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J. Phys. Chem. B 1997, 101, 11000-11003
Photodecomposition of a Langmuir-Blodgett Film of Stearic Acid on TiO2 Film Observed by in Situ Atomic Force Microscopy and FT-IR Phillip Sawunyama,† Lei Jiang,† Akira Fujishima,*,†,‡ and Kazuhito Hashimoto*,†,§ Kanagawa Academy of Science and Technology, 1583 Iiyama, Atsugi, Kanagawa 243-02, Japan, Department of Applied Chemistry, Faculty of Engineering, UniVersity of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113, Japan, Research Center for AdVanced Science and Technology, UniVersity of Tokyo, Komaba, Meguro-Ku, Tokyo 153, Japan ReceiVed: September 15, 1997; In Final Form: NoVember 3, 1997X
We have probed the TiO2-mediated photomineralization of Langmuir-Blodgett (LB) films of stearic acid via atomic force microscopy (AFM) and FT-IR. In situ AFM images revealed that at the submicrometer level the photodecomposition process of stearic acid molecules on a polycrystalline anatase TiO2 film is inhomogeneous, with the various reaction initiation centers or nucleation regions being randomly distributed throughout the photocatalyst surface. Furthermore, parallel FT-IR results showed that the reaction follows pseudo-first-order kinetics. To rationalize the observed random LB film photoetching and buckling behavior, we invoked a simple reaction model that incorporates the reactivity of the TiO2 film and LB film disorganization phenomenon during the photodegradation process.
Introduction The surface composition of TiO2 is very inhomogeneous and comprises a variety of defects such as Ti cation vacancies and oxygen anion vacancies.1 Consequently, these defects influence the reactivity of TiO2 in applications such as solar to chemical energy conversion,2 environmental decontamination,3 and more recently TiO2-based self-cleaning materials.4,5 Probe molecules, e.g., CO or NH3, are usually used to determine the nature and concentration of the surface active sites per unit area. In addition, the other fundamental details of reactions involving TiO2 are generally studied in aqueous or gaseous systems. The complexities of the involved photoprocesses, however, justify an investigation of the reactions from a different perspective. A fascinating approach involves the use of an organized molecular assembly such as a Langmuir-Blodgett (LB) film6,7 to probe the TiO2 photochemical reactions. A LB film offers several advantages over solution- or gas-based systems, for example, a rigid well-defined geometry and control over orientation of functional groups. In this investigation, we selected a simple stearic acid LB film/TiO2 assemblage as a model photocatalytic system. The TiO2-mediated photodecomposition of stearic acid8 can be summarized as follows: TiO2/hν
C17H35COOH + 26O2 98 18CO2 + 18H2O While low reagent and/or product concentrations present some difficulties with respect to monitoring this reaction in situ, it should be possible to indirectly observe the reaction using atomic force microscopy.9 AFM has been demonstrated to be a very versatile and powerful tool for surface imaging at the submicrometer level. For example, we have applied AFM to probe the surface structure and photoinduced surface morphological changes of a TiO2 crystal,10 an azobenzene LB film,11 and an azobenzene crystal.12 * Corresponding author: phone +81-(0)3-3481-4423; fax +81-(0)33481-4571; e.mail
[email protected]. † Kanagawa Academy of Science and Technology. ‡ Department of Applied Chemistry, University of Tokyo. § Research Center for Advanced Science and Technology, University of Tokyo. X Abstract published in AdVance ACS Abstracts, December 1, 1997.
S1089-5647(97)03009-5 CCC: $14.00
In this paper, we report for the first time the direct observation of a TiO2 photocatalytic reaction by in situ AFM and FT-IR. We show that at the submicroscopic level, the TiO2-mediated photodecomposition of LB films of stearic acid is an inhomogeneous process. Experimental Section The transparent polycrystalline anatase TiO2 film was deposited on polished CaF2 substrates (50 mm × 12.5 mm × 1.0 mm, Pier Optics Co.) via a dip-coating technique using a commercial TiO2 isopropoxide sol (NDH-520C) obtained from Nippon Soda Co.13 Prior to LB film deposition, the substrates were washed with ethanol, chloroform, and water, respectively. Film deposition was carried out as follows: 100 µL of a 2 mmol stearic acid in chloroform solution was spread over a 1 × 10-4 M BaCl2 aqueous subphase (Milli-Q H2O, 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 Chemical Co. and were used without further purification. The monolayers of stearic acid/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). Film deposition was reproducible, with transfer ratios better than 0.9. Photocatalytic experiments were carried out as soon as the LB films were dry. Irradiation of the as-obtained LB/TiO2 film assemblage was performed using filtered UV light (λmax ≈ 365 nm, power output ) 0.5 mW cm-2) under ambient conditions. The difference IR spectra were obtained by transmission mode FT-IR spectroscopy (Bio Rad model FTS-40A) using a liquid nitrogen cooled 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. The AFM experiments were performed using a SPA300 (Seiko Instruments) system with commercially available triangular Si3N4 sharpened cantilevers. The force between the tip and sample was typically 1 nN. The images shown were obtained in “constant height” mode, while precise topographic information was obtained from the simultaneous “constant force” mode images. In situ imaging was © 1997 American Chemical Society
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J. Phys. Chem. B, Vol. 101, No. 51, 1997 11001
Figure 2. FT-IR spectra for the photodecomposition of a five-layer stearic acid LB film mediated by anatase TiO2 film. The spectra were recorded after 0, 3, 7, 15, 25, 40, 60, 90, and 120 min of irradiation, respectively. The inset shows a plot of absorbance (CH2 asymmetric stretch, 2922.8 cm-1) as a function of irradiation time.
