Highly Ordered Arrays of Mesoporous Silica ... - Wiley Online Library

Highly Ordered Arrays of Mesoporous Silica Nanorods with Tunable Aspect Ratios from Block Copolymer Thin Films** By Aihua Chen, Motonori Komura, Kaori Kamata, and Tomokazu Iyoda* Mesoporous silica thin films have received much attention in applications as diverse as separation devices, sensors, and optoelectronic devices.[1] The evaporation-induced self-assembly method (EISA) has been established as an efficient process for the rapid preparation of mesoporous silica thin films.[2] Usually, beginning with a highly dilute homogenous solution of a soluble silica species and a surfactant in an ethanol/water mixture, preferential evaporation of ethanol concentrates the nonvolatile surfactant and silica species in water, thereby inducing the self-assembly of silica-surfactant micelles and their further organization into liquid-crystalline mesophases.[2] Recently, this efficient EISA method has been applied to prepare ordered mesoporous materials within the confined channels of anodic alumina membranes (AAM), silicon membranes, and other resist molds.[3-13] Compared to mesoporous powders and thin films, a hierarchically ordered mesoporous material is advantageous for self-assembly on a macroscopic scale and allows easy control over the morphology. A reduction in diameter below 50 nm to obtain mesoporous nanowires and -fibers is believed to offer even more benefits. However, studies of this type of materials have been limited so far due to the size limitation of the templates available. This shortcoming can be overcome by using the block copolymer lithography technique, which provides a powerful way to fabricate highly ordered arrays of inorganic nanoparticles with diameters of 5 – 50 nm.[14-26] Both the diameter and interparticle distance of the resulting nanoparticle arrays can be tuned by adjusting the volume fractions of the block copolymer templates. Unfortunately, control of the aspect ratio (or the height) still remains a challenge. Herein, we report for the first time preparation of a highly ordered array of mesoporous silica nanorods with tunable as-

– [*] Prof. T. Iyoda, Dr. A. Chen, Dr. M. Komura, Prof. K. Kamata Division of Integrated Molecular Engineering, Chemical Resources Laboratory, Tokyo Institute of Technology R1-25 4259 Nagatsuta, Midori Ku, Yokohama 226-8503 (Japan) E-mail: [email protected] Prof. T. Iyoda CREST, Japan Science and Technology Agency 4-1-8 Hon-cho, Kawaguchi, Saitama 332-0012 (Japan) [**] This work was partly supported by a Grant-in-Aid for Scientific Research (S) (No. 90168534). We are grateful to Prof. Takashi Tatsumi for the use of the FE-SEM. A. Chen acknowledges support from the Japan Society for the Promotion of Science (JSPS) through a postdoctoral fellowship for foreign researchers. Supporting Information is available online from Wiley InterScience or from the author.

