Heteroepitaxial Formation of Aligned Mesostructured Silica Films with

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Heteroepitaxial Formation of Aligned Mesostructured Silica Films with Large Structural Periodicities From Mixed Surfactant Systems Hirokatsu Miyata, Saeko Hayase, Yosuke Kanno, Masatoshi Watanabe, Masahiko Takahashi, and Kazuyuki Kuroda Langmuir, Just Accepted Manuscript • DOI: 10.1021/la401342q • Publication Date (Web): 13 May 2013 Downloaded from http://pubs.acs.org on May 19, 2013

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Heteroepitaxial

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Formation

Mesostructured

Silica

Films

of with

Aligned Large

Structural Periodicities from Mixed Surfactant Systems Saeko Hayase,1 Yosuke Kanno,1 Masatoshi Watanabe,2 Masahiko Takahashi,3 Kazuyuki Kuroda1, 4* and Hirokatsu Miyata,3*

1. Department of Applied Chemistry, Waseda University, 3-4-1 Ohkubo, Shinjuku-ku, Tokyo 169-8555, Japan 2. Nanomaterials R&D Center, Canon Inc., 3-30-2 Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan 3. Frontier Research Center, Canon Inc., 3-30-2 Shimomaruko, Ohta-ku, Tokyo 146-8501, Japan 4. Kagami Memorial Research Institute for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan

E-mail: [email protected], [email protected]

1 ACS Paragon Plus Environment

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Abstract Liquid crystal phases consisting of cylindrical micelles of amphiphilic block copolymers and silica precursors are epitaxially built up on aligned surface micelles formed by an alkyl-PEO surfactant, Brij56, irrespective of the large difference in the intrinsic structural periodicities, resulting in the formation of fully aligned mesostructured silica films with large lattice constants.

Brij56 works as an "alignment

controlling agent" on rubbing-treated polyimide through selective adsorption from a precursor solution containing the two surfactants, a block copolymer and Brij56, through strong hydrophobic interactions to form an anisotropic surface micelle structure. Aligned mesostructured silica layers with larger periodicities, which dominantly consist of block copolymers, form on these aligned surface micelles by gradually changing the vertical periodicity keeping the lateral intermicelle distance constant. This can be regarded as a kind of heteroepitaxy because the lattice constant at the surface is different from that of the bulk of the film.

Based on this new concept, highly aligned

mesostructured silica films with structural periodicities as large as 10 nm are successfully formed, which has never been achieved when the block copolymers are used alone as the structure-directing agent.

The periodicity of the aligned films can

precisely be controlled by an appropriate choice of block copolymers and the mixing


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ratio of the two surfactants, which increases the opportunity for applications of these films with highly anisotropic meso-scale structure. Introduction The surface nature of substrates significantly influences the structure of the films of mesostructured materials, which are prepared through self-assembly of surfactants, formed thereon.1-3

Typically, the crystallographic orientation of the films along the

normal direction to the substrate is spontaneously controlled.4-10

The orientation in the

plane of the films can also be finely controlled by using surfaces with in-plane structural anisotropy, such as polymer coatings with the polymer chains on the surface aligned in one direction.11-19

Consequently, mesostructured films with single-crystalline

crystallographic features are obtained.

One of the most meaningful features of these

films with a fully controlled in-plane crystallographic orientation is structural anisotropy, which is strong when the films have a two-dimensional hexagonal (2D-hex) structure consisting of cylindrical building units.20

Various nanocomposite films with

remarkable anisotropic properties have been reported so far.21-27

An additional

important feature in these single-crystalline films is a remarkable reduction of structural imperfections, such as dislocations, disclinations and distortions, which can be fatal for some applications of mesostructured films to devices that use the properties related to


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their structure.

However, only the surfactants with hydrophobic alkyl chains allow the

formation of the films with highly controlled in-plane structural orientations on flat substrates,11-19 unless external forces are applied.28-30

This has limited the available

pore sizes and periodicities because of lack of solubility with increasing alkyl chain length.

This limit of the sizes narrows possible applications of these anisotropic

mesostructured films, and a breakthrough for the preparation of highly aligned films with large periodicities has been required based on the proper design of the substrate-film interface. In this paper, we report on the common use of Brij56 (polyoxyethylene-10-cetyl ether) for the formation of highly aligned mesostructured silica films with large structural periodicities, which are prepared using block copolymers as a structure-directing agent, on a substrate with a rubbing-treated polyimide (PI) coating. For the formation of the films, Brij56 works as an “alignment-controlling agent” through selective adsorption from the mixed surfactant systems and the consequent formation of the aligned surface micelles on the rubbed PI surface.

