Single-crystalline nanoporous Nb 2 O 5 nanotubes

Single-crystalline nanoporous Nb2O5 nanotubes were fabricated by a two-step solution route, the growth of uniform single-crystalline Nb2O5 nanorods and the following ion-assisted selective dissolution along the [001] direction. Nb2O5 tubular structure was created by preferentially etching (001) crystallographic planes, which has a nearly homogeneous diameter and length. Dense nanopores with the diameters of several nanometers were created on the shell of Nb2O5 tubular structures, which can also retain the crystallographic orientation of Nb2O5 precursor nanorods. The present chemical etching strategy is versatile and can be extended to different-sized nanorod precursors. Furthermore, these as-obtained nanorod precursors and nanotube products can also be used as template for the fabrication of 1 D nanostructured niobates, such as LiNbO3, NaNbO3, and KNbO3. Introduction Nanomaterials, which have received a wide recognition for their sizeand shape-dependent properties, as well as their practical applications that might complement their bulk counterparts, have been extensively investigated since last century [1-8]. Among them, one-dimensional (1D) tubular nanostructures with hollow interiors have attracted tremendous research interest since the discovery of carbon nanotubes [1,9-14]. Most of the available single-crystalline nanotubes structurally possess layered architectures; the nanotubes with a non-layered structure have been mostly fabricated by employing porous membrane films, such as porous anodized alumina as template, which are either amorphous, polycrystalline, or only in ultrahigh vacuum [13,14]. The fabrication of single-crystalline semiconductor nanotubes is advantageous in many potential nanoscale electronics, optoelectronics, and biochemical-sensing applications [1]. Particularly, microscopically endowing these single-crystalline nanotubes with a nanoporous feature can further broaden their practical applications in catalysis, bioengineering, environments protection, sensors, and related areas due to their intrinsic pores and the high surfaceto-volume ratio. However, it still remains a big longterm challenge to develop those simple and low-cost synthetic technologies to particularly fabricate 1 D nanotubes for functional elements of future devices. Recently, the authors have rationally designed a general thermal oxidation strategy to synthesize polycrystalline porous metal oxide hollow architectures including 1 D nanotubes [15]. In this article, a solution-etching route for the fabrication of single-crystalline nanoporous Nb2O5 nanotubes with NH4F as an etching reagent, which can be easily transformed from Nb2O5 nanorod precursors is presented. As a typical n-type wide bandgap semiconductor (Eg = 3.4 eV), Nb2O5 is the most thermodynamically stable phase among various niobium oxides [16]. Nb2O5 has attracted great research interest due to its remarkable applications in gas sensors, catalysis, optical devices, and Li-ion batteries [9-11,16-21]. Even monoclinic Nb2O5 nanotube arrays were successfully synthesized through a phase transformation strategy accompanied by the void formation [10], which can only exist as non-porous polycrystalline nanotubes. In this study, a new chemical etching route for the synthesis of single-crystalline nanoporous Nb2O5 nanotubes, according to the preferential growth habit along [001] of Nb2O5 nanorods, is reported. The current chemical etching route can be applied to the fabrication of porous and tubular features in single-crystalline phase oxide materials. Experimental section