0 Copyright 1995 American Chemical Society
SEPTEMBER 1995 VOLUME 11,NUMBER 9
Letters Layer-by-Layer Construction of SiO, Film on Oxide Semiconductors Hiroaki Tada Nippon Sheet Glass Techno-Research Co. Ltd., 1, Kaidoshita, Konoike, Itami, Hyogo 664, Japan Received March 3, 1995. In Final Form: May 22, 1995@ A novel method for preparing SiO, (2 x 3) monolayers on TiO2, consisting of the chemical vapor surface modification with 1,3,5,7-tetramethylcyclotetrasiloxane(TMCTS,process I) and subsequent ultraviolet photoirradiation (process 111, is reported. X-ray photoelectron spectroscopy revealed that SiO, monolayers form layer-by-layerby cycling process 1-11. The surface oxidation of the Si-H and Si-CH3 groups to Si-OH groups during process I1was examined by differencediffuse reflectance Fourier-transformed infrared spectroscopicmeasurements. The monolayer of SiO, (-0.2 nm, coverage -0.86) drasticallychanged the acidic nature of the Ti02 surface, maintaining- the photocatalytic activity, while multilayers (-3 nm) reduced it greatly.
Introduction Photocatalytic reactions induced by semiconductors have recently received much attention owing to a variety of applications including syntheses of useful organic compounds' and decontamination of the environment.2It is generally of great importance to control the activity3 and increase the selectivity of the photocatalysts in compliance with different needs. One of the strategies is surface modification, whose effects can be viewed in both the heterogeneous photoexcited charge transfer process and thermal catalytic process. First, it has the possibility of enhancing the reactions by removing the surface electronic states within the band gap4and controlling the level of the flat band potential of semi~onductors.~ Second, there is a strong impetus for a selective reaction path to Abstract published in Advance A C S Abstracts, September 1, 1995. (1)Fox, M. A. In Photocatalysis Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; J o h n Wiley: New York, 1989. (2) In Photocatalytic Purification and Treatment of Water and Air; Ollis, F. D., Al-Ekabi, H., Eds., Elsevier Science: Amsterdam, 1993. (3) In most cases, the higher photocatalytic activity is desired; however, it has been a main subject in the field of paints to suppress the activity leading to their degradation. (4)SandoriT, C. J.;Hegde, M. S.; Forrow, L. A.; Bhat, R.; Harbison, J. P.; Chang, C. C. J.Appl. Phys. 1990,67, 586. ( 5 )Chazalviel, J. N. J. Ekctroanul. Chem. 1987,233, 37. @
be promoted by reducing its activation energy. In addition, the adsorption energy of the reactants may be changed with the treatment. It is well-known that the incorporation ofpromoters such as noble metals on T i 0 2 remarkably increases its photocatalytic activity in many systems.6 The platinization is usually achieved by photodeposition where Pt4+ions adsorbed are reduced by excited electrons generated at the conduction band of Ti02 in the presence of sacrificial reductants.' On the other hand, this report describes a novel method of preparing ultrathin SiO, (2 < x < 3) film on TiOz, originating from the photoinduced oxidation of a chemically adsorbed methylsiloxane monolayer. The method, consisting of the sequential chemical vapor surface modification (CVSM18with 1,3,5,7-tetramethylcyclotetrasiloxane (TMCTS) followed by photoirradiation (Scheme 11, enables the SiO, film to form with thicknesses controlled at the molecularlevel. It is further shown that the monolayer of SiO, (-0.2 nm) drastically changes the acidic nature ofthe T i 0 2 surface, maintaining the photocatalytic activity, while the multilayers (-3 nm) suppress it to a great extent. ( 6 )Energy Resources through Photochemistry and Catalysis; Gratzel, M., Ed.; Academic Press: New York, 1983. (7)Kraeutler, B.; Bard, A. J. J.Am. Chem. SOC.1978, 100, 4317. (8) Tada, H.; Nagayama, H. Langmuir 1994, 10, 1472.
