Visible light photodegradation of formaldehyde over TiO₂ nanotubes synthesized via electrochemical anodization of titanium foil

In this study, a series of TiO₂ nanotubes (NTs) were synthesized employing electrochemical anodization of titanium foil in an ionic liquid solution containing a mixture of glycerol and choline chloride, acting as electrolyte. The as-synthesized TiO₂ NTs were calcined at 350, 450, or 550^◦ C for a 2 h duration to investigate the influence of calcination temperature on NTs formation, morphology, surface properties, crystallinity, and subsequent photocatalytic activity for visible light photodegradation of gaseous formaldehyde (HCHO). Results showed that the calcination temperature has a significant effect on the structure and coverage of TiO₂ NTs on the surface. Freshly synthesized TiO₂ NTs showed better-ordered structure compared to calcined samples. There was significant pore rupture with increasing calcination temperature. The transformation from anatase to rutile phase appeared after calcination at 450^◦ C and the weight fraction of the rutile phase increased from 19% to 36% upon increasing the calcination temperature to 550^◦ C. The band gaps of the TiO₂ NTs were in the range from 2.80 to 2.74 eV, shifting the active region of the materials to visible light. The presence of mixed anatase–rutile TiO₂ phases in the sample calcined at 450^◦ C showed enhanced photoactivity, which was confirmed by the 21.56 mg·L^−1·g−1 removal of gaseous formaldehyde under 120 min of visible light irradiation and displayed enhanced quantum yield, θ_HCHO of 17%.