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Langmuir 2002, 18, 9632-9634
Patterning of Solid Surfaces by Photocatalytic Lithography Based on the Remote Oxidation Effect of TiO2 Tetsu Tatsuma,*,† Wakana Kubo,† and Akira Fujishima‡ Institute of Industrial Science, University of Tokyo, Komaba, Meguro-ku, Tokyo 153-8505, Japan, and Department of Applied Chemistry, School of Engineering, University of Tokyo, Hongo, Bunkyo-ku, Tokyo 113-8656, Japan Received July 16, 2002. In Final Form: October 11, 2002 A novel technique for solid surface patterning is developed on the basis of the remote oxidation effect of TiO2 photocatalysts. A TiO2-coated quartz plate was faced to a solid substrate, that is, a glass plate modified with an ultrathin organic layer or silicon, copper, or silver plate, separated by a small gap, and the TiO2 was irradiated with UV light in air through a photomask. As a result, two-dimensional images corresponding to the photomask are obtained. Those images are based on the contrasts of nonoxidized to oxidized surfaces.
Introduction 1
TiO2 photocatalysts are known to exhibit a strong oxidizing ability, where most organic species are oxidized to CO2 and some inorganic species are also oxidized to their most stable forms. On the basis of this property, TiO2 films have been used as self-cleaning, deodorizing, self-sterilizing, and air-cleaning coatings.2-4 All of these functions are due to oxidation of molecules or ions at the TiO2 surface. The substrates react with holes photogenerated on the TiO2 or hydroxyl radicals (HO•) generated as a result of water oxidation by the hole (eq 1).
H2O + h+ f HO• + H+
(1)
Holes cannot leave the TiO2 surface, and HO• is shortlived in most liquids. However, recently we found remote oxidation of organic materials by TiO2 via the gas phase.5,6 A TiO2-coated glass plate was faced to an organic film, separated by a small gap (10 µm to a few millimeters), and the TiO2 was irradiated with UV light. As a result, aromatic and aliphatic substances were oxygenated and decomposed to CO2 probably by active oxygen species that were generated at the TiO2 surface and transported in air. The active oxygen species involved in this remote oxidation reaction were presumed to be HO• on the basis of various experimental data.6 Some research groups7,8 have provided interesting data reproducing or supporting the remote oxidation. Regardless of the active species involved, the remote oxidation effect facilitates development of new uses of TiO2 photocatalysts. Those may include modification, etching, † ‡
Institute of Industrial Science, University of Tokyo. Department of Applied Chemistry, University of Tokyo.
(1) Fujishima, A.; Honda, K. Nature 1972, 238, 37-38. (2) Fujishima, A.; Hashimoto, K.; Watanabe, T. TiO2 Photocatalysis: Fundamentals and Applications; BKC: Tokyo, 1999. (3) Photocatalytic Purification and Treatment of Water and Air; Ollis, E. E., Al-Ekabi, H., Eds.; Elsevier: Amsterdam, 1993. (4) Linsebigler, A. L.; Lu, G.; Yates, J. T., Jr. Chem. Rev. 1995, 95, 735-758. (5) Tatsuma, T.; Tachibana, S.; Miwa, T; Tryk, D. A.; Fujishima, A. J. Phys. Chem. B 1999, 103, 8033-8035. (6) Tatsuma, T.; Tachibana, S.; Fujishima, A. J. Phys. Chem. B 2001, 105, 6987-6992. (7) Haick, H.; Paz, Y. J. Phys. Chem. B 2001, 105, 3045-3051. (8) Cho, S.-M.; Choi, W.-Y. J. Photochem. Photobiol., A 2001, 143, 221-228.
