Frank Krumeich

Oxidic Nanotubes and Nanorods—Anisotropic Modules for a Future Nanotechnology

The discovery of carbon nanotubes in 1991 is a milestone in nanomaterials research. Since then, more and more anisotropic nanoparticles have been detected and characterized. The development of nanodevices might benefit from the distinct morphology and high aspect ratio of nanorods and nanotubes as these can be functionalized in unique ways such as incorporation of nanorods in nanotubes. Downscaling a broad range of materials to 1D nanoscopic structures is currently the focus of a rapidly growing scientific community. Developing general pathways to this goal would transfer a wide variety of properties to the nanoscale-a spectrum of phenomena so diverse that it would cover not only inorganic systems but all of materials science. Synthesis of real functional materials, however, always involves considerable synthetic ingenuity, interdisciplinary collaboration, as well as technological and economical realism. The major topic of this review is to provide a survey of recent progress in the synthesis of oxidic nanotubes and nanorods-with their non-oxidic counterparts briefly highlighted-and to outline the major synthetic routes leading to them. With the challenges of synthesizing bulk oxidic materials in mind, the establishment of trustworthy and uncomplicated ways of providing them as anisotropic nano-modules on an industrial scale appears to be more or less serendipity. Of the methods utilized in nanotube and nanorod synthesis solvothermal processes have emerged as powerful tools for generalizing and systematizing controlled syntheses of nano-morphologies. The flexibility and reliability of this synthetic approach is demonstrated here for the transformation of transition-metal oxides into high-quality anisotropic nanomaterials.

REDOX-ACTIVE NANOTUBES OF VANADIUM OXIDE

Unlike many small carbon nanotubes, VOx nanotubes (shown on the right) are obtained as the main product of a direct chemical synthesis at relatively low temperatures. The multiwalled material contains template molecules between the individual shells, which by a simple cation exchange can be removed without destruction of the tubes.

Nanoparticle cytotoxicity depends on intracellular solubility: comparison of stabilized copper metal and degradable copper oxide nanoparticles

Metal nanoparticles have distinctly different chemical and physical properties than currently investigated oxides. Since pure metallic nanoparticles are igniting at air, carbon stabilized copper nanoparticles were used as representative material for this class. Using copper as a representative example, we compare the cytotoxicity of copper metal nanoparticles stabilized by a carbon layer to copper oxide nanoparticles using two different cell lines. Keeping the copper exposure dose constant, the two forms of copper showed a distinctly different response. Whilst copper oxide had already been reported to be highly cytotoxic, carbon-coated copper nanoparticles were much less cytotoxic and more tolerated. Measuring the two materials intra- and extracellular solubility in model buffers explained this difference on the basis of altered copper release when supplying copper metal or the corresponding oxide particles to the cells. Control experiments using pure carbon nanoparticles were used to exclude significant surface effects. Reference experiments with ionic copper solutions confirmed a similar response of cultures if exposed to copper oxide nanoparticles or ionic copper. These observations are in line with a Trojan horse-type mechanism and illustrate the dominating influence of physico-chemical parameters on the cytotoxicity of a given metal.

Monodisperse and Inorganically Capped Sn and Sn/SnO2 Nanocrystals for High-Performance Li-Ion Battery Anodes

We report a facile synthesis of highly monodisperse colloidal Sn and Sn/SnO2 nanocrystals with mean sizes tunable over the range 9-23 nm and size distributions below 10%. For testing the utility of Sn/SnO2 nanocrystals as an active anode material in Li-ion batteries, a simple ligand-exchange procedure using inorganic capping ligands was applied to facilitate electronic connectivity within the components of the nanocrystalline electrode. Electrochemical measurements demonstrated that 10 nm Sn/SnO2 nanocrystals enable high Li insertion/removal cycling stability, in striking contrast to commercial 100-150 nm powders of Sn and SnO2. In particular, reversible Li-storage capacities above 700 mA h g(-1) were obtained after 100 cycles of deep charging (0.005-2 V) at a relatively high current of 1000 mA h g(-1).

