Preparation of Self-Supporting Ultrathin Films of Titania by Spin

Oct 21, 2003 - Precursor mixtures of Ti(OnBu)4 and small organic molecules were similarly used to obtain self-supporting ultrathin PVA/(organic compou...
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Preparation of Self-Supporting Ultrathin Films of Titania by Spin Coating Mineo Hashizume† and Toyoki Kunitake* Frontier Research System, The Institute of Physical and Chemical Research (RIKEN), 2-1 Hirosawa, Wako, Saitama 351-0198, Japan Received June 18, 2003. In Final Form: August 22, 2003 A spin-coating process was developed for the preparation of self-supporting ultrathin films of titania. First, an ultrathin layer of poly(vinyl alcohol) (PVA) was formed by spin coating on the surface of an ethanol-soluble polymer that had been preformed on the surface of a Si wafer as a polymer underlayer. Titanium tetrabutoxide [Ti(OnBu)4] in chloroform was then spin-coated onto the PVA layer. The polymer underlayer was dissolved in ethanol, and a self-supporting ultrathin PVA/titania composite film was finally detached from the substrate. Precursor mixtures of Ti(OnBu)4 and small organic molecules were similarly used to obtain self-supporting ultrathin PVA/(organic compounds + titania) composite films. The organic molecules could be removed from the ultrathin film under mild conditions such as aqueous ammonia extraction and low-temperature oxygen plasma treatment. Nanopores were formed by the O2 plasma treatment, as evidenced by transmission electron microscopy. When (4-phenylazo)benzoic acid was used as a small-molecule component, the resulting film contained nanopores that selectively bound the template molecule. It is remarkable that a titania film with thickness of only 10-20 nm can be macroscopically uniform and self-supporting. The imprinted nanopore should be useful for development of permselective ceramic membranes.

Introduction Fundamental features that are desired for the separation membrane are defect-free uniformness, macroscopic robustness, high permeability, and tailor-made selectivity.1 Such often conflicting targets may be achieved in an ultimate form, if nanometer thick membranes with macroscopic mechanical stability and permeant-specific nanopores are prepared. Ion channels and other transport systems in the biological membrane are ideal in this respect. Polymeric materials and inorganic materials have been used most frequently to fabricate practical separation membranes. However, their thicknesses usually remain in the micrometer range, and the nanometer thickness is difficult to attain as uniform, self-supporting membranes. Recently, Nardin et al. reported a 10 nm thick, selfsupporting (1 mm2 size) film by a unique approach, in which they used self-assembly of an ABA triblock copolymer followed by polymerization to a stable film.2 Other novel approaches are also being developed for molecularly thin layers. One of the methods is electrostatic layer-bylayer assembly.3 This method is now widely used for the lamination of oppositely charged polymers,4 polymers with proteins,5 and organic with inorganic compounds6 in the molecular thickness. The preparation of self-supporting * To whom correspondence should be addressed. E-mail: [email protected]. Fax: +81-48-464-6391. † Present address: Graduate School of Materials Science, Nara Institute of Science and Technology (NAIST), 8916-5 Takayama, Ikoma, Nara 630-0192, Japan. (1) Mulder, M. Basic Principles of Membrane Technology, 2nd ed.; Kluwer Academic Publishers B.V.: Dordrecht, The Netherlands, 1996. (2) Nardin, C.; Winterhalter, M.; Meier, W. Langmuir 2000, 16, 77087712. (3) (a) Decher, G.; Hong, J.-D. Ber. Bunsen-Ges. Phys. Chem. 1991, 95, 1430-1434. (b) Decher, G.;. Hong, J.-D. Makromol. Chem., Macromol. Symp. 1991, 46, 321-327. (c) Decher, G. Science 1997, 277, 12321237. (d) Decher, G. In Comprehensive Supramolecular Chemistry; Atwood, J. L., Davies, J. E. D., MacNicol, D. D., Vo¨gtle, F., Lehn, J.-M., Eds.; Pergamon: Oxford, 1996; Vol. 9, pp 507-528. (e) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319-348.

