Preparation of Nanoporous Titania Films by Surface Sol− Gel Process

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Preparation of Nanoporous Titania Films by Surface Sol-Gel Process Accompanied by Low-Temperature Oxygen Plasma Treatment Jianguo Huang,† Izumi Ichinose,† Toyoki Kunitake,*,† and Aiko Nakao‡ Topochemical Design Laboratory, Spatio-Temporal Function Materials Research Group, Frontier Research System, and Surface Characterization Division, Characterization Center. The Institute of Physical and Chemical Research (RIKEN),Wako, Saitama 351-0198, Japan Received June 19, 2002. In Final Form: August 20, 2002 Nanoporous ultrathin films of titania were prepared. First, a titania/poly(acrylic acid) (PAA) nanocomposite film was prepared via the surface sol-gel process, with the thickness of the titania gel layer to be 0.44 ( 0.10 nm/cycle. The organic species in the film were then removed by low-temperature oxygen plasma treatment, as monitored by quartz crystal microbalance frequency measurement and Fourier transform infrared and X-ray photoelectron spectroscopies. The plasma treatment was effective at least up to the depth of 10-12 nm. The resulting ultrathin titania film showed a UV-visible absorption with the edge at about 330 nm, which is considerably blue-shifted from that of bulk titania. The film surface was flat and uniform over a large area, and its density was as low as 0.54 g/cm3. Transmission electron microscopy revealed the presence of pores with diameters of ca. 2 nm. The pore existence was confirmed by the formation of silver nanoparticles inside the film by photoreduction of silver ions.

Introduction Porous materials have become the focus of many recent investigations from academic and industrial viewpoints because of their potential applications in electronics, photoelectronics, catalysis, and solar energy conversion.1-5 Titanium dioxide is a wide-band gap semiconductor that is well-known as a stable and highly reactive photocatalyst6-8 and is widely used for optical coatings and sensors.9,10 The energy band structure becomes discrete for titanium dioxide of nanometer scale, and its photophysical, photochemical, and surface properties are quite different from those of the bulk one due to the quantum size effect. The difference is also illustrated by enhanced catalytic activity.11,12 Porous titanium dioxide in a film form provides a suitable surface for bioanalytical applications.13 * To whom correspondence should be addressed: Tel +81-48467-9601; fax +81-48-464-6391; e-mail [email protected]. † Topochemical Design Laboratory. ‡ Surface Characterization Division. (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) Hagfeldta, A.; Gra¨tzel, M. Chem. Rev. 1995, 95, 49. (3) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, A. Z. Nature 1996, 379, 703. (4) Stathatos, E.; Lianos, P.; Couris, S. Appl. Phys. Lett. 1999, 75, 319. (5) Vichi, F. M.; Tejedor-Tejedor, M. I.; Anderson, M. A. Chem. Mater. 2000, 12, 1762. (6) Nazeeruddin, M. K.; Kay, A.; Rodicio, I.; Humphry-Baker, R.; Mu¨ller, R.; Liska, P.; Vlachopoulos, N.; Gra¨tzel, M. J. Am. Chem. Soc. 1993, 115, 6382. (7) Sasaki, T.; Nakano, S.; Yamauchi, S.; Watanabe, M. Chem. Mater. 1997, 9, 602. (8) Sauve´, G.; Cass, M. E.; Coia, G.; Doig, S. J.; Lauermann, I.; Pomykal, K. E.; Lewis, N. S. J. Phys. Chem. B 2000, 104, 6821. (9) Brown, G. E.; Henrich, V. E.; Casey, W. H.; Clark, D. L.; Eggleston, C.; Felmy, A.; Goodman, D. W.; Gra¨tzel, M.; Maciel, G.; McCarthy, M. I.; Nealson, K. H.; Sverjensky, D. A.; Toney, M. F.; Zachara, J. M. Chem. Rev. 1999, 99, 77. (10) Bilmes, S. A.; Mandelbaum, P.; Alvarez, F.; Victoria, N. M. J. Phys. Chem. B 2000, 104, 9851. (11) Papavassiliou, G. C. J. Solid State Chem. 1981, 40, 330. (12) Kormann, C.; Bahnemann, D. W.; Hoffmann, M. R. J. Phys. Chem. 1988, 18, 5196.