These aligned

surface micelles, mainly consisting of Brij56, work as a scaffold for the totally aligned mesostructures of the films, although the periodicity of the mesostructures formed with block copolymers is much larger than that formed using Brij56 alone.8,31


This

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alignment mechanism is directly confirmed by cross-sectional images obtained by scanning transmission electron microscopy (STEM), which clearly show the specifically small periodicity in the vicinity of the substrate surface. The periodicity along the thickness direction monotonically increases from the 1st to the 6th layer to reach to a constant value, whereas the lateral periodicity is kept constant over the entire thickness of the film.

This shows an epitaxial formation of the 2D-hex phases of the

mixed surfactant systems on the aligned surface micelles dominantly consisting of Brij56, which has much smaller lattice constant.

Therefore, the formation of the

aligned mesostructured silica films using the mixed surfactant systems in this study is regarded

a

kind

of

heteroepitaxy,

because

the

lattice

constants

of

the

alignment-controlling layer are different from that of the formed mesostructure. This concept of this paper “separation of functionality” can be applied to a wide variety of block copolymers and consequently, fine control of the periodicity of the mesostructured films with single-crystalline features is possible over a wide range either by the choice of block copolymers and the variation of the mixing ratio.

The method

in this study will increase the possible applications of aligned mesostructured films through flexible control of periodicity and pore size, which is required for achieving the practically useful properties.


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Experimental Section Chemicals Tetraethoxysilane (TEOS) was purchased from Kishida Chemical Co. Hydrochloric acid and 2-propanol were supplied from Wako Chemical Co. were purchased from Aldrich.

Brij56, P123 and F127

P84 and PEO98-b-PPO60 were purchased from BASF

and Polymer source, respectively.

All the chemicals were used as received.

Preparation of Mesoporous Silica Films The details of the preparation of the substrate coated with a rubbing-treated poly(hexamethylenepyromellitimide) film are shown in our previous paper.11

When

P123 was used as a block copolymer, the precursor solution was prepared by mixing Brij56, P123, TEOS, hydrochloric acid, water and 2-propanol with the following molar ratios: TEOS 1.0: P123 8.810-3 : Brij56 x: HCl 4.010-3 : water 5: 2-propanol 16.6. The Brij56/P123 ratio was changed from 2 to 12, which corresponds to a range for x (molar ratio of Brij56) from 1.810-2 - 1.110-1. dip-coating with a withdrawal speed of 1 mm s-1.

The films were prepared by

For the other block copolymers, P84,

F127 and PEO98-b-PPO60, the films were prepared by spin coating at 3000 rpm.

The

molar ratios of the precursor solutions were optimized for each block copolymer.

The

detailed information about the composition of the precursor solutions are shown in the


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Supporting Information (Table S1). Characterization The structure of the films was characterized by X-ray diffraction (XRD).

The XRD

measurements with the Bragg-Brentano geometry were conducted using a RIGAKU Ultima III and a MAC Science M03X-HF diffractometers using FeK radiation. The in-plane diffraction patterns were recorded with a Rigaku ATX-G diffractometer equipped with a 4-axes goniometer using monochromated CuK radiation. incidence of 0.2

o

Grazing

was commonly employed for the in-plane XRD measurements. The

details of the in-plane XRD are shown in our previous paper.11

Cross-sectional images

of scanning transmission electron microscopy (STEM) were recorded on a Tecnai F30 at an accelerating voltage of 300 kV in a high angle annular dark field (HAADF) mode. The specimens for cross-sectional STEM observation were prepared by focused ion beam (FIB) process to thicknesses below 100 nm and were transferred to a copper grid for observation. Results and Discussion Alignment of mesochannels in the mesostructured silica films prepared using P123 The XRD profiles of the mesostructured silica films prepared using Brij56, P123, and a mixture of Brij56 and P123 (Brij56/P123 = 4/1, molar ratio) on a substrate coated with


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Figure 1. XRD profiles of the aligned mesostructured silica films. (A)  scanning profiles recorded under the Bragg-Brentano geometry and (B) in-plane  scanning profiles with respect to the ( 11). Profiles (a), (b) and (c) are for the films prepared using Brij56, P123 and Brij56/P123 = 4 mixture, respectively. rubbing-treated polyimide are shown in Figure 1.

