Exploring the Si-precursor composition for inline coating and agglomeration of TiO2 via modular spray-flame and plasma reactor

Inline particle coating after the particle formation process to preserve its specific properties is hardly investigated scientifically. Tackling that issue, we have studied the use of three different vaporized organo-siloxanes (tetraethyl orthosilicate TEOS, hexamethyldisiloxane HMDSO, and octamethylcyclotetrasiloxane OMCTS) as precursors for direct inline coating of pristine titanium dioxide (TiO ₂) nanoparticles made via spray-flame synthesis. The inline silica (SiO ₂) coating of the formed titanium dioxide nanoparticles is achieved by vaporizing and sending the chosen organo-siloxane precursors into a cylindrical coating nozzle downstream the particle formation zone of the spray-flame. To further explore the effects on morphology and the quality of the resultant TiO ₂|SiO ₂ core-shell nanoparticles, a plasma discharge – i.e., dielectric barrier discharge source – is applied after the coating step. The TiO ₂|SiO ₂ core-shell nanoparticles are characterized using Transmission Electron Microscopy (TEM) and Scanning Transmission Electron Microscopy (STEM), X-Ray Diffraction (XRD), Fourier-Transform InfraRed spectroscopy (FTIR), Brunauer-Emmett-Teller surface area analysis (BET), elemental analysis, and dynamic light scattering (DLS). Results showed distinct core-shell nanoparticles with shell thicknesses of around 1.5 nm alongside the formation of unattached SiO ₂ nanoparticles due to homogenous nucleation of SiO ₂. As the precursor silicon content increased (TEOS < HMDSO < OMCTS), the homogenous nucleation rose to generate materials with high BET surface areas. When employing OMCTS, the high homogeneous nucleation rate led to SiO ₂ agglomeration, which resulted in large TiO ₂|SiO ₂ agglomerates. Morphologically, the phase composition of anatase/rutile of the produced coated nanoparticles did not vary significantly when compared with the reference uncoated TiO ₂ nanoparticles, indicating that the SiO ₂ coating is purely a surface phenomenon. Plasma discharge was shown to reduce coated particle agglomeration up to certain extend. Based on these findings, we conclude that the best studied parameters to benefit the synthesis of homogeneously coated TiO ₂|SiO ₂ nanoparticles are (i) using TEOS as a coating precursor to minimize SiO ₂ homogeneous nucleation and (ii) applying a plasma discharge to slightly reduce coated particle agglomeration.