Nanoparticles are coated in-flight with a plasma-enhanced chemical vapor deposition (PECVD) process at ambient or elevated temperatures (up to 300 °C). Two silicon precursors, tetraethyl orthosilicate (TEOS) and hexamethyldisiloxane (HMDSO), are used to produce inorganic silica or silica-organic shells on Pt, Au and TiO2 particles. The morphology of the coated particles is examined with transmission electron microscopy (TEM) and the chemical composition is studied with Fourier-transform infrared spectroscopy (FTIR), Energy-dispersive X-ray spectroscopy (EDX) and X-ray photoelectron spectroscopy (XPS). It is found that both the precursor and certain core materials have an influence on the coating composition, while other parameters, such as the precursor concentration, aerosol residence time and temperature, influence the morphology, but hardly the chemical composition. The coated particles are used to demonstrate simple applications, such as the modification of the surface wettability of powders and the improvement or hampering of the photocatalytic activity of titania particles.

The plasma-based aerosol process developed for the direct coating of particles in gases with silicon oxide in a continuous chemical vapor deposition (CVD) process is presented. It is shown that non-thermal plasma filaments induced in a dielectric barrier discharge (DBD) at atmospheric pressure trigger post-DBD gas phase reactions. DBD operating conditions are first scanned to produce ozone and dinitrogen pentoxide. In the selected conditions, these plasma species react with gaseous tetraethyl orthosilicate (TEOS) precursor downstream of the DBD. The gaseous intermediates then condense on the surface of nanoparticles and self-reactions lead to homogeneous solid SiOx coatings, with thickness from nanometer to micrometer. This confirms the interest of post-DBD injection of the organo-silicon precursor to achieve stable production of actives species with subsequent controlled thickness of SiOx coatings. SiOx coatings of spherical and agglomerated metal and metal oxide nanoparticles (Pt, CuO, TiO2) are achieved. In the selected DBD operating conditions, the thickness of homogeneous nanometer sized coatings of spherical nanoparticles depends on the reaction duration and on the precursor concentration. For agglomerates, operating conditions can be tuned to cover preferentially the interparticle contact zones between primary particles, shifting the sintering of platinum agglomerates to much higher temperatures than the usual sintering temperature. Potential applications for enhanced thermal stability and tunable photoactivity of coated agglomerates are presented.

Ein plasma-unterstützer Aerosolprozess wird vorgestellt, der die kontinuierliche Beschichtung von Partikeln mit Siliziumoxid bei Umgebungstemperatur ermöglicht. Dabei wird eine dielektrische Barriere-Entladung genutzt, um verschiedene reaktive Spezies zu erzeugen. Tetraethylorthosilikat dient als Precursor. Da der Prozess bei Umgebungstemperatur erfolgt, können auch temperaturempfindliche Stoffe beschichtet werden. Ein weiterer Vorteil ist die weitgehende Unabhängigkeit des Prozesses von der Partikelquelle. Die erfolgreiche Beschichtung wird auf unterschiedlichen Partikelgeometrien und -materialien, wie Metallen, Salzen und Kunststoffen demonstriert und die Abhängigkeit der Schichtdicke von der Partikeloberfläche gezeigt. Zudem wird die im Prozess auftretende Ä nderung der Partikelanzahlkonzentration, die im Wesentlichen durch Agglomeration verursacht wird, diskutiert.A plasma-assisted aerosol process is presented, which allows the continuous coating of particles with silicon oxide at ambient temperature. A dielectric barrier discharge plasma is applied to produce different reactive species. Tetraethyl orthosilicate is used as precursor. No elevated temperatures are necessary to perform the coating so that even temperature-sensitive materials can be coated. Another advantage is the independence of the process from the particle source. The successful coating is demonstrated on different particle geometries and materials such as metals, salts and polymers and the dependence of the coating thickness on the particle surface area is shown. Furthermore, the change in particle number concentration along the coating reactor is primarily caused by agglomeration.

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