Novel Silica-Sol Mediated Synthesis of Nanoporous Carbons
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to produce porous carbons [2]. The synthesis of mesoporous M41S silica materials by the Mobil researchers stimulated explosive research on the preparation of inorganic porous materials through template approaches [3]. Wide ranges of mesoporous inorganic materials have been prepared using various types of organic templates including surfactant self-assemblies and block copolymers [4]. Uniform sized polymer latex spheres and emulsions have been also utilized to produce mesoporous inorganic materials with larger pores of > 10 nm [5]. In this presentation, we introduce a new synthetic methodology to produce nanoporous carbons using inorganic silica particles as templates. EXPERIMENT Aqueous colloidal silica sol solutions, Ludox HS-40 (40 wt. % silica in water, average particle size - 12 nm) and SM-30 (30 weight percent of silica with average particle size of 8 nm) were purchased from the Aldrich Chemicals Co. Resorcinol and formaldehyde (RF) were
93 Mat. Res. Soc. Symp. Proc. Vol. 593 © 2000 Materials Research Society
polymerized in the presence of silica sol particles to generate silica-RF composites. Carbonization followed by the HF etching of the composites produced porous carbons. In a typical synthesis, a mixture of 1:2 molar ratio of resorcinol and formaldehyde was added to a commercially available Ludox HS-40 silica sol solution. The molar ratio of the starting mixture is 1 Resorcinol: 2 Formaldehyde: 4.5 Si0 2:86 H20. The reaction mixture with an initial pH of 8.0 was aged at 85 'C for one week to get a silica-RF gel composite. For carbonization, the composite was heated under nitrogen atmosphere from room temperature to 850 'C with a heating rate of 5 °C/min and held at that temperature for 3 hours. The resulting silica-carbon composite was stirred in 48 wt. % aqueous HF solution for 12 hours, resulting in the formation of a carbon material. The carbon materials produced were designated as SMC (Silica solMediated synthesized Carbon)-1. The carbon content of SMC-1 was over 93 weight percent
after HF etching (a typical elemental analysis result: 93.5 % C, 0.58 % H, and 0.08 % N), demonstrating that most of silica template has been removed. In order to get more uniform pore size distribution, isolated silica particles stabilized by surfactant were applied as templates. In a typical synthesis, 5 g of cetyltrimethylammonium bromide (CTAB) was added to 100 mL of an aqueous Ludox HS-40 silica sol solution (40 weight percent of silica with average particle size of 12 nm), and the mixture was stirred for 20 minutes at 50 "C, resulting in a muddy slurry. The stabilized silica particles were retrieved by filtration under vacuum, and washed with double-distilled water to remove un-adsorbed surfactant. Polymerization, carbonization, etching steps is similar to those for SMC-1 syntheses. The resulting carbons were designated as SMC-2. RESULTS Synthesis of Nanoporous Carbons using Ludox silica sols as Templates Nanoporous SMC-l carbons have been synthesized using various amount of silica template at the initial pH of 8 a
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