Highly crystalline niobium oxide converted from flux-grown K₄Nb₆O₁₇ crystals

Highly crystalline niobium oxide (Nb₂O₅) nanotubes without defects such as bent and node were successfully prepared by a two-step process. The first step entails making high quality, layered K₄Nb₆O₁₇ crystals as a precursor material. In this study, well-developed, highly crystalline, layered K₄Nb₆O₁₇ crystals were readily grown by the rapid cooling of a KCl flux at a holding temperature of 800 °C and a cooling rate of 300 °C h⁻¹. The grown layered crystals of K₄Nb₆O₁₇ were transparent-colorless and had a median diameter of 530 nm. They were plate-like with well-developed faces. The second step is to transform the layered K₄Nb₆O₁₇ crystals into highly crystalline Nb₂O₅ nanotubes. In order to make the nanotubes, an intercalation–exfoliation process using tetra(n-butyl)ammonium hydroxide (TBA⁺OH⁻) aqueous solution was carried out, and highly crystalline Nb₂O₅ nanotubes having a uniform diameter were successfully fabricated in this medhod. The crystallinity, uniformity and size (diameter and length) of nanotubes were significantly dependent on those of the precursor crystals. The flux-grown crystals, therefore, played a very important role in the nanotube fabrication. The average length and outer diameter were, respectively, about 100–500 nm and 15–25 nm. The photocatalytic properties of the layered K₄Nb₆O₁₇ crystals and the Nb₂O₅ nanotubes were basically almost the same, although their Brunauer–Emmett–Teller (BET) surface areas were quite different from each other. The BET surface area of the Nb₂O₅ nanotubes (108.71 m² g⁻¹) was ca 20 times larger than that of the layered K₄Nb₆O₁₇ crystals (5.14 m² g⁻¹). As compared with the flux-grown K₄Nb₆O₁₇ crystals, the Nb₂O₅ nanotubes exhibited high photocatalytic activity for the photodegradation of trichloroethylene. The grown layered K₄Nb₆O₁₇ crystals and Nb₂O₅ nanotubes were investigated thoroughly by means of field emission scanning electron microscopy, transmission electron microscopy, powder X-ray diffraction analysis, energy-dispersive X-ray spectrometry, BET surface area and pore size distribution analysis, and spectrophotometry.