not only on the particle size but also on the dip-coating cycle. It was found (Fig. 4) that the thickness of TiO2 film increases as the number of dip-coating cycles increases and becomes saturated at certain thickness (depending on the particle size of TiO2 ). Fig. 4 also showed that the thickness increments are similar for each dip-coating cycle, except the first one. This finding suggested that the sulfonate group on the substrate interacted with only the first layer of TiO2 . Therefore, only TiO2 films obtained from one dip-coating cycle were used for studying the catalytic activity. A TiO2 film obtained from one dip-coating cycle is very
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Fig. 2. X-ray powder diffraction patterns (a) Degussa P-25. (b and c) TiO2 powder prepared from Ti(O´ı-Pr)4 at 80 ◦ C and dried at 50 ◦ C (the particle size of (b) is 150 nm; the particle size of (c) is 800 nm).
stable; it remains intact after reacting in a salicylic acid aqueous solution for 100 days. Adding of Au, Pd, and Ag particles in TiO2 films was achieved easily by dipping the film on the substrate in AuCl3 /CH3 CN, PdCl2 /CH3 CN, and AgNO3 /H2 O solution, respectively and exposing the film to the UV light for 24 h at room temperature. The metal ions were photo-reduced to metal particles, which were confirmed with the ESCA data. Au and Pd metals distributed evenly and adhered tightly on TiO2 films deposited on both sulfonate-grafted and unmodified glass. Nevertheless, the process for deposition of Ag metal on TiO2 film also damages the catalyst film, probably because of the NO3 − in AgNO3 was decomposed to form a reactive gaseous product when excited with UV photons. The Ag/TiO2 film comes off the substrate when it was deposited on unmodified glass; nevertheless, only broken film was observed on Ag/TiO2 deposited on sulfonate-grafted glass. The insertion of sulfonate–silane compound in between TiO2 film and glass substrate can enhance the
stability of TiO2 film not only in the film preparation but also during the modification procedure. 3.4. The effects of the sulfonate group on the performance of TiO2 catalyst films deposited on it The mechanism of photocatalytic degrading of organic compounds is believed to involve absorption of an UV photon by TiO2 to produce an electron–hole pair. Both hole and electron can react with water to yield hydroxyl and superoxide radicals which oxidize the organic molecules. The ultimate products of these reactions are CO2 and water [38,39]. The photodecomposed products of salicylic acid are also CO2 and H2 O; therefore, the amount of salicylic acid decomposed can be monitored by the decreasing of the absorption intensity of salicylic acid at 298 nm. Fig. 5 shows a plot of salicylic acid concentration versus time of exposure of the solution to UV light, in the presence of TiO2 film deposited on sulfonate-grafted glass. The figure revealed that this new type TiO2 film has good
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Fig. 3. SEM micrographs of TiO2 films deposited on (a) sulfonate-grafted glass and (b) unmodified glass.
photocatalytic reactivity on the degrading of salicylic acid in water. The kinetics of photodecomposition of organic molecules on TiO2 has been explained in terms of a Langmuir model [40]. At a low concentration of substrate (organic molecules), this model predicts
simple pseudo-first-order kinetics with respect to the substrate concentration. A plot of the concentration of salicylic acid versus reaction time gives a straight line (Fig. 5); suggesting that the same model can be applied to the TiO2 catalyst film fabricated with our
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Fig. 4. Dependence of the thickness of TiO2 films on the number of dip-coating cycles.
Fig. 5. UV–VIS absorption change due to the catalytic photodeposition of salicylic acid in water (insertion: a plot of the concentration of salicylic acid vs. reaction time).
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method. We are interested in understanding the effects of the sulfonate group on the catalytic performance of the TiO2 films. Therefore, only the activity of the first 24 h was used to evaluate the catalytic performance of TiO2 films deposited on both sulfonate-grafted and unmodified glass. The catalytic activity of TiO2 films is affected by several parameters, such as particle size, crystallinity, morphology, surface area, fabrication procedure, and even trace amounts of contamination. Therefore, the preparation and catalytic reaction of TiO2 film were carried out at well-controlled environments and the experiment conditions were kept as similar as possible. The effects of the sulfonate group on the catalytic activity of the TiO2 film deposited over it are quite complicated. They depend also on the particle size of the TiO2 in stock solution, film thickness, modification of TiO2 , as well as aging of the TiO2 stock solution and TiO2 film. In general, thin films (made from small particles) of TiO2 deposited on SO3 − grafted glass has a higher catalytic activity compared to that of those deposited on unmodified glass. For big TiO2 particles, the function of the small sulfonate group underneath is insignificant. The parameters which affected the catalytic performance of the TiO2 films deposited on sulfonate-grafted glass are discussed individually at the following paragraphs. 3.4.1. The particle size of TiO2 The catalyst performance for the gas-phase photooxidation degradation of trichloroethylene exhibited strong dependence on the particle sizes of TiO2 [41]. We also found that the particle size of TiO2 in stock
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solution affects the catalytic activity of the TiO2 film in the photodecomposition of salicylic acid in water, as listed in Table 1. The results showed that smaller TiO2 particles have a higher catalytic activity. Moreover, when the particle size of the TiO2 is smaller than 200 nm, TiO2 deposited on sulfonate-grafted glass has a higher activity than those deposited on unmodified glass for the films with similar thickness. The increasing of the catalytic activity for TiO2 deposited on sulfonate-grafted glass may be due to the fact that the sulfonate group is able to react with the surface OH groups of TiO2 particles. If the surface of TiO2 particles has too many OH groups, the catalytic activity will decrease [42]. 3.4.2. The thickness of TiO2 film The dependence of catalytic activity on the film thickness of TiO2 is shown in Fig. 6. The thickness of TiO2 film was calculated from the absorption intensity at 280 nm using the equation mentioned in the experimental section. It was found that the catalytic activities of the films increase as the film thickness increases for TiO2 deposited on both sulfonate-grafted and unmodified glass. Nevertheless, the slope of curve ‘a’ is higher than that of curve ‘b’, indicating that the activity increment per unit amount of TiO2 is higher for the film deposited on sulfonate-grafted glass. These results suggested that TiO2 film deposited on sulfonate-grafted glass may has a sparser structure. For a sparse film, light penetration and oxygen diffusion are easier; therefore, it will have higher catalytic activity compared to the denser film for the same amount (the same absorption intensity) of TiO2 .
