Surface Fabrication of Hollow Nanoarchitectures of Ultrathin Titania

Langmuir 2003, 19, 6545-6552

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Surface Fabrication of Hollow Nanoarchitectures of Ultrathin Titania Layers from Assembled Latex Particles and Tobacco Mosaic Viruses as Templates† Shigenori Fujikawa and Toyoki Kunitake* Frontier Research System (FRS), The Institute of Physical and Chemical Research (RIKEN), Hirosawa 2-1, Wako-shi, Saitama, 351-0198 Japan Received December 11, 2002. In Final Form: March 19, 2003 A novel procedure for fabricating nanoarchitectures of ultrathin titania layers was developed by using assembled latex particles and tobacco mosaic viruses (TMV) as templates. Latex particles and TMVs were assembled on the surface and then covered with ultrathin titania films by the surface sol-gel process. When the surface-covered latex template was subjected to an oxygen plasma treatment, hollow titania spheres connected with each other via nanotubes were formed at the original deposition of the particles. The titania-coated TMV produced tubular structures after the oxidative removal of the template. It is clear that the as-prepared titania shells replicated the original shape of the templates after its removal. The hollow structures were preserved despite the thickness of titania films being only a few nanometers. The use of oxygen plasma was indispensable for maintaining a three-dimensional structure because calcinations of the same sample gave flattened disks as a result of the collapse of the hollow shells.

Introduction Fabrication of three-dimensional architectures in the nanometer regime is an indispensable, yet largely unexplored, field of nanotechnology. Among the three-dimensional nanoarchitectures, the formation of the hollow structure has been rather limited in its scope (tubes and spheres) despite its attractive applications, such as drugdelivery materials and microreactors. The template approach has been widely utilized to prepare hollow structures in most of the previous studies. In these studies, nanosized objects such as a virus1, molecular assembly2, and simple sphere3 were employed as representative templates, and most of the templates were used as dispersions in solution. Organic molecules, inorganic compounds, polymers, and organic and inorganic particles can be candidates for the shell component if they are firmly attached to the template surface. After the template is covered with a shell component, hollow structures are created by removing the inner template, and their shapes have morphologies the same as those of the original template (positive copy of the template shape). Unfortunately, the shape of the hollow structure is rather restricted in this approach because it is still difficult to freely design three-dimensional nanosized templates that can be dispersed in solutions. * To whom correspondence should be addressed. Telephone: +81-48-467-9601. Fax: +81-48-464-6391. E-mail: kunitake@ ruby.ocn.ne.jp. † Part of the Langmuir special issue dedicated to David O’Brien. (1) (a) Shenton, W.; Douglas, T.; Young, M.; Stubbs, G.; Mann, S. Adv. Mater. 1999, 11, 253. (b) Flowler, C. E.; Sheton, W.; Stubbs, G.; Mann, S. Adv. Mater. 2001, 13, 1266. (2) (a) Jung, J. H.; Ono, Y.; Hanabusa, K.; Shinkai, S. J. Am. Chem. Soc. 2000, 122, 5008. (b) van Bommel, K. J. C.; Jung, J. H.; Shinkai, S. Adv. Mater. 2001, 13, 1472. (3) (a) Caruso, F.; Caruso, R. A.; Mo¨hwald, H. Science 1998, 282, 1111. (b) Donath, E.; Sukhorukov, G. B.; Caruso, F.; Davis, S. A.; Mo¨hwald, H.; Angew. Chem., Int. Ed. 1998, 37, 2201. (c) Caruso, F.; Shi, X.; Caruso, R. A.; Susha, A. Adv. Mater. 2001, 13, 740. (d) Imhof, A.; Pine, D. J. Nature 1997, 389, 948. (e) Caruso, F. Chem.sEur. J. 2000, 6, 413. (f) Caruso, F. Adv. Mater. 2001, 13, 11. (g) Zhu, G.; Qiu, S.; Terasaki, O.; Wei, Y. J. Am. Chem. Soc. 2001, 123, 7723. (h) Cassagneau, T.; Caruso, F. J. Am. Chem. Soc. 2002, 124, 8172.