The patterns in Figure 1(A)

recorded under the Bragg-Brentano geometry (the projection of the incident X-rays is perpendicular to the rubbing direction) give the structural information of the films predominantly along the thickness.

These profiles are consistent with the 2D-hex

mesostructure consisting of cylindrical micelles.13

Figure 1(B) shows the profiles of

in-plane rotation (-scanning) with respect to the ( 1 1) lattice plane measured under the in-plane geometry, which profiles represent the distribution of the alignment of the mesochannels in the plane of the films. The traces (a) in Figures 1(A) and (B) show that Brij56 provides a mesostructured


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silica film with a relatively small periodicity (d01 = 5.1 nm), while the mesochannels are highly aligned perpendicularly to the rubbing direction. On the other hand, P123 provides a film with a much larger d01 value of 9.0 nm, but the orientation of the mesochannels is not controlled at all (traces (b) in Figure 1(A) and (B)). Our previous study shows that the surfactants whose hydrophobic parts are alkyl groups allow the in-plane orientation of the mesochannels over a wide range of the alkyl chain length, from C10H25 to C22H45, on the rubbing-treated polyimide.14,15 However, the fine alignment has never been achieved on this substrate using PEO-b-PPO-b-PEO block copolymer surfactants such as P123, which are widely used for the preparation of mesostructured materials with large periodicities, no matter how the preparation conditions were explored in detail.

It is likely that the PPO blocks have weaker

interactions with the anisotropic polymer surface than alkyl chains due to smaller hydrophobicity, which leads to the failure of the alignment. We considered that the surfactant that contributes to the alignment through the interfacial interactions has not necessarily to be the same as that, which forms the bulk structure of the film.

That is to say, it would be possible to control the alignment of the

mesostructure made of surfactants that do not directly undergo the anisotropic interactions by using those that allow the formation of anisotropic surface micelle


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structure through strong anisotropic interfacial interactions. The design of such an interface could be realized simply by adding the alkyl-PEO surfactants because they would be selectively adsorbed on the polymeric surface from the mixed surfactant systems because of the strong hydrophobicity of the alkyl chains. The traces (c) in Figure 1(A) and (B), recorded for the film prepared using a 4:1 mixture of Brij56 and P123 demonstrate the formation of a finely aligned film with a d01 value of 8.0 nm.

The diffraction pattern (c) in Figure 1(A) proves the formation of a

uniform mesostructure wherein the two surfactants are evenly mixed.

The observed

decrease in the periodicity is caused by the reduction of the micelle size by the addition of the small alkyl-PEO surfactant, and is consistent with previous reports.32

Although

the molar ratio of Brij56 is four times larger than that of P123, the d01 value is closer to that of the film prepared using P123 alone.

This is ascribed to the large hydrophobic

PPO block of P123, which occupies the major part of the hydrophobic moiety in the micelles made of the two surfactants with this mixing ratio. Thus, it is demonstrated that the addition of the surfactant that strongly interacts with the anisotropic polymeric surface enables the alignment of the mesochannels dominantly consisting of block copolymers. Characterization of the mesostructured films by STEM


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9 8 7 6 5 4

0

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25

Layer number Figure 2. Cross-sectional STEM images of the films prepared using (A) Brij56/P123 = 4 mixture and (B) P123. (C) Interval of the layers along the thickness direction: red and blue lines represent the results of (A) and (B), respectively. In order to clarify the structural differences in the aligned and non-aligned films, we did high-resolution STEM observations of the cross-sections.

Figure 2 shows the

cross-sections of the aligned film prepared using a Brij56/P123 = 4 mixture (A) and the non-aligned film prepared using P123 alone (B).

Both of the films are formed on the

substrate with rubbing-treated polyimide. Apparently, the periodicity of the aligned


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film is smaller than that of the non-aligned film, which is consistent with the XRD results. The image of the aligned film has two important features. First, the periodicity along the thickness direction as well as the channels' cross-section in the vicinity of the substrate surface is distinctively smaller than for the rest of the film.

