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Photocatalysis on TiO2 Surfaces Investigated by Atomic Force Microscopy: Photodegradation of Partial and Full Monolayers of Stearic Acid on TiO2(110) Phillip Sawunyama,† Akira Fujishima,*,†,‡ and Kazuhito Hashimoto*,†,§ Kanagawa Academy of Science and Technology, 1583 Iiyama, Atsugi, Kanagawa 243-0297, Japan, Department of Applied Chemistry, University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan, and Research Center for Advanced Science and Technology, University of Tokyo, Komaba, Meguro-Ku, Tokyo 153-8904, Japan Received October 15, 1998. In Final Form: March 8, 1999 We have studied the nature and surface morphological changes associated with the photodegradation of stearic acid LB films on TiO2(110). Interestingly, submonolayers of stearic acid consisted of circular domains of various sizessa feature very attractive for monitoring TiO2 photocatalysis by AFM. We noted that there was no bulk differential reactivity at island edges compared to the interior. This suggests that the rate of photodegradation of the stearic acid molecules is independent of their location in the island. Accordingly, the overall surface reactivity trends were similar for both partial films and complete films. Likewise, the observed inhomogeneous reactivity patterns appear to be a reflection of the transient distribution of the reaction centers.
* To whom correspondence should be addressed at the University of Tokyo. Fax: +81-3-3481-4571. E-mail: kazuhito@ fchem.chem.t.u-tokyo.ac.jp. † Kanagawa Academy of Science and Technology. ‡ Department of Applied Chemistry, University of Tokyo. § Research Center for Advanced Science and Technology, University of Tokyo.
others, dominate the surface chemistry of TiO2. Likewise, the hydroxylated surface is composed of two types of hydroxyl groups,18,21,22 and the degree of hydroxylation influences its photoactivity. The direct observation of the TiO2 photocatalytic surface reaction might provide insight into the mode of action of TiO2. The power and versatility of scanning probe techniques such as scanning tunneling microscopy (STM)23 or atomic force microscopy (AFM)24 are indispensable in this respect. For example, Onishi et al. have employed STM to follow O2 adsorption on TiO2(110).25 Further, Onishi et al. have demonstrated the power of STM to monitor the thermal decomposition of the formate ion26,27 and the acetate ion28 on TiO2(110), respectively. Accordingly, we have shown that the photoreactivity of TiO2 can be directly “visualized” by AFM.29 In the previous study, we followed the photodegradation of Langmuir-Blodgett (LB) films of stearic acid mediated by a polycrystalline
(1) Bahnemann, D.; Cunnigham, J.; Fox, M. A.; Pelizzetti, E.; Pichat, P.; Serpone, N. In Aquatic and Surface Photochemistry; Helz, G. R., Zepp, R. G., Crosby, D. G., Eds.; Lewis Publishers: Boca Raton, FL, 1994; p 261. (2) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. Rev. 1995, 95, 69. (3) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735. (4) Mills, A.; Le Hunte, S. J. Photochem. Photobiol., A: Chem. 1997, 108, 1. (5) Pichat, P. In Handbook of Heterogeneous Catalysis; Ertl, G., Knozinger, H., Weitkamp, J., Eds.; Wiley-VCH: New York, 1997; Vol. 4, p 2111. (6) Kikuchi, Y.; Sunada, K.; Iyoda, T.; Hashimoto, K.; Fujishima, A. J. Photochem. Photobiol., A: Chem. 1997, 106, 51. (7) Sunada, K.; Kikuchi, Y.; Hashimoto, K.; Fujishima, A. Environ. Sci. Technol. 1998, 32, 726. (8) Watanabe, T.; Kitamura, A.; Kojima, E.; Nakayama, C.; Hashimoto, K.; Fujishima, A. In Photocatalytic Purification and treatment of Water and Air; Ollis, D. E., Al-Ekabi, H., Eds.; Elsevier: New York, 1993; p 747. (9) Paz, Y.; Luo, Z.; Rabenberg, L.; Heller, A. J. Mater. Res. 1995, 10, 2842. (10) Heller, A. Acc. Chem. Res. 1995, 28, 503. (11) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature (London) 1997, 388, 431. (12) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Adv. Mater. 1998, 10, 135.
