Chem. Mater. 2010, 22, 4117–4119 4117 DOI:10.1021/cm1009527
Hierarchically Ordered Structures by Converging Holographic Lithography and Surfactant Templating Seung-Kon Lee,† Hyosung Park,† Gi-Ra Yi,‡ and Seung-Man Yang*,† †
National CRI Center for Integrated Optofluidic Systems and Department of Chemical & Biomolecular Engineering, KAIST, Daejeon 305-701, Republic of Korea, and ‡ Department of Industrial Engineering Chemistry, Chungbuk National University, Cheongju, 361-763, Republic of Korea Received April 6, 2010 Revised Manuscript Received June 7, 2010
Hierarchically ordered porous structures and particles have long attracted interest from researchers seeking to develop high-performance catalytic materials because of their high mass- and heat-transfer rates as well as their extremely high surface areas for chemical reaction.1 Recently, the multiscale features of such hierarchically ordered materials have been applied for other applications, such as photocatalysts, cathode materials for batteries, and substrates for bioanalysis.2 In particular, structures containing arrays of macropores with periods in the optical wavelength range as well as mesopores can be used as substrate materials for high-throughput colorimetric analysis because the presence of mesopores increases the surface area enormously, and at the same time, ordered macropores provide unusual optical responses such as strong light scattering and photonic bandgap properties.3 Because conventional top-down process requires many fabrication steps together with precise integration of those steps, biomimetic or artificial self-assembly has been developed as an alternative route to such multiscale materials.1,4-6 In the selfassembly process, however, the precise control of feature scale and defect formation remains a challenge, particularly at the micrometer scales. To avoid such problems, twodimensional lithographic patterns have served as templates *Corresponding author.
(1) (a) Wan, Y; Zhao, D. Chem. Rev. 2007, 107, 2821. (b) Yang, P.; Deng, T.; Zhao, D.; Feng, P.; Pine, D.; Chmelka, B. F.; Whitesides, G. M.; Stucky, G. D. Science 1998, 282, 2244. (2) (a) Yi, G.-R.; Moon, J. H.; Manoharan, V. N.; Pine, D. J.; Yang, S. -M. J. Am. Chem. Soc. 2002, 124, 13354. (b) Liu, J.; Li, M.; Wang, J.; Song, Y.; Jiang, L.; Murakami, T.; Fujishima, A. Environ. Sci. Technol. 2009, 43, 9425. (c) Doherty, C. M.; Caruso, R. A.; Smarsly, B. M.; Adelhelm, P.; Drummond, C. J. Chem. Mater. 2009, 21, 5300. (d) Slowing, I. I.; Trewyn, B. G.; Giri, S.; Lin, V. S.-Y. Adv. Funct. Mater. 2007, 17, 1225. (e) Hagfeldt, A.; Gr€atzel, M. Acc. Chem. Res. 2000, 33, 269. (3) (a) Lim, S. H.; Feng, L.; Kemling, J. W.; Musto, C. J.; Suslick, K. S. Nature Chem. 2009, 1, 562. (b) Skrabalak, S. E.; Suslick, K. S. J. Am. Chem. Soc. 2006, 128, 12642. (4) Parker, A. R.; Townley, H. E. Nat. Nanotechnol. 2007, 2, 347. (5) Chiu, J. J.; Pine, D. J.; Bishop, S. T.; Chmelka, B. F. J. Catal. 2004, 221, 400. (6) (a) Rider, D. A.; Chen, J. I. L.; Eloi, J.-C.; Arsenault, A. C.; Russell, T. P.; Ozin, G. A.; Manners, I. Macromolecules 2008, 41, 2250. (b) Chae, W. -S.; Braun, P. V. Chem. Mater. 2007, 19, 5593. r 2010 American Chemical Society
for guiding three-dimensional self-assembled structures at submicrometer or nanometer scales.7 In this communication, we report a simple and controllable two-step method for fabricating hierarchical porous structures in threedimensional lithographic templates. In the proposed method, a 3D macroporous polymeric structure is first prepared by holographic lithography with array of microprisms, then a mesoporous structure is created within the macroporous polymeric scaffold by surfactant templating, and finally the polymeric macropores are removed, leaving behind hierachical macro-mesoporous structures. Inherently, holographic lithography (HL) is an ideal platform for creating periodic macroporous structures without defects. In this method, a 3D electromagnetic field distribution is formed by the constructive and destructive interference of multiple coherent beams with different wave vectors and polarization products. The symmetries and lattice parameters of the resulting interference pattern can be modulated by controlling the wavelength, polarization and beam arrangement.8 In conventional HL, the bulky and complex combination of mirrors and beam splitters are typically used to induce multidimensional light interference patterns. Instead, prism-based HL uses a single refracting prism, thus simplifying the overall beam setup and alignment. Recently, HL with a single prism was extended to HL with array of microprisms (μHL) to fabricate 2D array of 3D macroporous structures, in a single exposure process.9 Unlike the expensive materials and equipment used in the conventional method, the array system can use microprisms made of transparent elastomer (polydimethylsiloxane, PDMS) that can be easily replicated from a master with an inverted pyramid-shaped groove array. On the top and bottom surface of each microprism, the refraction and recombination of a laser beam occurs, respectively, creating a 2D array of 3D interference patterns. Finally, these structures can be transferred into well-defined polymeric macroporous structures by using a photosensitive resin (SU-8). The average exposure time for μHL was 0.15 s and the size of expanded laser beam covered more than 10000 microprisms with 100 μm 100 μm size. Therefore, the production rate of polymeric macroporous particles was much more than 6 10 4 per second. Optical setup including laser, electronic shutter, and microprism for μHL is provided in Figure S1a and S1b of the Supporting Information. (7) (a) Lin, K.-H.; Crocker, J. C.; Prasad, V.; Schofield, A.; Weitz, D. A.; Lubensky, T. C.; Yodh, A. G. Phys. Rev. Lett. 2000, 85, 1770. (b) Yang, S. M.; Miguez, H.; Ozin, G. A. Adv. Func. Mater. 2002, 12, 425. (c) Choi, D. -G.; Yu, H. K.; Jang, S. G.; Yang, S. -M. Chem. Mater. 2003, 15, 4169. (d) Kim, S. O.; Solak, H. H.; Stoykovich, M. P.; Ferrier, N. J.; de Pablo, J. J.; Nealey, P. F. Nature 2003, 424, 411. (e) Cheng, J. Y.; Ross, C. A.; Smith, H. I.; Thomas, E. L. Adv. Mater. 2006, 18, 2505. (8) Moon, J. H.; Ford, J.; Yang, S. Polym. Adv. Technol. 2006, 17, 83. (9) (a) Wu, L. J.; Zhong, Y. C.; Chan, C. T.; Wong, K. S.; Wang, G. P. Appl. Phys. Lett. 2005, 86, 241102. (b) Lee, S. -K.; Park, H. S.; Yi, G. -R.; Moon, J. H.; Yang, S. -M. Angew. Chem., Int. Ed. 2009, 48, 7000.
Published on Web 06/18/2010
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Figure 1. Fabrication scheme and the simulated evolution of the hierarchically ordered structures. (a) Brief scheme for the preparation of (i) hierarchical ordered array and (ii) free-floating microparticles. (b) Structure evolution calculated by solving interference equation: a polymeric HL structure featured by microprism array, a macroporous structure filled with precursor, and a hierarchically ordered macro-mesoporous structure obtained after removal of the polymer template.
The minimum size of the 3D structures was 20 μm using μHL with an array of microprisms of 50 μm in size. (see Figure S2 of the Supporting Information) Inorganic replica can be produced from the macroporous templates by infiltration of a precursor followed by calcination.2,10 Instead of conventional precursors for the sol-gel reaction, a silica precursor with a lyotropic surfactant phase was used for creating ordered mesopores in the replicated macroporous scaffold. The lyotropic silica precursor solution consists of tetraethylorthosilicate (TEOS), and a triblock copolymer (PEO106 PPO70 PEO106, Pluronic F127). The procedures used to fabricate arrayed and particulate hierarchical structures are shown schematically in Figure 1a. Figure 1b shows the simulated morphological evolution of macroporous templates and its inverse replica, which were determined by solving the interference equation for given electromagnetic waves (for details, see experimental section of the Supporting Information). Prior to infiltrating the precursor, the template was treated with oxygen plasma to make its surface hydrophilic and hence to promote capillary suction of the precursor solution. In addition, the structure of surfactant self-assembly can be improved by creating a surface that has a greater affinity for the hydrophilic ethylene oxide/silica blocks than the hydrophobic propylene oxide/silica blocks at the inner wall of macropores.6b,11 After oxygen plasma treatment, the precursor was infiltrated into the polymeric template and the system was annealed in a convection oven at 70 °C to activate hydrolysis reaction. Then, controlled calcination was performed at 500 °C for 10 h, leading to the creation of multiscale ordered structure with a triple hierarchy: a micrometer-scale particle array from the array of microprisms, a submicrometer-scale macroporous structures created by multibeam interference, and nanoscale mesoporous structures arising from self-assembled surfactant templates. Figure 2a-d shows various images of these (10) Villaescusa, L. A.; Mihi, A.; Rodrı´ guez, I.; Garcı´ a-Bennett, A. E.; Mı´ guez, H. J. Phys. Chem. B 2005, 109, 19643. (11) Sen, T.; Tiddy, G. J. T.; Casci, J. L.; Anderson, M. W. Chem. Mater. 2004, 16, 2044.