Surface-fabrication techniques made rapid progress, as is exemplified by photo- and electron lithographies, microcontact printing,4 and micromolding.4 These techniques provide an alternative to the nanofabrication of hollow structures on the substrate surface. The nanometer-sized objects are created on the surface by the surfacefabrication technique and covered with ultrathin films, and the inner template is subsequently removed to form hollow structures of the corresponding shape. The morphology of the hollow structure would be much richer in this new approach. The inverse opal structure is known as a typical example of the hollow structure that is prepared by a closely related approach. It is based on the polymerization of monomers in the interparticle void of the ordered particle array on a solid substrate and subsequent removal of the template particle.5 However, the resulting frame has an inversed morphology of the original template (negative copy). Our long-term target is to establish a new methodology for the preparation of three-dimensional nanoarchitectures based on the positive copying of nanosized templates. The surface sol-gel process is considered most appropriate for this purpose because it traces the sizes and shapes of the template molecules. We briefly reported in our previous communication that this approach is applicable to latex templates.6 In this report, we describe a detailed account of the positive copying of surface-organized latex particles and tobacco mosaic viruses (TMVs), a biological nanotube, on the basis of the surface sol-gel method that can produce ultrathin films of metal oxides by the layer-by-layer chemisorption of metal alkoxide precursors. Experimental Section The latex particle we employed (Polyscience; polybeads carboxylate) has carboxyl groups on the surface. The diameter is 500 nm with a deviation of 10 nm, and it is supplied as a 2.61 (4) Xia, Y.; Whitesides, G. M. Angew. Chem., Int. Ed. 1999, 37, 550. (5) (a) Gates, B.; Yin, Y.; Xia, Y. Chem. Mater. 1999, 11, 2827. (b) For a comprehensive review, please refer to Xia, Y.; Gates, B.; Yin, Y.; Lu, Y. Adv. Mater. 2000, 12, 693. (6) Fujikawa, S.; Kunitake, T. Chem. Lett. 2002, 11, 1134.

10.1021/la026979e CCC: $25.00 © 2003 American Chemical Society Published on Web 05/06/2003

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Fujikawa and Kunitake

Figure 1. Preparation of titania-latex core/shell structure on solid substrates and the removal of latex template by oxygen plasma treatment. (a) Precoating of a solid substrate by PDDA and PSS alternate layers to generate a cationic surface. (b) Adsorption of latex particles through the electrostatic interaction. (c) Formation of ultrathin titania layers on a substrate surface by the surface sol-gel process. (d) Removal of the inner template particles by the oxygen plasma process. wt % aqueous dispersion. It was used after dilution to 1/10 by ion-exchanged water. The concentration of the TMV in water is 0.24 mg/mL. The silicon wafers (Tokyo Ohka Kogyo), mica (Okabe Mica; natural mica grade), silicon oxide-coated transmission electron microscopy (TEM) grid (Ted Pella; silicon monoxide type A), and gold-coated quartz crystal microbalance (QCM) resonator (USI System, 9 MHz) were used as the solid substrates. A goldcoated QCM resonator was immersed in an ethanol solution of 3-mercaptopropionic acid (1 mM) overnight and rinsed with ethanol. To prepare a positively charged surface, these solid substrates were immersed in an aqueous solution of poly(diallyldimethylammonium chloride) (PDDA, Polymer Source; molecular weight ) 240 000, 1 mg/mL) and in ion-exchanged water for 1 min each. They were then immersed in an aqueous solution of poly(sodium styrene-4-sulfonate) (PSS, Polymer Source; molecular weight ) 127 600, 1 mg/mL) and in ionexchanged water for 1 min each. These successive operations were repeated three times, and the modified substrate was further immersed in aqueous PDDA (Figure 1a). The latex particles and TMVs were adsorbed by dipping the substrates for 10 min into their aqueous dispersions (Figure 1b). They were then soaked in ion-exchanged water for 1 min, and the substrate was dried by nitrogen-gas flushing. To prepare the multilayers of the latex particles, 20 µL of the dispersed particles was spread on a silicon wafer (about 3 × 8 mm) and was allowed to dry at room temperature. The surface sol-gel process was conducted by dipping the substrate in 100 mM of titanium(IV) isopropoxide (Azmax) in ethanol for 1 min, rinsing by ethanol and ionexchanged water for 1 min, and drying by nitrogen-gas flushing. This process was repeated several times (Figure 1c). In the QCM measurements, the frequency of the QCM resonator was monitored at each adsorption step of the titania layer. The oxygen plasma treatment (South Bay Technology; PE-2000 plasma etcher) was carried out by placing the sample directly on the radio frequency (RF) electrode (Figure 1d). The applied RF was 13.56 MHz, and the base pressure in the reactor was 75 mTorr. The oxygen (industrial grade) pressure during the plasma treatment was about 180 mTorr, and the RF power dissipated to the sample was 20 W for several minutes at room temperature. In the calcination experiments, the substrate was placed on a bench inside a muffle furnace (Denken; KDF S-70), and the temperature was raised from room temperature to 450 °C in 25 min or in 150 min, kept at 450 °C for 4 h, and finally allowed to cool to room temperature. Scanning electron microscopy (SEM) images were obtained by using a Hitachi S-900 scanning electron microscope operated at an acceleration voltage of 25 kV. Some of the samples were coated with platinum by an ion-sputtering coater (Hitachi; E-1030, 15 mA, 30 s). TEM observation was conducted on a JEOL JEM-2000EX instrument. The sample prepared on silicon oxide grids was subjected without staining to TEM observation with an acceleration voltage of 100 kV and an original magnification of 10 000.

