Thermostable Nitrogen-Doped HTiNbO₅ Nanosheets with a High Visible-Light Photocatalytic Activity

Nitrogen-doped HTiNbO₅ nanosheets have been successfully synthesized by first exfoliating layered HTiNbO₅ in tetrabutylammonium hydroxide (TBAOH) to obtain HTiNbO₅ nanosheets and then heating the nanosheets with urea. The resulting samples were characterized by X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray (EDX), X-ray photoelectron spectroscopy (XPS), UV–vis spectroscopy and N₂ adsorption–desorption measurements. It was found that N-doping resulted in a much higher thermostability of the layered structure, intrinsic bandgap narrowing and a visible light response. The doped nitrogen atoms were mainly located in the interstitial sites of TiNbO₅⁻ lamellae and chemically bound to hydrogen ions. Compared with N-doped HTiNbO₅, N-doped HTiNbO₅ nanosheets had a much larger specific surface area and richer mesoporosity due to the rather loose and irregular arrangement of titanoniobate nanosheets. Both N-doped layered HTiNbO₅ and HTiNbO₅ nanosheets showed a very high visible-light photocatalytic activity for the degradation of rhodamine B (RhB) aqueous solution. Moreover, due to the considerably larger surface area, richer mesoporosity and stronger acidity, N-doped HTiNbO₅ nanosheets had an even higher activity than N-doped HTiNbO₅, although the latter had a stronger absorption in the visible region. The dye molecules were mainly degraded to aliphatic organic compounds and partially mineralized to CO₂ and/or CO, rather than being simply decolorized. The effect of photosensitization was insignificant and RhB was degraded mainly via the typical photocatalytic reaction routes. Two different reaction routes for the photodegradation of RhB under visible light irradiation over N-doped HTiNbO₅ nanosheets have been proposed. The present method can be extended to a large number of layered metal oxides that have the characteristics of intercalation and exfoliation, thus providing new opportunities for the fabrication of highly effective and potentially practical visible-light photocatalysts.