Photoinduced Oxidation of Methylsiloxane Monolayers Chemisorbed

Feb 1, 1996 - Photoinduced Oxidation of Methylsiloxane Monolayers. Chemisorbed on TiO2. Hiroaki Tada*. Nippon Sheet Glass Techno-Research Co...
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Photoinduced Oxidation of Methylsiloxane Monolayers Chemisorbed on TiO2 Hiroaki Tada* Nippon Sheet Glass Techno-Research Co. Ltd., 1, Kaidoshita, Konoike, Itami, Hyogo 664, Japan Received May 25, 1995. In Final Form: October 26, 1995X Photoinduced oxidation of chemisorbed 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) in air at room temperature yielded a uniform SiOx monolayer with a thickness of approximately 0.2 nm on TiO2. The oxidation was suggested to be caused by the band gap excitation of TiO2 leading to the generation of activated oxygen species on the surface. The sequential cycles of chemisorption of TMCTS (process I) followed by photoirradiation (wavelength < 330 nm, process II) grew the SiOx (2 < x < 3) film 0.4 ( 0.1 nm per cycle, whereas the rate of the oxidation exponentially decreased with an increase in the film thickness. The contact angle of H2O, being almost constant at 90° ( 3° after each process I, decreased as the photoirradiation time increased during the subsequent process II. The gas phase adsorption experiments of pyridine further demonstrated that the surface acidic nature drastically changes from Lewis acid character to Bronsted acid character upon formation of the SiOx monolayer on TiO2. The photocatalytic activity was maintained to an appreciable extent in this state. Both the analyses of the diffuse reflectance infrared Fourier transform spectra and the data on the contact angle of H2O indicated that the surface oxidation obeys the Langmuir-Hinshelwood mechanism.

Introduction Since the discovery of water photolysis using semiconductor electrodes by Fujishima et al.,1 a number of investigations on photocatalytic reactions have been carried out in particulate systems as well as electrode systems.2 A current topic in this field is the application to the destruction of environmental pollutants in water and air.3 In essence, these applications are based on the strong redox power of hole-electron pairs generated by photoillumination of wide band gap semiconductors such as TiO2. Because of their large specific surface area, most of the studies have so far been concerned with semiconducting powders. However, the development of semiconducting films with high photocatalytic activity has become more important than the development of powders, since a variety of needs, including decomposition and removal of organic soils from glass substrates4 and disinfection of window glass,5 exist in the fields of electronics and medical treatments, respectively. On the other hand, it is known that TiO2 particles, prevailing popularly as pigments, cause serious degradation of paints due to photocatalytic activity. The coating of the particles with oxide layers such as silica and/or alumina has been widely employed to suppress this degradation.6 A fundamental understanding of the reactivity of organic compounds strongly adsorbed on solids is essential * FAX: 0727-81-4132. TEL: 0727-81-7251. X Abstract published in Advance ACS Abstracts, February 1, 1996. (1) (a) Fujishima, A; Honda, K. Bull. Chem. Soc. Jpn. 1971, 44, 1148. (b) Fujishima, A.; Honda, K. Nature 1972, 238, 37. (2) (a) Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic Press: New York, 1983. (b) Photocatalysis; Serpone, N., Pelizzetti, E., Eds.; John Wiley & Sons, Inc.: New York, 1989. (3) (a) Bard, A. J. J. Photochem. 1979, 10, 59. (b) Matthews, R. W. J. Phys. Chem. 1987, 91, 3328. (c) Tunesi, S.; Anderson, M. A. Chemosphere 1987, 16, 1447. (d) Jackson, N. B.; Wang, C. M.; Luo, Z.; Schwitzebel, J.; Ekerdt, J. G.; Brock, J. R.; Heller, A. J. Electrochem. Soc. 1991, 138, 3660. (e) Fox, M. A. CHEMTECH 1992, 22, 680. (f) Tada, H.; Honda, H. J. Electrochem. Soc. 1995, 142, 3438. (g) Photocatalytic Purification and Treatment of Water and Air; Ollis, F. D., Al-Ekabi, H., Eds.; Elsevier Sci.: Tokyo, 1993. (4) Soejima; Oikawa, M.; Nakai, H.; Fujii, T.; Suzuki, H.; Kashima, H.; Sakamoto, K. Proceedings of 12th ISCC in Yokohama; 1994; p 615. (5) Fujishima, A.; Hashimoto, K.; Kubota, Y. J. Surf. Sci. Soc. Jpn. 1995, 16, 188.

