Chalcogenide aerogels based entirely on semiconducting II-VI or IV-VI frameworks have been prepared from a general strategy that involves oxidative aggregation of metal chalcogenide nanoparticle building blocks followed by supercritical solvent removal. The resultant materials are mesoporous, exhibit high surface areas, can be prepared as monoliths, and demonstrate the characteristic quantum-confined optical properties of their nanoparticle components. These materials can be synthesized from a variety of building blocks by chemical or photochemical oxidation, and the properties can be further tuned by heat treatment. Aerogel formation represents a powerful yet facile method for metal chalcogenide nanoparticle assembly and the creation of mesoporous semiconductors.

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The synthesis and characterization of porous nanostructured inorganic polymers (gels and xerogels) of CdS, ZnS, PbS, and CdSe is described. Primary particles are synthesized via water-in-oil microemulsions, surface complexed with thiolate ligands, and dispersed in acetone. Oxidation of the thiolate ligands with H 2 O 2 leads to particle aggregation and the formation of wet gels, which are then dried under ambient conditions to produce xerogels. The xerogels exhibit optical band-edges that are intermediate between bulk precipitates and the more porous wet gels and aerogels, suggesting the extent of quantum confinement in such colloidal networks is intimately related to the pore structure and surface area (dimensionality). Heating of the xerogels in vacuo results in a continued decrease in the energy of absorption onset due to sintering of the nanoparticle network, and this is also reflected in a growth in crystallite size as probed by powder X-ray diffraction. The resulting xerogel networks demonstrate modest Brunauer-EmmettTeller (BET) surface areas of 29-65 m 2 /g with Barrett-Joyner-Halenda (BJH) average pore sizes of 3-15 nm, considerably smaller than values obtained for corresponding aerogels and consistent with increased density (decreased dimensionality).

A systematic study of the effects of pH, precursor salt, and temperature on the surface area, particle size, and copper speciation in 3 wt % CuO in silica aerogels and xerogels has been conducted. The materials were synthesized by copolymerization sol−gel reactions, followed by either supercritical solvent extraction using CO2 (aerogels) or conventional drying (xerogels) and annealing in air. Among the six different copper precursors examined, copper(II) acetate was found to be the best in terms of its stability, solubility, and binding capacity of copper in the silica matrix. Aerogels prepared with copper(II) acetate were found to have considerably higher surface areas than the corresponding xerogels, and the observed formation of CuO particles occurred at higher temperatures (900 vs 650 °C, by TEM analysis). X-ray diffraction of gels annealed at 900 °C likewise revealed the presence of nanocrystalline CuO, with larger particles observed in acid-catalyzed aerogels vs base-catalyzed aerogels (∼26 vs ∼15 nm, respectively). Additionally, in 900 °C annealed acid- and base-catalyzed xerogels, the silica matrix crystallized as crystobalite and quartz, respectively. EPR data suggest that the Cu2+ environment goes through a series of structural changes upon annealing. These changes are significantly different in acid- vs base-catalyzed systems because of differences in the internal structures and pore distributions of the materials. In both cases, a broad isotropic component, attributed to CuO, is observed upon heating. By 900 °C, the hyperfine peaks originally attributed to dispersed Cu2+ are absent in base-catalyzed systems, but they are still present as a small fraction in acid-catalyzed systems, suggesting incomplete transformation in the latter case.

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