Systematic study of high surface area aerogels: Copper oxide/silica composites and metal sulfides (metal = cadmium, zinc, and lead)

This dissertation research has focused on exploring two methods of aerogel property modification: (1) framework alteration by composite formation using an existing aerogel matrix, and (2) new framework development. Specifically, the first approach is adopted for the synthesis of Cu(OAc) 2/silica aerogels using sol-gel chemistry and supercritical fluid extraction techniques. A systematic study was conducted to test the effect of processing parameters (copper precursors, pH, and annealing temperature) on structural characteristics, homogeneity, crystallinity, copper speciation, particle size and distribution, and surface area. This study has found that the incorporation of copper(II) acetate leads to the anchoring of an octahedrally distorted Cu2+ species on the gel surface, retardation of CuOx segregation with temperature, and creation of aerogels with higher surface areas and pore volumes compared to the pristine silica aerogel. Gels synthesized under varying pH conditions exhibit different rates of CuOx crystallization, aggregate size, and structural transformations during thermal treatments. In this regard, the base catalyzed composite xerogels and aerogels produce a greater number of smaller crystallites that approach bulk CuO; whereas, the corresponding acid catalyzed gels exhibit larger crystallites with some degree of stranded Cu2+ even after annealing up to 900°C.; The second approach for modifying aerogel properties was based on developing new aerogel frameworks using metal chalcogenides. This has led to the successful synthesis of the first chalcogenide aerogel, CdS. The synthesis was achieved by the controlled aggregation of CdS nanoparticles to form a wet gel, and the subsequent supercritical fluid drying to generate a monolithic aerogel. The CdS aerogel surface area (250 m2/g) is comparable to that of a silica aerogel (601 m2/g) on a per mol basis. It also has a low skeletal density (0.07 cc/g) and pores ranging from the micro-to-macro regime. Unlike silica aerogels, the nanoparticle components of the CdS aerogels are crystalline and exhibit quantum confinement effects manifested in a sharp band edge luminescence, and a higher energy band gap compared to bulk samples.; Two additional chalcogenide aerogels (ZnS and PbS) were prepared using the methodology developed for the CdS system, thus demonstrating the generality of the procedure. These new aerogels are comparable to the CdS aerogels with respect to surface areas, crystallite dimensions and quantum confinement effects. Together, PbS, ZnS, and CdS aerogels have band gaps that span a range of energies from the IR (PbS) to the visible (CdS) to the UV (ZnS).