Figure 1. Low-resolution AFM images of the polycrystalline anatase TiO2 surface: (a) an (8 µm × 8 µm) image showing surface morphology with the corresponding vertical height-distance profile below and (b) a zoom-in image (500 nm × 500 nm) of (a) showing clearly the grain size and grain boundaries and a corresponding vertical height-distance profile.
done before and then at regular intervals after irradiating the sample, respectively. Results and Discussion AFM images of the TiO2 film surface are shown in Figure 1. The topographic vertical height profile in Figure 1a shows that the substrates were fairly “uneven.” A zoom-in image, Figure 1b, clearly reveals the surface composition or inhomo-
geneous grain structure of the TiO2 particles. The grain size is in the range of 10-20 nm with vertical height values in the range of 0.3-0.7 nm (cf. a monolayer of stearic acid is ca. 2.5 nm thick).14 The successive FT-IR investigation of the photomineralization of mono- to multilayered (1-7 layers) LB films of stearic acid revealed that the reaction proceeds via first-ordertype kinetics of the form: [SA]t ) [SA]0 exp(-kobst), where [SA]0 and [SA]t are the concentrations of stearic acid at time t ) 0 and t, respectively, and kobs is the pseudo-first-order rate constant. A value of 0.055 ( 0.011 min-1 was calculated for kobs. A typical IR decomposition profile for a five-layer LB film in the C-H stretching region is presented in Figure 2. The band at 2957.5 cm-1 is the asymmetric in-plane C-H stretching mode of the CH3 group. The bands at 2922.8 and 2853.4 cm-1 are the asymmetric and symmetric C-H stretching modes of the CH2 group, respectively.14 A plot of absorbance (CH2 asymmetric stretch, 2922.8 cm-1) as a function of time is shown in Figure 2 (inset). In the parallel AFM experiments, repeated imaging over the same sample area for various LB films produced no discernible changes in the images, thus indicating that tip-induced damage to the mechanically soft organic film was negligible. The corresponding low-resolution images obtained for the photodegradation of a five-layer LB film described above are shown in Figure 3. Prior to irradiation, the organic film surface is fairly smooth and homogeneous as depicted in the surface morphology image (top) and vertical height-distance profile (bottom) in Figure 3a. Upon sample irradiation, the decomposition reaction is effected through interfacial redox reactions involving photogenerated charge carriers and adsorbed molecules. Consequently, these changes are reflected structurally as shown in Figure 3b. Noteworthy is the clear contrast between regions where film damage has occurred (dark areas) and regions where little reaction has taken place (light areas). Perhaps, this distinction is more apparent in the vertical height-distance profile in Figure 3b (IR data show that up to 50% of the film has been destroyed). The discrepancy in the theoretical height/ thickness of the film is apparent for regions of the film exhibiting the islandlike structures. A five-layer stearic acid LB film has a thickness of ca. 12.5 nm assuming that all molecules
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Figure 3. Low-resolution AFM images (8 µm × 8 µm) and the corresponding topographic cross-sectional profiles for the photodecomposition of a five-layer stearic acid LB film mediated by anatase TiO2 film. Images were obtained after (a) 0, (b) 10, and (c) 60 min of irradiation, respectively.
irrespective of the layer in which they are located are aligned vertically. The AFM data (Figure 3 b) show peak-valley values of up to 30 nm over island distances of up to 265 nm. The disparity in film thickness can be explained by assuming that, prior to film degradation or as soon as the decomposition reaction has been initiated, there is a disordering event that eventually lead to the collapse of the film structure. This can be likened to the phenomenon of monolayer collapse observed at very high pressures during the compression of Langmuir films. Consequently, film collapse leads to aggregation of the stearic acid molecules and any intermediates thereof as evidenced by the AFM data. We also note that there is no clear pattern or trend in surface reactivity order; the whole process is random. Figure 3c shows that all most all the film was degraded after irradiating for 60 min (cf. FT-IR data in Figure 2). The irregularity in the vertical height-distance profile is attributed to the long-range roughness of the TiO2 film (as already discussed above) and not to the presence of organic materials. We will invoke a simple reaction model to rationalize the observed random film photoetching behavior. This model is illustrated in Figure 4. XRD data revealed that the anatase polycrystalline film consisted of many randomly oriented crystallites; with the majority oriented along the (101) plane. Moreover, the rutile phase was also present, albeit in minute quantities. Considering the surface of the TiO2 film, the different reaction initiation centers (which can be likened to reaction nuclei) are shown as alternating light and dark areas. It should be noted, however, that the actual scenario on the photocatalyst surface at the molecular level is much more complex than this simple model seems to portray. On contact with the photocatalyst surface, stearic acid molecules are either dissociatively or molecularly adsorbed onto the surface sites depending on the physical nature of the respective sites. Upon irradiation, the order and periodicity of the LB film are destroyed with the inherent buckling of the film and hence the observed height differences in the AFM images. Despite the transition from an ordered state to a disordered one, stearic acid molecules located within the most reactive centers are believed to undergo mineralization at a faster rate than those located in the least
Figure 4. Schematic of a model for the photodecomposition of an organic LB film mediated by a heterogeneous TiO2 film highlighting the aggregation/buckling effect (step 2) prior to or during the degradation process (step 3). The TiO2 surface is depicted as alternating very reactive regions (light areas) and least reactive regions (dark areas). The reaction is shown at arbitrary times.