0743-7463/95/2411-3281$09.00/00 1995 American Chemical Society
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3282 Langmuir, Vol. 11, No. 9, 1995
Scheme 1
TMCTS
-
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P i 0 - O - T - -0 CHJ
I -
,3,-7 H
Mmolayer tormation
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O
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0
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y ( T 4 o d
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Experimental Section Soda-lime-silicateglass plates (50 x 25 x 1.1 mm) with a Ti02 film (TiOdglass, anatase polycrystal, thickness = 55 f 5 nm) were used as substrates. The Ti02 film was coated by a chemical vapor deposition m e t h ~ d .In ~ this process, a solution of tetraisopropylorthotitanate(ll7) (Tokyo Kasei Chem.) dissolved in isopropyl alcohol was sprayed by a Nz 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 0 by the X-ray photoelectron spectroscopic (XPS)measurements. T i 0 2 particles (P-25,Degussa, BET surface area = 37.2 m2 g-l), preserved in a vacuum oven at 50 "C in order to prevent them from contamination, were used for diffise reflectance (DR) FTIR measurements. The TiOdglass substrates were soaked in a KOH alkaline solution (pH = 13.7) for 30 s and subsequently rinsed by sonification in distilled water (conductivity < 1 pS cm-l) for 10 min. A fully wetting surface against HzO was obtained by this treatment. TMCTS (200pL)(>98%,Shin-Etsu Chem.) 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, evacuating for an additional 0.5 h to remove the physisorbed TMCTS (processI). The samples were irradiated in air with a 500-W high-pressure mercury arc (wavelength 2330 nm) whose light intensity a t 365 nm was 635 pW cm+ (process 11). The surface acidity was checked by DRFT-IR spectra of pyridine adsorbed on the particles from the gas phase under 10Torr a t 30 "C for 0.5h.l0 In the cycle experiments of process 1-11, the n-time process I was followed by the n-time process I1 after complete oxidation of the CHs and Si-H groups of TMCTS by prolonged photoirradiation (>25h). Below the cycle numbers (n)of 4, the TMCTS layer was confirmed to be entirely oxidized within 25 h of photoirradiation by the HzO contact angle measurements. However, above 5 of n, the oxidationwas not accomplished even after 50h of photoirradiation
(9)Hardee, K. L.; Bard, A. J. J. Electrochem Soc. 1976,122, 739. (10)Parry, E.P.J. Cutul. 1963,2,371.
~
, I 3 6 b ~ 32bo
28bo
2bbo
20b0
16b0
12b0
80b
Wavenumber/cm"
Figure 1. (A)Difference DR-FT-IR spectra of Ti02 particles treated with TMCTS before and after the first process I. Difference DR-FT-IR spectra before and after photoirradiation (time = t ) during the first process 11: (B)t = 0.33 h;(C) t = 1.5 h; (D)t = 22.1h. because the photocatalytic activity was remarkably decreased in these states. DR-FT-IR spectra of the Ti02 particles (52.5 mg) packed in a stainless pan (7 mm diameter, 2 mm depth) were recorded on a JIR 5500 FT-IR a t 4 cm-l resolution from 4000 to 400 cm-l with 200 coadded scans. XPS spectra were measured with a Shimadzu electron spectrometer (ModelESCA 750) using a Mg Ka X-ray source (hv = 1253.6eV). 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 lO-'Torr. Incident and detected angles were fixed at 90"and irradiation area was ca. 19.6mm2. The binding energy scales were referenced by setting the hydrocarbon (CH,) peak maxima in the C1, spectra to 284.6 eV. The precision of the binding energy with respect to this standard value was within f0.3eV. Static contact angles were measured by using acontact angle meter (Model CA-D, Kyowa Interface Science Co.) at room temperature (20 & 1 "C). Water droplets with a diameter of approximately 2 mm were placed at six positions for one sample and the average was adopted as the value.
Results and Discussion Figure 1A shows the difference DR-FT-IR spectrum of T i 0 2 particles before and after the first process I. Two negative peaks due to the stretching vibrations of the surface isolated Ti-OH groups (v(Ti,-OH)) are observed at 3691 and 3631 cm-l. Positive peaks appear at 2970, 2912,2168,and 1267 cm-', which can be assigned to the antisymmetric(v,(C&)) and symmetric (vs(CH3))stretching vibrations of the CH3 groups, the stretchingvibration of Si-H groups (v(Si-H)), and the symmetricdeformation (6,(CH3))of the CH3 groups, respectively.ll A strong and broad peak near 1100 cm-' is due to the stretching vibration of the Si-0-Si bonds (vas(Si-O-Si)) of the TMCTS adsorbed. Also, a shoulder at 1061 cm-l is tentatively assigned to the stretching vibration of the (11) Smith,A. L.Anulysis ofSilicones;Wiley-Interscience: New York,
1974.
