The Critical Effect of Niobium Doping on the Formation of Mesostructured TiO₂: Single-Crystalline Ordered Mesoporous Nb-TiO₂ and Plate-like Nb-TiO₂ with Ordered Mesoscale Dimples

Highly ordered mesoporous niobium-doped TiO₂ with a single-crystalline framework was prepared by using silica colloidal crystals with ca. 30 nm in diameter as templates. The preparation of colloidal crystals composed of uniform silica nanoparticles is a key to obtain highly ordered mesoporous Nb-doped TiO₂. The XPS measurements of Nb-doped TiO₂ showed the presence of Nb⁵⁺ and correspondingly Ti³⁺. With the increase in the amount of doped Nb, the crystalline phase of the product was converted from rutile into anatase, and the lattice spacings of both rutile and anatase phases increased. Surprisingly, the increase in the amount of Nb led to the formation of plate-like TiO₂ with dimpled surfaces on one side, which was directly replicated from the surfaces of the colloidal silica crystals.