0743-7463/96/2412-0966$12.00/0

for these applications; however, there are few reports on such photodriven surface reactions. In a previous paper, a methylcyclosiloxane monolayer with the all-trans conformation was revealed to form on flat oxides by chemisorption of 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) from the gas phase.7 This paper describes a novel method, which consists of the sequential chemical adsorption of TMCTS followed by ultraviolet photoirradiation, giving rise to the SiOx monolayer with thickness controlled at the monolayer level on TiO2. The photoinduced oxidation of the methylsiloxane monolayers having a well-defined surface on flat substrates of TiO2 was examined in detail. Particular emphasis is placed on the kinetics of the reaction. Experimental Section Soda lime-silicate glass plates (50 × 25 × 1.1 mm) with a TiO2 film (TiO2/glass, anatase polycrystal, thickness ) 55 ( 5 nm) were used as substrates. The TiO2 film was coated by a chemical vapor deposition method.8 In this process, a solution of tetraisopropyl orthotitanate(IV) (Tokyo Kasei Chem.) dissolved in isopropyl alcohol was sprayed with a N2 carrier gas on a hot substrate (∼450 °C). The surface was found to be quite smooth by scanning electron spectroscopy. Some carbon contamination was only detected from the surface except for Ti and O by X-ray photoelectron spectroscopic (XPS) measurements. TiO2 particles (P-25, Degussa, BET surface area ) 37.2 m2 g-1), preserved in a vacuum oven at 50 °C in order to prevent them from contamination, were used for diffuse reflectance infrared Fourier transform (DRIFT) spectroscopic measurements. The TiO2/glass substrates were soaked in a KOH alkaline solution (pH ) 13.7) for 30 s and subsequently rinsed by sonification in distilled water (conductivity < 1 µS cm-1) for 10 min. A fully wetting surface against H2O was obtained by this treatment. TMCTS (>98%) (200 µL) was allowed to react with the substrates placed in a vacuum chamber under ca. 10 Torr at 80 °C for 0.5 h. Then the temperature was raised to 100 °C, and evacuation was continued for an additional 0.5 h to remove the physisorbed TMCTS (process I). The samples were irradiated in air with a 500-W high-pressure mercury arc (wavelength > (6) (a) Sullivan, W. F. Prog. Org. Coat. 1972, 1, 157. (b) Cundall, R. B.; Hulme, B.; Rundham, R.; Salim, M. S. J. Oil Colour Chem. Assoc. 1978, 61, 351. (c) Day, R. E.; Egerton, T. A. Colloids Surf. 1987, 23, 137. (7) Tada, H.; Nakamura, K.; Nagayama, H. J. Phys. Chem. 1994, 98, 12452. (8) Hardee, K. L.; Bard, A. J. J. Electrochem. Soc. 1975, 122, 739.

