Porous niobia spheres with large surface area: alcothermal synthesis and controlling of their composition and phase transition behaviour

Submicron-sized niobia (Nb₂O₅) porous spheres with a high specific surface area (300 m² g^-1) and nano concave-convex surfaces were synthesized via a rapid one-pot single-step alcothermal reaction. Prolonged reaction time or high reaction temperatures resulted in a morphology change of Nb₂O₅ from amorphous sphere to rod crystals with hexagonal crystal phase. A similar alcothermal reaction yielded TiO₂-Nb₂O₅ composite porous spheres, whose Ti : Nb molar ratio was controlled by changing the precursor solution component ratios. A simple thermal treatment of amorphous TiO₂-Nb₂O₅ porous spheres consisting of 1 : 2 (molar ratio) Ti : Nb at 600 °C for 2 h induced crystal phase transfer from amorphous to a monoclinic crystal phase of submicron-sized TiNb₂O₇ porous spheres with a specific surface area of 50 m² g^-1.