ultrathin films by this approach has been studied by several groups. Onda et al. prepared enzyme-containing alternate films on porous filter membranes and examined sequential enzyme catalysis.7 Preparation of ultrathin hollow polymer shells (or nanotubes) is extensively investigated by Caruso et al.8,9 and other groups.10 The polymeric shell is considered to be self-supporting though its area is quite small. Caruso et al. also reported ultrathin (4) (a) Lvov, Y.; Decher, G.; Mo¨hwald, H. Langmuir 1993, 9, 481486. (b) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3422-3426. (c) Schlenoff, J. B.; Dubas, S. T. Macromolecules 2001, 34, 592-598. (5) (a) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. J. Am. Chem. Soc. 1995, 117, 6117-6123. (b) Caruso, F.; Niikura, K.; Furlong, D. N.; Okahata, Y. Langmuir 1997, 13, 3427-3433. (c) Lvov, Y. M.; Lu, Z.; Schenkman, J. B.; Zu, X.; Rusling, J. F. J. Am. Chem. Soc. 1998, 120, 4073-4080. (6) (a) Kleinfeld, E. R.; Ferguson, G. S. Science 1994, 265, 370-373. (b) Keller, S. W.; Kim, H.-N.; Mallouk, T. E. J. Am. Chem. Soc. 1994, 116, 8817-8818. (c) Lvov, Y.; Ariga, K.; Ichinose, I.; Kunitake, T. Langmuir 1996, 12, 3038-3044. (d) Ichinose, I.; Tagawa, H.; Mizuki, S.; Lvov, Y.; Kunitake, T. Langmuir 1998, 14, 187-192. (e) Ostrander, J. W.; Mamedov, A. A.; Kotov, N. A. J. Am. Chem. Soc. 2001, 123, 1101-1110. (f) Fu, Y.; Xu, H.; Bai, S.; Qiu, D.; Sun, J.; Wang, Z.; Zhang, X. Macromol. Rapid Commun. 2002, 23, 256-259. (g) Fan, X.; Locklin, J.; Youk, J. H.; Blanton, W.; Xia, C.; Advincula, R. Chem. Mater. 2002, 14, 2184-2191. (7) Onda, M.; Lvov, Y.; Ariga, K.; Kunitake, T. J. Ferment. Bioeng. 1996, 82, 502-506. (8) (a) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111-1114. (b) Caruso, F. Chem.sEur. J. 2000, 6, 413-419. (c) Caruso, F. Adv. Mater. 2001, 13, 11-22. (9) (a) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H. Angew. Chem., Int. Ed. Engl. 1998, 37, 2201-2205. (b) Park, M.-K.; Xia, C.; Advincula, R. C.; Schu¨tz, P.; Caruso, F. Langmuir 2001, 17, 7670-7674. (10) (a) Marinakos, S. M.; Shultz, D. A.; Feldheim, D. L. Adv. Mater. 1999, 11, 34-37. (b) Marinakos, S. M.; Novak, J. P.; Brousseau, L. C., III; House, A. B.; Edeki, E. M.; Feldhaus, J. C.; Feldheim, D. L. J. Am. Chem. Soc. 1999, 121, 8518-8522. (c) Marinakos, S. M.; Anderson, M. F.; Ryan, J. A.; Martin, L. D.; Feldheim, D. L. J. Phys. Chem. B 2001, 105, 8872-8876. (d) Hotz, J.; Meier, W. Langmuir 1998, 14, 10311036. (e) Discher, B. M.; Won, Y.-Y.; Ege, D. S.; Lee, J. C.-M.; Bates, F. S.; Discher, D. E.; Hammer, D. A. Science 1999, 284, 1143-1146. (f) Sanji, T.; Nakatsuka, Y.; Ohnishi, S.; Sakurai, H. Macromolecules 2000, 33, 8524-8526. (g) Obare, S. O.; Jana, N. R.; Murphy, C. J. Nano Lett. 2002, 1, 601-603.

10.1021/la035079a CCC: $25.00 © 2003 American Chemical Society Published on Web 10/21/2003

