Unique three-dimensional nano-/micro-textured surfaces consisting of highly crystalline Nb₂O₅ nanotubes

Three-dimensional (3-D) Nb₂O₅ nanotube layers with honeycomb structures were successfully synthesized using polystyrene (PS) microsphere templates and NbO_x nanocrystals. A densely packed monolayer of monodisperse PS microspheres was deposited onto a silica glass substrate using the LangmuirBlodgett film technique. NbO_x nanocrystals were prepared via proton-exchange and exfoliation processes of highly crystalline, layered K₄Nb₆O₁₇ crystals obtained by flux-growth. The 3-D crystal layers were fabricated by coating the pastes containing NbO_x nanosheets onto the PS microsphere templates, followed by a heating treatment in an electric furnace. During the heating process, the PS microsphere templates were thermally decomposed, and the NbO_x nanosheets were transformed into Nb₂O₅ nanotubes. As a result, transparent/colorless, nano-/micro-textured surfaces were formed on the silica glass substrates. The surface structure clearly varied depending on the nanosheet concentration of the pastes. The rough, solid surfaces modified with fluoroalkylsilane were found to be highly hydrophobic (water contact angle=148°). The 3-D nano-/micro-textures of the Nb ₂O₅-layered surfaces enhanced their hydrophobicity. The highly hydrophobic surfaces were converted to ultrahydrophilic surfaces by photocatalytic oxidation using Nb₂O₅ nanotube layers.