The distribution

of the intervals is taken from the contrast profile of Figure 2(A) and is shown as the red profile in Figure 2(C). It is clearly shown that the interval is quite small in the vicinity of the surface, especially for the first 3 layers, and it monotonically increases to the 6th layer to reach to a constant value of ~7 nm.

This suggests a local difference in the

mixing ratio of the two surfactants, and it is likely that the micelles near the substrate surface are rich in Brij56.

This is reasonable because of the expected higher affinity of

Brij56 to the surface of the polyimide containing hydrophobic hexamethylene groups, due to the long alkyl chains, as described before.

It should be noted that the lateral

periodicity is kept constant over the entire thickness irrespective of the difference in the micelle size near the surface.

This can be explained by the elastic property of the

silicatropic liquid crystal (LC), that is, the possible change of the shape of the micelles. When LC layers with larger periodicity and micelle size are continuously formed on those with smaller ones keeping the lateral periodicity constant, the larger LC phase


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inevitably distort to reduce the lateral distance to match the periodicity. In the present case, the surface layers consisting of smaller micelles deform from the ideal hexagonal structure to increase the lateral distance, which is caused by the interfacial interactions with the anisotropic surface (green hexagons in Figure 2(A)).

The layers with a larger

periodicity formed above these surface layers are forced to match the fixed lateral periodicity, and consequently, the deformation from the ideal hexagonal structure becomes smaller than in the surface layers (yellow hexagons in Figure 2(A)).

The

small periodicity in the vicinity of the substrate surface is also confirmed by cross-sectional transmission microscopy (See Supporting Information Figure S1). In contrast to the aligned films, the cross-sectional image of the non-aligned film (Figure 2(B)) shows the uniform periodicity, micelle size and the degree of deformation of the ideal structure over the entire thickness.

The same yellow hexagons fit the

structure irrespective of the positions in the film.

The distribution of the intervals

shown as the blue profile in Figure 2(C) confirms the uniform periodicity. The second feature is the undulation of the substrate surface, which is highlighted by the yellow oval in Figure 3(A).

Such a surface undulation is not observed in the image

of the non-aligned film (Figure 3(B)). This undulation has to be formed from an organic material because of a similar contrast to the polyimide layer. It is most likely


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that the aligned hemicylindrical surface micelles (Figure 3(C)) are directly observed. They dominantly consist of Brij56, which is responsible for the in-plane alignment. This strongly supports the proposed alignment mechanism of selective adsorption of Brij56, which works as an alignment-controlling agent.

A

B

20 nm

20 nm

PI Si

PI Si

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Figure 3. Enlarged cross-sectional STEM images of the films prepared with (A) Brij56/P123 = 4 mixture and (B) P123 in the vicinity of the film-substrate interfaces. (C) Schematic illustrations of the film-substrate interfaces of the two films: blue and red represent the PPO blocks of Brij56 and the block copolymers, respectively. The silica walls and the PEO groups are not shown in the figure. Dependence of the film structure on the mixing ratio of the surfactants To make the role of Brij56 clearer, we prepared the films on the same substrate with 14 ACS Paragon Plus Environment

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rubbing-treated polyimide from the precursor solutions with different Brij56/P123 ratios and investigated their structure.

The formation of the aligned mesostructured silica

films is confirmed with the Brij56/P123 mixing ratios between 4 and 8 (See Supporting Information Figure S2). The film prepared with the mixing ratio of 2 has the same 2D hex structure, but the in-plane alignment of the cylindrical micelles is not controlled. This would be caused by insufficient amount of Brij56 that determines the alignment at the substrate surface. On the other hand, a regular 2D hex structure is not obtained when the Brij56/P123 ratio is larger than 10, probably due to the instability of the cylindrical micelles. The dependences of the d01 value and the full width at half maximum of the

-scanning profiles on the mixing ratio, taken from the obtained XRD patterns (Supporting Information Figure S2), are summarized in Figure 4. It is clearly shown that both of the structural parameters strongly depend on the mixing ratio.

The d01

value monotonically decreases with the increase of the Brij56 ratio. As described before, this is caused by the reduction of the micelle size by mixing with the small surfactant. Thus, it is demonstrated that the precise control of the periodicity of the highly aligned mesostructured films is possible simply by controlling the mixing ratio of the two surfactants.