(13) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1216. (14) Primet, M.; Pichat, P.; Mathieu, M.-V. J. Phys. Chem. 1971, 75, 1221. (15) Munuera, G.; Rives-Arnau, V.; Saucedo, A. J. Chem. Soc., Faraday Trans. 1 1979, 75, 736. (16) Gonzalez-Elipe, A. R.; Munuera, G.; Soria, J. J. Chem. Soc., Faraday Trans. 1 1979, 75, 748. (17) Lu, G.; Linsebigler, A.; Yates, J. T., Jr. J. Phys. Chem. 1994, 98, 11733. (18) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. Rev. 1996, 27, 61. (19) Hadjiivanov, K. I.; Lamotte, J.; Lavalley, J.-C. Langmuir 1997, 13, 3374. (20) Rusu, C. N.; Yates, J. T., Jr. Langmuir 1997, 13, 4311. (21) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, U.K., 1994. (22) Barteau, M. A. Chem. Rev. 1996, 96, 1413. (23) Binning, G.; Rohrer, H.; Gerber, Ch.; Weibel, E. Phys. Rev. Lett. 1982, 49, 57. (24) Binning, G.; Quate, C. F.; Gerber, Ch. Phys. Rev. Lett. 1986, 56, 930. (25) Onishi, H.; Iwasawa, Y. Phys. Rev. Lett. 1996, 76, 791. (26) Onishi, H.; Iwasawa, Y. Chem. Phys. Lett. 1994, 226, 111. (27) Onishi, H.; Iwasawa, Y. Langmuir 1994, 10, 4414. (28) Onishi, H.; Yamaguchi, Y.; Fukui, K.; Iwasawa, Y. J. Phys. Chem. 1996, 100, 9582. (29) Sawunyama, P.; Jiang, L.; Fujishima, A.; Hashimoto, K. J. Phys. Chem. B 1997, 101, 11000.
Introduction The photocatalytic properties of TiO2 are of practical significance in applications ranging from solar energy conversion through environmental remediation1-5 to antibacterial,6,7 self-cleaning,8-10 and antifogging functions.11,12 Extensive studies on the surface chemistry of TiO2 have established that the chemical and morphological form of the TiO2 surface, kinetic and thermodynamic constraints, crystalline modification, and so forth are of considerable influence in the overall photoreactivity.1-5,13-20 In particular, defects and hydroxyl groups,13,16 among
10.1021/la9814440 CCC: $18.00 © 1999 American Chemical Society Published on Web 04/24/1999
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Figure 1. Typical AFM image (10 µm × 10 µm) of a partial film of stearic acid on a rutile TiO2(110) surface. Distinct features in part a include the sizes and circular shapes of the coalesced domains. Also clearly visible are film defects or pinholes. (b) Three-dimensional image (4 µm × 4 µm) of the framed island. The partial film was transferred at a surface pressure of 15 mN m-1 at 20 °C.
