Effect of Co-Dopants in TiO<Subscript>2</Subscript>–SiO<Subscript>2</Subscript> Thin films on the Formaldehyde Degradation

Titanium dioxide (TiO₂)–silicon dioxide (SiO₂) thin films were synthesized using the peroxo titanic acid approach (PTA) combined with the sol–gel method at low temperature around 100°C. The effects of type and amount of dopants of ferric (Fe³⁺) or thiourea (N-S) and co-dopants of Fe³⁺ and N-S on the films physicochemical properties and on the photocatalytic degradation of the methylene blue and formaldehyde under UV and visible light irradiation were investigated. Physicochemical properties of photocatalysts were characterized by X-ray diffraction, transmission electron microscopy, wavelength-dispersive X-ray fluorescence spectrometry, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and UV–Vis spectroscopy. The results showed that the TiO₂ crystal phases obtained from this method were exclusively anatase and the needle-like crystals have an average diameter of 10–25 nm. Compared with the single dopant of 1.0 wt.% Fe³⁺ or 0.125 wt.% N-S that was the optimal concentration for photocatalytic degradation of methylene blue and formaldehyde, the co-dopants of 0.125 wt.% N-S + 1.0 wt.% Fe³⁺ furthermore increased the degradation efficiency. Co-dopants of 0.125 wt.% N-S + 1.0 wt.% Fe³⁺ in TiO₂–SiO₂ films were considered to play synergistic roles in narrowing TiO₂ band gap resulting in the higher methylene blue and formaldehyde degradation efficiency. Since the crystal grain size of TiO₂–SiO₂ films synthesized by the PTA method is small, in the visible light region, the high transmittance was attainable to 80% with no-doped and dropped to 50–60% with doped thin films.