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DOI: 10.1002/adma.200702010

pect ratios, through an integrated strategy of block copolymer lithography and EISA. Block copolymer thin films with normal, cylindrical domains are used as external scaffolds to define the morphology of SiO2 nanorods. The surfactant cetyltrimethylammonium bromide (CTAB) is used as an internal template to form mesoporous structures inside the SiO2 nanorods. The control of the diameter, the center-to-center distance, and the height of SiO2 nanorods were studied systematically. Microphase-separated diblock copolymer films of amphiphilic PEOm-b-PMA(Az)n (m and n denote repeated units of the individual segments), consisting of hydrophilic poly(ethylene oxide) (PEO) and hydrophobic poly(methacrylate) with azobenzene mesogens (PMA(Az)) in the side chain,[27-29] can be prepared by spin-coating 1 ∼ 3 wt % toluene or chloroform solutions thereof on silicon wafers substrates followed by annealing at 140 °C for 24 h in vacuum. Figure 1A–C show typical atomic force microscopy (AFM) height images of PEO114-b-PMA(Az)45, PEO272-b-PMA(Az)116, and PEO454b-PMA(Az)184 thin films, respectively, on a silicon wafer after annealing. Dark dots and bright surroundings in the images can be assigned to cylindrical PEO microdomains and PMA(Az) matrix, respectively. The insets showing fast Fourier transformed (FFT) images indicate a hexagonal or quasihexagonal arrangement of the PEO domains. The average diameter of PEO cylinders and their average center-to-center distance are (11.2 ± 1.6) nm and (22.1 ± 1.4) nm for PEO114-bPMA(Az)45, (18.6 ± 1.8) nm and (33.4 ± 1.9) nm for PEO272-bPMA(Az)116, and (23.7 ± 4.0) nm and (51.7 ± 3.1) nm for PEO454-b-PMA(Az)184, respectively. When the silicate sol was introduced into the PEOm-bPMA(Az)n thin film, the swollen PEO domains had been selectively doped, which was confirmed by TEM (See Supporting Information Figure S1). Through the sol-gel process driven by the solvent evaporation, the silicate sol will penetrate into the swollen PEO cylindrical domains to form one dimensional SiO2 nanoparticles. Figure 2A shows an AFM height image of the silicate sol hybridized with the PEO domains in a PEO272-b-PMA(Az)116 thin film (thickness: ∼ 150 nm) after immersion in the silicate sol for 3 h. A regular nanoparticle array protruded from the surface of the PEO272-bPMA(Az)116 thin film (height: ∼ 2 nm), which was not observed in cases with thicker PEOm-b-PMA(Az)n film templates or shorter immersion times. As shown by cross-sectional AFM image (Figure 2B) the cylindrical structure was still present after immersion, but most of the SiO2 nanorods appeared to lie flat on the substrate after calcination (Fig. S2).