anatase TiO2 film using FT-IR and AFM. Our results revealed that at the submicrometer level the TiO2 surface reactivity pattern is very complex. In the present study, we have investigated the photodegradation of partial and full monolayers of stearic acid on a rutile TiO2(110) single crystal. AFM observation of the surface morphology of submonolayer films also provided further insight into the reactivity of molecules in different regions of the islands. Experimental Section Polished rutile TiO2(110) single-crystal substrates (dimensions: 20 mm × 10 mm × 1 mm) used in this work were obtained from Nakazumi Crystal Laboratory. Prior to LB film deposition, the substrates were washed with ethanol, chloroform, and water, respectively, to give a water contact angle of about 60°. Film deposition involved a standard procedure. Briefly, 100 µL of a 2 mM stearic acid in chloroform (spectroscopic grade) solution was spread over a 1 × 10-4 M BaCl2 aqueous subphase (Milli-Q H2O with a resistivity of 18.3 MΩ cm, pH ≈ 6.0) maintained at 20 °C in a LB trough (USI). The stearic acid and BaCl2 (both 99.9% pure) were obtained from Wako Chemicals and were used without further purification. The monolayers of stearic acid and or barium stearate were transferred (Y-type) by a vertical dipping technique at 5 mm min-1 and at a pressure of 20 mN m-1 (Wilhelmy plate method). Submonolayers of stearic acid were transferred at 15 mN m-1 (area ≈ 0.201 nm2/molecule). In addition, all film transfer was performed under diffuse light (stearic acid photodegradation during film transfer was negligible). Photocatalytic experiments were carried out as soon as the LB films were dry. Illumination of the as-obtained LB/TiO2 film assemblage was performed using filtered UV light from a Hypercure 200 UV lamp (Yamashita Denso, Long wave UV λmax ≈ 365 nm) equipped with a light guide. The incident UV light intensity (measured using a UV radiometer: Topcon UVR-1) at the sample surface was approximately 2.5 mW cm-2. The AFM experiments were performed using a SPA300 system and a SPI3700 controller (Seiko Instruments) under ambient conditions (temperature ≈ 20 °C, relative humidity ) 40%). Commercially available triangular, Si3N4-sharpened cantilevers with a spring constant of 0.02 N m-1 were used. The force between the tip and
Figure 2. Island size distribution obtained by visually counting islands of a given size in a 20 µm × 20 µm area. It is obvious that the bulk of the islands had diameters in the range 1-4 µm. sample was typically 1 nN. A 20 µm scanner was used in all the AFM experiments. The AFM images were recorded simultaneously in “constant height” and “constant force” modes. In a typical experiment, the tip was withdrawn from the sample before UV irradiation. After irradiating for a predetermined period, the light was switched off and the tip was re-engaged for image recording. Repeated AFM imaging of the mechanically soft organic films did not result in film damage at forces e 1 nN. Difference IR spectra were obtained by transmission mode FTIR spectroscopy (Bio Rad model FTS-40A) using a liquid-nitrogencooled MCT detector. The sample compartment was purged with dry air continuously. In a typical IR experiment, 400 scans were coadded to give the final spectrum and all spectra were recorded at a resolution of 4 cm-1.
Results and Discussion Submonolayers. Due to the absence of a distinct twophase coexistence region in the surface pressure-area
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Figure 3. Sequential AFM images (3 µm × 3 µm) of the TiO2(110)-mediated photodegradation of stearic acid molecules that constitute the framed island in Figure 1. Images were obtained after (a) 5, (b) 10, and (c) 20 min of UV irradiation, respectively. In parts a and b, the images are displayed in two dimensions and three dimensions to highlight morphological changes (incident UV light intensity ) 2.5 mW cm-2).
(π-A) isotherm for stearic acid and other fatty acids, observation of phase-separated domains at the air-water interface is difficult. Nevertheless, by transferring the film at 15 mN m-1, island structures of stearic acid were obtained. A typical AFM image of the as-obtained island structures is shown in Figure 1. Noteworthy is the perfectly circular and polydisperse nature30 of the coalesced islands. The smallest islands were approximately 1 µm in diameter with the larger islands having diameters well over 20 µm. An island-size distribution is presented in Figure 2. It should be noted that in two-dimensional phase-separated systems, circular domains are thermodynamically favorable and coalescence of the islands is a result of the (30) Mulder, W. H. J. Phys. Chem. B 1997, 101, 7744 and references therein.
minimization of surface energy through a reduction in interfacial length and curvature.31,32 The circular domains are believed to have formed at the air-water interface and were preserved upon film transfer to the TiO2(110) surface. Nonetheless, we do not rule out the possibility of a postdeposition molecular rearrangement event as has been suggested by other workers.33,34 The thickness of a typical island (measured at the edges) was approximately 2.13 ( 0.12 nm (cf. a fully extended stearic acid molecule (31) Knobler, C. M. Science 1990, 249, 870. (32) Adamson, A. W.; Gast, A. P. Physical Chemistry of Surfaces; Wiley-Interscience: New York, 1997; Chapter 4. (33) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213. (34) Chi, L. F.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Thin Solid Films 1994, 242, 151.
