Monolithic Zinc Oxide Aerogels from Organometallic Sol−Gel Precursors

Aerogels belong to the large class of porous solids. They are characterized by a network of a mechanically stable solid, most likely inorganic in nature comprising a large gas volume in comparison to the volume of the solid material. Although a large variety of aerogels with silicate networks already exist, examples for materials with transition metal oxide networks are rare. One particularly interesting target is zinc oxide because of its semiconducting and multifunctional character. A sol−gel process facilitating an organometallic precursor system is established. The mechanism of gelation has been studied in detail. Unlike most other sol−gel processes, at first a large number of nanocrystalline ZnO particles are formed that rapidly agglomerate to secondary aggregates instead of forming a network directly. Thus, these secondary aggregates determine the textural properties of the pore walls as they assemble into the final, highly cross-linked network. A monolithic ZnO aerogel with porosities greater than 99% could be received after solvent extraction with supercritical CO2. Furthermore, the porosities could be tuned via a combination of conventional drying and supercritical solvent extraction by a process that we call scalar drying. Finally, one of the potential functional properties of the new ZnO aerogels was proven, its application in photocatalysis.