Structural studies of carbon aerogels and their metal-loaded derivatives

Date of Completion

January 2006

Keywords

Engineering, Materials Science

Degree

Ph.D.

Abstract

Carbon aerogels (CAs) are a class of mesoporous materials derived from the sol-gel polymerization of resorcinol (R) and formaldehyde (F) under base catalyse (C) in an aqueous solution, followed by supercritical drying and pyrolysis. CAs are characterized by highly-uniform pore sizes that can be tailored according to the reaction conditions, high surface areas, low densities, and high electrical conductivities. Due to the combination of these properties, CAs receive wide attention for such applications as the electrodes in fuel cells and supercapacitors, and supports for catalytic noble metals, to name but a few. Their high infrared optical absorption coefficients also make them promising materials for high-temperature thermal insulation and black broad-band absorption. ^ This dissertation is a comprehensive study of the structural aspects of naked CAs and their metal-loaded derivatives. As-prepared CAs were also subjected to a series of post-pyrolysis treatments, such as high-temperature heat-treatment and in-situ electron irradiation in TEM. In the first part of this study, it is shown using nitrogen physisorption and transmission electron microscopy (TEM) that the pore characters of CAs can be engineered by systematically adjusting the key parameters of the initial R-F reaction. These parameters include the ratio of resorcinol to catalyst (R/C) and the ratio of resorcinol to water (R/W). Such adjustments resulted in the preparation of a series of CAs with average pore diameter ranging from 4 to 22nm. In a parallel study, it is shown that the use of acidic catalyst caused a drastic change in the pore type of the resulting materials. In this case, macropores are dominant. ^ A selected group of noble metals (ruthenium and platinum) were incorporated into CA substrates via a novel supercritical carbon dioxide deposition, followed by a thermal reduction of the metal precursor. TEM results show that the metal nanoparticles thus prepared exhibit surprisingly uniform size distributions with no sign of coalescence. The character of the metal-CA nanocomposites was investigated using a range of instrumentation such as TEM, hydrogen chemisorption, XRD, and EDXS. ^ The microstructure of the as-synthesized CAs is modified significantly by heat-treatment at temperatures up to 2250°C in an inert atmosphere. Both the TEM and Raman IR data confirm that the structural modification is characterized by the graphitization of the initial CA structure, which is an intimate mixture of sp2- and sp 3-bonded carbons. The heat-treated CAs show improved oxidation resistance, as indicated by the thermogravimetric analyses (TGA). Additional studies on the radiolytic behavior of as-synthesized and heat-treated CAs as well as their metal-loaded derivatives show that they are all capable of undergoing further structural re-ordering to yield the common product of carbon onions. Furthermore, metal nanoparticles (ruthenium and cobalt) were found to be encapsulated tightly by multiple layers of hermetic graphene layers when the starting metal-bearing nanocomposites were subjected to heat-treatment at 2000°C under an inert environment. ^

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