Fabrication of Ordered Macroporous Structures Based on Hetero-Coagulation Process Using Nanoparticle as Building Blocks Fengqiu Tang, Hiroshi Fudouzi, Tetsuo Uchikoshi, Toru Awane, and Yoshio Sakka  National Institute for Materials Science, 1-2-1, Sengen, Tsukuba, Ibaraki 305-0047 (Received October 2, 2002; CL-020846) Asimplemethodbasedonhetero-coagulationprocessforthe preparation of well-defined ordered macroporous inorganic materials from nanoparticles and spherical polymer templates is reported. Thesynthesisofmacroporousmaterialswithahighlydefined pore distribution have attracted much attention due to their potential applications as separation and absorbent media, cata- lytic supports, insulators, biomaterials, lightweight structural materials and photonic crystals. 1{6 These materials are created typically by colloidal crystals such as silica beads or polymer latexes, which are often the best templates used to prepare macroporous materials. 4{10 The template is first infiltrated with various materials, such as ceramic precursors, metals and polymers and subsequently removed by calcination or solvent extraction to create macroporous materials. To date, inorganic materials 11;12 such as silica, titania, zirconia, and some organic materials 6;7;13 with well-defined submicron-sized macropores have been successfully synthesized. However, it has proved difficult to fully infiltrate the liquid precursors into the voids by capillary action, as a result, the frameworks of porous materials have been mechanically fragile, easily cracked and difficult to prepare large-sized bulk materials. Moreover, one drawback of this method is that the quality of the porous structure is highly dependent on the quality of the template crystal itself. The fabrication of highly ordered macroporous materials using particles instead of liquid precursors is of particular interest since many oxide and semiconductor materials are now easily available as nanoparticles with lower cost, which might be advantageous for some applications. Recently, some studies have focused on the fabrication of macroporous thin films on a glass or silicon substrates using nanoparticles as building blocks upon water evaporation by means of capillary forces, 14{16 but it was very time-consuming for solvent evaporation and difficult to prepare thick deposits of this kind of structures. Here we demonstrate a straightforward way to prepare large- sized macroporous materials based on the hetero-coagulation process of oppositely charged core and shell materials in a suspension. First, by appropriate surface charge modification, both template spherical polymers and inorganic nanoparticles are dispersed in aqueous media with opposite surface charges separately. In a second step, the two suspensions are well mixed to form core-shell composites via electrostatic attraction. The hetero-coagulated mixture is then filtered to form an ordered structure of inorganic-organic composites. Finally, the polymer templateisremovedtoproduceanordered3Dmacroporousmetal oxide after calcination. In our experiments, the spherical polymer PMMA with an average diameter of 350 nm (P350) was used as the template material; g -Al 2 O 3 powder with an average particle size of 34 nm was used as the inorganic building blocks. The structures of the macroporous materials are highly dependent on the properties of the starting materials and suspensions, such as zeta potential, particle size and volume ratio of the two powders. In order to fabricate the core-shell composite with uniform structures via our strategy, the key point is to prepare well-dispersed suspensions of both the template and the nanoparticle materials in a same pH range. Figure 1 shows the zeta potential (z)of g -Al 2 O 3 and P350 versus pH measured by a laser-doppler electrophoresis analyzer. P350 is negatively charged in the measured pH range of 3–12. A relatively high z value can be obtained between pH 7 and 11, indicating that the P350 suspension is well-dispersed in this pH range. On the other hand, the g -Al 2 O 3 with a highly positive surface charge (z  30 mV) can be obtained below pH 8. Hence, well-dispersed suspensions of both the g -Al 2 O 3 and the P350 can be obtained in the pH range of 7–8 as shown by the dotted line in figure. In a typical synthetic procedure, 1.5 g of the g -Al 2 O 3 powder was dispersed in 40 ml of distilled water at pH 7. Thirty ml of aqueous suspension containing 1 g of P350 was also adjusted to pH 7. The suspensions were ultrasonicated for 10 min and then stirred for 1 h to ensure their good dispersal. Afterwards, the Al 2 O 3 suspensionwasslowlyaddedtotheP350suspensionunder stirring so that the smaller Al 2 O 3 particles can uniformly adhere to the polymer surface to form the core-shell structure via electrostatic attraction. The resulting flocculated mixture was then vacuum filtrated to produce the close packed deposits of the core-shell structure, which can be finished in several tens of minutes. After drying, the polymer spheres were removed by calcinationat500  C for4hinair,thenfurtherheat-treatmentwas continually conducted for 2 h at 850  C at a heating rate of 1  C/ min to enhance the mechanical strength of materials. 2 4 6 8 10 12 pH -40 -20 0 20 40 60 80 Zeta potential / mV y-Al2O3 PMMA350nm Figure 1. Zeta potentials of g -Al 2 O 3 and polymer P350 in aqueous suspensions. 276 Chemistry Letters Vol.32, No.3 (2003) Copyright Ó 2003 The Chemical Society of Japan