Origin of Photocatalytic Deactivation of TiO2 Film ... - ACS Publications

The deactivation origin of the TiO2 film coated on an Al2O3-based ceramic substrate was revealed via investigating the physicochemical behaviors of th...
0 downloads 0 Views 316KB Size
Download PDF
9968

J. Phys. Chem. C 2007, 111, 9968-9974

Origin of Photocatalytic Deactivation of TiO2 Film Coated on Ceramic Substrate Tai-Hua Xie and Jun Lin* Department of Chemistry, Renmin UniVersity of China, Beijing 100872, People’s Republic of China ReceiVed: March 24, 2007

The deactivation origin of the TiO2 film coated on an Al2O3-based ceramic substrate was revealed via investigating the physicochemical behaviors of the mixtures of TiO2 with different oxides of various species found in the film such as Al, Si, and Ca. The production of these species mainly present as individual oxides at the boundaries among the TiO2 particles in the film originated from their thermal diffusion from substrate during the formation of TiO2 film at calcination temperature. It was demonstrated that Al present as Al2O3 with TiO2 caused a remarkable reduction in the production of hydroxyl radicals over the UV-irradiated TiO2, whereas the presence of SiO2 or CaO, especially SiO2, together with TiO2 gave a rise in the formation of hydroxyl radicals. The investigation also showed that a part of Al3+ substitutes for Ti4+ in the TiO2, resulting in the formation of oxygen vacancies in the lattice for the charge balance. The oxygen vacancies present in the TiO2 lattice acting as effective combination centers of photogenerated electrons and holes were proposed to be responsible for the deactivation. In addition, the presence of an additional SiO2 layer between the TiO2 film and the ceramic substrate, effectively inhibiting the diffusion of Al from the ceramic substrate into the TiO2 film, was found to recover the photocatalytic activity. The fabrication procedures and characterization results of the TiO2 films on the ceramic substrate were also described in details. This work would provide some useful information on the selection of support material for TiO2 photocatalysis application in environmental pollution remediation.

1. Introduction TiO2 has been intensively investigated as an efficient photocatalyst for air and water purification in the past three decades.1-3 Generally, the widely reported working forms of TiO2 in photocatalysis include highly dispersed fine particles and suspended particles in liquid medium. For a practical application, however, there is a serious drawback that the recycling of particulate TiO2 after treatment is very difficult.4 Therefore, great efforts were undertaken to immobilize TiO2 on a substrate to form a thin film. Several methods for immobilizing TiO2 have been well documented, including solgel process,5 chemical vapor deposition,6 liquid-phase deposition,7 and magnetic sputtering.8 The fabrication of an optical transparent, mesoporous, and crystalline TiO2 thin film with high adhesion on various substrates became feasible using these methods. It was also reported that TiO2 films coated on glass, tiles, and stainless steel possess some special functions such as antibacterial, antifogging, and self-cleaning under ultraviolet light.9-11 Although TiO2 thin film on a substrate displays its great advantages and becomes a promising form for its application in environmental pollution remediation, it cannot exhibit a high photocatalytic activity on all substrates and suffers from deactivation and efficient charge recombination. Recently, many research groups started to devote themselves to investigating the deactivation of TiO2 thin film coated on a substrate because this problem is indispensable for the future applications of TiO2 photocatalysis technology. The TiO2 coating on glass is an intensely and typically investigated topic. Tada et al.12 reported that the photocatalytic activity of a TiO2 film was lower on a * To whom correspondence should be addressed. E-mail: jlin@ chem.ruc.edu.cn. Phone: (+8610)-62516222. Fax: (+8610)-62516444.

soda-lime glass substrate than on quartz plate. It was attributed to Na+ ions diffused from the soda-lime glass, which acts as the recombination center or disorders the crystallinity of TiO2.12 Similar conclusions that Na+ ions from glass substrate are detrimental to the photocatalytic activity of the TiO2 thin film were supported by several reports.13-15 More recently, Nam et al. revealed that Na+ and Si4+ ions diffused from the glass substrate raised the anatase formation temperature and gave a rise to the particle size of TiO2 thin film, resulting in the reduction of the photocatalytic activity.16 The deactivation of a TiO2 thin film was also reported when a metal such as aluminum and stainless steel was used as a substrate.17-19 Ceramics is extensively used as the materials for the outer walls of building and the production of various hygienic facilities. To keep these ceramic walls and facilities from biological and chemical contamination, a self-cleaning surface on them is necessary. Currently, the coating of TiO2 films on these ceramic materials became a feasible solution. However, except for a few practical applications, not much attention was paid to the deactivation of TiO2 films on ceramics. In this work, a transparent crystalline TiO2 thin film on an Al2O3-based ceramic tile was fabricated by brushing with a TiO2 sol solution before a thermal treatment. The fabricated TiO2 film was shown to be poorly active for the degradation of formaldehyde in air. To reveal the deactivation origin of the TiO2 film, the mixtures of TiO2 with SiO2, Al2O3, or CaO that are present in the film were prepared, respectively. These mixtures were characterized with X-ray photoelectron spectroscopy (XPS), X-ray diffraction (XRD), and electron spin resonance (ESR). Finally, a better comprehension on the deactivation origin of TiO2 film coated on the ceramic substrate was offered according to the physicochemical behaviors of these mixtures.