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Figure 2. Optical and electron micrographs from hierarchical structures matched with calculated morphologies. (a) Optical image of arrayed multiscale structures. Inset is the scanning electron microscope (SEM) image of the overall object. (b) SEM image shows morphology of macroporous structure with 400 nm feature size. Structure slightly distorted during calcination process. Inset is a calculated morphology of the macroporous structure after calcination. To simulate the structure closer to the actual morphology after calcination, we tilted the interference pattern 15° along the x- and y-axes. Distortion filter of the graphic software imitated the thermal distortion effect of the structure. Final processed image is very well coincident with (b) SEM image of the real feature. (c, d) TEM images showing both of submicrometer scale structure and nanoscopic mesoporous structures. Inset of c is the simulation graphic of the cross-sectional morphology. Pillar-like morphologies (red box) are well-matched with TEM and SEM images of b-d. (d) Magnified shot taken from the yellow box in c.
hierarchical structures at the order of increasing magnifications, taken using optical microscopy (Figure 2a), scanning electron microscopy (SEM, inset of Figure 2a and Figure 2b), and transmission electron microscopy (TEM, Figure 2c and 2d), respectively. These microscopic images were matched well with 3D graphic simulation images (inset of Figure 2b and 2c). Simulation images in the inset of Figure 2b and 2c are calculated from the interference equation presented in the experimental section. These images show well-arrayed microparticles on a silicon wafer substrate (Figure 2a), each of which has a macroporous structure (Figure 2b) as well as mesoporous structure on the macroporous scaffolds. (Figure 2c and 2d) (see also Figure S3, S5 and S6 of the Supporting Information). It is well-known that the surface area and pore-size distribution can be controlled by adjusting the surfactant/TEOS ratio and concentrations. Among the precursor compositions used in the present work (F127:TEOS with the mass ratio of 1:1.4, 1:2, 1:5), the 1:5 sample exhibited the most stable and well-defined features. The pore size of the sample was 5.0 nm and the estimated surface area (250 m2/g) was 28 times higher than that of polymeric macroporous structure formed by μHL alone.9b,12 It is well-known that the Fm3m or Im3m symmetry is the more probable and favorable in self-assembly of the TEOS/F127 system without any external potential. In the bulk phase diagram of the F127 system, the area of 2D hexagonal (12) Wang, X.; Ma, J.; Liu, J.; Zhou, C.; Zhao, Y.; Yi, S.; Yang, Z. Nanotechnology 2006, 17, 3627.