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d01 / nm

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2

Brij56 / P123 molar ratio Figure 4. Dependences of the d01 value (blue plots) and the full width at half maximum of the  scanning profiles (red plots) on the Brij56/P123 molar ratio. This reduction of the d01 value is accompanied by the narrowing of the peak width of the in-plane -scanning profile, which means that the distribution of alignment becomes narrower with the decrease of the structural periodicity.

This is consistently explained

by the matching of the lateral periodicity of the film with the surface layer with a smaller periodicity, as described in the previous discussion.

The decrease of the

intrinsic periodicity of the silicatropic LC by the increase of the ratio of Brij56 results in the reduction of the degree of periodicity mismatch with the surface layer. Consequently, the LC layer is less distorted, which should reduce the structural undulation and narrows the alignment distribution of the mesochannels. 16 ACS Paragon Plus Environment

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B rubbing

(01)

(01)

(02)

(12)

a

(02)

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(01)

b (12)

(02)

a b

c 1

2

3

4

c -90

5

2 / o

0

90

180

/o

Figure 5. XRD profiles of the aligned mesostructured silica films obtained using three kinds of block copolymers. (A)  scanning profiles recorded under the Bragg-Brentano geometry and (B) in-plane  scanning profiles with respect to the ( 1 1). Profiles (a), (b) and (c) are for the films prepared using Brij56/P84, Brij56/F127 and Brij56/PEO98-b-PPO60 mixtures, respectively. Availability of other block copolymers The concept of the present paper, that is the addition of an alkyl-PEO surfactant as an alignment controlling agent to a silicatropic LC phase dominantly consisting of an amphiphilic block copolymer is applicable to other block copolymers when the two surfactants are evenly mixed to give a uniform LC phase.

We used three different

block copolymers, P84 (PEO19-b-PPO43-b-PEO19), F127 (PEO106-b-PPO70-b-PEO106) and PEO98-b-PPO60 instead of P123. controlling agent.

Brij56 is used, in all cases, as the alignment

The XRD patterns of the films are shown in Figure 5(A)

(Bragg-Brentano geometry) and 5(B) (in-plane -scanning). It was confirmed that all 17 ACS Paragon Plus Environment

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the block copolymers form aligned 2D-hex mesostructured films on the rubbing-treated polyimide by the addition of Brij56.

The d01 values are estimated to be 6.4 nm, 9.6 nm

and 10.8 nm for P84, F127 and PEO98-b-PPO60, respectively.

The alignment

distribution of the films prepared using F127 and PEO98-b-PPO60 are relatively wide (over 10 o), while P84, which gives the LC phase with a small intrinsic periodicity, shows remarkably narrow distribution of 3.0 o. These are consistent with the above discussion based on the matching of the lateral periodicity. Conclusion An alkyl-PEO surfactant, Brij56, enables the alignment control of mesochannels in mesostructured silica films with large periodicities, which are prepared using block copolymers as a structure-directing agent.

Brij56 is selectively adsorbed on the

anisotropic polymer surface and acts as an alignment controlling agent, which is proved by

the

existence

of

surface

layers

with

distinctively

small

periodicities.

Mesostructured silica with large periodicity, which dominantly consists of block copolymer, is heteroepitaxially formed on these aligned surface layers by matching the lateral periodicity.

Thus, through the different roles of the mixed two surfactants, fully

aligned mesostructured silica films with large structural periodicity are obtained. In addition to the alignment, the periodicity of the aligned mesostructured silica films can


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precisely be controlled by the choice of block copolymers and the mixing ratio of the two surfactants.

The method shown in this paper will increase the opportunity for

applications of these aligned mesostructured films through flexible control of periodicity and the pore size which are required for achieving the desired properties. Acknowledgement The authors acknowledge Dr. O. Albrecht (Canon Inc.) for careful reviewing of the manuscript.

The authors thank Mr. Y. Shibuya (Waseda Univ.) for the preparation of

the STEM specimens.

Supporting Information Available: Compositions of the precursor solutions using mixtures of Brij56 and P84, F127 and PEO98-b-PPO60, TEM image of the cross section of the aligned mesostructured silica film prepared using a Brij56/P123 = 4 mixture sliced perpendicular to the rubbing direction, XRD profiles of the films prepared using mixtures of Brij56 and P123 with different mixing ratios. This material is available free of charge via the Internet at http://pubs.acs.org.