10.1021/jp072334h CCC: $37.00 © 2007 American Chemical Society Published on Web 06/08/2007

Deactivation Origin of TiO2 Film Coated on Ceramic Substrate

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9969

2. Experimental Section 2.1. Fabrication of TiO2 Film Coated on Ceramic Substrate. The sol solution for coating TiO2 on ceramic substrates was prepared using the method similar to that reported by Legrand-Buscema et al.20 Titanium isopropoxide and acetylacetone with a molar ratio at 2.85 were mixed together to form a complex. Acetic acid and isopropanol were then added under ambient conditions with vigorous stirring to make a stable sol solution in which the molar ratio of acetic acid to titanium alkoxide was controlled at 0.2 and the concentration of titanium alkoxide could be 0.25 M. In this system, acetic acid substituted for water to initiate hydrolysis via an esterification reaction

R-OH + R′-COOH S H2O + RCOOR′

(1)

The substrate employed here was Al2O3-based ceramic tile with a dimension of 10 × 10 cm2. Prior to coating TiO2 thin film on it, a series of pretreatment procedures were taken first. The tiles were put into a diluted detergent solution and a 95% ethanol solution to go through sonication respectively for 30 min. Then, another sonication was taken in deionized water for 30 min as well. Next, the tiles went through a thermal treatment in an oven at 100 °C for 2 h. After the pretreatment processes, TiO2 gel film coated on the ceramic tiles was prepared from the above TiO2 sol solution by brushing in an ambient atmosphere. The TiO2 gel film on the ceramic tile was calcined in air at 520 °C for 1 h for organics removal and crystallization. After the calcination, a transparent polycrystalline TiO2 film on the ceramic tile was obtained. The SiO2 layer on the substrate ceramic tile was prepared from the SiO2 sol solution according to the literature.21 With the above TiO2 sol solution, a transparent polycrystalline TiO2 film on the ceramic tile precoated with a SiO2 layer was also prepared in the same manner above. 2.2. Preparation of Mixtures of TiO2 with Al2O3, SiO2, or CaO. To investigate the effects of various species such as Al3+, Si4+, and Ca2+ diffusing from the ceramic substrate and being present as individual oxides in the TiO2 film on the photocatalytic activity, the mixtures of TiO2 with Al2O3, SiO2, or CaO were prepared by the following methods. The crystalline Al2O3, SiO2, or CaO powders were added into the TiO2 sol solution above under vigorous stirring at the ratio of Ti/cation similar to that in the TiO2 thin film on the ceramic tile, as measured by XPS. The resulting suspension was heated in a water bath at 50 °C to remove volatile substances. The resultant was further dried at 100 °C into TiO2 xerogel along with the oxide and was grounded into powder before calcined in air at 520 °C for 1 h. Meanwhile, pure TiO2 powder was also prepared in the same manner using the TiO2 sol solution for coating TiO2 on ceramic substrates. 2.3. Characterization of TiO2 Films, Pure TiO2, and Mixtures of TiO2 with Al2O3, SiO2, or CaO. Scanning electron microscopy (SEM) observation was carried out on a JEOL JSM7401F field emission scanning electron microscope. UV-vis diffuse reflectance spectra of the films were recorded on a Hitachi U-3310 spectrophotometer equipped with a diffuse reflectance accessory. XPS data was obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300W Al KR radiation. The base pressure was about 3 × 10-9 mbar. The binding energies were referenced to the C1s line at 284.8 eV from adventitious carbon. XRD patterns of the TiO2 films coated on substrates were recorded on a Bruker D8 Advance X-ray diffractometer with Cu KR radiation with the operation voltage and current maintained at 40 kV and 40 mA, respectively. XRD analysis of all powder samples was

Figure 1. Photocatalytic degradation of formaldehyde in air over TiO2 films coated on ceramic tile and SiO2-precoated ceramic tile under UV irradiation. Starting concentration of formaldehyde in air (Co): ∼0.350 mg/m3. Starting relative humidity in the reactor: 42%. Temperature in the reactor: 25 °C.

