Morphology of and surface modification by TiO2 deposits on a porous ceramic substrate

We investigate the morphology of and surface modification by titanium dioxide (TiO2) deposits on porous ceramic substrates placed in a horizontal, tubular, hot-wall, atmospheric pressure chemical vapor deposition (CVD) reactor with titanium tetraisoperoxide (TTIP) as the precursor. The TiO2 particles are produced from TTIP through two routes: first by thermal decomposition which then kicks off hydrolysis. The deposit characteristics is found to be location dependent. Those at the reactor entrance and exit are different from that within the reactor. Within the reactor, the deposit characteristics is further found to depend strongly on the deposition temperature (T-d) and is almost independent of the flow-rate and concentration of the reactant. With a T-d of about 380 degrees C, the deposited TiO2 forms a forest-like structure with poor adhesivity to the substrate and results in an insignificant pore size reduction for the substrate. If T-d drops down to about 300 degrees C, the deposited TiO2 forms a rock-packing structure with good adhesivity to the substrate and can reduce the pore size of the substrate from about 1-2 mu m down to about 64 nm. At both the reactor entrance and exit, the deposited TiO2 are loosely-packed spherical particles of average diameter of 140-400 nm. A theory, based on whether or not the reaction is gas-phase or surface dominated, is proposed to explain the dramatic effect of T-d. (C) 1999 Kluwer Academic Publishers.