© 1996 American Chemical Society

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330 nm) whose light intensity at 365 nm was 635 µW cm-2 (process II). In the cycle experiments of processes I and II, the n-time process I (TMCTS/SiOx(n-1)/TiO2) was followed by the n-time process II (SiOx(n)/TiO2) after complete oxidation of the CH3 and Si-H groups of TMCTS by prolonged photoirradiation (>25 h). The surface acidity was checked by DRIFT spectra of pyridine adsorbed on the particles from the gas phase under 10 Torr at 30 °C for 0.5 h. DRIFT spectra of the particles (52.5 mg) packed in a stainless pan (diameter ) 7 mm, depth ) 2 mm) were recorded on a JIR 5500 FT-IR at 4-cm-1 resolution from 4000 to 400 cm-1 with 200 co-added scans. All the DRIFT spectra of adsorbed species in this study were obtained by subtracting reference spectra of TiO2 from the spectra of respective samples (sample - TiO2). The absorbances of peaks were computed using the Kubelka-Munk function. TiO2 particles were not diluted with KBr; however, the absorption spectrum might be expected to be quite similar to the absorption spectrum of the same sample measured by the transmission KBr disk technique, particularly in the 3200-1600-cm-1 range where there is no absorption of TiO2.9 The spectrum of TMCTS was obtained using the normal incidence transmission spectrum of pressed KBr disks prepared in a dry air atmosphere. Static contact angles were measured by using a contact angle meter (Model CA-D, Kyowa Interface Science Co.) at room temperature (20 ( 1 °C). H2O droplets with a diameter of approximately 2 mm were placed at five positions for one sample, and the average was adopted as the contact angle (reproducibility within (2.5%). X-ray photoelectron spectra (XPS) were measured with a Shimadzu electron spectrometer (Model ESCA 750) using a Mg KR X-ray source (hν ) 1253.6 eV). The X-ray source was operated at 30 mA and 8 kV. The residual gas pressure in the spectrometer chamber during data acquisition was less than 10-7 Torr. Incident and detected angles were fixed at 90° and the irradiation area was ca. 19.6 mm2. The binding energy scales were referenced by setting the hydrocarbon peak maxima in the C1s spectra to 284.6 eV. The precision of the binding energy with respect to this standard value was within (0.3 eV. Assuming that the average mean free path of photoelectrons escaping from TiO2 with a kinetic energy of 799.8 eV (λ) is 1.23 nm, one can calculate the film thickness (d) by the equation d ) (λ(E) sin θ) ln(I0/I). θ ()90°) is the angle between the sample plane and the detector of the photoelectrons. I and I0 are the intensities of the Ti2p photoelectron with and without the SiOx film, respectively. The error in this analysis is estimated to be ca. 40% for nonfree electrons.10

Results and Discussion Surface Reaction. Figure 1a shows the difference DRIFT spectra of TiO2 particles before and after the first process I (TMCTS/TiO2 - TiO2). Two negative peaks due to the stretching vibrations of the surface isolated Ti-OH groups (ν(Tis-OH)) are observed at 3691 and 3631 cm-1. Positive peaks appear at 2970, 2912, 2168, and 1267 cm-1, which are assigned to the antisymmetric (νas(CH3)) and symmetric (νs(CH3)) stretching vibrations of the CH3 groups, the stretching vibration of Si-H groups (ν(SiH)), and the symmetric deformation (δs(CH3)) of the CH3 groups, respectively.11 A peak due to hydrogen-bonded Ti-OH units is observed at 3550 cm-1. Since it is very weak, the contribution of hydrogen bonds to the decrease in the Tis-OH groups must be small. A negative peak at 1620 cm-1 is assigned to the deformation of H2O, which seems to originate from partial desorption of physisorbed H2O during the process. A strong and broad peak near 1100 cm-1 is based on the antisymmetric stretching vibration of the Si-O-Si bonds (νas(Si-O-Si)) of the TMCTS adsorbed, whose position is in good agreement (9) Griffiths, P. R.; Fuller, M. P. Advances in Infrared and Raman Spectroscopy; Clark, R. J. H., Hester, R. E., Ed.; Heyden & Son, Ltd.: London, 1982. (10) Penn, D. R. J. Electron Spectrosc. Relat. Phenom. 1976, 9, 29. (11) Smith, A. L. Analysis of Silicones; Wiley-Interscience: New York, 1974.

Figure 1. Difference DRIFT spectra of TiO2 particles: (a) TMCTS/TiO2 - TiO2; (b) SiOx(1)/TiO2 - TiO2 (t ) 15.3 h); (c) TMCTS/SiOx(1)/TiO2 - TiO2; (d) SiOx(2)/TiO2 - TiO2 (t ) 15.8 h); (r) FT-IR spectrum of TMCTS.