Table 1 The particle sizes and the corresponding catalytic activities of TiO2 films Particle size of TiO2 sol (nm)a
Activity of TiO2 deposited on sulfonate-grafted glassb
Activity of TiO2 deposited on unmodified glassb
Average film thickness of TiO2 (nm)
1900 900 300 200 150 100 80 445
3.60 5.93 10.5 12.1 12.2 20.2 18.3 13.6
5.76 7.23 10.8 12.0 12.0 17.5 15.8 14.3
89 94 150 155 200 89 121 130
a b
The TiO2 sols are made from Ti(O´ı-Pr)4 except the last one, which is Degussa P-25. The activity was represented as mmole of salicylic acid decomposed per gram of TiO2 after reacting for 24 h.
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Fig. 6. The dependence of catalytic activity on the thickness of TiO2 films deposited on (a) sulfonate-grafted glass and (b) unmodified glass.
Table 2 The changes of catalytic activity of TiO2 film after calcination Substrate
Sulfonate-grafted glass Glass a b
Catalysta TiO2 film
TiO2 film heated at 400 ◦ C
TiO2 film heated at 500 ◦ C
TiO2 film heated at 900 ◦ C
19.8b 19.6
15.5 8.0
14.9 8.4
10.3 9.0
TiO2 films were made from Ti(O´ı-Pr)4 , the thickness of the film before metal deposition is 0.54 m. The catalytic activity was represented as mmol salicylic acid decomposed per gram of TiO2 after reacting for 24 h.
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Fig. 7. Electron diffraction patterns of TiO2 sol (a) as prepared and (b) after aging for 70 days.
3.4.3. Calcination of the TiO2 films The changes of catalytic activity of TiO2 film after calcination at 400, 500 and 900 ◦ C are listed in Table 2. It was found that the catalytic activity of TiO2 decreased after calcinations; the decreasing of the activity is less significant for the film deposited on sulfonate-grafted glass. It was known that calcination of TiO2 increases its crystallinity, but at the same time also increases the particle size due to thermal-induced aggregation. The aggregation of TiO2 particles under calcination was revealed by UV–VIS spectra. It was found that the absorption intensity of TiO2 film deposited on unmodified quartz changed after calcination. On the other hand, after calcination, the absorption intensity of TiO2 film deposited on sulfonate-grafted quartz did not change obviously. The finding indicated that the underneath sulfonate groups, although they were oxidatively decomposed upon heating in air, 1 can hinder the gathering of the TiO2 particles during thermal treatment. 1 We did not find the S single on the ESCA spectrum of the sulfonate-grafted substrate after calcination. This result suggested that the alkylsulfonate molecules on the surface of the substrate were oxidatively decomposed during calcination.
3.4.4. The aging of TiO2 stock solution and TiO2 film We found that TiO2 film prepared from the aged stock solution showed higher catalytic activity. The activity enhancement is more significant for TiO2 deposited on sulfonate-grafted glass than that deposited on unmodified glass. It was reported [43] that Ti alkoxy sol solution aging in acidic solution increased the crystallinity. We also found that the crystallinity of TiO2 particle increased after aging, as revealed by the electron diffraction micrographs shown in Figs. 6 and 7. Surprisingly, although the aging of the stock solution increased the crystallinity of TiO2 , the particle size did not increase. For big particles, the particle size even decreased after aging, indicating that the increasing crystallinity is not due to the aggregation of small particles to form big grains. The high crystallinity of the aged sol comes from the restructuring of the TiO2 in the acidic solution. It seems that the sulfonate group underneath has a stronger effect on the catalytic activity of small TiO2 particles with high crystallinity (consequently high activity). This phenomenon could be due to the fact that the density of OH groups on a TiO2 surface is higher for smaller particles; therefore, the effect of the sulfonate group on the catalytic activity of TiO2 film is greater. Aging
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TiO2 film in ambient atmosphere also enhances its catalytic activity slightly. The catalytic activity increment of aged TiO2 film deposited on sulfonate-grafted glass is also slightly better than that deposited on unmodified glass. Ding and He [44] have shown that aging the dry gel in room temperature will increase the crystallinity of anatase phase. Maybe the crystallinity of TiO2 film also increased upon aging, although the film is too thin to give a good X-ray diffraction pattern. 3.4.5. The lifetime of the TiO2 catalyst films TiO2 films deposited on sulfonate-grafted glass are very stable. They can be used continuously for 100 days in aqueous solution without losing their catalytic activity. No damage of the film was observed and the IR spectrum of TiO2 film did not show any organic contamination after reacting for 100 days in salicylic acid aqueous solution.
