Pore Structure and Interconnectivity of CdS Aerogels and Xerogels by Hyperpolarized Xenon NMR

Abstract

Metal chalcogenide aerogels and xerogels are unique materials that combine semiconducting nanostructures with porosity and are thus of interest for photocatalysis and sensing. To assess the feasibility for applications dependent on molecular transport, the pore structure and interconnectivity in CdS aerogels and xerogels were probed by a combination of conventional techniques and hyperpolarized (HP) 129Xe NMR. HP 129Xe NMR of the aerogels was consistent with two distinct types of accessible pores (adsorption sites), and 2-D EXSY NMR data suggest that these are connected. In contrast, a single resonance of low intensity with a temperature dependence consistent with the dissolution of Xe within a polymer matrix, that is, transport facilitated at high temperatures due to chain motion, is observed in xerogels. 13C NMR and thermal gravimetric analysis data reveal the presence of residual organics in both xerogels and aerogels, but a relatively higher proportion in the former, due, in part, to the surfactant used in their preparation. These data, combined with the absence of any meaningful adsorption isotherm for N2 at 77 K (where limited thermal chain motion precludes gas adsorption) are consistent with a dynamic blocking of the xerogel pores. The results reveal the important role of residual organics and drying regimens on the accessibility of pores in metal chalcogenide gel architectures.