Porous titania films have been produced by deposition of titania particles or by removal of organic components from inorganic/organic hybrids. For example, macroporous titania films were prepared from a polymer-containing sol-gel system as a result of phase separation of the two components and calcination,14,15 and recently mesoporous titania films of submicrometer thickness were produced by dip-coating or spin-coating of titania sol solution with amphiphilic polymers as templates.16,17 Low-temperature oxygen plasma etching is employed as a milder method for removal of organic components from bulk hybrids18 and surface layers.19-23 We have shown that the surface sol-gel process is an excellent chemical method for the fabrication of polymer/metal oxide nanocomposite films.24,25 The organic moieties in such composite films may be readily removed by the oxygen plasma treatment.26 In the present study, we combined the surface sol-gel process and low-temperature oxygen plasma treatment and (13) Topoglidis, E.; Cass, A. E. G.; Gilardi, G.; Sadeghi, S.; Beaumont, N.; Durrant, J. R. Anal. Chem. 1998, 70, 5111. (14) Kajihara, K.; Nakanishi, K. J. Mater. Res. 2001, 16, 58. (15) Kajihara, K.; Nakanishi, K.; Tanaka, K.; Hirao, K.; Soga, N. J. Am. Ceram. Soc. 1998, 81, 2670. (16) Grosso, D.; Soler-Illia, G. J. de A. A.; Babonneau, F.; Sanchez, C.; Albouy, P.-A.; Brunet-Brumeau, A.; Balkenende, A. R. Adv. Mater. 2001, 13, 1085. (17) Yun, H.; Miyazawa, K.; Zhou, H.; Honma, I.; Kuwabara, M. Adv. Mater. 2001, 13, 1377. (18) Loy, D. A.; Shea, K. J. U.S. Patent 5321102, June 1994. (19) Kalachev, A. A.; Wegner, G. Makromol. Chem. Macromol. Symp. 1991, 46, 229. (20) Kalachev, A. A.; Mathauer, K.; Ho¨hne, U.; Mo¨hwald, H.; Wegner, G. Thin Solid Films 1993, 228, 307. (21) Hessel, V.; Detemple, P.; Geiger, J. F.; Keil, M.; Scha¨fer, R.; Festag, R.; Wendorff, J. H. Thin Solid Films 1996, 286, 241. (22) 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. (23) Jang, H. K.; Lee, S. K.; Lee, C. E.; Noh, S. J.; Lee, W. I. Appl. Phys. Lett. 2000, 76, 882. (24) Kleinfeld, E. R.; Ferguson, G. R. Mater. Res. Soc. Symp. Proc. 1994, 351, 419. (25) Ichinose, I.; Kawakami, T.; Kunitake, T. Adv. Mater. 1998, 10, 535. (26) Huang, J.; Ichinose, I.; Kunitake, T.; Nakao, A. Nano Lett. 2002, 2, 669.

10.1021/la026091q CCC: $22.00 © 2002 American Chemical Society Published on Web 10/17/2002