4. Conclusions We have synthesized a new type of TiO2 catalyst film, in which the TiO2 particles were adhered on the glass substrate via sulfonate groups. TiO2 films deposited on sulfonate-grafted substrates have a sparse structure; the reagents, UV light, and oxygen gas can penetrate inside the film and the photocatalytic decomposition products can diffuse out of the film easily, thus they have not only a good adhesion but also high stability and catalytic activity. The underneath sulfonate groups can also avoid the aggregation of the particles when the film on substrate was calcinated at high temperature. The major function of sulfonate groups is believed to be a dispersion agent for TiO2 film deposition and calcination. Another function of the sulfonate groups is that they are able to react with the OH groups on TiO2 surface. Those OH groups will react with the electron–hole pairs of the catalyst after irradiating with an UV light, consequently decreasing the catalytic activity.
Acknowledgements Financial support of this work by the Chinese Petroleum Company and National Science Council
of the Republic of China (Grant no. NSC-89-CPC7-008-008) is gratefully acknowledged. References [1] B.E. Yoldas, T.W. O’Keefe, Appl. Opt. 18 (1979) 3133. [2] T. Fuyuki, H. Matsunami, Jpn. J. Appl. Phys. 9 (1986) 1288. [3] M. Lottiaux, C. Boulesteix, G. Nihoul, F. Varnier, F. Flory, R. Galindo, E. Pelletier, Thin Solid Films 170 (1989) 107. [4] A. Fujishima, K. Honda, Nature 238 (1972) 37. [5] M.R. Hoffmann, S.T. Martin, W. Choi, D.W. Bahnemann, Chem. Rev. 95 (1995) 69. [6] A. Braun, in: E. Pelizzetti (Ed.), Photochemical Conversion and Storage of Solar Energy, Kluwer, Dordrecht, 1991, p. 551. [7] A.P. Rivera, K. Tanaka, T. Hisanaga, Appl. Catal. 3 (1993). [8] S.-J. Tsai, S. Cheng, Catal. Today 33 (1997) 227. [9] D.S. Muggli, J.L. Falconer, J. Catal. 175 (1998) 213. [10] V. Brezová, M. Jankoviæová, M. Soldan, A. Blazcová, M. Reháková, M. Surina, M. Ceppan, B. Havlinová, J. Photochem. Photobiol. A 76 (1993) 103. [11] R.W. Matthews, Water Res. 25 (1991) 1169. [12] M. Anpo, N. Aikawa, Y. Kubokawa, M. Che, C. Louis, E.J. Giamello, Phys. Chem. 89 (1985) 5017. [13] H. Yoneyama, S. Haga, J. Phys. Chem. 93 (1989) 4833. [14] Y.M. Xu, C.H. Langford, J. Phys. Chem. 99 (1995) 11501. [15] S. Cheng, S.-J. Tsai, Y.-F. Lee, Catal. Today 26 (1995) 87. [16] D. Matthews, A. Kay, M. Grätzel, Aust. J. Chem. 47 (1994) 1869. [17] K. Vinodgopal, P.V. Kamat, Sol. Energy Mater. Sol. Cells 38 (1995) 401. [18] A. Fernández, G. Lassaletta, V.M. Jiménez, A. Justo, A.R. González-Elipe, J.-M. Herrmann, H. Tahiri, Y. Ait-Ichou, Appl. Catal. 7 (1995) 49. [19] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Appl. Catal. 17 (1998) 25. [20] J.A. Byrne, B.R. Eggins, N.M.D. Brown, B. McKinney, M. Rouse, Appl. Catal. 17 (1998) 25. [21] J. Sabate, M.A. Anderson, H. Kikkawa, M. Edwards, C.G. Hill Jr., J. Catal. 127 (1991) 167. [22] M.A. Aguado, M.A. Anderson, Sol. Energy Mater. Sol. Cells 28 (1993) 345. [23] V.N. Parmon, Catal. Today 39 (1997) 207. [24] D.H. Kim, M.A. Anderson, Environ. Sci. Technol. 28 (1994) 479. [25] A. Sobzynsky, A.J. Bard, A. Campion, M.A. Fox, S.E. Malloux, S.E. Webwer, J.M. White, J. Phys. Chem. 91 (1987) 3316. [26] R.W. Matthews, Sol. Energy 38 (1987) 405. [27] Y.H. Heung, M.A. Anderson, J. Environ. Eng. 122 (1996) 217. [28] A. Fujishima, K. Hashimoto, Y.J. Kubota, Surf. Sci. Soc. Jpn. 16 (1995) 188. [29] X.Z. Fu, L.A. Clark, W.A. Zeltner, M.A. Anderson, J. Photochem. Photobiol. A: Chem. 97 (1996) 181. [30] P. Pichat, J. Disdier, C. Hoang-Van, D. Mas, G. Goutailler, C. Gaysse, Catal. Today 63 (2000) 363.
