Insights into the annealing process of sol-gel TiO 2 films leading to anatase development: The interrelationship between microstructure and optical properties

•As the titania thin films are annealed at higher temperatures, larger anatase crystallites appear.•Optical behavior of sol-gel titania thin films is well described by a Tauc Lorenz oscillator model.•Microstuctural and optical properties are well correlated by using Wemple-DiDomenico equation•In the 400–850 °C annealing range , static refraction index increases and average bandgap decreases.•The free carrier concentration maximum is reached for an annealing temperature of 750 °C.