6451
2006, 110, 6451-6453 Published on Web 03/15/2006
NC-AFM Observation of Atomic Scale Structure of Rutile-type TiO2(110) Surface Prepared by Wet Chemical Process Yoshimichi Namai* and Osamu Matsuoka Interfacial Science Group, Material Science Laboratory, Mitsui Chemicals, Inc., 580-32 Nagaura, Sodegaura, Chiba 299-0265, Japan ReceiVed: January 25, 2006
We succeeded in observing the atomic scale structure of a rutile-type TiO2(110) single-crystal surface prepared by the wet chemical method of chemical etching in an acid solution and surface annealing in air. Ultrahigh vacuum noncontact atomic force microscopy (UHV-NC-AFM) was used for observing the atomic scale structures of the surface. The UHV-NC-AFM measurements at 450 K, which is above a desorption temperature of molecularly adsorbed water on the TiO2(110) surface, enabled us to observe the atomic scale structure of the TiO2(110) surface prepared by the wet chemical method. In the UHV-NC-AFM measurements at room temperature (RT), however, the atomic scale structure of the TiO2(110) surface was not observed. The TiO2(110) surface may be covered with molecularly adsorbed water after the surface was prepared by the wet chemical method. The structure of the TiO2(110) surface that was prepared by the wet chemical method was consistent with the (1 × 1) bulk-terminated model of the TiO2(110) surface.
1. Introduction Titanium dioxide (TiO2) has been used in a variety of industrial applications such as catalysis, photocatalysis, solar cells, gas sensors, pigments, cosmetics, corrosion-protective coatings, optical coatings, etc.1,2 So the atomic scale structures and the properties of TiO2 surfaces have been extensively studied using scanning tunneling microscopy (STM) and noncontact atomic force microscopy (NC-AFM) under ultrahigh vacuum (UHV).2 To prepare atomically flat TiO2 surfaces, the surfaces have often been cleaned with cycles of Ar+ ion sputtering and thermal annealing under UHV conditions. However this sample preparation method requires a lot of time and work. The TiO2 surfaces prepared under UHV are quite different in structure and properties from those in actual catalyst systems such as photocatalyst, in which the TiO2 surfaces are in contact with aqueous solutions or air.3 Therefore, the preparation method of atomically flat TiO2 surfaces under the atmosphere will play a critical role in studying atomic scale structures of TiO2 surfaces in actual catalyst systems. Recently, Nakamura et al.3,4 and Yamamoto et al.5,6 succeeded in preparing atomically flat TiO2 surfaces under atmospheric conditions. They prepared atomically flat surfaces of rutile-type TiO2(100), (001), (110), (101), and (111) by an alternative method of chemical etching of HF solution and thermal annealing under atmospheric conditions, and confirmed that the surfaces were atomically smooth using tapping mode atomic force microscopy (TM-AFM), low energy electron diffraction (LEED), and reflection high energy electron diffraction (RHEED). However, observations of the atomic scale structures of the TiO2 surfaces by this wet chemical method have not been reported * Corresponding author. Tel: +81-438-64-2315. Fax: +81-438-64-2373. E-mail:
[email protected].
10.1021/jp068012f CCC: $33.50
so far. The wet chemical method is very useful for observing the surface structure in atomic scale. The UHV-NC-AFM method is a promising tool for observing surface structures at atomic scale, dynamic behaviors of atoms and molecules, and surface reaction mechanisms of conductive and nonconductive materials.7 In the wet chemical method, since the TiO2 surfaces are prepared by thermal annealing at 873 K in air, the surfaces are insulators with a nearly stoichiometric composition.1,2 Therefore, UHV-NCAFM can apply to observe the atomic scale structures of the TiO2 surfaces prepared in air. In this study, we tried to observe the atomic scale structure of the rutile-type TiO2(110) surface prepared by the wet chemical method of chemical etching in an acid solution and thermal annealing in air. In the UHV-NC-AFM measurements at 450 K, we succeeded in observing the atomic scale structure of the TiO2(110) surface prepared by the wet chemical method. 2. Experimental Section Rutile-type TiO2(110) wafers (6.5 × 1 × 0.25 mm3, Furuuchi Chemicals) were prepared by a procedure of washing with acetone, immersing in an acid solution (SH-303, Kanto Chemicals) for 10 min, washing with ultrapure water, drying under a nitrogen atmosphere, and annealing at 873 K for 1 h in air. An acid solution composed of a mixture of sulfuric acid and hydrogen peroxide water is commonly used as a cleaning solution or a resist stripper for the manufacturing process of semiconductors at room temperature (RT). The surface morphology of the prepared sample was confirmed by TM-AFM. TM-AFM (NanoScope IIIa, Digital Instruments); measurements were performed under a nitrogen atmosphere at RT. UHV-NC-AFM (JAFM-4610, JEOL) was a base pressure of 1.5 × 10-10 Torr. AFM cantilevers (MikroMasch) were © 2006 American Chemical Society
6452 J. Phys. Chem. B, Vol. 110, No. 13, 2006
Figure 1. UHV-NC-AFM image (116 × 116 nm2) of a TiO2(110) surface prepared by the wet chemical method in air. ∆f ∼ 87 Hz. The UHV-NC-AFM measurement was performed at RT.