carried out at room temperature with Rigaku D/max-2500 using Cu KR radiation under 40 kV and 200 mA. All of the powder samples were scanned from 20 to 60° for 2θ angle. ESR experiments were performed at room temperature with a Brucker ESR 300E spectrometer. The irradiation source (λ ) 355 nm) was a Quanta-Ray Nd: YAG-pulsed laser system. The settings for ESR spectrometer were centerfield ) 3480 G; sweep width ) 100.0 G, microwave frequency ) 9.77 GHz and power ) 12.71 mW. The aqueous suspensions of various powder samples were prepared in 10 mL vessels and then were sonicated for 2 h first. The volume of the suspensions needed for measurement was 0.3 mL. The volume of 5,5-dimethyl-1pyrroline-N-oxide (DMPO) with concentration at 0.4 M, a kind of hydroxyl radicals trapper usually used in ESR measurement, was 14 µL each time. To minimize experimental errors, the same quartz capillary tube was used for all ESR measurements. 2.4. Measurements of Photocatalytic Activity. The photocatalytic activity experiments of TiO2 films for the degradation of formaldehyde in air were performed in a 1.2 m3 reactor at an ambient temperature. A fan installed inside the reactor was used to agitate the air. In this reactor, two pieces of the ceramic tile coated with TiO2 film were put into a small stainless steel box with two opened ends. A circle 8W UV light with an emission peak at 254 nm was horizontally positioned between two TiO2-coated ceramic plates in the box. The distance between the UV light and each ceramic tile is about 1.8 cm. With a small fan connected with one end of the box, the air in the reactor was flowed through two ceramic plates at a rate of 12 m3/hr. A small amount of formaldehyde aqueous solution (∼38%) was injected into the reactor. The formaldehyde vapor was allowed to reach an adsorption/desorption equilibrium under the agitation of the fan inside the reactor. The formaldehyde concentration remained until the UV light inside the box was switched on. At the given interval, 2 L air in the reactor was collected for analysis. The concentration of formaldehyde was analyzed by the 3-methyl-2-benzothiazolone hydrazone (MBTH) spectrophotometric method,22 in which the absorbance of the solution sample at 630 nm was recorded using a Hitachi U-3310 spectrophotometer. 3. Results and Discussion 3.1. Photocatalytic Performance for the Degradation of Formaldehyde in Air. Figure 1 illustrates the photocatalytic degradation of formaldehyde in air over the UV-irradiated TiO2 films coated on the substrates calcined at 520 °C for 1 h. On

9970 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Figure 2. Time profiles for the cyclic degradation of formaldehyde in air over the TiO2 film coated on SiO2-precoated ceramic tile. Starting concentration of formaldehyde (Co) ranging from 0.345∼ 0.364 mg/m3.

the basis of Figure 1, little degradation of formaldehyde is observed in the case of UV irradiation alone (without TiO2coated ceramic tiles). The TiO2 film coated on the SiO2precoated ceramic tile exhibits significantly higher photocatalytic activity than that on the ceramic tile. Less than 20% formaldehyde is photocatalytically degraded over the TiO2 film coated on ceramic tile after UV irradiation for 7 h, whereas the photocatalytic degradation of formaldehyde occurs over the TiO2 film coated on the SiO2-precoated ceramic tiles with an appreciable rate, causing a removal yield of near 80% within UV irradiation for 7 h. Such a big difference between two kinds of TiO2 films in the photocatalytic activity seems to be attributed to the presence of the SiO2 layer between the TiO2 film and the ceramic substrate as the fabrication conditions for coating both TiO2 films are totally the same. Bard et al.23 demonstrated an enhanced photocatalytic decomposition of phenol on TiO2/ SiO2. They ascribed the enhanced photocatalytic activity to the TiO2/SiO2 interface region where the TiOSi phase serves to activate the photocatalytic process via strong Bronsted acid sites.24,25 The studies by Espinos et al.26 also showed the presence of new oxygen ions in the form of Ti-O-Si crosslinking bonds at the interface acting as a bridge between TiO2 and SiO2. Further comprehension for the photocatalytic performances of the TiO2 films coated on the ceramic tiles with and without SiO2 layer will be provided below based on the characterization results. Moreover, recycling capability of the TiO2 film coated on the SiO2-precoated ceramic tiles was also evaluated by an additional photocatalytic degradation of formaldehyde at the given time intervals (one week). The TiO2 film coated on the SiO2-precoated ceramic tiles was reutilized over six times (one time/per week), each time the reaction was followed for 7 h. As shown in Figure 2, the photocatalytic activity was kept like the freshly prepared. 3.2. SEM Observation and UV-vis Spectra. SEM observations indicate that both TiO2 films coated on ceramic tile and SiO2-precoated ceramic tile have similar morphology and surface roughness. Cross-sectional view and morphology of TiO2 film coated on the ceramic tile are observed by SEM as seen in Figure 3. It was found that the rigid surface of the ceramic substrate is covered by TiO2 film. A few small holes on the film are observed, which might be caused by the removal of organic composition in the sol solution during calcination at high temperature. The thickness of both TiO2 films coated on ceramic tile and SiO2-precoated ceramic tile is estimated to be in the range of 0.8∼1.0 µm. It was reported that TiO2 film thickness greatly affects its photocatalytic activity since TiO2

