Modern Inorganic Aerogels.

Essentially, the term aerogel describes a special geometric structure of matter. It is neither limited to any material nor to any synthesis procedure. Hence, the possible variety of materials and therefore the multitude of their applications are almost unbounded. In fact, the same applies for nanoparticles. These are also just defined by their geometrical properties. In the past few decades nano-sized materials have been intensively studied and possible applications appeared in nearly all areas of natural sciences. To date a large variety of metal, semiconductor, oxide, and other nanoparticles are available from colloidal synthesis. However, for many applications of these materials an assembly into macroscopic structures is needed. Here we present a comprehensive picture of the developments that enabled the fusion of the colloidal nanoparticle and the aerogel world. This became possible by the controlled destabilization of pre-formed nanoparticles, which leads to their assembly into three-dimensional macroscopic networks. This revolutionary approach makes it possible to use precisely controlled nanoparticles as building blocks for macroscopic porous structures with programmable properties.

Alloying Behavior of Self-Assembled Noble Metal Nanoparticles.

The atomic redistribution processes occurring in multiparticle nanostructures are hardly understood. To obtain a more detailed insight, we applied high-resolution microscopic, diffraction and spectroscopic characterization techniques to investigate the fine structure and elemental distribution of various bimetallic aerogels with 1:1 compositions, prepared by self-assembly of single monometallic nanoparticles. The system Au-Ag exhibited a complete alloy formation, whereas Pt-Pd aerogels formed a Pd-based network with embedded Pt particles. The assembly of Au and Pd nanoparticles resulted in a Pd-shell formation around the Au particles. This work confirms that bimetallic aerogels are subject to reorganization processes during their gel formation.

Simple and Sensitive Colorimetric Detection of Dopamine Based on Assembly of Cyclodextrin-Modified Au Nanoparticles.

A controlled assembly of natural beta-cyclodextrin modified Au NPs mediated by dopamine is demonstrated. Furthermore, a simple and sensitive colorimetric detection for dopamine is established by the concentration-dependent assembly.

Function-Led Design of Aerogels: Self-Assembly of Alloyed PdNi Hollow Nanospheres for Efficient Electrocatalysis.

One plausible approach to endow aerogels with specific properties while preserving their other attributes is to fine-tune the building blocks. However, the preparation of metallic aerogels with designated properties, for example catalytically beneficial morphologies and transition-metal doping, still remains a challenge. Here, we report on the first aerogel electrocatalyst composed entirely of alloyed PdNi hollow nanospheres (HNSs) with controllable chemical composition and shell thickness. The combination of transition-metal doping, hollow building blocks, and the three-dimensional network structure make the PdNi HNS aerogels promising electrocatalysts for ethanol oxidation. The mass activity of the Pd83 Ni17 HNS aerogel is 5.6-fold higher than that of the commercial Pd/C catalyst. This work expands the exploitation of the electrocatalysis properties of aerogels through the morphology and composition control of its building blocks.

Noble metal aerogels-synthesis, characterization, and application as electrocatalysts.

CONSPECTUS: Metallic and catalytically active materials with high surface area and large porosity are a long-desired goal in both industry and academia. In this Account, we summarize the strategies for making a variety of self-supported noble metal aerogels consisting of extended metal backbone nanonetworks. We discuss their outstanding physical and chemical properties, including their three-dimensional network structure, the simple control over their composition, their large specific surface area, and their hierarchical porosity. Additionally, we show some initial results on their excellent performance as electrocatalysts combining both high catalytic activity and high durability for fuel cell reactions such as ethanol oxidation and the oxygen reduction reaction (ORR). Finally, we give some hints on the future challenges in the research area of metal aerogels. We believe that metal aerogels are a new, promising class of electrocatalysts for polymer electrolyte fuel cells (PEFCs) and will also open great opportunities for other electrochemical energy systems, catalysis, and sensors. The commercialization of PEFCs encounters three critical obstacles, viz., high cost, insufficient activity, and inadequate long-term durability. Besides others, the sluggish kinetics of the ORR and alcohol oxidation and insufficient catalyst stability are important reasons for these obstacles. Various approaches have been taken to overcome these obstacles, e.g., by controlling the catalyst particle size in an optimized range, forming multimetallic catalysts, controlling the surface compositions, shaping the catalysts into nanocrystals, and designing supportless catalysts with extended surfaces such as nanostructured thin films, nanotubes, and porous nanostructures. These efforts have produced plenty of excellent electrocatalysts, but the development of multisynergetic functional catalysts exhibiting low cost, high activity, and high durability still faces great challenges. In this Account, we demonstrate that the sol-gel process represents a powerful "bottom-up" strategy for creating nanostructured materials that tackles the problems mentioned above. Aerogels are unique solid materials with ultralow densities, large open pores, and ultimately high inner surface areas. They magnify the specific properties of nanomaterials to the macroscale via self-assembly, which endow them with superior properties. Despite numerous investigations of metal oxide aerogels, the investigation of metal aerogels is in the early stage. Recently, aerogels including Fe, Co, Ni, Sn, and Cu have been obtained by nanosmelting of hybrid polymer-metal oxide aerogels. We report here exclusively on mono-, bi- and multimetallic noble metal aerogels consisting of Ag, Au, Pt, and Pd and their application as electrocatalysts.

Kinetically controlled synthesis of PdNi bimetallic porous nanostructures with enhanced electrocatalytic activity.

A class of 3D PdNi bimetallic nano-materials with porous nanostructures is synthesized using a facile and versatile approach at room temperature. Due to their porous nanostructures, their clean surfaces, as well as the synergistic effect between their compositions, the as-prepared PdNi exhibit greatly enhanced activity and stability towards methanol electrooxidation in an alkaline medium, holding great promise in fuel cells.

A membraneless glucose/O(2) biofuel cell based on Pd aerogels.

In this study, we introduce the first membraneless glucose/O2 biofuel cell using Pd-based aerogels as electrode materials. The bioanode was fabricated with a coimmobilized mediator and glucose oxidase for the oxidation of glucose, in which ferrocenecarboxylic acid was integrated into a three-dimensional porous beta-cyclodextrin-modified Pd aerogel to mediate the bioelectrocatalytic reaction. Bilirubin oxidase and Pd-Pt alloy aerogel were confined to an electrode surface, which realized the direct bioelectrocatalytic function for the reduction of O2 to H2 O with a synergetic effect at the biocathode. By employing these two bioelectrodes, the assembled glucose/O2 biofuel cell showed a maximum power output of 20 µW cm(-2) at 0.25 V.

Controlling the growth of palladium aerogels with high-performance toward bioelectrocatalytic oxidation of glucose.

We report the controllable synthesis of Pd aerogels with high surface area and porosity by destabilizing colloidal solutions of Pd nanoparticles with variable concentrations of calcium ions. Enzyme electrodes based on Pd aerogels co-immobilized with glucose oxidase show high activity toward glucose oxidation and are promising materials for applications in bioelectronics.

Decoration of diatom biosilica with noble metal and semiconductor nanoparticles (<10 nm): assembly, characterization, and applications.

Diatom-templated noble metal (Ag, Pt, Au) and semiconductor (CdTe) nanoparticle arrays were synthesized by the attachment of prefabricated nanoparticles of defined size. Two different attachment techniques-layer-by-layer deposition and covalent linking-could successfully be applied. The synthesized arrays were shown to be useful for surface-enhanced Raman spectroscopy (SERS) of components, for catalysis, and for improved image quality in scanning electron microscopy (SEM).