(5du) Flame Synthesis and Coating of Nanostructured Particles in One Step

Direct synthesis of in-situ coated nanostructured particles made by flame spray pyrolysis (FSP) was explored. Typically such coatings are achieved by tedious multi-step wet-phase processes. Thus there is a strong interest to develop simple, one-step gas-phase processes to coat TiO₂ and other oxide materials. So emphasis was placed here on developing a one-step process for synthesis and coating of TiO₂ nanoparticles by inert and transparent SiO₂. The aim is to passivate the photocatalytically active TiO₂ surface by hermetic SiO₂ coatings, to prevent the degradation of their host liquid or polymer matrices in the final suspension or nanocomposite products. Material properties were characterized with focus on the coating quality and optimized with the aim to significantly reduce the photocatalytic activity.

Thus, a process was developed for in-situ coating of flame-made particles. A mostly rutile TiO₂-producing spray flame (25 g/h) was enclosed by a quartz glass tube while hexamethyldisiloxane (HMDSO) vapor carried by N₂ was T-mixed downstream of the flame by a metal torus pipe ring placed at various heights above the FSP burner. Premature HMDSO vapor injection resulted in separate SiO₂ and poorly-coated TiO₂ particles, while smooth and homogeneous coatings, 2 - 4 nm thick, were formed as HMDSO was added further downstream at high enough temperatures for complete oxidation of HMDSO to SiO₂. The coating thickness was controlled by the silica content and the coating quality was evaluated in the photocatalytic oxidation of isopropanol (IPA) to acetone. Addition of 10 wt% SiO₂ or more resulted in limited photoactivity. In contrast, co-oxidizing all precursor for Si/Al/Ti in the flame, results in large SiO₂ domains and thus only partial coatings on TiO₂ particles which exhibit a high photocatalytic activity.

The effect of mixing intensity between the freshly-formed TiO₂ aerosol and the HMDSO-laden N₂ gas on product particle coating morphology was investigated experimentally and by computational fluid dynamics (CFD). The exit velocity or the number of jet outlets for HMDSO-laden N₂ gas was varied. The CFD showed that low flowrates or few jet outlets resulted in incomplete mixing across the radius of the reactor. At such conditions the coating efficiency decreased: separate SiO₂ and poorly coated TiO₂ particles were observed in transmission electron microscopy (TEM) images. In contrast, SiO₂-encapsulated TiO₂ particles were obtained at high mixing intensity or with multiple coating vapor jets which exhibited low photoactivity in UV-irradiated IPA suspensions.

Finally, the coating quality of the partially and hermetically SiO₂-coated TiO₂ product particles was assessed. The extent of SiO₂ surface coverage could be determined electrophoretically and chemically. Silica shifted the isoelectric point (IEP) in aqueous suspensions to lower pH, however, complete SiO₂ (≥ 10 wt%) coatings resulted in negative zeta potentials at all pH and thus no IEP was obtained. Similarly, isopropanol chemisorption could be applied to determine the coating quality. Pure titania chemisorbed isopropanol and converted a large fraction to propene, while no chemisorption took place on pure SiO₂. Thus, insufficiently coated particles still chemisorb isopropanol while complete coatings do not. Hermetic and partial coatings also yielded distinctly different FT-IR spectra in the region of Si-O-Si asymmetric vibrations.