Self-Supporting Ultrathin Films of Titania

hollow organic/inorganic (or metal) hybrid shells11 and nanotubes.11c Huck et al. reported the preparation of a self-supporting ultrathin film of alternate polyions by dissolving a glass substrate under strongly acidic conditions.12 Manadov and Kotov prepared self-supporting ultrathin films that incorporated magnetite nanoparticles by dissolving a cellulose acetate support layer on a glass substrate.13 The other method for preparation of nanometer thick films is the sequential sol-gel process, as reported by our group14 and by Kleinfeld and Ferguson15 independently. In our process, it was possible to laminate metal oxide layers and organic layers in designed compositions and sequences. An attempt to prepare self-supporting metal oxide nanofilms has been conducted by us by taking advantage of the surface sol-gel process.16 In this study, a composite (polymer/metal oxide) thin layer was prepared on a polymer underlayer,14,17 and was then detached from the substrate by dissolving the polymer underlayer. Unfortunately, the size of the obtained ultrathin PAA/(TiO2/ PAA)10 films could not be made larger than approximately 15 mm2. The original film was fragmented readily upon dispersion in solvents. This process was additionally tedious and time-consuming. We wanted to solve these problems in the current study by a new approach that includes designed underlayers and spin coating, and finally succeeded in developing self-supporting ultrathin titania films. Experimental Section General Procedures. A silicon wafer (diameter 200 mm, thickness 0.6 mm) precoated with 500 nm thick photoresist TDUR-P015 was obtained from Tokyo Ohka Kogyo, Japan. Poly(vinyl alcohol) (PVA) (98 mol % hydrolyzed, MW ≈ 78000) was purchased from Polyscience. Poly(acrylic acid) (PAA) (Mn ) 1070, Mw/Mn ) 1.16) and poly(methyl methacrylate) (PMMA) (Mn ) 69000, Mw/Mn ) 1.03) were purchased from Polymer Source, Canada. Titanium tetrabutoxide [Ti(OnBu)4] was purchased from Gelest. (4-Phenylazo)benzoic acid (4-PABA) and poly(4-vinylphenol) (PVP) (average Mw ) ca. 8000) were purchased from Aldrich. 3-Methylsalicylic acid (3-MSA) and carbobenzyloxy-L-phenylalanine (CbzPhe) were purchased from Acros Organics, Belgium. Chemical structures of these organic compounds are shown in Figure 1. Chloroform and ammonia solution were purchased from Junsei Chemical, Japan. A porous alumina membrane (ANODISC, pore size 0.02, 0.1, and 0.2 µm, diameter 13 and 25 mm, thickness 60 µm) was purchased from Whatman International. Other chemicals were purchased from Kanto Kagaku, Japan. All chemicals were used as received. Preparation of Self-Supporting Ultrathin Films. A schematic illustration of the preparation of self-supporting ultrathin films by spin coating is shown in Scheme 1. In this study, photoresist TDUR-P015 was used as the ethanol-soluble polymer underlayer. The photoresist-coated Si wafer was cut into approximately 3 cm × 4 cm pieces, and nitrogen gas was blown to dry and remove dusts on the surface. This solid substrate was set on the sample stage of a spin coater (ABLE Co. Ltd.). (11) (a) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Chem. Mater. 1999, 11, 3309-3314. (b) Caruso, F.; Schu¨ler, C.; Kurth, D. G. Chem. Mater. 1999, 11, 3394-3399. (c) Mayya, K. S.; Gittins, D. I.; Dibaj, A. M.; Caruso, F. Nano Lett. 2001, 1, 723-730. (12) Huck, W. T. S.; Stroock, A. D.; Whitesides, G. M. Angew. Chem., Int. Ed. 2000, 39, 1058-1061. (13) Mamadov, A. A.; Kotov, N. A. Langmuir 2000, 16, 5530-5533. (14) (a) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 831832. (b) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296-1298. (15) Kleinfeld, E. R.; Ferguson, G. S. Mater. Res. Symp. Proc. 1994, 351, 419-424. (16) (a) Hashizume, M.; Kunitake, T. RIKEN Rev. 2001, 38, 36-39. (b) Hashizume, M.; Kunitake, T. Polym. Prepr. Jpn. 2001, 50, 747. (17) Ichinose, I.; Kawakami, T.; Kunitake T. Adv. Mater. 1998, 10, 535-539.

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Figure 1. Chemical structures and abbreviations. PVA (5 mg mL-1 in H2O) was spin-coated onto the substrate surface at 3000 rpm for 2 min and dried in air for about 8 h. Then, a chloroform solution of Ti(OnBu)4 (100 mM, stirred several hours before use) was spin-coated onto the PVA surface at 3000 rpm for 2 min. The substrate was allowed to stand in air for 10 h to hydrolyze the unreacted metal alkoxides by air moisture. For easier penetration of the solvent, the spin-coated film about 1 mm from the periphery was scratched by a needle. Nitrogen gas was blown to dry the surface and to remove dusts. This substrate was placed in a Petri dish, and ethanol was gently added to the dish from the outside of the substrate to cover the substrate surface totally. Within a few minutes, the photoresist started to dissolve from the scratched part of the substrate, and finally an ultrathin PVA/titania composite film was detached from the substrate. A mixed solution of Ti(OnBu)4 and the organic molecule (not only small molecules but also polymers) was used to prepare self-supporting ultrathin films of titania/organic nanocomposites. For example, to a chloroform suspension of 20 mM 4-PABA was added neat Ti(OnBu)4, so that the concentration of the latter was 100 mM. The mixture was stirred for 8 h to give a clear solution. A 9/1 chloroform/ethanol mixture was used as solvent, when glucose and PAA were used as organic components. Removal of Organic Moieties from Ultrathin Films. Ultrathin composite films were transferred onto ANODISCs. To remove organic components, some of the samples were immersed in 1% aqueous ammonia for 1 h at room temperature, and then rinsed with ethanol and with Milli-Q water, and nitrogen gas was flushed for drying.18 Other samples were treated with oxygen plasma with an RF power of 30 W for 30 min by using a PE-2000 plasma etcher (South Bay Technology), to carry out oxidative degradation of organic molecules in the ultrathin film.19-21 Film Morphology. Macroscopic images of self-supporting ultrathin films were photographed by a digital camera, RICOH RDC-7, with 640 × 480 pixels. Scanning electron micrographs were obtained by using a Hitachi S-900 instrument at an (18) (a) Lee, S.-W.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 2857-2863. (b) Lee, S.-W.; Ichinose, I.; Kunitake, T. Chem. Lett. 1998, 1193-1194. (19) Huang, J.; Ichinose, I.; Kunitake, T.; Nakao, I. Nano Lett. 2002, 2, 669-672. (20) (a) Huang, J.; Ichinose, I.; Kunitake, T.; Nakao, I. Langmuir 2002, 18, 9048-9053. (b) Huang, J.; Ichinose, I.; Kunitake, T. Chem. Commun. 2002, 2070-2071.