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Chart 1

developed a new process for the controllable preparation of nanoporous titania films of thickness within tens of nanometers. Experimental Section Chemicals. Titanium(IV) n-butoxide (Gelest Inc.), PAA (Chart 1, molecular weight 15 500; Polymer Source, Inc.) and 2-mercaptoethanol (Kanto Chemicals) were used as received. All solvents were guaranteed reagents and used without further purification. The water used in all experiments was purified by a Milli-Q system (Millipore Corp.; resistivity 18.2 MΩ‚cm). Preparation of Nanoporous Titania Films. Gold-coated quartz crystal microbalance (QCM) resonators (9 MHz, USI System, Japan), mica plates, and quartz plates were used as substrates for film fabrication. QCM electrode was modified with 2-mercaptoethanol.27 Mica plate was cleaved freshly, and its surface was modified by adsorption of a poly(ethyleneimine) layer (immersion in 2.0 mg/mL aqueous solution for 30 min). The first layer was prepared from Ti(OnBu)4 for QCM resonator and quartz substrate, and from PAA for modified mica substrate. QCM frequency was measured with a Hewlett-Packard 5313A counter (225 MHz), where a 1 Hz frequency decrease corresponds to a 0.9 ng mass increase, according to the Sauerbrey equation. The density of adsorbed film is given as follows:25

F)

∆F 3.664d

(1)

where F is density of the film in grams per cubic centimeter, ∆F is the frequency decrease due to adsorbed mass, and d is the film thickness in angstroms. As-prepared films were subjected to oxygen plasma treatment on a PE-2000 plasma etcher (South Bay Technology) at room temperature. The sample was placed directly on the RF electrode. The applied radiofrequency (RF) was 13.56 MHz. The base pressure obtained in the reactor was 75 mTorr, and the oxygen (industrial grade) pressure during plasma treating was 176 mTorr. The RF power dissipated to the sample was 10 W, and the treatment time was varied between 30 s and 30 min, depending on the number of titania/PAA layers in a film. Silver Nanoparticle. Quartz or mica plates covered with plasma-treated nanoporous titania films were immersed in 0.2 M AgNO3 ethanol/water (1:1 v/v) solution for 2 h, then rinsed thoroughly with deionized water and dried in a stream of nitrogen. Photoreduction of silver was achieved by subjecting the films to UV irradiation (365 nm, 4 W) for 1 h. Characterization. XPS spectra were obtained on a VGESCALAB 250 spectrometer, employing Al KR radiation (anode operated at 15 kV, 20 mA) with the analysis chamber pressure less than 10-8 Pa. The C(1s) spectrum of the characteristic components (C-C, C-H bonding 285 eV) was used as standard. UV-vis spectra were measured with a Shimadzu UV-3100 spectrophotometer in the transmission mode. Top-view scanning electron microscopy (SEM) was conducted with a Hitachi S-900 instrument at an acceleration voltage of 25 kV. The samples were coated with 2-nm thick platinum by use of a Hitachi E-1030 ion sputter (15 mA, 10 Pa). Cross-sectional views were taken with a Hitachi S-5200 field emission scanning electron microscope without platinum coating at an acceleration voltage of 2.0 kV. Transmission electron microscopy (TEM) images were acquired on a JEOL JEM-2000 electron microscope at a 100 kV acceleration voltage. For TEM observation, the film was scraped off from the substrate in ethanol, and a dispersion of the thin film flakes was (27) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296.

Figure 1. QCM frequency shifts (-∆F) during alternate adsorption of Ti(OnBu)4 [100 mM in 1:1 (v/v) toluene/ethanol, 25 °C] (b) and PAA (1 mg/mL in ethanol, 25 °C) (O) and frequency shift after oxygen plasma treatment (RF power 10 W, 10 min). then dropped on a 200 mesh carbon-coated copper grid and left to dry in air.28

