Bastian Barton

Anhydrous Amorphous Calcium Oxalate Nanoparticles from Ionic Liquids: Stable Crystallization Intermediates in the Formation of Whewellite

The mechanisms by which amorphous intermediates transform into crystalline materials are not well understood. To test the viability and the limits of the classical crystallization, new model systems for crystallization are needed. With a view to elucidating the formation of an amorphous precursor and its subsequent crystallization, the crystallization of calcium oxalate, a biomineral widely occurring in plants, is investigated. Amorphous calcium oxalate (ACO) precipitated from an aqueous solution is described as a hydrated metastable phase, as often observed during low-temperature inorganic synthesis and biomineralization. In the presence of water, ACO rapidly transforms into hydrated whewellite (monohydrate, CaC2 O4 ⋅H2 O, COM). The problem of fast crystallization kinetics is circumvented by synthesizing anhydrous ACO from a pure ionic liquid (IL-ACO) for the first time. IL-ACO is stable in the absence of water at ambient temperature. It is obtained as well-defined, non-agglomerated particles with diameters of 15-20 nm. When exposed to water, it crystallizes to give (hydrated) COM through a dissolution/recrystallization mechanism.

Hydrothermal growth mechanism of SnO2 nanorods in aqueous HCl

Abstract Rutile-type nanorods of SnO2 were obtained in a one-pot hydrothermal synthesis starting from SnCl4·5H2O and HCl in a temperature range between 200 and 240°C. Although the nanorods are polydisperse, the average length of the nanorods could be adjusted from 13 to 65 nm by varying of the reaction temperature. The resulting anisotropic nanocrystals were characterized using powder X-ray diffraction (PXRD), (high resolution-) transmission electron microscopy (HR-TEM), and selected area electron diffraction (SAED). The particle growth proceeds via a dissolution-recrystallization process with soluble [SnCl5(H2O)]− intermediates, as confirmed by PXRD, Raman spectroscopy, and magic angle spinning nuclear magnetic resonance (MAS-NMR).

Ab initio structure determination and quantitative disorder analysis on nanoparticles by electron diffraction tomography

Nanoscaled porous materials such as zeolites have attracted substantial attention in industry due to their catalytic activity, and their performance in sorption and separation processes. In order to understand the properties of such materials, current research focuses increasingly on the determination of structural features beyond the averaged crystal structure. Small particle sizes, various types of disorder and intergrown structures render the description of structures at atomic level by standard crystallographic methods difficult. This paper reports the characterization of a strongly disordered zeolite structure, using a combination of electron exit-wave reconstruction, automated diffraction tomography (ADT), crystal disorder modelling and electron diffraction simulations. Zeolite beta was chosen for a proof-of-principle study of the techniques, because it consists of two different intergrown polymorphs that are built from identical layer types but with different stacking sequences. Imaging of the projected inner Coulomb potential of zeolite beta crystals shows the intergrowth of the polymorphs BEA and BEB. The structures of BEA as well as BEB could be extracted from one single ADT data set using direct methods. A ratio for BEA/BEB = 48:52 was determined by comparison of the reconstructed reciprocal space based on ADT data with simulated electron diffraction data for virtual nanocrystals, built with different ratios of BEA/BEB. In this way, it is demonstrated that this smart interplay of the above-mentioned techniques allows the elaboration of the real structures of functional materials in detail - even if they possess a severely disordered structure.

Investigation of layered and porous nanomaterials by electron diffraction tomography

Yasar Krysiak1, Haishuang Zhao1, Bastian Barton1, Jürgen Senker2, Reinhard B. Neder3, Ute Kolb1 1Inst. Of Inorg. Chemistry And Analyt. Chemistry, Johannes Gutenberg University, Mainz, Germany, 2Inorganic Chemistry III, University of Bayreuth, Bayreuth, Germany, 3Chair for Crystallography and Structural Physics, Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen, Germany E-mail: krysiak@uni-mainz.de

Optimization of automated electron diffraction tomography for challenging applications

Ten years after the development of automated electron diffraction tomography (ADT) [1, 2] the analysis of the sequentially scanned reconstructed reciprocal space and the use of extracted reflection intensities for direkt crystal structure analysis has gained strong attention in many subjects. Especially in combination with precession electron diffraction allowing for reflection integration (ADT/PED) it was possible to extract fine structural details often explicitly important for physical properties of the material. The variety of nanocrystalline materials solved with ADT on the basis of quasi-kinematical electron diffraction intensity data covers alloys, large cell porous minerals, zeolites, beam-sensitive metal-organic frameworks and small organic molecules. Some originating from single nanocrystals down to 30 nm, strongly agglomerated particles or FIB lamellae. In combination with other approaches like HR-TEM imaging, X-ray diffraction methods analysing reflections (XRPD) or total scattering information (PDF) and neutron diffraction (ND) or spectroscopic measurements like solid state nuclear magnetic resonance (SS-NMR) it was possible to describe the crystal structure solutions gained for twinned, pseudo-symmetric or disordered material even more detailed [3]. ADT data can be in principle collected using standard TEMs but in order to collect electron diffraction data in a quality necessary to achieve high-end crystal structure solutions the data collection should be tailored to the material and the problem to be solved. Here we focus on the description of different approaches and the applicability to the various material classes.

Solving Challenging Crystallographic Problems with Automated Electron Diffraction Tomography (ADT)

Many important materials, ranging from minerals or catalysts to framework compounds and pharmaceuticals are not suitable for growing large crystals prohibiting single crystal X-ray analysis. Yet, introduction of nano crystallinity and special crystallographic features like disorder, defects, pseudo symmetry or stress/strain effects creates new or allows optimizing existing physical properties. With increasing complexity of the structures and special structural features as well as with decreasing size of crystalline domains, X-ray powder diffraction becomes more and more difficult for structural characterization, which is fundamental for understanding material properties. High-resolution transmission electron microscopy (HR-(S)TEM) allows visualizing structural features directly at the atomic scale but requires high electron dose of several thousand e/Ås causing beam damage. In contrast, electron diffraction needs only a fraction of this electron dose. For a complete structure solution, delivering atomic positions in sub Ångstrom accuracy, needs three-dimensional experimental data with high completeness. Data collection from oriented nano crystals limits the amount of measurable reflections significant and thus, delivers mostly heavy atom positions but hardly lighter atoms. Dynamical scattering effects strongly enhanced in oriented zones and may be reduced by electron beam precession technique [1]