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Enhancement of the photocatalytic activity of TiO2 film via surface modification of the substrate Chun-Guey Wu∗ , Liang-Feng Tzeng, Yen-Ting Kuo, Chung Hsien Shu Department of Chemistry, National Central University, Chung-Li 32054, Taiwan, ROC Received 1 May 2001; received in revised form 23 August 2001; accepted 5 September 2001
Abstract A new type of TiO2 film is prepared by insertion of a bifunctional silane agent bearing sulfonate group in between TiO2 and the substrate. The alkoxyl silane is bound to the surface of the substrate via molecular self-absorption and the terminal sulfonate group is used to grasp the TiO2 particles to form stable film. Modifications of the TiO2 catalysts, such as metal deposition and calcination, were carried out simply with the films on substrates. The activity of this new type TiO2 film is tested by catalytic photodegrading of salicylic acid in water. It was found that when the particle size of the TiO2 is smaller than 200 nm, the catalytic activities of the TiO2 films deposited on sulfonate-grafted glass are better than those of the films deposited on unmodified glass. Increasing the crystallinity of TiO2 particles, depositing the precious metals (such as Au or Pd) on TiO2 , or aging the TiO2 film will enhance the catalytic activity of TiO2 . The enhancement is better for the film deposited on sulfonate-grafted substrates. In spite of calcining the TiO2 film decreased its catalytic activity due to the particle aggregation, the decrease is also less significant for TiO2 films deposited on sulfonate-grafted substrates. The sulfonate group can be regarded as a dispersion agent for TiO2 film deposition and calcination. © 2002 Elsevier Science B.V. All rights reserved. Keywords: Titanium oxide; Sulfonate; Catalyst film; Photodegrading; Organic waste
1. Introduction Titanium oxide (TiO2 ) is a well-known pigment; its high refractive index (2.57 for anatase and 2.74 for rutile at 550 nm) and excellent transmittance to the visible light make it useful in antireflective coatings [1], dielectric layers [2], and optical filters [3]. TiO2 is also known as a useful photosensitive material, such as for photoanodes [4] and photocatalysts [5]. In fact, TiO2 is the most investigated photocatalyst for the degradation of pollutant chemicals. The advantages of using TiO2 in the photocatalytic decomposition of organic pol∗ Corresponding author. Tel.: +11-886-3-4227151x5903; fax: +11-886-3-4227664. E-mail address: [email protected] (C.-G. Wu).
lutants are based on its remarkable activity, low cost, chemical and radiation stability; no strong oxidizing agents, such as O3 or H2 O2 , are required [6]; and also its non-toxic properties are important. Many studies have reported about the relations between crystallographic structure and surface properties and the effect of these properties on the catalytic properties [7–9] of TiO2 . Nevertheless, the immobilization of TiO2 on various supports remains one of the prerequisites to obtain an effective catalyst, because it is a technological requirement to avoid the separation/filtration step. Different supports and different immobilization techniques have been investigated, such as immobilization of TiO2 powder in a polymer matrix [10], sand [11], Vycor glass [12], clay [13], zeolites [14,15] or electrophoretic deposition over conducting glass
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[16,17], stainless steel [18,19], Ti–Al alloy, titanium foil or tin oxide-coated glass [20]. Another route is the coating of the support by in situ catalyst generation via a sol–gel technique [21–25]. However, in spite of so many efforts, it is still unclear which methods and supports are the most convenient in terms of mechanical stability and photocatalytic activity. On the other hand, titanium oxide could be readily attached to a glass surface by allowing the TiO2 to be illuminated as a stationary phase with water passing over the catalyst. Thus open system continuous operation is possible [26]. It was reported [27] that TiO2 film coated on metal substrate was a catalyst film for photon decomposition of formic acid in water. Fujishima et al. [28]. proposed a system using a film of TiO2 coated on the inside of window glass for air purification of buildings. Fu et al. [29] coated the reactor wall with TiO2 gel to study the photon decomposition of benzene and ethylene in air. Pichat et al. [30] showed that TiO2 coated on glass mesh can be used for purification/deodorization of indoor air. It seems that TiO2 catalyst in the film form has the advantages of not only easy fabrication/separation and good catalytic activity but also suitability for some special practical applications. Nevertheless, the film thickness of the semiconductors is though to be an important factor affecting the photocatalytic activity, particularly when the catalyst is exposed to back face illumination. It was reported [31] that an optimal thickness of TiO2 exists for back face illumination. Cui et al. [32] also found that thicker TiO2 film has higher catalytic activity but the activity saturated at the thickness of 0.6 m. Therefore, a strategy for preparing thick TiO2 film with good stable and high activity is necessary. Furthermore, the catalytic activity of TiO2 film decreases when it is coated on glass substrate due to the contamination of the catalyst with the Na+ ions in the glass. This result indicated that the interface between catalyst film and substrate also plays an important role in the catalytic performance of the TiO2 . The use of functionalized self-absorbed monolayers (SAMs) as organic templates for the controlled deposition of the TiO2 films has been reported [33]. Here, we report a new synthetic strategy to prepare the homogeneous, well-adhered TiO2 film without the contamination from the substrate. The novel synthetic strategy is based on the insertion of a SAM of bifunctional alkoxylsilane bearing a sulfonate group (–SO3 H) in between the substrate
and TiO2 film. The alkoxylsilane group reacted with the hydroxyl group on the substrate to form a SAM. The terminal sulfonate groups were then used to fix the TiO2 particles to the substrate. In this article, the detailed preparation of new type TiO2 catalyst film is reported; its catalytic activity in the photodecomposition of salicylic acid in water was also tested.
2. Experimental 2.1. Chemicals TiCl4 , Ti(O´ı-Pr)4 , Degussa P-25 (particle size 440 nm, shown by X-ray diffraction analysis to be mostlyof the anatase form), salicylic acid, 3-mercapto-propyltrimethoxysilane, HCl (aq), CH3 COOH (aq), methyl alcohol, H2 SO4 (aq), H2 O2 (aq), HNO3 (aq), and potassium bromide were purchased from commercial resources and used without further treatment. Glass slides (trademark) were obtained from a local manufacturer and silicon wafers ((1 0 0) oriented, p-type) were obtained from Topsil Co. 2.2. The cleaning procedure for glass and silicon substrates The glass and silicon substrates were cut into 2 cm× 2 cm slides, then dipped in a mixture (7/3 in volume) of concentrated H2 SO4 and 30% H2 O2 at 90 ◦ C for 30 min (caution: piraha solution is a very dangerous solution with high oxidation power). The acid treated substrates were then washed with copious amounts of water and, dried with nitrogen gas. The cleaned glass and silicon substrates were used right after cleaning or stored in methanol prior to use. 2.3. The surface modification of glass and silicon substrates In a glove box, the cleaned glass and silicon substrates were dipped in 7 mM HSC3 H6 Si(OMe)3 / MeOH solution for 14 h, then washed with MeOH thoroughly, and dried with nitrogen gas at room temperature. The terminal –SH groups on the substrate were converted to –SO3 H by dipping the silane modified substrate in the mixture of acetic acid and hydrogen peroxide (30 ml CH3 COOH and 6 ml,
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30% H2 O2 ) for 30 min at 80 ◦ C. The oxidant-treated substrate was then washed extensively with distilled water and dried with a stream of nitrogen gas.