Results and Discussion Preparation of Titania/PAA Films. It is indicated by our previous study that titania and PAA produce tightly bound alternate layers.25 This composite film had a flat, uniform surface. Hybrid films of small organic molecules gave less regular structures. Therefore, PAA was considered to be a good candidate for nanopore precursors. In the present work, nanocomposite titania/PAA ultrathin films were prepared on QCM resonators according to the previous procedure.25 The typical procedure is given briefly as follows. A 2-mercaptoethanol-modified gold-coated QCM resonator was immersed in 100 mM Ti(OnBu)4 in 1:1 (v/v) toluene/ethanol for 3 min, followed by rinsing in ethanol for 1 min, and hydrolysis in pure water for 1 min, and the frequency was measured. The resonator was then immersed in 1 mg/mL ethanolic PAA for 10 min, followed by rinsing with ethanol for 1 min and drying with nitrogen gas, and the frequency was measured again. This alternate adsorption was repeated for given cycles. Figure 1 shows QCM frequency shifts upon alternate adsorption of Ti(OnBu)4 and PAA for a 15-cycle composite film. The frequency decreased linearly during film fabrication, indicating regular growth of titania/PAA multilayers on the resonator.25 In each adsorption cycle, the frequency decrease is 28 ( 6 Hz for Ti(OnBu)4 adsorption and 20 ( 7 Hz for PAA. The corresponding mass is estimated by Sauerbrey equation.29 In the current system, 1 Hz frequency decrease corresponds to a thickness increase of 0.273/F (in angstroms), where F (in grams per cubic centimeter) is the density of the adsorbed film.25 The thickness of a single titania gel layer is estimated to be 0.44 ( 0.10 nm by using the bulk density of titanium dioxide-based gel (1.7 g/cm3), while the thickness of a PAA layer is 0.38 ( 0.14 nm on the basis of its bulk density (1.4 g/cm3).25 Since the bond length of Ti-O is 1.748(4) Å,30 the observed thickness of the titania gel layer corresponds to 2.5 Ti-O linkages, indicating that titanium dioxide gel layer of a minimal thickness was deposited. The unit thickness in the current process is similar to that in the surface sol-gel deposition of titania thin film24 and is (28) Yoshida, T.; Terada, K.; Schlettwein, D.; Oekermann, T.; Sugiura, T.; Minoura, H. Adv. Mater. 2000, 12, 1214. (29) Sauerbrey, G. Z. Phys. 1959, 155, 206. (30) Matilainen, L.; Klinga, M.; Leskela, M. Polyhedron 1996, 15, 153.

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Figure 2. XPS spectra of a 10-cycle titania/PAA nanocomposite film deposited on quartz substrate: (a) as-prepared film; (b) oxygen-plasma-treated film (RF power 10 W, 10 min). Insets: XPS spectra in the energy regions of O1s, Ti2p, and C1s.

comparable to that of layerwise chemisorption by the CVD technique.31 Oxygen Plasma Treatment. A QCM resonator covered with a titania/PAA film was placed on the electrode of the plasma etcher and subjected to oxygen plasma treatment at room temperature. The frequency was found to increase during the process of plasma treatment. For a 15-cycle titania/PAA film shown in Figure 1, the QCM frequency increment was 300 Hz after 10-min treatment with RF power of 10 W and was unchanged upon further etching. In the as-prepared film, the total frequency decrement due to PAA and titania gel adsorption was 293 and 413 Hz, respectively. Therefore, the plasma treatment removed the mass corresponding to the total PAA adsorption. The plasma etching is mainly caused by direct interaction of active atoms and molecules in the plasma (O+, O-, O2+, O2-, O, O3, ionized ozone, metastably excited O2, free electrons, etc.) with organic species in the sample, and the photon in the UV region can break the C-C and C-H bonds.19 The product includes CO2, CO, H2O, and hydrocarbons of low molecular weight, and they are removed in vacuo. Oxygen plasma treatment was also carried out as control on titania films prepared by means of stepwise adsorption of titanium n-butoxide alone. For such a 10-cycle film deposited on QCM electrode, the frequency increased by about 5 Hz upon plasma treatment with RF power of 10 W for 10 min. This much smaller change suggests removal of 2-mercaptoethanol layer and the small amount of unhydrolyzed butoxide groups that remained in the original titania gel film.27 Further plasma treatment with much higher RF power for a much longer time resulted in a frequency decrease of about 15 Hz, probably due to oxygen incorporation into the film.32 The removal of PAA from the titania/PAA composite was separately confirmed by Fourier transform infrared (FT-IR) spectroscopy. The spectrum of an as-prepared 5-cycle titania/PAA film possesses absorption bands at ca. 1712 cm-1 due to CdO stretching vibration of the free (31) Desu, S. B. Mater. Sci. Eng. B 1992, 13, 299. (32) Clark, D. T.; Dilks, A. J. Polym. Sci., Polym. Chem. Ed. 1979, 17, 957.

