Strict In-Plane Alignment Control of Block-Copolymer-Templated

Letter pubs.acs.org/Langmuir

Strict In-Plane Alignment Control of Block-Copolymer-Templated Mesostructured Silica Films Using an Alignment-Controlling Agent Wataru Kubo,*,† Masahiko Takahashi, Kohei Okamoto,‡ Shin Kitamura, and Hirokatsu Miyata Frontier Research Center, Canon Inc., 3-30-2, Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan S Supporting Information *

ABSTRACT: An alkoxysilane with an alkyl chain is introduced as an alignment-controlling agent of a blockcopolymer-templated mesostructured silica film. Use of the alkylalkoxysilane achieves the alignment of the mesochannels of a triblock-copolymer-templated film by an intermolecular interaction with a rubbing-treated polyimide film. Co-use of an alkoxysilane with a hydroxymethyl group as a hydrophobicity reducing agent improves the alignment close to that of the film prepared using an alkyl surfactant. This concept widens the range of the structural period of aligned mesoporous films and thus widens the useful range of the anisotropic optical properties.

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period of the mesostructured silica film (MSF) with a 2D-Hex structure, a triblock copolymer, poly(ethylene oxide)-b-poly(propylene oxide)-b-poly(ethylene oxide) (PEO-PPO-PEO) with various molecular weights, is well known as an excellent template.25,26 However, the copolymer, which has no alkyl chain in the molecule, only weakly interacts with the polyimide and thus cannot form the aligned structure. Considering the function of the alkyl surfactant, it plays two roles: mesostructure formation and induction of alignment. In the former role, the surfactant molecules are assembled to form a micelle in an aqueous solution and thus work as a template for the mesostructure. In the latter role, the surfactant molecules interact with the polyimide and induce the mesostructure to align in one direction. Here, we try to transfer the latter role to the other component forming the film, a silica precursor, to address this issue. Specifically, ndodecyltriethoxysilane (C12TES) is used as an alignmentcontrolling agent to provide the film using PEO-PPO-PEO with the alignment.

INTRODUCTION Film is an advantageous form of mesoporous materials with respect to applying their attractive features to optical and electronic devices.1−3 Among mesoporous films, a film with a structure consisting of packed tubular oxide walls aligned all over a substrate exhibits useful anisotropic optical properties such as optical birefringence4 and polarized emission.5 These properties are important for optical devices such as phase plates and displays. Several groups have developed methods to control the alignment.2,6−21 Among them, an evaporation-induced selfassembly process using a combination of a rubbing-treated polyimide and an alkyl surfactant can provide the film with a strictly aligned structure.20 The mechanism of the alignment is summarized as follows. An alkyl surfactant, a template of the mesostructure, is aligned along the rubbing direction by the hydrophobic interaction and van der Waals force with the methylene chains of the polyimide molecules because these are aligned by the rubbing treatment. A semicircular columnshaped micelle is formed on the first aligned surfactant molecule layer, and thus is aligned perpendicular to the rubbing direction. The micelles of a 2D hexagonal (2D-Hex) structure are arranged parallel to the aligned micelles and then fixed with the progress of the condensation of the silica species.20 To use the anisotropic optical and electronic properties of the film, such as polarized emission5 and anisotropic charge transport,22 control of the structural period (pore size) is important.23 The combination of the alkyl surfactant and the rubbing-treated polyimide can control the structural period of the aligned film precisely in the smaller region (d01 < 6 nm) by changing the alkyl chain length of the surfactant.24 However, in the larger region (d01 > 6 nm) this does not work because of the surfactant’s solubility problems. To control the structural © 2013 American Chemical Society

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EXPERIMENTAL SECTION

MSFs are prepared by spin coating (5000 rpm, 10 s) a precursor solution onto a rubbing-treated polyimide on a silicon substrate.20 The precursor solution is prepared as follows. Alkoxysilanes (tetraethoxysilane (TEOS), C 12 TES, and hydroxymethyltriethoxysilane (HMTES)), ethanol, and hydrochloric acid are mixed for 15 min, and then an ethanolic solution of PEO-PPO-PEO (trade name Pluronic P-123) is added to the mixture. The solution is aged for 1.5 h after the addition of the copolymer. A typical composition of the precursor solution without HMTES is 0.7/0.3/5.2/1.1 × 10−3/6.0/9.6 Received: April 1, 2013 Revised: June 6, 2013 Published: June 18, 2013 8193