Xie and Lin film’s ability to absorb UV light is strongly dependent on its thickness.27,28 As shown in Figure 4, two films display similar optical properties with an absorption onset at about 380 nm. This result indicates that the thickness difference between two TiO2 films is negligible. The extra peaks observed at the wavelength range of 400∼500 nm could be due to the substrate itself and interference caused by the film’s surface. 3.3. X-ray Photoelectron Spectroscopy (XPS). Figure 5 displays the XPS survey spectra for the surface of TiO2 films coated on the ceramic tile and SiO2-precoated ceramic tile, both of which were calcined at 520 °C for 1 h. The binding energies and atom percentage of various elements in the TiO2 films coated on the two substrates and all powder samples are given in Tables 1 and 2, respectively. From XPS measurements, it was found that the TiO2 film on the ceramic tile contains elements Ti, O, C, Al, Si, and Ca. The photoelectron peaks for Ti2p appear clearly at a binding energy Eb of 458.5 eV, O1s at Eb ) 529.9 eV, and C1s at Eb ) 285.0 eV. The peaks for Al2p, Si2p, and Ca2p are at binding energies 74.8, 103.2, and 347.3 eV, respectively. The presence of Al, Si, and Ca in the XPS spectra suggests that these elements migrate from the ceramic substrate into the TiO2 film during calcination. The element C shown in the XPS spectra is attributed to residual carbon on the film. As shown in Figure 5 and Table 1, only can the elements Ti, O, C, Si, and Ca be detected in the TiO2 film coated on SiO2-precoated ceramic tile. The results indicate that the SiO2 layer on the ceramic tile effectively prevents the diffusion of Al from the ceramic substrate into the film. It is not a surprise that Ca is still found in the TiO2 film coated on SiO2-precoated ceramic tile since CaO in the ceramic tile could be dissolved in the acidic SiO2 sol solution. Taking the binding energies shown in Table 1 into account, the elements Si and Ca in two TiO2 films coated on ceramic tile and SiO2-precoated ceramic tile are supposed to have the same present forms. Although these signals associated with the ceramic substrate elements such as Al, Si, and Ca are observed together in the film, no reaction among these elements occurs because the binding energies of these elements are found to be almost the same as those of their individual oxides.29 Therefore, we may conclude here that these elements from the substrate are mainly present as their individual oxides at the boundaries among the TiO2 particles of the film. On the basis of the XPS analysis (Table 1 and 2), there are almost no differences between the two kinds of TiO2 films in impurity element composition and their binding energies with an exception of element Al that was only detected in the TiO2 film coated on ceramic tile. Thus, the presence of the element Al would be responsible for the poor photocatalytic activity of the TiO2 film coated on the ceramic tile. Considering the occurrence of no reactions among these substrate elements in TiO2 film and the difficulties in a careful analysis of the TiO2 film using current instruments, we may be allowed to investigate the physicochemical behaviors of the mixtures of TiO2 with Al2O3, SiO2, or CaO to reveal how their presence in TiO2 film influence the photocatalytic activity. In accordance to XPS measurements on pure TiO2 and the mixtures of TiO2 with Al2O3, SiO2, or CaO, shown in Table 1 and 2, the binding energies for Al2p, Si2p, and Ca2p in the mixtures are almost the same as those in the TiO2 films. The results indicate that these elements in the mixtures are present as the same forms as those in the TiO2 films. Moreover, the composition of Al or Si in the TiO2/Al2O3 or TiO2/SiO2 mixture is also designed to be similar to that in the TiO2 film coated on ceramic tile. To obtain obvious measurement results below, the

Deactivation Origin of TiO2 Film Coated on Ceramic Substrate

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9971

Figure 3. Cross-sectional (a) and surface (b) SEM micrographs of TiO2 film coated on ceramic tile.

as Si, Al, and Ca present in the TiO2 thin film on photocatalysis via understanding physicochemical behaviors of the mixtures of TiO2 with Al2O3, SiO2, or CaO.