carboxyl group (PAA), together with vibrational modes of titanium carboxylate at ca. 1551, 1449, and 1416 cm-1.33 In contrast, the corresponding oxygen-plasma-treated film does not give such bands, indicating the complete removal of PAA. XPS Analysis. Figure 2 displays XPS spectra of asprepared and oxygen-plasma-treated 10-cycle titania/PAA films. As(3d) and Si(2p) peaks that arise from the quartz substrate are seen. In a spectrum of the as-prepared film, a broad unsymmetrical O(1s) peak at around 532 eV contains oxygen signals coming from titanium dioxide gel (at low binding energy), PAA, and quartz (at high binding energy). For the oxygen-plasma-treated film, the O(1s) peak for titanium dioxide becomes much more obvious as a shoulder at 530.0 eV, and the O(1s) peak of substrate quartz remains at 532.2 eV. In both cases, Ti(2p1/2) and Ti(2p3/2) peaks are present at binding energies of 464.2 and 458.4 eV, respectively, in consistence with characteristic XPS binding energies for titanium dioxide.34 Their band intensities increase in the plasma-treated film. It is reported that the symmetric Ti(2p) peaks are indicative of titanium atoms bonded stoichiometrically to oxygen atoms.35 The peak separation between Ti(2p1/2) and Ti(2p3/2) signals is 5.8 eV, which is in excellent agreement with the reported literature value.36 C(1s) peaks corresponding to COOH (288.4 eV) and -CH2-CH- (285.0 eV) are strong for the as-prepared film (Figure 2, inset). Upon plasma treatment, the carboxyl C(1s) peak disappeared completely, and the peak for hydrocarbon chains decreased remarkably. The latter peak is believed to come from adsorption of low molecular weight hydrocarbons produced during the treatment. The third C(1s) peak at 282.5 eV can be associated with C-Si bonds in the quartz substrate,37 and it remains unchanged after plasma treat(33) Lee, S.-W.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 2857. (34) Hofmann, S.; Sanz, J. M. J. Trace Microprobe Tech. 1982/83, 1, 213. (35) Li, Q. W.; Baer, D. R.; Engelhard, M. H.; Shultz, A. N. Surf. Sci. 1995, 344, 237. (36) McCafferty, E.; Wightman, J. P. Surf. Interface Anal. 1998, 26, 549. (37) Senemaud, C.; Gheorghiu-de La Rocque, A.; Dufour, G.; Herlin, N. J. Appl. Phys. 1998, 84, 4945.

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Figure 3. QCM frequency shifts (-∆F) of 5-, 10-, 15-, and 20-cycle titania/PAA nanocomposite films upon oxygen plasma treatment (RF power 10 W). Different reaction times were needed, depending on samples, until the QCM frequency became stable.

Figure 5. (A) SEM top-view image of a plasma-treated 5-cycle titania/PAA film deposited on mica substrate; (B) SEM crosssectional view image of a plasma-treated 15-cycle titania/PAA film deposited on a QCM resonator. (C) TEM micrograph of an as-prepared titania/PAA film; (D) TEM micrograph of the sample after plasma treatment.