Persistence of engineered nanoparticles in a municipal solid-waste incineration plant

More than 100 million tonnes of municipal solid waste are incinerated worldwide every year. However, little is known about the fate of nanomaterials during incineration, even though the presence of engineered nanoparticles in waste is expected to grow. Here, we show that cerium oxide nanoparticles introduced into a full-scale waste incineration plant bind loosely to solid residues from the combustion process and can be efficiently removed from flue gas using current filter technology. The nanoparticles were introduced either directly onto the waste before incineration or into the gas stream exiting the furnace of an incinerator that processes 200,000 tonnes of waste per year. Nanoparticles that attached to the surface of the solid residues did not become a fixed part of the residues and did not demonstrate any physical or chemical changes. Our observations show that although it is possible to incinerate waste without releasing nanoparticles into the atmosphere, the residues to which they bind eventually end up in landfills or recovered raw materials, confirming that there is a clear environmental need to develop degradable nanoparticles.

Harnessing Defect-Tolerance at the Nanoscale: Highly Luminescent Lead Halide Perovskite Nanocrystals in Mesoporous Silica Matrixes

Colloidal lead halide perovskite nanocrystals (NCs) have recently emerged as a novel class of bright emitters with pure colors spanning the entire visible spectral range. Contrary to conventional quantum dots, such as CdSe and InP NCs, perovskite NCs feature unusual, defect-tolerant photophysics. Specifically, surface dangling bonds and intrinsic point defects such as vacancies do not form midgap states, known to trap carriers and thereby quench photoluminescence (PL). Accordingly, perovskite NCs need not be electronically surface-passivated (with, for instance, ligands and wider-gap materials) and do not noticeably suffer from photo-oxidation. Novel opportunities for their preparation therefore can be envisaged. Herein, we show that the infiltration of perovskite precursor solutions into the pores of mesoporous silica, followed by drying, leads to the template-assisted formation of perovskite NCs. The most striking outcome of this simple methodology is very bright PL with quantum efficiencies exceeding 50%. This facile strategy can be applied to a large variety of perovskite compounds, hybrid and fully inorganic, with the general formula APbX3, where A is cesium (Cs), methylammonium (MA), or formamidinium (FA), and X is Cl, Br, I or a mixture thereof. The luminescent properties of the resulting templated NCs can be tuned by both quantum size effects as well as composition. Also exhibiting intrinsic haze due to scattering within the composite, such materials may find applications as replacements for conventional phosphors in liquid-crystal television display technologies and in related luminescence down-conversion-based devices.

Flame spray-pyrolyzed vanadium oxide nanoparticles for lithium battery cathodes

Vanadium pentoxide (V2O5) nanoparticles (30-60 nm) were made by a one-step and scalable flame spray pyrolysis (FSP) process. Optimization of the FSP processing conditions (precursor concentration and injection rate) enhanced the electrochemical performance of these nanoparticles. Increasing the cut-off potential for discharging from 1.5 to 2.5 V vs. Li/Li+ improved the cycle life of these V2O5 nanoparticles. Particles with the lowest specific surface area (approximately 32 m2 g(-1)) and highest phase purity (up to 98 wt%) showed excellent cyclability between 2.5 and 4.0 V vs. Li/Li+, retaining a specific charge of 110 mAh g(-1) beyond 100 cycles at a specific current of 100 mA g(-1), and also superior specific charge of 100 mAh g(-1) at specific current up to 20C rate (or 2000 mA g(-1)).

Opposite face sensitivity of CeO2 in hydrogenation and oxidation catalysis

The determination of structure-performance relationships of ceria in heterogeneous reactions is enabled by the control of the crystal shape and morphology. Whereas the (100) surface, predominantly exposed in nanocubes, is optimal for CO oxidation, the (111) surface, prevalent in conventional polyhedral CeO2 particles, dominates in C2H2 hydrogenation. This result is attributed to the different oxygen vacancy chemistry on these facets. In contrast to oxidations, hydrogenations on CeO2 are favored over low-vacancy surfaces owing to the key role of oxygen on the stabilization of reactive intermediates. The catalytic behavior after ageing at high temperature confirms the inverse face sensitivity of the two reaction families.