Figure 4. UV-vis diffuse reflectance spectra of the TiO2 films coated on (A) SiO2-precoated ceramic tile and (B) ceramic tile.

ratio of Ti to Ca in the TiO2/CaO is increased by about two times of that in TiO2 films. Thus, to a great extent, it is reasonable to reveal the individual effects of these elements such

3.4. X-ray Diffraction (XRD). XRD is used to investigate the crystalline structures of the TiO2 films, pure TiO2, and the TiO2 in various mixtures. XRD results reveal that the TiO2 films on the two substrates exhibit anatase structure with a similar crystallinity (see Supporting Information below), which rules out the possibility that the poor photocatalytic activity of the TiO2 film coated on ceramic tile is caused by the phase form and crystallinity of TiO2. Figure 6 displays X-ray diffraction patterns of pure TiO2 and various mixtures. It is obviously observed from Figure 6 that all TiO2 in various powder samples calcined at 520 °C for 1 h are present as anatase form only, no rutile or brookite phase are detected. Besides, the mixtures of TiO2 with Al2O3 or CaO do not show measurable Al2O3 or CaO diffraction peaks. The pattern of the TiO2/SiO2 mixture shows an impurity peak indexed to SiO2 along with anatase TiO2. The

9972 J. Phys. Chem. C, Vol. 111, No. 27, 2007

Xie and Lin TABLE 3: Crystal Size, Lattice Parameter, and Cell Volume of Anatase TiO2 in Various Samples

Figure 5. XPS survey spectra for the surface of the TiO2 films coated on (A) SiO2-precoated ceramic tile and (B) ceramic tile.

TABLE 1: Binding Energies for Various Species Detected in TiO2 Films Coated on Two Substrates and Various Powder Samplesa sample TiO2 film 1a TiO2 film 2b TiO2/Al2O3 TiO2/SiO2 TiO2/CaO a

Ti2p (eV) O1s (eV) Si2p (eV) Ca2p (eV) Al2p (eV) 458.5 458.5 458.0 459.0 458.6

529.9 529.9 529.4 530.2 529.9

103.2 103.2

347.3 347.1

74.8 74.9

103.0 346.6

1a, coated on ceramic tile; 2b, coated on SiO2-precoated ceramic

tile

TABLE 2: Composition (atom percentage) of TiO2 Films and Various Mixtures Based on XPS Analysisa sample

O

Ti

C

Al

Si

Ca

TiO2 film 1a TiO2 film 2b TiO2/Al2O3 TiO2/SiO2 TiO2/CaO

55.79 50.34 56.81 63.98 61.78

12.08 13.25 19.99 19.16 24.42

20.40 31.35 17.39 10.01 11.24

6.09

5.16 4.61

0.47 0.55

a

5.81 6.85 2.56

1a, coated on ceramic tile; 2b, coated on SiO2-precoated ceramic

tile

sample

crystal size (nm)

a ) b (Å)

c (Å)

cell vol (Å3)

Pure TiO2 TiO2/Al2O3 TiO2/SiO2 TiO2/CaO

36.82 33.19 22.52 17.20

3.7859 3.7815 3.7859 3.7919

9.6154 9.5209 9.6154 9.5735

137.82 136.15 137.82 137.65

and coexisting oxides in these mixtures. For pure TiO2 sample, an anatase TiO2 grows well during calcination at 520 °C and reaches the crystal size as large as 36.82 nm after 1 h. In the mixtures of TiO2/SiO2 and TiO2/CaO under the same thermal treatment, the crystal sizes of the formed anatase TiO2 are 22.52 and 17.20 nm, respectively, both of which are much smaller than that of pure TiO2. It was reported that the anatase phase could be stabilized by the surrounding SiO2 through the TiOSi interface in the TiO2/SiO2 mixture.23 Therefore, we believe that the presence of SiO2 or CaO inhibits the crystallite growth of anatase by providing dissimilar boundaries, which results in the formation of anatase TiO2 with small size. Among three mixtures, the anatase TiO2 in the TiO2/Al2O3 mixture has the largest crystal size (33.19 nm), close to that of pure TiO2. This indicates the effect of Al2O3 on the growth of anatase TiO2 seems to be much less than that of SiO2 or CaO. Figure 7 exhibits the enlarged X-ray diffraction peaks of crystal plane (101). As shown in Figure 7, it is clear that the (101) X-ray diffraction peak position of the anatase TiO2 in the TiO2/Al2O3 mixture shifts slightly toward a higher diffraction angle while the diffraction peak positions of the anatase TiO2 in other mixtures are almost kept unchanged as compared to that in pure TiO2 sample. The shift of the (101) diffraction peak shows the lattice distortion of the anatase TiO2 in the TiO2/ Al2O3 mixture. This conclusion is supported by the determination results of the lattice parameters and cell volume of anatase TiO2 in various samples below. The X-ray diffraction peaks of crystal plane (101) and (200) in anatase are selected to determine the lattice parameters of TiO2 in all powder samples using the following equations:

Bragg’s law: d(hkl) ) λ/2sinθ

(3)

d-2(hkl) ) h2a-2 + k2b-2 + l2c-2

(4)

where d(hkl) is the distance between crystal planes of (hkl), λ is the used X-ray wavelength, θ is the diffraction angle of crystal plane (hkl), and a, b, and c are lattice parameters (in TiO2 anatase form, a ) b * c). The obtained lattice parameters and cell volumes of anatase TiO2 in various samples are given in Table 3. According to Table 3, it can be found that there is an

Figure 6. X-ray diffraction patterns of pure TiO2 and various mixtures.

crystal size shown in Table 3 is determined from the broadening of corresponding X-ray spectral peaks by Scherrer’s formula

D ) Kλ/(βc - βs) cosθ

(2)

where D is the crystal size, λ is the wavelength of the X-ray radiation, and βc and βs are the full widths at half-maximum height of the sample and standard (single crystal silicon). The variation in the crystal sizes of the anatase TiO2 in all powder samples could partly demonstrate the interaction between TiO2

Figure 7. X-ray diffraction peak positions of crystal plane (101).

Deactivation Origin of TiO2 Film Coated on Ceramic Substrate

Figure 8. DMPO spin-trapping ESR spectra of pure TiO2 and various mixtures aqueous suspensions under UV irradiation.

obvious decrease in the lattice parameters and cell volumes of the TiO2 in the TiO2/Al2O3 mixture compared to those in pure TiO2. These results indicate that part of Al3+ ions enters TiO2 lattice to substitute for Ti4+ during the formation of crystalline TiO2 along with Al2O3 at the calcination temperature. To charge compensate in the crystal, the oxygen vacancies are also formed. The ionic radius of Al3+ (0.67 Å) is smaller than that of Ti4+ (0.745 Å).30 Both the small size Al3+ substitution and oxygen vacancy contribute to the decrease in the lattice parameters and cell volumes of the anatase TiO2 in the TiO2/Al2O3 mixture. Because of the big ion radium difference between Ti4+ and Si4+ (0.40 Å) or Ca2+(1.14 Å),30 it is difficult for them to enter the TiO2 lattice to substitute for Ti4+; instead, all of them are just present as oxides dispersed at the boundaries among TiO2 particles. 3.5. ESR Assays in Pure TiO2 and Various Mixtures. An examination of the formation of active oxygen species can offer useful information on the fate of the photogenerated electronhole pair and a greater comprehension on the photocatalysis of the various TiO2 mixtures in general. The ESR spin-trapping technique, a useful method for detecting various active radicals such as •OH and •OOH, was employed to examine the hydroxyl radicals produced in the aqueous suspension of pure TiO2 and various TiO2 mixtures under UV laser irradiation using DMPO as a hydroxyl radical trapper. The ESR measurements were performed under UV irradiation, and the results were shown in Figure 8. As shown in Figure 8, ESR signals are clearly observed in TiO2, TiO2/SiO2, and TiO2/CaO systems, respectively. With the increase of UV irradiation time, the intensity of all observed ESR signals is slightly enhanced. The observed signals with intensity of 1:2:2:1 are totally consistent with the characteristic peaks of DMPO-•OH adducts,31 suggesting the production of hydroxyl radicals in these systems. The formation of superoxide anion radical is expected due to the scavenging of the photogenerated electron by O2 dissolved in the aqueous suspension. However, no spin adduct DMPO-•OOH /O2•- was detected in these systems. This is because superoxide anion radical is produced and remains stable in an organic solvent medium rather than in our aqueous system.32 The peak intensity of •OH generated in the TiO2/SiO2 system is the highest among the four detected systems. The TiO2/CaO system seems to be more favorable for the production of hydroxyl radical than pure TiO2.

