2011, Lizandara Pueyo, Carlos, Siroky, Stephan, Wagner, Markus R., Hoffmann, Axel, Reparaz, Juan S., Lehmann, Michael, Polarz, Sebastian

The shape of crystalline particles is recognized as one important parameter for the adjustment of functional properties of inorganic materials. The surfaces of a thermodynamically stable crystal correspond to a set of lattice planes determined by the minimum interface energy. Thus, a morphology deviating from the most stable state correlates to either a change of the proportion of those surfaces to each other or ultimately a new set of surfaces emerges. At the nanoscale, when the surface-to-volume ratio is large, it is expected that a change in morphology implies a measurable alteration of properties. Here, the synthesis of nanocrystalline ZnO nanoparticles possessing a new non-equilibrium shape is presented. The reaction of special organometallic precursors at the interface of a water-in-oil emulsion facilitates the synthesis of fairly monodisperse prismatic ZnO nanocrystallites with an adjustable aspect ratio in gram amounts. It is found that the special morphology influences the bulk properties of the ZnO materials. Contrary to the well-known quantum size effect (smaller particles produce a blue-shift), a shortening of the ZnO nanoprisms induces a decrease in the bandgap (red-shift). This effect is due to the influence of an electric field inside the particles caused by the polarity of the surfaces terminating the nanoprisms (the quantum-confined Stark effect).

2010-03-07, Dreher, Marian A., Krumm, Michael, Lizandara Pueyo, Carlos, Polarz, Sebastian

Zinc oxide has become one of the most important semiconductor materials and it possesses a multitude of properties and applications. An even wider spectrum of properties can be envisioned if an additional element is introduced. On the cation side there is large interest in the combination of ZnO with paramagnetic metal centres like Cr-III. Two new single source precursors containing "ZnO" and chromium in the ratios 1:1 and 1:2 are presented. Advantages and disadvantages of using these precursors are reported. One of the advantages is that the geometric organization of the magnetic centres in the ZnO matrix can be preorganized on the molecular scale.

2010, Lizandara Pueyo, Carlos, Siroky, Stephan, Landsmann, Steve, van den Berg, Mauritz W.E., Wagner, Markus R., Reparaz, Juan S., Hoffmann, Axel, Polarz, Sebastian

Important changes in properties are observed in many instances when, at constant composition, materials possessing different crystal structure, so-called polymorphs are considered. Because many viable polymorphs have not yet been realized experimentally, it is an important task to learn about the factors which determine the formation of metastable phases. The preparation of such phases is particularly challenging when the thermodynamically stable phase forms already under mild conditions. Zinc oxide with Wurtzite structure represents such a case, and because of its multifunctional character, it is currently in the focus of research in many areas. Thus, it can be envisioned that zinc oxide materials with a structure different to Wurtzite will exhibit new, exciting, and eventually unforeseeable properties. The preparation of ZnO under kinetically controlled conditions using an organometallic precursor system is presented here. The formation of a new, nanocrystalline phase with 99% purity has been observed at low temperatures (T = 2 degrees C). The analysis of the new phase with a variety of analytical methods including powder X-ray diffraction (PXRD), differential scanning calorimetry (DSC), X-ray absorption near-edge spectra (XANES)/extended X-ray absorption fine structure (EXAFS), high-resolution transmission electron microscopy (HRTEM), FT-Raman and optical spectroscopy leads to the conclusion that a metastable ZnO with a crystal structure resembling the alpha-boron nitride structure has been prepared. The formation of this material represents a novel example for the application of the Ostwald step-rule in materials science.

2011, Lizandara Pueyo, Carlos

2010, Krumm, Michael, Lizandara Pueyo, Carlos, Polarz, Sebastian

Aerogels belong to the large class of porous solids. They are characterized by a network of a mechanically stable solid, most likely inorganic in nature comprising a large gas volume in comparison to the volume of the solid material. Although a large variety of aerogels with silicate networks already exist, examples for materials with transition metal oxide networks are rare. One particularly interesting target is zinc oxide because of its semiconducting and multifunctional character. A sol gel process facilitating an organometallic precursor system is established. The mechanism of gelation has been studied in detail. Unlike most other sol gel processes, at first a large number of nanocrystalline ZnO particles are formed that rapidly agglomerate to secondary aggregates instead of forming a network directly. Thus, these secondary aggregates determine the textural properties of the pore walls as they assemble into the final, highly cross-linked network. A monolithic ZnO aerogel with porosities greater than 99% could be received after solvent extraction with supercritical CO2. Furthermore, the porosities could be tuned via a combination of conventional drying and supercritical solvent extraction by a process that we call scalar drying. Finally, one of the potential functional properties of the new ZnO aerogels was proven, its application in photocatalysis.

2010, Dilger, Stefan, Hintze, Christian, Krumm, Michael, Lizandara Pueyo, Carlos, Deeb, Salem, Proch, Sebastian, Polarz, Sebastian

Titania materials with structures resembling aerogels are presented in the current contribution. In contrast to the classical routes the aerogel-like TiO2 materials discussed here are prepared exclusively by a high-temperature aerosol process. Thus, the titania particles forming the network are highly pure, highly crystalline and their surfaces are free of any additives. It is shown that the structure of the materials (particle-size, porosity, surface area, crystallinity, and phase composition) depends on a complex interplay of different parameters (temperature gradients, precursor concentration, residence time, etc.). However, the understanding of those correlations enables the preparation of tailor made titania materials with aerogel-like structure. The size of the primary, nanocrystalline particles can be varied systematically from 13 nm to 70 nm. The composition regarding the two different crystalline phases anatase and rutile can be adjusted as an independent parameter. Finally, the aerogel-like TiO2 materials were loaded with Au clusters and the activity in oxidation catalysis was investigated.

2010-04-14, Landsmann, Steve, Lizandara Pueyo, Carlos, Polarz, Sebastian

The main ability of amphiphilic molecules is to alter the energy of interfaces. They aid in the formation of various materials characterized by a high surface to volume ratio. Furthermore, amphiphiles tend to self-organize into structures of higher complexity. In the current study anionic surfactants containing a purely inorganic multinuclear head group of the polytungstate type R-[PW11O39](3-) were synthesized. Alkyl chains of different length were attached to the head group via siloxy bridges. Furthermore, the counterions could be varied. Ultimately, a heteropolyacid surfactant (H+ as the counterion) could be prepared. The self-assembly behavior of the polyoxometalate surfactants into micelles and even lyotropic phases was studied. For instance, the formation of a phase with P6/mm symmetry containing hexagonally packed cylinders has been observed. Finally, it was possible to extend the functionality of classical amphiphiles. The polyoxometalate amphiphiles have been used for the emulsification of and, at the same time, as the initiator for the cationic polymerization of styrene. As a result, interesting organic inorganic hybrid polymer latexes with surfaces containing heteropolyacid entities were prepared.

2010, Polarz, Sebastian, Lizandara Pueyo, Carlos, Krumm, Michael

In the current article, we present a concept for the synthesis of complex nanoscaled materials. The synthetic strategy involves a stepwise assembly of materials starting from special molecular precursors possessing multiple information. Therefore, the article focuses on a strong pervasion of inorganic materials chemistry, solid-state chemistry and molecular chemistry. The concept introduced is finally highlighted by examples from our current research in the field of zinc oxide materials. (C) 2010 Elsevier B. V. All rights reserved.