conductive silicon cantilevers with a typical resonance frequency of 315 kHz, spring constant of 48 N m-1, and radius of curvature less than 10 nm. Because the cantilever tips were not treated by Ar+ ion sputtering, they were probably initially covered with a native oxide. The resonant frequency shift (∆f) of the cantilever was regulated by using the FM detection method.8 Topographic images were obtained by applying a voltage to the z-piezo to keep a constant frequency shift. The sample bias voltage (Vs) was applied to the sample to compensate the average contact potential between the tip and the sample surface.9,10 The Vs was between +1.5 V and +2.0 V. The UHV-NC-AFM measurements were done at RT and at 450 K. The oscillation amplitude (∆A) of the cantilevers was approximately 15 nmp-p. 3. Results and Discussions In a TiO2(110) surface prepared by the wet chemical method, the atomically flat surface with mono atomic steps of 0.3-0.4 nm height was observed by UHV-NC-AFM at RT (Figure 1). Our results were consistent with previously reported results obtained by TM-AFM observations of TiO2(110) surfaces that were prepared by the treatment of HF solution and thermal annealing under atmospheric conditions.3-6 Although we tried to perform high-resolution observations of the TiO2(110) surface at RT, the atomic scale structure of the surface was not observed. In the UHV-NC-AFM measurements at 450 K, we could observe the atomic scale structures of the TiO2(110) surface that was prepared by the wet chemical method. Figure 2a shows a high-resolution UHV-NC-AFM image of the TiO2(110) surface. We succeeded in observing the atomic scale structure of the TiO2(110) surface by the UHV-NC-AFM measurements
Letters under the heating condition. In Figure 2a, the bright rows correspond to bridge oxygen rows protruding over the TiO2(110) surface, and the dark spots correspond to oxygen vacancies on the surface.11 Figure 2b shows a model of the bulk-terminated TiO2(110)-(1×1) surface. Since the TiO2(110) surface prepared by the wet chemical method could not be measured by STM, the TiO2(110) single crystal was an insulator with nearly stoichiometric composition. In Figure 2a, the distance between bright rows was 0.66 ( 0.02 nm, and this was in good agreement with the unit cell dimension of 0.65 nm along the [1-10] direction in the bulk-terminated model of the TiO2(110)-(1×1) surface (Figure 2b). Therefore, we succeeded in observing the atomic scale structure of the TiO2(110) surface prepared by the wet chemical method. Previously reported TPD measurements indicated that molecularly adsorbed water are removed perfectly by annealing above 350 K.12,13 The annealing temperature at 450 K was sufficient to remove the molecularly adsorbed water on the TiO2(110) surface. So we consider that the TiO2(110) surface may be covered with molecularly adsorbed water after the wet chemical preparation. After UHV-NC-AFM measurements at 450 K, we tried to perform the high-resolution observations at RT again, and confirmed the reproducibility of the atomic scale structure of the TiO2(110) surface. Therefore, after the wet chemical preparation, molecularly adsorbed water on the TiO2(110) surface may protect the atomic scale structure of the surface. The TiO2(110) surface prepared only by the acid solution was not an atomically flat surface but a very rough surface. This result suggests that the thermal annealing in air is an important factor to make the atomically flat surface by the wet chemical method. As described in the Introduction section, the preparation method of the TiO2(110) surface under UHV condition requies a lot of time and work, whereas the preparation method of the TiO2(110) surface under atmospheric conditions is not so difficult by comparison with that under UHV conditions. So we consider that the wet chemical method is a very effective way to obtain the atomic scale structure of the TiO2(110) surface in air. Nakamura et al. described that preparing the atomically flat TiO2 surfaces by the method of HF etching and annealing under atmospheric conditions had enabled us to open a new way to investigate the photocatalytic reactivity.3,4 Therefore, the wet chemical method may play a critical role in preparing atomic scale structures of TiO2 surfaces in actual catalyst systems. Since actual catalysis surfaces of TiO2 are in contact with aqueous solutions or air, observations of the atomic scale
Figure 2. (a) High-resolution UHV-NC-AFM image (21.1 × 21.1 nm2) of a TiO2(110) surface prepared by the wet chemical method in air. ∆f ∼ 110 Hz. The UHV-NC-AFM measurement was done at 450 K. (b) Bulk-terminated model of a TiO2(110)-(1×1) surface.