with that in the spectrum of pure TMCTS (Figure 1r). Also, a shoulder at 1061 cm-1, which is absent in spectrum r, is tentatively assigned to the stretching vibration of the interfacial Si-O-Ti bonds (ν(Si-O-Ti)).12 The data indicate the immobilization of TMCTS molecules through chemical bonding on the surface of TiO2 after the first process I (Scheme 1b). With adsorption of TMCTS, the absorbance ratio of ν(Si-H) to νas(Si-CH3) decreases from 4.4 (Figure 1r) to 3.7 (Figure 1a), which further supports this conclusion. The spectral change after 15.3 h of photoirradiation is shown in Figure 1b, the difference DRIFT spectrum of (SiOx(1)/TiO2 - TiO2). A new peak assignable to the stretching vibration of the isolated Si-OH groups (ν(SisOH)) grows at 3740 cm-1, and simultaneously all the absorption peaks due to the CH3 and Si-H groups weaken. On the other hand, no change was observed in the spectrum of the TMCTS monolayer on SiO2 particles even after 2.5 h of photoirradiation (not shown). The νas(Si(12) It was reported for the interfacial Si-O-Si bonds that the peak due to the stretching vibration is situated at 1060 cm-1: Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993, 9, 3518.

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Scheme 1. Experimental Procedure for Preparing SiOx Monolayers on TiO2

O-Si) band becomes broader, and the position shifts approximately 40 cm-1 toward higher wavenumber. Also, the increase in its integrated area points to the formation of Si-O-Si bonds as the result of condensation between the Sis-OH groups. From these findings, it is clear that a surface-hydroxylated SiOx (2 < x < 3) film is formed on TiO2 (Scheme 1c). Figure 1c is the difference DRIFT spectrum of (TMCTS/ SiOx(1)/TiO2 - TiO2). The intensities of the absorption peaks due to the TMCTS monolayer increase again, while the ν(Sis-OH) band at 3740 cm-1 grown in the first process II (Figure 1b) disappears completely. It is evident that the adsorption sites of TMCTS change from the Tis-OH groups in the first process I to the Sis-OH ones in the second process I. Further, the fact that the change of strength of the broad peak in the 3600-3200-cm-1 region due to hydrogen-bonded Si-OH groups is small indicates that TMCTS preferentially reacts with not the hydrogenbonded Si-OH groups but the Sis-OH groups.13 The difference spectrum of (SiOx(2)/TiO2 - TiO2) (photoirradiation time )15.8 h) is also shown in Figure 1d. The same oxidation occurs as in the first cycle (Scheme 1e). The increase in intensity of the νas(Si-O-Si) band and the shift of its peak position toward higher wavenumber point to the stimulation of the condensation between the Sis-OH groups with photoirradiation. Further, the absorbances of νas(Si-O-Si) bands near 1100 cm-1 in (13) Tada, H.; Hyodo, M.; Kawahara, H. J. Jpn. Soc. Colour Mater. 1991, 64, 621.

Figure 1c,d increase compared to those in Figure 1a,b by a factor of 2.13 ( 0.13, and the interfacial νas(Si-O-Ti) band is concealed by them in Figure 1c,d. These findings clearly indicate the rechemisorption of TMCTS on the SiOx monolayer and the growth of Si-O-Si networks during the second cycle of processes I and II. No intermediates could be detected by the DRIFT measurements, suggesting that the rate of their transformation to Si-OH groups is fairly rapid. Figure 2A shows the variations in the absorbances (A) of the νas(Si-CH3) (b), ν(Si-H) (a), and ν(Sis-OH) (c) bands as a function of photoirradiation time. Both A(Si-CH3) and A(Si-H) monotonously decrease as time increases, while A(Sis-OH) increases to a greater extent. Apparently, the rate of decrease in A(Si-H) is greater than in A(Si-CH3). Figure 2B shows the relation between A(SiH) and the sum of the decreasing amounts of A(Si-H) and A(Si-CH3). Since good linearity is observed between them (R > 0.98), both the Si-H and Si-CH3 groups prove to be oxidized to Sis-OH groups. In the absence of photoilluminated TiO2 on the surface of the substrate, the oxidation did not occur. The TMCTS monolayer on TiO2 can be considered as being oxidized by activated oxygen species such as O and OH radicals, which are generated by the reaction of the adsorbed oxygen or Tis-OH groups and the photoexcited carriers of TiO2.14 The contributions of adsorbed oxygen to the oxidation seem to be predominant since the rate of oxidation was (14) Formenti, M.; Teichner, S. J. Catalysis 1978, 2, 87.

