Highly Ordered Arrays of Mesoporous Silica Nanorods with Tunable Aspect Ratios from Block Copolymer Thin Films

Abstract

Mesoporous silica thin films have received much attention in applications as diverse as separation devices, sensors, and optoelectronic devices. The evaporation-induced self-assembly method (EISA) has been established as an efficient process for the rapid preparation of mesoporous silica thin films. 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. 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. 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. 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 aspect 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, 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). C O M M U N IC A IO N