Sol−Gel Processing of Semiconducting Metal Chalcogenide Xerogels:  Influence of Dimensionality on Quantum Confinement Effects in a Nanoparticle Network

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−Emmett−Teller (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).