Results A. Preparation of Titania Ultrathin Films on Surface-Assembled Latex Particles. The formation of titania thin films on the template was monitored by the frequency measurement of QCM. A QCM resonator, which

Figure 2. QCM frequency shifts due to the adsorption of the titania layers.

has C-500 latex particles on its surface, was subjected to the surface sol-gel process with Ti(O-iPr)4. The resonator surface was then dried by nitrogen-gas flushing. This series of operations was repeated, and the frequency change was monitored each time after nitrogen drying (Figure 2). In the first cycle of titania adsorption, the frequency increase (mass decrease) was reproducibly observed. This seems to arise from the desorption of weakly adsorbed latex particles from the resonator surface. After the second cycle, the frequency shifts decreased regularly, indicating that regular titania layers were formed. The total frequency shift after 10 cycles was 54 Hz. This frequency shift corresponds to a calculated thickness of about 1 nm based on the density (1.7 g/cm3) of bulk titania.7 Interestingly, this frequency shift is much smaller than that which we have previously reported.7 In our previous study, titanium tetra-n-butoxide was used as the titania precursor and the average frequency shift in one cycle of chemisorption was 61 ( 37 Hz.7 The latter frequency change is about 10 times larger than that of the present result, and the discrepancy may be attributed to the probable aggregation of titania butoxide in solution and the lower polarity of the washing solvent. Titanium tetran-butoxide forms oligomers, whereas titanium tetraisopropoxide exists as a monomer in a solution.8 Therefore, titanium tetra-n-butoxide as a precursor would give a thicker layer in a single operation. We already reported that washing by a polar solvent led to much less adsorption of titania compared to washing by a less-polar solvent.9 Apparently, polar solvents are more effective for removing weakly adsorbed titania species. The morphology of the latex surface was observed by SEM before and after the given cycles of the surface sol(7) (a) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Mater. 1997, 9, 1296. (b) Ichinose, I.; Senzu, H.; Kunitake, T. Chem. Lett. 1996, 10, 831. (8) Bradley, D. C. Chem. Rev. 1989, 89, 1317. (9) He, J.-H.; Ichinose, I.; Nakao, A.; Kunitake, T. RIKEN Review 2001, 37, 34.

Hollow Nanoarchitectures of Ultrathin Titania Layers

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Figure 3. SEM images of the latex surface before and after the sol-gel process. Parts 1 and 2 are obtained before the surface sol-gel process at low (×6000) and high (×60 000) magnification, respectively; parts 3-5 are obtained after 5, 10, and 20 cycles of the surface sol-gel process, respectively (scale bar ) 500 nm).

