A new addition to the aerogel community: unsupported CdS aerogels with tunable optical properties

Aerogels composed of interconnected networks of CdS building blocks have been prepared from controlled aggregation of discrete nanoparticles followed by supercritical fluid extraction. The resultant materials have surface areas up to 242 m2/g and average pore diameters in the range of 15–25 nm. Powder X-ray diffraction indicates that the average crystallite size is on the order of 2 nm in 100 °C heated aerogels, but increases to nearly 16 nm upon heating at 500 °C. This increase in size is accompanied by a phase change from the cubic to the hexagonal structure. The optical bandgap (measured by absorption onset) is also tunable with heat treatment, and was found to range from 2.2–2.8 eV. The photoluminescence excitation spectrum (λex = 400 nm) of a 100 °C-heated CdS aerogel is similar to that of the nanoparticulate starting materials, exhibiting both a sharp excitonic peak near 450 nm corresponding to band edge emission, and a broad feature in the region 500–700 nm, attributed to electron–hole recombination at trap states.