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Figure 3. Dependence of contact angles on time for the TiO2/ glass with TMCTS chemisorbed: (a) n ) 1; (b) n ) 2; (c) n ) 3; (d) n ) 4; (r) n ) 1 (substrate ) Si wafer).

Figure 2. (A) Plots of A(Si-H) (a), A(Si-CH3) (b), and A(SisOH) (c) vs time in the first cycle treatment. (B) Relation between A(Sis-OH) and the sum of the decreasing amount of A(Si-H) and A(Si-CH3).

remarkably reduced in the absence of O2 in the atmosphere. As the result of extraction of electrons from the excited TiO2, O2 must also have an effect on improving the efficiency of the charge separation of hole-electron pairs. Thus another possibility that the direct oxidation of the monolayer by holes occurs during the initial stage of the reaction cannot be ruled out. Gas chromatography confirmed the evolution of CO2, and its quantity increased with increasing photoirradiation time. The mechanism is complicated as suggested by the difference in oxidation rates of the Si-H and SiCH3 groups; however, the overall photoinduced oxidation of the methylcyclosiloxane monolayer by TiO2 can be represented by eq 1. TiO2

dSi(CH3)H + 5/2O2 9 8 dSi(OH)2 + CO2 + H2O (1) hν Surface Properties. Figure 3 is the photoirradiation time dependence of the contact angle for the TiO2/glass with TMCTS chemisorbed. In the first cycle (a), the contact angle value, being 90 ( 3° at the initial time, sharply decreases with an increase in time. The initial value is almost restored repeatedly after each process I in the cycle treatments of processes I and II. In all cases, they decrease with increasing time, whereas the decreasing rate is retarded as the cycle number increases (b-d). Figure 4 shows variations of Ti2p and Si2p XPS spectra with increasing cycle number (n). The signal intensities of Ti2p3/2 (458.4 eV) and Ti2p1/2 (464.3 eV) significantly

Figure 4. Variation of Ti2p and Si2p XPS spectra with increasing n.

decrease with increasing n, while the signal of Si2p intensifies. This suggests that the SiOx monolayers are piled up during the process. It is also noteworthy that the binding energy (BE) of Si2p rises from 102.6 eV (n ) 1) to 103.1 eV (n ) 4) monotonously with an increase in n. Since the BE of SiO2 is reported to be 103.35 eV, the shift is due to the development of the Si-O-Si networks with the sequential treatments as confirmed by the DRIFT spectra. The thicknesses of the SiOx films were determined from these data. The dependence of the SiOx film thickness (d/nm) on n could be given by d ≈ 0.48n - 0.43 (2 < n < 5), except for n ) 1 (d ≈ 0.2). The contact angle for the Si substrate with the TMCTS monolayer is invariant with photoirradiation within at

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Figure 6. Plots of ln(f0/f) vs t: (a) n ) 1; (b) n ) 2; (c) n ) 3; (d) n ) 4 (substrates ) TiO2/glass); (r) n ) 1 (substrate ) Si wafer).

Figure 5. Difference DRIFT spectra before and after adsorption of pyridine: (a) TiO2 particles; (b) SiOx(1)/TiO2 particles.