Figure 4. (A) Absorption spectra of a 5-cycle titania/PAA nanocomposite film before (a) and after (b) oxygen plasma treatment. (B) Absorption spectra of oxygen-plasma-treated 2(a), 6- (b), and 10-cycle (c) titania/PAA nanocomposite films. Oxygen plasma treatment: RF power 10 W, 10 min.

ment. Compositions of the as-prepared and plasma-treated film were Ti:C:O ) 1.0:5.5:7.0 and 1.0:1.3:6.3, respectively. The concentration ratio of carbon atoms to titanium atoms became rather low by the treatment. From these results, it is clear that PAA in the as-prepared film was completely removed by oxygen plasma treatment. Penetration Depth of Oxygen Plasma. Subsequently, we examined the depth profile in the plasma treatment since the effective depth of plasma etching is known to depend on the film composition and structure.19,20 Figure 3 shows QCM frequency shifts of serial titania/ PAA nanocomposite films upon oxygen plasma treatment. For the as-prepared 5-, 10-, 15-, and 20-cycle films, the QCM frequency decrement due to the total PAA adsorption

was 75, 176, 293, and 451 Hz, respectively, whereas the corresponding QCM frequency increment after the samples were treated with oxygen plasma was 79, 179, 300, and 296 Hz, respectively. It is clear that the oxygen plasma treatment removed all the PAA component for the 5-, 10-, and 15-cycle titania/PAA films. However, this is probably not the case with the 20-cycle film, which displayed a close QCM frequency increase with that of the 15-cycle film. It means that the PAA components in the first several cycles could not be removed. Therefore, the penetration depth of oxygen plasma etching was estimated to be 1012 nm on the basis of the film thickness, which was calculated from density of titanium dioxide-based gel and the bulk density of PAA. Organic species in deeper locations could not be removed under the current conditions. It has been reported that cold plasma can affect 25-30 layers (50-60 nm) of a LB film of polysiloxane.19 The temperature during the plasma treatment is lower than 100 °C,18,22 and thermal oxidation can be excluded under the present conditions. Morphology of Titania Nanofilms. Nanosized titanium dioxide possesses physical and photochemical properties different from those of the bulk material,11,12 as typically expressed by a blue-shifted energy gap.38 Figure 4A shows UV-vis spectra of a 5-cycle titania/PAA nanocomposite film before and after oxygen plasma treatment. The absorption threshold was estimated by extrapolating the steep part of the rising absorption curve. It is 332 and 333 nm for the as-prepared and oxygen-plasma-treated films, respectively. Both of these absorption onsets are considerably blue-shifted from those of crystalline titanium dioxides (anatase, 387 nm; rutile, 413 nm) and nanosized titanium dioxide particles (2.4-nm anatase, 370 nm; 3.8-nm anatase, 375.1 nm; 5.5-nm rutile, 398 nm),12,39 implying the nanosized structure unit in the current (38) Brus, L. E. J. Chem. Phys. 1984, 80, 4403. (39) Anpo, M.; Shima, T.; Kodama, S.; Kubokawa, Y. J. Phys. Chem. 1987, 91, 4305.

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Figure 6. Schematic illustration of low-temperature oxygen plasma treatment on titania/PAA nanocomposite film.