References


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1. Innocenzi, P.; Malfatti, L.; Kidchob, T.; Falcaro P. Order-Disorder in Self-Assembled Mesostructured Silica Films: A Concepts Review. Chem. Mater. 2009, 21, 2555–2564 2. Sanchez, C.; Boissière, C.; Grosso, D.; Laberty, C.; Nicole, L. Design, Synthesis, and Properties of Inorganic and Hybrid Thin Films Having Periodically Organized Nanoporosity. Chem. Mater. 2008, 20, 682–737. 3. Ariga, K.; Vinu, A.; Yamauchi, Y.; Ji, Q.; Hill, J. P. Nanoarchitectonics for Mesoporous Materials. Bull. Chem. Soc. Jpn. 2012, 85, 1-32. 4. 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. 5. 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. 6. Yang, H.; Coombs, N.; Sokolov, I.; Ozin, G. A. Registered Growth of Mesoporous Silica Films on Graphite. J. Mater. Chem. 1997, 7, 1285-1290. 7. 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.


Langmuir

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8. Zhao, D.; Yang, P.; Melosh, N.; Feng, J.; Chmelka, B. F.; Stucky, G. D. Continuous Mesoporous Silica Films with Highly Ordered Large Pore Structures. Adv. Mater. 1998, 10, 1380-1385. 9. Klotz, M.; Albouy, P.-A.; Ayral, A.; Ménager, C.; Grosso, D.; Van der Lee, A.; Cabuil, V.; Babonneau, F.; Guizard, C. The True Structure of Hexagonal Mesophase-Templated Silica Films As Revealed by X-ray Scattering: Effects of Thermal Treatments and of Nanoparticle Seeding. Chem. Mater. 2000, 12, 1721-1728. 10. Nishiyama, N.; Tanaka, S.; Egashira, Y.; Oku, Y.; Ueyama, K. Vapor-Phase Synthesis of Mesoporous Silica Thin Films. Chem. Mater. 2003, 15, 1006-1011. 11. Miyata, H.; Kuroda, K. Preparation of Mesoporous Silica Films with Fully Aligned Large Mesochannels Using Nonionic Surfactants. Chem. Mater. 2002, 14, 766-772. 10. 12. Miyata, H.; Suzuki, T.; Fukuoka, A.; Sawada, T.; Watanabe, M.; Noma, T.; Takada, K.; Mukaide, T.; Kuroda, K. Silica Films with a Single-Crystalline Mesoporous Structure. Nat. Mater. 2004, 3, 651-656. 13. 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. 14. Komoto, A.; Kubo, W.; Miyata, H. Control of Periodicity in Mesoporous Silica


Page 26 of 32

Page 27 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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Films with Aligned Mesochannels Prepared by a Sol-Gel Process. Chem. Lett. 2012, 41, 741-743. 15. Miyata,H.; Kobori, S.; Kubo, W.; Watanabe, M.; Kuroda, K. Lattice Matching in the Epitaxial Formation of Mesostructured Silica Films. Langmuir 2013, 29, 761-765. 16. Kawashima, Y.; Nakagawa, M.; Seki, T.; Ichimura, K. Photo-orientation of Mesostructured Silica via Hierarchical Multiple Transfer. Chem. Mater. 2002, 14, 2842-2844. 17. Kawashima, Y.; Nakagawa M.; Ichimura, K.; Seki, T. Photo-orientation of Mesoporous Silica Materials via Transfer from an Azobenzene-Containing Polymer Monolayer. J. Mater. Chem. 2004, 14, 328-335. 18. Fukumoto, H.; Nagano, S.; Kawatsuki, N.; Seki, T. Photo-Alignment Behavior of Mesoporous Silica Thin Films Synthesized on a Photo-Cross-Linkable Polymer Film. Chem. Mater. 2006, 18, 1226-1234. 19. Fukumoto, H.; Nagano, S.; Kawatsuki, N.; Seki, T. Photo-orientation of Mesoporous Silica Thin Films on Photocrosslinkable Polymer Films. Adv. Mater. 2005, 17, 1035-1039. 20. Wu, C.-W.; Jiang, X.; Yamauchi, Y. New Trend on Mesoporous Films: Precise Controls of One-Dimensional (1D) Mesochannels Toward Innovative Applications. J.