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× 10−3/3.5 (molar ratio) TEOS/C12TES/EtOH/HCl/H2O/P-123/ EtOH. HMTES is used as a hydrophobicity reducing agent of the silica species. A typical composition of the precursor solution with HMTES is 0.7/0.3/0.1/5.2/1.1 × 10−3/6.0/1.5 × 10−2/3.5 (molar ratio) TEOS/C12TES/HMTES/EtOH/HCl/H2O/P-123/EtOH, and the aging time is 3 h. After storage under 25 °C at 40% RH for a few days, an MSF is treated with TEOS (1 mL) in a sealed perfluoroalkoxy resin jar (180 mL) at 135 °C for 3 h and then taken out before the jar is cooled. The film is calcined at 350 °C for 1 h (rate 2 °C min−1) in air to remove the template. X-ray diffraction (XRD) patterns and SEM images are obtained using previously reported instruments.24

Scheme 1. Conceptual Drawing of the Aligned Mesostructure Formed in the Vicinity of the Substrate

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in the rubbing direction as in the alkyl surfactant case. The alkyl chain is considered to be extended toward the hydrophobic part of a micelle because of its high hydrophobicity. As a result, a semicircular column-shaped micelle is formed on the substrate perpendicular to the rubbing direction. Micelles, forming a 2DHex structure, are arranged on the aligned micelles and then fixed with the progress of the condensation of the siliceous species. The use of alkylalkoxysilane with a longer alkyl chain is advantageous to increasing the interaction with the substrate while unfavorable to suppressing the formation of aggregation in an alcohol-based solution. The film using C12TES shows a higher degree of the alignment than these using noctyltriethoxysilane or n-octadecyltriethoxysilane probably because of the balance of the two effects described above. The structural period of the MSF using C12TES (d01 = 8.5 nm) is comparable to that prepared without C12TES. This confirms that the transfer of the alignment function from a surfactant to a Si source widens the range of the structural period of uniaxially aligned MSFs. The inset of Figure 1c shows the full width at half-maximum (fwhm) of the peak of the rocking curve of MSF prepared with various C12TES/total Si ratios. The MSF prepared with the ratio 0.3 gives the highest degree of alignment. The alignment cannot be confirmed when the ratio is lower than 0.1 and higher than 0.5. If the ratio is lower than 0.1, then the number of silica species having an alkyl chain, which interacts with the polyimide, is not enough to induce the alignment with the film. These results indicate that C12TES induces the alignment and prove the concept of the alignment-controlling agent. In the case of a ratio higher than 0.5, we propose two reasons for the failure to form the aligned structure. One is the collapse of the mesostructure caused by a weakened silica network. Because C12TES has a terminal alkyl group, the increase in the ratio suppresses the development of the silica network. The other reason is an ill-defined mesostructure caused by the increase in the hydrophobicity of the silica species. The increase of the ratio decreases the hydrophilic−hydrophobic contrast between the hydrophilic part of the micelle (consisting of the hydrophilic domain of a surfactant and siliceous species) and the hydrophobic part of the micelle (consisting of the hydrophobic group of a surfactant) and blurs the border between the hydrophilic and hydrophobic parts of the micelles. The lowest peak fwhm, corresponding to the highest degree of alignment, is far wider than that prepared using an alkyl surfactant.20 This indicates that the alignment of the mesochannels of the MSF using a triblock copolymer is achieved but the degree of alignment is inferior to the MSF using alkyl surfactants. We considered that this inferior alignment is caused by the low hydrophilic−hydrophobic contrast described as the second reason in the above discussion. Therefore, we introduced an alkoxysilane having a hydroxyl group that does not react with a

RESULTS AND DISCUSSION Two-dimensional XRD patterns of an MSF using C12TES obtained with X-rays in parallel (ϕ = 0°) and perpendicular (ϕ = 90°) incidence geometry are shown in Figure 1a,b,

Figure 1. XRD profiles of an MSF prepared using C12TES ([C12TES]/ [total Si] = 0.3). Two-dimensional XRD pattern of the MSF obtained with X-rays in (a) parallel (ϕ = 0°) and (b) perpendicular (ϕ = 90°) incidence geometries. The area surrounded by dotted lines is the shadow of a beam stopper. (c) In-plane rocking curve profile. (The rubbing direction is indicated by an arrow, marked as R.) (Inset) Peak fwhm of the MSF using three different [C12TES]/[total Si] ratios.

