Coating of gasborne nanoparticles with silica and silica-organic shells in a post-plasma CVD process

Post, Patrick; Weber, Alfred P.

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 low pressure impactor, where agglomerates were impacted on a TEM sample grid and the size of the resulting fragments was determined. Both the mechanical and the thermal stability were found to be improved even by thin coatings. The photocatalytic activity could either be reduced or improved depending on the process parameters.

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Post, Patrick / Weber, Alfred: Coating of gasborne nanoparticles with silica and silica-organic shells in a post-plasma CVD process. 2019.

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