Langmuir

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Mater. Chem. 2011, 21, 8934-8939. 21. Molenkamp, W. C.; Watanabe, M.; Miyata, H.; Tolbert, S. H. Highly Polarized Luminescence from Optical Quality Films of a Semiconducting Polymer Aligned within Oriented Mesoporous Silica. J. Am. Chem. Soc. 2004, 126, 4476. 22. 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. 23. Suzuki, T.; Miyata, H.; Noma, T.; Kuroda, K. Platinum Thin Film Consisting of Well-Aligned Nanowires and Its Optical Behavior. J. Phys. Chem. C 2008, 112, 1831-1836. 24. Rice, R. L.; Arnold, D. C.; Shaw, M. T.; Iacopina, D.; Quinn, A. J.; Amenitsch, H.; Holmes, J. D.; Morris, M. A. Ordered Mesoporous Silicate Structures as Potential Templates for Nanowire Growth. Adv. Func. Mater. 2007, 17, 133-141. 25. Cui, W.; Lu, X.; Su, B.; Lu, Q.; Wei, Y. Anisotropic Electronic Properties of Ni Nanowires in Oriented Mesoporous Silica Films. Appl. Phys. Lett. 2009, 95, 153102. 26. Kawamura, G.; Hayashi, I.; Muto, H.; Matsuda, A. Anisotropically assembled gold nanoparticles prepared using unidirectionally aligned mesochannels of silica film. Scripta Mater. 2012, 66, 479–482.


Page 28 of 32

Page 29 of 32

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Langmuir

27. Fukumoto, H.; Nagano, S.; Seki, T. In situ Polymerization of Liquid Crystalline Monomers within Photoaligned Mesoporous Silica Thin Film. Chem. Lett. 2006, 35, 180-181. 28. 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. 29. Melosh, N. A.; Davidson, P.; Feng, P.; Pine, D. J.; Chmelka, B. F. Macroscopic Shear Alignment of Bulk Transparent Mesostructured Silica. J. Am. Chem. Soc. 2001, 123, 1240-1241. 30. 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. 31. Grosso, D.; Balkenende, A. R.; Albouy, P. A.; Ayral, A.; Amenitsch, H.; Babonneau, F. Two-Dimensional Hexagonal Mesoporous Silica Thin Films Prepared from Block Copolymers: Detailed Characterization and Formation Mechanism. Chem Mater. 2001, 13, 1848-1856.


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32. Chen, L.; Xu, J.; Zhang, W-H.; Holmes, J. D.; Morris, M. A. Syntheses of complex mesoporous silicas using mixtures of nonionic block copolymer surfactants: Understanding formation of different structures using solubility parameters. J. Colloid Interface Sci. 2011, 353, 169–180.

Figure Captions Figure 1. XRD profiles of the aligned mesostructured silica films.

(A)  scanning

profiles recorded under the Bragg-Brentano geometry and (B) in-plane  scanning profiles with respect to the ( 11).

Profiles (a), (b) and (c) are for the films prepared

using Brij56, P123 and Brij56/P123 = 4 mixture, respectively. Figure 2. Cross-sectional STEM images of the films prepared using (A) Brij56/P123 = 4 mixture and (B) P123.

(C) Interval of the layers along the thickness direction: red and

blue lines represent the results of (A) and (B), respectively. Figure 3. Enlarged cross-sectional STEM images of the films prepared with (A) Brij56/P123 = 4 mixture and (B) P123 in the vicinity of the film-substrate interfaces. (C) Schematic illustrations of the film-substrate interfaces of the two films: blue and red represent the PPO blocks of Brij56 and the block copolymers, respectively. walls and the PEO groups are not shown in the figure. 25 ACS Paragon Plus Environment

The silica

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Figure 4. Dependences of the d01 value (blue plots) and the full width at half maximum of the  scanning profiles (red plots) on the Brij56/P123 molar ratio. Figure 5. XRD profiles of the aligned mesostructured silica films obtained using three kinds of block copolymers.

(A)  scanning profiles recorded under the

Bragg-Brentano geometry and (B) in-plane  scanning profiles with respect to the ( 11). Profiles (a), (b) and (c) are for the films prepared using Brij56/P84, Brij56/F127 and Brij56/PEO98-b-PPO60 mixtures, respectively.


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Heteroepitaxial Formation of Aligned Mesostructured Silica Films with

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