In many applications using nanoparticles, the particles require a defined coating to achieve the desired product properties, such as a reduced photocatalytic activity or a controlled drug delivery. Especially silica shells are interesting, because silica is an inert and stable material. There are multiple methods to produce such core-shell structures, many of which use liquid phase sol-gel processes. However, gas phase processes have many advantages over liquid phase ones, such as a reduced complexity and the possibility of continuous core-shell particle production. There are different methods to apply coatings to nanoparticles in the gas phase. Most of these processes are chemical vapor deposition methods, where a precursor reacts to the desired coating material. The initiation of these reactions can be done in a multitude of ways, but the use of a non-thermal plasma discharge requires no high temperatures, which opens up interesting material combinations. Such a plasma was used here in the form of a dielectric barrier discharge to coat particles with silica-like coatings. The coating took place in the post-discharge environment outside of the plasma itself, which has significant advantages regarding the process stability. The process had two basic modes of operation, defined by the temperature during coating, which was either room temperature or between 100 and 300 °C. The particles were introduced continuously and coated gasborne at atmospheric pressure. Two precursors were used, tetraethyl orthosilicate for the production of inorganic silica shells and hexamethyldisiloxane for silica-organic coatings. The coatings produced in the process were quite homogeneous and the coating thickness could be controlled well. An important factor for a successful coating was found to be the species in the post-discharge environment, primarily defined by the discharge characteristics. The coating thickness could be controlled by the precursor concentration, the residence time in the system, the core particle surface area and the temperature during coating. Some changes in the core particle concentration were observed during the residence time, which seemed primarily related to agglomeration and less to actual losses by deposition on walls. The process was used to produce different combinations of core and shell materials and seemed very flexible in this regard. In fact, no core material was found that could not be coated. The connection between core and shell seemed to be mostly physical, but in the case of titania core particles, evidence for chemical bonds was found. The silica coatings were hydrophilic, while the silica-organic coatings were very hydrophobic. The hydrophobic property was preserved even after outgassing of the samples or tempering. Some applications for the core-shell particles were studied, such as the improvement of the mechanical and thermal stability of metals and the control of the photocatalytic behavior of titania. For the measurement of the mechanical stability, the coating process was combined with a…

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.

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.