reactive centers. Furthermore, it is considered that a parallel random stearic acid molecule attack by oxygen and H2O-derived free radicals diffusing within the organic film is also possible. Thus the inhomogeneous LB film surface morphological changes observed by AFM suggest that the stearic acid photodecomposition process on a polycrystalline TiO2 film is a heterogeneous process.
Letters Due to the uneven nature of the TiO2 film substrates, it was difficult to follow the reaction at higher AFM resolutions. Therefore, further AFM imaging at monolayer to submonolayer levels15 of films on very smooth surfaces such as polished TiO2 single crystals as well as the coupling of AFM with friction force microscopy10 and kelvin probe force microscopy16 should further aid us in elucidating the surface reactivity mechanism. In summary, we have demonstrated for the first time that an interfacial photocatalytic reaction can be directly observed by in situ AFM. At the microscopic level, the TiO2-mediated photodecomposition of a stearic acid film is not a uniform process but occurs randomly throughout a LB film on a polycrystalline anatase TiO2 substrate. There is also a LB film disorganization process that leads to total film collapse and the aggregation of the stearic acid molecules prior to or as soon as the decomposition reaction has been initiated. In addition, parallel FT-IR results showed that the reaction follows pseudofirst-order kinetics. The observed random LB film photoetching and buckling behavior was rationalized through a simple reaction model that merges the reactivity of the TiO2 film and LB film disorganization phenomenon during the photodegradation process. Acknowledgment. P.S. thanks the Science and Technology Agency for financial support. References and Notes (1) Hadjiivanov, K. I.; Klissurski, D. G. Chem. Soc. ReV. 1996, 27, 61.
J. Phys. Chem. B, Vol. 101, No. 51, 1997 11003 (2) Parmon, V. N.; Zamareav, K. I. In Photocatalysis: Fundamentals and Applications; Serpone, N., Pelizzetti, E., Eds.; Wiley: New York, 1989; Chapter 17, p 565. (3) Hoffmann, M. R.; Martin, S. T.; Choi, W.; Bahnemann, D. W. Chem. ReV. 1995, 95, 69. (4) Fujishima, A.; Nagahara, L. A.; Yoshiki, H.; Ajito, K.; Hashimoto, K. Electrochim. Acta 1994, 39, 1229. (5) Heller, A. Acc. Chem. Res. 1995, 28, 503. (6) Blodgett, K. B.; Langmuir, I. Phys. ReV. 1937, 51, 964. (7) Spooner, S. P.; Whitten, D. G. In Photochemistry in Organized and Constrained Media; Ramamurthy, V., Ed.; VCH: New York, 1991; Chapter 15, p 691. (8) Sitkiewitz, S.; Heller, A. New J. Chem. 1996, 20, 233. (9) Binning, G.; Quate, C. F.; Gerber, Ch. Phys. ReV. Lett. 1986, 56, 930. (10) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature (London) 1997, 388, 431. (11) Wang, R.; Jiang, L.; Iyoda, T.; Tryk, D. A.; Hashimoto, K.; Fujishima, A. Langmuir 1996, 12, 2052. (12) Nakayama, K.; Jiang, L.; Iyoda, T.; Hashimoto, K.; Fujishima, A. Jpn. J. Appl. Phys. 1997, 36, 3898. (13) To obtain TiO2 films that adhered well onto CaF2 plates, the commercial sol was diluted to 50% of its original concentration using an ethyl acetate/ethanol solvent obtained from the same company. Dip-coating was done at a pull rate of 1.5 mm s-1. The cycle, dip-coating and drying at 120 °C for 30 min, was repeated four times, before calcination at 500 °C for 1 h to give clear films ca. 50 nm thick. (14) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: Boston, MA, 1991; Chapter 2. (15) Chi, L. F.; Anders, M.; Fuchs, H.; Johnston, R. R.; Ringsdorf, H. Science 1993, 259, 213. (16) Semenikhin, O. A.; Jiang, L.; Iyoda, T.; Hashimoto, K.; Fujishima, A. J. Phys. Chem. 1996, 100, 18603.
