The present study investigates the impact of post annealing of TiO2 nanoparticulate films on their crystallinity, mechanical strength, and morphology. Non-agglomerated and amorphous TiO2 nanoparticles of 46 nm diameter were synthesized in a plasma field, and were subsequently deposited on substrates to form nanoparticulate films. The films were annealed at various temperatures in the range of 100–1,200°C. Phase transformations from amorphous-to-anatase and from anatase-to-rutile were observed at 400°C and at 1,000°C, respectively. The high rutile transformation temperature was considered to be due to a tensile field induced by shrinkage of the film. The as-deposited film and the films annealed at below 400°C had poor mechanical strength. Conversely, the films annealed at over 500°C were strengthened by necking of the nanoparticles. The size of nanoparticles changed with increasing temperature. Annealing at 100–300°C caused the nanoparticles to shrink to approximately 30 nm. The nanoparticle diameters changed only slightly when annealed at 400–600°C because the annealing time was insufficient for changes to manifest. Annealing at 700–900°C caused the nanoparticle diameter to increase to approximately 50 nm because of sintering and coalescence of the nanoparticles. The diameter of the nanoparticles annealed at over 1,000°C became approximately 200 nm because of densification during the anatase-to-rutile transformation. The porosities of the films annealed at below 900°C were over 80%. However, the porosities of the films annealed at over 1,000°C decreased significantly due to densification.
2015 The Society of Chemical Engineers, Japan