Hydrogels and aerogels from noble metal nanoparticles.

Aerogels are fine inorganic superstructures with enormously high porosity and are known to be exceptional materials with a variety of applications, for example in the area of catalysis. The chemistry of the aerogel synthesis originated from the pioneering work from the early 1930s and was further developed starting from the 1960s. Attractive catalytic, thermoresistant, piezoelectric, antiseptic, and many other properties of the aerogels originate from the unique combination of the specific properties of nanomaterials magnified by macroscale self-assembly. Currently, the most investigated materials that form fine aerogel superstructures are silica and other metal oxides together with their mixtures. Recently, the possibility of creating aerogels and even light-emitting monoliths with densities 500 times less than their bulk counterparts from colloidal quantum dots and clusters of metal chalcogenides has attracted attention. These developments may open opportunities in areas such as semiconductor technology, photocatalysis, optoelectronics, and photonics. Quite a number of different approaches have focused on modifying oxide-based aerogels (silica, titania, alumina, etc.) with metal nanoparticles (such as of platinum) to carry the catalytic properties from the metal 15] into the porous structures of the aerogels. 16,17] Fine mesoporous assemblies of catalytically active metal nanoparticles were also created by using artificial opals and fungi as templates. Other superstructural materials derived from metal nanoparticles include mesoporous platinum–carbon composites, gold nanoparticles interlinked with dithiols, necklace nanochains of hybrid palladium–lipid nanospheres, electrocatalytically active nanoporous platinum aggregates, foams, and highly ordered twoand three-dimensional supercrystals. The creation of non-supported metal aerogels has however not been reported to date. Recently, the formation of highly porous spherical aggregates (“supraspheres”) of several hundred nanometers in diameter, where nanoparticles from one or two different metals were cross-linked with dithiols, was reported. 31] The metal aerogels presented herein exhibit an average density two orders of magnitude lower than that of the reported foams. Their primary structural units match the size range of single nanoparticles (5–20 nm), which is an order of magnitude smaller than that of the self-assembled supraspheres. Moreover, in the present case, no chemical cross-linkers are involved in the self-assembly process. The formation of such noble-metal nanoparticle-based mesoporous monometallic and bimetallic aerogels is an important step towards self-supported monoliths with enormously high catalytically active surfaces. Considering that metal nanoparticles possess very specific optical properties owing to their pronounced surface plasmon resonance, aerogels from metal nanoparticles may also find future applications in nanophotonics, for example, as advanced optical sensors and ultrasensitive detectors. In terms of size, shape, and composition control, the synthesis of colloidal metallic nanoparticles is nowadays a well-developed research field. For gel formation, various methods of slow destabilization, developed previously for quantum-dot-based gels, were systematically applied to aqueous colloidal solutions of gold, silver, and platinum nanoparticles. Supercritical drying of the hydrogels with liquid CO2 finally produces aerogels. Aqueous colloidal metal solutions are normally very stable in the dilute as-prepared state (below ca. 10 m particle concentration). To gelate these sols, efficient destabilization is initiated by concentrating the sols (see the Supporting Information). Gel formation is achieved by, for example, the addition of ethanol or hydrogen peroxide to the concentrated colloids. Different morphologies of the gels can be obtained depending on the type and amount of destabilizer, and also on the metal colloid. Figure 1 shows scanning electron microscopy (SEM; A and B) and transmission electron microscopy (TEM) images (C and D) of an aerogel manufactured from platinum nanoparticles with the use of ethanol as [*] A. -K. Herrmann, M. Vogel, Dr. N. Gaponik, Prof. Dr. A. Eychm ller Physical Chemistry/Electrochemistry, TU Dresden 01062 Dresden (Germany) Fax: (+ 49)351-37164 E-mail: Alexander.eychmueller@chemie.tu-dresden.de Homepage: http://www.chm. tu-dresden.de/pc2/index.shtml