Acidity-Controlled Crystallization, Microstructural and Optical Behavior in Sol–Gel-Derived TiO2 Thin Films

Titanium dioxide thin films were synthesized by the sol–gel dip-coating technique under different acidic conditions (pH 5, 3, and 1) to investigate the influence of solution acidity on their structural, morphological, and optical properties. The deposited layers were annealed in air at 400°C for one hour and characterized by X-ray diffraction, Raman spectroscopy, scanning electron microscopy, atomic force microscopy, and ultraviolet–visible spectrophotometry. Structural analyses confirmed that all films crystallize in the anatase phase with a preferred orientation along the (101) plane, without any evidence of anatase–rutile transition. The diffraction and Raman peak intensities, as well as the crystallite size, increased with decreasing pH, indicating enhanced crystallinity under more acidic conditions. Surface imaging revealed that both the grain coarsening and surface roughness increased slightly with increasing sol acidity, in agreement with the observed structural evolution. All films exhibited high optical transparency, with average transmittance values exceeding 70% in the visible region, while the optical band gap slightly decreased with increasing acidity. These results demonstrate that controlled sol acidity effectively tailors the microstructure and optical response of sol–gel-derived titanium dioxide thin films for optoelectronic and photonic applications.