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Scheme 1. Preparative Procedure of a Self-Supporting Ultrathin Film by Spin Coating

acceleration voltage of 25 kV. Self-supporting ultrathin films suspended in ethanol were transferred onto ANODISCs, which were then cut into small pieces and set on a sample holder of the scanning electron microscope. To image the cross section of the ultrathin film, the sample pieces were kept perpendicular to the sample holder surface and fixed with carbon tape. The samples were coated with a 2 nm thick Pt layer using a Hitachi E-1030 ion sputter under an argon atmosphere. Self-supporting PVA/(PAA + titania) films were transferred onto a Cu grid (no support membrane type, diameter 3.0 mm, pitch 167 µm, mesh size 117 µm × 117 µm, grid width 50 µm) for transmission electron microscopy (TEM). This specimen was treated with oxygen plasma with an RF power of 30 W for 30 min to remove PAA, and then observed by a transmission electron microscope (JEM-2000, JEOL) at an acceleration voltage of 100 kV and a magnification of 50 K. UV-Vis Spectroscopy. A self-supporting ultrathin PVA/ (4-PABA + titania) composite film was transferred onto a quartz plate (7 mm × 25 mm × 0.5 mm) for UV-vis spectroscopy (Shimadzu UV-PC3100). The plate was then immersed in 1% aqueous ammonia for 1 h at room temperature with gentle stirring, followed by rinsing with ethanol and Milli-Q water, and dried by flushing nitrogen gas. After UV-vis spectral measurement, the quartz plate was immersed in a THF solution of 4-PABA (10 mM) for a rebinding experiment for 1 h at room temperature with gentle stirring. It was rinsed with THF and ethanol, dried by N2 gas flushing, and examined by UV-vis spectroscopy. These removal and rebinding experiments of 4-PABA were repeated two times each. Permeation Experiment. A PVA/(4-PABA + titania) composite film was transferred onto an ANODISC (diameter 25 mm) to completely cover one side of the disk surface. The disk was immersed in 1% aqueous ammonia for 2 h at room temperature, rinsed with ethanol and Milli-Q water, and dried by N2 flushing. This sample was mounted onto a stainless steel filter holder (Millipore microsyringe 25). UV-vis spectra of aqueous solutions of benzoic acid (1 × 10-4 M), azobenzene (1 × 10-6 M), and 4-PABA (1 × 10-6 M) were measured prior to filtration. The filtration was performed by placing each solution in a 5 mL syringe attached to the filter holder. The sample flow was induced manually (ca. 1 mL/s) at room temperature. This filtration process was repeated two times for given solutions, and UV-vis spectra of the filtrate solutions were measured at each filtration step. The result was represented as the relative absorbance at the absorption maximum of the solution after and before the filtration. (21) (a) Kalachev, A. A.; Mathauer, K.; Ho¨hne, U.; Mo¨hwald, H.; Wegner, G. Thin Solid Films 1993, 228, 307-311. (b) Chan, C.-M.; Ko, T.-M.; Hiraoka, H. Surf. Sci. Rep. 1996, 24, 1-54. (c) Spatz, J. P.; Mo¨ssmer, S.; Hartmann, C.; Mo¨ller, M.; Herzog, T.; Krieger, M.; Boyen, H.-G.; Ziemann, P.; Kabius, B. Langmuir 2000, 16, 407-415. (d) Jang, H. K.; Lee, S. K.; Lee, C. E.; Noh, S. J.; Lee, W. I. Appl. Phys. Lett. 2000, 76, 882-884.

Table 1. Self-Supporting Composite PVA/Ti(OnBu)4 Films film no.

compounds and concentrations of the spin-coating solution

thicknessa/ nm

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17

100 mM Ti(OnBu)4 10 mM Ti(OnBu)4 20 mM 4-PABA + 100 mM Ti(OnBu)4 20 mM 4-PABA + 100 mM Ti(OnBu)4 20 mM 4-PABA + 100 mM Ti(OnBu)4 1 mM 4-PABA + 5 mM Ti(OnBu)4 20 mM CbzPhe + 100 mM Ti(OnBu)4 20 mM 3-MSA + 100 mM Ti(OnBu)4 20 mM 4-PAR + 100 mM Ti(OnBu)4 10 mM glucose + 200 mM Ti(OnBu)4b 1 mM glucose + 20 mM Ti(OnBu)4b 5 mM PAA + 100 mM Ti(OnBu)4b 0.5 mM PAA + 10 mM Ti(OnBu)4b 10 mM PVP + 200 mM Ti(OnBu)4c 0.5 mM PVP + 10 mM Ti(OnBu)4c 5 mM PMMA + 100 mM Ti(OnBu)4 0.5 mM PMMA + 100 mM Ti(OnBu)4