Letters structures of the surfaces in various environments such as air and liquid are very important. As described in the Introduction section, the atomic scale imaging of TiO2 surfaces has been mostly performed under UHV. Recently, Sasahara et al. reported that the atomic scale structures of a TiO2(110) surface that was prepared under UHV condition were observed by NC-AFM measurements in atmospheric pressure of ultrapure N2 gas.14 In the future, further study is needed to understand the atomic scale structures and properties of TiO2 surfaces in actual catalyst systems. 4. Conclusions We succeeded in observing the atomic scale structure of the TiO2(110) surface prepared by the wet chemical method of chemical etching in acid solution and thermal annealing in air. The UHV-NC-AFM measurements at 450 K, which is above the desorption temperature of molecularly adsorbed water on the TiO2(110) surface, enabled us to observe the atomic scale structure of the TiO2(110) surface prepared by the wet chemical method. The TiO2(110) surface structure was consistent with the (1×1) bulk-terminated model of the surface. After the wet chemical preparation, the TiO2(110) surface may be covered with molecularly adsorbed water since the atomic scale structure of the surface was not observed at RT. Finally, we consider
J. Phys. Chem. B, Vol. 110, No. 13, 2006 6453 that the wet chemical method is an important method in preparing atomic scale structures of TiO2 surface under atmospheric conditions. References and Notes (1) Henrich, V. E.; Cox, P. A. The Surface Science of Metal Oxides; Cambridge University Press: Cambridge, 1994. (2) Diebold, U. Surf. Sci. Rep. 2003, 48, 53. (3) Nakamura, R.; Ohashi, N.; Imanishi, A.; Osawa, T.; Matsumoto, U.; Koinuma, H.; Nakato, Y. J. Phys. Chem. B 2005, 109, 1648. (4) Nakamura, R.; Okamura, T.; Ohashi, N.; Imanishi, A.; Nakato, Y. J. Am. Chem. Soc. 2005, 127, 12975. (5) Yamamoto, Y.; Matsumoto, Y.; Koinuma, H. Appl. Surf. Sci. 2004, 238, 189. (6) Yamamoto, Y.; Nakajima, K.; Ohsawa, T.; Matsumoto, Y.; Koinuma, H. Jpn. J. Appl. Phys. 2005, 44, L511. (7) Morita, S., Wiesendanger, R., Meyer, E., Eds. Noncontact Atomic Force Microscopy; Springer: New York, 2002. (8) Alberecht, T. R.; Gru¨tter, P.; Horne, D.; Rugar, D. J. Appl. Phys. 1991, 69, 668. (9) Meyer, E.; Howald, L.; Lu¨thi, R.; Haefke, H.; Ru¨etschi, M.; Bonner, T.; Overney, R.; Frommer, J.; Hofer, R.; Gu¨ntherodt, H. J. J. Vac. Sci. Technol. B 1994, 12, 2060. (10) Howald, L.; Lu¨thi, R.; Meyer, E.; Gu¨ntherodt, H. J. Phys. ReV. B 1995, 51, 5484. (11) Fukui, K.; Onishi, H.; Iwasawa, Y. Phys. ReV. Lett. 1997, 79, 4202. (12) Henderson, M. A. Surf. Sci. 1996, 355, 151. (13) Henderson, M. A. Surf. Sci. Rep. 2002, 46, 1. (14) Sasahara, A.; Kitamura, S.; Uetsuka, H.; Onishi, H. J. Phys. Chem. B 2004, 108, 15735.
