Felix Rechberger

Three-Dimensional Assembly of Yttrium Oxide Nanosheets into Luminescent Aerogel Monoliths with Outstanding Adsorption Properties

The preparation of macroscopic materials from two-dimensional nanostructures represents a great challenge. Restacking and random aggregation to dense structures during processing prevents the preservation of the two-dimensional morphology of the nanobuilding blocks in the final body. Here we present a facile solution route to ultrathin, crystalline Y2O3 nanosheets, which can be assembled into a 3D network by a simple centrifugation-induced gelation method. The wet gels are converted into aerogel monoliths of macroscopic dimensions via supercritical drying. The as-prepared, fully crystalline Y2O3 aerogels show high surface areas of up to 445 m(2)/g and a very low density of 0.15 g/cm(3), which is only 3% of the bulk density of Y2O3. By doping and co-doping the Y2O3 nanosheets with Eu(3+) and Tb(3+), we successfully fabricated luminescent aerogel monoliths with tunable color emissions from red to green under UV excitation. Moreover, the as-prepared gels and aerogels exhibit excellent adsorption capacities for organic dyes in water without losing their structural integrity. For methyl blue we measured an unmatched adsorption capacity of 8080 mg/g. Finally, the deposition of gold nanoparticles on the nanosheets gave access to Y2O3-Au nanocomposite aerogels, proving that this approach may be used for the synthesis of catalytically active materials. The broad range of properties including low density, high porosity, and large surface area in combination with tunable photoluminescence makes these Y2O3 aerogels a truly multifunctional material with potential applications in optoelectronics, wastewater treatment, and catalysis.

Assembly of BaTiO3 nanocrystals into macroscopic aerogel monoliths with high surface area.

Aerogels with their low density and high surface area are fascinating materials. However, their advantageous morphology is still far from being fully exploited owing to their limited compositional variety and low crystallinity. Replacing the sol-gel process by a particle-based assembly route is a powerful alternative to expand the accessible functionalities of aerogels. A strategy is presented for the controlled destabilization of concentrated dispersions of BaTiO3 nanoparticles, resulting in the assembly of the fully crystalline building blocks into cylindrically shaped monolithic gels, thereby combining the inherent properties of ternary oxides with the highly porous microstructure of aerogels. The obtained aerogels showed an unprecedentedly high surface area of over 300 m(2)  g(-1).

Direct Imaging of Dopant Clustering in Metal–Oxide Nanoparticles

Dopant atoms are used to tailor the properties of materials. However, whether the desired effect is achieved through selective doping depends on the dopant distribution within the host material. The clustering of dopant atoms can have a deleterious effect on the achievable properties because a two-phase material is obtained instead of a homogeneous material. Thus, the examination of dopant fluctuations in nanodevices requires a reliable method to chemically probe individual atoms within the host material. This is particularly challenging in the case of functionalized nanoparticles where the characteristic length scale of the particles demands the use of a high-spatial-resolution and high-sensitivity technique. Here we demonstrate a chemically sensitive atomic resolution imaging technique which delivers direct site-specific information on the dopant distribution in nanoparticles. We employ electron energy-loss spectroscopy imaging in a scanning transmission electron microscope combined with multivariate statistical analysis to map the distribution of Ba dopant atoms in SrTiO(3) nanoparticles. Our results provide direct evidence for clustering of the Ba dopants in the SrTiO(3) nanoparticles outlining a possible explanation for the presence of polar nanoregions in the Ba:SrTiO(3) system. The results we present constitute the first example of site-specific atomic resolution spectroscopy of foreign atoms in doped nanoparticles and suggest a general strategy to ascertain the spatial distribution of impurity atoms in nanocrystals and hence improve the performance of nanoparticle-based devices.

Synthesis of aerogels: from molecular routes to 3-dimensional nanoparticle assembly

Colloidal nanocrystals are extensively used as building blocks in nanoscience, and amazing results have been achieved in assembling them into ordered, close-packed structures. But in spite of great efforts, the size of these structures is typically restricted to a few micrometers, and it is very hard to extend them into the macroscopic world. In comparison, aerogels are macroscopic materials, highly porous, disordered, ultralight and with immense surface areas. With these distinctive characteristics, they are entirely contrary to common nanoparticle assemblies such as superlattices or nanocrystal solids, and therefore cover a different range of applications. While aerogels are traditionally synthesized by molecular routes based on aqueous sol-gel chemistry, in the last few years the gelation of nanoparticle dispersions became a viable alternative to improve the crystallinity and to widen the structural, morphological and compositional complexity of aerogels. In this Review, the different approaches to inorganic non-siliceous and non-carbon aerogels are addressed. We start our discussion with wet chemical routes involving molecular precursors, followed by processing methods using nanoparticles as building blocks. A unique feature of many of these routes is the fact that a macroscopic, often monolithic body is produced by pure self-assembly of nanosized colloids without the need for any templates.