140 ( 2.5 13 ( 1.2 190 ( 32 110 ( 47 140 ( 40 34 ( 20 73 ( 14 83 ( 7.5 81 ( 4.8 21 ( 4.3 32 ( 19

21 ( 10

Mean value with standard deviation. b Chloroform/ethanol ) 9/1 was used as the solvent. c Although the spin-coating solution was turbid, an almost transparent film was obtained. a

Results and Discussion Preparation of Self-Supporting Ultrathin Films. Thick self-supporting films are readily available by casting of precursor solutions and subsequently peeling off the resulting film. This straightforward procedure is, however, not directly applicable to nanometer thick films, if selfsupporting, due to limited mechanical strength of the very thin film. We, therefore, used polymer underlayers that are readily dissolved in organic solvents to facilitate separation of the solid substrate and thin film in our previous surface sol-gel approach.16 However, the resulting metal oxide film was still fragmented to small pieces, and it was clear that we had to devise much improved processes. We tested a photoresist as a new underlayer in the current study. Some of the photoresists are highly soluble in ethanol, and can be uniformly spin-coated onto a silicon wafer. The actual procedure is as follows. The titania layer was spin-coated onto a PVA underlayer which is lined with a photoresist lower layer on a Si wafer. This sample was placed in a Petri dish, and ethanol was added gently and slowly. The photoresist was readily dissolved, and an ultrathin PVA/titania film (film no. 1 in Table 1) was detached from the substrate. Figure 2 shows macroscopic images of this transparent, uniform

Self-Supporting Ultrathin Films of Titania

Figure 2. A PVA/titania film (sample no.1, Table 1) in ethanol. The vial diameter is 27 mm.

Figure 3. SEM images of the top view (a) and side views (b, c) of a PVA/titania film (sample no. 1, Table 1) on an ANODISC.

film. To see the microscopic morphology of the PVA/titania film, it was transferred onto an ANODISC, an alumina porous membrane, and subjected to scanning electron microscopy (SEM) examination. The SEM image in Figure 3a is a top view of the PVA/titania. Its surface is smooth and uniform. Its cross section images (Figure 3b,c) show the film thickness to be constant at about 170 nm. When the thickness was estimated from the cross section from different areas of the same specimen, a mean value of 140 nm with a standard deviation of 25 nm was obtained (film no. 1 in Table 1). The whole size of the film reproducibly corresponded to that of the original Si wafer (3 cm × 4 cm). Similar results were obtained, when cyclohexane, instead of chloroform, was used as the solvent of the metal alkoxide. A similar ultrathin film was also formed from PVA, in the absence of a titania layer: Si/photoresist/ PVA, thickness < about 15 nm (data not shown). Therefore, the PVA layer is not dissolved by the ethanol treatment and attached to the titania layer within the self-supporting film. The above procedure was subsequently applied to film preparation under different conditions. It was expected that the use of more dilute precursor solutions would produce thinner self-supporting films. In fact, a selfsupporting PVA/titania film with macroscopic morphology similar to that of Figure 2 was obtained from 10 mM titanium butoxide, with a film thickness of 13 ( 1.2 nm as estimated from an SEM image of Figure 4. As far as

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Figure 4. SEM images of the top view (a) and side view (b) of a PVA/titania film (sample no. 2, Table 1) on an ANODISC.