Figure 4. SEM images of the titania-coated particles by oxygen plasma treatment. Series A and B were obtained from samples for which the surface sol-gel process was repeated for 5 and 20 cycles, respectively. (1) Before the exposure of oxygen plasma, (2) after 10 min of exposure to oxygen plasma, (3) after 30 min of exposure to oxygen plasma, and (4) after 60 min of exposure to oxygen plasma (scale bar ) 500 nm).

gel process (0, 5, 10, and 20 cycles; Figure 3). The latex particles are randomly adsorbed onto the whole substrate surface, as was observed before the surface sol-gel process. Although some particles are isolated, most of the adsorbed particles form domain structures (Figure 3-1), and the particles within the domain are in contact with each other (Figure 3-2). The number of particles included in one domain is several to several tens. The structures of these domains was seemingly not changed after the surface solgel operation. The particle surface is very smooth after repeated sol-gel processes, and no change is found in the shape of the latex. As was mentioned previously, the thickness of the titania layer is estimated to be less than a few nanometers after 10 cycles of adsorption. The standard deviation of the diameter of the original latex particle is about 10 nm. Therefore, it is not possible to distinguish the shapes and sizes of the latexes before and after adsorption of the titania layer. B. Removal of the Template Latex Particle by the Oxygen Plasma Treatment. Oxygen plasma was used to remove organic templates. In the low-temperature

oxygen plasma process, oxygen plasma and related active species attack organic components to decompose them into lower-molecular-weight materials, such as carbon dioxide and water.10 The latex samples on which the surface solgel process was repeated for 5 and 20 cycles were exposed to oxygen plasma for different periods of time and subjected to SEM observation (Figure 4). Before the plasma treatment, the particle surface was very smooth (Figure 4a1,b-1). After 10 min of oxygen plasma exposure, the particle diameter in these samples decreased to 350-400 nm and about 400 nm, respectively, and the particle surface became rugged (Figure 4a-2,b-2). In the five-cycle sample, rodlike structures with diameters of 10-30 nm in width appeared between the shrunken particles (Figure 4a-2, arrows). In the case of the 20-cycle sample, each particle was connected by tubular structures with about a 100-nm width (Figure 4b-2, arrow). After 30 min of plasma (10) (a) Kalachev, A. A.; Wegner, G. Makromol. Chem., Macrol. Symp. 1991, 46, 229. (b) Boenig H. V. Fundamentals of Plasma Chemistry and Technology; Technomic Publishing: Lancaster, PA, 1988.

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Figure 5. TEM images of titania-coated latex particles. (1) After 10 min of exposure to oxygen plasma, (2) after 30 min of exposure to oxygen plasma, and (3) after 70 min of exposure to oxygen plasma.

Figure 6. SEM images of the calcined titania-coated latex. (a) After five cycles of the surface sol-gel process and a temperatureincreasing rate of 450 °C/25 min. (b) After 10 cycles of the surface sol-gel process and a temperature-increasing rate of 450 °C/150 min. (c) After 20 cycles of the surface sol-gel process and a temperature-increasing rate of 450 °C/150 min (scale bar ) 1000 nm).

treatment, the particle diameter decreased to about 200250 nm in the five-cycle sample (Figure 4a-3), and the junction length was elongated to several tens of nanometers (Figure 4a-3, arrow). In the 20-cycle sample, the particles were slightly shrunken (the particle diameter is 300-400 nm), and the connecting tubular structures were also elongated as a result of the shrinking of the particle upon 30 min of plasma treatment (Figure 4b-3). In either sample, the large changes in appearance could not be observed after 30-min plasma treatments (Figure 4a-4,b4). It is suggested from the results of the SEM and QCM experiments that very thin titania layers were formed on the surface of the template particle. The internal structure of the modified latex cannot be directly examined in the SEM observation of the outer layer. The five-cycle sample was, thus, subjected to TEM observation (Figure 5). Upon 10 min of plasma treatment, dark spherical structures 500 nm in diameter were seen, consistent with the original latex (Figure 5-1). After a longer treatment of 30 min, the particles were converted to spherical shells with diameters of about 300 nm and thicknesses of several nanometers (Figure 5-2, arrows), and the shell contained particles with diameters of about 250 nm. This inner particle appears to be composed of partially decomposed latex particles. After 70 min, this inner particle was not observed anymore, and the morphology of the interconnected spherical shell remained unchanged (Figure 5-3). Clearly, the inner organic component was totally decomposed at this stage, leaving only the titania shell behind. The TEM observation indicates the thickness of the shell wall to be less than 10

