Vacuum Ultraviolet Spectroscopic Analysis of Structural Phases in TiO2 Sol–Gel Thin Films

This study investigates the structural and electronic transitions of sol–gel derived titanium dioxide (TiO2) thin films using vacuum ultraviolet (VUV) spectroscopy, to elucidate the impact of annealing-induced phase evolution. As the annealing temperature increased from 400 °C to 800 °C, the films transitioned from amorphous to anatase, mixed anatase–rutile, and finally rutile phases. VUV spectroscopy revealed distinct absorption features: a high-energy σ → π* transition below 150 nm, associated with bonding to antibonding orbital excitations, and lower-energy absorption bands in the range 175–180 nm and near 280 nm, attributed to π → π* and t2g(π) → t*2g(π*) transitions, respectively. These spectral features highlight the material’s intrinsic electronic states and defect-related transitions. A slight redshift of the absorption band from 176 nm to 177 nm with annealing reflects bandgap narrowing, attributed to increased rutile content, crystallite growth, and defect-induced effects. Broadening and additional absorption features around 280 nm were attributed to oxygen vacancies and reduced titanium oxidation states (Ti3⁺), as corroborated by X-ray photoelectron spectroscopy (XPS). XPS further confirmed the presence of Ti3⁺ species and oxygen vacancies, providing complementary evidence of defect-mediated transitions observed in the VUV spectra. While complementary techniques such as X-ray diffraction (XRD) and Fourier Transform Infrared Spectroscopy (FTIR) confirmed phase transitions and the reduction of hydroxyl groups, respectively, VUV spectroscopy uniquely captured the dynamic interplay between structural defects, phase evolution, and optical properties. This study underscores the utility of VUV spectroscopy as a powerful tool for probing the electronic structure of TiO2 films, offering insights critical for tailoring their functional properties in advanced applications.