Christoph Ziegler

The Route to Functional Graphene Oxide

We report on an easy-to-use, successful, and reproducible route to synthesize functionalized graphite oxide (GO) and its conversion to graphene-like materials through chemical or thermal reduction of GO. Graphite oxide containing hydroxyl, epoxy, carbonyl, and carboxyl groups loses mainly hydroxyl and epoxy groups during reduction, whereas carboxyl species remain untouched. The interaction of functionalized graphene with fluorescent methylene blue (MB) is investigated and compared to graphite, fully oxidized GO, as well as thermally and chemically reduced GO. Optical absorption and emission spectra of the composites indicate a clear preference for MB interaction with the GO derivatives containing a large number of functional groups (GO and chemically reduced GO), whereas graphite and thermally reduced GO only incorporate a few MB molecules. These findings are consistent with thermogravimetric, X-ray photoelectron spectroscopic, and Raman data recorded at every stage of preparation. The optical data also indicate concentration-dependent aggregation of MB on the GO surface leading to stable MB dimers and trimers. The MB dimers are responsible for fluorescence quenching, which can be controlled by varying the pH value.

Micro- and mesoporous carbide-derived carbon prepared by a sacrificial template method in high performance lithium sulfur battery cathodes

Polymer-based carbide-derived carbons (CDCs) with combined micro- and mesopores are prepared by an advantageous sacrificial templating approach using poly(methylmethacrylate) (PMMA) spheres as the pore forming material. Resulting CDCs reveal uniform pore size and pore shape with a specific surface area of 2434 m2 g−1 and a total pore volume as high as 2.64 cm3 g−1. The bimodal CDC material is a highly attractive host structure for the active material in lithium–sulfur (Li–S) battery cathodes. It facilitates the utilization of high molarity electrolytes and therefore the cells exhibit good rate performance and stability. The cathodes in the 5 M lithium bis(trifluoromethanesulfonyl)imide (LiTFSI) electrolyte show the highest discharge capacities (up to 1404 mA h gs−1) and capacity retention (72% after 50 cycles at C/5). The unique network structure of the carbon host enables uniform distribution of sulfur through the conductive media and at the same time it facilitates rapid access for the electrolyte to the active material.

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.

Cold Flow as Versatile Approach for Stable and Highly Luminescent Quantum Dot-Salt Composites.

Since the beginning of the 1980s, colloidally synthesized quantum dots (QDs) have been in the focus of interest due to their possible implementation for color conversion, luminescent light concentrators, and lasing. For all these applications, the QDs benefit from being embedded into a host matrix to ensure stability and usability. Many different host materials used for this purpose still have their individual shortcomings. Here, we present a universal, fast, and flexible approach for the direct incorporation of a wide range of QDs into inorganic ionic crystals using cold flow. The QD solution is mixed with a finely milled salt, followed by the removal of the solvent under vacuum. Under high pressure (GPa), the salt powder loaded with QDs transforms into transparent pellets. This effect is well-known for many inorganic salts (e.g., KCl, KBr, KI, NaCl, CsI, AgCl) from, e.g., sample preparation for IR spectroscopy. With this approach, we are able to obtain strongly luminescent QD-salt composites, have precise control over the loading, and provide a chemically robust matrix ensuring long-term stability of the embedded QDs. Furthermore, we show the photo-, chemical, and thermal stability of the composite materials and their use as color conversion layers for a white light-emitting diode (w-LED). The method presented can potentially be used for all kinds of nanoparticles synthesized in organic as well as in aqueous media.

Core–Shell Structuring of Pure Metallic Aerogels towards Highly Efficient Platinum Utilization for the Oxygen Reduction Reaction

The development of core-shell structures remains a fundamental challenge for pure metallic aerogels. Here we report the synthesis of Pdx Au-Pt core-shell aerogels composed of an ultrathin Pt shell and a composition-tunable Pdx Au alloy core. The universality of this strategy ensures the extension of core compositions to Pd transition-metal alloys. The core-shell aerogels exhibited largely improved Pt utilization efficiencies for the oxygen reduction reaction and their activities show a volcano-type relationship as a function of the lattice parameter of the core substrate. The maximum mass and specific activities are 5.25 A mgPt-1 and 2.53 mA cm-2 , which are 18.7 and 4.1 times higher than those of Pt/C, respectively, demonstrating the superiority of the core-shell metallic aerogels. The proposed core-based activity descriptor provides a new possible strategy for the design of future core-shell electrocatalysts.

Synthesis and Agglomeration of Silver Nanoparticles Stabilized with 5-R-Tetrazoles

Abstract In this work we present a simple method to obtain silver sols stabilized by tetrazole derivatives. The well established metal precursor silver nitrate and the reductant sodium borohydride have been combined with 5-R-tetrazoles (R=CH3, NH2) acting as stabilizing ligands. Depending on the utilized tetrazole, different agglomeration kinetics could be observed. Long term stability and agglomeration processes have been in situ investigated by the use of UV/Vis- and Raman-spectroscopy.

Absolute Energy Level Positions in CdSe Nanostructures from Potential-Modulated Absorption Spectroscopy (EMAS)

Semiconductor nanostructures such as CdSe quantum dots and colloidal nanoplatelets exhibit remarkable optical properties, making them interesting for applications in optoelectronics and photocatalysis. For both areas of application a detailed understanding of the electronic structure is essential to achieve highly efficient devices. The electronic structure can be probed using the fact that optical properties of semiconductor nanoparticles are found to be extremely sensitive to the presence of excess charges that can for instance be generated by means of an electrochemical charge transfer via an electrode. Here we present the use of EMAS as a versatile spectroelectrochemical method to obtain absolute band edge positions of CdSe nanostructures versus a well-defined reference electrode under ambient conditions. In this, the spectral properties of the nanoparticles are monitored with respect to an applied electrochemical potential. We developed a bleaching model that yields the lowest electronic state in the conduction band of the nanostructures. A change in the band edge positions caused by quantum confinement is shown both for CdSe quantum dots and for colloidal nanoplatelets. In the case of CdSe quantum dots these findings are in good agreement with tight binding calculations. The method presented is not limited to CdSe nanostructures but can be used as a universal tool. Hence, this technique allows the determination of absolute band edge positions of a large variety of materials used in various applications.

A fast route to obtain modified tin oxide aerogels using hydroxostannate precursors

Nanostructured tin oxide materials with a high specific surface area and porosity are promising for applications such as electrocatalysis, lithium ion batteries or sensors. Here, we present a facile strategy for the synthesis of tin oxide aerogels using inexpensive hexahydroxostannate as a tin precursor. This easy and scalable method yields tin oxide aerogels with a high specific surface area and a wide pore size distribution. The method can be modified by adding hexahydroxoantimonate to obtain antimony-doped tin oxide aerogels that exhibit electrical conductivity after annealing. Cogelation with other preformed nanoparticles (e.g. Au, Pt) leads to mixed gels. Both modifications do not have a large impact on the porous properties of the obtained aerogels. Tin oxide materials prepared via this route can be tailored to a specific application by versatile modification possibilities.