J. Phys. Chem. C, Vol. 111, No. 27, 2007 9973 No •OH radical signals were observed after a 40 s preirradition period in the TiO2/Al2O3 system, and very weaker •OH radical signals do not appear until the irradiation extends to 80 s. Hydroxyl radical •OH originates from the oxidation of chemisorbed OH or H2O by the hole photogenerated on the valence band of UV-irradiated TiO2. The ESR measurement results show that the presence of SiO2 together with TiO2 enhances the formation of hydroxyl radical •OH, which would be achieved via strong Bronsted acid sites at the TiO2/SiO2 interface region.23 Moreover, it was demonstrated that a larger particle size gives a lower photocatalytic activity. In other words, a smaller partical size would favor the generation of hydroxyl radical.33,34 Thus, the slightly higher intensity of •OH generated in the TiO2/CaO system could be attributed to its smaller particle size as compared to that of pure TiO2. Oppositely, the Al2O3 present in the TiO2/Al2O3 plays a detrimental role in the production of the hydroxyl radical. The cell volume measurements above show that the Al3+ ions substitute for Ti4+ in the anatase TiO2 of TiO2/Al2O3 mixture, causing the formation of oxygen vacancy for a charge balance. The oxygen vacancy state in anatase TiO2 is below the lower end of the conduction band at 0.75-1.18 eV and acts as a recombination center for the photogenerated holes and electrons.35,36 Irie et al. also reported that the oxygen vacancies increase with the increase in the doped N of TiO2-xNx. The oxygen vacancies promote the recombination of holes and electrons.37 Therefore, we could conclude that the formation of oxygen vacancy caused by Al3+ ions substitution for Ti4+ in the anatase TiO2 gives a rise to the recombination of the holes and electrons, which significantly reduces the production of hydroxyl radicals. 3.6. Deactivation Origin of the TiO2 Film Coated on Ceramic Substrate. The individual effects of SiO2, Al2O3, or CaO on the phase formation and crystal structure of the TiO2 and the production of •OH radical signal are well described above because the Si, Al, and Ca species measured by XPS in the TiO2 film are present in the same forms as those in individual TiO2 mixtures. Therefore, on the basis of the physicochemical behaviors of the mixtures of TiO2 with SiO2, Al2O3, or CaO above, we reveal the reason why the TiO2 film coated on ceramic substrate is poorly active for degradation of formaldehyde. When the crystalline TiO2 film was fabricated on the ceramic tiles at a high calcination temperature, the elements Si, Al, and Ca were detected to migrate into the film. The Si and Ca species are totally present as the forms of oxides SiO2 and CaO to incorporate with the TiO2 particles in the film. Such incorporation could produce Bronsted acid sites at the TiO2/ SiO2 interface and inhibit the crystal growth of anatase TiO2, both of which are found to favor the generation of hydroxyl radical, whereas the Al species, a part of it in the TiO2 film at least, enters the TiO2 lattice to be present as Al3+ form at Ti4+ site. The Al3+ substitution for Ti4+ causes the formation of oxygen vacancies in the anatase TiO2. The oxygen vacancies function as the recombination center of the photogenerated electrons and holes. Thus, charge transfer to the surface is largely inhibited, resulting in a significant decrease in the production of hydroxyl radicals over the UV-irradiated TiO2 film coated on the ceramic tile. Most of the proposed mechanisms of the photocatalytic degradation of organic compounds mediated by TiO2 semiconductor upon UV irradiation regard the highly oxidizing hydroxyl radical as the main oxidative species responsible for the degradation. As a result, the TiO2 film coated on the ceramic tile exhibits a poor photocatalytic activity for the degradation of formaldehyde. As shown in Figure 1 and Table 1, a SiO2 layer precoated on the ceramic tile inhibits the

9974 J. Phys. Chem. C, Vol. 111, No. 27, 2007 migration of Al into the TiO2 film and significantly enhances the photocatalytic activity. This result further supports the proposed deactivation origin of the TiO2 film coated on the ceramic tiles. Acknowledgment. This work was financially supported by the starting foundation of Renmin University of China and the National Natural Science Foundation of China (20673145). We are also grateful to Professor Jincai Zhao at the Institute of Chemistry, The Chinese Academy of Sciences, for ESR measurements. Supporting Information Available: X-ray diffraction peak of crystal plane (101) of the TiO2 film coated SiO2-precoated ceramic tile and ceramic tile. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Ollis, D. F., Al-Ekabi, H., Eds. Photocatalytic Purification and Treatment of Water and Air; Elsevier: Amsterdam, 1993. (2) Linsebigler, A. L.; Lu, G. Q.; Yates, J. T. Chem. ReV. 1995, 95, 735. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (4) Calza, P.; Minero, C.; Pelizzetti, E. EnViron. Sci. Technol. 1997, 31, 2198. (5) Kim, D. J.; Hahn, S. H.; Oh, S. H.; Kim, E. J. Mater. Lett. 2002, 57, 355. (6) Lee, J. S.; Song, H. W.; Lee, W. J.; Yu, B. G.; No, K. Thin Solid Films 1996, 287, 120. (7) Pizem, H.; Sukenik, C. N. Chem. Mater. 2002, 14, 2476. (8) Weinberger, B. R.; Garber, R. B. Appl. Phys. Lett. 1995, 66, 2409. (9) Sunada, K.; Watanabe, T.; Hashimoto, K. EnViron. Sci. Technol. 2003, 37, 4785. (10) Sirghi, L.; Aoki, T.; Hatanaka, Y. Thin Solid Films 2002, 422, 55. (11) Mills, A.; Elliott, N.; Parkin, I. P.; O’Neill, S. A.; Clark, R. J. J. Photochem. Photobiol., A 2002, 151, 171. (12) Tada, H.; Tanaka, H. Langmuir 1997, 13, 360. (13) Herrmann, J.-M.; Tahiri, H.; Guillard, C.; Pichat, P. Catal. Today 1999, 54, 131.

