Highly crystalline niobium oxide (Nb 2 O 5 ) nanotubes without defects such as bent and node were successfully prepared by a two-step process. The first step entails making high quality, layered K 4 Nb 6 O 17 crystals as a precursor material. In this study, well-developed, highly crystalline, layered K 4 Nb 6 O 17 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 À1 . The grown layered crystals of K 4 Nb 6 O 17 were transparentcolorless and had a median diameter of 530 nm. They were plate-like with well-developed faces. The second step is to transform the layered K 4 Nb 6 O 17 crystals into highly crystalline Nb 2 O 5 nanotubes. In order to make the nanotubes, an intercalation-exfoliation process using tetra(n-butyl)ammonium hydroxide (TBA + OH À ) aqueous solution was carried out, and highly crystalline Nb 2 O 5 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 K 4 Nb 6 O 17 crystals and the Nb 2 O 5 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 Nb 2 O 5 nanotubes (108.71 m 2 g À1 ) was ca 20 times larger than that of the layered K 4 Nb 6 O 17 crystals (5.14 m 2 g À1 ). As compared with the flux-grown K 4 Nb 6 O 17 crystals, the Nb 2 O 5 nanotubes exhibited high photocatalytic activity for the photodegradation of trichloroethylene. The grown layered K 4 Nb 6 O 17 crystals and Nb 2 O 5 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.
Environmentally friendly, high-quality layered K 4 Nb 6 O 17 crystals were easily grown by two different flux methods involving supersaturation, that is, flux evaporation and cooling of the solution. To fabricate them, potassium chloride (KCl) was chosen as the flux. Transparent and colorless plate-like layered crystals of K 4 Nb 6 O 17 were prepared with well-developed {010} faces of the orthorhombic system. The crystal sizes obviously depended on the evaporation loss (evaporation method), the holding temperature, and the cooling rate (cooling method). Transmission electron microscopy (TEM)
Idiomorphic and prismatic K 2 Nb 8 O 21 crystals were successfully grown by the isothermal evaporation of KCl flux. On the basis of their aspect ratio, the crystals were divided into two morphological types, that is, whisker (needle) and prism. Growth was induced by heating mixtures of KCl and Nb 2 O 5 at 1100 C for 60 h followed by quenching in a furnace with the electric power turned off. The obtained K 2 Nb 8 O 21 crystals were colorless and transparent. Whiskers having an average size of approximately 160 Â 11:1 mm were grown from high-temperature solutions containing Nb at 1-30 mol %. The grown crystals had roundish or prismatic one-dimensional forms, and their surfaces were very smooth. Their generation and sizes were obviously dependent on the Nb concentration of the starting mixtures. The K 2 Nb 8 O 21 crystals exhibited high activity for dye adsorption and degradation. The degradation occurred via a photocatalytic process under ultraviolet light irradiation.
Flux. -Transparent and colorless plate-like title crystals are prepared from a KCl flux of K2CO3 and Nb2O5 (800-1100°C, 5-60 h) by either a rapid cooling or an evaporation method. The samples are characterized by XRD, SEM, TEM, and diffuse reflectance spectroscopy. The crystals exhibit good photocatalytic activity for the degradation of organic dyes such as methylene blue and methyl orange under UV light irradiation. -(TESHIMA*, K.; NIINA, Y.; YUBUTA, K.; SUZUKI, T.; ISHIZAWA, N.; SHISHIDO, T.; OISHI, S.; Eur.
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