films.40 Figure 4B shows absorption spectra of oxygenplasma-treated 2-, 6-, and 10-cycle titania/PAA nanocomposite films. The absorbance due to titanium dioxide increased linearly with increasing cycles of adsorbed layers, and all the absorption thresholds remained constant at around 330 nm. It appears that the titania film prepared by the current process is amorphous and is formed simply by removal of PAA and cross-linking of the original titanium dioxide gel layers. Figure 5A shows the top-view SEM photograph of an oxygen-plasma-treated film, from which a flat surface can be seen over a large area. The island structures on the film surface could be formed by platinum particles sputtered for SEM observation. This SEM image is similar to that of pure mica substrate measured under the same microscope parameters, indicating that the film surface is extremely smooth. Thermal treatment of titania films often results in rugged surfaces with crystalline particles (anatase or rutile) of a few tens of nanometers.41,42 Similar crystallite formation was observed for heat-treated titania ultrathin films that were prepared via surface sol-gel process. However, such crystalline morphologies are not found in the present case. Therefore, low-temperature oxygen plasma treatment is an effective method for removal of organic components from metal oxide/polymer nanocomposite ultrathin films without changing the structural feature of the matrix. Thin films of amorphous metal oxides are thus obtained. Figure 5B shows the crosssectional view of a plasma treated 15-cycle film on QCM resonator. The treatment was continued for this sample for an additional 10 min after mass equilibration was achieved (RF power, 10 W), to ensure complete removal of the organic components. It is seen that the film surface is rather flat despite the surface roughness of the gold electrode. The average film thickness is estimated to be 25 nm. Since the whole film thickness for the as-prepared sample was estimated to be 13 nm, film thickness increased as a result of the plasma treatment. As schematically illustrated in Figure 6a, the asprepared film is composed of loose networks of separate titania and PAA layers with titanium atoms coordinated to the carboxylate groups in the adjacent PAA layer, giving tightly complexed titania-PAA nanocomposite layers.25 PAA is decomposed upon oxygen plasma treatment to give gaseous products that would cause expansion of titania gel layers due to its flexibility. The originally separate plastic titania gel layers would be partially cross-linked with each other; hence, a highly porous, self-supporting amorphous titania film with increased thickness is produced (Figure 6b). (40) Moriguchi, I.; Maeda, H.; Teraoka, Y.; Kagawa, S. J. Am. Chem. Soc. 1995, 117, 1139. (41) Stathatos, E.; Lianos, P.; Laschewsky, A. Langmuir 1997, 13, 259. (42) Hagfeldt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269.

From the film mass due to titanium alkoxide adsorption in the as-prepared film (444 ng; ∆F, 493 Hz) and the film thickness estimated by SEM observation (25 nm), the film density is calculated as 0.54 g/cm3. This density is only one-seventh that of anatase (3.84 g/cm3), one-eighth that of rutile (4.26 g/cm3), and one-third that of bulk titanium dioxide-based gel (1.7 g/cm3),25 indicating the high porosity of the plasma-treated film. Figure 5 panels C and D show TEM images of a titania/ PAA film before and after plasma treatment. Both micrographs were taken under the same conditions. The asprepared film specimen was obtained as large flakes when scraped off from the original substrate, thanks to its selfsupporting property. On the other hand, the plasmatreated specimen was obtained as small flakes that were overlapped with each other on a copper grid. The asprepared sample shows a uniform and dense structure without any porosity. In contrast, numerous pores and channels can be seen for the plasma-treated sample. The pores have diameters of around 2 nm, which is similar to the pore size of surfactant-templated mesoporous silica films at the smallest.43,44 The mesopores are distributed randomly in the film without identifiable order or packing, as is the case with those of non-surfactant-templated mesoporous silica45,46 and polymer-silica composites.47 As a crude estimate from Figure 5D, more than 50% of the film is occupied by nanopores. This is in a qualitative agreement with the decrement of the film density mentioned above. Control TEM observation was also carried out on an oxygen-plasma-treated titania film prepared without PAA. Porous structures were not observed in this case. Formation of Silver Nanoparticles within Nanoporous Titania Film. Syntheses of metal nanoparticles are of great current interest because of their unique sizeand shape-dependent physical and chemical properties.48-52 The nanopores in the porous titania film may be used as templates for preparation of metal nanoparticles, as in the case of mesoporous silica.53 At the same time, silver nanoparticles, if formed, would be a clear proof of the (43) Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Nature 1996, 381, 589. (44) Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024. (45) Wei, Y.; Jin, D. L.; Ding, T. Z.; Shih, W. H.; Liu, X. H.; Cheng, S. Z. D.; Fu, Q. Adv. Mater. 1998, 10, 313. (46) Wei, Y.; Xu, J.; Dong, H.; Dong, J. H.; Qiu, K.; Jansen-Varnum, S. A. Chem. Mater. 1999, 11, 2023. (47) Wei, Y.; Feng, Q.; Xu, J.; Dong, H.; Qiu, K.-Y.; Jansen, S. A.; Yin, R.; Ong, K. K. Adv. Mater. 2000, 12, 1448. (48) Henglein, A. Chem. Rev. 1989, 89, 1861. (49) Ozin, G. A. Adv. Mater. 1992, 4, 612. (50) Valden, M.; Lai, X.; Goodman, D. W. Science 1998, 281, 1647. (51) Claus, P.; Bru1ckner, A.; Mohr, C.; Hofmeister, H. J. Am. Chem. Soc. 2000, 122, 11430. (52) Rodrı´guez-Sa´nchez, L.; Blanco, M. C.; Lo´pez-Quintela, M. A. J. Phys. Chem. B 2000, 104, 9683. (53) Plyuto, Y.; Berquier, J.-M.; Jacquiod, C.; Ricolleau, C. Chem. Commun. 1999, 1653.