that the film was coated on both side of the substrate, We can simplify Eq. (1) as Eq. (2)
2.4. Preparation of TiO2 stock solution
The weight of TiO2 film was calculated from Eq. (3)
TiCl4 , Ti(O´ı-Pr)4 , and Degussa P-25 were used as Ti sources for preparing the TiO2 stock solution. • Using TiCl4 as a Ti source: 1.89 g of TiCl4 was dissolved in 20 ml 6 M HCl (aq), then heated at 80 ◦ C for 2 h. • Using Ti(O´ı-Pr)4 as a Ti source: a proper amount of Ti(O´ı-Pr)4 was mixed well with iso-proponal, then added slowly into 0.15 M HNO3 (aq). The mixture was stirred at room temperature for 1-day, kept at room temperature for 2 days without disturbance, and then heated at 80 ◦ C for 10 h. The mole ratio of Ti:´ı-PrOH:H2 O is 1:3:100–400. • Using Degussa P-25 as a Ti source: a proper amount of Degussa P-25 powder was dispersed in water to form TiO2 stock solution, P-25.
2t = 2.303
Abs α
wt = 2tAd
(2)
(3)
where wt is the weight of TiO2 on the substrate, A the area of the substrate (4 cm2 ) and d the density of TiO2 = 3.9 g/cm3 . 2.6. Modification of TiO2 films TiO2 films on substrates were dipped in 0.01 M AuCl3 /CH3 CN, PdCl2 /CH3 CN, or AgNO3 /H2 O, then exposed to the UV light (30 W, λ > 300 nm) for 24 h. The deposition of metal particles on the TiO2 film was confirmed with the ESCA spectra. Calcination of the TiO2 films was accomplished by heating the films at 400, 500, or 900 ◦ C in air for 2 h. 2.7. Photocatalytic activity of TiO2 films
2.5. Fabrication of titanium oxide catalyst films The sulfonate-grafted glass substrates (or unmodified glass substrates) were dipped in a TiO2 stock solution for 5 min, then pulled up with a speed of 5 cm/min, and finally dried in air. The thickness of the TiO2 films was controlled by the concentration of the TiO2 stock solution and the dip-coating cycles. The homogeneity of the film was tested by the UV–VIS absorption. Only homogenous films (the surface roughness is less than 10% of the film thickness) were used for studying the catalytic activity. The weight of the TiO2 on the substrate was calculated from the absorbance of the UV light at 280 nm using the following equation [34] α = 2.303 × 133 ×
Abs LCM
(1)
where α is the absorption coefficient e12 (∼160,000 cm−1 ), Abs the absorption intensity at 280 nm, L the optical path, C the molar concentration and M is the molecular weight (79.9 g/mol for TiO2 ). Assuming that the TiO2 film under discussion is a nonporous film with average thickness of t (cm) and
The catalytic activities of TiO2 films deposited on glass were investigated using the photodegrading of salicylic acid in water as test reactions. The experiments were carried out by dipping the TiO2 film on glass substrate in a salicylic acid aqueous solution (40 ml, 5×10−4 M) and stirring slightly. The solutions were irradiated with a 30 W high-pressure mercury arc (λ > 300 nm) in air. A blank reaction (salicylic acid solution without TiO2 catalyst) was run in parallel to determine the background decomposition of salicylic acid by UV photons. The decreasing of the salicylic acid after reacting for 24 h was used to calculate the catalytic activities of TiO2 films. The concentration of salicylic acid was monitored via its absorption intensity at 298 nm. The photocatalytic activity of the TiO2 film was evaluated as the amount of salicylic acid decomposed for 24 h irradiation at room temperature per gram of TiO2 used. 2.8. Physicochemical measurements Fourier transform infrared (FTIR) spectra were recorded as catalyst films on Si substrate using a
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Bio-Rad 155 FTIR spectrometer. The attenuated total reflectance (ATR) spectra of silane monolayer on silicon substrates were obtained from Bio-Rad FTS185 FTIR with a linearized MCT detector and a reflection accessory from Specac Analytic Inc. UV–VIS–NIR spectra were obtained using a Varian Cary 5E spectrophotometer in the laboratory atmosphere at room temperature. Static contact angles were measured with a homemade goniometer at room temperature and at ambient humidity. Water was used as a probe liquid. A 2 l water droplet was placed on the substrate with a syringe. The angle was obtained by estimating the tangent to the drop at its intersection with the surface, and three measurements were taken for the reported contact angle readings. X-ray photoelectron spectroscopy studies were carried out on a Perkin-Elmer PHI-590AM ESCA/XPS spectrometer system with a cylindrical mirror electron (CMA) energy analyzer. The X-ray sources were Al K␣ at 600 W. and Mg K␣ at 400 W. X-ray diffraction studies were performed with a Shimadzu XRD-6000 X-ray diffractometer using Cu K␣ radiation at 30 kV and 30 mA. Scanning electron micrographs (SEM) were recorded with a Hitachi S-800 at 15 kV. The samples for SEM imaging were mounted on metal stubs with a piece of conducting tape then coated with a thin layer of gold film to avoid charging. Transmission electron microscopy (TEM) was performed with a JEOL Jem-2000, FXII microscope at 120 kV. Dynamic light scattering measurements of the TiO2 particles were carried out with a Malvern Zetasizer 3000 laser light scattering spectrometer equipped with a He–Ne laser operated at λo = 633 nm.
