Facile synthesis of highly ordered mesoporous silver using cubic mesoporous silica template with controlled surface hydrophobicityw

Recently, there has been great interest in the synthesis and application of various kinds of nanostructured noble metals such as platinum, gold and silver. Nanostructured silvers have been widely investigated as antimicrobial materials, bio-sensors, catalysts, and advanced materials exhibiting surface plasmon resonance and surface enhanced Raman scattering. All this research is mainly based on the successful synthesis of nanostructured silvers such as spheres, triangles, cubes, rods, etc. Porous metallic materials have also attracted considerable attention because they have several advantages such as increased catalytic capabilities due to high surface area, enhanced transport of reactants, and ease of catalyst recovery. Mesoporous materials, obtained by favourable self-assembly between organic templates and inorganic precursors, have opened up many new possibilities for applications in catalysis, separation, and nanoscience due to their large, controllable pore sizes, high surface areas, and easy functionalization. Synthesis of mesoporous materials has been extended from silica-based materials to non-siliceous materials such as carbon, metal oxides and metals. Even though there have been some successful attempts to obtain non-siliceous materials through direct synthesis using organic templates, it is still challenging because the mesostructures of such materials may collapse during the removal of templates. Recently, a nano-replication route, which utilizes mesoporous silica or carbon materials as a hard template has been regarded as a promising method for the preparation of non-siliceous mesoporous materials. However, the preparation of mesoporous metals through the nano-replication route has been less successful. Only the preparation of mesoporous metals such as platinum, osmium and palladium has been reported. Even though there are some reports on the formation of small organized silver network domains in MCM-48 and the preparation of silver nanowires using SBA-15 template, controlled growth of silver networks within the pores of a mesoporous silica template seems to be more difficult. Here, we demonstrate a facile method for the preparation of highly ordered mesoporous silver using cubic mesoporous silica (KIT-6) with controlled hydrophobicity as a hard template. It is well-known that the surface properties of supports are of much importance for the formation of metallic nanoparticles and their stabilities. Recently, mono-dispersed silver nanoparticles with high thermal stability within the pores of mesoporous silica materials have been synthesised by the modification of a silica surface with organic moieties that are able to strongly interact with silver nanoparticles. However, in the present work, we have modified the silica surface with hydrophobic methyl groups in order to decrease the interaction between the silver precursors and pore surfaces, resulting in the easy aggregation of precursors within the mesopores before reduction to the metallic domain. The mesoporous silica template, KIT-6, was synthesized following methods reported elsewhere. A triblock copolymer (Pluronic P123, EO20PO70EO20, Aldrich) and tetraethylorthosilicate (TEOS, Aldrich) were utilized as the structure-directing agent and framework source, respectively. After calcination, the silica surface of KIT-6 was modified with methyl groups by refluxing a mixture containing 0.6 g hexamethyldisilazane (HMDS, 99%, Fluka), 150 mL of n-hexane and 3.0 g of calcined KIT-6. The material thus obtained is denoted as HP-KIT-6 (hydrophobic KIT-6). BET surface areas and BJH pore sizes decreased upon surface modification (Fig. S1 and Table S1w). TGA data showed that the organic content of HP-KIT-6 was about 2.7 wt% (Fig. S2 and Table S1w). C solid state NMR data (Fig. S2w) also indicated the existence of carbon species after modification. In order to synthesize the mesoporous silver, a solution, containing 1.18 g of AgNO3 (99%, Aldrich), 0.9 g of EtOH and 0.9 g of distilled water, was impregnated into 3.0 g of the HP-KIT-6 through an incipient wetness method. After drying at 353 K for 24 h, the sample was heated to 573 K under nitrogen flow for 2 h. After the reduction process, the silica template was completely removed by treating the composite material with 3 M NaOH solution three times at 273 K. Finally, the mesoporous silver material thus obtained was washed with distilled water and acetone several times and dried at room temperature. Fig. 1 shows X-ray diffraction (XRD) patterns for the HP-KIT-6 template and the replicated mesoporous silver material. As shown in Fig. 1a, the HP-KIT-6 exhibits typical Department of Chemistry, BK21 School of Chemical Materials Science and SKKU Advanced Institute of Nanotechnology, Sungkyunkwan University, Suwon, 440-746, Korea. E-mail: [email protected]; Fax: +82 31 299 4174; Tel: +82 31 299 4177 b Separation Process Research Center, Korea Institute of Energy Research, Daejeon, 305-343, Korea. E-mail: [email protected]; Tel: +82 42 860 3112 w Electronic supplementary information (ESI) available: N2 sorption data, TGA data, C solid state NMR spectra, SEM images, XRD patterns, and schematic diagram. See DOI: 10.1039/b811718g