"We have 'opened up' a new class of aerogels: chalcogenide aerogels," researcher Stephanie Brock told nanotechweb.org. "Previously, aerogels were limited to metal oxides and carbon."
According to the scientists, the combination of high surface area, quantum confinement effects and photoluminescence make metal chalcogenide aerogels potential candidates for photocatalytic, photovoltaic and sensing applications.
The researchers demonstrated their technique for cadmium sulphide, cadmium selenide, zinc sulphide and lead sulphide nanoparticles. What's more, they believe they will be able to extend the method to other materials.
To prepare the aerogels, Brock and colleagues made nanoparticles capped with thiolates. Then they oxidized the thiolate capping groups. This controlled loss of surface groups revealed reactive sites for nanoparticle condensation and caused the nanoparticles to form a gel. Finally, the researchers dried the gel with supercritical carbon dioxide to maintain the pore architecture.
"The kinetics are important: if the condensation occurs too quickly, a precipitate will form instead of a gel," said Brock. "Subsequently, the wet gels have to be dried under supercritical conditions or else the pore volume will be lost."
The resulting structures contained mesopores - i.e. pores 2-50 nm in diameter. This gave them surface areas as high as 250 m2/g, compared to a benchtop-dried sample (with a structure in which the pores had collapsed) which had a surface area of 55 m2/g. The cadmium sulphide aerogel had a bulk density of 0.07 g/cm3, just 1.4% of the density of a single crystal of CdS.
"Unlike conventional silica aerogels and M41S-type materials, the framework is crystalline, consisting of quantum dots assembled into a pearl necklace-type morphology," said Brock. "Bulk monoliths exhibit the characteristic optical properties of these nanoscale building blocks - remaining quantum confined."
According to Brock, the materials displayed sharp, band-edge photoluminescence at energies significantly blue-shifted from single crystals. "Post-annealing permits the absorption energy of the material to be tuned to the red as the framework coarsens and the average crystallite size grows," she added.
"Finding ways to assemble nanoparticles into actual functional devices - without losing the characteristic nanoscale properties - is one of the current challenges for nanotechnology," said Brock. "This sol-gel route provides a simple, versatile method to do this, and should be amenable for the creation of more complex composite materials too."
Now, the team is working to characterize the electrical and surface characteristics of the materials and generate them as thin-films to evaluate their potential for photovoltaic and sensing applications. "We are also trying to see how extensive the chemistry is - how many different kinds of materials it can be applied to," said Brock.
The researchers reported their work in Science.