nm. The QCM data suggested that the thickness of the titania shell on the latex particles was close to 1 nm. However, this figure is an estimate from an overall frequency change and may not give a precise thickness. C. Removal of the Latex Template by Calcination. Calcination is most frequently employed for the removal of template particles in the general template synthesis. A latex particle can be burned and removed at temperatures over 400 °C. Thus, the five-cycle sample possessing the thinnest titania shell of a few nanometers was placed in a calcination oven. The temperature was raised to 450 °C within 25 min and was held for 4 h to remove the template. SEM observation of the calcined sample indicated the presence of flattened disks (Figure 6a), instead of hollow shells, in the oxygen plasma-treated sample. The diameter of the disk is about 500 nm, in close agreement with the original diameter of the latex particle. A titania shell surrounding the template surface appears to have collapsed as a result of the rapid removal of the calcined organic moiety. In hopes to obtain spherical hollow structures by calcination, we employed a slower temperature rise with a thicker titania shell. The surface solgel process was repeated for 10 and 20 cycles (sample-10 and sample-20, respectively), and both samples were heated in the oven to 450 °C more slowly (within 150 min) and kept at this temperature for 4 h. Again, the spherical shell was not observed (Figure 6b,c). The thick edge of the collapsed titania shell is seen as rings in Figure 6b, and the thicker layer in Figure 6c clearly shows overlapped layers upon collapse. In a comparison experiment, a titania hollow structure was first prepared by oxygen plasma


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Figure 7. SEM images of the titania-coated multilayers of latex particles. (1) Schematic representation of the sampling area. The series of parts 2-4 and 5-7 were obtained before and after the oxygen plasma process, respectively. The images in parts 2 and 5 are obtained at the edge of the cast area (area 1). The images in parts 3 and 6 are obtained around the intermediate cast area (area 2). The images in parts 4 and 7 are obtained in the center of cast area (area 3).

treatment and was then calcined with a temperature rise from room temperature to 450 °C in 150 min and aging at this temperature for 4 h. SEM observation proved that the spherical shell was preserved even after calcination in this case (data not shown). In short, once a hollow structure is formed, it is not destroyed under the calcination conditions. Therefore, it is suspected that rapid combustion (bursting) of the organic latex breaks the stillflexible titania shell in the calcination process. The more gradual oxidative degradation of the latex will not destroy the shell wall. Interestingly, there are earlier reports in which spherical structures of metal oxides are retained after calcination. For example, Imhof reported that an inner polystyrene particle covered with titania films was successfully removed by calcination without collapse of the titania shell.11 It should be noted that the thickness of the titania shell in this case is several tens of nanometers, in contrast to a few nanometers of thickness in our experiments. Apparently, the mechanical strength of the titania shell in a few nanometers of thickness is not sufficient to maintain the structure in the calcination process. A much thicker titania shell would resist collapse in the rapid combustion process. In contrast, oxygen plasma can remove the inner template particle without destroying the titania hollow shell with thicknesses as small as a few nanometers. D. Replication of the Multilayers of Close-Packed Latex Particles. In the preceding experiments, the titania replica of the latex particle on a solid substrate is described. It is possible to extend this approach from surface-assembled single-layer architectures to multilayer assemblies. As a first example, we employed multilayered particles of C-500 latex on the solid substrates. A dispersion of the latex particle was cast on a silicon wafer and (11) Imhof, A. Langmuir 2001, 17, 3579.

allowed to dry at room temperature. Because there is no attractive interaction between the silicon wafer and the latex particle, the specimen was manipulated very carefully during the surface sol-gel process to avoid disturbance of the particle array. The surface sol-gel process was repeated for five cycles. Three representative areas of the titania-covered cast film were subjected to SEM observation. They are the center area of the film, film edge, and intermediate area (Figure 7-1). According to SEM images of Figure 7, mono- and multilayers of particles were formed in the whole substrate area of the cast film. Multilayers of the close-packed particles were mainly observed around the edge of the cast film (Figure 7-2). The number of layers decreased as it approached the film center, and the particle existed as packed monolayers or in isolation near the film center (Figure 7-4). In the intermediate area, the number of layers included from one to several (Figure 7-3). The surface of the particle is invariably smooth after the surface sol-gel reaction. In all cases, the particle sizes are indistinguishable from those of the original uncoated particles. Such layer structures of the particle were not essentially altered upon the subsequent oxygen plasma processing (power 20 W, 1 h). In addition, all particles became shrunken at the original position, and their diameters decreased from 500 to about 300 nm. The individual particles are connected to each other through a tubelike structure of several tens of nanometers in width (Parts 5-7 of Figure 7, indicated by open triangles). The side view of the particle array in area 1 (Figure 7-5, area inside the broken circle) has a morphology similar to that of the top view of the intermediate layer. The second layer is observed under the top layer in area 2 (Figure 7-6, arrows), and it has a pattern of connection essentially identical to those of the top layer. It is inferred from these results that the internal hollow shells in the multilayer are intercon-