we know, this is the thinnest self-supporting film that maintains a size as large as ∼10 cm2. The macroscopic film image was retained over several months. The film was successfully transferred onto a solid substrate as well as onto a porous ANODISC. SEM pictures show that the pores of the ANODISC (200 nm maximum) are fully covered with the film without any defect. We previously reported the preparation of self-supporting PAA(TiO2/PAA)10 films by the surface sol-gel process.16 However, the whole film was broken into pieces of up to about 15 mm2 during detachment form the substrate. In this case, PMMA was spin-coated as an underlayer on a substrate such as a slide glass, and a PAA layer was adsorbed from solution. It is conceived that the PAA layer did not particularly promote development of an extensive two-dimensional titania network during the subsequent adsorption. The advantage of the current approach over the previous surface sol-gel approach is probably 2-fold. First, the photoresist we used as the polymeric underlayer was highly soluble in ethanol, and the composite film was detached from the solid substrate with minimal disturbance. In fact, ultrathin titania films directly prepared on a Si wafer and ultrathin PVA/titania films prepared on a glass substrate could not be detached from solid substrates without fragmentation. Second, the spin-coated PVA layer appears to play a preferable role in the formation of large-sized, ultrathin films. We already reported that a dense titania composite film was prepared in the form of the alternate layers with PAA or PVA rather than as single-component layers in the surface sol-gel process.17 Two-dimensional (2D) development of the titania layer was promoted due to reaction of the titania layer with hydroxyl groups of the polymer layer. A similar mechanism must be operative in the current system. The spin-coated PVA layer possesses densely populated hydroxyl groups, and they would promote 2D networking of the nascent titania layer. This effect can lead to growth of defect-free 2D networks of titania on a macroscopic scale. Apart from layer-by-layer assembly and the surface sol-gel process, there have been reports on the formation of ultrathin metal oxide films at ambient conditions: silica films (thickness ca. 90 nm) by spin coating of silica sol solutions.22 Another example is the preparation of ultrathin titania films by drain coating of a titania nanocrystal dispersion.23 In the latter, the resulting film was an accumulation of titania nanocrystals rather than homogeneous (continuous) titania layers. Moriguchi et al. reported the preparation of ultrathin TiO2 gel films (22) Thompson, W. R.; Pemberton, J. E. Chem. Mater. 1995, 7, 130136. (23) Peiro´, A. M.; Peral, J.; Domingo, C.; Dome`nech, X.; Ayllo´n, J. A. Chem. Mater. 2001, 13, 2567-2573.

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Figure 5. SEM images of the top view (a) and side view (b) of a PVA/(4-PABA + titania) film (sample no. 3, Table 1) on an ANODISC.

Figure 6. SEM images of the top view (a) and side view (b) of a PVA/(4-PABA + titania) film on an ANODISC after oxygen plasma treatment (sample no. 4, Table 1).

(2-3 Å) at the air/water interface.24 It is clear from these limited numbers of examples that preparation of nanometer thick metal oxide films in self-supporting form have not attracted much attention, despite their outstanding application potentials. Self-Supporting Ultrathin Nanocomposite Films. As mentioned in the Introduction, a variety of organic components are readily incorporated into ultrathin titania films via the surface sol-gel process. Therefore, it is interesting to test if the current process is extended to such nanocomposite systems. Novel optic, electronic, and magnetic properties may be realized by incorporation of functional organic compounds into self-supporting films. In addition, the molecular-imprinting mechanism may produce unique cavities in such films, leading to a new type of separation membrane. The first organic component we employed is 4-PABA. We have shown before that this compound is suitable for examination of the imprinting effect in the titania layer.18a Thus, a mixed solution of 20 mM 4-PABA and 100 mM Ti(OnBu)4 was spin-coated onto the Si/photoresist/PVA substrate. After exposure to air moisture, the substrate was immersed in ethanol and a PVA/(4-PABA + titania) film was detached from the substrate (no. 3, Table 1). The size of the film was close to that of the Si substrate. SEM images of this film (Figure 5) show that the film morphology is essentially identical to that of the PVA/titania film with a film thickness of 190 ( 32 nm. The self-supporting PVA/(4-PABA + titania) film appears less flexible than the PVA/titania film by the naked eye. A much thinner (34 ( 20 nm) film was obtainable by lowering the concentration of the precursor solution (no. 6, Table 1). Other small molecules were similarly incorporated into the titania film without damaging the self-supporting property of the film. They are an amino acid derivative (CbzPhe), aromatic dyes (3-MSA and 4-PAR), and glucose. In all cases, uniform self-supporting films were obtainable, although whitening was found at the film center for sample no. 11, Table 1. We have shown that alternate films of PAA and titania layers are denser than the single-component titania film due to strong interaction of PAA and titania layers.17 To take advantage of this observation, we carried out incorporation of such interacting vinyl polymers, PAA, PVA, and PMMA, in the titania layer, in addition to the PVA underlayer: sample nos. 12-17. Uniform, ultrathin films thus obtained were self-supporting. It is interesting that PMMA can be incorporated into the titania layer uniformly, although strong attraction is not expected between titania and PMMA. Information on how PMMA

