A Scrolled Sheet Precursor Route to Niobium Oxide Nanotubes
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A Scrolled Sheet Precursor Route to Niobium Oxide Nanotubes Yoji Kobayashi, Hideo Hata, and Thomas E. Mallouk Department of Chemistry, The Pennsylvania State University, University Park, PA, 16802
ABSTRACT Potassium hexaniobate (K4Nb6O17) is one of the few relatively well-studied oxides which, upon exfoliation, rolls up into scrolls almost quantitatively with monodisperse length (~300 nm) and diameter (30 nm). The tubes have high surface area (250-300 m2/g) and a wall thickness of 2-3 nm. These H4Nb6O17 scrolls were converted to Nb2O5 via a thermal dehydration process, yielding high surface area (150-200 m2/g) Nb2O5 nanotubes. Despite extensive atomic rearrangement during dehydration at 400-450 ˚C, little sintering is occurs, and so the tubular morphology is retained. Attempts to conduct further reactions to obtain LiNbO3 and KNbO3 nanotubes from reaction with molten alkali salts failed to yield the intended nanotubular oxides. INTRODUCTION Layered materials that can be exfoliated chemically to make unilamellar colloids possess the interesting possibility of re-stacking by scrolling into tubular morphologies. This has already been demonstrated in oxides such as K4Nb6O17 [1, 2], and with non-oxide layered materials such as carbon[3] and the layered chalcogenides[4]. However, few studies exist on the transformation of these scroll structures, which are still layered in nature, to closed nanotubes, where the layer registry is lost. Crystallographically, the scroll-to-nanotube transformation is quite interesting. The precursor nanosheets are almost always two-dimensional single crystals, since they are produced by exfoliating a single grain of a layered crystal structure. If the subsequent scroll-to-nanotube conversion proceeds topochemically, then it may be possible to form a single, albeit somewhat strained, crystal nanotube. The nanotube’s crystal structure would be entirely dependent on the direction or helicity of scrolling in the precursor nanosheets. Whether the helicity can be controlled, and how this will affect the resulting structure and properties of the nanotube is an intriguing question. Single crystalline oxide nanotubes would be interesting since their usual properties such as high surface area, etc, can be coupled with their exposure of only one crystallographic plane to the outside. If there is residual strain within the nanotube, this too may introduce new properties. Several papers have been published on the autonomous scrolling of K1-xHxNb6O17 upon exfoliation by aqueous tetra(n-butyl)ammonium hydroxide (TBAOH) [1, 2, 5]. Because this is one of the few oxide materials that is known to scroll readily, we chose it to study the transformation into Nb2O5 nanotubes. Unfortunately, this transformation cannot happen topochemically, and the resulting nanotubes are not single crystalline. However, we still find that despite the extensive rearrangement of atoms, the nanotubular morphology is still preserved after conversion to Nb2O5. This is an encouraging result as we attempt to identify other
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