Deposition of nanostructured titania films by particle-assisted MOCVD

Nanostructured TiO₂ films and particles were prepared in a tubular flow reactor at atmospheric pressure using titanium tetraisopropoxide (TTIP) as precursor. The morphology of the films and particles was characterised with scanning and transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) and atomic force microscopy analysis of the deposits were also performed. Depending on the location of the substrate in the reactor, i.e., the film forming species (vapour, monomers, particles), the morphology of the deposits was very different. In this paper, a qualitative explanation is given for the different film morphologies. The formation and growth of the particles are also explained. Close to the inlet, diffusion-controlled chemical vapour deposition (CVD) took place resulting in a dendritic (leaf-like) structure. Once particles are formed in the gas phase, they deposit together with vapour resulting in granular, nanostructured films with incorporated particles. After the point where all the vapour is consumed, separate, nearly monodisperse, 15-nm-sized particles deposited due to a competition between diffusion and thermophoresis. Downstream of the reactor, porous deposits consisting of agglomerated nanosized particles formed. The influence of the reactor temperature and the inlet precursor concentration on the morphology of the deposits was also investigated and is explained.