chains are distributed in the titania layer is not available at this moment. Removal of Organic Molecules by Oxygen Plasma Treatment. Recently, we developed a novel method for the preparation of nanoporous titania thin films19,20 by combination of the surface sol-gel process and lowtemperature oxygen plasma treatment.21 The resulting cavity reflects the size of the original organic template, and the titania layer remains amorphous during the lowtemperature plasma treatment. It is interesting if we can produce similar cavities in the self-supporting ultrathin films. For this purpose, one of the self-supporting films was transferred onto an ANODISC and treated with oxygen plasma at an RF power of 30 W for 30 min. SEM observation of the resultant film (sample no. 4, Table 1) as shown in Figure 6 indicates the film thickness as 110 ( 47 nm. There appear to be no morphological changes upon oxygen plasma treatment. Since the SEM observation gives the morphology of the film surface and cannot provide information on the nanopores buried in the film interior, we then conducted TEM observation of a plasma-treated film. A self-supporting film was prepared by spin coating of a mixed solution of 0.5 mM PAA (Mn ) 1070) and 10 mM Ti(OnBu)4. The self-supporting ultrathin PVA/(PAA + titania) film thus obtained (sample no. 13, Table 1) was transferred onto a copper grid, and subjected to the standard oxygen plasma treatment. Figure 7a,b compares TEM images of the film before and after oxygen plasma treatment. The as-prepared film of Figure 7a possesses gray domains with sizes of 2-3 nm scattered in the titania matrix. Each domain may consist mostly of PAA molecules, though they are buried in the matrix. The TEM detection of the pore contrasts with our related study on a PAA/TiO2 alternate film as prepared by the surface sol-gel process.19b In the latter, we could not detect the presence of nanopores for the as-prepared film. The surface sol-gel process gives regular, alternate layers of PAA and TiO2, whereas spin coating may produce ultrathin films in which the two components produce nanodomains. The plasma-treated sample shows much clearer pore structures, as formed from removal of PAA by oxygen plasma. The latter pores with sizes of 1-2 nm are smaller than those of the gray domains seen in the as-prepared sample. In the case of the layer-by-layer film, wormlike pores were formed with a diameter of ca. 2 nm. Different distributions of the PAA chain between the layer-by-layer film and the spin-coated film are apparent in TEM images of the plasma-treated films. The difference may be caused by the method of preparation, though the difference in the polymer chain length can give altered chain conformation in the films. Figure 7c shows a TEM image of a plasma-treated PVA/ titania film. No template organic molecules are included

(24) Moriguchi, I.; Maeda, H.; Teraoka, Y.; Kagawa, S. Chem. Mater. 1997, 9, 1050-1057.

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Figure 7. TEM images of a PVA/(PAA + titania) film (sample no. 13, Table 1) on a Cu grid before (a) and after (b) oxygen plasma treatment. (c) TEM image of a PVA/10 mM titania film on a Cu grid after oxygen plasma treatment. Figure 9. (A) UV-vis spectra of a PVA/(4-PABA + titania) film (sample no. 3, Table 1) on a quartz plate: (a) as-prepared sample, (b) after immersion in aqueous ammonia, (c) after immersion in a THF solution of 4-PABA, (d) after a second immersion in aqueous ammonia, (e) after a second immersion in a THF solution of 4-PABA. (B) Sequential change of the absorbance at 325.5 nm in (A).

Figure 8. SEM images of the top view (a) and side view (b) of a PVA/(4-PABA + titania) film on an ANODISC after aqueous ammonia treatment (sample no. 5, Table 1).

in this case. However, randomly distributed nanopores can be seen with a much larger size variation. It is suggested that the PVA underlayer creates less homogeneous phases in this case. Extraction and Rebinding of Organic Molecules: Molecular Imprinting. Organic small molecules incorporated into ultrathin titania films can be removed by treatment with aqueous ammonia.18 The resulting film contains cavities that selectively bind the original template. We treated a self-supporting PVA/(4-PABA + titania) film on an ANODISC with 1% aqueous ammonia for 1 h (sample no. 5, Table 1), and then observed it with SEM. As seen from Figure 8, the film thickness was 140 ( 40 nm, and the overall morphology remained unchanged upon ammonia treatment. Removal of organic molecules from the film is confirmed by UV-vis spectroscopy (Figure 9). A dye-containing selfsupporting film (sample no. 3, Table 1) was prepared and transferred onto a quartz plate. Figure 9A shows the change of UV-vis spectra of the PVA/(4-PABA + titania) film after two cycles of removal and rebinding of 4-PABA were repeated, as mentioned in the Experimental Section. The corresponding absorbance change at 325.5 nm is given in Figure 9B. These spectral changes suggest that release and binding of 4-PABA proceed reversibly, as was already confirmed for the layer-by-layer titania film. The imprinted

cavity must be similarly formed in the spin-coated film. However, a quantitative estimate was difficult from these data. Trapping Experiments. We performed permeation experiments to see if the imprinting effect is operative in filtration of organic molecules. A self-supporting PVA/ (4-PABA + titania) film was transferred onto an ANODISC to completely cover the disk surface, and the disk was immersed in aqueous ammonia to remove template 4-PABA molecules. The covered disk was set on a membrane filter holder, and used for filtration of THF solutions of benzoic acid, azobenzene, and 4-PABA. Figure 10 shows absorbance changes of each solution after the filtration was repeated two times for the same sample solutions. It is clear that the filter cannot trap azobenzene. In the case of benzoic acid, the absorbance decreased to 70% of the initial solution by the repeated filtration. The decrease was much enhanced in the case of 4-PABA, and the absorbance decreased to only 20% of the initial value after repeated filtration. The observed difference may reflect the imprinting effect and the consequent selective trapping of template 4-PABA. We can make a rough estimate of how many organic molecules are trapped in the composite films we employed. The molar extinction coefficient of 4-PABA is estimated to be ca. 2 × 104 mol-1‚L‚cm-1 from the absorbance (0.02) of 10-6 M 4-PABA at 325.5 nm. The absorbance change at 325.5 nm between removal and binding of 4-PABA on the same film on a quartz plate is ca. 0.015 (from Figure 9B), which corresponds to a solution of 7.5 × 10-7 mol‚L-1. Assuming that all the 4-PABA molecules are retained in the film, this concentration is equivalent to 7.5 × 10-10

