Deposition of nano-size titania-silica particles in a hot-wall CVD process

The deposition of nano-size titania-silica particles is carried out in a hot-wall CVD reactor by using two premixed precursors, titanium tetraisopropoxide (TTIP) and tetraethyl ortho-silicate (TEOS). The deposition occurs mostly in two regions, one near the reactor entrance and the other near the reactor exit. Deposits collected near the reactor entrance are densely packed micrometer-size aggregates/particles of good adhesion to the substrate, while those near the reactor exit are loosely packed nano-size particles with a poor adhesion to the substrate. It is conjectured that the more reactive TTIP reacts first and produces TiO2 particles later covered by SiO2 formed via a catalytic surface reaction of the less reactive TEOS on TiO2 particle surfaces. The presence of surface SiO2 retards the growth of TiO2 particles, leading to deposits formed by micrometer-size aggregates containing nano-size primary particles of 30 to 40 nm in the first deposition region. With less or no SiO2 present on TiO2 particle surfaces, the deposits formed in the first deposition region are densely packed micrometer-size particles. The Ti/Si ratio of the produced particles, for a furnace temperature of 750 °C, increases with increasing TTIP/TEOS concentration ratio, and is lower than the Ti/Si ratio of the incoming reactant stream. The Ti/Si ratio of the particles is also found to decrease with increasing furnace temperature.