Anne-Kristin Herrmann

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

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.

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.

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.

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.

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).

A Membraneless Glucose/O2 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.

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.

Structural Analysis and Electrochemical Properties of Bimetallic Palladium–Platinum Aerogels Prepared by a Two‐Step Gelation Process

Multimetallic aerogels have emerged as promising unsupported and high‐surface‐area metal materials for different applications in heterogeneous catalysis and electrochemistry, which are fabricated using a gelation process characterized by the controlled aggregation of metallic nanoparticles to form a macroscopic network structure in aqueous solution. However, the achievement of the structural homogeneity of the multimetallic aerogels in terms of the diameter of the nanochains and the chemical composition at the nano‐ and the macro‐scale is still a great challenge. In this paper, we investigated two Pd‐Pt aerogels prepared by the two‐step gelation method. The structural homogeneity and chemical distribution of both metals inside the aerogels were analyzed by using high‐resolution (scanning) transmission microscopy, energy‐dispersive X‐ray spectroscopy, extended X‐ray absorption fine structure spectroscopy and cyclic voltammetry. The Pd‐Pt aerogels show the presence of Pd/Pt‐rich domains inside the long‐range framework. It is evident that the initial monometallic features dominate over alloying during the gelation process. Although the same synthetic approach for Pd‐Pt aerogels with different atomic ratios was used, we observed that the sizes of these monometallic domains varied strongly between the Pd‐rich and Pt‐rich aerogels. The presence of such metal clusters influenced the electrochemical robustness of the Pd‐Pt aerogels dramatically. Electrochemical durability investigations revealed that the aerogels with a high content of Pd are less stable because of the gradual dissolution of the less noble metal particularly inside the Pd‐rich domains. A chemical and structural homogeneity might improve the lifetime of the Pd‐Pt aerogels under electrochemical conditions. In this work, we provide a better understanding of the structure and chemical distribution of the bimetallic aerogel framework prepared by the two‐step gelation process.