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Figure 8. Electron micrographs of the TMV-templated titania structures. The images in parts 1-3 and 4-5 were obtained by SEM and TEM, repectively, (1) after adsorption of the TMV, (2 and 4) after five cycles of the surface sol-gel process, and (3 and 5) after exposure to oxygen plasma.

nected three-dimensionally with each other via the wire structure. It is possible that the latex component in the underlayer is not completely removed under the plasma condition used. Even if that is the case, prolonged plasma treatment would remove all the organic components in the multilayer. E. Preparation of Nanotubes from the TMV Superstructure as the Template. Single and multilayers of spherical particles were shown to be effective as the template in the preceding experiments. This approach will be extended to other types of the nanosized morphology. TMV is a supramolecular assembly composed of thousands of coated proteins and a single RNA chain. These coated proteins are helically organized along the RNA axis to form a tubular structure with a diameter of 18 nm. We used TMV templates that are randomly adsorbed on a substrate. The surface sol-gel process was repeated five times on the TMV adsorbed on a silicon wafer on which alternate layers of PDDA and PSS had been adsorbed. The surface-coated TMV was then exposed to oxygen plasma to remove the organic moiety. Parts 1-3 of Figure 8 show SEM images of the TMV at each step. Prior to the surface sol-gel process, it is seen that individual TMVs of several tens of nanometers to 1 µm in length and about 20 nm in width are scattered on the whole substrate surface. Multilayered TMVs were not observed under the conditions used. Upon the surface solgel reaction, the spherical particulates with sizes of several nanometers (Figure 8-2, inset, arrows) are seen in addition to the original TMV. They appear to be titania particles. The rodlike structures were observed (Figure 8-3, arrows) after the oxygen plasma treatment. The diameter of the rod is about 20 nm. These rodlike structures are not very smooth and appear to be composed of linearly-connected particles that are similar to the scattered particle in size.

To elucidate the structural details, we conducted TEM observation on these samples, as is shown in Parts 4 and 5 of Figure 8. Because staining was not applied, the TMV itself cannot be observed, and the titania moiety is seen as shadowy figures. Tubular structures with outer diameters of 20-30 nm were formed before the oxygen plasma process. In addition, there are clearly seen inner channels with widths of several nanometers (Figure 8-4, arrows). The TMV itself has a channel with an inner diameter of several nanometers. It is certain that the TMV structure is replicated as a titania tube. These inner channels become observable more clearly after oxygen plasma processing, with a diameter of about 10 nm (Figure 8-5, arrows). In a preceding experiment, the template latex with a 500-nm diameter was removed by treating 20 W of oxygen plasma for 70 min. The TMV is much smaller, and removal of the organic moiety should be complete under these experimental conditions (for 1 h at 10 W). The outer diameter of the tubular structure is about 2030 nm and is not affected by the plasma treatment. The thickness of the titania layer is estimated to be several nanometers and is much thicker than the titania wall of the latex particle. Shrinking of the titania shell was, therefore, not significant in the case of the TMV. Discussion Robustness and Plasticity of the Ultrathin Titania Shell. The thickness of titania shell that surrounds the latex particles and TMV is estimated to be a few nanometers (less than 10 nm) from the QCM and TEM data. It is remarkable that such ultrathin shells maintain their morphologies after oxygen plasma treatment. They have a mechanical strength sufficient to maintain hollow spherical and tubular morphologies. An additional feature of the titania film is its plasticity. In oxygen plasma


