Chalcogel

A chalcogel or properly metal chalcogenide aerogel is an aerogel made from chalcogens (the column of elements on the periodic table beginning with oxygen) such as sulfur and selenium, with cadmium, tellurium, platinum, and other elements.[1]

Chalcogels preferentially absorb heavy metals,[2] showing promise in absorbing pollutants mercury, lead, and cadmium from water.[3] In addition, scientists have made one that is claimed to be twice as effective at desulfurization as any current methods. This is a very impressive feat, particularly because until the discovery of chalcogels' properties, scientists were beginning to consider desulfurization an optimized process.[4]

Semiconducting metal chalcogenide aerogels, first demonstrated by Professor Stephanie Brock at Wayne State University, show great promise for use in chemical sensors, solar cells, and photoelectrolysis of water.

Metal chalcogenide aerogels can be prepared from thiolysis[5] or nanoparticle condensation[6][7] contain crystalline nanoparticles in the structure.[7] The chalcogels are made with the approach and belong to a different class of materials. The method used is based on the metathesis (or partner-switching) reaction, makes use of molecular chalcogenide anions and linking metal cations. These reactions give a random network, which does not have long-range periodic structure. By its nature this method gives the advantage of tuning the resulting material properties through appropriate selection of anions and cations. At the same time a judicious match of the anion and metal components is needed in order to force the building block and linker metals to engage in a controlled self-assembly process so a gel can be obtained. The key is to avoid rapid precipitation or a permanent solution where with no gelation is taking place. Based on this chemical approach chalcogels were first demonstrate using platinum linking ions, and thiogermanate or selenogermanate anions. The synthetic method can be extended to many thioanions including tetrathiomolybdate-based chalcogels.[8] Different metal ions have been used as linkers Co²⁺, Ni²⁺, Pb²⁺, Cd²⁺, Bi³⁺, Cr³⁺.[8][9][10]

When the gels are dried aerogels with high surface areas are obtained and the materials have multifunctional nature. For example, chalcogels are especially promising for gas separation. They were reported to exhibit high selectivity in CO₂ and C₂H₆ over H₂ and CH₄ adsorption.[8][10] The latter is relevant to exit gas stream composition of water gas shift reaction and steam reforming reactions (reactions widely used for H₂ production). For example, separation of gas pairs such as CO₂/H₂, CO₂/CH₄, and CO₂/N₂ are key steps in precombustion capture of CO₂, natural gas sweetening and postcombustion capture of CO₂ processes leading ultimately at upgrading of the raw gas. The above mentioned conditioning makes the gas suitable for a number of applications in fuel cells.

Chalcogels were shown to be very effective at capturing ionic forms of Tc-99 and U-238, as well as nonradioactive gaseous iodine (i.e., a surrogate for I-129(2)), irrespective of the sorbent polarity. The capture efficiencies for Tc-99 and U-238 varied between the different sorbents, ranging from 57.3-98.0% and 68.1-99.4%, respectively. All chalcogels showed >99.0% capture efficiency for iodine over the test duration.[11]