Dealloying of Platinum-Aluminum Thin Films: Dynamics of Pattern Formation

The application of focused ion beam (FIB) nanotomography and Rutherford backscattering spectroscopy (RBS) to dealloyed platinum-aluminum thin films allows for an in-depth analysis of the dominating physical mechanisms of nanoporosity formation during the dealloying process. The porosity formation due to the dissolution of the less noble aluminum in the alloy is treated as result of a reaction-diffusion system. The RBS and FIB analysis yields that the porosity evolution has to be regarded as superposition of two independent processes, a linearly propagating diffusion front with a uniform speed and a slower dissolution process in regions which have already been passed by the diffusion front. The experimentally observed front evolution is captured by the Fisher-Kolmogorov-Petrovskii-Piskounov (FKPP). The slower dissolution is represented by a zero-order rate law which causes a gradual porosity in the thin film.

Translucent nanoparticle-based aerogel monoliths as 3-dimensional photocatalysts for the selective photoreduction of CO2 to methanol in a continuous flow reactor

The selective photoreduction of CO2 to methanol is an energy efficient way to transform a harmful greenhouse gas into a hydrocarbon of great industrial importance. However, the search for efficient photocatalysts able to reduce CO2 with water turned out to be extremely challenging. Besides studying new materials compositions and combinations, architectural design involving the structuring of catalysts from the nanometer to the centimetre scale and over three dimensions represents a much less studied, but nevertheless highly promising approach for the development of new photocatalysts. Translucent nanoparticle-based aerogels with their large surface areas, high porosity, well-developed crystallinity, multiscale structural features and their excellent light absorption properties offer all the advantages of a successful photocatalyst. Unfortunately, the mechanical fragility strongly limited their use in monolithic form. Here we present a prototype of a flow reactor, which enables the direct exploitation of cylindrical aerogel monoliths in various gas phase reactions. In particular, we studied the performance of TiO2–Au nanoparticle based aerogels for the photoreduction of CO2 with water to methanol, resulting in a conversion rate of 2.58 μmol g−1 h−1 with high selectivity. The assembly of nanoparticles into three-dimensional macroscopic bodies, which can directly be introduced in a flow reactor, opens up unique opportunities for the development of new and efficient photocatalysts beyond powders and films for a wide range of gas phase reactions.

Dealloying of Platinum-Aluminum Thin Films Part II. Electrode Performance

Highly porous Pt/Al thin film electrodes on yttria-stabilized zirconia electrolytes were prepared by dealloying of co-sputtered Pt/Al films. The oxygen reduction capability of the resulting electrodes was analyzed in a solid oxide fuel cell setup at elevated temperatures. During initial heating to 523 K, exceptionally high performances compared to conventional Pt thin film electrodes were measured. This results from the high internal surface area and large three phase boundary length obtained by the dealloying process. Exposure to elevated temperatures of 673 or 873 K gave rise to degradation of the electrode performance, which was primarily attributed to the oxidation of remaining Al in the thin films.

Dealloying of platinum-aluminum thin films: Electrode performance

Highly porous Pt/Al thin film electrodes on yttria-stabilized zirconia electrolytes were prepared by dealloying of co-sputtered Pt/Al films. The oxygen reduction capability of the resulting electrodes was analyzed in a solid oxide fuel cell setup at elevated temperatures. During initial heating to 523 K, exceptionally high performances compared to conventional Pt thin film electrodes were measured. This results from the high internal surface area and large three phase boundary length obtained by the dealloying process. Exposure to elevated temperatures of 673 or 873 K gave rise to degradation of the electrode performance, which was primarily attributed to the oxidation of remaining Al in the thin films.

Colloidal Nanocrystal-Based BaTiO3 Xerogels as Green Bodies: Effect of Drying and Sintering at Low Temperatures on Pore Structure and Microstructures

Although aerogels prepared by the colloidal assembly of nanoparticles are a rapidly emerging class of highly porous and low-density materials, their ambient dried counterparts, namely xerogels, have hardly been explored. Here we report the use of nanoparticle-based BaTiO3 xerogels as green bodies, which provide a versatile route to ceramic materials under the minimization of organic additives with a significant reduction of the calcination temperature compared to that of conventional powder sintering. The structural changes of the xerogels are investigated during ambient drying by carefully analyzing the microstructure at different drying stages. For this purpose, the shrinkage was arrested by a supercritical drying step under full preservation of the intermediate microstructure, giving unprecedented insight into the structural changes during ambient drying of a nanoparticle-based gel. In a first step, the large macropores shrink because of capillary forces, followed by the collapse of residual mesopores until a dense xerogel is obtained. The whole process is accompanied by a volume shrinkage of 97% and a drop in surface area from 300 to 220 m2 g-1. Finally, the xerogels are sintered, causing another shrinkage of up to 8% with a slight increase in the average pore and crystal sizes. At temperatures higher than 700 °C, an unexpected phase transition to BaTi2O5 is observed.

Processing of Cr doped SrTiO3 nanoparticles into high surface area aerogels and thin films

We present the nonaqueous sol–gel synthesis of crystalline SrTi1−xCrxO3 (x = 0, 0.3, 2, 5, 10%) nanoparticles and their processing into highly concentrated dispersions in ethanol by surface functionalization with 2-[2-(2-methoxyethoxy) ethoxy] acetic acid (MEEAA). These stable nanoparticle dispersions can then be assembled into 2- and 3-dimensional architectures such as films and aerogels. Homogeneous transparent films with a compact microstructure and a thickness of 140 nm are prepared from the dispersion by dip coating, while efficient destabilization and supercritical drying results in nanostructured bulk aerogels with a high surface area of up to 370 m2 g−1.