Langmuir, Vol. 11, No. 9, 1995 3283
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3 2.5
0.5 0
102
0
1
2
3
4
5
6
7
Treatment cycles Figure 2. Dependences of the SiO, film thickness and the binding energy of the Si2p-xPSsignal on the cycle number. interfacial Si-0-Ti bonds (v(Si-0-Ti)).l2 The data above indicate that TMCTS molecules are chemically immobilized on the surface of T i 0 2 after the first process I (Scheme 1B). The spectral change with photoirradiation during the first process I1is shown in parts B-D in Figure 1. A new peak assignable to the stretching vibration of the isolated Si-OH groups (v(Si,-OH)) grows at 3740 cm-l and simultaneously all the absorption peaks due to the CH3 and Si-H groups weaken as the photoirradiation time increases. Also, the strength of the v,,(Si-O-Si) band, which becomes broader and its position shifts approximately 40 cm-' toward higher wavenumber (Figure lD), increases. The presence of the surviving Si-H and Si-CH3 groups after 22.1 h of photoirradiation is ascribable to the dead space incapable of absorbing light, because the photoabsorption length ofTiO2 is on the order of micrometers. On the other hand, the surviving fraction of the Si-CH3 groups cfs) on the TiOdglass sample with the TMCTS layer could be estimated to be 0.82 (photoirradiation time ( t ) = 0.25 h), 0.66 (t = 0.5 h), 0.47 (t = 1 h) and -0 ( t > 3 h) in the first process I1 by analyzing the data on the HzO contact angles on the basis ofthe CassieBaxter theory.13J4 These facts suggest that a part of the Si-OH groups produced by the oxidation ofthe Si-H and Si-CH3 groups reacts to yield Si-0-Si bonds. No change was observed in the spectrum ofthe TMCTS layer on Si02 particles even after 2.5 h of photoirradiation (not shown). The TMCTS layer on T i 0 2 can be considered as being oxidized by activated oxygen species such as 0 and OH radicals, which are generated by the reaction of the adsorbed oxygen and the photoexcited carriers (h+ and e-) 0fTi02.l~From these findings, it is clear that surfacehydroxylated SiO, film is formed on T i 0 2 (Scheme 10. The difference DR-FT-IR measurements further confirmed the same reaction occurring during the sequential cycles of processes 1-11, whereas the rate of oxidation decreases with increasing cycle number. It was also confirmed by the sharp decrease in the contact angle of water that the same oxidation takes place on the surface of the TiOdglass substrates.14 Figure 2 shows the cycle number dependence of the SiO, film thickness determined ~~
(12) It was reported for the interfacial Si-0-Si bonds that the peak due to the stretching vibration situates at 1060 cm-': Tripp, C. P.; Veregin, R. P. N.; Hair, M. L. Langmuir 1993,9, 3518. (13) Baxter, S.;Cassie, A. B. D. J. Text. Znst. 1945, 36, T67. (14) Tada, H. To be submitted for publication in Langmuir. (15) Formenti, M.; Teichner, S.J. Catalysis 1978,2, 87.
from the reduction in the XPS TiZp signal intensity with growing Si0,-overlaid film.16 In the range of n below 4, the film thickness linearly increases 0.4 f 0.1 nm per cycle, which is comparable to the literature value of Si02 monolayer." The film thickness of SiO,(l) (ca. 0.2 nm) is smaller than that of the Si02 monolayer, suggesting the presence of the surface region not covered with TMCTS in the first process I. Assuming the closest packing of TMCTS chemisorbed on Si02,l8the fraction covered with TMCTS of this sample (0) is determined to be 0.86 from the analysis of the H2O contact angle. Further, the possibility that the film thickness is underestimated because of the use of the L(SiO2) for its calculation of the surface hydroxylated SiO, must also be taken into account. The degree of the error is unclear; however, this must be particularly significant in the case of n = 1. In the particulate system, Fukui et al. insisted that the TMCTS monolayer is produced on T i 0 2 by the chemisorption from the gas phase on the basis of the weight measurements.lg An upward deviation from the straight line in the region of n greater than 5 may be caused by the surviving CH3 groups on the surface as stated above. The binding energy of SiZp(BE) is also shown in Figure 2 as a function of n. The BE increases with the cycle number, approaching a constant value of ca. 103 eV above four cycles. Since the BE of Si02 is reported to be 103.35 eV,20the development of Si-0-Si networks with the sequential treatments seem to be responsible for the shift of the BE. With photoirradiation, the average formal charge of Si may increase from 2+ (TMCTS)toward 4+ (SiO2). Evidently, SiO, films having thickness controlled on a monolayer level can be formed on T i 0 2 . 