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tion FE-SEM images of SiO2 nanorod arrays templated from PEO114-bPMA(Az)45, PEO272-b-PMA(Az)116, and PEO454-b-PMA(Az)184 thin films, respectively. In all cases the average diameter and the center-to-center distance of SiO2 nanorods correlated well with the dimensions of the PEO cylindrical domains of the corresponding PEOm-b-PMA(Az)n thin-film template (Fig. 3F and G). Figure 1. Atomic force microscopy (AFM) height images of PEOm-b-PMA(Az)n thin films on silicon wafers (images: 500 nm 6 500 nm, insets: fast Fourier transformed patterns). A) PEO114-bThe height of the SiO2 nanorods was PMA(Az)45. B) PEO272-b-PMA(Az)116. C) PEO454-b-PMA(Az)184. investigated by section-view FE-SEM, as shown in Figure 4. The block copolymer template used to obtain the images shown in Figure 4A and B was PEO114-b-PMA(Az)45 with a film thickness of ∼ 500 nm. The height of the SiO2 nanorods was measured to be ∼ 75 nm after immersion in the silicate sol for 2 h, increasing to 180 nm after 3 h immersion(Fig. 4C). Additionally, a crosslinked structure amongst neighboring SiO2 rods developed at longer immersion times (5 h, Fig. S4). Furthermore, the height of the SiO2 nanorods seemed to be limited by the thickness of the mother block copolymer thin films (Fig. S5). Therefore, it can be concluded that the height of the SiO2 nanorods is determined by the amount of the siliFigure 2. AFM images of PEO272-b-PMA(Az)116 thin films (∼ 150 nm cate sol incorporated into the PEO domains, which can be thickness) immersed in the silicate sol for 3 h. A) Height image. controlled by immersion time and block copolymer thickness. B) Cross-sectional image. As mentioned above, two kinds of templates were used during array fabrication: i) PEOm-b-PMA(Az)n thin films as This may be due to the disappearance of the phase-segregated external templates to form the ordered SiO2 nanorod arrays, nanostructure in the template above the isotropic temperaand ii) CTAB as an internal template to form a mesostucture of the PEOm-b-PMA(Az)n thin film (120 °C) . ture inside the SiO2 nanorods. Figure 5A and B show transIn order to improve the thermal stability of the PEO cylinmission electron microscopy (TEM) images of SiO2 nanodrical scaffold, the PEOm-b-PMA(Az)n film was cured using rods templated from PEO114-b-PMA(Az)45 and PEO272-ban electron beam (EB) at 60 kV, 30 lA for 200 s to crosslink PMA(Az)116 thin films, respectively. Depending on the exterthe polymer chains prior to the immersion. Figure 3A shows nal template film used, mesochannels with a mean diameter a typical field-emission scanning electron microscopy (FEof ∼ 2 nm were found in individual SiO2 nanorods of 13 nm SEM) image of the SiO2 array obtained from an EB-cured and 20 nm diameter, respectively. In both cases, the mesoPEO114-b-PMA(Az)45 thin film. A highly ordered SiO2 nanochannels were aligned along the longitudinal axis of the SiO2 rod array was found to have formed on a large scale after nanorods. complete removal of the template. The inset shows an FFT It is well known that EISA is an effective way to prepare image indicating a hexagonal arrangement for the SiO2 nanomesoporous silica films. The CTAB surfactant in the silicate rods. In order to investigate the morphology of SiO2 nanorods sol solution self-assembles to form micelles during the evapoon a scale of several millimeters, samples were characterized ration of the solvent. However, the solvent evaporation not by grazing-incidence small-angle X-ray scattering (GI-SAXS), only drives the self-assembly of the silica-CTAB micelles, but as Figure 3B shows (2D patterns are shown in Fig. S3.). Solid also results in a concentration gradient of nonvolatile silica dot (䊉) and hollow dot (䊊) curves are GI-SAXS in-plane proand CTAB between the interior of PEO cylindrical domains files of a PEO114-b-PMA(Az)45 thin film and the resulting and the exterior of the block copolymer film, which subseSiO2 nanorod array, respectively. Theprelative peak positions quently drives the penetration of swollen PEO by the silica– p p of the two curves can be assigned to 1: 3: 4: 7, which correCTAB nanocomposites. Nonetheless, mechanistic considerasponds to a hexagonal arrangement of PEO cylindrical dotions, such as silicate–CTAB micelle formation and penetramains and SiO2 nanorods with periodicities of 22.6 and tion into the PEO domain, and vertical mesochannel forma23.1 nm, respectively. This indicates that the morphology of tion, still remain unsolved. A further study is in progress. the PEO114-b-PMA(Az)45 thin-film template was successfully In conclusion, we propose a facile and effective method to transcribed to the SiO2 nanorod array on a larger scale(sevfabricate highly ordered arrays of mesoporous SiO2 nanorods eral square millimeters). Figure 3C–E are higher magnificawith tunable aspect ratios by EISA using a series of PEOm-b-

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Figure 3. Field-emission scanning electron microscopy (FE-SEM) images and grazing incidence small angle X-ray scattering (GI-SAXS) profiles of SiO2 nanorod arrays. A) Lower magnification FE-SEM image of a SiO2 nanoarray obtained using a PEO114-b-PMA(Az)45 thin film template. B) GI-SAXS profiles of a PEO114-b-PMA(Az)45 thin film (䊉) and the corresponding SiO2 nanorod array(䊊). C–E) Higher magnification FE-SEM images of SiO2 arrays templated from PEO114-b-PMA(Az)45, PEO272-b-PMA(Az)116 and PEO454-b-PMA(Az)184 thin films, respectively. The scale bar is 60 nm. F,G) Plots of the average diameter and the average center-to-center distance of SiO2 nanorods versus the PEO cylindrical domains of the PEOm-b-PMA(Az)n thin films, respectively.

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applications, as well as being used as secondary templates for more elaborate nanostructures.