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has a chain length35 of 2.5 nm). In addition, a careful examination of the domains revealed pinholes or film defects with a mean diameter of approximately 35 nm. The surface morphological changes for the photodegradation of stearic acid molecules in a typical island (framed in Figure 1) are depicted in Figure 3. In Figure 3a, the inhomogeneous pitting or etching of the island is very conspicuous. The estimated diameters of the pits were in the range 24-100 nm. A simple calculation shows that this corresponds to approximately 2250-40000 stearic acid molecules having undergone photodegradation, respectively. Also the average depth of a pit was approximately 0.9 nm. Clearly, this shows that, at this stage of the reaction, the organic film comprised a mixture of stearic acid and intermediate molecules. Likewise, imaging of the island edges at higher resolutions revealed no evidence of preferential reaction at island edges compared to the interior regions. On analyzing numerous other islands, we obtained the same randomization phenomenon. With further irradiation, an even more peculiar reactivity pattern unfolded, as depicted in Figure 3b and c. The surface morphology after illuminating for 10 min revealed a startling merging of pits to generate a mosaic which eventually disappeared with progress of the reaction. At 20 min of reaction (Figure 3c), the island structure was no longer discernible. Prolonged irradiation resulted in the complete photodegradation of the stearic acid molecules (vide infra).36 Monolayers. Next, we studied the characteristics and photodegradation of complete stearic acid monolayers. In a typical 1 µm2 area, pinholes or film defects not larger than approximately 55 nm in diameter were visible. The film thickness (estimated from the depth of the deepest holes) was approximately 2.15 ( 0.13 nm. This value is in good agreement with the value obtained for submonolayer islands. Molecular resolution images revealed the presence of locally ordered and disordered domains. In Figure 4, rows of stearic acid molecules are clearly visible; however, there is no long-range order as shown in Figure 4b. On UV irradiation, we observed the characteristic inhomogeneous surface reactivity pattern witnessed for submonolayers. The organic film surface morphological changes with irradiation time are presented in Figure 5. The AFM images are presented together with crosssectional profiles along the lines indicated in the images. (Note that the line profiles show relative height differences only.) Prior to UV irradiation, the film morphology was fairly uniform. However, after illumination for 3 min, numerous randomly distributed shallow pits approximately 24 nm in diameter and approximately 0.15 nm deep were observed. Further irradiation (5 min) generated even deeper pits extending to well over 64 nm in diameter. The spatial localization of the decomposition process is also striking. Moreover, it also appears as though the widening of existing holes is now superseding the creation of new ones. Figure 5e shows that further decomposition of the organic film resulted in the merging of the pits to generate peninsula-like structures or a mosaic. Typical peninsula-like structures are depicted in Figure 5f and g. Beyond 20 min, continually merging holes resulted in the eventual generation of randomly scattered island structures as shown in Figure 5h. Peak-to-valley values for these island structures were typically 0.8 nm and their diameters were approximately 40 nm. (35) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991. (36) Sitkiewitz, S.; Heller, A. New J. Chem. 1996, 20, 233.
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Figure 4. Molecular resolution image (18 nm × 18 nm) of a monolayer of stearic acid on TiO2(110) prior to UV illumination. Parallel molecular rows are discernible in part a. (b) Filtered image (6 nm × 6 nm) of part of part a. Local molecular positional order is apparent; however, there is no long-range order.
Notwithstanding the complexity of the photodegradation process, it was also evident that film decomposition was accompanied by a parallel film disordering or aggregation event. We noticed a roughening of the surface when the film/TiO2 system was illuminated with UV (even for very brief periods, e.g., a few seconds). Also the AFM data in Figure 5 suggest that up to approximately 10 min, film aggregation and photodegradation appeared to be the major events. After this period only photodegradation predominated, as shown in Figure 5f-i. The bulk of the stearic acid and intermediate molecules were completely degraded after approximately 90 min. Also noteworthy was the initiation and subsequent propagation of the reaction. The reaction appeared to spread from randomly distributed surface active sites. This can be likened to a “ripple” effect; once the reaction had been initiated, adjacent molecules were attacked by photogenerated oxidants. We also performed control experiments in which monolayers of stearic acid were deposited onto CaF2 substrates. In contrast to the case of films on TiO2(110), no surface morphological changes or FT-IR spectral differences were
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Figure 5. Large-scale (1 µm × 1 µm) AFM images showing the surface morphology of a monolayer of stearic acid on TiO2(110) at different stages of reaction. Below each image is a cross-sectional profile along the white line. The images were obtained after (a) 0, (b) 3, (c) 5, (d) 7, (e) 10, (f) 15, (g) 20, (h) 30, and (i) 60 min of UV irradiation, respectively. The characteristic inhomogeneous surface reactivity pattern is conspicuous (incident UV light intensity ) 2.5 mW cm-2).
