Variable Angle Spectroscopic Ellipsometry of thick TiO₂-P25 films: graded-layer modeling for depth-dependent optical analysis

Titanium dioxide (TiO₂) is a highly versatile material extensively used due to its non-toxic nature, chemical stability and high photocatalytic activity. These features make TiO₂ particularly promising for a wide range of applications, especially as a film coating on various substrates. TiO₂ is commercially available in many forms, including pure anatase and mixed-phase products like P25, which is a mixture of anatase (80%) and rutile (approximately ≤20%), ideal for photocatalysis.

In this study, TiO₂-P25 films on FTO substrates were synthesized using the sol-gel process and studied using Variable Angle Spectroscopy Ellipsometry (VASE) to determine their optical constants and thickness. The measurements were carried out at room temperature in the wavelength range (300–900) nm at incident angles varying from 55° to 70°. The resulting thicknesses were found to be ranging from 1000 nm to 10000 nm.

Scanning Electron Microscope (SEM) measurements confirmed the porous nature of the films, suggesting an inhomogeneous structure with depth-dependent compositional gradients. The novelty of this study lies in the structural complexity of these films that presented unique challenges for VASE measurements, which are typically straightforward for thinner films under a few hundred nanometers.

A graded layer model, which allowed for an accurate representation of the depth-dependent optical variations, was employed to model the properties of these TiO₂-P25 films. This advanced modeling approach provided deeper insights into the internal structure of the films, particularly how the graded structural characteristics impact the overall optical behavior. The bottom layers of the films typically exhibited a significant increase in both refractive index and extinction coefficient, indicating greater density and higher absorbance compared to the top layers. Understanding these depth-dependent variations is essential for optimizing the use of TiO₂-P25 films in technologies such as solar cells and optical devices, where precise control over material properties is critical.