least 2.5 h (Figure 3r). The decrease in the contact angle is ascribable to the photoinduced oxidation of the Si-H and Si-CH3 groups to the Si-OH groups by TiO2. It is a well-known fact that the Si-CH3 groups show hydrophobic character, while the Si-OH groups are highly hydrophilic.15 The acidic nature of both the TiO2 and the SiOx(1)/TiO2 particles was examined by adsorbing pyridine as a probe molecule.16 Figure 5a shows the difference DRIFT spectrum of TiO2 particles before and after adsorption of pyridine. Two negative peaks assignable to ν(Tis-OH) are observed at 3691 and 3631 cm-1. Also, four sharp positive peaks at 1606 (8a), 1574 (8b), 1491 (19a), and 1444 cm-1 (19b)17 indicate the presence of only Lewis acid sites on the pristine particles. Figure 5b is the difference DRIFT spectrum of the SiOx(1)/TiO2 particles before and after adsorbing pyridine. New negative and positive peaks appear at 3740 and 1545 cm-1, which are due to ν(SisOH) and pyridinium ions (19b), respectively. Concurrently, the intensities of the four peaks observed in Figure 5a decrease significantly. These findings demonstrate the drastic change in the surface acidic nature from Lewis acid character to Bronsted acid character upon the formation of the SiOx monolayer.18 It can be considered that pyridine coordinates to Ti4+ ions having coordinative unsaturations on the surface of TiO2, while it adsorbs at Sis-OH groups in the case of the SiOx(1)/TiO2 particles, leading to pyridinium ions. Kinetics of the Reaction. The TMCTS molecules chemically adsorbed on oxides take the all-trans conformation with all of the CH3 groups toward the gas phase.7 The contact angle of the TiO2 surface just after chemisorption of TMCTS is comparable to that of polypropylene (∼94°).19 Then it is assumed to be in a close packing state of CH3 groups. The Si-CH3 groups are oxidized to SiOH groups by photoirradiation, and the concentrations of (15) Tada, H.; Nagayama, H. J. Electrochem. Soc. 1993, 140, L140. (16) Parry, E. P. J. Catal. 1963, 2, 371. (17) The notations 8a, 8b, 19a, and 19b express the vibrational assignments of the IR spectrum of pyridine; see ref 14. (18) It has recently been reported by Niwa et al. that the Bronsted acidity is generated on the SiO2 monolayer with the network of SiO-Si, which was prepared by a thermal reaction (T > 493 K) on the surface of Al2O3: Katada, N.; Toyama, T.; Niwa, M. J. Phys. Chem. 1994, 98, 7647. (19) Nakamae, K. In Kobunshi Hyomen no Kiso to Oyo; Ikada, Y., Ed.; Kagakudojin: Kyoto, 1990.

intermediates are negligibly small. Also, the influence of Si-H groups on the contact angle would be small, because they situate beneath the outermost Si-CH3 groups.20 Then, the surface subjected to partial oxidation can be regarded as a composite of CH3 and OH groups. According to the Cassie-Baxter theory based on thermodynamics,21 the contact angle of liquid droplets placed on the composite surface consisting of two components (θc) is written as

cos θc ) f cos θ1 + (1 - f) cos θ2

(2)

where θ1 and θ2 are the contact angles for the surface with the closest packing states of CH3 and OH groups (for water, θ1 ) 97.4°, θ2 ) 0°),7 and f is the fraction of the surviving CH3 groups. From the variation of θc with photoirradiation (Figure 3), the corresponding f value is calculated as a function of time using eq 2. Further, assuming that the oxidation of the CH3 groups takes place via a Langmuir-Hinshelwood mechanism under conditions such that the concentration of the activated oxygen species on the surface is in excess with respect to that of the CH3 groups and the resultant intermediates are rapidly transformed to SiOH groups, rate eq 3 is valid,

ln(f0/f) ) kt

(3)

where k is an apparent rate constant of the oxidation and t is the photoirradiation time. Figure 6 shows the plots of ln(f0/f) vs t for the samples treated with TMCTS repeatedly. As expected from eq 3, a linear relation is obtained for each case: the slopes, k (h-1), are 0.75 (a), 0.31 (b), 0.077 (c), 0.065 (d), and 0.0008 (r), respectively. Also, the half-lives of the CH3 groups given by (ln 2)/k are 0.92 h (a), 2.24 h (b), 9.0 h (c), and 10.7 h (d), respectively. When n was greater than 5, a completely wetting surface could not be obtained even after 50 h of photoirradiation, because the photocatalytic activity remarkably decreased. It should be noted that the rate of the oxidation decreased with increasing n; however, it remained fairly large in the second cycle (b). If the same Langmuir-Hinshelwood mechanism is presumed to be true for the TiO2 particulate system, eq 4 can be derived by applying a dead space model,

ln{(C0-Cd)/(C-Cd)} ) kt

(4)

where C0, C, and Cd are the surface concentrations of CH3 or Si-H groups at t ) 0, 0 < t < ∞, and t ) ∞, respectively. (20) Tada, H.; Tanaka, M. Thin Solid Films, in press. (21) Adamson, A. W. Physical Chemistry of Surfaces; John Wiley and Sons, Inc.: New York, 1982.