treatment, the individual titania shell shrunk at the position where the latex particle was originally situated. The titania layer near the contact area of the neighboring shells was not broken during the shrinkage. Instead, the separated shells became connected by titania tubes. This is produced as a result of elongation of the interconnecting titania layers. A similar situation is found in the case of multilayers of the latex particles, giving three-dimensionally interconnected titania capsules. It is clear that the titania layer is deformed in the elongation process without the destruction of the layer morphology. This unique plasticity and resilience of the ultrathin metal oxide film have not been noted before, to the best of our knowledge. This film robustness implies that we can construct selfsupporting three-dimensional architectures with a few nanometers of precision on the surface. Metal oxides appear to be unique in this respect because other materials such as metals and polymers would not produce robust, self-supporting films with this ultimate thickness. The biological system provides ultrathin films by the ordered assembly of lipid molecules. The thickness of the lipid bilayer is known to be 5-7 nm and is in the same range of thickness as that of the present titania film. The lipid bilayer membrane displays many unique features that arise from its molecular organization and ultimate thickness. Properties of the lipid bilayer that are closely associated with its molecular organization may not be reproduced by the titania layer because the latter is made of a dense network of metal oxides. However, the thicknessdependent properties of the lipid bilayer may be reproduced by the metal oxide layer of a similar thickness. We will pursue this exciting possibility in future research. Adaptability to the Sizes and Shapes of the Templates. The surface sol-gel process has been successfully applied to covering the surface of the latex particles with the diameter of 500 nm and of the cylindershaped virus with the diameter of about 20 nm. The morphologies of these templates are closely reflected in the resulting hybrid structures because the thickness of the titania shell is very small. Thus, the precision of the replication of the template morphologies is outstanding. The titania layer can replicate the molecular details of the template. In fact, this possibility has been implied in our previous studies on the wrapping of a polymer chain with a silicate layer12 and the molecular imprinting of organic molecules in titania films.13 In polymer wrapping, oligomeric sodium silicate is bound to a positively charged polymer, and the subsequent condensation leads to a silicate-wrapped polymer chain. In the case of molecular imprinting, titania films can maintain cavities due to imprinted guest molecules after the removal of the guests, and the cavities can selectively recognize the respective imprinted molecules. We also succeeded in the chiral recognition of amino acid derivatives by a similar approach.13a These results indicate that the metal oxide network can accurately trace the shapes of the template molecules, and the cavities created after the removal of the template maintain shapes and functionalities complementary to the template with molecular precision. Such precise replication would be achieved for the much larger templates employed in the present study, although the molecular detail could not be examined. This wide range (12) Ichinose, I.; Kunitake, T. Adv. Mater. 2002, 14, 344. (13) (a) Lee, S.-W.; Kunitake, T. Mol. Cryst. Liq. Cryst. Sci. Technol., Sect. A 2001, 371. (b) Lee, S.-W.; Ichinose, I.; Kunitake, T. Chem. Lett. 1998, 12, 1193. (c) Lee, S.-W.; Ichinose, I.; Kunitake, T. Langmuir 1998, 14, 2857.

