New challenges in nanotechnology arise in the assembly of nanoobjects into three-dimensional superstructures, which may carry synergetic properties and open up new application fields. Within this new class of materials nanostructured, porous functional metals are of great interest since they combine high surface area, gas permeability, electrical conductivity, plasmonic behavior and size-enhanced catalytic reactivity. Even though a large variety of preparation pathways for the fabrication of porous noble metals has already been established, several limitations are still to be addressed by research developments.
The new and versatile approach that is presented in this work makes use of a templatefree self-assembly process for the fabrication of highly porous, metallic nanostructures. Thereby, nanochains are formed by the controlled coalescence of noble metal NPs in aqueous media and their interconnection and interpenetration leads to the formation of a self-supported network with macroscopic dimensions. Subsequently, the supercritical drying technique is used to remove the solvent from the pores of the network without causing a collapse of the fragile structure. The resulting highly porous, low-weighted, three-dimensional nanostructured solids are named aerogels. The exceptional properties of these materials originate from the conjunction of the unique properties of nanomaterials magnified by macroscale assembly.
Moreover, the combination of different metals may lead to synergetic effects regarding for example their catalytic activity. Therefore, the synthesis of multimetallic gels and the characterization of their structural peculiarities are in the focus of the investigations. In the case of the developed preparation pathways the gelation process starts from preformed, stable colloidal solutions of citrate capped, spherical noble metal (Au, Ag, Pt, Pd) NPs. In order to face various requirements several methods for the initiation of the controlled destabilization and coalescence of the nanosized building blocks were developed and synthesis conditions were optimized, respectively. Multimetallic structures with tunable composition are obtained by mixing different kinds of monometallic NP solutions and performing a joint gel formation.
The characterization of the resulting materials by means of electron microscopy reveals the formation of a highly porous network of branched nanochains that provide a polycrystalline nature and diameters in the size range of the initial NPs. Furthermore, synthesis conditions for the spontaneous gel formation of glucose stabilized Au and Pd NPs were investigated. In order to gain a detailed knowledge of the structural properties of bimetallic aerogel structures a versatile set of characterization techniques was applied. A broad pore size distribution dominated by meso- and macropores and remarkably high inner surface areas were concluded from the N2 physisorption isotherms and density measurements. As investigated, a specific thermal treatment could be used to tune the ligament size of Au-Ag aerogels, whereas Au-Pd and Pt-Pd structures provide thermal stability under mild conditions.
Further investigations aimed to the enlightenment of the elemental distribution and phase composition within the nanochains of multimetallic gel structures. The different approaches provide complementary and consistent results. Phase analyses based on XRD measurements revealed separated phases of each metal in the case of Ag-Pd and Au-Pd aerogels. They further proved the possibility of temperature induced phase modifications that lead to complete alloying of Au and Pd. In addition, separated domains of Pt and Pd were established from the EXAFS analysis of the corresponding aerogel. STEM EDX high resolution elemental mappings confirmed the separated domains of different metals in the case of Au-Pd and Pt-Pd aerogels. Moreover, a complete interdiffusion and alloy formation of Au and Ag within the corresponding aerogel structure is suggested from STEM EDX results.
Finally, the presented investigations further promote the field of metallic aerogels by addressing the challenging issue of processability and device fabrication. Hybrid materials with organic polymers as well as various kinds of coatings on glass substrates and glassy carbon electrodes were prepared whereas the network structure was preserved throughout all processing steps. Moreover, it was illustrated that the NP-based aerogels carry metallic properties as expressed by their low Seebeck coefficients and high electrical conductivities.
| Identifer | oai:union.ndltd.org:DRESDEN/oai:qucosa.de:bsz:14-qucosa-149672 |
| Date | 05 January 2015 |
| Creators | Herrmann, Anne-Kristin |
| Contributors | Technische Universität Dresden, Fakultät Mathematik und Naturwissenschaften, Prof. Dr. rer. nat. habil. Alexander Eychmüller, Prof. Dr. rer. nat. habil. Alexander Eychmüller, Prof. Dr. rer. nat. habil. Stefan Kaskel |
| Publisher | Saechsische Landesbibliothek- Staats- und Universitaetsbibliothek Dresden |
| Source Sets | Hochschulschriftenserver (HSSS) der SLUB Dresden |
| Language | English |
| Detected Language | English |
| Type | doc-type:doctoralThesis |
| Format | application/pdf |
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