respectively. The pattern obtained with the perpendicular incidence (Figure 1b) shows three diffraction spots, (01), (10), and (1̅1), which are characteristic of a 2D-Hex structure, whereas only the (01) spot, which is attributed to a plane parallel to the substrate, is found in the pattern obtained with the parallel incidence (Figure 1a). The observed anisotropy in the 2D XRD profiles suggests the alignment of mesochannels in the plane of the film. The in-plane rocking curve (ϕ scanning, Figure 1c) profiles at the (10) and (11̅ ) positions confirms the alignment of mesochannels perpendicular to the rubbing direction all over the substrate. The alignment direction is the same as the MSF using the alkyl surfactant.20,24 This consistency suggests that the alignment mechanism of the MSF using C12TES is similar to that using the alkyl surfactant. A conceptual drawing of the aligned mesostructure formed in the vicinity of the substrate of the MSF using C12TES is shown in Scheme 1.In this case, an alkyl chain of C12TES interacts with the methylene chains of the rubbing-treated polyimide on the substrate by the hydrophobic interaction and van der Waals force and is aligned 8194

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silanol group, HMTES, as an additional Si source to reduce the hydrophobicity of the silica species. Two-dimensional XRD patterns of a MSF using HMTES obtained with X-rays in parallel and perpendicular incidence geometries are shown in Figure 2a,b, respectively. Similar to

Figure 3. Cross-sectional SEM image of an MSF prepared using C12TES and HMTES. The MSF is pretreated with TEOS vapor at 135 °C for 4 h prior to the observation. Cross-sections (a) parallel (ϕ = 0°) and (b) perpendicular (ϕ = 90°) to the rubbing direction.

template by calcination. Two peaks at ϕ = ±90° in the rocking curve profile having 9.3° fwhm (Figure S1) and anisotropic cross-sectional SEM images (Figure S2) indicate the lowering of the structural regularity by calcination but confirm that the film retains the aligned mesostructure even after calcination. The IR spectrum of the calcined film reveals that almost all dodecyl groups are lost from the film during the calcination process.

Figure 2. XRD profiles of an MSF prepared using C12TES and HMTES. Two-dimensional XRD pattern of the MSF obtained with Xrays in (a) parallel (ϕ = 0°) and (b) perpendicular (ϕ = 90°) incidence geometries. The area surrounded by dotted lines is the shadow of a beam stopper. (c) In-plane rocking curve profile. (The rubbing direction is indicated by an arrow, marked as R.)

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CONCLUSIONS The introduction of the alignment-controlling agent achieves the alignment of mesochannels of the block-copolymertemplated mesostructured silica film. A uniaxially aligned mesostructured silica film with a larger structural period is obtained using the combination of C12TES and PEO-PPOPEO triblock copolymer. HMTES improves the hydrophilic− hydrophobic contrast between the template and silica species and thus improves the structural regularity and the degree of alignment of the film. The improvement in the alignment is confirmed by the narrow peaks in the rocking curve profile, almost as narrow as those of the film prepared with the alkyl surfactant. This concept widens the range of the structural period of aligned mesoporous films and thus contributes to a wider range of applications.

Figure 1, the pattern obtained with perpendicular incidence (Figure 2b) shows three diffraction spots whereas only one spot is found in the pattern obtained with parallel incidence (Figure 2a). The stronger and smaller spots observed for the MSF prepared with HMTES compared to those prepared without HMTES suggest that the structural regularity of the MSF is improved by HMTES. The 4.8 times larger peak intensity of the rocking curve profile (Figure 2c) with HMTES confirms the above explanation. The peak fwhm of the MSF prepared with HMTES (6.3°) is one-fourth that without HMTES and close to that prepared with the alkyl surfactant. The structural period of the MSF prepared with HMTES (d01 = 9.5 nm) is larger than that without HMTES. This could be explained by the improved hydrophilic−hydrophobic contrast, which should extend the center-to-center distance of the cylinders. The coexistence of HMTES is advantageous to suppressing the formation of the aggregation in the precursor solution. This could be the reason that the optimized aging time for the solution with HMTES is longer than that without it. Cross-sectional images of the MSF prepared using C12TES and HMTES are shown in Figure 3. The film is treated with TEOS vapor prior to the observation to improve the structural stability. Cross-sectional images, parallel (Figure 3a) and perpendicular (Figure 3b) to the rubbing direction, show the radial and longitudinal sections of the cylindrical structure, respectively, and confirm the alignment of the mesochannels perpendicular to the rubbing direction. It is difficult to obtain the SEM images of the fine structure of the film without HMTES, probably because of its low stability. With the TEOS vapor treatment, the MSF prepared using C12TES and HMTES retains the aligned mesostructure after the removal of the

