This study investigates the structural and optical properties of titanium dioxide (TiO₂) thin films synthesized via the sol–gel spin-coating technique at varying spin speeds. Unlike the conventional power-law relationship between film thickness and spin speed in sol–gel spin coating. These results exhibited a shifted exponential decay described by \(d\left(\omega \right)= {d}_{\infty }+{A}_{0}{e}^{-k\omega }\) . X-ray diffraction (XRD) analysis confirmed the formation of the anatase phase with a preferred orientation along the (101) plane. The crystallite size increased with spin speed, reaching a maximum of 20 nm at 4000 rpm before decreasing at higher speeds. Raman spectroscopy revealed characteristic anatase peaks at 144 cm⁻1 and 639 cm⁻1, corresponding to O–Ti–O vibrational modes, while Fourier-transform infrared (FT-IR) spectroscopy confirmed the presence of strong Ti–O bonding. Optical measurements showed high transmittance exceeding 90%, and the minimum optical band gap of 3.49 eV was recorded at 4000 rpm. Photoluminescence (PL) spectra exhibited emission peaks at 460 nm, 480 nm, and 525 nm, attributed to oxygen vacancies and Ti4⁺ ions, indicating the presence of defect-related energy states that enhance photocatalytic activity. The photocatalytic performance, assessed through the degradation of methylene blue, showed a peak degradation efficiency of 84% at 4000 rpm, followed by a decline at higher spin speeds. The enhanced degradation is attributed to the increased density of oxygen vacancies and active Ti4⁺ sites. These findings highlight the critical role of spin speed in tailoring the structural and functional properties of TiO₂ thin films for optimized photocatalytic applications.
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