Control of Periodicity in Mesoporous Silica Films with Aligned Mesochannels Prepared by a Sol–Gel Process

Abstract

Periodicity of mesoporous silica films with aligned cylindrical pores formed on a substrate with a rubbing-treated polyimide through a solgel process can be finely controlled using nonionic surfactants with different alkyl chains. The degree of alignment increases with the alkyl-chain length of the surfactant due to increase in anisotropic hydrophobic interaction between the oriented polyimide chains and the alkyl groups, which is responsible for the alignment.Mesoporous silica (MPS) films are promising materials for application in optical and electronic molecular devices because they have highly ordered nanostructures and are synthesized through a simple self-assembly process.15 Especially, MPS films with highly aligned cylindrical mesochannels are expected to be used for anisotropic accommodation of various guest species, e.g., metals, 6 dyes, 7 and polymers. 8,9 Several methods to control the alignment of the mesochannels have been reported. 1017 Among them, the method in our previous report using a substrate with a rubbing-treated polyimide layer combined with evaporation-induced self-assembly (EISA) can control the mesochannel direction within the º distribution of 3°at macroscopic scales.11 The method, however, has not achieved to control the periodicity of the MPS films, which associates with the pore size. The optimum conditions for the preparation of aligned MPS films using different surfactants have been difficult to find because it is necessary to take the interactions between the oriented polyimide chains and the surfactant molecules into account in addition to the self-assembly of the surfactant. The control of the pore size of the aligned mesochannels enables the fine tuning of the properties of the guest species in the mesochannel through changes of the amount and the conformation. In view of this situation, we report the control of the periodicity and the pore size of the MPS films with aligned mesochannels by varying alkyl-chain length of nonionic surfactants.The MPS films with the aligned mesochannels were prepared based on our previous method 11 after careful optimization for each surfactant. The polyimide with a 8-nm thickness on a Si wafer underwent rubbing treatment using a cottoncovered cylindrical roller. Nonionic surfactant (poly(oxyethylene)-10-alkyl ether, C n EO 10 , n = 10, 12, and 16) was dissolved in 2-propanol (IPA), and then ultrapure water (Milli-Q), 0.1 M HCl, and tetraethoxysilane (TEOS) were added to the solution. The final molar ratio of the chemicals was C n EO 10 8.00 © 10 ¹2 0.120/TEOS 1.00/IPA 16.7/H 2 O 5.00/HCl 4.00 © 10 ¹3. After stirring and aging the solution for 0.53.0 h, the Si wafer was dip-coated with the solution at withdrawal speed of 1.5 mm s ¹1 at 25°C and 3943% RH and subsequently dried at 25°C and 40% RH over 16 h. The surfactants were removed by calcination at 400°C for 10 h in air. Structural analysis of the synthesized films was performed by X-ray diffractometry (XRD). Conventional ª2ª scanning XRD and in-plane XRD were performed with Philips X'Pert Pr...

Heteroepitaxial Formation of Aligned Mesostructured Silica Films with Large Structural Periodicities from Mixed Surfactant Systems

“…However, P123 provides a film with a much larger d 01 value of 9.0 nm, but the orientation of the mesochannels is not controlled at all (traces (b) in parts A and B of Figure ). 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 C 10 H 25 to C 22 H 45 , on the rubbing-treated polyimide. , 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.…”

“…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. − Typically, the crystallographic orientation of the films along the normal direction to the substrate is spontaneously controlled. − The orientation in the plane of the films can also be finely controlled using surfaces with in-plane structural anisotropy, such as polymer coatings with the polymer chains on the surface aligned in one direction. − 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 .…”

“…Various nanocomposite films with remarkable anisotropic properties have been reported so far. − 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 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 − unless external forces are applied. − 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.…”

Mesoporous TiO2 films with regularly aligned slit-like nanovoids

“…The observed difference undoubtedly comes from the structural anisotropy in the plane of the film, and these observed patterns are consistent with a uniaxially aligned 2D-hexagonal structure, which has been successfully formed for mesostructured silica films using the same substrate with a rubbing-treated polyimide coating. 22,26 The lattice planes (% 12) and (% 13) are not parallel to the substrate surface, and therefore, do not satisfy the diffraction condition under the ideal Bragg-Brentano geometry. However, the broadened reciprocal lattice points of mesostructured materials, which appear in the low-angle region, and the finite divergence angle of X-rays make these spots detectable.…”

Strict In-Plane Alignment Control of Block-Copolymer-Templated Mesostructured Silica Films Using an Alignment-Controlling Agent

“…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 (d 01 < 6 nm) by changing the alkyl chain length of the surfactant. 24 However, in the larger region (d 01 > 6 nm) this does not work because of the surfactant's solubility problems. To control the structural 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.…”