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diffusion of •OH radicals and other photogenerated oxidizing species to attack molecules farther away from the nucleation sites is also likely. The tendency for the number of reaction nucleation sites to saturate with illumination time is probably due to the passivation of active sites by the products of the reaction. On the whole, the observed morphological changes are attributable to a number of factors such as the intrinsic surface properties of the photocatalyst, the behavior of free radicals, and the rate of creation of new nucleation sites versus the passivation of existing sites by the products of the reaction. Further work on other TiO2 single-crystal surfaces should help us better understand the complex surface photoreactivity. Conclusions
Figure 6. (a) FT-IR spectral changes (in the C-H stretching region) for the photodegradation of a monolayer of stearic acid on TiO2(110). The spectra were recorded at 0, 10, 15, 20, 30, 90 and 120 min, respectively. (b) Normalized absorbance versus time profiles for a monolayer of stearic acid on TiO2 (b) and CaF2 (O) with UV irradiation time, respectively (incident UV light intensity ) 2.5 mW cm-2).
observed for films on CaF2 upon UV irradiation. A representative FT-IR decomposition profile for a film on TiO2(110) is shown in Figure 6. All organic compounds were photodegraded in about 90 min. The photodecomposition mechanisms are considered to involve two main oxidative routes; direct hole oxidation37-39 and •OH radical oxidation40-43 of molecules located at or near reaction nucleation sites. Surface migration or (37) Hashimoto, K.; Kawai, T.; Sakata, T. J. Phys. Chem. 1984, 88, 463. (38) Stafford, U.; Gray, K. A.; Kamat, P. V.; Varma, A. Chem. Phys. Lett. 1993, 205, 55.
The surface morphological changes that were observed during the photodecomposition of partial and full monolayers of stearic acid on TiO2(110), respectively, have provided further insight into the course of TiO2 photocatalytic reactions. Several factors are believed to be responsible for the observed inhomogeneous nature of the reaction. (a) The intrinsic surface properties of the photocatalyst such as acidic and basic sites affect the photocatalytic reactivity differently. (b) The products of may passivate the surface active sites. For example, carbonates generated by the reaction of TiO2 surface oxygen and CO and CO2 can suppress the surface activity. (c) The rate of desorption of the products may also affect the rate of regeneration of the surface active sites. What is not clear, however, is whether desorption of products necessarily restores the surface active sites to their prereaction condition. In all, the AFM observations suggested that the inhomogeneous reactivity patterns are probably a reflection of the momentary distribution of reaction centers. While this study has been largely of a qualitative nature, quantitative AFM studies are expected to provide further understanding of the relationship between nucleation sites or active centers, the number of molecules undergoing reaction at any given time, and the overall surface photoreactivity. Acknowledgment. Financial support from the Science and Technology Agency, Japan, in the form of a research fellowship to PS is gratefully acknowledged. Further support in the form of a Grant-in-Aid for Scientific Research on Priority Area of Electrochemistry of Ordered Interfaces was provided by Ministry of Education, Science, Sports and Culture, Japan. LA9814440 (39) Carraway, E. R.; Hoffmann, A. J.; Hoffmann, M. R. Environ. Sci. Technol. 1994, 28, 786. (40) Okamoto, K.; Yamamoto, Y.; Tanaka, H.; Tanaka, M. Bull. Chem. Soc. Jpn. 1985, 58, 2015. (41) Turchi, G. S.; Ollis, D. F. J. Catal. 1990, 122, 178. (42) Lawless, D.; Serpone, N.; Meisel, D. J. Phys. Chem. 1991, 95, 5166. (43) Upadhya, S.; Ollis, D. F. J. Phys. Chem. B 1997, 101, 2625.