3. Results and discussion 3.1. Surface modification of the glass and silicon substrates
alkoxylsilyl (or trichlorosilyl) group with the hydrated surface of the substrate [35]. The terminal –SH groups were further oxidized to –SO3 H with an oxidant in acidic solution. The terminal sulfonate group provides both a high degree of local (surface) acidity and net negative charge, thus promoting the hydrolysis and/or surface attachment of the solvated titanium-containing species. Sulfonate groups dissociate even in strong acid (6 M HCl) environment to –SO3 − and these sites are then available to react with the cationic complexes of Ti, promoting the attachment of the Ti complexes onto the SAM. The SAM also presents a more uniform and complete array of such sites, leading to adherent films with uniform thickness. The formation of a SAM thiol-silane on the substrate was confirmed with the contact angle, XPS, and ATR-IR data. After treating with the silane compound, the surface contact angle of the silicon substrate increased from 18 to 30◦ . This result was consistent with the deposition of organic molecules on an inorganic surface [36]. The terminal thiol group is quite hydrophilic; therefore, after deposition of the SAM, the contact angle of the substrate increases only 12◦ . XPS spectrum of the thiol-silane modified surface revealed an S2p peak at 167–169 eV, which is close to the binding energy of sulfur in a thiol compound. ATR-IR spectra of thiol-silane modified silicon wafer showed two well-defined broad peaks at 2874 and 2948 cm−1 . These two peaks are similar to the absorption peaks of bulk thiol-silane molecules, as shown in Fig. 1. The absorption intensity of the peaks is reasonable for the absorption of an organic monolayer. The surface –SH groups were further oxidized to –SO3 H with a H2 O2 /CH3 COOH solution. The formation of –SO3 H groups on the surface of the substrate was also confirmed with the ESCA spectra, which showed a peak at 169.5 eV. This peak is reasonably assigned to the binding energy of the S2p in a sulfonate group [37]. 3.2. Preparation of TiO2 stock solution
To attach the –SO3 H group on a solid substrate, we grafted the surface of the substrate with a silane compound bearing a thiol group. This reaction was carried out by absorbing the SHC3 H6 Si(OMe)3 on the surface of the substrate via molecular self-absorption. The chemical attachment of the self-absorption monolayer (SAM) of silane on glass substrate is based on the siloxane network formed by the reaction of the tri-
The Ti sources used in our experiments are TiCl4 , Ti(O´ı-Pr)4 and commercial Degussa P-25 (P-25). TiO2 films obtained from stock solution made from TiCl4 showed very low catalytic activity compared to those made from Ti(O´ı-Pr)4 and P-25. Therefore, the following discussions are focused on the TiO2 made from Ti(O´ı-Pr)4 , and P-25 is used for comparison. The
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Fig. 1. ATR-IR spectra of (a) bulk thiol-silane. (b) thiol-silane monolayer.
particle size of TiO2 sol obtained from Ti(O´ı-Pr)4 depends on the experimental conditions. In general, big TiO2 particles were obtained at lower acid concentration, higher water content and temperature. TiO2 with particle sizes in the range of 80–2000 nm was prepared from different reaction conditions. However, a homogeneous clear sol solution exists only when the particle size is smaller than 200 nm. White precipitate was formed when the average particle size is greater than 200 nm. X-ray powder diffraction pattern, Fig. 2, of TiO2 powder prepared from Ti(O´ı-Pr)4 at 80 ◦ C showed predominantly the anatase phase. The crystalline domain size is close to the size of the TiO2 sol in stock solution (measured with a Dynamic Light Scattering Spectrometer) when the particle size is smaller than 150 nm. In fact, exact control of the size-homogeneity and crystallinity of TiO2 via reaction conditions is not easy; therefore, the comparisons of the catalytic activity were based on the same batch of TiO2 stock solution. 3.3. The deposition and modification of TiO2 films on glass substrates The thickness of TiO2 films deposited on sulfonategrafted and unmodified substrates, obtained from one
dip-coating cycle, is similar. However, TiO2 films deposited on sulfonate-grafted substrates have a better adhesion and are more reproducible in film thickness for every dipping cycle. Under SEM (Fig. 3), TiO2 film deposited on sulfonate-grafted glass is smoother than that deposited on unmodified glass. The surface roughness of TiO films deposited on unmodified and sulfonate-grafted quartz (estimated from the atomic force microscope (AFM) data) are 27 and 20 nm, respectively. Nevertheless, both of the films are rather compact and homogeneous, therefore, the absorbance of the film can be used to calculate the average thickness and the weight of the TiO2 film on the glass substrates. The thickness of TiO2 film depends not only on the particle size but also on the dip-coating cycle. It was found (Fig. 4) that the thickness of TiO2 film increases as the number of dip-coating cycles increases and becomes saturated at certain thickness (depending on the particle size of TiO2 ). Fig. 4 also showed that the thickness increments are similar for each dip-coating cycle, except the first one. This finding suggested that the sulfonate group on the substrate interacted with only the first layer of TiO2 . Therefore, only TiO2 films obtained from one dip-coating cycle were used for studying the catalytic activity. A TiO2 film obtained from one dip-coating cycle is very
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Fig. 2. X-ray powder diffraction patterns (a) Degussa P-25. (b and c) TiO2 powder prepared from Ti(O´ı-Pr)4 at 80 ◦ C and dried at 50 ◦ C (the particle size of (b) is 150 nm; the particle size of (c) is 800 nm).
