Search

Self-assembly of nanoparticles is a promising route to form complex, nanostructured materials with functional properties. Nanoparticle assemblies characterized by a crystallographic alignment of the nanoparticles on the atomic scale, i.e. mesocrystals, are commonly found in nature with outstanding functional and mechanical properties. This thesis aims to investigate and understand the formation mechanisms of mesocrystals formed by self-assembling iron oxide nanocubes. We have used the thermal decomposition method to synthesize monodisperse, oleate-capped iron oxide nanocubes with average edge lengths between 7 nm and 12 nm and studied the evaporation-induced self-assembly in dilute toluene-based nanocube dispersions. The influence of packing constraints on the alignment of the nanocubes in nanofluidic containers has been investigated with small and wide angle X-ray scattering (SAXS and WAXS, respectively). We found that the nanocubes preferentially orient one of their {100} faces with the confining channel wall and display mesocrystalline alignment irrespective of the channel widths. We manipulated the solvent evaporation rate of drop-cast dispersions on fluorosilane-functionalized silica substrates in a custom-designed cell. The growth stages of the assembly process were investigated using light microscopy and quartz crystal microbalance with dissipation monitoring (QCM-D). We found that particle transport phenomena, e.g. the coffee ring effect and Marangoni flow, result in complex-shaped arrays near the three-phase contact line of a drying colloidal drop when the nitrogen flow rate is high. Diffusion-driven nanoparticle assembly into large mesocrystals with a well-defined morphology dominates at much lower nitrogen flow rates. Analysis of the time-resolved video microscopy data was used to quantify the mesocrystal growth and establish a particle diffusion-based, three-dimensional growth model. The dissipation obtained from the QCM-D signal reached its maximum value when the microscopy-observed lateral growth of the mesocrystals ceased, which we address to the fluid-like behavior of the mesocrystals and their weak binding to the substrate. Analysis of electron microscopy images and diffraction patterns showed that the formed arrays display significant nanoparticle ordering, regardless of the distinctive formation process. We followed the two-stage formation mechanism of mesocrystals in levitating colloidal drops with real-time SAXS. Modelling of the SAXS data with the square-well potential together with calculations of van der Waals interactions suggests that the nanocubes initially form disordered clusters, which quickly transform into an ordered phase. At the time of the doctoral defense, the following papers were unpublished and had a status as follows: Paper 4: Manuscript. Paper 5: Manuscript.

A novel method for synthesising rare earth oxide (RE2O3) nanopowder was applied to several rare earths including Nd, Eu, Gd, Ho, Y and Yb. The process was successful in producing homogeneous, fine dispersed and deagglomerated powder particles with uniform morphology for all rare earth oxides, except neodymia. In the first process step, rare earth nitrate solutions were mixed with glycine and subsequently heated in a furnace. Glycine acted as fuel to initiate the combustion synthesis (SHS) in order to obtain nanocrystalline and weakly agglomerated particles. Powder samples were produced at three different glycine-to-nitrate ratios (G/N), involving stoichiometric ratio (G/N = 0.56) and two fuel lean ratios (0.32 and 0.25). As a consequence of low combustion temperature at fuel lean ratios were carbonate structures found and their formation was correlated to the size of the rare earth cation. Resulting cellular structures were analysed by X-ray diffractometry (XRD) and transmission electron microscopy (TEM). The cellular structure is unsuitable to be processed to a final product. The resulting powder has a very large specific volume and is comprised of strongly agglomerated crystallites, which renders pressing and sintering of these structures difficult. Therefore an additional synthesis step was employed with the addition of ammonium sulfate and calcination for 4 h in a tube furnace at 1100°C. In order to understand the influence of the initial, cellular structure on the morphological and microstructural powder properties different sulfate concentrations were used. Samples were characterized by XRD and scanning electron microscopy (SEM). The average crystallite size increased during calcination from below 15 nm in the initial structure to 50 nm in the final product. However, the resulting particles were loosely agglomerated. A benefit of sulfate-doped rare earth compounds under heat treatment is the formation of nearly isotropic surfaces, which might be related to the promotion of surface diffusion and partial inhibition of densifying transport mechanisms such as volume diffusion. Validerat; 20110923 (anonymous)