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 work presented here describes a facile approach toward the in situ coating of various inorganic semiconductor nanoparticles (ISNs) including ZnO, TiO₂, and Fe₃O₄ with a hydrophobic polymer shell by a surfactant-free aerosol-based synthesis method. In this regard, fresh spherical ZnO nanoparticles were initially generated by spark discharge followed by a sintering process in the gas phase. Then, the resulting ZnO nanoparticles exhibit excellent performance in photocatalytic surface-initiated polymerization of the butyl acrylate monomer in the continuous aerosol-photopolymerization, within the average aerosol residence time of 35 s in the photoreactor. This method is based on heterogeneous condensation of monomer vapor around the surface of gas-borne ISNs, which is then polymerized “in flight” under UV light irradiation. In this one-pot synthesis, ISNs not only act as inorganic cores, but also at the same time are able to release charge carriers (electron–hole pairs) upon photoexcitation which drive the free radical polymerization near their surface. Therefore, no added initiator is required. To further investigate the basis and the mechanism of our experimental results, the optical properties of ZnO nanoparticles during the UV irradiation in the gas phase process were simulated by a three-dimensional finite difference time-domain (FDTD) method. Moreover, the coating efficiency was evaluated by aerosol photoemission (APE) and the results demonstrate that monomer and polymer coating efficiencies are 99% and 80%, respectively. Then, to verify the generality of this method toward coating, different kinds of ISNs, such as rod-like ZnO, TiO₂ P25, and magnetic Fe₃O₄ nanoparticles, were used as inorganic cores and photoinitiators. More importantly, with proper control of the experimental conditions, the polymer shell thickness can be easily tuned at the nanoscale. Finally, the successful change in the surface hydrophobicity of ISNs after the encapsulation with the polymer shell was confirmed by their preference for an organic solvent. Compared with conventional wet methods, the presented aerosol-photopolymerization process is fast, avoids the need for surfactants, co-initiators and surface modification of ISNs before the polymerization and produces core–shell nanostructures continuously.

A catalyst production method that enables the independent tailoring of the structural properties of the catalyst, such as pore size, metal particle size, metal loading or surface area, allows to increase the efficiency of a catalytic process. Such tailoring can help to make the valorization of $\mathrm{C}{\mathrm{O}}_2$ into synthetic fuels on Ni catalysts competitive to conventional fossil fuel production. In this work, a new spray-drying method was used to produce Ni catalysts supported on $\mathrm{S}\mathrm{i}{\mathrm{O}}_2$ and $\mathrm{A}{\mathrm{l}}_2{\mathrm{O}}_3$ nanoparticles with tunable properties. The influence of the primary particle size of the support, different metal loadings, and heat treatments were applied to investigate the potential to tailor the properties of catalysts. The catalysts were examined with physical and chemical characterization methods, including X-ray diffraction, temperature-programmed reduction, and chemisorption. A temperature-scanning technique was applied to screen the catalysts for $\mathrm{C}{\mathrm{O}}_2$ methanation. With the spray-drying method presented here, well-organized porous spherical nanoparticles of highly dispersed NiO nanoparticles supported on silica with tunable properties were produced and characterized. Moreover, the pore size, metal particle size, and metal loading can be controlled independently, which allows to produce catalyst particles with the desired properties. $\mathrm{N}\mathrm{i}/\mathrm{S}\mathrm{i}{\mathrm{O}}_2$ catalysts with surface areas of up to 40 m² g⁻¹ with Ni crystals in the range of 4 nm were produced, which exhibited a high activity for the $\mathrm{C}{\mathrm{O}}_2$ methanation.

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 SiO_x 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 SiO_x coatings. SiO_x 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.

Nanoparticles produced in technical aerosol processes exhibit often dendritic structures, composed of primary particles. Surprisingly, a small but consistent discrepancy was observed between the results of common aggregation models and in situ measurements of structural parameters, such as fractal dimension or mass-mobility exponent. A phenomenon which has received little attention so far is the interaction of agglomerates with admixed gases, which might be responsible for this discrepancy. In this work, we present an analytical series, which underlines the agglomerate morphology depending on the reducing or oxidizing nature of a carrier gas for platinum particles. When hydrogen is added to openly structured particles, as investigated by tandem differential mobility analysis (DMA) and transmission electron microscopy (TEM) analysis, Pt particles compact already at room temperature, resulting in an increased fractal dimension. Aerosol Photoemission Spectroscopy (APES) was also able to demonstrate the interaction of a gas with a nanoscaled platinum surface, resulting in a changed sintering behavior for reducing and oxidizing atmospheres in comparison to nitrogen. The main message of this work is about the structural change of particles exposed to a new environment after complete particle formation. We suspect significant implications for the interpretation of agglomerate formation, as many aerosol processes involve reactive gases or slightly contaminated gases in terms of trace amounts of unintended species.