Hydrogels and Aerogels from Noble Metal Nanoparticles

Abstract

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. [1] The chemistry of the aerogel synthesis originated from the pioneering work [2] from the early 1930s and was further developed starting from the 1960s. [1,3] 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. [4][5][6][7][8][9][10][11][12][13] 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 [14,15] into the porous structures of the aerogels. [1,16,17] Fine mesoporous assemblies of catalytically active metal nanoparticles were also created by using artificial opals [18] and fungi [19] as templates. Other superstructural materials derived from metal nanoparticles include mesoporous platinum-carbon composites, [20] gold nanoparticles interlinked with dithiols, [21] necklace nanochains of hybrid palladium-lipid nanospheres, [22] electrocatalytically active nanoporous platinum aggregates, [23] foams, [24] and highly ordered two-and three-dimensional supercrystals. [25][26][27][28][29] 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. [30,31] The metal aerogels presented herein exhibit an average density two orders of magnitude lower than that of the reported foams.[32] 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.[31] 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 nano...

Catalytic Properties of Cryogelated Noble Metal Aerogels

“…However, the fabrication methods for these kinds of structures are tedious and intrinsically limited, thus calling for novel preparation methods that could be applied to other metallic NCs. A way to overcome these problems could be hydro-or aerogelation of metals and metal oxides [14][15][16][17][18]. It is reported that high voluminous structures out of NCs, which also partly possess the properties of their respective colloidal solution, can be achieved [16,19,20].…”

Nanostructuring Noble Metals as Unsupported Electrocatalysts for Polymer Electrolyte Fuel Cells

“…Citrate and cyclodextrin are frequently utilized as the stabilizers to narrow the size distribution of the metal NPs. [44] The relatively weak coordination interaction between the stabilizer and metal NPs is critical for the gelation step (from B1 to H1 in Figure 1), which can be realized via an spontaneous gelation process [25,26,38] or via controlled destabilizations (by heating [45] or adding salt [46] and cross-linkers [47] Besides the porous and interconnected 3D network structure, hierarchical aerogels also possess backbones with locally modified morphologies instead of solid nanochains. As shown in Figure 1, this sub-structure can be obtained either by tailoring the hydrogel via strategy II (B1-H1-H2-A2) [43] or engineering the NBBs via strategy III (B1-B2-H3-A3).…”

“…The nanowirelike backbones are derived from the fusion and connection of the particulate NBBs (mostly noble metal NPs) during the gelation process. Depending on the elemental composition and distribution, these aerogels can be divided into three types: monometallic aerogels (Pd, Pt, Au, Ag), [38,39,46,47] bimetallic alloy aerogels (Pd x Pt y , Pt x Ni, Pt x Cu, etc), [25,26,49] and mixed multi/bimetallic aerogels (Au-Pd, Pd-Pt, Au-Ag-Pt, etc.). [45,50] Figure 2 shows the representative scanning electron microscopy (SEM) and transmission electron microscopy (TEM) images of the wire-based aerogels.…”

“…[47] Following a similar strategy, Ag and Pt aerogels were fabricated using H 2 O 2 and ethanol as the destabilizer. [46] It should be pointed out that the concentration step of as-prepared NPs required for these approaches, however, is usually time-consuming, thereby limiting scale-up of the synthesis. To address Figure 1.…”

Synthesis of noble metal nanoparticles and their non-ordered superstructures

“…Since a technique was introduced very recently to make aerogels from metal chalcogenide nanoparticles (Eychmüller 2005;Mohanan et al 2005;, 2007aBrock et al 2006;Bag et al 2007Gaponik et al 2008), a lot of effort has been made to create monoliths from nanoparticles. A variation of this technique was recently applied to sols of metal nanoparticles by Bigall et al (2009). While previous approaches for metal aerogels were based on the modification of oxide-based aerogels with metal nanoparticles for obtaining highly catalytically active materials (Pajonk 1997;Hüsing & Schubert 1998;Campbell 2004;Jiang & Gao 2006;Vallribera & Molins 2008), with this approach the resulting highly porous macroscopic monoliths are completely non-supportedly built only from metal nanoparticles.…”