In Situ Coating of Flame-Made TiO2 Particles with Nanothin SiO2 Films

Rutile TiO2 particles made by flame spray pyrolysis (FSP) were coated in a single step with SiO2 layers in an enclosed flame reactor. This in situ particle coating was accomplished by a hollow ring delivering hexamethyldisiloxane (HMDSO) vapor (precursor to SiO2) through multiple jets in swirl cross-flow to Al-doped nanostructured rutile TiO2 aerosol freshly made by FSP of a solution of titanium tetraisopropoxide and aluminum sec-butoxide in xylene. The as-prepared powders were characterized by (scanning) transmission electron microscopy (STEM and TEM), energy dispersive X-ray analysis, X-ray diffraction, nitrogen adsorption, electrophoretic mobility, DC plasma optical emission (DCP-OES), and Fourier transform infrared (FT-IR) spectroscopy. The coating quality was assessed further by the photocatalytic oxidation of isopropyl alcohol to acetone. The effect of HMDSO injection point and vapor concentration on product particle morphology was investigated. The titania particles were uniformly SiO2-coated with controlled and uniform thickness at a production rate of about 30 g h(-1) and exhibited limited, if any, photoactivity. In contrast, spraying and combusting equivalent mixtures of the above Si/Al/Ti precursors in the above reactor (without delivering HMDSO through the hollow ring) resulted in particles segregated in amorphous (SiO2) and crystalline (TiO2) domains which exhibited high photocatalytic activity.

Nanoröhren und Nanostäbe auf Oxidbasis – anisotrope Bausteine für künftige Nanotechnologien

Die Entdeckung der Kohlenstoff-Nanorohren im Jahr 1991 ist zu einem Meilenstein der Forschung an nanoskopischen Materialien geworden. Mehr und mehr anisotrope Nanopartikel sind seither entdeckt und auch charakterisiert worden. Besonders die Entwicklung neuer Baueinheiten auf Nanobasis kann von den morphologischen Charakteristika und Vorteilen der Nanorohren und Nanostabe profitieren, denn diese konnen auf einzigartige Weise funktionalisiert werden – man denke nur an die Moglichkeit, Nanostabe in bestehende Nanorohren einzufugen. Gegenstand zahlreicher aktueller Forschungsaktivitaten ist die Herstellung eines ganzen Spektrums von nanoskopischen Materialien in eindimensional strukturierter Form. Sind erst einmal generelle Wege zur Herstellung eindimensionaler Nanostrukturen gebahnt worden, so eroffnet sich der Zugang zu einer ganzen Reihe von Eigenschaften in neuartiger nanoskopischer Form, der nicht auf anorganische Systeme beschrankt ware, sondern auch grose Gebiete der Materialwissenschaften einschliesen wurde. Zur Herstellung anwendungsreifer funktioneller Materialien bedarf es jedoch immer noch eines grosen Mases an Einfallsreichtum fur die Synthese, der durch interdisziplinare Zusammenarbeit gefordert wird, sowie einer realistischen Einschatzung des technischen und wirtschaftlichen Umfeldes. Diese Ubersicht informiert uber die neuesten Fortschritte in der Synthese oxidischer Nanorohren und Nanostabe und zeigt aus der Vielzahl der veroffentlichten Arbeiten die wichtigsten generellen Synthesetrends auf. Zudem werden kurz die pragnantesten Neuerungen bei den nichtoxidischen Nanomaterialien erwahnt. Die Aufgabe, ebenso zuverlassige wie unkomplizierte Wege zur Herstellung anisotroper Nanomodule in technisch relevantem Umfang aufzufinden, ist eine grose Herausforderung und scheint nur durch oft zufallige und uberraschende Entdeckungen losbar zu sein. Aus der Vielzahl der Methoden, die bislang zu Nanorohren und Nanostaben fuhrten, konnen Solvothermalsynthesen als eine besonders leistungsfahige Methode hervorgehoben werden, da sie in einzigartiger Weise Moglichkeiten zur Generalisierung und Systematisierung der Synthesen bieten. Ihre Flexibilitat, die mit einer zuverlassigen und bestandigen Qualitat der Produkte einhergeht, wird im Folgenden anhand der quantitativen Umwandlung von Ubergangsmetalloxiden in anisotrope Nanomaterialien demonstriert, die fur hochste technische Anforderungen konzipiert sind.