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of adaptability is a great advantage of metal oxide ultrathin films as building blocks for constructing nanostructures. Oxygen Plasma Treatment versus Calcination. The oxygen plasma treatment can remove the organic template without the collapse of the titania layer that is attached to the template surface, despite the thickness of titania films being only a few nanometers. The ultrathin titania layer thus formed is self-supporting in the micrometer to nanometer dimension. In contrast, calcination in air could not keep the hollow structure of the titania shell, and the spherical shell collapsed into the disklike plate. According to a previous report,11 thicker metal oxide films are seemingly required to maintain their threedimensional hollow structures. It is interesting to note that the plasma-treated hollow structure, once formed, did not collapse under the calcination conditions. The collapse proceeded during direct calcination, but not after plasma treatment. Oxygen plasma decomposes the organic components rather gradually, whereas rapid combustion may proceed in the calcination process. The gradual removal of gaseous products will not cause destruction of the titania shell because the plasma treatment concomitantly promotes the condensation of unreacted titanium alkoxides to enhance the mechanical strength of the titania film. On the other hand, the thermal decomposition of organic moieties in the calcination process proceeds first gradually and then reaches an ignition point. The oxygenated gases will burst out of the titania shell, thus destroying spherical titania shells. We conclude from these considerations that the oxygen plasma treatment is highly effective for removing the organic template without losing ultrathin (a few nanometers) shell structures. Surface Covering against Void Filling. In our approach, ultrathin titania films are formed on the surfaces of the latex particles through the surface sol-gel process (Figure 9a-1). Titanium alkoxide can react with the surface functional groups such as the carboxyl and hydroxyl groups. The active functional groups are regenerated by hydrolysis and the condensation reaction of surface alkoxides for the subsequent surface sol-gel process.7,14 The latex particles that we employed here have carboxyl groups on their surfaces, and the TMV has many charged amino acid residues, such as glutamate and aspartate.15 These functional groups undergo a facile reaction with titanium alkoxides, and as-prepared titania films can precisely trace the outer shape of the template as a result of chemical bonding with the surface functional group of the template. The thickness of the titania layer can be adjusted by the number of surface sol-gel cycles. The as-prepared titania shell can reproduce the original shape of the template after its removal (Figure 9a-3). These unique features contrast with the preparation of inverse opals in which the monomers are infiltrated into the voids of the layered nanoparticles (Figure 9b-1) and then polymerized to form a solid support of the pore structure (Figure 9b-2). The interparticle void is indispensable as the monomer reservoir, and after the monomer is polymerized in the void, the inner template is removed to form the pore structure. The shape of the solid support obtained by void filling is a negative copy of the original template and is called an inverse opal (Figure 9b-3). The inter(14) (a) Ichinose, I.; Kawakami, T.; Kunitake, T. Adv. Mater. 1998, 10, 535. (b) Huang, J.; Ichinose, I.; Kunitake, T.; Nakao, A. Langmuir 2002, 18, 9048. (15) Namba, K.; Stubbs, G. Acta Crystallogr., Sect. A 1985, 41, 252.

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Figure 9. Schematic representations of the formation of the interconnected hollow structures and inverse opals.

particle void is not essential in our approach. Because the titania layer is uniform and only a few nanometers thick, the current process gives a positive copy of the template morphology. Surface Fabrication. It has been concluded from the above discussion that the titania layer obtained by the surface sol-gel process has unique properties as the ultrathin film. First, it is robust and plastic with a thickness of less than 10 nm. As a consequence, it is adaptable to different sizes and varied shapes of templates by covering the template surface with nanometers precision and without measurable defects. The use of oxygen plasma was essential for maintaining hollow structures with the ultrathin layer. These outstanding features make the titania layer an attractive tool for the template-based construction of complex three-dimensional architectures on the surface. The three-dimensional submicron architecture on the surface may be produced by physical methods such as lithography, microcontact printing and micromolding, as was mentioned in the Introduction. Chemical methods for this purpose include molecular assembly and template synthesis. The template synthesis may provide architectures that are either positive copies or negative copies of the template structure. When a selfsupporting ultrathin layer is employed for the positive copy, a richer variety of submicron architectures will be formed because the hollow spaces of definite morphologies are created within the architecture. Recently, Ferry and Okuyama reported the formation of metallic domes by

colloidal templating on substrates and over-sputtering.16 Although this approach produced a positive copy of the particles, sputtered metal could not cover the whole surface uniformly, and the dome structure (semisphere) resulted, instead of complete spherical structures. In contrast, the surface sol-gel process based on the chemisorptions of substrates onto templates produced films nanometers in thickness with a mechanical strength sufficient to maintain three-dimensional structures. Therefore, the surface fabrication based on the surface sol-gel process is much more promising as a means of constructing more complex micro- and nanoarchitectures. A more flexible design of the template morphology is strongly desired. In conclusion, it is clear that the current approach offers a great potential for surface fabrication in the nanometer regime. Acknowledgment. We thank Professor Y. Takanami (Department of Applied Genetics and Pest Management, Faculty of Agriculture, Kyushu University, Japan) for providing the TMV. We also appreciate the contributions of Professor N. Kimizuka and Dr. T. Kawasaki (Department of Chemistry and Biochemistry, Graduate School of Engineering, Kyushu University, Japan) to the virus multiplication and its purification. LA026979E (16) Ferry, M.; Okuyama, K. Adv. Mater. 2002, 14, 930.

Surface Fabrication of Hollow Nanoarchitectures of Ultrathin Titania

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