Phase formation of nano-TiO₂ particles during flame spraying with liquid feedstock

The nanostructured TiO₂ deposits can be synthesized through flame spraying with liquid feedstock based on our previous result. The deposit consisted mainly of anatase TiO₂ with a small fraction of rutile TiO₂. But the crystalline size of rutile TiO₂ was larger than that of anatase TiO₂. To clarify the phase formation during the deposition, the nanostructured TiO₂ deposits formed by flame spraying with liquid feedstock were annealed at different temperatures from 200 to 800°C. The microstructure of the as-deposited and annealed TiO ₂ deposits was characterized by transmission electron microscopy (TEM), X-ray diffraction (XRD) and thermal analysis. The transformation of the anatase crystalline to rutile crystalline in the nanostructured TiO₂ was also examined through investigating photocatalytic performance of the TiO₂ coatings to degrade the acetaldehyde under the ultraviolet illumination. XRD analysis showed that the significant transformation of anatase crystalline to rutile crystalline occured at the temperature higher than 600°C. It was found that the grain size of the rutile phase was larger than that of the anatase phase in the deposits annealed at different temperatures. The deposits annealed at temperature lower than 450°C presented a good photocatalytic activity. However, although the deposit annealed at temperature of 500°C contained about 95% anatase crystalline, it became photocatalytically inactive. Despite the anatase contents from 95% to 0% in the deposits annealed at temperatures higher than 500°C, those deposits presented no significant difference in the photocatalytic activity. On the other hand, thermal analysis suggested that the phase transformation of anatase to rutile occurred at 400 to 500°C. Based on the experimental findings, a model for the phase formation in flame-sprayed nano-TiO₂ deposit with liquid feedstock and transformation of the nanostructured TiO ₂ is proposed.