and patterning of solid surfaces. Physical and chemical characteristics and morphology of organic and inorganic surfaces should be modified by a simple and noncontacting dry process based on the remote oxidation without any complicated or expensive equipment. In the present work, surface patterning was conducted by a technique similar to conventional photolithographic processes, although the present technique, photocatalytic lithography, does not require a photoresist. A solid surface that is subjected to the modification is faced to a TiO2 thin film, and the film is irradiated with UV light through a photomask. Thus, an irradiated TiO2 region is excited to cause the remote oxidation, while the other nonirradiated region is not excited so that no photoelectrochemical process proceeds. This technique would be applied to fabrication of biosensors, biochips, chemical chips, and display devices. Experimental Section Materials. A TiO2 aqueous sol STS-21 (Ishihara Sangyo, Japan) was diluted with water (80 vol %), sonicated for 1 h, and coated on a Pyrex or quartz plate by spin-coating at ∼1500 rpm for 15 s, unless otherwise noted. The TiO2 was calcined at 400 °C for 60 min to obtain an anatase TiO2 coated glass plate. The resulting TiO2 coating was irradiated with a black-light-type lamp (∼300-400 nm, ∼1 mW cm-2) overnight before each experiment to clean the surface. An octadecyltriethoxysilane (ODS) film was prepared as follows: A glass plate was treated with 1 M aqueous NaOH for 1 h with sonication and rinsed thoroughly with water. The glass plate was dried and then soaked in a 10 vol % toluene solution of ODS for 30 min. An n-type conducting silicon single crystal was used as obtained. Copper and silver plates were polished to a mirror finish with alumina slurry prior to use. Surface Modification and Patterning. The surface modification was carried out as shown in Figure 1a. The ODS-coated glass plate was faced to the TiO2 coating with a small intervening gap (12.5-50 µm). The gap was controlled by use of polyimide films. A photomask (Fuji Film, Japan) with thin slits (slit width, 100-500 µm) was mounted on the back side of the TiO2-coated glass plate. The TiO2 film was irradiated with a Hg-Xe lamp through the slits. A control experiment was carried out by mounting the TiO2-coated glass plate upside down, as shown in Figure 1b. Evaluation of Surface Patterning. After the UV irradiation, if necessary, a ca. 0.3 µM methanol solution of rhodamine G (RhG) was cast on the surface, and ethanol was evaporated under
10.1021/la026246u CCC: $22.00 © 2002 American Chemical Society Published on Web 11/05/2002
Letters
Langmuir, Vol. 18, No. 25, 2002 9633
Figure 1. Experimental setups for the photocatalytic lithography based on the remote oxidation (a) and the control experiment (b).
Figure 2. Time courses of the water contact angle of the ODScoated glass surface during the remote oxidation experiment (gap, 12.5 µm; irradiated with the Hg-Xe lamp). (a-c) The thicker TiO2 coated on a glass plate or (d) thinner TiO2 coated on a quartz plate was used (see text for details). Light intensity: (a) 10, (b) 30, and (c, d) 100 mW cm-2. the ambient condition. Then, the substrate was observed with a fluorescence microscope (BX51, Olympus, Japan) with a WIG filter.
Results and Discussion Optimization of Experimental Conditions. As we have reported previously,6 it is possible to remove an ODS layer on a glass substrate by the remote oxidation. This has been verified by water contact angle measurements and X-ray photoelectron spectroscopy (XPS). In the present work, the conditions for the remote oxidation were optimized for the efficient removal of ODS. In the previous work,6 the UV light intensity was ∼10 mW cm-2. Here we checked whether the removal is facilitated by using stronger light. Figure 2a-c shows the changes in the water contact angle at the ODS-coated glass surface under different UV light intensities. It is clear that the removal of ODS is faster under stronger UV light, suggesting that the rate of the remote oxidation of ODS is controlled by the supply of photons. Thus, in the present work, UV light of 100 mW cm-2 was employed. Next, the substrate for the TiO2 film was changed from a Pyrex glass plate to a quartz plate to increase transparency to UV light, because the UV light is incoming from the back side, through the substrate (Pyrex or quartz) to the TiO2 film. In addition, since UV light has to pass through the TiO2 film to the TiO2 surface that is faced to the ODS layer, UV light absorbed at the TiO2 surface, which contributes to the remote oxidation, decays if the TiO2 film thickens. Therefore, thinner TiO2 films were prepared on a quartz plate with diluted aqueous sols and
higher spin rates. Among them, a film prepared with a 67 vol % diluted TiO2 sol at the spin rate of ∼2500 rpm (15 s) was the best one; under more extensive conditions, the resulting film was not adhesive enough to the substrate. By using the quartz plate coated with the thinner TiO2 film, the ODS removal was further accelerated as Figure 2d shows. Thus, the thinner film on a quartz plate was used in the rest of this work. Control experiments were carried out by mounting the TiO2-coated quartz plate upside down (Figure 1b). Since no significant changes (
