Surfactant-free synthesis of ellipsoidal and spherical shaped TiO₂ nanoparticles and their comparative photocatalytic studies

The work presents surfactant-free synthesis of ellipsoidal and spherical TiO₂ nanoparticles (NPs) and their photocatalytic activities are compared with commercially available cubic shaped titania. The ellipsoidal TiO₂ NPs having average length of ∼21 nm and width ∼10 nm were synthesized via a facile solvothermal method. The spherical TiO₂ NPs were prepared by the solvent evaporation method having average diameter of ∼8 nm. We effectively control the shape and size of TiO₂ NPs under mild reaction conditions without using any surfactant. XRD patterns show diffraction peak at 25.3° corresponding to (1 0 1) crystalline plane, which confirmed the anatase phase of TiO₂ NPs. BET surface area of the ellipsoidal and spherical TiO₂ NPs are 2.22 and 1.85 times higher than cubic TiO₂, respectively. Kubelka-Munk plots demonstrate significant decrease in the band gap of ellipsoidal (3.0 eV) and spherical (3.17 eV) NPs as compared to cubic shaped TiO₂ (3.24 eV). The photocatalytic activities of these materials investigated by decolorization/removal of rhodamine-6G (R6G) and methylene blue (MB) under UV-visible light irradiation. In first 15 min of photo-degradation experiments, the results indicated 98, 86 and 71% decolorization and 90, 80 and 60% total organic carbon (TOC) removal of R6G by ellipsoidal, spherical, and cubic shaped TiO₂ NPs, respectively. Similar findings were obtained for the photo-degradation of MB solution. This confirms the superior photocatalytic characteristics of the ellipsoidal NPs among the reported TiO₂ shapes.