Communication
phase is very small compared with the areas of Fm3m, Im3m and disordered phase.1a However, the confinement of polymeric macroporous templates prepared by HL provides more favorable effect for forming 2D hexagonal phase during the self-assembly of F127 surfactant molecules. TEM images show hexagonal, tubular, and heterogeneous morphologies coexist among the resulted structures even at the fixed mass ratio of 1:5 (see Figure S7 of the Supporting Information). High-resolution TEM images (Figure 2c,d) disclosed that the hexagonal domains of mesopores formed and completely enveloped the surface of the macroporous structures. Outside the dotted area in Figure 2d, the morphology changed from hexagonal to polycrystalline tubular domains with significant border area (∼50 nm) against the hexagonal phase. These results suggest that the formation of hexagonal phase near the surface is obviously due to the confinement effect and the selective affinity of the surfactant molecules with the hydrophilic macropores during self-assembly. To achieve a complete transition of the mesostructures in the macropores, further study of the morphology evolution in the smaller macropores is required.6b,11,13 TEM images at different mass ratio can be seen in Figure S8 of the Supporting Information. Additionally, small-angle X-ray diffraction data taken from the 2D array of hierarchical structures confirmed that the mesoporous structures were successfully formed (see Figure S9 of the Supporting Information). The presence of the mesopores is expected to greatly enhance the loading capacity of the materials, whereas the macroscopic structure should afford a high mass-transfer rate. To compare the loading capacity of the hierarchically featured sample with that of holographically featured polymeric particles and structureless particles, all samples were immersed in an 80 μM solution of the fluorescent dye fluorescein isothiocyanate (FITC) for 5 min then repeatedly washed with pure ethanol for 24 h. Although the holographically featured polymeric particles showed about 10 times higher fluorescent loading compared with structureless particles, our hierarchical structure exhibited an additional 15-fold amplification compared with the holographically featured macroporous structure.9b This enhancement in fluorescent intensity arises from the difference in surface areas among the particles with and without additional mesoporous structures (see Figure S10 of the Supporting Information). Instead ot typical sol-gel reaction generally used for templating, our approach can be applied for the formation of metallic structures via deposition or etching techniques (see Figure S11 of the Supporting Information). Additionally, free-floating particles with internal periodic structures exhibit interesting properties and find potential applications.2a,14 As shown in Figure 3a, macroporous polymeric particles filled with lyotropic precursor could be detached from the microarray by means of a sacrificial release layer and formed a colloidal dispersion of hierarchical particles. After removing the polymer template by calcination on the silicon wafer substrate, the resulting particles finally have (13) (a) Li, F.; Wang, Z.; Ergang, N. S.; Fyfe, C. A.; Stein, A. Langmuir 2007, 23, 3996. (b) Sakurai, M.; Shimojima, A.; Heishi, M.; Kuroda, K. Langmuir 2007, 23, 10788. (14) Jang, J. -H.; Dendukuri, D.; Hatton, T. A.; Thomas, E. L.; Doyle, P. S. Angew. Chem., Int. Ed. 2007, 46, 9027.
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Figure 3. Scheme for free-floating particles and its differential image contrast (DIC) image dispersed in the aqueous medium. (a) Experimental scheme for the preparation of the free-floating hierarchical particles. (b) Silicate macro-mesoporous particles whose refractive index matches with the surrounding aqueous media. Inset shows a mixture of polymeric macroporous particles (opaque) and silica macro-mesoporous particles (transparent) with significant contrast discrepancy.
both well-organized macropores and mesopores. Figure 3b shows the optical micrographs of the free-standing silicate particles with dual length scale pore structures. When such particles were immersed in an aqueous medium, the multiscale silica particles with macro- and mesopores became transparent because of the refractive index matching with the surrounding solvent medium (see also Figure S3 of the Supporting Information). Inset of Figure 3b shows the significant contrast difference between holographically featured polymeric macroporous particles and hierarchically ordered silicate particles from the mixture of those in aqueous phase. As a result, the transparent silicate particles could be easily distinguished from the dark polymeric particles through the differential interference contrast (DIC) images. (TE-2000U, Nikon) In summary, microarrayed and free-standing particles with multiple hierarchies were successfully created by combining μHL and surfactant templating. The combined scheme provides definite advantages for generating hierarchically ordered structures, including simple and defectfree fabrication of hierarchical structures in a controlled manner. An array-based feature has advantages for the parallel analysis of chemical and biomolecular detection and screening. The dual-length-scale structure of the macropores and mesopores in a single microparticle gives not only high surface areas but also high mass- and heat-transfer rates. These characteristics are ideal for a wide range of potential applications including catalytic supports, drug carriers, and photovoltaic and battery materials. In addition, a simple HL with array of microprisms provides mass productivity in practical synthesis of these novel structures. Acknowledgment. This work was supported by a grant from the CRI Program of the MEST for “Complementary Hybridization of Optical and Fluidic Devices for Integrated Optofluidic Systems.” Partial support from the Brain Korea 21 Program is also appreciated. G.R.Y. acknowledges the support by the NRF grant (2009-0082451) funded by the MEST. We thank T. S. Bae at KBSI for help with UHR-SEM analysis. Supporting Information Available: Detailed experimental method and data (PDF). This material is available free of charge via the Internet at http://pubs.acs.org..