Oxidation of Methylsiloxane Monolayers

Figure 7. Plots of ln{(C0-Cd)/(C-Cd)} vs time: (a) Si-H; (b) Si-CH3.

In the particulate system, Cd resulting from a dead space incapable of absorbing light should be present even at n ) 1 because the thickness of the powder-packing sample (ca. 2 mm) is far greater than the absorption length of TiO2 in the range of wavelength less than 330 nm (ca. several micrometers).22 The peaks due to the residual Si-H and CH3 groups after photoirradiation for ca. 15 h are evidence for the presence of the dead space (Figure 1b,d). Figure 7 shows the plots of ln{(C0-Cd)/(C-Cd)} vs t for the reaction of the two functional groups, i.e., Si-H (a) and Si-CH3 (b). The values of A(Si-H) and A(SiCH3) in the DRIFT spectra (Figure 2A) were used for the calculation of ln {(C0-Cd)/(C-Cd)}. Both plots provide two straight lines (R > 0.99) whose slopes are 1.53 (a) and 0.76 (b), respectively. The rate of oxidation of the Si-H groups is ca. 2 times greater than that of the Si-CH3 groups, for which two factors seem to be responsible. One is the greater reactivity of the former for the activated oxygen species (O and OH radicals), and the other is the inward orientation of the Si-H groups favorable to the surface oxidation on TiO2. Also, it should be noted that the slope of plot b in Figure 7 is in good agreement with the slope of plot a in Figure 6 (0.75). It seems reasonable that since P-25 contains 70% of anatase (the remainder is rutile) and the photocatalytic activity of the former is generally far greater than that of the latter, the rate constant for P-25 is close to that for pure anatase. This (22) Gratzel, M. In Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic Press: New York, 1983.

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fact strongly supports the validity of the kinetic analysis, based on the data of contact angle, of the surface oxidation in spite of the fact that the contact angle data generally give only indirect information on the surface; however, they are quite sensitive to its state. These results also indicate that the oxidation kinetics of the functional groups of the methylsiloxane monolayer obeys the Langmuir-Hinshelwood mechanism; i.e., the well-ordered Si-H and Si-CH3 groups are oxidized by the activated oxygen species on the surface of TiO2. The dependence of k on d was expressed as k ) exp(-1.73d0.09). The exponential reduction in k with increasing n can be accounted for by the increase of d, which decreases the probability that the photocarriers reach the surface through the tunneling effect.23 Conclusions The photooxidation of the well-ordered methylsiloxane monolayer by TiO2 was examined by analyzing the DRIFT spectra and the contact angles. The surface reaction was revealed to obey the Langmuir-Hinshelwood mechanism. If TMCTS is regarded as a model substance of organic adsorbates, the present contact angle analysis provides a ready method for assessing the photocatalytic activity of semiconducting thin films. This would be useful for developing photocatalysts with high performance characteristics, leading to a realization of their possible wide application described in the Introduction. Pyridine adsorption experiments indicated that the acidic nature is changed from a Lewis acid character to a Bronsted acid character upon formation of the SiOx monolayer on TiO2. Since the photocatalytic activity is appreciably maintained in this state, TiO2 covered with the SiOx monolayer has the potential to improve the selectivity and activity in some reaction systems. On the other hand, the multilayers of SiOx markedly decreased the photocatalytic activity of TiO2 in the range of cycle numbers above 5. This also provides fundamental information on the suppression of paint degradation. Acknowledgment. The author expresses his sincere gratitude to K. Shimoda (NSG Techno-Research) for experimental support. LA950404I (23) Somorjai, G. A. Introduction to Surface Chemistry and Catalysis; John Wiley and Sons, Inc.: New York, 1994.