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ASSOCIATED CONTENT

S Supporting Information *

An in-plane rocking curve profile and a cross-sectional SEM image of the calcined film prepared with C12TES and HMTES and a detailed explanation of the XRD experiment. This material is available free of charge via the Internet at http:// pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Tel: +81-3-3758-2111. Fax: +81-3-3757-3047. E-mail: kubo. [email protected]. Present Addresses †

Nanomaterials R&D Center, Canon Inc., 3-30-2, Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan. ‡ Optics R&D Center, Canon Inc., 3-30-2, Shimomaruko, Ohtaku, Tokyo 146-8501, Japan. 8195

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Notes

Substrates with Designed Conical Holes. J. Am. Chem. Soc. 2008, 130, 10165−10170. (18) Farrell, R. A.; Petkov, N.; Morris, M. A.; Holmes, J. D. SelfAssembled Templates for the Generation of Arrays of 1-Dimensional Nanostructures: from Molecules to Devices. J. Colloid Interface Sci. 2010, 349, 449−472. (19) Su, B.; Lu, X.; Lu, Q. A Facile Method to Prepare Macroscopically Oriented Mesostructured Silica Film: Controlling the Orientation of Mesochannels in Multilayer Films by Air Flow. J. Am. Chem. Soc. 2008, 130, 14356−14357. (20) Miyata, H.; Kawashima, Y.; Itoh, M.; Watanabe, M. Preparation of a Mesoporous Silica Film with a Strictly Aligned Porous Structure through a Sol−Gel Process. Chem. Mater. 2005, 17, 5323−5327. (21) Radhakrishnan, L.; Wang, H.; Yamauchi, Y. Precise Manipulation of One-Dimensional Mesochannel Alignments in Mesoporous Silica Films by Novel Rubbing Method Utilizing Lyotropic Liquid Crystals. Chem. Asian J 2010, 5, 1290−1293. (22) Rauda, I. E.; Sentera, R.; Tolbert, S. H. Directing Anisotropic Charge Transport of Layered Organic−Inorganic Hybrid Perovskite Semiconductors in Porous Templates. J. Mater. Chem. C 2013, 1, 1423−1427. (23) Cadby, A. J.; Tolbert, S. H. Controlling Optical Properties and Interchain Interactions in Semiconducting Polymers by Encapsulation in Periodic Nanoporous Silicas with Different Pore Sizes. J. Phys. Chem. B 2005, 109, 17879−17886. (24) Komoto, A.; Kubo, W.; Miyata, H. Control of Periodicity in Mesoporous Silica Films with Aligned Mesochannels Prepared by a Sol−Gel Process. Chem. Lett. 2012, 41, 741−743. (25) Wan, Y.; Zhao, D. On the Controllable Soft-Templating Approach to Mesoporous Silicates. Chem. Rev. 2007, 107, 2821−2860. (26) Zhao, D.; Feng, J.; Huo, Q.; Melosh, N.; Fredrickson, G. H.; Chmelka, B. F.; Stucky, G. D. Triblock Copolymer Syntheses of Mesoporous Silica with Periodic 50 to 300 Angstrom Pores. Science 1998, 279, 548−552.

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We are grateful to Dr. Kazunori Fukuda for help with the X-ray measurements and Dr. Otto Albrecht for reviewing the manuscript.

