Electronic and optical properties of TiO 2 and its polymorphs by Z-scan method

TiO 2 is a material which has attracted considerable attention from the scientific community for its innumerable properties. TiO 2 is known to exist in nature in three different crystalline structures: rutile, anatase, and brookite. Anatase and rutile TiO 2 films have been widely characterized for their potential applications in solar cells, self-cleaning coatings, and photocatalysis. In the present report, the third-order nonlinear susceptibilities of TiO 2 and its polymorphs, anatase, and rutile, prepared by the sol—gel technique followed by heat treatment are investigated using the Z-scan technique at a wavelength of 532 nm with a duration of 7 ns. Imaginary and real values of χ (3) for amorphous, anatase, and rutile are also calculated and found to be 5 × 10 −19 m 2 /V 2 , 27 × 10 −19 m 2 /V 2 , 19 × 10 −19 m 2 /V 2 , respectively. It is found that the values of the optical constants of amorphous TiO 2 after heat treatment vary considerably. It is assumed that this could be due to the variation in the electronic structure of TiO 2 synchronous with the formation of its polymorphs, anatase, and rutile. Amorphous TiO 2 is marked by the localization of the tail states near the band gap, whereas its crystalline counterparts are characterized by completely delocalized tail states.