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Figure 10. Absorbance changes at the respective absorption maxima of (a) benzoic acid (1 × 10-4 M, λmax ) 271.5 nm), (b) azobenzene (1 × 10-6 M, λmax ) 318.5 nm), and (c) 4-PABA (1 × 10-6 M, λmax ) 325.5 nm) in THF after filtration. Filter: an ammonia-treated PVA/(4-PABA + titania) film (sample no. 5, Table 1) on an ANODISC.

mol for the solution included in a 1 cm path length and 1 cm2 area. The film area on the alumina disk is ca. 4.9 cm2, which possess the binding site of 3.7 × 10-9 mol () 4.9 (cm2) × 7.5 × 10-10 (mol‚cm-2)). The 4-PABA molecule that is contained in the 3 mL sample solution is 3 × 10-9 mol, and therefore, all of the 4-PABA molecules can be retained in the film. The actual retention was, however, limited to 80% of 4-PABA. The rest may be permeated through the film. The selective retention of 4-PABA relative to azobenzene and benzoic acid is consistent with the imprinting effect we observed previously for aromatic carboxylic acids.18a We conclude that the imprinting cavity can be created also in a self-supporting titania film. The results indicate that these molecular-imprinted films would be suitable for the use like affinity column chromatography rather than a molecular-shape-selective permeable membrane, which is our original target. However, we believe that the molecular-imprinting technique can add shape-selective ability to these self-supporting polymer/metal oxide films. For example, the use of derivatives of the target molecule, such as esters or other functional substitutions, as the imprint is an expectable approach, which is also usually employed in the case of molecular imprinting for organic polymer materials. Conclusions As we discussed in the Introduction, research efforts to prepare self-supporting ultrathin films have been conducted by using alternate assembly, polymerization, and the sol-gel process. In these studies, the thinness and size of the films are usually in a tradeoff relationship. The thinnest self-supporting film ever reported for linear macromolecules was of 5 nm thickness with µm2 size,12

Hashizume and Kunitake

and the size can be increased to 1 mm2 when the film thickness is about 10 nm.2 The submicrometer thickness was required to achieve a larger film size (cm2 scale).13 We encountered a similar result, when we prepared nanometer thick metal oxide films by the surface sol-gel process. The fundamental characteristics of these selfsupporting films, such as flatness, uniformity, and mechanical strength, have not been investigated, and their preparative procedure is often complicated and timeconsuming. In the present study, we developed a novel approach for the preparation of self-supporting ultrathin films of titania. This method is based on a simple, fast spin-coating process under mild conditions. We can readily prepare self-supporting titania films with 10 nm thickness and ∼10 cm2 size. The use of a photoresist caused facile detachment of the metal oxide film from the substrate, and the use of the PVA underlayer was probably critical for extensive 2D development of a titania network within the ultrathin film. This may be considered to be a template effect by the surface hydroxyl group of the PVA layer that promotes the sol-gel reaction at the interface. The formation of the imprinted cavity by small organic compounds is a further advantage of the current process. It is contrary to our original expectation that molecular imprinting gave rise to adsorption membranes rather than permselective membranes. The imprinted cavity apparently trapped the template molecule effectively, so that it was not readily released. The adsorption function itself is interesting, and we may devise novel uses for such membranes. However, we need new ideas to create permselective membranes. Molecular imprinting will still be a powerful method, but different kinds of template molecules may be required to realize permselectivity. Other ways of forming nanopores in the ultrathin film will also have to be tested in the future. There are two interesting directions of research as a direct extension of the current study. One of them is to test other closely related preparative processes.25 Another extension is the use of metal oxides other than titania, as is the case with typical surface sol-gel processes.14,17 Furthermore, incorporation of functional organic molecules should be promising as a means to explore novel optical, electronic, and magnetic properties in the form of self-supporting films. Acknowledgment. We thank Dr. T. Ishihara (Frontier Research System (FRS), RIKEN) for the use of a spin coater. M.H. thanks Dr. S.-W. Lee (Kitakyushu University), Mr. T. Ogata (Tokyo Ohka Kogyo Co., Ltd.), and Dr. J. Huang (FRS, RIKEN) for helpful discussions. This work was partly supported by a Grant-in-Aid for Encouragement of Young Scientists (13750642). LA035079A (25) Hashizume, M.; Kunitake, T. Manuscript in preparation.