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REFERENCES

(1) Ogawa, M. Formation of Novel Oriented Transparent Films of Layered Silica-Surfactant Nanocomposites. J. Am. Chem. Soc. 1994, 116, 7941−7942. (2) Yang, H.; Kuperman, A.; Coombs, N.; Mamiche-Afara, S.; Ozin, G. A. Synthesis of Oriented Films of Mesoporous Silica on Mica. Nature 1996, 379, 703−705. (3) Lu, Y.; Ganguli, R.; Drewien, C. A.; Anderson, M. T.; Brinker, C. J.; Gong, W.; Guo, Y.; Soyez, H.; Dunn, B.; Huang, M. H.; Zink, J. I. Continuous Formation of Supported Cubic and Hexagonal Mesoporous Films by Sol−Gel Dip-Coating. Nature 1997, 389, 364−368. (4) Miyata, H.; Fukushima, Y.; Okamoto, K.; Takahashi, M.; Watanabe, M.; Kubo, W.; Komoto, A.; Kitamura, S.; Kanno, Y.; Kuroda, K. Remarkable Birefringence in a TiO2−SiO2 Composite Film with an Aligned Mesoporous Structure. J. Am. Chem. Soc. 2011, 133, 13539−13544. (5) Martini, I. B.; Craig, I. M.; Molenkamp, W. C.; Miyata, H.; Tolbert, S. H.; Schwartz, B. J. Controlling Optical Gain in Semiconducting Polymers with Nanoscale Chain Positioning and Alignment. Nat. Nanotechnol. 2007, 2, 647−652. (6) Aksay, I. A.; Trau, M.; Manne, S.; Honma, I.; Yao, N.; Zhou, L.; Fenter, P.; Eisenberger, P. M.; Gruner, S. M. Biomimetic Pathways for Assembling Inorganic Thin Films. Science 1996, 273, 892−898. (7) Hillhouse, H.; Okubo, T.; van Egmond, J. W.; Tsapatsis, M. Preparation of Supported Mesoporous Silica Layers in a Continuous Flow Cell. Chem. Mater. 1997, 9, 1505−1507. (8) Trau, M.; Yao, N.; Kim, E.; Xia, Y.; Whitesides, G. M.; Aksay, I. A. Microscopic Patterning of Orientated Mesoscopic Silica through Guided Growth. Nature 1997, 390, 674−676. (9) Kawashima, Y.; Nakagawa, M.; Seki, T.; Ichimura, K. PhotoOrientation of Mesostructured Silica via Hierarchical Multiple Transfer. Chem. Mater. 2002, 14, 2842−2844. (10) Yamauchi, Y.; Sawada, M.; Noma, T.; Ito, H.; Furumi, S.; Sakka, Y.; Kuroda, K. Orientation of Mesochannels in Continuous Mesoporous Silica Films by a High Magnetic Field. J. Mater. Chem. 2005, 15, 1137−1140. (11) Yamauchi, Y.; Sawada, M.; Komatsu, M.; Sugiyama, A.; Osaka, T.; Hirota, N.; Sakka, Y.; Kuroda, K. Magnetically Induced Orientation of Mesochannels in Mesoporous Silica Films at 30 T. Chem. Asian J. 2007, 2, 1505−1512. (12) Wu, C.-W.; Ohsuna, T.; Edura, T.; Kuroda, K. Orientational Control of Hexagonally Packed Silica Mesochannels in Lithographically Designed Confined Nanospaces. Angew. Chem., Int. Ed. 2007, 46, 5364−5368. (13) Bolger, C. T.; Farrell, R. A.; Hughes, C. M.; Morris, M. A.; Petkov, N.; Holmes, J. D. Pore Directionality and Correlation Lengths of Mesoporous Silica Channels Aligned by Physical Epitaxy. ACS Nano 2009, 3, 2311−2319. (14) Walcarius, A.; Sibottier, E.; Etienne, M.; Ghanbaja, J. Electrochemically Assisted Self-Assembly of Mesoporous Silica Thin Films. Nat. Mater. 2007, 6, 602−608. (15) Richman, E. K.; Brezensinski, T.; Tolbert, S. H. Vertically Oriented Hexagonal Mesoporous Films Formed through NanometreScale Epitaxy. Nat. Mater. 2008, 7, 712−717. (16) Yamauchi, Y.; Suzuki, N.; Kimura, T. Formation of Mesoporous Oxide Fibers in Polycarbonate Confined Spaces. Chem. Commun. 2009, 5689−5691. (17) Yamauchi, Y.; Nagaura, T.; Ishikawa, A.; Chikyow, T.; Inoue, S. Evolution of Standing Mesochannels on Porous Anodic Alumina 8196

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