Wendt, Robert: Formation and Growth Studies of Microwave-assisted Nanoparticle Syntheses. , 2019
Humboldt-Universität zu Berlin

Solvothermal syntheses are one of the most applied methods for the preparation of monodisperseand crystalline metal oxide nanoparticles. Nevertheless, the drawbacks of this method are longreaction times, expansive autoclaves and difficulties to investigate the nanoparticle growth in situ. Microwave-assisted solvothermal synthesis techniques have become subject of renewed fundamental and applied interests due to distinguished control and exact online determination of pressure and temperature inside the sealed reaction vessel. Especially the efficient internal volumetric "in-core" heating by directly coupling the microwave field to the reaction solution allows for high heating rates with small thermal gradients and strongly decreased reaction times. However, microwave-assisted nanoparticle syntheses do not yet allow in situ access for the characterization of the reaction solution. In this thesis in situ tandem experiments on the microwave-assisted nanoparticle evolution in solution are performed by a new self-developed and self-constructed modular pressure-stable setup, which features the possibility to simultaneously perform various characterization techniques. For the evolution of the synthesized nanoparticles, in situ UV-vis spectroscopy, in situ small-angle X-ray scattering and in situ wide-angle X-ray scattering are carried out in parallel on pressurized reaction solutions in a time-resolved manner with high temporal resolution. The feature is facilitated by the modification and self-design of two different flow cells for UV-vis spectroscopy and X-ray scattering characterization techniques. Within this work, the modular setup constitutes a novel and essential tool to investigate and verify the complex physicochemical processes behind nanoparticle formation, growth and ripening with a high temporal resolution so that even processes during fast microwave-assisted nanoparticle syntheses are revealed. Supporting complementary ex situ measurements are provided by transmission electron microscopy and X-ray absorption near edge spectroscopy, performed by the implementation of an optional time-resolved sample separation feature. The evolution of nanoparticles in the setup is examined on the basis of two model systems for the synthesis of iron oxide nanoparticles. Studies at different final reaction temperatures reveal significant dependencies of the physicochemical processes on final temperature and heating rate. It is verified that the final reaction temperature determines growth and ripening processes as well as nanoparticle crystallization and thus the final nanoparticle size. The postulated mechanisms reveal temperature-dependent fundamental classical and non-classical growth and ripening processes during nanoparticle evolution that are identified by the deviations of the classical nucleation and growth theories from Lifshitz, Slyozov and Wagner. This work provides a comprehensive basis for optimizing and manipulating of the reaction and physicochemical processes required for future preparation of new, tailor-made and desired nanomaterials, especially energy-relevant materials.