Formation of graphitic structures in cobalt- and nickel-doped carbon aerogels.
ABSTRACT We have prepared carbon aerogels (CAs) doped with cobalt or nickel through sol-gel polymerization of formaldehyde with the potassium salt of 2,4-dihydroxybenzoic acid, followed by ion exchange with M(NO3)2 (where M = Co2+ or Ni2+), supercritical drying with liquid CO2, and carbonization at temperatures between 400 and 1050 degrees C under a N2 atmosphere. The nanostructures of these metal-doped carbon aerogels were characterized by elemental analysis, nitrogen adsorption, high-resolution transmission electron microscopy (HRTEM), X-ray photoelectron spectroscopy (XPS), and X-ray diffraction (XRD). Metallic nickel and cobalt nanoparticles are generated during the carbonization process at about 400 and 450 degrees C, respectively, forming nanoparticles that are approximately 4 nm in diameter. The sizes and size dispersion of the metal particles increase with increasing carbonization temperatures for both materials. The carbon frameworks of the Ni- and Co-doped aerogels carbonized below 600 degrees C mainly consist of interconnected carbon particles with a size of 15-30 nm. When the samples are pyrolyzed at 1050 degrees C, the growth of graphitic nanoribbons with different curvatures is observed in the Ni- and Co-doped carbon aerogel materials. The distance of graphite layers in the nanoribbons is approximately 0.38 nm. These metal-doped CAs retain the overall open cell structure of metal-free CAs, exhibiting high surface areas and pore diameters in the micro- and mesoporic region.
- SourceAvailable from: Ana Arenillas12/2011; , ISBN: 978-953-307-913-4
- [Show abstract] [Hide abstract]
ABSTRACT: To aim at thermal insulator applications, the shrinkage and the pore structure of resorcinol–formaldehyde (RF) aerogels and carbon aerogels were investigated during the supercritical drying and the carbonization process. The water (W) molar ratio has small effects on the surface area or the particle size, but has significant effects on the density of the aerogel. Higher W/R ratio leads to lower density and larger pore size, and leads to less shrinkage during the carbonization process. The molar ratio of catalyst sodium carbonate (C) has significant effects on the shrinkage, pore size, and particle size of the aerogel. Lower R/C ratio leads to smaller particle size and smaller pore size, and thus induces more shrinkage both in the supercritical drying and in the carbonization, the obtained CA is much denser. The R/C ratio should be higher than 300 to prevent excessive shrinkage. In order to synthesize carbon aerogels combining with small shrinkage, low density (less than 0.1g/cm3), and small pore size (less than 150nm) for thermal insulators, the preferred W/R ratio is between 90 and 100, and the preferred R/C ratio is between 300 and 600. KeywordsCarbon aerogels–Supercritical drying–Linear shrinkage–Pore sizeJournal of Sol-Gel Science and Technology 01/2011; 59(2):371-380. · 1.66 Impact Factor
- [Show abstract] [Hide abstract]
ABSTRACT: Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Materials Science and Engineering, 2006. Includes bibliographical references (leaves 219-233). Carbon aerogels offer several unique advantages which make them ideal for evaluating a metal's ability to catalyze nanotube growth, including in situ carbothermic reduction of oxidized nanoparticles to their catalytic metallic phase as they form and production of a bulk quantity of nanoparticles which can be easily characterized. In this work, metal-doped carbon aerogels of seven transition metals were synthesized, characterized, and evaluated for their ability to catalyze growth of carbon nanotubes by thermal chemical vapor deposition (CVD). It was found that carbon aerogels doped with Fe, Rh, Re, Au, and Nb all catalyzed the formation of nanotubes in moderate to high yields, resulting in a direct growth of nanotubes on the exterior surfaces of aerogel monoliths. Ta was found to grow nanotubes only after thorough reduction of its oxides. Growth with W was inconclusive. CVD growth of nanotubes throughout the interior porosity of metal-doped carbon aerogels was also achieved by templating a network of interconnected macropores into the monoliths. Surface-based nanoparticles composed of rhenium, gold, and varying combinations of gold and rhenium were investigated for their ability to catalyze carbon nanotube growth. (cont.) Nanoparticles of these metals were nucleated onto silicon wafers from solutions of anhydrous ReCI5 and AuC13. After deposition, the nanoparticles were reduced under hydrogen for 10 min and then oxidized in air for 4 min. The samples were then processed by CVD employing hydrogen and ethanol-saturated Ar for 10 min. Nanoparticles deposited from metal chloride solutions with a 1:1 molar ratio of gold to rhenium or higher were found to result in high yields of single-walled nanotubes, where nanoparticles deposited from solutions with less than a 1:4 gold-to-rhenium ratio resulted in no nanotube growth. Lastly, a new low-pressure CVD system specialized for nanotube growth was developed. The objectives of the system are to provide a flexible architecture for developing new nanotube growth techniques and to lower the minimum temperature required for nanotube growth. The system features a separate sample heating plate for thermally activating nanoparticles and hot filament for carbon feedstock cracking. The system also features the ability to easily install or remove modules for electric field- and plasma-assisted growths. by Stephen Alan Steiner, III. S.M.