Experimental

Height of SiO2 nanorods / nm

Preparation: Amphiphilic liquid-crystalline block copolymers, PEO114-b-PMA PEO272-b-PMA (Az)116, and (Az)45, PEO454-b-PMA (Az)184 were synthesized by atom transfer radical polymerization C 400 (ATRP) of 11-[4-(4-butylphenylazo)- phenoxy]undecyl methacrylate from PEO-based 350 macroinitiators as described elsewhere [27]. 300 1 – 3 wt % solutions of PEOm-b-PMA (Az)n block copolymers in toluene or chloroform 250 were spin-coated onto Si wafer substrates and subsequently annealed for 24 h in vacu200 um, at 140 °C. In order to form silica-surfac150 tant nanocomposites, a precursor solution was prepared as described in the following 100 two-step procedure: First, a mixture of etha50 nol (5.5 g), tetraethylorthosilicate (TEOS, 2.08 g), water (0.5 g), and 0.4 g of aqueous 0 hydrochloric acid solution (0.1 M) was heated 0 1 2 3 4 5 at 70 °C for one hour. Subsequently, cetyltriImmersion time / h methylammonium bromide (CTAB, 0.78 g) and ethanol (10 g) were added to the mixFigure 4. Section-view FE-SEM images of SiO2 nanorod array. A,B) PEO114-b-PMA(Az)45 thin films ture, which was stirred at room temperature (thickness ∼ 500 nm) immersed in the silicate sol for 2 and 3 h, respectively (samples tilted by for a further 60 min. The PEOm-b200). C) Plots of SiO2 nanorod height vs. immersion time. PMA(Az)n films were then immersed in the precursor solution and held at room temperature for 2–5 h. The samples were then rinsed with water and dried at 50 °C overnight, before being heated to 550 °C at a rate of 1 °C min–1 at which temperature they were kept for 6 h. Measurement: AFM images were taken with a Digital Instruments Dimension 3000 scanning force microscope. Imaging was conducted in tapping mode using a silicon cantilever with a resonance frequency of 300 kHz. TEM images were obtained with a Hitachi 7000 transmission electron microscope. In order to further characterize the inside structure of the silica nanorods by TEM, they were removed from the silicon wafer substrate by sonication in ethanol solution for 30 min and the resulting nanorod suspension was dropped onto a copper grid and subjected to TEM analysis. Prior to the recording of FE-SEM images on a Hitachi S-5200 field-emission scanning electron microscope a Pt/Au layer (thickness ∼ 2 nm) was deposited onto the surface of the samples using a HITACHI E-1010 ion sputter. Grazing-incidence Figure 5. Transmission electron microscopy images of SiO2 nanorods small angle X-ray scattering (GI-SAXS) measurements were pertemplated by block copolymer thin films of A) PEO114-b-PMA(Az)45 and formed on a Rigaku MicroMaxTM-007HF instrument with a monoB) PEO272-b-PMA(Az)116 , respectively. The scale bar is 20 nm. chrome X-ray wavelength of 1.54 Å. The incidence angles were 0.200 and 0.210 for PEOm-b-PMA(Az)n template film and resulting SiO2 nanorod array, respectively. The exposure times were 60 min and 30 min, respectively. A USHIO Min-Electron Beam was used to cure PMA(Az)n thin films as template. The diameter and the cenblock copolymer thin films at 60 KV, 30 lA for 200 s.

ter-to-center distance of the SiO2 nanorods can be tuned by selecting suitable block copolymer thin films while the height of the SiO2 nanorods can be controlled by variation of immersion time and thickness of the block copolymer thin films. In the present study nanorods of nearly 200 nm height were achieved, with an aspect ratio of 15. Furthermore, mesochannels with a diameter of 2 nm were formed inside and aligned along the longitudinal axis of the SiO2 nanorods. Highly ordered tunable SiO2 nanorod arrays have a tremendous potential for sensory, optoelectronic, and on-chip separation

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Received: August 11, 2007 Revised: October 11, 2007 Published online: January 29, 2008

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