Xie and Lin (14) Fujishima, A.; Rao, T. N.; Proc. Indian. Acad. Sci., Chem. Sci. 1997, 109, 471. (15) Yu, J.; Zhao, X. Mater. Res. Bull. 2000, 35, 1293. (16) Nam, H.-J.; Amemiya, T.; Murabayashi, M.; Itoh, K. J. Phys. Chem. B 2004, 108, 8254. (17) Chen, S.-Z.; Zhang, P.-Y.; Zhu, W.-P.; Chen, L.; Xu, S.-M. Appl. Surf. Sci. 2006, 252, 7532. (18) Dai, W.; Wang, X.; Liu, P.; Xu, Y.; Li, G.; Fu, X. J. Phys. Chem. B 2006, 110, 13470. (19) Yu, J. C.; Ho, W.; Lin, J.; Yip, H.; Wang, P. K. EnViron. Sci. Technol. 2003, 37, 2296. (20) Legrand-Buscema, C.; Malibert, C.; Bach, S. Thin Solid Films 2002, 418, 79. (21) Duran, A.; Femandez-Navarro, J. M.; Casariego, P.; Joglar, A. J. Non-Cryst. Solids 1986, 82, 391. (22) Rodriguez, I. C.; Bravo, S. R.; Bendito, D. P. Int. J. EnViron. Anal. Chem. 1995, 61, 331. (23) Anderson, C.; Bard, A. J. J. Phys. Chem. B 1997, 101, 2611. (24) Lassaletta, G.; Fernandez, A.; Espinos, J. P.; Gonzalez-Elipe, A. R. J. Phys. Chem. 1995, 99, 1484. (25) Galan-Fereres, M.; Alemany, L. J.; Mariscal, R.; Banares, M. A.; Anderson, J. A.; Pierre, J. L. G. Chem. Mater. 1995, 7, 1342. (26) Espinos, J. P.; Lassaletta, G.; Caballero, A.; Fernandez, A.; Gonzalez-Elipe, A. R. Langmuir 1998, 14, 4908. (27) Tada, H.; Tanaka, N. Langmuir 1997, 13, 360. (28) Langlet, A.; Permpoon, S.; Riassetto, D.; Berthome, G.; Pernot, E.; Joud, J. C. J. Photochem. Photobiol., A 2006, 181, 203. (29) Moulder, J. F.; Stickle, W. F.; Sobol, P. E.; Bomben, K. D. Handbook of X-ray Photoelectron Spectroscopy; Perkin-Elmer Corporation: Eden Prairie, MN, 1992. (30) Shannon, R. D. Acta. Crystallogr. 1976, A32, 751. (31) Hunag, Y.; Li, J.; Ma, W.; Cheng, M.; Zhao, J. J. Phys. Chem. B 2004, 108, 7263. (32) Chen, C.; Zhao, W.; Lei, P.; Zhao, J.; Sperpone, N. Chem.sEur. J. 2004, 10, 1956. (33) Oh, S.-M.; Kim, S.-S.; Lee, J.-E.; Ishigaki, T.; Park, D.-W. Thin Solid Films 2003, 435, 242. (34) Nam, H.-J.; Amemiya, T.; Murabayashi, M.; Itoh, K. J. Phys. Chem. B 2004, 108, 8254. (35) Nakamura, I.; Negishi, N.; Kutsuna, S.; Ihara, T.; Sugihara, S.; Takeuchi, K. J. Mol. Catal. A: Chem. 2000, 161, 205. (36) Kato, H.; Kudo, A. J. Phys. Chem. B 2002, 106, 5029. (37) Irie, H.; Watanable, Y.; Hashimoto, K. J. Phys. Chem. B 2003, 107, 5483.