Langmuir 2006, 22, 2311-2322
2311
Principles of Hierarchical Meso- and Macropore Architectures by Liquid Crystalline and Polymer Colloid Templating Ozlem Sel,† Daibin Kuang,† Matthias Thommes,‡ and Bernd Smarsly*,† Max-Planck Institute of Colloids and Interfaces, Am Mu¨hlenberg 1, D-14424 Potsdam, Germany, and Quantachrome Instruments, 1900 Corporate DriVe, Boynton Beach, Florida 33426 ReceiVed July 29, 2005. In Final Form: NoVember 24, 2005 The generation of porous silica with hierarchically organized bimodal mesoporosity of adjustable size and welldefined shape was investigated by using surfactant mixtures and the nanocasting procedure (liquid crystalline templating). A systematic study of combinations of various block copolymers (Pluronics F127, KLE (poly(ω-hydroxypoly(ethyleneco-butylene)-co-poly(ethylene oxide))) and SE (PS-co-PEO)) with smaller surfactants (Pluronics P123, C16mimCl, and CTAB) revealed that hierarchical bimodal mesopore architectures could only be obtained by the usage of block copolymers with a strong hydrophilic-hydrophobic contrast, such as KLE and SE, giving rise to pores between 6 and 22 nm. Furthermore, the ionic liquid (IL) C16mimCl appeared to have advantageous templating properties, resulting in 2-3-nm pores being located between the block copolymer mesopores, whereas phase separation was observed for Pluronics and CTAB as small templates. Thereby, the study provided also general insights into the mixing and co-self-assembly behavior of block copolymers and ionic surfactants in water and confirmed the special templating properties of ILs, as recently proposed. In addition to the bimodal mesoporosity, additional tunable macroporosity was created by the presence of poly(styrene) or poly(methyl methacrylate) spheres, leading to well-defined trimodal hierarchical pore architectures with the small pores being located in the walls of the respective larger pores. As a major improvement, due to the pore hierarchy, these large-pore materials showed relatively large surface areas and pore volumes, and the size of densely packed macropores could even be decreased down to 90 nm. The materials were characterized by electron microscopy, small-angle X-ray scattering, and nitrogen sorption using a proper NLDFT (nonlocal density functional theory) approach for calculations of the pore size distribution in the entire range of microand mesopores.
Introduction Hierarchical porous materials are of interest in fundamental research and for practical applications in catalysis, separation, adsorption, or electrode materials.1-3 Various types of hierarchical bi- or trimodal porous materials have been reported recently, in particular, micro-macro,4 micro-meso,5 meso-macro,6a,b small meso-large meso,7a,b micro-meso-macro,8a,b,c and small mesolarge meso-macro,9a using ionic liquids as templates for small mesopores.9a,b,c In this context, we define a hierarchical pore architecture as a 3D arrangement of well-defined pores of different sizes, the smaller ones being located in the walls between the larger pores, thereby also establishing the connectivity. (See * Corresponding author. E-mail:
[email protected]. Tel: +49 331 567 9509. Fax: +49 0331-567-9502 † Max-Planck Institute of Colloids and Interfaces. ‡ Quantachrome Instruments. (1) Hagfeldt, A.; Gratzel, M. Chem. ReV. 1995, 95, 49. (2) Chiu, J. J.; Pine, D. J.; Bishop, S. T.; Chmelka, B. F. J. Catal. 2004, 221, 400. (3) Pauly, T. R.; Liu, Y.; Pinnavaia, T. J.; Billinge, S. J. L.; Rieker, T. P. J. Am. Chem. Soc. 1999, 121, 8835. (4) Zhou, Y.; Antonietti, M. Chem. Commu. 2003, 2564. (5) Newalkar, B. L.; Katsuki, H.; Komarneni, S. Microporous Mesoporous Mater. 2004, 73, 161-170. (6) (a) Antonietti, M.; Berton, B.; Goltner, C.; Hentze, H. P. AdV. Mater. 1998, 10, 154-159. (b) Mann, S.; Burkett, S.; Davis, S.; Fowler, C.; Mendelson, N.; Sims, S.; Walsh, D.; Whilton, N. Chem. Mater. 1997, 9, 2300-2310. (7) (a) Sun, J. H.; Shan, Z.; Maschmeyer, T.; Coppens, M. O. Langmuir 2003, 19, 8395-8402. (b) Okabe, A.; Niki, M.; Fukushima, T.; Aida, T. J. Mater. Chem. 2005, 15, 1329-1331. (8) (a) Sen, T.; Tiddy, G.; Casci, J.; Anderson, M. W. Angew. Chem. Int. Ed. 2003, 42, 4649-4653. (b) Sen, T.; Tiddy, G. J. T.; Casci, J. L.; Anderson, M. W. Chem. Mater. 2004, 16, 2044-2054. (c) Brandhuber, D.; Huessing, N.; Raab, C. K.; Torma, V.; Peterlik, H. J. Mater. Chem. 2005, 15, 1801. (9) (a) Kuang, D. B.; Brezesinski, T.; Smarsly, B. J. Am. Chem. Soc. 2004, 126, 10534-10535. (b) Smarsly, B.; Kuang, D.; Antonietti, M. Colloid Polym. Sci. 2004, 282, 892-900. (c) Brezesinski, T.; Erpen, C.; Iimura, K. I.; Smarsly, B. Chem. Mater. 2005, 17, 1683-1690.
Scheme 1; the varying degree of additional micropores (