stable; it remains intact after reacting in a salicylic acid aqueous solution for 100 days. Adding of Au, Pd, and Ag particles in TiO2 films was achieved easily by dipping the film on the substrate in AuCl3 /CH3 CN, PdCl2 /CH3 CN, and AgNO3 /H2 O solution, respectively and exposing the film to the UV light for 24 h at room temperature. The metal ions were photo-reduced to metal particles, which were confirmed with the ESCA data. Au and Pd metals distributed evenly and adhered tightly on TiO2 films deposited on both sulfonate-grafted and unmodified glass. Nevertheless, the process for deposition of Ag metal on TiO2 film also damages the catalyst film, probably because of the NO3 − in AgNO3 was decomposed to form a reactive gaseous product when excited with UV photons. The Ag/TiO2 film comes off the substrate when it was deposited on unmodified glass; nevertheless, only broken film was observed on Ag/TiO2 deposited on sulfonate-grafted glass. The insertion of sulfonate–silane compound in between TiO2 film and glass substrate can enhance the
stability of TiO2 film not only in the film preparation but also during the modification procedure. 3.4. The effects of the sulfonate group on the performance of TiO2 catalyst films deposited on it The mechanism of photocatalytic degrading of organic compounds is believed to involve absorption of an UV photon by TiO2 to produce an electron–hole pair. Both hole and electron can react with water to yield hydroxyl and superoxide radicals which oxidize the organic molecules. The ultimate products of these reactions are CO2 and water [38,39]. The photodecomposed products of salicylic acid are also CO2 and H2 O; therefore, the amount of salicylic acid decomposed can be monitored by the decreasing of the absorption intensity of salicylic acid at 298 nm. Fig. 5 shows a plot of salicylic acid concentration versus time of exposure of the solution to UV light, in the presence of TiO2 film deposited on sulfonate-grafted glass. The figure revealed that this new type TiO2 film has good
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Fig. 3. SEM micrographs of TiO2 films deposited on (a) sulfonate-grafted glass and (b) unmodified glass.
photocatalytic reactivity on the degrading of salicylic acid in water. The kinetics of photodecomposition of organic molecules on TiO2 has been explained in terms of a Langmuir model [40]. At a low concentration of substrate (organic molecules), this model predicts
simple pseudo-first-order kinetics with respect to the substrate concentration. A plot of the concentration of salicylic acid versus reaction time gives a straight line (Fig. 5); suggesting that the same model can be applied to the TiO2 catalyst film fabricated with our
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Fig. 4. Dependence of the thickness of TiO2 films on the number of dip-coating cycles.
Fig. 5. UV–VIS absorption change due to the catalytic photodeposition of salicylic acid in water (insertion: a plot of the concentration of salicylic acid vs. reaction time).
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method. We are interested in understanding the effects of the sulfonate group on the catalytic performance of the TiO2 films. Therefore, only the activity of the first 24 h was used to evaluate the catalytic performance of TiO2 films deposited on both sulfonate-grafted and unmodified glass. The catalytic activity of TiO2 films is affected by several parameters, such as particle size, crystallinity, morphology, surface area, fabrication procedure, and even trace amounts of contamination. Therefore, the preparation and catalytic reaction of TiO2 film were carried out at well-controlled environments and the experiment conditions were kept as similar as possible. The effects of the sulfonate group on the catalytic activity of the TiO2 film deposited over it are quite complicated. They depend also on the particle size of the TiO2 in stock solution, film thickness, modification of TiO2 , as well as aging of the TiO2 stock solution and TiO2 film. In general, thin films (made from small particles) of TiO2 deposited on SO3 − grafted glass has a higher catalytic activity compared to that of those deposited on unmodified glass. For big TiO2 particles, the function of the small sulfonate group underneath is insignificant. The parameters which affected the catalytic performance of the TiO2 films deposited on sulfonate-grafted glass are discussed individually at the following paragraphs. 3.4.1. The particle size of TiO2 The catalyst performance for the gas-phase photooxidation degradation of trichloroethylene exhibited strong dependence on the particle sizes of TiO2 [41]. We also found that the particle size of TiO2 in stock
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solution affects the catalytic activity of the TiO2 film in the photodecomposition of salicylic acid in water, as listed in Table 1. The results showed that smaller TiO2 particles have a higher catalytic activity. Moreover, when the particle size of the TiO2 is smaller than 200 nm, TiO2 deposited on sulfonate-grafted glass has a higher activity than those deposited on unmodified glass for the films with similar thickness. The increasing of the catalytic activity for TiO2 deposited on sulfonate-grafted glass may be due to the fact that the sulfonate group is able to react with the surface OH groups of TiO2 particles. If the surface of TiO2 particles has too many OH groups, the catalytic activity will decrease [42]. 3.4.2. The thickness of TiO2 film The dependence of catalytic activity on the film thickness of TiO2 is shown in Fig. 6. The thickness of TiO2 film was calculated from the absorption intensity at 280 nm using the equation mentioned in the experimental section. It was found that the catalytic activities of the films increase as the film thickness increases for TiO2 deposited on both sulfonate-grafted and unmodified glass. Nevertheless, the slope of curve ‘a’ is higher than that of curve ‘b’, indicating that the activity increment per unit amount of TiO2 is higher for the film deposited on sulfonate-grafted glass. These results suggested that TiO2 film deposited on sulfonate-grafted glass may has a sparser structure. For a sparse film, light penetration and oxygen diffusion are easier; therefore, it will have higher catalytic activity compared to the denser film for the same amount (the same absorption intensity) of TiO2 .