2 1 When multilayers of SiO, (-3 nm) were formed after six cycles, the rate of oxidation markedly decreased and the XPS signal intensity of Ti2p almost disappeared. These facts are strong evidence for the layerby-layer structure of the SiO, film (Scheme 1E). It was previously concluded from the spectroscopicand theoretical results that the methylcyclosiloxane monolayer (-0.49 nm in thickness) on oxides has an orientation of its ring parallel to the surface and all the CH3 groups outward (Scheme lB), while the most stable conformation of TMCTS in the gas phase is the all-trans form (Scheme 1A).18 The drastic conformational change may be caused by the formation of the interfacial covalent bonds (Si0-M, M = metal ions on the surface of substrates) and the low surface energy of the film with the outermost CH3 groups. Upon completion of the monolayer formation during the first process I, the successive multilayer formationwould be inhibited because ofthe disappearance of the adsorption sites of TMCTS molecules, Le., Ti,-OH groups (Figure 1A). In the first process 11, the adsorption sites of TMCTS for the next monolayer formation are reproduced, i.e., Si,-OH groups (Figure lD),and the same (16)Assumingthe average mean free path ofphotoelectronsescaping from T i 0 2 with kinetic energy of 799.8 eV (L(799.8eV))is 1.23 nm, one can calculate the film thickness ( d ) by the equation, d = ME) sin 8 ) ln(Z&. B (=90")is the angle between the sample plane and the detector ofthe photoelectrons.ZandZo are the intensities of the Tizp photoelectron with and without the SiO, film, respectively.The error in this analysis is estimated to be ca. 40%for non free electrons: Penn, D. R. J.Electron Spectrosc. Relat. Phenom. 1976, 9, 29. (17) Niwa, M.; Kato, S.; Hattori, T.; Murakami, Y. J . Chem. SOC., Faraday Trans. I 1984,80,3135. Nakamura, K.; Nagayama, H. J. Phys. Chem. 1994, (18) Tada, H.; 98, 12452. (19) Fukui, H.;Ogawa,T.;Nakano, M.;Yamaguchi,M. In Controlled Interphases in Composite Materials; Ishida, H., Ed.; Elsevier: Amsterdam, New Yorki 1990. (20) Briggs, D.;Seah, M. P. Practical Surface Analysis by Auger and X-ra.y Photoelectron Spectroscopy; .. John Wiley and Sons: New York, 1983. (21) Although the value of x cannot be determined experimentally, it must theoretically decrease with the growth of Si-0-Si networks (the trend was confirmed by XPS) and be in the range of 2 -= x < 3.
Letters
3284 Langmuir, Vol. 11,No. 9, 1995 reaction cycle is repeated. It follows that the monolayer of SiO, is thought to yield during each cycle. The reduction in the rate of oxidation with cycle number can also be explained in terms of the increase in the SiO, film thickness, which decreases the probability that the photocarriers generated in T i 0 2 reach the surface of the substrate through the tunneling effect. From another point of view, the rate of the methylsiloxane oxidation becomes a good index of the photocatalytic activity of Ti02.14 Pyridine adsorption experiments further demonstrate the appearance of the Bronsted acid sites on the surface of the T i 0 2 particles covered with the SiO, monolayer (Si0,(1)iTi02; difference DR-FT-IR, 1545 cm-l(19b)),whereas only Lewis acid sites are present on the surface of the pristine particles (1606 (Ba), 1574 (Bb), 1491 (19a), and 1444 cm-' (19b)),22 It can be thought that pyridine coordinates to Ti4+ions with coordinative unsaturations on the surface of TiOz, while it adsorbs at the Si-OH groups in the case of SiO,(l)/TiOZ, leading to pyridinium ions.23 The detail data will be reported in a subsequent paper.14 In conclusion, the SiO, (2 < x < 3) monolayers could be (22) Morterra, C.; Cerrato, G.; Pinna, F.; Signorretto, M. J. Phys. Chem. 1994,98,12373.
piled up layer-by-layer at ambient temperature by repeating the CVSM with TMCTS and successive photoirradiation. When the layer of SiO,(l) (6 0.86) is formed on TiOz,the acidic nature of the surface is found to be remarkably changed, while the photocatalytic activity is retained owing to the tunneling effect. On the other hand, the multilayers of SiO, (n =- 5) remarkably reduce the photocatalytic activity. Consequently, the possibility of controlling the photocatalytic reactions with thickness of SiO, deposited on oxide semiconductors was indicated. It is the author's opinion that this study contributes to the modification of the reactivity and selectivity of the oxide semiconductorphotocatalysts, leading to the extension of their applications. The investigations on the detail reaction mechanism and the application to the photocatalytic reactions are now in progress in this laboratory.
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Acknowledgment. The author expresses his sincere gratitude to K. Shimoda (NSG Techno-Research) for experimental support. LA9501694 (23) It has recently been reported by Niwa et al. that the Bronsted acidity is generated on the Si02 monolayer with the network of Si0-Si. which was ureDared bv the thermal reaction (T> 493 K)on the surface of Al203: *Kaiada, N.; Toyama, T.; Niwa, M. J. Phys. Chem. 1994,98, 7647.
