Enhanced photocatalytic activity of bismuth-doped TiO₂ nanotubes under direct sunlight irradiation for degradation of Rhodamine B dye

Bismuth-doped TiO₂ nanotubes (Bi-TNT) were successfully synthesized by combination of sol–gel and hydrothermal methods. The synthesized photocatalyst was efficiently used for degradation of rhodamine B (RhB) dye under direct sunlight irradiation. Subsequent characterization of synthesized photocatalysts was carried out using PXRD, SEM, TEM, EDX, FT-IR, Raman, N₂ adsorption, TPD-NH₃, UV–Vis DRS, XRF and ICP techniques. The surface area of the TiO₂ nanoparticles increased after tubular structure formation (TiO₂ nanoparticles—114.21 m²/g, TiO₂ nanotube—191.93 m²/g). The degradation studies revealed that initial rate of photocatalytic degradation of RhB dye using Bi-TNT was 5.56, 4.16, 1.30 and 2.38 times higher as compared to TNP, Bi-TNP, TNT and Degussa P-25 TiO₂ (P-25), respectively, under direct sunlight irradiation. The enhanced photocatalytic activity of Bi-TNT may be due to the increase in the surface area and Bi doping, which leads to effective separation of photogenerated carriers. The degradation was confirmed by chemical oxygen demand, total organic carbon and total inorganic carbon analysis of the degraded dye solutions. The probable degradation mechanism of RhB dye has also been proposed using liquid chromatography-mass spectrometry analysis of degraded samples.