Monodisperse ZnO Micro and Nanoparticles Obtained by Micro Segmented Flow Synthesis

Mikro- und Nanopartikel aus Zinkoxyd (ZnO) besitzen bemerkenswerte Eigenschaften für Applikationen im Bereich der Elektronik, Optik und Photonic. Als ein Halbleitermaterial mit großer Bandlücke ist ZnO ebenfalls für die Entwicklung von Sensoren, Light Emitting Diodes (LEDs) und Solarzellen von hohem Interesse. Die Herstellung definierter Materialien mit einheitlicher Morphologie und enger Partikel-Größenverteilung ist hierzu eine wichtige Voraussetzung. Verschiedene Verfahren zur Herstellung entsprechender Partikel sind in der Vergangenheit untersucht worden. Die tropfenbasierte Mikrofluidik bietet die Möglichkeit einer exzellenten Reaktionskontrolle durch die Verwendung eines Tropfens als Reaktionsgefäß. Kurze Mischzeiten, hohe Heiz-/Kühlraten sowie eine definierte Verweilzeit ermöglichen so neben stöchiometrischen Parametern eine exakte Reaktionsführung. Ziel der hier vorliegenden Dissertationsschrift ist die Untersuchung der ZnO-Präzipitation in entsprechenden mikrofluidischen Systemen sowie die Charakterisierung der hergestellten Materialien.

ZnO micro and nanoparticles have attracted considerable interest because of their remarkable performance in electronics, optics and photonics. As a wide band gap semiconductor material, ZnO is also a potential candidate for various applications including gas sensing, light emitting devices and solar cells. Although some technologies have been developed to produce well-defined ZnO particles of different shapes and sizes, ZnO particles prepared by micro segmented flow synthesis have been rarely reported. The aim of this work was to develop a microfluidic system based on the micro segmented flow method and to test whether the microfluidic components are suitable for the generation and investigation of ZnO particles with improved homogeneity.In order to optimize the experimental conditions, ZnO particles were first synthesized in batch. The optimized batch conditions were then adapted to two microfluidic arrangements for continuous synthesis of ZnO particles below 100°C. The set-ups included computer-controlled syringe pumps, T-injectors, PTFE tubings and PTFE knot mixers in a thermostat water bath. The ZnO particles were obtained under strong alkaline conditions at elevated temperature in aqueous solution and DMSO solution. Needle-like, flower-like and compact ZnO particles were obtained. In nearly all cases, a strong effect of the flow conditions on the homogeneity of the formed particles was observed. The higher quality of the particles can be attributed to the fast mixing and enhanced heat transfer caused by segment-internal convection.In addition, two other microfluidic set-ups were developed to control the ZnO formation reaction at temperature up to 150°C. A static micromixer was used for mixing the reactants at room temperature. The formation of segmented flow was realized by injection of the reaction mixture into a carrier stream. The particle growth took place in PTFE tube coils inside a thermostat, which allowed to heat up to 150°C. By using this set-up, flower-like, star-like, and spherical ZnO particles were successfully synthesized. The shape and size of the formed particles were strongly dependent on the reactant concentration and the molar ratio of NaOH/Zn(Ac)2. The total residence time for preparation of these particles was only 9.3s, which is very short compared to the most conventional methods.The effect of the solvent on the formation of ZnO particle has also been investigated using this microfluidic set-up. Two different experimental conditions were applied to prepare ZnO particles, where Zn(Ac)2 and NaOH in ethylene glycol (EG) were mixed with water or water/EG mixing solvent to achieve different water contents in the final mixture solution. The formation of homogeneous particles was characterized by SEM and TEM. A stronger dependence of the particle size and shape on the water content was observed. Furthermore, the water content can be used for tuning the optical absorption spectra of the formed ZnO particles. Besides the ZnO microparticles, ZnO nanoparticles with an average diameter around 4-5nm have been synthesized using Zn(Ac)2 and LiOH in ethanol. The prepared nanoparticles exhibited green and blue emission under excitation at 325nm. In order to understand the size-dependent optical properties of ZnO nanoparticles, extended X-ray absorption fine structure (EXAFS) spectroscopy was applied to study their local structure properties and compared with that of ZnO flower-like microparticles. The EXAFS measurements revealed higher vacancies and a higher degree of structural disorders in the nanoparticles than the microparticles. These disorders and vacancies could contribute to the blue shift of the visible emission from ZnO nanopartilces.Due to the potential applications of semiconductor-metal composite particles in diverse areas, the flower-like ZnO microparticles obtained by micro segmented flow synthesis were used to fabricate ZnO/4-MBA/Au composite particles using a simple strategy. The formed composite particles were very homogeneous in shape and size. The surface coverage of Au nanoparticles on ZnO/4-MBA particles can be adjusted by changing the molar ratio of ZnO/4-MBA to Au. In order to study the interaction of 4-MBA molecules with ZnO and Au particles, Raman spectra of ZnO/4-MBA and ZnO/4-MBA/Au particles were analysed.In summary, the segmented flow technique is suitable to generate ZnO particles with controlled size and morphology. Compared to most conventional methods, this technique offers several advantages, and it provides a new insight into material synthesis under environmentally friendly conditions.


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