Highly crystalline niobium oxide converted from flux-grown K4Nb6O17 crystals

Abstract

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 Nb2O5nanotubes. 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 Nb2O5nanotubes 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 Nb2O5nanotubes 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 Nb2O5nanotubes (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 Nb2O5nanotubes exhibited high photocatalytic activity for the photodegradation of trichloroethylene. The grown layered K4Nb6O17 crystals and Nb2O5nanotubes 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.