Studies on the synthesis and in-line coating of nanoparticles from the gas phase

Functional core-shell nanoparticles find a plethora of applications in diverse fields, from medical applications such as bio-imaging and drug delivery to energy applications such as anode materials for Li-ion batteries or materials to improve the storage capacity of capacitors. Since the properties of the core must be retained or deactivated by coating with a shell, such functional nanoparticles are usually synthesized by wet chemical means and for some inorganic materials also by fluidized bed methods with the greatest possible reaction control. Wet-chemical processes are being increasingly replaced with gas-phase processes in order to reduce the need for solvents and avoid typical problems such as the formation of undesirable liquid by-products. The synthesis and functionalization of nanoparticles usually still form two separate steps in the production chain. Recently, the inline synthesis and coating of nanoscale materials has come under the spotlight as it enables a continuous, one-step process in the same aerosol reactor. Nevertheless, inline aerosol synthesis and coating in the gas phase is still not extensively explored. Therefore, in this work, three different gas-phase reactor concepts for the synthesis and coating of nanoparticles in a single step are investigated: spray flame reactor, microwave plasma reactor, and a spray flame reactor with downstream dielectric barrier discharge (DBD). Since siloxane-based precursors such as HMDSO, TEOS, and OMCTS are non-toxic, chemically versatile, and easy to use, they are widely used as precursors for coating substrates, especially in the gas phase such as CVD or powder ALD. Therefore, in this study, the use of the above siloxane-based precursors to coat nanoparticles such as silicon and TiO2 with SiO2 to form core-shell structures is investigated. For this purpose, special coating nozzles have been developed, which are integrated downstream of the aerosol flow (from the reactors mentioned above). The main goal of this work is the development of an inline coating process in which the core particles are first synthesized and then directly coated in the gas phase. A major challenge is to ensure good mixing between the aerosol laden core particles and the coating precursors. For this purpose, the mixing flows between the core particles and the coating precursors as well as the concentrations of the coating precursors must be matched to ensure that the core particles are covered with a shell as uniformly and completely as possible. In order to ensure good and uniform mixing between the particle-laden aerosol and the coating precursor, fluid mechanical (CFD) simulations were carried out to design and optimize a reasonable geometry for the coating nozzle and the required coating gas flows.

The resulting coated nanoparticles were analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM), Fourier transform infrared spectroscopy (FTIR) and Brunauer, Emmet and Teller (BET) specific surface area determination to gain an understanding of their morphology and the coating efficiency and characteristics. Depending on the reactor design and the coating precursor used, coating thicknesses between 1.1 and 10 nm were found. It was found that the use of high coating precursor flows negatively affects the morphology and composition of the final product as it leads to undesired homogeneous nucleation rather than forming thicker shells around the core nanoparticles. In addition, it was shown that with a larger number of silicon atoms and Si–O–Si bonds in the precursor (TEOS vs. HMDSO), homogeneous nucleation of the coating precursor was observed, so that small SiO 2 nanoparticles were also present in addition to the particles of the core material and the BET surface increased. It turned out that TEOS was an optimal choice as a precursor, as it showed only a low tendency towards homogeneous nucleation compared to HMDSO. It was also found that OMCTS tends to heterogeneously nucleate quite well at low flow rates, but forms very unevenly thick shells, which was observed in most of the experiments regardless of the reactor type. There were also areas that were not coated.

The analysis of coated nanoparticles with uncoated reference materials revealed that their shape, size and composition do not change significantly as a result of the coating process . This applied regardless of the reactor system examined. Using a downstream DBD plasma source showed that the aggregation of the coated particles tended to decrease. The result that the composition of the core nanoparticles remained unchanged indicates that the inline coating investigated here is purely a surface phenomena.


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