Table 1 The particle sizes and the corresponding catalytic activities of TiO2 films Particle size of TiO2 sol (nm)a
Activity of TiO2 deposited on sulfonate-grafted glassb
Activity of TiO2 deposited on unmodified glassb
Average film thickness of TiO2 (nm)
1900 900 300 200 150 100 80 445
3.60 5.93 10.5 12.1 12.2 20.2 18.3 13.6
5.76 7.23 10.8 12.0 12.0 17.5 15.8 14.3
89 94 150 155 200 89 121 130
a b
The TiO2 sols are made from Ti(O´ı-Pr)4 except the last one, which is Degussa P-25. The activity was represented as mmole of salicylic acid decomposed per gram of TiO2 after reacting for 24 h.
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Fig. 6. The dependence of catalytic activity on the thickness of TiO2 films deposited on (a) sulfonate-grafted glass and (b) unmodified glass.
Table 2 The changes of catalytic activity of TiO2 film after calcination Substrate
Sulfonate-grafted glass Glass a b
Catalysta TiO2 film
TiO2 film heated at 400 ◦ C
TiO2 film heated at 500 ◦ C
TiO2 film heated at 900 ◦ C
19.8b 19.6
15.5 8.0
14.9 8.4
10.3 9.0
TiO2 films were made from Ti(O´ı-Pr)4 , the thickness of the film before metal deposition is 0.54 m. The catalytic activity was represented as mmol salicylic acid decomposed per gram of TiO2 after reacting for 24 h.
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Fig. 7. Electron diffraction patterns of TiO2 sol (a) as prepared and (b) after aging for 70 days.
3.4.3. Calcination of the TiO2 films The changes of catalytic activity of TiO2 film after calcination at 400, 500 and 900 ◦ C are listed in Table 2. It was found that the catalytic activity of TiO2 decreased after calcinations; the decreasing of the activity is less significant for the film deposited on sulfonate-grafted glass. It was known that calcination of TiO2 increases its crystallinity, but at the same time also increases the particle size due to thermal-induced aggregation. The aggregation of TiO2 particles under calcination was revealed by UV–VIS spectra. It was found that the absorption intensity of TiO2 film deposited on unmodified quartz changed after calcination. On the other hand, after calcination, the absorption intensity of TiO2 film deposited on sulfonate-grafted quartz did not change obviously. The finding indicated that the underneath sulfonate groups, although they were oxidatively decomposed upon heating in air, 1 can hinder the gathering of the TiO2 particles during thermal treatment. 1 We did not find the S single on the ESCA spectrum of the sulfonate-grafted substrate after calcination. This result suggested that the alkylsulfonate molecules on the surface of the substrate were oxidatively decomposed during calcination.
3.4.4. The aging of TiO2 stock solution and TiO2 film We found that TiO2 film prepared from the aged stock solution showed higher catalytic activity. The activity enhancement is more significant for TiO2 deposited on sulfonate-grafted glass than that deposited on unmodified glass. It was reported [43] that Ti alkoxy sol solution aging in acidic solution increased the crystallinity. We also found that the crystallinity of TiO2 particle increased after aging, as revealed by the electron diffraction micrographs shown in Figs. 6 and 7. Surprisingly, although the aging of the stock solution increased the crystallinity of TiO2 , the particle size did not increase. For big particles, the particle size even decreased after aging, indicating that the increasing crystallinity is not due to the aggregation of small particles to form big grains. The high crystallinity of the aged sol comes from the restructuring of the TiO2 in the acidic solution. It seems that the sulfonate group underneath has a stronger effect on the catalytic activity of small TiO2 particles with high crystallinity (consequently high activity). This phenomenon could be due to the fact that the density of OH groups on a TiO2 surface is higher for smaller particles; therefore, the effect of the sulfonate group on the catalytic activity of TiO2 film is greater. Aging
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TiO2 film in ambient atmosphere also enhances its catalytic activity slightly. The catalytic activity increment of aged TiO2 film deposited on sulfonate-grafted glass is also slightly better than that deposited on unmodified glass. Ding and He [44] have shown that aging the dry gel in room temperature will increase the crystallinity of anatase phase. Maybe the crystallinity of TiO2 film also increased upon aging, although the film is too thin to give a good X-ray diffraction pattern. 3.4.5. The lifetime of the TiO2 catalyst films TiO2 films deposited on sulfonate-grafted glass are very stable. They can be used continuously for 100 days in aqueous solution without losing their catalytic activity. No damage of the film was observed and the IR spectrum of TiO2 film did not show any organic contamination after reacting for 100 days in salicylic acid aqueous solution.
4. Conclusions We have synthesized a new type of TiO2 catalyst film, in which the TiO2 particles were adhered on the glass substrate via sulfonate groups. TiO2 films deposited on sulfonate-grafted substrates have a sparse structure; the reagents, UV light, and oxygen gas can penetrate inside the film and the photocatalytic decomposition products can diffuse out of the film easily, thus they have not only a good adhesion but also high stability and catalytic activity. The underneath sulfonate groups can also avoid the aggregation of the particles when the film on substrate was calcinated at high temperature. The major function of sulfonate groups is believed to be a dispersion agent for TiO2 film deposition and calcination. Another function of the sulfonate groups is that they are able to react with the OH groups on TiO2 surface. Those OH groups will react with the electron–hole pairs of the catalyst after irradiating with an UV light, consequently decreasing the catalytic activity.
Acknowledgements Financial support of this work by the Chinese Petroleum Company and National Science Council
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