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resonance absorption band of silver nanoparticle around 450 nm [Figure 7B(b)]. TEM observation provides additional support. A TEM image of the scraped-off film is displayed in Figure 7C, where a large amount of spheroidal silver nanoparticles are seen. A size distribution histogram plotted in Figure 7D reveals that the size of the individual particle is 2-4 nm. A few large particles, up to 3-5 times mean size, are also detected, probably due to aggregation of small particles. These TEM and UV data indicate the formation of silver nanoparticles inside the titania film.54-56 When the same experiment was carried out on a titania film prepared without PAA, no surface plasmon absorption could be observed, indicating the absence of silver nanoparticles. Thus, we conclude that the nanoporous structure derived from removal of PAA is essential for formation of silver nanoparticles. Conclusions

Figure 7. Silver nanoparticle formation within a plasmatreated 8-cycle titania/PAA film: (A) SEM top-view image of the film deposited on mica substrate, (B) absorption spectra of the titania film deposited on quartz substrate before (a) and after (b) silver nanoparticle formation, (C) TEM micrograph of the obtained silver nanoparticles, and (D) histogram of the corresponding particle size distribution.

presence of nanopores. The obtained nanoporous titania film was dipped into a solution of silver nitrate to bring about filling of the pores by AgNO3, followed by thorough rinsing with pure water and drying with nitrogen gas. It was then subjected to irradiation with UV light. The results in the case of a plasma-treated 8-cycle titania/ PAA film are displayed in Figure 7. The surface topography of the silver nanoparticle incorporated film (Figure 7A) does not show much difference from that of the pure porous titania film (Figure 5A), and no silver particle can be observed on the film surface. On the other hand, a UVVis spectrum of the film shows strong surface plasmon

Low-temperature oxygen plasma treatment of titania/ PAA nanocomposite films yields amorphous titania films with high nanoporosity, and the film thickness can be controlled on the nanometer level. The nanoporous film possesses pores with diameters of around 2 nm inside the film, and the film surface is uniform and flat over a large area. In contrast, porous structures were not formed for a titania film assembled without PAA. Silver nanoparticles are readily formed in the nanoporous structures. The current approach is generally applicable to syntheses of nanoporous films of various metal oxides. We have shown in our molecular imprinting studies33 that a large variety of imprinted cavities are formed in ultrathin metal oxide films. Removal of organic templates may be achieved either by extraction or by plasma treatment. It should be possible to design sizes and shapes of the nanopore by appropriate combination of these processes. LA026091Q (54) Fujii, M.; Nagareda, T.; Hayashi, S.; Yamamoto, K. Phys. Rev. B 1991, 44, 6243. (55) Kume, T.; Nakagawa, N.; Yamamoto, K. Solid State Commun. 1995, 93, 171. (56) Itoigawa, H.; Kamiyama, T.; Makamura, Y. J. Non-Cryst. Solids 1997, 220, 210.